Catalysis by Acids and Bases (Studies in Surface Science and Catalysis)

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Studies in Surface Science and Catalysis 20 CATALYSIS BY ACIDS AND BASES

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Studies in Surface Science and Catalysis 20

CATALYSIS BY ACIDS AND BASES Proceedings of an International Symposium organized by the lnstitut de Recherches sur la Catalyse - CNRS - Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Villeurbanne (Lyon), September 25-27,1984

Editors

B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine lnstitut de Recherches sur la Catalyse, CNRS, 69626 Villeurbanne, France

ELSEVIER

Amsterdam - Oxford

- New York - Tokyo

1985

ELSEVIER SCIENCE PUBLISHERS B.V. Molenwerf 1 P.O. Box 21 1,1000 AE Amsterdam, The Netherlands Distributors for h e United States and Canada: ELSEVIER SCIENCE PUBLlSHiNG COMPANY INC.

52, Vanderbilt Avenue New York, N Y 10017

ISBN 044442449-0 (Vol. 20) ISBN 044441801-6 (Series) 0 Elsevier Science Publishers B.V., 1985 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./Science &Technology Division, P.O. Box 330,1000 AH Amsterdam, The Netherlands. Special regulations for 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 to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified. Printed in The Netherlands

V

CONTENTS Studies in Surface Science and Catalysis ............................

IX

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

XI

Pr~face........

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

Catalysis by solid bases and related subjects (K. Tanabe) ...........

XI11 1

A TPD, FT-IR and catalytic study on the interaction of methanol with pure and KOH doped Ti02 anatase (G. Busca, P. Forzatti, J.C. Lavalley and E. Tronconi) ......................................

15

Acid and base strength of alumina-magnesia mixed oxides (J.A. Lercher, Ch. Colombier, H. Vinek and H. Noller) ...............

25

Influence of the operating conditions on the morphology and acidity o f K2C03/y A1203 ( X . Montagne, C. Durand and G. Mabilon)

33

....

Acidic reactions on some transition metal oxide systems (8. Grzybowska-Swierkosz).

45

Modification of the acidity and basicity o f the surface oxide catalysts (S. Malinowski) ...........................................

57

Basic molecular sieve catalysts/side-chain alkylation of toluene by methanol (J.M. Garces, G.E. Vrieland, S . I . Bates, F.M. Scheidt).. .

67

Importance of the acid strength in heterogeneous catalysis (D. Barthomeuf) .....................................................

75

Structure and acidic properties of high silica faujasites (F. Maug6, A. Auroux, J.C. Courcelle, Ph. Engelhard, P. Gallezot and J. Grosmanginl .......................................................

91

Acidity in zeolites (A.G. Ashton, S. Batmanian, D.M. Clark, J.Dwyer, F.R. Fitch, A . Hinchcliffe and F.J. Machado)

101

Acidic and basic properties of aluminas in relation to their properties as catalysts and supports (H. Knozinger) .................

111

Reactivity of isopropanol on K- and Cs-exchanged ZSM-5 and mordenite (J.B. Nagy, J.-P. Lange, A. Gourgue, P. Bodart and Z. Gabelica)

.....

127

Quantitation and modification of catalytic sites in ZSM-5 (E.G. Derouane, L. Baltusis, R.M. Dessau and K.D. Schmitt) ..........

135

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

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

VI

C h a r a c t e r i z a t i o n o f a c i d i c p r o p e r t i e s o f h e t e r o p o l y compounds i n r e l a t i o n t o heterogeneous c a t a l y s i s (M. Misono) .................

147

H e t e r o p o l y compounds : s o l i d a c i d s w i t h guarded p r o t o n s (J.B. M o f f a t ) ......................................................

157

H e t e r o p o l y a c i d s as s o l i d - a c i d c a t a l y s t s (Y. Ono, M. Taguchi, G e r i l e , S. Suzuki and New

T. Baba)

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

167

c o v a l e n t boron (111)-molybdenum ( V I ) mixed 0x0 model compounds

as e l i g i b l e h e t e r o b i m e t a l l i c c a t a l y s t s f o r p r o p y l e n e e p o x i d a t i o n ( E . Tempesti, L. G i u f f r e , C . Mazzocchia and F. D i Renzo) ...........

177

C a t a l y t i c a c t i v i t i e s and s e l e c t i v i t i e s o f c r y s t a l l i n e E-Zr(HP04)2 (K. Segawa, Y.

Kurusu and M. K i n o s h i t a )

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

183

C a l o r i m e t r i c s t u d y o f a d s o r p t i o n o f ammonia a t 420 K on bismuth molybdate ( 2 : 1 )

(L.

Stradella)

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

191

S k e l e t a l i s o m e r i z a t i o n o f n-butene o v e r m o d i f i e d boron phosphate (B.P.

N i l s e n , M. S t o e c k e r and T. R i i s ) .............................

197

C a t a l y t i c a p p l i c a t i o n o f hydrophobic p r o p e r t i e s o f h i g h - s i l i c a z e o l i t e s . 11. E s t e r i f i c a t i o n o f a c e t i c a c i d w i t h b u t a n o l s ( S . Namba, Y. Wakushima,

T.

Shimizu, H. Masumoto and T. Yashima) ...

205

The mechanism o f n-pentane t r a n s f o r m a t i o n o v e r s o l i d superacids A1 *03/A1 C1

(M. Marczews k i ) ........................................

213

F a c t o r s a f f e c t i n g t h e d e a c t i v a t i o n o f z e o l i t e s by c o k i n g (E. G. Derouane).

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

221

V a l o r i s a t i o n des o l i i f i n e s : o l i g o m 6 r i s a t i o n c a t a l y s i i e p a r l e t r i f l u o r u r e de b o r e (C. M a r t y

e t Ph. Engelhard)

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

241

Upgrading o f C4 c r a c k i n g c u t s w i t h a c i d c a t a l y s t s (B. J u g u i n ,

B. T o r c k and G. M a r t i n o )

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

H y d r o c r a c k i n g o f n-heptane on Pt-HZSM-5.

253

E f f e c t of c a l c i n a t i o n and

r e d u c t i o n c o n d i t i o n s (G. G i a n n e t t o , G. P e r o t and M. G u i s n e t )

.......

265

T r a n s i t i o n i o n s exchanged z e o l i t e s as c r a c k i n g c a t a l y s t s (0. Cornet and A. Chambellan)..

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

273

VII

C h a r a c t e r i z a t i o n o f a c i d c a t a l y s t s by use o f model r e a c t i o n s

(14. G u i s n e t ) ....................................................

283

A p p l i c a t i o n de l a resonance magnetique n u c l e a i r e ?il ' e t u d e de l a d i s t r i b u t i o n e t de l ' a c i d i t e de l ' e a u de c o n s t i t u t i o n des s o l i d e s (C. Doremieux-Morin e t J. F r a i s s a r d ) .

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

299

M i c r o c a l o r i m e t r i c c h a r a c t e r i z a t i o n o f a c i d i t y and b a s i c i t y o f v a r i o u s m e t a l l i c o x i d e s (A. Auroux and J.C.

Vedrine)

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

311

D e t e r m i n a t i o n de l ' a c i d i t e de c a t a l y s e u r s s o l i d e s en m i l i e u aqueux

a

l ' a i d e d ' u n marqueur c i n e t i q u e (R. Durand, P . Geneste,

C. Moreau e t S. Mseddi)

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

319

D e g r a d a t i o n mechanism o f 3-methyl-pentane on a supported s u p e r a c i d c a t a l y s t s t u d i e d b y t h e 13C i s o t o p i c (F. Le Normand.and F. F a j u l a ) .

t r a c e r technique

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

325

R e l a t i o n s h i p between c a t a l y t i c a c t i v i t y and a c i d s t r e n g t h o f LaHY z e o l i t e s i n cumene c r a c k i n g and o - x y l e n e i s o m e r i z a t i o n (She L i - Q i n , Hung Su and L i Xuan-Wen)

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

335

A c i d p r o p e r t i e s o f a b i d i m e n s i o n a l z e o l i t e (D. Plee, A. Schutz, G. P o n c e l e t and J.J. F r i p i a t )

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

343

Thermal s t a b i l i t y and a c i d i t y of A13+ c r o s s l i n k e d s m e c t i t e s (D. T i c h i t , F. F a j u l a , F . F i g u e r a s , J. Bousquet and C. Gueguen)

....

351

Mechanisms o f t h e a c i d - c a t a l y z e d is o m e r i z a t i on o f p a r a f f i n s (F. F a j u l a )

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

361

A c i d i c c a t a l y s i s and r a d i c a l a s s i s t a n c e (D. Brunel, H. Choukroun,

A. Germain and A. Commeyras).

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

371

A l k y l a t i o n o f benzene w i t h propene on benzyl s u l f o n i c a c i d s i l o x a n e c a t a l y s t s (A.Saus,

B. Limbacker, R. B r t i l l s and R. Kunkel).

The c o n v e r s i o n of d i m e t h y l e t h e r o v e r Pt/H-ZSMS, c a t a l y z e d r e a c t i o n (C.W.R.

J.H.C.

Van H o o f f )

383

A bifunctional

Engelen, J.P. W o l t h u i z e n and

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

391

A c i d - c a t a l y z e d c o n v e r s i o n o f n-decane o v e r h i g h - s i l i c a f a u j a s i t e s (P.A. Jacobs, J.A. Martens and H.K.

Beyer)..

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

399

VIII

A

new approach t o t h e c r a c k i n g o f alkanes as a t e s t r e a c t i o n

f o r solid acid catalysts

(A.

Corma and V.

Forn6s)

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

409

Comparison of t h e r e a c t i o n s o f e t h y l c y c l o h e x a n e and 2-methyl heptane on Pd/LaY z e o l i t e ( J . Weitkamp and S. E r n s t ) .......................

419

P r i m a r y c r a c k i n g modes o f l o n g c h a i n p a r a f f i n i c hydrocarbons i n open a c i d z e o l i t e s ( J .

A.

Martens, J . Weitkamp and P.A.

Catalyseurs i s o l a n t s e t a c i d i t e

-

Jacobs)

....

427

l e s a c i d e s paradoxaux

( Y . Trambouze) .....................................................

437

IX

Studies in Surface Science and Catalysis Volume

1

Preparation of Catalysts I. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium held a t the Solvay Research Centre, Brussels, October 14-1 7, 1975 edited by B. Delrnon, 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 i n 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 Soci6tte'de Chirnie physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon

Volume 5

Catalysis by Zeolites. Proceedings of an International Symposium organized by the lnstitut de Recherches sur la Catalyse - CNRS - Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, 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 the International Symposium, Antwerp, October 13-1 5,1980 edited by B. Delrnon and G.F. Froment

Volume 7

New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, 30 June-4 July 1980 edited by T. Seiyama and K. Tanabe

Volume

Catalysis by Supported Complexes by Yu.1. Yerrnakov, B.N. Kuznetsov and V.A. Zakharov

8

Volume 9

Physiaof Solid Surfaces. Proceedings of the Symposium held i n Bechyfie, Czechoslovakia, September 29-October 3, 1980 edited by M. L&niEka

Volume 10

Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium held in Aix-en-Provence, September 21 -23, 1981 edited by J. Rouquerol and K.S.W. Sing

Volume 11

Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium organized by the lnstitut de Recherches sur la Catalyse - CNRS Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallerot, G.A. Martin and J.C. Vedrine

Volume 12

Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings o f a Workshop, Bremen, September 22-24,1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jiru and G. Schulz-Ekloff

Volume 13

Adsorption on Metal Surfaces. An Integrated Approach edited by J. &nard

Volume 14

Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, California, U.S.A., 1 - 4 September 1982 edited b y C.R. Brundle and H. Morawitz

Volume 15

Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets

Volume 16

Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, louvain-la-Neuve, September 6-9, 1982 edited b y G. Poncelet, P. Grange and P.A. Jacobs

X Volume 17

Spillover of Adsorbed Species. Proceedings of the International Symposium, Lyon-Villeurbanne, September 72--16,1983 edited by G.M. Pajonk, SJ. Teichner and J.E. Gerrnain

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. Jiru, V.B. Kazansky and G. Schulz-Ekloff

Volume 19

Catalysis on t h e Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, QuCbec, P.O., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay

Volume 20

Catalysis by Acids and Bases. Proceedings of an International Symposium organized by the lnstitut de Recherches sur la Catalyse-CNRS-Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Villeurbanne (Lyon), September 25-27,1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine

XI

FOREWORD

The i n i t i a l d i s c o v e r y t h a t s e v e r a l hydrocarbon r e a c t i o n s can be c a t a l y z e d b y a c i d s s t i m u l a t e d g r e a t i n t e r e s t i n academic as w e l l as i n d u s t r i a l l a b o r a t o r i e s . Acid-catalyzed

petroleum chemistry. halides,

especially i n

r e a c t i o n s a r e now by f a r t h e most developed, Initially,

soluble

s u l f u r i c and phosphoric acids,

alkanes, o l e f i n s ,

acid catalysts,

such

as

aluminium

were used f o r c a t a l y t i c r e a c t i o n s o f

a r o m a t i c s . However t h e s e s o l u b l e c a t a l y s t s c o u l d n o t be used

a t h i g h t e m p e r a t u r e . Hence, a major development i n a c i d - c a t a l y z e d r e a c t i o n s was t h e discovery o f s o l i d acid c a t a l y s t s .

Alumina and a c i d - t r e a t e d c l a y s were

found t o be a c t i v e f o r most o f t h e r e a c t i o n s u s u a l l y c a t a l y z e d by a c i d s i n solution.

I m p o r t a n t advances i n a c i d - c a t a l y s i s o c c u r r e d when i t was d i s c o v e r e d

t h a t t h e i n c o r p o r a t i o n o f alumina i n s i l i c a produced h i g h l y a c i d i c m a t e r i a l s and l a t e r when i t was found t h a t p r o t o n a t e d z e o l i t e s behaved as h i g h l y a c i d i c solutions. Because of

t h e great

importance o f

acid catalysts

i n t h e petrochemical

i n d u s t r y e x t e n s i v e r e s e a r c h work has been c a r r i e d o u t d u r i n g t h e l a s t 30 y e a r s c o n c e r n i n g t h e fundamental contrast,

and a p p l i e d aspects o f

base-catalyzed

reactions

have

received

c a t a l y s i s b y acids. little

attention

In in

heterogeneous c a t a l y s i s , a l t h o u g h i t has been r e c o g n i z e d f o r a l o n g t i m e t h a t hydrocarbons

may undergo v a r i o u s

reactions

i n s o l u t i o n i n t h e presence o f

bases. An i n t e r e s t i n g and i m p o r t a n t f e a t u r e o f s o l i d acid-base c a t a l y s t s i s t h a t i n many cases, b o t h a c i d i c and b a s i c s i t e s e x i s t s i m u l t a n e o u s l y on t h e s u r f a c e . It was

argued t h a t

t h e s e dual

acid-base

sites

could provide

new r o u t e s

for

hydrocarbon r e a c t i o n s i n v o l v i n g a dual s i t e mechanism.

In

addition

considerable

interest

has

been

directed

to

the

possible

c o r r e l a t i o n between c a t a l y t i c a c t i v i t y and t h e a c i d i c and/or b a s i c p r o p e r t i e s o f the catalyst. appropriate

The search f o r c o r r e l a t i o n s

measurements

environment o f t h e a c i d

of

the

( o r base)

number,

has been implemented t h r o u g h

nature,

active sites.

strength,

location

and

A number o f chemical

and

p h y s i c a l methods have been developed and have p r o v i d e d v a l u a b l e i n f o r m a t i o n on t h e i n t e r p r e t a t i o n o f the c a t a l y t i c e f f e c t s . Because a c i d i c z e o l i t e s have been f o u n d much more a c t i v e and more s e l e c t i v e t h a n amorphous s i l i c a - a l u m i n a , t h e r e has been an i n c r e a s e i n r e s e a r c h a c t i v i t y

on t h e s e m a t e r i a l s w i t h t h e aim o f d e s c r i b i n g more p r e c i s e l y t h e a c t i v e s i t e s .

As a r e s u l t t h e r e has been a decrease i n r e s e a r c h a c t i v i t y on o t h e r i n o r g a n i c c a t a1ys t s However

.

l a r g e surface

area

inorganic solids

acid

have been used as c a t a l y s t

s u p p o r t s . Although t h e s u p p o r t has o f t e n been c o n s i d e r e d as an i n e r t m a t e r i a l , this

i d e a has been c o n t r a d i c t e d

by experimental

results.

The concept

Of

b i f u n c t i o n a l o r d u a l - s i t e c a t a l y s t s has l e d t o improved c h a r a c t e r i z a t i o n of t h e chemical n a t u r e o f t h e s u r f a c e s i t e s . These s i t e s have been found t o i n t e r a c t w i t h t h e supported c a t a l y s t and/or w i t h t h e r e a c t a n t s , d u a l - s i t e mechanism f o r t h e r e a c t i o n .

t h u s again p r o v i d i n g a

It i s c l e a r from t h e l i t e r a t u r e t h a t t h e r e i s a renewed i n t e r e s t i n l a r g e s u r f a c e area i n o r g a n i c s o l i d s e x h i b i t i n g a c i d i c o r b a s i c p r o p e r t i e s . It appears a l s o t h a t e x i s t i n g methods f o r c h a r a c t e r i z i n g t h e a c i d i t y o r b a s i c i t y o f s o l i d s s t i l l have t o be improved and t h a t new methods must be developed. The aim o f t h i s symposium was t o e v a l u a t e o u r knowledge o f t h i s i m p o r t a n t area o f a c i d and base c a t a l y s i s and t o cover a broad range o f s o l i d s ,

z e o l i t e chemistry being

o n l y one aspect of heterogeneous c a t a l y s i s . The symposium was sponsored and funded by t h e Centre N a t i o n a l de l a Recherche S c i e n t i f i q u e w i t h i n t h e frame o f 'IColloques I n t e r n a t i o n a u x " . We are g r a t e f u l t o Prof.

R.

CNRS,

f o r h i s encouragement t o o r g a n i z e t h e symposium. We a l s o thank a l l t h e

Maurel,

f o r m e r S c i e n t i f i c D i r e c t o r o f t h e Chemistry Department o f

a u t h o r s and p a r t i c i p a n t s f o r t h e i r i n t e r e s t . hes.

L.

Badolo and B.

We a r e p a r t i c u l a r l y i n d e b t e d t o

Barsan f o r t h e i r h e l p i n t h e p r e p a r a t i o n o f t h e s e

Proceedings. The o r g a n i z i n g committee wishes t o thank a l l t h e people who have c o n t r i b u t e d t o t h e o r g a n i z a t i o n o f t h e meeting.

B. IMELIK, C. NACCACHE,

J.C. VEDRINE

Y. BEN TAARIT, G. COUDURIER

XI11

PREFACE

A la suite de la d6couvertr du r6le jou6 par les acides dans la catalyse des transformations des hydrocarbures, de nombreuses applications potentielles ont vu le jour aussi bien dans les laboratoires universitaires qu'industriels. Actuellement, les reactions catalysees par les acides sont de loin les plus d&elopp6es, en particulier, dans 1 'industrie pgtrochimique. Les acides solubles tels que les halog'enures d'aluminium, les acides sulfurique et phosphorique ont constitu6 la premiere g6nCration des catalyseurs utilis6s pour la transformation catalytique des hydrocarbures saturCs, olefiniques et aromatiques. Cependant leur utilisation est lirnitee par leur instabilitg thermique et ce n'est qu'avec la dCcouverte des catalyseurs solides acides que les &actions acides ont connu leur plus grand essor. L'alumine, les argiles acidifiees se sont r6vel6es actives dans la plupart des &actions catalysees par les acides en solution. Des proqr6s considerables ont 6t6 obtenus lorsqu'il a 6t6 decouvert que l'addition d'alumine 'a la silice produisait des materiaux tres acides et plus tard lorsqu'il a 6te' montr6 clue les formes protonees des zeolithes se comportaient comme les acides en solution. L' importance consid6rable de la catalyse acide dans l'industrie p6trochimique justifie la somme des recherches entreprises au cours des trente derni'eres ann6es tant du point de vue fondamental qu'applique. Les reactions catalysees par les bases ont, par contre, peu retenu l'attention dans le domaine de la catalyse h6t6rogene bien que de nombreuses transformations d'hydrocarbures catalysees par des bases en solution soient connues depuis fort longtemps. Les catalyseurs solides presentent frequemment la particularit4 tres intgressante d'avoir simultan6ment > leur surface des sites acides et basiques. I1 apparait que ces sites doubles acido-basiques ouvrent la route 'a de nouvelles transformations d'hydrocarbures par un mecanisme bifonctionnel La connaissance des propri6tCs acides et/ou basiques des catalyseurs sol ides et 1 a recherche de relation entre ces propri6tes et les propriet6s catalytiques ont mobilisg un grand nombre de chercheurs. Pour determiner le nombre, la nature, la force, la localisation et l'environnement du site acide ou basique, i l a Pt.6 necessaire de developper ou d'adapter des methodes chimiques et physiques et 1 'ensemble des informations recueillies a Dermis d'interDr6ter les effets cat a1 yt iques.

.

XIV

Parce que les zeolithes acides se sont revCl6es beaucoup plus actives et selectives que 1 a si 1ice-a1umine amorphe, 1 a -recherche s 'est cons i dCr ab 1 ement developpee dans ce domaine avec comme objectif la description precise des sites actifs au detriment des etudes relatives aux autres catalyseurs acides mine'raux. Toutefois, les solides mineraux trks disperses sont souvent utilises comme supports de catalyseur. En general, ces supports sont consid6rCs comme des materiaux inertes, cependant dans certains cas, i l a et6 necessaire de reviser la notion "d'innoncence" du support pour expliquer les resultats experimentaux. A la lumi'ere du concept de bifonctionalite des catalyseurs et grzce 'a la caractdrisation de la nature chimique superficielle, i l a et6 montre que certains sites d u support reagissent avec la phase active et/ou avec les reactifs conduisant ainsi 'a un mecanisme bifonctionnel de la reaction. On assiste donc, et la litterature scientifique recente en est le reflet, a un regain d'interdt pour ces solides mineraux divise's presentant des proprietes acides ou basiques, avec pour consequence une amelioration des techniaues de caracterisation de l'acidite et de la basicit6 des solides et le developpement de nouvelles methodes. Ce colloque a pour objectif de faire le point de nos connaissances dans le vaste domaine de la catalyse acide et basique et des catalyseurs solides. I1 a 6t.6 organis6 par 1'Institut de Recherches sur la Catalyse dans le cadre des Colloques Internationaux du C.N.R.S. et finance par le C.N.R.S. Le Comite d'organisation est reconnaissant au C.N.R.S. et B Monsieur MAUREL, Directeur Scientifique de la Chimie, pour leurs encouragements. I1 nous est particuliGrement agreable de remercier tous les auteurs de communications et tous les participants. Que Mesdames Lydie Badolo et Biserka Barsan trouvent ici nos remerciements pour l'aide qu'elles ont apportee 'a la realisation technique de cet ouvraqe.

8. IMELIK, C. NACCACHE, J.C. VEDRINE, Y. BEN TAARIT, G . COUDURIER.

1

B. Imelik e t al. (Editors), Catalysis b y Acids and Bases

a 1985 Elsevier Science Publishers B.V., Amsterdam - h i n t e d

in The Netherlands

CATALYSIS BY SOLID BASES AND RELATED SUBJECTS

KOZO TANABE Department o f Chemistry, F a c u l t y o f Science, Hokkaido U n i v e r s i t y , Sapporo 060 (Japan)

Resume L ' i n t C r @ t de l a c a t a l y s e p a r l e s bases s o l i d e s e s t d6montrd a t r a v e r s l ' a n a l y s e de p l u s i e u r s exemples i m p o r t a n t s pour l a synthese o r g a n i q u e e t pour 1 ' i n d u s t r i e chimique. Les aspects c a r a c t h r i s t i q u e s e t l e s avantages des r e a c t i o n s c a t a l y s g e s p a r l e s bases e t m e t t a n t en j e u un mecanisme a n i o n i q u e s o n t exposes e t l a n a t u r e s t r u c t u r a l e des s i t e s basiques a c t i f s e s t d e c r i t e . LeS s u j e t s connexes t e l s que l a c a t a l y s e b i f o n c t i o n n e l l e acid-base e t l l u t i l i s a t i o n de s o l i d e s basiques comme s u p p o r t s de c a t a l y s e u r s s o n t d i s c u t d s . E n f i n l e s problemes d ' a v e n i r de l a c a t a l y s e p a r l e s s o l i d e s basiques s o n t developpds

.

ABSTRACT The s i g n i f i c a n c e o f c a t a l y s i s by s o l i d bases i s emphasized f i r s t by d e m o n s t r a t i n g s e v e r a l examples w h i c h a r e i m p o r t a n t f o r o r g a n i c s y n t h e s i s and chemical i n d u s t r y . The c h a r a c t e r i s t i c f e a t u r e and t h e advantages o f h e t e r o geneous b a s e - c a t a l y z e d r e a c t i o n s which t a k e p l a c e by a n i o n mechanism and t h e s t r u c t u r a l n a t u r e o f b a s i c a c t i v e s i t e s a r e r e v e a l e d . As i n t r i g u i n g r e l a t e d s u b j e c t s , acid-base b i f u n c t i o n a l c a t a l y s i s and a p p l i c a t i o n o f s o l i d bases as c a t a l y s t s u p p o r t a r e discussed. F i n a l l y , f u t u r e problems i n c a t a l y s i s by s o l i d bases a r e o u t l i n e d . INTRODUCTION There a r e many i n d u s t r i a l l y i m p o r t a n t r e a c t i o n s c a t a l y z e d by homogeneous bases such as i s o m e r i z a t i o n , o l i g o m e r i z a t i o n , a1 k y l a t i o n , a d d i t i o n , hydrogena t i o n , dehydrogenation, c y c l i z a t i o n , o x i d a t i o n , e t c . ( r e f . 1 ) .

Replacement o f

homogeneous l i q u i d bases by heterogeneous s o l i d bases as c a t a l y s t s i n chemical i n d u s t r y i s expected t o b r i n g a b o u t t h e f o l l o w i n g m e r i t s ; no c o r r o s i o n o f r e a c t o r , no environmental problem f o r d i s p o s a l o f used c a t a l y s t , p o s s i b l e r e peated use o f c a t a l y s t , easy s e p a r a t i o n o f c a t a l y s t a f t e r r e a c t i o n , and l o w energy s y n t h e s i s .

However, n o t much work has been made on heterogeneous r e -

a c t i o n s c a t a l y z e d by s o l i d bases ( r e f . 2 - 4 ) .

Here, t h e c h a r a c t e r i s t i c f e a t u r e o f

2

s o l i d b a s e - c a t a l y z e d r e a c t i o n s which a r e i m p o r t a n t f o r o r g a n i c s y n t h e s i s and chemical i n d u s t r y i s demonstrated f i r s t by t a k i n g s e v e r a l examples i n v e s t i g a t e d i n our laboratory.

One o f t h e c h a r a c t e r i s t i c s o f s o l i d b a s e - c a t a l y z e d r e a c t i o n s

which t a k e p l a c e v i a a n i o n i n t e r m e d i a t e s i s t o e x h i b i t i n t r i g u i n g a c t i v i t y and s e l e c t i v i t y d i f f e r e n t from s o l i d a c i d - c a t a l y z e d r e a c t i o n s o r m e t a l - c a t a l y z e d r e a c t i o n s which proceed v i a c a t i o n i n t e r m e d i a t e s o r r a d i c a l i n t e r m e d i a t e s .

Another

c h a r a c t e r i s t i c s i s t h a t t h e f o r m a t i o n o f by-products h a r d l y occurs g i v i n g h i g h s e l e c t i v i t y , which i s d i f f e r e n t from t h e case o f a c i d - c a t a l y z e d r e a c t i o n s .

The

mechanism o f b a s e - c a t a l y z e d r e a c t i o n s and t h e a c t i v e s i t e s on s o l i d bases a r e discussed.

In c o n n e c t i o n w i t h s o l i d base c a t a l y s i s , importance o f acid-base b i f u n c t i o n a l c a t a l y s i s and a p p l i c a t i o n o f s o l i d bases as c a t a l y s t s u p p o r t s a r e a l s o discussed. F i n a l l y , f u t u r e problems i n c a t a l y s i s by s o l i d bases a r e p o i n t e d o u t . 1 . C h a r a c t e r i s t i c Feature o f S o l i d Base C a t a l y s i s a ) Double-Bond I s o m e r i z a t i o n o v e r A l k a l i n e E a r t h Metal Oxides. Calcium o x i d e s c a l c i n e d i n a i r a t 350-900°C were c a t a l y t i c a l l y i n a c t i v e f o r t h e hydrocarbons whose a c i d s t r e n g t h i s weaker t h a n t h a t o f C02 because o f p o i s o n i n g o f t h e b a s i c s i t e s w i t h C02. b u t found r e c e n t l y t o e x h i b i t an e x t r e m e l y h i g h a c t i v i t y f o r i s o m e r i z a t i o n o f 1-butene when C02 adsorbed on t h e b a s i c s i t e s was removed b y e v a c u a t i n g a t 600°C as shown i n Table 1 ( r e f . 5 ) . TABLE 1 I s o m e r i z a t i o n o f 1-butene o v e r CaO. Catalyst React ion React ion weiqht(mg) t i m e ( m i n ) temp.("C)

Catalyst CaO c a l c i n e d a t 600°C i n a i r CaO c a l c i n e d a t 600°C i n vacuo

140 17

120 20

200 30

Con ve r s ion (%) 0 63

The a c t i v i t y of t h e b a s i c CaO c a t a l y s t was about one hundred t i m e s h i g h e r t h a n t h a t o f an a c i d i c Si02-A1203 c a t a l y s t .

For t h e i s o m e r i z a t i o n o f 1,4-

pentadiene, t h e a c t i v i t y d i f f e r e n c e was t e n thousand t i m e s ( r e f . 6 ) .

The

s e l e c t i v i t y ( t h e r a t i o o f c i s - 2 - b u t e n e t o t r a n s - 2 - b u t e n e ) was 7 f o r CaO and 16 f o r MgO ( r e f . 7 )

i n c o n t r a s t w i t h 1 f o r Si02-A1203, i n d i c a t i n g an a n i o n i c

mechanism o v e r t h e b a s i c c a t a l y s t s .

The i s o m e r i z a t i o n by i n t r a m o l e c u l a r

hydrogen t r a n s f e r o v e r s o l i d bases was r e v e a l e d by t h e experiment o f coisomerization o f cis-2-butene d -d ( r e f . 8 ) . 0 8 I n i s o m e r i z a t i o n o f a-pinene t o p-pinene,

S r O e x h i b i t e d a h i g h a c t i v i t y and

s e l e c t i v i t y compared w i t h t h e o t h e r b a s i c and metal c a t a l y s t s as shown i n Table 2 (ref.9).

Over SrO, t h e i s o m e r i z a t i o n t o o k p l a c e a t room temperature and

a t t a i n e d i t s e q u i l i b r i u m i n o n l y 15 min.

Some s o l i d bases a r e r e p o r t e d t o be

3

h i g h l y a c t i v e and s e l e c t i v e a l s o f o r double-bond i s o m e r i z a t i o n s o f 3-carene (ref.lO), A7(13)-protoilludene ( r e f . l l ) ,

and A 2(3) ' ( ' 3 ) - i 11udadiene ( r e f .11)

.

TABLE 2 I s o m e r i z a t i o n o f a-pinene t o B-pinene. Cat a1y s t Ca ( NH2) Pd/Al203 t-BuOK i n DMSO SrO

Reaction temp. ("C) 170-220 200 65 room temp.

Reaction t i m e

Selectivity

(%I

-

85 85

-

-

s e v e r a l hours 15 m i n

100

The s o l i d bases such as MgO and CaO a r e good c a t a l y s t s p a r t i c u l a r l y f o r i s o m e r i z a t i o n s o f t h e compounds c o n t a i n i n g b a s i c n i t r o g e n o r b a s i c oxygen such as a l l y l a m i n e (ref.12)

o r 2-propenyl e t h e r s (ref.13,14),

n o t i n t e r a c t w i t h b a s i c group o f r e a c t i n g molecule.

s i n c e t h e b a s i c s i t e s do I n t h e case o f s o l i d a c i d ,

t h e a c i d s i t e s a r e poisoned w i t h t h e b a s i c group and l o s e t h e c a t a l y t i c a c t i v i t y . Recently, s o l i d super bases were found t o e x h i b i t p o w e r f u l 1 c a t a l y t i c a c t i v i t i e s f o r double-bond i s o m e r i z a t i o n o f o l e f i n s ( r e f . 1 5 ) .

For example, Na-

MgO whose b a s i c s t r e n g t h was H-235 was much more a c t i v e t h a n MgO f o r i s o m e r i z a t i o n o f 1-hexene and 1-pentene ( r e f . 1 6 ) and Na-NaOH-A1203 (A1203 t r e a t e d w i t h NaOH and t h e n w i t h Na, H-137) was much more a c t i v e t h a n NaOH-A1203 o r NaA1 203 f o r i s o m e r i z a t i o n o f 5 - v i n y l - b i c y c l o [ 2 . 2 .l]hepta-2-ene

bicyclo[Z.Z.l]hepta-2-ene

and 5 - i s o p r o p e n y l -

The c a t a l y s t l i f e i s s a i d t o be l o n g e r t h a n

(ref.17).

t h a t o f Na-A1 203. I t s h o u l d be p o i n t e d o u t here t h a t b a s i c p r o p e r t i e s and c a t a l y t i c a c t i v i t i e s o f s o l i d bases and super bases change depending on t h e p r e p a r a t i o n methods ( r e f .4,18,19). b) A l k y l a t i o n o f Aromatics; Syntheses o f 2,6-Xylenol

and S t y r e n e .

A l k y l a t i o n o f phenol w i t h methanol i s i n d u s t r i a l l y i m p o r t a n t as a r e a c t i o n t o s y n t h e s i z e 2,6-xylenol

which i s a monomer o f a good h e a t - r e s i s t i n g poly-(2,6-

d i m e t h y l ) phenylene o x i d e r e s i n .

The r e a c t i o n has been known t o be e a s i l y

c a t a l y z e d by s o l i d a c i d s such as Si02-A1203, A1203, e t c . forms v a r i o u s p r o d u c t s such as o-,m-,p-cresol, d e r i v a t i v e s , 2,4,6-trimethylpheno1, f o r 2,6-xylenol (several

X).

etc.,

However, Si02-A1203

o-,m-,p-xylenol,

anisole

as shown i n F i g . 1 . and t h e s e l e c t i v i t y

formed by m e t h y l a t i o n a t o r t h o - p o s i t i o n s o f phenol i s v e r y l o w I n 1965, General E l e c t r i c found t h a t MgO i s h i g h l y s e l e c t i v e (more

t h a n 90%) f o r t h e f o r m a t i o n o f 2,6-xylenol

(ref.20).

What causes such a b i g

difference i n the o r t h o - s e l e c t i v i ty? An i n f r a r e d s t u d y o f phenol adsorbed on Si02-A1203 and MgO r e v e a l e d t h a t t h e o r t h o - s e l e c t i v i t y i s s t r o n g l y c o n t r o l l e d by t h e adsorbed s t a t e s o f phenol as

4

F i g . 1. A l k y l a t i o n o f phenol w i t h methanol o v e r a c i d i c and b a s i c c a t a l y s t s . seen i n F i g . 2 ( r e f . 2 1 ) . Since, i n t h e case o f Si02-A1203, t h e p l a n e of t h e benzene r i n g o f p h e n o l a t e i s c l o s e t o t h e c a t a l y s t s u r f a c e , any o f t h e o-,m-, p - p o s i t i o n s c a n be a t t a c k e d b y a methyl c a t i o n formed from methanol.

On t h e

o t h e r hand, o n l y t h e o - p o s i t i o n can be m e t h y l a t e d i n t h e case o f MgO, because t h e o - p o s i t i o n i s near t o t h e c a t a l y s t s u r f a c e .

H

O

H

O

Fig. 2 . Adsorbed s t a t e o f phenol on MgO and Si02-A1203. Then why is phenol adsorbed i n t h e form o f ( b ) i n F i g . 2 on S i O 2 - A I 2 O 3

and i n

t h e form o f ( a ) on MgO? The d i f f e r e n c e i s c o n s i d e r e d t o depend on t h e a c i d strength o f the catalysts.

S i n c e t h e a c i d s t r e n g t h o f Si02-A1203 i s v e r y h i g h ,

t h e a c i d s i t e s i n t e r a c t w i t h t h e r - e l e c t r o n s o f t h e benzene r i n g o f phenolate,

g i v i n g t h e adsorbed form ( b ) .

However, such an i n t e r a c t i o n does n o t o c c u r on

v e r y weakly a c i d i c MgO, and t h e adsorbed form ( a ) i s produced. The MgO-Ti02 c a t a l y s t showed h i g h e r a c t i v i t y t h a n MgO, b u t t h e s e l e c t i v i t y was l e s s because o f i t s h i g h e r a c i d i t y ( r e f . 2 1 ) .

The Fe203-ZnO c a t a l y s t which

e x h i b i t e d a s u r p r i s i n g l y h i g h s e l e c t i v i t y (more t h a n 99%)(ref.22) adsorbed phenol i n t h e f o r m ( a ) i n F i g . 2 ( r e f : 2 3 ) .

However, t h e decomposition o f

methanol c o u l d n o t be a v o i d e d o v e r t h e c a t a l y s t c o n t a i n i n g i r o n . For t h e s y n t h e s i s o f s t y r e n e by a l k y l a t i o n o f t o l u e n e w i t h methanol ( c f . Fig. 3), b a s i c c a t a l y s t s such as RbX z e o l i t e ( r e f . 2 4 ) ,

MgO ( r e f . 2 5 ) ,

and Cs-C (ref.26)

have been r e p o r t e d t o be a c t i v e , though t h e a c t i v i t y was n o t so h i g h .

It i s

i n t r i g u i n g t h a t t h e a d d i t i o n o f b o r i c a c i d t o RbX enhanced t h e a c t i v i t y ( r e f . 2 7 ) , s u g g e s t i n g an acid-base b i f u n c t i o n a l c a t a l y s i s .

F i g . 3. A l k y l a t i o n o f t o l u e n e w i t h methanol o v e r a c i d and base c a t a l y s t s . c ) Cannizzaro R e a c t i o n and Tishchenko Reaction Benzaldehyde i s known t o form benzoic a c i d and benzyl a l c o h o l i n t h e presence o f sodium h y d r o x i d e i n aqueous s o l u t i o n ( C a n n i z z a r o r e a c t i o n ) and t o f o r m benzyl benzoate i n t h e presence o f metal b e n z y l a t e (Tishchenko r e a c t i o n ) . r e a c t i o n s a r e homogeneous b a s e - c a t a l y z e d r e a c t i o n s .

Both

How i s t h e c a t a l y t i c

a c t i o n o f s o l i d bases i n t h e absence o f any s o l v e n t ? Calcium o x i d e was found t o form m a i n l y benzyl benzoate.

The r e a c t i o n r a t e

w e l l c o r r e l a t e d w i t h t h e b a s i c i t y on t h e s u r f a c e o f CaO, as shown i n F i g . 4. The mechanism e l u c i d a t e d by k i n e t i c and s p e c t r o s c o p i c s t u d y i s i l l u s t r a t e d by t h e scheme o f F i g . 5 ( r e f . 2 8 ) .

The a c t i v e species o f CaO f o r t h e e s t e r

formation a r e t h e c a l c i u m b e n z y l a t e s whose f o r m a t i o n i s f a c i l i t a t e d by b o t h t h e 2+ b a s i c s i t e s ( 0 2 - ) and a c i d i c s i t e s (Ca ) on t h e s u r f a c e . The mechanism o f t h e f o r m a t i o n o f b e n z y l a t e i s v e r y s i m i l a r t o t h e homogeneous Cannizzaro r e a c t i o n .

6 However, t h e d i f f e r e n c e i s t h a t a Lewis a c i d s i t e as w e l l as a b a s i c s i t e p l a y s an i m p o r t a n t r o l e as an a c t i v e s i t e i n t h e heterogeneous r e a c t i o n .

0.8

0,6

-

1015

-

1014

H

0 E E

0.4 0.2 0-

Y-

O

-

1

700 900 1100 Pretreatment temperature ("C)

300

500

F i g . 4. Change of s u r f a c e p r o p e r t y and c a t a l y t i c a c t i v i t y o f CaO w i t h change o f c a l c i n a t i o n temperature O : B a s i c i t y , U : A c t i v i t y f o r r e a c t i o n o f benzaldehyde, +-:Amount o f r e d u c i n g s i t e s , - & : A c t i v i t y f o r s t y r e n e p o l y m e r i z a t i o n , -A-: A c t i v i t y f o r hydrogenation of propylene.

-+ '0-b-H'gH5

-+ O=C-H

I -Ca-0-

-Ca-0-

(1)

!sH5

I -Ca-0-

-+

I

?

-

16H5

C16H 5 +

H-C-H I

?

(3)

7

Y

C6H5-C=0

+ C6H5CH20-k

F i g . 5 . Mechanism o f r e a c t i o n o f benzaldehyde o v e r CaO. d) M i s c e l l a n e o u s Base-Catalyzed Reactions. A d d i t i o n o f amines t o dienes o c c u r r e d e f f e c t i v e l y o v e r s o l i d bases such as MgO, CaO, S r O , La203, and Tho2.

I n p a r t i c u l a r , CaO e x h i b i t e d an e x t r e m e l y h i g h

a c t i v i t y f o r a d d i t i o n o f dimethylamine t o 1,3-butadiene ( r e f . 2 9 ) . C

H t C~ H ~ = ~C H - C~ H =-+ C H ~ CH ~ N - C H ~ - C H = C H - C H ~

CH

CH 3

3/

The MgO c a t a l y s t p r e t r e a t e d a t 1000°C showed a h i g h a c t i v i t y f o r decomposition o f methyl formate t o methanol aiid carbon monoxide, t h e s e l e c t i v i t y b e i n g 100% (ref.30). HCOOCH3

A d d i t i o n o f Na t o MgO b r o u g h t about a g r e a t i n c r e a s e i n t h e a c t i v i t y . MgO A

CO + CH30H

Rearrangement o f 2-carene o x i d e o v e r Zr02-Ti02 which possesses h i g h b a s i c i t y gave an a l l y 1 a l c o h o l ( c i s - 2 , 8 ( g)-p-menthadiene-l-ol)

w i t h 100% s e l e c t i v i t y

( r e f .31).

Hydrogenation o f b u t a d i e n e e a s i l y o c c u r r e d o v e r MgO evacuated a t 1000°C, c i s 2-butene b e i n g formed s e l e c t i v e l y (ref.32,33).

The h y d r o g e n a t i o n i s c o n s i d e r e d

t o t a k e p l a c e v i a an a n i o n i n t e r m e d i a t e o f s t a b l e c i s - f o r m which i s formed from adsorbed b u t a d i e n e and h y d r i d e i o n as shown i n F i g . 6. C h a r a c t e r i s t i c n a t u r e o f MgO evacuated a t 1100' is summarized i n Table 3 i'n comparison w i t h m e t a l and m e t a l o x i d e c a t a l y s t s . On t h e b a s i s o f t h e knowledge t h a t hydrogen s p l i t s h e t e r o l y t i c a l l y i n t o Ht and

H- and carbon monoxide forms [(C0),l2-

on t h e s u r f a c e o f MgO p r e t r e a t e d a t 1000°C,

we have observed by temperature-programmed d e s o r p t i o n and i n f r a r e d spectroscopy

8

CH- CH H3C/ 'CH3

F i g . 6. Mechanism o f h y d r o g e n a t i o n o f 1,3-butadiene

o v e r MgO evacuated a t 1100°C.

TABLE 3 Hydrogenation o f 1,3-Butadien Catalyst Metals ZnO, CrZO3,

C0304

over various c a t a l y s t s . Hz-Dz

Molecular

Position o f

iden t i t y

D addition

Equilibration

Not m a i n t a i n

1,Z-,

1,4-

Active

Maintain

1,4-,

1,2-

Active

MgO( 600OC)

Not m a i n t a i n

1,4-,

1,Z-

Active

MgO(1100"C)

Mai n t a i n

1,4-

Inactive

t h a t CO adsorbed on MgO Surface r e a c t s w i t h H2 t o form adsorbed HCHO i n t h e t e m p e r a t u r e range o f 70-310°C ( r e f . 3 4 ) .

The r e a c t i o n i s c o n s i d e r e d t o proceed

by t h e scheme o f F i g . 7 .

I n f a c t , HCHO was d e t e c t e d as a p r o d u c t o f t h e r e a c t i o n o f CO ( 5 0 T o r r ) w i t h H2 (100 T o r r ) a t 210°C o v e r MgO and Na/MgO, w h i l e CH30H o v e r Zr02 and La203. A t h i g h e r temperature (3OO0C), CH30H was formed o v e r MgO ( r e f .35). 2. S o l i d Acid-Base B i f u n c t i o n a l C a t a l y s i s

Even i n t h e r e a c t i o n s w h i c h have been r e c o g n i z e d t o be c a t a l y z e d o n l y b y a c i d s i t e s on c a t a l y s t s u r f a c e , b a s i c s i t e s a l s o a c t more o r l e s s as a c t i v e s i t e s i n cooperation w i t h a c i d s i t e s .

The c a t a l y s t s h a v i n g s u i t a b l e acid-base p a i r s i t e s

sometimes show pronounced a c t i v i t y , even i f t h e acid-base s t r e n g t h o f a b i f u n c t i o n a l c a t a l y s t i s much weaker t h a n t h e a c i d o r base s t r e n g t h o f s i m p l e a c i d o r base.

For example, Zr02 which i s weakly a c i d i c and weakly b a s i c shows

h i g h e r a c t i v i t y f o r C-H bond cleavage t h a n h i g h l y a c i d i c S i O Z - A l 2 O 3 o r h i g h l y b a s i c MgO ( r e f . 3 6 ) as summarized i n Table 4.

The c o o p e r a t i o n o f a c i d s i t e s w i t h

b a s i c s i t e s i s s u r p r i s i n g l y powerful f o r p a r t i c u l a r r e a c t i o n s and causes h i g h l y selective reactions.

T h i s k i n d of r e a c t i o n i s o f t e n seen i n enzyme c a t a l y s i s .

9

Mg2+ 02-

0'-

Mg2+

0

I Mg

I I

I I

I

0

Mg

0

Mg

-'A

j.lg2+

0

!-

0

Mg

H

H

H-

I 0

Mg

HCHO

HCHO

0

Mg

0

Mg

0

Mg

PO

Mg

0

F i g . 7. Mechanism o f f o r m a t i o n o f HCHO from CO and He o v e r MgO. TABLE 4 Heterogeneous acid-base b i f u n c t i o n a l c a t a l y s i s .

