Heterogeneous Catalysis and Fine Chemicals II: Proceedings of the 2nd International Symposium, Poitiers, October 2-5, 1990 (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 59 HETEROGENEOUS CATALYSIS AND FINE CHEMICALS II This Page Intentionally Lef...

Author: M. Guisnet | J. Barrault | C. Bouchoule | D. Duprez

20 downloads 817 Views 22MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!

Report copyright / DMCA form

DOWNLOAD PDF

Studies in Surface Science and Catalysis 59 HETEROGENEOUS CATALYSIS AND FINE CHEMICALS II

This Page Intentionally Left Blank

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

Vol. 59

HETEROGENEOUS CATALYSIS AND FINE CHEMICALS II Proceedings of the 2nd International Symposium, Poitiers, October 2-5, 199 0

Editors

M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Pbrot, R. Maurel and C. Montassier Laboratoire de Catalyse en Chimie Organique (URA CNRS 3501,UFR Sciences, Universitb de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers, France

ELSEVIER

Amsterdam - Oxford - New York

-Tokyo

1991

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1 , 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada. ELSEVIER SCIENCE PUBLISHING COMPANY INC 655, Avenue of the Americas New York, NY 10010, U.S.A.

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

Data

Heterogeneous c a t a l y s i s and f i n e c h e m i c a l s I 1 p r o c e e d i n g s o f t h e 2nd i n t e r n a t l o n a l s y m p o s i u m . P o l t i e r s . O c t o b e r 2-5, 1 9 9 0 / e d i t o r s . M. Guisnet Let al.1. 59) p. cm. -- ( S t u d i e s i n s u r f a c e s c i e n c e a n d c a t a l y s i s P a p e r s from t h e 2nd I n t e r n a t i o n a l Symposium on H e t e r o g e n e o u s C a t a l y s i s and F i n e Chemicals. I n c l u d e s b i b l i o q r a p h i c a l r e f e r e n c e s and i n d e x e s . ISBN 0 - 4 4 4 - 8 8 5 1 4 - 5 1. G u i s n e t . M . 1. H e t e r o g e n e o u s c a t a l y s i s - - C o n g r e s s e s . 11. I n t e r n a t i o n a l S y m p o s i u m o n H e t e r o g e n e o u s C a t a l y s i s a n d F i n e 111. S e r i e s . Chemicals (2nd 1990 Poltiers. Francel 00505.H463 1991 541,3'95--dc20 9 1-9044

...

.

CTP

ISBN 0-444-885 14-5

0 Elsevier Science Publishers B.V.. 199 1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Academic Publishing Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulationsfor readers in the USA -This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred t o the publisher. No responsibility is assumed by the Publisher for any injury and/or damage t o 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. Although all advertising material is expected t o conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. This book is printed on acid-free paper Printed in The Netherlands

V

CONTENTS

Foreword ...................................................................

XI11

Preface ......................................................................

XV

Scientific Committee .......................................................

XVII

Organizing Committee .......................................................

XVII

Financial Support .........................................................

XVIII

PLENARY

LECTURES AND

INVITED

PAPERS

Gas-1 iquid-solid reactors for hydrogenation in fine chemicals synthesis J.F. Jenck ...........................................................

1

Structure-reactivity in the hydrogenation of alkenes. Comparisons with reductions by diimide and the formation of a Ni(0) complex S. Siege1 ............... ............................................

21

Heterogeneous catalytic oxidat on and fine chemicals R.A. Sheldon.. .......... ............................................

33

Solids for catalysis and control in organic synthesis K. Smith .............................................................

55

Enantioselective catalysis by chiral sol ids : approaches and results H.U. Blaser and M. Muller.. ..........................................

73

Catalysis with immobilized enzymes : hydrolysis and esterification by Rhizopus arrhizus C. Gancet ............................................................ 93

\' I

RESEARCH PAPERS

I . HYDROGENATION AND RELATED REACTIONS Hydrogenation o f benzaldehyde t o b e n z y l a l c o h o l i n a s l u r r y and f i x e d - b e d reactor

M. H e r s k o w i t z .......................................................

105

S t r u c t u r e and c a t a l y t i c p r o p e r t i e s i n h y d r o g e n a t i o n o f v a l e r o n i t r i l e o f Raney n i c k e l prepared f r o m C r and Mo doped Ni2A13 a l l o y s

M. Besson, 0. Djaouadi, J.M. Bonnier, S. H a m a r - T h i b a u l t and M. J o u c l a ...........................................................

113

Selective preparation o f c h l o r o a n i l i n e s from chloronitrobenzenes over s u l f i d e d hydrotreating catalysts C . Moreau, C . Saenz, P. Geneste, M. Breysse and M. L a c r o i x

..........

121

The a p p l i c a b i l i t y o f d i s p e r s e d m e t a l s as c a t a l y s t s f o r o r g a n o m e t a l l i c r e a c t i o n s R.L. Augustine, S.T.

O'Leary, K . M . Lahanas and Y.-M.

Lay ............ 129

S u r f a c e organometall i c c h e m i s t r y on m e t a l s : s e l e c t i v e h y d r o g e n a t i o n o f c i t r a l i n t o g e r a n i o l and n e r o l on t i n m o d i f i e d s i l i c a s u p p o r t e d rhodium

B. O i d i l l o n , A . El Mansour, J.P. Candy, J.P. B o i i r n o n v i l l e and J.M. Basset .........................................................

137

S e l e c t i v e h y d r o g e n a t i o n o f u n s a t u r a t e d aldehydes o v e r z e o l i t e - s u p p o r t e d m e t a l s D.G.

Blackmond, A. Waghray, R . Oukaci, B. Blanc and P. G a l l e z o t . .

...

145

The mechanism o f h y d r o g e n o l y s i s and i s o m e r i z a t i o n o f o x a c y c l o a l k a n e s on m e t a l s .

X. Nature o f t h e a c t i v e s i t e s i n the r e g i o s e l e c t i v e hydrogenation o f oxiranes F . Notheisz, A.G.

Zsigmond, M. Barto'k, 0. Ostgard and G.V.

Smith .... 153

Chemo-, r e g i o and s t e r e o s e l e c t i v i t y i n s t e r o i d h y d r o g e n a t i o n w i t h Cu/A1203. Intra-

and i n t e r m o l e c u l a r hydrogen t r a n s f e r r e a c t i o n s

N. Ravasio, M. Gargano, V . P .

q u a t r a r o and M. Rossi .................. 161

S e l e c t i v e h y d r o g e n a t i o n o f a r o m a t i c and a1 i p h a t i c n i t r o compounds b y hydrogen t r a n s f e r over MgO

J. K i j e < s k i , M. G l i i s k i , R . WiSniewski and S. Murghani .............. 169

VII

Mass t r a n f e r c o n s i d e r a t i o n s f o r t h e e n a n t i o s e l e c t i v e h y d r o g e n a t i o n o f a - k e t o e s t e r s c a t a l y z e d b y cinchona m o d i f i e d Pt/A1203

M. Garland, H.P. J a l e t t and H.U. B l a s e r ..............................

177

S e l e c t i v e carvone hydrogenat i o n on Rh supported c a t a l y s t s

R. Gomez, J. Arredondo, N. Rosas and G. Del Angel ....................

185

S e l e c t i v e h y d r o g e n a t i o n o f c i t r a l i n t h e 1 i q u i d phase o v e r unsupported n i c k e l molybdenum c a t a l y s t s N i l - x Mo, J. Court, F. J a n a t i - I d r i s s i and S. V i d a l

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

193

S u l f u r removal f r o m terpenes by h y d r o d e s u l f u r i z a t i o n on c a r b o n - s u p p o r t e d catalysts

F . Casbas, D. Duprez and J. O l l i v i e r .................................

201

S t u d i e s on t h e c a t a l y t i c h y d r o g e n a t i o n o f r e s i n a c i d s d e r i v a t i v e s : s y n t h e s i s o f a benzoxazol e B. Gigante, A.M.

Lobo, S. Prabhakar, M.J. M a r c e l o - C u r t o and

D.J. W i l l i a m s ........................................................

209

The i s o m e r i s a t i o n o f l a c t o s e t o l a c t u l o s e c a t a l y s e d by a l k a l i n e ion-exchangers

B.F. K u s t e r , J.A.W.M.

Beenackers and H.S.

van d e r Baan ............... 215

F u r a n i c d e r i v a t i v e s s y n t h e s i s f r o m p o l y o l s b y heterogeneous c a t a l y s i s o v e r metals C . Montassier, J.C. Menezo, J . Moukolo, J. Naja, J. B a r b i e r and

J.P. B o i t i a u x

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

223

A c t i v i t y and s t a b i l i t y of promoted Raney-nickel c a t a l y s t s i n g l u c o s e hydrogenation P.J. Cerino, G. Fleche, P. G a l l e z o t and J.P. Salome

.................. 231

T r a n s f o r m a t i o n o f sugar i n t o g l y c o l s on a 5 % Ru/C c a t a l y s t P. M u l l e r , P. Rimmelin, J.P. Hindermann, R. K i e f f e r , A. Kiennemann and J. C a r r e .........................................................

237

S e l e c t i v e h y d r o g e n a t i o n o f acetophenone on unpromoted Raney n i c k e l : influence o f the reaction conditions J. Masson,

P. C i v i d i n o , J.M. Bonnier and P. F o u i l l o u x ................ 245

VIIl

Chemoselective r e d u c t i o n o f enones t o a l l y 1 i c a l c o h o l s J. Kaspar, A. T r o v a r e l l i , F . Zamoner, E. F a r n e t t i and M. G r a z i a n i

... 253

Comparison o f homogeneous and heterogeneous p a l l a d i u m c a t a l y s t s i n t h e c a r b o n y l a t i o n o f a1 l y l e t h e r s M.M. Barreto-Rosa, M.C. Bonnet and I . Tkatchenko ....................

263

L i q u i d - p h a s e s e l e c t i v e h y d r o g e n a t i o n o f 1 , 4 - b u t y n e d i o l on s u p p o r t e d N i and N i Cu c a t a l y s t s

F.M. B a u t i s t a , J.M. Campelo, A. G a r c i a , R. GuardeKo, D. Luna and J.M. Marinas ........................................................

269

C a t a l y t i c p r o p e r t i e s o f t r a n s i t i o n metal s u l p h i d e s f o r t h e d e h y d r o g e n a t i o n o f s u l p h u r c o n t a i n i n g molecules

M. L a c r o i x , H. M a r r a k c h i , C. C a l a i s , M. Breysse and C . Forquy

....... 277

React i o n s o f u n s a t u r a t e d e t h e r s on a copper-chromium c a t a l y s t

R . Hubaut and J.P. B o n n e l l e .

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

287

Hydrogenation o f methyl -3 b u t e n a l on p o l y c r y s t a l l i n e p l a t i n u m C.-M. P r a d i e r , E. Margot, Y. B e r t h i e r , G. C o r d i e r ................... 295 Surface c h e m i s t r y and c a t a l y s i s w i t h o r g a n i c n i t r o compounds. L o o k i n g f o r t h e key t o h i g h e r s e l e c t i v i t i e s

P.A.J.M.

Angevaare, A. Maltha, T.L.F.

Favre, A . P . Zuur and V . Ponec. 305

P r e p a r a t i o n o f orthophenylenediamine f r o m 4 - c h l o r o - 2 - n i t r o a n i l i n e J.L. M a r g i t f a l v i , M. Hegedus, S. Gdbolos and E. Talas ............... 313 Chemoselective h y d r o g e n a t i o n o f a r o m a t i c c h l o r o n i t r o compounds w i t h amidine modified nickel catalysts P. Baumeister, H.U.

B l a s e r and W . S c h e r r e r ..........................

321

Intermediates formation i n t h e c a t a l y t i c hydrogenation o f n i t r i l e s Ph. Marion, P. G r e n o u i l l e t , J. Jenck and M. J o u c l a . ................. 329 R e d u c t i v e a m i n a t i o n o f acetone on t i n m o d i f i e d s k e l e t a l n i c k e l c a t a l y s t s S. Gdbolos, E. Talas, M. Hegedus, J.L. M a r g i t f a l v i and J. Ryczkowski 335

IX

S y nt h es is o f d i m e t h y l a l k y l a m i n e s f r o m a c i d s and e s t e r s over promoted copper catalysts J. B a r r a u l t , G. Delahay, N. Essayem, Z. G a i z i , C. Forquy and

R. Brouard

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

343

T e r t i a r y amine p r e p a r a t i o n by r e d u c t i v e a l k y l a t i o n o f a1 i p h a t i c secondary amines w i t h ketones R.E.Malz, Jr. and H. G r e e n f i e l d

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

351

E f f e c t o f promoters on Pt/SiO2 c a t a l y s t s f o r t h e N - a l k y l a t i o n o f s t e r i c a l l y hindere d a n i l i n e s i n t h e vapor phase M. Rusek

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

359

P o l y f u n c t i o n a l i t y o f Zn-Cr-0 (Pd) c a t a l y s t f o r t h e s y n t h e s i s o f p y r a z i n e s f rom diamines and g l y c o l s L. F o r n i and R. M i g l i o

.............................................. 11.

367

OXIDATION

From s urf ac es t o d i s c r e t e molecules as c a t a l y s t s f o r alkene e p o x i d a t i o n K.A. J ~ r g e n s e n

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

377

On t h e oxygen t o l e r a n c e o f n o b l e metal c a t a l y s t s i n l i q u i d phase a l c o h o l o x i d a t i o n s . The i n f l u e n c e o f t h e s u p p o r t on c a t a l y s t d e a c t i v a t i o n P. Vinke, W. van d e r Poel and H. van Bekkum.........................

385

I r o n - p h t h a l l o c y a n i n e s encaged i n z e o l i t e Y and VPI-5 m o l e c u l a r s i e v e as c a t a l y s t s f o r t h e o x y f u n c t i o n a l i z a t i o n o f n-alkanes R.F. Parton, L. Uytterhoeven and P.A.

Jacobs..

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

395

M i l d o x i d a t i o n o f c y c l i c C6-clO hydrocarbons i n l i q u i d phase a t room temperature b y heterogeneous p h o t o c a t a l y s i s J.M. Herrmann,

W. Mu and P. P i c h a t ..................................

405

O x i d a t i v e dehydrogenation o f 3-hydroxy-4-methyl-4-penten-2-one t o 4-met hyl-4pent e n-2 , 3 -d ion e over CuO-based c a t a l y s t s H.G.-J. Lansink R o t g e r i n k , G. Penn, P.C. A. B a i k e r

F u n f s c h i l l i n g and

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

413

P a r t i a l o x i d a t i o n o f t o l u e n e t o benzaldehyde M. A i ...............................................................

423

New p o l y d e n t a t e M o ( V 1 ) - g r a f t e d p o l y ( amido amine) r e s i n s as heterogeneous epoxidation c a t a l y s t s P. F e r r u t i , E . Tempesti, L. G i u f f r e , R . Ranucci and C . Mazzocchia ... 431

S e l e c t i v e o x i d a t i o n o f methyl e t h y l k e t o n e t o d i a c e t y l o v e r vanadium phosphorus oxide c a t a l y s t s E. McCullagh, J.B. McMonagle and B.K. H o d n e t t

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

437

(Heterogeneous) p h o t o c a t a l y t i c o x i d a t i o n o f t o l u e n e u s i n g p u r e and i r o n - d o p e d t i t a n i a catalysts J.A. Navio, M. G a r c i a Gomez, M.A.

Pradera A d r i a n and J. Fuentes Mota 445

S y n t h e s i s o f n i t r i l e s b y r e a c t i o n o f p - x y l e n e w i t h NO o v e r Cr203-Al203 catalysts S. Z i n e and A. Ghorbel

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

455

S e l e c t i v e e l e c t r o c a t a l y t i c o x i d a t i o n o f g l y o x a l i n aqueous medium E.M.

B e l g s i r , H. Huser, C . Lamy and J.-M. Leger .....................

463

111. ACID-BASE CATALYSIS N i t r i c a c i d a s s o c i a t e d w i t h i n o r g a n i c s o l i d s : a v e r s a t i l e r e a g e n t and c a t a l y s t i n t h e chemistry o f aromatics M.H.

Gubelmann, C . Doussain, P.J. T i r e l , J.M. Popa .................. 471

D e h y d r a t i o n o f carboxamides t o n i t r i l e s u s i n g s u l p h a t e d z i r c o n i a c a t a l y s t R.A.

Rajadhyaksha and G.W.

J o s h i ....................................

479

S a t u r a t e d and u n s a t u r a t e d ketones manufactured b y heterogeneous c a t a l y s i s W . R e i t h , M. Dettmer, H. Widdecke and B . F l e i s c h e r .................. 487

Condensation o f methyl N-phenyl carbamate w i t h sol i d a c i d c a t a l y s t s J.S. Lee, C.W.

Lee, S.M.

Park ................. 495

Lee, J.S. Oh and K.H.

Z e o l i t e s as base c a t a l y s t s . P r e p a r a t i o n o f c a l c i u m a n t a g o n i . s t s i n t e r m e d i a t e s b y condensation o f benzaldehyde w i t h e t h y l a c e t o a c e t a t e

A . Corma, R.M. M a r t i n - A r a n d a and ,F. Sa'nchez..

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

503

XI

Mechanism o f p h e n y l a c e t a t e t r a n s f o r m a t i o n on z e o l i t e s Y. P o u i l l o u x , J.P. Bodibo, I. Neves, M. Gubelmann, G. P e r o t and M. Guisnet ..........................................................

513

O r t h o s e l e c t i v e a l k y l a t i o n o f 2 - e t h y l a n i l i n e w i t h methanol on f e r r i c o x i d e catalysts J. Valyon, R.M. M i h a l y i and 0. K a l l o ................................

523

Rearrangement o f c y c l ohexanone oxime t o c a p r o l actam o v e r s o l i d a c i d c a t a l y s t s

T. C u r t i n , J.B. McMonagle and B.K. H o d n e t t ..........................

531

Beckmann rearrangement r e a c t i o n s on a c i d i c s o l i d s

E . G u t i e r r e z , A.J. Aznar and E . R u i z - H i t z k y .........................

539

S e l e c t i v e r i n g - o p e n i n g o f i s o m e r i c 2-methyl - 3 - p h e n y l o x i r a n e s on o x i d e c a t a l y s t s

A. Molna'r, I . Bucsi and M. B a r t i k . .

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

549

Mono and t r i d i r e c t i o n a l 12-membered r i n g z e o l i t e s as a c i d c a t a l y s t s f o r c a r b o n y l group r e a c t i o n s M.J. Climent, A.Corma, H. Garcia, S. I b o r r a and J . Primo

....... ....

557

T r i p l e bond h y d r a t i o n u s i n g z e o l i t e s as c a t a l y s t s

A. F i n i e l s , P. Geneste, M. Lasperas, F. Marichez and P. Moreau.

.... 565

Rearrangement o f epoxides u s i n g m o d i f i e d z e o l i t e s M. Chamoumi, 0. Brunel, P. Geneste, P. Moreau and J . S o l o f o

..... ..... 573

The gas phase i s o m e r i z a t i o n o f s u b s t i t u t e d halobenzenes on z e o l i t e s 6. Coq, J. P a r d i l l o s and F. Figueras

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

581

K a o l i n promoted W i t t i g o l e f i n a t i o n and a r o m a t i c n i t r a t i o n

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

589

A u t h o r Index ..............................................................

597

S u b j e c t Index .............................................................

601

S t u d i e s i n Surface Science and C a t a l y s i s ( o t h e r volumes i n t h e s e r i e s )

.... 605

C . C o l l e t and P. L a s z l o

This Page Intentionally Left Blank

XI11

FOREWORD

The Second International Symposium on Heterogeneous Catalysis and Fine Chemicals was held in Poitiers (Futuroscope), France, October 2-5, 1990. Just as in the first symposium (held in Poitiers, March 1988) the aims were to illustrate the present day role played by heterogeneous catalysis in the synthesis of functional compounds and to discuss methods of research in this field. Since the first Symposium was held, research activities have rapidly expanded as has the support provided by industry. Thus 104 Abstracts were submitted (an increase of 44 on the first symposium), closely related to the symposium theme. Moreover, the scientific contribution made by industry was also greater than at the previous symposium since this time a third of the papers was presented by researchers solely from industry (5 papers) or in collaboration with researchers from Universities (16 papers). The transformations studied were this time much more complex, covering all aspects of selectivity : chemo, regio and stereo-selectivity having been frequently considered. The three themes of the symposium : selective hydrogenation, selective oxidation and acid-base catalysis were introduced by four plenary lectures and two invited communications, A panel concerned with the future of zeolites and other shape-selective materials for fine chemical synthesis was conducted by specialists in the field : 0. Barthomeuf (University of Paris 6 ) , E. Derouane (University of Namur), L. Forni (University of Milan), M. Gubelmann (RhBnePoulenc, St Fons), W. Hoelderich (BASF, Ludwigshafen) and G. Perot (University of Poitiers). An exhibition of equipment was held during the symposium on October 3 and 4 . Over 20 firms exhibited equipment, chemicals and catalysts which were of interest to researchers involved with the synthesis of functional compounds by heterogeneous catalysis. The Organizing Committee would 1 ike to thank all the participants, particularly the authors of the communications, the various session chairmen and the members of the panel on zeolites. Special thanks are due to the members of the Scientific Committee who accomplished the difficult task of selecting the communications and reviewing the papers. Their suggestions allowed us to improve the quality and presentation of the communications. We would also like to thank all the members of the Laboratory of Catalysis in Organic Chemistry and the members of Atlas (the Association of postgraduate students and doctors of this laboratory) for their enthusiastic help. We hope the symposium provided all the participants with an opportunity to establish fruitful social and scientific ties.

This Page Intentionally Left Blank

xv PREFACE

Le second Colloque International CNRS sur le theme Catalyse Heterogene et Chimie Fine s’est tenu a Poitiers (Futuroscope) du 2 au 5 octobre 1990. Comme pour le premier Colloque (Poitiers 15-17 mars 1988), l’objectif etait de montrer le r6le joue aujourd’hui par la Catalyse Heterogene dans la Synthese des composes fonctionnels et de discuter des strategies de recherche a developper dans ce domaine. Depuis le premier Colloque, 1 ’activite de recherches s’est fortement accrue et le soutien de 1’Industrie est alle croissant. C’est ainsi que 104 resumes (au lieu de 60 lors du premier Symposium) en rapport etroit avec le theme nous ont ete soumis. Par ailleurs, la contribution scientifique de 1‘Industrie a enormement progresse puisqu’a ce Colloque, 5 communications venaient de 1’Industrie et 16 resultaient de collaborations entre chercheurs de 1’Industrie et de 1’Universite. Enfin les reactions presentees deviennent de plus en plus complexes, tous les aspects de la selectivite : chimio, regio et stereoselectivite etant souvent examines. Les trois grands themes du Symposium : hydrogenation, oxydation et catalyse acidobasique furent introduits par 4 conferences plenieres et 2 communications invitees. Une table ronde sur l’avenir des zeolithes et des autres materiaux a selectivite de forme en synthese organique a ete animee par des specialistes du domaine : D. Barthomeuf (Universite de Paris 6 ) , E. Derouane (Universite de Namur), L. Forni (Universite de Milan), M. Gubelmann (RhBnePoulenc, St Fons), W . Hoelderich (BASF, Ludwigshafen), G. Perot (Universite de Poitiers). Une exposition de materiel s’est tenue en parallele avec le Symposium les 3 et 4 octobre. Plus de 20 Societes y ont presente materiel, produits chimiques et catalyseurs de grand inter6t pour les chercheurs concernes par la synthese de composes fonctionnels par Catalyse Heterogene. Le Comite d’organisation remercie tous les participants et particul ierement les auteurs de communication, les Presidents de Seance et les animateurs de la table ronde. Des remerciements particuliers sont dijs aux membres du Comite Scientifique qui ont eu la tiche delicate de choisir les communications et d’examiner les articles. Leurs suggestions et leurs critiques ont incontestablement permis d’amel iorer la qua1 it6 et la presentation des communications. Nos remerciements vont aussi a tous les membres du Laboratoire de Catalyse en Chimie Organique et d’Atlas (Association des Chercheurs et Anciens Chercheurs de ce Laboratoire) qui ont participe avec enthousiasme et efficacite a l’organisation de ce Symposium. esperons que ce Colloque a donne l‘opportunite a tous les participants d’etablir des relations a la fois amicales et scientifiques. Nous

This Page Intentionally Left Blank

XVII

SCIENTIFIC COMMITTEE J.E. BACKWALL, U n i v e r s i t y o f

P . C . GRAVELLE, PIRSEM (CNRS), P a r i s ,

Uppsal a, Sweden

France

G. BALAVOINE, Departement Chimie

G. HECQUET, NORSOLOR, Mazingarbe,

CNRS, France

France

J. BARBIER, U n i v e r s i t y o f P o i t i e r s ,

W. HOELDERICH, BASF, Ludwigshafen, RFA

France

M. BLANCHARD, U n i v e r s i t y o f P o i t i e r s , France

J.C. JACQUESY, U n i v e r s i t y o f

H.U.

G. MARTINO, I n s t i t u t F r a n c a i s du

BLASER, Ciba-Geigy, Basel,

P o i t i e r s , France

S w i t z e r l and

P e t r o l e , Rueil-Malmaison, France

J. BOUSQUET, E l f A q u i t a i n e , P a r i s ,

G. MATTIODA, Hoechst, S t a i n s , France

France

C. MERCIER, Rhijne-Poulenc, S a i n t -

A. CORMA, I n s t i t u t e o f C a t a l y s i s ,

Fons, France

Madrid, Spain

D. O L I V I E R , I n s t i t u t de Recherches

B. DELMON, U n i v e r s i t y o f L o u v a i n - l a -

sur l a Catalyse,Villeurbanne,

France

Neuve, Belgium

Y. ONO, I n s t i t u t e o f Technology,

G. DESCOTES, Un i v e r s it e C1 aude

Tokyo, Japan

Bernard, Lyon, France

K. SMITH, U n i v e r s i t y o f Swansea,

G. FLECHE, Roquette, Lestrem, France

L. FORNI, U n i v e r s i t y o f M i l a n , I t a l y

U n i t e d Kingdom H. VAN BEKKUM, U n i v e r s i t y o f D e l f t ,

P. GENESTE, U n i v e r s i t y o f

The Netherlands

M o n t p e l l i e r , France

ORGANIZING

COMMITTEE

M. GUISNET

Chairman

3. BARRAULT and 0 . DUPREZ

Secretaries

C . BOUCHOULE, R. MAUREL, C. MONTASSIER and G. PEROT

Members

ATLAS 86 ( A s s o c i a t i o n o f s t u d e n t s o f t h e C a t a l y s i s Group o f P o i t i e r s )

XVlll

F I N A N C I A L SUPPORT

The O r g a n i z e r s a r e g r a t e f u l t o t h e i r Generous Sponsors :

- CENTRE NATIONAL DE LA RECHERCHE S C I E N T I F I Q U E (CNRS) - CONSEIL GENERAL DE LA VIENNE - SOCIETE FRANCAISE DE C H I M I E - D I V I S I O N CATALYSE - UNIVERSITE DE P O I T I E R S AND UFR SCIENCES FONDAMENTALES E l

APPLIQUEES - ATOCHEM - BASF - CIBA-GEIGY

-

DEGUSSA

- DERIVES RESINIQUES ET TERPENIQUES - I N S T I T U T FRANCAIS DU PETROLE - JONHSON MATTHEY - RHONE-POULENC - ROQUETTE FRERES

M. Guisnet et al. (Editors), Heterogeneous Catalysis andFine Chemicals II

1

0 1991 Elsevier Science Publishers B.V.,Amsterdam

-

-

GAS LIQUID SOLID REACTORS FOR HYDROQENATION IN FINE CHEMICALS SYNTHESIS

Jean F. JENCK Unit6 Mixte CNRS - R h h e Poulenc (UMR BP 166 F 69151 DECINES (FRANCE)

-

45)

ABSTRACT :

Although the presence of a liquid phase in heterogeneous hydrogenation catalysis is useful for chemical reactivity COIItrol, it introduces considerable engineering complexity. Different types of triphasic hydrogenation reactors, with moving or immobilized catalyst, in continuous or batch mode, are compared. Coupling of intrinsic kinetics with mass and energy transfer determines reactor performances, in rate as well as in selectivity. Reactor design and scale-up require the knowledge of numerous physico-chemical parameters, whose acquisition by measurement or correlation is briefly presented. INTRODUCTION :

Solid catalysts are commonly used in reactions of gaseous dihydrogen with liquid substrates, particularly in the field of fine chemicals. By I1fine" , we usually mean organic molecules exhibiting structural complexity, related to polyfunctionality and/or the presence of heteroatoms (O,S,N,P,X, etc ...), involved in small production processes (less than a few thousand T/yr, and down to the kg/yr scale), with a high production cost (over 20 FF/kg). The question of selectivity in these fine hydrogenations is frequently raised : for instance, in the last symposium in this series (Poitiers, March 1988), the following topics were presented : hydrogenation of unsaturated aldehydes [la], of sugars [lb] : alcaloid modifiers to introduce chirality on a Pt catalyst [lc] ; regioselective hydrodechlorination of polychloroaromatics [Id] : Pb alloying to modify Pd [le] or Ni [If] hydrogenation catalysts. Concerning activity, most studies focus on intrinsic (chemical) kinetics, with little consideration to the apparatus and its possible physical limitations. In fact,the design and selection of a catalytic hydrogenation reactor (hydrogenator) is not a trivial problem at all, owing to the broad range of process conditions encountered.

2

The presence of liquid phase introduces engineering complications : the interactions between transport phenomena, both for mass and energy, and intrinsic kinetics play a vital role in determining reactor performances, both for activity and selectivity, catalyst stability, etc...

-

/ BOLID BYBTEMB Although other methods, such as stoechiometric iron reduction [2], are still practised, gaseous dihydrogen is widely used, as documented in the reference books by Augustine [ 3 ] , Freifelder and Cerveny [6]. Recent patents and articles [ 4 1 Rylander [ 5 ] will be quoted throughout this article, more by way of illustrative examples than for the sake of exhaustivity. 1-1 Polyphasic systems in catalytic hydroqenations 1

TRIPHABIC GAB / LIQUID

Hydrogen

I

substrate

!catalyst

I

field :

It is worthwhile mentioning that : - triphasic also means G / L / L , for instance in homogeneous catalysis where H2 is contacted with two immiscible liquids [ 7 ] ) - G / L / S processes exist where S is a reagent or a G / L contact promotor. Here we will discuss G / L / S reactors with S solid catalysts (almost always a supported or massive metal). 1-2 A liquid medium in conjunction with G reactant and S catalyst : Two major reasons can be put forward for the presence of a liquid. First, a high temperature may not be suitable : to prevent damages to thermosensitive fine molecules or catalysts : to improve hydrogenation selectivity. Low volatility and/or high concentration of the organic sustrate, under reaction conditions, then lead to the appearance of a liquid phase Example [ E l : hydrogenation a bulky molecule of like

X

W

0

"

X

Fine chemical hydrogenations are G / S processes : Ar

H2 + A r d o

H2

+

NQ

sometimes still carried out

JoH Cu/Borosilicate N

COOR

zrOz CHO

+

,260.C

Cr203 3 4 0 ' C

[9]

[lo]

in

3

The second reason is that a liquid layer may be desired, either to provide an environment around the catalytic sites avoiding deposits and thus ensuring higher effectiveness, even chemically modifying the site, or to improve temperature control (no "hot spots11because liquids have a much higher conductivity and heat capacity than gases). Furthermore, a liquid layer can also help to control the reactivity scheme, for instance by inhibiting or promoting secondary reactions inside the L phase. Among the considerable number of cases, here are some intentional additions of L component : for acido-basic properties : acidity for hydrodehalogenations [Id], pyridine-ring protection [llJ, p-aminophenol from nitrobenzene [12], basicity for triazoles [13] for dielectric properties : hydrogenolysis (of C - 0 , C-X bonds) increases vs hydrogenation (of double bonds) with higher z [14] for site modification : control of hydrogenolysis by sulfides [15], formamidine acetate [16] ; partial reduction of nitro to hydroxylamines in presence of sulfoxide [17], of alkynes with quinoline promotor [18] ; enantioselective a-ketoesters hydrogenation with alkaloid modified catalysts [lc] water, even in small quantities, sometimes promotes (dinitriles hydrogenation on Raney cobalt [19]), sometimes poisons (acetophenone hydrogenation on Raney nickel [20]). Dilution with a solvent causes however a lower productivity and, at times, downstream purification problems : solventless I1neat1l processes are occasionally claimed [21]. Besides these positive effects, a major disadvantage is introduced : a liquid barrier to direct access of gaseous Ha to the catalyst particle ! The rheological properties of the fluid are also deeply modified, because the viscosity of liquids is many orders of magnitude higher than for gases. Finally, properties such as solubility, molecular diffusivity, etc.. of H2 in organic mixtures, difficult to measure and even to estimate, have a vital influence on the mass transport phenomena, which can be schematized as follows :

'

-

Energy transfer limitations can also appear, as all tions are fairly to highly exothermic,

hydrogena-

4

-

G / L /S HYDROGENATION REACTOR8 To understand and ultimately to forecast the performance of a reactor, it is essential to study the coupling of lltruell(intrinsic) kinetics with mass and energy transport, and to determine the flow regimes of the three phases (hydrodynamics). Modelling a reactor involves :

2

r

Nature of p W s conversion Ploducldislribulicn

Hydrodynamics : solid +

llua phases circulation

2-1 Classification of G/L/S hydroqenators The fundamental discrimination lies in the flow of solid phase : - moving catalyst (fine particles) : stirred @Is1urryg1 tank reactor STR jet-loop llVenturill reactor JLR bubbling column reactor BCR fluidized slurry reactor FSR - fixed bed (large pellets) : submerged fixed bed reactor FBR trickle-bed reactor TBR A second consideration is the operating mode : continuous, batch, or semi-continuous. An extensive textbook on theory, design and scale-up of multiphase reactors was published by Gianetto and Silveston in 1986 [ 2 2 ] , supplementing "Three-phase catalytic reactors1' (1983, by Ramachandran and Chaudhari [ 2 3 ] ) . General books on reactor engineering 1 2 4 1 give few details on G/L/S systems. 2 - 2 Characteristics of G/L/S hydroqenators : 2.2.1 stirred slurry tank reactor BTR This llworkhorsell for industry is extensively used for batch hydrogenations (1 to 100+ m3, up to 100 bar). Very fine (1 to 2 0 0 pm) solide particles are suspended in L, almost perfectly mixed by a mechanical agitator. STR .. can accommodate different agitators : the 6-bladed Rushton turbine is very popular [ 2 5 ] . Recent developments focus on hollowshaft turbines.

. . .

.

.

.

:.

.

,

5

Heat removal is accomplished by internal cooling coils or wall jacket exchangers. Hydrodynamic regimes are complex, because of complicated flow patterns, prone to quick and dramatic changes. Usually a few overall parameters are considered, such as : gas residence time and holdup, solid suspension, energy input, volumetric mass transfer coefficient (sec 5 3.2.3). 2.2.2.

Jet-loop (venturi) reactor JLR

Using the same slurry, JLRs tend to replace S T R s in the most recent fine chemical hydrogenations [26]. The L/S slurry is circulated back at high flow in a loop connected to a Venturi. The local underpressure in the neck causes gas to be sucked in : the intense turbulence achieves a very large interfacial area between tiny bubbles and the slurry. An external heat exchanger on the loop enables an almost unlimited heat removal, convenient for extremely high exothermic reactions, and isothermal operations. On the other hand, JLRs are restricted to a batch mode and can only accommodate catalysts compatible with the pump (low hard ness, low attrition). 2.2.3.

Bubbling column reactor BCR c

Also called "gas sparged reactor", it is little used in hydrogenations. Gas is fed, with partial recycling to increase turbulence, at the bottom of a virtually stationary L phase. Mixing is by far less efficient than in S T R or JLR. BCR is preferred only when the overall reaction is slow ; it is an alternative for TBR ( 5 2.2.6) with better temperature control as a result of higher liquid holdup.

C

2.2.4

Fluidized slurry reactor FBR

L

a

It only operates in continuous mode and uses catalyst particles of a slightly larger size than in BCR : an upward flow of L maintains S in suspension, but the L velocity should be slower than the S settling velocity. Stability also requires a very narrow particle size distribution. Hydrodynamics and mass transfer depend on G/L flow ratio. G velocity is usually rather slow, with bubbles rising through a continuous L phase. Heat removal is restricted to use of wall exchangers.

6

Submerged fixed bed reactor FBR S is immobile : fixed bed reactors always operate in continuous mode, which is not quite suitable for small fine chemicals pro0x0; 0 duction. 280 -80" In F B R s , particles are significantly larger than in slurry (1 to 10 mm) and packed in a fixed bed. A slowly moving L wholly wets the catalyst bed, giving excellent temperature stability and a close to perfect piston flow, whereas small gas bubbles ascend through the bed. The low gas flow makes F B R s not quite adapted for hydrogenation St.*"."t Liquid 2.2.6 Trickle-bed reactor TBR ton. flov TBR is in fact a version of FBR without submersion, but with a downward flow of L through the bed, in most cases co-currently to G. dry ,pot Quite different is the wetting of particles : here G is the continuous phase and the 5 to 5 0 mm particles may not be completely wetted by the downstream rivulets, and thus may develop "dry spotsp8. Stagnant pockets fill the interstices. Other operating difficulties are : wall-bypassing and, above all, stiff temperature control related to intricate hydrodynamic behavior. On the other hand, intense fluid phases interaction is achieved, (at the cost of increased energy consumption) and TBRs are more and more used in fine chemicals, for hydrogenations at higher pressures than in STRs. 2.2.5

B

:3

,

2-3 Which technology in industrial hydroqenation ? [ 2 7 1 FBR : commonly applied in petrochemicals and bioprocesses, it only has few applications in hydrogenations : phenylacetylene,

dinitriles. TBR : widely used for all sorts of hydrotreatments in petro and

chemicals. commodity chemicals, it is now adopted in fin= Intermediates hydrogenation includes : quinones, sugars, lactones, functional aromatics, etc... Despite continuous operation, small size TBR can be adapted to batch-wise synthesis by multiple recycling of L product. Example : trifluoracetic acid hydrogenation [ 2 8 ] . STR and JLR : batch hydrogenators are generally used : a technological comparison is given in J 2 . 6 below.

7

The difficulty of making the right choice is illustrated by the following table : continuous high pressure hydrogenation of adiponitrile in ammonia (obviously not fine chemistry) gives a meaningful example : company

raactor

BASP

trickle-bad

Philips Du Pont

slurry-loop reactor co-current upflow FBI?

ICI

fixed bed

VickersZimmr

downward cocurrent tube-bundla reactor

tamperature control by cooling and partial recycling of L serial arrangement of beds vith intermediate L cooling by cooling oe racyclad offgases evaporative cooling by inert diluant

2-4 Slurry or fixed bed ? advantages and disadvantages Glucose hydrogenation to sorbitol, ester hydrogenolysis to alcohols are good examples to depict the dilemma : formerly performed in a slurry technology (Raney nickel or copper chromite powders), they are now processed in TBRs, with new supported precious metal catalysts. Advantages are said to be : - no loss of metal, better quality of product (no contamination) - reduced side-reactions in L layer, due to smaller L holdup. The big drawback is the risk of a glpathologicalll loss of temperature control, related to the appearance of "hot spotsf1. Other examples of this : - in cyclohexene hydrogenation, benzene is coproduced [29] - due to decarboxylation risk of cyclohexane carboxylic acid, STR cascade is preferred to FBR in (old) benzoic acid + H2 process. The following table gives a selection of advantages and drawbacks [30] : techno

slurry phase

advantages

disadvantages

. continuous or batch mod, . mechanical stirring : E . T stability, easy heat expenditure, maintenance removal catalyst crushing . handling of viscous . high L hold-up, side liquids reactions . good L/S wetting. good . operation at outlet concatalyst life centration poor productl . good G/L L/S mass Vity (CSTR) transfer . difficult catalyst sepa. possible removal of ration expensive Filtracatalyst

. plug-Flow operation close to : high proFixed bed

. . .

..

ductivity low catalyst loss, precious metals low maintenance cost higher P and T, larger volumes low liquid holdup lower investment

tion product pollution by fines

. no viscous liquids . poor catalyst effectiveness (size) . risks of pressure drop . performances depend dramatically on hydrodynamics (narrow range) strength of catalyst + especially eor TBR difficult heat removal, difficult T control incomplete catalyst wetting

. mechanical . .

8

2-5 Consequences of technological choices on catalyst design Blurry phase (BTR, JLR, BCR, FBR)

2.5.1 A

good powder catalyst has the following properties :

- high resistance to attrition, to avoid the generation of fines - suspension characteristics (size, shape, density, material base. .)

- filterability (narrow and mono-dispersity, agglomeration,...) which is conflicting with suspension ! Especially important in JLR is a low hardness to preserve the fragile recirculation pump : a carbon support is favored. It should also be recalled that even in a L environment, the nature of the support can exert a chemical influence ; examples from patents : Na-exchanged silica + alumina is preferred to carbon for hydrogenation of trimethylquinone 1311 ZnO is preferred to alumina in long chain aldehydes hydrogenation 1321 2 . 5 . 2 Fixed bed (FBR, TBR) catalyst pellet requirements are : high mechanical strength, compatible with packing homogeneity in shape (spheres, cylinders, extrudates,...) control of metal impregnation Alumina under &form is the most common support. For hydrogenation in fine chemistry, in small scale TBRs, granular carbon increasingly raises interest on account of better heat conductivity, better wetting and also easy metal recovery in ashes after catalyst burning. 2 - 6 Batch hydrogenators For the reasons explained in the 2 previous paragraphs, most applications in fine chemicals are run in batch mode, where STR, JLR and BCR may be chosen [ 3 3 ] : the performances of these batch hydrogenators, as shown below in 5 3 , hinge on G/L mass transfer capability, and above all, on interfacial area a : m2 of bubbles area per m3 slurry :

. .

. . .

reactor

movement of bubbles

Venturi neck to reaction zone

2000 to 3000

For exothermal reactions like hydrogenations, usually the second limiting parameter :

heat transfer

~~

reactor BCR STR

JLR

cooling system

wall jacket and cooling coil, poor mixing same, with Intense m ixin g tube and shell exchanger on external loop

h (WU-~K-~)

=

500

900 > 1300

is

9

2-7 Laboratory G/L/S hydrogenators Hereafter is a random selection of lab hydrogenators : type hydroqenation of

STR BCR

FSR FBR TBR

STR

3 As

-

nitrobenzene cottonseed oil a-methylstyrene acetone 1-heptene styrene, phenylacetylene cyclohexene a-methylstyrene crotonal

pressure ranqe 2

to 21 bar 5 bar 1 bar c 1 bar 2 bar

1 to 1 1 to 1

5 bar bar 15bar bar

catalyst (size, conc.) a 1 2 to 7 dp-SSyn, dp = 10 dp 100

dp dp

--

gl-1 8 w/w 0 . 4 to 3 q1-1 to 65 pm to 200 pn

dp * 3 . 5 10111 0 , s to 4 mP 0,4 to 2 om dp-5nm

ref (341

(351 (361

137) (381 (391 1401 ~ 4 1 1 (421

DESIGN AND MODELLING OF Q/L/B HYDROGENATION REACTORS

mentioned in the introduction, the liquid medium negatively acts on the transport of H2. 1

L

The overall process cdnsists of the following successive steps : 1 - mass transfer from the bulk bubble to the interface 2 - mass transfer from interface to the bulk liquid phase 3 - mixing and diffusion in the bulk liquid 4 - mass transfer to the external surface of particle 5 - mass transfer inside the particle porosity 6-7-8 - catalysis (adsorption, reaction, desorption) Obviously the L substrate and product(s) follow similar processes (3 to 8 ) . Reactor design requires extensive knowledge of 3 aspects, wh i.ch are raised in this chapter : chemical kinetics

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

(E and m transfer)

'4 reactor

hydrodynamics 1

-

1

a pratical illustration, experiments performed in our lab (UMR CNRS-Rh6ne Poulenc) will be pointed out as an approach to global modelling of industrial G/L/S hydrogenators.

As 45

10

3-1 Hydrodynamics By hydrodynamics, we mean the movements of L and G phases through the S packing or with the S particles inside the reactor ; it - firstly the flow regime deals with : - then for STR : interfacial area, gas holdup, pumping flow of stirrer, power input, etc.. for TBR : pressure drop, solid wetting, liquid holdup etc. Physical properties ( L density, viscosity, tension,. . . ) strongly affect hydrodynamics,which in turn affect mass and heat transfer. For the flow regime, the most useful notion is "residence time distributiong1(R.T.D). A small STR is perfectly mixed and a lab TBR is perfectly piston plug-flow, but for larger equipments, inadequate baffling or stirring, the presence of coils, wall bypassing, axial and radial dispersions, etc.. make the real reactor far from ideal ! An assembly of ideal plug-flow reactors and continuous stirred reactors in series and parallel can be derived by a "simulation" procedure (see e.g. [ 4 5 ] ) to give the same R.T.D as the actual reactor. In our lab, R.T.Ds are experimentally determined for the 3 phases by pulse injections of radioactive tracers in the real medium : gaseous Ar or Kr : liquid organometallic complexes, or halides : solid neutron-irradiated metal catalysts. 3-2 Mass-transfer The organic sustrate is normally at much higher molar concentration than H2 dissolved in L. Therefore, the first limitation can be predicted to be on &, especially at low pressure : it is called the "limiting reactant". 3.2.1 Overall model Concentration profile, for limiting HZ :

C ( i s constant

due

to excellent

mixing in liquid

:

By definition of steady rates : r = kla (ci -c1) = to bulk L transfer

G

mass : rng

state, all ksas (cl

steps take

- c,)

L to S surface

transfer

=

place at

equal

msq k* OH eL overall grain reaction

11

The physical significance of the parameters will be briefly discussed later. kl, ks : mass transfer coefficients a, as : interfacial areas q : efficiency factor (see : 5 3.2.6)k* : rate constant 0 : coverage of active surface, with hypothesis of Langmuir Hinshelwood. It is essential here to focus not only on the rate, but on understanding that the H2 local concentration at the catalytic site, where selectivity is also settled, totally depends on preliminary transport phenomena. A striking example is [ 4 6 ] :

H2+cH30a - cH'0aN c1

c1

NO2

it is better not to have any H2 transfer limitations, because the site depletion in H2 would cause more C-0 and C-C1 hydrogenolysis, lower activity and more Pt detachment from carbon support. The overall model, too complex, can be converted in the case of limited Ha pressure, where Langmuir-Hinshelwood kinetics simplify to 1st order [ 4 7 ] (more complicated mathematical treatments can nevertheless be made, as shown by Aris [ 4 8 ] ) . Ci 1 1 1 1 - =

r r e s i stames :

- + kla

(-

ms

gas absorption

kSaS

+ -

1 '1 k*

external and i n t e r m l catalyst

Knowing r, the overall (slurry phase) hydrogenation rate for various catalyst loading, the plot of 1 vs 1 , if linear, allows r mg to calculate kla, the volumetric G/L mass transfer coef. We used this method in our lab for Raney metals catalyzed hydrogenation of ____................. cyano functions, but with rather I l/m, large imprecision. 3.2.2. G/L mass tranefer The vital and sometimes overlooked factor is the equilibrium solubility of H2 in L : Ci = P/& (Henry's law). The Henry constant is a function of temperature and the nature of the liquid. Values can be found, or estimated by "solubility parameters of Hildebrand" found in classical engineering handbooks [ 4 9 ] . P d,

ci'rPpl CI

12

On mixtures, except for one review [50], little information is available and measures are required : both indirect (physical absorption) and direct (chromatographic) methods are possible. In UMR45, by high pressure adjustment of a chromatographic method, we proved that water in organic media, even in trace amount, has a dramatic negative influence on H2 solubility. H2 is generally poorly soluble ; among the best solvents are : apolar (low E , [14]), volatile, low cohesion energy density. Unlike for other gases, HI for hydrogen decreases with T. The G/L resistance is "film" : r = kla . (Ci-Cl)

located only

on the

liquid side, in

kl : ms-1

a: m-1

c: mol. m-3

a

represents film thickness and properties : mass transfer coefficient. Different physical models corroborate experimental findings : kl cc D1 1/2. D1 molecular diffusivity (from first Fick law) is a function of L viscosity and temperature. Adapting the Stockes-Einstein law to real media led to WilkeChang correlation [51], among others : 1 D1 p . - = f (physical L parameters) T Direct D1 measuring remains more accurate, but expensive : we used Taylor's method [52] (pulse injections in capillary column). The rheological properties of L exert a huge influence on kl and hence must be apprehended in the model. A s a striking example, we discovered that adding a catalyst promotor made the L behavior change from ideal (Newtonian) to pseudo-plastic : viscosity p , very high at the beginning of the stirring, only goes down with increasing shear.

.

@ interfacial area (m2 par m3) gives a picture of how bubbles are spread. Extensive litterature is available on methods to measure kl, a and kla : see e.g. [53]. The physical absorption / desorption method was developped in UMR45 for H2 in organic L + catalyst S medium (see I 3.2.4. below). Typical values in G/L contactors are : cyclones

Vanturi

STR

2 0 to 50

100 t o 2500

100 t o 2000

[541 FBR 50 t o 1 7 0 0

(a in m - 1 )

13

The volumetric mass transfer coefficient can be correlated with EL the energy dissipation. A wealth of scattered data for G/L/S hydrogenation reackla’s’l [ I tors [ 551 , are summarized in the following graph. EL reaches a few kW.m-3 on plant, but 10 times that level 10-1 in lab reactors. kla is sometimes correlated 1 0 -2 , for with ( E L D L ) ~ / ~ especially - 1 ( 0 . 3 1 .-

3.2.3

10.1

100

101

Scaling-up G/L mass transfer

STRs are frequently used, and often limited by G/L transfer.

Extrapolation raises serious questions : which is the best scaling-up criterion ? rotation of stirrer N ? diameter D of the turbine ? a function N a D P ? Typical design (even for lab hydrogenators) of a 6-bladed Rushton turbine 4-baffle equipped STR : optimal geometry is : 0.3 < D/T < 0.5 H/T = 1 T/B = 10 H/S = 3 H/P = 6 The power input of such agitation is : P* = ct. N3D5 EL being P*/Volume, and kla being best n correlated to ( E l ) 1 / 2 , it is concluded that a I1good1lextrapolation criterion is constant N3/2D.

.

-

4

0

.

:

3.2.1 Influence of suspended 8 on kla : There is strong experimental evidence, but no agreement on effects [56] that S loading (and size, shape, density ...) affects G/L transfer in slurry. For these uppermost important effects, two lltheoriesll have been put forward : for decrease of kia : a decrease of kl through increased film viscosity or a reduction of area (surface phenomena, coalescence, turbulence dampening, bubble surface rigidity) for increase in kla : by collision effects, by stretching otherwise spherical bubbles by llshuttlegl effect, where adsorbing S (with dp < film) penetrates the film, loads transferring H 2 , returns to bulk L , then desorbs H2, thus enhancing the transfer process.

14

We recorded dramatic, reverse and still unforeseable effects ; the tendency of S to act either as an isolated particle or to agglomerate seems to be a key factor. This is connected to recent work on the adhesion of S to G bubbles [ 5 7 ] ; it is shown that i 20 pm Pd/C considerably enhances G/L transfer, since Pd/A1203 is inert. 3.2.5

L/B

mass transfer r = ksas . (C1

-

Cs)

L/S interfacial area can be calculated from loading, particle size and geometry H

m

A

L

a

turbulent liquid lamina film

Solid

can (seldom) be measured, but dimensionless group correlations are available

As conventional Re # cannot be computed, a concept of local tropic turbulence is introduced :

Rei #

iso-

T : energy dissipation dp : particle size y : L viscosity Practically, in STRs with dp < 5 0 pm, L/S transfer is almost never limiting even with viscous liquids. In TBRs, as k, is down to 10-5 ms-1, it may become limiting ; but in general, when limitation appears at the S level, the intragranular phenomenon prevails.

= T

dp4 Y-3

3.2.6 Intragranular mass transfer The approach is similar to G / S hydrogenation, but here the pores are filled with a stagnant liquid. H2 molecules move by a pure diffusional process : no Knudsen diffusion. Modelling remains basically the same as in G/S, with notorious differences : - D1 around 10-9 m2s-1, some 104 times less than in G phase ; - H2 concentration in L lower than the equivalent pressure G ; - 102 times better thermal conductivity in L : with the exception of ##dryspotsi8in TBR, beds and particles in G/L/S/ reactors can reasonably be assumed to be isothermal. The procedure for checking intraqranular diffusion is : - record apparent rate r, measure (or estimate) C1 and Deff, the effective diffusivity (see next 5 3.2.7) - record S characteristics : diameter dp, specific area Sp, density Pp

15

- suppose 1st order kinetics (other mathematical treatments are available), compute : 4 = (dp2.sP*pp.r)/ (Deff-C1) Thiele modulus and the effectiveness factor +I (--> l/+ when +anh

+

-->

m)

- the "truell (non diffusion disturbed) rate is r*

= r/,, : use with care for q < 0.7 due to errors on Deff. r.c dp2 ! It is The pratical usefulness is straightfoward : experimentally found that : - for dp < 100 pm, intragranular transport is very rarely 1imiting - for dp > 5 mm, intragranular transport always limits, which is the reason for the "egg shell8*design of catalyst for TBR Carberry et a1 [58] found for a-methylstyrene hydkogenation on = 0.007 with 8.25 mm Pd/A1203: q = 1 with 30 pm slurry (STR), pellet (TBR) Recall that kinetics are tgfalsifiedol in diffusion regime [59]. 3.2.7 Effective diffusivity in catalyst pores Deff can be measured, either directly by the flux through a catalyst pellet (Wicke-Kallenbach diffusion cell [60]), or by transient pulse method [61]. It is easier, but less accurate, to relate Deff to molecular diffusivity. E : porosity (fraction of S consisting of void) Deff = E / r ( 7 : tortuosity (can be viewed as the angle any pore makes with a straight line Usual values are : 0.25 < E < 0 . 5 ) ) => Deff g D1/10

+

.

3 < r < 3

)

In fine chemistry hydrogenation, the diffusional limitation

can

result not from H2 but from bulky (slowly diffusing) organic substrate ! Two examples [62] : linoleate (18 carbon chain) + H2 : preferably on a Itegg shellg1 Pd on granular carbon . 12 to 22 C nitriles t H2 : on a high porosity, large pores (low tortuosity) Ni -+ MgO + Si02 catalyst. The consequence of low diffusivity can be detrimental in the (common) case of consecutive hydrogenation : A --> B --> C : if the movement of desired B out of the porosity is slow, C by-product will increase, with a rapid selectivity drop in B. "Egg shelltt,uniform and "egg yolko8Ni/AlzOj catalysts [63] behave very differently for alkyne --> alkene (--> alkane) hydrogenations

.

16 3 . 3 Kinetics of G/L/S hydroqenations Transition from G/S to G/L/S cannot be done in a formal way, a detailed and comprehensive analysis is necessary [64]. Gathering reliable data, designing kinetic experiments taking into account side-reactions occurring homogeneously in the L medium, demands strenuous work. Then, if one wants to investigate liquid-phase effects and their kinetic complications, analysis of the results is laborious : even the mathematical data treatment is often difficult, because most experimental data is collected in closed systems (integral). Open (differential) reactors would be more adapted although costly and arduous to operate cleanly. Also, this time-consuming process to establish intrinsic kinetics is rarely realistic for a small company involved in fine chemicals [65]. It appears that Langmuir-Hinshelwood models frequently fit the data from serious studies, i.e with variety range of operating conditions ( P I TI concentrations) [64]. Dissociative adsorption of hydrogen is common, but in many instances on a different site than that which adsorbs the organic substrate

[661.

In our own lab experiments with various cyano compounds and nickel catalysts, we concluded on a l-site L.H type catalysis [67] but we had to introduce corrective parameters for substrate interactions, indicating failure of the basic assumption of surface ideality, i.e equal adsorption energy whichever coverage is reached. 3 . 4 Mass transfer eliminatin in laboratory hydroqenation To know how transport phenomena intervene, the criterion is to compare the observed rate to the maximum possible rates for G/L, L/S, S/pore mass transfer, as shown in 5 3.2. This procedure requires knowledge of a large set of values : diffusivity, Henry constant, kla etc.. The detection of intragranular diffusion is the most difficult path : - the Koros-Novak test [68] proposes dilution of the catalyst particles with inert material : it is however unable to discriminate an extragranular L/S transfer limitation - the Madon-Boudart test [ 6 9 ] works with constant size but different metal loading ("dispersion") on the support : it requires preparations of reproducible catalysts. - lately, methods based on increased poisoning of one single catalytic material have been proposed [ 7 0 ] . mfunnhr

I--\-

In our group, for slurry-phase hydrogenations, we use the wing diagram to check experimental regimes [71] :

-

follo-

17

.. . .. .. . . - ,

inIragranular

US lransler or US intragranular

-

l/dp

CONCLUSION

The use of solid catalysts, mostly supported and massive metals, for liquid phase hydrogenation of functional, complicated, expensive, fragile fine chemicals, has already led organic synthetic chemists t o cooperate w i t h catalysis experts, in order to design highly specific materials and reaction conditions, and tune-up the catalytic site activity and selectivity in the light of coordination chemistry concepts. The engineering complexity of triphasic gas-liquid-solid media makes the catalytic hydrogenation reactors troublesome to model and scale-up. The goal of this paper is to convince that a reactor engineering specialist must be involved in a "tri-expertll cooperation.

18

REFERENCES

6

M-Guisnet, J.Barrault, C.Bouchoule, D.Duprez, C. Montassier and G.PBrot (Eds.), Heterogeneous Catalysis and Fine Chemicals, Elsevier, Amsterdam, 1988 a) p.123 and 171 b) p.165 and 189 c) p.153 d) p.19 e) p.197 f) p. 145 Eastman Kodak, EP 347136 (14.6.88) RL.Augustine, Catalytic hydrogenation, Marcel Dekker, New York, 1985 M.Freifelder, Practical catalytic hydrogenation, Wiley, New York, 1971 PN.Rylander, Catalytic hydrogenation in organic syntheses, Academic Press, New york, 1979 : PN.Rylander, Hydrogenation methods. Academic Press, London. 1985 L.Cerveny (Ed.) , Catalytic hydrogenation, Elsevier, Amsterdam,

7 8

RhBne Poulenc EP320339(1.12.87); Henkel DE 3841698(10.12.88) R.Jacquot(Rh6ne Poulenc), communication at GECAT, Belgodere,

9 10 11 12 13 14

BASF, EP 325141(16.1.88) Mitsubishi Kasei, EP 343640(25.5.88) F.W.Vierhapper and E.L.Elie1, J.Org.Chem40(1975)2729 Technical Research Institute, US 4885289(8.6.87) Ciba Geigy, EP 363318(28.9.88) PN.Rylander, Chemical Catalyst News, Engelhardt Corp., October 89 and ref. herein Bayer, EP 355351(20.7.88) and DE 3824625(20.7.88) Ciba Geigy, EP 325892(31.12.87) M.Von Pierre, Helv. Chim. Acta 72(1989)1554 J.G.Ulan and W.F. Maier, J.Mol.Cat. 54(1989)243 Du Pont, US 4885391(14.1.88) Hoechst Celanese, EP 358420(7.9.88) Hoechst Celanese, EP 353898(19.7.88) A.Gianetto and P.Silveston (Eds.),Multiphase chemical reactors, Hemisphere, Washington, 1986 P.A.Ramachandran and R.V.Chaudhari, Three phase catalytic reactors, Gordon and Breach, New york, 1983 CG.Hi11 Jr, An introduction to chemical engineering kinetics and reactor design, Wiley, New york, 1977 : GF.Froment and KB. Bischoff, Chemical reactor analysis and design, Wiley, New york, 1979 : O.Levenspie1, The chemical reactor omnibook, OSU Book, 1979 : J.Villermaux, Genie de la reaction chimique, Lavoisier, Paris, 1982 : H.I.De Lasa (Ed.), Chemical reactor design and technology, NATO AS1 Serie E 110, Martiniis Nijhoff,

1

2 3 4 5

1986 1990.

15 16 17 18 19 20 21 22 23 24

1986. 25 26 27 28 29 30 31 32 33 34 35

JH.Rushton, Chem.Eng.Progr. 46 (1950), 395 RJ.Malone (Herzog-Hart Corp), CEP june 1980, 53 A.Gianetto and P.L.Silveston, chapter 16 in ref. 22 Rh6ne Poulenc, EP 365403(21.10.88) V.Stanek and J.Hanika,Eth Congress CHISA,Prag, September 84 J.Hanika and V.Stanek, chapter 16 in ref. 6 Mitsubishi, EP 264823(1986) Eastman Kodak, US 4837368(6.6.89) J.J.Concordia (Herzog-Hart Corp), CEP, march 90, 50 F.Turek and R.Geike, Chem.Technik 33 (1981) 24 J.Marangozis, 0B.Keramidas and G.Paparisvas, IEC PRD 16 (1977)

36 37 38

T.K.Sherwood and E.J.Farkas, Chem.Eng.Sci 21(1966)573 N.O.Lemcoff and G.J.Jameson, AIChE J. 21(1975)730 A.N.Gareman, A.Emakova, V.P.Bachvalova and N.I. Rassadnikova, Hung.J.Ind.Chem. 3(1975)37 S.Mochizuki and T.Matsui, AIChE J. 22(1976)904

361

39

19

40 J.Hanika,K.Sporta,Z.Ulbrichova,J.Novak and V.Ruzicka, Coll. Czech. Chem. Comm. 39(1974),240 41 F.Turek, R.Lange, A.Busch and R.Loewe, Chem.Technik 27 (1975), 149 42 C.N.Kenney and W.Sedriks, Chem.Eng.Sci. 27(1972),2029 43 F.Turek, R.Chakrabarti, R.Lange, R.Geike, W.Flock, Chem.Eng. Sci 38(1983),275 44 JM.Lambert Jr, in D.W.Blackburn (Ed.), Catalysis in organic Reactions, Marcel Dekker, New york, 1990, p. 97 45 JM.Smith, Chemical engineering kinetics, Mac Graw Hill, New York, 1981 46 G.Leuteritz, ACHEMA, Frankfurt a.M., june 1985 47 C.N.Satterfield, Mass transfer in heterogeneous catalysis, MIT Press, Cambridge USA, 1970 48 R.Aris, Mathematical theory of diffusion and reaction in permeable catalysis, Clarendon, Oxford, 1975 49 Perry, Chemical Engineering Handbook, Mac Graw Hill, New York; R.C.Reid,J.M.Prausnitz, B.E.Poling, Properties of gases and liquids, Mac Graw Hil1,New york, 1987 50 H.Battino and H.Clever, Chem.Rev. 60(1966)395 51 C.R. Wilke and P. Chang, AIChE J. 1(1955)264 52 G.Taylor, Proc. Royal SOC. London, GB A 219(1953)186 and 225 (1954)473 53 J.C.Charpentier, chapter 4 in ref. 22 54 A.Laurent and J.C.Charpentier, 1ntern.Chem.Eng. 3(1983) 265 ; 55 H.J.Warnecke and P. Hussmann, Chem.Eng.Comm.78(1989)131 J.Voigt and K.Schueger1, Chem.Eng.Sci. 34(1979)1221 ; LL.Van Dierendonck, G.W.Meindersma and GM.Leuteritz, 6 th Euro Conf. on Mixing, Pavia, may 1988 56 J.C.Lee, S.S.Ali and P.Tasakorn, 4 th Euro Conf. on Mixing, Noordwijkerhout, april 82 : G.E.Joosten, JG. Schilder and J.J.Jansen,Chem.Eng.Sci. 32(1977)563 : E.Alper, T.Wichtendah1 and D.Deckwer, Chem.Eng.Sci.35(1980)217 : S.K.Pa1, MM. Sharma and VA Juvekar, Chem.Eng.Sci.37(1982)327 : E.Sada, H.Kumazawa and I.Hashizume, Chem.Eng.J.26(1983)239 ; E.Alper, Chem.Eng.Comm.36(1985)35 57 0.J.Wimmers and J.M.Fortuin, J.Eng.Sci. 43(1988)313 58 N.D.Sylvester, K.I.Kulkami and J.J.Carberry,Can.J.Chem. Eng. 53 (1975)313 59 A.Wheeler in Advances in Catalysis, vol. 3, Academic Press, New york, 1951 60 E.Wicke and P.Kallenbach, Kolloid 2. 97(1941)135 61 N.Wakao and S.Kaguei, Heat and mass transfer in packed beds, Gordon and Breach, New york 1980 62 W.A.Cordova and P.Harriott, Chem.Eng.Sci.(1975)1201 Unilever, EP 340848(6.5.88) 63 Y.Uemura and Y.Hatate, J.Chem.Eng.Jap. 22(1989)287 64 S.L. Kiperman, chapter 1 in ref. 6 65 H.J.Janssen,AJ.Kruithof,G.J.Steghuis andK.R. Westerterp, Ind. Eng.Chem.Res 29 (1990) 754 66 OM.Kut, F.Yuecelen and G.Gut, J.Chem.Tech.Biotech 39 (1987),107 67 C.Mathieu, E.Dietrich, S.Indey, H.Delmas and J.Jenck, RQcents Progres en Genie des ProcQdBs, Lavoisier, Paris, in press 68 R.M.Koros and E.J.Novak, Chem.Eng.Sci. 22(1967)470 69 R.J. Madon and M. Aoudart, 1nd.Eng.Chem.Fund. 21(1982) 438 70 G.W.Smith,D.J.Ostgard,F.Notheisz,A.Zsigmond,I.Palinko and M.Bartok, in D.W. Blackburn (Ed.) Catalysis in Organic reactions, Dekker, New york, 1990, p. 157 71 J.Breysse, RhBne Poulenc Industrialisation, private communication

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine @ 1991 Elsevier Science Publishers B.V., Amsterdam

Chemicals I1

STRUCTURE-REACTIVITY IN THE HYDROGENATION OF ALKENES. COMPARISONS WITH REDUCTIONS BY DIIMIDE AND THE FORMATION OF A Ni(0) COMPLEX

S . SIEGEL

Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas, 72701 (USA)

ABSTRACT The effect of structure on the rates of hydrogenations catalyzed by Pt, Pd, and Ni is compared with the effects upon the rates of reduction by diimide (diazene) (Garbisch) and the association constants with a Ni(0) complex (Tolman). These later reactions serve as models for the effect of structure on certain of the elementary reactions of catalysis by metals. Some of the factors which determine the selectivity of a catalyst are reviewed including the kinetics, the metal, and the importance of isomerization as a competing reaction. INTRODUCTION The rate of hydrogenation of an alkene depends upon the catalyst, the reaction conditions, and the structural environment of the double bond. That substituting alkyl groups for vinyl hydrogens lowers reactivity has been known for many years (refs. 1-3). The individual and competitive rates of hydrogenation (Pt/SiOz) of alkenes which represent a broad range o f structure has been reported by Tellier and Maurel (ref. 3 ) . Other structural effects on reactivity have been recognized but no comparable study embraces structures which include vinyl substituted polar groups as well as unsaturated hydrocarbons. In this paper we shall compare the effect of structure on hydrogenations on Pt, Pd and Ni catalysts with the structural effects on reductions with diimide (diazene) (ref. 6) and the equilibrium constants for the association of substituted ethylenes with a Ni(0) complex (ref. 7). These particular reactions were chosen because of our perception of their relation to the mechanisms of catalytic hydrogenation, and the insightful analysis of the relationship between structure and reactivity provided by the authors of these studies. KINETICS AND MECHANISM ON Pt AND Pd Maurel and Tellier showed that the variation in the structure of alkenes has a much smaller effect on the individual rates than on the competitive rates of hydrogenation on Pt/SiOz (refs. 4 . 5 ) . For a group of 24 compounds, the individual rate constants, kA differ by less than 10 whereas the competi-

21

22 6 tive rates span a range of 10 . Following the procedure of Wauquier and Junger

(ref. 8). they extracted the relative adsorption constants, KA, from the competitive rates of hydrogenation with the aid of the individual rate constants, eqn. (l), where I is the slope of the plot of log[A] vs log[B]. Seven alkyl

substituted ethylenes furnish a linear plot of log

K.4

against the summation of

the Taft polar substituent constants, sigma* (ref. 4 ) . But the polar and steric substituent constants of alkyl groups are intercorrelated and therefore the result does not reveal the relative contributions of these effects (refs. 2,3). The method of Wauquier and Jungers assumes Langmuir-Hinshelwood kinetics in

which the fraction of the surface, which is occupied by an adsorbed reactant, is governed by the Langmuir adsorption isotherm. Hussey et al., however, argue that the relative competitive rates on platinum catalysts are measures of the competitive rates of alkene adsorption (ref. 9). They note the near absence of isomerization in Pt catalyzed hydrogenations and, when Dz is used in place of

Hz, little of the exchanged alkene is formed although the distribution of deuterium in the product alkane indicates a rapid interconversion of the alkyl intermediate and the adsorbed alkene. These interpretations of the kinetics differ in that in one, the rate of desorption of the alkene is assumed to be fast relative to the conversion of adsorbed alkene to the alkyl intermediate, while in the other, desorption is assumed to be relatively slow. The appropriate interpretation is likely to depend upon the structure of the unsaturated compound, the catalyst and the conditions employed, all variables which affect the product controlling step. The mechanisms of hydrogenating cyclohexene on Pt and Pd differ. Madon, O'Connell and Boudart found the kinetics of hydrogenation of cyclohexene on platinum in the liquid phase is zero order in cyclohexene and first order i n Hz; the rate constant is independent of the solvent providing that the concentration of Hz is used in the rate expression (ref. 10). They concluded that the rate is determined by the dissociative adsorption of HZ which reacts rapidly with the alkyl intermediate, presumed to be the main form of adsorbed cyclohexene on the surface. I n contrast, Gonzo and Boudart showed that the rate of the gas-phase or the liquid-phase hydrogenation of cyclohexene on palladium, supported on silica gel or alumina, is zero order in cyclohexene but one half order in hydrogen pressure (ref. 11). Recalling that a large amount of exchanged alkene and some HD is formed when deuterium is used in place of hydrogen (ref. 1 2 ) they showed that the results are consistent with a mechanism in which adsorption of Hz is reversible and the reaction of an

23

adsorbed hydrogen atom with the alkyl intermediate is the rate controlling step. Similar conclusions were drawn by Lee, Inoue and Yasumori from their studies of the kinetics of the gas-phase hydrogenation of cyclohexene on highly dispersed Pd on ZrOz or A1203 (ref. 13). The distribution of deuterium in the products when Hz was replaced with either Dz or mixtures of Hz and Dz furnished supporting evidence for their mechanism. From studies of the Pt/SiOZ and Pd/SiOz catalyzed reactions of various alkenes and alkynes with Hz and Dz in a liquid-phase batch reactor, Kung and Burwell, Jr. concluded that "adsorbed hydrogen was in preequilibrium on neither catalyst and adsorbed olefin was not in preequilibrium on Pt/SiOz and probably not on Pd/SiOz except for trans-di-t-butylethylene", the most sterically hindered olefin in their study (ref. 1 4 ) . Their conclusions do not deny that the elementary reactions which precede the rate or product controlling reaction are reversible; only that they are not in equilibrium. The Langmuir-Hinshelwood treatment of the kinetics of surface catalyzed reactions affords a useful representation of some of the characteristics of catalytic hydrogenation. It is a limiting form of more exact equations which recognize that, even though the elementary steps are reversible, few if any will be at equilibrium (ref. 15). Not surprisingly, alternative assumptions regarding the relative rates of the forward and reverse elementary reactions can lead to approximate equations of the same form. Individual rates usually are determined under conditions in which the rate is zero order in alkene. In the competitive reactions, however, the relative

rates are proportional to the relative concentrations of the competing unsaturated compounds, i.e. first order. Presumably, the mechanisms are unchanged by the competitor, and accordingly, the relative individual rates and the relative competitive rates are determined by the difference in energy of the same transition states but different ground states; in the former the alkene is bound to the catalyst, in the latter it is free. The effect of alkene structure on relative reactivity indicates that a much greater structural change in the alkenic moiety occurs on adsorption than in the change from adsorbed alkene to the transition state of the rate controlling surface reaction. Moreover, where measures indicate appreciable differences in adsorption energy, the more strongly adsorbed compound often exhibits the smaller zero order rate. STRUCTURE-REACTIVITY IN RELATED REACTIONS Some understanding of the effect of structure on the rate of catalytic hydrogenation has been sought through comparisons with structural effects in other types of reactions. The attempt to find linear free energy relationships

24 between recorded substituent constants and either the reaction rates or the apparent relative adsorption constants have had some success (refs. 2,3). We believe, however, that the effect of structure on the association constants of substituted ethylenes with Ni[P(O-o-toly1)3]3 and the reduction of the double bond by diimide are particularly useful models. The first models the effect of structure upon the adsorption of substituted ethylenes on a metal, the second involves the

syll

transfer of hydrogen to the carbon-carbon double bond and

models either the adsorption of the alkene or the transfer of the first hydrogen atom to the adsorbed alkene to form the alkyl intermediate (ref. 16). These hypotheses are supported by recent ab initio quantum mechanical calculations for reductions by diimide (refs. 17,18), and for hydrogenations catalyzed by ClRh(PH3)3

(ref. 19). In both reactions, electron density is trans-

ferred to the alkenyl double bond, the bond is lengthened and the attached groups are bent out of the plane in an eclipsed conformation. Because in both model reactions the rate is proportional to the concentration of the unsaturated compound, neither reaction represents directly the effect of structure upon the individual rates of hydrogenation when zero order in alkene; correlations with the model reactions are seen within groups of structurally related compounds. Complexation with Ni[P(O-o-tolyll& Tolman has shown that the equilibrium constants for the reactions of 38 substituted ethylenes with Ni[P(O-o-tolyl3)]3 in benzene, to form (ENE)bis(tri-o-toly1phosphite)nickel complexes, is sensitive to the ethylene's struc-

ture, eqn. (2) (ref. 7 ) . Values of

NiL3 + ENE

Ki =

(ENE)NiL2

+L

K1

at 25'

where L

=

-4 vary from 10 for cyclohexene to

P(O-o-tolyl)s.

8 4 x 10 for inaleic anhydride. The stability of the complex is enhanced by electron withdrawing substituents, such as cyano and carboxyl, and lowered by alkyl groups. That resonance involving unshared electrons on the oxygen of an alkoxy group overpowers the inductive effect is indicated by the relative values of K1 for ally1 methyl ether, 1-hexene, and vinylbutyl ether which diminish in that order by factors of 3:1:0.006. Log K1 correlates well with the sum of the substituent parameters, sigmap+,as defined by Swain and Lupton (ref. 20). Tolman notes that the high sensitivity of the Ni(0) equilibrium constants to structural modifications of the alkene is due to the low ionization potential of Ni(0)

and the resulting small energy separation between the HOMO

of the metal and the pi* orbital of the alkene. Steric effects of substituents

25 are relatively unimportant compared to electronic effects and resonance is more important than inductive interactions. The ability of the metal to back bond is lowered progressively in the series Ni(0) > Pt(0) > Rh(1) > Pt(I1) > Ag(1) which reduces the importance of resonance and decreases the selectivity of the metal for different substituted alkenes. The relative importance of sigma donation from the occupied pi orbital of the alkene to an empty metal orbital compared to back donation from the metal to the alkene‘s pi* orbital determines the geometry of the alkene moiety which can vary from close to the planer alkene to a structure best described as a rnetallacyclopropane (ref.

21). The later structure might explain why trans- disubstituted ethylenes form more stable complexes than their

cis- isomers (see following section).

Diimide reductions 6

The relative reactivities toward diimide cover a range of -10 , from 1 , 2 dimethylcyclohexene to norbornene (ref. 6). Electron attractive substituents increase the reactivity of the double bond towards diimide although the data to place compounds such as maleic acid or acrylonitrile on the scale for Garbisch’s hydrocarbons is lacking (ref. 21b). Garbisch et al. found that the main factors that contribute to the observed reactivities in diimide reductions of unsaturated hydrocarbons, eqn. ( 3 ) , are torsional strain, bond angle

+ H

‘N_N/

H

,

,

NEN

NEN 3)

strain, and alpha-alkyl substituent effects as indicated by the good agree ent between calculated and observed relative reactivities. In their calculations, they assumed that the transition state occurred early along the reaction coordinate, about one third of the change to the saturated product, and that the pi-bond order is fairly large. Steric effects, between diimide and the alkene, are assumed to be negligible. Resonance or polar interactions between vinyl substituents and the double bond affect the ground state energy which decreases to zero in the product. Using the same structural parameters in the calculations, the agreement with the observed relative reactivities of cycloalkenes for different addition reactions indicates that the model is qualitatively correct (ref. 22). This treatment was applied also to stereo

26

selectivities (refs. 6,16). The effect of polar groups on the diimide reaction is sensitive to the configuration of the attached groups. For example, fumaric acid (trans) is ten times as reactive as maleic acid (&)

and the ratio of reactivities of the

geometrical isomers of cinnamic acid, trans/cis, is 10:3 (ref. 21b). In comparison, &-and trans-2-butene have almost identical reactivities. The difference may be explained by a change in the degree of advancement of the transition state towards the saturated product where the eclipsed conformation would result in a greater non-bonded repulsive interaction between the

a-

substituents than the trans. A correlation of the effect of structure on the Ni(0) association constants

and reductions by diimide is displayed in Fig. 1. Unfortunately, none of the negatively substituted ethylenes in Tolman’s series are included in Garbisch et al.’s study.

Log k (Diimide)

Fig. 1. Correlation of l o g k (Diimide). (a) vs l o g I (Pt/A1203) ( * ) ; (b) vs log KAB ( P t / A l z 0 3 ) (0). ComDarison of catalytic hydrogenation on metals with the model reactions The correlation between the apparent association constants, KA, which are derived from the competitive rates on Pt and reductions by diimide indicates that structural changes in the alkene generally have parallel effects on these reactions, Fig. 2 .

Because the diimide reduction is essentially free of

steric effects, this effect is liable to account for some of the differences which are observed in extended groups of compounds. The small range of individual reactivities on Pt, which are zero order in alkene, can be understood in that the variation in structure which increases the driving force towards

27

Log k (Diimide)

Fig. 2 . Correlation of log k (Diimide) vs l o g K (Ni(0) complex). the addition of hydrogen, also increases the strength of adsorption on the metal. The latter is a function of the metal that apparently diminishes in the order Pt> P&

Ni (refs. 3 , 1 4 , 2 3 ) .

Within a limited group of hydrocarbons, cycloalkenes, the kinetically derived association constants on Pt/Alz03 correlate with both model reactions and the strain in the double bond which suggests that the relief of strain is a principal factor in determining relative reactivity in this series, Table 1. TABLE 1 Structure-Reactivity of cycloalkenes. Comparisons of individual and competitive hydrogenation rates on Pt with related reactions.

Hz,Pt'

Ni(ENE)b

STRAINh

Diimidec

~~

Compound

h

Bicycloheptene Cis-cyclooctene Cyclopentene Cycloheptene Cyclohexene

223 10 121

78 113

KAB

--25. 7.5 6.3 (1.0)

Keq

4.4

6.Z X ~ O - ~ 2.6~10-~ 2.3~10-~

3.5~10-~

a0.52% Pt/Alz03 at 250C, 1 atm, (ref. 3 ) . b(Ref. 5 ) . c(Ref. 4 ) .

H ,Kcal

krel

27.2 7.4

4 . 5x102 17.

6.8 6.7 2.5

12.

15.5 (1.0)

28

Interestingly, the order of the reactivity in the individual rates on Pt, Pd, and Ni exhibit similar patterns except for the placement of cyclohexene and cyclooctene, Table 2 . The relative reactivity of cyclooctene is low because, other than norbornene, it is more strongly attracted to the metal than are the others in the group. TABLE 2 . Effect of structure of cycloalkenes on the individual rates of hydrogenation (relative to cyclopentene) on metal catalysts compared to diimide reductions. Compound Bicycloheptene Bicyclooctene Cyclopentene Cyclohexene Cycloheptene Cyclooctene

Diimidea

Pt/AlzO~b

Pd/SiOzb

29 1.9

1.8

1.9 2.0

1.4

1.4

(1.0)

(1.0)

(1.0)

(1.0) 0.43 0.96 0.13

0.90 0.64 0.08

0.065 0.078

1.1

0.5

___

0.05

NiCc

___

NiBd 3.8

___

(1.0) 0 .O l e

0.6 0.2

'(Ref. 1). b(Ref. 14). c(Ref. 27). d(Ref. 23). RRelative to cyclooctene, the value would be 0.16 from ref. 29 Although alkenes appear to be less tightly bound to Pd than to Pt, the relative individual rates in the two series differ little. The competitive rates on Pd were not determined so a comparison with the kinetically derived relative adsorption constants on Pt is unavailable. The zero order rate of norbornene relative to cyclohexene on 1-5% Pd/A1203 at 30 OC is 3.4 while the relative competitive rate is 4.7 which increases to 7 . 6 in the presence of triphenylphosphine (ref. 24). STRUCTURE AND REACTIVITY ON NICKEL Nickel affords selective catalysts for the hydrogenation of alkenes, dienes, and alkynes. When catalyzed by C. A. Brown's P - 2 nickel, prepared by the reduction of Ni(0Ac)z

with NaBHb in ethanol, the individual rates as well

as the competitive rates appear to be sensitive to the alkene structure as judged by the reported initial rates of hydrogen addition (ref. 23). Alkene isomerization is relatively slow. Except for the most reactive alkenes such as norbornene. the individual hydrogenations seem to be first order in alkene. This indicates that alkenes are more weakly bound to Ni than to Pt or Pd. Similar selectivities are reported by Brunet, Gallois, and Caubere for a catalyst prepared by the reduction of Ni(0Ac)z (ref. 27).

with NaH and t-amyl alcohol in THF

29

The order in which the reactivity of these cycloalkenes fall on these nickel catalysts may be compared with the relative reactivities on Pt, with diimide and with the Ni(0) association constants measured by Tolman, Tables 1 and 2. The place of cyclooctene in these orderings is particularly noteworthy. Recall that cyclooctene is the better competitor in hydrogenations on Pt which is reflected in the relative apparent adsorption constants, Table 1. The two nickel catalysts mentioned above show opposite relative reactivities for these cycloalkenes although the illustrated plots of the progress of the reactions on both catalysts suggest that the rates are approximately first order in alkene. Interestingly, Brown reports that over his P-2 Ni, the selectivity of cyclooctene over cyclohexene is larger in competitive hydrogenations than in individual reductions (ref. 23). An explanation for this difference in selectivity of the Ni catalysts is suggested by the studies of Okamoto et al. who correlated the difference in the X-ray photoelectron spectra of various nickel catalysts with their activity and selectivity in hydrogenations (ref. 28,29). They find that in individual as well as competitive hydrogenations of cyclohexene and cyclooctene on Ni-B, cyclooctene is the more reactive while the reverse situation occurs on nickel prepared by the decomposition of nickel formate (D-Ni). On all the nickel catalysts the kinetically derived relative association constant favors cyclooctene (ref. 29). The boron of Brown’s P-2 nickel donates electrons to the nickel metal relative to the metal in D-Ni. The association of the alkene with the metal is diminished which indicates that, in these hydrocarbons, the electron donation from the HOMO of the alkene to an empty orbital of the metal is more important than the reverse transfer of electron density from an occupied d-orbital of the metal into the alkene’s pi* orbital. APPLICATIONS TO SYNTHESIS There is general recognition that selectivity for the addition of hydrogen to one compound rather than another in a mixture, or to a particular double bond in a compound which has multiple unsaturation, depends upon the catalyst and the conditions. The illustrated structure-reactivity correlations afford an estimate of the degree of selectivity which may be achieved when adsorption is adequately reversible. The later is aided by a weakening of the attraction between the double bond and the metal center of the catalyst. There are circumstances when the opposite selectivities are desired and kinetic control of adsorption may be required. This aspect of selectivity is not addressed here. For example, Tolman found that the Ni(0) association constant of vinyl 6

methyl ketone is 5 x 10 greater than 2-methyl-1-pentene.Accordingly, it is

30

not surprising that Pd catalyzes the highly selective addition of hydrogen to the double bond in eremophilone which is conjugated with the carbonyl group leaving the methylene group untouched, eqn. (4) (ref. 3 2 ) . In contrast, tristriphenylphosphinerhodium(1) chloride, (PhsP)RhCl, catalyzes the

hydrogenation of the methylene group exclusively. This illustrates Tolman’s note that the oxidation state of the metal affects the selectivity of a metal for different alkenes (ref. 7).

0

0

Eremophi lone

The possibility that isomerization may effect the selectivity is important to note. A convenient method o f removing 1,9-octalin from a mixture of the 1 , 9 - and the 9,lO-octalinsis to hydrogenate the mixture over a Pt catalyst.

Pd is ineffective because it is more active than Pt in catalyzing the inter-

conversion of the isomers, eqn. (5) (procedure suggested by Hussey given in ref. 3 0 ) .

11 [HI

031 , g - o c t a l i n

(5)

Double bond migration can be inhibited by amines and other nucleophilic agents and lead to higher selectivities in the reduction of dienes such

as

1,4-cyclooctadieneor 1,3-cyclopentene to their respective monoenes (ref. 33)

31

If alkene isomerization is not possible or degenerate as in cyclohexene, the relative individual rates are not greatly different (Table 1) (ref. 14). The literature indicates that selectivity often can be improved, particularly with Ni and Pd catalysts by the use of promoters such as amines (ref. 34). Presumably, the amine competes for reactive sites with the alkenes and is effective if its adsorption constant lies between the constants of the competing alkenes. The effectiveness of the promoter is not diminished with the depletion of the more reactive alkene and is most useful with a supported catalyst where the concentration of molecules near a reactive site may be limited by pore diffusion. Selectivity would also improve if the promoter increases the rate of desorption of the alkenes (ref. 35). An interesting means of improving the selectivity of Pd for the conversion o f unconjugated dienes, such as 1,4-cyclooctadieneto the monoene is to add

phenylacetaldehyde to the mixture undergoing reaction (ref. 36). The mechanism of action is not established but it may involve aldehyde decarbonylation to form adsorbed CO; but the addition of small amounts of CO to the reactants does not reproduce the effect of the aldehyde (ref. 37). Means to modify the metal suface in other ways can prove effective, the studies of Ni catalysts by Okamoto et al. afford an interesting example of an attempt to reach a more fundamental understanding of catalyst selectivity. SUGGESTIONS FOR FURTHER STUDY Some suggestions are in order for anyone beginning a kinetic study of the effect of structure on reactivity. Aside from the precautions to assure that the rates are free from transport limitations, consideration should be given to the effect of alkene concentration on the individual rates as well as the competitive rates in light of the studies of Boitiaux et al. who found that the more strongly bound unsaturated compounds inhibit reduction at high concentrations, particularly on catalysts with low metal loadings and high dispersions (ref. 25). Furthermore, Kung et al. have shown that the competitive reactivity of sterically hindered hydrocarbons relative to cyclopentene on Pt catalysts is sensitive to the dispersion of the metal (ref. 26). REFERENCES 1 2

3

4 5 6

S. V. Lebedev, G. G. Kobliansky and A. 0. Yakubchik, J. Chem. SOC., 127 (1925) 417. M. Kraus, Adv. Catal., 29 (1980) 151. L. Cerveny and V. Ruzicka, Adv. Catal., 30 (1981) 335. R. Maurel and J. Tellier, Bull. SOC. Chim. France (1968) 4650. R. Maurel and J. Tellier, Bull. SOC. Chim. France (1968) 4191. E. W. Garbisch, Jr., S . M. Schildcrout, D. B. Patterson and C. M. Sprecher, J . Am. Chem. SOC., 87 (1965) 2932.

32

7 8 9

10 11 12 13 14 15

16 17 18 19 20 21 21b 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

C. A . Tolman, J . Am. Chem. SOC., 96 (1974) 2780. J. P. Wauquier and J . C. Jungers, Bull. SOC. Chim. France, 24 (1957) 1280. A. S. Hussey, R. H. Baker and G. W. Keulks, J. Catal., 10 (1968) 258. R. J. Madon, J . P. O'Connell and M. Boudart, AIChE J., 24 (1078) 904. E. E. Gonzo and M. Boudart, J. Catal., 52 (1978) 462. R. L. Burwell, Jr., Acc. Chem. Res., 2 (1969) 289. B. Y. Lee, Y. Inoue and I. Yasumori, Bull. Chem. SOC. Jpn., 54 (1981) 13. H. H. Kung and R. L. Burwell, Jr., J. Catal., 63 (1980) 11. M. Boudart and G. Djega-Mariadassou,Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Princeton, N.J., 1984, Chap. 3. S . Siegel, G. M. Foreman and D. Johnson, J. Org. Chem., 45 (1975) 3589. E. Flood and P. N. Skancke, Chem. Phys. Letters, 54 (1978) 53. D. J. Pasto and D. M. Chipman, J . Am. Chem. SOC., 101 (1979) 2290. N. Koga, C. Daniel, J. Han, X. Y. Fu and K. Morokuma, J. Am. Chem. SOC., 109 (1987) 3455. C. G. Swain and E. C. Lupton, Jr., J. Am. Chem. SOC., 90 (1968) 4328. J. P. Collman, L. S . Hegedus, J . R. Norton and R. G. Finke, Principles and Applications o f Organotransition Metal Chemistry, University Science Books, Mill Valley, California, 1987. S. Hunig, H. R. Miller and W. Thier, Angewandte Chemie, Int. Edn., 4 (1965) 271. E. W. Garbisch, Jr., J . Am. Chem. SOC., 87 (1965) 505. C. A . Brown and V. K. Ahuja, J. Org. Chem., 38 (1973) 2226. J. A . Hawkins, Doctoral Dissertation, University, University of Arkansas, Fayetteville, AR, 1983, pp. 87, 102, 136. J. P. Boitiaux, J. Cosyns and E. Robert, Applied Catal., 32 (1987) 145. H. H. Kung, R. Pellet and R. L. Burwell, Jr., J. Am. Chem. SOC., 98 (1976) 5603-56117 J - J . Brunet, P. Gallois and P. Caubere. J. Org. Chem., 45 (1980) 1937. Y. Okamoto, Y. Nitta, T. Imanaka and S . Teranishi, J . C. S . Faraday I , 75 (1979) 2027. Y. Okamoto, Y. Nitta, T. Imanaka and S . Teranishi, J . Catal., 64 (1980 397. G. V. Smith and R. L. Burwell, Jr., J . Am. Chem. SOC., 84 (1962) 925. A. W. Weitkamp, J . Catal., 6 (1966) 431. M. Brown and L. W. Piszkiewicz, J . Org. Chem., 32 (1967) 2013. H. Hirai, H. Chawanya and N. Toshima, Bull. Chem. SOC. Jpn., 58 (1985) 682. P. N. Rylander, Hydrogenation Methods, Academic Press, Inc. (London) Ltd., 1979. J. P. Boitiaux, J . Cosyns and S . Vasudeuan, Applied Catal., 15 (1985) 317326. S . Nishimura, M. Ishibashi, H. Takamiya, N. Koike and T. Matsunaga, Chem. Lett., (1987) 167. S . Nishimura, personal communication.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

33

HETEROGENEOUS CATALYTIC OXIDATION AND FINE CHEMICALS R.A.

Sheldon

5900 AB

R&D D e p a r t m e n t , Andeno B.V.,

VENLO (The N e t h e r l a n d s )

ABSTRACT The use o f s o l i d c a t a l y s t s i n l i q u i d p h a s e o x i d a t i o n s o f r e l e v a n c e t o f i n e chemicals manufacture i s reviewed. Heterogeneous c a t a l y s t s o f f e r obvious a d v a n t a g e s : ease o f p r o d u c t a n d c a t a l y s t r e c o v e r y a n d s u i t a b i l i t y f o r continuous processing. Moreover ' s i t e i s o l a t i o n ' o f redox m e t a l i o n s i n o x i d a t i v e l y r e s i s t a n t inorganic matrices a f f o r d s s t a b l e c a t a l y s t s w i t h unique a c t i v i t i e s a n d s e l e c t i v i t i e s . The v a r i o u s t y p e s o f o x i d a t i o n p r o c e s s e s a r e r e v i e w e d on t h e b a s i s o f t y p e o f mechanism, o x i d a n t a n d c a t a l y s t . The l a t t e r i s d i v i d e d i n t o t h r e e categories : supported metals, metal i o n s and o x i d i c ( o x o m e t a l ) c a t a l y s t s . Emphasis i s p l a c e d on s y s t e m s e x h i b i t i n g u n u s u a l substrate, chemo-, regioand s t e r e o s e l e c t i v i t i e s , e s p e c i a l l y on new developments such as redox z e o l i t e s and redox p i l l a r e d c l a y s . INTROOUCTION C a t a l y t i c o x i d a t i o n i s t h e most i m p o r t a n t technology f o r t h e c o n v e r s i o n o f hydrocarbon feedstocks ( o l e f i n s , a r o m a t i c s and alkanes)

t o a variety of bulk

I n g e n e r a l , two t y p e s o f processes a r e used : h e t e r o -

i n d u s t r i a l chemicals.'

geneous, gas p h a s e o x i d a t i o n and homogeneous l i q u i d p h a s e o x i d a t i o n . The f o r m e r tend

to

involve

supported

metal

o r metal

oxide

catalysts

e.g.

in

tne

maflUfaCtUre o f e t h y l e n e o x i d e , a c r y l o n i t r i l e a n d m a l e i c a n h y d r i d e w h i l s t t h e l a t t e r generally

employ d i s s o l v e d m e t a l

salts,

e.g.

i n the production of

t e r e p h t h a l i c a c i d , b e n z o i c a c i d , a c e t i c a c i d , phenol and p r o p y l e n e o x i d e . I n t h e f i n e c h e m i c a l s i n d u s t r y t h e r e i s a l s o c u r r e n t l y much i n t e r e s t i n t h e use o f c a t a l y t i c o x i d a t i o n a s an e n v i r o n m e n t a l l y more a c c e p t a b l e a l t e r n a t i v e f o r o x i d a t i o n s e m p l o y i n g c l a s s i c a l s t o i c h i o m e t r i c o x i d a n t s such a s p e r m a n g a n a t e and d i c h r o m a t e .

Since the m a j o r i t y o f

f i n e c h e m i c a l s a r e complex,

multi-

f u n c t i o n a l molecules h a v i n g h i g h b o i l i n g p o i n t s and l i m i t e d thermal s t a b i l i t y , processing i s l a r g e l y

limited to the

l i q u i d phase.

E i t h e r homogeneous o r

h e t e r o g e n e o u s C a t a l y s t s can b e employed a n d b o t h h a v e t h e i r a d v a n t a g e s and disadvantages : HOMOGENEOUS ADVANTAGES

-

Mild conditions

HETEROGENEOUS

- Easy s e p a r a t i o n o f

- High a c t i v i t y / s e l e c t i v i t y DISADVANTAGES

catalyst & product

- E f f i c i e n t heat t r a n s f e r

-

-

C a t a l y s t recovery

- Heat t r a n s f e r problems

Not r e a d i l y adapted t o

-

continuous processing

Continuous processing Low a c t i v i t y l s e l e c t i v i t y

34

Indeed, t h e i d e a l c a t a l y s t s a r e t h o s e t h a t combine t h e h i g h a c t i v i t y and s e l e c t i v i t y u s u a l l y a s s o c i a t e d w i t h homogeneous c a t a l y s t s w i t h t h e ease o f r e c o v e r y and r e c y c l i n g t h a t i s c h a r a c t e r i s t i c o f s o l i d c a t a l y s t s . F u r t h e r m o r e , heterogeneous c a t a l y s t s a r e g e n e r a l l y s t a b l e towards d e a c t i v a t i o n by o x i d a t i v e d e s t r u c t i o n o f the ligands surrounding the metal i o n and/or the formation o f unreactive

p-0x0

dimers

(oligomers)

that

characterizes

many

homogeneous

o x i d a t i o n c a t a l y s t s ( s e e l a t e r f o r a more d e t a i l e d d i s c u s s i o n ) . Hence, i n t h e Context o f f i n e chemicals m a n u f a c t u r e t h e r e i s c o n s i d e r a b l e i n t e r e s t i n t h e development o f o x i d a t i v e l y s t a b l e , s o l i d c a t a l y s t s t h a t e x h i b i t h i g h a c t i v i t i e s and s e l e c t i v i t i e s i n l i q u i d phase o x i d a t i o n s . TYPES OF

MECHANISM

Both homogeneous and heterogeneous

c a t a l y t i c o x i d a t i o n s can be d i v i d e d

i n t o t h e same t h r e e c a t e g o r i e s based on t h e t y p e o f mechanism i n v o l v e d : ( a ) autoxidation,

(b)

direct

oxidation

of

(coordinated)

substrates

and

(c)

c a t a l y t i c oxygen t r a n s f e r . Autoxidation

In c o n t r a s t t o c a t a l y t i c h y d r o g e n a t i o n , where no r e a c t i o n t a k e s p l a c e i n t h e absence o f a c a t a l y s t ,

c a t a l y t i c o x i d a t i o n s w i t h m o l e c u l a r oxygen a r e

c o m p l i c a t e d by t h e f a c t t h a t oxygen r e a c t s w i t h o r g a n i c s u b s t r a t e s even i n t h e absence o f a c a t a l y s t . T h i s i n v o l v e s t h e s o - c a l l e d f r e e r a d i c a l a u t o x i d a t i o n mechanism w i t h t h e f o l l o w i n g as key s t e p s :

R.

r

0, fast

ac,.

(2)

A m a j o r problem a s s o c i a t e d w i t h such a u t o x i d a t i o n s i s t h a t t h e y a r e l a r g e l y

i n d i s c r i m i n a t e , i.e.

t h e y e x h i b i t p o o r chemo- and r e g i o - s e l e c t i v i t i e s .

They

a r e s y n t h e t i c a l l y u s e f u l o n l y w i t h r e l a t i v e l y s i m p l e s u b s t r a t e s c o n t a i n i n g one r e a c t i v e p o s i t i o n , e.g.

the o x i d a t i o n o f toluene t o benzoic a c i d o r p-xylene

t o t e r e p h t h a l i c a c i d . Any c a t a l y t i c o x i d a t i o n has t o complete w i t h t h i s nonc a t a l y t i c pathway. Moreover, t h e s i t u a t i o n i s f u r t h e r c o m p l i c a t e d b y t h e f a c t that

transition

decomposition

of

metal trace

ions

also catalyze

amounts

of

a u t o x i d a t i o n s by m e d i a t i n g t h e

hydroperoxides

r a d i c a l s , v i a t h e s o - c a l l e d Haber-Weiss mechanism :

into

chain-initiating

35

Metal(i0n) oxidations o f coordinated substrates The key s t e p i n t h i s c a t e g o r y i n v o l v e s t h e o x i d a t i o n o f a c o o r d i n a t e d s u b s t r a t e by a m e t a l i o n or an oxometal s p e c i e s (see l a t e r ) . Examples i n c l u d e t h e p a l l a d i u m ( I 1 ) - c a t a l y z e d o x i d a t i o n of

olefins

(Wacker p r o c e s s ) and t h e

o x i d a t i v e dehydrogenation o f a l c o h o l s , where t h e key s t e p s a r e r e a c t i o n s ( 5 ) and ( 6 ) , r e s p e c t i v e l y . RCH-CHz

+

Pd"X,

+

H,O

___)

RCCCH,

I

2HI

Fin

The o x i d i z e d f o r m o f t h e metal i o n i s subsequently r e g e n e r a t e d by r e a c t i o n o f t h e reduced f o r m w i t h m o l e c u l a r oxygen. A s p e c i a l case o f r e a c t i o n ( 6 ) i s

i n v o l v e d i n t h e o x i d a t i v e dehydrogenation o f a l c o h o l s o v e r supported m e t a l s (see l a t e r ) . C a t a l y t i c oxygen t r a n s f e r 3 T h i s i n v o l v e s t h e r e a c t i o n o f an oxygen donor w i t h an o r g a n i c s u b s t r a t e i n t h e presence o f a metal c a t a l y s t a c c o r d i n g t o t h e g e n e r a l scheme :

s +

x-0--y

X-0-Y

=

*

oxygen donor; S

so =

c

(7)

Tf

substrate; H

=

catalyst

When t h e oxygen donor i s H202 o r ROZH, t h e a c t i v e o x i d a n t i n these processes i s an oxometal o r a peroxometal s p e c i e s formed as shown below : M-O,?.

&

PEROXOMETAL

HX + RG,H

HOR

T

30

36 I n t e r e s t i n g y, the so-called

the f i r s t

examples o f c a t a l y t i c oxygen t r a n s f e r i n v o l v e d

reagent^,^

Milas

formed b y m i x i n g heterogeneous metal

c a t a l y s t s with a s o l u t i o n o f hydrogen p e r o x i d e i n t e r t - b u t a n o l .

oxide

These r e a g e n t s

w e r e u s e d f o r -he v i c i n a l h y d r o x y l a t i o n o f o l e f i n s ( r e a c t i o n 1 0 ) . OH

I I

cdtalyst R'CIH-CHR'

H,O?

t

OH

b

(10)

RICH-CHR-

Many a c i d i c m e t a l o x i d e s s u c h a s Os04, Moog, W03, V205 a n d Cr03 c o n s t i t u t e effective catalysts f o r t h i s reaction.

A l t h o u g h many o f t h e s e o x i d e s , 2.3. t h e a d d i t i o n o f H202 r e s u l t s i n t h e

Moo3, V2O5, a r e i n s o l u b l e i n t e r t - b u t a n o l f o r m a t i o n o f s o l u b l e p e r a c i d s (e.g.

I t was s u b s e q u e n t l y f o u n d t h a t many

HV04).

o f these reactions proceed v i a epoxide intermediates t h a t a r e hydrolyssd t o v i c i n a l d i o l s u n d e r t h e a c i d i c r e a c t i o n c o n d i t i o n s . When c e r t a i n c a t a l y s t s . e.g.

o r Na2Mo04, w e r e u s e d u n d e r b a s i c o r n e u t r a l c o n d t i o n s s e l e c t i v e

Na2W04

e p o x i d a t i o n was ~ b s e r v e d . ~ Thus,

t h e M i l a s r e a g e n t s may b e c o n s i d e r e d t o b e t h e p r o g e n i t o r s o f t h e

metal c a t a l y s t / a l k y l

hydroperoxide reagents5-8 t h a t were l a t e r developed oy,

i n t e r a l i a , Halcon, Arco and S h e l l w o r k e r s and c u l m i n a t e d i n t h e r e a l i z a t i o n o f c o m n e r c i a l p r o c e s s e s f o r t h e e p o x i d a t i o n o f p r o p y l e n e ( r e a c t i o n 11). T n 2 s 2 r e a g e n t s i n v o l v e t h e v e r y same m e t a l c a t a l y s t s , e.g.

Movl,

W"',

V v and

Ti"',

as t h e M i l a s r e a g e n t s and t h e y a r e m e c h a n i s t i c a l l y c l o s e l y r e l a t e d .

CH,CH==CH,

+

c a t a I ys t

RO,H

b

CH,CH-CH,

ROH

A R C 0 P r o c e s s : Homogeneous c a t a l y s t (Mo")

RO,H

=

(CII,),CO,H

(TBHP)

SHELL P r o c e s s : Heterogeneous c a t a l y s t ( T i T V / S i 0 2 ) R02H

=

PhCIf(CH,)O2i1

(E3HP)

B i o c a t a l y t i c oxygen t r a n s f e r The c y t o c h r o r n e P 4 5 0 - c o n t a i n i n g m o n o ~ x y g e n a s e s ~c a t a l y z e t h e o x i d a t i o n o f a w i d e v a r i e t y o f o r g a n i c s u b s t r a t e s v i a t h e g e n e r a l scheme :

S

-

s u b s t r a t e ; DH,

=

hydrogen d o n o r ( N A D H , e t c . )

The p r o s t n e t i c g r o u p o f t h e s e enzymes c o n t a i n s a n i r o n ( I I 1 ) p o r p h y r i n complex a n d t h e a c t i v e o x i d a n t can b e f o r m a l l y r e g a r d e d a s a h i g h - v a l e n t o x o i r o n ( V -

37 p o r p h y r i n species formed as shown i n f i g u r e 1. PFe I : I

peroxide shun t

PFs"

IRO,H

or tlaOC1

!02

p-o:io (inactive)

FIGURE 1. Mechanism of cytochrome P450 catalyzed oxidation. A l t e r n a t i v e l y , t h e a c t i v e o x o i r o n ( V ) s p e c i e s can be g e n e r a t e d d i r e c t l y f r o m r e a c t i o n o f t h e i r o n ( I I 1 ) p o r p h y r i n w i t h an oxygen donor'', NaOCl, e t c . ,

such as H202. ROzH,

o b v i a t i n g t h e need f o r a c o f a c t o r as hydrogen donor.

Although

these model systems, employing i r o n ( 111) o r manganese( I 1 I ) p o r p h y r i n s , o b v i a t e t h e need f o r a r e d u c i n g agent ( c o f a c t o r ) t h e y v i r t u a l l y a l l s u f f e r f r o m t h e same

disadvantage

as

the

in

vivo

system,

i.e.

they

contain

expensive,

o x i d a t i v e l y u n s t a b l e 1 igands. Moreover, t h e model systems tend t o s u f f e r from an e x t r a

disadvantage

: active

f o r m a t i o n of d i m e r i c p-0x0

oxornetal

comp1exes.l'

species are deactivated v i a We s h a l l

the

r e t u r n t o these problems

later. O b v i o u s l y i t would be advantageous i f i t was p o s s i b l e t o d e v i s e a C a t a l y s t that

i s able t o mediate the transfer

of b o t h oxygen atoms o f d i o x y g e n t o

o r g a n i c s u b s t r a t e s , w i t h o u t r e q u i r i n g t h e consumption o f a r e d u c i n g agent. I n p r i n c i p l e , t h i s can be achieved a c c o r d i n g t o r e a c t i o n 13.

Indeed, t h i s pathway i s f o l l o w e d i n many gas phase o x i d a t i o n s o v e r metal oxide Catalysts.

such as vanadium p e n t o x i d e and b i s m u t h molybdate.

It

i S

g e n e r a l l y r e f e r r e d t o as t h e Mars-van K r e v e l e n mechanism a f t e r i t s o r i g i n a l Such a scheme i s f e a s i b l e i n gas phase o x i d a t i o n s where adsorbed s u b s t r a t e m o l e c u l e s can r e a c t w i t h s u r f a c e oxometal s o e c i e s t o f o r m r a d i c a l

38 intermediates t h a t are r a p i d l y f u r t h e r converted t o products.

I n l i q u i d phase

o x i d a t i o n s , i n c o n t r a s t , any r a d i c a l s t h a t a r e formed w i l l r e a c t r a p i d l y w i t h surrounding substrate molecules and/or dissolved dioxygen i n the b u l k l i q u i d leading

to

difference

free

radical

between

autoxidations.

heterogeneous

gas

This phase

is

an

important

oxidations

and

fundamental liquid

phase

oxidations. Nevertheless,

there

are

scattered

reports o f

homogeneous

systems

that

appear t o i n v o l v e t h e t r a n s f e r o f b o t h oxygen atoms o f d i o x y g e n t o o r g a n i c s u b s t r a t e s . Thus, Groves and Q u i n n r e p o r t e d 1 3 t h e c a t a l y t i c a e r o b i c e p o x i d a t i o n o f o l e f i n s mediated b y a d i o x o (tetramesitylporphyrinato)ruthenium(VI) complex Two e q u i v a l e n t s o f epoxide w e r e formed

a t ambient temperature and pressure.

for each m o l e c u l e o f d i o x y g e n consumed.

F u r t h e r m o r e , i t was shown t h a t t h e

d i o x o r u t h e n i u m ( V 1 ) complex was a competent s t o i c h i o m e t r i c o x i d a n t under anaer o b i c c o n d i t i o n s . The f o l l o w i n g mechanism was proposed t o e x p l a i n t h e r e s u l t s :

PRu"

11-

/"

[

+ 0,

PRII:",

t Interestingly,

V i

P[u=o 0

lc=c t h e analogous t e t r a - p - t o l y l

p o r p h y r i n a t o complex, which i s

known t o f o r m a p-0x0 dimer upon o x y g e n a t i o n , was i n a c t i v e as an o x y g e n a t i o n Catalyst.

T h i s l e d t h e a u t h o r s t o c o n c l u d e t h a t i n h i b i t i o n o f p-0x0 dimer

formation, v i a s t e r i c hindrance f r o m bulky s u b s t i t u e n t s i n the porphyrin r i n g , i s essential f o r c a t a l y t i c a c t i v i t y . More

recently,

Ellis

and

Lyons

have

i r o n ( I 1 I ) p o r p h y r i n complexes a r e s t a b l e , that

reported14

that

polyfluorinated

highly a c t i v e oxidation Catalysts

c a t a l y z e t h e unprecedented s e l e c t i v e h y d r o x y l a t i o n

of

isobutane w i t h

m o l e c u l a r oxygen a t ambient temperature. TFe'

Temp. 2 4 O

TPPF,,

Time (h)

Conversion

Selectivity

143

18%

95%

17%

8 7 "6

3

80' =

(TPPF,,)OH]

trtr~kis:pentafLuorophenyl)p~~phyKln~to

39 I n o r d e r t o e x p l a i n t h e i r h i g h a c t i v i t y and s t a b i l i t y i t was p o s t u l a t e d t h a t p o l y h a l o g e n a t i o n o f t h e p o r p h y r i n r i n g system n o t o n l y s t a b i l i z e s t h e l a t t e r towards o x i d a t i v e d e s t r u c t i o n b u t a l s o s t a b i l i z e s t h e o x o i r o n i n t e r m e d i a t e w i t h r e s p e c t t o p-0x0 dimer f o r m a t i o n .

I n p r i n c i p l e , i t should a l s o

be p o s s i b l e t o d e s i g n s t a b l e s o l i d c a t a l y s t s capable o f m e d i a t i n g analogous s e l e c t i v e o x i d a t i o n s i n t h e l i q u i d phase. TYPES

OF OXIDANT

In bulk chemicals manufacture economic c o n s i d e r a t i o n s u s u a l l y d i c t a t e t h e use o f m o l e c u l a r oxygen as t h e o x i d a n t . o t h e r o x i d a n t s may be c o m n e r c i a l l y o x i d a n t s (e.g.

I n f i n e chemicals, on t h e o t h e r hand,

feasible

(see t a b l e

1).

Indeed,

other

30% hydrogen p e r o x i d e ) may even be p r e f e r r e d for reasons o f

s e l e c t i v i t y and ease o f h a n d l i n g ,

i.e.

i t is not a question o f p r i c e

per

b u t p r i c e / p e r f o r m a n c e r a t i o . A l t h o u g h m o l e c u l a r oxygen i s t h e l e a s t expensive o x i d a n t i t r e q u i r e s e l a b o r a t e s a f e t y p r e c a u t i o n s , and t h e a s s o c i a t e d c o s t s , i n order t o avoid working w i t h i n explosion l i m i t s . TABLE 1. Single oxygen donors. % ACTIVE OXYGEN DONOR

-

COPRODUCT

H202

47.01

H20

03 t -Bu02H

33.3 17.8

02 t BuOH ~

NaClO

21.6

NaC1

NaC102

19.2

NaCl

NaBrO

13.4

NaBr

HN03

25.4

NOX

13.7

C5H1 lNO

KHS05

10.5

KHSOL

NaI04

7 . 2’

Na I

PhIO

7.3

PhI

C

~ 1 H~ 0 2~ 3

1. Based on 100% H,O,; 2. Assuming that onLy one oxygen a t o m is utilized; 3. N-Nethylmorpholine-N-oxide

In a d d i t i o n t o p r i c e and ease o f h a n d l i n g t h e n a t u r e o f t h e c o p r o d u c t and t h e Percentage a v a i l a b l e oxygen a r e i m p o r t a n t c o n s i d e r a t i o n s .

The f o r m e r i s

i m p o r t a n t from an environmental v i e w p o i n t and t h e l a t t e r i n f l u e n c e s t h e volume y i e l d ( k g p r o d u c t p e r u n i t r e a c t o r volume p e r u n i t t i m e ) .

The o x i d a n t o f

c h o i c e f o r f i n e chemicals manufacture i s o f t e n 30% H202 s i n c e i t i s r e l a t i v e l y

40

cheap, easy t o h a n d l e and i t s c o p r o d u c t i s water.

Moreover, u n l i k e m o l e c u l a r

oxygen i t g e n e r a l l y does n o t r e a c t w i t h o r g a n i c s u b s t r a t e s i n t h e absence o f a catalyst. CATALYST TYPES Heterogeneous c a t a l y s t s f o r l i q u i d phase o x i d a t i o n s can be d i v i d e d i n t o t h r e e d i f f e r e n t c a t e g o r i e s : ( a ) s u p p o r t e d m e t a l s (e.9. m e t a l i o n s (e.g.

Pd/C),

( b ) supported

i o n exchange r e s i n s , m e t a l i o n exchanged z e o l i t e s ) and ( c )

s u p p o r t e d oxometal

( o x i d i c ) c a t a l y s t s (e.g.

TiIV/SiO2,

redox z e o l i t e s ,

redox

p i l l a r e d c l a y s ) . T h i s d i v i s i o n o f t h e v a r i o u s c a t a l y s t t y p e s w i l l be used as a framework f o r t h e ensuing d i s c u s s i o n .

SUPPORTED METALS AS CATALYSTS

-

O X I D A T I V E DEHYDROGENATION

I n 1845 D o b e r e i n e r noted15 t h a t e t h a n o l i s o x i d i z e d t o carbon d i o x i d e and w a t e r by oxygen i n t h e presence o f aqueous a l k a l i and a p l a t i n u m C a t a l y s t . The c a t a l y t i c e f f e c t o f p l a t i n u m on t h e a e r o b i c o x i d a t i o n o f cinnamyl a l c o h o l d e s c r i b e d 1 6 by S t r e c k e r

Wieland

t h a t f i n e l y divided palladium catalyzes

showed''

i n 1855.

I n t h e p e r i o d 1912-1921

was subsequently

the o x i d a t i o n of

p r i m a r y a l c o h o l s and aldehydes t o aldehydes and c a r b o x y l i c a c i d s , r e s p e c t i v e l y , i n aqueous s o l u t i o n . S i n c e oxygen c o u l d be r e p l a c e d b y o t h e r hydrogen a c c e p t o r s

i t was concluded t h a t these r e a c t i o n s i n v o l v e a d e h y d r o g e n a t i o n mechanism, f o l l o w e d by o x i d a t i o n o f h y d r i d e by oxygen, e.g.

More r e c e n t l y , n o b l e m e t a l - c a t a l y z e d widely

appiied

to

che

selective

o x i d a t i v e d e h y d r o g e n a t i o n s have been

oxidations

of

alcohols18

and

(18)

.. 0

OH

vicinal

C a t a l y s t : Pt. Pb/C o r Pt. Bi/C

In

particular,

catalytic

aerobic

oxidations

of

carbohydrates,

using

s u p p o r t e d n o b l e m e t a l s i n t h e l i q u i d phase, have been e x t e n s i v e l y s t u d i e d by

41 Heyns and c o w o r k e r s 2 2 7 2 3 a n d , m o r e r e c e n t l y , b y g r o u p s a t t h e u n i v e r s i t i e s o f E i n d h c ~ v e n ~a ~n d- ~D~e l f t . 3 2 - 3 6

I n p r i n c i p l e , t h e s e r e a c t i o n s can i n v o l v e f o u r

d i f f e r e n t types o f chemoselective o x i d a t i o n : 1. C1 a l d e h y d e ( h e m i a c e t a l ) o x i d a t i o n CHO

2 . P r i m a r y CH20H

+

3. S e c o n d a r y CHOH

C=O

their

C=O

+

+

r e s u l t s with Pt/C

O=C as

proposed the f o l l o w i n g r e a c t i v i t y scale COCH20H

>

CH20H

>

C02H

C02H

+

4. D i o l c l e a v a g e -CH(OH)CH(OH)

Based on

+

CHOHaxial

>

catalyst,

Heyns a n d P a ~ l s e n ~ ~ ? ~ ~

for t h e d i f f e r e n t g r o u p s : CHO

)

CHOHeaUatorial

C1 o x i d a t i o n One

reaction that

has

been e x t e n s i v e l y

studied,

as

established i n d u s t r i a l processes i n v o l v i n g fermentation, catalyzed oxidation o f

0-glucose

to

D-gluconate

an a l t e r n a t i v e

to

i s the noble metal-

( r e a c t i o n 19)

i n aqueous

a1 k a l i.22-25

Palladium selectivity trimetallic

is in

superior this

(Pd, P t ,

to

process.

platinum with More

respect

recently,

bi-

to (e.g.

both Pd,

activity

ana

Bi/C)37

and

B i / C ) 3 8 c a t a l y s t s have been d e s c r i b e d t h a t a f f o r d v e r y

high s e l e c t i v i t i e s t o gluconic acid.

F o r e x a m p l e , Degussa w o r k e r s 3 * o b t a i n e d

g l u c o n i c a c i d i n 96% s e l e c t i v i t y u s i n g a 4% Pd, c a t a l y s t a t pH 10, 55°C a n d 1 0 mbar O2 p r e s s u r e .

1% P t ,

5% B i - o n - c h a r c o a l

P t enhances t h e a c t i v i t y and

B i t h e s e l e c t i v i t y o f t h e Pd c a t a l y s t . T h a t C1 o x i d a t i o n p r o c e e d s v i a i n i t i a l d e h y d r o g e n a t i o n , a s shown i n r e a c t i o n

42 19, was d e m o n s t r a t e d some y e a r s ago by t h e D e l f t g r o u p who showed32i33

that

t r e a t m e n t o f a l d o s e s w i t h n o b l e m e t a l c a t a l y s t s ( P t a n d Rh w e r e s u p e r i o r t o Pd, N i a n d

R u ) a t h i g h pH ( > 1 2 ) r e s u l t s i n t h e s i m u l t a n e o u s f o r m a t i o n o f t h e

corresponding a l d o n i c a c i d and m o l e c u l a r hydrogen.

i t was shown t h a t

Indeed,

t h e a l d o s e can f u n c t i o n a s a n e f f i c i e n t h y d r o g e n d o n o r f o r c a t a l y t i c h y d r o g e n t r a n s f e r reactions. C1-oxidation

o f aldoses i s a general

reaction3'

and has

been s u c c e s s f u l l y a p p l i e d t o t h e o x i d a t i o n o f g a l a c t o s e , mannose a n d xylOSe to

the

corresponding

aldonic

acids.

More

recently,

the

oxidation o f the disacharide lactose t o lactobionic acid, PdiC c a t a l y s t ,

impregnated i n - s i t u

with Bi,

selective

(>99%)

u s i n g a commercial

h a s been d e m o n s t r a t e d b y

the

E i ndhoven g r o u p . 2 9

C g vs C2 o x i d a t i o n F u r t h e r r e a c t i o n o f g l u c o n i c a c i d w i t h o x y g e n o v e r P t / C o r Pd/C C a t a l y S t S l e a d s t o t h e o x i d a t i o n o f t h e c 6 p r i m a r y CH20H g r o u p t o a f f o r d D - g l u c a r i c a c i d v i a t h e c o r r e s p o n d i n g a l d e h y d e ( L - g u l o r o n i c a c i d ) a s i n t e r m e d i a t ? (See f i g u r e 2). due Tne

t3

Unfortunately,

t h e r e a c t i o n e x h i b i t s o n l y moderate s e l e c t i v i t i e s

competing degradation o f t h e carbon c h a i n t o lower d i c a r b o x y l i c acids.

best

results

(55-60%

selectivity

to

glucarate)

obtain

with

?t/C

c a t a 1 ys t 5.30 931

i"

HC HO

-+Hy/ co,-

CHC

OH

co, -

L-guloronate

HO

OH

D-glucarate

2-keto-D-gluconate

FTGvRE 2. 3:tidation of D-gluconate. E f f o r t s t o increase t h e s e l e c t i v i t y of

t h i s r e a c t i o n by doplng the Pt/C

c a t a l y s t w i t h Pb r e s u l t e d i n t h e s e r e n d i p i t o u s d i s c o v e r y , group,

o f t h e s e l e c t i v e Pt,Pb/C-catalyzed

b y :he

Eindhoven

o x i d a t i o n o f g l u c o n i c a c i d t o 2-

43 k e t o g l u c o n i c a c i d i n a l k a l i n e medium.z6-z8

The r e a c t i o n i s a g e n e r a l one a n d

can b e a p p l i e d t o t h e s e l e c t i v e o x i d a t i o n o f a v a r i e t y o f a - h y d r o x y a c i d s t o t h e c o r r e s p o n d i n g 2 - k e t o a c i d s , e.g.

l a c t i c a c i d a f f o r d s p y r u v i c a c i d i n >95%

s e l e c t i ~ i t y . * ~ The * ~ ~r a t i o o f C2 t o C6 o x i d a t i o n i n g l u c o n i c a c i d i n c r e a s e d b y a f a c t o r o f 140 on d o p i n g t h e P t / C c a t a l y s t w i t h a n i n s o l u b l e l e a d S a l t . 2 7 The s e l e c t i v i t y e n h a n c i n g e f f e c t o f

t h e l a t t e r was p o s t u l a t e d z 6 t o i n v o l v e

c h e l a t i o n o f t h e a-hydroxy a c i d t o l e a d ( I 1 ) on t h e s u r f a c e o f t h e C a t a l y s t thus

facilitating

transfer

of

the

hydrogen

of

the

hydroxyl

at

C2

to

platinum(0). Catalyst deactivation

A l l o f t h e r e a c t i o n s d e s c r i b e d above s u f f e r f r o m t h e same d r a w b a c k : r a p i d catalyst deactiviation.

I n noble metal-catalyzed o x i d a t i v e dehydrogenations

t h e m e t a l must p e r f o r m two f u n c t i o n s : s u b s t r a t e dehydrogenation and subsequent o x i d a t i o n o f t h e s u r f a c e m e t a l h y d r i d e s p e c i e s b y a d s o r b e d oxygen. The s u c c e s s o f a p a r t i c u l a r c a t a l y s t depends on a d e l i c a t e b a l a n c e b e t w e e n t h e s e t w o Steps. U n f o r t u n a t e l y , t h e m o l e c u l a r oxygen t h a t i s n e c e s s a r y f o r t h e d e s i r e d r e a c t i o n i s a l s o responsible f o r the d e a c t i v a t i o n of

the catalyst,

t h e mechanism o f

w h i c h i s b y no means f u l l y u n d e r s t o o d . I t i s g e n e r a l l y n o t o b s e r v e d w i t h gas phase r e a c t i o n s b u t i s c h a r a c t e r i s t i c of

(aqueous)

l i q u i d phase o x i d a t i o n s

o v e r n o b l e m e t a l c a t a l y s t s , w h e r e a d s o r b e d o x y g e n atoms r e a c t w i t h W a t e r t o form adsorbed hydroxyl species. a d s o r b e d o x y g e n atoms,

I t i s thought30i31

t o i n v o l v e m i g r a t i o n of

v i a t h e formation of adsorbed h y d r o x y l species,

the P t l a t t i c e , a process r e f e r r e d t o as 'dermasorption'. minimized,

but not eliminated,

into

D e a c t i v a t i o n can be

by u s i n g l o w oxygen p a r t i a l p r e s s u r e s ,

low

s t i r r i n g speeds a n d s o - c a l l e d d i f f u s i o n - s t a b i l i z e d c a t a l y s t s . 3 4 ~ 3 5 The l a t t e r c o n c e p t i n v o l v e s t h e u s e o f l a r g e u n i f o r m p a r t i c l e s (e.9.

e x t r u d a t e s ) i n which

oxygen d i f f u s i o n l i m i t a t i o n leads, a t a c e r t a i n d e p t h i n t h e p a r t i c l e ,

to a

p r o p e r t u n i n g o f r e a c t i o n s and consequently t o a h i g h e r s t e a d y s t a t e a c t i v i t y . Recently,

van

Bekkum a n d

coworkers39

studied the

oxygen

tolerance

of

v a r i o u s n o b l e m e t a l / c a r b o n c a t a l y s t s i n l i q u i d phase o x i d a t i v e dehydrogenation o f a l c o h o l s . The o r d e r o f s t a b i l i t y t o w a r d s p o i s o n i n g by o x y g e n was f o u n d t o be P t

> Ir >

Pd

>

Rh

>

Ru (Ru/C was i n a c t i v e ) . O f p r a c t i c a l i m p o r t a n c e

i S

the

maximum t u r n o v e r number d i v i d e d b y t h e p r i c e o f t h e m e t a l a n d i t was s u g g e s t e d that

Pd p r o b a b l y has a b e t t e r p r i c e / p e r f o r m a n c e

ratio,

i.e.

' v a l u e f o r money' t h a n P t even t h o u g h t h e l a t t e r i s more s t a b l e .

gives

better

44

V i c i n a l d i o l cleavage In

the

noble metal-catalyzed

oxidations

described

above

vicinal

diol

cleavage i s sometimes observed as a s i d e - r e a c t i o n b u t never as a main r e a c t i o n . Oxidative

diol

periodate

(Malaprade

cleavage

usually

oxidation)

involves

stoichiometric

and t h e r e

oxidants

i s a g r e a t need f o r

such as catalytic

p r o c e d u r e s employing i n e x p e n s i v e , c l e a n o x i d a n t s such as O2 o r H202. Recently,

heterogeneous

catalytic

systems were

described41 t h a t

employ

m o l e c u l a r oxygen f o r t h e l i q u i d phase o x i d a t i v e c l e a v a g e o f v i c i n a l d i o l s . A l t h o u g h t h e c a t a l y s t s appear t o be m i x e d m e t a l o x i d e s r a t h e r t h a n supported metals

t h e method

resembles

closely

the

noble metal-catalyzed

oxidations

d e s c r i b e d above, hence t h e i r i n c l u s i o n i n t h i s s e c t i o n . The c a t a l y s t s a r e h i g h s u r f a c e a r e a r u t h e n i u m p y r o c h l o r e o x i d e s h a v i n g t h e g e n e r a l f o r m u l a A2+XRu2-X07-y (A = Pb, B i ; 0

< x <

1; 0

<

They were

y 2 0.5).

successfully applied t o the s e l e c t i v e o x i d a t i o n o f cyclohexane-1,2-diol a d i p a t e ( r e a c t i o n 20) under m i l d c o n d i t i o n s (25-95"C,

2 b a r 02, pH

>

to

13).

R e a c t i o n s were c a r r i e d o u t i n b a t c h a u t o c l a v e s and i n a c o n t i n u o u s t r i c k l e bed r e a c t o r .

I n t h e l a t t e r e x p e r i m e n t s a t 55-95°C a Bi2,39Ru1.6107-y

catalyst

gave no evidence o f d e a c t i v a t i o n , l e a c h i n g by t h e a l k a l i n e s o l u t i o n , or change i n b u l k s t r u c t u r e a f t e r 180 hours o f o p e r a t i o n . A d i p i c a c i d s e l e c t i v i t i e s were of

t h e o r d e r o f 8 1 4 7 % a t complete c o n v e r s i o n .

observed,

in initial

experiment^^^,

I n c o n t r a s t , we

have n o t

any cleavage w i t h c a r b o h y d r a t e s under t h e

same c o n d i t i o n s . SUPPORTED METAL IONS AS CATALYSTS I o n exchange r e s i n s as s u p p o r t s A s i m p l e b u t e f f e c t i v e means o f p r e p a r i n g s u p p o r t e d m e t a l i o n c a t a l y s t s i s

t o employ i o n exchange r e s i n s .

F o r example, a cobalt-exchanged H-type r e s i n

(Dowex 5 0 ) was shown43 t o be an e f f e c t i v e s o l i d c a t a l y s t f o r t h e a u t o x i d a t i o n o f a c e t a l d e h y d e t o a c e t i c a c i d a t 20°C.

No l e a c h i n g o f c o b a l t i o n s f r o m t h e

r e s i n was observed and t h e c a t a l y s t was used r e p e a t e d l y ( 5 x ) w i t h o u t any significant

loss of activity.

More

recently

t h e use o f weak a c i d r e s i n s

exchanged w i t h c o b a l t i o n s as c a t a l y s t s f o r t h e a u t o x i d a t i o n o f cyclohexane

45

o r cyclohexanone t o d i b a s i c a c i d s , i n a c e t i c a c i d s o l v e n t a t 85-105°C and 5-20 b a r , has been described.44 Similarly.

cobalt(I1)-pyridine

(Copy)

s t y r e n e and a c r y l i c o r m e t h a c r y l i c a c i d ,

complexes bound t o copolymers o f cross-linked w i t h divinylbenzene,

c a t a l y z e t h e a u t o x i d a t i o n o f t e t r a l i n d i s p e r s e d i n w a t e r a t 50°C and 1 bar.45 The r a t e o f o x i d a t i o n w i t h t h e c o l l o i d a l Copy c a t a l y s t was t w i c e as f a s t as w i t h homogeneous Copy and n i n e t i m e s as f a s t as w i t h c o b a l t ( I 1 ) a c e t a t e i n a c e t i c acid. I n a v a r i a t i o n on t h i s theme cobal t p h t h a l o c y a n i n e t e t r a s u l f o n a t e (CoPcTs) was bound v i a t h e a n i o n i c s u l f o n a t e groups t o s t y r e n e - d i v i n y l b e n z e n e copolymer l a t e x e s c o n t a i n i n g q u a t e r n a r y amnonium i o n s . 4 6 The r e s u l t i n g c o l l o i d a l C a t a l y s t was used t o e f f e c t solution,

to

the

the a u t o x i d a t i o n o f 2.6-di-tert-butylphenol

c o r r e s p o n d i n g diphenoquinone

( r e a c t i o n 21).

i n aqueous The

rate of

o x i d a t i o n was t e n times f a s t e r than w i t h homogeneous CoPcTs i n water.

T r a n s i t i o n m e t a l i o n s i m n o b i l i z e d on i o n exchange r e s i n s have a l s o been used as CatalYStS i n oxygen t r a n s f e r r e a c t i o n s . For example, t h e homogeneous Mo and V-based c a t a l y s t s for e p o x i d a t i o n o f o l e f i n s w i t h a l k y l h y d r o p e r o x i d e s have been h e t e r o g e n i z e d by imnobi 1 i z a t i o n on ion-exchange r e ~ i n s . ~ ~Thus, - ~ ' Ivanov e t a147 d e s c r i b e d a resin-bound molybdenyl (Moo2*+) c a t a l y s t t h a t showed a 7% decrease i n a c t i v i t y a f t e r r e c y c l i n g f i v e t i m e s . L i n d e n and Farona4' a r e s i n bound vanadyl Of

CYCliC

recycling

(VO")

prepared

t h a t i s an a c t i v e c a t a l y s t f o r t h e e p o x i d a t i o n

and a c y c l i c o l e f i n s and showed no n o t i c e a b l e decrease i n a c t i v i t y on several

times.

More

recently,

the

c h e l a t i n g polymer

resins of

s t r u c t u r e ( I ) 4 9 and ( H ) ~ ' and t h e c o m n e r c i a l l y a v a i l a b l e d i h y d r o x y b o r y l s u b s t i t u t e d r e s i n (111)50 have been used t o i m n o b i l i z e vanadyl a n d / o r molybenyl epoxidation catalysts. Such oxometal c a t a l y s t s can a l s o be i n m o b i l i z e d as a n i o n s on a n i o n exchange r e s i n s as r e p o r t e d r e c e n t l y by Kurusu and M a ~ u y a m awho ~ ~ used t e t r a b r o m o oxomolybdate(V) bound t o a t e t r a a l k y l amnoni urn-contai n i ng s t y r e n e l d i v i n y l benzene copolymer as a c a t a l y s t a l c o h o l s w i t h TBHP.

for

t h e e p o x i d a t i o n o f o l e f i n s and o x i d a t i o n o f

46

Chromium( 111)

and

cerium( I V )

impregnated

NafionRK

(a

perf1uarinated

s u l f o n i c a c i d r e s i n ) were used a s c a t a l y s t s f o r t h e c h e m o s e l e c t i v e o x i d a t i o n o f a v a r i e t y o f a l c o h o l s u s i n g TBHP

or NaBr03 a s t h e oxygen d o n o r , 5 2 e.g.

TBHP ..

OH

&OH

80% yield

Catalyst TBHP

0

Catalyst : ~

C e X V / N A F K : 98% y i e l d

C ~ ~ ~ ' / N A F: K82% y i e l d

A Ph

4 CH,OH -

(Cr

'

'INAFK Ph

TBHP

-

AA 81% yield

(Ce"/NAFK) NaBrO,

O

CH,CH =

O

-

73% yield

n

OH

A

(CH,),CH,OH

(Ce"/NAFK) NaBrO,

;CH,),CH,OH

81% yield I n principle, alternatives

to

t h e s e systems c o n s t i t u t e c o m n e r c i a l l y i n t e r e s t i n g c a t a l y t i c classical

Cr-

and

Ce-based

oxidants.

From

a

practical

v i e w p o i n t , however, i t i s e s s e n t i a l t h a t t h e c a t a l y s t r e t a i n i t s a c t i v i t y o v e r l o n g p e r i o d s o f time.

I n one e x p e r i m e n t w i t h 1 - p h e n y l e t h a n o l

and C r I ' I i N A F K

and TBHP t h e c a t a l y s t was r e c o v e r e d , d r i e d a n d r e u s e d w i t h o u t l o s s o f CrilI,

47

a l t h o u g h t h e y i e l d o f acetophenone decreased s l i g h t l y f r o m 95 t o 92%. A palladium(I1)-exchanged

form)

p o l y s t y r e n e s u l f o n i c a c i d r e s i n (Dowex 50W, H

c a t a l y z e s t h e o x i d a t i o n o f 2-methylnaphthalene w i t h 60% aqueous H202

( r e a c t i o n 27),

a f f o r d i n g 2-methyl-1,4-naphthoquinone

(menadione)

i n 55-60%

y i e l d a t 90-972 c o n v e r ~ i o n . Menadione ~~ i s a comnercially important v i t a m i n K i n t e r m e d i a t e and these existing

industrial

results

processes

compare f a v o u r a b l y that

employ

with

those o b t a i n e d

stoichiometric

quantities

in of

chromium t r i o x i d e i n s u l f u r i c a c i d .

mCH3 60% H,O,

(Pd"

resin)

0

( 5 5 - 6 0 % yield

Metal i o n exchanged (impregnated) z e o l i t e s Redox m e t a l i o n s can a l s o be i m m o b i l i z e d by exchanging them w i t h t h e sod urn i o n s i n z e o l i t e s . F o r example, a cerium(1V)-exchanged z e o l i t e NaY was used as a heterogeneous,

regenerable ( i n a separate step) oxidant f o r the o x i d a t ve

cleavage o f p i n a c o l .54 S i m i l a r l y , a CoII-exchanged z e o l i t e NaX c a t a l y z e d t h e o x i d a t i o n o f 2,6-dialkylphenols

w i t h TBHP,

a f f o r d i n g t h e corresponding 1,4-

benzoquinone i n h i g h ~ e l e c t i v i t i e s:~ ~

'bR. OH

'i

TBHP

(Co' 'NaX)

0

SUPPORTED OXOMETAL (OXIDIC) CATALYSTS Metal o x i d e s have o f t e n been used as c a t a l y s t s f o r t h e a u t o x i d a t i o n o f hydrocarbons.'

I n many cases t h e metal p r o b a b l y d i s s o l v e s i n t h e r e a c t i o n

medium and c a t a l y s i s i n v o l v e s homogeneous m e t a l complexes. H o w e v e r , a c c o r d i n g t o a r e c e n t r e p o r t 5 6 c e r i u m o x i d e c a t a l y z e s t h e l i q u i d phase o x i d a t i o n o f cyclohexanone i n a c e t i c a c i d (5-15 b a r and 98-118°C) w i t h o u t d i s s o l v i n g i n t h e r e a c t i o n medium. Metal

oxide-based

c a t a l y s t s were a l s o

s t u d i e d 5 7 i n t h e e a r l y days

development o f o l e f i n e p o x i d a t i o n w i t h a l k y l

hydroperoxides.

Of

Moo3 was an

e x c e l l e n t c a t a l y s t , W03 showed moderate a c t i v i t y and o t h e r o x i d e s (V205, SeOp,

48

CrO3, Cr203, Nb205) n e g l i g i b l e a c t i v i t y . S u p p o r t i n g Moo3 on ~ i l i c a ~ ~l e -d ~t o' a s i g n i f i c a n t increase i n a c t i v i t y .

However,

i t was shown t h a t a c t i v i t y was

v i r t u a l l y e n t i r e l y due t o r a p i d l e a c h i n g o f molybdenum f r o m t h e s u r f a c e t o g i v e a homogeneous c a t a l y s t .

Thus,

epoxidation continued a t v i r t u a l l y

the

same r a t e when t h e m i x t u r e was f i l t e r e d and t h e f i l t r a t e a l l o w e d t o r e a c t f u r t h e r . The f u n c t i o n o f t h e s i l i c a s u p p o r t i s p r i m a r i l y t o promote d i s s o l u t i o n by d i s p e r s i n g t h e Moo3. I n c o n t r a s t , t h e t i t a n i u m - s i l i c a c a t a l y s t developed by S h e l l 5 i s a t r u l y heterogeneous and h i g h l y a c t i v e c a t a l y s t t h a t i s used i n a c o m n e r c i a l process

f o r t h e epoxidation o f propylene w i t h ethylbenzene hydroperoxide (see e a r l i e r ) . C a t a l y s t s c o n t a i n i n g a wide v a r i e t y o f o t h e r o x i d e s s u p p o r t e d on Si02 have been described but

i t i s only the T i I V / S i O 2

catalyst

t h a t d i s p l a y s t h e Unique

c o m b i n a t i o n o f h i g h a c t i v i t y and t r u e h e t e r o g e n e i t y . a c t i v e c a t a l y s t contains tetrahedral Ti1"

We suggested5 t h a t t h e

c h e m i c a l l y bound t o s i l o x a n e 1 igands

( s t r u c t u r e I V ) and t h a t t h e T i - 0 - S i bonds a r e v e r y r e s i s t a n t t o h y d r o l y s i s .

\

\

\

- Q, Ti'

(:I/!

The s i l o x a n e l i g a n d s a r e presumed t o i n c r e a s e t h e e l e c t r o p h i l i c i t y

(1.2.

Lewis a c i d c h a r a c t e r ) o f t h e t i t a n y l ( T i I V = O ) group t h u s f a c i l i t a t i n g complex f o r m a t i o n w i t h R02H and subsequent

oxygen t r a n s f e r

f r o m the e l e c t r o p h i l i c

peroxometal complex t o t h e o l e f i n . I n a d d i t i o n , i s o l a t i o n o f r e a c t i v e monomeric t i t a n y l (Ti"=O)

s p e c i e s i n t h e s i l i c a framework s t a b i l i z e s them w i t h r e s p e c t

t o d e a c t i v a t i o n v i a t h e f o r m a t i o n o f p o l y m e r i c p-0x0 ( T i - 0 - T i )

species. I t i S

w e l l known t h a t t i t a n y l complexes, such as T i O ( a ~ a c ) ~have , a propensity for p o l y m e r i z a t i o n , which e x p l a i n s why t i t a n i u m compounds a r e g e n e r a l l y n o t v e r y a c t i v e as homogeneous e p o x i d a t i o n c a t a l y s t s . Redox z e o l i t e s as s t a b l e , s e l e c t i v e o x i d a t i o n c a t a l y s t s Another

approach

to

isolating

redox

metal

ions

in

stable

inorganic

matrices, thereby c r e a t i n g o x i d a t i o n c a t a l y s t s w i t h unique a c t i v i t y / s e l e c t i v i t y relationships,

i s t o i n c o r p o r a t e them i n a z e o l i t e l a t t i c e framework. T h i s

f u n d a m e n t a l l y d i f f e r e n t t o t h e metal i o n exchanged ( i . e .

i S

impregnated) z e o l i t e s

d e s c r i b e d e a r l i e r and t h e ' s h i p i n t h e b o t t l e ' t y p e z e o l i t e s 6 1 1 6 2 where a m e t i l l

49

complex i s t r a p p e d w i t h i n t h e pores o f a z e o l i t e . The f i r s t example o f such a 'redox z e o l i t e '

i s the synthetic

( T S - l ) , developed by Enichem

titanium(1V)

worker^.^^-^'

zeolite,

titanium s i l i c a l i t e

T S - 1 was shown t o c a t a l y z e a v a r i e t y

of s y n t h e t i c a l l y u s e f u l o x i d a t i o n s w i t h 30% H202, such as o l e f i n e p o x i d a t i o n , oxidation

of

primary

alcohols

to

aldehydes,

aromatic

hydroxylation

and

amnoximation o f cyclohexanone t o cyclohexanone oxime (see f i g u r e 3 ) .

FIGURE 3. Oxidation catalyzed by titanium silicalite (TS-1). The TS-1 c a t a l y z e d h y d r o x y l a t i o n o f phenol t o a 1:l m i x t u r e o f c a t e c h o l and hydroquinone has a1 ready been comnercial i z e d by Enichem. Another r e a c t i o n o f c o n s i d e r a b l e comnercial

importance i s t h e above mentioned amnoximation

of

cyclohexanone t o cyclohexanone oxime66, an i n t e r m e d i a t e i n t h e manufacture o f caprolactam. I t c o u l d f o r m an a t t r a c t i v e a l t e r n a t i v e t o t h e e s t a b l i s h e d process that

i n v o l v e s a c i r c u i t o u s r o u t e v i a o x i d a t i o n o f amnonia t o n i t r i c a c i d

f o l l o w e d by r e d u c t i o n o f t h e l a t t e r t o h y d r o x y l a m i n e ( f i g u r e 4 ) . The T S - I t i v i t i e s , e.g.

catalyst exhibits

some q u i t e remarkable a c t i v i t i e s and s e l e c -

e t h y l e n e i s e p o x i d i z e d w i t h 30% H202 i n aqueous t e r t - b u t a n o l a t

ambient temperature, a f f o r d i n g e t h y l e n e o x i d e i n 96% s e l e c t i v i t y a t 97% H202 conversion.

50

E X I S T I N C ROUTE

NE'd ROUTE

I

F I G U R E 3 . Two routes to cyclohexanone o x h e Shape s e l e c t i v e o x i d a t i o n I n c o r p o r a t i n g redox c a t a l y t i c

sites within a zeolite

l a t t i c e framework

s h o u l d a l s o p r o v i d e a b a s i s f o r e f f e c t i n g shape s e l e c t i v e o x i d a t i o n s .

i t has

recently

been

reported6'

that

TS-1

catalyzes

the

shape

Indeed, selective

o x i d a t i o n o f a l k a n e s w i t h 30% H202. L i n e a r a l k a n e s w e r e o x i d i z e d much f a s t e r than branched o r c y c l i c alkanes, sieving action of

TS-1.

presumably as a

The p r o d u c t s w e r e

result

o f the molecular

t h e c o r r e s p o n d i n g a l c o h o l s and

k e t o n e s f o r m e d b y o x i d a t i o n a t t h e 2- a n d 3 - p o s i t i o n s ,

e.g.,

From a m e c h a n i s t i c v i e w p o i n t i t i s w o r t h n o t i n g t h a t t h e TS-1 c a t a l y s t c o n t a i n s t h e same c h e m i c a l e l e m e n t s i n r o u g h l y t h e same p r o p o r t i o n s a s t h e She1 1 amorphous T i 1 ' / S i 0 2 displays

catalyst referred to earlier.

a much b r o a d e r r a n g e o f a c t i v i t i e s

e x p l a n a t i o n may be t h a t isolated titanyl

the TS-1

catalyst

than

the

However, latter.

c o n t a i n s more

c e n t r e s t h a n t h e amorphous T i 1 ' / S i 0 2 .

t h e former A

possible

( o r more a c t i v e )

Based on t h e q u i t e

r e m a r k a b l e r e s u l t s o b t a i n e d w i t h TS-1 we e x p e c t many m o r e e x a m p l e s of r e d o x z e o l i t e s , i.e.

z e o l i t e s , a l p o s , etc. m o d i f i e d by isomorphous s u b s t i t u t i o n w i t h redox m e t a l i o n s i n t h e c r y s t a l l a t t i c e , as s e l e c t i v e o x i d a t i o n c a t a l y s t s . 66

51

Redox p i l l a r e d c l a y s a s shape s e l e c t i v e o x i d a t i o n c a t a l y s t s Of

considerable

intercalating agents.69

current

clay minerals

interest of

is

the

the

design

smectite

of

type w i t h

new

catalysts

various

by

pillaring

P i l l a r i n g o f c l a y s w i t h redox m e t a l i o n s can l e a d t o t h e f o r m a t i o n

o f o x i d a t i o n c a t a l y s t s w i t h i n t e r e s t i n g p r o p e r t i e s . The f i r s t e x a m p l e o f such a redox p i l l a r e d clay,

vanadium-pillared montmorillonite

(V-PILC)

h a s been

V - P I L C was s y n t h e s i z e d b y r e f 1 u x i n g a

r e p o r t e d b y C h o u d a r y a n d coworkers.70

s o l u t i o n o f V0Cl3 i n d r y benzene w i t h H - m o n t m o r i l l o n i t e .

I t p r o v e d t o b e an

e f f e c t i v e heterogeneous c a t a l y s t f o r t h e e p o x i d a t i o n o f a l l y l i c a l c o h o l s w i t h a1 k y l h y d r o p e r o x i d e s , d i s p l a y i n g r a t e s c o m p a r a b l e t o t h e homogeneous VO(aCaC)2 c a t a l y s t and i n t e r e s t i n g r e g i o s e l e c t i v i t i e s :

TBHP OH

OH

(V-PILC)

.

no reaction in

15 h at R.T.

(V-PILC)

2 . 5 h ; R.T.

OH 944 yield

TBHP

+

( v - P ILC) 7 h ; R.T.

(22) , 91% y i e l d

Such r e g i o s e l e c t i v i t i e s a r e u n i q u e a n d s u g g e s t t h a t r e d o x p i l l a r e d C l a y s may have b r o a d scope and u t i l i t y a s s e l e c t i v e ,

heterogeneous c a t a l y s t s f o r

l i q u i d p h a s e o x i d a t i o n s . I n d e e d , V-PILC a l s o c a t a l y z e s t h e o x i d a t i o n o f b e n z y i alcohol

( t o a m i x t u r e o f benzoic a c i d and benzylbenzoate) w h i l s t a-methyl

b e n z y l a l c o h o l i s l e f t c o m p l e t e l y untouched.71 S i m i l a r l y , p - s u b s t i t u t e d b e n z y l alcohols a r e o x i d i z e d w h i l s t o-substituted benzyl a l c o h o l s a r e i n e r t . 7 1 Finally, prepared.71

a

titanium(iV)

pillared

clay

(Ti-PILC)

catalyst

has

been

I n t h e presence o f t a r t a r i c a c i d e s t e r s a s c h i r a l l i g a n d s Ti-PILC

i s an e f f e c t i v e ,

heterogeneous c a t a l y s t f o r t h e a s y m n e t r i c e p o x i d a t i o n nf

a l l y l i c alcohols.

E n a n t i o s e l e c t i v i t i e s were comparable t o those observed i n

t h e homogeneous system7* and r e a c t i o n s c o u l d b e c a r r i e d o u t a t c o n c e n t r a t i o n s up t o 2M w i t h a s i m p l e w o r k - u p v i a f i l t r a t i o n o f t h e c a t a l y s t .

52

CONCLUSIONS AN0 FUTURE PROSPECTS As

a

result

of

i n c r e a s i n g environmental

pressure the

substitution of

out-dated o x i d a t i o n technologies i n v o l v i n g c l a s s i c a l s t o i c h i o m e t r i c oxidants w i t h cleaner, near

future.

c a t a l y t i c a l t e r n a t i v e s w i l l c o n t i n u e t o g a t h e r momentum i n t h e The

days

of

such

antiquated,

environmentally

technologies as chromic a c i d o x i d a t i o n s a r e c l e a r l y over.

unacceptable

It i s ,

therefore,

e n c o u r a g i n g t o b e a b l e t o c o n c l u d e t h a t a t t h e moment when we need them such efficient,

c a t a l y t i c a l t e r n a t i v e s a r e b e g i n n i n g t o emerge.

We c o n f i d e n t l y e x p e c t , t h e r e f o r e ,

t h a t t h e use o f heterogeneous c a t a l y s t s

i n l i q u i d phase o x i d a t i o n s w i l l p l a y a n i m p o r t a n t r o l e i n t h e s e developments. N o t o n l y b e c a u s e h e t e r o g e n e o u s c a t a l y s t s h a v e t h e a d v a n t a g e o f ease o f r e c o v e r y and r e c y c l i n g and s u i t a b i l i t y f o r c o n t i n u o u s p r o c e s s i n g b u t a l s o because t h e y o f f e r t h e p o s s i b i l i t y o f designing s i t e - i s o l a t e d redox metal c a t a l y s t s (redox zeolites,

redox p i l l a r e d c l a y s ,

etc.)

d i s p l a y i n g unique s u b s t r a t e ,

chemo-,

r e g i o - and s t e r e o s e l e c t i v i t i e s . Perhaps t h i s w i l l a l s o h e r a l d t h e l o n g - a w a i t e d coming t o g e t h e r o f h e t e r o g e n e o u s c a t a l y s i s w i t h t h e d i s c i p l i n e s O r g a n O m e t a l l i C c h e m i s t r y , homogeneous c a t a l y s i s and o r g a n i c c h e m i s t r y . REFERENCES 1.

2. 3. 4.

5. 5.

7.

a.

R.A. S h e l d o n and J.K. K o c h i , " M e t a l - C a t a l y z e d O x i d a t i o n s o f O r g a n i c Compounds", Academic P r e s s , New Y o r k , 1981. R.A. S h e l d o n , S t u d . S u r f . S c i . C a t a l . , 5 5 (1990) 1-32. R.A. S h e l d o n , B u l l . SOC. Chim. B e l g . , 94 (1985) 651-659. N. M i l a s , J. Am. Chem. SOC., 59 (1937) 2342; N. M i l a s and S. SuSSman. J . Am. Chem. S O C . , 58 (1936) 1302; 59 (1937) 2345. R.A. S h e l d o n , i n " A s p e c t s o f Homogeneous C a t a l y s i s " , V o l . 4 (R. Ugo, Ed.), R e i d e l , D o r d r e c h t , 1981, pp. 1-70, a n d r e f e r e n c e s c i t e d therein. R.A. Sheldon, i n "The Chemistry o f F u n c t i o n a l Groups, P e r o x i d e s " , S. P a t a i , Ed., W i l e y , New Y o r k , 1982, pp. 161-200. K.A. J o r g e n s e n , Chem. Rev., 89 (1989) 431-458. H. Mimoun, Angew. Chem. I n t . Ed. E n g l . , 21 (1982) 734; H. Mimoun, i n C o m p r e h e n s i v e C o o r d i n a t i o n C h e m i s t r y , V o l . 6 , G. W i l k i n s o n , R . D . G i l l a r d a n d J.A. M c C l e v e r t y , Eds., Pergamon, New Y o r k , 1987, pp.

317-410. 9. P . O r t i z de M o n t e l l a n o , Ed.,

" C y t o c h r o m e P450 : S t r u c t u r e , Mechanism and B i o c h e m i s t r y " , Plenum, New Y o r k , 1986. 10. F o r l e a d i n g r e f e r e n c e s see : 0. Mansuy, P u r e A p p l . Chem., 59 (1987) 759; P.L. A n e l l i , S. B a n f i , F. M a n t a n a r i a n d S. Q i c i , S t u d . S u r f . S c i . C a t a l . , 5 5 (1990) 63-70. 11. A.W. van d e r Made, M.J.P. Gerwen. W. O r e n t h a n d R.J.M. Nolte. J. Chem. SOC. Chem. Cornnun., 888 (1987). 12. P. M a r s a n d D.W. v a n K r e v e l e n , Chem. Eng. S c i . S p e c i a l S u p p l e m e n t ,

3 (1954) 41-57. 13. J.T. Groves and R. Q u i n n , J. Am. Chem. Sac., 107 (1985) 5790-5792. 14. P.E. E l l i s a n d J.E. L y o n s , C a t a l y s i s L e t t . , 3 (1989) 389-398; P.E. E l l i s a n d J.E. L y o n s , J. Chem. SOC. Chem. Cornnun.. 1189-1190, 1187il88, 1315-1316 (1989).

53 15. J.W. D o b e r e i n e r , Ann., 53 (1845) 145. 16. A. S t r e c k e r , Ann., 93 (1855) 370. 17. H. Wieland, Ber., 45 (1912) 484, 2606; 46 (1913) 3327; 54 (1921) 2353. 18. M. Hronec, Z. Cvengrosova, J. T u l e j a and J. I l a v s k y , Stud. Surf. S c i . C a t a l . , 55 (1990) 169-178. 19. M.I.A. Khan, Y. Miwa, S. M o r i t a and J. Okada, Chem. Pharm. B u l l . , 3 1 (1983) 1141, 1827. 20. M. C o s t a n t i n i and L. Krumenacker, French P a t e n t Appl., 82 (1982) 09525 t o RhBne Poulenc. 21. H. F i e g e and K. Wedemeyer, German P a t e n t , 2, (1980) 836, 327 t o Bayer. 22. K. Heyns and H. Paulsen, Angew. Chem., 69 (1957) 600; Advan. Carbohydrate Chem., 17 (1962) 169. 23. K . Heyns, H. Paulsen, G. Ruediger and J. Weyer, F o r t s c h r . Chem. Forsch., 11 (1969) 285. 24. H.G.J. de W i l t , J. L i n d h o u t and B.F.M. K u s t e r , Carbohydrate Res., 19 (1971) 5; 23 (1972) 333; 23 (1972) 343. 25. J.M.H. D i r k x and H.S. van d e r Baan, J. C a t a l ., 67 (1981) 1, 14; J.M.H. D i r k x , H.S. van d e r Baan and J.M.A.J.J. van den Broek, Carbohydrate Res., 59 (1977) 63. 26. P.C.C. S m i t s , B.F.M. K u s t e r , K. van d e r W i e l e and H.S. van d e r Baan, Appl. C a t a l . , 33 (1987) 83-96. 27. P.C.C. S m i t s , B.F.M. K u s t e r , K. van d e r W i e l e and H.S. van d e r Barn, Carbohydrate Res., 153 (1986) 227-235. 28. P.C.C. S m i t s , Ph.0. T h e s i s , Eindhoven U n i v e r s i t y o f Technology, 1984. 29. H.E.J. H e n d r i k s , B.F.M. K u s t e r and G.B. M a r i n , Carbohydrate R?s., i n press. 30. P.J.M. D i j k g r a a f , M.J.M. R i j k , J. M e u l d i j k and K. van d e r W i e l e . J. C a t a l . , 112 (1988) 329; P.J.M. D i j k g r a a f , H.A.M. D u i s t e r s , B.F.M. K u s t e r and K. van d e r Wiele, J. C a t a l . , 112 (1988) 337. 31. P.J.M. D i j k g r a a f , Ph.0. T h e s i s , Eindhoven U n i v e r s i t y o f Technology, 1989. 32. G. de W i t , J.J. de V l i e g e r , A.C. Kock van Dalen, A.P.G. Kieboom and H. van Bekkum, T e t r a h e d r o n L e t t . , 1327-1330 (1978). 33. G. de W i t , J.J. de V l i e g e r , A.C. Kock van Dalen, R. Heus, R. L a r o y , A.J. van Hengstum, A.P.G. Kieboom and H. van Bekkum, Carbohydrate Res., 9 1 (1981) 125. 34. H.E. van Dam, A.P.G. Kieboom and H. van Bekkum, Appl. C a t a l . , 33 (1987) 361-372; H.E. van Dam, P. Duijverman, A.P.G. Kieboom and H. van Bekkum, Appl. C a t a l . , 33 (1987) 373-382. 35. H.E. van Dam, Ph.0. T h e s i s , D e l f t U n i v e r s i t y o f Technology, 1989. 36. A.J. van Hengstum, A.P.G. Kieboom and H. van Bekkum, S t a r c h , 36 (1984) 317. 37. H. S a i t o , S. Ohnaka and S. Fukuda, Eur. P a t e n t , 0142725 (1984) t o Kawaken F i n e Chemicals/Kao Corp. 38. b.M. Despeyroux, K. D e l l e r and E. Peldszus, Stud. S u r f . S c i . C a t a l . , 55 (1990) 159-168. 39. H.E. van Dam, L.J. Wisse and H. van Bekkum, Appl. C a t a l . , 6 1 (1990) 187-197. 40. A.S. P e r l i n , in 'Carbohydrates, C h e m i s t r y and B i o c h e m i s t r y , 2nd edn., W. Pigman and 0. H o r t o n , Eds., Academic Press, New York, 1980, V O l . 113, pp. 1167-1215 and r e f e r e n c e s c i t e d t h e r e i n . 41. T.R. F e l t h o u s e , J. Am. Chem. SOC., 109 (1987) 7566-7568. 42. R.A. Sheldon and C. Emons, u n p u b l i s h e d r e s u l t s .

54

43. T.C. Chou and C.C. Lee, I n d . Eng. Chem. Fundament., 24 ( 1 9 8 5 ) 32-39. 44. H.C. Shen and H.S. Weng, Ind. Eng. Chem. Res., 27 ( 1 9 8 8 ) 2246-2254 and (1988) 2254-2260; see a l s o F. W a l l e r , J. C a t a l . Rev. S c i . Eng., 28 (1986) ( 1 ) 1-12. 45. R.S. Chandran and W.T. Ford, J. Chem. SOC. Chem. Comnun., 104-105 (1988). 46. H. Turk and W.T. Ford, J. Org. Chem., 53 ( 1 9 8 8 ) 460-462; a l s o see W.M. Brouwer, P. P i e t and A.L. German, J. Mol. C a t a l . , 3 1 (1985) 169. 47. S. Ivanov, R. Boeva and S. T a n i e l y a n , J. C a t a l . , 56 ( 1 9 7 9 ) 150. 48. G.L. L i n d e n and M.F. Farona, I n o r g . Chem., 16 ( 1 9 7 7 ) 3170. 49. T. Yokoyama, M. Nishizawa, T. Kimura and T.M. S u z u k i , Chem. L e t t s . , 1703-1706 (1983). 50. E. Tempesti, L. G i u f f r i , F. OiRenzo, C. Mazzocchia and G. Modica, J. Mol. C a t a l . , 45 (1988) 255-261. 51. Y. Kurusu and Y. Masuyama, J. Macromol. S c i . Chem., A24 ( 1 9 8 7 ) 389-401. 52. S. Kanemoto, H. Saimoto, K. Oshima and H. Nozaki, T e t r a h e d r o n L e t t . , 25 (1984) 3317-3320. 53. S. Yamaguchi, M. Inoue and S. Enomoto, Chem. L e t t s . , 827-828 ( 1 9 8 5 ) . 54. M. F l o o r , A.P.G. Kieboom and H. van Bekkum, Rec. Trav. Chim. PaysBas, 108 (1989) 128-132. 55. J.C. Oudejans and H. van Bekkum, J. Mol. C a t a l . , 12 ( 1 9 8 1 ) 149-157. 56. See H.C. Shen and H.S. Weng, Ind. Eng. Chem., 29 (1990) 713-719 and references c i t e d therein. 57. F. Mashio and S. Kato, Mem. Fac. Ind. A r t s K y o t o Tech. Univ. S c i . Technol., No. 16, pp. 79-95 (1967) (Chem. A b s t r . , 69 ( 1 9 6 8 ) 68762e. 58. F. T r i f i r o , P. F o r z a t t i and I . Pasquon, i n " C a t a l y s i s , Heterogeneous and Homogeneous", (8. Oelmon and G. Jannes, Eds.), Elsevier, Amsterdam, 1975, pp. 509-519). 59. P. F o r z a t t i and F. T r i f i r o , React. K i n e t . C a t a l . L e t t . , 1 (1974) 367; P. F o r z a t t i , F. T r i f i r o and I . Pasquon, Chim. I n d . ( M i l a n ) 56 ( 1 9 7 4 ) 259. 60. J. Sobczak and J.J. Z i o l k o w s k i , React. K i n e t . C a t a l . L e t t . , 11 ( 1 9 7 9 ) 359-363. 61. N. Herron, G.D. S t u c k y and C.A. Tolman, J. Chem. SOC., Chem. Comnun., 1 5 2 1 (1986). 62. N. Herron and C.A. Tolman, J. Am. Chem. SOC., 109 ( 1 9 8 7 ) 2837; C.A. Tolman and N. Herron, C a t a l y s i s Today, 3 (1988) 235-243. 63. 8. N o t a r i , Stud. S u r f . S c i . Catal., 37 (1988) 413-425. 64. G. Perego, G. B e l l u s s i , C. Corno, M. Taramasso, F. Buonomo and A. E s p o s i t o , Stud. S u r f . S c i . C a t a l . , 28 ( 1 9 8 6 ) 129-136. 65. U. Romano, A. E s p o s i t o , F. Maspero, C. N e r i and M.G. C l e r i c i , Stud. S u r f . S c i . C a t a l . , 55 (1990) 33-41. 66. P. R o f f i a , G. L e o f a n t i , A. Cesana, M. Mantegazza, M. Padovan, G. P e t r i n i , S. T o n t i and P. G e r v a s u t t i , Stud. S u r f . S c i . C a t a l . , 5 5 ( 1 9 9 0 ) 43-52. 67. T. Tatsumi, M. Nakamura, S. N e g i s h i and H. Tominaga, J. Chem. SOC. Chem. Comnun., 476-477 ( 1 9 9 0 ) . 68. See f o r example, K. Habersberger, P. J i r u , Z. Tvaruzkova, G. C e n t i and F. T r i f i r o , React. K i n e t . C a t a l . L e t t . , 39 ( 1 9 8 9 ) 95-100. 69. F. F i g u e r a s , C a t a l . Rev. S c i . Eng., 30 (1988) 457. 70. B.M. Choudary, V.L.K. V a l l i and A. Durga Prasad, J. Chem. SOC. Chem. COmun., 721-722 (1990). 71. B.M. Choudary, p r i v a t e comnunication. 72. T. K a t s u k i and K.B. S h a r p l e s s , J. Am. Chem. SOC., 102 ( 1 9 8 0 ) 5976-5978.

55

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

SOLIDS FOR CATALYSIS AND CONTROL IN ORGANIC SYNTHESIS

Keith Smith Department of Chemistry, University College of Swansea, Swansea, SA2 8PP, U.K.

summary The presence of solids such as clays, zeolites, silica or ion-exchange resins may allow catalysis or control of organic reactions. Often, yields are higher and work-up procedures simpler than for the corresponding homogeneous reactions, and product distributions may also be improved. Examples of selective substitution reactions in aromatic and heteroaromatic systems and of selective reactions of alkenes are discussed, and the wider potential for synthesis of fine chemicals is discussed.

Introduction Our entry into the field of fine chemicals and catalysis owes much to chance. Some years 1

ago, during studies of the reaction of organoboranes with dichloramine-T @CT, Fig. 1) we utilized column chromatography over silica gel to purify the chloroalkane product. During the

course of the chromatography additional minor products were formed, indicating that the silica had in some way brought about a reaction of one or more of the components of the reaction product mixture. DCT, 25OC

R3B

> RC1

Figure 1. Reaction of a trialkyborane with dichloramine-T Out of sheer curiosity we decided to investigate the reactive system generated and utilized toluene as a model subsmate. It was soon established that DCT,which is unreactive towards toluene at ambient temperature and effects side-chain chlorination at elevated temperature, 2 became an electrophilic chlorinating agent in the presence of silica gel (Fig.2).

56

6

DCT, SiO,

____)

CCL,, 2 5 O C

&

+

l65%1

6 /

Cl (35 %I

Figure 2. Chlorination of toluene with DCT-silica. Dichlorourethane and ten-butyl hypochlorite (TBH) were even more reactive in the presence of silica, chlorination of toluene with TBH being complete in just one hour at ambient temperature with 3.8g of BDH silica for a 5 mmol reaction in 10 ml of tetrachloromethane solvent.

Intrigued by these observations, we decided to undertake a more systematic

investigation of the potential of solid catalysts for organic synthesis. Development of Optimal Catalytic Systems for Chlorination of Aromatics 3 Silica had previously been shown to catalyse the reaction of toluene with chlorine or 4

sulphuryl chloride, but it was not clear what factors were influential in producing the catalytic

5

effect. We therefore investigated this aspect (Fig. 3).

It was clear that whereas mean pore diameters and surface areas may have some effect on reaction rates, the major influences were the water contents and acidities of the silicas. These factors combine in determining surface acidity, and it is presumably this which is the real cause

of the catalysis. Chlorinated hydrocarbons were the best solvents, and as a general method for chlorination of aromatic hydrocarbons the use of TBH (5 mmol) in tetrachloromethane (10 ml) in the presence of BDH silica (3.8g) can be recommended. Such conditions result in rapid and quantitative monochlorination. Isomer dismbutions are similar to those obtained by traditional 2 chlorination methods. More reactive substrates like phenol react rapidly with TBH in the absence of any added catalyst, but use of less reactive chlorinating agents such as N-chlorodialkylamines permits silica to have an effect6 Interestingly, this process provides somewhat higher mono : dichlorination and ortho :para chlorination ratios than traditional methods of chlorination, presumably because of the bringing together at the surface of the hydroxyl group of the phenol and the active chlorinating species.

BDH 15049 " + 1%H20

100 95

1 48

4.7

1.8 (2.8)

773

18

Mallinckrodt silicic acid " (dried)

0 100

48 2

5.3 4.5

16

585

34

Kieselgel60 (dried)

80

2

6.2

(6.6f

(490f

(581'

10

24

6.9

(5.2f

(300f

(146f

Davison 57 (dried)

%or reaction with TBH at 2 0 ' ~for time indicated. bOf a 10% aqueous slurry. C

Measurement recored prior to drying.

Figure 3. Catalytic effectiveness of several silicas. Benzene reacted only slowly at ambient temperature with TBH in the presence of silica and less reactive substrates such as halogenobenzenes failed to react under such conditions. Thus, attention was turned to aluminosilicatesas potential catalysts. These solids may be considered to have a silica-like lattice in which some of the Si atoms have been replaced by A1 atoms. This requires the Al atoms to be tetracoordinate and consequently negatively charged. Electrical neutrality is achieved through the presence of accompanying cations (Fig. 4). The countercations may be protons and in this case the solids are much more smngly acidic than silica and might be more effective catalysts.

58

Figure 4. charged nature of aluminium in aluminosilicates. Furthermore, aluminosilicates are available in a variety of different structural types including lamellar clays and three-dimensional microcrystalline zeolites. catalysts of typical organic petrochemicals industry.

Such solids can be useful

although their main applications to date have been in the Zeolites offer particular promise because they possess pores and

channels of molecular dimensions and can therefore act as molecular sieves and display shape9 selective catalysis. We experimented with a number of different zeolites and obtained excellent results with the proton-exchanged form of zeolite X (faujasite)."

Reactions were more rapid than with silica as

the catalyst and occurred reasonably rapidly even with halogenobenzenes. Furthermore the selectivity was outstanding, giving the highest para:orrho ratios ever achieved. For toluene, the selectivity was highest in diethyl ether as solvent (Fig. 5 ) , but some reagent was destroyed by reaction with the solvent. With less reactive substrates the destruction of the reagent was paramount and for such cases it was preferable to use acetonitrile as a general solvent. Under 10 these conditions good selectivities were obtained with a range of aromatic substrates (Table 1).

Figure 5. Highly para-selectivechlorination of toluene.

59

Table 1

orrho :para Selectivities in chlorination of monosubstituted benzenes with TBH

-

zeolite HNaX in acetonitrile

R +

t

H ,Na Faujasite I Bu tOCl,CH3CN

CI

R

O:D

Me

18 : 82

Et

13 : 87

Pri

20 : 80 t

Bu

2 : 98

Ph

14 : 86

c1

3 :97

Br

3 :97

Significant para-selectivities in the chlorination of toluene have been obtained previously using chlorine on substrate entirely preadsorbed on zeolite CaX,ll but the yields of monochlorotoluenes were less and the selectivities were still short of those obtained with the TBH-HNaX system. In other cases zeolites have catalysed reactions but with little benefit on

para : ortho selectivities.12 Thus, the system reported here remains the best for providing quantitative yields and conversions with the highest selectivities. We have made no attempt to investigate the detailed mechanism of the reaction. However, the following points are noteworthy: (a) larger crystals of the zeolite produce even higher selectivities (94%para on chlorination of toluene) under otherwise comparable conditions; (b) the reaction with DCT rather than TBH is not substantially catalysed by zeolite HNaX and the

60

low yield of chlorotoluenes obtained shows no exceptional selectivity; (c) no isomerization of the products occurs under the reaction conditions. Thus, it appears likely that the reaction with TBH takes place inside the pores of the zeolite and that the regioselectivity is kinetically conmlled by the differing spatial constraint on the two transition states. It is clear that alurninosilicates can act as effective acidic catalysts for the reaction and also provide shape selectivity. We used rather large amounts of zeolites in these reactions. In principle, it is quite possible

to use much smaller quantities of the catalyst. However, the rate is then less and the contribution of free solution ("uncatalysed') reaction increases, thus lowering the selectivity. Nevertheless, for a small trade off in selectivity lower quantities of catalyst may be preferable. The demonstration of the synthetic potential of solid catalysts for selective chlorination led us to consider other possible applications.

The rest of this review will deal with other

applications. Bromination of aromatics and heteroaromatics N-Bromosuccinimide (NBS) is a convenient organic brominating agent. In combination with BDH silica gel it proved to be capable of rapid and quantitative room temperature brornination of anisole and phenetole (Fig. 6).13 However, this sytern did not react readily with unactivated aromatics. Thus, attention was turned to its possible application for brominaion of electron-rich heterocycles such as indoles. It should be noted that solids have previously been 14 used to catalyse aromatic brorninations with bromine. OR

011

(K=Me. EU

Figure 6 . Broniination of alkoxybenzenes with NUS-silica. Traditional methods for bromination of indoles are not very satisfactory. For example, use of NBS in aqueous acetic acid generally gives r i s e to oxindoles or indolenines rather than to b r o m o i n d o l e ~ .Bromination ~~ is better in aprotic solvents16 but yields are still often poor and/or conditions vigorous. By contrast, NBS-silica brominated skatole rapidly at room temperature in

61

dichloromethane, giving 2-bromoskatole or 2,6-dibromoskatole depending on the stoichiometry.

