Heterogeneous Catalysis and Fine Chemicals IV

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

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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

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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.

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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 con^ete 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 fi^om 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:

^eO,

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.

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

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

jL^om^

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 '^i ^ . .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

fK^OU

^

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.

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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 Pck^atalyst / 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.

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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 t5^e 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

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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

HaN^A^

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

<3.2

1

<5 <5

1 1

<7.5

<6.6

1

9.9 g of Deloxan (on a dry basis) per 100 g of benzamide ( quantity was not optimized), The level of detected Pd is dependent on the quantity of the sample assayed. The above levels would have been lower if larger samples were assayed.

44

Symmetrical Urea During the course of the piloting an intermediate in the synthetic sequence, 5% of symmetrical urea impurity was formed (see structure below). Various attempts to remove the impurity at the subsequent step were unsuccessful.

Symmetrical urea Fortunately, based on our earlier experience with sulfonic acid resins, it was decided to try a resin treatment of the hydrogenation filtrate. The strategy being that the acid resin would adsorb the synmietrical urea in preference to benzamidines. Several acid resins were tried, which included Dowex 5x8-200, Dowex 5x2-200, Aberlite 15, and Bio-Rad Chelex 100. The resin which was found to be the most effective was the Dowex 5x2-200. The resin treatment was successful in lowering the symmetrical urea to < 1%. New Hydrogenation Process In our continued effort to improve the hydrogenation/isolation step, a hydrogenation process that employed water as a solvent with 5 equivalents of glacial acetic acid at 60 psig hydrogen pressure and 50 °C (vs 60 °C) using a more active 5% Pd/C catalyst was developed. The new process had the following advantages over the acetic acid solvent method: 1. Shorter hydrogenation time, i.e., 2 h vs 16 h 2. Palladium levels < 1 ppm without Deloxan treatment

Isolation Process The isolation of the benzamidines as done originally by Searle's Discovery group was carried out by removal of the glacial acetic acid by vacuum distillation followed by the crystallization from a mixture of MeOH, EtOH and CH3CN. An intermediate process that was developed for this isolation involved the removal of the majority of the glacial acetic acid by vacuum distillation and recystallization of the residue from a mixture of glacial acetic acid and IPA in the ratio of 1 g of crude residue to 2 mL of glacial acetic acid to 12 mL of IPA. While this process was doable it was less than ideal. Subsequently, a new process employing water/acetone instead of acetic acid/IPA for the crystallization/isolation of benzamidines was developed. This hydrogenation was run more concentrated, i.e., 28% concentration (based on grams of hydroxybenzamidines/mls water) as compared to 10-20% used in the previous process. The optimum ratio for the crystallization i.e., addition of acetone to the aqueous filtrate was 1 g of benzamidines to 3.8 mL of water to 9 mL of acetone. The advantages of this change were:

45 3. 4.

1. Higher isolation throughput. 2. Eliminated water distillation.

Reduced processing time. Higher purity and yield.

Table III reports the results of yield and purity profile of the new hydrogenation process. The results of the isolated benzamidine clearly demonstrate the advantages of the new hydrogenation/isolation process as shown by the higher yields and purities. An additional problem has been with the aromatic amide (see structure below) that is produced in an earlier step. Table III clearly indicates an almost total elimination of aromatic amide in benzamidine when the water/acetone method is used.

Aromatic Amide

Table III

Typical hydrogenation results[a] Analysis after Hydrogenation (crude) HPLC area%

Lot

Benzamidine

5

60

50

5

60

50

5% Pd/C 10 5% Pd/C 10

(%)

1 ^ 1 ^ a

Hydrogenation pressure (psig)

Temp. (°C)

Eq. of Acetic Acid

Catalyst (Loading)

Analysis after crystallization HPLC area%

Aromatic Amide

Benzamidine

Aromatic Amide

98.53

1.14

99.71

0.06

86.2 1

98.54

1.18

99.71

0.09

84.5 II

Yield

(%)

The hydroxybenzamidine was 97.59% (area) pure by HPLC and contained 1.43% aromatic amide.

Experimental Typical preparation of the Benzamidines Procedure: A 250 ml Parr Shaker bottle was charged with 0.05 moles of hydroxybenzamidine, 4.0 g of 5% Pd/C (50% water wet) and 60 ml of deionized water containing 0.25 moles of acetic acid. The Parr shaker was purged with nitrogen and hydrogen. The reaction was shaken and hydrogenated under a constant hydrogen pressure of 60 psig and 50 °C for 4 h. The reaction was initially a slurry but as hydrogenation proceeded the product dissolved in the water. Upon completion, the hydrogen was vented and a nitrogen atmosphere was applied to the reaction vessel. A sample was then taken for HPLC analysis to confirm reaction completion. The catalyst was then removed by hot filtration (50 °C) through a layer of powdered cellulose and the catalyst was washed with 10 ml of hot deionized water. The filtrate was placed in 500 ml 3neck RB flask fitted with a thermometer, addition funnel and reflux condenser and heated to 50-

46 55 °C. Then 160 ml of room temperature acetone was added over 30 min while maintaining 5055 °C. The suspension was cooled to 5 °C after acetone addition and maintained at 5 °C for 1 h. The resulting crystallized product was collected by filtration and washed with 30 ml of acetone. The product was dried at 50 °C in a vacuum oven (-80 kpa) for 24 h to give the corresponding benzamidine in yields of 80-85% with a purity of >99% by HPLC. Conclusion An improved procedure for the hydrogenation of hydroxybenzamidines to benzamidines and their isolation was developed. The advantages are: 1. 2. 3. 4. 5.

Shorter hydrogenation time. Elimination of colloidal palladium contamination. Higher hydrogenation and isolation throughput. Reduced processing time. Higher purity and yield.

The improvements addressed above were instrumental in making large scale hydrogenation/isolation feasible and economical. Acknowledgements The 4% Pd/C and 5% Pt/C catalysts were obtained from Degussa and the 5% Pd/C catalyst was obtained from Johnson Matthey. The authors would like to thank James Groce and Richard Frost for performing the HPLC analyses. References 1.) P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, New York, 1967, p. 18. 2.) B. D. Judkins, D. G. Allen, T. A. Cook, B. Evans, and T. E. Sardharwala, Synthetic Communications, 26, 4351-4367 (1996). 3.) C. D. Eldred, B. Evans, S. Hindley, B. D. Judkins, H. A. Kelly, J. Kitchin, P. Lumley, B. Porter, B. C. Ross, K. J. Smith, N. R. Taylor, and J. R. Wheatcraft, /. Med. Chem., 37, 3882 (1994). 4.) A. J. Bird, and D. T. Thompson, Catalysis in Organic Reactions (W. H. Jones ed.). Academic Press, New York, 1980, pp. 61-106.

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

47

Catalytic hydrogenation reactors for the fine chemicals industries. Their design and operation K.R. Westerterp", E.J. Molga^ and K.B. van Gelder' ^) Chemical Reaction Engineering Laboratories, Faculty of Chemical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netheriands ^) Chemical and Process Engineering Department, Warsaw Technical University, ul. Warynskiego 1, 00-645 Warsaw, Poland ') Dow Benelux N. V., PDCD, P.O. Box 48, 4530 AA Terneuzen, the Netheriands ABSTRACT Design and operation of reactors for catalytic hydrogenation in the fine chemical industries are discussed. The requirements for a good multiproduct catalytic hydrogenation unit as well as the choice of the reactor type are considered. Packed bed bubble column reactors operated without hydrogen recycle are recommended as the best choice to obtain a flexible reactor with good selectivities. The results of an experimental study of the catalytic hydrogenation of 2,4-dinitrotoluene (DNT) in a miniplant installation are presented to prove that the maximum yield in such a reactor can be achieved without a hydrogen recycle and with a hydrogen supply somewhat higher than the stoichiometric amount. Some characteristic properties of the reactor system and the influence of the reactor pressure and the hydrogen supply ratio are elucidated. Introduction In fine chemicals industries the so-called Bechamps reaction was widely spread for hydrogenation reactions. Here hydrogen is produced by the interaction of Fe or Zn powder and hydrochloric acid, producing very active hydrogen. This reaction is executed in the batch mode in the liquid phase. The reaction rate can be controlled by the addition rate of the metal powder and the reaction occurs at low temperatures. These properties made it the most important hydrogenation tool for fine chemicals. Regretfully large amounts of waste materials are being coproduced after neutralization, like iron hydroxide sludges and neutralization salts, both often contaminated with organic chemicals. The amounts of waste materials are often 5 to 20 times larger than the amount of desired chemicals produced. This waste problem demanded for environmental protection measures and therefore catalytic hydrogenation came under consideration also in the fine chemicals industries. Catalytic hydrogenation, of course, for many years has been applied in bulk chemical processes in specially designed reactors. For fine chemicals a reactor installation is normally used for many different chemicals and production series usually last only from a few days to a few weeks, so that in the same installation often 10 to 30 different chemical reactions are executed in the course of a year. Production quantities are so small, that the construction of a unit dedicated to a single product is economically not feasible. So multiproduct use is a conditio sine qua non, also for a catalytic hydrogenation unit for fine chemicals. In our laboratories therefore a large experimental and theoretical study has been made to determine the most economical method to change from Bechamps to catalytic hydrogenation reactors [1]. To this end a number of possible reactor types has been tested and here, we will report on our initial considerations to come to a selection, discuss the arguments pro and contra for certain solutions and the final choice made. We will also report on experimental work to corroborate our selection, the continuously operated, packed bubble column reactorfilledwith a packed catalyst bed and with hydrogen feed rates, slightly higher than the stoichiometrically required amount.

48

Requirements for a good multiproduct catalytic hydrogenation reactor for fine chemicals For catalytic hydrogenations we usually are concerned with three phases: gaseous hydrogen, an, often dissolved, component to be hydrogenated in the liquid phase and a heterogeneous solid catalyst. Homogeneous catalysis is not very widespread. A number of good three phase reactors is available in process industries. Therefore we will restrict ourselves to the classical three phase reactors which already have proven their value in bulk chemicals processes. However, for fine chemicals applications a number o^special requirements have to be met, which will be discussed in detail below. Catalyst, suitable for many reactions In a multiproduct unit a multiproduct catalyst should be applied. Quick changing of catalyst is difficult. Slurry catalysts, of course, can be washed out and filtered off, but also here their removal is time consuming and specially for expensive catalysts,filtrationwith a high efficiency is difficult to achieve and slow. Filtration can be avoided if a stationary bed of catalyst particles is used, but also the change of afixedbed catalyst is time consuming. Therefore, it is preferred to keep the catalyst in the reactor system. In order to avoid the replacement by each time different catalysts, peferably one catalyst must be used for all different hydrogenation reactions to be executed. Ni, Pt and Pd catalysts are all rather universal and the choice of the most suitable catalyst depends on the production programme. We tested all three and selected Pd. Ni catalysts are cheaper, but for our applications demanded for higher temperatures and hydrogen pressures to achieve the same conversion rates as with Pd catalysts. In the selection of a catalyst also possible deactivation must be considered and methods be developed to regenerate the catalyst in an economic way. A universal method of heat removal Hydrogenation reactions usually are very exothermic. Therefore much cooling area must be installed in the reactor per unit of volume and also agitation levels must be high in order to obtain high heat transfer coefficients. This demands for extensive and complicated cooling coils or externally located circulating coolers for batch equipment. Further mixing and/or recirculation may influence the selectivity in a process. As the method to remove the reaction heat under all circumstances and at all rates, we opted for heat withdrawal by evaporation of a solvent. Many reactions are executed in dissolution or the reactants are volatile at reaction conditions. A wide range of temperatures and solvents must be handled For many different hydrogenation reactions also various solvents with different properties have to be used, specially with respect to vapour pressures and boiling points. Each reaction requires its own temperature range and also this demands for a greatflexibilityof a multiproduct reactor. Good selectivities Many hydrogenations are carried on up to completion, but as soon as only partial hydrogenation of a molecule is required, also selectivity plays a major role. For multicomponent reactions of the consecutive type it is known that the batch or plug flow reactor gives higher yields and selectivities than mixed reactors, so mixing should be suppressed. This is often more complicated if competing reactions are of different reaction orders, but in general mixing should be avoided as much as possible. Ease of operation also without much information on kinetics The determination of adequate and reliable kinetic data is laborious and time consuming. To test three phase catalytic reactors we used as a test reaction the hydrogenation of 2.4. dinitrotoluene to intermediate andfinalproducts. Langmuir-Hinselwood expressions were developed for the five main reactions and it took us several man years to obtain accurate rate equations valid over a wide range of temperatures and pressures [2,3,4]. Such a large input of manhours cannot be afforded in fine chemicals industries, where often only a few tonnes of product are made in a short period of time. Therefore units must be well operated with the highest possible selectivities and yields without detailed information on kinetics. Ease of control For a wide range of process conditions the reactor must be reliably controlled and kept at the desired conditions under all circumstances avoiding runaways or other instabilities. The control

49 system must be easy to understand and to handle, so that operators can dedicate their attention above all to the execution of the ever changing recipes for the production of desired chemicals. Process economics It is evident that the process economics must be sound. This not only demands for much detailed attention to equipment selection and mechanical design, but also to fiirther process development and easy possibilities for modifications. Hydrogen excess and recirculation is costly, the gas carries away evaporated liquid, which has to be condensed in condensors with low heat transfer coefficients due to the presence of gas; after that the gas must be recompressed and recycled to the reactor. Compression is an expensive unite operation and, moreover, in recirculation systems a purge is required to prevent accumulation of inert components: this inevitably leads to hydrogen losses. Short switch over times must be feasible to reduce losses to off*-spec products to a minimum. For the same reason also rapid startup and shut down methods must be developed. This also leads to long on-stream-times and a good productive use of the plant. Also the catalyst life is extremely important. The change of catalyst batches should be avoided as much as possible. The replacement of a fixed catalyst bed is cumbersome and involves a loss of possible production time. So some kind of rapid in situ regeneration is to be preferred. Choice of reactor type and batch or continuous mode of operation In fine chemicals industries the batch reactor is used almost exclusively. For catalytic hydrogenations the batch reactor has a number of inconveniences in case a slurry catalyst is used. The complicated cooling coils, dead spaces behind baffles, etc. are difficult to clean if the catalyst has to be removed from the vessel. After each batch the vessel has to be emptied, which also implies that the catalyst falls dry. It is known that specially in this period all reactants and products, still contained in the pores of the catalyst, rapidly may deteriorate via unwanted side reactions and so deactivate the catalyst. Usually deactivation starts as soon as the catalyst is no more protected by the solvent. As a production series of one product consists of a number of batches, the exposure of the catalyst to deactivation conditions is frequent. Despite the experience with batch reactors it may be worthwhile to operate continuous reactors also for fine chemicals. Continuously operated reactors only demand for one start-up and one shut-down during the production series for one product. This increases the operating time efficiency and prevents the deactivation of dry catalysts; this implies that the reactor volume can be much smaller than for batch reactors. As to the reactor type for three phase systems an agitated slurry tank reactor [5,6] is not advisable, because of the good mixing characteristics. Specially for consecutive reaction systems the yields to desired products and selectivities will be considerably lower than in plug flow type reactor. The cocurrent down flow trickle flow reactor [7,8,9,10] at high flow rates approaches best the plug flow character. Regretfially, for a good wetting of the catalyst particles, liquid velocities must be high so that residence times usually are limited and not higher than a few minutes to a maximum of about a quarter of an hour. Many reactions demand for much longer reaction times. For that reason the packed bubble column reactor remains; this reactor has been tested by us and its many features were amply discussed [11,12,13]. In this reactor the liquid and the gas flow cocurrently upward through the packed catalyst bed. It exhibits axial dispersion in the liquid phase [13], but by choosing a high and slender column the influence of the axial dispersion can be largely suppressed: under normal operating conditions and with not too large catalyst particles several "reaction units" per meter column height can be obtained. By changing the liquid flow rate a any residence time for the liquid phase can be achieved, a residence time of several hours is not a problem. This implies that also selectivities approaching those in a plug flow reactor can be expected in this type of reactor, as is desired. For the case of heat removal in the reactor by evaporation of the reactant or a solvent and condensing the vapours a cooler-condensor in the exit vapour line, a wide range of operating temperatures can be chosen by setting the reactor pressure. For these reasons we feel the packed bubble column reactorfilledwith a stationary solid catalyst bed is the best suited reactor with a very high versatility for application in the fine chemicals industries.

50 Operation of the continuous packed bubble column reactor A number of the previously mentioned points will now be further elaborated. For further information see o.a. [14]. Start-up and shut-down The solvent, which is heated up via a feed preheater to a temperature at which the reaction starts, can be put in the reactor and the reactor completelyfilledwith liquid. After that the liquid reactant feed and the hydrogen gas are supplied to the reactor in the desired ratio. The reaction starts, the reaction mixture heats up and at reaching a temperature somewhat below the boiling point of the solvent at the set reactor pressure, the evaporation will become so high that a stable operating point is reached. Solvent vapours are condensed and returned to the reactor, the liquid phase leaves the top of the catalyst bed via an overflow. For stopping the reaction the catalyst bed is washed out with pure solvent and the catalyst is kept covered with liquid. Switch over to a new product If for the new product a different solvent is used, first the old solvent is washed out over the top of the catalyst bed by the new solvent and the solvent mixture sent to the solvent recovery system of the plant. After that the same start-up procedure as before is followed and the reactor pressure adjusted if necessary. The catalyst is kept wet under liquid all the time in order to prevent decomposition reactions or coke formation at the catalyst surface if exposed to gas and/or air. Catalyst life and reactivation We observed that for our reactions the catalyst was prone to deactivation if the catalyst was exposed to gas or air. Via polymerisation and condensation reactions tar and coke was formed. We could reactivate our catalyst bed by burning off of cokes and tar [11] with air diluted with steam or nitrogen. The bed had to be preheated to a temperature where the tar and coke combustion started. This empirically determined temperature in our case was 130°C. In [15] we have described how the oxygen content in the regeneration gas can be determined and where thermocouples in the catalyst bed must be placed to be sure that always at least one thermocouple is located in the zone with the highest temperature in the bed, where the combustion takes place. Ease of control Besides rate controllers for the gas and liquid feed flows and liquid level controllers, the most important instrument is the reactor pressure controller. By setting the pressure also the maximum temperature is set. The liquid will evaporate at a temperature somewhat below the boiling temperature of the solvent or reactants at the set pressure. At the boiling temperature itself no hydrogen can dissolve anymore in the liquid and so the reaction would automatically stop. This also insures that a runaway never can occur; the temperature never can surpass the solvent boiling temperature. By adjusting the pressure we also automatically adjust the temperature level in the reactor. A large temperature range can be covered depending on the vapour pressure curve of the liquid reaction phase and on the maximum allowable pressure of the reactor vessel. Hydrogen excess and gas recirculation In [1] and [12] on the basis of the analysis of a mathematical model describing the packed bubble column reactor for catalytic hydrogenation applications, it was suggested - in order to produce an intermediate product in a consecutive reactions system - to supply a hydrogen stream slightly larger than stoichiometrically required amount to hydrogenate the feedstock to the desired intermediate product. It was suggested that in this way also automatically the highest yield was obtained. The reaction stops as soon as almost all hydrogen supplied has been consumed. This implicates that the feed rate is so low that the reaction stops already before the top of the catalyst bed is reached. Or in other words the catalyst bed is larger than required for the desired conversion at the feed rate set. The hydrogen stream in such a case must be diluted e.g. with nitrogen in order to carry the solvent vapours to the condensor. The same holds, of course, also for a complete hydrogenation. This mode of operating a packed bubble column reactor has two great advantages.

51 The first one is that we do not use an excess of hydrogen. This implies that we do not need to recirculate the excess of hydrogen and that the very costly hydrogen recycle compressor can be avoided, resulting in important investment and energy savings. The second advantage is that we do not need to know the reaction kinetics, we only need to know the stoichiometry. Of course, in preliminary laboratory tests a suitable solvent has to be found and also a minimum temperature level has to be determined at which sufficiently high reaction rates are obtained. In the upper part of the catalyst bed, where the reaction has stopped, no temperature increase takes place anymore. In order to increase the capacity of the reactor the gas and liquid flow rates can be raised simultaneously as long as the last two thermocouples in the upper part of the reactor indicate equal temperatures, that is the reaction is complete before the end of the catalyst bed. These suggestions based on a study of the mathematical model have been verified experimentally. Therefore an experimental program as explained in the following part has been executed. We present results of the hydrogenation of 2,4-DNT obtained in a miniplant packed bed bubble column reactor and in methanol as the evaporating solvent. The corresponding reaction network consisting of parallel and consecutive pathways is shown in Fig. 1. The aim of our work is to determine the best method of operating the reactor to obtain the maximal yield of an intermediate product.

Fig. 1. Reaction scheme for the catalytic hydrogenation of 2,4 - DNT

Experimental installation and procedure The flow scheme of the miniplant installation is shown in Fig. 2. The reactor consists of a cylindrical stainless steel shell H and an internal tube 12 which contains the catalyst bed.

Fig. 2. Flowsheet of the miniplant installation. PI - pressure indicator, TI - temperature indicator, MFC - mass flow controller.

52 Both gas and liquid enter the reactor at the bottom and flow cocurrently to the top of the bed. The gas leaves the reactor at the top, whereas the liquid flows over the edge of the inner tube and downward in the annulus between the shell and the inner tube and then leaves the reactor at the bottom. A heating jacket is installed around the reactor shell to shorten the time required to reach steady state conditions and to maintain an almost adiabatic operation of the reactor. For further details and drawings see [11]. The liquid feed is stored in a jacketed supply vessel 13 equipped with an U-shaped agitator. This vessel is heated by means of hot water from a thermostat. Before starting an experiment oxygen is stripped out of the solution by slow bubbling through of nitrogen. The liquid is fed to the reactor by means of Lewa membrane pump 14 and the flow is controlled by changing the stroke length. By means of two burettes 22 in the liquid line the pump can be calibrated. The liquid reaction product is collected in a buffer vessel H . The gas feed, being either hydrogen or a hydrogen-nitrogen mixture, is taken from high pressure feed lines. Magnetic valves are installed in both feed lines and the flow rates of hydrogen and nitrogen are controlled by Brooks 581 Mass Flow Controllers 16. Gas and liquid are mixed before entering the reactor bed in a gas-liquid chamber 17 at the inlet of the reactor. The gas leaving the reactor flows through a condenser 18, where evaporated solvent is condensed, and then to the gas-liquid separator 19. The condensed solvent is collected in a separate buffer vessel 20 and returned to the reactor if necessary, the gas leaves the system through a backpressure valve controlling the pressure in the system. The volumetric flow of the effluent gas is measured with a gas meter 21. A system of sampling tubes and thermocouple tubes, inserted directly into the bed, enables us to determine the temperature as well as the liquid phase composition at four locations along the reactor. The sampling tubes are equiped with magnetic valves installed outside the reactor. The reactor system is automated with a computer HP 9816 and a data acquisition and control unit HP 3497A of Hewlett Packard. The programme a.o. checks if all variables are within preset safety limits. In case of emergency the programme switches the gas feed from H2 to N2 and stops the liquid feed. The key variables are the temperatures in the catalyst bed, the gas and the liquid inlet temperatures as well as the gas and the liquid outlet temperatures andfrirtherthe reactor pressure, the inlet and the outlet gas flow rates and the liquid feed flow rate. They are temporarily stored in the computer memory and later transferred on floppy disk. The temperatures at all other locations are monitored and recorded with a Philips PM 823 7A multipoint data recorder. The catalyst used is a shell catalyst, contains 0.08 % wt. of Pd on 4*4 mm porous cylindrical alumina pellets and is manufactured by Girdler Slid Chemie A.G.; the penetration depth of palladium in the cylindrical support is estimated to be about 100 mm. Methanol is used as an evaporating solvent. A solution of DNT usually in the range of 0.11 to 0.20 kmol/m^ in methanol is used as a feed. About one hour after reaching steady state conditions liquid samples are collected and immediately analysed with a Varian 3300 gas chromatograph using a 6ft*8" Tenax 60/80 mesh column and a HP 3 3 92A integrator. The entire sampling and analyzing sequence at all detection points takes between two and three hours, during which period all operating parameters are kept constant. The whole experiment, including warming up of the reactor, stabilizing conditions, sampling procedure and cooling down of the reactor, takes usually about 7-8 hours. Temperatures measured in the reactor bed during the entire experiment are plotted in Fig.3. We can see that steady state is reached after 1.5-2 hours from the start of the process, which is equivalent to 4-5 times the liquid residence time. At a hydrogen supply ratio - defined as molar hydrogen feed rate over the stoichiometric molar rate needed to convert all DNT to DAT - of OCDAT < 1 equilibration of the reactor took usually as much as 2 to 3 hours.

53

100-

o-o^c

80 H

- + - T 3

-%-

T

M-o 100

200

300

t [inin]

Fig. 3 Measured reactor bed temperatures. Locations of thermocouples: 3 - points (+), 2 - points (*), 1 - points (o) - see Fig. 2. Experimental results To describe the reactor performance the following aspects have been taken into account: 1. The pressure drop is small in comparison to the reactor pressure: the reactor is practically isobaric. 2. Because of the heating jacket temperature following the reactor temperature we can also assume that the reactor is nearly adiabatic. 3. The temperatures of the gas and the liquid phases are equal to each other over the crosssection of the reactor bed. 4. The flow regime in our reactor was checked with criteria given in literature. The gas mass velocity has never been higher than 2,5 10"^ kg/m^ s and the mass flow ratio \\}X\J{jQViQ) varied between 10 and 35, so according to the flow map by Turpin and Huntington [16] all our experiments have been performed in the bubble flow regime. With a minimum gas velocity of UG,min = 0.3 10" m/s according to the criterion of Saada [17] two-phase flow must take place in all voids for all operating conditions. So the available catalyst area is fully wetted and utilized. The most important experimental data are given in Figures 3, 4 and 5 and further in [18]. The hydrogen conversion has been defined as the ratio of the amount of hydrogen actually consumed over the amount of hydrogen consumed for a complete conversion of DNT to DAT. According to this definition experimental values of the hydrogen conversion at the j-th measurement point have been determined as follows: J

1

^. -1

^

-(Xc

XD)

+

XE

(1)

where Xi are relative molar fractions of the i-th reactant in the liquid phase, respectively, defined as follows:

54

X, = ^

(2)

The temperature of samples taken at different locations in the packed bed was sometimes as high as 393 K, so a certain amount of methanol vapour wasflashedoff and consequently the measured molar concentrations of the organic compounds were higher than in the reactor. On the basis of Eq. 1 and 2 we still obtain the correct hydrogen conversions, automatically compensated for the evaporation loss during sampling. The overall hydrogen conversion Ztop,cai as measured at the top of the catalyst bed can also be calculated according to the equation: - ^H,out ' top.co/

6 (lu,

(3)

CAO

where fno and f^out are the inlet and the outlet molar hydrogen flow rates and qLo the inlet flow rate of the liquid. The hydrogen supply ratio aoAi is defined as: CCDAT -

-r-——

(4)

The hydrogen conversions determinedfi-omconcentration measurements, Ztop (Eq. 1) compared well to those calculated with Eq. 3, Ztop,cai; only for a few experiments the differences exceeded 10%. The amount of gas measured in the outlet mainly consisted of nitrogen, non-converted hydrogen and not condensed methanol vapours. The amount of methanol vapours condensed has been measured and varied between 1.3 to 127 * lO'^NmVs. Deactivation of the catalyst Initially the deactivation of the catalyst is very fast. The decrease in the catalyst activity becomes smaller adfter several weeks. As catalyst deactivation may be caused by an irreversible adsorption of reaction products we washed the catalyst bed after every experiment with methanol with a volume of five times the liquid hold-up of the reactor; in between two experiments we kept the reactorfilledwith methanol and under nitrogen. We observed that the activity can be stabilized in this way and that deactivation does not occur any more. Based onfiartherstudies performed to investigate the influence of the hydrogen supply ratio and the reactor pressure under conditions where aDAi < 1, we are sure that the catalyst activity is almost constant and reproducible, provided the catalyst is kept wetted al the time. Reactor operation Observing the dependence of the total hydrogen conversion and the reactor pressure in Figure 4 only at P = 0.8 MPa almost all supplied hydrogen is consumed. At lower hydrogen partial pressure, the reaction rate significantly decreases, so not all supplied hydrogen can be converted in the reactor: the total hydrogen conversion Ztop is smaller than the hydrogen supply ratio aoAT- For a hydrogen supply ratio of aoAT < 1 and at long enough residence times we can expect the total hydrogen conversion Ztop equals aoAi, because all supplied hydrogen is converted.

55

',.,,' I

Fig. 4 Hydrogen conversions as a function of the reactor pressure P. Fixed hydrogen supply ratio aoAT = 0.62.

Fig. 5 Yield of the intermediate C at the reactor outlet as a function of the hydrogen supply ratio aDAx- Fixed reactor pressure P = 0.81 MPa.

Yield of the intermediate product C The optimization of the production of intermediates without an expensive recycle of recompressed hydrogen, can be achieved at a hydrogen supply ratio aDAx < 1. The yield of the intermediate product C, he, can be determined directlyfromthe concentration measurements because he = Xc. For economical reasons it is also important that all supplied hydrogen is converted. The influence of the hydrogen supply ratio on the yield towards the intermediate compound C at P = 0.81 MPa is shown in Fig 5. A distinct maximum in the yield of component C is noticed in thisfigureat a fixed value of the hydrogen supply ratio. At aoAx = 0.52 and at P = 0.81 MPa the highest value of he = 0.67 is obtained, also almost dl supplied hydrogen is converted at these conditions. In Fig. 5 we have indicated with a full line the yields as calculated with the model discussed in [12] and with the kinetics given in [4]; with a band has been indicated the range, where the yield deviates less than five per cent of the maximum yield. It demonstrates that by setting a stoichiometric hydrogen supply ratio of a little above 0.50 the optimum yield is already approached almost correctly. For consecutive reactions, of course, it is impossible to know what the correct ratio is without knowing the kinetics. In our case of the hydrogenation of 2.4 DNT we only can be sure it must be beyond a = 0.50 because in the maximum also already part of the C has been converted into E. So in practice the operator would set the ratio e.g. at 0.55 and then empirically increase it slowly step by step till the maximum yield eventually has been found. In our case with ratios between 0.49 and 0.59 we loose less than 5% of our raw material, which loss must be acceptable in the initial period of a run. We also have seen that a certain minimum pressure is required. Also this has to be determined step by step, in our case pressures of 0.2 and 0.4 MPa, see Fig. 4, are still somewhat too low; at 0.8 MPa the reactor runs fine. Summary and conclusions Experimental results obtained in a miniplant installation with a packed bed bubble column reactor for the catalytic hydrogenation of 2,4- DNT with an evaporating solvent have been presented and discussed. A deactivation of the catalyst has been noticed and the method to stabilize the catalyst activity by a special treatment of the bed after each experimental run has been developed. The influence of operating parameters as the hydrogen supply ratio and the pressure of the reactor has been discussed. The method, as suggested by Westerterp et al [1] and by Van Gelder et al [12], to obtain the maximal yield of an intermediate product without hydrogen recycle has been experimentally evaluated. The packed bubble column reactor can be run with a correct amount of hydrogen supplied to achieve the maximum yield of intermediate product at a certain feed rate of reactant in the liquid phase. A small amount of the inert gas - in our case nitrogen - must be added to the reactor to assure a stable performance of the reactor in case almost all supplied hydrogen is consumed. The reactor pressure

56 must be high enough to maintain a sufficiently high hydrogen consumption rate and assure almost complete conversion. Both the hydrogen supply ratio as well as the required reactor pressure can be determined experimentally without studying reaction kinetics. For the investigated system and after stabilizing the catalyst activity good conditions were aoAi = 0.53 and P = 0.81 MPa at which the maximal yield of the intermediate product C is obtained without losses of not consumed hydrogen. This proves the suggestion of Westerterp, Van Gelder et al [1,12] that the maximum yield of a desired product can be obtained Avith a supply of hydrogen slightly higher than stoichiometric, provided the reactor pressure is set high enough for sufficiently high reaction rates to convert the hydrogen supplied. We also discussed the choice of the reactor. A batch reactor has a much larger volume per unit of reaction product and tank like pressure vessels are much more expensive than cylindrical vessels. This combined with the difficulties of handling catalyst slurries and above all of preventing losses of the often rather expensive catalysts made us consider continuously operating reactors with fixed catalyst beds too. We eventually chose for the packed bubble column as a well suited reactor. We also should mention some drawbacks of the proposed reactor. Axial mixing is not fiilly suppressed, so additional bed height is required to compensate for it. Further additional bed height is required for the production of intermediate products in a consecutive reaction system to ensure the hydrogen is almost completely consumed. Further we have to realize that the evaporation of solvent or reactant reduces the partial pressure of hydrogen, above all in the upper part of the reactor. Also this aspect demands for additional catalyst. As a consequence the productivity of the reactor per unit of catalyst bed will be only afi-actionof a reactor with cooling coils or catalyst in wall-cooled, small diameter tubes without evaporation. However, at the expense of productivity the reactor has a simple construction and does not need a hydrogen recycle compressor. We therefore expect it to be also a very cheap if not the cheapest reactor. Acknowledgement. This work has been supported by the Netherlands Foundation for Chemical Research (SON). Mr A. Pleiter for his technical support and advice is gratefiilly acknowledged and Prof S. Wronskifi-omCPE Department of Warsaw Technical University for his favourable interest and support, and also dr.ir. M.H. Oyevaar, dr.ir. W. Wammes, dr.ir. H.J. Janssen en dr.ir. D. Stegeman, who also participated at some stage in the project. Notation Ci

P QL

q T To

u Xi

Xi

z

concentration of component i, mole/m^ reactor pressure, MPa liquidflowrate, m^ / s gasflowrate, Nl/min temperature, K temperature of gas and liquid entering the reactor bed, K gas or liquidflowvelocity, m / s molefi'actionof i-th component in liquid phase,relative molefi-actionof i-th component axial coordinate, m

Greek letters OCDAT hydrogen supply ratio,h yield,Zj hydrogen conversion at j-th measurement point.,f molar flow rate, mol/s Subscripts ccnd superscripts in inlet G gas

57 N2 o out H L

nitrogen initial outlet hydrogen liquid

Abbreviations A 2,4-DNT, B 4-HA-2-NT, C 4-A-2-NT, D 2-A-4-NT, E 2,4-DAT,

2,4-dinitrotoluene 4-hydroxylamino-2-nitrotoluene 4-amino-2-nitrotoluene 2-amino-4-nitrotoluene 2,4-diaminotoluene

Literature 1. K.R. Westerterp, KB. vanGelder, HJ. JanssenandM.H. Oyevaar, Chem. Eng. Sci., 43 (1988) 2229-2236. 2. H.J. Janssen, A.J. Kruithof, G.J. Steghuis and K.R. Westerterp, Ind. and Eng. Chem. Res., 29 (1990) 754-766. 3. H.J. Janssen, A.J. Kruithof, G.J. Steghuis and K.R. Westerterp, Ind. and Eng. Chem. Res., 29 (1990) 1822-1829. 4. E.J. Molga and K.R. Westerterp, Chem. Eng. Sci., 47 (7) (1992) 1733-1749. 5. K.R Westerterp, H.J. Janssen and H.J. van der Kwast, Chem. Eng. Sci., 47 (1992) 4179-4189. 6. H.J. Janssen, H.J. Vos and K.R. Westerterp, Chem. Eng. Sci., 47 (1992) 4191-4208. 7. W.J.A. Wammes, S.J. Mechielsen and K.R. Westerterp, Chem. Eng. Sci., 45 (1992) 3149-3158. 8. W.J.A. Wammes and K.R. Westerterp, Chem. Eng. Sci., 45, (1990) 2247-2254. 9. W.J.A. Wammes, J. Mddelkamp, W.J. Huisman, CM. de Baas and K.R. Westerterp, AIChE Journal, 37 (1991) 1849-1854. 10. W.J.A. Wammes and K.R. Westerterp, Chem. Eng. and Techn., 14 (1991) 406-413. 11. K.B. van Gelder, J.K. Damhof, P.J. Kroijenga and K.R. Westerterp, Chem. Eng. Sci., 45 (1990) 3159-3170. 12 K.B. van Gelder, P.C. Borman, RE. Weenink and K.R. Westerterp, Chem. Eng. Sci., 45 (1990) 3171-3192. 13. K.B. van Gelder and K.R. Westerterp, Chem. Eng. and Techn., 13, (1990) 27-40. 14. M.H. Oyevaar, T. de la Rie, C.L. van der Sluijs and K.R. Westerterp, Chem. Eng. and Proc, 26 (1989) 1-14. 15. K.R. Westerterp, H.J. Fontein and F.P.H. van Beckum, Chem. Eng. and Techn., H (1988) 367375. 16. J.L. Turpin and R.L. Huntington, AIChE Journal, 13 (1967) 1196-2001. 17. M.Y. Saada, Chem.Ind. Genie Chem., 105 (1972) 1415-1418. 18. K.R. Westerterp, E.J. Molga and K.B. van Gelder, Chem. Eng. & Proc, 36 (1997) 17-27.

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

59

Selective Catalytic Hydrogenation of 2-Butyne-l,4-diol to ci5-2-Butene-l,4diol in Mass Transfer Efficient Slurry Reactors J.M. Winterbottom*, H. Marwan, J. Viladevall, S. Sharma, and S. Raymahasay School of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

Abstract - The hydrogenation of 2-butyne-l,4-diol over a commercial 5% Pd/charcoal and a 5% Pd/Ti02 catalyst prepared in-house was investigated and the selectivity towards cis-lbutene-l,4-diol under various reaction conditions is reported. The experiments were carried out in a glass stured tank reactor (STR) and a cocurrent downflow contactor reactor (CDCR). It was found that besides c/5-2-butene-l,4-diol and 1,4-butanediol, some side products such as cis- and trans-crotyl alcohol, n-butanol, and n-butyraldehyde were also observed. A reaction scheme is proposed. In the STR, the effects of catalyst loading, temperature, base, poisons and metal additives on the selectivity were studied. The results indicate that triethyl phosphite and lead acetate led to inhibition of fiuther hydrogenation. The use of lead poisoned Pd/charcoal catalyst gave partial hydrogenation with selectivity towards c/5-2-butene-l,4-diol of 97.498.8%. The use of Pd/Ti02 catalyst exhibited higher selectivity, more than 99%. In the reaction with Pd/charcoal, CDCR has shown its potential giving increased selectivity (up to 100%) to c/5-2-butene-l,4-diol, at the end of first hydrogenation step, even with no addition of poison and with much higher reaction rates. 1. INTRODUCTION The sohd-catalysed reaction of a gas with a hquid is often carried out in a slurry reactor andfindsfi-equentapphcation in fine chemical and pharmaceutical synthesis. A typical exanq)le is found in the selective hydrogenation of acetylenic conq)ounds. The selectivity of the net formation of the intermediate conq)ounds in these reactions has been of scientific and economic concern for a long time. For a variety of reaction conditions, it is not surprising to find that selectivity is sometimes markedly influenced by small variations in the amount of hydrogen adsorbed, by numerous additives that change the rate of hydrogenation relative to the rate of mass transfer, the rate of acetylene hydrogenation relative to that of the olefin, and also by reaction variables such as the concentration of catalyst, agitation, tenq)erature, solvent and catalyst support [1]. The selective catalytic hydrogenation of 2-butyne-l,4-diol to cw-2-butene-l,4-diol over palladium represents a common situation involving a multifimctional conq)ound. Fortunately, the reduction of triple bonds is very selective over palladium [1,2]. Butenediol is an in^ortant chemical intermediate due to its use in the production of several insecticides and pharmaceuticals (i.e. endosulfan and vitamin B6). The use of supported palladium [3-8], Raney nickel [9] or nickel [10] catalyst has been reported for this reaction under relatively mild operating conditions. However, due to the possibihty of several side reactions [3-4], the problem of selectivity towards c/^-butenediol becomes inq)ortant. Over Pd/C, it was reported * To whom correspondaice should be addressed. E-mail: [email protected]

60 that the formation of butanediol during the course of the first step, i.e. butynediol to cisbutenediol indicated the possibility of a sequential reaction without preferential formation and/or desorption of desired intermediate product. Besides cw-butenediol and butanediol, the formation of side products such as ;^hydroxybutyraldehyde, n-butyraldehyde, n-butanol, and an acetal also were observed in small amounts during first reaction step, and in significant quantities at conq)lete reduction stage [4]. Fukuda and Kusama [11] successfiiUy partially hydrogenated butynediol by enq)loying PdCaCOg and quinoline as poison. Li later work of Fukuda [3], the combination effect of lead acetate and quinoline on Pd/BaCOs gave partial hydrogenation as well. In order to selectively produce cw-butenediol, Chaudhari et al. [12] reported the use of Lindlar catalyst doped with zinc acetate that gave partial reduction with selectivity as high as 99.8% toward cw-butenediol. The aim of this work is to produce cw-butenediol using a commercial Pd/charcoal catalyst in high yield and purity by inhibiting the production of 1,4-butanediol. The influence of reaction conditions and poisoning on the selectivity has been investigated in a 0.5 dm^ glass stirred tank reactor (STR) operated at atmospheric pressure and in the tenq)erature range 1555^*0. Preliminary experiments were carried out by varying catalyst loading and ten[q)erature, and solvent. The use of various additives was investigated, either by adding the additive into the reaction solution or by pre-inq)regnating the additive onto the catalyst surface. As a corq)arison, a Pd/Ti02 catalyst was also prepared ia-house by the incipient wetness inq)regnation method and then used for the reaction. The pilot plant scale experiments were studied in a 50 dm^ cocurrent downflow contactor reactor (CDCR) operated at pressure 5-15 bara and in the tenq)erature range 40-110''C. The CDCR allows greater operational flexibility for a wide range of reactions [13]. As reported elsewhere, the CDCR has been proved to offer mass transfer performance superior to that of not only conventional bubble columns but also STR's for three-phase catalytic hydrogenation reactions [14,15]. 2. EXPERIMENTAL METHODS The commercial catalyst used for hydrogenation was a palladium on charcoal catalyst (type 37), manufactured by Johnson Matthey (UK)) with metal content 4.89% and suppHed as powder. The Pd metal was deposited on the exterior surface of the charcoal. The Pd/Ti02 catalyst was prepared using sodium tetrachloropalladate (U), suppHed by Johnson Matthey (UK) and titanium dioxide,firomDegussa (Germany). The hydrogen gas was supplied by BOC (UK), with >99.98% purity and was used directlyfi-omcyhnder. The reactant, butyne-l,4-diol 99%, was procured fi'om Aldrich (UK) and water and 2-propanol, obtainedfi:omFisons (UK), were used as solvents. Additives such as lead acetate, quinoline, thiophene and triethyl phosphite were suppUed by Aldrich (UK) and cupric acetate, zinc sulphate, ferric nitrate and potassium hydroxide were provided by Fisons (UK). A glass stirred tank reactor (STR), 0.5 dm^ capacity of solution, with four baffle plates was used. The agitation device consisted of glass inq)ellers with six flat turbine blades. The STR was operated as a dead-end system, meanwhile hydrogen was suppUed continuously according to consun:q)tion rate by a hydrogenation control unit (HCU). The HCU was set to maintain the reactor pressure at 3.5" water gauge. A 50 dm^ stainless steel cocurrent downflow contactor reactor (CDCR) system was also enq)loyed and operated as a slurry system with the catalystfiillywetted. The CDCR was equipped with a control unit capable of allowing operation at pressures up to 30 bara and temperatures up to 250*'C. The flow regime

61 of reactor is a combination of mixed and plug flow, based on residence time distribution studies [14, 17]. The system was purged by flowing nitrogen and then hydrogen before reaction started. For the reaction in the STR, the reactor ten^erature was controlled by immersing the reactor base into a water bath. Afl;er the desired ten^erature was attained, the e?q)eriment was started by switching the stirrer on and then by opening low pressure outlet of the HCU. Runs were performed at a catalyst loading in the range of 0.5 to 4.0 g/1 of solution, initial butynediol concentration of 20 g/1 of solution, and temperatures in the range of 15 to 55°C under atmospheric pressure. The stirring speed was kept constant at 1500 rpm. In the case of reactions in the CDCR, the slurry containing butynediol was charged into the feed tank and then pun]5)ed into the reactor. The slurry was recirculated within a siagle loop, using a slurry punq), through the reactor and the receiver. The operational principles of the CDCR have been reported elsewhere [13, 17]. In these experiments, Uquid san^les were collected at different times during the reaction and analysed for the products. The analysis was carried out by gas chromatography. A type 94M Ai Cambridge gas chromatograph (GC) equipped with FID, an electronic integrator, and a printer was used. The column used was a capillary column of DB-WAX with 30 m long and 0.25 microns of fihn thickness. The ten^erature column was set at three ranq)s, namely 80 to 120T, 120 to 195T, and 195 to 220''C with different rate for each ranq). A carrier gas (heUum) flowrate of 0.7 ml/min and injection tenq)erature of 240*'C were used. The products such as cw-butenediol, butanediol, cis- and tram-cxotyX alcohol, n-butyraldehyde and n-butanol were identified by comparing with standard samples in GC and confirmed using a GC-MS technique. The 5% Pd/Ti02 catalysts were prepared by the incipient wetness ur^regnation method. In each preparation, initially 10 g of the support was weighed out. The mass of sodium tetrachloropalladate (11) required was calculated to give 0.5 g of palladium metal and the weighed palladium salt was dissolved in the volume of water that is needed just to fill the pores of the support. The weighed support was added into the solution and mixed for 2 hours. The water was evaporated to give a dry sanq)le in a Gallenkanq) evaporator. The dried catalyst was calcined in a fiimace at 200*'C overnight. The inq)regnation of the Pd/charcoal catalyst with various metal salts was effected by dissolving the metal salt in 200 cm^ deionized water with 50 cm^ acetone to which the Pd/charcoal catalyst was added. The slurry was stirred for 30 minutes at room tenq)erature, then dried on a hot plate with stirring to a paste. The later was dried in an oven to remove remaining water and calcined in an air fiimace at 200°C overnight. 3. RESULTS AND DISCUSSION 3.1 Product Identification and Reaction Route Analysis of the reaction products in the STR indicated that the solution predominantly contained c/5-butene-l,4-diol as the desired product during the first reaction step. In the second step, besides the formation of butanediol the other side products, namely cis and transcrotyl alcohol, n-butyraldehyde and n-butanol, were observed in significant quantities. The formation of these side reactions was not surprising due to the possibility of hydrogenolysis and isomerization of alkenes over palladiimi [1, 16]. Figure 1 shows the reaction scheme proposed based on the reaction mechanism as suggested by other workers [6, 16].

62 +2H2 Ho

butyiie-l,4-diol

Ho

=^-> c/.s-butene-l,4-diol -H2O

>

1,4-butaiiediol

+H2

CIS- or tranS'CTOtyl alcohol -> n-butyraldehyde

i+H2

+H2

"^

n-butanol

Figure 1. Reaction Scheme in Hydrogenation of Butynediol over Pd/Charcoal Catalyst 3.2 Preliminary Results in the STR The hydrogenation of butynediol using a commercial 5% Pd/charcoal was carried out in the STK It was found that in the solvent 2-propanol, the selectivity to butenediol was 73.587.1% at the end of first stage and the selectivity to butanediol was 4.6-21.1% on conq)lete hydrogenation, depending on reaction conditions. A typical plot of product distribution against conversion is presented in Figure 2. The results revealed a reduction in selectivity with 100

80

CB = 20g/L; w = 1 g/L; T = 1 5 T .^^ solvent = 2-propanol

P

butynediol cw-butenediol butanediol cis-croty\ alcohol trans-CTotyl alcohol n-butyraldehyde n-butanol ^

200

Conversion (%)

Figure 2. A Typical Products Distribution in Hydrogenation of Butynediol over Pd/Charcoal (200% conversion corresponds to 2 moles of hydrogen) increasing catalyst loading and ten^erature. In such conditions, the availabihty of hydrogen on the catalyst surface becomes less and as a consequence it could promote hydrogenolysis reaction products [1]. The pH measurements in the presence of butynediol and/or butenediol indicated that the solutions were acid with pH values in the range 4-5.5. Hence, the formation of butyraldehyde is likely due to acid catalysed homogeneous transformation of trans-ciotyl alcohol. Solvent variations studies showed that water gave higher selectivity toward butenediol

63 conq)ared with 2-propanol, even though the solubiHty of hydrogen in 2-propanol is greater than that in water. The addition of base (KOH) reduced considerably the production of side products both in alkyne hydrogenation stage and in alkene hydrogenation stage by ehminating hydrogenolysis [6], presumably because the presence of KOH increased the pH of the solution, resulting in an elimination of butyraldehyde via acid catalysed isomerization. The selectivity to butenediol was increased to 98.9% by increasing the amount of base in the STR, and the selectivity to butanediol was increased from 14.1% (without base) to 79.4%. Figure 3 shows the distribution of products in reaction using KOH. This effect was more marked in 2propanol than in water since more side products found in 2-propanol. Without poison or additive, the reaction using Pd/Ti02 catalyst prepared by incipient wetness method exhibited higher selectivity towards butenediol conq)ared to the selectivity using a commercial Pd on 100 <>

CB = 20g/L;w = 4g/L ^ ^ . T = 35T;KOH = 0.1g/ / osolvent = 2-propanol ^

O ::

80 -\

o

I

/

40

20

1

\ 1 \^

Y

/

//

/ ^

1 20

/

/

^ V ^ ° 0

\\ ^\

/ 60

s

\

\

/

/ X 0-

A 40

v^

60

^

= ? ^ ^-&<^

80

\

/ y

butynediol cis-butenediol A butanediol /M[ unknown ^ n-butanol/^

^y \

/^

^\

~R

^

/

\

-nr"'--ff--""

100

/

\

T 'MJ^ 1—'-'—1 ^^

—u—1

120

\

140

160

180

200

Conversion (%)

Figure 3. Products Distribution in Hydrogenation of Butynediol over Pd/Charcoal with KOH (200% conversion corresponds to 2 moles of hydrogen) Table 1. Selectivity in Hydrogenation of Butynediol Using Pd/Ti02 Catalyst CB = 20 g/L, T = 25T, w = 5 g/L, solvent = water, 100 ml STR Selectivity (%) no modifier 40 mg KOH 0.4 ml triethyl phosphite 0.25 ml quinohne 0.5 g lead acetate 0.25 ml quinohne + 0.07 g lead acetate

Butenediol*

Butanediol**

95.84 99.49 97.89 99.05 96.84 99.23

29.16 91.71

* selectivity in the end offirstreaction step ** selectivity in the end of second reaction step

-

64 charcoal catalyst, possibly due to stronger adsorption of butenediol into the charcoal support than that into the titania support. Table 1 gives some results using Pd/Ti02 catalyst. By adding KOH to the reaction, the selectivity using Pd/Ti02 catalyst rose to 99.49%, whereas the second step of reaction gave mainly butanediol with less side product. 3.3 Partial Hydrogenation of Butynediol In attempts to selectively produce cw-butenediol without further hydrogenation, two approaches were enq)loyed, namely: (i) by adding the poison into the reaction solution and (ii) by pre-inq)regnating the poison onto the catalyst surface. Figure 4 shows the selectivity measured when the reaction ceased consuming hydrogen. Triethyl phosphite prevented reaction proceeding beyond one mole hydrogen uptake, but thiophene and quinohne did not and the reaction proceeded into the second mole hydrogen uptake and ceased dunng this stage. In these cases, the effect of poison in inhibiting fiuther hydrogenation of butenediol increased in the following order (2-propanol used as appropriate solvent for dissolving the additives): quinoline < thiophene < triethyl phosphite. Surprisingly, addition of small amounts of triethyl phosphite to Pd/charcoal catalyst successfully gave partial hydrogenation with selectivity as high as 94%. In the reaction in water, the use of 7.5 ml of quinohne led to selectivity of 99.23% with no subsequent reduction of butenediol. However, the system gave agglomeration of the catalyst in the presence of quinoline due to problem of immiscibihty of water/quinoline system

Volume (ml) Figure 4. Effect of Different Poisons on Butenediol Selectivity on Conq)lete Reaction The pre-inq)regnation method was enq)loyed also for metal ion additives such as Pb, Cu, Zn, and Fe. The use of a lead poisoned Pd/charcoal catalyst for partial hydrogenation showed excellent selectivity. With the weight ratio of catalyst and lead acetate of 1:0.1, the reaction in water solvent gave selectivities of 97.4-98.8% with neghgible effects of tenq)erature and catalyst loading. In the case of Pd/Ti02 catalyst, as shown in Table 1, the use of some additives such as triethyl phosphite, quinohne, and lead acetate which were dissolved in the reaction solution also gave partial hydrogenation with high selectivity toward c/^-butenediol.

65 Table 2. Cw-butenediol Selectivity Using Metal Salts Pre-inq)regiiated onto Pd/Charcoal CB = 20 g/L, solvent = v^ater, 500 ml STR Metal additives

Ratio*

(CH3COO)2Pb

1 1 1 1 1 1 1 1 1 1

(CH3COO)2Pb + 0.1 ml quinoline (CH3COO)2Pb + 0.2 ml quinoline ZnS04 (CH3COO)2Cu Fe(N03)3

0.025 0.05 0.10 0.10 0.125 0.05 0.05 0.10 0.10 0.10

TCC)

w(g/L)

Selectivity (%)

55 55 55 35 55 55 55 55 55 55

4 4 2 4 4 4 4 4 4 4

2.47 92.20 98.78 97.76 96.45 96.41 93.11 11.69 28.05 3.83

* weight ratio of Pd/charcoal catalyst and metal salt 3.4 Reactions in the CDCR The reaction carried out in the high pressure CDCR using Pd/charcoal catalyst conjQrmed the initial findings in the STR as given in Table 3. A slightly different trend, however, was found as selectivity towards butenediol increased with increasing tenq)erature. The selectivities obtained varied between 92-100% even without the use of poisons, depending on reaction conditions. The production of by-products was observed to be predominant at higgler Table 3. Cw-butenediol Selectivity at the End of First Step for Reactions in the CDCR CB = 10 g/L, KOH = 0.2 g/L, solvent = water P (barg) 1 5 10 10 10 10 10 10 5 10 10 5

T(oC) 50 50 50 50 65 65 65 35 50 50 65 50

w(gfL) 0.01 0.01 0.01 0.01 0.01 0.10 0.25 0.01 0.005 0.005 0.005 0.25

Conversion (%) 65 65 65 92 80 95 99 99 90 75 95 99

Selectivity (%) 98.7 99.2 99.7 97.4 100.0 97.3 94.1 92.1 99.2 99.6 98.3 98.9

catalyst loading and only when butynediol was ahnost totally reacted. In the second step, butanediol was produced in higher selectivity than found in the STR, When the reaction was allowed to continue to conq)letion, the by-product formation also reduced and was practically ehminated as the reaction achieved concpletion. However, in the case of the CDCR significantly greater rates were obtained with much lower catalyst loadings, i.e. the lowest

66 catalyst loadings were respectively 0.005 g/L and 0.2 g/L in the CDCR and the STR. It is suggested that the greater reaction rates are a direct result of the in^roved hydrogen mass transfer achieved in the CDCR and when con^ared on the same basis, i.e. without addition of KOH at 35''C and 5 barg, the reaction rate observed in the CDCR was over thirty tunes greater than that in the STR The greater selectivity observed without recoiu-se to poison addition in the CDCR is attributable also to the greater availabihty of hydrogen at the catalyst surface as provided by the CDCR and its superior mass transfer capabihties. 4. CONCLUSIONS Partial hydrogenation of butynediol with high selectivity to c/5-butene-l,4-diol has been achieved using poisons such as triethyl phosphite and lead acetate, and the CDCR has shown its potential givhig increased c/^-butenediol selectivity with no addition of poison even with the Pd/charcoal catalyst, which appears to give lower selectivity than the Pd/Ti02 catalyst. It is possible that significant inq)rovements in selectivity towards c/5-butene-l,4-diol may be achieved by enq)loying lower metal (palladium) loading on the support and this is being investigated. The CDCR also gave rise to greater reaction rates conq)ared with the STR and this attributed to its superior mass transfer capacity. ACKNOWLEGDEMENT All of us kindly acknowledge Syiah Kuala University (Indonesia) for H.Marwan's scholarship, Johnson Matthey (UK) for loan of the precious metal catalysts, EPSRC (UK) and ICI-Katalco (UK) forfinancialsupport.

REFERENCES 1. Rylander, P.N. (1990); Hydrogenation Methods, Academic Press, New York. 2. Bond, G.C., Webb, G., and Winterbottom, J.M. (1962); J. Catal., 1, 74. 3. Fukuda, T. (1958); Bull. Chem. Soc. Japan, 31, 343. 4. Chaudhari, RV., Jaganathan, R, Kolhe, D.S., Emig, G., and Hofinann, H. (1987); Appl Catal., 29, 141. 5. Russel, T.W., and Duncan, D.M. (1974); J. Org. Chem., 39, 3050. 6. Tedeschi, RJ., McMahon, H.C., and Pawlak, M.S. (1967); Annals New York Academy of Sciences 145 91 7. McQuiUin, KJ.Tand Ord, W.O. (1954); J. Chem. Soc, 2902 8. Jardine, L, Howsam, RW., and McQiulKn, F.J. (1969); J. Chem. Soc. (C), 260. 9. General Aniline and Fihn Corp. (1962); in Alcohols, J. A. Monick, Reinhold, New York. 10. Canq)elo,J.M., Guardeno,R, Luna,D., and Marinas, J.M. (1993); J. Molec.Catal, 85, 305. 11. Fukuda, T., and Kusama, T. (1958); 5w//. Chem. Soc. Japan, 31, 339. 12. Chaudhari, RV., Parande, M.G., Elamachandran, P.A., Brahme, P.H., Vadgaonkar, H.G., and Jaganathan, R {19^5), AIChE J, 31, 1891. 13. Sharma, S., Wiaterbottom, J.M., Boyes, A.P., Raymahasay, S., and Khan, Z. (1996); The 1996 IChemE Research Event, vol. 2, Rugby (UK), 829 14. Lu, X.X., Boyes, A.P., and Winterbottom, J.M. (1994); Chem. Eng Sci., 49, 5719. 15. Boyes, A.P., Chughtai, A., Lu, X.X., Raymahasay, S., Sarmento, S., Tilston, M.W., and Winterbottom, J.M. (1992); Chem. Eng Sci., 47, 13/14, 3729 16. Bond, G.C. (1987); Heterogeneous Catalysis, 2nd ed.. Clarendon Press, Oxford. 17. Lu, X.X. (1988); PhD Thesis, The University of Bhmingham.