-CH2

t

t H-

-CH3 + -CH;

+ H

t

S t r o n g a c i d s (Si02-A120 A1203) S t r o n g bases (MgO, CaO)3' Weak acid-base (Zr02, Tho2)

CH3-D exchange X X 0

Not o n l y t h e a c i d and base s t r e n g t h b u t a l s o t h e o r i e n t a t i o n o f a c i d and base s i t e s a r e i m p o r t a n t f o r 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 .

Although b o t h t h e

a c i d i t y and b a s i c i t y o f Zr02 do n o t change much w i t h t h e change o f e v a c u a t i o n temperature ( r e f . 3 7 ) ,

Zr02 evacuated a t 6OOOC shows maximum a c t i v i t i e s f o r

h y d r o g e n a t i o n o f l Y 3 - b u t a d i e n e w i t h H2 and exchange between H2 and D2, whereas

10

Zr02 evacuated a t 800°C g i v e s maximum a c t i v i t i e s f o r i s o m e r i z a t i o n o f 1-butene and h y d r o g e n a t i o n o f 1,3-butadiene w i t h cyclohexadiene as seen i n F i g . 8 ( r e f . 38).

S i n c e i t i s known t h a t t h e l a t t i c e constant c o n s i d e r a b l y changes w i t h t h e

change o f e v a c u a t i o n temperature, t h e appearance o f two k i n d s o f maximum a c t i v i t i e s i s c o n s i d e r e d due t o t h e d i f f e r e n c e i n d i s t a n c e between a c i d s i t e

2

( Zr4+) and base s i t e ( 0 - )

.

4 ,

Pretreatment temperature ("C) Fig. 8. C a t a l y t i c a c t i v i t i e s o f ZrOZ p r e t r e a t e d a t d i f f e r e n t temperatures. 0; h y d r o g e n a t i o n o f 1,3-butadiene w i t h H , 0 ; H - D exchange, A ; i s o m e r i z a t i o n o f 1-butene, A ; h y d r o g e n a t i o n o f 1 , 3 - b u t i d i e n e &ti; cyclohexadiene 3. S o l i d Bases as C a t a l y s t Supports Metal o r m e t a l o x i d e s u p p o r t e d on a s o l i d base sometimes shows a h i g h c a t a l y t i c a c t i v i t y . For example, N i s u p p o r t e d on MgO showed a h i g h a c t i v i t y and a l o n g l i f e f o r h y d r o g e n a t i o n o f t h e o l e f i n s c o t a i n i n g n i t r o g e n o r oxygen as shown i n Table 5 (ref.39,40).

T h i s i s due t o no i n t e r a c t i o n between t h e s u p p o r t

(MgO) and b a s i c N o r 0 group o f t h e o l e f i n s . a c i d s i t e s i n t e r a c t w i t h t h e b a s i c groups.

I n t h e case o f a c i d i c s u p p o r t , t h e Such an i n t e r a c t i o n i n t e r f e r e s t h e

approach o f double bond o f o l e f i n toward N i s i t e s . Another example i s t h a t t h e a c t i v i t y o f Mo03-A1203 f o r h y d r o c r a c k i n g o f t h i o p h e n e can be enhanced by t h e a d d i t i o n o f MgO t o A1203 ( r e f . 4 1 ) . Moo3 s u p p o r t e d on MgO has been found t o be h i g h l y e f f i c i e n t o f e t h y l benzene ( r e f .42)

.

Recently,

f o r dehydrogenation

TABLE 5 C a t a l y t i c a c t i v i t i e s o f N i s u p p o r t e d on d i f f e r e n t o x i d e s f o r h y d r o g e n a t i o n o f

N,N-dimethyl-2-propenylamine. Catalyst

Activity

N i f MgO N i / ZrOp N i /A1 203 N i / T i O2 Ni/SiO,

100 70 60 40 5

4. F u t u r e Problems o f S o l i d Base C a t a l y s i s

i)Device o f New Measurement Method o f B a s i c P r o p e r t y . T i t r a t i o n w i t h benzoic a c i d using a c i d i c i n d i c a t o r s (ref.43,44), w i t h t r i c h l o r o a c e t i c a c i d u s i n g b a s i c Hammett i n d i c a t o r s ( r e f . 4 5 ) , w i t h s u l f u r i c acid solution (ref.46), ( r e f .49),

diphenylamine method ( r e f .37),

method (ref.51,52) s o l i d surface.

titration

potentiometric t i t r a t i o n (ref.471,

C02 o r NO a d s o r p t i o n ( r e f .37,48),

exchange method ( r e f . 4 1 )

titration anion

c a l o r i m e t r i c method

XPS method ( r e f .50), and t e s t r e a c t i o n

have been used f o r c h a r a c t e r i z a t i o n o f b a s i c p r o p e r t y on

However, each method has b o t h advantage and disadvantage and

t h e r e i s no a b s o l u t e l y r e l i a b l e method.

Thus, t h e j o i n t use o f s e v e r a l methods

i s necessary f o r more p e r t i n e n t c h a r a c t e r i z a t i o n a t p r e s e n t and t h e k i n d s o f a p p r o p r i a t e probe molecules s h o u l d be expanded i n f u t u r e .

ii)S y n t h e s i s o f S o l i d Super Bases. A t p r e s e n t , we have s e v e r a l k i n d s o f s o l i d super bases ( T a b l e 6) a c c o r d i n g t o t h e d e f i n i t i o n d e s c r i b e d below and a l r e a d y r e a l i z e d t h e p o w e r f u l c a t a l y t i c a c t i v i t y i n the foregoing section. TABLE 6 Kinds o f S o l i d Super Bases. Starting material, P r e p a r a t i o n method CaO SrO Mg0-NaOH Mg0-Na A1 0 -Na Al$O:-NaOH-Na

Ca C03 Sr(OH)2 (NaOH impregnated) (Na v a p o r i z e d ) (Na v a p o r i z e d ) (NaOH, Na impregnated)

Pretreatment temp .( "C)

H-

900 850 550 650 550 500

26.5 26.5 26.5 35 35 37

Ref. 44 44 53 54 3 5 55 56

The d e f i n i t i o n o f s u p e r base was proposed i n Japanese i n 1980 ( r e f . 1 5 ) a substance whose b a s i c s t r e n g t h i s h i g h e r t h a n H-=26. d e f i n i t i o n i s as fo1,lows.

t o be

The b a s i s o f t h e

As t h e a c i d s t r e n g t h o f s u p e r a c i d i s h i g h e r t h a n Ho=

-12 ( a c i d i t y f u n c t i o n o f 1010%H2S04), i t s s t r e n g t h i s 19 u n i t s h i g h e r t h a n H0=7 o f n e u t r a l substance.

Therefore, i t seems r e a s o n a b l e t h a t a substance whose

12 b a s i c s t r e n g t h (expressed by b a s i c i t y f u n c t i o n , H-) i s more t h a n 19 u n i t s h i g h e r t h a n H-=7 o f n e u t r a l substance s h o u l d be c a l l e d a superbase. The s y n t h e s i s o f much s t r o n g e r s o l i d super bases s h o u l d be e x p l o r e d by combining v a r i o u s components and by changing t h e p r e p a r a t i o n method. i n ) Development o f Acid-Base B i f u n c t i o n a l C a t a l y s i s . Complex o x i d e s seem t o be p r o m i s i n g f o r t h e development.

Hitherto, only five

k i n d s o f complex o x i d e s which possess b o t h a c i d i c and b a s i c p r o p e r t y have been r e p o r t e d : A1203-Mg0 (ref.41,57), ZnO ( r e f . 6 0 ) ,

Mg0-Ti02 ( r e f . 5 8 ) ,

Ti02-Zr02 ( r e f . 5 9 ) ,

A1203-

and Zr02-Sn02 ( r e f . 5 1 ) .

i v ) Development o f S o l i d Base C a t a l y s t w h i c h can n o t be Poisoned by H20 and C02. S y n t h e s i s o f t h e acid-base b i f u n c t i o n a l c a t a l y s t which i s weakly a c i d i c and w e a k l y b a s i c i s emphasized f o r t h i s purpose.

v) D e t e r m i n a t i o n o f S u r f a c e S t r u c t u r e o f S o l i d Acid-Base. I n p a r t i c u l a r , t h e d e t e r m i n a t i o n o f t h e d i s t a n c e between a c i d s i t e and base s i t e i s i m p o r t a n t f o r d e s i g n i n g e f f i c i e n t acid-base b i f u n c t i o n a l c a t a l y s t . S p e c t r o s c o p i c method i s recommended t o be a p p l i e d .

vi) S t u d y on Role o f B a s i c P r o p e r t y o f C a t a l y s t s f o r O x i d a t i o n , Hydrogenation, H y d r o c r a c k i n g , C1 c h e m i s t r y , e t c . Any k i n d s o f c a t a l y s t s have more o r l e s s b a s i c p r o p e r t y .

I n oxidation o f

p r o p y l e n e o v e r Sn02, i t i s known t h a t f o r m a t i o n o f a c r o l e i n i s enhanced by i n c r e a s i n g a c i d i t y o f t h e c a t a l y s t , w h i l e f o r m a t i o n o f benzene b y i n c r e a s i n g t h e b a s i c i t y (ref.61).

The r o l e o f b a s i c s i t e s on MgO f o r hydrogenation, hydro-

c r a c k i n g , and r e a c t i o n o f CO and H2 was d e s c r i b e d a l r e a d y .

This k i n d o f study

w i l l provide useful informations f o r design o f c a t a l y s t f o r various reactions. vii) E l u c i d a t i o n o f S t r u c t u r e s o f A c t i v e B a s i c S i t e s . Some model s t r u c t u r e s o f b a s i c s i t e s on MgO and CaO have been proposed r e c e n t l y on t h e b a s i s of s p e c t r o s c o p i c s t u d y ( r e f . 4 , 6 2 ) .

However, more d e t a i l e d

s t u d y of a c t i v e b a s i c s i t e s ( e l e c t r o n p a i r donor s i t e s ) i n r e l a t i o n t o r e d u c i n g s i t e s ( s i n g l e e l e c t r o n donor s i t e s ) w i l l be necessary.

A l t h o u g h t h e mechanism

o f a c i d i t y g e n e r a t i o n and model s t r u c t u r e s o f a c i d s i t e s on a c i d i c m i x e d metal o x i d e s have been proposed ( r e f . 6 3 ) ,

n o t h i n g i s known o f b a s i c s i t e s on b a s i c

mixed metal o x i d e s (see group 6 i n Table 7 ) .

Thus, t h e s t r u c t u r a l s t u d y o f t h e

b a s i c s i t e s i s encouraged.

viii) Development o f New Type S o l i d Bases. The k i n d s o f u p - t o - d a t e s o l i d bases shown i n Table 7 a r e l e s s numerous t h a n those o f s o l i d acids.

The development o f new t y p e s o l i d bases i s d e s i r a b l e .

13 TABLE 7 Sol i d bases 1. Mounted bases: NaOH, KOH mounted

on s i l i c a o r alumina; A l k a l i metal and

a l k a l i n e e a r t h m e t a l d i s p e r s e d on s i l i c a , alumina, carbon, K2C03 o r i n o i l ; NR3, NH3, KNH2 on alumina; Li2C03 on s i l i c a

2. Anion exchange r e s i n s 3. Charcoal h e a t - t r e a t e d a t 1173K o r a c t i v a t e d w i t h N20, NH3 o r ZnC12-NH4C1-C02 4. Metal o x i d e s : BeO, MgO, CaO, S r O , BaO, ZnO, A1203, Y203, La203, Ce02, Tho2, Ti02, Zr02, Sn02, Na20, K20 5. Metal s a l t s : Na2C03, K2C03, KHC03, KNaC03, CaC03, SrC03, BaC03, (NH4)2C03,

Na2W04-2H20, KCN 6. Mixed o x i d e s : Si02-Mg0, Si02-Ca0, Si02-Sr0, Si02-Ba0, Si02-Zn0, Si02-A1203, Si02-Th02, Si02-Ti02, Si02-Zr02, A1 203-Ti 02, A1203-Zr02

Si02-Mo03, Si02-W03, A1203-Mg0,

, A1 203-Mo03,

A1203-W03,

Zr02-Zn0,

A1203-Th02

,

Zr02-Ti 02, Ti02-Mg0,

Zr0,-SnO, -~ ~~

7. Various k i n d s o f z e o l i t e s exchanged w i t h a l k a l i metal o r a l k a l i n e e a r t h m e t a l

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., .,

14

1 8 H. H a t t o r i , K . Shimazu, N. Y o s h i i and K. Tanabe, B u l l . Chem. SOC. Jpn., 49 (1976) 969, 19 K. Tanabe, K . Shimazu and H. H a t t o r i , Chem. L e t t . , (1975) 507. 20 General E l e c t r i c Co., U.S. Patent, 3,446,856 (1964); Neth. Appl. 6,506,830, ( 1 965) 21 K. Tanabe and T. N i s h i z a k i , Proc. 6 t h I n t e r n . Congr. C a t a l y s i s , 2 (1977) 863. 22 T. Kotanigawa, M. Yarnamoto, K . Shimokawa and Y . Yoshida, B u l l . Chem. SOC. Jpn., 44 (1971) 1961. 23 Unpublished r e s u l t s . 24 T. Yashima, K. Sato, T. Hayasaka and N. Hara, J. Catal., 26 (1972) 303. 25 K. Tanabe, 0. Takahashi and H. H a t t o r i , React. K i n e t . C a t a l . L e t t . , 7 (1977) 347. 26 M i t s u b i s h i Petrochemical Co., Japan P a t e n t Appl Sho 52-133,932 ( 1 9 7 7 ) . 27 Monsanto Co., US Patent, 4,115,424 ( 1 9 7 8 ) . 28 K. Tanabe and K. S a i t o , J. Catal., 35 (1974) 2'47. 29 Y . Kakuno, H. H a t t o r i and K. Tanabe, Chem. L e t t . , (1982) 2015. 30 T. Ushikubo, H . H a t t o r i and K. Tanabe, Chem. L e t t . , (1984) 649. 31 K. Arata, J. 0. Bledsoe and K. Tanabe, Tetrahedron L e t t . , 43 (1976) 3861; J. Org. Chem., 43 (1978) 1660. 32 Y. Tanaka, H. H a t t o r i and K . Tanabe, Chem. L e t t . , (1976) 37. 33 H. H a t t o r i , Y. Tanaka and K. Tanabe, J . Am. Chem. SOC., 98 (1976) 4652. 34 G. Wang, H. H a t t o r i , H. I t o h and K. Tanabe, J. Chem. SOC. Chem. Commun., (1982) 1256. 35 G. Wang, H . H a t t o r i and K. Tanabe, "Shokubai ( C a t a l y s t ) " , 25 (1983) 359. 36 T. Yamaguchi, Y. Nakano, T. I i z u k a and K . Tanabe, Chem. L e t t . , (1976) 677. 37 Y . Nakano, T. I i z u k a , H . H a t t o r i and K. Tanabe, J. Catal., 57 (1979) 1 . 38 Y. Nakano, T. Yamaguchi and K. Tanabe, 3. Catal., 80 (1983) 307. 39 H. I m a i , H. H a t t o r i and K. Tanabe, Chem. L e t t . , (1979) 1001. 40 H. H a t t o r i and K. Tanabe, H e t e r o c y c l e s , 16 (1981) 1863. 41 N. Yamagata, Y . Owada, S . Okazaki and K. Tanabe, J . Catal., 47 (1977) 358. 42 Unpublished r e s u l t s . 43 K. Tanabe and T. Yamaguchi, J. Res. I n s t . C a t a l . Hokkaido Univ., 11 (1964) 179. 44 J. Take, N. K i k u c h i and Y . Yoneda, 3. Catal., 21 (1971) 164. 45 T. Yamanaka and K, Tanabe, J. Phys. Chem., 79 (1975) 2409. 46 S . Malinowski and S. Szczepanska, J. Catal., 2 (1963) 310. 47 H. K i t a , N. Henmi, K . Shimazu, H . H a t t o r i and K . Tanabe, J. Chem. S O C . Faraday Trans. I , 77 (1981) 2451. 48 T. I i z u k a , Y. Endo, H. H a t t o r i and K. Tanabe, Chem. L e t t . , (1976) 803. 49 K . Tanabe and T. Yamaguchi, J . Res. I n s t . C a t a l . Hokkaido Univ., 14 (1966) 93. 50 H. Vinek, H . N o l l e r , M. Ebel and K . Schwarz, J. Chem. SOC. Faraday Trans, I, 73 (1977) 734. 51 G. Wang, H. H a t t o r i and K. Tanabe, B u l l . Chem. SOC. Jpn., 56 (1983) 2407. 52 M. A i , B u l l . Chem. SOC. Jpn., 49 (1976) 1328. 25 (1977) 329. 53 J. K i j e n s k i and S . Malinowski, B u l l . Acad. P o l o n a i s e S c i 54 J. K i j e n s k i and S. M a l i n o w s k i , B u l l . Acad. P o l o n a i s e S c i . , 25 (1977) 427. 55 J. K i j e n s k i , M. Marczewski and S. M a l i n o w s k i , React. K i n e t . C a t a l . L e t t . , 7 (1977) 151. 56 P r i v a t e communication from T. Suzukamo. 57 S . Miyata, T. Kumura, H . H a t t o r i and K . Tanabe, Nippon Kagaku Zasshi, 92 (1971) 514. 58 K. Tanabe, T. Sumiyoshi, H. H a t t o r i , K . Tamaru and T. Kondo, 3. C a t a l . , 53 (1978) 1 . 59 K. Arata, S. Akutagawa and K. Tanabe, B u l l . Chem. SOC. Jpn., 49 (1976) 390. 60 K . Tanabe, K . Shimazu, H . H a t t o r i and K e i . Shimazu, J. Catal., 57 (1979) 35. 61 T. Seiyama, M. Egashira, T. Sakamoto and I. Aso, J . Catal., 24 (1972) 76. 62 S . C o l u c c i a and A. J . Tench, Proc. 7 t h I n t e r n . Congr. Catal., Kodansha, Tokyo, 1980, 6-35. 63 K. Tanabe, T. Sumiyoshi, K. S h i b a t a , T. K i y o u r a and J. Kitagawa, B u l l . Chem. SOC. Jpn., 47 (1974) 1064.

.

.

.,

15

B. Imelik et al. (Editors), Cutalysis b y Acids and Bases o 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

A TPD, FT-IR AND CATALYTIC STUDY OF THE INTERACTION OF METHANOL WITH PURE AND

KOH DOPED Ti02 ANATASE 1 3 2 G. BUSCA , P. FORZATT12, J . C . LAVALLEY and E. TRONCONI 'Istituto Chimico, Facolts di Ingegneria, Universitz di Bologna, Viale Risorgimento 2 - 40136 Bologna (Italy) 2 . Dipartimento di Chimica Industriale e Ingegneria Chimica del Politecnico, P.zza Leonard0 da Vinci 32 - 20133 Milano (Italy) 31.S.M.Ra., Universitg de Caen, 14032 Caen CGdex (France)

ABSTRACT The interaction of methanol with pure and KOH doped Ti02 anatase has been studied by means of TPD and FT-IR techniques, and by pulse reactor measurements. Three different samples of Ti0 have been considered. FT-IR spectra have allowed H-bond&l and chemisorbed species present on the identification of a number of Ti0 surface. The nature of such species has been related to the results of TPD 2 . experiments and of pulse reactor measurements. By taking into account the different experimental conditions of the three techniques, a unitary picture of the CH OH-Ti0 surface interaction is arrived at. 3 2 RESLW L'interaction du m6thanol avec le Ti02 anatase pure et dopGe au KOH a btb btudie6 par les techniques TPD et FT-IR, et par des nesures dans un rgacteur b impulsions. Trois diffbrents Gchantillons de Ti02 ont 6tE considGr6s. Les spectres FT-IR ont permis d'identifier un certain nombre d'esphces physisorbegs et chimisorbges, prssentes P la surface de Ti02. La nature de ces espsces a Bt6 mise en rapport avec les rbsultats des expbriences de TPD et des mesures 5 impulsion. En tenant compte des conditions expgrimentales diffbrentes pour les trois techniques, on obtient une vision unitaire de l'interaction de surface CH OH-Ti02. 3

INTRODUCTION The interaction of lower aliphatic alcohols with oxide surfaces has been extensively studied by means of IR spectroscopy, TPD, and kinetic measurements. A l s o , the effect of doping with alkaline compounds onto the acid-base properties

of oxide surfaces has been investigated. Most of the literature, however, refers to alumina, silica, and silica-alumina, but only a few data refer to Ti02. Fur-

thermore, different behaviors have been reported for different samples of Ti0 2 [l]. In this paper we present the results of a study on the interaction of methanol with pure and KOH doped Ti02 anatase, aimed at characterizing the acid-base properties of this oxide compound, which is being more and more extensively used as a s u p p o r t or as a component in many commercial catalysts [2]. Several complementary techniques, namely FT-IR, TPD, and pulse reactor measurements, are em-

16 ployed in order to arrive at a more complete description of the interactions. EXPERIMENTAL Three TiO, samples have been considered: a Degussa P25 sample (Ti02-D),

a

Tioxide CLDD 1587/1 sample (Ti02-T), and a sample prepared in our laboratory by hydrolysis of TiC14 (Carlo Erba RP

reagent)followed by drying overnight and

calcination at 973 K for 3 hours (Ti02-P).

Main properties of the three samples

are given in Table 1. TABLE 1 Characteristics of catalyst samples Sample

2

BET surf. area (m /g)

Phase composition by XRD

Ti02-D

50

Ti02-T

170

Anatase

Ti0 -P 2

20

Anatase

Anatase

90%; rutile

Main impurities

10% HC1=0.3%;SiU2=0.2%

so3=5.7% -

KOH doped samples were obtained by impregnation from water solution, followed by calcination at 673 K for 2 hours. KOH content is given as K'

% by weight. FT-

IR spectra were recorded using a Nicolet MX1 spectrometer (11. Experimental details on TPD runs are the same as reported elsewhere (31. Pulse reactor experiments were performed in a standard pulse apparatus with methanol-nitrogen mixtures (CH30H = 4%). RESULTS AND DISCUSSION Ti0

-2

FT-IR. Fig. -

1A and Fig. 1B show the FT-IR spectra of Ti0 -D after contact 2 two doublets in the vCH

with methanol at low pressure. A vOH band at 3470 cm-',

region and a 60H band at 1365 cm-l appear at room temperature (r.t.), evenafter evacuation (see curves b and c vs. a in Fig. l A , and curve a in Fig. IB). Evacuation at 473 K causes the vOH and 60H bands to disappear, along with the higher frequency components of the two vCH doublets. The bands still present after evacuation at 473 K are all and only those expected for adsorbed methoxy groups -1 due to heating points to a con(see Fig. 2, species b). The shift of a few cm formational rearrangement of this species. The bands which disappear after evacuation at 473 K may be assigned to an undissociated form of CH OH interacting 3 with a Lewis acid center (species a). Notice that the vOH and 60H frequencies of species a agree with those of alcohols interacting with Lewis acids [4]. The existence of both dissociated and undissociated chemisorbed species has been observed on Ti0 -D after interaction at r.t. with CH SH [5]. Species 5 and 2 3

b

are also

17

of an activated Ti0 Fig. 1. (A) Transmittance FT-IR spectra (3800-2700 cm-') 2 disc (a), in contact with methanol vapor (up to 0.1 Torr) (b), and after eof the vacuation at 473 K (c). (B) Absorbance FT-IR spectra (1700-1000 cm-') species formed on Ti02 anatase: b and c, same as above; d, after evacuation at 523 K. Spectra of Fig. 1 B are plotted in absorbance after subtracting the spectrum of the starting sample in order to point out the bands near the cut-off due to bulk Ti-0 vibrations.

observed on Ti0 -T, but species 5 is relatively more abundant, in agreement with 2 the lower basicity of this sample. Heating at 523 K causes the progressive formation of intense bands at 1560, 1378, and 1360 cm-', heating

typical of formate ions

COO, 6CH, U s C O O ) . Further as at temperatures up to 723 K attenuates the intensity of the bands of (V

both formate and methoxy groups, which are however still present. Fig. 3 and 4 show the FT-IR spectra of Ti0 -D after contact with methanol at 2 high pressure and subsequent evacuation at increasing temperatures. The simple admission of CH OH at 10 Torr causes the disappearance of the bands associated 3 with surface OH groups of anatase and perturbation of the VOH and &OH bands of species 2. Thus the presence of

H-bonded

methanol species

< and e can be in-

Fig. 2. Proposed structures of adsorbed methanol forms on Ti02 anatase.

18

-1 Fig. 3. Transmittance FT-IR spectra (3800-2600 cm of Ti02 disc after contact with methanol vapor (10 Torr) at r.t. (a), evacuation at r.t. (b), evacuation at 373 K (c) and evacuation at 473 K (d); Ti0 +2% K+ after contact with methanol vapor at 10 Torr and evacuation at 573K (e3

.

ferred. Besides, several VOH, VCH and V C O bands are apparent in Fig. 3 (curve a) and Fig. 4 (curve a), indicating that a number of different adsorbed species are present. Prolonged evacuation at r.t. causes the almost complete disappearance of components at 3150 (very broad) and 1032 cm-', absorbance near 1460 cm-'.

as well as the decrease of the

These features may be assigned to VOH, VCO and &OH of

a CH30H molecule acting a s a proton donor in hydrogen bonding with basic sites of the surface (species

c ) . The

corresponding VCH bands are observed near 2950

(shoulder), 2910 and 2815 cm-'. Under the same conditions the 60H band of spe-1 d decomposes is restored, thus indicating that species cies 5 at 1365 cm through methanol desorption. On rising the evacuation temperature from room up to 473 K the bands at 3420 and 1060 cm-t which can be assigned to v0H and VCO of species $ and

e, progressively disappear. At

473 K all and only the bands due to

methoxy groups are present, and bands due to surface OH groups are not restored.

*

This indicates that also species 5 transforms into methoxy groups. Therefore, methoxy groups on anatase are formed

different mechanisms, namely

*

methanol

dissociation on acid-base pairs at r.t., as already reported for alumina [6]; reaction of CH30H with surface hydroxy groups at 373-473 K, as reported for silica [7] ; and possibly

via

decomposition of chemisorbed species

a at

373-473 K.

Starting from 523 K formate ions begin to appear. Strictly similar results have been obtained for Ti02-P.

19

Fig. 4 . APsorbance FT-IR spectra (1700 -1000 cm ) of the species formed on the surface of anatase after the same treatments as in Fig. 3. TPD. -

Fig. 5 . TPD curve of Ti02-P.

The full line curve in Fig. 5 shows the results of methanol TPD from

Ti0 -P. Similar results are obtained with Ti0 -D. On the basis of FT-IR data and 2

2

on-line gaschromatographic analysis the full line curve has been decomposed into four single TPD peaks: peak I, associated with the evolution of weakly adsorbed methanol species 2; peak 11, associated with the evolution of methanol from d and 2; peak 111, associated with the decomposition of methoxy groups, species followed by CH20 evolution; peak IV, associated with methanol evolution interpreted as the result of the recombination of methoxy groups with surface mobile protons, made available through the oxidation of methoxy and/or formate species.

Also the decrease of the formate species, after evacuation up to 723 K, is likely to result in the evolution of CO during TPD measurements in this temperature region (81, which however could not be detected by FID. During TPD runs performed with Ti0 -T (containing 5.7% SO ) ethane and formaldehyde were observed in 3

2

addition to methanol, with peak maximum temperatures at 650 and 560 K , respectively. Diiaethylether

-

detected in the temperature range 550

(DME) was

650 K.

These results are somehow intermediate between those obtained for Ti0 -P and 2

Ti0 -D, and those reported by Carrizosa et al. [ 9 ] for anatase prepared by hydro2 lysis of titanyl sulphate. In addition to methanol evolution, these authors observed evolution of DME at 573

-

673 K, and of C H at 573 2 6

-

723 K. This compari-

son indicates a likely correlation between ethane evolution and the presence of sulphate impurities.

20

The decomposition of the full line curve in Fig. 5 has been made under the assumption that curves I, IV are symmetrical, and curve I1 is obtained by difference. Both a priori and experimental criteria have shown that diffusional resistances and readsorption could be neglected during the analysis of TPD results [3,10] Curves I, 111, IV in Fig. 5 have been analyzed on the basis of a homogeneous surface model and first order desorption kinetics [ll], and the following energies of desorption calculated: curve I, E =10 Kcalfmol; curve 111, E =27 Kcal/mol; d d curve IV, Ed=32 Kcal/mol. The analysis of curve I1 could not be performed under the above assumptions, even in the framework of a heterogeneous surface model, because the requirement of a single adsorption state is not satisfied (compare FT-IR results). Pulse reactor experiments. Fig. 6 presents the results of pulse measurements carried out on Ti0 -P at different temperatures with a methanol-nitrogen mixture. 2 At 373 K and typical pulse conditions methanol is likely to be mainly physisorbed due to the poisoning effect of water molecules on Lewis acid sites, the interaction is almost completely reversible. In the range 423

-

so

that

573 K water

desorbs and methanol is totally irreversibly adsorbed on the Ti02 surface. FT-IR and TPD measurements pointed out that different chemisorbed and physisorbed species are formed. These are responsible for methanol evolution from 373 up to 523 -573 K, and/or can transform into methoxy species which are stable up to 573 K. Considering that pulse reactor measurements are carried out at conditions far from saturation, contrary to typical TPD conditions (surf. conc 'pulse/surf. -2 ) ) , methanol is expected to adsorb on the most active sites duconc.TpD=O(10 ring pulse runs. Besides, the higher contact time and the greater particle size of pulse experiments (t /R =lo) will favor the transpulse/tTPD=20; p' pulse p TPD formation of adsorbed methanol into methoxy species. Both these facts are in line with the absence of any product during pulse experiments in 4 2 3

ocwn

0

Fig. 6 . Results of pulse reactor experiments with Ti02-P.

-

573 K.

21 Above 573 K the lattice oxygen becomes sufficiently reactive, as indicated by the formation of GO in pulse experiments. This effect is consistent with the fo2mation of formate adsorbed species; as detected by FT-IR, and with the evolution of formaldehyde, as obtained during TPD. The absence of formaldehyde (and methanol) evolution during pulse experiments can be explained by further oxidation to G O , owing to much higher contact times, greater R values and strong interaction P with acid Ti02 surface sites, and/or methanol readsorption. The carbon balance is never fulfilled, thus confirming that methanol still remains irreversibly adsorbed in this high temperature region in the form of methoxy and formate species. Reoxidation at 773 K is required to clean the surface completely. This agrees with the FT-IR observation that methoxy and formate species are still present even after evacuation at 723 K. Ti0 + K" -b FT-IR. Fig. 7 shows the effects of KOH doping on the spectra of hydroxy groups and of adsorbed GO on anatase. Only one vOH band is observed on KOH doped Ti02, compared with five on the pure sample [l]; its frequency shifts downwards (3720 on 1%K+ sample, 3708 cm-I on 2%Kf sample) and its intensity is progressi-

vely diminished. Simultaneously, adsorption sites of GO, which are responsible for the Lewis acidity of anatase [l] , are also progressively poisoned.

.€ I 3735 b

'

cm? cr

2'200' 2.10 I

Fig. 7. Transmiitance FT-IR spectra of pure Ti02 (a), 1%K+ on Ti02 ( b ) , 2%K+ on Ti0 ( c ) and 3%K on Ti0 (d), activated in vacuo at 673 K (A) and in contact wit2 100 Torr of CO gas BB).

22

In agreement with these results, adsorption of methanol on KOH doped samples -1

causes the formation of a very broad band centered at 3100 cm by evacuation at 373 milar to species

-

c but

(VCO)

that disappears form

si-

interacting with a stronger basic site. An adsorbed spe-

cies responsible for bands at 2910, 2800 ( v C H ) ,

1125 cm-'

,

423 X. This feature may be due to adsorbed

1 4 6 5 , 1443 (ACH), 1151 (PCH) and

is also formed (curve e in Fig. 3 and 4 ) , and resists evacuation

at 573 K. These features agree with those of a methoxy group, even if the low values of $ C H and the high value of vCO cleyrly indicate that it is more anionic than the previous species

owing to the weakening of Lewis sites induced by KOH

doping. The impregnation of Ti0 -P or Ti0 -D with KOH solution results in a pro2 2 gressive disappearance of the high temperature desorption peaks, as shown inFig. TPD. -

8 for Ti0 -P. This is related t o the poisoning of Lewis acid sites, and of surfa2 ce Ti02 hydroxy groups, due to the reaction with KOH. By this way the formation of species 5 and g is prevented, and their transformation into methoxy or fcrmate groups cannot occur any longer, as confirmed by FT-IR spectra. Doping with KOH also causes a progressive shift of the temperature of the maximum of peak I

from 353 K to 388 K. This indicates that stronger basic sites are formed after reaction of KOH with Ti02 surface hydroxyls, and it is further consistent with the assignment of peak I to the desorption of

adsorbed

methanol species 5.

Pulse reactor measurements. Fig. 9 presents the results of pulse measurements for Ti0 -P + 2%K+ with methanol-nitrogen mixture. Methanol is partially irrever2 sibly adsorbed at 373 K . This i s in line with the poisoning effect of KOH on the

+

Fig. 8. Effe+ct of KOH doping on the TPD spectra of pure Ti02-P ( a ) , Ti02+1%K (b), TiO2+2%K (c), TiO2+3ZK (d).

23

Fig. 9 .

Results of pulse reactor experiments with Ti02-P + 2%K+.

Lewis and Broensted acid sites of Ti02, and with the slightly more acidic nature of methanol with respect to water, so that methanol is preferentially adsorbed on the basic sites, and still remains partially irreversibly adsorbed at 3 7 3 K. Above 3 7 3 K all the methanol is desorbed, the carbon balance is fulfilled, and formaldehyde is observed for T > 4 7 3 K. These effects are consistent with KOH doping preventing the formation of chemisorbed 5 methanol species. Therefore, strongly adsorbed methoxy and formate species are not formed at higher temperatures. Also, the formation of

4

and

e

adsorbed

methanol species is prevented.

The formation of formaldehyde in pulse reactor experiments is likely related to the presence of more anionic methoxy groups; it is made possible by much longer contact times than in TPD experiments, and is preserved due to poisoning of acid reactive sites of Ti02. This behavior becomes apparent at temperatures where the lattice oxygen is sufficiently reactive ( above 4 2 3 K). ACKNOWLEDGMENT Two of the authors thank M.P.I. (Rome) for financial support (P.F. and E.T.). REFERENCES G. Busca, H. Saussey, 0. Saur and J.C. Lavalley, submitted for publication. S. Matsuda and A. Kato, Appl. Catal. 8 ( 1 9 8 3 ) 1 4 9 . P. Forzatti, M. Borghesi, I. Pasquon and E. Tronconi, submitted for publication. J.P. Gallas, Thesis, Universit6 de Caen ( 1 9 8 4 ) . H. Saussey, 0. Saur and J.C. Lavalley, J. Chem. Phys., in press. R.G. Greenler, J. Chem. Phys. 37 ( 1 9 6 2 ) 2094. B.A. Morrow, J. Chem. SOC. Faraday Trans. 70 ( 1 9 7 4 ) 1 5 2 8 . R.P. Goff and W.H. Manogue, J. Catal., 79 ( 1 9 8 3 ) 4 6 2 . I . Carrizosa, G. Munuera and S . Castanar, 3. Catal. 49 (1977) 265.

24 10 R. Gorte, J . Catal. 75 ( 1 9 8 2 ) 164. 11 P. F o r z a t t i , M. Borghesi, I. Pasquon and E. Tronconi, Surface Sci. 137 (1984) 595.

B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

25

ACID AND BASE STRENGTH OF ALUMINA-MAGNESIA MIXED OXIDES J.A. L E R C H E R , Ch. COLOMBIER, H. VINEK and H. NOLLER Technische Universitat Wien, I n s t i t u t f u r Physikalische Chemie, Getreidemarkt 9 , A-1060 Vienna, Austria

ABSTRACT Acid and base strength of alumina magnesia mixed oxides was investigated by adsorption of various molecules. The change of the i . r . spectra of acetone, pyridine and carbon dioxide a f t e r adsorption was used t o estimate the strength of Lewis acid and base s i t e s which interacted w i t h the adsorbed molecules. Gutmann's electron p a i r donor acceptor model served to explain the shifts of the i . r . bands. The strength of the acid s i t e s decreased t h a t of the basic s i t e s increased with increasing magnesia content. While three different kinds of Lewis acid s i t e s were observed (OH groups, Mg2+ and A13+ cations) only one k i n d of Lewis base s i t e s (oxygen) was detected. RESUME La force acido-basique des oxydes mixtes d'aluminium e t de magnesium a ete etudige par adsorption de differentes mol@cules. Les modifications des spectres IR de 1 'acetone, de l a pyridine e t du dioxyde de carbone aprPs adsorption ont permis d'6valuer l a force des s i t e s acides de Lewis e t des s i t e s basiques en interaction avec l e s molecules adsorbees. Le mod@le"donneur-accepteur de paire d'electrons" de Gutmann a servi d expliquer les deplacements des bandes IR. La force des sites basiques c r o i t avec la teneur en magnesium alors que c e l l e des s i t s acides decroit. Trois types de s i t e s de Lewis (groupes O H , cations Mg2+ e t Al?+) e t un s i t e base de Lewis o n t ete mis en evidence.

INTRODUCTION As reactions over polar catalysts need acid as well as basic s i t e s i n order t o proceed ( l ) , the investigation of b o t h s i t e s i s a crucial task and subtle variations in t h e i r strength may provide new c a t a l y t i c routes as well as insight into c a t a l y t i c mechanisms. We studied therefore how addition of a basic oxide t o an acidic affects the strength of acidic and basic s i t e s . I t has been p u t foreward t h a t the strength of such s i t e s should be i n between those of the components ( 2 ) . However, many authors have observed higher s i t e strengths than those of the components, i f oxides formed mixed phases ( 3 ) . We used the Mg0-A1203 system as Mg i s incorporated easily i n t o the defect spinel structure of Y-A1203 u p t o 50 mol % MgO and formed a separate MgO phase a t higher concentration so introducing additional heterogeneity to probe. The main question t o be asked was how the

26

different sites would vary with composition and to what extent surface heterogeneity is manifested. METHODS Infrared spectroscopic measurements were performed using the conventional transmission absorption mode, the oxides being pressed to thin self-supporting wafers. The instrument used was a Perkin Elmer 325, the resolution was 3 cm-' at 3600 cm-'. Experimental details are described in (4). Catalytic measurements Elimination reactions with alcohols were carried out in pulse or continuous flow mode. 100 mg of the catalyst sample was pretreated in a flow of He at 773 K, then cooled to the reaction temperature. Butan-2-01 (p.a. Merck) was used as reactant. Analysis was performed by a Perkin Elmer F11 gas chromatograph. The column used for separation of butenes, butanone and butan-2-01 was carbowax, 1.5 m, 1/8 inch, 50' C. Decomposition of diacetonealcohol was studied i n a micro slurry reactor, 0.5 to 0.04 g of catalyst was charged and evacuated at 773 K for 10 hours.Then the reactor was purged with nitrogen (99.995 vol % ) and 1 ml diacetonealcohol was injected via a septum. Details of analysis are given in (5). Oxides Catalysts were prepared by adding ?-Al2O3 to a solution of Mg(N03)2 containing the desired amount o f MgO. The suspension was evaporated and the remainciertempered at 773 K for 24 hours. The composition of the mixed oxides, their BET surface area and the X-ray diffraction results are cornDiled in table 1. TABLE 1 mole % MgO

Oxide A1203 A1203/Mg0 A1203/Mg0 AI2O3/Mg0 A1203/Mg0 A1203/Mg0 MgO

1 2 3 4 5

0 5 25 50 75 95 100

BET surface (m2/g) 148 152 144 132 82 14 119

27

RESULTS AND INTERPRETATION The activated surface After evacuation a t 873 K f o r 1 hour pure alumina and A1203/Mg0 1 showed very similar i . r . spectra (3795, 3730, 3680 f - Al20-3, 3730,3680 A1203/Mg0 1). Addition of 25 and 50 mol % MgO led t o one band between 3735 and 3740 cm-l, while further MgO caused an a d d i t i o n a l band near 3685 cm-l increasing i n intens i t y w i t h MgO content. The OH group of pure MgO was, however, found a t 3740 cm-l. T h i s suggests three different types of A1203/Mg0 oxides, those w i t h very small amounts of MgO and OH stretching bands similar t o those off-A1203, those w i t h MgA1204 dominating, which have only one OH-stretching band and those of h i g h MgO content with a new type of hydroxyl group, apparently associated with an MgO phase. A detailed description can be found i n ( 4 ) . Adsorption of acetone Acetone interacted with the surfaces i n two ways : i ) w i t h OH groups, i i ) with accessible metal cations and surface oxygens, the molecules being parallel t o the surface. I t has been shown t h a t hydrogen bond w i t h OH group increases with the OH acidity strength ( 6 ) . Hence the OH frequency s h i f t due t o the acetone adsorption would r e f l e c t the strength of the interaction and thus enables t o scale the OH acid strength. T h i s frequency s h i f t decreased from 290 cm-I (forf-Al203) t o 260 cm-l ( f o r MgO) indicating decreasing acid strength of the hydroxyl groups i n t h a t order. The spectra of the f l a t form of adsorption, suggesting carboxylate l i k e structures, led us t o a classification of the mixed oxides similar t o t h a t obtained from the spectra of free OH groups. A1203 and mixed oxides 1 and 2 had very similar spectra in the carbonyl region (doublet a t 1630 and 1610-1620 cm-I). More magnesia caused additional band (1580 cm-1). T h i s indicates t h a t the surface of the oxides w i t h low magnesia content i s weakly basic and resembles the properties of r-A1203 surface, the surface of MgO rich oxides is inhomogeneous and has one rather strongly basic component and t h a t f i n a l l y the mixed oxides with h i g h MgAl2O4 content have properties i n between. Adsorption of pyri d i ne Pyridine adsorption resulted i n three types of?19b bands w i t h different wavenumbers characteristic f o r adsorption on Lewis acid s i t e s (see fig.1). The higher the wavenumber of the7196 band, the higher i s the strength of ( 7 , 8 ) . Provided no s t e r i c a f constraints interaction with a Lewis acid s i t e e x i s t ( 9 ) and the Lewis s i t e s have a similar number of neighbouring oxygens the wavenumber indicates the strength of a Lewis acid site. Since these re-

28

1L40

i

I * 5

25

50

75

95

m o l % MgO

Fig. 1. y19b band of pyridine adsorbed on the oxides (evacuation at 473 K) quirements are fullfilled with A13+ and Mg2+ cations for the investigated oxides we conclude that the sites of highest acid strength are found with alumina rich, the weakest with MgO rich oxides, sites o f intermediate strength beeing most abundant on MgA1204 rich oxides. Adsorption of C02 C02 was either adsorbed via its donor function (oxygen) on accessible cations or via its acceptor function (carbon) on surface oxygen forming various carbonates. With increasing MgO content, the two most abundant carbonates, bicarbonate and monodentate carbonate, decreased in their wavenumbers of Symmetric stretching (bicarbonate) and antisymmetric stretching vibration (moncdentate carbonate), which can be seen in figure 2. Although it is not clear at present, as to why the antisymmetric vibration (of bicarbonate) or symmetric (of monodentate carbonate) do not yarj their wavenumbers in a similar way, the shift indicates increasing strength of interaction o f C02 with surface oxygen and hence increased base strength. Detailed description of C02 adsorption can be found in (10). Catalytic reactions The elimination reactions o f butan-2-01 over the mixed oxides showed increasing selectivity towards dehydrogenation with increasing magnesia content (11). It has been reported (12, 13) that this is accomplished by increasing strength of interaction of surface oxygen with hydrogen in B position to the OH group and weaker interaction with the OH group itself. Therefore increasing

29 lL60

-

-1580

..

- 1570

a 3 1420

5

25

50

75

95

m o l % MgO

Fig. 2. Wavenumbers of symmetric stretching vibration of bicarbonates ( 0 ) and antisymmetric stretching vibration of monodentate carbonates ( w ) versus composition of the oxide dehydrogenation selectivity suggests increasing base strength of the oxygen. With increasing content of magnesia also the rate constant for dissociation of diacetonealcohol,a base catalyzed reaction (14, 151, increased by nearIy three orders o f magnitude (5). As the number o f basic sites per surface area unit will not vary markedly over the oxides studies this can be taken as evidence that the activity of the basic sites and hence their strength increased. DISCUSSION AND CONCLUSION Interpreting the i.r. spectra we have assumed that all interactions at the surface of theseoxides take place between electron pair donor (EPD, Lewis base) and electron pair acceptor (EPA, Lewis acid) sites. We are aware that radicals may exist at the surfaces, but suggest that they play no major role for acidbase catalyzed processes (16, 17). If that is assumed one can utilize Gutmann's "Donor-Acceptor Approach" (18). It suggests that the bonds near an EPD-EPA interaction will be the more elongated the stronger the interaction is. Moreover it implies that there is not only charge transfer from the EPD site to the EPA site but also electron redistribution within both partners of interaction. Charge shifts from a rather negatively charged atom to a rather positively charged will lead to shortening of the bond, while the reverse shift will lead to elongation of the bond. Noller and Gutmann (19) have introduced this approach to surface chemistry and catalysis. Therefore looking at the changes of polar molecules when adsorbed on polar surfaces one has a subtle mean to describe qualitatively strengths and modifications of EPD and EPA sites.

30

While it appears that the surface is quite heterogeneous in structure and that for many catalysts two solid phases are present some properties (acid strength of hydroxyl groups and base strength of surface oxygen) seem to be duite homogeneous. If the wavenumber shifts of hydroxyl groups after acetone adsorption (representing OH acid strength) are plotted against the wavenumbers of antisymmetric stretching vibrations of monodentate carbonate (representing base strength) a reasonably good correlation is obtained (figure 3).

1580 r

-

I

I

V: 260

270 280 290 wavenumbers I cm" )

300

Fig. 3. Wavenumber shift of OH bands after acetone adsorption versus wavenumber of antisymetric stretching vibration of monodentate carbonate This suggests not only that both properties have a distribution in strength with only one maximum, but also that the acid strength of hydroxyl groups increases with decreasing base strength of the oxygen and vice versa. Especially the first result accords nicely with Sanderson's ideas that electronegativity equalizes i n a mixture of elements of different atomic electronegativity (20). This is achieved by redistribution of electrons. For oxygen, if must result in similar electmn density and hence similar base strength. According to Sanderson's model ( Z O ) , also the electronegativities of the cations equalize, but as they have different numbers of electrons and electronegativities in the atomic state, it will require varying electron depletion to reach t h e same electronegativity, which leads to different electron densities at the cations. This i n turn causes Lewis acid sites (cations) of different electron pair acceptor strengths, which are manifested by three different bands of Lewis acid bound pyridine (4). Thus it appears that Lewis acid strength can be tuned rather subtle by varying the surface concentrations, while this will be difficult with base strength being an overall property.