17

Similar results were obtained with 2-methylindole (Fig. 7) and indole-3-acetonitrile.

mar 2 NBS

jr

H

'H3

50,. CH,CL, 30min.

9 5a/'

H

CH3

H

15min.

>90%

Figure 7. Bromination of 2-methylindole with NBS-silica. Attempts to polybrominate indoles by use of excess NF3S resulted in slow reactions and mixtures of products. Attention was therefore turned next to other heterocycles. Reaction of benzimidazole gave 2-bromobenzimidazole with one equivalent of NBS and 2 5 dibromobenzimidazole with two equivalents while 2-rnethylbenzimidazole readily gave 5-bromo2-methylbenzimidazole (Fig. 8).17 The formation of 2-bromobenzimidazole is interesting since maditional bromination of benzimidazole gives 5-brom0benzimidazole.~~ Presumably, the production of 2-bromobenzimidazole is another example of the silica surface binding the substrate, via H-bonding, and the active reagent in close proximity.

H

H

Figure 8. Bromination of benzimidazoles with NBS-silica.

H

62

The novel bromination method also works well for carbazole (Fig. 9).19 Because there is little influence on the reactivity of the second benzene ring when the first is brominated monobromination is not particularly selective. However, dibromination is highly selective, as is tribromination. Tetrabromination is very slow, even with excess NBS. The behaviour of Nethylcarbazole is similar, except that monobromination is slightly less selective (50% 3-bromo-Nethylcarbazole with one equivalent of NBS), while there is essentially zero tendency to

(!lo%,

161kl

H

I

\BS. I 9 h equiv.

(Si’III

Figure 9. Bromination of carbazole with NBS-silica. Iminodibenzyl is somewhat more reactive than carbazole.

Appropriate choice of

stoichiometry allows ready production of 3-bromoiminodibenzyl (80% with one equivalent of NBS), 3,8-dibromoiminodibenzyl (98% with two equivalents), 1,3,8-tribromoiminodibenzyl 19 (86% with three equivalents) or 1,3,8,1O-tetrabromoiminodibenzyl(90% with excess reagent). The case of N-ethyliminodibenzyl is interesting. Mono- and dibromination occur satisfactorily to give 63% and 80% yields respectively (Fig. 10). However, with excess reagent unexpected The dibromination dealkylation occurs, leading to the production of tetrabr~moiminodibenzyl.~~ process can also be carried out successfully with imipramine.

63

(80%)

1

(63%)

> 6 equiv. N E S SO,.

CH,CL,

BrQBr Br

H

Br

Figure 10. Bromination of N-ethyliminodibenzyl with NBS-silica. Although the NBS-silica system has useful potential for bromination of electron rich heterocycles, it has limited application to non-activated aromatics and sometimes meets problems with polybromination even for the activated heterocycles. Thus, it was of interest to investigate the potential of different brominating agents and different solids. Bromination of anisole with NBS at ambient temperature was used as a test reaction to test the effectiveness of different catalysts."

The effectiveness decreased in the order montmorillonite K10 > synclyst 25 (25% t

alumina) > synclyst 13 > H Mordenite, Amberlyst A125, silica > Amberlite IRA 120 >> Nafion

+

(H ). The order suggests that surface acidity is a major factor in the catalysis, but the mordenite

and Nafion were far less active than their acidities would suggest. Presumably this results from failure of the reagent to penetrate the catalyst interiors to reach the active sites in these cases. Under comparable conditions with the same catalyst (silica), the order of activity of various brominating agents was acetyl hypobromite > TBH, bromine, N&-dibromamine-T > 1,3-

dibromo-5,5-dimethylhydantoin (DBDMH) > NBS > Nfl-dibromo-ten-butyylamine. Thus, it is now possible to select a combination of reagent and catalyst to provide a highly active system, a very weakly active system, or an intermediate system according to the needs of the substrate. Some of the difficult brominations encountered previously can now be tackled. For example, tetrabromination of carbazole occurs readily at ambient temperature with silica as catalyst if DBDMH is used instead of NBS.19 In another example, attempts to polybrominate indole-3-acetonitrile with NBS over a silica catalyst give rise to complex mixtures but use of the more active catalyst, montmorillonite K10, allows the clean production of an unstable

64

intermediate. Depending upon the work-up conditions good yields of either of two stable 19 products can be obtained (Fig. 11).

3NBS

@ H C Z JN

KlO

H

Figure 11. Brornination of indole-3-acetonitrilewith NBS-K10. The range of brominated indoles now available provides opportunities for convenient syntheses of a number of potentially useful compounds. For example, many marine natural products contain a bromoindole fragmen?'

derivable from 6-bromotryptamine, but 6-

bromotryptamine has been obtained in only 6% yield following a multistage synthesis involving build up of the indole nucleus2l Reduction of the products in Figure 11 may provide a much more direct approach and we are currently investigating this po~sibility.'~In addition, catalytic tritiation of bromoindoles provides an opportunity for synthesis of radiolabelled compounds (e.g.

19

Fig. 12).

rnBr CH&N

Br

T,, complex catalyst*

wNH

i t \

H

H (76 Ci rnmol-')

Figure 12. Synthesis of tritiated tryptamine. Reagent systems suitable for bromination of deactivated aromatics such as nitrobenzene can be devised, as can mild reagent systems suitable for selective bromination of highly activated 19 aromatics such as phenols (Fig. 13).

Supported Bromine

6'

*

65

OIH

+

dr (80% - 100~0)

(X=H, OMe. CH, CI)

(0- 20%)

Figure 13. Selectivepara-brominationof phenols. Other Electrophilic Substitution Reactions The principle of combination of an organic reagent and a solid acid catalyst can be usefully applied in different types of electrophilic substitutionreactions. For example, we have found that use of an acyl nitrate in combination with the proton or aluminium form of mordenite or zeolite X leads to good yields of nitroarenes having much higher para:ortho isomer ratios than those obtained via traditional nitration methods (Fig. 14)22 Laszlo's group have achieved similar 23 results by use of metal nitrates supported on K10 clay in the presence of acetic anhydride. R

6

R

PhC02N02 AI3+,H+Mord.

=-

I

o : m : p

Yield (%)

time

99

10 rnin

32 : 1 : 67

97

80 rnin

25 : 2 :73

86

2h

26 : 2 : 72

86

70 rnin

14:2:84

96

70 rnin

5 : 3 :92

Figure 14. Selective para-nitrationof alkylbenzenes. The group of Geneste in Montpellier has studied Friedel-Crafts acylation of aromatic hydrocarbons by carboxylic acids in the presence of solid acid ~atalysts?~ whilst Friedel-Crafts alkylationhas receivedextensive study over both

andclays26 It is clear that such solid

66

acid catalysts have substantial potential for electrophilic aromatic substitution. It would be of interest to know if similar benefits could be obtained in nucleophilic aromatic substitution reactions by means of solid bases.

The Ullmann Synthesis of Diary1 Ethers A useful reaction for investigation is the Ullmann synthesis of diaryl ethers:7

which has

potential importance for synthesis of a number of commercial products in the agrochemical and pharmaceutical fields, but which suffers from serious drawbacks, particularly high temperatures o 28 (typically 180-220 C) or long reaction times (up to 20 hours) and modest yields (typically 4030 60%).29We hoped that a powerful solid base such as fluoride-impregnated alumina might prove advantageous. However, in practice it appeared that simple, solid potassium carbonate was as good a base as any. Nevertheless, we have been successful in developing conditions which give

substantial practical advantages by using ultrasonic irradiation of solventless reaction mixtures (Fig. 19.”

Ultrasound has previously been found to effect rate enhancement in the Ullmann 31 synthesis of biaryls.

(excess) (X,y = H, OMe, CH3)

(70- 95%)

Figure 15. Ultrasound-assisted synthesis of diaryl ethers. Applications of Solids in Aliphatic Chemistry Although this review has concentrated primarily on aromatic chemistry, there are numerous potential applications of heterogeneous catalysts in aliphatic chemistry. Any acidcatalysed reaction may in principle be catalysed by the acid of a zeolite or clay and many such examples have been reported. 8’32-34 Base-catalysed reactions could benefit from utilization of fluoride-impregnated alumina3’ However, few of the reported examples in aliphatic chemistry appear to have made use of the shape-selectivity properties of the catalysts. One example where such a factor might be useful is in the Diels-Alder reacti0n.3~ For example, if the reaction between acrylonitxile and cyclopentadiene could be catalysed within the

67

pores of an appropriate zeolite it might be possible to gain control of the exo:endo ratio and cause the em-product, unusually, to predominate. Our first attempt at this reaction seemed to give complete selectivity in favour of the e~o-product,'~but the yield was only ca. 35%. However, breakdown of the zeolite liberated further adduct, which turned out to be almost entirely endoproduct. Thus, the reaction itself did not produce the desired selectivity (no transition state control), but there was impressive separation of the products, which may warrant further investigation.

In one final example from the work carried out in our laboratories, it is interesting to consider the possibility of shape selection between chemically similar functional groups. This would be especially useful when catalytic sites within the pores of a zeolite were essential for reaction, but the selectivity could still be demonstrated even on uncatalysed reactions. We therefore considered the addition of bromine to alkenes.

In principle, a zeolite with an appropriate pore structure should be able to select between a straight chain and a branched or cyclic alkene because only the straight chain compound could enter the pores. If an equimolar mixture of the two alkenes were treated with one equivalent of bromine in different ways differences should become evident. In the absence of zeolite, little selectivity would be expected. If the zeolite were present in sufficient quantity to absorb all of the straight chain alkene and then the bromine were added, preferential reaction should take place on the alkene not absorbed. On the other hand, initial absorption of the bromine followed by addition of the alkene mixture should give rise to preferential reaction on the alkene which can enter the pores to meet the bromine. The validity of this proposition is demonstrated in Figure l6.l' successfully applied to a range of other examples.

the process has been

68

0

+

+ Br

65 : 35

no zcolitc prcsciit

zeolite p r e s c i i t , re;iclioii 'iiulsitle' pores 96 : 4 aviililc ~ i r c s c i i l , rc;icliiiii 'iiisidc'

Iiiirvh

17 : X 3

Figure 16. Selective bromination of alkenes in the presence of a zeolite. A selective bromination of styrene with bromine absorbed into a molecular sieve was we believe this to be the f i s t claimed once befor?6 but the claim was later r e f ~ t e d . 3 Thus, ~ genuine demonstration of this effect. Conclusion Zeolites and clays are extensively used as catalysts in the bulk chemicals and petrochemicals industries. temperatures.

Typically, reactions are carried out in the gas phase at high

Such conditions become unreasonable when liner chemicals are involved.

Instead, low temperatures and liquid phase reactions become necessary. The costs of the reagents and catalyst are also less important as the product values increase, but product selectivity and good yields are more important. Thus, there is considerable interest in the development of mild, selective, liquid phase reactions such as those described here. Although this review has concentrated particularly on the work of our own gronp, many others are working in the field and their contributions are referred to in the review articles cited. Acknowledgements The contributions of an able group of research students - Michael Butters, Karl Fry, Mark Hammond, Anil Mistry, Martin James, Lysanne Pearce, Vincent Boschat and Dennis Jones - and of industrial collaborators

-

Barry Nay, David Walker, Martin Atkins (BP), Martin Bye

(Amersham International), Derek Basset, Paul Ashworth and Janet Chetland (Associated Octel) are gratefully acknowledged. The companies and the S.E.R.C. are thanked for financial support.

69

Finally, I am grateful to Professor M. Guisnet and the organizers of the Conference on Fine Chemicals and Catalysis for the invitation to present this work in Poitiers.

References 1.

V. B. Jigajinni, W. E. Paget and K. Smith, J. Chem. Res. (S),1981, 376.

2.

K. Smith, M. Butters, W. E. Paget and B. Nay, Synthesis, 1985, 1155.

3.

C. Yaroslavsky, Tetrahedron Lett., 1974,3395.

4.

M. Hojo and R. Masuda, Synth. Commun., 1975,5, 169.

5.

M. Butters, Ph.D. Thesis, Swansea, 1986.

6.

K. Smith, M. Butters and B. Nay, Tetrahedron Lert., 1988,29, 1319; see also UK Patent 2 165 244A (1986).

7.

P. Laszlo, Accounts Chem. Res., 1986,19, 121; Science, 1987,235, 1473.

8.

W. Holderich, M. Hesse and F. Nlumann, Angew. Chem. Int. Ed. Engl., 1988, 27, 226; H. van Bekkum and H.W. Kouwenhoven, Recl. Trav. Chim. Pays-Bas, 1989,108,283.

9.

D. W. Breck, '2eolite Molecular Sieves", Wiley, New York, 1974.

10.

K. Smith, M. Butters and B. Nay, Synthesis, 1985, 1157; see also U.K. Patent 2 155 009A (1985).

11.

J. Van Dijk, J. J. Van Daalen and G.P. Paerels, Recl. Trav. Chim. Pays-Bas, 1974, 93,72; see also H. van Bekkum, T. Huizing, J. J. F. Schotten and T. M. Wortel, Terrahedron Letr., 1980,21,3809.

12.

For an extensive discussion of the use of zeolites for catalysis of aromatic chlorinations see L. Delaude and P. Laszlo, J . Org. Chem., in press. We thank Professor Laszlo for a preprint of this publication.

13.

See A. G. Mistry, K. Smith and M. R. Bye in D. Price, B. Iddon and B. J. Wakefield, Ed., "Bromine Compounds: Chemistry and Applications", Elsevier, Amsterdam, 1988 (contains reports of a conference held in Salford, September 1986), p.277. The process has recently been developed into a general method for para-selective bromination of aromatic ethers; H. Konishi, K. Aritoni, T. Okano and J. Kiji, Bull. Chem. Soc. Japan, 1989,62,591.

70

14.

See

Th. M. Wortel, D. Oudijn, C. J. Vleugel, D. P. Roelofsen and H. van Bekkum, J.

Caral., 1979, 60, 110, and references cited therein; F. de la Vega and Y. Sasson, J.C.S. Chem. Comm., 1989,653. 15.

T. Hino, M. Nakagura and S. Akaboshi, Chem. Phurm. Bull (Japan). 1967,15, 1800; R. L. Hinman and C. P. Bauman, J. Org. Chern., 1964, 29, 1206; J. Parrick, A. Yahya and Y.

Jin, Terrahedron Lerr., 1984, 25, 3099, W. B. Lawson, A. Patchornik and B. Witcop, J. Am. Chem. SOC.,1960,82,5918; T. Hino and M. Nakagawa, Heterocycles, 1977.6, 1680.

16.

N. Putokhin, J. Gen. Chem., (U.S.S.R.), 1945, 15, 332; R. S. Phillips and L. A. Cohen,

Tetrahedron Lerr., 1983,24,5555; and references cited therein. 17.

A. G. Mistq, K. Smith and M. R. Bye, Terrahedron Lerr., 1986,27,1051.

18.

D. J. Evans, H. F. Thimm and B. A. W. Collier, J . Chem. SOC.Perkin Trans. 11, 1978,865.

19.

Unpublished observations.

20.

See D. J. Faulkner in D. Price, B. Iddon and B. J. Wakefield, Ed., "Bromine Compounds:

Chemisrry and Applications", Elsevier, Amsterdam 1988, p. 121. 21.

C. Grgn and C. Christophersen, Acra Chem. Scand. B , 1984,38,709.

22.

K. Smith, K. Fry, M. Butters and B. Nay, Terrahedron Len., 1989, 30, 5333; see also European Patent Number EP 0356091 (1990).

23.

A. Cornelis, L. Delaude, A. Gerstmans and P. Laszlo, Terrahedron Lerr., 1988, 29, 5657; A. Cornelis, A. Gerstmans and P. Laszlo, Chem. Lerr., 1988, 1839; P. Laszlo and J.

Vandormael, ibid, 1988, 1843. 24.

B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille and D. Pioch, J. Mol. Caral., 1987, 42 , 229; B. Chiche A. Finiels, C. Gauthier and P. Geneste, J. Org. Chem., 1986, 51,2128.

25.

N. S. Gnep, J. Tejada and M. Guisnet, Bull. SOC.Chim. Fr., 1982,5.

26.

For a recent example, see J. A. Clark, A. P. Kybett, D. J. Macquarrie, S. J. Barlow and P. Landon, J. Chem. SOC.Chem. Commun., 1989, 1353.

27,

A. A. Moroz and M. S. Shvarstberg, Usp. Khim., 1974,43, 1443.

28.

T. Y'amamoto and Y. Kurata, Can. J. Chem., 1983,61, 86.

29.

H. E. Ungnade and E. F. h o l l , Org. Synrh., 1946,26,50.

71

30.

T. Ando, S. J. Brown, J. H. Clark,D. G . Cork, T. Hanafusa, J. Ichihara, J. M. Miller and M. S. Robertson, J . Chem. SOC.Perkin Trans. II, 1986, 1133.

31.

J. Lindley, J. P. Lorimer and T. J. Mason, Ultrasonics, 1987,25,45; 1986,24,292.

32.

P. Laszlo, Science, 1987,235, 1473.

33.

P. Laszlo, Accounts Chem. Res., 1986,19, 121.

34.

F. Figueras, Caral. Rev. Sci. and Eng., 1988,30,457.

35.

J. Ipaktschi, Z. Natutforsch. B;Anorg. Chem., Org. Chem., 1986,41B, 496.

36.

P. A. Risbood and D. M.Ruthven, J . Am. Chem. SOC.,1978,100,4919.

37.

R. M. Dessau,J. Am. Chem. SOC.,1979,101, 1344.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

73

ENANTIOSELECTIVE CATALYSIS BY CHIRAL SOLIDS: APPROACHES AND RESULTS. HANS-ULRICH BLASER* and MANFRED M a L E R Central Research Laboratories, CIBA-GEIGY AG, R 1055.6, CH-4002 BASEL

ABSTRACT The application of solid chiral catalysts for the enantioselective synthesis of chiral molecules is reviewed. An attempt has been made to classify the different types of catalytic systems and to discuss the approaches and methods which have been used for the investigations. Enantioselectivities observed for several reaction types (hydrogenation/hydrogenolysis/dehydrogenation; electrochemical reactions; base catalysis; miscellaneous reactions) are summarized according to substrates and catalytic systems. The influence of system parameters and mechanistic investigations are reviewed for the following catalyst systems: Tartrate modified catalysts, cinchona modified catalysts and electrochemical systems. Conclusions concerning synthetic and commercial-scale applications of chiral solid catalysts are presented. I. INTRODUCTION /SCOPE

Enantioselective synthesis is a topic of undisputable importance in current chemical research and there is a steady flow of articles, reviews and books on almost every aspect involved. The present overview will concentrate on the application of solid c h i d catalvsts for the enantioselective synthesis of chiral molecules which are a special class of fine chemicals. Included is an account on our own work with the cinchona-modified Pt catalysts. Excluded is the wide field of immobilized versions of active homogeneous complexes or of bio-catalysts. During the preparation of this survey, several reviews have been found to be very informative [l-141. if its Let us start off with a few fundamental concepts and definitions. A molecule is image and mirror-image are not superimposable and therefore enantiomeric. A reaction or a catalyst is called enantioselective (or asymmetric or enantioface-differentiating [4]) if one of the enantiomers is produced preferentially starting from non-chiral substrates. If a reaction occurs faster with one enantiomer of a racemic substrate we speak of kinetic resolution (or enantiomerdifferentiation [4]). Enantioselectivity (or enantiomeric excess (ee) or optical yield) is only possible if a chiral agent is present during the reaction and interacts with the substrates in the product-determining step. It is a kinetic phenomenon, due to the difference in activation energy between the diastereomeric transition states leading to the two enantiomers (distinguished by the prefix R and S or d and 1). The enantioselectivity is defined as ee (%) = 100 x I[R]-[S]l/ ([R]+[S]). At 25 OC, an energy difference of 1.5 kcal/mol and 3 kcal/mol leads to about 80% (90%:10%) and 98% (99%:1%) enantiomeric excess, respectively. The observed ee will be below the inherent catalyst selectivity if the racemic reaction occurs uncatalyzed or on non-chiral sites as well. An enantioselective catalyst has two functions: First, it has to perform what one could call the chemical catalysis, here named activating function. In addition, it has to control the

74

stereochemical outcome of the reaction and we term this the controlling function. The two functions can be performed by the same or by two different agents. In the following scheme we have tried to classify the different types of catalytic systems described in Section I1 where an inherently chiral or a chirally modified solid catalyst is involved. At least two extreme cases can be distinguished, one where reactions are being catalyzed only at the surface of a "hard" solid (e.g. a metal) and the other where the reaction occurs inside a "soft" material (e.g. an organic polymer). An attempt will be made to discuss critically the approaches and methods which have been applied during various phases of the investigations, because each phase has its own type of problems and therefore requires a different strategy. Actkratlng function

Controlllng tunctlon

Reactlon type

metallic surface

modifier or polymer

metallic surface

chiral support

metal salt or oxide

modifier or polymer

chiral metal salt

chiral metal salt

chiral polymer

chiral polymer

none

crystal

hydrogenation erectrochemistry hydro enation de h ycfogenat ion isomerization polymerization isornerization pol merization caAene addition SN2 reaction nucleophilic addition oxidation bromination dimerization h drogenolysis

Our review will be organized as follows: 11

I11 IV V VI

EARLY INVESTIGATIONS / EXPLORATORY PHASE: THE SEARCH FOR CATALYTIC SYSTEMS. INFLUENCE OF SYSTEM PARAMETERS: THE SEARCH FOR BETIER ENANTIOSELECTIVITIES. SYNTHETIC AND COMMERCIAL-SCALE APPLICATIONS. MECHANISTIC INVESTIGATIONS: THE SEARCH FOR UNDERSTANDING. CONCLUSIONS.

11. EARLY INVESTIGATIONS / EXPLORATORY PHASE: THE SEARCH FOR CATALYTIC

SYSTEMS. During the early phase of an investigation there is usually very little information available and therefore, the goal is to find a "lead" which then can be developed and improved further to give e.g. a synthetically useful system. The approach most often used is what one could call "screening with a concept": experiments are set up in order to test an intuitive idea or a more or less well defined hypothesis. A good illustration for this approach is described by Izumi [4]: "we expected simply that an optically active product should be produced from the influence of the optically active environment, like baking a waffle. We used silk fibroin as a 'waffle iron"'.

75

Random screening is usually too time consuming but a certain randomness is desirable during the exploratory phase because unexpected effects are bound to occur. Again a citation: Tai [I] says regarding the discovery of the Ni/tartrate/NaBr catalyst and its application to 0-keto ester hydrogenation: "It was sheer luck that methyl acetoacetate was employed as a substrate...". The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally!) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried: First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. We carried out a thorough literature search and the results are summarized in Figs. 1-4 (substrates), Fig. 5 (important modifier structures) and Tables 1-4 (catalytic systems). We are confident that most of the relevant investigations and catalytic systems are reported here. Some papers are available only in Russian or Japanese and in these cases we either cite the Chemical Abstract reference or a review. Results are classified according to reaction type: a) Hydrogenation/hydrogenolysis/dehydrogenation;b) Electrochemical reactions; c) Base catalysis; d) Miscellaneous reactions. If available, the best optical yield for a substrate type is included in the Figures, together with the best catalyst system. a) Hydrogenation / hydrovenolysis / dehydrogenation This is clearly the most important application of chiral solid catalysts and Fig. 1 and Table 1 show an impressive number of entries. As will be seen in Sections In.-V., much of the available information is concentrated on very few catalyst systems and substrates. The first successful experiments were reported by Schwab [ 161: Cu, Ni and Pt on quartz were used to dehydrogenate racemic 2-butanol 2. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 2 had reacted preferentially (ee
a

76

TABLE 1 Enantioselective heterogeneous hydrogenation catalysts (for substrates see Fig. 1). Year No. Catalytlclcontrol functlon

Substrates (ee)

1932 fl 1939 g 1939 1940 @ 1956 g ! 1958 @ 1958 1959 @ 1964

Cu, Ni, Pd, Wquartz PtO2/cinchonine Raney Ni/glucose PI blackkhiral acids Pdsilk fibroin Raney Ni/camphor (Raney) Ni/amino acid Pd, Pt-colloid/polysaccharide (Raney) Niitartrate/(NaBr)

14 (n.d.),

1966 1967 J4!l 1970 1970 H13 1979 H14

Pd/C/amine or amino acid Pd, Ni, Ru/polypeptide Pd/cellulose Pd/ion exch.resin/amino acid Wcinchona alkaloid

1985 1985 1987

Pdkinchona alkaloid Wzeolitekhiral amine Pd/C/cyclodextrine

H17

Ref.

(n.d.), 8 (-10)

[7,8, 161 I171 14 ( 4 ) 16 (4) I19.201 r2 (18)'I181 5(15),ll(30),l3(n.d.),l5(66),l8(23)[21,22] (24) I221 -8 (10)J.Q(51,Is(50) I1 221 6 (l), (n.d.1 17,231 -1 (88),2(74),3 (71), 4 (70). 2 (74) Ill I!!(17). (5) [79,811 [24-261 6 (8) 11 (10) 12 (4). 14(2). 16 (4) 127,281 14 (6i 6 ( 4 , Is(<:1) 1291 6 (4),S ( 4 ) 1291 5 (12), 6 (92),7 (47),3 (43),l3 (15) [30-321 14 (51, ( 3 ) , 3 (11 ) , g (25) 5 (5),= (30). 22 (50) [32,331 5 (n.d.), (n.d.) I341 5 (1) I351

I!!(8)

s

I

a (i)Ta a

R 1

R,R1 = alkyl n

2 -

3 R = alkyl -

4 R=H,OMe - R 1 = H, Me

5 R = alkyl, aryl

-

Fig. 1. Substrates used i n hydrogenation reactions (the reacting function i s highlighted). The number of the catalyst system (see Table 1) and the best optical yield i s given as w e e ) .

77

modify J3@ l- tP and Ni-catalysts (H7) leading eventually to the best known enantioselective heterogeneous catalytic systems: Ni/tartrate/NaF%r(H9) which are able to hydrogenate P-ketoesters with optical yields up to 88% [l, 41 (see Sections III-V). Other modifiers were not very effective (H6, H10, H17) and not further investigated. A notable exception are the cinchona alkaloid modified Pt catalysts (H14) first described by Orito for the asymmetric hydrogenation of a-keto ester with optical yields reaching 87% [31] (see Sections HI-V). Later, Pdcinchona catalysts (H15) were also described [32, 331 (see Sections 111, IV). An interesting approach is the but no experimental details and no ee’s have been reported. use of modified Wzeolites Just as very few catalyst/modifiercombinations give good optical yields, only certain types of substrate are suited (see Fig. 1). Reproducibly good ee values have been reported for a-and P-ketoesters @, IJ, a-ketolactone 2, P-diketone 2, P-ketosulfones (3J and methylketones (4, 9. C=N- and C=C-bonds are hydrogenated with p r to moderate optical yield. The hydrodehalogenation of 22 is a unique case of substrate specificity [33]. b) Electrochemical reactions. It is debatable whether electrochemical reactions should be discussed here. There are two reasons for including them: First, electrochemical and catalytic reductions have many common features and parallels [37]. Second, electrochemical methods have been used to determine the amount of adsorbed tartrate on Ni [12] and it might be possible to study the adsorption behavior of certain modifiers on metallic electrodes using methods recently described by Soriaga et al. [38]. The results of the application of chiral electrodes are summarized in Table 2 and Fig. 2. We can keep our comments short because an excellent review by Tallec [36]gives a thorough discussion of scope and limitations of enantioselective electrochemistry. Three types of systems have given the best results: metal electrodes with strongly adsorbing chiral modifiers (El, E3), the use of chiral electrolytes (E2, preferentially ephedrinium salts) and electrodes coated with polypeptides (E4, E5). It is interesting to note that different substrate and modifier types give the best enantioselectivities in catalytic hydrogenations compared to electrochemical reductions. Good results are obtained for acetyl pyridines (25-22, note the effect of the acetyl position!) with a Hg electrode modified by the addition of strychnine or brucine; for a C=C reduction (33) with a Hdsparteine and the dehalogenation reaction (29) (Hglemetine). On the other hand, a m a t e modified Ni-electrode reduced a P-ketoester with only 6% ee. In some cases coupling

a

TABLE 2 Enantioselectiveelectrochemicalsystems (substrates see Fig. 2). Year

1970 1973 1982 1983 1983

No.

Electrode/controi1ing function

Substrates (ee)

Ref.

Hg/alkaloid

25 (a), 3(O), 27 (4O), 28 (1% 29 (45)

1361

Hg/chiral electrolyte Nihartrate graphite/polypeptide Wpolypeptide

1(6)

30 (5), a(20) 32 (4,s (20) 24 (20), (26)29 (IT),2 (21),33(43), %Po) 4(93)

Fig. 2. Substrates employed for asymmetric electrochemistry (reduced function is highlighted). The number of the modified system (see Table 2) and the best optical yield are given as W e e ) . reactions compete with the desired reduction, leading to dimers which can be chiral as well (35). Very high optical yields are reported for the oxidation of hindered arylsulfides (34, R = t-buty1)with an interesting, polypeptide coated graphite or Pt electrode (E4.E5). Tallec concludes in his review that, with few exceptions, enantioselective electrochemistry is at the moment not a competitive method for the preparation of chiral molecules. c ) Base catalysis Reactions catalyzed by solid bases were obvious candidates for testing hypotheses on the nature and the mode of action of enzymes. Bredig [40] used aminated cellulose (B2) as a model because an enzyme was thought to consist of “a specific active function and a colloidal carrier”. Indeed, cyanohydrin 40 was formed with an enantiomeric excess of 22%!Fig. 3 and Table 3 contain a summary of the reported results for base-catalyzed reactions. It is not clear whether the ZnO/fructose catalyst (B1) described by Erlenmeyer [39] is really heterogeneous but it is the first report on using sugars as modifiers. Some reactions are probably just curiosities (39,4lJ, but two

TABLE 3 Solid enantioselective base catalysts (substrates see Fig. 3). Year

No.

1922 1932

82

1955 1957 1962 1968 1984 1984

83 84 85 86

87

Catalytlc/controliing function

Substrates (ee)

ZnO/d-fructose amino cellulose LiOIquartz A1203/alkaloid polyamine polypeptide Ca(OH)2/chitosane Zn-tartrate

38 (n.d.) 40 (22) 37 (n.d.) 39 (n.d.) 40 (21) 42 (96) 41 (n.d.)

6 (85)

Ref. 1391 1401 17,411 1421 1431 110, 111 1441 [451

79

0

+

R lAFi

0

0

R, R,

= alkyl

+

F C N

+

-coo"

O C H O H 0

NuR

k o H

Rl

CN

Br2

+ HCN

ofcoo"

le5/21)

b

(B7)

(86/96)

bph + H202lNaOH Ph phase trakfer

various sugars O PhJ

H

q o Ph

Fig. 3. Reactions catalyzed by solid basic catalysts. The number of the modified catalyst (see Table 3) and the best optical yield are given as w e e ) . deserve special mentioning: The Zn-tartrate (BS) catalyzed nucleophilic ring opening 3(eeup to 85%) and the epoxidation 42 carried out under phase transfer conditions, where a polypeptide0 controls the stereochemistry (ee up to 96%!). From the observation that the preferred conformation of the polymer (a-or P-helix) has an effect on the enantioselectivity of the catalyst, it is deduced that the local ordering of the polypeptide mamx might be important (for reviews on chiral polymers see [ 10, 111). Solid enantioselective base catalysts have not been investigated systematically and at the moment only reactions 6 and 42 seem to be suitable for synthetic application. d) Miscellaneous ractions Fig. 4 and Table 4 summarize some other reaction types and also some very special cases of enantioselection. Quartz catalysts (M1) have been used for dehydration (45) and isomerization (44) reactions (again with very low ee) and Cu-tartrate (M6) catalyzes the carbene addition 9 with an acceptable optical yield, giving an intermediatein a steroid synthesis [49]. Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9]. Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the active site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of "R"- and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate

m.

80

TABLE 4 Miscellaneous solid catalytic systems (substrates see Fig. 4). Year

NO.

Catalytic system or contr. funct.

Substrates (ee)

1932 1969 1976 1977 1979 1985 1990

M1 M2 M3

Ag, Cu, Ni/-quartz

44

M4 M5 M6 M7 -

Ref.

(n.d.),& ( 4 )

18, 16,411 (461 1471 191 1481 1491 (501

ss (6)

chid crystalline environment chid crystalline environment TiClg/AIR+hiral polymer Cd-tartrate Cu-tartrate a-cyclodextrine crystals

so (90) s (30)

47 (37) 43 (46) SS (64)

"Q + N2CHCOCH2CH,Br 43

OW

44 isomerization

COOR

kin. resolution

COOH

+ HX

OH

45 dehydration

-

kin. resolution

48

AS

e a i k y l

s

47

polymerization kin. resolution

X C O O H

a!E3

polymerization

yo @qvi

p-tol

lM7/64)

OMe

-

1 \

dimerization

kin. resolution

Fig. 4. Substrates used with miscellaneous solid catalytic systems. The number of the modified system (see Table 4) and the best optical yield are given as wee). Enantioselection by a chiral crystalline environment might again be a borderline case for catalysis by solids. But some of the reactions described give quite good optical yields (M7) and are also very interesting from a theoretical point of view, especially when achiral molecules crystallize in a structure (like the well known quartz) which then can undergo stereocontrolled reactions (for a review see [51]). If the right- and left-handed crystals are separated "B la Pasteur", products with an enantiomeric excess can be obtained (M2, M3). This fact has been used as a mechanistic tool in order to demonstrate that hydrogenation reactions can occur in the solid phase via a spillover mechanism [52]. Chiral micelles [53] or liquid crystals [54] as controlling medium for asymmetric synthesis have also been described. In. INFLUENCE OF SYSTEM PARAMETERS: THE SEARCH FOR BETI'ER ENANTIOSELECI'IVITIES. Gathering more qualitative and quantitative information concerning a new catalytic system is the central point during the next phase of investigation. This information is then applied either for

81 COOH & COOH

NH2

clnchona alkalolds

R-COOH

HO OH (R,R)-tartarlc acld (natural form)

I-amino aclds (natural form)

R

0 0 R = H strychnlne R OMe bruclne

N-dlmethyl-ephedrlnlumsalts

(+)-emetlne

(-)-sparteine

Fig. 5. Structures of the most important chiral modifiers. Not shown are the sugars and biopolymers used. In some cases the naturally occuring compound was derivatized for best effects but very often this was not even necessary. optimizing a catalytic system or as basis for further mechanistic studies. The approach most often used in order to learn about the influence of system parameters is the time honored method of "change one thing at a time and keep everything else constant". Sometimes, this leads to wrong conclusions because the results can be strongly dependent on the choice of the other parameters. Factorial design experiments with qualitative and/or quantitative parameters are often helpful. An interesting strategy is the random choice of combinations of several qualitative parameters (an example is given below). This section will be organized according to catalyst systems: a) Tartrate modified catalysts, b) Cinchona modified catalysts and c) Electrochemical systems. We are aware of no other reaction types where systematic investigationshave been reported. a) Tartaric acid (tartrate) modified catalysts As already pointed out this is the best studied family of catalysts and processes. Since very good reviews [ 1,4, 6, 14,551 cover almost all aspects of these catalytic systems we will only give a sort of inventory and will cite the pertinent reference. Most investigations have been carried out with just one substrate: methyl acetoacetate. Therefore, the following statements can only be generalized with caution. Substrates. P-ketoesters, acetylacetone and methylketones are preferred substrates. See Section 11. [ 1,4]. Modifier structure. Tartaric acid is clearly superior to a-aminoacids or other a-hydroxy acids [ 1,4] (see also Section V).

82

Co-modifiers. NaBr is the most important co-modifier as it enhances the optical yields by 10-30%.Others have been studied [l, 41. Catalyst type. Raney nickel is the preferred catalyst for preparative purposes. Other Ni catalysts are suitable [I, 41. Bimetallic and noble metal catalysts have been studied [6, 141. Catalyst preparation. Freshly prepared Raney nickel gives the best results. For other Ni catalysts the preparation method has been shown to be very important [l, 41. Catalyst structure. For supported Ni catalysts the optimal Ni particle size was estimated to be 10-20 nm. But there is no correlation between ee and any catalyst parameter which is valid generally [I, 4.61. Modifying conditions. Modifier concentration, pH, temperature, time and sometimes procedures are crucial for a good catalyst performance [l, 41. Conditions must be optimized for each type of catalyst [33]. Catalyst stability. Embedding the modified catalyst in a silicone polymer [4, 561 or treatment with an amine enhances the catalyst stability and repeated use is possible [57]. Solvent and additives. Aprotic semipolar solvents, especially methyl propionate, give the highest ee's. But other trends have been reported [4, 121. The purity of the solvent is important 1331. The addition of weak acids increases the ee's, especially pivalic acid in the hydrogenation of methyl ketones, while water is demmental [ 1, 41. The reaction can also be carried out in the gas phase but optical yields are lower [55]. Reaction conditions. Temperatures between 60-100 OC and H2 pressures between 80-120 bar give good results. No simple correlation has been found between ee and p or T [ 1.41. b) Cinchona modified Pt catalysts Since these catalysts have never been reviewed, we will discuss the reported results in somewhat more detail. With few exceptions all the following statements are valid only for the hydrogenation of a-ketoesters 6 (mostly with R = methyl, phenyl and R, = methyl, ethyl). Substrates. Preferred substrates are a-ketoesters [30]. See Section 11. Modifier structure. Naturally occuring cinchona alkaloids give best results [31, 581 (see also Section V). Ephedrine derivatives give low to moderate optical yields [33]. Catalyst type. Pt catalysts on various supports are suitable. Rh catalysts give moderate ee, Pd, Ru and Ni are not effective [30]. Catalyst preparation. Activity and selectivity of the cinchonidine modified Pt catalysts have been shown to depend primarily on the platinum salt used for impregnation and the reduction method. Support material and platinum content also influence the catalyst performance [31, 591. The best results (optical yields S O % ) are obtained when catalyst precursors, made by impregnation of the alumina with 5% H2PtCI6, are reduced in aqueous solutions of Na(HCOO), K(HCOO) or CHzO [59]. Commercial catalysts can be used [30,31]. Catalyst pretreatment. Already in the first reports by Orito et al. [31], the beneficial effects of preheating the catalyst in hydrogen at 300-400 "C, followed by soaking in a solution of the modifier were described. Later, it was found that the H2treatment is crucial for Pr/AI2O3 catalysts

83

while the premodification was important for W C and it was demonstrated with factorial design experiments that the combined effects of H2 treatment, premodification and modifier addition to the reaction solution is much larger than the sum of the individual effects [30]. The effect of the thermal pretreatment of Pt/Al2O3 has been studied. It has been found that Pt dispersion and A1203 texture are not affected but the degree of reduction and maybe the crystallinity of the Pt is improved [601. Catalyst structure. The investigation of a large series of WA1203 catalysts showed that catalysts with dispersions > 0.4 give lower optical yields and lower turnover numbers. However, the platinum dispersion is not a sufficient parameter to explain the enantioselectivities observed for the different catalysts. Other factors such as texture of the support, morphology and size distribution of the platinum particles may affect the catalyst performance as well [30,58,59,61]. Catalyst stability. Under the conditions used for preparative experiments, the optical yield remains constant up to complete conversion, suggesting that the modified catalyst is rather stable [62]. However, experiments at low modifier concentration indicate that the cinchona alkaloid deteriorates slowly and its enantioselectiveeffect is lost [33,63]. Solvent and additives. Several systems have been studied concerning solvent effects. Fig. 6 shows that quite small changes in substrate, modifier or reaction conditions can lead to rather different results. Generally, very good results are obtained in apolar solvents with dielectric constants of 2-6. But in some cases alcohols can give equally high ee's. An important conclusion is that the optimal modifier concentration is dependent on solvent, modifier and substrate type [33]. The addition of amines and weak acids can affect the enantioselectivity [31,33].

ee (%) 80

a)

60 40

20 20 40 60 80 100

20 40 60 80 100

20 40 60 80 100

20 40 60 80 100

dlelectrlc constant

Fig. 6. Influence of the solvent polarity on the optical yield of various a-ketoester hydrogenations. a),b) catalyst not pre-modified, cinchonidine (Cd) added to the reaction mixture [33, 621; c) catalyst pre-modified with Cd, no Cd added, d) catalyst pre-modified with Cd and Cd added [31].

Reaction conditions. Temperatures between 20-50 OC and pressures >10 bar give good results. Usually higher pressures lead to slightly higher ee's and an increase in rate, while an increase of the temperature also leads to higer rates but to a decrease in selectivity. Typical results are depicted in Fig. 7 where it can be seen that the situation is sometimes more complex [30,33].

84

ee (%)

rate (m0Vmin.g)

80

60

.

40

lobar 100 bar

-

(XI

ao 60 40

20

20

0

20

40

60

80

I 0

OC

20

40

60

80

*c

Fig. 7. Influence of temperature and pressure on rate and optical yield. Catalyst not pre-modified, cinchonidine (Cd)and dihydrocinchonidine ( H a ) added to the reaction mixture. a) solvent EtOH 1301,-ee, --- rate; b) -benzene, 75 bar; toluene, 50-150 bar [33].

---

Modifier concentration. Preliminary experiments indicated that catalyst and modifier concentration have a strong effect on rate and ee [30]. In a detailed investigation this was confirmed (see Section V). Dehalogenation of 22 with cinchona modified Pd catalysts. Here we give a brief illuswtion of how we proceeded in order to improve the enantioselectivity for this hydrodehalogenation. Our first experiments using Pd/C/cinchonidine resulted in ee's of 2-4%, while the chemoselectivity (total dehalogenation) was >90%.Based on our experience with dehalogenation reactions and enantioselective catalysts we investigated the following parameters: catalyst type (Pd/C, Pd/CaC03, Pd/BaS04, Pt/13aS04, Pt/C), modifier (several cinchona derivatives), solvent (THF,methyl acetate, EtOH, MeOtBu) and base (NaOAc, N ( B u ) ~ MgO). , In a first series only cinchonidine was used as modifier and 15 of the 60 possible catal yst/solvent/base combinations were chosen randomly. Optical yields between 0 and 30% were obtained and first trends indicated that P ~ / B ~ S O J H F / N ( B U was ) ~ the best combination to continue. Screening of ca. 20 cinchona derivatives and a brief paxameter optimization resulted in an enantioselectivity of 50% (Pd/BaSO&inchonine/ THF/N(Bu)3,25 "C, 4 bar) [33]. c) Electrochemical systems. Again we refer to the review by Tallec 1361 where the effect of system parameters on enantioselectivity is summarized. For a given substrate, the most important factors are electrode material, electrode pretreatments, modifier structure, solvent, electrolyte, pH and buffer system, voltage and temperature.

IV. SYNTHETIC AND COMMERCIAL-SCALE APPLICATIONS. While the former three sections deal with investigations of model substrates and reactions, this part is dedicated to applications of enantioselective heterogeneous catalysts to solve "real" synthetic problems both on a laboratory and on a commercial scale. With one exception, all the

85

examples reported here are hydrogenation reactions, the only reaction type developed to any kind of maturity. As a rule, synthetic chemists will consider only those new reactions and catalysts for preparative purposes where the enantioselectivityreaches a certain degree (e.g. S O % ) and where both the catalyst and the technology are readily available. For heterogeneous catalysts this is not always the case because the relevant catalyst parameters are often unknown. It is therefore of interest that two types of modified Nickel catalysts are now commercially available: a Raney nickeVtartrateMaBr from Degussa [64] and a nickel powder/tartrate/NaBrfrom Heraeus [65, 661. It was also demonstrated that commercial Pt catalysts are suitable for the enantioselective hydrogenation of a-ketoesters [30, 311. With some catalytic experience, both systems are quite easy to handle and give reproducibleresults. In addition to good enantioselectivity and availability, a viable production catalyst has to meet further requirements e.g. activity, productivity, price, handling and separation (for a discussion of these problems see [67, 681). Heterogeneous catalysts have an inherent advantage concerning handling and separation. But in the case of the nickeutartrate system productivity and price of the modified catalyst can be a problem and successful attempts have been made in order to re-use the catalyst either by coating with a polymer [56] or by adding certain amines [57]. We have found that the development of such a process is more demanding than a classical heterogeneous hydrogenation reaction because so many additional reaction parameters are involved. The use of statistical optimization methods can be of advantage and in addition, rigorous quality control (substrates,catalyst, solvent etc.) is necessary to garantee reproducibility. The first example, a multistep synthesis of several isomers of the sex pheromone of the pine sawfly, starts with the nickel catalyzed hydrogenation of methyl 2-methyl-3-oxobutyrate(1) with fair stereoselectivitywhich later was further improved [69,70]. 1. Niltartrate 100°C, 100 bar

L

(l)

ervthro/threo = 3/1 ee = ca. 60%

'

and stereoisomers

The same catalyst system was also reported to lead to biologically active Cl&,6 3-hvdroxvacids starting from the corrresponding ketoesters with optical yields of 83-87%, which can be increased to >99% with a simple crystallization [71]. Cu-tartrate has been used to catalyze a carbene addition (seereaction 43 in Section XI) with 46% optical yield, giving an intermediate in a steroid synthesis [49]. A convenient and efficient ligand synthesis for homogeneous enantioselective hydrogenation is described [72] starting with the stereoselective hydrogenation of acetylacetone (2)

--

1. Raney nickeVtartrate/NaBr d

ee > 97% de not given

& x

x

X = OPPh2, PPh2

86

that was developed by Izumi's group and commercialized by Wako Pure Chemicals Ind. [l , 731. Kawaken Fine Chemicals Co. has also indicated that similar catalytic reactions are under development and that certain optically pure intermediates will be produced [74]. The next example originates from our own laboratory: Two potential intermediates for the angiotensinconverting enzyme inhibitor benazevril can be synthesized using cinchona modified noble metal catalysts (3). While the hydrogenation of the a-ketoester has been developed and scaled-up into a production process (10-200 kg scale, chemical yield >98%, ee 79-82%), the novel enantioselective hydrodechlorination reaction (see Section In) could be a potential alternative to the established synthesis where the racemic a-bromobenzazepinon is used [75]. At the moment both selectivity and productivity of the catalyst are too low and substitution reactions occur less readily with the chloro analog.

Finally, Raney nickel modified by (R,R)-tartaric acid/NaBr has been shown to be an efficient catalyst for the asymmetric hydrogenation of an intermediate in the synthesis (4) of tetrahydrolipostatin, a pancreatic lipase inhibitor developed by Hoffmann-LaRoche (100% chemical yield, ee 90-92%,6-100 kg scale) [76].

(CH2)FH3

1. Raney NiRarIratelNaBr

V. MECHANISTIC INVESTIGATIONS: THE SEARCH FOR UNDERSTANDING. Even though it is quite obvious that the empirical strategies described above are very effective for improving a catalytic system, understanding how a catalyst works is certainly the ultimate challenge. This is difficult for any heterogeneous catalyst and even more so for an enantioselective one. For the tartrate modified catalysts a large series of investigations have been reported by several research groups. These are summarized and commented in the following reviews [ l, 4, 6, 12, 14, 551. We will attempt to compare some aspects investigated for both the

87

NVtartrate and the Pt/cinchona systems and describe the pmposed conclusions concerning their mode of action. As usual the first hypotheses were based on qualitative and unsystematic observations as described in Section III. These were then refined or rejected in the course of further investigations. Effect of modifier and substrate structure. Very often structural effects are the first factors to give an idea on the mode of action of an enantioselective catalyst. From the observed dependence of the optical yield on modifier (see Fig. 8) and substrate structure (see Table 1) it was soon concluded that the interactions in the product determining step must be very specific. For Ni catalysts the two carboxyl and at least one OH group are essential for an effective modifier while preferred substrates must have an oxygen function in P-position of the keto group 11, 41. For the cinchona modified Pt catalysts the quinuclidine-nitrogen is considered essential and the configuration at C, of the alkaloid determines which enantiom of the a-hydroxyester is formed preferentially. Substrates with an additional carboxyl group a to the ketone (or the CClz-group in are suitable [30,581. COOH

COOH

ee(%) 75

COOH COOH 83

COOH

COOH

COOH

COOH

CH,

OMe

H

OMe

OH COOH

OH COOH

OM.

COOH

COOH

OH OH COOH

H OH OH COOH

65

68

61

0.2

1.2

0.0

ee(%) HOCH,

79

clnchonldlne derlvatlves excess R-lactate

-

-

clnchonlnederlvatlves excess Slactate

Fig. 8. Effect of the modifier structure on the optical yield for the hydrogenation of methyl acetoacetate [4] and ethyl pyruvate 1581. Adsorption of modifier and substrate. This aspect has been very well studied for the nickel catalysts: IR,UV, X P S , EM, electron diffraction and electrochemical investigations were carried out, very often using model catalysts. But also more conventional investigations like the effect of pH on the amount of adsorbed tartrate have been reported. There is a general consent that under the optimized conditions a corrosive modification of the nickel surface occurs and that the tartrate molecule is chemically bonded to Ni via the two carbonyl groups. There is also agreement that during the hydrogenation (which is carried out in an organic solvent) the adsorbed tartrate does

88

not leave the surface. There are two suggestions as to the exact nature of the modified catalyst: Sachtler [55] proposes an adsorbed [Ni2tartrate.J, complex; japanese [l, 41 and russian [14] groups prefer a direct adsorption of the tartrate on the Ni surface. In the gas phase, it has been shown that methyl acetoacetate is adsorbed as enolate and there are indications that the adsorption of the substrate is stronger if the catalyst is modified [55]. For the Wcinchona catalysts only preliminary adsorption studies have been reported [30]. From the fact that in situ modification is possible and that under preparative conditions a constant optical yield is observed we conclude that in this case there is a dynamic equilibrium between cinchona molecules in solution and adsorbed modifier. This is supported by an interesting experiment by Margitfalvi [63]: When cinchonine is added to the reaction solution of ethyl pyruvate and a catalyst pre-modified with cinchonidine. the enantiomeric excess changes within a few minutes from (R)-to (S)-methyl lactate, suggesting that the cinchonidine has been replaced on the platinum surface by the excess cinchonine. Kinetic studies and mechanistic schemes. With this paragraph we will conclude our survey on the mechanism of chirally modified hydrogenation catalysts. Several kinetic studies have been carried out using various Ni catalysts both in the liquid and the gas phase [l, 4, 551. Activation energies were found to be 10-15 kcal/mol. The reaction was first order in catalyst. Reaction orders for H2 ranged from 0 to 0.2 in the gas phase and from 0 to 1 in liquid phase while for methyl acetoacetate values of 0.4-1 (gas phase) and 0.2-0.8 (liquid phase) were determined. Based on these findings and on many other observations two mechanistic schemes were proposed: Izumi's and also Klabunovskii's groups favor a classical Langmuir-Hinshelwood approach: the adsorbed substrate reacts with activated hydrogen on the nickel surface in a stepwise fashion. The orientation of the adsorbed f3-ketoester is controlled by the tartrate via hydrogen bonding. There are results which suggest that the enantio-differentiation is determined in the adsorption step of the ketoester and not by the addition of hydrogen, but without structural evidence this is just a hypothesis. The important NaBr effect is explained as blocking of non-modified sites since the ratio of modified and non-modified sites determines the resulting optical yield [ 1.4, 141. Sachtler proposes a "dual site" mechanism where the hydrogen is dissociated on the Ni surface and then migrates to the substrate which is coordinated to the adsorbed nickel-tartrate complex. In this context it is of interest that the well known Sharpless epoxidation probably takes place on a dimeric tartrate complex of Ti. Sachtler suggests that both the anion and the cation have a function which vanes according to the conditions used. It is not clear whether the spillover mechanism is also proposed for the reaction in solution [55]. In our laboratory a kinetic study is in progress with a Pt/A1203 catalyst, modified with 10.1 1-dihydrocinchonidine(HCd) using ethyl pyruvate (Etpy) as substrate and ethanol or toluene as solvent. We are studying both the modified and the unmodified systems and it was demonstrated in both cases that the rate of reaction was not transport controlled [77]. The reaction for the unmodified catalyst was found to be first order in the Pt/A1203 catalyst. Depending on H2 pressure the following reaction orders were determined:

89

Unmodified WAI 03

Hp pressure <20 bar >40 bar

4

WA1203/HCd

Etpy

EtPY

H2

0 0-0.4

>O

0

0.8 0.8

0

0-0.5

In addition, we are investigating the influence of low modifier concentration. Because of problems with the stability of the hydrocinchonidine, rates and optical yields have to be determined at very low conversions. During these studies the large accelerating effect by the cinchona modifier was confirmed. Fig. 9 summarizes our first results which show that, especially in toluene, extremely low modifier concentrations (corresponding to a ratio of HWP&,,& OS!) are necessary to obtain maximum ee and rate. The dependence of both rate and optical yield can be explained by a "ligand accelerated" type of catalysis [78] where a slow unselective (unmodified catalyst) and a fast enantioselective reaction cycle (adsorbed cinchona modifier) are assumed to be in a dynamic equilibrium. This mechanistic scheme predicts an interdependence between enantioselectivity and reaction rate either as ee vs l/rate (linear) or ee vs rate (hyperbole). Fig. 10 shows two cases where we find just this type of correlation: for the experiments with varying modifier concentrations and - interestingly enough - also for the turnover frequencies (TOF)of the various Pt/Al2O3catalysts tested (see Section HI). One possible explanation for this difference in catalyst performance is that the portion of the metal surface which can be modified is dependent on the nature of the Pt crystallites. At the moment we have no good explanation for the observed acceleration except that it has a connection to the basic character of the quinuclidine part and the adsorption behavior of the cinchona molecule. In addition, we think that the rate and product determining steps occur on the platinum surface and that well defined interactions between the platinum surface (ensembles), one cinchona molecule and the a-ketoester are crucial. There are, of course, other possible explanations for the observed enantioselection. Wells and Thomas [80] have proposed that an array of

ee (%)

0.1

0.2

1

1

0.3 0.4 1

,

HCdlPt 0.5

,

40 20

toluene

robs

ee (%I

0.2 0.4

0.6 0.8

HWlPl 1.0 robs

(mow

. 8~1O-~

70

. 6

50

. 4

30

. 2

0.02 0.04 0.06 0.08 0.1

[HW] (mmol/l)

10

' 4

ethanol

' 2

0.04 0.080.12 0.16 0.2

[HWl (mmolfl)

Fig. 9. Dependence of initial rate (.) and optical yield (+) on 10,lldihydrocinchonidine concentration and H W t ratio in toluene and EtOH (Pt/Al2O3, RT, 20 bar) [78b].

90

wate (simol)

ma(%)

8

4

12

x104

80 60

40 20 I 2

4

6

8

10 x10-5

rate (mous)

. . 20

.

60

.

. . 100 TOF (1/s)

Fig. 10. Interdependence of rate and enantioselectivity for the hydrogenation of ethyl pyruvate with WAI2O3 catalysts. a) For varying HCd concentrations (results from Fig. 9) --- ee versus l/rate and -ee versus rate; b) For different Pt/A1203 catalysts modified with cinchonidine [59]. cinchona molecules controls the stereochemistry. Interactions between substrate and modifier could also occur in solution. We think that especially the results with the very low dihydrocinchonidineconcentrations make these alternatives less likely. VI. CONCLUSIONS From a theoretical or conceptional point of view, enantioselective catalysis with chiral solids is a fascinating and challenging area of chemistry. The polymeric heterogeneous catalysts described in this review can be regarded as enzyme models. The catalysis very likely occurs inside the chiral mamx and the reaction is controlled by supramolecular interactions. For the case of the

rnodi3ed hydrogenation catalysts we propose that the metal surface must have a suitable structure to allow exactly the right interactions between the metal, the adsorbed modifier and the adsorbed substrate. This would explain the observed requirements for high enantioselectivity: two functional pans for the modifier (for adsorption on the catalytic surface and for interactions with the substrate) as well as for the prochiral subsfrate (binding function and reaction site). From a synthetic point of view, there are a few reaction types catalyzed by chiral heterogeneous catalysts which are useful for preparative chemists. But it is also evident that the scope of most catalytic systems is rather narrow and very high substrate specificity is observed. Compared to homogeneous or bio-catalysis, enantioselectivities are usually lower but there are exceptions. From a technical or commercial point of view, enantioselective heterogeneous catalysts would be preferable to homogeneous catalysts because of their handling and separation properties, but only if their catalytic performance is satisfactory. It has been demonstrated that this is indeed possible. ACKNOWLEDGMENTS We would like to thank E. Broger. K. Deller and J. Smtz for providing information on technical aspects of asymmetric hydrogenations, M. Garland and J. Margitfalvi for preliminary

91

results and R. Bader, H.P. Jalett, I. Mergelsberg, B. Pugin and A. Togni for critical discussions and support during the preparation of this manuscript. REFERENCES A. Tai and T. Harada, in Y. Iwasawa (Ed.), Taylored Metal Catalysts, D. Reidel, Dordrecht, 1 1986, p. 265. J. D. Momson (Ed.), Asymmetric Synthesis, Vol. 5, Academic Press Inc., London, 1985. 2 J.D. Momson and H.S. Mosher, Asymmetric Organic Reactions, Amer. Chem. Soc., 3 Washington DC, 1976. 4 Y. Izumi,Adv. Cat., 32 (1983) 215. H. Brunner, Topics in Stereochemistry, 18 (1988) 129. 5 M. Bartok, in Stereochemistry of heterogeneous metal catalyts, chapt. XI, J. Wiley, New 6 York, 1985, p. 511. H. Pracejus, Fortschr. Chem. Forsch., 8 (1967) 493. 7 E.I. Klabunovskii, "Asymmetric Synthesis", Goskhimizdat, Moscow, 1960, german 8 translation by G. Rudakoff, VEB Deutscher Verlag der Wissenschafkn, Berlin, 1963. C. Carlini and F. Ciardelli, in Y. Yermakov and Likholobov (Eds.), Homogeneous and 9 Heterogeneous Catalysis, VNU Science Press, Utrecht, 1986, p. 471. For a review see P. Pino and R. Miihlhaupt, Angew. Chem. 92 (1980) 869. 10 S. Inoue, Adv. Polym. Sci., 21 (1976) 78. 11 M. Aglietto, E. Chinellini, S. D'Antone, G. Ruggeri and R. Solaro, Pure & Appl. Chem., 60 (1988) 415. 12 M.J. Fish and D.F. Ollis, Cat. Rev.-Sci. Eng., 18 (1978) 259. 13 J. Mathieu and J. Weill-Raynal, Bull. Soc.Chim. Fr., (1968) 1211. 14 E.I. Klabunovskii, Izv. Akad. Nauk. SSSR. Ser. Khim., (1984) 505 (engl. 463). 15 F. Rost, Angew. Chem. 48 (1935) 73. 16 G.M. Schwab and L. Rudolph, Natunviss., 20 (1932) 362; G.M. Schwab, F. Rost and L. Rudolph, Kollooid-Zeitschrift, 68 (1934) 157. 17 D. Lipkin and T.D. Stewart, J. Amer. Chem. Soc.,61 (1939) 3295. 18 Y. Nakamura, Bull. Chem. Soc.Jpn., 16 (1941) 367. 19 T.D. Stewart and D. Lipkin, Amer.Chem.Soc., a(1939) 3297. 20 M. Nakazaki, J. Chem.Soc. Japan, Pure Chem. Sect., 25.(1954) 831. 21 S. Akabori, Y. Izumi, Y. Fuji and S. Sakurai, Nature, 178 (1956) 323. 22 T. Isoda, A. Ichikawa and T. Shimamoto, Rikagaku Kenkyusho Hokuku, 34 (1958) 134, 143. C.A., 54 (1958) 285. See also [4]. 23 A.A. Balandin, E.I. Klabunovskii and Y.I. Petrov, Dokl. Akad. Nauk. SSSR, 127 (1959) 557 (engl. 57 l), 24 T. Yoshida and K. Harada, Bull. Chem. Soc.Jpn., 44 (1971) 1062. 25 E.S. Neupokoeva, E.I. Karpeiskaya, L.F. Godunova, E.I. Klabunovskii, Izv. Akad. Nauk SSSR, Ser. Khim., (1975) 2354 (engl. 2241). 26 Asahi Patent, JP 13307 (1963). C.A., 60 (1966) 3092. 27 H. Hirai, J. Polymer. Sci. B (Polymer Letters), 9 (1971) 459. 28 R.L. Beamer, R.H. Belding and C.S. Fickling, J. Pharm. Sci., 58 (1967) 1142 and 1419. 29 K. Harada and T. Yoshida, Natunviss., 57 (1970) 131 and 306. 30 H.U. Blaser, H.P. Jalett, D.M. Monti, J.F. Reber and J.T. Wehrli, in Studies in Surface Science and Catalysis 41 (Heterogeneous Catalysis and Fine Chemicals), M. Guisnet et al. (Eds.), Elsevier, Amsterdam, 1988, pp. 153-163. 31 Y. Orito, S. Imai, S. Niwa and Nguyen G-H, J. Synth. Org. Chem. Jpn., 37 (1979) 173. Y. Orito, S. Imai and S. Niwa, J. Chem. Soc.Jpn., (1979) 1118., (1980) 670 and (1982) 137. 32 J.R.G.. Perez, J. Malthete and J. Jacques, C. R. Acad. Sc. Paris Sene 11, (1985) 169. 33 H.U. Blaser, M. Garland, H.P. Jalett, M. Miiller and U. Pittelkow (Ciba-Geigy), unpublished work. 34 R.M. Dessau, Mobil Oil Co. US 4,554,262 (1985). 35 R. Fomasier, F. Marcuzzi and D. Zorzi, J. Mol. Catal., 43 (1987) 21. 36 A. Tallec, Bull. Soc. Chim. Fr., (1985) 743. 37 F. Beck, Chem.-1ng.-Tech., 48 (1976) 1096. 38 M.P. Soriaga, E. Binamira-Soriaga, A.T. Hubbard, J.B. Benziger and K.W.P. Pang, Inorg. Chem., 24 (1985) 65 and 73.

92

39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81

E. Erlenmeyer and H. Erlenmeyer, Biochem. Zeitschr., 233 (1922) 52. G. Bredig and F. Gersmer. Biochem. Zeitschr.. 250 (1932) 414. A.P. Teren’tev and E.I. Klabunovskii, C. A., 49 (1955) 5263. T.L. Jacobs and D. Danker, J. Org. Chem.. 22 (1957) 1424. S. Tsuboyama, Bull. Chem. Soc. Jpn., 35 (1962) 1004. A.G. Osinovski and B.V. Erofeev, Dokl. Akad. Nauk. BSSR, 28 (1984) 006. C. A. 102 (1985) 95926. H. Yamashita. Bull. Chem. Soc.Jpn., 61 (1988) 1213. K. Penzien and G.M.J. Schmidt. Anpew. Chem.. 81 (1969) 628. M. Lahav, F. h u b , E. Gati, L. his&witz and.Z. Ludmer, J. Amer. Chem. Soc.,98 (1976) 1620. M. Marchetti, E. Chiellini, M. Sepulchre and N. Spassky. M h m o l . Chem.. 180 (1979) 1305. A.R. Daniewski and T. Kowalczyk-Przewloka, J. Org. Chem.. 50 (1985) 2976. Y. Tanaka, H. Sakuraba and H. Nakanishi. J. Org. Chem., 55 (1990) 564. B.S. Green, R. Arad-Yellin and M.D. Cohen, Topics in Stereochemistry, 16 (1986) 131. R. Lamartine, R.Pemn, A. Thozet and M. Pemn. Mol. Cryst. Liq. Cryst.. 96 (1983) 57. J.M. Brown, Further Perspective in Organic Chemistry, Ciba Foundation Symp. 53, Elsevier, Amsterdam, 1978, p. 149. V.A. Pavlov. N.I. Spitsina and E.I. Klabunovskii, Dokl. Akad. Nauk. SSSR, Ser. Khim., (1983) 1653(engl. 1501). W.M.H. Sachtler, in L.Augustine (Ed.), Catalysis in Organic Reactions, Chem. Ind., 22 (1985) 189. A. Tai, K. Tsukioka, Y. Imachi, Y. Inoue, H. Ozaki, T. Harada and Y.Izumi, Proc. 8th Int. Congr. Cat. (1984) 531 A. Tai, K. Tsukioka, H. Ozaki, T. Harada and Y.Izumi, Chem. Lett., (1984) 2083. H.U. Blaser, H.P. Jalett, D.M. Monti, A. Baiker and J.T. Wehrli, ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, 1990, Boston. Manuscript in print. J.T. Wehrli, A. Baiker, D.M. Monti, H.U. Blaser, J. Mol. Catal.. 61 (1990) 207. H.U. Blaser, H.P. Jalett, D.M. Monti and J.T. Wehrli. Appl. Catal., 52 (1989) 19. J.T. Wehrli, A. Baiker. D.M. Monti and H.U. Blaser, J. Mol. Catal., 49 (1989) 195. J.T. Wehrli, A. Baiker, D.M. Monti, H.U. Blaser and H.P. Jalett, J. Mol. Catal., 57 (1989) 245. J. Margitfalvi, Federal Institute of Technology, Ziirich, personal communication. K. Deller, Degussa, Hanau, personal communication. J. Strutz, W.C. Heraeus GmbH, Hanau, personal communication. H. Brunner, M. Muschiol. T. Wischert and J. Wiehl, Tetr. Asymm., 1 (1990) 159. J.W. Scott, Topics of Stereochemistry, 19 (1989) 209. R. Sheldon, Chem. Ind. (London), (1990) 212. A. Tai, M. Imaida, T. Oda and H. Watanabe, Chem. Lett.,(1978) 61. A. Tai, H. Watanabe and T. Harada, Bull. Chern. Soc.Jpn., 52 (1979) 1468. M. Nakahata, M. Imaida, H. Ozaki, T. Harada and A. Tai, Bull, Chem. Soc.Jpn., 55 (1982) 2186. J. Bakos, I. Toth and L. Marko, J. Org. Chem, 46 (1981) 5427. Catalogue of Wako Pure Chemicals Indusaies (Osaka), 22. Ed. p.471 and 547 (cited in [l]). M. Ishii, Kawaken Fine Chemicals Co., personal communication, S.K. Boyer, R.A. Pfund, R.E. Pomnann, G.H. Sedelmeier and Hj. Wetter, Helv. Chim. Acta, 71 (1988) 337. G.H. Sedelmeier, H.U. Blaser and H.P. Jalett, EP 206993 (1986). E. Broger, Hoffmann-LaRoche, Basel, personal communication. M. Garland, H.P. Jalett and H.U. Blaser, these prepxints. a. E. N. Jacobsen. I. Marko, W. S. Mungall. G. Schrijder and K. B. Sharpless, J. Amer. Chem. Soc., 110 (1988) 1968 and 111 (1989)737. b. M. Garland and H.U. Blaser, J. Amer. Chem. Soc., 112 (1990) 7048. R.M. Laine, G. Hum, B.J. Wood and M. Dawson, Stud. Surf. Sci. Catal., 7 (1981) 1478. P.B. Wells, Faraday Discuss. Chem. Soc.. 87 (1989) 1; J.M. Thomas, Angew. Chem. Adv. Mater., 101 (1989) 1105. M. Bartok, G. Wittmann, G.B. Bartok and G.Gondos, J. Organomet. Chem., 384 (1990) 385.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

93

CATALYSIS WITH IMMOBILIZED ENZYMES : HYDROLYSIS AND ESTERIFCATION BY RHlZOPUS ARRHlZUS

C. GANCET Groupement de Recherches de Lacq. Elf Aquitaine, BP 34,64170 Artix, France

SUMMARY The dead cells of the mycelium of Rhizopus arrhizus constitute a naturally immobilized lipase very active in organic solvents. This immobilized enzyme was used for hydrolysis and synthesis of ester bonds : triglycerides hydrolysis. and interesterification, esters and glycerides synthesis. More recently, the catalytic system has been applied in drug synthesis to the resolution of racemic esters with a good enantioselectivity. Under non-aqueous or micro-aqueous conditions, this fungal catalyst shows high efficiency and good operational stability. As neither purification, nor immobilization step are needed, total cost is low, and fully compatible with industrial uses.

INTRODUCTION In standard aqueous media, hydrolases are enzymes which are able to hydrolyse

covalent bonds (Fig. 1.). Three classes of hydrolases are used industrially : osidases (glycosidic linkage hydrolysis). proteinases (peptidic linkage hydrolysis) and esterases (ester linkage hydrolysis). Lipases are triglyceride esterases, and as these substrates are insoluble in water, lipases are interfacial enzymes. Under these conditions, the hydrolytic reaction versus the synthetic reaction is favoured'. However. ester bond synthesis in aqueous conditions has been reported, but low yields were obtained. Since 1978, numerous works that show the ability of lipolytic enzymes to be active in organic solvents have been ~ u b l i s h e d ~Moreover. -~. it appears that in certain conditions the stability of the enzyme is enhanced b y the low content of water. In such conditions, as the organic substrates are soluble in the reactional media which is therefore homogeneous, continuous processes can be designed in a very classical way.

94

Fig. 1. Hydrolases

Lipases, Esterases E.C.3.3.1,

Most of the lipases are Serine enzymes, and it is clearly admitted today that the mechanism of the enzyme includes an acyl-enzyme intermediary which occurs between the fatty acid of one substrate and the Serine hydroxyl group of the active site (Fig. 2,). Following the reaction. this intermediary is attacked by a nucleophile which is for example water in the case of hydrolysis. Alcohols. thiols. and sometimes amines have been used according to the same scheme. Lipases are proteins which molecular weight is in most cases between 40 and 50 kDa. which corresponds about 300 amino-acid residues. Isoelectric points are between 4 and 7. and optimal p H values are between 5 and 8. according to the origin. On the other hand, lipases are glycoproteins which glycosylated hydrophile part is located opposite to the hydrophobic zone around the active site. The lipase from the yeast Candido. for example, contains 4.2 % b y weight of sugars. Lipases may show different types of specificity towards their substrates. Position on the alvcerol The enzyme can be 1.3 specific. like the mammals pancreatic lipase. or aspecific. like the Pseudomonos fluorescens one. Chain lenath of the fattv

a

The formation of the acyl-enzyme intermediary is more or less rapid according to the affinity between the lipase site and the considered chain. For example, Penicillium and Aspergillus lipases prefer short chains, when Rhizopus or Pseudomonas have broader

mectra.

95

R-C-OH

8

Fig. 2. Acyl-enzyme mechanism

Chain unsaturation of the fattv acid In a very similar way. acyl-enzyme formation depends upon the unsaturation level of the chains, and upon the position of the double bond(s) on the chains. Geotrichum condidum lipase is known to prefer fatty acids with a double bond on the C9, like oleic acid. Nature and structure of the nucleoDhile In the case of Rhizopus arrhizus, primary alcohols have a good reactivity when they are not too much sterlcally hindered, but secondary alcohols are less reactive and tertiary ones do not react at all. Enantioselectivity As lipases are proteins, they are able to act as chiral catalysts, and for example to

hydrolyse specifically one of the isomers in a racemic mixture of esters. Lipases can be found in animals, vegetals and microorganisms. Historically the pancreatic lipase of mammals was the first to be studied, but today, only the enzymes produced by microorganisms are susceptible to industrial development, under different forms according to the considered process.

RHlZOPUS ARRHlZUS MYCELIAL LIPASE

Rhizopus arrhizus (ATCC 24563) is a filamentous fungus, known to be producer of an exocellular lipase. According to the culture medium and especially to the carbon and nitrogen sources the lipolytic activity can remain bound to the cells.

96 After drying and delipidation this biomass c a n be considered as a naturally immobilized enzyme (Fig. 3.). The hydrolytic activity measured on olive oil in di-isopropylether is around 75 to 200 micromoles per g of dry mycelium and per mn. The chain length specificity spectrum is broad. as shown before (from C14 to C22)

300

5 250

c 1 mn

Oil Water Acetone MTBE

10 01.5 15 73.5

10 ml

. Mycelium

0

10

20

30

40

50

60

Time, mn 1 Unit = 1 Frnole FA / rnn (typically,75-200 U/g)

Fig.3. Mycelium activity ORGANIC MEDIA Organic solvents used must be compatible with the enzymatic activity. not take place in the reaction, be good solvent of the substrates, have a low cost, and be the more harmless possible. The solvents commonly used in the literature are aliphatic alkanes, or similar compounds of low polarity. Aliphatic ethers that were used in this work, show less hydrophobicity than alkanes, and thus allow introduction of water for hydrolysis reactions. For interesterification experiments, trichloro-trifluoro-ethane was used, as water is not a substrate for the reaction (Fig. 5,). Tertio-amylic alcohol was retained for glycerides synthesis, as a solvent able to dissolve either fatty acids or triglycerides and glycerol. ENZYMATIC VERSUS CHEMICAL CATALYSIS Enzymes must be preferred in the following situations : -low stability of the substrateW -needed cleanliness of the reaction -needed "natural" character of the reaction6

97 Fig. 4. Production of fatly acids by triglycerides hydrolysis

Triglycbrides + H20

Fatty acids + glycerol

-b

MTBEIAcetone 85-1 5 v/\ 30°C 4 x 7 5 m n Yield : 88-97 Yo

1

b

Water

T..

n

Segmented reactor : (a) triglycerides solution : tallow 20, water 1.5, acetone 15, MTBE 63.5(Yop/p) ; (b) adjustment of water content.

.*.....

807

; !i

a >

C

8

3 20 1

ol

1

0

0.5

1

1.5

2

2.5

3

3.5

4

Time, months Operational stability of Rhizopus arrhizus mycelium on continuous hydrolysis of triglycerides

98

-needed selectivity Fig. 5. Relative activity of Rhizopus arrhizus mycelium in organic solvents I

?rificationof oleic acid and octar Solvent Freon 11

Activity %

103

Freon 112

995

Freon 113

97,2

Perfluoroheptane

55

Diphenyl ether

89

Dibutyl phtalate

79.6

Hexane

732

Methyl-t-butyl ether (MTBE)

66.2

Dimethoxy propane

52.6

Tributyl phosphate

51,7

Dioxane

132

DMF

0

TRIGLYCERIDES HYDROLYSIS The reaction is carried out in a continuous fixed-bed reactor with several segments. Each segment contains a load of dry Rhizopus arrhizus mycelium added with silica to ensure good flow properties. The substrate solution is injected through the first segment, then water content is adjusted as water solubility increases when diglycerides appear, and the reaction goes on through the following segments (Fig. 4.). Estimated residence time is about 1 hour per segment. and obtained conversion rates are as follow. Total water added reaches about 5 times the stoechiometry; e.g. in case of primrose oil hydrolysis, water added at each step was 7.5. 11, 5.6 and 4.4 g per 100 g of triglycerides. Operational stability was tested through tallow hydrolysis. It appears that constant yield was maintained during several months, showing the high stability of the mycelial lipase in these conditions4. Initial water solubility in the reactional medium is a key point, and two systems were designed to increase this parameter. The first solution is to use micro-emulsions of water in the solvent with the help of di-octyl-sulfosuccinate(AOT) as tensio-active agent. Good results con be obtained, but separation and recycling of the detergent are difficult to extrapolate at larger scales.

99 The solution we use is the addition of a polar co-solvent as a ketone. In this case, the activity of the catalyst is decreased by the co-solvent, but a convenient compromise with the increase of water solubility can be found.

INTERESTERIFICATIONOF TRIGLYCERIDES5 The interesterification of fats and oils is the only way to create new hybrid products with new physical, and especially

new rheologlcal properties. Chemical

interesterification is well known, but has no position or chain specificity, and is not very clean. With lipases in micro-aqueous media, the exchange of acyl groups between the different triglycerides may be oriented, and designed according to the specificity of the enzyme. A single segment mycelium reactor was used with trichloro-trifluoro-ethane as

solvent (Fig. 5.). For a concentration of 25 % (V/V) of triglycerides, full interesterification was obtained within 1 hour of residence time. The productivity of the system can be estimated to 1.5 kg of interesterifiedproduct per hour and per kg of dn/ mycelium. The operational stability was measured through monitoring of a triglyceride probe during 2.5 months on a continuously running reactor. The decrease of activity was about 15 % per month.

As in the case of hydrolysis, water plays a role. But here, it's only a catalytic role, and the water concentration needed is about 100 ppm for good results. If it increases, hydrolysis takes place, and if it decreases, the activity of the enzyme can literally be switched off.

MONO AND DIGLYCERIDES SYNTHESIS For hydrolysis or synthesis of esters, and for interesterification of triglycerides, solubility of the substrate is good in the usual low polarity solvents. When dealing with glycerides synthesis either by direct esterification of fatty acids b y glycerol or by glycerolysis of triglycerides, glycerol is poorly soluble in these solvents, and another medium must be used. Tertiary alcohols, and especially tertio-amylic alcohol were found to give homogeneous solutions of both glycerol and fatty acids.

100 Fig. 6. Mono and diglyceridessynthesis by direct coupling of acid and glycerol or by glycerolysis of triglycerides

Fatty acid + glycerol or Glycerol + triglycerides

-

Mono and diglycerides

t-amyl alcohol

Direct svnthesk 1 mole C18:l for 3 moles glycerol

Conversion in monoolein : 44.7 %(molar) diolein : 2.0 %

1 "mole" of tallow for 10 moles of glycerol

Conversion in monolein : 38.8 %(p/p, monoolein/suif) Operational stability Glycerolysis of tallow

$2

50

0

20

40

60

Time, days

00

I

101 Two segments of the same fixed bed reactor were used. with dehydration of the reactional medium on molecular sieve between each of them. With oleic acid, and an excess of 3 moles of glycerol per mole of acid, the yield was 44.7 % (molar) of monoolein after 2 hours. At the same time, only 2 % of di-olein were obtained (Fig. 6,). With the same system applied to the transesterification of tallow, and an excess of 10 moles of glycerol per mole of triglyceride. a conversion of 38.8 % (weight) in

monoglycerides was observed, the rest being diglycerides from the initial substrate. The stability of the mycelium under these conditions is also very interesting : a glycerolysis reactor was run continuously during 3 months with very little loss of activity.

RESOLUTION OF RACEMICESTERS~

Enzymes can be stereospecific. and lipases as esterases can act as very efficient catalysts even on molecules which ester groups are not glycerides. The hydrolysis of such esters is not always possible, especially if they are sterically hindered, or if the carboxylic acid involved is aromatic, and the carbonyl group conjugated. Esters of benzo'ic acid, for example are very difficult to hydrolyse. The stereospecificity may be carried. either by the carboxylic acid moiety, or by the alcohol part of the molecule. There is no rule up to now to predict if a given molecule will be a substrate, and if the enzyme will express its stereospecificity toward it. Screening of lipases and esterases is the only method to select firstly the active enzymes, and secondly the specific ones that give the wanted isomer. Very often, the enantiospecificity is not absolute, and kinetics play a major role in the efficiency of the enantiomer selectivity. The example we show here is the resolution of a racemic mixture of epoxy-esters according to the following reaction :

0

Lipase

RLCOOCH3 H20 O H Rl,/-kOOCH3 H

+

H O R q o C O O H H

102

Fig. 7. Resolution of a racemic mixture of esters

0 R ~ ~ C O O C --w H ~

H H

0 R-/~_H

+...

mycelium MTBEIAcetone 85-15

Best enantioselectivity factor E in these conditions :

E = k+k-

Ln(1- p ) (1- ee ) =

Ln(1- p ) (l+ee )

avec ee = enantiomeric excess of the substrate et p = global hydrolysis yield

Variation of the enantioselectivity factor E

103 Among the 50 enzymes screened, Rhizopus arrhkus mycelial lipase showed the best results in terms of E, which is the enantioselectivity coefficient depending of the enantiomeric excess and the conversion (Fig. 7J7. Methyl-tertiobutyl-ether1 acetone was found to be a good solvent of the racemic

epoxy-ester, and thus is used for the reaction, as for triglycerides hydrolysis. Batch conditions are used with a ratio enzyme/substrateof 2 (weight), during 1 to 24 hours at 25 "C. Progress of the reaction is measured b y proton NMR with an internal standard, and enantiomeric excess is obtained through chiral HPLC analysis of the product. A coefficient E as high as 37 has been obtained with a yield of 02 % of the theoretical

maximum, and an enantiomeric excess of 99.9b.

CONCLUSION Rhizopus arrhizus dead mycelium was found to be very active in organic solvents as a naturally immobilized lipase. Triglycerides hydrolysis and interesterification, esters and glycerides synthesis, natural flavour esters preparation and racemic mixtures resolution in pharmaceutical drugs synthesis are among the successfully designed processes, each of one with a specific reactional medium. Under these different conditions, the fungal catalyst shows high efficiency and stability, and as either purification, or immobilization are avoided, operational cost is low, and thus compatible with industrial use.

REFERENCES P. Desnuelle, The Enzymes, P. Boyer (Ed.),Vol. VII, 1972,575. G. Bell, J.A. Blain, J.D.E. Patterson, C.E.L. Show and R. Todd, Ester and glyceride synthesis b y Rhizopus arrhizus mycelia, Fems Microbiol. Lett., 3, 1978,223-225. G. Bell, R. Todd, J.A. Blain, J.D.E. Patterson and C.E.L. Show, Hydrolysis of triglycerides by solid phase lipolytic enzymes of Rhizopus orrhizus in continuous reactor systems, Biotech. Bioeng., 23,1981,1703-1719. C. Gancet and C. Guignard. Proc. Int. Symp. Biocat. in Org. Media, Wageningen. The Netherlands, December 7-10,1986.Studies in organic Chemistry, C. Laone (Ed.),Elsevier, 29,1987,261-266. C. Gancet. C. Guignard and P. Fourmentraux. Process for carrying out enzymatic reactions in an organic solvent, US. Pat. 4855233,1989.

C. Gancet, Synthese enzymatique d'esters naturels, Societt5 Francaise de

104

Chimie, Third national meeting, Nice, France, 1988. 7

C.S. Chen. Y. Fujimoto, G. Girdaukas and C.J. Sih. Quantitative analysis of

biochemical kinetic resolution of enontiomers. J. Am. Chem. SOC.. 104. 1982, 7294.

7299. 8

C. Gancet, J.A. Laffitte. C and C. Soccol, Procede de preparation d'un

diastereoisomere de derives glycidiques, Demande de Brevet Francais 8914938.1989.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

105

HYDROGENATION OF BENZALDEHYDE TO BENZYL ALCOHOL IN A SLURRY AND FIXED-BED REACTOR M. Herskowitz

Dept of Chem. Eng., Ben Gurion University , Beer Sheva, Israel ABSTRACT A kinetic model was developed based on data obtained over a range of temperatures and hydrogen pressures. The kinetic parameters were expressed as a function of temperature. The kinetic model was applied to the analysis of the trickle-bed data. Predictions of a matheniatical model of the trickle-bed reactor were compared with data obtained at two temperatures and a range of pressures. The intraparticle mass transfer resistance was very important. INTRODUCTION The catalytic hydrogenation of benzaldehyde is a model reaction of hydrogenations of aromatic aldehydes. The principal reaction is: C7H,0+H2

+ C,H,O

There are two side reactions which are thermodynamically feasible: C,H,O

+ 2H2 + C7H, + M, 0

C7H8+ Id2

-+

C,H,

+ CH,

Benzaldehyde has been hydrogenated on Pd/C( l), Raney nickel and nickel boride (2) catalysts. Baltzly (Ref. 1) measured the rate of hydrogen pressure decrease as a function of time in a batch reactor. He found that the rate of reaction was zero order for both reactants at hydrogen pressures above 3 atm. and benzaldehyde concentrations above 1.O gniol/l. The rate data was obtained at 22°C in various solvents. No measurements of the products and the benzaldehyde were reported. For the 3% Pd/C catalyst, the rate of reaction was 1.6 x gniol/g.min, independent of thc type of solvent. Schreifels et al.( Ref. 2) measured the rate of benzaldehyde hydrogenation at 70°C and 6 atm., using Raney nickel. They found that the rate of reaction

106

depended strongly on the reactant to catalyst ratio. The reported rate of reaction was in the range 1.7 x

-

1.3 x 10-3 gmol/g.min. No information on the

selectivity of the products was given. Industrial gas-liquid hydrogenation reactions are carried out in slurry and trickle-bed reactors (Ref. 3). Modeling of the latter has been advanced significantly in the last two decades (Refs. 4-6). Predictions of trickle-bed reactors performance were in good agreement with experimental data (Ref.7). The purpose of this study is twofold: to develop a kinetic expression for nickel catalysts and to test the perfomiance of a trickle-bed reactor as compared with model predictions. EXPERIMENTAL Hydrogenation runs were carried out in a dead-end 300 cm3 batch autoclave manufactured by Autoclave Engineers, Erie, Pa. Description of the experimental setup has been given elsewhere (Ref. 8). The bakh -recycle

Q I

Figure I : Schematic diagram of trickle-bed system: 1. trickle-bed. 2. separator.

3. liquid pump. 4. back pressure regulator. 5 . thermostatic bath.

107

trickle-bed reactor is described schematically in Figure 1. The liquid was pumped from a 2-liter glass separator through a rotameter to the reactor, a 2.54 ID stainless steel tube equipped with a jacket. The reactor was packed with a layer of alumina pellets, a layer of nickel catalyst pellets and another layer of alumina pellets. The pressure in the reactor was controlled by a back pressure regulator. The reactor temperature was maintained by circulating oil through the jacket from a themiostatic bath and it was measured by a thermocouple. The benzaldehyde was 99.5% pure, as measured by GC and HPLC. Its contact with air was avoided so as to eliminate the possibility of oxidation. The purity of hydrogen was better than 99.7%. Two nickel catalysts were used, both provided by Engelhard, de Meem B.V. Their properties are given in Table 1. RESULTS AND DISCUSSION Kinetic Study: The operating conditions in the kinetic study are given in Table 2. In all experiments the overall mass balance was checked, retaining only samples which gave deviations of less than 3%. Plots of the benzaldehyde concentration against time of reaction yielded a linear dependency at concentrations above about 1.5 g.mol/l. Below this value, the pseudo-zero-order with respect to benzaldehyde changed to a pseudo-first-order, as illustrated in Figure 2. The kinetic data was obtained only in the zero-order range. The kinetic data were measured at an impeller speed of 2000 RPM. In the range of 1200-2000 RPM no changes in the rate of reaction were measured indicating that the gas-liquid mass transfer resistance was negligible. Furthermore, the rate of reaction increased linearly with catalyst concentration, as shown in Figure 3. The catalyst particle size was in the range of 35-70 pm. Several runs carried out with 10 pm particles gave similar rates of reaction, which means that intraparticle mass transfer resistance was negligible. A semilogarithmic plot of the rate constant against the reciprocal of the absolute temperature presented in Figure 4 yielded an activation energy of 13.2 kcal/gmol. Kinetic model The rate of reaction order with respect to benzaldehyde was found to

108

Tn353.1 K P:446 kPa 2.5 56 catalyst

0.024 I

Time

20

, min

.

. 30

.

, 40

,

50

catalyst concentration, g/l

2.Typical hydrogenation run. 3.Effect of catalyst concentration on reaction rate change from zero to one. Benzyl alcohol has no effect on the rate. The effect of hydrogen concentration was studied by measuring the rate of reaction as a function of hydrogen pressure. The rate data are plotted in Figure 5 . On the basis of those results, a kinetic Langniuir-Hinshelwood model is proposed, which assumes that the surface reaction is rate limiting.

r=

KHPH 1 + KB CB (1 +.I KH PH)2 kKBCB

where C , is the benzaldehyde concentration, P, is the hydrogen pressure, K, and KH are the benzaldehyde and hydrogen adsorption constants, respectively and k is the rate constant. In the range of conditions studied here, K, is of the order of 1 (gmol/l)-' which yields a zero-order at high benzaldehyde concentration. r=k

K H pH

(1

+ .IKH PH)'

= k,,

This expression was employed in the analysis of the rate data in Figure 5. The two parameters k and KH were expressed as a function of temperature, by

109

fitting simultaneously the rate data at three temperatures (Figure 5). The lines in Figure 5 are the predicted rate constants using the best values of the constants:

(3)

k = 2.18 x lo8 exp (-lOOOO/T) kgnio1kg.s

temperature ,K C

,013

-E

I

C C

I

E

P=446 kPa 2.5 % catalyst

.

353.1 343.1

CI

5

A

5

-

2

0

2

.MoI i 0.0028

.

, 0.0027

.

,

.

0.0028

,

.

0.0029

0.0030

0

200

400

800

1000

BOO

-1

1IT ,K

4. Rate constant dependency on temperature.

Hydrogen pressure, kpa

5. Effect of hydrogen pressure

on the reaction rate.

K,

= 1.85 x 1O-Io exp (5500/T) kPa-'

(4)

As expected, k increases significantly with temperature while K,

decreases with temperature. Trickle-bed studv The operating conditions are listed in Table 2. The results given in Table

3 were obtained with one batch of catalyst packed in the reactor. The liquid was drained from the system and replaced with pure benzaldehyde three times. After an initial decrease in catalyst activity - of about 30% -no significant decay was measured during the run. The limiting reactant in the reactor is hydrogen. All mass transfer resistances have to be accounted for. The hydrogen flux from the gas to the liquid, to the external pellet surface and inside the pellet are equal, assuming complete wetting of h e pellets, as expected under those conditions (Ref. 4).

1200

110

CH, and CHs are the hydrogen concentration in the liquid and the external pellet surface, respectively.

H is the Henry’s constant, estimated to be 2.3 x 1 0 4

kPa/(kmol/m3) (9). k,a, and k, as are the gas-liquid and liquid-solid mass transfer coefficients, respectively. q is the effectiveness factor, which can be expressed as a function of the Thiele modulus:

- 6 (KH H CHs)l/2 + 6 (1 + K H H CHS)

- 112 112

) In (1+ (KIj H CHS) 1’2

1

(7 1

The details are given elsewhere (Ref.lO). The mass balance for the benzaldehyde is:

where m is the mass of the catalyst and V is the liquid volume. Over the range of zero-order with respect to benzaldehyde, equation (Ref.9) can be integrated to give:

The calculation of ro in equation (9) requires the estimation of k,a,,

D,. kLaL and k, as

k, as and

were estimated using the correlations recommended

elsewhere (Ref. 4). The value of D, was calculated from the equation:

(10) where

E,,

is the pellet porosity, DH is the hydrogen diffusivity and T is the

tortuosity factor. Their values are given in Table 3. The predicted values are in

111

TABLE 1 Properties of nickel catalysts Name

Composition %Ni

Surface Area m2/g

Pellet density kg/m3

Porosity

1404 5852

68 56.7

130 239

1.72 1.34

0.67 0.66

TABLE 2 Range of operating conditions 1. kinetic studv: catalyst conc. (kg/m3) :25 - 50 Temp (K) 1343 - 373 Pressure (kPa) A70 - 1120 2.trickle-bed study: Liquid velocity (m/s) :0.004 Gas velocity (m/s) :0.004 -0.008 Temp.(K) :353 - 373 Pressure (kPa) 1220- 580 170 g catalyst and 1000 cm3 liquid.

TABLE 3 Comparison of trickle-bed data with model predictions k,a, = 0.12 s-l ksaS= 0.70 s-l D, =8 x m2/s

T,K

353.1

373.1

P, kPa

360 580 220 360 580

z =3

- AC/At, measured

kmol/m3.s predicted

17

7.8 x 1.3 x 8.8 x 1.5 x 2.5 x

8.2x 1.1 x 9.0 x 1.4 x 2.2 x

0.042 0.04 1 0.043 0.04 1 0.040

10-2 10-1

lo-' 10-1

10-2 10-2 10-1 10-1

112

good agreement with the data. The effectiveness factor is very low, indicating that intraparticle mass transfer resistance is vcry significant. The gas-liquid mass transfer resistance is also important, as expected. On the other hand, the liquid-solid mass transfer resistance is negligible. As a result, the rate of reaction i n the slurry reactor is about 50 times higher than that in the trickle-bed. Thereforc, i n cases of such high rates of reaction, the slurry reactor is a better choice, although the gas-liquid mass transfer and the filtration of the catalyst may be a problem. CONCLUSIONS The kinetic model developed in this study can be used to design and analyze various chemical reactors for the hydrogenation of benzaldehyde. Although it is based on a Langmuir-Hinshelwood mechanism, it does not prove that this is the correct mechanism. The analysis of the trickle-bed runs indicate that intraparticle mass transfer resistance is very significant. Gas-liquid mass transfer

may ~ I S O have a

significant resistance. This is an important consideration in the decision proccss of using a slurry or a trickle-bed reactor. REFERENCES 1. R. Baltzly, Studies on catalytic hydrogenations, J. Org. Cheni., 41 (G), (1976), 920-28. 2. J.A. Schreifels, P.C. Maybury and W.E. Swartz, Comparison of the activity and lifetime of Raney nickel and nickel boride in the hydrogenation of various functional groups, J. Org. Chem., 46(7), (1 98 I ) , 1263-69. 3. P.A. Ramachandran and R.V. Chaudhari, Three-Phase Catalytic Reactors, Gordon and Breach Science Publ., New York, 1983. 4. M. Herskowitz and J.M. Smith, Trickle-bed reactors: a review, AIChE J., 29, (1983), 1-18. 5 . R.M. Koros, Engineering aspects of trickle-bed reactors, in :H.I. dc Lasa (Ed.), Chemical Reactor Design and Technology, M. Nijhoff Publ., Dordrecht, 1986, pp.579-630. 6. A. Gianetto and F. Berruti, Modelling of trickle-bed reactors, ibid., pp.631-685. 7. S. Goto and J.M. Smith, Trickle-bed reactor performance, AIChE J., 21, (1975), 706-19. 8. J. Wisniak, M. Herskowitz, K. Leibowitz and S. Stein, Hydrogenation of xylose to xylitol, Ind. Eng. Chem., Prod. Res. Dev., 13, (1974), 75-80. 9. M. Herskowitz, J. Wisniak and L. Skladman, Hydrogcn solubility in organic liquids, J. Chem.Eng. Data, 28, (1 983), 164-6 10. M. Herskowitz, Modelling of a trickle-bed reactor - the hydrogenation of xylose to xylitol, Chem. Eng. Sci., 40(7), (1983, 1309-1 1.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 o 1991 Elsevier Science Publishers B.V., Amsterdam

113

STRUCTURE AND CATALYTIC P R O P E R T I E S I N HYDROGENATION OF VALERONITRILE OF RANEY NICKEL PREPAKED FROM C r AND Mo DOPED Ni2A13 ALLOYS.

M. BESSON',

D. DJAOUADI',

J.M. BONNIER',

S. HAMAR-THIBAULT'

and M. JOUCLA3

' L a b o r a t o i r e d ' E t u d e s Dynamiques e t S t r u c t u r a l e s de l a S e l e c t i v i t e ( L E E S 1 ) CNRS URA 332 - U n i v e r s i t 6 Joseph F o u r i e r - BP 53X - 38041 GRENOBLE CEDEX (France). L a b o r a t o i r e Therrnodynamique e t P h y s i c o - C h i m i e M e t a l l u r g i q u e s (LTPCM) - CNRS URA 29 - BP75 - 38402 SAINT-MARTIN-D'HERES CEDEX ( F r a n c e ) . l l n i t e M i x t e Rh6ne-Poulenc I n d u s t r i a l i s a t i o n 166 - 69151 DECINES-CHARPIEU CEDEX ( F r a n c e ) .

-

24, Avenue Jean J a u r e s - BP

SUMMARY Raney n i c k e l c a t a l y s t s , unpromoted o r doped w i t h molybdenum o r chromium, The s t r u c t u r e and were p r e p a r e d f r o m t h e p r e c u r s o r a l l o y s o f t h e t y p e N i A1 phase c o m p o s i t i o n o f t h e c a t a l y s t s have been d e t e r d n e 3 d . H y d r o g e n a t i o n o f v a l e r o n i t r i l e a t 90°C and 1 . 6 MPa i n c y c l o h e x a n e was p e r f o r m e d t o e v a l u a t e c a t a l y s t a c t i v i t i e s and t h e r e l a t i v e amounts o f amines f o r m e d . D o p i n g c a t a l y s t s b y chromium i m p r o v e d r e a c t i o n r a t e s and y i e l d s o f p r i m a r y amine, whereas molybdenum a d d i t i o n was i n e f f e c t i v e .

.

INTRODUCTION The h e t e r o g e n e o u s c a t a l y t i c h y d r o g e n a t i o n o f n i t r i l e s has been u s e d i n amine p r e p a r a t i o n f o r a l o n g t i m e . G e n e r a l l y t h e p r o d u c t s a r e a m i x t u r e o f p r i m a r y , s e c o n d a r y and t e r t i a r y amines,

t h e n a t u r e o f w h i c h depends on t h e

c a t a l y s t used as w e l l as on r e a c t i o n c o n d i t i o n s ( r e f s . 1,Z).

The s e l e c t i v i t y

o f n i t r i l e hydrogenation i s o f importance, p a r t i c u l a r l y i n the production o f p r i m a r y amines. I n such r e a c t i o n s t h e c a t a l y s t s most o f t e n p r o p o s e d a r e Raney nickel catalysts (refs. 1-3). To promote

the

activity

and

selectivity

of

Raney n i c k e l

catalysts,

a l l o y i n g o f t h e s t a r t i n g Ni-A1 a l l o y w i t h m e t a l was o f t e n used. F o r i n s t a n c e , Montgomery ( r e f . 4 ) p r e p a r e d c a t a l y s t s b y a c t i v a t i n g t e r n a r y a l l o y powders o f A1 ( 5 8 w t % ) - N i ( 3 7 - 4 2 w t

X) -

M (0.5 w t % ) where M

A l l promoted c a t a l y s t s t e s t e d were

=

Co, C r ,

Cu, F e and Mo.

more a c t i v e t h a n t h e r e f e r e n c e c a t a l y s t ,

i n h y d r o g e n a t i o n o f b u t y r o n i t r i l e . Molybdenum was t h e most e f f e c t i v e p r o m o t e r . W i t h Cr o r T i ,

h y d r o g e n a t i o n o f i s o p h t a l o n i t r i l e on Raney n i c k e l o c c u r r e d a t

l o w e r optimum t e m p e r a t u r e t h a n w i t h non a c t i v a t e d n i c k e l

(ref.

5).

I t was

shown t h a t a d d i t i o n o f T i o r Co t o Raney n i c k e l s u p p r e s s e d t h e f o r m a t i o n o f secondary amine ( r e f . 6 ) . T h i s work has been u n d e r t a k e n t o compare promoted Raney n i c k e l c a t a l y s t s

114

( M = Cr o r Mo). The

o b t a i n e d f r o m s t a r t i n g a l l o y s of c o m p o s i t i o n N i 2 - x M x A 1 3

m i c r o s t r u c t u r e o f t h e c a t a l y s t s was determined and t h e b e h a v i o u r o f t h e s e c a t a l y s t s i n terms o f r a t e o f h y d r o g e n a t i o n and s e l e c t i v i t y was i n v e s t i g a t e d i n t h e h y d r o g e n a t i o n o f v a l e r o n i t r i l e as a model m o l e c u l e . EXPERIMENTAL

Preparation o f c a t a l y s t s The undoped c a t a l y s t was prepared f r o m t h e monophasic c r y s t a l l i z e d Ni2A13 a l l o y ( r e f . 7 ) . The molybdenum and chromium promoted c a t a l y s t s were prepared from a l l o y s w i t h t h e composition Ni2-xMxA13 M = C r ( x = 0.07 o r 0.11)

where M = Mo (0.05,(x,(0.4)

and

( r e f . 8 ) . The c a t a l y s t s were t h e n prepared

d e s c r i b e d p r e v i o u s l y ( r e f . 91,

as

by l e a c h i n g t h e crushed a l l o y s i n a 6N sodium

h y d r o x i d e s o l u t i o n a t b o i l i n g temperature.

The c a t a l y s t s were k e p t under a

molar s o l u t i o n o f NaOH. Characterization o f catalysts The

specific

adsorption

and

surface the

areas

metallic

were

determined

surface

areas

by

by

means

using

of

nitrogen

adsorption

of

3 - m e t h y l t h i o p h e n i n l i q u i d phase ( r e f . 9 ) . The b u l k c o m p o s i t i o n o f each sample was determined by chemical a n a l y s i s and expressed by t h e atomic r a t i o s A l / N i and M / N i

.

The c a t a l y s t s were observed by t r a n s m i s s i o n e l e c t r o n microscopy

(JEOL 200 C X - T E M )

and analysed e i t h e r g l o b a l l y

o r at point

-

l a t e r a l r e s o l u t i o n o f 1 . 5 nm by means o f a STEM ( V G energy

-

level with a

HB 501) connected t o an

d i s p e r s i v e X-ray a n a l y s e r ( E D A X ) .

V a l e r o n i t r i l e hydrogenation The procedure was d e s c r i b e d i n d e t a i l i n a p r e v i o u s work ( r e f s . 1 0 - 1 1 ) . Hydrogenation was c a r r i e d o u t i n l i q u i d phase i n a 250 m l a u t o c l a v e w i t h a magnetic s t i r r e r (1600 rpm), a t c o n s t a n t p r e s s u r e ( 1 . 6 MPa) and t e m p e r a t u r e (90°C).

The

catalyst

was

carefully

washed

with

cyclohexane. A f t e r l o a d i n g t h e c a t a l y s t ( 0 0 . 5

water,

isopropanol

and

g ) and cyclohexane (135 m l ,

HPLC grade and d i s t i l l e d ) t h e a u t o c l a v e was f l u s h e d w i t h hydrogen. The m i x t u r e was p r e t r e a t e d under hydrogen p r e s s u r e (1.6. MPa) a t room t e m p e r a t u r e f o r 1 h. Temperature was r a i s e d t o 90°C and f r e s h l y d i s t i l l e d v a l e r o n i t r i l e ( 1 0 m l ) was i n t r o d u c e d . The s t a r t o f hydrogenation.

Samples

of

s t i r r i n g was c o n s i d e r e d t o be t h e s t a r t o f t h e 0.5

ml

were

taken

chromatography equipped w i t h a 10 % Carbowax 20 M

and 7-

analysed

by

FID

gas

10 % KOH on Chromosorb WHP

80-100 packed column ( 4 m x 1 / 8 " ) . Hexadecane was u s i d as i n t e r n a l s t a n d a r d .

115 RESULTS Characterization o f c a t a l y s t s The d e t a i l e d c h a r a c t e r i z a t i o n o f t h e c a t a l y s t s was d e s c r i b e d elswhere ( r e f s . 12,13). We summarize some o f t h e s e r e s u l t s . (i ) Composition o f p r e c u r s o r a1 l o y s

I n t h e s e doped a l l o y s , t h e major phase (P,) t h e N i 2 A 1 3 phase and a small amount o f d i s s o l v e d C r (

had t h e c o m p o s i t i o n of

'L

W)

1.5 a t

and Mo ('L

0.2 a t % ) . T h i s p r i m a r y phase was surrounded by a small amount o f a b i n a r y

i n t h e case o f C r a d d i t i o n , and o f two phases i n t h e case o f

phase (-Cr4A19)

+ 0.3 % Mo and a t e r n a r y phase P3

= NiAl

Mo a d d i t i o n (P,

(NiMo)A13). The

=

p r o p o r t i o n s o f t h e v a r i o u s phases i n t h e a l l o y s v a r i e d and t h e q u a n t i t y of phase P1 decreased s i g n i f i c a n t l y when t h e Mo c o n t e n t i n c r e a s e d . (ii) Composition o f c a t a l y s t s -

When t h e s e doped a l l o y s were leached, t h e d i f f e r e n t phases p r e s e n t i n t h e p r e c u r s o r a l l o y gave r i s e t o d i f f e r e n t agglomerates i n t h e c a t a l y s t . EDX m i c r o a n a l y s i s performed on Cr-doped c a t a l y s t showed a l a r g e number o f

agglomerates formed f r o m t h e p r i m a r y N i 2 A 1 3 phase.

(Al/Ni

0.22,

Cr/Ni-

0.08). However some C r r i c h zones were a l s o observed, formed f r o m t h e C r - A 1 rich

phase mentionned.

The c a t a l y s t s

contained

oxidized

chromium

(Cr

+3

s t r o n g l y segregated a t t h e s u r f a c e . On t h e

contrary,

i n t h e case o f

Mo a d d i t i o n ,

three well

defined

agglomerates were analysed and were r e l a t e d t o t h e phases observed a l l o y . The A1 and A,

-

0.60, Mo/Ni 0.2-2.0)

0.04).

-

agglomerates i s s u e d r e s p e c t i v e l y f r o m t h e P1

phases, had a low molybdenum c o n t e n t ( A l / N i

CL

0.25,

Mo/Ni

CL

The A 3 t y p e s were r i c h i n Mo ( A l / N i

and were r e l a t e d t o t h e P 3 phase.

i n the and P,

0.05 and A l / N i 0.3,

'L

Mo/Ni%

The amount o f t h e s e d i f f e r e n t

agglomerates depended on t h e c o m p o s i t i o n o f t h e p r e c u r s o r a l l o y ,

and t h e

amount o f A1 decreased when t h e Mo c o n t e n t i n c r e a s e d . The o t h e r physico-chemical s u r f a c e area SBET,

metallic

characteristics of the catalysts (specific

s u r f a c e area SNi

and chemical c o m p o s i t i o n i n

volume) a r e g i v e n i n Table 1. The i n t r o d u c t i o n o f C r i n c r e a s e d b o t h SBET and SNi Mo had h a r d l y any e f f e c t on t h e s e s u r f a c e areas, increasing,

they

decreased

considerably.

With

Cr,

b u t w i t h promoter l e v e l t h e r e was

promoter d u r i n g t h e l e a c h i n g t r e a t m e n t ; i n t h e case o f promoter was d i s s o l v e d ,

; small additions o f

Mo, -65-80

no l o s s o f % of the

confirming observations o f o t h e r i n v e s t i g a t o r s ( 4 ) .

The e x t e n t o f removal o f aluminium was v e r y low and decreased w i t h i n c r e a s i n g promoter c o n t e n t .

116

TABLE 1 Physicochemical c h a r a c t e r i s t i c s o f t h e c a t a l y s t s Ni2A13

Precursor a l l o y

-1) *g 2 -1 SNi(m .g 1 Chemical c o m p o s i t i o n at % Al/Ni a t % M / N i x 100

Ni2-xMoxAl

0.07

0.11

0.05

0.1

0.17

0.4

80

122

113

78

74

68

23

65

83

77

59

56

42

6

0.51 5.4

0.28 0.49

0.36 1.07

0.62 3.70

1.02 8.70

X

2

N i 2-xCrxA1

0.28

0.38 4.0

_C_a_ _t a_ _l y- t i c h y d r o g e n a t i o n o f v a l e r o n i t r i l e C a t a l y t i c h y d r o g e n a t i o n o f v a l e r o n i t r i l e w i t h a commercial Raney n i c k e l c a t a l y s t under d i f f e r e n t r e a c t i o n c o n d i t i o n s was d e s c r i b e d i n p r e v i o u s papers ( r e f s . 10,111.

I t occurs as f o l l o w i n g ( s e e F i g . 1 ) .

@PA)

PENTYLAMINE(PA)

Fig. 1

TRI PENTYLAM I NE (TPA 1

ENAMINE

.

Reaction network o f v a l e r o n i t r i l e h y d r o g e n a t i o n

The r e d u c t i o n o f t h e n i t r i l e proceeds s t e p w i s e w i t h f o r m a t i o n o f a p r i m a r y a l d i , n i n e which t h e n i s hydrogenated t o t h e p r i m a r y amine ( p e n t y l a m i n e ) . P a r t o f t h e a l d i m i n e condenses w i t h p r i m a r y amine a l r e a d y formed t o produce t h e unstable aldimine

aminal.

This

intermediate

(dipentylimine)

(dipentylamine).

which

looses

ultimately

ammonia leads

to to

yield

a

secondary

secondary

amine

The r e a c t i o n o f t h e same p r i m a r y i m i n e w i t h t h e secondary

amine g i v e s r i s e t o the

t e r t i a r y amine ( t r i p e n t y l a m i n e ) , a f t e r h y d r o g e n a t i o n .

The p r o d u c t s d i s t r i b u t i o n as a f u n c t i o n o f t i m e i s i l l u s t r a t e d i n F i g . 2 f o r t h e unpromoted c a t a l y s t d e r i v e d f r o m t h e N i 2 A 1 3 a l l o y . The r a t e o f disappearance o f v a l e r o n i t r i l e remained c o n s t a n t w i t h t i m e up to-75

% c o n v e r s i o n . As soon as t h e r e a c t i o n s t a r t e d ,

t h e presence o f PA and

D P I were observed. D P I reached a maximum and was g r a d u a l l y hydrogenated t o DPA

117

or

r e a c t e d back t o g i v e p e n t y l a m i n e ( r e f . 1 1 ) . When t h e r e was n o more VN,

DPI

had d i s a p p e a r e d . The amount o f TPA f o r m e d was s m a l l .

Fig.2. Hydrogenation o f valer o n i t r i l e : p r o d u c t s d i s t r i but i o n as a f u n c t i o n o f t i m e o n the catalyst derived from N i *A1 3 .

40

20

60

reaction t i m e (min) I n f l u e n c e o f t h e promoters A s shown i n F i g . 3, t h e k i n e t i c s o f v a l e r o n i t r i l e h y d r o g e n a t i o n on t h e d i f f e r e n t c a t a l y s t s d i f f e r e d w i t h a d d i t i o n o f a p r o m o t e r . The i n i t i a l s p e c i f i c r e a c t i o n r a t e s were determined by t h e slopes o f t h e conversion c u r v e s o f v a l e r o n i t r i l e a t i n i t i a l time. E f f e c t o f chromium a d d i t i v e s on h y d r o g e n a t i o n a c t i v i t y was r a t h e r good : t h e i n i t i a l s p e c i f i c a c t i v i t y i m p r o v e d w i t h a d d i t i o n o f C r ( x = 0.071, f u r t h e r increase o f C r ( x

=

0.11)

but

d i d n o t change s i g n i f i c a n t l y t h e i n i t i a l

r a t e . A s t h e a d d i t i o n o f chromium i n c r e a s e d , t h e t i m e n e c e s s a r y t o o b t a i n h a l f c o n v e r s i o n and a l s o t h e t o t a l t i m e o f h y d r o g e n a t i o n i n c r e a s e d m a r k e d l y . Jhen t h e v a l e r o n i t r i l e had been c o m p l e t e l y t r a n s f o r m e d , t h e r e r e m a i n e d DPI. The m o d i f i c a t i o n o f t h e Raney n i c k e l w i t h l o w

Mo amount ( x

= 0.05

o r 0.1)

l e a d t o c a t a l y s t s w h i c h had r o u g h l y t h e same k i n e t i c b e h a v i o u r as t h e undoped. F u r t h e r i n t r o d u c t i o n o f molybdenum i n t h e Ni2A13 a l l o y had a s u b s t a n t i a l n e g a t i v e e f f e c t on t h e p r o p e r t i e s o f t h e c a t a l y s t s ,

w i t h a drop

a c t i v i t y o f t h e c a t a l y s t w i t h t h e h i g h e s t Mo c o n t e n t ( x = 0 . 4 ) .

i n the

Though t h e

amount o f c a t a l y s t was s i x t i m e s more t h a n f o r t h e o t h e r c a t a l y s t s ,

the

c o m p l e t i o n o f h y d r o g e n a t i o n was n o t o b t a i n e d . T a b l e 2 summarizes t h e r e s u l t s o f t h e hydrogenation r a t e s surface area),

(based on weight

o f c a t a l y s t and

on

t h e t i m e s f o r 50% c o n v e r s i o n and f o r t o t a l r e a c t i o n ,

s e l e c t i v i t i e s expressed

as

percentage

of

VN transformed

into

a

metallic and t h e reaction

product. The t a b l e 2 shows moreover t h a t t h e e f f e c t s o f chromium ( x molybdenum a d d i t i v e s were b e n e f i c i a l f o r t h e i n t r i n s i c a c t i v i t y

=

0 . 1 1 ) and o f

Via.

The chromium doped c a t a l y s t s had a l s o a marked i n f l u e n c e o n t h e p r o d u c t s obtained,

a f f o r d i n g a h i g h e r s e l e c t i v i t y i n p e n t y l a m i n e : i t was i n c r e a s e d

118 from 79% (undoped c a t a l y s t ) chromium ( x = 0.11)

t o 83% ( x = 0.07)

resulted s t i l l

; further

introduction o f

i n an improvement (85% i n PA)

and t h e

f o r m a t i o n o f t e r t i a r y amine was n o t d e t e c t e d . I n t r o d u c t i o n o f molybdenum i n t o t h e N i 2 A 1 3 a l l o y r e s u l t e d i n d e c r e a s i n g o f t h e s e l e c t i v i t y i n p r i m a r y amine t o ~ 7 7 6%.The c o n t e n t o f molybdenum had no a p p r e c i a b l e e f f e c t on t h i s s e l e c t i v i t y , ( x = 0.4).

even i n r e l a t i v e l y l a r g e amounts

I n t h i s l a t e case, D P I accumulated i n t h e r e a c t i o n medium and

reached 17 % o f VN transformed i n t o D P I a t t h e maximum, compared t o 8-9.5 w i t h t h e o t h e r Mo promoted c a t a l y s t s .

A Ni1.93cr0,07A13 NiI.B9Cr0.11 A'3

A -6 -4

20

1ooa

io

$0

ioo

n n

-

1-20

reaction time ( m i n 1

reaction t i m e ( m i n )

F i g . 3. E v o l u t i o n o f v a l e r o n i t r i l e (-1 and d i p e n t y l i m i n e (---I t i m e o v e r C r and Mo promoted c a t a l y s t s . R e a c t i o n c o n d i t i o n s : 0.5 g c a t a l y s t , T = 9O"C, PH* = 1.6 MPa.

with reaction 0.1 mol VN,

%

119 TABLE 2

Hydrogenation o f v a l e r o n i t r i l e

on t h e c a t a l y s t s

prepared f r o m Ni2-xMxA13

p r e c u r s o r a1 l o y s .

v0

A1 l o y precursor

mmo1.s N i2Al

Nil .93Cr0.07A13 N i .89Cr0.

lAl

-1

103

.g

-1

vlOx mmol .s

103

-

t50%tlO0% S e l e c t i v i t y

-2

-1

(min)

" Ni

%PA

%DPA

%TPA

79

1.215

20

-

65

79.2

20.7

100

1.205

15

-

70

83.1

16.9

1.43

18

-

120

85.1

14.9 t r a c e s

2,110

<0.1 0.1

Nil .95M00.05A13

80

1.355

15 -

65

76.7

23.0

0.3

Nil .9UM00.10A13

77

1.375

17

-

75

75.3

24.4

0.3

Ni 1 .83M00. 1 7A13

60

1.43

21

-

110

76.0

23.6

0.4

8

1.33

50 *340a

.

Ni 1 60M00. 40A13

74.15 16.15

0.2

aThere was s t i l l 15% untransformed VN and 9.35% D P I . 3.3 g o f c a t a l y s t used. DISCUSSION

The i n t r o d u c t i o n o f f u r t h e r a l l o y i n g components i n t o a N i 2 A 1 3 a l l o y had a s u b s t a n t i a l e f f e c t on t h e p r o p e r t i e s o f t h e Raney n i c k e l c a t a l y s t . C o n t r a r y t o t h e l i t e r a t u r e ( r e f . 41, molybdenum promotion d i d n o t b r i n g t o a r i s e i n t h e s p e c i f i c a c t i v i t y o f t h e c a t a l y s t i n our r e a c t i o n conditions. I n f a c t t h e r e were decreases i n t h e s p e c i f i c r e a c t i o n r a t e s . Adding h i g h l e v e l o f molybdenum ( x = 0.4) made t h e r a t e d r o p c o n s i d e r a b l y . However t h e a c t i v i t y o f t h e molybdenum promoted c a t a l y s t s was f o u n d p r o p o r t i o n a l t o t h e i r m e t a l l i c s u r f a c e area. Moreover, i t was shown ( r e f . 13) t h a t t h i s m e t a l l i c s u r f a c e area was a d i r e c t f u n c t i o n o f t h e P1 phase c o n t e n t i n t h e a l l o y (Mo-doped Ni2A13 w i t h a low Mo c o n t e n t 0.2 a t % ) . The agglomerates i s s u e d from P1 seem t o be t h e o n l y a c t i v e phase i n t h e h y d r o g e n a t i o n o f v a l e r o n i t r i l e . T h i s e x p l a i n s why all

Mo-promoted

catalysts

had t h e

same

intrinsic

activity

but

a l s o why

i n c r e a s i n g t h e Mo promoter l e v e l d i d n o t a f f e c t t h e s e l e c t i v i t y . There i s evidence o f a promoting a c t i o n o f chromium on n i c k e l c a t a l y s t s for

the

reaction

of

hydrogenation o f

valeronitrile

I n t r o d u c t i o n o f chromium i n c r e a s e d t h e i n i t i a l

in

specific

our

conditions.

activity

and t h e

s e l e c t i v i t y . The promoting e f f e c t o f chromium on a c t i v i t y c o u l d be c o r r e l a t e d t o t h e i n c r e a s e o f t h e m e t a l l i c s u r f a c e . Another e x p l a n a t i o n c o u l d be t h a t t h e Cr+3 segregated a t t h e surface o f t h e c a t a l y s t may p l a y t h e r o l e o f a Lewis a c i d c e n t e r and may be r e s p o n s i b l e f o r a b e t t e r c h e m i s o r p t i o n o f v a l e r o n i t r i l e on t h e c a t a l y s t s , through n i t r o g e n l o n e p a i r e l e c t r o n s o r t h e n o r b i t a l o f t h e

CN bond. However, f u r t h e r examination o f t h e r e s u l t s o b t a i n e d ( s e e F i g . 3 )

120 shows t h a t

with

the

catalyst

with

the

higher

content, t h e r a t e o f r e l a t i o n w i t h t h e degree disappearance o f t h e n i t r i l e decreased g r a d u a l l y i n Cr

o f c o n v e r s i o n : t h i s suggests t h a t t h e h y d r o g e n a t i o n p r o d u c t s o r i n t e r m e d i a t e s are

adsorbed more s t r o n g l y

on t h e

c a t a l y s t . There a r e a l s o d i f f e r e n t

Cr-doped

catalyst

than

on

unpromoted

a b i l i t i e s o f t h e c a t a l y s t s t o adsorb and

t r a n s f o r m t h e i n t e r m e d i a t e r e a c t i o n p r o d u c t s : t h i s i s c o n f i r m e d by t h e f a c t t h a t on Cr-promoted c a t a l y s t s ,

a t t h e end o f v a l e r o n i t r i l e h y d r o g e n a t i o n ,

t h e r e was s t i l l DPI t o be t r a n s f o r m e d . T h i s l a t e r r e a c t i o n occured a t a l o w e r rate. CONCLUSION The m o d i f i c a t i o n o f

the

Ni2A13 a l l o y

by

addition

of

molybdenum

chromium has a s i g n i f i c a n t e f f e c t on t h e p r o p e r t i e s o f t h e c a t a l y s t i n t h e r e a c t i o n o f hydrogenation o f v a l e r o n i t r i l e .

or

Raney n i c k e l

I n t h e case o f

molybdenum, t h e c a t a l y t i c p r o p e r t i e s may be c o r r e l a t e d t o t h e p h y s i c o - c h e m i c a l characteristics

of

the catalysts.

Chromium

is

an e f f e c t i v e

promoter

for

i n i t i a l a c t i v i t y and f o r s e l e c t i v i t y . The mechanism f o r p r o m o t i o n o f chromium i n Raney n i c k e l i s n o t known e x a c t l y . ACKNOWLEDGEMENTS The a u t h o r s w i s h t o thank Rhdne-Poulenc f o r f i n a n c i a l

s u p p o r t and P .

C I V I D I N O f o r h e r c o n t r i b u t i o n i n d e t e r m i n i n g t h e s u r f a c e areas. REFERENCES

1 2 3 4

5 6 7

8 9 10

11 12 13

J. V o l f and J . Pasek, i n L . Cerveny (Ed.), S t u d i e s i n S u r f a c e Science and C a t a l y s i s 27 : C a t a l y t i c Hydrogenation, E l s e v i e r , Amsterdam, 1986, Ch. 4, p . 105. P.N. Rylander and D.R. S t e e l e , Engelhard I n d . Tech. B u l l . 1 1 (1970) 19-24. H. G r e e n f i e l d , I n d . Eng. Chem. Prod. Res. Develop. 6 (1967) 142-4. S.R. Montgomery i n W.R. Moser ( E d . ) , Chem. I n d u s t r i e s 5 : C a t a l y s i s o f Organic Reactions, M. Dekker New York, 1981, p. 383. N . I . Schcheglov and D.V. S o k o l ' s k i i , Met. i Khim. Prom. Kazakhstana, Nauchn. - Tekhn. Sbornik, 3 (1961) 68-71. USSR P a t e n t 237150 (1960) 14. Khaidar, C. A l l i b e r t , J. D r i o l e and P. Germi, M a t e r . Res. B u l l . 17 (1982) 329. J . Gros, S. Hammar-Thibault and J.C. Joud, J . Mat. S c i . 24 (1989) 2987-98. S. Sane, J.M. Bonnier, J.P. Damon and J . Masson, Appl. C a t a l . , 9 (1984 69-83. M. Besson, J.M. Bonnier and M. J o u c l a , B u l l . SOC. Chim. F r . 127 (1990 5-1 2. M. Besson, J.M. Bonnier, D. Djaouadi and M. Joucla, B u l l . SOC. Chim. F r 127 (1990) 13-19. J.M. Bonnier, J.P. Damon and J. Masson, A p p l . C a t a l . 42 (1988) 285-97. S. Hamar-Thibault and J. Masson, accepted i n J . Chem. Phys..

M. Guisnet et al. (Editors ), Heterogeneous Catalysis and Fine Chernicaki I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

SELECTIVE

PREPARATION

OF

CHLOROANILINES

FROM

121

CHLORONITROBENZENES

OVER

SULFIDED HYDROTREATING CATALYSTS C.MOREAU~,

c .SAENZ’,

P. G E N E S T E ~ , M. BREYS S E ~and M. LACROIX2

’ L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimique Appliquees, URA CNRS 418, Ecole N a t i o n a l e Superieure de Chimie de M o n t p e l l i e r 8, r u e Ecole Normale, 34053 M o n t p e l l i e r Cedex 1 ( F r a n c e ) ‘ I n s t i t u t de Recherche sur l a Catalyse, 2 Avenue A l b e r t E i n s t e i n , 69626 V i l leurbanne Cedex (France) SUMMARY The r a t e c o n s t a n t s f o r t h e h y d r o p r o c e s s i n g o f c h l o r o n i t r o b e n z e n e s over i n d u s t r i a l s u l f i d e d c a t a l y s t s l i k e NiO-Moo /y-A1203 and COO-MOO / Y-A1 0 have been measured a t temperatures r a n g i n g &om 50 C t o 250°C and $0 b a r 2 0 ? hydrogen pressure. The r e a c t i o n proceeds t h r o u g h t h e r a p i d h y d r o g e n a t i o n o f t h e n i t r o group f o l l o w e d by t h e slow cleavage o f t h e C-C1 bond o f t h e i n t e r m e d i a t e c h l o r o a n i l i n e s . The l a r g e d i f f e r e n c e i n t h e apparent a c t i v a t i o n e n e r g i e s o b t a i n e d f o r b o t h r e a c t i o n s shows t h a t t h e b e t t e r s e l e c t i v i t y t o c h l o r o a n i l i n e s i s o b t a i n e d a t low temperatures. Moreover, t h i s s e l e c t i v i t y i s improved o v e r t h e COO-MOO /Y-A1 0 c a t a l y s t which t e n d s t o i n c r e a s e t h e r a t e o f h y d r o g e n a t i o n o f &e n i ? r $ group and t o reduce t h e r a t e of h y d r o g e n o l y s i s o f t h e C - C 1 bond.

INTRODUCTION I n a p r e v i o u s paper c o n c e r n i n g t h e a p p l i c a t i o n o f s u l f i d e d h y d r o t r e a t i n g c a t a l y s t s t o t h e s y n t h e s i s o f f i n e chemicals we have a l r e a d y shown t h a t i t was p o s s i b l e t o o b t a i n s e l e c t i v e r e a c t i o n s by t a k i n g advantage o f t h e

d i f f e r e n c e s i n r e a c t i v i t y observed

for

h y d r o g e n a t i o n and

hydrogenolysis

reactions ( r e f . ] ) , A new i l l u s t r a t i o n o f t h i s approach i s g i v e n i n t h i s paper w i t h t h e hydroprocessing o f c h l o r o n i t r o b e n z e n e s o v e r t h e s u l f i d e d i n d u s t r i a l h y d r o t r e a t i n g c a t a l y s t s , Co0-MoO3/y-Al2O3 HR 306 and Ni0-MoO3iy-Al2O3 HR 348 f r o m P r o c a t a l y s e . These c a t a l y s t s have been shown t o d i f f e r i n t h e i r hydrogenol y s i s r a t h e r than i n t h e i r hydrogenation properties ( r e f . 2 ) .

EXPERIMENTAL Experiments

were

carried

out

in

a

0.3

litre

stirred

autoclave

( A u t o c l a v e Engineers t y p e Magne-Drive) o p e r a t i n g i n a b a t c h mode andequipped with

a

system

for

sampling

o f l i q u i d during t h e course o f t h e r e a c t i o n

122 w i t h o u t s t o p p i n g t h e a g i t a t i o n . T y p i c a l procedure was as f o l l o w s : a 0.06M s o l u t i o n of o r g a n i c r e a c t a n t i n 60ml o f decane ( a n a l y t i c a l g r a d e ) was poured i n t o t h e autoclave.

The s u l f i d e d c a t a l y s t

( 0 . 6 9 ) was t h e n added t o t h i s

s o l u t i o n . A f t e r p u r g i n g w i t h n i t r o g e n , t h e t e m p e r a t u r e was i n c r e a s e d under n i t r o g e n up t o t h e r e q u i r e d w o r k i n g t e m p e r a t u r e . N i t r o g e n was t h e n removed and hydrogen was i n t r o d u c e d a t t h e r e q u i r e d p r e s s u r e ( 2 0 b a r ) . Zero t i m e was taken when t h e a g i t a t i o n began. Analyses were performed on a

D e l s i DI200 gas

chromatograph equipped

w i t h a flame i o n i z a t i o n d e t e c t o r u s i n g hydrogen as c a r r i e r gas and adequate c a p i l l a r y columns.

Products were i d e n t i f i e d by comparison w i t h a u t h e n t i c

samples and/or by GC-MS a n a l y s i s . The c a t a l y s t s used were i n d u s t r i a l P r o c a t a l y s e HR 348 (2.7%

NiO ;

16.5% Moo3 ; 80.8% promoted alumina and HR 306 ( 3 % COO ; 14% Moo3 ; 83% Y-alumina)

presulfided

at

atmospheric

pressure

using

a

fluidized-bed

t e c h n i q u e w i t h a gas m i x t u r e o f 15% H2S and 85% H2 by volume (gas f l o w : 120ml/min ; 400°C f o r 4 h ) . The r a t e c o n s t a n t s were deduced f r o m t h e e x p e r i m e n t a l c u r v e s by c u r v e f i t t i n g and s i m u l a t i o n u s i n g t h e AnaCin S o f t w a r e ( r e f . 3 1 ,

assuming a l l t h e

r e a c t i o n s t o be f i r s t o r d e r i n t h e o r g a n i c r e a c t a n t . The c a l c u l a t e d r e a c t i o n r a t e constants ( i n min-’)

depend on t h e w e i g h t o f c a t a l y s t and a r e t h e n

referred t o l g catalyst. RESULTS AND DISCUSSION The h y d r o p r o c e s s i n g o f c h l o r o n i t r o b e n z e n e s has been i n v e s t i g a t e d o v e r t h e s u l f i d e d HR 306 and HR 348 c a t a l y s t s a t t e m p e r a t u r e s r a n g i n g f r o m 50°C t o 250°C under 20 bar o f hydrogen p r e s s u r e . The r e a c t i o n has been shown t o proceed t h r o u g h t h e c o n s e c u t i v e r e a c t i o n network g i v e n i n Scheme 1, i n v o l v i n g r a p i d h y d r o g e n a t i o n o f t h e n i t r o group o f t h e s t a r t i n g c h l o r o n i t r o b e n -

LI

Cl

Scheme 1. Reaction network f o r t h e h y d r o p r o c e s s i n g o f c h l o r o n i l r o b e n z e n e s over HR 306 and HR 348 c a t a l y s t s .

123

zenes followed by slow hydrogenolysis of the C-C1 bond of the intermediate chloroanilines. Such a reaction scheme was not completely unexpected from the individual behavior of the monosubstituted compounds, nitrobenzene, chlorobenzene and aniline toward both sulfided catalysts. Indeed, for the first step, the nitro group of nitrobenzene is more rapidly hydrogenated than the C-C1 bond o f chlorobenzene is cleaved whatever the catalysts ; for the second step, the C-C1 bond of chlorobenzene is more rapidly hydrogenolyzed than the C-NH2 bond of aniline, whatever again the catalysts. I

CI

CI

Experimental concentration vs time plots for the hydroprocessing of 4-chloronitrobenzene over the sulfided HR 306 and HR 348 catalysts at 200°C and 20 bar H2 are presented in Fig.1 and Fig.2, respectively.The curves drawn in Fig.1 and Fig.2 are computer-simulated based on the consecutive reaction network shown in Scheme 1.

,tlOO 'A / O

1OOhC

,o-o-oO \O

Fig.1. Concentration vs time plot for hydroprocessing of 4-chloronitrobenzene over sulfided CoMo HR 306 catalyst at 200°C, 20 bar HE. 4-chloronitrobenzene ( A), 4-chloroaniline ( 0 1, aniline ( 0 ) .

oL 0

Fig.2. Concentration vs time plot for hydroprocessing of 4-chloronitrobenzene over sulfided NiMo HR 348 catalyst at 2OO"C, 20 bar H2. 4-chloronitrobenzene ( A1, 4-chloroaniline ( 0 ) aniline ( 0 )

124

The

r a t e constants

c a l c u l a t e d from

this

kinetic

scheme

hydrogenation o f t h e n i t r o group o f 4 - c h l o r o n i t r o b e n z e n e ( kl

for

the

and hydroge-

n o l y s i s o f t h e C-C1 bond o f 4 - c h l o r o a n i l i n e ( k 2 ) a r e r e p o r t e d i n Table 1.

TABLE 1 Rate c o n s t a n t s ( x zene ( k l )

lo3

mind' . g . c a t - l )

f o r hydrogenation o f 4-chloronitroben-

and h y d r o g e n o l y s i s o f 4 - c h l o r o a n i l i n e ( k 2 1 over s u l f i d e d CoMo HR

306 and NiMo HR 348 c a t a l y s t s a t t e m p e r a t u r e s r a n g i n g f r o m 50°C t o 250°C and 20 b a r o f hydrogen p r e s s u r e

200

250

85

250

349

149

21 5

235

12

49

3

25

T"C

50

kl ( N i Mo )

10

38

k,(CoMo)

17

92

k ( N i Mo )

1

k ( Co Mo )

0.5

100

150

Although small d i f f e r e n c e s i n t h e r a t e c o n s t a n t s a r e observed f r o m a c a t a l y s t t o t h e o t h e r , i t i s seen t h a t t h e CoMo HR 306 c a t a l y s t i s s l i g h t l y more

hydrogenating than

( < 200°C).

the

NiMo

A t h i g h e r temperatures,

HR

348

c a t a l y s t a t low temperatures

t h e expected h i g h e r h y d r o g e n a t i o n p r o -

p e r t i e s o f t h e NiMo HR 348 c a t a l y s t a r e found again. As f a r as i t was exper i m e n t a l l y p o s s i b l e t o c a l c u l a t e t h e h y d r o g e n o l y s i s r a t e c o n s t a n t k2, t h e NiMo c a t a l y s t appears t o be a b e t t e r c a t a l y s t t h a n t h e CoMo HR 306 c a t a l y s t f o r t h e cleavage o f t h e C-C1 bond o f 4 - c h l o r o a n i l i n e . T h i s agrees w e l l w i t h t h e p r e v i o u s f i n d i n g s observed on m o n o f u n c t i o n a l i z e d compounds ( r e f . 2 ) . Combining t h e h y d r o g e n a t i o n and h y d r o g e n o l y s i s p r o p e r t i e s o f t h e c a t a l y s t s , t h e b e t t e r s e l e c t i v i t y t o 4 - c h l o r o a n i l i n e from 4-chloronitrobenzene i s o b t a i n e d o v e r t h e CoMo c a t a l y s t a t l o w t e m p e r a t u r e . Indeed, t h e k i n e t i c

kLB

c o n s e c u t i v e scheme A chloronitrobenzenes

k2

>C

hydrogenation

of

(scheme 1 ) a l l o w s t o c a l c u l a t e t h e c o n c e n t r a t i o n

in

c h l o r o a n i l i n e intermediate, maximum c o n c e n t r a t i o n

(61,

for

f r o m which i t can be deduced b o t h t h e

i n t h e intermediate,

when d ( B ) / d t = O . t i m e tmax

observed

(Blmax,

and t h e c o r r e s p o n d i n g

125

The d i f f e r e n t e x p r e s s i o n s l e a d i n g t o (BImax and tmax a r e g i v e n below :

Thus t h e maximum c o n c e n t r a t i o n i n 4 - c h l o r o a n i l i n e

( g i v e n as

molar

p e r c e n t ) i n c r e a s e s f r o m 72% a t 250°C t o 86% a t 200°C and 95% a t 150°C o v e r t h e s u l f i d e d NiMo HR 348 c a t a l y s t . The r e s u l t s a r e b e t t e r o v e r t h e CoMo HR 306 c a t a l y s t ,

77% a t 250"C,

94% a t 200°C

and 98% a t

150°C.

A

better

i l l u s t r a t i o n o f t h e i n c r e a s e i n s e l e c t i v i t y i s g i v e n i n F i g . 3 and F i g . 4 w i t h

Fig.3. A r r h e n i u s p l o t f o r h y d r o -

Fig.4. Arrhenius p l o t f o r hydro-

processing o f 4-chloronitrobenzene

processing o f 4-chloronitrobenzene

over s u l f i d e d CoMo HR 306 c a t a l y s t .

o v e r s u l f i d e d NiMo HR 348 c a t a l y s t .

126

the Arrhenius p l o t s obtained f o r both c a t a l y s t s .

The apparent

activation

e n e r g i e s c a l c u l a t e d f r o m t h e s e p l o t s a r e o f t h e same o r d e r o f magnitude f o r t h e two c a t a l y s t s , 6-7 kcal.mo1-'

f o r t h e h y d r o g e n a t i o n o f t h e n i t r o group

f o r t h e h y d r o g e n o l y s i s o f t h e C - C 1 bond.

and 18 kcal.mo1-'

The l a r g e d i f -

f e r e n c e s i n t h e apparent a c t i v a t i o n e n e r g i e s i l l u s t r a t e unambiguously t h e increase i n t h e s e l e c t i v i t y t o c h l o r o a n i l i n e , t h e e f f e c t o f t h e temperature b e i n g more i m p o r t a n t f o r t h e h y d r o g e n o l y s i s t h a n f o r t h e h y d r o g e n a t i o n r e a c tion. A t lOO"C,

o n l y c h l o r o a n i l i n e was d e t e c t e d .

The v a l u e o f t h e apparent a c t i v a t i o n energy i s r a t h e r low f o r t h e hydrogenation s t e p . T h i s v a l u e i s o f t h e same o r d e r o f magnitude t h a n t h o s e encountered f o r alkenes h y d r o g e n a t i o n o v e r m e t a l o r s u l f i d e d c a t a l y s t s , k c a l .mol-'

(refs.4,5).

3-8

Alkenes a r e w e l l known t o be r e a d i l y hydrogenated.

The low a c t i v a t i o n energy observed f o r t h e h y d r o g e n a t i o n o f t h e n i t r o group can t h u s be r e l a t e d t o i t s g r e a t a b i l i t y t o undergo h y d r o g e n a t i o n r a t h e r than t o d i f f u s i o n a l phenomena except maybe a t h i g h t e m p e r a t u r e s where some d e v i a t i o n s f r o m t h e A r r h e n i u s e q u a t i o n a r e observed. to

avoid d i f f u s i o n

considered,

limitation

in

the

liquid

Classical requirements

phase

have

already

been

p a r t i c u l a r l y the proportionality o f the reaction r a t e t o t h e

c a t a l y s t w e i g h t , t h e c o n c e n t r a t i o n o f t h e a c t i v e component, speed, and c a l c u l a t i o n s made w i t h t h e T h i e l e modulus, 0

the agitation

r (k/D)"

=

assuming

a p a r t i c l e r a d i u s r o f .0045 cm and a d i f f u s i o n c o e f f i c i e n t 0 f o r t h e l i q u i d phase l y i n g between

and

cm2/s ( r e f s . 6 , 7 ) .

Another p o i n t w o r t h m e n t i o n i n g i s t h e r e d u c t i o n o f s u b s t i t u t e d n i t r o benzenes by sodium d i s u l f i d e i n aqueous m e t h a n o l i c s o l u t i o n . T h i s r e a c t i o n was shown t o be l a r g e l y i n f l u e n c e d by t h e presence o f e l e c t r o n - d o n a t i n g o r electron-withdrawing substituents ( r e f . 8 ) .

The e f f e c t o f s u b s t i t u e n t s f i t s

t h e Hammett e q u a t i o n w e l l , t h e s l o p e 0 - v a l u e b e i n g a u t h o r s t o propose t h e d i s u l f i d e a n i o n S2-

thus leading t h e

i. 3.55,

as t h e r e d u c t i v e s p e c i e s . 2- and

3 - c h l o r o n i t r o b e n z e n e s have been s t u d i e d over t h e CoMo HR 306 and NiMo HR 348 c a t a l y s t s under o p e r a t i n g c o n d i t i o n s s i m i l a r 4-chloro

derivative.

No s i g n i f i c a n t

t o those

difference

observed from i n d i v i d u a l o r c o m p e t i t i v e e x p e r i m e n t s . absence

of

substituent

effects

that

the

reported

for

the

i n t h e r e a c t i v i t i e s was It r e s u l t s from t h e

hydrogenation

mechanism

over

s u l f i d e d CoMo HR 306 and NiMo HR 348 c a t a l y s t s d i f f e r s f r o m t h e r e d u c t i o n mechanism over

sodium d i s u l f i d e .

As

a

consequence,

the

S2-

species,

sometimes i n v o k e d as t h e a c t i v e s p e c i e s o f s u l f i d e d c a t a l y s t s l i k e RuS2 and NbS3 ( r e f s . 9,101,

would n o t be t h e a c t i v e s p e c i e s f o r t h e s u l f i d e d h y d r o -

t r e a t i n g NiMo and CoMo c a t a l y s t s . The presence o f d i s u l f i d e i o n s p e c i e s f o r both l a t t e r c a t a l y s t s c o u l d n o t be supported by any e x p e r i m e n t a l method.

127

Experiments a r e b e i n g c o n s i d e r e d t o complete t h e s e

preliminary results

concerning t h e e f f e c t o f s u b s t i t u e n t s . CONCLUSION Hydrogenation

of

chloronitrobenzenes

to

chloroanilines

achieved over c o n v e n t i o n a l s u l f i d e d h y d r o t r e a t i n g c a t a l y s t s .

is

easily

Large d i f f e -

rences i n t h e a c t i v a t i o n e n e r g i e s a r e observed f o r h y d r o g e n a t i o n o f t h e n i t r o group o f c h l o r o n i t r o b e n z e n e and c l e a v a g e o f t h e C-C1 bond o f c h l o r o a n i l i n e . It i s thus possible t o increase t h e s e l e c t i v i t y t o c h l o r o a n i l i n e by o p e r a t i n g a t low temperatures.

T h i s s e l e c t i v i t y i s i n c r e a s e d a g a i n by

u s i n g t h e Co-promoted r a t h e r t h a n t h e Ni-promoted c a t a l y s t . properties

of

sulfided

hydrotreating

catalysts

can

be

The s e l e c t i v e advantageously

compared t o t h o s e o f o t h e r c a t a l y t i c systems ( r e f . 1 1 ) .

REFERENCES 1. C.Moreau, R.Durand, P . G r a f f i n and P.Geneste, Stud.Surf.Sci.Catal., 41 (1988) 139. 2. n o r e a u , J . J o f f r e , C.Saenz and P.Geneste, J.Catal.,122 (1990) 448. 3. J . J o f f r e , P.Geneste, A.Guida, G.Szabo and C.Moreau, 5 d . P h y s . Theor. Chem., ,71 990) 409. 4. S. J .Thornson a n d T . Webb, J.Chem.Soc. ,Chem.Commun., ( 1976) 526. 5. Z . P o l t a r z e w s k i , S.Galvagno, R . P i e t r o p a o l o and P . S t a i t i , J.Catal., 102 (1986) 190. 6. %.Bond, "Heterogeneous C a t a l y s i s : p r i n c i p l e s and a p p l i c a t i o n s " , 2 e d i t i o n , Clarendon P F ~ S S , Oxford, 1987, p.48. 7. P.B.Weisz, Proceedings 7 I n t e r n a t i o n a l Congress on C a t a l y s i s , T.Sziyama and K.Tanabe, Eds., Kodanska-Elsevier, Tokyo,1981,p.3. 8. M.Hojo, Y.Takagi and Y.Ogata, J.Am.Chem.Soc., 82 (1960) 2459. 9. J.B.Goodenough, Proceedings o f t h e F o u r t h I n t e r n a t i o n a l Conference on t h e Chemistry and Uses o f Molybdenum (M.F.Barry and P.C.H. M i t c h e l l , Eds.) Ann Arbor, M I , Climax Molybdenum Company, 1982,p.19. 10. M.Vrinat, C . G u i l l a r d , M.Lacroix and M.Breysse, Bull.Soc.Chim.Belg., 96 (1987) 1017. 11. 6-r e c e n t r e v i e w s on h y d r o g e n a t i o n o f n i t r o groups, see f o r example : Compendium o f o r g a n i c s y n t h e t i c methods, J.Wiley, N.Y., Vo1.6 (1988) and p r e c e d i n g volumes i n t h e s e r i e s , and J.R.Kozak i n " C a t a l y s i s o f Organic Reactions", P.N.Rylander, H . G r e e n f i e l d and R.L. Augustine, Ed., M.Dekker, N.Y. , 1 9 8 8 , ~ . 135.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

129

THE APPLICABILITY OF DISPERSED METALS AS CATALYSTS FOR ORGANOMETALLIC REACTIONS R.L. Augusthe*, S.T. O'Leary,K.M. Lahanas and Y.-M. Lay Department of Chemistry, Seton Hall University, South Orange, New Jersey 07079 USA SUMMARY Initial work indicates that dispersed metals may be used to promote a variety of organometallic reactions. The Heck Arylation proceeds smoothly over supported Pd catalysts while diene cyclizations can be catalyzed by dispersed Rh metal. The use of these heterogeneous species facilitates product isolation and permits the application of flow systems rather than batch reactors for these reactions. Frontier Molecular Orbital and mechanistic considerations indicate that these reactions take place on the coordinately unsaturated comer atoms on the metal surface.

INTRODUCTION The use of heterogeneous catalytic reactions in the fine chemical industry is usually limited to the modification of functional groups by hydrogenation or oxygenation reactions. If, however, heterogeneous catalysts could also be used to promote synthetically useful C-C bond forming reactions, such processes would have significant practical, economical and environmental importance. At the present time there are a number of these synthetically useful reactions which are catalyzed by soluble organometallic complexes (ref. 1) but, the large scale use of such soluble species to make compounds of interest to the fine chemical industry is not practical. The primary problem associated with the use of these homogeneous catalysts is the separation of the product not only from the organometallic species but also from ligands which may have dissociated from the catalyst during the reaction. If one could use heterogeneous catalysts such as dispersed metals to promote this type of reaction, product separation would be facilitated and the more efficient flow processes could be used instead of the commonly employed batch mode. There are, however, a number of problems which must be overcome before such systems can be used practically. In the first place it must be shown that dispersed metals can promote these reactions and, secondly, a more detailed knowledge must be acquired of substrate adsorption on the catalyst and the interaction of the adsorbed species to give the product. The first of these problems appears to have a reasonable expectation of solution. There are a few publications which state that supported metals can be used to promote some of the organometallic reactions commonly run with soluble catalysts (refs. 2-4). In these reports, though, the use of the supported metal is generally included only as an entry in a table describing the effect of changing reaction parameters on product yield and/or selectivity. The fact that a heterogeneous catalyst was used is seldom discussed. When it is mentioned, it is

130

usually assumed that the metal promotes the reaction because some of it is "solubilized" to give the active catalytic species. We describe here some of our initial efforts concerned with the use of dispersed metals as catalysts for organometallic reactions and the development of a Frontier Molecular Orbital description of the reactions taking place on the metal surface. RESULTS AND DISCUSSION

Oreanometallic reactions An interesting organometallic reaction is the Heck Arylation (Eqn. 1) (ref. 5 ) , which is commonly run using a Pd(OAc)2 catalyst. This reaction is used to prepare aryl enol ethers which can be valuable synthetic intermediates in that they can be hydrolyzed to aldehydes or ketones, species which can be useful themselves or as intermediates in further reactions. The influence of reaction parameters on the rate and selectivity of this reaction was reported in a series of papers (refs. 2, 3). In these a brief mention in some tables was made that Pd/C was able to catalyze this reaction but no discussion of the use of this catalyst was included, We have found, though, that this reaction is readily promoted over dispersed Pd catalysts. When run with Pd(OAc)2 as the catalyst, the Heck reaction gives as the primary products the E (1) and Z (2) aryl /3 enol ethers in about a 2: 1 ratio. The u isomer, 3, and ester, 4, are also produced but in much smaller amounts. When the reaction is run over Pd/A1203, the same products are obtained but the /3 enol ethers 1 and 2 are produced in nearly a 3:l ratio. Table 1 lists the product compositions of these reactions. TABLE 1 p-Nitrobenzoyl Chloride Reaction Run Over Various Palladium Catalystsa Percent Yield Catalyst P~(OAC)~~ Pd/ y- A12 0 3

b-E (1)

b-Z (2)

E/Z Ratio

a (3)

Ester(4)

50.4 42.9

27.1

1.86 2.75

6.3 5.8

4.4

15.6

3.2

aThe reactions were run with 2.5 mmol of p-nitrobenzoyl chloride, 5.0 mmol of butyl vinyl ether, 3.75 mmol of n-ethylmorpholine, and catalyst in 0.25 mol % (based on p-NBC) in 25 ml of dioxane. Dodecane was used as an internal standard. The experiment was performed under a blanket of N2 at the reflux temperature of the solvent. bl mol % (based on p-NBC). Xylene was the solvent. The most striking comparison between the homogeneous and heterogeneous catalysts was that four times more palladium was required in the homogeneously catalyzed reaction to give about the same rate as that of the Pd/A1203 promoted reaction. A tertiary amine is present in the reaction mixture to remove the HCl from the catalyst and regenerate the catalytically active

131

species. When the heterogeneously catalyzed reaction was run in toluene, the solvent commonly used in homogeneously catalyzed reactions, the catalyst was rapidly deactivated by the precipitation of the amine hydrochloride. To prevent this, dioxane was used as the solvent to keep the salt in solution. To establish that the Pd/A1203 was responsible for the reaction and not some "solubilized" species, the catalyst was separated from the reaction mixture after 10% conversion and the resulting solution heated under conditions known to promote the homogeneous reaction. No further reaction was observed until the Pd/A1203 was reintroduced to the reaction mixture.

ICOJ 5

- WtHtM Rh

0

0

0

6

7

8

Eqn.

1

Eqn.

2

The diene cyclization shown in Eqn. 2, has been reported to take place only over RhC13 and Wilkinson's catalyst (ref. 6). We have found that it also occurs when run over supported Rh catalysts. The heterogeneously catalyzed reaction is particularly sensitive to the nature of the solvent used. With alcohols or other solvents which can adsorb on the catalyst, there is an apparent competition with the adsorption of the double bonds and the cyclization does not take place. In alkane solvents, which do not interact with the catalyst, the reaction occurs with reasonable facility. This cyclization is run routinely at 145°C in a flow system with a decane solution of 5 passing through a small column containing a Rh/A1203 catalyst. The product composition was related to the time 5 was in contact with the catalyst. With fast flow rates (short contact times) 6 was the primary product of the reaction but the isornerized species, 7 and 8, were produced when slower flow rates were used. This indicates that 6 was the primary product of the reaction but that it was isomerized over the catalyst to 7 and 8.

132

In neither of these reactions was it necessary to add any ligands or modifiers to the system to promote the reaction. Active sites While these results indicate that a supported metal can be used to promote organometallic reactions, there are a number of questions which must be answered before their use in this way can become routine. One of the most important of these considerations concerns the nature of the "active site" on the metal which promotes the reaction and the process by which the reaction takes place on this site. Data are available which indicate that groups or "ensembles" of surface atoms are used to promote reactions involving C-C bond cleavage or hydrocarbon rearrangements while single atom sites are responsible for C-H bond forming and breaking reactions (refs. 7, 8). Further results show that the "ensemble" sites responsible for C-C bond breaking are primarily groups of atoms on the 111 faces of the metal particles (ref. 9). The single atom sites which promote C-H bond formation or cleavage, on the other hand, are the more coordinately unsaturated corner atoms (refs. 10, 11). Other single atom sites are the edge atoms which presumably can promote double bond isomerizations (ref. 11).

X

Side

V i e w

Z

X

Top

-Y

V i e w

Fig. 1. Top and side views of a bulk atom in an fcc crystalline lattice shown as a twelve coordinate complex.

133

Most catalytically active metals have the fcc crystal lattice. Examination of crystal models shows that there are at least 13 different types of surface atoms possible with the fcc crystal arrangement (ref. 12). In these metals each bulk atom is surrounded by twelve nearest neighbors as depicted in Fig. 1 for that orientation viewed from the 100 face. This entity can be thought of as a twelve coordinate "complex" of the central atom, M, surrounded by twelve "ligand" atoms. The different single atom surface sites can be derived from this twelve coordinate species by removing varying numbers of the "ligand" atoms. The 100 face atom "complex" is produced by removing "ligands" 4, 9, 10, and 12 from the species shown in Fig. 1. This results in a surface "complex" composed of the central atom, M, surrounded by "ligands" 1, 2, 3, 5, 6, 7, 8, and 11. The octahedral comer atom depicted in Fig. 2 is composed of the metal atom with "ligands" 2, 3, 6, and 7. We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomenzation sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10).

X

-:j

l p z

Top

Vi e w

e v

5PX

5 P x 5P, 5s

-1 0

4 d X42d- ,=22

-

#

S i d e

View

I

Fig. 2. Energy levels of the 5s, 5p, and 4d electrons of a Pd octahedral comer atom

- 1

134

When a series of STO characterized PdlA12Q catalysts were used to promote the Heck reaction (Eqn. 1) the amount of the /? aryl enol ethers, 1 and 2, formed after a 60 minute reaction was directly related to the comer site densities on these catalysts. Thus, this reaction and presumably, others such as the diene cyclization shown in Eqn. 2, which require the adsorption of two reactive species on a single surface atom, must take place on the more coordinatively unsaturated comer atoms. Frontier Molecular Orbital mechanistic treatment In order to utilize these heterogeneously catalyzed reactions more fully it is necessary to develop an understanding of the mode of substrate adsorption and interaction on these sites. While the octahedral orientation is common to most soluble organometallic catalysts, surface species with this arrangement are not possible on fcc metals. Surface complexes having the octahedral orientation cannot be produced regardless of which "ligands" are removed from the twelve coordinate species shown in Fig. 1. However, if the electronic character of these sites were determined, it should be possible to use reaction sequences similar to the mechanisms proposed for the soluble species as long as the surface orbitals of the site which are involved in the interactions have the correct symmetry and are available for substrate bonding. Scheme

1

10 Slde

Vlew

111 ' ' Q

13

12

EHMO calculations on 111 and 100 metal planes have indicated that the surface electron orbitals are quite localized (refs. 14-16). This supports the premise that these surface sites can be considered as "surface complexes". With this assumption classical inorganic techniques can

135

be used to determine the electron distribution at each of these sites. We have developed an Angular Overlap Method (ref. 17) approach to this problem and have calculated the s, p, and d electron energies for each of the possible surface sites on a number of different fcc metals. The s, p, and d electron orbital energy values for the Pd octahedral comer atom is shown in Figure 2 along with the side and top views of the d orbital arrangement for this site. Using Frontier Molecular Orbital considerations it can be seen that the 5 s orbital is the LUMO and the degenerate 4dx, and 4dyz orbitals are HOMO. Thus, a substrate can adsorb on this site by electron donation to the 5s orbital with back bonding from either the 4dx, or 4dYr Only one of these can be used since when adsorption occurs further interaction with another species from the z direction is blocked. The adsorption of a second species must take place in the x-y plane and involves electron donation to the LUMO 5px or 5py. Backbonding from the HOMO 4d,2 orbital is not possible since it is out of the x-y plane. The 4d,2.y2 orbitals are coincident with the px and py so they cannot take part in the adsorption either. Instead the 4dxy orbitals which do have the proper orientation are used for backbonding. Scheme 1 illustrates these points for an alkene hydrogenation on a Pd octahedral comer site, 9. Adsorption of H2 occurs with D donation to the 5s orbital and back bonding from the 4dx, to the D* orbitals of the b,as in 10, to give the dihydride, 11, shown in both top and side views. Alkene adsorption now can only take place by IT donation to the 5py with backbonding to the IT*orbital from the 4d,, as in 12. Hydrogen insertion gives the hydrido metalalkyl, 13, again depicted in both side and top views, Reductive elimination gives the alkane and regenerates the active site, 9. Scheme 2 shows a similar mechanistic pathway for a Heck reaction taking place on a Pd octahedral comer. This mechanism is based on that established for soluble Pd catalysts (ref. 5). Adsorption of the aryl halide (or aryl acid chloride after decarbonylation) gives the aryl Pd halide, 15, by way of the adsorbed intermediate, 14. Vinyl ether adsorption, as in 16, takes place as described in Scheme 1. Aryl insertion gives the halometalalkyl, 17, which on B elimination to the available 4dxy orbital gives the aryl enol ether, 2 (or 1 depending on which hydrogen is eliminated in 17). The resulting halo palladium hydride, 18, then reacts with the tertiary amine to give the amine hydrochloride and regenerates the octahedral comer for further reaction. CONCLUSIONS

It appears that supported metal catalysts can be used to promote synthetically useful organometallic reactions. The utilization of such reactions can be of practical, economic, and environmental importance to the fine chemical industry. Frontier Molecular Orbital and mechanistic considerations indicate that these reactions, along with hydrogenations and, presumably, oxygenations, take place on the coordinately unsaturated comer atoms present on the surface of these dispersed metal catalysts.

136

2

s c heme

9

R,NH+

14 Side

Vlew

11 '

Ill

HZ

uo E

H

2

18

16

ACKNOWLEDGEMENT

This research was supported by Grant DE-FG02-84ER45120 from the U.S. Department of Energy, Office of Basic Energy Science. The metal salts were obtained through the JohnsonMatthey Precious Metal Loan Program. REFERENCES 1 J.P. Collmann, L.S. Hegedus, J.R. Norton and R.G. Finke, Principles and Applications of Organotransition Metal Chemistry, University Science Books, Mill Valley, CA, 1987. 2 C.M. Andersen, A. Hallberg and G.C. Daves, J. Org. Chem., 52 (1987) 3529. 3 C.M. Andersen and A. Hallberg, J. Org. Chem., 53 (1988) 235. 4 D.L. Bergbreitner and B. Chen, J. Chem. Soc., Chem. Commun. (1983) 1238. 5 R.F. Heck, Acc. Chem. Res., 12 (1979) 146. 6 A. Bright, J.F. Malone, J.K. Nicholson, J. Powell and P.L. Shaw, J. Chem. SOC.,Chem. Commun. (1971) 712. 7 J.H. Sinfelt, J.L. Carter and D.J.C. Yates, J. Catal., 24 (1972) 283. 8 P.S. Kirlin and B.C. Gates, Nature (London), 325 (1987) 38. 9 D.W. Goodman, Chem. Ind. (Dekker) 22 (Catal. Org. React.) (1985) 171. 10 R.L. Augustine and M.M. Thompson, J. Org. Chem., 52 (1987) 1911. 11 M.J. Ledoux, J. Catal., 70 (1981) 375. 12 R.L. Augustine and P.J. O'Hagan, Chem. Ind. (Dekker) 40 (Catal. Org. React.) (1989) 11 1. 13 R.L. Augustine and R.W. Warner, J. Catal., 80 (1983) 358. 14 J.Y. Saillard and R. Hoffmann, J. Am. Chem. Soc., 106 (1984) 2006. 15 S.S. Sung and R. Hoffmann, J. Am. Chem. SOC.,105 (1985) 578. 16 J. Silvestri and R. Hoffmann, Langmiur, 1 (1985) 621. 17 R.S. Drago, Physical Methods in Inorganic Chemistry, Saunders NY 1977.

M. Guisnet et al. (Editors ), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

SURFACE

ORGANOMETALLIC

CHEMISTRY

ON

METALS:

137

SELECTIVE

HYDROGENATION OF CITRAL INTO GERANIOL AND NEROL ON TIN MODIFIED SILICA SUPPORTED RHODIUM.

B. DIDILLON*, A~ EL MANSOUR**, J.P. CANDY*, J.P. BOURNONVILLE*** 2nd J.M. BASSET I.R.C. 69626 Villeurbanne Cedex, France ** Universiti? Mohamed V, Facult6 des Sciences de Rabat, Maroc *** I.F.P. 92506 Rueil-Malmaison Cedex, France ABSTRACT

A bi-metallic Rh-Sn(n-C4Hg)2/SiO2 catalyst, obtained by the organometallic route, has been found to be extremely active and selective in the hydrogenation of citral ( A : geranial and b: neral) to the corresponding unsaturated alcohols (geraniol and nerol). The synthesis of the Ilbimetallic catalyst" from the organometallic precursor as well as the kinetics of hydrogenation is described. A tentative explanation for the extremely high chemoselectivity (96% at 100% conversion) for the hydrogenation of the C=O double bond is given. INTRODUCTION

Surface organometallic chemistry on metals is a new method to obtain well defined bimetallic catalysts (1). For example, the reaction of tetra-n-butyl tin with the surface of group VIII metals leads to bimetallic catalysts which exhibit very high selectivities and activities for the hydrogenolysis of ethyl acetate into ethanol (2-4). However the high temperature treatment of the solids obtained by this way totally removes the butyl groups and the catalytic active phase is a new bimetallic material, which is very likely an alloy. Careful studies of the reaction between tetra-n-butyl tin and silica supported group VIII metal M ( M = R h ( ' ) , Ru(O) or Ni(')) catalysts (5), indicate that the reaction proceeds stepwise via a surface intermediate complex which can be formulated as MaSn(C4Hg)x (1<x<3, a>l). To our knowledge, such surface organometallic complex was never mentioned or even characterized. Consequently, no catalytic studies have ever been carried out on metallic surfaces modified on DurDose by a grafted organometallic fragment In this work, we report the catalytic activities and selectivities of Rh,Sr~(cqHg)~ in the selective hydrogenation of citral. Citral is a member of the alpha-beta unsaturated aldehydes family; It offers three kinds of unsaturations: (i) an aldehydic

138

function, a conjugated double bond and an isolated double bond. Rhodium or platinum supported on silica are totaly unselective for the hydrogenation of citral to unsaturated alcohols (6-12). EXPERIMENTAL The silica support (small pellets, 1 nun in diameter, surface area of 250 m2.g-l) was purchased from SHELL. Rhodium was grafted on silica by cationic exchange between [RhC1(NH3)5I2+ ions and surface GSi-O'NH4' groups at pH 10. The SSi-O-NH4+ groups were obtained by exchange between fSi-0-H+ groups and NH4' ions in ammonia solution. [RhCl(NH3)5](0H)2 was obtained by contact of [RhC1(NH3)5]C12 with an anionic exchange resin (IRA 400) in aqueous solution. The surface complex obtained by this route was decomposed by calcination at 570K in flowing dry air and then reduced in flowing hydrogen at 570K to give catalyst A. This catalyst contains 1 wt% of rhodium. A could be transformed into B by treatment with dry air at 300K for 1 hour. The particle size of B was in the range of 1.0-1.5 nm as determined by electron microscopy. Hydrogen-oxygen interaction on B indicates (13) that reduction of B occurs at 300K and could be complete at 400K. The reaction between tetra-n-butyl tin [Sn(n-C4H9)4] and B is achieved in the liquid phase, directly in the reaction vessel (stainless steel autoclave, well stirred by a magnet). A given amount of B (typically 250 mg, 2.5.10-5 no1 of Rh) is placed, under argon, in the autoclave, with 10 ml of n-heptane and x ml of Sn(n-C4H9)4. The slurry is then stirred under 6 MPa of hydrogen, at 370K, during 60 mn, to obtain catalyst C. The reduction of citral is performed in situ, in the same autoclave, without any exposure of the catalyst to air. After cooling down the reactor to room temperature and reducing the hydrogen pressure, a solution of 0.9 nl of citral and 0.4 nl of tetradecane (internal standard) in 10 ml of n-heptane is introduced under hydrogen in the autoclave. The temperature and the hydrogen pressure are then raised to respectively 340K and 7.6 MPa. The kinetic of the reaction is followed with time by analysis of samples of the liquide phase. The selectivity for a product X at 100% conversion (S,) is defined by: Sx = [X]100/[Citra1]0. [Citral]~ represents the initial concentration of Citral ( 2 and E) and and [XI100 represents the concentration of X at 100% conversion.

139

RESULTS

a) Prevaration and characterization of the surface orsanometallic catalvst. Catalyst C is obtained by reaction of Sn(n-C4H9)4 with B in the liquid phase (n-heptane) under a hydrogen atmosphere (vide supra). The amount of tin fixed on the catalyst depends on the amount of Sn(n-C4H9)4 introduced (Table 1).

The maximum quantity of tin fixed corresponds to a Sn/Rh ratio of ca. 1. During this reaction n-butane is evolved. The amount of remaining butyl groups has been quantitatively measured by subsequent hydrogenolysis at 630K which leads to the formation of n-butane. The value obtained was found to be 2.1 C4/Sn fixed on the cat.alyst. The catalyst C seems to be best described by the formula: RhaSn(C4Hg)2/Si02. Such surface organometallic fragment has been characterized by M.A.S H' and 13C NMR (14) as well as volumetric measurements, electron microscopy and IR spectroscopy (15). It has been checked that during catalysis the butyl groups are not removed from the surface : after catalytic reaction a subsequent hydrogenolysis at 630K indicates that 1.93 C4 equivalent/Sn still remain on the catalyst. b) Citral hvdrosenation on catalvst C The overall reaction path for citral Z is represented in scheme 1. Typical kinetic measurements for citral (2 and E mixture 35/65) hydrogenation on catalyst C (Sn/Rh=l) are shown on figure la. In every case, a first order in citral is observed (figure lb), assuming that: d[citral]/dt = k[citral]. The constant (k) is directly proportional to the amount of catalyst (figure 2a) and to the hydrogen pressure PH2 (figure 2b). The apparent activation energy calculated from the slope of the Arrhenius plot in the temperature range 273-313K is 49 kJ.mol-l. The overall reaction rate can be represented by the following equation: d[citral]/dt

=

-K.e-49/RT[citral].

[c].pH2

(1)

140

2pi$j

[Citr.] (mol/l)

-Ln [Citr.] 1

3

0

1

2

3

4

5

0

1

2

3

time (h)

4

5

time (h)

Figure 1: Kinetics measurements of citral hydrogenation T=310K; RhlCitral=O.005; Hydrogen pressure:

(*I

3 MPa, (+) 2 MPa, (x) 1 MPa

1.0 0.8

0.6

(Rh/Cit r a I).10

Py(MPa)

Figure 2 Variation of the constant (k) versus: (a) RhlCitral ratio; T=310K; Hydrogen pressure=5 MPa (b) Hydrogen pressure; T=310K; RhlCitral=0.005

Sn /Rh Figure 3: Selectivity and activity of citral hydrogenation, versus SnlRh ratio Hydrogen pressure=7.6 MPa, T=350K. RhlCitral=0.005

141

Scheme 1 trans DIMETHYL-3.7

OCTENE-2 AL

DIMETHYL-3.7 OCTANAL

-A+B

/

CITRAL TRANS

A

-

roH-B+C trans DIMETHYL-3.7

OCTENE-2 OL

CITRONELIAL

/

DIMETHYL-3.7 OCTANOL

CITRONELLOL

The chemoselectivity of the reaction depends on the Sn/Rh ratio (Table 2 and Figure 3).

The selectivity for citronella1 increases up to a value of (at 100% conv.) for a Sn/Rh ratio of 0.12. Above these values, the selectivity for citronella1 decreases and the selectivity for geraniol and nerol increases up to 96% (at 100% conv.) for a Sn/Rh ratio of 0.92. Significant variations of activities are simultaneously observed suggesting selective metallic surface poisoning followed by enhanced catalytic activity due to a new catalytic material which contains a tin din-butyl fragment. It is possible to transform this kind of organometallic catalyst in an bimetallic rhodium-tin alloy by treatment under 81%

142

hydrogen at 630K ( 3 ) . Citral hydrogenation on this catalyst is totally unselective (Table 2) It is interesting to discuss those results in the light of the various hypothesese which were previously proposed for the selective hydrogenation of alpha-beta unsaturated aldehydes. It is known that Pt-Sn, Pt-Fe, Pt-Ru, Pt-Co or Pt-Ge supported on nylon (7-9) or carbon (10-11) give for certain compositions high yields of unsaturated alcohols. In addition to a geometric effect of the second metal, which prevents hydrogenolysis by decreasing (16), two hypothesese based on a the size of the Pt @@ensembles@@ possible electronic effect of the second metal are proposed to explain the high selectivities and activities. In the case of PtFe (10), the authors suggest that the two metals are in the metallic state and belong to an alloy. They observe, by EXAFS, an electron transfer from Fe to Pt (12) and suggest that the aldehydic double bond is then more easily adsorbed on the Ilinduced sites@@. In the case of Pt-Sn and Pt-Ge (7-9), the authors propose that the Snn+ or Gen+ ions (Lewis acids) activate the carbonyl group by enhancing the positive charge of the C=O carbon atom. The effect of electrophiles on the lone pair of electrons of a carbonyl group is a well known process in coordination chemistry (18); Electrophiles are known to increase the rate of CO insertion in metal alkyl bonds. In the hydrogenation of alpha-beta unsaturated aldehydes they would favor the coordination of the multifunctionnal molecule by its aldehydic fragment. These ions would then be responsible for the increase of selectivity for the C=O hydrogenation In our case a similar explanation can be advanced. Preliminary IR data (15) indicate that CO can be chemisorbed on catalyst C (RhSn(C4H9)2/SiO2) in a linear and bridged manner, suggesting that the dialkyl tin fragment and CO could be adsorbed on rhodium in a close vicinity. It is also reasonnable to assume that tin is in the divalent oxidation state. In this case, the mechanism of the citral hydrogenation could be the following:

143

In conclusion this paper reports, for the first time, that an organometallic fragment grafted on the surface of a metal particle can render the metallic catalyst selective for a given reaction The implications in catalysis can be very wide since it could be possible to change the organometallic fragment at will for a given reaction as it is currently done in homogeneous catalysis.

.