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

67

Replacing Liquid Acids in Fine Chemical Synthesis by Sulfonated Polysiloxanes as Solid Acids and as Supports for Precious Metal Catalysts Stefan Wieland and Peter Panster DEGUSSA AG, Inorganic Chemical Products Division, Research and Applied Technology Chemical Catalysts and Organosilanes P.O.Box 1345, D-63403 Hanau

The use of solid acid catalysts, consisting of polysiloxanes bearing alkylsulfonic acid groups, is described, in organic synthesis and in technical applications . Although sulfonated polysiloxanes have been reported in the literature and have been found to show excellent activities in comparison to conventional polystyrene based cation exchange resins, no large scale technical use has become known thus far. We now report on the esterification of free fatty acids and phthalic anhydride to obtain the corresponding esters in quantitative yield and high purity (acid number < 0.1 mg KOH/g) as well as on acid catalyzed fat splitting at temperatures from 140-200°C. The condensation of phenol with acetone was also extensively studied and a catalyst prepared that gives high conversion and good selectivity to the desired product p-,pbisphenol-A. The alkylation of phenol was studied with isobutene in the temperature range from 90 - 130°C and high conversion was found as well as in the alkylation of a-methyl-styrene with 2,3-dimethyl-l-butene. Finally, the catalytic hydrogenolysis of the model substance 1-phenyl-ethanol to ethylbenzene proceeds quantitatively at 30°C without addition of a soluble acid. 1. Introduction Sulfonated polysiloxanes have been described and reported in the literature for their excellent activities in comparison with polystyrene based cationic exchange resins ^. Inorganic materials with the exception of zeolites, have been used less often than organic polymers as solid acid catalysts, although the better physical properties of the former in general more than compensate for the better chemical properties and simplicity of synthesis of the latter. Advantages of the oxide supports are their rigid structure, which prevents deactivation, and a higher temperature, solvent and aging stability. Finally, a defined pore structure independent of solvent, temperature and pressure gives greater control over diffusional factors. Polycondensation (sol-gel process) of suitable alkyl sulfonic acid functionalized organosilane monomers leads to solids with a siliceous matrix and a high concentration of anchored ligand groups , thus leading to the development of specially designed polysiloxane based solid acid catalysts Deloxan" ASP (like in formula 1, 2a, 2b, and 3).

68

0

/

\ / 0—Si — 0 / \

— O—Si —O

/ \

\

— 0—Si —CH2CH2R

0

0—Si— CH2CH2CH2SO3H

/ \

0

0

— 0—Si— CH2CH2CH2SO3H

c/

\

'4m

— O—Si— CH2CH2CH2SO3H

/

(2a R=CH3, 2b R=CH2SH) These organofUnctional polysiloxanes overcome the drawbacks of organic polymers by virtue of their inert matrix material and excellent compatibility with almost all organic solvents. The sol-gel process upon synthesis of the Deloxan® catalysts allows to obtain products of consistently controllable size with a relatively narrow particle size distribution, e.g. from 100 |im to 400 |im, for use in suspension or as spheres with diameters up to 1.4 mm for fixed-bed applications. Furthermore, the catalysts are characterized by their high porosity, large pore diameters (> 20 nm) and high BET surface areas (300-600m^/g). These catalysts are extremely stable in gas phase reactions (like the splitting of MTBE to isobutene) and in organic media. Special advantages of the catalysts include: High structural stability: no swelling and no shrinking in organic media, in contrast to polystyrene based cation exchange resins, high temperature stability (e.g. 200°C), High catalytic activity, comparable to soluble acids as H2SO4 or p-toluene sulfonic acid, based on a high Bronsted acid strength, High selectivity. 2. Results and Discussion 2.1 Esterification of free fatty acids An illustrative example for the use of the Deloxan® ASP solid acid catalyst is the esterification of free tallow fatty acid (acid number: 205) with high boiling alcohols like 2ethylhexanol. In the initial stages of the esterification reaction, the rate of 1000 reaction is determined by the B effectiveness of water removal, an a increase of catalyst concentration has only a minor effect on the rate under the selected test conditions within the first hour. For the sake of selectivity, i.e. decreasing side product formation by dehydration of the used alcohol, the catalyst concentration was kept low. The reaction readily goes to completion Q Qg ^ -,5 2 2,5 3 and the acid number in the raw product time [h] is found to drop to < 0.2 mg KOH/g. pig. 1. Esterification of free tallow fatty acid

69 2.2 Esterification of acid anhydrides to form the corresponding diesters The high thermal stabihty of the Deloxan ASP catalysts is also a key factor when looking at the achieved conversion rates in the esterification of phthalic anhydride with various alcohols, e.g. 1-octanol or 2-ethyl-hexanol, to yield di-octyl-phthalates (reaction 1) that are used as plasticizers. Product yields of > 99% for the diester are obtained for residence times between 30-50 minutes, at temperatures between 150°C and 180°C.

o

o6'

o 2

CgHiyOH

O C8Hi7

+

H2O

(1)

O CsHiy

O 4

phthalic anhydride

1 -octanol or 2-ethyl-hexanol

di-octyl-phthalate

O B

§

0,01 7

8

batch time [h] Fig. 2: Esterification of phthalic anhydride with 2-ethyl-hexanol. 293.3g 2-ethyl-hexanol and 111.2 g phthalic anhydride were placed in a round flask and heated up. After one hour, 20.Og catalysts 2a (Deloxan® ASP IV) with a capacity of 0.9 mmol [H"^] / g were added.

Typically the reaction was conducted in a round flask. The reaction water was removed by azeotropic distillation using a water separator. The raw product was separated from the catalyst by filtration and excess of alcohol was removed by distillation. The recovered catalyst was re-used up to five times with no loss of activity being observed. With the use of the temperature stable catalysts 1 and 2a it is possible to develop alternative processes (cascade like processes) for 6 besides the established routes, which use e.g. amphoteric compounds such as titanates"*.

70

2.3 Acid catalyzed fat splitting Transesterification of fats into its corresponding methylesters, as well as fat splitting to produce the free fatty acids is of great industrial relevance. Solid acid catalyzed fat splitting was achieved by reacting rape seed oil with a large excess of water (weight-ratio= 2:25) in an autoclave at 180°C and 11 atm. The amount of catalyst 1 used was 25 wt.-% of the used rape seed oil, the reaction time for complete splitting was about 12 h and the final acid number of the free fatty acids after separation from the water-glycerol phase was determined to 193.5 mgKOH/g. 2.4 Condensation reaction to form bisphenol-A For special applications it is of interest to incorporate also non-functional, e.g. propyl-, or functional siloxane units with cocatalytic properties, e.g. mercapto-functionalized siloxanes, into the polysiloxane matrix. One example in which this is very important is the industrial production of bisphenol-A (eq. 2). OH

HOv.

/ \

^^

/OH

O

H^ H^C

(2)

CH3 H3C

8 acetone

7 phenol

CH3

p-,p-bisphenol-A

Improvements with respect to activity, selectivity, and over-all productivity (see fig. 3) of the catalyst in the condensation reaction of acetone with phenol have been demonstrated using specially designed Deloxan® ASP catalysts (10). 100 95,6 95,4 95

90

^^^^H

95,2 170

^H

200

a to

150

^^

93^^H

^H

--

O

100

0

85

50

o

o

250

230

0

80 phenol / acetone 10:1

\

IS OH

0^

^

phenol / acetone 7:1

conversion selectivity productivity (right side axes) T= 70°C, WHSV = 0.83 h"^ Fig. 3: Condensation of phenol and acetone; conversion and selectivity to bisphenol-A and production output using Deloxan ASP V

71 A co-catalytically active mercapto group was incorporated within the polysiloxane matrix. The desired 4,4'-bisphenol-A is produced with a selectivity of 95-96% whereas the total selectivity for the 4,4'- and 2,4'-bisphenol-A is higher than 99%. These results are achieved with extremely short residence times as compared to the state-of-the-art^ technology. Whereas the use of polystyrene resins gives selectivities of about 4 to 5% less at WHSV = 0.2 - 0.5 K\ the above reported excellent selectivities obtained with the Deloxan" catalysts are reached at space velocities from 0.5 up to 2.0 h" . Increasing the feed rate increases the overall production output. The latter can be improved further when using a lower molar excess of phenol over acetone like 7 : 1 instead of the technically used ratio of 10 : 1. Using polystyrene resins, this leads to an intolerable decrease in selectivity, whereas selectivity remains at a constant high level for the Deloxan® catalyst. State-of-the-art technology uses conventional polystyrene ion exchange resins that have been modified by ionic fixation of mercaptoalkylamines (11). The differences with our catalyst are shown schematically in figure 4.

^^3^ 9*^2 I

^^ CH2

I

I

CH2

H3N^-CH2CH2SH sOo" . .^

CH2

o /

SO3H

o /

- CH2 - CH2 ~ CH2 " CH2 - CH2 -

10 Deloxan" ASP V

n ionically modified polystyrene ion exchange resin

Fig. 4: Schematic representation of bifunctional polysiloxane catalysts and ionically modified polystyrene cation exchange resins.

Beside the increase in productivity, a higher quality of the raw-product (colorless, reduced amount of traces of several side-products) is obtained using catalyst 10, which is of great importance for the application of bisphenol-A, e.g. for the production of polycarbonates. 2.5 Alkylation of phenols with branched and linear terminal olefins Alkylation of phenols and phenol derivatives with olefins like isobutene, octene, and nonene to synthesize industrially very important alkylphenols, -cresols, or -xylenols is typically performed at temperatures between 80°C and 120°C. High yields and good selectivities are obtained at these temperatures by using Deloxan" catalysts. The very good thermal stability of the Deloxan" catalysts allows their use at even higher operating temperatures, like 110°C to 150°C, which results in a higher productivity. The reaction of phenol with isobutene was studied by introducing the olefin into the phenol melt in a rate that was comparable to the reaction rate. Samples were taken at constant time

72

intervals and analyzed by gas chromatography. Addition of isobutene was stopped when the stoichiometric amount for disubstitution was consumed. It is interesting to note, that in the alkylation of phenol with isobutene, a change in temperature seems also to change the course of the alkylation reaction with the polysiloxane based catalysts. The principal reaction pathways to the main substitution products (according to the theory for electrophilic substitution, only very small amounts of meta-alkylation products are observed) are given in fig. 5.

Fig. 5:

Reaction pathways for the alkylation of phenol with isobutene

Results in table 1 show, that a higher reaction temperature gives a greater selectivity to the p-substituted alkylphenol, thus increasing the rate of para-substitution as opposed to orthoalkylation and increases the 2,4-di-tert-butyl-phenol. The tested zeolite BETA has a pronounced selectivity for the mono-alkylated para-substitution product as would be expected from its shape-selectivity.

catalyst

ASP I ASP II (Al) zeolite BETA Table 1:

temperature [°C] 80 90 90 130 90

2-tert.-butylphenol [GC area-%] 8.2 1.3 16.1 13.3 2.5

4-tert.-butylphenol [GC area-%] 4.1 11.1 5.4 27.5 73.2

2,4-di-tert.butyl-phenol [GC area-%] 61.6 70.5 51.8 54.5 15.1

2,4,6-tri-tert.butyl-phenol [GC area-%] 18.8 12.1 12.7 2.6 0.3

Alkylation of phenol with isobutene. Values reported correspond to a stoichiometry of phenol: isobutene = 1 : 2 and complete conversion of the phenol

73

2.6 Selective alkylation of a-methylstyrene as test reaction Using the catalyst 1, the alkylation reaction (3) which is a probe reaction for the synthesis of fragrances proceeded with better yields compared to various other solid acid catalysts (see figure 6). The drying method of the catalyst and the amount of residual water is critical for the reaction rate.

O 12 a-methylstyrene

(3)

13 2,3-dimethyl-1 -butene

14 1,1,2,4,4-pentamethyltetralene

30,0

„

25,0

I

20,0

o

15,0 10,0

O OH

5,0

6,0

7,0

reaction time [h]

Fig. 6: Yield [area-%] of 1,1,2,4,4-pentamethyltetralene as function of solid acid catalysts. ^ ^ Deloxan® ASP I/7;^o- Amberlyst ®15;-«- Lewatit® SP 120;-^Deloxan® ASP 1/7, extracted with ethanol before drying. solvent: cyclohexane, equimolar amounts of reactants (50 mmol each), catalyst weight: 12,5g, temperature: 60°C The Deloxan® ASP 1/7 catalyst has a [H^]-capacity of 1.2 mmol/g which is lower than the typical [H ]-capacity of about 2-4 mmol/g determined for organic (polystyrene based) ion exchange resins. Nevertheless, the Deloxan catalyst gives the highest product yield, which is even higher than that obtained either by using H2SO4 / glacial acetic acid (homogeneous) according to a published procedure or by synthesis according to the standard Friedel Crafts procedures with AICI3. The used Deloxan" ASP 1/7 catalyst had a surface area (BET) of 510 m^/g and a pore volume of 0.6 ml/g (maximum in the pore size distribution at 5 nm). 2.6 Hydrogenolysis reactions using acidic Pd catalysts The alkyl sulfonic acid bearing polysiloxane is also used as a support for powder type precious metal catalysts 3 (particle size < 200 jiim) that can be used in hydrogenolysis reactions. The conversion of 1 -phenylethanol to ethylbenzene proceeds quantitatively (>99%) with the same rate as achieved with the combination of H2SO4 and a Pd/C catalyst. Therefore,

74

no homogeneous acid is necessary for the hydrogenolytic cleavage of the carbon-oxygen bond. This is of great importance in fine chemistry, e.g. for debenzylation. Hydrogenolysis reaction

ASP /2% Pd, H2 (1 atm) suspension, T =

15 phenylethanol

Lv^J

(4)

C

16 ethylbenzene

3. Experimental The synthesis of organoflinctional polysiloxanes and the preparation of these catalysts were conducted following published procedures ' ' . The reactions reported were carried out in standard glass apparatus or in thermostated glass columns, where data from continuous testing are reported (conditions reported together with results). GC or HPLC was used for analysis. 4. Conclusion Comparison data were presented, that demonstrate the use of the polysiloxane material as an advantageous substitute for organic cation exchange resins, sulfuric acid, p-toluene sulfonic acid and acidic zeolites. It is demonstrated, that materials like 1 and 2 are costefficient and reliable catalysts in esterification, alkylation, and condensation, whereas use of the bifunctional catalyst 3 gives excellent conversions in hydrogenolysis reactions in general. The use of the described solid acid catalysts helps to minimize waste of production, as it reduces the amount of salts formed upon neutralization of the former used liquid acids. Furthermore, when selectivity is increased as in the synthesis of bisphenol-A, the use of the Deloxan® catalyst results in a better (i.e. higher) utilization of the raw material. Altogether, it is seen that improvements of the catalysts as described in this contribution are a significant contribution to the further development of the currently needed ecology-minded processes. References ^ ^ ^ "^ ^

^

Yoshio Ono, Sadakatsu Suzuki in: Acid-Base Catalysis, Proceedings of the Int. Symp. on Acid-Base Catalysis, Sapporo, 1988 pp. 379-396. P. Panster, European patent EP 548 821 to Degussa AG. P. Panster, German patent DE 42 23 589 to Degussa AG. F. K. Towae, W. Enke, R. Jackh, N. Bhargava in: UUmann's Encyclopedia of Industrial Chemistry, 5'*" Ed., 1992, Vol A 20, pp. 181-211. K. Berg, German patent DE 43 12 039 to Bayer AG; K. Berg, European patent EP 620 041 to Bayer AG; K. Berg, European pat. EP 583 712 to Bayer AG; J. B. Powell, United States pat. US 5,105,026 to Shell; M. J. CipuUo, United States pat. US 5,315,042 to General Electric. T.F. Wood, W.M. Easter, M.S. Carpenter, J. Angiolini: J. Org. Chem. 28, (1963), 2248-55.

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

75

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. Rodri^ez, A. Cauvel and D. Brunei* UMR 5618-CNRS-ENSCM Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique. Ecole Nationale Superieure de Chimie 8, rue Ecole Normale, 34296 - MONTPELLIER Cedex 5- (France)

MCM-41 type silica surfaces were grafted with primary amino groups through silanation process. Tertiary amino groups covalently bound to the surface were prepared through displacement of previously anchored halogen atom by piperidine. The catalytic activities of the immobilized amino groups were studied in the Knoevenagel condensation of benzaldehyde with ethylcyanoacetate. The mechanism of the catalyst action is discussed 1. INTRODUCTION Widespread interest is now devoted to solid catalysts possessing both active sites and high surface area because of their potential in the development of clean processes for the production of fine chemicals. Besides acid catalysts, recyclable solid base catalysts need to be explored. Such catalysts we have examined [1-4] for this purpose included zeolites containing occluded alkali oxide species [5-9]. However, the constraints imposed by the framework limit their use to organic molecules of small dimensions. That promped us to recently develop the design of base catalysts featuring large mesoporous surfaces [10] in order to perform base-catalyzed reactions in the fields of perfumes, pharmaceuticals and agro-chemicals which include high molecular weight compounds. In this respect, the recently discovered family of materials synthetized by silicate condensation around surfactant micelles (Micelle-Templated Silica, MTS) provides unique inorganic support [11,12] due to their regular arrays of uniformly sized channels in the mesopore range of 20-100 A. This solid was recently impregnated with cesium oxides with the aim to obtain superbase catalysts [13]. Nevertheless, the leaching of the basic particles cannot be excluded. In view to avoid such possible phenomena, we have studied the covalent attachment of basic functions such as amino groups on the MTS surface

76 taking into account the use of piperidine as base catalyst in homogeneous conditions [14,15]. The primary amino groups bound on silica was previously reported to be an efficient catalyst for the Knoevenagel condensation reaction in mild conditions [16]. However the mechanism of the action of the catalyst was not demonstrated. The aim of our study is to understand the role of the amine function United on MTS surface with the opportunity of benei&tting from the unique pore structure and surface of this recently discovered family of mesoporous materials. Our previous works on the Knoevenagel reaction (scheme 1) have born witness to its model behaviour for testing the base catalyst activity. Moreover they have emphasized the unique properties of DMSO as solvent for this kind of catalytic reaction as it was shown for homogeneous reactions [17]. In this work, we report the study of the catalytic activity of immobilized amino groups in the Knoevenagel condensation of benzaldehyde (BA) with ethylcyanoacetate (EGA) in DMSO solution (scheme 1). Scheme 1

H^

^CN BA

-H2O

H^

N^N

EGA

A special attention was focused on the catalyst reuses. Simultaneously we examined the mechanistic aspects of carbonyl activation by the primary amine via activated imine intermediate on the condensation reaction in considering also the activity of a tertiary amine group covalently grafted to the surface. 2. RESULTS AND DISCUSSION 2.1 Catalyst preparation The primary amino groups were anchored to the MTS sihca surface through the grafting of 3-aminopropylsilane to the siloxy groups (silanol, siloxane) by t r e a t m e n t with refluxing anhydrous toluene solution of 3aminopropyltriethoxysilane (scheme 2, eq. 2). The tertiary piperidino moieties linked to the MTS surface were obtained through the grafting of 3-chloro- or 3-iodo-propylalkoxysLlane (scheme 2, eq.2) followed by the nucleophilic displacement of the halogen atom by piperidine (scheme 2, eq. 3).

77

The organic chains linked to the mesoporous surface thus obtained have been characterized by IR and l^C CP-MAS-NMR spectroscopies. The textural characteristics were determined by X-ray diffraction and nitrogen sorption isotherm analysis. Scheme 2

/J—OH

(EtO)3Si (CH2)3 X (2) refluxing toluene

Piperidine (3) refluxing toluene

The number of grafted moieties have been calculated from both elemental analysis and thermogravimetric data reported on table 1. Table 1 Textural characteristics and compositions of the catalysts 1-4. Surface Area Mesoporous Diameter Chain content Solids (m2 g-1) Volume (ml g-1) (A) x 103(mol g" 1) 1 0.76 958 32 2a 821 0.60 29 0.8 2b 3a 3b 4a 4b 4b»

698 827 721 740 657 639

0.45 0.55 0.43 0.49 0.36 0.34

26 27 24 26 22 21

1.7 1 1 0.7 1 0.9

It should be noted that the average diameter determined acccording to the relation 4V/S, monotically decreases with the steric hindrance of the organic chains. In addition, the results obtained with 2a and 2b possessing the same

78

anchored moiety show the same trend related this time with the surface organic coverage. Moreover, the best efficiency of the nucleophiUc displacement of iodine compare to that of chlorine by piperidine provided immobilized N-piperidinyl chains on mesoporous silica having two different contents of tertiary amino groups in addition with different surrounding fiinctions since 3a contains some residual chloropropyl chains. On the other hand the washing of 4b with hot ethanol did not significantly alter the grafted amino group content. Nevertheless, the higher % C observed by elemental analysis of 4b* suggests that some additional ethoxy groups would be grafted on the residual silanols during the washing proccess. 2.2. Catalytic Knoevenagel reaction The Knoevenagel condensation reaction of equimolar solution of BA and EGA in DMSO was carried out on the various catalysts 1-4 in batch conditions. During all the experiments, the selectivities observed in the Knoevenagel condensation product are nearly 100% (>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 catal5^ic 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

<.

H2C O2C2H5

o <. O2C2H5 H^

^02C2H5

o O2C2H5

This assumption is also argued by the possible role played by DMSO which is the well-adapted solvent for this condensation reaction, in the assistance at N-H bond polarization [17] during the imine formation from carbonyl group of the benzaldehyde. 3. EXPERIMENTAL 3.1. Materials. 3.1.1. MTS silica (OH-MTS) 1. MCM-41 type-silica (MTS) 1 was prepared by addition of Zeosil 175 MP precipitated silica (Rhone-Poulenc, 0.17 weight % Al) to a stirred solution of cetyltrimethylammonium (CTMA) bromide (Aldrich), sodium hydroxide and deionized water. The reaction mixture was heated in a stirred autoclave at 393 K for 16 hours. After filtration and washing with deionized water to pH 9, then with ethanol, the soHd phase was dried at 353 K in air then calcined at 823 K in flowing air for 7 hours to eliminate the occluded organic template. X-ray powder diffiraction shows a main pic at 2 0 = 2.4° using Cu Ka radiation corresponding to hexagonal lattice parameter equal to 46.2 A. The nitrogen sorption isotherm

of MTS is a type IV isotherm, showing a sharp, reversible step at P/PQ 0.38 characteristic of capillary condensation within a regular mesoporous system. Composition of the OH-MTS soUd: Al/(AH-Si): 0.003; Na/(A1+Si): 0.0014 2.1.2. 3-aminopropylsilyl-MTS (NH2-MTS) 2a and 2b 2a and 2b were prepared by addition of 3-aminopropyltrialkoxysilane (3 g, 13 mmol. and 0.7 g, 3 mmol. respectively) to a suspension of freshly activated MTS silica (3 g) in refluxing toluene (50 ml), then stirred for 5 hours. The modified solid was filtered, washed in a soxhlet apparatus with di-ethyl ether and dichloromethane, then dried at 393 K. Elem. anal.:2a % C 3.48, %N 1.18, %Si 40.54; 2b % C 7.07, %N 2.22, %Si 37.60. % organic weight / dry mineral weight: 2a: 6.8; 2b: 11 IR (cm-l): 3732, 3650, 3375, 3306, 2979, 2936, 2867, 1974, 1836 . 13C CP-MAS-NMR (ppm): = 2b: Si -aCH2-PCH2-YCH2-NH2: «£: 8.9; P^: 26.4; Y£l: 43.3 2.1.3. 3-chloropropylsilyl-MTS (Cl-MTS) 3a and 3-iodopropylsilyl-MTS (IMTS) 3b. 3a and 3b were prepared using 3-chloro- and 3-iodopropyltriethoxysilane as silanating agents, respectively, according to the previous procedure. Elem. a n a l : 3a % C 7.22, %C1 5.23, %Si 38.43; 3b C 6.65, %C1 0.62, %I 10,%Si 35. % organic weight / dry mineral weight :3a: 12.9 ; 3b: 20.3. IR (cm-l): 3a 3732, 3650, 2970, 2940, 1974, 1836 . 13C CP-MAS-NMR (ppm) : 3a: = Si (OCH2CH3)aCH2-PCH2-YCH2-Cl: 0£H2CH3: 58.7; OCH2CH3: 16.3; ^Q: 8.9; P£: 26.3; Y£: 46.3. 2.1.4. 3-N-piperidinopropylsilyl-MTS (Cl-PipMTS) 4a and (I-PipMTS) 4b. A suspension of activated 3a or 3b (3 g) in toluene (30 cc) was refluxed and stirred in an excess of piperidine (1 g) for 6h. The modified solid 4a or 4b was then washed in succession with water, then with di-ethyl ether-dichloromethane mixture in a soxhlet aparatus overnight. 4b was subsequently washed with ethanol in a soxlhet apparatus overnight leading to 4b'. Elem. anal. : 4a % C 6.86, %N 0.86, %C1 0.20, %Si 38.54; 4b %C 9.72, %N 1.42, %C1 0.12, %I 0.24, %Si 38.05; 4a' % ClO.26, %N 1.27, CI 710 ppm, %I 0.47, %Si 35.55. % organic weight / dry mineral weight: 4a: 12.5; 4b: 15.0; 4b': 15.7 IR (cm-l): 4a: 3731, 3649, 2945, 2892, 2867, 2818, 1974, 1866 . 2.2. General condensation procedure Benzaldehyde (1.75 g, 16.5 mmol.) and ethylcyanoacetate (1.87 g, 16.5 mmol.) was added to the modified catalyst (0.22 g) previously activated at 150°C under vacuum overnight and stirred in suspension in DMSO ( total volume : 55 ml) under nitrogen atmosphere at 80°C. The reaction progress was

82

monitored by periodically withdrawing samples which were analyzed by G.C. using decane as internal standart for mass balance. 4. CONCLUSION MTS silicas functionalized with amino groups exhibit interesting base catalytic properties in condensation reaction which are well-correlated with their physico-chemical characteristics. The greater activity of the immobilized primary amine sites compare to the tertiary amine sites in the Knoevenagel condensation is explained by the imine intermediate formation which is much more reactive than the parent carbonyl group of the reactant. ACKNOWLEDGEMENTS The authors thanks Francesco Di Renzo for fruitful discussions on the characterizations of mesoporous materials. One of them (A.C.) is indebted to ADEME (Agence de TEnvironnement et de la Maitise de I'Energie) for a doctoral grant. REFERENCES 1 I. Rodriguez, H. Cambon, D. Brunei, M. Lasp6ras, P. Geneste, Stud. Surf.Sci.Catal., 78(1993)623 2 M. Lasperas, H. Cambon, D. Brunei, I.Rodriguez and P. Geneste, Microporous Mater., 1(1993)343 3 M. Lasperas, I. Rodriguez, D. Brunei, H. Cambon, P. Geneste, Stud. Surf. Sci.Catal. 97(1995)173; 4 M. Lasperas, H. Cambon, D. Brunei, I.Rodriguez and P. Geneste, Microporous Mater., 7(1996)61 5 C. Naccache, D. Archier and G. Coudurier, Abstr. IX^^ IZC, Montreal Eds. JB Higgins, R. von Ballmoos and M.M. J. Treacy, (1992) FP18. 6 P.E. Hathaway and M.E. Davis, J. Catal., 116 (1989) 263 and 279 7 J.C. Kim, H.-X. Li, C.-Y. Chen and M.E. Davis, Microporous Mater., 2 (1994) 413 8 H. Tsuji, F. Yagi and H. Hattori, Chem. Lett. (1991) 1881 9 F. Yagi, N. Kanuka, H. Tsuji, H. Kita and H. Hattori, Stud. Surf. Sci. Catal., 2Q (1994) 349 10 A. Cauvel, G. Renard and D. Brunei, Europacat II, Maastricht (1995); J. Org. Chem. (in print) 11 D. Brunei, A. Cauvel, F. Fajula and F. Di Renzo, Stud. Surf. Sci., Catal. 22, (1995) 173 12 A. Cauvel, D. Brunei, F. Di Renzo and F. Fajula, Amer Inst.of Phys., 254 (1996) 477. 13 K.R. Kloetstra and H. van Bekkum, J. Chem. Soc. Chem. Commun.,(1995) 1005 14 Knoevenagel, Ber., 22 (1896) 172 15 A. Corma, V. Fom6s, R.M. Martin-Aranda, H. Garcia and J. Primo, Applied Catal., 52(1990)237 16 E. Angeletti, C. Canepa, G. Martinetti and P. Venturello, Tetrahedron. Lett., 22 (1988) 2261 17 A.Reyes and R.M. Scott, J. Phys. Chem., M (1980) 3600 18 G. Charles, Bull. Soc. chim. Fr., (1963) 1559

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

83

Modified clay catalysts for acylation of crown compounds S. Bekassy^ K. Bir6^ T. Cseri''^'^ B. Agai' and F. Figueras^ ^Department of Organic Chemical Technology, Technical University of Budapest, H-1521 Budapest, Hungary ^Institut de Recherches sur la Catalyse du CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France

1. SUMMARY Heterogeneous catalytic acetylation of benzo-15-crovm-5 was investigated using different ion exchanged KIO catalysts. The nature of the transition metal ions introduced in the mesoporous clay plays an important role in the activity of the catalysts. Fe^^-KlO is particularly advantageous for practical preparative purposes. 4'-Acetyl-benzo-15-crown-5 was isolated with 55% yield under optimal conditions.

2. INTRODUCTION Crown ethers are well known for their ability to form strong complexes with alkali metal and organic cations. Introduction of lipophilic substituents into crown ethers has a great importance e. g. for their application as electroactive substances in ion-selective electrodes [1] and as complexing agents for recovering precious or radioactive metal cations [2]. These substituents can be introduced into benzo annelated crown ethers by S^ reactions (alkylation, acylation, nitration, halogenation). The introduced substituents modify the complexing properties of the crown ether macroring through the aromatic ring. We have taken an advantage of this phenomenon at the planning of the K-selective ionophore BME-44 (2) and at its synthesis starting from benzo-15-crown-5 (B15C5) (1). In order to study how the NH group of the molecules of type 2 influences the complexation it was necessary to synthesize the analogue of type 4 [3]. To clear up the role of the NO2 group we have synthesized the derivatives of type 5 [4,5] having the similar electron-withdrawing ester group which is also sufficiently lipophilic in the case of an appropriate R substituent. Both these types as target molecules have the same 3 acetyl derivative (4'Ac-B15C5) as key intermediate. The obvious way to prepare the latter compound is the acetylation of _!, therefore it was extremely important to develop a well controllable procedure for the preparation of 3. Acylation of B15C5 in a Friedel-Crafts type reaction with the classical Lewis acid AICI3 can be performed only with poor yields (about 30%) in the presence of a large excess of ' present address: Institiit FrauQais du Petiole, F-92506 Rueil-Mabnaison, France s work was supported by the VARGA JOZSEF Foundation of the Technical University of Budapest and by the Hungarian Science Foundation OTKA (grant No. T-015673 and T-015677).

84 catalyst (catalyst/substrate ratio is about 5.5) [6] because of the formation of a stable AlCl,crown ether complex [7]. In reality the acetylation can be carried out with P2O5 or in polyphosphoric acid [8,9]. Both homogeneous catalytic reactions give rise to serious corrosion and environmental problems and their practical application is therefore very limited. Substitution of mineral acids by solid acids as catalysts can reduce these difficulties. Moreover solid catalysts offer by their nature other advantages such as an easier separation and work-up because of the heterogeneous system.

C"" N^V^"^ """^W

"^

<^o

^

^^^^"

H N ^ o

BME.44 FLUKA product No. 60397

.y...

o - c^ -?0

85 Mesoporous clays, such as the commercial catalyst KIO, ion exchanged with transition metal cations (Me"^-K10) were active for benzylation of aromatic compounds [10]. The reaction rate appeared to be related not to the acidity of the solid, but to its redox properties. In other words, the formation of the carbocation involved a redox step of the type: O-CH2CI + Fe^^ -^ Fe"^ + O-CHjCl"^"

a)-CH2Ci'^" -^ o-CH2'^ + c r Cr + F e ' ^ - > F e ' V c r Such a mechanism for C-Cl activation should be valid for acyl chlorides and we reported recently a heterogeneous catalytic acetylation of B15C5 with an acceptable 32% yield of isolated product, using Cu^^-KlO [11].

C "

AcCl, 83°C (ill 1,2-dichloroethane)

»

o

CH,

Cu^^-K 10 catalyst

This work presents an optimization of the reaction and an extension to other catalysts in order to clarify the role of acidity and achieve a yield high enough for production purposes. 3. RESULTS AND DISCUSSION Previous work was performed with a fairly diluted reaction mixture, and it was first attempted to increase the total concentration in order to reduce the volume/product ratio. Under unchanged component ratios two steps of increase could be realized (Table 1) with the limiting amount of the solid material (about 17 g catalyst for 100 cm^ solvent) provided that sufficient stirring was used. The gained 9% B15C5 concentration in the reaction mixture is already a useful level for fine chemical synthesis. Table 1 Development in reaction conditions for Cu^^-KlO catalyst B15C5 concentration g/lOOcm^

Component ratio mol/mol CU^VB15C5

ACC1/B15C5

4'Ac-B15C5 % By-products (time to reach the final level) %

0.18

0.2

20

42

(3h)

traces

3.6

0.2

20

66

(Ih)

11

oa

5

60

(2h)

10

0.1

5

64

(1.5h)

liiH^^^^

8

After the first concentration jump it was established that both the catalyst/B15C5 ratio and the AcCl excess could be reduced, at practically unchanged yield (determined by HPLC) and by-product level. The amount of the by-products could not be reduced by decreasing the reaction temperature to 60°C. We have also examined how catalysts exchanged with other transition metal cations can be used for this acetylation reaction. ZnClj is good catalyst of special Friedel-Crafts reactions (activated aromatic compounds, e.g. phenols) therefore Zn -exchanged KIO was one of the selection. The results of two hours reaction time are presented in Table 2. The reaction conditions are the same as found best for Cu'^^-KlO (3.6 g B15C5/100cm', 0.1 mol Me"Vmol B15C5, 5 mol AcCl/mol B15C5). The optimum conditions for Fe^^-KlO were identical to those determined with Cu^^-KlO. Table 2 Comparison of catalysts containing different exchanged metals 4'Ac-B15C5 By-products

Acidity

rel. int.

%

%

Bronsted

Cu^'^-KlO

60

10

0.13

0.68

Zn^'^-KlO

55

10

0.16

0.73

Fe^'^-KlO

85

14

0.35

0.34

KIO(H^)

45

8

0.33

0.28

Lewis

Cu^^-KlO and Zn^^-KlO catalysts result in practically the same reaction rate and product yield. On the other hand Fe ^-KIO leads the reaction substantially faster (Fig. 1) and till a practically complete conversion, although the amount of the by-products is also higher. With a double quantity of Fe -KIO catalyst the reaction is completed in 30 minutes. For comparison the performance of the original KIO (without ion exchange) is also presented in Table 2. The differences are always significant in favour of the ion exchanged catalysts, so the introduced metal ions play an important role in the activity of the catalyst. These metal ions increase the Lewis acidity of the catalysts [10] but the efficiency does not depend directly on this acidity: Fe -KIO is particularly active although it has a lower Lewis acidity than the two other ion-exchanged catalysts. At the same time the Lewis acidity of the salts in homogeneous phase diminishes in the sequence Fe^^> Zn^^> Cu^^; CuCl2 itself is not used in this type of organic reactions. Nevertheless, the Lewis acidity could play a role because the heat treatment of the catalysts at 250°C is advantageous: in the case ofFe^'^-KlO the reached product yield increases from 55% (with only dried catalyst) to 85%. The question arises why Fe -KIO catalyst has a special reactivity in this reaction. The activity pattern observed on the same series of catalyst samples for the alkylation of aromatics by benzyl chloride i.e. Fe^^-KlO > 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.

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

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 <

^

jT^

10 -

X

sulfolane D dodecane

5 A J

P

1

()

5

1

1

1

1

10 15 20 25

time (h) Figure 1 : Transformation of the phenolphenyl acetate mixture over 500 mg of HBEA-10. Yield in HAP (%) as a function of time (h) in sulfolane and in dodecane solvents

10 20 30 40 50 PA conversion (%) Figure 2 : Transformation of the phenolphenyl acetate mixture over 500 mg of HBEA-10. Yield in heavy products (%) as a function of PA conversion (%) in sulfolane and in dodecane solvents

The initial rates of HAP production are similar in sulfolane and in dodecane (Fig 1). However, after 5 hours reaction in dodecane a constant value of the HAP yield of around 12% is obtained. In sulfolane the HAP yield increases more significantly with time and reaches a constant value, at a longer reaction time (afl:er approximately 20 hours HAP yield = 23 %). Therefore, the catalyst deactivation is faster in dodecane than in sulfolane. Figure 2 shows that the selectivity to heavy reaction products (bisphenol A derivatives etc.) which can poison the

93 active sites or block their access, is higher in dodecane than in sulfolane, which could explain the more rapid deactivation observed in dodecane. The effect of the solvent is different for the production of o- and p-HAP (Fig 3): the rate of o-HAP production is greater in dodecane than in sulfolane (about 6 times initially) while it is the contrary for p-HAP production particularly for long reaction times. The consequence of this solvent effect is that, whatever the PA conversion, the p/o HAP ratio is much greater in sulfolane than in dodecane e g around 8 in sulfolane at 30%PA conversion against 1 in dodecane.

6 -[

a

5] ^"^

^ 4

n

<

^ 2 o 1 n

]f

sulfolane 1 D dodecane | 1

U i*

1

c)

2

1

1

1

1

1

4 6 time (h)

8

10

sulfolane D dodecane

6 time (h)

8

10

Figure 3 : Transformation of the phenol-phenyl acetate mixture over 500 mg of H-BEA-10. Influence of the solvent on the yields in o-HAP (a) and in p-HAP (b). This contrary effect of solvent on the productions of o-HAP and p-HAP is in favour of the recent proposal of different mechanisms for their formation [4] : intramolecular rearrangement of PA for the formation of o-HAP

OAc

o„o

ar

-CH3

(1)

and acylation of P by PA for that of p-HAP OH

OAc (2)

p-AXAP would result from the autoacylation of PA with simultaneous formation of phenol OAc

'^ 0 II

C-CH3

OH

+

(3)

94 An excess of phenol is formed, due either to the decomposition of PA (reaction 4) and/or to the reaction of PA with water (reaction 5) or with the zeolite hydroxyl groups.

QAc

OH OH

6^

CH2=C=0 (ketene) (4)

OAc

6^

H20

*-\( ) \ + CH3COOH (5)

Water can result, for instance, from the formation of bisphenol A and of its derivatives. Indeed bisphenol A could result from the acid catalyzed condensation of two molecules of phenol with one molecule of acetone (reaction 6) resulting from the bimolecular condensation of acetic acid. Through this scheme one water molecule per molecule of bisphenol A is formed. HO

o

O + CH3-e-CH3

(6)

For simplification the production of this excess of phenol will be henceforth called PA 'hydrolysis'. The initial rate of p-AXAP formation is greater in sulfolane than in dodecane. Whatever the solvent, a maximum is observed in the yield of p-AXAP. This maximum can be explained by a secondary transformation of this product probably by 'hydrolysis' into p-HAP. At low PA conversion the ratio of the rates of p-HAP and p-AXAP productions is equal to 5 in sulfolane and to 7 in dodecane. These high values are expected from the better donor effect of the OH group compared to the OAc group. An increase in this ratio is observed at high PA conversion due to 'hydrolysis' of p-AXAP into p-HAP. The negative effect of sulfolane on the formation of o-HAP, already observed during the transformation of pure PA, could be explained by a competition between sulfolane and PA for adsorption on the acid sites. However, if this were the case, sulfolane would also decrease the initial rates of p-HAP and p-AXAP formations which involve adsorbed PA species (for the acylation of P or of PA). The initial rates of production of p-HAP and p-AXAP in sulfolane and in dodecane are less easy to compare than the rates of o-HAP production. Indeed the apparent greater production of p-HAP and p-AXAP in sulfolane could be due to the greater catalyst stability in this solvent. However, from the two experiments with H-BEA-10 (lOOmg and 500 mg) and from the experiment with H-BEA-70 it can be concluded that the initial production of p-HAP and p-AXAP is faster in sulfolane than in dodecane. Moreover, the same conclusion can be drawn from acylation of anisole with phenyl acetate or p-tolyl acetate : the initial rate of formation of p-methoxyacetophenone is higher in sulfolane than in dodecane. Therefore sulfolane would have, besides a negative effect on the reaction rates due to its

95 competition with PA for adsorption on the acid sites, a large positive effect (larger than the negative one) on the formation of p-HAP and of p-AXAP. This positive effect of sulfolane could be related to better dissociation of phenyl acetate into acylium ions.

H^

+ CH3-C=0

^

(7)

The solvation of the acylium ions would displace to the right the thermodynamic equilibrium of reaction 7. This better dissociation of phenyl acetate would favour the bimolecular formation of p-HAP and p-AXAP and would be unfavorable for the intramolecular formation of o-HAP. Experiments with equimolar amounts of P and PA (2.2 M) in sulfolane - dodecane mixtures of different compositions were carried out to specify the effect of the sulfolane concentration (Csuif) on the formation of products. The rate of o-HAP formation decreases with the introduction of sulfolane into the reaction mixture then becomes constant for Csuif ^ 1.6 M. On the other hand the rates of p-HAP and p-AXAP formation first increase with the introduction of sulfolane into the reaction mixture then decrease to remain constant for Csuif > 1.6 M (Fig 4). The curves with a maximum obtained for the latter products indicate that sulfolane has two effects on their formation : i) sulfolane increases their rate of formation owing to a solvation effect ii) sulfolane competes with the reactants for adsorption on the active acid sites, hence limiting the rate of formation of p-HAP and p-AXAP. Furthermore the decrease in the rate of o-HAP formation when Csuif increases can be explained either by the competition between sulfolane and PA for adsorption on the acid sites or by this competition plus a solvation effect. To discriminate between these two proposals the effect of the concentration of PA (CPA) and of P (Cp) on the initial rates was determined.

O)

o O-HAP

"o E E^

6 -

a p-HAP A p-AXAP

4 -

2 m E 2 4 Csuif (mol/l)

b

8 -[

2^ 0-

A~_

H

1

-\

2 4 Csuif (mol/l)

Figure 4 : Transformation of the phenol-phenyl acetate mixture over 100 mg of H-BEA-10. Rates of o-HAP (a), p-HAP and p-AXAP (b) formations as a function of the concentration of sulfolane Csuif.

96 2. Kinetic study of the transformation of the phenol-phenyl acetate mixture. The effect of Cp and CPA was determined under the following conditions : a) effect of C?, CpA = 2.2 moU'\ Cp = 0.6, 1.2 and 2.2 mol.r\ Csuif being then equal to 7.0, 6.5 and 5.5 mol.l"^ respectively b) effect of CPA, CP = 2.2 moU'\ CPA = 1.1, 2.2 and 3.2 mol.l'^ and Csuif = 6.9, 5.5 and 4.1 mol.l'V Very high sulfolane concentrations were chosen so that their change did not affect the initial reaction rates. Table 1 shows that the reaction orders are not very different in sulfolane and in dodecane for the formation of p-HAP and p-AXAP. On the other hand the effect of Cp and CPA on the rate of o-HAP formation is completely different in sulfolane and in dodecane. Table 1 : Transformation of the phenol (P), phenyl acetate (PA) mixture on a H-BEA-10 zeolite sample. Reaction orders. Formation of With respect to o-HAP p-HAP p-AXAP Solvent 2 PA Sulfolane 1 1 1.5 0.3 0.6 Dodecane P Sulfolane -1 0.1 0.1 -1 -0.5 0.2 Dodecane The following scheme in which the surface reaction (step b) is the limiting step has been previously proposed [4] to explain the formation of o-HAP. X is the active acid site. PA+X PA-X o-HAP-X

^

^

^ ^

PA-X

(a)

o-HAP-X

(b)

o-HAP+ X

(c)

This reaction scheme which does not take into account solvation effects leads to the following equation of the initial rate Vo

1+ Kp^ CpA + K p Cp + K s Cs with kb the rate constant of step b, C^ the total concentration in acid sites, KPA, KP and Ks the equilibrium constants of adsorption of PA, P and S (the solvent). Equation 8 can explain the effect of Cp and CPA on the reaction rate found in dodecane : KPA CPA, in comparison to 1+Kp Cp, is not negligible i.e that PA is strongly adsorbed. Phenol is also strongly adsorbed (reaction order around-0.5). This equation can also explain the effect of Cp and CPA on the rate of o-HAP formation in the sulfolane solvent if we admit that sulfolane is strongly adsorbed on the acid sites. Indeed, in this case, Ks Cs will be greater than the other terms of the denominator, which leads to reaction orders with respect to PA equal to 1 and to P equal to 0 i.e practically to the experimental values (Table 1). Therefore, if a solvation effect on the rate of o-HAP formation cannot be excluded, all the kinetic resuks can be explained just by the competition between sulfolane and the reactants for adsorption on the acid sites.

97 3. Influence of the acylating agent. A preliminary study of the effect on the anisole acylation rate of the aromatic acetate used as acylating agent, was carried out in sulfolane under the standard conditions. Phenyl acetates substituted with donor groups (acetate, methyl, methoxy) were chosen. Indeed these donor groups should increase the basicity of the oxygen atoms of the acetyl group, hence favouring the formation of the acylium ion. Whatever the acetate, p-methoxyacetophenone was practically the only product of anisole transformation (selectivity >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.

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

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 (lO^Og-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 (<10%) of the non desorbed products are soluble in methylene chloride when the zeoUte is treated by soxhlet. However, after dissolution of the zeolite in a hydrofluoric acid solution and extraction with methylene chloride all of the products were recovered. This shows that most of the non desorbed products are located inside the zeoUte pores. With all the zeolites these products are constituted of mono, bi and triheptyltoluenes.

110

2.1. Activity and stabiliiy of the zeolite samples. The initial rates of 1-heptene isomerization and of alkylation were estimated from curves : the conversion of 1-heptene into isomers and of n-heptenes into alkylation products plotted as a function of time. To obtain accurate values the experiments were carried out over three different weights of catalysts. The initial rates obtained with the series of FAU zeolites are plotted as a function of nH+, i.e. the theoretical number of acid sites estimated from the unit cell formula (Figure 2). With both reactions the most active catalyst is HFAU30 (nH+=:3.2 1020g^l). The shape of the curve represented in figure 2 is the one predicted by the topologic model of Barthomeuf (11-12). For low values of nH+ (hence of NAI the number of framework aluminium atoms per unit cell) the A104" tetrahedra have no close Al neighbour atom in the second layer of T atoms and the acidity of the protonic sites is maximal. Above a certain value of NAI, the aluminium atoms are no longer isolated and the strength of the corresponding protonic sites, hence their activity is lower. However, the value of NAI for which this situation occurs in the case of FAU zeolites was estimated to be equal to 28, which corresponds to a framework Si/Al ratio of 5.8 i.e. a Si/Al ratio much lower than the one corresponding to the maximum activities. Moreover, the alkylating activity of HFAU4 is abnormally low. All this indicates that acidity is not the only parameter determining the HFAU activity. i

^Ao (10"^mol h''' g"'')

600 -

/V

400

1-/ X. /

200

IKA

0 -^

MOR\

MAZ BE)^^s^ T| ^^1 1 10 + 20 nH (10

Figure 2 : Initial rates R of isomerization (I) and of alkylation (A) as a function of the theoretical number of protonic sites (nH+).

-P-

>

15 -1

sites g )

Figure 3 : Initial activities (Ao) of toluene alkylation as a function of the theoretical number of protonic sites (nH+).

The same occurs with HMOR zeolites : the maximum activity is found for a Si/Al ratio of 80 (Figure 3) while the aluminium atoms become isolated for Si/A1^9.4 (11, 12). Moreover the alkylating activity of HMORIO, the non dealxmiinated sample, is very low, much lower than that of a HFAU zeolite with the same number of acid sites. This is also the case for the non dealuminated HMAZ and HBEA samples. However, figure 3 shows that the greater the Si/Al ratio of the HMOR samples (hence the more pronounced their degree of

Ill

dealumination) the smaller the difference in activity between the HMOR and the HFAU samples. This can be explained by the presence of mesopores created by dealumination. Indeed these mesopores sJlow a quasi tridimensional diffusion of the reactant molecules (13) favouring the desorptionof the alkylates. The activity of non dealuminated HMAZ and HBEA samples is also very low, and with HMFI, an average pore size zeolite, alkylation occurs with the alkylation products remaining trapped inside the pores (13wt% of non desorbed mono and biheptyltoluenes after 10 hours reaction) thus no products appear in the liquid phase. Limitations in the desorptionof alkylation products from non dealuminated large pore zeolites could also be responsible for their low activity. In agreement witfithis, the lower the amount of non desorbed products retained in the HFAU or HMOR zeolites at 100% conversion of heptene into alkylation products, the faster the alkylation (Table 2). The increase in the alkylation activity caused by dealumination can be related to the creation of mesopores which would favour the desorption of the alkylation products. However, with this hypothesis a h i ^ alkylating activity should be found with the HBEAIO sample whose crystallite size is very small (around 200A). This is not the case, most likely because the extraframework aluminium species present in this sample Umit the desorption of alkylation products. In agreement with this proposal the amount of products retained in the pores of HBEAIO after complete conversion of heptenes into alkylation products is approximately 10 times greater than that retained in the pores of HFAU30 (Table 2). Table 2 Initial rates of alkylation of toluene, A (10'^ mol.h"l.g"l) and percentage of non desorbed products, C(wt%) at complete conversion of alkenes into alkylation products. Zeolite HFAU HFAU HFAU HFAU

4 16 30 100

A 20 350 600 40

C 23.0 8.1 2.3 9.6

Zeolite HMOR HMOR HMOR HBEA

10 20 80 10

A 7 27 120 13

C 4.4 2.8 2.2 23.5

2.2. Product distribution As shown in figures 4 for HFAU30, double bond shift and toluene alkylation occur through consecutive schemes with all the zeoUte samples: 1- Heptene ,^ ^ 2- Heptenes .^ ^ 3- Heptenes Toluene ^ ^ = = ^ Monoheptyltoluenes (M) ^

^

Biheptyltoluenes (B)

However the product distribution depends on the zeoUte. Thus, with HFAU30, biheptyltoluenes appear at a heptene conversion of 30%, 40% with HBEA, 60% withHMORSO and 80% with HMAZ. Whatever the heptene conversion, M l in the monoheptyltoluene fraction is more favoured with HMOR80 than with the other zeoKtes.

112

FAU30 Vo

100

M 80

60 '

40 B

20 f 20

0

1

i

40

60

^ 80

100

X(%)

Figure 4a : Percentages of 2-heptenes Figure 4b : Yields in mono (M) and (2-C7=)and3-heptenes(3-C7=)inthe biheptyltoluenes (B) as a function of heptene mixture as a function of the ^^P^^^ conversion into alkylates (X). percentage of isomerized 1-heptene (Xi):Xi = 2C7= + 3C7=. Figure 5 illustrates the change in the monoheptyltoluene distribution on the more active faiyasite sample (HFAU30) as a function of n heptene conversion. At low conversion, Ml is formed with a high selectivity (>90% at zero conversion). The percentages of M2 and M3 increase with conversion at the expense of Ml, which can be related to the increase in the amount of 2-heptenes and 3-heptenes in the heptene mixture (Table 3). 80 T

FAU30

MOR80

00 -1 Ml

60 +

-*M1

80 60

5^ 40 I 40 20

20

M2

0

-I I I

_

20

40

60

80

100

X (% heptenes)

Figure 5 : Distribution of monoheptyltoluenes on HFAU30 as a fimction of heptenes conversion into alkylation products (X).

29

1

40

60

"T"- ^ 80

M3 1

100

X (% heptenes)

Figure 6 : Distribution of monoheptyltoluenes on HMOR80 as a function of heptenes conversion into alkylation products (X).

113

By considering the relative distributions of heptenes and monoheptyltoluenes (Table 3) with respect to the alkylation scheme (Figure 1) it can be concluded t h a t the monoheptyltoluene distribution is determined by the heptene distribution. There is no shape selective effect: steric constraints on certain alkylation steps or limitations of the desorption of the bulkier monoheptyltoluenes. The absence of limitations in the monoheptyltoluenes (M) desorption is confirmed by the very low amount of M in the non desorbed products (5%) and by the fact that the distribution of these monoheptyltoluenes is identical to that found in the desorbed products. Table 3 Comparison of the compositions of the mixtures of heptenes and of monoalkylation products at 40 and 90% conversion (X) of heptenes into alkylation products. X(%)

HFAU30

HMOR80

40

1-C7=:71% ; 2-C7=:24% ; 3-C7=:5% Mi:70% ; M2:25% ; M3:5%

1-C7=:50% ; 2-C7=:45% ; 3-C7=:5% Mi:88% ; M2:10% ; M3:2%

90

1-C7=:50% ; 2-C7=:40% ; 3-C7=:10% 1-C7=:30%; 2-C7=:60% ; 3-C7=:10% Mi:60% ; M2:30% ; M3:10% Mi:85% ; M2:10% ; M3:5%

Whatever the n-heptene conversion (Figure 6) the percentage of M l in the monoheptyltoluene fi*action found with the more active mordenite sample (HMOR80) is higher than the one found with HFAU30 (Figure 5). This observation is unexpected when the heptene distributions are compared at identical conversions (Table 3). Therefore this high selectivity to Ml can only be explained by the shape selectivity of mordenite. This selectivity could be partiy due to difficulties in the desorption of the more bulky M2 and M3 products. Indeed non desorbed products contain a larger amount of M products with HMOR80 than with HFAU30 (56% against 5%). However this diffusion control plays only a limited role, for non desorbed monoheptyltoluenes have practically the same composition as the desorbed monoheptyltoluenes (85% of M l instead of 90%). Therefore the high selectivity to M i is most likely due to steric constraints affecting the alkylation of toluene into M2 and M3 in the mordenite pores. EXPERIMENTAL The zeoUte samples were suppUedfi^om PQ (HFAU, HBEA, HMFI), fi-om Zeocat (HMOR) and fix)m Elf Aquitaine (HMAZ). These samples were characterized by IR spectroscopy (structure bands), by nitrogen adsorption at 77 K and by NH^ stepwise TPD. Alkylation of toluene with heptene was carried out in liquid phase under the following operating conditions : 90''C, toluene/alkene molar ratio of 3, 0.07 to 0.5 g powdered zeoUte with a stirring rate of 200 RPM. Before introduction into the toluene/alkene mixture the zeoUte samples were activated under dry air

114

flow at 500^*0 for 12 hours. Small samples of the reaction mixture (approximately 50 jil) were taken at various reaction times, diluted with methylene chloride and analj^ed by gas chromatography on a 25 m column of fused silica (DB5). The reaction products were identified by gas chromatography/mass spectrometry coupling. The experimental methods used to recover and to analyze the non desorbed products are described in ref. 14. CONCLUSION The activity of zeolites for alkylation of toluene with 1-heptene depends on their acidity and on the ease of desorption of alkylates. Thus average pore size zeolites such as HMFI and monodimensional large pore zeolites are practically inactive. Extraframework aluminium species which Hmit the product desorption have a negative effect, while mesopores (in particular with monodimensional zeolites such as HMOR) have a positive effect. HMOR zeolites are more selective for 2phenylalkanes than HFAU zeolites. This is mainly due to shape selective preference via transition state control. REFERENCES 1. P.R. Pujado, in "Handbook of Petroleum Refining Process" (R.A. Meyers, ed.) McGraw-Hm,1986. 2. J.L.G. de Almeida, M. Dufaux, Y. Ben Taarit and C. Naccache, J. Am. Oil Chem. Soc, 71 (1994) 675. 3. R.J. Larson, T.M. Rothgeb, R.J. Shimp, T.E. Ward and R.M. Ventullo, J. Am. Oil Chem. Soc, 70 (1993) 645. 4. J.L.G. de Almeida, M. Dufaux, Y. Ben Taarit and C. Naccache, Appl. Catal. A : General 114 (1994) 141. 5. S. Sivasanker, A. Thangar^], J. Catal. 138 (1992) 386. 6. S. Sivasanker, A. Thangar^', R.A. Abulia and P. Ratnasamy, in L. Guczi et al. (Eds.), Stud. Surf. Sci. Catal. vol. 75, Elsevier, Amsterdam(1993) p.397. 7. A. Mourran, P. Magnoux and M. Guisnet, J. Chim. Phys., 92 (1995) 1394. 8. L.B. Zinner, K. Zinner, M. Ishige and A.S. Araujo, J. Alloys and Compounds, 193 (1993) 65. 9. P. Magnoux, A. Mourran, S. Bernard and M. Guisnet. in J.Weitkamp and B. Liicke (Eds.), Proceedings of the D.G.M.K. Conference "Catalysis on SoUd Adds and Bases". Berlin (1996) p.49. 10. C. Coutanceau, J.M. Da Silva, M.F. Alvarez, F.R. Ribeiro and M. Guisnet, J. Chim. Phys., to be published. 11. D. Barthomeuf, Mater. Chem. Phys., 17 (1987) 49. 12. D. Barthomeuf, in J.W. Ward (Eds.), Stud. Surf. Sci. Catal. vol. 38 Elsevier, Amsterdam (1987) p.l77. 13. N.S. Gnep, Ph. Roger, P. Cartraud, M. Guisnet, B. Juquin and Ch. Hamon, C. R. Acad. Sci., 309 (1989) 1743. 14. M. Guisnet and P. Magnoux, Appl. Catal., 54 (1989) 1.