31

REFERENCES 1 H. Noller and W. Kladnig, Catal.Rev.-Sci. Eng., 13, (1976) 149. 2 H. Vinek, H. Noller, M. Ebel and K. Schwarz, J.Chern.Soc. Faraday I, 73, (1977) 734. 3 4 5

P.G. Rouxlet and R. Sernples, J.Chern.Soc. Faraday I, 70,(1974) 2021. J.A. Lercher, Z.Phvs.Chern. N.F. 129,(1982) 209. J.A. Lercher; Ch. Colornbier and H. Noller, React. Kinet. Catal. Lett., 23, (1983 ) 365.

6 7 8 9 10

M.L. E.P. J.A. J.A. J.A. (1984

Hair and W. Hertl, J.Phys.Chern., 74, (1970) 91. Parry, J. Catal., 2, (1963) 371. Lercher, React. Kinet. Catal. Lett., 20,(1982) 409. Lercher, H. Vinek and H. Noller, J.Chern.Soc. Faraday I, 80, (1984) 1239. Lercher, Ch. Colornbier and H. Noller, J.Chern. SOC. Faraday I , 80, 949.

H. Vinek, Z. Phys.Chern. N.F. 120,(1980) 119. H. Pines and J. Manassen, J.Advan. Catal. Relat. Subj. 16,(1966) 49. H. Knozinger, H. Buhl and K. Kochloefl, J.Catal., 24,(1972) 57. A.A. Frost and R.G, Pearson, Kinetics and Mechanism, John Wiley & Sons New York, 1961. 15 K. Tanabe and Y. Fukuda, React. Kinet. Catal. Lett., 1,(1974) 21. 16 H. Noller and J.M. Parera, J.Res. Inst. Catal. Hokkaido University, 29,

11 12 13 14

(1981 .) 95. 17 H. Noller, Acta Chirn. Acad. Scient. Hung., Tornus 109 (41, (1982) 429. 18 V. Gutrnann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York 1978. 19 V. Gutrnann and H. Noller, Mh-Chernie 102,(1971! 22. 20 R.T. Sanderson, Chemical Bonds and Bond Energy, Academic Press, New York 1971.


B. ImeUk et 01. (Editon), Cotolyrkt by Acidr and Bow# 0 1986 Ekevier Science P u b 1 i . h ~B.V., Anuterdun -Printed in The Netherndr

INFLUENCE OF THE OPERATING CONDITIONS ON THE MORPHOLOGY AND ACIDITY OF K2C03/~A1 203 X. MONTAGNE, C. .DURAND and G. MABILON I n s t i t u t F r a n ~ a i sdu Petrole, B.P. .311

-

92506 RUEIL-MALMAISON CEDEX (FRANCE)

RESUME Une alumine y c a

ete

impregnee par K2C03

des teneurs comprises e n t r e 1

e t 20 % K. On a mis en evidence p a r DRX. e t I.R. I ' a p p a r i t i o n d'une phase de t y p e K A1C03(0H)2 dans l e s e c h a n t i l l o n s contenant au moins 4 % K. La synthese e t l a decomposition thermique de c e t t e phase o n t Bt& r e a l i s @ e s . L'etude I.R. e t D.T.P. de NH3 des OH e t des s i t e s acides r e s i d u e l s met en evidence l a decroissance de l ' a c i d i t e en f o n c t i o n du taux de potassium e t l'importance du t r a i t e ment thermique sur 1 'alumine e t l e s p r o d u i t s formes a I'impr15gnation. ABSTRACT Alumina samples have been impregnated w i t h K2C03 a t K contents ranging from

1 t o 20 wt. %.A potassium hydroaluminocarbonate has been evidenced by X.R.D. and 1.R. i n the samples c o n t a i n i n g a t l e a s t 4 wt. % K. T h i s phase has been synthesised and thermally decomposed. The I . R . and T.P.D. of NH3 study o f the r e s i d u a l a c i d i c s i t e s and of t h e OH evidences t h e decrease o f t h e a c i d i t y w i t h the K - c ~ n t e n tincrease and the importance o f the thermal treatment on alumina and t h e products formed during impregnation. INTRODUCTION I n a recent review MROSS (1) underlined t h a t many i n d u s t r i a l l y important r e a c t i o n s are catalyzed by alkali-doped c a t a l y s t s . Among them a m n i a synthesis, n-hexane dehydrocyclization, iso-synthesis and water-gas s h i f t r e a c t i o n s are performed over c a t a l y t i c systems i n c l u d i n g a t l e a s t y alumina associated w i t h potassium oxide. The potassium can a c t i n t h e n e u t r a l i z a t i o n o f a c i d i c s i t e s , t h e formation o f basic s i t e s , t h e m o d l f i c a t i o n o f t h e e l e c t r o n i c s t r u c t u r e of a nearby metal o r t h e s t a b i l i z a t i o n o f some c r j r s t a l l o g r a p h i c phases. Previous studies (2) o f t h e impregnation o f K2C03 on y A1203 had shown t h a t the a l k a l i i o n was w e l l dispersed on the surface. Though no c r y s t a l l i s e d compound such as KA102 o r B ( K ) A1203 was detected below 900°C (3,4) i t seems poss i b l e t h a t some potassium could migrate through t h e surface above 180°C (5).

34

I n t h i s respect we have examined t h e mechanism o f a l k a l i doping o f yc alumina a t various contents o f K2C03 i n order t o i d e n t i f y t h e l o c a t i o n o f t h e potassium on the surface o f alumina, and s p e c i f y t h e r e s u l t i n g e f f e c t s on t h e a c i d i t y of the support. EXPERIMENTAL Preparation yc alumina (Rhbne-Poulenc, 208 m2/g, pore volume =0.6 cm3/g)

calcined a t

450°C was impregnated w i t h K2C03 ( a n a l y t i c a l grade) aqueous s o l u t i o n

by two

methods :

-

method A = excess o f s o l u t i o n ; 40 g o f alumina were soaked w i t h 500 rnl

o f s o l u t i o n d u r i n g 24 h, then washed and d r i e d a t 120°C. Potassium content was determined by X-ray fluorescence a f t e r c a l c i n a t i o n a t 250°C. Nomenclature A-1.4-K = 1.4 weight % K on a sample A.

-

method 8 = d r y impregnation ; 24 m l o f K2C03 s o l u t i o n were completely ab-

sorbed by 40 g o f alumina i n a spinning bowl. A f t e r 24 h t h e samples were d r i e d a t 120°C. As the decomposition o f carbonates i s markedly dependent upon t h e K c o n t e n t i t i s n o t p o s s i b l e t o have a standard c a l c i n e d s t a t e . Therefore the K c o n t e n t g i v e n i s t h a t added t o t h e alumina: B-8-K = 8 g K f o r 100 g A1203.

K c o n t e n t ranging from 1 t o 20 w t . %. A n a l y t i c a l methods X-ray d i f f r a c t i o n

(X.R.D)

Powdered samples were s t u d i e d u s i n g a Siemens d i f f r a c t o m e t e r

D 501 w i t h a Cu tube

and a germanium primary monochromator.

I . R. spectroscopy The samples a r e s t u d i e d i n s i t u i n a c e l l described p r e v i o u s l y ( 6 ) on a D i g i l a b FTS-15-E spectrophotometer. Thermal a n a l y s i s About 100 mg o f powdered sample were p u t i n a p l a t i n u m c r u c i b l e and heated under n i t r o g e n flow a t 4"C/mn i n a M e t t l e r thermoanalyzer TA1. Temperature programmed d e s o r p t i o n o f ammonia = T.P.D.N. 1 g o f sample i s c a l c i n e d a t 400"C, cooled down t o room temperature and f l u s h e d under ammonia d u r i n g 5 minutes. A f t e r f l u s h i n g w i t h helium d u r i n g 2 hours, the temperature was r a i s e d up t o 600°C a t 5"C/mn and the gaseous phase analyzed by G.C. Temperature programmed decomposi t i o n o f carbonates = T.P.D.C. I n t h e same device as T.P.D.N. 800°C w h i l e a n a l y z i n g by G.C.

t h e sample was heated a t a constant r a t e up t o t h e C02 e v o l u t i o n i n the helium a t t h e e x i t .

35

RESULTS Characterization

of t h e d r i e d samples and t h e i r thermal e v o l u t i o n

Values l i s t e d i n t a b l e I show t h a t t h e h i g h e s t observed potassium 2 c o n t e n t i n A samples i s o n l y 1 . 4 w t . % t h a t i s t o say 1.1 at.K/nm .

TABLE I e v o l u t i o n w i t h A impregnation

Potassium and aluminum

170 XRD

of

260

444

t h e s e samples a r e v e r y s i m i l a r t o t h o s e o f t h e s t a r t i n g alumina.

Some o f them show v e r y weak l i n e s o f b a y e r i t e t h a t have a l r e a d y been d e t e c t e d i n a l k a l i - t r e a t e d alumina ( 7 ) . I n B samples f o r t h e l o w K-contentssome weak l i n e s may be those o f pseudo-boehmite g e l ( f i g . 1,c)

F i g . 1. X-Ray d i f f r a c t o g r a m m s of samples d r i e d a t 120°C : ( a ) s t a r t i n g a l u m i n a ; ( b ) A-1.4-K w i t h b a y e r i t e ( ) ; ( c ) B-4-K w i t h pseudoboehmite ( T ) ; ( d ) B-8-K w i t h KA1C03(0H) ; ( e ) B-20-K w i t h KA1C03(0H) and K-carbona?es ( * ) . ( f ) KA1C03(0H) ; ( g f KA1C03(0H)2 + pseudo-boehmite g e l 7 ) .

f

F i g . 2. D i f f e r e n t i a l t h e r m a l a n a l y s i s and thermogravimetry f o r B-8-K, B-20-K and KA1C03 (OH) 2.

36

When t h e concentration o f potassium increases t o 8 w t . % some peaks appear ( f i g . l,d),

which are very c l o s e t o those o f t h e JCPOS f i l e no 22-791 which

r e f e r s t o a compound KA1C03(0H)2 published by TOMILOV e t a l . ( 8 ) . Nevertheless 0

t h e f i r s t peak ( I = 100, d = 6.66 A) i s n o t detected i n our samples and t h e i n t e n s i t i e s are n o t e x a c t l y those o f the f i l e . When the K-content increases some o t h e r peaks appear, which can be a t t r i b u t e d t o KHC03 and perhaps t o K2C03, 1.5 H20. When s t u d i e d by samples show

I.R. i n a i r a t ambient temperature a l l t h e d r i e d

hydrogenocarbonate

bands. These bands disappear on outgassed

samples. For K-content as low as 4 w t . %, bands a r e found a t 3440 ( w i t h a shoulder a t 3410), 1975, 1825, 1405 and 1100 cm-l which are s i m i l a r t o those o f KA1C03(OH)2 (9). Potassium hydroalumino carbonate x As t h e X.R.D.

peaks o f t h e compound detected i n B-8-K do n o t e x a c t l y f i t

t h e p r e v i o u s l y published p a t t e r n o f KA1C03(0H)2 we have s y n t h e t i z e d i t according t o GROOTE (10) by m i x i n g a KHC03 s o l u t i o n w i t h aluminium t r i - i s o p r o p o x i d e . The

I.R.

spectrum o f t h e s y n t h e t i c compound i s p e r f e c t l y i d e n t i c a l t o

those published i n ( 9 ) . Moreover the X.R.D.

peaks a r e t h e same as those appea0

r i n g i n B-8-K ( f i g . 1) b u t t h e r e i s no l i n e a t 6.66 A, and the r e l a t i v e i n t e n s i t i e s a r e n o t t h e same as i n t h e J.C.P.D.S.

f i l e . Changing s l i g h t l y t h e o p e r a t i n g

c o n d i t i o n s o f preparation, pseudo-boehmite i s obtained i n m i x t u r e w i t h 0

KA1C03(0H)2 ( f i g . 1, 9). This pseudo-boehmite presents a f i r s t l i n e a t 6 . 6 A as a l r e a d y assessed by (11). Thermal a n a l y s i s o f KA1C03(0H)2 shows T.G.

and D.T.A.

curves ( f i g . 2)

s i m i l a r t o those o f TOMILOV e t a l . ( 8 ) b u t w i t h some d i f f e r e n c e s . Those authors found a weight loss o f 50 % w i t h an i m p o r t a n t c o n t r i b u t i o n below 200°C, which i s n o t recorded f o r our sample. The f i n a l weight l o s s a t 900°C i s 36.3 % which i s s l i g h t l y less

than t h e t h e o r e t i c a l value of 38.7 % f o r a pure KA1C03(0H)2

compound decomposing t o KA102 and C02 p l u s H20. I n T.P.D.C.

t h e C02 e v o l u t i o n

shows a peak a t 320°C f o l l o w e d by a continuous t a i l up t o 760°C where another smaller peak r i s e s . According t o l i t t e r a t u r e (10) KAlC03(OH)2 can a l s o be prepared from an aluminumhydroxide

and KHC03. Consequently we impregnated y c A1203 w i t h KHC03

i n s t e a d o f K2C03 i n order t o prepare a 6-8-K sample. Asa r e s u l t we observed a s t r o n g enhancement o f t h e T.P.D.C.

peak a t 320°C and of the I.R.

bands of

KA1C03( OH)2.

x: Though our data do n o t e x a c t l y agree w i t h those o f ( 8 ) i n t h e f o l l o w i n g we s h a l l reference our product as KA1C03(0H)2.

31

Heating o f the impregnated samples Results o f T.G. f o r the A-1.4-K

sample show t h a t t h e weight l o s s i s c o n t i -

nuous as f o r the s t a r t i n g alumina. For samples r i c h e r i n

K ( f i g . 2 ) the curves

resemble those of KA1C03(OH)2, b u t w i t h some d i f f e r e n c e s : t h e r e are two endotherms near 300"C, one a t about 250°C and t h e o t h e r a t about 320°C. The weight l o s s a t about 700°C i s more important than i n t h e pure KA1C03(0H)2, b u t w i t h o u t n o t i c e a b l e thermal e f f e c t . When u s i n g T.P.D.C.

i t i s shown t h a t

f o r K-content lower than 2 w t . % (A o r B impregnation) small amounts o f C02 evolve o n l y i n t h e range 100 t o 400°C ( f i g . 3). This i s c o n s i s t e n t w i t h a c a t i o n i c exchange of potassium, t h e carbonate anion remaining i n t h e s o l u t i o n o f impregnation. A t h i g h e r concentrations a peak appears a t 250°C f o l l o w e d by a n e a r l y continuous e v o l u t i o n up t o 760°C. Above 7 w t . % K two peaks r i s e a t 325°C and 660°C. The X.R.D.

o f samples whose T.P.D.C.

was stopped a t 300 and

380°C shows t h e disappearance of t h e l i n e s o f t h e KA1C03(0H)2 phase. This a l l o w s us t o conclude t h a t t h e e v o l u t i o n o f C02 a t about 320°C i s r e l a t e d t o t h e decomposition of KA1C03(0H)2. The o t h e r COP releases do n o t a r i s e from t h e decomposition o f c r y s t a l l i n e compounds.

200

600

400

800

T ("C)

Fig. 3. Temperature programmed decomposition o f carbonates : (a) B-1-K ; (b) B-2-K ; ( c ) B-3-K ; (d) B-4-K ; (e) B-5-K ; (f) B-6-K ; (9) B-7-K ; (h) B-8-K. The I.R. study o f t h e thermal decomposition o f 6-8-K ( f i g . 4) shows t h a t t h e bands a t 3440 (shoulder a t 3410), 1975, 1825, 1405 and 1100 cm-l a t t r i b u t e d t o KAlC03(0H)2 disappear a t about 300°C. Above t h i s temperature two bands

remain a t 1350 ( w i t h a shoulder a t 1420) and 1550 cm-' which, according M0-C t o (12) and (5) a r e those o f K b / C = 0

.

38 I

I

I

I

2000

t

1500

1000

F i g . 4. I n f r a r e d s p e c t r a (carbonate s t r e t c h i n g r e g i o n ) o f 8-8-12 d u r i n g thermal treatment : (a) sample evacuated 2 h a t 25°C ; ( b ) - ( i ) evacuation f o r 30 mn a t 50°C (b), 100°C ( c ) , 150°C (d), 200°C (e), 250°C ( f ) , 300°C (g), 350°C ( h ) , 400°C (i). X-Ray d i f f r a c t i o n under d r y N2 o f KA1C03(OH)2 heated 2 hours i n d r y a i r shows t h a t t h e c r y s t a l l i n e s t r u c t u r e i s destroyed a t 300"C, t h i s agrees w i t h t h e D.T.G.

and D.T.A. 0

curves. The r e s u l t i n g diagram presents a very broad band b e t 0

0

ween 4 A and 2.5 A, w i t h a peak a t 2.8 A , and broad weak l i n e s which may be those o f pseudo-boehmite.

When heated 2 hours a t 500°C t h e sample shows l i n e s

o f KA102 and some o t h e r u n i d e n t i f i e d l i n e s .

I f t h e heated sample i s a m i x t u r e o f KA1C03(0H)2 and pseudo-boehmite prepared as r e f e r r e d t o p r e v i o u s l y , t h e broad band becomes 0

even broader,

0

0

between 4 A and 2 A, t h e r e i s no more peak a t 2.8 A, b u t t h e r e are weak l i n e s 0

0

a t 2 A and 1.4 A, which a r e t h e main l i n e s o f y o r q alumina. X-Ray diagram o f B20K ( f i g . 5, e) heated a t 400°C shows t h a t t h e peaks of KA1C03(0H)2 have disappeared b u t t h e r e i s an increase o f i n t e n s i t y i n t h e r e g i o n 0

0

between 4 A and 2 A, w i t h a peak a t 2.8 A. This seems t o be t h e s u p e r p o s i t i o n o f t h e diagrams o f alumina and o f heated KA1(C03)(0H)2(fig. 5, a, f ) . For B-8-K (fig.5,d)

t h e r e areno more l i n e s o f KA1C03(0H)2, n e i t h e r l i n e a t 2.8 A, t h e d i a -

y a m looks l i k e a s u p e r p o s i t i o n o f alumina and o f t h e heated m i x t u r e KA1C0,(OH)2 pseudo-boehmite ( f i g . 5,a,g).

For K-contents l e s s

t h a n 8 w t . % (fig.5,b,c),

t h e r e i s a l s o an increase o f t h e r e l a t i v e i n t e n s i t i e s i n the r e g i o n o f the (220) l i n e o f alumina, b u t i t i s l e s s important. T h k may be r e l a t e d

t o the s m a l l e r

content i n K, and perhaps a l s o t o d i f f e r e n c e s i n t h e l o c a l i z a t i o n o f K i n t h e alumina. There was indeed no c r y s t a l l i n e K-carbonate detected under 8 w t . % K. I t seems t h a t t h e r e may be a r e a c t i o n between alumina and t h e by-products of t h e

decomposition of KA1C03(0H)2 o r o f the o t h e r carbonates detected i n d r i e d B - 2 5 - K . 0

This r e a c t i o n gives r i s e t o an amorphous phase, p l u s the d i n g of t h e p r o p o r t i o n s o f a v a i l a b l e K

l i n e a t 2.8 A, depen-

and A ? . Heating t h e samples a t 700°C

39 shows t h a t t h e r e i s no more c r y s t a l l i n e compound o t h e r t h a n m o d i f i e d alumina. Above t h i s temperature W102

0

2

I

appears.

I

22

I

I

42

I

1

62

1

I

82

28

+

F i g . 5 X-Ray d i f f r a c t o g r a m n s o f samples c a l c i n e d 2 h a t 400°C under d r y a i r : ( a ) s t a r t i n g a l u m i n a ; ( b ) A-1.4-K ; ( c ) B-4-K ; (d) B-8-K ; (e) B-20-K ; ( f ) KA1C03(0H)2 ; ( 9 ) KA1C03(0H)2 + pseudo-boehmite.

S.T.E.M.

a n a l y s i s o f samples c a l c i n e d a t 450°C shows an a l m o s t homogeneous

l o c a t i o n o f potassium. A c i d i t y o f K-impregnated alumina The n e u t r a l i z a t f o n o f t h e a c i d i t y o f t h e y c alumina by K2C03 has been f o l l o w e d by T.P.D.N.

In T.P.D.N.

and I . R .

spectroscopy o f t h e OH and o f adsorbed p y r i d i n e .

i t must be emphasized t h a t t h e a c t i v a t i o n temperature was o n l y

400°C, which i s r e a l i s t i c f o r c a t a l y t i c uses b u t i s t o o low t o cause t h e t o t a l decomposition o f carbonates i n h i g h K-content samples. A t i n c r e a s i n g K-content a double e f f e c t appears i n t h e T.P.D.N.

-

curve :

t h e s t r e n g t h o f t h e more a c i d i c s i t e s d i m i n i s h e s a b r u p t l y f r o m p u r e

,alumina t o B-8-K : t h e f i n a l d e s o r p t i o n t e m p e r a t u r e decreases f r o m 460°C down t o 160°C ( f i g . 6 )

-

t h e t o t a l q u a n t i t y o f desorbed NH3 decreases r a p i d l y b u t c o n t i n u o u s l y

(fig. 6).

L 100

200

300

400

T ("C)

F i g . 6. Temperature programmed d e s o r p t i o n o f NH3 under h e l i u m and t o t a l amount o f desorbed NH3 : ( a ) s t a r t i n g alumina ; ( b ) 6-1-K ; ( c ) B-2-K ; ( d ) B-3-K ; ( e l B-4-K ; ( f ) 6-8-K.

The I . R .

spectrum o f fcA1203 a f t e r t r e a t m e n t under vacuum (PJU-~

Torr) at

480 "C shows f i v e bands a t 3790, 3770, 3730, 3680 and 3590 cm-' i n dgreernent w i t h (13). The sum o f t h e OH bands i n t e n s i t i e s decreases s t r o n g l y up t o 2 wt,% K ( f i g . 71, w h i l e t h e 3790 cm-' band disappears. The presence o f potassium seems 1 t o s h i f t t h e bands ( e x c e p t t h e one a t 3680 cmtowards low f r e q u e n c i e s : A S o f about 10 t o 20 cm-l. Between 2 and 6 wt,% K t h e s m a l l decrease i n OH d e n s i t y o c c u r s s i m u l t a n e o u s l y w i t h a decrease of t h e (3770 - A h cm-l and 3680 cm- 1 bands

.At

6 wt.% K t h e o n l y w e l l d e f i n e d band i s a t (3730 - 4 s ) cm- 1 which i s

i n good agreement w i t h t h e r e s u l t s o f ( 4 ) . T h i s band s t i l l decreases a t h i g h e r K content. P y r i d i n e a d s o r p t i o n was c a r r i e d o u t on t h e r m a l l y a c t i v e d samples by i n j e c t i o n Of

Small q u a n t i t i e s (50pmoles/g,AP~

0.05 t o r r ) a t room temperature. The

spectrum o f adsorbed p y r i d i n e was o b t a i n e d by s u b s t r a c t i n g t h e c o n t r i b u t i o n o f t h e gaseous phase. The f i r s t i n j e c t i o n evidences t h e Lewis s i t e s (band a t 1605 cm- 1 1. A t i n c r e a s i n g p y r i d i n e p r e s s u r e appears t h e band o f H-bonded p y r i d i n e a t

1590 cm- 1

.

41

0

3

6

9

wt. % K

F i g . 7. OH d e n s i t y a t i n c r e a s i n g K-content f o r samples evacuated 3 h a t 450 " C . The simultaneous o b s e r v a t i o n i n t h e OH-region shows t h a t t h e bands a t 3790 1 1 and 3770 cm- a r e t h e f i r s t t o disappear when p y r i d i n e p r e s s u r e i n c r e a s e s . 1 A t s t i l l h i g h e r p r e s s u r e t h e band a t 3730 cm-' s h i f t s t o 3720 cm- and t h e n 1 d i s a p p e a r s w h i l e t h e bands a t 3680 cm-l and 3590 cm- remain q u i t e u n a f f e c t e d . cm-

As t h e K-content i n c r e a s e s up t o 3 w t % K, t h e number o f Lewis a c i d i c s i t e s 1 (band a t 1605 cm- ) decreases s t r o n g l y . The q u a n t i t y o f H-bonded p y r i d i n e decreases more s l o w l y t o be n e a r l y n u l l a t 10 W t % K.

DISCUSSION Pot as s i um 1o c a t ion When 'bc alumina i s impregnated w i t h K2C03 and t h e n d r i e d t h e r e i s no l o n g e r K2C03 n o r KHC03 d e t e c t e d by X.R.D.

except f o r v e r y h i g h K-content samples. The

main c r y s t a l l i n e potassium compound d e t e c t e d i s KA1C,03(OH)2 whose f o r m a t i o n i s r e l a t e d t o t h e presence o f KHC03. We m i g h t e x p l a i n t h e f o r m a t i o n o f t h e hydrogenocarbonate e i t h e r f r o m t h e exchange o f a p r o t o n o f t h e alumina w i t h a potassium c a t i o n o f K2C03 o r e i t h e r f r o m t h e d i s s o l u t i o n o f atmospheric C02 i n t h e b a s i c s o l u t i o n occluded i n t h e pore volume (K2C03 + H20 + C02+

2 KHC03).

Though t h e f i r s t process m i g h t be predominant f o r samples d r i e d d u r i n g 24 h, t h e second one i s p r o b a b l y r e s p o n s i b l e o f t h e s t r o n g enhancement o f t h e T.P.D.C peak r e l a t e d t o KA1C03(OH)2 i n a sample d r i e d d u r i n g 2 months. However t h e major p a r t o f potassium i n t h e low K-content samples m i g h t belong t o p o o r l y o r non c r y s t a l l i n e compounds. I n X.R.D.

t h e enhancement o f t h e

(220) l i n e o f alumina on heated samples seemed t o be i n agreement w i t h t h e l o c a t i o n o f potassium on t e t r a h e d r a l s i t e s a t t h e s u r f a c e o f alumina. However t h e study o f t h e decomposition p r o d u c t s o f KA1C03(0H)2 i n m i x t u r e w i t h alumina

42

shows an i m p o r t a n t s i g n a l due t o an amorphous phase m a i n l y i n t h e range 4 A 2

. For

-

h i g h K-content samples t h e c o n t r i b u t i o n o f t h e s e decomposition p r o d u c t s

i s t h e main probable e x p l a n a t i o n t o t h e enhancement o f t h e (220) compared t o t h e o t h e r l i n e s . F o r low K-content samples t h e f i r s t e x p l a n a t i o n i s l i k e l y b u t t h e e f f e c t i s much s m a l l e r t h a n t h e e f f e c t r e l a t e d t o t h e amorphous KA1C03(0HJ2 deconposition products. Acidity C a l c i n e d ',fc

alumina p r e s e n t s a s t r o n g a c i d i t y as r e v e a l e d by t h e f i n a l

d e s o r p t i o n temperature o f NH3 o f 460 "C.

The Lewis a c i d i t y i s r e s p o n s i b l e f o r

t h e main p y r i d i n e a d s o r p t i o n . Nevertheless t h e r e e x i s t s H-bonded s p e c i e s o f F y r i d i n e weakly adsorbed on alumina : t h e s e species can be e l i m i n a t e d by e v a c u a t i n g t h e sample. N e u t r a l i z a t i o n o f alumina by potassium carbonate a t low K-content causes t h e sumultaneous removal o f t h e most a c i d i c s i t e s (T.P.D.N.) o f t h e Lewis s i t e s 1 ( I R o f p y r i d i n e ) and o f t h e OH band a t 3790 cmIt i s obvious t h a t t h e f i r s t

.

potassium adducts n e u t r a l i z e t h e most a c i d i c s i t e s which a r e o f Lewis t y p e . 1 Nevertheless i t a l s o remove t h e OH v i b r a t i n g a t 3790 cm- as does p y r i d i n e when i t i s adsorbed on alumina. These OH has been s a i d t o be b a s i c and t o be t h e

p r e c u r s o r s o f t h e f o r m a t i o n o f o( p y r i d o n e upon p y r i d i n e a d s o r p t i o n on

9

and

6

alumina ( 1 4 ) . As t h e c a r b o n y l band o f q p y r i d o n e has n o t been d e t e c t e d and as t h i s OH band i s removed by two b a s i c compounds such as potassium carbonate and p y r i d i n e i t seems t o have a r a t h e r a c i d i c c h a r a c t e r . According t o t h e o r d e r o f removal o f t h e o t h e r h y d r o x y l bands a t i n c r e a s i n g K-content we can propose t h e f o l l o w i n g a c i d i t y s c a l e : 3790

>

3770

-u

3680

Y

3590

5

3730 cm-

1

When c o n s i d e r i n g p y r i d i n e as another b a s i c probe we f i n d another s c a l e : 3790 5

3770

>

3730

> 3680

3590 cm-l

T h i s l e a d s t o t h e c o n c l u s i o n t h a t t h e a c i d i t y s c a l e o f t h e h y d r o x y l i s dependent upon t h e n a t u r e of t h e probe

m o l e c u l e and t h e method o f p r o b i n g : K2C03 i s

impregnated on f u l l y h y d r o x y l a t e d alumina whereas p y r i d i n e i s adsorbed on a c t i v a t e d sample. T h i s may a1so)explain t h e d i s c r e p a n c y w i t h t h e a c i d i t y s c a l e proposed i n ( 1 3 ) f r o m a c r y s t a l l o g r a p h i c model of s u r f a c e s i t e s .

CONCLUSION The i n t e r a c t i o n between K2C03 and [cAl 0 l e a d s t o t h e f o r m a t i o n of 2 3 KA1C03(OH)2 f o r K-content h i g h e r t h a n 4 w t %.Nevertheless no s p e c i f i c e f f e c t o v e r

43 t h e n e u t r a l i z a t i o n o f t h e a c i d i t y o f alumina has been observed. T h i s may be r e l a t e d t o t h e decomposition o f t h i s phase d u r i n g t h e c a l c i n a t i o n s t e p p r e c e d i n g t h e a c i d i t y measurement. ACKNOWLEDGEMENTS We a r e i n d e b t e d t o f o r T.G.,

L. BARRE, B.

REBOURS f o r X.R.D.

measurements, M.C.

POUZET

S. DEBOUDAUD f o r T.P.D.N.

REFERENCES 1

W.D.

2

P.O. Scokart, A. Amin, C. Defosse and P.G. Rouxhet, J. Phys. Chem., 85 (1981)

Mross, C a t a l . Rev. - S c i . Eng., 25 (19831, 591.

3

W.H.J.

4

B.W.

1406.

5

S t o r k and G.T.

P o t t , J. Phys. Chem.

Krupay and Y . Amenomiya, J . Catal.,

78 (1974)

2496.

67 (19811, 362.

M. Kantschewa, E.V. Albano, G. E r t l and H. Knoezinger, App. C a t a l . 8 (19831, 71.

6

X. Montagne, IFP, Report 31 796, (1983).

7

J.P. Franck, E . Freund and E . Quemere, J.C.S.

3

N.P. Tomilov, A.S.

9

A.S. Berger, N.P. (19711, 42.

Berger and A . I .

-

Chem. Comm. 10, (19841, 629.

Boikova, Russ. J. I n o r g . Chem., 14, (19691,

352.

i(JI . W .

Groote, U.S.

T o n i l o v and I . A .

Vorsina, Russ. J. I n o r g . Chem.,

Pat. 2 783 124, ( 1 9 5 7 ) .

11 D. Papee, R. T e r t i a n and R. B i a i s , B u l l . SOC. Chim., 12 G. Busca and V. L o r e n z e l l i , M a t e r i a l s Chem.,

(19581, 1301.

7, (19821, 89.

13 H. Knoezinger and P. Ratnasamy, C a t a l . Rev. - S c i . Eng.,

17, (19781, 31.

14 C . M o r t e r r a , A. C h i o r i n o , G. G h i o t t i and E . Garrone, J.C.S. ( 1 9 7 9 ) , 271.

16,

Faraday I, 75,


45

B. Imelik et at. (Editom), Cntniysia by Acid8 and B w s GI 1986 Elsevier Science Publishers B.V.. Amsterdam -Printed in The Netherlands

ACIDIC REACTIONS ON S O M E TRANSITION M E T A L OXIDE S Y S T E M S B. GRZYBOWSKA-SWIERKOSZ

Institute of Catalysis and Surface Chemistry. Polish Academy of Sciences, 30-239 Krak6w (Poland)

ABSTRACT

La decomposition du propanol-2 et le craquage du cum6ne ont 6td dtudigs sur quelques oxydes mixtes, catalyseur d'oxydation m6nag6er presentant differents modes d'organisation des oxydes composants. Les molybdates bismuth-fer et cobat-tellure ont fourni des exemples de systemes monophasiques, V,Oi~-Ti02 et Sn02-Sb 0 des exemples de systemes multiphasiques. On ddmontre que les r&%!ions acides peuvent Stre utilis6es pour caracteriser le mode de dispersion des oxydes dans les systemes multiphasiques. La correlation entre l'acidite des systemes etudi6s et leur selectivite dans les rdactions d'oxydation menagde est discutee. Decomposition of isopropanol and cumene cracking have been studied on mixed transition oxide systems, catalysts for selective oxidation, of different mode of arrangement of the two oxide components. Bismuth-iron and cobalt-tellurium molybdates w e r e taken as examples of monophasic systems, V2O5 'Ti02 and SnO2 Sb04 as examples of multiphasic ones. T h e change of acidIc properties with the different mode of dispersion of the two oxides 'has been shown In the two latter cases, and the possibility of applying the acidic reactions to characterization of this parameter is discussed. Correlatton between acidic properties, measured by rate of these reactions, and selectivity to partial oxidation products a r e considered.

-

-

INTRODUCTION

Transition metal oxide systems exhibit also acid-basic

- catalysts

for oxidation processes

-

properties, being capable of sorption of acids

and/or b a s e s , as weU as of catalysing some acidic reactions s u c h a s dehydration of alcohols. isomerization and cracking of hydrocarbons b

(I). Higkvalent, not fully coordinated metal ions or

anionic vacancies

have been proposed as acidic centres in this case and basic character of oxide ions, 02- has been claimed to account for basicity of these

s y s t e m s . T h e presence of Brbnsted centres is also possible. Several works, in particular on mixed oxide catalysts for selective oxidation of hydrocarbons, have been concerned with searching correlations between the acidc-basic

properties of these systems and

activily/setectivity in oxidation reactions (2-6).

46

Analysis of oxidation reactions h a s indeed suggested that some of elemenhry steps in these processes. s u c h as for instance sorption and activation of a hydrocarbon molecule, can be considered as an acidbasic reaction ( 6 ) , thus implying a direct correlation between activity and acidity for the reactions in which this s t e p is rate determining.

Addo-basic properties can be also invohred In regulating energetics of the sorption-desorption s t e p s of both substrates (hydrocarbons) and products (aldehydes, acids) of these reactions, if w e consider the above compounds in terms of their electron donor or accelptor

propedes. In this approach acid-basic

centres have been identirmd

wIth the centres on which selective oxidation takes place. Another possibility i s the participation of acidic centres In some side reactions of hydrocarbons, analogous to those observed on typical acidic catalysts, which invoke formation of carbocations and Lad to undesirabk for selective oxidation destruction processes. In spite of numerous works no general correlation between acid-basic

properties

and performance in oxidation reaction h a s been so f a r formulated. The

reasons of this failure m a y come from: (a) lack of an appropriate method to determine acidity and basicity of oxidation catalysts in the conditions close to those of oxidation reactions: low specific surface 2

area of these systems (0.5-5 m 1% in most cases) makes difflcult classical or s p e c t ral sorption measurements, hlgh reaction temperature 623 K) may change the state of easily reducible oxide systems ( with respect to that at which the acidity measurements are performed,

>

( b ) the presence of reactive adsorbed oxygen which can react with acid or base probe molecules obscuring the real sorption processes, (c) participation of other but acldo-basic reaction in the rate determining step of the oxidation. Besides. in most of the cases in which acid-basic

properties of mixed transition oxide systems are studied,

little, o r no attention i s being paid to the mode of mutual arrangement of the two oxide components and their morphology. the discussion being limited to the effect of particular Ions (added to an oxide very often in an ill-defined way) on acid-basicity, In the present work acidic properties, measured by the rate of isopropanol dehydration and cumene cracking, have been determined for

several mixed oxide systems active in selective oxidatlon of olefhs and alkyhrornatics, which provide examples of different modes of dispersion of the component oxides. They include: (a) monophasic, definite compounds In the Bi-Mo-Fe-0 systems, V205

and Co-Mc-Te-0

- Ti2 and Sn02 - Sb20q of

systems, (b) multiphasic

different ratio of the two

47

oxide components and thus of different fashion of the oxide phase interact-ions.

T h e behaviour in the acidity test reactions is compared

with the selectivity in some oxidation reactions determined previously. EXPERIMENTAL Samples T h e compounds of the Bi-Mo-FcO s y s t e m comprised d -bismuth molybdate. B12 a (Moo4)

and two bismuth-iron molybdates derived

from its structure: Bi2Bi (Mo2,3Fel/304)

(Bi2M02FeO12) and

Bi2Fe (Mo2,3Fel1304) 3. Their preparation, characterization and catalytic properties in propene oxidation were described in (7,8 ) . Characteristics and preparation method of cobalt molybdate and cobalt

VI

N

and C O e Moo6 can be found in

telluromolybdates. Co4Te Mo3OI6 ( 9 , 10). T h e samples in the V205

- Ti02 system comprised the

preparations obtained by impregnation, (I) of anatase, (AN) and mtile, (R?f 2 modifications of titania of low specific surface area (10 m /g) with different amounts of precursor of vanadla phase followed by calcination

at 773 K. Their physicochemical and catalytic properties in o-xylene oxidation were reported in (11. 12). The samples of V205

- Ti02

(AN)

oE a monolayer type obtained by chemical grafting method (13) and

a solid solution of V4'

ions in RT were a l s o included in the studies.

T h e samples of SMb-0 system of different S b content were those prepared and thoroughly characterized by Figueras, Portefaix, V o l t a and others (14-18)

in the Institute of Catalysis in Villeurbanne.

Measurements of acidic reactions Dehydration of isopropanol at 473 K and cumene cracking at 6 2 3 K were applied as test reactions for the presence of weak and strong acidic centres respectively. T h e isopropanol dehydrogenation product. acetone was also determined. Only trace amounts of di-isopropyl ether were observed. In the case of cumene decomposition. the dehydrogenation product,& -methylstyrene w a s formed in small quantities on the samples active in the isopropanol-acetone conversion. The reactions were studied with the pulse method using 0.5 pl

-

pulses of the reactants on the samples (0.1-0.5 in a stream of dried helium ( F R 30 ml min-').

g of the catalysts T h e data presented

furiher in the text pertaln to the first pulse introduced after pretreatment

of the samples in a stream of d r at 7 7 3 K. followed by a stream of helium at the reaction temperature. T h e activity V S .

number of pulses

dependence varied with the type of samples: practically no change of

TABLE 1

b P

03

Activity in acidic reactions on compounds In BI-Mo-Fe-0

Isopropanol decmpn. Preparatlon

-H2 0 7 10 mole C3H6

and C-Mo-Te-0

Cumene cracking

systems Selectivity In oxidation of propene

(%I.

-H2 8 ioclmole C ~ H ~ O 10 mole C3H6

Acr

c02

2 m a

2 m s

2 m e

1.4

1.9

0.01

88

12

Bi3M02FeO12

3.3

2.1

0.9

83

15

Bi2M 02Fe2012

7.3

2.0

2.5

70

28

11.5

0.1

0.1

5

90

Co4TeMo 0

0.1

5.3

tr

77

16

Cdl’eMo06

0.6

1.0

tr

88

10

CoMo04

3 16

49

activity with the number of pulses w a s observed for the V205

- T102

preparations, w h e r e a s activity d e c r e a s e d markedly in the case of Sn02S b 0 system. and slightly In the case of the other samples. It h a s 2 4 been checked that pre-adsorption of pyridine, introduced in the form of 1 pl pulses before the pulses of isopropanol or cumene. s u p r e s s e d almost completely dehydration of the alcohol and cumene cracking, indicating that the acidic

we

c e n t r e s are i n v o k e d In t h e s e two

reactions. T h e yield of the isopropanol dehydrogenation product, acetone. did not change after the pyridine s o r p t i o n

R E S U L T S AND DISCUSSION T a b l e 1 summarizes the data obtained for monophasic systems. In the case of bismuth-molybdate samples, the incorporation of iron with the formation of mixed bismuth-iron molybdates i n c r e a s e s the activity in the both acidic type reactions 1.e. dehydration of isopropanol and cumene cracking, the dehydrogenation activity remaining practically unchanged. S i n c e F e 2 0 3 is only slightly active in the both acidic reactions, the i n c r e a s e in activity could be ascribed to the change in structure of bismutkiron rnolybdates as compared to pure Biz (Moo4) 3 , rather than to intrInsIc properties of the Fe ions. Indeed, the p r e s e n c e of d e s c r e t e Ri:o04 tetraedra w a s suggested for the Bi3FeMo2012

compound, which

r e p l a c e s the complex s y s t e m of octa- a n d tetracoordtned molybdenum in pure

& -bismuth molybdate. T h e ir s p e c t r a

of the mixed molybdates

indicate moreover the a b s e n c e of terminal Mo-0 bonds ( 7 ) : molybdenum atoms exposed therefore o n the s u r f a c e could give rise to new acidic centres. \\'ith the i n c r e a s e in the rate of the acidic reactions the selectivity to the partial oxidation product, acrolein, in the propene oxidation d e c r e a s e s , with the simultaneous i n c r e a s e in the selectivity to COz. S i n c e the rate determining s t e p in selective oxidation of propene on bismuth morybdates i s the activation of the hydrocarbon molecule and the oxygen incorporation h a s the same rate on the three compounds ( B ) , the increase in the selectivity to C 0 2 indicates that the increased acidity leads to a different route of the propene activation which conducts the formation of the degradation products. In the case of the Co-hIo-Te

system, the tellurornolybdates show

markedly lower r a t e s of the isopropanol dehydration and higher rates of

i t s dehydrozenation as compared with pure cobalt molybdate, and practically no activity in the cumene cracking. T h e lower activity in the acidic type reactions i s again accompanied b y the Increase in selectivity to partial oxidation product of propene oxidation. LOW activity

TABLE 2

01

Activity of V 0

2 5

- Ti02

0

catalysts in decomposition o f isopropanol and cumene cracking

Isopropanol decomposition

Cumene cracking

Preparation 8 10 mole 2 m s

mL's

20% 0.4%

- m o 2 (AN) v205 - Trio2

v205

(AN)

10 0.01

Selectivity to C 8 In o-xylene oxidationX (ref. )

%

5.0

0.5

1.4

69(11)

5.3

0.002

-

80

4.9

0.006

-

74

2.5

9.6

4.7

48

(13)

manolayer, grafting 1.2% V205

- T102 (AN)

~

~

0.03

(11)

impregnation 20%

v205

V 0

f+

2 5'

-

- <no2 (RT)

20

0.07

3.9

plates

4-

f -

100 001

x maximal selectivity to partial oxidatlon at converdon

100%

(11)

64

(22)

51 in selective oxidation of stoichiometrk cobalt molybdate h a s been ascribed to the formation of carbonaceous deposit owing to the strong sorption of propene: no s u c h deposit was observed on telluromolybdates (10). T h e results of the isopropanol dehydration illustrate then in this

c a s e the correlation between the acidity of the system and the strength of sorption of the hydrocarbon species. High activities in the isopropanol

dehydrogenation could be at the s a m e time correlated with the high activity in selective oxidation. as the latter reaction requires the presence of dehydrogenating centres for abstraction of hydrogen from the olefin molecule. N o detailed discussion about the modification of acidic centres on introduction of tellurium can be proposed in this case

as the structure of cobalt tellurium molybdates h a s not been resolved so far. Table 2 presents the results obtalned for the V205

- T i 0 2 system,

the samples differing by the mode of arrangement (dispersion) of the two component oxides and by the modification of Ti02. Since titanla is practically inactive in the studied reactions under the adopted conditions

(ll), the data reflect the changes in acidic properties of vanadia phase in contact with Ti02. As seen, the mechanism of isopropanol decornposition i s markedly changed when vanadia i s deposited on AN modification of T102: the dehydration, which i s the main reaction on pure V205 i s

suppressed, whereas dehydrogenation to acetone increases. This effect i s particularly distinct at low vanadia content Fig, 1 illustrates the

changes in amounts of propene and acetpne with vanadia concentration for samples prepared by grafting and impregnation techniques. Practically no dehydration product i s found at low vanadia content up to about 1 monolayer. When this monolayer coverage i s exceeded and the phase of

V205 appears. the rapid increase in the propene yield is observed. T h e amount of acetone increases with the vanadia content up to a monolayer coverage and then slightly decreases. Similar effect i s observed for the samples prepared b y the impregnation techrdque, though in this case the maximal yield of acetone i s observed at higher amounts of V205. T h e acetone formation and suppression of dehydration can be then proposed as an indicator of the formation of a monolayer dispersion of vanadia on

AN-Ti02. Such the monolayer catalysts have been shown to exhibit the highest selectivity and activity in o-xylene oxidation to phthalic anhydride ( 1 3 ) . The decomposition of isopropanol can be then applied in this

system as a method of characterizing the phase composition and the mode of dispersion of active phase on titanfa: it provides a quick test for evaluation of catalysts, and for checking the adequacy of a preparation

52

mono la 25

240

0,

z

SZ

-

P-

20

-30

0

Q 0;

& C 0

Q, d

E"

d

C

-

15

c)

0

10

Q,

-20

g L

a

- 10

L

5

1

2

5

10 O/O

20 mote V20,

Fig. 1 Activity in isopropanol decomposition of V 0

-

Ti0 catalysts. Triangles: propene, circles: acetone, 0, A 2- 'graftin2 technique, .,A impregnation technique.