REFERENCES Y.A. Ryndin and Y.I. Yermakov in Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis, NATO AS1 Series, J.M. Basset et al. (Edit.), Kluwer (Publ.), 1987,pp 127-1 55 O.A. Ferretti, L.C. Bettega de Pauli, J.P. Candy, G. Mabilon and J.P. Candy, in Preparation of Catalysts IV, B. Delmon et al. (Edit.), Elsevier (Publ.), 1987 pp 713-723 (3) J.P. Candy, O.A. Ferretti, G. Mabilon, J.P. Bournonville, A. El Mansour, J.M. Basset and G. Martino, J. Catal., 112,210 (1988) (4) P. Louessard, J.P. Candy, J.P. Bournonville and J.M. basset, in Structure and Reactivity of Surfaces, C. Morterra, A. Zechina and G. Costa (Edit.), Elsevier (Publ.), 1989, pp 591-600 M. Agnelli, P. Louessard, A. El Mansour, J.P. Candy, J.P. Bournonville and J.M. Basset, Catalysis Today, 5,63 (1989) P.N. Rylander, Catalytic Hydrogenation in Organic Syntheses, Academic Press, New York, 1979, pp 72-80 S. Galvagno, 2. Poltarzewski, A. Donato, G. Neri and R. Pietropaolo, J. Chem. SOC., Chem. Comm., 1729 (1986) 2 . Poltarzewski, S. Galvagno, R. Pietropaolo and P. Saiti, J. Catal., 190 (1986) S. Galvagno, 2. Poltarzewski, A. Donato, G. Neri and R. Pietropaolo, J. Mol. Cat., 35, 365 (1986) D. Goupil, P. Fouilloux and R. Maurel, React. Kinet. Catal. Lett., 35, 185 ( 1 987) P. Fouilloux, in "Heterogeneous Catalysis and Fine Chemicals", M. Guisnet e t al. (Edit.), Elsevier Science, Amsterdam (Publ.), 1988, pp 123-129. 6. Moraweck, P. Bondot, D. Goupil, P. Fouilloux and A.J. Renouprez, Journal de Physique, 48,297 (1987) J.P. Candy, O.A. Ferretti, G. Mabilon, J.P. Bournonville, A. El Mansour, J.M. Basset and G. Martino, J. Catal 112,201 (1988) B. Didillon, P. Lesage, J.P. Candy, J.P. Bournonville and J.M. Basset, to be published B. Didillon, J.P. Candy, J.M. Basset, F. Lepeltier and J.P. Bournonville, to be published in Preparation of Catalysts V. M. Ichikawa, A.J. Lang, D.F. Shriver and W.H. Sachtler, J. Amer. Chem. SOC.107,7216 (1985) A. El Mansour, J.P. Candy, J.P. Bournonville, O.A. Ferretti, G. Mabilon and J.M. Basset. Angewandte Chemie, 361 (1989) Angew. Chem. Int. Engl. 28, 347 (1989) (18) F. Correa, R.Nakamura, R.E. Stimson, R.L. Burwell Jr. and D.F. Schriver, J. Am. Chem. SOC.lJ22, 5112 (1980)

m,

m,

This Page Intentionally Left Blank

M . Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

145

SELECTIVE HYDROGENATION OF UNSATURATED ALDEHYDES OVER ZEOLITESUPPORTED METALS

D.G. BLACKMOND,' A. WAGHRAY,' R. OUKACI,', B. BLANC,' and P. GALLEZOT' 'Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261 (USA) *Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex (FRANCE)

SUMMARY The hydrogenation of 3-methyl crotonaldehyde was investigated over Ru supported on NaY and KY zeolites in both liquid- and gas-phase reactions. Significant effects of the nature of the support on the product selectivity were observed. It was suggested that increased basicity of the zeolite resulted in increased selectivity towards the unsaturated alcohol product. INTRODUCTION Selectivity in the hydrogenation of =,p-unsaturated aldehydes has become an important topic in heterogeneous catalysis (refs. 1-4). Unsaturated alcohols, important in the synthesis of fine chemicals, may be produced selectively over certain supported group VIII-metal catalysts (refs. 5,6), but the general problem of the selective intramolecular hydrogenation of carbonyl groups remains a challenging task. Studies by this group have suggested (ref. 7) that an electronic interaction between a functionalized graphite support and small Pt or Ru particles enhances the selectivity of these metals towards the production of unsaturated alcohols, the desired product. Comparison with results f o r these metals on a carbon support demonstrated the significance of the support in this electronic effect. This paper discusses an extension of those studies to investigate the selectivity of Ru supported on NaY and Kexchanged NaY zeolites in the hydrogenation of 3-methyl

146

crotonaldehyde. Support effects have been observed for similar Ru/Y catalysts in the Fischer-Tropsch synthesis (ref. 8 ) , where modifications in the zeolite resulted in changes in catalyst activity for hydrogenation of olefins formed as primary products. EXPERIMENTAL Supported Ru catalysts were prepared using NaY zeolite (Strem Chemicals) as a support. The Ru/NaY catalyst was prepared using the zeolite directly as received. Potassium-exchanged NaY (KY) was prepared by ion-exchange of the NaY zeolite with potassium nitrate (Alpha Products, ultrapure) (refs. 10,ll). Ru-loaded catalysts were then prepared by ion-exchange of the two zeolite supports with Ru(NH,),Cl, (Strem Chemicals) to a nominal weight loading of 3% as described in detail in (9,ll). Samples were pretreated by heating in hydrogen at 75 cc/min to 573 K at 0.5 K/min and holding there for two hours. Samples were passivated and stored in air until use. Crystallite size was examined by TEM (JEOL 100 CX equipped with high-resolution pole pieces). The high pressure, liquid-phase hydrogenation of 3-methyl crotonaldehyde was carried out in a well-stirred batch autoclave under 4 MPa H, (Air Liquide, 99.995% purity) pressure using 0.1 mol of 3-methyl crotonaldehyde (UAL) (Merck) and 0.6 g catalyst. Isopropanol (37.5 cc) was used as a solvent. The catalyst was activated by stirring under 4 MPa H, pressure at 373K for two unsaturated aldehyde UAL hours prior to introduction of the reactant at the same temperature. The reaction products were monitored by repetitive sampling and gas chromatographic analysis. Since this was a batch reaction, data are reported as selectivity vs. conversion. Time of reaction to reach about 30% conversion was close to 6 0 minutes for Ru/NaY and 150 minutes for Ru/KY. Gas phase studies were performed at 0.1 MPa total pressure with a flow rate of 100 cc/min H, and 100 cc/min He (Linde). The helium stream was diverted through a saturator containing the liquid reactant UAL (Aldrich) such that its volume fraction in the inlet to the reactor was 0.5%. The gas hourly space velocity was 120 lh'lg'' catalyst. Prior to reaction, the catalyst was pretreated at 673 K for at least 2 hours following a 1 K/min rise to the reduction temperature. The reactor was cooled under

147

flowing hydrogen to 313 K prior to introduction of the UALsaturated He stream. Reaction products were analyzed by repetitive sampling and chromatographic analysis. After one hour on stream, the catalysts reached a steady state of about 1-3% conversion of UAL to three products, the unsaturated alcohol (UOL), the saturated alcohol (SOL), and the saturated aldehyde (SAL)

.

RESULTS AND DISCUSSION Characterization of the Ru/KY and Ru/NaY catalysts used in this study by transmission electron microscopy showed that the metal was well-dispersed within the zeolite as particles small enough to fit inside the supercages, less than 1 nm in diameter. Figure 1 compares the selectivities of hydrogenation products as a function of UAL conversion for liquid-phase reaction over the two RU catalysts. Both catalysts produced significant amounts of SAL. However, selectivity towards UOL, the desired product, increased threefold for the K-exchanged catalyst compared to Ru/NaY, Results of the continuous flow gas-phase reaction of UAL over the two Ru zeolite catalysts are given in Figure 2 , where the product selectivity is plotted against time of reaction. In this reaction mode the differences in product selectivity between the two catalysts were even more striking than those observed in the liquid-phase reactions described above. Once again the K-exchanged catalyst offered higher selectivity towards UOL compared to Ru/NaY. In fact, UOL was the major product for Ru/KY, surpassing SAL in contrast to the results for the liquid-phase reaction on these catalysts. Modification of the zeolite appears to have affected the selectivity of Ru in these hydrogenation reactions. Exchange of K cations for Na cations in Y zeolite increases the basicity of the support (ref. 9). In Fischer-Tropsch reactions over similar catalysts, Ru/Y catalysts so modified yielded significant increases in -the olefinic product fraction at the expense of paraffins. Olefins are believed to be primary products in F-T synthesis, with paraffins being produced from olefins in secondary hydrogenation reactions. In an analogous fashion, the Ru/KY catalyst used in the present study might also be expected to

148

inhibit intramolecular C=C hydrogenation reactions in molecules such as 3-methyl crotonaldehyde. This was indeed the case for both gas- and liquid-phase reactions, as can be seen from Figures 1 and 2. In accordance with interpretation of the F-T data, the increased basicity of the KY compared to Nay, possibly resulted in a transfer of charge from the support to the metal which in turn decreased the capability of C=C hydrogenation. Similar electronic effects were invoked in previous work by this group (7) to explain high selectivity of cinnamaldehyde to cinnamyl alcohol over Ru supported on functionalized graphite. A promoting effect on both the activity and selectivity for the production of unsaturated alcohol was also observed previously by this group (refs. 13,14) for hydrogenation of cinnamaldehyde over Pt-Fe catalysts. It was suggested that a dual-site mechanism operated in the bimetallic system whereby cationic Fe electron acceptor species preferentially activated the C=O bond while hydrogenation of the reduced Pt sites provided hydrogen for aldehyde to the alcohol. A similar mechanism might explain the increased UOL formation over Ru/KY. If the small crystallites of Ru are located within the zeolite supercages in close proximity to the K' neutralizing cations, the result may be a system with both metallic sites to provide hydrogen and a cationic site to activate Interactions between CO and alkali species whereby the CO bond. a direct K--O=C interaction causes weakening of the C=O bond have been suggested for alkali-promoted single crystal (refs. 15,16) and supported metal (refs. 17-19) systems. Recent gas-phase studies of crotonaldehyde hydrogenation over Pt/TiO, by Vannice and Sen (ref. 20) suggested a similar mechanism for TiO, moieties interacting with CO on Pt. The reaction network for hydrogenation of the unsaturated aldehyde crotonaldehyde was studied extensively by Simonik and They observed isomerization of the unsaturated Beranek ( 2 0 ) . alcohol to the saturated aldehyde at a reaction temperature of 433 K:

149

80

. A

-D

SAL

-+SOL

-

4ucc

91

Q u)

V

T

.

0

1

10

.

I

20

-

I

30

-

I

40

’

I

513

Conversion (“A)

. B 60

Q

SAL

+see 4 L K x

Figure 1. Product selectivities as a function of conversion in liquid-phase hydrogenation of 3-methyl crotonaldehyde over A ) Ru/NaY: and B) Ru/KY. UOL = unsaturated alcohol: SOL = saturated alcohol: SAL = saturated aldehyde. Pressure = 4 MPa; Temperature = 373K.

150

0

I

I

I

I

I

60

100

160

200

260

300

Tlrne (rnlna) too

13

10 -

a

ro

A A

90

-

-

M

UOL SOL SAL

Figure 2. Product selectivities as a function of time in gas-phase hydrogenation of 3-methyl crotonaldehyde over A ) Ru/NaY; and B) Ru/KY. UOL unsaturated alcohol; SOL = saturated alcohol; S A L = saturated aldehyde. Pressure = 0.1 MPa, Temperature = 313K.

-

151

In the present study, liquid-phase reactions over both NaY and KY showed significant production of SAL compared to the gas-phase studies. The higher temperature of the liquid-phase work might account for this increased SAL production through UOL isomerization as shown above. Another possibly important factor is the residence time of the products in the batch liquid-phase reactions. In the continuous flow mode, products may not be in contact with the catalyst long enough to undergo significant secondary isomerization reactions, a limitation which does not exist in batch reactions. Further work is underway in both liquid- and gas-phase hydrogenation studies aimed at achieving a better understanding of both the kinetics of the reaction network and the role of the zeolite support in altering the product selectivity of the metal. CONCLUSIONS Preliminary results for Nay- and KY-supported Ru catalysts demonstrate significant effects of the nature of the zeolite cation for the selectivity of 3-methyl crotonaldehyde hydrogenation. It was suggested that increased basicity of the zeolite resulted in increased selectivity toward the unsaturated alcohol product. These results agree with earlier suggestions that the nature of the support can have significant influence on the product distribution in the hydrogenation of -,p-unsaturated aldehydes. ACKNOWLEDGMENTS Support from NATO Scientific Affairs Division (Brussels, Belgium), the Centre International des Etudiants et Stagiaires (C.I.E.S., Paris, France) and the U.S. National Science Foundation (Presidential Young Investigator Program, CBT-8552656) is gratefully acknowledged.

152

REFERENCES

9

P.N. Rylander, IIHydrogenation MethodsI1l Academic Press, London, 1985. J.E. Germain, I1Catalytic Conversion of Hydrocarbons,I1 Academic Press, New York, 1969. R.L. Augustine, Catal. Rev. Sci.-Eng., u,285 (1976). R.L. Augustine, in IIAdvances in CatalysisI1l (D.D. Eley, P.W. Selwood, and P.B. Weisz, Eds.), V o l . 25, p. 56, Academic Press, New York, 1976. M.L. Khidekel, A.S. Bakhanov, A.S. Astakhova, K.A. Brikenshtein, V.I. Savchenko, I.S. Monakhova, and V.G. Dorokov, Izv. Akad. Nauk. SSR, Ser. Chem. (1970), p. 499. P.N. Rylander, and D.R. Steele, Tetr. Lett., 1579 (1969). A . Giroir-Fendler, D. Richard, and P. Gallezot, in tlHeterogeneousCatalysis and Fine Chemicals,1o (M. Guisnet et al., Eds.), p.171, Elsevier, Amsterdam, 1988. F.A.P. Cavalcanti, D.G. Blackmond, R . Oukaci, A. Sayari, A. Erdem-Senatalar, and I. Wender, J. Catal. , 113,1 (1988). Y.W. Chen, H.T. Wang, and J.G. Goodwin, Jr., J. Catal., B,

10

R. Oukaci,

1 2 3 4 5 6 7 8

499 (1984). A.

Sayari, and J.G. Goodwin,

Jr., J. Catal.,

102, 126 (1985). 11 12 13 14

15 16 17 18 19 20 21

J.Z. Shyu, E.T. Skopinski, A . Sayari, and J.G. Goodwin, Jr., Appl. Surf. Sci., a,297 (1985). Jacobs et al. JCS Far. 1 7 4 , 403 (1980). D. Richard, J. Ockelford, A. Giroir-Fendler, and P. Gallezot, submitted to Catal. Lett. D. Richard, P.Fouilloux, and P. Gallezot, in IICatalysis: Theory to Practice,Il Proc. 9th Intl. Congr. Catal., (M. J. Phillips and M. Ternan, Eds.), Vol. 3, p. 1074, The Chemical Institute of Canada, Ottawa, 1988. D. Lackey, M. Surman, Jacobs, S . Grider, D. and D.A. King, Surf. Sci. , 152-153, 513 (1985). K.J. Uram, L. Ng, and J.T. Yates, Jr., Surf. Sci., 177, 253

.

(1986) S. Kesraoui, R. Oukaci, and D.G. Blackmond, J. Catal., 432 (1987).

105,

P.A.J. Angevaare, H.A.C. Hendrickx, and V. Ponec, J. Catal. , 110, 11 (1988). P.A.J. Angevaare, H.A.C. Hendrickx, and V. Ponec, J. Catal. , 110, 18 (1988). M . A . Vannice, and B. Sen, J. Catal., 115, 65 (1989). J. Simonik, and L. Beranek, Collection Czechoslov. Comm.,

z, 353

(1972).

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

153

THE : lECHANISM OF HYOROGENOLYSIS AN0 I S O M E R I Z A T I O N OF OXACYCLOALKANES ON METALS, PART X? NATURE OF THE A C T I V E S I T E S I N THE REGIOSELECTIVE HYDROGENATION OF OXIRANES F. NOTHEISZ’,

A.G.

ZSIGMOND1,

M. BARTOKI, 0. OSTGARO 2 and G.V.

SMITH2

‘Oepartment o f Organic Chemistry, A t t i l a Jozsef U n i v e r s i t y , 06m t e r 8, H-6720 Szeged (Hungary) 20epartment o f Chemistry and Biochemistry and Molecular Science Program, Southern I l l i n o i s U n i v e r s i t y , Carbondale, I L 62901 (USA)

SUMMARY The hydrogenolysis and i s o m e r i z a t i o n of methyloxirane were s t u d i e d over various P t c a t a l y s t s i n order t o determine t h e number and nature of the act i v e s i t e s . The steps were found t o be t h e probable a c t i v e s i t e s and the transformation i s s t r u c t u r e - s e n s i t i v e . The r e g i o s e l e c t i v i t y is n o t a f f e c t e d by v a r i a t i o n i n the c a t a l y s t s t r u c t u r e , so i t i s determined by t h e nature of t h e metal.

INTRODUCTION

It are

i s w e l l known t h a t n o t a l l o f the exposed atoms of a metal

catalytically

tual

active

crystallite

a c t i v e . Establishment of the number and nature o f t h e

s i t e s i s an important step i n the determination of the

ac-

ccmplete

of surface r e a c t i o n s and i n p r e d i c t i o n and p r e p a r a t i o n of nore ac-

riiechanisms

t i v e and s e l e c t i v e metal c a t a l y s t s . Outing

studies

cycloalkanes

o f the hydrogenolysis and i s o m e r i z a t i o n o f

the

2-Me-oxa-

on t r a n s i t i o n metal c a t a l y s t s , i t was found t h a t d i f f e r e n t

met-

a l s have d i f f e r e n t r e g i o s e l e c t i v i t i e s ( r e f s 1,2). On Cu and N i c a t a l y s t s , p r i marily mation lysts

the C-0 bond adjacent t o the s u b s t i t u e n t is s p l i t , l e a d i n g t o t h e for-

of

a primary alcohol or aldehyde ( r e f . 3 ) , w h i l e on P t and

secondary a l c o h o l or ketone (Scheme

MeC(0)Me + MeCH(0H)Me

1).

mMe

+!!!k a

a ’ g b

Scheme 1 8

Pd

cata-

mainly t h e more d i s t a n t C-0 bond undergoes cleavage (ref. 4 ) y i e l d i n g a

Part I X . :

ref. 6

+PrOH

+ EtCHO

154

An explanation o f why the r e g i o s e l e c t i v i t y on Cu and N i i s d i f f e r e n t

from

on P t or Pd demands a knowledge o f the mechanism o f the r e a c t i o n . On

that and

Pd

c a t a l y s t s , a C-0 bond i n the oxiranes undergoes cleavage through

participation of site

chemisorbed hydrogen; the r e g i o s e l e c t i v i t y i s

governed

f a c t o r s . On N i and Cu, i o n i c i n s e r t i o n takes p l a c e and

stereochemical

regioselectivity

i s observed. Through deoxidation, o x i r a n e i s

Pt the by

oppo-

able

to

the Cu and N i surfaces, r e s u l t i n g i n i o n i c s u r f a c e s i t e s (Lewis acid-

oxidize

base s i t e s ) . I n t h e cases o f P t and Pd, the oxide formed i n t h i s way i s r a p i d l y reduced i n t h e presence o f hydrogen. results

Our

explained

by

show t h a t the observed d i f f e r e n c e i n r e g i o s e l e c t i v i t y can

regioselectivity

is

determined

oxygen,

by t h e n a t u r e o f the metal.

However,

other

r e g i o s e l e c t i v e r i n g opening on P t c a t a l y s t s ? This paper r e p o r t s

the

be the

responsible

metal by v a r y i n g i t s s t r u c t u r e ? What k i n d of s u r f a c e s i t e s are

for

i.e.

questions a r i s e : Is i t p o s s i b l e t o a f f e c t t h e r e g i o s e l e c t i v i t y o f

interesting a

d i f f e r e n t a f f i n i t i e s of t h e metals f o r

the

new

r e s u l t s concerning the above questions. EXPERIMENTAL Methods The

investigations o f 175 cm3

volume

. The

were performed i n a closed c i r c u l a t i o n r e a c t o r with a 3 volume o f the r e a c t a n t sample was 0.3 cm The c a r r i e r

.

was helium, f r e e d from oxygen with an A l l t e c h Oxy-Trap. The hydrogen used

gas in

the

measurements was produced by a Matheson 8326

equipped

electrolysis

apparatus

with a Pd d i f f u s i o n c e l l . 2-5 mg c a t a l y s t samples were used. D e t a i l s

on the experimental procedure were r e p o r t e d e a r l i e r ( r e f s . 5,6). Catalysts 0.48%, 0.83%, 1.17%, and 1.91% Pt/Si02 c a t a l y s t s o r i g i n a t e d from

The

laboratories o f pregnating

Burwell and B u t t ( r e f . 7). They were prepared e i t h e r by

H2PtC16

or by ion-exchanging Pt(NH3)4C12 on Davison s i l i c a

o f exposed P t atoms was determined by hydrogen

the im-

gel.

chemisorption

The

percentage

and

H2-02 t i t r a t i o n . The 0.13% Pt/Si02 c a t a l y s t was prepared i n our l a b o r a t o -

ry

from

platinum acetylacetonate ( r e f . 8). I t s d i s p e r s i o n was determined

CO

chemisorption and H2-02 t i t r a t i o n . A d d i t i o n a l l y , t h e p a r t i c l e s i z e s o f a l l

catalysts

were

examined

by e l e c t r o n microscopy on a H i t a c h i H

by

500H t r a n s -

mission e l e c t r o n microscope. For other d e t a i l s , see r e f s . 7,8. Analysis

A The 1.5m

Carlo

Erba Fractovap 2150 gas chromatograph was used f o r the

chromatographic

columns were 0.5 m 20% OOPN/Chromosorb

15% Reoplex 400/Chromosorb

W a t 293

analysis. K,

and

W a t 313 K.0ata were processed with a Perkin-

155

Elmer Sigma 10 integrator. Material The methyloxirane used in this study was a product of BOH (GC purity: 9 9 % ) ; prior to use, it was double distilled. RESULTS AN0 DISCUSSION Metal surfaces have a variety of sites which differ in the extent of coordinative unsaturation. The various sites are planes ( 1MI, steps (’M) and corners (3M) (ref. 9 ) . Much work has been done to correlate these surface sites with a set of interrelated reactions occurring simultaneously on the same surface (refs. 8,10,11). In order to determine the active sites in the transformations of methyloxirane, we studied its hydrogenation and isomerization on a set of Pt/Si02 catalysts with different dispersions. The results can be seen in Table 1. (Data were obtained at a hydrogen pressure of 33.3 kPa. The partial pressure of methyloxirane was 1.6 kPa.) TABLE 1 Turnover frequencies in transformations of methyloxirane and correlation between the fraction of active sites and the rate of transformation on Pt/Si02 catalysts. Loading Oispersion

(%I

(%)

1.91 1.17 0.48

7.1 40.7 62.1

0.28

0.83 0.13

80.9 100.0

0.22 0.04

MP Ac 2P

IP Na

= =

= = =

Ns =

MP 0.17

1.52

Ac Turnover 2Pfrequencies 1P (s-l) Total

10.10 1.54

1.23 5.22 11.70 2.36

0.19

0.18

0.88 2.86

0.19

0.37 0.60

0.19 0.03

2.47 8.73 23.90 4.31 0.44

N,”S

0.09 0.20

0.34 0.14 0.04

turnover frequency of the minor products. turnover frequency of acetone. turnover frequency of 2-propanol. turnover frequency of 1-propanol. number of active sites, determined by carbon disulfide poisoning in alkene hydrogenation. number of surface sites, determined by chemisorption methods.

The data in Table 1 show that the transformation of methyloxirane is a structure-sensitive reaction, since the total turnover frequency (TOF) in the transformation of methyloxirane varies appreciably with increasing dispersion. The rate of transformation passes through a maximum as a function of dispersion. Since the theoretical calculations of van Hardeveld and Hartog (ref. 12) indicate that the number of step sites changes via a maximum curve a? a function of the dispersion, this maximum curve may be an indication that

156

the steps are the a c t i v e s i t e s i n t h i s r e a c t i o n . found ( r e f . 5 ) t h a t t h e hydrogenolysis o f methyloxirane on

We e a r l i e r

Pt

c a t a l y s t s takes place v i a an a s s o c i a t i v e mechanism (Scheme 2). Me CH M e

Scheme 2 mechanism i n v o l v e s an a s s o c i a t i v e a d s o r p t i o n o f the oxirane; the r a t e -

The

step i s t h e r i n g opening, with t h e p a r t i c i p a t i o n o f hydrogen. The

determining following

steps

acetone.

are

relatively

I n deuterium, acetone-dl

rapid, resulting i n

secondary

was the main product, t h a t

is

or

alcohol

intramole-

H-migration does n o t p l a y a s i g n i f i c a n t role i n acetone formation.

cular

reduction

of

acetone t o 2-propanol does n o t take p l a c e as l o n g as

The

unreduced

methyloxirane i s present i n t h e m i x t u r e . The observed i n v e r s i o n o f t h e c o n f i g u r a t i o n is i n good accordance w i t h t h e p a r t i c i p a t i o n o f hydrogen i n t h e t r a n s i t i o n complex ( r e f . 1 3 ) . This two

mechanism can proceed e a s i l y on t h e s t e p s i t e s . Since t h e steps

c o o r d i n a t i v e unsaturations, they are a b l e t o adsorb hydrogen and

have

methyl-

oxirane simultaneously. The

same

Pt/Si02

c a t a l y s t s were poisoned by CS2;

the

hydrogenation

(+)-apopinene was used as an i n d i c a t o r r e a c t i o n ( r e f . 1 4 ) . The amount o f necessary t o e l i m i n a t e the hydrogenation a c t i v i t y p e r m i t s c a l c u l a t i o n o f fraction between in

CS2 the

corre-

t h e number o f s t e p s i t e s . S i m i l a r l y , a good c o r r e l a t i o n i s

found

t h i s f r a c t i o n and t h e r a t e o f methyloxirane t r a n s f o r m a t i o n (Table 1).

results

These

tion

o f metal s i t e s a c t i v e i n o l e f i n hydrogenation. This f r a c t i o n

with

lates

of

r e v e a l t h a t t h e s t r u c t u r e - s e n s i t i v i t y i s caused by t h e

the

number o f a c t i v e s i t e s , and t h e steps appear t o be

the

variaactive

s i t e s f o r the r e g i o s e l e c t i v e hydrogenation o f methyloxirane. Burwell propane

and coworkers ( r e f . 15) s t u d i e d t h e t r a n s f o r m a t i o n o f methylcyclo-

on t h e same series o f P t c a t a l y s t s , and found i t t o be m i l d l y

ture-sensitive. continuously

struc-

The TOF i n t h e hydrogenolysis of methylcyclopropane increased

as a f u n c t i o n o f the dispersion. The t o t a l TOF v a r i e d by a

two, w h i l e t h e a c t i v a t i o n energy of t h e r e a c t i o n was

independent

fac-

tor

of

the

percentage o f metal exposed.These f a c t s o f f e r e d a simple geometric expla-

of

157 nation of TOF.

t h e i r r e s u l t s : the more numerous the a c t i v e s i t e s , t h e h i g h e r

persion,

the

the numbers o f step and k i n k s i t e s increase with i n c r e a s i n g

Since

they

considered t h a t the hydrogenation o f methylcyclopropane

distakes

place on these s i t e s . In

s p i t e o f i t s close s i m i l a r i t y t o methylcyclopropane, methyloxirane

hibits sites

different are

behavior.

The observed maximum curve means t h a t

i n a c t i v e s i t e s i n t h e transformation of oxiranes. The

ex-

the

kink

reason

for

i n a c t i v i t y is n o t completely c l e a r , b u t i t i s very probable t h a t CO

the

poi-

soning i s responsible. The on

main minor product i s ethane. (The d i s t r i b u t i o n of t h e minor

0.48% Pt/Si02

catalyst:

CH4 = 2.6%, CO = 10.6%, C2H4 =

products

32.4%, C2H6

=

54.4%.) The minor products are produced by t h e decarbonylation o f methyloxirane,

but

is

sur-

only t h e hydrocarbons desorb, the CO remaining adsorbed on t h e

During the decarbonylation process, C-0 and C-C bond r u p t u r e s occur. I t

face.

w e l l known t h a t k i n k s i t e s are the a c t i v e s i t e s o f C-C hydrogenolysis,

SO

i t i s understandable t h a t decarbonylation w i l l poison t h e k i n k s i t e s . have a l s o s t u d i e d the change i n r e g i o s e l e c t i v i t y and t h e s e l e c t i v i t y o f

We

acetone

of

I t i s seen t h a t the r e g i o s e l e c t i v i t y does n o t depend on t h e d i s p e r s i o n :

data. on

formation as f u n c t i o n s o f the dispersion. Table 2 shows b o t h s e t s

all

c a t a l y s t s , the s t e r i c a l l y less hindered bond breaks. I n o t h e r

words,

r e g i o s e l e c t i v i t y i s n o t affected by the v a r i a t i o n i n t h e metal s t r u c t u r e .

the

This observation c o r r e l a t e s w e l l w i t h our former r e s u l t s : the d i f f e r e n t r e g i o selectivities nisms

are

due t o t h e d i f f e r e n t types o f mechanism, and these

mecha-

are governed by the d i f f e r e n t a f f i n i t i e s o f the metals f o r oxygen ( r e f s

3,4). TABLE 2 R e g i o s e l e c t i v i t y data f o r the transformation o f methyloxirane and s e l e c t i v i t y of acetone formation, Catalyst 1.91% P t / S i O 2 1.17% Pt/Si02 0.48% Pt/Si02 0.83% Pt/Si02

Hydrogen pressure (kPa 1

Regioselectivity

33.3.

0.92

33.3

0.96

1.8

1.8

33.3 1.8 33.3 1.8

1.00

1.00 0.97

1.00 0.95

Acetone selectivity

0.38 0.66 0.34 0.51 0.45 0.62

1.00

0.38 0.57

0.93

0.48 0.61

1.00 R e g i o s e l e c t i v i t y = TOFAc+TOF2p/TOFAc+TOF2p+TOFlp. S e l e c t i v i t y of acetone formation = TOFAc/TOFAc+TOF2p.

the

158

The

selectivity

of

acetone f o r m a t i o n e x h i b i t s a

curve

with

a

slight

minimum

character as a f u n c t i o n o f t h e d i s p e r s i o n ( F i g u r e 1, curve "a").

possible

e x p l a n a t i o n o f the minimum curve is r e l a t e d t o t h e mechanism.

the

two

main products (acetone and 2-propanol) have a

the

selectivity

(the

higher

prove at

common

intermediate,

is determined by t h e hydrogen a v a i l a b i l i t y on the s u r f a c e

the hydrogen pressure, t h e h i g h e r t h e a l c o h o l

selectivity).

the correctness o f t h i s explanation,we a l s o determined the availability,

the

a more c h a r a c t e r i s t i c change should be observed a t

lower

hydrogen pressure.) The r e s u l t s ( F i g u r e 1, curve "b") c o n f i r m t h e

idity

of

curve

"a".

the

the explanation: t h e minimum i n curve "b" is deeper, than

curves.

dispersion). others,

this

I n b o t h cases t h e r e is an experimental p o i n t which does

hythe val-

that

in

not

These are the s e l e c t i v i t y data on t h e most a c t i v e c a t a l y s t Since

To

selectivity

hydrogen pressure. ( I f t h i s s e l e c t i v i t y is determined by

lower

drogen

The Since

fit (62%

t h e r e a c t i o n is much f a s t e r on t h i s c a t a l y s t than on

leads

t o a hydrogen d e p l e t i o n on t h e surface, which

the

causes

a

higher 0x0 s e l e c t i v i t y .

SAC

0.7

: I

J

1.6 kPa

0.5

a

-

1

0.3

0.1

3 33.3 kPa

20

40

60dispersion 80 (%)

100

F i g . 1. The s e l e c t i v i t y o f acetone formation as a f u n c t i o n o f the d i s p e r s i o n . The sites surface

d i s s o c i a t i o n o f the hydrogen molecule i s thought t o occur on t h e (ref.

16) and t h e hydrogen atoms can m i g r a t e t o any o t h e r s i t e on

step the

( r e f . 17). I n other words t h e hydrogen a v a i l a b i l i t y on t h e s u r f a c e i n

connected w i t h the number o f s t e p s i t e s .

159

The experimental r e s u l t s p e r m i t the f o l l o w i n g conclusions: -The transformation of methyloxirane on various P t c a t a l y s t s i s a s t r u c ture-sensitive curve

reaction.

t o t a l TOF o f the r e a c t i o n e x h i b i t s

The

a

maximum

as a f u n c t i o n of dispersion. The s t r u c t u r e s e n s i t i v i t y i s caused by the

change i n the number o f a c t i v e s i t e s . -The

hydrogenolysis

and i s o m e r i z a t i o n o f methyloxirane take p l a c e

via

a

mechanism i n which hydrogen i s i n v o l v e d i n t h e rate-determining step. This rea c t i o n occurs on the step s i t e s . -The pendent The

regioselectivity

of

the transformation o f

methyloxirane

is

inde-

o f the c a t a l y s t s t r u c t u r e , b u t i t depends on t h e nature o f t h e metal.

selectivity

character

as

of acetone formation e x h i b i t s a curve with a s l i g h t minimum

a f u n c t i o n o f dispersion, since t h i s s e l e c t i v i t y i s

determined

by the hydrogen a v a i l a b i l i t y on the surface. REFERENCES

1 F . Notheisz, M. Bartok and A . G . Zsigmond, React. K i n e t . Catal. L e t t . , 29 (1985) 339-343. 2 G. Senechal a?d 0. Cornet, 81. SOC. Chim. France, (1971) 773-783. 3 F. Notheisz, A. Molnar, A.G. Zsigmond and M. Bartok, J. Catal., 98 (1986) 131-137. 4 F . Notheisz, A.G. Zsigmond, M. Bartok and G.V. Smith, J. Chem. SOC., Faraday Trans. l . , 83 (1987) 2359-2363. 5 M. Bartok, F. Notheisz, A.G. Zsigmond and G.V. Smith, J. Catal., 100 (1986) 39-44. 6 0. Ostgard, F . Notheisz, A.G. Zsigmond, G.V. Smith and M. Bartbk, J.Catal., (submitted f o r p u b l i c a t i o n ) . 7 T. Uchijima, J.M. Hermann, Y . Inoue, R.L. Burwell,Jr., J.B. Butt and J.B. Cohen, J. Catal., 50 (1977) 464-478. 8 F. Notheisz, M. Bartok, 0. Ostgard and G.V. Smith, J. Catal., 101 (1986) 212-217. 9 S. Siegel, J. Outlaw,Jr. and N. G a r t i , J. Catal., 52 (1978) 102-115. 10 M.J. Ledoux. Nouv. J . Chim.. 2 (1978) 9-15. 11 G.V. Smith, ' A . Molnar, M.M. 'Khan, 0.. Ostgard and T . Yoshida, J. Catal., 98 (1986) 502-512. 1 2 R. van Hardeveld and F. Hartog, Adv. Catal., 22 (1972) 75-113. 13 M.Bartdk, Stereochemistry of heterogeneous metal c a t a l y s i s , Wiley (1985) 14 G.V. S m i t h , F. Notheisz, A.G. Zsigmond, 0. Ostgard, T. Nishizawa and M. Bartok, i n : M.J. P h i l l i p s and M.Ternan (Eds), Proc. 9 t h I n t . Congr. Catal., Calgary, Canada, June 26-July 1, 1988, The Chem. I n s t . o f Canada, Ottawa, 1988, Vol. 4, pp 1066-1072. 15 P.H. Otero-Schipper, W.A. Wachter, J.B. B u t t , R.L. Burwel1,Jr. and J.B. Cohen, J. Catal., 50 (1977) 494-507. 16 G.A. Somorjai, Chemistry i n Two Dimensions: Surfaces, C o r n e l l U n i v e r s i t y Press, I t h a c a , N.Y., 1981. 17 K. Tanaka, Adv. Catal., 33 (1985) 99-158.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chernicak I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

161

CHEMO-, REGIO- AND STEREOSELECTIVITY IN STEROID HYDROGENATION WITH CdA1203. INTRA- AND INTERMOLECULAR HYDROGEN TRANSFER REACTIONS.

N.RAVASIO1, M.GARGANO1, V.P.QUATRARO1 and M.ROSS12 1 Centro C.N.R. sulle Metodologie Innovative di Sintesi Organiche, Dipartimento di Chimica dell'Universita, via Amendola 173, 701 26 BAR1 (Italy) 2 Centro C.N.R. e Dipartimento di Chimica lnorganica e Metallorganica, Universita di Milano, via Venezian 21, 20133 MILANO (Italy)

SUMMARY The hydrogen transfer from different secondary alcohols to a steroidic conjugated enone and a saturated ketone in the presence of Cu/A1203 has been investigated. The stereochemical pathway of the reaction has been studied and the results are compared with those obtained by conventional molecular hydrogen addition in the presence of the same catalyst. While chemo- and regioselectivity are essentially unaffected by the hydrogen source, the stereoselectivity of the hydrogen addition to the conjugated olefinic moiety depends upon the alcohol used as hydrogen donor, and 5p:5cr isomer ratios ranging between 48 and 85% were observed. In the second step of H2 addition, the reduction of the 3-keto-group, a strong effect of the donor molecule on the stereochemistry was observed moving from 2-propano, to 2-octanol. However, an excess of the equatorial alcohol was obtained in every case. INTRODUCTION Supported copper catalysts are widely used in industrial chemical processes for the hydrogenation of different compounds. Of great importance are the synthesis of methanol in the presence of CuO/ZnO/A1203 catalyst and hydrogenation of fat oxo-aldehydes to alcohols with mixed copperchromium oxides. On the other hand, the use of copper catalysts in laboratory scale hydrogenations is little known. We recently found that prereduced 7.5% Cu on Al2O3, easy to prepare by conventional techniques, can b e conveniently used under very mild conditions (60°C, 1 atrn of H2) for the selective hydrogenation of steroidic enones. Thus, Cu/A1203 allows the chemospecific reduction of a,p

162

unsaturated carbonyl groups when other saturated keto-groups or isolated olefinic bonds are also present in the moleculel. Moreover, the enone olefinic bond is reduced with total regioselectivity and AIB cis, 5P-3-0XO derivatives are produced with stereoselectivity values up to 89%. The % O X 0 group is hydrogenated according to a subsequent, well separated step (mono:dihydrogenation selectivity up to 95%), giving equatorial alcohols. In the particular case of 1,4-androstadien-3,17-dioneregioselectivity promoted by copper was better than that exhibited by any catalytic system previously reported, giving up to 72% of 4-androsten-3,17-dione (93% regioselectivity).

However, during this study we have discovered, besides the expected hydrogenation produced by gaseous He activation, a secondary hydrogen source originated from alcoholic groups present in the substrate molecule or in the solvent. The formal hydrogen transfer from the alcoholic function to the enone one, parallels the conventional hydrogenation reaction as both processes are catalyzed by the same copper catalyst. Catalytic hydrogen transfer reactions are well known2 but they have been underutilized in the reduction of organic compounds with respect to the conventional use of molecular hydrogen or metal hydrides. These reactions promise potential advantages if compared with catalytic hydrogenations as the absence of gaseous hydrogen which involves considerable hazards. Moreover, transfer methods could afford new and not yet explored selectivities in the reduction of organic molecules. Direct comparison of reaction products by using gaseous H2 or a hydrogen donor are needed not only to get an insight into the reaction mechanism but also to evaluate the advantages or disadvantages of the two methods. Preliminary investigation on the reduction of steroidic molecules via hydrogen transfer showed new and interesting features of selectivity3. We report here the results obtained by using secondary alcohols as hydrogen donors towards steroidic conjugated enones and saturated ketones in the presence of 7.5% Cu on alumina.

163

RESULTS Unsaturated A5-3P-hydroxy steroidic molecules undergo a facile isomerization in toluene solution at 60°C in the presence of Cu on alumina under inert atmosphere. The saturated ketones derived through a formal hydrogen transfer from the alcoholic to the olefinic moiety are formed, besides small amounts of A4-3-ketone.

The internal hydrogen exchange favours the formation of the 50 (A/B cis) isomer which was obtained with 72-88% stereoselectivity, w h e r e a s conventional catalytic hydrogenation of A 5 olefinic bonds in the presence of Pd catalysts gives only the 5a isomer and homogeneous hydrogenation catalysts are totally inactive4. This observation can be used to derive a possible reaction mechanism for the hydrogen transfer process. We assume that the first step is the dehydrogenation of the alcoholic group, followed by isomerization of the unsaturated ketone to the conjugated one:

&-& CU/A1203

0

0

This latter point was experimentally proved by converting As-cholesten3-one into the conjugated A 4 isomer in the presence of Cu/A1203. The subsequent hydrogen addition to the A4-3-one follows the stereochemistry expected in the presence of heterogeneous catalysts as Cu/A1203 which produces an excess of the 5p isomer also with gaseous H2. The synthetic value of this reaction can be outlined.

164

In fact, the production of 5P-steroids from A5-3P-ols, readily available and cheap starting materials, requires preliminar oxidation through the Oppenauer reaction or, more recently, fermentation to the A 4 - 3 - k e t o derivatives. From this one 5p steroids are readily obtained through catalytic hydrogenat i o n4. On the other hand, the use of Cu/A1203 allows the production of 5p derivatives (cardioactive agents) in one step. Moreover this result suggests the investigation of the role of different external hydrogen donors on the selectivity in the hydrogenation reaction of steroidic molecules. Therefore we used 4-androsten-3,17-dione 1 and 5 a - a n d r o s t a n - 3 , 1 7 d i o n e 2 as model substrates to investigate the chemo- regio- and stereoselectivity of hydrogen transfer from different secondary alcohols , 2-propano1, 2-octano1, cyclohexanol, 1-phenyl-ethanol and diphenylmethanol in the presence of Cu/A1203. In particular, hydrogenation of 1 allowed to determine the selectivity towards 5p isomers, whereas the percent of axial alcohol was derived from the hydrogenation of 2 . These results can be compared with those obtained with the same catalyst in the presence of molecular hydrogen.

All the examined alcohols were active as hydrogen transfer reagents both towards the olefinic and the carbonylic double bond in a wide range of temperatures (60"-140°C). Under these conditions preliminar tests showed that selectivity does not depend on the temperature. Blank experiments using A1203 pretreated as the Cu containing catalyst at 270°C showed that the hydrogen transfer in the presence of the support alone is negligible. On the other hand prereduced Cu on Si02 gave the same results as Cu/A1203 in the hydrogenation of 2. All hydrogen transfer reactions were carried out under N2 at 90°C. According to GLC analysis the time required for the reduction of 1 to the corresponding ketones ranged between 20 min and 1.5 hours, whereas the reduction of 2 to the corresponding alcohols required 2-4 hours. Table I and Fig.la and b collect the experimental results obtained in the hydrogenation of 1 and 2 in the presence of 7.5% Cu/A1203.

165

The chemoselectivity in the hydrogen transfer reaction is the same already observed in H2 addition conditions in the presence of Cu/AI203. Thus, the conjugated enone is selectively hydrogenated while the saturated keto-group remains unchanged. Also regioselectivity is the same and the olefinic bond is reduced before the carbonylic one. However mono:dihydrogenation selectivity is very poor and saturated alcohols begin to form almost at the same time as saturated ketones. This situation is very different from that observed in the presence of molecular H2, when two well separated hydrogenation steps are present, as evidenced by Fig.la and Fig.1b. By comparing the results on stereochemistry obtained in the reduction of 1 and 2 by using alcohols and molecular hydrogen, a great flexibility of the hydrogen transfer reaction is apparent. Thus, selectivity towards the formation of 5p isomer can be modified from 48% to 85% and towards the formation of the axial alcohol from 15 to

45%.

0

1

Fig.1. Products distribution vs. equivalents of H2 or alcohol consumed during the hydrogenation of 1 in the presence of Cu/A1203 and molecular H2 (a) or 1-Ph-ethanol (b). ~ = 1 , 0 = 5 ~ - d i o n e , ~ = 2 , r = 5 ~ -x=5a-ols, ols, n=5P-diols.

166

Therefore by changing the nature of the reagent we can move the reaction from non selective conditions to highly directed stereo addition of hydrogen. In particular, 85% of the 5p isomer with respect to the 50: one represents the highest observed value which can not be obtained with gaseous H2 either changing operative conditions (T and P) or the nature of the copper catalyst. .................................................................. Table la Stereochemical course during the hydrogenation of 1 and 2 by hydrogen transfer in the presence of Cu/A1203

2-propanol 2-octanol cyclo hexanol 1(Ph)ethanol diphenylmethanolb

48 55 85 72 73

15 45 42 16 15

a = donor alcohol as solvent, 90°C b = l g in toluene (5 mL), 900c

Concerning the hydrogen transfer mechanism, the formation of surface copper hydrides as intermediates can be suggested also in the case of alcohols as donors1 This would explain the observed regioselectivity according to a 1 , 4 addition process followed by a prototropic rearrangement, as discussed by Bonnelle et al. for the hydrogenation of simpler molecules on copper chromites. Unfortunately, not simple correlations between molecular structure of the donor alcohol and stereoselectivity can be actually derived and more experiments with different alcohols are required. However, the products stereochemistry found when using 1-phenylethanol and diphenylmethanol as donors is close to that observed in H2 addition conditions, thus suggesting the occurrence o f two consecutive steps,

167

dehydrogenation of the donor alcohol and hydrogenation of the sterooid substrate: donor ketone + H2 Substrate + H2---+ Substrate.H2

--------,

In the case of 2-propanol and 2-octanol a direct surface hydrogen transfer reaction may take place, as was demonstrated by Burwell for the reaction between 2-propanol and 2-butanone on copper oxide’. According to this hypothesis, it is not surprising that the donor size influences the product stereochemistry. It is worth saying that both lacking of selectivity in the A 4 olefinic moiety hydrogenation and increasing of the axial epirner in 3 ketoderivatives saturation following the donor steric demand are typical aspects of steroid reduction by means of complex hydridess.

EXPERIMENTAL IR spectra were recorded on a Perkin-Elmer 577 instrument; 1 H and 1% NMR spectra were recorded on a Varian XL 200 instrument. GC analysis were performed on a Hewlett-Packard 5880 instrument, FI detector, equipped with a Methyl Silicone fluid capillary column (35 m), by using n-esadecane as internal standard. GC-MS analysis were performed using a HewlettPackard 5995 C instrument. Reaction products were identified by comparison with authentic samples, 1% NMR and MS data. Catalyst preparation: to a solution of Cu(N03)2.3H20 (16g ) in 150 ml H20, 30% NH40H was added till dissolution of the hydroxide initially formed. To the clear solution 20 g of alumina (Riedel-De-Haen), pH 4.5, surface area 200 m2/g, particle size 70-290 mesh, were added. The suspension was stirred for 10 minutes and diluted, very slowly,to 2L volume, stirred for 30 minutes and filtered off. The solid was dried at 120°C for 4 hours and heated in air at 350°C for 6 hours. The catalyst was pretreated at 270°C with H2 at atmospheric pressure (prereduced catalyst: Cu/A1203) following the procedure previously reported for copper chromiteg. Thus, a sample of CuO/A1203 was introduced in a glass reactor and heated at 270°C in a thermostattic device for 20 min with the reactor open, then under vacuum for 20 min. Hydrogen at 1 atm was carefully introduced: a fast reduction took place as indicated by water condensation on the cold arm of the reactor. Water was removed from time to time under vacuum and finally the catalyst was cooled under H2 to the

168

temperature choosen for the reaction. Hydrogen was removed under vacuum and the reaction vessel washed several times with N2 before its use in the hydrogenat ion react io n . Cu/AI203 obtained in this way has a copper content of 7-8%, determined by atomic absorption, surface area before the reduction treatment 220-250 m*/g (BET methodlo), specific Cu(0) area after pretreatment 20-30 m2/g (N20 decomposition1 '). Hydrogenation procedure. The steroid (0.2 mmoles) was dissolved in the donor alcohol (6 ml) and the solution heated to 90°C, transferred, under N2, in the reaction vessel where the catalyst (150 mg) had been previously pretreated and stirring was begun. To monitor the products distribution versus the alcohol equivalents consumed (Fig.l), 20 p L samples were withdrawn from the reacting solution through a viton septum and analyzed by GC. EquatoriaVaxial ratio was determined on the mixtures coming from the hydrogenation of 2 , by digitonide precipitation1 2, GC quantitative determination after silyl derivatives formationi 3 and CAD-MIKE spectroscopy following the procedure already described14 . REFERENCES 1) N.Ravasio and M.Rossi, J. Org. Chem. submitted for pubblication 2) R.A.W.Johnstone, A.H.Wilby and I.D.Entwistle, Chem. Rev. 85 (1985) 129 3) M.Gargano, V.P.Quatraro, N.Ravasio and M.Rossi, V IUPAC Symposium on Organometallic Chemistry directed towards Organic Syntheses, October 1-6, Firenze (Italy), Abstracts P S I -55 4) R.L.Augustine, in J.Fried and J.Edwards (Eds.), Organic Reactions in Steroid Chemistry, Van Nostrand-Reinhold, New York, 1972, chapt. 3. 5) D.Lednicer and L.A.Mitscher, Organic Chemistry of Drug Synthesis, vol.1 and Vol. II, John Wiley &Sons, New York, 1977 and 1980 6) a)R.Hubaut, M.Daage and J.P.Bonnelle, Applied Catalysis 22 (1986) 231 ; b) R. Hubaut, J.P.Bonnelle and M.Daage, J. Mol. Catalysis 55 (1989) 170. 7) J.Newham and R.L.Burwell, Jr., J. Am. Chem. SOC.86 (1964) 1179 8) D.M.S.Wheeler and M.M.Wheeler in ref.4, chapt.2 9) C.Fragale, M.Gargano and M.Rossi, J. Am. Oil Chem. SOC.59 (1982) 465 10) S.Brunauer, P.H.Emmett, E.Teller, J. Am. Chem. SOC. , 60 (1938) 309 11) T.J.Osinga, B.G.Linsen, W.P. Van Beek, J. Catalysis 7 (1967) 277 12) L.F Fieser,.M. Fieser, "Natural Products Related to Phenanthrene", Reinhold Pub. Corp., New York (1949), 3rd Ed., pp. 102-104 13) E. M. Chambaz, E. C. Homing, Analyt. Biochem. 30 (1969) 7 14) B. Pelli, P.Traldi, M. Gargano, N. Ravasio, M. Rossi, Org. Mass Spectrom. 22 (1987) 183.

169

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

SELECTIVE HYDROGENATION OF AROMATIC AND ALIPHATIC NITRO COMPOUNDS BY HYOROGEN TRANSFER OVER MgO

J. KIJENSKI, M. GLINSKI, R. WTSNIEWSKI and S. MURGHANI The Laboratory of Catalytic Synthesis, Institute of Organic Technology Warsaw Technical University (Politechnika), 00 662 Warsaw, Poland SUMMARY The possibility of using of aliphatic alcohols as hydrogen donors for the catalytic transfer reduction of nitro group over MgO was examined. Catalytic hydrogen transfer was found to be effective and selective method f o r reduction of nitrobenzene, 4-nitrotoluene, 4-chloronitrobenzene, 4-nitro-m-xylene, -nitrostyrene, 3-nitrobenzaldehyde, 1-nitropropane, and 1-nitrobutane. Conversion of starting nitro compound into desired product depended on the alcohol used as a donor. Adsorption of reactant and catalyst deactivation were studied by esr. New aspects of a role of one-electron donor sites in hydrogen transfer over MgO were demonstrated.

p

INTRODUCTION Commercially nitroarenes arid nitroalkanes are reduced to corresponding amines on nun-catalytic (using Bechamp o r Zinin method) or on catalytic way (hydrogen gas over metals). The catalytic transfer hydrogenation is an effective alternative for the above methods. Reductions of nitroarenes to aminoarenes by hydrogen transfer was reported for a wide range of metallic catalysts such as Pd, Cu, Fe, Ni, Rh and Ru (ref. 1). In a search for active hydrogen donors, it was found that formic, phosphinic and phosphorous acids, and their salts, unsaturated hyddrocarbons as cyclohexene, and especially hydrazine would reduce nitro compounds to amines with satisfying yield (ref. 1). The only but important limitation of the process is the high price of donors used. In the present paper we have examined the possibility of using of alcohols as hydrogen donors for catalytic transfer reduction (CTR) of nitro group. This paper is the continuation of our previous efforts at studying the synthetic application of hydrogen exchange on oxides. Our recent results concerned the reduction of aldehydes (saturated and unsaturated), epoxides, and nitriles, as well as the dehydrogenation of long chain aliphatic alcohols and alkylaromatics (ref. 2 ) . The reduction of a series of aromatic and aliphatic nitro compounds with various alcohols was studied over magnesium oxide as the catalysts. The hydrogen transfer reaction between alcohols and nitrocompounds should proceed according to the following equations:

170

RN02

+

RNOz

+ 3R1R2CHOH-RNHz

3R1CH20H-RNH2

”

3R C H‘

+

+

+ 2H20 f o r primary and

3R1RzC = 0

+

2H20

f o r secondary a l c o h o l s .

I n b o t h cases the minimum donor - acceptor molar r a t i o demanded f o r t h e reduct i o n o f n i t r o - t o amino group equals 3.

EXPERIMENTAL Reactions were c a r r i e d o u t i n a continuous f i x e d bed r e a c t o r a t atmospheric pressure i n t h e temperature range 350-450°C w i t h MgO as a c a t a l y s t . The r a d i c a l p r o p e r t i e s of the f r e s h , deactivated and regenerated c a t a l y s t , as w e l l as t h e adsorbed s t a t e s o f r e a c t a n t s were s t u d i e d by esr u s i n g Radiopan SE/X 2547 spectrometer. The adsorption o f r e a c t a n t s was performed according t o t h e procedure described elsewhere ( r e f . 3 ) . The magnesia c a t a l y s t p r e p a r a t i o n was described p r e v i o u s l y ( r e f . 3 ) . The l i q u i d product mixtures were analysed by gc (Chrom 5) using 4-m g l a s s column f i l l e d with 20 % OV-101 on Gas Chrom Q . Products were characterized by comparison w i t h a u t h e n t i c samples !ir, gc) and by m e l t i n g p o i n t s o f t h e i r hydrochlorides. RESULTS AND DISCUSSION values f o r the r e a c t i o n s of hydrogen t r a n s f e r between primary and seP condary alcohols and n i t r o compounds were c a l c u l a t e d u s i n g Van Krevelen and The K

Chermin procedure ( r e f . 4 ) . S i m i l a r l y as the simple r e d u c t i o n w i t h hydrogen the c a t a l y t i c hydrogen t r a n s f e r from alcohols t o n i t r o d e r i v a t i v e s is s t r o n g l y favor e d thermodynamically. For a l l s t u d i e d r e a c t i o n s w i t h primary a l c o h o l s the K values were o f the range o f h o l s K N 1044 P

P w h i l e f o r processes i n v o l v i n g secondary a l c o -

.

Reduction o f n i t r o a r e n e s Magnesium oxide e x h i b i t e d h i g h a c t i v i t y and h i g h s e l e c t i v i t y i n t h e hydrogen t r a n s f e r from alcohols t o s t u d i e d n i t r o a r e n e s . Because o f t h e l i m i t e d space o f the paper t h e complete amine y i e l d

-

temperature dependence was shown only

for

nitrobenzene r e d u c t i o n (Table 1). However, a l s o f o r o t h e r r e a c t a n t s the y i e l d o f the aminic product increased c o n t i n o u s l y between the values obtained a t the l o west (350°C) and the h i g h e s t (450°C) r e a c t i o n temperatures. Below 350°C t h e comp l e t e l a c k of a c t i v i t y o f MgO i n the s t u d i e d t r a n s f o r m a t i o n was noted. The same was observed by us e a r l i e r ( r e f . 2) i n the case t h e c a t a l y t i c t r a n s f e r r e d u c t i o n

of o t h e r f u n c t i o n a l groups. (i) Reduction o f nitrobenzene. The conversion o f nitrobenzene i n t o a n i l i n e depended s t r o n g l y on t h e a l c o h o l used as a hydrogen donor. Unexpectedly, methan o l was the most e f f e c t i v e donor molecule (91.4 % o f a n i l i n e ) . The order of ac-

171

tivity for hydrogen donation was found to be methanol-isopropanol > s-butanol n-butanol > i-butanol. >n-propanol >ethanol (ii) Reduc-tionof other nitroarenes. Table 2 depicts the yields of amines formed by reduction of the studied nitroarenes. No general rule of usefulness of particular alcohols for the reduction process was observed. Maximum yield of p-tohidine (39.8 % ) was gained in the reaction of 4-nitrotoluene with n-butanol. Highest conversion of 4-nitro-m-xylene into 4-amino-m-xylene was obtained using isopropanol as a donor molecule, while the most effective action of ethanol was noted in the reduction of 4-chloronitrobenzene (62.8 % of 4-chloroaniline). It should be wnderlined that all studied reactions occured with the selectivity higher than 99 %, only traces of condensation tar-like products were detected in the products mixture. The ease of reduction decreased in the order 4-chloronitrobenzene > 4-nitrotoluene >2,4-dimethylonitrobenzene. The above observation led us to the preliminary conclusion that the electronegative character of the substituent diminishes fitness of nitro group in nitroarene on the reduction. (iii) Reduction of nitroarenes possessinq second reducible group. The p r o ducts distributions of the redilction of -nitrostyrene and 3-nitrobenzaldehyde with various alcohols are listed in Table 3. -Phenylethylarnine (I) - the product of total reduction of side chain as well as both products of its partial reduction: -phenylvinylamine (11) and p-phenylnitroethane (111) were obtained in the reaction of P-nitrostyrene. The type of a donor used strongly affected the reaction selectivity. E.g. using methanol as hydrogen donor the ratio of I : II : I11 (450'C) was 41.2 : 49.0 : 8.4 (at the conversion of reactant - 100 a), the same ratio was 12.1 : 67.1 : 18.8 (at the reactant conversion of 98.0 % ) €or isopropanol (35OoC), and 30.1 : 42.3 : 15.4 (at the conversion of /3-nitrostyrene of 97.8 %) for s-butanol. The greater ease of reduction of nitro group in comparison with C=C bond reduction is obvious, however, the presence of remarkable amounts of -phenylethylamine in reaction products indicates that exchange of -NO2 group accelerates the reduction of a neighbour vinyl group. It should be emphasized that the reduction of 13-nitrostyrene by catalytic transfer reduction leads to the products completely different than these obtained in hydrogen transfer over metals. Namely, reduction of -nitrostyrene with formic acid over palladium gave the oxime of phenylacetaldehyde (ref. 5). Much more spectacular were the selectivity variations in the case of 3-nitrobenzaldehyde reduction (Table 3 ) . Depending on the hydrogen donor used 3-nitrobenzyl alcohol (methanol, 450°C) or 3-aminobenzaldehyde (i-propanol, 450°C) were the main reaction products.

>

/3

p

P

Reduction of nitroalkanes The effectiveness of catalytic transfer hydrogenation of nitroparaffins over MgO is demonstrated in the Table 4. At 450°C 1-nitropropane yielded 94.9 % of

TABLE 1

F

N 4

The y i e l d s of a n i l i n e formed i n t h e r e a c t i o n of n i t r o b e n z e n e w i t h v a r i o u s a l c o h o l s o v e r MgO, HLSV-1 Reaction temperature "C

Y i e l d of a n i l i n e u s i n g a g i v e n donor mol %

Oonor/acceptor ratio methanol

ethanol

n-propanol

i-propanol

n-butanol

s-butanol

12.3 18.2 22.6 25.6 37.1

30.0 33.1 41.2 50.0 54.4

i-butanol

______

350 375 400 425 450

3 3 3 3 3

: : : : :

77.3 77.8 78.4 81.5 91.4

l l l l l

15.8 26.5 33.6 39.3 51.3

26.1 30.9 38.3 39.1 47.3

62.5 73.9 74.5 78.3 91.0

tr 5.9 12.3 14.1 29.9

~~

TABLE 2

The y i e l d s o f amines formed i n t h e r e a c t i o n s of c o r r e s p o n d i n g n i t r o a r e n e s w i t h v a r i o u s a l c o h o l s o v e r MgO, donor : a c c e p t o r ratio-3,

HLSV-1

Reactant

Reaction temperature "C

Y i e l d o f amine u s i n g a g i v e n donor rnol % methanol

ethanol

n-propanol

i+ropanol

n-butanol

i-butanol

~~

4-nitrotoluene

350 450

18.1 24. 2

14.4 33.1

28.3

12.5 29.6

13.9 39.8

2,4-dimethylnitrobenzene

350 450

7.3 13.7

16.6 31.0

6.3 18.2

tr 16.0

tr 20.7

4-chloronitrobenzene

350 450

19.0 32.4

38.0 58.1

11.7

25.1 63.6

13.8 27.4

18.4 45.1

TABLE 3 The p r o d u c t s of t h e r e d u c t i o n of n i t r o a r e n e s p o s s e s s i n g a second r e d u c i b l e group w i t h v a r i o u s a l c o h o l s over MgO, donor : a c c e p t o r r a t i o - 6 , Hydrogen donor

React i o n temperature

"C

HLSV-1 P r o d u c t s of

pe t h-phenylylamine

P-nitrostyrene reduction mol %

P r o d u c t s of 3 - n i t r o b e n z a l d e h y d e r e d u c t i o n rnol %

P-phenylvinylamine

3-aminobenz y l alcohol

lI)-phenylnitroethane

3-aminobenzaldehyde

3-nitrobenzyl alcohol

methanol

350 450

41.3 41.2

35.8 49.0

14.3 8.4

2.8 13.6

-

82.6

ethanol

350 450

18.7 20.4

44.0 49.9

21.2 29.0

2.0 35.1

5.7 8.9

36.0

n-propanol

350 450

19.4 36.4

45.1 28.2

30.5 20.3

i-propanol

450 350

12.1 22.1

61.1 47.1

18.8 25.2

2.7 14.8

1.5 51.6

7.0

15.4 20.2

42.3 46.4

30.1 32.3

s-butanol

350 450

-

-

TABLE 4 The y i e l d s of c o r r e s p o n d i n g arnines formed i n r e a c t i o n s of 1 - n i t r o p r o p a n e and 1 - n i t r o b u t a n e w i t h v a r i o u s a l c o h o l s ,

donor

: acceptor r a t i o - 3 ,

Reactant

HLSV-1 Reaction temper a ture

Y i e l d of amine u s i n g a g i v e n donor rnol % methanol

350

1-nitropropane

450

4.3 67.5

1-nitrobutane

350 450

11.1 83.7

ethanol

n-propanol

i-propanol

n-butanol

s-butanol

i-butanol

16.5

10.6 41.2

9.3 94.9

35.5 84.9

3.7 76.3

21.7 90.0

23.8 87.9

9.6 90.9

9.8 79.0

8.7 16.9

6.3 53.8

13.6 93.5

18.6

W

174

1-propylamine with ~ 1 0 %0 selectivity (isopropanol). At the same temperature 1-nitrobutane was converted to 1-butylamine with yield of 93.5 % and 100 % selectivity (isobutanol). Catalyst deactivation and regeneration The catalyst decay during nitrobenzene reduction was studied in long-time experiments. The gradual poisoning of the catalyst was observed (Table 5) which led in 4-5 hrs to the significant diminishing of reactant conversion. TABLE 5 The decrease of aniline yields (mole %) during nitrobenzene reduction with various alcohols, temperature - 450°C, donor : acceptor ratio-3, HLSV-1 Time on stream hr 0

1.0

2.0

3.0

4.0

5.0

20.0

91.4 91.0 91.0 91.0 51.3 54.4

69.1 46.5 91.3 90.7 31.4 32.6

57.7 25.3 90.8 91.4 16.5 17.4

36.0 13.8 54.0 90.3 15.3 11.2

18.6 12.0 32.1 91.0 9.5 8.8

10.6 9.6 21.0 90.9

10.0 10.4

Hydrogen donor methanol isopropanol 1 isopropanol(N2)2 isopropanol(02) n-propanol s-butanol

90.7 9.1 8.3

1-catalyst regenerated by nitrogen (450°C) treatment during 10 min after each 33 min of reaction, 2-catalyst regenerated by air treatment according the same procedure Tndepending on the used alcohol the deactivation profiles reached the plateau corresponding the yield of aniline in the range of 8-10 mol %.Various regeneration procedures have been applied to preserve the catalyst activity on the high level. The calcination of used catalyst during 10 min in air at 450°C following each 0.5 hr of catalyst work was found to be the optimum regeneration mode (Table 5). The heating in neutral gas (nitrogen or argon) did not result in satisfying activity stability. The same regeneration procedure as f o r nitrobenzene was successfully adopted in reduction of other investigated nitroarenes. Esr studies of surface intermediates In our previous paper (ref. 2) we demonstrated the particular role played by one-electron donor centres on magnesia surface in catalytic transfer hydrogenation. Moreover, nitroarenes exhibit high tendency to convert themselves into corresponding anion radicals during adsorption on MgU. Thus, it was expected that esr spectroscopy would reveal new data concerning the reactants activation.

175

Esr i n v e s t i g a t i o n s were done of c a t a l y s t s samples with r e a c t a n t s adsorbed a t room and a t r e a c t i o n temperature. Also the p r e p a r a t i o n s o f d e a c t i v a t e d and regenerated c a t a l y s t were studied. From a l l s t u d i e d n i t r o compounds o n l y t h e f o l l o wing: nitrobenzene (parameters o f esr s i g n a l : g = 2.0031; A Hmax = 7 Gs; i n t e n -

.

1

.

spin g- 1, m-dinitrobenzene (2.0043; 9 Gs; 1.9 10" s p i n s i t y 1.2 g-'), 4 - n i t r o t o l u e n e (2.0051; 10 Gs; 6.1 10" s p i n . g -1), 4-nitro-m-xylene (2.0031;

-

1

13 Gs; 1 . 8 . 1019 s p i n - g- ) , formed t h e corresponding anion r a d i c a l s .

None from the used a l c o h o l s was converted i n t o paramagnetic species on MgO s u r f a ce. New evidence for t h e importance o f one e l e c t r o n donor centres f o r c a t a l y t i c t r a n s f e r r e d u c t i o n has a r i s e n from esr i n v e s t i g a t i o n s . Both, h e a t i n g o f anion r a d i c a l o f nitrobenzene on MgO surface from room temperature t o 350"C, or a d s o r p t i o n o f nitrobenzene a t 350°C on f r e s h MgO r e s u l t e d i n the new paramagnetic species. Esr s i g n a l ( A on F i g . 1) o f t h i s species d i f f e r e d i n shape ( l a c k o f h . f . c .

s t r u c t u r e ) and i n g va-

l u e (g = 2.0023) from t h e s i g n a l o f t h e par e n t i o n r a d i c a l , i n t e n s i t y remained o n l y s l i g h t l y changed. During t h e r e a c t i o n o f n i trobenzene with a l c o h o l surface species un'

1

derwent f u r t h e r e v o l u t i o n and

esr spectrum o f

MgO a f t e r 5 h r s o f r e a c t i o n revealed t h e preF i g . 1. E s r o f paramagnetic spesence o f a narrow s i g n a l ( A H m a x = 4 Gs, c i e s on deactivated and regeneg = 2.0023) o f the i n t e n s i t y c.a. 600 times r a t e d c a t a l y s t surface. higher than.the one measured f o r t h e i o n r a d i c a l (B on F i g . 1). Most probably the new s i g n a l d e r i v e d from r a d i c a l s formed i n a surface c h a i n r e a c t i o n o f adsorbed reactants. During regeneration by a i r treatment the number o f s u r f a c e r a d i c a l species remarkably diminished, the esr spectrum (C on F i g . 1) o f regenerat e d c a t a l y s t consisted from t h e narrow s i g n a l (AHmax = 3 Gs, g = 2.0030) which i n t e n s i t y corresponded t o only 8 . 10''

s p i n . g - I . The one-electron donor proper-

t i e s of deactivated and regenerated c a t a l y s t were c o n t r o l l e d u s i n g nitrobenzene ( e l e c t r o n a f f i n i t y 0.7 eV) and m-dinitrobenzene (E.A.

1 . 4 eV) adsorption. The

adsorption o f nitrobenzene on b o t h deactivated and regenerated surfaces d i d n o t l e a d t o the appearance o f a new paramagnetic surface species. The same r e s u l t was noted when n-dinitrobenzene adsorbed on deactivated magnesia. However, m-dinitrobenzene adsorption on regenerated MgO surface r e s u l t e d i n the formation o f 18 a t y p i c a l r a d i c a l species ( g = 2.0043,aHmax = 12 Gs, i n t e n s i t y 1.2. 10

1

s p i n . g- ) (0 on F i g . I ) . This observation l e d us t o the conclusion t h a t from s t r o n g and moderate donor s i t e s present on MgO surface (Ref. 31, o n l y t h e second one would be e a s i l y regenerated and e x h i b i t a c t i v i t y i n s t u d i e d r e a c t i o n s .

176

Strong centres, forming anion radical even from nitrobenzene molecule are poisoned irreversibly, however, their presence is not necessity for the preservation of catalytic activity. Taking into consideration that regenerated MgO which is not ahle to ionize nitrobenzene molecule is still active in its reductiori by hydrogen transfer and that only a few from reduced nitro compounds form ion radicals on catalyst surface one can ascertain that ion radicals formation is not necessary step in nitroarenes (or nitroparaffins) activation. Probably, one-electron donor sites take part only in activation of alcohol what was demonstrated by us earlier. CONCLUSION The main conclusions wolild be summarized as following: (i) the reduction of nitro compounds with alcohols by catalytic hydrogen transfer is a very selective process; (ii) the conversion of starting nitro compounds into desired products depends on the alcohol used as a donor. Each reaction should be individually optimized to find the most effective donor molecule. The substitution in nitroarene molecule diminishes its reactivity in catalytic transfer reduction (CTR); (iii) the previously demonstrated (ref. 2) action of one-electron donor sites on MgO surface is limited to the donating alcohol transformation; ionization of nitro compound molecule is not necessary step of its activation for CTR; (iiii) the simplicity of reaction, accessibility of reactants and ease of catalyst regeneration make CTR of nitro group with alcohols over MgO useful method for the commercial selective synthesis of aryl and alkylamines. REFERENCES 1 R.A.W. Johnstone, A.H. Wilby and 1.0. Entwistle, Heterogeneous catalytic transfer hydrogenation and its relation to other methods f o r reduction of organic compounds, Chem. Rev., 85 (1985) 129-1.70. 2 J. Kijehski, M. Glifiski and J. Reinhercs, Hydrogen transfer over MgO. An alternative method for hydrogenation-dehydrogenation reactions, in: M. Guisnet, J. Barrault, C. Bouchoule, 0. Ouprez, C. Montassier and G. Perot ( E d s . ) , Studies on Surface Science and Catalysis, V o l . 41, Elsevier Amsterdam, 1388, pp. 231-240. 3 J. Kijefiski, 5. Malinowski, Influence of sodium on physico-chemical and catalytic properties of MgO, J.C.S. Faraday I, 74 (1978) 250-262. 4 G.J. Janz, Estimation of Thermadynarnic Properties of Organic Compoimds, Academic Press, New York, 1958, pp. 183-197. 5 1.0. Entwistle, A.E. Jackson and R.A.W. Johnstone, Reduction of nitro-compounds, J.C.S. Perkin I, (1977) 443-444.

M. Guisnet et al. (Editors), Heterogeneous Catalysisand Fine Chemicals IZ 0 1991 Elsevier Science Publishers B.V., Amsterdam

177

MASS TRANSFER CONSIDERATIONS FOR THE ENANTIOSELECTIVE HYDROGENATION OF a-KETO ESTERS CATALYZED BY CINCHONA MODIFIED Pt/A1203 M. GARLAND*, H.P. JALElT and H.U. BLASER Central Research Laboratories, R-1055, Ciba-Geigy AG, 4002 Basel, Switzerland

ABSTRACT For the enantioselective hydrogenation of ethyl pyruvate catalyzed by a commercially available Pt/A1203 powder catalyst modified with dihydrwinchonidine, turnover frequencies of up to 50 s-' at 20 OC and 10.0 MPa were observed. Generally, the optical yields were S O % but under certain conditions lower enantioselectivities were observed. An integrated program of catalyst characterization, transport calculations and kinetic experiments was undertaken to quantify the mass transfer parameters. Catalyst characterization suggested that the powder catalyst was in fact of 'eggshell' design. By using catalyst fractions of varying mean particle diameter, negligible intraparticle resistance was found (Koros/Nowak and Madofloudart criterion). Further, calculations and experiments indicated that, under specific conditions, the lower ee's were due to liquid-solid transport resistance. Conditions can now be identified where intrinsic kinetics, not affected by transport problems, can be measured for future mechanistic studies. INTRODUCTION Recently, the enantioselective hydrogenation of ethyl pyruvate catalyzed by cinchona modified Pt/A1203 (ref. 1) was shown to be a ligand accelerated reaction (ref. 2). The rate of reaction for the fully modified system is more than 10 times faster than the racemic hydrogenation using unmodified catalyst. Under certain reaction conditions, this liquid phase hydrogenation exhibits a turn-over frequency of up to 50 s'l (3.4 mol/kg-cat s). Emphasis until now has been directed at empirically increasing optical yields (ref. 3,4).

CATALYST

CH3q o \ c * H s

0 Ethyl Pyruvate

+

H2

MODIFIER

'so

H+*+o, OH CH,

OH

C2HS

0 (R)-Ethyl Lactate

+

CH,

\

CZHS

0 (S)-Ethyl Lactate

Such a high reaction rate strongly suggested the potential for mass transport problems. Indeed, a turn-over frequency on the order of 1 s-l is considered appropriate for the purpose of mechanistic studies normally conducted in the gas phase (ref. 5). At higher rates, various complications including intraparticle diffusion problems, often arise. The situation is even more severe in the liquid phase where the bulk diffusivity of species is considerably reduced. A

178

thorough discussion of the inherent transport problems in heterogeneous hydrogenations in the liquid phase can be found in the literature (ref. 6). The goal of the present study was to identify regions of negligible transport control for future mechanistic studies. In the following, a systematic approach to the current transportlreaction problem is presented. EXPERIMENTAL Reactions All kinetic experiments were carried out in a double-walled 50 ml batch reactor (3.2 cm diam.). The reactor was equipped with baffles, a 3 cm magnetic stimng bar, a thermocouple, and a capillary sampling line. The reactor was connected to a 45 ml reservoir, pressure regulators, transducers. and a cryostat. The system was designed to operate at reaction conditions of T=273-303K, A T d . 3 C and P=O-15.OMPa, AP=kO.lIWa. Typically, the reactor was loaded with 50 mg of 5% Pt/A1203 catalyst (prereduced 2 hours at 400OC under Hi),and 10 mg dihydrocinchonidine (Hcd). 10 ml ethyl pyruvate (freshly distilled) and 20 ml toluene (Fluka puriss) were then added to the reactor. The autoclave was sealed and the system was purged with argon (2.0 MPa) 5 times while stimng. Reactions were initiated by pressurizing both the reservoir and reactor with hydrogen in the absence of stimng, waiting for 2-3 minutes for thermal effects to subside and then starting the stirrer. Approximately 30 second were needed to saturate the liquid phase with dissolved hydrogen. Rates were measured from the pressure drop in the reserviour after this initial saturation period. Optical yields were determined by derivatizing the ethyl lactate with isopropyl isocyanate followed by glc on a Chirasil-Val column (ref. 9). This method has been shown to give accurate and reproducible results (ref. 10). Catalyst A commercially available 5% Pt/AI2O3 catalyst (Engelhard Industries 4759) was used in this study. The catalyst sample had a mean particle size of 55 pm as measured by light scattering, a BET surface area of 140 m2/g, a mean pore radius of 50 A and a density of 5.0 dml. The platinum loading was 4.65%, and the platinum dispersion was 0.28 as measured by static CO titration (ref. 11). RESULTS AND DISCUSSION Catalyst Characterization For subsequent tests of intraparticle transport resistance, the catalyst was dry sieved into seven fractions. The mean particle size of these seven fractions were 18,29, 35.44.57,81, and 93 pm. The particle size distributions of these seven fractions are shown in Figure 1, the platinum loading and dispersion are depicted in Figure 2. It is clear from Figure 2 that the platinum loading is a strong function of the catalyst particle size i.e. 5.42% for the 18 pm fraction and 3.25% for

179

the 93 pm fraction. The dispersion varies less than 10% over the seven fractions. The remaining physical properties of the fractions are listed in Table 1. Incidence %

70 I

e

60

0.3

I

5-

- 0.25

4-

- 0.2 0.15

3-

0

50 100 150 Particle Diameter (micron)

2-

-

1-

- 0.05

0'

'0

200

0.1

0 20 40 60 80 100 Mean Particle Diameter (micron)

Fig. 1. Particle size distributions of the sieved catalyst fractions. Fig. 2. Platinum loading and dispersion of the sieved catalyst fractions. Platinum %.

----

Dispersion, -

TABLE 1 Texture parameters of the individual catalyst fractions Mean Particle Size

Pt

(Pm)

%

18 29 35 44 57 81 93

5.46 5.19 4.95 4.76 4.30 3.83 3.25

Dispersion

0.257 0.274 0.276 0.283 0.286 0.281 0.288

Surface Area

Real Density

Apparent Density

Pore Volume

Mean Pore Radius

(m%)

(S/ml)

(@mi)

(ml/g)

(A)

1.17 1.66 1.84 1.90 1.91 1.94 1.92

0.61 0.37 0.34 0.36 0.31 0.29 0.30

122 145 151 159 144 130 116

4.10 4.25 4.84 5.94 4.61 4.52 4.56

100 51 44 45 42 45 52

The decreasing platinum loadings with increasing particle diameter strongly suggest that the catalyst is of "egg-shell'' design. In other words, there is an enhanced concentration of the metal in the outermost layer of the catalyst particles. The preferential deposition close to the exterior surface of A1203 particles is well documented for a variety of metal salts (ref. 12). This is

180

particularly the case for catalysts prepared from HzPtCI, (ref. 13). However, as far as we are aware, the preparation of egg-shell Pr/A1203 catalysts with particle sizes of 20-100 microns has not been documented in the open literature. Attempts to verify this structure by direct measurements ( E M ) were inconclusive, but by using simple geometrical arguments we estimate that a shell thickness of approximately 10 pm is consistent with the observed platinum loadings. The mechanical strength of the unsieved catalyst was tested in stirring experiments. These attrition tests were canied out with 200 mg catalyst in 30 ml toluene at 900 RPM. The tests were conducted for 0, 1, 2, 5, 10, and 30 minutes, the stimng stopped and the suspension filtered over a 5 km porous glass filter. The results as mean particle size versus stimng time are presented in Figure 3. A 50% reduction in the mean particle size occured in approximately 8 minutes. In order to check whether the high rate of attrition also occurs under normal catalyst loadings (50 mg), we collected the individual fractions after 15-30 minutes of reaction (see Fig. 6-8) and the particle size distributions were determined. The 18,28, 35,44,57,81 and 93 pm particles were reduced to 18, 30, 33, 35, 38, 36 and 41 pm respectively. Clearly, there is attrition of the bigger catalyst particles but a significant difference in size still existed between the smallest (18 pm) and largest (93/41 pm) fractions even at reaction times considerably greater than the 1-5 min that were used to determine the initial rates and optical yields. Mass Transfer Studies Suspension of Catalyst Particles. There were concerns about complete catalyst suspension due to the density of the particles. Calculations indicated that total suspension of the dense A1203 particles should occur by 600 RPM (ref. 14). Such calculations are normally valid for agitated reactors with 1:l height to diameter ratios, and a turbine impeller at 1/4 height. Lower clearance in the reactor (the present case) will decrease the impeller speed required for complete suspension of the particles. Visual inspection of the open reactor confirmed complete suspension of the catalyst at 450-600 RPM. Gas-Liquid Hydrogen Transport. Using a dynamic method (ref. 15), the gas-liquid mass transfer coefficient KLa for hydrogen into toluene was measured in the 50 ml reactor. The autoclave was pressurized to an initial pressure PI and then stimng was started. The rate of mass transfer as a function of time and in terms of P, and the final pressure P2, is given by Equation 1.

(P2/pl)ln[(Pl-Pi)/(P~-P2)1= KLa x t

(1)

Four experiments were conducted under an initial pressure of 10.0 MPa hydrogen at 150, 300, 450, and 600 RPM. Plots of the left hand side of equation 1 versus time for these experiments are shown in Figure 4. The resulting numerical values of KLa (slope of the straight lines) were 0.0025, 0.005, 0.015, and 0.06 s1respectively. The data shows that the mass transfer coeffent is roughly proportional to (WM)2. Thus at 900 RPM. a stimng speed which will be subsequently used, the predicted value of KLa is calculated to be 0.14 s-'. This corresponds to a maximum rate of hydrogen transfer of 1 . 2 ~ 1 mol/s. 0~

181

In order to avoid mass transfer effects in an agitated reactor, the rate of reaction should not exceed 10% of the maximum rate of gas-liquid mass transfer, KLaxC(H2), where KLa is the diffusion coefficient and C(H2) is the solubility of hydrogen (ref. 16a). This guarantees that the liquid phase is essentially saturated with hydrogen. Preliminary experiments at low concentrations of modified catalyst gave an activity of 8.5~10"mol/(g-cams) at 10 MPa and 20 OC. Assuming a H, solubility in toluene of 0.3 mom (ref. 17). we calculate that the maximum loading of catalyst should not exceed 0.14 grams at 900 RPM. Size (micron)

60 I

0 0

5

10

15

20 25

30 35

Time (minutes)

0' 0

I

100

200

300

400

500

Time (seconds)

Fig. 3. Mean particle size versus stirring time (200 mg unsieved catalyst; toluene). Fig. 4. Determination of KLa (toluene; 10.0 MPa; 2OOC) Liquid-Solid Transport. The transport of hydrogen from the bulk liquid phase through the liquid film to the external catalyst surface was also a concern. Again, the rate of reaction inside a catalyst particle should not exceed 10% of the maximum liquid-solid mass transfer rate (a,,k,C(Hz)) (ref. 16b). For the unsieved catalyst (mean particle size = 55 km, apparent density = 2 g/cm3) the external surface area % was estimated to be 600 cm2/g and the liquid-solid mass transfer coefficient k,(Hz) was calculated as 0.12 cm/s at a stirring speed of 900 RPM (ref. 16~). After taking into account the hydrogen solubility, the measured rate of reaction represents 4% of the corresponding maximum mass transfer rate. Therefore the condition for negligible liquid-solid mass transfer resistance is met. To confirm these calculations, stirring experiments were conducted under the standard conditions at 300, 450, 600, 900, and 1200 RPM. The results presented as pressure drop in the reservoir versus time together with the optical yields are shown in Figure 5. There is no increase in the reaction rate for the system above 600 RPM, consistent with the assumption that rates determined at 900 RPM should be essentially free of both gas-liquid and liquid-solid mass transport control. However, significantly lower reaction rates and optical yields were observed for

182

the experiments conducted at stirring speeds less than 600 RPM.Since we have observed that lower hydrogen pressures lead to lower enantioselectivities (ref. 18). the stirring experiments indicate that there is a lower effective hydrogen concentration at low impeller speeds. Given the previous transport considerations, this could be due to gas-liquid resistance and/or liquid-solid resistance. Intrauarticle Resistance. The Koros/Nowak (ref. 7) or Madofloudart (ref. 8) criterion states that, in the absence of mass transfer influences, the activity of a heterogeneous catalyst should be proportional to the number of active sites. In other words, the observed turn-over frequency (TOF) should be independent of the particle size if there is negligible intraparticle resistance since all active sites are fully effective. Such experiments with the different catalyst fractions were conducted in toluene at both 2.0 and 10.0 MPa hydrogen and at 900 RPM. The initial rates of the reaction as well as the initial ee’s for these two sets of experiments are shown in Figures 6 and 7. In both cases, essentially constant TOF’s as well as constant ee’s are obtained, indicating a complete absence of intraparticle control.

Pressure Drop (bar)

30 I

25 20

09 (yo)

---Iioo 0

0 v

-90

0

- 80

15 A

10

5

0

10

20 30 40 50 Time (minutes)

60 70

-50

-0

20 40 60 80 100 Initial Particle Size (micron)

Fig. 5 . Influence of stirring on rate of hydrogen uptake and optical yield (unsieved catalyst; toluene; 10.0 MPa; 20T) Fig. 6.Effect of mean particle size on optical yields and turnover frequencies (individual catalyst fractions; toluene; 2.0 MPa; 2OoC; 900 RPM).---- TOF; -ee.

In addition to these experiments, the effectiveness factors (ref. 20) were also calculated for the smallest and largest catalyst particles (assuming a uniform distribution of active sites). The

183

effective diffusivity of Hz ( 2 . 5 ~ 1 0cm2/s) ~ was calculated using the Wike-Chang equation (ref. 19) and a tortuosity of 3 (ref. 16d). The reaction is first order in hydrogen (ref. 18). For particle diameters of 18. 55 and 93 pm, effectiveness factors of q(l8 Fm) 1, q(55 pn) = 0.7 and q(93 pm) < 0.5 were obtained which means that the TOF's observed for the smallest particles should be about 2 times higher than those measured for the largest particles. This is clearly not what we find experimentally! Either abrasion or a non-uniform distribution of platinum could be responsible for this discrepancy. Since the observed rates are determined during initial reaction times (1-5 min), abrasion can not be the main reason for the independence of TOF on particle size. Hence, these experiments suggest that the catalyst used has indeed an "egg-shell" structure. Hydrogenations were also carried out in 100% ethyl pyruvate and the results are shown in Figure 8. It should be noted that the turnover frequencies are lower than those observed in toluene (compare Fig. 7. with Fig. 8.). But more importantly. both TOF and ee's increase with particle size! Gas-liquid resistance can be excluded since KLa for H2 transfer into ethyl pyruvate has been measured and is also about 0.06 s-l at 600 RPM.Further, intraparticle control is unlikely because the TOF's should decrease with particle size. Hence, we think that these observations can be explained by liquid-solid mass transport effects as follows. The apparent density of the catalyst

-

ee (%) I100

TOF (11s) 1001

80 -

0

0

n u

1

TOF(l/s) 0 0

~

ee (%) 1

0

90

TL:

0

0

60 -

A

_ _A_ A

40 -

A

- 80 - 70

20 -

- 60

0 0

50 40 60 80 100 Initial Particle Size (micron) 20

Oo:/:ll 40 60 20

0

--

01

0

-A-

0

pi'

A A"

70

a- 60

'50 20 40 60 80 100 Initial Particle Size (micron)

Fig. 7. Effect of mean particle size on optical yields and turnover frequencies (individual catalyst fractions; toluene; 10.0 MPa; 20°C; 900 RPM).---- TOF; -ee. Fig. 8. Effect of mean particle size on optical yields and turnover frequencies (individual catalyst fractions; no solvent; 10.0 MPa; 20% 900 RPM).---- TOF; -ee.

0

184

particles decreases with decreasing particle size (see Table 1). Therefore, the density difference between reaction medium and catalyst particles decreases with decreasing size as well, leading to a reduced relative velocity and consequently to a reduced H2 transfer (ref. 21). The observed effect on the optical yield also points to an apparent lower hydrogen concentration on the catalyst, analogous to the effects observed for the stimng experiments. CONCLUSIONS We have identified reaction conditions where intrinsic kinetics can be obtained for the very fast enantioselective hydrogenation of ethyl pyruvate using a commercially available Pt/Al203 powder catalyst, modified with dihydrocinchonidine. We conclude that this is in part due to i) the egg-shell structure of the catalyst, ii) the high turbulence achieved in the reactor and iii) the density and/or the viscosity of the solvent used. In solvents like ethyl pyruvate, liquid-solid transport problems can arise. ACKNOWLEDGMENTS We would like to thank Dr. H.H. Fuldner and Mr. R. Miiller for the determination of the texture parameters, Ms. R. Gosteli for the measurements of the platinum dispersion and Dr. O.M. Kut for valuable discussions. REFERENCES 1 Y. Onto, S . Imai and S . Niwa, J. Chem. SOC. Jpn., (1979) 1118. M. Garland and H.U. Blaser, J. Amer. Chem. SOC., 112 (1990) 7048. 2 3 J.T. Wehrli, A. Baiker, D.M. Monti and H.U. Blaser, J. Mol. Catal., 49 (1989) 195. 4 H.U. Blaser, H.P. Jalett, D.M. Monti, J.F. Reber and J.T. Wehrli, in: M. Guisnet (Ed.) Heterogeneous Catalysis and Fine Chemicals, Elsevier, Amsterdam, 1988, pp. 153-163. M. Boudart, and R.L. Burwell, Jr., in: E.S, Lewis (Ed.) Techniques in Chemistry, Vol. VI, 5. Wiley, New York, 1974, pp. 693-740. 6. G. Gut, O.M. Kut, F. Yuecelen and D. Wagner, in: L. Cerveny (Ed.) Catalytic Hydrogenation, Elsevier, Amsterdam, 1986, pp. 5 17-545. R.M. Koros and E.J. Nowak, Chem. Eng. Sci., 22 (1967) 470. 7 8 R.J. Madon and M. Boudart, Ind. Eng. Chem. Fundam., 21 (1982) 438. W. A. Konig, I. Benecke and S. Sievers, J. Chromatogr. 238 (1982) 427. 9 10 J.T. Wehrli, Dissertation No. 8833, ETH-Zurich, 1989. 11 R.J. Farrauto, AIChE J. Symp. Ser., 70 (1978) 9. 12 R.L. Moss, in: R.B. Anderson, P.T. Dawson (Eds.), Experimental Methods in Catalytic Research, Vol. II., Academic, New York, 1976, pp. 43-91. 13 C.N. Satterfield, Heterogenous Catalysis in Practice, McGraw-Hill, New York, 1980. 14 G . Baldi, R. Conti and E. Alaria, Chem. Eng. Sci., 33 (1978) 21. 15 A. Deimling, B.M. Karandiker, Y.T. Shah, and N.L. Cam, Chem. Eng. J., 29 (1984) 127. 16 P.A. Ramachandran and R.V. Chaudhari, Three-Phase Catalytic Reactors, Gordon and Breach, New York, 1983. a) p. 190, b) p. 191, c) p. 179. 17 E. Brunner, J. Chem. Eng. Data, 30 (1985) 269. 18 M. Garland, H.P. Jalett and H.U. Blaser, Manuscript in Preparation. 19 R. Wilke and P. Chang, AIChE J., 1 (1955) 264. 20 C.G. Hill Jr, An Introduction to Chemical Engineering Kinetics and Reactor Design, Wiley, New York, 1977, p. 448. 21 C.N. Sattertield, Mass Transfer Effects in Heterogeneous Catalysis, MIT Press, Cambridge, 1970, p. 115.

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine ChemicalsII

185

0 1991 Elsevier Science Publishers B.V., Amsterdam

SELECTIVE CARVONE HYDROGENATION ON Rh SUPPORTED CATALYSTS

R. Gomez, J. Arredondo, N. Rosas and G. Del Angel Universidad Autonoma Metropolitana-Iztapalapa Dept. of Chemistry, P. 0. BOX 55-534. Mexico 09340 D.F.

SUMMARY The catalytic properties of rhodium supported on MgO, SiOz and Ti02 had been studied for the carvone hydrogenation reaction. Catalysts prepared in basic medium result to be more active than the ones prepared in acid medium. The main reaction products are carvotanacetone, carvomenthone and carvomenthol. In all cases, the main product is carvotanacetone, when the support is MgO the selectivity towards that product is even higher (92YY). The hydrogen stereoaddition towards the axial-equatorial carvomenthol formation is higly selective in Rh/MgO catalysts (100%). The particular Rh/MgO behavior can be explained by a deposit of MgO support over the metallic particles, diminishing the size of the Rh atoms ensembles.

INTRODUCTION

It is well known that catalytic processes employing soluble catalysts are more selective in the hydrogenation of poly-unsaturated compounds than those using solid catalysts. However the continuous demand of chemical products obtained by selective hydrogenation of unsaturated molecules, invite

to

study

solid catalysts, since their advantages for industrial application are well known. Nevertheless few attempts in this way have been done, eventhough the promising results reported with metal supported catalysts: high selectivity in the partial hydrogenation of 1,4 cyclohexanedione (ref. 1 ) employing Ru/SiOz

catalysts (up to 70% of 4-hydroxycyclohexanone), and up to 90%

selectivity in the hydrogenation of the double bond of 2-cyclohexenone for Pt/SiO

2

catalysts (ref. 2). Additional examples for selective hydrogenation

on supported catalysts are given elsewhere (refs. 3-61. In the mentioned studies the selective hydrogenations were made with the aim to obtain kinetical data, and the catalysts characterization was scarce. On the other hand, it is known that metal supported catalysts exhibit important particle size and support effects in the selectivity patterns (ref. 7).

Therefore it seems to be interesting to study such effects in the selective hydrogenation of a poly-unsaturated molecule as carvone. It was reported that the partial hydrogenation of this molecule is very sensitive to different homogeneous catalysts: organometallic compounds (refs. 8-10), Zn/OH (re:.

111,

NaBH

(ref.121, and Zn-NiC1

(ref. 13) as examples.

186 The purpose of the present work is to study: the precursor (metallic chlorides or carbonyl compounds), particle size

and support (silica,

magnesia and tltania) effects in the selective hydrogenation of carvone employing rhodium as active metal. EXPERIMENTAL Preparation. The catalysts were prepared by impregnation from aqueous solution of RhC13.3 H20 (ICN Pharmaceuticals) and n-hexane solutions of the complexes, Rh2(CO)4C12, Rh4(CO)12 and Rh6(CO)16. prepared in our laboratory (ref 14). The supports were silica (KetJen F-2, 380 m2/g), Titania (Degussa, 60 m2/g) and MgO (ICN, Pharmaceuticals, 40 m2/g).

The supports had been

previously calcined in air at 450 OC for 12 h, and reduced in flowing hydrogen for 2 h at 400 OC. Dispersion Measurements. chemisorption at' 25

Dispersions

were

determined

by

hydrogen

0

C in a conventional glass volumetric apparatus. The

amount of uptaken hydrogen was obtained by extrapolating to zero pressure the linear portion of the isotherm. The stoichiometric ratio H/Rh = 1 . 0 , was used for dispersion calculations in agreement with previous results (ref. 7 ) . The mean crystallite size was calculated assuming a simple spherical particle shape and equipartition of the dense crystal planes (1.33

x

10''

atoms/m2).

For carbonyl clusters impregnated type catalysts, the particle size was determlnated by electron microscopy (only particles smaller than 20

A

were

observed). Catalytic Experiments. Activities were performed

in a 1 liter Parr

reactor. A typical experiment was performed as follows: at a temperature of 100 OC, 100 mg of the catalyst and 1.5 X wt of (-1-carvone (Aldrich) in

n-hexane solution (100 ml) were introduced in a high pressure Parr reactor equipped with mechanical stirring and automatic temperature control. Before introducing the hydrogen the system was purged 2 or 3 times with N2, The total hydrogen pressure was 21 atm. The reaction products were analysed by gas chromatography, M41 Hnd Mass Spectrometry and ldentifled as: unreacted carvone, carvotanacetone, carvomenthone and three carvomenthol stereoisomers (axial-equatorlal, equatorial-equatorial and equatorial-axial). RESULTS The dispersion values, particle size and metal content for the various catalysts are reported in Table 1. The results show a high dispersion on most of the cataysts and particle sizes going from 11 to 42 A. It can also be seen that the two ammonlacal preparations of Rh/SIO do not change the 2 dispersion.

187 The initial rate (ro) and activity per site (TOF) are reported in Table 1. In contrast with the dipersion results, the low values obtained for the ammoniacal preparations show an important precursor effect and a small one on the nature of the support. However, the selectivity values for the formation of the three hydrogenated products reported in Table 2, demonstrate that selectivity depends on the nature of the support. Magnesia support presents the highest selectivity (90%) to the carvotanacetone formation. Particle size effects in selectivity were not detected, since, the small changes observed

TABLE 1 Dispersion, particle size and activity of Rh catalysts for carvone hydrogenation. Catalysts

Rh

Dispersion

(wt % I

(XI

0.5 1.0 1.0 2.0 1.0 2.0 1.0 2.0

94 79 93 46 42 25 57 44

RNSiOza RNSiOza Rh/SiOz Rh/si02 Rh/MgO Rh/MgO RNTiOz RWTiOz

TOF. 10'

Particle

ro.10'

(A)

(b)

(C)

0. 18 0.27 2.70 2.50

0.26 0.35 3.06

size 11 13 11 23 25 42 19 26

0.96

2.75 2.32

1.85

3.38

__

__

a) Sol. NHrOH. b) mol/g cat min. c) molecule/site min.

TABLE 2 Selectivity

(XI

Catalysts

Rh/SiOz RWS102 Rh/SiOz Rh/SiOz Rh/MgO Rug0 Rh/TiOz RNTi02

for carvone hydrogenation on Rh catalysts.

Carvotanacetone

Carvomenthone

0.5a 1. Oa 1.0 2.0 1.0

79 83 83 75 92

2.0 1.0 2.0

90

19 15 16 18 6 6 15 20

a) Sol. NHiOH

75 71

Carvomenthol

2

2 1 7 2 4 10 9

188

on silica and magnesia supports at different dispersions do not justify any speculation about it; two catalysts with the same dispersion value (SiO 2.0% and MgO 1.0%) have different selectivity patterns. The support effect in terms of selectivity can be observed in Table 3. The results show that the axial-equatorial carvornenthol is the only product when the support is magnesia. Rh/MgO catalysts results stereoespecific for the hydrogen carbonyl addition.

to

be

highly

DISCUSSION The results of Table 1, show that the preparation method does not affect the metallic dispersion

.

However, the catalysts prepared

in ammoniacal

solution have the lowest activity per site, showing that

in carvone

hydrogenation an important precursor effect in activity is obtained. Nevertheless, in the hydrogenation of poly-unsaturated molecules the catalyst effects are more evident in the selectivity patterns, as is shown in Table 2 and 3. The selectivity behavior for the various catalysts, show that R M g O is the most selective for carvotanacetone formation. The addition is mainly limited, in these catalysts, to one hydrogen molecule, although in carvone there are three possible sites at which reduction can occur. Though the magnesia effect is detected in hydrogen addition, this effect is most remarkable in the stereospecificity towards the axial-equatorial

carvomenthol formation (Table 3 ) . TABLE 3 Stereoisomers selectivity (%I of carvomenthol of carvone hydrogenation on Rh Catalysts. Catalysts

(a)

Rh/SiOz 1.Od RWSi02 1.0

29 30 100

Rh/MgO

RWMgO

1.0 2.0

100

a) axial-equatorial carvomenthol. b) equatorial-equatorial carvomenthol. c) equatorial-axial carvomenthol. dl s o l . NHIOH.

71 70

--

189

Fig. 1. Consecutive mechanism in carvone hydrogenation. In

carvone

hydrogenation a

consecutlve

mechanism without

desorption from the surface is expected (Fig. 1).

molecule

The high selectivity

towards the carvotanacetone observed in magnesia supports suggest that the consecutive mechanism is not completed in this support. Addition of three hydrogen molecules to carvone to obtain the cavomenthol without desorption of the molecule requires at least three adjacents sites, resulting in a particle size sensitive reaction; this sensitivity could not be observed in our catalyts, probably because the particle size in SiO supports are very close. However two catalysts showing comparable dispersions, Rh/Si02 2% and Rh/MgO 1%

give

different

selectivity

to

carvotanacetone

and

carvomenthol

stereoisomers. This implies that an unusual effect operates in magnesia support.

TABLE 4

Selectivity ( X )

for carvone hydrogenation on Rh catalysts prepared

from carbonyl clusters. Catalysts (a)

Carvotanacetone

a) 1% wt Rh content.

Carvomenthone

Carvomenthol

20 27 76 16

68

95

1

2

3 13

190

TABLE 5 Stereoisomers selectivity (%I of carvomenthol in carvone hydrogenation on Rh catalysts prepared from carbonyl clusters. Catalytst

(a)

(b)

(C)

e OH

Rh4(CO)iz/Si02 Rh6 (CO)16/SiO2 Rhz(CO)rCla/MgO Rhr (CO112AgO

13 51

4

--

--

--

---

87 49 96 100

a) axial- equatorial. b) equatorial-equatorial. c) equatorial-axial. Recently Poels et a1 (ref. 151, in the "syngas" reaction study shown that over Rh/MgO catalysts, a partial blocking of the metal surface occurs by effect of MgO hydrolysis. Similar effects have been also reported in benzene hydrogenation and methylcyclopentane hydrogenolysis (refs. 16,171 over Ru/MgO catalysts. It could then be possible that the same effect operates in carvone hydrogenation over the magnesia support. In this case the hydrogen addition stops at the first step and the stereospeclficlty to the axial-equatorial carvomenthol formation could be due to the blockage of the adjacents sites by MgO support deposited on the metallic particles. Additional evidence of that hypothesis is given in Tables 4 and 5. The catalysts prepared with carbonyl clusters ln n-hexane medium must avoid the MgO

hydrolysis. The selectivity patterns for such catalysts show notable

differences in comparison with the aqueous impregnated type catalysts. The carvotanacetone formation is largely dlmlnlshed and the stereospecificity to axial-equatorial carvomenthol is totaly inhlbited. However in Rhodium silica supported catalysts the selectivity to carvotanacetone practically does not change. The effects in stereospeciflty towards the carvomenthol product may be due to a small silica hydrolysis effect. CONCLUSIONS

The following important conclusions emerge from this study: ( i ) precursor effect is exhibited in carvone hydrogenation activity, ( i i l

the R W g O

catalysts results to be more selective towards carvotanacetone formation than

191 Rh/Si02and

Rh/T1O2

catalysts.

(ill)

the

stereospecificity

to

hydrogen

addition in the carvomenthol formation is higher in MgO supported catalyst. (iv) magnesia support effect is found due to the blockage of the metal particles by the MgO. REFERENCES 1 M. Bonnet, P. Geneste and M. Rodriguez, J. Org. Chem., 45 (19801 40. P. Geneste, M.Bonnet and C. Frouin, J. Catal., 64 (1980) 371. 2 3 P. Geneste, M Bonnet and M. Rodriguez, J. Catal., 57 (1979) 147. 4 A. A. Pavia, P. Ceneste and J. L. OLive, Bull. Soc. Chim., (1981) 24. 5 G. C. Accrombessi, P. Ceneste, J. L. Olive and A. A. Pavia, Tetrahedron, 18 (1981) 3135.

6

G. C. Acrombessi, P. Ceneste, J. L. Olive and A. A. Pavia, J. Org. Chem. 45 (1980) 4139.

7

G. Del Angel, B. Coq, R. Dutartre and F. Figueras, J. Catal., 87 (1984) 27.

R. E. Ireland and P. Bey, Org. Synth., 53 (1973) 63. Ch. Larpent, R. Dabard and H. Patin, Tetrahedron Lett., 28 (1987) 2507. A. J. Birch and K. A. M. Walker, J. Chem. SOC., (c) (1966) 1894. 11 J. C. Fairlle, C. L. Hdgson and T. Money, J. Chem SOC., Perkin I (1973)

8

9 10

2109. 12 13 14

N. R. Natale, Org. Prep. Proc. Int., 15 (1983) 389. Ch. Petrler and J. L. Luche, Tetrahedron Lett., 28 (1987) 2351. N Rosas, C. Marquez, H. Hernandez and R. Comez, J. Mol. Catal., 48 (1988) 59.

E. K. Poels, P. J. Mangnus, J. Van Welzen and V. Ponec, In Proc. Int. Cong. Catal., 8 th Berlin, 1984, 2 (1984) 59. 16 M. Viniegra, R. Comez and R. D. Conzalez, J. Catal., 111 (1988) 429. 17 P. Villamil, J. Reyes, N. Rosas and R. Gomez, J. Mol. Catal., 54 (1989) 15

205.

This Page Intentionally Left Blank

M. Guisnet et a]. (Editors),Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

S E L E C T I V E HYDROGEii,iTlO,i

C!T,hL

df

N I C K E L - k O L Y B D E ~ ~ U ! .C~t \ T A L Y S T S N i

J.COUHT,

F.JUNATI-IDKISSI

111 T H E

193

L I Q U I D P H A S E OVER

UIWIPPORTED

-x~40x.

and S . V I D A L

L a o o r a s o i r e d ' E i u d e s Dynawiques ec S.ti-ucturales de l a S e l e c - i i v i . i @ (LEOSS-1) CIWS g?A 332 - U n i v e r s i - i 6 Joseph F o u r i e r - BP 53X - 38041 G R E i W B L t C E X X (France).

SUII;YA t? Y The h y d r o g e n a i i o n o f ci.;ral has 3een i n v e s c i g a c e d i n cyclohexane and i n Sy c o - r e d u c c i o n o f 2-propanol , w i ih un-supported ili d-x i40x ca'ialys-is, prepared m i x t u r e s o f i o d i d e s o f a p p r o p r i a L e d conposi,ion w i t h napnchalene-sodium as r e d u c i n g agent. i i i g h y i e l d s i n c i ; r o n e l l o l were observed i n 2 - p r o q a n o l . The s e l e c t i v i s y o f t h e d i f f e r e n t steps i n so;h s o l v e n t s i s d i s c u s s e d u s i n g as s e l e c t i v i t y c r i , e r i a ,he r a i i o s k . D /k .o f o r each r e a c t i o n s i e p ( c o m p e x i ' i i v e N o r c o n s e c u z i v e ) . These r a t i o s $a& deen conpuied oy f i . i . i i n g t h e neasured produc-c conposiLions t o .ihe f u n c t i o n s obzained by a n a l y x i c a l i n t e g r a t i o n o f .the LkNGivlUIX-HIl\lSELi1OUD raee expressions.

INTROOUCTION Lie prepared a se:-ies o f o i n e x a l l i c , co-reduction

of

mixiures

sodiua-napniAalene

as

of

dry

reducing

un-supporeed

iodides agen'i.

of

Nii40x

appropiaie

Catalyst

ca,:alys';s

oy

comqosi-Lion w i ;h

cnaracieriza.iion

and

.
i n 'ine 1 i q u i d phase hydrogena'cion o f ace.iopnenone have

c a i a l y c i c proper,ies

oeen d e s c r i b e d p r e v i o u s l y ( r e f . 1 ) . I n o r d e r

improve o u r knowledge ; o f xhe

'LO

p r o p e r - i i e s o f i h e s e c a ' i a l y s t s i n t h e h y d r o g e n a t i o n o f compounds h a v i n g C = U and C=C

oonds,

we have s i u d i e d t h e h y d r o g e n a t i o n o f

3,7dimethyl-Z,5occadienal

(cieral ) . Cici-cil i s converi;iDle i n t o c i r r o n e l l a l and c i i r o n e l l o l ( r e f . 2 ) which a r e very

valuaole

ingredienis

3,7-di:~eshylocxanal

detracis

of

.ihe

perfunery

f r o m che

odor

of

induscry.

The

ciironellal

presence which

is

of its

p r i n c i p a l l y d e s i r e d p-oper'iy. S i m i l a r l y t h e presence o f 3,7-dimethyloc~canol i n ci,ronellol

has zhe same disavan'iages. Therefore, a s e l e c i i v e ca-ialys; n u s i De

used, so -chat o f .[he two o l e f i n i c douole sonds e x i s i i n g i n xhe c i - i r a l n o l e c u l e i n i h e 2 ( a l l y l i c ) and 6 p o s i t i o n s , o n l y i h e a l l y l i c dou312 3ond i s reduced and t h e aldehyde grou:,

begins ^LO oe reduced o n l y a f c e r .ine c o n j u g a i e d douale

sond is hydrogenated. I n o r d e r t o achieve .chis goal, i.ii s necessary ' t o opl;iioize condi.iions,

i.e.

.
i o determine xhe e f f e c t s o f numerous paraiile'cers on t h e

selec.civi.cy o f each r e a c i o n s'iep. ide r e p o r i e d xhe i n f l u e n c e o f -ihe c a x a l y s t

194 preparation

and o f

che na'iure o f

ihe

solven'c

on .ihe a c t i v i x y

s e l e c t i v i i y , u s i n g as s e l e c t i v i t y c r i t e r i a t h e r a t i o s kibM/kjblrl. previously (ref,

l;ne

3 ) t h a i f o r each r e a c i i o n s t e p ( c o m p e t i ~ c i v e o r c o n s e c u t i v e )

where k i ,

these ra'cios,

and

;.le have shown

k.

J

are t h e

r a t e constants

and

bH,

bN a r e .the

a d s o r p t i o n e q u i l i b r i u m constanzs, a r e u s e f u l s e l e c t i v i t y c r k e r i a . They can be computed by obtained

f i t r i n g t h e measured pr0duc.c

by

analytical

integration

of

conposilions ,he

to

.ine

functions

LANGMUIR-HINSELW000

rate

expressions.

1-5- I ,,C HO

CH,OH

k?bB

A

B

E

A : Citral (Z + E isomers) B : Citronella1 C :Citronellol D :3,7-dirnethyloctanal E :3,7-dirnethyloctanol F :Geraniol, nerol

F i g . 1 . R e a c t i o n scheme EXPERIMENTAL Hydrogenations i n 2-propanol o r i n cyclohexane were c a r r i e d ou'i a i 353 K i n a 250 m l s c a c i c r e a c e o r under constanc hydrogen p r e s s u r e (1.01 kPa) w i t h a s t i r r i n g speed o f 1300 r p m, so c h a t d i f f u s i o n l i r n i c a t i o n d i d noc a f f e c t t h e

195 r e a c t i o n . The ca.ia1ys.i and c i c r a l ( r e f . 4 ) c o n c e n x r a t i o n s were 1.33 g . 1 - I

and

0.195 mol .l-’,r e s p e c t i v e l y . The r e a c e i o n k i n e t i c s was s t u d i e d oy GLC a n a l y s i s o f samples withdrawn f r o m .the r e a c x i o n m i x t u r e . F o r t h e GLC a Supelcowax 10 wide-bore c a p i l l a r y column ( 3 0 m x 0.75 mm ID, 1.0 m f i l m t h i c k n e s s ) was used w i t h h e l i u m as t h e c a r r i e d gas a’;

ml/min.

a flow rare o f 5

The column

tempera‘Lure was i s o t h e r m a l a t 403 K. The n i c k e l s u r f a c e area was measured b y ,he

r h i o p h e n e mernod ( r e f . 5 )

alchough 3 m e t h y l t h i o p h e n e was used f o r g r e a t e r accuracy. DETEHMINATION OF THE SELECTIVITY CRITERIA C i c r a l has xhree s i i e s o f h y d r o g e n a t i o n and as shown i n ( F i g . mechanism f o r i t s r e d u c t i o n i s complex.

1 ) the

However we nave shown p r e v i o u s l y

( r e f . 3 ) thaL, a f t e r t h e e l i m i n a t i o n o f .Lime as an independent v a r i a b l e , LANGiWIR-HINSELYOOD ra,e be i n i e g r a i e d .

:he

equa*Lion f o r each componenc i n r h e l i q u i d phase can

The i n x e g r a i e d e q u a t i o n s I B ( = f ( l A I ) ,

h ( l B l ) , [ E l = i ( l B l ) and IF1 = k40C/kl bA, k3bIB/kloA .k7bF/kloA

. ..

ICI

= g(ldl),

10; =

j ( l S l ) depend on s i x r a t i o s

: k2DB/kloA, which have been computed s i m u l i a n e o u s l y .

The agrement between t h e c a l c u l a t e d p r o d u c i c o m p o s i t i o n s as a f u n c t i o n o f hydrogen consumed and t h e e x p e r i m e n t a l d a r a i s e x c e l l e n r as i l l u s x r a i e d i n ( F i g . 21. T h e r e f o r e t n e computed r a t i o kio,,,/kjbN

f o r each s t e p has been used

as s e l e c t i v i t y c r i i e r i a .

- -1 0

,x I s z

El c

U p:

e z W V

z

0 V W

> CI

+

5 w p:

0

ioo

0 PERCENT OF HYDROGEN CONSUMED

F i g . 2 . C i t r a l hydrogenation over pure n i c k e l i n cyclohexane. R e l a t i v e c o n c e n t r a x i o n I X l / l A l as a f u n c t i o n o f t h e p e r c e n t o f t h e hydrogen consumed, .the measured d a t a p o i 8 t s and t h e computed r e l a t i v e c o n c e n t r a t i o n c u r v e s .

196 RESULTS AND D I S C U S S I O N Influence o f the catalyse preparation Molybdenum

doped

catalysts

which

presented

.
ci.;ronel l o 1

s e l e c t i v i . i y i n cyclohexane ( r e f . 3 ) have been p r e p a r e d as d e s c r i b e d p r e v i o u s l y ( r e f . 11, excepr Lhac a d d i t i o n a l a l c o h o l washings were s u b s c i w c e d co .ihe f o u r w i e r washings. The c a i a l y s s s p r e p a r e d wi.ihout waxer washings i v i 11 oe r e f e r r e d 'LO

as "Un-water-\.lashed

: Uiil"

(i) Physico-chemical c h a r a c t e r i s t i c s o f

A ba'ich o f NillloOal2

c a t a l y s t o o t a i n e d a f . t e r f o u r washings wieh THF and f o u r washings w i c h 99.5% e.ihanol was d i v i d e d i n t o ewo p a r r s . The more w i s h 99.5 % e i h a n o l ,

one

fir5.i

t h e second one (;lid)

(did) was washed f o u r r i m e s

was wasned f o u r .cines w i - i h

w a i e r . The physico-cheinical c h a r a c x e r i s t i c s a r e g i v e n i n T a b l e 1 .

TABLE 1 Physico-cheni c a l charac i e r i s'ci c s o f Ilio. 881yoo.

N i o. 881qoo.

Caralysrs

( UId )

N i o. 88i400.

44

N i c k e l s u r f a c e area ( m 2 g - l ) Ni

Chemical a n a l y s i s (acorn % )

ivlo

IUi

Surface composirion ( a i o n %)

1/10

(:ill)

55

88.1 11 .Y

Ni I40

75 25

Ni IYO

One can n o e i c e ehac i n t h e UU cacalys:,

Y0.5 Y.5 85 15

molyJdenum i s segrega-ied a t -che

s u r f a c e s i n c e t h e a-coiilic s u r f a c e c o n c e n i r a x i o n o f nolyodenuin i s almost ';hree 'cines g r e a i e r

ehan 'ihe a c o n i c concen.iracion

decrease s i g n i f i c a n t l y :he

i n rhe

oulk.

daeer

washings

inolybdenum l e v e l a t .
c h e r e f o r e molyodenun i s leached o u t by water. s u r f a c e area a f - i e r w a t e r washings

The s l i g h t i n c r e a s e i n n i c k e l

i s i n accordance w i x h .
nolybdenun. A c c o r d i n g co -che o x i d i s a b i l i i y o f rnolyodenum, one can a s s m e .
(ii) A c t i v i c y o f c a t a l y s t s . The i n i x i a l hydrogenacion r a ' i e s expressed i n mol.s-l

per square mezer o f n i c k e l s u r f a c e a r e a a r e g i v e n i n Taole 2., a l s o

included f o r

';he

purpose o f

comparison,

are

.
oixained

wi-ih

unpromoted n i c k e l . The w a t e r washing decreases .the a c t i v i . c y o f p u r e n i c k e l , i l h i s observa.cion c o u l d be ae'iribuced e o t h e o x i d a . i i o n of N i ax t h e s u r f a c e . On t h e opposise,

i.lil molybdenum promoted c a i a l y s e s a r e more ac,ive

than 'ine Ui.l

ones, fur.chermore t h e a c L i v i - i y o f -
197 c o m p o s i i i o n o f ,nolyodenum a i i n e s u r f a c e . //hereas t h e a c r i v i t y o f t h e UA i s a l n o s i independenL o f ;he aconic c o n p o s i r i o n i n i h e D u l k . Since w a t e r wasning leacnes

iaolyodenun, s u r f a c e analyses w i l l oe necessary D e f o r e a c o n c l u s i o n

OUL

c o u l d oe reached. TABLE 2 I n i r i a l hydrogena.iion ra;es

z01 s-lm-'

expressed i n

( n i c k e l )xlOG

Solvenx

Ca.ialyszs

INi

Moo.03

#oo.06

Moo.12

Cyclohexane Cyclohexane 2-propanol

kJi.1

1.5 2.5 6

4.5 3.5

8 5 36.5

11.5 4 42

iJ ;.I UiJ

26.5

( i i i 1 S e l e c c i v i x y . Tne coapu.ied r a t i o s kibil/kjbN xhe d i f f e r e n ,

and .the s e l e c t i v i x y o f

s'ceps i n cyclohexane a r e g i v e n i n Table 3.

The s e l e c t i v i c y o f

t h e o v e r a l l h y d r o g e n a i i o n r e a c - i i o n depends on t h e s e l e c t i v i t y o f t h e d i f f e r e n t sxeps, which we w i 11 d i s c u s s s e p a r a t e l y . F o r t n e sequence A + B -s reactions,

i h e maxirnum concen;ration

the r a r i o k

2 8/k 10 k Z a

C,

i.e.

two c o n s e c u t i v e ,

of cixronellal

first

order

( Bmax ) i s a f u n c z i o n o f

D

a -

lalhlax For molyodenun c a i a l y s x s ,

=

lAlo

a l-a

i h e suppression o f ehe washings wi.ih wa'ier

r e s u l t s i n a i h r e e f o l d i n c r e a s e i n .the r a t i o klbA/k2bB, xhe n a x i n u a c i i r o n e l l a l y i e l d over

t h u s as an example

r i s e s f r o m 88% LO 95.5%.

Alehough

s m a l l e r .ihe sane e f f e c - i i s ooserved w i t h p u r e n i c k e l , .chis e f f e c r c a n n o i De a t . i r i i l u x e d 'io r e s i d u a l sodium i o d i d e i n U d c a - i a l y s i s , s i n c e t h e a d d i t i o n o f ria1

LO

1JJ caialys.;s

does noc m o d i f y t h e s e l e c t i v i , c y .

The r e a c r i v i x y o f .the C=O bond re1ai;ive co t h a x o f .the o l e f i n i c d o u o l e

1, i s two t i n e s g r e a t e r o v e r UJ molyodenum

bond i n .
I

D

c a L a l y s c s Lhan o v e r Lhe y/J ones. Thus t h e s e l e c t i v i t y o f i h e hydrogena.cion o f ci.ironella1

LO

ci,ronellol

i n .the x h i r d seep, which i s h i g h o v e r Wd molybdenum

ca.calys.is, reaches Y8% over il;J 140 c a x a l y s - i s . The suppression o f cne washings w i t h wa'ier has a l s o a o e n e f i c i a l e f f e c r on .;he

sequence B --., C

--+

E , xhe maxinun y i e l d i n c i r r o n e l l o l over

Nii400.06

and iiii?oo.12 i s s i g n i f i c a n i l y i n c r e a s e d . I n f l u e n c e o f .ihe n a u r e o f ,the s o l v e n t . I n order

.LO

determine -ihe i n f l u e n c e o f che n a i u r e o f t h e solvenx on t h e

s e l e c t i v i t y o f each r e a c t i o n s.tep ,and w i t h t h e aim t o enhance xhe c a t a l y t i c

198 properties

of

t h e Uil caralyszs

h y d r o g e n a t i o n s have been c a r r i e d

out

in

Good y i e l d s o f c i r r o n e l l o l a r e g e n e r a l l y observed i n a l c o h o l s

2-propanol. ( r e f s . 6-11).

(i) A c i i v i r y . I n 2-propanol molybdenum has a l a r g e p r o m o t i n g e f f e c r on the i n i t i a l

As compare w i t h cyclohexane t h e a t e n - f o l d increase

h y d r o g e n a t i o n r a t e T a b l e 2.

a c t i v i t i e s o f t h e UY c a e a l y s z s a r e enhanced i n 2-propanol,

b e i ng observed over N i o. 8 8 M ~ U . 2. Furehermore, t h e a c t i v i t y o f t h e molybdenum c a t a l y s t s i n c r e a s e s wieh t h e molybdenum c o n c e n r r a c i o n i n t h e b u l k . ( i i ) S e l e c t i v i r y . The y i e l d s i n c i r r o n e l l o l f o r the o v e r a l l hydrogenacion r e a c t i o n i n 2-propanol

PI^^.^^

o v e r UW c a r a l y s t s were :

96%, ivloo.06

98% and

Mo0.12 96%. t h i s v e r y h i g h s e l e c t i v i t y i s t h e r e s u l t o f s e v e r a l e f f e c t s we

w i 11 d i s c u s s s e p a r a c e l y

.

For t h e sequence A

4

B + C,

t h e r a c i o s klbA/k2DB

Table 3. do n o t d i f f e r s i g n i f i c a n t l y f r o m t h e r a t i o s klbA/k2bB

i n 2-propanol i n cyclohexane,

t h i s resuli: h o l d s f o r pure n i c k e l and molybdenum promoted c a t a l y s t s .

Taking

i n r o account c h a t .che i n i t i a l h y d r o g e n a t i o n r a t e which i s p r o p o r t i o n a l 20 klbA is

notably

enhanced

in

2-propanol,

one

can

assume

thac

the

rare

of

hydrogenation o f c i r r o n e l l a l t o c i e r o n e l l o l increases i n s i m i l a r proportion. TABLE 3 Computed

raiios

kibM/kjbN

and

seleceivity

of

the

different

sreps

in

cyclohexane and i n 2-propanol.

Steps Catalysis

kl bA/k2bB

Bmax

Cycl cyc 1 2 Pr

48 62 40

92 93.5 91

21 Y 125

95 YO 99

6 40

69 91

WW

1;

cyc 1 cyc 1 2 Pr

26 100 48

88 95.5 92

21 48 5 00

95 98 100

4.4 3.2 117

65 60 96

ww

1;

cyc 1 cyc 1 2 Pr

32 Y1 42

89 95 91

14 3Y 500

93 97.5 100

4.0 15.5 475

63 83 98.5

J,.I

cyc 1 cyc 1 2 Pr

23 42

87 91 93

37 47 500

97 Y8 100

4.2 20 900

klU

N i Ud UiJ

'qo0.03

M00.06

ko0.12

Sol vene

;;

55

k2bB/k3b'B

s1

64 85 100

i s ,the maximum c i t r o n e l l a 1 y i e l d i n t h e second seep.

i s ene s e l e c Z i v i L y of i h e h y d r o g e n a r i o n o f c i r r o n e l l a l t o c i t r o n e l l o l i n t i e t h i r d s t e p expressed i n p e r c e n t . Cmax i s i h e maximum c i t r o n e l l o l y i e l d i n t h e 4''h

step.

199

On t h e o t h e r hand, f o r t h e sequence B

<

, the

selectivity i n citronellol

D

i s rremendously i n c r e a s e d i n 2-propanol.

Over molybdenum promoted c a t a l y s t s ,

t h e r a t i o s k2bB/k3b'B a r e g r e a t e r t h a n 500, i . e . t h e amount o f

D i s t o o small

A s i m i l a r e f f e c e i s observed o v e r p u r e

t o allow s i g n i f i c a n t determination.

n i c k e l s i n c e t h e r a t i o k2bB/k301B r i s e s f r o m 9 (S1 = 90%) i n cyclohexane t o 125 (S1 = 99%) i n 2-propanol. Similarly sequence B

--+

2-propanol C

+ E,

exalts

the

selectivity i n

citronellol

for

.ihe

f r o m t h e values o f t h e r a t i o s k2bB/k40C, i.i appears

.chat t h e maximum y i e l d i n c i z r o n e l l o l over p u r e n i c k e l r i s e s f r o m SY% K O 91%. Over molybdenum promoted ca'ialysxs t h e s e l e c t i v i x y o f t h i s sxep i n c r e a s e s w i t h ,cne molyodenum c o m p o s i t i o n i n t h e o u l k '5.0

reach 100% w i r h NilyloOSl2,

i.e.

c i t r o n e l l o l i s n o t hydrogenared 'to 3 , 7 d i m e t h y l o c t a n o l . I n our o p i n i o n t h i s remarkable e f f e c t c o u l d be a t t r i o u c e d t o a lower v a l u e o f t h e p r o d u c t k4bC i n 2-propano1, o f t h e a d s o r p t i o n cons'iani catalysr

surface

than

D ~ .An

an

and more p r e c i s e l y t o a l o w e r v a l u e

a l c o h o l b e i n g l e s s s t r o n g l y adsorbed on t h e

aldehyde

in

2-propanol

observaeion c o u l d s u s t a i n t h i s assump.iion, geraniol

(F) which

as

solvenx.

Another

t h e hydrogenaxion o f n e r o l

i s r a p i d i n cyclohexane i s r a t n e r low i n 2-propanol,

and

since

t h e small amount o f F formed i n i t i a l l y i s s t i l l p r e s e n t i n .the mixeure whereas

B has been t o t a l y hydrogenated. CONCLUSION The s o l v e n t s used t o e l i m i n a r e sodium i o d i d e a f t e r c o - r e d u c e i o n o f t h e mixcures o f n i c k e l and molybdenum i o d i d e s , activiry

and

the

selectivity

of

the

have a sxrong effec,

caealysts.

As

reported

on r h e

previously

( r e f . 12) t h e washings w i t h waxer i n c r e a s e t h e a c t i v i t y o f t h e c a t a l y s i s , b u t

t o reach h i g h c i t r o n e l l o l s e l e c t i v i t y , w a t e r n u s t be avoided. J i t h 2-propanol

as s o l v e n t ,

.ihe .iendency ro hydrogenate t h e o l e f i n i c

double bond i n t h e 6 p o s i c i o n i s reduced, t h u s v e r y good y i e l d s i n c i t r o n e l l o l were observed i n t n i s solvenz. The f o r m a t i o n o f 'ihe two i n d e s i r a o l e producxs : 3 , 7 d i r , i e ~ ~ h y l o c t a n aand l 3,7 d i m e t h y l o c t a n o l b e i n g almost a n n i h i l a t e d . ACKNOULEDGEMENTS : ;Je a r e i n d e b t e d 'io S. HAifiAR-THIBAULT f o r Auger a n a l y s e s . REFERENCES and NOTES 1 2

J.W. Bonnier, 3. Court, P.T. i4ierzcnowski and S . Hamar-Thibault, Appl. Cae. 53 (1989) 217-31. L. Cerveny and V . Ruzicka, S e i f e n . Ole. F e t t e . Washe, 114 (1988) 605-609.

200 J . Court, F . J a n a t i - I d r i s s i , S. V i d a l and P.T. d i e r z c h o w s k i , 2. Chirn.Phys. 87 (19901, 379-391. 4 C i e r a l f r o m ALDRICH was a m i x t u r e t h e E-isomer ( g e r a n i a l ) and t h e Z-isomer ( n e r a l ) , t h e i n i t i a l r a r i o E / Z = U.51 remained alinosz conscan'i d u r i n g hydrogenarion, t h e r e f o r e we o n l y r e f e r r e d ro .
3

.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

201

SULFUR REMOVAL FROM TERPENES BY HYDRODESULFURIZATION ON CARBON-SUPPORTED CATALYSTS F. CASBAS', D. DUPREZ2 and J. OLLIVIER3 'SociCtC des DCrivCs Resiniques et Terpeniques, Vielle St Girons 40560, France. 'Laboratoire de Catalyse en chimie Organique, Poitiers 86022, France. 3Groupement de Recherche de Lacq, SNEA, BP 34 Artix 64170, France.

ABSTRACT The purpose of this study was to design a catalyst for the sulfur removal from turpentine fractions by hydrodesulfurization (HDS) avoiding isomerization and cracking of the terpenes. p-pinene, one of the most fragile tevenes, was used as a reference throughout the study. Carbons present, alone, a significant HDS activity but the degradation of p-pinene varies from less than 1% for the most inert su port to about 100% for the most active one. On these carbon supports, dipentene is t e main product of transformation of p-pinene. Impregnation of cobalt and molybdenum between two layers of sodium ions (sodium molybdate, cobalt nitrate and finally sodium hydroxide) give the best results in HDS of p-pinene : less than 10% degradation and 80% desulfurization.

I:

INTRODUCTION Terpenes constitute a large class of fine chemicals widely used in perfumery, food and pharmaceutical chemistry, adhesives .... Among the monoterpenes (CloH16) , apinene, p-pinene and d-carene fractions are the most used in chemical industry.

a-pinene

O-pinene

A3-carene

Turpentine oils extracted from pine resins are now marginal sources of terpenes : pinenes, carene and some other monoterpenes are essentially obtained from the paper oils produced in the KRAFT process. Nevertheless this process is based on a treatment of wood chips in sodium sulfide, which leaves significant fractions of sulfur compounds in the paper oils. For the use of terpenes, these sulfur compounds must be eliminated. The process requires : * a good efficiency in desulfurization * a total neutrality of the catalyst with respect to terpenes. In particular the doublebond isomerization of p-pinene into a-pinene as well as the skeletal isomerization of pinenes have to be avoided.

202

Recently we have proposed an HDS catalytic treatment based on sodium-doped CoMo catalysts [Ref.l3]. Previous studies concerned essentially alumina-supported catalysts. As carbon was shown to be a good support for sulfided CoMo catalysts [Ref.4], we decided to investigate the performance of carbon-supported catalysts in terpene HDS. EXPERIMENTAL Seven active carbons were used as supports. The origin and the main characteristics of these carbons are given in Table 1. TABLE 1. Carbons used in HDS of terpenes. I

I

Carbon Nr

Commercial names

BET area m2 g-1

Density kg m-3

Pore volume cm3 g-1

1 ! I

I

I

I

CECA NC 35

1200

550

CHEMVIRON FILTRASORB FS-400

1100

425

0.9

D EGUSSA AS IV/420

1200

420

1.0

I

~

D E GUSSA F 401430

1200

420

PICATAL ELP 612

900

240

v1

PICATAL E L 612

900

500

VII

PICATAL E 612

970

538

IV

I v

I

1.0

I

II I

As the series of PICATAL 612 carbons comprised the most interesting support for the terpene HDS (E 612), a detailed chemical analysis of these carbons was carried out (Table 2). PICATAL E 612 was used as a support of CoMo catalysts. Sodium-free catalysts were prepared, according to Massoth [RefS] and Chung and Massoth [Ref.6] by successive impregnations of molybdenum and cobalt using aqueous solutions of ammonium heptamolybdate and cobalt nitrate. Sodium-doped catalysts were prepared by successive impregnations of sodium molybdate (instead of heptamolybdate), cobalt nitrate and eventually sodium hydroxide (Table 3). All the catalysts contained 7 wt.-% COO and 4.4 wt.-% Moo3 corresponding to a Co/Cot Mo atomic ratio of 0.75. In sample 2 the Na20

203

content resulting from the impregnation of 4.4 wt.-% Moog as sodium molybdate is 1.9 wt.-%. In sample 3, the amount of sodium hydroxide used for the fourth impregnation was calculated so as to have a total Na20 content of 2.5 wt-%. The catalysts were dried at 110°C in each impregnation and calcined in helium at 500°C. TABLE 2. Chemical analysis of the PICATAL 612 carbons (ppm).

I

Carbon

Na

K

Ca

Al

Mn

Fe

Ni

cu

ELP612 EL 612 E 612

192 52 527

209 131 5005

2822 310 465

75 73 58

25 11 16

260 86 104

10 6 6

-3 14

I

TABLE 3. Carbon-supported Co-Mo catalysts used in terpene hydrodesulfurization (SM = sodium molybdate, AHM = ammonium heptamolybdate, CN = cobalt nitrate). All the catalysts contain 7 wt.-% COO and 4.4 wt.-% Moo3 corresponding to a Co-to-Co+Mo atomic ratio of 0.75.

I

Catalvst no

Successive impregnations

Composition 1 Na

1 2 3

CO-MO Co-Mo-Na Na-Co-Mo-Na

None SM SM

4

2 Mo

3 co

Na

AHM

CN CN CN

None None NaOH

r1

The terpenes used were mainly p-pinene fractions provided by DRT (SociBtB des Dtrivks Rtsiniques et Terpeniques, Vielle-St-Girons) and, for certain experiments a turpentine oil containing the main three terpenes : a-pinene, P-pinene, and A 3-carene. The p-pinene fractions contained 80-90% ppinene, 2% a-pinene, 4 5 % myrcene, 2-3% dipentene and 700-1500 ppm S. GC-MS analyses showed that sulfur impurities were composed of alkyl and alkenyl sulfides (mainly dimethyl sulfide), alkyl and alkenyl disulfides (mainly dimethyl disulfide), trisulfides, thiophene and alkylthiophenes (methyl, dimethyl, acetyl and tertiobutyl). Terpene catalytic hydrodesulfurization was performed in a pilot-plant reactor at 200°C, 1 atm. The volume of catalyst was 70 cm3; the liquid hourly space velocity of terpene 0.4 h" and the hydrogen to terpene molar ratio of 7. The catalysts were pretreated in situ in a flow of N2 at 260°C then sulfided in H2S/H2 (1:9) from 260°C to 370°C. After 5 h-on-stream, the catalysts were cooled down to the reaction temperature in H2S/H* The hydrocarbons were analyzed by GC on a CP Wax 57 CB capillary column at 65°C and the sulfur contents were determined by microcoulometry using the ASTM D312077 norm. The

204

performance of the catalysts was evaluated by the percentage of desulfurization (HDS%) and by the percentage of degradation (DEG%) of the principal terpene.

RESULTS AND DISCUSSION .. carbons in HDS of ~-~lnene. ctivitv The p-pinene fraction was used as a reference to determine the isomerization activity of the supports. Results given in Table 4 show that carbon VII is particularly inert with respect to p-pinene. This behaviour is certainly related to the high content of this carbon in potassium (0.5 wt.-96). On the contrary, the CaO impurities present in carbon V seem to increase the isomerization activity of this carbon. It is well-known that the double bond shift isomerization of hydrocarbons can proceed via carbocation intermediates (protonic catalysis) or via allylic carbanion intermediates (acido-basic or purely basic catalysis) [Ref.7]. The results obtained with potassium-doped carbons show that in ppinene isomerization during HDS, the protonic mechanism predominates. TABLE 4 Catalytic behaviour of commercial carbons in HDS of p-pinene (200"C, 1 atm). (DEG% = conversion of b-pinene; HDS% = percentage of sulfur removal). Carbon

time-on-stream (h)

DEG %

HDS Yo

I

4 25

34 6

45 39

I1

16 24

97 91

76 71

111

4 24

73 40

77 72

IV

6 14

61 50

73 82

V

22

97

40

VI

14 21

99 99

VII

8 24

0.6 0.5

%i03

24

17

Si02

24

2

Other supports [ref.3]

15 10

205

Fig. 1 shows the selectivities of the main isomerization products : a-pinene, dipentene and camphene during the HDS experiment with carbon I and 111. Similar results were obtained with other carbon samples. In fig. 1 these results are compared with the isomerization behaviour of A1203 (RhBne-Poulenc SCS 79 delta-alumina) and Si02 (Rhane-Poulenc DBM 250 silica), which were studied previously [Ref.3,4]. Carbon induces profound changes in the spectrum of isomerization products : dipentene is the major

'%' 40

20

Carbon

I

--.

-*

I

/

1 @t'

I

' */*, I

I -0

0-

1 --*& I

I

! 40

I I

I

-

--.

0

:

I

I

I

I

40

I

--.>

20

I 0-

-0-

e-

0

I

I

10

20

--rh

0

Fig.1 Selectivities of the main isomerization products during the P-pinene HDS on the bare supports (200"C, 1 atm). a-pinene ;0 dipentene ; ncamphene.

*

206

product, whereas this is a-pinene on alumina and silica. The protonic mechanism of ppinene isomerization is likely to proceed through the carbocation intermediates shown in Fig. 2 [Ref.8]. C-C bond shifts in the pinenium ion leading to the A ion and then to the camphenium ion are relatively difficult to occur. This explain why the selectivity to camphene remains low. On the contrary the p scission leading to the B ion is much more probable since the constraint in the pinene bicycle is then suppressed.

a pinene

p pinene

I\

6z=+b 0.Q A

B

II

*

d ipen t e n e

- H' c--

Camphene

Fig2 Protonic mechanism for P-pinene isomerization. It is not easy, however, to correlate the nature and the surface state of support with the selectivity to dipentene : a plausible hypothesis would be that, for steric reasons, the dipentenium is favoured in the microporosity of the transformation pinenium -> carbons.

..

c t i v u c a r b o n - W t e- d c The results obtained on the CoMo/Carbon VII catalysts are given in Table 5.

207

Table 5 Activity of CoMo/C catalysts in 8-pinene HDS (200"C, 1 atm).

I Catalyst

Time-on-stream (h)

DEG %

I

HDS %

I

CO-MO

6 30

99 99

60 64

Co-Mo-Na

6 30 50

53 32 24

71 77 78

Na-Co-Mo-Na

6 30

26 10

74 77

Impregnation of cobalt and molybdenum (without sodium) increases largely the isomerizing activity of the catalyst : the p-pinene is then completely converted. The catalysts prepared with sodium molybdate and sodium hydroxide (Co-Mo-Na and Na-CoMo-Na) have lower isomerizing activities while their HDS activities are significantly increased. As in the case of alumina supported catalysts the sulfided CoMo phase protected by a double layer of alkaline ions on the carbon support gives the best results in HDS of p-pinene. The behaviour of this catalyst was examined in desulfurization of the turpentine oil (40% a-pinene, 25% p-pinene, 25% A3-carene and 10% camphene t dipentene + myrcene, 1500 ppm S). The results are recorded in Table 6. Table 6 Desulfurization of a turpentine oil. Time a-pinene 6

-1 + 1.2 +8

DEG % p-pinene

+ 15 + 26 + 30

HDS % 3-carene

- 2.6 + 0.8 + 1.5

77 77 79

The negative values for the degradation of a-pinene and of A3-carene at 6h-onstream correspond to an increase of these terpenes in the products. After 30 h-on-stream all the main terpenes are degraded during the HDS treatment. It should be noted that the conversion of p-pinene is higher in the turpentine oil than in the 8-pinene fraction. This is not due to a change in the terpene concentration : in an experiment of HDS carried out with a mixture of a-pinene, p-pinene and A3-carene (in the same proportion as in the turpentine oil), the conversion of p-pinene was 12%, very close to the conversion recorded

208

with the p-pinene fraction. It seems that the turpentine oil contain impurities increasing the isomerization activity of the catalysts. ACKNOWLEDGEMENTS The authors thank Mr Agouri (CETRA) for constant interest concerning this work. Thank are due to the Ministtre de la Recherche et de la Technologie for financial support (Contracts 84-F-0963 and 84-F-0964). REFERENCES 1 2 3 4 5

6 7 8

F. Casbas, D. Duprez, J. Ollivier and R. Rolley, Eur. Pat. 243238 (A1 . F. Casbas, D. Duprez, J. Ollivier and R. Rolley, Eur. Pat. 267833 (All. F. Casbas, D. Duprez and J. Ollivier, Appl. Catal., 50 (1989) 87. J.C. Duchet, E.M. van Oers, V.H.J. de Beer and R. Prins, J. Catal., 80 (1983) 386. F.E. Massoth, in "Adv. in Catalysis (D.D. Eley, H. Pines and P.B. Weisz, Eds.) Vo1.27 p265. Acad. Press, New York (1978). K.S. Chung and F.E. Massoth, J. Catal., 64 (1980) 320 and 332. M. Guisnet, J.L. Lemberton, G. Perot and R. Maurel, J. Catal., 48 (1977) 166. J.E. Germain and M. Blanchard, in "Adv. in Catalysis" (D.D. Eley, H. Pines and P.B. Weisz, Eds.) V01.20 p267. Acad. Press, New York (1969).

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

209

STUDIES ON THE CATALYTIC HYDROGENATION OF RESIN ACIDS DERIVATIVES: SYNTHESISOFABENZOXAZOLE

B. GIGANTE,l A. M. LOB0,2 S. PRABHAKJ~R,~ M. J. MARCELO-CURTO,' and D. J. WILLIAMS3

Laboratbrio Nacional de Engenharia e Tecnologia Industrial, Departamento de Tecnologia de lndustrias Quimicas, ServiCo de Quimica Fina, Estrada das Palmeiras, 2745 Queluz (Portugal) 2SecC%ode Quimica OrgAnica Aplicada, Departamento de Quimica, FCT, Universidade Nova de Lisboa, Quinta da Torre, 2825 Monte da Caparica, and Centro de Quimica Estrutural, Complexo I, IST, Av. Rovisco Pais, 1096 Lisboa Codex (Portugal) 3Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY (United Kingdom)

SUMMARY In the course of reduction experiments of methyl 12,14-dinitrodehydroabietate3, a new benzoxazole resin acid derivative 6 was synthesized and its structure established by spectroscopic data, chemical derivatization and X-ray analysis. INTRODUCTION Dehydroabietic acid 1 , the main resin acid of disproportionated rosin, is a readily available hydrophenanthrene derivative and a useful starting material for the synthesis of industrial and/or physiologically important products (I), by introduction of suitable substituents in the aromatic ring, such as the nitro or amino groups. Nitration

of

dehydroabietic

acid

methyl

ester

2

yields

methyl

12,14-dinitrodehydroabietate3 (2-3). We have previously reported (4) that the reduction

of 3 with tin/hydrochloric acid (5) afforded essentially methyl 12,144iaminodehydroabietate 5. However, work up proved troublesome due to the formation of tin derivatives, consisting mainly of I)-stannic acid (6), and as a consequence gave low yields of isolated product. This led us to look for alternative reducing agents.

Me

1 R, 2 R, 3 R, 4 R, 5 R, 7 R,

= COOH,

R,

= R, = H

6

H = COOMe, R, = R,= NO, = COOMe, R, = NO, R3= NH, = COOMe, &, R, = NH, = COOMe, R, = NHCOMe, R, = NO, = COOMe,

= R,=

EXPERIMENTAL The following commercial powder catalysts (all from Degussa) were used in the screening runs: rhodium-platinum oxide (45,65% Rh, 19,8% Pt), rhodium oxide (58,2% Rh), platinum oxide (8O,6% Pt), palladium-on-carbon (5% Pd) and platinum-on-carbon(5% Pt). The reactions were carried out in the liquid phase in a well stirred reactor at room temperature and under hydrogen pressure (40 psi). a) For reactions with metal oxides as catalysts the standard conditions were: the catalyst (8.3% w/w) was reduced to metal with hydrogen in the glacial acetic acid (6 ml), the starting material 3 (0.25 mmoles) was added and shaken with hydrogen. b) For reactions with metal on carbon as catalysts the standard conditions were: the catalyst (16% w/w) and starting material 3 (0.5 mmoles) in glacial acetic acid (34 ml ) were shaken with hydrogen. During the course of the reaction samples were withdrawn at appropriate intervals and analysed by GLC.

211

RESULTS AND DISCUSSION

In the course of experiments conducted with other reducing agents (viz. Rh2O3, PtO2, Pd/C, PVC) on methyl 12,14-dinitrodehydroabietate 3,the formation of varying proportions

of

methyl

12-amino-14-nitrodehydroabietate

4 and methyl

12,14-diaminodehydroabietate 5, along with the benzoxazole derivative 6 (Fig. 1), was

detected. The formation of a benzoxazole ring under these conditions has not been hitherto reported and experimental conditions necessary for maximum yield were studied. The highest yield of 6, ca. 58%, was obtained with Rh203/Pt02.xH20 in acetic acid and took over 6 days. When acetic anhydride or mixtures

of acetic acid/acetic

anhidride were used as the solvent, only the acetamide derivative 7 was formed. The yield of 6 was not improved by the use of catalysts such as Rh2O3, Pt02, Pd/C or

W C in

acetic acid (Table 1). TABLE 1 Catalytic Hydrogenation of Methyl 12,14-Dinitrodehydroabietate3 Catalyst

Solvent

Time (h)

7

3 AcOH Ac20 AcOH/Ac20(1:2) AcOH AcOH glac. AcOH glac. AcOH

160 160 180 162 162 48 45

0 0 42 0 0 0 0

0 0 0 0 68

3 39

20

58

0 0

0 0

42 21 74 20

24 0 12c) 23c)

a) Relative percentages by gc analysis of the end product.

b) Below detection limits; c) Not observed when conc. H2SO4 (0.1O/O v/v) was added to the solvent (4).

212

It is interesting to speculate that the formation of 6 with Rh2O3 is a result of the intermediate phenylhydroxylamine 8, the precursor for the amine 5, being 0-acylated to

9 and suffering a 3,3'-azaoxy Cope rearrangement. The product, an o -acetoxyaniline 10, eliminates a molecule of water and generates 6 (Fig. 2).

All the compounds were identified by physical and spectroscopic data and comparison with literature data (3,6). The structure of the new benzoxazole 6 was based on spectroscopic data and synthesis of its acetylated derivatives, and confirmed by X-ray crystallographic analysis.

pgH2 Ho\ H , N

'C0,Me

COPMe

8

9

CO, Me

%OOMe

6

10

Fig. 2. Proposed pathway for the formation of the benzoxazole derivative 6.

213

REFERENCES E. Schroder, R. Albrecht and C. Rufer, Dehydroabietylamin-Derivate und ihre antibakteriellen Eigenschaften, Arzneim.-Forsch., 20 (1970) 737-743. L. F. Fieser and P. Campbell, Concerning Dehydroabietic Acid and The Structure of Pine Resin Acids, J. Am. Chem. Soc., 60 (1938) 159-170. E. R. Littmann. The Resin Acids. The Action of Palladium on Abietic Acid, J. Am. Chem. Soc., 60 (1938) 1419-1421. 8. Gigante, A. M. Lobo, S. Prabhakar and M. J. Marcelo Curto, Studies on The European Catalytic Hydrogenation of DehydroabieticAcid Derivatives, in: Proc. !jth Conference on Biomass for Energy and Industry, Lisbon, Portugal, October 9-13, 1989, Elsevier, Amsterdam, in press. H. Becker, W. Berger, G. Domschke, E. Fanghanel, J. Faust, M. Fisher, F. Gentz, K. Gewald, R. Gluch, K. Schwetlick, E. Seiler, and G. Zeppenfeld, in: P. A. Ongley (Ed.) Organicum: Practical Handbook of Organic Chemistry, Pergamon Press, New York, 1973, p. 552. A. Tahara, M. Shimagaki, M. Itoh, Y. Harigaya and M. Onda, Diterpenoids. XXXVIII. Conversion of I-Abietic Acid into Steroidal Skeletons: Formation of the D-Ring, Chem. Pharm. Bull., 23 (1975) 3189-3202.

This Page Intentionally Left Blank

M.Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chernicak II 0 1991 Elsevier Science Publishers B.V., Amsterdam

215

THE ISOMERISATION OF LACTOSE TO LACTULOSE CATALYSED BY ALKALINE ION-EXCHANGERS B.F.M. KUSTER, J.A.W.M. BEENACKERS, H.S. VAN DER BAAN, Laboratorium voor Chemische Technologie, Technische Universiteit Eindhoven,

P.O. Box 513, 5600 MB Eindhoven (The Netherlands). SUMMARY

Lactulose is industrially produced by the homogeneous alkali catalysed isomerisation of lactose. The use of heterogeneous alkaline ion-exchangers as catalyst can result in a simpler overall process and in higher selectivities. The effect of the type of ion-exchanger, particle diameter, temperature and sugar concentration on the kinetics has been studied. A kinetic model is given which can be used to describe the experimental results. Loss of selectivity can occur due to a limiting diffusion rate. High sugar concentrations are beneficial for a high yield. Some proposals are given for process improvements. INTRODUCTION Lactulose, 4-O-~-D-galactopyranosyl-D-fructose,

is a synthetic

disaccharide which is commercially produced from lactose, 4-0-P-Dgalactopyranosyl-D-glucose, by alkali catalysed isomerisation. Production

estimates are 8000 tons per year and prospects are steady expansion (ref.1). A recent review has been written by Mizota et.al.

(ref.2).

In

most commercial procedures homogeneous alkalis are added t o catalyse the isomerisation. The use of alkaline ion-exchangers as catalysts is described by Demaimay and Baron (ref.3).

Recently Shukla et.al. (ref.4)

mentioned the use of zeolites. Up to now most patents and publications only give recipees. We did some kinetic studies on the heterogeneous alkaline isomerisation of carbohydrates, which have been described in the thesis of Beenackers (ref.5).,

a part of which will be presented here.

Experimental procedures are given there. Details on the physical properties of the ion-exchangers used have been published before (ref . 6 ) . Reaction Network Sugars behave as weak acids (pKA

-

12) and at high pH ionisation

occurs. Ionisation in its turn induces enolisation of the aldehydo and keto functions in sugars and via these enolate intermediates sugars are mutually interconverted. The enolate intermediates not only reconvert t o

216

sugars but also enter into degradation reactions, ultimately leading to acidic products by splitting and rearrangement reactions. The enolate ions are unstable intermediates, hence the pseudo steady state approximation can be applied to these intermediates, resulting in a kinetic model in which only stable components figure. It also can be proven (ref.5) that such a model will be mathematically equivalent to the one as follows from the network presented in figure 1. LA \

kLA

LA-

Lu-

/

LU

LA(-) = lactose (anion) LU(-) = lactulose (anion)

ku.\

kLu/

kLUD

GAL

=

k

= rate constants

D-galactose

GAL

+ ACIDS Fig. 1.

Simplified reaction network of lactose isomerisation.

The network as depicted in the figure is a very simplified one. It does not incorporate epi-lactose (C-2 epimer of lactose, 4-0-P-D-galacto-

pyranosyl-D-mannose) and further reactions of D-galactose. However, it does describe the major events: the interconversion of lactose and lactulose, the formation of D-galactose by an elimination reaction and the simultaneous formation of acidic by-products from the glucose moiety. The major acidic product is iso-saccharinic acid. The consequence of the formation of acidic by-products is that the reaction is self-poisoning: the acids neutralize the alkaline catalyst and the reaction virtually stops as soon as an amount of acid has been formed equivalent to the amount of alkali added as catalyst. This is true for the commercial homogeneous alkali catalysed reaction as well as for the heterogeneous alkali catalysed reaction. RESULTS AND DISCUSSION Batch experiments have been carried out with several anion-exchangers in the OH--form, starting with lactose as well as with lactulose at temperatures in the range 303 to 363 K. Final conversions, which depend

on the catalyst to sugar ratio, generally were reached within 8 ks, depending on temperature. Using anion-exchangers as a heterogeneous

217 catalyst results in a neutral, ion-free sugar mixture as the product, while all degradation acids are retained within the catalyst. Samples of the sugar mixture were analysed at different reaction times by ionexchange chromatography (ref.7). The catalyst could be regenerated with alkali, washing out the degradation products. A typical composition of the washings, as determined with isotachophoresis (ref .5),

is presented in

table 1. TABLE 1 Composition (mol fraction) of acid product mixture washed out of the deactivated catalyst. Conditions: Lactose, 200 mourn3; Amberlite IRA 904, 40 mourn3; T, 333 K. formic acid

.21

dihydroxybutyric acid

.12

acetic/glycollic acid

.05

deoxypentonic acid

.04

lactic acid

.03

meta-saccharinic acid

.ll

glyceric acid

.01

iso-saccharinic acid

.43

From the reaction network we derive the following kinetic equations

(N = number of moles):

the initial selectivity for lactulose from lactose, S o , is given by:

In order to evaluate the rate constants from the analysed sugar concentrations, we need information on the amount of sugar present in the ionized form inside the ion-exchanger as a function of these external concentrations. These relations have been determined separately and we previously reported on ionisation (ref.8) and adsorption (ref.9).

Using

these relations rate constants were calculated from initial rate data (ref.5) and the results will be presented in the following sections.

218

I n f h n c e o f t h e tvDe o f a t - a l y s t Using s e v e r a l ion-exchangers ( r e f . 6 ) , s t a r t i n g w i t h l a c t o s e , t h e s e l e c t i v i t y f o r l a c t u l o s e has been determined. A t 313 K as well as a t 333 K t h e m a c r o r e t i c u l a r r e s i n s IRA 904 and IRA 938 gave t h e h i g h e s t v a l u e s ,

b o t h r e s i n s have an e x c e p t i o n a l l y h i g h p o r o s i t y .

TABLE 2 I n f l u e n c e of p a r t i c l e s i z e and p o r o s i t y of t h e ion-exchanger on t h e a p p a r a n t r a t e c o n s t a n t s and i n i t i a l s e l e c t i v i t y . C o n d i t i o n s : L a c t o s e , 600 m o l d ; Ion e x c h a n g e r , 40 mol/m3; T, 313 K.

IRA 401 (gel type) .47 .05

dp'

This f a c t

IRA 904 (macroporous) .50 .05

.21

.41

.54

-05

.022

-063 .052

.80

.95

.90

.51

.91

i d i c a t e s I a t s e l e c t i v i t y w i l l b e lowered due t o pore

d i f f u s i o n l i m i t a t i o n . T h i s i s i l l u s t r a t e d i n t a b l e 2 , where a p p a r a n t r a t e c o n s t a n t s and i n i t i a l s e l e c t i v i t y a r e g i v e n f o r IRA 401 g e l t y p e and IRA 904 macroporous type ion-exchanger i n normal s i z e ,

- 0.05

m. The macroporous ion-exchanger

-

0 . 5 mm, and ground,

as w e l l as t h e ground g e l t y p e

do n o t c a u s e d i f f u s i o n problems, however, t h e unground g e l t y p e does. Because l a c t u l o s e i s more s e n s i t i v e t o d e g r a d a t i o n t h a n l a c t o s e , a low d i f f u s i o n r a t e c a u s e s l a c t u l o s e t o b r e a k down b e f o r e i t l e a v e s t h e c a t a l y s t r e s u l t i n g i n a high a p p a r e n t v a l u e f o r kUD. The r e s u l t s a l s o show t h a t t h e g e l t y p e c a t a l y s t g i v e s a h i g h e r s e l e c t i v i t y provided t h a t d i f f u s i o n c o n t r o l can b e a v o i d e d . More d e t a i l s on d i f f u s i o n of s u g a r s i n ion-exchangers can b e found elswhere ( r e f . 1 0 ) .

Inf l u e n c e ~ f - ~ h ~ - ~ ~ m ~ ~ ~ r ~ u ~ Some d a t a on t h e e f f e c t of t e m p e r a t u r e a r e g i v e n i n f i g u r e s 2a and b . Below 313 K an a c t i v a t i o n energy of 90 KJ/mol i s e s t i m a t e d f o r b o t h kU and kLU. Above 313 K d i f f u s i o n l i m i t a t i o n o c c u r s , r e s u l t i n g i n a d e c r e a s i n g a p p a r e n t a c t i v a t i o n energy and a l s o i n a lower s e l e c t i v i t y a t higher temperature.

2 19

b

a -3

1.00

s,

C

-6

.

-7

.

W Y

Y

5

0.80

-8. -9

.

-

2.70

2.80

2.90

3.00

3.10

3.20

300 310

320 330 340 350 360

1000/T (T in K )

T (T in K )

Fig. 2a. Arrhenius plot of kU and kLU, 2b. Selectivity of the lactose isomerisation. Conditions: Amberlite IRA 9 0 4 , 40 m o u r n 3 ; lactose: 600 mourn3. Influence of the concentration At two different concentrations isomerisation experiments have been carried out with lactose as well as with lactulose. The results are shown in table 3 , together with the initial catalyst condition data as calculated from separate absorption and ionisation measurements (refs.8,9). It appears that a higher selectivity can be obtained at higher initial concentrations. TABLE 3 Influence of lactose/lactulose concentration on initial catalyst condition. apparent rate constants and initial selectivity. Conditions: Amberlite IRA 9 0 4 , 40 mol/m3; T, 313 K. c

~ in solution, , ~ mol/m-l ~

CLA,Lu in ion exchanger, mol/m3 CHz0 in ion exchanger/CH pure 2

90

5 70

250

5 70

0.94

0.87

13.43

13.15

0.60

0.54

0.20

0.27

0.15

0.064

0.67

0.51

0.81

0.90

220

From the rate constants it can be noticed that a lower C

and a H20 lower pH inside the ion-exchanger result in a shift of equilibrium backwards to lactose and it must be concluded that our system deviates from an ideal one. We further see that this slightly negative effect is overcompensated by a decrease of the rates for degradation. Apparently, the ratio of rates for reverse enolisation (sugar formation) and degradation of the enolate intermediates is influenced by CHz0 and pH. This is not yet accounted for in our kinetic model. INDUSTRIAL APPLICATION Because lactulose has a high degradation rate, good selectivities can only be obtained at low lactose conversions. In industrial practice concentrated lactose solutions are heated with approximately 0.04 mol alkali per mol lactose. This results in a final conversion of about 20% and a selectivity of 80’1. which is the point where the alkali is neutralized by the acid degradation products (i.e. 0.04 = 0.2 x (1-0.8)). By concentrating and cooling most of the residual lactose can be crystallized and recycled, leaving a product mixture consisting of lactulose, D-galactose, lactose, epi-lactose and salts. For further work-up the salts are removed by cation- and anion-exchange. Using alkaline ion-exchangers as the catalyst can result in a cheaper process because only one type of exchanger is needed. From our results it appears that good selectivities, generally higher than those reported in the patent literature, can be obtained, provided that diffusion control is avoided. Especially high sugar concentrations are beneficial and first trials in our laboratory have shown that also at high temperature and high concentration good selectivities are reached using less active catalysts which, therefore, do not cause diffusion problems. Although up to now the commercial product generally is a sugar mixture containing lactulose, there is a trend to further purification. We recently developed an oxidative procedure using a Pd-catalyst, in situ doped with Bi, as described elswhere (ref.11). Using this catalyst all aldose sugars i.e. D-galactose, lactose and epi-lactose, in an isomerisation mixture could be oxidized with air or oxygen to the corresponding aldonic acid salts, without any breakdown of lactulose. The salts could be removed by ion-exchange resulting in a high-purity lactulose solution.

221 REFERENCES

1.

2. 3. 4. 5.

6. 7. 8. 9. 10.

11.

H . Pluim, Duphar BV, p e r s o n a l communication. T. M i z o t a , Y. Tamura, M. Tomita, S. Okonogi, B u l l . I n t . D a i r y Fed., 212 (1987) 69-76. M. Demaimay, C. Baron, L a i t , 58 (575-576) (1978) 234-45. R. S h u k l a , X.E. V e r y k i o s , R. M u t h a r a s a n , Carbohydr. R e s . , 143 (1985) 97-106. J.A.W.M. B e e n a c k e r s , Ph.D. T h e s i s , May 1980, T e c h n i s c h e U n i v e r s i t e i t Eindhoven (The N e t h e r l a n d s ) . J.A.W.M. B e e n a c k e r s , B.F.M. K u s t e r , H.S. van d e r Baan, Appl. Catal., 16 (1985) 75-87. L.A.Th. V e r h a a r , M . J . M . v a n d e r Aalst, J.A.W.M. Beenackers, B.F.M. K u s t e r , J . of Chromatogr., 170 (1979) 363-370. J.A.W.M. B e e n a c k e r s , B.F.M. K u s t e r , H.S. van d e r Baan, Carbohydr. R e s . , 140 (1985) 169-183. J.A.W.M. B e e n a c k e r s , B.F.M. K u s t e r , H.S. v a n d e r Baan, Appl. C a t a l . , 23 (1986) 183-197. J.A.W.M. B e e n a c k e r s , B.F.M. K u s t e r , H.S. van d e r Baan, Appl. Catal., 23 (1986) 199-206. H.E.J. H e n d r i k s , B.F.M. K u s t e r , G.B. Marin, Carbohydr. R e s . , i n p r e s s .

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Hetrrogeneous Catalysis and Fine Chemicals I1 1991 Elsevier Science Publishers B.V., Amsterdam

223

Q

FURANIC DERIVATIVES SYNTHESIS FROM POLYOLS BY HETEROGENEOUS CATALYSIS OVER METALS. C. MONTASSIER, J.C. MENEZO, J. MOUKOLO, J. NAJA, J. BARBIER, URA CNRS 350 Laboratoire de chimie 4 40, Avenue du Recteur Pineau 86022 POITIERS CEDEX J.P. BOITIAUX Institut Fragais du Petrole 1-4 Avenue de Bois Preau 92506 RUEIL-MALMAISON CEDEX

ABSTRACT Raney copper catalyst is modified by mixing with aqueous salts of metals (Ir, Rh, Ru, Pd, Pt, Au) having a standard oxido-reduction potential higher than that of Cu. On pure Raney copper it is assumed that 10-15% of the surface copper atoms are hydroxylated and that these sites are responsible for the C-C and C - 0 bonds breakin observed during polyols conversion (dehydroxylation, retro-CLAISEN, retro-MICHAELf These sites are the first exchanged during the oxido-reduction modification, so that the above reactions are poisoned. For higher coverages of the additives, new sites appear that catalyze polyols cyclodehydration into furanic derivatives (glucitol is mainly converted into 1,43,6dianhydro-D-glucitol called isosorbide). This phenomenon is interpreted through an electronic transfer from copper to the second metal, leading to electrophilic copper. INTRODUCTION In a previous study (1) dealing with polyols conversion (glycerol, erythritol, glucitol) in neutral aqueous phase, between 453 and 533 K, under hydrogen pressure (3-6 MPa), we showed that ruthenium hydrogenolyses drastically both C-C and C - 0 bonds. Raney copper has not such a property but is able to convert polyols through its hydro-dehydrogenation activity according to the following scheme. Adsorption of polyols over copper surface and dehydrogenation, favoured at the end of the carbon chain, lead to unsaturated species strongly adsorbed which can undergo the nucleophilic attack of adsorbed hydroxyls ("OH), leading to dehydroxylation (DOH) :

224

H ., C=CH2

I OH /

I

2

H2C -CH

-CH,

I 1 OH OH

61

OH OH

If more intensive dehydrogenation occurs, for instance at 1 and 3 positions, the reaction of a "OH"group can lead to retro-CJAISEN products (RC) : H

OH

Formic acid is quickly dehydrogenated into carbon dioxide. If the carbon chain of the polyol contains 5 or more carbon atoms, a retroMICHAEL reaction (RM) can occur from a 1-5 didehydrogenated species :

Finally, unsaturated species are hydrogenated by copper and can desorb. This last reaction (RM) enables us to understand the formation of products containing 3 carbon atoms (glycerol, 1,Zpropanediol) from glucitol (sorbitol) but is always in competition with the two other ones (DOH, RC). The ratio of these three reactions, determining the conversion selectivity, depend widely on the copper origin (Raney, deposited on a support, impurities, activation process). So, we studied the influence of additives deposited on Raney copper on these reaction selectivities. EXPERIMENTAL The experiments were carried out in a static reactor (300 ml autoclave) with aqueous polyols solutions (0,ll to 0,44M). Hydrogen pressure was set in the 3,O-6,0 MPa range and temperature between 483 and 533 K. Samples drawn from the liquid and gaseous phases in

225

the course of the reaction were analyzed by HPLC (BIO-RAD HPX 87H and 87C columns, refractometric detector) and by CPV (PORAF'AK Q column). Raney copper is prepared by intensive leaching of a commercial copper-aluminum alloy (50-50 wt%) washed with water until neutral. Bimetallic catalysts are obtained using an oxido-reduction method : summarized as : n CU-s + 2 M"+ ---> n cu2+ + 2 M, in which M may be : Ir, Rh, Ru, Pd, Pt, Au whose standard oxido-reduction potentials are higher than that of Cu. This reaction is carried out by simply contacting (15 hours) an aqueous suspension of a known amount of Raney copper stirred by nitrogen bubbling with a known amount of a second metal salt solution. Knowing the number of copper surface mol.Cug-' mesured from N20reaction), the amount of salt (chlorides) atoms. (4,36 x is set so as to obtain the desired coverage : M/Cus, that is the atomic ratio second metal/initial surface copper. The following bimetallic catalysts were prepared : bimetals

Cu-Ir

Cu-Rh

M/Cus

0,lO

0,17

CU-RU

0 to 0,35

Cu-Pt

Cu-Pd

CU-AU

0 to l,o

0,80

0,90

RESULTS AND DISCUSSION Selectivitv of Dolyols conversion over the Cu -Ru bimetallic catalysts Ru/Cu, = 0.33 Glycerol (0,44M), at 533 K under 4MPa of hydrogen, is not converted after 6 hours of reaction (4g of catalyst). mol.min-I gIn the same conditions, erythritol (0,33M) is transformed (2,52 x 'cat) with a high selectivity (near 100%) into 1,4-anhydroerythritoI without any isomerisation.

mol.min-' g-l Likewise 1,4-butanediol is cyclised into tetrahydrofuran (2,93 x cat.at.493 K). Xylitol is transformed (2,O x 104mol.min-1g-1cat. at. 533 K) into 1,4-anhydro-(D,L)xylitol :

226

Glucitol ("sorbitol") is converted at the initial rates of 0,135 x 104mol.min-1g-1c;,t.nt 493 K and 8,8 x 104mol.min'1g-1cat. at 533 K. The main path is its cyclization into, first, (isosorbide) (figure 1). 1,4-anhydro-D-glucitol and then to 1,4-3,6-dianhydro-D-glucitol Two secondary ways of conversion are observed leading to 2,s-anhydro-L-iditol and 2,sanhydro-D-mannitol. All these cyclodehydration products are the result mainly of an inside chain oxygen attack on a primary carbon : the chiral carbon configurations remain unchanged. l h e only exception to this rule concerns the secondary products obtained from glucitol. 2.5-anhydro-L-iditol arises from the attack of the oxygen at C-2 on C-S which configuration is inversed. 2,s-anhydro-D-mannitol arises from the attack of the oxygen at C-5 on C-2 which configuration is inversed. In the case of glucitol conversion, the anhydroalditols total yield is 71% at 533 K, the other products obtained arising from degradations (breaking of C-C and C - 0 bonds). klectivity of b c i t o l conversion over the Cu-M bimetallic catalvsb Results are presented in table 2 for glucitol conversion at 493 K. TABLE 2 Glucitol (0,ll M) conversion over various bimetallic catalysts. Initial rate : molmin- lg- ]cat lo4. Products distribution in mole % of initial glucitol. GCL = glucitol, MAH = monoanhydrohexitols, ISOS = isosorbide. Catalyst

Cu-Pt CU-AU Cu-Pd CU-RU Cu-Rh Cu-Ir

M/Cu,

18 0,YO

0,80 0,35 0,17 0,lO

Initial rate 2,0 1,22 0,4 1 0,135 0,02

Products distribution after 360 min. %GCL %MAH 7,3 0 67 20.1 56,s 47,7

76,3 61,4 753 72,7 23,9 42,6

%lSOS 16,5 34,l 18,O 7,2 45

2,8

227

Figure 1 : Glucitol conversion over Cu-Ru (Ru/Cus = 0,33) at 533 K. The product distribution indicated is in mole percent in the mixture of all anhydrohexitols which total yield relative to initial glucital is 71 mole%

228

Initial conversion rates are not easy to compare because of the very different coverages obtained according to the second metal used. We can only notice that for M/Cus = 0,lO and 0,17 (Cu-Ir and Cu-Rh), these rates are very low. The main important feature i , that the same cyclodehydration reactions are observed, like that for Cu-Ru catalyst, whatever the nature of the second metal is. Activity and selectivitv variations according to &metal covera& As shown in figure 2 for glucitol conversion at 493 K for platinum and ruthenium as additives, the first atoms exchanged with copper are strong poisons for the copper catalyst until M/Cus = 0,lO to 0,15. In the range (A), the selectivity observed is that of Raney copper (DOH mainly, RC, RM). No cyclodehydration products have been detected. Above M/Cus = 0,lO to 0,15 (C) the activity increases and the cyclodehydration reaction leading to furanic products is the main one. This cyclodehydration is well known, especially for the synthesis of isosorhide (2-5) and is usually carried out in strongly acidic medium. Its mechanism was proved (4) to be a SN2 intramolecular reaction catalyzed by protonation of the alcoholic functions. The products distributions we obtained are close to those obtained by acidic catalysis.

?5

RU

Pt +

3 0 -

' 0

1

4

05

00

I

I

0

07

0

08

08

1

M/Cu (surf.)