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

115

Reductive O- and N-alkylations. Alternative catalytic methods to nucleophilic substitution. F. Fache, V. Bethmont, L. Jacquot, F. Valot, A. Milenkovic and Marc Lemaire* Institut de Recherches sur la Catalyse, Laboratoire de Catalyse et Synthese Organique, Universite Claude Bernard Lyon I, CPE, 43 bd du 11 nov. 1918, 69622 Villeurbanne CEDEX France. Different amides and anilines have been selectively mono-N-alkylated using catalytic heterogeneous palladium and carbonyl compounds as alkylating agents. The same method has been applied to the synthesis of ethers from alcohols. Reaction parameters have been studied in details and a mechanism is proposed. 1. INTRODUCTION The increase of environmental awareness forces chemists to reconsider the way they used to work and to take into account the concept of atom economy 1. One solution to the waste problem is the replacement of stoichiometric reagents generating organic or inorganic by-products by catalytic methods. Thus, nowadays, the main challenge is not only the complexity of the target molecule but the way to synthesize it. There is a need for simple, selective and efficient catalytic methods allowing to perform whatever modification of basic structures we need. Therefore, we studied two well-known and extremely useful classical organic reactions, the N-alkylation of anilines and amides and the Williamson's synthesis of ethers in order to turn them into catalytical ones. Moreover, due to the advantages of heterogeneous catalysis versus homogeneous one in terms of catalyst separation and recycling, we searched for heterogeneous catalytic systems. We have reconsidered the old Eschweiler-Clarke procedure^ which allowed the reductive alkylation of amines with aldehyde under hydrogen with P d / C . This method is widely used in organic synthesis for the N-alkylation of amines^. Generaly speaking it is considered to be unefficient on amides. Analysis of the reaction mechanism made us hope a possible transposition to amides, anilines and alcohols (Figure l).The first step of the reaction is the nucleophilic attack of the amine on the carbonyl compound leading to the hemiaminal A^ in equilibrium with the starting materials. Then, there are two possibilities : the first one is the hydrogenolysis of the hemiaminal A into the corresponding amine C. The second one is the dehydration of A into the related imine B. which after hydrogenation leads to the amine C.. This compound can undergo further Nalkylation via the hemiaminal D which can give after hydrogenolysis either the

116 tertiary amine F directly or first be dehydrated into the enamine E if at least one of the substituents on the nitrogen atom is a primary or secondary alkyl group. O R1NH2 R2

R2

Dehydration^

R,—N

Rl—N R'l

R3

R2

R3

Hydrogenation

Imine B

Hemiaminal A HO ^2

H

-H2O

Hydrogenolysis

^^\0^

H Ri — N Monoalkylation Secondary amine C O

R2

A' R3

Ro

Y

R3 Dehydration OH

R2

R,—N

R3

R2 R3 Enamine E if R2 primary or secondary alkyl

Hemiaminal D

-H2O

R2

r

R3

Hydrogenolysis

R3

Rl—N^

p R2

Dialkylation

Tertiary amine^

Figure 1 : mechanism of the N-mono or di-alkylation of amines.

117 In the reverse case, for the dimethylation with formaldehyde for example (R2 = R3 = H) , the dehydration is impossible. Thus, in this last case, the only possible pathway is the hydrogenolysis of the hemiaminal D. The competitive hydrogenation of carbonyl must be considered carefully because it influences the choice of the reaction conditions. We assume that with amides, anilines and alcohols similar mechanisms should be possible. 2. RESULTS and DISCUSSION 2.1.N-alkylation of amides Table 1 : influence of the substrate and of the alkylating agent. Substrate

isolated yield

Product

Alkylating agent

(%)

Q.

octanal H

Ok N

^ 0

1

octanal

Ok

N

^ 0

1

butanal

a

acetone

Ok

N 1 C4H9

H

Ok 1

N

A

H

Ok N

^ 0

1

N

^ 0

1

0

Ok

0

N 0 1 CHj-Ph

1 CH3 f

butanal acetone

V

C-N—C4H0

—

/

butanal

97

0 >—O-N

II H 0

(

81*

\

> - C - N - Q H ,

}—C-NH. 0

81

formaldehyde

>—C—NH2 0

0

93

7 — C— NH2 ^^^"^

98

Ok

H \

0

benzaldehyde

H

Ok

93

N 0 1 QH17

H

Ok

96

89

0

Conditions : Carbonyl/amide = 4(molar ratio), Pd/C (10%) = 2%mol/amide, Na2S04 (99.99%)/amide = 1 (molar ratio), 40 bar H2, 100°C, AcOEt = 10ml, [substrate] = IM, 4h.M8h.

118 The N-alkylation of amides is an important reaction involved in the synthesis of numerous amines^. Most of the time, strong bases and alkyl halides are required to perform such a reaction^. These methods are not chemioselective mainly due to the competitive O- or C-alkylation. Moreover, mono- and di-Nalkylated amides are generally obtained. In the literature, methods using phase transfer catalysis or inorganic solids were proposed but they are limited to aromatic amides^ or lead to poor yields^. In both cases these methods generate salts due to the use of alkylhalides and bases. Catalytic methods have also been reported. Ruthenium^ has been used with alcohols as alkylating agents generating only water but this procedure could not be applied to lactames and the yields fell down when the chain length of the alcohol diminished. We have developed the reductive alkylation of amides with carbonyls^ and optimized the reaction parameters (Table 1). This work is based on the assumption that the hemiaminal form of the amide is reduced faster than carbonyls. We went to the conclusion that a polar aprotic solvent is necessary as well as an excess of the alkylating agent to go to complete conversion. We have also to shift the reaction towards the proposed intermediate, the hemiaminal, and therefore we need to add Na2S04 as dehydrating agent. When using a preactivated palladium (Pd/C 10% Aldrich treated at 100°C, 2h, under H2), the reaction can be performed using 0.2% of Pd/substrate whereas 2.5% are necessary with a non activated one. In order to make our method easy to reproduce in a classical organic laboratory, we describe here results obtained with an inactivated catalyst, which is also much easier to handle. This method allowed the selective mono-N-alkylation of primary amides and lactames with good isolated yields (>80%) using aldehydes or ketones as alkylating agents. No O- or C- alkylation was detected. Above all, it generated only little amount of by-products as both the catalyst and the dehydrating agent could be recycled. Finally, if we consider the mechanism, there are evidences of hydrogenolysis in the case of several products but the pathway dehydrationhydrogenation could not be excluded. 2.2. N-alkylation of anilines The N-alkylation of aniline derivatives provides access to numerous interesting products such as fungicides or herbicides^^. Most of the time, the alkylating agent is a chloro or bromo reagent which generates halogenated byproducts^O'ii. Several other methods, catalytic or not, have been reported but they either lead to poor yields or need harsh conditions'^"!"*. The procedure reported by Watanabe et al. '^ using RuCl2(PPh3)3 as catalyst is much more convenient but limited to primary alcohols. We have used the same methodology as for amides in order to obtain selective alkylation of aniline(Table 2). Unnatural N-aryl dipeptides can be easily obtained using our procedure direct acylation of a-aminoacid esters with nitro-benzoyl chlorides followed by reductive alkylation leads to the preparation of derivatives of methyl prolinate 1 and methyl valinate 2 (Table 3). A small but significant diastereoisomeric excess (d.e.) of 20-30% is measured by ' H NMR. Optimisation of the reaction conditions as well as of the nature of the inductor may generate better d.e.. These

119 Table 2 : N-alkylated anilines obtained by reductive alkyktion. Product Isolatedyield (%) Starting material Alkylating agent

o'

V

aK

V

o'

95

88

CyHgCHO CH3CHO CH3CHO

O'

85

o a'

r

86

78

78 u

78 NH2

H N-

u

^ COOMe

COOMe

60

COOMe NH,

u

67 NHj

'

78

83 Conditions : Carbonyl/aniline= 4 (molar ratio), P d / C (10%) = 2.5%mol/aniline, N a 2 S 0 4 (99.99%)/aniline = 1 (molar ratio), 50 bar H2, 20°C, cyclohexane= 20ml, [substrate] = 2M, 4h-24h.

120

preliminary results show that our method can lead to the formation of new unnatural aminoacid precursors (Table 3). Table 3 : diastereoselective experiments. Synthesis of unnatural dipeptides d.e. (%) Product Starting material Alkylating Isolated agent yield (%) H,COOC

HjCOOC*^

Y ^ OMe 0

1

4(|V'''^ H 6

0

6

60

20

COOCH, 13 16

COOCH3

^

30

H ^

6 1

0

93

H 7^ " ' COOCH, 9 10' 1

Conditions : see Table 2.

2.3. Synthesis of ethers One of the oldest reaction used by organic chemists is the synthesis of ethers according to Williamson. Thus, since 1850, a great variety of bases and alkyl halides had generated the desired ethers but also their counterparts, the corresponding inorganic salts^^. Moreover, the reaction conditions are generally unusable for base sensitive molecules. Industrially, diisopropylether is synthesized from isopropanol in strong acidic conditions!^. Metal from group VIII have been used for catalytic reaction of alcohols on olefins^^ or on carbonylsl^ but they either led to poor yields or (and!) required stoichiometric amounts of acids. Using the same approach as for the N-alkylation of amides^ we performed the catalytic synthesis of ethers from both aldehydes or ketones with palladium as catalyst^O. Nevertheless, in the case of ethers, the proposed intermediate, the hemiketal is easy to synthesize and it is not necessary to find special conditions for its formation. This method allowed the selective formation of ethers from both aldehydes or ketones with good to excellent isolated yields (up to 95%) and the catalyst could be recycled. There are evidences for the hydrogenolysis pathway (Table 4, last entry) but nothing can be excluded. We have described the synthesis of dissymmetric ethers because of their higher synthetic interest but, of course, symmetric ones could be obtained in the same way, with probably higher selectivity as the only observed by-product is due to carbonyl reduction. 3. CONCLUSION We have shown that heterogeneous catalysis can be applied to reductive alkylation with success in reactions such as ether synthesis or N-alkylation of amides and anilines. Concerning the mechanism, several pathways are in competition depending on the structure of the substrate and of the alkylating agent. The important point is that both the product of addition (the hemiacetal or hemiaminal) and the product of elimination (imine, enamine or enolether)

121

which are the probable intermediates of the reaction, are reduced faster than the carbonyl compound, which makes the reactions possible. We plane to go further and to extend the scope of application of heterogeneous catalysis in organic synthesis by systematically reconsidering the classical synthetic methods Table 4 : synthesis of ethers Substrate

Solvent

Product

Isolated yield(%)

Octanal

1-butanol

C4H9-O-C8H17

95

Octanal

2-butanol

80

0 CgHn

methanol

Octanal Octanal

CH3-O-C8H17

n

2methoxyethanol

—0

O-CgHn

95 95

Hi3C6^^ 1-butanol

2-octanone

0

92

1

C4H9 Hi3C6v^^ 2-octanone

2-butanol

1-octanol

acetone

CgHn-O-iPr

90

r^r-butanol

octanal

-

0

octanal

-

0

octanol

CF3(CF2)2CHO

-

0

octanol

(CH3)3CCHO

^o-c8Hi7

75

0

3 r

50

Conditions : [substrate] = 0.2M, Pd/C = 2.5%mol./substrate, 100°C when the substrate was an alcohol, 50°C when the substrate was a carbonyl compound, 40 bar H2.

4. TYPICAL PROCEDURE Amide, aniline or alcohol, carbonyl compound, N a 2 S 0 4 (leq. molar/substrate) and P d / C 10% (2. 5% molar/substrate) were mixed in the

122 appropriate solvent and put under H2 pressure (40 bar) at the appropriate temperature until complete conversion of the starting material. After filtration and evaporation of the solvent the crude product was obtained (for more details see tables). The reaction is monitored by gas chromatography (J&W, DB1701, 25 m X 0.25 mm0). In the case of : -amides : 100°C, solvent AcOEt, -anilines : 20°C, solvent: cyclohexane, -ethers : 50°C or 100°C, see table, solvent: the alcohol or the carbonyl compound. P d / C (10%): Acros (19503-0500); Na2S04 (99.99%+): Aldrich, 20,444-7. REFERENCES 1. B.M. Trost, Science, 254 (1991) 1471; R.A. Sheldon, Chem. Ind., (1992) 903. 2. J. March, "Advances in Organic Chemistry", third Edition, John Wiley and Sons, eds., Wiley Interscience Publication, N. Y. 1985, p. 798. 3. P.N. Rylander, Academic press. Inc. London limited, 1985. 4. P.G. Mattingly and M.J. Miller, J. Org. Chem., 46 (1981) 1557. 5. W. Fones, J. Org. Chem., 14 (1952) 1099; J. Park and R. Englert, J. Am. Chem. Soc, 74, (1952) 1010; D.W. Slocum and F.E. Stonemark, J. Org. Chem., 38 (1973) 1677. 6. T. Gajda and A. Zwierzak, Synthesis, (1981) 1005. 7. J. Yamawaki, T. Ando and T. Hanafusa, Chem. Lett., (1981) 1143; K. Sukuta, Bull. Chem. Soc. Jpn, 58 (1985) 838. 8. Y. Watanabe, T. Ohta and Y. Tsuji, Bull. Chem. Soc. Jpn, 56 (1983) 2647. 9. F. Fache, L. Jacquot and M. Lemaire, Tetrahedron Lett., 35 (1994) 3313; F. Fache, V. Bethmont, L. Jacquot and M. Lemaire, Reel. Trav. Chim. Pays-Bas, 115 (1996) 231. 10. W. Lunkenheimer and W. Brand, Ger. Offen., 2,915, 026 (1979); R. Bader, P. Flatt, P. Radimerski, Eur. Pat., 605,363 (1994); Ibid, U.S. Pat., 5,430,188 (1995). 11. M. Onaka, A. Umezono, M. Kawai and Y. Izumi, J. Chem. Soc, Chem. Commun., 17 (1985) 1202. 12. Y. Watanabe, N. Suzuki, S.C. Shim, M. Yamamoto, T. Mitsudo and Y. Takegami, Chem. Lett., 4 (1980) 429. 13. H. Hauck and H.J. Nestler, CA 95 : 42611s, Ger. Offen. 2, 941, 070 (23 apr. 1981). 14. H.R. Morales, M. Perez-Juarez, L. Cuellar, L. Mendoza, H. Fernandez and R. Contreras, Synth. Commun., 14(13) (1984) 1213. 15. Y. Watanabe, Y. Tsuji, H. Ige, Y. Ohsugi and T. Ohta, J. Org. Chem., 49 (1984) 3359. 16. S.D. Burke, F.J. Schoenen and M.S. Nair, Tetrahedron Lett., 28 (1987) 4143. 17. A. P. Lurie, Kirk-Othmer Encyclopedie, 2^ Edition, John Wiley and Sons, eds., Wiley Interscience Publication, N. Y. 1971, vol. 8, p.470. 18. J. Ehlers, G.J. Hochstadt, P. Greenough, M.P. Atkins and W.J. Ball, Ger. Offen. DE 3, 813, 689; 17 Nov. 1988. 19. M. Verzele, M. Acke and M. Anteunis, J. Chem. Soc, (1963) 5598. 20. V. Bethmont, F. Fache and M. Lemaire, Tetrahedron Lett., 36(24) (1995) 4235.

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

123

N-methylation of aniline over AlPO^ and AlP04-metal oxide catalysts F. M. Bautista, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas and A. A. Romero Organic Chemistry Department, Faculty of Sciences, University of Cordoba, Avda. S. Alberto Magno s/n, E-14004 Cordoba, Spain.

Summary The alkylation of aniline with methanol was studied over a series of middly acidic AlPO^ and AlP04-metal oxide (25 wt% AI2O3. TiOj or Zr02) catalysts prepared in different ways. Surface acidity was measured by pyridine adsorption and its effect on aniline alkylation activity is discussed. The influence of feed rate, time-on-stream and temperature on the activity and selectivity of the products, namely N-methyl (NMA) and N,N-dimethylaniline (NNDMA) were also investigated. Selectivity to N-alkylation (NMA+NNDMA) attained 100 mol% in the temperature range 523-623 K. Besides, N-methylaniline decreased as contact time/temperature increased. At 673 K, N,N-dimethyltoluidines (NNDMT, p->o-) appeared, although in very small amounts. AIPO4-AI2O3 proved to be a better catalyst for aniline methylation than AIPO4. Moreover, AlP04-Zr02 shows little acidity and hence decreased methylation ability with respect to AIPO4.

1. INTRODUCTION Aniline alkylation is an industrially important reaction due to the fact that alkylanilines form the basic raw materials for the synthesis of organic chemicals and chemical intermediates or additives in dyes, synthetic rubbers, explosives, herbicides and pharmaceuticals. Vaporphase aniline alkylation over environmentally safe solid catalysts is an answer to the conventional method of producing alkylanilines using mineral acids and Friedel-Crafts type catalysts. Metal oxides [1] and more recently zeolites [1-3] are conmionly used as catalysts. Previous studies indicated that the major factors influencing the activity and selectivity of vapor-phase aniline methylation are acid-base properties (number and strength) and shapeselectivity in the solid catalyst as well as reaction conditions (temperature, composition and feed rate). The activity of acid catalysts for this reaction might be suppressed by the adsorption of aniline since the aniline itself has a strong basic characteristic. Moreover, aniline molecule can protonate reducing the effective concentration of reactant. It therefore seems that the use of a strong acid catalyst is not suitable for this reaction. In the present work, we have carried out the vapor-phase N-methylation of aniline over middle acidic AIPO4 and AlP04-metal oxide (25 wt% AI2O3. Ti02 or Zr02) catalysts [4]. A comparison with strongly acidic SAPO-5 [5] and a commercial Si02-Al203 (13 wt% AI2O3, Si-235 Harshaw Chemie) is also drawn. Our results indicate that N-methyl (NMA) and N,N-dimetylaniline (NNDMA) are almost exclusively formed by a sequential reaction path.

124

2. EXPERIMENTAL 2.1. Catalysts AIPO4 (AP) catalysts were obtained by precipitation, from aluminum chloride and H3PO4 aqueous solutions, with aqueous ammonia (A), ethylene oxide (E) or propylene oxide (P). AIPO4-AI2O3 (25 wt% AI2O3,) catalysts were obtained by adding aluminum hydroxide to a reaction medium where the precipitation of AlPO^, from aluminum chloride and H3PO4 aqueous solutions was initiated by the addition of aqueous ammonia (APAl-A), ethylene (APAl-E) or propylene oxide (APAl-P). The total precipitation of AIPO4 was carried out in all cases with aqueous ammonia. AlP04-Ti02 (25wt% Ti02) and AlP04-Zr02 (25wt% Zr02) were prepared by adding, respectively, commercial Ti02 (anatase) or Zr02 (monoclinic, ceramic grade) to a reaction medium where the precipitation of the AIPO4 was initiated, and later completed, by the addition of aqueous ammonia, ethylene or propylene oxide (A, E or P samples). After filtration, washing and drying at 390 K, all the solids were calcined at 923 K for 3 h. Details on the characterization of all catalysts have been previously described [4]. A comparison with strongly acidic SAPO-5 [5, 7] and a commercial Si02-Al203 (13 wt% AI2O3, Si-235 Harshaw Chemie) is also drawn. The surface area (Sggj), pore volume (V ) and mean pore radius (r ) are collected in Table 1. 2.2. Surface acidity measurements The surface acidity was measured in a dynamic mode by means of the gas-phase adsorption of pyridine (PY) using a pulse chromatographic technique [11,12]. 2.3. Catalytic activity measurements The alkylation of aniline with methanol was carried out in a down-flow reactor (Ld, 6 mm, length 120 mm) and at atmospheric pressure. Catalyst samples (ca. 120 mg, <0.149 mm) were pretreated for 1 h at 523 K in a nitrogen flow (3 L h" , 99.999%). The aniline-methanol mixture (1-10 M) was feed from the top through a calibrated motorised syringe (Harward 44) to achieve different WHSV and different aniline conversion levels. Experiments were carried out at temperatures in the range 523-673 K (mostly 523 K). Blanks runs at 673 K showed that the thermal reaction could be neglected. The products were sampled on-line every 15 min and analyzed by GLC by using a column (2nix3mm) of 10% Carbowax 20M/2% KOH on Chromosorb W-AW 80/100. The alkylated anilines were identified (HR-MS, VG AutoSpec) as being mainly N-methyl (NMA) and N,N-dimethylaniline (NNDMA) and minor amounts of N,N-dimethyltoluidines (NNDMT, p- > 0-). The conversions reported here are on a methanol free aniline basis and selectivities are expressed as the ratio of moles of a given product to the sum of moles produced (mol%). AN and methanol (99.5 %, Merck) were used without further purification.

3. RESULTS AND DISCUSSION 3.1. Surface acidity Although pyridine is not specific because it interacts with both Bronsted and Lewis acid sites, the conversion of Lewis sites into Bronsted ones cannot be ruled out since the alkylation reaction generates water. So, the pyridine adsorption can still be useful for surface acidity measurements. Thus, the surface acidity of catalysts is given in Table 1 as the amount of

125 Table 1 Textural properties and surface acidity v^:. PY of AlPO^ and AlPO^-metal oxide catalysts Catalyst

^BET

P

Pyridine (mol/g)

p

(m^/g)

(mL/g)

(nm)

373 K

AP-A

109

0.48

8.8

AP-P

228

APE

573 K

673 K

135

1^

~

0.75

6.6

336

23

a

242

0.52

4.3

417

33

a

APAl-A

244

0.37

3.1

234

41

12

APAl-P

319

0.67

4.2

247

70

30

APAl-E

242

0.54

4.5

272

67

35

APTi-A

192

0.73

7.6

195

9

a

APTi-P

258

0.81

6.3

321

20

a

APZr-A

82

0.71

17.3

_b

7

a

APZr-P

193

0.69

7.1

_b

21

a

APZr-E

204

0.57

3.6

_b

11

a

Si-235

318

0.57

3.6

192

81

47

1.7

b

L60

141

SAPO-5

214

0.18

There is no adsorption of the probe molecule.

Not measured.

pyridine (PY) adsorbed at saturation at a given temperature. As expected, on increasing temperature from 373 to 673 K, the surface acidity gradually decreases since only the strongest acid sites retain the adsorbed base. The acidity measurements show that the total acid amount value of AIPO4 (AP) and AIPO4AI2O3 (APAl) catalysts depends upon the gellification medium, increasing in the order ammonia < propylene oxide < ethylene oxide. Besides, total acidity in AP catalysts is great compared to that of APAl ones except when the catalyst is obtained in aqueous ammonia. Notwithstanding, APAl catalysts display a greater number of medium and strong acid sites (measured vs. PY at 573 and 673 K, respectively) regardless of the precipitation medium. Also, it can be seen that the surface acidity is almost the same for APAl-P and APAl-E catalysts while for the APAl-P sample, the decrease in acidity, as adsorption temperature increases is faster than that of the APAl-E sample, suggesting a lower acid strength of its acid sites, and supporting the higher catalytic activity achieved with the APAl-E catalyst in typical acid-catalyzed reactions, like cyclohexene isomerization [6] and toluene methylation [4]. Moreover, APTi and APZr catalysts are less acidic than AP and APAL ones. Furthermore, SAPO-5 and Si-235 catalysts displayed a greater number of medium and strong acid sites than AP and APAl catalysts. Thus, SAPO-5 is the most acidic catalyst used here.

126 3.2. Catalytic measurements The alkylation of aniline with methanol on AIPO4 and AlPO^-metal oxide catalysts produces NMA and NNDMA as principal products in all cases. NNDMT (p->o-) appeared only at the highest reaction temperature and in small amounts. In the absence of boundary layer, inter- and intraparticle diffussional influences (feed rates over 3 10" mol/s and catalyst particle sizes lower than 0.149 mm) aniline conversion data (X^j^) are fitted in a first-order rate equation In [1/(1-X^)] = k (W/F) where W is the catalyst weight and F the feed rate. The slope of the straight lines yields the values of the pseudo-first rate constants in mol/g s. At least three feed rates in the range 3.06.7 10"^mol/s are used for k calculations. The values obtained at 523 K after 2 and 8 h on stream are listed in Table 2 for the different catalysts. Calculations are performed only in order to compare the reactivities of the different catalysts and are not aimed at finding the detailed rate equations. A least squares regression analysis and a Students' ^test of significance show, in all instances, that k values are significant at levels over 1%. All values are reproducible to within about 8%. X^^^ and selectivities to N-methylaniline (Sj^^j^y^) and N,N-dimethylaniline (Sjyfjyfj^j^^) are also collected in Table 2. On AIPO4 the conversion varies from 4 to 20 mol% and follows the order AP-A < AP-P < AP-E, i.e. as the surface acidity increases. The incorporation of AI2O3 to the AIPO4 increases the conversion except in the case of the APAl-E catalyst in which it remained almost unchanged in relation to the AP-E one; the increase in activity is more significant in the case of APAl-A. Moreover, the incorporation of Ti02 develops an APTi catalyst that is more active than the AP one when ammonia is used as the gellification agent while the reverse is true when propylene oxide is used instead ammonia. Furthermore, the incorporation of Zr02 brings down the aniline activity in relation to AP catalysts, due to the low surface acidityof APZr catalysts, regardless of the precipitation method. The order of activity does not agree with that of acidity obtained for PY adsorption at 673 K (strong acid sites). This may be because as aniline and its derivatives are basic, they could be strongly adsorbed at strong acid sites and thus deactivate the catalyst. Nevertheless, the results obtained for PY adsorption at 373 (weak acidic) and 573 K (medium acidic) generally agree with the activity. On the other hand, the use of stronger acid SAPO-5 [5, 7] and Si-235 catalysts, not only supposed any increase in AN alkylation but also decreased it. A similar decrease in aniline alkylation activity with an increase in acidity has been reported [8,9]. In this sense. Woo et al. [8] suggested that strong, medium and weak acid sites are active sites to produce C-alkylate and coke, NNDMA and NMA, respectively, so that strongly acidic catalysts ought to be modified by alkali metal species to increase Sj^^^^j^j^. So, our results corroborate that strong acid sites are not required for the N-alkylation of aniline and that weak to moderate acid sites are responsible for the reaction.

3.2.1. Effect of feed rate, time-on-stream and temperature The effect of feed rate (WHSV: 1.9-18.9 h"^) on the aniline conversion and product selectivities for APAl-P catalyst is shown in Fig. 1. The trends observed are comparable for other catalysts. Thus, both aniline conversion and Sj^^^^j^^ decrease with WHSV whereas ^NMA continuously increase. At the low feed rate studied here (1.9 h'^), NNDMT (p- > 0-)

127 Table 2 Aniline conversion (X^^^, mol%), reaction rate constant (k, mol/g s) and product selectivities (S, mol%) in aniline alkylation with methanol over AlPO^ and AlPO^-metal oxide catalysts^ Catalyst

8 h on stream

2 h on stream ^AN

^NNDMA

^AN

k 10^

^NMA

^NNDMA

AP-A

4.6

1.34

90.9

9.1

4.4

1.30

91.6

8.4

AFP

9.7

2.91

86.3

13.7

9.2

2.73

86.7

13.3

APE

20.2

6.40

74.3

25.7

19.0

5.96

74.7

25.3

APAl-A 15.6

4.84

79.7

20.3

15.4

4.76

79.0

21.0

APAl-P

16.8

5.25

73.7

26.3

16.8

5.25

75.4

24.6

APAl-P*' 38.3

13.78

57.3

42.7

37.7

13.49

56.7

43.3

APAl-P^ 57.1

24.10

38.8

59.7

55.6

23.20

40.1

58.5

APAl-E

19.7

6.27

78.8

21.2

19.8

6.28

78.3

21.7

APTi-A

8.8

2.61

88.0

12.0

8.8

2.61

88.5

11.5

APTi-P

9.3

2.77

88.2

11.8

7.1

2.10

88.7

11.3

APZr-A

3.9

1.12

91.1

5.9

2.7

0.78

95.0

5.0

APZr-P

7.8

2.32

89.4

10.6

7.3

2.14

89.3

10.7

APZr-E

4.4

1.29

81.6

18.4

4.3

1.25

85.5

14.5

Si-235

11.1

5.10

83.1

16.9

8.2

3.67

83.3

16.7

SAPO-5

1.6

0.45

96.0

4.0

_d

^ T: 523 K; F = 3.36 10"^ mol/s; WHSV = 9.53 h"^ 5 M aniline in methanol. ^ T: 573 K. T: 623 K. "^ Not measured. also appears although in small amounts. A WHSV of 5.93 h"^ is chosen to study the effect of time on stream and reaction temperature. The effect of time on stream is shown in Table 2. X^^^ and each of the S^^j^^ and ^NNDMA 1*^^1^1115 almost unchanged during the initial 8 h on stream. Besides, the selectivity of N-alkylation remains 100 mol% during this period of process time. However, Si-235 catalyst deactivates more rapidly; this can be associated to its strong acid sites. The effect of reaction temperature (523-673 K) on aniline methylation (after 2 h on stream) over AP-P, APAl-P and Si-235 catalysts is shown in Fig. 2. It is clearly seen that conversion increases continuously up to 673 K. As regards reaction selectivity, as temperature and/or conversion increases, S^^j^^ steadily decreases and Sj^^j^j^^^ increases. More or less the same trend is maintained on remaining catalysts. Moreover, C-alkylated anilines (mostly N,N-dimethyl-/?-toluidine) are formed only at the highest temperature (673 K) and, even with the more active catalysts, never surpass 20 mol% of the total selectivity.

128

^AN A

A S NNDMA

o

T SNNDMT(p-)

CO

><

''NNDMT(O-)

10 WHSV(h-i)

15

Figure 1. Effect of feed rate on the aniline conversion and selectivity over APAl-P catalyst (T: 573 K; time-on-strean: 2h).

AP-P X'AN o E^

o

APAI-P Si-235 © Q

-'NMA

CO

X'

250

300

350

^NNDMA

^

^NNDMT

V

400

T(K) Figure 2. Effect of temperature on the aniline conversion and selectivity over AP-P, APAl-P, and Si-235 catalysts (F: 3.36 10"^ mol/s; WHSV = 9.53 h"^ 5 M aniline in methanol).

129 In order to obtain more knowledge about the reaction sequence, we have constructed the Optimum Performance Envelope (OPE) curves [10] by plotting fractional conversion to each reaction product against aniline conversion, for different weight ratios of catalyst with respect to introduced aniline (Figure 3). In these curves, we have included experimental data corresponding to different temperatures and feed rates on the same diagram. Using such a procedure, an insignificant scattering of the data is evident in the selectivity diagrams and clear tendencies can be observed from these curves. OPE selectivity curves indicated, in all cases, that N-methylaniline is a primary product coming from aniline by alkylation although it is an unstable product since a maximum on its OPE curve is found. Moreover, N,N-dimethylaniline is a secondary unstable product and, furthermore, N,N-dimethyltoluidines (p- > 0-) seem to be secondary reaction products. From the data on feed rate, temperature and OPE curves, it can be concluded that aniline methylation on AIPO4 and AlP04-metal oxide catalysts follows a sequential reaction path of formation of NMA, to NNDMA and then C-alkylated products: MeOH

AN

NMA

MeOH

-^ NNDMA

MeOH

-

NNDMT(p->o-)

as occurs on other acid catalysts [1,2,8,9]. N-methylaniline react faster than aniline due to the its greater negative charge on the nitrogen-atom, by the electron donating effect on the methyl group, that facilitates its alkylation. Moreover, dimethylsubstitution on nitrogen-atom facilitates ring alkylation at o- or p-position although the steric hindrance in the o-isomer is responsible for the bigger formation of /7-isomer. Notwithstanding, N-methylation always predominates.

o ^NMA I-

o

X.NNDMA

<

^NNDMT(p-)

o z z

X,NNDMT(o-)

X <

40

60

X^N (mol %) Figure 3. OPE curves for aniline methylation: fractional conversion at a particular reaction product (X) vs. aniline conversion (X^j^) for APAl-P catalyst.

130 The formation of N,N-ciimethylaniline via disproportionation is less favorable than that of its formation via successive alkylation [2].

4. CONCLUSIONS AIPO4 and AlP04-metal oxide catalysts exhibited high activity and selectivity in aniline alkylation to produce N-alkylated products in a consecutive first-order reaction process. N-methylaniline was easily formed at low temperatures/contact times and converted to N,Ndimethylaniline as temperature/contact time increased. N-alkylated products remained 100 mol% at 523-623 K. At 673 K, N,N-dimethyltoluidine (p->o-) also appeared although in very small amounts. The results obtained for pyridine adsorption at 373 (weak acidic) and 573 K (medium acidic) generally agreed with catalytic activity. 5. ACKNOWLEDGMENTS The authors acknowledge the subsidy received from the DGICYT (Project PB92/0816), Ministerio de Educacion y Ciencia, and from the Consejeria de Educacion y Ciencia (Junta de Andalucia). They also acknowledge the linguistic revision of the manuscript carried out by Prof. M. Sullivan

6. REFERENCES 1 A.N. Ko, C.L. Yang, W. Zhu and H. Un, Appl. Catal. A, 134 (1996) 53, and references cited therein. 2 P.S. Smg, R. Bandyopadhyay and B.S. Rao, Appl. Catal. A, 136 (1996) 177, and references cited therein. 3 S.M. Yang and T.W. Pan, J. Chin. Chem. Soc, 42 (1995) 935, and references cited therein. 4 A. Blanco, J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and A.A. Romero, J. Catal., 137 (1992) 51, and references cited therein. 5 J.M. Campelo, F. Lafont and J.M. Marinas, Zeolites, 15 (1995) 97. 6 J.M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M.I. Martinez, Mater. Chem. Phys., 21 (1989) 409. 7 J.M. Campelo, F. Lafont and J.M. Marinas, Proc. 13th Iberoamerican Symp. Catal., Segovia (Spain), 1992, p. 1031. 8 S.I. Woo, J.K. Lee, B.S. Hong, Y.K. Park and Y.S. Uh, Stud. Surf. Sci. Catal., 49 (1989) 1095. 9 Y.K. Park, K.Y. Park and S.I. Woo, Catal. Lett., 26 (1994) 169. 10 D.A. Best and B. W. Wojciechowski, J. Catal., 47 (1977) 343. 11 A.K. Ghosh and G. Curthoys, J. Chem. Soc, Faraday Trans. I, 79 (1983) 2569. 12 J.M. Campelo, A. Garcia, D. Luna and J. M. Marinas, J. Mater. Sci., 25 (1990) 2513.

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

131

P R E P A R A T I O N OF S Y M M E T R I C A L AND MIXED SECONDARY A L K Y L A M I N E S O V E R RANEY N I C K E L AND SUPPORTED C O P P E R CATALYSTS S. G6b616s, M. Hegediis, E. Tfilas and J. L. Margitfalvi Central Research Institute for Chemistry of the Hungarian Academy o f Sciences, 1525 Budapest, OB 17, Hungary Summary Raney nickel catalyst was modified with 0.5 wt % V or Mg to increase the selectivity to secondary amines in the alkylation of ammonia with n-propanol or i-butanol. Due to the modification selectivities around 70-80 % were obtained at 90-95 % conversions. The mixed secondary alkylamine, N-ethyl-N-n-butylamine was prepared from ethylamine and n-butanol on a commercial CuO-ZnO-AI20 3 catalyst. The highest yield of EtNHn-Bu around 76 % was obtained at 190 °C and EtNH2/n-BuOH molar ratio 5 or above. Introduction

Symmetrical and mixed secondary alkylamines with the general formula of R-NH-R and RNH-R', respectively are used as epoxy hardeners and plant protecting agents. Lower aliphatic secondary amines are frequently prepared by the alkylation of a primary amine or ammonia with an alcohol on a nickel or copper catalyst [1]. In this work highly selective preparation of di-npropylamine (n-Pr2NH), di-i-butylamine (i-Bu2NH) and N-ethyl-N-n-butylamine (EtNH-nBu) is described. Di-n-propylamine can be produced in industrial scale by the alkylation of ammonia with npropanol on Ba(OH)2 modified Ni/A1203 catalyst [2], by the reductive

amination of

propionaldehyde over a cobalt-containing catalyst [3] or by the hydrogenation acrylonitrile in nexane on Ni/AI20 3 catalyst [4,5]. However, only scare data is available about the alkylation of ammonia or i-butylamine with i-butanol [6-10]. Di-i-butylamine was prepared from i-butanol over alumina at 370-380 °C with 28 % yield [7]. 20 wt% Co - 5 wt% Ni catalyst supported on alumina was used to prepare di-i-butylamine from i-butanol and i-butylamine at 200 °C with a yield o f 60 .6 [10]. Aliphatic mixed secondary amines prepared from a pimary amine and an alcohol were usually obtained on copper-containing catalysts at 190-200 o c with 30-50 % yields [11-13]. In this paper the effect of modification of Raney nickel catalyst with V or Mg on the selectivity of secondary amine in the alkylation of ammonia with n-propanol and i-butanol is studied. In addition, we report the preparation of N-ethyl-N-n-butylamine over a commercial CuOZnO-AI203 catalyst. The effect of reaction temperature and ammonia or primary amine to alcohol molar ratio on the selectivities and yields of symmetrical and mixed secondary amines will be discussed.

132

Experimental The skeletal nickel catalyst (Ni) was prepared by leaching a 50 w4% Ni-Al alloy with 20 wt% NaOH-H20 solution at 50 °C as desrcibed elsewhere [14]. Modification of Raney Ni catalyst with 0.5 wt% V or Mg was carried out by adsorption using an aquoeus solution of NH4VO3 or MgCl2, respectively [15]. V an Mg modified catalysts will be referred as Ni(V) and Ni(Mg), respectively. Prior to activity test the catalysts were heated to 250 ^C at a rate of 2 ^C/min in a flow of 75 % H2 - 25 % N2 mixture and kept at 250 ^C for 3 hours. A commercial CuO-ZnO-Al203 catalyst (referred as CuZn/Al, with composition: 37 wt% CuO, 36 wt% ZnO, 27 wt% AI2O3 and particle size: 0.31-0.63 mm) treated in 75 %H2 - 25 % N2 mixture at 250 ^C for 3 hours was used in the preparation of mixed amine. The composition, the textural and surface properties of the catalysts were studied by AAS, mercury porosimetry, XRD, XPS and TG-DTA [16,17]. The amount of metallic copper on the surface of catalyst was determined by titration with N2O [18]. The characterization of bulk and surface properties of the catalysts is given elsewhere [14,16,17]. The alkylation of NH3 with n-PrOH and i-BuOH and that of EtNH2 with n-BuOH was carried out at 0.45 MPa partial pressure of ammonia, under 1.3 or 2.1 MPa total pressure and at a H2/NH3 or H2/EtNH2 molar ratio of 3 in a continuous-flow reactor charged with 20 g of skeletal or 13 g of CuZn/Al catalyst (WHSV = 0.7-1.5 h'^). The reaction products were analysed by GC using FID and a glass column (3 m x 3 mm) filled with 60/80 mesh Chromosorb P NAW containing 5 % wt% KOH and 18 wt% Carbovax 20M. The conversion of alcohol and the quantitative yields of amines were determined using i-propanol as an internal standard. Results and Discussion Conversion and selectivity data obtained in the alkylation of ammonia with n-propanol over different Raney nickel catalysts are listed in Table 1. Data given in Table I indicate that n-Pr2NH can be obatined on unmodified and vanadium modified Raney nickel catalyst with 70-72 % selectivities at 92-95 % conversions. Upon modifying the Raney nickel catalyst with Mg the selectivity of the secondary amine increased to 74-75 % at 94-97 % conversions. The increase of the reaction temperature from 225 to 245 ^C resulted in slight increase of the conversion and the selectivity to the primary amine both on unmoified and Mg modified catalysts. The introduction of V or Mg modifiers affected both the selectivity of the secondary amine and the ratio of primary to tertiary amines. The effect of NH3/n-PrOH molar ratio on the selectivites in the alkylation of ammonia with npropanol is shown in Fig. 1. The selectivity of n-Pr2NH is represented by the difference of values of the two curves shown in the figure. Upon increasing the ratio of ammonia to alcohol in the range of 1.4-2.0 the selectivity of the desired secondary amine was only slightly altered, whereas the selectivity of the primary amine increased and that of the tertiary amines decreased as expected from the thermodynamics. The effect of reaction temperature on the conversion of n-PrOH in its reaction with ammonia is shown in Fig. 2a. As seen in the figure 215 and 240 ^C is required to achieve 90 and 95 % conversions, respectively. In this reaction upon increasing the reaction

133

temperature in the range 190-250 ^C the selectivity of n-Pr2NH slightly increased at the expense of the primary amine (see Fig.2b). Table 1 Alkylation of ammonia with n-propanol over different Raney nickel catalysts (P=2.1 MPa, WHSV=1.5 h-^ NH3/n-propanol molar ratio) N^ Catalyst

Xa % 92.1 94.6 94.7 93.5 97.5

T OC

225 1 Ni 245 2 Ni 225 3 Ni(V) 225 4 Ni(Mg) 245 5 Ni(Mg) a) X = conversion of n-propanol

Selectivities, % n-Pr2NH n-PrNH2 9.2 69.8 9.5 70.7 11.3 71.7 9.3 75.3 10.0 74.4

Yield, % n-Pr^N 21.0 64.3 19.8 66.9 17.0 67.9 15.4 70.4 15.6 72.5

100

CO

0)

o 0)

CO

1.5

1.7

1.9

2.1

NHg/nPrOH molar ratio

Fig.l Effect of NH3/n-PrOH molar ratio on the selectivities in the alkylation of ammonia with npropanol over vanadium modified Raney nickel catalyst (T= 225 ^C, WHSV=1.4 h'^, H2/NH3=3, conversion of n-PrOH=93-95 %). Conversions, selectivities and yield of i-Bu2NH obtained in the alkylation of ammonia with ibutanol are summarized in Table 2. The modification of the Raney nickel catalyst with vanadium resulted in 4-5 % increase in the selectivity of secondary and primary amine, whereas the conversion only slightly decreased (compare exp.s N^l and N^2 in Table 2.).

134

135

an ammonia/alcohol ratio of about 1.4. Further increase of the NH3/i-BuOH ratio resulted in an increase of the selectivity to the primary amine and decrease of the selectivities o f the secondary and tertiary amines, especially above an ammonia to alcohol ratio of 2. The mixed N-ethyl-N-n-butylamine was prepared 60 either from ethylamine or its 70 wt% go .~ aqueous solution. The results obtained in the alkylation of 70 wt% ethylamine-water solution with ~ 40 n-butanol on CuZn/AI catalyst are listed in Table 3. Table 3 Alkylation of ethylamine in 70 wt% aqueous solution with n-butanol on CuZn/Al catalyst (P=l.3 MPa, WHSV=0.7 h -1) N°

T na Xb Selectivities, % Yield c °C % EtNI-InBu nBuNI-I 2 nBu2NH % Fig. 2 190 5 83.1 82.5 10.9 5.7 68.6 Effect of reaction temperature on the conversion of n-PrOH (a) and the selectivities (b) in the alkylation 195 of ammonia 5 90.9 with n-propanol 79.6 over magnesium 13.0 modified 5.4 Raney nickel 72.8 catalyst (WHSV= 1.25 h- 1,5NH3/n_PrOH 200 93.0 molar ratio 77.7= 1.7, H2/NH3=3 15.5 ). 5.9 72.3 203 5 93.6 76.8 16.5 6.1 71.9 5 96.6 71.5 19.7 7.7 69.1 Table 2 207 209of ammonia 5 96.8 68.6 21.0 8.1 with vanadium 66.4 Alkylation with i-butanol over Raney nickel catalyst modified (P=2.1 10 83.4 83.1 15.8 1.1 69.2 MPa) 180 186 10 91.5 78.8 19.5 1.7 72.1 10 78.4 20.0 Selectivities, 1.6 % 72.7 N ° T 190 WHSV n a 92.7 X b Yield c 0 96.9 % 71.6 iBuNH 224.0 iBu2NH 3.8 iBu3N 69.4 o C 205 h-1 10 % 11d 220225 1.0 10 65.1 13.5 26.5 70.4 7.0 16.1 63.4 1.597.9 88.0 62.0 a)2n=EtNH2/n-butanol molar ratio, b) X = conversion of n-butanol, c) yield o f EtNHn-Bu 220 1.0 1.5 86.6 18.2 74.7 7.1 65.0 3 240 1.0 1.0 85.5 14.5 75.7 9.8 64.7 4 given 240 in Table 1.0 3 indicate 1.2 16.3 6.7 the molar 69.6 Data that89.8 depending on the reaction77.5 temperature and ratio of 5 240 1.0 1.5 91.5 19.6 78.0 2.4 71.4 amine to alcohol the mixed secondary amine can be obtained over the copper-containing catalyst 6 240 1.0 2.5 95.2 32.2 62.4 5.4 59.4 with 65-83 % selectivities at 98-83 % conversions, respectively. Upon increasing the reaction 7 240 1.6 1.3 86.4 17.2 75.5 8.3 65.2 temperature conversion alcohol monotonously increased, whereas 8 240 the 1.6 1.4 of the90.1 21.9 75.0 3.1 the selectivity 67.6 of EtNHn-Bu The 72-73 % yield 9 240 decreased. 1.6 1.6 highest91.2 25.4 of EtNHn-Bu 72.9 was achieved 1.7 at 190-195 66.5 °C reaction It 2.0 is worthwhile that 68.0 upon increasing1.2the molar 64.5 ratio of 10 240temperature. 1.6 94.8 for mentioning, 30.8 11 240 to alcohol 2.2 1.3 5 to 78.0 20.0 increase 76.7 3.3 and the 59.8 ethylamine from 10 no significant in the selectivity yield of 12 240 2.2 1.6 85.9 24.9 73.0 2.1 62.7 EtNHn-Bu was observed. The high amine to alcohol ratio provided very low selectivity o f tertiary

a) n=NH3/i-butanol molar ratio, X = conversion of i-butanol, of i-Bu2NH d) exp. and N° amines (Et2Nn-Bu, EtNn-Bu 2 andb)n-Bu3N ). Correlation between c) theyield conversion o f n-butanol carried out on unmodified Raney nickelofcatalyst. thewas selectivity of EtNHn-Bu in the alkylation ethylamine with n-butanol over CuZn/AI catalyst is shown in Fig 3. It is interesting to note, that above 95 % conversions the selectivity o f the desired increasing reaction temperature from 220 to 240 oc, both the conversion of i-BuOH mixed Upon secondary amine the sharply decreased.

andThe the results yield of significantly increased from 86.6 %onand 65.0 %,respectively, and the o f i-Bu2NH the alkylation of ethylamine with n-butanol CuZn/AI catalyst at different yield of the secondary amine reached 71.4 % at 91.5 % conversion (compare exp.s N°2 and N°5 in temperatures and amine/alcohol ratios are listed in Table 4. Upon increasing the reaction Table 2). Itfrom is noteworthy, increasing the NH3/i-BuOH molar at constant of WHSV temperature 175 to 205that°Cupon at EtNH2/n-BuOH molar ratio of 1.3 ratio the selectivity the values (1.0 secondary and 1.6 h "l) bothsharply the selectivity andat the of di-i-butylamine reached maximum symmetrical amine increased the yield expense of the mixed amine andathe yield ofat

135 an ammonia/alcohol ratio of about 1.4. Further increase of the NH3/i-BuOH ratio resulted in an increase of the selectivity to the primary amine and decrease of the selectivities o f the secondary and tertiary amines, especially above an ammonia to alcohol ratio of 2. The mixed N-ethyl-N-n-butylamine was prepared either from ethylamine or its 70 wt% aqueous solution. The results obtained in the alkylation of 70 wt% ethylamine-water solution with n-butanol on CuZn/AI catalyst are listed in Table 3. Table 3 Alkylation of ethylamine in 70 wt% aqueous solution with n-butanol on CuZn/Al catalyst (P=l.3 MPa, WHSV=0.7 h -1) N°

T na Xb Selectivities, % Yield c °C % EtNI-InBu nBuNI-I2 nBu2NH % 190 5 83.1 82.5 10.9 5.7 68.6 195 5 90.9 79.6 13.0 5.4 72.8 200 5 93.0 77.7 15.5 5.9 72.3 203 5 93.6 76.8 16.5 6.1 71.9 207 5 96.6 71.5 19.7 7.7 69.1 209 5 96.8 68.6 21.0 8.1 66.4 180 10 83.4 83.1 15.8 1.1 69.2 186 10 91.5 78.8 19.5 1.7 72.1 190 10 92.7 78.4 20.0 1.6 72.7 0 205 10 96.9 71.6 24.0 3.8 69.4 1 225 10 97.9 65.1 26.5 7.0 63.4 a) n=EtNH2/n-butanol molar ratio, b) X = conversion of n-butanol, c) yield o f EtNHn-Bu Data given in Table 3 indicate that depending on the reaction temperature and the molar ratio of amine to alcohol the mixed secondary amine can be obtained over the copper-containing catalyst with 65-83 % selectivities at 98-83 % conversions, respectively. Upon increasing the reaction temperature the conversion of the alcohol monotonously increased, whereas the selectivity of EtNHn-Bu decreased. The highest 72-73 % yield of EtNHn-Bu was achieved at 190-195 °C reaction temperature. It is worthwhile for mentioning, that upon increasing the molar ratio of ethylamine to alcohol

from 5 to 10 no significant increase in the selectivity and the yield of

EtNHn-Bu was observed. The high amine to alcohol ratio provided very low selectivity o f tertiary amines (Et2Nn-Bu, EtNn-Bu 2 and n-Bu3N ). Correlation between the conversion o f n-butanol and the selectivity of EtNHn-Bu in the alkylation of ethylamine with n-butanol over CuZn/AI catalyst is shown in Fig 3. It is interesting to note, that above 95 % conversions the selectivity o f the desired mixed secondary amine sharply decreased. The results o f the alkylation of ethylamine with n-butanol on CuZn/AI catalyst at different temperatures and amine/alcohol ratios are listed in Table 4. Upon increasing the reaction temperature from 175 to 205 °C at EtNH2/n-BuOH molar ratio of 1.3 the selectivity of the symmetrical secondary amine sharply increased at the expense of the mixed amine and the yield of

136 EtNHn-Bu varied between 54-60 %. The incerase of the amine/alcohol molar ratio to 5 or 10, resulted in high selectivity and yield of the mixed amine (see exp.s N®6, N^7 and N^IO in Table 4).

100 Conversion, % Fig. 3 Correlation between conversion and selectivity 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.) 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

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

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 ; ^i—^ ^ 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 (<^) T : 210°C, PH2 10 bar, Catalyst weight: 5g, Ts : 1.3 s. This brief part of our work on the reactivity of intermediates or of by-products shows why an excess of methanol is necessary for the synthesis of DMEA from [MEA,MeOH,H2]. On the one hand, it acts as a methylating agent of monoethylamine and on the other hand, it inhibits strongly the alkylation reactions i.e the MEA disproportionation into DEA and TEA From this study the following reaction scheme describing the transformation of ethylamine to the main product DMEA and by-products was established. From a kinetic point of view, steps 2 and 3 are the rate determining reactions. It follows that the DMEA selectivity is increased by modifying the acido-basicity of copper chromite used as a catalyst. In fact, the change of the catalyst basicity can decrease the MEA condensation to form DEA without modification of the hydro-dehydrogenating properties of the catalyst which are necessary for the methylation of ethylamine with methanol (steps 1 and 3).

146 TEA + MEA -NH3 + H2/-H2O

+ MeOH

DEA MEA .NH3

^

DEFA

^

DEMA

(2)

+ MeOH/-H20

EMA (3)

DMEA + MeOH

EtOH + TMA

+ H2O

+ H2O -MeOH

EtOH + DMA ACKNOWLEDGMENTS The authors from the University of Poitiers are very grateful to ELF-ATOCHEM for their financial support and fruitful discussions.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

DE 3 627 592 (1986), BASF EP 0 107 457 (1982), ICI EP 0 103 176 Al (1979) DE 3 010 791 (1979), UCB SA H.W. Chen, J.M. White, J.G. Ekerdt, J. Catal., 99 (1986) 293 J.W. Evans, M.S. Wainwright, N.W. Cant, D.L. Trimm, J. Catal., 88 (1984) 203 A.K. Agarwal, N.W. Cant, M.S. Wainwright, D.L. Trimm, J. Catal., 43 (1979) 79 J.C. Lee, D.L. Trimm, M.A. Kohler, M.S. Wainwright, N.W. Cant, Catal. Today, 2 (1988)643 J. Volf and J. Pasek, Studies in Surface Science and Catalysis, 27 (1987) 105. Ed C. Cerveny A. Baiker and J. Kijenski, Catal. Rev. Sci. Eng., 27 - 4 (1985) 653 Z. Gaizi, Thesis, Poitiers, France (1990) A. Baiker, D. Monti and Y.S. Fan, J. Catal., 88 (1984) 81 H. Abe, Y. Yokota and K. Okabe, Appl. Catal., 52 (1989) 171

147 14. 15. 16. 17.

J. Barrault, S. Brunet and N. Essayem, J. Mol. Catal., 77 (1992) 321-332 Y. Pouilloux, V. Doidy, S. Hub, J. Kerveimal and J. Barrault, J. Mol. Cat., submitted to publication N. Essayem, Thesis, Poitiers, France (1991) J. Barrault, S. Brunet, N. Essayem, A. Piccirilli and C. Guimon, Studies in Surface Science and Catalysis, 78 (1993) 305

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

149

Catalyst acid / base properties regulation to control the selectivity in gas-phase methylation of catechol L. Kiwi-Minsker, S. Porchet, R. Doepper and A. Renken Institute of Chemical Engineering, Swiss Federal Institute of Technology (EPFL) CH-1015 Lausanne (Switzerland) The reaction of catechol methylation by methanol in gas-phase over modified y-alumina was studied with the aim to correlate catalytic activity and selectivity to acid/base properties of the catalysts. Catalytic activity and selectivity towards guaiacol formation (0-alkylation) was found to increase with surface acidity. A 20-fold change in the 0/C-methylation ratio was achieved by varying the catalyst acid/base properties, keeping constant the other reaction parameters.

1. INTRODUCTION Alkylation of catechol is an important industrial reaction. Until now only very limited information regarding this reaction can be found, mainly in the patent literature. Monomethylated derivatives of catechol are useful organic intermediates in the pharmaceutical and agrochemical industries [1]. Previous studies in our laboratory [2-4] revealed that vapour phase catechol alkylation over y-alumina in the temperature range 260300°C is a promising synthetic approach to produce selectively guaiacol and 3-methyl catechol. These studies show a competition between O- and C-methylation depending on reaction parameters such as temperature, reactants concentration, conversion and acid-base properties of the catalyst used. Mg^"^-modified y-alumina was observed to be a suitable catalyst to produce 3-methyl catechol. The dependence of C-alkylation selectivity on the amount of magnesia added to y-alumina and the optimal catalyst composition has been reported [4]. The study also showed that basic oxides (MgO, CaO) were not active for the methylation reaction at a temperature below 300°C. A certain basicity seems to be necessary at the catalyst surface for the formation of C-alkylated products. The reaction of phenol alkylation with methanol over oxides and zeolites to produce anisol and cresols presents some of the features of the reaction under investigation. Studies of phenol alkylation revealed that the reaction was sensitive to the acid-base properties of the catalyst [5-13]. The catalytic activity increased with acidity, but the selectivity towards O- or Cmethylated products did not follow a simple correlation with observed acid-base properties. According to [7,12,16] the catalysts with basic sites favour C-methylation. Other authors [6,11] recently reported that an increase in catalyst acidity promote C-methylation. Therefore, a variance in the results concerning acidity and catalytic properties exists in the literature.

150 This work intends to correlate the catalytic activity and selectivity of catechol methylation to catalyst acid-base properties under constant reaction operating conditions. The work also aims at the study of the reaction selectivity towards O- or C-alkylated product formation through suitable catalyst modification.

2. EXPERIMENTAL 2.1. Catalyst preparation As a starting material y-alumina from Engelhard (Al-3982) was used. The aluminas modified by cations were prepared by wet capillary impregnation with aqueous solutions of nitrate salts. H3BO3 and H3PO4 aqueous solutions were used in the case of modification by anions. The concentrations of the solutions were adjusted to the required ratio cation(anion)/AP"^ in atomic percentage (at.%) by varying the amount of nitrates or acids added to y-alumina. Before the impregnation alumina was dried at 200°C for 2 hours. After impregnation (2 h, 25°C) and drying (50°C, 12 h) the samples were calcined in air for 8 hours at 620°C.

2.2. Catalytic test The catalytic reaction was carried out at 270°C and 101.5 kPa in a stainless steel tubular fixed-bed reactor. The premixed reaction solution, with a molar ratio catechol : methanol : water of 1:1:6, was fed into the reactor using a micro-feed pump. Detailed description of kinetic experiments and the experimental set-up have been recently reported in [2-4] . Catechol conversion (Xi) was always less than 0.05, allowing the reaction to be carried out in the differential kinetic region. Before reaction, the catalyst was activated in N2 flow at 270°C for 30 min. The yields (Yi) of the monomethylated products were measured under steady-state conditions after 8-10 hours of the catalyst activity stabilisation. The rate of catechol transformation (-Ri) over different catalysts was used to compare catalytic activity.

2.3. Catalyst characterisation Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing the BET method with Sorptomatic 1900 (Carlo-Erba). X-ray diffraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (6/29) diffractometer with Cu Ka monochromatic radiation. The total amount of surface basic sites was determined via surface titration by benzoic acid from dry hexane solution as reported elsewhere [14]. Before the titration catalyst samples were dried at 300°C for 2 hours. IR spectra of the samples were obtained in the range of 4000 cm'^ to 900 cm'^ by a Nicolet 710 (Nicolet Analytical Instruments) FT-IR spectrometer with a MCT detector. All spectra were recorded after 100 scans. A self-supporting disk of catalyst sample was mounted in a transmission IR cell. Prior to the methanol adsorption experiment the sample was dried under vacuum at different temperatures, then exposed to the methanol vapour for 60 minutes and dried under vacuum again.