-

method In producing the optimal catalyst. T h e structure of this monolayer on the low specific surface a r e a Ti02 is not known so far. T h e ammeliorating effect of AN on vanadia properties h a s been interpreted by p r e f e r e n q l exposition of 001 plane of V205 on the corresponding planes of Ti02-AN. with exposition of vanadyl bonds (20). In this approach the monolayer of vanadia could be then envisaged as a bidimensional 001 plane. T o check the effect of exposition of different planes in V205 on its acidity, the decomposition of isopropanoI was studied for vanadia samples of different morphology (21). T h e last rows in Table 2 give some data of this study. As seen, the samples of the lower morphological factor f corresponding to the higher extent of the participation of 001 plane favour dehydrogenation of isopropanol, still the changes in the dehydration/ dehydrogenation ratio with the changes in the value of morphological factor a r e small (w2x) when compared with the dramatic change of this ratio when a monolayer structure of vanadia is formed. Thus, though the effect of morphology of the oxide on i ts acidic properties is distlnct, the results obtained indicate that the monolayer of vanadia phase on anatase h a s the different structure than the 0 0 1 plane of V205. Vanadla deposited

53 on rutite preserves the properties of the bulk V205.

dehydration being

the main reaction path In the isopropanol decomposition. T h e acidity-selectivity correlations in the case of V205

- Ti02 s y s t e m

resemble the pattern observed in the BI-Mo-Fe and Co-Mo-Te compounds, the catalysts of the higher rates of the both acidic reactions being less selective in partial oxidation of propene and o-xylene. T h e activity in the acidic reactions on the samples of Sn02

- Sb204

system of different composition and different calcination temperatures i s reported in Table 3 and Fig. 2.

/

/

5

40

10 20

60

80

100

Sb

"'Sb+Sn

Fig. 2 Activity in isopropanol dehydration (circles) and cumene cracking (triangles) for SnO2 Sb2O4 catalysts.O,A -samples calcined at 773 K samples calcined at 1023 K. I region of soUd solution Sb/SnO2, I1 biphasic region: solid solution + S b204

-

-

-

In the case of the isopropanol dehydration the activity increases considerably with the S b content beginning from the concentration a t which the phase of SbZ04 appears (18). Higher activities of the samples calcined a t 1023 K could be ascribed to the enrichment of the surface with S b observed previously (14). The high activity in this reaction appears then to be a n indicator of the presence of antimony oxide. A different dependence of the acidity on the sample composition i s observed

TABLE 3 Activity in acld-base reactions on Sn02 Isopropanol decrnpn. Calcination

%Sb

temp.

7 1 0 mole C H

(K1

773

3 g

Sb204

Selectlvlty ln oxidation

Cumene cracking

(%) 2

7 1 0 m o l e C3H60

8 1 0 mole C3H6

propene ( 4'

o-oxylene ( 1 9 )

2 m s

rns

m

s

Acr

0

0.28

1.3

0.14

37

1.5

0.66

1.1

0.08

35

5

0

86

5.0

0.45

1.0

0.88

40

27

11

65

2

2

c8

PA

C02

10.0

0.42

0.50

0.96

50

59

30

38

20.0

0.27

0.17

0.76

53

57

13

45

70.0

2.35

1.22

0.20

90

80

0

20

2.16

0.06

42

71

0

19

2

0

90

60

12

0

80

27

0

73

100

1023

-H

-Fl 0 2

-

u1 A

10.9

1.5

1.13

5

0.51

1.87

10

2.0 2.0

20

1.04

0.44

0.4

84

40

1.54

0.36

0.9

€30

70

4.20

0.37

1.0

92

55

in the case of the cumene cracking, the increase of the rate of this reaction being observed at the Sb concentration region in which the solid solution of antimony ions in Sn02 i s present. The acidic reactions can b e then also in Ws case applied to characterization of the phase composition on the surface of the mixed oxide system. No simple correlation is found, however, in the case of Sn02

- Sb204 s y s t e m between the

activity in the acidic reactions and selectivity to partial oxidation products. T h i s may be due to different types of acidic centres present in different composition regions as shown by norbparallel changes of cracking and dehydration activities with the change in S b content, which may influence various steps of the oxidation reactions in a different way.

ACKNOWLEDGEMENTS The technical assistance of Mrs. I. G r e s s e l in performing the measurements of the both reactions under study is gratefully acknowledged. T h e author is also grateful to Dr. R. Kozlowski for the samples of V205

-

TI02 prepared by the grafting technique, and to Dr. J. Sloczyriski for the samples of cobalt telluromolybdates. REFERENCES

1

D. Barthomeuf and F. Figueras, Chemical and Physical Aspects of

Catalytic Oxidation, ed. by J.L. Portefaix and F. Figueras, Editions du CNRS, 1980, pp. 241-269. 2 M. Ai and S. Suzuki J.CetaL, 30 (1973) 3 6 2 3 7 1 . M. A1 ibid. 40 (1975) 31-26. 49 (1977) 305-312. 49 (1977) 313319. 54 (1978) 4 2 6 4 3 5 , 60 (1979) 306-315. J.-E. Germain, Intra Science Chem.Rep., 6 (1973) 101-112. J.&. Germain in (1) pp. 207-237. P. Forzatl, F. Trifiro and P.L. Villa, J.CataL, 52 (1978) 389-396. J. Haber, Proc. 8th Intern. Congress on Catalysis, Berlin 1984. Verlag Chemie, 1984 pp. I 85-112. B. Grzybowska, E. Payen, L. Gengembre and J.-P. Bonnelle, BulL Acad.Polon.Sci. ser. sci.chim., in print. 8 K. Bdckman and B. Grzybowska, React.Kin.CataLLett., in 9 J. StoczytiskI and B. Sliwa, Z.anorg.allg.Chemie, 438 (1978j%k5-304. 1 0 J. F o e , B. Grzybowska and J. Sloczytiski, BuILAcadSolonSci., ser. sci. chirn.. 24 (1976) 975-980. 11 M. Gqsior, 1. Gpsior and B. Grzybowska, Applied CataL, 10 (1984) 87-100. 1 2 M. Rusiecka, B. Gnybowska and M. Gqsior ibid. 10 (1984) 101-110. 13 G.C. Bond and K. BrUckman, Farday Discuss., 72 (1981) 235-248. 14 Y. Boudeville, F. Figueras, M. Forissier, J.-L. Portefaix and J.C. Vedrine, J. Cat& 5 8 (1979) 52. 1 5 J.-M. Herrmann, J.-L. Portefaix, M. Forissier, F. Figueras, and P. Pichat J.C.S. Faraday I (1979) 1346. 16 J.4.Volts, G. Coudurier, I. Mutin and J.C. Vedrine, J.Chem.Soc.Chem. Comm. 1982 p. 1044-1045. 1 7 J.-C. Volt+ B. Benaichouba, I. Mutin and J.C. Vedrine, Applied.CataL 8 (1983) 215-237.

56

18 19 20

21 22

J . Z . Volta, P. B u s s i e r e , G. Coudurier, J.-M. Herrmann and J.C. Vedrine. IX Iberoamerican Symposium o n Catalysis, 1981 F. F i g u e r a s , M. Gqsior, B. G r z y b o w s k a and J.-L. Portefaix, React. KhCataLLett. 20 (1982) 367-371. A. Vejux and P. Courtine, J. S o l i d State Chem., 23 (1978) 93-101. B. G m y b o w s k a srd M. G q s i o r submitted to R e a c t K i n . and CataL Lett. M. Gqsior and T. Machej, J.CataL 83 (1983) 472-475.

57

B. Imelik et al. (Editors), Cataiysis b y Acids and Bases 0

1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

MODIFICATION OF THE ACIDITY AND BASICITY OF THE SURFACE OF OXIDE CATALYSTS STAHISLAW MALINOWSKI Chemistry Dept. T e c h n i c a l U n i v e r s i t y ( P o l i t e c h n i ka)

, 00-662

Warsaw (Poland)

ABSTRACT The i n c r e a s e i n t h e a c i d i t y o f o x i d e s u r f a c e i s achieved by t h e d e p o s i t i o n o f atoms o r groups o f e l e c t r o n - a c c e p t o r c h a r a c t e r . The i n c r e a s e i n b a s i c i t y r e s u l t s f r o m t h e d e p o s i t i o n o f donor elements. The d e p o s i t i o n of H3P04, PCl3,. PCl5, P2O5 b r i n g s a b o u t a n i n c r e a s e i n t h e a c i d i t y . The d e p o s i t i o n o f a l k a l i ions increased the b a s i c i t y , w h i l e the evaporation o f a l k a l i metals r e s u l t s i n super b a s i c s u r f a c e f o r m a t i o n . The d e p o s i t i o n o f MgTet, Znmet r e s u l t s i n surfaces c o n t a i n i n g a l a r g e number o f one e l e c t r o n donor cen r e s . RESUME L ' a c c r o i s s e m e n t de 1 ' a c i d i t @ s u p e r f i c i e l l e des oxydes e s t obtenu p a r depBt d'atomes ou de groupes d'atomes de c a r a c t e r e a c c e p t e u r d ' 6 l e c t r o n s . L ' a c c r o i s sement de l a b a s i c i t 6 p r o v i e n t du depdt d ' e l e m e n t s donneurs. Le d @ p d t de H3 PO4, PC13, PC15, P2O5 augmente a u s s i l ' a c i d i t b . Le d e p d t d ' i o n s a l c a l i n s augmente l a b a s i c i t e , t a n d i s que l ' e v a p o r a t i o n de metaux a l c a l i n s c o n d u i t 1 l a f o r m a t i o n de s i t e s s u p e r f i c i e l s superbasiques. Le d e p d t de Mgmet> Znmet c o n d u i t d des s u r f a c e s c o n t e n a n t un nombre e l e v e de s i t e s donneurs. INTRODUCTION The a c i d i t y and b a s i c i t y as w e l l as t h e a c i d and b a s i c s t r e n g t h of t h e s u r f a c e o f s o l i d o x i d e s i s dependent upon :

1. t h e k i n d o f c a t i o n i n t h e c r y s t a l l a t t i c e o f t h e o x i d e ; 2 . t h e k i n d o f two o r more c a t i o n s i n t h e c r y s t a l l a t t i c e o f t h e o x i d e 3. t h e p r e p a r a t i o n method o f t h e c a t a l y s t ( c o - p r e c i p i t a t i o n , m e l t i n g , c a l c i n a tion) ; 4. t h e means o f a c t i v a t i o n . I n t h i s paper, t h e p r o p e r t i e s r e s u l t i n g f r o m t h e mentioned d a t a w i l l be r e f e r e d t o as " n a t u r a l " . The paper i s concerned w i t h t h e m o d i f i c a t i o n , m a i n l y t h e i n c r e a s e i n t h e o v e r a l l a c i d i t y o r b a s i c i t y as w e l l as t h e a c i d and b a s i c s t r e n g t h , o f t h e c e n t r e s on t h e s u r f a c e . T h i s i s achieved by a r t i f i c i a l l y chang i n g t h e p r o p e r t i e s of t h e s u r f a c e o f n a t u r a l s o l i d o x i d e s . The m o d i f i c a t i o n of t h e a c i d - b a s i c p r o p e r t i e s o f t h e s u r f a c e can be accompl i s h e d by d e p o s i t i n g atoms o r groups o f atoms o f h i g h a c i d i c o r b a s i c c h a r a c t e r on t h e s u r f a c e o f t h e o x i d e s .

58

I n o t h e r words : a. atoms o f s t r o n g a c c e p t o r p r o p e r t i e s . They i n c r e a s e t h e a c c e p t o r a b i l i t y , and i n consequence o f t h a t , t h e a c i d i c s t r e n g t h o f t h e s u r f a c e by p u l l i n g away e l e c t r o n s f r o m t h e a c t i v e c e n t r e s . T h i s i s a c h i e v e d by d e p o s i t i n g on t h e s u r f a c e , such atoms as C1, F o r t h e i r compounds, f o r example CCl4 ; b. atoms o f donor c h a r a c t e r . They i n c r e a s e t h e donor c h a r a c t e r , t h e b a s i c s t r e n g t h o f t h e s u r f a c e b y crowding e l e c t r o n s towards t h e o x i d e i o n s T h i s i s achieved by d e p o s i t i n g m e t a l l i c sodium o r potassium e t c . on t h e o x i de s u r f a c e . T h i s g e n e r a l p r i n c i p l e has s e v e r a l

variations

. As

an example, one can depo-

s i t on s u r f a c e , a group o f atoms o r whole molecules c o n t a i n i n g i n t h e i r s t r u c t u res : a. atoms, w h i c h a r e s t r o n g acceptors, t h e y can be i n t h e f o r m o f p r o t o n i c a c i d s : H3P04, HC1, FS03, FP03H e t c . ,

o r such Lewis a c i d s as f o r example AlC13,

SbF5,

P2O5 o r f i n a l l y b o t h o f them t o g e t h e r . U s u a l l y , c a t a l y s t s p r e p a r e d i n t h i s manner a r e c h a r a c t e r i z e d by t h e i r h i g h a c i d i t y and h i g h a c i d s t r e n g t h . b. an atom o r atoms which a r e s t r o n g donors, f o r example : NaOH, Na-O-C2H5, Na-naphthalene, Na", Mg", Zn". C a t a l y s t s p r e p a r e d i n such a way a r e c h a r a c t e r i z e d b y t h e i r h i g h b a s i c i t y and h i g h b a s i c s t r e n g t h .

I n o r d e r t o o b t a i n e s p e c i a l l y h i g h - a c i d i c o r h i g h - b a s i c systems, o x i d e s w h i c h a r e a l r e a d y c h a r a c t e r i z e d b y t h e i r own h i g h a c i d i t y o r b a s i c i t y a r e used as t h e i n i t i a l s u p p o r t . F o r example, mixed o x i d e s a r e used as t h e s u p p o r t i n o r d e r t o o b t a i n c a t a l y s t s o f v e r y h i g h a c i d i t y . The r a t i o o f m e t a l s i n t h e mixed o x i d e s

i s such as t o produce t h e most a c i d i c s u r f a c e , f b r example A1203-Si02 w i t h a 15-22% A1203 c o n t e n t , o r Ti02-A1203 e t c . S i m i l a r l y , o x i d e s which have a h i g h b a s i c s u r f a c e , f o r example CaO, BaO, MgO a r e used as t h e i n i t i a l s u p p o r t i n order t o obtain catalyst o f very high basicity.

In

a d d i t i o n t o t h i s , t h e m a t t e r must be c o n s i d e r e d f r o m t h e p r a c t i c a l p o i n t

o f view. As an example, t h e c a t a l y t i c system c a n n o t be t o o s e n s i t i v e t o steam, oxygen, C02 e t c .

, it

has t o have a w e l l developed surface. The d e p o s i t i o n o f

AlF3 i s n o t used, because i t has a tendency t o produce l a r g e c r y s t a l l i t e s o f a small surface etc. S k i l l f u l l y a p p l y i n g t h e s e r u l e s , one can o b t a i n c a t a l y s t s o f i n c r e a s e d b a s i c i t y o r a c i d i t y w i t h i n c e r t a i n l i m i t s . The s u r f a c e c e n t r e s o f v e r y h i g h a c i d i c s t r e n g t h a c q u i r e o n e - e l e c t r o n a c c e p t o r p r o p e r t i e s . T h i s phenomenon c o n s t i t u t e s in a

sense t h e upper l i m i t . As an example, t h e s u r f a c e o f aluminum o x i d e , on

w h i c h P205 has been d e p o s i t e d , c o n t a i n s c e n t r e s o f s u p e r a c i d i c p r o p e r t i e s , as w e l l as s t r o n g o n e - e l e c t r o n a c c e p t o r p r o p e r t i e s . By analogy, a System composed o f magnesium o x i d e , on which m e t a l l i c sodium has been d e p o s i t e d , has superb a s i c p r o p e r t i e s , and besides t h a t , c e n t r e s o f o n e - e l e c t r o n donor p r o p e r t i e s .

59

The c a t a l y t i c p r o p e r t i e s are a f f e c t e d by t h i s phenomenon. Reactions are c a t a l y zed by superacidic o r superbasic surfaces,

according t o an i o n i c as w e l l as a

free r a d i c a l mechanism. Superbasic c a t a l y s t s MgO-K, f o r example, a r e very a c t i v e i n the r e a c t i o n o f hydrogenation. They.are a l s o very s e l e c t i v e . The increase o f a c i d i t y and a c i d i c s t r e n g t h o f oxide surfaces The c h l o r i n a t i o n o f aluminum oxide has been f o r a l o n g time, a w e l l known example of i n c r e a s i n g the a c i d i t y o f i t s surface. L i t t l e data, d e a l i n g w i t h the m o d i f i c a t i o n o f t h e s u r f a c e p r o p e r t i e s under d i f f e r e n t c o n d i t i o n s o f c h l o r i n a t i o n i s a v a i l a b l e ( r e f . 1 ) . There i s an increase i n c h l o r i n e content, which i s

-

dependent upon the temperature o f c h l o r i n a t i o n 25

550". The number o f a c i d i c

centres on the surface increases up t o a temperature o f 350" and then drops (table 1). TABLE 1 Alumininm oxide

Temperatures "C

-

c h l o r i n a t e d a t d i f f e r e n t temperatures

C1 mmol/g

20 150 350 550

0.20 0.44 0.56 0.58 __

H, - 3

Acidity 1.01 1.31 1.40 0.91

Basicity

H- 12.9

1.27 1.30 1.21 1.08

__

Bronsted centres are produced as a r e s u l t o f c h l o r i n a t i o n a t lower temperat u r e s w h i l e Lewis centres develop, a t h i g h e r temperatures. The s t r u c t u r e o f new centres produced d u r i n g c h l o r i n a t i o n i s n o t d e f i n i t e l y established. I t i s assumed t h a t the increase i n a c i d i t y takes p l a c e as a r e s u l t o f t h e formation o f A1-C1 u n i t s . The a c t i o n o f gaseous c h l o r i n e on a l u m i n a s i l i c a t e g e l s o f d i f f e r e n t Al2O3/ Si02 r a t i o s i s an example o f the i n f l u e n c e o f a c i d i t y o f the support o f d i f f e r e n t a c i d i t i e s ( r e f . 2 ) . C h l o r i n a t i o n was c a r r i e d o u t a t t h e temperature o f 550" f o r 1 hour. W i t h i n t h e f i r s t minutes, t h e adsorption o f c h l o r i n e i s very e f f i c i e n t , b u t afterwards t h e r a t e o f a d s o r p t i o n drops. Most probably, i n i t i a l l y c h l o r i n e r e a c t s w i t h t h e c o o r d i n a t e l y unsaturated aluminum atoms on t h e surface, and then w i t h l e s s a c i d i c centres o f the surface

-

f o r example by r e p l a c i n g the

OH groups. The a c i d i t y o f the sample being c h l o r i n a t e d i s s i m i l a r t o t h e a c i d i t y of the i n i t i a l sample ( f o r samples c o n t a i n i n g up t o 65% A l ) , and then r a p i d l y increases. The b a s i c i t y a l s o changes i n s i g n i f i c a n t l y f o r samples up t o 75% A l , i n t h e case o f h i g h e r aluminum content i t r a p i d l y decreases. The comparison o f the i n f l u e n c e o f d e p o s i t i o n o f phosphoric a c i d and Lewis

60

acids c o n t a i n i n g phosphorus, on the surface o f A1203 i s an example o f the depos i t i o n o f Bronsted and Lewis acids. The a c i d i t y o f t h e surface increases i n the f o l l o w i n g order : A1203-H3P04

< A1203- PC13 < A1203-PC15 35

are produced as a r e s u l t o f the evaporation o f

m e t a l l i c Na, K, Rb o r Cs on MgO (calcined under oxygen a t 550°C). Bases, as s t r o n g as those are n o t known i n the l i q u i d phase. The very strong bases, which were prepared have been named by us superbases (by analogy t o superacids). The p r o p e r t i e s o f s o l i d superbases s t r o n g l y depend on the temperature i n which the support was calcined before t h e evaporation o f sodium. The highest concentration of b a s i c centres i s achieved by evaporating a l k a l i metal Gn Mg3 which was c a l cined a t 650°C. MgO, which has been c a l c i n e d a t a temperature above 650°, a f t e r the evaporation of m e t a l l i c sodium, gives r i s e t o systems which surfaces contain a small amount of centres o f h i g h basic strength 27< H-
LHSV 4 hr-l, 1 atm. k 2 To1 . Cow.% (0); MeOH s e l .%(a); 2o s t y . se1.X (0)

-

k

-I

-

W

10

0

EB

f

St

= ConsumeYd MeOH

0 900

800

700

600

x 100

s t y . s e l . % = , h - x 100

REACTION TEMPERATURE O K

Effect of Added C02 Addition of COP t o the toluene/methanol feed a t a 2/1 C02/MeOH molar r a t i o , reduced the a c t i v i t y of the CsNaX c a t a l y s t as shown i n Figure 3. T h i s e f f e c t was almost r e v e r s i b l e . Similar r e s u l t s were observed using the CsB-carbon c a t a l y s t . The reduction in a c t i v i t y on addition of COP was accompanied by an increase i n styrene f r a c t i o n i n the alkylated product. These e f f e c t s a r e probably due t o the adsorption of COP on the s o l i d s . IR data reported by Unland ( 1 4 ) show t h a t t h e carbonate band above 1650 cm-' i n samples o f z e o l i t e X exposed t o methanol a t 400"C, decreases i n i n t e n s i t y when the cation s i z e increases from Na t o Cs. Weaker carbonate and formate bands were observed i n

1

o+

5t n 3.

-

E4-

> z

g 3 - o r 0 o w Z w 2 -

-

r0.

3 -I

-

P I -

0

Fig. 3. Effect of C02 on toluene conversion on CsNaX. Conditions: 5 tol/MeOH; LHSV 4 hr-l; 1 atm, 698°K 2 COJMeOH ( 0 ) ;NO C02 (0)

u 0

50

100

150

200

PROCESS TI ME C MI N!

250

71 b o r a t e m o d i f i e d z e o l i t e , BCsNaX, t h a n i n CsNaX.

Apparently both the c a t i o n

t y p e and t h e C02 p a r t i a l p r e s s u r e a f f e c t t h e a c t i v i t y o f t h e c a t a l y s t . Metal Vapors As R e a c t i o n Intermediates, Wood, e t a l . ( 1 9 ) used a Knudsen c e l l mass s p e c t r o m e t e r t o s t u d y t h e gaseous s p e c i e s i n e q u i l i b r i u m w i t h a l k a l i metal s a l t s and t h e i r m i x t u r e s w i t h carbon, a l o n e o r i n t h e presence o f added gases.

The m a j o r vapor species o v e r K2C03(s)

Analogous p r o d u c t s were f o u n d o v e r

a r e K2C03(g), K ( g ) , C02(g) and 0 2 ( g ) . C S ~ C O ~ . I n t h e i r m i x t u r e s w i t h carbon, t h e gaseous m e t a l , CO and CO2 were observed, s u g g e s t i v e o f c a r b o t h e r m i c r e d u c t i o n o f t h e s a l t s .

I n contrast, only

KBr(g) was observed o v e r p u r e K B r o r K B r admixed w i t h carbon.

A d d i t i o n o f COY

C02 and H20 l e d t o a r e d u c t i o n o f t h e K(g) p a r t i a l p r e s s u r e o v e r K2C03-carbon mixtures.

T h i s e f f e c t was t o t a l l y r e v e r s i b l e f o r CO and o n l y p a r t i a l l y f o r

CO2.

Sancier, w o r k i n g w i t h t h e same t y p e o f carbon as Wood e t a l . (ZO), f o u n d by ESR, a l a r g e and i r r e v e r s i b l e i n c r e a s e i n t h e resonance l i n e w i d t h o f carbon mixed w i t h K2C03 at>65O0K.

I t was suggested t h a t t h e i n c r e a s e i n l i n e w i d t h may

be 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 potassium i o n t o z e r o v a l e n t potassium by carbon. The temperatures a t which t h e ESR l i n e - b r o a d e n i n g began and a t which gaseous p o t a s s i u m and cesium a t o m were d e t e c t e d o v e r m i x t u r e s o f carbon and t h e a l k a l i metal carbonates, 600-700°K,

a r e v e r y c l o s e t o t h e o n s e t temperature f o r t h e

s i d e c h a i n a l k y l a t i o n o f a l k y l a r o m a t i c s o v e r KNaX and CsNaX and o v e r CsB-carbon c a t a l y s t s , 600°K.

Presumably, t h e unique a c t i v i t y o f these c a t a l y s t s c o u l d be

connected w i t h t h e f o r m a t i o n o f a l k a l i m e t a l s by r e d u c t i o n o f a l k a l i m e t a l i o n s w i t h i n t h e i r pores. To determine whether s i m i l a r r e s u l t s w o u l d be f o u n d w i t h z e o l i t e s , samples o f Cs2C03 and KNaX, RbNaX and CsNaX z e o l i t e s were l o a d e d i n a mass s p e c t r o m e t e r and t h e i r s p e c t r a were r e c o r d e d f r o m 423°K up t o 800°K. CO2

M e t a l l i c cesium and

were t h e m a j o r s p e c i e s d e t e c t e d o v e r Cs2C03 i n good agreement w i t h Wood

e t a l . (19).

A p p a r e n t l y , Cs2C03 decomposes f i r s t t o C02 and cesium o x i d e ,

f o l l o w e d by t h e e v o l u t i o n o f cesium metal vapor f r o m t h e o x i d e i t s e l f .

Like-

w i s e Cs, Rb and K m e t a l vapors were d e t e c t e d o v e r t h e c o r r e s p o n d i n g - z e o l i t e s above 600°K.

I n c o n t r a s t , no Na metal vapor was d e t e c t e d above t h e same

z e o l i t e s under 800°K.

The temperature a t which t h e f i r s t metal vapors a r e

d e t e c t e d o v e r KNaX, RbNaX and CsNaX, 60OoK, c o i n c i d e s w i t h t h e o n s e t temperature f o r t h e a l k y l a t i o n o f t o l u e n e by methanol o v e r these c a t a l y s t s .

We suggest t h a t

t h e metal o x i d e s and t h e metal vapors c o n s t i t u t e t h e b a s i c s i t e s needed f o r t h e a c t i v a t i o n o f t h e a l k y l groups i n t h e s i d e c h a i n a l k y l a t i o n o f a l k y l a r o m a t i c s . The r e d u c t i o n i n a c t i v i t y o f t h e CsNaX and CsB-carbon c a t a l y s t s by a d d i t i o n o f C02 d u r i n g t h e a l k y l a t i o n o f t o l u e n e by methanol and t h e mass s p e c t r a l r e s u l t ' s suggest a c l o s e r e l a t i o n s h i p between t h e c a t a l y s t s b e h a v i o r and a1 k a l i metal carbonate e q u i l i b r i a .

To t e s t t h i s i d e a , an e q u i l i b r i u m model was

72

developed as follows : M2C03(~)= M2O(s) + C02(9) (1 1 M2C03(~)= 2M(g) + C O 2 ( g ) + %02(g) (2) (3) M2C03(~)+ C(S) = 2M(g) + CO2(g) + C O ( g ) Calculated Gibbs energy changes ( 2 1 , 2 2 ) f o r the model reactions were compared with a c t i v i t y data f o r z e o l i t e X c a t a l y s t s ( s e e Table 1 ) . The calculated values show a large energy gap between Li2C03 and the other s a l t s f o r the three r e a c t i o n s . Only the values f o r reactions 2 o r 3 (Fig. 4 ) produce a c o r r e l a t i o n c o n s i s t e n t with the a c t i v i t y d a t a . Consequently, they provide a b e t t e r model t o describe the c a t a l y s t s . O f course, we cannot r u l e out the r o l e o f metal oxides as reaction intermediates. I

I

I

I

I

I

I

I

Fig. 4 . Free energy change f o r reaction 2 as a function of temperature. Li (01, Na (el,K (01, R b (01,

cs

400 200

I

I

400

I

I

600

I

I

800

I

(0)

I

1000

TEMPERATURE,OK

TABLE 1 Activity o f a l k a l i forms of z e o l i t e X f o r the a l k y l a t i o n o f toluene with methanol. %Selectivity Cation* %Exchange %To1 Conversion+ EB Sty Xyl enes Li 28 3.0 0 100 0 Na 100 0.3 99 Trace Trace K 88 2.7 90 10 0 Rb 60 2.8 96 4 0 cs 39 7.1 96 4 0 *All samples adjusted t o pH13 with metal hydroxide. t Conditions: 5 Tol/l MeOH, LHSV 4 hr-1, 6 9 8 " ~ , 1 atm. All values represent peak a c t i v i t y .

73

SUMMARY The c l o s e a c t i v i t i e s o f CsB-carbon and CsNaX z e o l i t e , w i t h s i m i l a r p o r e s t r u c t u r e s , i n t h e a l k y l a t i o n o f t o l u e n e by methanol g i v e f u r t h e r s u p p o r t t o a r e a c t i o n pathway a f f e c t e d by t h e s p a t i a l f e a t u r e s o f t h e c a t a l y s t p o r e s . Furthermore, t h e a l u m i n o s i l i c a t e backbone i s n o t e s s e n t i a l t o c r e a t e a h i g h l y active catalyst.

Besides, t h e presence o f carbonaceous d e p o s i t s on t h e z e o l i t e

c a t a l y s t s c o u l d mean t h a t t h e s i m i l a r i t y w i t h carbon supported c a t a l y s t s i s g r e a t e r t h a n expected a t f i r s t s i g h t .

A model based on t h e r e d u c t i o n o f m e t a l carbonates t o a l k a l i m e t a l vapor, carbon o x i d e s and O2 i s proposed t o a c c o u n t f o r t h e r e l a t i v e a c t i v i t y o f these c a t a l y s t s , t h e common o n s e t temperature f o r v a r i o u s a l k a l i metal

forms o f t h e

z e o l i t e s and t h e e f f e c t o f v a r i o u s gases on t h e c a t a l y s t s performance.

The

temperature f o r peak a c t i v i t y i s most l i k e l y determined by t h e e q u i l i b r i a between t h e decomposition p r o d u c t s o f methanol, m e t a l vapors and t h e s u r f a c e o x i d e , carbonate and carbon s p e c i e s p r e s e n t w i t h i n t h e p o r e .

The f o l l o w i n g

r e a c t i o n p a t h i s proposed:

A.

M 2 C 0 3 ( ~ )M20(s) z

B.

M 2 0 ( s ) 2 2M(g) M20(s)

C.

@CH3

+

+

' co2(g)

402(,)

C(s)22M(g)

+ M +@CH2-Mt

F. $CH=CH2 + CH30H*+

f

co(g) 4H2

@CH2CH3+ CO + H2

F i n a l l y , we propose t h a t p o l y m e r i z a t i o n o f s t y r e n e c a t a l y z e d by metal vapors

(23) c o u l d l e a d t o d e a c t i v a t i o n o f t h e c a t a l y s t by f o r m a t i o n o f carbonaceous d e p o s i t s w i t h i n i t s pores.

I t i s a l s o l i k e l y t h a t m e t a l vapors and h i g h l y

r e a c t i v e metal o x i d e s c o u l d l e a d t o s t r u c t u r a l d e g r a d a t i o n o f t h e c a t a l y s t i n l o n g t e r m use. ACKNOWLEDGMENT The a u t h o r s thank The Dow Chemical Company f o r p e r m i s s i o n t o p u b l i s h t h i s article.

We r e c o g n i z e t h e a s s i s t a n c e o f M r . R. Bolenbaugh, M r . L. K r e s s l e y

and t h e c o n t r i b u t i o n s o f many c o l l e a g u e s .

S p e c i a l l y we thank Drs. M. Chase f o r

thermodynamic c a l c u l a t i o n s and D. Z a k e t t f o r mass s p e c t r a l work.

74

REFERENCES 1 Y. Ono, i n C a t a l y s i s by Z e o l i t e s , B. I m e l i k , e t a l . (Eds.), E l s e v i e r , Amsterdam ( 1 980) . 2 H. Pines and W.M. S t a l i c k , Base-catalyzed Reactions o f Hydrocarbons and Related Compounds, Academic Press, New York, 1977, p.240. 3 Y.N. Sidorenko, P.N. Galich, V . S . Gutyrya, V . G . I l i n and I . E . Neimark, Dokl. Akad. Nauk SSSR, 173 (1967) 132. 4 T. Yashima, H. Suzuki and N. Hara, J. Catal., 26 (1972) 303. 5 M.L. Unland and G.E. Barker, US P a t 4,115,424 assigned t o Monsanto Co., (1978). 6 H . I t o h , A. Miyamoto and Y . Murakami, J. Catal., 64, (1980) 284. 7 H. I t o n , T. H a t t o r i , K. Suzuki, A. Miyamoto and Y . Murakami, J. Catal, 72 (1981) 170. 8 H . I t o h , T. H a t t o r i , K. Suzuki and Y . Murakami, J. Catal, 79 (1983) 21. 9 K. IJda, A. Hindkiya and R. K i t o , Japan Pat.No. 57-68144, assigned t o UBE Kosan Co. (1982). 10 Japan Pat. No. 58-189039, assigned t o M i t s u b i s h i Chemical I n d . CO. L t d . (1983). 11 K. Tanabe, 0. Takahashi and H. H a t t o r i , React. Kin. Cat. L e t t . 7(3), (1977) 347. 12 K. Mori and T. Yokoi, Japan Pat. 52-133932, assigned t o M i t s u b i s h i CO., L t d . (1977). 13 T. Sodesawa, I . Kimura and F. Nozaki, B u l l . Chem. SOC. Jap. 52(8), (1979) 2431. 14 M.L. Unland, J. Phys. Chem. 82 (1978) 580. 15 J.J. Freeman and M.L. Unland, J. Catal. 54 (1978) 183. 16 M.L. Unland and J.J. Freeman, 3. Phys. Chem, 82 (1978) 1036. 17 M.D. S e f c i k , J. Amer. Chem. SOC. 101(9), (1979) 2164. 18 G.E. Vrieland, Unpublished Results. 19 8.3. Wood, R.D. B r i t t a i n , and K.H. Lau, Am. Chem. SOC. Fuel D i v . Chem., 28(1), (1983) 55, p r e p r i n t . 20 K.M. Sancier, Fuel, 62 (1983) 331. 21 Y.A. Chang and N. Ahmad, Thermodynamic Data on Metal Carbonates and Related Oxi des , AIME, Wassendal e , PA 1982. 22 JANAF Thermochemical Tables, The Dow Chemical Company, Midland, Michigan, Current Tabu1 a t i o n . 23 H. Pines and W.M. S t a l i c k , op. c i t . , p.205.

B. Imelik e t al. (Editors), Catalysis b y Acids and Bases 0

75

1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

IMPORTANCE OF THE ACID STRENGTH IN HETERGNEOUS CATALYSIS

D. wmoMEuF

Laboratoire de Chimie des Solides, ER 133 du CNRS, Universit6 Paris VI, 4 Place Jussieu, 75230 PARIS Cedex 05

RESUME De tr& nombreuses relations qualitatives ont 6t6 obtenues, en catalyse h6t6rog&ne, entre la vitesse d’une ri‘action et la force acide du catalyseur. Des classements, selon les forces acides mises en jeu, de r6actions types et de catalyseurs sont d6crits. Par ailleurs des 6tudes quantitatives permettent de relier avec succ&s l’activit6 catalytique soit 2 une fonction thermodynamique telle que Ho (comme dans une solution acide) soit 2 des propri6t6s massiques telles que 1’6lectron6gativit6. Cette dernisre approche a 6t6 appliqu6e 2 des catalyseurs tr&s divers (oxydes, sulfates, phosphates, h6t6ropolyacides, z6olithes). ABSTRACT

Many qualitative relationships have been obtained in heterogeneous catalysis between a reaction rate and the acid strength of the catalyst. Series of reaction types and catalysts classified according to the acid strength involved are described. Several quantitative studies relate successfully the catalytic activity either to thermodynamic parameters such as the Ho function (such as in an acidic solution) or to bulk properties such as electronegativity. This last case as been applied to various catalysts (oxydes, sulfates, phosphates, heteropolyacids, zeolites). INTRODUCTION For a long time the acid strength has been recognized as an important parameter in the field of acidic catalysis. In the 1920’s, BrEnsted and Pedersen (ref. 1) and further Hammett and Deyrup (ref.2) quantified the relationship between the acid strength of an acid in solution and the rate of the reaction catalyzed by this acid. The stronger the acid, the easier it is to activate a bond to form a transition state complex. As soon as acid solids were used in heterogeneous catalysis similar attempts were made. The two main problems in that case are first the characterization and measurement of the acid strength of solids and the establishment of a unique scale of acidity strength similar for instancc to the pKA one in solution. Secondly, the reaction mechanism may be directed differently from what occurs in solution, only because the reaction involves a site tied to a surface. In the broad field bf research bn heterogeneous acid catalysis, the present

76 paper is devoted to the search of relationships between catalytic properties (reaction rate, selectivity) and properties characterizing the solid acid strength.

IN I-KNCCENEOUS CATALYSIS

ACID S~~

The main features of the correlations between acid strength and reaction rate in homogeneous catalysis are of interest for potential application to reactions in heterogeneous phase. Brijnsted relation In 1924 BrCnsted and Pedersen proposed a relation on the basis of their experimental work (ref.1). It relates the effectiveness of a catalyst to its acidbase strength

kA = GA Ka A

(1)

where kA is the specific rate constant for catalysis by the acid whose acid dissociation constant is KA, G and a are constants for a series of similar cataA lysts but depend on khe nature of the reaction (i.e. on the reaction mechanism) and also on the solvent and the temperature. The exponent a is usuallypositive and less than unity.Attempts have been made\@ -lain

the meaning crfI3is exponent

(ref.3). It has been shown qua1it;atively and quantitatively that it measures the extent of proton transfer at the transition state of proton transfer reactions. It then reflects the transition state structure. It should be low (a generally less than 0.5) in the reactant

-

like transition states of exothermic reactions

and it should be high (generally greater than 0 . 5 ) in the product

-

like transi-

tion state of endothermic reactions. It has also been observed than when a large range of pK

A

values is considered, a may vary since the structure of the transi-

tion state and therefore the degree of proton transfer at the transition state cannot be expected to remain constant if ffi'

(i.e.-RT In K ) is varied over a A

sufficiently large range (ref.3).

Hammett relation Hamnett and Deyrup (ref.2) noted that the rate of certain reactions cofirelates with the acidity function Ho of the acid they defined as :

where B is a neutral base (colored indicator) and BH'

its conjugate acid. pK is A

77 p s H + , IBI and IBH+l a r e the concentrations i n solution and a +, yB and y

H

BH+

+

are respectively the proton a c t i v i t y and the a c t i v i t y c o e f f i c i e n t s of B and BH The H

.

a c i d i t y function evaluates the trend of a solution t o give a proton t o

t h e indicator. Since the i n d i c a t o r s a r e chosen such a s yB/yBH+ i s i d e n t i c a l f o r a l l of them i n the same a c i d s o l u t i o n ( s i m i l a r indicator s t r u c t u r e ) , H

is in-

dependent of the p a r t i c u l a r indicator which is used t o measure it and i s a char a c t e r i s t i c property of the medium. The H

a c i d i t y function can a l s o be defined

f o r L e w i s acids, IBH+I being replaced by IABl which i s the concentration of the n e u t r a l base B reacting with t h e L e w i s acid A. With a s e r i e s of inorganic acids (HC104, H2S04, HN03,

HC1) a s c a t a l y s t s Hammett and Deyrup observed t h a t t h e

hydrolysis of sucrose i s consistent with the equation

loglo k

+ H = constant

(3)

where k i s the r a t e constant. Such a r e l a t i o n s h i p i s less general than the Br6nsted r e l a t i o n s i n c e f o r some reaction r a t e s log k follows t h e proton concentration rather the H

fuhction. Zucker and Hammett proposed t h a t t h i s was as-

sociated with precise difference i n mechanism ( r e f . 4 ) . Examples a r e s t i l l found i n the recent l i t t e r a t u r e f o r instance isopropylation of benzene i n s u l f u r i c a c i d medium, which i l l u s t r a t e the H a m m e t t equation ( r e f . 5 ) . Attempts have been made t o extrapolate those r e l a t i o n s t o heterogenous catal y s i s . They w i l l be discussed l a t e r .

Hard and s o f t acids. Lewis a c i d strength In a general concept of acids and bases, protons a r e hard acids while f o r instance t r a n s i t i o n metal ions (Lewis a c i d s ) a r e s o f t acids. The former i n t e r a c t with an adsorbate through a charge-control process while the latter imply an o r b i t a l - c o n t r o l process ( r e f . 6 ) . N o c o r r e l a t i o n s between c a t a l y t i c properties and the strength of Lewis acids has been presented f o r several reasons. The main one i s t h a t t h e r e i s no unique s c a l e of t h e strength of L e w i s acids since t h e i r order of r e a c t i v i t y depends strongly on t h e reaction considered. There would be as many s c a l e s a s t h e r e are reaction types. The very broad subject t o cover would involve t h e whole f i e l d of c a t a l y s i s (metal, t r a n s i t i o n metal ions, oxides...

as c a t a l y s t s ) .

ACID SOLIDS AS CATALYSTS

A l a r g e v a r i e t y of acid s o l i d s a r e used a s c a t a l y s t s . The main c l a s s e s a r e

reported i n t a b l e 1. Each s o l i d has a c h a r a c t e r i s t i c acid s t r e n g t h , usually a

78 TABLE 1 S o l i d a c i d s as c a t a l y s t s Clays and z e o l i t e s Mounted a c i d s on oxides, halogenated oxides and superacids Cation exchange r e s i n s and Nafion H Charcoal v a r i o u s l y modified Metal oxides and s u l f i d e s Mixed oxides Metal salts Heteropolycompounds d i s t r i b u t i o n of s t r e n g t h s . The s t r o n g e r a c i d s (modified oxides or r e s i n s ) have a s u r f a c e c o n t a i n i n g u s u a l l y halogenated compounds and they behave as superacids i n solution.

ACIDITY MEASUREMENT

S e v e r a l methods (chemical t i t r a t i o n , s p e c t r o s c o p i c and thermal meth ods...)

are used t o e v a l u a t e t h e a c i d i t y number and s t r e n g t h of sites. Only t h e f i r s t one, t i t r a t i o n with a base i n t h e presence of colored i n d i c a t o r s , is b r i e f l y d e s c r i b e d here.

It is t h e only one which measures t h e a c i d s t r e n g t h i n t e r m s of

pK or H used i n q u a n t i t a t i v e r e l a t i o n s (1) t o ( 3 ) . The s o l i d i s suspended i n A 0 a d r y o r g a n i c medium and t i t r a t e d with a base ( u s u a l l y n-butylamine) u n t i l t h e end-point given by a colored i n d i c a t o r . A series of H a m m e t t i n d i c a t o r s , able t o be adsorbed on t h e s o l i d , i s used. T h i s g i v e s t h e d i s t r i b u t i o n of acid s t r e n g t h s

(ref.7). For microporous c a t a l y s t s , i t has t o be checked t h a t at least t h e base e n t e r s i n t o t h e pores (ref. 8 ) . The r e s u l t s are expressed as f o r i n s t a n c e i n t a b l e 2. Because of t h e simultaneous presence of d i f f e r e n t a c i d s t r e n g t h s , t h e

TABLE 2 D i s t r i b u t i o n of a c i d s t r e n g t h i n NaHY z e o l i t e s (number of e q u i v a l e n t p e r u n i t c e l l ) ( f r o m ( r e f . 9 ) )

Zeolite Na3@26 Na H Y 13 4 3

3 . 3 ' a ) 3 9 > -5.6 ( b )

-5.6

2.4 2.5

3 Ho> 7.3 7.2

-8.2(c)

-8.2 >Ho 6 16.2

( a ) Dimethylaminoazobenzene ( b ) Benzalacetophenone ( c ) Anthraquinone r e s u l t s are d i v i d e d i n domains of a c i d s t r e n g t h whose l i m i t s are defined by t h e pKATS of t h e i n d i c a t o r s used (-8.2,

-5.6...).

From equation ( 2 ) it comes t h a t

79

function existing in each domain is equals to or lower than the pK A Of the indicator which gives the color of its acid form (i.e. when B I / IBH+/< 1) the H

1

and equals to or higher than the pK

A

I

when BI /IBH+I),l). sive pK

A

IS

of the indicator in its basic form (i.e.

It follows that Ho' lies in a domain defined by two succes-

in the Hammett indicators series, for hstance -8.2GH ( - 5 . 6 . '0

DEPENDENCE OF REACTION RATE AND SELECTIVITY ON ACID STRENGTH. QUALITATIVE

APPROACH For a long time it has been observed that changing the acidity of a solid modifies its catalytic properties (ref. 10, 11). It was also early recognized that a change in the acid strength distribution influences the selectivity as for instance for the cracking of cumene with silica-alumina catalysts (ref.12).

Let us consider a reaction rate

where 1 Ns

1

and I Sl are respectively the concentration of active sites and of kT the substrate and k is the rate constant, with - the universal frequency, h

ASo#the standard entropy change at the transition state and E the apparent activation energy. Values of r (or of percentage of reactant transformation), de-

I I

pends on both the number of sites Ns

and on tkeir "energy" reflected by ASo

and E. The number of total acid sites of a given strength can be measured but the fraction of them IN

[

which is active in a given catalysis is usually unk-

nown. Moreover solid catalysts don't usually have a unique acid strength but show a distribution of strengths. All those parameters make difficult to see what are the separate effects of the number and of the strength of sites, particularly when only the reaction rate r is known but k unknown. An overall change in catalytic properties cannot be ascribed to the acid strength alone. Improvements are, when possible, to express the catalytic activity as the turn over number per acid site (i.e. '/INs[

=

k IS/ ) if one want to characterize a

typical acid strength by its selectivity in a given reaction. This last approach is useful when different reaction products are formed each of them requiring a different acid strength. A change in the distribution of acid strength is reflected in a change of the individual rates, i.e. in the overall selectivity.

80

Generation and modification of acid strength Several parameters are known to modify the surface of solids i.e. their acid strength and their catalytic behavior. For protonic solids they may be classified as follows. Modification by chemicals. They cover acid site generation as well as poisoning of existing sites. The well known poisoning of protonic sites by cation exchange or adsorption of bases (usually containing nitrogen (ref.10,ll) has been and is still used extensively. It has been applied to almost all acidic solids

of the protonic or Lewis type. It shows that poisoning of the stronger sites by a cation or a base decreases specifically the reactions which involve this acid strength. Generation of acids sites by chemical treatment of acidic surfaces, like A 1 0 fluorination for instance (ref.12,13) has been reported for a long 2 3

time. The formation of solid superacids by treatment of a protonic solid by a Lewis acid is also very successful. For instance various metal oxides treated with SbF exhibit activity for the conversion of n-alkanes (ref.14). 5

Removal of acid sites.