I

A

0 4

c

Figure 2 : Initial rate variations of glucitol conversion at 493 K in function of M/Cus for M = Ru (m) and Pt (+). From all these results we suggest the following scheme :

229

OH

I

C u Raney

c u - c u - c u - c u - c u - c u CL-CU-CU-CU

A

+H,-H

DOH,RC,Xbl

CU-M M/Cu s

0,lO

CU-M M/Cu s > 3,lO

CU-CU-M CU Cu-C u - C U - C U - C U - C U

-

+H, H

Cu C U \, Gu-CU \: Cu-Cu-M-CU u -2

'\

B

C

//H.-H CDOIl I

,

In A, a Raney copper catalyst would be able to hydro-dehydrogenate alcoholic functions (-H, + H ) on metallic copper sites. About 10 to 15% of the copper would be hydroxylated copper able to catalyze the degradation reactions DOH, RC, RM. These sites would be more reactive than Cu towards Mn -t in the oxido-reduction modification of the initial Raney copper, so that, beeing first exchanged, the rates of DOH, RC, RM decrease. In C, further exchange between metallic copper and M"' would create new sites able to catalyze the cyclodehydration reaction (CDOH). These sites must be, as protons, electrophilic centers able to weaken C-0 bonds enough to allow intramolecular SN2 reactions mainly on primary carbon atoms. As the standard oxido-reduction potential of copper is lower than that of the second metals used in this work we suggest that an electronic transfer from copper to M (Ir, Rh, Ru, Pd, Pt, Au) could generate electrophilic copper M- - Cu' able to catalyze the cyclodehydration according to the scheme :

CONCLUSION The catalytic properties of copper during polyols conversion in aqueous phase may be drastically modified by some additives. Metals having a standard oxido-reduction potential higher than that of copper (Ir, Rh, Ru, Pd, Pt, Au) can be deposited on it by oxidoreduction reaction. The first atoms of second metal deposited exchange with hydroxylated

230

copper which amount to 10 to 15% of the initial superficial copper. The hydroxylated copper would be the sites active for dehydroxylation, retro-CLAISEN and retroMICHAEL reactions, that, so, are poisoned by the additive. When the exchange is higher than 10-15% of the surface copper, new sites appear, able to catalyze cyclodehydration reactions leading to furanic derivatives. We suggest that an electronic transfer from copper to the additive undergoes electrophilic copper sites able to adsorb alcohols and to initiate intramolecular SN2 reactions as it is well known for acidic catalysis of the cyclodehydration. Despite the practical interest of the good selectivity we obtained for these reactions (6) the variations in selectivity and activity according to the additive coverages show that relatively complex molecules such as natural polyols, which configurations are well established, are useful probe molecules able to bring out important informations about catalytic sites and their properties. ACKNOWLEDGMENTS We thank the Institut Francais du PCtrole for their financial support. REFERENCES 1 a) C. Montassier, D. Giraud, J. Barbier, Proc. “1st Int. Symp. Heterogeneous Catal. and Fine Chemicals”. Poitiers, France, 1988, Elsevier, Amsterdam, 41 (1988), 165-70. b) C. Montassier, D. Giraud, J. Barbier, J.P. Boitiaux, Bull. Soc. Chim. Fr. (1989, 148-55. R.M. Goepp, H.G. Fletcher, J. Am. Chem. SOC.,68 (1946), 939-41. 2 3 B.G. Hudson, R. Barker, J. Org. Chem. 32 (1967), 3650-8. 4 K. Bock, C. Pedersen, M. Thogersen, Act. Chem. Scand., B 35 (1981), 441-9. 5 F. Jacquet, A. Gaset, J.P. Gorrichon, Information Chimie, 246 (1984), 155-8. 6 J. Barbier, J.P. Boitiaux, P. Chaumette, S. Le Pors, J.C. Menezo, C. Montassier, Eur. pat. 90.400.177.3 (1990)

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

231

Q 1991 Elsevier Science Puhlishers B.V., Amsterdam

ACTIVITY AND STABILITY OF PROMOTED RANEY-NICKEL CATALYSTS IN GLUCOSE HYDROGENATION P.J. CERlNOl , G. FLECHE2. P. GALLEZOTl and J.P. SALOMEZ lnstitut de Recherches sur la Catalyse, C.N.R.S.. 2 avenue Albert Einstein, 69626 Villeurbanne CBdex - (France) 2 Societe Roquette Freres, 62136 - Lestrem (France)

-

-

SUMMARY The activity and stability of Raney-nickel catalysts promoted with molybdenum, chromium and iron have been studied in glucose hydrogenation. There are several causes of deactivation. Sintering plays a minor role after four recyclings. Leaching of iron atoms from the nickel surface IS the main cause of the rapid deactivation of ironpromoted catalysts. Poisoning by organic fragments produced by glucose cracking on the nickel surface could be the main cause of deactivation. The presence of chromium or molybdenum on the nickel surface is beneficial for the stability because they decrease the crackin activity and favor the activation and hydrogenation of the carbonyl group of the aldehyjc form of glucose. INTRODUCTION It is well known, even from old literature data (ref. 1) that the presence of metal promotors like molybdenum and chromium in Raney-nickel catalysts increases their activity in hydrogenation reactions. Recently Court et al (ref. 2) reported that Mo, Cr and Fe-promoted Raney-nickel catalysts are more active for glucose hydrogenation than unpromoted catalysts. However the effects of metal promotors on the catalytic activity after repeated recycling of the catalyst have not been studied so far. Indeed, catalysts used in industrial operation are recycled many times, stability is then an essential criterion for their selection. From a more fundamental standpoint, the various causes of Raneynickel deactivation have not been established. This work was intended to address two essential questions pertinent to the stability of Raney-nickel in glucose hydrogenation namely what are the respective activity losses experienced by unpromoted or by molybdenum, chromium and iron-promoted catalysts after recycling and what are the causes for their deactivation ? EXPERIMENTAL Precursor alloys of composition Ni40-~M~A160 were prepared by cooling metal melts under argon atmosphere. The ingots were annealed for three weeks at 950°C under argon, then ground and sieved to keep grains smaller than 40 pm. Composition of the alloys are given in table 1. Batches of 100 g were leached by 500 cm3 of soda (6N) added slowly at room temperature. The suspension was refluxedfar two hours and washed with

232

Table 1 : Composition and characterizationof Raney-nickel catalysts

Catalysts

Alloy composition

MINI ( x i 02)

RNi Ni39.2Al60.8 RNiMo Ni38.2A160.8MOl 2.6 RNiCr Ni38.3A160.6Crl 2.7 RNiFe Ni34.gAI58.7Fe6.4 18.5 RNilndl a RNilnd2a -

Catalyst composition AI/Nib M/Nib X(I

0.19-0.1 8 0.25-0.21 0.28-0.24 0.45-0.34 0.11-0.09 0.13-0.1 1

02)

0.89-0.84 2.6-2.2 18.5-6.9 0.80-0.81 0.1 -0.1 1

BET areab (m2/g)

Crystallite sizeb (A)

82-64 79-70 116-73 97-75 77-74 101-61

43-47 38-45 34-39 45-36

a - industrial catalyst promoted with molybdenum b - before and after five hydrogenation cycles soda (1N). The powder was then submitted to three refluxing treatments in 6N, 4N and 2N soda solutions and finally kept under 1N soda. The composition of the different catalysts thus prepared and of two industrial catalysts are given in table 1. BET surface areas were measured after outgassing the catalysts under vacuum at 120°C for 4h. X-ray patterns were recorded with catalysts kept under water. Crystallite sizes were obtained with the Scherrer formula. The local composition of catalysts was measured by energy-dispersive X-ray emission (EDX) associated with a VG HB501 scanning transmission electron microscope (STEM). The spatial resolution of analysis is 1.5 nm. The STEM-EDX study was performed on ultramicrotome sections of Raney-nickel grains embedded in an epon resin. Before hydrogenation the pH of a D-glucose solution (3.37 mol-1) was adjusted at 6.5 with acetic acid. The solution was heated at 60°C and poured in an autoclave containing 2 wt % of Raney-nickel with respect to glucose monohydrate. The autoclave was pressurized under 45 bars of hydrogen and the temperature was increased from 60 to 130°C. The reaction was started after pressurization at 50 bar and stirring at 1400 rpm. Samples of the reaction medium were periodially taken for HPLC analysis (Column Biorad HPX 87C at 85°C). After the first hydrogenation the reactor was purged, the catalyst was washed and a new charge of glucose was hydrogenated. All these steps were conducted under H2-pressure. RESULTS Table 1 shows that the precursor alloys have the expected composition Ni40-~M,A160 within analytical errors. The X-ray pattern of the precursor alloy of RNi and RNiCr is characteristic of the Ni2A13 hexagonal phase. In the case of iron-promoted alloy there are weak additional reflections corresponding to the A15Fe2 phase. In the case of molybdenum, A13Mo and AlgMog phases are detected in agreement with literature data showing that molybdenum has a low solubility in the Ni2A13 phase (ref. 3). The fresh

233

catalyst RNiMo has lost a large amount of molybdenum with respect to its precursor alloy (table 1). This is in contrast with RNiCr and RNiFe where the M/Ni ratios did not change after soda attack. This lost is probably due to the large fraction of molybdenum not associated with nickel in the Ni2A13 phase. As noticed previously (ref. 3) the presence of a third metal increases the retention of aluminum. The local composition of the catalysts was measured by STEM-EDX on different zones of a given ultramicrotome section (edge, core), on different areas (1 nm2-1 pm2) and on different sections. In all catalysts except RNilnd2, the promotors are distributed throughout the nickel grain on a nanometer-sized scale. In RNiMo, inclusions of a Morich phase have been detected, they could result from the attack of the A13Mo and AlgMog phase detected in the precursor alloy. In RNilnd2, the concentration of molybdenum is very heterogeneous the promotor being concentrated near the external surface of the catalyst grains. Figure 1 (a-f) gives the conversion of glucose as a function of time for the different catalysts, fresh and during four successive recyclings. The initial rates expressed per catalyst weight are given for the first and fifth hydrogenation.The selectivity to sorbitol was always higher than 97 %. the less active catalysts giving the lowest selectivity because a fraction of dextrose isomerizes into fructose which is subsequently hydrogenated into mannitol. After five hydrogenation runs, the BET area decreases for all catalysts (table 1) which could be attributed to a partial sintering. However the area loss could be due to a poisoning of the catalyst. Indeed, the presence in Raney nickel grains of strongly bound organic residue, which could not be washed out or outgassed, would decrease the amount of physisorbed nitrogen. Table 1 gives the average sizes of nickel crystallites measured by X-ray line broadening analysis on (1 11) reflections, before and after the five hydrogenation runs. They increase moderately and even decrease for RNiFe. This confirms that the BET area loss could be due in part to a poisoning which reduces the capacity of nitrogen adsorption. However, measurements of the metallic surface area should also be done to confirm possible surface poisoning. DISCUSSION From figure 1 (a - e) it is clear that the activities of Raney-nickel catalysts increase with the addition of promotors, iron producing the largest rate enhancement in agreement with previous reports (ref. 3). These effects can be tentatively interpreted by the following mechanism. The promotor atoms on the surface of nickel crystallites are more electropositive than nickel since the electron affinities (ref. 4) are in the series Fe < Cr < Mo < Ni (15.7:64.2:71.9:115.5 kJ mol-1). Even under H2-pressure. they can be positively charged and act as adsorption sites for the glucose aldehydic form via the oxygen atom of the carbonyl group. The polarization of the C = 0 bond favors a

234 Conventon tx)

100 . I -

o

M

40

MI

60

100 im

140

160 IMI

Time (min) Conmnlon (X) -

loor

mo

--

~

80

60

i 40

lo

50

----

1

o

m

40

Conraalon

60

I%)

ro

1.375

1.96

01

7

(LO ioo I M Time (min)

160 180

140

lo

5O

2.98

0.59

ao

mo

o

1

T

I

r

m

40

60

MI

,

7- 7--,-7--

loo

im

140 ibo IM

mo

I

20

After indwtrlnl

0

30

00

80

120

I

1

160

180

1

1

I

8

0

30

Time (min)

- 1 Hydro.

+-

2 Hydro.

80

80

-

120

160

180

Time ( min ) --C

3 Hydro.

G --I

4 Hydro.

-

5 Hydro

Fig. 1 : Conversion of glucose as a function of time in five successive hydro enations for the different catalysts (a) RNi ; (b) RNYo ; (c) RNiCr ; (d) RNiFe ; (e) WNilndl ; (f) RNilnd2. The initial rates ro (mol h-lg- ) are given for the first and fifth reactions. The lower curve in (e) corresponds to a catalyst after many industrial hydrogenations.

235

nucleophilic attack of the carbon atom by hydrogen dissociated on neighbouring nickel atoms as suggested by the following scheme.

HO

After successive recyclings the catalyst activities decrease at different paces. Thus, during the fifth run the hydrogenation rate on RNiFe is five times smaller than during the first, whereas the rate of RNiMo and RNiCr are divided by 2 and 1.4 respectively. Catalyst RNilndl promoted with molybdenum deactivates almost like RNiMo (figures 1b, 1e) whereas RNilnd2 deactivates rapidly (figure 1f). One cause of catalyst deactivation is the sintering of the metal phase. This is obvious for RNilndl which after much recycling in industrial conditions has lost both its activity (lower curve in figure 1e) and its BET area (14 m2g-1). It was checked by electron microscopy that the nickel crystallites are large and agglomerated in this aged catalyst. Although in the other catalysts there is a simultaneous decrease of BET area and activity, it cannot be concluded that deactivation after five runs is due mainly to metal sintering. As mentionned above, the decrease in BET area could be partly due to the presence of organic residues indeed the little increase of nickel crystallite sizes points at a moderate sintering. Besides, there is no proportionality between the BET area and the activity, thus in RNiCr, there is a large apparent loss of BET area without much deactivation. Clearly there are other factors contributing to catalyst deactivation. Thus in RNiFe the rapid aging after successive recycling has nothing to do with sintering since the crystallite sizes even decrease. It can be attributed to an extraction of the iron atoms from the surface which are solubilized in the reaction medium as shown by the decrease of the Fe/Ni ratio after reactions (table 1). Then the activity is no longer promoted by the mechanism discussed above. Another cause of deactivation for all the catalysts and especially for the unpromoted ones is the poisoning of active sites by side reaction products. Indeed nickel is well known for its cracking activity producing organic fragments which remain adsorbed strongly on the nickel surface. The presence of chromium or molybdenum on the nickel surface (and also of aluminum which is in larger amount in

236

promoted-catalysts) could reduce this cracking activity either by geometric effect (size of nickel ensemble) or by electronic effects (electron transfer to nickel). Thus the RNilnd2 catalyst deactivates rapidly because molybdenum is not distributed throughout catalyst grains as shown by STEM-EDX analysis, whereas the other Mo-promoted catalysts and RNiCr keep their activities. The presence of organic fragments poisoning the surface has been confirmed by magnetic measurements which will be reported elsewhere. Indeed the magnetization of nickel was found smaller after reaction on RNi but not on RNiCr, indicating that surface nickel atoms in the former catalyst are "demetallized" by chemisorbed species. CONCLUSION Raney nickel catalysts promoted by molybdenum, chromium and iron exhibit higher initial activities in glucose hydrogenation because electropositive atoms on the nickel surface polarize the carbonyl group of the glucose molecule adsorbed via the oxygen atom. The larger the electropositivity, the larger the activation of the carbonyl group. The activities decrease progressively after successive recyclings. Threee factors are involved in the deactivation process (i) a leaching of the promotor atoms from the nickel surface. This process is specially marked on iron-promoted catalysts which deactivate rapidly after the first reaction. (ii) a poisoning of the catalyst by organic fragments produced by side cracking reactions. This would be the main cause of deactivation during the first few runs. However there is little deactivation for molybdenum and chromium promoted catalysts (iii) a decrease in the surface area due to nickel crystallite sintering. This is the major cause of deactivation after a large number of recycling e.g. after a long period of industrial operations. REFERENCES 1 2

3 4

R. Paul, Bull. SOC.Chim. Fr. 13 (1946) 208. J. Court, J.P. Damon, J. Masson and P. Wierzchowski in : M. Guisnet et al (Ed.), Heterogeneous catalysis and fine chemicals, Elsevier, Amsterdam, 1988, pp 189196. L. Kaufman and H. Nesor, Metallur ical Trans., 5 (1974) 1627. H. Hotop and W.C. Lineberger, J. Fhys. Chem. Ref. Data, 14 (1985) 731-750.

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

237

1991 Elsevier Science Publishers B.V., Amsterdam

TRANSFORMATION OF SUGAR I N T O GLYCOLS ON A 5% Ru/C CATALYST

P. MULLER, P. RIMMELIN, J.P. HINDERMANN, R . KIEFFER, A. KIENNEMANN L a b o r a t o i r e de Chimie Organique Appliquee E H I C S URA CNRS 469 1, r u e B l a i s e Pascal 67000 STRASBOURG FRANCE J . CARRE' SUCRERIES ET RAFFINERIES 67150 E R S T E I N FRANCE

SUMMARY Saccharose h y d r o g e n o l y s i s was performed i n a s l u r r y t y p e r e a c t o r i n presence o f a 5% Ru/carbon c a t a l y s t . M o d i f i c a t i o n o f pH d u r i n g t h e r e a c t i o n can i n c r e a s e t h e y i e l d o f 1,Z-propane d i o l and g l y c e r o l n o t i c e a b l y . An adsorbed complex i s proposed t o account f o r t h e d i f f e r e n c e i n s e l e c t i v i t y f o r v a r i o u s C 5 and C sugars. 6 INTRODUCTION

The p r o d u c t i o n excess o f saccharose i n t h e w o r l d i n g e n e r a l and i n Europe i n p a r t i c u l a r l e a d s t o t h e r e s e a r c h o f i t s v a l o r i z a t i o n by chemical methods. A p a r t t h e hydrogenation o f aldose o r c e t o s e t o t h e c o r r e s p o n d i n g p o l y o l s ( r e f s . 1-11) t h e o b t e n t i o n o f lower p o l y o l s l i k e g l y c e r o l , and

ethylene-glycol

is

also

an

interesting

process

1,2-propane

(refs.

12-15).

diol The

m u l t i p l i c i t y o f t h e r e a c t i o n s i n b a s i c media when saccharose i s c o n s i d e r e d (inversion

e.g.

hydrogenolysis

ose e.g.

formation, glycol

hydrogenation

formation,

e.g.

polymerization)

hexitol makes

formation, the

process

d i f f i c u l t t o c o n t r o l . Furthermore, t h e h y d r o g e n a t i o n process i s known t o be r i n g s t r u c t u r e dependant ( r e f s . 2,4,11)

i n aqueous s o l u t i o n . The s t u d y o f t h e

c a t a l y t i c h y d r o g e n o l y s i s o f saccharose was undertaken i n o u r l a b o r a t o r y i n a t h r e e phase s l u r r y t y p e r e a c t o r .

Monomers o f v a r i o u s

s t e r e o c h e m i s t r y were

s u b m i t t e d t o h y d r o g e n o l y s i s i n o r d e r t o g a i n some i n s i g h t i n t o t h e mecanism i n v o l ved. EXPERIMENTAL Experiments were c a r r i e d o u t i n a t h r e e phase s l u r r y t y p e r e a c t o r w i t h aqueous sugar s o l u t i o n s device

(A).

. Hydrogen was

f e d i n t o t h e r e a c t o r through a f l o w r e g u l a t i o n

The s t a i n l e s s s t e e l

r e a c t o r was

temperature c o n t r o l l e d by a p o w e r s t a t .

heated e l e c t r i c a l l y

and t h e

I t was equipped w i t h a m a g n e t i c a l l y

d r i v e n v a r i a b l e speed s t i r r e r . The equipment was f i t t e d w i t h i n s t r u m e n t s f o r measuring t e m p e r a t u r e (C) ( i n

238

Fig. 1 : Hydrogenolysis r e a c t o r

t h e r e a c t i o n medium and i n t h e h e a t i n g d e v i c e ) . Flow r a t e and p r e s s u r e were m o n i t o r e d by a computer. Hydrogenolysis r u n s were s t a r t e d by c h a r g i n g t h e r e a c t o r w i t h sugar s o l u t i o n s containing

the

appropriate

amount

of

catalyst.

predetermined p r e s s u r e and t h e f l o w was s e t on.

Hydrogen was

fed

to

the

Then h e a t i n g and s t i r r i n g

begun. A d d i t i v e s were l e d i n t o t h e r e a c t o r under p r e s s u r e .

A sampling t u b e

p e r m i t t e d w i t h d r a w a l o f l i q u i d a t s p e c i f i e d i n t e r v a l s and t h e e x i t hydrogen c o u l d be analyzed by on l i n e GPC t h r o u g h a sampling l o o p ( B ) . Chemically p u r e monosaccharides and d i s a c c h a r i d e s were used. The c a t a l y s t ( 5 $ Ruthenium on c a r b o n ) was purchased f r o m ALDRICH. Chromatographic c o n d i t i o n s : Sugar and p o l y o l s i n aqueous media were analyzed by HPLC. (column, sugar pack waters;

e l u a n t H20; f l o w r a t e 0.5 m l / m i n .

: t e m p e r a t u r e 90°C;

differential

refractometer d e t e c t o r ) . G l y c o l s and monoalcohols were s e p a r a t e d by c a p i l l a r y column GPC (column,

WAX 58 CHROMPAC,

flame i o n i s a t i o n detector,

CP

programmed t e m p e r a t u r e : 60 t o

.

250°C, 5"C/mi n ) . Gaseous samples were determined by GC (column s t a t i o n a r y phase,

carbosieve;

d e t e c t o r , catharometer, programmed t e m p e r a t u r e : 30 t o 230"C, 4"C/min.). The s t a r t i n g t i m e f o r t h e experiments was t a k e n a t t h e s t a r t o f t h e h e a t i n g o f t h e r e a c t o r . A f t e r one hour t h e r e a c t i o n t e m p e r a t u r e was reached (220°C). The

yields

are

given

as

the

following

ratio

:

weight

of

considered

239

p r o d u c t / w e i g h t o f s t a r t i n g p r o d u c t x 100. RESULTS AND D I S C U S S I O N Table 1 p r e s e n t s t h e r e s u l t s o b t a i n e d i n t h e saccharose h y d r o g e n o l y s i s . I n p a r t A,

i t means s t a r t i n g w i t h a b a s i c medium, t h e s t u d y shows t h a t many

r e a c t i o n s a r e r u n n i n g i n t h e same t i m e : f i r s t i n v e r s i o n o f saccharose t o f o r m glucose and f r u c t o s e , t h e n h y d r o g e n a t i o n o f t h e monosaccharides t o h e x i t o l s and u l t i m a t e l y h y d r o g e n o l y s i s i n t o 1,2-propane d i o l . The y i e l d s i n 1,2-propane d i o l and i n g l y c e r o l a r e n o t v e r y h i g h (17 and 7% r e s p e c t i v e l y ) a f t e r 4 hours. Table 1 : Y i e l d o f saccharose h y d r o g e n o l y s i s .

............................................................................. Time on stream ( h ) Remaining Saccharose % Yields G F

1 71 2 2

m A

S

3

M 1,2-PG GLY

2

3

4

5 3 2 10 1 15

2

1

7 2 23 17

2 1 17 7

12 5 26 6

5 24 7

17

8

46

35 12

____---_____--__________________________-----------------~-----~------

B

Remaining Saccharose % Yields G F m S M 1 ,Z-PG GLY

9 30 22

5

7 2 9

17 9 20

________________________________________---------------~-----------Remaining Saccharose % G Yields F m C

S

M 1,2-PG GL Y

5

a

2 1

3

52 30

26 10 27

________________________________________----------------------------A : Reaction c o n d i t i o n s : T = 220°C, P = 5,5 MPa, pH = 10 sugar c o n c e n t r a t i o n 40 g/L, s u g a r / c a t a l y s t w e i g h t r a t i o 16. B : Same as f o r A b u t s u g a r / c a t a l y s t w e i g h t r a t i o 32. C : Same as f o r A b u t pH = 6 f o r t h e f i r s t two hours t h e n a d j u s t e d t o pH = 10. G : glucose S : sorbitol 1,2-PG : 1,2-propylene g l y c o l F : fructose M : mannitol GLY : g l y c e r o l m : mannose

Some r e a c t i o n parameters were t h e r e f o r e changed i n influence o f t h e d i f f e r e n t steps

order

to

study

the

on t h e o b t a i n e d y i e l d s . Thus t h e c a t a l y s t

c o n c e n t r a t i o n was lowered and t h e i n v e r s i o n c o u l d p a r t l y be separated f r o m t h e h y d r o g e n a t i o n and t h e h y d r o g e n o l y s i s ( p a r t B ) . T h i s r e s u l t e d i n a b e t t e r y i e l d i n 1,2-propane d i o l (24% a f t e r 4 h o u r s ) . Decreasing t h e c a t a l y s t c o n c e n t r a t i o n

240

d i m i n i s h e s t h e h y d r o g e n a t i o n r a t e . The pH o f t h e s o l u t i o n d r o p s d u r i n g t h e f i r s t hour when s t a r t i n g a t PH

10. T h i s pH d r o p i s f a s t e r i n presence of

=

l e s s c a t a l y s t p r o b a b l y because o f secondary r e a c t i o n s .

Indeed t h e a n a l y s i s

shows t h a t much more a c i d i c compounds a r e p r e s e n t i n t h e case o f l o w c a t a l y s t c o n t e n t . T h i s can e x p l a i n t h a t saccharose i s much f a s t e r c o n v e r t e d t o g l u c o s e and f r u c t o s e and why saccharose i s a p p a r e n t l y t r a n s f o r m e d f a s t e r t o p o l y o l s i n presence o f l e s s c a t a l y s t . The h y d r o g e n o l y s i s o f t h e C 6 oses o r p o l y o l s i s thought

to

proceed

reaction i s thus

through

faster

an e n e d i o l

than

in

part

(ref.

A

15-1 7 ) .

since

one

dehydrogenate t h e h e x i t o l s . The y i e l d s i n 1,2-propane

The need

hydrogenolysi s not

at

first

d i o l and g l y c e r o l a r e

t h u s enhanced. The s e p a r a t i o n between h y d r o g e n a t i o n and h y d r o g e n o l y s i s can be o b t a i n e d by m o d i f i c a t i o n o f t h e pH d u r i n g t h e r e a c t i o n . a c i d i c pH (pH 5 t o 6 ) . F o r h y d r o g e n o l y s i s , e n e d i o l which s t a r t e d a t pH

Indeed h y d r o g e n a t i o n o c c u r s a t t h e r e a c t i o n i n t e r m e d i a t e i s an

i s u n f a v o r e d i n a c i d i c media. =

I n p a r t C,

t h e r e a c t i o n was

6. The base (Ca(OHI2) was added up t o pH 10 a f t e r two hours,

i t means a f t e r t h e h y d r o g e n a t i o n s t e p . I t can c l e a r l y be seen t h a t i n t h e f i r s t h o u r d u r i n g t h e t e m p e r a t u r e i n c r e a s e

f r o m 20 t o 220°C,

h y d r o l y s i s o f saccharose and h y d r o g e n a t i o n o f t h e formed

g l u c o s e and f r u c t o s e t o s o r b i t o l

and m a n n i t o l

p o l y o l s a r e hydrogenolysed t o 1,2-propane

proceeds.

Then,

t h e formed

d i o l and g l y c e r o l e s s e n t i a l l y . T h i s

r e s u l t s i n an i n c r e a s e o f t h e y i e l d i n 1,2

propane d i o l and g l y c e r o l .

This

enhancement can be e x p l a i n e d p a r t i a l l y by a decrease o f secondary r e a c t i o n (e.g.

p o l y m e r i s a t i o n o f t h e oses)

s i n c e now t h e p r o d u c t s a r e p r e s e n t

as

p o l y o l s and n o t as oses. A n e a r e r e x a m i n a t i o n o f t h e r e s u l t s ( T a b l e I , p a r t C ) shows t h a t t h e formed m a n n i t o l i s t r a n s f o r m e d more e a s i l y t h a n s o r b i t o l .

It

appears t h u s c l e a r l y t h a t t h e h y d r o g e n o l y s i s r e a c t i o n i s s e n s i t i v e t o t h e s t r u c t u r e o f t h e h e x i t o l . The h y d r o g e n o l y s i s o f p o l y o l s and t h e c o r r e s p o n d i n g oses has t h u s been undertaken t o g e t a b e t t e r i n s i g h t i n t o t h e r e a c t i o n process ( T a b l e 2 ) . TABLE 2. Y i e l d s of p o l y o l s and oses h y d r o g e n o l y s i s (1,2-propanediol g l y c e r o l 1. POLYOLS YIELDS % OSES YIELDS %

t

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

MANNITOL GALACTITOL SORBITOL IDITOL

56 44 35 35

ARABITOL X Y L ITOL

46 39

MANNOSE GALACTOSE GLUCOSE I DOSE FRUCTOSE ARABINOSE XYLOSE

57 49 41 48 43 37

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

R e a c t i o n c o n d i t i o n s l i k e i n T a b l e 1 ( p a r t A) f o r t h e p o l y o l s and i n Table 1 ( p a r t C) f o r t h e oses. The p r o d u c t s ware analyzed a f t e r 4h.

241

0 II

0 II

Rnlacrirol

Fig.

2

: S ' r u c t u r r s of

the proposed complexes.

242

For t h e C 5 compounds e s s e n t i a l l y 1,2-propane

d i o l and g l y c e r o l a r e observed. T h i s w i l l be d i s c u s s e d

Only s m a l l amounts o f e t h y l e n e g l y c o l a r e d e t e c t e d . here

after.

From Table

2

clearly

it

results

that

the

oses

and

the

c o r r e s p o n d i n g p o l y o l s r e a c t i n t h e same manner. However t h e s e l e c t i v i t y o f t h e h y d r o g e n o l y s i s d i f f e r s f o r t h e v a r i o u s p o l y o l s used i n t h i s s t u d y . The problem remains t o e x p l a i n t h i s d i f f e r e n c e i n s e l e c t i v i t y .

I t would be d i f f i c u l t t o

g i v e h e r e a l l t h e d e t a i l s about t h e s t r u c t u r e o f sugars and r e l a t e d p o l y o l s . The i n t e r e s t e d r e a d e r m i g h t f o u n d more i n f o r m a t i o n s i n r e f . ( 1 8 - 1 9 ) . Andrews and K l a e r e n ( 2 0 ) have proposed a r e a c t i o n scheme w i t h a c o o r d i n a t e d sugar a l k o x y a n i o n as i n t e r m e d i a t e i n r u t h e n i u m based homogeneous c a t a l y s i s o f monosaccharide s e l e c t i v e h y d r o c r a c k i n g . However t h e p r o d u c t s o b t a i n e d i n t h e r e a c t i o n o f g l u c o s e ( e t h y l e n e g l y c o l and e r y t h r i t o l ) a r e n o t i n concordance w i t h our r e s u l t s .

In t h e h y d r o g e n a t i o n o f g l u c o s e and f r u c t o s e on copper

c o n t a i n i n g c a t a l y s t , Makkee e t a1 ( r e f . 4 ) proposed t h a t 0 - f r u c t o s e (and o t h e r k e t o s e s ) formed i o n i s e d f u r a n o s e s p e c i e s adsorbed on copper by c o o r d i n a t i o n o f 0-1, 0-2 and 0-5 t o t h e s u r f a c e . An a d s o r p t i o n o f t h e p o l y o l s on t h e c a t a l y s t s s u r f a c e i n a t h r e e f o l d c o o r d i n a t i o n s i m i l a r t o t h a t proposed by Makkee e t a1 ( r e f . 4 ) i s p o s s i b l e as shown on f i g . 2. I t can be seen t h a t t h e 2-0, 3-0, 4-0 and 3-0, 4-0, 5-0 c o o r d i n a t i o n s a r e most s t a b l e f o r m a n n i t o l . As shown on t h e f o l l o w i n g scheme ( f i g . give 2 C3

3 ) , t h e 2-0,3-0,4-0

4-0,

and 3-0,

5-0 complexes can

species.

OH

\ ri2 c -

OH

\I/

c;\

+ CHz OH-CHOH-CI10

4

I

I

C-

C-

I

M F i g . 3 : Proposed r e a c t i o n scheme.

CIlzOH

I

t

2

CHZ OH-CHOH-CHO

243

Experiments on t h e h y d r o g e n a t i o n o f g l y c e r o l show t h a t i t i s n o t c o n v e r t e d into

1,Z-propane

diol

in

our

reaction

conditions.

Unlike

glycerol,

g l y c e r a l d e h y d e i s c o n v e r t e d t o a m i x t u r e o f 1,2-propane d i o l and g l y c e r o l . I t seems t h u s t h a t g l y c e r a l d e h y d e which i s d e t e c t e d i n s m a l l amounts

i n our

h y d r o g e n o l y s i s experiments c o u l d be a r e a c t i o n i n t e r m e d i a t e . For g a l a c t i t o l t h e same c o o r d i n a t i o n s a r e p o s s i b l e b u t a s t e r i c i n t e r a c t i o n e x i s t s between t h e s u r f a c e and t h e CHOH-CH20H c h a i n . The 2-0,3-0,4-0

and 3-0,

4-0, 5-0 c o o r d i n a t i o n s a r e t h u s l e s s f a v o u r e d t h a n f o r m a n n i t o l . F o r s o r b i t o l , t h e 3-0,

4-0 and 5-0 c o o r d i n a t i o n i s t h e same as f o r m a n n i t o l .

However f o r t h e 2-0,3-0

and 4-0 complex t h e two c h a i n s (CH20H and CHOH-CH20H)

a r e on t h e

same

s i d e and

therefore

unfavoured.

It

results

from

these

c o n s i d e r a t i o n s t h a t t h e s e l e c t i v i t y t o 1,2 propane d i o l and t o g l y c e r o l s h o u l d be m a n n i t o l

galactitol

sorbitol.

T h i s i s what was

observed

i n our

experiments. The r e s u l t s f o r t h e C 5 p o l y o l s o r oses show t h e same v a r i a t i o n o f s e l e c t i v i t y w i t h s t r u c t u r e as f o r t h e C6 compounds. As f o r t h e C6 oses m o l e c u l a r models f o r C5 oses show t h a t t h e

proposed

c o o r d i n a t i o n i s a l s o f a v o u r e d f o r a r a b i t o l b u t n o t f o r x y l i t o l . The a r a b i t o l complex has t h e same c o n f i g u r a t i o n as m a n n i t o l , b u t t h e 3-C, 4-C bond cleavage g i v e s 1 C2 and 1 C3 i n s t e a d o f 2 Cj molecules. T h e r e f o r e t h e y i e l d o f 1,2propane d i o l and g l y c e r o l i s l o w e r f o r a r a b i t o l t h a n f o r m a n n i t o l . G l y c o l a l d e h y d e which would be i n o u r r e a c t i o n scheme a r e a c t i o n i n t e r m e d i a t e i n t h e f o r m a t i o n o f g l y c o l f o r C 5 compounds i s o n l y s l i g h t l y c o n v e r t e d t o g l y c o l b u t i s r a t h e r decomposed i n t o hydrocarbons (methane, c o u l d e x p l a i n why almost o n l y C 3

p o l y o l s are obtained.

ethane).

This

Comparing now t h e

r e a c t i v i t y t o t h e s t a b i l i t y o f t h e s e complexes a good c o r r e l a t i o n can be o b t a i n e d : mannose

galactose

arabinose

sorbose = i d o s e

xylose.

The

same r e s u l t s were o b t a i n e d f o r h e x i t o l s . F r u c t o s e which i s hydrogenated t o m a n n i t o l and s o r b i t o l has an i n t e r m e d i a t e s e l e c t i v i t y

(between g l u c o s e and

mannose 1. CONCLUSION The study o f t h e h y d r o g e n o l y s i s o f saccharose t o 1,2-propane

d i o l has shown

t h a t a b e t t e r y i e l d can be o b t a i n e d by t h e s e p a r a t i o n o f h y d r o l y s i s and hydrogenation steps, f r o m t h e bond cleavage. T h i s can be achieved by a d j u s t i n g t h e pH d u r i n g t h e r e a c t i o n . The h y d r o g e n o l y s i s o f d i f f e r e n t oses has shown a d i f f e r e n c e i n t h e s e l e c t i v i t y o f t h e r e a c t i o n . An adsorbed f o r m o f t h e p o l y o l s can account f o r these d i f f e r e n c e s . Indeed some oses l i k e mannose and g a l a c t o s e have no h i n d r a n c e t o f o r m t h e proposed complex.

244

A f u r t h e r improvement o f t h e t r a n s f o r m a t i o n o f saccharose t o 1,2-propane can p r o b a b l y be o b t a i n e d f r o m i t s s e l e c t i v e h y d r o g e n a t i o n t o m a n n i t o l e.g.

diol in

presence o f molybdate i o n s ( r e f . 21.

REFERENCES 1 J.W. Green, The carbohydrates, Chemistry and B i o c h e m i s t r y , Acad. Press, New York, 2nd Ed 16, 1980, 989. 2 M. Makkee, A.P.G. Kieboom, H. van Bekkum, S t a r c h / S t a r k e , 37, (19851, 133. 3 J. Wisniak, M. Hershkowitz, R. L e i b o w i t z , S. S t e i n , I n d . I n g . Chem. Prod. Res. Dev., 13, (19741, 75. 4 M. Makkee, A.P.G. Kieboom, H. van Bekkum, Carbohydr. Res., 138, (19851, 225. 5 J. Wisniak, R. Simon, I n d . Eng. Chem. Prod. Res. Dev., 18, (19791, 50. 6 F.B. Bishanov, R.B. Drozdova, React. K i n e t . C a t a l . L e t t . , 21, (19821, 35. 7 A.H. Germain, M.L. Wauters, G.A. L'Homme, Stud. S u r f . S c i . C a t a l . 7, (19811, 1492. 8 A.A. W i s m e i j r , A.P.G. Kieboom, H. van Bekkum, React. K i n e t . C a t a l . L e t t . , 29, (19851, 311. 9 J.M. Bonnier, J.P. Damon, J . Masson, Appl. C a t a l . 30, (19871, 181. 10 G. Vanling, A.J. Driessen, I n d . Engng. Chem. Prod. Res., 9, (19701, 212. 11 J. Ruddlesden, A. Stewart, 0. Thompson, R. Whelan, Faraday Discuss. Chem. SOC. , 72, (19811, 397. 12 C . M o n t a s s i e r , D. Giraud, J. B a r b i e r , Heterogeneous C a t a l y s i s and F i n e Chemicals M. GUISNET e t a l . ( E d i t o r s ) , 1988 E l s e v i e r Science P u b l i s h e r s B.V., Amsterdam p. 165 13 D. Ariono, C . Moraes, A. Roesyadi, G. Declercq, A. Z o u l a l i a n , B u l . SOC. Chim., 5, (19861, 703. 14 I . T . C l a r k , I n d . & Eng. Chem., 50, (19581, 1125. 15 I . D . Rozhdestvenskaya, T.N. Fadeeva, L.V. S h i l e i k o , K i n e t . C a t a l . 11, (19701, 568. 16 D.K. Sohounloue, C . M o n t a s s i e r , J. B a r b i e r , React. K i n e t . C a t a l . L e t t . , 22, (19831, 391. 17 A.P. Sergev, B.L. Lebedev, Uspekhi Khimi, 28, (19591, 669. 18 S.J Angyal, Angew. Chem. 81, (1969), 172. 19 S. J. Angyal, Adv. Carbohydr. Chem. Biochem. 42, (19841, 15. 20 M.A. Andrews, S.A. K l a e r e n , J. Am. Chem. SOC., 111, (1989), 4131.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Puhlishers B.V., Amsterdam

SELiCTIVE HYDROGENATION OF ACETOPHENWE GI1 I"O#OTtD

245

HAllEY INICKEL : 1i.IFLUENCE

OF THE ; l i A C T I O N CONDITIONS J. MASSON',

P . CIVIDINO',

J.W. BONNIER

1

and P . FOilILLOiJX

2

' L a o o r a t o i r e d ' E t u d e s Llynamiques e i S z r u c t u r a l e s de l a S e l e c c i v i i s (LtOSS-1 ) CiWS URA 332 - I l n i v e r s i ' c 6 Joseph F o u r i e r - BP 53X - 38041 GRENOBLE CEDEX (France). 2

U n i e e lvlixte }
-

24, Avenue Jean J a u r e s

B P 165

SUlYlMAHY The i n f l u e n c e o f t h e r e a c t i o n c o n d i t i o n s o n -ihe a c t i v i t y and s e l e c t i v i t y for t h e hydrogenation of acetophenone t o 1-phenylethanol has been d e m o n s t r a t e d . The f a c t o r s i n c l u d e d t e m p e r a t u r e , h y d r o g e n p r e s s u r e , n a t u r e of t h e s o l v e n t . I n c y c l o h e x a n e , l o w t e m p e r a t u r e s and h i g h h y d r o g e n p r e s s u r e s were f o u n d t o oe o e n e f i c i a l i n o o t a i n i n g 1 - p h e n y l e t h a n o l . i.le e s . c a o l i s h e d c h a t l o w e r a l c o h o l s g i v e r i s e t o good s e l e c r i v i r i e s and we o o s e r v e d a c o r r e l a t i o n oerween s e l e c t i v i t y and d i e l e c t r i c c o n s t a n t o f solven.cs.

I INTKOUUCT I 0 IN Arorna'iic

ketones a r e o f t e n hydrogenated w i t h

corresponding aromaeic a l c o h o l s complex

molecule

eo e f f e c e

-

the

aim t o

prepare

the

Acetophenone was chosen h e r e as a r e a s o n a o l y

a modelizaeion

of

rhis

family

of

catalytic

h y d r o g e n a i i o n s . P r e v i o u s p a p e r s have p r o v e d t h a t Haney n i c k e l i s an e f f e c s i v e cacalysi i n this

type

of

reaction

hydrogenaeed TO 1 - p h e n y l e t h a n o l

(refs.l-4).Acetophenone

which i s a

very

valuasle

is

principally

product

of

ihe

perfumery i n d u s t r y . The h y d r o g e n a t i o n o f ace'iopnenone w n i c h h a s an a r o n a e i c u n s a e u r a i e u r i n g and a c a r o o n y l f u n c t i o n a l

g r o u p i n v o l v e s a sequence o f s e v e r a l c o n p e i i i i v e

p a r a 1 l e l and c o n s e c u L i ve r e a c ' i i o n s . T h i s work was devosed i o s t u d y i h e i n f l u e n c e o f x e m p e r a i u r e , o f h y d r o g e n p r e s s u r e and o f t h e n a t u r e o f i n e s o l v e n x o n a c c i v i r y and s e l e c r i v i r y o f 2aney nickel i n t h i s reaccion E X P E t? IPIE NT AL

Ca.; a 1.-y s~ The i?aney n i c k e l ca'ia1ys.c was o b t a i n e d o y l e a c h i n g a c o m m e r c i a l a l l o y (5O-Sd w i 2 P r o l a o o ) w i s n sodiuio h y d r o x i d e s o l u c i o n

Ili-A1

( 6 mol.1-'1

ai

a o i l i n g ceinperarure f o l l o w i n g a me-chod a l r e a d y d e s c r i o e d e l s e w h e r e ( r e f . 5 ) . The s p e c i f i c s u r f a c e a r e a measured oy BET meixod,

a f i e r a d e s o r p t i o n a.i

100°C, was 80 m 2 g - l and t h e m e k a l l i c s u r f a c e a r e a d e t e r m i n e d i n t h e l i q u i d 2 -1 phase oy r e a c t i v e a d s o r p t i o n o f 3-me-chylctiiophen ( r e f . 5 ) was 50 m g .

246

S t a r~ ting m a r e r_ i a1 s _ _ Acetophenone

( 9 9 % ) and cyclohexane (HPLC g r a d e ) were d i s-ci 1 l e d b e f o r e

use. I n a d d i t i o n Haney n i c k e l was soaked i n t h e cyclohexane used as a s o l v e n r i n o r d e r t o remove any s u l f u r compounds. HPLC grade a l c o h o l s were used. Experimental procedures Due t o

the

possiole

influence of

sodium

hydroxide

on

the

reaction

oehaviour, a s p e c i a l c a r e must De t a k e n f o r c a t a l y s t washing ( r e f . 6 ) . Lihen t h e r e a c t i o n t a k e s p l a c e i n an a l c o h o l t h e m e t a l l i c powder i s washed t h r e e r i m e s w i t h d i s t i l l e d warer, t h r e e r i m e s w i r h a b s o l u t e methanol and t h r e e t i m e s w i t h the alcohol.

I f cyclohexane i s t h e h y d r o g e n a t i o n s o l v e n t ,

rhe operation i s

f o l l o w e d by an a d d i t i o n a l s t e p i n t o l u e n e . B e f o r e i n r r o d u c c i o n o f acetophenone, t h e r e a c s o r c o n t a i n i n g a suspension o f r n e c a t a l y s t ( 0 . 2 g ) i n -che s o l v e n t (135 r n l ) was purged w i t h a f l o w o f hydrogen, f o l l o w e d by pretreatmenx a'c room cemperature f o r 1 h w i r h s t i r r i n g under a hydrogen p r e s s u r e o f 0.9 MPa. Then t h e t e m p e r a t u r e i s r a i s e d t o 80°C and acetophenone i s i n t r o d u c e d .

The s t i r r i n g i s

s.iarred e s t a o l i s h i n g

the

o r i g i n o f the kinetics. A t t h e chosen acetophenone c o n c e n r r a i i o n (0.3mol .l-l),t h e r e a c s i o n i s no'c l i m i r e d by g a s - l i q u i d t r a n s f e r f o r a s t i r r i n g speed comprised oetween 1500 and 2000 rpm and t h e r e a c x i o n r a t e i s independent o f t h e c a t a l y s t w e i g h t

(O.l$n

40.4 9 ) . A n a l y s i s o f t h e producss -~ Samples o f t h e r e a m i o n produc-cs were w i t h d r a w n o u t o f r h e r e a c t o r a t r e g u l a r r i m e i n t e r v a l s and analysed oy gas chroraarography. The c o n d i r i o n s f o r t h e GC measuremen.Ls were as f o l l o w s : a packed column ( 4 m

-

1 / 8 " ) w i t h UV 210

10 % + XE 60 5 % on Chromosorb WHP 80/100 and column remperaeure i s o t h e r m a l a t 110°C

-

Dodecane was used as a s t a n d a r d .

RESULTS ANU U I S C U S S I U N

C a t a l y t i c h y d r o g e n a t i o n o f acetophenone Under t h e m i l d c o n d i t i o n s used i n t h i s work (temperacure o e i n g s e t oetween 50 and 9O0C), t h e p o s s i b l e network i s g i v e n on r h e f i g u r e 1. The h y d r o g e n a t i o n

o f t h e c a r o o n y l f o n c t i o n i n acerophenone (AC) g i v e s 1-phenylecnanol ( P E ) , t h e h y d r o g e n a t i o n o f t h e a r o m a t i c r i n g g i v e s m e t h y l c y c l o h e x y l k e t o n e (IYICK). compounds undergo f u r c h e r h y d r o g e n a t i o n t o 1 - c y c l o h e x y l e t h a n o l h y d r o g e n o l y s i s o f t h e C-OH oond i n PE a f f o r d s ethylbenzene ( E B ) ,

Boxh

(CE), whereas subsequently

hydrogenaxed t o e i h y l c y c l o h e x a n e ( E C ) . F i g u r e 1 i l l u s t r a r e s t h e ooserved p r o d u c r c o m p o s i t i o n as a function o f r i m e i n srandard c o n d i x i o n s . I n t h e presence o f Raney n i c k e l , i f AC i s s t i l l present

(step 11,

t h e hydrogenaxion g i v e s

almost e x c l u s i v e l y PE and v e r y

l i t t l e I N K . When a l l AC has disappeared (st,ep 2 ) t h e rat.e of h y d r o g e n o l y s i s o f

247

xne C-UH oond i s noLaoly increased. A t t h e same time IMCK i s nydrogenaced 'io CE .ine l a t e r does nor give E C in .;he reaccion condicions, o u c E d i s iransformea very slowly i n i o E C which i s t h e ultimate nydrogenation producc i r e f . 7 1 . de defined V o A c as the i n i r i a l nydrogenacion r a t e o f aceeopnenone, Vopt and VoNCK as rhe i n i t i a l hydrogenation r a t e s of t h e C = 0 douole oond and of t h e aromatic ring respectively. VoEd and VEB represent ehe hydrogenolysis race of t T a-i the oeginning of s.;ep 1 and s i e p 2 . I n cne sxandard conditions of f i g u r e 1 , t h e r a t e s expressed i n mmol nin-lg-' are VoAC = 5.Y0, VopE = 5.31) Vo,VICK = 0.48, VoE3 = 0.15 and VEB = U.64. One can nocice 'chat t h e r a t e of P E hydrogenolysis i s four rimes greater in ihe second scep o f t h e reaction than in ehe f i r s c . Tnis r e s u l t leads 'io the conclusion inai A C i s more strongly adsoroed on 9aney nickel surface tnan P E . Tnis assumption i s susrained oy t n e influence of ehe AC i n i t i a l concentration : hydrogenolysis oeing more important a t low AC concencration.

--Step

1 --+,-Step

2 ----t

45

-

PE

Time

a

Fig. 1

:

min

t

Hydrogenation of acetophenone i n cyclonexane (80" C a - Reaction network i n mild conditions D - Product distrioueion as a funciion of time

-

0.Y IViPa)

Influence of xhe reaction remperaiure and t h e maximum The i n i t i a l raxe of ace'iopnenone hydrogenazion (V",) y i e l d in 1-pheny1e:hanol were measured f o r 'zemoerazures ranging from 50 'Lo 9 0 ° C . The s e l e c t i v i c y decreases when ,;he cemperaiure i s increased (Taole 1 j .

248 TAdLE 1

Effect of

i e m p e r a e u r e on aceLopnenone n y d r o g e n a r i o n r a c e

and

on

1-pnenyl

e t h a n o l maximum y i e l d (PEmax).

T

'%ax

V"AC

("cj

(mmo1.min.g

-1

J

5.Y

94 Y2 8Y 85

7.7

83

2.9 3.7 4.9

50 OU 7u 80 YO

The a p p a r e n t a c L i v a t i o n e n e r g y f o r acetophenone h y d r o g e n a r i o n i n P i i s 21 kJ.iilO1-l.

Such

a

low

value

could

be

due

'to

diffusion.

t3y

accivizy

OecerminaLions as f u n c i i o n o f s - c i r r i n g speed and c a c a l y s r w e i g m i , we r u l e d ouc ;he

possioility o f g a s - l i q u i d - i r a n s f e r 1iriiJsa.iion. U u i we a r e n o ' i s u r e .iha.i

i n e r a x e i s n o t l i m i t e d oy l i q u i d - s o l i d L r a n s f e r i n ;ne Naney p a r i i c l e s .

Tne a c r i v a e i o n e n e r g y ,

s l i g n - i l y n i g h e r and ii r e a c h e s 71 k c l . n o l - '

29 kJ.mo1-'

p o r e n e i w o r k o f ehe f o r IMCK f o r m a ' i i o n

f o r etnylaenzene.

is

Tnus -cne l o s s i n

s e l e c i i v i c y a - i h i g h iempera-cure i s due i o an i n c r e a s e o f h y d r o g e n o l y s i s r a c n e r .inan t o a grea;er

_I n_f l_u _e n_c e

produciion o f IKK.

o f ____ .
The effec.;

: i n e nyarogen pressure

i n v e s x i g a d e d i n c y c l o h e x a n e as w e l l as i n i s o p r o p a n o l i s v a r i e d f r o n 0.3

.LO

6.1 IWa ( T a s l e

2). The i n i - c i a 1 r a c e ( V U A c J

increased

w i i h n y d r o g e n p r e s s u r e i n c y c l o n e x a n e and i n i s o p r o p a n o l . F o r ;he r e a c . c i o n i n c y c l o n e x a n e , we c a n see ,ha.; i m p r o v e d oy a r i s e i n n y d r o g e n p r e s s u r e . cons'can;:

inerefore

';he

selec-iiviey increase,

n y d r o g e n o l y s i s . Un L1ie c o w c r a r y ,

i h e s e l e c i i v i z y i n PE i s

Tne r a t i o V"pE/VoiMCX resul-is from

a

i s

almos-c

decrcas?

of

i n is o p r o p a n o l i h e n y d r o g e n p r e s s u r e nas n o

i n f l u e n c e on ;he s e l e c . i i v i ' i y i n PE : t h e s l i g n t i n c r e a s e i n r i n g hydrogena.cion o e i ng e x a c r l y cornpensaied OY i h e d e c r e a s e i n h y d r o g e n o l y s i s . de have a l s o c o n t r o l l e d t h e e f f e c t o f h y d r o g e n p r e s s u r e o f p r e t r e a e n e n x o n I n i s o p r o p a n o l 'ine

h y d r o g e n p r e s s u r e o f p r e i r e a e r n e n i nas n o -1 - 1 . on i h e i n i i i a l r a t e i V o H C = 11 + 0 . 5 nmol rnmn g I DeLween U.2 and

t n e ac-Liv.i i y . effece

4.5 NPa.

On

;he

con'irary,

i n cyclonexane

( T a o l e 3 ) we o b s e r v e d a s l i g h i

d e c r e a s e i n a c e i v i . i y as t n e h y d r o g e n p r e s s u r e i n c r e a s e s . T n i s a c r i v i e y l o s s i s accompanieo

OY

a decrase i n s p e c i f i c

surface area deiernined

oy

surface

methyl cniopnene

a s s m e rha- m e a c e i v i i y l e s s c o u l d r e s u l ,

fro;:,

area

(SdET),

reaciion a par>:jal

D U ~L n e

remains

nickel

cons'iani.

s u r f a c e poison:ng

Ne zy

249 a s p e c i e s s t r o n g l y adsorbed s i n c e i t r e m a i n s a f r e r a d e s o r p t i o n a - i l U O ° C , w h i c h c a n b e d i s p l a c e d oy r l i e t h y l t h i o p h e n . cyclohexane occurs i n our c o n d i t i o n s ,

Je think

chat hydrogenolysis

then the p a r t i a l

oux of

p o i s o n i n g c o u l d be

a z z r i o u t e d t o a s u r f a c e c a r b i d i c s p e c i e s . The f o r m a x i o n o f such s p e c i e s i s t h e r a t e - l i m i t i n g s t e p i n t h e hydrogenolysis o f alkanes on n i c k e l c a t a l y s t ( 8 ) . TABLE 2 I n f l u e n c e o f h y d r o g e n p r e s s u r e on h y d r o g e n a t i o n o f acetophenone i n c y c l o h e x a n e and i s o p r o p a n o l .

-_

___ Solvent

Pressure

Cyclohexane

Isopropanol

(rnnol.nin

'

" PE "'~KK

v'AC

(IvlPa)

-1 -1 g )

'Lax %

u.3 0.6 U.Y 2.1 3.1 4.6 6.1

3.1 4.1 5.9 7.4 Y.4 Y.6 10.6

13 12.5 11 Y.6 11 12.4 11

81 84

0.3 U.6 0.9 3.1 4.6 6.1

7.2 10.6 12.4 16.7 18.5 18.3

31.8 30 27.7 21.8 19.5 16.8

Y1 92 92 92 92 92

a5 85 87 90 YO

TABLE 3 I n f l u e n c e o f h y d r o g e n p r e s s u r e d u r i n g p r e t r e a t m e n t ( c y c l o h e x a n e ) an i n i t i a l r a t e ( a ) and on s u r f a c e a r e a ( o ) . a Pressure

0

Pressure

'"AC

(MPa)

(rnmol m i n - l g - l )

0.2

7.7 5.9 5.5

O.Y

4.6

SdET

(#Pa)

'iii h2g-l

no p r e t r e a t e d 0.2 0.9

80 74 65

1 6U 60 60

I n f l u e n c e o f t h e s o l v__ ent -~ To s t u d y t h e i n f l u e n c e o f -ihe s o l v e n r o n che a c t i v i t y and t h e s e l e c x i v i c y o f t h e r e a c c i o n , we have chosen s o l v e n t s w i i h d i f f e r e n r p o l a r i t y : a l c o h o l s o f various

seructure

(Cl-C4)

and

c y c l o h e x a n e . It

can

oe seen

(Table 4) that

250

a c r i v i e y i s s t r o n g l y a f f ? c L e d ~y .
rates

are

very

low.

de

ootain

a nlgh

acciviry

in

i s o p r o p a n o l . Dehydrogenation o f a l c o h o l s on n i c k e l s u r f a c e nave oeen repor-ced for

temperatures

formaldehyde

i n the

range

50-200°C

(ref.

can oe s t r o n g l y adsoroed on

Y).

nickel

and

Aldehydes

and

p o i s o n che

mainly metallic

s u r f a c e . I n o r d e r t o v e r i f y t h i s h y p o t h e s i s , t h e e f f e c t o f added formaldehyde, acetaldehyde and acetone on t h e r e a c r i o n has oeen i n v e s t i g a t e d . The r e a c t i o n i s almosr r o t a l l y i n h i o i t e d ~y 2 . w 3

mol of

formaldehyde.

The i n h i o i r i n g

e f f e c t o f acetaldehyde i s weak and we ooserve no i n f l u e n c e w i t h acetone. The d i f f e r e n c e i n ac-civi.iy i n r h e d i f f e r e n t a l c o h o l s can De e x p l a i n e d oy .cne poisoning

effects

of

the

Dy-products

coming

from

the

alcohol

solvent

dehydrogenation.

TABLE 4 I n f l u e n c e o f s o l v e n t on t h e r e a c t i o n .

iqet h ano 1 Ethanol N-Propanol Isopropanol

0.6 1.1 3.3 11

Cyclohexane

5.9

94 93 92

0.1 u.2 0.5 2.8

32.5 24.3 20.1 18.3

85

4.2

2

94

a : d i e l e c t r i c c o n s t a n t o f .the s o l v e n t

The s e l e c r i v i e y i s a l s o s e r o n g l y a f f e c i e d by t h e s o l v e n t : t h e maxiilium y i e l d i n PE i s n o t i c e a b l y lower i n cyclohexane t h a n i n a l c o h o l s ( T a o l e 4 ) . The a n a l y s i s of che r e a c r i o n p r o d u c t s shows t h a t t h e r i n g h y d r o g e n a t i o n and t h e h y d r o g e n o l y s i s o f t h e C-UH oond a r e f a v o u r e d i n cyclohexane ( F i g . 2 ) . As shown i n t a b l e 4 t h e s e l e c t i v i t y i n PE i s c h i e f l y reduced b y t h e f o r m a t i o n o f I N K . It can be seen zha-i t h e y i e l d i n IMCK decreases as t h e d i e l e c t r i c c o n s t a n t

(6-1

o f rhe solvent increases. One can n o t i c e t h a t t h e a d d i t i o n o f w a t e r ( s o l v e n t w i t h h i g h d i e l e c t r i c constant)

ro i s o p r o p a n o l , i n c r e a s e s t h e r a r e as w e l l as t h e s e l e c t i v i t y i n PE.

The maximum y i e l d i n ivlCK decreases as t h e w a t e r coneent i n c r e a s e s

(Fig. 3).

251 A s i m i l a r e f f e c t i s oocained i n cyclohexane. The a d d i t i o n o f a v e r y s m a l l amount o f water 'io cyclohexane (0.1 % v o l ) i n c r e a s e s t h e i n i t i a l r a - i e f r o m 5 . 9

ro 9.8 rnmol m i n - l g - ' and The s e l e c s i v i . i y i n PE f r o m 85 t o 93 % . T h i s i n c r e a s e o f selec.;ivity

r e s u l c s f r o m a decrease of -;he

r i n g hydrogenaiion, since f o r

example t h e maximum y i e l d i n MCK f a l l s f r o m 4.2 LO 1 . 1 %.

A l l .;hese r e s u l c s c l e a r l y show 'chat r h e r i n g hydrogenacion i s i n h i b i t e d oy p o l a r molecules. The r i n g adsorpcion i s p r o b a b l y i n h i b i t e d ~y t h e e l e c c r i c d i p o l e i n che so1ven.c

l a y e r adherent t o t h e m e t a l l i c

s u r f a c e whi1s.i

the

carbonyl adsorption i s favoured.

7----

98.

J

60

s K m

80

100

,% MCt'

2 E

Y

0

E 1

0 0

10

20

30

H20 yo v o l

F i g . 3 A c t i v i t y and maximum y i e l d i n I N K and PE i n i s o p r o p a n o l as a f u n c r i o n o f water c o n t e n r .

0

50

100

A c conv,%

F i g . 2 Product d i s t r i o u r i o n as a f u n c t i o n o f acetophenone c o n v e r s i o n . Sol v e n t : Isopropanol(1) cyclohexane(2).

252 COIKLUS I ON The s e l e c c i v i t y o f Lhe h y d r o g e n a t i o n o f acetophenone depends o n s e v e r a l factors :

- The s e l e c t i v i x y i n PE d e c r e a s e s w i t h i n c r e a s i n g T,empera'iure

.

The l o s s i n

s e l e c t i v i r y r e s u l r s f r o i n an i n c r e a s e o f h y d r o g e n o l y s i s r e a c t i o n r a c h e r r h a n r i n g hydrogenarion.

-

An

opposite

effect

i s ootained

with

increasing

hydrogen

pressure

c y c l o h e x a n e s o l v e n t . Due 'io t h e d i f f e r e n c e i n ene a d s o r p t i o n cons'iant,

in

PE i s

more e a s i l y d i s p l a c e d t h a n AC oy hydrogen, t h e r e f o r e y i e l d i n EB d e c r e a s e s as i n e hydrogen pressure increases.

- The n a t u r e o f r h e s o l v e n i h a s a h i g h e f f e c t on s e l e c c i v i t y . F o r -cne t e s r e d s o l v e n t s , we o o s e r v e d t h a t ene n e t h y l c y c l o h e x y l k e t o n e y i e l d i s c o r r e l a . i e d w i t h d i e l e c t r i c c o n s t a n t . The r i n g a d s o r p t i o n i s i n h i o i t e d oy p o l a r s o l v e n t . We have shown acxivity.

The

t h a t the nature o f

l a r g e a c r i v i - i y varia.cion

i n e s o l v e n i s t r o n g l y m o d i f i e s The

i s e x p l a i n e d oy a p a r r i a l

surface

p o i s o n i n g a y .ihe d e h y d r o g e n a t i o n p r o d u c t s o f a l c o h o l s o l v e n . i s on Raney n i c k e l . The e f f e c t o f t h e n y d r o g e n p r e s s u r e i n p r e t r e a m e n ' c suggescs iha.c c y c l o h e x a n e can a l s o r e a c t w i t h t h e c a t a l y s t s u r f a c e .

T h e r e f o r e c y c l o n e x a n e c a n no l o n g e r

be r e e a i n e d as an i n e r t s o l v e n t . F i n a l l y ii: was p o s s i b l e

-LO

o o . i a i n a 1 - p h e n y l e t h a n o l y i e l d o f 97 % w i . i h a

n i g h r e a c t i o n r a c e i n i s o p r o p a n o l c o n t a i n i n g 30 % waxer. REFERENCES K a j i c a n i , N. S u z u k i , T . Aoe, Y. Kaneko, K . Kasuya, X . T a k a h a s h i and A. S u g i m o r i , B u l l . Chern. SOC. Japan 52 ( 1 9 7 9 ) 2343-48. J.ii. B o n n i e r , J.P. Uamon and J . iyasson, A p p l . C a t . 42 (1988) 285-97. T. K o s c i e l s k i , J.N. B o n n i e r , J.P. Uailion and J. Masson, A p p l . Ca;. 4Y ( l W 9 91-99. J.iY. B o n n i e r , J. C o u r t , P.T. d i e r z c h o w s k i and S. Hamar-Thioaul.i, A p p l . C a t . 53 (1Y8Y) 217-31. S. Sane, J.M. B o n n i e r , J.P. Uanon and 3 . Idasson, A p p l . Ca.c. Y ( 1 Y 8 4 ) 5 9 - 8 3 . A.P.G. Kieboom and F . Van Rantwi j k , D e l f U n i v e r s i t y P r e s s , H y d r o g e n a x i o n and h y d r o g e n o l y s i s i n s y n t h e - i i c o r g a n i c c h e m i s t r y ( 1 9 7 7 ) p . 70. P. Geneste and Y . Lozano, C.R. Acad. Sc. 28i) ( 1 9 7 5 ) 1137-40. G.A. M a r t i n , J. C a t . 60 ( 1 9 7 9 ) 345-55. 14.E. K r a f t , J.J. Crooks, 8 . Zoac, S.E. i q i l c z a n o w s k i , J . Org. Chem. 53 ( 1 9 8 8 ) 3158-63. 111.

253

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

CHEMOBELECTIVE REDUCTION OF ENONEB TO ALLYLIC ALCOHOLS

Jan Kaspar, Farnetti and

Alessandro Trovarelli*, Franco Mauro Graziani.

Zamoner, Erica

Dipartimento di Scienze Chimiche, Universita' di Trieste, via A.Valerio, 34127 Trieste, Italy. *Istituto di Chimica, Facolta' di Ingegneria, Universita' di Udine, Viale Ungheria 43, 33100 Udine, Italy. SUMMARY Chemoselective reduction of conjugated enones to allylic alcohols via hydrogen transfer from propan-2-01 over metal oxides is investigated in vapour phase conditions. The unique ability of MgO to reduce exclusively carbonyl group is observed. However, because of the high basicity of MgO side reactions are present. It is shown that by doping the MgO catalyst with HC1 a significant decrease of its basicity occurs and consequently side reactions are minimized. INTRODUCTION Chemoselective catalytic reduction of a l p unsaturated ketones to allylic alcohols is a challenging problem since, but a few exceptions [l-31, this reaction generally proceeds with formation of saturated ketones or saturated alcohols [ 4 ] . This reduction indeed is best carried out with stoicheiometric hydrides [4] but even in this case overreduction products are often obtained [5]. Recently, we reported in a preliminary communication [6] the unprecedented observation that a,@ unsaturated ketones are reduced to the corresponding allylic transfer from propan-2-01 over MgO as alcohols by hydrogen catalyst according to the following scheme:

MgO, 250 Flow conditions

OC

R,

OH

Several metal oxides such as A1203, CaO, Ba0...[71, are known to catalyze the hydrogen transfer reduction of simple ketones. In this study we report the catalytic activity and the

254

chemoselectivity in the reduction of the carbonyl group of 4hexen-3-one over various oxides. Among the catalysts tested, MgO shows the highest chemoselectivity towards formation of unsaturated alcohols. However, besides the chemoselective reduction, several side reactions occur, lowering the selectivity towards allylic alcohols. Therefore the effects of addition of dopants on the selectivity of the MgO catalyst is also investigated. METHODS 4-hexen-3-one (Aldrich, trans 98&, 2%) and propan-2-01 (Baker) were distilled before use and stored under nitrogen. Commercial oxides, i.e. A1203, CaO, MgO and SrO (C.Erba) were used as purchased. Hydrotalcite was synthesized according to a were prepared by published procedure [ 8 3 Mg (OH) and Ca (OH) hydration at 100°C for 2 hours of the corresponding oxides (15 g in 200 ml of water) followed by filtration. The powder was dried overnight at 12OoC. The resulting hydroxides contained less than 2% of the starting oxide as checked by powder x-ray diffraction. The doped catalysts were prepared by adding the dopants during the hydration process and the excess of water was successively eliminated on a rotary evaporator. The catalytic runs were carried out in a stainless steel tubular flow reactor (length = 100 mm, i.d. 4 . 6 mm) which was connected to a stainless steel air-cooled condenser. Liquid reactants were fed by means of a Waters M-45 high pressure pump. Typically 0.29 g of the catalyst were placed in the reactor between two layers of granular quartz which acts as a preheater. The catalysts were activated at 35OoC for 4 hours in a flowing mixture of O2 in Ar (15%, 15 ml/min) [ 9 ] , the temperature was then lowered to 25OoC and the catalytic reaction started by introducing the liquid reactants. Temperature programmed desorption (TPD) of C02 ( 5 O/min, flow of He, 15 ml/min) was carried out on a conventional flow apparatus. In a typical experiment, 0.29 g of the catalyst were activated as above reported, then the system was cooled to 25OC and approximately 2*10-5 mol of C 0 2 were injected by means of a gas sampling valve. After degassing in flow of helium for 60 min the amount of the irreversibly adsorbed C02 was determined with an on-line g.1.c. equipped with a thermal conductivity detector,

.

255

using a wide bore Porapak Q capillary column (Chrompack). Surface areas were determined by single point B.E.T. method of nitrogen adsorption. RESULTS AND DISCUSSION Reduction of ketones and aldehydes by hydrogen transfer from alcohols is catalyzed by a variety of oxides [7,10]. For example, alumina catalysts turned out to be active in this reaction both in the gas-solid [11,12] and liquid-solid conditions with the alcohol preadsorbed on the catalyst [13]. Also zeolites [14] and hydroxyapatite [15] have been employed in this reaction. However, a l p conjugated ketones resulted to be quite resistant to the reduction and poor yields of allylic alcohols or only saturated ketones could be obtained [lo-131. Chemoselective catalytic reduction of conjugated enones to allylic alcohols is a challenging problem in heterogeneous catalysis since this reaction generally proceeds with formation of saturated ketones or alcohols. Our recent findings that MgO promotes chemoselective reduction of conjugated enones to allylic alcohols [6] prompted us to investigate the hydrogen transfer reduction of a conjugated enone (4-hexen-3-one) using propan-2-01 as hydrogen donor and metal oxides as catalysts. The results are reported in Table 1. The reaction is carried out in vapour phase (250OC) using a flow system (see methods section). This procedure turned out to be essential in order to mantain the hydrogen transfer as the main reaction pathway. A batch experiment carried out in an autoclave actually showed a wide range of condensation products besides some saturated ketone [6]. Reactions of ketones over oxide catalysts can lead to a variety of products due inter alia to aldol condensation, intramolecular dehydration and intermolecular disproportionation [16]. However, the presence of a good hydrogen donor such as a secondary alcohol and vapour phase conditions favour the transfer hydrogenation as the major reaction [16,17]. In our reaction conditions, products attributable to crotonic condensations and subsequent 1,4 Michael addition [18] were observed by g.1.c.-m.s. (Table 1). Typically at the beginning of the reaction (first 15 minutes) a strong adsorption of the reactants is observed before the steady state conditions are attained. Then, after 1.5-3 hours

256

a slow deactivation of the catalyst is observed. Thus in the run 1.1, Table 1, a conversion of 78% is observed after 1 hour of reaction which decreases to a value of 11% after 3 hours. However the catalyst can be easily reactivated in flow of 02/Ar at 35OoC (compare runs 2 and 2.1, Table 1) using the procedure reported in the previous section. Catalytic activity can be restored also by a thermal treatment in flow of He (35OoC, 15 h), and this suggests that strongly adsorbed produts could be responsible for catalyst deactivation. The amount of 4-hexen-3one converted depends on the nature of the catalyst precursor and on its thermal pretreatment. Thus, over a non activated commercial MgO (obtained by thermal decomposition of MgC03, surface area 17 m2/g) , 0.5 moles of 4-hexen-3-one/mole MgO are converted, while when the same MgO was activated at 35OoC (surface area 34 m2/g), 2 moles of 4-hexen-3-one/mole MgO are converted. Over a high surface area MgO (prepared by thermal decomposition of Mg(OH)2, surface area 281 m2/g) up to 5 moles of 4-hexen-3-one/mole MgO can be converted. Conversion of 4-hexen-3-one depends also on reaction temperature: 25OoC is found to be the best one, since both at higher and lower temperatures side reaction are favoured (runs 2.2 and 2.3, Table 1). Since different oxides were employed, the product distributions reported in Table 1 were measured in stationary conditions after 1 hour of reaction. Surface properties and catalytic activity of oxides depend on the nature of the precursor. In the isomerization of cis-2butene, the maximum of activity and selectivity (ratio of t-2butene to 1-butene) of MgO ex Mg(OH)2 is about fourfold of that observed with MgO ex 4MgC03.Mg(OH)2.5H20 [19]. Similar effects have been observed in the hydrogenation of 1.3butadiene catalyzed by MgO [20]. By thermal treatment of Mg(OH)2 at 300-400°C, MgO crystallites grow topotaxially with their (111) plane parallel to the basal (0001) plane of Mg(OH)2 [21] and the recrystallization is slow even at 8OO0C [9], giving MgO of different surface morphology In our case the ratio hexan3-one/hexen-3-ols is unaffected by surface morphology (runs 1.1 and 2, Table 1). On the other hand, side reactions increase on increasing the surface area of MgO; this result could be associated with a higher number of surface basic sites in the

.

257

TABLE 1. Hydrogen transfer reduction of 4-hexen-3-one catalyzed by metal oxides, Mg(0H) and MgC12 .a Run Catalyst

Surface

arY (m /g) 1. MgO 1.1

17 34

W/$ Conversion (*102) (t) IS

2.4

l.ld 2.e MgO 2.d

281

2.4

2.1

11 78 42

94 61 93 41 96

2.2 2.3

10 5

8 1

6.5 1 6 1

Product distribution (%)' SK SA UA Others

0.5 <0.5

69

3

32

2

<0.5 <0.5 <0.5

72 40 75

20

-

-

<0.5

1

0.5

63

13.5 15.5 34.5 29

3.

CaO

17

2.3

17

13

2

2

-

4.

cao

22

2.3

51

10

4

4

33

5.

sro

2.4

41

22

1.5

1

16.5

163

2.4

30

9

12.5

8

Hydrotalcite 250

2.4

100

0

45

8

27

2.4

31

14

<0.5

1

16

2.4

27

15

<0.5

1

11

6.g A1f3

7.

8.i Mg(OH)2 9.

M@12

38

a. Reaction conditions: temperature 250°C, propan-2-01/4-hexen-3-one = 20. Catalysts were activated at 350°C (see methods section) except for runs 1,3 where no activation was used. Product distribution was measured in steady state conditions after 1 hour of reaction. b. Time factor: W/F g(catalyst)$min*ml( liquid reactants)-'. c. IS (isomers): percentage of s-4-hexen-3-one + 5-hexen-3-one. Typically a ratio of 2 : l was observed; SK (saturated ketone): hexan-3-one; SA (saturated alcohol): hexan-3-01; UA (unsaturated alcohols): percentage of t-4-hexen-3-01 + _c-4-hexen-3-01 + 5-hexen-3-01. Typically a ratio of 15:1:2 was observed; Others: mainly products of crotonic and aldol condensation as identified by g.1.c.-m.s. d . Product distribution relative to the total amount of products obtained after 3 hours of reaction. e. Prepared i n s i t u from ME(OH)~. Run 2.1: catalyst from run 2 reactivated as reported in note a; run 2.2: reaction temperature 150OC; run 2.3: reaction temperature 30OoC. f. Prepared i n s i t u from Ca(0H) by activation at 45OoC. g. Reaction temperature 18OOC. f t 25OoC only saturated ketone and saturated alcohol are formed. h. Hydrotalcite: Mg AlZ(0H) s(C%) .4&0. Activated at 45OoC. i . Activated at 2d0C for hours. At. the end of the react.ion, only ME(OH)~ is present as checked by powder X-ray diffraction.

1

258

high surface area catalyst. Surface basic OH sites have been acetone over group 2 metal oxides [22]. The high basicity of MgO is associated with the presence of surface 02-cus (cus= coordinatively unsaturated site) : their concentration depends on thermal pretreatment and it shows a maximum around 7OO0C [23]. A sample of MgO catalyst pretreated at 65OoC showed only a negligible enhancement of the are not amount of condensation products suggesting that 02-cus responsible for them (however during the condensation reaction water is produced which could saturate 02-cus). Higher basic strength of surface OH group of CaO and SrO [22] should promote the condensation reactions. This is the case as shown in Table 1, runs 4,5, provided that basic sites are free from adsorbed H20 and C02 (compare runs 3 and 4, Table 1). Notably, once again the ratio hexan-3one/hexen-3-ols appears unaffected by the catalyst precursor and/or pretreatment (runs 3,4, Table 1). These observations suggest that in the transfer hydrogenation of 4-hexen-3-oneI the substrate is coordinated on a weak acid site while propan-2-01 must be coordinated on an adjacent surface basic site [7,24]. This is confirmed by the lack of reduction products observed over Mg(OH)2 and MgC12 (runs 8,9, Table 1). Conjugate carbonyl compounds contain centers of different hardness (hard CO and soft C=C), consequently hard hydrides favour conversion of enones to allylic alcohols [25]. Such a concept could be operative also in our case since C=C bond is preferably reduced on going from MgO to SrO. Besides oxides of group 2, A1203 and hydrotalcite have also been tested, and they turned out to be the most active (Table 1, runs 6 and 7 respectively). (Upon activation at 450°C, hydrotalcite looses reversibly water and C 0 2 , and Mg6A1208(OH)2 is formed [ a ] ) . However the high catalytic activity of aluminumcontaining catalysts is accompanied by a low selectivity towards formation of unsaturated alcohol. In the reduction of methyl vinyl ketone over A1203, mainly saturated products were observed, and it was shown that the saturated ketones resulted from a rearrangement of the unsaturated alcohol [ll]. This could apply also in our case since on A1203 in different catalytic conditions obtained by changing reaction temperature (18025OoC) and time factor (W/F, 17-2.4 g(catalyst)*

259

min*ml (liquid reactants)-l) good yields in the unsaturated alcohols could not be obtained. Notably, using hydrotalcite as catalyst a large amount of by-products is formed in the reaction. Presence of more basic sites in the latter catalyst could account for such behavior [ 2 6 ] . With the aim of suppressing the concurrent condensation reactions a number of doped MgO catalysts was prepared and their catalytic activity investigated (Table 2 ) . Surface basicity of these catalysts was measured by means of temperature programmed desorption of irreversibly adsorbed C 0 2 (see methods section). Desorption peaks in the TPD experiments are considered to appear at higher temperatures as the basic sites on the surface become stronger, while peak area is considered to be related to the number of these sites [ 2 2 ] .

TABLE 2 .

Hydrogen transfer reduction of 4-hexen-3-one catalyzed by doped MgO catalysts.a Run Dopant (% wt)

Surface Conv. (%)

IS

Product distribution (%) COz desorptionb SK SA UA Others temp. area (OC)

-

281

61

6.5

<0.5

1

40

26 20

26 13

3 3.5

0.5 0.5

-

-

20 9

-

3. Ce(N03)3 (2.5) 227

51

9

7

-

13

22

4. HC1 (1.1) (0.2) 4.1 4.2 (0.05)

27 26 25

3.5 11 13

0.5

-

20 6 2

3 9

1.

2. CeC13 (2.5) 2.1 (5)

29 241 271

<0.5

-

-

-

(a.u.1

13.5

500

54000

2.5

520

200

545 420 450

< 100 5400 13300

10

a. Reaction conditions as reported in Ta le 1, note a. W/F = 2.4*10-2 g(catalyst)Smin*ml( liquid reactants)-'. The reported product distribution is relative to the total amount. of the products obtained after 3 hours of reaction; product distribution os reported in Table 1, note c. b. Temperature of the mnximum dosorption rate and the area (arbitrary units) of the relative peak.

Complexation of substrate with lanthanide ions favours the attack of the carbonyl group in the reduction of conjugated

260

enones by borohydride [5,25a]. We have therefore tested the effects of addition of CeC13 to the MgO catalyst. The results reported in Table 2, runs 2, show that increasing the loading of CeC13, the condensation reactions are suppressed (runs 2, 2.1, Table 2). At the same time, both surface area and total conversion drop significantly. Over these catalysts , isomerization products and unsaturated alcohols are formed almost exclusively. The lack of side reactions, however, does not appear to be related to the presence of the Cerium cation, since a Ce(N03)3 doped catalyst produces a large amount of byproducts (run 3, Table 2). Transformation of Mg(OH)2 to MgO is heavily influenced by the presence of HC1. On increasing the chloride content a sensible drop of the surface area is observed [27], which was attributed to a post-dehydration sintering process aided by the chloride ion [28]. We prepared three HC1 doped catalysts with different loading of the C1- ion (see methods section). On increasing the C1- content, surface area strongly decreases, and so does the basicity of the catalyst as measured by the C02 desorption (runs 4-4.2, Table 2) (There are some strong basic sites present in HC1 (1.1% wt) catalyst, but their number is at least of two orders of magnitude lower than in the other samples). The amount of condensation products follows the same trend, and this is consistent with the role of the basic sites in promoting the condensation reactions. However, the overall selectivity towards the unsaturated alcohols appears to be improved only in the case of the HC1 (1.1 %wt) sample, since a discrete amount of isomerization products is present when the high surface area doped catalysts are used (runs 4.1 and 4.2, Table 2). The edge of (100) plane has been proposed as catalytic site for isomerization of olefins over MgO [29]. Consistently, in the low surface area sample (run 4, Table 2) , the isomerization is depressed, and unsaturated alcohols become the main produts. Finally, comparison of the catalytic activity of the CeC13 (2.5% wt) and HC1 (1.1% wt) catalysts, which contain the same amount of chloride, confirms the role of C1- in modifying catalyst basicity and morphology, hence minimizing the by-products. Acknowledsments. The authors thank CNR (Roma) IIProgetto Finalizzato Chimica Fine 11" and M.U.R.S.T. (Fondi 40%) for financial support.

261

REFERENCES 1. E.Farnetti, G.Nardin and M.Graziani, J.Chem.Soc.Chem.Commun., (1989) 1264. 2. M.Shibata, N.Kawata, T.Masumoto and H.Kimura, J.Chem.Soc. Chem.Commun., (1988) 154. 3. C.S.Naramsimhan, V.M.Deshpande and K.Ramnarayan, J.Chem.Soc. Chem.Commun., (1988) 99. 4. M.Hudlicky in Reductions in Organic Chemistry, ed. Ellis Horwood Limited, Chichester (United Kingdom), 1984, pp.120-121

.

and refs therein. 5. J.Luche, J.Amer.Chem.Soc.,

1 0 0 (1978) 2226; A.L.Gema1 and J.Luche, J.Amer.Chem.Soc., 1 0 3 (1981) 5454. 6. J.Kaspar, A.Trovarelli, M.Lenarda and M.Graziani, Tetrahedron Lett. , (1989) 2705. 7. R.A.W.Johnstone, A.Wilby and I.D.Entwistle, Chem.Rev., 85 (1985) 129 and refs. therein. 8. W.T.Reichle, S.Y.Kang and D.J.Everhardt, J.Catal., 1 0 4 (1986)

352. 9. A.G.Shastri, H.B.Chae, M.Bretz and J.Schwank, J.Phvs.Chem., 8 9 (1985) 3761. 10. -M.Shibagaki, K.Takahashi and H.Matsushita, Bull.Chem.Soc. &, 6 1 (1988) 3283. 11. W.Z.Sharf, L.Kh.Freidlin and N.K.Vorob'eva, 1zvest.Akad.Nauk SSSR. Ser.Khim., 8 (1972) 1846. 12. R.V.Ramana and C.V.Pillai, Canad.J.Chem., 47 (1969) 3705. 13. G.H.Posner, A.W.Runquist and M.Chapdelaine, J.Oru.Chem., 42 (1977) 1202. 14. J.Shabtai, R.Lazar and E.Biron, J.Mol.Catal., 2 7 (1984) 35. 15. C.L.Kibby and W.K.Hal1, J.Catal., 3 1 (1973) 65. 16. K.Ganesan and C.N.Pillai, J.Catal., 118 (1989) 371. 17. D.V.Ramana and C.N.Pillai, Indian J.Chem., 8 (1970) 1106. 18. G.SD.Salvapati, k.V.Ramqanamurty and M.Janardanarao, J.Mol.Catal., 54 (1989) 9. 19. H.Hattori, K.Shimazu, N.Yoshii and K.Tanabe, Bull.Chem. SOC.JaR., 4 9 (1976) 969. 20. K.Myiahara, Y.Murata, I.Toyoshima, Y.Tanaka and T.Yokoyama, J.Catal., 6 8 (1981) 186. 21. L.Volpe and M.Boudart, Catal.Rev.Sci.Enq., 2 7 (1985) 515. 22. G.Zhang, H.Hattori and K.Tanabe, A~~l.Catal.,3 6 (1988) 189. 23. M.Che, C.Naccache and B.Imelik, J.Catal., 2 4 (1972) 328. 24. Y.Okamoto, T.Imanaka and S.Teranishi, Bull.Chem.Soc.JaD., 4 5 (1972) 3207. 25. a) T-L.Hok, Tetrahedron, 4 1 (1985) 1; b) J.Bottin, 0.Eisenstein, C.Minot and T.A.Nguyen, Tetrahedron Lett., (1972) 3015. 26. W.T.Reichle, J.Catal., 94 (1985) 547. 27. G.Leofanti, M.Solari, G.R.Tauszik, F.Garbassi, S.Galvagno and J-Schwank, AvDl.Catal., 3 (1982) 131. 28. T.E.Holt. A.D.Logan, S.Chakraborti and A.K.Datye, Ap~l.Catal., 34 (1987) 213. 29. H.Hattori in Adsoprtion and Catalysis on oxide Surfaces, M.Che and G.C.Bond (eds.), Stud.Surf.Sci.Catal., 2 1 (1985) 319. ~

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine ChemicalsXI 0 1991 Elsevier Science Publishers B.V., Amsterdam

263

COMPARISON OF HOMOGENEOUS AND HETEROGENEOUS PALLADIUM CATALYSTS IN THE CARBONYLATION OF ALLYL ETHERS

M. M. BARRETO-ROSA, M. C. BONNET and I. TKATCHENKO* Laboratoire de Chimie de Coordination, UPR CNRS 8241 like par conventions 51 1’Universitt Paul Sabatieret 2 1’Institut National Polytechnique de Toulouse, 205 Route de Narbonne, 31077 Toulouse Cedex (France)

SUMMARY The carbonylation of allylic methoxyoctadienes has proved to be very efficient in the presence of soluble palladium(0) and -(II) complexes to give selectively the linear methyl nona-3,8-dienoate, provided that hydrochloric acid is added to the reaction medium. Under the same conditions, 10% palladium deposited on charcoal exhibits similar activity and selectivity to soluble complexes. In both cases, careful examination of the results seems to agree with homogeneous catalytic pathways.

INTRODUCTION Palladium has been extensively used in organic syntheses and in homogeneous catalysis (ref. 1-3), but industrial applications have remained relatively rare so far (ref. 4).The main reason lies in the de-activation of the catalyst by precipitation of metallic palladium under catalytic conditions. Such a process is actually observed in the carbonylation reactions under CO pressure. We have studied the carbonylation of various allylic ethers in the presence of transition metal complexes (ref. 5) with special emphasis on the reaction of methoxyoctadienes 1 , 2 catalyzed by palladium complexes (ref. 6). With bis[(methallyl)chloropalladium(II)], the best ether conversion (97%) and methyl nona-3,8-dienoate 3 yield (95%) are obtained under 30 bar of carbon monoxide (eqn. 1).

+

-

Pd(dba), 2MeOH

+ 2 PPh3

-c

‘

OMe 90%

1 OMe

8OOC

10%

4 30 bar CO, 100°C OMe

3

0

Mechanistic studies (ref. 6, 7) suggest the intermediacy of a zerovalent palladium complex produced by reductive elimination of an acid chloride, and the need for hydrogen chloride in order to cleave the C-OMe bond by preliminary protonation of the methoxy group leading to an allylpalladium complex which undergoes carbonylation (Scheme 1).

264

Scheme 1 The occurrence of a palladium(0) intermediate prompted us to examine the same reaction with a supported palladium catalyst with the aim of preventing the loss of palladium and separating more easily the catalyst from the reaction products. EXPERIMENTAL Starting materials Palladium(I1) chloride (Johnson Matthey) and 10% Pd/C (Aldrich) were used as received. Bis[ (methallyl)chloropalladum(II)] (ref. 8), bis(dibenzylideneacetone)palladium(O) (ref. 9), methoxyoctadienes (ref. lo), 1-methoxy-3-octene (ref. 1l), methoxyallyle and cis + trans-methoxycrotyle (ref. 12) were prepared as described in the literature. All reactions were carried out following the Schlenk tube technique. All solvents were distilled under argon. Analytical Drocedures GC analyses were performed on an Intersmat IGC 121 FL instrument equipped with a FID detector (column: 1.5 m x 1/8" of 10% SE30 on Chromosorb Q (80-100 mesh); camer gas: nitrogen (20 mlmin-I); temperature programming: 9.5 min at 100°C, then increase to 200OC at a rate of lS°C min-1) using decane as internal standard.

IH and I3C NMR spectra were obtained on a Bruker AC 200 spectrometer. Catalytic reactions Catalytic reactions were performed in a 100-ml glass-lined stainless-steel autoclave contnining a magnet bar. In a typical run, 10% Pd/C (0.5 mmol of Pd) was weighted, then introduced into the autoclave. Two argon-vacuum cycles were perfomied before addition of 8 ml of toluene, 50 mmol (7.01 g) of substrate and S mmol of hydrochloric acid (CH30H + CH3COCI). The autoclave was pressurized with 30 bar of carbon monoxide, and heated to 100OC. After 24 hours, the autoclave was cooled and the remaining carbon monoxide was vented. The reaction mixture was transferred by means of a syringe and centrifugated (600rpm during 6 min) in order to separate the solid. The liquid phase was analyzed by GC, after dilution in ethoxyethane. The solid was washed with toluene, dried under vacuum and examined by electron diffraction microscopy and EDAX.

265

RESULTS AND DISCUSSION Inspection of Table 1 indicates that the presence of both proton and chloride is required for the catalytic conversion of 1 and 2 into the linear ester 3. The most appropriate co-catalyst for this reac tion is therefore hydrogen chloride whose excess is necessary to observe noticeable conversions. Although we have not carried out a systematic study as in the case of soluble palladium catalysts (ref. 6), the reaction in the presence of Pd/C is also sensitive to the nature of the counter anion used. Runs 6, 7 lead to similar conclusions to those reported for homogeneous reactions. Moreover, run 10 strongly suggests that soluble species are operating also with the use of Pd/C precursors. The tetrafluoroborate counter-anion may therefore stabilize a soluble cationic palladium species. Nevertheless, the chloride anion must intervene at some stage of the catalytic process in order to obtain the expected ester. It is worthy of note that the catalyst precursor is recovered in good yield when the reaction proceeds in the presence of HCI.

TABLE 1 Carbonylation of 1- and 3-methoxyoctadienes (isomer ratio: 97/3) in the presence of 10% Pd/charcoal (Experimental conditions: P(C0) = 30 bar; T =lOO°C; Pd/C = 532 mg (0.5 mmol); substrate / Pd = SO; reaction time = 24 h; solvent = 10 ml toluene; stirring speed = 500 rpm) Run

b c

Additive/Pd

% Conversion

% Yield

% Pd/C recovered

0

0

99 85

1

-

2

HBF,

1/1

228

0

3

PPNClb

1/1

0

0

4

HBF,:PPNCl

1:1/1

22

20

5

HC1

1/1

7

2

84

6

HC1

5/1

32

30

97

7

HC1:n-Bu4BF4

5:5/1

81

66

63

8

HCl:n-Bu,BF,

10:5/1

95

83

45

9

HClc

5/ 1

29

10

80

n-BudBF4

5/1

9

0

10 a

Additive

Polymers PPNCI: [Ph,P=N=PPh,]+CIReaction with P d C recovered after the run 6 (0.25 mmol Pd)

The reaction exhibits an induction period. Comparison of different catalyst precursors shows that the behaviour of Pd/C is similar to that of PdC12 (Table 2). Examination of the medium at the first stages of the reaction indicates in both cases the formation of dimethyl carbonate. This product is known to be formed by the reaction of CO and MeOH in the presence of O2 and PdCl, (ref. 13).

266

TABLE 2 Carbonylation of methoxyoctadienes in the presence of soluble or supporred palladium catalysts (Experimental conditions: P(C0) = 30 bar; T = 100OC; substratepd = 50) Time (h) Conversion (%) Yield (%)

AdditivePd

Run

Catalyst

Additive

11

[(q3-C4H,)PdCl],

HCI

511

24

97

95

12

PdCl,

HCP

511

24

88

81

HCI

2011

8

66

49

HCl

511

24

32

30

HCl

2011

8

50

45

111

24

0

0

24

0

0

13

10%PdlC

6

14

PPNCl

3

Pd(dba),

15

a

16

HCI

5/1

24

85

76

17

nBu4Cl

111

24

22

0

18

HBF4

111

24

17

0

Na2[PdC14]instead of PdClz

In the case of experiments performed under the conditions of run 6, but in the presence of 1 ml of methanol, 1.6 equivalent of dimethyl carbonate was obtained according to GC analysis. No dimethy1 carbonate was observed in the absence of hydrogen chloride. Therefore, in the early stage of the carbonylation of 3, Pd/C is partly oxidized to palladium chloride (eqn. 2). This compound reacts in turn with CO and MeOH to give, according to one of the routes described in Scheme 2, dimethyl carbonate and a zerovalent palladium complex (noted [Pd]).

+

[Pd]

2HCl

+

1/20,

PdClz

+ I320

(2)

This oxidation is made feasable due to the presence of trace amounts of oxygen in the reaction medium, since no attempts have been made to deoxygenate the ether mixture and the solvent. route a

,cooMe
II

[Pdl,

c1

0

-+

[Pd]

+

route b [pd],,c1

c1

co ---+

[Pd]

+ C0Cl2

ClCOOMe

MLQH

- HCl

MeOH

---+

ClcoOMe

Scheme 2

HCI

+

MeOCOMe

II

0 MeOH

- HC1

MeOCOMe

II

0

267

The analogous behaviour of PdC12 and PdJC is further demonstrated by the reactivity of allylic ethers of various nature (Table 3). In the carbonylation of I-methoxy-3-octene, the generation and the stabilization of the Pd(0) active species are more difficult than in the case [(q3-C4H,)PdC1l2and Pd(dba),. The stability of this zerovalent species is also important in the carbonylation of cis and rrans methoxycrotyle. Actually, for all such ethers, there is no extra-stabilization as observed with 1 and 2, where the terminal double bond could be coordinated to the metal centre (ref. 6). TABLE 3 Carbonylation of various allylic ethers RCH=CHCH20Me: conversions and ester yields (Experi mental conditions: P(C0) = 30 bar; T = 100°C; substrate/ Pd = 50; HCl/Pd = 5; t = 24 h) Runs

Ether

[(q3-C4H7)PdC1l2

19-22

1-methoxy-3-octene

92(63)

22-25

cis + ?ram-methoxycrotyle

50(45)

26-28

methoxyallyle

a determined

PdCl2

Pd/C

Pd(dba)2

37(18)

28(5)

53(15)

2(2)

2(<1)

2(<0

43 (43)

22(22)a

61(61)a

by *HNMR spectroscopy

Soluble catalysts are deactivated by palladium precipitation (ref. 6). The main purpose in using Pd/C was to avoid catalyst de-activation. The catalyst recovered by centrifugation from run 5 has been used for another carbonylation of 1 and 2 under standard conditions (Table 1, run 9). Although the conversion is similar to that observed for a fresh catalyst (run 6), the ester yield is much lower. Heavy products are formed, arising, as in run 2, from the oligomerization of the starting ethers. Examination of the catalyst prior to and after a carbonylation reaction was untertaken by transmission electron microscopy and EDAX. No difference in the chemical composition is observed. Figures 1 and 2 show the complete transformation of the well dispersed fresh catalyst (particle size ca. 3 nm) into large, ill-shaped particles. We suggest that the processes leading first to soluble palladium active species are followed by precipitation of palladium.

I

Fig. 1: E M of the fresh Pd/C catalyst

Fig. 2: TEM of the P#C catalyst from run 6

268

To conclude, palladium on charcoal is only a precursor for soluble palladium species which are active in the carbonylation of ally1 ethers. The formation of these species requires the presence of chloride and proceeds jointly with the generation of dimethyl carbonate. This catalyst could be reused but we have been unable so far to recover the ester selectivities achieved with fresh catalysts. The observation of a higher content of oligomeric materials in the reaction mixture resulting from the use of the recovered catalyst led us to suggest that an increase of the surface acidity of the carrier is presumably responsible for this increase in the oligomer content.

REFERENCES 1 P.M. Maitlis, The Organic Chemistry of Palladium, Volume I1 : Catalytic Reactions, Academic Press, London, 1971. 2 J. Tsuji, Organic Syntheses by Means of Transition Metal Complexes, Springer Verlag, Berlin, 1975; Organic Syntheses with Palladium Complexes, Springer Verlag, Berlin,1980. 3 R.F. Heck, Palladium Reagents in Organic Synthesis, Academic Press, London, 1985. 4 G.W. Parshall, Homogeneous Catalysis, Wiley-Interscience, New-York, 1980; G.W. Parshall and R.E. Putscher, J. Chem. Educ., 63 (1986) 189-191. 5 M.C. Bonnet, D. Neibecker, B. Stitou and 1. Tkatchenko, J. Organomet. Chem., 366 (1989) C9-Cl2. 6 M.C. Bonnet, J. Coombes, B. Manzano, D. Neibecker and I. Tkatchenko, J. Mol. Catal., 52 (1989) 263-276. 7 M.C. Bonnet, M.M. Barreto and I. Tkatchenko, J. Organomet. Chem., submitted. 8 W.T. Dent, R. Long and A.J. Wilkinson, J. Chem. SOC.,(1964) 1585. 9 M.F. Rettig and P.M. Maitlis, Inorg. Synth., 17 (1977) 134. 10 D. Neibecker, J. Poirier and I. Tkatchenko, J. Org. Chem., 54 (1989) 2459-2462. 11 B. Stitou, Thesis, Toulouse (1988) p. 130. 12 W.T. Olson, H.F. Hipsher, C. M.Buess, LA. Goodman, I. Hart, J.H. Lamneck Jr. and L.C. Gibbons, J. Am. Chem. SOC.,69 (1947) 2451-2453. 13 R.P.A. Sneeden in: G. Wilkinson, F.G.A. Stone and E.W. Abel (Eds.), Comprehensive Organometallic Chemistry, Pergamon Press, Oxford, 1982, pp. 20- 100. ACKNOWLEDGEMENTS The authors wish to thank the CNRS for supporting this research.

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

269

0 1991 Elsevier Science Publishers B.V., Amsterdam

LIQUID-PHASE SELECTIVE HYDROGENATION OF 1 ,GBUTYNEDIOL ON SUPPORTED Ni and Ni-Cu CATALYSTS

.

F.M.

BAUTISTA, J.M.

CAMPELO, A. GARCIA, R. GUARDERO, D. LUNA and J. M. MARINAS.

Department of Organic Chemistry, University o f Cordoba, San Albert0 Magno Av., E-14004 Cordoba, Spain.

ABSTRACT Liquid-phase hydrogenation of 1,4 butynediol to cis-l,4-butenediol and 1,4butanediol has been carried out on nickel catalysts supported on thirteen different supports. Some comnercial nickel catalysts were used as references. furthermore, metal loading and Ni-Cu alloying have also been studied. The results obtained indicates that catalytic activity, selectivity and metal surface area of catalysts are closely correlated to some textural and/or acidbase properties of the corresponding support. Similarly, the influence o f Cu as a second metal in catalyst behaviour is also related to the nature of the support. INTRODUCTION As catalytic semihydrogenation of alkynes to Cis-alkenes is not only a very important synthetic operation (ref. 1) but also of industrial interest, it is a challenging task for both synthetic and catalytic chemists. For instance, the importance of the problem is illustrated by numerous recent publications on different aspects of the selective hydrogenation of many compounds related to the propargyl alcohol structure (refs. 2-7). In this respect, 1,4-butenediol, obtained by the liquid-phase semihydrogenation of 1,4-butynediol, is a raw material for insecticides and Vitamin 86 (refs. 2,8,9). Furthermore, the total and selective liquid-phase hydrogenation of this compound is one of the procedure for making butanediol, the top 95 chemical produced in the United States (refs. lO,ll), whose major use is in the manufacture of polyesters. The present work reports on results of the liquid-phase catalytic hydrogenation of butynediol on supported nickel catalysts specifically tailored for these processes. In this respect, we have studied support effects, the influence of nickel loading as well as the influence o f Cu as a second metal. EXPERIMENTAL Supports Thirteen different supports have been used: commercial silica (Si02) from Merck (Kieselgel 60, 70-230 mesh); a commercial alumina (A1203) (aluminum oxide active acidic for chromatography) from Merck; three AlP04 prepared according to

270

Kearby (ref. 12) by precipitation from aluminum chloride and phosphoric acid (aqueous solutions 85 wt.%) , using ammonium hydroxide solution (AlP04-F), ethylene oxide (A1P04-E) and propylene oxide (A1P04-P); three A1P04-A1203 (75-25 wt.%) systems similarly obtained (A1P04-A1203-F, E and P systems, respectively) and an AlP04-Si02 (20:80 wt.%) obtained by precipitation with ethylene oxide. The resulting powders screened to < 0.149 mm were calcined at 923 K for 3 h. Both commercial supports, A1203 and Si02, were subjected to the same calcination treatment. Besides, A1PO4-P was also calcined at 773 and 1073 K for 3 h so obtaining the A1P04-P5 and -P8 systems, respectively. The commercial active carbon (C) from Panreac and a natural sepiolite (S) from Vallecas (Madrid) supplied by Tolsa S.A. were used as received. The nominal chemical composition of the sepiolite is: Si02 62.0, MgO 23.0, A1203 1.7, Fez03 0.5, CaO 0.5, K20 0.6, Na20 0.3, weight loss from 293 to 1273 K 10.5 %. The detailed synthesis procedure and textural properties (surface area, SBET in in2 g-l; pore volume, V in ml l'g and main pore diameter, d in nm), determined by nitrogen adsorption from B.E.T. method have been published elsewhere (refs. 13-18) and are summarized in Table 1, where the surface acidity and basicity of supports are also collected. These values were determined by a spectrophotometric method described elsewhere (ref. 191, that allows titration of the amount (in vmol g-l) of irreversibly adsorbed benzoic acid (BA, pKa= 4.191, pyridine (PY, pka= 5.25) or 2,6-diterbutyl-4-methylpyridine (DTMPY, pKa= 7.5) employed as titrant agents of basic and acid sites, respectively. Furthermore, the apparent rate constant values of different supports in the gas-phase skeletal isomerization of cyclohexene (CHSI), in ,,mol atm" 9-l s-', at 673 K, are also collected in Table 1, because these values are another way of measuring the stronger acid sites of supports (ref. 19). Catalysts The synthesis of Ni and Ni-Cu supported catalysts was carried out by impregnation of the supports to incipient wetness with aqueous solutions of nickel or nickel and copper following the previously described procedures (refs. 20-23). They were dried, crushed, and screened to a particle size (0.149 mm (100 mesh size), reduced in an ultrapure hydrogen stream (1.7 cm3 s") at 673 K for 3 h, and then cooled to room temperature in the same hydrogen stream. Bulk nickel, in the same conditions obtained by reduction of nickel oxide (Merck, p.a.1 employed with the supported systems, was also used as catalyst. Final metal loading for catalysts supported on sepiolite were determined by atomic absortion spectrometry and the values obtained are summarized in Tables 2-4. Metal surface areas, SNi, for the different catalysts were determined from the average crystallite diameter, obtained by X-Ray Diffraction meaSUremnts as reported in a previous paper (ref. 20). Their corresponding values are collected in Tables

271

TABLE 1 Textural and acid-base properties of different supports. SUPPORT SBET V d Acidity Basicity PY DTMPY BA AlP04-P 228 0.94 2.5 227 166 78 4.3 242 0.52 267 AlP04-E 90 266 156 0.68 3.6 AlPO4-F 190 53 200 AlP04-P5 151 3.0 0.68 201 37 150 AlP04-P8 11 a a 10 a a AlP04-A1203-P 319 0.68 4.2 326 79b 774 AlP04-A1203-E 242 0.54 4.5 208 52b 577 AlP04-Al 03-F 244 0.37 3.1 187 3Zb 535 A1P04-Siij2 327 0.46 3.0 380 8 70 A1 O3 72 0.24 2.7 23 191 Sd2 366 0.68 3.5 206 164 Sepiolite 203 0.54 5.3 31 9b I74b C 743 0.55 1.5 124b -132b a --Ni-bulk 16 a a There is no adsorption of the titrant agent. Actually determined.

--

-__

---

CHSI

Ref

3.2 5.0 3.3 1.9

13,14 14-16 13,14 17 17 14 14 14 b 13 13 18

---

18.0 13.0 15.0

---------0.9 -------

b

2-4. Besides, four commercial nickel catalysts from Harshaw Chemie B. V. were used as a reference: a Ni-5333 T (20 wt% Ni), a Ni-5132 P (64 wt% Nil, a Ni-3210 T (35 w t % Ni) and a Ni-6458 T (60 wt% Nil. Apparatus, Materials and Procedure According to the procedure previously employed (refs. 20-231, hydrogenations were carried out i n a closed vessel, vigorously shaken in a conventional lowpressure hydrogenator (Parr Instruments Co., Md. 3911 ). Cis-2-Butene-l,4-diOl and 2-Butyne-1 ,$-dial were obtained from Merck p.a. and purified by distillation under reduced pressure and low temperature. Hydrogen (99.999 %, S.E.O.) and solvent methanol, ethanol, I-propanol, 1-butanol, 2-butanol and 3-butanol (p.a. 99%; Panreac); 1 -pentanol, 1 -hexanol, I -heptanol and 1 -octanol (Merck for synthesis) and N,N-Dimethylformamide (Merck, Lab.) were used without further purification. Most hydrogenation reactions were carried out in 20 ml of 0.5 M methanolic solutions of substrate, at 313 K , under initial hydrogen pressure of 0.41 MPa with 0.3 g of catalyst. However, one set of reactions was carried out with the catalyst Ni/A1P04-P in the hydrogenation pressure range of 0.3-0.7 MPa, temperature range of 303-333 K, substrate concentration of 0.5-3 M, catalyst weight range of 0.05-1 g and several solvents, in order to test the influence of these parameters on the catalytic activity and selectivity. The initial reaction rates in triple and double bond hydrogenation, (rT and rD in pmol s-l ,l'g respectively), were calculated by taking the initial slope of the plot of the hydrogen pressure decrease on the manometer versus time. Hydrogenation runs were followed by GLC analysing the reaction mixtures at the

272

TABLE 2 Support influence on the catalytic activity and the selectivity, S, in the catalytic hydrogenation of 2-Butyne-l,4-diol, rT, and cis-2-Butene-l,4-diol, rD, on 20 wt% supported nickel catalysts. SUPPORT ’Ni rT rD S (m2 gNi-’) (pmol s-1 9-11 (pmol s-1 g-1) (%I 32 10.30 40.29 92.5 AlP04-P 66 9.00 76.10 92.2 AlPO4-E 27.92 87.5 56 13.50 AlP04-F 123.87 91.2 AlP04-P5 69 12.13 55.42 90.3 AlP04-P8 17 7.42 42 0.88 9.81 90.2 AlP04-A1203-P 32 0.76 6.80 87.6 AlP04-A1203-E 1.66 86.0 103 0.31 AlPO -A1 0 -F 108.55 82.7 84 16.37 AlPOi-Sd; A1 03 27 6.26 27.98 93.9 159.23 86.1 Si% 45 66.70 40a 11.64 102.91 85.5 Sepiolite 145.84 84.4 C 26 22.51 28.27 91.7 Ni-bulk 13 2.19 84.75 73.4 Ni-5333 T 130b 68.32 45.07 95.3 Ni-5132 P 193b 42.01 1 25b 8.73 53.03 75.7 Ni-3210 P Ni-6458 P 139.45 278.75 83.9 a metal loading 28.3 w t X . b BET Surface area in m2 g-1.

---

appropriate intervals of hydrogen uptake to obtain a very low concentration o f alkyne. Thus, the relative concentration of alkene with respect to alkene and alkane let us determine the selectivity, S (ref. 24). GLC analysis were performed with a Hewlett-Packard 5830 gas chromatograph fitted with an H.P. 18850 GC Terminal, equipped with a column packed with 5 % Carbowax-20M in 80/lOO Chromosorb GAW-DMCS. Neither isomerization nor hydrogenolysis products were detected in any of the cases. RESULTS AND DISCUSSION The selected working conditions are the same as in previous works (refs. 2023) in order to avoid internal and external hydrogen diffusion control in the liquid phase hydrogenation o f the olefinic double bond. Similarly, results now obtained indicate that the kinetic data determined in the range of the studied operation variables are also free from transport influences. Besides, under the present standard conditions at 313 K, the reaction order in the initial hydrogen pressure, determined between 0.3 and 0.7 MPa, and in the substrate concentration determined in the range 0.5-3 M, was zero order for alkyne hydrogenation as well as for the corresponding alkene. Consequently, identical kinetic behaviour is found in the liquid-phase hydrogenation of the double and triple bond so that both hydrogenation mechanisms must be very closely related.

273

TABLE 3 Metal loading influence on catalytic activity and selectivity, S, in the catalytic hydrogenation of 2-Butyne-I,4-diol, rT, and cis-2-Butene-I ,4-diol, rD, on supported nickel catalysts. N’ i rT rD S SUPPORT %Ni (m2 SNi”) (mol s-1 9-11 (Irmol s-1 9-11 (XI AlP04-P 10 1 72 0.27 0.62 92.7 AlP04-P 20 160 10.67 16.34 90.4 AlP04-P 30 87 11.58 71.92 90.3 AlP04-P 35 25 19.37 121.16 87.1 AlP04-P 40 21 50.94 147.22 83.8 C 5 64 6.33 94.80 83.6 C 10 46 13.27 154.46 81 .I C 20 26 22.51 145.84 84.4 Sepiolite 6.3 118 1.21 23.63 82.6 Sepiolite 14.1 170 4.46 24.28 83.2 Sepiolite 28.3 40 11.64 102.91 85.5 Sepiolite 38.3 40 17.88 113.24 90.5 Sepiolite 39.4 24 9.69 133.94 92.3 Sepiolite 41.8 27 7.38 164.74 92.4

TABLE 4 Influence o f copper loading on catalytic activity and selectivity, S, in the catalytic hydrogenation o f 2-Butin-l,4-diol, rT, and cis-2-Buten-1 ,4-diol, rD, on supported nickel catalysts. S SUPPORT rT %Ni %CU SN~ (m2 gNi-1) (mol s-1 9-11 (wol S- g-l) (%I AlP04-P 20 20 18 9.51 20.02 97.9 32.37 97.2 AlP04-P 20 15 25 2.41 10 72 3.42 36.43 97.5 AlP04-P 20 90.5 66 18.05 44.35 8 AlP04-P 20 88.3 84 20.20 61 .I1 AlP04-P 20 7 AlP04-P 20 5 75 25.32 69.16 91.4 AlP04-P 20 4 84 12.22 114.38 91.9 117.54 88.2 AlPO4-P 20 3 117 15.58 AlP04-P 20 2 99 17.81 105.45 91.4 34.53 93.78 87.7 1 75 AlP04-P 20 86.8 12.11 72.26 AlPOq-P 20 0.6 82 84.6 16.38 23.37 AlP04-P 20 0.3 65 88.4 16.02 42.94 AlP04-P 20 0.1 85 94.3 10.41 35.08 Sepiolite 22.6 7.9 72 8.81 60.87 90.5 0.6 70 Sepiolite 20.3 87.8 50.98 71.22 Sepiolite 18.5 0.3 77 86.4 9.16 53.43 0.1 61 Sepiolite 16.3 147.00 91.2 4.71 C 20 7 59 84.28 86.7 0.6 52 41.88 C 20 26.83 84.62 90.9 20 0.3 57 C 86.3 11.77 124.24 0.1 50 20 C

‘3

Taking into account the zero-order kinetics obtained (refs. 20-23) with respect to the olefin concentration as well as to the hydrogen pressure, the

274

initial reaction rates (rD and rT, respectively) can be considered the corresponding reaction constant values. So, the Arrhenius equation can be applied to the rT and rD data to obtain for Ni20/A1P04-P the corresponding values of Apparent Activation Energies, for the triple bond (Ea = 28.1 2 KJ mol”) as well as for the double bond (Ea = 26.5 2 3 KJ mol-I). Selectivity values, S, are steadily increased with temperature in the interval studied from 88.9 to 94.7, respectively. Changes in hydrogen pressure or in substrate concentrations do not promote appreciable modifications in selectivity values. According to the results in Table 5, we see that increasing the dielectric constant values of solvents increases not only catalytic activity rT, as previously obtained for rD in several olefinic compounds (ref. 25), but also S values. In order to determine the potential influence of textural and acid-basic properties of the supports on the catalytic properties of supported nickel catalysts, we built a correlation matrix using all the data in Table 1 and Table 2. Results obtained in the regression analysis of those well correlated parameter pairs are shown in Table 6. According to these results, we see that the metal surface area of catalysts increases on increasing the surface acidity as measured with pyridine. Besides, catalytic activity for triple and double bond hydrogenation decreased in those catalysts whose supports were more active in the CHSI process and/or exhibited a higher number of basic sites titrated with benzoic acid. Finally, selectivity in the semihydrogenation of the alkyne bond increased when supports exhibited a low BET surface area and/or high surface acidity, as measured with DTMPY. In summary, different textural properties of supports promote important changes in selectivity, catalytic activity and metal surface area, the first ranging between 82 and 93% following the sequence A1203 = AlP04 ) Ni-bulk ) A1P04-A1203 ) SiOz > Sepiolite > Carbon ) A1P04-Si02. However, catalytic activity decreased in the order Si02 > Carbon > A1P04-Si02 > Sepiolite > A1PO4 ) A1203 ) Ni-bulk > AlP04-Al203 the first of these being 175 times more active than the last. However, with respect to catalytic activity, the best catalyst was the commercial Ni-6458 T, which, however, exhibited a relatively low selectivity. All these facts, as well as the influence of metal loading on catalytic activity and selectivity, may be associated with metal-support interaction effects. Thus, differential effects on catalytic activity and selectivity promoted by differences in the metal/support ratio, are related to the nature of the supports, as shown in Table 3. In this respect, on going from 10 to 40 w t % in Ni/A1P04, catalytic activity increased by a factor higher than 150 and selectivity decreased from 92.7 to 83.8%. In Ni/C, an increase in nickel loading leads to a slight increase in activity although the selectivity does not change, while in Ni/Sepiolite, the optimun catalytic activity in triple bond hydrogenation was obtained at 38.3 w t X Ni, an intermediate value, and the f

275

TABLE 5 Solvent effect on selectivity, S, and catalytic activity in the liquid-phase hydrogenation of 2-Butyne-l,4-diol, rT, on the Catalyst Ni20/AlP04-P. PT S SOLVENT Dielectric constant (Debye1 (Pmol ,-I g-1) (XI METHANOL 32.6 10.30 92.5 ETHANOL 24.3 10.66 84.2 20. I 8.06 92.7 PROPANOL 1 -BUTANOL 17.1 4.06 83.0 2-BUTANOL 17.7 3.90 86.6 3-BUTANOL 10.9 5.15 77.4 1-PENTANOL 13.9 2.29 78.7 1 -HEXANOL 13.3 2.56 79.3 2.66 76.4 1-HEPTANOL 10.3 3.46 75.4 1 -0CTANOL N, N-DIMETHYLFORMAMIDE 7.3 4.96 86.8

----

TABLE 6 General expression of the correlation y = ax + b obtained between some surface properties of the supports in Table 1 and the corresponding catalytic properties o f supported nickel catalysts in Table 2. Y X a b Significance(%) PY 0.1215 26.903 94.4 sN i CHSI -0.0998 1.338 99.8 log rT log rD CHSI -0.0822 1.994 98.1 BA -0.0024 I .442 99.9 log rT BA -0.0021 2.131 log r 99.8 Selec?i v i ty S( BET) -0.0110 91.263 95.3 Selectivity DTMPY 0.0785 84.880 91 .I

selectivity increased with metal loading. In bimetallic catalysts, the influence of Cu as a second metal in catalytic activity and selectivity is also closely related to the nature of the supports. However, as a general rule Ni-Cu alloying promotes an improvement in selectivity values. In this respect, the most interesting results were obtained in NiCu/A1PO4 catalyst containing 20 w t X of both Ni and Cu because the selectivity increased to 98% and the catalytic activity maintained the same level as in Ni /A1PO4 monometa1 1 i c catalyst. CONCLUSIONS AlP04 could be an adequate support component to enable a tailored Ni-Cu catalyst to obtain the most appropriate activity and selectivity in the semihydrogenation of 1,4-butynediol, not only due to its high degree o f selectivity toward the olefinic compound, but also because there was no formation of other side reaction products, as described (refs. 2 and 1 1 ) in the literature.

276

ACKNOWLEDGEMENTS This work was subsidized by a grant from the Direccidn General de Investigaci6n Cientffica y Tecnica (DGICYT, Project PA86-00651, Ministerio de Educacidn y Ciencia. Furthermore, financial aid from the Consejerfa d e Educaci6n y Ciencia de la Junta de Andalucfa is gratefully acknowledged. The authors would also like to thank Harshaw Chemie B. V. for providing some catalyst samples and wish to acknowledge the grammatical revision of the manuscript carried out by Prof. M. Sullivan. REFERENCES 1 P. Rylander, Catalytic Hydrogenation in Organic Syntheses, Academic Press, New York, 1979, Ch. 2, p. 13. 2 R. V. Chaudhari, R. Jagernathan, D. S. Kolhe, 6. Erning and H. Hofman, Appl. Catal., 29 (1987) 141. 3 0. V. Sokolskii, T. 0. Omarkulov, Zh. Mukataev, L. K. Zhubanova and L. V. Babenkova, Kinet. Catal. Engl. Trans., 26 (1985) 643. 4 T. L. Ho and S. H. Lin, Syn. Commun., 17 (1987) 969. 5 J. Rajaram, A. P. S. Narula, H. P. S. Chawla and S. Dev, Tetrahedron, 39 (1983) 2315. 6 A. Heath, Chem. Eng., (1973) 48. 7 Y. Izumi, Y. Tanaka and K. Urabe, Chem. Lett., (1982) 679. 8 H. Hoffman, 6. Boettger, K. Boer, W. Wache, H. Grafje and W. Koerning, (BASF) Germ. Offen. 2.451.929 (22.1.1976). 9 A. S. Wood and J. M. Reitz, (GAF) Germ. Offen. 2.605.241 (18.11.76) and Germ. Offen. (18.11.1976). 10 P. J. Chenier and D. S. Artibee, J. Chem. Educ., 65 (1988) 433. 11 R. Del ROSSO, C. Mazzochia, P. Grouchi and P. Centola, Appl. Catal., 9 (1984) 269. 12 K. Kearby, in: Proc. 2nd. Inter. Congr. Catal., (Technip. Ed.), Paris, 1961, D. 2567. 13 F. Bautista, J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, J. Catal., 107 (1987) 181. 14 J. M Campelo, A. Garcia, D. Luna , J. M. Marinas and M. I. Martinez, Mater. Chern. Phys., 21 (1989) 409. 15 A. Blanco, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M. S. Moreno, React. Kinet. Catal. Lett., 38 (1989) 223. 16 A. Blanco, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M. S. Moreno, React. Kinet. Catal. Lett., 38 (1989) 237. 17 J. M. Carnpelo, A. Garcia, D. Luna, J . M. Marinas and M. S. Moreno, J. Colloid Interface Sci., 118 (1987) 98. 18 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, Mater. Chem. Phys., 24 (1989) 51. 19 J. M.. Campelo, A. Garcia, J. M. Gutierrez, D. Luna and J. M. Marinas, Can. J. Chem., 61 (1983) 2567. 20 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, J.Catal., 97 (1986) 108. 21 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, Appl. Catal., 3 (1982) 315.

Campelo, A. Garcia, J. M. Gutierrez, D. Luna and J. M. Marinas, Appl. Catal.. 7 (1983) 307. 23 J. M. Campelo, A. Garcia, 0. Luna and J. M. Marinas, J. Chem. SOC., Faraday Trans. I, 80 (1982) 223. 24 L. Cerveny, J. Vopatova and V. Ruzicka, React. Kinet. Catal. Lett., 19 22 i . - M .

(1982) 223. 25 F. M.. Bautista, J. M. Campelo, A. Garcia, R. GuardeRo, D. Luna, and J. M. Marinas, J. Chem. SOC. Perkin Trans. 11, (1989) 493.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chernicak I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

277

CATALYTIC PROPERTIES OF TRANSITION METAL SULPHIDES FOR DEHYDROGENATION OF SULPHUR CONTAINING MOLECULES. M. LACROIX',

c.

H. MARRAKCHIl, C. CALAISl, M. BREYSSE'

THE

and

FORQUY~.

'Institut de Recherches sur la Catalyse, Einstein, 69626 Villeurbanne CBdex, France.

2

avenue

Albert

2Groupement de Recherches de Lacq, BP34, 64170 Artix.

SUMMARY The catalytic properties of unsupported transition metal sulphides have been examined for the reaction of dehydrogenation of tetrahydrothiophene. This study has shown that a selectivity higher than 90% for thiophene formation can be obtained for the most active catalysts, essentially the second row sulphide catalysts. The comparison between the catalytic activities in both dehydrogenation of tetrahydrothiophene and hydrodesulphurization of thiophene suggests that the sites involved in these reactions are comparable.

INTRODUCTION Transition metal sulphides are able to catalyze a very large number of reactions. The most important utilization concerns catalytic hydrotreating, but many others can be foreseen due to the resistance of these catalysts towards sulphur. For example, recent studies have demonstrated the interest of such catalysts for the selective conversion of carbon monoxide into hydrocarbons [l] or alcohols [2]. Until now, only few papers and patents report on the utilization of sulphides for fine chemical applications [3-61. Nevertheless, this type of solids fits well to catalyze the reactions dealing with sulphur containing molecules. Therefore, it appeared interesting to apply to thiochemistry the knowledge acquired in the hydrotreating field concerning sulphide catalysts. The reaction chosen was the dehydrogenation of tetrahydrothiophene (thiacyclopentane) into thiophene. This reaction is interesting from the practical and fundamental standpoint. Concerning this latter aspect, correlation can be

278

made with previous results obtained for hydrodesulphurization and hydrogenation reactions [7]. The objective of the present work is to classify the catalytic performances of unsupported transition metal sulphides (TMS) for these different reactions : dehydrogenation of a sulphur containing molecule without sulphur elimination, hydrodesulphurization and hydrogenation.

EXPERIMENTAL Catalvtic activity measurements The catalysts were tested in the dehydrogenation of tetrahydrothiophene (DHN of THT), the hydrodesulphurization of thiophene (HDS of thiophene) and the hydrogenation of biphenyl (HN of BP). The reactions were carried out in the vapor phase using dynamic flow microreactors equipped with an automatic online analysis. Reaction conditions are given in Table 1. TABLE 1 Experimental conditions catalytic activities.

HN of BP HDS of thiophene DHN of THT (1) H2 atmosphere

533

used

for

the

determination

8

4

623

23(l) l(1)

24

0

653

1(2)

6.6

6.6

of

the

(2) N2 atmosphere

All the reaction products were analyzed by gas chromatography with a FID detector. The specific activity As is determined by the relation A,=(Qx)/m, where Q = hydrocarbon flow, m = catalyst weight and x = total conversion. For all the samples studied, the conversion was kept lower than 15% by adjusting the catalyst weight.

279

Under these conditions, the HN of BP led essentially to phenylcyclohexane and dicyclohexyl, the HDS of thiophene to C4 hydrocarbons and the DHN of THT to thiophene and C4 hydrocarbons. Catalvst mewaration and characterization The synthesis of the different sulphides has been described previously [7]. The freshly-prepared sulphides were treated at 673K in an H2-H2S flow. The aim of this sample pretreatment was to stabilize each catalyst at a temperature higher than the reaction temperature. The structure and surface areas (after catalytic tests) of the different solids are given in Table 2. XRD and sulphur to metal ratios indicate that the techniques used for the lowtemperature synthesis allow the preparation of the expected most stable binary sulphides. These solids are generally stable under the test conditions, but structural changes occur for some of them in an H2 atmosphere : Nil Co and Pd monosulphides are transformed into Ni3S2, Cogs8 and Pd4S. In order to compare the catalytic properties of the different sulphides, the activity are normalized by unit area of the used samples. The proportionality between catalytic activities and surface areas was checked for several samples [7].

RESULTS The catalytic properties in the different reactions are given in Table 2 and shown for the second row sulphides in Fig.1 and 2. The second row TMS are active for all the reactions. Fig.1 represents the comparison between the activities for the HDS of thiophene and for the DHN of THT. This figure evidences the strong similarities between the results obtained for both reactions : very high activity for RuS2 (roughly 10 times more active than MoS2) and Rh2S3, medium activity for MoS2 and low activity for NbS3. The only difference is the activity presented by palladium sulphide, similar to that of molybdenum sulphide in DHN and very low in HDS.

280

This difference is ascribed to different stoichiometries, i.e. Pd4S for HDS and PdS for DHN, due to the absence of H2 inthe gas flow for this latter reaction (except the relatively low partial pressure of H2 formed by the reaction). TABLE 2 Crystalline phases, surface areas (SBET), specific and intrinsic activities of transition metal sulphides

60

0.35

46.3

41

1.13

27.6

46

0.60

Cr2S3 a-MnS

21.0 3.20 0.10

32.5 10

0.10

103

261

0.39

172

160

1.08

0.01

0.07

1.40

5.5

0.25

0.16

6

0.03

0.70 7.62

10

Fel-xS

1.5

5.08

3.00

4.5

0.67

Cogs8

1.20

32

0.04

50.4

43

1.17

8.00

28

0.29

Ni3S2 CU~S

0.76 0.25

20

0.04

2.94

72

0.07

0.10

1.0

0.10

5

0.05

0.50

4.5

0.11

E

1.5

€

O-ZnS

E

32

E

1.70

16.5

0.10

5.20

10.5 0.50

V2S3

NbSx(6) 2.30

2.5

0.92

4.80

5.5

0.87

4.00

15

0.27

MoS2

5.70

38

0.15

23.4

11

2.13

12.0

5

1-50

RUS2

26

26

1.00

1590

53

30.0

490

40

12.3

Rh2S3

135

55

2.50

915

45

20.3

56.0

9.5

5.89

PdxS(7) 1.25

22

0.06

0.80

4

0.20

24.0

13.5 1.78

CdS

6

0.01

0.07

3

0.02

E

-

0.07

(1) At 533 K

(5

(2) At 653 K

(6) Mixture of NbS2 and NbS3

( 3 ) 10-8mol.s-1.g-1

(7) Pd4S after biphenyl HN and thiophene HDS ; PdS after

(4) m2.g-1

€

10-8mo1.s-1. m-2

tetrahydrothiophene DHN The comparison between the properties for dehydrogenation and hydrogenation reactions is shown in Fig.2. The general trend is similar : high activity for ruthenium and rhodium sulphides,

281

Ai (10-9mol/s.m2)

100

10

0.1

J -

Nb

’

I

I

I

Mo

(Tc)

Ru

Rh

Pd

Fig.1. Comparison between the intrinsic activities for thiophene HDS ( * ) and THT DHN (0). Ai (10-gmol/s.m2)

I

I

I

I

I

I

Nb

Mo

(Tc)

Ru

Rh

Pd

Fig.2. Comparison between (9) and THT DHN(o).

the

intrinsic

activities

for BP HN

282

but some differences appear : palladium sulphide is active in as above, DHN and not in HN of BP due to the same reasons niobium sulphide presents a low activity in DHN and a very high one in the conversion of biphenyl, but in this latter case the reactant is converted into benzene and cyclohexane, which can be considered as cracking products, instead of phenylcyclohexane and dicyclohexyl (hydrogenated products) for the other sulphides. Moreover, Rh sulphide appears more active than Ru sulphide for the hydrogenation reaction whereas the latter presents a better activity than Rh for the dehydrogenation of THT. This inversion of activity observed also in HDS seems to be dependent on the experimental conditions used for the determination of the catalytic properties [ 8 - 9 1 . Selectivity (%) 1001 I

I

80'

I

60 I

I

40

1

1

I

20

0

1

I

1 L i Ilr. 1

Nb Mo Ru Rh Pd

V

Cr Mn Fe Co Zn

Fig. 3 Thiophene selectivity of transition metal sulphides. For the most part, the first row TMS are less active. Results on Table 2 indicate that for the DHN of THT, only CrZS3 presents a noticeable activity which is also found for the other

283

reactions. Nevertheless, in thiophene HDS and BP HN, V2S3 is more active than Cr2S3. For all the other sulphides, the comparison is hazardous because low activity and/or low surface areas are observed. The selectivity towards thiophene formation, defined by the ratio S=thiophene/(C products) is represented in Fig.3. These results show that except the Zn, Fe, Mn and Co sulphides which are among the less active solids, selectivities higher than 9 0 % are usually observed.

DISCUSSION The strong similarities between HDS and DHN results suggest that the type of catalytic sites involved in these two reactions are similar. This would mean that the routes for DHN of THT and HDS of thiophene include some identical reactional step(s), at least for the kinetically limitative one(s). Angelici and coworkers, in a detailed work [ l o ] concerning the HDS of thiophene, THT, 2,3- and 2,5-dihydrothiophene showed that the dihydrothiophenes are much more reactive than the other molecules. On this basis and an additional study [ l l ] on organometallic compounds, these authors have suggested that dihydrothiophenes are reaction intermediates and proposed a mechanistic pathway including this step. Our results concerning the comparative study of thiophene HDS and THT DHN lead us to think that there is (are) a (some) common reaction intermediate(s) for the two reactions, presumably dihydrothiophene(s). This proposal is supported by the study of thiophene selectivity in DHN of THT which was performed over unsupported RuS2 (the most active and selective catalyst), by following the evolution of this selectivity with the total conversion. For this purpose, the total conversion was changed by modifying either the reactant flow or the weight of the catalyst. The data, reported in Table 3 , clearly evidence that the selectivity is constant whatever the total conversion is, even at a conversion as high as 97%.

284

This result suggests that thiophene and C4 hydrocarbons come from the same intermediate. Thus, a formal mechanism can be schematized as following :

THT

Int

H %

Thiophene C4 hydrocarbons

where Int represents the reaction intermediate. TABLE 3 Thiophene selectivity and total conversion over unsupported RuS2 in tetrahydrotiophene DHN. TOTAL CONVERSION

THIOPHENE SELECTIVITY

0.10 0.23

0.98 0.98 0.98 0.98 0.95 0.97

0.35 0.53 0.61 0.97

Taking into account Angelici's work, Int could be the 2,3and (or) 2 , 5 - dihydrothiophene which can be formed either by addition of two adsorbed H* to thiophene or by H* abstraction from adsorbed THT. This hydrogen transfer can be considered as the limiting step of the overall mechanism. Such an hydrogen transfer from the catalyst to the molecule to be transformed exists also in the case of aromatic hydrogenation which probably explains the similar activity trend observed during BP HN for most of the catalysts. These hypothesis suggest that the differences in activity observed for the various TMS consist mainly in their ability to transfer hydrogen from the catalyst to the molecule, even if some differences can be easily foreseen due to the differences of the reacting molecules (BP is much more different from THT than thiophene). However, on the basis of the results described, the latter hypothesis seems reasonable and further work concerning the ability of TMS to activate H2 is undertaken.

285

CONCLUSION The catalytic activities of transition metal sulphides were classified in the DHN of THT, the HDS of thiophene and the HN of BP. The results clearly evidence strong similarities between the activities of the most active catalysts in the three reactions, which implies that the catalytic sites involved in each of these reactions are comparable. Moreover, this study points out the ability of sulphides to catalyze either reactions involving the conversion of sulphur containing molecules without desulphurization (DHN of THT) or desulphurization of these molecules (HDS of thiophene) as well as classical hydrogenation reactions (HN of BP). These properties make the sulphide catalysts interesting to be applied to organic chemistry, particularly in thiochemistry. REFERENCES 1 2

3

4 5

C. Mauchausse, H. Mozzanega, P. Turlier and J.A. Dalmon, in M.J. Phillips and M. Ternan (Eds.), Proc. 9th Int. Congress on Catalysis, Calgary, 1988, Vo1.2, pp.775-782. J.G. Santiesteban, C.E. Bogdan, R.G. Herman and K. Klier, in M.J. Phillips and M. Ternan (Eds.), Proc. 9th Int. Congress on Catalysis, Calgary, 1988, Vo1.2, pp.561-568. A. Onopchenko, E.T. Sabourla, C.M. Selwitz, U.S. Patent 4, 219, 679 (1978). Malz Jr., E. Russel, H. Greenfield, EP80100813 (1980). R. Braden, H. Kaupfer, S. Hartung, US Patent 4, 002, 673 (1977).

6

7 8

C. Moreau, R. Durand, P. Graffin and P. Geneste, Studies in Surface Science and Catalysis, M. Guisnet et al. (Eds.), Elsevier, Amsterdam, 1988, Vo1.41, p.139. M. Lacroix, N. Boutarfa, C. Guillard, M. Vrinat and M. Breysse, J. Catal., 120, 473 (1989). J. P. R. Vissers, C. K. Groots, E. M. Van Oers, V. J. H. De Beer and R. Prins, Bull. SOC. Chim. Belg., 93 (8-9), 813 (1984).

9

M.J. Ledoux, 0. Michaux and G. Agostini, J. Catal., 102, 275 (1986).

E.G. Markel, G.L. Schrader, N.N. Sauer and R.J. Angelici, J. Catal., 116, 11 (1989). 11 N.N. Sauer, E.J. Markel, G.L. Schrader and R.J. Angelici, J. Catal., 117, 295 (1989). 10

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

287

REACTIONS OF UNSATURATED ETHERS ON A COPPER-CHROMIUM CATALYST R. HUBAUT and J.P. BONNELLE L a b o r a t o i r e de C a t a l y s e Heterogene e t Homogene,

U.R.A.

C.N.R.S.

N’

04020,

U n i v e r s i t e des Sciences e t Techniques de L i l l e F l a n d r e s - A r t o i s F - 59655 V i l l e n e u v e d’Ascq Cedex (France).

ABSTRACT The copper-chromium o x i d e has two d i f f e r e n t a c t i v e s i t e s i n a reduced s t a t e . The cuprous i o n s a s s o c i a t e d w i t h a h y d r i d e and two a n i o n i c vacancies a r e t h e hy dro genat io n (HYD) s i t e s . The chromium i o n s i n t h e same environment a r e t h e s i t e s where o c c u r t h e i s o m e r i z a t i o n ( I ) and t h e hydrodeoxygenation (HDO) r e a c t i o n s . The use o f uns a t u r a t e d e t h e r s p e r m i t s t o c o n f i r m and t o p r e c i s e t h e n a t u r e and t h e r o l e o f t h e a c t i v e s i t e s . Wi t h t h e compounds which have t h e oxygen atom k e p t away o f t h e c a t a l y s t ’ s surface, t h e HYD a c t i v i t y i s v e r y l o w and t h e HDO/I r a t i o too, whereas, i n t h e o p p o s i t e case, t h e s e values increase. W i t h t h e v i n y l i c e t h e r s , t h e s a t u r a t e d compound i s t h e main p r o d u c t because t h e I and t h e HDO r e a c t i o n s proceed v i a a c o n c e r t e d mechanism w i t h a common p r e l i m i n a r s t e p and an a l l y l i c rearrangement which i s i m p o s s i b l e w i t h geminate f u n c t i o n s . INTRODUCTION I t has been r e p o r t e d r e c e n t l y , t h a t t h e copper chromium, a wellknown hy dro genat io n c a t a l y s t ( l ) , has two d i f f e r e n t a c t i v e s i t e s i n i t s reduced s t a t e ( 2 ) . The cuprous i o n s a s s o c i a t e d w i t h a h y d r i d e i o n and two a n i o n i c vacancies

- t h e s o - c a l l e d CH s i t e s f o l l o w i n g t h e SIEGEL’s

nomenclature ( 3 ) -

are the

h y dro genat io n s i t e s . The chromium i o n s i n t h e same environment a r e r e s p o n s i b l e f o r t h e o t h e r r e a c t i o n s which o c c u r w i t h u n s a t u r a t e d compounds. Because o f t h e weak a c i d i c c h a r a c t e r o f t h e f i r s t s i t e s , o n l y s t r o n g l y b a s i c s u b s t r a t e s a r e adsorbed and can be hydrogenated.

T h i s i s t h e case f o r conjugat ed dienes,

a , P - e t h y l e n i c ca r b o n y l compounds, a l l y 1 i c a l c o h o l s and carbonyl compounds, b u t i t i s n o t t h e case f o r monoenes. Th i s i s t h e reason why t h e monohydrogenation s e l e c t i v i t y i s v e r y h i g h w i t h t h e c o n j u g a t ed dienes ( 4 ) , b u t l o w f o r t h e a , P - e t h y l e n i c c a r b o n y l compounds (5). On t h e c o n t r a r y , t h e chromium i o n s a r e more a c i d i c s i t e s and t h e y p e r m i t t h e a d s o r p t i o n o f l e s s b a s i c molecules. On t h e o t h e r hand, o n l y t h e r e a c t i o n s where t h e h y d r i d e i o n i s exchanged a g a i n s t an a n i o n i c p a r t o f t h e s u b s t r a t e a r e p o s s i b l e . Thus, we observe on t h e Cr3+ s i t e s : t h e c i s - t r a n s i s o m e r i z a t i o n o f e t h y l e n i c compounds (4) and t h e a l l y l i c a l c o h o l s l e a d t o , a t l e a s t , two d i f f e r e n t p r o d u c t s : a c a rbonyl compound f rom an i s o m e r i z a t i o n r e a c t i o n (I), some hydrocarbons f r o m a h y d r o d e h y d r o x y l a t i o n r e a c t i o n (HDOH) (6). The I and HDO r e a c t i o n s have a common i n i t i a l s t e p ( 7 ) , t hen, t h e a n i o n i c p a r t

288

which l e a v e s t h e s u b s t r a t e depends on i t s s p a t i a l c o n f o r m a t i o n and on i t s adsorbed s t a t e . I t i s u s u a l l y assumed t h a t t h e f r a c t i o n n e a r e s t t o t h e c a t a l y s t s u r f a c e i s p i c k e d up by t h e s o l i d . T h i s a b s t r a c t i o n t a k e s p l a c e a f t e r an a l l y l i c m i g r a t i o n o f t h e e t h y l e n i c d o u b l e bond i n a c o n c e r t e d mechanism. U n f o r t u n a t e l y , t h e p r e v i o u s models do n o t p e r m i t t o v e r i f y t h e s e assumptions and t o compare t h e a c t i v i t y o f each a c t i v e s i t e . The u n s a t u r a t e d e t h e r s can l e a d t o t h e same t y p e o f r e a c t i o n . Moreover, an a p p r o p r i a t e c h o i c e o f t h e s e m o lecules enables us t o p r e c i s e t h e mechanism o f t h e r e a c t i o n s because t h e y p r e s e n t some advantages : The s p a t i a l p o s i t i o n of t h e oxygen atom can be f r o z e n , i . e . t h e c y c l i c

i)

ethers ii)

The anchorage by t h e f r e e e l e c t r o n s o f t h e het eroat om i s s t r o n g e r because o f t h e donnor e f f e c t o f t h e v i c i n a l a l k y l group.

i i i ) The v i n y l i c p o s i t i o n o f t h e a l c o x y l group i s s t a b l e . i i i i ) T h e l o s s o f t h e a l c o x y l group i s n o t p o s s i b l e as a d e h y d r a t i o n r e a c t i o n . EXPERIMENTAL The copper chromium o x i d e (Cu/Cr = 1) has been prepared by c o p r e c i p i t a t i o n o f copper and chromium n i t r a t e s w i t h ammonium hydroxide, f o l l o w e d b y t hermal dec o mp os it io n i n f l o w i n g n i t r o g e n up t o t h e f i n a l t emperat ure (370'C), according t o a p r e v i o u s l y d e s c r i b e d method ( 8 ) . The apparatus and t h e c a t a l y t i c procedure have a l s o been d e s c r i b e d elsewhere i n case o f gas phase r e a c t i o n s ( 5 ) and l i q u i d phase r e a c t i o n s (7). RESULTS Reac t io ns

of

a l l v l i c ethers

F i r s t o f a l l , we have t o n o t e t h e v e r y weak r e a c t i v i t y o f a l l y l i c e t h e r s i n comparison w i t h a l l y l i c a l c o h o l s ( Ta b l e 1).At a r e a c t i o n t emperat ure o f 60°C, 2 , 5 - d i h y d r o f u r a n (V) - t h e most r e a c t i v e e t h e r among t hose t e s t e d - i s about f i v e t i m e s l e s s r e a c t i v e t h a n 2 - m e t h y l - 2 propen-1-01 ( 1 1 ) .

"\ ,

SCHEME 1 /

2-CYCLOHEXEN-1-OL

I

289

i f t h e HYD/ItHDO and t h e HDO/I r a t i o s a r e v e r y weak f o r 2, 5 -d ihy dro f u ra n (V), 0.04 and 0.1 r e s p e c t i v e l y , t hese values i n c r e a s e w i t h e t h y l - 2 - p r o p e n y l e t h e r , (III), up t o 0.12 and 11. These r e s u l t s a r e s i m i l a r t o o t h os e obt a ined f o r 2-cyclohexenol, ( V I I I ) , which present s weak hydroge n a t i o n and HDO a c t i v i t i e s ( Ta b l e 2 ) . On t h e o t h e r hand,

TABLE 1 R e l a t i v e a c t i v i t i e s o f a l l y l i c and v i n y l i c oxygenated compounds

a) 0.34

a)

5x10-3

)

b) 16x10-3

70x10-3

) l l O ~ l O - ~b) 18x10-3

c ) 27x10-3 a) 0.3

a) 0 . 8 ~ 1 0 - 3 b) 1 . 7 ~ 1 0 - 3

) 1.6~10-3 ) 3 . 2 ~ 1 0 - 3 b) 14x10-3

c ) 23x10-3 3)

0.004

t a) 0.45

I)

0.036

a) 4 . 2 ~ 1 0 - 3 b ) 1 2 . 8 ~ 1 0 ' 3 )) 0 . 3 1 ~ 1 0 ' 3 :) 0 . 5 6 ~ 1 0 - 3

=

Hydrogenation ; HDO

-

:)

=

) 9 . 2 ~ 1 0 - 3 b) 4 . 0 ~ 1 0 - : c ) 4.4x10-: ) 66x10-3

a) b) 1 . 5 ~ 1 0 - 3

gas phase : m c a t a l y s t = 100 mg ; P s u b s t r a t e a) T = 60'C b) T = 1OO'C HYD

) 0.85~10-3

-

) 9 8 ~ 1 0 - ~ b)

c)

-

10 t o r r s ; QH = 60 ml/min C ) T = 140'C

Hydrodeoxygenation ; I = I s o m e r i z a t i o n

Reactions of v i n v l i c e t h e r s These compounds a r e v e r y i n t e r e s t i n g because t h e corresponding a l c o h o l s a r e n o t s t a b l e . The most s t r i k i n g r e s u l t i s t h e extreme weakness o f t h e i r r e a c t i v i t y , even a t h i g h temperature ( Ta b l e 3) : a t 14OoC, e t h y l 1-propenyl e t h e r , ( I V ) , l e a d s t o 1% o f co n v e r s i o n and 2 , 3 - d i h y d r o f u r an ( V I ) i s o n l y j u s t more r e a c t i v e . I n b o t h cases, t h e hydrogenated compound i s t h e main p r o d u c t . The HYD/ItHDO r a t i o reaches 2.3 f o r t h e f i r s t r e a c t a n t and 5.2 f o r t h e second one (T able 2 ) . I t i s a l s o i n t e r e s t i n g t o n o t e t h a t these molecules a r e n o t isomerized.

290

TABLE 2 HYD/HDOtI and HDO/I r a t i o s f o r a l l y l i c alcohols conversions ; comparison w i t h

a l l y l i c and v i n y l i c ethers

(VIII)

coH ' 7 0"" c)

liYD 1iDO+1

7.5

1

0.38

E

0.11

0.11

0.05

0.01

2.3

5.2

~

I

b

IlDOfl IlYD

HDO I

0.12

0.04

11

0.1

a ) gas phase 60°C ; b) gas phase 140'C ; c ) l i q u i d phase 140°C

2.5-DIHYDRGFURAN

Tension i n the cycles SCHEME 2

W

29 1

TABLE 3 Conversion (%) and p r o d u c t d i s t r i b u t i o n (%) o f a l l y l i c and v i n y l i c e t h e r s a t v a r i o u s temperatures

DISCUSS I O N The hy dro g e n a t i o n (HYD) and t h e hydrodeoxygenation (HDO) r e a c t i o n s o f a - u n s a t u r a t e d compounds need a simultaneous a d s o r p t i o n o f t h e n e l e c t r o n s o f t h e e t h y l e n i c double bond and o f t h e f r e e e l e c t r o n s o f t h e heteroatom (6). When t h i s double anchorage i s hindered, b o t h r e a c t i v i t i e s must decrease. A t t h e same t ime, t h e HDO/I r a t i o s h o u l d decrease s i n c e t h e oxygenated group i s k e p t away f r o m t h e surface o f t h e c a t a l y s t and cannot r e p l a c e t h e h y d r i d e on t h e chromium s i t e . Indeed, i n t h e case o f a l l y l i c a l c o h o l s , t h e r e i s a d r a s t i c decrease o f b o t h HYD/ItHDO and HDO/I r a t i o s as t h e s t e r i c hindrance increases f rom 2-methyl -2 propen-1-01 (11) t o 2-cyclohexenol (VIII) t h rough 1-met hyl-2 propen-1-01 ( I ) and n e r o l ( V I I ) (T a b l e 2 ) . To t h i s purpose, 2 - c yclohexenol i s a good model because t h e hy dro x y l group t a k e s a pseudo a x i a l p o s i t i o n (9) (scheme l), and i n such a p o s i t i o n , t h e anchor e f f e c t as w e l l as t h e c a t a l y s t hindrance should be considered. On a m e t a l l i c c a t a l y s t , l i k e n i c k e l , t h e anchor e f f e c t seems more e f f e c t i v e t han

292

the c a t a l y s t hindrance ( l o ) , b u t the opposite e f f e c t i s observed w i t h mixed oxide c a t a l y s t s , probably because the s t r u c t u r e o f t h e a c t i v e s i t e i s more r e s t r i c t i n g towards adsorption. These assumptions are confirmed by comparing two a l l y l i c ethers : e t h y l 2-propenyl ether ( 1 1 1 ) and 2,5-dihydrofuran ( V ) . This l a s t molecule has some s i m i l a r i t i e s w i t h 2-cyclohexenol ( V I I I ) because t h e a l k o x y l group i s n o t e a s i l y d i r e c t e d towards t h e c a t a l y s t surface (scheme 2) ; t h i s i s the reason why hydrogenation and hydrodealkoxylation a c t i v i t i e s are weak. The recovering o f the free r o t a t i o n around the o bond i n e t h y l 2-propenyl e t h e r leads t o an increase of the HYD a c t i v i t y and o f t h e HDO/I r a t i o . Moroever, the strong donnor e f f e c t of the a l k y l group, which supplies a strong n u c l e o p h i l i c character t o t h e oxygen atom, enhances the HDO reaction. The HDO and isomerization r e a c t i o n s were p r e v i o u s l y described as bimolecular nucleophil i c s u b s t i t u t i o n s w i t h a l l y l i c m i g r a t i o n s - t h e s o - c a l l e d SN2' mechanism ( 7 ) . The f i r s t common step i s the f i x a t i o n o f t h e hydride on the carbon sp2 o f the substrate. The l o s s o f t h e hydroxyl group o f t h e alcohols could not be a simple dehydration -a p r e l i m i n a r e l i m i n a t i o n r e a c t i o n - as t h e 3-butene-1-01 leads t o n e i t h e r isomerization nor hydrodehydroxylation ( 6 ) . The r e s u l t s observed w i t h v i n y l i c ethers confirm t h a t o n l y a l l y l i c oxygenated compounds are able t o undergo e a s i l y isomerization and HDO r e a c t i o n s . Moreover, we can note t h a t f u r a n t e t r a h y d r o and furan do n o t r e a c t a t a l l even a t high temperature (200'C). A t l a s t , because o f the strong donnor e f f e c t o f t h e a l k y l group, the ethers

r e a c t d i f f e r e n t l y than the corresponding alcohols.

It i s wellknown t h a t the

hydrogenation s i t e s are poisoned by t h e oxygenated groups (11) ; the s t r e n g t h o f t h i s poison depends on the b a s i c i t y o f t h e group : a c e t i c a c i d poisons d e f i n i t e l y , whereas a l l y l i c alcohols only p a r t l y . Ethers are an average between these compounds, so the HYD a c t i v i t y i s very weak but never n i l assuming t h a t the f r e e e l e c t r o n s o f the heteroatom, a c t u a l l y ,

take p a r t i n t h e adsorption phenomenon. On the contrary, we can forecast t h a t the stronger t h e basic character o f t h e oxygenated group i s , the easier the l o s s o f t h i s one i s . Ethers, e f f e c t i v e l y , c o n f i r m t h a t because the HDO r e a c t i o n becomes more important w i t h these a l l y l i c compounds.

CONCLUSION

The reactions observed on a copper chromium oxide w i t h unsaturated ethers permit t o c o n f i r m and precise t h e nature and t h e r o l e o f t h e d i f f e r e n t a c t i v e s i t e s o f the c a t a l y s t . On the copper ions, only s t r o n g l y basic substrates are able t o adsorb and t o be hydrogenated. So, the monoenes and t h e unsaturated oxygenated molecules w i t h t h e oxygen atom kept away from the c a t a l y s t ' s surface are n o t very r e a c t i v e . On the contrary, when the anchoragewith the 0-group i s possible, the hydrogenation a c t i v i t y increase, but, i n t h e same time, t h e poisoning too. On the chromium ions, the r e a c t i o n s which occur need a concerted mechanism.

293

Thus, t h e y a r e e a s i e r w i t h t h e 0-group i n an a l l y l i c p o s i t i o n . I s o m e r i z a t i o n and hydrodeoxygenation r e a c t i o n s have a common p r e l i m i n a r s t e p and t h e p a r t which l e a v e s t h e s u b s t r a t e i s depending on t h e s p a t i a l p o s i t i o n o f t h e 0 - g r o u p by r e s p e c t t o t h e c a t a l y s t . When t h i s group i s f a r f r o m t h e c a t a l y s t ’ s s u r f a c e , t h e i s o m e r i z a t i o n r e a c t i o n i s preponderant. I n t h i s o p p o s i t e case, because o f t h e s t r o n g donnor e f f e c t o f t h e a1 k y l group, t h e hydrodeal k o x y l a t i o n becomes more important.

REFERENCES

1 2 3 4 5 6 7 8 9a)

b)

10 11

H. Adkins and R. Connors, J. Am. Chem. SOC., 53 (1931), 1091. R. Bechara, G. Wrobel, M. Daage and J.P. Bonnelle, A p p l . C a t a l . 16 (1985) 15. S. S i e g e l , J. C a t a l . , 30 (1973) 139. M. R. R. R.

Daage and J.P. Bonnelle, Appl. C a t a l . , 16 (1985 355. Hubaut, M. Daaqe and J.P. Bonnelle, Appl. C a t a l , 22 (1986) 231. Hubaut, M. Daage and J.P. Bonnelle, Appl. C a t a l 22 (1986) 243. Hubaut and J.P. B o n n e l l e , J. o f Mol. C a t a l . , 55 {l-3) (1989 170. G. Wrobel, P. W a l t e r and J.P. B e a u f i l s , C.R. Acad. S c i . C 283 i976) 335. K. Manaya; J. Chem. SOC. Japan, 91 (1970) 82 Y. Senda, S. Imatzumi, S. O c h i a i and K. F u j i t a , Tetrahedron, 30 (1974) 539. S. M i t s u i , M. I t o , A. Nanbo and Y. Senda, J. o f C a t a l . , 36 (1975) 119. R. Hubaut, M. Daage and J.P. Bonnelle, Proceedings du l l h e C o l l o q u e Franco-Polonais s u r l a Catalyse, septembre, Caen, 1985.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals IZ 0 1991 Elsevier Science Publishers B.V., Amsterdam

295

HYDROGENATION OF METHYL-3, BUTENAL ON POLYCRYSTALLINE PLAT1NUM C.-M. PRADIER’, E. MARGOT’, Y . BERTHIER’,G. CORDIER2 ‘Laboratoire de Physico-Chimie des Surfaces, associk au CNRS Ecole Nationale Supkrieure de Chirnie de Paris 11, rue Pierre et Marie Curie - 75005 PARIS - FRANCE ‘Rhdne Poulenc lndustrialisation 24, Av. Jean Jaurbs 69151 DECINES-CHARPIEU

SUMMARY The activity and selectivity of a C, unsaturated aldehyde (prenal) has been investigated in gaseous phase on a platinum polycrystalline sample. The aim of this work was the study of the respective effects of the pretreatment of the sample by oxygen and of the presence of a small partial pressure of H,S in the gas phase. The results gave us a better understanding of the surface mechanism and proved the importance of the chemisorption mode of the molecule. INTRODUCTION The selective hydrogenation of organic compounds which contain a C=C and a C=O double bonds into unsaturated alcohols is a very challenging subject from an industrial and from a fundamental point of view. Recent studies have shown that somecatalyticsystems are suitable for a selective hydrogenation in particular bi or trimetallic supported catalysts but no clear conclusion could be drawn concerning the mechanism and the respective effects of the support and the alloying (ref.1). The idea of this work was the realization of a not easily selective hydrogenation under conditions relevant to the surface science, in order to get a better understanding of the mechanism. Such an approach already led to interesting conclusions concerning the hydrogenations of butadiene and isoprene (ref. 2). We chose the methyl-3, butenal or prenal as a-P unsaturated aldehyde and we carried out the reaction in gaseous phase on a polycrystalline sample of pure platinum in order to avoid any support effect. The objectives of this study were the following : i)a precise definition of the activity of the surface under easily reproducible conditions; ii) an investigation of the influence of - the reaction parameters, i.e. temperature, reactant pressure; -the pretreatment of the surface; - the presence of a small H,S partial pressure in the reactionnal gas phase upon the activity and the selectivity of the reaction. The results of these points led us to propose some hypothesis concerning the Surface mechanism

296

EXPERIMENTAL The sample was a4cmzpiece of high purity (5N) polycrystalline platinum; it was polished and annealed under hydrogen at T=l400K for 24 hours before the first experiment. Astandard regeneration procedure has been defined and used before every catalytic test leading to reproducible results :the sample was heated to 600K and kept under an oxygen Stream for 1 to 2 mn then heated to 1100 K and kept under an hydrogen stream for 1h. After cooling under hydrogen flow and the temperature beeing stabilized to the desired value, the reaction mixture containing a large excess of hydrogen was introduced and circulated in the closed System according to the principle of a batch reactor. The hydrogenations were carried out under quasi atmosphericpressure. All thetubesofthe reactorwere madeofglassand permanently heated to 350K to avoid any condensation of the reactants; this was made necessary because of the relatively high boiling point of the prenal, T,=406K. As a consequence, reactant and product gases were kept at temperature close or equal to the catalyst one, no thermalisation phenomena occured in the gas adsorption step. The time-course of the hydrogenation was obtained from gas chromatography and the selectivity to a given product was defined as the partial pressure of the latter in all the converted products. For a better characterization of the catalytic surface, the sample has been transferred through a glove box into an ultra high vacuum device and Auger electron spectroscopy was carried out.

RESULTS 1 - TemDerature and Dressure effects

UDon

the activitv and the selectivitv of the reactiM

Fig. 1 shows a typical time-course of the hydrogenation of prenal achieved under the following conditions : PH, = 400 torr = 10' torr Pprsnal T = 350 K Saturated aldehyde (isovaleraldehyde), unsaturated and saturated alcohols (prenol and isoamylic alcohol) simultaneously appear at the initial stage of the reaction ; prenol is the majoritary product of the reaction but it rapidly undergoes a second hydrogenation. The rate of conversion progressively decreases with increasing conversion; the reaction appears to be blocked for a conversion around 80%. The activity has been characterized by the turnover number, T.O.N., or number of molecules of prenal converted per surface atom and per second at the origin of the reaction: = 2.10-3mol. at-' s-l T.O.N.,

297

pH2=400Torr pPrensl = IO-’Torr

T-350K

Fig. 1 : Hydrogenation of prenal on polycr-ystallineplatinum; time-course Ofthe reaction at T=350 K. Temperature effect The reactionwas carried out in the 300 to 420 K temperature range, an activation energy has been deduced from the linear variation of the initial activity as a function of l / l : Ea= 14 Kj mole-’

It has to be added that the influence of temperature variations is considerable upon the final conversion and upon the selectivity. As a matter of fact, the final conversion increases with T, approaching 100% when T is above 370 K. In the 300K - 360 K temperature range, a slight decrease of the selectivity to unsaturated alcohol has been observed with increasing temperature; it occurs to the benefit of the saturated alcohol. From 370 K, the increase of conversion goes with a drastic change of selectivity to the benefit of unexpected alight. products, as hydrocarbon compounds (fig.2). The formation of these 4ight. product is 370 K in the standard conditions of pressure above defined. negligible ( ~ 4 %below )

7

$00

Fig. 2 :

350

400

l.K

Initial formation of <
298

Hydroaen and DrenaI Dressure effects The hydrogen pressure effect has been investigated in the following range : 300 torr c PH, c 700 torr at the standard temperature of 350 K. The linear increase of the initial rate was observed with increasing PH,; some punctual experiments performed by varying Ppmna,led to the kinetic orders : dH, = 1 dprenal 0 or possibly slightly positive No considerable variations of the selectivity could be detected in the investigated ranges of pressures.

-

2 - Pretreatment effects UDOn the activitv and selectivitv of the reaction The standard regenerationtreatment described in the experimental section appeared to be necessary to recover the catalytic properties of the platinum surface. Varying the duration of the oxygen pretreatment induces no change of either the initial rate or of the final conversion but it considerably modifies the selectivity of the reaction (see figure 3) :

Fig. 3 :

Hydrogenation of prenal after oxygen pretreatment of 0,3,20mn.

299

At the origin of the reaction,the rates of formation of both the saturated alcohol and the saturated aldehyde decrease to the benefit of the unsaturated alcohol. The unsaturatedalcohol or prenol starts to be hydrogenated into methyl butanol after a lapse of time which increases with the duration of oxygen pretreatment.

The following treatment under hydrogen stream at high temperature appeared to be necessary but no systematic study has been made.

3 - J-LSeffects UD . on the activitv and selectivitv of the reaction

The ratio PH,S/PH, which represents the sulfur chemical potential under equilibrium conditions has been varied from 2.10-5 to 2.104. No important variation of the initial rate of conversion appearsasfaras PH,S/PH,is below 1.5.10d,thenthe reaction issuddenly blocked for PH,S/PH,=1.6.1 Od. Important changes of the initial selectivity have been made clear and reported on fig. 4.

p n Z i 400Torr pPrenal.10-'Torr

T = 350K

Fig. 4 :

Initial selectivity of the hydrogenation of prenal as a function of PH,S/PH,.

The results correspondingto PH,S/PH,=1.6.1Od are less precise than others since the reaction is blocked a very short time after the start. Important and complementary variations of the unsaturated alcohol and the saturated aldehyde appear, the formation of saturated alcohol slightly increases in presence of sulfur, and,assoonasalowPH,S/PH,ratio is present, -light., productsappearinthegasphase,their concentration seems not to be very PH,S/PH, sensitive. In order to precise the effect of sulfur, an Auger analysis of the surface has been performed by transferring the sample in an ultra high vacuum chamber right before the

300

catalytic test. The obtained spectra are not simply interpretable since considerable differences have beenobserved whenchangingthe impact pointoftheelectronicbeamonthesurface of the polycrystalline sample (fig.5).

h

Fig. 5 :

Auger spectrum of the sample before a catalytic test.

Anyway one has to notice the following points : i) there is never a high quantity of adsorbed sulfur, 8,<0.2monolayer accordingto the average of the calibrations wellknown for the low index planes of platinum (ref. 3). ii) carbon and oxygen coverages are high, far beyong the monolayers whereever occurs the analysis; ii) another element sometimes appear, calcium, which is a platinum impurity. The important feature is the considerable observed effects of sulfur upon the Selectivity even at a very low surface concentration.

DISCUSSION The activity and the selectivity of the hydrogenation of prenal were observed to vary depending not only on the experimental reaction conditions but also on the preparation conditions and on the ccimpurity), added in the gas phase. The values of the orders towards the reactants indicate that the reaction occurs on a surface saturated with prenal and that the hydrogen supply is a limiting factor. The Auger analysis performed right before the catalytic test confirms that the reaction is not poisoned by either an oxygen or a carbon monolayer on the surface. Our discussion will concern the range of temperature below 370 K for which cracking

301

phenomena are negligible. From the timecourse of the reaction presented on fig.1, its appears that the saturated alcohol is a primary product of the reaction indicating that the molecule of prenal can be flat adsorbed on the surface even at the very beginning of the reaction. The amount of prenol starts to decrease before the one of saturated aldehyde; that means that the adsorptionof saturatedaldehyde is weakerthan the adsorption of unsaturated aldehyde or alcohol. The fact that the selectivty is not either pressure or temperature dependent indicates that in the investigated range of pressures, at T c 370 K, the surface keeps saturated with prenal and that the ratio of the product adsorption strenghts is the selectivity determining factor. The evolution of the selectivity observed when increasing the oxygen pretreatment Suggests that only the formation of prenol which results from an 1,2 addition of hydrogen is enhanced, the 1,4,3,4 and 1,2,3,4 additions leadingtothe other productsbeeing disfavoured. As a matter of fact, a rather long oxygen pretreatment may have two consequences :a partial restructuringof the surface at the atomic scale and (or) the presence of oxygen atoms in the superficial layer which have not been removed during the hydrogen treatment (Oxygen dissolved in the grain boundaries for example). Both phenomena may disfavour the flat adsorption of the molecule of prenal to the benefit of an adsorptionthroughthe C=O double bond with a partial raising up of the molecule. Ifoxygen atoms are actually present on the surface as suggestedby the Auger Spectrum, they

4 c

c ~ -

\\ 'CH

I

'CH"

are likely to increase the acidity of the platinum surface atoms and enhance a positive change on the C=O carbon atom leading to the following reaction scheme : This positive effect of oxygen upon the selectivity to prenol can be related to a recent work by Gallezot who has shown that a restructurationof divided platinum catalysts induces

302

a better selectivity (ref. 4). A similar change of selectivity has also been reported in the literature with Snz*ions which directly activate the carbonyl group (ref. 5). Figure 3 also shows that the prenol formation curve goes through a sharp maximum. After a certain reaction time, (conversion 70%), the C=C bond of the prenol is preferentially hydrogenated with regard to the C=O bond of the isovaleraldehyde. This result proovesthat the C=O of a saturated molecule and the C=O of an unsaturated aldehyde behave differently, and a sequence of hydrogenation probabilitiescan be defined : unsaturated C=O bond > alcohol C=C bond > saturated C=O bond. The conjugation effect upon the activity of a C=O bond is reinforced by the oxygen pretreatmentand the longerthe oxygen pretreatment is, the strongerthe differences between the above considered hydrogenations are. When a high enoughconversion has been reached,the coverage of prenal (conjugated C=O) is low and the prenol (C=C bond) comes to be hydrogenated before the isovaleraldehyde (saturated C=O) in agreement with the above sequence of hydrogenation. As for the influence of a very little partial pressure of H,S in the gas phase, the negligible variation of the initialactivity is in agreement with a very small steric poisoningeffect indicated by the low sulfur coverage. This result has been deduced from Auger analysis. It appears not to be in agreement with the extrapolation to 350 K of the sulfur adsorption isotherms determined with a pure H,-H,S mixture on platinum (ref. 6). The latter would indicate a saturation of the surface by sulfur for PH,S/PH,=10.5. Nevertheless in presence of another reactive molecule, the isotherms are likely to be displaced towards higher values PH,S/PH, by several orders of magnitude;such a phenomenon has already been shown in presence of butadiene (ref. 7). The changes in selectivity as a function of PH,S/PH,, in particular the variations of unsaturated alcohol and saturated aldehyde which are opposite to the ones observed with oxygen show that oxygen preatment and sulfur from H,S in the gas phase act differently. As a matter of fact, H,S, beeing present in the gas phase, may have two different effects : - First a surface reaction involving sulfur and oxygen atoms may occur and decrease the surface oxygen concentration and consequently the above discussed effect upon the prenol selectivity; - Secondly, H,S may involve adsorption of sulfur on the surface which modifies the adsorptionmode of prenal (poisoningeffect); one has then a preferentialhydrogenationof the C=C bond as it was observed in absence of sulfur, at high conversion. Ifthe effect of the pretreatments was only a restructuring of the surface, one can also admit that H,S in the gas phase tends to reduce this restructuration. At last, another explanation could be a possible interaction of H,S with the molecule of prenal which would modify the electronic density and the electron delocalization on the molecule, to the benefit of the C=C bond hydrogenation. One can notice a break of the evolution of the selectivity for PH,S/PH,S=I Od.At this level

-

303

of contamination, comparative adsorption studies of sulfur, prenal and reaction products would be interesting for the understanding of the phenomenon. CONCLUSION The hydrogenation of methyl-3,butenalhas been carried out in gaseous phase on anon dispersed platinum catalyst. The initial rate of conversion, of the.order of 10-3moleculesper second and per surface atom, is very slightlydependent on the preparation of the surface, on the reaction conditions, on the presence of a small partial pressure of H,S in the gas phase. The selectivity, in particularthe formation of unsaturatedalcohol is strongly dependent on the pretreatment of the surface and on PH,S : -it increaseswithincreasingtheduration of theoxygen pratreatment(restructuring or oxygen contamination of the surface ). - it decreases with increasing sulfur chemical potential in the gas phase (change of the chemisorption of prenal induced by adsorption of sulfur). These results lead us to propose that the adsorption mode of the molecule of prenal and consequently the hydrogenation selectivity strongly depend upon structural and electronic modifications of the surface. Further experiments on single crystals, low index and complex planes, and in situ characterizationof the active surface would be of great interest for the understanding Of the effects discussed in this paper. REFERENCES

- Y. Nitta, K. Veno, T. Imanaka, Applied Catal. dec.89 (56) nol. - C.-M. Pradier, E. Margot, Y. Berthier and J. Oudar, Applied Catal. 43 (1989) 177-192. D. Vassilakis, E. Margot, C.-M. Pradier and Y. Berthier, J. of Molec. Catal. accepted. 3 - Y. Berthier, M. Perdereau and J. Oudar, Surf. Sci. 36 (1973) 225-240. 4 - P. Gallezot, 13 rd, O.R.C.F. Conference, 21-23 May 90, USA 5 - 2. Poltarzewski, S. Galvagno, R. Pietropaolo,and P. Staiti, J. of Catal. 102, 190-198 (1 986). 6 - N. Barbouth and M. Salame, J. of Catal. 104 (1987) 240-245. 1 2

7 -

J. Oudar, S. Piiiol, C.-M. Pradier and Y. Berthier, J. of Catal. 107, (1987) 445-450.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

305

SURFACE CHEMISTRY AND CATALYSIS WITH ORGANIC NITRO COMPOUNDS, LOOKING FOR THE KEY TO HIGHER SELECTIVITIES. I'.A..J.M.Angevaare, A.Maltha, T.L.F.Favre, A.P.Zuur, and V.Ponec, Leiden University, Gorlaeus Laboratories, P.O.Box 9502, 2300 RA Leiden, The Netherlands.

Summary In this paper the differences between the behaviour of aliphatic and aromatic nitro compounds adsorbed on a-Mn,O, are discussed. The presence of a hydrogen atom on the a carbon of aliphatic nitro compounds prevents their selective reduction to the nitroso analogues. Suggestions are made concerning the mechanisms of the reduction of nitrobenzene to nitrosobenzene and of the formation of some side products of the reduction (azobenzene and azoxybenzene).

Introduction The selective introduction of an oxygen atom into an organic molecule (selective oxidation) and the selective removal of oxygen from oxygen containing organics (selective hydrogenation, reduction) are attractive steps in organic syntheses. By oxygen introduction

or removal interesting products can be obtained. In this way it is also possible to make molecules more reactive for subsequent reactions. Aliphatic and aromatic nitro compounds are such molecules that can produce interesting products by removal of an oxygen atom. The most desired reaction pathway should lead to the corresponding nitroso compounds"]. The wideIy applied processes for converting organic compounds into partially oxidized analogues are heterogeneous catalytic ones, with the use of solid oxides as catalysts. Most of the processes can be described by the so called "Mars and van Krevelen" mechanism, also called the "redox" mechanism. The essential feature of this mechanism is the introduction of lattice oxygen of a catalyst into a reactant, thereby creating oxygen vacancies in the catalyst. The replenishment of these oxygen vacancies is mostly performed by molecular oxygen. An interesting question is whether the reoxidation step can be used to obtain valuable products. There are patents known, which suggest a heterogeneous catalytic reduction of nitrobenzene to nirro~obenzene'~'~'~' or of aromatic carboxylic acids to the corresponding a l d e t i y d e ~ ' ~by~ ,the ' ~ ~use of a suitable oxide as a catalyst.

306

The one-step synthesis of nitroso compounds by selective reduction of the corresponding nitro compound is a commercially interesting route. Some relevant information on this reaction can be already found in the literature. So, for example, KishiI6]studied the adsorption of nitrobenzene on Fe/Ni( 100) surfaces with different pretreatment conditions and found that the extent of preoxidation of the Fe/Ni surface controls the dissociative chemisorption of nitrobenzene into nitrosobenzene, C,H,NO(,,,, and nitrene, C,H,N(,,,. The dissociation of nitrobenzene into nitrosobenzene takes place when the surface is oxidized to an Fe,O,-like species. This species appears, in contrast to the pure Fe/Ni( 100) surface, to be unable to dissociate nitrosobenzene further into nitrene. Several spinels like a-Mn,O, and Co,OF1 are known to be good catalysts for the transformation of nitrobenzene into nitrosobenzene. It has been found that in these catalysts the active site is related to the octahedral Mn(ll1) and Co(II1) position, respectively[*]. On the other hand, reactions of aliphatic nitro compounds never lead to the formation of nitroso compounds. Although Benzigei" suggests formation of an aliphatic nitroso surface species as an intermediate in the nitromethane decomposition on clean Ni(l11) and on Ni(lIl)-p(2x2)0, there is no definitive evidence supporting this suggestion. By means of TPD the author observed scission of the N - 0 bonds on the mentioned surfaces. This scission leads to the formation of primarily HCN, H,, and adsorbed oxygen. The major reaction upon decomposition of nitromethane on alumina supported NiO and Cr,O,, is also an N - 0 bond scission"']. Finally. as described in a previous paper'"], the admission of nitromethane to y-Al,O, and a-Mn,O, at room temperature leads to the formation of nitronate, and subsequently, to the appearance of formate surface species. If we assume that it is possible to reduce an aliphatic nitro compound to the correspon-

ding nitroso compound, the possibility of a simultaneous isomerisation to an oxime has to be kept in mind. It is known from the reduction of primary and secondary nitro paraffins by stannous chloride[12],that the reduction does not proceed to form amines, which could be explained by a rapid isomerisation of the adsorbed intermediate to an oxime. The aim of the present study is to cleanfy which factors play a key role in the differences between the surface chemistry of aliphatic and aromatic nitro compounds on various adsorbents.

Experimenta1 The oxide, a-Mn,O,, used in this study was prepared by thermal decomposition of manganese(1I)hydroxide in air at 390K. The mentioned hydroxide was precipitated with ammonia from a manganese(1l)nitrate solution (Mn(NO,), .4H20, Fluka, Switzerland) at

307

pH=9. Three nitro compounds were used, having hydrocarbon groups with totally different characters: one with an aromatic character, one with an aliphatic hydrocarbon ring and the third one is the simplest aliphatic nitro compound: nitrobenzene (over 99.5% pure, J.T.Baker Chemicals B.V., the Netherlands), nitrocyclohexane (97% pure, Aldrich Chemical Company, U.S.A.) and nitromethane (over 99% pure, Aldrich Chemie, West-Germany), respectively.

The catalytic measurements were performed in a continuous flow system (23.5 cm3/min total flow) in a fixed bed reactor. In this reactor a standard amount of (0.30 g) catalyst was used. The camer gas used was helium. The partial pressure of nitrobenzene was 1.2x103Pa (total pressure 1.01x10'Pa). The composition of the reaction mixture was determined by GC analysis. Self-supporting pellets of a-Mn,O,, as used in the infrared spectrocopic part of the present study, were obtained by pressing of about 50 mg of this oxide using pressures of 1xlO7Pa. These pellets were placed in an all-metal transmission

mounted with

CaF,-windows, which are transparent down to 1000 cm". The cell was connected to a conventional gas manipulation evacuation system. The pellet could be heated and cooled in the beam and the oxidation, reduction and adsorption procedures could also be performed in situ. After the pretreatment a-Mn,O, was evacuated in the cell at 575K for about 15 hrs. The standard pretreatment consisted of an oxidation step in pure 0, (1 hr at 575K) and a reduction step in pure H, (1 hr at 575K). All used nitro compounds are liquids at room temperature with vapour pressures of 1-SxlO'Pa, and they are degassed prior to use. The infrared spectra were recorded by an evacuable m-IR spectrometer, Bruker IFS113v, equipped with a liquid nitrogen cooled MCT (mercury cadmium telluride) detector. All infrared spectra showed were obtained by substraction of the background (oxide) spectrum, recorded at the same temperature.

Results For a better understanding of the factors that play a role in the catalytic selective reduction of nitrobenzene to nitrosobenzene some pieces of relevant information from previous work have to be considered. Favre et aLi6]found that oxides of various transition metals show catalytic activity in the mentioned reaction and a-Mn,O, (Hausmannite) appeared to be the most active and selective catalyst. The function of nitrobenzene as an internal reducing agent has already been suggested by Zengel"' and is confirmed by Favre et al. Nitrobenzene can thus reduce as well as oxidize the catalyst.

308

The time dependence of the selectivity upon reaction of nitrobenzene at a-Mn,O, (S7SK) shows a typical behaviour, as shown in figure 1.

0

10

20

30

40

-->time (hr) nilrosobenzene 0

aniline

a

azo/azox benzene

d

Figure I . Time dependence of the selectivities of reduction products upon reaction of nitrobenzene at a-Mn,O, (575K).

It can be seen from this figure that the steady state production of nitrosobenzene is preceded by an induction period, in which aniline is the main product. Further, small amounts of azobenzene and azoxybenzene are formed throughout the reaction. The existence of an autoredox reaction implies that a selectivity of 100% from nitrobenzene to nitrosobenzene is impossible. After the induction period the selectivity of nitrobenzene to nitrosobenzene becomes above 90% of the reduction products. The extent of conversion of nitrobenzene is also time dependent. In the steady state situation about 20% of the nitrobenzene is converted, after an initial conversion of 65%. It is the main aim of this study to establish which factor is responsible for the very different behaviour of aliphatic and aromatic nitro compounds. An earlier study of the interaction of various nitro compounds with the a-Mn,O, surface in the temperature range 300-600K has indeed revealed variations depending on the

hydrocarbon. The adsorption and surface reactions of nitromethane have already been reported in an earlier paper‘”’. It has been found that the adsorption of CH,NO, on a-Mn,O, at 300K is dissociative, whereby a proton splits off and a surface nitronate ion is formed upon adsorption. At the same temperature, the nitronate ion is very easily oxidized to formaldehyde. This formaldehyde either desorbs or stays in the adsorbed phase, forming a formate

309

species observable by infrared spectroscopy. The oxygen vacancies created in this way reduce 'NO,' species to NO, and the latter is observed in the gas phase after its desorption. If the temperature is subsequently increased, the nitronate ion is also converted into an

(iso)cyanate surface species. At least one additional independent experiment (labelled molecules, substituted analogues) has been camed out to support the existence of each above mentioned reaction intermediate.'"' The surface chemistry of the cyclic aliphatic nitro compound (nitrocyclohexane), shows some important similarities with that of nitromethane. After adsorption of nitrocyclohexane at room temperature a number of infrared bands is observed which disappear upon raising the temperature in the range 300-475K. These bands, with maxima around 1611(s), 1262(m-s), 1236(m-s), and 1129(vs) cm-' are, according to

Feuer et aI[l4],due to vibrations of nitronate surface species (see structure I). The bands can be attributed to v(C=N), u,,(NOJ (a

0

N t 0-' 0-

structure I

splitted band), and u,(NO,), respectively. The spectrum that remains after disappearance of the nitronate bands upon increasing temperature, is very similar to the spectrum obtained after adsorption of nitrobenzene on a-Mn,O, at 300K, followed by a temperature increase to 575K (see figure 2(a)). The adsorption of nitrobenzene at 300K is accompanied by only a slight shift (around 10 cm-') of the symmetric and antisymmetric NO, stretch absorptions with respect to liquid nitrobenzene, whereas the other absorptions show no observable shift at all. This result supports the assumption that the observed interaction of nitrobenzene with the transition metal oxide surface occurs through the nitro group. The absorption bands characteristic of nitrobenzene disappear when the temperature is raised in the range 300-475K. In this temperature range the spectrum shown in figure 2(b) develops. In order to get information on the surface species responsible for the spectra obtained after adsorption of both cyclic nitro compounds, the adsorption of several potential intermediates has been investigated. It can be concluded from the adsorption of isocyanic acid that the absorption around 2190 cm-' can be due to surface isocyanate species. To explain the absorption bands in the range 1600-1200 cm-l, the first idea was a surface amide sbucture, such as was observed by Krietenbrink et al."" However, considering the low stability of this adsorption complex and the disappearance of the band around 1475 cm" when going from C,H,NO, to C,D,NO, (a shift down to approximately 1395 cm-I), the conclusion is arrived at that such an assignment would be incorrect.