151

3. RESULTS AND DISCUSSION 3.1. Kinetic reaction scheme The kinetics of the gas-phase methylation of catechol (Ai) over 'y-Al203, with methanol (methylating agent), showed [2,3] that the reaction leads to a mixture of different alkylated products. The reaction conditions have been optimised via preliminary experiments [2] to avoid the formation of polymethylated products. Three monomethylated products were observed: guaiacol (A2), 3-methyl catechol (A3), 4-methyl catechol (A4) at low catechol conversion (Xi < 0.05) and in the temperature range 260-300°C since the rates of rearrangement and consecutive steps were negligible. Fig.l presents the dependence of monomethylated product yields on the catechol conversion for Mg^'^(5at.%)/y-Al203. It seen that the three monomethylated products were formed in parallel pathways.

0.05Y3

0.041

>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 ™^^"

A^OH (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)

<

3109

2963

2817

1149

1109

1069

Wavenumber [cm-^]

Figure 3. IR-spectra of adsorbed methanol: (A) y-Al203; from B to E: Mg^'^(n at.%)/ y-AliOs, where n = 5 ( B ), 10 ( C ), 20 ( D ), 30 ( E ). This observation suggests that the O-C bond of the methoxy group gains in strength with concomitant weakening of the oxygen bond with Lewis acid site ( O-M""^) indicating a decrease in the acidity for the surface Lewis sites. This leads to an increase of the energy needed to activate the methyl group. Our results of methanol TPD experiments [4] seems to confirm this suggestion. Fig. 4 presents the shift Avco (characteristic for surface acidity) on the right hand side axis and the catalytic activity on the left hand side axis as a function of the Mg^"*" concentration. Figure 4 shows that the catalytic activity goes down simultaneously with surface acidity under constant reaction conditions. The reaction selectivity also depends on catalyst modification. We compared the reaction selectivity over modified catalysts with the selectivity observed for pure Y-AI2O3 Boron modified Y-AI2O3 shows a small decrease of selectivity towards 3-methyl catechol in favour of guaiacol formation without any change in 4-methyl catechol selectivity (Table 1).

154

10

15

20

Mg^''/Al^''[at.%] Figure 4. Catalytic activity and basicity (shift Avco» see text) as a function of the Mg^"*" concentration. Modification of Y-AI2O3 by phosphoric acid was observed to lead to a significant increase in O/C alkylation ratio: up to 7.33 from 2.45 for Y-AI2O3. The selectivity towards 4-methyl catechol formation is seen to be only slightly affected. The rise in surface basicity of Y-AI2O3 by incorporation of Mg^'^ and Li"^ cations into the lattice leads to the increase of 3-methyl catechol selectivity up to 0.65 at 7.5 at.% of Mg^"^ added. On Fig. 5 the selectivities towards O- and C-monomethylated products are plotted as functions of the amount of benzoic acid adsorbed per unit surface area, [m^], of the catalyst (characteristic of surface basicity). The O/C alkylation ratio is seen to go down when basicity increased. 0-alkylation is reduced with a concomitant increase in C-alkylation, but the ring methylation stays preferentially ortho-selective. It was possible to change the selectivity towards 3-methyl catechol about 10 times, from 0.06 to 0.65, by varying the acid/base character of the catalyst surface without observing any significant change in the selectivity towards 4-methyl catechol. 1.0 H

O-methylation

0.8 H

. 0.6-1

O

0.4-

C/5

0.02.0

2.2

2.4

2.6

2.8

2 3-0

3.2

Basicity [^mol^^^i^^^.m- ] Figure 5. Selectivity towards O- and C-methylated products as a function of surface basicity.

155 It is known that boric and phosphoric acids supported on Y-AI2O3 introduce Br0nsted acidity on the catalyst surface. Borated alumina has mostly strong acid sites [15] and phosphated alumina from medium to weak ones [17,18]. In agreement with Tanabe and Nishuzaki [16] the orientation of reactant molecules during the adsorption plays an important role in the reaction selectivity. Strong Br0nsted acid sites interact with n- electrons of the ring, resulting in horizontal catechol adsorption which leads to the destruction of the molecule and coke formation. The formed coke blocks the surface active sites for further reactant adsorption. This process results in deactivation of borated alumina. Probably, under steady state conditions, the distribution in the strength of the acid sites does not differ from those found in Y-AI2O3. That is why the observed selectivity changed very little with boron modification. Phosphoric acid introduces Br0nsted acid sites of weak to medium strength on the Y-AI2O3 surface which avoid the coke formation. The y-AliOs and Mg^^ modified Y-AI2O3 has been known to have moderate acid/base sites [22-23]. No significant deactivation of these catalysts was observed. During the catechol methylation under the conditions used, three parallel reactions take place (see Figure 2). The rates of these pathways (Ri) can be easily calculated from the data of Table 1, since Ri = Si(-Ri), where Si is the selectivity towards the monomethylated product Ai. The rise of acidity affects differently the rates of the reaction pathways: for the phosphated alumina the rate of guaiacol formation (R2, 0-methylation ) doubled and the rate of Cmethylation pathways simultaneously decreased 1.5 times, if compared to pure y-Al203. The increase in surface basicity due to incorporation of Mg^"^ in the Y-AI2O3 lattice affects the reaction as shown below ( Fig. 6). 2.0- ^ J7^

^'

1.6- \ - ^ l

s0 6 1.2-

b -0.80) i 15 0.4-

0.0-

x^'^^^-^ .^3

X 1

1

0

,

1

4

-~-~~^

T-

6

10

Mg^'^/Al^"' [at.%] Figure 6. Rate dependencies on the amount of Mg added to Y-AI2O3. As is seen from Fig. 6, the increase in basicity leads to the important decrease in Omethylation rate (R2) without significant changing in the rate of 3-methyl catechol formation (R3) until 7.5 at.% of Mg^"^. At higher magnesium concentration the rate R3 goes down more rapidly, than for guaiacol formation. It results in the observed maximum for 3-methyl catechol selectivity at 7.5 at.% Mg^V Y-AI2O3, reported as an optimal catalyst composition [4]. The acid/base properties of the surface active sites, their surface density and the distribution in their strength seems to govern the selectivity towards O- or C-methylated products and will be further investigated.

156 4. CONCLUSIONS The reaction of catechol methylation in gas phase at temperatures below 300°C is seen to proceed efficiently over modified y-aluminas with moderate acid sites. At low catechol conversion (Xi < 0.05), O- and C-methylated products are formed in parallel reaction pathways. The 0/C methylation ratio has been regulated by varying acid/base properties of the catalyst. Modification of Y-A^OB by phosphoric acid was observed to increase the selectivity towards guaiacol formation (0-methylation) up to S2=0.89. The catalyst Mg^"^ (7.5 at.% )/ Y-AI2O3 showed the maximum selectivity towards 3-methyl catechol formation (Cmethylation) to be 83=0.65. It means that a 20-fold change in the 0/C methylation ratio was achieved, when the catalyst acidity was modified, keeping constant other reaction conditions. ACKNOLEDGMENT The financial support provided for this work by the Swiss National Science Foundation is gratefully acknowledged.

REFERENCES 1. H. Fiege, et al., Phenol Derivatives, in Ullmann's Encyclopaedia of Industrial Chemistry, Vol. A19, VCH Verlagsgesellschaft, Weinheim, 1991, p. 313. 2. S. Porchet, S. Su, R. Doepper, A. Renken, Chem. Eng. TechnoL, 17 (1994) 108. 3. S. Porchet, L. Kiwi-Minsker, R. Doepper, A. Renken, Chem. Eng. Sci., 51 (1996) 11 2933. 4. L. Kiwi-Minsker, S. Porchet, P. MoeckU, R. Doepper and A. Renken, in: "Studies in Surface Science and Catalysis", vol. 101 A, Elsevier Sci. B. V., 1996, p. 171. 5. V. Durgakumari, S. Narayanan, J. Molec. CataL, 65 (1991) 385 and references cited therein. 6. S. Velu., C. S. Swamy, Appl. Catal. A: General, 145 (1996) 141 and references cited therein. 7. E. Santacesaria, D. Grasso, D. Gelosa, S. Carra, Appl. Catal. A: General, 64 (1990) 83. 8. E. Santacesaria, M. Diserio, P. Siambelli, D. Gelosa, S. Carra, Appl. Catal. A: General, 64 (1990) 101. 9. M. Marczewski, J. P. Bodibo, G. Perof and M. Guisnel, J. Molec. Catal, 50 (1989) 211. 10. Z. H. Fu and Y. Ono, Catal. Lett., 21 (1993) 43. I L L . Garcia, G. Giannetto, M.R. Goldwasser, M. Guisnet, P. Magnoux, Catal. Lett., 37 (1996) 121. 12. C. Benzuhanava and M. A. Al-Zihari, Appl. Catal, 83 (1992) 45. 13. R. T. Tiemat-Manzalji, D. B. Bianchi and G. M. Pajonk, Appl Catal, 101 (1993) 339. 14. A. Gervasini, A. Auroux, J. Phys. Chem., 97 (1993) 2628. 15. S. Sato, M. Kuroki, T. Sodesawa, F. Nozaki, G. E. Maciel, J. Molec. Catal A: Chemical, 104 (1995) 171. 16. K. Tanabe and T. Nishizaki, in Proc. 6'*' ICC, G. C. Bond, P. B. Well, F. C. Tompkins (Eds.), The Chemical Society, London (1977) p.863. 17. S. Malinowski, in "Catalysis by Acids and Bases", B. Imelik et al. (Eds.), Elsevier, Amsterdam (1985) p.57. 18. E. C. DeCanio, J. C. Edwards, J. W. Brunot, J. Catal, 148 (1994) 76. 19. H. Knozinger, AJv. Catal Relat. Subj., 25 (1976) 184. 20. C. Lahousse, F. Mauge, J.-C Lavalley, Proceedings of 11* ICC, Baltimore, 1996. 21. G. Busca, P.F. Rossi, V. LorenzeUi, J. Phys. Chem., 89 (1985) 5433. 22. J. A. Lercher, React. Kinet. Catal Lett., 20 (1982) 409. 23. J. A. Lercher, C. Colombier, H. Noller, J. Chem. Soc, Faraday Trans.l, 80 (1984) 949.

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

157

Enantioselective hydrogenation of ethyl pyruvate and isophorone over modified Pt and Pd catalysts Antal Tungler^ ,Karina Fodor^, Tibor Mathe^, Roger A. Sheldon'' ^Technical University of Budapest, Department of Organic Chemical Technology 1521 Budapest ^Hungarian Academy of Sciences, Res. Group Organic Chemical Technology ''Delft University of Technology, Lab. for Organic Chemistry and Catalysis The present study aimed at revealing the mode of enantiodifferentiation in the asymmetric hydrogenation of ethyl pyruvate and isophorone over platinum and palladium catalysts. The effect of adding the modifier after an initial phase of racemic hydrogenation and the combined use of different vinca and cinchona type modifiers on enantioselectivity and activity were studied. A mechanistic rationale is proposed to account for the experimental observations. 1. INTRODUCTION The most well-known heterogeneous catalysts for enantioselective hydrogenation are Ni modified with tartaric acid for the hydrogenation of P-keto esters [1] and Pt modified with cinchona alkaloids for the hydrogenation of a-keto esters [2]. Premodification of the catalyst prior to the hydrogenation (Ni/tartrate) or in situ modification, i.e. simple addition of the modifier to the reaction mixture (Pt^cinchona), can ensure enantioselectivities up to 95 % under optimised conditions. In recent years new chiral modifiers [3-9] have been found and new prochiral substrates [8-12] have been used in enantioselective heterogeneous catalytic hydrogenations with promising ee's. Several mechanistic proposals have been made to explain the enantiodifferentiating processes. For example, it was proposed that P-keto esters are hydrogenated on chirally modified nickel according to the Langmuir-Hinshelwood mechanism. Competitive reaction on unmodified sites affords racemic product [13]. It was assumed that the interaction between the modifier and the substrate involves hydrogen bonding through hydroxyl and carbonyl groups. In addition to the enantioselective effect, cinchona alkaloids also produce a rate acceleration , i.e. this is an example of ligand accelerated catalysis [14]. The model of a nonclosepacked ordered array of cinchonidine molecules adsorbed on platinum, proposed by Wells and co-workers, was abandoned in their later study [15]. Augustine [16] deduced from the behaviour of this system at low modifier concentrations that the chiral sites are formed at the edge and comer platinum atoms, which involve the adsorbed cinchonidine and a metal adatom. The different authors agreed that the quinoline ring of the modifier is responsible for the adsorption on platinum, the quinuclidine part, through the nitrogen atom, interacts with

158 tbe carbonyl group of the pyruvate, and the product configuration is determined by the C8 and C9 geometry of the modifier molecule. Protonation of the basic nitrogen of the modifier increases the enantiomeric excess. Baiker and co-workers [17] carried out molecular modelling calculations in order to find out the likeliest conformation of the substrate-modifier adduct on the platinum surface. Margitfalvi [18] concludedfiromNMR, kinetic measurements and molecular modelling calculations that the interaction between the modifier and the substrate, which also exists in solution, changes the reactivity of the pyruvate and the conformation of the cinchonidine. The quinoline ring of the latter exerts a shielding effect on the pyruvate determining the direction of the entering of the hydrogen. The emergence of new prochiral substrates and chiral modifiers and new enantioselective reactions have created a need for identifying common mechanistic features in all of these reactions: the structural, reactivity- and affinity characteristics of the effective modifiers, the interactive functional groups of the prochiral substrates and the appropriate catalysts. A vinca-type alkaloid, dihydroapovincaminic acid ethyl ester (dihydrovinpocetine, DHVIN, I) (Fig. 1), proved to be an effective chiral additive in the Pd mediated hydrogenation of C=C and in the Pt mediated reduction of C=0 double bonds [8].

EtOOC

Figure 1. Structure of dihydrocinchonidine and (-)-dihydroapovincaminic acid ethyl ester. The discovery of the new additive afforded the opportunity for the combined use of different chiral modifiers in the asymmetric hydrogenation of ethyl pyruvate and isophorone. The present study aimed at revealing the mode of enantiodifferentiation through the variation of modifiers and catalysts. 2. RESULTS AND DISCUSSION The model reactions were the following:

Pd catalysts H2 MeOH,AcOH modifier

^ o

o

Pt catalysts

H2 ^

\*^i<^^Et

o-^* MeOH,AcOH modifier

^^

Figure 2. Model reactions.

^

159 In both reactions (Fig. 2), different Pd and Pt catalysts (Fig. 3, 4) and the combination of vinca and cinchona modifiers (dihydrocinchonidine (DHCND) and dihydrocinchonine (DHCNN)) were tested (Fig. 5, 6, 7). The effect of hydrogen and product adsorption was investigated; for this reason the hydrogenation was allowed to proceed to 15-30% conversion before addition of the modifier (Table 1, 2). e.e. (%)

(-)-DHVIN DHCND 11 DHCNN

Pd black

Pd/TiOz

Pd/C

Pd/AlgOa

Pd/SiOa Pd powder

Figure 3. Hydrogenation of isophorone with different catalysts and modifiers. Conditions: 0.05 mol isophorone, 0.1 g modifier, 0.5 g acetic acid, 25 ^C, 0.5 g catalyst, 100 ml MeOH, 40 bar. e.e. (%) (-)-DHVIN DHCND DDHCNN

Pt/AlaOa

Pt/C

Adams Pt

Pt/TiOg

Pt/SiOa

Figure 4. Hydrogenation of ethyl pyruvate with different catalysts and modifiers. Conditions: 0.1 mol ethyl pyruvate, O.lg catalyst, O.lg DHCND, 0.1 g DHCNN, 0.2 g (-)-DHVIN + 0.2 g acetic acid, 100 ml MeOH, 250C, 10 bar.

160 The same substrate-modifier-metal systems give different enantiomeric excesses using catalysts on different supports and different preparation methods (Fig. 3, 4). In some cases even the product configuration changed with the catalyst. In the hydrogenation of isophorone, the catalysts of small or moderate dispersion (Pd black, D<0.05, and Pd/Ti02, D<0.1) resulted in the highest enantioselectivities[ll]. In the hydrogenation of ethyl pyruvate the two cinchona alkaloid derivatives (differing in the absolute configuration at C8 and C9) afford the opposite enantiomer in excess. The resulting optical yields are substantially higher than in the hydrogenation of isophorone, where the product configuration depends on the catalyst used (see Fig. 3). Some hydrogenations were carried out in the presence of two modifiers, a vinca type producing, for example, the (S) enantiomer and a cinchona type producing the (R) enantiomer in excess. The catalysts used were Pd black and Pd on titania in the hydrogenation of isophorone, and Pt on carbon, Pt on alumina and Adams Pt in the hydrogenation of ethyl pyruvate. The amount of vinca alkaloid was kept constant, while the amount of the other was increased in each experiment. The results are shown in Figure 5, 6 and 7.

e.e. (%) 20

20 mg (-)-DHVIN and X mg DHCNN 4

10

DHCNN (mg)

Figure 5. Hydrogenation of isophorone over Pd black in the presence of (-)-DHVIN and increasing amount of DHCNN. Conditions: 0.05 mol isophorone, 0.3 g Pd black catalyst, (-)-DHVIN constant 20 mg, 6th bar 0 mg, 50 ml MeOH, 25 ^C, 45 bar. When isophorone was hydrogenated over Pd black, the vinca type alkaloid controlled the enantioselection even if the amounts of the modifiers were equal in the reaction mixture. These results are in good agreement with the considerably higher enantiomeric excesses induced by dihydroapovincaminic acid ethyl ester compared with that of cinchona alkaloids. Using Pd/C catalyst, the cinchona alkaloid dominated with small enantioselectivities, indicating that the combined effect of the modifiers depends also on the catalyst type. In contrast, in the hydrogenation of ethyl pyruvate [19], the cinchona alkaloid controlled the enantiodifferentiation with every platinum catalyst used, as well as inducing much higher optical yields than dihydroapovincaminic acid ethyl ester. In the hydrogenation of ethyl pyruvate using Pt on carbon and Adams Pt, which has a much higher activity, the resulting

161 enantioselectivity significantly decreased with increasing conversion when dihydrocinchonidine was used. The smaller the amount of dihydrocinchonidine, the greater the decrease of the enantiomeric excess. The reason for this behaviour probably is that the quinoline ring of dihydrocinchonidine can be more easily hydrogenated over these catalysts than the indole ring of dihydroapovincaminic acid ethyl ester and as a result, the adsorption ability of molecules and their role in determining the enantioselectivity becomes smaller. e.e. (%)

0.521

1.04

5.21

10.4

DHCND (mmol/l)

Figure 6. Hydrogenation of isophorone over Pd/C in the presence of (-)-DHVIN and increasing amount of DHCND. Conditions: 0.05 mol isophorone, (-)-DHVIN constant 17.4 mmol/l, 0.2 g Pd/C catalyst, 25 ml MeOH, 25 °C, atm. press. e.e. (%) 60 40 (R) 20 0 20 (S) 40

ffll

100 mg (-)-DHVIN and X mg DHCND

20

50

100

100

DHCND (mg)

Figure 7. Enantiomeric excess in the hydrogenation of ethyl pyruvate with (-)-DHVIN and increasing amount of DHCND. Conditions: 0.1 mol ethyl pyruvate, 0.1 g Pt/Al203 thermal-treated (Engelhard), 100 mg (-)DHVIN, 6th bar 0 mg, 0.5 g acetic acid, 100 ml MeOH, 250C, 50 bar.

162 From the results described above, the catalyst-modifier-substrate interaction on and with the catalyst surface seems to be the most important in the enantioselection process. This finding led us to examine the influence of the chiral product molecules and the hydrogen covering the catalyst surface on the asymmetric effect of the chiral modifiers. For this purpose, the modifier was added to the reaction mixture not at the beginning of the hydrogenation, but at 15-30% conversion, thus a racemic mixture of the product molecules was produced in the initial stages of the reaction. The results of these experiments and the ones where the chiral modifier was present from the start of the reaction were compared and are summarised in Table 1 and 2 (ee values were corrected with respect to the modified course of the reactions). The catalysts used were Pd on titania and Pd black in the hydrogenation of isophorone, Pt on carbon and Adams Pt in the hydrogenation of ethyl pyruvate. Table 1. Effect of racemic starting in the hydrogenation of isophorone (e.e. %) Pd black Pd/Ti02 * ** * ** 40 34 19 (-)-DHVIN 40 15 10 5 DHCND 20 <5 5 DHCNN 15 7 * start after modification, ** racemic starting: modification after 15-30% conversion Conditions: 0.05 mol isophorone, catalyst 0.3 g Pd black, 20 mg modifier, 0.5 g acetic acid; or 0.5 g Pd/Ti02 and 100 mg modifier, 50 ml MeOH, 25 *"€, 45 bar. Table 2. Effect of racemic starting in the hydrogenation of ethyl pyruvate ([e.e. %) Adams Pt Pt/C * * ** ** 30 14 12 (-)-DHVIN 10 37 25 5 DHCND 20 15 25 5 DHCNN 12 * start after modification, ** racemic starting: modification after 15-30% conversion Conditions: 0.1 mol ethyl pyruvate, 0.1 g Pt/C catalyst, 100 mg modifier, 0.2 g acetic acid, 10 bar; or 0.05 g Adams Pt and 50 mg modifier, 100 ml MeOH, 25 °C, 50 bar. One general observation is that the chiral effect of both the cinchona and vinca type alkaloids appears to change, in most cases to decrease, if the reaction is started as a racemic hydrogenation compared with the case when the chiral modifier is added at the beginning of the hydrogenation. But no clear conclusion can be made as to whether or not the modifier molecules interact less with the catalyst surface, which is covered by hydrogen and chiral product molecules, and as a result exert less asymmetric effect. 3. CONCLUSION Our results clearly show that the behaviour of the same modifier-substrate system strongly depends on the catalyst used (the same metals on different supports or Adams Pt or Pd black), indicating that the catalyst-modifier-substrate interaction on the catalyst surface is a crucial factor in the process of enantioselection and the observed rate acceleration of pyruvate hydrogenation. On the other hand, it has been shown by CD and NMR measurements that the

163 modifier and the substrate molecules also form a complex or associates in the liquid phase [18, 8]. Nevertheless the formation of the substrate-modifier complex is a necessary but not a sufficient condition for inducing enantioselectivity. We suggest that the modifier-substrate complex is in an equilibrium between the liquid phase and the catalyst surface as well as the "free" substrate and modifier molecules. Whereas it is uncertain that the separately adsorbed substrate and modifier molecules can react on the catalyst surface to give adsorbed substratemodifier complex (Scheme 1). The observed enantioselectivity depends on the equilibrium constants and reaction rates. The differences in the performance of the different catalysts cannot simply depend on the adsorption strength of the modifiers on them, but also on their ability to accommodate the substrate-modifier adduct in an appropriate conformation. To summarise, the most important conditions for enantioselection in precious metal mediated hydrogenations are the following: - condensed aromatic part of the modifier, responsible for the adsorption on the metal, secondary or tertiary nitrogen atom in rather rigid chiral environment to interact with the carbonyl group of the substrate, and protonation of the modifier with a weak acid, - conjugation in the substrate, containing one of the two structural units consisting of two carbonyls or a carbonyl and a C=C double bond: - besides the activity of the metal to saturate the C=0 or C=C double bond, its ability to accommodate the substrate-modifier adduct in an appropriate conformation on its surface. In solution

*- optically active catalyst surface

catalyst surface In adsorbed state

Scheme 1. EXPERIMENTAL The catalysts used were partly commercial products: 5 % Pt/C Merck, 5 % Pd/Al203 Aldrich, 5 % Pd/BaS04 Aldrich, Pd powder Degussa and 10 % Pd/C Selcat of Fine Chemical Co. Budapest. 10 % Pd/Ti02, 10 % Pd/Si02 were prepared as follows. The calculated amount of the catalyst precursor (K2PdCl4) was added to the aqueous suspension of the support. The pH value of the solution was adjusted to 10-11 by addition of KOH. The suspension was boiled for 1 hour then Na(HCOO) was added to the boiling mixture. After half an hour the suspension was cooled, the catalyst was filtered and washed with distilled water.

164 Pd black catalysts were prepared according to the following procedures: 18 mmol (6.0 g) K2PdCl4 was dissolved in 50 ml water and reduced at boiling point with 74 mmol (5.0 g) Na(HCOO) dissolved in 20 ml water. When the reduction was complete, the pH of the suspension was basic (pH 11 ). The catalyst was filtered and washed several times with distilled water. The platinum catalysts used were also partly commercial products: 5% Pt/C Heraeus, Aldrich and Janssen, 5% Pt/Al203 Engelhard, Aldrich, Janssen. Pt02 was the product of Degussa. Pt/Si02, Pt/Ti02 were prepared as follows. The calculated amount of the catalyst precursor ( (NH4)2PtCl5) was added to the aqueous suspension of the support. The pH value of the solution was adjusted to 10-11 by addition of KOH. The suspension was boiled for 1 hour then Na(HCOO) was added to the boiling mixture. After half an hour the suspension was cooled, the catalyst was filtered and washed with distilled water. Some catalyst samples (tt) were heat-treated three hours in hydrogen stream at 400°C in a glass reactor, they were cooled down in nitrogen to room temperature. Ethyl pyruvate, cinchonidine and cinchonine were supplied by Merck. Vinpocetine® was supplied by Richter Gedeon Co. (-)-Dihydrovinpocetine was prepared in our laboratory by catalytic hydrogenation of vinpocetine followed by separation of the epimers[8]. Hydrogenation The hydrogenation of isophorone and ethyl pyruvate was carried out in methanolic solution at 25°C and 1-50 bar hydrogen pressure in a conventional apparatus or in a Biichi BEP 280 autoclave equipped with a magnetically driven turbine stirrer and a gas-flow controlling and measuring unit. Before hydrogenation the reaction mixtures were stirred under nitrogen for 10 minutes in the reaction vessel. During the hydrogenations samples were taken. These samples were analysed with GC on a P-cyclodextrine capillary column (ethyl lactate on 9(fC, dihydroisophorone on llO^'C). The analysis provided base-line separation of the enantiomers. The chromatograms were recorded and the peak areas were calculated with a CWS (chromatography work station). Enantimoric excess values were calculated fi'om the peak areas of the enantiomers with the usual method: [R]-[S]/[R]+[S]. Acknowledgements The authors gratefully acknowledge the financial support of the Commission of European Communities, COST PECO 12382 and the support of the Hungarian OTKA Foundation under No. 6 and 1/3/2239, they are grateful to Gedeon Richter Co. for supplying vinpocetin.

REFERENCES 1 2 3 4 5 6

Y. Izumi, M. Imaida, H. Fukawa and S. Akabori, Bull. Chem. Soc. Jpn., 36 (1963) 21. Y. Orito, S. Imai and S.Niwa, J.Chem.Soc.Jpn., 8 (1979) 1118. T. Heinz, G. Wang, A. Pfaltz, B. Minder, M. Schiirch, T. Mallat, A. Baiker, J. Chem. Soc, Chem. Commun. (1995) 1421. B. Minder, T. Mallat, A. Baiker, G. Wang, T. Heinz, A. Pfaltz, J. Catal. 154 (1995) 371. B. Minder, M. Schiirch, T. Mallat, A. Baiker, Catal. Lett. 31 (1995) 143. K. E. Simons, G. Wang, T. Heinz, T. Giger, T. Mallat, A. Pfaltz, A. Baiker, Tetrahedron: Asymmetry 6 (1995) 505.

165 7 8 9 10 11 12 13 14 15 16 17

18 19

G. Wang, T. Heinz, A. Pfaltz, B. Minder, T. Mallat, A. Baiker, J. Chem. Soc, Chem. Commun. (1994)2047. A. Tungler, T. Mathe, T. Taraai, K. Fodor, J. Kajtar, I. Kolossvary, B. Herenyi, R. A. Sheldon, Tetrahedron: Asymmetry 6 (1995) 2395. A. Tungler, T. Tamai, T. Mathe, G. Vidra, J. Petro, R. A. Sheldon in G. Jannes and V. Dubois, Eds., 'Chiral Reactions in Heterogeneous Catalysis", Plenum Press, New York, (1995) p. 201. H. U. Blaser, H. P. Jalett, Stud. Surf. Sci. Catal. 78 (1993) 139. T. Tamai, A. Tungler, T. Mathe, J. Petro, R. A. Sheldon, G. Toth, J. Mol. Catal. 102 (1995)41. W. A. H. Vermeer, A. Fulford, P. Johnston, P. B. Wells, J. Chem. Soc, Chem. Commun. (1993) 1053. E. I. Klabunovszkii, A. A. Vednyapin, Asymmetricheskii Kataliz. Gidrogenizatsiya na Matallakh, Nauka, Moszkva (1980). M. Garland, H. U. Blaser, J. Am. Chem. Soc. 112 (1990) 7048. 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, A. Ibbotson, Reel. Trav. Chim. Pays-Bas 113(1994)465. R. L. Augustine, S. K. Tanielyan, L. K. Doyle, Tetrahedron: Asymmetry 4 (1993) 1803. a. K. E. Simons, G. Wang, T. Heinz, T. Giger, T. Mallat, A. Pfaltz, A. Baiker, Tetrahedron: Asymmetry 6 (1995) 505. b. A. Baiker, T. Mallat, B. Minder, O. Schwalm, K. E. Simons, J. Weber in G. Jannes and V. Dubois, Eds., "Chiral Reactions in Heterogeneous Catalysis", Plenum Press, New York, (1995) p. 95. c. O. Schwalm, B. Minder, J. Weber, A. Baiker, Catal. Lett. 23 (1994) 271. d. O. Schwalm, J. Weber, B. Minder, A. Baiker, J. Mol. Struct. (Theochem) 330 (1995) 353. a. J. L. Margitfalvi, M. Hegedus, E. Tfirst, Tetrahedron: Asymmetry 7 (1996) 571. b. J. L. Margitfalvi, J. Catal. 156 (1995) 175. J.L.Margitfalvi, P. Marti, A. Baiker, L. Botz, O. Sticher, Catal. Lett., 6, (1990) 281.

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

167

Controlling the Enantioselective Hydrogenation of Ethyl Pyruvate Using Zeolites as Catalyst Support K. Morgenschweis, E. Pdlkiehn and W. Reschetilowski Karl-Winnacker-lnstitut der DECHEMA e. V., Theodor-Heuss-Allee 25, D-60486 Frankfurt am Main, Germany 5 wt.-% Pt/carrier catalysts (carrier materials: NaY, mordenite, erionite and NaX as well as Y-AI2O3 as a microporous, non-zeolitic support) combined with (-)cinchonidine as the chiral auxiliary were used successftiUy for the enantioselective hydrogenation of ethyl pyruvate to R(+)ethyl lactate. With acetic acid as the solvent, the catalysts Pt/NaY, Pt/mordenite and Pt/erionite produced high enantiomeric excesses (86 to 90% ee), whereas Pt/NaX (77% ee) and Pt/y-AljOg (67% ee) gave comparatively poor optical yields. With regard to the k^ values, which were obtained from kinetic data of the hydrogenation related to the mass of the catalysts only, a correlation between enantioselectivity and catalytic activity was not discernible. This changed significantly, if referring to the k value related to the mass of the catalyst and its specific Pt surface area (derived from volumetric CO chemisorption measurements). Then, the catalysts producing the highest enantiomeric excesses also resulted in the highest catalytic activities. Therefore, catalytic activity and enantioselectivity essentially depend on the specific Pt surface area of the catalyst. This means that Pt particles of appropriate geometry and size are necessary to generate high enantioselectivity and catalytic activity. Zeolites Y, mordenite and erionite turned out to be appropriate templates for the creation of such Pt particles during catalyst preparation. This was confirmed by temperature-programmed CO desorption measurements. Zeolite X was partially destroyed by acid solution during catalyst preparation, which was proved by DTA and XRD measurements. Therefore, zeolite X, as well as Y-AI2O3, could not act as a template for the creation of appropriate Pt particles. 1. INTRODUCTION In syntheses offinechemicals, heterogeneously catalyzed enantioselective hydrogenations have been gradually developing into a topic of great interest over the past few years, as enantiomerically pure substances are required in pharmaceuticals, biochemistry and food technology. For these purposes, chirally modified metal/carrier catalysts have been used, but the effect of the catalytic system is little known [1-8]. Having chosen appropriate chiral auxiliary, solvent and reaction parameters, it is necessary to apply tailor-made metal/carrier catalysts in order to optimize enantioselectivity and catalytic activity. Since the carrier material significantly influences the properties of the active Pt particles.

168 it is interesting to investigate its effect on enantioselectivity and catalytic activity. This paper deals with the asymmetric hydrogenation of ethyl pyruvate to ethyl lactate showing a high enantiomeric excess in favour of the R-enantiomer over (-)cinchonidine modified Pt/carrier catalysts. Due to their regular structures, zeolites in particular have been used as carrier materials.

2. EXPERIMENTAL 2.1. Catalyst preparation The commercial zeolites mordenite, X, Y and erionite as well as Y-AI2O3 (microporous material with an irregular pore system) were used as supports in catalyst preparation, having comparable crystallite and particle sizes (1-2 ^im) that were determined by SEM. The materials were impregnated with 5 wt.-% Ft using an aqueous solution based on H2Pt(OH)6 and HNO3. The precursors were conditioned in a nitrogen stream (16 h at 523 K, flow: 10 1 h"*) and afterwards reduced in a hydrogen stream (3 h at 523 K, flow: 8 1 h"^). The catalysts were used for the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate immediately after having been reduced. 2.2. Catalyst characterization The specific surface areas of the Pt/carrier catalysts were determined by volumetric N2 adsorption at 77 K, and the specific Pt surface areas were derived fi"om volumetric CO chemisorption measurements at 298 K (as described in Ref [8]). The resuhs are shown in Table I. The values of the pore access diameters are obtained from Ref [9]. Only the Pt particles sited at the external surface of the zeolites can participate in the enantioselective hydrogenation, because the cinchonidine used as chiral auxiliary is too large to penetrate the zeolitic pore systems. Table 1 Textural data of the carriers and the catalysts S catalyst ^cat. / '/ m^g-^ m^g-^

m^g-^

dporet^]/ nm

Pt/NaY

970

490

5.0

0.74

Pt/mordenite

440

250

2.5

0.65x0.70 0.26x0.57

Pt/erionite

250

22

8.3

0.36x0.51

Pt/NaX

435

215

4.1

0.74

Pt/Y-Al203

235

205

12.6

irregular

^carrier^cat.'

Specific Surface area of the carrier specific surface area of the catalyst

Spt/

Sp^: specific Pt surface area ^pore- pore access diameter [9]

169 2.3. Catalytic and analytic procedure The hydrogenations were performed at 293 K and an initial hydrogen pressure of 7.1 MPa under constant stirring at 1200 min"^; 10 ml ethyl pyruvate were hydrogenated in a mixture of 20 ml solvent (acetic acid or cyclohexane), 5 mg (-)cinchonidine (chiral auxiliary) and 100 mg Pt/carrier catalyst. The conversion of ethyl pyruvate was monitored by measuring the pressure drop in relation to time. The amount of ethyl pyruvate converted per time unit was derived from this drop in pressure. It provides the initial reaction rate constant k^, if related to the mass of the catalyst only, and the initial reaction rate constant k', if related to both the mass of the catalyst and its specific Pt surface area. Final conversion and enantiomeric excess were determined using GC-MS as described in [8].

3. RESULTS AND DISCUSSION 3.1. Influence of the carrier on the catalytic activity, related to mass^ and on the enantioselectivity 1«-1 ko/mmols"^g

solvent: cyclohexane

NaY

mordenite erionite NaX carrier of the catalyst reaction rate constant kg Q

y-Al203

enantiomeric excess ee

Figure 1. Dependence of A:^, and ee on the catalyst support (solvent: cyclohexane)

170 Fig. 1 shows the dependence of the initial reaction rate constant k^ (related to mass) and the enantiomeric excess on the support of the catalyst. In this case, cyclohexane is used as the solvent. The catalysts Pt/NaY, Pt/mordenite and Pt/erionite yield the highest enantiomeric excesses (75 to 80% ee), whereas Pt/NaX (64% ee) and Pt/Y-Al203 (54% ee) give comparatively poor optical yields. On the other hand, the high ko value of Pt/Y-Al203 is remarkable. Therefore a correlation between enantioselectivity and the mass-related k^ values is not discernible. The influence of the catalyst carrier on the initial reaction rate constant k^ and on the enantiomeric excess with acetic acid as the solvent is depicted in Fig. 2. 1/1-1 ko/mmols'^g

NaY

ee / %

mordenite erionite NaX carrier of the catalyst

Y-AI2O '2^3

reaction rate constant kg EH enantiomeric excess ee

Figure 2. Dependence of ^^ ^^^ ^^ ^^ ^^e catalyst support (solvent: acetic acid) By comparison with the series of measurements using cyclohexane as the solvent, the enantiomeric excesses are generally about 10% higher when acetic acid is used as the solvent, whereas the ko values derived from the catalysts Pt/NaY, Pt/mordenite, Pt/erionite and Pt/NaX diminish significantly but, in relation, analogously. On the contrary, the mass-related catalytic activity of Pt/Y-Al203 is much reduced when acetic acid is used as solvent. This disproportionate diminution of the k^ value might be induced by

171 metal/solvent interaction in acid solvents, e.g. acetic acid, which partially conceals the metal/support interaction between Pt and Y-AI2O3, featuring Lewis acid properties. This metal/support interaction probably does not occur between Pt and the commercial zeolites X, Y, mordenite and erionite. Generally, a correlation between enantioselectivity and catalytic activity is not identifiable yet. Therefore, catalytic activity has been related to a further parameter, the specific Pt surface area of the catalyst, giving the k' value that is discussed in section 3.2. 3.2. Influence of the carrier on the catalytic activity, related to mass and specific Pt surface area, and on the enantioselectivity The series of measurements portrayed in Fig. 3 is the same as given in Fig. 2, but a new measure of catalytic activity has been employed, considering both the mass of the catalyst and its specific Pt surface area: the initial reaction rate constant k'.

k*/mmols"'*m"^

ee/% 100

NaY

mordenite erionite NaX carrier of the catalyst reaction rate constant k' O

y-Al20 2^3

enantiomeric excess ee |

Figure 3. Dependence of A:' and ee on the catalyst support (solvent acetic acid) By establishing the initial reaction rate constant k', the correlation between enantioselectivity and catalytic activity becomes obvious. This is in accordance with the "ligand-accelerated"

172

character of the reaction, established by Blaser and co-workers [6]. Thus, it is concluded that Pt particles of appropriate size and geometry are necessary to generate high enantioselectivity and catalytic activity. Furthermore, zeolites Y, mordenite and erionite seem to act as appropriate templates for the creation of such Pt particles during catalyst preparation. However, these theses have to be proved, and the question has to be answered, why NaX does not act as an appropriate template ahhough isomorphous to NaY. 3.2. Differential thermal analysis (DTA) The catalysts and the carrier materials were investigated by differential thermal analysis to ascertain whether the carrier materials had been affected by the catalyst preparation.

Pt/NaX exothermic

200

400

600

800 1000 temperature / K

1200

1400

Figure 4. DTA thermograms of Pt/NaX and Pt/NaY compared to the pure supports (heating rate: 20 K min'^; gas flow: Nj (7 1 h'^); reference: a-AljOj) The reason for the very different catalytic performances of Pt/NaX and Pt/NaY can be derived from Fig. 4: Whereas the broad peak at 400 K (desorption of water) and the sharp peak at 1200 K (destruction of the zeolitic lattice) have vanished during the catalyst preparation in the case of Pt/NaX, they have been preserved for Pt/NaY. As these peaks are typical of zeolites X

173 and Y, it is concluded that NaX was partially destroyed during the catalyst preparation. This conclusion was supported by XRD measurements indicating a loss of crystallinity by more than 20% in the case of NaX only. The experimental findings can be explained by the high susceptibility of NaX (Si/Al = 1.35) to aciJs, whereas NaY is more stable towards acids (Si/Al = 2.6). 3.3. Temperature-programmed CO desorption (CO TPD) Before performing the CO TPD measurements, the catalysts were calcined for 24 hours at 623 K and 10"^ Pa. Afterwards they were charged for 1 hour with CO at 10^ Pa and 323 K, followed by thefinalevacuation at 323 K and 10"^ Pa for 16 hours. The CO TPD measurements were carried out with a heating rate of 5 K min"\

Pp/10^Pag"^ 1E-6 Pt/NaY Pt/mordenite Pt/erionite Pt/NaX PVy-MzOzl

_

3E-7 //* * 'x til f'y ' * ^

1E-7

-

. .- .^'""''^ *N

H n

'7/ | / iu / |S 11

'' ^

Ul1

1

I

\

400

^

^

\

\ -

\\ .^ ' . \ \ \ ^ \ ^ \ \ \ \ 1

500

\

\

JL

/ \ 1\J

\

, /

/ V \J /

\ V-. \

/ I

» * '

**

*

\y^^

y ,1 A]/'.

N

**

\

^

" x

*

\

11 A1

1E-8

.'^

y. » . - . A.

ill 3E-8

*\

\

^ ^

» . ' ' "' '. \ '. ' 1

'

/

/

/ ; /

\ /

600 temperature / K

/ /ii 7/s

/

'\

/'

;

* / /:

/I

' w i

/ *

.' '*;/ 1 / 1 ' ' ij j / » ' ' 1 :

/

*i ' / I, ;

700

1

\\

800

Figure 5. CO TPD spectra of the prepared Pt/carrier catalysts (Pj^: specific desorption pressure) Fig. 5 illustrates that the catalysts Pt/NaY, Pt/mordenite and Pt/erionite give rise to defined peaks between 370 K and 570 K. This is due to an appropriate range of Pt particle sizes, which are induced by the regular structures of the supports during the catalyst preparation. Pt/support interaction is negligible in this case (commercial zeolites as supports).

174

The broad desoq)tion peaks between 370 K and 730 K, which are related to Pt/NaX and Pt/Y-Al203, result mainly from the irregular Pt particle sizes of these catalysts and partially from Pt/support interaction. As it was partially destroyed by the acid solution during the catalyst preparation, zeolite X, similar to Y-AI2O3, could not act as a template for the creation of appropriate Pt particles.

4. CONCLUSIONS Enantiomeric excess and catalytic activity of the asymmetric hydrogenation of ethyl pyruvate over (-)cinchonidine modified Pt/carrier catalysts depend significantly on the specific Pt surface area. This is due to the morphology of the Pt particles and to surface chemical Pt/support interaction. Thus, reaction pathway control is possible by varying these parameters. Zeolites NaY, mordenite and erionite are appropriate carrier materials, because they induce the formation of Pt particles with fairly appropriate geometry and size during the catalyst preparation.

5. ACKNOWLEDGEMENTS The authors thank the AiF for support of this project, which was funded by the BMWi.

REFERENCES 1. Y. Orito, S. Imai, S. Niwa and G.H. Nguyen, J. Synth Org. Chem. Jpn., 37 (1979) 173. 2. A. Baiker and H.U. Blaser, "Enantioselective Catalysts and Reactions", in "Handbook of Heterogeneous Catalysis" (eds. G. Ertl, H. Knozinger, J. Weitkamp) VCH, Weinheim, in press. 3. U.K. Singh, R.N. Landau, Y. Sun, C. LeBlond, D.G. Blackmond, S.K. Tanielyan and R.L. Augustine,./. Catal, 154 (1995) 91. 4. K.E. Simons, G. Wang, T. Heinz, T. Giger, T. Mallat, A. Pfaltz and A. Baiker, Tetrahedron:Asymmetry, 6 (1995) 505. 5. J.L. Margitfalvi, Chem. hid., (Dekker), 62 (Catal. Org. Reactions) (1995) 189. 6. M. Garland and H.U. Blaser,./. Am. Chem. Soc, 112 19 (1990) 7048. 7. W. Reschetilowski, U. Bohmer and K. Morgenschweis, Chem.-Ing.-Tech., 67 2 (1995) 205. 8. U. Bohmer, K. Morgenschweis and W. Reschetilowski, Catal. Today, 24 (1995) 195. 9. W.M. Meier and D.H. Olson, "Atlas of Zeolite Structure Types", 2"^ edition, Butterworth, London (1987).

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

175

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 and M. Studer* Central Research Services, CfflA-GEIGY AG, R-1055.6, CH-4002 Basel, Switzerland 1. SUMMARY The modifier concentration dependence of rate and enantioselectivity (e.e.) was investigated for the hydrogenation of ethyl pyruvate catalyzed by hydrocinchonidine (HCd) modified platinum catalysts. A model assuming the reversible adsorption of one (described by Km) or two molecules of HCd (Kna) on unmodified active sites (Ptu) was used. Good agreement between measured and calculated data was obtained for different types of Pt catalysts, several solvents and also for experiments in presence of additives. It was shown that high enantioselectivities were obtained only for catalytic systems with a high km/ku ratio (rate constants of the modified and unmodified sites) and high intrinsic selectivity (s). No correlation exists between the observed maximum e.e. values and the absolute size of Km or km. The results in presence of small amounts of quinoline and triethylamine indicate a competitive adsorption with HCd on modifiable sites, whereas thiourea seems to adsorb irreversibly. 2. INTRODUCTION

©0(3©©©-"r unmodified (Pt^)

OH

o rate^ = k^' [PtuJ

HCd

p^

O

modified (Pt^)

2^

H

o rate^ = k ^ [ P t J intrinsic selectivity s

Figure 1. Reversible adsorption of modifier on catalyst and reaction scheme

176 There is an ongoing interest in the mode of action of the enantioselective hydrogenation of ethyl pyruvate (etpy) with modified Pt-catalysts [1-3]. Garland and Blaser described a kinetic model which is able to explain the dependence of rate and e.e. on the modifier concentration [4]. The model assumes that the modifier adsorbs reversibly on modifiable sites (Figure 1). With ethanol as solvent, the measured data is well described. For toluene and AcOH, however, adsorption of a second HCd molecule had to be introduced to explain the effects at high modifier concentration. With the work presented here, we wanted to answer the following questions: Is this model also useful for other catalytic systems (different catalyst type, solvent etc.)? Which parameters make a 'good' or a 'bad' system? Which modifier concentration is optimal for screening experiments? This paper describes the application of the kinetic model to catalytic systems with different types of Pt catalysts, solvents and additives. Conclusions are drawn concerning the limitation of the model and the parameters that are important for obtaining a selective catalyst. 3. EXPERIMENTAL All reactions were carried out in a thermostated, magnetically stirred 50 ml autoclave (baffles, two thermocouples, X-shaped stirrer, 1=25 mm, max. 900 rpm). A reservoir and a pressure regulator allowed experiments to be carried out under isobaric conditions, p and T in the autoclave and in the reservoir were recorded every 10 s. Usually, the reactions were stopped after 10 min corresponding to conversions of 10 - 50%. Conversion and e.e. were determined by GLCanalysis of the crude reaction mixture (30 m capillary P-Dex 100, 75°C). In all experiments with HCd modifier, the R-enantiomer was the major enantiomer. The initial rates of hydrogenation were calculated with a linear least square method from the pressure drop in the reservoir, accounting for the compressibility of hydrogen. In all series, one experiment was carried out without modifier to determine ku (see below). 100

75

75 -

^ '50 4

50 — # — 0 . 1 mgHCd

1 mgHCd :

--»--

-

1.2mgHCd

25 -

25 -f

r ^

::S:

1.2 mgHCd

- 5.0 mgHCd

---A--- 5.0 mgHCd

" 1—

\

10 20 Conversion (%)

0 - ___________v_

30

\ 10 20 Conversion (%)

30

Figure 2: Dependence of e.e. and A e.e. on conversion at different modifier concentrations. (50 mg 5% Pt/AlzOs (JMC 94), 10 ml e^y, 20 ml toluene, 20X, 100 bar).

177 Etpy was purchased from Fluka (purum). Before use, it was distilled and kept at 5 °C for a maximum of 3 weeks. 10,11-Dihydrocinchonidine (HCd) was prepared from commercially available cinchonidine by hydrogenation with Pd/C. All catalysts had a Pt-loading of 5% and are commercially available from Engelhard (E 4759, E 4762) Johnson Matthey (JMC 94) and Degussa (F 407, F 209, PtSi). The dispersion D of the catalysts was determined as described in [5]. The catalysts were pre-reduced for 2 h at 400^C under H2. They could be stored for 2 weeks under air with no significant loss in selectivity or activity. Because e.e.-values can strongly depend on conversion [3], e.e. vs. conversion was measured in cases where considered critical and incremental e.e. values (Ae.e.) were determined (Figure 2). At low modifier concentration, the Ae.e. decrease after 10% conversion. This means, that the modifier is unstable under the reaction conditions. Therefore, e.e.'s were measured at about 10% conversion in this case. For higher modifier concentration, the stability of the modifier is not critical. In these cases, e.e.'s were measured after 30-50% conversion, ku was determined for every series independently with an experiment without modifier. All curve-fitting was done using the EXCEL-Solver Program (MS EXCEL 4.0, least squares method) with the settings TSfewton' method, Torward' derivatives and 'Tangent' estimates. 4. RESULTS AND DISCUSSION

4.1. Catalyst types Table 1 and Figure 3 show the results obtained with different catalyst types. As seen in Figure 3, a good fit of the measured data was obtained for the different catalysts. It is obvious that both the acceleration factors (km/ku ratio) and the intrinsic selectivity (s) have to be optimal in order to get high e.e.max The reason for the observed differences is not clear, however. The following points are noteworthy: High e.e.max are only observed with high kjku ratio and high s (entries 3, 4). e.e.max are low when km/ku is low (1, 2, 5), irrespective of the absolute values of the kinetic parameters. Similar ku-values are found for all catalysts except Pt/Si02: This indicates that the support, the dispersion, and so on have little influence on the activity of the unmodified sites. With Pt/Si02, the highest values for Km, ku and km are observed. However, kjk^ as well as s are low. This means that this catalyst is very active, but not particularly selective. Km/Km2 is similar for all catalysts (17-35) except for 4 (78). With this catalyst adsorption of the second modifier molecule is less favored than on the others. This gives a very high e.e.maxTable 1. Parameters obtained for various types of Pt catalysts with differort dispersion (D). No.

catalyst

type

D

ku

*^m

'^ny'^u

K-m

s

Km2

6-®-inax

1

F407

Pt/CaCOs

0.06

1.6

24

15

14'000

0.92

860

72%

2

PtSi

Pt/Si02

0.69

5.6

89

16

17'000

0.87

480

68%

3

JMC 94

WM2O3

0.19

1.8

53

29

32'000

0.94

r400

83%

4

E4759

WM2O3

0.37

1.3

28

22

37*000

0.94

480

84%

5

E4762

WM2O3

0.22

1.8

27

15

14'000

0.92

860

76%

6

F209

WM2O3

0.22

1.0

21

21

16'000

0.88

670

75%

50 - 200 mg catalyst, [HCd] = 0-0.02 M, 20 ml toluene, 10 ml etpy, 100 bar H2.

178

T 4.E-04

100 X

o

ee caic JMU 94 ee obs F 407 -eecalcF407 ee obs RSi D —eecalcRSi r obs RSi

+ 3.E-04

A

+ 2.E-04 o E

1.E-08

I i.f^^ I - - y ^ p 1.E-07 1.E-06 1.E-05

1 1.E-04

O.E+00 1.E-02

1.E-03

eeobsJMC94

A - - - - -

" r caic rUoi robsJMC94 -rcalcJMC94 r obs F 407 - i fja\G r

'iuf

HCd concentration (mol/l)

Figure 3. Measured and calculated ee. and rate curves for different catalysts.

4.2. Solvent Table 2 and Figure 4 show the results obtained with different solvents. A good fit of the measured data was obtained for different solvents. AcOH is the outstanding solvent with high s and high acceleration kJK [4]. Table 2. Kinetic parameters for different solvents. Solvent

Kuj/Ku

Km

s

53

30

32'000

93 38

53 87

4'000 16'000

6

15

78'000

14

31

7'000

97

240'000

k„ 1.8

k„

EtOH acetone CH3CN

1.8 0.4 0.4

DIPE

0.4

AcOH§

0.9

85

toluene

Km2

Kn/Kni2

0.94

r400

24

83%

0.86 0.86

2'900 2'400

1 7

67% 74%

0.58

180

440

18%

0.85

r5oo

5

65%

0.96

r800

130

92%

C'6'max

§ k^ =140, S2=0.96. 50 mg JMC 94, [HCd] = 0 - 0.02 M, 20 ml solvait, 10 ml etpy, 100 bar H2. The following points are noteworthy: High e.e.max are observed with toluene and AcOH. Both solvents have a high kjku, a high s, andK„/K„a>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 omc^itration and ten^erature. 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.

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

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^^a, 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 ^^e 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 ©^EP.mod ^^^ ®^EP,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 0^EP,mod ^ ^ ®^EP,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 ^ ^ 0^EP,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: ^^

k^EP] 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

k^EP]

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

^EP

L

h

R

(16)

\H,

"^

KH}^[H2^ KEP[EP]

jcnm

k^EP]

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.

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

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'n<^honidine

^

.^^^^

5wt5K Pd/support

2. EXPERIMENTAL The supported Pd catalysts (Pd 5wt%) were prepared by a precipitation-deposition method unless otherwise noted. The standard procedures were as follows. An aqueous suspension of PdCl2 solution, containing 0.05g of Pd in 0.72wt% HCl, and 0.95g of a support, was gently stirred for 15 min at 348 K. A Na2C03 solution was added dropwise to the suspension under vigorous stirring. The final pH value of the solution was adjusted to 10-11. Afl;er gentle stirring for 15 min at the same temperature, the precipitate wasfiltered,washed three times with distilled water, and dried in air at 383 K for 20 h. A portion of the catalyst precursor (0.06 g) was reduced immediately before use by stirring in a solvent under an atmospheric pressure of hydrogen at 298 K for 1 h, or by heating at 473 K for 1 h in a hydrogen flow of 8 1/h. The catalysts prepared are listed in Table 1, together with the supports employed. Commercial Pd/C catalysts were also used for comparison. Cinchonidine and 1 were used as received. The hydrogenation reactions were carried out in 10 ml of a mixed solvent (A) ethyl acetate and 95% ethanol (3:2 in volume) [3], or (B) A^,A^-dimethylformamide (DMF) and water (9:1 in volume) [13], both at 298K under an atmospheric pressure of hydrogen. Cinchonidine (usually 0.06 mmol) was added to the suspension of the reduced catalyst and, by stirring in a hydrogen atmosphere, immediately hydrogenated to dihydrocinchonidine. After 20 min, the substrate 1 (usually 2 mmol) was added and the hydrogenation was started with the stirring speed of 1200 rpm. The reactions were kinetically controlled under our reaction conditions. After the hydrogen uptake finished, the products were isolated from the reaction mixture according to the procedure described before [12], esterified to the methyl ester by the reaction with CH3OH/BF3 CH3OH, and analyzed by HPLC equipped with a chiral detector (JASCO OR990) on a chiral column (CHIRALCEL OJ-R). The optical yield was expressed as the enantiomeric excess (ee) of 2 : ee (%) = 100 X (S—R) I (S+R). The initial hydrogenation rate (rg) was measured at 20% conversion based on the hydrogen uptake. Adsorption measurements with different supports or catalysts were carried out by using a mixed solution of cinchonidine and substrate 1 (4 mmol/1 for each) in solvent B. After stirring at 298 K for 1 h, the amount of each compound adsorbed was determined from the decrease in the concentration of the solution. The concentrations were monitored by HPLC. The mean crystallite sizes of Pd in the reduced catalysts were obtained from XRD line broadening. The total surface areas (SBET) and the areas of Pd metal surface (Spd) were determined using the nitrogen adsorption at 77 K and by the CO chemisorption at 323 K, respectively.

193 Table 1 Supported 5wt% Pd catalysts prepared in this work and the supports used Catalyst

Support

Source of support

Super M-30 Pd/C-1 Osaka Gas Company Pd/Si02 Wakogel C-200 Wako Pure Chemical Ind. JRC-ALO-4 Catalysis Society of Japan Pd/Al203 EP Daiichi-Kigenso Pd/Zr02 Pd/Ti02-2 JRC-TIO-2 Catalysis Society of Japan Pd/Ti02-3 JRC-TIO-3 Catalysis Society of Japan Pd/Ti02-4 JRC-TIO-4 Catalysis Society of Japan Pd/Ti02-5 JRC-TIO-5 Catalysis Society of Japan Pd/Ti02-3n ^ JRC-TIO-3 Catalysis Society of Japan JRC-TIO-3 Pd/Ti02-3i' Catalysis Society of Japan ^ Specific surface area of support. ^ PrecipitatedfromPd(N03)2 instead of PdCl2. Prepared by an impregnation method with Pd(NH3)4Cl2.