Dealumination of amorphous silica-alumina or zeolites

performed by chemical treatment or steaming results in a selective removal of sites of specific strength and catalytic properties. Depending on which solid is used or which treatment is performed the A1 sites linked to the weakest or the strongest sites are removed, changing or not the catalytic cracking and the selectivity (ref.15-18). Hydrochloric acid dealumination of amorphous silica-aluminas removes first the very strong sites which improves the thermal stability and does not affect the cracking properties until sites of the ri&t

strength

for cumene or n-octane cracking are eliminated. By contrast, zeolite dealumination with EDTA, acetylacetone or other A1 complexing agents removes first the weaker sites which also improves the stability and does not change much the isooctane cracking (ref.18). Steaming ofbare earth X or Y zeolites decreases mainly the number of strong acid sites. This\dkppar-

results in an increa-

sed gasoline yield and decreased coke and gas make and not much changes in conversion (ref.16). Variously preatreated zeolites give a selectivity, expressed as the research octane number, which increases with the acid strength (decrease in unit cell size which means increase in Si/Al ratio hence in acid strength). Such types of modifications have been applied mainly to amorphous Or crystalline aluminosilicates. They would also very likely change the acidity distribution and catalytic performances of other mixed oxides or acid s o l i d s . Effect of temperature. Two temperature effects have to be considered, that of pretreatment and that of the catalytic reaction conditions. For any protonic acid solid it is well known that increasing dehydration temperature removes pro-

81

tons possibly leaving Lewis acid sites on the surface. The weakest protons are first eliminated. Moreover the acid strength of each of the remaining protons may be increased due only to the lower inductive influence of close neighbors.

As a consequence the catalytic properties depend on the pretreatment conditions (yield and selectivity)(ref.l9,20). For instance n-hexane cracking increases up to pretreatment temperatures of 820 K while proton number starts to decrease at 700 K (ref.19). Pretreatment may also change the nature of the acid solid aswith

phosphoric acids. Simultaneously the selectivity in 1-butene isomerization to 2-butene or isobutene is modified (ref.21). For non protonic acids such as metal sulfates the effect of pretreatment is versatile enough so that sites of the moderate acid strength required for a particular reaction can be readily formed (ref.22).

As to the effect of reaction temperature it has been found for cumene cracking that sites of decreasing strength as measured at room temperature, are active for the catalysis of a given reaction when the temperature is raised (ref. 23). This is explained in terms of an easier breaking of the C-C bond and/or

in that the intrinsic acid strength of the active sites is increased. General trends The very large amount of work done during the last 30 or 40 years constitute a priceless data base from which general rules have been drawn. For instance, Tanabe (ref.24) summs up correlations between the catalytic properties and the acid strength of a large number of solids, A1 0 -Si02, Zr -SO2, phosphorous 2 3 2 acid on silica gel, M@-SiO2, metal sulfates... Scales of the acid strength (H Hammett function) required for each reaction are proposed. It was observed (ref. 25) that the change in reaction rate with acid strength varies from reaction to

reaction : it is much more marked for propylene than for isobutylene dimerization for example. The dependence with acid strength is less pronounced than for homogeneous acid-catalyzed reactions (ref.25). Table 3 reports results obtained for amorphous silica-aluminas progressively poisonned with pyridine and shows that the most difficult reaction is the skeletal isomerization of olefins (ref. 26). For HY zeolites, table 3 adds some other reactions, the general order being

the same (ref.27). The comparison of those aluminosilicates results with other acid catalysts mentionned above (see ref.24) shows that the order for the scale of reactions is not much dependent on the catalysts. The scale is extended towards the range of catalysis by weak acids when using metal sulfates and phosphates (ref.22,24). Besides this reactions classification, it has been for a long time observed that for a given reaction such as paraffins or alkylaromatics cracking, the rate

82

TABLE 3 Catalytic activity of amorphous silica-aluminas versus the sites acid strength (from ref.26).

1

. dehydration of tert-butanol to butenes . cracking (diisobutylene to butenes and others) . double bond migration and cis-trans isomerization of n-butenes

C

$

5

2 8

. cracking (dealkylation of tert-butylbenzene) . skeletal isomerization (isobutylene to n-butenes)

8$

,$ 4

s

Catalytic activity of HY zeolite versus the sites acid strength (from ref. 2 7 )

I

. alcohol dehydration

. olefin isomerization

c iJ 0 m alci v)

sd

.. aronatics alkylation alkylaromatics isomerization

. alkylaromatics transalkylation . alkylaromatics cracking . paraffins cracking

U

8$ 2 ‘cf

‘‘ ’‘

2

is raised upon increase of the catalysts acid strength (removal of poisons, increase in zeolite Si/Al ratio,..). In zeolites the effect is not structure dependent. This has been also observed recently in olefin hydration (ref.28).

QUANTITATIVE REIATIONSHPS B E m E N CATALYSIS AND SURFACE ACID STRENXH Attempts to quantitatively correlate the reactions rates to acid strength have to take two points into account. Firstly the exact value of the catalysts pK

A

cannot be determined directly or calculated as in a solution. The almost

quantitative but undirect way is its evaluation from the n-butylamine titration with colored indicators as described above. Secondly, for almost any acid solid a range of sites of different strengths exist. As said previously it is possible to determine the number of sites of each domain defined by its range of acid strength H

0’

It is not possible to know the exact H

0

value of each site. The ra-

te of a reaction on such a solid depends as in a solution (equation 4) both on the number and on the strength of acid sites. As seen above only domains of acid strengths can be defined, therefore only domains of number of sites belonging to this family of acid strength are consequently considered. A n attempt to obtain quantitative relationship have been proposed by Yoneda (ref.29). The “regional analysis” considers” region of acid strengths” in the catalyst. The analysis is

83 based upon the use of least squares method to find "regional rates" for those acid strength families. It requires a large set of data for the mathematical calculation. It is also necessary for any catalyst under study to know what is the strongest acid strength. Limiting H

determination to -8.2 as usually done with

Hammett indicators is not precise enough since the very strong acid sites contribute for the major part to the reaction rate. The study assumes that the Brijnsted law holds among regions on a heterogeneous catalyst. An increase in the Itregionalrate constant" is observed for depolymerization of paraaldehyde (ref. 29) and o-xylene isomerization (ref.30) fig.(l). This is in agreement with equa-

I -8

I

1

-1 0

Acid rtrenqth

-1 I

IHd

1 -1 4

Fig 1. o-xylene isomerization regional rate constants and energies of activation at 500 "C as a function of average regional acid strength (Ho) of silica-alumina (from ref.30).

tion (3) proposed for solutions. Recently the thermodesorption of ammonia divided in four temperatures regions has been used to evaluate the regional acid strength (ref.31). Nevertheless, since the acid strength so measured is not related to a thermodynamic or fundamental property the relationship obtained with regional rate constant is more qualitative than quantitative. Other ways of evaluating the surface acid strength such as zero point of charge (ref.32) or H value (ref.33,34) have been applied to a large variety o,max of oxides. For the time being no correlation has been presented with catalytic properties very likely because those catalysts constitute a heterogeneous class of solids. Parameters other than acid strength, for instance surface structure, may interfere. It is well accepted that the surfaceproperties are different from those of

84

the bulk. Since the acid catalysis proceeds on the surface one would expect correlations mainly with the surface acid strength as it has been described above. Nevertheless the surface properties may result from overall IjIloperties and several examples are known of bulk acidic properties giving good correlations with catalytic properties.

QUANTITATIVE CORRELATIONS BElWEEN CATALYTIC ACTIVITY AND CATALYSTS BUu( PROPERTIES Such correlations have been established for catalysts containing a surface cation acting as a Lewis site (sulfates, phosphates, oxides) or for solids with a very high surface area (zeolites). In the first case the influence of the cation is probably strong enough as to overcome other parameters in directing the bulk acidity strength. In the very open structure of zeolites, the major part of surface A10 and SiO tetrahedra belongs also to the bulk. The surface and 4

4

bulk properties are superimposed. A long time ago, Tanaka et a1 (ref.35) observed correlations between catalytic reactions such as dehydrogenation of formic acid or isobutylene polymerization and the enthalpy of formation of the oxides used as catalysts. The basic idea was to compare the acid strength concept in oxides to that established for oxy-acids in solution and which relates an increase in the acid electronegativity to an increase in acid strength (ref.36). Sanderson (ref.36) showed that in oxyacids a high acid strength is associated with a high degree of deficiency of electrons on the oxygen, i.e. a partial charge on the oxygen closer to zero. When the electronegativity of the acid increases, for instance by addition of an electronegative atom, the partial charge on the oxygen is lowered and the strength of the acid is increased. Based on this idea, Tanaka et a1 (ref.35) related the electronegativity of oxides to the enthalpy of their formation through the oxygen partial charge. The correlation holds well for some oxides. Improving the search of quantitative relationships. Tanaka et a1 (ref.32) considered families of catalysts either phosphates or sulfates which then differ only by the nature of the cation associated to the anion. Starting again from the electronegativity concept they consider that the acidity of an anion in an aqueous solution increases with the electronegativity of the central metal ion according to the reaction.

85

They calculate the electronegativity

where Z is the ion charge and 0

7.

of metal ions from:

the electronegativity of the metal atom(2 = 0).

p

0

10

5

X

in aqueous medium as a function of electroFig 2. Dissociation constant (FA) negativity for metal ions (from ref.32).

The figure 2 confirms for a large number of metal ions that the pK

A

corres-

ponding to equation (6) decreases as the electronegativity of the cation increases. The authors consider catalytic results obtained by others in hydration of propylene or acetaldehyde polymerization on sulfates, in isobutylene polymerization on phosphates. All the results show a good correlation with the

x.

electronegativity they calculate as seen for example in figure 3. The higher

a,,

the stronger the metal ion attracts electrons which increases the protonic acidity (due to ionization of adsorbed water on sulfates or unneutralized acidity function in acid phosphates). Tanabe (ref.34) extended the idea to the results of Seiyama et a1 (ref.37). It was shown that the selectivity in oxidation of propylene on Sn02 based binary oxides depends strongly on the metal oxides electronegativity. Figure 4 shows that the formation of acrolein requires high electronegativity while that of benzene production is related to low electronegativity of the metal oxide mixed with SnO

2'

Using the same electronegativity concept in order to evaluate the acid strength of metal-cation substituted heteropolycompounds, Niiyama et a1 found

86

1501 0

I

1 10

5

x

xi.

Fig. 3. Catalytic activity of sulfates in propylene hydration as a function of The conversion is 1 % at the temperature reported and at a given flow rate (from ref.32).

50 7.

!.

10

U

kL-!J

O L 0

0

5

10

To TO hcroleio

&I1 10

15

Xi

Fig. 4. Catalytic activity of various binary oxides including Sn02 for oxidation of propylene as a function of electronegativity of metal oxides which are mixed with Sn02. Oxides mixed with Sn02 : 1 : K20, 2 : Na20, 3 : Li20, 4 : BaO, 5 : CaO, 6 : Mg0, 7 : Cr203, 8 : no, 9 : Sb2O4, 10 : W03, 11 : As2O5, 12 : P205, 13 : M"03 (from ref .34,37).

a filear relationship between the 2-propanol dehydration and the metal cation electronegativity for some specific catalysts (ref.38). By contrast with the above catalysts, zeolites have a heterogeneous acid distribution which makes difficult the search of quantitative relationship with catalysis. Mortier (ref.39) applied successfully Sanderson electronegativity equa-

87 lization principle to the calculation of charges on atoms in the zeolite framework. This was further extended to catalysis and a linear relationship was obtained between the calculated charge on the proton and the turn over number in isopropanol dehydration (ref.40). Considering in the next step, not the proton charge, but the zeolite electronegativity Jacobs (ref.41) proposed a linear relationship between the values calculated for a series of different zeolites structures and the turn over number in isopropanol decomposition and n-decane hydroconversion. A s pointed out by the authors the Sanderson electronegativity so calculated is an average property which also reflects the average acid

strength evaluated by hydroxyl group wavenumbers shifts. The approach allows to expla5n and to predict rates of reactions like isopropanol dehydration or n-decane hydroconversion and also changes in selectivity (isomerization-cracking of n-decane). It should also apply to the hydration of olefins on zeolites which rate was found to increase with A1 framework content (ref.28). A very practical correlation has been proposed recently (ref.17) between on the one side the catalytic activity and selectivity (Mat activity, octane number, C gas yield ...) 3

and on the other side the unit cell parameter in a series of highly siliceous faujasites. In fact the unit cell parameter as other bulk average properties (for instance infra red T-0 wavenumbers) varies in a rather smooth way with the framework A1 content. Then any overall property which dependence on chemical composition parallels that of acid strength should provide a probe to compare catalytic behavior. It turns out that zeolites, more than other acid solids show such kinds of overall properties.

COMPARISON OF THE VARIOUS APPROACHS

Compared to the Yoneda analysis (ref.29) which considers experimental “regional” acid strengths and the related catalytic activities, the Sanderson electronegativity approach involves only overall properties and then does not detect changes in acid strength due to short range interactions as they can occur in the zeolite cages. In that sense it cannot replace the experimental acid strength measurement when local disturbances become of importance (high Si/Al range, cations in the faujasite supercage, new superacid sites...). The linear relationship obtained between electronegativity and reaction rates means that for these catalytic reactions the overall rate constant k in equation 4 does not depend on the acid site distribution of each zeolite (or that a l l the zeolites have the same acid strengths distribution) (ref.41). It would depend on the average strength of this distribution. This implies that the BrGnsted relation, if valid, would involve an ”average” dissociation constant KAav for a k rate consav

88

kav

= G Ka A Aav

tant (equation 8). This is not probably true for any catalytic reaction. It was said above that Tanaka et a1 (ref.35) based their concept of protonic acidity of oxides on the behevior of oxyacids in solution studied by Sanderson. According to Sanderson results (ref.36) the acid electronegativity i.e. its acid strength should increase when an electronegative atom is added to the acid. This occurs in fact when the zeolite framework becomes sicher in silicium (Si Sanderson electronegativity equals 2.84 compared to 2.22 for Al). This is also in line with what was said about strength of oxyacids and zeolites in a general analogy between those solids and solutions (ref.42). Not only for wide based catalysts but also for other acidic catalysts (sulfates, phosphates ...) it was shown that the electronegativity is a very valuable tool for the evaluation of the acid strength and its quantitative correlation with catalysis. The question then arises to know in more details how this property can be related to usual acidity characteristics. It was shown that the negative of electronegativity is the chemical potential of a molecule (ref.43). From the Briinsted and Hammett relations (equations (1) and (3)) one would expect that log k rather than k is related to the electronegativity. A

more accurate

definition of solid acid strength and a thorough theoretical correlation with electronegativity would be very helpful.

RJZFEFSNCES 1 J.N. Bronsted, K. Pedersen, J. Phys, Chem., 108 (1924) 185. 2 L.P. Hammett, A.J. Deyrup, J.A.C.S., 54 (1932) 2721.

3 R.P. Bell, "The Proton in Chemistry", Chapman and Hall, London, 1973, p. 194 and p. 204. A.J. Kresge in "Proton-Transfer Reactions", (E.F. Caldin and V. Gold edit.). Chapman and Hall, London, 1975, p. 179. 4 L. Zucker, L.P. Hammett, J.A.C.S., 61 (1939) 2791. 5 A.A. Zerkalenkov, 0.1. Kachurin, Kin i Kat., 21 (1980) 1442. 6 W.B. Jensen, "The Lewis Acid-Base Concepts. An Overview", John Wiley and Sons, New York, 1980. 7 C. Walling, J . A . C . S . , 72 (1950) 1164. 0. Jdhnson, J. Phys. Chem., 59 (1955) 827. H.A. Benesi, J.A.C.S., 78 (1956) 5490. 8 W.F. Kladnig, J. Phys. Chem., 83 (1979) 765. D. Barthomeuf, J. Phys. Chem., 83 (1979) 766. 9 R. Beaumont, D. Barthomeuf, J. Catal., 26 (1972) 218.R.Beaumont,?hesisLyon 10A.G. Oblad, T.H. Milliken, G.A. Mills, Rev. Inst. Fr. Petrole, 6 (1951) 343. 11 K.V. Topchieva, I.F. Moskovskaia, Dokl. ACdd. Nauk , SSSR, Ser. Phys. Chem., 123 (1958) 891. 12 A.E. Hirschler, A. Schneider, J. Chem. Engineering Data,$ (1961) 313.

89 13 V.C.F. Holm, A. C l a r k , A.C.S. Meeting, A t l a n t i c C i t y , Sept. 1962, paper A-45. 14 K. T a n a b e , H. H a t t o r i , Chem. L e t t . , ( 1 9 7 6 ) 625. 15 D. B a r t h o m e u f , C.R. A c a d . Sci. Paris, 259 (1964) 3520 a n d E C ( 1 9 7 0 ) 1549. 16 L. Moscou, R. Mone, J. C a t a l . , 30 (1973) 417. 17 L.A. Pine, P.J. Maher, W.A. Wachter, J. C a t a l , 85 (1984) 466. 18 R. B e a u m o n t , D. B a r t h o m e u f , C.R. A c a d . S c i . , 272C (1971) 363. 19 P.D. H o p k i n s , J. C a t a l . , 12 (1968) 325. 20 P.A. Jacobs, M. T i e l e n , J.B. Uytterhoeven, J. C a t a l . , 50 (1977) 98. 21 A. M i t s u t a n i , Y. H a m a m o t o , K o g y o K a g a k u Z a s s h i , 67 (1964) 1231. 22 K. Tanabe, T. T a k e s h i t a , Adv. i n C a t a l . , 17 (1967)315. 23 L i X u a n w e n , She L i q i n , L i u X i n g y u n , J. C a t a l . C h i n a , 4 (1983)43. 24 K. Tanabe, "Solid A c i d s and B a s e s " , K o d a n s h a , Tokyo, (1970). 25 V.A. D z i s k o , Proc. I n t e r n . C o n g r . C a t a l y s i s , 3rd, A m s t e r d a m , 1 (1964) 422. 26 M. Misono, Y. S a i t o , Y. Y o n e d a , Proc. I n t . C o n g . C a t a l . , 1 (1964) 408. 27 P.A. Jacobs i n " C a r b o n i o g e n i c A c t i v i t y of Zeolites", E l s e v i e r , A m s t e r d a m , (1977). 28F. F a j u l a , R. Ibarra, F. F i g u e r a s , C . G u e g u e n , J. C a t a l . , (1984), i n press. 29 Y. Y o n e d a , J. C a t a l . , 9 (1967) 51. 30 Y. Y o n e d a , D a i 5 - K a i H a n n o K o g a k u S h i n p o j i u m K o e n y o s h i - S h u ( J a p a n e s e ) , SOC. Chem. Engr. Japan, (1965) n014. 31 B.V. R o m a n o v s k i i , Yu N. K a r t a s h e v , K i n . i K a t a l . , 24 (1983)n03 758. 32 K . I . Taraka, A. O z a k i , J. C a t a l . , 8 (1967) 1. 33 T. Y a m a n a k a , K. Tanabe, J. Phys. Chem., 79 (1975) 2409. 34 K. Tanabe i n " C a t a l y s i s Science and Technology", ( E d . J.R. Anderson, M. B o u d a r t ) , S p r i n g e r - V e r l a g , B e r l i n , 2 (1981) 231. 35 K.I. Tanaka, K. T a m a r u , B u l l . Chem. SOC. Jap., 37 (1964) n012,1862. 36 R.T. Sanderson, " C h e m i c a l Periodicity", R e i n h o l d Pub. C o r p . , New Y o r k , (1960). 37 T. S e i y a m a , M. Egashira, T. S a k a m o t o , I. A s o , J. C a t a l . , 24 (1972) 76. 38 H. N i i y a m a , Y. Saito, E. Echigoya i n l l C a t a l y s i s l f , ( T . S e i y a m a , K. Tanabe edit.), Elsevier, A m s t e r d a m and K o d a n s h a , Tokyo, (1981) 1416. 39 W.J. M o r t i e r , J. C a t a l . , 55 (1978) 138. 40 P.A. Jacobs, W.J. M o r t i e r , J.B. U y t t e r h o e v e n , J. Inorg. Nucl. Chem., 40 (1978) 1919. 41 P.A. Jacobs, C a t a l . R e v . S c i . Eng., 24 (1982) 415. 42 D. B a r t h o m e u f , J. Phys. Chem., 83 (1979)249. 43 R.G. Parr, R.A. D o n n e l l y , M. Levy, W.E. Palke, J. Chem. Phys., 68 (1978) 3m1. R.A. D o n n e l y , R.G. Parr, J. Chem. Phys., 69 (1978) 4431.


91

B. Imelik et al. (Editors), Catalysis b y Acids and Bases 0 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

STRUCTURE AND A C I D I C PROPERTIES OF H I G H SILICA FAUJASITES F.

MaugGl,

A.

Aurouxl,

J.C.

Courcelle2,

Ph.

Engelhard2,

P.

Gallezotl et

J. Grosmangin2. 1 I n s t i t u t de Recherches s u r l a C a t a l y s e , CNRS, 2 avenue A l b e r t E i n s t e i n , 69626 V i 1 leurbanne Cgdex. Compagnie Francaise de Raffinage, Centre de Recherches, 76700 H a r f l e u r .

ABSTRACT High s i l i c a f a u j a s i t e s (Si/A1=19-78) have been prepared b y repeated steamings o f HY z e o l i t e f o l l o w e d b y a c i d e x t r a c t i o n . A l a r g e p o r t i o n o f t h e z e o l i t e l a t t i c e i s d e s t r o y e d d u r i n g steam t r e a t m e n t l e a v i n g p o r e s t h r o u g h o u t t h e c r y s t a l . The A 1 atoms e x t r a c t e d from t h e l a t t i c e ( e i t h e r f r o m t h e c r y s t a l l i n e f r a c t i o n o r f r o m t h e destroyed f r a c t i o n ) are hexacoordinated. They a r e replaced b y S i atoms i n t e t r a h e d r a l s i t e s . Measurements o f t h e a c i d i t y b y c a l o r i m e t r y and i n f r a r e d spectroscopy i n d i c a t e t h a t t h e number o f Bronsted s i t e s can be s m a l l e r t h a n l / u n i t c e l l . T h i s accounts f o r t h e v e r y l o w c r a c k i n g a c t i v i t y measured on h i g h s i l i c a f a u j a s i t e s embedded i n k a o l i n . RESUME Des z 6 o l i t h e s de t y p e f a u j a s i t e 'a haute t e n e u r en s i l i c i u m o n t 6 t 6 pr6par6es p a r t r a i t e m e n t 'a l a v a p e u r d ' e a u e t e x t r a c t i o n a c i d e . L e t r a i t e m e n t h y d r o t h e r m i q u e d g t r u i t une p a r t i e de l a s t r u c t u r e en l a i s s a n t des pores. Les atomes A1 e x t r a i t s du r 6 s e a u ( p a r t i e c r i s t a l l i n e e t p a r t i e d 6 t r u i t e ) s o n t h e x a c o o r d i n 6 s Y i l s s o n t rernplac6s par des atornes S i au c e n t r e des t 6 t r a g d r e s . Les mesures d ' a c i d i t e p a r c a l o r i m g t r i e e t s p e c t r o m g t r i e I R m o n t r e n t que l e nombre de s i t e s de B r o n s t e d e s t i n f e r i e u r 'a 1 p a r m a i l l e . C e t t e t r e s f a i b l e a c i d i t 6 r 6 s i d u e l l e permet d ' i n t e r p r g t e r l a f a i b l e a c t i v i t e c r a q u a n t e de c e s zeolithes. INTRODUCTION The p r e p a r a t i o n o f

u l t r a s t a b l e f a u j a s i t e s b y h i g h temperature c a l c i n a t i o n o f

Y z e o l i t e and treatment i n s a l t s o l u t i o n s has been r e p o r t e d e a r l y ( r e f . 1 - 3 ) .

A

c o m p r e h e n s i v e r e v i e w on t h e 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 o f aluminum d e f i c i e n t z e o l i t e s has been p u b l i s h e d r e c e n t l y ( r e f . 4 ) . (ref.5)

that

the

zeolite

stability

is

due

I t h a s been s u g g e s t e d

to

the

formation

of

t r i o x o - t r i a l u m i n u m c a t i o n s i n t h e soda1 i t e cages and t h a t aluminum atoms a r e p a r t i a l l y r e i n s e r t e d i n t h e vacant t e t r a h e d r a l s i t e s . I n c o n t r a s t , d e m o n s t r a t e d ( r e f . 6,7)

i t has been

t h a t h i g h t e m p e r a t u r e t r e a t m e n t s o f NH4Y z e o l i t e s i n

presence o f water vapor and e x t e n s i v e aluminum e x t r a c t i o n i n a c i d i c s o l u t i o n s r e s u l t i n v e r y s t a b l e h i g h s i l i c a f a u j a s i t e s h e r e t h e aluminum atoms o f t h e l a t t i c e have been r e p l a c e d b y s i l i c o n atoms.

92 The f i r s t aim o f t h e p r e s e n t work i s t o prepare h i g h s i l i c a f a u j a s i t e s by r e p e a t e d h e a t i n g s o f NH4Y z e o l i t e s under well d e f i n e d steam pressure, then t o c h a r a c t e r i z e t h o r o u g h l y t h e s t r u c t u r e o f t h e steamed and o f t h e e x t r a c t e d z e o l i t e s i n o r d e r t o c h a r a c t e r i z e b e t t e r t h e s t a b i l i z a t i o n process. T h i s has been achieved b y combining c r y s t a l s t r u c t u r e a n a l y s i s p r o b i n g t h e s t r u c t u r e of t h e z e o l i t e framework and r a d i a l e l e c t r o n d i s t r i b u t i o n ( R E D ) p r o b i n g t h e s t r u c t u r e o f b o t h t h e c r y s t a l and amorphous phases. The second aim i s t o c h a r a c t e r i z e t h e number and s t r e n g t h o f t h e a c i d s i t e s p r e s e n t a f t e r steam treatments and a f t e r aluminum e x t r a c t i o n . M i c r o c a l o r i m e t r i c measurements o f t h e h e a t o f ammonia a d s o r p t i o n and i n f r a r e d absorption spectroscopy using p y r i d i n e as probe molecule were used f o r t h a t purpose. F i n a l l y c a t a l y t i c c r a c k i n g t e s t was p e r f o r m e d t o assess 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 h i g h s i l i c a f auj a s i t e s .

EXP ER I MENTAL A

L i n d e NaY z e o l i t e

2.2-2.6

was f i r s t

exchanged i n NH4C1 s o l u t i o n s t o g e t

wt % Na z e o l i t e s . H e a t i n g a t 600°C under 1 0 0 % H20 r e s u l t e d i n a 0

u n i t - c e l l s h r i n k a g e f r o m 24.75

t o 24.45

A .The z e o l i t e was re-exchanged t o

wt % Na z e o l i t e s . A f t e r h e a t i n g u n d e r 1 0 0 % H20 w i t h i n t h e

o b t a i n 0.2-0.27

0

temperature range 82O-92O0C, t h e u n i t c e l l shrank t o 24.32-24.18 upon t h e t e m p e r a t u r e o f t h e f i n a l t r e a t m e n t .

A,

depending

The steamed samples were then

t r e a t e d i n H C l (1N) a t 90°C. The u n i t c e l l constant d i d not change s i g n i f i c a n t l y b u t t h e Na content c o u l d decrease t o 0.04 wt%. The treatment and composition of t h e samples i n v e s t i g a t e d are g i v e n i n Table 1. The

crystal

(Deal 9-2,ext)

structure

of

the

steamed

and

acid-extracted

zeolite

has been d e t e r m i n e d f r o m X - r a y powder d a t a (CuKa). The d a t a

c o l l e c t i o n and t h e s t r u c t u r e refinement procedure have been described p r e v i o u s l y (ref.8).

I n t e r a t o m i c distances were determined b y r a d i a l e l e c t r o n d i s t r i b u t i o n

from wide angle X-ray s c a t t e r i n g

data (ref.9,lO).

The d i f f e r e n t i a l h e a t s o f ammonia a d s o r p t i o n were measured b y m i c r o c a l o r i m e t r y w i t h a Thian-Calvet c a l o r i m e t e r

as described p r e v i o u s l y ( r e f .11).

The s t u d y b y i n f r a r e d s p e c t r o s c o p y was c a r r i e d o u t w i t h z e o l i t e w a f e r s t r e a t e d i n t h e I R c e l l . The s p e c t r a were r e c o r d e d w i t h a P e r k i n - E l m e r 125 spectrometer. The c a t a l y t i c a c t i v i t i e s o f z e o l i t e s were measured a f t e r i n c o r p o r a t i o n i n a k a o l i n m a t r i x (20 w t % o f z e o l i t e ) . The m i c r o a c t i v i t y t e s t (MAT) ( r e f . performed on a vacuum d i s t i l l a t e feedstock.

1 2 ) was

93 TABLE 1 Treatments and composition o f z e o l i t e s Samples

Treatments

Na %

S i / A l ( b ) a/A(a)