310

As can be derived from figure 2 the most likely assignment available at this moment

for the bands in the range 1600-1200 cm.', is to assume that they originate from two different surface species: a carboxylate and an azoxy species. Absorption bands ascribed to a carboxylate would correspond with an adsorbed acetate. However, the presence of other carboxylate compounds can not be excluded yet.

h

i

m

v

I aha

----> wavenumber (cm-')

Figure 2 . FT-IR spectra of (a) nitrocyclohexane (575K),( b ) nitrobenzene (575K),( c ) acetic acid (575K),and (d) nitrosobenzene (475K)adsorbed at a-Mn,O,. (Spectrum ( d ) is found to be assignable to surface azoxybenzene species.) The transformation of nitrocyclohexane into an azoxy surface complex needs a dehydrogenation step. The proven formation of a nitronate species indicates that dehydrogenation easily happens, indeed.

Discussion Information obtained from the systems studied, leads to the assumption that the time dependence of the product distribution of selective nitrobenzene oxidation (fig.1) has to be explained in the following way:

311

During the induction period, removable oxygen of the transition metal oxide attacks the ring of nitrobenzene and leads to the formation of the reducing fragments C, H, and CH, at the surface. Oxidation of these fragments produces carboxylate species at the surface. With nitrocyclohexane or nitromethane this oxygen leads also to the formation of carboxylate species. In the course of the induction period, the reducing fragments accumulate on the surface and create a situation at the surface, which leads to the formation of aniline. When reaching steady state conditions, the catalyst is partially reduced and selective removal of one of the oxygen atoms from the nitro group can be accomplished, resulting in nitrosobenzene production. During the period of selective reduction of nitrobenzene to

Q O \

nitrosobenzene, the most likely intermediate is a surface species of which only one of the two oxygen atoms interacts with the oxide surface, possibly as shown in smcture 11. Direct evidence for this adsorption mode could not be obtained by FT-IR spectroscopy, most probably because the lifetime of this mode is too short to allow its observation.

0

0

structure I1

It is likely that on highly reduced materials, like metals, a nitrene intermediate is formed upon reduction of nitr~benzene[~~. Although direct evidence for nitrene formation has not been obtained in this study, an indirect indication for such an intermediate can be found in the production of azobenzene and azoxybenzene. Coordination chemistry reveals how two ArN species can be coupled into one azobenzene molecule. In the case of the reaction of Fe,(CO),, with aromatic nitro compounds in benzene[I6’,formation of derivatives such as in structure I11

N

/.‘ (C0)s Fe

\

e(CO)s

.$e(co)3

has been proven by X-ray diffraction. Azoxybenzene can be formed by reaction of nitrene with nitrosobenzene, formed by reduction of nitrobenzene.

structure 111

The above mentioned process of the transformation of nitro compounds into nitroso compounds should also be possible for the other nitro compounds studied, nitromethane and nitrocyclohexane. However, no aliphatic nitroso compounds (or oximes obtained by isomerisation of the aliphatic nitroso compound) have been detected either in the gas or in the adsorbed phase. This is obviously caused by the easy migration of the a-hydrogen along

312

the molecules of nitroalkanes, which leads to the formation of a very reactive, easily oxidizable nitronate species. Once this pathway is opened, a very reactive C=N bond is formed and subsequently oxidized. This sequence of reaction steps prevents the competitive reduction to the corresponding nitroso compounds.

References [ 1] H.G.Zenge1, Chem.Ing.Tech. 2 (1983) 962

[2] [3] [4] (51 [6] [7] [XI [9] [ 101

1111 1121 [ 131 [ 141 [ 151

161

H.G.Zengel and M.Bergfeld, Ger.Offen 2939692 (1981) D.Dodman, K.W.Pearson, and J.M.Woolley, Brit.App1. 1322531 (1973) N.L.Holy, A.P.Gelbein, and R.Hansen, U.S.Patent 4585900 (1986) C.S.John, European Patent 0 178 718 A 1 (1986) K.Kishi, Surf.Sci. 192 (1987) 210 T.L.F.Favre, P.J.Seijsener, P.J.Kooyman, A.Maltha, A.P.Zuur, and V.Ponec, CataLLett. 1 (1988) 457 T.L.F.Favre, A.Maltha, P.J.Kooyman, A.P.Zuur, and V.Ponec, Proc.XI Symp.lberoam. (1988) 807 J.B.Benziger, Appl.Surf.Sci. (1984) 309 J.B.Benziger, Combust.Sci.Techno1. 29 (1982) 191 P.A.J.M.Angevaare, E.J.Grootendorst, A.P.Zuur, V.Ponec, Stud.Surf.Sci.Catal. 55 (1990) 861 Interscience J.H.Boyer, The chemistry of the nitro and nitroso group, H.Feuer (4.). Publishers, U.S.A. (1969) 244 H.A.C.M.Hendnckx, PhD Thesis 1988, Leiden, the Netherlands H.Feuer, C.Savides. C.N.R.Rao, Specmhim.Acta 19 (1963) 431 H.Knetenbrink and ILKnozinger, Zeitschr.Physik.Chemie, Neue Folge 102 (1976) 43 J.M.Landesberg, L.Katz, and C.Olsen, J.Org.Chem. 2 (1972) 930

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

313

T fi IJ A id I L I ‘1i I> 17 E P AK A T 1 UiJ U F I1[<TII0 1’ IE N Y LEN E D I A M IN E FROM 4 - C H LOR 0 - 2- N I

J.L.Margitfalvi,

M.HegedLis,

S.Gtlb616s

a n d E.TAlas

C e n t r a l R e s e a r c h I n s t i t u t e f o r C h e m i s t r y o f t h e H u n g a r i a n Academy o f S c i e n c e s , 1 5 2 5 B u d a p e s t , POB 1 7 , H u n g a r y

SUMMARY I n t h i s w o r k t h e p r e p a r a t i o n o f o r t h o p h e n y l e n e d i a m i n e (OPDA) f r o m 4 - c h l o r o - 2 - n i t r o a n i l i n e was s t u d i e d o n a l u m i n a s u p p o r t e d p a l l a d i u m c a t a l y s t s . H i g h OPDA y i e l d s w e r e o b t a i n e d o n c a t a l y s t s c o n t a i n i n g s t a b i l i z e d i o n i c palladium. I n the preparation o f t h e given palladium containing c a t a l y s t s anchoring type surface reactions were u s e d . The e x i s t e n c e o f p a l l a d i u m i n i o n i c f o r m was e v i d e n c e d b y X P S a n d EI’R m e a s u r e m i i n t s .

1N T R 0 OU C T I0N Urthophenylenediamine

(OPOAI

i s an i m p o r t a n t i n t e r m e d i a t e of

s e v e r a l b i o l o g i c a l l y a c t i v e compounds,

i n c l u d i n g t h e benzimidazene-

- c a r b a r n a t e t y p e f u n g i c i d e s [ I ] .T h e r e a r e d i f f e r e n t s y n t h e s i s o f UPUH.

I n t h e present work an d t t q i t

p a r e OPDA f r o m 4 - c h l o r o - 2 - n i t r o ~ n i l i n e ( C N A ) . by n i t r d t i o n and subsequent The p r e p a r a t i o n of i c steps,

i.e.

routes for the

was done t o p r e -

CNA c a n b e o b t a i n e d

a r n m o n o l y s i s o f p a r a - d i c h l o m - b e n z e n e [I].

UPOA f r o m CNA r e q u i r e s t w o d i f f e r e n t c a t a l y t -

d e c h l o r i n a t i o n and r e d u c t i o n .

Both o f these rcac-

t i o n s r e q u i r e s h y d r o g e n a n d a r e c a t a l y z e d b y Group V I I I m e t a l s . Supported p a l l a d i u m i s considered as one o f t h e most a c t i v e c a t a l y s t b o t h f o r hydrodehalogenation [ ‘ , 3 ] and r e d u c t i o n o f t h e n i t r o group 1-

1.

I t i s a l s o known,

that in addition t o the catalytic reaction

r o u t e t h e dehalogenation o f aromatic h a l i d e s can be c a r r i e d o u t stoichiometric reaction LiA1H4 o r NaBH4).

[;I

i n t h e presence o f metal hydrides

Further c h a r a c t e r i s t i c feature o f hydrodehaloge-

n a t i o n r e a c t i o n s is t h e r e q u i r e m e n t f o r a d d i t i o n o f the reaction

i n

le.g.

rree bases i n t o

m i x t u r e t o f i x t h e formed h y d r o c h l o r i c a c i d [ L I l ] .

CATALYST U E S I G N R e a c t i o n s i n v o l v e d i n t h e f o r m a t i o n o f OPDA a r e g i v e n i n Scheme I. T h e m o s t i m p o r t a n t i s s u e i n t h e p r e p a r a t i o n o f OPOA f r o m C N A i s t o find a specific catalyst,

which can c a t a l y z e b o t h t h e hydrodechlo-

r i n a t i o n and t h e r e d u c t i o n s t e p s .

I n t h i s case the conversion o f

314

qNo2 GNH2 L

by - products

slow

Cl

SCHEME I CNA to OPOA can be carried out in one step. Further technological improvement will be if the use of free base could be avoided. The basis for the design of catalysts f o r Scheme I was the need to obtain higher rates f o r the hydrodechlorination step than f o r the reduction of the nitro group of CNA. If rate of conversion of CNA to 4-chloro-1,2-orthophenylenediamine ( L P D A I is high i n this case the formation of OPDA is strongly hindred a s , due to the substituent effect, the rate of hydrodechlorination of Lr'UA is low. Characteristic feature of catalyst designed f o r the hydrodehalogenation and reduction is the p r e s e n c e of a stabilized, anchored form of ionic palladium o n the alumina support. T h e idea to have ionic forms of palladium on the support is tiased o n homogeneous catalytic analogies. It has been reported that i n hydrodehalogenation o f arylhalides the rate limiting step is the oxidative addition of the ArCl to the palladium [5]. Based on this knowledge it has been suggested that the introduction of palladium into the heterogeneous catalyst not i n metallic but i n ionic form should i n crease the rate of hydrodehalogenation reaction provided the mode of stabilization of the ionic form of palladium can b e found. SURFACE CHEMISTRY T h e r e are different approaches to introduce ionic species onto silica or alumina supports. In this work an anchoring process via lithiated alumina was used as follows:

315 S u r f a c e r e a c t i o n s ( 1 ) and (21 have been w i d e l y used t o p r e p a r e s i l i c a s u p p o r t e d m e t a l complex c a t a l y s t s [ 6 ] . The s t a b i l i z a t i o n o f i o n i c f o r m s o f p a l l a d i u m w a s strurif;ly i n c r e a s e d b y t h e p r e s e n c e of -0Li moiety. Simultaneous presence o f - O L i and (-0InPd s u r f a c e s p e c i e s r e s u l t e d i n a very a c t i v e hydrodehalogenation and r e d u c t i o n catalyst.

EXPERIMENTAL C a t a l y s t p r e p a r a t i o n and c h a r a c t e r i z a t i o n

I n t h i s w o r k K e t j e n CK 3 0 0 t y p e a l u m i n a w a s u s e d a s s u p p o r t . 0 i f f e r e n t p a r t i c l e s i z e were u s e d d e p e n d i n g o n t h e a p p l i c a t i o n ( 0 . 0 3 - 0 . 0 5 mm a n d 0 . 3 1 - 0 . 6 3 mm f o r s t i r r e d t a n k a n d t r i c k l e b e d r e a c t o r s , r e s p e c t i v e l y ) . P r i o r t o t h e c a t a l y s t p r e p a r a t i o n t h e s u p p o r t was t h e r m a l l y t r e a t e d i n v a c u u m a t I x I O - ~ b a r . S o l v e n t s a n d g a s e s were c a r e f u l l y d r i e d and

deoxygenated p r i o r t o t h e i r use.

R e a c t i o n ( 1 ) was c a r r i e d o u t i n n-hexane s o l v e n t . The e x c e s s but y l l i t h i u m was e i t h e r removed by w a s h i n g w i t h n-hexilne o r decompos e d b y t h e r m a l t r e a t m e n t . A n c h o r i n g o f p a l l a d i u m ( r e a c t i o n (2)) w a s c a r r i e d o u t i n a c e t o n e s o l u t i o n f o l l o w e d by w a s h i n g w i t h a c e t o n e and methanol.

T h e formed S u r f a c e Complex (SC) was s t a b i l i z e d by

t h e r m a l t r e a t m e n t i n n i t r o g e n a t 1O0-30O0C f o r 3 h o u r s . F u r t h e r d e t a i l s on c a t a l y s t p r e p a r a t i o n w i l l b e g i v e n i n t h e R e s u l t s a n d Uiscussion. S o m e o f t h e c a t a l y s t s a m p l e s p r e p a r e d were c h a r a c t e r i z e d b y E P R

a n d XPS.

E P R s p e c t r a were r e c o r d e d a t 2 0 a n d -196OC u s i n g a JEOL

JES-FE3X s p e c t r o m e t e r .

XPS m e a s u r e m e n t s were t a k e n by u s i n g a V G

ESCA 3 s p e c t r o m e t e r w i t h a n a l u m i n i u m K a

radiation source. A l l bin-

d i n g e n e r g i e s w e r e r e f e r r e d t o t h e A12p l i n e ( B E

=

74.7 eV).

Catalytic reactions

The conversion of 4-chloro-'-nitroaniline

(CNA)

( F l u k a , purum,

'98%) w a s c a r r i e d o u t i n s t i r r e d t a n k o r t r i c k l e b e d r e a c t o r s under r e l a t i v e l y mild reaction condtion (P =

35-120OC).

=

2-30 b a r and T =

Gas C h r o m a t o g r a p h y was used f o r p r o d u c t a n a l y s i s .

RESULTS AND DISCUSSI0N S u r f a c e r e a c t i o n s and c a t a l y s t p r e p a r a t i o n

I n order t o obtain different extent of lithiation i n reaction (1) t h e f o l l o w i n g experimental v a r i a b l e s were changed: t e m p e r a t u r e

o f d e h y d r o x y l a t i o n . t h e amount o f b u t y l l i t h i u m u s e d , t e m p e r a t u r e and t i m e o f t h e r e a c t i o n . The p a l l a d i u m c o n t e n t o f t h e c a t a l y s t was

316 Table 1 Condition of t h d preparation of ionic palladium c a t d l y s t s React ion (11

Ta

N o [OC]

1 2 3L

150 150 150

4c

300

5

200 150 150 150 150 250 150

6

7 8d gd 10 11

Reaction

EuLib A1203

time, min

2.3 2.3 2.3 0.9

60

1.5

90

2.3 1.5 2.5 2.5 2.3 2.3

60

60 60 60

time, min

XPS d a t a

Concentration

(2)

W%

Li

Pd

60 1440 60

0.31 0.61 0.46

60

0.52 0.51

60

30 135 900

85

60

0.92

85 85

60 60

85

GO

0.84 0.24 0.49

0.55 0.70

1.2 1.2 1.2 0.3 1.2 l.G 0.4 0.6 1.3 0.4 0.6

Pd3d5/2 binding FWHM energy

Cl n.a n.a n.a 0.3 0.3 n.a 0.5 n.a n.a n.a n.a

336.6 3 3 5 . ~ 3 3 ~ J.

3.7 3.0 3.2 -

336.6 336.3 334.9

11.0 4.3 2.8 -

Temperature of dehydroxylation; G i v e n i n mmol/g; A f t e r r e a c t i o n ( 1 ) t r e a t m e n t a t 150°C f o r 1 h o u r a t I x I O - ~ b a r P a r t i c l e s i z e t0.045 mm, i n o t h e r s a m p l e s : 0 . 3 1 - 0 . 6 3 mm c o n t r o l l e d by ( i ) t h e e x t e n t of

lithiation,

(ii) the temperature

a n d [ i i i ) t h e t i m e o f r e a c t i o n (21. C h a r a c t e r i s t i c p r o p e r t i e s o f c a t a l y s t s p r e p a r e d and c o n d i t i o n s o f p r e p a r a t i o n a r e g i v e n i n T a b l e I.

T h e i o n i c c h a r a c t e r o f c a t a l y s t s p r e p a r e d s t r o n g l y depended on t h e e x p e r i m e n t a l c o n d i t i o n s u s e d i n s u r f a c e r e a c t i o n s ( 1 ) a n d (2) and on t h e modes o f t h e removal o f t h e u n r e a c t e d b u t y l f a r a s i n reaction

i m p o r t a n t t o remove u n r t d c t e d b u t y l l i t h i u m ing w i t h n-hexane.

lithium.

As

( 1 ) a n e x c e j s b u t y l l i t h i u m was u s e d i t was v e r y by

washing o r e x t r a c t -

I n t h i s way u n d e s i r e d r e d u c t i o n o f rdCl2 u n d e r

c o n d i t i o n o f r e a c t i o n (21 c o u l d b e p r e v e n t e d .

T h e t h e r m a l t r e a t m e n t o f t h e l i t h i a t e d a l u m i n a dirnod t o d e c o m pose unrracted butyl lithium resulted

(see catalysts

NO3

i n reduction o f palladium

and 4 ) . I n t h i s c a s e u n d e s i r e d r e d u c t i o n o f pal-

l a d i u m w a s d t t r i b u t e d t o t h e p r e s e n c e o f h i g h l y d i s p e r s e d metallic l i t h i u m or l i t h i u m h y d r i d e f o r m e d f r o m C 4 H g L i

during the thermal

trcatment . T h e f o r m a t i o n o f m e t a l l i c p a l l a d i u m w a s a l s o o b s e r v e d in t h e p r e s e n c e o f s m a l l amount o f w a t e r i n t r o d u c e d i n t o t h e a c e t o n e t o i n c r e a s e t h e s o l u b i l i t y o f PdCl?. o f reaction

(LdtalySt5

S i g n i f i c a n t i n c r e a s e of t h e time

(2) resulted also i n partial reduction NOZ

and 31.

o f palladium

317 C h a r a c t e r i z a t i o n o f c a t a l y s t s b y EPR a n d XPS The EPR s p e c t r a o f a l i t h i a t e d a l u m i n a a n d t w o t y p e s o f p a l l a d -

1 . C a t a l y s t NO5

i u m c a t a l y s t a r e shown i n F i g . by a n c h o r i n g t e c h n i q u e , pregnation.

has been p r e p a r e d

w h e r e a s c a t a l y s t NO7 b y c o n v e n t i o n a l i m -

A n a r r o w EPR s i g n a l ( w i t h g = 2 . 0 0 4 1

was d e t e c t e d o n l y

on c a t a l y s t c o n t a i n i n g i o n i c p a l l a d i u m . B a s e d on t h e g v a l u e s a n d t h e absence of a n i s o t r o p y and h y p e r f i n e s t r u c t u r e i n s p e c t r a b and c t h e EI’R s i g n a l s c a n b e a t t r i b u t e d t o f r e e e l e c t r o n o r i g i n a t e d f r o m t h e e l e c t r o n i c i n t e r a c t i o n o f i o n i c p a l l a d i u m and t h e a l u m i n a

171. I t i s w o r t h f o r m e n t i o n i n g , t h a t t h e r e d u c t i o n

support

t a l y s t N O 5 i n h y d r o g e n a t 200°C

ance o f t h e n a r r o w l i n e i n s p e c t r u m c . s i g n a l s c o r r e s p o n d i n g t o Pd+’ spectra

.

o f ca-

does n o t r e s u l t i n t h e d i s a p p e a r -

I t i s a l s o noteworthy,

that the

a n d Pd+3 c a n n o t b e d e t e c t e d i n

H

I H

1 g = 2 ,0046 C

100 G

100G

U

I

Fig.

1 . Li’R sli e c t r a o f L i / A 1 , 0 3 and d i f f e r e n t P d - c o n t a i n i n ( a ) L i / A 1 2 0 3 p r e c u r o s o r o f C a t a l y s t N o 5 j ( b ) C a t a l y s t N 5 c ap trael py satrse. d b y a n c h o r i n g p a l l a d i u m ; ( c l C a t a l y s t N O 5 t r e a t e d i n H 2 a t 200°C;

i

( d ) C a t a l y s t p r e p a r e d b y i m p r e g n a t i o n l s p e c t r a t a k e n a t 20°C. X P S d a t a a r e g i v e n i n T a b l e 1 . B a s e d on l i t e r a t u r e d a t a [ d , g I

t h e b i n d i n g e n e r g i e s around 335.0 m e t a l l i c and i o n i c p a l l a d i u m . d i n g e n e r g y o f t h e Pd 3dSl2 lysts

NO1,

J ,

:I

and 336.7 eV were a t t r i b u t e d t o

respectively.

The v a l u e s o f t h e D i n -

bond s t r o n g l y i n d i c a t e t h a t i n c a t a -

palladium i s i n i o n i c form.

Contrary t o that,

c a t a l y s t NO7 m e t a l l i c p a l l a d i u m was e v i d e n c e d .

i n

I n t h e case o f c a t a -

l y s t NO7 t h e FWHM o f t h e Pd 3 d 5 / 2 b o n d h a d t h e l o w e s t v a l u e .

I n ca-

t a l y s t s a m p l e s c o n t a i n i n g i o n i c Pd t h e FWHM v a l u e s a r e much l a r g e r .

These d a t a r e v e a l t h e h o m o g e n i t y o f s u r f a c e s p e c i e s w i t h m e t a l l i c c h a r a c t e r and c e r t a i n i n h o m o g e n i t y o f t h e i o n i c p a l l a d i u m

introduLed

318

by anchoring. I n the latter case the inhomogenity can be attributed either to different valent states or ligand environment of the ionic palladium anchored to the alumina. It has also been reported that in the presence of chlorine the FWHM values o f the P d 3dSl2 bonding energy is higher than in its absence [ e l . Conversion of 4-chloro-2-nitroaniline to orthophenylenediamine Kinetic curves o f the products formation obtained on ionic palladium catalyst (NO91 are shown in Fig. 2. T h e formation of OPOA on catalysts containing both ionic and metallic palladium is shown i n Fig. 3. These results reveal the high activity of alumina supported ionic palladium catalysts both in the hydrodehalogenation reaction and reduction o f the nitro group. As seen i n Fig. 3.. the addition o f metallic palladium strongly dechreases the initial hydrodechlorination activity of catalysts containing ionic palladium. In the latter catalysts the metallic palladium was introduced prior t o the lithiation (reaction ( 1 ) ) .

0

20

40

60 80 time, rnin

100

0

20

40 60 time, min

80

Fig. 2. Kinetic curves of the products formation from CNA. D-OPDAI + - ONA; o - CPDA; Catalyst N09, isopropanol-water (90:10),?OO cm3, CNA: 5 g; CNA/catalyst: 8; P: 1 bar; T : GOOCJ stirred tank reactor. Fig. 3. Influence o f the metallic palladium on the formation of OPDA; Metallic palladium [w%l: - 0.0 [catalyst N o 8 1 , o - 0.05, 0 - 0 . 1 0 1 Conditions: s e e Fig. 2.; CNA/catalyst = 12.

319

100

s

40 -

D

a

b

A / -A-A

.-b)

1

SO 20

0

a

0

- A

time, min

time, min

F i g . 4. I n f l u e n c e o f t h e amount o f c a t a l y s t on t h e f o m t i o n o f OPDA and CPOAI A - 0 . 9 3 g, x - 1 . 8 6 g, # - 3 . 2 g~ C a t a l y s t N ' l l , CNA: 30 g P H ~ : 3 0 b a r ; T: 9 5 C; NH3-HzO ( 5 0 - 5 0 % ) , 3 0 0 cm3; s t i r r e d t a n k reactor.

100

1

100

1

w.

a 0 a U x-x-x

0

30

60

90

0

120

time, min

30

60

90

120

time, min

F i g . 5. R o l e o f t h e temperature o f pretreatment on t h e f o r m a t i o n o f OPDA and CPOA; o - 100°C, 0 - 200°C, x - 30OoC; c a t a l y s t : N O 1 O j c a t a l y s t : 1.86 gl CNA: 30 g; PH2: 30 bar; T: 95OC. NH3-H20 (50-50%) 300 cm3; s t i r r e d t a n k r e a c t o r . As shown i n F i g .

4.,the

r a t e s o f OPOA a n d CPOA f o r m a t i o n a n d

thus y i e l d s were s t r o n g l y i n f l u e n c e d by t h e amount o f c a t a l y s t optimized r e a c t i o n c o n d i t i o n

used.

Under

and a t 100 % o f conversion OPDA s e l e c t i v i t y around

96 % could be obtained. Heat treatment i s used t o s t a b i l i z e t h e i o n i c p a l l a d i u m formed i n s u r f a c e r e a c t i o n (2). A l l o f t h e c a t a l y s t s were t r e a t e d in

n i t r o g e n a t 100°C f o r 3 h o u r s .

of

treatment resulted i n s i g n i f i c a n t increase i n t h e hydrodechlo-

r i n a t i o n a c t i v i t y a s showri i n F i g . Results obtained i n a t r i c k l e

The i n c r e a s e o f t h e t e m p e r a t u r e

5. b e d r e a c t o r a r e s u m m a r i z e d i n Tab-

320 Table 2

Conversion of CNA in trickle bed reactor Catalysta'

v

T

Inlet, cmj.min-"

bar liquid"

OC

gas

Selectivity, 2

Conversion c

OPUA

CPUA

unknown

34.4 10.7

Ill.