Sa^'/mV^ 3000 370 170 25 16 51 49 2.7 51 51

Type

y-alumina anatase rutile anatase rutile rutile rutile

3. RESULTS AND DISCUSSION First, support effects were studied with the catalysts reduced in situ at 298 K in solvent A. As shown in Fig. 1, the ee values for all the catalysts except Pd/C-1 were between 20 and 25% irrespective of the support used for the catalyst preparation, whereas the activity varied significantly, depending on the catalysts. Pd/Ti02-3 was found most active in the catalysts examined. The unexpectedly low activity and selectivity of Pd/C-1 suggest that the Pd particles incorporated in micropores of the support cannot play a role in this reaction; both the substrate and the modifier are too bulky to reach the surface of Pd in micropores. The XRD measurements showed that the mean crystallite sizes of Pd were smaller than 3 nm for all the catalysts reduced at 298 K. Since the relatively high optical yield of 2 was reported with a Pd/C catalyst [3], we tested four kinds of commercial Pd/C catalysts, after reduction at 298 K for 1 h, under the same reaction conditions as above. As shown in Table 2, the activity and selectivity of these catalysts considerably varied depending on the catalyst origin. Pd/C-NE exhibited the highest enantioselectivity, i.e., similar selectivity to that reported by Perez and co-workers [3]. Pd/CNt and Pd/C-W showed higher selectivities but lower activities than Pd/C-1 prepared in this study. The amount of chlorine remaining in the catalysts could be a factor which decreases the activity. The catalysts prepared by an impregnation method using PdCb or Pd(NH3)4Cl2 had lower activities than those prepared by a precipitation method [12]. On the other hand, the favorable effects of remaining CI on the enantioselectivity have been reported with cinchona alkaloid-modified Pt and Ir catalysts [16,17]. Also with Pd/C catalysts of similar surface area, shown in Table 2, and Pd/Ti02-3 catalysts shown in Figs, land 2, it can be said that catalysts having higher chloride contents provide higher optical yields of 2. Further studies on the effects of the CI content are now underway. Published data on enantioselective catalysts for the hydrogenation of ketoesters indicate that the high metal dispersions are detrimental for achieving high enantioselectivities [7, 19-21]. In

194 60 50 ^ o

40

I 30

iftftftliiiyl

20 l5

10


«s O CO

m

<S

o

9

^

OH

<S fS

ro

Tf


O

o §

o ^

o

OH OH

OH

§ OH

PH

OH

«s

s OH

Figure 1. Hydrogenation activity, TQ, ( D ) and enantioselectivity, ee, ) of different Pd catalysts reduced in situ at 298 K for 1 h. Reaction conditions: 0.06 g catalyst, 2mmol 1, 10 ml solvent A, atmospheric pressure of H2, 298 K. Table 2 Commercial Pd/C catalysts and their properties Catalyst Pd/C-NE Pd/C-K Pd/C-W Pd/C-Nt

Source (5wt%Pd) (7.5wt%Pd) (5wt%Pd) (5wt%Pd)

N. E. Chemcat Co. Kawaken Fine Chemicals Wako Pure Chemical Ind. Nacarai tesqu Inc.

Sa/mV^ Clcontent/% 1200 1100 800 760

0.13 0.01 0.74 0.17

ro

ee/%

30.1 25.1 0.8 1.1

32.2 26.0 24.9 15.8

^ specific surface area. ^ Initial hydrogenation rate (mmol/g h) observed in the reaction in solvent A. Optical yield of 2 obtained in the reaction in solvent A. the previous paper [12], we reported that the pre-reduction of the precipitated precursors at elevated temperatures much improved the selectivity of Pd catalysts in the enantioselective hydrogenation of 1. The reduction of Pd/Ti02-3 at 473 K resulted in the highest optical yields of 2, up to 72%ee, for the reactions carried out in solvent B under optimal reaction conditions [13, 15]. Therefore, we examined the support effects with the catalysts reduced at 473 K for 1 h, for the reactions carried out in solvent B. As shown in Fig. 2, the enantioselectivity, as well as the activity, varied significantly depending on the support employed. Similar dependencies were observed for the reactions in solvent A. With most catalysts, the heat treatment resulted in much increased selectivities and decreased activities. The increase in the mean crystallite size of Pd was not obvious in XRD measurements except for Pd/Ti02-5, Pd/Ti02-3n, and Pd/Ti02-3i catalysts. Although the degree of Pd dispersion, determined from the CO chemisorption, slightly decreased after the heat treatment of the catalysts as shown in Table 4, it is difficult to explain the effects of the

195

/o 60

-

50 40

>

30 20 10

0 O CO OH OH

OH

"w

'O

"O

i i

9 PH

Figure 2. Hydrogenation activity, FQ, ( D ) and enantioselectivity, ee, (H) of different Pd catalysts reduced at 473 K for 1 h. Reaction conditions: 0.06 g catalyst, 2nimol 1, 10 ml solvent B, atmospheric pressure of H2, 298 K. *: Measured in solvent A. heat treatment on the catalysts performance only in terms of the decrease in Pd dispersion. It is likely that the pretreatment removes the surface contamination, promoting the adsorption of reactant and modifier, as will be shown below. The catalysts prepared with the support JRC-TIO-3, a titania with a moderate specific surface area, exhibited relatively high enantioselectivities. Alumina, zirconia, and another titania (JRC-TIO-4) with a moderate surface area, were also effective. However, the heat treatment of Pd/Ti02-5, supported on a titania with a very small surface area, resulted in considerable decreases both in the enantioselectivity and in the activity, accompanied by a great increase in the mean crystallite size of Pd up to 25 nm. The very low dispersion of Pd seems to be the reason not only for the low activity but also for the poor selectivity of this catalyst, because a very slow reaction may lead to the desorption of the modifier from the catalyst surface and thereby lead to the decrease in the enantioselectivity [15]. On the other hand, the drastic decreases in the activity and selectivity of Pd/Si02 catalyst are hardly attributed only to the decrease in Pd dispersion after the heat treatment, because the increase in the mean crystallite size of Pd obtained from the XRD measurement was not so significant. The Pd particles may migrate into the micropores of Si02 during the heat treatment and become inaccessible to the bulky modifier and substrate, as mentioned above for Pd/C-1 catalyst. Dramatic decreases in the activities were also observed with commercial Pd/C catalysts reduced at elevated temperatures. Therefore, a support with a moderate surface area seems to be preferable. Similar results were reported for the hydrogenation of ethyl pyruvate on cinchonidine-modified Pt/Al203 catalysts [21]. It is not clear at this moment if the dispersion of Pd is a factor which determines the enantioselectivity of the catalyst. However, the positive effect of the heat treatment on the selectivity of most catalysts suggests that the clean Pd surface of a relatively large ensemble size is advantageous for both the modifier and substrate molecules to adsorb strongly enough.

196 interacting with each other in an adequate conformation. In order to obtain the information on the adsorption ability of the catalysts, the extent of adsorption of cinchonidine and the substrate 1 were estimated using solvent B. The amounts adsorbed on supports only, without Pd, are shown in Table 3. Comparison of the amounts adsorbed per unit surface area of supports indicates that the substrate (acid) is adsorbed on AI2O3, Zr02, and Ti02 in appreciable amounts, as expected from the acid-base properties of the supports. On the other hand, cinchonidine (base) was adsorbed in a larger amount on Ti02 than on AI2O3 and Zr02. The amounts adsorbed on Si02 were negligible for both compounds. Titania, especially JRC-TIO-3, was reported to have enough amounts of both acid and base sites [22]. This explains the adsorption behavior of this kind of Ti02. Table 3 Amounts of (E)-a-phenylcinnamic acid (PCA) and cinchonidine (CDN) adsorbed on supports Support

A.A.(mmol/g) PCA CDN

A.A.(|amol/m^) PCA CDN

cSi02

0.28 0.001 0.22 0.019 0.021 0.027

0.09 0.002 1.29 0.76 0.41 0.55

A1203 Zr02 Ti02-3 Ti02-4

0.51 0.008 0.019 0.006 0.026 0.014

0.17 0.02 0.11 0.24 0.51 0.28

Supports listed in Table 1. Amounts adsorbed per imit mass of support. ^Amounts adsorbed per unit surface area of support. The adsorption ability for both compounds was significantly enhanced by the introduction of Pd to supports, indicating that these compounds are adsorbed mainly on Pd metal or that Pd facilitates the adsorption onto the support. Table 4 shows the amounts of cinchonidine and 1 adsorbed on the catalysts reduced at 298 K and 473 K, together with the results of CO chemisorption. The amounts adsorbed on Pd metal were estimated from the difference between the amounts adsorbed on supports and on catalysts, tentatively assuming that no spillover occurred from Pd onto the supports. Although these measurements were prone to relatively large errors up to 20%, there are three tendencies to be noted in Table 4. First, the amounts of cinchonidine adsorbed on Pd in the catalysts reduced at 473 K are much larger than those on Pd reduced at 298 K, whereas the adsorbed amounts of 1 do not vary so much with the heat treatment. This explains the results of hydrogenation experiments shown in Fig. 2. The remarkable increase in the enantioselectivity, as well as the decrease in the activity, of the catalysts reduced at 473 K, is attributable to the increase in surface concentration of the modifier adsorbed on Pd; modification causes a drastic decrease in the activity of this catalytic system [15], contrary to the cinchona-modified Pt system [1,7]. Second, the order of the catalysts adsorbing the modifier is different from the order for the substrate. That is, the amount of cinchonidine adsorbed per unit surface area of Pd metal is decreased in the order Pd/Al203>Pd/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

<3 <3 <3 <3

0.22 0.79 0.63 0.68

0.003 0.266 0.098 0.120

0.013 0.097 0.072 0.042

0.58 3.8 6.9 7.9

1.3 5.4 5.7 1.3

0.03 0.18 0.33 0.37

0.06 0.26 0.27 0.06

473K Pd/Si02 Pd/Al203 473K 473K Pd/Zr02 PdA^i02-3 473K

1.1 13.9 11.4 9.7

19 1.5 1.8 2.1

5 <3 <3 <3

0.06 0.75 0.61 0.52

0.005 0.238 0.065 0.106

0.011 0.221 0.114 0.087

4.2 2.0 4.1 8.9

3.1 14.6 9.5 6.4

0.20 0.10 0.20 0.42

0.15 0.69 0.45 0.30

Pd/Si02 Pd/Al203 Pd/Zr02 Pd/Ti02-3

"" Nominal palladium loading 5%. Mean particle size of palladium estimatedfromCO uptake assuming a 1:2 CO:Pd ratio. ^ Mean crystallite size of palladium obtainedfromXRD measurements. Degree of dispersion estimatedfromCO uptake. ^Amounts adsorbed per unit mass of catalyst. / Amounts adsorbed per unit surface area of palladium estimated by deducting the amounts on supports. ' Calculated extent of adsorption as a ratio of substrate 1 or modifier to Pd metal atom. of Pd surface for cinchonidine and 1 are strongly influenced by the support materials. Third, the amounts of both cinchonidine and 1 adsorbed per gram of Pd/Si02 are almost negligible, unlike the other three catalysts. This finding is in harmony with the very low activity and selectivity of the Pd/Si02 catalyst, especially when reduced at 473 K. Taking into account the molecular sizes of the adsorbed compounds, the extents of adsorption, calculated as molar ratios of them to surface Pd atom, seems to be too large for most of the catalysts. Therefore, it is considered that the spillover from Pd onto support takes place with both compounds. The amounts concentrated on the surface of surrounding support will influence the surface concentration on Pd, thus affecting the hydrogenation on Pd surface. These preliminary results of adsorption measurements suggest that a support which can adsorb both modifier and substrate is favorable. In other words, a support with appreciable amounts of both acidic and basic sites, such as Ti02, seems to be preferable. 4. CONCLUSIONS Different supported Pd catalysts, pre-reduced in situ at 298 K and modified with cinchonidine, show similar enantioselectivities in the hydrogenation of 1 to 2, irrespective of the support used. With the catalysts pre-reduced at elevated temperatures, however, significant support effects are observed. Most catalysts reduced at 473 K exhibit much higher enantioselectivity but lower activity than those reduced at 298 K . The activity and selectivity of commercial Pd/C catalysts considerably varied depending on the origin. The amounts of the modifier and the substrate adsorbed on Pd are influenced by the reduction temperature of the catalysts and by the support employed, which may explain the hydrogenation activity and

198 enantioselectivity of the catalysts. A support having appreciable amounts of both acidic and basic sites with a moderate specific surface area seems to be preferable; Pd/Ti02, prepared with JRC-TIO-3, exhibited the best results. The effects of the Pd dispersion and the CI content in the catalyst remain to be clarified. The distribution of Pd metal particles in the pores of the support should also be studied. ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research No. 07650944 from the Ministry of Education, Science and Culture, Japan. We would like to thank Dr. K. Okuda of Rigaku Industrial Co. for the measurements of the CI content in Pd/C catalysts. REFERENCES 1. H.-U. Blaser, Tetrahedron: Asymmetry, 2 (1991) 843. 2. Y. Izumi, Adv. Catal., 32 (1983) 215; A. Tai and T. Harada, "Tailored Metal Catalysts," ed by Y. Iwasawa, Reidel Publ. Co., Dordrecht, 1986, p.265. 3. J. R. G. Perez, J. Malthete, and J. Jacques, C. R. Acad. Sc. Paris, t.300, Serie II (1985) 169. 4. Y. Orito, S. Imai, and S. Niwa, J. Chem. Soc. Jpn., (1979) 1118; (1980) 670; (1982) 137. 5. Y. Nitta and Y. Ueda, unpubHshed results. 6. M. Bartok, G. Wittmann, G. B. Bartok, and G. Gondos, J. Organomet.Chem., 384 (1990) 385. 7. H.-U. Blaser, H.-P. Jallet, M. Muller, and M. Studer, to be pubUshed in Catal. Today. 8. T. Tarnai, A. Tungler, T. Mathe, J. Petro, R. A. Sheldon, and G. Toth, J. Mol. Catal. A:Chemical, 102(1995)41. 9. A. Tungler, T. Mathe, T. Tarnai, K. Fodor, G. Toth, J. Kajtar, I. Kolossvary, B. Helenyi, and R. A. Sheldon, Tetrahedron: Asymmetry, 6 (1995) 2395. 10. C. Thorey, F. Henin, and J. Muzart, Tetrahedron: Asymmetry, 7 (1996) 975. 11. T. J. Hall, P. Johnston, W. A. H. Vermeer, S. R. Watson, and P. B. Wells, Stud. Surf. Sci. Catal, 101, Part A (1996) 221. 12. Y. Nitta, Y. Ueda, and T. Imanaka, Chem. Lett., (1994) 1095. 13. Y. Nitta and K. Kobiro, Chem. Lett., (1995) 165. 14. Y. Nitta, K. Kobiro, and Y. Okamoto, Proc. 70th Ann. Meeting Chem. Soc. Jpn., I (1996) 573. 15. Y. Nitta and K. Kobiro, Chem. Lett., (1996) 897. 16. J. T. Wehrli, A. Baiker, D. M. Monti, and H.-U. Blaser, J. Mol. Catal., 61 (1990) 207. 17. K. E. Simons, A. Ibbotson, P. Johnston, H. Plum, and P. B. Wells, J. Catal, 150 (1994) 321. 18. H.-U. Blaser and M. Muller, Stud. Surf. Sci. Catal, 59 (1991) 73; H.-U. Blaser and H. P. Jalett, ihid, 78 (1993) 139. 19. Y. Nitta, F. Sekine, T. Imanaka, and S. Teranishi, J. Catal, 74 (1982) 382. 20. G. Webb and P.B. Wells, Catalysis Today, 12 (1992) 319. 21. H.-U. Blaser, H. P. Jalett, D. M. Monti, A. Baiker, and J. T. WehrU, Stud. Surf. Sci. Catal, 67(1991) 147. 22. Reference Catalyst Committee in Catalysis Society of Japan, Catalysts & Catalysis, 38 (1996) 370.

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

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Enantio-differentiating hydrogenation of 3-alkanones with asymmetrically modified fine nickel powder Tsutomu Osawa , Tadao Harada , Akira Tai, Osamu Takayasu , and Ikuya Matsuura Faculty of Science, Toyama University, Toyama 930, Japan Faculty of Science and Technology, Ryukoku University, Otsu 520-21, Japan Faculty of Science, Kanaji, Himeji Institute of Technology, Hyogo 678-12, Japan The enantio-differentiating (e.d.) hydrogenation of 3-alkanones was carried out over a tartaric acid-NaBr-modified nickel catalyst. An optical yield of 44% was attained in the hydrogenation of the 3-alkanones after an intensive investigation of the catalyst preparation conditions and hydrogenation conditions. The following points were the special features of the hydrogenation of the 3-alkanones. (i) Fine nickel powder was a better source of the e.d. catalyst than Raney nickel which was a suitable source of the e.d. catalyst for p-ketoesters and 2-alkanones. (ii) The addition of a highly bulky carboxyUc acid such as 1-methyl-1cyclohexanecarboxylic acid or 1-adamantanecarboxylic acid was necessary for attaining the high optical yield. (iii) The optimal hydrogenation temperature was 100°C. The differentiation between the ethyl and other alkyl groups (3-alkanones) was much more difficuh than that between the methyl and other alkyl groups (2-alkanones). However, tartaric acid-NaBr-modified fine nickel powder was a unique catalyst giving good optical yield in the enantio-differentiating hydrogenation of the 3-alkanones. 1. INTRODUCTION The tartaric acid-NaBr-modified nickel catalyst (TA-NaBr-MNi) is an excellent enantiodifferentiating (e.d.) heterogeneous catalyst which gives around 90% optical yield (OY) for the e.d. hydrogenation of various p-ketoesters [1] and 2-alkanones [2]. Especially in the e.d. hydrogenation of 2-alkanones, this catalyst is unique by producing a high OY among all heterogeneous catalysts and homogeneous catalysts [3]. As the optical purity of the hydrogenated products was high enough to obtain optically pure compounds by the preferential recrystallization of the hydrogenated products or their derivatives [4], this catalyst is a promising catalyst for the production of chiral building blocks for the syntheses of optically active fine chemicals. In order to extend the substrate specificity (meaning specificity for the substrate giving a high OY) of this catalyst, we attempted the e.d. hydrogenation of 3-alkanones. During the hydrogenation of the 3-alkanones, the catalyst should differentiate a smaller difference between the two carbon chains attached to the

200

carbonyl group than that in the 2-alkanones. It is expected that this catalyst would be applied to various kinds of substrates when it gains high differentiating ability for alkyl groups. In this paper, we wish to present our recent findings concerning the hydrogenation of 3alkanones and discuss the important factors that affect the differentiating ability of the various alkyl groups. 2. RESULTS AND DISCUSSION In order to develop a high performance catalyst (the catalyst with high hydrogenation activity and high e.d. ability (e.d. ability was represented by the OY of the reaction)) for the e.d. hydrogenation of 3-alkanones, the various catalyst preparation conditions and reaction conditions were investigated using 3-octanone as a standard substrate. 2.1. Conditions of catalyst preparation It has been demonstrated that there are two types of sites on nickel surface [5]. (1) The site with affinity for TA where e.d. hydrogenation takes place (e.d. site). This site would locate on the surface of the crystallite nickel. (2) The site without affinity for T A where racemic products are produced (non-enantio-differentiating site (non-e.d. site)). This site would locate on the surface of aluminum derivatives or the amorphous nickel. The e.d. ability of the catalyst was represented by both the intrinsic e.d. ability of the modification reagent and the ratio of the optically active compounds produced on the e.d. site to the racemates produced on the non-e.d. site. Therefore, one of the effective strategies for raising the e.d. ability of the catalyst is to investigate the various catalyst preparation conditions in order to remove the non-e.d. site from the catalyst surface. 2.1.1. Types of nickel catalyst source Modified nickel catalysts were preparedfi-omthe various nickel sources (Table 1). In the case of the modified nickel catalysts preparedfi-omNiO, e.d. ability of the catalyst depended on the NiO manufacturers. These phenomena were also observed for the e.d. hydrogenation of methyl acetoacetate (MAA) [6]. Since the method of the NiO preparation presumably affect the percentage of crystallite part of reduced Ni, the types of NiO would reflect the e.d. ability of the catalyst. On the other hand, fine nickel powder (FNiP) gave the e.d. catalyst Table 1 Effect of the source of the catalyst on e.d. ability in 3-octanone hydrogenation^ Source of catalyst Catalyst preparation Hydrogenation temp. / °C e.d. ability / % RNi lOOT, Ih, digestion 100 10 FNiP^ 280T,0.5h,H2 treatment 120 30 NiO'^ 350T, Ih, H2 treatment 120 4 NiO^ 350T, lh,H2 treatment 120 13 NiO' 350T, Ih, H2 treatment 120 23 NiO^ 350T, Ih, H2 treatment 120 3 ^ Pivalic acid (PA) (8.15 g) was added to the reaction mixture. ^ Modification conditions: TA 1 g, NaBr 0.1 g, pH 3.5, lOOT. ' 99.9%, Wako Pure Chemical Industry, light grayish green ^Wako Pure Chemical Industry, green ' Kanto Chemical Co., gray ^Nakarai tesque, grayish green

201 with higher e.d. ability than Raney nickel (RNi) which was a good source of the e.d. catalyst for p-ketoesters and 2-alkanones. Furthermore, modified FNiP (MFNiP) also showed better reproducibility of e.d. ability than RNi for the hydrogenation of 3-octanone. Therefore, FNiP was the most suitable catalyst source for the preparation of an e.d. catalyst for 3-octanone hydrogenation. 2.1.2. Hydrogen treatment temperature of FNiP It is essential for FNiP to be activated in a hydrogen stream to reduce nickel oxide on the surface before modification. Table 2 indicates the effect of the H2-treatment temperature on e.d. ability and on the mean crystallite size of the catalyst [7]. The e.d. ability value and mean crystallite size increased with the elevation of the H2-treatment temperature in the range between 200-300°C. These phenomena would indicate that nickel particles with a larger crystallite size would provide a surface suitable for the TA adsorption of the effective enantio-differentiation. Table 2 Effects of H2 treatment temperature on e.d. ability and mean crystallite size^ H2-treatment temp. / °C e.d. ability / %^ Mean crystallite size / nm 11.6 200 23 13.6 250 25 14.2 300 31 17.5 "Ref [7], ''e.d. ability of TA-NaBr-modified FNiP (TA-NaBr-MFNiP); Modification conditions: TA Ig, NaBr 0.1 g, pH 3.5, 100°C. Reaction mixture: 3-octanone (32 m mol), THF (10 ml), and PA (8.15 g). Hydrogenation temp.: 120T, "" Without H2 treatment and modification 2.1.3. Conditions for the Ni-surface modification In the hydrogenation of MAA and the 2-alkanones, the pH of the modification solution had a significant influence on e.d. ability of the catalyst [8, 9], because it changed the posture of the adsorbed modification reagents [8]. The pH dependence of the modification solution on e.d. ability in the 3-octanone hydrogenation revealed that a pH of 3.2-3.5 gave the maximum e.d. Table 3 Effects of modification temperature on e.d. ability Catalyst Modification conditions Additives / g e.d. ability / % Temp / T pH 0 4.0 TA-NaBr-MFNiP^ PA/8.15 11 TA-NaBr-MFNiP^ 50 PA/8.15 4.0 17 100 TA-NaBr-MFNiP^ 4.0 24 PA/8.15 100 41 TA-NaBr-MFNiP*' 3.5 MCAV7.28 110 3.5 43 TA-NaBr-MFNiP^ MCA / 7.28 120 43 TA-NaBr-MFNiP^ 3.5 MCA / 7.28 TA-NaBr-MFNiP^ 130 3.5 13 MCA / 7.28 ^ FNiP was treated with a hydrogen stream at 200°C. ^ FNiP was treated with a hydrogen stream at 280®C. "^ MCA: 1-methyl-1-cyclohexanecarboxylic acid Modification conditions: TA 1 g,NaBr 0.1 g. Hydrogenation temperature: lOOT

202

ability similarly to the case of MA A and the 2-alkanones. This means that the posture of the adsorbed tartaric acid on the surface suitable for the e.d. hydrogenation of MAA and the 2alkanones was also appropriate for that of 3-octanone. The temperature of Ni surface modification is also an important factor for increasing the e.d. ability. The high temperature modification improved the e.d. ability of RNi in 2-alkanone hydrogenation [10], RNi [6] and reduced Ni (HNi) [11] in MAA hydrogenation. When RNi was used as the source of the e.d. catalyst, the high temperature modification removed the aluminum and related metal compounds from the surface [6]. The high temperature modification was also considered to remove amorphous Ni from the HNi surface [11]. For the hydrogenation of 3octanone over MFNiP, the modification temperature also affected the e.d. ability (Table 3). The value of e.d. ability increased with the increase in modification temperature. The modification at 110120°C gave the maximal e.d. ability. It was 100 150 200 demonstrated that the mean crystallite size NaBr / mg of FNiP did not change with the modification temperature. [7] Therefore, Fig. Effect of the amount of NaBr on e.d. ability these temperature effects would be attributed to the surface conditioning FNiP was treated with a hydrogen stream at 200"C. Modification pH: 3.5 Reaction mixture: 3-octanone which preferentially removed amorphous (32m mol), THF (10ml) and pivalic acid (8.15g) Ni from the surface and provided a smooth Hydrogenation temperature: 100°C surface appropriate for the regularlyarranged adsorption of TA. Inorganic salts are known to increase the e.d. ability when they co-exist with TA in the modification solution. They selectively adsorb and deactivate the non-e.d. sites on the catalyst surface [5]. Especially, NaBr effectively increased the e.d. ability in the e.d. hydrogenation of MAA and the 2-alkanones. Figure 1 shows the effects of the amount of Table 4 Effects of inorganic salts on e.d. ability Inorganic salt Amount / mg e.d. ability/% 100 NaBr 39 100 NaCl 38 100 NaC104 38 100 Na2S04 37 100 NaF 18 100 KBr 5 100 LiBr FNiP was treated with a hydrogen stream at 320T. Modification conditions: pH 3.5, lOOX Reaction mixture : 3-octanone (32 mmol), THF (10 ml), and MCA (7.3g) Hydrogenation temperature : 120°C

203

NaBr on e.d. ability of the catalyst in the hydrogenation of 3-octanone. The addition of NaBr increased the e.d. ability and reached a plateau for more than 50 mg of NaBr. The amount of NaBr required for the maximum e.d. ability for FNiP was much smaller than that for RNi (2.5g of NaBr is required for Ig of RNi catalyst for the hydrogenation of 2-octanone [10] ). This means that the ratio of the non-e.d. site of FNiP to e.d. site was much smaller than that of RNi. Table 4 indicates the effects of the addition of various inorganic salts on e.d. ability. Similarly to the hydrogenation of MAA and the 2-alkanones, the addition of NaBr gave the highest e.d. ability. The effects of NaCl, NaC104, and Na2S04 were comparable to NaBr. 2.2. Hydrogenation conditions 2.2.1. Additives to the reaction mixture Table 5 indicates the effects of the addition of carboxylic acid to the reaction system on OY in the hydrogenation of 3-octanone. Similarly to the hydrogenation of the 2-alkanones, the addition of a carboxylic acid was indispensable to increase OY. However, the effect of the acid variety in the hydrogenation of 3-octanone was unique for the following reasons: (i) Branching at the a-carbon of the acid increased OY, but the degree of the branching had no influence on OY (Entries 3 and 5). (ii) Branching at the P-carbon (Entry 4) gave ahnost the same OY as did the a-branched carboxylic acid, (iii) A carboxylic acid with branched bulky alkyl groups at the a-carbon (Entries 6 and 7) gave the highest OY. Thus, the effective differentiation between the ethyl and the other alkyl groups requires highly bulky carboxylic acids. Table 5 Effects of the carboxylic acids on OY in the 3-octanone hydrogenation Entry Carboxylic acid Amount / m mol OY / % Configuration 1 None — 6 S 2 17 22 Acetic acid S 40 29 3 Isobutyric acid S Isovaleric acid S 4 20 30 5 77 30 Pivalic acid S 40 6 77 l-Me-l-cyclohexanecarboxylic acid S 42 7 1-Adamantanecarboxylic acid 51 S FNiP was treated with a hydrogen stream at 280T. Modification conditions: pH 3.5, lOOT Hydrogenation temperature: 120T 2.2.2. Hydrogenation temperature The other characteristic of the hydrogenation of alkanones was the effect of hydrogenation temperature on OY. It was demonstrated that over a TA-NaBr-modified RNi (TA-NaBrMRNi), the OY of the hydrogenation of the 2-alkanones increased at the lower temperature when pivalic acid (PA) was added to the reaction system. On the other hand, the OY remained constant regardless of the reaction temperature in the hydrogenation of MAA. Figure 2 shows the effect of hydrogenation temperature on OY in the hydrogenation of 2octanone over a TA-NaBr-modified FNiP (TA-NaBr-MFNiP) in the presence of 1-methyl-1cyclohexanecarboxylic acid (MCA). Optical yield increased with the lower hydrogenation temperature and maximal OY was attained at 60T over MFNiP. This temperature dependence on OY was the same as that obtained over MRNi in the presence of PA. The

204 dependence of the hydrogenation temperature on OY was investigated in the presence of MCA or acetic acid (Fig. 3) in the hydrogenation of 3-octanone. In this case, OY increased with an increase in temperature and reached maximum at 100°C, but it decreased beyond this temperature. Thus, this relationship between OY and hydrogenation temperature was not determined by the types of the added carboxylic acid or the catalysts, but by the types of the ketones. 80 r 50 70

40

60 > ;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.

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

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^^u ^^^ ^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

L^o=-^ 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.LabeLCon^unds 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.

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

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 ^hlPA^O^,, 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 RhlKl^O^, 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 rfV^o ^°^^^ (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
O —

To

-A '

0

100

200

pretreatment T

^-20

'

300

)

Rh/AI203 Rh/AI203,EDCA

400

0

100

200

pretreatment T

300

400

)

Figure 5. Initial rates of hydrogenation of 1 in ethanol with or without EDCA (a) and final diastereoisomeric execess d.e. (b) for Rh/Al203 samples pretreated in hydrogen at different temperatures. With or without EDCA present, the ex-situ treatment in hydrogen at room temperature produced a large decrease of the rates of reaction. At higher temperatures, the rates increased to attain a plateau (figure 5a). These changes could be related to the different valence states of the

221 rhodium evidenced by XPS (Rh°, Rh', and Rh"^). Indeed, the rate of hydrogenation of aromatic compounds depends primarily upon their adsorption coefficient on the surface, which, in turn, depends upon the electronic state of rhodium. Figure 5b shows that the diastereoselectivity was modified by thermal pretreatments. When the reaction was performed in the absence of EDCA, the d.e. increased from 28% to 50% after pretreatment at 50°C; it decreased after prehydrogenation at higher temperatures. Similarly, with EDCA present, the d.e. increased from ca. 38% to a maximum of ca. 68% for pretreatments in the range 30 to 100°C; then, it decreased to 50% after treating at 300°C. These results can again be accounted for in terms of the various factors discussed in the previous section, viz: (i) changes in the oxidation states of rhodium as a function of the temperature of treatment under hydrogen. The selectivity, governed by the interactions between the substrate and the rhodium surface, would be maximum for an optimum reduction state or hydrogen coverage of the surface. The effects of various reductive and oxidative catalyst pretreatment temperatures were reported earlier in the palladium catalysed reduction of Schiff bases [11] or in the enantioselective hydrogenation of a-ketoesters on alumina-supported platinum catalysts [12]. (ii) As the catalysts were heated under hydrogen, the morphology of the particles may change. Thus, it has been shown [13] that after heating a VxJPA^O^ catalyst in hydrogen from room temperature to 400°C, the initial bidimensional morphology of the platinum particles changed to a polyhedral morphology which was correlated with the increase of the enantioselectivity in ethyl pyruvate hydrogenation. Similar morphological changes with pretreatments in hydrogen could also occur in the present system which might account for the diastereoselectivity improvement, (iii) Thermal treatment could also change the state of the alumina surface, e.g., by eliminating a monolayer of water possibly associated with the alumina surface, or changing the nature of acid sites. In both cases, this would modify the interaction of the proline moiety with alumina and thus the diastereoselective control. 3.4. Effect of the nature of the solvent The effects of the nature of the alcoholic solvent on the activity and diastereoselectivity, have been studied on catalysts Rh/C and ^thlkip-^ pretreated at 100°C (Table 2). Without EDCA, the nature of the solvent had little effect on the activity of Rh/C and RhlkX^Oj^ catalysts. In the presence of EDCA, the initial rates on both catalysts were in the following order: MeOH > iPrOH > EtOH. In methanol, a gradual deactivation was observed, so that the reactions could not be run to completion. In the presence of EDCA, the d.e. were clearly lower in isopropanol since a value of ca. 25% was observed compared to ca. 42% and 68% over Rh/C and RYdkl^O^, respectively. This could be partly due to the formation of larger amounts of intermediate cyclohexenic compound 6 in isopropanol, which was hydrogenated preferentially into 2, thus decreasing the final d.e. values. Table 2. Initial reaction rates (mol.h'.molRh'^) and d.e (preferential configuration in parentheses) in different alcohols. Rh/Al,0/ solvent Rh/C d.e. (%) initial rate d.e. (%) initial rate EDCA' EDCA^ EDCA" EDCA^ MeOH EtOH iPrOH

15 15.5 15

5.5 2.3 4.5

17.5(2) 17(2) 18(2)

41(3) 43(3) 26(3)

5.0 7.8 5.8

4.6 2.0 3.7

50(3) 42(3) 19(3)

67(3) 68(3) 25(3)

a: pretreated at 100°C, b: EDCA/Rh = 3.5, c: EDCA/Rh = 5. CONCLUSIONS The diastereoselective hydrogenation of N-(2-methylbenzoyl)-(5j-proline methyl ester 1 into optically active 2-methylcyclohexane carboxylic acids was studied on rhodium catalysts supported on active carbon, graphite and alumina. The following points were highlighted:

222

1. EDCA addition had comparatively little effect on the initial rates and d.e. of the ^(hIAlf>^ catalyst used without preliminary thermal treatment under hydrogen. This is probably due to the low coverage of the metal surface by the amine which is preferentially adsorbed on the acidic sites of the alumina support. 2. The diastereoselectivity was highly dependent upon the nature of the support. In the absence of EDCA, a 40% selectivity in favour of 3 was observed over Rh/AljOg in contrast with Rh/C or Rh/G catalysts which gave only the racemic mixture, at least before the cyclohexenic intermediate started to hydrogenate. This means that substrate 1 was preferentially adsorbed via one specific face of the aromatic ring. The driving force controlling this specific adsorption could be due to at least three effects, viz: (i) The stronger interaction of the oxidized rhodium species present in ^hlAiflj^ with die oxygen-containing groups or with the aromatic ring of the substrate; (ii) Steric effect imposed by a more flat rhodium surface; (iii) A possible interaction of the oxygen-containing groups of the proline moiety with the oxygen anions at the surface of alumina. None of these factors can be discarded on the basis of present results and further studies on model catalysts are required to give preference to one of them. 3. The diastereoselectivity of Rh/AljOg catalyst depended upon the thermal treatments under hydrogen prior to reaction. Again the three factors discussed above may at this stage account for the changes observed and a definitive interpretation must await additional studies. 4. From the standpoint of applied research, the optimization of the rhodium catalyst (support and thermal pretreatment) and of reaction conditions (solvent, addition of amine) have led to a final d.e. of 68%, which is much higher than those obtained in our previous investigations and is encouraging for further studies in view of the difficulties to achieve the chiral synthesis of cyclohexyl derivatives.

REFERENCES 1. R.A. Sheldon, Chirotechnology: industrial synthesis of optically active compounds, Marcel Dekker, New York, 1993. 2. J.K. Whitesell, Chem. Rev., 92 (1992) 953. 3. The Merck Index, 12th ed, Merck Research Lab., Div. of Merck and Co, Inc (eds), 1996. 4. for a review, see H.U. Blaser and B. Pugin, G. Jannes and V. Dubois (eds), Chiral Reactions in Heterogeneous Catalysis, Plenum Press, pp. 33, 1995. 5. A. Tungler, T. Tamai, T. Mdthe, J. Petro and R.A. Sheldon, G Jannes and V. Dubois (eds), Chiral Reactions in Heterogeneous Catalysis, Plenum Press, pp. 121, 1995. 6. K. Nasar, M. Besson, P. Gallezot, F. Fache and M. Lemaire, G Jannes and V. Dubois (eds), Chiral Reactions in Heterogeneous Catalysis, Plenum Press, pp. 141, 1995. 7. M. Besson, B. Blanc, P. Gallezot, K. Nasar and CPinel, R.Malz (ed.). Catalysis of Organic Reactions, Marcel Dekker, New York , pp.177, 1996. 8. M. Besson, B. Blanc, M. Champelet, P. Gallezot and K. Nasar, J. Catal., in press. 9. C. Exl, E. Fersti, H. Honig and R. Rogi-Kohlenprath, Chirality, 7 (1995) 211. 10. A. Giroir-Fendler, D. Richard and P. Gallezot, M. Guisnet et al (eds.). Studies in Surface Science and Catalysis 41: Heterogeneous Catalysis and Fine Chemicals, Elsevier, pp. 171, 1988. 11. A. Tungler, M. Acs, T. Mdth6, E. Fogassy, Z. Bende and J. Petro, Appl. Catal., 17 (1985) 127. 12. B. Minder, M. Schiirch, T. Mallat, A. Baiker, T. Heinz and A. Pfaltz, J. Catal., 160 (1996) 261. 13. A. Tungler,T. Math6, K. Fodor, R.A. Sheldon and P. Gallezot, J. Mol. Cat. A, 108 (1996) 145. ACKNOWLEDGEMENTS Thanks are due to C. Leclerq for TEM experiments and to P. Delichere for XPS measurements. The authors are grateful to the Conmiission of European Union, COST Chemistry D2 for financial support.

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

223

Stereoselective reductions of aromatic compounds Emmanuel Auer, Andreas Freiind, Peter Panster, Gemot Stein and Thomas Tacke Degussa AG, Silicas and Chemical Catalysts Division, Applied Research & Development Chemical Catalysts and Zeolites, P.O. Box 1345, D-63403 Hanau, Germany. Mono- and bimetallic Rh catalysts were evaluated for the stereoselective reductive amination of alkylphenols to the corresponding alkylcyclohexylamines. Rhodium catalysts as well as bimetallic catalyst formulations with platinum were found to be the most suitable for this one-step hydrogenation. Both the chemo- and the stereoselectivity of that reaction depend on the amount of base added and other parameters such as kind of solvent and/or support. 1. INTRODUCTION Research activities on heterogeneously-catalyzed stereoselective reductions have generally increased during the past years[l]. Among various functionalities the stereoselective aromatic ring reduction is of inportance in the synthesis of fine chemicals such as pharmaceuticals, herbicides, fragrances and liquid crystals. Particularly, substituted phenol derivatives offer the possibility to control the product selectivity via reduction of the prochiral intermediate cyclohexanone [2,3]. In addition to the cycloaliphatic alcohols, the corresponding amines are accessible by reacting the same substrate in the presence of ammonia. In principal, the latter products can also be obtained by reduction of a subtituted nitrobenzene derivative (see Figure 1). However, this kind of reduction gives rise to additional side reactions known for nitro groups [4] and for the subsequent hydrogenation of aromatic amines [5], e.g. coupling or hydrogenolysis. Furthermore, it is preferred to perform these reactions in two separate steps in order to minimize by-product formation. This is also valid for the two step formation via the alkylcyclohexanone and reductive amination of the latter. Both single steps require different catalysts. The first step is preferably catalyzed by palladium catalysts, whereas Raney nickel, palladium or platinum catalysts are commonly used for the reductive amination. In particular, the latter reaction yields a fairly low stereoselectivity (cis/trans ratio of 1.27 for 4-tert. alkylcyclohexylamine) [6]. In the present work, the influences on the activity and selectivity of the direct hydrogenation of mono-substituted alkylphenols to the corresponding alkylcyclohexylamines have been studied. Although this one-pot reaction has been reported in the literature, almost no information was given with regard to the stereoselectivity [7]. The reactions described were carried out at rather high temperatures in the range of 500 K. 2. EXPERIMENTAL Precious metal catalysts were prepared by precipitation of one or two different metal precursors selected from the group M = Pt, Rh and Ru on various powder supports (e.g.

224 activated carbon, alumina, silca, titania or organofunctionalized polysiloxanes). Subsequent reduction with reducing agents such as formaldehyde, sodium tetrahydridoborate, hydrogen etc. conq)letes the catalyst preparation, because unreduced catalysts have been found to be considerably less active under identical reaction conditions than their reduced counterparts. Catalytic tests were performed in 500 ml autoclaves using various a]ky]phenols (R = 4C2H5, 4-CH3O, 4-i-C3H7, 2-t-C4H9, 4-t-C4H9, 4-t-C5Hii ). The reactions were carried out typically at 398 K and under hydrogen pressures of 5 MPa in isopropanol using an alkylphenol concentration of 80 g/1, a catalyst concentration of 1 wt.-% with regard to the substrate and a mo]ar ratio of ammonia to alkylphenol of 6. These initial parameters were used as a starting set which was refined during the investigations (see Tables 1 - 5). Particularly, influences of parameters such as type of catalyst, solvent and additional inorganic and organic bases have been investigated.

NO2

NH2 3H2 cat.

3H2 cat. H

NH^

NH2 '

^

^

:

:

=

^

'

'

H

H cis

trans

3H2 cat. NH3

Figure 1. Principle catalytic pathways for the manufacture of substituted cyclohexylamines

225 3. RESULTS AND DISCUSSION For the given mild reaction conditions it was necessary to use Rhodium catalysts or Rh containing catalysts together with inorganic bases, e.g. alkali hydroxides, in order to facilitate the aromatic ring reduction of 4-alkyl-phenols. Without any alkali hydroxide addition, the reaction is very sluggish or does not occur. This is in agreement with earlier results which suggest the use of additional bases, preferably lithium or sodium based [7,8]. Although both hydroxides are suitable for the investigated reaction, lithium has been selected for the present study. Depending on the amount of lithium hydroxide added, the reaction is accelerated first and with increasing amounts of base retarded again. The optimum range of lithium hydroxide (< 0.2 mol/mol phenol) favored not only the chemoselectivity for the formation of the alkylcyclohexylamines, but also the cis-products were obtained more readily . By decreasing the molar ratio of LiOH to phenol to 0.05, more than 80% of the products were present in cisconfiguration (see Table 1). The literature known for ring hydrogenation of aromatic amines or phenols offer two possible explanations about the function of alkali promotors: On the one hand, the phenol can be converted to the phenolate anion facilitating the hydrogenation of two double bonds of the aromatic ring [1,8]. On the other hand, the lithium can moderate the acidity of the catalyst support minimizing hydrogenolysis reactions [7]. Table 1 Influence of LiOH concentration on chemo- and stereoselectivity Run

LiOH

Reaction time

Conversion

Selectivity to amines^

[mol/mol phenol]

[min]

[%1

[%1

cis/trans ratio amines

1

0.05

300

71.6

71.8

4.19

2

0.1

120

99.7

74.4

2.48

3

0.15

120

99.9

62.6

2.17

4

0.2

180

99.3

76.0

2.32

5

0.3

300

99.8

61.8

2.09

6

1

180

95.9

17.3

1.68

Reaction parameters: alkylphenol R = 4-t-C4H9, 5% Rh/C catalyst; ^ Residual composition: mainly alkylcyclohexanols, traces of alkylcyclohexane. Addition of further bases like triethylamine or N-ethyl morpholine allowed the reduction of the level of alkylcyclohexanols (see Table 2). Whereas small amounts of the tertiary amine yielded a cis/trans ratio of about 3, a higher ratio was obtained at a N-ethyl morpholine concentration of 0.2 mol/mol phenol. However, under the latter conditions the formation of alkylcyclohexanols was increased in favor of the desired alkylcyclohexylamines. As in the case of the inorganic bases, larger amounts of tertiary amines added to the reaction led to a decrease of the catalyst activity.

226 Table 2 Influence of tertiary amine concentration on chemo- and stereoselectivity Run

N-ethyl morpholine [mol/mol phenol]

Reaction time

Conversion

[min]

cis/trans ratio amines

[%1

Selectivity to amines^ [%]

1

-

330

78.9

88.7

2.94

2

0.015

330

87.1

89.1

3.21

3

0.05

180

93.9

70.7

3.36

4

0.1

300

99.8

69.2

2.27

5

0.2

180

26.0

66.4

5.98

Reaction parameters: alkylphenol R = 4-t-C4H9, 0.1 mol LiOH/mol phenol, 1% Pt + 4% Rh/C catalyst; ^ Residual composition: mainly alkylcyclohexanols, traces of alkylcyclohexane. Amino-functionalized polysiloxanes (Deloxan®) with tertiary amines incorporated in the silica matrix, show a similiar behaviour. The tertiary amine can be omitted, but the stereoselectivity of this catalyst was lower than that of the conventional catalysts (see Table 3). Table 3 Influence of the nature of the catalyst on chemo- and stereoselectivity Run

Catalyst

Reaction time

Conversion

[min]

_[%]_

Selectivity to amines

cis/trans ratio ammes

1

2% Rh 3% Ru/C

30

> 99.9

23.9

1.10

2

3% Rh/Deloxan®

60

> 99.9

58.6

2.40

3

5%Rh/Al2 03

45

> 99.9

50.9

2.98

4

5%Rh/Al2 03^'^

180

> 99.9

66.2

3.68

5

5% Rh/Si02

180

99.9

75.7

2.67

6

l%Pt4%Rh/Si02

200

> 99.9

78.1

3.19

7

5% Rh/Ti02

90

> 99.9

63.9

2.86

8

l%Pt4%Rh/Ti02

90

> 99.9

64.6

3.11

9

l%Pt4%Rh/Ti02^

120

> 99.9

67.7

3.48

10

l%Pt4%Rh/Ti02^'^'^

240

99.2

70.7

3.64

Reaction parameters: alkylphenol R = 4-t-C4H9, 0.1 mol/mol phenol LiOH. Residual composition: mainly alkylcyclohexanols, traces of alkylcyclohexane. ^0.05 mol LiOH/mol phenol. ^ 0.1 mol N-ethylmorpholine/mol phenol was added. "^ Support was rutile. ^ T = 423 K.

227

Apart from monometallic rhodium catalysts, the best results were obtained with bimetallic formulations, e.g. 4% Rh + 1% Pt (see Table 3). Rh-Ru catalysts were even more active, but rather unselective. Support properties such as low surface acidity are also inq)ortant. In the order AI2O3 > Ti02 (anatase/rutile 73/27) > Si02 > Ti02 the surface acidity decreases while the stereoselectivity increases. The con^arison 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 con^osition: 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).

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

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 catalj^ic 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

/^om-CF3

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

-2.8

P-CH3 <4i-NH2

g>

/

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/ia^i/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/[%]<: 97.0 99.0 98.0 98.0 98.0 87.0 91.0 89.0 64.0

a Nitro compound: 10 mmoles, hydrazine monohydrate: 15 mmoles, amount of catalyst: 0.05 g. All reactions were carried out in 100 ml ethanol at 70°C. ^ The purity of the reactants and the products was checked by GC-MS, IH- and l^C-NMR and elementary microanalyses. c Yields of pure, recrystalUzed products (based on the nitro compoimd).

236 The reaction times were between 2 hours (m-Cl) and 6 hours (p-OH). The reactions were stopped, when all of the reactants had disappeared. The reason for the better yields in the reduction of 4-nitroazobenzenes with electron-withdrawing substituents is the higher selectivity for the reduction of the nitro group vis-a-vis that of the azo bridge, as well as the fact that the positive slope of the pa-relationship is higher for reaction (1) than for the reactions (2) and (3) (Table 2). 3.3.

Arrhenius activation energy for the nitro group reduction and the reductive azo cleavage By variation of the temperature from 50 to 70°C the Arrhenius activation energies were determined for the reduction of the unsubstituted 4-nitroazobenzene using a tenfold excess of hydrazine hydrate. For this compound the nitro group reduction at 55°C is about ten times faster than the reductive azo cleavage. For this reason, the rate constants for the nitro group reduction (1) were determined at the very beginning of the reduction. Reaction (2) could not be observed. The rate constant of the azo cleavage (3) in 4-aminoazobenzene was evaluated after all 4-nitroazobenzene had disappeared and only reaction (3) was taking place. The results are shown in Figures 3 and 4.

Ea(l)=81.1kJ/mole

Ea(3) = 46.6 kJ/mole 11 J

8.5 /-^v

e

S 7.5

10 ^

1

6.5 0.34

y

r

q

0.36

0.38

1000/(R-T) [mole/J]

Figure 3. Arrhenius activation energy for the nitro group reduction (1) in 4nitroazobenzene. ko = 1 mole/(l-min), r2 = 0.994.

0.34

0.36

0.38

1000/(R-T) [mole/J]

Figure 4. Arrhenius activation energy for the reductive azo cleavage (3) in 4aminoazobenzene. ko = 1 mole/(l-min), r2 = 0.983.

The Arrhenius activation energies Ea were calculated from the temperature dependence of the measured pseudo 0*^*^-order rate constants by linear regression. The Arrhenius activation energy for the nitro reduction (1) is Ea(l) = 81.5 kJ/mole, that for the azo cleavage (3) is Ea(3) = 46.6 kJ/mole.

237

4. EXPERIMENTAL 4.1. Preparation, characterization and handling of the catalyst The iron oxide hydroxide modification was precipitatedfi*oman aqueous solution of Fe(III)Cl3 with sodium hydroxide at 60°C. The pH-value of the solution dtiring the precipitation should not exceed 8. After centrifugation and drying, the catalyst was redispersed and milled to a fine powder. The characterization of the iron oxide hydroxide catalyst and the influence of the surface structure on its H-transfer activity is being examined in the research group of R. Prins [8]. Half an hour before the reaction was started, the catalyst had to be activated by adding some drops of distilled water to develop its full catal3^ic activity in organic solvents. After the reaction the catalyst can be filtered off, washed and reused for fiirther reductions. At temperatures above 70°C, the catalyst changes its colour fi-om reddish-brown to black. After this colour change, most of its activity is lost. Obviously the active iron oxide hydroxide modification is transformed into a thermodjmamically more stable modification with lower H-transfer activity. 4.2. Kinetic measurements All experiments were carried out in a lOOml-thermostatted glass vessel with a cooling jacket. The reaction solution was stirred vigorously with a magnetic stirrer at 1000 rpm. All the ap-relationships for the various reduction steps were determined in ethanol at 55°C and with a tenfold excess of hydrazine hydrate. The concentrations were O.lAf of substituted nitrobenzenes with 0.1 g catalyst per 100 ml ethanol and 0.03Af of substituted 4-nitroazobenzenes with 0.05 g catalyst per 100 ml ethanol. After addition of the catalyst to the reaction solution, the reactions were started by adding the hydrazine hydrate. For the evaluation of the reduction kinetics, samples from the reaction mixture were drawn at different times. Their composition was immediately measured by UV-Vis spectroscopy, after filtering off the catalyst by a microfilter of 0.2 iim pore diameter. The UV-Vis measurements were carried out on a Shimadzu UV-260 recording spectrophotometer with a SOW halogen- and a D2-lamp. The concentrations of the nitrobenzenes and the corresponding anilines were determined by measuring the absorbance between 270 and 350 nm. The concentrations of the nitroazo compounds were monitored above 350 nm. In this range, only the azo dyes but not the products of a possible azo cleavage absorb. The concentrations of the nitro- and aminoazo compoimds were determined by measiiring the absorbance of the reaction mixture at two different wavelengths. The concentrations of the cleavage products were calculated by subtracting both the concentration of the nitroazobenzene and that of the corresponding aminoazobenzene fi'om the initial concentration. The molar extinction coefficients of the nitro and the amino compounds were determinedfi*ompure reference substances. Under the reaction conditions used for the kinetic measurements, no formation of intermediates (e.g, nitroso, hydroxylamino or hydrazo compounds) coxdd be observed. The composition of reaction mixtures was checked by thin layer and gas chromatography, monitored by UV-Vis spectroscopy and finally verified by GCMS, IH- and^^c.NMR.gpectroscopy. The initial rate constants of the nitro group reduction were determined to be 0*^*^ order with respect to the nitro compound and -0.5^*^order in hydrazine hydrate.

238 5. CONCLUSIONS A great variety of nitrophenylazo derivatives with electron-withdrawing substituents R in the aromatic ring not carrying the nitro group can be reduced selectively to the corresponding aminophenylazobenzenes using the catalytic Htransfer system presented here. For nitrophenylazo derivatives with electrondonating substituents no selective reduction is observed because the rate constants of the azo cleavage and that of the nitro group reduction are of the same order of magnitude. The selectivity for the reduction of the nitro group vis-a-vis that of the azo fimction can be enhanced by increasing the reaction temperature as the Arrhenius activation energy for the nitro reduction (Ea(l) = 81.5 kJ/mole) is about twice that for the azo cleavage (Ea(3) = 46.6 kJ/mole). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

Furst A., Berlo E.G., Jtlooton S., Chem. Rev., 65 (1965) 51. Brieger G., Nestrick T.J., Chem. Rev., 74 (1974) 567. Wissmann J., Ph.D. thesis ETH Zurich No. 11719, (1996). Miyata T., Ishino Y., Hirashima T., Synthesis, (1978) 834. Bechamp A.J., Anal. Chim. Phys., 42 (1854), 186. Zinin N., J. Prakt. Chem., 27 (1842) 140. Jafife H.H., Chem. Rev., 53 (1953) 191. Benz M., Ph.D. thesis ETH Zurich, in preparation, (1996).

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

Selective catalytic hydrogenation of 2,4-dinitrotoluene nitroarylhydroxylamines on supported metal catalysts

239

to

M. G. Musolino\ C. Milone^ G. Neri^, L. Bonaccorsr, R. Pietropaolo^ and S. Galvagno'' * Department of Industrial Chemistry, University of Messina, 1-98166 Messina, Italy ^ Faculty of Engineering, University of Reggio Calabria, 1-89100 Reggio Calabria, Italy

Abstract The selective liquid phase hydrogenation of 2,4-dinitrotoluene (2,4-DNT) to the corresponding 2,4-nitroarylhydroxylamines has been studied over supported Pd, Pt, and Ru catalysts. Pt and Pd samples were found more active and selective than Ru. On the palladium catalysts the influence of metal particle size, temperature and nature of the support on the catalytic activity and selectivity has been also investigated. Both specific activity and selectivity were found to be dependent on the palladium particle size. Larger Pd particles were found more active and selective towards the formation of the nitroarylhydroxylamines The results reported have been interpreted on the basis of a different geometry and strength of adsorption of the substrate on the active sites. The products distribution is influenced also by the acid-base properties of the support used. 1. INTRODUCTION Aromatic hydroxylamines are generally prepared by chemical reduction or selective hydrogenation of aromatic nitrocompounds by using metal catalysts promoted with dimethylsulfoxide. However, such methods of synthesis are characterized by difficult products purification and low yields /1,2/. The low cost production of arylhydroxylamines can be of great practical interest because these compounds can undergo rearrangement to yield a variety of important chemicals 111. Due to the complex reaction mechanism and to the difficulty to identify all reaction products, few studies have been reported in literature on the selective hydrogenation of dinitrocompounds. In previous papers we have investigated the 2,4-DNT hydrogenation over Pd/C catalysts. This reaction is industrially important for the production of 2,4diaminotoluene (2,4-DAT) which is used in the synthesis of toluenediisocyanate. We have reported that by carrying out the reaction under mild conditions high yields to the corresponding 2,4-nitroarylhydroxylamine isomers can be obtained /3-5/. In this work we have investigated the selective hydrogenation of 2,4-DNT over different noble metals (Pd, Pt and Ru) supported on active carbon with the aim of maximizing the yields to the arylhydroxylamines. The effect of metal particle size, nature of the support and temperature on the selectivity to 2,4-nitrohydroxyaminotoluenes over Pd supported catalysts is also reported.

240

2. EXPERIMENTAL Pd, Pt and Ru supported on carbon (Chemviron SCXII, 100-200 mesh, surface area 1100 m'^/g) were prepared by incipient wetness impregnation of the support with aqueous solutions of PdCb, H2PtCl6 and RuCb, respectively. Metal loading was varied between 0.5 and 5 wt.%. After impregnation, the catalysts were dried at 120 °C and reduced under flowing hydrogen. The Pd samples were reduced between 200 - 600 °C for Ih, whereas the Pt and Ru samples were reduced at 350 °C for 2h and a 400 °C for 3h, respectively. 5% Pd samples supported on y-alumina (Rhone-Poulenc GFSC, surface area 220 m^/g), titania rutile (Riedel, surface area 10 m^/g ) and silica (Grace type 432, surface area 300 m^/g ) were also prepared and reduced at 200 °C in flowing H2. Chemisorption of CO was measured at room temperature by using a pulse flow technique. The metal dispersion was calculated assuming a stoichiometry CO/Me = 1. Metal dispersion was also checked by XRD and TEM. A good agreement between the different methods was found; details will be reported in a forthcoming paper 161. Table 1 shows the main characteristics of the catalysts used. Table 1 Main characteristics of the metal supported catalysts Catalysts code

MGPd05 MGPdla MGPdlb MGPdlc MGPd3 MGPd5 MGPd5a PdA PdT PdS PtC RuC

Metal

Metal loading (wt %)

Support

Reduction temp. (°C)

CO/Me

Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pt Ru

0.5 1 1 1 3 5 5 5 5 5 5 5

carbon " " " " " " AI2O3 Ti02 Si02 carbon

200 200 400 600 200 200 200 200 200 200 350 400

0.16 0.17 0.13 0.01 0.31 0.27 0.38 0.02 0.01 0.02 0.07 0.06

The hydrogenation of 2,4-DNT was carried out in a 100 ml five necked flask, equipped with a reflux condenser, a thermocouple and a stirrer head. Constant temperature ( 0.5 °C) was maintained by circulation of silicone oil in an external jacket connected with a thermostat. The catalyst was added to the required amount of solvent (25 ml of 95% ethanol) and reduced "in situ" at 50 °C for Ih under H2 flow. After cooling to reaction temperature, 25 ml of a solution (0.07 - 0.1 M) of 2,4-DNT in ethanol, containing hexadecane as an internal standard, was added through one arm of the flask. The progress of the reaction was followed by analyzing a sufficient number of samples withdrawn from the reaction mixture. Products analysis was performed by gas-chromatography and liquid

241 chromatography. Details on the GC and HPLC analysis of the reaction mixture are reported elsewhere /3,5/. Preliminary mns carried out at different catalyst loading, stirring rate and catalyst grain size have shown the absence of diffusional limitations. It is useful to note that particular care should be paid in the execution of experiments, because arylhydroxylamines and aromatic amines are carcinogens compunds. Arylhydroxylamines are also thermally unstable compounds.

3. RESULTS AND DISCUSSION Under the experimental conditions used, the reaction was found to proceed, on all the catalysts tested, through a complex consecutive/parallel reaction network (Scheme 1). The reaction pathway involves the formation of 4-(hydroxyamino)-2-nitrotoluene (4HA2NT), 2(hydroxyamino)-4-nitrotoluene (2HA4NT), 4-amino-2-nitrotoluene (4A2NT) and 2-amino4-nitrotoluene (2A4NT) as relevant reaction intermediates. No significative formation of the hydroxyamino-aminotoluene isomers was instead observed. Most likely, this is due to the high reactivity of these intermediates.

N02(NH0H)

NHQH(N02)

2,4-nitn) hydro xyamino to luene isomers ^OZ

2,4-dimtn)toIueiie

^.^^^^^^

^

f^^

y ^

^

NH2

2,4-diainmotoIuene

NH2(N02)

2,4-nitroaminotoluene isomera Scheme 1.