NH4Y

NaY exchanged i n NH4Cl

2.2

2.4

24.70

Deal 6-2

NH4Y steamed 600'C 100 % H20, exchanged w i t h NH4C1, steamed 820'C 60 % H20

0.27

2.4

24.35

Deal 6-3

Same b u t f i n a l treatment 890'C 100 % H20

0.27

2.4

24.32

Deal 6-4

Deal 6-3 steamed 920'C

0.27

2.4

24.22

Deal 9-2

Same as Deal 6-2 b u t f i n a l treatment 100 % H20 890°C

0.26

2.4

24.1 9

Deal 9-2,ext

Deal 9-2 t r e a t e d w i t h HC1 (1N) a t 9O'C

0.07

19

24.20

Deal l 0 , e x t

Same as Deal 9-2,ext

0.07

49

24.18

Deal 16,ext

Same as Deal 9-2,ext

0.04

78

24.20

HY

NaY exchanged i n NH4Cl ( e i g h t times) outgassed a t 300'C i n vacuum

1

-

-

-

- 100 % H20

-

b u t steaming and

e x t r a c t i o n repeated t w o times

~

~

2.4

~~~~

( a ) u n i t c e l l constant ; ( b ) from chemical a n a l y s i s . RESULTS AND DISCUSSION Texture o f steamed z e o l i t e s The t e x t u r e o f z e o l i t e s was s t u d i e d b y t r a n s m i s s i o n e l e c t r o n microscopy w i t h a JEOL l O O C e l e c t r o n microscope. T h i n s e c t i o n s o f z e o l i t e s embedded i n epon r e s i n were c u t w i t h an u l t r a m i c r o t o m e . F i g u r e 1 g i v e t h e t r a n s m i s s i o n b r i g h t f i e l d s image o f z e o l i t e s Deal 9-2 and Deal 9-2,ext. respectively).

( M i c r o g r a p h s a and b

T h e r e are many areas o f low c o n t r a s t corresponding t o amorphous

domains o r more probably t o holes and c a v i t i e s p u n c t u r i n g t h e z e o l i t e c r y s t a l . Thus, t h e l a t t i c e has been destroyed a t many places b y t h e steam treatment a t h i g h t e m p e r a t u r e . However t h e l a t t i c e images o f t h e ( 1 1 1 ) p l a n e s a r e w e l l r e s o l v e d and o r i e n t e d t h r o u g h o u t t h e c r y s t a l i n s p i t e o f t h e destroyed areas. This i n d i c a t e s t h a t t h e c r y s t a l l i n e f r a c t i o n i . e .

t h e undestroyed p o r t i o n o f

t h e z e o l i t e r e m a i n w e l l ordered. The micrographs are q u i t e s i m i l a r i n Deal 9-2 and Deal 9-2,ext.

h i c h i n d i c a t e s t h a t t h e a c i d t r e a t m e n t does n o t change t h e

p o r o s i t y o f t h e z e o l i t e created d u r i n g t h e hydrothermal treatment. The c r y s t a l f r a c t i o n e s t i m a t e d f r o m a d s o r p t i o n c a p a c i t y measurement using C 6 h 2 as probe molecule can be as small as 20% o f t h a t o f t h e parent z e o l i t e (ref.13).

94

F i u r e 1 : E l e c t r o n microscope t r a n s m i s s i o n view through a t h i n section o f b ( l e f t f i e l d ) and o f Deal 9-2,ext ( r i g h t f i e l d ) , c u t w i t h an ultramicrotome ( m a g n i f i c a t i o n : 5x105). C r y s t a l s t r u c t u r e analysis A p r e l i m i n a r y X - r a y d i f f r a c t i o n s t u d y p e r f o r m e d w i t h a h i g h temperature

Guinier-Len&

camera, has shown t h a t under f l o w i n g d r y h e l i u m t h e i n t e n s i t i e s o f

t h e X-ray l i n e s o f Deal 9-2,ext

do n o t change up t o 1060'C which was t h e h i g h e s t

temperature a t t a i n a b l e . Therefore t h e Deal 9-2,ext

z e o l i t e i s s t a b l e a t l e a s t up

t o 1060'C. The c r y s t a l s t r u c t u r e o f t h e dehydrated Deal 9-2,ext from t h e X-ray p a t t e r n r e c o r d e d a t 25'C.

z e o l i t e was determined

The s t r u c t u r e was r e f i n e d w i t h 1 6 1

s t r u c t u r e f a c t o r s corresponding t o t h e h k l r e f l e c t i o n s w i t h h2 + k 2 + l2 6 299 e x c e p t t h e 111 l i n e . T (Si, Al),

01,02,

The a t o m i c c o o r d i n a t e s

and t e m p e r a t u r e f a c t o r s of

03 and 04 atoms were r e f i n e d , then t h e occupancy f a c t o r s .

A f t e r f i v e refinement c y c l e s t h e occupancy f a c t o r o f T atoms decreased t o 0.99 which c o r r e s p o n d s t o 1 9 0 T / u n i t c e l l i n s t e a d o f 1 9 2 T / u n i t c e l l f o r f u l l occupancy o f t e t r a h e d r a l s i t e s . This

discrepancy i s w i t h i n t h e standard e r r o r s .

S i n c e 46 o u t o f 56 aluminum have been e x t r a c t e d from t h e z e o l i t e , t h e occupancy f a c t o r would be 0.76 i f t h e aluminum s i t e s were l e f t vacant. Therefore i t can be c o n c l u d e d t h a t d u r i n g steaming treatments, A1 atoms are replaced by S i atoms i n tetrahedral sites. Deal 9-2,

T h i s i s c o r r o b o r a t e d b y t h e s e t o f T-0 d i s t a n c e s i n

e x t compared t o t h a t o f a reference HY sample ( t a b l e 2 ) . The mean T-0

value i s 1.60 A & i c h i s t y p i c a l o f pure Si-0 distances as i n q u a r t z . I t can b e c o n c l u d e d t h a t t h e r e c r y s t a l l i s a t i o n p r o c e s s o f Y z e o l i t e i n t o pure s i l i c a f a u j a s i t e i s almost completed. S i m i l a r c o n c l u s i o n s were r e p o r t e d p r e v i o u s l y ( r e f . 8 ) i n t h e case o f dealumination by chemical e x t r a c t i o n .

95

TABLE 2 I n t e r a t o m i c d i s t a n c e s ( S i , A1)-0 (Angstrom). Reference sample HY

Distances T-01

1.644

T-02 T-03 T-04 Mean v a l u e

1.669 1.652 1.602 1.642

Deal 9-2,ext 1 .bZb 1.595 1.634 1.542 1.600

Radial Electron D i s t r i b u t i o n Study and s o l i d s t a t e NMR ( r e f . 1 5 )

B o t h X - r a y e m i s s i o n spectroscopy (ref.14)

give

e v i d e n c e f o r t h e f o r m a t i o n o f o c t a h e d r a l l y c o o r d i n a t e d aluminum i n NH4Y z e o l i t e heated i n presence o f steam. The r a d i a l e l e c t r o n d i s t r i b u t i o n h a s b e e n u s e d t o c o n f i r m t h e s e p r e v i o u s f i n d i n g s and t o c h a r a c t e r i z e t h e s t r u c t u r e o f t h e alumina phase

formed.

F i g u r e 2 g i v e s t h e d i s t r i b u t i o n s c a l c u l a t e d f r o m X-ray s c a t t e r i n g d a t a o f HY ( c u r v e l ) , Deal 9-2 ( c u r v e 2 ) and Deal 9-2,ext

(curve 3). It i s noteworthy t h a t

t h e a v e r a g e T-0 d i s t a n c e s g i v e n b y t h e f i r s t peak on t h e

d i s t r i b u t i o n increase

0

f o r m 1.65 t o 1.67 A a f t e r t h e steaming t r e a t m e n t . T h i s can be a t t r i b u t e d t o t h e 0

f o r m a t i o n o f h e x a c o o r d i n a t e d A1 (1.92 t e t r a c o o r d i n a t e d A1 (1.77

w,

A,

T-0 d i s t a n c e s ) a t t h e expense o f

T-0 d i s t a n c e s ) . The a c i d t r e a t m e n t r e s u l t s i n

s m a l l e r T - 0 d i s t a n c e s ( c u r v e 3 ) which means t h a t p a r t o f t h e h e x a c o o r d i n a t e d A1 have been e x t r a c t e d so t h a t o n l y S i - 0 and f e w A l ( V 1 ) - 0 d i s t a n c e s r e m a i n . The o b s e r v e d average d i s t a n c e 1.62 that

the

zeolite

lattice

compares w e l l w i t h t h a t c a l c u l a t e d b y assuming comprises

192

SiO4

t e t r a h e d r a and

that

10

h e x a c o o r d i n a t e d A1 atoms a r e l e f t i n t h e cages. By s u b t r a c t i n g c u r v e 3 f r o m c u r v e 2 t h e i n t e r a t o m i c d i s t a n c e s p r e s e n t i n

Deal 9-2 t h e peak

and absent t r o m Deal 9-2,ext a t 1.92

A

appear as p o s i t i v e peaks ( c u r v e 4 ) . Thus,

corresponds t o Al(V1)-0

d i s t r i b u t i o n a l s o e x h i b i t s p e a k s a t 2.94,

3.30

distances.

The d i f f e r e n c e

and 4.92

which a r e q u i t e

and A l ( V I ) - A l ( V I )

distances found i n

s i m i l a r t o t h e Al(V1)-Al(VI),

Al(V1)-AT(1V)

t r a n s i t i o n aluminas ( r e f . 1 6 ) .

Therefore these r e s u l t s p r o v i d e d i r e c t e v i d e n c e

f o r t h e p r e s e n c e o f a l u m i n a - l i k e s p e c i e s i n Y z e o l i t e heated i n steam a t h i g h temperature. Alumina fragments a r e generated b y d e a l u m i n a t i o n o f t h e f r a m e w o r k , t h e y are probably

e n t r a p p e d i n t h e z e o l i t e c a g e s . On t h e o t h e r hand, l a r g e

amount o f alumina must be i s s u e d f r o m t h e d e s t r o y e d p a r t s o f t h e z e o l i t e w h i c h a c c o u n t f o r as much as 80% o f t h e p a r e n t z e o l i t e . These species can g a t h e r on t h e e x t e r n a l s u r f a c e o r i n t h e o u t e r l a y e r s o f t h e z e o l i t e c r y s t a l s . The s u r f a c e enrichment i n A1 observed b y XPS i n s t e a m - t r e a t e d z e o l i t e s ( r e f . 1 7 ) be e x p l a i n e d b y t h i s process.

could well

96

IYY

m

YY

.c

0

a

a

i z, -

F i ure 2

m

curve 2

-

-

R a d i a l e l e c t r o n d i s t r i b u t i o n o f z e o l i t e . 1 HY (NH4Y outgassed a t Deal 9-2 ; 3 Deal 9-2,ext ; 4 Difference distribution : curve 3.

-

-

Acidic properties The d i f f e r e n t i a l h e a t o f NH3 a d s o r p t i o n was measured on t h e d e a m n i a t e d NH4Y z e o l i t e , on a steamed z e o l i t e ( D e a l 6-3) and on a steamed and e x t r a c t e d z e o l i t e (Deal 10, e x t ) . The number o f a c i d s i t e s can be e s t i m a t e d b y assuming t h a t each s i t e i s n e u t r a l i z e d b y one amnonia molecule. The heat o f adsorption on

HY i s i n i t i a l l y w i t h i n 130-140 k J molee1 and decreases s l o w l y w i t h t h e amount o f NH3 adsorbed ( f i g u r e 3,

c u r v e 1). I t can b e c o n c l u d e d t h a t adsorption takes

place on Bronsted s i t e s o f homogeneous s t r e n g t h and d i s t r i b u t i o n . The a c i d s i t e s on Deal 6-3 are stronger than i n HY since t h e heat o f adsorption i s i n i t i a l l y as h i g h as 170 k J mole-I,

b u t decreases r a p i d l y t o 45 k J mole-l ; t h i s corresponds

t o 8 cm3 g - l NH3 coverage o r about 4 Ht/unit c e l l . These s t r o n g a c i d s i t e s are probably associated w i t h t h e aluminum t e t r a h e d r a remaining i n t h e l a t t i c e . The most s t r i k i n g r e s u l t i s t h e d i s a p p e a r a n c e o f t h e a c i d i t y i n Deal 10, e x t . ( c u r v e 3 ) . The h e a t o f a d s o r p t i o n d r o p s from 90 k J mole-1 t o 15 k J mole-1 as

1.6 cm3 o f NH3 i s adsorbed. T h i s means t h a t t h e r e i s s t a t i s t i c a l l y l e s s than one Bronsted s i t e per u n i t c e l l . Moreover t h e heat o f adsorption i s s m a l l e r t h a n o n t h e deammoniated NH4Y z e o l i t e o r on Deal 6,3. However t h i s could be an apparent v a l u e because t h e f i r s t NH3 increment, how small i t might be, c o u l d i n t e g r a t e a c i d s i t e s o f decreasing strengths.

91

1

150 c

e

*

I

20

0

40

80

(10

100

Ammonk covorw Icm3.g-'

F i g u r e 3.:

M i c r o c a l o r i m e t r i c measurements o f t h e d i f f e r e n t i a l heat o f adsorption

o f NH3 a t 150'C.

1

- HY

;2

-

Deal 6-3 ; 3

-

Deal l0,ext.

These conclusions are supported b y t h e i n f r a r e d study. F i g u r e 4 ( l e f t f i e l d ) gives t h e i n f r a r e d spectra o f t h e z e o l i t e s i n t h e region o f the stretching frequencies o f t h e hydroxyl groups. A l l t h e s p e c t r a a r e t a k e n a f t e r o u t g a s s i n g t h e z e o l i t e a t 200'C

u n d e r vacuum. Curve a corresponding t o t h e NH4Y z e o l i t e

deammoniated a t 2OO'C

e x h i b i t s t h e t w o bands a t 3540 and 3640 cm-l bhich are

a t t r i b u t e d t o t h e OH groups formed b y t h e a s s o c i a t i o n o f p r o t o n s w i t h l a t t i c e oxygen

anions

(Deal 9-2,ext)

(ref.18).

After

steaming

at

890'C

and a c i d e x t r a c t i o n

both bands vanish ( c u r v e b ) which i n d i c a t e s t h a t t h e B r o n s t e d

s i t e s a s s o c i a t e d w i t h t h e former l a t t i c e OH groups have almost disappeared. The band a t 3740 cm-l a t t r i b u t e d t o t e r m i n a l S i - O H groups developped i n d i c a t i n g t h a t t h e z e o l i t e has a l a r g e e x t e r n a l surface. T h i s i s i n agreement w i t h t h e e l e c t r o n m i c r o s c o p y and a d s o r p t i o n s t u d y showing t h a t a l a r g e number o f mesopores have been created. The a l m o s t complete disappearance o f Bronsted a c i d i t y i s confirmed b y t h e I R s t u d y u s i n g p y r i d i n e as p r o b e m o l e c u l e .

F i g u r e 4 ( r i g h t f i e l d ) shows t h e

spectrum o f t h e NH4Y z e o l i t e outgassed a t 2OO'C, and o u t g a s s e d a t 150'C.

contacted w i t h p y r i d i n e a t 25'C

The band a t 1548 cm-1 corresponds t o p y r i d i n i u m ions

c h a r a c t e r i s t i c f o r t h e p r e s e n c e o f B r o n s t e d a c i d i t y . The i n t e n s i t y o f t h e 1548 cm-1 band i n Deal 9-2,ext

i s reduced d r a s t i c a l l y i n d i c a t i n g t h a t Bronsted

s i t e s have been a l m o s t c o m p l e t e l y e l i m i n a t e d . The band n e a r

1450 cm-l i s

generally attributed t o pyridine i n t e r a c t i o n w i t h Lewis a c i d s i t e s . small f n t e n s i t y means t h a t t h e r e are also v e r y few Lewis s i t e s l e f t .

I t s very

98

trequencylcm

1600

1400

F i u r e 4 I n f r a r e d a b s o r p t i o n s p e c t r a o f deammoniated NH4Y ( c u r v e a ) and e x t ( c u r v e b ) . L e f t f i e l d : absorption bands o f h y d r o x y l groups a f t e r a c t i v a t i o n a t 2OO'C. R i g h t f i e l d : 1400-1700 cm-l r e g i o n a f t e r adsorption o f p y r i d i n e a t 25°C and outgassing a t 150°C.

b,

Cat a1y t i c a c i t iv i t 1 The c a t a l y t i c a c t i v i t i e s o f t h e h i g h s i l i c a f a u j a s i t e s (Si/A1?19) embedded i n a k a o l i n m a t r i x h a v e been measured

on a vacuum d i s t i l l a t e f e e d s t o c k .

The

c o n v e r s i o n i s l o w e s p e c i a l l y i f t h e a c t i v i t y o f t h e m a t r i x p e r se i s t a k e n i n t o a c c o u n t . The s e l e c t i v i t y i n g a s o l i n e w i t h r e s p e c t t o t h e t o t a l (gas + coke + g a s o l i n e ) i s h i g h (70%) because o f l o w coke y i e l d s . S i m i l a r o b s e r v a t i o n s were r e p o r t e d b y Scherzer and R i t t e r (ref.19).

The c r a c k i n g a c t i v i t y p a t t e r n i s v e r y

d i f f e r e n t from t h a t o f r a r e earth zeolites o r o f l e s s dealuminated f a u j a s i t e s because t h e r e are t o o f e w a c i d s i t e s present i n t h e s o l i d as shown p r e v i o u s l y . CONCLUSIONS

-

T r e a t m e n t s a t 9OO'C

u n d e r 100% steam of l o w sodium f a u j a s i t e s i n i t i a t e a

r e c r y s t a l l i s a t i o n process whereby a l l t h e A1 atoms are r e p l a c e d b y S i atoms i n t h e t e t r a h e d r a l s i t e s . T h i s i s evidenced b y t h e u n i t c e l l shrinkage t o 24.18 b y t h e average T-0 d i s t a n c e o f 1.60

k

i,

c h a r a c t e r i s t i c o f p u r e s i l i c a and b y t h e

i n v a r i a n c e o f t h e occupancy f a c t o r s o f t h e t e t r a h e d r a l s i t e s . L a r g e domains o f t h e z e o l i t e c r y s t a l s h a v e been d e s t r o y e d d u r i n g t h e

-

process y i e l d i n g

-

mesopores throughout a well ordered s i l i c a framework.

The alumina atoms e x t r a c t e d from t h e l a t t i c e are h e x a c o o r d i n a t e d and f o r m

a l u m i n a - l i k e f r a g m e n t s . T h i s i s evidenced b y t h e r a d i a l e l e c t r o n d i s t r i b u t i o n showing Al(V1)-0 and Al(V1)-Al(V1)

d i s t a n c e s . However a l u m i n a s h o u l d a l s o b e

issued f r o m t h e destroyed p a r t s o f t h e c r y s t a l s .

99

-

The e x t r a c t i o n o f a l u m i n a i n a c i d s o l u t i o n y i e l d s h i g h s i l i c a f a u j a s i t e s

w i t h Si/A1 r a t i o s as h i g h as 78. The v e r y h i g h t h e r m a l and a c i d r e s i s t a n c e o f t h e s e m a t e r i a l s are o b s v i o u s l y due t o t h e p u r e s i l i c a framework.

-

Only very few acid s i t e s

a r e l e f t on h i g h s i l i c a f a u j a s i t e s . T h i s accounts

f o r t h e i r l o w c a t a l y t i c a c i t i v i t y i n cracking reactions. Clearly, i n order t o o p t i m i z e c r a c k i n g c a t a l y s t s a compromise s h o u l d b e made t o l e a v e enough a c i d s i t e s w i t h o u t d e c r e a s i n g t o o much t h e t h e r m a l s t a b i l i t y ,

and t h e g a s o l i n e

selectivity. REFERENCES

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

18 19

C.V. Mc D a n i e l and P.K. Maher, M o l e c u l a r S i e v e s , S o c i e t y o f Chemical I n d u s t r y , London, 1967, p.186. G.T. K e r r , Adv. Chem. Ser. 121, American Chemical SOC., Washington, 1973, p.219. P.K. Maher, F.D. H u n t e r and J. S c h e r z e r , Adv. Chem. Ser. 101, American Chemical SOC., Washington, 1971, p.226. J. S c h e r z e r , i n T.E. Whyte J r . e t a1 ( E d s ) , C a t a l y t i c M a t e r i a l s R e l a t i o n s h i p betrreen S t r u c t u r e and r e a c t i v i t y , ACS Symp. Ser. 248, American Chemical SOC., 1984, p.157, D.W. B r e c k and G.W. Skeels, M o l e c u l a r Sieves 11, ACS Symposium S e r i e s 40, American Chemical SOC., Washington, 1977, p.271. J. Scherzer, 3. Catal., 5 4 (1978) 285. V. Bosacek, V. Patzelova, Z. Tvaruzkova, D. Freude, V. Lohse, W. S c h i r m e r , H. Stach and H. Thamm, J. Catal., 61 (1980) 435. P. G a l l e z o t , R. Beaumont and D. Barthomeuf, J. Phys. Chem., 78 (1974) 1550. A.J. Leonard and P. Ratnasamy, C a t a l . Rev. - S c i . Eng., 6 (1972) 293. P. G a l l e z o t , A. Bienenstock and M. Boudart, Nouv. J. Chim., 2 (1978) 263. A. Auroux, P. Wierzchowski and P.C. G r a v e l l e , Thermochimica Acta, 32 (1979) 165. Standard method f o r t e s t i n g f l u i d c r a c k i n g c a t a l y s t s b y m i c r o a c t i v i t y t e s t , ASTM D 3907-80. F. Maug6, J.C. C o u r c e l l e , Ph. Engelhard, P. G a l l e z o t , J. Grosmangin and M. P r i m e t , t o be p u b l i s h e d . R.L. P a t t o n , E.M. F l a n i n g e n , L.G. Dowel1 and D.E. Passoja, i n J.R. K a t z e r (ed.), M o l e c u l a r S i e v e s 11, ACS Symposium S e r i e s 40, A m e r i c a n C h e m i c a l Societv. 1977._ D. . 64. J. K l i n o w s k i , J.M. Thomas, C.A. F y f e and G.C. Gobbi, Nature, 296 (1982) 533. A.J. Leonard, F. Van Cauwelaert and J.J. F r i p i a t , J. Phys. Chem., 71 (1967) 695. Th. Gross, V . Lohse, G. E n g e l h a r d t , K.H. R i c h t e r and V . P a t z e l o v a , Z e o l i t e s , 4 (1984) 25. J.W. Ward, Adv. Chem. Ser. 101, American Chemical Sot., 1971, p. 380. J. Scherzer and R.E. R i t t e r , I n d e t Eng. Chem., Prod. Res. Dev., 1 7 (19781, 219.


101

B. Imelik e f al. (Editors), Catalysis b y Acids and Bases 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

ACIDITY IN ZEOLITES

A.G. Ashton2, 5. Batmanian F.J. Machado 4

1, D.M. Clark', J. Dwyerl, F.R. Fitch3, A. Hinchcliffe 1 and

I Department of Chemistry, UMIST, P.O. Box 88, Manchester, M60 IQD, UK

* BP R e s e a r c h C e n t r e , Sunbury-on-Thames,

Middlesex, UK

L a p o r t e Industries, Widnes, Cheshire, UK (from 1.10.84) University of C a r a c a s , Venezuela

ABSTRACT T h e acidity of f a u j a s i t i c z e o l i t e s as defined by c a t a l y t i c measurements is discussed in t e r m s of a structure/composition p a r a m e t e r a s s o c i a t e d with t h e distribution of aluminiums i n t h e framework. C h a n g e s in t h e a c i d i t y a n d c a t a l y t i c a c t i v i t y of H-ZSM-5 cons e q u e n t upon hydrothermal t r e a t m e n t a r e examined. C a t a l y s t s a r e c h a r a c t e r i s e d by sorption measurements, magic angle spinning NMR, s u r f a c e analysis and by t e m p e r a t u r e programmed desorption of ammonia. Activity is s e e n to depend on an a p p r o p r i a t e bala n c e b e t w e e n non-framework and framework aluminium.

INTRODUCTION Acidity in z e o l i t e s a r i s e s from t h e bridging a n d terminal hydroxyls which provide t h e B r b s t e d s i t e s and from Lewis s i t e s which c a n involve c a t i o n i c s p e c i e s b u t a r e not, in a l l cases, well defined.

Additionally B r b s t e d s i t e s of enhanced a c t i v i t y , sometimes

c a l l e d superacid s i t e s have been proposed (ref.1).

I t is well known t h a t properties,

including a c i d i c properties, a r e r e l a t e d to s t r u c t u r e and composition (ref.2), and s e v e r a l t h e o r e t i c a l approaches h a v e been advanced to explain experimental observations. Z e o l i t e properties have b e e n c o r r e l a t e d with i n t e r m e d i a t e e l e c t r o n e g a t i v i t y (ref.31, pseudo-electric

field

parameters

(ref.4),

aluminium coordination p a r a m e t e r s (refs.6,7).

orbital

electronegativities

(ref.5)

and

Additionally, empirical a p p r o a c h e s have

been made (ref.8) and M.O. theory has been utilised (ref.9). More recently discussion of a c i d i t y has involved both s t r u c t u r a l , chemical and quantum chemical considerations (ref.10). I n p r a c t i c e t h e main i n t e r e s t in z e o l i t e a c i d i t y is in r e g a r d to catalysis, and by using 'test' r e a c t i o n s d i r e c t c o r r e l a t i o n s b e t w e e n ' c a t a l y t i c acidity' and composition c a n b e established.

Considerable success has been achieved in c o r r e l a t i o n of c a t a l y t i c

a c t i v i t y and composition via a single p a r a m e t e r , t h e i n t e r m e d i a t e e l e c t r o n e g a t i v i t y (ref.11).

However, this p a r t i c u l a r p a r a m e t e r may n o t b e a d e q u a t e at very low levels of

102

aluminium (ref.12) and since e l e c t r o n e g a t i v i t y is determined by composition it c a n n o t a c c o u n t for properties which a r e structurally dependent. A previous paper (ref.7) by t h e authors considered t h e correlation of k i n e t i c p a r a m e t e r s for cyclopropane isomerisation with t h e composition of f a u j a s i t i c zeolites. In this previous work composition was varied by s e v e r a l dealumination procedures including EDTA e x t r a c t i o n and steam/acid leaching, and a framework coordination p a r a m e t e r (PI was suggested as a basis for expressing e f f e c t i v e acidity. In t h e present paper this approach is e x t e n d e d to make use of framework coordination p a r a m e t e r s c a l c u l a t e d for s e v e r a l s t r u c t u r a l models (ref.l3), and t h e effect of non-framework aluminium in g e n e r a t i n g s i t e s of enhanced a c t i v i t y in H-ZSM-5 is examined.

EXPERIMENTAL S y n t h e t i c faujasitic z e o l i t e s w e r e provided by L a p o r t e Industries L t d (Widnes) and Dealumination of z e o l i t e Y utilised EDTA

H-ZSM-5 by BP R e s e a r c h C e n t r e , Sunbury.

(ref.14) and hydrothermal t r e a t m e n t of H-ZSM-5 involved h e a t i n g t h e c a t a l y s t slowly, under nitrogen flow, to 600 "C, holding for 2-3 hours, and t h e n admitting s t e a m at fixe d partial p r e s s u r e for 2.5 hours at 600 OC. Magic-angle-spinning NMR (MASNMR) were kindly provided by BP R e s e a r c h C e n t r e .

S u r f a c e analysis by fast atom bombardment

mass s p e c t r o m e t r y (FABMS) utilised procedures described previously (refs.15,16) and a n e l e c t r o n i c microbalance and a q u a r t z spring balance were used for sorption measurements.

T e m p e r a t u r e programmed desorption of ammonia (TPD) involved a t e m p e r a t u r e

programmed f u r n a c e with mass s p e c t r o m e t r i c analysis of desorbed gases.

Chemical

composition of t h e zeolites was determined by w e t analysis, by X R F and by e l e c t r o n microprobe. C a t a l y t i c results utilised a pulsed microreactor to d e t e r m i n e s u r f a c e a c t i v a t i o n e n e r g i e s in cyclopropane isomerisation and in ortho-xylene isomerisation, and a simple flow r e a c t o r was used for n-hexane cracking. RESULTS AND DfSCUSSION Acidity a n d f r a m e w o r k composition In f a u j a s i t i c z e o l i t e s a l l t e t r a h e d r a l positions (Fig.1) a r e topologically identical forming p a r t of t h r e e 4-rings, t w o 6-rings and one 12-ring. distinct first-neighbour

Each t e t r a h e d r o n has four

t e t r a h e d r a and nine distinct second neighbour t e t r a h e d r a .

Because of Loewnstein's rule t h e f i r s t t e t r a h e d r a l neighbours of a given aluminium must be silicon a t o m s whereas t h e second neighbours may b e e i t h e r aluminium o r silicon. However t h e nine second neighbour t e t r a h e d r a l a t o m s a r e not equidistant from t h e given aluminium.

The t h r e e second neighbours in t h e t h r e e four rings a r e closer

(w4.4A) t h a n t h e remaining six ( ~ 5 . 5 A ) . The f r a c t i o n a l occupancy by aluminium of o n e

of t h e t h r e e second coordination t e t r a h e d r a l a t o m s in t h e t h r e e 4-rings (P) and t h e

103 f r a c t i o n a l occupancy by aluminium of one of t h e nine second coordination t e t r a h e d r a l a t o m s (Q) has been c a l c u l a t e d for a wide range of compositions (Si/Al) and for s e v e r a l models (ref.13).

These p a r a m e t e r s r e f l e c t t h e self shielding of an aluminium s i t e by

neighbouring aluminiums and hence provide a measure of e f f e c t i v e acidity (refs.6a,7). In previous work (ref.7) t h e s u r f a c e a c t i v a t i o n energy for cyclopropane isomerisat i o n was used as a measure of e f f e c t i v e acidity.

Accepting t h e view t h a t stabilisation

of a carbenium ion is enhanced by stronger a c t i v i t y (refs.7,16,17,18),

t h e n application

of t h e c o n c e p t of linear f r e e energy relationships allows for estimation of this stabilisat i o n from measurements of s u r f a c e a c t i v a t i o n energies. In previous work t h e s u r f a c e a c t i v a t i o n e n e r g i e s for cyclopropane isomerisation w e r e c o r r e l a t e d with single composition parameters, aluminium f r a c t i o n and intermedi-

ate e l e c t r o n e g a t i v i t y , and i t was suggested t h a t t h e p a r a m e t e r 'PI might b e applicable (ref.7). However, in ?his previous work, d a t a w e r e wide ranging and included results from several sources o n z e o l i t e s dealuminated by various methods. In t h e p r e s e n t work results, determined by a single o p e r a t o r , a r e confined mainly to s y n t h e t i c faujasites, in t h e Na/H form, provided by t h e same manufacturer ( L a p o r t e Industries Ltd.). results for faujasitic zeolites daluminated using EDTA a r e included.

Some

The EDTA process

is used since t h e resulting c a t a l y s t s do not r e t a i n aluminium dislodged during t h e dealumination which c a n modify c a t a l y t i c properties as discussed subsequently. sequently b e t t e r c o r r e l a t i o n of

Con-

c a t a l y t i c and bulk properties is to be e x p e c t e d .

However, it is now c l e a r (ref.15) t h a t t r e a t m e n t with EDTA c a n cause composition g r a d i e n t s (and secondary p o r e systems (refs.19,20)) but t h e c a t a l y t i c consequences of this a r e not y e t evaluated. Fig.2a shows plots of s u r f a c e a c t i v a t i o n energy f o r cyclopropane isomerisation a g a i n s t (I-P) c a l c u l a t e d for ordered s t r u c t u r e s as proposed by Englehardt (ref.21), and Fig.2b shows t h e same d a t a plotted against t h e Sanderson electronegativity. I t c a n b e concluded, t h e r e f o r e , t h a t t h e s t r u c t u r a l l y determined p a r a m e t e r s

'PI

(and also IQI) do provide a basis for defining e f f e c t i v e acidity in zeolites. Their sensit i v i t y to s t r u c t u r a l changes allows g r e a t e r scope t h a n is avaiiable for p a r a m e t e r s d e p e n d e n t only upon composition, for example i n t e r m e d i a t e electronegativity. However, no a c c o u n t c a n be t a k e n of c a t i o n s or o t h e r non-framework species, as is t h e case with e l e c t r o n e g a t i v i t y , so t h a t c o r r e l a t i o n s with p a r a m e t e r s 'P' and 'Q' should be improved by considering homoionic zeolites.

In t h e present work all t h e z e o l i t e s were in t h e

Na/H forms but t h e r a t i o N a / H was not c o n s t a n t and this was presumably r e f l e c t e d in residual variation associated with t h e linear correlations.

Q u a l i t a t i v e examination of

linear f i t s such as t h o s e shown in Fig.2 (i.e. consideration of residuals) suggests t h a t a b e t t e r f i t is obtained using a model based on o r d e r e d (ref.21) r a t h e r than on random distribution of aluminiums.

A f u r t h e r improvement in linear f i t is observed when a

mixed model, which assumes ordered s t r u c t u r e s at higher aluminium c o n c e n t r a t i o n s and a random distribution at lower concentrations, is used. The improved f i t with t h e mixed model suggests t h a t o r d e r e d s t r u c t u r e s may be increasingly favoured at Al/uc

>

56, in

104

Fig.1. Faujasite structure showing first (a,b,c,d) and second (1 to 9)coordination spheres of a framework'T'atom.

-L.

1 I && 0.0

100

Parent NHo-ZSH-5

Ff

Y k

80

activation energies. Pulsed microreactor. NaHY zeolites. 170

160

2

i -60

1"

o lbo

too SCALE ppn REFERENCE TO lAllHlO#&

Fig.4.27AI MASNMR of H-ZSM-5 steamed at 60O0C for 2.5 hours. 910 90

110

-

12: E/kJ mo1ISOMERISATION OF CYCLOPROPANE SURFACE ACTIVATION ENERGY

Fig.3.

100

0

parent NaHY(Si/AI=255). NaHY dealuminated using EDTA. isomerisation of cyclopropane over faujasitic zeolites. osynthetic. dealuminated (EDTA).

' L

0

E

h

c

w

I-P S Fig.2. Dependence of surface activation energy on (a) parameter pfordered structures). (b) Sanderson electronegwty.

105 a g r e e m e n t with a previous suggestion (ref.13). R e s u l t s in Fig.2, and previous results (ref.7) r e f e r to isomerisation of cyclopropane, which is s e l e c t e d b e c a u s e i t is a f a c i l e r e a c t i o n and is consequently less likely to be dependent o n t h e e x i s t e n c e of small numbers of highly acltive s i t e s and t h e r e f o r e more likely to r e f l e c t changes i n t h e bulk properties of t h e zeolite. I t is of i n t e r e s t to know whether conclusions based on cyclopropane isomerisation also hold for more demanding reactions. Fig.3 shows t h e c o r r e l a t i o n of s u r f a c e a c t i v a t i o n energies f o r isomerisation of cyclopropane and t h e more demanding isomerisation of ortho-xylene, and implies t h a t , for t h e present c a t a l y s t s , t h e conclusions hold more generally. R e s u l t s shown in Figs.2 and 3 r e f e r to c h a n g e s in composition g e n e r a t e d e i t h e r during synthesis or by post synthesis dealumination using EDTA. Zeolites so p r e p a r e d do not have aluminium present in non-framework postitions and only one signal, assigned

to t e t r a h e d r a l framework aluminium, is s e e n in t h e 27Al MASNMR spectra. Conversely, during hydrothermal t r e a t m e n t , which is used to e n h a n c e t h e stability and a c t i v i t y of z e o l i t e c a t a l y s t s , aluminium dislodged from t h e framework may be retained.

However,

t h e c a t a l y t i c consequences of hydrothermal t r e a t m e n t a r e n o t completely understood. In this study, H-ZSM-5 was hydrothermally t r e a t e d at 600 "C using a nitrogen s t r e a m containing fixed partial pressures of w a t e r ( b e t w e e n 0 and 357 rnmHg). Increase d s t e a m pressures resulted in increased dislodgement of aluminium from t h e framework

as revealed by 27Al MASNMR.

In t h e s t e a m e d z e o l i t e s peaks at 0.0 ppm (ref.

AI(H 0 ) 3+), corresponding to o c t a h e d r a l aluminium, and at 50 ppm, assigned to low2 6 symmetry 'polymeric' aluminohydroxyl species, a r e p r e s e n t i n addition t o t h e peak at 53 pprn d u e to framework t e t r a h e d r a l aluminium. T h e i n c r e a s e in t h e amount of dislodged aluminium with increased severity of steaming is s e e n clearly in Fig. 4. The major p a r t of t h i s non-framework aluminium, at higher values of PH20 is associated with a broad band, around 50 ppm, assigned to lower-symmetry polyaluminohydroxyl species which presumably a r i s e from condensation and agglomeration of aluminium species dislodged from t h e framework by steam.

Fig. 5 shows results of s u r f a c e analysis using FABMS.

These, and previous results (refs.15,16,22)

show increased s u r f a c e enrichment of

aluminium with increased severity of steaming d u e to migration of dislodged aluminium s p e c i e s (refs.15,16,22).

Sorption measurements at room t e m p e r a t u r e show virtually no

c h a n g e in sorption c a p a c i t y for n-hexane and only a slight change in c a p a c i t y for p-xylene (ref.23). C a t a l y t i c studies of n-hexane cracking at 285 "C show t h a t initial r a t e s pass through a maximum (Fig.6).

Only one previous r e p o r t (ref.24) appears to r e p o r t this

p a t t e r n and gives no explanation for it. Since in H-ZSM-5 steaming dislodges aluminium from t h e framework and c a t a l y t i c a c t i v i t y in n-hexane d e c r e a s e in framework aluminium (refs.12,25),

cracking d e c r e a s e s with

an increased a c t i v i t y is unexpected.

T h e s e results t h e r e f o r e strongly imply t h a t dislodged aluminium species c a n play a role in n-hexane cracking.

Increased c a t a l y t i c a c t i v i t y may involve synergism b e t w e e n

framework Brbnsted s i t e s and dislodged aluminium species (ref.26) or i t may involve

106

Fig.5. FABMS profiles of steamed HZSM-5.

200

400

600 800 TEMPERATUREI'C

Fig.7.TPD of NH3 .Steam treated HZSMS W A I = 19) NH,-form. xxxxx60mm steam. ............ cbmmsteam.---. 100mm steam. ___-__40mm steam.-.---- W m m steam. *

Fig.6. n-hexane cracking Over HZSM-5 steamed at 600°C for 25hours.

AMOUNT OF16MASS PEAK AT 440°C (arb units

Fig.8. Rate of n-hexane cracking (at 285OC) over steamed H-ZSM-5 (steam treatment at 600°C for 2.5 hours).

107 generation of additional a c t i v e sites unconnected with framework Bryhsted sites. However, synergistic aluminium species (Lewis s i t e s ) c a n provide a possible explanation

H

f o r t h e observed results (Fig. 6).

Since increased s e v e r i t y of steaming increases t h e

amount of dislodged aluminium at t h e e x p e n s e of framework aluminium an optimum balance b e t w e e n framework and non-framework aluminium is to be expected.

In t h e

p r e s e n t study t h e optimal a c t i v i t y o c c u r s when t h e s t e a m pressure is around 60 mmHg w h e r e t h e r a t i o of non-framework to framework aluminium is close to unity.

If, f o r

example, e a c h aluminium dislodged under t h e conditions used provided one Lewis s i t e this would suggest a one-to-one correspondence of Brbnsted and Lewis s i t e s f o r optimum a c t i v i t y in H-ZSM-5.

However, results for f a u j a s i t i c zeolites where dislodged

aluminium may be distributed b e t w e e n both small and l a r g e c a g e s suggest t h a t t h e optimum distribution of aluminium between framework and non-framework s i t e s is not unity (ref.27) so t h a t s t r u c t u r a l considerations c a n n o t b e neglected.

For H-ZSM-5 at higher

p a r t i a l pressures of s t e a m (P H 0 > 100 mmHg), a c t i v i t y d e c r e a s e s and this coincides 2 (Fig. 5) with t h e a p p e a r e n c e of significant amounts of aluminium on t h e e x t e r n a l surf a c e (ref.22) due to migration during steaming (refs.15,15,22).

The consequences of

migration a r e not y e t e v a l u a t e d but i t could clearly r e d u c e activity by d e s t r u c t i o n of a n y synergism or perhaps i n some instances might "free" s i t e s blocked by aluminohydroxy s p e c i e s resulting in increased activity. Accumulation of aluminum at outer-surfaces would clearly modify o u t e r s u r f a c e sites. In order to d e m o n s t r a t e t h a t c a t a l y t i c results a r e dependent on acidity, temperat u r e programmed desorption of ammonia (TPD)was examined. Results showed a progressive reduction in t h e main desorption preaks for relatively weakly sorbed ammonia ( ~ 1 5 0 ' C ) and for strongly sorbed ammonia b 3 4 O o C ) with progressive i n c r e a s e in s e v e r i t y of steaming (Fig. 7). However t h e portion of t h e TPD c u r v e above 400°C is of p a r t i c u l a r i n t e r e s t since t h e amount of ammonia r e t a i n e d between 400°C and 6OO0C, o r t h e amount r e t a i n e d at 4OOOC is in linear c o r r e l a t i o n with t h e initial c a t a l y t i c a c t i v i t y (Fig. 8) in a g r e e m e n t with t h e production of s i t e s of enhanced activity by g e n e r a t i o n of an optimum distribution of aluminum between framework and non-framework positions.

Fig. 6 shows c l e a r e v i d e n c e of d e a c t i v a t i o n with increased time-on-stream

and

implies t h a t t h e more a c t i v e s i t e s a r e t h e more readily d e a c t i v a t e d presumably by c o k e deposition. However, i t is unlikely t h a t t h e p a t t e r n for results at o n e minute o n s t r e a m

108 is significantly a f f e c t e d by d e a c t i v a t i o n since results at 1 minute correspond to t h o s e using a pulsed r e a c t o r containing fresh c a t a l y s t (ref.28). S e p a r a t e work (ref.22) showed a similar p a t t e r n for c a t a l y t i c conversion of toluene at 600°C o v e r t h e same catalysts.

T h e s e results (ref.22) were based on analysis at I5

minutes on s t r e a m and values e x t r a p o l a t e d t o z e r o time.

P u l s e studies (ref.28) confirm

t h e observed maximum in r a t e . However t h e pulsed studies w e r e made over a r a n g e of t e m p e r a t u r e s from 350 to 550 "C.

No d e a c t i v a t i o n was observed up to 20 pulses and

a p p a r e n t a c t i v a t i o n energies were obtained by measurements a t randomised t e m p e r a t u r e levels.

A simple f i r s t order model was used for t h e s e relatively low-conversion d a t a .

R e s u l t s i n d i c a t e t h a t a p p a r e n t a c t i v a t i o n energies, which r e f l e c t both sorption energet i c s and s u r f a c e r a t e s , d e c r e a s e with increased amounts of dislodged aluminium, as do pre-experimental f a c t o r s . Consequently t h e d e t a i l of t h e p a t t e r n for r e a c t i o n r a t e as a function of P H 2 0 or r a t i o of dislodged to framework aluminium depends somewhat o n t h e t e m p e r a t u r e chosen for study.

However, for toluene disproportionation over t h e

r a n g e 350-550 OC t h e maximum is in t h e r a n g e of PH20 b e t w e e n 40 and 100 mmHg (corresponding to a r a t i o of non-framework to framework aluminium b e t w e e n 0.4 and 1.4).

T h e s e results a r e consistent with a model based on development of more a c i d i c

s i t e s by i n t e r a c t i o n b e t w e e n framework hydroxyls and non-framework aluminium species b u t t h e p i c t u r e is complicated by possible changes in sorption energetics.

Note t h a t

t h e p a t t e r n s obtained c a n n o t b e explained by significant c h a n g e s in micropore volume s i n c e sorption c a p a c i t i e s at room t e m p e r a t u r e a r e hardly a l t e r e d by t h e steaming process (ref.23).

Additionally i t is c l e a r t h a t t h e optimal steaming regime for a given

z e o l i t e c a t a l y s t depends upon t h e s t r u c t u r e and composition of t h e p a r e n t z e o l i t e and on t h e p a r t i c u l a r r e a c t i o n conditions.

REFERENCES P.A. Jacobs, H.E. Leeman and J.B. Uytterhoeven, J.Catalysis, 33 (1974) 17-30. 8 3 (1979) 249-256. W.J. Mortier, J.Catalysis, 5 5 (1978) 138-45. N-Y. Topsoe, K. Pederson and E.G. Derouane, J.Catalysis, 7 0 (1981) 41. Kyoung Tai No, Hakze Chon, Talkyue R e e and Mu Shik Jhon, J.Phys.Chem., 85 2065. 6 (a) E. Dempsey, J.Catalysis, 33 (1974) 497-9; idem, ibid, 39 (1975) 155-7. (b) R.J. Mikowsky and J.F. Marshall, %Catalysis, 44 (1976) 170-3; R.J. Mikowsky, J.F. Marshall and W.P. Burgess, ibid, 58 (1979) 489-92. 7 S.H. Abbas, T.K. Al-Dawood, J. Dwyer, F.R. F i t c h , A. Georgopoulos, F.J. Machado and S.M. Smyth, "Catalysis by Zeolites" (Ed. B. Imelik et al.) Elsevier (Amsterdam) (1980) 127-134. 8 R. Beaumont and D. Barthomeuf, %Catalysis, 26 (1972) 218. 9 (a) V.B. Kazanskii, in "Proc 4th. Nat.Symp.Catalysis", Ind.Inst.Technol., Bombay (1948) 14. (b) W.J. Mortier, P. Geerlings, C. van Alsenoy and H.P. Figeys, J.Phys.Chern., 83 (1979) 855-61. 10 W.J. Mortier, in "Proc. 6 t h 1nt.Zeolite Conf.", R e n o Nevada (1983) in print. I1 P.A. Jacobs, Cataly.Rev.Sci.Eng., 24 (1982) 415.

D. Barthomeuf, J.Phys.Chem.,

109 12 J. Dwyer, F.R. F i t c h and E. Nkang, J.Phys.Chem., 87 (1983) 5402-5404. 13 B. Beagley, J. Dwyer, F.R. F i t c h , R. Mann and J. Walters, J.Phys.Chem., 88 (1984) 1744-1751. 14 G.T. Kerr, A. C h e s t e r and D. Olson, A c t a Phys.Chem., 24 (1978) 169. 15 J. Dwyer, F.R. F i t c h , G. Qin and J.C. Vickerman, J.Phys.Chem., 86 (1982) 4574-4578. 16 A.G. Ashton, J. Dwyer, 1.S. Elliott, F.R. F i t c h , G. Qin, M. Greenwood and J. Speakman, in "Proc. 6 t h Int. Zeolite Conf." R e n o Nevada (1983) in print. 17 M. Zhavoronkov, Kin.Katal., 24 (1973) 3322-7. 18 J. Dwyer, Chemistry and Industry, 7 (1984) 258-269. 19 F.J. Machado, Ph.D. Thesis, UMIST, 1983. 20 U. Lohse and Mildenbrath, Z.Anorg.Allg.Chem., 476 (1981) 126-135. 21 G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak and M. Magi, Z.Anorg.Allg.Chem., 482 (1982) 49. 22 A.G. Ashton, S. Batmanian, J. Dwyer, I.S. Elliott and F.R. F i t c h , in "Proc. 9 t h Canad.Symp. on Catalysis", Quebec C a n a d a (1984). To b e published. 23 S. Batmanian, 3. Dwyer and F.R. F i t c h , unpublished work. 24 W.O. Haag, R.M. Lago, E.P. 0 034 444 (1981). 25 D.H. Olson, W.O. Haag and R.M. Lago, J.Catal., 61 (1980) 390. 26 C. Mirodatos and D. Barthomeuf, J.C.S., Chem.Commun, (1981) 29-40. 27 I.S. Elliott, Ph.D. Thesis, to be submitted, UMIST 1984. 28 S. Batmanian, J. Dwyer and F.R. F i t c h , unpublished work.


B. Imelik et al. (Editors), Catalysis b y Acids and Bases @

111

1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

ACIDIC AND BASIC PROPERTIES OF ALUMINAS IN RELATION TO THEIR PROPERTIES AS CATALYSTS AND SUPPORTS

H. K N ~ Z I N G E R I n s t i t u t f u r Physikalische Chemie, Universitat Miinchen, Sophienstr. 11 8000 MUnchen 2 , FRG.

ABSTRACT In this paper structural aspects of transitional aluminas and surface struct u r e models are briefly reviewed. Acidic and basic surface properties of aluminas can be interpreted on the basis of surface groups suggested by structure models. When aluminas are used as supports, t h e i r acidic and basic properties play a decisive role in the i n i t i a l interaction w i t h catalyst precursors. For impregnation from aqueous solutions the isoelectric point IEPS which i s a t pH7-9 f o r aluminas , determines the optimal conditions f o r adsorption of ionic precursor species. This i s exemplified f o r the adsorption of molybdates and Na' i o n s , and f o r anionic and cationic Pt-complexes. The use of organometallic compounds such as Mo(v-C3H )4, Mo(C0) , and carbonyl clusters i s interesting, since t h e i r reactions w i f h alumina $emonstrate the wide range of polyfunctionality of alumina surfaces. The importance of acidic and basic properties in relation t o c a t a l y t i c behaviour of aluminas i s discussed f o r a few model reactions.The two precesses of industrial importance which use aluminas as the c a t a l y s t , are the Claus reaction and COS hydrolysis.

RESUME Dans c e t a r t i c l e on donne u n resume bref de l a constitution des alumines de transition e t des modeles de structure de leur surface. Les modeles de structure proposes permettent egalement de f a i r e des speculations sur l a nature des groupes 8 la surface e t de leur comportement s o i t acide ou bastque. Les proprietes acido-basique des alumines utilis@cscomme support jouent un rdle decisif dans l ' i n t e r a c t i o n i n i t i a l e du precurseur avec le catalyseur. Quand l'impregnation s e f a i t 8 p a r t i r d'une solution aqueuse l e point iso@lectrique IEPS ( q u i s e trouve entre pH7-9 pour les alumines) determine les conditions optimales d'adsorption des precurseurs ioniques. Ceci e s t evident pour T'adsorption des molybdates, d e s ions Na+ e t pour les formes anioniques e t cationiques des complexes de P t . L'etude des complexes organom&talliques, comme par exemple Mo(v -C3H5)4, Mo(C0)6 e t des clusters carbonyles, e s t aussi interessante car les reactions entre ceux-ci e t l'alumine demontrent l a t r e s grande game de comportement polyfonctionnel des surfaces des a1 umi nes. Les exemples c i t e s dans c e t a r t i c l e permettent de preciser dans quelques cas exemplaires 1 'importance des proprietes acido-basiques en ce q u i concerne l e comportement catalytique des a1 umines. Deux exemples de procedes d'importance i n d u s t r i e l l e qui u t i l i s e n t des alumines comme catalyseurs sont la reaction de Claus e t l'hydrolyse COS.

112 1. INTRODUCTION Transitional aluminas, namely

q - and

g-Al2O3, are used i n industrial processes

primarily as catalyst supports, whereas only few processed apply aluminas as the catalyst. Typical processes in which aluminas are used as catalyst supports are hydrotreatment andcatalytic reforming; in terms of catalyst volume or weight the petroleum industry appears t o be the biggest user of alumina-supported catalysts. The market for US manufacturers of HDS (hydrodesulfurization) catalysts i n 1978 was estimated t o be in the 10,000 t o 12,000 tons/year range (ref. 1). This increased t o 16,000 tons/year in 1980 and i s predicted t o double again until 1986 (ref. 2). The estimate for the U S market in 1980 for catalytic reforming was 4,000 tons/year (ref. 2). These figures may serve as an indication of the enormous importance of aluminas in industrial catalysis. This importance of alumina is due to various factors. Aluminas are easily available i n large quantities and i n high purity. They are thermally very stable and develop

2

reasonable surface areas in the 100 t o 250 m /g range. Pore volumes can be controlled during fabrication and bimodal pore size distributions can be achieved. However, besides these textural aspects, the surface chemical properties of aluminas play a major role, since these are involved i n the formation and stabilization of catalytw cally active components supported on their surfaces. Needless t o say that the surface chemical properties determine the catalytic properties of pure catalytic aluminas or of bifunctional catalysts such as R / A I 0 reforming catalysts. 2 3 Aluminas are amphoteric, hence, they possess acidic and basic properties which are controlled by the surface groups or ions which terminate the microcrystallites. The acidic and basic properties of these materials can be modified by the heat treatment conditions and by incorporating additives, such a s e.g.