~

~~~~

35

2

NU 5

10 70

NO5

70

2 2 5

NO5 N%

117

5

80

NO6

80

CJ 5

NO5 No5

0.7 0.7 0.7 0.7 0.4

0.7 0.7

110 110 110 110 110 120 60

100 100 100 100 Yti

100

100

b5.b

80.C

Y8.2 96.2 17.1 96.9 98.0

0.5 1.1 7.5 3.1 2 .0

U.Ci

1.3 2.7 75.4 U.L< I1.L

See T a b l e 1

Amount of catalyst: 5 g 5 wt% CNA i n ethanol l e 2. These investigations were aimed t o obtain high UPDA selectiv-

ities at I O U % of conversion. The selectivity of the OPUA formation strongly depended on the reaction temperature. A t temperature abovc llO°C condensation of the OPOA and CPDA wdb o b s t r v e d , howw,sI, dt rr lLitively low temperature and hydrogen pressure 1 0 0 % conversion and 98 % OPUA selectivity could be achieved. N o significant ageing of

the catalysts up to 1 0 0 hours rzartion t i n i t .

zuulu

tt:

ubsirvtd.

CONCLUSION

Results obtained in this work strongly indicate that catalyst design b a s e d on the primary knowledge o f the reaction network and the mechanism of the given reactions can be used to obtain highly active and selective catalysts f o r hydrodechiorination reaction and reduction o f the nitro group. It w a s also demonstrated that upon using anchoring type surface reaction palladium can b e stabilized on the alumina i n ionic form. REFERENCES

1

2 3

H.W.Layer, Arnines, aromatic-phenylenediamines, i n M.Garyson and 0.Eckroth ( E d s . ) "Kirk-IIthmtr Encyl.Chern.Technol., 3 r d Ed., Wiley. New York, 1 Y 7 6 , 2, 3 4 8 - 3 5 4 . A.R.Pinder, Synthesis, I Y t i U , 1125. P.h.Rylander "Catdlytic Hydrogenation i n Organic Synthesis", Academic Press, New York, 1979, p p . 2 3 5 - 2 4 6 . G.Elrieger and T.J.Nestrick, Chemical Reviews, 74 ( 1 9 7 4 ) 5 6 7 . J.K.Stille and K.S.Y.Lau, J.Am.Chem.So~., 98 ( 1 9 7 F ; I 5 8 4 1 . Yu.l.Yermakov and V.A.Likholobov, Kinet.Katal., 2 1 ( 1 9 8 0 ) 1206. P.A.Uerger and J.F.Roth, J.Catal., 4 ( 1 9 G 5 ) 7 1 7 . F.Ro7on-Verduraz, A.Omar, J.Escadr and B.Pontvainne, J.Cata1. 53 ( 1 9 7 0 ) 1 2 6 . A.I.Lapidus, V.V.Maltsev, E.S.Spiro, G.V.Antoshin, V.I.Garanin arid H.M.Minachev, 17v.Akad. N a u k S S S R , Ser.Khim. 1 9 7 7 , 2 4 5 4 .

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

32 1

CHEMOSELECTIVE HYDROGENATION OF AROMATIC CHLORONFTRO COMPOUNDS WITH AMIDINE MODIFIED NICKEL CATALYSTS. P. BAUMEISTER, H.U. BLASER and W. SCHERRER Central Research Laboratories, Ciba-Geigy AG, CH-4002 Basel, Switzerland

ABSTRACT The chemoselective hydrogenation of halogen substituted aromatic nitro compounds is described using Raney nickel modified with amidine derivatives. Screening of a wide variety of amidines suggests that the C- and N-substituents and the type of anion have a strong influence on the inhibiting properties of the modifier. Formamidine acetate has been shown to be the most effective dehalogenation inhibitor. With this modifier even very sensitive substrates like halogenated dinitro benzene can be hydrogenated with selectivities > 97%. Investigation of the over-all kinetics and measurements of the catalyst potential are reported. From these results it is concluded that the dehalogenation occurs as a consecutive reaction after the halogenated aniline has been formed and that the dehalogenation is suppressed either by the presence of the strongly adsorbing nitro compounds and the corresponding reaction intermediates or by the effective modifiers. INTRODUCTION The hydrogenation of halogen substituted aromatic nitro compounds to the corresponding halogen substituted anilines very often is accompanied by an undesired hydrodehalogenation reaction. Two strategies have been used in order to find selective catalytic systems which produce high yields of the halogenated product: either the properties of the catalyst were optimized using various production methods (ref. 1-3). or commercially available catalysts were made more selective by adding specific modifiers (or inhibitors) to the reaction solution (ref. 4-6). The following two factors have been shown to influence the sensitivity of the C-X bond to hydrogenolysis: the type of halogen (I>Br>Cl>>F) and the position of X in relation to the nitro group (ortho>para> meta) (ref. 5). Additional nitro groups in ortho and para position to the halogen increase the tendency for hydrogenolysis even further. To date, the best studied modified systems are Pt catalysts inhibited with sulfur compounds, morpholine or phosphorous compounds (ref. 5). Raney nickel modified with dicyandiamide has also been reported to be able to hydrogenate aromatic chloronitro compounds with very good selectivities and activities. Since nickel is an attractive alternative to precious metal catalysts we decided to search for other types of inhibitors and to investigate the stage at which dehalogenation occurs.

u R

Raney nickel ____)

modifier

322

EXPERIMENTAL Reaction conditions All experiments were carried out in well stirred three-phase-slurry reactors at constant temperature and hydrogen pressure. Hydrogen consumption and hydrogenation time were determined by recording the pressure drop in a reservoir of known volume as a function of time. In special cases the catalyst potential was also measured during the reaction using the following system: gold electrode / lithium acetate bridge / Ag-AgC1 reference electrode (ref. 7). Typical hydrogen uptake and potential curves are depicted in Figure 1. For the test of the various modifiers the following conditions were used: 300 ml stainless steel autoclave; 40.8 g (0.2 mole) l-chloro-2,4-dinitrobenzene;2.0 g Raney nickel (60% in water); 0.012 moles of modifier; 120 ml methanol; temperature 60°C; H2 pressure 10 bar; 1100 rpm. Different substrates have been tested under the following conditions: 300 ml stainless steel autoclave; 0.25 moles of substrate: 2.0 g Raney nickel (60% in water); 1.5 g (0.014 moles) of formamidine acetate; 120 ml methanol; temperature 80°C;H2 pressure 12 bar, 1500 rpm. The catalyst potential was measured using the following conditions: 750 ml glass reactor, 15.8 g (0.1 mole) 2-chloro-nitrobenzene; 3.0 g Raney nickel (60% in water); 0.02 moles modifier, 550 ml methanol; temperature 30°C; H2 pressure 1.1 bar; 1500 rpm. Reagents and catalysts The halonitro compounds and the methanol used were of purum or pract. quality from Fluka. The amidine derivatives (in the form of their salts) were purchased from Fluka or Aldrich (purum or pract.). The N,N-dialkyl-formamidine acetates were prepared by analogy to a published procedure (ref. 8) from cyanamide and used as isolated (containing ca. 10% ammonium acetate). Two commercially available types of Raney nickel were used as 60% aqueous suspensions: B 113 W (Degussa) and M (Doduco) with no difference in the hydrogenation performance. Analytics The reaction solutions were analyzed as follows: I-chloro-2,4-dinitrobenzene:the reaction mixture was treated with acetic anhydride and the acetylated products analyzed by HPLC (Hypersil ODS 5 pm; eluent water/acetonitrile; UV detector 254/300nm). The other reaction mixtures were analyzed by GLC (OV 101; FID). In these cases it was possible to identify the following compounds: substrate, dehalogenated nitro compound, desired aniline; dehalogenated aniline. Two reaction intermediates (hydroxylamine and azo- or hydrazo-compound) were determined as a sum. The selectivities given were determined at the end of the hydrogenation and are defined as S (%) = 100 x (desired aniline / I:anilines)

323

RESULTS AND DISCUSSION Influence of modifier structure We decided to use as a very sensitive model reaction the hydrogenation of I-chloro-2,4-dinitrobenzene in order to find new and efficient dehalogenation inhibitors. For comparison, dicyandiamide was used as standard inhibitor. We concentrated our search of new modifiers on nitrogen containing compounds and found the most interesting effects with compounds of the following general structure

-

i H

\

R2- NH

R2

general structure of

effective inhibitors

Some results of this inhibitor screening are summarized in Table 1. It is immediately clear that formamidine acetate and its N-alkylated derivatives are good dehalogenation inhibitors. The selectivity achieved is comparable to that of dicyandiamide while the hydrogenation time observed is considerably lower. Compared to the unmodified system, the other inhibitors do not show a significantly improved selectivity but have a positive effect on the activity. The following structural effects can be distinguished: - The substituent R1 at the central carbon atom has to be either H or NHCN. Replacing it by either a methyl, phenyl or amino group leads to a sharp decrease in chemoselectivity. - The N-substituents R2 are less critical as long as R2 = H or alkyl. The selectivity decreases when R2 is phenyl. - The anion also has some influence on the performance of the catalytic system and acetate is clearly the better choice. At the moment we do not have a convincing explanation for the observed influence of the inhibitor structure. The basicity can not account for the effect of the substituents because both the more basic guanidine and the less basic N,N’-diphenyl formamidine are less selective than formamidine itself. A positive cooperative effect of formamidine and acetic acid is possible because we have found that acetic acid acts as a promotor in various types of hydrogenations. It was shown in separate experiments that anilines can react with formamidines to give both N-mono- and N,N-diary1 formamidines which no longer are effective inhibitors. Another way of modifier deactivation is the hydrolysis to ineffective formamide by water formed during the hydrogenation of the nitro group. This could explain the rather high modifier concentration needed in order to get good selectivities. Similar observations of instability under reaction conditions have also been reported for dicyandiamide (ref. 4).

324

TABLE 1 Influence of the modifier structure on reaction time and chemoselectivity of the hydrogenation of

l-chloro-2,4dinitrobenzene(Raney nickel; methanol; 60°C; 10 bar). Modifier

Rl

Formamidine acetate

H

N,N'-Dibutyl- acetate formamidine

H

N,N'-Ditetbutylformamidine acetate

Reaction time [min]

Selectivity Remarks

acetate

75

97,5

(CHZ),CH3

acetate

80

97

CH3 d-CH3

acetate

80

96

acetate

120

R2

H

X

[%]

standard

LH3

N,N'-Dimethylformamidine acetate

H

CH3

N,N'-Diphenylformamidine

H

phenyl

75

94,5 82

Guanidine acetate

NHz

H

acetate

135

66

Acetamidine acetate

CH3

H

acetate

75

86

Benzamidine

phenyl

H

180

76

H

H

CI

140

86

CH3

H

CI

150

71

240

70

210

99

Formamidine hydrochloride Acetamidi ne hydrochloride no modifier

Dicyandiamide

NH-CN

H

free base

free base

Reference experiment Reference experiment

Effect of substrate structure Different substrates were hydrogenated in presence of formamidine acetate in order to test the scope of this new dehalogenation inhibitor. The results are summarized in Table 2. In all cases where comparable results are available, formamidine acetate performs with selectivities and activities as high as those of the catalytic systems described in the literature for Pt catalysts. The different sensitivity of the C-X bond towards hydrogenolysis for varying substitution patterns is also clearly seen with this modifier. In addition, the hydrogenation of a C=O bond is inhibited as well (4-chloro-3-nitroacetophenone).These results demonstrate that we have found a new, universal modifier for the selective hydrogenation of substituted aromatic nitro compounds. While for most halogenated nitro compounds investigated very selective catalysts (both platinum and nickel) can be found, this is not the case for the dinitro substrate. Here only Raney nickel modified by either dicyandiamide or formamidine has been described to give satisfactory selectivities.

325

TABLE 2 Reaction time time and chemoselectivity for the hydrogenation of different substrates (formamidine acetate; Raney nickel; methanol; 80°C; 12 bar)

Substrate 1-Chloro-2,4dinitrobenzene

l-Chloro-2nitrobenzene l,Chloro-2nitrobenzene l-Chloro-3nitrobenzene 1-Chloro-4nitrobenzene 1 2-Dichloro-3nitrobenzene 1,4-Dichloro-2nitrobenzene 1 2-Dichloro-4nitrobenzene l-Bromo-3nitrobenzene 1-Bromo-3nitrobenzene

Reaction time [ min ]

Selectivity

75

97,5

from Table 1

90

99,4

(98.3%;110°C) (ref. 5)

100

88,O

no modifier

90

99,4

95

99,7

70

99,7

65

[“/,I

”16

65

99,7

110

98,7

165

91,o

4-Chloro-3nitroacetophenone

60

97,2

4-Chloro-3nitroacetophenone

75

93,l

Remarks (literatureresults)

(99.6%;1lo°C) (ref. 5) (99.3%;40°c) (ref. 2) 99.1%;6OoC (ref. 2) /99.8%;13O0k) (ref. 1) (99.8%;10O0C) (ref. 5) (99.5%;6O0C) (ref.2) no modifier

IK)

modifier

Over-all kinetic studies The question at what stage of the nitro group reduction the undesired dehalogenation occurs has been addressed by several research groups (ref. 3,9). It has been demonstrated that the dehalogenation of bromo- and chloro-nitrobenzene using platinum or nickel catalysts mainly occurs as a consecutive reaction of the haloaniline. Palladium catalysts act differently and can even preferentially hydrogenolyze the C-Xbond (ref. 10). For both theoretical as well as practical reasons it was of interest to us to investigate this problem for the modified nickel catalysts. We decided to use 2-chloronitrobenzene as model substrate and to cany out carefully controlled experiments with several modifiers. In order to determine the composition of the reaction solution, samples were withdrawn during the reaction and analyzed by GLC. In addition, hydrogen uptake and the catalyst potential via a gold electrode were measured. This method is not applied widely and the theoretical basis is still not fully developed but there is agreement that the following statements can be made (see Figure 1) (ref. 7, 11): the potential which is measured using the electrode described above depends on the oxidation potential of the substrates present and on their adsorption on the catalyst surface. When there is no reducible substrate, the SO called

326

hydrogen uptake

catalyst potential (mW . .

[YO)

0

-400

4

:

+

:

. *: d

20

-500

40

-600

60

hydrogen uptake hydrogen uptake catalyst potential catalyst potential

.

(selective) (unselective) (selective) (unselective)

Fig. 1 80

-700

loo

I

I reaction time

Schematic representation of typical curves for hydrogen uptake and catalyst potential for the hydrogenation of aromatic halonitro compounds.

reversible potential is measured, which is attributed to a hydrogen covered catalyst. When for example a nitro compound is added, the potential shifts to positive values of about 400 rnV (( 1 ) in Figure 1). When all the nitro groups are converted it is sometimes possible to detect the partially reduced intermediates (hydroxylamine, azoxy, azo, hydrazo etc), indicated by a potential drop of 100-200 rnV (21. When these are completely reduced to the aniline the reversible potential is reached again (3).Since the observed potential is also influenced by the pH value. the final reading may be somewhat more negative because the anilines act as a weak base. If dehalogenation occurs, hydrogen consumption continues and the potential rises again [ 4). corresponding to the formation of HX. Figures 2a-d show plots of catalyst potential and substrate concentration (GLC) versus hydrogen consumption for the hydrogenation with formamidine acetate (a), with dicyandiamide (b), with guanidine acetate (c) and without modifier (d). This type of presentation allows to standardize and to compare reactions with different reaction times, which are indicated at the upper edge of the graphs. As expected by analogy with the results reported for the unmodified nickel catalysts (ref.9), no dehalogenation is observed as long as either nitro compounds or partially reduced intermediates are present in solution. Using the two most effective modifiers the reaction stops after consumption of the theoretical amount of hydrogen. This is indicated by a sharp decrease of the catalyst potential and no further hydrogen uptake. In the case of the 2-chloro aniline no dehalogenated products are detectable by GLC even after prolonged hydrogenation times. The unselective catalysts show different behavior: while again no hydrogenolysis occurs before all reducible species have disappeared, the reaction goes on and dehalogenated anilines are detected by GLC. If one interrupts the hydrogenation at this point, no dehalogenated aniline can be detected by GLC (c 0.1%). In addition, the catalyst potential rises again because of the HCL produced. This negative potential peak represents a much more precise endpoint for the nitro

327

reduction than the hydrogenation curve which shows only a slight inflection when 100% of the theoretical amount of H2is consumed.

readion Dime (min.) 36 75 90 150

reaction time (min.) 160 240 3001360

60

potential

I

I

DLC-area

GLC-area

I%)

(mv) -300

100

-400

80

100

I%)

60 -590 40

-600

M

A,.' 0

'

'

'

20

40

60

'X

80

x 100

'

-700

0

hydrogen uptake (%) a) formamidine acetate

c) guanidlne acetate

reaction time (min.)

r e a c h time (min.) 45

110

260 315 GLC-area 100

(%I

BO

-600 60

60 -700

40

40 -800

.

.. .. .

..,

20

20

0

0

0 2 0 4 0 6 0 B O l 0 0 hydrogen uptake (%)

b) dicyandiamide

8

* A

8

0

20

60 80 100 105 hydro(lm uptake (%) 40

d) without modifier

catalyst potential (selective) catalyst potential (unselective) intermediates chloronitrobenzene chloroaniline aniline

Fig. 2a-d: Hydrogenation of 2-chloro-nitrobenzene in presence of different modifiers. Catalyst potential and concentration of reactants and products versus hydrogen uptake (conversion). (Raney nickel; methanol; 30°C; 1.1 bar). The observation that no hydrogenolysis of the C-X bond takes place as long as either nitro compounds or reaction intermediates are present can be explained by the strong adsorption of these molecules, thereby preventing the interaction of the C-Cl bond with the catalyst. The mode of action of the modifiers is less clear. It could be due to a modification of the catalytic properties of the Raney nickel or also to a competitive adsorption between the effective modifiers and the

328

haloanilines. CONCLUSIONS Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted fomamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. ACKNOWLEDGMENTS The authors would like to thank Mr. R. Miiller and Mr. R. Juanes for the experimental work, Ms. A. Lutterotti for the analytical support and Ms. E. Scherrer for critical comments. REFERENCES 1 J.B.F. Anderson, K.G. Griffin and R.E. Richards, Chemie-Technik, 18/5 (1989) 40-44. 2 J. Strutz and E. Hopf, Chem.-1ng.-Tech., 60/4 (1988) 297-298. 3 W. Pascoe, Catalysis of Organic Reactions, P.N. Rylander et al., Dekker Inc., NY, (1988) pp. 121-134. 4 DE 2’441’650 to Nippon Kayaku KK, (1973). 5 J.R. Kosak, Catalysis in Organic Synthesis, W.H. Jones ed., Academic Press, NY, (1980) pp. 107-117.

EP 325’892 to Ciba-Geigy, (1987). 7 F. Beck, Chem.-1ng.-Tech, 48/12 (1976) 1096-1105. 8 C. Kashima, M. Shimizu, T. Eto, Y . Omote, Bull. Chem. SOC. Jpn., 59 (1986) 3317-3319. 9 V.I. Savchenko, T.V. Denisenko, S. Ya. Sklyar, V.D. Simonov, Journal of organic chemistry of the USSR, 11 (1975) 2183-2186. 10 J. Margitfalvi, private communication, Chemisch-technisches Laboratorium, ETH Zurich, Switzerland. 11 F. Wolf, H. Fischer, Journal f. prakt. Chemie, 317/2 (1975) 247-251. 6

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

329

0 1991 Elsevier Science Publishers B.V., Amsterdam

INTERMEDIATES NITRILES

FORMATION

IN

THE

CATALYTIC

HYDROGENATION

OF

by Ph. MARION, P. GRENOUIILET, J. JENCK and M. JOUCLA UMR

45

- CNRS - RP RHONE POULENC INDUSTRIALISATION 24 rue J. Jaures 69151 DECINES-CHARPIEU Cedex FRANCE

ABSTRACT :

In the course of the catalytic hydrogenation of u , w dinitriles over Raney nickel, by-products are obtained from C-N and C-C bond formation. The mechanism of the formation of these compounds was investigated. Cyclic and linear secondary amines can result from the same secondary imine through a transimination process involving a ring-chain tautomerism. Stereochemical results for 2-aminomethyl-cyclopentylamine (AMCPA) are in accordance with a specific cyclisation pathway favored by an intramolecular hydrogen bond giving rise to the cis isomer from aminocapronitrile, unfavored in the case of adiponitrile which leads to the trans AMCPA as the major isomer. INTRODUCTION : Catalytic hydrogenation of nitriles over Raney nickel leads to primary amines with variable amounts of secondary and tertiary amines depending on reaction conditions (Ref. 1). These by-products result from hydrogenation of secondary imines and enamines respectively. We have investigated the catalytic hydrogenation of u , w dicyanoalkanes and we report results with 1,4-dicyanobutane (ADN). Scheme 1 details the intermediates involved in a step-by-step addition of hydrogen to the dinitrile molecule adsorbed on the metal surface. Some of them are to be considered as precursors for the formation of by-products (detected in the liquid phase after desorption).

330

/

A ADN

c

ACN

(adiponilrile)

(aminocapronitrile)

HMD (hexam4lhyldne diamine)

Scheme 1

We will focus on two types of compounds arising from :

.

C - N bond formation hexamethylene triamine (BHT)

.C-

C

: 1-azacycloheptane

(HMI) and

bis

bond formation : 2-(aminomethyl)-cyclopentylamine

( AMCPA)

EXPERIMENTAL PART

Hydrogenation procedure : Adiponitrile was hydrogenated over Raney nickel catalyst in a 150 ml autoclave equipped with a magnetic stirring under constant pressure and temperature. The catalyst was washed then weighted by pycnometry and charged into the reactor with the solvent. The dinitrile was introduced in a special container. The reactor was closed and purged repeatedly with nitrogen first and then with hydrogen. The autoclave was heated to the reaction temperature and the nitrile was introduced. Gas chromatography was carefully carried on the liquid mixture at the end of the hydrogenation. Transimination : Pure 1-aza-1-cycloheptene was introduced in a NMR probe at a known concentration and primary amine was gradually added. The equilibrium was determinated by 1H and 13C NMR analysis.

331 RESULTSAND DISCUSSION :

-

N bond formation As secondary amines proceed from hydrogenation of secondary imines, the behaviour of these compounds is of importance in the reaction mixture. Secondary imines can result from the condensation of primary amino group with primary imines cornparables in reactivity with a carbonyl function. C

Only in the case of a , w - dicyanoalkanes hydrogenation, there is a possibility of intramolecular condensation of intermediate [&I between amino and imino group. Such a condensation leading to an aminal [z] prone to ammonia loss, gives rise to l-aza-l-cycloheptene 2. Hydrogenation of this intermediate leads to azacycloheptane 5 . 1-aza-1-cycloheptene is a key-intermediate for the formation of azacycloheptane 5 and bis hexamethylene triamine 8 [R = (CH2)6 NHz]. In fact 2. can react in three different ways : - Addition of hydrogen leading to secondary amine 5 Addition of ammonia going backwards to [L] - Addition of any amino group to give, via aminal [ 5 ] , amino imine (scheme 2).

-

Scheme 2

332

This last point has been examplified through transimination reactions and ring-chain tautomerism between cyclic aminals and open chain amino imines. We have prepared 2 following the synthetic sequence (scheme 3)

1 ) NaN3, DMSO, Nal cat.

Schema 3

Compound 1 polymerizes slowly at room temperature and can be kept in solution for several hours. Add tion of pr mary amine shows an equilibrium between 2 - and 7 -. Diamine 8 is obtained from hydrogenation of am no imine which can be formed by direct addition of primary arnine to amino imine [A] or to 1-aza-1-cycloheptene 3 . As it is generally accepted, formation of l-aza-l-cycloheptene in the course of the hydrogenation reaction leads to the cyclic amine 4. We have demonstrated its ability to generate also diamine 8 .

z

2-(aminomethyl)- cyclopentylamine (AMCPA) The well-known Thorpe-Ziegler condensation reaction (Ref. 2) involves the nucleophilic addition of a carbanion to an electrophilic center. Starting from adiponitrile, enamino nitrile 2 is recovered. Catalytic hydrogenation of this compound gives trans AMCPA as the major isomer (scheme 4 ) :

cis (minor) Scheme 4

333

In order to determine the course of the formation of AMCPA we have investigated the catalytic hydrogenation of amino capronitrile (ACN) over Raney nickel. GC analysis of the crude material indicates that AMCPA is present in small amount and moreover that the cis isomer predominates. The Thorpe-Ziegler reaction requires the presence of two electron withdrawing groups in the reactant. In accordance with the microreversibility principle, dehydrogenation of a primary amino group has been demonstrated (Ref. 3). Imino nitrile A can be the intermediate producing the cis observed. Two condensation products can arise starting from this compound : amino nitrile 11 and enamino imine lo (scheme 5 ) :

AMCPA

aNH2 CN

11

Lp Scheme 5

The cis stereochemistry observed is better supported starting from rather than from 11. In the first case intramolecular hydrogen bonding between amino and imino group favors the cis configuration leading to cis AMCPA as the major compound. Minor trans isomer can be issued from the slow isomerization of 12a (scheme 6). With enamino nitrile 2 intramolecular hydrogen bonding cannot take place so a rapid isomerization occurs between 13a and p J giving rise to the trans AMCPA as the major isomer after hydrogenation. Isomerization to 13b is reinforced also by the slower rate of hydrogenation of a nitrile group than an aldimine one.

334

AMCPA bans

AMCPA cI3

AMCPA cis

AMCPA lrans

Scheme 8

Conclusion The synthesis of 1-aza-1-cycloheptene and the study of its reactivity allows us to propose this compound as an important intermediate for the generation of secondary amines in the course of the catalytic hydrogenation of adiponitrile to hexamethylene diamine. Concerning 2-(aminomethyl)-cyclopentylamine formation, some dehydrogenation reaction of amino capronitrile can occur on the catalytic surface. Stereoselectivity can be interpreted in terms of chelating effect leading to the cis isomer. REFERENCES [l] RYLANDER P.N. "Hydroqenation Methods, Academic Press, 1985, p. 94 [2] J. March "Advanced Organic Chemistry" 3 th Ed. Wiley and

Sons,

(31

1985, p. a54 M. Besson, J.M. Bonnier and M. Joucla, Bul. Soc. Chim. F. Part. 1990, 127,5-12 and 13 19.

-

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

335

REOUCTIVE A M I N A T I O N OF A C E T O N E O N T I N M O O I F I E O SKELETAL N I C K E L CATALYSTS S.Gllbolos,

E.Tdlas,

M.HegedLis,

J.L.Margitfalvi

and J .Ryczkowski*

C e n t r a l R e s e a r c h I n s t i t u t e f o r C h e m i s t r y o f t h e H u n g a r i a n Academy o f S c i e n c e s , 1 5 2 5 B u d a p e s t , POB 1 7 , H u n g a r y ABSTRACT C o n t r o l l e d S u r f a c e R e a c t i o n s (CSKs) b a s e d on t h e r e a c t i v i t y o f a d s o r b e d hydrogen t o w a r d s t i n a l k y l compounds were used f o r t h e modification of a skeletal nickel catalyst. The modification of t h e s k e l e t a l n i c k e l c a t a l y s t w i t h t i n was a i m e d t o s u p p r e s s t h e formation of i s o p r o p y l a l c o h o l i n t h e r e d u c t i v e amination of acetone. The introduction of t i n t e t r a a l k y l s i n t o t h e c a t a l y s t res u l t e d i n an i n c r e a s e of t h e f o r m a t i o n o f d i i s o p r o p y l a m i n e and s l i g h t decrease of t h e formation of alcohol. Upon u s i n g t i n dibenzyl dichloride f o r the modification o f the c a t a l y s t s t r o n g supp r e s s i o n of t h e s e l e c t i v i t y towards isopropanol could be achieved. X-Ray d i f f r a c t i o n r e v e a l e d t h e p r e s e n c e o f A 1 3 N i 2 a n d A l N i a l l o y s , m e t a l l i c N i , N i O , Al(OH)3 and A l O [ O H I i n t h e h e a t t r e a t e d c a t a l y s t , t h e l a t t e r c o u l d be r e s p o n s i b l e f o r t h e entidii~.~cl s e l e c t i v i t y t o w a r d s t h e f o r m a t i o n o f s e c o n d a r y a m i n e a t 190-2OOOC.

INTRODUCTION Lower a l i p h a t i c a m i n e s a r e w i d e l y u s e d a s i n t e r m e d i a t e s f o r t h e synthesis of herbicides,

i n s e c t i c i d e s a n d d r u g s or c a n b e a p p l i e d

a s r u b b e r a c c t ~ l e r a t o r s ,c o r r o s i o n i n h i b i t o r s , etc.

[I].

surface active agents

T h e most w i d e s p r e a d method f o r t h e p r e p a r a t i o n o f l o w e r

a l i p h a t i c a m i n e s i n v o l v e s t h e r e a c t i o n o f ammonia w i t h a n a l c o h o l

o r a c a r b o n y l c o m p o u n d i n t h e p r e s e n c e o f h y d r o g e n . T h e m o s t common c a t a l y s t s u s e d f o r r e d u c t i v e a m i n a t i o n o f a l c o h o l s , a l d e h y d e s and k e t o n e s c o n t a i n n i c k e l , component 11-31.

platinum, palladium or copper as a c t i v e

One o f t h e m o s t i m p o r t a n t i s s u e s i n t h e r e d u c t i v e

amination is t h e s e l e c t i v i t y c o n t r o l a s t h e product d i s t r i b u t i o n , i.e.

the ratio

o f primary t o secondary

o r tertiary amines, is

s t r o n g l y a f f e c t e d by t h e r m o d y n a m i c s . I n t h i s work r e s u l t s o b t a i n e d i n a c a s e s t u d y , i . e .

the reduct-

i v e a m i n a t i o n o f a c e t o n e on a s k e l e t a l n i c k e l c a t a l y s t t o i s o p r o p y l a m i n e a n d d i i s o p r o p y l a m i n e w i l l b e g i v e n a n d d i s c u s s e d . Keact i o n s involved i n the r e d u c t i v e amination of a c e t o n e a r e g i v e n i n

*On l e a v e f r o m U e p a r t m e n t o f C h e m i c a l T e c h n o l o g y , Faculty of Chemistry, U n i v e r s i t y of Maria Curie-Sklodowska, 20-021 L u b l i n , Poland

336 Scheme 1.

+H 2

+NH3,

+HZ

(CH312CH-Otl

-H20

+H2

-

(CH.3 )' -2C = O

(ClH3)2C=NH

[Cli

+(CH3I2C=n, -H20 ( C H 3 1 2 C H - N H 2 +(CH3I2C=NH, - N H 3

+HZ

).C=N-CH(CH3)2

3.2

+I I

(CH312 CH-NH-CH(CH312 Scheme 1 .

I t i s known t h a t u p o n d i s t i l l a t i o n o f t h e r e a c t i o n m i x t u r e o f t h e r e d u c t i v e a m i n a t i o n o f a c e t o n e d i i s o p r o p y l a m i n e and i s o p r o p a n o l forms ficult.

i 7 ~ 1 1 t r om p~ aking t h e separation extremely d i f -

a binary

Therefore,

t h e g o a l o f t h i s w o r k was t o f i n d t h e modes a n d

ways f o r t h e s u p p r e s s i o n o f t h e f o r m a t i o n o f i s o p r o p a n o l t i v e poisoning the

shclital

nickel catalyst

via selec-

by a second m e t a l such

as t i n . I m p r e g n a t i o n w i t h t h e s o l u t i o n o f t h e compound o f a p o i s o n i n g element

[Sn,

Pb,

Bi,

P etc.)

t o poison n i c k e l catalysts.

i s c o n s i d e r e d a s t h e m o s t g e n e r a l way However,

upon u s i n g c o n v e n t i o n a l i m -

p r e g n a t i o n t e c h n i q u e s t h e o p t i m a l s e l e c t i v e p o i s o n i n g o f t h e supp o r t e d o r s k e l e t a l c a t a l y s t s c a n n o t b e g u a r a n t e i i l ~ 4[ 1 . I n t h i s w o r k t i n o r g a n i c compounds w i t h g e n e r a l f o r m u l a o f SnRnC14-n

[R =

dhyl

o r b e n z y l l were used f o r t h e s e l e c t i v e p o i -

soning o f the s k e l e t a l n i c k e l catalyst.

A c c o r d i n g t o o u r a, p r o a c h

t h e s e l e c t i v e p o i s o n i n g o f a Group VIII m e t a l can b e c o n s i d e r e d a s dn a n c h o r i n g p r o c e s s

a r e used t o

i n w h i c h C o n t r o l l e d S u r f a c e R e a c t i o n s (CSRs)

i n t r o d u c e t h e p r e c u r s o r o f t h e second m e t a l ( S n l

the surface o f the f i r s t

I:II~L~

one 151.

SURFACE CHEMISTRY Kecently,

we h a v e d e m o n s t r a t e d t h a t h y d r o g e n p r e a d s o r b e d o n p l a -

t i n u m o r n i c k e l can r e a c t w i t h t i n o r l e a d t e t r a d l k y l

compounds r e -

sulting i n the formation o f bimetallic surface e n t i t i e s w i t h metali n t e r a c t i o n [5-81.

-metal

CSRs l e a d i n g t o t h e f o r m a t i o n o f b i m e t a l -

l i c s u r f a c e s p e c i e s can b e w r i t t e n as f o l l o w s : xNitl

+

SnRnC14-n

-

solvent

Ni -SnR

-

(I1

(n-xIR1-l

+

- xC

1

+

xRkI

(1)

(4-n)HCl

(21

20 - 5OOC

Nix-SnKn-xC14-n

Nix-Sn

+

A

I n reaction

( 1 ) hydrugen adsurbed on n i c k e l r e a c t s s e l e ~ l i v e l y

337

w i t h t h e t i n o r g a n i c c o m p o u n d r e s u l t i n g i n a P r i m a r y S u r f a c e Comp lex (PSC)

( I ) , which can be decomposed i n hydrogen a t m o s p h e r e i n

t h e t e m p e r a t u r e i n t e r v a l b e t w e e n 50-3OO0C ( r e a c t i o n (2)I . EXPERIMENTAL C a t a l y s t p r e p a r a t i o n and m o d i f i c a t i o n Granular skeletal nickel catalyst w i t h p a r t i c l e s i z e of d mm w a s p r e p a r e d by l e a c h i n g a N i - A 1

=

3-5

a l l o y c o n t a i n i n g 50 w t % n i c k e l .

H a l f o f t h e a m o u n t o f a l u m i n i u m was l e a c h e d o u t w i t h 3 w t % N a O H w a t e r s o l u t i o n a t 50'C

f o r 12 hours. A f t e r leaching tbe c a t a l y s t

was washed w i t h d i s t i l l e d w a t e r a n d was k e p t u n d e r a n a q u e o u s s o l u t i o n h a v i n g pH

=

9.

Prior t o t h e m o d i f i c a t i o n w i t h t i n t h e c a t a l y s t w a s d r i e d i n f l o w i n g n i t r o g e n a t 120°C f o r 4 h o u r s . A f t e r d r y i n g t h e c a t a l y s t w a s t r e a t e d i n h y d r o g e n a t 200 or 3OO0C f o r 2 h o u r s f o l l o w e d b y c o o l i n g t o room t e m p e r a t u r e i n h y d r o g e n . t a l y s t w i t h t i n compounds, i . e . a t 50°C vent.

The modification of the ca-

surface reaction

( 1 ) was c a r r i e d out

u s i n g 2 0 g o f g r a n u l a r s a m p l e a n d 100 cm3 o f b e n z e n e s o l -

T h e c o n c e n t r a t i o n o f t i n c o m p o u n d s was

varied. Reaction

( I ) w a s m o n i t o r e d by g a s v o l u m e t r y a n d G C a n a l y s i s a s d e s c r i b e d e l s e w h e r e [ 6 ] . U e c o m p o s i t i o n o f PSC (11, i . e .

surface r e a c t i o n

(2

was p e r f o r m e d i n hydrogen i n t h e c a t a l y t i c r e a c t o r p r i o r t o t h e a c t i v i t y test u s i n g a h e a t i n g rate of Z°C/min

and a f i n a l tempera

t u r e o f Z5OoC. Catalyst characterization T h e n i c k e l a n d a l u m i n i u m c o n t e n t s o f t h e c a t a l y s t s were d e t e r m i -

ned by a t o m i c a b s o r p t i o n s p e c t r o s c o p y (AAS). T i n a n d c h l o r i n e c o n -

t e n t s o f t h e m o d i f i e d c a t a l y s t s g i v e n i n T a b l e 2 were d e t e r m i n e d by AAS a n d c h e m i c a l a n a l y s i s , r e s p e c t i v e l y . T h e p h a s e c o m p o s i t i o n o f c a t a l y s t s was s t u d i e d by X - r a y d i f f r a c -

tion

[ X R O ) t e c h n i q u e . X R O s p e c t r a were r e c o r d e d by u s i n g a P h i l l i p s

1700 powder d i f f r a c t o m e t e r equipped c h r o m a t o r a n d CuK,

w i t h a g r a p h i t e c r y s t a l mono-

radiation.

Catalytic reaction A s t a i n l e s s s t e e l f l o w r e a c t o r c h a r g e d w i t h 20 g c a t a l y s t was

used t o study t h e r e d u c t i v e amination o f a c e t o n e ( A C ) .

Further

d e t a i l s on e x p e r i m e n t a l c o n d i t i o n s c a n b e f o u n d i n T a b l e 2 . R e a c t i o n p r o d u c t s w e r e a n a l y s e d by g a s c h r o m a t o g r a p h y u s i n g a g l a s s column f i l l e d w i t h 18 w t % Carbowax 2000

+

5 w t % KOH o n C h r o m o s o r b P

338

NAW s u p p o r t . T h e f o l l o w i n g r e a c t i o n p r o d u c t s were a n a l y s e d : i - p r o pylamine (IPA), di-i-propylamine amine [ I P P A ) ,

i-propylalcohol

(DIPA), i - p r o p y l i d e n e - i - p r o p y l -

(IPALI.

KESULl'J ANU UISLUSSlUN

P r e p a r a l i r i n arid c h a r a c t e r i ~ a t i o no f t h e s k e l e t a l n i c k e l c a t a l y s t s

I t i s known t h a t s t a r t i n g a l l o y s o f s k e l e t a l n i c k e l c a t a l y s t s a r e u s u a l l y a m i x t u r e of A 1 3 N i , 19-11].

A13Ni2,

A l h i and e u t e c t i r . phdscs

D e p e n d i n g on t h e c o n d i t i o n s o f l e a c h i n g t h e c a t a l y s t s c o n -

g e n e r a l l y of m e t a l l i c n i c k e l , s m a l l a m o u n t o f N i O anti been found t h a t t h e

IIIUIL

UI

r e s i d u a l aluminium, A13r'li2

lks5 h y d r a t e d a l u m i n a [ 9 , I 1

1. It

alloy, has

lower t h e c o n c e n t r a t i o n o f a l k a l i n e s o l u t i o n

a n d t h e lower t h e t e m p e r a t u r e o f l e a c h i n g . t h e h i g h e r a r t ! t h e a a l l o y a n d A1(OHI3 p h a s e s r e m a i n i n g i n t h e c a t a l y s t mounts of A 1 N i 3 2 [ 9 - 1 1 ] . The c r y s t a l l i n i t i y and t h e e x t e n t o f h y d r a t i o n o f a l u m i n a p h a s e s c a n b e a l t e r e d by t h e r m a l t r e a t m e n t of t h e s k e l e t a l n i L k e l catalyst.

I t has a l s o been observed t h a t hydrated alumina can sup-

pressed the s i n t e r i n g of the c a t a l y s t s [12,

131.

A s t h e a i m of t h e p r e s e n t work was t o p r e p a r e a s k e l e t a l n i c h e 1

c a t a l y s t s t a b l e a t high temperature i n the reductive amination o f a c e t o n e , d i l u t e d NaOH s o l u t i o n w a s u s e d f o r l e a c h i n g o f t h e s t a r t i n g a l l o y . B a s e d on l i t e r a t u r e d a t a i t was e x p e c t e d t h a t t h e leached o u t c a t a l y s t would conkdin s u f f i c i e n t amount o f a l u m i n a p h a s e s f o r s t a b i l i z a t i o n of t h e c a t a l y s t . Indeed, t h e chemical a n a l y s i s o f t h e u n m o d i f i e d c a t a l y s t by AAS c o n f i r m e d t h a t o n l y a b o u t h a l f o f t h e a l u m i n i u m was l e a c h e d o u t u s i n g d i l u t e d NaOli s o l u t i o n . T h e A 1 a n d t h e N i c o n t e n t o f t h e c a t a l y s t w a s 2 2 a n d 54 w t % , r e s p e c t i v e l y . T h e presence of metallic A 1 i n s o l i d solu t i o n w i t h nickel. A1-Ni

residual

a l l o y s a n d h y d r a t e d a l u m i n a p h a s e s c o u l d a c c o u n t f o r t h e high

A 1 content of the catalyst.

I n t h e X R D s p e ~ t r u m( n o t s h o w n ) o f t h e

fresh catalyst lines characteristic of A13Ni2

a l l o y and A l ( O I i 1

p l i d s e s i n Ltie f o r m s o f D a y e r i t e a n d G i b b s i t e h a v e a p p e a r e d .

sencc o f A13Ni2

Thepre-

a l l o y i n t h e c a t a l y s t can b e e x p l a i n e d by t h e f a c t

t h a t t h i s a l l o y i s much m o r e p a s s i v e t h a n o t h e r s a n d r e a c t s v e r y s l o w l y i n d i l u t e d a l k a l i n e s o l u t i o n a t 50°L Al(Oli)

[lo].

The presence of

p h a s e s i n t h e c a t a l y s t can b e a t t r i b u t e d t o t h e h y d r o l y s i s

3 of a l u m i n a t e s formed i n t h e l e a c h i n g p r o c e s s [ 9 ] .

XRD d a t a g i v e n i n

T a b l e 1 i n d i c a t e t h e p r e s e n c e o f m e t a l l i c Ni, A l 3 N i 2 .

AlNi.

A1(011)3

a n d AlO(0H) p h a s e s i n t h e t h e r m a l l y t r e a t e d a c t i v i t y t e s t e d unmodi f i e d c a t a l y s t . The p r e s e n c e o f A l U l U H )

i n t h e c a t a l y s t can b e e x -

p l a i n e d by p a r t i a l d e c o m p o s i t i o n o f A l ( O H 1 3 p h a s e s d u r i n g

protmit-

339 TABLE 1 XRD d a t a o f u s e d u n m o d i f i e d s k e l e t a l n i c k e l c a t a l y s t a

I/Io

d,8

I/Io

6.13

27

2.35

15

AlO(0H)

A l ( O H 1 3(B1

Phases

d,a

Phases

4.85

57

2.22

15

4.73

29

2.10

4

4.3Y

29

2.05

92

A13Ni2,

AlNi

3.50

0

2.04

92

A13Ni2,

N i

3.35

5

2.02

100

A13Ni2

3.17

21

1.92

5

A13Ni2

2. 86

3

1.77

24

2.45

6

1.45

9

AlNi,

i. 39

10

1.42

9

A13Ni2

NiO

N i NiO

a A f t e r t r e a t m e n t i n h y d r o g e n a t 25OoC. B = Bayerite,

G = Gibbsite

m e n t i n h y d r o g e n a t 25OoC. Modification of the s k e l e t a l nickel c a t a l y s t with t i n I n t h i s series o f experiments t h e r e a c t i v i t y o f hydrogen c h e m i s o r b e d on g r a n u l a r s k e l e t a l n i c k e l c a t a l y s t t o w a r d s d i f f e r e n t t i n a l k y l compounds, i . e . =

e t h y l , Bu

=

S n E t 4 , SnBu4, SnEt2C12 a n d SnBz2C12 ( E t

b u t y l , Bz

=

=

b e n z y l ) h a s b e e n s t u d i e d . Upon m o d i

-

f y i n g t h e s k e l e t a l n i c k e l c a t a l y s t w i t h d i f f e r e n t t i n a l k y l s surface reaction

( 1 1 a p p e a r e d t o be v e r y s e l e c t i v e . Only s a t u r a t e d

h y d r o c a r b o n s c o r r e s p o n d i n g t o t h e a l k y l g r o u p o f t i n p r e c u r s o r compound h a s b e e n f o r m e d . D e t a i l s on s u r f a c e r e a c t i o n s i n v o l v e d i n t h e m o d i f i c a t i o n of t h e c a t a l y s t w i l l be g i v e n elsewhere [151. t.itlciu c t

i v e arni n d t i o n o f a c e t o n e I n the r e d u c t i v e amination of a c e t o n e c a r r i e d o u t i n t h e tempe-

r a t u r e r a n g e o f 169-21OoC t h e f o c u s w a s l a i d on t h e s e l e c t i v i t y changes a t high conversion l e v e l .

Experimental conditions, charac-

t e r i s t i c f e a t u r e s o f c a t a l y s t s and c o r r e s p o n d i n g a c t i v i t y and sel e c t i v i t y data obtained i n t h e reductive amination of acetone a r e given i n Table 2. It is noteworthy t h a t i n o r d e r t o d e c r e a s e the f o r m a t i o n o f i s o p r o p y l a l c o h o l b y - p r o d u c t a r e l a t i v e l y low h y d r o g e n / /ammonia m o l a r r a t i o ( H 2 / N H 3

=

0.5)

h a s t o b e c h o s e n . As s e e n i n

340

F i g . 1 . R e l a t i o n s h i p b e t w e e n s e l e c t i v i t i e s a n d H,/NH3 molar r a t i o i n t h e r e d u c t i v e a m i n a t i o n o f a c e t o n e o n u r i ~ i i u u l~tt d s k t i ~ . L d l r ~ i t : h 1 L a t a l y s t [T = 1 1 I " I , P = 0.0 MPa, WHSV = 0 . 0 h K 1 , N H 3 / A C = 2 1 .

Fig.

I , t h e s e l e c t i v i t i e s t o w a r d s aniines and i s o p r o p a n o l can b e

s t r - u r i t . 1 ~ a l t e r e d by li2/Nli3

c h a n g i n g t h c H /NH3 r a t i o . The h i g h e r the 2 r a t i o , t h e higher is the s e l e c t i v i t y towards isopropanol.

U a t a g i v u r i i n T a b l e 2. i n d i c a t e d t h a t u p o n i n c r e a s i n e t h e r c a c t i o n ternperaturc i n the reductive amination of acetone the s e l e c t i v i t y t1;wdrds

D I P A e s p e c i a l l y o n tiri m o d i f i e d c a t a l y s t s s i g n i f i -

caritly increased, whereas s e l e c t i v i t i e s towards o t h e r products dec r e a s e d i n d i f f e r e n t e x t e n t . S u c h an i n c r e a s e i n t h e s e l e c t i v i t y o f f o r m a t i o n o f UIF'A u p o n i n c r e a s i n g t h e r e a c t i o n t e m p e r a t u r e c a n n o t be e x p l a i n e d by thermodynamics 1 1 4 1 , and i t h a s n o t been o b s e r v e d

it is s u g g e s t e d t h a t t h e e r ~ l i a r i c e ds e l e c t i v i t y t o w a r d s U I P A o b t a i n e d a t on c o n v e n t i o n a l

1YU-?I3UUC

Cdfl

skeletal nickel catdlysts [ 2 , 1 4 ] . Therefore,

b e d L L r i b u t e d t o t h e h i g h AlO(0H) c o n t e n t o f t.tiF: cc3-

t a l y s t 1 2 1 . However,

the mechanism of t h e f o r m a t i o n o f t h e second-

a r y arriinil r e q u i r e s f u r t h e r e l u c i d a t i o n . A s seen from t h e d a t a g i v e n i n T a b l e 2 t h e s e l e c t i v i t y o f t h e

unmodified n i c k e l c a t a l y s t towards t h e formation o f isopropanol is higher than /

:.

Upun m o d i f y i n g t h e c a t a l y s t w i t h t i n ,

r e l a t i v e l y s m a l l amount o-f t i n i n t r o d u c e d ,

despite the

t h e amount o-f i s o p r o p -

a n u l formed d e c r e a s e d and s i g n i f i c a n t s e l e c t i v i t y c h a n g e s were obdecrease i n the activity. tin from t i n t e t r a a l k y l s t h e s e l e c t i v i t y t o -

s e r v e d w i t h o u t noticeable lJpon i n t r o d u c i n g

wards t h e furrnation o f secondary amine s i g n i f i c a n t l y i n c r e a s e d a t

341 Table 2 R e d u c t i v e a m i n a t i o n o f a c e t o n e on s k e l e t a l n i c k e l c a t a l y s t s a

'

Sn c1 wt% wt% oc

Catalyst

0

0

n. 082 0 N i -SnE t4- b

0.13

Ni-SnBu4-lC

0.025

bi-SnBua-2"

0.076

Ni-SnEt2C12- i t'

0.12

0.07

0.27

0.14

0.30

n. 17

170 190 172 192 200 171 192 170 191 169 193 200 190 1goe 210e 170 190 171 190 202

Conversion

0 98.6 99.3 59.1 99.2 99.5 58.1 Yii.4 98.0 58.5 98.2 98.5 99.0 98.2 98.9 99.0 95.6 96.9 93.4 95.0 96.2

se

1 e c t i v i t i e s,+ % IPA OIPA IPPA IPAL

85.1 83.' 78.1 68.2 64.7 75.4 66.3 83.6 74.7 84.5 70.9 68.0

82.11 85.3 71.6 83.3 84.7 86.4 87.0 85.4

3.6 8.6 11.6 24. ,, 27.5 14.5 26.6 7.e 20.6

5.2 20.7 25.4 12.6 9.5 23.2 7.0 9.4 3.4

8.8 12.1

1.0 0.5 3.4 1.7 1.4 4.6 2.3 1.0 n.6 5.0

2.9 1.9 1.4 0.6 0.6 6.9 3.4 10.2 4.2 2.5

10.3 7.3 5.9 6.0 6.0 5.5 4.8 7.5 4.0 5.3 5.5 4.7 4.0 4.2 4.6 2.8 2.5
P = 0 . 0 MPA, WHSV = 0 . 8 h - ' I , m o l a r r a t i o H2:NH3:AC = 2 : 4 : 1 and T h e p r e t r e a t m e n t o f t h e c a t a l y s t i n h y d r o g e n was c a r r i e d o u t a t 3 0 0 a n d 200°C, r e s p e c t i v e l y . I n s t e a d o f benzene a c e t o n e was used a s s o l v e n t i n r e a c t i o n ( I ) . P = 1 . 4 MPa S e l e c t i v i t y d a t a w e r e c a l c u l a t e d on t h e b a s e o f t h e c o n t e n t O F CH - C H - C H moieties i n t h e reaction products. 3 3 t h e expense of t h e primary one. T h e decrease of t h e s e l e c t i v i t y of

t h e f o r m a t i o n o f i s o p r o p a n o l was o n l y s m a l l on c a t a l y s t s p r e p a r e d by u s i n g t i n t e t r a a l k y l s a s p r e c u r s o r c o m p o u n d s . T h e i n c r e a s e o f

t h e t i n c o n t e n t o f c a t a l y s t (see c a t a l y s t s Ni-SnEL4-l,-2

drld r d i -11Lu4-

,

- I

r e s u l t e d i n only s l i g h t change i n the s e l e c t i v i t y p a t t e r n . A slight i n c r e a s e i n t h e f o r m a t i o n of S c h i f f b a s e (IPPA) was a l s o o b s e r v e d on t i n m o d i f i e d c a t a l y s t s . T h i s i n d i c a t e d t h a t s i t e s r e s p o n s i b l e

f o r t h e hydrogenation of t h e carbonyl group of acetone'and

t h e dou-

IPPA have been s l i g h t l y p o i s o n e d .

b l e bond 0 1

T h e s e l e c t i v i t y p a t t e r n o f c o t a l y s t s p r e p a r e d by u s i n g S n E t 2 C l 2 showed t h a t t h e I P A / O I P A

r a t i o h a s i n c r e a s e d compared t o t h a t ob-

t a i n e d on c a t a l y s t s m o d i f i e d by t i n t e t r a a l k y l s .

Idi-;ntt

L1

.'

On c a t a l y s t s

t h e s e l e c t i v i t y o f t h e f o r m a t i o n o f i s o p r o p a n o l tdecrtdsei!

. U p o n i n t r o d u c i n g t i n f r o m S n B z2 C 12 t h e a c t i v i t y o f the s k e l e t a l nickel c a t a l y s t s l i g h t l y decrLased. On t h i s i r o J i f i w l

Lo

-

,.I

342

catalyst the IPA/DIPA ratio was c l o s e to the value obtained on the unmodified one. This catalyst showed the lowest selectivity value f o r the formation o f isopropanol. It cannot be excluded that in Ni-SnEtZC12 and Ni-SnBzZClZ catalysts the mutual effect of chlorine and tin or the stabilization of tin in its ionic form i s r e sponsible f o r the suppression of the selectivity towards isopropanol i n the reductive amination of acetone. CONC LUS ION It has been demonstrated that skeletal nickel catalysts can b e modified with tin by using CSRs taking place between tin alkylsand hydrogen adsorbed on nickel. Upon applying this type of modification the selectivity pattern of the catalysts i n the reductive amination of acetone can be tailored. Selective poisoning of sites responsible f o r the formation of isopropanol could be achieved by using tin dibenzyl (or diethyll dichloride a s tin precursor cornpound. REFERENCES 1

2 3 4

5 6

7 8

Y 10 11

12 13 14 15

E.A.Schweizer, R.L.Fowlkes, J.H.McMakin and T.E.Whyte, Jr. (Eds.). Kirk-Othmer Encyclopedia o f Chemistry and Technology, Vol. 2, Wiley, New York, 1987, pp. 272-283. M.V.Klyuev and M.L.Khideke1, Russ.Chem.Rev., 49 (1980) 14. A.Baiker and J.Kijenski, Catal.Rev.Sci.Eng., 27 (19051 653. J.Petrd in Z.G.Szabd and D.Kalld (Eds.1 ContaLt Catalysis, Vo1.2, Elsevier, Amsterdam, 1976. pp. 65-75. J.Margitfalvi, M.Hegedijs, S .G Clbolos , E.Kcrn-Tdlas, P.Szedlacsek, S.Szabb and F.Nagy, Proc. 8 t h International Congress on Catalysis, Vol. IV. Verlag Chemie, Weinheim, 1984, pp. 903-914. E.Kern-Tdlas, M.Hegedijs, S. GClbGl6s. P.Szedlacsek and J . Margitfalvi, in B.Oelmon, P.Grange and G.Poncelet (Editors) Preparation of Catalysts IV. Stud.Surf.Sci.Catal., Vol. 31, Elsevier, Amsterdam, 1 9 8 7 , pp. 689-700. J.Margitfalvi, E.Tdlas and S.GBb6lijs, Catal. Today, 6 (19791 73. J.Margitfalvi, S . G d b i j l o s , M.Hegedds and E.Tdlas, in M.Guisnet e t al. (Editors) Heterogeneous Catalysis and Fine Chemicals I, Stud.Surf.Sci.Cata1.. Vol. 41, Elsevier, Amsterdam, 1 9 8 8 , p p . 145-152. P.Fouilloux, App.Cata1.. 8 (19831 1. J.Free1. M.3.W.Piete1-s and R.B.Anderson, J.Catal., 14 (1969) 247 * S.Sane, J.M.Bonnier, J.P.Damon and J.Masson, Appl.Catal., 9 (1984) 69. A.I.Savelov and A.B.Fasman, Kinet.Katal., 1 5 (1974) 994. A.I.Savelov and A.B.Fasrnan, Zh.Fiz.Khim., 56 (1982) 1 8 3 8 . J.Pasek. P.Richter, J.Volf, M.Matous and P.Kondelik, Chem. Prurnysl. 21/46 (1971) 491. E . T d l a s and J.Margitfalvi, submitted f o r publiLation.

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

343

SYNTHESIS O F DIMETHYLALKYLAMINES FROM ACIDS AND ESTERS OVER PROMOTED COPPER CATALYSTS.

J. BARRAULT, G. DELAHAY, N. ESSAYEM, Z. GAIZI, C. FORQUYl and R. BROUARD~ Laboratoire de Catalyse en Chimie organique, URA CNRS 350, 40, Av. du Recteur Pineau, 86022 Poitiers Cedex, France. ELF AQUITAINE, Groupement de Recherches de Lacq, 64170 Artix, France. CECA-ATOCHEM, 95, Rue Danton, 92303 Levallois-Perret, France. ABSTRACT The one-step synthesis of N-dimethyl dodecylamine from dodecanoic acid (or methyl dodecanoate), ammonia, hydrogen and methanol is investigated on supported copper-chromium catalysts. The selectivity in RNHCH and RN(CH3)2 is greatly enhanced when i) these catalysts are promoted with Ca or A n ii) the activation process is well chosen. Moreover it is also observed that a large excess of hydrogen or and methanol to obtain a significant selectivity with unprogramed catalysts. e first physic0 is c aracterizations seem to indicate that the promoters increase the stability of the chemical catalyst with regard to ammonia and water.

d

neceSSaX

INTRODUCTION Copper-catalysts promoted with i) other group VIA or VIIIA metals and ii) alcaline or alcaline earth elements (IA or IIA) are used for selective hydrogenation of various organic compounds (1). Moreover Cu(Co) Zn-Al catalysts were extensively studied for the synthesis of methanol and of light alcohols (2,3). More recently, due to the development of fine chemical processes, detailed studies of copper catalysts were carried out in order to show, like for noble metals, the effect of supports (SMSI), of promoters and of activation ... on metal dispersion or reduction, on alloy formation ... For example modified copper catalysts are known for their utilization in the dehydrogenation of esters (4-6), in the hydrolysis of nitriles (7), in the selective hydrogenation of nitriles (8), in the amination of alcohols (9)... In our laboratory we have shown that such catalysts could also be used in a one-step amination of esters into primary amines (10). Proceding in this work we investigated the possibility of a one-step synthesis of methyl-alkylamines starting from esters or acids with ammonia, methanol, hydrogen : CllH23COOCH3

+ NH3 + H2 + CH30H->

C12H25NHCH3 + H20

or C11H23COOH

C12H25N(CH3)2

(1)

344

METHODS * The amination of esters or acids was carried out in a dynamic fixed-bed reactor under pressure (3-8 MPa) described elsewhere (11). * TPR, N20 decomposition, H2 adsorption, TPD experiments were done with a pulse chromatographic reactor. * Catalysts were prepared by i) coimpregnation of copper and chromium salts on the supports or ii) coprecipitation of copper and chromium hydroxides in the presence of the support. A1203 GFSC from RhBne Poulenc and T i 0 2 P25 from DEGUSSA were used as supports. Additives such as calcium or manganese were incorporated by further impregnation. The powders were then dried in a oven at 393°K for 12h and calcinated in air at diffrent temperatures before being reduced "in situ" with hydrogen at 623°K for 10h. RESULTS AND DISCUSSION 1) Amination of methyl dodecanoate or dodecanoic acid in the presence of supported Cu-Cr catalysts. TABLE 1 Amination and N methylation of methyl dodecanoate or dodecanoic acid, hydrogenation and N methylation of dodecylnitrile, N methylation of dodecylamine in presence of copperchromium catalysts. P = SO bars, T = 250"C, (LHSV)reagent = 1/6 h-' ; Reagent : NH3 : CH30H : H2 = 1 : 10 : 40 : 100 (a) Reagent : NH3 : CH30H : H2 : 1 : 10 : 40 : 400. REAGENT

SELECTIVITY (%)

CONVERSION (7%)

RNH2 RNHCH3 RN(CH3)2

ESTER CllH23COOCHg

Cr20

R2NH R2NCH3

R3N othcrs

Ti02

Catalyst

C1120

97.5 (a) 90.0

31.9 5.2

30.7 16.5

20.0 61.0

2.5 1.5

2.0 3.5

2.0 2.0

10.7 9.9

2.0

12.9

78.0

2.7

2.9

1.5

-

7.3

83.3

0.8

0.9

1.9

5.X

NITRILE C1 H23C=N

1(M

AMINE C11H23CH2NH2

100 Calaly.rt

CuIO

CrlO

AIZOj

84

12.3

30.4

36.6

ESTER C11H23COOCH3

2.6

18.3

345

Table 1 shows that methyl dodecanoate is easily converted into amine in the presence of CuCr deposited on alumina or on titania. Nevertheless one can observe that the methylation reaction is rather difficult and favoured by alumina. Moreover, a significant increase of N-dimethyl dodecylamine is obtained when the reaction is carried out with a large excess of hydrogen. Due to the mechanism of the reaction this is unexpected : indeed it is generally considered that the methylation of primary amine with methanol requires i) the dehydrogenation of alcohol into a carbonyl compound and ii) a further reaction of this compound with primary amine or secondary amine via imine or enamine intermediates. (i) C1 lH23COOCH3 t NH3

+ H2 -> C12H25NH2 t H 2 0

4 2 (ii) C12H25NH2 + (CH30H .$HCHO]

> C12H25NHCH3 -

+ H 2 0 ------>

In order to explain this hydrogen effect it can be supposed that i) the hydrogen coverage in normal conditions is not sufficient to maintain the catalyst in the adequate reduced state, ii) the excess of hydrogen inhibits the formation of carbonaceous deposits (and the modification of the catalyst) or the strong adsorption of some reagents and products ... In an effort to understand better the catalytic chemistry associated with this reaction, the reactivity of dodecylnitrile or dodecylamine was measured under the same experimental conditions. The results listed in Table 1 show that the nitrile and the primary amine are much more easily transformed into N-dimethylalkylamine than the ester or the acid (Table 2). The rate determining step in the methylation process is directly related to one of the first reactions converting the ester or the acid into nitrile. It can be assumed that: 1) The adsorption of the reagent is not quite effective on the catalyst or/and ii) the water formed during the reaction could lead to a superficial (or a bulk) modification of the catalyst and of the adsorption properties of some of the reagents.

346

TABLE 2 Influence of promoters in the amination of dodecanoic acid. P = 50 bars, T = 300"C, (LHSV),,id = 1/6 h-', Acid : NH3 : CH30H : H2 = 1 : 10 : 40 : 100 (ester) Catalyst

Selectivity (%)

Acid or estcr Conversion (%) RNH2

RNHCH3

RN(CH3)2

RCOOMe

others

Reaction : acid/NH3/CH30H/H2

I

CuO 43-Cr203 39

100

21.2

26.0

37.1

7 .0

8.7

Cu15 Crl5-AI2O3

100

20.0

32.4

26.0

8.0

13.6

Cu15Cr15 Ca2-AI-203

100

8.4

24.0

67.0

0.6

Cu15Cr15 Mn2-AI2O3

100

16.0

36.0

46.0

2.0

Cu15Cr15 Ca2-Ti02

100

27.0

31.0

35.6

Unsupported catalyst

2.1

4.3

Reaction : cstcr/NH3/CH30H/H2 Cu15Cr15 Ca2-AI2O3

100

10.0

20.8

65.7

3.5

2) Influence of (Ca or Mn) additives on the catalytic properties of CuCr/A1203 (Ti02) in the amination of dodecanoic acid : The effect of adding Ca or Mn to CuCr/Al203 (TiOZ) catalysts presented in Table 2 demonstrate that i) the selectivity in N-dimethyldodecylamine is much enhanced, the effect being rather more significant with alumina than with a titania support; ii) the total amine selectivity is particularly high, above 98% instead of 80% without promoter. A similar result also presented in Table 2 is obtained when the acid is replaced by methyl dodecanoate. 3) Effect of calcination pretreatment The influence of additives was investigated after changing the conditions of catalyst activation especially after modifying the calcination temperature. It can be seen in Table 3 that the selectivity varies very much with this activation step and also that the final result depends on the nature of the additive;

347

TABLE 3 Influence of calcination temperature on the catalytic properties of Cu-Cr/support catalysts in the amination of dodecanoic acid. P = 50 bars, T = 300"C, (LHSV),,id = 1/6 h-', Acid : NH3 : CH30H : H2 = 1 : 10 : 40 : 100

Catalyst

Selectivity (%)

Calcination temperature ("C) RNH2

Cu15 Cr15 Mn2/AI2O3

Cu15Cr15 Ca2/AI2O3

Cu15Cr15 Ca.-JTi02

RNHCH3

RN(CH3)2

RCOOCH3

others

120

9.0

23.0

66.5

1.5

380

16.0

36.0

46.0

2.0

120

10.0

28.0

54.0

330

8.4

24.0

67.0

120

21.0

48.0

27.0

1.0

3.0

330

27.0

31.0

35.6

2.1

4.3

3.2

5.0 0.6

* When the catalyst is promoted with manganese, an increase of calcination temperature from 120°C to 380°C (followed by the reduction step at 350°C) decreases the selectivity in methylated products. Moreover figure l a shows a rapid decrease, with time on stream, of the selectivity into N-dimethyldodecylamine. * Contrary to the previous situation when the catalyst is promoted with calcium, the selectivity into the desired product increases with the calcination temperature and there is no significant change of selectivity with reaction time (figure lb) if there is an increase of RN(CH3)z in the first hours of the reaction.

348

Fig1 Influence of calcination temperature on the catalytic properties of a) CulSCrlS Mn2A1203 ; b) Cu15Cr15 Ca2-Al203 catalysts in the amination of dodecanoic acid. (-_-_---_) calcinated at 120°C ) calcinated at 380°C (a) or 330°C (b). (

4)Catalyst characterization In table 4 the modifications of the reduction rate and of the adsorption properties after the addition of Ca or Mn are presented. If is evident from these results that the reducibility of the Cu-Cr-AI203 catalyst especially when promoted with manganese is reduced. Nevertheless the accessible copper surface and the hydrogen adsorption are not modified by additives; but the hydrogen storage, which appears from TPD measurment, is decreased by Ca and Mn. From the TPD curves, it appears that there are three hydrogen desorption steps at 130, 260 and 35OoC, the two last ones are preponderant with unpromoted catalysts while it is the contrary with promoted catalysts. Therefore the addition of Ca or Mn to Cu-Cr catalysts inhibits the adsorption of strongly bonded hydrogen. On the other hand these catalysts have been studied for other hydrogenation reactions and we have also observed a decrease of hydrogenation activity when Cu-Cr catalysts are modified with Ca or Mn (13).

349

TABLE 4 Influence of promoters on the reducibility and adsorption properties of Cu-Cr/Al203 catalysts. (a) The reduction is calculated in assuming that all Cu(I1) and Cr(VI) species are reduced into Cu(o) and Cr(II1) states. Catalyst Cu15-Cr15

Cu15-Cr15-Ca2

310-400

430

Cu15-Cr15-Mn2

Cu-Cr-X/Al203 TPR T m a i C'C) Reduction rate (%)

92

83

350-400 67

(a) Hydrogen adsorption pmo~eg - l catal.

3.4

2.7

4.1

Hydrogen TPD pmole g- 1 catal.

12.8

4.1

3.9

Copper area m2 g-1 catal.

2.6

2.0

2.0

Now the influence of water or ammonia on copper catalysts is being investigated. Previously A. BAIKER and coll. have shown that ammonia could modify the catalytic properties of copper catalysts used in the amination of alcohols (9). These authors noticed the formation of copper nitride after NH3 exposure at a temperature of about 300°C which is the reaction temperature of our study. The first results that we obtained in our study showed that both H 2 0 and NH3 decrease significantly the copper dispersion in unpromoted catalysts and that this modification is less significant when Ca or Mn are added to the Cu-Cr catalyst. We are now studying what are the superfical modifications consecutive to the addition of promoters or/and water and ammonia. 5) Conclusion To summarize, we demonstrated in this study that the addition of a small amount of calcium or manganese increases the rates of the amination of ester and of acid and the

350

N-methylation with methanol. These results can be obtained without increasing the partial hydrogen pressure as was observed for unpromoted catalysts. On the other hand we noticed that these compounds don't modify the metallic area but decrease the reducibility which means that copper oxide and chromium (VI) oxide are only partially reduced. Moreover as the highly adsorbed hydrogen is also inhibited and as these catalysts are more stable in the presence of H 2 0 or NH3 than unpromoted catalysts, one can also deduce that one of the important roles of the hydrogen during the reaction is to prevent the modification of catalysts or/and the amination reaction by ammonia and water. REFERENCES H. Adkins, "Reactions of hydrogen with organic compounds"; The university of 1 Winconsin Press, 1938,65. 2 J.C.J. Bart and R.P.A. Sneeden, Catal. Today, 1987,2, 1. a) P. Courty, D. Durand, E. Freund and A. Sugier; J. Mol. Catal. 1982, 17,241. 3 b) N. Mouaddib, Thesis, Lyon, 1989. H.W. Chen, J.M. White, J.G. Ekerdt, J. catal., 1986,99, 293. 4 J.W. Evans, M.S. Wainwright, N.W. Cant, D.L. Trimm, J. Catal. 1984,88, 203. 5 A.K. Agarwal, N.W. Cant, M.S. Wainwright, D.L. Trimm, J. Mol. Catal., 6 1987,43,79. J.C. Lee, D.L. Trimm, M.A. Kohler, M.S. Wainwright, N.W. Cant, Catal. Today, 7 1988,2, 643. J. Volf, J. Pasek, Studies in Surface Science and Catalysis 1987,27, 105 8 Ed. L. Cerveny. 9 A. Baiker, J. Kijenski, Catal. Rev. Sci. Eng., 1985,27-4, 653. J. Barrault, M. Seffen, C. Forquy, R. Brouard, "Heterogeneous Catalysis and Fine 10 Chemicals" in Stud. In Surf. Science and Catalysis, 1988,41,361. 11 M. Seffen, Thesis, Poitiers, 1986. 12 L. Jalowiecki, G. Wrobel, M. Daage, J.P. Bonnelle, J. Catal., 1987, 107,375. (and previous paper in Appl. Catal.). Z. Gaizi, Thesis, Poitiers, 1990. 13

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

351

TERTIARY AMINE PREPARATION BY REDUCTIVE ALKYLATION OF ALIPHATIC SECONDARY AMINES WITH KETONES R. E. MALZ, Jr.1 and

H. GREENFIELD2

SUMMARY This paper discusses the need for more stringent catalyst requirements for the reductive alkylation of secondary to tertiary amines. We illustrate the major importance of steric factors, with respect to both the amine and ketone and discuss the relative effectiveness of several catalysts. One obtains excellent yields with the more reactive and unhindered ketone, such as cyclohexanone and acetone, and relatively unhindered secondary amines.

INTRODUCTION We developed a process of preparative and potential commercial utility for the production of tertiary aliphatic amines by the reductive alkylation of dialkyl amines and of alicyclic secondary amines with ketones in the presence of hydrogen and a catalyst3. Such tertiary amines have at least one secondary alkyl group. The reductive alkylation of primary alkylamines with ketones is a well-known and useful method for the preparation of secondary amines4. Major side reactions are hydrogenation of the ketone to the alcohol and, at higher temperatures, condensation reactions of the ketones and of ketone-amine addition products. One may drastically reduce these side reactions by the proper choice of catalysts and conditions. There are many examples of the preparation of tertiary aliphatic amines by the reductive alkylation of dialkylamines or secondary non-aromatic heterocyclic amines with ketones using platinums-'3, palladium12-l7, mixtures of platinum and palladiuml*,and nickel12. 1 3 . 1 9 - 2 2 catalysts.

352

The literature reports yields which decrease with increasing size and complexity of the groups attached to the nitrogen atom of the amine and the carbonyl group of the ketone7.23. One sees much slower reductive alkylation of secondary amines with ketones to tertiary amines than the corresponding transformation of primary to secondary amines7. 2 4 . The increase in by-product formation is the result of the need for the more severe operating conditions, particularly higher temperatures. The reductive alkylation reaction consists of a sequence of steps in which the hydrogenation is preceded by chemical processes. For primary amines, one forms the alcoholamine, which could proceed on to the ketimine. Hydrogenation of either the alcoholamine or the ketimine produces the secondary amine product.

The secondary amine product probably is derived from the ketimine rather than by hydrogenolysis of the alcoholamine25.

353

Since ketimine formation is not possible in the reductive alkylation of secondary amines, this reaction must involve the hydrogenolysis of an alcoholamine. However, if either carbon a to the starting carbonyl has a hydrogen available, the enamine formation is possible.

This enamine can be reduced to the tertiary amine product.

Thus, two major differences between the reductive alkylation of primary and secondary amines are the increased steric hindrance in the latter case, and the fact that tertiary amine formation cannot proceed through a ketimine intermediate.

EXPERIMENTAL Dibutylamine, piperidine, N-ethylcyclohexylamine, N-ethyldicyclohexylamine, and the ketones were reagent grade chemicals. The 5% palladium on carbon, 5% platinum on carbon, sulfided 5% platinum on carbon and sulfided 5% rhodium on carbon catalysts were obtained from Engelhard Industries. The 2 0 % molybdenum sulfide on alumina (Girdler T-318) was obtained from the Chemetron Corp. Palladium chloride was obtained from Matheson, Coleman and Bell. Ruthenium trichloride was obtained from Ventron. A bulk ruthenium sulfide catalyst was prepared by bubbling

354

gaseous hydrogen sulfide for 1 h into a solution of 10.0 g of ruthenium trichloride hydrate (RuC1,.1-3 H,o) in 1 liter of distilled water. The black precipitate was filtered, washed with 2 liters of distilled water, then with 500 ml of 2-propanol, and then with 1 liter of cyclohexanone. A bulk palladium sulfide catalyst was prepared by substantially dissolving 10.0 g of palladium chloride dihydrate in 1 liter of 0 . 3 N hydrochloric acid with stirring and then bubbling in gaseous hydrogen sulfide for 0.2 h. The black precipitate was filtered, washed with 2 liters of distilled water, then with 5 0 0 ml of 2-propanol, and then with 500 ml of cyclohexanone. Example 1 N,N-Dibutylcyclohexylamine by reductive alkylation of dibutylamine with cyclohexanone. The results are shown in Table 1. A detailed description of one experiment illustrates the procedure. In all other experiments we list the starting materials, the autoclave and the experimental conditions. To a 1.7 liter autoclave were added 64.6 g ( 0 . 5 0 mole) of dibutylamine, 250 ml (ca. 2.4 mole) of cyclohexanone, and 3.5 g of 5% palladium on carbon. The autoclave was sealed, purged first with nitrogen and then with hydrogen, and hydrogen added to a pressure of 500 psig. The reaction mixture was heated with agitation for 4.3 h at 45-500 and 350-500 psig. The autoclave was cooled and depressurized, and the reaction product was removed. The catalyst was removed by filtration through Celite filter-aid. A pure sample of N,N-dibutylcyclohexylamine26 was obtained by preparative GC of a portion of the filtrate. Anal. Calcd for C,,H,,N: MW, 211. Found by titration with 0.1 N perchloric acid in acetic acid: 212. Analysis of the filtrate by quantitative GC indicated the presence of 103.5 g (98% yield) of N.N-dibutylcyclohexylamine, no detectable dibutylamine, and 14% reduction of the excess cyclohexanone to cyclohexanol. Example 2 N,N-Dibutyl-1,3-dimethylbutylamine by reductive alkylation of dibutylamine with methyl isobutyl ketone A _ We reacted 64.6 g ( 0 . 5 0 mole) of dibutylamine, 250 ml (ca. 2.0 mole) of methyl isobutyl ketone, and 3.5 g of a sulfided platinum on carbon catalyst in a 1-liter autoclave for 5.0 h at

355

2000 and 500-800 psig. A pure sample of N,N-dibutyl-1,3-dimethylbutylamine was obtained by preparative HPLC. Anal. Calcd for C,,H,,N: C , 78.79; H, 14.64; N, 6.56. Found C, 78.82; H, 14.58; N , 6.55. A quantitative GC analysis indicated the presence of 64 g (60% yield) of N,N-dibutyl-1,3-dimethylbutylamine and no detectable dibutylamine. We reacted 129.2 g (1.00 mole) of dibutylamine, 430 ml (ca. 3.4 mole) of methyl isobutyl ketone, and 12.0 g of a 5% palladium on carbon catalyst in a 1.7 liter autoclave for 2.6 h at 190-2050 and 600-800 psig. A quantitative GC analysis indicated the presence of 114 g (54% yield) of N,N-dibutyl-1,3-dimethylbutylamine and no detectable dibutylamine.

Example 3 N-Isopropylpiperidine by reductive alkylation of piperidine with acetone. We reacted 42.6 g (0.50 mole) of piperidine, 250 ml (ca. 3.4 mole) of acetone, and 3.5 g of a sulfided 5% platinum on carbon catalyst for 1.3 h at 90-1000 and 400-700 psig. A portion of the N-isopropylpiperidine27. 2 8 was distilled at 149-1500. Anal. Calcd for C,H,,N: MW 127. Found by titration with 0.1N perchloric acid in acetic acid: 127. A quantitative GC analysis indicated the presence of 59 g (93% yield) of N-isopropylpiperidine and no detectable piperidine. -B We reacted 85 g (1.0 mole) of piperidine, 515 ml (ca. 7.0 mole) of acetone, and 6.0 g of a 5% palladium on carbon catalyst for 1.0 h at 60-650 and 500-800 psig. GC analysis as in A indicated the presence of 107 g (84% yield) of N-isopropylpiperidine and no detectable piperidine. Example 4 N-Ethyldicvclohexylamine by reductive alkylation of N-ethylcyclohexylamine with cyclohexanone. The results are shown in Table 2.

RESULTS AND DISCUSSION The results summarized in Table 1 illustrate the successful preparation of a trialkylamine by the reductive alkylation of

356

dibutylamine, a dialkylamine, with cyclohexanone using a palladium catalyst and a number of metal sulfide catalysts. Excellent yields of the tertiary amine were obtained. ApproxTcble 1. REDUCTIVE ALKYLATION OF D18UTYLPbllblE WlTH CYCLOHEX!.NOt.Ifa

'lield, mJeX

Catalyst type

Pd

2d PtS,

Pt;,

wt,q

3.5 3.5 35 3.5

T47p %

FreTsUre (psiq)

Tim, h

%mineb

cyclohexandC

45-50 85-95 45-5C 195-1 10

350-m

4.3

600-800 509-800

14 lG.3

500-71'lfi

07

a5

---

r,

?,

0

96 84d

RhS, FdS,

3.5

95-100

500-800

1355145

RuS,

f

MOSX

70

75-80 240-253

500-800 500-800

3.8 7.0 33

I00

P

600-1000

43

14

6 '3

94

__

36

_-

89

II

a. Each experiment was run with 64.6 q (050) m d e of dihutvlornrne, 250 r r i (co. 2.4 m d e cysloherancre'l. b. N.N-dibutylcyclohex~lomine .: E k e d cr excess (1 9 d e ) cvclchhermme d. Dihitylamine, recovered cnly in this experiment, was 17%. Yield bfl5ed cn Cmver5icr was 101 Y e Prepwed frcm 10. q palladium chlor;de hydrate f prepared frm I09 ruthenlorn trichlwids hydrate

imately 5 to 15% of the excess cyclohexanone was hydrogenated to cyclohexanol. We estimate ketone condensation occurred accounting for about 5% of the excess ketone. Reacting acetone, MEK and MIBK at the same catalyst level and pressure, we observed it took a temperature of 9 5 o C with acetone, 145oC with MEK and 200OC for MIBK to achieve a significant reaction. The cycle time varied somewhat, but the general trend showed the more hindered gave a marked decrease in reaction rate. The details of the rection with dibutylamine with methyl isobutyl ketone (MIBK) are give in example 2. Experiments with MIBK required much higher temperature than with cyclohexanone and gave 54 and 60% yields of desired tertiary amine, using palladium and platinum sulfide catalysts, respectively. The absence of starting dibutylamine in the reaction product indicated that side reactions involved the amine as well as the ketone. Hydrogen absorption data showed that only about 5 to 10% of the excess MIBK had been reduced to the corresponding alcohol. Platinum sulfide appeared superior to palladium for the reductive alkylation of piperidine with acetone. A more carefully controlled comparison of platinum sulfide with palladium and with platinum is shown in Table 2 for the reaction of N-ethylcyclohexylamine with cyclohexanone. Platinum gave a very poor conversion of the starting secondary amine (27%) and a correspondingly low yield of the tertiary amine product (22%), although the yield based on conversion was good (81%). The

357 TABLE 2 REDUCTI r E ALh YLATION Of N-ELH (LCYCLGHEXYLAMINE WITH CYCLOHEXANONEa Yield, mde % recwwd 3O amneb rrcovaed Catalyst 2Quntne 3ctudc 6 0 C A cyclohexanme cyclohexade

0

PtS, Pt

49

42

73

22

Pd

56

30

51 51 68

16 16

35

23

35

61

Each Pxperimnt was run with 636 q 1050 mde) of N-ethylcylccheqlmne. 54 0 g (055 mde) r y c l d e i a n m g 1 15 nj nf mthand and 050 9 of 5%catalyst M r a r h fw7 5 h at 1620 and in the rarqe of 600-763 psi9 Eased on dirycld-texylamine E m 4 x starting ~ v c l o h ~ ~ o i c n ~ Eased wl cwgerted cvcloh~xannne Determtn,?l by quntitdiw 3' andysi:

of

b r

d e

undesired reduction of ketone to alcohol was much more pronounced with the platinum than with the palladium or platinum sulfide catalysts. The platinum sulfide gave 42% tertiary amine while the palladium resulted in 30% tertiary amine. Both gave the same amount of ketone reduction. These results illustrate the practicality of preparing trialkylamines by the reductive alkylation of dialkylamines with aliphatic ketones. Excellent yields are obtained, particularly with the more reactive and less hindered ketones, such as cyclohexanone and acetone, and with the less hindered secondary amines. Platinum sulfide, or other platinum metal sulfides, are the catalysts of choice when more hindered reagents require more severe operating conditions.

Uniroyal Chemical Co., Naugatuck, Conn., 06770, U.S.A. 2 Presently, First Chemical Corp., Pascagoula, Miss. 39581, U.S.A. 3 R.E. Malz, Jr.and H. Greenfield (to Uniroyal) Eur. Pat. Appl. 14985 (Sep. 3 , 1980); Chem. Abstr. 1981, 94, 102811y.) 4 W . S . Emerson, Organic Reactions, Vol IV, John Wiley and Sons, New York, 1948, pp 174 to 255 5 A . Skita, F. Keil with L. Boente, Chem. Ber. 1929, 62B, 1142. 6 A. Skita, F. Keil with H. Havemann, K.P. Lawrowsky, Chem-Ber. 1

1930, 638, 34. 7 8 9

A. Skita, F. Keil, H. Havemann, Chem. Ber. 1933, 66B 1400. R.V. Heinzelman, B.D. Aspergren, J. Am. Chem. SOC., 1953, 75, 3409. A. Skita, W. Stuhmer, Ger. 932,677 (Sep. 5, 1955), Chem.

358

Abstr.; 1958, 52, 20200h. 10 D.E. Ames, D. Evans, T.F. Grey, P.J. Islip, K.E. Richards, J.

Chem. SOC. 1965, 2636. 11 E. Seeger, A. Kottler (to Dr. K. Thomae G.m.b.H.) Ger. 1,255,646 (Sep. 29, 19661, Chem. Abstr. 1966, 65, 18564. 12 M. Freifelder, Practical Catalytic Hydrogenation, Wiley-Interscience, New York, 1971, p 376,377 (reference 3) 13 B.A.O. Alink, N.E.S. Thompson (to Petrolite) U.S. 3,994,975 (Nov. 30, 1976). B.A.O. Alink, N.E.S. Thompson, R.P. Hutton, (to Petrolite) U.S. 4,040,799 (Aug. 9, 1977). 14 A. Skita, F. Keil, E. Baesler, Chem. Ber. 1933, 66B. 858. 15 R.M. Robinson, (to Abbott) U . S . 3,314,952 (Apr. 18, 1957). 16 S. Wolownik, (to Abbott) U.S. 3,432,508 (Mar. 11, 1969). 17 P.F. Jackisch, (to Ethyl), U.S. 4,521,624 (Jun. 4, 1985) 18 W.B. Wright, Jr, J. Org. Chem. 1959, 24, 1016. 19 H.A. Shonle, J.W. Corse, (to Eli Lilly) U.S. 2,424,063, (July 15, 1947). 20 F.J. Villani, N. Sperber, (to Schering) U.S. 2,852,526 (Sep. 16, 1958). 21 L.F. Kuntschik, O.W. Rigdon, (to Texaco) U.S. 3,976,697 (Aug. 24, 1976). 22 Q.W. Decker, E. Marcus, (to Union Carbide) U.S. 4,190,601 (Feb. 26, 1980). 23 Ref. 4, p. 195. 24 Ref. 12, p. 359, 376. 25 Ref. 4, p. 181 26 0. Stichnoth, W. Schmidt, (to BASF) Ger. 851,189 (Oct. 2, 1952), Chem. Abstr. 1953, 47. 112394. 27 H. Thies, H. Schoenenberger. P.K. Qasba, Arch. Pharm. 1969, 302, 610, Chem. Abstr. 1969, 71, 124154 (bp 145-1480 at 720 mm). 28 R.A.Y. Jones, A.R. Katritzky, A.C. Richards. R.J. Wyatt, J. Chem. SOC. (B) 1970, 122 (bp 510 at 21 mm).

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

359

EFFECT OF PROMOTERS ON Pt/Si02 CATALYSTS FOR THE N-ALKYLATION OF STERICALLY HINDERED ANLINES IN THE VAPOR PHASE Milos Rusek, Central Research Laboratories, R 1055, CIBA-GEIGY AG, CH-4002 Basel, Switzerland ABSTRACT We have developed a multimetallic catalyst for the large scale synthesis of sterically hindered mono-N-alkylanilines with very good selectivity and high catalytic activity. In contrast to copper chromite catalysts which allow the N-alkylation only with primary alcohols, the doubly promoted Pt/SiO2 catalysts described here are useful for the reaction of ortho-substituted anilines with both primary and secondary alcohols. The catalyst must activate three reaction steps: Dehydrogenation of the alcohol, condensation of the aniline with the carbonyl compound produced and hydrogenation of the resulting imine to the desired N-alkylaniline. In the vapor phase the hydrogenation step is the most difficult to achieve under our reaction conditions. The effects of different metallic promoters (Sn, Ge, Re etc.) and of various basic additives on the performance of the catalyst are discussed. The best catalyst developed is a Pt-Sn/SiOl catalyst pretreated with Ca2+ which is able to catalyze the alkylation of several ortho-disubstituted anilines with high conversions and selectivities. INTRODUCTION Most multipromoted catalysts have been described for the catalytic reforming of petroleum. For this process it is typical, that several reactions take place simultaneously: dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes and dehydrocyclization of alkanes. Isomerization, hydrogenolysis, and hydrocracking are also involved in the process. In fine chemical manufacturing, the application of promoted platinum catalysts is less known. Maxted and Akhar have reported that the addition of stannous, manganous, ceric and ferric chloride to platinum oxide (Adams catalyst) facilitates the hydrogenation of aldehydes, ketones and olefins (ref. 1). The selective hydrogenation of unsaturated aldehydes or ketones to unsaturated alcohols has been achieved by the addition of ferrous sulfate and zinc acetate to platinum catalysts (ref. 2). Ortho substituted N-alkyl anilines are intermediates for an important class of pesticides. They can be synthesized by the reaction of the aniline with the appropriate alcohol:

‘Et

MEA

MOIP

‘Me

AA

The following multi-step mechanism is proposed for this transformation: First, the dehydro-

360

genation (2a) of the alcohol to the corresponding carbonyl compound takes place. Condensation (2b) of this carbonyl compound with the aniline follows and the last step is the hydrogenation (2c) of the imine. Ho\c/R1 H/

\

R2

R

@HC:R’ R

H

R2

A

All three steps are reversible under the conditions normally used. We found that with most catalysts the third step - the hydrogenation of the imine - is the slowest reaction. If this step is hindered, the first two steps will remain far from equilibrium. It is therefore important to find catalysts with enhanced hydrogenation activity because this leads to an increase of the over-all conversion. Experiments with catalysts which are known to catalyze the alkylation reaction in the liquid phase (ref. 3), showed that the desired gas-phase reaction of substituted anilines with alkoxyalcohols occurs, but with very low yield. Pd promoted copper chromite catalysts which are able to catalyze the alkylations of sterically hindered anilines with primary alkoxyalcohols (ref. 4, 5) showed only very low activity and selectivity when secondary alcohols were used. We found a solution with new, doubly promoted platinum catalysts on silica and reported their scope and limitations for the synthesis of various aniline derivatives (ref. 5). In the following communication we describe the development of the most effective catalyst using as model reaction the N-alkylation of 2-methyl-6-ethylaniline with methoxy-2-propanol. EXPERIMENTAL Materials. The catalysts were prepared by impregnation of SiOz with an aqueous solution of HzPtC16 and the appropriate promoting metal salts, using the incipient wetness technique. SiOz, type M from Chemische Werke Uetikon, Switzerland, was used (20-35 mesh (ASTM), BET surface area 470 m2/g, pore volume 0.38 ml/g, composition: 41.9% Si, 860 pprn Ca, 150 ppm Mg, <200 ppm Na, <200 pprn Al, <200 ppm Ti,
361

Mg are important. The impregnated precursors were dried stepwise at 90' (2,5h) and at 160' (2,5h) in vacuum and at 350-450°C (5h) in presence of air. Catalyst activation 3,l ml dried catalyst precursor was activated in situ in a microreactor quartz tube (0,9 cm i.d., 17 cm long) at 140°C with a Hz/Nz gas mixture (1 bar, 50 mumin). The concentration of hydrogen was increased from 5% to 100% in 1,5 h. Then, the temperature of the catalyst was raised under H2-flow (50 mumin) to the desired reaction temperature. Reaction procedure The N-alkylation of 2-methyl-6-ethylaniline (MEA) with methoxy-2-propanol (MOIP) was investigated in the same flow microreactor under atmospheric pressure. Feed: MEA : MOIP = 0.5 (3 ml/h) and hydrogen (4,7 mumin). The reaction product was condensed in a cooling trap. Each catalyst was tested for 24 h and 7 samples were collected and analyzed separately by GLC on a fused silica capillary column with methylsilicon fluid (Hewlett Packard) as stationary phase. Data evaluation. For the comparison of the different catalysts the GLC values obtained from the samples after 18, 21 and 24 hours were averaged. The following quantities were detennined: Conversion of MEA Conversion of MIOP Selectivity to N-alkylated products*)

C ~ = CMOIP= SMEA = SMIOP=

Yield of N-alkylated products*)

Y

Yield of N-alkylated aniline Hydrogenation activity

A100 [ 1-MEA/C(MEA+aromatic products)] 100 [ 1-MOIP/Z(MOP+aliphaticproducts)] 100 [(I+AA)/Zaromatic products] 100 [(I+AA)naliphatic products]

~ = AC M E A . S M E A / ~ ~ ~

YMOIF CMOIP.SMOLP/~OO SAA = 100 [AAfEaromatic products] a = SAA/SMEA = AA/(I+AA)

*)imine + AA Because the gas chromatograms showed traces of various products which could not be identified, the values described above are not absolutely exact, but in general the agreement between the values based on MEA and MOIP was satisfactory. RESULTS AND DISCUSSION Effect of non-alkali metal promoters These tests were carried out at 275OC with a 5% platinum loading and various Pt/promotor ratios. At these high temperatures the exothermic hydrogenation reaction is expected to be hindered and under these conditions even small differences in activity will give visible effects. The results summarized in Table 1 were obtained under the following conditions: MOP/MEA = 2; 275OC; in presence of 1 mol hydrogen / mol aniline. Because of the high temperature used in

362

TABLE 1 Effect of promotors on activity and selectivity of the N-alkylation.

363

these tests, the hydrogenation activities a and the over-all selectivities SAA are much lower than those obtained under optimal conditions. Optimum temperatures for the N-alkylation of ortho-disubstituted anilines are 190-200OC. Under these conditions a = 1 and selectivities = 90%(ref. 2). Generally, we have found that the catalysts with a Pf/M ratio of 3 show the highest hydrogenation activity a. There are exceptions: for Re the highest a are observed at an atomic ratio of 1 and for Ti and Ru the value of a is not dependent on the Pt:M ratio. The following metals reduce both the over-all and the hydrogenation activity: Zn, Cd. Fe. The behavior of the catalysts with Pb is rather surprising because the selectivity and hydrogenation activity are high while the conversions are lower than without promotor. Co, Ni, Cr,Mo and W have very little effect. Uranium enhances the hydrogenation activity, but the selectivity is very low. Niobium gives somewhat improved results while the lanthanides do not show any effect. The best over-all results, where the unprornoted catalyst is improved in every aspect, are obtained for the addition of Sn, Ge and Mn.Because of practical considerations we selected the Sn-promoted type to look for further improvements.

Yo Mn

20

0.8

8

’

\

0

\

0

15

-

’.

8 \

SW’ Ru ’

Ge, G

\

‘.?i

I

0

I

5

-

‘0

’ ’c;

8

Nb

0

10

w

\

.cr

\

‘Pb

. N;

co

Ce

.Mo

P7

‘Cd

. . Fe

I

I

I

100

200

300

. ;‘\,

La

*u Zn I

400

I

500

I

600 kJ/rnol

Heat of Formation (-AHMO,/x) Fig. 1. Plot of the yield of N-alkylated anilines AA (= YMvIEA.a) versus heat of formation of the oxides of the promoting metals. Data from Table 1 and ref. 7. The promoting effects of the various metals can be explained by their electronic interactions with the platinum. Many attempts have been made to find correlations between the catalytic performance and thermodynamic properties of different metals (ref. 6). One such correlation is

364

shown in Figure 1. Here the yield of N-alkyl aniline (Y,=YMua) is plotted versus the heat of formation of the bivalent oxides (in some cases of closely related oxides) (ref. 7). The result is a correlation of triangular shape where metals having heats of formation lower than 250 or higher than 550 kJ/mol per M - 0 show very low performances. We presume that metals with intermediate values are capable of an electronic interaction with platinum which leads to higher hydrogenation activity. For some of the exception we can offer the following explanations. Fe, Co, Ni, Mo, W, Cr and U which are known to have a strong hydrogenolysis capability are well below the envelope because the selectivities are lower. Cd and Zn which very often act as catalyst poisons are also below the correlation lines because they exhibit both very low conversions and hydrogenation activity. At the moment the position of Ru cannot be explained. Interestingly, we have found that the catalysts on the left hand side of the graph are easily regenerated while those on the right are not. Alkali metal promoters Because alkoxy substituted alcohols are acid sensitive the support must be either neutral or even basic. That the presence of a basic promoter is needed can be seen by a strongly decreased activity when the Ca and Mg, which are present originally in the carrier, are removed by washing with 2N HCl. Catalysts prepared with additional basic metals as chlorides (1% metal on catalyst) were tested at 200OC. At these temperatures there are only very small differences between the metals. Figure 2 shows a plot of ionic radius versus conversion of MEA. The best promotors are potassium and calcium.

Conversion of MEA %

Fig. 2. Plot of conversion of MEA versus ionic radius of the

60

metal of the basic promotor

40 20

Metal conc. 1% wlw; 200 'C; MEA : MOIP = 2

washed with HCI p a ] < 10 ppm

0.5

1.o

1.5

(A)

Ionic Radius

On the other hand, we know that depending on the basicity of the aniline a different alkali metal is required for optimal performance. E.g. for the N-alkylation of 2,6-diisopropylaniline barium is a better promoter than Ca. In comparison with calcium it enhanced the conversion of

365

the aniline from 36.2% to 53.7% and the the selectivity from 79.1% to 86.0% We propose that the alkali metal neutralizes the hydroxyl groups on the catalyst surface and prevents the chemisorption of substrates on the acidic center of the support. CONCLUSIONS From our results we conclude that doubly-promoted Pt-SiO, catalysts are useful for the N-alkylation of hindered anilines in the gas phase. In most cases studied the addition of a promoting metal is necessary in order to get high activities and selectivities. If the silica does not contain enough basic trace metals, a second promotor has to be added. Because three reaction steps must occur on the same catalyst, the individual effects of the promoting metals are not easy to explain. In most cases the hydrogenation activity is the crucial parameter. Because the properties of the catalyst can be influenced by the choice of the promotors, they can be optimized for different substrates. ACKNOWLEDGMENTS I would like to thank Mr. J. Kaufmann for carrying out the GLC analyses and Mr. B. Casagrande, Ms. M.Ch. Dimg and Mr. A. Schindler for their experimental contributions. REFERENCES 1 E.B. Maxsted and S. Akhar: J. Chem. Soc.,(1959) 3130. 2 P.N. Rylander, N. Himmelstein and M. Kilroy: Engelhard Ind. Tech. Bull. 4 (1963) 49. 3 a) W.S. Emerson in Organic Reactions 4 (1948) 174 b) P. Rylander, Catalytic Hydrogenation in Organic Synthesis, Academic Press Inc., London, 1979, pp. 165. c) M.V. Kljuner and M.L. Khidekel, Russ. Chem. Rev. 49 (1980) 14. d) R.G. Rice and E.J. Kohn: J. Amer. Chem. SOC. 77 (1955) 4052. e) F.S. Dovell and H. Greenfield: Ind. Eng. Chem., Prod. Res. Dev. 1 (1962) 179. 4 M. Rusek, 1st International Symposium on 'Organic Chemistry in Technological Perspective', Jerusalem 1986. 5 M. Rusek, 9th International Congress on Catalysis, Calgary 1988, p. 1138; EP 0,190.101 (Ciba-Geigy, 1986) 6 (3.1. Golodec and V.A. Rojter, Ukrain. Khim. Zh. 29 (1963) 667 7 D.D. Wagman et al., The NBS Tables of Chemical Thermodynamic Properties, J. Phys. Chem. Ref. Data 1 1 (1982), Suppl. 2.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors ), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

367

POLYF"CTIONAL1TY OF ZN-CR-O(PD)CATALYSTFOR THE SYNTHESIS OF PYRAZINES FROM DIAMINES AND GLYCOLS Lucio Forni and Roberta Miglio Dipartimento di Chimica Fisica ed Elettrochirnica Universita' di Milano, Via C.Golgi, 19

20133 Milano, Italy

ABSTRACT A mechanistic study of the cyclisation of ethylene diamine and ProPylene glycol to 2-methylpyrazine is reported, baaed on a detailed GC-MS analysis of the reaction product. Most of the more than thirty different species, detected in the reactor effluent, were recognised, allowing to individuate the reaction paths taking place in the present system and showing that many different functions must be attributed to the Zn-Cr-O(Pd) catalyst. such as dehydration, hydro-dehydrogenation, aldolic condensation, double bond isomerisation and migration, hydro- and steam-dealkylation, disproportionation of methyl groups and oxidation. A general reaction scheme is proposed, taking into account all the most important products and intermediates.

INTRODUCTION The

cyclisation

(PG) to

of

ethylenediamine

2-methylpyrazine

(MP)

is

a

catalytic synthesis of 2-amidopyrazine tubercular drug.

(ED) and

propyleneglycol

fundamental

step

for

(AP), a well-known

the

anti-

The formation of MP takes place in vapour-phase

at ca. 650 K, atmospheric pressure and in excess of steam over a Zn-Cr-O(Pd) catalyst. The exploratory research and optimisation of the catalyst El-61 the

latter

and

to

led to a proper procedure for the activation of the

determination

of

the

optimal

range

of

reaction conditions. The present work describes an extended study, aiming at throwing light on the intimate mechanism o f the process.

EXPERIMENTAL Catalyst.

The catalyst was a mixture of Zn and Cr oxides (Zn/Cr =

3/1 atomic ratio) prepared by coprecipitation as reported [l], which BET

1 wt

%

surface

respectively.

Pd was added by area

and

impregnation with aq.

porosity

It has been pressed

were

52.9

(2 tons/cm

2

rn / a 2

and

PdSO, 0.4

to [5]. 3

cm /g.

in wafers, crushed

and sieved, collecting the 0.18-0.15 m m fraction.

368 Apparatus,

procedure and

analysis.

microreactor assembly employed,

The

f ixed-bed,

continuous

the procedure and the G C analysis

of the reactor effluent are described

in detail elsewhere

[1,71.

The main species detected by G C have been identified by GC-MS and through a comparison with preformed mixtures of known composition.

RESULTS AND D I S C U S S I O N

GC-MS analysis.

The complete

list of

the compounds detected

the effluent mixture is shown in Table 1. mass

spectra

with

literature

data

The comparison of

individuation of all the species indicated

by

one

asterisk

definitive,

can since

be

considered

pure

samples

reaSGnablY of

such

Table 1 List o f the substances detected by GC-MS (see Fig.1).

No.

*"(confirmed) MW *(hyPothesised)

1 2 3 6

** ** ** **

** *t

5 6

7 8 9

10 11 12 13 16-15 16 17-13 19 20 21 22 23 26 25 26

** ** ** *" ** ** ** ** ** f

** * * **

* ** ** ** *

* *

18 32 66 65 58 58 58 60 55 72 60 86 80

76 94 98

81 86 96 108

108 108 100 110 124 122 126 138 120

the

permitted

the

two asterisks.

As

:8,9]

for the remaining species, the attribution o f

in

those indicated by safe,

althcugh

substances

were

not not

in the reactor effluent

Substance

water methylalcohol acetaldehyde e t h r laliohol acetone propionaldehyde 2-propen-1 -01 n-propanol propionitrile methyl-ethyl-ketone ( M E K ) ethvlenediamine ( E D ) diet hy 1ketone pyrazine ( P I propyleneglycol ( P G ) 2-methrlpyrazine (MP) t e t i - a t - l r d r o m c t r r y l p y r a z ine ( T H M P ) 2- and ?.-methylpyrrole tetrahydropyrazine ( T H P ) dihydromethylpyrazine (DHMP) 2,5-dimethylpyrazine pyrazin-2-carboxyaldehyde 2,3-dimeth~ lpyraz ine 2-methylpiperazine ( M P I P ) 2,3-dimethyldihydropyrazine

2,3,5-trimethyldih~dro~~razine tetramethyldihydropyrazine 2-propen-pyrazine

369

- 0 5

€6

93

0 120

40

1

loo

?'

lo:L

d)

50

40

d

r

T

120

1 ].?;,I,

-

7

200

I

loo 50

LW 40

14

5:-

loo 132

SlO

0

200

, 120

,

,

,

40

200

loo

04

55 67

110 121

120

200

100 2j

n)

121

f)

40

2W

110120 121

40

loo 112

y;

111

120

70

h)

5 40 ; 120 L ZW0

118

200

370

available for a direct matching of

their spectra.

The

spectra

recorded for the most important species are shown in Fiy.1. The attribution of the single-asterisk-marked spectra was based on many different observations, taking into account some specific experimental

features, besides

the

comparison

spectra. For instance, the trend of

with

literjture

the GC peak area of no.17

species vs. time factor is characteristic of a primary product or, better, of a reaction intermediate.

The comparison between the

spectra of methyl- piperazine (MPIP) (Fi.z.lk) and of MP (Fig.1~) shc.ws that no. 17 species (Fig. 19) should possess an intermediate structure. The principal fragments in the no.17

spectrum can be

explained only b y admitting that this species is l e s s stable than

M P and decomposes similarly, so that the most probable structure Zhould be the dihydromethylpyrazine (DHMP). the

attribution of

the

spectrum

of

As a further example,

no.17

species to pyrazin-

2 -carboxyaldehyde can be cited. Although the mass spectrum of the

aldehyde was not found in the literature for a direct comparison, our spectrum (Fig.l j ) presents the characteristic features of the aromatic aldehydes 181, the

tendency

to

such as strong M * and ( M - 1 ) '

form

the

benzoilic

cation,

signals and frequently

corresponding to the strongest signal of these substances. Indeed, in our case M t = 108 and (M-l)*= to

the

pyrazinoilic

cation,

107 and the latter, corresponding is

the

strongest

signal

of

this

spectrum. Furthermore, the typical fragments of the pyrazinic ring (a.m.u. = 6 0 , 6 2 , 53. 67 and 80) are present. Owing to the complex nature of our system, some

Auxiliary runs.

auxiliary runs (see Table 2 ) have been carried out with different feeding mixtures and under different reaction conditions, in order to collect a wider information, useful for writing a reliable set o f stoichiometric equations.

Absence

of

When feeding an aqueous solution of pure PG, a

ED.

very high numbei abundant

ones,

of products were noticed, among which the most i.e.

methanol,

acetaldehyde, ethanol,

propanal, a1 lyl alcohol, n-propanol, propionitri le, k?t

X ~ E

(MEK)

and

3-ptntanc~ne were

acetone,

methylethyl -

rrcognised positively.

These

specie5 were found also among the reaction products obtained under

quite stressed Conditions (390 "C).

At

such a temperature, the

selectivity to light alcohols and aldehydes may attain 20%. study

1101

carried

out

on

a

similar,

unpalladiated

A

Zn-Cr-0

371

Table 2 Experimental Conditions for auxiliary runs

* Feed

F U ~no. I

T,

**

0

C

mg cat.

carrier gas

A1

PG 6 . 4 %

390

70

3

A2

ED 3.5%

390

70

3

A3

MP

52

390

70

3

A4

MP

5%

370

40

6

A5

MP 5%

360

40

6

A6

EG 3.6%

t

PD 4 . 3% 3 9 0

70

3

A7

MeOH 3%

t

MP 5%

GOO

40

3

AS

MPIP 1%

370

GO

3

A9

MPIP 5%

390

10

3

A10

MPIP 5 %

390

lo***

3

* Wt % in the aq. solution fed at * * Volumetric flow rate in scc/min * * * Unpalladiated catalyst ~

3

cm /hr

2

~

catalyst, showed that the latter can promote a wide spectrum of reactions,

such

as

dehydrogenation,

condensation

(mainly

decarboxylation,

aldolic),

dehydration,

migration and isomerisation. In the present case,

hydro-

double by

bond

taking into

account that our catalyst possesses a stronger hrdro-dehvdrosenating activity, owing to the presence of Pd, a r e t of possible stoichiometric relations could

be

written

(Table

31,

for

the

formation of the species noticed. The formation o f

acetone, for

instance,

simultaneous

can

take

place

through

at

least

two

reactions: dehydration o f PG and kstonisation o f preformed ethyl alcohol.

The

dehydration

or

approximate

of

ketonisation was

results of run A 6 pr-

ratio

the

amounts

determined

coming

from

comparing

the

(Table 21, in which ethyleneglycol ( E G )

and

by

i,ylenediamine (PD) were fed, with those o f a run carried out 0

with standard feed and at the fame temperature ( 3 9 0 C). BY feeding EDtPG the selectivity to acetone ( S A ) was ca. feeding EG+PD it was ca. 0.05 m o l

%.

0.83 m o l

%;

by

Since in the second case

acetone can form only through ketonisation of ethylalcohol, it was concluded that no more than ca. 6 form through such a route.

%

of

the overall acetone can

372 Tat.1.:

J

possible stoichiometris products from PF alone.

+

CII j - C H O H - C l i L - O H

++

C H d - C H L OH

H,

+

CHS-CHO

C H -CHOH-CH,-OH

CHL-OH

CHL=CH

CHJ-CHOH

--t

CHL=CH-CHO

CH,,-OH

CHJ-CO

--t

CH,-@H

+

+

OH t CHJ-CH,-CH,-OH

CIdj

t

CO

+

2 C l j J CO Cli-,

(CH,),C=CH-CO-CH3

+

CHO

Absence c.f

the

CHJ-OH

t H1O

H2

HLO i

1

2

3 HL

t

::L

+

C H S CHL-C@-CHL-Cl:J

+

CO

3 H,

t HLO

+ HzO

CHJ-CHI-CH=C(CHJ)-CHO

-f

+ CO

CH3-C@-CHL-CHJ

2 C H J - C H L CHL-OH

2 CHJ-CHL

of

CH,-CH,-C:i@

4

+

2 C l i A CHL-OH

formation

CHJ-CHL-CHL-OH

W

?:i - C : i @ H - C H L - O H

C H A CHI

i

thc

+ HL

H,O

+

HL

t

CH,

for

HLO t C H J - C O - C H A

C H A C I i O H - C H L OH

CHA-CH,-CHO

relations

When feeding an aq. solution of pure ED, only P

PG.

(Fig.1a) and tetrahydroprrazine ( T H P , reactor efflusnt. T h e selectivity

Fig. 1 Y )

t o THP

were found in t h e

( S T H P at ) very low values

,of time-on-streamwas about twice that relative tG P (S,), but the ti,

figures tci-dde.:

the

5arne

value

when

attaining

the

steady

.;s,nditianc.Iiowever, under standard rzaction conditions, i. e. when

fe,z!, E,,,jSP

L o t h PG and ED are

that

in na-rnal

'* 0.1,

reaction no more

50

than

that it can he concluded 1/10 of

the P

forms by

condensatioii of ED. Tr-ansfoi-matici7

>:slues of tim;

cf -6t-I

MP.

IJith fresh catalyst and for very

ztream, br feeding an aq.

Some t e t r a h v d r o r n e t h r l ~ r i - c ; = i n e ( T t l M P ) mett,ane.

TIji

~

short

solution of Pure M P

formed, accompanied by P and

is p i - ~ > L , s b l , r due tch thc residual hrdr-oean, remaining

In t t , e ,catalj.zt afttr tl-je activation procedure [S].

Indeed, these

hrdrogeiiat ion-hi.drc,jetii.lrsi; riactions ai-e no more cobserved at the .--t;-s,j; state, whcrd t h e mctst pi-cJbat,lereaction leading to P bccornes

the stcamdealkylatien of the methyl group. Another C~C~Ssibleroute to

I" c o u l d be the disproportionation of MP.

Hcuever,

the absence

of dirrccthrlcyrazinc (DMP) in all of the A 3 - - A 5 runs ~ x c l u d e s that

MP car, undergo such a reaction in significant amount. Indeed, at 3CIO"C

and under steady conditions, o n l y 0.12 mol of P formed b y

feeding 100 r n o l

of M P and no DMP was observed, while in normal

373 r e a c t i o n a n d u n d e r t h e same c d o t i d i t i m i s , 8? i n n 1 o f 111’ c ~ t i ~I IIP Lo 2 . 9 mol

of

stable

These d a t a <.orifit

C o t rned.

P

molecule,

but

that

0190

d i s p r o p o r t i o n a t i o n o f methyl g r o u p s the

formation

of

curve,

amounts

of

iri

t o DMP

selectivity sliaped

and

2,3-

s h o u l d be e x p l a i n e d

I V J ~ atiily

tti

it

(Tig.1.f)

a d i f f e r e n t way.

vs.

time

a

of

So, a

the

secondary (DHDMP)

dihydt.odimetliylpyt’azirle

o b s e r v e d among out- p r o d u c t s .

in

I r i faLC,

shows

factor,

characteristic

t i

uridergo

riot

does

vei y

is a

I1P

arwi-cciaLtle omwitit.

it1

2,5-DMP

II>ill

Hence,

reaction

tiormal

the trend of t h e sigrnoid-

typical

product,

and

( F i g . 11) a r e

small

usually

a n s a l k y l a t i o n of the adsorbed

r e a c t i o n i n t e r m e d i a t e c a n b e h y p o t h e s i z e r l a s a p o s s i b l e r-nute f o r

tt>e f o r m a t i o n o f DMP. Another reacting

important

system

byproducts, and

of

a

observed,

obcervation

towards

concerns

c.teadj

tho

the

state.

evolution

Ccsidcs

P

of

arid

ttie

liglit

t h e i n i t i a l f o r m a t i o n of v c r y small a m o u n t s o f methane with

species

parent

p-ak

r a p i d l y disappeot i t i y w i t l i i t l

at

a.m.u.

98

(riu.1d)

4 ht o m i - s t r c a m .

The

was ratio

b e t w e e n t h e G C peak a r e a ~ > ft h i s s p e c i e s a n d o f P w a s 16.7, 0.66, 0.60 a n d 0.55,

after

T h e analysis of

tttc

1,

tilass

t o be a s s i g n e d t o THMP.

2 , 3 a n d 4 h r s on-str-eani.

respectively.

spcr-Lrum i n d i c a t e s t h a t vet-Y l i k e l y i t i s This s p e c i e s is c l e a r l y v e r y u n s t a b l e and

r a p i d l y d e h y d r o g e n a t e s t o DIIIIP ( F i g . la). on v e r y f r e s h , hydrogeri-riclt

Latalyst,

It

i.e.

c a n be o b s e r v e d o n l y when a r c l w l i v e l y d e e p

h y d r o g e r i a t i o t i o f t h e p y r a z i t i i c r I n g i s Cavc)uI ed.

Reaction

mentioned,

of

the

EG

A6

+

PD.

Besides

o u x i l i a t - y t-un

tlw

illfot-matiun

(Table 2 )

previously

sliowed a l s o

that

y i e l d t o MP t-eoclied o n l y 4 h . r ~ mol X o f the r e a c t e d d i a r n i n c ,

DHMP

P

DHDMP

DMP

the

while

374 in

normal

y i s l d s as high

reaction

the

formation

which

DMP

is

of

Fesdii779 of MeUH

t h e most

a much

about

higher

three

+ MP a7d

more

of

MPIP.

abundant polyalb.ylates

of

that

same of one.

t h e c u r v e of of

A pozsible

p o l ~ I r l ~ t t ~ f l e t i -among s,

Dimethylpsraziniz

i n our effluent,

t h e i r concentratiorr

MP, s u g g e s t i n g route

i 8 fGoth

formrsti:-n. factor

tiii,c

parallel

be ; x p l a i r , e d

Since

ED.

this

P were

i f

:stel, st

d i s p r o p , : , r - t i o n a t i o n does

surface,

probably

through

r c ~ ! l d b-: ~ 3 i m c d i s t r a t e d c ~ n l y by

fram

which c o u l d

d e h y d r o c y c l i s a t i z ~o f

ncst

take

place

with

MP

interaction

of

the catalyst.

f s e d i n g p u r e DHMP.

the

sccondsri.

T h i s mechanism Unfortunately,

s u b s t a n c e ic. c c l a b i l e t h a t all o u r a t t e m p t s t o i s o l a t e i t

the

reactor r f f l u e n t o r to prepare

MPXP ( r u n s AS-AlO, Alterimtiveli., t d . e l j l i i ~ e thrc,u.;h

one 5

msy

hypothesize

cur

attempts

react

methanol

and

MP

polyeeth,lation

ttlet

r L-1 c1- Il -._- a l - t y p e m e c h a r ; i m ,

t e t i A - m c t h i l - d i h y d r - o p y r a z i n e s were t.2

i t b s d e h y d i - 3 3 i n a t i o r I af

T a b l e 2 ) were u n f r u i t f u l .

and

51,. ..led

t'y

DHMP a d s o r b s q u i t e s t r c m g l r on t h e

r i i t r o y e r i atom w i t h t h e a c i d s i t e s of

:.LLC~5

main

explairi

DHDW s a d DMP, t , u t a l e o thr ,3rowii-19

f o r m i n g onli.

alc,i-,c, i t m a y t x deduced t h a t

8:dn

The

i s ttle

to t h e

T h i s i-,:,ut.:

amount of P f o u n d a t i n c r e a s i n g , v a l u e s of t i m e f Z f : t . j r , i w t

(DMP) t c i n a

i z important

it

thili-

'9.5.

a reacticm

is shown iri F i o . 2 .

n c t c r ~ l rt h e p r e z e n i e

attained i 2 d ~ ~t ue

abundant.

t o t h r o w some l i g h t on t h e s t o i c h i c m z t r y o f s h a p e of

were

k

T h i s l o w Yielcl

amount

times

mol

as 86.5

under i d e n t i c a l experimental conditiolls.

s i n i e t r a c e s of

also (e.9-

detected. i'un

A7,

can tri-

How~:,fer,

Table

2)

~ : , ~ ~ i u s i ~ :t ihle, u n r e a c t e d r e a g e n t s i n t h e r e a c t o r e f f l u c n t .

375 confirmed by the presence of the substance with apparent parent 138. This spectrum (Fig.lp) is very likely the sum

peak at a.m.u.= of

two substances:

and

dihydro-2,4,5,6-tetramethyl-pyrazine

2-propen-pyrazine

accompanied Among

the

by

(MW=120),

with

M+=120

and

(MW=138)

(M-l)+=

119.

the characteristic fragments of the prrazinic ring.

fragments

from

the

pyrazinic

ring,

propionitrile

is

usually present.

Fig.3

-

Possible route to pyrozin-2-cai.toxy-ald~t~yde.

(CH,),=CH-CO-CH,

CH,-CO-CH3

f

CH,-CO-CH,-CH,

Fig.4 - General reaction scheme of the process. Last, but not (Fig. lk)

-

This

least, is the mentioned recognition of the MPIP identification. is

important

for

conf irinine

main sequence of reactions leading t o the desired product arrows in Fis.4).

the

(bold

A l l the intermediates of the sequence have been

detected, although in low concentration, owing to their tendency to quickly dehydrogenate towards the final product.

376

M. Guisnet et a]. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

371

FROM SURFACES TO DISCRETE MOLECULES AS CATALYSTS FOR ALKENE EPOXIDATION Karl Anker Jorgensen Department of Chemistry Aarhus University DK-8000 Aarhus C, Denmark. SUMMARY - Different forms of adsorbed molecular and atomic oxygen on an Ag(ll0) surface are analyzed from a theoretical point of view. It is proposed that the active intermediate for the epoxidation of ethylene is an Ag-0 surface species. A parallel is made to discrete silver complexes as catalysts for alkene epoxidation as it is experimentally shown that these have similar properties at the silver surface. l8O Labelling studies indicate an Ag-0 complex as the reactive catalyst. INTRODUCI?ON Oxidation of organic substrates with molecular oxygen as the oxygen source and catalyzed by metal surfaces is industrially very important reactions. E.g. is ethylene oxide is produced in about 1 x 1O'O kg/year on a silver surface with ethylene and molecular oxygen as reactants, phthalic anhydride and maleic anhydride are produced in about 2 x lo9 and 4 x lo8 kg/year on a vanadyl pyrophosphate surface with o-xylene and n-butane, respectively, as substrates and molecular oxygen as the oxygen donor (ref. 1). The silver-surface catalyzed epoxidation of ethylene (reaction la) has been the subject of very intensive investigations and serves as a challenge to both the industry and the academic field, as the different steps involved in the reaction path are not fully resolved (ref. 2).

R, R

,c=c

R,

\R

/"\

PhlO Ag-complex)

R-c-c-R

I

I

R

R

(Ibi

Although the silver-surface catalyzed epoxidation has been the subject of many investigations there still remain many unanswered questions: (i)How is molecular oxygen activated on a silver surface? (ii) What is the nature of the oxygen that is transferred to ethylene? and (iii) How does the oxygen transfer step take place? If we move to discrete systems as catalysts for alkene epoxidation there has been an increased interest during the last decade in the development of new epoxidation catalysts, where a variety of transition-metal systems have been investigated (ref. 3 ) . But, according to our knowledge there has been no attempt to build a bridge from the

378

silver-surface catalyzed epoxidation (reaction l a ) to the epoxidation catalyzed by discrete silver complexes (reaction lb). This paper will show that a such a bridge exists. We will start with an analysis of how molecular and atomic oxygen can be adsorbed on an Ag(ll0) surface. This is followed by a model for the oxygen transfer step to ethylene; for these purposes we have used extended-Huckel tight-binding calculations (ref. 4). Then, it will be shown that both Ag20 and AgN03, the latter in the presence of tertiary amines, such a pyridine, can achieve the epoxidation of different alkenes with iodosylbenzene as the oxygen donor. RESULTS AND DISCUSSION 1. Silver-Surface Catalyzed Epoxidation. The Ag(ll0) surface has been found to be a good model for the silver-surfacc catalyzed epoxidation of ethylene (ref. 5) and we have therefore used this surface in the theoretical study. Three possible candidates for adsorbed molecular oxygen on a Ag(ll0) surface are shown in 1 - 3.

1

2

3

The binding of molecular oxygen in 1 is end-on with the oxygen bound to one silver atom; in 2 and 3 molecular oxygen is bridging between two silver atoms. Thc relative binding energies, Fermi levels, Ag-0 and 0-0 overlap populations for 1 - 3 are given in Table 1. Table 1. Relative Binding Energies, Fermi levels, Ag-0 and 0-0overlap populations for 1 - 3. Binding energy (kcal/ mol)

Fermi level (eV)

Overlap population 4-0 00

-11.02 -10.99

0.298 0.128 0.240

@(g) 1 2 3

0.0 -8.4 -1.1

-10.90

0.817 0.433 0.452 0.445

It appears from Table 1 that 2, in which molecular oxygen is oriented along the [110] direction, is the most stable form. Though, 1 and 3 are only about 7-8 kcal/mol more unstable than 2. The Fermi levels for the 1- 3 are all found at about -11 eV. The Ag-0 overlap populations show that the Ag-0 bond is weakest in 2 and strongest in 1, whereas the 0-0 overlap populations are nearly identical for the three different adsorption types.

379

The orbital interaction which is responsible for the adsorption of molecular oxygen on the Ag(ll0) surface is mainly between silver d,, and n*o-o.The interaction between the silver surface and molecular oxygen leads to a donation of 1.55 electrons into the K*O.O from the surface because the Fermi level of the surface is located higher in energy than x*o-o. In Figure 1 is shown the contribution for the adsorbed molecular oxygen in 2 to the total density of states (DOS) and also the Ag-0 and 0-0 crystal orbital overlap population (COOP) curves.

Figure 1. (a) Total DOS for 2. The contribution from molecular oxygen is shown as the shaded area. The dotted line is the integration of the contribution from molecular oxygen. (b) The COOP curves for the 0-0 bond (dotted line) and the Ag-0 bond (full line) The DOS curve for 2 splits in three peaks, one at -13 eV and two just above and just below -16 eV. The 0-0 COOP curve shows that the two latter originate from bonding 0-0 and Ag-0 orbitals, whereas the former is the antibonding part. From the Ag-0 COOP curve it is seen that bonding Ag-0 orbitals only are located below the Fermi level. The DOS and COOP curves for 1 and 3 are very similar to those presented in Figure 1. The adsorption geometry 2 could be the one which is observed by NEXAFS (ref. 6). Adsorbed molecular oxygen on Ag(ll0) dissociates into atomic oxygen above 170 K.2Considering the population of the x* orbital in molecular oxygen this is not surprising because this leads to a weakening of the 0-0 bond. Thus, 2 and 3 could be the precursors to the atomic species observed on an Ag(ll0) surface. The binding energies, Fermi levels and Ag-0 overlap populations for different adsorption geometries, 4 - 7,of atomic oxygen on an Ag(ll0) surface are given in Table 2.

380

7

6

Table 2. Relative Binding Energies, Fermi levels, Ag-0 overlap populations for 4 - 7. Binding energy (kcal/ mol) -

Fermi level (ev)

Overlap population Ag-O

~~

4

0.0

5

51.1

-11.39 -10.79

6 7

-2.5

-11.25

44.6

-11.00

0.280 0.233 0.187 0.217

The binding energy for 4 is found to be about 70 kcal/mol lower than those found for 1- 3, indicating that adsorbed atomic oxygen should be more stable on a silver surface. The adsorption sites for the most stable forms of adsorbed atomic oxygen on an Ag(l10) surface are the terminal A g - 0 species, 4, and oxygen bridging to two silver atoms in the [110] direction, 6. These are about 50 kcal/mol more stable than the two others. The DOS and COOP curves of interest for 4 are shown in Figure 2 (next page). It appears from Figure 2a that the oxygen p levels are spread over approximately 2.5 eV, from about -13.5 to -16 eV. Figures 2b and 2c show the DOS curvc’s for oxygen px and pz orbitals which interact with the metal d,, and dZ2orbitals producing the Ag-0 K and x* orbitals, and the (T and (T* orbitals, respectively. The COOP curvc’ (Figure 2d) shows that both the bonding and the antibonding Ag-0 orbitals i n 4 are located below the Fermi level.

381

Figure 2. (a) Total DOS for 4. The contribution for atomic oxygen is showed as the shaded area. The dotted line is the integration of the contribution from atomic oxygen. (b) The contribution from the oxygen px orbital. (c) The contribution from the oxygen pz orbital. (d) The COOP curve for the Ag-O bond. The present results point from an energetic point of view towards atomic oxygen as the active oxygen species on a Ag(ll0) surface. In an attempt to build a bridge from a surface to discrete molecules as catalysts for alkene epoxidation we will in the following use the Ag-0 surface species, 4, although we can not, of course, exclude the others from being involved in the oxygen transfer reactions to ethylene. Other theoretical investigations of oxygen transfer from silver surfaces to ethylene have also used atomic oxygen as a model (ref. 7). For epoxidation of alkenes catalyzed by metal complexes an 0x0-metal species have become widely accepted as the active intermediate (ref. 3). With an Ag-0 surface species such as 4, and with an orbital picture as shown in Figure 2d, and the the n: and JC* frontier orbitals of ethylene, two types of interaction between oxygen in 4 and ethylene are possible. The asymmetric approach 8 corresponds to an interaction of the cs* Ag-0 orbital with the x* of ethylene, whereas the symmetric approach, 9, is an interaction of the n* A g - 0 with the 7 ~ *orbital of ethylene. Besides the attractive interaction between the HOMOS of the oxygen and the LUMO of ethylene in 9, repulsive interactions between the Ag-0 (T*orbitals and the HOMO of ethylene are also observed.

8

9

382

Calculation of the total energy of 8 and 9 shows that 8 is about 8 kcal/mol more stable than 9. Bending the hydrogens up in 9, to make the carbons tetrahedral leads to an even more unfavourable system. With this favouring of the assymmetric intermediate, free rotation around the C-C bond becomes possible. This can cause the change in stereochemistry observed for the epoxidation of deuterio substituted ethylene (ref. 8). 2. Epoxidation of Alkenes Catalyzed by Discrete Silver Complexes. Some attempts to use simple Cu and Ag salts as catalyst for alkene epoxidation have been performed. Both silver nitrate and triflate in acetonitrile as the solvent were unsuccessful (ref. 9). However, we have found that Ag20 as well as AgN03, the latter in the presence of tertiary amines, can catalyze the oxidation of alkenes with iodosylbenzene as oxygen donor (ref. 10). The results for the epoxidation of different alkenes catalyzed by Ag20 and with iodosylbenzcnc as oxygen donor at 60 O C in CHC13 are given in Table 3.

Table 3. Epoxidation of different alkenes with Ag2O as catalyst and iodosylbenzene as oxygen donor. Alkene PhCH=CH2 Cyclohexene cis-PhCH=CHPh trans-PhCH=CHPh cis-PhCH=CHCH3 trans-PhCH=CHCHj

Epoxide/ % yield 15 2 12 (cis:truns = 1:5) 15 (trans) 18 (&:trans = 1:17) 25 (trans)

By-products in the reactions in Table 3 are mainly cleavage products (carbonyls) An interesting by-product is found in the oxidation of styrene, as phenyl acetate is produced. It has been found that phenyl acetate is formed via oxidation of either styrene oxide or acetophenone. Nearly the same yield and stereochemical mixture of the epoxides, as in Table 3, can be obtained by oxidation with Ag202. It appears from Table 3 that the epoxidation of alkenes catalyzed by discrete silver complexes also takes place without maintaining the stereochemistry of the alkene, as in the silversurface catalyzed reaction. AgN03, in the presence of excess pyridine (AgN03:pyridine = l:lO), produces a complex which also can act as a catalyst for the epoxidation with iodosylbenzene as the oxygen source. It has furthermore been found that the presence of pyridines substituted with electron-withdrawing groups increase the yield of the epoxides, whereas pyridines substituted with electron-donating groups decrease the yields. Epoxidation of styrene gives in the presence of pyridine only 2 % styrene oxide, whereas 18 % is obtained with the presence of 4-nitropyridine. The presence of 4-methoxypyridine causes a decrease in the styrene oxide formed as less than 1 % is observed. In the silver-surface catalyzed epoxidation of ethylene it has been found that electronegative moderators such as chlorine also lead to an increase in ethylene oxide yield (ref. 2).

383

The course of the oxygen atom transfer during the catalysis has been investigated by the incorporation of l 8 0 into the epoxide by isotopic l8O labelled iodosylbenzene or l80labelled water. The incorporation of l8O has been tested for both AgzO and AgNO~/pyridine.The reaction of styrene with l80labelled iodosylbenzene and AgzO as the catalyst (reaction 2) gives an incorporation of l8O into the epoxide which is nearly identical to the composition in iodosylbenzene indicating that the oxygen in the styrene oxide is derived from l8O labelled iodosylbenzene. Epoxidation of styrene by l60 labelled iodosylbenzene in the presence of labelled water and the AgN03/pyridine as catalyst gives about 40 % of l8O labelled oxygen in the styrene oxide formed.

The isotopic l80labelling experiments for the incorporation of oxygen into the epoxide, sup ort mainly an Ag-0 complex as the reactive species, because the incorporation of 0 labelled oxygen from H2l80 into the epoxide is consistent with an independent Ag-0 intermediate which undergoes isotopic exchange with l80enriched water (ref. 10) The experimental results for the silver-catalyzed epoxidation support also an intermediate where free rotation around the C-C bond is possible, 10. This intermediate lives long enough to allow rotation around the C-C bond which gives the thermodynamically more stable intermediate, 11, from which the trans-epoxide is formed (reaction 3). The intermediate, 10, is very similar to the one which was proposed for the silver-surface catalyzed epoxidation, 8.

f

R’

‘1”

C

-Ag

-

13)

/

10

11

Although studies of chemical reactions on transition-metal surfaces take place under very different conditions than chemical reactions catalyzed by discrete transition-metal complexes, several similarities are apparent, as indicated here. We are now in a periode of time where both experimental techniques and theoretical calculations are beginning to provide us with a picture of how chemical reactions on transition-metal surfaces take place - hopefully these studies will provoke others to try to develop discrete systems with similar properties, and thus show us that reac-

384

tions taking place on either a surface or in the coordination sphere of a transition metal are not that different. ACKNOWLEDGEMENTS Thanks are expressed to Professor Roald Hoffmann and 1 3 . Erik Larsen for cooporation in this work. REFERENCES. I. M. Campbell Catalysis at Surfaces, Chapman and Hall, London, 1988, 5. 1. For some recent reviews about the silver-surface catalyzed epoxidation: 2. (a) W.M.H. Satchler, C. Backx and 1I.A. van Santen Catal. Rev. 1981, 127. (b) M.A. Barteau a n d R.J. Madix In The Chemical Physics of Solid Surfaces arid Heterogeneous Catalysis; D.A. King and D.P. Woodruff (Eds.); Elsevier Scientific: Amsterdam, 1982, 45. (c) R.A. van Santen and H.P.C.E. Kuipers Adv. Catal. 1987, & 265. For some recent reviews: 3. (a) R.A. Sheldon and J.K. Kochi Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (b) B. Meunier Bull. SOC.Chim. Fr. 1986, 578. 431. (c) K.A. Jerrgensen Chem. Rev. 1989, 4. (a) R. Hoffmann 1. Chem. Phys. 1963,39- 1397. (b) M.-H. Whangbo, R. Hoffmann and R.B. Woodward Proc. R. SOC.London 1979, -231. (c) R. Hoffmann Solids and Surfaces: A Chemist's View on' Bonding in Extended Structures; VCH: New York 1988. C.T. Campbell and M.T. Paffeit Surf. Sci. 1984,139,396. 5. 6. (a) D.A. Outka, J. Stohr, W. Jark, P. Stevens, J. Solomon and R.J. Madix Phys. Rev. B 1987,175, 101. (b) K.C. Prince, G. Paolucci and A.M. Bradshaw Surf. Sci. 1986,375,101. 7. (a) E.A. Carter and W.A. Goddard 1111. Catal. 1988,112,80. (b) E.A. Carter and W.A. Goddard 111 Surf. Sci. 1988, 243. (c) P.J. van der Hoek, E.J. Baerends and R.A. van Santen I. Phys. Chem. 1 9 8 9 , 6469. ~ (d) K.A. Jerrgensen and R. Hoffmann 1. Phys. Chem. 1 9 9 0 , s 3046. 8. (a) W.F. Richey I. Phys. Chem. 1972, & 313 (b) A.L. Larrabee and R.L. Kuczkowski I. Catal. 1978, 72. (c) N.W. Cant and W.K. Hall 1. Catal. 1978,3 2 -! 81. 9. (a) R.B. VanAtta, C.C. Franklin and J.S. Valentine Inorg. Chem.. 1 9 8 4 , a 412. (b) C.C. Franklin, R.B. VanAtta and J.S. Valentine I. Am. Chem. Soc.. 1984,108, 814. 10. K.A. Jerrgensen and E. Larsen 1. Chem. SOC., Dalton Trans. 1990, 1053. 11. (a) J.T. Groves and W.J. Kruper I. Am. Chem. SOC.. 1979,101, 7613. (b) K. Srinivasan and J.K. Kochi Inorg. Chem.. 1985, 4671.

a

a

a

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1

385

0 1991 Elsevier Science Publishers B.V., Amsterdam

On the oxygen tolerance of noble metal catalysts

in liquid phase alcohol oxidations The influence of the support on catalyst deactivation P. Vinke, W. van der Poel and H. van Bekkum, Laboratory for Organic Chemistry, Delft University of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands. SUMMARY During noble metal catalyzed liquid phase oxidation of alcohols and related compounds, catalyst poisoning by oxygen often occurs. This catalyst deactivation was studied for ten different catalyst systems, using two different substrates. In the oxidation of 5-hydroxymethylfurfural (HMF) -a versatile intermediate obtainable from carbohydrates- the noble metal catalyst surface was protected by the strongly adsorbing substrate, thus preventing poisoning by oxygen. In the oxidation of methanol, the various noble metal catalysts display four types of oxygen tolerance, depending on the choice of metal and dispersion. Several supports were studied, including so-called poly-alumazane, which is prepared by subsequent treatment of silanol rich silica with aluminum trichloride and ammonia. With the resulting support palladium catalysts with very high dispersion were obtained. INTRODUCTION Liquid phase alcohol oxidation towards carbonyl compounds plays an important role e.g. in carbohydrate chemistry. Many stoichiometric oxidation procedures are known’, mostly without recycling of the oxidant2. Catalytic oxidation using oxygen (air) or hydrogen peroxide as the oxidant are attractive options towards clean oxidation technologies. In many cases supported noble metals are used, because of their high activity and selectivity in oxidation reactions. A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. Especially in the oxidation of low-reactive and weakly adsorbing substrates like nonreducing carbohydrates and carbohydrate derivatives this deactivation is enco~ntered”~. Possible ways to circumvent these problems include (i) the use of ’diffusion stabilized catalysts’ in which the outer shell of the catalyst particles serves as a diffusion barrier for oxygen’ and (ii) keeping the oxygen concentration in the liquid phase low, by applying a low oxygen partial pressure or a low stirring speed.

386

It was reported recently6 that, during methanol oxidation, activated carbon supported noble metal catalysts show quite different oxygen tolerances in the liquid phase. Some noble metals, like rhodium and ruthenium, had hardly any or no activity at all, whereas others like platinum and iridium had a maximum activity for methanol oxidation at oxygen concentrations of more than 2 ppm. We also showed7 that HMF, though relatively difficult to oxidize, protects a palladium surface during oxidation. In this way, catalyst poisoning by oxygen could be prevented. This protection was thought to be caused by strong HMF adsorption at the noble metal surface due to its aromatic nucleus. This paper reports on the oxygen tolerance of a series of catalyst systems, by subjecting two different substrates to oxidation: (i) methanol as an easily oxidizable model compound and (ii) HMF, a versatile intermediate, as a more difficult substrate with the advantage of the presence of two different oxidizable groups. Catalyst variables included the metal as well as the support. We also studied the influence of catalyst structure on the selectivity of the oxidation of HMF. As was reported before7, HMF can be oxidized into several different intermediates (cf. Scheme 1). Depending on noble metal and catalyst support the relative amounts of the intermediate oxidation products can be adjusted.

HMF

!iFCA

Scheriie 1. Oxidation products derived from HMF: 2,5-furundicarboxaldellyde (FDC), 5Iiydroxyrnetliyl-2-furuncarboxylicacid (HFCA), 5-formyl-2-furancarboxylicacid (FFCA) rod 2,S-furandicarboxylic acid (FDCA).

Besides some conventional supports, a new carrier material, poly-alumazane, was tested. Poly-alumazane, which was recently reported by Chinese researchers', is a silica carrier, the surface of which is modified by subsequent treatments with aluminum chloride and ammonia, forming an A - N phase on the silica surface. This coating is able to interact with two-valent noble metal ions (like Pd") which is found to result -after reduction- i n a catalyst with very high dispersion. MATERIALS AND METHODS Preparation of polv-alumazane. Poly-alumazane (denoted as AI-N) was prepared according to literature data' with some minor alterations. 20 g aluminum trichloride (AICI,) (Merck, PA) was dissolved in 500 ml nitromethane (Merck) in a 1 1 three-necked flask and 20 g silica (C 560, 200-500 bm, Uetikon, Switzerland, dried in vacuum at 120 "C) was added. After impregnation of the silica with AICI, the silica was treated with gaseous ammonia, followed by heating in nitrogen gas to 500 "C. The poly-alumazane was further prepared according to literature. Noble metal catalyst preparation. Platinum on AI-N (Pt/AI-N) was prepared according to

387

ref. Palladium on AI-N (Pd/AI-N) was prepared by adding 10 g Al-N to a solution of 0.836 g palladium chloride (PdCI,, Johnson Matthey) in 200 ml water. The suspension was stirred for 5 h. After separation in a centrifuge and drying at 100 "C under vacuum a dark brown powder was obtained. The catalyst was reduced with H, in cyclohexane at 20 "C. Nitrogen porosimetry. Nitrogen adsorption isotherms were measured at liquid nitrogen temperature on a Carlo Erba Sorptomatic 1800 apparatus. From these results BET surfaces ),S( and pore volumes and distributions could be determined. Carbon monoxide adsorption measurements. To determine the dispersion of the noble metal CO adsorption measurements were performed using a modified Quantasorb apparatus. The samples were reduced in flowing hydrogen for 16 h at 20 "C prior to the analysis. Then, the noble metal dispersion was determined by injection of pulses of CO and measuring the amount of CO which was not adsorbed. A blank experiment showed that A - N itself did not adsorb any CO. XPS analysis. XPS spectra were recorded on a Kratos X-SAM 800 apparatus. The signal was calibrated by adjusting the Cls signal of carbon to 284.6 eV. The Al-K, line was used as X-ray source. For Pt analysis the 4d5/, line (about 315 eV) was used instead of the 4f7/, line as was done before", because the 2p line of aluminum interferes with this platinum signal (about 75 eV). IR analysis, Infrared spectra were recorded on a Bruker IFS-66 lT-IR apparatus, using pellets of 1 to 2 mg substrate dispersed in 200 mg potassium bromide. TGA analysis. The amount of surface silanol groups present on the silica surface was determined using a TGA apparatus as described in literature". The sample was heated to 1000 "C at a rate of 10 "C/min in a flow of nitrogen. HPLC analvsis. The HPLC system consisted of a Waters 590 chromatography pump, a Waters R401 differential refractometer and a Perkin Elmer ISS-100 autosampler. A Biorad HPX87H column (strong ion exchange resin in Ht form) was used with 3*10" M trifluoro acetic acid as mobile phase at 60 "C. Oxidation eauimnent, The oxidation experiments were performed in the equipment described previously7. The glass batch reactor was thermostatted and had a volume of 300 ml. During reaction the solution was stirred with a gastight stirrer at 1500 rpm. The experiments were performed at constant pH. The oxygen partial pressure could be adjusted to any desired value between 0.05 and 1.0 atm and was kept constant during the reaction using an automatic oxygen supply system. The oxygen concentration in the liquid phase could be measured between 0.05 and 14 ppm in the temperature range from 10 to 40 "C using an Orion 970899 oxygen electrode. Oxidation procedure with measurement of the oxyeen concentration in the liauid phase. A calculated amount of catalyst, in order to obtain 12.5 pmol surface atoms noble metal, was introduced into the reactor. 50 ml of water was added and the system was flushed with nitrogen (500 ml/min) for 5 rnin to remove oxygen from the reactor. Then the reactor was flushed with hydrogen for 5 min, followed by 25 min at low gas flow and stirring at 1500 rpm. Finally the system was flushed with nitrogen for 5 min. In the case of methanol oxidation 1.28 g methanol in 30 ml water was added to the

388

reaction mixture (yielding a 0.5 M solution) under a low nitrogen flow. In the case of HMF oxidation, 2.02 g HMF, which resulted in a 0.2 M solution, was used. After equilibration of the reaction mixture at the preset temperature (30 "C) the desired oxygen partial pressure could be adjusted by withdrawing a calculated amount of gas from the reactor, which was automatically replaced by pure oxygen. In this way, the oxygen concentration in the gas phase was set to 0.05 atm. After adjusting the pH to 9 the reaction started. After a suitable time (depending on the rate of reaction) the oxygen concentration in the gas phase was increased by withdrawing another amount of gas from the reactor. This procedure was repeated several times until the oxygen concentration in the liquid phase reached a value of 14 ppm. At last, the reactor was flushed with pure oxygen for 3 min and the rate of reaction at 1 atm oxygen was measured. Procedure for the selective oxidation of HMF. The oxidation was performed as described before7. Reaction conditions were: 0.1 M HMF, 60 "C, pH 9, 1500 rpm, 80 ml water, p(0,) 0.2, 1 g dry powdered catalyst. The reactions were performed with catalysts 1, 3, 6 and 11 (see below). Samples were analyzed by HPLC. Oxidation catalysts. Table 1 lists the catalysts used for these reactions, together with some of their properties and the amount of catalyst needed for 12.5 fimol surface atoms noble metal. The catalysts were obtained from: 1, 5, 9, 11 Degussa A.G., Germany; 2, 6 Janssen Chimica, Belgium: 4 Strem Chemicals Inc., USA; 8 Drijfhout, The Netherlands; 10 Engelhard, The Netherlands; 12 Alfa Products, USA. Catalysts 3 and 7 were prepared as described above. Table 1. Oxidation catalysts used in this study. catalyst

no.

Pd/C Pd/A1 203 Pd/A1 -N Pd b l a c k Pt/C Pt/A1 203 Pt/A1 - N Pt black Rh/C Rh/A1 203 Ru/C Ir/C

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

metal (%) 5 5 4 100 5 5 1.7 100 5 5 5 5

dispersion 0.26 0.07 0.40 0.030 0.51 0.30 0.15 0.044 0.20 0.39 0.13 0.47

d r y m a t t e r (%) 47.2 92.7 90.0 100.0 52.3 98.8 92.2 100.0 49.6 97.4 48.1 100.0

mass ( g l l 0.222 0.397 0.103 0.045 0.183 0.165 1.074 0.055 0.251 0.076 0.405 0.100

'Amount of catalyst needed for 12.5 pmol surface atoms noble metal. RESULTS AND DISCUSSION Preparation of poly-alumazane. During impregnation the surface silanol groups of the silica are thought to react with AICI, yielding Si-0-AI bond structures under evolution of hydro-

389

chloric acid. Accordingly silica's with a relatively small amount of silanol groups, gave only a low conversion of AICI,. The silica selected had a large number of surface silanol groups (4.5 nm'2, as determined by TGA). After reaction with NH, and sublimation of excess NH,CI evidence for the presence of nitrogen containing groups is obtained from IR spectroscopy (see Figure 1) and elemental analysis (4.2% w/w). In the not calcined sample a signal at 3150 cm-' is found, indicating the presence of a quaternary ammonium compound. The absorption at 1400 cm-' is assigned to an N - H bond and the absorption at 630 cm-' is ascribed to an AI-CI bond12. In the calcined sample, the absorptions at 3150 and 1400 cm-' disappeared, indicating that quaternary nitrogen is not . 3000 present in the sample. The absorption at 630 4;oo 2000 1000 wavenurnber c r n - l cm-' is shifted to 700 cm-', indicating the formation of an A1,0, phase. Elemental Figure 1. IR spectra of (a) silicu, (b) polyanalysis of the calcined sample showed a as and (c) calcined nitrogen content of 0.23%. This indicates that poly-alummane. during calcination most of the N is oxidized under formation of an alumina phase. The initial overall surface reactions until the sublimation stage are shown in Scheme 2. The resulting product is a silica support covered by an alumina/poly-alumazane layer. The specific surface area and pore volume decreased significantly during preparation of the alumazane (silica 450 m2/g and 0.84 ml/g, polyalurnazane 290 m2/g and 0.75 ml/g). 27Al NMR did not reveal much information. TWO AICI,

, , Si

OH

OH

I

1

, ,,Si,

SI

AICI,

,d -HCI

SI,

CI

AICI,

I

I

0

0

0

I

I

I

1

Si,

Si,

,Si ,.O,

Si \

/A[\

t

SI,

0 NH,

+

-NH,CI

Scheme 2. Reaction pathway of the formation of poly-alurnazane until the sublimation stage.

390

broad peaks were found, corresponding with an asymmetric octahedral (0 ppm) and a tetrahedral (60 ppm) environment. Pt/AI-N and Pd/AI-N catalvsts. The amounts of noble metal added as PdCI, and H,PtCl, during catalyst preparation were adjusted in order to obtain a 5% metal loading. The adsorption of noble metal onto the Al-N did not proceed quantitatively, resulting in a 4% metal loading for Pd, whereas for Pt the loading was only 1.7% (determined by ICP and AAS). The Pd" and Pt'" complexes are thought to interact especially with nitrogen or disturbed alumina sites present in the surface layer of the modified support. During adsorption of the Pd and Pt compounds, the color of the catalyst changed from yellow to dark gray, indicating the formation of small reduced noble metal particles. Moreover XPS measurements indicated the presence of Pt" at the catalyst surface. In the case of Pd XPS results were similar: the surface contained Pd" also. Probably, surface oxide layers are responsible for these MI' species. The dispersion of the resulting Pt and Pd catalyst as measured by CO adsorption is given in Table 1. The noble metal catalvzed oxidation of HMF and methanol. In order to he able to study the oxygen tolerance for the different catalyst systems, the oxygen partial pressure in the gas phase was increased in steps of 0.05 atm. Meanwhile the oxygen consumption and oxygen concentration in the liquid phase [O2IL were monitored. Figure 2 gives, as an example, the resulting graphs for methanol oxidation over Pd/C. In the left graph the oxygen partial pressure in the gas phase and the corresponding [O2IL are shown and in the right graph the oxygen consumption as a function of time is shown. These graphs illustrate the course of the oxidation up to a certain p(0,) -in this example 0.35 atm- where surface oxidation leads to an inactivated catalyst and oxygen consumption ceases, followed by an increase in oxygen concentration in the liquid phase to the equilibrium value. From these results, the turnover number (TON, defined as mol 0, consumed per mol of surface noble metal per minute) as a function of [O2IL can be calculated for every experiment, revealing information about the oxygen tolerance of the different catalyst systems.

50

0 40

I

100,

1

1

/' 'I

"'"I..: 1 '

6.

,

,

000

,

,

~

0

100

200

t (rnin)

300

,

125

00

400

Lri-'

'10 0

0

100

200

300

400

t (rnin)

Figure 2. Course of the oxidation of methanol over a Pd/C catalyst under standard conditions (pH 9, 30 "C,155 m o l Pd surface atoms, 0.5 M methanol, 1500 tprn). Left gruph: (- - - -) = P ( 0 2 ) > (-)=

l02lL.

391

Four types of oxygen tolerance can be distinguished: (1) a very low activity over the whole range

P

.-L E

of oxygen partial pressures is observed, (2) a relatively high maximum activity occurs = at very low [O2IL ( < O S ppm) followed by ? rapid deactivation at higher values, (3) maximum activity is found at moderate values of [02],(between 1 and 10 ppm),

4 0

30 20

10

oo 0

2

4

6

8

1 0 1 2 1 4

++

35

[O*l, (PPm) followed by deactivation and (4) oxidation is essentially first order in [O,], Figure 3. The 4 types of oggen tolerance with a maximum activity at 35 ppm (water illustrated by methanol oxidations performed standard conditions. (1) A Pd/Al-N, (2) o saturated with atm 02). Pd/C, (3) Pd/A1203 and (4) v Pt/Al,O, The four types of oxygen tolerance are exemplified with four experiments of the oxidation of methanol as shown in Figure 3. Table 2 summarizes the results of the oxidation of methanol and HMF for the different catalyst systems. Methanol oxidation. In the interpretation of the oxidation results, two aspects should be considered separately, ( i ) the type of oxygen tolerance for each single catalyst system in connection with the dispersion of the catalyst and (ii) the change in TON as a function of noble metal dispersion for one type of oxygen tolerance. As can be seen from Table 2 the catalysts gain stability upon lowering the dispersion. Probably, this is caused by the fact that small noble metal particles loose their 'noble character' and can be oxidized relatively easy. The particle size at which this loss of metal character may occur depends on the Fermi level of the noble metal13. In general, particles of 400 atoms or less (k 2 nm diameter and D,= 0.4) can be considered to be non-metallic. In the case of the Pt catalysts, a change in oxygen stability and TON can be observed between Pt/C and the other Pt catalysts (entries 5 and 6, 7, 8). The Pt/AI-N, for instance, appears to be an active and oxygen tolerant catalyst. For Pd no type 4 tolerance is observed, but here a significant change in activity can be seen, too. Another important result in the oxidation of methanol, is the decrease in TON upon increasing metal dispersion (entries 6, 7 and 8). Apparently, the rate of reaction is higher at crystal planes with low indices (which are predominantly present at larger metal particles). This surprising also applicable to methanol oxidation over Pd catalysts (entries 2 and 4). In order to obtain a better insight in the influence of dispersion on catalyst stability and activity, some experiments using catalysts with different dispersions but the same support are in progress. In the case of Rh, Ru and Ir the results are in accordance with literature data6. Rh and particularly Ru show hardly any activity for methanol oxidation, due to rapid oxidation of the metal surface. This is in harmony with the general thought that Rh and Ru are somewhat 'less noble' (thus are easier to oxidize) than the other metals tested. The stability of Ir is good, although the metal is not very active.

392

Tuhle 2. [OJL at maximum TON for the oxidation experiments of HMF and methanol at stundard reaction conditions (water, 30 "C, p H 9, 1500 tpm, 0.5 M methanol or 0.2 M HMF).

catalyst Pd/C

Pd/Al 203 Pd/A1 -N Pd black Pt/C Pt/Al 203 Pt/A1 - N P t black Rh/C Rh/A1 203 Ru/C I r/C a

metal di sp.

0.26 0.07 0.40 0.030 0.51 0.30 0.15 0.044 0.20 0.39 0.13 0.47

no.

1 2 3 4 5 6 7 8 9 10

11 12

methanol oxidation TON [021La typeb Dom /min 0.45 3.66 2 3 2.0 1.72 1 0.9 0.15 3 12.5 3.6 3 4.9 9.4 4 4.95 35 4 10.6 35 15.1 4 35 0.09 1.6 1, 3 1 .o 1 0.04 0.0 1 3 1.1 1.6

no.

HMF oxidation TON [021La typeb

13 14 15 16 17 18 19 20 21 22 23 24

/min 1.53 0.77 0.29 0.83 0.24 0.19 0.14 1.35 0.13 0.09 0.0 0.0

ppm 35 35 35 35 35 35 35 35 1.6 1.6

4 4 4 4 4 4 4 4 3 1, 3

1 1

[O2ILat max TON, type of oxygen tolerance (cf. Figure 3).

HMF oxidation. In comparison with the oxidation of methanol, the picture changes significantly when oxidizing HMF. First of all, the type of oxygen tolerance changes for all Pd and Pt catalysts; the catalyst is not poisoned by oxygen anymore, indicating a protection of the noble metal surface by HMF. Also in the case of Rh, however, the catalyst is not protected by HMF, so this catalyst shows the same stability as for the oxidation of methanol. In the cases of Ru and Ir the catalyst is deactivated, which means that these metals only have a small interaction with the substrate and/or are unable to oxidize HMF at these reaction conditions. Secondly, compared to methanol oxidation , the rates of reaction decrease significantly upon oxidation of HMF for the Pt and Pd catalysts. In the case of Pt this decrease is drastic under the mild conditions applied here, indicating a strong interaction of HMF with the Pt surface. In the case of Pd the decrease is less pronounced and for Pd/Al-N even an increase in rate of reaction is found (entries 3 and 15). Probably the interaction of HMF with Pd is less strong, especially in the case of highly dispersed Pd. In order to study the interaction of HMF with noble metals, DRIFT (diffuse reflectance infrared fourier transform) measurements are in progress. The selective oxidation of HMF. In general, in HMF oxidation, a surprisingly high selectivity towards 5-formylfuran carboxylic acid (FFCA) is found, showing preferential oxidation of the primary alcohol in the presence of the aldehyde group. Normally, aldehydes are oxidized selectively in the presence of primary alcohol groups. The 'reversed' oxidation sequence for HMF is explained by assuming a stabilization of the non-hydrated form of the aldehyde groups caused by its resonance with the aromatic nucleus. Dehydrogenation of the

393

aldehyde is possible only in the hydrated form. The intermediate dialdehyde FDC is less stabilized, allowing hydration of an aldehyde group followed by oxidation towards FFCA. Although the high selectivity towards FFCA is inherent to HMF itself, the catalyst also influences the course of the reaction. In Figure 4 the course of the oxidation of HMF is shown for four different catalysts. Under the conditions applied here (60 "C) Ru/C (graph c) showed a relatively small activity for HMF oxidation, whereas at lower temperatures (30 "C, see Table 2) the catalyst was totally inactive. The increase in stability at higher temperatures is probably caused by higher overall rates of dehydrogenation of the substrate together with a decrease in oxygen solubility. At 60 "C HMF still protects the noble metal surface of the Pd and Ru catalysts (graphs b, c, d), while catalyst deactivation occurs after consumption of the HMF. Only in the case of Pt/Al,O, (graph a) the catalyst remains stable and active, yielding the final oxidation product FDCA in quantitative yield. The selectivity towards FFCA is dependent on the type of catalyst used. For example the maximum yield in the case of Pd catalysts is about SO%, whereas Pt and Ru reach a maximum of 75%. In the case of Pd, oxidation of the aldehyde group as first step of the reaction (leading to HFCA) plays an important role. This might be due to a higher concentration of hydrated HMF present on the Pd surface. Also the catalyst support influences the selectivity of the oxidation. For Pd/Al-N the selectivity towards HFCA is higher than for Pd/C, with the maximum yield of FFCA being almost the same. 0 10 F

2

-'t

-D

b

0' l o0/ , 8

008

E

OOG

0 06

I\

9

004

C

; 002 c 0

000 0

100

200

300

400

0

500

100

200

300

400

500

400

500

t (min)

t (mi n)

0.04

0 02 0 00 0

100

200

300

t (mi n)

400

500

0

100

200

300

t (min)

Figure 4. Oxidation of HMF over different catalysts under standard conditions (60 "C). v 5hydroxymethylfurfural (HMF), U 2,s-furandicarboxaldehyde (FDC), A 5-hydronymethyl-2furancarboxylic acid (HFCA), o 5-formyl-2-furancarboxylicacid (FFCA), + 2JfurandicarOoxylic acid (FDCA). Catalysts:graph a Pt/A120i b Pd/C; c Ru/C; d 3 Pd/Al-N

394

CONCLUSIONS The measurement of TON versus [O2IL is a useful tool for studying the oxygen tolerance of oxidation catalyst systems. The oxidation of methanol over a series of noble metal catalyst systems reveals large differences in catalyst stability and TON. Pt catalysts appears to be most stable and active, whereas Pd shows less stability, depending on metal dispersion. Upon oxidation of HMF, all Pd and Pt catalysts show a very good tolerance against oxygen. Pd catalysts are the most active in this case, and here also, metal dispersion plays an important role in rate of reaction. Upon oxidation of HMF, the selectivities towards intermediate oxidation products depend primarily on the catalyst system. Pt appears to be a stable and active catalyst with a good selectivity towards FFCA. The use of Pd results in a relative high yield of HFCA. The use of AI-N as catalyst support yields interesting catalysts with improved characteristics (especially in the case of Pd/AI-N). This support proves to be an interesting variable in catalyst design. ACKNOWLEDGEMENTS We want to thank Dr. Mulder of KSLA (Shell Laboratory Amsterdam) for performing the XPS measurements, Siidzucker A.G. for providing samples of HMF and Johnson Matthey for providing samples of H,PtCI,. We also want to thank Mr. Teunisse and Mr Van Westen for performing the nitrogen porosimetry and CO adsorption measurements. The Netherlands Organization for Scientific Research (NWO/SON) is gratefully acknowledged for financial support. REFERENCES 1. W.J. Mijs, C.R.H.I. de Jonge (editors), Organic Syntheses by Oxidation with Metal Compounds, Plenum Press, New York & London, 1986. 2. M. Floor, A.P.G. Kieboom and H. van Bekkum, Red. Truv. Chirn. fuys-Bus 108 (1989)

128. 3. H.E. van Dam, A.P.G. Kieboom and H. van Bekkum, Appl. Cutul. 33 (1987) 361. 4. P.J.M. Dijkgraaf, M.J.M. de Rijk, J. Meuldijk and K. van der Wiele, J. Cutul. 112 (1988) 329. 5. H.E. van Dam, P. Duijverman, A.P.G. Kieboom and H. van Bekkum, Appl. Catal. 33 (1987) 373. 6. H.E. van Dam, L.J. Wisse and H. van Bekkum, Appl. Cutul. 61 (1990) 187. 7. P. Vinke, H.E. van Dam and H. van Bekkum, in ’New Developments in Selective Oxidation’, Studies in Surface Science and Catalysis Vol. 55, G. Centi, F. Trifero’ (Editors), Elsevier, Amsterdam (1990) 147. 8. Y.X. Yuan, M.Y. Huang and Y.Y. Jiang, J. Mucrornol. Sci.-Cliern. A24 (1987) 261. Or S.K. Cao, M.Y. Huang and Y.Y. Jiang, Polymer Bulletin 19 (1988) 353. 9. Y.X. Yuan, M.Y. Huang and Y.Y. Jiang, J. Cutul. (Cuilluu Xuebuo) 9 (1988) 223. 10. M.Y. Huang, C.Y. Ren, Y.Y. Jiang, J. Mucrornol. Sct-Chern. A24 (1987) 269. 11. G. Hakvoort, Thesis Delft University of Technology, 1978. 12. K. Nakamoto, Infrared and Raman spectra of inorganic and coordination compounds, 4th edition, Wiley, New York etc., 1986. 13. M. Che, C.O. Bennet, Adv. Cutul. 36 (1989) 55.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

395

IRON-PHTHALLOCYANINESENCAGED IN ZEOLITE Y AND VPI-5 MOLECULAR SIEVE AS CATALYSTS FOR THE OXYFUNCTIONALIZATION OF n-ALKANES RUDY F. PARTON, LIEVE UYTTERHOEVEN and PETER A. JACOBS K.U. Leuven, Dept. Biotechnische Wetenschap en, Laboratorium voor Oppervlaktechemie, Kardinaal Mercierlaan 92, B-3030, HeverEe (Leuven), Belgium. ABSTRACT The preparation of iron-phthallocyanines encaged in molecular sieves was made from ferrocene impregnated in zeolite Y and VPIJ molecular sieve. Both catalysts exhibit shape selectivity in the oxyfunctionalization of alkanes with tertiary butyl hydroperoxide as oxy en donor at 298 K and 0.1 MPa. As shown by turnover numbers resistance of the zeofite encaged corn lexes against oxidative destruction and activity in the oxidation of nalkanes exceed by ar those of free iron-phthallocyanines. Molecular graphics analysis shows the complex in both molecular sieves and presents a qualitative interpretation of the higher shape selectivity in the regioselective oxidation of n-octane on ironphthallocyanine encaged in zeolite Y than in V P I J .

P

INTRODUCTION Enzymes are catalytic proteins that control the rates of biological reactions. The active site of metallo-enzymes is a metal ion, whereas the major function of the protein is the construction of a tridimensional mantle which causes selective conversion of substrates and creates the right environment for the metal. Zeolites seem to be promising catalysts for the conversion of fine chemicals and organic intermediates [ 1-31, Metallo-phthallocyanines encaged in zeolites Y have been proposed as enzyme mimics [4-71. Zeolites can replace the protein portion of natural enzymes and modify the reactivity in the same way as enzymes do by imposing steric constraints on the environment of the active metal ion site. In the present work, the construction of a mimic of cytochrome P-450 is attempted by in situ synthesis of iron-phthallocyanines in the supercages of zeolite Y and in the channels of VPI-5.Its catalytic activity and selectivity is tested in the oxyfunctionalization of n-alkanes with tertiary butyl hydroperoxide. EXPERIMENTAL Materials A sample of NaY with a Si/Al ratio of 2.46 was purchased from Ventron. VPI-S was synthesized according to a recent literature procedure [8]. 1,2-Dicyanobenzene (DCB) (+98%), ferrocene (Cp2) (t 98%), tertiairy butyl hydroperoxide (t.BHP) (70% in water), acetone, dichloromethane (CH2C12), n-pentane, n-hexane, n-heptane, n-octane, n-nonane and n-decane (all t 99%) were purchased from Aldrich. H2-Phthallocyanine (H2Pc) (t 98%) was purchased from Aldrich and Fe-phthallocyanine (FePc) (+98%) from Strem Chemicals.

396

Ferricenium-Y is prepared by adding 5 g of air dried NaY to 50 ml of a solution of ferrocene in acetone containing 84 mg ( = 1 complex per unit cell) of C p , followed by air drying at 343 K. The latter solid is mixed with 5 g of DCB and heated under He atmosphere at 423 K for 4 hours. This khaki-coloured solid is succesively soxhlet extracted with acetone, pyridine and again with acetone, until a colourless extract is obtained. The final catalyst is air dried at 343 K. FePcVPI-5 is synthesized at 523 instead of 423 K according to the same procedure. Characterisation I.R. characterisation of the samples is carried out using the KBr technique. U.V. spectroscopy was used for the semi-quantitative determination of the amount of intracrystalline phthallocyanines, after dissolution of the zeolite in concentrated sulfuric acid (0.1 g of catalyst in 10 ml of concentrated H2S04 for 4 h). The iron content of the samples is determined by chemical analysis. X-ray powder diffraction is used to ensure good crystallinity of the zeolite after the synthesis and purification procedures. Molecular eraDhicS A molecular graphics analysis of phthallocyanines in faujasite and VPIJ is performed with a ChemX-software package from Chemical Design Ltd. Structural data of the molecular sieves and the phthallocyanine complexes were taken from the literature [9111. Reaction DroOxyfunctionalization reactions of n-alkanes are carried out at room temperature and atmospheric pressure with t.BHP as oxidans and acetone as solvent. Product analysis was done with GC on a 50 m CP Sil-88 capillary column from Chrompack. RESULTS AND DISCUSSION Characterisation of encaeed FePc When dichloromethane solutions of Cp2 or FePc are impregnated on NaY or VPI-5, and heated at 423 and 523 K respectively, the application of the standard soxhlet extraction procedure removes all iron from the solids. The same is true for ferricenium-Y and ferrocenium-WI-5, When in situ synthesis of FePc is made in both molecular sieve structures, the extraction procedure removes only part of the iron. Thus all residual iron present is associated with encaged FePc. I.R. data shown in Fig. 1 confirm this. Indeed, after extraction the characteristic lines of Cp2 (814 cm-l) and DCB (965 cm-l) are absent. Surprisingly, the major I.R. bands, which are present in the sample after synthesis and extraction in zeolite Y, are not those of Fe-phthallocyanine but those of H2-phthallocyanines. Doublets of some I.R. bands (Fig. 1) are characteristic for H2Pc. The presence of H2Pc is not unexpected as the initial amount of DCB is largely in excess over Cp2. A mixture of FePc and H2Pc exists also in the samples of FePcVPI-5, the latter being present in excess. Chemical analysis combined with U.V. spectroscopy of samples dissolved in concentrated sulfuric acid allow to determine the amount of FePc and H2Pc present in the molecular sieves. The NaY samples used for the catalytic experiments have about 1 FePc for every 77 supercages and about 1 H2Pc for every 8 supercages. In case of VPI-5 samples the amount of intracrystalline FePc and H2Pc corresponds to 0.021 and 0.2 for each unit cell, respectively.

397

1500

1000

1500

1000

Wave number (ern-') Fig. 1. LR. spectra of KBr pellets of (A) FePc, (B) HzPc, (C) FePcY, (D) DCB, (E) Cp2 and (F) FePcVPI-5. Molecular hics analvsis of Dhthallwnines in molecular sieves FePc was located in the faujasite structure with the centre of the molecule at the centre of the supercage, the bridging N-atoms being oriented to the four-rings of the cubooctahedra. To avoid overlap between the molecule and the zeolite structure, the planarity of the complex had to be disturbed. This was done by rotating the aromatic rings around an axis through the two carbon atoms of the pyrrole rings bonded to the bridging N-atoms (Fig. 2).

Fig. 2. Representation of the FePc molecule and its deformation in the supercage of zeolite Y (axis A: downward rotation and axis B: upward rotation) and in VPIJ (axis C is parallel to the channel axis).

398

The aromatic rings are lifted out of the original plane and stick through the 12-ring pores of the supercage. The minimum deviation from the original plane is found to be 37.27O, based on an ionic radius of 0.026 nm and 0.136 nm for Si and 0, respectively [ 121 and a van der Waals radius of 0.1 and 0.16 nm for H and C, respectively. The saddleconformation thus obtained is shown in Fig. 3A. Skeletal flexibility of porphyrines and saddle-type conformations are known from literature and can be imposed by crystal packing, steric effects or protein constraints [13-141. If the aromatic rings of FePc encaged in zeolite NaY are oriented to the centre of the 12-rings a deformation of 67.27' out of the original plane is required.

A

B

Fig. 3. Molecular graphics representation of encaged FePc in zeolite NaY (A) and VPI-5 (B). The deformation of the planar FePc molecule determines the free apertures to the active site. The latter can be derived from the distance between the benzene rings and the zeolite framework. The distance varies from 0.65 nm, for a deformation of 37.27', to 0.52 nm, for a deformation of 67.27O. The distance between the centre of the molecule and the lattice oxygens is between 0.74 and 0.77 nm. In the same way, FePc in VPI-5 was analysed. The molecule was oriented parallel to the axis of the VPI-5 pores with two bridging N-atoms on the channel axis as shown in Fig. 3B. No overlap between the molecule and the molecular sieve occurs, provided the plane of FePc intersects two opposite six-rings in the 18-membered ring pore of the molecular sieve (Fig. 3B). The shortest distances between iron and framework oxygen is 0.76 nm.

399

Catalvtic activitv with FePcY Zeolite-enclosed FePc are suitable catalysts for the selective oxidation of paraffines to ketones and alcohols at ambient temperature and pressure with t.BHP as oxygen atom donor. Oxidation reactions are performed both in air and in nitrogen atmosphere without significant changes in conversion and product distribution. Fig. 4 shows the conversion of a series of n-alkanes with different chain length on FePc and FePcY catalysts. FePc dissolved in dichloromethane is shown to be much less active than when it is encaged in a zeolite framework. The yield of alcohols and ketones on peroxide basis on FePcY and the FePc catalysts is 75% and less than lo%, respectively. The major side reaction is the decomposition of the organic peroxide to molecular oxygen and t.butano1.

15 n

8

W

10

fi

0 e n

I I I

h Q)

P

fi

5

f 0

C-5

C-6

C-7

C-8

C-9

C-10

Substrate Fig. 4. Conversion after 2 hours of vigorous stirring of CS to C10 n-alkanes to alcohols and ketones, carried out at 298 K and 0.1 MPa in a microreactor of 3 ml with 2.4 mmol t.BHP, and 6 mmol paraffin. 8.103 mmol FePc was used in 1.5 ml dichloromethane and 0.1 g FePcY in 1.5 ml acetone. A second difference between the homogeneous and heterogeneous FePc catalysts is their catalytic stability. The latter catalyst does not undergo any colour change and can be reused. In contrast the homogeneous FePc catalyst is completely destroyed during the first run, while changing colour from blue-green to light yellow. Turnover numbers for the selective oxygen insertion reaction vary between 20 and 30 for the FePc catalyst and between 180 and 260 for the zeolite enclosed phthallocyanines. Fe3+-Y, ferricenium-Y, ferrocenium-Y as well as H2Pc impregnated on NaY are inactive, indicating that the low amounts of FePc present in the supercages of zeolite Y are the active sites. Fig. 4 also shows a shallow maximum in activity for an alkane chain length of 6 to 7 carbon atoms. As secondary carbon atoms are more reactive than primary ones, the activity is expected to increase with chain length. To explain the maximum, the diffusivity of the n-alkanes should decrease with chain length.

400

Fig. 5 shows for both catalysts the distribution of ketunes and alcohols against the chain length of the n-alkanes. Both catalysts exhibit a high selectivity towards the formation of ketones. The selectivity for the formation of alcohols seems to decrease somewhat with chain length. The ketone to alcohol ratio which ranges from about 3 to 10, is typical for free radical chain oxidation reactions. It suggests that the active site is not an "oxenoTd species as in enzymes [15] and their mimics [16-181, but that the reaction rather occurs via a free radical chain mechanism. This chain mechanism seems to be disturbed by the zeolite framework where the reaction yield on a peroxide basis exceeds that of free FePc by a factor of 10. However, Haber et al. [19] using the oxidation of cyclohexane with hydrogen peroxide over chloro-tetratolylporphyrinatochromium(III), obtained under comparable conditions ketone to alcohol ratios between 1.4 and 4, and claimed that the reactions were not radicalar in nature. The enhanced ketone/alcohol ratio observed with the heterogeneous catalyst, points again to the existence of diffusion limitations. Catalyst: FePcY

Catalyst: FePc

Lotone

100

alcohol

100 A

dc

v

--

80

w

dc

80

60

." .-*

60

*

Y

L

*

C

-

40

u

U

40

0

al 0 v)

I

20 0

0

rn

20

*

c-5 c-6 c-7 c-8 c-9 c-I0

C-5

C-6 C-7 C-8 C-9 C-10

Substrate

Substrate

Fig. 5. Selectivity for alcohols and ketones in the oxidation reaction of C5 to C10 nalkanes. Conditions are the same as in Fig. 4. 3 R

CI

. I

c

* 0

. I

2

8 m 8

v1

d .C1

M

1

2

0

C-6

C-7

C-8

C-9

c-10

Substrate Fig. 6. Re ioselectivi in the ketone fraction expressed as standarized molar ratios of C2/C3ancfC2/(C4+ 5). Corrections are made so as to have an equal number of C2 and

2

(C4 t C5) positions in the n-alkane chain, irrespective of its chain length. Conditions are the same as in Fig. 4.

401

On homogeneous FePc no regioselectivity is observed as the C2/C3 and C2/(C4+C5) ratios are all around 1. The same is true for n-hexane on FePcY (Fig. 6). However, for longer chains the zeolite enclosed FePc exerts a shape selective effect of increasing intensity on the insertion of oxygen. C2 carbon atoms are preferentially oxidized over C3, which in turn are oxidized faster than those at position 4. Thus the regioselectivity increases with chain length, obviously, because longer alkyl chains are more liable to steric constraints exerted by the zeolite framework at the level of the active site. Regioselectivityin the alcohol fraction is insignificant.

f mparison l $ Fig. 7 shows the conversion of octane to ketones and alcohols on FePcY and FePcVPI-5 molecular sieves at a constant rate of t.BHP addition. In FePcY initial conversions approach the limit imposed by reaction stoichiometry. Upon further addition of oxidans the conversion curve gradually deviates from the theoretical line, pointing to catalyst deactivation. At this stage, turnover numbers (N)per FePc molecule amount to 6,000 and exceed those of homogeneous FePc (N ranging from 20 to 30) as well as those reported in the literature on FePcY catalysts with iodosobenzene as oxidans (N=5.6) [6, 91. It is clear that the zeolite protects the phthallocyanine structure against oxidation compared to the homogeneous case, just as the protein mantle does in the enzyme cytochrome P-450. It also follows that the stability of the presently prepared FePcY is by far superior to previously reported attempts.

/

PcPcVPI-5

0

100% ketone formation

I

0.00

0.20

1

0.40

1

1

,

1

1

0.60

1

1

1

1

1

1

0.80

1

1

1

1

,

1.00

,

,

1.20

Ratio of t.BHP to octane (moI/moI) Fig. 7. Oxidation conversion of n-C8 with t.BHP to ketones and alcohols at 298 K and 0.1 MPa. With FePcY 0.5 g catalyst (1FePc/7 supercages), 200 mmol n-C8, lOOml acetone and a t.BHP injection rate of 21.9 mmo1.h- is used. With FePcVPI-5 0.1 g catalyst (0. 21 per unit cell), 50 mmol n-C8,5Oml acetone and a t.BHP injection rate of 8.76 mmo1.h- is used. The straight line represents the maximum possible conversion when only ketones are formed and t.BHP conversion is complete.

1

9

402

Even after correction for differences in experimental conditions, FePcVPI-5 remains less active than FePcY. In the former system, free t.BHP accumulates gradually while with FePcY all t.BHP added is consumed at once. The linear increase in conversion observed with FePcVPI-5, suggests the absence of catalyst deactivation, at least up to turnover numbers of about 1,500. At this stage of the work, two possible explanations can be advanced to rationalize the higher activity of FePcY and the superior stability of FePcVPI-5. As a result of the tubular nature of the pores of VPI-5 and eventual diffusion limitations of the reaction, only the FePc complexes at the external rim of the crystals are active initially but are gradually consumed during reaction. Consequently, FePc complexes located more towards the centre of the crystals become active. Secondly, it can be speculated that the saddleconformation of FePc in NaY changes the electronic environment of the active iron, thus increasing not only its catalytic activity but also its vulnerability towards self-oxidation. Fig. 8A shows that the selectivity for ketone and alcohol formation is quite similar for both molecular sieves. Fig. 8B indicates that regioselectivity exists for both molecular sieves, possibly due to the encaged nature of the complex. However, lower values of the C2/C3 and C2/C4 ratios are obtained in VPI-5 compared to zeolite Y, pointing to the existence of shape selectivity. The molecular graphics analysis, which enabled quantification of the free pore apertures, shows that the difference in selectivity can hardly be caused by differences in the zeolitic environment. The enhanced constraint observed for FePcY should then be related to the saddle-type deformation of the complex.

B

A EFa

alcohol

ketone

3

Y

= Y V

c

.n

.-E

40

1.00

lid

Y

I

;

1.50

Y

60

. Y n

t

C2IC4

h

;.

I0

Y

h

C2lC3

2.00

100

20 0

0.50

0.0 0

FePcY

FePcVPI- 5

Citalyst

FePcY

FePcVPI- 5

Catalyst

Fig. 8. Selectivity for ketone and alcohol formation (A) and regioselectivity (B) in the oxidation of n-octane at 5% conversion over FePcY and FePcVPI-5 catalysts. Conditions are those of Fig. 7.

403

CONCLUSION Iron-phthallocyanines are encaged in molecular sieves by an in situ synthesis from ferrocenium-Y and ferricenium-VPI-5. Molecular graphics analysis of the encaged ironphthallocyanines shows a saddle-type deformation of the complex in zeolite Y and no deformation in VPI-5 molecular sieve. The activity of iron-phthallocyanines in zeolite Y is higher than in the VPI-5 catalyst which in turn exceeds those of the free complexes as shown by turnover numbers of respectively, 6,000, 1,500 and 25. However, ironphthallocyanine-VPI-5 doesnot seem to deactivate, whereas the FePcY catalyst after turnover numbers higher than 6000 becomes completely inactive and the free complex suffers from very fast oxidative destruction. Shape selectivity in the regioselective oxidation of alkanes is higher on FePcY than on FePcVPI-5, which in turn is higher than on the non-selective homogeneous catalyst. This can as explained by a combined effect of steric constraint imposed by the framework of the molecular sieve and the deformation of the phthallocyanine. ACKNOWLEDGMENTS The authors are rateful to the Belgian Fund of Scientific Research for a grant. One of us (RFP) acknowlegkes a fellowship from the same institution. REFERENCES 1. R.F. Parton, J.M. Jacobs, D.R. Huybrechts and P.A. Jacobs, Stud. Surf. Sci., 46 (1989) 163-193. H. van Bekkum and H.W. Kouwenhoven, Recl. Trav. Chim. Pays-Bas, 108 (1989) 2. 283-294. 3. W.F. Holderich, Stud. Surf. Sci., 49 (1989) 69-93. N. Herron, G.D. Stucky and C.A. Tolman, J. Chem. SOC.Chem. Commun., (1986) 4. 1521-1522. G. Meyer, D. Wohrle, M. Mohl and G. Schulz-Ekloff, Zeolites, 4 (1984) 30-34. 5. 6. B.V. Romanovsky, Proceed. 8th Int. Congr. Catal., Verlag Chemie, Weinheim, 4 (1984) 657-667. N. Herron, J. Coord. Chem., 19 (1988) 25-38. 7. P.J. Grobet, J.A. Martens, I. Balakrishnan, M. Mertens and P.A. Jacobs, Appl. 8. Catal., 56 (1989) L21-L27. K.A. Van Genechten and W.J.Mortier, Zeolites, 8 (1988) 273-283. 9. 10. C.E. Crowder, J.M. Garces and M.E. Davis, Adv. X-Ray Analysis, 32 (1988) 5075 14. 11. R. Mason, G.A. Willems and P.E. Fielding, J. Chem. SOC. Dalton Trans., (1979) 676-683. 12. R.D. Shannon, Acta Cryst., A32 (1976) 751-767. 13. KM. Barki ia, L. Chantranupong, K.M. Smith and J. Fajer, J. Am. Chem. SOC.,110 (1988) 756l7567 14. A. Forman, M.W. Renner, E. Fujita, K.M. Barkigia, M.C.W. Evans, K.M. Smith and J. Fajer, Isr. J. Chem., 29 (1989) 57-64. 15. ochrome P-450: Structure, Mechanism and Biochemistry, Ed. P.R. Ortiz de

16. 17. 18. 19.

J. Haber, R. Iwanejko and T. Mlodnicka, J. Molec. Catal., 55 (1989) 268-275.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

405

MILD OXIDATION OF CYCLIC C6-clO HYDROCARBONS IN LIQUID PHASE AT ROOM TEMPERATURE BY HETEROGENEOUS PHOTOCATALYSIS J.M. HERRMANN, W. MU and P. PICHAT Photocatalyse, Catalyse et Environnement, URA au CNRS no 1385, Ecole Centrale de Lyon, B.P. 163 - 69131 ECULLY Cedex, France. SUMMARY The mild oxidation of various liquid hydrocarbons (cyclohexane, methyl- and 1,4dimethylcyclohexane, decalin, cyclohexene and tetralin) by 0 2 (or air) over UV-illuminated Ti02 at room temperature has been studied in a static sluny reactor using wavelengths h > 300 nm. Some photochemical transformations (i.e. in the absence of titania) are only observed with the unsaturated hydrocarbons, but remains limited compared with the photocatalytic transformation. The main photocatalytic products are the corresponding ketones : cyclohexanone and its methylated derivatives, 2-decalone, and 1-tetralone. Cyclohexene is also oxidized into cyclohexenone mainly, but some cyclohexene oxide is also formed. The influence of various parameters (mass of catalyst, initial concentration in acetonitrile used as an inert solvent, temperature, radiant flux, wavelength) has been studied, as well as the modification of titania by a metal deposit (0.5 < wt % Pt < 10) and by ion doping (Cr3+, Ga3+, Sb5+, V5+). The mechanism of oxidation, at least in the initial attack of the organic molecule, involved a dissociated surface oxygen species in interaction with the photoproduced electrical charges. INTRODUCTION When a semiconductor oxide is illuminated with photons whose energy is equal to or greater than its band gap energy, it can induce, in the presence of oxygen or air, the catalytic oxidation of orgamc substrates provided the recombination of electron-hole pairs is not too rapid. It has previously been shown (refs 1 - 3) that alkanes are thus photocatalytically oxidized into aldehydes and/or ketones mainly. Benzene withstands oxidations, but disubstituted aromatic hydrocarbons are easily oxidized in the less substituted branch. For instance, alkyltoluenes are selectively oxidized into alkylbenzaldehydes (refs 4, 5), i.e. no oxidation of the aromatic ring is detected. For the first time, the photocatalytic oxidation of a hydrocarbon was carried out in the neat-liquid phase, when 4-tertiobutyltoluene was converted into 4tertiobutylbenzaldehyde (ref. 5). In this paper, we report a study of the photocatalytic oxidation in the neat-liquid phase and at room temperature of various model hydrocarbons containing one or two (26 rings (cyclohexane, methyl- and dimethyl-cyclohexane, cyclohexene, decalin and tetralin) over T i 0 2 to further investigate the possibilities of heterogeneous photocatalysis in the field of fine chemicals. Bibliographic investigations gave no references on the photocatalytic oxidation of the various hydrocarbons cited above, except for cyclohexane (refs 6 - 8) with which wa have initiated the study of liquid cycloalkanes (ref. 9). The second part of this paper presents results that aim at showing the photocatalytic character of these oxidations and the effects of titanium dioxide modifications (Pt deposition, substitutional doping) on the conversion of cyclohexane.

406

EXPERIMENTAL Most experiments were carried out with Degussa P-25 Ti02 (ca. 70 % anatase, specific area 50 m2 g-1, non porous). Cyclohexane (Rathburn), methylcyclohexane (Merck), cis- and transdimethylcyclohexane (Aldrich), cyclohexene (Fluka), cis-decalin (Aldrich) and tetralin (Merck) of reagent grade quality were employed as received. The cylindrical static slurry photoreactor of 90 cm3 had a bottom optical window of ca. 4 cm in diameter transmitting wavelengths > 300 nm. Generally, titania was suspended in 10 cm3 of pure liquid organic phase. Illumination was provided by a Philips HPK 125 W mercury lamp, whose radiant flux was measured with a calibrated radiometer (United Technology, model 21 A). The reactions were performed at room temperature. The amounts of carbon dioxide formed were measured with a catharometer gaschromatograph, whereas the liquid phase was analyzed by flame-ionization gas chromatography after ultra-centrifuation of the suspension. RESULTS AND DISCUSSION 1 - Product distribution of the photocatalvtic oxidations For all the compounds studied, no transformation was detected i n the absence of UVillumination. Since the reactions were camed out in neat-liquid phase, the initial number of moles of reactant (ca. 0.8) is orders-of-magnitude greater than the radiant flux (7 10-7 Einstein.s-l ; 1 Einstein = 1 mole of photons). This means that the conversion remains small ( 2 1%) for an illumination time of a few hours, given the quantum yield (see section 2.4). The advantage of these low conversions is that true kinetics can be determined because of the absence of inhibition by the products or of possible photochemical transformations of the products. For preparative purposes, increased conversions can be obtained by increasing the radiant flux with a corresponding adaptation of the size and geometry of the photoreactor and by diluting the reactant in an inert solvent such as acetonitrile. 1 - 1 - Cyclohexane. The selectivities of the photocatalytic oxidation of neat-liquid cyclohexane are indicated in the scheme below :

0

85.4%

OH

2.6%

12%

These selectivities remain constant within the duration of the experiments (3 h). Because of its high selectivity in cyclohexanone, this reaction was chosen to study the influence of various parameters on the photocatalytic activity (see section 2). 1 - 2 - Methvl-cvclohexane. The photocatalytic oxidation of neat-liquid methyl-cyclohexane gives selectivities equal respectively to 67 % in ketones, 4 % in alcohols, 2 % in cyclohexylformaldehyde and 27 % in C02. The following scheme indicates the percentages in the various

407

mild oxidation products (in brackets are given the relative percentages in the different methylcyclohexanone isomers)

5%

92 %

53 %

28%

19%

The secondary ring carbon atoms are preferentially oxidized with respect to that of the methyl group. A simple statistical calculation demonstrates that they are 6.5 times more reactive. This behaviour is opposite to the gas phase photocatalytic oxidation of toluene (ref. 4), which produces only traces of benzaldehyde, whereas the aromatic ring withstands oxidation, at least in pure gas or liquid organic phase and in the absence of water. The above selectivities seem to be correlated to steric factors governing the mode of adsorption of methylcyclohexane on the surface of titania. 1 - 3 . Dimethvl-1.4-cvclohexane.A disubstitution by methyl group in symmetrical positions produces only one ketonic isomer. The selectivities in the following scheme :

5 3.3% 15.3 % 33.3% show that, although the ketone remains the main product, the relative amount of the corresponding alcohol is ca. three times greater than for methylcyclohexane. Moreover the selectivity in C 0 2 seems to increase with the degree of substitution, i.e. with the number of tertiary carbon atoms. 1 - 4 - Decalin. Decalin was chosen to study the reactivities and selectivities of and a-P disubstituted cyclohexane ring as well as the influence of the adjonction of a second saturated cfj ring. The cis- and trans-isomers were photoxidized in the same conditions. The cis-isomer was found at least 10 times more reactive with a selectivity of 81% in mild oxidation products and 19% in C02. The following relative selectivities for the main mild oxidation products (2-decalone, with small amounts of 1-decaloneand 2-decal01 (decahydro-2-naphtol) were found :

408

a

1

'CO:

6

6

Ti02

+ 02

c

hV

0

3

O

"

4

7%

7%

86%

The preferential attack at position 2 would to be due to the steric mode of adsorption of cisdecalin at the surface of Ti@.

1 - 5 - Cvclohexene. This molecule was chosen to compare its reactivity with that of cyclohexane and to determine the influence of a double bond in the cfj ring. The main oxidation

products with their relative selectivities are given in the following scheme :

0

56%

OH

27%

9%

8%

Contrarily to the saturated hydrocarbons studied above, cyclohexene is not photochemically inert (i.e. i n the absence of titania). However the amount of 2-cyclohexenone obtained is ca. 10 times smaller than that produced photocatalytically. Considering only the three mild oxidation products in the liquid phase, it can be inferred that the photocatalytic oxidation of cyclohexene occurs at two sites of the molecule : mainly the allylic position (86 %), giving the cyclohexenone and the cyclohexenol, and, to much lesser extent, the double bond (14 %) yielding the epoxide. 1 - 6 - Tetralin. The presence of an aromatic ring in the molecule induces important changes with respect to the saturated homologue (decalin). Firstly, the photochemical oxidation (i.e. in the absence of titania) is very important (see Fig. 1) as expected from the absorption spectrum ; it produces principally 2-tetralone as the main product and 1-tetralone. Secondly, the photocatalytic oxidation occurs principally at position 1 (see. Fig. I and the following scheme). Note that this scheme refers to both photocatalytic and photochemical products ; however, because of the role of inner filter played by T i q , the former products should be predominant.

34.1%

0.4%

409

The presence of an aromatic ring in the molecule favours the oxidation in the allylic position. By constrast with the selectivity pattern of photochemistry (see Fig. l), the formation of I-tetralone would be due to the mode of adsorption of tetralin at the surface of titania and/or to the mode of attack of photoactivated oxygen.

4.0

I

E

0

1

time

/h

3

Fig. I. Oxidation of tetralin by a) photocatalysis : I-tetralone (A) ; 2-tetralone (B) ; C02 (C) - b) photochemistry (i.e. without Ti02) : I-tetralone (A') ; 2-tetralone (B').

In addition, this study confirms that, in neat-liquid phase, the aromatic ring is not affected as previously observed in the case of alkyltoluenes (refs 4, 5). Moreover, the presence of an aromatic ring seems to stabilize the molecule with respect to total oxidation since a very small amount of C02 is produced (0.4 %). 2 - Effect of various uarameters on the photocatalytic activity The influence of various physical parameters was studied in the case of the oxidation of cyclohexane selected as a model reaction. Since the main product is cyclohexanone, its rate of formation was chosen as representative of the photocatalytic activity of the system.

2 - 1 - Effect of the mass of catalyst. As usual for photocatalytic reactions, the reaction rate was found to increase with the mass of catalyst up to a plateau, which corresponds to the full absorption of photons by Ti02.

410

2 - 2 - Effect of the initial concentration in acetonitrile.Acetonitrile was chosen as a solvent because of its stability under photocatalytic conditions (ref. 10). The initial rate of production of cyclohexanone as a function of the initial concentration followed the Langmuir-Hinshelwood mechanism :

ro= k K Co/(l + K CO) whose linear transform enables one to determine both constants : k = 9.1 x 10-5mo1.h-1 (with a light flux of 46 m W cm-2) ; K = 4.3 I.mol-1. This dependency is one of the criteria of the photocatalytic character of a reaction. 2 - 3 - Effect of temoeraturc. The temperature was vaned from 10 to 75°C by using a jacketted photoreactor connected to a Huber HS cryostat. The temperature had almost no influence upon the photocatalytic activity between 20 and 60°C. Above 6o°C, the activity begins to decrease with a negative apparent activation energy. This is interpreted by the fact that at temperaturesclose the boiling point of cyclohexane (81OC) the rate-limiting step becomes the adsorption of the reactant. The apparent activation energy of the reaction Ea thus contains a negative term, which includes the heat of adsorption QA of cyclohexane : Ea = Et - aQA

E

-am< 0

witha
Et is the true activation energy, which is close to zero for a photocatalytic reaction. 2 - 4 - Effect of the radiant flux. Ouantum yield. The variation in the reaction rate r as a function of the radiant flux $ is linear for < 25 m W cm-2. Above this value, r tends to vary as which indicates that the electron-holerecombination became predominant, as demonstrated in ref. 11 for the liquid-phasephotocatalyticoxidation of isopropanol. Differential quantum vield. It is considered as a dynamic characteristics of the system and is defined as the ratio of the reaction rate to the photon flux (calculated from the radiant flux, the intensities of the rays of the UV lamp, the transmittance of the optical window of the reactor, and the absorbance of Ti02). A quantum yield of ca. 0.1 was found in the region where r varies Linearly with $. The effects described in sections 2.1 to 2.4 are in agreement with the photocatalytic character of the cyclohexane oxidation chosen as model of the oxidation of cyclic hydrocarbons. 2 - 5 - Effect of titania modificatioq. Ti02 was modified by deposition of platinum or by pand n-type doping with heterovalent cations. 2 - 5 - 1 - Effect of platinum content. Since a beneficial role of platinum deposited on titania had been reported for the photocatalytic oxidation of some organic compounds (refs 6, 7), several catalysts, from 0.5 to 10 wt % Pt with a constant particle size, were prepared and studied. The variations of the initial rate of formation of cyclohexanoneas a function of Pt contents are shown in Fig. 2. There is not only no beneficial effect of Pt as mentioned in refs.(6, 7) but a

411

hyperbolic-like decrease in r vs Pt wt %. The curve parallels the variations in the photoconductance (3 vs Pt wt % under vacuum. This is interpreted by the transfer of photoelectrons from the semiconductor to the metal particles (ref. 12). Also, the quantity of oxygen photoadsorbed on Ti02 as negatively charged species is diminished for increasing Pt contents (ref. 13).

1.0

0.5

0

I

c

5

10

Pt W t U

Fig. 2. Effect of the percentage of platinum deposited on Ti02 on the initial formation rate of cyclohexanone. From the relationship between (3 of 5 wt % P o i 0 2 and oxygen pressure Pg;!: (3 = k

PO^-^/^

it is inferred that the species that control the equilibrium between gaseous 0 2 and UV-illuminated pt/Ti02 are 0-species formed by :

The existence of O-(ads) species substantiates previous conclusions which indicated that these entities are determinant, once activated by photoproduced holes, in the initial oxidation step of pure gaseous or liquid organic compounds (refs 14, 15). 2 - 5 - 2 - Effect of ion dnplne * . Since (photo)-electronic processes are involved at the surface of titania, this oxide was modified by ion doping, both of p- and n-type, by dissolving either tri-(Ga3+, Cr3+) or pentavalent (Sb5+, V5+) heterocations during the preparation by the flame reactor method (ref. 16), which produced homodispersed and homogeneously doped non-

412

porous Ti02 particles. Both p- and n-type doped samples exhibit smaller photocatalytic activities that that of pure titania of similar texture. The strongest inhibiting effect is observed for Cr3+ Ti@, in agreement with previous results in the case of other photocatalytic reactions (ref. 17). This is interpreted by the fact that tri- and pentavalent ions create acceptor and donor centers respectively that attract charge carriers of the opposite sign. Consequently, they behave as recombination centers of the photoproduced charges. Despite their detrimental effect on the photocatalytic oxidation reactions, both types of titania modification (doping and metal deposition) have the interest of showing the role of photoproduced charges. CONCLUSION The first part of this paper shows that interesting selectivities in the oxidation of model cyclic hydrocarbons can be obtained by heterogeneous photocatalysis, despite the absence of optimization. This method is applicable at room temperature with neat-liquid compounds, which avoids any thermal degradation. It could be extended to more complex molecules of high added value. The use of photons as the activation mode is not redhibitory especially for fine chemicals, and, moreover, substantial advances in UV-lamp technology have been accomplished (light power output > 20 %). Additionally, the energy cost is compensated by the absence of heating, the possibility of using air as the oxidizing agent, the low price of pure TiO2, and finally the absence of environmentally damaging wastes. REFERENCES 1 M. Formenti and S.J. Teichner, in "Photoelectrochemistry, Photocatalysis and Photoreactors, ed. M. Schiavello, D. Reidel PubLCo., Dordrecht, 1985, pp. 457-489. 2 P. Pichat in "Photoelectrochemistry, Photocatalysis and Photoreactors, Ed. M. Schiavello, D. Reidel, Publ. Co., Dordrecht, 1985, pp. 425-455. 3 P. Pichat, A.C.S. Symp., Ser. 278 (1985) 21-42. 4 M.-N. Mozzanega, J.-M. Herrmann and P. Pichat, Tetrahedron Lett., 34 (1977) 29652966. 5 P. Pichat, J. Disdier, J.-M. Herrmann and P. Vaudano, New J. Chem., 10 (1986) 545. 6 C. Giannotti, S. Le Greneur and 0.Watts, Tetrahedron Lett., 24 (1983) 5071 7 I. Izumi, W.W. Dunn, K.O. Wilbourn, F.R. Fan and A.J. Bard, J. Phys. Chem., 84 (1980) 3207. a) S. Kaliaguine, A. Mahay and P.C. Roberge, Roc. 2nd World Congr. Chem. Eng., 8 Mondal, Vol. 3 (1981) 272. b) A. Mahay, S. Kaliaguine and P.C. Roberge, Can. J. Chem., 60 (1982) 27 19. 9 W. Mu, J.-M. Herrmann and P. Pichat, Catal. Lett. 3 (1989) 73-84. 10 D.D. Sacket and M.A. Fox, J. Phys. Org. Chem., 1 (1988) 103. 11 T.A. Egerton and C.J. King, J. Oil Col. Chem. Assoc., 62 (1979) 386-391. 12 J. Disdier, J.-M. Herrmann and P. Pichat, J. Chem. SOC.Faraday Trans. 1, 77 (1983) 651. 13 H. Courbon, J.-M. Herrmann and P. Pichat, J. Phys. Chem., 88 (1984) 5210. 14 J.M. Hemnann, J. Disdier, M.N. Mozzanega and P. Pichat, J. Catal., 60 (1979) 369-377. 15 P. Pichat, J.-M. Herrmann, H. Courbon, J. Disdier and M.-N. Mozzanega, Canad. J. Chem. Eng., 60 (1982) 27-32. 16 F. Juillet, F. Lecomte, H. Mozzanega, S.J. Teichner, A. ThCvenet and P. Vergnon, Faraday Symp. Chem. Soc.,7 (1973) 57. J.-M. Henmann, J. Disdier and P. Pichat, Chem. Phys. Lett., 108 (1984) 618-622. 17

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1

413

0 1991 Elsevier Science Publishers B.V., Amsterdam

O X I D A T I V E DEHYDROGENATION OF 3-HYDROXY-J-METHYL-4-PENTEN-2-ONE TO 4-METHYL-J-PENTEN-2,3-DIONE O V E R C U O - B A S E D C A T A L Y S T S Hans G.-J.Lansink RotgerinP*d,Gerhard Pennb, Peter C. FiinfschillingC and Alfons BaikeP a) Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH-Zentmm, CH-8092 Zurich, Switzerland b) SANDOZ Pharma AG Chemical Development Department PharmdAgro, CH-4002 Basle, Switzerland c) SANDOZ Production Pharma International, Department of Chemical Engineering Technique, CH-4002 Basle, Switzerland d) Present address: DEGUSSA AG, FCPH-K, Postfach 1345, D-6450 Hanau, FRG ABSTRACT 3-Hydroxy-4-methyl-4-penten-2-one (I)is converted in the gas phase over CuO-based catalysts to 4-niethyl-4-penten-2,3-dione (2) in >90% yield. The unsaturated dione 2 is a highly reactive compound that can serve as an intermediate in the synthesis of heterocyclic molecules. It is shown that 2 is produced by oxidative dehydrogenation, i.e. in the presence of oxygen. In the absence of oxygen the major product is 4-methyl-pentan-2,3-dione(3),the saturated equivalent of 2. Catalyst deactivation is observed both in the presence as well as in the absence of oxygen. The deactivation can be partially suppressed by deliberately poisoning acidic surface sites by adding pyridine to the reactant feed. Since the catalyst can be regenerated by treatment in air at 25OoC, a feasible process would consist of alternate reaction/regeneration cycles. INTRODUCTION Oxidation reactions are important methods in organic synthesis. However, these reactions often use metal ions in high oxidation state such as chromium, manganese or lead and cause ecological problems. In modem technique clean and selective reaction pathways are required. As

an example of such an approach we report here the oxidative dehydrogenation of 3-hydroxy4-methyl-4-penten-Z-one (1) to 4-methyl-4-penten-2,3-dione(2) [ref. 11. This reaction can be carried out under classical conditions [ref.2,3] in a methanol/acetic acid/water mixture and with CU(OAC)~ as the oxidation agent. Although the yield is reasonable (70-80%),this process is not attractive on a technical scale because of the handling of copper salts and the formation of Cu20 and acetic acid as by-products. OH

With this in mind we found it worthwhile to investigate this oxidation reaction in the gas phase over a solid catalyst, thereby avoiding the formation of the by-products mentioned above. In this paper we describe some catalyst screening experiments and the development of a process using an heterogeneous catalyst. As will be shown, copper oxide based catalysts exhibit high

414

activity and selectivity towards the desired product in the presence of oxygen and thereby offer an attractive alternative to the classical synthesis route. EXPERIMENTAL Preparation of starting material and reference compounds 3-Hydroxy-4-methyl-4-penten-2-one (1) was prepared from 3-chloro-mesityl oxide using a modified procedure from the literature [ref.4]. A reference sample of 4-methyl-4-penten-2,3-dione

(2) was prepared from 1 using the copper acetate method [ref.2]. 4-methyl-pentan-2,3-dione( 3 ) was purchased from Wiley & Co. 2-Hydroxy-4-methyl-4-penten-3-one (4)was synthesized in 30% yield from 2-propenyl-magnesium bromide and 2-hydroxy-propionitrile in tetrahydrofurane. The structures of all compounds were confirmed by 'H-,"C-NMR- and mass spectroscopy [refS]. Catalvsts A series of different catalysts were tested (table 1). Most of them were commercially available (Harshaw). CuO/A120, was prepared according to the procedure reported in ref.6 and contained 50.9 wt% CuO. CuO/SiO, was prepared by ion exchange according to ref.7 and contained 4 wt% CuO. The BET areas of the Cu0/A2O3 and CuO/Si02 catalysts were 166 m2/g and 197 m2/g, respectively. Catalytic experiments A standard experimental set up was used for the catalytic experiments. The flow rates of helium, which was used as carrier gas and oxygen were regulated by mass flow controllers (Brooks). Both gases were used as delivered, without any further purification. The pressure at the reactor inlet was measured by means of a mercury manometer. The liquid reactant was pumped to the reactor by means of an injection pump. The tubing from the pump to the reactor was made of teflon, all others were stainless steel (i"o.d., 2 mm i.d.) The reactor was made of glass (10 mm id.). The catalyst was used in the form of coarse particles (sieve fractions mostly between 0.355 and 0.710). These were supported by a quartz frit. A bed of glass granules (length ca. 10 cm) was placed on the top of the catalyst and some quartz wool was placed over the glass granules. This configuration ensured complete vaporisation of the reactant I . The reactor was heated with an oven (length 30 cm) which was regulated by B

temperature controller (Tecon). The temperature was measured with a thermocouple which was placed in the small space between the walls of the oven and the reactor. A second thermocouple was placed inside the catalyst bed. The gases left the reactor at the bottom and flew to a 4-way valve. The tubes after the reactor were heated with heating jackets (60-80OC). The products were either analysed on-line or collected in a condenser for off-line analysis (Varian 3400 gas chromatograph with a packed column, 10% OV-17 on chromosorb W; detector TCD).

415

DEFINITIONS The terms conversion, selectivity and yield are defined as follows: conversion

=

‘l,in

- (‘1 + ‘4)out

. 100%

C1,in Ci. out

=

selectivity for product i

‘1.h

yield of product i

=

Ci. out ~

- (‘1

. 100%

+ c4)out

.loo%

C1.h

in which

ci = concentration of compound i in the gas stream

1 = compound I 4 = compound 4 (see results of screening experiments) RESULTS AND DISCUSSIONS Screening experiments, reaction products Initial screening tests were performed without oxygen in the reactant feed. Table 1 lists the results obtained with different catalysts. Depending on the catalyst used, the GLC-analysis of the condensed reactor effluents showed the presence of at least 4 compounds, which could be separated by preparative GLC. Structure elucidation of the isolated compounds was done by GLC-MS analysis and NMR-spectroscopy.

0

0

0

4

In addition to unconverted 3-hydroxy-4-methyl-4-penten-2-one (I)the desired unsaturated diketone

2 and two other compounds were found in the product mixture: 4-methyl-

pentan-2,3-dione (3), which is the corresponding saturated diketone and 2-hydroxy-4-methyl4-penten-3-one (d), an isomer of 1. Upon prolonged storage of the reaction mixtures at room temperature, 6-acetyl-2,5-dimethyI2-(2,3-dioxo-l-propy1)-3,4-dihydro-[2H]pyrane(5) is formed. As was shown in separate experiments, 5 is a dimerisation product of the unsaturated diketone 2 and can be converted back to 2 in quantitative yield by passing it through a hot glass tube at 400°C. In table 1 the 4 was not considered to be conversion, since it was shown in a separate isomerization of 1 experiment that 4 is converted into 2 in a similar rate to that of 1 2.

-

-

416

TABLE 1 Results of screening tests with different catalysts in the absence of oxygen in the gas phase. Conditions: T = 230”C, 6.0 g of catalyst, 2.9 g of 1 was passed through the catalyst bed within 20-25 min. The catalyst was preheated 30 min prior to its use.

Catalyst

GLC-analysis (area%) ~

CuO-Cr207(H-Cu-1808)’) CuO-Cr2O7(H-Cu-I 808)‘) CuO-Cr,O, (H-CU-I~O~)’.~) CuO-Si02 Cu0-Cr20,-Ba0 (H-Cu-I 230E)’) cuo-AI,o~ Cu0-Cr203-Ba0(H-Cu-1184T)’) CuO-Cr203( H - C U O ~ ~ ~ T ) ~ ) Ni-SiO, (H-Ni- 1404T)’) Ni-SiO, (H-Ni-3288E)’)

1+J

2

82 70 69 64

9 16 17

55

8 1 8 7 2

8 10 11 17 35 29

18

67 55

89 49 25

recovery”)

3

-

35 1 46 66

5

0.92 0.90 0.79 0.80 0.87 0.82 0.68 0.74 0.79 0.74

a) The recovery is defined as weight of isolated liquid product divided by the weight of reactant passed through the catalyst bed. b) Harshaw catalyst. c ) 12.0 g of catalyst was used. d ) T=330”C Experiments with the commercial H-Cu-0203T catalyst The initial experiments discussed in this section were performed without oxygen in the gas phase. Figures l a and Ib show conversion of

1 and selectivity

towards

2 and 3 as a function of

time, respectively. Initially, the conversion (65%) as well as the selectivity were stable. However, after 6 hours on stream the catalyst started to be deactivated. The decrease in conversion was accompanied by a decrease in selectivity towards the unsaturated diketone 2 and an increase in that for the saturated diketone

3. The ratio of the hydroxy ketone isomers 1 and 4 was not

influenced by the deactivation. After 16 hours on stream, no more unsaturated diketone 2 was formed and the addition of I to the reactant gas feed was stopped. The oven was maintained at 200°C and 0, was added to the gas feed. The temperature in the catalyst bed increased instantaneously by ca. 30°C. The temperature decreased gradually during the following 1.5 hours reaching a final value of 200°C. This behaviour is attributed to the reduction of the catalyst during the 16h period in which it was on stream and exothermic reoxidation of Cu-metal to copper oxide. This indicates that oxygen of the catalyst is consumed in the conversion process of 1. In the reaction of

1

+

2, two hydrogen

atoms have to be removed. Oxygen from the copper

oxide is assumed to react with the hydrogen atoms and water desorbs. As the oxygen is removed gradually from the catalyst, the production of the unsaturated diketone 2 decreases and the selectivity to the corresponding saturated diketone 3 increases. This scenario is suppolted by the

417

following ohservarions:

-

the highest ratio 2J3 was found for the H-Cu-0203T catalyst having the highest amount of

CuO (79%) of all the Cu-catalysts tested -

in the experiment with the double amount of H-Cu-1808 catalyst, the amount of

2

had

doubled whereas the amount of 3 was increased by merely 25%.

I

100 $

80-

U

60-

maa

a

a

b)

3

5 40-W

d

0000

20-

0

v)

0

Fig. 1.

I

10 TIME ON STREAM (hrs)

0

n

20

Conversion of 1 and selectivity towards 2 and 3 as a function of time on stream (hrs). a) conversion, b) selectivity; compound 2 (o), compound 3 (e).

Thus, the reaction

I

-

2 cannot be explained as simple dehydrogenation process since

removable lattice oxygen is necessary for the formation of the unsaturated diketone 2. The fonnation of

3 can be explained by an isomerisation via endiol 6 [ref.8]. This isomerisation takes

418

also place when a solution of 3 in toluene is heated in the presence of catalytic amounts of an acid [ref.9]. Based on the observations described above, we propose the following reaction pathway:

*. l 4 OH

H -O

6

f0,

-

Kib OH

-

0

+ ! 0,

F

1 -

2

-

Further experiments were therefore carried out with oxygen in the gas phase in order to maintain a high concentration of lattice oxygen in the solid phase. In fig.2 conversion vs time plots are shown for experiments in which the molar ratio 02/1was lower than 0.5. After each experiment (2a and 2b) the catalyst (1.Og of H-Cu-0203T) was reactivated in an 02-He gas stream which restores its initial activity.

100 n

8. v

10

z

0

z r W

>

z

0 0 0 Fig. 2.

1 2 3 TIME ON STREAM (hrs)

4

Conversion (%) as a function of t h e on stream (hrs) at different temperatures for 1.OOg of H-Cu-0203; ratio O J l = 0.34. a) T = 350°C, b) T = 200°C.

419

Adding 0, to the gas stream drastically increases the selectivity towards the unsaturated diketone

2 up to 295% selectivity

at 200°C. The conversion, however, decreases with time on

stream. At 350" the selectivity is only 30% and the catalyst deactivates rapidly. In table 2, the yield of 2 is given as a function of the concentration of 0, in the gas stream. The amount of I was ca. 1 vol% and 0, was present in excess. TABLE 2 Yield of 2 (%) as a function of the amount of O,(vol%) in the gasphase; feed : ca. 1 vol% of 1. entry

Val% 0,

T,,,("C)

Conversion of 1 (%)

Selectivity of 2 (%)

1 2 3 4 5 6

0.5 2.0 4.0 6.0 8.0 8.0

24 1 244 241 25 1 255 236

28 35 45 52 58 44

86 93 95 97 97 97

In entry 1 to 5 (table 2), the temperature of the oven was kept constant and the increase in the catalyst bed temperature from 241 to 255°C was caused by an exothermic reaction, presumably the formation of water and small amounts of CO, (max. 15 mol% at 255°C). However, increasing the amount of oxygen did lead to an increased yield of the diketone

2 even at lower temperatures

(entry 6). Influence of the support

To get an idea about the influence of the support on the selectivity a series of support materials were tested with and without copper loading. Only some qualitative results will be given here. All copper oxide based catalysts exhibit a high selectivity for 2 in the presence of oxygen. High conversions could be obtained by taking an appropriate amount of catalyst. All of the bare supports tested (SiO,, Al,O,, TiO, and ZrO,) produced the unsaturated diketone 2, but the main product in each case was the saturated compound 3. During the tests, all of the initially white supports tumed dark yellow or brown due to the deposit of residues. The formation of residues was confirmed by the mass balance (amount product divided by amount of feed). For the reaction over y-Al,O, the mass balance reached only 12%. Each of the CuO/Al,O, samples that were tested exhibited a high selectivity towards 2, but all samples produced also a little of 3 (selectivity 4 0 % ) .On CuO/Si02 the unsaturated diketone 2 was observed exclusively. Increasing the residence time of the reactants in the catalyst bed (by increasing the amount of catalyst) led to higher conversion without substantial loss of the selectivity of 2.

420

Catalyst deactivation Long-term runs were performed in which the activity was monitored as a function of time on stream of the catalyst. An experiment with 50 wt% CuO/AI,O, showed that the conversion decreased after a few hours on stream. The spent catalyst was characterized by TGA-DTA and EDAX. Upon heating in air the TGA-DTA measurement showed that the spent catalyst sample lost 5% weight and two exothermal peaks (at 300°C and 400°C) were observed. Similar measure-

showed only one peak in the DTA diagram (405°C). A ments with spent alumina (without (21.10) fresh and spent CuO/AI2O3 sample were characterized with EDAX. The analysis showed that chlorine was present on the spent catalyst, whereas it was totally absent in the fresh sample. The origin of chlorine was from a chlorine containing impurity in the hydroxy ketone 1 which irreversibly adsorbed on the catalyst. Adsorbed chlorine is known to increase the acidity of the alumina support and thereby may enhance cracking or polymerisation processes which finally lead to catalyst deactivation. The dark-yellow colour of the initially white supports after use in the reaction indicated that residues were retained on the catalyst. The yellow colour disappeared after calcination in air at 500°C. In some experiments we have deliberately poisoned acidic sites on the catalyst by adding pyridine to the gas phase. In table 3, two experiments are compared which were perfonned under identical conditions, apart from the presence of pyridine. As can be seen from the results, the presence of pyridine does significantly slow down the rate of deactivation. TABLE 3 Effect of continuous addition of pyridine to the gas phase on the production-rate of 2 (moles/unit of time); T=140°C; feed 1 vol% 1; 15 vol% 0,

Pyridine/l (g/g) 0.00 0.02

r2 at t=O 4.8 4.5

r2 at t=28h 2. I 3.6

Catalyst regeneration A few experiments were performed in which a partly deactivated CuO/Si02 catalyst was treated in an 02/He mixture at 25OOC (6h) for regeneration. In all cases this treatment was sufficient to regain the initial activity. A technical process for the production of 4-methyl4-penten-2.3-dione from 3-hydroxy-4-methyl-4-penten-2-one consisting of alternate reaction and subsequent regeneration cycles seems therefore feasible. CONCLUSION 4-Methyl-4-penten-2,3-dione (2) can be obtained in high selectivity (>95%) by oxidative

(1) over copper oxide based catalysts. dehydrogenation of 3-hydroxy4-methyl4-penten-2-one

421

This heterogeneous process may constitute an interesting alternative to the classical synthetic route. Catalyst deactivation can be slowed down by deliberately poisoning the acidic surface site with added pyridine in the reactant feed. A feasible operation mode for a continuous heterogeneous process consists of reaction and subsequent reoxidation cycles of the catalyst. ACKNOWLEDGEMENTS Thanks are due to Victor Bassili for preparing some of the catalyst, to RenC Koeppel and Robert Vultier for their help with several experimental techniques and to Edward Jobson for valuable discussions. We are also grateful to Jean-Paul Mutz and Christian Reithmaier for performing some screening experiments and to Ramsay Richmond for GLC-MS measurements. REFERENCES AND NOTES 1 For The preparation of 4-methyl-4-penten-2,3-dione from 3,4-epoxy-mesityl oxide see A.L. Shabanov M.M. Movsumzade, S.S. Muradova and I. Rats, DOH. Acad. Nauk. Azerb. SSR 22 (1971), 42; Chem. Abstr. 76 (1972). 153169f. 2 P. Ruggli and P. Zeller, J.Chem.Soc. 1951,741. 3 Other methods for the preparation of 1,2-diketones see E.R. Freiter, US Pat. 4,107,210 (Aug. 15th 1978). 4 A. Egner and J. Pete, Bull. SOC.Chim. Fr. 1975,1681. 5 P.C. Funfschilling, J.-P. Mutz, G. Penn, C. Reithmaier and E. Terpetschnig, unpublished results. 6 A. Baker and W. Richarz, Synth. Conunun. 8 (1978), 27. 7 J.E. Lee, D.L. Trimm, M.A. Kohler, M.S. Weinwright and N.W. Cant, Catalysis Today 2 (1988),643. 8 R.M. Pollak, P.L. Bounds and C.L. Bevins in: The Chemistry of Enones, Part 1 ( S . Patai and Z.Rappoport Eds.), p 559, J.Wiley & Sons Ltd, 1989. 9 E.A. Braude and C.J. Timmons, J. Chem. SOC.1953,3144.

This Page Intentionally Left Blank

M. Guisnet e t al. (Editors), Heterogeneous Catalysis and Fine Chemicals I I 0 1991 Elsevier Science Publishers B.V., Amsterdam

PARTIAL OXIDATION

OF

423

TOLUENE TO BENZALDEHYDE

M. A1 Research L a b o r a t o r y of Resources U t i l i z a t i o n . Tokyo I n s t i t u t e o f Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227 (Japan) ABSTRACT Vapor-phase o x i d a t i o n o f t o l u e n e t o benzaldehyde was s t u d i e d w i t h v a r i o u s Mo-, U-, and Sb-based mixed-oxide c a t a l y s t s . The s e l e c t i v i t y t o benzaldehyde f e l l w i t h i n c r e a s i n g t h e t o l u e n e conversion. The b e s t performances were o b t a i n e d w i t h Mo-P and U-Mo o x i d e c a t a l y s t s : t h e one-pass y i e l d o f benzaldehyde reached 40 mol% w i t h a s e l e c t i v i t y o f about 60 mol%. The c a t a l y t i c a c t i v i t y o f t h e U-Mo oxides was more s t a b l e t h a n t h a t o f t h e Mo-P oxides. The e f f e c t s o f t h e r e a c t i o n v a r i a b l e s on b o t h t h e r a t e and s e l e c t i v i t y were a l s o s t u d i e d .

INTRODUCTION Vapor-phase o x i d a t i o n o f t o l u e n e t o benzaldehyde i s a c l a s s i c a l s u b j e c t i n t h e f i e l d of p a r t i a l o x i d a t i o n .

Indeed, i t has a l r e a d y been s t u d i e d w i t h v a r i -

ous V- and Mo-based o x i d e c a t a l y s t s [l-181.

However, t h e one-pass y i e l d o f

benzaldehyde was s t i l l lower t h a n t h a t o f o t h e r oxygenated compounds o b t a i n e d i n o x i d a t i o n o f o l e f i n s and a r o m a t i c hydrocarbons.

For example,

t h e maximum y i e l d

of benzaldehyde o b t a i n e d w i t h Bi-Mo o x i d e s was around 10 mol% [7,11,12]. I n o u r p r e v i o u s s t u d i e s [19-211,

i t was found t h a t t h e y i e l d o f benzaldehyde

reaches 40 mol% w i t h Mo-P-based o x i d e c a t a l y s t s and t h a t b e t t e r performances a r e o b t a i n e d a t 500 t o 550°C w i t h a s h o r t c o n t a c t time, though t h e c a t a l y t i c a c t i v i t y f a l l s g r a d u a l l y w i t h time-on-stream. I n t h i s study, we attempted t o e x p l o r e more e f f e c t i v e c a t a l y s t s f o r product i o n o f benzaldehyde and a l s o t o c l a r i f y t h e c h a r a c t e r i s t i c f e a t u r e s o f t h i s o x i d a t i o n r e a c t i o n , s i n c e d e t a i l e d r e p o r t on t h e y i e l d has n o t been p u b l i s h e d . EXPERIMENTAL Ca t a l ys t s The c a t a l y s t s used i n t h i s s t u d y were Mo-.

U-,

and Sb-based mixed-oxides.

They were supported on 8- t o 20-mesh s i z e pumice o r i g i n a t i n g from v o l c a n i c stone. follows.

For example t h e U/Mo atomic r a t i o = 85/15 c a t a l y s t was prepared as U(CH3C00)2*2H20 (54.2 g ) was d i s s o l v e d i n w a t e r and (NH4)6M07024*4H20

(4.0 g ) was d i s s o l v e d s e p a r a t e l y i n h o t water.

The two s o l u t i o n s were mixed and

t h e excess water was evaporated, y i e l d i n g a s t i c k y syrup. pumice was added t o t h e syrup and t h e m i x t u r e was d r i e d .

Thereafter,

100 m l o f

The o b t a i n e d s o l i d was

d r i e d f u r t h e r a t 200°C and t h e n i t was c a l c i n e d a t 550°C i n a stream o f a i r .

424 R e a c t i o n Procedures The vapor-phase c o n t a c t o x i d a t i o n o f t o l u e n e was conducted i n a c o n v e n t i o n a l f l o w system.

The r e a c t o r was made o f a s t e e l tube, 50 cm l o n g and 1.8 cm I.D.,

mounted v e r t i c a l l y and immersed i n a l e a d bath.

A i r o r a m i x t u r e o f oxygen and

n i t r o g e n was i n t r o d u c e d from t h e t o p o f t h e r e a c t o r , w i t h t o l u e n e b e i n g i n j e c t e d i n t o t h e p r e h e a t i n g s e c t i o n o f t h e r e a c t o r by means o f a s y r i n g e pump. Unless i n d i c a t e d o t h e r w i s e , t h e feed r a t e s were f i x e d as f o l l o w s : 1 ( a t 20"C)/min

(ca. 3.5 mol/h);

a i r , 1.40

t o l u e n e , 36 mmol/h (ca. 1.03 mol% i n a i r ) .

The e f f l u e n t gas from t h e r e a c t o r was l e d s u c c e s s i v e l y i n t o f o u r c h i l l e d scrubb e r s c o n t a i n i n g 2-propanol

s o l u b l e compounds.

t o r e c o v e r t h e 2-propanol

The

r e a c t i o n p r o d u c t s and u n r e a c t e d t o l u e n e were analyzed by gas chromatography. The y i e l d and s e l e c t i v i t y o f a p a r t i c u l a r p r o d u c t were d e f i n e d as mole p e r centage y i e l d and s e l e c t i v i t y on a carbon-accounted-for y i e l d of " o t h e r acid",

basis.

As f o r t h e

t h e y i e l d was c a l c u l a t e d b a s i n g on t h e asumption t h a t

t h e a c i d was a c e t i c a c i d o r m a l e i c anhydride, because t h e main a c i d s , b e s i d e s benzoic acid, were found t o be a c e t i c a c i d and m a l e i c anhydride. t i m e was d e f i n e d as (volume o f c a t a l y s t used [ m l ] ) / ( t o t a l

The c o n t a c t

flow rate [ml/s]).

RESULTS Performance o f v a r i o u s m e t a l - o x i d e c a t a l y s t s Various metal-oxides were t e s t e d as c a t a l y s t s a t a t e m p e r a t u r e o f 550"C, s i n c e b e t t e r performances had been o b t a i n e d a 500 t o 550°C [19-21]. a r e l i s t e d i n Tables 1-3,

The r e s u l t s

according t o the c l a s s i f i c a t i o n o f metal oxide i n

view o f b o t h acid-base and o x i d i z i n g f u n c t i o n s

[22.23].

Table 1 shows t h e r e s u l t s o b t a i n e d w i t h c a t a l y s t s c o n s i s t i n g o f Moo3 and another o x i d e w i t h o u t o x i d i z i n g f u n c t i o n . oxidation a c t i v i t y .

The Moo3 c a t a l y s t shows a v e r y low

A t a c o n t a c t t i m e o f 1.3 s, t h e y i e l d o f benzaldehyde i s

9.2 mol% w i t h a s e l e c t i v i t y o f 60 mol%. those a l r e a d y r e p o r t e d [10,14,20].

The r e s u l t s a r e i n c o n f o r m i t y w i t h

The a d d i t i o n o f SO3,

B203, and Te02 t o Moo3

decreases t h e o x i d a t i o n a c t i v i t y , w h i l e t h a t o f W03, Sb205. and A1203 i n c r e a s e s the a c t i v i t y , t o a small extent. l y the a c t i v i t y . of 58 mol%.

The a d d i t i o n o f P205 t o Moo3 enhances marked-

The y i e l d o f benzaldehyde reached 42 mol% w i t h a s e l e c t i v i t y

The r e s u l t s a r e a l s o i n c o n f o r m i t y w i t h t h e p r e v i o u s r e s u l t s [ Z O ] .

A t e r n a r y o x i d e w i t h a Mo/W/Te atomic r a t i o o f 10/4/4 which had shown a good performance i n t h e o x i d a t i o n o f 4 - m e t h y l s t y r e n e t o p h e n y l a c r o l e i n [23]. was a l s o t e s t e d as a c a t a l y s t f o r t h i s o x i d a t i o n .

A t a c o n t a c t t i m e o f 1.3 s, t h e

y i e l d o f benzaldehyde reached 11.4 mol% w i t h a s e l e c t i v i t y o f 48 mol%. performance i s almost t h e same as t h a t of t h e Mo/W

=

The

8 / 2 b i n a r y oxide.

Table 2 shows t h e r e s u l t s o b t a i n e d w i t h c a t a l y s t s c o n s i s t i n g o f Moo3 and another oxide w i t h o x i d i z i n g function.

The a d d i t i o n o f t h e s e o x i d e s t o Moo3

enhances markedly t h e o x i d a t i o n a c t i v i t y .

However, t h e s e l e c t i v i t y t o benzal-

425 TABLE 1 Performance o f Mo-based b i n a r y o x i d e c a t a l y s t s ( 1 ) Cata 1y s t (atomic r a t i o ) Mo a l o n e Mo-S (10-8)

Mo-B Mo-Te Mo-W Mo-Sb Mo-A1 Mo-P

(10-8) (10-8) (8-2 ) (10-4) (8-2) (10-2)

c.t. (sec)

1.3 1.3 1.3 1.3 1.3 1.3 1.3 0.65 1.3 2.6

Conv. (%)

Bald

15.3 9.1 9.0 6.0 26.8 22.4 25.7 27.7 42.2 72.0

9.2 5.8 3.6 4.9 14.1 14.6 13.2 17.1 31.0 42.0

Y i e l d (mol%) o f 8aci COX O.A.

0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.4 1.1 2.6

0.6 0.5 0.7 0.2 1.2 1.2 1.2 1.0 2.3 5.5

S other

(mol%)

2.9 2.0 3.2 0.1 5.5 2.2 3.7 7.3 2.8 6.1

60 63 40 82 53 65 51 62 73 58

2.6 0.7 1.5 0.8 6.0 4.4 7.6 1.9 5.0 15.8

c.t., c o n t a c t time: S, s e l e c t i v i t y t o benzaldehyde: Bald, benzaldehyde; Baci, benzoic a c i d : O.A., o t h e r acids: COX. carbon oxides: o t h e r , [ ( o v e r a l l conversion o f t o l u e n e ) ( y i e l d s o f Bald + Baci + O.A. + COX)].

-

Ca t a 1ys t c.t. Conv. Y i e l d S ( % ) (mo1Z) (mol%) (atom r a t i o ) ( s e c ) Mo-a1 one 1.3 15.3 9.2 60 Mo-V (10-2) 0.032 46.5 10.8 23

Mo-U Mo-Ti

(8-2) (9-1) (8-2) (6-4)

0.065 0.16 0.32 0.32 0.65 0.13 0.32 0.4 0.8

58.1 46.5 68.6 40.1 55.0 22.0 65.2 50.8 70.6

13.5 28.0 35.3 20.3 22.5 17.8 23.0 23.7 23.2

23 60 52 50 41 81 35 47 33

Catalyst c.t. (atom r a t i o ) (sec) Mo-Sn (9-1) 0.16 . .

(7-3) (3-7) Mo-Fe (9-1)

(7-3) (8-2) (6-4) Mo-Bi (9-1) (8-2) (6-4) MO-CO

0.32 0.032 0.065 0.013 1.3 0.96 1.3 1.3 0.32 1.3 1.3

Conv. Y i e l d S (%) (mo1X) (mol%)

40.0 70.0 54.5 77.0 60.0 26.7 45.6 30.0 38.8 20.5 48.7 52.4

26.3 31.1 18.3 22.2 16.2 19.0 26.4 20.3 19.8 14.3 21.5 22.2

66 45 34 29 27 71 58 68 51 70 44 42

Yield, y i e l d dehyde decreases as t h e c o n v e r s i o n increases. w i t h t h e Mo-U oxides.

The b e s t r e s u l t s a r e o b t a i n e d

The n e x t b e s t r e s u l t s a r e o b t a i n e d w i t h t h e Mo-Ti oxides.

The r e s u l t s o b t a i n e d w i t h

U-based o x i d e s a r e shown i n Table 3.

The U308

c a t a l y s t i s v e r y a c t i v e and t h e y i e l d o f benzaldehyde reaches 36 rnol%.

The b e s t

performance i n b o t h a c t i v i t y and s e l e c t i v i t y i s o b t a i n e d w i t h t h e o x i d e w i t h a U/Mo atomic r a t i o o f 8 / 2 t o 9/1.

The y i e l d reaches 40 mol%.

This f i n d i n g i s i n

c o n f o r m i t y w i t h t h e r e s u l t s r e p o r t e d [2.11]. The r e s u l t s o b t a i n e d w i t h Sb-based o x i d e s a r e a l s o shown i n Table 3.

It i s

c l e a r t h a t t h e Sb-based o x i d e s a r e n o t e f f e c t i v e as c a t a l y s t s f o r t h i s r e a c t i o n . The V-P o x i d e w i t h a P / V atomic r a t i o o f 1.06 [ 2 4 ] showed t h e y i e l d o f below

12 mol%.

On t h e o t h e r hand, t h e y i e l d reached 14.5 mol% w i t h a P/V

These r e s u l t s a r e a l s o i n c o n f o r m i t y w i t h t h o s e r e p o r t e d [19].

=

1.6 oxide.

426

S c.t. Conv. Y i e l d Catalyst S Conv. Y i e l d c.t. Cata 1y s t ( 2 ) (mol%) (mol%) ( 5 ) (mol%) (mol%) (atom r a t i o ) ( s e c ) (atom r a t i o ) ( s e c ) U-MO (9-1) 0.065 46.5 32.4 63 U alone 0.065 21.4 19.3 90

U-P

(8-2)

U-W U-V

(9-1) (9-1)

0.2 0.32 0.4 0.032 0.065 0.032 0.032 1.3 ~~

)

alone

47.6 66.0 77.4 37.8 54.2 38.4 48.4 0.0

32.1 32.0 35.8 17.6 15.4 26.4 26.4

69 49 47 47 38 69 54

0.0

--

0.13 0.065 0.13 0.26 0.39 0.26 0.52

(8-2)

(7-3)

66.7 36.3 44.6 66.1 76.7 30.1 55.0

39.2 26.8 31.2 38.2 40.0 18.2 28.8

59 74 71 58 52 61 53

I .

I

.

Sb-U (4-1) 1.3 11.0 5.6 50 Sb-K (9-1) 0.065 14.0 4.2 30 Y i e l d , y i e l d o f benzaldehyde: o t h e r a b b r e v i a t i o n s a r e t h e same as f o r Table 1. Effects o f reaction variables The s t u d y i n t h e p r e c e d i n g s e c t i o n r e v e a l s t h a t t h e b e s t performances f o r t h e p r o d u c t i o n of benzaldehyde a r e o b t a i n e d w i t h t h e Mo-P and U-Mo o x i d e c a t a l y s t s . I n t h i s s e c t i o n . t h e c h a r a c t e r i s t i c f e a t u r e s o f t h i s o x i d a t i o n r e a c t i o n were s t u d i e d u s i n g a U-Mo o x i d e w i t h a U/Mo atomic r a t i o o f 85/15, s i n c e t h e r e a c t i o n w i t h Mo-P-based

o x i d e s had been s t u d i e d p r e v i o u s l y [20,21].

Product d i s t r i b u t i o n s .

The r e a c t i o n was conducted a t 550°C by changing t h e

c o n t a c t time, w h i l e f i x i n g t h e o t h e r c o n d i t i o n s as p r e s e n t e d under Experimental. The main p r o d u c t s were benzaldehyde and carbon oxides.

The f o r m a t i o n o f benzo-

i c acid, a c e t i c a c i d , and m a l e i c a n h y d r i d e was a l s o detected, were much s m a l l e r .

b u t t h e i r amounts

The y i e l d s o f each p r o d u c t a r e shown as a f u n c t i o n o f t h e

t o l u e n e c o n v e r s i o n i n Fig. 1 . t h e o r i g i n (dashed l i n e s ) .

The s e l e c t i v i t i e s a r e g i v e n b y t h e s l o p e s f r o m

The s e l e c t i v i t y t o benzaldehyde decreases w i t h an

i n c r e a s e i n conversion, w h i l e t h a t t o carbon o x i d e s i n c r e a s e s , i n d i c a t i n g t h a t t h e benzaldehyde formed i n i t i a l l y i s o x i d i z e d g r a d u a l l y t o carbon oxides. Stability of catalytic activity.

The s t a b i l i t y o f a c t i v i t y was checked.

F i g u r e 2 shows t h e c o n v e r s i o n o f t o l u e n e and t h e y i e l d o f benzaldehyde o b t a i n e d a t 550°C as a f u n c t i o n o f time-on-stream.

I t was found t h a t t h e a c t i v i t y o f t h e

U/Mo = 85/15 o x i d e i s more s t a b l e t h a n t h a t o f t h e Mo-P-based E f f e c t o f temperature.

o x i d e s [20,21].

The r e a c t i o n was conducted by changing t h e tempera-

t u r e and t h e c o n t a c t time, w h i l e f i x i n g t h e o t h e r c o n d i t i o n s .

I n order t o

compare t h e s e l e c t i v i t y a t t h e same l e v e l o f t h e t o l u e n e conversion,

the yields

o f benzaldehyde a r e p l o t t e d as a f u n c t i o n o f t h e c o n v e r s i o n i n Fig. 3. s e l e c t i v i t y i n c r e a s e s w i t h r a i s i n g t h e temperature. m i t y w i t h t h a t o b t a i n e d w i t h V- and Mo-P-based E f f e c t o f oxygen c o n c e n t r a t i o n .

The

This f i n d i n g i s i n confor-

o x i d e c a t a l y s t s [19-211.

The r e a c t i o n was conducted by changing t h e

427

1

I

0' Conversion

(%)

Fig. 1. O x i d a t i o n o f t o l u e n e on t h e U/Mo = 85/15 o x i d e c a t a l y s t . T =550°C. S = s e l e c t i v i t y t o benzaldehyde (molX).

2

4

6

8

Time on stream

10

12 14

(h)

F i g . 2. S t a b i l i t y o f c a t a l y t i c a c t i v i t y . Temperature = 550°C: c o n t a c t t i m e = 0.13 s.

i n i t i a l c o n c e n t r a t i o n o f oxygen, w h i l e f i x i n g t h e o t h e r c o n d i t i o n s :

t h e sum o f

t h e feed r a t e s o f oxygen and n i t r o g e n was f i x e d a t 1.40 l / m i n (ca. 3.5 mol/h). F i g u r e 4 shows t h e t o l u e n e c o n v e r s i o n o b t a i n e d a t t h e c o n t a c t t i m e o f 1.3 s and 550°C as a f u n c t i o n o f t h e oxygen c o n c e n t r a t i o n .

The c o n v e r s i o n i n c r e a s e s

s t e a d i l y w i t h t h e oxygen c o n c e n t r a t i o n up t o t h e c o n c e n t r a t i o n o f 50 mol%. The y i e l d s o f benzaldehyde o b t a i n e d a t 550°C f o r f o u r d i f f e r e n t oxygen con-

L 0' On 40 60 80 Conversion

(%)

Fig. 3. E f f e c t o f t h e temperature on t h e y i e l d o f benzaldehyde.

10

20

Oxygen

30

40

50

(mol%)

Fig. 4. E f f e c t o f t h e oxygen conc e n t r a t i o n on t h e conversion. Cont a c t t i m e = 1.3 s , t o l u e n e = 1 mol%.

428

(X)

Conversion

F i g . 6. E f f e c t o f the toluene c o n c e n t r a t i o n on t h e r a t e . T = 550°C, oxygen = 20 mol%.

Fig. 5. E f f e c t o f t h e Oxygen concent r a t i o n on t h e y i e l d o f benzaldehyde. T = 550°C, t o l u e n e = 1.0 mol%.

c e n t r a t i o n s a r e p l o t t e d i n Fig. 5 as a f u n c t i o n o f t h e t o l u e n e c o n v e r s i o n : t h e c o n v e r s i o n was v a r i e d by changing t h e c o n t a c t t i m e .

The s e l e c t i v i t y i s s c a r c e l y

a f f e c t e d by t h e oxygen c o n c e n t r a t i o n , when t h e e x t e n t o f t h e r e a c t i o n i s low. However a t h i g h e r t o l u e n e conversions,

t h e y i e l d f a l l s because o f a l a c k i n

oxygen, when t h e oxygen c o n c e n t r a t i o n i s low. E f f e c t o f toluene concentration.

The r e a c t i o n was conducted by changing t h e

i n i t i a l concentration o f toluene, w h i l e f i x i n g the o t h e r conditions:

,o-Tolu;ne

:rnol%: 0;5,

:;1

,O:

contact

1

.r

>

O20

40

60

Conversion

80 (X)

F i g . 7. E f f e c t o f t h e t o l u e n e concent r a t i o n on t h e y i e l d o f benzaldehyde. T = 550°C. oxygen = 20 mol%.

-

0 20

40 Conversion

60

80 (X)

F i g . 8. E f f e c t o f t h e f e e d r a t e on t h e y i e l d o f benzaldehyde. T = 550 "C, t o l u e n e = 1.0 mol% i n a i r .

429 t i m e = 0.065 s, temperature = 550°C.

F i g u r e 6 shows t h e r a t e o f benzaldehyde

f o r m a t i o n as a f u n c t i o n o f t h e t o l u e n e c o n c e n t r a t i o n .

The r a t e i n c r e a s e s almost

p r o p o r t i o n a l l y w i t h the toluene concentration. The y i e l d s o f benzaldehyde o b t a i n e d w i t h t h r e e d i f f e r e n t t o l u e n e concentrat i o n s a r e shown i n Fig. 7; t h e c o n v e r s i o n was v a r i e d by changing t h e c o n t a c t time.

The s e l e c t i v i t y decreases s l i g h t l y w i t h an i n c r e a s e i n t h e c o n c e n t r a t i o n .

E f f e c t o f feed r a t e .

The r e a c t i o n was conducted a t a f i x e d t o l u e n e concen-

t r a t i o n o f 1.0 mol% i n a i r , w h i l e changing t h e f e e d r a t e . aldehyde a r e shown i n Fig. 8.

The y i e l d s o f benz-

The s e l e c t i v i t y i n c r e a s e s w i t h t h e feed r a t e .

The r e s u l t s i n d i c a t e t h a t t h e d e g r a d a t i o n o f t h e benzaldehyde produced t a k e s place. i n p a r t , a t t h e p o s t - c a t a l y s t

zone i n t h e r e a c t o r .

DISCUSSION Toluene i s l e s s r e a c t i v e t h a n o l e f i n i c compounds such as propylene, butenes, and a - m e t h y l s t y r e n e .

Therefore, o x i d e s w i t h a r e l a t i v e l y h i g h o x i d i z i n g func-

t i o n may be r e q u i r e d as c a t a l y s t s f o r t o l u e n e o x i d a t i o n .

Certainly, the oxides

used u s u a l l y i n o x i d a t i o n o f ethylbenzene t o s t y r e n e [ 2 5 ] a r e s c a r c e l y a c t i v e f o r toluene oxidation.

The Bi-Mo-based

oxides, which show an e x c e l l e n t p e r f o r -

mance i n o x i d a t i o n o f propylene and butenes, a r e n o t e f f e c t i v e f o r t o l u e n e o x i d a t i o n , because t h e y promote a l s o t h e d e g r a d a t i o n o f t h e produced benzaldehyde. The Mo-W and Mo-W-Te oxides, which show a good performance i n o x i d a t i o n o f d - m e t h y l s t y r e n e t o p h e n y l a c r o l e i n [ 2 3 ] a r e n o t s e l e c t i v e f o r t h e benzaldehyde formation.

These f i n d i n g s suggests t h a t t h e c a t a l y t i c f u n c t i o n s r e q u i r e d f o r

y i e l d i n g benzaldehyde a r e d i f f e r e n t from those r e q u i r e d f o r oxydehydrogenation o f o l e f i n i c compounds. It has been r e p o r t e d t h a t Mo-P oxides show a good performance i n o x i d a t i o n

o f butenes t o m a l e i c anhydride [ 2 6 ] .

On t h e o t h e r hand, Bordes e t a l .

[ 2 7 ] have

r e p o r t e d t h a t U-Mo o x i d e s w i t h Mo-rich c o m p o s i t i o n s a r e e f f e c t i v e as c a t a l y s t s f o r o x i d a t i o n o f butenes t o m a l e i c anhydride.

These f i n d i n g s suggest t h a t t h e

functions required f o r oxidation o f toluene a r e s i m i l a r t o those required f o r o x i d a t i o n o f butenes t o m a l e i c anhydride. f e c t i v e f o r toluene oxidation.

However, t h e V-P o x i d e s a r e n o t e f -

P o s s i b l y , t h e c o n s e c u t i v e o x i d a t i o n o f benzalde-

hyde cannot be suppressed w i t h V2O5-conttaining c a t a l y s t s .

Even o v e r t h e Mo-P

and U-Mo oxides, benzaldehyde i s degraded, t o a c e r t a i n e x t e n t . The main by-product w i t h Mo-P-based o x i d e s i s m a l e i c a n h y d r i d e [20,21]. w h i l e t h a t w i t h U-Mo o x i d e s i s carbon oxides. t h e c o n s e c u t i v e o x i d a t i o n o f benzaldehyde.

These by-products a r e formed by

P o s s i b l y , m a l e i c anhydride formed

i n i t i a l l y i s decomposed p r o m p t l y t o carbon o x i d e s o v e r U-Mo oxides, w h i l e Mo-Pbased o x i d e s a r e i n a c t i v e f o r t h e decomposition o f m a l e i c anhydride. The s e l e c t i v i t y t o benzaldehyde i s s c a r c e l y a f f e c t e d w i t h t h e v a r i a t i o n i n t h e c o n c e n t r a t i o n s o f oxygen and t o l u e n e , b u t i t i n c r e a s e s as r a i s i n g t h e tem-

430

p e r a t u r e , i n d i c a t i n g t h a t t h e r e a c t i v i t y o f benzaldehyde r e l a t i v e t o t h a t o f t o l u e n e decreases as r a i s i n g t h e temperature. The c o n d i t i o n s o f h i g h temperat u r e s and s h o r t c o n t a c t t i m e s a r e s u i t a b l e f o r t h e benzaldehyde f o r m a t i o n .

It

should a l s o be n o t e d t h a t t h e s e l e c t i v i t y i n c r e a s e s w i t h an i n c r e a s e i n t h e f e e d r a t e ( F i g . 8).

The d e g r a d a t i o n o f benzaldehyde t a k e s p l a c e a t t h e p o s t - c a t a l y s t

zone i n t h e r e a c t o r , because benzaldehyde i s a v e r y r e a c t i v e compound. The r a t e o f r e a c t i o n i n c r e a s e s w i t h an i n c r e a s e i n b o t h t h e oxygen and t o l u e n e c o n c e n t r a t i o n , b u t i t decreases w i t h t h e a d d i t i o n o f w a t e r vapor, sugg e s t i n g t h a t t h e r e a c t i o n i s c o n t r o l l e d by b o t h t h e r e g e n e r a t i o n o f reduced c a t a l y s t and t h e a c t i v a t i o n o f t o l u e n e on a c i d i c s i t e s o f c a t a l y s t , which may be h i n d e r e d by w a t e r vapor. It may be concluded t h a t o x i d a t i o n o f t o l u e n e t o benzaldehyde i s more d i f f i -

c u l t t h a n o x y d a t i v e dehydrogenation o f o l e f i n s , because t o l u e n e i s r e l a t i v e l y s t a b l e and, f u r t h e r more, benzaldehyde i s v e r y u n s t a b l e .

R e l a t i v e l y good p e r -

formances a r e o b t a i n e d w i t h Mo-P and U-Mo o x i d e s , because t h e c o n s e c u t i v e o x i d a t i o n o f benzaldehyde i s suppressed s a t i s f a c t o r i l y w i t h t h e s e oxides. REFERENCES 1 2

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

W.G. Parks and R.W. Yula, Ind. Eng. Chem.. 33 (1941) 891. W.L. F a i t h , D.B. Keyes and R.L. C l a r k , I n d u s t r i a l Chemicals, 3 r d edn., J. Wiley. Nwe York, 1957, p.120. J.K. Oixon and J. L o n g f i e l d , C a t a l y s i s , 7 (1960) 207. J. Downie, K.A. Shestad and W.F. Graydon. Can. J. Chem., (1961) 201. N.I. Popova and B.V. Kabakoba, K i n e t . K a t a l . , 5 (1964) 289. N . I . Volynkin, Zh. P r i n k l . Khim., 39 (1966) 2783. C.R. Adams. J. Catal., 10 (1968) 355. K.A. Reddy and L.K. Doraiswamy, Chem. Eng. Sci., 24 (1969) 1415. D.L. Trimm and M. I r s h a d , J. Catal.. 18 (1970) 142. J.E. Germain and R. Laugier, B u l l . SOC. Chim. F r . , (1971) 650. J.E. Germain and R. Laugier, C. R. Acad. S c i . P a r i s , C, 276 (1973) 1349. K. Van d e r Wiele and P.J. Van den Berg, J. Catal.. 39 (1975) 439. R.K. Sharma and R.D. S r i v a s t a v a , J. Catal., 65 (1980) 481. N.K. Nag, T. Fransen and P. Mars, J. C a t a l . , 68 (1981) 77. S.L.T. Andersson, J. Catal., 98 (1986) 138. A.J. Van Hengstum, J. Pranger, S.M. Van Hengstum-Nijhuis, J.G. Ommen and P.J. G e l l i n g s , J. Catal., 101 (1986) 323. B. Jonson, R. Larsson and B. Rebenstorf. J. C a t a l . , 102 (1986) 29. B. Grzybowska. M. Czerwenka and J. S l o c z y n s k i . C a t a l . Today, 1 (1987) 157. M. A i , Kogyo Kagaku Zasshi. 73 (1970) 946; Chem. Abstr.. 73 (1970) 76790k. M. A i . Kogyo Kagaku Zasshi, 74 (1971) 1636; Chem. Abstr.. 75 (1971) 1 0 9 9 9 2 ~ . M. A i , Nippon Kagaku k a i s h i , (1972) 1151: Chem. Abstr.. 77 (1972) 66559k. M. A i . i n T. Seiyama and K. Tanabe (Eds), Proc. 7 t h I n t e r n . Congr. Catal., Tokyo, 1980. Kodansha-Elsevier, Tokyo-Amsterdam, 1981, p. 1060. M. A i , J. Catal., 120 (1989) 206. M. A i , J. Catal.. 100 (1986) 336. G. Emig and H. Hofmann, J. Catal., 84 (1983) 15. M. A i and S. Suzuki. J. C a t a l . , 32 (1973) 362. E. Bordes, S.J. Jung and P. C o u r t i n e . i n M.F. P o r t e l a (Ed.), Proc. 9 t h Ibero-american Symp. Catal.. Lisbon. 1984, P. 983; i n G. C e n t i and F. T r i f i r o (Eds.), S t u s i e s i n S u r f a c e Science and C a t a l y s i s , E l s e v i e r , Amsterdam, 55 (1990) 585.

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

431

0 1991 Elsevier Science Publishers B.V., Amsterdam

NEW POLYDENTATE Mo(V1) - GRAFTED POLY(AMID0 AMINE) RESINS AS HETEROGENEOUS EPOXIDATION CATALYSTS

PFERRUTI, E.TEMPESTI, L.GIUFFRE, R.RANUCC1 and CMAZZOCCHIA Dipartimento di Chimica Industriale, Politecnico di Milano, Piazza Leonard0 da Vinci 32, Milano 20133 - ITALY

ABSTRACT New heterogeneous oxygen-transfer catalysts have been prepared by Mo(V1) grafting on suitably functionalized poly(amido amine) resins containing units derived from carboxylic aminoacids. One of these catalysts prepared starting from N,N'-ethylene-diaminoacetic acid proved particularly successful in typical liquid-phase oxidation reactions.

INTRODUCTION It is well known that homogeneous compounds of Mo(V1) with carboxylic acids are powerful catalysts for the epoxidation of olefins. In order to heterogenize the Mo(V1) derivatives, a commercial carboxylated resins has been used by Ivanov et al. (ref. 1). A weak point of using a commercial resin is that it is not possible to control the number and distribution of carboxylic groups. This results in a poor flexibility of structure and properties of the final catalyst. On the other hand it is known that poly-amidoamines, either linear or crosslinked, provide a convenient way t o obtain multifunctional macromolecular substances with well defined structures. We have prepared a wide series of Mo(V1)-grafted poly(amido a m i n e ) resins based on aminoacids with varying distances between the aminic and carboxylic groups. We have found that the catalytic activity of these resins is strongly dependent on that distance, maximum activity within this series being obtained when the resin supposedly co-ordinates to a Mo(VT)02 c o r e in a quadridentate manner throught the use of two suitably located adjacent carboxylic functions.

432

EXPERIMENTAL The synthetic procedures which preside over the preparation of a typical resin (EGDA) obtained from 1,4-bis acryloylpiperazine (BP), N,N'ethylenediaminoacetic acid (EDDA) and triethylamine (TEA) and subsequently crosslinked with vinylpyrrolidone (VIP) in the presence of 2,2-azo-bis (2-methylpropionitrile) (AIBN) according to the following Scheme

:p

0 II

X CH2=CH-C-

N-C-CH=CHz

w

+

0.7 X H-N-CHz-CHz-NH + 0.7 X(CzH&N I I CH2 CH2 I COOH COOH

HzO, room to. 24 hrs

COOH I

COOH I

I

+*IBN Crosslinked resin

have already been reported in a preliminary communication (ref. 2) together with the adopted Mo(V1) grafting procedures. According to the reported results we assume that the interaction of molybdic acid with the surface acid groups occurs on the basis of an acidbase interaction accompanied by water elimination which leads to a structure such as

433

This assumption stems from the fact that under the same conditions adopted to heterogenize conventional molybdenum catalysts using differently functionalized polymeric supports such as surface boronic (ref. 3) or phosphonic (ref. 4) acid groups, the Mo(V1)-fixation on a support such as (EGDA) invariahly leads to a metal to acid group ratio equal to 2.0 rather than 1.0. Although it cannot be excluded, no evidences have been found of a possible stabilizing effect of the complex due to backdonation by the aminonitrogen

as evidenced with molybdenum peroxo complexes stabilized by picolinato and pyridine-2,6-dicarboxylato ligands (ref. 5 ) . RESULTS AND DISCUSSION In the i.r. spectra of oxomolybdenum complexes the strong metal-oxygen absorption usually stands out from the ligand bonds and the type of metaloxygen core is identifiable from the special features. W e have sought to identify by i.r. the molybdenum-oxygen core in the new (EGDA)-Mo(V1) complex since, in agreement with literature finding (ref. 6 ) , we have found that the type of core may depend both on the pH of the mother liquor used for Mo-fixation since the pH obviously affects the relative stability of t h e co-ordinated molybdenum and on the number and kind of functional groups present in the ligand aminoacid since they determine their demand for coordination sites on the metal. Once again contrary to earlier results obtained with d i f f e r e n t l y functionalized polymeric matrixes, for the same (EDGA)-Mo(V1) complex different core structures have been observed with time. Indeed, while n o

434

substantial modifications with time are observed for the co-ordinated (COO) bands (ca. 1640 cm-1), the complex exhibits a very strong doublet which is initially centered at 975 and 9 1 0 cm-1. On increasing time at room temperature the 975 cm-1 band progressively shifts to lower frequencies and after four months of conditioning at room temperature under inert atmosphere the doublet is definitely centered at 955 and 910 cm-1. The displacement observed (from 975 to 955 cm-1) has been attributed to lower oxidation states of the metal core. In order to confirm this assumption we have: - fixed and characterized by i.r. relative to known molybdenum ( V ) oxocomplexes reported in literature (ref. 7 ) , the molybdenum ( V ) oxocomplex derived from (EGDA); - followed by i.r. the modifications of the (EGDA)-Mo(V) oxygen core after conditioning at room temperature or u n d e r s i m u l a t e d oxidation conditions, i.e., in the presence of active oxygen. When definitely stabilized, the (EGDA)-Mo(V1) complex shows a doublet ;it 955-910 c m - 1 which may be attributed to symmetric (Mo=O); the antisymmetric mode was not detected. T h e appearance of the symmetric stretching-mode is consistent with a cis disposition of two terminal Mo=O bonds (ref. 8). In concomitance with the displacement observed by i.r., an evolution of the catalytic activity has been observed while studying the liquid-phase epoxidation of cyclohexene in the presence of (EGDA)- Mo(VI), freshly prepared or after four months of conditioning a t room temperature under inert atmosphere. As usual, the appearance of epoxide was followed by gas chromatographic analyses or by direct titration of oxirane oxygen and the disappearance of hydroperoxide was monitored by iodometric titration. In figure we report concentration-time for typical runs in ethylbenzene at 80°C obtained with the experimental procedure already described (ref. 9). It may be seen that with a freshly prepared catalyst an induction period is observed which lowers the initial catalytic activity. Our modified MichaelisMenten type model equation (ref. 9 ) cannot adequately fit the kinetic curves obtained due to the absence of kinetic parameters which account for the apparent initial induction period (see Figure). After conditioning, when all the molybdenum i s Mo(VI), the observed catalytic activities are comparable t o those found with conventional catalysts such as M o 0 2 ( a c a c ) 2 over the whole range of experimental conditions considered. SignQcantly, however the autoinhibitory nature of the

435

reaction considered, evaluated as usual by optionally adding known amounts of the same alcohol or epoxide formed during the course of the reaction, cannot be adequately described by using our modified MichaelisMenten type model equation (ref.9).

-

-,-

I

or X or A

After Conditioning Freshly prepared Catalyst

T

01

U

c

2

0,l

I

80°C

(Olefine) I 0.3 - 0.5 M (Catalyst) 0.0019 - 0.0021 M (Hydroperoxide) 0.19 - 0.2 M

-.

- -

-

10

0

20

30

40

TIME (mln)

-

Figure Variation in the concentration of reactants (hydroperoxide) and products (epoxide) during the epoxidation of cyclohexene with (EGDA) - Mo(VI)

Work is in progress in order to satisfactorily define the kinetic approach.

REFERENCES

1 S.Ivanov, R.Boeva and S.Tanielyan, J.Cata1. 56 (1979) 150. 2 P.Ferruti, E.Tempesti, L.Giuffr6, P.Arlati, E.Ranucci and G.Airoldi, J.Appl.Pol.Sc., in press. 3 E.Tempesti, L.Giuffr6, F.Di Renzo, C.Mazzocchia and G.Modica, J.Mol.Catal., 45 (1988) 255. 4 E.Tempesti, L.Giuffr6, C.Mazzocchia, P.Gronchi and F.Di Renzo, J.Mol.Catal., 55 (1989) 371. 5 S.E.Jacobson, R.Tang and F.Mares, J.C.S. Chem. Commun., (1978) 888. 6 R.J.Butcher, H.K.J.Powell, C.J.Wilkins and S.H.Yong, J.C.S. Dalton, (1976) 356.

436

7 P.Alonso, I.deFrutos, T.GutiCrrez ana A.Doadrio Lbpez, Transition Met.Chem., 12 (1987) 133. 8 F.A.Cotton, D.L.Hunter, L.Ricard and E.Weiss, J.Coord.Chem., 3 (1 974) 259. 9 E.Tempesti, L.Giuffr6, F.Di Renzo, C.Mazzocchia and G.Airoldi, Applied Catalysis, 26 (1986) 285.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

437

SELECTIVE OXIDATION OF METHYL ETHYL KETONE TO DIACETYL OVER VANADIUM PHOSPHORUS OXIDE CATALYSTS. E McCULLAGH, J B McMONAGLE and B K HODNETT Dept of Materials and Industrial Chemistry University of Limerick Plassey Technological Park Limerick, Ireland

ABSTRACT Selective oxidation of methyl ethyl ketone to diacetyl has been studied by passing a mixture of the ketone in artificial air over vanadium phosphorus oxide catalysts in the temperature range 200-350°C. Products observed included diacetyl, methyl vinyl ketone, acetaldehyde, acetic acid and carbon dioxide. C4 products were favoured at low temperatures and at low or zero oxygen partial pressures. These results are rationalised in terms of two pathways for C2 products, namely oxidation of the double bond in the enol form of methyl ethyl ketone to yield acetic acid and acetaldehyde, and acid catalysed hydration of the keto form to yield acetaldehyde only. The C4 roducts are envisaged to go through a common intermediate, namely, CH3COCHOH Hg, formed by interaction between methyl ethyl ketone and lattice oxygen.

8

INTRODUCTION Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moilerate conversions of MEK have been reported for catalysts based on Cog04 as a pure oxide and with basic oxides added; conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. In this study we examine the role of lattice oxygen from VPO catalysts in the oxidation of MEK. EXPERIMENTAL Catalyst DreDaration (i) Oraanic medium: A mixture of 9.lg of V2O5 and 13.849 of 85% H3PO4 was refluxed for 3 hours in 200 ml of isobutanol, allowed to cool, and filtered. The residue was dried in a vacuum oven at 12OoC for 2 hours. This catalyst precursor was subsequently calcined at

438

450°C for 2 hours in air yielding a vanadium phosphorus oxide catalyst with a nominal P:V atomic ratio of 1.2:l. The final product was dark green in colour. This catalyst will be identified below as 1.2PV(dir). (ii)Aaueous medium: Using a method developed by Bordes and Courtine (ref. 7) whereby oxalic acid is used as the reducing agent in aqueous medium, catalysts were prepared with nominal P:V ratios of 0.8:l and 1.2:l. The method involved heating 0.5 mol oxalic acid to reflux in 150 ml water, adding 0.1 mol V2O5 and the required amount of NHqH2P04 to yield the desired P:V atomic ratio. After 3 hours under reflux a deep blue solution formed, which was allowed to cool, and then filtered. The filtrate was held at 8O'C in a rotary evaporator at reduced pressure to remove the water, leaving typically a green solid precursor. Finally, the catalyst was formed by calcination of the precursor at 550'C for 5 hours. These catalysts will be referred to as 0.8PV(ox) and 1.2PV(ox) below. Catalvst Testing All catalysts to be tested were initially pelletised, and sieved between 106 and 850pm meshes; 0.59 was placed in a fixed-bed U-tube reactor. The testing apparatus was a continuous flow system operated at ambient pressure. Nitrogen was used to entrain MEK into the vapour phase from its adsorbed state on a molecular sieve. The sieve was contained in a stainless steel tube held at 1.6'C by means of an ice-bath. The exit from the MEK saturator merged with an air stream forming a mixture with the molar ratios of MEK:02:N2 of 1:8:54. The feed gas entered a four-way valve which allowed bypass and reactor effluent streams to be analysed by gas chromatography. The reactor was placed in an oven and the temperature monitored using a thermocouple situated in a thermowell inside the catalyst bed. A total gas flow of 25 ml min-l was employed when testing 0.8PV(dir) and 50 ml min-' for 0.8and 1.2PV(ox). DA, MEK, methyl vinyl ketone (MVK), propionaldehyde (PrH), and acetaldehyde (AcH) were analysed by on-line gas chromatography using a Varian 3400 GC equipped with a thermal conductivity detector and a 2m column containing 25% w/w I3,O'-oxydipropionitrile on Chromosorb W (80-100 mesh) operated at 6OoC; He was used as the carrier gas. Acetic acid (AcOH) was collected in 2ml of water from the effluent stream over a period of 1 hour and later analysed on ii Porapak QS column at 15OoC. C02 was tested by removal of 2ml samples from the exit of the reactor with a gas syringe and injecting them onto a Porapak QS column operated at 6OoC. The stability of reaction products in the reaction conditions was assessed by placing each in turn in the saturator, mixing them with nitrogen and passing them along with air over 1.2PV(ox) at a total gas flow of 50ml min-l between 200 and 35OOC. Testina in the absence of oxvaen The ability of MEK to extract lattice oxygen, thus reducing the catalyst, was examined by passing MEK in N2 (10ml min-l) over 0.5g of l.PPV(dir) at 30OoC. Reaction products

439

were followed by on-line GC analysis only. Initial levels of MEK transformation were high but declined. When negligible activity was attained the system was flushed with N2 overnight. Air at a flow rate of 15ml min-l was admitted for 3 hours at 3OO0C to regenerate the catalyst and exposed again to a flow of MEK in N2 for 0.5 hours. This procedure was repeated for a number of cycles. Selectivity and yield were calculated on a carbon number basis.

RESULTS Figure 1 reveals that steady state activity was established after approximately 30 hours of reaction over 1.2PV(dir) at 3OO0C under standard conditions. Initially there was almost total conversion of MEK which fell to ca. 60% when at the steady state. In these conditions AcH appeared in highest yield. Yield of DA reached a maximum of 10% after 10 hours on stream but gradually fell to 6.5% at the steady state. The other major products, C02 and AcOH, gave steady state yields of ca. 11.5% and 19% respectively.

y00

Figure 1: MEK conversion (.),and yield of DA (+), AcH (lf),MVK (0)and PrH ( x ) at 300'C over l.PPV(dir) ( Po, = 96 torr, PMEK = 12.2 torr, W/F = 1.2 g s rn1-l)

Data for three catalysts are presented in figure 2 as a function of temperature. As expected the conversion of MEK (fig 2(a)) increased exponentially with temperature on all samples tested. For the aqueous-based catalysts 1.2PV(ox) was more active than 0.8PV(ox); 1.2PV(dir) gave the highest conversion. The yield of DA increased with temperature on the aqueous-based catalysts and reached a maximum at 25OoC on l.ePV(dir). Figure 2(c) shows yields of all major products and conversion of MEK on 1.2PV(dir). AcH and AcOH formed in highest abundance with maxima at about 30OoC. The ratio AcH:AcOH was greater than unity below 3OO0C and less than unity above this temperature. C02 and PrH increased with temperature while MVK and DA peaked at 3OO0C and 25OoC respectively. Table 1 shows that under typical operating conditions DA and AcH were oxidised to AcOH, particularly at low temperature, while C02 became the more predominant product

440

I

MEK Conversion (%)

1001

v

40

20160

200

260

300

350

400

Temperature ("C)

Yield of DA (YO)

I

400

Yield (YO)

1

40 I

30

,Ot

400

160

Temperature ("C)

Figure 2(a): Influence of temperature on the conversion of MEK over l.PPV(dir) (*), l.PPV(ox) (+) and 0.8PV(ox) (a). Figure 2(b): Influence of temperature on the yield of DA. Symbols as in fig. 2(a). Figure 2(c): Influence of temperature on the ield of DA (+), AcH (k), MVK (o), PrH (x), C o g ( ) and Ac H (A).

t:

441

at higher temperatures. No detectable conversion of AcOH occurred over the temperature range studied. Butane-2,S-diol gave AcH as the principal product at low temperature, while 3-hydroxybutan-2-one yielded DA with trace amounts of MVK,AcH and AcOH up to 300%. TABLE 1 Stability of products in standard reaction conditions. TEMP.R ANG EPC

FEED DA AcH AcOH Butane-2,3-diol 3-hydroxybutan-2-one

PR 0DUCTS

200-250 250-350 200-250 250-350 200-350 200-250 250-300 300-350 200-300 300-350

AcOH,C02 AcOH,C02 COP AcH AcH,AcOH,C02

E2

Figure 3 shows that during contact of MEK with 1.2PV(dir) in the absence of 0 2 , production of DA and MVK both reached a maximum between 10 and 20 minutes on stream and decayed to negligible levels after 100 minutes, whereas the rate of AcH formation fell from the beginning of the reaction. In subsequent trials after regeneration the rate of formation of all products was less but remained constant; AcH production ceased before the third cycle. DA was always more abundant than either MVK or AcH and this effect was more marked in the first 50 mins of reaction. Rate of Formation (lOE+GmoI/g/min) I

I

0

20

40

60

80

100

Time ( m i d Figure 3: R6 e of formation of DA (+), AcH ( # ) and MVK (0) over l.PPV(dir) at 300 (Po* = 0 torr, 2,,,EK = 30.5torr, W/F = 3.0 g s ml-1)

L.

Y

Figure 4 relates selectivity of all of the major products on 1.2PV(dir) with partial pressure of oxygen at 30OoC. AcH was always produced in higher molar quantities than AcOH but particularly at low oxygen ptessures. Below ca. 10 torr of 0 2 , C4 product formation

442

became more favourable than C2 scission products. PrH selectivity remained low throughout the range of 0 2 pressures examined. Selectivity (910)

40

MEK Conversion (90),oo

1

-X

n 0

20

40

60

80

100

Partial Pressure of Oxygen (torr) Figure 4: Influence of oxygen partial pressure on the conversion of MEK ( ), and selectivit to DA (+), AcH (+), MVK (o),PrH (x), COP ( 0 ) and AcOH (A) over l.SPV(dir) = 12.2 torr, WIF = 1.2 g s ml-1). at 300'C [P, DISCUSSION This discussion will concentrate on two aspects of MEK oxidation over VPO catalysts, namely, the establishment of a reaction network and the respective roles of gas phase or adsorbed 0 2 and lattice oxygen in this network. It is clear that oxidation of MEK can be catalysed by a range of oxides, acidic and basic. Within the VPO class of catalysts similar performance was observed from catalysts prepared in organic and aqueous media, and with varying P:V ratios so it appears that MEK oxidation cannot at this point be uniquely associated with particular phases, e.g., OVOPO4 (present at low P:V ratios) or (V0)2P207 (present at high P:V ratios), nor can it be associated uniquely with a particular cleavage plane (for example the (020) plane in (VO)2P2O7 which predominates in alcoholic preparation media). Preceding work on MEK oxidation has identified a number of possible reaction networks whereby formation of DA and the numerous observed by-products could be rationalised. Yamazoe et al. (ref. 2) and Ai (ref. 5) have proposed the formation of peroxy intermediates formed by reaction with 02-(ads) to explain the formation of DA and C2 scission products. Several aspects of these networks appear inconsistent with this study. The first obvious feature is the occurrence of greater amounts of AcH than AcOH, particularly at low temperatures and oxygen partial pressures. AcH:AcOH ratios greatly in excess of 1:l have been noted for C03O4 catalysts to which acidic dopants have been added. This ratio was less than 1 :I when basic oxides were used. This point is important because according to both networks described above AcH and AcOH should be produced in equal quantities initially (refs. 2,5); Table 1 shows that AcH is unstable in our reaction conditions whereas AcOH is stable; and Table 1 also shows that AcH and DA convert into AcOH in our

443

reaction conditions. These data imply that if MEK decomposes to form AcH and AcOH an imbalance in favour of AcOH should be observed, whereas an excess of AcH was sometimes observed. Above 3OO0C DA decomposition (fig 2c) contributes to the observed yield of AcOH. In conventional organic chemistry scission of MEK to AcH and AcOH would proceed through an enol intermediate, i.e., CH3COH=CHCH3, with scission of the C=C bond. Although about ten times less stable the alternative enol intermediate CH2=COHCH2CH3 would be expected to form also; the scission products would be propionic acid and formaldehyde (ref. 11). The former was never observed in this study although propionaldehyde and formaldehyde were. These findings argue against straightforward C=C oxidative bond scission as the only source of C2 products. The second feature of our results at variance with the literature networks is the observed relationship between oxygen partial pressure and product distribution; low oxygen partial pressures favoured DA and MVK formation over C2 products (see figs. 3 and 4). These data taken with the ability of MEK to reduce the VPO catalyst, and in so doing generate DA, MVK and AcH (AcOH was not analysed for in these experiments), imply that lattice oxygen must play a role in the reaction. A more likely network consistent with the product distribution observed above would involve acid catalysed hydration of MEK, followed by oxidative cleavage of the intermediate diol, i.e., CH3 C CH2 CH3

a

+ H'eCH3

+9 CH2 C H 3 e CH3 YH +CH CH3 OH

H20

OH

CH3 YH YH C H 3 e CH3 F H qHCH3OH OH2 OH OH t

2 CH3CHO + H20

In support of this proposed network sufficient Bronsted and Lewis acidity have been associated with the surface of VPO catalysts (refs. 8-10): Indeed in the presence of water some Lewis acidic sites convert into Bronsted sites. Sufficient water is produced via complete oxidation products to support the reaction network. Finally diol oxidation is a well known reaction in conventional organic chemistry (ref. 1l), and from Table 1 butane-2,3diol, under our conditions, readily formed large amounts of AcH at low temperature. Oxidation of the enol form of MEK i.e., CH3C(OH)=CHCH3+CH3COOH

+

CH3CHO

must also be included in any reaction mechanism, but on our catalysts operation in the reaction conditions described above must only represent a partial pathway for AcH formation.

444

The observation of DA and MVK in similar conditions point to a possible common intermediate. A working hypothesis may be outlined in reference to the analagous SeO2 oxidation of methylene groups alpha to a carbonyl group (ref.12). In this hypothesis the enolate anion of MEK becomes susceptible to attack by a surface V=O species, with V in the 5+ or 4+ oxidation state, i.e., CHCJ-~=CH-CH~

CHs-C-CH-CH3 I\

P

I

O ? V' \ H:O-P

L O \ H:O-P Structure [ I ]

This species may be formally regarded as a common intermediate to DA and MVK. This may be more clearly seen if the VPO moiety in structure [ I ] is replaced by a hydrogen atom. This structure then becomes CH3C(O)CH(OH)CH3,i.e.,3-hydroxybutan-2-one.This compound was tested under standard reaction conditions (Tablel), and at low to moderate temperatures formed large arriounts of DA, by oxidation of the alcohol function, and traces of MVK, by dehydration. It is clear that VPO catalysts possess the acidic properties necessary for the latter and the Lewis base properties for the former (refs. 8-10). Reaction networks can be described for the oxidation of MEK to DA and MVK in which monatomic oxygen species, probably lattice oxygen species are involved. By implication total oxidation species are probably associated with direct interaction between dioxygen species and MEK although some C 0 2 was observed when MEK was contacted with 1.2PV(dir) in the absence of 02.

REFERENCES 1 Kirk-Othmer, Encyclopedia of Chemical Technology, Wileylnterscience, 10 (1984) 462.

2 N. Yamazoe, S. Hidaka, H.Arai and T. Seiyama, Oxidation Communications, 4 (1983) 287. 3 Y. Takita, K. Inokuchi, 0. Kobayashi, F. Hori, N. Yamazoe and T. Seiyama, J. Catal., 90 (1984) 232. 4 Y. Takita, F. Hori, N. Yamazoe and T. Seiyama, Bull. Chem. SOCJpn, 60 1987) 2757. 5 M. Ai, J. Catal., 89 [I 984) 413 6 B.K. Hodnett, Catal. Rev. Sci. Eng., 27 (1985) 373. 7 E. Bordes and P. Courtine, J. Catal., 57 (1979) 236. 8 S.J. Puttock and C.H. Rochester, J. Chem SOC.Faraday Trans 1,82 (1986) 2773. 9 S.J. Puttock and C.H. Rochaster, J. Chem SOC.Faraday Trans 1,82 (1986) 3013. 10 S.J. Puttock and C.H. Rochester, J. Chem SOC.Faraday Trans 1,82 (1986) 3033. 11 A. Streitwieser and C. H. Heathcock, Introduction to Organic Chemistry, McMillan, 12 J. March, Advanced Organic Interscience, (1985) 1078.

445

M.Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science PublishersB.V., Amsterdam

(HETEROGENEOUS)PHOTOCATALYTIC OXIDATION OF TOLUENE USING PURE AND IRON-DOPED TITANIA CATALYSTS J ,A.NAVIO'

, M. GARCIA COMEZ2,

2

MS

A. PRADERA ADRIAN

and J .FUENTES MOTA

2

'Instituto de Ciencias de Materiales-CSIC/Dpto. de Quimica 1norgBnica.Facultad de Quimica.Universidad de Sevilla.41012-Sevilla (Spain). 2Dpto. de Quimica 0rgBnica.Facultad de Quimica.Universidad de Sevilla. 41012-Sevilla (Spain). SUMMARY We report here results on the photocatalytic oxidation by oxygen of neat-liquid toluene, under UV-irradiation, by using unloaded and iron-loaded titania catalysts. Several experimental conditions have been selected to investigate parameters which can influence the chemical yields and the distribution of products; particular attention has been devoted to investigate the evolution of chemical yields and selectivity by extending the irradiation times up to 12 h. A discussion on the effects of structural features and the concentration of catalysts is reported. The role of water has been also investigated.

INTRODUCTION Heterogeneous photocatalysis, applied to the transformations of organic molecules, has become an exciting and rapidly growing area of research i n the last few years (refs.1-3). The interest of this study arise from synthetic, mechanistic or environmental purposes. Into the wide groups of aromatic hydrocarbons, the heterogeneous oxidation of toluene,in the liquid phase, using irradiated semiconductor materials,has been studied by Fujihira et al.(refs.4-6) using different experimental condit i o n s ; thus, for example, in aqueous media, the total proportion of products,

stemming from the photo-oxidation of the side-chain (benzaldehyde and benzyl alcohol), versus those obtained from hydroxylation of the aromatic ring (cresols) dicreases when increasing the pH and also in the presence of oxidants (ref.4); the influence, on the products distribution, of the type of catalyst used, has been studied by the same authors (ref.5). Studying the heterogeneous photocatalitic oxidation of neat toluene, by air (ref.6), benzaldehyde was the only product, arising from the side-chain photo-oxidation; depending upon the conditions, 1,2-diphenylethane ( 1 ) was a l s o detected. On the other hand, the photocatalyN

tic oxidation of toluene, in the gas phase, has been studied by Pichat and collab.(ref.7) using Ti02 as catalyst ; these authors reported that only traces of benzaldehyde were detected.

We study here the heterogeneous photocatalytic oxidation of neat toluene, using unloaded and iron-loaded Ti02 as photocatalysts. The influence on the

446

product

distribution

of several parameters as the concentration of catalyst,the

presence of water and the irradiation times, are also analysed.In particular,the evolution with irradiation times, of chemical yield and selectivity, have been investigated by extending the irradiation duration up to 12 h. EXPERIMENTAL Materials Two types of catal.ysts were used in this study: p u r e Ti02 (Degussa,P-25), previously calcined in air at 500cC for 24 h, and iron-doped Ti02. The iron-doped Ti02 catalysts, Fe/TiO

2

used in the present study (iron contents: 0.5 wt

% or 5 wt %, both calcined at 5 0 0 c C , 24 h), involves the dispersion of iron (as 3t

Fe

)

in the Ti02 matrix; the preparation of these catalysts, by the wet impreg-

nation method, as well as the characterization, have been previously described (ref.8). Distilled toluene was reagent grade. Method and techniques

A 400-W medium-pressure mercury-arc lamp, radiating predominantly at

365-

-366 nm, served as a light source; this lamp (Applied Photophysic Ltd.) produces -1

more than 5x1o1'

photons s

within the reaction flask. It was contained in a

double-walled quartz-glass immersion well, through which water was passed for cooling. A borosilicate glass sleeve was used to remove short wavelenth radiation (less than 300 nm). A gas inlet reaction flask (400 m l ) was used; a double surfa -

ce reflux condenser fitted to the reaction flask, was used in order to prevents "creep" and l o s s of vapour when low boiling-point solvents are used. In all of thc experiments, the catalyst was suspended in 375 m l of neat-liquia toluene, and oxygen was bubbled through the suspension; after purging with oxygen, for 15 min., the suspension was subjected to UV-irradiation with continuous bub bling of oxygen. Portions of

-

3 ml were extracted, from the reaction flask, at

several times during the irradiation. The photocatalyst was separated by centrifugation, to analyse the liquid phase. Identification of products was performed by GC-MS using a KRATOS-MS 80 RFA instrument; separations were achieved on a CPSIL-5 Chrompac column whose temperature was programmed up to 280cC. The method of external standards was used for semiauantitative determinations. RESULTS AND DISCUSSION Experiments were carried out under six different conditions in order to

447 a c h i e v e i n f o r m a t i o n about some p a r a m e t e r s which can i n f l u e n c e t h e s e l e c t i v e con v e r s i o n o f t o l u e n e t o benzaldehyde. For each c o n d i t i o n , t h e e f f e c t o f i r r a d i a t i o n time was a l s o i n v e s t i g a t e d . Table 1 summarizes t h e changes, d u r i n g t h e i r r a d i a t i o n t i m e s , o f t h e products q u a n t i t i e s and t h e chemical y i e l d s . I n g e n e r a l , benzaldehyde, benzyl a l c o h o l and benzoic a c i d were d e t e c t e d a s t h e main p r o d u c t s . During e x p e r i m e n t s 1 and 5 , benz y l a l c o h o l was d e t e c t e d a s t r a c e s o n l y ; a l s o i n experiment 3 , benzoic a c i d was detected a s traces

,

and t h a t o n l y a f t e r prolonged i r r a d i a t i o n t i m e s . I t i s i n t e

r e s t i n g t o mention h e r e , t h a t , i n a l l e x p e r i m e n t s , t r a c e s o f 2-,

3-, and 4- c r e -

s o l s were a l s o d e t e c t e d . I n a d d i t i o n , under t h e e x p e r i m e n t a l c o n d i t i o n s 2 , t r a c e s o f benzyl benzoate ( 2 ) and o-benzyl benzoic a c i d ( 3 ) were d e t e c t e d . N

cy

0

2

-

3

5

For t h e h i g h e s t c o n c e n t r a t i o n of c a t a l y s t u s e d [ 2 . 5 g p e r l i t e r o f t o l u e n e ] , pure Ti02 was more a c t i v e than iron-doped Ti02 c a t a l y s t s , ( compare e x p e r i m e n t s 1 , 3 and 5 i n Table 1 ) ; however, t h e iron-doped T i 0 2 c a t a l y s t , c o n t a i n i n g 0 . 5 w t %

of i r o n , showed h i g h e r p h o t o a c t i v i t y than p u r e T i 0 2 and than iron-doped

Ti0 2 with a c o n t e n t of i r o n of 5 wt%, when a lower c o n c e n t r a t i o n o f c a t a l y s t was used [ 1 . 2 5 g p e r l i t e r o f t o l u e n e ] ( compare, chemical y i e l d s from e x p e r i m e n t s 2 , 4 and 6 i n Table 1 ) ; i n f a c t , a r e l a t i v e l y i m p o r t a n t chemical y i e l d i n benzaldehy de ( - 0 . 9 % ) was o b t a i n e d by u s i n g Fe/Ti02 ( 0 . 5 w t % o f i r o n ) c a t a l y s t a t t h e con centration of 1.25 g per l i t e r of toluene. S t r u c t u r a l d a t a f o r pure and iron-doped

t i t a n i a powders have been r e p o r t e d

elsewhere ( r e f s . 9 - 1 0 ) showing t h a t a t 5000C Ti02 i s mainly i n t h e form o f p u r e a n a t a s e phase i n which Fe3+ h a s a g r e a t e r s o l u b i l i t y t h a n i n r u t i l e . Data o b t a i ned by Navio e t a l .

( r e f . 1 0 ) have shown t h a t a t 500'C,

samples c o n t a i n i n g i r o n

c o n c e n t r a t i o n s ~ 1 produces % s o l i d s o l u t i o n s . However, w i t h l a r g e r c o n c e n t r a t i o n o f i r o n ( e . g . 5 w t % ) t h e s o l i d s o l u t i o n i s s a t u r a t e d . I r o n which c a n n o t be a c c o -

modated i n s o l i d s o l u t i o n , is s e g g r e g a t e d , t o form a s u r f a c e l a y e r o f i r o n o x i d e a n d / o r r e a c t s , by thermal t r e a t m e n t , w i t h Ti02 forming p s e u d o b r o o k i t e , F e T i 0

2 5' a s a s e p a r a t e d phase ( r e f . 1 0 ) . For t h e low c o n c e n t r a t i o n o f c a t a l y s t [ 1 . 2 5 g p e r

l i t e r of t o l u e n e

1,

t h e p r e s e n c e o f i r o n i o n s i n s o l i d s o l u t i o n seems t o i n c r e a s e

t h e benzaldehyde p r o d u c t i o n , b u t t h e c o n t e n t o f i r o n must b e l i m i t e d ; i n p a r t i -

P

00 Q

TABLE

Data f o r t h e p h o t o c a t a l y t i c o x i d a t i o n o f Toluene o n p u r e o r iron-doped

1

I r r a d i a t i o n time

Main Expriments

pho t o g e n r r a t c d

1

5

3

t i t a n i a .-atal:,sts.

(Hours)

7

9

12

llf

la, d

Benzaldrhyde Benzoic a c i d

3.9

0.1

9.5

0.3

11.1

0.3

15.4

0.4

8.4 1.5

0.2 0.04

9.1 3.7

0.3 0.1

8.4 2.0

0.2

2.1

0.06

2.9

0.08

3.6 1.7

0.1

2a,e

Benzaldehyde Benzyl a l c o h o l Benzoic a c i d

3.6 2.1 3.8

0.1 0.06 0.1

3.3 1.9 4.4

0.09 0.05 0.1

4.4 2.8 5.8

0.1 0.08 0.2

4.4 2.3

6.5

0.1 0.07 0.2

Benzaldehyde

1.0

0.03

3.9

0.1

4.4

0.1

6.8

0.2

5.6

0.2

6.6

0.2

4.9

0.1

0.04

3.3

0.09

2.0

0.06

1.9

0.05

2.2

0.06

11.2

0.3 0.04

14.2 1.9 1.5

0.4 23.2 0.05 5.6 0.04 5.0

0.7 0.2 0.1

30.9 9.7 7.5

0.9 0.3 0.2

32.2 0.9 13.5 0.4 8.4 0.2

-b. d

3 '

Benzyl a l c o h o l

4b e

Benzaldehyde Benzyl a l c o h o l Benzoic a c i d

5C 9 d

Benzaldehyde

6 c Ie

Benzaldehyde Benzyl a l c o h o l Benzoic a c i d

ch.y.

0.05

1.4 1.5

0.5

0.01

0.6

0.02

0.9

0.02

1.0

0.03

1.2

0.03

1.3

1.9

0.05

2.9

0.08

3.3

0.09 0.05

3.0

0.09 0.05 0.03

4.0

1.6

1.1

Chemical y i e l d .

a Ti0 ;

2

Fe/Ti02

(0.5 w . t .

% of i r o n ) ;

2.5g o f c a t a l y s t p e r l i t e r of T o l u e n e ; Data showed f o r e x p r r i m r t

1.7

Fe/TiO ( 5 w . t . %

of i r o n ) .

2 1.25 g of c a t a l y s t p e r l i t e r of T o l u e n e .

3 a r e c o r r e s p o n d i n g t o 10 h o u r s o f i r r a d i a t i o n .

0.04

0.1 2.9 0.08 2.4 0.07

0.06

23.2 0.7 9.7 0.3 6.7 0.2

1.3

0.04

3.7 0.1 2.9 0.08 1.5 0.04

449

cular, specimens which contains the pseudobrookite phase (Fe/TiO

2’

5 wt% of iron)

are less photoactive. The energy band-gap of Fe Ti05 is 2.18 eV, comparable 2

with that of Fe203 ( E = 2.2 eV) but smaller than that of Ti0 ( E = 3 . 0 eV); mo g 2 g reover and d u e to the poor mobility of the electrons photogenerated in Fc Ti0 2

5

phase, i t is very likely that the rate of recombination of the electron-hole pairs is greater than that of trapping by substrates. By applying laser techniques, Rothemberg et al. (ref.11) in colloidal semiconductors, and Navio et al. t

(ref.12) in powdered semiconductors, have shown that the charge carriers recombination time i s drastically retarded in iron-doped Ti02. These observations,can explain the differences in chemical yields observed. On the other hand, the catalyst concentration, seems to be one other important factor which can influence the chemical yields. This factor required more attention. Table 2 summarizes the average values of the quantum yields (q.y.) for the different experimental conditions. These quantum yields have been estimated by assuming that all the photons supplied by the lamp are absorbed by the

catalyst grains; these estimations are not strictly correct, because of light scattering and reflexion by the catalyst grains. In fact, according to results reported in Table 2, the variation of quantum yields seems to be related to the nature of the catalyst and/or with its concentration. TABLE 2 Data of the average values of quantum yields(*)for experiments reported on Table 1 Experiments

Quantum Yield

aTi02; bFe/Ti02 (0.5 wt.% of iron) ; ‘Fe/TiO ( 5 wt.% of iron); 2 d2.5 g of catalyst per liter of toluene e1.25 g of catalyst per liter of toluene (*)

Values obtained as an average of the quantum yields for each ones of experiments.

For pure Ti02, the q.y. is higher, for the more concentrated suspension; however for the case of both iron-doped Ti02, the higher q.y. is obtained for the lower concentration of catalyst. The differences can be explained in terms of differences in particle

size of the catalysts. Recent results by Navio et al.

450 (ref.10) have shown that

,

while pure Ti02 is constituted by small free indivi-

dual grains (1-5 p m , diam.), the Fe/TiO grains are formed by aggregation of seve 2 ral particles; for Fe/TiO (0.5 wt.% of iron) the average size of aggregates are 2 about 50 m diam.. whereas for Fe/TiO ( 5 wt.% of iron), the average size of

r

2

aggregates has been estimated to be about

-

180 m diam.

Y

The influence of the size particles of catalysts on the chemical yields of heterogeneous photocatalytic reactions, has received little attention, however our results suggest that both factors, the size of grains and the concentration of catalyst, must play an important role for light scattering and reflexion processes; these factors can influence the quantum yields and consequently the chemica1 yields of the photogenerated products. About

the selectivity, it is interesting to comment on that from the

three main products, benzaldehyde is the only one that can be obtained in the absence

of the other two

for short irradiation times ( see Table 1).

The heterogeneous photocatalytic oxidation of toluene, in non-aqueous sys-

terns and in the presence of oxygen, has been previously investigated by Fujihira et al. (ref.6) up to 2 h, under UV-irradiation. A mechanism has been proposed by these authors, assuming the possibility that the formed benzyl radicals can readily reacts either with oxygen leading to a peroxiradical which is reduced to benzaldehyde or either with 0; species to give directly benzaldehyde (ref.13). It is interesting to mention, that from results reported by Fujihira et al. (ref. 6), after UV-irradiation up 2 h, only benzaldehyde was detected, when using pu-

re Ti02. These observations are in accordance with our results, below 3 h of irradiation, for experiments 1 and 2 in which Ti02 were used as photocatalyst; in all of our experiments the benzyl alcohol only appeared after prolonged irradiation times, at least above 3 h. On the other hand, the heterogeneous photocatalytic oxidation of toluene, in aqueous suspensions of Ti02, has been studied with some details, by Fujihira et al. (refs.4-5). These authors have reported that, in aqueous media, up to 2 h of UV-irradiation, benzaldehyde was one of the main products; benzyl alcohol was de tected as traces and benzoic acid was not detected (refs.4-5). However, in o u r experimental conditions using neat-liquid toluene, not only benzaldehyde but both benzyl alcohol and benzoic acid were detected as main products. According to o u r results, the formation of benzyl alcohol, generated from the photooxidation of benzyl radicals (ref.4) could be associated to the presence of water. It is of primary importance to be considered that in aqueous systerns, due to solubility reasons, benzyl alcohol must be easier solved than benzaldehyde; then benzyl alcohol, in the aqueous phase could be photocatalytically destroyed by a drastic photooxidation to C02 and water. In fact, the photocatalytic oxidation of the aromatic ring to C02 under UV-irradiation in aqueous emulsions of Ti02, have been observed by Izumi et al. (refs. 14-15). It is worthy

451

of note that in all our experimental conditions (neat-liquid toluene) only traces of water can be generated during the photoreaction. These little amounts of water, could explain the increases in benzyl alcohol with irradiation times, because the photocatalytic degradation of benzyl alcohol is drastically retarded.

In fact, the photodegradation processes of organic molecules seems to be associa ted with the presence of hydroxy radicals OH' which can be generated from water (refs. 4-5, 14). The formation of benzoic acid, from benzaldehyde, could be explained by the autooxidation of benzaldehyde by direct reaction with molecular oxygen which is very well known (ref.16). Benzoic acid is detected after prolonged irradiation times and could also be photodegraded to C 0 2 into the aqueous phase (ref.15). The formation of cresols as traces, are also the consequence of the presence

of water generated during the photoreaction; in fact hydroxy radicals photogenerated from water can be attached to the aromatic ring of toluene leading to the formation of an intermediate radical which is further aromatized (refs.4 and 1 6 ) according to equation 1. The formation of cresols, from toluene, in non-aqueous media, have been also observed by other authors (ref.18) but after prolonged irradiation times.

Equation 1 Finally, the formation of products ( 2 ) and ( 3 ) could be explained by the N

hr

presence of benzyl radicals which can be attached to the benzoic acid molecules, either to the ring o r to the chain, according to equation 2. We does not exclude the formation of very amall amounts of other products which could be formed by benzyl radicals attacking other compounds. In fact, Fujihira et al. (refs.4-5) have found the compound

(2) as

a product during the

irradiation of toluene in aqueous systems and also in non-aqueoiis systems in the absence of air (ref.6).

In summary, although the heterogeneous photocatalytic oxidation of toluene has been previously investigated by several authors, our investigation, has attempted to further correlate experimental conditions ( such as structural aspects and concentration of photocatalysts, irradiation times, etc.) with chemi-

452

COOH I .

4

Equation 2 cal yields and selectivity of products. In particular, our results show that water which can be generated during the: c o u r s e of the photorpaction, play an i m portant role in the distribution of products. These basic studies could be% of importance in scaling-up such type of process

to a s i z e acceptable for indus-

trial development.

ACKNOWLEDGEMENTS This work was supported by "JUNTA DE ANDALUCIA" ( R e s . SEPT.88). One of us (M.G.G.) wishes to thank the Ministry of Education and Science of Spain for the: award of a scholarship.Authors are very gratefull to Mr. F.J.Marchena (University of Sevilla,Spain) for the preparation of iron-doped titania catalysts. Fina Ily, we are gratefull to Dr. Pierre Pichat (CNRS,Ecole Crntrale de Lyon,France) for helpful discussions and continuing collaboration on this and related photocatalytic transformations. REFERENCES M.A.Fox (Ed.); "Organic Phototransformations in Non-homogeneous Media",Am. Chem.Soc.Symp. Ser. 278, American Chemical Society, Washington,l985. M.A.Fox;"Photocatalytic Oxidation of Organic Substrates", in: "Photocatalysis and Environment: Trends and Applications", M.Schiavel10 (Ed.), NATO-ASI, S e ries C, Vol. 237, by Kluwer Academic Publishers, Dordrecht, The Netherlands; 1988, pp. 445-467. P.Pichat and M.A.Fox;"Photocatalysis on Semiconductors", in: "Photoinduced Electron Transfer", Part D, Elsevier S c i . Publishers, The Nethcrlands,l988, pp. 241-302. M.Fujihira, Y.Satoh and T.Osa, Nature,293 (1981) 206-208. M.Fujihira,Y.Satoh and T.Osa, Chem. Letter (1981) 1053-1056. M.Fujihira,Y.Satoh and T.Osa,J.Electroanal. Chem. 126 (1981) 277-281. M.N. Mozzanega,J.M. Herrmann and P.Pichat, Tetrahedron Letters,34 (1977) 2965 -2966. R.I.Bickley,T.Gonzalez-CarreRo and L.Palmisano, in: "Preparation of Catalysts IV", B.Delmon, P.Grange,P.A.Jacobs and G.Pocelet (Eds.), Elsevier,Amsterdam,

453 The Netherlands,l987, pp.297-306. 9 D.Cordishi, N.Burriesca,F.D'Alba,M.Petrera,G.Polizzotti and M.Schiavello, J.Solid State Chem.,56 (1985) 182-186. 10 J.A.Navio, M.Macias,F.J.Marchena and A.Justo (submitted for publication to J. of Catalysis). Preliminary results were presented at the 2nd. Internatio nal Symposium on Solid State Chemistry,Pardubice,Czechoslovakia, M.Frumar(Ed.) Institute of Chemical Technology,Pardubice,l989, pp. 155-156. 11 G.Rothemberg,J.Moser, M.GraStze1, N.Serpone and D.K.Sharma,J.Am. Chem. S O C . 107 (1985) 8054-8059. 12 J.A.Navio, F.J.Marchena,M.RonceI and M.A. De La Rosa, J.of Photochemistry and Photobiology:A,Chemistry (in press).Preliminary results were presented at the I International Symposium of Photochemistry in Synthesis and Catalysis, Ferra ra (Italy), 1989,pp. 87-89. 13 T.Kanno, T.Oguchi,H.Sakuragi and K.Tokumaru, Tetrahedron Letters,21 (1980) 467-470. 14 I.Izumi,W.W.Dunn,K.O.Wilbourn,F.F.Fan 15 16

17 18

and A.J.Bard, J.Phys.Chem., 84 (1980) 3207-3210. I.Izumi, F.F.Dunn and A.J.Bard, J.Phys. Chem.,85 (1981) 218-223. N.S.Isaacs, in: "Reactive Intermediates in Organic Chemistry", John Wiley and Sons (Eds.), London, 1974, pp.334-335. C.Walling and R.A.Johnson, J.Am. Chem. S O C . 97 (1975) 363-367. R.Ogushi,T.Kanno, H.Sakuragi and K.Tokumaru; Abstract of Annual Meeting ofthe Chemical Society of Japan,I (1981) p.256.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 1991 Elsevier Science Publishers B.V., Amsterdam

455

SYNTHESIS OF NITRILES BY REACTION OF p-XYLENE hlITH NO OVER Cr2!I3-Al2O3 S. ZINE'

CATALYSTS

and A. GHORBEL'

t n s t i t u t P r P p a r a t o i r e aux Etudes d ' I n g 6 n i e u r de Nabeul

-

Tunisie

' L a b o r a t o i r e de Chimie des F ' a t W a u x e t ' C a t a l y s e , Departement de Chimie, Facul ti. des Sciences de Tunis T u n i s i e

-

SUb!?"ARY

The s y n t h e s i s o f p a r a t o l u n i t r i l e (PTN) and t e r e p h t a l o n i t r i l e (TPN) b y react i o n o f paraxylene w i t h n i t r o g e n monoxide was s t u d i e d o v e r a s e r i e s o f a e r o g e l chromium o x i d e alumina c a t a l y s t s . The s t a b i l i z a t i o n o f t h e a c t i v e phase was i n t e r p r e t e d on t h e b a s i s o f Cr203 s u ? p o r t i n t e r a c t i o n s . K i n e t i c s t u d i e s show t h a t t h e r e a c t i o n f o l l o w s a "redox" mechanism f o r t h e f o r m a t i o n o f PTM and a Langmuir Hinshelwood mechanism f o r t h e p r o d u c t i o n o f TPN. IrJTRODUCTION E a r l i e r work i n t h i s l a b o r a t o r y showed t h a t chromium o x i d e supported on a l u mina i s a good c a t a l y s t f o r t h e c o n v e r s i o n o f o l e f i n s ( r e f . 1) as w e l l as p a r a f f i n s ( r e f . 2) t o n i t r i l e s w i t h h i g h s e l e c t i v i t i e s , by r e a c t i o n o f NO w i t h t h e hydrocarbons ( n i t r o x i d a t i o n ) . Recent work ( r e f . 3) r e p o r t e d ? r e l i a i n a r y r e s u l t s o f t h e n i t r o x i d a t i o n o f p a r a x y l e n e as an e x t e n s i o n o f t h e use o f Cr203-A1203 t o t h e c a t a l y t i c s y n t h e s i s o f a r o m a t i c n i t r i l e s . It s h o u l d be mentioned t h a t o n l y few d a t a a r e a v a i l a b l e i n t h e l i t e r a t u r e r e l a t e d t o t h e n i t r o x i d a t i o n o f aromatic hydrocarbons. T e i c h n e r e t a1 ( r e f . 4

) reported interesting results o f s e l e c t i -

ve s y n t h e s i s o f b e n z o n i t r i l e b y n i t r o x i d a t i o n o f t o l u e n e on Ni0-A1203 c a t a l y s t s . Improvements o f t h e c a t a l y t i c a c t i v i t y and s e l e c t i v i t y i n t h i s r e a c t i o n were reached by use o f Cr203-A1203 which a l s o e x h i b i t s s t r i k i n g p r o o e r t i e s i n t h e synt h e s i s o f p a r a t o l u n i t r i l e by c o n t a c t o f NO w i t h p a r a x y l e n e ( r e f . 3). I n c o n t r a s t w i t h chromia supported on alumina, pure chromium o x i d e i s a p o o r c a t a l y s t f o r t h e n i t r o x i d a t i o n o f hydrocarbons as i t d e a c t i v a t e d r a p i d l y w i t h t i m e on stream and favoured deep o x i d a t i o n a t t h e steady s t a t e ( r e f . 3), althourlh i t e x h i b i t s good dehydrogenation p r o p e r t i e s ( r e f . 2 ) . I t was concluded t h a t a l u -

mina p r e v e n t s t h e s e g r e g a t i o n o f chromia phase and t h u s f a v o u r s t h e f o r m a t i o n o f low c o o r d i n a t e d Cr3+ i o n s which a r e e a s i l y c o n v e r t e d t o C r 5 + i n presence o f oxygen o r n i t r o g e n monoxide ( r e f . 2 ) . The p r e s e n t paper d e a l s w i t h t h e e x t e n s i o n o f t h e p r e v i o u s and p r e l i m i n a r y s t u d y o f t h e n i t r o x i d a t i o n o f paraxylene on Cr203-A1203 ( r e f . 3), i n o r d e r t o

456

b e t t e r understand t h e r o l e o f t h e s u p p o r t i n t h e s t a b i l i z a t i o n o f t h e a c t i v e phase and i n v e s t i g a t e t h e r e a c t i o n mechanism. Hence, t h e e f f e c t o f chromium cont e n t on t h e t e x t u r a l , s t r u c t u r a l and c a t a l y t i c p r o p e r t i e s o f Cr203-A1203 a e r o g e l s i s s t u d i e d . The sample c o n t a i n i n g 10 % o f chromium i s t h e n s e l e c t e d t o p e r form t h e m e c h a n i s t i c s t u d y o f t h e c a t a l y t i c n i t r o x i d a t i o n o f p a r a x y l e n e . EXPERIMENTAL

A s e r i e s o f Chromia-Alumina a e r o g e l c a t a l y s t s c o n t a i n i n g d i f f e r e n t c o n t e n t s o f chromium was prepared by a u t o c l a v e method. The s p e c i f i c areas o f t h e c a t a l y s i s were measured w i t h N2 a t 77°K a c c o r d i n g t o t h e BET method. T h e i r s t r u c t u r a l p r o p e r t i e s were determined f r o m t h e X r a y d i f f r a c t i o n p a t t e r n s r e c o r d e d on a p h i l i p s d i f f r a c t o m e t e r PW 1050/70. EPR measurerents were performed w i t h a B r u k e r 200 TT spectrometer a t 77'K o p e r a t i n g i n X band. DPPH was used as t h e g v a l u e s t a n d a r d . K i n e t i c d a t a were o b t a i n e d i n dynamic p y r e x m i c r o r e a c t o r o p e r a t i n g a t atmospheric p r e s s u r e as d e s c r i b e d elsewhere ( r e f . 3 ) . RESULTS AND DISCUSSIONS E f f e c t o f t h e c o n t e n t o f chromium on t h e t e x t u r a l and s t r u c t u r a l p r o p e r t i e s o f C r 0 -A1 0 -2-3-2-3 Table 1 g i v e s c o m p o s i t i o n , s t r u c t u r a l and t e x t u r a l c h a r a c t e r i s t i c s o f t h e

c a t a l y s t s . The e v o l u t i o n o f t h e BET s u r f a c e areas w i t h t h e chromium c o n t e n t as w e l l as t h e r e s u l t s o f X r a y a n a l y s i s o f t h e samples p r e t r e a t e d i n a i r a t 410'C. TABLE 1 BET s u r f a c e areas and XRD c h a r a c t e r i z a t i o n o f Cr203-A1203 c a t a l y s t s atomic c o n t e n t Cr/Cr

+ A1 % 1 6 10 20 30 40 50

S(BET) 2 m /g

429 358 290 240 222

XRD

XRD

a t 410°C

a t 460°C

amorphous amorphous amorphous amorphous amorphous Cr203 Cr203

amorphous amorphous amorphous Cr203 p o o r l y c r y s t a l 1 i z e d Cr2O3 Cr203 Crp03

shows t h a t alumina e x e r t s an i m p o r t a n t e f f e c t on chromium o x i d e w h i c h d e p o s i t e s on t h e s u p p o r t i n a d i s p e r s e d and amorphous phase when a t o m i c chromium p e r c e n t a ge i n t h e c a t a l y s t s does n o t exceed 30 %. Samples c o n t a i n i n g more chromium e x h i b i t a b e g i n n i n g o f c r y s t a l l i z a t i o n o f Cr203. I f t h e c a t a l y s t s a r e h e a t e d i n a i r a t 46OOC i n s t e a d o f 410"C, t h i s c r y s t a l l i z a t i o n b e g i n s e a r l i e r s t a r t i n n f r o m 20%

457

o f c hromi unl. These r e s u l t s i n d i c a t e t h a t alumina a c t s on Cr203 phase t o p r e v e n t i t s c l u s t e r i n g and s e g r e g a t i o n w i t h h i g h c o o r d i n a t e CrSt

ions. This dispersive e f f e c t o f

t h e s u p p o r t p r o v i d e s a s u i t a b l e environment f o r t h e f o r m a t i o n on t h e s u r f a c e o f low c o o r d i n a t e chromium i o n s ( r e f . 2 ) . However t h i s e f f e c t o f alumina t e n d s t o depress when t h e c o n t e n t o f chromium exceeds 3C

I

a t 410'C.

T h i s r e s u l t seems t o

i n d i c a t e a s a t u r a t i o n o f the surface s i t e s o f t h e support which i n t e r a c t

with

t h e chromium ( s u r f a c e o f alumina covered w i t h a l a y e r o f chromium o x i d e ) . Then an excess o f Cr203 d e p o s i t e d l e a d s t o i t s c l u s t e r i n g and c r y s t a l l i z a t i o n . Conseq u e n t l y t h e c o o r d i n a t i o n o f chromium i o n s chancles f r o m t e t r a h e d r a l ( l o w c o o r d i nation) t o octahedral (high coordination).

Furthermore, t h e h e a t t r e a t m e n t o f c a t a l y s t s i n oxygen a t 410°C generates a 5t paramaonetic s p e c i e s w i t h a sharp EPR s i g n a l c o r r e s p o n d i n g t o t h e y l i n e o f C r ( r e f . P I . T h i s s i g n a l i s superimposed on a l a r g e band due t o Cr3' clumps ( r e f . 2 ) , which reduces t h e accuracy o f t h e base l i n e d e t e r m i n a t i o n f o r samoles c o n t a i n i n g more t h a n 30 % o f chromium (see F i g . 1). The r e l a t i v e e v o l u t i o n o f t h e number o f s p i n s Cr5'

versus t h e c o n t e n t o f Cr2D3 i n t h e samples (see F i g . 2) shows t h a t

F i g . 1. EPR S p e c t r a o f s a n p l p s Cr,,o -,r,i 9 F i g . 2. E v o l u t i o n o f t h e number o f s p i n s ~ r 5 +versus Cr20g c o n t e n t i n (content , ) p r e t r f d t c d I n oxpyzn 410°C catalysts.

458

the number o f i o n s Cr5' increases f i r s t w i t h t h e percentage o f chromium, reaches a maximum f o r 10 % and then decreases f o r c a t a l y s t s r i c h e r i n chromia. This maxi-

mum o f the number o f s p i n s seems t o c o r r e l a t e v i t h t h e s a t u r a t i o n o f t h e s u r f a c e s i t e s o f alumina which i n t e r a c t w i t h chromium. These s i t e s a r e l i k e l y Lewis a c i 3t d i c centers presumably aluminium i o n s (A1 ) generated by h e a t dehydration o f alumina a t 410°C. This assumption i s strengthened by the s t r i k i n g closeness o f the number o f s t r o n g a c i d i c s i t e s (Lewis c e n t e r s ) c a l c u l a t e d from t h e measure o f 20 the i r r e v e r s i b l e a d s o r p t i o n o f ammonia by alumina a t 200°C(L7610 s i b s p e r aram

(ref.5)) w i t h the number o f chromium i o n s C r 5 + e s t i m a t e d a t f o r t h e sample c o n t a i n i n g 10 % of chromia a c c o r d i n i t o

M.

1 .ZD1020

s i t e s p e r gram

Sbelef ( r e f , 6 ) .

E f f e c t o f t h e c o n t e n t o f chromium on t h e c a t a l y t i c p r o p e r t i e s o f Cr20,-A1?03 aerogels The increase o f t h e c o n t e n t o f chromium from 6 % t o 30 % enhances the c a t a l y t i c a c t i v i t y o f Cr203-A1203 ; whereas t h e sum o f s e l e c t i v i t i e s toward p a r a t o l u n i t r i l e (PTN) and t e r e p h t a l o n i t r i l e (TPN) remains almost c o n s t a n t (see Table 2 ) . TABLE 2 N i t r o x i d a t i o n o f Daraxylene ( p . x y l ) a t 450°C on mixed Cr203-A1203 aerogels Selectivities i n %

t = 450°C

PTN TPN Cr/Cr+Al 6

10 20 30 40 50

looa a

Activities i n

lo-'

mol/aram.s

FA Kj/mol

@-CN

C02

p-xyl

PTN

TPN

6-CN

C02

PTN p - x y l

2.8 2.9 2.9 3.0 3.8 3.1

2.8 2.5

76.3 91.5 134.6 169.2 57.9 47.8 7.4

65.7 77.8 115.8 149.6 49.8 39.2 2.1

7.0 9.1 13.4 13.1 4.6 2.2

2.2 3.0 3.9 5.2 2.2 1.5

1.4 1.6 1.5 1.3 1.3 4.9 5.3

94.6 94.6 94.6 93.8 64.0

% 85.3 84.8 85.5 88.0 85.0 78.8 28.5

9.1 9.P. 9.9 7.7 7.8 4.3

-

-

1.7 1.3 3.4 14.8 71.5

-

-

42.7

-

99.2 99.2 99.2 99.6 69.9 51.5

-

r e s u l t s obtained a t 440°C

T h e s e l e c t i v i t y o f c a t a l y s t s i n C02 i s always lower than 3 % i n a l l the temp e r a t u r e ranqe i n v e s t i g a t e d (410

-

430°C). !,n

excess o f chromia deoosited on t h e

support reduces the a c t i v i t y o f t h e c a t a l y s t s which become more o x i d a n t and l e s s s e l e c t i v e i n n i t r i l e s . Sample

c o n t a i n i n q 50 % o f chromium tends t o behave

as pure Cr203 which favours deep o x i d a t i o n and y i e l d s 71.5 % o f C02 a t 440°C (see Table 2). On the b a s i s o f t h e l i k e l y assumption t h a t the s i t e s o f f o r m a t i o n o f n i t r i l e s a r e d i f f e r e n t from those r e p o n s i b l e of degradation o f t h e a r o v a t i c hydrocarbon ( s i t e s o f deep o x i d a t i o n ) , the e v o l u t i o n o f a c t i v i t y and s e l e c t i v i t y s e e m t o i n d i c a t e a change i n the same p r o p o r t i o n o f t h e two types o f s i t e s when t h e

459 percentage o f chromium i n t h e samples v a r i e s f r o m 6 X t o 30 X . F o r these c a t a l y s t s , t h e r e a c t i o n mechanism i s p r o b a b l y t h e same as i t c o u l d be deduced a l s o f r o m t h e i n v a r i a n c e o f t h e apparent a c t i v a t i o n energy (EA) f o r t h e f o r m a t i o n o f p a r a t o l u n i t r i l e o r t h e disapperance o f p a r a x y l e n e (see T a b l e 2 ) .

A l l these r e s u l t s a r e e a s i l y e x p l a i n e d on t h e b a s i s o f s t r u c t u r a l p r o p e r t i e s o f samples p r o v i d e d by X r a y and EPR s t u d i e s . I t was a c t u a l l y p o i n t e d o u t t h a t f o r l o w c o n t e n t s o f chromium i n t h e c a t a l y s t s , alumina i n t e r a c t s w i t h t h e amorphous a c t i v e phase and f a v o u r s t h e s t a b i l i z a t i o n o f low c o o r d i n a t e chromium i o n s These i o n s a r e known as a c t i v e s i t e s i n t h e dehydrooenation o f h y d r o c a r -

Cr3+.

bons ( r e f . 2 ) and a r e c o n v e r t e d e a s i l y t o Cr5+i o n s which a r e needed t o r e a l i z e t h e n i t r o x i d a t i o n o f o l e f i n s and p a r a f f i n s . I t i s l i k e l y t h a t these chromium 5+ i o n s ( C r 3 + and C r ) a l s o p l a y an i m o o r t a n t r o l e i n t h e s e l e c t i v e p r o d u c t i o n o f a r o m a t i c n i t r i l e s b y n i t r o x i d a t i o n o f t h e p a r a x y l e n e . Hence, t h e improvementof c a t a l y t i c performance o f Cr203-A1 203 when t h e c o n t e n t o f chromium i n c r e a s e s f r o m 6 % t o n e a r l y 30 % i s c o r r e l a t e d t o a c o r r e s p o r d i n g i n c r e a s e i n t h e number o f a c t i v e s i t e s C r 3 + and C r 5 + on t h e s u r f a c e o f c a t a l y s t s . F o r samples r i c h i n chromium t h e s e g r e g a t i o n and c r y s t a l l i z a t i o n of C r p O g g r a d u a l l y reduce t h e number o f low c o o r d i n a t e C r 3 + which a r e t h e source o f Cr5+ions. S i m u l t a n e o u s l y t h e number o f h i g h c o o r d i n a t e C r 3 + i n Cr203 c l u s t e r s which a r e r e s p o n s i b l e f o r deep o x i d a t i o n i n c r e a s e s . T h e r e f o r e , c a t a l y t i c a c t i v i t y as w e l l as s e l e c t i v i t y toward a r o m a t i c n i t r i l e s decrease. M E C H A N I S T I C STUDY

The n i t r o x i d a t i o n o f p a r a x y l e n e on Cr203-A1203 c a t a l y s t s g i v e s m a i n l y parat o l u n i t r i l e (PTN) i n accordance w i t h t h e c o n s e c u t i v e e q u a t i o n : p-xylene

NO

p a r a t o l u n i t r i 1e

NO

t e r e p h t a l o n i tri l e

k i n e t i c s t u d y o f t h i s r e a c t i o n was undertaken a t 4 4 O O C on Cr203-A1203 a e r o g e l c o n t a i n i n g 10 % o f chromia i n a l a r g e range o f p a r t i a l p r e s s u r e s o f r e a c t a n t s . The r e s u l t s o b t a i n e d c o r r e s p o n d i n g t o t h e consumption of p a r a x y l e n e and t h e f o r m a t i o n o f PTN (see F i g s . 3 and 4) a r e i n t e r p r e t e d by a r e d o x mechanism w h i c h i m p l i e s a s t a t i o n a r y s t a t e w h i t h e q u a l i t y between t h e r a t e s o f t h e two s t e p s o f r e d u c t i p n o f t h e c a t a l y s t by p a r a x y l e n e and i t s o x i d a t i o n b y n i t r o g e n monoxide (ref.

3 ) . Table 3 g i v e s t h e values of t h e r a t e c o n s t a n t s o f t h e two s t e p s and

shows t h a t KO and Kr c a l c u l a t e d f o r t h e consumption o f p a r a x y l e n e o r t h e format i o n o f p a r a t o l u n i t r i l e , t h e main p r o d u c t , a r e v e r y s i m i l a r i n agreement w i t h t h e redox mechanism.

460

t

OPp-xyl=10 t o r r =20 t o r r

ul

=25 t o r r =50 t o r r

3

0

E

=150torr

.r 0

1-

-1

l/PNoin t c l r r - ' Fig. 3.

Variation o f 1 / A C versus l / P p o @ P N O = 12.5 t o r r )lePNo=

7

.PNO= 150 torr

*

I

0

Fig. 4.

25 t o r r

l / p p - x y ~ i n torr - 1

9 :05 Variation o f 1 / A C versus 1/P

P-XYl

AC = a c t i v i t y toward t h e consumption o f paraxylene PNo= p a r t i a l pressure o f nitrogen monoxide P = p a r t i a l pressure o f paraxylene

P-XYl

TABLE 3

Kinetic values o f Kr and KO in mol g - l s-'

torr-I

r a t e constants

consum?tion o f p-xylene

Kr

1.06 10-7 0.75

KO

Formation of oaratol u n i t r i l e 0.80 10-7 0.67 IO-'

I n order t o confirm t h i s mechanistr, EPR study o f t h e adsorption o f r e a c t a n t s i s performed on the c a t a l y s t i n the same conditions o f the r e a c t i o n . The r e s u l t s obtained (See Fig. 5 ) show t h a t the adsorotion o f Faraxylene a t 440°C on the c a t a l y s t pretreated i n oxyaen a t 410°C removes the EPR y l i n e o f Cr5+ ions and

461

generates a new s h a r p s i g n a l with a g f a c t o r o f 2.002 p r o b a b l y due t o a s u p e r f i c i a l complex formed between C r 5 + and chemisorbed p a r a x y l e n e (Crnt with

n

<

-

paraxylene

5 ) . The i n t e n s i t y o f t h i s s i o n a l i s n o t changed by a f o l l o w i n g desorp-

t i o n i n h e l i u m a t 440°C, i n d i c a t i n g t h a t t h e complex i s s t a b l e . The i n t e r a c t i o n of t h i s complex w i t h NO a t 440°C removes from EPR spectrum t h e new sharp s i g n a l and p r a c t i c a l l y r e s t o r e s t h e i n i t i a l

y

l i n e due t o Cr5+i o n s . Hence, t h e s h i f t o f Cr5'

t o Crnt

(n

<

5 ) i n r e d u c t i o n con-

d i t i o n s and i t s r e v e r s e i n o x i d i z i n g cond i t i o n s show t h e i n t e r c o n v e r s i o n o f c h r o mium i o n s between two o x i d a t i o n s t a t e s i n good agreement w i t h redox r e a c t i o n t a k i n p p l a c e on t h e s u r f a c e o f Cr203-A1203 d u r i n g the c a t a l y t i c synthesis o f F a r a t o l u n i t r i l e . Nevertheless, whereas t h e i n c r e a s e o f paraxylene p a r t i a l p r e s s u r e f a v o u r s t h e f o r m a t i o n o f PTN, i t c o n v e r s e l y decreases t h e c a t a l y t i c a c t i v i t y and s e l e c t i v i t y o f TPN (See F i g s . 6 and 7 ) . T h i s r e s u l t c o u l d be e x p l a i n e d i f t h e mechanism

o f t h e second

0.34

0.36

Tesla

i s performed a t 440°C on t h e same c a t a l y s t

F i g . 5 . EPS s p e c t r a o f Cr203-A1203 (10%) catalvst a- ti-eatmeni o f c a t a l y s t v i t h pxylene a t 440°C b - treatment of catalyst I.,ith NO a t 440°C.

F i g . 6. Rate o f t h e f o r m a t i o n o f TPN as a f u n c t i o n o f Pp-xy, f o r v a r i o u s f i x e d PNo

F i g . 7. E v o l u t i o n o f s e l e c t i v i t i e s toward PTN and TPN a t 440°C f o r v a r i o u s f i x e d PNo.

step i n the consecutive r e a c t i o n i . e . react i o n o f NO w i t h p a r a t o l u n i t r i l e , i s known, Thus, t h e n i t r o x i d a t i o n o f F a r a t o l u n i t r i l e

462

i n a p r e s s u r e range o f PTN near f r o m t h a t o b t a i n e d d u r i n g i t s p r o d u c t i o n by n i t r o x i d a t i o n o f paraxylene (0.5

-

3 t o r r ) . The r e s u l t s s h o w t h a t p a r a t o l u n i t r i l e

degrades a t t h e b e g i n n i n g o f t h e r e a c t i o n on t h e f r e s h c a t a l y s t and t h e b a l a n c e sheet o f carbon i s o n l y s a t i s f i e d a t t h e s t e a d y s t a t e , i n d i c a t i n g t h a t p a r a t o l u n i t r i l e adsorbs s t r o n g l y on t h e c a t a l y s t . E x p e r i m e n t a l d a t a show, t h a t when t h e p a r t i a l p r e s s u r e o f NO i n c r e a s e s , t h e r a t e o f appearance o f TPN does, t o o ; whearas t h e v a r i a t i o n o f p a r t i a l p r e s s u r e o f PTN has no e f f e c t on t h i s r a t e . Hence, t h e o r d e r w i t h r e s p e c t t o PTN i s z e r o . Experimental o r d e r with r e s p e c t t o NO i s f o u n d t o be 0.48. T h e r e f o r e , t h e k i n e t i c e q u a t i o n o f t h e r a t e r e a c t i o n i s AC = K Pi048. T h i s k i n e t i c l a w i s i n t e r p r e t e d by a Lanomuir-Hinshelwood mecha-

nism which i n v o l v e s a s t r o n g a d s o r p t i o n o f PTN on s i t e s S1 d i f f e r e n t f r o m t h e s i t e s S 2 o f r e t e n t i o n o f NO. T h i s mechanism accounts a l s o f o r t h e k i n e t i c r e s u l k o f t h e f o r m a t i o n o f TPN by c a t a l y t i c n i t r o x i d a t i o n o f p a r a x y l e n e assumina i n t h i s case t h a t p a r a x y l e n e as w e l l as PTN a r e s t r o n g l y and c o n c u r r e n t l y adsorbed on S1 s i t e s ; whearas NO i s always r e t a i n e d on S 2 s i t e s . Thus s u r f a c e r e a c t i o n concerning the formation o f t e r e p h t a l o n i t r i l e

o c c u r s between adsorbed PTN and

NO.

CONCLUSION Alumina supported chromium o x i d e c a t a l y s t i s h i g h l y s d e c t i v e i n p a r a x y l e n e n i t r o x i d a t i o n . The s u p p o r t i n t e r a c t s w i t h chromia phase and s t a b i l i z e s p o o r l y c o o r d i n a t e d Cr3+ i o n s w i c h c o n v e r t s e a s i l y t o h i g h o x i d i z e d C r 5 + i o n s r e q u i r e d f o r t h e r e a c t i o n t o t a k e p l a c e . K i n e t i c d a t a a r e i n t e r p r e t e d i n terms o f a r e d o x mechanism f o r t h e s y n t h e s i s o f PTN and a Langmuir-Hinshelwood mechavisn f o r t h e f o r m a t i o n o f TPN. REFERENCES

A. S a y a r i , A . Ghorbel, G. M. Pajonk and S. J . T e i c h n e r , R i t r o x y d a t i o n C a t a l y t i q u e p a r NO du Propene e t 1 ' I s o b u t e n e s u r l e s C a t a l y s e u r s Aeroaels P i x t e s a base d ' 0 x y d e de Chrome e t d ' A l u m i n i u m ; B u l l . SOC. Chim. France 1981 pp 220 224. H. Zarrouk, A. Ghorbel, G. M. Pajonk and S. J. T e i c h n e r , EPR I n v e s t i g a t i o n o f Chromia Alumina Aerogel C a t a l y s t s f o r t h e T r a n s f o r m a t i o n o f I s o b u t a n e by NO i n t o M e t h a c r y l o n i t r i l e , Proc. 9 t h I b e r o A m e r i c a i n Symposium on C a t a l y s i s , Lisbon, J u l y 16-21, 1984, pp 339 - 348. S . Zine, A . S a y a r i and A . Ghorbel, C a t a l y t i c Y i t r o x y d a t i o n o f Paraxylene o v e r chromium o x i d e Based c a t a l y s t s ; t h e camdian J o u r n a l o f chemical Engeneering, v o l 65 Februarv. 1957 DD 127 - 131. S. Abou Arnadasie, G. M'.' Pajonk, J . E. GermaiF and S. J. Teichner, C a t a l y t i c n i t r o x i d a t i o n o f Toluene i n t o B e n z o n i t r i l e o v e r N i c k e l o x i d e Alumina Xero o r Aero-gels c a t a l y s t s , A p p l i e d C a t a l y s i s , V o l . 9, 1984 pp 119-128. A. S a Y a r i - A . Ghorbel, I;. M . Pajonk and S. J . ' r e i c h n e r , b u l l . S O C . c h i m , , France 1980, pp 7-15. M. S h e l e f , E l e c t r o n Paramaqnetic Resonnance o f hlIr) Adsorbed on QedL'ced ChromiaPlLimina. J o u r n a l o f C a t a l y s i s , r o l . 15, 1969 pp 1P9 792

.

-

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1

463

0 1991 Elsevier Science Publishers B.V.,Amsterdam

SELECTIVE ELECTROCATALYTIC OXIDATION OF GLYOXAL I N AQUEWS MEDIUM E.M. BELGSIR, H. HUSER, C . LAMY and J.-M. LEGER L a b o r a t o i r e de Chimie I , E l e c t r o c h i m i e e t I n t e r a c t i o n s , URA CNRS 350, U n i v e r s i t e de P o i t i e r s , 40 Av. du Recteur Pineau, 86022 P o i t i e r s Cedex (France) SUMMARY I n a c i d i c medium, t h e e l e c t r o c a t a l y t i c o x i d a t i o n o f g l y o x a l on p l a t i n u m i n t h e p o t e n t i a l range 1 t o 1.5 V/RHE l e a d s m a i n l y t o f o r m i c a c i d (60%) and CO? (40%). With l e a d adatoms, i t becomes p o s s i b l e t o o x i d i z e g l y o x a l between 0.4 and 1.0 V/RHE l e a d i n g m a i n l y t o C02 f o r m a t i o n (46%), w h i l e t h e s e l e c t i v i t y towards g l y o x y l i c a c i d i s s e n s i b l y increased (28%). A t pH=7 and 1,9 V/RHE, t h e main o x i d a t i o n pro duc t i s f o r m i c a c i d (99%). Otherwise, i n a c i d i c medium t h e o x i d a t i o n i s more s e l e c t i v e towards g l y o x y l i c a c i d (70%), when t h e appl iecl p o t e n t i a l i s i n t h e range o f 1.80 t o 2.13 V/RHE. INTRODUCTION D urin g

the

last

decade

great

attention

has

been

paid

to

the

e l e c t r o s y n t h e s i s o f g l y o x y l i c a c i d f r o m g l y o x a l [1-61. D i f f e r e n t e l e c t r o c h e m i c a l systems have been s t u d i e d m a i n l y under g a l v a n o s t a t i c c o n d i t i o n s i n o r d e r t o i n v e s t i g a t e t h e r o l e o f t h e e l e c t r o l y t e and t h e e l e c t r o d e m a t e r i a l on t h e s e l e c t i v i t y . The c o n c l u s i o n s p a r t i c u l a r y those concerning

drawn f r o m t hese works were n o t c o n v i n c i n g , t h e adatom e f f e c t s [ 5 9 6 1 , which showed that.

a d d i t i o n o f small amounts o f adatoms (Ag, Sn, B i , T1 o r Au) c o u l d increase t h e y i e l d of g l y o x y l i c a c i d f o r m a t i o n on p l a t i n u m a t a c o n t r o l l e d c u r r e n t d e n s i t y o f 1 A dm-2 ( i . e . a t e l e c t r o d e p o t e n t i a l s E=1.5-2.0 V/RHE). These r e s u l t s a r e co mp let e ly

at

variance w i t h

numerous

investigations,

which

concluded

that

adatoms are desorbed above 1 V/RHE f 7 - 9 1 . On t h e o t h e r hand, a c c o r d i n g t o t h e s t u d y o f Horanyi e t a 1 . [ l o ] on t h e o x i d a t i o n o f g l y o x a l and g l y o x y l i c a c i d a t p l a t i n u m e l e c t r o d e s , t h e C - C bond b r e a k i n g may p l a y a s i g n i f i c a n t r o l e d u r i n g t h e a d s o r p t i o n process. The aim o f t h i s paper i s t o show how t h e s e l e c t i v i t y

of

the

e l e c t r o c a t a l y t i c o x i d a t i o n o f g l y o x a l can be m o d i f i e d b y v a r y i n g t h e e l e c t r o d e s u r f a c e c h a r a c t e r i s t i c s , t h e pH o f t h e s o l u t i o n and t h e e l e c t r o l y s i s p o t e n t i a l . EXPERIMENTAL a f i x e d p o t e n t i a l b e f o r e oxygen e v o l u t i o n , t h e a c t i v i t y e l e c t r o d e f o r t h e o x i d a t i o n o f small o r g a n i c molecules decreases At

of as

the the

consequence o f po i s o n f o r m a t i o n . These p o i s o n i n g species can be o x i d i s e d o n l y at. h i g h p o t e n t i a l s , which a l l o w s t o r e g e n e r at e t h e e l e c t r o d e s u r f a c e d u r i n g operation.

Therefore,

t h e e l e c t r o l y s e s were c a r r i e d o u t u s i n g a c o n t r o l l e d -

464

[I11 i n o r d e r t o m a i n t a i n t h e e l e c t r o d e a c t i v i t y a t t h e

programmed p o t e n t i a l

h i g h e s t p o s s i b l e l e v e l . E l e c t r o l y s e s were performed i n a two compartments g l a s s ( w o r k i n g and r e f e r e n c e e l e c t r o d e compartment and c o u n t e r e l e c t r o d e

cell

compartment)

separated

by

an

ionic

exchange

membrane.

The

supporting

e l e c t r o l y t e s were prepared f r o m Merk " s u p r a p u r " p r o d u c t s and f r o m u l t r a p u r e water ( M i l l i p o r e system). The g l y o x a l was a 30% s o l u t i o n i n w a t e r ( F l u k a ) . A l l t h e s o l u t i o n s were desoxygenated b y b u b b l i n g u l t r a p u r e n i t r o g e n working

electrode

(specpure q u a l i t y )

was

a

smooth

platinum

foil

provided

by

(U Qual i t y ) . The Johnson-Mattey

and t h e c o u n t e r e l e c t r o d e a v i t r e o u s carbon p l a t e .

The

r e f e r e n c e was a mercury-mercurous s u l f a t e e l e c t r o d e (MSE), b u t a 1 p o t e n t i a l s a r e quoted i n t h e r e v e r s i b l e hydrogen e l e c t r o d e (RHE) s c a l e . Qual i t a t i v e and q u a n t i t a t i v e analyses o f t h e r e a c t i o n medium were made by chromatographic t e c h n i q u e s (HPLC and GC) [71. RESULTS AND DISCUSSION

( 1 ) E l e c t r o c a t a l v t i c o x i d a t i o n o f alvoxal on platinum at 1.2 V/R H € i n perchloric: acid s o l u t i o n (0.1 M ) at room temperature. The p o t e n t i a l programme used f o r t h i s e l e c t r o l y s i s c o n s i s t s i n a p o t e n t i a l p l a t e a u a t 1.18 V/RHE d u r i n g 30 s f o l l o w e d b y a r a p i d t r i a n g u l a r sweep, between 0 and 1 . 8 V/RHE a t 500 mV s - l (see i n s e r t i n F i g . 1 ) .

vs.

p o t e n t i a l curves

i(E),

Recording o f t h e i n t e n s i t y

t h e s o - c a l l e d voltammograms,

a l l o w s us t o f o l l o w

q u a l i t a t i v e l y t h e e v o l u t i o n o f t h e s o l u t i o n composition w i t h time (Fig.1).

0

0.4

0.8

1.2

EIVRHE

F i g . 1. E v o l u t i o n o f t h e voltammograms o f a p l a t i n u m e l e c t r o d e r e c o r d e d d u r i n g t h e p r o l o n g e d e l e c t r o l y s i s o f g l y o x a l a t a p o t e n t i a l p l a t e a u Ep=1.18 V/RHE. The main o x i d a t i o n peaks a r e l a b e l l e d A , B and C. The p o t e n t i a l programme i s shown i n t h e i n s e r t . Time o f e l e c t r o l y s i s : (-) 0 h r , (....-.-..) 25 h r s , (----) 50 h r s . The e v o l u t i o n o f t h e p r o d u c t s c o n c e n t r a t i o n , w i t h t h e e l e c t r o l y s i s t i m e , i s shown i n F i g u r e 2.

as analysed b y HPLC and GC,

465

F i g . 2. Prolonged e l e c t r o l y s i s o f g l y o x a l a t p l a t i n u m (0.1 M HC104, 0.1 PI g l y o x a l , room t e m p e r a t u r e ) ; ( 0 ) g l y o x a l (G); (+) g l y o x y l i c a c i d (GA); (A) f o r m i c a c i d (FA); (0) carbon d i o x i d e (CO2). G ly o x al ( G ) i s c o m p l e t e l y e l e c t r o l y s e d a f t e r 30 hours. The d e t e c t e c produc t s are g l y o x y l i c a c i d (GA), f o r m i c a c i d (FA), carbon d i o x i d e (COz), and t r a c e s o f o x a l i c a c i d (OA). I n p r e v i o u s work, a k i n e t i c s t u d y has shown t h a t t h e t r a n s f o r m a t i o n can be d e s c r i b e d b y a successive r e a c t i o n s scheme where GA and FA. a r e p r i m a r y pro d u c t s respectively

to

OA

171.

Then t h e s e s p e c i e s undergo f u r t h e r o x i d a t i o n l e a d i n g

and

CO2.

After

electrolysis

during

7

hours

(time

corresponding t o t h e maximum p r o d u c t i o n o f GA), t h e conversion o f G i s about 3% and t h e pro duc t y i e l d s a r e 6% f o r GA, 80% f o r FA and 13% f o r CO2. GA i s formed w i t h a low s e l e c t i v i t y as t h e r e s u l t o f t h e a d s o r p t i o n process o f g l y o x a l on plat in um, where C - C bond b r e a k i n g occurs [10-121. T h i s i s conf irmed by t h e h i g h y i e l d o f FA which i s t h e main o x i d a t i o n product . Moreover, t h e f u n c t i o n a l symmetry o f g l y o x a l suggests a p a r a l l e l approach o f t h e two hydrat ed carbonyl groups on t h e e l e c t r o c a t a l y t i c s u r f a c e . a t t h e e l e c t r o d e p o t e n t i a l used (1.18 V/RHE),

On t h e o t h e r hand,

the

p l a t i n u m s u r f a c e i s covered b y h y d r o x y l and oxyhydroxyl species [131. T h i s means t h a t t h e i n t e r a c t i o n between g l y o x a l and t h e c a t a l y s t s u r f a c e i s an " o r g a n i c species-metal hydroxyde" one. One o f t h e p o s s i b l e s t r u c t u r e s o f t h e adsorbed species c o u l d be t h e f o l l o w i n g :

2 Pt-OH

+ ,.,H,,Ho\ HC f

OH C,**-O H

-

H\

H0,C-Cd 0

/

OH H

0

H'

The i n s t a b i l i t y o f such a c y c l i c adsorbed i n t e r m e d i a t e b r e a k i n g o f t h e C - C bond :

l e a d s t o t he

466

H, HOrF/

,PtQ

OH

q'd

H

P

Pt

-

2

H\

q/p +

2Pt

OH

I n t h i s mechanism, t h e e l e c t r o n t r a n s f e r occurs d u r i n g t h e r e g e n e r a t i o n step o f the e l e c t r o c a t a l y t i c surface :

Pt

+

H20

-

and t h e o x i d a t i o n o f one n e c e s s i t a t e s two e l e c t r o n s .

R-OH

molecule o f

+

H+ + le-

glyoxal

i n t o two

molecules o f

FA

The f o r m a t i o n o f GA presumably i n v o l v e s a p e r p e n d i c u l a r approach o f t h e glyoxal t o t h e electrode surface.

( 2 ) Electrocatalytic oxidation o f glyoxal on a platinum electrode modified by

lead adatoms. I t has been shown t h a t t h e m o d i f i c a t i o n o f t h e p l a t i n u m e l e c t r o d e by

u n d e r p o t e n t i a l d e p o s i t i o n (upd) o f l e a d adatoms improves g r e a t l y thc! e l e c t r o c a t a l y t i c a c t i v i t y f o r g l y o x a l o x i d a t i o n a t l o w p o t e n t i a l s (between 0.4 M Pb(C104)~. and 0 . 8 V/RHE). The l e a d s a l t i s added t o t h e e l e c t r o l y t e as 5 The experiment i s c a r r i e d o u t u s i n g a p o t e n t i a l programme w i t h t h r e e p l a t e a u x . The upd o f t h e adatom i s made a t 0.0 V/RHE d u r i n g 1 s and t h e e l e c t r o l y s i s i s performed a t 0.5 V/RHE d u r i n g 10 s ( b e f o r e t h e d e s o r p t i o n o f l e a d which occurs around 0.8 V/RHE). Then a s h o r t p o i s o n i n g s pec ies t o be o x i d i s e d .

potential

pulse

at

1.4 V/RHE

allows

the

Data on t h e e l e c t r o c a t a l y t i c o x i d a t i o n o f g l y o x a l under t hese c o n d i t i o n s a r e shown i n F i g u r e 3. I n t h i s case, t h e r e a c t i o n g i v e s m a i n l y CO2 (46%), which corresponds t o t h e t o t a l o x i d a t i o n p r o d u c t , and a l s o FA (18%). However, t h e amount o f GA (28%) i s s e n s i b l y i n c r e a s e d compared t o t h e preceding e l e c t r o l y s i s .

?-O ‘H

203

Vd

vo

V9

L9P

468

( 3 ) E l e c t r o c a t a l y t i c o x i d a t i o n o f g l y o x a l on p l a t i n u m i n a b u f f e r phosphate medium (pH=7). The e l e c t r o l y s i s was c a r r i e d o u t w i t h a p o t e n t i a l programme i n c l u d i n g o n l y two p l a t e a u x , because o f weak p o i s o n i n g o f t h e e l e c t r o d e s u r f a c e , w i t h a f i r s t . one a t 0.2 V/RHE, d u r i n g 0.2 s f o r t h e a d s o r p t i o n o f g l y o x a l , and a second one a t 1.9 V/RHE c o r r e s p o n d i n g t o t h e e l e c t r o l y s i s p o t e n t i a l ( 2 0 s ) .

OA

GA

c02

FA

F i g . 4. D i s t r i b u t i o n o f t h e r e a c t i o n p r o d u c t s o f g l y o x a l e l e c t r o l y s i s on p l a t i n u m a t 1.9 V/RHE i n a phosphate b u f f e r medium (pH=7), a f t e r 7 h o u r s of' operat ion. R e s u l t s a r e shown i n f i g u r e 4. hours o f e l e c t r o l y s i s .

The c o n v e r s i o n reaches 66% a f t e r sever1

The main p r o d u c t i s FA (9YL) and no s e l e c t i v i t y

i5,

observed towards t h e f o r m a t i o n o f g l y o x y l i c a c i d . Otherwise, c o n v e r s e l y t o t h e p r e v i o u s e l e c t r o l y s i s i n p e r c h l o r i c a c i d medium ( p H = l ) , no t r a c e o f C02, n e i t h e r i n t h e gas phase n o r i n s o l u t i o n as CO3=, i s observed. T h e r e f o r e , t h e o x i d a t i o r i o f g l y o x a l s t o p s a t t h e FA s t a g e . T h i s was c o n f i r m e d by a v o l t a m m e t r i c s t u d y , which showed t h a t FA i s n o t e l e c t r o r e a c t i v e above 1.0 V/RHE.

The FA produced

d u r i n g t h e e l e c t r o l y s i s a t 1.9 V/RHE does n o t undergo f u r t h e r o x i d a t i o n .

A

small

amount

of

glycolic

acid

is

produced

as

the

result

of

the

Cannizzaro t r a n s f o r m a t i o n (non e l e c t r o c h e m i c a l r e a c t i o n ) . ( 4 ) E l e c t r o c a t a l y t i c o x i d a t i o n of g l y o x a l on p l a t i n u m i n p e r c h l o r i c a c i d medium

a t f i x e d p o t e n t i a l i n t h e oxygen e v o l u t i o n r e g i o n . I t was observed t h a t , when t h e a p p l i e d p o t e n t i a l was chosen i n t h e oxygen

e v o l u t i o n r e g i o n , t h e c u r r e n t was s t a b l e a t around 10 mA cm-2.

469

Two e l e c t r o l y s e s were t h u s c a r r i e d o u t a t d i f f e r e n t p o t e n t i a l s i n o r d e r t o c o n f i r m t h e e l e c t r o r e a c t i v i t y o f g l y o x a l i n t h i s p o t e n t i a l range and t o st udy t h e p o t e n t i a l e f f e c t on t h e s e l e c t i v i t y .

60

5

40

20

0

FA

0.4

GA

c02

F ig . 5. D i s t r i b u t i o n o f t h e r e a c t i o n p r o d uct s o f g l y o x a l e l e c t r o l y s i s or1 p l a t i n u m i n t h e oxygen e v o l u t i o n r e g i o n i n a c i d i c medium ( p H = l ) , a f t e r 7 hours o f operation. E=2.03 V/RHE, E=2.13 V/RHE.

e)

m)

R es ult s o f t h e s e two e l e c t r o l y s e s a r e summarized i n F i g u r e 5.

In all

cases, an inc re as e o f t h e s e l e c t i v i t y i s observed f o r t h e p r o d u c t i o n o f GA. The y i e l d o f GA, r e f e r r e d t o g l y o x a l conversion, reaches n e a r l y 70%. Smaller amounts o f FA and C02 (<1%), t h e C - C bond b r e a k i n g product s, a r e d e t e c t e d d u r i n g t hese e l e c t r o l y s e s and t h e amount o f OA remains weak (= 5%). On t h e o t h e r hand,

f o r t h e h i g h e s t e l e c t r o l y s i s p o t e n t i a l (2.13 V/RHE),

g l y o x a l c onv ers io n i s more i m p o r t a n t , and t h e amount o f o x a l i c a c i d increases, whereas

the

concentration

of

FA decreases.

T his

confirms

the

selectivity

dependence o f t h e C - C bond b r e a k i n g w i t h t h e e l e c t r o d e p o t e n t i a l , and shows t h a t t h e o pt imal p o t e n t i a l f o r a maximal s e l e c t i v i t y towards GA p r o d u c t i o n i s c l o s e t o 2 . 1 3 V/RHE. These r e s u l t s are, i n some sense, c o n s i s t e n t w i t h t hose o f P i e r r e et. a l . [31. These a u t h o r s showed t h a t g l y o x a l e l e c t r o l y s i s under g a l v a n o s t a t i c c o n d i t i o n s ( 1 A dm-2) on p l a t i n u m i n p e r c h l o r i c a c i d medium leads t o 39% of GA, t h e p o t e n t i a l a p p l i e d between t h e two e l e c t r o d e s b e i n g i n t h e range of 2 6 V/RHE.

However, remarks

must

simu lt a neous ly

t h e i n t e r p r e t a t i o n of be

made.

and

this

Glyoxal competition

t h e s e r e s u l t s i s n o t obvious, oxidation may p l a y

and a

oxygen

b u t somcb

evolution

significant

role

occur on

the

s e l e c t i v i t y . I n t h e s t u d i e d p o t e n t i a l range and a t t h e b e g i n n i n g o f oxygeri e v o l u t i o n , t h e c o m p e t i t i o n appears t o be, a t f i r s t , f a v o u r a b l e f o r G k pro duc t io n.

b u t t h i s seems t o b e maximum f o r an o p t i m a l p o t e n t i a l .

I n other.

470

i f t h e p o t e n t i a l i s t o o p o s i t i v e (E > 2.5 V/RHE), t h e e l e c t r o c a t a l y t i c i s s a t u r a t e d by r e a c t i v e oxygenated species and t h e o x i d a t i v c ! degradation o f g l y o x a l i s more important. The r e a c t i o n mechanism can be summarised as f o l l o w s : terms,

surface

~t

of01 +

(OH),HC-CH(OH)~

--. pto

+

0 (OH)~HC-C~

+ H20

' O H

I n conclusion, t h i s fundamental study showed t h a t i t i s p o s s i b l e t o o b t a i n s e l e c t i v e l y chemical products by e l e c t r o c a t a l y t i c t r a n s f o r m a t i o n i n aqueou:, medium. I t was a l s o p o s s i b l e t o b e t t e r understand t h e r e a c t i o n mechanisms, but. o n l y under c o n t r o l l e d experimental c o n d i t i o n s ( e l e c t r o d e s t r u c t u r e , e l e c t r o d e p o t e n t i a l , s o l u t i o n pH,...).

REFERENCES ( 1 ) C h l o r i n e Engineers Corp. L t d , p a t e n t s : Ger. Offen. 2,940,379 ( c l .C25B3/02), 12-06-80, Japan, Appl. 78/150,570 - 7-12-78. ( 2 ) Societe Francaise Hoechst, p a t e n t s : F r . 2 569762, 84 13607, I n t . C14 : C 25 B 3/02 ; C 07 C 59/153,4. Sept 84. ( 3 ) G. P i e r r e , M. E l Kordy, G. Cauquis, G. M a t t i o d a and Y . C h r i s t i d i s , J. Electroanal Chem., 186( 1985)167. ( 4 ) G. P i e r r e , M. E l Kordy and G. Cauquis, Electrochim. Acta, 30(1985)1219. ( 5 ) G. P i e r r e , M. E l Kordy and G. Cauquis, Electrochim. Acta, 30(1985)1227. ( 6 ) G. P i e r r e , M. E l Kordy and G. Cauquis, Electrochim. Acta, 32(1987)601. B e l g s i r , H. Huser, C. Lamy and J.-M. Leqer, J . E l e c t r o a n a l . Chem., (71 . . E.M. 270( 1989y151. (8) R.R. Adzic, i n H. Gerischer and C.W. Tobias (Eds.), Advances i n E l e c t r o c h e m i s t r y and Electrochemical Engineering, v o l 13, Wiley, New York, 1984,- P. . 159. ( 9 ) D.M. Kolb, i n H. Gerischer and C.W. Tobias (Eds.), Advances in E l e c t r o c h e m i s t r y and Electrochemical Engineering, v o l . 11, Wiley, New York, 1978, p. 125. (10) G. Horanyi, G. I n z e l t and Z. Szetey, Acta. Chim. Acad. Sci. Hung., 98( 1978)49 ; 98( 1978)403. (11) E.M. B e l g s i r , H. Huser, J.-M. Ldger and C . Lamy, J . E l e c t r o a n a l . Chem., 225( 1987)281. (12) E.M. B e l g s i r , Ph.D. Thesis, U n i v e r s i t y o f P o i t i e r s , 1990. (13) M.R. Tarasevich, A. Sadkowski and E. Yeager, i n B.E. Conway, J . O'M. Bockris, E. Yeager, S.H.M. Khan and R.E. White (Eds.), Comprehensive T r e a t i s e of E l e c t r o c h e m i s t r y , vol.7, Plenum Press, New York and London, 1983, p. 301. (14) N. Furuya and S. Motoo, J. E l e c t r o a n a l . Chem., 98(1979)189. (15) P.C.C. Smits, B.F.M. Kuster, K. van d e r Wiele and H.S. van d e r Baan, Carbohydr. Res., 153( 1986)227.

.

.

M. Guisnet et al. (Editors ), Heterogeneous Catalysis and Fine Chemicab I1

471

0 1991 Elsevier Science PublishersB.V., Amsterdam

NITRIC ACID ASSOCIATED WITH INORGANIC SOLIDS : A VERSATILE REAGENT AND CATALYST IN THE CHEMISTRY OF AROMATICS M.H. GLIBELMA"*, C. DOUSSAIN*,P.J.TIREL*, J.M. POPA** *RHONEPOULENC RECHERCHES. BP 62. F-69192 SAINT FONS (France) **RHONEPOULENC RECHERCHES, F-93308 AUBERVILLlERS (Prance)

ABSTRACT The behaviour of the "nitric acid-solid acid' couples in the functionalization of aromatic substrates depends essentially on their ability to trap water produced by the nitration process. In the liquid phase applications, the solid acts either as an acid catalyst or as a dessicant. It is shown, that accumulation of water rapidly inhibits the acid component and leads toward oxidation patterns, whereas a good dessicant allows interesting nitration performances.

INTRODUCTION From an industrial point of view, the applications of nitric acid in inorganic chemistry can be divided into oxidations' and nitrations2*" The nitration of an aromatic nucleus is probably one of the earliest procedures used to functionalize basic petrochemicals into starting materials for chemically more elaborate structures and has been known since 1834 and was first employed on tin industrial scale in 1847 (eq l)4.

ArH

+ HNO,

-

ArNO,

+

H2O

(1)

Still today, this reaction is an industrial reality of crucial importance. Essential basic chemicals such as TDI (toluenediisocyanate) and MDI (methylenediisocyanate) are both obtained by initial nitration of toluene and benzene respectively. Nitration, are highly exothermic (30-70 k ~ a l / m o l e ) and ~ . ~ have to be conducted with great care. Usually they are performed using the classic sulfonitric mixtures ("mixed acid"), in which a slight molar excess of concentrated nitric acid toward substrate and a molar equivalent of concentrated sulfuric acid are u ~ e d ~ ~ ' - ~ . From a chemical standpoint, these processes perform well and the aromatic balance is very close to 100 %.

472

Despite the high reactivity and the low cost of the reagents, the use of sulfuric acid is however connected with some disadvantages. First of all, because it captures all the water liberated, it cannot be recycled directly without a high temperature dehydrating step (sulfuric concentration : SC). If no SC is operated, the outcoming aqueous sulfuric acid has to be neutralized and produces highly salty effluents, non desirable for the environment. Furthermore, dilute sulfuric acid can produce severe corrosion problems of the reaction vessels. We have been looking for new technologies, which could bring some solutions to these problems, either by changing the nature of the nitrating agent or by using solid acids. The later are quite promising, because they are can be operated in the fixed bed mode and regenerated continously. Different types of nitrating agents have appeared over the years". For industrial purposes, only nitric itself and/or its oxides have any economical and technical reality. Recent work from academia and industry has been devoted to the replacement of sulfuric acid by solid acids such as sulfonated resins" in the liquid phase, or oxides'*, supported phosphoric and sulfuric acids" as well as claysI4 and ~ e o l i t e s ' ~in * 'the ~ vapour phase. Aromatic nitrations are highly exothermic, as shown by a simple thermodynamic analysis using basic data in normal conditions (eq. 2,3)6. LIQUID PHASE NITRATION

VAPOUR PHASE MTRATION

+

3N02 d

+ NO + H20

A H R = -7lkcalmole

(3)

For security reasons and specially in the cases where the substrate contains akylsubsituents, we have chosen to operate in the liquid phase and to use nitric acid itself.

473

When one deals with the use of nitric acid in conjunction with a solid acid, one has to keep in mind that the solid may play two fundamentally different roles, which frequently are strongly connected. One of being a CATALYTIC ACID, and the other of being a STOICHIOMETRIC DESSICANT. We tried to separate these two basic functions. The differences observed by working either in the presence of a solid acid or a dessicant are shown in this communication. EXPERIMENTAL Reactions were carried out in the liquid phase using 100 % HN03 from MERCK. Acidic clays (Siid CHEMIE - Munich FRG) and dessicants ( P R O M O ) were of commercial grade and used without further purifications, besides thermal treatments if needed. All compounds were analysed by gaz chromatography and their structures confirmed by mass spectrometry and infrared spectroscopy. RESULTS AND DISCUSSION Use of the “nitric acid-inorganic soIid“ couple without trapping of the water produced The experimental conditions and some results obtained in the presence of an acidic montmorillonite type clay are summarized in table 1. TABLE 1 : Formation of MNT (mononitrotoluenes), DNT (dinitrotoluenes) and MDPM (methyldiphenylmethanes) from toluene by using nitric acid in conjunction with a commercial acidic clay of the montmodonite type (toluene 1.45 mole ; 100 % HNO, 21 m o l e ; clay 90 g. Reaction time 3.5 hr).

TOL.+ HN0-j + CLAE Ident.

p -

Selectivity

Selectivity

O+P

DNT/HNO,VW

MDPM/HNO~(%)

41

0.41

0

0

95

0.50

0

0

64

0.52

0

32

0.49

0.46

3

@

0.44

0

‘lo

474

It appears, that by using toluene itself as a solvent and by increasing the temperature, the "acid clay-HN03" system permits the formation of MDPM with interesting selectivities. A striking feature is that nitrated MDPM are not formed and that arylation does not occur (no biphenyls). Moreover, the selectivity toward MDPM increases with temperature and no significant oxidation of MDPM is detected. Complementary experiments, conducted in the same conditions but in the absence of nitric acid, show that MDPM are not formed neither from toluene nor from nitrotoluenes. In the case of an acid catalysis by protons only, condensation of toluene should lead primarly to the isomers of dimethylbiphenyls by an "arylation - dehydrogenation" sequence. However, these products are not observed. Accordingly, the role of water might be explained in the following way : in the absence of water, protonation of toluene can induce arylation, whereas, in the presence of water, the acidity of the clay is just sufficient to protonate nitric acid and to favour the formation of an ipsosubstituted Wheland intermediate. The most reasonable reaction sequence compatible with our observations is depicted in scheme 1 and eq. 4. SCHEME 1 : Mechanistic hypothesis for the oxidative coupling of toluene by the "HN03-clay" system.

Formation of nitronium ion t

HONO,

" I P S 0 attack of NO2 Formation of benzylium cation by elimination of H N 0 2 Benzylation of toluene (SEAr)

OVERALL REACTION

2

6

+

HN03

CLAY-H

___)

o^Q,

+ HN02

Me

t H20

(4)

475

In the presence of a clay, it therefore seems possible to change nitric acid into an oxidative coupling agent. The role of the clay is to provide a constant hydrated environment of its acidic sites (regulation of acid strength) and to avoid overcondensation reactions, which could lead to polyaromatic compounds containing more than two aromatic nuclei. The synthesis of MDPM by this route is unprecedent and is an example for a new application of nitric acid. In fact, so far MDPM have been produced either by oxidative coupling in the presence of iron(II1) salts"or air on V20518, or by Friedel-Crafts alkylations catalyzed by Lewis acidslg or zeolites". Use of the "nitric acid - inorganic solid" couple with trapping of produced water Because of the inherent difficulties of eliminating water azeotropically during nitration reactions, due to the nitric acid insitu techniques.

-

nitric oxides equilibria, we searched for more convenient

Usually nitric acid alone is very soluble in aromatic hydrocarbons, e.g. toluene. One can think of a device, where it would be possible to use a liquid "mechanical stirring agent" as a matrix in which both the reagents (substrate and HNO,) and the solid, in this case a dessicant, are not soluble. Hence, the matrix should favour the close contact between the reagents and the dessicant and avoid any problems of dilution and partial conversion due to the use of the substrate itself as a solvent. Obviously the matrix has to be inert toward nitric acid. This new concept is rationalized in figure 1.

FIGURE 1

: The concept of the "Liquid Matrix Device" (LMD)

+

INWT LIQUID MATRIX*

+

HNo~-suBsmTEMMTuRE

f-

INORGANIC DESSICANT

* in which neither the nitric acid nor the substrate arc soluble These considerations lead us to a class of compounds which one could call liquid "teflons", i.e. perfluoroalkanes. Commercially interesting examples include perfluorodecaline (PFD), perfluoromethylcyclohexane and FC 72, a mixture of C&,,

isomers. Furthermore,

besides being inert toward HNO,, these liquids strongly solubilize nitric oxides, which avoids losses of the gases into the vapour phase. The use of a liquid matrix is particularly useful in cases where the substrate is a liquid and the product is a solid, e.g. nitration of nitrotoluene to dinitrotoluene21*22. In the case of paranitrotoluene (PNT), the experimental conditions as well as some results obtained in the presence of inorganic anhydrous sulfates are shown in table 2.

476

TABLE 2 : Nitration of PNT by the "nitric acid - dessicant (anhydrous sulfate)" couple (p.nitrotoluene 20 m o l e ; 100 9% HNO, 21.4 m o l e ; perfluoroalkane 6 cm3 ; dessicant 7-8 g. Temperature 6OoC; reaction time 1-1.5 hr.). Anhydrous salt or preferentially used hydrnte

Sulfate

- I x

5

7 1 0 5 x50.5

-

PNT paranitrotoluene

;

-1

Dehydratationconditions duration (h)

1 {::

PNT conversion

2.4 DNT selectivity

(%)

(%)

27

76

57

94

48

100

71

98

54

95

2.4-DNT = dinitrotolucne

These results show that by using the LMD technique it is possible to nitrate even less reactive molecules such as PNT with a high degree of conversion, under mild conditions and with high selectivities. It is noteworthy that this system nitrates toluene quantitatively. The reason why performances are not as good with MNT (mononitrotoluenes), probably lies in the fact that theire exists a competitive adsorption between HNO, and MNT ont the surface of the dessicant. The most probable mechanistic hypothesis is summarized in equations 5 to 9. Preliminary studies in situ by IR spectroscopy are confirming that scheme.

Autoprotolvsis

Nitric anhydride formation

L

N205

Dehydration of the medium Dcssicant

+

H20

a

hydrate

Aromatic nitration (electrophilic substitution

Rearomatization

Wheland interniedinte

417

Furthermore, when the "HN03- dessicant" system is used in the mononitration of different substrates, it permits to obtain excellent yields and modified regioselctivities in favour of the para isomer (table 3). TABLE 3 : Mononitrations (MN) of different substrates by the "HN03- anhydrous CaS04" system (substrate 47 mmole ; 100 % HN03 5.3 m o l e ; anh. CaS04 2 g. ; temperature 6OoC) ISOMER DISTRIBUTION6 )

SUBSTRATE

REACTION TIME (mn)

HNO3 - C d O4

YIELD of MN (%)

0

m -

0

P

60

90

45

2

58

38

150'

64

16

3

26

66

60

89

7

0

13

86

60

94

0

0

43

56

60

83

2.3-DCNB

2.3-DCNE

10

10

3.4-DCNB

* Temperature = 45'C The observed para-selectivity might be due either to the bulkiness of the non dissociated N0,@N03@ ion pair in solution or to the adsorption of the same nitrating agent onto the polar surface of the dessicant. The later would indeed produce a strong increase in steric hindrance and could explain the para-attack of the incoming substrate.

478

CONCLUSION Replacement of sulfuric acid in “mixed acid’ aromatic nitrations by inorganic solids, with accumulation or elimination of produced water results in a fundamentaly different behaviour of the “HNO, - solid” couple. If water accumulates, nitric acid becomes a selective oxidative coupling agent, whereas when water is eliminated efficiently, nitric acid alone behaves as a strong nitrating agent, with increased paraselectivity as compared to the sulfo-nitric system. REFERENCES 1

2 3 4

5 6 7 8 9 10 11 12

13 14 15 16 17 18 19

20 21 22

a) K. Weissermel, H.J. Arpe, Industrial Organic Chemistry. Verlag Chemie, Weinheim, 1978, pp. 211-213. b) Kirk-Othmer Encyclopedia of Chemical Technology, 3rd edn. vol.1, John Wiley, New-York, 1978, pp.518-520 ; ibid., vo1.15, 1981, pp. 856, 869. Reference la, pp. 326-334. H.G. Franck, J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer Verlag, Berlin, 1988, pp. 13-17 a) G.A. Olah, R. Malhotra, S.C. Narang, Nitration Methods and Mechanisms. Organic Nitro Chemistry Series, Verlag Chemie, Weinheim 1989, Chapter 1, ref. 2 to 6. b) L. Bretherick, J. Chem. Educ., 66 (1989) A220. Kirk-Other Encyclopedia of Chemical Technology, 3rd edn., vo1.15 ; John Wiley, New-York, 1981, pp. 842-845. D.R. Stull, E.F. Westrum, G.C. Sinke, The Chemical Thermodynamics of Organic Compounds, John Wiley, 1969. P. Pascal, Nouv. Tr. Chim. Min., Masson ed. Vol.10, 1956, pp. 518-523 Ref. la, pp. 331-332. Ref. 3, pp. 237-238. Ref. 4a, chapter 2. T. Kameo, T. Hirashima, 0. Manabe, Nippon Kagaku Kaishi, 3 (1983) 414. a) US patent, 2 109 873,1936, see also R.H. McKee, R.H. Wilhelm, Ind. Eng. Chem., 28 (1936) 662. b) Japan patent, 58 157 748,1982, to SUMITOMO. c) European patent, 182 771, 1984, to MONSANTO. a) US patent, 4 112 006,1976, to HOECHST. b) US patent, 4 347 389, 1980,to MONSANTO. J.M. Bakke, J. Liaskar, G.B. Lorentzen, J. Prakt. Chem., 324 (1982) 488. Japan patent, 59 216 851,1983, to JAP. SYNTH. RUB. a) US patent, 4 426 543, 1984, to MONSANTO. b) European patent, 78 247, 1981, to MONSANTO. S. Uemura et al. J. Chem. SOC.Perkin Trans. I, (1976) 1966. US patent, 4 727 208, 1986, to DOW CHEMICAL. a) German patent, 3 544 733,1985, to BAYER. b) US patent, 3 679 760, 1972, to U.O.P. c) US patent, 3 714 280,1973, to PHILIPPS PETROLEUM. d) G. Olah et al., J. Org. Chem., 39 (1974) 2430. US patent, 4 01 1 278,1977, to MOBIL OIL. French patent application, 14 288, 1988, to RHONE POULENC. French patent application, 2 468, 1989, to RHONE POULENC.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

479

DEHYDRATION UF CARBOXAMIDES T O NITRILES USING SULPHATED Z I R C O N I A CATALYST

R.A. RAJADHYAKSHA and G . W . JOSH1 Department o f Chemical Technology, University o f Bombay, M a t u n g a , Bombay-400 0 1 9 , India. SUMMARY Dehydration o f benzamide, nicotinam is studied using sulphated zirconia a s sion to corresponding nitriles could b e w e r e carried out in water immiscible s o vity o f the different amides seems t o t h e dissociation o f the C = O bond o f t h e

d e , stearamide and o l e a m i d e catalyst. Complete c o n v e r achieved when the r e a c t i o n s vents. T h e order o f r e a c t i be governed b y t h e e a s e o f amide.

INTRODUCTION rvitriles a r e very versatile intermediates which c a n be used in the synthesis o f a wide variety o f organic compounds including amines, aldehydes, amidines and heterocyclic compounds. A number o f nitriles are, therefore, important industrial chemicals. Nitriles can be manufactured by two common routes; substitution using alkali cyanides and dehydration o f carboxamides. T h e latter r o u t e is normally favoured due t o toxicity o f alkali cyanides. T h e dehydration o f carboxamide has been investigated extensively using w i d e variety o f catalysts and dehydrating a g e n t s (ref.1). When t h e desired nitrile is suffciently volatile, t o a l l o w r e c o v e r y b y d i s t i l l a t i o n , phosphorous pentoxide can be employed a s a dehydrating agent. T h e reaction can also be carried out using phosphorous pentahalides, thionyl chloride and acid anhydrides. T h e conventional acid c a t a l y sts like aluminium chloride and zinc chloride have a l s o b e e n shown to be effective for t h i s class o f reactions (refs.2,3). All t h e a b o v e reagents, however, are required t o be employed in molar o r m o r e than molar proportions with t h e reactant and a r e consumed during reaction or in the subsequent recovery o f t h e product. T h e byproducts formed can pose problems o f w a s t e disposal. Use of solid acid catalysts for t h i s reaction can be a d v a n t a g e ous, since they can be r e a d i l y separated f r o m t h e product and t h e reaction medium remains noncorrosive. However, very f e w reports

480

have appeared o n t h e use o f solid acid catalysts f o r dehydration o f carboxamides. Rama Rao et al. (ref.4) have reported vapour phase dehydration o f carboxamides using a z e o l i t e catalyst. T h e method r e q u i r e s high reaction t e m p e r a t u r e ( 4 0 O O C ) and is restricted t o volatile arnides only. In a r e c e n t communication (ref.51 t h e a u t h o r s have reported t h e use o f sulphated z i r c o n i a catalyst f o r t h i s r e a c t i o n in liquid phase a t much lower temperture. T h e present work r e p o r t s a detailed investigation o n t h e s u i t a b i l i t y o f s u l p h a t e d z i r c o n i a a s catalyst for t h i s c l a s s o f reactions. Dehydration o f benzarnide, nicotinamide, o l e a m i d e and steararnide h a s been investigated using t h i s catalyst. T h e corresponding nitriles o f a l l t h e s e amides a r e important industrial chemicals. T h e reactions a r e a l s o carried out using polystyrene s u l p h o n i c acid, Nafion-H and t r i f l u o rornethane sulphonic acid (triflic acid) a s catalysts f o r comparison. EXPERIMENTAL Preparation o f zirconia c a t a l y s t h a s been previously d i s c u s s e d (ref.6). Untreated zirconia w a s prepared by similar p r o c e d u r e excluding the treatment with s u l p h u r i c acid. T h e physical c h a r a c t e r i s t i c s o f these m a t e r i a l s a r e r e p o r t e d r e c e n t l y (ref.7). Nafion-H w a s procurerd f r o m D u Pont. It w a s r e p e a t e d l y treated with 25% nitric acid a t r o o m temperature, washed f r e e o f a c i d , dried a t llO°C for 16 hours and stored in airtight g l a s s t u b e until use. Polystyrene sulphonic acid r e s i n (Indian 130) w a s o b t a i n e d f r o m Ion Exchange (India) Ltd. Triflic acid w a s obtained f r o m Fluka. All t h e chemicals used in the investigation w e r e o f 'Analytical Reagent' grade. T h e reactions w e r e carried out in a 150 ml glass r e a c t o r equipped with a turbine stirrer and a r e f l u x condenser. P r o d u c t samples were withdrawn periodically and analysed b y g a s c h r o m a t o graphy. T h e stationary phase used for analysis w a s 5 % mixed c y a n o propyl si licane Ihilar-5 cp + S i l a r - 7 cp) on chromosorb W ( A W ) treated with dirnethyl chlorosilane (column length 1.1 meter). R E S U L T S AND DISCUSSION Dehydration o f benzamide In t h e preliminary e x p e r i m e n t s dehydration o f benzamide w a s f i r s t investigated without using a n y solvent. T h e reaction w a s carried out using 40% w/w sulphated z i r c o n i a in molten benzamide a t

481

2 I TIME ( H O U R S )

2

6

T I M E (HOURS)

F i g . 1 . Dehydration of benzamide i n d i p h e n y l e t h e r ( a ) w i t h sulphated zirconia (b) w i t h untreated zirconia (c) without catalyst. 230

t.

Fig.2. Dehydration of benzamide using sulphated zirconia i n (a) diphenylether (b) 2-nitrotoluene (c) tetralin.

The r e a c t i o n d i d n o t p r o c e e d b e y o n d 3 0 % c o n v e r s i o n . T h i s

was a t t r i b u t e d t o p o i s o n i n g o f t h e c a t a l y s t b y t h e w a t e r p r o d u c e d d u r i n g r e a c t i o n . T h e s u b s e q u e n t i n v e s t i g a t i o n was t h r e f o r e c a r r i e d out

using

water

immiscible solvents

under r e f l u x i n g

A l l

t h e e x p e r i m e n t s were c a r r i e d o u t u s i n g 25% w/w

conditions. of

amide

in

s o l v e n t u s i n g 10% w / w o f s u l p h a t e d z i r c o n i a . Figure

1 shows

t h e r e s u l t s of

the reaction carried out using

sulphated and untreated z i r c o n i a T h e r m a l d e h y d r a t i o n does zirconia

show

very

i n diphenylether

(b.p.

259' C ) .

o c c u r t o a f i n i t e e x t e n t . The s u l p h a t e d

significant

activity

for

the

reaction

as

compared t o t h e u n t r e a t e d z i r c o n i a . Figure different

shows

2

the

results

temperatures

c o n v e r s i o n of

using

of

the

different

reaction

carried out a t

solvents.

b e n z a m i d e was o b t a i n e d a t 259OC a f t e r

r e a c t i o n seems t o f o l l o w

zero order k i n e t i c s

More

than

a t 205'

a n d 227OC,

h o w e v e r a t 259OC t h e r e a c t i o n o r d e r a p p e a r s t o b e d i f f e r e n t zero order. ssion

r

from

T h e d a t a a t 259OC c o u l d b e f i t t e d t o a k i n e t i c e x p r e =

kc/(ltKc).

temperature are given o u t under

90%

12 h o u r s . T h e

The

i n Table

kinetics

1.

constants

at

differnt

T h e r e a c t i o n was a l s o c a r r i e d

i d e n t i c a l c o n d i t i o n s i n t h e absence of c a t a l y s t .

A t the

-

e n d of t e n h o u r s t h e o b s e r v e d c o n v e r s i o n s w e r e a s f o l l o ~ s : 2 5 9 ~ C

-

19.1%

, 227OC

-

1 3 . 7 % a n d 205OC

-

5.0%.

482

TABLE 1 K i n e t i c c o n s t a n t s f o r d e h y d r a t i o n of benzamide ~

Temperature