Typical conveVsion-time plots showing the course oi the hydrogenation of 2,4- DNT over the investigated supported noble metals are reported in Figure 1. The formation of the end product of reaction (2,4-DAT) starts only v^hen almost all the 2,4-DNT has been converted. The specific rate of disappearance of 2,4-DNT, ri (expressed per atom of metal on the surface), measured on the 5% metal supported on carbon catalysts is reported in Table 2. In the same table the selectivity to the intermediate products is also shown. In a previous paper we have reported an HPLC analysis able to detect all the reaction intermediates involved in the reaction /5/. Unfortunately, the separation of the peaks of the two 2,4nitroarylhydroxylamines is not sutTicient to give a reliable quantitative determination of their

242

relative abundance. Therefore, only the overall selectivity to the two isomers, 4HA2NT+2HA4NT, is reported. The selectivity values v^ere determined at a conversion of about 50%. It should be noted that on Pd and Pt catalysts, the selectivity to the intermediate products remains fairly constant within a large range of conversion (15-90%), in agreement with previous results /3,4/. On the Ru/C sample, instead, the selectivity to 2,4nitroarylhydroxylamines decreases as the conversion increases, due to the relatively higher hydrogenation rate of these intermediates on Ru. OU ' h

—

—

^a): /

8060Q.

E

o O

40-

^X /' "

20-

^ - ^—©—

n I rf^JT^""^^^^

40

—

—

80

—— 120

¥ 160

200

Time (min)

100 ^

80

O

120

180

300

Time (min)

Figure I. Hydrogenation of 2,4-DNT over supported metal catalysts. Tr = 50 °C; PH2 = 0.1 MPa. a) MGPd5, 0.05 g, [2,4-DNT] = 0.1 M; b) RuC, 0.412 g, [2,4-DNT] = 0.07 M. ) 2,4-DNT; (A) 4HA2NT4-2HA4NT; (v) 4A2NT; (o) 2A4NT; (4) 2,4-DAT.

243

Table 2. Catalytic properties of 5% metal supported carbon catalysts in the hydrogenation of 2,4DNT. Tr = 50 ^C; PH2 = 0-1 MPa. Selectivity (%) at 50% of conversion r, (molDNTs'' Me(s)'') Catalysts 4HA2NT+2HA4NT 4A2NT 2A4NT 2,4-DAT 0.20 82.8 10.4 5.6 1.2 MGPd5 0.41 87.2 9.4 3.4 PtC 0.02 43.6 40.6 15.8 RuC

Table 2 shows that the carbon supported palladium and platinum catalysts are the most actives for the 2,4-DNT hydrogenation. Over these samples, 4HA2NT and 2HA4NT isomers are the main intermediates which are formed with an overall selectivity higher than 80 %. The preferential formation of phenylhydroxylamine on platinum catalysts has been also reported by Rylander et al. in the nitrobenzene hydrogenation 111. It has been suggested that the observed high selectivity is due to a major tendency of platinum to adsorb the 2,4DNT. Notwithstanding the higher selectivity of platinum, Rylander et al 111 have shown that a small amount of dimethylsulfoxide (DMSO) had to be added to the reaction mixture in order to obtain a high selectivity. The role of DMSO is to favor the desorption of the intermediate reaction products thus decreasing the probability for the nitroaiylhydroxylamines hydrogenation. It should be noted that in our case no additive was necessary for the selective hydrogenation of 2,4-DNT. The remarkable lower specific activity, ri, observed on Ru/C is in agreement with previous literature results which indicate mthenium as the less active metal for the reduction of aromatic nitrocompounds /I/. The lower selectivity can be related to the faster hydrogenation of the nitroarylhydroxylamines on this catalyst compared to Pt or Pd. In fact on Ru/C the conversion of the nitroaiylhydroxylamines to nitroamino isomers starts even in the presence of unreacted 2,4-DNT (see Figure lb).

3.1. Effect of metal particle size The effect of metal particle size on the selective hydrogenation of 2,4-DNT has been investigated on a series of Pd/C catalysts having different metal loading. Figure 2 shows the specific activity, ri, and the selectivity to the reaction intermediates as a function of the metal dispersion. In the range of dispersion studied (0.01 - 0.4), the specific activity was found to increase as the Pd dispersion decreases. Similar results have been previously reported and attributed to the formation of p-hydrides on the larger particles /8/. On the other hand, it can be excluded that the higher activity observed on the larger particles is due to a variation of the average coordination number, N, of the palladium surface atoms. The largest change of N is in fact expected at the highest dispersion were instead the catalytic activity was found almost constant (Figure 2). The formation of p-hydrides cannot however explain the lower selectivity to arylhydroxylamines and the preferential formation of the 4A2NT observed at higher dispersion, therefore other factors should be taken into account. While this point needs further investigations, at this stage it can be suggested that, due to steric constrains, the adsorption of 2,4-DNT on the small particles occurs mainly through the P-NO2 group with

244

the aromatic ring oriented away from the active sites (Figure 3). On the flat surface of the larger particles, it is instead more likely an adsorption geometry where both the nitrogroups of the 2,4-DNT molcules interacts with the Pd surface. The interaction of the aromatic ring with the Pd surface causes however a destabilization of the adsorption strength of the nitrogroups and, therefore, favours the desorption of the arylhydroxylamine intermediates.

2.00

1.50 1

T3 DL

1.00

^

0.50 ]

0.00 0.00

0.10

0.20

0.30

0.40

0.50

0.40

0.50

CO/Pd

CD

CO

0.00

0.10

0.20

0.30

CO/Pd

Figure 2. a) Specific activity for the 2,4-DNT hydrogenation over carbon supported Pd catalysts vs. the CO/Pd ratio, b) Selectivity to the intermediate products at 50% of conversion vs. the CO/Pd ratio, (A) 4HA2NT+2HA4NT; (v) 4A2NT; (o) 2A4NT.

245 CH3

n NOo

Pd

Pd

Figure 3. Schematic view of the adsorption of 2,4-DNT on small and large Pd particles. 3.2. Effect of support The influence of the nature of the support on the 2,4-DNT hydrogenation is reported in Table 3. Catalysts with similar dispersion have been compared in order to neglect the effect of metal particle size. The specific activity of palladium on the different supports investigated is similar. A different products distribution is instead observed. The results reported in Table 3 suggest that the acid-base characteristics of the support play an important role in determining the products distribution. The higher selectivity to 2,4-nitrohydroxyaminotoluenes was in fact observed on the less acid supports, such as carbon and Ti02. Table 3 Catalytic properties of Pd on different supports in the hydrogenation of 2,4-DNT. Tr = 50 °C; PH2 = 0-1 MPa. Catalysts

CO/Pd

MGPdlc PdT PdA PdS

0.01 0.01 0.02 0.02

fi (mol

DNTs- Pd(s) '') 1.52 1.12 2.30 1.28

Selectivity (%) at 50% of conversion 4HA2NT+2HA4NT 4A2NT 2A4NT 4.7 89.2 6.1 79.3 4.7 15.9 54.3 21.7 18.9 62.3 16.7 17.2

It is known that the formation of arylhydroxylamines is favored in neutral or weakly alkaline conditions /9/. It should be recalled that under these conditions the formation of byproducts, such as azocompounds, is strongly enhanced. In our case, none of these side reactions occurs in a significative way. On the more acidic supports such as silica and alumina, the selectivity to 2,4-nitrohydroxyaminotoluenes is significatively lower.

246 3.3. Effect of reaction temperature The influence of temperature on the products distribution has been investigated on Pd supported on AI2O3 and carbon in the temperature range between 10 and 50 °C. On Pd on alumina, a decrease of the reaction temperature causes an increase of the selectivity to 2,4nitrohydroxyaminotoluenes from about 54% at 50 °C to 69% at 35 °C up to 80% at 10 °C. Correspondingly, a decrease of the selectivity to the 2,4-nitroaminotoluene isomers occurs. A similar trend was observed also on Pd/C samples. A detailed kinetic analysis of this reaction carried out on Pd/C catalysts /4/ has shown that the increase of the selectivity to 2,4-nitrohydroxyaminotoluenes is due to the higher apparent activation energy of the step involving the reduction of the hydroxylamino- into the aminogroup (-NHOH -^ -NH2). Therefore, the higher kinetic stability of the 2,4-nitrohydroxyaminotoluenes at the lower temperatures allows a higher yield to these intermediates. 4. CONCLUSIONS The results obtained in this work indicate that in the selective catalytic hydrogenation of 2,4-DNT over metal supported catalysts the activity and selectivity to 2,4nitroarylhydroxylamines is strongly dependent on the nature of the noble metal used. Platinum and palladium supported on carbon give a higher activity and selectivity as compared to Ru catalyst. The different products distribution observed on this latter catalyst is related to a different rate of hydrogenation of the intermediates. The metal particle size, temperature and nature of support also influence the products distribution. Larger Pd particles were found more active and selective towards the formation of the 2,4-nitrohydroxyaminotoluenes. This behavior has been explained by assuming a different geometry and strength of adsorption of the nitrocompounds as a function of the particle size. High selectivity to nitroarylhydroxylamines can be obtained by carrying out the reaction under mild conditions of temperature and by using non-acid supports.

REFERENCES 1. P.N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, New York, 1967. 2. S.L. Karwa, R.A. Rajadhyaksha, I&C Res., 26 (1987) 1746. 3. G. Neri, M.G. Musolino, C. Milone, A.M. Visco and A. Di Mario, J. Mol. Cat. A: Chemical, 95(1995)235. 4. G. Neri, M.G. Musolino, C. Milone and S. Galvagno, I&C Res. 34 (1995) 2226. 5. G. Neri, M.G. Musolino, E. Rotondo, and S. Galvagno, J. Mol. Cat. A: Chemical 111 (1996)257. 6. G. Neri et al., (in preparation). 7. P.N. Rylander, I.M. Karpenko and G.R. Pond, Ann. N. Y. Acad. SCI 172 (1970) 266. 8. A. Benedetti, G. Fagherazzi, F. Pinna, G. Rampazzo, M. Selva and G. Strukul, Catal. Lett. 10(1991)215. 9. A.M. Stratz, Catalysis of Organic Reactions, J.R. Kosak (ed.), M. Dekker, New York, 1984.

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

247

Kinetics and Pathways of Selective Hydrogenation of l-(4-Nitrobenzyl)-l,2,4-Triazole C. LeBlond, J. Wang, R.D. Larsen, C.J. Orella, A.L. Forman, F.P. Gortsema, T.R. Verhoeven, Y.-K. SUIT , Merck Research Laboratories, Merck & Co., Inc., P.O. Box 2000, RY55-228, Rahway, NJ 07065, U.S.A. SUMMARY The nitro group in l-(4-nitrobenzyl)-l,2,4-triazole (4NBT) was selectively hydrogenated to the amine using a Pd/C catalyst. Kinetics and reaction pathways were studied using in-situ kinetic tools, i.e., measurements of heat flow by reaction calorimetry and rate of hydrogen uptake, in addition to analysis of samples taken from the reactor. The hydrogenation reaction follows predominandy a two-step monomeric pathway from 4NBT to the hydroxylamine intermediate and then to the aniline. The first step, hydrogenation of 4NBT to the hydroxylamine, follows nearly zero-order kinetics. Activation energies and heats of hydrogenation associated with each step in the consecutive hydrogenation were determined. The heat of hydrogenation of the second step, hydrogenation of the hydroxylamine (-58 kcal/mol), is slighdy less than that of the first step (-65 kcal/mol). However, the activation energy associated with the second step is higher than that associated with the first step. Consequently, the rate of hydrogenation of the hydroxylamine increases with increasing temperature at a faster pace than that of hydrogenation of the 4NBT does. Hydrogenation via the dimeric pathway, as evidenced by presence of ati*aceamount of the azoxy intermediate (formed from a coupling reaction of the nitroso and the hydroxylamine intermediates) contributes only -0.1% to the overall hydrogenation reaction. The low probability of the reaction following the dimeric pathway may be attiibuted to the high rate of hydrogenation of the nitroso on the catalyst. 1. INTRODUCTION Selective catalytic hydrogenation of aromatic nitro compounds finds many applications in fine and specialty chemical industiies (1). This class of hydrogenation reactions has been studied extensively using various solvents, catalysts and under various reaction conditions (1). The hydrogenation reaction has been found to follow mainly a mechanism that was dehneated by Haber in 1898 from his study of electi*ochemical reduction of nitrobenzene (2). The mechanism, consisting of two types of reaction pathways, is schematically described in Fig. 1. The first pathway is a "monomeric" one that proceeds in three consecutive steps: (a) hydrogenolysis of one of the N-0 bonds in the nitro group to produce the nitroso intermediate; (b) hydrogenation of the nitroso to the hydroxylamine intermediate; and finally (c) hydrogenolysis of the remaining N-0 bond in the hydroxylamine to produce aniline. TTie second pathway is a "dimeric" one in which dimers derived from coupling between intermediates (nitroso and the hydroxylamine), and between intermediate and final product (nitroso and aniline) are formed and then hydrogenated. By proper selection of reaction conditions, various hydrogenation products, both monomeric and dimeric, may be obtained in addition to the aniline (1,3). l-(4-Aminobenzyl)-l,2,4-triazole is an intermediate in the synthesis of a new Merck drug substance, MK-462, a selective agonist for the 5-HTij) receptor used forti*eatmentof migraine. It ' Corresponding author.

248

Hg

H2

H2O

1

(|)-NO-

1^

nitroso

2

H2O

-(|>-NHOHhydroxylamlne

1*

^

aniline

HI H

Fig. 1. A general description of the major reaction pathways for the catalytic hydrogenation of aromatic nitro compounds. The non-shaded pathway is referred to as the "monomeric" pathway, whereas the shaded one is referred to as the "dimeric" pathway. is synthesized by selective hydrogenation of the nitro group in l-(4-nitrobenzyl)-l,2,4-triazole,

3H2

2H20

5% Pd/C

1 -(4-Nitrobenzyl)-1,2,4-Tilazole 4NBT

1 -(4-Aminobenzyl)-1,2,4-Triazole Aniline

over supported palladium catalysts. Rational design of a process to ensure production of highquality l-(4-aminobenzyl)-l,2,4-triazole in high yield and with a low impurity profile requires a comprehensive picture of the reaction pathways, kinetics, and thermodynamics associated with the catalytic reaction. An accurate measurement of kinetics is essential for investigation of the reaction mechanism since kinetics are a "reflection" of the mechanism. It is also particularly important for this process because of the high degree of exothermicity of the reduction reaction. In addition, a determination of the fractions of the reaction following the monomeric and the dimeric pathways, and a determination of the factors that may affect the partition between these two pathways would help the development of an optimal process for production. In this study, the reaction was characterized using a combination of in-situ kinetic probes, heat flow from reaction calorimetry and measurement of the hydrogen uptake rate, in addition to the commonly-used method of analysis of samples taken from the reactor. Heat flow from the reaction calorimetry and measurement of rate of hydrogen uptake are intrinsically superior kinetic tools in that they both provide rate data directly and in a quasi-continuous fashion. Consequently, they are capable of producing clear and detailed kinetic pictures which offers hints on reaction pathways and mechanism (4,5). It will be shown that thermodynamic information regarding each step in the hydrogenation reaction network may also be obtained directly from a combination of the heat flow and the hydrogen uptake data.

249 2. EXPERIMENTAL DETAILS The reaction calorimeter used in this study was a Mettler's RCl with a 1 L reactor vessel. Both heat flow and hydrogen uptake were measured. The heat flow rate measured under isothermal conditions is directiy proportional to a summation of the rate of each reaction step weighted by heat of reaction Af/^ of the corresponding step, i.e.,

where V^ is the volume of the contents in the reactor. Similarly, rate of hydrogen uptake is also directly proportional to a summation of the rate of each reaction step weighted by the hydrogen stoichiometry of the corresponding step, i.e.,

The substrate was synthesized in-house at Merck. The catalyst used was 5% Pd/C (JM Type 21R, 50% moisture). It was characterized by TPR and by pulse chemisorption of CO. The dispersion of Pd was found to be 9%, assuming a CO to Pd ratio of unity. In each experiment, the substrate (40 g), catalyst (0.2 g, dry basis), aqueous ammonia (24 ml), and methanol were charged to the RCl reactor. The mixture was stirred at 1600 rpm under Nj at the reaction temperature to dissolve the substrate. The substrate was fully soluble in tiie solvent under these conditions. To initiate tiie hydrogenation reaction, Nj ^^s rapidly evacuated by a mechanical pump, followed by pressurizing the reactor to 40 psig Hj. This evacuation-and-fill procedure was conducted with the agitation speed at 1600 rpm to ensure rapid and complete degassing of the inert gas from, and rapid incorporation of Hj into the Hquid phase. The process usually took < 10 s. The composition of components in the reactor was determined periodically by sampling from the reactor. Sampling was conducted under pressure without interfering with the hydrogenation reaction. Unless otherwise specified, the sampUng and subsequent sample workup for analysis were conducted in a nitrogen-purged environment to prevent the air oxidation of the major hydrogenation intermediate (vide infra). Samples were analyzed by an HPLC metiiod that allows qunatification of both the monomeric (nitro, nitroso, hydroxylamine and aniline) and the dimeric (azoxy, azo and hydrazo) species. 3. RESULTS AND DISCUSSION 3.1. Hydrogenation Pathways: Monomeric vs. Dimeric Hydrogenation intermediates observed in the liquid phase during the hydrogenation reaction were identified and quantified to help determine the fractions of the hydrogenation reaction that proceeds via the monomeric and dimeric hydrogenation pathways described in Fig. 1. Concentration profiles of components in the reactor during isothermal hydrogenation of 4NBT at 25 °C are shown in Fig. 2 (top panel), where it is seen that the hydroxylamine, an intermediate of the monomeric pathway, is the main intermediate species. The precursor to the hydroxylamine, the nitroso, was not observed. The top panel in Fig. 2 also shows the presence of the azoxy intermediate (0.02 M) which would be indicative of the presence of the dimeric pathway. These results were obtained from HPLC analysis of samples taken and handled in air.

250

It was soon realized that the hydroxylamine undergoes a facile oxidation reaction in air, producing the azoxy. As a result, data in the top panel of Fig. 2 does not reflect the true concentrations of the intermediates in the reactor during the reaction. To determine the true level of the azoxy in the reactor, sampling and sample workup were conducted in a nitrogen-purged environment. The new profiles, shown in the lower panel of Fig. 2, show that the true level of the azoxy is only 0.0002 M, two orders of magnitude lower than the value obtained when sampling was conducted in air. Note that the level of the true value of the hydroxylamine is also correspondingly higher. Under the reaction conditions at 25 °C, disproportionation reaction of the hydroxylamine was not observed (6). Other dimeric species such as the azo or the hydrazo were not detectable. Aniline

Ȥ9mplinginAlr [Hydroxylamine]^ax - ^-28 W [Azoxy]m3x = 0.02(M)

Sampling in N^ [HydroxylamineJ^ax = 0.31 (M) [Azoxy]rnax< 0.0002 (M)

100 150 Time (min)

250

Fig. 2. Concentration profile of components in the reactor during isothermal (25 °C) hydrogenation of 4NBT in CH3OH over Pd/C, determined by sampling from reactor followed by HPLC analysis. Sampling was conducted in air (top panel) and in Nj-purged environment (lower panel). The maximum concentrations of the azoxy aie shown to therightof each diagram.

The results in Fig. 2 (lower panel) indicate that under the reaction conditions, the reaction almost exclusively follows the monomeric pathway, with hydrogenation via the dimeric pathway contributing to only -0.1% of the reaction. The low probability of the reaction following the dimeric pathway is consistent with the absence of the nitroso species in the liquid phase. Apparendy, the nitroso has high reactivity on the catalytic surface to hydrogenation reaction, producing the hydroxylamine. The significant distortion of the level of the azoxy due to sampling in air would have misled one to believe that a substantial fraction of the reaction follows the dimeric pathway. This underscores the need to exclude air during sampling when studying catalytic hydrogenation of aromatic nitro compounds. The results in Fig. 2 (lower panel) also indicate the sequential nature of the monomeric pathway, i.e., 4NBT is hydrogenated first to the hydroxylamine, followed by a subsequent hydrogenation of the hydroxylamine to the aniline.

251

dX KineUc$ Rates of isothermal hydrogenation at 25 and 50 °C were measured independently both by the heat flow and by the rate of H2 uptake. The results are displayed in Fig. 3 (25 °C) and Fig. 4 (50 °C), respectively, along with corresponding profiles of components in the reactor. Fig. 3 shows that the kinetics at 25 °C are characterized by two distinct rate regimes with a sharp transition between them, indicative of two different reactions occurring in each regime. The two regimes correspond very well to the two sequential steps involved in the monomeric pathway. The first regime corresponds to hydrogenation of 4NBT to the hydroxylamine, whereas the second corresponds to hydrogenation of the hydroxylamine to the aniline.

01 o 0

100 Time (min)

Fig. 3. (a) Rates of heat flow and hydrogen uptake for isothennal (25 °C) hydrogenation of 4NBT to the aniUne in CH3OH over Pd/C. [4NBT] o=0.46 M, 1600 rpm, 40 psig Hj, 4NBT/catalyst (dry)=200 (WAV), (b) Profile of components in the reactor determined by sampling from reactor followed by HPLC analysis.

Fig. 3a shows that the kinetics of hydrogenation of 4NBT to the hydroxylamine are nearly zeroorder in [4NBT], i.e., the rate remains rather constant with decreasing [4NBT] in the liquid phase. The zero-order kinetics reflect properties of processes on the catalyst, rather tfian extraneous factors such as hydrogen gas/liquid mass transfer limitations, since the rate of maximum Hj mass transfer at 1600 rpm was over 100-fold greater than the rate of the reactive hydrogen uptake. The zero-order kinetic behavior suggests that the catalytic surface is saturated with 4NBT, which further suggests that 4NBT chemisorbs on the catalytic surface much more strongly than other species, such as the hydroxylamine and the aniline. The sharp transition in rate in Fig. 3 occurs at a time when 4NBT is nearly depleted from the solution, and the concentration of the hydroxylamine reaches a maximum. The transition into a

252 lower rate regime after depletion of 4NBT shows that at 25 ^C the rate coefficient associated with the hydroxylamine hydrogenation is lower than that associated with the 4NBT hydrogenation, which also accounts for the buildup of the hydroxylamine (to 65% of the initial 4NBT concentration) in the liquid phase. The overall rate of hydrogenation is much faster at 50 ""C (Fig. 4). However, the rate associated with hydrogenation of the hydroxylamine is accelerated at a much greater pace with increasing temperature than that of the 4NBT hydrogenation. This is evident both from the disappearance of the sharp transition in kinetics, and from a lower buildup of the hydroxylamine (17% at 50 "^C vs. 65% at 25 °C). These observations demonstrate that the activation energy associated with the hydroxyalmine hydrogenation is much higher than that associated witii the 4NBT hydrogenation. Indeed, kinetic modeling using the kinetic data in Figs. 3 and 4 (7) shows that the activation energy is approximately 6 and 12 kcal/mol for the first and the second step, respectively. 35J

??H

^.r-'-''''""''*^

«

iJ j ^ « 20J

X

1

1

= sJ 0-11

\ \

10J

S

*®**^

,

,

,

V

l"~^

40

Time (min)

Fig. 4. (a) Heat flow rate for isothennal (50 °C) hydrogenation of 4NBT to the anUine in CH3OH over Pd/C. [4NBT1 (pO.46 M, 1600 rpm, 40 psig Hj, 4NBT/catalyst (dry)=200 (W/W). (b) Concentration profile of components in the reactor detennined by sampling from reactor followed by HPLC analysis.

3.3. Stepwise Kinetic and Thermodvnamic Parameters The overall heat of hydrogenation from 4NBT to the aniline can be convenientiy determined to be 4 kcal/mol by integrating the heat flow curve in Fig. 3 or 4. The stepwise heats of hydrogenation of this two-step consecutive hydrogenation may be determined from a profile of the instantaneous heat of hydrogenation per mole of H2 reacted.

253

Using Eqs. (1) and (2), the instantaneous heat of hydrogenation per mole of Hj reacted, defined as the ratio of rate of heat flow to the corresponding rate of H2 uptake, may be expressed as ZAH, AHH,

= ^H,

dt

(3)

Hfi

Due to the sequential nature of the hydrogenation, the heat flow in the initial stage of the reaction (Fig. 3) may be attributed mainly to the hydrogenation of 4NBT to the hydroxylamine, whereas in ti^e fmal stage mainly to the hydrogenation of the hydroxylamine to the aniline. At these two extremes, Eq. 3 becomes simplified to AHi . .„ A//jj « — L and AH^ AA^i

A//2 «—AA^,

in the initial and fmal stage of the reaction, respectively. The instantaneous heat of hydrogenation per mol of hydrogen reacted obtained from Fig. 3 according to Eq. 3 is plotted in Fig. 5. Corresponding to the two distinct kinetic regimes in Fig. 3, there are also two distinct regimes in profile of the heat of hydrogenation, each associated with one of the two steps.

I I I I I

200

250

Time (min) Fig. 5. Instantaneous heat of hydrogenation per mol of hydrogen reacted as a function of time. Derived using Eq. 3 by taking the ratio of the heat flow curve to the reactive hydrogen uptake curve in Fig. 3.

In the initial stage of the first regime, AH^ was —32.5 kcal/moLHj, whereas in the second regime it increased to —58 kcal/mol_H2. Because the hydrogen stoichiometry in the first step (4NBT hydrogenation to the hydroxylamine) is two {AN^ = 2), AH^ = 2 Af/y = -65 kcal/mol. Similarly, because the hydrogen stoichiometry in the second step (the hydroxylamine to the aniline) is one (AA^2 = 1)' the heat of hydrogenation shown in Fig. 5 in the second regime (-58 kcal/mol) equals AH2. Heat of hydrogenation from 4NBT to the aniline is a sum of these two stepwise heats, i.e., -123 kcal/mol. These thermodynamic results, along with the activation

254

energies associated with each step obtained from kinetic modeling of the results in Figs. 3 and 4 (7), are summarized in a one-dimensional potential energy diagram in Fig. 6. Energy (kcal/mol)

Fig. 6. One-dimensional potential energy diagram depicting kinetic and thermodynamic information of 4NBT hydrogenation to the aniline.

AH =

4

4. CONCLUSIONS Hydrogenation of 4NBT on the Pd catalyst follows predominandy the monomeric pathway, with the dimeric pathway contributing only 0.1% to the overall hydrogenation reaction. The first step (from 4NBT to the hydroxylamine) has associated with it nearly zero-order kinetics and AHi ~ -65 kcal/mol. The second step (from the hydroxylamine to tiie aniline) has a slightly lower heat of hydrogenation, AHj ~ -58 kcal/mol. On the other hand, the second step has a higher activation energy for hydrogenation than the first step. As a result, the rate of the second hydrogenation step increases with mcreasing temperature more rapidly than rate of the first step does. REFERENCES 1) See for example, (a) P.N. Rylander, I.M. Karpenko and G.R. Pond, Ann. N. Y. Acad. ScL, 172, 266 (1970). (b) W.H. Jones, S.H. Pines and M. Sletzinger, Ann. N. Y. Acad. Sci., 214, 150 (1973). (c) A.M. Stratz, in Catalysis of Organic Reactions, Ed. J.R. Kosak, Marcel Dekker, New York, 335 (1984). 2) (a) F. Haber, Z Electrochem., 4, 506 (1898). (b) F. Haber and C. Schmidt, Z. Physik. C/im.,32, 171 (1900). 3) (a) S.L. Karwa and R.A. Rajadhyaksha, Ind Eng. Chem. Res., 26, 1746 (1987). (b) S.L. Karwa and R.A. Rajadhyaksha, Ind. Eng. Chem. Res., 27, 21 (1988). 4) R.N. Landau, U. Singh, F. Gortsema, Y.-K. Sun, S.C. Gomoka, T. Lam, M. Futran, and D.G. Blackmond, /. Cat., 157, 201 (1995). 5) C. LeBlond, J. Wang, R.D. Larsen, C.J. Orella, A.L. Forman, J. Sowa, Jr., R.N. Landau, D.G. Blackmond, and Y.-K. Sun, Thermochimica Acta (in press). 6) J.R. Kosak, in ''Catalysis of Organic Reactions,'' Eds. P.N. Rylander, H. Greenfield, R.L. Augustine, Marcel Dekker, (1988). 7) C. LeBlond, J. Wang, C.J. Orella and Y.-K. Sun, unpublished results.

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

255

Design of selective l-ethyl-2-iiitromethylenepyiTolidine hydrogenation for pharmaceuticals production V.A. Semikolenov, I.L.Simakova, A.V.Golovin, O.A.Burova, N.M.Smirnova Boreskov Institute of Catalysis, Prospekt Akademika Lavrentieva 5, Novosibirsk, 630090, Russia The peculiarities of l-ethyl-2- nitromethylenepyrrolidine (ENMP) hydrogenation over Pd/C catalyst were studied. Two routes of ENMP conversion resulting in l-ethyl-2aminomethylpyrrolidine (EAMP) and side products formation were found out. It was shown that the side reaction is the ENMP hydrolysis. This reaction proceeds via the acid-base mechanism and is accelerated by EAMP, producing the basic medium. The new method of EAMP preparation by ENMP hydrogenation in presence of CO2 was proposed. The kinetic peculiarities of this process were studied and parameters determining the EAMP selectivity were clarified. The role of CO2 in selective ENMP hydrogenation was studied using IR, ' H and ^^C NMR methods. It was found that the reversible interaction of EAMP with CO2 produces the intermolecular salt of EAMP carbamic acid and thus prevents ENMP hydrolysis by maintaining the neutral pH of the reaction medium. The catalytic setup for the ENMP hydrogenation and the new method of EAMP separation and purification, which provides the high-purity EAMP for medicines synthesis, have been developed.

INTRODUCTION l-Ethyl-2-aminomethylpyrrolidine (EAMP) is a key semi product for synthesis of pyrrolidylquinolines, pyrrolidylquinazolines and benzamides - a new group of high-efficient medicines. In particular, one of them, "Sulpyride", is applied in the treatment of nervous disorders caused by drugs and alcohol as well as of stomachial diseases [1]. N-substituted 2-aminomethylpyrrolidines can be prepared by the reduction of the corresponding N-substituted 2-nitromethylenepyrrolidines with hydrogen "in situ" [2 ], or by the catalytic hydrogenation [2, 3, 4] according to the scheme:

(!^=^"N^^

^

^

( ^ - C H . N H , ^ 2Hp0

Platinum metals such as Pt-black, Rh/Al203, Pd/C [2, 3], as well as Raney-Ni [2, 3] are used as the catalyst. The hydrogenation of l-ethyl-2-nitromethylenepyrrolidine (ENMP) is complicated with intensive side products formation and, therefore, the huge catalyst loading (1/5^10/1 g.catalyst/g.ENMP) is used to achieve 60-65% yield of EAMP. The ENMP molecule contents the conjugated system including 71-electrons of -C=C- and -N=0 double bonds and the electron pair of N-atom of pyrrolidine ring and due to that ENMP

256 is very sensitive towards the electrophilic or nucleophilic attacks. The hydrogenation mechanism of such type conjugated nitroolefms is not studied in details, but it was noted [5, 6] that formation of various products depends on reaction conditions. The main problem in the selective synthesis of EAMP is to diminish the role of undesirable reactions of ENMP conversion. We have solved this problem and the original method of EAMP synthesis by the selective hydrogenation of ENMP on Pd/C catalyst has been proposed [7]. The aim of the present work is to illustrate the steps of designing of the high-purity EAMP preparation process.

RESULTS AND DISCUSSION 1. Preliminary experiments. The influence of the solvent used on the hydrogenation rate and selectivity of EAMP formation has been studied. The high selectivity and low hydrogenation rate are observed when the process is carried out in non aqueous and aprotonic solvents (Table 1). The maximum hydrogenation rate, and yet minimum selectivity are achieved in the aqueous solution. Table 1 Effect of solvent in ENMP hydrogenation Hydrogenation Solvent time*, min 30 Water 90 Ethanol 102 Toluene 185 1,4-Dioxane

EAMP Selectivity % 27.14 63.21 79.03 39.18

Solvent - 20 ml, ENMP - 20 mmol, catalyst 4%Pd/C - 0.5 g., 80T, H2 pressure 10 atm. *-Experiments were performed up to complete of H2 uptake. This results show that the side reaction is promoted by the presence of water in the solution. But the application of water-free solvents can not completely resolve the problem of enhancing of selectivity, because on the hydrogenation of nitro-group two H2O molecules appear, that, in their turn, can participate in the side reaction. The promising approach to the resolution of this problem is to clear up the peculiarities of the side reaction occurring and then to find out the reaction conditions to suppress the side process. Thus the following experiments were carried out in water because it is the best solvent for clarification of the side reaction nature. 2. The reaction products analysis. Conversion of ENMP in the presence of H2 and the Pd/C catalyst leads to formation of EAMP, 1-ethylpyrrolidinon (EP) and methylamine. The reaction products were isolated from the reaction mixture, and their structure was confirmed by H, C NMR; IR and GLC methods. The scheme of two routes of ENMP conversion has been proposed: -ENMP hydrogenation to form EAMP and H2O {main rout).

257 -ENMP hydrolysis to form l-ethylpyrrolidinon and nitromethane, which can be catalytically reduced to methylamine {side reaction). 3. Study of ENMP hydrolysis. ENMP is quite stable in neutral aqueous solutions, but in a basic medium it is easily hydrolyzed to form l-ethylpyrrolidinon and nitromethane. ENMP is a strong organic base [8] (pKa=10.5 for diluted aqueous solution). The acid-base mechanism with ENMP behaving as a base can be proposed for the ENMP hydrolysis. The kinetic peculiarities of ENMP hydrolysis were studied. The time dependence of EP concentration curve increases continuously and tends to the value of ENMP complete ^ :<. conversion. In semilogarithmic coordinates this dependence is described by linear function, that indicates the first order with respect to ENMP concentration (Fig.l). The dependence of ENMP hydrolysis Fig.l. ENMP hydrolysis. Kinetic curves for EP rate on [EAMP] is described by the formation. Current EP concentration - [A], final curve sharply increasing in the initial EP concentration - [A]o = 0.5 mol/1. Reaction part (Fig.2a). The logarithmic conditions: water - 50 ml, [ENMP] = 0.5 mol/1, dependence of ENMP hydrolysis rate on [OH'] is linear with tangent a=2.05 [EAMP] = 0.1 mol/1, 80°C. (Fig.2b). These data indicate a specific

g

acid-base catalysis mechanism [9]. 0.020

0,015

O.OIOH

o (N

X

0,0051

? O.OOOi 0,05

0.10

EAMP, mol/1

a)

0.5

[OH], mmol

b)

Fig.2. Effect of EAMP concentration (a) and [OH"] (b) on ENMP hydrolysis rate constant kH20. Reaction conditions: water - 50 ml, [ENMP] = 0.5 mol/1, 80°C.

258 The reaction rate (WH2O) is of the second order with respect to [OH'] and can be expressed as: WH2O = kH20x[ENMP]x[0H-]^. (where [OK] = 2.10-^

^ mol/1.)

It means that the mechanism of ENMP hydrolysis probably includes two consequent steps catalyzed by OH': a) an attachment of H2O molecule to -C=C- double bond; b) a removal of CH3N02to form EP. Thus, the ENMP hydrolysis is an acid-base process, where H2O (solvent or a product of -NO2 group reduction) can act as a reagent and the strong base EAMP (product of ENMP hydrogenation) - serves as a catalyst. 4. Search of conditions for selective ENMP hydrogenation The results of ENMP hydrolysis study show that the contribution of side route can be diminished by decreasing of [OH']. The effect of H2SO4/ENMP ratio on hydrogenation process was studied (Table 2). It was found that an increase in this ratio results in increase in selectivity of EAMP formation. The excess of sulfuric acid stopped the process. It may be attributed to a significant influence of pH on the rate and the selectivity of ENMP hydrogenation. The titration of ENMP and EAMP with H2SO4 was performed and it was found out that at pH = 6.5-7.5 the optimum in hydrogenation rate and selectivity can be achieved. Table 2 The effect of sulfuric acid/ENMP ratio on hydrogenation rate and selectivity of EAMP formation M

H2SO4/ENMP mol/mol

Hydrogenation rate (Wh), n mmol ENMP/g Pd

Selectivity of EAMP formation, %

1

0.000

34.6

27.14

2

0.125

25.0

65.48

3

0.250

26.3

91.23

4

0.375

24.5

95.93

5

0.500

18.0

95.77

6

0.750

<1

-

Solvent water - 20 g, ENMP - 20 mmol, 4% Pd/C - 0.5 g, 80^C, H2 pressure 11 atm. To create a nearly neutral pH during the conversion of ENMP to EAMP, the method based on ENMP hydrogenation in the presence of CO2 has been proposed. The selectivity of EAMP formation increases drastically in presence of CO2 (Fig.3). This indicates that CO2 participates in suppressing of the side process.

259 5. Role of CO2 in selective ENMP hydrogenation It was found that the initial EAMP solution (1 mol/1, pH=12.06) can reversibly absorb 1 mol CO2 per 1 mol EAMP giving a viscous solution with pH=7.2. The interaction of EAMP with 1

13

CO2 was studied using IR, H and C NMR methods. It was found that as a result of the reaction of EAMP with CO2 the intermolecular salt of EAMP carbamic acid is formed.

100

80

r

604

40-^

20

4

6. Kinetic principles of ENMP atm ^o^ hydrogenation The dependence of hydrogen uptake on Fig.3. ENMP hydrogenation. Effect of CO2 reaction time is linear up to 80% ENMP pressure on selectivity of EAMP formation. conversion (Fig.4) that indicates the zero Reaction conditions: catalyst 4%Pd/C - 0.5 order of hydrogenation rate with respect g, water - 20 ml, [ENMP] - 1.0 mol/1, to [ENMP]. This observation can be PH2 = (ll-PC02)atm, 80'C. explained by the strong ENMP adsorption on the catalyst surface due to the tight bounding of ENMP conjugated 7c-electron system to the surface Pd atoms. In this case the catalyst surface is covered with ENMP and hydrogenation rate is determined by the concentration of the catalyst but not by ENMP concentration in the solution. The effect of hydrogen pressure (Fig. 5) and catalyst concentration (Fig. 6) on the selectivity and rate of EAMP formation has been studied. The ENMP hydrogenation rate linearly increases with the hydrogen pressure 30 growth up to 10 atm both at constant PCO2 E =2 atm (fig.5-l-0-) and at the varying of partial CO2 pressure (PC02=11-PH2) at constant total pressure (PH2+PCO2 = 11 \Q\ atm) (fig.5-l-A-). Thus CO2 has no effect on the ENMP hydrogenation rate which is of 0620 40 60 80 the first order in respect to PH2. The 100 Time, min enhancement of catalyst loading up to 0.5 g results in linear increase of the ENMP Fig.4. Kinetic curves for ENMP hydrogenation rate that corresponds to the hydrogenation in the presence of CO2. kinetic regime (fig.6). If the catalyst loading Reaction conditions: catalyst 4%Pd/C - 0.5 g, is greater then 0.5 g the process proceeds to water - 20 ml, ENMP = 10 mmol, PH2 = 9 atm, diffusion regime and the ENMP PCO2 = 2atm,80"c. hydrogenation rate is determined by hydrogen dissolution rate.

260 The kinetic equation of ENMP hydrogenation rate (Wh) has been proposed. Wh = khxPH2x[kat]x[ENMP]^ (where PH2 = 1-10 atm, [kat] = 1^25 g/l)

0.0

02

0,4

0.6

0,8

1.0

Pd/C catalyst loading, g.

Fig.5. ENMP hydrogenation. Effect of H2 pressure on ENMP hydrogenation rate Wh (1) and selectivity of EAMP formation S (2). Reaction conditions: catalyst 4%Pd/C 0.5 g, water - 20 ml, [ENMP] = 1.0 mol/1, 80"C, (-0-,-B-)PC02 = 2atm (-A-) PC02= (11-PH2) atm .

Fig.6. ENMP hydrogenation. Effect of catalyst content on ENMP hydrogenation rate Wh (-0-) and selectivity of EAMP formation S(-B-). Reaction conditions: catalyst 4%Pd/C, water - 20 ml, [ENMP] = 1.0 mol/1, 80°C, PH2 = 9 atm, PCO2 = 2 atm.

7. Selectivity of ENMP hydrogenation The comparative analysis of the kinetic principles relating to the conversion of ENMP by hydrogenation and hydrolysis routes has been performed, and the parameters determining the selectivity of ENMP hydrogenation have been established. S = (Wh/WH20)x//l[(WH20/Wh) + 1] It has been found that the decrease of [Off], [ENMP] as well as increase of catalyst content and hydrogen pressure result in the growth of ENMP hydrogenation selectivity. 8. Scheme of selective ENMP hydrogenation On the basis of the results obtained the main routes of ENMP conversion in presence of Pd/C, H2and CO2 were suggested.

261 Scheme of ENMP conversion: (A) H2. Pd/C^ ^ ^ /S-CH2NH2 (I) 2^ \_/

I

^ ^^^^ > / S - C H 2 N H - C O O H .^Q^ V_^

(B)

0=

CHNO2

OH + ^ - C H 2 N H 3

H2O, OH" (II)

CH3NO2

ENMP is converted by the two independent routes: I) ENMP hydrogenation with EAMP and H2O formation via heterogeneous catalysis route. CO2 can reversibly interact with -NH2 group of EAMP giving carbamic acid (equilibrium A). Intermolecular protonation of pyrrolidine ring nitrogen atom leads to the salt formation. Thus, organic base EAMP is converted to the neutral salt. II) ENMP hydrolysis with the formation of 1-ethylpyrrolidinon and nitromethane via homogeneous asid-base catalysis. The basic medium is formed via EAMP protonation (equilibrium B). 9. The pilot experiments To meet the high selectivity demand, the new technology for ENMP hydrogenation process realization has been developed [10]. The catalytic setup, which provides a means for the continuous solid-state reagent supply into the flow reactor (10^20 ml/min) loaded with the spherical Pd/C catalyst, the liquid phase circulation through the reactor and maintenance of constant hydrogen pressure (1-i-lO atm), has been designed. The regimes for selective ENMP hydrogenation have been worked out. 10. The separation and purification of EAMP The new method of EAMP isolation from the reaction medium and purification has been proposed. This method includes water evaporation under the reduced pressure at 30 °C with the following EAMP carbamic acid salt thermodecomposition at 65-70 °C (equilibrium A, scheme) and product distillation.

EXPERIMENTAL Catalytic tests were effected both in a stainless steel autoclave (kinetiks of ENMP hydrogenation) and in a flow reactor with fixed catalyst bed. Samples of 4%Pd/C (5^50 mm) and 0.5% Pd/C (1.6-^2.0 mm) were used as catalysts. The examples with detailed description of the experimental details are given in [7,10].

262 IR spectra were recorded using a Specord 75-IR spectrometer. H and C NMR spectra were recorded on a MSL-400 Bruker spectrometer with TMS and butanol-1 as internal references. Catalytic tests were performed in a slurry reactor and a trickle flow column reactor in the presence of granulated (1.0-^3.0 mm) and powdered 4.0%Pd/C catalysts. The reaction products were analyzed chromatographically (2 m x 3 mm column, [15% PEG-20M + 2.5% KOH]/Chromaton-N).

CONCLUSION The complex process of ENMP conversion including the routes of hydrogenation on Pd/C catalyst and hydrolysis catalyzed by OH' has been studied. The main parameters governing the selectivity of EAMP formation were found to be pH, catalyst activity and concentration, H2 pressure, ENMP concentration and contact time. The new method of selective ENMP hydrogenation has been designed to prepare the highpurity EAMP (98.5-99.0%) for medicines synthesis.

REFERENCES 1. Negwer M., Organic chemical drugs and their synonyms (an international survey), Akademic - Verlag, Berlin, 1987, Vol. 1-3. 2. Fratmann S.A. et al., GB Patent No. 1 374 818 (1974). 3. Takashi Kamiya at al., US Patent No. 3 748 341 (1973). 4. Tomine M. at al., USSR Patent No. 607 551, (1978). 5. P.N.Rylander, Catalytic hydrogenation over platinum metals, A.P., N-Y, London, 1967, P.176. 6. Rajappa S., Zondler H., DE Patent No. 2 800 311 (1978). 7. Semikolenov V.A. at al, Russian patent application No. 95 100 785 (1996). 8. Arnold J.Gordon, Richard A.Ford, The chemist's companion. New -York - London Sydney - Toronto, 1972. 9. Kojevnikov LV., Catalysis by acids and bases, Nauka, Novosibirsk, 1991. 10. Semikolenov V.A., Russian patent application No. 96 111 047 (1996).

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

263

Kinetic Study of a Nitroaliphatic Compound Hydrogenation V. Dubois, G. Jannes and P. Verhasselt Physical Chemistry and Catalysis, Institut Meurice Avenue Gryzon,! - B-1070 Brussels, Belgium

1. SUMMARY 2-methyl-2-nitropropane hydrogenation is used as a probe reaction for the study of the role of the catalyst support. Indeed, the support may influence the reaction in a favorable or unfavorable way. The surface of this solid (naked or as metal support) can be modified by an appropriate treatment. This modification induces a variation in its catalytic behavior. After a first surface characterization and considering the complete reaction scheme and the results of the kinetic experiments, we propose to consider that the support effects influence the ^butylhydroxylamine disproportionation and the coupling reactions between intermediates.

2. EVTRODUCTION Precious metal catalysts are widely used in fine chemistry for their activity and selectivity. Both properties lead researchers to look for the more appropriate metal, but also for the best support. Very recently, the effects of supports in fine chemical catalysis have been systematically studied, revealing the essential role it may play in some reactions [1]. It is important to choose the right support because it may influence the structure and the electronic state of the final catalyst: it would a mistake to consider carbon as an inert matrix [2-4]. The reaction we propose to study is the hydrogenation of 2-methyl-2-nitropropane into ^butylamine through the nitroso and the hydroxylamine intermediates (scheme 1). This sequence is called "the main reaction". A preliminary study, encompassing temperature, catalyst mass, hydrogen pressure, initial reactant concentration, metal and support effects has already been carried out [5]. For this nitroaliphatic compound, palladium appears to be more active than platinum, and less active than rhodium. On the other hand, carbon seems to be more appropriate as support than alumina or calcium carbonate.

NO2 ^3C-C-CH3 CH3

NO _H"O "

H3C-C-CH3

NHOH -i^^2_^

H3C-C-CH3

CH3

CH3

Scheme 1.

NH2 .H^Q ^

H3C-C-CH3 CH3

264 Addition of active carbons or modified active carbons as a mechanical mixture with the catalyst to the reaction heel can modify, in several cases, the activity and/or the selectivity (fig 1). This experimental fact led us to consider, as developed for aromatic series (fig 2), a reaction scheme taking into account coupling reactions between intermediates and between nitroso and amine, as well as hydroxylamine disproportionation. In this paper, we will characterize the surface of the active carbons, and we will try to explain (by correlation) the role of those solids when added to the reaction heel. 0.16

150

200 Time (min)

250

400

-MNP(std)

-TBHA(std)

-MNP(+solid)

-TBHA(+solid)

-MNP(+solidHN03)

-TBHA(+solid HN03)

Figure 1. MNP and TBHA curves (obtained with Pd/C 5%, from C.C.E.) in the case of i) standard experiment (std), ii) an experiment carried out in presence of 0.4 g of active carbon "Darco" (+solid) and iii) an experiment led in presence of 0.4 g of nitric acid treated "Darco" (+solid HNO3) [6].

3. EXPERIMENTAL 3.1. Experimental Setup A 1 dm^ batch reactor (SFS) is filled with 0.4 dm^ of solvent. The reactor is operated at isothermal conditions. Stirring rate is 750 rpm. The analysis is carried out by gas chromatography (HP 5890 series II) with FID detector and capillary column (Chrompack CP-Wax 52 CB). Integration is performed by the software HP Chemstation. Oven temperature is programmed between 90°C and 120°C. The infrared analysis carried out on active carbons is performed by a Perkin Elmer 1600 FTIR in the transmission mode.

265 N02

OH

NH2

+ 3H,

' "'"-{yij^ Figure 2. Complete reaction scheme for nitrobenzene hydrogenation [7], showing main reaction, coupling reactions (c), disproportionation (d) proposed by Kosak [8], and other potential reactions.

The metallic surface area and dispersion are measured by CO pulse chemisorption, performed on a Micromeritics Pulse ChemiSorb 2700. 3.2. Reactants and Materials Solvent is methanol 99%+; 2-methyl-2-nitropropane and 1-propanol (chromatographic standard) are 99% purity. Helium is 99.999% purity while hydrogen is 99.9997% and carbon monoxide 99.997%. Catalyst used is Johnson Matthey type 487 (Pd/C 5%, 57.8% moisture). The active carbon is the same as the support of this catalyst. In this paper, 2-methyl-2-nitropropane will be noted MNP, 2-methyl-2-nitrosopropane MNoP, ^butylhydroxylamine TBHA and ^butylamine TBA. 3.3. Experimental Procedure for the Hydrogenation Experiments The main part of the solvent, the catalyst, and the chromatographic standard are put in the reactor and gently stirred. Oxygen is removed by nitrogen purging. The reactor heel is heated to 80°C and is pressurized to 15 bar with hydrogen for 1 hour, to ensure a standard reduced

266 surface state. The reactor is then cooled down to the experimental temperature (typically 60°C) and pressure is decreased to 1 bar. The initial reactant is dissolved in a small amount of solvent (50 ml) and flushed with nitrogen. It is then transferred to a small steel cylinder, pressurized under 15 bar hydrogen, and injected into the reactor taking advantage of the pressure difference between the cylinder and the reactor. This is considered as the beginning of the reaction ("zero time"). Hydrogen pressure and stirring rate are adjusted to their nominal value (9 bar and 750 rpm). Just before each sample is taken, hydrogen isflushedthrough the sampling tube to purge it. The first few milliliters of sample are discarded, and the last 2 ml are immediately analyzed by gas chromatography. After each sample is taken, hydrogen pressure is readjusted to its nominal value. 3.4. Experimental Procedure for FTIR The active carbon or the catalyst is mixed (0.25% wt.) to KBr. Afl:er grinding and drying (2h, 120°C) the mixture, the disks are prepared (10^ Pa, 15 min). The thickness is around 0.5 mm and the mass is about 0.12 g. Each disk is scanned 16 times, with a resolution of 16cm-49-ll]. 3.5. Experimental Procedure for Dispersion Measurement The catalyst is first dried, reduced under hydrogen flow (1 h, 150°C), then flushed with helium (2 h, 200°C) to avoid chemisorbed or dissolved hydrogen. A series of CO pulses is passed at room temperature on the sample (0.1 g). When the amount of unadsorbed gas reaches a constant value, this saturation value and the loop volume lead us to the total volume of CO chemisorbed. Dispersion is then calculated with the following expression :

^ ^

V-M Y„-m-C-P m

where V is the adsorbed volume in standard conditions, M is the atomic weight of the metal in consideration, Vn, is the standard molar volume of the active gas, m is the dry sample mass, C is the stoichiometric coefficient of the adsorption, P is the metal loading of the catalyst (%).

4. RESULTS AND DISCUSSION As we have shown in fig 1, the effect of different active carbons depends on their nature (preparation, activation, treatment, ...) of the added solid. For this series of experiments, we have used a Johnson Matthey catalyst and its support. Fig 3 displays the curves of MNP disappearance, TBHA accumulation and disappearance and TBA production, obtained in standard conditions (9 bar hydrogen pressure, 60°C, 0.6 g of catalyst, initial MNP concentration 0.15 M).

267

150

200

350

Time (min) -MNP

-TBHA

-TBA

Figure 3. Concentrations vs time diagram of the MNP hydrogenation, on Pd/C 5% (Johnson Matthey 487), at 60°C, under 9 bar hydrogen.

Fig 4 compares kinetics in the presence of the catalyst and the support treated separately with HNO3 0.25 M with the curve obtained under the same experimental conditions with the catalyst directly treated with HNO3 0.25 M. However, this nitric acid treatment of the catalysts could resuh in a dissolution of the metal, and a modification of the metallic dispersion. ICP measurement shows that the metal loading remains unchanged after treatment, while dispersion decreased from 12% to 8%. As shown in fig 4, this lower dispersion is surprisingly accompanied by an increase in activity. The easiest explanation of this experimental fact, confirmed by the results obtained with a mechanical mixture of carbon and catalyst, consists in the catalytic role of the support. The questions that are coming in mind are i) in which elementary step does carbon (as support or as added solid) play a role ? and ii) what are the parameters governing this behavior ? The answers lie in the surface characterization and in a step by step study led in presence and absence of active carbon. 4.1. In which Elementaiy Step does Carbon play a Role ? Main reaction. If the active carbon catalyzes the first elementary step of the main reaction, more free metallic sites will remain free for the other compounds. But experiment shows that the hydrogenolysis of MNP to MNoP occurs exclusively on the metallic phase. Indeed, no any conversion into MNoP is observed either in homogeneous phase, either in presence of naked carbon.

268 0,16

- MNP (+solid HN03) - B - TBHA (+solid HN03)

— MNP (cata treated) - o — TBHA (cata treated)

Figure 4. Comparison of the MNP and TBHA curves obtained with a mechanical mixture of catalyst and treated support (+solid HNO3) and with a catalyst directly treated by HNO3.

If Starting the hydrogenation with MNoP or TBHA in presence of active carbon, a conversion is observed into TBHA and TBA while no conversion is observed in homogeneous phase. Those transformations are slower than in presence of a metallic catalyst: if the rate is standardized to the mass of solid (catalyst or active carbon), the experiment led in presence of metal is 300 times faster for the MNoP hydrogenation, and 20 times faster for the TBHA reduction. Hence, the action of the added carbon can not be explained by a catalytic role in the main reaction. Therefore, we have to adapt the complete reaction scheme of nitrobenzene reduction for nitroaliphatic compounds hydrogenation and to consider both coupling reactions and TBHA disproportionation. DisproportionatiotL The hydrogenation of MNoP into TBHA is so fast that the disproportionation of TBHA may be approximated by 2 TBHA -> TBHA + TBA instead of 2 TBHA -^ MNoP + TBA. This reaction decreases the TBHA concentration and increase the TBA formation. Depending on the adsorption coefficient value for each organic compound, this process may free a part of the metallic surface area, and therefore favor the MNP conversion. However, this hypothesis can not explain a slowing down of the reaction due to the addition of another active carbon.

269 Coupling reactions. If the added solid favors the coupling reactions, the abundance of the intermediary compounds (MNoP, TBHA) in the main reaction sequence will be decreased in the liquid phase as well as on the metal. This will leave more space on the surface for MNP reduction and the reaction rate of this first step will increase. But, to explain the increase in overall reaction rate - i. e. the amine production - we have to propose that other pathways yield the same final product. The simplest proposals is that the hydrogenation/hydrogenolysis of coupling products (azoxy, azo and hydrazo) contribute to the amine production i) by reaction in the homogeneous phase or on other sites (support or other metallic sites) or ii) by reaction on the same sites but at a higher rate than the main reaction sequence. This hypothesis of the contribution of at least two pathways : the main reaction sequence and the coupling products hydrogenation/hydrogenolysis explains also the variation in selectivity observed when some added solid influences the rate constants ratios between these different pathways. The same explanation holds for the variation in activity, all the more if one considers that the coupling reactions already occur in the reference experiment and that they significantly contribute to the overall reaction rate. These coupling reactions may play a second role if they yield heavy products that can strongly adsorb on the catalyst surface. Addition of active carbons that enhance production of those species - probably basic carbons [12] - would decrease the metallic surface available for the reduction reactions, and lead to an apparently lower activity. Conversely, addition of carbons that prevent the formation of these heavy products will lead to an apparent higher activity. On the whole, interpretation of the modification in selectivity and activity observed when adding active carbon should rest on a combination of the hydroxylamine disproportionation and variability in the importance of the coupling products pathway. 4.2 What are the Parameters governing this Behavior ? Surface characterization is needed to answer the question of the parameters that favor such a behavior. As we mentioned just before, we think that that acid/base character is the main responsible of the modifications carried by a solid. Boehm and Johnson Matthey titrations. Both titrations give information on the acidic or basic character. The procedures are described in [13] and [14]. Boehm method classifies the surface acidic sites in three types (called phenolic, lactonic, carboxylic), by growing strength of acidity. However, this arbitrary classification fails to attribute one surface acidic group to one type of acidity. Moreover, this method does not give very accurate results with solids containing few acidic sites. Nevertheless, it appears for our active carbon that "phenolic" sites are a majority. The amount of each type of acidic sites does not change very much with the nitric treatment. Johnson Matthey method gives information on the acidity and on the basicity of the solids but does not give any information on different type of acidity or basicity. The results show that the effect of the carbon modification is mainly observed in the basic part of the curve (fig 5). It means that the nitric acid treatment does not create supplementary acidic sites on the surface, but suppresses basic sites, those ones that are probably responsible for the coupling reactions. The nitric treatment of the active carbon probably decrease the formation of heavy products.

270

FTIR spectroscopy. Several absorption bands are mentioned in the literature [9-11] and attributed to vibrational properties of different surface groups. It is important to stress that this method does not make the difference between identical groups of different strength. In this section, we will only focus on the common features of the support and the catalyst. The phenolic groups seem to be more numerous than the others (quinonic, carboxylic, lactonic). The nitric treatment seems to increase the amount of quinonic groups on the support, and the quinonic and phenolic ones on the catalyst. Those results are in accord with these obtained in the Boehm titration. At this moment, we have no direct information on the basic surface groups.

«/

/J

^A

J\

25

-25

J. I

/ I «

-50

1

T \

\-

1

\

8

10

12

PH I

— non treated active carbon —«—active carbon treated with HN03 0,25 M |

Figure 5. Johnson Matthey titration of the non treated and nitric acid treated active carbon. The upper part of tlie graph is due to the addition of NaOH, the lower one to the addition of HCl.