halogen or alkali. It is the aim

of this paper t o relate these surface properties t o the nature of the interaction of precursors in the preparation of supported catalysts and t o catalytic properties of pure aluminas.

2. STRUCTURE

OF

ALUMINAS

The transitional aluminas ?-and

r - A l 2 O 3 are formed during thermal dehydration

of bayerite and boehmite, respectively (ref. 3,4), according to the following transformat ions:

boehmite bayerite

- - 720 K 500 K

r-A1 0

1000 K

6-AI2O3

1270 K

1470 K

8 + a - A l 0 3-

?-Al2O3 1 1 2 0 K 0-A1203 1470 K =-A1

x-A1203

0

2 3

The structural characteristics of the various alumina phases have been described by Lippens and de Boer (ref. 3). The most widely used transitional phases,

12-

and

r-

Al2O3, both have defect spinel lattices that differ in disorder. 2 - A l 2 O 3 has a strong

113 one-dimensional disorder o f t h e cubic close-packed stacking, while f o r

y-A1203 t h e

oxygen sub-lattice is f a i r l y well-ordered w i t h the tetrahedral A l l a t t i c e being strongl y disordered. Radial electron distribution studies and AIK,

fluorescence l i n e s h i f t

measurements (ref. 5) have shown t h a t the oxygen sub-lattice of densely packed than t h a t of

9-AI2O3 was less

r - A 1 2 0 3 , and t h a t octahedral sites were p r e f e r e n t i a l l y

occupied by A l , the f r a c t i o n o f cations in tetrahedral position being slightly higher in x - A I 2 O 3 than in y - A 1 2 0 3 .

M o r e recently, John e t al. (ref. 6) measured t h e cation

distributions as a f u n c t i o n o f dehydration temperature b y means o f 27Al MASNMR. They found a f r a c t i o n of (0.25'0.04) Y-Al2O3

tetrahedrally coordinated aluminium cations in

which was temperature-independent between 700 and 1100 K. In contrast,

i n Q -A1203 the f r a c t i o n of tetrahedrally coordinated aluminium cations varied somewhat w i t h temperature f r o m (0.37'0.04)

a t 720 K t o a value o f (0.27'0.04)

a t tem-

peratures between 870 and 1100 K.

3. SURFACE STRUCTURE MODELS Based on t h e s t r u c t u r e analyses b y Lippens (ref. 3,4), it was suggested t h a t t h e most densely packed (111)-face

would preferentially t e r m i n a t e crystallites o f

A1203 and (110)- o r (100)-faces those o f

x-AI2O3.

7~

Peri (ref. 7) and B u t t and co-

workers (ref. 8,9) used t h e (100)-face when they were modeling t h e surface dehydroxylation process by a Monte Carlo method. M o r e recently, Soled (ref. 10) described K - A I 2 O 3 as a d e f e c t oxyhydroxide o f the stoichiometry A 1 2 ~ 5 ~ o ~ 5 0 3 ~ 5 ( O H ) o ~ 5 whereby the OH groups are considered t o be located i n t h e surface o f t h e microcrystallites. It was concluded f r o m this stoichiometry t h a t

r - A I 2 O 3 should consist o f

particles which have t h e shape o f octahedra and are t e r m i n a t e d by t h e most densely packed ( 1 l l ) b f a c e s . The model predicted an edge length o f t h e idealized p a r t i c l e o f 2 11.5 n m , a surface area near 200 m / g , and pore volumes ranging between 0.04 and

3 1.3 c m /g in f a i r agreement w i t h t e x t u r a l data o f t y p i c a l aluminas. It is w e l l known t h a t aluminas (and oxides i n general) are terminated b y O H groups t o minimize the surface energy (ref. 11). These surface O H groups can be considered as intrinsic surface probes which provide i n f o r m a t i o n on their coordination and hence, on t h e local surface s t r u c t u r e , via their infrared stretching frequencies. A l l transitional aluminas have O H stretching spectra w i t h five more or less w e l l resolved individual bands w i t h i n t h e frequency range 3700-3800 cm-'.

Table 1 is a summary o f

t h e t y p i c a l frequency ranges o f these five bands which were observed f o r various alum i n a modifications b y d i f f e r e n t research groups. The assignment o f these bands has been made (ref. 12) on t h e basis of t h e f o r m a l n e t charge located on t h e OH group

3t

depending on i t s coordination t o A1

cations in t h e surface, t h e net charge being

calculated by means o f Pauling's electrostatic valence rule. As a consequence o f t h e various possible coordination numbers o f the O H groups and of the A l

3t

cations, the

five possible configurations described i n Table 1 can be expected on (111) faces o f

114 TABLE 1 Hydroxyl Group Configurations and OH Stretching Frequencies o f Transitional Aluminas OH-type

Coordination number

N e t charge a t surface anion

0 la Ib Ila Ilb Ill

'2-

1 1 2 2 3

and r - A I 2 O 3 ,

1 1 2 3

1

-

1

-

-1.25 -1.5 -0.75 -1 -0 -0.5

-

VOH/cm

-1

OH

-0.25 -0.5 +0.25 0 +0.5

3760-3780 3785-3800 3730-3735 3740-3745 3700-371 0

whereas only configurations la, Ib, and I l b are possible on (110)

faces, and t h e (100) f a c e would bear exclusively I b t y p e O H groups (ref. 12). We have previously concluded f r o m these considerations, t h a t t h e particles o f transitional aluminas must presumably be t e r m i n a t e d by these t h r e e low-index planes their relative contributions probably being dependent o n various preparation parameters and on the p a r t i c u l a r crystallographic modification. Qualitatively, one would expect increasing 0 - H stretching f o r c e constants w i t h increasing negative charge density located on t h e O H group. The assignments in Table 1 were hence made accordingly (ref. 12). Based on this model, the surface dehydroxylation could be described by condensat i o n o f adjacent OH groups, whereby t h e m o r e negatively charged (more basic) groups would combine w i t h the proton provided b y the more positively charged (acidic) groups (ref. 12). In t h e temperature range below 670 K , dehydroxylation w i l l f o r m surface oxide ions (following the notation introduced by Burwell (ref. 131, the surface oxide ions w i l l be represented

d-0-, t h e hydroxyl groups d - O H ) and anion vacancies

which expose coordinatively unsaturated Al

3+

(cus) ions, the coordination of the 6 - 0 -

species and the degree o f unsaturation o f t h e AI3+(cus) ion being determined b y t h e o f t h e 6 - O H groups which are removed as water. A p a r t i a l l y hydro3+ xylated alumina surface is t h e r e f o r e constituted of OH groups, oxide ions, and A l

configuration

(cus) ions exposed in vacancies as depicted schematically in Fig. 1.

Fig. 1. Schematic representation of p a r t i a l l y hydroxylated A1203 surface.

115 A f u l l y hydroxylated (111) face would bear 14.5 x

loi4

d-OH's/cm2,

and densities

o f oxide ions and vacancies o n p a r t i a l l y hydroxylated alumina surfaces must b e in t h e same order of magnitude (ref. 12). However, densities o f c a t a l y t i c a l l y active sites are n o r m a l l y lower by one or t w o orders o f magnitude (ref. 14), indicating t h a t t h e y must be related w i t h d e f e c t sites which may f o r m during dehydroxylation a t temperatures above 570 K. These sites can probably be described as m u l t i p l e vacancies associated w i t h "islands" o f oxide ions (ref. 12). Charge defects o f this t y p e m a y be removed a t heat t r e a t m e n t s above approximately 870

K

when surface m o b i l i t y o f

oxide ions becomes activated.

4. ACIDIC A N D BASIC PROPERTIES OF A L U M I N A S The constituents o f alumina surfaces as described i n the previous section must 3+ determine t h e surface acidic and basic properties. Beyond doubt, the A l (cus) sites

d -0- sites should func-

have Lewis acid (electron pair acceptor) charact.er, whereas t i o n as Lewis base ( e l e c t r o n pair donor) sites. The

d - O H species may principally de-

velop basic or proton acidic properties, t h e O H group behaving the m o r e l i k e a hydro-

I

xide ion t h e higher the negative charge density located on it. Hence, t h e type

d - O H ' S which are characterized by t h e highest 0 - H stretching frequency should develop the strongest basic properties among t h e surface hydroxyls. Lewis acid sites have been largely characterized by IR spectroscopy using L e w i s bases ( l i k e pyridine, NH3, e t c ) as probe molecules (ref. 14-16). The acid strength o f these Lewis sites is significant and shows a broad distribution. f o r pyridine (on k J mo1-l

Heats of chemisorption

x - A I 2 O 3 a f t e r dehydroxylation a t 770 K ) range f r o m 90 t o over 120

(ref. 17), and chemisorbed pyridine cannot be quantitatively desorbed a t

temperatures below 750 K when i t begins t o decompose (ref. 14). The acid strength 3c 3+ . ion (Altet ex-

o f the Lewis sites depends on t h e degree o f unsaturation o f the A l

posed in a vacancy is a stronger site than A13+ ). The strongest Lewis acid sites are oct provided b y m u l t i p l e d e f e c t sites (ref. 12). The choice o f an appropriate probe molecule f o r Lewis acid s i t e t i t r a t i o n is c r i t i c a l , since many Lewis bases undergo surfacechemical transformations when they are brought in c o n t a c t w i t h alumina surfaces (see below). Steric hindrance has been shown t o be c r i t i c a l w i t h bulky probe molecules (ref. 18). As shown by Kazansky e t al. (ref. 19), t h e low temperature adsorpt i o n of dihydrogen is a promising approach f o r the characerization o f Lewis acid sites. A frequency shift of 180 cm-'

towards lower values relative t o the H - H

stretching frequency of the f r e e molecule was observed for

?-AI2O3

pretreated at 3+ (CUS)site.

870 K. This value should be a measure of the polarizing power o f the A l

I t has been a long-lasting debate o f whether alumina surfaces possess protonic (Br6nsted) a c i d i t y or not. Beyond doubt, protonation of basic probe molecules according t o d-OH

+

B

-

d -0-

+

BHf

(1)

116 could not be observed by I R spectroscopy on pure alumina except for very strong bases such as NH3 (ref. 14-16).

Even at temperatures up t o 570

K , pyridinium ion

formation could not be detected (ref. 20). A low B r h t e d acidity was associated with the presence of traces of moisture (ref. 15,211; an H 2 0 molecule can be coordinated t o A13+ (cus) whereby it i s polarized and may provide acidic protons:

+

A?+-/J

-

H ~ O

AI~+L-

.

0'::

These qualitative indications of negligible intrinsic protonic acidity of alumina surfaces under "dry"

conditions finds support by a reported pK value of 8.5 (ref. 221, which a was determined from the shift of the 0 - H frequency induced b y adsorption of benzene f r o m the gas phase. H-bonding properties of

d - O H species have been reviewed

(ref. 16,23). It is clear that type Ill groups are the strongest H-bond donors and type I groups

the strongest H-bond acceptor sites among the d - O H species. The basic properties of alumina surfaces have been investigated less extensively. Scokart and Rouxhet (ref. 24) measured the frequency shift of the N H stretching mode of pyrrole adsorbed on alumina surfaces. This frequency shift was considered as a measure of the basicity of the H-bond acceptor site on the surface. A base strength comparable t o that of pyridine and

was estimated; a distinction between d - O H

d-0- acceptor sites, however, was not possible. The use of deuteriochloroform

was proposed as a probe for the distinction between these two types of basic sites, the position of the C-D stretching band being considered as a measure of the base strength (ref. 25). Bands a t 2253 and 2225 c m - l were detected when CDC13 was adsorbed on alumina after dehydroxylation a t 770 K (ref. 26). These bands were associated w i t h weak basic sites (considered t o be (considered t o be bridging

d - O H ) and stronger basic sites

d - 0 - species), respectively.

Alumina surface hydroxyl groups may also function as nucleophiles which is documented i n various surface-chemical transformations of adsorbed molecules. For example, carboxylate species are formed when ketones (e.g.

acetone) are adsorbed on

partially dehydroxylated surfaces (ref. 14,16):

d -OH

+

(CH312C0

d-(CH3-COO)

+ CH4

(3)

Other examples for nucleophilic attack of 6 - O H onto adsorbed molecules are the formation of bicarbonate from C02, of carboxylates from alcohols, of amides from nitrites, and of pyridone from pyridine (ref. 14,16). A l l these reactions, the surface products of which were identified by IR spectroscopy nucleophilic attack by the most basic (high frequency)

can be explained assuming a

d -OH-species, whereby coor-

dination of the adsorbed molecule to an adjacent Lewis acid site most likely assists

117 this reaction (ref. 27). These reactions are therefore typical examples for the concerted action of acid-base pair sites on alumina surfaces. Acid-base pair sites also play an important role as catalytically active sites (ref. 12,16,21) as w i l l be discussed i n section 6.

5. ALUMINAS AS CATALYST SUPPORTS The by far prevailing use of catalytic aluminas i s their application as supports. Oxides, sulfides, and metals are among the most important active components which are dispersed on aluminas. Precursors are anchored on the support surface either f r o m aqueous solutions by various procedures (ref. 28), or by reaction of organometallics (including carbonyls) i n non-aqueous media. It must be emphasized that the initial interaction between catalyst precursor and support surface may critically determine the distribution and dispersion of the active component in the final catalyst.

5.1 Adsorption of Catalyst Precursors f r o m Aqueous Solution Alumina surfaces are almost completely hydroxylated when they are suspended in water. The surface chemistry of the oxide in contact with an aqueous solution is largely determined by the dissociation of the d - O H species (ref. 29). Equilibria can

-

be expressed as follows:

AI-OH;

AI-OH

AI-0- +

Decreasing pH of the solution

H+

(4)

shifts the equilibrium t o the left, increasing pH t o

the right. The point of zero charge (isoelectric point IEPS) for aluminas was reported t o bepH 8-9 (ref. 30). Hence, adsorption of anions must be performed at pH values

< 8,

whereas cations will be adsorbed at pH values>

of equilibrium loadings of molybdate and Na'

9. Fig. 2 shows the dependence

ions on Al2O3 as a function of the

final pH of the solution. The data support the above prediction; molybdate (anion) loadings are high at low pH values and fall t o low loadings between pH 6-8 when the isoelectric point is reached. The reverse is observed for Na+ cations, the equilibrium loading of which rises when the pH is increased above the isoelectric point (

> pH 8).

In the case of molybdate anion adsorption, the pH of the solution not only determines the amount adsorbed at a given concentration but also the nature of the molybdate species in solution. The equilibrium (ref. 32)

7 MOO:-

+

8 H+

====

Mo 06- + 7 24

4 H20

is shifted to the right with decreasing pH. Consequently, polymolybdate anions should be adsorbed under the conditions of low pH favorable for anion adsorption. Raman and U V spectroscopy indeed proved the presence of polyanions on the face after equilibrium adsorption at

pH

-= 6

x - A I 2 O 3 sur-

(ref. 31). The electrostatically adsorbed

polyanions react with the alumina surface during the drying and calcination procedures

118

Final pH Fig. 2. Equilibrium loadings o f molybdate (1) and Na' (2) ions adsorbed on f-A1203 as a f u n c t i o n o f the f i n a l p H o f t h e aqueous solution ( f r o m ref. 34).

and substitute t h e reactive

6 - O H . These are the most basic (high frequendy) t y p e I

O H groups as shown by the significant erosion o f the 0 - H stretching bands a t 3800 and 3780 cm-',

whereas the less basic groups remain on t h e surface (ref. 33). The

basic d - O H species are the exchangeable hydroxide-like surface anions.

Molybdate

anions are therefore linked t o t h e support surface i n patches o f three-dimensional species which probably have structures analogous t o those o f t h e originally adsorbed polymolybdate anion (ref. 31,34,35). In the preparation o f alumina-supported m e t a l catalysts, impregnation f r o m aqueous solutions containing anionic or cationic complex ions o f t h e particular m e t a l is frequently applied (ref. 28). Typical catalyst precursor complexes f o r the preparat i o n o f e.g.

supported platinum catalysts are H2[ PtC16]

or

[Pt(NH3)4]

C12. For

favorable adsorption o f the hexachloroplatinum anion and the tetramine platinum c a t ion, low and high p H conditions, respectively, must be applied. Although t h e interact i o n mechanism o f these complex ions w i t h alumina surfaces are f a r f r o m being understood in detail,

[ P t ( N H 3 ) 4 ] 2+

seems t o react w i t h the support surface without

ligand exchange, whereas CI- ions were detected i n solution when contacted w i t h alumina (ref. 36). Hydrolysis of t h e

[ PtC16]

*-

[ PtCI6]

2-

was

anion may occur as

follows:

[ PtCI6]

2-

+ n OH

w

[PtC16-n(OH)n]

2-

+

nCI-,

(6)

although under the low p H conditions required f o r anion adsorption this equilibrium would be shifted t o t h e left. Unless additional complex solution equilibria occur, anion

119 adsorption on aluminas a t p H < 7 can be described as follows (ref. 37):

+

~AI-OH;

M"-

===

(7)

(AI-OH+) M"2Y

The nature of the complex ion (charge, size, ligand sphere) present in sol-ution is c r i t i c a l for the adsorption strength. Moreover, foreign ions adsorbed on oxide surfaces influence their IEPS (e.g.

anions reduce the IEPS) (ref. 29,30) and hence, the optimal

p H conditions for the complex noble metal ions. Particularly, when alumina pellets are used for impregnation, the adsorption of complex noble metal ions w i l t be controlled by i t s diffusion rate which competes with the adsorption rate (ref. 36). Adsorption strength and rate of the complex respond sensitively and selectivley toward the presence of foreign ions i n the impregnating solution (ref. 36,38,39). It is therefore possible t o control the distribution of the active component within an alumina pellet by addition of appropriate acids and salts as coingredients t o the impregnating solution. In conclusion, the deposition of metal precursors onto aluminas by impregnation from aqueous solution i s an extremely complex process i n which colloid and surface chemistry are strongly involved. I t is presumably correct t o say that this area of catalyst preparation is s t i l l more an a r t than science (even more so with materials other than aluminas f o r which the surface chemistry i s frequently less well understood).

5.2 Organometall ic Compounds as Catalyst Precursors Although less feasible for large-scale commercial catalyst preparation, active components may be deposited on alumina surfaces by reaction of organomatallics (including metal carbonyis) w i t h surface groups from the gas phase or from non-aqueous solutions. Since rehydration of the support surface w i l l not occur under such conditions, the surface state of the alumina surface can be predetermined by i t s degree of h yd rox y lat ion. Yermakov and his coworkers (ref. 40-42) and Iwasawa e t al. (ref. 43) have advocated the use of ally1 complexes for the preparation of supported catalysts. These complexes are anchored onto the support surface b y reaction with d-OH species as follows:

2 d-OH Since the

+

M o ( ~ - C ~ Hpentane 270 ~ ) ~ D ( d -O-)2Mo( k - C 3 H 5 ) 2

+

2 C3H6

ce-ally1 ligands have basic character and behave formally as monoanions,

the reaction proceeds preferentially with the more acidic

&OH

species (low 0 - H

stretching frequency). Zr(BH4I4 reacts with alumina surfaces with Formation of ( d - o - ) Zr(BH4)4-n surface coordination compounds which were identified by inelastlc electron tunneling spectroscopy (ref. 44). The surface chemistry involved in reactions of group Vlb metal carbonyts with

120 alumina surfaces was reviewed b y B u r w e l l (ref. 13). Substitution o f CO b y surface ligands o f alumina started near room temperature when Mo(CO16 was contacted w i t h p a r t i a l l y or strongly dehydroxylated aluminas. It was suggested t h a t basic d - O H species f o r m e d a COOH- group by nucleophilic a t t a c k on a CO ligand. The COOH- I i gand would than labilize a cis-CO ligand t o f a c i l i t a t e i t s substitution by a

d - 0 - or

d - O H surface ligand (ref. 45). A M o ( C O ) ~surface complex was shown t o be the result o f t h e low temperature reaction between M o ( C 0 1 6 and alumina surfaces (ref. 13). The tricarbonyl complex was then oxidized on p a r t i a l l y dehydroxylated alumina sur-

-

faces a t 570 K i n helium; this process could roughly be described as follows: Mo(CO)~

+

2 6 -OH

( d -0-)2M02+

+

3 CO

+

H2

(9)

(somewhat less than 3 moles CO were evolved and some methane was also found). The importance o f basic d - O H species in reactions o f m e t a l carbonyls is also demonstrated b y t h e oxidation o f zerovalent rhodium i n Rh6(C0),6

t o f o r m a surface

R h + ( C 0 l 2 complex w i t h simultaneous evolution o f hydrogen (ref. 46). Oxidative addi~ ~ observed w i t h t i o n reactions of d - O H groups t o an 0 s - 0 s bond o f O S ~ ( C O )were HOS~(CO)~~(O d -)- (ref. 47-50).

t h e f o r m a t i o n o f a surface-bound trinuclear cluster

A t elevated temperatures this anchored cluster is oxidized b y 6 - O H groups and 2+ 2+ 0 s (CO)2 and 0 s (CO)3 are most l i k e l y formed which are anchored t o surface oxygen ions giving ensembles o f t h r e e osmium ions w i t h interionic distances o f approxim a t e l y 0.6 n m (ref. 49,501. When Fe3(C0),2

was contacted w i t h p a r t i a l l y dehydroxylated alumina surfaces,

basic d - O H species behaved as nucleophiles t o w a r d a coordinated CO w i t h concomi-

-

tant f o r m a t i o n of t h e cluster anion

d -OH

+

Fe3(C0)12

6'-

[HFe3(CO)11]

-

[: HFe3(CO)11] -

(ref. 51):

+ CO;dS

(10)

CO

produced in t h i s reaction is n o t released b u t rather f o r m s a surface carbonate. 2 The bonding o f t h e cluster anion t o t h e alumina surface is interesting i n t h a t it invol-

ves presumably coordination t o a Lewis acid site:

t

rn,%Z?4 as indicated b y a carbonyl stretching band a t 1598 acceptor interactions were observed when

[C p N i ( C 0 ) J 2

ern-'

[CpFe(CO)

(ref. 51). Analogous donor-

14,Cp3Ni3(C0)2,

and

were brought in contact w i t h nearly f u l l y dehydroxylated r - A I 2 O 3

-

(ref. 52). This t y p e o f interaction o f CO ligands, namely f o r m a t i o n o f adducts of t h e t y p e Me-C=O

AI3+%has strong implications f o r a possible route f o r activation of

121 carbon monoxide on alumina supported metal catalysts (ref. 53). These fascinating surface-organometallic reactions which may be utilized f o r catalyst preparations, demonstrate the wide polyfunctionality of alumina surfaces, which could be predicted on the basis of the surface structure model discussed i n sections 3 and 4. Hence, the chemistry involved in catalyst preparation f r o m organometallics is probably much b e t t e r understood than t h a t occurring in impregnation f r o m aqueous solutions. However, f o r large scale commercial catalyst manufacture, the organomet a l l i c route i s less favorable.

6. ALUMINAS AS CATALYSTS Table 2 is a summary o f reaction classes which are catalyzed by aluminas. The c a t a l y t i c properties of aluminas f o r reactions of simple hydrocarbons and alcohols have been reviewed by John and Scurrell (ref. 86). In addition, Posner (ref. 87) described the use of aluminas as catalysts for synthesis of more complicated organic

TABLE 2 Reactions Catalyzed by Aluminas

Reaction o - H / p - H conversion 2 2 H2/D2 exchange Alkene D-exchange Alkene double-bond isomerization Cyclopropane isomerization Alcohol dehydration Claus reaction COS hydrolysis Alkene, skeletal isomerization o-xylene isomerization

Temp./K

78 150 300 300 375 350 > 400 > 500 600 770

Refs.

54 54-57 58-63 58, 64-72 73, 74 75- 77 78, 79 80, 81 67, 82-85 67

chemicals. This l i s t o f reactions catalyzed by aluminas again emphasizes the significant polyfunctionality o f alumina surfaces. Nevertheless, the only t w o alumina catalyzed reactions of Table 2, which receive industrial application on a larger scale are the modified Claus process and the COS hydrolysis. I t has been well established and reviewed (ref. 12,14,21 ,861, that reactions such as o-H2/p-H2 conversion, H / D exchange, D-exchange w i t h hydrocarbons, and double2 2 bond isomerization of alkenes require m u l t i p l e acid-base sites on the c a t a l y t i c alumina surface. These are t o be described as defect sites consisting o f m u l t i p l e vacancies (Lewis acid sites) w i t h adjacent islands of basic oxygen ions and hydroxyl groups (ref. 12). The l a t t e r cannot play the role o f Brdnsted sites in these reactions because of their low protonic a c i d i t y and the low reaction temperatures. Reactions which proceed via cationic mechanisms and therefore require protonic acidity, are only cata-

122 lyzed a t higher temperatures; it had been argued (ref. 15,211 t h a t the a c t i v i t y of aluminas f o r skeletal isomerizations was not due t o t h e presence o f intrinsic Brdnsted a c i d i t y b u t rather t o t h e presence o f traces o f moisture which led t o a transformat i o n o f Lewis acid sites t o Brdnsted sites according t o eq. (2). I t could be shown (ref. 74), t h a t the coordination o f undissociated alcohol on Lewis acid sites of

-A1203 enhanced t h e a c t i v i t y f o r isomerization o f cyclopropane significantly, presumably because o f induced Brdnsted a c i d i t y provided b y t h e polarized alcohol 0 - H bond. Recent research devoted t o an elucidation of t h e mechanisms of the Claus reaction (ref. 78.79)

+

2 H2S

SO2

312 S2

+

2 H20

(11)

and t h e COS hydrolysis (ref. 80,81) COS

+

H20

====co2

+

H2S

(12)

indicated t h a t basic sites o f alumina surfaces (presumably adjacent d-0-1 played t h e major role in b o t h reactions. In alcohol dehydration

6-0-(H-bond

sites are involved (ref. 75-77). abstraction o f the

/J-proton

acceptor) sites and d - O H (H-bond donor)

In addition, strong basic sites ( d - O - 1 are needed f o r f r o m t h e alcohol molecule. The reaction proceeds via

an €2 (concerted) eliminationi mechanism (ref. 75-77).

However, at increasing reaction

temperatures t h e mechanism becomes more E l - l i k e , particularly f o r secondary and even m o r e pronounced f o r t e r t i a r y alcohols. This apparent discrepancy w i t h t h e experience o f negligible Brdnsted a c i d i t y can be removed by l i f e t i m e considerations. The f a i l u r e o f observing pyridinium ions on aluminas can be related t o a residence t i m e o f the proton i n a

d -OH ....N bond on the N-side being too low f o r detection b y

infrared spectroscopy. It was suggested (ref. 74,881 t h a t t h e activation of an alcohol molecule f o r water elimination was achieved by proton fluctuations in an H-bonded alcohol molecule:

I

I1

As emphasized b y F r i p i a t (ref. 89), there are four t i m e parameters which w i l l determine t h e mode o f a c a t a l y t i c transformation. These are (1) t h e l i f e t i m e o f a protonated s i t e ( C ) ;(2) the residence t i m e o f an adsorbed molecule ( t i m e @f t h e protonated molecule

( T );

zA),( 3 ) t h e I i f e -

and (4) t h e t i m e required f o r t h e chemical

P transformation ( T c ) . Assume, Z and TA be long as compared t o T and T''. Hence, P structure 11 I S a very short-lived species as compared t o s t r u c t u r e I. However,during

123 the l i f e t i m e of 11 chemical transformations may occur, provided the respective t i m e For an E l - l i k e mechanism ‘c ( E l ) would parameters obey the condition Tc Cu(60) > H(60) > Fe(48) > Al(36) > Pd(26) > La(24) > Zn(13) For s a l t s of HTS Ag(79) > Cu(61) > H(39) > Fe(24) > Al(15) > Zn(7) > La(2) Numbers i n parentheses i n d i c a t e t h e hydrocarbon y i e l d a t 2-6 h of running time. In general, t h e metal s a l t of HTP i s more a c t i v e than t h e corresponding metal

s a l t of HTS. The d i s t r i b u t i o n s of hydrocarbons over v a r i o u s metal s a l t s a r e very similar t o t h a t over HTP, i n d i c a t i n g t h a t t h e r e a c t i o n mechanism i s common t o parent heteropolyacids and t h e i r metal s a l t s . Thus, t h e a c t i v e c e n t e r s f o r methanol conversion should be common, and t h e y a r e presumably Bronsted a c i d s i t e s . I t should be noted t h a t s i l v e r and copper s a l t s a r e more a c t i v e among

169 me t a l s a l t s and even more a c t i v e t h a n p a r e n t a c i d s . Th er ef o r e, t h e mechanism o f a c i d s i t e s formation o f s i l v e r dodecatungstophosphate(AgTP) and CuTP were studied i n detail. FORMATION OF A C I D SITES I N AgTP

I n t h e methanol co n v e r s io n o v e r n e a t AgTP a t 513 K , a long i n d u c t i o n time

was observed as shown i n F i g . 1. S i n c e t h e i n d u c t i o n time i s o f t e n r e l a t e d t o t h e for m at i o n o f a c t i v e c e n t e r s (H'),

t h e examination o f t h e f a c t o r s i n f l u -

e nc ing t h e i n d u c t i o n time may g iv e a c l u e f o r t h e mechanism o f a c i d s i t e format i o n . E f f e c t o f hydrogen was examined as a p o s s i b l e so u r ce o f p r o t o n s , s i n c e small amount o f hydrogen was always found i n t h e r e a c t i o n p r o d u c t s ( r e f . 1 3 ) . The c a t a l y s t was k e p t i n a hydrogen stream ( 4 . 1 x lo-* mol h - l ) a t 523 K f o r 1 h and t h e n r e a c t i o n s t a r t e d . A s i s shown i n F i g . 1 t h e i n d u c t i o n time almost

disa ppear ed by hydrogen p r e t r e a t m e n t . I t i s c l e a r t h a t hydrogen p l a y s an e s s e n t i a l r o l e i n t h e f o r m at i o n o f Bronsted

a c i d s i t e s . P r o t o n s may b e g e n e r a te d by t h e r e a c t i o n o f s i l v e r c a t i o n s w i t h hydrogen molecules.

Ag'

+

1/2 H2 ( o r H)

Ago

+

H+

(1)

During t h e methanol c o n v e r s io n , hydrogen m o le c u le s (or atoms) may be p r o v i d ed by t h e decomposition o f methanol. I n d u c t io n time i s supposed t o b e t h e p e r i o d which i s r e q u i r e d f o r t h e e s t a b li s h m e n t o f t h e e q u i l i b r i u m o f Reaction ( 1 ) . The i n d u c t i o n t i m e was a l s o observed i n methanol conversion o v er CuTP a t 523 K , and i t d i s ap p ear ed by t h e p r e t r e a t m e n t o f CuTP by hydrogen. Thus, t h e mechanism s i m i l a r t o Reaction (1) i s o p e r a t i v e a l s o i n CuTP ( r e f . 1 4 ) . Reaction (1) e x p l a i n s why s i l v e r ( 1 ) and c o p p e r (I 1 ) s a l t s a r e t h e most a c t i v e among t h e metal s a l t s o f h e te r o p o l y a c i d s ,

s i n c e t h e s e s a l t s a r e known t o be t h e

one which a r e most e a s i l y reduced by hydrogen ( r e f . 1 5 ) . The g e n e r a t i o n of Bronsted a c i d s i t e s by t h e i n t e r a c t i o n o f hydrogen and AgTP

o r CuTP i s confirmed by examining t h e c a t a l y t i c a c t i v i t y f o r t h e i s o m e r i z a t i o n o f o-xylene, which i s t h e r e a c t i o n c a t a l y z e d by Bronsted a c i d s i t e s ( r e f . 1 6 ) . The r e a c t i o n was c a r r i e d o u t a t 573 K by u s i n g AgTP o r CuTP (30 wt%) on a c t i v e carbon a s c a t a l y s t . AgTP showed no a c t i v i t y f o r o-xylene i s o m e r i z a t i o n , b u t t h e a c t i i r i t y developed when t h e c a t a l y s t was p r e t r e a t e d i n a hydrogen o r methanol stre a m f o r 2 h a t 573 K . These f a c t s show t h a t AgTP, as p r ep ar ed , has mrfiransted a c i d s i t e s , b u t t h e a c i d i t y i s induced by i t s i n t e r a c t i o n w i t h hydrogen o r methanol. F u r t h e r evidence o f t h e i n t e r a c t i o n o f AgTP w i t h hydrogen was o b t ai n ed from i n f r a r e d s p ect r o s co p y o f adsorbed p y r i d i n e ( r e f . 1 6 ) . AgTP evacuated a t 573 K d i d n o t g i v e t h e bands due t o pyridinium i o n , w h i le AgTP t r e a t e d by hydrogen

170

or methanol at 573 K gave them. Similar results are obtained also for CuTP. Thus, the effects of the treatments by hydrogen and methanol on the Bronsted acidity of AgTP as observed by infrared spectra of adsorbed pyridine are in complete conformity with the effects of the pretreatments by the substances on the catalytic activity of o-xylene isomerization. AgTP was exposed to deuterium of 7.5 kPa at 563 K for 1 h and evacuated at 573 K for 30 min; new bands appeared at 2542 and 2641 cm-’, which are ascribed to the streching of 0-D groups. The sample was then exposed to pyridine vapor at 393 K for 1 h and evacuated at 393 K for 2 h. The 0-D bands completely dis1 appeared and the band due to deuterated pyridiniwn ion (C5H5ND+) at 1482 cmappeared. These results clearly demonstrate that hydroxyl groups are formed by the interaction of hydrogen and AgTP and they are acidic. While hydrogen pretreatment eliminates the induction period in the methanol conversion, the presence of gaseous hydrogen enhances the reaction rate (ref.13). The methanol conversion was carried out with AgTP (30 wt%) on active carbon as catalyst with the initial partial pressure of methanol of 5 1 kPa and with varying partial pressure of hydrogen. The hydrocarbon yield increased as the increase in the partial pressure of hydrogen. Thus, without hydrogen, the hydrocarbon yield was 24%, while it was 43% under the hydrogen partial pressure of 51 kPa. The effect of hydrogen was reversible as is shown in Fig. 2. After carrying out the run under a hydrogen pressure of 51 kPa for 2 h, hydrogen was replaced by nitrogen. The hydrocarbon yield was reduced to the value which would be expected when the reaction was started without gaseous hydrogen. Then, nitrogen was again replaced by hydrogen, the hydrocarbon yield being back to the original value. Thus, it is concluded that Reaction (1) is really operative and reversible under the conversion conditions. Oxygen was found to depress the catalytic activity. Thus, a small amount of oxygen (9.8 x lo-’ mol) was pulsed into the feed during the run in the presence and in the absence of hydrogen. The activity was sharply depressed, but gradually returned to that before adding oxygen. The retardation by oxygen may be caused by oxidation of silver metal to the cation. 2 Ago

+ 2 H+

+

1/2

O2

>-,

2 Ag’

+

H20

(2)

The recovery of the activity may be due to the reduction of silver cation to the metal by Reaction (1). Effect of hydrogen is not restricted to methanol conversion. The catalytic activities of AgTP for the synthesis of methyl t-butyl ether from isobutene and methanol (ref.17) and the esterification of acetic acid with ethanol are greatly enhanced by hydrogen pretreatment and also by the presence of hydrogen

171

H2 1

30

o--o-o~o,

\

f \

z.z

.'0'-8

20

1.

C 0

n

1 j L

:: 10

0

TJ

-0

I

I

L

2

ZI

0

/

-00

,

,

,

,

,

,

,

L

10

5

0 Time

on S t r e a m

0

10

5 Time on S t r e a m

/ h

F i g. 1 . Change i n hydrocarbon y i e l d w i t h time on stream i n methanol conv e r s i o n o v e r AgTP w i t h ( 0 ) o r witho u t ( 0 ) hydrogen p r e t r e a t m e n t . 513 K , methanol: 30.4 k P a , W/F = 57 g.h.mo1-l.

F i g . 2. E f f e c t o f co f eed i n g gas on hydrocarbon y i e l d i n methanol conv e r s i o n o v er AgTP/C a t 573 K. Cofeed g a s : hydrogen ( o ) , n i t r o g e n ( 0). The g a s was changed from hydrogen t o n i t r o g e n (J) and from n i t r o g e n t o hydrogen ( + ) . 573 K, methanol: 51 kPa, W/F = 50 g - h - m o l - I .

100

.

s

/ h

loo

7

80

TJ

2 60 >C

40 0 U

E!

D, 20

0

1

2

3

Time on Stream

4 / h

Fi g. 3. E f f e c t o f c o f e e d i n g gas on C 2 + y i e l d i n methanol conversion

o v e r PdTP/Si02. 573 K , ( a ) H 2 , (b) (C) N 2 4 H 2 , (d) H 2 + N 2 * methanol: 51 kPa.

N2'

5

0

10

20

Hydrogen

30 Pressure

40

50

I kPa

F ig . 4. Ef f ect o f hydrogen p r e s s u r e on product d i s t r i b u t i o n i n methanol c o n v e r s i o n o v er PdTP. 573 K , methanol: 51 k P a .

172 i n t h e gas phase. Reduction o f metal c a t i o n s i s n o t only way o f a c i d s i t e formation. For examp l e , i n t h e c a s e o f t h e A 1 s a l t , t h e mechanism o f t h e a c i d s i t e g e n e r a t i o n i s e n t i r e l y d i f f e r e n t ( r e f . 1 6 ) . Hydrogen h a s no e f f e c t on t h e c a t a l y t i c a c t i v i t y f o r o-xylene i s o m e r i z a t i o n . The c a t a l y t i c a c t i v i t y and t h e c a p a c i t y f o r p y r i d i nium i o n f o r m at i o n a r e enhanced by t h e p r e t r e a t m e n t w i t h water. The p l a u s i b l e mechanism f o r p r o t o n f o r m a ti o n may be a s s o c i a t e d w i t h d i s s o c i a t i o n o f w at er , as sugge s t ed by Niiyama ( r e f . 1 0 ) .

HYDROGEN SPILLOVER IN METAL-HETEROPOLYACID SYSTEM

When methanol conversion was c a r r i e d o u t o v e r p al l ad i u m s a l t o f h et er o p o l y a c i d s u p p o r t ed on s i l i c a a s c a t a l y s t , t h e g r e a t e f f e c t o f hydrogen was observed.

As shown i n Fig. 3, t h e y i e l d o f hydrocarbons w i th carbon numbers more t h an one (C2+ y i e l d ) was about 70% when p a ll a d iu m dodecatungstophosphate(PdTP) was p r e t r e a t e d w i t h hydrogen a t 570 K , and t h e r e a c t i o n was c a r r i e d o u t by co f eed i n g hydrogen (51 kPa) (Curve a ) . The C 2 + y i e l d was about 10% when PdTP was p r e t r e a t e d under n i t r o g e n and t h e r e a c t i o n was c a r r i e d o u t by co f eed i n g n i t r o g e n (Curve b ) . When t h e cofeed-gas was changed from n i t r o g e n t o hydrogen (Curve c ) o r hydrogen t o n i t r o g e n (Curve d ) , t h e C2+ y i e l d g r a d u a l l y changes t o t h e v al u e which was supposed t o be o b t a i n e d i f t h e r e a c t i o n was c a r r i e d by co f eed i n g t h e second g as from t h e beginning. The e f f e c t o f hydrogen i s r e v e r s i b l e . The a c t i v i t y o f PdTP i n t h e p r e s e n c e o f hydrogen i s much h i g h e r t h a n HTP o r AgTP i n t h e pr e se nce o f hydrogen. Because o f high

hydrogenation

a c t i v i t y o f Pd m e t a l , no o l e f i n i c p r o d u ct s

were observed i n t h e p r e s e n c e o f hydrogen, a l l t h e hydrocarbon p r o d u ct s b ei n g a l k a n e s . Decomposition o f methanol i n t o carbon monoxide and hydrogen,and hydroge na t i o n o f methanol i n t o methane and w a te r a l s o o ccu r r ed . Fig. 4 shows t h e e f f e c t o f t h e p a r t i a l p r e s s u r e of hydrogen on t h e p r o d u ct y i e l d . The C2+ y i e l d i n c r e a s e s almost l i n e a r l y w i t h hydrogen p a r t i a l p r e s s u r e . On t h e o t h e r hand, t h e y i e l d o f carbon monoxide d i d n o t depend on t h e p a r t i a l p r e s s u r e o f hydrogen. A p l a u s i b l e mechanism f o r t h e enhancement of t h e a c i d i t y by hydrogen may be a s f o l l o w i n g . Palladium c a t i o n s a r e completely reduced t o t h e metal by t h e p r e t r e a t m e n t w i t h hydrogen. Hydrogen molecules from t h e gas phase may d i s s o c i a t e i n t o hydrogen atoms o v e r t h e m et al , and hydrogen atoms t h u s formed may s p i l l o v e r and i n t e r a c t w i th surrounding h et er o p o l y an i o n s converted i n t o p r o t o n s . The p r o c e s s e s i s r e v e r s i b l e . H2,

p e 2 H (over

Pd m e t a l )

t o be

173 H

+

[

~

w

~

~

c

~ H+~ +~ [PW ~ 1 2-c40-14-

Ifpalladium metal i s t h e c e n t e r f o r hydrogen d i s s o c i a t i o n and n o t t h e d i r e c t f o r methanol conversion, t h e a c t i v i t y was expected t o b e n o t n eces-

active s i t e

s a r i l y p r o p o r t i o n a l t o t h e number o f Pd c o n t e n t i n t h e c a t a l y s t . T h e r e f o r e , t h e c a t a l y t i c a c t i v i t y o f PdxH3-2xPW12C40 s u p p o r te d on s i l i c a was examined. The r e s u l t s i s given i n F ig . 5 . A s i s shown i n F ig . 5 , t h e C2+ y i e l d g r e a t l y i n c r e a s e d w i t h a d d i t i o n o f small amount o f p a l la d i u m (x = 1/16) t o HTP. Only a s l i g h t i n c r e a s e i n t h e C 2 + y i e l d was a t t a i n e d by f u r t h e r i n c r e a s e o f x. The y i e l d o f carbon monoxide i n c r e a s e d w i th i n c r e a s i n g c o n t e n t o f palladium, conf i r m i n g t h a t t h e a c t i v e c e n t e r s f o r t h e decomposition o f methanol i s m e t a l l i c pa l l a di u m . Now, i t i s c l e a r t h a t t h e c a t a l y s t i s n o t n e c e s s a r i l y prepared from m et al s a l t s o f h e t e r o p o l y a c i d . T h e r e f o r e , a m i x t u r e o f HTP and c h l o r o p l a t i n i c a c i d was sup p o r t ed on s i l i c a . By 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 by hydrogen, pla ti num metal i s ex p e c t e d t o be formed and t o a c t a s c e n t e r f o r hydrogen d i s s o c i a t i o n . Thus, methanol conversion o v e r 30 w t % HTP t o g e t h e r w i t h 0.07% P t suppor t ed on s i l i c a gave t h e C Z c y i e l d o f 50% w i t h t h e n e g l i g i b l e f o r m at i o n o f carbon monoxide a t 570 K. T h i s t y p e o f t h e c a t a l y s t p r e p a r a t i o n may open up a novel method f o r o b t a i n i n g h i g h l y a c i d i c c a t a l y s t . HETEROPOLYACID AS A COMPONENT OF BIFUNCTIONAL CATALYST

Isom er i zat i o n o f a l k a n e s i s an i n d u s t r i a l p r o c e s s , which u s e s p l at i n u m i n combinationwith a c i d i c carriers s u c h a s f l u o r i n a t e d alumina and z e o l i t e s . A s f o r the r e a c t i o n mechanism, t h e d u a l f u n c t i o n a l i t y i s g e n e r a l l y accep t ed . The i s o m e r i z a t i o n o f a l k a n e s was a tt e m p te d by u s i n g palladium dodecatungstophosphate [Pd3(PW12040)2, PdTP] s u p p o r t e d on s i l i c a - g e l ( r e f . 1 7 ) . P r i o r t o t h e r e a c t i o n , t h e s a l t was h e a t e d i n a hydrogen stream a t t h e r e a c t i o n t em p er at u r e (443-523 K ) .

By t h i s t r e a t m e n t , Pd(I1) c a t i o n s a r e reduced t o metal and p r o t o n s

a r e c r e a t e d by t h e r e a c t i o n . P d( I1)

+

H2

>-

Pd(0)

+

2 H+

I t i s n o t ed t h a t PdTP i s h i g h l y a c t i v e f o r a c i d - c a t a l y z e d r e a c t i o n such as

e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h e t h a n o l and MTBE s y n t h e s i s even w i t h o u t hydrogen p r e t r e a t m e n t , i n d i c a t i n g t h a t t h e r e i s a n o t h e r way o f a c i d - s i t e format i o n . Moreover, as d i s c u s s e d i n a p r e v i o u s s e c t i o n , t h e a c t i v i t y of PdTP as s o l i d - a c i d i s g r e a t l y enhanced by t h e p r e s e n c e o f hydrogen i n t h e gas phase. T he re fo r e, PdTP a f t e r t h e r e d u c t i o n i s expected t o be a p o t e n t i a l c a t a l y s t f o r a l k a n e i s o m e r i z a t i o n , s i n c e it would c o n t a i n b o t h metal (Pd) and s t r o n g a c i d

174

loo

.

0'

80 -

0

C

60 -

QJ

.-0 ul

.

60

-

5

LO

L

01

>

0

20

V

-*-o--.

,*'

-100

-

,,*-

- 80

,a'

- 60

P-o-o-o

-/"

40-

20-

> .d

\o

/"

U

-40

-cc

-

v)

20

P 0 0

1 .o

0.5

1.5 Reaction Temperature / K

X

wXH3-2Xpw120W

F i g . 6 . Effect o f r e a c t i o n temper a t u r e on t h e a c t i v i t y and t h e s e l e c t i v i t y i n hexane isomer o v e r PdTP(50 wt%)/SiOZ. hexane: 30 kPa,

Fig. 5. C a t a l y t i c a c t i v i t i e s o f 'OnPdxH3-2xPW12040 for v e r s i o n . 573 K, methanol: 51 kPa,

W/F = 50 g.h.mol- 1

.

hydrogen: 71 kPa, W/F = 100 geh-mol- 1

.

80

V

0

0.5

x

1

1.5

0

450

475

500

525

Reaction Temperature

550

575 J

/ K

p d H~3 - h P W 1 $ ~

F i g . 7. C a t a l y t i c a c t i v i t i e s o f PdxH3-2xPW12040 f o r i s o m e r i z a t i o n of hexane. 443 K , hexam: 30 kPa, -1 hydrogen: 71 kPa, W/F = 100 g.h.mol

.

F i g . 8 . E f f e c t o f r e a c t i o n temper a t u r e on i s o m e r i z a t i o n o f hexane o v e r HTP s u p p o r t e d on Pd/C. hexane: 30 kPa, hydrogen: 71 kPa, W/F = 100 g.h.mol -1

.

o

175

centers (H+). The reaction was carried out with a continuous flow reactor operating at atmospheric pressure. Table 1 shows the effect of hydrogen on the conversion of hexane and the selectivity to hexane isomers together with detailed product distribution. As shown in Table 1, both the activity and the selectivity depend very strongly on hydrogen pressure. Besides hexane isomers, methylcyclopentane and cyclohexane were also found in the products. Formation of aromatic compounds was not observed. The effect of hydrogen is reversible; elimination of hydrogen from the gas-phase depressed the conversion sharply.

Fig. 6 shows the effect of the reaction temperature on the conversion and the selectivity in isomerization of n-hexane. The conversion increases with reaction temperature up to 500 K, but it decreases at higher temperature. The decrease in the activity at higher temperatures may be due to loss of protons as water. The selectivity is constant (94%) below 450 K, but decreases at higher temperatures. The similar trend was observed in isomerization of pentane. Thus, at 453 K, the selectivity of 97% was obtained at the pentane conversion of 40%. Under the same reaction conditions, the selectivity of 92% and the conversion of 58% were obtained at 473 K. Isomerization of heptane is more difficult than that of pentane o r hexane. Thus, the selectivity to hexane isomers was 70% at the conversion of 20% at 423 K. Since the presence of two components (Pd metal and H+) are essential f o r the reaction, there must be

-an optimum ratio of Pdo and H+

f o r the catalytic

TABLE 1 Effect of hydrogen partial pressure on the conversion of hexane and the product distribution. Partial pressure of H2 / kPa Conversion / % Selectivity / % Product distribution / % Ethane Propane Butanes Pentanes 2,2-Dimethylbutane 2 3-Dimethylbutane 2-Methvlventane . * 3-Methylpentane Methylcyclopentane Cyclohexane

0 2.1 41.9

30 7.2 82.5

71 29.8 89.6

0.0 3.3 4.8 3.3 trace

0.0 1.1 4.2 2.1 1.0

trace 1.5 4.5 2.4 3.1

28.1

57.2

59.7

13.8 46.7 0.0

24.3

26.9 1.0 0.9

8.0

2.1

Catalyst 50 wt% PdTP/Si02, Reaction temperature 483 K, W/F = 47.7 g.h.mol

-1

Hexane pressure 30 kPa, The data are average o f 1-5 h of the process time.

,