~~~

k

K

g mole/g,

OC

205

1.19

227

1.74

259

2.64

T h e r e a c t i o n was c a r r i e d o u t

hr

l i t gmole-l

0.424 u s i n g Nafion-H and p o l y s t y r e n e s u l -

p h o n i c a c i d r e s i n c a t a l y s t a t 18OoC a n d 14OoC, r e s p e c t i v e l y , w h i c h a r e t h e i r maximum t e m p e r a t u r e s

of

use 2 - n i t r o t o l u e n e s o l v e n t and

10% w / w c a t a l y s t . C o n v e r s i o n s o f 2 0 % a n d 1.5% w e r e o b t a i n e d a t t h e end o f s i x hours i n d i c a t i n g t h a t h i g h e r temperatures w i l l be neces s a r y t o a c h i e v e a p p r e c i a b l e r a t e s on t h e s e c a t a l y s t s .

Since these

c a t a l y s t s a r e n o t s t r u c t u r a l l y s t a b l e above t h e r e s p e c t i v e temperatures,

t h e y appear

r e a c t i o n was

t o be unsuitable for

also carried out

various temperatures.

this

using t r i f l i c

application.

The

a c i d as c a t a l y s t a t

The r e s u l t s a r e shown i n F i g . 3 .

Surprisingly,

t h e r e a c t i o n d i d n o t p r o c e e d beyond 40% c o n v e r s i o n i n s p i t e o f t h e h i g h a c i d i t y of t h e c a t a l y s t . Dehydration o f pyridine-3-carboxylic

a c i d amide ( N i c o t i n a m i d e )

D e h y d r a t i o n o f n i c o t i n a m i d e was c a r r i e d o u t a t d i f f e r e n t ratures

u s i n g c o n d i t i o n s s i m i l a r t o t h o s e employed f o r

The r e s u l t s a r e shown i n F i g . 4 .

The r e a c t i o n f o l l o w e d

tempe-

benzamide. zero order

k i n e t i c s and r a t e of r e a c t i o n i s v e r y comparable t o t h a t o f dehyd r a t i o n of benzamide. In

contrast

with

The k i n e t i c c o n s t a n t s a r e g i v e n i n T a b l e

benzamide

dehydration

of

nicotinamide

a b s e n c e o f c a t a l y s t was n e g l i g i b l e . TABLE 2 K i n e t i c c o n s t a n t s f o r d e h y d r a t i o n of n i c o t i n a m i d e Temperature OC

k g mole/g,hr

227

1.49

205 186

9.86 2.05

E

kcal/gmole

16.09

2.

i n the

483

6

2 4 TIME (HOURS

TIME (HOURS)

1

F i g . 3. D e h y d r a t i o n o f benzamide u s i n g t r i f l i c a c i d 20% w / w i n (a) diphenylether (b) 2-nitrotoluene (c) 2-dichlorobenzene

F i g . 4 . D e h y d r a t i o n on n i c o t i n a mide i n (a) 2 - n i t r o r o l u e n e (b) tetralin (c) 2-dichlorobenzene

Dehydration o f stearamide and oleamide D e h y d r a t i o n of

s t e a r a m i d e was f i r s t i n v e s t i g a t e d i n t h e a b s e n c e

o f c a t a l y s t . The r e a c t i o n d i d n o t p r o c e e d t o a n y a p p r e c i a b l e e x t e n t (conversion l e s s than 5% a f t e r 6 h r ) a t

18OoC a n d 2 0 5 O C . H o w e v e r ,

80% c o n v e r s i o n

259'

product, The

could

however, progress

shown i n F i g . 5 .

be

achieved

at

after

C

6

hours.

The

c o n t a i n e d b y p r o d u c t s t o t h e e x t e n t o f 15%. of

the

reaction

i n t h e presence o f c a t a l y s t

The r e a c t i o n c l e a r l y f o l l o w s

is

zero order k i n e t i c s .

A t 259OC q u a n t i t a t i v e c o n v e r s i o n o f s t e a r a m i d e c o u l d b e o b t a i n e d i n

4

No b y p r o d u c t s w e r e o b s e r v e d i n t h e p r o d u c t m i x t u r e .

hours.

i n v e s t i g a t e t h e e f f e c t of

s o l v e n t on

the

rate

the

reaction

To was

c a r r i e d o u t i n t e t r a l i n , 2 - n i t r o t o l u e n e and d i p h e n y l e t h e r a t 205OC. TABLE 3 K i n e t i c constants f o r dehydration o f stearamide

k

Temperature OC

g mole/g,

227

9.49

205

6.03

180

1.64

E

hr

kcal/qmole

14.52

--

484

The r e a c t i o n r a t e s w e r e o b s e r v e d t o b e i d e n t i c a l in a l l t h e t h r e e c a s e s i n d i c a t i n g no e f f e c t o f s o l v e n t o n t h e r a t e . T h e k i n e t i c s c o n s t a n t s f o r t h e d e h y d r a t i o n s of s t e a r a m i d e a r e g i v e n i n T a b l e 3 . R e u s a b i l i t y o f z i r c o n i a c a t a l y s t was i n v e s t i g a t e d b y r e c y c l i n g t h e catalyst

without

any

intermitant

No s i g n i f i c a n t

washing.

decline

i n a c t i v i t y c o u l d be observed a f t e r t h r e e r e c y c l e s . The c a t a l y t i c b e h a v i o u r f o r identical.

The

kinetic

d e h y d r a t i o n o f o l e a m i d e was

constants

were

also

identical

almost

to

those

r e D o r t e d i n T a b l e 3. Mechanism o f r e a c t i o n The

structure

and

nature

of

a c i d i t y of sulphated z i r c o n i a has been r e c e n t l y i n v e s t i g a t e d

8-10).

(ref.

Untreated zirconium hydro-

xide which llises

i s amorphous,

crysta-

i n t h e m o n o c l i n i c form a t

35O0C.

On t h e c o n t r a r y ,

the

sul-

phate treated zirconium hydroxide crystallises

(5OOo0 C )

ture form,

3

which

greater surface

S

6

at

T I M E (HOURS 1

to

the

infra Fiq.5. Dehydration o f stearamide using sulphated zirconia i n (a) diphenylether, (b) 2 - n i t r o t o l u ene, ( c ) t e t r a l i n , ( d ) o - d i c h l o robenzene

higher into

has

tetragonal

significantly

area

monoclinic red

tempera-

as

compared

zirconia.

spectra

of

The

absorbed

of pyridine indicate Dresence only coordinative;y bonded p y r i d i n e and n o t t h e p r o t o n a t e d pyridine

implying

that

the

surface a c i d i t y of t h e sulphated z i r c o n i a i s predominantly of Lewis type. The

dehydration

i t s enol form

of

carboxamide

(ref.11).

i s believed t o proceed through

On a L e w i s a c i d c a t a l y s t t h e r e a c t i o n

i s

l i k e l y t o f o l l o w t h e f o l l o w i n g mechanism. 0 II R-C-NH~

R-C=NH

R-;=NH

tH LA

OH

#

The r e a c t i o n w o u l d t h u s w h i c h c o u l d r e c e i v e OH-

R-CEN

I

LA

i n v o l v e a Lewis a c i d and a b a s i c s i t e

a n d Ht

respectively

i n consecutive steps.

485

T h e requirement o f a strongly acidic catalyst would s u g g e s t t h a t t h e first step is likely t o be r a t e controlling. T h e s u r f a c e s i t e s will be generated by subsequent generation o f water. T h e r e s u l t s o f the present work indicate t h e following order of r e a c t i v i t y f o r t h e carboxamides. benzamide nicotinamide 1 s t e a r a m i d e N oleamide T h e higher r a t e o f dehydration o f benzamide and nicotinamide appears t o be d u e t o electron withdrawing effect of t h e a r o m a t i c r i n g which facilitates dissociation of t h e C = O bond. CONCLUSION T h e present study demonstrates suitability of sulphated z i r c o n i a for dehydration of carboxarnides. It needs t o be emphasised t h a t t h e common inorganic acid catalysts a r e not sufficiently a c i d i c t o catalyst the reaction below 4 O O 0 C w h i l e t h e strongly acidic r e s i n catalysts a r e not structurally s t a b l e a t temperature a t w h i c h t h e reaction would occur at appreciable rate. T h u s t h e sulphated zirconia appears t o be a unique c a t a l y s t for this application. REFERENCES

7

K. Friedrich and K. Wallenfels, Introduction o f c y n o - g r o u p s into t h e molecules, in : Z . R o p p o Port (Ed.), T h e Chemistry of C y a n o Group, Interscience, New York, 1970, pp 67-122. J.A. Norris and B.M. Sturgis, T h e preparation o f nitriles and amides, Reactions of e s t e r s with a c i d s and with a l u m i n i u m c h l o ride. J.Am.Chem.Soc., 61 (1939) 1413-14. J.A. Norris and A.J. Klemka, T h e preparation o f n i t r i l e s and amides, Reactions of e s t e r s with a c i d s and with a l u m i n i u m c h l o ride. T h e use of the s a l t NaCl.AlC13 in t h e Friedel and Crafts Reaction. J.Am.Chem.Soc., 62 (1940) 1413-35. A.V. Rama Rao, N.H. Rao, K. Gariyali and P. Kumar, Synthesis of nitriles f r o m Carboxamides with Zeolites, Chem.Ind., (1984) 270. G.W. Joshi and R.A. Rajadhyaksha, Dehydration o f C r b o x a m i d e s t o nitriles with Zirconia Catalyst, Chem.Ind., (1986) 876-77. D . D . Chaudhari and R.A. Rajadhyaksha, Alkylation of o - x y l e n e by styrene using superacid catalysts, Ind.Engg.Chem.Res., 26 (1987) 1743-45. P . S . Kumbhar and G.D. Yadav; Catalysts by Sulfur promoted s u p e r acidic zirconia, condensation r e a c t i o n s o f hydroquinone with aniline and substituted anilines, Chem.Engg.Sci. 44 (1989) 2 5 3 5 2643. T . Yamaguchi, K. Tanabe cd Y.C. Kung, Preparation and c h a r a c t e promoted Z r 0 2 , Mat.Chem.Phys., 16 rization of Z r 0 2 and SO% (1986) 6 7 - 7 7 . T. Yamaguchi, T. Jin, T. Ishida, K. Tanabe, Structural identification o f acid sites o f sulfur promoted solid superacid and construction o f its structure o n Silica Support, Mat.Chern.Phy., 17 (1987) 3-19.

486

10 M.

Bensitel, 0. Saur and J.C. Lavalley, A c i d i t y of z i r c o n i u m o x i d e and sulfated Zr@ samples, Mat.Chem.Phy., 17 ( 1 9 8 7 ) 2 4 9 258. 1 1 J. March, Advanced Organic Chemistry, 2nd end., M c G r a w Hill Kogakusha, Tokyo, 1977, p p 953.

487

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicaki II @ 1991 Elsevier Science Publishers B.V., Amsterdam

SATURATED AND UNSATURATED KETONES MANUFACTURED BY HETEROGENEOUS CATALYSIS

W.

Reith’,

M. Dettnier’,

H. Widdecke‘,

B. F l e i s c h e r ‘

1 RWE-DEA Ah f u e r M i n e r a l o e l und i n e m i e , P.0.Box 1d1420, U ( k e s t tiemany) Technische U n i v e r s i t i t Braunschweig, Hans-Somner-Str. 10, 0 33U0 Braunschweig (West Lermany)

*

-

4130 Moers l

-

SUMMARY A c i d i c i o n exchange r e s i n s a r e used f o r m a n u f a c t u r i n g b o t h b u l k chemicals and f i n e chemicals. The p r e s e n t paper r e l a t e s t o d i f f e r e n t r o u t e s o f p r o d u c i n g methyl i s o b u t y l ketone ( P l I B K ) , methyl i s o p r o p y l k e t o n e (MIK) and methyl i s o propenyl k e t o n e ( M I P K ) u s i n g a palladium-doped i o n exchange r e s i n as a c a t a l y s t . A new process v a r i a n t f o r a l t e r n a t i v e l y m a n u f a c t u r i n g MIPK and MIK w i t h t n e same equipment i s d e l i n e a t e d . INTRODUCTION Since t h e m i d - f i t t i e s s u l f o n a t e d r e s i n s oased on s t y r e n e / d i v i n y l b e n z e n e copolyiners, i n i t i a l l y developed as i o n exchangers m a i n l y t o r w a t e r t r e a t m e n t , nave a l s o been used as s t r o n g l y a c i d i c s o l i d c a t a l y s t s . Witn few exceptions, i n d u s t r i a l a p p l i c a t i o n i n continuous processes i s l i m i t e d t o t h e manufacture o f b u l k chemicals, sucn as Disphenol A, ( m e t h ) d c r y l a t e s , m e t h y l e t h e r s o f brancned o l e f i n s (MTBE, TAME) and secondary a l c o h o l s (IPA, SBA). F o r i n s t a n c e , methyl t e r t - b u t y l e t h e r (MTdE) used w o r l d w i d e as an octane improver i n g a s o l i n e i s produced a t a s t i l l growing g l o b a l c a p a c i t y o f approx. 8 m i l l i o n mtlyr u s i n g s u l f o n a t e d r e s i n s as c a t a l y s t s ( r e f . I ) . HWE-DEA ( f o r m e r Ueutsche Texaco Ab) i s one o f t h e l e a d i n g companies i n t h e development ana commercial a p p l i c a t i o n o f processes u s i n g i o n excnange r e s i n s as a c i d i c c a t a l y s t s . Our e x p e r t i s e comprises p r o d u c t i o n o f b u l k chemicals, sucn as MTBE ( r e f s . 2-31,

i s o p r o p y l a l c o h o l ( r e f s . 2 , 4-5) and s e c - b u t y l

a l c o h o l ( r e f s . 2, 6-7) as w e l l a s manufacture o f low-volume chemicals s o l d a t n i g h e r p r i c e s , sucn as methyl i s o b u t y i ketone, methyl i s o p r o p e n y l Ketone and methyl i s o p r o p y l k e t o n e ( r e f s . 8 , Y-10):

488

fl

CH3-C-CH

Methyl i s o b u t y l ketone

0 CH II I

ZH

Methyl isopropenyl Ketone

3

f

ZH 1 3 -CH-CH3

MIBK

3

1

MIPK Z

-C-Z=CH2 0 ctl. II I

3

MIK

CH3-C-CH-iH3

Methyl i s o p r o p y l ketone

3

MANUFACTURE OF MIBK MIBK

1,a

s o l v e n t f o r i n k s and lacquers, i s formed oy r e a c t i n g two equiv-

a l e n t s o f acetone ff. v i a i t s i n t e r m e d i a t e s diacetone a l c o h o l oxide

5 and m e s i t y l

2.

!

2 CtlJ-C-CH3

0

->

II

CH3-C-LH

fl

utl I

-C-CH 2 1 Ctij

-> CH -C-CH=C-CH 3

I 3 CHJ

-> 1

5

4

This process can be performed i n d i f f e r e n t ways: Three-Step Process t o MIBK The c l a s s i c a l r o u t e uses t h r e e steps: p r o d u c t i o n o f

2 using

a s t r o n g base

as a c a t a l y s t , denydration by a c i d i c c a t a l y s i s y i e l d i n g 6, and hydrogenation w i t h a noble-metal c a t a l y s t s e l e c t i v e l y y i e l d i n g

1.

Two-step Process t o MIBK

I n t h i s v a r i a n t an a c i d i c i o n exchange r e s i n c l i r e c t i y c a t a l y z e s t h e forination of 6 w i t h i n s i g n i f i c a n t formation o f

2.

Hydrogenation i s t h e same as i n

t h e three-step process. One-Step Process t o RlBK I n a one-step process a Pd-doped s u l f o n a t e d r e s i n (e.g.

a standard macro-

porous t y p e w i t h 0.1-5 % Pd) c a t a l y z e s o o t h t h e condensation o f ff y i e l d i n g

6

1

and t h e hydrogenation o f ! = t o i i n a s i n g l e r e a c t o r . RWE-DEA has been producing

1for

many y e a r s by t h i s process developed i n t h e i r l a b s and p i l o t p l a n t s .

MANUFACTURE OF MIPK AND M I K MIPK 2 and M I K

2 are

f i n e chemicals used as raw m a t e r i a l s i n t h e p r o d u c t i o n

of dyes, agrochemical s, pharmaceuticals, s p e c i a l t y polymers e t c . A t f i r s t s i g h t s i m i l a r processes as f o r t h e manufacture o f MIBK seem t o be a p p r o p r i a t e . Methyl e t h y l ketone (MLK) 7 r e a c t s w i t h an aqueous s o l u t i o n o f formaldehyde

489

-8 y i e l d i n g genation t o

t h e hydroxyketone

2 that

can be dehydrated t o

4

f o l l o w e d oy hydro-

3.

H

0 II

CH -Z-CH + CH 0 - > CHJ-C-CH-CH2-OH 3 1 2 L I LH3 CH3 7 -

8 -

-> 2 ->

3 -

9 -

One-Step Process t o M I P K / M I K Even triough t n e above r e a c t i o n e q u a t i o n suggests use o f t h e e l e g a n t s i n g l e s t e p process, t h i s v a r i a n t i s n o t a p p l i c a b l e i n t h e case o f t h e M I K s y n t h e s i s . Oue t o unavoidable s i d e r e a c t i o n s t h e c a t a l y s r l i f e t i m e would be v e r y s h o r t r e s u l t i n g i n an i n t o l e r a b l e c o s t i n c r e a s e . MEK

1 nas

f i v e hydrogen dtonis i n a l p n a p o s i t i o n t o t h e c a r b o n y l group. Each

o f them i s a b l e t o r e a c t w i t h formaldehyde t o f o r m n o t o n l y t h e d e s i r e d monon y a r o x y r w t h y l a t e d ltetone b u t a l s o d i - , tri-, t e t r a - and pentahydroxymethylated ketones. These hydroxyketones as w e l l as t h e c o r r e s p o n d i n g u n s a t u r a t e d ones form s o - c a l l e d ' k e t o n e r e s i n s ' oy polycondensation, p o l y a d d i t i o n and p o l y m e r i z a t i o n . Because o f t h e p o l a r i t y o f t h e i o n exchange r e s i n , t h e concent r d t i o n o f w a t e r and formaldehyde, r e f e r r i n g t o IUIEK, i s h i g h e r i n s i d e t h e oeads than i t i s o u t s i d e r e s u l t i n g i n an i n c r e a s e i n s i d e r e a c t i o n s . F u r t h e r more, t r a n s p o r t a t i o n o f t h e ' k e t o n e r e s i n s ' formed i n s i d e i s nindered. Conseq u e n t l y , t h e beads a r e d e s t r u c t e d ( s e e f i g s . 1 and 2 ) which r e s u l t s i n a n i n c r e a s e i n p r e s s u r e drop over t h e r e a c t o r .

E l e c t r o n Scan M i c r o s c o p i c Shot o f C a t a l y s t Beads F i g . I Used C a t a l y s t F i g . 2 Fresh C a t a l y s t

490

The problem i s enhanced during the r e a c t i o n as the i n i t i a l l y homogeneous l i q u i d mixture becomes heterogeneous. This i s i l l u s t r a t e d i n Fig. 3 showing the ternary phase diagram o f t h e feed components MEK/water/formal dehyde w i t h i t s m i s c i b i l i t y gap a t 105 "C/10 bar. The two s t r a i g h t l i n e s mark t h e hypot h e t i c a l mixtures o f 40 % f o r m a l i n w i t h t h e MEK/water azeotrope ( l i n e I ) and a molar 4 : l m i x t u r e o f MEK/formaldehyde w i t h water ( l i n e 11). The l i n e i n t e r section represents t h e composition o f the mixture a t t h e r e a c t o r i n l e t . During the r e a c t i o n t h e MEK concentration decreases s l i g h t l y , whereas the formaldehyde concentration approaches zero. The m i s c i b i l i t y gap widens due t o format i o n o f the nonpolar proaucts

2 and 3.

The r e a c t i o n mixture i n the r e a c t o r

thus r a p i d l y separates i n t o a formaldehyde-poor organic phase and a formaldehyde-rich aqueous phase. The d e s i r e d molar MEK excess thus i s o f f s e t i n the aqueous phase. Since the c a t a l y s t s t i l l has a higher a f f i n i t y f o r t h e aqueous phase, formation o f ketone r e s i n s increases i n s i d e t h e beads. As a r e s u l t o f t h i s unavoidable f o u l i n g the c a t a l y s t l i f e time decreases r a p i d l y . This may be acceptable f o r a cheap c a t a l y s t , b u t i s h i g n l y uneconomic f o r an expensive noble metal doped c a t a l y s t . Iormaldahyda

,7;/5do 0.0

MEK

Fig. 3

line II

0.1

0.3

0.5

0.7

0.9

Phase Diagram (Mass F r a c t i o n ) o f t h e Ternary Feed w i t h M i s c i b i l i t y Gap a t 105 "C/10 bar

Two-step Acid-Catalyzed Process t o MIPK/MIK Since a Pd-doped r e s i n would be t o o expensive even i f the palladium was recovered f o r reloading, a two-step process using an inexpensive r e s i n and subsequent hydrogenation w i t h a comnon c a t a l y s t i s s u i t a b l e f o r t h e rnanufact u r e o f M I K and M I P K (Fig. 4 ) . A molar excess o f MEK (approx. 4 : l ) i s f e d together w i t h an aqueous s o l u t i o n of formaldehyde (approx. 40 X ) t o t h e f i r s t r e a c t o r R-1 c o n t a i n i n g a nondoped r e s i n . Here the same f o u l i n g problem occurs

49 1

besides the desired condensation and dehydration, b u t the c a t a l y s t i s f a r l e s s expensive and can be replaced occasionally. The heterogeneous e f f l u e n t obt a i n e d a f t e r the r e a c t i o n i s separated and the almost formaldehyde-free aqueous phase i s freed from organic compounds i n column C-1. A t t h i s p o i n t o f the process there are two options: t o feed t h e organic phase d i r e c t l y t o the d i s t i l l a t i o n section f i n a l l y y i e l d i n g pure MIPK o r t o l e a d i t t o hydrogenat i o n i n a second reactor R-2 using a comnon Pd c a t a l y s t (e.g.

0.5

-

5 X Pd on

A1203) y i e l d i n g pure MIK. The p u r i f i c a t i o n o f the two crude products i s b a s i c a l l y the same. Surplus MEK i s recovered and i s recycled as an azeotropic mixture w i t h water t o column C-2, while h i g h - b o i l i n g by-products are separated i n column C-3.

hydrogen 7

c-2

waste water

Fig. 4

Two-step Process t o MIPK/MIK

The cheapest c a t a l y s t f o r the f i r s t reactor i s a gel-type sulfonated r e s i n (e.g. Amberlite@IR 12U, Kohm and Haas). The k i n e t i c s i s c o n t r o l l e d by pore d i f f u s i o n as i l l u s t r a t e d i n Fig. 5 showing t h a t the r e a c t i o n r a t e decreases as the c a t a l y s t g r a i n size increases. l.lE-4

*-*:

loOOC 105OC

A-A:

110%

0-0: c

I a Ec

-E

9.OE-5.-

\8

.-.-

._ E

'a 5

7.OE-5.

*\*

5.OE-5-

I

---•-*-------.

o'o\o__o

+.o -.3.OE-5,::

-

: . . : : : .

0 :

:

: : : : : . :

~.

.

~

i0

Fig. 5

Reaction Rate as a Function o f C a t a l y s t Grain Size

492

Using macroporous i o n exchange r e s i n s t h e e f f i c i e n c y o f t h e a c t i v e s u l f o n i c a c i d groups i s s l i g h t l y improved. When comparing a comnercial g e l - t y p e i o n exchange r e s i n and a macroporous one (Lewatit@SPC 1U8, Bayer) manufactured w i t h t h e same amount of divinylbenzene, t h e r e a c t i o n r a t e , r e f e r r i n g t o t h e amount o f acid, i s h i g h e r w i t h a macroporous r e s i n ( c f . Table 1 ) . The s t a r t i n g mater i a l s may d i f f u s e unhindered through t h e permanent pores and t h e t r a n s p o r t a t i o n d i s t a n c e through t h e gel phase i s s h o r t e r . A t a h i g h e r c r o s s l i n k i n g degree t h e r e a c t i o n i n t h e gel phases i s repressed. Therefore, Lewatit@SPC 118 and A m t ~ e r l y s t @ l Sa t t a i n o n l y slow r a t e s . Using a surface-sulfonated i o n exchange r e s i n ( r e f s . 11-12],

optimum acces-

s i b i l i t y o f t h e a c t i v e groups which a r e almost e x c l u s i v e l y l o c a t e d a t t h e i n n e r surface o f t h e m i c r o p a r t i c l e s i s a t t a i n e d . The measured r e a c t i o n r a t e (see Table 1 ) i s about ten times h i g h e r than t h a t o f t h e i d e n t i c a l l y cross1 inked Lewati t @ S P C 118. However, when u t i l i z e d on a cormnercial scale, p r i m a r i l y t h e r e a c t i o n r a t e s , r e f e r r i n g t o c a t a l y s t q u a n t i t y o r b u l k volume, a r e i m p o r t a n t because they determine r e a c t o r s i z e and amount o f c a t a l y s t r e q u i r e d . R e f e r r i n g t h e r e a c t i o n r a t e i n t a b l e 1 t o one gram o f r e s i n , t h e s u r f a c e - s u l f o n a t e d i o n exchange r e s i n F 027 i s t h e l e a s t e f f i c i e n t because t h e g e l areas do n o t c o n t a i n any s u l f o n i c a c i d groups. The o t h e r f o u r r e s i n s , however, a r e q u i t e s i m i l a r : they a r e homogeneous and almost completely s u l f o n a t e d . Macroporous and surfaces u l f o n a t e d r e s i n s a r e h i g h e r p r i c e d , b u t n o t more e f f i c i e n t i n comnercial application. TABLE 1 Keaction r a t e s a t t a i n e d w i t h d i f f e r e n t r e s i n s

( T = 105 "C; MEK:HCHO = 4 : l ; F o r m a l i n 20 wt.%) Resin

D i v i n y l benzene

u u a n t i ty

'HCHO

m o l h i n meq

molhin g

120

8

b.3 X

3.1

SPZ 108

8

7.0

3.5

SPC 118

18

3.7

1.6

Amberlyst 15

20

2.8

1.4

F 027

18

IR

38.7 x

1.0

w4

493

New Base-Catalyzed Two-step Process t o M I P K / M I K Besides c a t a l y s t f o u l i n g , t n e g r e a t e s t disadvantage when u t i l i z i n g t h e two processes d e s c r i b e d b e f o r e i s t h e energy-consuming r e c o v e r y o f u n r e a c t e d MEK f r o m M I P K o r ivlIK t h e b o i l i n g p o i n t s o f which a r e c l o s e t o t h a t o f MEK. To overcome b o t h c a t a l y s t f o u l i n g and s e p a r a t i o n d i f f i c u l t i e s a new two-step process has been developed ( c f . F i g . 6 ) : i n r e a c t o r R-1

2,

NaOH as a homogeneous c a t a l y s t y i e l d i n g

L

reacts with

wnich excess 7 can e a s i l y be removed by d i s t i l l a t i o n i n tower C-1. second s t e p

2

2

using

a h i g h - b o i l i n g interiliediate from I n the

i s f e d t o a second r e a c t o r (K-2) c o n t a i n i n g t h e same Pd-doped

a c i d i c exchange r e s i n used i n t h e one-step process f o r h I B K manufacture. Depending on m a r k e t requirements, t h e o u t p u t o f

2 and 3 can

be v a r i e d by

opening o r c l o s i n g t h e hydrogen v a l v e on t h e second r e a c t o r .

I

I

hydrogen

New Base-Catalyzed Two-step Process t o rvlIPK/ivllK

Fig. o

dust-in-time production i s essential, p a r t i c u l a r l y f o r t n e unsaturated ketone

2,

because d u r i n g s t o r a g e a t 2U

OL

the content o f

2 in

the finished

p r o d u c t decreases by (1.4 % p e r day due t o CyClOdimeriZatlOn as shown i n f i g .

0

MIP

0

DMIP

2 -

Fig. 7

10 -

Z y c l o d i m e r i z a t i o n o t IbIIPK

7.

494

Hydroxymethylation o f

I

to

2 in

t h e f i r s t r e a c t o r i s very f a s t a t 40 "C.

In

p r i n c i p l e , a s t r o n g l y b a s i c i o n exchange r e s i n may be used as a s o l i d catal y s t , b u t as expected t h e c a t a l y s t f a i l e d i n t e s t r u n s due t o i n a c t i v a t i o n by f o r m i c a c i d produced by Cannizarro r e a c t i o n . 2 CH20

-> CH30H + HCOOH

CONCLUSION The b e s t way t o a l t e r n a t i v e l y manufacture MIPK and M I K i n t h e same equipment i s the combination o f a hanogeneous and a heterogeneous c a t a l y t i c step.

No c a t a l y s t f o u l i n g occurs, MEK recovery i s easy and cheap and p l a n t f l e x i b i l i t y i s high. REFERENCES 1 2 3 4

D. Rohe, Chemische I n d u s t r i e 4 (1990) bU-63 M. P r e z e l j , Hydrocarbon Processing 9 (1987) 68-70 Deutsche Texaco AG, DE 3322753 Deutsche Texaco AG, DE 2233967

5

Petrochemical Handbook '89, Hydrocarbon Processing 11 (1989) 111 Deutsche Texaco AG, DE 3040997 M. P r e z e l j , W. Koog, M. Dettmer, Hydrocarbon Processing 11 (1988) 75-78 Rheinpreussen AG, UE 1260454 Rheinpreussen AG, DE 1193931 Rheinpreussen AG, DE 1198814 U. Haupt, PhD Thesis, Technical U n i v e r s i t y o f Braunschweig (198b) 6 . Halim, MS Thesis, Technical U n i v e r s i t y o f Braunschweig (1988)

6 7 8

9 10 11 12

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1

495

0 1991 Elsevier Science Publishers B.V., Amsterdam

CONDENSATION OF METIIYL N-PHENYL CARBAMATE WITH SOLID .4CID CATALYSTS

JAE S. LEE', CHUL W. LEE', SANG M. LEE2, JAE S. OH2 and KWANG H. PARK2 ll)cpartnient of Chemical Engineering, Poliang Institute of Science and Technology, P.O. Box 125, Poliang (Korea) 2 Lucky R Sr D Center, P.O. Box 10, Science Town, Daejeon (Korea)

A B ST R .4 CT The coildensation of irietliyl N-plienylcarba,mate with IICHO to metliylcne diphcnyl diuretliane has been studied in a batch reactor in the presence of cation excha,nged resins. llnlilte conventional II2SO4 catalyst, fresh resin catalysts did not form a byproduct N-bcnzyl compound. However, accunnilation of water from repea,ted uses of the catalyst caused a decreased activity and the formation of the byproduct. The deactivated catalyst could he conipletely regenerated by drying is uucuo. Ethylacetate and toluene were found to be efficient solvent,s with the resin catalysts. INTI10 DU CTIO N Processes a.re under development to manufacture metliylene diplienyl diisocyanate (MDI) without using toxic and corrosive phosgene. The proposed process schemes usually consist of three st,cps: alkoxycarbonylation of nitrobenzene or aniline with CO and an alcoliol to alliyl plieiiylcarbamate, contlensation of the carbamate, and then tlierinal deconipositioii of the result,ing u r e t h n e to MDI. For exitmple, the condensation of methyl N-~~IicnyIca~rba.mate (nilPC),and NCIIO into inethylene diphenyl diurethane (MDLJ) is carried out in t,he presence of an acid catalyst. 2 CGIISNHCOOCH~ HCHO CHz(C&NHCOOCH3)2 H2O (I) Liquid acids such as HCI or H1SO4 h e been found to be efficient catalysts. However, the catalysts prodnce interniediate compounds having methylene-amino bonds, such as N-benzyl compound, CsHsN( COOCII3)(C H ~ C ~ H S N H C O O C [l]. H ~ These ) intermediat.es do not produce isocyanates and have a harmful influence on the next thermal decomposition step. Since these compounds must be catalytically transformed to desired MDU, the condensation reaction has to be carried out in two steps to use tlie best chara.cterist,ics of each cataIyst. Solid a.cid catalys1,s such as a perfluorinated ion-exc11a.ngeresin (Nafion) li,zvc received limited attention as catalysts for the condensation reaction 121. In a st,utly of solid acid catalysts for tlie c,ondensation of MPC and HCHO, we found that strongly acidic cation-exclianged resins with macropores were active and selective catalysts for tlie reaction [3]. In particular, the fresh catalyst did not form the N-benzyl compound, and thus suggested the possibility that tlie condensation reaction could be

+

-

+

496

accoinplislietl in a single step. Such solid acid catalysts are expected to provide several advantages over liquid counterparts including decreased corrosion, facile product rccovt~y and safer handling. EX P ER Ihl ENT A I, Methyl N-plieiiylcarbamatc wa,s synthesized by a stoichioinetric react.ion between aniline and methyl chloroformate according to a known procedure [4]. After vacuum distillation at !)0-95 '<: and 0.5 tmr, MPC was obtained with a 97% yield and a 99% purity. Aniherlyst 15 antl 3G, a,nd Amberlite 120, are commercial products of Ilohm and Haas. Tlicir characteristics are sumniarizetl i i i Table 1. Aiiiberlyst 36 is known to have properties similar to Amberlyst 15 and a better tlierinal sta.bility. Nafion N R 5 0 were purcliascd from Alfa. The catalysts were dried a t 100 'C i i i a11 oven for at least a day before use. The condciisation reactioii was carried out in a stirred 50 cm3 glass flask undcr a rcllux condition. l'hc rca.ctioii tcinperaturc was miiintaiiied by a heated silicoii oil batli surrounding thc reactor, and measured by a mercury thermometer imincrsed iiito the reaction mixture. After the reaction, the solid product and ca.tAyst were separated from the reaction solutioii by filtration. The protluct was dissolved i n ethyl acetate and separated from the catalyst particles by anotlicr filtra.tion. Tlic product was washed by wat.er antl tllcn dried over hlgS04. Aiialysis of tlic product was made with a IIPLC (Wat,ers Y90) eqiiippetl with a v-Porasil coliumn. A 30% ethyl acetate i n hexane was used as an elrieiit. 'I'lie conversion of HCHO, the limiting reactant, was calculated from t,he isolated solid protluct. 'Ylie selectivity was calculated by comparing the peak a.rea of the niain product with that, for aii authentic 4,4'-hlDU sample.

TABLE 1 C'liaract,eiistics of Am1)crlyst 15 aiid Ambcrlytc IR-120a

'I'ypc Surface area/m?g-l Radius at inas. pore volume/nni Pore 1acIius raiige/iiiii

Apparent tleiisi ty/g ciir3 SkeIetaI density/g an-' Porosit,y/cm3 g-l I011 exchange capacit,y/meq g" hloisture Iiolding capacity/%H~O >I

Amherlyst, 15

Amberlitc IR-120

Macroreticular 54.8 8

Gel

6-30 0.9s2

<0.1

None None 1.483

1.527

1.488

0.363

0.005

4.8

4.G 46

49

Source: 1i.A. I i u i i atid It. liuniii, J. Polymer Sci. C, 16 (1967) 1457.

497

ILESU LTS I n our initial scrcaiing of liquid acid catalysts, H2S04 was found to be the best in the contlcnsatioii of hlPC and IICIIO. I n Table 2, its performance is compared with solid acid catalysts. At 90 O C , GO g of 50% I-IaS04 and 1.9 g of 37% FICIIO were used. More than 99% of the product was accountcd for by 4,4'-MDU, 2,4'-MDU, trimeric urethanes, and the N-bciizyl compound. Following a procedurc rccommendcd in literature [l],the recovercd solid product was dissolved in CF3COOH and refluxed at 75 O C for 0.5 h. As shown in the second entry of Table 2, all N-benzyl compound was transformed to MDU by this second st.cp rca.ction. Among the hetcrogcncous catalysts, the macroreticular type cation-exchange polystyrene resins, Amlicrlysl 15 and 36 showed good performance. Compared to the case of 11$3'04,the HCIIO conversion was slightly small, but selectivity to 4,4'-MDU was about the same. Rlost noteworthy wa,s that tlic N-benzyl compound was not formed. Instead, the content of 2,4'-MDU and trimcr increased. Nafion NR-50, a. pcrfluorinated sulfonic acid resin, showed a similar selectivity, but the HCHO conversion was less. The gel-type hmberlite IlL-120 was almost inact.ivc. The effect of solvent i n thc RlPC condensation reaction in the presence of Amberlyst 15 is sliowii i n 'I'ablc 3 . Two moles of solvent werc employed for a mole of MPC. In all cases, t lie N-henzyl coiril)ound was not formed. In general, nonpolar and aprotic solvents showed

'I~Ar3LE2 Coiitlcimtion of mctliyl N-phenyl carbainate at 90 'Ca C'a tklyst

50% I I ~ S O ~ -(I Atiibcrlile IR-120

Nai'ion NR-50 Aiiiberlyst 1 5 i\mbcrlyst 36

IrCriob conversion( %) 85.9 s5.9 3 27 70 65

4,4'-MDU

Selectivity(%)c 2,4'-MDU N-bcIlZyl

S6.0 90.4

7.0 s.0

5.7 0

-

-

-

7s SO 79

12 11 11

0 0 0

"The sourcc of IICFIO, 37% aqucons solution; mole ratio of methyl N-phenylcarbamate to 1IC:IIO red to the rwctor, 4; holding time, 4 h , solvent for solid catalysts, etliylacetate. Ii(:alcrilat,ecI cronr isola.tctl solid protlucts. c,1he rest was inostly the trimers. '1.4!'tcr the rcarraiigemcnt reaction in CF3COOH at 75 OC for 0.5 h. 1

498

TABLE 3 Effects of solvent i n methyl N-phenyl carbarnate condensation with Arribcrlyst 15" Reaction rreinp.(°C)

IICIIO conversion(%)

Selectivity (%) 4,4-'MDU 2,4'-MDIJ

triiners

DhlF C'IIsCN DhlSO THF nil iol)enzene ethylacelate t olr1ene

95

11.1'.

-

-

-

75

40

87 -

8

5

-

-

61 80 80

39 11

0

11

73

20

TO

18

9 7 12

cyclollcxallc

GO

Sol Ven t I'

60-120

11.1'.

GO

37 49 70 51 TOC -d

GO

GO (i0

9

aIIolding time= 4 11, hIPC/IIC'HO= 4 , mole iatio of solvent i o MPC'= 4. ')DhlF( Dimctliylformamidc), DhISO(Diniethyl sulfoxide), TIIF( tetialiydrofuran). 'RIole ratio of the solvent to MPC of 2 was employed. (Ipioduct solitis coatetl catalyst particles.

Wlicii toluciic was used, the products were not, dissolved arid the reaction uinetl a n cmulsion forin as the reaction proceeded. Analysis of the product

good results.

particles by a particle size analyzer and a scanning electron niicroscope indicated that slah-sha.ped particles were foriiicd with an a.verage size of 1.4 pin. This represents a market1 contrast, to the H?SO.I cathlyst wliirh forms solid products that stick to the wall of the reactor a.nd on impeller. The solid formation was much sevcrcr with cyclohexane as solvent, in which case the reaction stopped because the product solids coated the catalyst beads complet,ely. I'igurc 1 shows deterioration of catalyst performance with repeated uses. After a lxitcli rcactioii a t GO OC: for 4 11, the catalyst was separated, rinsed with etliylacetate solvent, and reused with fresh cha.rge of the reactants. The conversion of HCHO continuously tlecreased. hilost significant change in selectivity was emergence of the N-benzyl rompouiid starting froni the sccoiid batcli reaction. When the used catalyst was rinsed with water, and dried at 100 'C: i n a vaciiuiii oven overnight, the catalyst recovered the performance of the fresh catalyst hoth in IICIIO conversion and selectivity (the fifth batch).

499

9 0,

L1/;

a 8 0.

fi 4 0.

II

,

El 4,4’-MDU

2.4-MDU

Trim

0 N-benzyl 3 02 0. 10.

0 , .

,

.

,

. Batch

I

.

I

.

t

1

2

3

4

5

Batch

Fig. 1. Variation of 1IC:FIO convcrsioti (a) and selectivity (I)) upon repeated uses and rcgcncratioti of Anibcrlyst 15. DISClTSSION The contlensat,ion reaction of methvl N-ihenvl carbamate over solid acid catalysts The present study tlcmonstrated that, in MPC condensation reaction, Ainberlyst-type catalysts were as active as and more selective than the best homogeneous catalyst, II?sO4. Aniberlyst 15 and 36 are macroreticular type polystyrene sulfonic acid resins partially cross-linlicd with divinylbenzene. The absence of the N-benzyl product wlien solid acid catalysts were employed suggests the possibility that the reaction could be carried out in a siiiglc stcp. It is also cxpect,ed to provide all the aforementioned advantages of solid catalysts ovcr liquid ca,talysts. Strongly acidic iori cxcliangc resins such as those used in the present study have been employed as replacements for homogeneous acids in a host of chemical reactions [ 5 ] .Among t h e , t-butyl alcoliol dehydration, hydration of olefins, Bisphenol-A synthesis and methyl t-butyl ether synthesis are conimercial technologies using this type of solid acid catalysts. The condensation of MPC distinguishes itself from these commercially successful processes in that it requires a solvent to dissolve the solid reactant, MPC. The good results for the Aml~erlyst15 and 36 catalysts appear to be due to the fact that they possess macropores t.liat allow tlic taransportof reactants and products to and from acid sites residing inside of pores. As slio\vn i n Table 1, Amberlyst 15 has the pores, most of which are greater than 6 nm in tlianieter. The gel type resin Amberlite IR-120, on the other hand, is nonporous. The Nafion NIL 50 is known to possess much stronger acidity than Amberlyst 15 or 36. The inferior performance of Nafion, hence, a.ppears due to the absence of pores. These nonporous resins would have needed a good solvent which can solvate and swell them to be effective catalysts. However, as discussed later, selection of solvent also affects the reaction itself.

500

Tlre most int,crcst,ing feature for Aniberlyst 15 in h~ll'C! condensation is tlic absence of t I I P N-l)cnzyl compound i n tlic products. A s i m p l e calculation shows that when 50%)

I12S04 is used antl conrplctcly dissociated, the system coiit,ains 2.S3 moles of water per iriolc of proton, SVX of wliicli coincs from t,he 50% II?SO4. After 100% conversion of IIC'HO, this to 2.87. For Aniberlyst 15, the tI?O/II ratio is initially 0.88, antl increases t o 1 . I S ripori r.oiirI)lctc IIC'HO conversion. This analysis indicates t,lrat at, least. i n t,Iie early stage of t,lie hlPC coiitlcnsation, Anil)crlyst 15 conld possess a significant amount of protons \vlriclr arc not, solvatcd with water, wlrilc this is not likely to lie the case with 50% II2SO4. Figure 1 sliows tliat tlic rcsiir catalyst, is tleterioratctl bot,li i l l activit,y and selectivity by rcpcatcd uses. I n particulu, the rcsiii catalyst also foriris the N-beiizyl compound from tlic sccond batch. Since tlic simple evacuation would recover its initial conversion and sclcct,ivity, tlic niost probahlc cause of the deterioration is tlie accumulation of water gcncratcd from the reaction itself. 'l'licse analysis and observation suggest that tlic content, of water may lie a critical factor contrihuting to the formation of tlie N-benzyl comporind in hll'c' con(1cwsal~ioiireact,ion.

+

a. Liquid Acid Catalyst 0

II

-

+

n+

OH c

II

0

0

(N-benzyl)

b. Sulfonic acid resin Catalyst u

Fig. 2. I'ioposed reaction mcclianism for inetliyl N-plieiiyl carbaillate condensation i n a acid catalyst ( a ) and over a sulfonic acid resiii(1)).

liqiiitl

501

Tliere have been a number of reports of improved selectivity with sulfonic acid resin catalysts compared wi tli conventional liquid acid catalysts[G-9]. Various explaiiations have also been proposed. If mechanisms usually postulated for condensation reactions with liquid Brdnsted acid [lo] and solid acid catalysts [ll] are adopted, the sequence of steps shown in Fig. 2 could be considered for the coiidcnsatioii of MI'C. Both mechanisms incorporate tlie cxcntial features of known carbenium ion chemistry, i . e . , electrophilic attack on tlie aromatic ring by polar carbcnium ion intcrmcdiatcs. Note that MDU is formed by this attack on tlie benzene ring of MPC, while tlie N-bcnzyl compound by the attack on nitrogen atom. An csscntial characteristic of solid catalysts is tliat acid sites are fixed on solid phase. Ilcncc, ratlicr tlian a free carbenium ion, an intermediate is formed with a hydrogen bond 1)ctweeii a car1)onyl group of IICIIO and a resin -SO3H group [ l l ] . The species is like a clicmisorl~cdspccics on a solid surface [l I ] . The occurrence of hydrogen bonding lias bccn cstablislictl by an iiifrarcd spectroscopy [la]. Subsequcnt steps also take place on t,liis fixed site. Due to a steric hindrance imposed on this fixed acid site and MPC molecule itself, it would have been very difficult that the carbeniuni intermediate attacks the nitrogen atom of RIPC i i i the last step of Fig. 2b to form the N-benzyl compound. Since tliere is no such a sbcric hindrance for a free carlxniuni ion, tliere must exist a better chance of tlic attacks OH iiit,rogcii atom and tlie formation of t,he N-benzyl compound with liquid a.cid catalysts. \Vlieii an excess water is present i n a solid acid system, liowever,it would solvate the -S03H to form -S03-1{30+ or dissociate t,o form a free H30' ion in an extreme case [13]. In surli cases, catalytically active species changes from a fixed proton to a fixed or free II3O . Not only this would reduce tlie rcactioii rate [14], but tlic steric hindrance is also relaxed. Hence, tlic lowcrcd HCNO conversion and the formation of the N-benzyl compound as shown in Fig. 2 for tlie reused resin ca,talyst could be understood.

+

Effect of solvent i n metlivl N-i)lieiiyl condensation over Aniberlvst 15 As mentioned, a solvent is required i n tlie MPC condensation reaction i n order to dissolve MPC and MDU. When 50% IIySO4 is iised as a catalyst, it also serves as a solvent. Siiicc aqiieous 112SO.1foriris a lioinogeiieous phase with MCHO and MPC nielts above 40 O C , tlir rcactioii mixture would form a. homogeneous phase under tlie reaction condition. Ilowevcr, tlic product MDlJ is not dissolved and is obtained as a precipitate. The formation of solid products provides a favorable equilibrium condition, and may be partly responsible for the high yield with tlie 50% &SO4 catalyst. With Amhcrlyst 15, t,lie phase of tlic product should be t,he one easily separablc from the solid catalyst. This deprives us of the advantage in equilibrium by forming solid products. Polar solvents will dissolve tlie product MDU, but would solvate tlie proton and lower the catalyst efficicncy as discussed earlier. Protic solvents are not desirable because tlicy stabilize lICIl0 i n the solution by hydrogen bonding. Hence, it is natural tliat a solvent cannot satisfy a,ll the requirenre!it,s. As 4iowii in Table 3, ethylacetate and toluene aliowetl

502

tile bcst perforinance. Etliylacctatc dissolves MPC and MDU, and provides a lioinogencous solution. Tolucne, especially when used i n a small amount rclativc to MPC, sliowcd a good I-ICI-IO conversion, but provided solid products. Fortunately, the products was fine enough to form an cmulsion phase and allowcd easy separation from tlic catalyst particles. The good conversion with toluene a,gain a,ppears to arise from the favorable equilibrium. It, is iiitercsting to note that more 2,4'-MDU and triiners are formed wif,li toluene than wit,li et,liylaceta.te. Wlien cycloliexane was used as a solvent, llie formation of solid products appeared to be inore rapid, and the reaction stopped before completion due to the coating of tlic catalyst particle by solid products. Results of the prcsent study could be incorporaled into a new process using a fixcd bed rea.ctor paclted w i t h Amberlyst 15 ca.talyst. Ethylacctate could be tlic solvent wliich forms a lioinogencous liquid phase reaction mixture. Tolucnc sliowed the possibility that it could bc cmployctl i n a fixcd bcd operation bccausc tlic product is in an einulsion form easily sepa,rable from thc catalyst. The latter, however, must be vcrilkd i n a continuous reactor. AC:KNOWLEDGEMENT Aut,hors thanlt the Lucky R & D Center for the permission to publish this work. REFERENCES 1 S. Fultuoka,, M. Chono, and h.1. Kolino, Chemt,ech, 11 (1984) 670. 2 S. Fultuolta and R4. Teino, Japanese I
Tlieorctical and Applicd Concepts, Butterworths, Boston, 1987, 11187. R.A. R.einicker a,nd B.C. Gates, AICliE J., 20 (1974) 93. E. Knijzingcr, and 11. Noller, Z. Pliysik Chem. Frankfurt,), 55 (1967) 5'3. R . Tliornt,on and B.C Gates, .I. Catal. 34 (19740 275. V.J. Frilctte, E.B. Mowar, and M.K. Rubin, J . Catii1.J (1964) 25.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

503

ZEOLITES AS BASE CATALYSTS. PREPARATION OF CALCIUM ANTAGONISTS INTERMEDIATES BY CONDENSATION OF BENZALDEHYDE WITH ETHYL ACETOACETATE.

A. Corma", R.M. Martin-Aranda' and F. SAnchez2.

'Institute de Tecnologia Quimica, CSIC-UPV, Universidad Politkcnica d e Valencia, Camino Vera S/N, Valencia, Spain.

21nstituto de Quimica Orginica, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain. ABSTRACT The Condensation of benzaldehyde with ethyl cyanoacetate, ethyl malonate and ethyl acetoacetate were cam'ed out with high rutes and selectiviv promoted by lithium-, sodium, potassium-, and caesiumexchanged X and Y zeolites and on sodium-Gemaniurn substituted faujasite. The 2-nitrobeizzaldeliyde, 3-nitrobenzaldehyde, 2-trifluoromethylbenmldehyde and 2,4dichlorobenzaldehyde were condensed with ethyl acetoacetate to obtain precursors in the production of dihydropyridine derivatives in the presence of these base catalysts operating in a batch reactor at inoderate temperatures. The most active zeolites are more active than pyidine, and less active than piperidine, showing basic sites with pKb < 13.3. The selectivity to the desired condensation product when using zeolites is, at least, as high as in the case of the homogeneous catalyst. INTRODUCTION One of the main features of microcrystalline solids as catalysts is the possibility for inducing high selectivity to a given product as a consequence of the combination of strong electric fields and geometrical constraints. There is, however, another important property of these materials which is related to their power to achieve high concentration of reactants inside of the micropores. Then, the rate of bimolecular and, therefore, their selectivity when competing with monomolecular reactions, are enhanced in microcrystalline with respect to macroporous solid or homogeneous catalysts. Microcrystalline solids such as zeolites and zeolite like structures have shown the utility of those properties in the domain of acid catalysis. However, little is known on their possibilities as base catalysts. It has been shown [ref. 1,2] that zeolites have basic sites which are able to catalyze reactions needing weak and medium basic strengths. Moreover, a correlation between the basicity and the Sanderson's average electronegativity of the framework has been observed [ref. 31. Then, their activity as base catalysts can be modified by changing the countercation [ref. 41, the framework Si/Al ratio, or by introducing atoms other than Si and Al in the framework [ref. 51. Among the different reactions which have been studied on solid bases, Bsomerization of Linear butenes [ref. 61, aldolic condensation [ref. 71, and side chain alkylation in alkylaromatics [ref. 81,

504

have been the most widely used to measure the strength of the basic sites. There is another type of base catalyzed reactions, i.e.,the Knoevenagel condensation, which can also be used to form CC bonds through the reaction of a carbonyl with an activated methylenic group. This type of

reactions, which are of interest to prepare unsaturated esters and nitriles, could be used to “titrate” catalysts with different basicity by changing the pK of the reactant with the activated methylenic group. In this work we have studied the Knoevenagel condensation of benzaldehyde and benzaldehyde derivatives with ethyl cyanoacetate (pK<9), ethyl acetoacetate (pK= 10.7), and ethyl malonate (pK= 13.3). Thus, we have prepared compounds of the type 1, which are intermediates in the preparation of some dihydropyridines of type 2.

1

2

Dihydropyridines chemistry is of interest not only from the point of view of fundamental research on heterocyclic compounds [ref. 91, but specially because of expanding practical applications of 1,4dihydropyridine derivatives as pharmaceuticals in the line of calcium channel blockers [ref. 10,l I] EXPERIMENTAL Materials NaX and NaY zeolites (Union Carbide) were exchanged by lithium, potassium, and caesium in a 1 M solution of the corresponding metal chloride at 80QCfor 60 min, using a liquid to solid ratio of 10. The samples were then filtered and washed free of chlorides. After drying, the zeolite was pelletized, crushed, and sieved to different particle sizes. A N;i-Ge faujasite with

Ge/AI ratio of 1.1, with 5.3 meq.g of Na’, was prepared following the procedure described by Poncelet et al. [ref. 121. ;I

Reaction Procedure An equimolar solution of the two reactants, without any solvent, was kept in a batch reactor in a silicone bath under magnetic stirring, while heating up to reaction temperature (90- 170QC).Then, 1-2 wt% of catalyst was added and the reaction time started. Samples were taken periodically and the evolution of t h e reaction between 1 and 480 min was followed by gas-liquid chromatography on

505

TABLE 1.- Chemical analysis of the zeolite catalysts (mmol per 100 g of hydrated catalyst).

ZEOLITE

Li

Na

ZYLi ZYNa ZYK ZYCS ZXLi ZXNa ZXK zxcs

353 -

185

139 3 17 30 74 149 472 86 174

-

K

cs

-

-

275 350

-

200

-

19 3

TOTAL 324 3 17 305 274 502 472 436 367

a 60 m phenylsilicone capillary column. The ’H-NMR spectra of the products were obtained on a 90 MHz spectrometer using chloroform as solvent and tetramethylsilane as reference.

RESULTS AND DISCUSSION Influence of the cation and framework silicon-to-aluminium ratio on activitv and selectivitv. Previously to any kinetic experiment, we have studied the influence of external and internal diffusion by changing the stirring speed and the particle size respectively. It has been found that for more than 1000 r.p.m. and particle size < 0.25 mm the reaction is not controlled by either external or internal diffusion. We have studied the condensation of benzaldehyde (14 mmol) with ethyl cyanoacetate (14 mmol) in presence of lithium-, sodium, potassium-.and caesium- exchanged X and Y zeolites (0.03 I g) at 90, 120 and 140QC.As an example, the results obtained at 120QCare given in Figure 1 (left).

The results confirm that the order of reactivity is L i < N a < K < C s and for all cations zeolite-\( < zeolite-X. The same order of activity was also observed at 90 and 140QC.Then, the activity of the zeolites for these condensation reactions, increases when decreasing the framework Si/AI ratio

(X= 1.2 and Y=2.4), and increasing the radius of the countercation (Li < Na < K < Cs). In other words, the activity increases when the calculated average negative charge on the oxygen atom, i.e. the basicity, increases [ref. 2,131. The same order of basicity has been found by using pyrrole adsorption [ref. 3,4]. On the other hand, the substitution of Si by G e into framework during the synthesis results in a strong increase in activity with respect to the analogous silicon faujasite (Fig. 1, right), which would indicate a stronger basicity of the Ge faujasite. It is known that the basicity of the zeolites increases when increasing the negative charge on the framework oxygen atoms, and higher when lower is the average Sanderson’s electronegativity [ref. 2-4,161. However, in the case of the Ce and Si faujasite the average Sanderson’s electronegativity

506

are 3.28 and 3.25 respectively. Therefore, we can not explain the activity differences on the bases of the calculated electronegativities and, other factors, such as the differences in the T-0-T bond angles, may he determinant in this case. With practically all the catalysts, the conversion stabilizes at a relatively low value. This could be due to poisoning of the catalyst by bigger molecules formed during the consecutive Michael's reactions. To check the poisoning, a sample of used catalyst was reused again and much lower activity than that of the fresh catalyst was observed. no

c

50

-

v

40

-

A

H

0

N.l

~

I

0

o

20

40

80

no

iao

120

1.0

TIME (min)

o

20

10

80

80

tau

120

14"

TIME (min)

Fig.l.Condensation of benzaldehyde (14 mmol) and ethyl cyanoacetate (14 mmol) using X and Y zeolites (0.031 g) at 120T. (left) and NaGeX and NaSiX faujasitcs (0.031 g) at 140% (right).

In order to compare the catalytic behaviour of zeolites with organic bases in homogeneous

media, the reactions have also been carried out on two nitrogen bases currently used for catalyzing this type of reactions; pyridine (pKb=8.8) and piperidine (pKh= 11.12). The amount of these liquid bases used was 1.6 meq, which is the same as the number of cations present in the zeolite in similar experiments.

TABLE 2.- Comparative activity of pyridine and piperidine as catalyst and X Zeolite for the condensation of benzaldehyde and ethyl cyanoacetate. CATALYST

T(PC)

PYRIDINE

90

ZXNa

90

zxcs

90

PIPERIDINE

50

'.

TIME (min)

CONVERS ION^ ( % )

5 60 120 5 60 120

2.2 10 16.5 2.7 17.5 26.3

5 60 120

3.2 23 30.5

5 15

Yield of condensation product.

80 92

507

The results in Table 2 show that the pyridine is less active than any of the X zeolites and Ge faujasite except the lithium form which shows slightly lower activity, whereas all Y zeolites show lower activity than pyridine. Piperidine, however, is more active than any of the zeolite samples studied here. From this comparison, it appears that, most of the basic sites of the zeolites must have pK< 10.3. However, the fact that zeolites are also active for catalyzing the condensation of benzaldehyde with ethyl malonate, indicate that these samples have some basic sites with pK< 13.3. On a quantitative bases, and comparing the activity of zeolites for condensation with ethyl cyanoacetate, ethyl acetoacetate and ethyl malonate (Fig. 2), we can conclude that most of the basic sites of the zeolite have pKc9.0 with a sensible amount with 9.0
0

20

40

80

80

100

I20

140

0

20

TIME lmm)

,o

I0

60

IW

120

TIME ( m t n )

Fig. 2. Condensation of equimolar amounts of benzaldehyde and ethyl qanoacetate, ethyl acetoacetate and ethyl malonate at 140% using 1 %wt of zeolite. NaGeX (left) and ZXCs (right).

TABLE 3.- Piperidine and NaGeX catalyzed condensation of benzaldehyde and ethyl acetoacetate.

CATALYST

5 ' H6

NaGeX a

T(*C)

TIME (min)

CONV.a ( % ) OTHERS PROD.(%)

165

15 30 60

49 46 40

165

15 30 60

30 36 62

Product of Knoevenagel condensation

4 8 14

508

From the point of view of selectivity, results from Table 3, clearly show that zeolites are much more selective catalysts, without side reactions, for the desired condensation reaction with ethyl acetoacetate than either pyridine or piperidine. Preparation of calcium antaeonist intermediates. For the preparation of 4-aryl-1,4-dihydropyridinederivatives, there are two general approaches, shown in the Scheme I, which involve variants of the Hantzsch reaction [ref. 14,151. The route a where an alkyl aminocrotonate 3 was condensed with a substituted benzaldehyde 4 and an alkyl acetoacetate 5 at reflux in a suitable solvent (i.e. methanol, ethanol), to give the

corresponding 1,4-dihydropyridine 2 in c.a. 35% yields. Whereas the route b involves the reaction of an alkyl benzylideneacetoacetate 1 with an alkyl R-aminocrotonate 3 appears to afford

consistently better yields (34-71 96) and easier purification was generally used [ref. 161.

/

H

2

1

3

Scheme 1.

Based on the Knoevenagel reactions discussed above, we have studied the condensation of a series of benzaldehydes 4a-e with ethyl acetoacetate on a series of X, Y and sodium germanium faujasites.

509

I

Leo1

la-e

4a-e

Scheme 2.

Rl

R2

R3

a b

H

H H

C

H c1 CF3

H H H c1 H

d e

NO2

NO2

H H

CONVERSION ( % ) 66

50 55 56

25

The reaction was carried out in a batch reactor at 100 and 140QC.An equimolar solution of the two reactants (7 mmol), without solvent, was kept under stirring while heating up to reaction temperature. Then, 2 wt% of zeolite catalyst (0.0334 g) was added and the reaction started. The results obtained for the condensation of ethyl acetoacetate and the five benzaldehydes derivatives at 140°C on Na-Ge faujasite are given in scheme 2. High conversions with practically 100% selectivity are obtained with these catalysts. The order of activity of the zeolite samples is the same seen previously using benzaldehyde (Fig. 3).

Fig. 3. Condensationof 2-nilrobenzaldehyde (7 mmol) and ethyl acetoacetate (7 mmol) at 1 4 K using 0.0334 g) of zeolite.

As it was done before, the reactions have also comparatively been carried out on two bases currently used for catalyzing this industrial reaction; pyridine and piperidine. With all reactants it has been observed that, the most active zeolites are more active than pyridine, and less active than piperidine. The selectivity to desired condensation product when using zeolites is, at least, as high as in the case of the nitrogen bases. Some members of this family of 1,4-dihydropyridines [ref. 17,18,19] (nifedipine, nitrendipine, nicardipine etc...) have already passed through clinical trials and are now marketed in many countries for different therapeutic uses. In this paper we have developed a method to prepare the intermediates of type 1 with solid base catalysts it introduces advantages for industrial manufacture, such as an easily separation and regeneration of t h e catalyst and a more facile work-up and purification of products. CONCLUSIONS

X and Y zeolites exchanged with alkali metal cations are active and selective catalysts for the condensation of benzaldehyde derivatives with methylenic groups. The activity of the zeolites increases with decreasing the framework Si/AI ratio of the zeolite and, increasing the radius of the countercation, i.e., the activity increases when decreasing the average electronegativity of the framework. The suhstitution of Si by G e in the framework of a X type faujasite strongly increases the activity of the zeolite for these hase catalyzed reactions. The activity of the Na-Ge faujasite is higher than pyridine and lower than piperidine, indicating that the most of the basic sites of the NaG e X zeolite have a pKb 11.2. Nevertheless sites with strength in order of pKb= 13.3 must exist, since zeolite catalysts are able to abstract protons from ethyl malonate. Therefore, most of the basic sites in alkaline X and Y zeolites have pKb< 10.3 and, only a few sites with pKh< 13 are present in the more hasic CsX zeolite, from the point of view of their proton-ahstraction ability.

These zeolites are active and selective catalysts for the preparation of intermediates in the manufacturing of dihydropyridines. REFERENCES 1. D. Barthomeuf and A. de Mallman, P.J. Grobet et al, ed., Innovation in Zeolite Material Science, Amsterdam (1988).

2. D. Barthomeuf, G. Coudurier and J.C. Vedrine, Materials Chemistry and Phys., 18 (1988) 553.

3. D. Barthomeuf, J. Phys. Chem., 88 (1984) 42. 4. A. Corma, R.M. Martin-Aranda, H. Garcia and J. Primo, Applied Catal., 59 (1990) 237. 5. A. Corma, R.M. Martin-Aranda and F. SBnchez, J. Catal.. in press. 6 . L.R.M. Martens, P.J. Grobet, W.J.M. Vermein and P.A. Jacobs, Stud. Surf. Sci. Catal. 28 (1986)

935. 7. 0.1. Kuznetsov, G.M. Panchenkov, A.M. Guseinov and T.A. Chasova, Nefteperab. Neftekhim. (Moscow), 3 (1973) 28.

511

8. P.E. Hathaway and M.E. Davis, J. Catal., 116 (1989) 263, 279. 9. J. Kuthan and A. Kurfurst, Ind. Eng. Chem. Prod. Res. Dev., 21, (1982) 191 10. A. Schwartz, D.J. Triggle, Annual Repin Med. Chem., 35 (1984) 325. 11. T. Godfraind, R. Miller and M. Wibo, Pharmacol. Rev., 38 (1986) 321. 12. G. Poncelet and M.L. Dubru, J. Catal., 52 (1978) 321. 13. W.J. Mortier. J. Catal., 55 (1978) 138. 14. F. Bossert, H. Meyer and E. Wehinger, Angew. Chem. Int. Ed. Engl., 20 (1981) 762. 15. J. Prous, P. Blancafort, J. Castaner, M. Serradell and N. Mealy, Drugs Future 6 (1981) 427.

16. G. Marciniak, A. Delgado, G. Leclerc, J. Velly, N. Decker and J. Schwartz, J. Med. Chem., 32 (1989) 1402. 17. J.R. Lawrence, Drugs of Today, 22 (1986) 449. 18. R. Mannhold, Drugs of Today, 20 (1984) 69. 19. R.A. Janis and D.J. T r i d e , J. Med. Chem., 26 (1983) 775.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors),Heterogeneous Catalysis andFine C!hern~cakI1 0 1991 Elsevier Science Publishers B.V., Amsterdam

513

MECHANISM OF PHENYLACETATE TRANSFORMATION ON ZEOLITES

Y. POUILLOUX1, J-P. BODIBOl, I . N E V E S l , M. GUBELMANN2, G. PEROTl and M. GU IS N E T ~ lURA CNRS 350, C a t a l y s e en Chimie Organique, U n i v e r s i t e de P o i t i e r s , avenue du Recteur Pineau, 86022 P o i t i e r s Cedex, France 2Rhdne-Poulenc Recherches S a i n t - F o n s Cedex, France

-

40,

Centre de Recherches des C a r r i e r e s , BP 62, 69192

SUMMARY The t r a n s f o r m a t i o n o f p h e n y l a c e t a t e was c a r r i e d o u t o v e r two z e o l i t e s : HY ( S i / A l = 20) and HZSM5 ( S i / A l = 40) a t 400 C under atmospheric p r e s s u r e . The r e a c t i v i t i e s o f t h e p r o d u c t s and i n t e r m e d i a t e s were a l s o s t u d i e d under t h e same c o n d i t i o n s . On b o t h c a t a l y s t s t h e main p r o d u c t was phenol which means t h a t ketene was a l s o formed. HY l e d m a i n l y t o ortho-hydroxyacetophenone w h i l e HZSM5 gave b o t h o r t h o - and para-hydroxyacetophenones i n equal amounts. 2-Methylchromone and 4-methyl coumarine were formed on HY o n l y and supposedly r e s u l t e d f r o m an i n t r a m o l e c u l a r condensation o f ortho-acetoxyacetophenone which was n o t observed among t h e p r o d u c t s . On HZSM5 d e h y d r o a c e t i c a c i d and 4 acetoxy 6-methyl 2-pyrone were formed presumably t h r o u g h autocondensat i o n o f ketene. Para-acetoxyacetophenone was observed on b o t h c a t a l y s t s . Water had p r a c t i c a l l y no e f f e c t on t h e a c t i v i t y and s e l e c t i v i t y o f HY w h i l e i t i n c r e a s e d t h e s t a b i l i t y o f HZSM5 markedly. INTRODUCTION As r e p o r t e d i n t h e 1 i t e r a t u r e , t h e a c y l a t i o n o f a r o m a t i c hydrocarbons can be c a r r i e d o u t by u s i n g z e o l i t e s as c a t a l y s t s and c a r b o x y l i c a c i d s o r a c y l c h l o r i d e s as a c y l a t i n g agents.

Thus t o l u e n e can be a c y l a t e d b y c a r b o x y l i c

a c i d s i n t h e l i q u i d phase i n t h e presence o f c a t i o n exchanged Y - z e o l i t e s ( r e f . 1 ) . The a c y l a t i o n o f phenol o r phenol d e r i v a t i v e s i s a l s o r e p o r t e d . The a c y l a t i o n o f a n i s o l e b y c a r b o x y l i c a c i d s and a c y l c h l o r i d e s was o b t a i n e d i n t h e presence o f v a r i o u s z e o l i t e s i n t h e l i q u i d phase ( r e f . 2). The a c y l a t i o n o f phenol by a c e t i c a c i d was a l s o c a r r i e d o u t w i t h s i l i c a l i t e ( r e f . 3 ) o r HZSM5 ( r e f . 4 ) . The p a r a isomer has been g e n e r a l l y f a v o u r e d e x c e p t i n t h e l a t t e r case i n which ortho-hydroxyacetophenone was o b t a i n e d p r e f e r e n t i a l l y . One p o s s i b l e e x p l a n a t i o n f o r t h e h i g h o r t h o - s e l e c t i v i t y i n t h e case o f t h e acylation of intermediate

phenol by a c e t i c a c i d i s t h a t p h e n y l a c e t a t e c o u l d be an from which ortho-hydroxyacetophenone would be formed

i n t ramol e c u l a r l y .

514

Actually,

another

method

for

obtaining

hydroxyacetophenones

is

the

rearrangement o f p h e n y l a c e t a t e ( F r i e s rearrangement, scheme 1). ,OH

0

OCOCH3

+

@

COCH3

+ OH G

C O C H ,

Scheme 1 - F r i e s rearrangement. In

this

case

hydroxyacetophenones

are

supposed

to

be

formed

i n t r a m o l e c u l a r l y f r o m t h e s u b s t r a t e a l t h o u g h t h i s has n o t y e t been c l e a r l y e s t a b l i s h e d ( r e f . 5 ) . T h e r e f o r e i t i s most i m p o r t a n t t o i d e n t i f y t h e p r i m a r y p r o d u c t s o f t h e F r i e s rearrangement and t o i n v e s t i g a t e t h e i r mechanism o f formation.

I n a p r e l i m i n a r y s t u d y it was shown t h a t t h e n a t u r e o f t h e

p r o d u c t s as we1 1 as t h e ortho-/para-hydroxyacetophenone m o l a r r a t i o depended on t h e c a t a l y s t ( r e f . 6 ) .

Y z e o l i t e gave more o r l e s s t h e same p r o d u c t

as f l u o r i n a t e d alumina w i t h a h i g h p r o p o r t i o n o f o r t h o hydroxyacetophenone w h i l e HZSM5 gave a h i g h e r p r o p o r t i o n o f t h e p a r a isomer.

distribution

T h i s was a t t r i b u t e d t o shape s e l e c t i v i t y . We r e p o r t h e r e a d e t a i l e d s t u d y o f t h e t r a n s f o r m a t i o n o f p h e n y l a c e t a t e

o v e r HY and HZSM5 z e o l i t e s . A r e a c t i o n scheme i s proposed i n b o t h cases. EXPERIMENTAL Catal v s t s

HY

zeolite

framework S i / A l

( N a 0 . 2 H 1 1 . 8 A i 1 * . 0 s i 1 8 ~ . ~ o ~ ~ was ~) supplied

by

ZEOCAT.

Its

atomic r a t i o was deduced f r o m t h e u n i t c e l l parameter u s i n g

t h e e q u a t i o n proposed b y Breck and F l a n i g e n ( r e f . 7 ) . HZSM5 (H2.2Al2.2Si93.80192) was s y n t h e t i z e d a c c o r d i n g procedure ( r e f . 8).

to

the

Mobil

Both z e o l i t e s were c a l c i n e d a t 500'C f o r 12 hours under a d r y a i r - s t r e a m t h e n t r e a t e d a t 500'C f o r 12 h o u r s i n d r y n i t r o g e n j u s t b e f o r e use. Procedure The t r a n s f o r m a t i o n o f p h e n y l a c e t a t e was c a r r i e d o u t i n a f l o w r e a c t o r a t under atmospheric p r e s s u r e . The s u b s t r a t e ( A l d r i c h Chimie) was d i l u t e d i n n i t r o g e n ( 0 . 9 atm.). The p r o d u c t s were c o l l e c t e d i n an i c e t r a p and

400'C

analyzed by gas chromatography ( I n t e r s m a t I G C 16 equipped w i t h a CP S i l 5 c a p i l l a r y fused s i l i c a column).

515

RESULTS AND DISCUSSION Transformat ion of ahenvl acetate (i) Activitv-Aseinq. As can be seen from Fig. 1, the activity versus time on stream of both catalysts decreases very rapidly during the first two hours of the reaction. This is due to a deposit o f "coke" on the zeolite. A preliminary study of the deactivation process was carried out on HY. As can be seen from Fig. 2 a significant amount of "coke" (= 20 wt %) builds up in a few hours. The process by which "coke" is deposited is very fast during the first five minutes of the reaction. Actually during this time, the ratio between the rate of coke formation and the rate of transformation of phenylacetate into desorbed products is of about 0.1.

n

N

v

n

N

C

.-

U

0 Lo

0

L

1y

P)

0

> c

0

0

0

100 200 300 400 5b0 Time on stream (mn)

Fig. l.Transformat ion o f phenyl acetate (400 C, 1 atm. versus time-on-stream on HY ( U ) and HZSM5 (u).

0'

200 400 600 8 )O Time on stream (mn)

Fig.2. Transfoyation of phenylacetate on HY (400 C, 1 atm.). Amount of coke deposited versus time-onstream.

The coke composition was determined after 6 hours' reaction on a 0.5 g sample of HY-zeolite. The sample contained about 20 wt % "coke". However only 15-20 % of the total amount of coke could be recovered by extraction through methylene chloride after dissolution of the zeolite by using the procedure described by Magnoux et al. (ref. 9). This shows that the coke is highly polyaromatic in nature. The main compounds which could be identified by GC-MS analysis of the soluble coke are listed in table 1. As we shall see some of

516

them are found among the reaction products in the gas phase. These compounds are probably blocked inside the crystallites by pore plugging. TABLE 1

Compounds recovered after dissolution of a deactivated HY sample. Examples of poss i bl e structures

General formula

1
a) CnH2n-601 CnH2n-8023 CnH2n-1003 m/e

=

94

m/e

=

m/e

=

108,122,136,150

146,160,174,188,202,216,230

i i ) Product distribution. In addition to phenol, the main products which

are formed in variable quantities according to the catalysts used, are listed below : -

I and I 1

- 111 :

: 0-

and p-hydroxyacetophenone :

p-acetoxyacetophenone :

CH3CO

a

ococH3

-

I V and V : 2-methylchromone and 4-methylcoumarine :

-

V I : dehydroacetic acid :

-

VII

0

q' 0

: 4-acetoxy 6-methyl 2-pyrone :

517

As can be seen i n F i g s . 3a and 3b phenol i s c l e a r l y a p r i m a r y p r o d u c t on b o t h c a t a l y s t s . On HY a l l t h e o t h e r compounds seem t o b e secondary p r o d u c t s except t h e hydroxyacetophenones which c o u l d be formed a t l e a s t p a r t l y as p r i m a r y p r o d u c t s . Compounds 111, I V and V a r e formed o n l y a t h i g h c o n v e r s i o n . On HZSM5, hydroxyacetophenones a r e d e f i n i t e l y secondary p r o d u c t s w h i l e p a r a acetoxyacetophenone i s a p r i m a r y p r o d u c t . Compounds I V and V a r e n o t formed i n d e t e c t a b l e amounts. Compounds V I and V I I a r e formed i n small q u a n t i t i e s .

12

60

3,a

10

50

2,5

b

n

6

8

n

0 t50

K v

6

Q 0

I 4 2

10

0'

40

20

60

~

Conversion ( X )

P Conversion ( X )

F i g . 3. T r a n s f o r m a t i o n o f p h e n y l a c e t a t e on HY ( a ) and HZSM5 ( b ) (4OO0C, 1 atm.). Product d i s t r i b u t i o n versus c o n v e r s i o n . Phenol ( o ) ; h y d r o x y a c e t o phenones ( m ) ; para-acetoxyacetophenone ( n ) ; compounds I V and V ( + ) ; compounds V I and V I I ( x ) .

As

i n d i c a t e d i n a p r e l i m i n a r y s t u d y ( r e f . 6 ) compounds I V and V a r e

probably

formed

through

the

intramolecular

transformation

of

ortho-

acetoxyacetophenone. The f o r m a t i o n o f I V and V i m p l i e s t h e e n o l i z a t i o n o f t h e acetyl

and

of

the

acetoxy

groups

respectively.

On

HY

ortho-

acetoxyacetophenone would be transformed q u a n t i t a t i v e l y i n t o I V and V . fact

that

these

compounds

are

not

formed

on

HZSMS means

that

The

ortho-

acetoxyacetophenone i s n o t e i t h e r , o r t h a t t h e p r o d u c t s cannot desorb because o f s t e r i c constraints.

518

The presence o f phenol i n l a r g e amounts among t h e p r o d u c t s shows t h a t it does n o t r e s u l t o n l y f r o m t h e d i s p r o p o r t i o n a t i o n o f p h e n y l a c e t a t e (scheme 2 ) . Indeed

this

reaction

acetoxyacetophenones.

should

Actually

lead

to

equal

amounts

of

t h e phenol/( acetoxyacetophenone

phenol t

and

I V t V)

molar r a t i o i s much g r e a t e r t h a n one (between 30 and 80). T h i s means t h a t p h e n y l a c e t a t e decomposes e i t h e r i n t o phenol p l u s a c e t i c a c i d ( w h i c h would r e q u i r e m o i s t u r e ) o r more l i k e l y i n t o phenol p l u s ketene.

OCOCH3

PH

2@-@+

Scheme 2 - P h e n y l a c e t a t e d i s p r o p o r t i o n a t i o n . Compounds V I and V I I which a r e formed o n l y on HZSM5 r e s u l t p r o b a b l y f r o m on autocondensation o f a c e t i c a c i d o r k e t e n e ( r e f . 10). T h i s r e a c t i o n c o u l d be f a v o u r e d by t h e p o r e s t r u c t u r e o f t h e z e o l i t e . I f we examine t h e f o r m a t i o n o f hydroxyacetophenones more c l o s e l y , we can see ( F i g . 4a) t h a t on HY, a t l e a s t p a r t o f ortho-hydroxyacetophenone c o u l d be formed d i r e c t l y f r o m p h e n y l a c e t a t e ( i.e.

i n t r a m o l e c u l a r l y ) whereas t h e p a r a -

isomer i s c l e a r l y a secondary p r o d u c t . On HZSM5 b o t h compounds a r e secondary products (Fig.

4 b ) . Moreover,

one can see t h a t t h e o r t h o - / p a r a - h y d r o x y a c e -

tophenone molar r a t i o changes w i t h c o n v e r s i o n e s p e c i a l l y o v e r HZSM5 where it decreases f r o m 6 t o 1 as c o n v e r s i o n decreases.

Two e x p l a n a t i o n s

can be

c o n s i d e r e d : i ) a c o n s e c u t i v e t r a n s f o r m a t i o n o f para-hydroxyacetophenone which would n o t occur i n t h e case o f t h e o r t h o - i s o m e r ; i i ) a change i n t h e ortho-/para-selectivity points

at

high

o f t h e z e o l i t e i n t h e c o u r s e o f d e a c t i v a t i o n . The

conversion

being

obtained

on

the

fresh

catalyst,

a

preferential deactivation o f the sites located outside o f the p a r t i c l e s w i l l decrease t h e ortho-/para-hydroxyacetophenone m o l a r r a t i o i f one supposes t h a t t h e s e s i t e s which a r e e a s i l y a c c e s s i b l e f a v o u r t h e f o r m a t i o n o f t h e o r t h o isomer

.

519

12

a

Conversion

(X)

Conversion ( X )

F i g . 4 . T rans f o r m a t i o n o f p h e n y l a c e t a t e (4OO0C, 1 atm.). D i s t r i b u t i o n o f o r t h o - ( o)Oand p a r a - ( ) hydroxyacetophenones on HY ( a ) and HZSM5 ( b ) .

: The t r a n s f o r m a t i o n s o f hydroxyacetophenones and o f ort ho-acet oxyacet ophenone were s t u d i e d i n s o l u t i o n i n p h e n ylacet at e (10 mol %) under t h e same c o n d i t i o n s as t h e t r a n s f o r m a t i o n o f p h e n y l acet at e. The e f f e c t o f w a t e r on t h e t r a n s f o r m a t i o n o f p h e n y l a c e t a t e was a l s o s t u d i e d . (i)

Hvdroxvacetophenones.

Under

the

reaction

conditions

ortho-

hydroxyacetophenone was n o t t r a n s f o r m e d t o a n o t i c e a b l e e x t e n t on e i t h e r o f b o t h c a t a l y s t s . However about 20 % and 35 % para-hydroxyacetophenone was c onv ert ed o v er HY and HZSM5 r e s p e c t i v e l y i n t o para-acetoxyacetophenone. As i n d i c a t e d above,

t h i s may e x p l a i n a t l e a s t p a r t l y t h e

increase o f t h e

ortho/para-hydroxyacetophenone m o l a r r a t i o a t h i g h conversion. ( i i ) Orthoacetoxvacetoohenone. On b o t h c a t a l y s t s about 20 % o r t h o acetoxyacetophenone was c o n v e r t e d m a i n l y i n t o ortho-hydroxyacetophenone. On HY, compounds I V and V were a l s o formed. ( i i i ) E f f e c t o f water. Water has

p r a c t i c a l l y no e f f e c t on t h e t r a n s f o r m a t i o n o f p h e n y l a c e t a t e o v e r HY. However, it has a pronounced e f f e c t on HZSM5. The c a t a l y s t i s much more s t a b l e . A f t e r 5 hours' r e a c t i o n , t h e c onv ers io n o f p h e n y l a c e t a t e i s s t a b i l i z e d a t about 62 % i n t h e presence o f w at e r i n s t e a d o f a t about 15 % i n i t s absence. Moreover t h e p r o d u c t i o n o f hydroxyacetophenones i n c r e a s e s markedly ( m u l t i p l i e d by = 10). An i n t e r e s t i n g f e a t u r e i s t h a t t h e o r t h o - / p a r a - s e l e c t i v i t y changes w i t h time-on-stream. As i n t h e absence o f water,

i t passes f r o m a value o f about 6 t o a v a l u e of 1

520

a f t e r s t a b i l i z a t i o n . As a l r e a d y mentioned, t h i s i s p r o b a b l y due i n p a r t t o a p r o g r e s s i v e p o i s o n i n g o f t h e e x t e r n a l s u r f a c e o f t h e c r y s t a l l i t e s . On t h e o t h e r hand, t h e p r o d u c t s (VI and VII) r e s u l t i n g f r o m t h e t r a n s f o r m a t i o n o f ketene d i s a p p e a r and a c e t i c a c i d i s formed i n t h e expected amount (= 1 mol p e r mol o f phenol produced). T h i s suggests t h a t compounds VI and VII a r e probably responsible t o a l a r g e extent f o r t h e d e a c t i v a t i o n o f t h e z e o l i t e i n t h e absence o f water.

V I + VII

F i g 5 : R e a c t i o n scheme o f p h e n y l a c e t a t e t r a n s f o r m a t i o n on z e o l i t e s

CONCLUSIONS A l l t h e r e s u l t s can be summarized b y t h e r e a c t i o n scheme shown i n F i g . 5. On HY,

p h e n y l a c e t a t e d i s s o c i a t e s i n t o phenol and k e t e n e ( r e a c t i o n a ) . Ortho-hydroxyacetophenone i s produced p a r t l y b y t h e F r i e s rearrangement o f phenylacetate (intramolecular reaction,

r e a c t i o n b ) and by t r a n s - a c y l a t i o n

( r e a c t i o n c ) w h i l e para-hydroxyacetophenone i s e x c l u s i v e l y t h e r e s u l t o f t r a n s - a c y l a t i o n ( r e a c t i o n d).

P h e n y l a c e t a t e can a l s o d i s p r o p o r t i o n a t e i n t o

phenol and acetoxyacetophenones ( r e a c t i o n e)

.

Para-acetoxyacetophenone can

a1 so be formed t h r o u g h t r a n s e s t e r i f i c a t i o n between para-hydroxyacetophenone and p h e n y l a c e t a t e ( r e a c t i o n f ) .The f o r m a t i o n o f secondary p r o d u c t s 1 i k e 2 methylchromone and 4-methylcoumarine

i s consecutive t o t h e formation o f

521

Hi

-si -7

’

+

0‘Ai-

I

\\

1) FRIES (intramolecular)

2) - H+

J.

16“;

Fi ‘0

0-

aceiophenone

J( ketene auto condensation

Scheme 3 : Reaction mechanism on protonic z e o l i t e s

522

ortho-acetoxyacetophenone which does n o t appear among t h e p r o d u c t s ( r e a c t i o n 9) * On

HZSM5

both

hydroxyacetophenones

are

formed

by

trans-acylation.

D i s p r o p o r t i o n a t i o n ( r e a c t i o n e ) p r o b a b l y does n o t e x i s t because o f s t e r i c contraints.

phenylacetate

(probably

acetoxyacetophenone, formation

s i n c e ortho-hydroxyacetophenone does

Moreover

of

for

the

same

reason)

to

not

react with

give

ortho-

r e a c t i o n g cannot t a k e p l a c e . On t h e o t h e r hand, t h e

products

(dehydroacetic acid,

resulting

6-methyl

from

the

oligomerization

4 - a c e t o x y 2-pyrone,

of

ketene

r e a c t i o n h ) i s favoured

presumably because o f t h e confinement e f f e c t i n t h e z e o l i t e . These compounds a r e supposed t o be t o a l a r g e e x t e n t r e s p o n s i b l e f o r t h e d e a c t i v a t i o n o f HZSM5. The mechanism o f p h e n y l a c e t a t e t r a n s f o r m a t i o n can be d e s c r i b e d as shown i n scheme 3. P h e n y l a c e t a t e adsorbs on a p r o t o n i c c e n t e r and t h e n can e i t h e r rearrange phenol.

into The

ortho-hydroxyacetophenone

adsorbed

acylium

i o n can

(Fries then

rearrangement)

react

with

phenol

or

desorb ( i f the

c o n v e r s i o n i s h i g h enough) t o g i v e o r t h o - and para-hydroxyacetophenone which w i l l t h e n appear as secondary p r o d u c t s . I t can a l s o r e a c t w i t h p h e n y l a c e t a t e t o g i v e o r t h o - and para-acetoxyacetophenones o r r e a c t w i t h k e t e n e t o g i v e t h e condensat i o n p r o d u c t s .

REFERENCES 1 6. Chiche, A . F i n i e l s , C . G a u t h i e r and P . Geneste, J . Org. Chem. 51 (1986) 2128. 2 A. Corma, M.J. C l i m e n t , H. G a r c i a and J. Primo, A p p l . C a t a l . 49 (1989) 109. 3 U.S. Pat. 4 652 683 (1987) t o I. N i c o l a u and A. A g u i l o . 4 U.S. Pat. 4 668 826 (1987) t o B.B.G. Gupta. 5 P.B. Venuto and S . L a n d i s , Adv. C a t a l . 18 (1968) 259. 6 Y . P o u i l l o u x , N . S . Gnep, P. Magnoux and G. PCrot, J . Mol. C a t a l . 40 (1967) 231. 7 D.W. Breck and E.M. F l a n i g e n , M o l e c u l a r Sieves, S o c i e t y o f Chemical I n d u s t r y , London, 1968, p. 47. 8 U.S. Pat. 3 702 886 (1972) t o R.J. Argauer and G . R . L a n d o l t . 9 P. Magnoux, P. Roger, C . Canaff, V . Fouche, N.S. Gnep and M. G u i s n e t , i n : B. Delmon and G.F.Froment (Eds.), C a t a l y s t D e a c t i v a t i o n , Stud. S u r f . S c i . C a t a l . , V o l . 34, E l s e v i e r , Amsterdam, 1987, p. 317. 10 A.B. S t e e l e , A.B. Boese and M.F. D u l l , J. Org. Chem. 14 (1949) 460.

M. Guisnet et al. (Editors ), Heterogeneous Catalysis and Fine Chemicals II 0 1991 Elsevier Science Publishers B.V., Amsterdam

523

ORTHOSELECTIVE ALKYLATION OF 2-ETHYL A N I L I N E WITH METHANOL ON FERRIC O X I D E CATALYSTS

J . VALYON, R . M . M I H A L Y I a n d D. KALLO C e n t r a l R e s e a r c h I n s t i t u t e f o r C h e m i s t r y o f t h e H u n g a r i a n Academy o f S c i e n c e s , P.0.Box 1 7 , H - 1 5 2 5 B u d a p e s t ( H u n g a r y ) ABSTRACT Vapour phase m e t h y l a t i o n o f 2 - e t h y l a n i l i n e ( 2 - E t - A ) w i t h m e t h a n o l o v e r h e m a t i t e ( d - F e 0 ) a n d d - F e 0 c o n t a i n i n g 3,5 a n d 8 w t % GeO was s t u d i e d . T h e & a ? n r e a c t i o n 2 p $ o d u c t s a r e 2 - e t h y l - 6 methyl a n f l i n e (2-Et-6-Me-A) , N-methyl-2-ethyl a n i l i n e (N-Me-2-EtA) a n d N - m e t h y l e n e - 2 - e t h y l a n i l i n e ( N - M e e n - 2 - E t - A ) . Germanium promoted c a t a l y s t s a r e more a c t i v e , s t a h l e and s e l e c t i v e i n o r t h o methylation than pured-Fe 0 T h e c r y s t a l l i n e p h a s e s d e t e c t e d were O n l y L$w?s a c i d s i t e s w e r e o b s e r v e d o n t h e d-Fe 0 and&-GeO c a t a ? y $ t s . D e p o s i ? ; o n o f GeO d o e s n o t c h a n g e t h e a c i d i t y o f t h e d -Fe 0 however, i t decreages t h e h y d r o x y l c o n c e n t r a t i o n and t h e o x i d i G i 8 ; p o w e r o f t h e h e m a t i t e s u r f a c e . D a t a s u g g e s t t h a t germanium promoted c a t a l y s t s a r e l e s s s u s c e p t i b l e t o p o i s o n i n g than h e m a t i t e .

.

INTRODUCTION

The i n c r e a s i n g i n d u s t r i a l demand f o r a l k y l a t e d a r o m a t i c a m i n e s i n i t i a t e d research t o develop heterogeneous c a t a l y t i c process f o r t h e a l k y l a t i o n o f a n i l i n e and a l k y l a n i l i n e s . Some z e o l i t i c a n d n o n - z e o l i t i c m o l e c u l a r s i e v e c a t a l y s t s a r e c l a i m e d t o be c a p a b l e f o r o r t h o - a n d p a r a - s e l e c t i v e a1 k y l a t i o n u s i n g o l e f i n as a l k y l a t i n g a g e n t ( r e f s .

1,Z).

Zeolite catalysts

a r e l e s s a c t i v e and s e l e c t i v e i n t h e m e t h y l a t i o n o f a n i l i n e by methanol ( r e f s .

3,4).

Reaction i s usually c a r r i e d out w i t h a large

e x c e s s o f m e t h a n o l s i n c e a l a r g e f r a c t i o n o f t h e a l c o h o l decomposes without participating i n the alkylation.

Numerous N - a n d C-alkylated

a n i l i n e d e r i v a t i v e s appear i n t h e r e a c t i o n product.

I t was f o u n d

t h a t N-alkylation requires basic s i t e s while C-alkylation occurs m a i n l y on a c i d i c s i t e s ( r e f s . 5 - 7 ) . Various m e t a l l i c o x i d e s a r e of s a t i s f a c t o r y a c t i v i t y i n t h e C methylation ( r e f s . 7-9) and c e r t a i n mixed oxides,

such as, S i 0 2 -

Fep03, C r 0 - S i 0 2 - F e 2 0 3 , a n d Ge02-Fe203 show g o o d o r t h o - s e l e c t i v i t y 2 3 ( r e f . 10). Despite t h e i r increasing p r a c t i c a l importance n e i t h e r t h e c a t a l y t i c n o r t h e s t r u c t u r a l and s u r f a c e p r o p e r t i e s o f these mixed oxides c o n t a i n i n g i r o n o x i d e as a main c o n s t i t u e n t were studied. S i n c e i r o n o x i d e s a r e known a s c a t a l y s t s f o r d e h y d r o g e n a t i o n

524

a n d o x i d a t i o n o f some h y d r o c a r b o n s a n d a r e o f i n d u s t r i a l

importance

f o r many o t h e r r e a s o n s s p e c i a l a t t e n t i o n h a s b e e n p a i d t o t h e i n v e s t i g a t i o n o f t h e i r chemical,

s t r u c t u r a l and surface properties.

The p r e s e n t work r e l a t e s t o t h e e f f e c t o f germanium p r o m o t i o n on t h e c a t a l y t i c and s u r f a c e p r o p e r t i e s o f h e m a t i t e .

E X P E R I MENTAL H e m a t i t e was p r e p a r e d f r o m F e ( N 0 3 ) 3 . 9 H 2 0

(Reanal, Hungary)

F e r r i g e l was p r e c i p i t a t e d a t pH=7 f r o m a 0.25M F e ( N O 3 I 3 . 9 H 2 0 s o l u t i o n b y a d d i t i o n o f a 25 w t % NH40H s o l u t i o n .

Precipitate

ra s

d r i e d a t 370K f o r 24h a n d c a l c i n e d i n a i r a t 720K f o r I h .

washed,

F o r t h e p r e p a r a t i o n o f t h e d - F e 2 0 3 c a t a l y s t s o f 3,5

o r 8 w t % Ge02

c o n t e n t 209 o f h e m a t i t e was i m p r e g n a t e d s t e p - b y - s t e p c i e n t a m o u n t o f a q u e o u s Ge02 ( V e n t r o n , room t e m p e r a t u r e .

with suffi-

A l f a Product) s o l u t i o n a t

S a m p l e s w e r e d r i e d i n a i r a t 380K o v e r n i g h t a n d

a i r - c a l c i n e d a t 720K f o r I h . S u r f a c e a r e a s d e t e r m i n e d b y t h e BET-method u s i n g K r a d s o r p t i o n 2 a t 77K w e r e 2 3 . 3 , 2 3 . 5 , 23.6 a n d 20.3 m / g f o r t h e d - F e 2 0 j a n d f o r t h e 3,5 and 8 w t % Ge02-Fe203 c a t a l y s t s ,

respectively.

X-ray

d i f f r a c t i o n p a t t e r n s were r e c o r d e d by a d i f f r a c t o m e t e r ( P h i l l i p s t y p e P W 1050) u s i n g N i - f i l t e r e d C u b r a d i a t i o n . The v a p o u r p h a s e a l k y l a t i o n o f 2 - E t - A

w i t h m e t h a n o l was c a r r i e d

o u t i n a f i x e d b e d d o w n - f l o w r e a c t o r a t 643K a n d a t m o s p h e r i c pressure.

The c y l i n d r i c a l r e a c t o r o f 17-mm d i a m e t e r c o n t a i n e d 3 . 4 9

catalyst.

C a t a l y s t s w e r e p r e t r e a t e d i n f l o w i n g a i r a t 720K f o r I h

and c o o l e d t o t h e r e a c t i o n t e m p e r a t u r e i n N2. 2-Et-A 0.22,

m o l a r r a t i o a n d t h e WHSV ( g 2 - E t - A x

The m e t h a n o l t o

g i l t x h-')

were 5 and

respectively.

C a t a l y s t s w e r e c h a r a c t e r i z e d b y TPR.

H2-consumption o f t h e

s a m p l e s was m o n i t o r e d b y m e a s u r i n g t h e t h e r m a l c o n d u c t i v i t y o f t h e e f f l u e n t gas passed t h r o u g h a d r y - i c e t r a p t o c o l l e c t w a t e r p r o d uced.

2 0 mg o f m e t a l o x i d e was c a l c i n e d i n O 2 f l o w a t 723K f o r 3 h

e v a c u a t e d f o r 30 m i n and r e d u c e d i n a 10% H 2 / N 2 m i x t u r e f l o w i n g a t a r a t e o f 20 m l m i n - I .

T h e t e m p e r a t u r e c h a n g e d a t a r a t e o f 10K min-'

f r o m 373K t o 1 0 7 3 K . T h e H 2 / N 2

m i x t u r e (ODV,

H u n g a r y ) was p u r i f i e d

b y p a s s i n g t h r o u g h a d e - o x y g e n a t i n g c a t a l y s t (De-Ox c a t a l y s t , Ventron,

Alfa

P r o d u c t ) a n d a c o l u m n f i l l e d w i t h a c t i v a t e d 4A z e o l i t e .

A S a r t o r i u s 4 1 0 2 t y p e m i c r o b a l a n c e o p e r a t e d i n f l o w - t h r o u g h mode was u s e d t o d e t e r m i n e t h e w e i g h t c h a n g e o f t h e s a m p l e s c o n t a c t e d w i t h p y r i d i n e (Py) (Reanal, Hungary).

I R m e a s u r e m e n t s w e r e c a r r i e d o u t b y a N i c o l e t 170SX F T - I R

525

spectrometer. were used. ments.

S e l f - s u p p o r t i n g w a f e r s o f a b o u t 1 0 mg c m - 2

"thickness"

Spectra were r e c o r d e d a t room t e m p e r a t u r e a f t e r t r e a t -

1 0 0 0 s c a n s w e r e c o l l e c t e d a t a r e s o l u t i o n o f 2 cm-'

averaged.

and

T h e c o m p u t e r s u b t r a c t i o n t e c h n i q u e was u s e d t o g e t t h e

s b e c t r u m o f t h e a d s o r b e d Py. O x y g e n was p a s s e d t h r o u g h c o l u m n s f i l l e d w i t h d e h y d r a t e d CaC12 and Mg(C1O4l2 f o r d r y i n g .

T h e Py u s e d was d i s t i l l e d f r o m KOH a n d

s t o r e d over a c t i v a t e d 4A molecular sieve. RESULTS R e s u l t s o f TPR m e a s u r e m e n t s a r e p r e s e n t e d i n F i g . experiment oxide reduced t o metal.

1.

I n each

The T P R c u r v e o f h e m a t i t e shows

two c h a r a c t e r i s t i c peaks: a b r o a d a n d s t r o n g p e a k w i t h a maximum i n t h e r e g i o n o f 8 7 3 - 9 2 3 K a n d a smaller peak a t 623K.

The l o w - t e m p e r a t u r e

peak c o r r e s p o n d e s t o t h e r e d u c t i o n o f t h e Fe203 t o Fe304 ( F i g .

1,curve

0 ) . T h e TPR c u r v e o f t h e F e 2 0 3 - G e 0 2 m i x t u r e i s p r a c t i c a l l y t h e same a s t h a t o f hematite,

except the t a i l

b e t w e e n 973 a n d 1073K, w h i c h i s r e l a t e d t o t h e r e d u c t i o n o f t h e Ge02 I

373

673

1 0 7 3 ~ 30

.

60mln

phase ( F i g .

1,

cf.

c u r v e s C, D and E ) .

The g e r m a n i u m i n t r o d u c e d b y F i g . 1 . TPR p r o f i l e o f m i x t u r e composed f r o m e q u a l w e i g h t s o f d - F e 0 a n d 3 w t % GeO - F e 0 ( A ) , 8 ~ t % ~ G 2 0 ~ - F( Be) , ~ E Oi x~t 6 r - 2 o f d - F e 0 a n d 8 w t % Ge02 ( C ) , d-Fe 3 ( D ) , a n d Ge02 ( E ) . 2 3

6

impregnation s i g n i f i c a n t l y a l t e r s reduction

characteristics.

The l o w - t e m p e r a t u r e peak a p p e a r s a t 693K ( F i g .

1, curve B ) .

I d e n t i c a l c u r v e s were o b t a i n e d f o r c a t a l y s t s c o n t a i n i n g 3,5

o r 8wt%

Ge02.

and

M i x t u r e o f o(-Fe2Oj

3 w t % G e 0 2 - F e 2 0 3 was t o g e t h e r p r e t r e a t e d a n d r e d u c e d . T h e appearence o f two low-temperature peaks demonstrates t h a t no chemical i n t e r a c t i o n o c c u r s between t h e mixed components ( F i g .

1, c u r v e A ) .

X-ray a n a l y s e s c o n f i r m e d t h a t samples c o n t a i n o(-Fe203 o n l y as c r y s t a l l i n e phases.

andd-Ge02

No s i g n i f i c a n t d i f f e r e n c e was f o u n d i n

t h e 1a t t i c e p a r a m e t e r s o f t h e F e 2 0 3 a n d t h e G e 0 2 - F e 2 0 3 p r e p a r a t i o n s . X - r a y d i f f r a c t i o n a n d TPR r e s u l t s s u g g e s t t h a t g e r m a n i u m i s w e l l d i s p e r s e d and s t r o n g l y bound t o t h e surfac.e o f t h e d - F e 2 0 3 c a t a l y s t s and i t h i n d e r s t h e r e d u c t i o n o f i r o n .

526

I R - s p e c t r a o f t h e V(OH) r e o i o n a r e

-1 0

shown i n F i g . 2. I

Though a s t r i c t l y

q u a n t i t a t i v e c o m p a r i s o n o f t h e integrated

B

i n t e n s i t i e s i s n o t attempted i t can be concluded t h a t t h e OH-concentration i s s i g n i f i c a n t l y s m a l l e r i n t h e Ge02-Fe203 c a t a l y s t t h a n i n t h e h e m a t i t e . T h i s r e s u l t substantiate t h a t p a r t o f t h e s u r f a c e OH-groups o f the d-Fe203 were a n n i h i l a t e d i n t h e p r o c e s s

J

o f germanium f i x i n g . G r a v i m e t r i c and

A

3800

3400

I R - s p e c t r o s c o p i c e x a m i n a t i o n o f Py a d -

3000

WAVENUMEERS. CM

-1

sorption revealed not only the acidic

Fig.2. I R spectra o f d - F e 0 ( A ) a n d 3 w t % GeO

p r o p e r t i e s o f t h e c a t a l y s t s b u t provided

-

information also about the r e a c t i v i t y o f

Fe 02(') ni ti;e t h e i r s u r f a c e s . I R - s p e c t r a o f t h e adsorbed r e g i o n a f t e r 1-h t r e a t m e n t i n 0 f l o w a t 720K Py a r e s h o w n i n F i g . 3 . A f t e r a 1 - h e v a c a n d 20-min eva2uation at u a t i o n a t 373K Lewis-bound Py o n l y i s d e t e c t e d room t e m p e r a t u r e .

V(6H3

b o t h on o(-Fe203 and 3wt% Ge02-Fe203 ( F i g . a p p e a r a t 1610, 1594, 1132 cm-'. 19a,

19b,

1578,

3 , spectra A and a ) . Bands

1490, 1445, 1218,

1150, 1068,

1140 a n d

These f r e q u e n c i e s c o r r e s p o n d t o t h o s e o f t h e 8a, 8a, 8b, 19a,

15,

18a,

molecule, respectively.

12 a n d 1 2 v i b r a t i o n a l modes o f t h e f r e e Upon e v a c u a t i o n a t 4 7 3 K a n d 573K t h e

i n t e n s i t y o f t h e bands a t 1 5 9 4 a n d 1 0 6 8 c m - ' a p p e a r a t 1 6 5 0 a n d 1 5 5 2 cm-' o f 1 4 3 0 - 1 4 7 0 crn-l

d e c r e a s e s a n d new b a n d s

( F i g . 3 , spectrum C)

( F i g . 3, spectra

C and c ) .

and i n t h e r e g i o n

The a p p e a r a n c e o f t h e

new bands i n d i c a t e t h a t a t e l e v a t e d t e m p e r a t u r e i n i n t e r a c t i o n w i t h t h e c a t a l y s t new s u r f a c e c o m p l e x e s a r e f o r m e d .

ih/L &h T

"I

w

u z 4

No s p e c t r a c o u l d b e F i g . 3. I R s p e c t r a o f t h e Py adsorbed on d-Fe 0 ( A - C ) a n d 3 ~ t % ~ G 8- O Fe 0 (a-c) (a If ht e 0r p fr lzot w r e iai tti l 720K ent

I

b

and 26 min evac. a t room temp. ), Samples were c o n t a c t e d w i t h

IE

0

m d

1600

1200

and e v a c u a t e d 373K (A and a ) , (B and b ) and (C and c ) f o r

a

A

800

WAVENUMBERS. CM-'

1600

1200

800

at 473K 573K 1 h.

521

r e c o r d e d a f t e r e v a c u a t i o n a t 673K b e c a u s e t h e t r a n s m i t t a n c e o f t h e h e m a t i t e s a m p l e was c o m p l e t l y l o s t . A c i d i t y i s c h a r a c t e r i s e d w i t h t h e a m o u n t o f Py b o u n d b y t h e c a t a l y s t a t 298K a t a Py p a r t i a l p r e s s u r e o f 640Pa ( T a b l e 1 ) .

Py

u p t a k e s show t h a t c a t a l y s t s a r e p r a c t i c a l l y o f t h e same a c i d i t y . When s a m p l e i s l i n e a r l y h e a t e d Py d e s o r b s a n d t h e w e i g h t o f t h e c a t a l y s t decreases. A b o v e 473K TABLE 1 ( a 1 t h e w e i g h t s t a r t s t o increase, Weight g a i n s i n c o n t a c t with pyridine p r e s u m a b l y d u e t o t h e formation Catalyst W e i g h t g a i n , mg.g - 1 o f a new s u r f a c e complex ( F i g . 4 ) . a t 298K a t 473K The weight increase o f d -Fe203 o( - F e 2 0 3 7.3 2.6 b e g i n s a t 1o w e r t e m p e r a t u r e 3 w t % Ge02-Fe203 7.1 1.2 t h a n t h a t o f Ge02-Fe203 and i s 5 w t % Ge02-Fe203 6.7 1.4 much more pronounced. The weight 8 w t % Ge02-Fe203 6.5 1.1 g a i n s o f t h e samples were ( a ) S a m p l e s were a c t i v a t e d i n f l o w i n g 0 a t 720K f o r 2h c o o l e d t o t h e t g m p e r a t u r e o f t h e experiment, purged w i t h He a n d t h e n c o n t a c t e d w i t h a He f l o w s a t u r a t e d w i t h Py a t 273K.

determined i n separate experiments a t 473K (Table 1). Data show t h a t d - F e 2 0 3 r e a c t s more e a s i l y w i t h Py t h a n Ge02-Fe 0

2 3

I

t \

loo

\

I ’\

20

298

473

catalysts.

201

673

Desorption temperature .K

F i g . 4. The w e i g h t c h a n g e o f t h e c a t a l y s t s e q u i l i b r a t e d w i t h 0.63% Py/He m i x t u r e a t r o o m t e m p . - y p o n in h e a t i n g a t a r a t e o f 5K m i n t h e 0 . 6 3 % Py/He f l o w : d - F e 2 0 3 ( A ) , 3 w t % Ge02-Fe203(B), 5wt% GeO -Fe203(C) 8 w t % GeO -Fe 0 (0). The w e i g k c h a n g e c?f-&-h2O3 i s inserted f o r comparison.

Time on flow,h

F i g . 5. T h e e f f e c t o f germanium p r o m o t i o n on t h e a c t i v i t y and deactivation o f hematite i n t h e vapour phase m e t h y l a t i o n o f 2 - e t h y l a n i l i n e b y methanol. ( T r e a c t . =643K, WHSV=0.22 g2-Et-A.g-1

.h-’,

methanol t o 2-Et-A r a t i o = 5 ) d-Fe203 (A), 3 w t % Ge02-Fei03

(0).

528

I m p r e g n a t i o n b y germanium s u b s t a n t i a l l y i n c r e a s e s t h e a c t i v i t y of

t h e c a t a l y s t i n t h e a l k y l a t i o n o f 2-Et-A

increased conversion o f t h e 2-Et-A deactivation (Fig.

5).

C a t a l y s t e f f i c i e n c y i s c h a r a c t e r i z e d by

t h e y i e l d s (100 x moles o f product/moles

o f 2-Et-A

s e l e c t i v i t i e s (100 x moles o f product/moles f o r t h e main p r o d u c t s ; N-Meen-2-Et-A

b y m e t h a n o l . The

i s n o t accompanied by f a s t e r

i.e.,

feed)

and t h e

o f converted 2-Et-A)

f o r 2-Et-6-Me-A,

N-Me-2-E-A

and

( F i g . 6 ) . A t 3 w t % Ge02 c o n t e n t a l r e a d y a l l t h e

f a v o u r a b l e c a t a l y t i c e f f e c t s were a c h i e v e d . The y i e l d o f 2-Et-6and

Me-A i n c r e a s e d c o n s i d e r a b l y w h i l e t h e y i e l d s o f N - M e - 2 - E t - A N-Meen-2-Et-A

w e r e much l e s s i n f l u e n c e d ( F i g . 6 ) .

I

I

a 2-€1-6-Me-A

30.

T

E

-

b 2-Et-6-Me-A

40

N-Me-2-€1-A

c u 20 5 fn

0

3.0 5.0 8.0 G e 0 2 wt ?h

N Meen-2 -Et -A

3.0 5.0 Ge02 W t %

0

8.0

9

F i g . 6 . The e f f e c t o f g e r m a n i u m p r o m o t i o n o n t h e y i e l d ( a ) a n d s e l e c t i v i t y ( b ) o f t h e main p r o d u c t s i n t h e vapour phase m e t h y l a t i o n o f 2 - e t h 1 a n i l i n e b y m e t h a n o l ( T r e a c t =643K, WHSV-0.22 g g-y h - l , methanol t o 2-Et-A rati8=!fTAData r e f e r t o the composition o f the l i q u i d product i n the 48th hour o f the experiment. DISCUSSION The p a r t i c i p a t i o n o f t h e s u r f a c e o x y g e n a t o m s o f i r o n o x i d e i n o x i d a t i o n r e a c t i o n s even i n t h e absence o f gas-phase oxygen has been d e m o n s t r a t e d ( r e f s .

11-14).

I t was shown t h a t i r r e v e r s i b l y 20; a n d O 2 , h y d r o x y l g r o u p s a n d

chemisorbed O2 s p e c i e s , such as, 2lattice-oxygen 0 i o n s o f t h e u p p e r m o s t s u r f a c e l a y e r s may s e r v e as t h e source o f t h e r e a c t i v e oxygen ( r e f s .

14-17).

t h a t t h e o x i d i z i n g power o f t h e germanium-modified

We r e c o g n i z e d d-Fe203 i s

s u b s t a n t i a l l y weaker t h a n t h a t o f t h e d - F e 2 0 3 . The s h i f t o f t h e l o w - t e m p e r a t u r e

TPR peak t o h i g h e r t e m p e r a t u r e

r e f l e c t s t h e s m a l l e r r e a c t i v i t y of t h e oxygen atoms a c c e s s i b l e f o r

H2.

I n absence o f e a s i l y r e d u c i b l e m e t a l atoms h i g h e r t e m p e r a t u r e

529

i s r e q u i r e d t o a c t i v a t e H2 ( F i g .

1).

Py a d s o r p t i o n m e a s u r e m e n t s d i d n o t r e v e a l s i g n i f i c a n t d i f f e r -

ences i n t h e a c i d i c p r o p e r t i e s o f t h e samples.

No B r o n s t e d a c i d

s i t e s w e r e d e t e c t e d a n d a b o u t t h e same a m o u n t o f L e w i s a c i d s i t e s w e r e a v a i l a b l e f o r Py a d s o r p t i o n i n t h e d i f f e r e n t c a t a l y s t s . However,

s i g n i f i c a n t d i f f e r e n c e s were found i n t h e r e a c t i v i t y o f

t h e s u r f a c e s t o w a r d s Py ( T a b l e 1 ) . I t i s k n o w n t h a t Py a d s o r b e d o n h e m a t i t e r e a c t s w i t h t h e s u r f a c e

a t elevated temperature.

The a p p e a r a n c e o f new I R b a n d s a f t e r t r e a t -

m e n t a b o v e 4 2 3 K i n d i c a t e s t h e f o r m a t i o n o f a new s u r f a c e c o m p l e x . I t i s b e l i e v e d t h a t t h e new b a n d s s t e m f r o m s u r f a c e - b o u n d 2 , 2 ’ -

bipyridyl or

d

-pyridone

(refs.

18,19).

C o n c o m i t t a n t l y w i t h Py

o x i d a t i o n i r o n o x i d e becomes r e d u c e d a n d i s l o s i n g i t s transmission. Whatever i s t h e r e a c t i o n p r o d u c t Ge02-Fe203 c a t a l y s t s s t a r t t o o x i d i z e Py a t h i g h e r t e m p e r a t u r e a n d b i n d much l e s s n e w l y f o r m e d s u r f a c e complex than d - F e 2 0 3 ( F i g s .

3 and 4 ) .

Three d i f f e r e n t k i n d s o f hematite h y d r o x y l s were d i s t i n g u i s h e d : i s o l a t e d t e r m i n a l ( 3 6 7 0 cm-’1, h y d r o x y l groups (3460 cm-’)

b r i d g e d ( 3 6 4 0 cm-’)

(refs.

17,20).

and H-bonded

Only t h e broad band o f

t h e H-bonded OH-groups can be f o u n d i n t h e s p e c t r u m o f o u r Fe203 p r e p a r a t i o n . The b a n d a t 3 4 6 0 cm-’

i s much l e s s i n t e n s e i n t h e

s p e c t r u m o f t h e Ge02-Fe203 c a t a l y s t ,

but,

t h e bands o f t h e f r e e

and q u a s i - f r e e h y d r o x y l s a r e c l e a r l y observable. The b i n d i n g o f g e r m a n i u m o x i d e c a n b e d e p i c t e d o n t h e a n a l o g y o f t h e s u r f a c e c a r b o n a t e f o r m a t i o n . The a q u e o u s s o l u t i o n o f Ge02 u s e d f o r i m p r e g n a t i o n i s e s s e n t i a l l y a weak a c i d . Though H2Ge03 i s a weaker d i b a s i c a c i d than

H2C03 i t seems

reasonable t h a t i t reacts

w i t h t h e b a s i c OH-groups o f h e m a t i t e . The s t r o n g b a s i c i t y of

the

hydroxyl surfaces o f i r o n oxyhydroxides and t h e s t r o n g a d s o r p t i o n o f C O P was a l r e a d y d e m o n s t r a t e d ( r e f .

21).

The

f o r m a t i o n o f Fe-0-Ge

bonds can e x p l a i n mentioned s p e c t r a l d i f f e r e n c e s .

Such b o n d s c a n

s t a b i l i z e t h e h i g h l y d i s p e r s e d germanium o x i d e phase on thed-Fe203 surface. U n l i k e t o z e o l i t e c a t a l y s t s a c i d i c s i t e s h a r d l y p l a y any r o l e i n t h e C-methylation of 2-Et-A

on d - F e 2 0 3 o r Ge02-Fe203 s i n c e

germanium p r o m o t i o n induces a c o n s i d e r a b l e i n c r e a s e s i n t h e 2-Et-6-Me-A

y i e l d and s e l e c t i v i t y w h i l e a c i d i t y remains p r a c t i -

c a l l y unchanged.

Regarding t h a t t h e y i e l d o f t h e N - a l k y l

d e r i v a t i v e s i s o n l y s l i g h t l y i n f l u e n c e d by germanium t h e p a r t i c i p a t i o n o f t h e weak b a s i c o r a c i d i c s i t e s i n t h e N - m e t h y l a t i o n n o t be r u l e d o u t .

can

530

The m a i n e f f e c t o f Ge p r o m o t i o n i s t h a t i t d e c r e a s e s t h e r e a c t i v i t y o f t h e h e m a t i t e s u r f a c e and t h e b i n d i n g s t r e n g t h o f organ i c c o m p l e x e s . The d e c r e a s e o f s u r f a c e r e a c t i v i t y c a n b e a l s o a c h i e v e d by s i m p l y e v a c u a t i n g t h e h e m a t i t e a t e l e v a t e d t e m p e r a t u r e (ref.

1 3 ) . Such t r e a t m e n t d e p l e t e s t h e s u r f a c e o f o x y g e n j u s t a s

t h e f i x a t i o n o f Ge02, h o w e v e r ,

w h i l e h e a t t r e a t e d surface can g e t

r e a c t i v a t e d i n c o n t a c t w i t h t h e r e a c t a n t s t h e change caused by t h e germanium p r o m o t i o n i s i r r e v e r s i b l e . Ge02-Fe203

This e f f e c t implies that

catalysts are less s u s c e p t i b l e t o p o i s o n i n g t h a n

d

-Fe203.

We w o u l d 1 ik e t o t h a n k S p e c t r o s c o p i c L a b o r a t o r i e s o f t h e C R I C f o r t h e a s s i s t a n c e i n t h e I R - s p e c t r o s c o p i c measurements.

Support f o r

t h i s work was provided by t h e C o m m i t t e e f o r R e s e a r c h a n d D e v e l o p m e n t , H u n g a r y (OMFB,

P r o j e c t No.

213/87)

and t h e N i t r o c h e m i c a l Works,

Hungary. REFERENCES

1 2 3 4 5

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

E u r . P a t e n t A p p l . A 1 0226 781 ( 1 9 8 6 ) E u r . P a t e n t A p p l . A2 0 2 4 5 7 9 7 ( 1 9 8 7 ) G . O . C h i v a d z e , L . Z . C h k h e i d z e , I z v . A k a d . N a u k , G r u z . SSR, Ser. Khim., 1 0 ( 3 ) , ( 1 9 8 4 ) 232-234. G.O. Chivadze, L.Z. Chkheidze, L . I . B a l a k h i s h v i l i , V.V. K h a k h n e l i d z e , Ts.1. N a s k i d a s h v i l i , I z v . Akad. Nauk, G r u z . SSR, S e r . Khim., 1 1 ( 4 ) , ( 1 9 8 5 ) 262-266. P . Y . Chen, M . C . Chen, H . Y . Chu, N . S . C h a n g a n d T.K. C h u a n g , i n Y . M u r a k a m i , A . I i j i m a a n d J.W. Ward ( E d i t o r s ) , P r o c . 7 t h . I n t . Z e o l i t e C o n f e r e n c e , Tokyo, J a p a n , A u g u s t 17-22, 1986, Kodansha L t d , Tokyo, 1986, pp. 739-746. P . Y . Chen, S.J. Chu, N . S . Chang a n d T . K . Chuang, i n P . A . J a c o b s and R.A. van Santen ( E d i t o r s ) , Z e o l i t e s : F a c t s , F i g u r e s , F u t u r e , 1989, E l s e v i e r , Amsterdam, 1984, p p . 1105-1113. M. I n o u e a n d S. Enomoto, S e k i y u G a k k a i s h i , 1 5 ( 1 9 7 2 ) 2 8 - 3 4 . Ger. O f f e n . 2240362 ( 1 9 7 3 ) U S P a t e n t 4,721,810 (1988) U S P a t e n t 4,351,958 (1982) J.Novakova, P . J i r u and V.Zavadi1, J. Catal., 21 (1971) 143-148. V . L o r e n z e l l i , G . B u s c a and N. Sheppard, J . Catal., 66 (1980) 28-35. G.Busca a n d V . L o r e n z e l l i , J . C a t a l . , 66 ( 1 9 8 0 ) 1 5 5 - 1 6 1 . K . S a k a t a , F . Ueda, M. M i s o n o a n d Y . Y o n e d a , B u l l . Chem. S O C . J p n . , 53 ( 1 9 8 0 ) 3 2 4 - 3 2 9 . F . A1 M a s h t a a n d N . S h e p p a r d , J.Chem.Soc. Faraday Trans. I . , 78 ( 1 9 8 2 ) 9 7 9 - 9 8 9 . M. I w a m o t o , Y . Yoda, N . Yamazoe a n d T. S e i y a m a , B u l l . Chem.Soc. J p n . , 51 ( 1 9 7 8 ) 2 7 6 5 - 2 7 7 0 . C . H . R o c h e s t e r a n d S . A . Topham, J.Chem.Soc. F a r a d a y T r a n s . I., 75 ( 1 9 7 9 ) 1 2 5 9 - 1 2 6 7 . G.Busca, V . L o r e n z e l l i , M a t e r i a l s C h e m i s t r y , 6 ( 1 9 8 1 ) 1 7 5 - 1 8 6 . K . M o r i s h i g e , S . K i t t a k a , S . K a t s u r a g i a n d T . M o r i s m o t o , J . Chem. S O C . F a r a d . T r a n s . I . , 78 ( 1 9 8 2 ) 2 9 4 7 - 2 9 5 7 . C . H . R o c h e s t e r a n d S . A . Topham, J.Chem.Soc. Faraday Trans. I . , 75 ( 1 9 7 9 ) 872-882. 8 . J . L i a w , D.S. Cheng and B.L. Yang, J.Catal., 118 (1989) 312-326.

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

531

0 1991 Elsevier Science Publishers B.V., Amsterdam

REARRANGEMENT OF CYCLOHEXANONE OXIME TO CAPROLACTAM OVER SOLID ACID CATALYSTS. T.CURTIN, J.B.McMONAGLE and B.K. H O D N E T Dept. of Materials and Industrial Chemistry University of Limerick Plassey Technological Park Limerick, Ireland. ABSTRACT Beckmann rearrangement of cyclohexanone oxime to caprolactam in the gas phase over a boria catalyst has been invcstigated. Catalyst testing using a continuous flow fixed bed reactor indicated typical time on stream reaction profiles, to consist of an initiation period , a brief stable operating phase and finally slow deactivation. The fast deactivation of nonselective, strongly acidic catalyst sites IS thought to account for the initiation period. In addition to the mechanisms already known (coking and basic by-product re-adsorption) for the loss of caprolactam selectivity during the final deactivation phase, evidence is presented to suggest an additional process whereby some of the selective sites of intermediate acidity are converted to non-selective sites during reaction. The latter are thought to be due to the formation of some amorphous boron species. INTRODUCTION Caprolactam is an industrially important intermediate in the manufacture of Nylon-6. Conventional methods for the production of the compound involve the liquid phase Beckmann rearrangement of cyclohexanone oxime using fuming sulphuric acid as the catalyst. Although highly selective, there are a number of significant shortcomings associated with these existir,g processes, including the formation of large amounts of ammonium sulphate as a by-product, as well as corrosion, waste water disposal and heat removal problems. In order to circumvent some of these limitations, various workers have investigated the possibility of performing this rearrangement reaction in the vapour phase over solid acid catalysts such as zeolites (refs. 1-3) and boria supported on various materials (refs. 4-6). In the case of boria on alumina catalysts, preparation method, reaction temperature and surface acidity have all been shown to influence oxime conversion and lactam selectivity (refs. 6,7). Catalyst preparations which produce a uniform distribution of boria on the alumina surface have been observed to be most effective in catalysing this reaction. In addition it appears that a minimum boria content of around 15% is required : preparations with lower boria contents have been proposed to possess non-selective strongly acidic sites (pKa -5.6 to -8.2) as a result of the formation of a surface mixed oxide of boriaalumina, whereas catalysts which contain greater than around 15% boria show a predominance of sites giving intermediate Bronsted acid strength (pKa > -3.0). The latter type of sites are thought to be selective for the formation of caprolactam. Catalyst operation at high (>350"C) reaction temperatures has been reported (ref. 6) to give

decreased selectivity towards caprolactam, the suggested reason for this being an increase in side reactions rather than a change in the acid property of the catalyst. Catalyst deactivation as a function of operation time is perhaps the most serious limitation of this approach to the production of caprolactam from cyclohexanone oxime, and is a common problem with all calalyst types. Various reasons have been put forward to account for this effect including coke formation (ref. 6) and/or irreversible adsorption of basic reaction by-products (ref. 3). A detailed examination of the effects of operation conditions on boria on alumina catalyst performance and lifetime is reported in this paper, in an attempt to further elucidate the important parameters controlling optimisation and maintenance of caprolactam yield. EXPERIMENTAL The catalyst was made up according to established methods, (ref. 7), impregnating boric acid (BDH general purpose grade) on alumina (120m2/g surface area, pore volume o.95cm3/g) and calcining at 350'C before use. The percentage boria, as measured by Spectrophotometry, was found to be approx 13wt%. Characterisation by XRD showed that boria (B2O3), was the only crystalline phase present on the surface of the alumina after calcination.