5. CONCLUSIONS This work demonstrates that in fine chemicals reactions, the direct catalytic role of the support should not be downplayed. Beside its classical effect on the geometric and possibly electronic properties of the metalUc phase, even a carbon support may directly catalyze some reactions. In this way it will exert an influence on the balance of competing reactions, modifying the global activity and the selectivity as well, in a favorable or unfavorable way. Because of the intricate network of reactions in consideration, the global effect is very difficult to interpret without a detailed investigation on each step of the reaction. Kinetics of each reaction step should be studied with the standard catalyst, the mechanical mixture of catalyst and supplementary support (modified or not) and with the modified catalyst.

271 Modification of the catalyst may be done by nitric treatment of a standard catalyst or by preparation of a catalyst on a support treated with nitric acid. The last modification procedure has the drawback to affect also the geometric and electronic influence of the support on the metallic phase. The first one has the drawback to modify also the metallic phase. Both may contribute to the understanding of the phenomena. This kinetic analysis should rest on the complete rake scheme we have developed elsewhere [5]. Finally, a more precise method for the characterization of the acidity and the basicity of the active carbon sites is needed. TPD of weak acids and bases may contribute. Basicity is probably more important, since we have shown that i) global surface basicity is more affected by nitric acid treatment than surface acidity, and ii) the amount of the different surface acidic sites is not directly related to the catalytic effects.

REFERENCES 1. M. Spiro, Catalysis Today, 7 (1990), p 167. 2. A.S. Lisitsyn, P.A. Simonov, A.A. Ketterling and V.A. Likholobov, in "Preparation of Catalyst V" (G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, eds.), Elsevier, Amsterdam, Stud. Surf Sci. Catal. 63 (1991), p 449. 3. D.J. Suh, T.J. Park and S.K. Ihm, Ind. Eng. Chem. Res., 31 (1992), p 1849 4. P. Albers, K. Deller, B.M. Despeyroux, A. Schaffer and K. Seibold, J. of Catal., 133 (1992), p 467. 5. V. Dubois, G Jannes, J.-L. Dallons and A. Van Gysel, in "Catalysis in Organic Reactions" (J.R. Kosak and T.A. Johnson, eds.). Marcel Dekker, New York, 1994, p 1. 6. M. Adam, V. Dubois and G Jannes, unpublished results. 7. J.C. Jungers, L. Sajus, I. de Aguirre and D. Decroocq, in "L'Analyse Cinetique de la Transformation Chimique" tome 1, Editions Technip, Paris, 1967, p 22. 8. J.R. Kosak, in "Catalysis of Organic Reactions" (P.N. Rylander, H. Greenfield and R.L. Augustine, eds.), Marcel Dekker, New York, 1988, p 135. 9. C. Ishizaki and I. Marti, Carbon, 19 (1981), p 409. 10. J.M. O'Reilly and R.A. Mosher, Carbon, 2 (1983), p47. 11. F. Rositani, P.L. Antonucci, M. Minutoli, N. Giordano and A. Villari, Carbon, 25 (1987), p325. 12. P.N. Rylander, in "Ullmann's Encyclopedia of Industrial Chemistry", Vol. A13, via Engelhard Chemical Catalyst News (July 1990). 13. J.S. Bandosz, J. Jagiello and J.A Schwarz, Anal. Chem., 64 (1992), p 891. 14. D.S. Cameron, S.J. Cooper, I.L. Dodgson, B. Harrison,J.W. Jenkins, Catalysis Today, 7 (1990), p 113.

ACKNOWLEDGEMENTS We very much appreciated the generous gift of catalyst samples fi-om Johnson Matthey. We acknowledge Perldn Elmer for ICP measurement, J. Van Cautenberg and P. Uliana for their valuable help in preparing the manuscript.

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

273

Kinetics of the Hydrogenation of Citral over Supported Ni Catalyst Paivi Maki-Arvela, Lasse-Pekka Tiainen, Rivka Gil, Tapio Salmi Laboratory of Industrial Chemistry, Abo Akademi, FIN-20500 Turku, Finland Summary The hydrogenation kinetics of an unsaturated aldehyde, citral, was investigated in a semibatch reactor operating at 60 - 77 °C and at atmospheric pressure. Crushed alumina supported nickel was used as a catalyst and ethanol as a solvent. The kinetic experiments revealed that the conjugated double bond is the most reactive one, giving citronellal as a primary product, which is partially acetalized with ethanol. The acetalization can be suppressed by hydrogen spillover on the support. Hydrogenation of the carbonyl group in citronellal gave citronellol as the secondary reaction product with high yields (92.5 - 94.9 %). In long time experiments the hydrogenation of citronellol to 3,7-dimethyloctanol was confirmed. The parallel hydrogenation part of citral through nerol and geraniol was observed to be almost negligible. Based on the qualitative results, a reaction scheme was written and pseudo-first order rate equations were derived. Approximate values for the rate constants were obtained by applying non-linear regression analysis on the kinetic data.

1. INTRODUCTION Hydrogenation of citral is a suitable model system for the investigation of the selectivity and kinetics of the hydrogenation of unsaturated aldehydes containing isolated and conjugated double bonds as well as a carbonyl group. The principal primary reaction products are the unsaturated alcohols (nerol and geraniol) and the unsaturated aldehyde, citronellal. From the secondary hydrogenation products the unsaturated aldehyde, citronellol is of particular interest, since it is frequently used e.g, in the perfumery industry [1]. Citral is usually hydrogenated in an alcoholic solvent, but the drawback is the formation of citronellal acetal. In hydrophobic solvents, on the other hand, a part of citronellal isomerizes to a cyclic compound, isopulegol. — The aim of the present work was to determine the kinetics and mechanism of hydrogenation of citral over a commercial nickel catalyst.

2. EXPERIMENTAL Citral was hydrogenated isothermally in ethanolic solutions at 60-77 "'C, in a semibatch reactor working at atmospheric pressure. Hydrogen was dispersed into the reaction mixture through separate glass sinters. A reflux condenser was placed on top of the reactor in order

274

to prevent the escape of volatile liquid components. The crushed and sieved Ni/Al203 catalyst particles (Crosfield HTC-400, 16.7 % Ni, particle size 63-90 pm, dispersion 15.4 % (chemisorption), mean metal particle size 65.7 A) were transferred into the reactor, which was heated to the activation temperature (350 *C) under nitrogen flow. The catalyst was reduced with hydrogen (99.999 %, AGA) for two hours, after which it was cooled down under hydrogen flow. The solvent (ethanol, 100 %, Alko) was transferred on the catalyst and the activation was continued in wet conditions for one hour at 80 T in hydrogen flow. The desired amount of citral (Lancaster 5460, 33.9 mol-% cis-isomer, 66.1 mol-% trans-isomer) was injected into the reactor, and the hydrogenation was commenced. The mass of citral-to-mass of nickel ratio was in all experiments 25. Samples (50 pg) were withdrawn from the reactor for chemical analysis. The sample was injected into another flask, which contained a 1 ml solution of methylcyclohexane (>98 %, Merck 806145) and 1.5-2 mg cyclohexanone (>99 %, Merck 822269) as an internal standard. The analyses were carried out with a gas chromatograph (Varian 3300) using an HP-Innowax capillary column (polar polyethylene glycol column; length 30 m, internal diameter 0.32 mm and film thickness 0.25 pm) and a flame ionization detector. The temperature programme used was: 100 °C(30 min) - 5 °C/min - 160 °C. The injector and detector temperatures were 270 °C and 275 °C, respectively. Helium was used as a carrier gas (velocity 26 cm/s, split ratio 68). The peak identification was based on the retention times obtained from a reference solution containing the following hydrogenation products of citral: nerol (90 %, Fluka 72170), geraniol (tech. %, Lancaster 6238), citronellal (>98 %, Fluka 27468), citronellol (Lancaster 5743), isopulegol (>99 %, Ruka 59770) and 3,7-dimethyloctanol (99 %, Aldrich 30.577-4). The identification was confirmed with GC-MS and with NMR-techniques. The citronellal ethylacetal was identified with the GC-MS technique. Before the kinetic experiments some preliminary tests were performed with different hydrogen flows (75 and 100 ml/min) and catalyst particle sizes (<45 pm and 63-90 pm) to ensure the absence of mass transfer limitations in the kinetic experiments. 3. RESULTS AND DISCUSSION 3.1. Qualitative results The main experimental parameters being studied were: the reaction time, the temperature and the reagent concentration. The dominating primary products were always citronellal and its acetals. Gradually the carbonylic group of citronellal was hydrogenated to hydroxyl leading to the formation of citronellol as a dominating secondary product. The amounts of unsaturated alcohols, nerol and geraniol, were always very minor (about 2-3 mol-%), so they did not significantly contribute to the formation of citronellol. A long-time experiment (Fig. 1) showed the same trends, but the decrease of the citronellol concentration became visible after about 480 min and simultaneously the concentration of 3,7-dimethyloctanol increased, confirming the hydrogenation of the isolated double bond.

275

T

0

120

240

360

480

600

time/min

Fig. 1. Hydrogenation kinetics of citral at 70 'C, a long-time experiment. Symbols: ) citral, (cis and trans), ) citronellal, (A) citronellol, ) nerol and geraniol, ) 3,7-dimethyloctanol. The continuous lines represent the fit of the model to the experimental data. The temperature effect on the hydrogenation kinetics was examined at 60 ''C, 70 "C and 77 °C, The yield of citronellal and its ethylacetal attained its maximum most rapidly at the lowest temperature (60 °C). This is probably because the activation energy of the hydrogenation of the conjugated double bond is lower than that of the carbonyl group. The experiments with different initial concentrations of citral (Fig. 2) revealed an interesting effect: the hydrogenation of citronellal was practically stopped after a certain reaction time and no more citronellol was formed for the lowest initial concentration. The primary reason was that the major part of citronellal appeared as acetal: about 65 mol-% was in the form of acetal for the initial concentration of 0.05 M, whereas about 1-2 mol-% only were in the acetal form for the higher concentrations (0.1 M and 0.2 M). This effect might be explained by the spillover of citronellal on the support. The acetalization reaction [3] \

H^

C=0 + 2 ROH->

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, ^itrar ^'^

PB^I

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.

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

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^ ^Institut 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 (^Ag= 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.

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

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 (0
290 ethanol could be improved by high temperature reduction of the catalyst sample. ^^ This was attributed to electronic effect of partially reduced TiOx^+ located at the support-platinum interface. For Rh/Si02 catalyst, the addition of tin either by SnCl2 impregnation or Sn(CH3)4 chemical vapor deposition promote the rate of acetophenone hydrogenation when the active bimetallic ensemble have the composition of Sns/Rhs=l/1,5.^ ^ In this case also, the authors suggested that the oxygen atom of the carbonyl interacted with the surface tin atoms. In order to verify that this effect could be general, we have studied the hydrogenation of acetophenone into phenylethanol, on tetrabutyltin modified silica supported rhodium catalysts.

EXPERIMENTAL Monometallic starting catalyst: The silica (Degussa Aerosil, 200 m^/g) was impregnated by cationic exchange of the hydroxyl groups of the surface with chloropentammine-rhodium complex [RhCl(NH3)5Cl2]"'""'" in aqueous solution, containing NH4+ ions (pH 10) as competitors. After filtration and washing with water, the solid was calcinated in dry air at 573 K and treated under hydrogen at 573 K. The metal loading of the sample was 1 wt % and the metallic particle size of the catalyst was in the range of 1.0 to 1.5 nm as determined by electron microscopy. The dispersion of the catalyst (D=Rhs/Rh, where Rhg is the number of the surface rhodium atoms) measured by hydrogen adsorption ^^ is 0.8, a value which is in close agreement with that deduced from the metallic particle size observed by electron microscopy ^^. Preparation of bimetallic catalysts: After reduction under hydrogen at 573 K, 250 mg of the monometallic starting catalyst (2.5.10"^ mol of Rhs) was placed under argon in the reaction vessel (stainless steel autoclave, well stirred by a magnet), with 10 ml of n-heptane and a given amount of Sn(n-C4H9)4 (so that Sn/Rhs = 0.3 or 2). The hydrogen pressure in the autoclave was then raised to 8 MPa and the tempertature was regulated at 373 K. Depending on the desired value of Sn/Rhs, the reaction time was varied from 30 to 840 minutes. The blank experiment with the silica alone showed that under these conditions, the amount of Sn(n€4119)4 fixed on the support was negligible. Catalytic reaction: The reduction of acetophenone was performed in situ, in the same autoclave. After cooling down the reactor to room temperature and reducing the hydrogen pressure, a solution of 0.6 ml of acetophenone (acetophenone/Rhs = 200) and 0.4 ml of tetradecane (internal standard) in 10 ml of n-heptane was introduced in the autoclave containing the catalyst. The hydrogen pressure and the temperature was then raised to 8 MPa and 340 K, respectively. The change in the composition of the reaction mixture with time was followed by analysing the liquid samples using a gas chromatograph (G.C.).

291 The possible reaction scheme of acetophenone hydrogenation on a metallic catalyst could be described as follows:

OH

Ethyl benzene

(1)

OH Phenyl-1 ethanol

Acetophenone

(2)

Cyclohexyl-1 ethanol

Cyclohexyl methyl ketone

Methyl cyclohexane Scheme 1 For a given reaction time (t), a) the acetophenone conversion (Conv.)t is expressed as : [OC(0)CH3]o-[OC(0)CH3]t (Conv.)t= 100. [
292 50 min, as seen in figure la. Phenyl-1 ethanol and cyclohexyl methyl ketone are primary products. They are readily hydrogenated into cyclohexyl-1 ethanol. b) Sn(n-C4H9)4 modified Rh/SiOa catalysts The monometallic Rh/Si02 is modified by reaction of tetrabutyltin with the rhodium surface, in n-heptane, under hydrogen and at 373 K. Three different catalysts were prepared by varying the amount of Sn(n-C4H9)4 introduced and the reaction time as shown in table I. Table I: Bimetallic samples prepared by reaction of Sn(n-C4H9)4 and Rh/Si02. They are named RhSna^/Si02, where a is the amount of tin fixed and b is the reaction time Sample

Amount of i5n(n-C4H9)4

reaction time

introduced

fixed

RhsSno.30'VSi02

0.3 Sn/Rhs

0.3 Sn/Rhs

0,5 hours

RhsSno.8^'VSi02

2.0 Sn/Rhs

0.8 Sn/Rhs

0.5 hours

RhsSnojWSi02_

0.3 Sn/Rhs

0.3 Sn/Rhs

14 hours

When Rh/Si02 catalyst is modified by Sn(n-C4H9)4, there is a drastic improvement of the selectivity for phenyl-1 ethanol, as seen in figures lb, c and d. Depending on the amount of tetrabutyltin introduced and on the time of the reaction in the catalyst modification, the catalytic activity of the bimetallic sample were strongly reduced (figures Ic and Id) or clearly increased (figure lb). The initial rate of acetophenone consumption r(0) (mmol/min/gcatai.) and the selectivity of phenyl-1 ethanol at 90% of conversion (S^go) are given in table II. Table II: Initial rate of acetophenone consumption, r(0) and selectivity for phenyl-1 ethanol at 90% of conversion (SI90). Sample

r(0) (mmol/min/gcatai.)

SI90 (%)

Rh/Si02

0.66

30

RhsSno.30'VSi02

0.84

90

RhsSno.8^'^/Si02 RhsSno.3^4/si02

0.28

93

0.12

70

293 Concentration (mol/1) 30 T

0

50 100 time (min)

Concentration (mol/1) 0.3 f

Figure Ic

Concentration (mol/1) Figure lb

50 100 time (min)

150

Concentration (mol/1) T

Figure Id

100 200 0 100 200 time (min) time (min) Figure 1: Reaction of acetophenone (x) on various catalysts Rh/Si02 (la), RhsSno 3^'^/Si02 (lb), RhsSno.8^'VSi02 (Ic) and RhsSncs^^/SiOi (Id). Phenyl-1 ethanol (+), cyclohexyl methyl ketone (o), cyclohexyl ethanol (*), ethylbenzene (-).

We had previously studied the reaction between the tetrabutyltin and the rhodium surface in n-heptane and under hydrogen at 373 K.^^. For low Sn/Rhg ratio (c.a. 0.35), the butyl groups are fully removed after 20 minutes of reaction. For high amount of tetrabutyltin introduced (c.a. 2 Sn(n-C4H9)4/Rhs), the amount of tin fixed by surface rhodium atom after 20

294 minutes of reaction is about 1 and about 1 butyl group remains on the surface. On the basis of these data the results presented in this work can be tentatively explained on the following way: -For pure rhodium/silica, the hydrogenation of acetophenone is unselective that is both the aromatic ring and the ketone are simultaneously hydrogenated. For a particle of 15 A, there are both faces, edges and corners which are able to some extent to hydrogenate these two functions. It is likely that some flat planes will favor aromatic ring hydrogenation.!5'16 As soon as tin is introduced drastic changes of selectivity occur and no more aromatic ring hydrogenation is observed. -At low Sn/Rhs value (Sn/Rhs=0.3, figure lb), the organotin complex is usually fully dealkylated and tin is present on the surface as adatoms which are likely present on specific crystallographic positions where the hydrogenolysis reaction of the organotin occurs. ^^ This could well be faces of the particles. In this situation, the hydrogenation of the aromatic ring does not occur probably because a flat adsorption of the ring is needed. The selectivity for phenyl-1 ethanol is 90% with the formation of ethylbenzene as main by-product. The corresponding catalytic activity is increased as compared to that of the monometallic catalyst (figure la and table I). This is easily interpreted by the fact that there is no side reaction of ring hydrogenation. -Upon increasing the amount of tin (Sn/Rhs=0.8) both adatoms and alkyl-tin fragments are present on the surface. The selectivity of the desired product remains close to 90%, but the rate of the reaction is drastically decreased. More sites have been blocked by tin adatoms or tin alkyl fragments. Concerning the results of Figure Id (Sn/Rhs = 0.3 and reaction time =12 hours) the lower activity can only be explained by a progressive structural change of the tin-rhodium surface; perhaps the tin atoms slowly migrate on the most active sites of the particle. 4. REFERENCES 1) Travers, C ; Bournonville, J. P.; Martino, G.; Proceedings of the 8h International Congress on Catalysis ; Dechema; Frankfurt-an-Main, 1984, Vol. IV, pp 891-902. 2) Margitfalvi, J.; Hegedus, M.; Gobolos, S.; Kern-Talas, E.; Szedlacsek, P.; Szabo, S.; Nagy, F.; Proceedings of the 8h International Congress on Catalysis ; Dechema; Frankfurt-an-Main, 1984, Vol. IV, pp 903-914. 3) Candy, J. P.; Ferretti, O. A.; Mabilon, G.; Bournonville, J. P.; El Mansour, A.; Basset, J. M.; Martino, G.; /. Catal. 1988, 772, 210 4) Candy, J. P.; Didillon, B. D.; Smith, E. L.; Shay, T. B.; Basset, J. M.; /. Mol. Catal. 1994, 86, 179.

295 5) 6)

7) 8) 9)

10) 11) 12) 13) 14)

15) 16)

Didillon, B.; Candy, J. P.; El Mansour, A.; Houtman, C ; Basset, J. M.; J. Mol. Catal 1992, 74, 43. Didillon, B.; El Mansour, A.; Candy, J. P.; Basset, J. M.; Le peltier, F.; Boitiaux, J. P.; Proceedings of the 10th International Congress on Catalysis : New Frontiers in Catalysis ; Elsevier; Amsterdam, 1993, Vol. C, pp 2370-2374. El Mansour, A.; Candy, J. P.; Bournonville, J. P.; Ferretti, O. A.; Basset, J. M.; Angew. Chem. Int. Ed. Engl. 1989, 28, 347. Didillon, B.; Houmtan, C ; Shay, T.; Candy, J. P.; Basset, J. M.; J. Am. Chem. Soc. 1993, 775, 9380. Didillon, B.; El Mansour, A.; Candy, J. P.; Bournonville, J. P.; Basset, J. M.; Studies in Surface Science and Catalysis, Heterogeneous Catalysis and Fine Chemicals II; Elsevier, Amsterdam, 1991, Vol. 59, pp 137-143. Lin, S. D.; Sanders, D. K.; Vannice, M. A.; Appl. Catal. A: General 1994, 775, 59. Yoshikawa, K.; Iwasawa, Y. J. Mol. Catal. A: Chemical 1995,100, 115. Candy, J. P.; Fouilloux, P.; Renouprez, A. J. J.Chem.Soc, Faraday 119H0, 76, 616. Van Hardeleld, R.; Hartog, F. Surf Sci. 1969, 75, 189. Didillon, B.; Candy, J. P.; Le Peltier, F.; Ferretti, O. A.; Basset, J. M. Studies in Surface Science and Catalysis. Heterogeneous Catalysis and Fine Chemical III; Elsevier, Amsterdam 1993, Vol. 78, pp 147-154. Vishwanathan, V.; Rajashekar, M. S.; Sreekanth, G.; Narayanan, S.; J. Chem. Soc. Faraday Trans. 1991, 87,3449. Fuentes, S.; Figueras, F.; J. Catal. 1977, 46, 382.

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297

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*, and P. Salagre* Laboratoire des Materiaux Catalytiques et Catalyse en Chimie Organique. UMR 5618 CNRS, E.N.S.C.M. 8, rue Ecole Normale. 34053 Montpellier, France..

Hydrogenations of acetonitrile and pentylnitrile (valeronitrile) in gas and liquid phases respectively were carried out on catalysts obtained from Ni/Mg/Al layered double hydroxides (LDHs) precursors of various Mg/Ni molar ratios. Their catalytic properties were compared with those of a commercial Ni/Al203 catalyst. Selectivities to primary amines, higher than 90% were obtained on catalysts with Mg/Ni molar ratios in the range 0.3-1. This behaviour was correlated with the acido-basic properties of the solids characterized by TPD of NH3 and microcalorimetry of monoethylamine adsorption. Both studies show that upon Mg addition, the surface acidity, which is responsible for secondary amine formation decreases.

1. INTRODUCTION Amines can be prepared commercially by hydrogenation of nitriles [1,2]. Raney nickel, Raney cobalt and supported Ni catalysts are probably the most frequently used for the primary amine production from nitrile hydrogenation. Due to the high reactivity of imines, the partially hydrogenated reaction intermediates, a conventional hydrogenation process leads to a mixture of primary, secondary, and tertiary amines. Promotion by bases of the reaction medium was found to decrease the formation of secondary and tertiary amines, by a poisoning of the surface acid sites, which would be responsible, for a large part, of the coupling reaction between primary imine and primary amine to secondary amine [2,3]. It has therefore to be expected that, on tuning the acido-basic character of the support, a change in selectiviy should be found in favour of primary amines. It has been thus recently reported that the modification of the intrinsic acidity of catalysts by the addition of alkali metals promotes the selectivity to primary amines [3,4]. The Ni supported catalysts are usually prepared by precipitation of Ni salts with other cations [5]. A promising route is the use of layered double hydroxides (LDH), with the general formula (Mf_xM^(0H)2)''''(anion"~)^/^,mH20 , of the hydrotalcite type as precursors for well dispersed metallic particles with specific properties [6]. These compounds are easily decomposed into a mixed oxide of the Mi^(M^^J)0 type under calcination [7]. Basic properties were identified on someone of these mixed oxides with Ho values near 17 [8]. Departament d'Enginyeria Quimica, Universitat Rovira i Virgili, Carretera de Salou, 43006 Tarragona, Spain

298 We have prepared LDHs samples containing Ni and Mg as divalent cations, and Al as trivalent cation, with different Mg/Ni ratios to modify the basicity of the solids. These samples have been studied in the gas phase hydrogenation of acetonitrile, and the liquid-phase hydrogenation of valeronitrile. The catalytic properties were correlated with reducibility of Ni particles, and acido-basicity of the surface probed by NH3 and ethylamine adsorptions.

2. RESULTS AND DISCUSSION. The main characteristics of the catalysts are resumed in Table 1, details are given elsewhere [9]. The salient points which could be recalled are the following: 1) All samples show the typical XRD patterns of hydrotalcite type phases. They contain 2 both NO3 and CO3' compensating anions coming from the starting salts and the surrounding atmosphere respectively. The specific surface areas in the range of 15-35 m^g-i in the non calcined samples increase up to a maximum value of 190-210 m^ g-i after calcination at 623 K. This has been attributed to a craterisation phenomenon due to the decomposition of the vaporisable anions. No micropore was identified in the samples. 2) Upon calcination, the crystallinity of the lamellar structure progressively decreases while a mixed oxide phase of the NiO type appears for every samples. Only the mixed oxide is observed from calcination at 623 K.

Table 1 Main characteristics of the catalysts Ni/Mg/Al SBET Sample NHS^ desorption Heat of adsorption, O -^ 0.5 (kJ mol-1) CH3CN (m2g-l) T D MEA amount H2 (mol) 70 80 87 193 583 0.28 74/0/26 HA 85 40 210 576 0.16 72 50/15/35 HC 0.19 34/37/29 210 568 HG 553 Ni/Al203 20 wt% Ni 205 ^: temperature of maximum NH3 desorption rate (TD, /K), and amount desorbed (meq g"0 The TPR profiles of calcined LDHs precursors show two peaks of H2 consumption (Fig.l). The first peak around 570 K corresponds to the release of NO3 anions as NO2, and their subsequent reduction to NO and N2O as identified by mass spectrometry [9]. The second peak with maxima at 705,920 and 1000 K for HA, HC and HG samples respectively, corresponds to the reduction of NiO particles. These experiments show that the reducibility of the nickel oxide particles decreases when the Mg content increases. This could be compared with the decrease of the Ni^ crystal size measured by XRD in HC and the lack of detection of these particles in HG samples. This behaviour has been attributed to the formation of excess Ni aluminate and Ni spinel type phases decreasing the size of the mixed oxide particles and hindering their reducibility [6]. During the gas-phase hydrogenation of acetonitrile, MEA was the main organic compound formed, with selectivity normally higher than 70%. Diethylamine (DEA), diethylimine (DEI)

299 and triethylamine (TEA) usually appeared as by-products. The catalysts were first tested by temperature programmed reaction from 353 to 453 K by step of 10 K for 2 h. The plots of acetonitrile conversion and MEA selectivity as a function of reaction temperature are shown in Fig. 2. Whatever the catalyst, the MEA selectivity goes through a maximum value. The byproducts are mainly DEI at low temperature and conversion, but DEA and TEA at high temperature and conversion. The MEA selectivity obtained at medium conversion on HA sample, i.e. 80-85%, compares well with the values reported by Veerhak et al [3] for the gas phase hydrogenation of acetonitrile on Ni/Al203.

=3

400

600 800 1000 Temperature (K)

1200

Figure 1. Temperature programmed reduction profiles of HA (O), HC (O), and HG (A) samples.

340 360 380 400 420 440 460

Temperature (K) Figure 2. Acetonitrile conversion (open symbols), and MEA selectivity (full symbols), as a function of reaction temperature on HA , HC (0,#), and HG (A,A) samples.

After passivation of the catalyst at 393 K for 12 h under the reaction medium, the hydrogenation of acetonitrile was then carried out under isothermal conditions between 340 and 400 K. Table 2 reports some catalytic properties of the samples, obtained at low acetonitrile conversion. At steady state and 387 K, the MEA selectivity reached 92.6% at 99% acetonitrile conversion on HC sample.

Table 2 Main catalytic properties of Ni based catalysts for the reaction of acetonitrile with hydrogen; P(H2) = 88 kPa, P(acetonitrile) = 13 kPa. Temp. Conv. Sample Ea Products selectivity (mol%) (mol%) (kJmol-i) MEA DEI DEA TEA (K) 349 HA 10 51 79.2 15.0 5.5 0.2 353 5 HC 67 93.9 4.7 1.2 0.1 369 HG 10 70 90.1 2.8 6.6 0.3

300

Tables Main catalytic properties of Ni based catalysts for the reaction of valeronitrile with hydrogen; Sample Time to achieve ~ 99% Inter-amines selectivity (mol%) conversion (min) MPA DPI DPA HA 265 87.6 12 550 HC 92.6 7.4 650 HG 3.2 95.5 1.3 0.3 Ni/Al20 3 56 84.4 15

The same behaviour occured during the liquid-phase hydrogenation of valeronitrile where selectivities to pentylamine (MPA) higher than 80% were obtained whatever the catalyst (Table 3 and Fig. 3). Tripentylamine was never detected, and the main reaction by-products were dipentylimine (DPI) and dipentylamine (DPA). Taces of pentane were sometimes identified in the liquid phase. As for the hydrogenation of acetonitrile, the tendency is to an increase of MPA selectivity with Mg content, but in valeronitrile hydrogenation MPA selectivity does not reach a maximum value and is slightly higher on HG than on HC sample. Nevertheless, and with respect to the Mg free samples (HA, Ni/Al203), HC and HG samples behave in a very comparable fashion for the hydrogenation of both nitriles in gas- or liquidphase, with selectivities to primary amines higher than 90%. The maximum of MPA selectivity obtained on HG sample is higher to that found with Cr and Mo promoted Raney Ni [10], and similar to the value reached when the reaction is carried out under ammonia pressure [11] on the same catalyst.

0.25

0

100 200 300 400 500 600 Time (min)

Figure 3. Composition of the reaction mixture as a fiinction of time during the hydrogenation of valeronitrile on HG sample; ) valeronitrile, ) MPA, (A) DPI, (O) DPA.

500

600

700

800

Temperature (K) Figure 5. Temperature programmed desorption of NH3, (U) HA, (O) HC, (A) HG.

301

A mechanism for the formation of secondary and tertiary amines was proposed in the pioneer work of von Braun et al [12]. These compounds are coming ifrom a nucleophilic addition of the primary amine on the intermediate imine. The amino-dialkylamine thus formed decomposed in ammonia and secondary or tertiary imines promptly. According to von Braun et al [12], in liquid phase hydrogenations these reactions would occur in the homogeneous phase. However, Dallons et al [13] refuted this last proposition and claimed that and heterogeneous process through surface reactions was involved. These reactions between amine and imines could take place on the Ni surface purely, as on Raney Ni, or on both metal and support surfaces in the case of supported metal catalysts. In this last case Verhaak et al [3] proposed a bifunctionnal mechanism for the formation of secondary and tertiary amines over supported Ni catalysts (Fig. 4). The nitrile undergoes hydrogenation to primary imines and amines on the Ni sites. These compounds then migrate to acid sites where imine is protonated and then reacts with amine to yield secondary imines and NH3, then amines after hydrogenation. In this frame, the acidity of the support is a key factor which determines the formation of secondary and tertiary amines during nitriles hydrogenation.

Acidic function

Figure 4. Reaction scheme of the gas phase hydrogenation of nitriles as proposed by Verhaak ^ra/. (ref 3).

302

The inhibition of the acid sites can be reached by alkali metal addition [3], a NH3 partial pressure [12], but also by a self-promotion with the primary amine formed in the course of the reaction. Moreover, an influence of ammonia on the reverse reaction with DPI: RCH=NCH2R + NH3 <

> RCHNH2 <

> RCH = NH + RCH2NH2

NHCF^R cannot be ruled out. For these reasons, the effect of 'support' on Ni based catalysts is better shown when comparing the MEA selectivity at low acetonitrile conversions (Table 2). The improvement of primary amine selectivity upon Mg addition could arise from a modification of the acido-basic properties of the support surface. To check any differences in these properties, the acid sites were probed by TPD of NH3 and adsorption of MEA followed by calorimetry. Both studies showed that HC is less acid than HA due to Mg incorporation. Indeed, Table 1 reports the differential heat of MEA adsorption at about half-coverage of the adsorbate for HA and HC samples. These results show that MEA adsorption is more energetic by 40 kJ mol"^ on HA compared to HC sample. Since MEA, with basic properties, interacts essentially with the acid sites, it comes out that the strength of remaining acid sites, if any, of HC sample is lower than that of HA. This conclusion is furthermore substantiated by the results from TPD of NH3 (Fig.5 and Table 1). NH3 desorbs as a main peak around 580 K with tailing up to 800 K. The maximum of NH3 desorption rate is shifted to lower temperatures from HA to HG samples, i.e. upon Mg addition. Moreover, the amount of NH3 desorbed decreased concurrently (Table 1). A comparison with a well known catalyst with medium acidity (HY, Si/Al = 2.5) shows an amount of acid sites sixfold lower for the LDHs issued samples. For the same samples the temperature of maximum NH3 desorption is lower by 20 K which characterizes a lower acid strength. This decrease of acidity of the Ni based catalyst after a small Mg addition can be compared to that observed on Ni/Al203 upon K promotion [3]. Therefore, both MEA adsorption and NH3 desorption allows us to conclude to a decrease of acidity of the catalysts with increasing Mg content. The influence of Mg is not limited to the inhibition of acid sites of the support, but also acts on the metallic Ni particles. It seems that this action of Mg on Ni operates by two ways: 1) on the one hand, the opposite variations of heats of H2 and acetonitrile adsorption (G « 0.5) upon Mg addition (Table 1) could arise from a modification of the Ni d-band. The same phenomenon was observed for the initial heats of H2, CO and acetonitrile adsorption [15]. These same behaviours observed for CO and acetonitrile, both n acceptor molecules, are in agreement with the appearance of more tightly bound state of CO upon promotion of Ni/Si02byK[16]. 2) on the other hand, patches of MgOx species might decorate the surface of Ni particles, and interact with the -CsN bond [13]. These two phenomena could explain, with the difference of reducibility (Fig. 1), the change of reactivity between the samples HA and HC for nitrile hydrogenation. On the other hand, the nucleophilic addition of primary amines on the intermediate imine can probably takes place in part on the Ni surface, as on Raney Ni. Therefore, some modifications of Ni d-band could affect the reactivity of the intermediate imine and its interaction with amine. One could expect that an electron "enrichment" of Ni d-band will

303

decrease the electron donation from the unsaturated -CH=NH system, and possibly the nucleophilic attack at the C atom by amine. The basicity of the support will thus participate in decreasing the formation of secondary, and tertiary, amines on both Ni and support surfaces.

3. EXPERIMENTAL Three samples containing Ni2"^/Mg2+/Al3"^ cations were prepared (Table 1). They were obtained by coprecipitation at constant pH = 9 0.2 of an aqueous solution containing in appropriate amounts the nitrate precursors and a solution of NH4OH and (NH4)2C03. The precipitated gels were heated in air at 353 K for 14 h, filtered and washed several times at 353 K, dried in an oven at 393 K for 72 h, and then finally calcined under air at 623 K for 2 h (ramp: 2 K min'l). The catalysts were characterized by X-ray diffraction (XRD), N2 sorption at 77 K, thermo-gravimetry, temperature programmed reduction (TPR) by H2 and temperature programmed desorption (TPD) of NH3. Differential heats of H2, acetonitrile and ethylamine (MEA) adsorption were determined. An industrial Ni/Al203 catalyst (Engelhard Ni-5256 E 3/64) was tested as reference. TPR by H2 were carried out on the calcined sample with H2/Ar (3/97) from room temperature to 1073 K at 5 Kmin-^. H2 uptake was monitored by a thermal conductivity detector. Before TPD of NH3 and calorimetric experiments, the samples were reduced at 723 K for 2 h under H2/N2 flow (10/90, vol/vol) as done for the catalytic experiments. For TPD experiments, NH3 was adsorbed at 373 K on the reduced and outgassed catalyst. Heating was then started up to 873 K under He flow. NH3 evolution was monitored by conductimetry. Heats of hydrogen, acetonitrile and MEA adsorption were determined with a modified SETARAM microcalorimeter DSC-111. After reduction and outgassing under flowing He at 723 K, the sample was cooled to room temperature. The temperature was then fixed to 313 0.01 K where micropulses of the probe molecule were fed to the catalyst using a 6-way sampling valveflushedwith He. The thermal event in the calorimetric cell was then recorded as a fiinction of the adsorbate uptake, which was monitored by a thermal conductivity detector. The gas phase hydrogenation of acetonitrile has been studied in a differential microflow reactor operating at atmospheric pressure. Prior to any measurements, the sample was reduced under a diluted hydrogen flow (H2/N2: 10/90, vol/vol) at 723 K for 2h (ramp: 2 K min"i). The reaction temperature ranged from 343 to 453 K, and the H2/acetonitrile molar ratio was 6.8. The effluent was analyzed by sampling on line to a gas chromatograph (Perkin Ehner) equipped with a capillary column (30 m x 0.25 mm i.d., apolar phase) and a flame ionization detector. The liquid-phase hydrogenation of valeronitrile in cyclohexane was studied in a 100 cm"^ batch reactor. The catalyst (300 mg) was first reduced ex-situ at 723 K for 2 h, then reactivated in-situ at 423K in cyclohexane under 4.5 MPa H2 pressure. The valeronitrile was then charged and the reaction started at 363 K under 1.6 MPa H2 pressure, with a reactant concentration of 1 x 10"^ mol cm'^, total volume = 60 cm^.

304

4. CONCLUSION Supported Ni catalysts prepared by calcination and reduction of Ni-containing LDHs precursors are efficient for the hydrogenation of acetonitrile and valeronitrile in gas and liquid phases respectively. The selectivity to primary amines increases with Mg content and reaches values higher than 90%. This behaviour can be accounted for a part by the decrease of the amount of residual acid sites upon Mg addition. The secondary and tertiary amines are indeed issued from the coupling reaction between (RCHNH2)''' and RCH2NH2 on these acid sites. Calorimetric studies of MEA adsorption and TPD of NH3 provide evidence of a weaker interaction of RCH2NH2 with Mg-containing samples. HG sample thus retains the smaller amount of NH3 which desorbs 20 K lower than from the other solids. Moreover, the heat of MEA adsorption is lower by 40 kJ mol'^ on HC compared to HA sample, which does not contain Mg. By contrast, heats of CH3CN, CO and hydrogen adsorption are modified to a much lesser extent.

REFERENCES 1. M. Grayson, Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 2, 2^^ Ed. Wiley Intersciences, New York, 1983, p. 272. 2. J. Volf and J. Pasek, in Catalytic Hydrogenation (L. Cerveny, Ed.), Elsevier, Amsterdam, 1986,p.l05. 3. M.J.F.M. Verhaak, A.J. Van Dillen and J.W. Geus, Catal. Letters, 26 (1994) 37. 4. F. Medina, P. Salagre, J.L.G. Fierro and J.E. Sueiras, J. Catal., 142 (1993) 392. 5. J.A. Schwarz and J.L. Falconer, Catal. Today, 7 (1990) 22. 6. F. Trifiro, A. Vaccari and O. Clause, Catal. Today, 21 (1994) 185. 7. W. T. Reichle, J. Catal., 94 (1985) 547. 8. F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 11 (1991) 173. 9. D. Tichit, F. Medina, R. Dutartre and B. Coq, Appl. Catal. A: General, submitted 10 M. Besson, D. Djaouadi, J.M. Bonnier, S. Hammar-Thibault and M. Joucla, in Heterogeneous Catalysis and Fine Chemicals^ M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier (eds.), Elsevier, Amsterdam, 1991, p.l 13. 11 M. Besson, J.M. Bonnier and M. Joucla, Bull. Soc. Chim. Fr. 127 (1990) 5. 12. J. von Braun, G. Blessing and F. Zobel, Chem. Ber., 36 (1922) 1988. 13 J.L. Dallons, A. Van Gysel and G. Jannes, in Catalysis of Organic Reactions, W.E. Pascoe (ed.). Marcel Dekker, New York, 1992, p.93. 14 R. L. Augustine, Catal. Rev., 13 (1976) 285. 15 F. Medina, D. Tichit, B. Coq, A. Vaccari and N.T.Dung, J. Catal., submitted 16 M. Gravelle-Rumeau-Maillot, V. Pitchon, G.A. Martin, and H. Praliaud, Appl. Catal. A: General, 98 (1993) 45.

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

305

The effect of co-adsorbates on activity/selectivity in the hydrogenation of aromatic alkynes S. D. Jackson*, H. Hardy, G. J. Kelly, and L. A. Shaw. ICI Katalco, RT&E Group, PO Box 1, Billingham, Cleveland TS23 ILB, U.K.

The effect of co-adsorbed species on the liquid phase hydrogenation of higher molecular weight alkynes has been studied over palladium catalysts. Addition of 1:1 molar quantities of the respective alkenes resulted in a change in the rate of reaction of the alkyne. Addition of benzonitrile to a reaction system reduced the rate of alkyne hydrogenation. However the benzonitrile enhanced the rate of styrene hydrogenation while reducing the rate of cis-1 -phenyl-1 -propene hydrogenation.

1. INTRODUCTION The selective hydrogenation of higher molecular weight alkynes to alkenes has not been an area of extensive research [1 - 4], especially when compared to the wealth of publications on ethyne hydrogenation [5]. Competitive hydrogenation reactions are also rarely studied and yet in most cases where there is a need for hydrogenation of an alkyne there is a significant presence of the alkene. There have been reports on competitive hydrogenation between alkenes [6, 7] but studies of competitive hydrogenation between alkynes and alkenes are rare. Previously we have studied the hydrogenation of phenyl acetylene and the competitive hydrogenation of phenyl acetylene and styrene [8]. In this paper we have expanded the study to consider the effect of co-adsorbates (styrene (Ph-CH=CH2), trans-1-phenyl- 1-propene (Ph-CH=CH-CH3), and benzonitrile (Ph-CN)) on the activity/selectivity of phenyl acetylene (Ph-C2-H) and 1-phenyl-1-propyne (Ph-Cj-CHj) hydrogenation. Control of activity and selectivity independently is of considerable importance to the fine chemicals industry. In this paper we will show that by appropriate choice of catalyst and co-adsorbate such control can be achieved.

2.

EXPERIMENTAL

The catalysts used throughout this study were a 1% w/w Pd/Carbon and a 1% w/w Pd/Zr02. Sufficient palladium nitrate (PGP Industries, Pd assay 10.123%) solution was added to a quantity of carbon powder (Sutcliffe Speakman, Ref 9281) to thoroughly wet the carbon and achieve a 1% loading. The slurry was dried at 343 K for 5 h, then at 363 K

306 overnight and finally calcined in air at 423 K for three hours. The dispersion of the palladium was determined as 18% by carbon monoxide chemisorption assuming a 1:2 CO:Pd ratio. The Pd/Zr02 catalyst was prepared by impregnation. PdCl2 (PGP Industries Ireland) was dissolved in sufficient dilute hydrochloric acid to fully wet the zirconia (Degussa, S. A. 50m^ g'^). The resulting mixture was evaporated to dryness at 353 K. The weight loading obtained was 0.99 % w/w Pd/zirconia. The dispersion of the catalyst was determined by carbon monoxide chemisorption at 96 %, assuming a ratio of 1:2 for CO:Pd. The reaction chemistry was performed in a 0.5 1, stirred, autoclave which incorporated a hydrogen-on-demand system sparged through the solution. The reactor also had a sample point from which aliquots could be removed during the reaction. The following procedure was used to reduce the catalyst and introduce the reactants. The autoclave was purged with nitrogen and 170 ml of dodecane (Aldrich, >99%) added. A sample of catalyst (0.013 g) was added as a slurry in dodecane to the reactor. The reactor was then heated to 323 K, the system purged with hydrogen (BOC, >99.99%), and the stirrer set at 750 rpm. Over the following two hours the reactor was put through pressure/depressure cycles of increasing the hydrogen pressure to 0.2 MPa, then reducing to 0.1 MPa, hence generating a flow through the system. At the end of this procedure the reactor was cooled to the desired temperature (temperature control +/-1 K) and 0.4 ml of 1-phenyl-1-propyne (Aldrich, 98%) or phenyl acetylene (Aldrich, >99%) added in a solution of dodecane. The final volume of solvent was 200 ml. The hydrogen pressure was set at 0.2 MPa and this was maintained throughout the experiment. Samples (approximately 1 ml) were removed every five minutes for the first forty-five minutes of the reaction and then at one hour. Analysis of the products was by GC using a FID and a CPSIL 5CB column. The results were quantified by use of standard solutions. The absence of diffusion control in the system was confirmed by performing an experiment at 1500 rpm as well as 750 rpm; in both cases a similar rate of reaction was measured.

3.

RESULTS

The hydrogenation of phenyl acetylene, over both catalysts, and 1-phenyl-1-propyne, over the carbon supported catalyst, was performed in the absence of any other species to obtain a reference reaction profile. For both phenyl acetylene and 1-phenyl-1-propyne the order of reaction was zero. In the case of the competitive reactions the reaction of equimolar quantities of trans 1-phenyl-1-propene and 1-phenyl-1-propyne, equimolar quantities of phenyl acetylene and styrene, equimolar quantities of phenyl acetylene and benzonitrile, and equimolar quantities of 1-phenyl-1-propyne and benzonitrile, were studied over the Pd/Carbon catalyst. Only the equimolar reaction of phenyl acetylene and styrene was examined over the Pd/ZrOj catalyst (Figure 1). The same data for the Pd/Carbon catalyst is plotted in Figure 2. In each case 0.4 ml of each reactant was charged to the reactor at 303 K, and the reaction followed over a 1 h period. The effect of the co-adsorbates on activity is shown in Table 1. While the effect on selectivity is shown in Table 2.

307

Figure 1. Hydrogenation of Phenyl Acetylene, Styrene, and PA/S 1:1 mixture. Catalyst: Pd/Zirconia

2.5

1 2 ID JQ CO

^1.5

H

0)

- BS

0

0

N C 0

1 '

-

PA PA/S

1^0.5 UJ

-JIL J 10

15 20 25 30 35 Time of Reaction (min)

40

45

60

FIGURE 2. Phenyl acetylene and styrene hydrogenation over Pd/C 1

10

15 20 25 30 35 Reaction Tinne (min)

40

45

60

308

Table 1. Effect of co-adsorbate on activity^^l 1 Reaction Time min.

% Conversion of phenyl acetylene or 1-phenyl-l-propyne Pd/Carbon Pd/Zirconia PA/ST PA/BN PP PP/TPP PP/BN PA PA/ST 1 3 1 12 10 5 8 7 1 6 3 23 20 16 16 22 1

PA

1 5

29

10

48

20

83

18

10

45

32

23

37

1 40

99

37

70

76

54

38

79

49 1 89 1

(a) PA, phenyl acetylene; PP, 1-phenyl-l-propyne; ST, styrene; BN, benzonitrile; TPP, trans 1 -phenyl-1 -propene. Table 2. Effect of benzonitrile on selectivity^^^ 1 Reaction Time min.

% Selectivity for styrene or cis 1-phenyl-1-propene Pd/Carbon PA^N PP

PA

1 5

1 ^^

1 20 1 40

Pd/Zirconia PP/BN

PA

95 92

15 10

89 89

98

89

98

87

25

87

97

72

6

82

97

92 92 88

1 1 1 1

(a) PA, phenyl acetylene; PP, 1-phenyl-l-propyne; ST, styrene; BN, benzonitrile. No evidence was found for benzonitrile hydrogenation and the measured concentration of benzonitrile in solution was found to be constant throughout these experiments. Figure 3. Hydrogenation of 1-phenyl-1-propyne/benzonitrile.

100

10

20 30 Time of reaction (min)

Alkyne Trans alkene Cis alkene

40

50

309 Hydrogenation of 1-phenyl-1-propyne can, in principle, result in the formation of the cis and trans isomers of 1-phenyl-1-propene. However in practice only the cis isomer was formed (Figure 3). No isomerisation was observed, even when trans 1-phenyl-1-propene was hydrogenated in the absence of the alkyne. This is a slightly surprising result as usually cis/trans isomerisation is a lower energy process than hydrogenation [5]. Hence cis and trans alkenes try to attain equilibrium when a surface, on which they can adsorb, is present. The addition of benzonitrile or trans 1-phenyl-1-propene to the hydrogenation of 1-phenyl-1-propyne did not change this behaviour.

4.

DISCUSSION

Over the Pd/C catalyst, when there is a competitive reaction between the alkyne and the alkene in a 1:1 ratio, the results show that the reactivity of the alkyne is helped by having a methyl group in place of a proton. Indeed up to 10 min. the conversion of 1-phenyl-1-propyne is only lowered slightly by the presence of the alkene, at higher conversions a larger effect is seen, whereas with the phenyl acetylene/styrene reaction, the conversion of phenyl acetylene is reduced by almost an order of magnitude in the early stages. Hence with the phenyl acetylene/styrene reaction over the Pd/C catalyst it is apparent that the styrene is effectively competing with the phenyl acetylene. Indeed a plot of log([So]/[St]) vs log([PAo]/[PAt]) was found to be linear (Figure 4) indicating a competitive adsorption situation, whereas with the 1-phenyl-1-propyne /trans 1-pheny-l-propene couple, no correlation was found. Also the order of reaction for phenyl acetylene changed from zero ^ ^ order to first order when styrene was present as a Figure 4. competing reactant, whereas Phenyl acetylene/styrene co-hydrogenation. the reaction order for Catalyst: Pd/Carbon 1 -phenyl-1 -propyne 0.4 remained zero order even B 0.3 ^'^ when 1-pheny-l-propene was present. §^ 0.2 The 1-phenyl-1-propyne /trans 1-pheny-l-propene 2 0 couple behave in a classical manner with the alkyne -0.1 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 being more strongly LOG ([So]/[St]) adsorbed and hence blocking the surface for the alkene. In the case of the terminal alkyne/alkene we suggest that the alkene can compete because of perpendicular adsorbed states of both alkyne and alkene (Ph-C=C-M, Ph-CH=CH-M). The bond energy of the M-C bond will be similar for both the adsorbed alkyne and alkene. The lower hydrogenation activity may be due to a reduction in the available hydrogen on the surface. — 0.1 O

When we examine the results from the Pd/Zirconia catalyst it is apparent that the phenyl acetylene/styrene competitive reaction is behaving in a classical manner with the phenyl

310

acetylene blocking the adsorption site. This is Phenyl acetylene/styrene co-hydrogenation confirmed by a plot of Catalyst: Pd/Zirconia log([So]/[SJ)vs log([PAJ/[PAJ) which was found to be non-linear (Figure 5) and the reaction order with respect to phenyl acetylene remained zero order. Therefore in this case the change in the dispersion of the palladium removes the ability of the styrene to LOG ([So]/[St]) effectively compete with the ^"^ — ^ phenyl acetylene by changing the mode of adsorption. These results are typical of a reaction controlled solely by the strength of adsorption of the reactants. Figure 5.

More dramatic effects are seen when the competitor molecule is benzonitrile. With phenyl acetylene the rate of hydrogenation is reduced by almost a factor of thirty, whereas the rate of hydrogenation of the 1-phenyl-1-propyne is only reduced by only a factor of two. The effect of benzonitrile on selectivity is both dramatic and surprising. The selectivity to the alkene in phenyl acetylene hydrogenation changes from over 90% to less than 15%, whereas with 1-phenyl-1-propyne hydrogenation the selectivity is slightly increased. Therefore the effect of benzonitrile on phenyl acetylene hydrogenation is to change the main hydrogenated product to being the alkane. This may come about by the promotion of either the hydrogenation of the alkene or of the direct hydrogenation of the alkyne to alkane. (^. ECHANISMl.

Ph-C=CH

Ph-C=CH

Ph-C=C-H

Ph-C=CH

-^ Ph-C=N

Ph-C=N

Ph-CH=CH2

I

I

Ph-CH=NH

Ph-CH=NH

Ph-CH=CH2

^

I H

^'H

Ph-C=N^ *

*

IH 'H Ph-C=N

Ph-CH=CH2

I

I

Ph-C^N

Ph-CH2CH3

^

+ Ph-C=N *

*

In searching the literature we have not found any obvious examples of this behaviour in general hydrogenation chemistry, however in the area of asymmetric catalysis promotional effects of nitrogen containing groups have indeed been seen [9]. In these studies it was found that the addition of the "modifier" (a large organic molecule such as cinchonidine)

311 rather than lowering the rate of reaction, actually increased the rate. This rate enhancement was believed to be due to a hydrogen bonding interaction lowering the energy of the half-hydrogenated state and increasing its effective surface concentration. An alternative explanation can be found in hydrogenation of alkenes by diimide (HN=NH). A survey by Siegel [10] compared the rates of hydrogenation catalysed by metals to that of hydrogenation by diimide. This study showed that it was possible to enhance the rate by a factor of twenty, or decrease the rate by an order of magnitude, over that obtained with a metal catalyst when diimide was used. Controlling factors were the bond angle strain, torsional strain, and alpha-alkyl substituents. Replacement of a vinyl hydrogen by a methyl group reduced the rate by a factor of 0.19 [11]. Therefore in a similar manner we propose that the hydrogenation of styrene is enhanced by hydrogen transfer from the adsorbed benzonitrile (acting in a analogous fashion to diimide) to the styrene, which is faster than transfer from the metal itself, whereas with the addition of the methyl group the rate is decreased (Mechanism 1). At the stage where the alkene has been formed and is adsorbed on the benzonitrile there is a choice between further hydrogenation or desorption into the liquid phase. In comparing the two systems, the effect of changing from a terminal to an internal alkene is observed (i.e. from =CH2 to =CHR, where in this example R=CH3). In the case of styrene the rate of hydrogenation is faster than that of desorption whereas in the case of the cis 1-phenyl-1-propene, the rate of desorption is faster than the rate of hydrogenation. Indeed showing exactly the behaviour pattern observed with diimide hydrogenation. Therefore the phenyl acetylene is hydrogenated to ethyl benzene via an adsorbed styrene but with the styrene never entering the liquid phase.

References: 1. M. Terasawa, H. Yamamoto, K. Kaneda, T. Imanaka, and S. Teranishi, J. Catal., 57 (1979)315. 2. L. V. Mironova, B. L. Belykh, I. V. Usova, and F. K. Shmidt, Kinet. Katal., 26 (1985) 469 3. J. G. Ulan and W. F. Maier, J. Org. Chem., 52 (1987) 3132. 4. V. I. Parvulescu, G. Filoti, V. Parvulescu, N. Grecu, E. Angelescu, and V. loan, J. Mol. Catal., 89 (1994) 267. 5. G. Webb, in "Comprehensive Chemical Kinetics", Eds. C. H. Bamford and C. F. H. Tipper, Elsevier, Amsterdam, Vol. 21, 1978, pi. 6. J. A. Cabello, J. M. Campelo, A. Garcia, D. Luna, and J. M. Marinas, J. Catal, 94 (1985)1 7. J. K. A. Clarke and J. J. Rooney, Adv. Catal., 25 (1976) 125. 8. S. D. Jackson and L. A. Shaw, Appl. Catal. A, 134 (1996) 91 . 9. G. Bond, P. A. Meheux, A. Ibbotson, and P. B. Wells, Catal. Today, 10 (1991) 371. 10. S. Siegel, in "Heterogeneous Catalysis and Fine Chemicals 11", Studies in Surface Science and Catalysis, Eds. M. Guisnet et al., Elsevier, Amsterdam Vol. 59, 1991, p21. U . S . Siegel, G. M. Foreman, and D. Johnson, J. Org. Chem., 40 (1975) 3589.

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313

Simple preparation of bimetallic palladium-copper catalysts for selective liquid phase semihydrogenation of functionalized acetylenes and propargylic alcohols M.P.R. Speea, D.M. Grove^ G. van Koten^, J.W. Geus^ ^Debye Institute, Department of Metal-Mediated Synthesis, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands ^Debye Institute, Department of Inorganic Chemistry, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands

Silica supported palladium-copper catalysts were obtained in a fast and simple preparation method by reduction of bimetallic organometallic compounds on the support surface in the liquid phase at room temperature. The supported bimetallic particles were analysed by TEM and ED AX. Directly after preparation the silica supported palladium-copper catalysts could be used in the semihydrogenation of triple bonds. The catalysts are selective in the hydrogenation of acetylenes and propargylic alcohols giving high yields of either olefins or saturated hydrocarbons, depending on reaction time. In addition, the catalytic system shows reasonable selectivity towards c/s-olefins in the hydrogenation of disubstituted acetylenes.

1. INTRODUCTION Catalytic liquid phase semihydrogenation of acetylenes is an important industrial and laboratory reaction, especially in fine chemical synthesis [1]. The use of supported metal catalysts for this selective hydrogenation readily facilitates the separation of organic products from the catalyst. However, liquid phase reactions with supported catalysts tend towards mass transport limitation [2] and, therefore, the support particles should be between 1 and 10 fim in size; this avoids transport limitations and separation problems. With support particles of this size high temperature reduction in a flow of H2 gas is very difficult and to avoid this step it is possible to prepare supported metal particles by decomposing organometallic compounds under mild conditions [3-5]. As part of a major research topic concerning characterization and mode of operation of novel (bi-)metallic catalysts for liquid phase catalysis, we have now developed a fast and simple preparation method for supported bimetallic catalysts, even of less noble metals. This method involves reduction of bimetallic

314 organometallic compounds on the support surface in the liquid phase at room temperature. This method has been used to prepare silica supported palladiumcopper catalysts ( P d / C u / S i 0 2 ) which were tested in the liquid phase semihydrogenation of functionalized acetylenes and propargylic alcohols. 2. RESULTS AND DISCUSSION 2.1. Catalyst preparation The organometallic copper precursor is synthesized in two steps. First ptolylcopper, Cu4(p-tolyl)4, is prepared from p-tolyllithium and copper(I) bromide in diethyl ether. Reaction between p-tolylcopper and a second equivalent of ptolyllithium affords the cuprate [6-8]. (See equations 1-2.)

4 CuBr + 4 L i — ^ ^ - M e ^ ^ ^ ( C u " - ^ ^ - M e ) + 4 LiBr

(1)

^ - ^ ^ ^ ) 4 ^' L ^ ^ Q ^ ^ ^ j

(2)

^ ' Me^gX^^^^>
Addition of a toluene solution of the cuprate, Cu2Li2(p-tolyl)4-2Et20 (1), to a solution of palladium(II) acetate in toluene leads to the in situ formation of a thermally unstable organocopper complex in which the lithium atom of 1 has been replaced by the more electronegative precious metal. Subsequent reductive elimination of the organic tolyl group from the unstable bimetallic complex in the presence of silica affords supported bimetallic particles, which without further treatment are an active catalytic system. (Equation 3)

[Pd(OAc)2 + Si02] + 1

RT Pd/Cu/Si02 + 2 bitolyl Toluene

(3) ^^

315 Table I. Liquid phase semihydrogenation of monosubstituted acetylenes (5 mmol) catalysed by Pd(4 w%)/Cu(2 w%)/Si02 {circa 35 mg) in ethanol. Substrate

t^ax^ (min)

Ph-C=CH

5

OH Me-C-C=CH

11

Semihydrogenated product

Yield^'^ (%)

Ph-CH=CH2

95

OH Me-C-CH=CH2

98

I

I

Me

Me

OH

OH

I

I

Et-C-C=CH

2

Et-C-CH=CH2

I

97

I

Me

Me

O Me Me-C-0-C-CSCH

3

O Me Me-C-0-C-CH=CH2

I

I

Et

Et

95

PH 94 C=CH

^

^ ^CH=CH2

^Time for maximum percentage of semihydrogenated compound. t>Yield at 100% alkyne conversion determined by G.C. peak area analysis. ^In all cases the byproduct was the fully saturated compound. 2.2. Catalytic hydrogenations To test the selectivity of the bimetallic system, monosubstituted acetylenes (Table I) and disubstituted acetylenes (Table II) were hydrogenated with the use of the silica supported palladium-copper catalysts. The yields of the olefins at 100% conversion of the acetylene are given. However, in all cases with longer hydrogenation times it was possible to end up with the fully saturated product.

316 Table 11. Liquid p h a s e semihydrogenation of disubstituted acetylenes (5 mmol) catalysed b y Pd(4 w % ) / C u ( 2 w % ) / S i 0 2 {circa 35 mg) in ethanol. Substrate

tmax^ (min)

Ph-C=C-Ph

H

6

H

I I H O - CI - C =IC - C - O H

H

Semihydrogenated product

Ph-CH=CH—Ph

H 20

H

cis 85 trans 4 H

I I H O - C - C H =I C H - CI - O H

H

Yield'^'^ (%)

H

cis 99d trans

OH H~C-C=C~Ph I Me

3

OH H-C-CH=CH—Ph I Me

cis 80 trans 3

OH H-C-C=C-Ph I Ph

5

OH H-C~CH=CH—Ph I Ph

cis 88 trans d

6

OH H-C-CH=CH-C6H4-4-OMe Ph

cis 85 trans -"^

OH H-C-C=C-C6H4-4-OMe Ph

^Time for m a x i m u m percentage of semihydrogenated c o m p o u n d . t>Yield at 100% alkyne conversion determined b y G.C. peak area analysis. ^In 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.

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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.

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

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

^olefin

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.

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

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 probablyfi^omsubstantial 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, ^a«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 ^aw5^-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, R^O.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.

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

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


A

i Si^=5

/.A

^

..

.

Si/Ti=2 si^^5

Figure 3 X-ray powder diffraction patterns of titania/silicas 3 ^ The reactivity of titania/silica catalysts in the epoxidation of olefin Figure 4 shows the effect of preparation method on the reactivity of titania/silica catalysts in the epoxidation of cyclooctene. In the epoxidation of cyclooctene with tert-butyl hydroperoxide in tert-butanol, the sol-gel titania/silicas are much more active t h a n the precipitated ones. The conversions of cyclooctene and the selectivities to cyclooctene oxide are 41-42% and 97%, respectively, for the sol-gel titania/silicas (Si/Ti= 5, 10). On the other hand, conversions of cyclooctene and selectivities to cyclooctene oxide with precipitated titania/silicas (Si/Ti= 5, 10) are 10-14% and 89%, respectively. The difference in the catalytic activity must be caused by the difference in the dispersion of Ti component at the catalyst surface [1-3]. However, titania/silica with high Ti content gave different result. For example, the sol-gel titania/silica with Ti/Si ratio of 1 shows lower cyclooctene conversion than the corresponding precipitated one. Both samples show the diffraction patterns of anatase Ti02, that is, they have clear aggregates of Ti02. From the result of XPS, the titania concentration at the precipitated catalyst surface is higher than that at the sol-gel catalyst surface. If the titanium concentration at the surface is sufficiently high and aggregation of titania particles occurs easily, the conversion of olefin mainly not depends on the Ti dispersion but depends on the concentration of surface Ti. Thus, most Ti atoms in the sol-gel catalyst with

350 low titania content must be isolated from each other and the sol-gel titania/silica can give higher epoxide yield in the epoxidation of olefins. This result is supported by the difference of Ti-O-Si bond formation measured by 29Si MAS-NMR reported in our previous work [4].

o m

> 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> ^Institut de Recherches sur la Catalyse -CNRS, conventionne a TUniversite CI. Bernard, Lyon1, 2 avenue A. Einstein, 69 626 Villeurbanne Cedex, France ^Elf-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). ^ m^ata = 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; <^=353K.

All alcohols tested are oxidised, with moderate yields, but high selectivities (close to 100%). After 6h, an equilibrium is reached, except with phenylethanol for which 24h are necessary. 4methyl-2-pentanol is also oxidised; if one remembers that 4-methyl-2-pentanone is not reduced by 2-propanol, this result suggests that the important step is the reaction of substitution of the alkoxy ligand with the alcohol. Recycling catalyst 3 after simple filtration leads to poor results, which are dependent upon the substrate. But, treatment of the solid with boiling 2-propanol restores most of the activity (90%), suggesting that deactivation of the catalyst must be attributed to the formation of a stable benzyloxyzirconium complex in the case of phenylethanol for example. 2.5 Discussion All zirconium based solids synthesized share in common a number of features which are responsible for their catalytic properties and their stability. Thus, zirconium is anchored to a silica surface via a Zr-Og bond, known to be a strong bond. This bond intervenes in two ways: it stabilizes Zr as a mononuclear species (dimerization is further prevent from by the fact that the hydroxy] groups on the surface are isolated); it leads to a very reactive species, formally an 8e" species. Thus the siloxy ligand shows an immobilizing effect and an electronic effect.

359 The energy of dissociation of the Zr-Og bond is larger than that of the Zr-C and Zr-H bonds; this allowed the transformation of (sSiO)ZrNp3 into (=SiO)3ZrH, without rupture of the Zr0(Si=) bond, the anchoring bond [10]. We now show that even in the presence of oxygen containing molecules such as alcohols or hydrogen peroxide, this bond Zr-0(Si=) is stable, at least under the relative mild conditions of temperature under which reactions such as hydrogen transfer and epoxidation of cyclohexene take place. The reactions of alcohol interchange have been much studied [15]; in contrast, the substitution of a siloxy ligand by an alkoxy group is not well documented to our knowledge. One may expect that steric factors would favor substitution of an alkoxy ligand rather than a siloxy ligand by an alcohol. This seems to be the case for the reported systems. Although the conditions of catalysis have not yet been optimized, the results are interesting in the sense that these heterogeneous molecular catalysts all show properties different from their molecular counterparts, in terms of either activity (epoxidation of olefins, MPVO reactions, silane activation) or selectivities (epoxidation for example). These features need to be further investigated, by varying the number of solid ligands in the coordination sphere of Zr: preliminary experiments with epoxidation of olefins show that this parameter is important for the activity of Zr. Finally most interesting is the fact that all these catalysts can be recycled by simple washing/filtration without significant loss of their activity and selectivity, a fact which should give renewed interest for the search for supported molecular catalyst. 3. EXPERIMENTAL Tetraneopentylzirconium, ZrNp4, was synthesized according to Lappert et al. [16]. Silica (fi-om Shell, 350 mV^) was partially dehydroxylated at 450°C (10-4 IQ„^ 15^) Reactants (from Aldrich) i.e. ketones, alcohols, cyclohexene were used as received. H2O2 (70% wt) was fi-om Atochem. All manipulations were performed under dry Ar,when appropriate. Solid A was prepared by impregnation of a pretreated silica (2g) with a n-hexane solution of ZrNp4 (200mg, 5ml). After magnetically stirring this slurry (25°C, 0.5h), the solvent is removed via a canula and the solid evacuated (10"^ torr, 60°C, lOh) in order to achieve complete reaction of the complex with silica and removal of traces of unreacted complex. The solid is then contacted either with H2O (22 torr, 25°C, lOmin), or with 2-propanol, and then evacuated (25°C, 2h). Catalytic testing was performed in batch reactors under the conditions given under the corresponding tables. Products were analyzed by gas phase chromatography using an Intersmat chromatograph, equipped with a capillary column (Quadrex Q2, 0.32nm, 15m ) and a FID detector or a HP 5890 equipped with a BP20 column. Calibrations were performed against authentic samples. The amount of H2O2 not converted at the end of the reaction time (cyclohexene oxidation or phenol hydroxylation) was analyzed by iodometric titration [17]. Quantitative analysis of the zirconium content of the solutions at the end of reaction was performed by ICP (Perkin Elmer). 4. CONCLUSIONS These studies clearly show that a careful design of the supported molecular complexes leads to efficient and recyclable molecular heterogeneous catalysts. The important parameters are the nature of the metal and its ligands, the nature of the solid support, the reactants and the

360 products of the target reaction (very precisely): these parameters must be considered as a whole. One can consider that the reactivity of organometallic complexes with a number of conventional supports (inorganic oxides, organic polymers) is now fairly well known. But, the reactivity of supported complexes towards different classes of reactants is not so far well documented; this is yet very important if one is concerned with catalyst recycling. REFERENCES [I] see for example D. C. Bailey and S. H. Langer, Chem. Rev., 81 (1981) 109; Y. I. Yermakov, B. N. Kuznetsov and V. A. Zakharov, Stud. Surf Sci. Catal., 8 (1981) 1; C. U. Pittman, Jr. in. Comprehensive Organometallic Chemistry (Eds., G. Wilkinson, F. G. A. Stone and E. W. Abel), Pergamon Press, Oxford, 1982, Vol. 8, p. 553; J. Lieto, D. Milstein, R. L. Albright, J. V. Minkiewicz and B. C. Gates, CHEMTECH, 13 (1983) 46. [2] G. E. Maciel and D. W. Sindorf, J. Am. Chem. Soc., 102 (1980) 7606; L. Y. Hsu, S. G. Shore, L. D'Omelas, A. Choplin and J. M. Basset, J. Catal., 149 (1994) 159; L. T. Zhuralev, Langmuir, 3 (1987) 316. [3] F. Quignard, C. L6cuyer, C. Bougault, F. Lefebvre, A. Choplin, D. Olivier and J. M. Basset, Inorg. Chem., 31 (1992) 928. [4] T. D. Tilley, Ace. Chem. Res., 26 (1993) 22; H. G. Woo, J. F. Walzer and T. D. Tilley, J. Am. Chem. Soc, 114 (1992) 7047. [5] Y. Ishii, T. Nakano, A. Inada, Y. Kishigami, K. Sakurai and M. Ogawa, J. Org. Chem., 51 (1986) 240; T. Okano, M. Matsuoka, H. Konishii and J. Kiji, Chem. Lett., (1987) 181. [6] F. Quignard, C. Lecuyer, A. Choplin, D. Olivier and J. M. Basset, J. Mol. Catal., 74 (1992) 353; F. Quignard, C. L6cuyer, A. Choplin and J. M. Basset, J. Chem. Soc. Dalton Trans., (1994) 1153. [7] F. Quignard, A. Choplin and J. M. Basset, J. Chem. Soc., Chem. Commun., (1991) 1589. [8] P. L Watson, J. Am. Chem. Soc, 105 (1983) 6491. [9] B. Coutant, F. Quignard and A. Choplin, J. Chem. Soc, Chem. Commun., (1995) 137. [10] C. Lecuyer, F. Quignard, A. Choplin, D. Olivier and J. M. Basset, Angew. Chem. Intern. Ed. Engl., 30 (1991) 1660. [II] R. C. Rogers, Brit. Patent N° 1 249 079 (1971); H.P. Wulff, US Patent N° 3 923 843 (1975). [12] M. Taramasso, G. Perego and B. Notari, US Patent N° 44 10 501 (1983). [13] R.A. Sheldon and J.A.Van Doom, J. Catal. 31 (1973) 427; M. K. Dongare, P. Singh, P. P. Moghe and P. Ratnasamy, Zeolites 11 (1991) 690. [14] C. F. de Grauuw, J. A Peters, H. van Bekkum and J. Huskens, Synthesis, (1994) 1007. [15] Metal Alkoxides, R. C. Mehrotra, D. C. Bradley, R. C. Gaur, Acad. Press, New York, (1978) and references therein. [16] P. J. Davidson, M. F. Lappert and R. Pearce, J. Organomet. Chem. 57 (1973) 269. [17] G. H. Jeffery, J. Bassett, J. Mendham, R. C. Denney, Vogel's Handbook of Quantitative Chemical Analysis, Longman Scientific 8L Technical, 5th ed. (1989) pp.394.

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

361

Selective sulfoxidation of thioethers on Ti-containing zeolites under mild conditions V. Hulea", P. Moreau*' and F. Di Renzo^ ^Faculty of Industrial Chemistry, 71 Blv. D. Mangeron, P.O.Box 2007, 6600 lasi, Romania ^aboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, CNRS UMR 5618 Ecole Nationale Superieure de Chimie de Montpellier, 8 rue de I'Ecole Normale 34053 Montpellier, France

The oxidation of various thioethers with aqueous H2O2, in organic solvents, over two titanium containing zeolites, TS-1 and Ti-beta has been studied. The performances of these catalysts depend strongly on the thioether structure and the nature of solvent. Thus, the reactivity of organic substrates is in agreement with the nucleophilic character of the sulfiir atom and their molecular size: (Ethyl)2 S > (Propyl)2 S > (Allyl)2 S > (Butyl)2 S > Methyl-SPhenyl > (Phenyl)2 S. The conversion of sulfide in protic solvents is higher than that obtained in aprotic solvents. For all the solvents, Ti-beta is more active and selective than TS-1 in the oxidation of hindered molecules.

1. INTRODUCTION Sulfoxides and sulfones are important intermediates in fine organic synthesis. These products are formed by oxidation of sulfides with various agents, including peracids, hydrogen peroxide and organic hydroperoxides [1]. Hydrogen peroxide, H2O2, either alone or associated with various solvents or catalysts, is the most widely agent used for oxidizing organic sulfides [2]. H202has taken new importance in recent years, because of its environmental implications, water being the only chemical by-product of oxidation reaction; moreover, it is less expensive and more accessible than the other oxidizing agents, such as peracids or hydroperoxides. The use of transition-metal (Ti, Mo, V, Fe, W, Re, Ru) complexes as active catalysts for the selective oxidation of thioethers by H2O2 in homogeneous conditions has been reported by various authors [3-10]. Recently, it has been shown that some of these metals (Ti,V) incorporated in a zeoliticfi*amework,such as MFI or MEL, are able to catalyse the oxidation reaction of thioethers with H2O2 [11,12]. However, no detailed studies on this reaction are available. The scope of this work was to investigate the influence of thioether structure, solvent nature, reaction temperature and shape selectivity effect of catalyst on the performances of TS-1 and Ti-beta samples in the sulfoxidation of organic sulfides with H2O2.

362 2. RESULTS AND DISCUSSION The oxidation of various thioethers with hydrogen peroxide has been carried out using TS-1 and Ti-beta as catalysts, in an organic solvent. The concentration of both substrate and H2O2 was 0.19 M. In this section we report the catalytic performances of these catalysts with respect to thioether structure, solvent nature and reaction temperature. 2.1. Influence of the nature of catalyst TS-1 (pore diameter 0.55 nm) and Ti-beta (pore diameter 0.7 nm) have been tested in the oxidation reaction of two aliphatic sulfides, ethyl sulfide (Et2S) and butyl sulfide (BU2S), in methanol (MeOH) and tert-butanol (t-BuOH) as solvents. The results obtained show that thioether conversion highly depends on the nature of the catalyst (Fig. 1).

o E

30 Reaction time,min Figure 1. Influence of the nature of the catalyst on the oxidation reaction of Et2S (1,2,3,6) and BU2S (4,5) by H2O2 at 303 K: (l)-Ti-beta/MeOH, (2)-Ti-beta/t-BuOH, (3)-TS-l/ MeOH, (4)Ti-beta/MeOH, (5)-TS-l/MeOH and (6)-TS-l/t-BuOH. Thus, under the same conditions, Ti-beta is more active than TS-1. As well, the effect of the molecule size of the substrate and of the solvent on the catalyst performance is much more important in the case of TS-1 than of Ti-beta. For more bulky molecules (butyl sulfide as substrate or tert-butanol as solvent) TS-1 is much less active than for small molecules (ethyl sulfide in methanol). This difference is not as important for Ti-beta catalyst. Both Ti-beta and TS-1 show a high activity towards the oxidation of small size substrates such as ethyl sulfide in methanol as solvent, which can be the proof that the efficiency of the active Ti sites is the same for the two catalysts. In this context, the poor activity of the TS-1 catalyst in the case of the large molecules can be attributed both to a restricted transition state shape selectivity and to a diffiisivity effect of reagents and products. These effects are not as important for a large pore zeolite, such as Ti-beta. A similar resuh was

363 reported by Corma and al. [13] in the case of the epoxidation of cyclododecene with H2O2; under identical conditions, Ti-beta was found to be more active than TS-1, when diffusion limitations become the governing factor. 2.2. Influence of the nature of the thioether The results concerning the sulfoxidation reaction of some thioethers i.e. ethyl sulfide (Et2S), propyl sulfide (Pr2S), butyl sulfide (BU2S), allyl methyl sulfide (MeAllylS), allyl sulfide (AllybS), methyl phenyl sulfide (MePhS) and phenyl sulfide (Ph2S), over Ti-beta catalyst, in tert-butanol as solvent, at 303 K, are shown in Figure 2.

30

60

90

120

Reaction time,min Figure 2. Kinetic of sulfoxidation reactions of different thioethers by H2O2 (T=303 K, solvent; t-BuOH, catalyst: Ti-beta) : (l)-Et2S, (2)-Pr2S, (3)-MeAllylS, (4)-Allyl2S, (5)-Bu2S, (6)MePhS, (7)-Ph2S. It can be seen that, under similar conditions, the reactivity of the dialkyl sulfides is directly linked to their molecular size: Et2 S > Pr2 S > Bu2 S, and saturated sulfides are more reactive than allyl or aryl sulfides : Pr2 S > Me S Allyl > Allyb S > Me S Ph > Ph2 S. These results can be explained, first, if we take into account the relative easiness of thioethers accessibility to the Ti active sites of the catalytic species located in the zeolitefi-amework.The diflfiision of the bulkier molecules, such as Ph2S is very difficult even inside the large pores of Ti-beta zeolite. Secondly, the reactivity of thioethers is in agreement with the nucleophilicity of the sulfiir atom, so that alkyl sulfides are more easily oxidized than allyl or aryl sulfides by H2O2 (an electrophilic oxidant) in agreement with reported results [1-9]. It must be pointed out, that in the case of allyl methyl sulfide and di-allylsulfide, the epoxidation of the allyl system is not observed under our experimental conditions.

364 2.3. Influence of the nature of the solvent To study the influence of the nature of the solvent, the oxidation of ethyl sulfide was carried out at 303 K in the presence of methanol (MeOH), ethanol (EtOH), tert-butanol (tBuOH), acetonitrile (MeCN), acetone (Me2C0) or tetrahydrofliran (THF) as solvents. Figure 3 shows the sulfide conversion for a reaction time of 60 min, both over Ti-containing zeolites and without catalyst. 100 90 01

o E

withouttl

80

DTS-I

70

STi-beta

60 50

^

40 30 20 10 ' 0

M THF

Me2C0

t-BuOH

MeCN

E10H

MeOH

Figure 3. Effect of the nature of the solvent on Et2S conversion in the oxidation reaction by H2O2 at 303 K (reaction time 60 min). These results lead to the following remarks : (i)- ethyl sulfide can be oxidized with H2O2 under mild conditions, even in the absence of any catalyst; (ii)- the use of Ti-containing zeolites leads to a higher conversion of the thioethers whatever the solvent; (iii)- for all the solvents, Ti-beta is more active than TS-1 in Et2S oxidation; (iv)- thioether conversion highly depends on the nature of the solvent: - for TS-1, the order of efficiency of the solvents is the same as that without catalyst : MeOH > EtOH > MeCN > t-BuOH > Me2C0 > THF; - for Ti-beta, it can be seen that, in the three protic solvents, conversions of Et2S are very similar and much higher than those obtained in aprotic solvents.

SiO-

Ti

SiO^

\

.^' ^ ^

^

Figure 4. Proposed complex intermediate for the oxidation reaction of organic sulfides by H2O2 over Ti-containing zeolites, in ROH type solvent.

365 These results confirm the proposed mechanism for the sulfoxidation of thioethers by H2O2 catalyzed by Ti-containing zeolites [14], which involves the coordination of an alcohol molecule to the active titanium site of the hydroperoxo species, as shown in Figure 4. It is known that, in such a complex, the transition metal increases the electrophilicity of the peroxydic oxygen atom, which makes the nucleophilic attack of organic substrates easier. The coordination of a protic solvent to the active site leads to a stabilization of this intermediate and thus favors this type of mechanism. On the other hand, the results obtained over Ti-beta catalyst for ROH type solvent (Fig.3), i.e. MeOH « EtOH «t-BuOH, show that, due to the Ti-active species, the influence of the nature of the solvent becomes smaller, compared with TS-1, where MeOH > EtOH » tBuOH. Considering that the oxidation mechanism of ethyl sulfide by H2O2 is similar for both TS-1 and Ti-beta, such results confirm that medium pore Ti-containing zeolites, like TS-1, lead to a significant transition-state shape selectivity effect. Detailed studies on the solvent effect in the sulfoxidation of thioethers by H2O2 using Ti-containing zeolites as catalysts are in progress in our laboratory. 2.4. Influence of the reaction temperature The effect of the temperature in the sulfoxidation reaction has been investigated over Ti-beta catalyst. BU2S has been chosen as the model substrate, because of its lower reactivity under standard conditions (303 K). No direct decomposition of hydrogen peroxide was observed at the various temperatures used : 303, 313, 323 and 333 K respectively. A linear Arrhenius plot of the initial reaction rates (Figure 5) leading to a derived activation energy of 51 kJ.mol'^ is obtained.

/ -

6-

y /

5/

A -

*r

'

3,1

11

1

3,2

r

1

3,3

T

\

1

1

1

1

1

3,4

10^1/T

Figure 5. Arrhenius plot of the initial reaction rates for the BU2S oxidation by H2O2, over Tibeta, in t-BuOH.

366 These results show that the temperature influences strongly the catalytic activity of the Ti-beta samples in sulfoxidation reaction.

2.5. Selectivity of the sulfoxidation reaction Sulfoxidation reaction of organic sulfides with H2O2 leads to sulfoxides, which, upon further oxidation, can be converted to the corresponding sulfones (reaction 1). H2O2 R1SR2

H2O2 > R1SOR2

(a)

> R1SO2R2

(1)

(b)

The ratio between sulfoxide and sulfone depends on the reaction conditions (Table 1) Table 1 Selectivity (S) of the oxidation reaction of thioethers (R1SR2) with H2O2 over Ti-beta catalyst^. Ri

R2

Ethyl

Ethyl

Butyl

Butyl

Propyl Methyl Allyl Methyl

Propyl Allyl Allyl Phenyl

Solvent MeOH EtOH t-BuOH MeCN Me2C0 THF EtOH MeCN THF MeOH t-BuOH t-BuOH t-BuOH t-BuOH t-BuOH

H2O2/R1SR2'

1 1 1 1 1 1 2 2 2 1 1 1 1 1 1

S8o'',% 99.7 99.6 99.5 97.3 97.2' 93.3' 100 96.8 92.4 97.6 95.8 98.0 95.1 84.6 92.8

t'^,min 25 30 35 62 138' 155' 3 21 75 100 135 38 85 123 156

(a)-303 K, 2 mmol R1SR2, 10 ml solvent, 50 mg Ti-beta; (b)- molar ratio (c)-S8o= RiSOR2/(RiSOR2+RiSO2R2)xl00, for a conversion of R1SR2, a=80%; (d)- reaction time for a=80% (e)-S6o(a=60%). A good selectivity in sulfoxide is obtained at a 80% conversion of sulfides under all conditions. This selectivity depends on the sulfide and solvent nature, or the molar ratio H2O2 / thioether, and is directly linked to the reactivity : high sulfide conversions for a short reaction time lead to high selectivities in sulfoxides.

367 These results can be explained by the relative rates of the formation of the sulfoxide (step a) and of the corresponding sulfone (step b) in the oxidation reaction of thioethers (reaction 1). It is known that, for dialkyl sulfides, such as Et2S, Pr2S and BU2S, sulfoxide formation proceeds much faster than sulfone formation [1]. For the allyl sulfide the selectivity in sulfoxide is lower, because the difference in the rates of the two steps (a) and (b) of the oxidation reaction is less important, due to the conjugation of the lone electron pairs on the sulftir atom with the unsaturated system [1]. For an excess of hydrogen peroxide, the lower selectivity in sulfoxide observed in THF and MeCN, compared with EtOH (Table 1) can be attributed to the lower reaction rate of the first step in aprotic solvents (Figure 3).

3. EXPERIMENTAL 3.1. Materials Ethyl sulfide (98%), n-Propyl sulfide (97%), Allyl sulfide (97%), Allyl methyl sulfide (98%), n-Butyl sulfide (96%), Methyl phenyl sulfide (99%) and Phenyl sulfide (98%) fi-om Aldrich, were used as supplied. Hydrogen peroxide (aqueous solution 31wt.-%) was obtained from Prolabo. Methanol, analytical grade (SDS), ethanol, analytical grade (SDS), tert-butanol, HPLC grade (Sigma-Aldrich), acetonitrile, 99% (Aldrich), acetone, HPLC grade (Aldrich) and tetrahydrofuran, analytical grade (SDS) were used as solvents.

3.2. Catalysts TS-1 and Ti-beta, with structure MFI and BEA respectively, have been used as catalysts. TS-1 was provided by the Groupement de Recherche de Lacq ; Ti-beta was prepared in this laboratory according a recently reported procedure [15]. The composition of these zeolites (aluminiumfi-ee)was Ti / (Ti+Si)= 0.011 and 0.008 for TS-1 and Ti-beta respectively. Crystal size as determined by scanning electron microscopy (Cambridge Stereocam 260) was 0.1 |im for TS-1 isometric crystal and 0.4 jim for zeolite beta spheroidal grains. The DR-UV VIS spectra of the two catalysts exhibit only an absorption band at 48000 cm"\ The presence of this transition indicates that Ti(IV) is incorporated in thefi-ameworkof molecular sieves [16]. 3.3. Catalytic experiments The oxidation of thioethers was carried out in a glass flask equipped with a condenser, a magnetic stirrer and a thermometer. In a typical reaction, TS-1 (40 mg) or Ti-beta (50 mg) was stirred with sulfide (2 mmol), the solvent (10 ml), H2O2 (2 mmol) at a constant temperature. The products were analyzed using a GC, equipped with a capillary column (Methyl Silicone Gum, 25 m x 0.2 mm x 0.33 |Lim Film thickness). The hydrogen peroxide was measured by standard iodometric titration.

4. CONCLUSION Oxidation of organic sulfides with hydrogen peroxide, over Ti-containing zeolites, may be a selective, mild and facile method for preparing sulfoxides. The reactivity of thioethers is in

368 agreement with both the nucleophilic character of the sulfiir atom (alkyl sulfides are more easily oxidized than aryl sulfides) and their molecular size. The reaction rate greatly depends on the nature of the solvent. For TS-1, the order of efficiency of the solvents is the same as that observed without catalyst: MeOH > 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).

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

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)2P2^i 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" (NVPO^o) 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^.^'^'^O^

^ 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

"V^O " 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 "V^Oy 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, NVPOgy^O-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.

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

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 ^imol 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.

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

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.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

T. Mallat and A. Baiker, Catal. Today 19 (1994) 247. C. Bronimann, Z. Bodnar, P. Hug, T. Mallat and A. Baiker, J. Catal. 150 (1994) 199. T. Mallat, Z. Bodnar, P. Hug and A. Baiker, J. Catal. 153 (1995) 131. T. Mallat, Z. Bodnar and A. Baiker, in Catalytic Selective Oxidation, S.T. Oyama and J.W. Hightower, eds., ACS Symp. Ser. 523 (1993) 308. A. Abbadi and H. van Bekkum, Appl. Catal. A 124 (1995) 409. H. E. J. Hendriks, B.F.M. Kuster and G.B. Marin, Carbohydrate Res. 204 (1990) 121. T. Mallat, Z. Bodnar, A. Baiker, O. Greis, H. Strubig and A. Reller, J. Catal. 142 (1993) 237. R. Garcia, M. Besson and P. Gallezot, Appl. Catal. A 127 (1995) 165. T. Mallat, A. Baiker and J. Patscheider, Appl. Catal. A 79 (1991) 59. M. Besson, F. Lahmer, P. Gallezot, P. Fuertes and G. Fleche, J. Catal. 152 (1995) 116. A. Abbadi and H. van Bekkum, J. Mol. Catal. A 97 (1995) 111. B.M. Despeyroux, K. Deller and E. Peldszus , Stud. Surf. Sci. Catal. 55 (1990) 159. M. Wenkin, R. Touillaux, P. Ruiz, B. Delmon and M. Devillers, Appl. Catal. A, in press. H. Hayashi, S. Sugiyama, Y. Katayama, K. Kawashiro and N. Shigemoto, J. Mol. Catal. 91 (1994) 129. T. Mallat, Z. Bodnar, S. Szabo and J. Petro, Appl. Catal.A 69 (1991) 85. H. Hayashi, S. Sugiyama, N. Shigemoto, K. Miyaura, S. Tsujino, K. Kawashiro and S. Uemura, Catal. Lett. 19 (1993) 369. O. Tirions, M. Devillers, P. Ruiz and B. Delmon, Stud. Surf. Sci. Catal. 91 (1995) 999. M. Devillers, O. Tirions, L. Cadus, P. Ruiz and B. Delmon, J. Solid State Chem., in press. H. Okamoto, ASM Handbook, Alloy Phase diagrams, H. Baker and H. Okamoto, eds., 2.103, Ohio, 1992

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

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 temperatui^e. 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 fi^om 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

Toluene Toluene Toluene

Ih 1.5 h 4h

33% 43% 57%

85% 88% 86%

Methylisobutylketone Methylisobutylketone Methylisobutylketone

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

Toluene Toluene Toluene

Ih 1.5 h 4h

44% 61% 74%

32% 30% 22%

Methylisobutylketone Methylisobutylketone Methylisobutylketone

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.

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

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.

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

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.

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

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.

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

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^ ^O OH

1 4

OH

o^-^o

->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

<1

50

4

280

93

<1

46

5

630

97

<1

45

6

1420

81

53 (at 58% conversion) 64 (at 81% conversion) 74 (at 77% conversion) 63 (at 79% conversion)

<1

26

*molh -' mol"^ Pt The adsorption of acids is assumed to be responsible for the catalyst deactivation observed at pH 2 which blocks further conversion of glyceric acid. This problem is alleviated simply by neutralising the acids. Thus, at pH 4, 5 and 6, conversion proceeds smoothly and there is an increase in the initial rate of reaction as the pH is increased. The anomalously high rate at pH 2 is assumed to be due to the initial rapid adsorption of glyceric acid on the platinum metal which increases the reaction rate at first since it brings the substrate into the immediate proximity of the bismuth. Acidic conditions were shown earHer to favour oxidation of the secondary hydroxy function of glyceric acid on PtBi/C [12] and in the work reported here high selectivities for hydroxypyruvic acid were obtained across the acidic pH range. A complexing mechanism is beUevedto be responsible for the dramatic change in selectivity. Thus, as shown in Figure 4, the neighbouring carboxylate and secondary hydroxy function of the glycerate anion form a complex with a bismuth atom on the catalyst surface and the consequent proximity of the secondary hydroxy group to the platinum surface leads to its preferential oxidation. In addition, coordination with bismuth may enhance the susceptibility of the C2 hydrogen to being abstracted as a hydride ion.

H Figure 4: Proposed complexing mechanism for oxidation of the secondary hydroxy function of glyceric acid.

o^ /

c--c/

o \

—

OH

H

0

' \

i

\ /

^

\^y C / \

H

X

rp,,~^

^BiS^

The proposed complex is assumed to be stable over a certain pH range which thus corresponds with the pH range for maximum selectivity. Due to the competing mechanisms of deactivation by adsorbed acids and over-oxidation, the optimum pH is difficult to ascertain from the given data (a possible future way to determine this value would be to measure the stability of

434

the complex which is formed with the bismuth on the surface of the catalyst). In view of the reduced rate of conversion at pH of 6, it is tentatively proposed that -the optimum pH lies in the range pH 2-5. Over-oxidation of the hydroxypyruvic acid to oxalic and glycolic acids reduces the selectivity in all cases and there appears to be a dependence on pH since increasing quantities of these by-products are evolved at higher pH. This may be because the rate of decarboxylation of hydroxypyruvic acid increases with pH and/or that the rate of the main reaction decreases with an increase in pH. At pH 2, levels of these by-products are negligible. Data for the analysis of the reaction medium after reaction is also presented in Table 1. After 22 hours of reaction, levels of leached platinum were below the limit of detection of the analytical method. However, significant quantities of the bismuth promoter were leached (26%50%, across the pH range 2-6). In a separate experiment, the loading of the bismuth promoter on a platinum catalyst (4.6%Pt) was varied to see if this might reduce the degree of catalyst deactivation under acidic conditions by preventing the strong adsorption of acids on the platinum surface. Deactivation was indeed delayed with an increase in the loading of the bismuth promoter ft-om 1.4 %Bi (Figure 5) to 3.9%Bi (Figure 3a), but with an even higher loading (7.6%Bi) there was no further improvement. 100

^

80-1

o

60 J 40 J 20 0

Q^^

-r-Q-

0

= f = ^

4 time (h)

Figure 5: Product distribution for the oxidation of glyceric acid at pH 2 on Pt(4.6%)Bi(1.4%) as a function of time (A- glyceric acid, o- hydroxypyruvic acid, o- oxalic acid and # - criycolic acid). ^ -^ 3.2 Oxidation of tartronic acid Tartronic acid was oxidised to mesoxalic acid on Pt(5.7%)Bi(2.4%)/C at pH 1.5 (Figure 6(a)). The high initial selectivity was not maintained due to deactivation of the catalyst, by adsorption of the acid product, and over-oxidation (maximum yield: 29% at 53% conversion). Higher initial rates of conversion were obtained at pH 5 (Figure 5(b)) to give higher yields (39% at 79% conversion). However, the rate of over-oxidation was also increased. Thus, as was observed for the oxidation of glyceric acid, yields may be improved significantly by neutralising the adsorbed acids which are responsible for deactivation. However, over-oxidation, to carbon dioxide and water, is even more pronounced in this case so that the selectivity and hencefinalyields are reduced.

435

Figure 6: Product distribution for the oxidation of tartronic acid on Pt(5.7%)Bi(2.4%)/C at (a) pH 1.5 and (b) pH 5, as a function of time (A - tartronic acid and - mesoxalic acid). 4. CONCLUSIONS Under acidic conditions, bismuth-promoted platinum catalysts selectively oxidise the secondary hydroxy function of glyceric and tartronic acids to their respective keto-acids: hydroxypyruvic and mesoxalic acids. A complexing mechanism is proposed to increase the rate of oxidation of the secondary hydroxy function. By using close-to-neutral conditions for the catalytic oxidation of glyceric and tartronic acids on platinum-bismuth catalysts to their respective keto derivatives, deactivation of the catalyst by adsorbed acids may be reduced, leading to higher degrees of conversion and improved yields. In addition, higher loadings of bismuth promoter may also serve to reduce deactivation by adsorbed acids. In general, the platinum component of the catalyst appears to be highly stable under the conditions employed, with very little or no leaching observed. The bismuth promoter, however, is clearly prone to lixiviation. Unfortunately, it would appear that the conditions required to optimise selectivity also lead to dissolution of the promoter. In summary, this work demonstrates that high selectivities for oxygenated keto-acids derived from glycerol may be obtained by catalytic oxidation on bismuth-promoted platinum under acidic conditions. However, problems of catalyst deactivation by adsorbed acids, overoxidation of targeted products and leaching of the promoter need to be overcome to attain the ultimate goal of theoretical yield.

436 REFERENCES 1. J.W. DoborQinQr, Annalen der Chemie, 53, 145, (1845). 2. R.A. Sheldon, Chemtech, 566-76, (1991). 3. T. Mallat and A. Baiker, Catalysis Today, 19, 247-84, (1994). 4. K. Heyns and H. Paulsen, Advances in Carbohydrate Chemistry, 17, pp.169-221, (1962). 5. P. Vinke, D.D. Wit, A.T.J.W.D. Goede and H.V. van Bekkum (eds.j, Stud. Surf. Sci. Catai, 72, 1-20 (1992). 6. P. Gallezot and M. Besson, in Carbohydrates in Europe (Carbohydr. Res. Foundation, ed.), 13, 10-15, The Hague, Netherlands (1995). 7. P. Verdeguer, N. Marat and A. Gaset, Appl. Catal, 112, 1-11 (1994). 8. T. Imanaka, H. Terasaki, A. Fujio and Y. Yokota, Japanese Patent 05 331 100 (1993). 9. R. Garcia, M. Besson, and P. Gallezot, App/. Catal, 111, 165-76, (1995). 10. H. Kimura and K. Tsuto, Appl. Catal, 96, 217-28 (1993). 11. H. Kimura, T. Imanaka and Y. Yokota, Japanese Patent 06 279 352 (1993). 12. P. Fordham, M. Besson and P. Gallezot, Appl Catal, 133, L179-84 (1995). 13. P. Fordham, R. Garcia, M. Besson and P. Gallezot, Stud. Surf. ScL Catal, 101, 161-170 (1996). 14. P.C.C. Smits, B.F.M. Kuster, K. van der Wiele and H.S. van der Baan, Appl Catal, 33, 83-96 (1987). 15. A. Abbadi and H. van Bekkum, Appl Catal, 124, 409-17 (1995). 16. P. Gallezot, R. de Mesanstoume, Y. Christidis, G. Mattioda and A. Schouteeten, /. Catal, 133, 479-85 (1992). 17. P. Gallezot, F. Fache, R. de Mesanstoume, Y. Christidis, G. Mattioda and A. Schouteeten, Stud. Surf. Scl Catal, 75, 195-204 (1993). 18. M. Besson, F. Lahmer, P. Gallezot, P. Fuertes and G. Fleche, /. Catal, 152, 116-21 (1995).

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

437

PDMS occluded Ti-MCM-41 as an improved olefin epoxidation catalyst

Ivo F.J. Vankelecom, Nancy M.F. Moens, Karen A.L. Vercruysse, Rudy F. Parton, Pierre A. Jacobs

Centre for Surface Chemistry and Catalysis, Department of Interphase Chemistry, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium Telefax: 00-32-16.32.19.98; E-mail: [email protected]

Summary A composite membrane was prepared by incorporating Ti-MCM-41 in PDMS (polydimethylsiloxane). Using this catalyst, the epoxidation of 1-octene was studied with focus on the influence of the polymer environment on the actual catalytic performance. Three different oxidants were investigated, as well as the influence of the solvent. It was found that the newly developed catalyst is especially interesting under solvent free reaction conditions where it might suppress side reactions. Furthermore, the removal of reagents and catalyst from the reaction mixture after reaction is facilitated and epoxidation in a continuous counter current membrane reactor becomes feasible.

1. Introduction In Parton et al., a new type of heterogeneous catalyst was proposed consisting of a solid catalyst (iron phthalocyanine zeolite Y) dispersed in a dense PDMS (polydimethylsiloxane) polymer matrix. [1] The system resulted in strongly increased catalytic activities in the oxidation of cyclohexane.[2] Other systems, such as Mn(bipy)2-Y (manganese bipyridine zeolite Y) were also proven to benefit from such incorporation.[3,4] The results presented here using Ti-MCM-41 confirm this for the epoxidation of olefins, an important route for the production of fine chemicals.[5] The influence of the polymer on the reaction activity and selectivity is shown by using different oxidants and solvent conditions in the epoxidation of 1octene. It will enable the deduction of the advantages and limitations of the reported membrane occluded catalyst system.

2. Experimental Ti-MCM-41 (Ti02/Si02 = 52) was synthesized following Franke et al.[6] 18.8 g Ludox AS-40 (40 wt% colloidal silica in water) was mixed with 19.4 g tetraethylammoniumhydroxide (TMAOH 20 wt% in water) and 16.1 g cetyltrimetylammoniumchloride (CTMACl

438

25 wt% in water). After stirring for 15 minutes, a second portion (32.2 g) of cetyltrimetylammoniumchloride was added, followed by 3.64 g Ti(V)-«-butoxide dissolved in 3.60 g isopropylalcohol. The complete synthesis was done in ice cooled glass recipients. After homogenization, the synthesis gel was poured into stainless steel autoclaves which were placed in a rotating oven at 383 K during 24 h. After synthesis, the autoclaves were cooled down and the catalyst was filtered, washed several times and dried at 333 K during 10 h. TiMCM-41 was calcined at 820 K during 12 h, a temperature reached at a rate of 1 K/min. The sample was characterized by X-Ray Diffraction (XRD), Infra-Red (IR) and UV-Vis spectroscopy. In order to prepare the composite membrane, 1 g of this catalyst was mixed during 1 h with 4 g of a 15 wt% solution of prepolymerized PDMS (RTV-615, General Electric). The prepolymerization of 3.6 g PDMS-prepolymer (RTV-615A) with 0.36 g PDMS-crosslinker (RTV-615B) took place in 4-methyl-2-pentanone (99,5 %, Acros Chimica) during 1 h at 70 °C.[7] After casting the mixture in a petri dish and allowing the solvent to evaporate, the membrane was finally cured during 1 h at 150 °C under vacuo. Reactions were performed with 0.2 g catalyst, 75 mmol 1-octene (99 %, Across Chimica) and 30 mmol oxidant in a batch reactor at 56 °C under reflux during 24 h. Under solvent conditions, the minimal amount of solvent required to reach one phase was added: 3.12 g acetone (99.5 %, AnalaR BDH)/ g substrate and 14.62 g methanol (99.8 %, Lab-Scan)/ g substrate for the hydrogen peroxide (35 wt% in water, Acros Chimica) reactions and 4.5 g methanol / g substrate for the reactions with /-BHP (tertiary butyl hydroperoxide, 70 wt% in water, Acros Chimica). Chlorobenzene (99 %, Acros Chimica) was used as external standard in the gas chromatographical analysis of the reactions. Sorption in PDMS was determined by immersing pieces of PDMS in the pure liquids, as described by Vankelecom et al.|8] 3. Results and discussion 3.1. Characterization of Ti-MCM-41

The synthesized Ti-MCM-41 sample shows a typical XRD pattern, similar to that reported in literature.[9] The pore diameter is about 35 A and the peaks can be indexed on a hexagonal structure.

439

1200

1000

800

600

Wavenumber (cm'')

The IR spectrum shows a band around 960 cm' . Although this band is taken as an evidence for the presence of tetrahedral Ti(IV) in the framework of titanium siUcalites [10], this can not be said in the case of Ti-MCM-41 as this band also appears in the spectrum of siliceous MCM-41.

50000

40000

30000

20000

10000

wavenumber (cm-'') UV-Vis spectroscopy reveals the absence of anatase impurities. The spectrum shows a band at 40000 cm' due to the ligand-to-metal charge transfer of tetrahedral Ti in the structure and no band at 25000 cm' indicative for TiO^. 3.2. H2O2 as oxidant In a first series of experiments, an aqueous solution of hydrogen peroxide was applied as oxidant. As can be seen in Table 1, only low turnover numbers (T.O.N. = mmol products formed / mmol Ti sites in the catalyst) could be realised under solvent free conditions. As both reagent phases hardly mix. the blank reaction is completely negligible. The results obtained in the catalysed reactions can be explained by considering the position of the catalyst in the reaction mixture. In spite of the stirring, the catalyst powder is mainly present in the bottom layer of the reactor containing the peroxide phase. The low solubility of 1-octene in this aqueous layer, together with the preferential uptake of polar compounds in the Ti-MCM-41 pores, causes an olefin deficiency near the active sites. This situation changes when the catalyst is incorporated in PDMS. First of all, the density of this composite membrane makes the catalyst float at the interphase between both phases.

440

Secondly, the supply of l-octeiie is no longer a problem now due to the high affinity of the polymer for the olefin, reflected in its very high sorption in PDMS (Table 2).

Table 1: Turnover numbers after 24 h, epoxide yield (mmol) and product distribution (%) for the epoxidation of 1-octene with Ti-MCM-41 using aqueous hydrogen peroxide as oxidant. (B = blank reaction; P = catalyst as powder; M = membrane resident catalyst) H2O2 (35 wt% in water)

T.O.N/^^

Epoxide yield (mmol) Product distribution (%) Epoxide Heptanal Allylic products Hydroperoxides Other/^^

Solvent free M B P

B

14

25

21

66

23

13

42

17

0.19

0.44

0.40

0.95

0.42

0.25

0.56

0.55

22 8

28 6

30 5

23 7

29 4

31 12

19 9

55 5

34 37

36 31

46 19

44 27

50 17

23 34

36 37

23 18

1

Acetone P M

B

Methanol P M

(1) The T.O.N, of the blank reaction M^as calculated by conidering an imaginary amount of active sites, equal to the amount present in the Ti-MCM-41 catalysed reactions. (2) 1-octenyl-3-hydroperoxide and 2-octenyl-l-hydroperoxide. (3) Other allylic oxidation products: 2-octenal, l-octene-3-ol, l-octene-3-one, 2-octenel-ol A different situation arises when acetone or methanol is added as a solvent to this two phase system. In both cases, the blank reaction gains much importance as a homogeneous mixing of both reagents is realised now. The product distribution in the blank reaction is quite similar to the one from the catalysed reactions. The turnover number of the non membrane resident catalyst increases strongly. Indeed, by adding acetone or methanol to reach a homogeneous reaction mixture, the sinking of the catalyst into the polar peroxide phase is no longer a problem and both reagents can sorb easily from the homogeneous reaction mixture into the MCM-pores. The membrane resident catalyst has no benefits from the presence of a solvent. The turnover numbers decrease, because the main effect of the solvent here is a dilution of the reagents. Indeed, as explained already by Parton et al.[l], the PDMS phase constitutes a "solvent" phase on its own by sorbing both reagents from the reaction medium. It makes the use of a solvent redundant.

441

Table 2: Sorption (ml/g) of the reagents and solvents in PDMS. Sorpt ion (ml/g) t-BHP H2O2

c/5-cyclo-octene 1-octene Acetone Methanol H2O

0.070 0.025 1.270 1.560 0.062 0.022 0.022

These experiments prove that - apart from eliminating the need of a solvent - a membrane resident catalyst can be very useful when blank reactions are involved making undesired products. By leaving away the solvent in reactions involving two immiscible reagent phases, a membrane resident catalyst can allow reaction, while strongly suppressing this blank reaction.

3.3. ^BHP (70 wt% in water) as oxidant Even under solvent free reaction conditions, an aqueous solution of /-BHP as oxidant gives generally higher conversions than the best hydrogen peroxide system. As the catalyst powder still mainly remains in the aqueous phase at the bottom of the reactor, the better dissolution of the olefin in the /-BHP phase - being a less polar solution than the aqueous hydrogen peroxide solution - might explain the good performance of the non membrane resident catalyst and the relatively prominent blank reaction. For Ti-MCM-41/PDMS, the socalled "solvent phase" effect of the PDMS environment becomes obvious: the increased conversion - compared to the results with hydrogen peroxide as oxidant - is a result of the better sorption of /"-BHP in the membrane matrix in comparison with hydrogen peroxide (Table 2). Clearly, the ^BHP and 1-octene concentrations are adapted near the catalyst's active sites in a beneficial way thanks to the membrane phase. Furthermore, PDMS excludes water from interfering with the active sites. This might explain the higher epoxide yield for Ti-MCM-41/PDMS. This time, the addition of a solvent hardly improves the performance of the non embedded catalyst. Dilution of the reaction medium and a more imporant blank reaction are the only consequences. This confirms that a membrane resident catalyst under solvent free conditions can actually be used to reduce interferences from the blank reaction. The one phase reaction systems using methanol can be used to study the intrinsic reactivity of Ti-MCM-41 with hydrogen peroxide (Table 1) or t-BU? as oxidant. The results obtained with the non membrane resident Ti-MCM-41 as catalyst prove the higher intrinsic reactivity of Ti-MCM-41 with hydrogen peroxide than with r-BHP. Taking into account the threefold higher dilution of the hydrogen peroxide reaction, the difference is even more evident. On the other hand, tliis is not the case when considering the membrane resident

442

catalyst. From Table 2, it is clear that the "solvent phase" effect of the PDMS matrix induces this result: ^BHP is enriched near the active sites when using a PDMS resident catalyst so that this oxidant creates a higher reactivity in the membrane occluded catalyst. On the other hand, hydrogen peroxide is preferentially sorbed in the polar pores of Ti-MCM-41 and far less in the hydrophobic PDMS matrix.

Table 3: Turnover numbers (T.O.N.) after 24 h, epoxide yield (mmol) and product distribution (%) for the 1-octene epoxidation with Ti-MCM-41 using tertiary butylhydroperoxide as oxidant. ^BHP (in water) Solvent free B M P T.O.N/^^ Epoxide yield (mmol)

B

^BHP (in decane)

Methanol P M

29

67

69

15

0.15

0.45

1.01

0.21

11 3

23 3

22 3

36 2

74 9

59 19

64 11

62 9

Product distribution (%) Epoxide 8 2 Heptanal Allylic products 80 Hydroperoxides Other/^^ 9

22

<Solvent free P M B

26

60

84

113

0.50 0.42

0.38

3.88

3.47

26 3

10 3

70 0

48 3

60 11

71 14

22 6

40 8

(1) The T.O.N, of the blank reaction was calculated by conidering an imaginative amount of active sites, equal to the amount present in the Ti-MCM-41 catalysed reactions. (2) l-octenyl-3-hydroperoxide and 2-octenyl-l-hydroperoxide. (3) Other allylic oxidation products are 2-octenal, l-octene-3-ol, l-octene-3-one, 2octene-1-ol.

3.4. ^BHP (5.5 M in decane, Aldrich) In this system, the highest conversions can be reached due to the presence of only one phase, even without adding any solvent - for as far as decane can't actually be considered as a solvent in this reaction system. For non membrane resident Ti-MCM-41, this system is clearly the most promising. Probably this can be ascribed to the fact that decane is - in contrast with acetone and methanol - not competing with the reagent molecules for sorption on the active sites in the Ti-MCM-41 pores, hi spite of the high T.O.N, realised, the use of this oxidant has some important drawbacks. First of all, it is more expensive than the other peroxide solutions commercially available. Secondly, its use is less general since it cannot be applied when using catalysts that oxidise alkanes. Furhtermore, an important influence of the blank reaction has to be considered as well.

443

4. Product yields The reactions with /^-BHP in decane as oxidant don't only show the highest T.O.N., but also the highest epoxide selectivity due to the absence of water in the reaction mixture. As decane is actually a solvent in lliis reaction, the reaction with aqueous /^-BHP as oxidant and a membrane occluded catalyst is liy far the most promising system to be further developed in a continuous membrane reactor. The exclusion of water from the catalyst pores by PDMS (Table 2) induces a higher epoxide selectivity when using a membraen occluded Ti-MCM-41. However, the selectivity is still rather low. Lots of substrate molecules react via a radical mechanism towards 1-octeny 1-3-hydroperoxide and 2-octenyl-l-hydroperoxide. These hydroperoxides were identified using GC/MS and by adding triphenylphosphine as reductant. After addition of the phosphine to the reaction mixture at the end of the reaction, both hydroperoxide peaks disappeared completely on the GC-analysis, whereas the area of the peaks identified as the corresponding alcohols increased strongly. To make sure, the reaction with Ti-MCM-41 and /^-BHP in decane was repeated in the presence of 0.25 g 2,6-di-tertiary butyl phenol as a radical scavenger. The result was a reaction with a slightly lower conversion, but a 95 % epoxide selectivity.

5. Conclusions The incorporation of Ti-MCM-41 in a PDMS-matrix was proven to show interesting features in the epoxidation of olefins. First of all, the composite membrane plays the role of an interphase between two immiscible reagent phases, enabling solvent free reactions to take place with high conversions. Apart from facilitating the subsequent product removal and eliminating possible side reactions involving solvent molecules, these systems may also suppress undesired blank reactions. Furthermore, as water is excluded from the membrane, epoxide ring opening reactions are partly suppressed as well. The above results prove that PDMS resident Ti-MCM-41 opens interesting perspectives as catalyst in a continuous counter current membrane reactor for olefin epoxidations.

References 1. Parton, R.F., Vankelecom, I.F.J., Casselman, M.J.A., Bezoukhanova, C.P., Uytterhoeven, J.B., and Jacobs, P.A., Nature, 370 (1994) 541. 2. Vankelecom, I.F.J., Parton, K.F., Casselman, M.J.A., Uytterhoeven, J.B., Jacobs, P.A., J. Catal 163,2(1996)457. 3. Parton, R.F., Vankelecom, I.F.J., Tas, D., Janssen, K.B.M., Knops-Gerrits, P.-P., Jacobs, P.A., J. Mol Cat. in press. 4. Knops-Gerrits, P.P., Vankelecom, I.F.J., Beatse, E., Jacobs, P.A., Catalysis today in press. 5. Weissermel, K., Arpe, H.-J.. Industrial Organic Chemistry, 2nd Edition, VCH-Weinheim Germany (1993). 6. Franke O., Rathousky, J., Schulz-Ekloff, G., Starek, J., Zukal, A., Stud Surf. Sci. Cat., 84 (1994) 77. 7. Vankelecom, LF.J., Uytterhoeven, J.-B., Jacobs, P.A. Eur.Patent (1996). 8. Vankelecom, I.F.J.; Scheppers, E.; Reus, R.; Uytterhoeven, J.-B. J. Phys. Chem. 98 (1994) 12390.

444

9. Beck J.S.; Vartuli J.C.; Roth WJ.; Leonowicz M.E.; Kresge C.T. J.Am.Chem.Soc. 114 (1992) 10834. 10. Perego G.; Bellussi G.; Corno C; Taramasso M.; Buonomo F.; Esposito A. Stud. Surf. Sci.Catal 2^ {19U)\29.

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

445

Epoxidation with Manganese N,N'-bis(2-Pyridinecarboxamide) Complexes Encapsulated in Zeolite Y p.p. Knops-Gerrits*, M. L'abbe, \\A. Jacobs Centrum voor Oppervlaktechemie en Katalyse, KU Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium; * author for correspondence.

SUMMARY Adsorption of tetradentate ligands of the type N,N'-bis (2-pyridinecarboxamide)-l,2-R (bpR with R = ethane, cyclohexane, benzene) and -1,3-R (bpR with R = propane) on Mn^NaY zeoUtes yields [Mn(bpR)]-NaY catalysts for the selective epoxidation of olefins with hydrogen peroxide (H2O2) and terthutyl hydroperoxide (/^BHP). The (de)protonation and coordination geometry of the occluded complexes is probed by FT-IR, EPR and DRS spectroscopy. Their catalytic activity in cyclohexene epoxidation differs strongly with the nature of the bridging ligand. Heterogenisation by zeolitic occlusion leads to a decrease in allylic oxidation of olefins with H2O2 and ^BHP. Implications of ligand amide N-H protonation on hydrogen bonding with peroxides, catalyst activity and stability towards oxidation are shown. Solvent peroxide interactions and effects of residual zeolitic Bronsted acidity on epoxide selectivity are considerable.

INTRODUCTION Higher oxidation states of manganese (Mn , Mn

and Mn ) occur in polynuclear structures

such as photosystem II, which is a catalyst for water splitting, as well as in mononuclear complexes. Solutions of Manganese salts and bipyridine form polynuclear complexes [1] such as [Mn202(bpy)4] ^ in air under oxidative conditions. [Mn(bpy)2] ^ complexes can retain their mononuclearity only when encapsulated in zeolites [2-6]. Tetradentate ligands of the N,N'-bis(2-pyridinecarboxamide)-l,2-R type [7-10] either fully deprotonate, yielding N4-ligands or remain protonated, thus coordinating through pyridine N's and amide O's as a N202-ligand. Their Mn and Fe complexes are either bi- or mononuclear [11-12]. Hydrolysis stable complexes like [Mn"^(bpc)(02CMe)] (bpc= 4,5-dichloro-bp-l,2-benzene) oxidise olefins. In Fe"-Y zeolites mononuclear complexation with bpR (R = ethane, propane, cyclohexane and benzene) occurs to form [Fe(bpR)]-Y [13]. The synthesis and epoxidation catalysis of [Mn(bpR)]-Y is reported here. Previously the "ship-in-a-bottle" catalysts [Mn(L)]-NaY, with L = salen, salen-derived ligands as

446 well as [Mn(pyren)]^^-Y and |Mn(pyrpn)]^^-Y (pyren = bis(2-pyridinecarboxaldehyde)-l,2ethylenediimine ; pyrpn = bis(2-pyridinecarboxaldehyde)-l,3-propylenediimine), in which the phenol groups are replaced by oxidation-resistant pyridyls, have been studied in alkane oxidation [14-15]. Variation of the nature of the guest ligand in MnNaY using polyamines [16] and of the host by replacing the FAU by EMT topology in case of [Mn(bpy)2]^^ [17] have been examined.

—\ -N /

o N—^

bpenH2 \

/

'^ bppnH2 \\ / \ _ /

bpbH2 \

/

Figure 1. (a) Ligands used for complexation of manganese in Mn Y zeolite,

Figure 1. (b) molecular graphics picture of the supercage of zeolite Y, occupied by a Mn"(bpch) complex.

RESULTS AND DISCUSSION 1. Encapsulated Mn complexes in Y zeolite Electron Spin Resonance Spectroscopy on manganese exchanged zeolites can be used to probe the nuclearity, the valency and in some cases the local symmetry of Mn . The Mn-nucleus gives a six-line splitting for MnNaY (21+1,1=5/2) with g=2.01 and A= 95G [18]. For unchelated Mn^"" in faujasite type zeolites, D (axial parameter) and E (rhombic parameter) are quite small, so that only the A My = +1/2