The vapour phase Beckmann rearrangement reaction was carried out using a continuous flow system operated at ambient pressure. Helium was used to entrain the cyclohexanone oxime (Aldrich general purpose grade) into the vapour phase from a saturator. The vapour pressure of the oxime was controlled by adjusting the saturator temperature. The catalytic reaction took place in a fixed bed U-tube reactor. The temperature was controlled by a thermocouple placed in a thermowell in the catalyst bed. In normal operation the total gas flow rate was 30 ml min-l. On leaving the reactor the gas stream was cooled by passing it through a glass spiral trap immersed in an ice bath. At regular intervals the accumulated products in the trap were washed out with a known volume of methanol and analyzed by gas chromatography. The gas chromatography column, a 3m column of 3%. OV17, was operated at 170'C using a Flame Ionization Detector and nitrogen as carrier gas, with naphthalene as internal standard. Following testing, catalyst regeneration was attempted by heating in air at 500°C for 48 hours. All catalysts were crushed to a fine powder and analyzed by X-ray diffraction using a Phillips Diffractometer with nickel filtered CUK radiation as the source. Catalysts were analyzed for boria content according to the carminic acid spectrophotometric method (refA), using a Varian DMS 100s U.V. Visible Spectrophotometer. In preparation for total boria content analysis, 0.04g of crushed catalyst was added to 5mls of conc. H2SO4. lOmls of water was slowly added, stirring and heating until all the catalyst had dissolved. This solution was made up to 25mls with distilled water. Preparation for water soluble

boria analysis involved leachirig the boron oxide from the catalyst by stirring in warm water for 0.5 hours. RESULTS AND DISCUSSION Figure 1 shows the conversion, selectivity and yield when 0.lg of boria on alumina catalyst at 300'C is used to rearrange cyclohexanone oxime to caprolactam. Although a high oxime conversion was achievable on commencing the reaction, this was not matched by correspondingly high lactam yield and selectivities. Thus side reactions predominated at low values of reaction time. Lactam selectivity increased with time on stream, however, presumably as a result of rapid poisoning of the active sites for the faster non-selective side reactions. Similar "initiation" behaviour has been reported (ref. 9) for this reaction over zeolites. As time on steam increased further, decreases in both conversion and lactam selectivity were apparent due to further non-selective poisoning of all catalyst active sites.

40 -

30

I 1 0

1

2

3

1

I

1

I

I

4

5

6

7

8

9

Time (hours) Fig.1. (I) oxime conversion, (+) caprolactam selectivity and (u) caprolactam ield 00'(!. with time using 0.lg boria on alumina catalyst at a reaction temperature of 3

The effect of temperature on the conversion, selectivity and yield after 3 hours on stream is shown in figure 2. In each case a catalyst mass of 0.lg boria on alumina catalyst was tested. With increasing reaction temperature the oxime conversion increased, however, maxima in lactam selectivity and yield were observed at a reaction temperature of 300°C. At higher temperatures excessive coking and side reactions were thought to occur,

534

resulting in high conversion but poor lactam selectivity. Values of the latter parameter also decrease at low reaction temperatures. Although this is somewhat at variance with that reported by other workers (ref. 6), it is postulated that the longer residence times used in this study resulted in an increased likelihood of re-adsorption of the lactam on the catalyst at these temperatures. This hypothesis in borne out by the observation that total conversion at 250'C decreased much more rapidly than at higher reaction temperatures.

200

250

300

350

400

Reaction Temperature ('C) Fig.2. Diagram showing the effect of reaction temperature on ( I ) oxime conversion, (+) caprolactam selectivity and (*) caprolactam yield alter 3 hours on stream using 0.1 g boria on alumina catalyst. The effect of mass of catalyst on performance, shown in figure 3, indicated an optimum mass for maximum selectivity and yield. Small catalyst beds were observed to give a maximum in lactam yield at short reaction times, thus ending up predominantly in the deactivated state (both conversion and lactam selectivity low) after three hours on stream. On the other hand large catalyst beds required much longer to achieve maximum lactam production, and after three hours on stream are predominantly in the initiation phase (high conversion, low lactam selectivity).

535

100

80

60 O/O

40 'Ac

20

0 0

I

I

I

I

I

0.1

0.2

0.3

0.4

0.5

0.6

Mass of Catalyst (9) Fig. 3 Diagram showing the effect of catalyst mass on (.) oxime conversion, (+) caprolactam selectivity and ( J C ) caprolactam yield after 3 hours on stream, using boria on alumina catalyst at a reaction temperature of 300'C. Figure 4 is a plot of oxime conversion, caprolactarn selectivity and yield of different catalysts over a period of 30 hours. The three catalysts used were boria on alumina, alumina and a regenerated boria on alumina catalyst. The regenerated catalyst was previously on stream for 30 hours and then heated in air for 48 hours at 500'C in an effort to remove coke. The concept of an initiation period in which the non-selective, presumably strongly acidic, catalyst sites are initially deactivated (refs. 6,7)is given further credence by comparing the performance of the catalyst with that of the alumina support. It is obvious that the increased initiation period, in the latter case, is a result of the larger number of nonselective sites which must be deactivated. This is also indicated by the higher overall conversion of the alumina support. It is also noteworthy that there were a limited number of sites on the alumina which were selective for lactam formation. A comparison of the performance of the fresh and the regenerated catalysts can be made from figure 4. It is obvious that, although the regeneration process does not cause a large improvement in lactarn selectivity, the fact that higher overall conversions were attained means that lactam yield recovers somewhat. The increased conversions obtained with the regenerated catalyst is most likely due to the fact that loss of boron during the regeneration process (see tattle) results in the uncovering of additional nonselective sites on the alumina support.

536

100

C

90

0

n V

80

e r S

i

(4

7c

0

n 6(

o/o

S

e I

81

C

61

e t

(b) i V

i t Y

4

2

YO

E

%

(c)

y

t

e I d

1

F

0

ti

10

15

20

25

30

Time (h) Fig.4. (a) Oxime conversion, (b) Caprolactam selectivity, (c) Caprolactam Yield using 0.2g (.) boria on alumina, (u)regenerated boria on alumina and (+) alumina catalyst at 300'C for 30 hours.

537

TABLE Analysis of Catalyst Samples by Spectrophotometry %Boron (as B2O3)

Sample Total boron

Water soluble boron

Fresh BpO3lalumina

12.7

12.3

Regenerated catalyst1

11.1

7.0

Regenerated in air at 500'C for 48 hours.

In the case of the regenerated catalyst improved lactam yields (compared to that of the deactivated catalyst) were retained only for much shorter operating times compared to the performance of fresh catalyst, presumably because not all of the lactam-selective sites have been regenerated. There is ample evidence both from the present and other (refs. 3,6) work that both coking and re-adsorption of basic reaction by-products play a part in catalyst deactivation. In the former case the observed catalyst colour change on use and the fact that removal of the carbon by calcination increased both the lactam yield and total conversion would tend to indicate that coking is a fairly non-selective deactivation process. Spectroscopic evidence for the presence of aromatic amines on zeolite catalysts used for this reaction (ref. 3), as well as the fact that acidic carrier gases such as carbon dioxide are known (ref. 1) to give superior lactam selectivity, point towards the involvement of base readsorption in deactivation. Again in this case calcination would be expected to regenerate lactam yield. The melting and agglomeration of B203 has been proposed (ref. 10) as a deactivation mechanism for silica supported catalysts. This is not thought to be likely in the present case since XRD evidence on used catalysts showed little evidence of crystalline B2O3 being present. In addition boron analysis of the regenerated catalysts (see table) indicated a decrease in boron oxides (especially water soluble) present. Almost 100% of the total boron present in the fresh catalyst is in a water soluble form (B2O3), however only 70% of this latter form of boron is in the regenerated catalyst. It is apparent from the present results that coking and base re-adsorption cannot be the only catalyst deactivation mechanisms, since, if they were, it would be expected that the regenerated catalysts would have contained significant amounts of crystalline, water soluble B2O3 as did the fresh catalyst. The boron analyses (see table) and XRD data indicated that although crystalline water soluble B2O3 was present in the unused catalyst, it was significantly less in the regenerated samples, even though the total boron content (water soluble and water insoluble) was still high. This suggests a third deactivation mechanism (possibly linked to the other two) whereby the B2O3, present on the fresh

538

catalyst and partially responsible for the lactam selectivity, is converted during the catalytic reaction into an amorphous water-insoluble boron species which is not selective for lactam formation. This deactivation process is not thought to be caused by a temperature effect alone since unused catalyst samples which were prepared by calcination in air at 500'C (same conditions as for regeneration) showed lactam yields as good or better than conventional preparations (calcined at 350'C). In conclusion, decrease in cyclohexanone oxime yield and caprolactam selectivity with time on stream is a major factor in the use of boria on alumina catalyst in the rearrangement reaction. Coke deposition and basic by-product adsorption have been suggested as a means of deactivation. In addition the conversion of water soluble boron, which is selective to lactam formation, to an amorphous water insoluble boron species is another factor that can account for the catalyst deactivation.

REFERENCES

Sato,H., Ishii,N., Hirose,K. and NakamuraS., Studies in Surface Science and Catalvsis, . 28 (1 987)755 Aucejo,A., Burguet,M.C., Corma,A, and Fornes,V., ADDI.Catal.. 22 (1986)187 Bbrguet,M.C., Aucejo,A. and Corma,A., Can. J. Chern. Eng., 65 (1987)944 Izumi,Y., Sato,S. and Urabe,K., Chem. Letts. (Chem. SOC.JDn.1. I19831 1649 Sato,S., $akurai,H., Urabe,K. and Izumi,Y., Chem. Letts. (Chem. SOC.Jpn.), (1985)277 Sato,S., Hasebe,S., Sakurai,H., Urabe,K. and Izumi,Y., Ap LCatal., 29 (1987)107 Sakurai,H., ato,S., Urabe,K. and Izumi,Y., Chern Letts. Chern. SOC.Jpn.), (1985)1783 Pupha ,K.W., Merrill,J.A., Booman,G.L. and Rein,J.E., Anal. Chem,. 30 (1958)161 2 Landis,P.S. and Venuto,P.B., J.Catal., 6 (I 966)245 Sato,S., Urabe.K. and Izumi,Y., J. Catal., 99 (1986)102

\

l

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

539

BECKMANN REARRANGEMENT REACTIONS ON ACIDIC SOLIDS

E.GUTIERREZ, A.J.AZNAR and

E.RUI2-HITZKY

Instituto de Ciencia de Materiales, CSIC c/Serrano 115 bis.- 28006 Madrid (Spain) SUMMARY Oximes can be catalytically transformed to amides by different inorganic solid materials following molecular rearrangement processes (Beckmann reactions). These reactions take place in "dry media" conditions, i.e. without any solvent, at relative low temperature (10Oo-16O0C).The yield and the selectivity depend on the nature of the solids (structure, composition, texture, etc) differing, in some cases substantially, with respect to that obtained in the classical homogeneous conditions. Thus acidic solids, as alurninosilicates with layer structure (A13'-montmorillonite), proton-exchanged zeolite (HNaY) or amorphous silica-alumina materials, as well as sulphoarylcompounds (sulphopolystyrene and sulphoarylderivates of silica), have been employed as catalysts, showing good conversion rates and selectivities to the amide formation, in particular after elimination of adsorbed water molecules. The competitive reaction is the hydrolysis of the oxime to carbonyl compounds, which is enhanced by the presence of water at the surface of the solids, in particular when these materials are of less acidic character (silica, alumina, Na-exchanged zeolites and montmorillonites). INTRODUCTION We have recently reported (refs. 1-3) the catalytic rearrangement reactions of 1,2-glycols to carbonyl compounds ( "pinacol rearrangement") on different inorganic materials, the selectivity of the reactions being clearly dependent on the nature of the solid support. The topology of the solids shows a special influence in the development of those reactions. Thus, lamellar materials such as certain 2:l charged phyllosilicates ("clay minerals") give reaction products that are not obtained in the presence of other solids with different topology, or in homogeneous media. The selectivity of these molecular rearrangement processes is clearly determined by the layered structure of the solid catalysts. The purpose of the present work is to study comparatively the activity of different acidic solids as catalysts in other "classical" type of molecular rearrangement as it is the conversion of oximes to amides (Beckmann rearrangement, eqn. 1 ) , by adopting "dry media"

540

conditions and operating under soft experimental treatments. As it is well known these reactions, which are conventionally carried out in homogeneous media, are acid-catalyzed processes. H'

Rl( C=N-OH)R, oxime

R,-CO-NH-R, amide

(1)

Different attempts to use some solid acids -as zeolites and phosphates-as catalysts in gas-solid reactions, have been already described (refs. 4-6). Nevertheless, the control of the selectivity appears to be difficult because drastic experimental conditions are always required. EXPERIMENTAL PART The starting oximes (acetophenoneoxime and cyclohexanone oxime) were prepared by reaction of the corresponding carbonyl compound with hydroxylamine hydrochloride. The purity of the resulting oximes was checked by GC and IR spectroscopy after recrystallisation of the samples. The catalytic reactions were performed either on the hydrated solids (equilibrated with the relative humidity -about 55%- of the atmosphere) or on the dehydrated catalysts (heated at 160"C, during 3 hours). An intimate mixture of the inorganic solid (100 mg) and the oxime in solid state ( 2 0 mg) was introduced in a Pyrex glass reactor. Thus, the reaction was carried out in "dry media" conditions, i.e. without any solvent. The mixtures were either activated with a microwave oven or heated at 100, 130 or 160°C in a conventional oven, during variable times (in the standard procedure: 1 hour). The microwave oven used is a domestic (2450 MHz) Moulinex model FM 460, carrying out the experiments at 600 W of power and introducing a unic vessel in the oven in each experiment. The reaction products were extracted by treatment with a large excess ( 5 ml) of an appropriate solvent (methanol or chloroform), and the extracts were analyzed by GC. The GC analysis were effectued in a Perkin Elmer 8410 chromatograph with a 12 m capillary column of fused silica with associated BP-1 phase. IR spectra were recorded in the range of 40002 5 0 cm-l using K B r disks, fluorolube or nujol mulls on a Perkin-Elmer 580B spectrophotometer, coupled to a M-3500 data station.

54 1

The solids used as catalysts are: Al-mont and Cr-mont: homoionic A1 or Cr exchanged montmorillonite, prepared from a sample obtained from the sodium aluminosilicate of the natural deposit of Wyoming, USA, (sample 25 b, supplied by Word's Natural Science Stablishement Inc., Rochester, N.Y., USA). SAS: arylsulphonic derivative of silica obtained as described in ref.7. A15: commercial sulphopolystyrene resin (Amberlist-15), crosslinked with 8% divinylbenzene, obtained from Fluka. ZHY: partially proton-exchanged Y zeolite, supplied by Union Carbide. Si/A1 ratio=2.9. The exchange ratio of Na' by H': 40% Other solid catalysts: silica (silicagel 60, Merck, 230 mesh), alumina (neutral 90, Merck, 70-230 mesh), and silica-alumina (Akzo Chemie, Ketjen Catalysts, grade LA-3P, Al,O,= 13.8%, t200 mesh). RESULTS AND DISCUSSION Reactions of acetophenone oxime (1) When acetophenone oxime (1)is thermically treated with acidic solids in "dry media" under soft experimental conditions, two main products are obtained: the rearrangement one: acetanilide (N-phenyl acetamide) ( 2 ) obtained by Beckmann rearrangement with migration of the phenyl group, and the hydrolysis one: acetophenone ( 3 ) , obtained by the hydrolysis of the imino group (C=N) (eqn.2). N-OH Acidic solid I1 C,H,-C-CH, s acetoph.oxime

(1)

51

0 \\

CH,-C-NH-C,H, + CH,-C-C,H, acetanilide acetophenone

(2)

(3)

The yield of the reaction (Table 1) clearly depends or the nature of the solid and on the experimental conditions (temperature, time). Thus, with silica and alumina, amorphous solids with a relatively low acidity, the acetophenone oxime molecule reacts with a very low yield, being the only reaction product the hydrolysis one, acetophenone. With the synthetic mixed oxide silica-alumina, that possesses simultaneously Bransted and Lewis acidic centres, the conversion is quantitative, being also the major product the hydrolysis one (31, when the reaction is carried out at 160°C.

Nevertheless, at 130°C (3h), the silica-alumina does not produce appreciable conversion of 1. When the aryl sulphonic derivative of silica is selected as catalyst, quantitative yields are obtained for 3h at 130°C or for lh at 160'C, being in both cases the selectivity of the reaction directed to the hydrolysis product 3 . With the sulphonic resin Amberlist 15 the conversion of the oxime 1 depends on the temperature of the reaction: thus, at 130°C (3h) the yield is close to 50% being the major product the acetophenone ( z ) , while at 160°C (lh) the yield is 73%, and the two products 2 and 3 are detected. TABLE 1 Conversion ( % ) and selectivity ( % ) in the reaction of acetophenone oxime with different acidic solids (rate solid/oxime = 5:l) hydrated (50% r.h.). Reaction conditions: 160"C/lh and 130°C/3h.

Solid Silica Silica Alumina Alumina Si1-alum Si1-alum SAS SAS A15 A15 ZHY ZHY Al-mont Al-mont

Selectivity ( % ) Conditions Conversion (%) Acetanilide Acetophen.("C/hours) 160/1 130/3 160/1 130/3 160/1 130/3 160/1 130/3 160/1 130/3 160/3 130/3 160/1 130/3

6 0 8 0 100 6 100 98 73 50 100 15 100 100

2 2

100 0 88 0 82 50 78 98

57

40

2

90 65 100

0 0 0 0 0 25

33 0 68 17

22

81

-

0 0 12 0 18 25 20 0 3 8 2 0 10 2

In the same experimental conditions (lh, 160"C), the protonic zeolite NaHY gives rise to quantitative percentages of conversion, being the rate 2/3 = 33:65, while at 130'C, 3h, the conversion is of a 15% and the main product the ketone 3. Finally, when the lamellar phyllosilicate montmorillonite (homoionically exchanged with A13' cations) is selected as catalyst, great differences in the selectivity of the reaction are observed: thus when the reaction is carried out at 130°C, 3 hours, the rate of conversion is 100% and the selectivity acetanilide (z)/acetophenone ( 3 ) is 17/81, while at 16OoC, 1 hour, the conversion is likewise of 100% and the rate 2/3 is 68/22.

543

The obtained results point out that a great competition between the Beckmann rearrangement reaction and the hydrolysis one exists. Both reactions are acid catalyzed, but the rearrangement one is favoured when strong acidic solids are used as catalysts and when high reaction temperatures are selected. It is known from the literature that certain acidic solids as protonic zeolites (refs.4 and 5) or aluminium phosphates (ref.6) are used as solid catalysts in the Beckmann rearrangements reaction, but generally strong experimental conditions are required (temperatures higher than 300") In the present work we can deduce that the selectivity of the reaction can be easily changed by varying the experimental conditions (temperature/time) and/or the acidic solid used as catalyst. From Table 1 it can be observed that only the most acidic solids give rise to considerable conversion rates (silica-alumina, aryl sulphonic solids, protonic zeolite, A13*-montmorillonite), the selectivity of the reaction being in general favourable to the hydrolysis product 3. Only with the A13+-montmorillonitesolid at 160"C, the major product is the Beckmann rearrangement one, acetanilide ( 2 ) . This different behaviour of the selected substrates could be interpreted in terms of topology of them and of the strength of the acidic centres. Thus, it has been demonstrated that cations with high polarizing power (high charge and low radii) present in the interlayer space of phyllosilicates induce a great degree of dissociation in the water molecules coordinated with them (eqn.3) and give rise to selective reactions those does not take place with other acidic solids (refs. 1-3). A~(H,o),~+<

>

A1(H20),-n(OH),'3-n'*+ nH'

(3)

In this case, it has been likewise demonstrated that with A13'montmorillonite as acidic solid, the selectivity of the reaction can be easily modified to obtain the ketone ( 3 ) or the amide ( 2 ) . The fact that with the lowest temperature the percentage of hydrolysis product increases, could be attributed to the greatest content of interlayer water (not directly coordinated) present at that temperature (130°C) compared with the highest one (160°C). On the contrary, the rearrangement product, acetanilide, is the principal one for the highest temperature, which is in good agreement with the fact that the rearrangement reaction needs a greater acidity, that is increased when the content of water is low, therefore being favoured at highest temperatures.

544

To demonstrate that the water molecules coordinated to the interlayer cations are the responsible of the hydrolysis reaction, we have carried out the dehydration of the A13*-montmorillonitefor 3h at 160°C previously to the reaction. In this case, we have selected two experimental conditions, 160°C for 1 hour and 100°C for 2 hours, in order to find great differences between both reactions. The obtained results (Fig. 1) clearly shows that when the water content diminishes by the dehydratation process, the percentage of ketone also diminishes, increasing the content of amide in the reaction mixture, especially when the reaction is carried out at the higher temperature (160°C). a)

b)

100°C/2 hours

16O"Cll hour

Fig. 1. Conversion ( % ) and selectivity ( % ) in the reaction of acetophenone oxime with A13'-montmorillonite (rate solid/oxime = 5: 1 ) hydrated (50% r.h.) and dehydrated (3h at 160°C). Reaction conditions: 100°C/2h (a) and 160"C/lh (b). The decrease in the water content by the dehydration process also have influence on the by-products, that mainly correspond to the products from the hydrolysis of acetanilide acetic acid and ani1ine Beside, the yield of the reaction at 100 "C increase for the dehydratated solid, due to the increase in the rearrangement yield. On the other hand, we have compared the yield and selectivity in the reactions of acetophenone oxime with A13* and Cr" exchanged montmorillonites activated thermically and by microwaves (Fig. 2). From the obtained results, we can deduce that microwaves enhance significatively the conversion rate, being the conversion practically quantitative for only 10 minutes of treatment at 600W. The selectivity of the reaction can be directly correlated with that of the thermal treatment at 160°C. The effect of microwaves could be attributed, as we have previously demonstrated (ref.8) to the rapid

.

(z),

545

activation and consequently loss of the water molecules in the solid, that gives rise to a great activation of the organic molecules. The selectivity of the reaction is in good agreement with the diminishing in the water content, which increases the acidity of the solid and, as we have previously demonstrated, favours the rearrangement reaction. a)

b) 10 8

6 4 2

Fig. 2. Conversion ( % ) and selectivity ( % ) in the reaction of acetophenone oxime with A13+ and Cr"-montmorillonites (rate solid/oxime = 5:l) actived thermically (160"C/lh) (a) and by microwaves ( 600W. 10min) (b). Reactions of cyclohexanone oxime ( 4 ) It has been described the reaction of cyclohexanone oxime with solids of different nature, as zeolites (refs. 4 and 5) and aluminum phosphates ( ref. 6 ) , giving rise to the Beckmann rearrangement product, E-caprolactame when the reactions are carried out in the gas phase at relatively high temperatures (>300"C). In the present work we have carried out the reaction of cyclohexanone oxime in the presence of the selected acidic solids in dry media under soft experimental conditions (16OoC, 1 hour). The obtained results show a great complexity due to the presence of significants amounts of differents products (by-products and subproducts), accompanying to the main ones: €-caprolactame ( 5 ) formed by Beckmann rearrangement and cyclohexanone ( 6 ) obtained by hydrolysis of the oxime (eqn. 4 ) . Among the other products present in the reaction mixture, the aminoacid, 6-aminohexanoic acid and the nitrile, 5-hexanenitrile have been detected. We have compared the yield and the selectivity in the reaction f o r the solids in equilibrium with a relative humidity of 50% and dehydrated at 160°C during 3 hours (Table 2). In all cases the conversion rate is close to 100% as much f o r the hydrated solids as for the dehydrated ones. The selectivity of the reaction can be directed by changing the solid

546

and the experimental conditions. Thus with the hydrated A13’montmorillonite the yield in cyclohexanone is 728, while the sulphopolyestyrene resin Amberlyst 15 dehydrated 3h at 160°C. yields E-caprolactame in a 61%.

beozo 0

0 = ” - O H

acidic solid

cyclohexanone oxime

n

e- caprolactame 5

4

4-

+

others

cyclohexanone

(4)

6

From the results presented in Table 2 it can be again deduced that the dehydration process increase in general the selectivity towards the amide, while the presence of certain amounts of water in the solid favours the hydrolysis reaction. With the present data is not possible to establish correlations between conversion and selectivity of the reaction with the nature and topology of the different acidic solids, therefore further investigations are needed to go deeply into the mechanism of the reaction. TABLE 2 Conversion ( % ) and cyclohexanone oxime solid/oxime = 5:1 ) (160°C/3h). Reaction

selectivity ( % ) in the reaction of with different acidic solids (rate hydrated ( 50% r. h. ) and dehydrated conditions: 160”C/lhour. ~

Selectivity Solid

(%)

Hydratation Conversion (%) Cyclohexa. E-caprolac.Others state ~

Sil-alum Sil-alum SAS SAS A1 5 A1 5 ZHY Z HY Al-mont Al-mont

50% r.h. dehydrated 50% r.h. dehydrated 50% r.h. dehydrated 50% r.h. dehydrated 50% r.h. dehydrated

95 97 100 98 99 100 99 98 100 96

~

14 33 15 24 2 5 32 14 72 26

38 47 24 48 47 61 24 29

8 21

43 17 61 26 50 34 43 55 20 49

CONCLUSIONS The acid catalyzed conversion of oximes over different acidic solids in “dry media“ under soft conditions gives as major products the amide (Beckmann rearrangement) and the ketone (hydrolysis). The

547

selectivity of the reaction can be easily directed towards the amide or the carbonyl compound by selecting the experimental conditions (temperature, time) and the acidic solid used. Thus, the nature, topology and hydration state of the solid have direct influence in the conversion and selectivity of the reactions. In the same way, the reaction can be activated by microwaves, reacting quantitative yields for only 10 min. of activation at 600w. ACKNOWLEDGEMENTS The authors acknowledge their very fruitful discussions with Prof. G. Bram and Prof. J.M. Serratosa. This work has been partially financed by the CICYT, Spain. REFERENCES 1 E. Gutierrez and E. Ruiz-Hitzky, Mol. Crvst. Lia. Crvst. Inc. Nonlin. Opt., 161 (1988), 453-458. 2 E. Gutierrez, A.J. Aznar and E. Ruiz-Hitzky, Studies in Surface Science and Catalysis: Heteroaeneous Catalysis and Fine Chemicals, M. Guisnet et a1 (Ed.), Elsevier Sci. Pub., B.V., Amsterdam (1988), 211-219. E. Gutierrez and E. Ruiz-Hitzky, Pillared Lavered Structures: 3 Current Trends and ApDliCatiOnS, I.V. Mitchell (Ed.), Elsevier Appl. Sci., London (1990), 199-208. P.S. Landis and F.B. Venuto, J. Catal., 6 (1966), 245-247. 4 A. Aucej0,M.C. Burguet, A. Corma and V. FornBs, ADD. Catal., 22. 5 (2) (1986), 187-200. I.A. Costa,P.M. Deya, J.V. Sinisterra and J.M. Marinas, An. 6 Quim.. Ser. C, 28 (1) (19821, 43-47. A.J. Aznar and E. Ruiz-Hitzky, Mol. Crvst. Lip. 7 Cryst.Inc.Nonlin. Opt., 161 (1988), 459-469. E. Gutierrez, A. Loupy, G. Bram and E. Ruiz-Hitzky, Tetrahed. 8 Lett., 30(2) (1989), 945-948.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1

549

0 1991 Elsevier Science Publishers B.V., Amsterdam

ON OXIDE CATALYSTS

SELECTIVE RING-OPENING OF I S O M E R I C 2-METHYL-3-PHENYLOXIRANES

ARPAD MOLNAR, IMRE B U C S I and MIHALY BARTdK Department o f Organic Chemistry, A t t i l a Jozsef U n i v e r s i t y , 06m t 6 r Szeged (Hungary)

8,

H-6720

SUMMARY The rearrangement o f l i g h t gnd deuterium-labelled cis- and trans-2-methyl-3phenyloxiranes (1, 2 and 1 , 2 ) was s t u d i e d on ZnO, A 1 0 and WO , and i n t h e presence of BF Both i n the gas phase (473-673 K) and ?he l i q u j d phase (298413 K ) , 1-phdyl-2-propanone ( 3 ) and 2-phenylpropanal (4) were formed with h i g h s e l e c t i v i t i e s (0-90% and 11-80%, r e s p e c t i v e l y ) . Ring-opening was found t o occur by s e l e c t i v e f i s s i o n o f the benzyl C-0 bond. Mechanistic s t u d i e s revealed t h e formation of an open carbenium i o n or a double-bonded surface intermediate. The a c i d i c ( e l e c t r o p h i l i c ) and basic characters o f the oxides determine the product d i s t r i b u t i o n s by a f f e c t i n g the r e l a t i v e importance o f the competing mechanisms.

i6

.

INTROOUCTION The

ring-opening o f oxiranes, leading t o the formation o f isomeric

compounds continuing the

carbonyl

the a c t i o n of a c i d c a t a l y s t s as a r e s u l t of rearrangement, i s

by

i n t e r e s t ( r e f s . 1-4).

of

However, most o f these s t u d i e s focus mainly on

transformations o f terpene oxides

or oxiranes with o t h e r f u n c t i o n a l groups

i n the l i q u i d phase, under homogeneous r e a c t i o n c o n d i t i o n s . The

present

paper r e p o r t s r e s u l t s on the ring-opening o f

ence

of

pres-

d i f f e r e n t oxides both i n the l i q u i d phase and i n t h e gas phase.

Oxide

are known t o catalyse the rearrangement o f oxiranes ( r e f s . 2-4).

catalysts literature pointed

cis- and trans-2-

(1 and 21, c a r r i e d o u t f o r the f i r s t time i n t h e

methyl-3-phenyloxirane

data

( r e f s . 5-7) and our own s t u d i e s with 2-methyloxirane ( r e f .

the importance of acid-base p r o p e r t i e s i n determining

to

selectivities.

Some

8)

ring-opening

For t h i s reason, the oxides used were ZnO, A1203 and W03,

which

cover a wide range of acid-base p r o p e r t i e s . The study o f these simple model compounds

can provide important i n f o r m a t i o n about the r e g i o s e l e c t i v i t y and stereo-

chemistry

of

the ring-opening. A c o r r e l a t i o n can a l s o be expected between

the

a c t i v i t y / s e l e c t i v i t y of the c a t a l y s t s and t h e i r acid-base p r o p e r t i e s . EXPERIMENTAL Materials The

isomeric oxiranes prepared by r i n g c l o s u r e o f 1-bromo-1-phenyl-2-propan-

01 (ref. plate

9)

were separated on a Fischer c o n c e n t r i c tube

number: 90) (b.p.:

cis (1) 355

K/13 mm Hg,

trans (2)

column

(theoretical

361 K/13 mm Hg,

ac-

550

coi~iing to

r e f . 10: 356-357 and 361 Kf13 mm Hg, r e s p e c t i v e l y ;

97%, y i e l d :

58%). The isomers o f 2-methyl-3-phenyloxirane-

isomer

purity:

r 211 1: 2-

(I* and

H

2*)

.

were synthesized i n a s i m i l a r way from 1-bromo-1-phenyl-2-propanol- 2- H 11 C h a r a c t e r i s t i c data on the oxides (Strem Chemicals) are g i v e n i n Table 1. TABLE 1. C h a r a c t e r i z a t i o n of oxide c a t a l y s t s

-n/K

BET surface

value

(m’9-I)

wo,

423K

573K

0 0.10 0.72

20

27.0

0.1s

100 17

58.8 96.8

0.72 1.67

ZnO

A1203

p y r i d i n eC

Acid-base p r o p e r t i e s a Me2Znd nBuNH2e

benzo&c acid

5.25 5.15 4.80

1.96 1.20 3.54 ~

0.90 2.77 0.53

~~

a

Values i n d i c a t e number o f surface a c i d i c or b a s i c s i t e s , nm-’. bn i s the formal charge, and i s the i o n r a d i u s . %umber o f p y r i d i n e molecules adsorbed, as determined by p u l s e chromatographic d t i t r a t i o n i n helium; c a t a l y s t pretreatment: 773 K , 2 h. Number o f surface OH groups as determined by pulse t i t r a t i o n w i t h d i m e t h y l z i n c tetrahydrofuranate complex i n helium a t 363 K according t o r e f . 11; c a t a l y s t pretreatment: 773 K , 2 h. e T i t r a t i o n i n absolute benzene according t o r e f . 1 2 . Methods Reactions

o-xylene tions

with

tions

in

with

i n the l i q u i d phase were c a r r i e d out i n 1,4-dioxane (373 K ) or

(413

K ) ( 0 . 1 g c a t a l y s t and 0.1 g r e a c t a n t i n 0.5 m l s o l v e n t ) .

lpl

8F3.Et20 as c a t a l y s t were r u n a t room temperature.

Transforma-

the gas phase were s t u d i e d a t 473-673 K by u s i n g t h e GC-pulse

catalyst

2

q u a n t i t i e s corresponding t o 0.5 m s u r f a c e

area

in

Reacmethod

(pretreatment:

700

K , 1 h i n helium; 1,411 pulses). The t r a n s f o r m a t i o n o f t h e

led

compounds was c a r r i e d out i n a continuous f l o w r e a c t o r a t 523 K (1 g

deuterium-labelcata-

l y s t , 1 g h - l feeding r a t e o f r e a c t a n t s , f l o w r a t e o f helium: 50 ml min-’1. Analyses 20% polyethylene g l y c o l succinate on Kieselguhr column (1.2 m, 443 K ,

A

ml min-’ helium c a r r i e r gas) was used f o r GC analyses. Deuterium-labelled pounds

50 com-

were analysed by NMR spectroscopy (JEOL C 60-HL equipment) a f t e r separaw i t h a Carlo Erba Mod P p r e p a r a t i v e GC. Mass spectrometric analyses o f the

tion

reaction

mixtures

were c a r r i e d out with a Hewlett Packard 5890A GC

instrument

(25 m HP-20M column, 353-473 K ) coupled with a 5970 MSD quadrupole mass spectrometer ( E I source, 70 eV, 1-s scans, HP 59970 MS ChemStation data system). RESULTS The in

the

experimental r e s u l t s on the t r a n s f o r m a t i o n s o f the two i s o m e r i c oxiranes gas phase (Table 2) i n d i c a t e the formation of t h r e e

isomeric

carbonyl

551

compounds (Fig. 1). Results in the liquid phase in solvents (2-xylene, 1,4-dioxane) often used in homogeneous reactions are given in Table 3. TABLE 2 Selectivity of transformations of and trans-2-methyl-3-phenyloxirane (1 and 2) on oxide catalysts in the gase p h x r e a c t i o n temperature: 523 K )

a-

Catalyst Oxirane

ZnO

1

Conversion (mol%) 2-Phenylpropanal ( 4 ) 1-Phenyl-2-propanone (3) 1-Phenyl-1-propanone (5) Decomposition 4/3 ratio

A1203 1 2

2

46 13 83 76 2 2 0.32 1.22 0.16 17 24

15 55 45

wo3

1

40 48 49

100 11 87

2 98 13 84

3 2 3 0.98 0.13 0.15

TABLE 3 Selectivity of transformations of 1 and 2 on oxides in the liquid phase Solvent (reaction temp., K ) Catalyst Oxirane Conversion (reaction time, h) 2-Phenylpropanal ( 4 ) 1-Phenyl-2-p~opanone( 3 ) Unidentified

o-Xylene (413) b

1,4-~ioxane( 3 7 3 1 ~ A1

1

1

73(24) 62(24) 14 26 79 70 7 4

32

20(12) 14(12) 75 25

50 50

1

78(3) 20 80

62(3) 47 50 3

aNo transformation was observed n ZnO and A1 0 after 24 h. bNo transformation was observed on ZnO after 24 h. ‘Mainly polym&?c material.

phwMe a O b

&/I

H-+/l

Ph-CH2-C-Me

Me Ph-tH-C,

6 3

4 H

4

Ph-C-C%-Me

1

5

Fig. 1. Transformation directions of 2-methyl-3-phenyloxirane. Data on the transformations under homogeneous conditions with BF3 as catalyst can be found in Table 4. Because of the significant side-reactions taking place mainly in 2-xylene, only the results in l,4-dioxane can be considered characteristic of rearrangement.

552

Further important data concerning t h e r e a c t i v i t i e s of l i g h t and deuterium-laisomers are given i n Table 4. Oata on t h e deuterium d i s t r i b u t i o n i n

belled carbonvl

comoounds formed i n t h e t r a n s f o r m a t i o n s o f t h e isomers o f

[

phenyloxirane- 2- H

'11

the

2-methvl-3-

are t o be found i n Table 5.

TABLE 4. isomeric 2') o f transformations o f l i g h t ( 1 , 2) and l a b e l l e d (l*,

Selectivity

2-methyl-3-phenyloxiranes catalysed by BF3 under homogeneous c o n d i t i o n s a 1

Oxirane

1*

Solvent

2*

1*

1

(3)

16 22 20

2

2*

o-Xylene

1,4-Dioxane

Conversion (mol%) :-Ph,nyl-~-~E-;~n3riL: Unidentified a

2

11

35

24

90

23

10

12

97 3

21 40 60

14 7c 30

3 22 78

4

23 71

Room temperature, r e a c t i o n time: 30 min. bblainly polymeric m a t e r i a l .

TABLE 5 Tracer s t u d i e s w i t h C a t a l y s t Substrate

cis (1')

and

trans (2*)

c

2-methyl-3-phenyloxirane- 2- H

2-Phenylpropanal (4*Ia

1-Phenyl-2 propanone (3*)a

deuterium distribution i n content l a b e l l e d p o s i t i o n s

deuterium distribution i n content l a b e l l e d p o s i t i o n s C1-0 C3-0

c1-0

c2-0

l*

0.97 0.97 0.83 0.71 0.70 0.60 0.82 0.93

2*

BF3 ZnO

1*

0.96 0.97 0.90 0.93 0.83 0.91

2*

1* A1203

2*

w03

2'

1*

*I1

0.91 0.91 0.86 0.89 0.80 0.87

0.05 0.05 0.04 0.04 0.03 0.04 ~

0.97 0.97 approximately equal amounts i n t h e two positions

~~

a A s t e r i s k s i n d i c a t e l a b e l l e d compounds. The above experimental r e s u l t s can be summarized as f o l l o w s . Under homogeneous c o n d i t i o n s , e x c l u s i v e formation o f 1-phenyl-2-propanone (i)

( 3 ) took place, regardless o f t h e stereochemistry o f t h e oxiranes. On the oxides i n the gas phase t h e (ii) phenyl-2-propanone ited

similar

cis isomer

(1) was transformed i n t o 1-

( 3 ) with h i g h s e l e c t i v i t y , w h i l e t h e

s e l e c t i v i t i e s i n the formation o f b o t h

trans isomer

(2) exhib-

1-phenyl-2-propanone

(3)

and 2-phenylpropanal (4).

(iii) The oxide c a t a l y s t s e x h i b i t e d an i n c r e a s i n g a c t i v i t y p e r u n i t surface a r ea

i n the sequence ZnO< A1203<

W03. A s i m i l a r t r e n d o f i n c r e a s i n g s e l e c t i v i t y

was observed i n the formation o f 1-phenyl-2-propanone ( 3 ) . (iv)

I n 1-phenyl-2-propanone

formed i n t h e BF3-catalysed r e a c t i o n , e x c l u s i v e l y

553

position C1 is labelled, while on the oxides there was a significant deuterium loss and extensive randomization of the deuterium in positions C1 and C3 via enolization. In contrast, the formyl hydrogen in the 2-phenylpropanal formed was almost selectively labelled with deuterium. (v) Further important information not included in the Tables is that interconversion of the three carbonyl compounds was not observed under any experimental conditions. DISCUSSION The following main conclusions resulting from the experimental observations require interpretation. (i) Under homogeneous conditions, the selectivity does not depend on the steric structure of the oxiranes. (ii) On ZnO and Al2O3, the isomers exhibit different selectivities: the 4/3 ratio is always higher for the trans isomer. In contrast, on W03 the same 4/3 ratio is observed for the two isomers. (iii) The oxides exhibit different activities and selectivites. Theoretically, the ring-opening of 2-methyl-3-phenyloxirane can take place by rupture of either the benzyl C-0 (route a ) o r the alkyl C-0 (route b ) bond (Fig. 1). The consecutive or concerted migration of the corresponding groups lead to three isomeric carbonyl compounds. The negligible amount of l-phenyl-lpropanone (5) among the products indicates that route b can not play a significant r o l e in determining product selectivities. The almost selective deuterium labelling of the formyl hydrogen in the 2-phenylpropanal formed from 1' and 2' points to an exclusive methyl migration during ring-opening. On the basis of these two facts, alkyl C-0 bond rupture (route b) can be excluded, i.e. ringopening takes place by fission of the benzyl C-0 bond (route a ) . From a mechanistic point of view, the rearrangement of oxiranes can be interpreted in terms of either a concerted mechanism or the participation of an open carbenium ion. For oxirane rearrangements catalysed by BF3. Et20, a mechanistic pathway involving an open carbenium ion is generally accepted (refs. 2-4). Product selectivities are then determined by the migratory aptitude of the different groups, the ease of formation of the transition state and the rate of rotation about the C-C bond of the carbenium ion intermediate. Studies of 2-substituted oxiranes labelled with deuterium on C3 either cis or trans to the susbtituent revealed a slight diastereotopic selection of the trans H o r 0 in rearrangements (refs. 13, 14). This means that the rate of hydride/deuteride migration is comparable with the rate of rotation. The selective formation of ketone 3 in our experiments under homogeneous conditions reflects a fast rotation relative to migration. It allows the generation of a conformation which ensures the necessary alignment of the migrating hydrogen and the empty p orbital (Fig. 2).

554

Fig. 2 . Mechanism o f BF -catalysed rearrangement o f isomeric 2-methyl-3-phenyloxiranes t o 1-phenyl-2-bropanone ( 3 ) . On t h e oxides, aldehyde 4 was always formed t o some e x t e n t s . From a consideration

of

the

above arguments concernig the

mechanistic

considerations,

the

i n the r e l a t i v e r a t e o f r o t a t i o n about the C-C bond seems t o be the most

change

important reason f o r t h e decrease i n s e l e c t i v i t y o f t h e formation o f ketone 3. The ic

c a t a l y t i c a c t i v i t i e s o f the oxides are a t t r i b u t e d t o t h e i r surface a c i d -

and

nated

basic centres ( r e f . 15-17). The a c t i v e s i t e s are

metal

groups sites

can

ions also

and oxygen i o n s (Lewis a c i d and base

incompletely centres).

a c t as Bronsted centres. The mutual a c t i o n

of

coordiOH

Surface these

active

r e s u l t s i n ring-opening, l e a d i n g t o t h e formation o f a surface intermedi-

ate. I f t h i s i s bonded t o b o t h a c i d i c and b a s i c surface a c t i v e s i t e s , then r o t a about the C-C bond i s g r e a t l y hindered as compared t o r o t a t i o n i n the open

tion

carbenium i o n (Fig. 3). However, i t i s f l e x i b l e enough f o r t h e l i m i t e d conformat i o n a l motions r e q u i r e d f o r m i g r a t i o n of the s u b s t i t u e n t s .

F i g . 3. Surface-bound intermediate formed i n the ring-opening o f methyl-3-phenyloxiranes on oxides (M= metal i o n s ) . For aligned (Fig. the be

the surface species formed from the

cis isomer,

a l i m i t e d r o t a t i o n gives

conformations i n which e i t h e r the hydrogen or t h e methyl group i s

the

f o r m i g r a t i o n t o occur, y i e l d i n g ketone 3 and aldehyde 4 ,

4).

2-

isomeric

Because o f the s t e r i c r e p u l s i o n between t h e phenyl

suitably

respectively

susbtituent

and

surface i n the conformation f o r methyl m i g r a t i o n , an excess o f ketone 3 can expected. I n the surface species formed from t h e

trans isomer,

the situation

i s the opposite ensuring a higher p r o b a b i l i t y f o r t h e formation o f aldehyde 4.

555

H

Me

3-

- 4

Fig. 4. Conformational changes of surface-bound intermediate formed from 1, allowing H or Me migration. Changes in selectivities on the oxide catalysts cannot be interpreted in terms of the concerted mechanism. Since steric requirements for the formation of the transition state favour migration of the substituent trans to the bulky group on the migration terminus, large differences in product distributions should be expected, depending on the steric structures of the isomers. As regards the reactivities of oxides in catalysing the rearrangement, the trend ZnOC A1203 < W03 correlates well with their acidity, determined by titration with pyridine, and with their E/L value, which is a measure of the electrophilic character of the oxides (Table 1). The increasing acidic character results in increasing selectivity of the formation of ketone 3 (decreasing 4/3 ratios). In harmony with this, poisoning the acidic sites with pyridine leads to decreased activities, but gives rise to increasing selectivity for the formation of 4 (increasing 4/3 ratios) (see the example of W03 in Fig. 5).

xl-==-l 5

413 mtio

Fig. 5. Effects of pyridine on relative conversion and product selectivity of 1 ( a)and 2 ( A ) on W03 (reaction temperature: 473 K ) .

(x)

-6

25

Kl mol pyridine

Similar correlations between the acid-base properties of catalysts and activity/selectivity were earlier observed in the rearrangement of simple oxiranes (refs. 5-8). In our case it seems reasonable to suppose that the observed changes are due to the different competing mechanisms discussed above. W03, with strong acidic sites in high concentration, is able to form the carbenium ion. Since the density and the strength of the basic sites on W03 are low, f o r mation of the double-bonded surface species depicted in Fig. 3 has only a low probability. The single-bonded open carbenium ion is then mainly transformed to ketone 3. In harmony with this, the isomers exhibit identical selectivity, a

556

p i c t u r e c h a r a c t e r i s t i c o f the BF3-catalysed r e a c t i o n . On

A1203 and more t y p i c a l l y on ZnO, the s e l e c t i v i t y s h i f t s towards formation

the aldehyde ( i n c r e a s i n g 4/3 r a t i o s , Table 2). There are a l s o l a r g e

of

ences

in

differ-

s e l e c t i v i t y between the isomers. These are i n d i c a t i v e o f a change

in

mechanism o f the rearrangement. With i n c r e a s i n g b a s i c i t y of t h e oxides, the

the

relative

importance o f basic s i t e s i n the r i n g opening increases. This

results

i n an increased p r o b a b i l i t y o f formation o f t h e double-bonded surface i n t e r m e d i ate (Fig. 3 ) , which can be transformed t o b o t h isomeric carbonyl compounds. CONCLUSIONS The

rearrangements o f

found

(1 and 2) were

t o occur by s e l e c t i v e f i s s i o n o f the benzyl C-0 bond y i e l d i n g l-phenyl-2-

propanone selectivity ic)

cis- and trans-2-methyl-3-phenyloxirane

;3) and 2-phenylpropanal ( 4 ) . The a c t i v i t i e s o f t h e c a t a l y s t s and the o f formation o f 3 increased w i t h i n c r e a s i n g a c i d i c ( e l e c t r o p h i l -

character o f the oxides. The d i f f e r e n c e s i n s e l e c t i v i t y under d i f f e r e n t re-

action

c o n d i t i o n s were explained i n terms o f d i f f e r e n t r e a c t i o n mechanisms. Se-

lective

r i n g opening catalysed by EF3 and W03 occurs through an open

carbenium

ion. This allows f r e e r o t a t i o n around the C-C bond, p e r m i t t i n g the h i g h l y selective place acidic

formation via and

of

3

from b o t h isomers. Rearrangement on A1203 and ZnO

a double-bonded surface i n t e r m e d i a t e w i t h the p a r t i c i p a t i o n o f b a s i c s i t e s . The l i m i t e d conformational motion i n t h i s species

s u l t s i n the formation o f both

3 and

4

takes both

re-

.

REFERENCES 1 W.L.F. Armarego, i n Stereochemistry o f H e t e r o c y c l i c Compounds, P a r t 2, Wiley, New York, 1977, 13-32. The Chemistry o f F u n c t i o n a l 2 M. Bartok and K.L. L i n g , i n S. P a t a i (Ed.), Groups, Supplement E, Wiley, Chichester, 1980, Chapter 14, 609-681. 3 K . Arata and K. Tanabe, Catal. Rev.-Sci. Eng., 25 (1983) 365-420. 4 M. Bartok and K.L. L i n g , i n The Chemistry o f H e t e r o c y c l i c Compounds, Vol. 42, ( A . Hassner, Ed., Small Ring Heterocycles, P a r t 31, Wiley, New York, 1985, Chapter 1, 1-196. 5 T. Imanaka, Y. Okamoto and S. T e r a n i s h i , El. Chem. SOC. Jpn., 45 (1972) 1353-1357. 6 Y. Okamoto, T. Imanaka and S. T e r a n i s h i , El. Chem. SOC. Jpn., 46 (1973) 4-8. 7 E. Ruiz-Hitzky and B. Casal, J. Catal., 92 (1985) 291-295. 8 A. M o l n i r , G. Resofszki, I.Eucsi, Gy. G h t i and M. Eartok, i n p r e p a r a t i o n . 9 P.A. Marshall and R.H. Prager, Austr. J. Chem., 30 (1977) 151-159. 10 F. Fischer, Chem. Eer., 90 (1957) 357-362. 11 L. Nondek, React. K i n e t . C a t a l . L e t t . , 2 (1975) 283-289. 12 V.N. Borodin, Zh. F i z . Khim., 51 (1977) 928-929. 13 J.M. Coxon and C-E. Lim, Aust. J . Chem., 30 (1977) 1137-1143. 1 4 J.M. Coxon and O.Q. McOonald, Tetrahedron L e t t . , 29 (19EE) 2575-2576. 15 H. Knozinger, Adv. Catal., 25 (1976) 184-271. 16 H.A. Eenesi and B.H.C. Winquist, Adv. C a t a l . , 27 (1978) 97-182. 1 7 A. Zecchina, E. Garrone and E. G u g l i e l r n i n o t t i , i n G.C. Bond and G. Webb (Eds.), C a t a l y s i s , Vol. 6, Royal Society o f Chemistry, B u r l i n g t o n House, London, 1983, 90-143.

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1

557

0 1991 Elsevier Science Publishers B.V., Amsterdam

MONO AND TRIDIRECTIONAL 12-MEMBERED RING ZEOLITES AS ACID CATALYSTS FOR CARBONYL GROUP REACTIONS

M .J . Climent, A. Corm*, H . Garcia, S . Iborra and J. Primo Instituto de Tecnologia himica UPV-CSIC, Universidad Polit&mica, Apartado 22012, 46071 Valencia, Spain.

SUMMARY Monodirectional 12 membered ring zeolites (offretite, L, mordenite and a ) are very inefficient as catalysts for . formaldehyde benzene condensation to give diphenylmethane, esterification of phenylacetio acid with equimolar amounts of ethanol, Friedel-Crafts acylation of 3-phenylpropanoyl chloride with anisole and Claisen-Schmidt condensation of acetophenone with benzaldehyde. This fact has been attributed to diffusional constraints of organic compounds inside the channels. By contrast, the behaviour of the tridirectionalp zeolite is very similar to that of dealuminated HY zeolites, increasing the turnover of the acid sites with the framework Si-to-A1 ratio. INTRODUCTION Recently, we have shown that aaid faujasites can oatalyze reactions involving carbonyl groups in the liquid phase and at. moderate temperatures, and have observed that physicochemical modifications on the Y zeolite inf hence their catalytic activity (refs. 1-4). In this context, little information is currently available ooncerning the use of other aoidic large pore zeolites, especially

with

Beta

(P)

structure,

as acid catalysts

in

organic reaations. In the present paper we have studied four acid catalyzed reaotion8 involving aarbonyl compounds: alkylation of benzene with formaldehyde, esterifiaation of phenylacetic acid, FriedelCrafts acylation by phenylpropanoyl chloride, and the cross aldolic condensation of aaetophenone with benzaldehyde in the presence of

three HP zeolites with

different

framework

Si-to-A1

ratio and four monodirectional 12 membered ring zeolites (offretite, mordenite, L, and omega ( a ) ) .The aim is to determine the role played by the intrinsic acidity and the crystalline struature of the zeolite on this type of acid catalyzed reaot ions

558

EXPERIMENTAL Faujasite type zeolites were prepared from two NaY oommercial samples (,Si-to-A1ratio 2.4) with average orystal size of 0 . 8 0 (HY100) or 0 . 3 0 pm

by exchanging with solutions of ammonium acetate

(ref. 3). HP Zeolites were prepared starting from TEA-0 (ref. 5 ) by heating at 773 K in N2 stream,

followed by calcination in air at

823 K and twice NH4' exchange and calcination (823 K). Zeolites n (orystal size: fl-1 0.1 x 0 3 pm polycrystalline agregates, fl-2 0.1 x 4 . 0 )rm prismatio single arystals) and offretite were kindly given to us by professor F. Figueras (Montpellier). fl-1 and fl-2 were activated in the same way as zeolites Beta, while offretite was submitted to four exchange-caloinations. Zeolite L was synthesized following standard procedures ( r e f . 6), while mordenite was supplied by P.O.Industries.The A1 ocntent of the zeolites is given in Table 1

TABLE 1 Chemiaal composition ZEOLITE

(Al/Al + Si) of

From chemical analysis

From NMR MAS

0.294 0.286 0.091

0.098 0.142 0.069 0.045 0.027

HY-100

HY-P Ha-10 HB-18 Ha-23 Mordenite

0.052 0.034

L

0.164 0.190 0.250

0-2

0.227 0.22 1

offrerite

n-1

the zeolite studied.

A solution of the corresponding reagents and nitrobenzene (150 as internal standard, was poured onto the thermally The resulting aotivated (423 K, 1-3 Torr) aatalyst (1.00 g) suspension was magnetiaally stirred at 351 or 408 K and the aourse of the reaation periodlaally followed by GC (25 m oapillary column of cross-linked 5% phenyl methyl siliaone) analysis of the organio solution at times ranging from 0 1 to 20 h At the end of the reaction the aatalyst was filtered, and subrmtted to

mg),

559

oontinuous Soxhlet extraation with diohloromethane. The organia solutions were conaentrated in vaouum, weighted and analyzed by GC-

MS and IH-NMR (Varian 360 EM using deuterated trichloromethane as solvent). The total material reoovered accounted in all cases for more than 85% of the starting materials. RESULTS AND DISCUSSION of Zeolite StrThe stoiahiometry of the reaations studied and the results aohieved using different oatalysts are oontained in Tables 2-5, It has to be remarked the strong deareaee in reaotivity observed in theee liquid phase reaotions when moving from tri to monodireotional zeolites, the latest being muah more ineffioient as aatalysts for the reactions tested.

TABLE 2

Results of the condensation of formaldehyde ( 3 . 3 3 mmol) with benzene ( 5 0 ml) at 351 K aatalyzed by aaid zeolites (1.009).

CH20 + 2PhH

CATALYST

a-10

B-18 B-23 L n-1 n-2 Mordenite offretite HY-100 (Si/Al9.2) HY-P (SiIAl6.0)

YIELD OF I(%)

43.0 32.0 27.0 3.3 2.6 14.0

-

PhCH2Ph + H20 1

JNlTIAL RATE(h-l)

TOP

435

30.0 42.0 34.0 5.2 0.8 11.0

1027

61.0

77.0

785

63.0

91.0

637

922

<1
Calculated from A1

by NMR

560

TABLE 3 Results for the esterifioation of phenylaoetio aoid ( 1 , 8 mmol) with ethanol ( 2 . 1 mmol) in triohloromethane (50 ml) at 351 K catalyzed by acid zeolites (1.009).

PhCH2COOH + EtOH

c.rrrlyor 0-10 13-18 13-23

offmite

L

-D

Yield of 2 (%) ( d o n time 17 h) 82 15 48 <1 <1

PhCH2COOEt + H 2 0 2

Initialrate (U1)

21.0 11.3 12.4

TOF 261 247 374

In order to discuss possible differences in the intrinsic aotivity, the oatalytio isomerization of xylenes and oraoking of nheptane were carried out on these samples and the results given in Table 6 . As oan be seen, exoept for L and fl, there is no signifioant differences in intrinsic activity of these zeolites. Therefore, the absenoe of product formation during the condensation of formaldehyde with benzene (Table 2) on mordenite and offretite cannot be related with intrinsio activity but probably with diffusional problems. Moreover, the higher initial rate for the oondensation of formaldehyde with benzene for fl-2 than for 0-1 (Table 2 ) agrees with some kind of diffusion oontrol, since the diffusion rate is two orders of magnitude higher in prismatic single crystals ( f l - 2 ) than in polycrystalline agregates (n-1) (ref. 7). Therefore, it seems that tridirectional zeolites HY and H13 are the most promi sing ones for liquid phase reactions. In the case of H13 zeolites, two items deserve speoial oomments. Firstly, the yield (82%) of ethyl phenylacetate for the equimolar esterification of phenylacetic acid and ethanol in the presence of the B-10 sample is Rubstantially higher than that of the equilibrium (69%) at the same temperature and solvent (Table 3 ) , Analogous results have been already observed with dealuminated acidio Y faujasites and oan be due to zeolite water adsorption andlor to the hydrophobioity of the in surfaces (ref. 2). The hydrophobic oharacter of high silioa

561

zeolites makes very small the oonuentration of free water on the solid surface, the real reaction medium, thus shifting the equilibrium to further esterification.

TABLE 4 Friedel-Crafts acylation of 3-phenylpropanoyl chloride (0.59mmol) in anisole (50 ml) at 408 K in the presenue of acid zeolites (1.00 g) for 17 h of reaction time. Intramolecular aoylation

+

HC1

3

Intermolecular acylation

4

Produd (Yield %)

caratrsr a-lo

l3-18 U-23

Initial rates (h-l)

3

4

3

8.5 7.1 10.0

7 6 6

0.04 1.22 4.70

a 3-Phenylpropanoic acid is also formed

4

0.60 0.93 4.35

TOF 3 12.3 37.2 148.7

4

8.8 20.6 137.9

562

TABLE 5 Claisen-Sohmidt oondensation of aoetophenone and benzaldehyde (0.5 mmol) in benzene (10 ml) in the presenoe of aoid zeolites (1.009). PhCOCH3 + PhCHO

PhCOCH=CHPh

-

+ PhH

(0.5 mmol) at 351 K

PhCOCH=CHPh + H20 5 Ph / PhCOCH2CH \ 6 Ph

-

product Yield(%)

catalyst 13-10 1)-18

13-23 HY-100 (SiM9.2)

5

Initial tllre of 5 (h-l)

6

0 0 0 9

92 75 64 50

W C i S

TOFof 5

15 10 15

360

6

61

lati0

165

15.7 13.0 14.4 11.5

190

TABLE 6 Initial rate cracking.

for

m-xylene

isomerization,

and

for

n-heptane

REACTION m-xylene isomerizariona Zeolite HY-100 HY-P 0-10 L 0-1

0-2 Mordeilite

Offmite

:X

To

/@-1.s-lmo1)

E: ; 15.6 5.6 3.6 4.2 47.1 91.5

p-xylene/o-xylene 0.7

0.7 1.7 0.7 1.1 1.1 1.1 1.8

n-heptane crackingb To

(mill-') 1.50 2.34 0.70 0.02 0.07 0.90 1.00

a Calculated by extrapolation at zero time on stream, 6 2 3 K reaction temperature and 0.20 atm of m-xylene partial pressure. Caloulated at time on stream 2-5 minutes, 7 2 3 K of reaction temperature, and atmospheric pressure of n-heptane.

563

Secondly, the product distribution for the reaction of 3phenylpropanoyl chloride with anisole catalyzed by zeolite beta (Table 4 ) is very similar to that found for acid faujasites and quite different to the AlCl3 catalysis in which the ratio of 3 to 4 obtained is 7 . 0 .Taking into account the adsorption properties of zeolites, their enhancement of the intermolecular reaction could be attributed to a high concentration of both reagents inside the cavities, thus promoting more efficiently the formation of the propiophenone 4 than a conventional AlC13 catalyst. On the other hand, a remarkable difference between catalysis by Y and 0 zeolites has been found for the Claisen-Schmidt condensation of acetophenone and benzaldehyde (Table 5 ) . When the cross aldolic reaction is carried out in the presence of HY, together with the expected and chalcones 5 , the 3,3-diphenylpropiophenone 6 is also formed, this product being not detected on 13 zeolites. A likely explanation for the absence of 6 using zeolite beta is that the crystalline structure of this zeolite exerts a spatial constraint making difficult the formation of a big size molecule like 6 , especially in the smaller channel. Similar effects due steric limitations on 13 catalysis have been found €or the formation of multi-branched products during the cracking of alkanes (ref 8). Finally, the influence of the framework Si-to-A1 ratio is clearly observed by increases upon dealumination of the turnover frequency (TOF) per framework A1 calculated from the MAS NMR data, for the series of HYD and I3 zeolites. Moreover, similar TOF values are found for HYD and D zeolites, being in general slightly higher for the seaond zeolite. This would indicate that I3 has stronger acid sites than dealuminated HYD samples, something whiah agrees with I R data of both solids, where the wavenumber of the accesible acidic hydroxy groups is 3620-3630 cm-l on dealuminated HY, and 3612 cm-l in H13 zeolites (ref. 8). A similar conclusion was reached by measuring the desorption of pyridine on these two types of large pore zeolites. In conclusion, we have shown that due to the diffusional problems, monodirectional large pore zeolites are very inefficient to perform bimolecular reactions involving carbonylic reagents in the liquid phase. By contrast, catalysis by tridirectional Y and D zeolites show similar features, with a high catalytic activity which increases with the acid strength of the acid sites.

564

ACKNOWLEDGEMENT Finanoial support by the Comisi6n Interministerial de Cienoia y Teonologla of Spain (Project MAT 88-0147) is gratefully aoknowledged . REFERENCES 1 , A. Corma, M.J. Climent, H. Garoia and J . Primo, Appl. Catal,, 49 (1989), 109 2 . A. Corma, H. Garoia, S . Iborra and J . Primo, J. Catal., 120 (1989). 78. 3 . M.J. Climent, A. Corma, H . Garcia and J . Primo, Appl. Catal., 51 (1989), 113. 4. M.J. Climent, H. Garcia, J. Primo and A. Corma, Catal. Lett. 4 (1990), 85. 5. J . PBrez-Pariente, J. Martens and P.A. Jacobs, Appl. Catal., 31 (1987), 35, 6. R.M. Barrer and H . Villiger, Z. Kristallogr., 128 (1985). 35. 7 . B . Chauvin, F. Fajula, F . Figueras, C. Gueguen and J . Bousquet, J. Catal. 111 (1988), 94. 8. A. Corma, V. Forn6s. F. Melo, and J. Pbrez-Pariente in:

M.Ooelli (Ed,),Fluid Catalytio Craoking, Role in Modern Refinary, A.C.S. Symp. Ser. 375 (1988). 49, and J-Catal., 107 (1987). 288.

M. Guisnet et al. (Editors), HeterogeneousCatalysis and Fine ChemicalsII 0 1991 Elsevier Science Publishers B.V., Amsterdam

565

TRIPLE BOND HYDRATION U S I N G ZEOLITES AS CATALYSTS

A. FINIELS, P. GENESTE, M. LASPERAS, F. MARICHEZ and P. MOREAU L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimique Appliquees Ecole N a t i o n a l e Superieure de Chimie de M o n t p e l l i e r - 8, Rue de 1 ' E c o l e Normale 34053 M o n t p e l l i e r C6dex 1 France. SUMMARY H y d r a t i o n r e a c t i o n s o f alkynes and n i t r i l e s were s t u d i e d o v e r v a r i o u s z e o l i t e s i n l i q u i d phase, w i t h e t h a n o l as s o l v e n t . Alkyne h y d r a t i o n l e d t o expected carbonyl compounds whereas t h e f o r m a t i o n o f t h e amide and o f t h e c o r r e s p o n d i n g e s t e r was observed d u r i n g n i t r i l e h y d r a t i o n . I n t h e case o f alkynes, a l i n e a r c o r r e l a t i o n was f o u n d between t h e a c t i v i t y o f t h e z e o l i t e and t h e Si/A1 r a t i o . A d i f f e r e n t behaviour was observed f o r n i t r i l e s , which can be r e l a t e d t o t h e d i f f e r e n c e i n t h e a d s o r p t i o n o f t h e compounds. INTRODUCTION

The use o f z e o l i t e s as s e l e c t i v e c a t a l y s t s i n o r g a n i c syntheses i s a f i e l d o f growing importance. Z e o l i t e s a r e s a l t s o f s o l i d s i l i c o a l u m i n i c a c i d s c h a r a c t e r i z e d by a s t r i c t l y r e g u l a r s t r u c t u r e o f t h e i r c r y s t a l l i n e

lattice

( r e f . 1 ) and by t h e i r h i g h a c i d i t y and shape s e l e c t i v i t y ( r e f . 2 ) . A t t e n t i o n t o t h e broad p o t e n t i a l of z e o l i t e s i n o r g a n i c r e a c t i o n s was f i r s t drawn i n t h e s i x t i e s by Venuto ( r e f . 3 ) and v a r i o u s a p p l i c a t i o n s o f t h e i r c a t a l y t i c p r o p e r t i e s have been r e c e n t l y reviewed ( r e f . 4 ) . Our c u r r e n t i n t e r e s t i n t h e development o f heterogeneous c a t a l y s i s l e d us t o c o n s i d e r t h e use o f such z e o l i t e s as c a t a l y s t s i n t h e h y d r a t i o n r e a c t i o n o f C - C and C :N

t r i p l e bonds i n l i q u i d phase.

The h y d r a t i o n r e a c t i o n o f alkynes l e a d i n g t o c a r b o n y l compounds i s gener a l l y c a r r i e d out i n d i l u t e a c i d i c conditions w i t h mercuric i o n s a l t s ( o f t e n t h e s u l f a t e ) as c a t a l y s t s ( r e f . 5 ) . Only v e r y r e a c t i v e a l k y n e s ( p h e n y l a c e t y l e n e and d e r i v a t i v e s ) can be h y d r a t e d i n s t r o n g a c i d i c c o n d i t i o n s w i t h o u t mercury s a l t s

(ref.

6).

(H2S04)

Mercury exchanged o r impregnated s u l f o n i c

r e s i n s have a l s o been used i n such r e a c t i o n s ( r e f . 7 ) . N e v e r t h e l e s s , t h e l o s s o f t h e c a t a l y s t d u r i n g t h e r e a c t i o n and environmental problems due t o t h e use o f mercury make t h i s r e a c t i o n method n o t as c o n v e n i e n t as i t s h o u l d be f o r t h e p r e p a r a t i o n o f c a r b o n y l compounds. A v a r i e t y o f methods e x i s t f o r t h e h y d r a t i o n o f n i t r i l e s t o amides o r t o

a c i d s ( r e f . 8 ) . While a c i d i c and b a s i c c a t a l y s t s have l o n g been employed, t h e importance

of

catalytic

h y d r a t i o n , e s p e c i a l l y under n e u t r a l c o n d i t i o n s , i s

566

r e c o g n i z e d t o a v o i d such drawbacks as p r o d u c t i o n o f a l a r g e q u a n t i t y o f byp r o d u c t s a l t s . Under t h e c o n d i t i o n s commonly used, h y d r o l y s i s o f t h e i n t e r m e d i a t e amide i s as f a s t as, o r much f a s t e r t h a n , t h e i n i t i a l h y d r a t i o n o f t h e n i t r i l e depending on t h e n i t r i l e r e a c t i v i t y . However, s e v e r a l e f f e c t i v e p r o c e dures have been developped f o r i n t e r r u p t i n g t h e process a t t h e amide s t a g e i n homogeneous ( r e f . 9 ) o r heterogeneous c a t a l y s i s on m e t a l l i c o x i d e s ( r e f . 1 0 ) . I n b o t h cases, t h e h y d r a t i o n r e a c t i o n o f t r i p l e bonds o v e r z e o l i t e s has received l i t t l e a t t e n t i o n . Concerning alkynes, o n l y t h e h y d r a t i o n o f a c e t y l e n e t o acetaldehyde has been s t u d i e d on X z e o l i t e s i n Cu, Ag,

Zn, Cd forms ( r e f . 1 1 ) . These z e o l i t e s

are a c t i v e but they s u f f e r f a s t deactivation. I n t h e case o f n i t r i l e h y d r a t i o n , o n l y few examples a r e g i v e n u s i n g zeol i t e c a t a l y s t s . Whatever t h e n a t u r e o f t h e z e o l i t e ,

acidic

(ref.

12), basic

( r e f . 1 3 ) o r m e t a l - s u p p o r t e d ( r e f , 141, amide f o r m a t i o n i s always observed. We have a l r e a d y shown t h a t Y z e o l i t e s a r e e f f i c i e n t c a t a l y s t s f o r a l k y n e h y d r a t i o n ( r e f . 1 5 ) . Such z e o l i t e s can a l s o be used f o r n i t r i l e h y d r a t i o n , but,

i n t h i s case,

t h e occurence o f e s t e r f o r m a t i o n i n

alcoholic

medium

c o n s t i t u t e s a new and i n t e r e s t i n g r e s u l t . As p a r t of our programme on m e c h a n i s t i c s t u d i e s over z e o l i t e c a t a l y s t s , t h e p r e s e n t paper d e a l s w i t h t h e c o n f i r m a t i o n o f t h e a c t i v i t y o f

various

s t r u c t u r e z e o l i t e s i n t h e a l k y n e h y d r a t i o n , and w i t h t h e r e s u l t s o b t a i n e d i n t h e case of n i t r i l e behaviour o v e r t h e same z e o l i t e s . EXPERIMENTAL The r e a c t i o n s were c a r r i e d o u t i n a 0.1 l i t e r s t i r r e d a u t o c l a v e operat i n g i n a b a t c h mode and equipped w i t h a system f o r sampling o f l i q u i d d u r i n g the reaction.

The a u t o c l a v e was charged w i t h t h e s u b s t r a t e t o g e t h e r

with

s o l v e n t and t h e f r e s h l y c a l c i n a t e d z e o l i t e ( c a l c i n a t i o n o v e r n i g h t a t 500°C o r 400°C i n a i r , p r i o r t o u s e ) and t h e n heated t o t h e d e s i r e d t e m p e r a t u r e . The amounts o f z e o l i t e and o f d i s t i l l e d w a t e r depend b o t h on t h e n a t u r e and t h e c o n c e n t r a t i o n o f s u b s t r a t e . Samples were withdrawn p e r i o d i c a l l y and analyzed by g.l.c..Two alkynes

:

t y p i c a l examples a r e d e s c r i b e d below : l g (lo-' moll

0.5 g z e o l i t e , T

=

p h e n y l a c e t y l e n e , 50 m l E t O H , 1 m l (0.055 m o l ) H20,

200°C.

n i t r i l e s : 2.5 g (2.4.10-' H20, 1.25 g z e o l i t e , T

=

moll benzonitrile,

58 m l E t O H ,

The z e o l i t e HY w i t h S i / A l

= 2.5

m l (0.12 m o l l

was o b t a i n e d by deammoniating NH4Y

( L i n d e SK 41 f r o m Union C a r b i d e ) . The z e o l i t e s HY w i t h S i / A l p r o v i d e d by Zeocat

2.2

230°C.

=

10 and 15 were

(ZF 510 and ZF 515).

The H R z e o l i t e samples ( S i / A l = 14 and 1 5 ) were t h e method d e s c r i b e d p r e v i o u s l y ( r e f . 1 6 ) .

prepared a c c o r d i n g t o

567

The m o r d e n i t e HM ( Z e o l o n 100 H, Si/A1 = 6.9) was o b t a i n e d f r o m Norton Co. T h i s m o r d e n i t e was dealuminated by t r e a t m e n t i n 1M HC1 aqueous s o l u t i o n f o r 4.5 h a t 80°C t o o b t a i n a H-mordenite w i t h Si/A1 = 8. RESULTS AND D I S C U S S I O N Both r e a c t i o n s o f a l k y n e s and n i t r i l e s were s t u d i e d o v e r mordenite, B and Y - f a u j a s i t e t y p e z e o l i t e s i n t h e i r H-form. To p r e v e n t a t o t a l d e a c t i v a t i o n o f t h e z e o l i t e u s i n g w a t e r as s o l v e n t , t h e s e h y d r a t i o n r e a c t i o n s were c a r r i e d o u t i n e t h a n o l which i s e a s i l y m i s c i b l e w i t h water. I n these conditions,

t h e behaviour o f t h e s e

substrates

is

somewhat

different :

-

a l k y n e h y d r a t i o n g i v e s o n l y t h e expected c a r b o n y l compounds :

-C

"ZO

>-

C-H

-C-CH3 U

0

EtOH

- n i t r i l e h y d r a t i o n competes w i t h n i t r i l e s o l v o l y s i s which l e a d s t o t h e corresponding e s t e r together w i t h traces o f acid.

L

-C

N

7

EtOH

-C-NH2

t

II

-C-OEt

b

0

a/ H y d r a t i o n o f alkynes Over t h e z e o l i t e s mentioned above, acetophenone and propiophenone a r e e a s i l y and q u a n t i t a t i v e l y produced f r o m t h e h y d r a t i o n o f p h e n y l a c e t y l e n e and 1-phenylpropyne i n v e r y s h o r t p e r i o d s o f t i m e , whatever z e o l i t e i s used. T h i s r e s u l t i s i n agreement w i t h t h e s p e c i a l h i g h r e a c t i v i t y o f p h e n y l a c e t y l e n e and d e r i v a t i v e s towards h y d r a t i o n a l r e a d y found i n homogeneous c a t a l y s i s ( r e f .6). On t h e o t h e r hand, 1-hexyne and 2-hexyne, which a r e known t o be l e s s r e a c t i v e towards

hydration,

are

converted

into

the

corresponding

hexanones

in

r e l a t i v e l y convenient y i e l d s . For example, o v e r HM ( S i / A 1 = 8 ) , 30% 2-hexanone i s produced f r o m 1-hexyne a f t e r one hour r e a c t i o n . Except t h e H Y (Si/A1=2.5) which i s l e s s a c t i v e , t h e o t h e r z e o l i t e s w i t h h i g h e r S i / A 1 r a t i o s show s i m i l a r a c t i v i t i e s a t 200°C. The s t u d y o f h y d r a t i o n r e a c t i o n o f p h e n y l a c e t y l e n e a t lower temperatures p e r m i t s t o determine t h e o r d e r o f a c t i v i t y o f t h e s e z e o l i t e s , F i g . 1 f o r t h e r e a c t i o n a t 160°C.

as shown i n

568

.

F i g . 2. C o r r e l a t i o n between t h e hydration a c t i v i t y per a c i d s i t e and S i / A l r a t i o i n t h e v a r i o u s zeolites. XHMg, OHY10, +HY15, OH015

F i g 1. Acetophenone y i e l d a g a i n s t t i m e o v e r v a r i o u s z e o l i t e s a t 160°C. OHY2.5,

These z e o l i t e s can be arranged i n t h e o r d e r o f t h e i r t o t a l a c t i v i t y p e r gram o f c a t a l y s t , as f o l l o w s f o r t h e h y d r a t i o n o f p h e n y l a c e t y l e n e : H8 1 5 > H Y 1 5 > HYIO

> HM8 > HY2.5

T h i s o r d e r i s t h e same when t h e z e o l i t e s a r e arranged i n t h e o r d e r o f t h e i r a c t i v i t y per a c i d i c s i t e .

A g r a p h i c a l r e p r e s e n t a t i o n o f t h e dependence o f t h e a c t i v i t y p e r a c i d i c s i t e o f t h e z e o l i t e s on t h e i r S i / A 1 r a t i o i s g i v e n i n F i g . 2. It i s known t h a t zeolite

acidity

i s enhanced

by i n c r a s i n g t h e S i / A l

ratio

(ref.

17,181.

Therefore, t h e o b t a i n e d l i n e a r c o r r e l a t i o n s t r o n g l y suggests t h a t t h e a c t i v i t y o f z e o l i t e s i n h y d r a t i o n o f a l k y n e s i s a d i r e c t and s i n g l e f u n c t i o n o f t h e a c i d s t r e n g t h and i s n o t dependent o f t h e z e o l i t e framework.

Such a r e s u l t i s

consistent w i t h t h e c o r r e l a t i o n obtained i n o l e f i n h y d r a t i o n ( r e f . 19). b/ H y d r a t i o n o f n i t r i l e s I n t h e r e a c t i o n o f b e n z o n i t r i l e , a l l t h e z e o l i t e c a t a l y s t s s t u d i e d show a s i m i l a r behaviour which i s t h e f o l l o w i n g :

-

a s h o r t s t e p d u r i n g which t h e n i t r i l e disappearance obeys t o a f i r s t

o r d e r dependence a g a i n s t n i t r i l e c o n c e n t r a t i o n ;

-

a second s t e p f o r which t h e r a t e seems t o be z e r o o r d e r .

The r e l a t i v e r a t e o f amide and e s t e r f o r m a t i o n depends on t h e n a t u r e o f t h e z e o l i t e b u t we observe g e n e r a l l y a r a p i d s t a b i l i z a t i o n o f t h e amide conc e n t r a t i o n w h i l e t h e e s t e r c o n c e n t r a t i o n shows a l i n e a r dependence a g a i n s t t i i a e . A t y p i c a l example i s d e s c r i b e d f o r a HFI ( S i / A l = 8 ) ( F i g . 3 ) . The s i g n i -

569

f i c a n t r e s u l t s a r e summarized i n Table 1. TABLE 1

*

E s t e r and amide y i e l d s , e s t e r s e l e c t i v i t y a t 24 hours o v e r t y p i c a l z e o l i t e s % ester y i e l d

HY2.5

HBl 4 HM8

*

% amide y i e l d

% ester selectivity

40

5

88

20

10

67

13

17

43

e s t e r y i e l d = 100 x r e s t ] mol.L-’/

[nit]

e s t e r s e l e c t i v i t y = 100 x [ e s t ] m o l . L - l /

mo1.L-’ [ p r o d u c t s ] mo1.L-’

The i n f l u e n c e o f t h e z e o l i t e c a t a l y s t i s c l e a r l y shown by t h e change i n e s t e r s e l e c t i v i t y w i t h t h e n i t r i l e c o n v e r s i o n ( F i g . 4 ) : a t a 25% c o n v e r s i o n , t h e percentage e s t e r s e l e c t i v i t y v a r i e s f r o m 80% f o r t h e HY t o 50% f o r t h e Hg and 15% f o r t h e HM z e o l i t e s . O/a

90 h +J

.r

>

10

5

1

L-e

th

I

10

B

25

% n i t r i l e conversion

F i g . 3. B e n z o n i t r i l e c o n v e r s i o n a g a i n s t t i m e o v e r HM ( S i / A 1 ) = 8 : o % n i t r i l e , + % amide, x % e s t e r .

F i g . 4. E s t e r s e l e c t i v i t y as a f u n c t i o n o f n i t r i l e conversion o v e r o HY2,5, x Hg14, + HM8.

The o r d e r o f t o t a l a c t i v i t y p e r

Pam o f c a t a l y s t f o r t h e s o l v o l y s i

reaction i s the following : HY2.5 > HB14 > HM8 which l e a d s t o a d i f f e r e n t o r d e r i n a c t i v i t y p e r a c i d i c s i t e : H614

>

HY2.5

>

HM8

570

F o r t h e h y d r a t i o n r e a c t i o n o f b e n z o n i t r i l e , t h e arranged o r d e r , expressed i n terms of e i t h e r t o t a l a c t i v i t y per gram o r a c t i v i t y p e r a c i d i c s i t e ,

i s the

following :

HM8

>

Hel4

HYZe5

>

T h i s o r d e r o f a c t i v i t y i s , i n t h i s case,

d i f f e r e n t f r o m t h a t observed

w i t h a l k y n e s , e s p e c i a l l y t h e somewhat unexpected h i g h e r a c t i v i t y o f m o r d e n i t e

in n i t r i 1e h y d r a t i o n . The v a r i a t i o n s observed i n t h e o r d e r of a c t i v i t y o f t h e s e z e o l i t e s may be r e l a t e d ,

i n a f i r s t approach,

t o a d i f f e r e n c e i n t h e adsorption o f t h e

compounds l e a d i n g t o a d i f f e r e n t mechanism o f t h e h y d r a t i o n r e a c t i o n i t s e l f . It must be s t a t e d t h a t t h e use o f e t h a n o l as s o l v e n t i n t h e n i t r i l e

r e a c t i o n makes i t d i f f i c u l t t o u n d e r s t a n d t h e mechanism

. At

t h i s stage o f our

present r e s u l t s , i t i s p a r t i c u l a r l y not possible t o determine i f t h e e s t e r i s formed f r o m a d i r e c t a d d i t i o n o f e t h a n o l on t h e s t a r t i n g n i t r i l e o r f r o m t h e amide as i n t e r m e d i a t e .

As shown i n Table 1,

HYZm5 zeolite HM8 z e o l i t e s t h e amide i s

the less acidic

f a v o u r s e s t e r f o r m a t i o n w h i l e i n t h e case o f t h e p r e f e r a b l y formed.

Such r e s u l t s seem t o i n d i c a t e t h a t t h e e s t e r f o r m a t i o n

occurs o v e r weak a c i d i c s i t e s whereas t h e h y d r a t i o n r e a c t i o n needs s t r o n g e r acidic sites. Moreover, carbonyl

contrary

compound was

to

a1 kyne h y d r a t i o n where

detected,

the

adsorption

of

the

problem i s c o m p l i c a t e d h e r e

no

by

the

s a t u r a t i o n o f t h e s t r o n g a c i d i c s i t e s by t h e formed amide, t h e c o n c e n t r a t i o n o f which shows a r a p i d s t a b i l i z a t i o n a g a i n s t t i m e ( F i g . 3 ) .

Consequently t h e

r e a c t i o n s e l e c t i v i t y g r e a t l y depends on t h e e s t e r percentage. The b e h a v i o u r of t h e amide i t s e l f o v e r t h e s t u d i e d z e o l i t e s c o n f i r m s t h i s o b s e r v a t i o n c o n v e r s i o n o f t h e amide i n t o e s t e r goes f a s t e r on t h e t h e Hg 1 4 and on t h e comprehension o f

HM8 z e o l i t e s .

the different

This

mechanisms

: the

HY2.5 z e o l i t e t h a n on

later

point,

in

relation

together with

with

the

the

zeolite

p r o p e r t i e s , w i l l be d i s c u s s e d i n a f u r t h e r paper. CONCLUSION D e s p i t e t h e h i g h temperatures which a r e used i n t h e s e r e a c t i o n s ,

our

r e s u l t s show t h a t z e o l i t e s a r e e f f e c t i v e c a t a l y s t s f o r p r o m o t i n g h y d r a t i o n o f t r i p l e bond d e r i v a t i v e s i n r e s p e c t o f an easy h a n d l i n g and

avoidance o f

corrosion. The dealuminated z e o l i t e s , which possess h i g h a c i d i c p r o p e r t i e s ,

show a

r e l a t i v e l y good a c t i v i t y i n b o t h h y d r a t i o n r e a c t i o n s o f a l k y n e s and n i t r i l e s as a l r e a d y observed i n t h e case o f o l e f i n s ( r e f . 1 9 ) . measure

the

acidity

of

the

zeolites

understanding o f t h e d i f f e r e n t reactions.

used,

in

Work i s i n p r o g r e s s t o order

to

gain

further

571

REFERENCES

D. W. Breck, Z e o l i t e s M o l e c u l a r Sieves, J. W i l e y and Sons,Wiley I n t e r -

10

11 12 13 14

15 16 17 18 19

science, New York, 1974. J.A. Rabo, Z e o l i t e Chemistry and C a t a l y s i s , ACS Monograph 171, American Chemical S o c i e t y , Washington, 1976. P.B. Venuto and P.S. Landis, Adv. Catal., 18 (1968) 346 ; P.B. Venuto, Adv. Chem. Ser., 122 (1971) 260. W. H o l d e r i c h , M. Hesse and F . Naumann,Angew. Chem. I n t . Ed.Eng1.,27(1988) 266. M.M.T. Khan and A.E. M a r t e l l , Homogeneous C a t a l y s i s by Metal Complexes Academic Press I n c . New York, 1974 ; M. Miocque, N.M. Hung and V.Q. Yen, Ann. Chim., 8 (1963) 157. D.S. Noyce and M.D. S c h i a v e l l i , J. Am. Chem. SOC., 90 (19681372 D.S. Noyce and M.D. S c h i a v e l l i , J. Org. Chem., 33 (1968) 845. M.S. Newman, J. Am. Chem. SOC., 75 (1953) 4740 ; G.A. Olah and D. Meidar, S y n t h e s i s , 9 (1973) 671 ; F.C. M e n e g a l l i and R.E. Cunningham, L a t . Am. J. Chem. Engl. Appl., 10 (1980) 157. H. Rappoport, "The c h e m i s t r y o f t h e cyano group" i n S. PataT. I n t e r s c i e n c e P u b l i s h e r s , New York, 1970. B.F. Plummer, M. Menendez, M. Songster, J. Org. Chem., 54 (1989) 718 C.M. Jensen, W.C. T r o g l e r , J. Am. Chem. SOC., 108 (1986) 723. H. Hayashi, H. N i s h i , Y . Watanabe, T. Okazaki, J. C a t a l . , 69 (1981) 44 ; K. Sugiyama, H. Miura, Y. Watanabe, Y. Ukai, T. Matsuda, B u l l . Chem. SOC. Jpn., 60(1987) 1579 ; K. Sugiyama, H. Miura, Y. Nakano, H. Suzuki, T. Matsuda, B u l l . Chem. SOC. Jpn., 60 (1987) 453 ; K.-T. L i u , M.-H. Shih, H.-W. Huang, C.-J. Hu, Synthesis, 9 (1988) 715. E . Detrokoy, E. Onyestyak and D. K a l l o , React. K i n e t . C a t a l . L e t t . , l 5 (1980) 443. K. Konuma, T.Kaneko, A. Nakanishi, Jpn. Kokai Tokkyo Koho, J.P. 62,201,853, 1987; C.A. : 110 74838r. K. K . Dzkumakaev, Y . I . Isakov, G.T. Fedolyak, K. M. Minachev, T.A. I s a k o v a A.D. K a g a r l i t s k i i , K i n e t . K a t a l . , 28(4) (1987) 856. H. Miura, T. H a t t o r i , K. K e i t o k u , K. Sugiyama, T. Matsuda, Nippon Kagaku K a i s h i , 4 (1982) 692. A. F i n i e l s , P. Geneste, F. Marichez and P. Moreau, C a t a l . L e t t . , 2 (1989) 181. R.L. Wadlinger, G.T. K e r r , E.J. R o s i n s k i , US Pat. 3,308,069 (1967). V.R. Choudhary and D.B. Akolekar, J. C a t a l . , 119 (1989) 525. J . A. Rabo, P.E. P i c k e r t , D.N. Stamires and J.E. Boyle, Proc. 2nd I n t e r . Congr.Catal., P a r i s , France, 1960, Technip, P a r i s , 1961, p . 2055. F. F a j u l a , R. I b a r r a , F. F i g u e r a s and C. Gueguen, J. C a t a l . , 89 (1984) 60.

This Page Intentionally Left Blank

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

573

REARRANGEMENT OF EPOXIDES USING MODIFIED ZEOLITES

.

.

M. CHAMOUMI , D. BRUNEL , P. GENESTE, P MOREAU and J SOLOFO L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimique Appliquees URA 418 CNRS. Ecole N a t i o n a l e Superieure de Chimie de M o n t p e l l i e r 8, r u e Ecole Normale - 34053 MONTPELLIER Cedex 1. FRANCE

SUMMARY The rearrangement o f s t y r e n e o x i d e i n t o phenylacetaldehyde was s t u d i e d o v e r v a r i o u s z e o l i t e s (H-ZSM-5, HY, H - o f f r e t i t e ) . I t was f i r s t shown t h a t b o t h e x t e r n a l and i n t e r n a l a c i d i c s i t e s a r e i n v o l v e d i n t h a t easy i s o m e r i z a t i o n . Moreover, a comparative study o f t h e rearrangement o f t h i s epoxide and o f i t s h i n d e r e d analog, l-phenyl-1,2-epoxycyclohexene, on s i l a n a t e d o f f r e t i t e , a l l o wed a d i s c r i m i n a t i o n between t h e a c t i v i t i e s o f t h e s e s i t e s .

INTRODUCTION

Products formed by rearrangement o f epoxide compounds p r o v i d e u s e f u l i n t e r m e d i a t e s i n o r g a n i c syntheses, and some o f them a r e v a l u a b l e raw m a t e r i a l s i n t h e chemical i n d u s t r y . A c i d - c a t a l y z e d epoxide rearrangement leads, i n gener a l , t o carbonyl compounds, ketones and aldehydes, whereas t h e b a s e - c a t a l y z e d r e a c t i o n o f such d e r i v a t i v e s forms a l l y l i c a l c o h o l s as main p r o d u c t s ( r e f . 1 ) . Concerning t h e a c i d - c a t a l y z e d rearrangement, Lewis a c i d s , such as BFj,

FeC13,

MgBr2 o r ZnBr2, have been m o s t l y used as c a t a l y s t s i n t h e homogeneous r e a c t i o n s o f a l i p h a t i c ( r e f . 2,3) and v a r i o u s c y c l i c ( r e f . 4-7) o x i d e s . The h e t e rogeneous r e a c t i o n s over s o l i d a c i d i c c a t a l y s t s have been l a t e r i n v e s t i g a t e d . M e t a l l i c oxides,

m a i n l y s i l i c a and alumina,

b u t a l s o supported m e t a l s and

v a r i o u s phosphates have been s t u d i e d i n t h e i s o m e r i z a t i o n o f p r o p y l e n e o x i d e (ref.

8 1 , cyclohexene o x i d e and s u b s t i t u t e d analogs

(ref.

9,101

and some

o t h e r s ( r e f . 1 1 ) . On t h e o t h e r hand, o n l y a few r e s u l t s d e a l t w i t h t h e use o f z e o l i t e s as c a t a l y s t s i n epoxide rearrangements ( r e f . 10,12-14). I n a l l t h e i s o m e r i z a t i o n r e a c t i o n s c a r r i e d o u t i n heterogeneous c o n d i t i o n s , t h e n a t u r e o f t h e p r o d u c t s and p r o d u c t r a t i o depended l a r g e l y on t h e t y p e o f c a t a l y s t employed, and, moreover, i n most o f t h e cases no s e l e c t i v i t y was found. Papers have r e c e n t l y appeared c o n c e r n i n g t h e t r a n s f o r m a t i o n o f s t y rene o x i d e i n t o phenylacetaldehyde c a t a l y z e d by a s e r i e s o f n a t u r a l s i l i c a t e s and amorphous s i l i c a - a l u m i n a

(ref.

1 5 ) and by p e n t a s i l t y p e z e o l i t e s ( r e f .

1 6 ) . It i s s a i d t h a t , i n b o t h cases, i s o m e r i z a t i o n o c c u r s on t h e a c i d i c s i t e s ( s i l a n o l s ) o f t h e external surface, m i l d experimental c o n d i t i o n s .

which a c t as a c t i v e c e n t e r s even under

574

I n t h e case o f z e o l i t e s ,

i f i t can be assumed t h a t t h e weak e x t e r n a l

s i t e s a r e a c t i v e enough t o c a t a l y z e t h e easy s t y r e n e o x i d e i s o m e r i z a t i o n , t h e p a r t i c i p a t i o n o f i n t r a c r y s t a l l i n e a c i d i c s i t e s cannot be e x c l u d e d ( r e f . 1 7 ) . The aim o f t h e p r e s e n t paper i s t o demonstrate t h e i n v o l v m e n t o f b o t h i n t e r n a l and e x t e r n a l s i t e i n v o l v m e n t i n t h e rearrangement o f s t y r e n e o x i d e and s u b s t i t u t e d d e r i v a t i v e s o v e r v a r i o u s m o d i f i e d z e o l i t e s . EXPERIMENTAL

- General procedure The r e a c t i o n s were c a r r i e d o u t i n glassware equipment

(0.1

l i t e r two-

neck f l a s k equiped w i t h a w a t e r condenser) w i t h a magnetic s t i r r e r and under nitrogen

atmosphere.

In

a

typical

experiment,

a s o l u t i o n o f t h e epoxide

(8.3 mmol) i n 40 m l t o l u e n e (0.208 M ) was heated by means o f a t h e r m o s t a t e d oil

bath

to

the

appropriate temperature ; t h e f r e s h l y c a l c i n a t e d z e o l i t e ,

250 mg, ( c a l c i n a t i o n a t 500°C or 400°C i n d r y a i r o v e r n i g h t , p r i o r t o u s e ) was then added t o t h e s o l u t i o n , t h e moment o f a d d i t i o n c o r r e s p o n d i n g t o t h e s t a r t

o f t h e k i n e t i c measurements ( t = O ) . Samples were withdrawn analyzed by g.1 .c.

p e r i o d i c a l l y and

(DELSI 30, c a p i l l a r y column OV1, 30m, c a r r i e r gas H 2 ) .

- Catalysts The z e o l i t e HY w i t h Si/A1=2.5 was o b t a i n e d by deammoniating NH4Y ( L i n d e SK 41 f r o m Union C a r b i d e ) . The z e o l i t e HY w i t h S i / A l = 1 5 was p r o v i d e d by Zeocat

(ZFS15). The H-ZSM-5 z e o l i t e w i t h Si/A1=13 and t h e H - o f f r e t i t e w i t h Si/A1=3.8 were prepared a c c o r d i n g t o methods a l r e a d y d e s c r i b e d ( r e f . 18,191.

- Catalysts treatments

. Phenanthridine above,

poisoning :

I n a glassware

the freshly calcinated zeolite,

equipment

as

described

250 mg, was added t o a s o l u t i o n o f

p h e n a n t h r i d i n e ( i : 0.28 mmole ; ii : 0.56 mmole) i n 20 m l t o l u e n e ( i : 7.10-3M;

ii : 14.10-3M).

The m i x t u r e i s t h e n s t i r r e d f o r

appropriate temperature before t h e epoxide s o l u t i o n (8.3

15 min.

at the

mmoles i n 20 m l

t o l u e n e ) i s added.

. S i 1a n a t i o n The z e o l i t e s were m o d i f i e d by chemical vapour d e p o s i t i o n ( r e f .

20) o f

t e t r a a l k o x y s i l a n e s . The amount o f s i l a n a t i o n was c o n t r o l l e d by FT-IR s p e c t r o s copy and m i c r o a n a l y s i s ( r e f . 2 1 ) .

575

RESULTS AND D I S C U S S I O N S t y r e n e o x i d e i s e a s i l y and q u a n t i t a t i v e l y r e a r r a n g e d i n t o p h e n y l a c e t a l dehyde

over

the

zeolites

mentioned

above

in

r e l a t i v e l y mild conditions

( r e f . 17). Scheme 1

H

0

\

J-

CZH

F o r example, o v e r H-ZSM-5 ( S i / A 1 = 1 3 ) , a 100% c o n v e r s i o n o f s t y r e n e o x i d e is

obtained a f t e r o n l y -3 - 1 -1 4.10 m o l . mn . g

fifteen

minutes

at

95"C,

a t an i n i t i a l r a t e o f

.

The s t y r e n e o x i d e i s o m e r i z a t i o n i s known t o b e a n e a s y r e a c t i o n due t o t h e carbonium s t a b i l i z a t i o n by t h e aromatic nucleus.

I n t h e c a s e o f H-ZSM-5,

t a k i n g i n t o a c c o u n t t h e r e s p e c t i v e s i z e o f t h i s medium-pore z e o l i t e (5.5A) and t h e k i n e t i c d i a m e t e r o f t h e s t y r e n e o x i d e m o l e c u l e (5.9A),

i t was assumed t h a t

t h e weak e x t e r n a l a c i d i c s i t e s a r e a c t i v e enough t o c a t a l y z e t h e r e a c t i o n ( r e f . 1 6 ) . I f t h i s were t h e c a s e f o r a l l z e o l i t e s , be o b t a i n e d

for

any

epoxide

rearrangement.

no s h a p e - s e l e c t i v i t y c o u l d

Nevertheless,

for

large-pore

z e o l i t e s , t h e c o n t r i b u t i o n o f a l l t h e a c i d i c s i t e s c a n n o t be e x c l u d e d . I n o r d e r t o check t h e p o s s i b i l i t y o f i n t r a c r y s t a l l i n e c a t a l y s i s i n t h e s t y r e n e o x i d e i s o m e r i z a t i o n , we f i r s t s t u d i e d t h i s r e a c t i o n o v e r

Y

l a r g e pore

z e o l i t e s , e x t e r n a l s i t e s o f w h i c h were made i n a c t i v e . Two d i f f e r e n t methods were used t o s e l e c t i v e l y r e d u c e t h e s u r f a c e a c t i v i t y o f Y z e o l i t e s : i ) t h e a c t i o n o f a s t e r i c h i n d e r e d base such as phenant h r i d i n e and i i ) t h e c h e m i c a l v a p o u r d e p o s i t i o n o f a l k o x y s i l a n e ( r e f . 2 0 ) . i ) The s t e r i c h i n d e r e d p h e n a n t h r i d i n e ( k i n e t i c b a s i c enough (pKa = 6.31

diameter

: 7.12i)

t o neutralize the acidic external sites of the

is

Y

z e o l i t e b u t n o t b a s i c enough t o r e a c t w i t h t h e e p o x i d e ( a s shown b y a b l a n k reaction). As shown i n T a b l e 1, an i n c r e a s e i n t h e c o n c e n t r a t i o n o f p h e n a n t h r i d i n e leads t o a decrease i n t h e r a t e o f s t y r e n e o x i d e t r a n s f o r m a t i o n .

576

TABLE 1 E f f e c t o f p h e n a n t h r l d i n e p o i s o n i n g on s t y r e n e o x i d e c o n v e r s i o n a t 95°C p h e n a n t h r i d i ne 0

7

7.2

0.48

mol . L - ~ . I O ~

vo(mol .mn

-1

.g

-1

).lo 3

14

0.34

Although t h e amount o f p h e n a n t h r i d i n e i s t e n and t w e n t y t i m e s t h a t o f t h e amount necessary t o cover t h e t o t a l e x t e r n a l s u r f a c e o f t h e z e o l i t e ,

it

must be n o t e d t h a t t h e r e a c t i o n always t a k e s p l a c e . ii

Chemical vapour d e p o s i t i o n (CVD) o f t e t r a m e t h o x y s i l a n e a t i n c r e a s i n g

p e r i o d s o f t i m e l e a d s t o an i n c r e a s i n g amount o f s i l a n a t i o n ( r e f . 2 0 ) . Table 2 g i v e s t h e r e s u l t s o b t a i n e d i n terms o f i n i t i a l r a t e s o f s t y r e n e o x i d e c o n v e r s i o n and shows a s i m i l a r decrease i n t h e r a t e w i t h an i n c r e a s i n g amount o f s i 1a n a t i on. TABLE 2 E f f e c t o f s i l a n a t i o n o f Y z e o l i t e on s t y r e n e o x i d e c o n v e r s i o n a t 95°C s i l a n a t i o n a t i m e (mn)

-

-

vo (mo1.m l . g l 1 . 1 0 ~

0

5

18

180b

360

7.2

2.2

0.72

0.4

0.3

a ) C V D s i l a n a t i o n w i t h (MeOI4Si a t 300°C, t h e n t h e m o d i f i e d z e o l i t e c a l c i n a t e d o v e r n i g h t a t 400°C ; b ) m o d i f i e d z e o l i t e t o t a l l y i n a c t i v e towards n a p h t a l e n e a d s o r p t i o n

From t h e s e r e s u l t s , i t c o u l d be concluded t h a t t h e e x t e r n a l a c i d i c s i t e s are e f f e c t i v e l y involved i n t h e isomerisation r e a c t i o n , since t h e untreated z e o l i t e i s t h e most a c t i v e . But, when t h e s e s i t e s a r e t h o u g h t t o be t o t a l l y poisoned, t h e r e a c t i o n always t a k e s p l a c e even i f i t i s slowed down, so t h a t t h e i n v o l v m e n t o f i n t r a c r y s t a l l i n e a c i d i c s i t e s must be t a k e n i n t o account. Nevertheless, t h e i n t e r n a l a c i d i c s i t e s can a l s o be a l t e r e d e i t h e r by phenant h r i d i n e ( k i n e t i c d i a m e t e r : 7.12

w)

o r by t e t r a m e t h o x y s i l a n e ( r e f . 22) ; i t

i s t h e n d i f f i c u l t t o d e t e r m i n e t h e p r e c i s e cause o f t h e r e s i d u a l a c t i v i t y o f the zeolite.

511

Such a c o n c o m i t t a n t p o i s o n i n g o f b o t h e x t e r n a l and i n t e r n a l s i t e s may be

i)and

avoided by t h e use o f s m a l l e r p o r e - s i z e z e o l i t e s such as H-ZSM-5 ( 5 . 5 0

H - o f f r e t i t e (6.4A).

In t h e case o f H-ZSM-S,

as shown i n Table 3, a n e a r l y t o t a l i n h i b i t i o n

i s observed w i t h a t r e a t e d c a t a l y s t

corresponding t o

a

l a r g e amount

of

d e p o s i t e d s i l a n a t i n g agent. TABLE 3 E f f e c t o f s i l a n a t i o n o f H-ZSM-5 z e o l i t e on s t y r e n e o x i d e c o n v e r s i o n a t 95°C. S i l a n a t i o n a t i m e (mn)

0

150

360

a/ C V D s i l a n a t i o n w i t h (MeOI4Si a t 300°C, t h e n t h e m o d i f i e d z e o l i t e c a l c i n a t e d o v e r n i g h t a t 400°C.

I n t h e case o f t h e H - o f f r e t i t e , styrene

oxide

isomerization

over

the

a s i m i l a r decrease i n t h e r a t e o f modified

zeolite

compared

to

the

u n t r e a t e d one i s a l s o observed, as shown i n Table 4, b u t t h e r e a c t i o n i s n o t t o t a l l y i n h i b i t e d even w i t h h i g h l y s i l a n a t e d c a t a l y s t . Such a r e s u l t may be t h e consequence o f i n t r a c r y s t a l l i n e c a t a l y s i s . I n o r d e r t o chek t h i s p o s s i b i l i t y , we s t u d i e d t h e comparative

behaviour o f a

h i n d e r e d s t y r e n e o x i d e analog over u n t r e a t e d and s i l a n a t e d o f f r e t i t e s ( r e f . 23). TABLE 4 Effect

of

silanation

of

H-offretite

on

styrene

oxide

and

l-phenyl-1,2-epoxycyclohexene c o n v e r s i o n s a t 75°C. 3 vo(mol .mn '.g '1.10 vo u n t . Epoxi de

epoxy s t y r e n e

untreated

silanated

offretite

offretite

K = vo s i l a .

4.8

0.3

16

40

0.2

2 00

1-phenyl-l,2epoxycyclohexene

a/ C V D s i l a n a t i o n w i t h (MeO14Si a t 210°C f o r 5 hours, vacuum a t 200°C o v e r n i g h t .

t h e n a c t i v a t e d under

578

Over

the

untreated

zeolite,

the

rearrangement

of

1-phenyl-1,2-epoxycyclohexene (kinetic diameter : 6.5i) into 1-phenyl-cyclopentanecarboxaldehyde ( v o = 4.10-' mol .mn-l . g - l ) i s t e n t i m e s f a s t e r then t h e styrene oxide isomerization i n t o phenylacetaldehyde (vo = 4.8.1c1-~ mol. mn-l.g-'),

as expected t a k i n g i n t o account t h e f o r m a t i o n o f a

t e r t i a r y c a r b o c a t i o n i n t e r m e d i a t e (scheme

2).

Scheme 2

H ..

On t h e another hand, o v e r t h e m o d i f i e d z e o l i t e , t h e rearrangement o f t h e s u b s t i t u t e d epoxide i s s l i g h t l y s l o w e r t h a n t h e s t y r e n e o x i d e i s o m e r i z a t i o n (vo

=

0.2.10-3

versus 0.3.10-3

mol.mn-l.g-l).

Thus, i n s p i t e o f t h e h i g h r e a c t i v i t y o f t h e s u b s t i t u t e d epoxide,

its

rearrangement i s more a l t e r e d by t h e s i l a n a t i o n o f t h e e x t e r n a l s i t e s o f t h e H - o f f r e t i t e , as shown by t h e v a l u e s o f t h e r a t i o ( K ) o f i n i t i a l r a t e s o f t h e isomerization reactions (Table 4 ) . Such a r e s u l t can o n l y be e x p l a i n e d by a t r a n s f o r m a t i o n o f t h e s t y r e n e o x i d e o v e r t h e i n t e r n a l a c i d i c s i t e s , which a r e much l e s s a c c e s s i b l e by t h e more h i n d e r e d l-phenyl-l,2-epoxycyclohexene. Such r e s u l t s l e a d us t o assume t h a t shape s e l e c t i v i t i e s may o c c u r even i n easy r e a c t i o n s over m o d i f i e d z e o l i t e s . The s t u d y o f t h e i n f l u e n c e o f o t h e r z e o l i t e s on t h e r e a c t i v i t y o f progress.

styrene

oxide

and

h i n d e r e d analogs

is

in

579

REFERENCES

1

-

2

-

3 4 5 6

-

7

-

13 14 15 16 17

-

8 9 10 11 12 -

18 19 20 21 22 23

-

J.March, Advanced Organic Chemistry, 3 r d . ed., John W i l e y Ed., New York, 1985.. J.M. Coxon, E.Dansted, M.P.Harsthorn and K.E.Ritchards, Tetrahedron, 25 (1969) 3307. G . K o l a c z i n s k i , T.Mehren and W.Stein, F e t t e S e i f . Amqtrich.,g (1971) 553. S.M.Naqvi, J.P.Horwitz and R . F i l l e r , J.Am.Chem.Soc., 79 (1957) 6283. J.B.Lewis and G.W.Hedrik, J.Org.Chem., 30 (1965) 4271. M.P.Hartshorn, D.N.Kirk and A.F.A.Wallis, J.Chem.Soc., (19641, 5494. R.L.Settine, G.L.Parks and G.L.K.Hunter, J.Org.Chem., 29 (1964) 616. Y.Okamoto, T.Imanaka and S . T e r a n i s h i , Bull.Chem.Soc.Japan, 46 ( 1 9 7 3 ) 4. K.Arata and K.Tanabe, Bull.Chem.Soc.Japan, 53 (1980) 299. K.Arata, S,Akutagawa and K.Tanabe, B u l l .Chem.Soc.Japan, 48 (1975) 1097. K.Arata and K.Tanabe, Catal.Rev.Sci.Eng., 25 (1983) 365. T.Imanaka, Y.Okamoto and S.Teranishi, B u l l .Chem.Soc.Japan, 45 ( 1 9 7 2 ) 3251. S.Matsumoto, M . N i t t a and K.Aomura, B u l l .Chem.Soc.,Japan, 47 (1974) 1537. C.Neri and F.Buonomo, Eur.Pat.100117, ( 1 9 8 3 ) . E.Ruiz-Hitzky and B.Casa1, J.Catal., 92 (1985) 291. G.Paparatto and G.Gregorio, Tetrahedron L e t t . , 29 (1988) 1471. D.Brune1, M.Chamoumi, B.Chiche, A . F i n i e l s , C.Gauthier, P.Geneste, P . G r a f f i n , F.Marichez and P.Moreau, Stud.Surf.Sc.Catal., 52 (1989) 139. F. Remoue, Thesi s ,Montpel 1ie r ( 1989). F . F a j u l a , L.Moudafi and F.Figueras, U.S.Patent Appl. 676393 ( 1 9 8 5 ) . M.Niwa, M.Kato, T . H a t t o r i and Y .Murakami, J.Phys.Chem., 90 (19861 6233. J . S o l o f o , A . F i n i e l s , P.Geneste and P.Moreau, J.Catal., t o be p u b l i s h e d . T.Bein, R.F.Carver, R.D.Farlee and G.D.Stucky, J.Am.Chem.Soc., 110 (1988) 4546. D.Brune1, M.Chamoumi, S.Shie1, P.Geneste and P.Moreau, t o be p u b l i s h e d .

This Page Intentionally Left Blank

581

M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals I1 0 1991 Elsevier Science Publishers B.V., Amsterdam

THE GAS PHASE I S O M E R I S A T I O N OF SUBSTITUTED HALOBENZENES ON ZEOLITES B.COQ, J-PARDILLOS and F.FIGUERAS

L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimique Appliquees URA 418 CNRS,ENSCM, 8 Rue Ecole Normale - 34053 M o n t p e l l i e r Cedex 1 ( F r a n c e )

SUMMARY The gas phase i s o m e r i s a t i o n o f o-dichlorobenzene (odCB) was s t u d i e d over p r o t o n i c z e o l i t e s : HZSMS, HMOR, HMAZ, HOFF, HBETA and a p i l l a r e d c l a y H P I L C . A l l t h e c a t a l y s t s d e a c t i v a t e . The d e a c t i v a t i o n r a t e i s t h e h i g h e s t when d r y a i r i s used as c a r r i e r gas, and t h e l o w e s t with nitrogen c o n t a i n i n g water. The r a t e l a w o f odCB i s o m e r i s a t i o n obeys t h e LangmuirHinshelwood f o r m a l i s m : r = k A P / ( l + ,3P). The a d s o r p t i o n s t e p i s c o r r e l a t e d w i t h a Hammett e q u a t i o n 'log K = l o g k o t G'p, and t h e n e g a t i v e v a l u e o f p r e f l e c t s t h e development o f a p o s i t i v e charge i n t h e t r a n s i t i o n s t a t e . The r a t e o f t h e s u r f a c e r e a c t i o n , 1 , Z - s h i f t o f halogen s p e c i e s X, i s b e t t e r c o r r e l a t e d w i t h an h o m o l y t i c C - X cleavage t h a n w i t h an h e t e r o l y t i c cleavage. The i n i t i a l a c t i v i t y o f t h e f r e s h p r o t o n i c z e o l i t e depends on b o t h z e o l i t e s t r u c t u r e and A1 c o n t e n t . The c o n v e r s i o n o f odCB q i v e s p r e d o m i n a n t l y i s o m e r i s a t i o n . The b i m o l e c u l a r t r a n s h a l o g e r t i o n o f o d i B t o chlorobenzene and t r i c h l o r o b e n z e n e s remains lower t h a n i n omogeneous phase. INTRODUCTION

I n r e c e n t y e a r s , a l a r g e body o f work emphasized t h e use o f z e o l i t e s for

production

of

f ne

chemicals

(refs. -4).

The

interests

stand

in

replacement o f l i q u i d a c i d s t o lower c o r r o s i o n o f equipment and p o l l u t i o n , and t o reach s p e c i f i c s e l e c t i v i t i e s . However, t h e hopes r a i s e d up i n a r a p i d development o f processes seems r e s t r a i n e d _ .

nowadays. .

. . .

Many p a t e n t s c l a i m e d

. _

-.

.

z e o l i t e s as c a t a l y s t s b u t v e r y few have r e c e i v e d i n d u s t r i a l a p p l i c a t i o n s . A c t u a l l y , b a s i c r e s e a r c h on t h e s t a b i l i t y , t h e o r i g i n o f d e a c t i v a t i o n , t h e r e g e n e r a b i l i t y o f t h e c a t a l y s t s have t o be developed. Moreover, fundamental aspects o f t h e mechanism o f t h i s new k i n d o f r e a c t i o n s a r e l a c k i n g ,

in

p a r t i c u l a r , t h e p o s s i b i l i t y o f r a d i c a l mechanisms, which a r e r a t h e r scarce w i t h hydrocarbons, b u t can l i k e l y occur when heteroatoms a r e i n v o l v e d i n t h e r e a c t a n t . Those were our o b j e c t i v e s i n t h e s t u d y o f t h e i s o m e r i s a t i o n o f s u b s t i t u t e d halobenzenes on z e o l i t e s ( r e f s . 5 - 7 ) . claimed t o occur r e a d i l y on z e o l i t e s ( r e f s . 8 - 9 1 , i n d u s t r i a l development has f o l l o w e d .

Indeed t h i s r e a c t i o n was b u t i t i s supposed t h a t no

EX PER I MENTAL

Catalysts. Mordenite samples ( M O R ) were o b t a i n e d f r o m d i f f e r e n t sources ( r e f . 5 ) . Zeolites

beta

(BETA),

mazzites

(MAZ),

offretites

(OFF)

and

ZSM5

s y n t h e s i z e d a c c o r d i n g t o procedures d e s c r i b e d elsewhere ( r e f s . 1 0 - 1 2 ) .

were Some

parent o f f r e t i t e s , mordenites and m a z z i t e s s u f f e r e d hydrothermal t r e a t m e n t s

A wide

and a c i d l e a c h i n g s t o o b t a i n dealuminated m a t e r i a l s ( r e f s . 1 3 , 1 4 ) .

v a r i e t y o f samples were t h u s prepared w i t h S i / A l r a t i o s i n t h e r a n g e : BETA : 6.3 ( S i / A l { 31.5,

<

ZSM5 : 13.2 ( Si/A1

: 2.5 { Si/A1 ( 5, MOR : 4.4

<

Si/A1

<

44, OFF : 3.4 ( S i / A 1 ( 26, MAZ 39.5.

The p r o t o n i c forms o f t h e

z e o l i t e s , e.g. HZSM5, were o b t a i n e d by c a l c i n a t i o n o f t h e ammonium forms a t d i f f e r e n t temperatures. The chemical c o m p o s i t i o n s were determined by atomic a b s o r p t i o n .

The

c r y s t a l l i n i t y o f t h e samples was checked by XRD t e c h n i q u e . S i z e and morphol o g y o f t h e c r y s t a l s were examined by scanning e l e c t r o n microscopy. Catalytic tests. They were performed i n a f l o w m i c r o r e a c t o r o p e r a t e d a t atmospheric pressure between 548 K and 673 K. P r i o r t o any measurement, t h e c a t a l y s t was dehydrated i n s i t u , under f l o w i n g N2 a t 723 K o v e r n i g h t . The c a r r i e r gas, g e n e r a l l y n i t r o g e n , was s a t u r a t e d w i t h t h e vapor o f t h e r e a c t a n t and t h e n passed t h r o u g h t h e r e a c t o r .

The c o m p o s i t i o n o f t h e

e f f l u e n t was determined by an on l i n e gas chromatograph equipped w i t h a c a p i l l a r y column ( 3 0 m x 0.75 mm i . d . 1 ,

bonded w i t h an a p o l a r phase.

RESULTS AND D I S C U S S I O N

S t a b i 1 it y . I n t h e course o f t h e r e a c t i o n , deposition

of

coke.

Several

all

deactivation

the laws

catalysts

d e a c t i v a t e d by

were

with

fitted

the

o f t e n claimed t o represent experimental d a t a . The Voohries law, r = rotmn, ageing o f a c i d c a t a l y s t s d i d n o t a p p l y . The b e s t f i t f o r t h e r a t e l a w o f t h e d e a c t i v a t i o n process was o b t a i n e d w i t h - d r / d t

= kd.rq

,

with

o(= 120.2

depending on t h e c a t a l y s t . T h e r e f o r e a f i r s t o r d e r d e a c t i v a t i o n r a t e a p p l i e s which t a k e s t h e i n t e g r a l f o r m r

=

ro exp(-kd.t).

I n most cases, c o r r e l a t i o n

c o e f f i c i e n t s b e t t e r t h a n 0.9 were o b t a i n e d when d e t e r m i n i n g k d ( r e f s . 5 , 1 5 ) .

On z e o l i t e s , coke i s b u i l t by a mechanism i n i t i a t e d on a c i d s i t e s , and t h e e x t e n t o f t h i s d e p o s i t i s o f t e n c o r r e l a t e d w i t h t h e A1 c o n t e n t o f t h e s o l i d . I n odCB c o n v e r s i o n t h e s t a b i l i t y o f t h e c a t a l y s t , expressed as t h e k d c o n s t a n t , does n o t appear d i r e c t l y c o r r e l a t e d w i t h t h e aluminium c o n t e n t o f t h e s o l i d . A c t u a l l y , f o r t h a t r e a c t i o n , t a r r y m a t e r i a l s a r e formed b o t h by

583

an a c i d mechanism r e s p o n s i b l e f o r condensed p o l y a r o m a t i c s , and a r a d i c a l mechanism i n i t i a t e d by f e r r i c i m p u r i t i e s r e s p o n s i b l e f o r p o l y p h e n y l compounds ( r e f .6). The k d v a l u e s r e p o r t e d i n Table 1 evidence two main c o n t r i b u t i o n s t o t h e decay o f a c t i v i t y under stream. The f a s t e r d e a c t i v a t i o n i n t h e presence TABLE 1 D e a c t i v a t i o n r a t e c o n s t a n t k d a t 643 K as a f u n c t i o n o f c a t a l y s t and c a r r i e r gas n a t u r e s ; PodCB ~

=

8.42 kPa.

~~

c a r r i e r gas

HZSM5

HBETA

0.04tO. 01

0.05tO. 01

0.1 t o . 03

0.016

0.04

Air

0.65

N2 t

HPILC

0.62 0.09+0.01

high

0.1

H.7

N2

HMAZ

HMORD

k20a

sel e c t i v i t y f o r coke

b

1

2

6

100 Pa ; b : expressed i n grams o f coke per

a : P

100 g o f odCB

H2° converted. of

air,

and

(refs.5,6) The

on

iron-containing

HPILC,

confirms

the

previous

reports

on t h e r o l e o f Fe3' species i n t h e f o r m a t i o n o f t a r r y m a t e r i a l s .

influence

of

the

zeolite

structure

emphasizes

the

role

of

shape

s e l e c t i v i t y on b o t h coke f o r m a t i o n and t o x i c i t y . HZSM5 i s t h e l e s s s e l e c t i v e f o r coke f o r m a t i o n ( 1 % ) and t h e most s t a b l e .

HBETA i s t h e most s e l e c t i v e

( 6 % ) b u t t o l e r a t e s h i g h coke

to

amounts

owing

i t s open

structure.

At

v a r i a n c e , t h e t u b u l a r c h a r a c t e r o f t h e i r porous network makes HMOR and MAZ h i g h l y s e n s i t i v e t o coke p o i s o n i n g . TABLE 2 Reaction r a t e r o (mmole/g/h)

on t h e f r e s h and r e g e n e r a t e d HBETA sample

( S i / A l = 9.5) fresh r0

0.41

l r s t regen. 0.41

2nd.regen. 0.12

3 r d regen. 0.06

R e g e n e r a t i o n o f a HBETA sample was a t t e m p t e d b y c a l c i n a t i o n o v e r n i g h t a t 800

K under

dry a i r . The i n i t i a l a c t i v i t y r o o f t h e f r e s h and r e g e n e r a t e d

c a t a l y s t s a r e r e p o r t e d T a b l e 2. A f t e r t h e f i r s t r e g e n e r a t i o n t h e a c t i v i t y o f t h e f r e s h c a t a l y s t was r e c o v e r e d , b u t subsequent r e g e n e r a t i o n s t e p s d i d n o t succeed. Kinetics

and Mechanism.

The

odCB

isomeri s a t i on o b e y s

a

Langrnui r -

Yinshelwood mechanism w i t h a r a t e l a w : r o = k ? ~ P / ( l + i l P ) i n w h i c h -8 i s t h e e q u i l i b r i u m c o n s t a n t o f t h e odCB a d s o r p t i o n and surface r e a c t i o n (halogen s h i f t ) .

k

the r a t e constant o f the

The r e a c t a n t p r e s s u r e and t h e r e a c t i o n

t e m p e r a t u r e were v a r i e d i n a w i d e r a n g e : 133 Pa ( P 643 K.

<

K

13 kPa, 598

The t r u e a c t i v a t i o n e n e r g y E a o f t h e s u r f a c e r e a c t i o n ,

and t h e

e n t h a l p y AHa o f t h e a d s o r p t i o n s t e p c o u l d be t h u s e v a l u a t e d : E a = 104+_5 kJ/mol, AHa i.e.

AG

=

=

A t 613

2 5 t 1 0 kJ/mol ( r e f . 6 ) .

K t h e v a l u e o f 5 i s 16.65 atm-’,

-14.2 k J / m o l , t h e v a r i a t i o n o f e n t r o p y d u r i n g t h e a d s o r p t i o n c a n

be t h e n e s t i m a t e d t o A S a = - 7 i 3

e.u..

Such a v a l u e

l e t think

t o the

o c c u r e n c e o f a g r e a t m o b i l i t y o f odCB i n t h e a d s o r b e d phase. Studies

of

the

adsorbed

odCB

spectroscopies were c a r r i e d o u t

phase

(ref.4).

by

13C

NMR

MAS

and

FTIR

They e v i d e n c e t h e o c c u r e n c e o f

p r o t o n a t e d odCB s p e c i e s a t t h e s u r f a c e and t h e i r i n t e r a c t i o n w i t h B r o n s t e d sites o f the catalyst.

These

features

are

confirmed

by

the

very

good

a p p l i c a t i o n o f t h e Hammett e q u a t i o n t o t h e i s o m e r i s a t i o n o f p - s u b s t i t u t e d bromobenzenes ( F i g . l a ) . T h i s e q u a t i o n t a k e s t h e f o r m , l o g ( r a t e ) t

=

constant

G?, i n w h i c h r c h a r a c t e r i z e s t h e t y p e o f s u b s t i t u e n t s and p t h e n a t u r e o f

the reaction.

When a h i g h c h a r g e t r a n s f e r

between t h e i n i t i a l

and t h e

t r a n s i t i o n s t a t e s i s occuring, t h e use o f c + i s p r e f e r e d ( r e f . 16). I n t h a t c a s e b r o m i n e was t h e m i g r a t i n g s p e c i e s . The b e t t e r c o r r e l a t i o n with

ct

rather

than

O-

emphasizes

the

carbocationic

nature

of

adsorbed

intermediates. I n previous

reports

Olah e t

al.

(refs.17,18)

suggested

that

the

h a l o g e n s h i f t i s made e a s i e r b y a g r e a t e r s t a b i l i t y o f p o s i t i v e l y c h a r g e d halogen species

( i n t h e l i m i t i n g c a s e an X t

ion).

The v a l i d i t y o f t h i s

s t a t e m e n t was c h e c k e d f o r t h e i s o m e r i s a t i o n o f p - s u b s t i t u t e d f l u o r o b e n z e n e s on HBETA ; t h e s u b s t i t u e n t X i s t h e m i g r a t i n g s p e c i e s i n t h a t c a s e .

The

r e a c t i o n r a t e was c o r r e l a t e d w i t h e i t h e r t h e h o m o l y t i c o r t h e h e t e r o l y t i c bond s t r e n g t h o f t h e A r - X bond. These l a t t e r w e r e e v a l u a t e d a c c o r d i n g t o t h e f o l l o w i n g equations ( r e f . 1 9 ) :

585

FC6H4X

FCgHq

--

FC6H4 X*

-e-

t

X+

Ehetero

=

X'

Ehomo

FC6H4

- EA

e-

t

FC6H4X

+

I1

FC6H4

Ehomo

t

xt

Ehetero

' ll-EA

F i g u r e I b p o i n t s o u t t h a t l o g ( r a t e ) i s b e t t e r c o r r e l a t e d w i t h an homol y t i c t h a n an h e t e r o l y t i c cleavage o f A r - X bond. Therefore, we can p o s t u l a t e a low p o s i t i v e charge on t h e m i g r a t i n g s p e c i e s .

r

homolytic bond strength (kcal/mol) 100

50

- 1'-

I

\

\

300 I -03

-0.2

-0.1

0

0.1

r+

350 I

.

F-OQ (50

400

heterolytic bond strength (kcalhol)

F i g u r e 1. I s o m e r i s a t i o n of p a r a - s u b s t i t u t e d bromobenzenes ( a ) and f l u o r o b e n zenes ( b ) o v e r HBETA ( S i / A l = 8 ) ; r e a c t i o n temperature, 613 K ; r e a c t a n t p r e s s u r e = 130 Pa.(a) : w i t h d r a w i n g e f f e c t o f t h e s u b s t i t u e n t X . ( b ) : e f f e c t o f t h e h o m o l y t i c , o r h e t e r o l y t i c , s p l i t t i n g o f A r - X bond. The major p a r t o f isomers ( ) 90%) a r e formed b y an a c i d mechanism, according t o t h e c o n s e c u t i v e scheme odCB phase (ref.6).

However, t h e small odCB

mdCB

pdCB, as i n homogeneous

pdCB d i r e c t i n t e r c o n v e r s i o n , and

t h e appearance o f CB and TCBs, evidenced a dechlorination/chlorination p r o cess i n i t i a t e d on Fe3' i m p u r i t i e s by a r a d i c a l mechanism ( r e f . 6 ) . Activity

: The m e c h a n i s t i c and k i n e t i c s t u d y has shown t h a t odCB

i s o m e r i s a t i o n i s c a t a l y z e d by t h e a c i d c e n t r e s o f t h e s o l i d .

Furthermore,

586

t h e r e a c t i o n r a t e on HBETA i s f i v e f o l d l a r g e r t h a n on a c e r i u m exchanged z e o l i t e BETA. I t i s known t h a t Lewis s i t e s a r e f a v o u r e d on t h e l a t t e r , and Bronsted s i t e s on t h e f o r m e r . We can t h e n c o n c l u d e t h a t p r o t o n s a r e t h e most active c a t a l y t i c species i n z e o l i t e f o r t h a t r e a c t i o n . Among o t h e r f a c t o r s , t h e s t r e n g t h o f t h e p r o t o n s i n z e o l i t e depends on t h e framework A1 c o n t e n t , and should go t o w a r d a maximum around Si/A1 (ref.20). Al/(AltSi)

Indeed,

a volcano-shaped

dependency

10

=

between t h e r a t e and m

was r e p o r t e d f o r odCB i s o m e r i s a t i o n on HMOR,

=

HBETA and HOFF

( r e f . 7 ) . Moreover, t h e n a t u r e o f t h e z e o l i t e i n f l u e n c e s t h e p r o t o n s t r e n g t h t o o , Table 3 r e p o r t s t h e r e a c t i o n r a t e s and i n t r i n s i c a c t i v i t i e s , expressed as t u r n o v e r number (TON), on v a r i o u s z e o l i t e s a t a S i / A l t h e optimum.

content c l o s e t o

The TON was c a l c u l a t e d by d i v i d i n g t h e r a t e by t h e p r o t o n

concentration i n t h e z e o l i t e . The h i g h a c t i v i t y observed on HZSM5 i s t y p i c a l o f t h e s t r e n g t h o f t h e p r o t o n i n t h i s z e o l i t e , and t h e low a c t i v i t y o f HPILC o f i t s Lewis a c i d i t y . TABLE 3 Reaction r a t e s and TON ( h - ’ )

f o r t h e odCB i s o m e r i s a t i o n on v a r i o u s s o l i d

acids. HZSM5

HMOR

H MAZ

13.2

S i /A1

HBETA

HOFF

9.2

4.35

9.5

8.7

ro(mmol/g/h)

2.95

2.3

0.58

0.41

0.12

TON( h - l )

3.7

1.7

0.32

0.42

0.083

HPILC

0.01

S e l e c t i v i t y : When u s i n g z e o l i t e s as c a t a l y s t s one o f t h e o b j e c t i v e s i s t h e r e s e a r c h o f shape s e l e c t i v i t y t o p r e v e n t unwanted r e a c t i o n s . k i n d s o f shape s e l e c t i v i t y e x i s t t r a n s i t i on-s t a t e

Three

: on t h e r e a c t a n t s , t h e p r o d u c t s o r t h e

.

No r e a c t a n t shape s e l e c t i v i t y was evidenced d u r i n g t h e i s o m e r i s a t i o n o f odCB, mdCB and pdCB over HBETA s i n c e t h e r e a c t i v i t y o f pdCB i s o n l y t w o f o l d l a r g e r t h a n t h a t o f odCB. D u r i n g t h e A 1 C 1 3 c a t a l y z e d i s o m e r i s a t i o n o f odCB, Olah e t a l . ( r e f . 1 7 ) observed t h e occurence o f d i s p r o p o r t i o n a t i o n t o chlorobenzene (CB) and t r i c h l o r o b e n z e n e s (TCBs) (5-10% s e l e c t i v i t y ) . T h i s intermolecular

chlorine

migration could

be

formally

analogous

to

the

d i s p r o p o r t i o n a t i o n o f xylenes. When p r o c e s s i n g odCB on z e o l i t e s t h e amounts o f CB and TCBs were never balanced, observed.

As

d i s c u s s e d above

and l a r g e excess o f CB were

the formation

of

CB and

TCBs

is

always better

587

e x p l a i n e d by a dechlorination/chlorination o f odCB o b e y i n g t o a r a d i c a l mechanism. The occurence o f d i s p r o p o r t i o n a t i o n o f odCB i n z e o l i t e s can be t h u s d i s c a r d e d due t o t r a n s i t i o n - s t a t e

selectivity

(Table 4 ) .

The

high

s e l e c t i v i t y o f HPILC f o r CB and TCBs can be understood c o n s i d e r i n g t h a t HPILC e x h i b i t s a v e r y low a c t i v i t y f o r

i s o m e r i s a t i o n on B r o n s t e d s i t e s

( T a b l e 31, b u t f a v o u r s r a d i c a l r e a c t i o n s r e s p o n s i b l e f o r d e a c t i v a t i o n and chlorination/dechlorination,

owing t o i t s h i g h amount o f i r o n ( 2 4 % ) .

The shape s e l e c t i v i t y i n p r o d u c t s i s evidenced m a i n l y by t h e mdCB/pdCB r a t i o a t a g i v e n c o n v e r s i o n ( T a b l e 4). The behaviour o f HPILC cannot be taken i n t o account s i n c e t h e m a j o r i t y o f p r o d u c t s a r e formed by r a d i c a l mechanism. On t h e p r o t o n i c z e o l i t e s t h e i s o m e r i s a t i o n o f d i c h l o r o b e n z e n e s f o l l o w s a c o n s e c u t i v e r e a c t i o n scheme f o r t h e main p a r t , and t h e mdCB/pdCB r a t i o depends on t h e odCB c o n v e r s i o n . However, t h i s r a t i o can be m o d i f i e d when l i m i t a t i o n t o d i f f u s i o n occurs, and t h e f i n a l p r o d u c t pdCB w i l l be t h e n favoured. TABLE 4 S e l e c t i v i t y f a c t o r s i n t h e c o n v e r s i o n o f odCB a t 673 K over d i f f e r e n t s o l i d acids HZSM5

crystal size

0.3

20

HMOR

MAZ

HBETA

HOFF

HPILC

2

1.5

0.6

1

2

6

5

7

4

1.6

0.006

0.006

0.02

(pm) (mdCB/pdCBIa (CB + TCBs) (isomer.)

7 0.

l.gb

ooze

3 ~

~~~

a : a t 5% c o n v e r s i o n ; b : a t 2.5% c o n v e r s i o n ; c : o n l y CB d e t e c t e d Table 4 shows t h a t l i t t l e r e s t r i c t i o n t o d i f f u s i o n o c c u r s on small c r y s t a l ( 0 . 3 pm) o f HZSM5 and HBETA. S l i g h t l i m i t a t i o n s appear on HMOR, HMAZ and HOFF owing t o coke d e p o s i t near t h e pore mouth ( r e f . 3 ) .

However,

the

l a r g e s t e f f e c t o f p r o d u c t shape s e l e c t i v i t y i s observed when comparing small ( 0 . 3 pm) and l a r g e ( 2 0 pm) p a r t i c l e s of HZSM5. I n t h a t case t h e mdCB/pdCB r a t i o drops t o a v a l u e o f 1.5 a t 2.5% c o n v e r s i o n c l o s e t o t h e v a l u e a t thermodynamic e q u i l i b r i u m (1.161.

According t o t h e s i z e o f t h e c r y s t a l s , t h e

s e l e c t i v i t y can t h e n be d i r e c t e d t o one o f t h e isomers.

588

CONCLUSION The gas phase i s o m e r i s a t i o n o f s u b s t i t u t e d halobenzenes occurs r e a d i l y on z e o l i t e s . S i m i l a r i t i e s appear w i t h t h e same r e a c t i o n c a t a l y z e d by A1C13 i n t h e homogeneous phase, i n p a r t i c u l a r t h e a p p l i c a b i l i t y o f t h e Hammett equation.

However,

several

differences

stand

i n t h e mechanism and t h e

r e a c t i o n scheme. P a r t o f t h e t r a n s f o r m a t i o n o f halobenzenes o c c u r s by means o f a r a d i c a l dechlorination/chlorination mechanism. F o r t h e same reasons t h e formal

reaction

follows

a

triangular

scheme

which

differs

from

the

c o n s e c u t i v e scheme o c c u r i n g i n t h e l i q u i d phase. Moreover, s e l e c t i v i t y can be s t r o n g l y a f f e c t e d by t h e r e s t r i c t i o n t o d i f f u s i o n i n t h e porous volume o f the solid. REFERENCES 1 - P.B.Venuto and P.S.Landis,Adv.Catalysis,l8 (1968) 259. 2 - W.F. H o e l d e r i c h , Pure Appl. Chem., 58(1986) 1383. 3 - W.F. H o e l d e r i c h , M. Hesse and F. Naeumann, Angew. Chemie, 100( 1988) 232. 4 Heretogeneous C a t a l y s i s and F i n e Chemical s, M.Gui snet e t a1 .(Eds. 1, S t u d i e s i n Surface Science and C a t a l y s i s ; E l s e v i e r , Amsterdam, 1988. 5 J . P a r d i l l o s , B. Coq and F . F i g u e r a s , Appl. C a t a l . , 51(1986) 285. 6 - J . P a r d i l l o s , D. B r u n e l , B. Coq, P. M a s s i a n i , L.C. de Menorval and F. F i g u e r a s , J. Am. Chem. SOC., 112(1990) 1313. B. Coq, J . P a r d i l l o s and F. F i g u e r a s , Appl. C a t a l . , 62 ( 1 9 9 0 ) 281. 7 8 - Toray, Inc., Japan Patent 140, 123 ( 1 9 8 4 ) . 9 - L i t t e r e r , H., European P a t e n t 140,123 ( 1 9 8 4 ) . 10 - F . F a j u l a , F.Figueras, L.Moudafi, M.Vera Pacheco, S.Nicolas, P.Dufresne and C.Gueguen, French P a t e n t 850772 ( 1 9 8 5 ) . 11 - F . F a j u l a , F . F i g u e r a s and L.Moudafi, Eur.Pat. 0118,382 ( 1 9 8 7 ) . 12 - A.Albizzane, Ph.D.Thesis, M o n t p e l l i e r , l 9 8 8 . 13 - G.Ferre, P.Dufresne and C . M a r c i l l y , French P a t e n t 84 134 74 ( 1 9 8 4 ) . 14 - B.Chauvin, Ph.D.Thesis, M o n t p e l l i e r 1988. 15 - J . P a r d i l l o s , Ph.D.Thesis, M o n t p e l l i e r , l 9 8 9 . 16 - C.D. Jonhson, "The Hammett E q u a t i o n " , Cambridge U n i v e r s i t y Press, London, 1973. 17 - G.A.Olah,W.S.Tolgyesi and R.E.A.Dear, J.Org.Chem., 27 ( 1 9 6 2 ) 3449. 18 - G.A.Olah and M.W.Meyer, J.Org.Chem., 27 ( 1 9 6 2 ) 3464. 19 - E.M.Arnett, K.Amarnath, N.G.Harvey and J.P.Cheng, J.Am.Chem.Soc., 112 (1990) 344. D.Barthomeuf, Mater.Chem.Phys., 17 (1987) 49. 20

-

-

-

M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals II 1991 Elsevier Science Publishers B.V., Amsterdam

589

K A O L I N P R O M O T E D WITTIG O L E F I N A T I O N AND A R O M A T I C N I T R A T I O N

C.COLLET and P.LASZLO L a b o r a t s i r e s d e C h i m i e F i n e aux I n t e r f a c e s UnivcrsitC d e L i e g e , S a r t - T i l m a n , 6000 L i e g e ( B e l g i u m ) and E c o l e P o l y t e c h n i q u e , 91128 P a l a i s e a u ( F r a n c e )

3 U M I1A R 'r' B y i n c r c a s i n g the r e a c t a n t s m e e t i n g p r o b a b i l i t y a n d modifying t h e i r p o l a r i t y , k a o l i n o r m o n t m o r i l l o n i t e K 1 0 c l a y s c h a n g e t h e r a t e a n d t h e s e l e c t i v i t y of t h e t i t l e reactions.

INTRODUCTION Layered aluminosilicates catalyze chemical r e a c t i o n s in v a r i o u s w a y s . They s t a b i l i z e high-energy i n t e r m e d i a t e s , s t o r e e n e r g y in their l a t t i c e s t r u c t u r e s and c a t a l y z e redox r e a c t i o n s (ref. 1). h i g h s u r f a c e acidity

They often exhibit

(ref. 2 ) .

AROMATIC N I T R A T I O N A r o m a t i c n i t r a t i o n s p e r f o r m e d in t h e p r e s e n c e of K 1 0 m o n t m o r i l l o n i t e lead to i n c r e a s e d p a r a s e l e c t i v i t y . W i t h t o l u e n e as test m o l e c u l e , t h e p r o p o r t i o n o f

para-

n i t r o t o l u e n e r e a c h e s 798 w h e n u s i n g c l a y - s u p p o r t e d copper(I1)

nitrate

( " c l a y c o p " ) in t h e p r e s e n c e of a c e t i c

a n h y d r i d e u n d e r high d i l u t i o n c o n d i t i o n s i n C C l + ( r e f . 3 )-

We t l e c t s d to study t h e n i t r a t i o n o f

P-anisaldehyde

+ CHO 1

CHO 2

NO* 3

The m e t h o x r s u b s t i t u e n t d i r e c t s n i t r a t i o n to t h e .>,-tho

and t h e para p o s i t i o n s . In the f o r m e r c a s e , t h e

L

590

resulting Wheland

2.

product

intermediate

In t h e l a t t e r c a s e .

loses a proton e x p u l s i o n of

to yield

protonated

3. T h i s i p s o - s u b s t i t u t i o n f o r m s o n l y t r a c e s of p - n i t r o a n i s o l e 3 u n d e r homogeneous

carbon monoxide a f f o r d s product

reaction conditions

( T a b l e 1).

TABLE 1 Proportion o f p-nitroanisole function

formed

(21.2%) as a

the solid present.

01

Solid

support

( n o n eI

a

(%I

3

1.1

silica.

2.3

Kieselghur’

1. 6

Cu(N03)z.3HzOb

1.3

KIOa

9.6

”

C

1a y c op

10

”

K10-Cu’+

13

KlO-Al”

15

K10-Ti4+

16 a

21

Kaolinitem

27

K10-Zr4+

__---_--_-_-_--_------------------------------------a

1.5 g

the solid +

of

1 0 mmol

excess relative to HNOi clay

are

min., x

g

amount

0.5

of of

ml

+ 3.1

m l AczO

fuming HN03

(

i . e . an in

the

(25 ml)

and

plus the residual water

in t h a t o r d e r in C C l G

for 114 h while

refluxed %

introduced

L

~ . a - , Merck,

99.5

is a d d e d d r o p w i s e .

solid and

no HNO3

( x

value

i s function of

the

~03-/9).

CHO 2’ ~

The two Wheland

i n t e r m e d i a t e s 2’ a n d 3’ a r e h i g h l y

59 1

p o l a r , w i t h c a l c u l a t e d d i p o l e moment respectively

( r e f . 4-51.

The high acidity

(ref. 2 ) and the high electric a m o n t m o r i l l o n i t e clay

thus

diminishing

Electric

of 4.49 a n d 5.57 D

would

(

H o = -6 t o

-8)

f i e l d at t h e i n t e r f a c e of

increase the formation of

the a c t i v a t i o n e n e r g y b a r r i e r

a,

( r e f . 6).

f r o m G a u s s ’ s l a w , at a

field calculat3d

d i s t a n c e of o b, from a c l a y s h e e t w i t h a

charge density

a p p r o p r i a t e f o r a s m e c t i t e ( t h e U p t o n b e n t o n i t e ) of 6.8 m i l l i e l e c t r o n . ~ - ’ has a

l i m i t i n g v a l u e of 1.2

f o r a d i e l e c t r i c c o n s t a n t of u n i t y . e x p e c t e d f o r a n i n c r e a s e of p o l a r i t y

1Olo V . m - ’

The energy gain 1 D of

of

t r a n s i t i o n s t a t e s is t h u s 5.5 kJ.mol-’

the

for a local

d i e l e c t r i c c o n s t a n t of 5 (ref. d ) . In t h e

p r e s e n c e of m o n t m o r i l l o n i t e

K10,

the proportion

of P - n i t r o a n i s o l e 3 in t h e p r o d u c t m i x t u r e i n c r e a s e s by m o r e t h a n o n e o r d e r of m a g n i t u d e .

I t grows up a s the

B r s n s t e d a c i d i t y at t h e s u r f a c e is e n h a n c e d by acidic

surface intersticial

L i k e w i s e , yet

in a more

pronounced manner,

a kaolinite,

another clay with a strongly acidic s u r f a c e (Ho 6 ) ( r e f . 2).

a l s o favors ips0 nitration:

nitroanisole

a

compared

r i s e s t o 27%. b o o s t e d by

to homogeneous +

the Lewis-

c a t i o n s ( r e f . 7-81. = -3 t o

t h e y i e l d o f Pa f a c t o r 20 a s

reaction conditions!

Kaolin E T - 1 W/Q

0

Kaolin B.E.T=20m2/0

h (c

0

3

IGG

0 h h s s @f kaolin

Fig.

1.

Relationship

nitroanisole mass

L

3_

a m m t of anisaldehyde

between the Dercentage

formed and the ratio

of k a o l i n i t t

;in moll.

!

250

of P-

(g.mol-’) of

the

( i n J ) to t h e a m o u n t of p - a n i s a l d e h y d e

-

592

T h e proportion of ips0 n i t r a t i o n product first amount of clay present

depends u p o n the

mixture, then r e a c h e s a plate.

in the r e a c t i u n

This l i m i t c a n s q u a r e w i t h

a clay saturation. The r e s u l t s o b t a i n e d by X-ray s t u d i e s o n the k a o l i n i t e separated

diffraction reaction HNir3

mixture by

filtration

before

from

the

th+ acidition s ~ f

indicate that there is no i n t e r c a l a t i o n of

anisaldshyde r3r nitrating a g e n t , which would r-eiult in a n increase o f

d o o i , which

in our c a s e r e m a i n s .:onstant.

To sum u p , the adsorption of p-anisaldehyde particule o f the

clay r e s u l t s in a c o n s i d e r a b i t

z p s o substitution

on a

increase in

pathway during nitratiori.

WITTIG OLEFINATION Following S c h l o s s e r (ref. 9 ) . we divided alkylidene phosphoranes

(phosphorus

the

ylides)

L

used

in

W i t t i g r e a c t i o n s into three groups a c c o r d i n g to their

react i v i t Y -

+ RI-P-CH-R

<----)

R3

-P=CH-R

L Nonstabiiizcd

( R = a l k y l o r electron-donating

and " m o d e r a t e d " ( R = h a l o s e n , vinyl.

group)

arvl o r a l k y n v l )

f l i d e s r e a c t i o n s involve early t r a n s i t i o n s t a t e s h a v i n g phosphorus

in a distorted s q u a r e - p y r a m i d a l geometry

10). Under

k i n e t i c c o n t r o l , the d e c o m p o s i t i o n of

oxaphospnetane

l e a d s t o the

(2)-olefin.

(ref.

the

Stabilized ylides

(R=electron-withdrawing groups) react v i a a p r o d u c t - l i k e transition s t a t e , and k i n e t i c E s e i e c t i v i t v is e a s i l y understood o n t h e basis of o x a p h o s p h z t a n e - l i k e s t e r i c interactions

(Ref. i l l .

We have studied t h s influence of

the k a o l i n i t c upon

the W i t t i g r e a c t i o n vsing a s a base NaOH/HzO and t h e phosphonium

Nonstabilized

salt

L

where R ' =

[ref. 121

.

Ph

vlides react with the a l d e h y d e a s s o o n

a s they form ,on the surface of the b a s e , therefore t h e rcaction.

kaolinite , d o e s not influence the c o u r s e ~f Thc

r e a c t i o n r a t e s and the r a t i o o f

a r e modified

E

and 2 isomer;

when the r e a c t i o n involves a s t a b i l i z e d o r

" m o o e r a t a d " ylide and an aromatic a l d e h y d e .

+

RHC-CHR'

PhjP-O

Ph3P 0 R = aromatic R' = electron-withdrauwing group

R

1CCOq 1 I\

.+.. wih kaolin

:\\ 'I\ -':\\

80

-+ with kaoln (12.2s)

- & - .without

cond

kaclin

:\\ ', ! \

€a-

', \

\

i \,

%*,.

40

-

s o l v e n t = C H g C l2

4

I ,

'\~

k.

'$

'\

\,k Q<-.&

' a :

-,<

20

~~

+... 'C..

. . . b .

,+., '&

-

I--& ~: -&==& ----t' - -A -+

L.3

-7%

t - - - - -+.

...

__

s o lvent = dioxane A

WITHXJT kACUI

I

594

Using dioxane a s solvant.

we observe “ p s e u d o ” first-

order kinetic, and the rate equations of the c u r v e s are:

- for X=O,

+ 2.272

= -0.OI2.t

log[ben=.ldch~de’.1OO ~ b e n z a l d a h r d c ]

(1)

(t > 1 0 m i n )

- for ~

=

0

.

2

,

1

~

~

~

~

~= -0.0176.t ~ ~ ~ ~

+ ~ 2.217 ~ ~ ~( 2~) ~

C b e n z a l d e h r d c ]

( t > 20minl T h i s t y p e of b e h a v i o u r s (ln---k’.t)

obtained assumes

C A I

steady-state

( d c e y - O )of

the ylide concentration.

One can

d t

account for the time necessary to reach the steady-state by

i n t r o d u c i n g a s e c o n d t e r m in e q u a t i o n s

This time is shorter when kaolinite How can

we

( 1 ) and

(2).

is p r e s e n t .

e x p l a i n t h e i n c r e a s e in r e a c t i o n r a t e :

1. T h e e q u a t i o n s

(11 and

(2) account

“macroscopic” observations.

for

They correspond t o the

a c c u m u l a t i o n o f t h e r e a g e n t s o n k a o l i n s u r f a c e , so that t h e i n c r e a s e of increase o f

local concentration c a u s e s a n

reaction rate. d C A

1

--kl

.[ A l l o s r l

d t

2. A n

interaction between the clay and the reagents

and/or the reactions intermediates causes a d i m i n u t i o n of

the activation energy

( k ’ k a o l L n = 1.65

* k ’ u i t h o u t

of

the reaction

k a o l i n ) .

F i g . 3 C o m p a r i s o n of t h e F T I R s p e c t r a

--- b e n z a l d e h y d e -benzaldehvde

adsorbed on kaolinite

(2.10-~mole) 0. rig

’

.

595 T h e c o m p a r i s o n of

the benzaldehyde

shift

the carbonyl

frequency, reflecting an increase o f

bond polarity, carbon.

That

of

1 5 cm-'

IR spectra shows a

hypsochromic

thus a higher

phenomenon

( 1 7 0 3 cm-'--->1688

c m - l ) of the

charge on the carbonyl

favors the reactions with

the

anions.

CONCLUSION The clay

surface increases

reagents meeting This dual

the probability

of

and modifies the reagents polarity.

influence

f a v o r a b l e to t h e

is s t r o n g l y

r e a c t i o n s t h a t we h a v e s t u d i e d .

REFERENCES T.J.

Pinnavaia,

P.L.

Hall,

S.S.

Cady.

M.M.

Morthland.

Aromatic radical cation formation on the intracrystals s u r f a c e s ,of t r a n s i t i o n m e t a l

J.

Phys.

H.A.

Chem..

Benesi,

layers lattice silicates.

( 1 9 7 4 ) 994-999.

78

Acidity

of c a t a l y s t

strengh from colors adsorbed

I. A c i d

surfaces.

i n d i c a t o r s , J.

Am.

Chem.

S O C . 78 ( 1 9 5 6 ) 5 4 9 0 - 5 6 9 6 . H.A.

Benesi

a n d B.H.C.

solid catalysts.

A. A

C o r n e l i s , L.

Adv.

Winquest, Catal.

Delaude,

A.

27 (1978) 9 7 - 1 8 2 . C e r s t m a n s a n d P.

Lett.,

Collet,

R.C.

Tetr.

(1988) 5657-5660.

29

aromatic

of

Laszlo,

procedure for quantitative regioselective nitration

of a r o m a t i c h y d r o c a r b o n s in t h e l a b o r a t o r y ,

C.

of

Surface acidity

A.

D e l v i l l e a n d P.Laszlo.

nitration,

Bingham.

molecules.

Angew.

M.J.S.

Dewar

Chem.

a n d D.H.

MIND0/3,

XXVI.

J.

Clays directs

29 ( 1 9 9 0 ) 5 3 5 - 5 3 6 .

Am.

Lo,

Ground

Chem.

states

SOC. 97

(1975) 129L-1301. J - L . Andrss,

A.

Lledos,

M.

D u r a n , J.

Bertran,

Electric

f i e l d a c t i n g a s c a t a l y s t s in c h e m i c a l r e a c t i o n s . initio study

of

Phys.

153 ( 1 9 8 8 ) 8 2 - 8 6 .

P.

Lett.,

the Walden

inversion reaction, Chem.

Laszlo ed., Preparative chemestry

reagents. P. L a s z l o ,

Academic Press Catalysis

solids, Acc.

Chem.

: San

Dieoo

using

supported

(1987).

of o r g a n i c r e a c t i o n s by

Res.,

19

A n ab

( 1 9 8 6 ) 121-127.

inorganic

596

9

M.

S c h l o s s e r , in : Methodicum Chimicum. Georg T h i e m e

Verlag. 10 E.

Stuttgart. 7 ( 1 9 7 6 ) 529.

Vedjs and C.F.

M a r t h , Mechanism

of

the Wittig

reaction: the role of s u b s t i t u e n t s a t phosphorus.

J. Am. 11 E.

Chem. 3 o c .

Vcdjs, T .

1 1 0 (1,988) 3963-3958.

F l e c k and S. Hara, Evidence a g a i n s t

rrvsrsible W i t t i g r e a c t i o n

of

s t a b i l i s e d y l i d e : high

(E)-olefin selectivity under k i n e t i c c o n t r o l , 3. O r g . Chem.

52 ( 1 9 8 7 ) 6637-6639.

12 G. Gallagher and R . L .

W e b b , Tetrasubstitued

a c r v l a t e s : thc Wittig-Horner r e a c t i o n o f triethyl a-phosphonoPropionate,

k e t o n e s with

S y n t h e s i s ( 1 9 7 6 ) 122

-126. 13 Source Clay Minerals re posit or^. Dept.

University o f

of

Geology,

Missouri C o l u m b i a , Missouri 65201 U S A .

597

AUTHOR A

INDEX C a r r e , J.

237

A i , M.

423

Casbas, F .

201

Angevaare, P.A.J.M.

305

C e r i n o , P.J.

23 1

A r r e d o n d o , J.

Chamoumi, M.

573

Augustine, R . L .

1a5 129

Aznar, A.J.

539

C i v i d i n o , P. C l i m e n t , M.J.

245 557

Collet, C.

589

€3

Baiker, A.

413

Coq, 6 .

B a r b i e r , J.

223 343

Cordier, G.

Barrault, J. B a r r e t o - R o s a , M.M. Barto'k, M . B a s s e t , J.M.

263 153,549

Corma, A . C o u r t , J. C u r t i n , T.

58 1 295 503,557 193 53 1

Baumeister, P.

137 321

D

B a u t i s t a , F.M.

269

D e l a h a y , G.

343

Beenackers, J.A.

215

Del Angel, G.

185

B e l g s i r , E.M.

D e t t m e r , M.

B e r t h i e r , Y.

463 295

D i d i l l o n , 8.

487 137

Besson, M.

113

D j a o u a d i , D.

113

Blackmond, D.G. B l a n c , B. B l a s e r , H.U. Bodibo, J.P. B o i t i a u x , J.P. B o n n e l l e , J.P. Bonnet, M.C. B o n n i e r , J.M.

145 145 73,177,321 513 223 287 263

Doussain, C.

471

Duprez, D.

20 1

E E l Mansour, A

137

Essayem, N .

343

B o u r n o n v i l l e , J.P.

113,245 137

F a r n e t t i , E.

253

B r e y s s e , M.

121,277

F a v r e , T.L.F.

Brouard, R.

343

F e r r u t i , P.

305 43 1

Brunel , D.

573

Figueras, F .

Bucsi, I .

549

F

F i n i e l s , A. F l e c h e , G.

C Calais, C.

Fleischer, 6.

Campelo, J.M.

277 269

Forquy, C .

Candy, J . P .

137

F o u i l l o u x , P.

Forni, L.

58 1 565 23 1 487 367 277,343 245

Fuentes Mota, J. F u n f s c h i l l ing, P.C.

445 413

G

G a i z i , 2. G a l l e z o t , P. Gancet, C.

J Jacobs, P . A .

395

J a n a t i - I d r i s s i , F.

193

J a l e t t , H.P.

343 145,231

Jgrgensen, K.A.

Garcia, A.

93 269

G a r c i a , H.

557

G a r c i a Gomez, M.

445

K

Gargano, M.

161 177

K a l l o , D.

Garland, M. Geneste, P. Gigante, B. Ghorbel, A. G i u f f r g , L. G l i r f s k i , M. Gbbolos, S. Gomez, R .

121,565,573 209 455 43 1 169 313,335

177

Jenck, J.

1,329 377 479 113,329

J o s h i , G.W. J o u c l a , M.

523 253

Kaspar, J. K i e f f e r , R. Kiennemann, A. K i j e n ' s k i , J.

237 237 169 215

K u s t e r , B.F.

L

185

L a c r o i x , M.

G r a z i a n i , M.

253

Lahanas, K.M.

129

G r e e n f i e l d , H.

351 329

Lamy, C . Lansink R o t g e r i n k , H.G.J.

463 413

Lasperas, M.

565 129

G r e n o u i l l e t , P. Guardeco, R. Gubelmann, M. Guisnet, M. G u t i e r r e z , E.

269 471,513 513 539

H Hamar-Thibault, S. Hegedus, M. Herrmann, J.M. Herskowitz, M. Hindermann, J.P. Hodnett, B.K. Hubaut, R. Huser, H.

113 313,335 405 105 237 437,531 287 463

121,277

Lay, Y.M. Lee, C.W.

589 495

Lee, J.S.

495

L a s z l o , P.

Lee, S . M .

495

Leger, J .M.

463

Lobo, A.M.

209

Luna, D .

269

M M a l t h a , A. Malz, R.E.,

305 Jr.

M a r c e l o - C u r t o , M.J. Marg i t f a l v i, J. L.

I I b o r r a , S.

Margot, E.

557

Marichez, F .

351 209 313,335 295 565

599

Marinas, J .M. Marion, Ph.

269

Perot , G.

329

P i c h a t , P.

513 405

Ma rrak c hi, H. M a r t i n Aranda, R.M.

277 503

Ponec, V. Popa, J .M.

305 471

Masson, J .

245 43 1

P o u i l l o u x , Y. Prabhakar, S.

513 209

Pradera Adrian, M.A.

445

P r a d i e r , C.M.

295

Primo, J.

557

Mazzochia, C . McCullagh, E. McMonagle, J.B.

437 437,531

Menezo, J.C. M i g l i o , R.

223 367

M i h a l y i , R.M. Molnbr, A.

523 549

Montassier, C .

223 121

Moreau, C . Moreau, P. Moukolo, 3.

565,573 2 23

Q Q u a t r a r o , V.P

161

R Rajadhyaksha, R.A. Ranucci, R .

479 43 1

Mu, W. M u l l e r , M.

405 73

Ravasio, N. Reith, W.

161 487

M u l l e r , P. Murghani, S.

237 169

Rimmelin, P. Rosas, N.

231 185

Rossi, M. Ruiz-Hit zky, E .

539

Rusek, M. Ryczkowski, J.

359 335

N Naja, J. Navio, J.A. Neves, I . Notheisz, F.

223 445 513 153

S Saenz, C . Sal ome, J P.

.

0 Oh, J.S.

161

121 23 1

O’Leary, S.T

495 129

Sanchez, F. Scherrer, W .

321

011 i v i e r , J.

201

Sheldon, R.A.

33

Ostgard, 0.

153

Siegel, S.

21

Oukaci, R.

145

Smith, G.V. Smith, K.

153 55

Sol o f o , 3.

573

P

Pard ill o s , J.

581

Park, K.H. Parton, R.F.

495 395

T Ta’las,

Penn, G.

413

Tempesti, E.

E.

503

313,335 431

600

T i r e l , P.J.

47 1

W

Tkatchenko, I .

263

Waghray, A.

145

T r o v a r e l l i, A.

253

Widdecke, H.

487

W i l l i a m s , D.J.

209

Wigniewski, R.

169

U Uytterhoeven, L.

395

Z V

Zamoner, F .

253

Valyon, J.

523

Zine, S .

455

Van Bekkum, H.

385

Zsigmond, A . G .

153

Van d e r Baan, H.S

215

Zuur, A . P .

305

Van d e r Poel , W.

385

Vidal, S .

193

Vinke, P.

385

601

INDEX

SUBJECT

A Acyl a t i o n

Esterification

93,503,557

503,513,557

A1 k y l a t i o n o f

F

- , 2 - e t h y l a n i l ine

523

F r i e s rearrangement

513

N-alkylation o f -,amines w i t h ketones

351

H

-,anilines

359

Halogenation

Amination o f

55

Hydration o f

- , a c i d s and e s t e r s

343

- , a l k y n e s and n i t r i l e s

565

- ,acetone

335

Hydrodechl o r i n a t i o n

313

6 Beckmann rearrangement

531,539

Bromine a d d i t i o n t o alkenes

55

Hydrodeoxygenat ion

287

Hydrodesul f u r a t i o n

201

Hydrogen t r a n s f e r

161,169,253

Hydrogenation o f -,acetophenone

C Carbonyl a t i o n o f a1 l y l e t h e r s 263 C h i r a l sol i d s

73

Claisen-Schmidt condensation 557 Clays

55,471

- ,k a o l i n

589

-,montmorillonite -

539,589

, p i 11 ared

581

-,redox p i l l a r e d Condensation

33 495,503

245

- ,a1 kenes -,benzaldehyde -,butyned i o l - ,carvone - ,c h 1o r o n i t roaromat i c s - ,c i t r a l

21 105 269 185 121,321 137,193

-,glucose -

23 1

,a-ketoesters

177

- ,n itr i l e s

113,329

-,nitrocompounds

169

Conversion o f p o l y o l s

223

-,oxiranes

153

Cycl i z a t i o n o f dienes

129

-,resin acid derivatives

209

- ,s t e r o i d s

161

D Deactivation

- , u n s a t u r a t e d a1 dehydes 231,581

D e h y d r a t i o n o f amides

137,145,193,295

479

-,unsaturated ethers

277

Hydrogen01ys i s o f saccharose 237

Dehydrogenation o f

mass t r a n s f e r i n

- ,t e t r a h y d r o t h i o p h e n e

287 1,105,177

Hydrolysis

93

E E lectrocatalys i s

463

Enant i o s e l e c t i v e c a t a l y s i s

I I o n exchange r e s i n s

73,93,177 Enzyme Epoxidat ion

93 377,431

55,215,487,495 Isomerization o f - ,epox i d e

573

602

-,halobenzene

58 1

-,Ru/zeol i t e

145

,la c t o s e -,oxiranes

215

M o ( V 1 ) - g r a f t e d polymers

43 1

153,549

-,O-pinene

201

-,unsaturated ethers

287

-

Model r e a c t i o n s

21

N N i t r a t i o n o f aromatics

L

55,471,589

Label 1 i n g s t u d i e s

377,549

M

N i t r o x i d a t i o n o f p-xylene

455

0

Mechanisms

33,73,129,329,367,377

Organometallic r e a c t i o n s

129

Oxidation o f Metal c a t a l y s t s -

,Cu/A12O3

161

-,Cu, modi ied -

269,343

,I r/C

385

-,Ni -

21,269

,Ni/A1 PO4

-,Ni,

269

amid ne m o d i f i e d

- ,N i 1-xMox - ,Pd/A12O3 -

32 1

- ,a1 coho1 s

385

-,glyoxal

463

-,hydrocarbons

395,405,423,445

-,methyl e t h y l k e t o n e

437

O x i d a t i v e d e h y d r o g e n a t i o n 33,413 Oxide c a t a l y s t s Oxides o f

193

-,Ag

377

129,313,385

- ,A1

541,549

,Pd/C

- , Pd, unsupported - ,Pt/A12O3 - ,P t / C -,Pt,

cinchona m o d i f i e d

-,Pt,

polycrystall ine

263,385

-,B

53 1

129,385

-,Cr-A1

455

385

- ,cu

413

385

-,Cu-Cr

287

- ,Fe

523

73,177 295

- ,Ge

523

153

-,Mg

169,253

promoted

359

-,Mn

305

unsupported

209 223

- ,Mo - ,P t

423

-,Raney Cu, m o d i f i e d

- ,Pt/Si02 -,Pt/Si02,

-,Pt/Rh, - ,Raney

Ni

113,231,245,329

-,Raney N i , C r and Mo m o d i f i e d 113,231 -,Raney N i , Sn m o d i f i e d -

,Rh/A12O3

,Rh/C - ,Rh/Mg0

385

-

-,Rh/SiO2, -

,Ru/C

335 129,385

-,Si

55

-,Ti

405,445

-,v-P

137 237,385

437

- ,W

549

-,Zn

549

Poly-alumazane, as a s u p p o r t

185 Sn m o d i f i e d

463

385 Oxygen t r a n s f e r

33

603

P

Photocatalytic oxidation 405,445 Polyfunctional catalysis 367,487 Potential measurement 321

Synthesis of -,isosorbide -,pyrazines

R

W

Reactors, triphas ic 1,105 Reduction o f -,nitro to nitroso compounds 305 - ,enones 253

Witt ig olef inat ion

S Sol vent effects

193,245,495

Steric effects 351,359 Structure-react ivity 21,581 Sulphur removal from terpenes 201 Sulphated zirconia 479 Sulphided catalysts 121,201,277,351

Superacid sol ids 479 Surface organometallic chemistry 137

223 367

589

Z

Zeol ites 55,539,565 - ,1 arge pore zeol i tes 503,557 -,offretite 573 -,protonic zeolites 513,581 -,redox zeol ites 33 - , V P I 5 , iron phthallocyanines encaged in 395

- ,Y

513,573

-,Y, iron phthallocyanines encaged in 395 - ,ZSM5 513,573

This Page Intentionally Left Blank

605

STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U S A .

Volume 1 Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417,1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet Volume 2 The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation t o Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Volume 3 Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Volume 4 Growth and Properties of Metal Clusters. Applications t o Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9- 1 1, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Volume 6 Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Volume 8 Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechytte, September 29October 3, 1980 edited by M. GzniEka Volume 10 Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 2 1-23, 198 1 edited by J. Rouquerol and K.S.W. Sing Volume 11 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-1 6, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Volume 12 Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jirir and G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Volume 14 Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz

606 Volume 5 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Volume 6 Preparation of Catalysts Ill. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Volume 17 Spillover of Adsorbed Species. Proceedings of an International Symposium, LyonVilleurbanne, September 12-1 6, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9- 13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jirli, V.B. Kazansky and G. Schulz-Ekloff Volume 19 Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S.Kaliaguine and A. Mahay Volume 2 0 Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Volume 2 1 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Volume 22 Unsteady Processes i n Catalytic Reactors by YuSh. Matros Volume 23 Physics of Solid Surfaces 1984 edited by J. Koukal Volume 2 4 Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoroi-Portorose, September 3-8, 1984 edited by B. Driaj, S.HoEevar and S.Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Volume 26 Vibrations a t Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-windermere, September 15- 19, 1985 edited by D.A. King, N.V. Richardson and S.Holloway Volume 27 Catalytic Hydrogenation edited by L. Cervenp Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Volume 3 0 Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1, 1986 edited by A. Crucq and A. Frennet Volume 31 Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by 6 . Delmon, P. Grange, P.A. Jacobs and G. Poncelet Volume 3 2 Thin Metal Films and Gas Chemisorption edited by P. Wissmann Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 3 4 Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment

607 Volume 35 Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Volume 37 Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-1 7, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Volume 38 Catalysis 1987. Proceedingsof the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Volume 39 Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Volume 40 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Volume 4 1 Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-1 7, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Volume 42 Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Paal Volume 43 Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Volume 44 Successful Design of Catalysts. Future Requirementsand Development. Proceedingsof the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Volume 45 Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Volume 46 Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an InternationalSymposium, Wurzburg, September 4-8, 1988 edited by H.G. Karge and J. Weitkamp Volume 47 Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Volume 48 Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Volume 49 Zeolites: Facts, Figures, Future. Proceedings of the 8th InternationalZeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Volume 50 Hydrotreating Catalysts. Preparation, Characterizationand Performance. Proceedingsof the Annual InternationalAlChE Meeting, Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G. Anthony Volume 5 1 New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Volume 52 Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-1 9, 1989 edited by J. Klinowski and P.J. Barrie Volume 53 Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimrn, S.Akashah, M. Absi-Halabi and A. Bishara

608 Volume 54 Future Opportunities i n Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura Volume 55 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F. Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin PolymerizationCatalysts, Tokyo, October 23-25, 1989 edited by T. Kelli and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 578 Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction t o Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd InternationalSymposium, Poitiers, October 2-5, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 60 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Volume 6 1 Natural Gas Conversion. Proceedings of the Natural Gas Conversion Symposium, Oslo, August 12- 17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II.Proceedings of the IUPAC Symposium (COPS 11). Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso,J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of Catalysts V. Proceedings of the Fifth International Symposium on the Scientific Bases for the Preparation of HeterogeneousCatalysts, Louvain-laNeuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon

Heterogeneous Catalysis and Fine Chemicals IV

Read more

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

Read more

Catalysis by Zeolites: International Symposium Proceedings (Studies in surface science and catalysis)

Read more

New Developments in Selective Oxidation by Heterogeneous Catalysis: Proceedings (Studies in Surface Science and Catalysis)

Read more

Preparation of Catalysts: 2nd: International Symposium Proceedings (Studies in Surface Science and Catalysis)

Read more

Acid-Base Catalysis II: Proceedings of the International Symposium on Acid-Base Catalysis Ii, Sapporo, December 2-4, 1993 (Studies in Surface Science and Catalysis)

Read more

Catalysis: An Integrated Approach to Homogenous, Heterogenous and Industrial Catalysis (Studies in Surface Science and Catalysis)

Read more

Surface and nanomolecular catalysis

Read more

Surface and Nanomolecular Catalysis

Read more

Isotopes in Heterogeneous Catalysis

Read more

Isotopes In Heterogeneous Catalysis

Read more

Catalytic Polymerization of Olefins (Studies in Surface Science and Catalysis)

Read more

12th International Congress on Catalysis (Studies in Surface Science and Catalysis)

Read more

Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis: Proceedings of the Acs Symposium on Structure-Activity Relationships in (Studies in Surface Science and Catalysis)

Read more

Preparation of Catalysts VII (Studies in Surface Science and Catalysis)

Read more

Catalysts for Upgrading Heavy Petroleum Feeds, Volume 169 (Studies in Surface Science and Catalysis) (Studies in Surface Science and Catalysis)

Read more

Laboratory Studies of Heterogeneous Catalytic Processes (Studies in Surface Science and Catalysis)

Read more

Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces (Studies in Surface Science and Catalysis)

Read more

Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)

Read more

Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces (Studies in Surface Science and Catalysis)

Read more

Zeolite Chemistry and Catalysis Proceedings of an International Symposium Prague

Read more

Heterogeneous Catalysis Research Progress

Read more

Past and Present in DeNOx Catalysis: From Molecular Modelling to Chemical Engineering, Volume 171 (Studies in Surface Science and Catalysis) (Studies in Surface Science and Catalysis)

Read more

Mechanical Catalysis: Methods of Enzymatic, Homogeneous, and Heterogeneous Catalysis

Read more

Photochemistry on Solid Surfaces (Studies in Surface Science and Catalysis)

Read more

Fluid Catalytic Cracking V (Studies in Surface Science and Catalysis)

Read more

Heterogeneous Catalysis in Organic Chemistry

Read more

Fluid Catalytic Cracking V (Studies in Surface Science and Catalysis)

Read more

Catalysis by Metals and Alloys (Studies in Surface Science and Catalysis)

Read more

Catalysis and Automotive Pollution Control (Studies in Surface Science and Catalysis)

Read more

Recommend Documents

Heterogeneous Catalysis and Fine Chemicals IV

Studies in Surface Science and Catalysis 108 HETEROGENEOUS CATALYSIS AND FINE CHEMICALS IV This Page Intentionally Le...

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

Studies in Surface Science and Catalysis 78 HETEROGENEOUS CATALYSIS AND FINE CHEMICALS 111 This Page Intentionally L...

Catalysis by Zeolites: International Symposium Proceedings (Studies in surface science and catalysis)

Studies in Surface Science and Catalysis 5 CATALYSIS BY ZEOLITES Proceedings of an International Symposium organized by...

New Developments in Selective Oxidation by Heterogeneous Catalysis: Proceedings (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 72 NEW DEVELOPMENTS IN SELECTIVE OXIDATION BY HETEROGENEOUS CATALYSIS This ...

Preparation of Catalysts: 2nd: International Symposium Proceedings (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 3 PREPARATION OF CATALYSTS II Scientific Bases for the Preparation of Heterog...

Acid-Base Catalysis II: Proceedings of the International Symposium on Acid-Base Catalysis Ii, Sapporo, December 2-4, 1993 (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 90 ACID-BASE CATALYSIS I1 Proceedings of the International Symposium on Acid-B...

Catalysis: An Integrated Approach to Homogenous, Heterogenous and Industrial Catalysis (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 79 CATALYSIS An Integrated Approach to Homogeneous, Heterogeneous and Industri...

Surface and nanomolecular catalysis

DK3277_title 3/8/06 10:57 AM Page 1 SURFACE AND NANOMOLECULAR C ATALYSIS edited by Ryan Richards Boca Raton London N...

Surface and Nanomolecular Catalysis

DK3277_title 3/8/06 10:57 AM Page 1 SURFACE AND NANOMOLECULAR C ATALYSIS edited by Ryan Richards Boca Raton London ...

Isotopes in Heterogeneous Catalysis

Isotopes in Heterogeneous Catalysis This page intentionally left blank CATALYTIC sCIENCE SERIES— vol.4 SERIES eDIT...