176 a c t i v i t y . Th er ef o r e, t h e c a t a l y t i c a c t i v i t y o f PdxH3-2xPW12040 supported on s i l i c a f o r hexane i s o m e r i z a t i o n was examined a s a f u n c t i o n o f x. The r e s u l t i s shown i n F i g . 7. The c o n v e r s io n o f hexane o v e r HTP was 5 %. The i n c r e a s e i n Pd(I1) i n t h e s t a r t i n g c a t a l y s t c a u s e s t h e enhancement o f t h e a c t i v i t y up t o

x = 0.75. The f u r t h e r i n c r e a s e i n x d i d n o t affect t h e c a t a l y t i c a c t i v i t y . The s e l e c t i v i t y d i d n o t depend on t h e c o n t e n t o f p a l lad i u m . I n o r d e r t o confirm t h a t p a l la d i u m metal p l a y s an i m p o r t an t r o l e i n al k an e i s o m e r i z a t i o n , HTP was s u p p o r t e d o v e r Pd(5%) on carbon which was o b t ai n ed from

a commercial s o u r ce. The r e s u l t i s shown i n Fig. 8 which shows t h e e f f e c t o f t h e r e a c t i o n t em p er a t u r e on t h e conversion and t h e s e l e c t i v i t y i n hexane i s o m e r i z a t i o n . The comparison o f F i g . 6 w it h F ig . 8 shows t h a t HTP su p p o r t ed on Pd/C g i v e s b e t t e r performance. The h i g h e r s e l e c t i v i t y was a t t a i n e d up t o 532 K t o g e t h e r w i t h h i g h e r a c t i v i t y . Thus, t h e s e l e c t i v i t y o f 97% was o b t a i n e d a t hexane co n v er s i o n of 78% a t 523 K. AgTP on Pd/C a l s o gave t h e h i g h a c t i v i t y .

REFERENCES 1 Y . Ono and T. Mori, J . Chem. SOC., Faraday Trans. 1, 77 (1981) 2209. 2 Y. Ono, T. Mori and T. Keii, Proc. 7 t h I n t e r n . Congress. Catal., Kodansha, Tokyo, 1981, 1006 pp. 3 T. Baba, J . S ak a i , H. Watanabe and Y . Ono, B ul l . Chem. SOC. Jp n ., 55 (1982) 2555. 4 M. F u r u t a , K. S a k a ta , M. Misono and Y. Yoneda, Chem. L e t t . (1979) 31. 5 N. Hayakawa, T. Okuhara, M. Misono and Y. Yoneda, Nippon Kagaku K ai sh i (1982) 356. 6 T. Okuhara, A. Kasai, N . Hayakawa, M. Misono and Y . Yoneda, Chem. L e t t . (1981) 391. 7 T. Okuhara, A. Kasai, N. Hayakawa, M. Misono and Y. Yoneda, Bu l l . Chem. SOC. J p n . , 55 (1982) 400. 8 T. Baba and Y . Ono, J . Phys. Chem., 87 (1983) 2406. 9 M. Misono, Proc. Climax 4 t h I n t . Conf. on Chemistry and t h e Uses o f Molybdenum, Climax Molybdenum Company, p. 289. 10 H. Niiyama, Y. S a i t o , S. Yoshida and E . Echigoya, Nippon Kagaku K ai sh i (1982) 569. 11 Y. Ono, T. Baba, J. S a k a i and T. Keii, J. Chem. SOC., Chem. Comm. (1981) 400. 12 T. Baba, J . Sakai and Y. Ono, B u ll . Chem. SOC. J p n . , 55 (1982) 2657. 1 3 Y . Ono, M. Kogai and T . Baba, Proc. P a n - P a c if i c Synfuel Conference Vol. 1, p. 115, 1982, Tokyo, J a p a n Petroleum I n s t i t u t e . 14 S. Yoshida, H. Niiyama and E . Echigoya, J . Phys. Chem., 86 (1982) 3150. 15 T. Baba and Y. Ono, J . Appl. C a t a l . , 8 (1983) 315. 16 T . Baba and Y . Ono, J . Phys. Chem., 87 (1983) 2406. 1 7 Y. Ono and T. Baba, Proc. 8 t h I n t e r n . Congr. Catal., 1984, Vol. 15, p . 405, Verlag Chemie. 18 S. Suzuki, K. Kogai and Y. Ono, Chem. L e t t . (1984) 699.

177

B. Imelik et al. (Editors), Catalysis b y Acids and Bases

o 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

NEW COVALENT BORON(J.11) -MOLYBDENUM(VI) MIXED OX0 MODEL COMPOUNDS AS ELIGIBLE HETERO BIMETALLIC CATALYSTS FOR PROPYLENE EPOXIDATION

E.TEMPEST1, L.GIUFFRE', C.MAZZOCCHIA and F. DI RENZO Politecnico di Milano, Dipartimento di Chimica Industriale e Ingegneria Chimica, Piazza Leonard0 da Vinci, 32 - 20133 Milano, Italia

ABSTRACT New boron(II1)-molybdenum(V1) mixed covalent 0x0 compounds have been tested in order to assess the possibility of modifying the electronic requirements for hydroperoxide activation through its coordination to the metal centre prior to the oxygen-transfer step. SOMMAIRE Nous rapportons l'6tude de nouveaux composes mixtes covalents de type 0x0 du bore(II1) et du molybd&ne(VI) qui ont 6th etudies pour verifier la possibilit4 de modifier la disponibilite Blectronique lors de la coordination metal-oxygene hydroperoxydique avant que le transfert d'oxygene n'ait lieu.

INTRODUCTION It is known that Shell Oil has recently developed ( 1 ) a titanium/silica catalyst for the epoxidation of propylene with alkyl hydroperoxides which is highly active and truly heterogeneous. The active catalyst contains tetrahedral Ti(1V) chemically bonded to siloxane ligands ( ZSiO) which are assumed tentatively to increase the electrophilicity (Lewis acid character) of the Ti(IV) while stabilizing active monomeric titanyl (Ti=O) species (2). On the other hand for the same reaction many attempts have been made ( 3 )

in

order to heterogenize more conventional molybdenum catalysts. As yet these approaches did not help in casting new light on different aspects of the reaction mechanism (e.g., specific metal-support interactions) which are still controversial or neglected. We have found that by using new model compounds such as

178

which have already been tested as epoxidation catalysts (4) and which may be heterogenized, it is possible to modify the electronic requirements for hydroperoxide activation through its coordination to the molybdenum centre prior to the oxygen-transfer step. This coordination is rather uninfluenced by generic ligand effects which normally are observable only in the initial stages of the reaction but it is mainly affected by the proximity of a stable B-0 covalent bond.

REFERENCE MODEL COMPOUNDS

The purity of catalysts (I) and (11) has been checked by elemental B/Mo plasma analyses and by comparison of X-ray diffraction pattern intensities (see Table 1) and infrared spectra obtained with reference model compounds such as MOO (acac) or 2-acetylacetonate-l,3,2-benzodioxaborole 2 2

which has been synthetized according to known procedures ( 5 ) .

TABLE 1 X-ray diffraction patterns

(11)

(1) d(i)

8.292 7.900 7.462 6.033 4.092 3.849 3.474 3.381 3.182

111,

.3 .2 .4 1.

.2

.3 .2 .2 .5

d

(i)

8.308 7.886 7.468 7.296 6.671 6.025 5.374 5.021 4.098 3.510 3.184

11x1)

111, 1.

.9 .5 .5 .1 .4 .1

.l .3 .3 .3

d

(i)

8.177 7.036 6.491 6.262 5.925 3.776 3.414 3.387 3.368

111, .5

.4 .5 1 .o

.5 .3 .3 .5 .5

179 STRUCTURAL ASSIGNMENTS BY I R ANALYSIS

-1 For compound (111) we have found t h a t t h e r i n g B-0 a b s o r p t i o n band (1480cm ) i s s u b s t a n t i a l l y h i g h e r t h a n t h a t normally found i n t e r v a l e n t boron-oxygen compounds. This i m p l i e s a B-0 bond o r d e r h i g h e r t h a n normaland would b e c o n s i s t e n t w i t h o t h e r o-phenylenedioxyboron compounds ( 6 ) having c o n t r i b u t i n g c a n o n i c a l forms t o type

@jJ0$Z 0x w i t h t h e boron atom i n an aromatic-type r i n g d i s p l a y i n g 6 % - e l e c t r o n resonance. -1 Due t o t h e abnormally low carbonyl s t r e t c h i n g frequency observed (1570 cm ) f o r t h e carbonyl group of t h e a c e t y l a c e t o n a t e l i g a n d , i t i s e v i d e n t however

that

t h e phenyl r i n g , by a c t i n g a s an e l e c t r o n s i n k , f u r t h e r d e l o c a l i z e s t h e charge d i s t r i b u t i o n r e p o r t e d f o r t y p e (IV). A s a r e s u l t t h e boron atom i s s t i l l e l e c t r o n d e f i c i e n t and c h e l a t i o n by t h e a d j a c e n t carbonyl oxygen o c c u r s ( 7 ) . Other -1 c h a r a c t e r i s t i c a b s o r p t i o n bands a r e found a t 1360 and 1240 cm which may b e a t t r i b u t e d t o t h e alkylboron-oxygen and ring-C-0

stretching frequencies respecti-

vely. Considering now compound ( I ) r e l a t i v e t o compound ( I I I ) , we f i n d t h a t c o n t r i b u t i n g c a n o n i c a l forms t o t y p e (IV) a r e s t i l l a c t i v e b u t t o a l e s s e r degree a s implied by t h e ring-B-0

and ring-C-0

s t r e t c h i n g f r e q u e n c i e s which do n o t s h i f t

b u t a r e s e n s i b l y l e s s i n t e n s e . S i g n i f i c a n t l y however a broadening of t h e band -1 p r e v i o u s l y a s s i g n e d t o t h e alkylboron-oxygen s t r e t c h i n g (1360 cm ) i s observed: t h i s e f f e c t may be a t t r i b u t e d t o t h e presence of t h e Moo2 moiety whose synnnetric -1 and asymmetric s t r e t c h i n g v i b r a t i o n s a r e found a t 940 and 905 cm respectively Even i n t h i s c a s e an abnormally l o w carbonyl s t r e t c h i n g frequency i s ob-1 served a t 1555 cm which now concerns t h e molybdenum c e n t r e .

(8).

The most s t r i k i n g f e a t u r e observed w i t h compound (11) concerns t h e s h i f t t o -1 lower f r e q u e n c i e s of t h e r i n g B-0 s t r e t c h i n g v i b r a t i o n (1450 cm ) . This s h i f t s t i l l i m p l i e s a B-0 bond o r d e r h i g h e r t h a n normal b u t i t can only b e c o n s i s t e n t

( 6 ) with c o n t r i b u t i n g c a n o n i c a l forms t o type

180 where the oxygen back-donating to the boron is attached to the dioxo molybdenum group whose symmetric and asymmetric stretching vibrations are now found at 960 -1 and 915 cm respectively. Other CGaracteristic absorption bands are found at -1 1565 cm

-1

(carbonyl bonded stretching frequency) and 1060 cm

which may be at-

tributed to the ring C-0 stretching frequency (6). By comparing contributing canonical forms to type (1V)and

(V) it may be seen that for the latter there is

no possibility of further charge delocalization.

On this basis, considering now compounds (I) and (11) relative to a conventional molybdenum catalyst such as MOO (acacI2, it may be reasonably assumed 2 that in both compounds (I) and (11) the Lewis acid character of the molybdenum centre is increased due to the presence of a vicinal B-0 covalent bond. Significantly this effect is more pronounced for compound (I).

CATALYTIC ACTIVITIES IN THE DECOMPOSITION OF 1-PHENYLETHYLHYDROPEROXIDE The decomposition of 1-phenylethylhydroperoxide ( 0 . 4 mol/l in ethylbenzene)

under an inert atmosphere. The sam-

was carried out in a glass reactor at 90'C

ples were analyzed iodometrically for active oxygen content. For the different -3 catalysts ( 3 . 5 ~ 1 0 mol/l) used, we report in Table 2 the observed initial rates r,, (mol/l sec) of decomposition.

TABLE 2 Initial rates of decomposition (mol/ lsec)

Catalyst MOO (acacl2 2

r0 -4 4.9 x 10

(11)

5.9

(I)

-4 9.7 x 10

(111)

0

These results fairly agree with the assumption cited above since, by comparing the measured initial rates of catalysed hydroperoxide decomposition, the reported ro decrease in the order (I) > (11) > MOO (acac)2. Thus the appearan2

ce of

a synergistic effect for our mixed catalysts may be attributed, at least

as far as hydroperoxide activation prior to the oxygen transfer step is concer-

181 ned, to a change of electron density of the molybdenum coordinating centre induced by boron. The kinetic data reported above have been obtained on the basis of the initial rates of decomposition. Analysis of the experimental data of l-phenylethylhydroperoxide decomposition obtained over a broad range of catalyst and hydroperoxide concentrations has shown that these data do not fit a simplified kinetic model such as

where k is the decomposition constant and K

c

is the stability constant of the

catalyst-hydroperoxide complex respectively. This is not totally unexpected since such a deviation in the rate reaction in the later stages has already been observed in a few similar cases (3)

and

has been attributed to an inhibiting effect of the alcohol which results from the decomposition of the hydroperoxide. In this case the presence of a stage of complex formation between the catalyst and I-phenylethanol which reduces the active concentration of the catalyst-hydroperoxide complex and which ultimately leads to a significant decrease in the initial rate of hydroperoxide decomposition has been confirmed experimentally. For each tested catalyst the obtained kinetic data are satisfactorily described by the following model

where Ki

is the stability constant of the catalyst-alcohol complex.

Work is in progress in order to fully evaluate for each catalyst the extent of autoretardation induced by alcohols. This effect, if properly related to the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes, may cast new light on the mechanism of the elementary act of hydroperoxide activation. ACKNOWLEDGMENT The authors wish to thank Prof. M. Zocchi for X-ray analyses.

182 REFERENCES Brit.Pat. 1 249 079 (71) to Shell Oil; U.S.Pat. 3 923 8 4 3 ( 7 5 ) , H.P.Wulff to Shell Oil. R.A.Sheldon, J.Mol.Cat. 7 (1980) 107. S.Ivanov, R.Boeva and S.Tanielyan, J.Cat., 56 (1979) 150. E.Tempesti, L.Giuff~-4,C.Mazzocchia, G.Modica and E.Montoneri, submitted to the 4th 1nt.Symp. on Homogeneous Catalysis, 24-28 Sept. 1984, Leningrad. H.Sch2fer and O.Braun, Naturwissenschaften, 39 (1952) 280. J.A.Blau, W.Gerrard, M.F.Lappert, B.A.Mountfield and H.Pyszora, J.Chem.Soc. 380 (1960). L.A.Duncanson, W.Gerrard, M.F.Lappert, H.Pyszora and R.Shafferman, J.Chem. SOC. 3652 (1958). R.J.Butcher, H.P.Gunz, R.G.A.R.Maclagan, H.K.J.Powel1, C.J.Wilkins and Yong Shim Hian, J.Chem.Soc. Dalton Trans. 1223 (1975) and references c i t e d therein.

183

B. Irneiik e t al. (Editors), Catalysis b y Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

CATALYTIC ACTIVITIES AND SELECTIVITIES OF CRYSTALLINE c-Zr(HP04)2

K. SEGAWA, Y . KURUSU and M. K I N O S H I T A Department of Chemistry, F a c u l t y o f Science and Technology, Sophia U n i v e r s i t y , 7-1 K i o i c h o , Chiyoda-ku, Tokyo 102 (Japan)

ABSTRACT C r y s t a l l i n e E-Zr(HPOt,)2(abbreviated as E - Z r P ) i s o b t a i n e d d u r i n g d e h y d r a t i o n o f amorphous z i r c o n i u m phosphate g e l ( a b b r e v i a t e d as Z r P - g e l ) w i t h p h o s p h o r i c a c i d s o l u t i o n under reduced pressure, f o l l o w e d by r e f l u x i n g i n s o l u t i o n : i t has no w a t e r of c r y s t a l l i z a t i o n and shows o t h e r d i s t i n c t f e a t u r e s . When c-ZrP was 1000 K), most phosphate groups(s98 % ) evacuated a t h i g h e r temperatures(700 were removed w i t h consequent loss o f w a t e r due t o t h e condensation o f phosphate groups between each l a y e r . T h i s c-ZrP which had been evacuated a t h i g h e r temp e r a t u r e s showed good c a t a l y t i c a c t i v i t i e s f o r t h e i s o m e r i z a t i o n o f butenes and cyclopropane. R e s u l t s i n d i c a t e t h e presence o f s t r o n g Br'dnsted a c i d i t y , which d e r i v e s f r o m r e s i d u a l phosphate groups.

-

INTRODUCTION

A metal hydrogen-phosphate g e n e r a l l y shows t h e a c i d - c a t a l i s t

activities.

Most m e t a l phosphate c a t a l y s t s a r e h y d r a t e d forms o f amorphous g e l s o r a c i d salts.

F o r these m a t e r i a l s , i t i s r a t h e r h a r d t o d i s t i n g u i s h t h e s p e c i f i c c a t a -

l y t i c a c t i v i t i e s , due t o t h e c o m p l e x i t y o f t h e i r s t r u c t u r e s b r o u g h t a b o u t by heat treatments. Z i r c o n i u m phosphates a r e w e l l known as i o n i c exchangers[l], workers have r e p o r t e d them[2,3,4]

as s o l i d a c i d c a t a l y s t s .

b u t o n l y a few

C l e a r f i e l d e t a1.[2,

3 1 and H a t t o r i e t a l . [4] r e p o r t e d about t h e c a t a l y t i c a c t i v i t i e s on ~ x - z r ( H P 0 ~ ) ~ . H 2 0 ( a b b r e v i a t e d as a-ZrP).

A f t e r c a l c i n a t i o n a t an e l e v a t e d t e m p e r a t u r e ( ~ 7 0 0K),

w Z r P shows h i g h e r c a t a l y t i c a c t i v i t i e s t h a n t h e o r i g i n a l c r y s t a l s .

These

t

a u t h o r s proposed t h e presence o f two t y p e s o f a c i d s i t e s : one i s H on t h e phosp h a t e group and t h e o t h e r s i t e s a r e e l e c t r o n d e f i c i e n t s i t e s [ 2 , 3 ]

o r f r e e phos-

p h a t e groups[4] on t h e s u r f a c e . We prepared h i g h l y c r y s t a l l i n e E-ZrP, w h i c h i s supposed t o have a more simple s t r u c t u r e t h a n a-ZrP, s i n c e i t has no w a t e r o f c r y s t a l l i z a t i o n between each layer.

This

E-ZrP

showed remarkable c a t a l y t i c a c t i v i t i e s f o r t h e i s o m e r i z a t i o n

of butenes and cyclopropane, i n comparison w i t h a-ZrP and o t h e r c o n v e n t i o n a l s o l i d a c i d s such as A1203 and Si02-A1203.

The p r e s e n t work has been done t o

s t u d y t h e s t r u c t u r e s of E-ZrP a f t e r c a l c i n a t i o n a t v a r i o u s temperatures and t o compare them w i t h t h e c a t a l y t i c a c t i v i t i e s f o r t h o s e s p e c i f i c s u r f a c e s .

184 EXPERIMENTAL P r e p a r a t i o n o f c r y s t a l l i n e z i r c o n i u m phosphate a-ZrP can be o b t a i n e d by r e f l u x i n g t h e ZrP-gel i n p h o s p h o r i c a c i d s o l u t i o n

[5].

B u t f o r t h e p r e p a r a t i o n o f c-ZrP,

a c i d i s required[6]. removal

a high

concentration o f phosphoric

The procedure i n d e t a i l i s n o t c l e a r .

Finally, during the

o f t h e h y d r a t e d w a t e r ( p a r t o f which i s w a t e r o f c r y s t a l l i z a t i o n ) o f

ZrP-gel by r e f l u x i n g w i t h p h o s p h o r i c a c i d ,

E-ZrP

supposed t o be c r y s t a l l i z e d de-

pending on t h e temperature and t h e process t i m e w i t h a s p e c i f i c c o n c e n t r a t i o n o f phosphoric acid.

I n o r d e r t o complete t h e d e h y d r a t i o n process, we heated t h e

ZrP-gel w i t h c o n c e n t r a t e d p h o s p h o r i c a c i d under reduced p r e s s u r e .

By t h i s p r o -

cedure, h i g h l y c r y s t a J l i n e s-ZrP has been o b t a i n e d i n a s h o r t e r process time. E-ZrP.

The s t a r t i n g m a t e r i a l o f ZrP-gel was o b t a i n e d as a g e l a t i n o u s amor-

phous p r e c i p i t a t e when an excess o f p h o s p h o r i c a c i d was added t o a z i r c o n y l n i t r a t e aqueous s o l u t i o n .

The p r e c i p i t a t e was washed w i t h d i s t i l l e d w a t e r , f o l -

lowed by f i l t r a t i o n and d r y i n g a t 330 K f o r 50 h.

The r e s u l t i n g ZrP-gel has 7.8

mol o f h y d r a t e d w a t e r and w a t e r o f c r y s t a l l i z a t i o n p e r Z r . ZrP-gel was mixed 3 w i t h p h o s p h o r i c a c i d s o l u t i o n ( l 5 molwdm- ) , f o l l o w e d by h e a t i n g up t o 453 K a t a c o n s t a n t temperature i n c r e a s e r a t e f o r 180 m i l

under reduced p r e s s u r e ( 2 . 7 kPa).

The w a t e r which e v o l v e d d u r i n g d e h y d r a t i o n o f ZrP-gel was removed f r o m t h e s i d e arm a t t a c h e d t o t h e system, t h e n t h e g e l was r e f l u x e d more p h o s p h o r i c a c i d ( l 5 3 molsdm- ) f o r 4 h. The c r y s t a l s were washed w i t h d i s t i l l e d w a t e r and d r i e d a t 383 K f o r 50 h. a-ZrP.

a-ZrP was o b t a i n e d by t h e method o f C l e a r f i e l d C S ] .

The r e s u l t i n g

c r y s t a l s were washed and d r i e d a t room t e m p e r a t u r e under reduced p r e s s u r e . Catalytic reactions I s o m e r i z a t i o n o f 12 kPa o f butenes o r cyclopropane was c a r r i e d o u t a t 323 Q 3 453 K by u s i n g a c l o s e d r e c i r c u l a t i o n system(230 cm ) . P r i o r t o r e a c t i o n , t h e catalyst(25

-

250 mg) was evacuated a t a s p e c i f i e d temperature.

RESULTS AND DISCUSSION C h a r a c t e r i z a t i o n o f z i r c o n i u m phosphates The thermal g r a v i m e t r i c a n a l y s i s(TGA) c u r v e and t h e t e m p e r a t u r e programmed decomposition(TPDE) spectrum o f s-ZrP under vacuum c o n d i t i o n s showed one-stage d e h y d r a t i o n due t o t h e c o n d e n s a t i o n o f phosphate groups w i t h consequent l o s s o f

1 mol o f w a t e r ( a b o u t 6 % w e i g h t loss). s - Z r ( HP04 )

A

ZrP2O7

D u r i n g t h e e v a c u a t i o n up t o 770

K, ~ 9 %8

+ H ~ O

o f r e a c t i o n of e q . ( l ) proceeded.

However, t h e E - Z r P , which was evacuated a t 773

185

K f o r 4 h p r i o r t o t h e TPDE examination, showed a t r a c e amount o f w a t e r which evolved a t the higher temperature region(800

- 1000 K ) .

These r e s u l t s suggest

t h a t %2 % of phosphate groups s t i l l remained on t h e s u r f a c e even a f t e r evacuat i o n a t h i g h e r temperature.

Those r e s i d u a l phosphate groups would be on t h e

c o r n e r s and edges o f c r y s t a l s .

I n c o n t r a s t w i t h E-ZrP, a-ZrP showed a two-stage

e l i m i n a t i o n o f w a t e r : t h e 1 s t - s t a g e corresponds t o t h e e l i m i n a t i o n o f 1 mol o f w a t e r of c r y s t a l l i z a t i o n , and t h e 2nd-stage t o t h e condensation o f phosphate groups between each l a y e r . Scanning e l e c t r o n micrographs(SEM) o f

E-ZrP

powder d i f f r a c t o m e t r y ( X R D ) p a t t e r n s i n F i g . 2.

a r e shown i n F i g . 1 and X-ray The e x t e r n a l appearance o f

E-ZrP,

(A) i n F i g . 1 a r e hexagonal p l a t e s whose average c r y s t a l dimensions a r e : 4.0 urn i n l e n g t h , 1.0 um i n w i d t h and 0.5 pm i n t h i c k n e s s .

The shape o f t h e s e c r y s t a l s

d i d n o t change even a f t e r e v a c u a t i o n a t 523 K; t h i s r e s u l t was a l s o c o n f i r m e d by XRD examination, w h i c h i s shown as (A) i n F i g . 2.

However, ( B ) i n F i g . 1, about

10 % r e d u c t i o n o f c r y s t a l s i z e o c c u r r e d a f t e r e v a c u a t i o n a t h i g h e r temperatures (750

T,

1100 K ) .

The c o n s t a t e d

c r y s t a l s gave XRD r e s u l t s which a r e s i m i l a r t o

t h e p a t t e r n s o f z i r c o n i u m diphosphate(ZrP207); t h e s e a r e shown as ( C ) and ( D ) i n F i g . 2. From t h e XRD p a t t e r n s , t h e s t r u c t u r e o f E-ZrP can b e assigned t o be a l a y e r e d one making r e f e r e n c e t o a-ZrP[7,8].

Each l a y e r c o n s i s t s o f planes o f z i r c o n i u m

atoms b r i d g e d t h r o u g h phosphate groups which a1 t e r n a t e above and below t h e metal atom planes.

As was s t a t e d p r e v i o u s l y , a f t e r e v a c u a t i o n a t 873 K, a t r a c e

amount o f r e s i d u a l phosphate groups s t i l l remained on t h e s u r f a c e , even though t h e XRD p a t t e r n s a r e q u i t e s i m i l a r t o z i r c o n i u m diphosphate.

F i g . 1 SEM photographs o f E - Z r P ;

On t h e o t h e r hand,

( A ) evacuated a t 373 K, ( B ) evdcuated a t 773 K.

186

I

10

I

I

I

I

I

I

40

30

20 213

Fig. 2 XRD patterns of E - Z r P evacuated a t different temperatures; ( A ) 298 -523 K, (B) 573 K, (C) 623 - 773 K, (D) 873 1073 K.

-

ill05 v(P-0)

A B

cD t

4000

I

3000

I

I

2000 1500 Wave number / cm-’

I

1000

I

1

500 250

Fig, 3 XR spectra of E-ZrP evacuated a t different temperatures; ( A ) 298 - 573 K, ( 0 ) 773 - 1073 K.

K, (6) 623 K, (C) 673

187 a-ZrP a f t e r e v a c u a t i o n a t 500 K whose chemical c o m p o s i t i o n i s e q u i v a l e n t t o

ZrP became amorphous. e v a c u a t i o n a t 1000

E-

I n a d d i t i o n , c r y s t a l s were s t i l l amorphous even a f t e r

K.

The I R spectrum o f

E-ZrP

evacuated a t 298

- 573

K i s shown as ( A ) i n F i g . 3.

Four m a j o r bands were observed f r o m 4000 t o 600 cm-l wavenumber r e g i o n . erence t o t h e I R d a t a o f i n o r g a n i c phosphorus compounds[9],

By r e f -

t h e s e f o u r bands can

be assigned as f o l l o w s : PO-H s t r e t c h i n g which g i v e s a band a t 3435 cm-’,

P-0 s t r e t c h i n g a t 1105 cm-l and P-0-H

P-0- s t r e t c h i n g a t 1140 c m - l ( s h o u l d e r ) , bending a t 910 cm-’.

ionic

Observed PO-H s t r e t c h i n g i s about 200

-

300 cm-l l o w e r

t h a n t h e normal mode o f v i b r a t i o n s o f H20(3756 cm-l f o r v 3 and 3653 cm-l f o r vl) i n Czv symmetry[lO].

These r e s u l t s suggest t h a t each phosphate group has a hy-

drogen bonding w i t h a n o t h e r phosphate group between each l a y e r . t i o n a t h i g h e r temperatures, shown as ( B ) , ( C )

A f t e r evacua-

and (D) i n F i g . 3, i n t e n s i t i e s o f

PO-H s t r e t c h i n g and i o n i c P-0- s t r e t c h i n g a r e decreased c o n c o m i t a n t l y .

The

s t r e t c h i n g and b e n d i n g v i b r a t i o n o f P-0-P appeared a t 980 cm-l and 750 cm-’; t h e i r i n t e n s i t i e s were i n c r e a s e d w i t h i n c r e a s i n g e v a c u a t i o n temperatures. Catalytic reactions I s o m e r i z a t i o n o f cyclopropane.

The r i n g opening i s o m e r i z a t i o n o f c y c l o p r o p a -

ne i s known t o be c a t a l y z e d by Br‘dnsted a c i d s [ l l , l 2 ] .

Reaction r a t e s a t 453 K

f o r i s o m e r i z a t i o n o f cyclopropane were measured: on E-ZrP evacuated a t 523 K t h e v a l u e was 3.9 x 3.1 x

lo-’

sec-1*m-2.

sec-1*m-2, w h i l e on t h e c a t a l y s t evacuated a t 773 K i t was Apparent a c t i v a t i o n e n e r g i e s f o r t h i s r e a c t i o n were ob-

t a i n e d : 54.0 k J - m o l - ’ on t h e c a t a l y s t evacuated a t 523 K and 69.0 k J - m o l - ’ a t 773 K .

The c a t a l y s t evacuated a t 773 K has a much s m a l l e r number o f phosphate

groups t h a n t h e c a t a l y s t evacuated a t 523 K.

I t i s i n t e r e s t i n g t h a t , even

though t h e p r o t o n i c c o n c e n t r a t i o n s a r e g e t t i n g s m a l l e r , t h e r e a c t i o n r a t e f o r i s o m e r i z a t i o n was enhanced and became about 80 t i m e s f a s t e r t h a n on t h e c a t a l y s t evacuated a t l o w e r temperatures(373 I s o m e r i z a t i o n o f butenes.

-

573 K ) .

Table 1 shows t h e c a t a l y t i c a c t i v i t i e s and s e l e c -

t i v i t i e s f o r t h e i s o m e r i z a t i o n o f 1-butene a t 353 K.

The c-ZrP which was evacu-

a t e d a t 773 K shows h i g h e r c a t a l y t i c a c t i v i t i e s t h a n t h o s e o f o t h e r forms o f z i r c o n i u m phosphates, such as a-ZrP and ZrP-gel o r o t h e r s o l i d a c i d s , such as alumina and s i l i c a - a l u m i n a c a t a l y s t s .

For

E-ZrP

c a t a l y s t , t h e a c t i v i t i e s and

s e l e c t i v i t i e s o f i s o m e r i z a t i o n o f butenes a r e d r a s t i c a l l y changed f o r below and above t h e boundary o f e v a c u a t i o n temperature a t 680 K.

I n a l l butenes, a c t i v i -

K a r e h i g h e r by about 3 o r d e r s o f magn i t u d e t h a n those on t h e c a t a l y s t evacuated below 680 K. The t e m p e r a t u r e a t

t i e s on t h e c a t a l y s t evacuated above 680

which t h e condensation o f phosphate groups i s almost c o m p l e t e d ( ~ 9 8% ) i s c o n s i s t e n t w i t h t h i s temoerature.

188

Table 1 C a t a l y t i c a c t i v i t i e s and s e l e c t i v i t i e s f o r i s o m e r i z a t i o n o f 1-butene on v a r i o u s solid acid catalyst. Catalyst

Evacuation temp.

cis/trans** S u r f a c e a r e a

Reaction r a t e *

/K

r

1010/sec-1*m-2

/m2*g-l

E-ZrP

473 773

118 28300

2.1 1 .o

4.5 5.0

a-ZrP

373 773

1 594

1.2 1 .o

11.6 12.3

Z r P-gel

373 773

79 348

0.9 1.1

5.4 4.9

A1 203***

773

48

2.6

177.0

Si0,-Al,O,****

773

3000

1.1

560.0

*

I n i t i a l r a t e o f i s o m e r i z a t i o n : React. temp., I n i t i a l product r a t i o . JRC-ALO-4 ****JRC-SAL-2

** ***

353 K; P1-b=12 kPa

1 -butene

cis

0 100

80

60

40

F i g . 4 I s o m e r i z a t i o n o f butenes a t 373 K on (B) evacuated a t 700 1100 K.

-

20 E-ZrP;

0

loo trans

( A ) evacuated a t 300

-

600K,

189 The t i m e courses f o r i s o m e r i z a t i o n o f butenes on s-ZrP c a t a l y s t s a r e shown i n F i g . 4.

R e s u l t s show t h e t y p i c a l a c i d - c a t a l y z e d r e a c t i o n s f o r a l l b u t e n e s [ l 3 ] .

Reaction r a t e s obeyed good f i r s t - o r d e r - k i n e t i c s ,

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

f r o m each butene a r e c o r r e l a t e d by eq.(2) as independent o f e v a c u a t i o n temperat u r e s o f s-ZrP.

T h i s suggests a three-component k i n e t i c system w i t h c o m p e t i t i v e

reversible reactions[l4].

{*}{

trans

trans

1-butene

I

1-butene

=

(2)

Cis

Two d i s t i n c t i v e r e a c t i o n mechanisms f o r butenes can be proposed on t h e s e c a t a l y s t s : w h i c h one o c c u r s depends on t h e t e m p e r a t u r e s o f evacuation. c a t a l y s t evacuated a t l o w e r temperatures(300

--

For t h e

600 K), t h e i n i t i a l c i s / t r m s

r a t i o o f t h e r e a c t i o n o f 1-butene was about 2; t h e r e a c t i o n proceeds on t h e t e r m i n a l phosphate group and t h e double bond oxygen(P=O) a t t h e same t i m e i n a conc e r t e d mechanism; t h i s i s shown i n F i g . 5.

The r a t i o o f s t a t i s t i c a l c o n c e n t r a -

t i o n o f gauche- and a n t i - 1 - b u t e n e i s 2 t o 1[15]. Evacuated a t 300

-

600 K;

For t h e c a t a l y s t evacuated a t

cis/trans = 2.

H

cis

gauche - 1 -butene

H3&b t!

H

H

CH2

trans

a n t i - 1 -butene O.-Pi -

Evacuated a t 700

c=c-c-c 1-butene

Fig. 5

-

1100 K;

-

.-,

cis/trans = 1.

secondary b u t y l carbenium i o n

#

*

,H

c=c

CHC ,H CH
Ho 2 -3.0 i n w a t e r .

These z e o l i t e s a r e known

t o be hydrophobic [ 2 , 3 ] , and, t h e r e f o r e , have good a f f i n i t y f o r e t h y l a c e t a t e i n an aqueous s o l u t i o n . An e s t e r i f i c a t i o n i s one o f t h e most i m p o r t a n t r e a c t i o n s c a t a l y z e d by a c i d s i n t h e chemical i n d u s t r y .

In t h e liquid-phase e s t e r i f i c a t i o n o f a c e t i c a c i d

w i t h b u t a n o l s , w a t e r i s produced and, t h e r e f o r e , t h e common s o l i d a c i d c a t a l y s t s i n s o l u b l e i n w a t e r a r e t h o u g h t t o be i n a c t i v e .

However, t h e h i g h - s i l i c a

z e o l i t e s b e i n g hydrophobic a r e expected t o be a c t i v e f o r t h e e s t e r i f i c a t i o n . T h i s s t u d y has examined t h e l i q u i d - p h a s e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h b u t a n o l s on h i g h - s i l i c a z e o l i t e s .

206 EXPERIMENTAL Ma t e r i a 1s H i g h - p u r i t y grade a c e t i c a c i d , n - b u t a n o l , i - b u t a n o l and t - b u t a n o l were used without f u r t h e r p u r i f i c a t i o n . The c a t a l y s t s used were HZSM-5, dealuminated HY and H-form c a t i o n exchange r e s i n s ( A m b e r l i t e ZOOC and Amberlyst 1 5 ) .

ZSM-5 z e o l i t e s w i t h v a r i o u s S i / A l

a t o m i c r a t i o s were s y n t h e s i z e d by a method s i m i l a r t o t h a t d e s c r i b e d i n M o b i l ' s p a t e n t [4].

The NaZSM-5 t h u s p r e p a r e d was t r a n s f o r m e d i n t o H-form by a The d e a l u m i n a t i o n o f Y

c o n v e n t i o n a l c a t i o n exchange procedure w i t h 1N HC1.

z e o l i t e by t r e a t i n g NaY (Toyo Soda M a n u f a c t u r i n g ) w i t h s i l i c o n t e t r a c h l o r i d e was performed i n a manner s i m i l a r t o t h a t i n v e n t e d by Beyer e t a l . [5].

The

dealuminated NaY z e o l i t e s w i t h v a r i o u s Si/A1 atomic r a t i o s were t r a n s f o r m e d i n t o H-form (DA1-HY) by a c o n v e n t i o n a l c a t i o n exchange procedure w i t h 0.5N

NH4C1 f o l l o w e d by t h e c a l i c i n a t i o n a t 773 K.

A l l o f t h e c a t a l y s t s exposed t o

a i r were used w i t h o u t any d e h y d r a t i o n t r e a t m e n t s . Procedure The l i q u i d - p h a s e e s t e r i f i c a t i o n o f a c e t i c a i d w i t h b u t a n o l s was c a r r i e d o u t i n a flask.

Unless o t h e r w i s e n o t e d , t h e r e a c t i o n t e m p e r a t u r e was 313 o r 333 K

and t h e i n i t i a l m o l a r r a t i o o f a c e t i c a c i d t o b u t a n o l was 1.

The r e a c t i o n

p r o d u c t s were analyzed by gas chromatography. RESULTS AND DISCUSSION The e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h

n- o r i - b u t a n o l was c a t a l y z e d n o t o n l y

by HZSM-5, DA1-HY and t h e c a t i o n exchange r e s i n b u t a l s o by t h e r e a c t a n t a c e t i c acid.

F i g . 1 shows t h e t i m e dependence o f t h e c o n v e r s i o n o f a c e t i c a c i d w i t h

and w i t h o u t HZSM-5.

I t i s c l e a r t h a t HZSM-5 e x h i b i t s t h e c a t a l y t i c a c t i v i t y

f o r t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n-butanol. The i n i t i a l r a t e s o f t h e e s t e r i f i c a t i o n w i t h c a t a l y s t s ( r ) and w i t h o u t c a t a l y s t s (rself)

were measured.

These measurements were performed i n t h e

c o n v e r s i o n range w i t h i n 2 %. I n e v e r y case, a l i n e a r r e l a t i o n s h i p between t h e c o n v e r s i o n and t h e r e a c t i o n t i m e was observed.

The i n i t i a l r a t e o f t h e

e s t e r i f i c a t i o n c a t a l y z e d s o l e l y by t h e s o l i d a c i d c a t a l y s t s ( r c a t ) were o b t a i n e d as t h e d i f f e r e n c e between r and rself. The rcatvalues f o r t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n- o r i - b u t a n o l on t h e DA1-HY and HZSM-5 z e o l i t e s w i t h v a r i o u s Si/A1 r a t i o s a r e shown i n F i g s . 2 and 3, r e s p e c t i v e l y .

With i n c r e a s i n g Si/A1 r a t i o s , t h e s u r f a c e o f z e o l i t e

becomes more hydrophobic and, t h e r e f o r e , have more a f f i n i t y f o r t h e r e a c t a n t s , w h i l e t h e number o f a c i d s i t e s decreases.

Hence,

an optimum Si/A1 r a t i o may

I n t h e case o f DA1-HY, t h e a c t i v i t y was maximized a t Si/A1 = 8 f o r t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n- and i - b u t a n o l s . The a c t i v i t y o f t h e

exist.

201

p a r e n t HY was n e g l i g i b l e s m a l l , because t h e p a r e n t HY was h y d r o p h i l i c .

I n the

case o f HZSM-5, t h e a c t i v i t y d i d n o t change r e m a r k a b l y w i t h Si/A1 r a t i o s and a c l e a r optimum Si/A1 r a t i o was n o t o b s e r v e d .

As shown i n F i g . 2, t h e HZSM-5 w i t h

a l a r g e c r y s t a l l i t e s i z e e x h i b i t e d a very low a c t i v i t y .

T h i s f a c t suggests

t h a t t h e d i f f u s i o n o f t h e r e a c t a n t s o r p r o d u c t s t h r o u g h t h e p o r e i s v e r y slow and, t h e r e f o r e , t h e e s t e r i f i c a t i o n t a k e s p l a c e m a i n l y on t h e e x t e r n a l s u r f a c e o f HZSM-5 c r y s t a l l i t e s .

Reaction conditions: t e m p e r a t u r e ; 313 K [CH3COOH] [n-BuOH] 0;

= 2.36 m o l / l = 9.45 m o l / l

w i t h 0.60 g-HZSM-5 ( S i / A 1 = 49) i n

10 m l A; w i t h o u t c a t a l y s t

R e a c t i o n t i m e /10 3x min Fig. 1.

E s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n - b u t a n o l w i t h and w i t h o u t HZSM-5

!-

I

0 Si/A1 r a t i o F i g . 2. E f f e c t o f Si/A1 r a t i o o f OA1-HY on t h e a c t i v i t y f o r t h e e s t e r i f i c a t i o n of a c e t i c a c i d w i t h n - b u t a n o l ( 0 ) o r i - b u t a n o l (*). R e a c t i o n c o n d i t i o n s : temperature, 313 K; [CH3COOH]/[BuOH] = 1

20

A

40

I

I

60

80

Si/A1 r a t i o F i g . 3. E f f e c t o f Si/A1 r a t i o o f HZSM-5 on t h e a c t i v i t y f o r t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n-butanol (0,A) o r i-butanol ( 0 ) . Reaction conditions: see F i g . 1 . (o,.), small c r y s t a l l i t e (30 - 60 nm); (A) l a r g e c r y s t a l l i t e (280 nm)

208 The a c i d s i t e s on t h e e x t e r n a l s u r f a c e o f HZSM-5 c r y s t a l l i t e can be poisoned

w i t h 4 - m e t h y l q u i n o l i n e whose m o l e c u l a r s i z e i s t o o l a r g e t o e n t e r t h e pores o f HZSM-5 a t r e l a t i v e l y low temperatures. 4 - m e t h y l q u i n o l i n e i s shown i n F i g . 4.

The e f f e c t o f p o i s o n i n g o f HZSM-5 w i t h The r e a c t i o n r a t e o f t h e e s t e r i f i c a t i o n

o f a c e t i c a c i d w i t h n - b u t a n o l ( r c a t ) was reduced t o a b o u t h a l f t h e o r i g i n a l v a l u e by t h e p o i s o n i n g .

On t h e o t h e r hand, t h e r e a c t i o n r a t e o f t h e

e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h i - b u t a n o l was reduced t o about 16 % o f t h e o r i g i n a l v a l u e by t h e p o i s o n i n g .

The e x t e r n a l s u r f a c e area o f t h e HZSM-5

c a t a l y s t used i s about s e v e r a l % of t h e t o t a l s u r f a c e area [6].

Therefore, i t

is suggested t h a t t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h i - b u t a n o l proceeds m a i n l y on t h e e x t e r n a l s u r f a c e o f z e o l i t e c r y s t a l l i t e s .

Isobuthyl acetate

formed i n t h e p o r e o f z e o l i t e may h a r d l y d i f f u s e t h r o u g h t h e p o r e poening o f t h e z e o l it e . I n t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n - b u t a n o l a t 313 o f t h e c a t i o n exchange r e s i n ( A m b e r l i t e 200C, rcat= 4.9 x

K, t h e a c t i v i t y mol.min-lag-’)

was h i g h e r t h a n t h a t of HZSM-5 ( S i / A l = 49) by a f a c t o r o f about 70. a s m a l l amount o f w a t e r ([H,O]

By a d d i n g

= 3.7 mol/l) t o t h e r e a c t i o n system, t h e

r e a c t i o n r a t e f o r t h e c a t i o n exchange r e s i n was reduced t o 35 % o f t h e o r i g i n a l v a l u e , w h i l e t h a t f o r HZSM-5 was reduced t o only 82 % o f t h e o r i g i n a l one. T h e r e f o r e , t h e c a t i o n exchange r e s i n i s more s e v e r e l y poisoned by water t h a n HZSM-5.

The d i f f e r e n c e i n a c t i v i t y between t h e c a t i o n exchange r e s i n and HZSM5 may be e x p l a i n e d as f o l l o w s ; t h e number o f t h e a c i d s i t e s i n u n i t w e i g h t of t h e c a t i o n exchange r e s i n i s more t h a n t h a t o f HZSM-5 by a f a c t o r o f about 5, and, moreover, t h e e s t e r i f i c a t i o n may be c a t a l y z e d p r e d o m i n a n t l y by t h e a c i d

0

2 3 4 - M e t h y l q u i n o l i n e added 1

/ m l .g-’ F i g . 4.

P o i s o n i n g of HZSM-5 ( S i / A l

= 49) w i t h 4 - m e t h y l q u i n o l i n e .

Reaction c o n d i t i o n s : see F i g . 2.

[CH3COOH] F i g . 5.

/mol. 1-1

P l o t s o f rcatvs. [CH3COOH].

R e a c t i o n c o n d i t i o n s : c a t a l y s t , HZSM-5 (Si/A1 = 49); temperature, 313 K.

209

s i t e s on t h e e x t e r n a l s u r f a c e o f c r y s t a l l i t e s i n t h e case o f HZSM-5, whose e x t e r n a l s u r f a c e area determined by t h e f i l l e d p o r e method [6]

i s 5.5 % o f t h e

t o t a l s u r f a c e area. The k i n e t i c s t u d y on t h e e s t e r i f i c a t i o n of a c e t i c a c i d w i t h n - b u t a n o l on HZSM-5 was made.

The r a t i o o f t h e c o n c e n t r a t i o n o f a c e t i c a c i d , [CH3COOH],

t h a t o f n - b u t a n o l , [n-BuOH], measured.

to

was v a r i e d and t h e i n i t i a l r e a c t i o n r a t e was

As shown i n F i g . 5, t h e e s t e r i f i c a t i o n c a t a l y z e d s o l e l y by HZSM-5

i s o f t h e f i r s t o r d e r w i t h r e s p e c t t o [CH3COOH] r e s p e c t t o [n-BuOH].

and o f t h e z e r o o r d e r w i t h

Then t h e r e a c t i o n r a t e , rcat, i s expressed as f o l l o w s :

where kcat i s t h e r a t e c o n s t a n t .

On t h e o t h e r hand, t h e e s t e r i f i c a t i o n

c a t a l y z e d b y t h e r e a c t a n t a c e t i c a c i d was f o u n d t o be o f t h e second o r d e r w i t h r e s p e c t i v e t o [CH3COOH1 and of t h e f i r s t o r d e r w i t h r e s p e c t t o [n-BuOH], i n d i c a t i n g t h e r e a c t i o n between a c e t i c a c i d and n - b u t a n o l was c a t a l y z e d by a n o t h e r a c e t i c a c i d molecule.

From t h e k i n e t i c s t u d y d e s c r i b e d above, i t i s

suggested t h a t t h e mechanism o f t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n - b u t a n o l

on HZSM-5 i s d i f f e r e n t f r o m t h a t o f t h e homogeneous e s t e r i f i c a t i o n . The HZSM-5 e x h i b i t e d t h e c a t a l y t i c a c t i v i t y f o r t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h t - b u t a n o l a t 333 K, w h i l e t h e r e a c t a n t a c e t i c a c i d d i d n o t . r e a c t i v i t y o f t - b u t a n o l on HZSM-5 a t 333

K

The

was about 1 / 7 o f t h a t o f n - b u t a n o l .

The d e h y d r a t i o n o f t - b u a t n o l t o produce i - b u t e n e t o o k p l a c e s i m u l t a n e o u s l y w i t h the esterification.

F i g . 6 shows t h e t i m e dependence o f t h e c o n c e n t r a t i o n s o f

t - b u t y l a c e t a t e and water.

The r a t e o f t h e d e h y d r a t i o n a t t h e i n i t i a l s t a g e

was much h i g h e r t h a n t h a t o f t h e e s t e r i f i c a t i o n b y a f a c t o r o f about 50.

c I

Reaction c o n d i t i o n s :

7

temperature; 333 K [CH3COOH] = 6.8 m o l / l n

[t-BuOH] = 6.8 m o l / l

-3. m I V

n

v

c a t a l y s t ; 0 . 5 g i n 7.35 m l

0 cu I

V

0 0

U

V

m

I U V

Reaction t i m e /min F i g . 6.

E s t e r i f i c a t i o n o f a c e t i c a c i d w i t h t - b u t a n o l on HZSM-5 ( S i / A l = 48)

210 The e s t e r i f i c a t i o n of a c e t i c a c i d w i t h i - b u t e n e i n t h e presence o f w a t e r was c a r r i e d o u t a t 333 K. and w a t e r .

i - B u t e n e gas was bubbled i n t o t h e m i x t u r e o f a c e t i c a c i d

Not o n l y t h e e s t e r i f i c a t i o n b u t a l s o t h e h y d r a t i o n t o o k p l a c e .

F i g . 7 shows t h e t i m e dependence o f t h e c o n c e n t r a t i o n s o f t - b u t y l a c e t a t e , t - b u t a n o l and w a t e r .

The i n i t i a l r a t e o f t h e h y d r a t i o n o f i - b u t e n e was much

higher than t h a t o f t h e e s t e r i f i c a t i o n . e q u i l i b r i u m w i t h i n 200 min.

The h y d r a t i o n seems t o a t t a i n

From t h e s e r e s u l t s , t h e f o l l o w i n g r e a c t i o n scheme

i n c l u d i n g t - b u t y l carbeniurn i o n i s proposed.

7 + H,’ C-C-OH d ‘-Ht

$

- H20

C-C-OH I +2C + H20

- H+ C \L-.c-c-c c=c-c

i 1

I

+.

H+

+ CH3COOH

- Ht

~

CH3COOC(CH3)3

The r a t e d e t e r m i n i n g s t e p o f t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h t - b u t a n o l o r i - b u t e n e may be t h e r e a c t i o n o f t - b u t y l carbenium i o n w i t h a c e t i c a c i d . The a c t i v i t y o f t h e c a t i o n exchange r e s i n ( A m b e r l i s t 15, rcat= 4.8 x mol-min-l.9-l)

was much h i g h e r t h a n t h a t o f HZSM-5 ( S i / A l = 48, rcat= 4.5 x

mol . m i n - l - g - ’ ) = 1.

by a f a c t o r o f about 100 a t 333

K and a t [CH3COOH]/[t-BuOH]

I t has been r e p o r t e d t h a t a l k a n e w i t h a q u a r t e r n a r y carbon can n o t e n t e r

t h e p o r e o f HZSM-5 a t 273 K [7].

T h e r e f o r e , t h e p r o d u c t t - b u t y l a c e t a t e , whose

m o l e c u l a r dimension i s t o o l a r g e t o e x i s t i n t h e p o r e o f HZSM-5, may n o t be formed i n t h e pore.

The e f f e c t i v e a c i d s i t e s may e x i s t s o l e l y on t h e e x t e r n a l

s u r f a c e o f HZSM-5 c r y s t a l l i t e s . Xps measurements suqqest t h a t t h e s u r f a c e Si/A1

Reaction c o n d i t i o n s : temperature; 333 K

[CH3COOH] = 15.6 m o l / l [H20] = 1.49 mol/l c a t a l y s t ; 1.00 g i n 10 ml f l o w r a t e of i - b u t e n e ; 3 ml/min

R e a c t i o n t i m e /min F i g . 7.

E s t e r i f i c a t i o n of a c e t i c a c i d w i t h i - b u t e n e on HZSM-5 ( S i / A l = 48)

211 r a t i o i s almost t h e same as t h e b u l k Si/A1 r a t i o [ 8 ] .

I f t h e number o f t h e

a c i d s i t e s on HZSM-5 corresponds t o t h a t o f A1 atoms, t h e number o f t h e e f f e c t i v e a c i d s i t e s can be o b t a i n e d f r o m t h e Si/A1 r a t i o and t h e r a t i o o f t h e external surface area t o the t o t a l surface area.

The number o f t h e e f f e c t i v e

a c i d s i t e s on HZSM-5 [ S i / A l

= 48,

= 36/438] may be 2.6 x

mol/g, w h i l e t h e number o f t h e a c i d s i t e s on

A m b e r l i s t 15 i s 4.4 x

mol/g.

(External surface area)/(Total surface area) Then, t h e t u r n o v e r f r e q u e n c i e s f o r HZSM-5

and f o r A m b e r l i s t 15 can be c a l c u l a t e d t o be 2.9 x

s - l and 1 . 8 x

l o m 3 s-’,

respectively.

The t u r n o v e r frequency f o r HZSM-5 i s n o t l o w e r t h a n t h a t f o r

A m b e r l i s t 15.

I f a h i g h - s i l i c a z e o l i t e w i t h v e r y l a r g e pores can be

s y n t h e s i z e d , i t s a c t i v i t y w i l l be h i g h . I n c o n c l u s i o n , t h e h i g h - s i l i c a z e o l i t e as HZSM-5 e x h i b i t s t h e c a t a l y t i c a c t i v i t y f o r t h e e s t e r i f i c a t i o n o f a c e t i c a c i d w i t h n-, i-, and t - b u t a n o l s . However, t h e a c t i v i t y o f HZSM-5 i s much l e s s t h a n t h a t o f t h e c a t i o n exchange r e s i n , because t h e a c t i v i t y o f HZSM-5 corresponds t o t h e a c i d s i t e s s o l e y on t h e external surface o f t h e c r y s t a l l i t e s .

REFERENCES 1 2 3 4 5 6 7 8

S. Namba, N. Hosonuma and T . Yashima, J . C a t a l . , 72 (1981) 16. N.Y. Chen, J . Phys. Chem., 80 (1976) 60. D.H. Olson, W.O. Haag and R.M. Lago, J . C a t a l . , 61 (1980) 380. B r i t . Pat., 1402981. H.K. Beyer and I. B e l e n y k a j a , Stud. S u r f . S c i . C a t a l . , 5 (1980) 203. I . Suzuki, S. Namba and T. Yashima, J . C a t a l . , 81 (1983) 485. S. Namba, A. Yoshimura and T. Yashima, Chem. L e t t . , (1979) 759. S. Namba, A. Inaka and T . Yashima, u n p u b l i s h e d d a t a .


213

B. Imelik et al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

THE MECHANISM OF n-PENTANE TRANSFORMATION OVER SOLID SUPERACIDS

-

AlZO3/A1C13

M. MARCZEWSKI Chemistry Dept., Warsaw T e c h n i c a l U n i v e r s i t y , 00 662 Warsaw/ Poland /

ABSTRACT Superacid p r o p e r t i e s o f A1203/A1C13 c a t a l y s t were s t u d i e d . I t was f o u n d t h a t pentane low t e m p e r a t u r e i s o m e r i z a t i o n occurs i n presence o f a c c e p t o r s i t e s w i t h a c t i v a t i o n energy o f 10 Kcal/mol. I n presence o f t h i s s u p e r a c i d c a t a l y s t pentane a l s o decomposes t o f o r m isobut a n e . The mechanism o f i s o b u t a n e f o r m a t i o n c a t a l y s e d by s u r f a c e a t t a c h e d c a r b o c a t i o n s has been discussed. RESUME e t u d i e e s . On Les p r o p r i @ t @ ssuperacides du c a t a l y s e u r A l Z 0 /AlC13 o n t @t@ montre que 1 ' i s o n e r i s a t i o n du n-pentane ?I basse Pemperature e s t c a t a l y s e e p a r l e s s i t e s a c c e p t e u r s avec une C n e r g i e d ' a c t i v a t i o n de lOKcal/mole. En presence de ce c a t a l y s e u r s u p e r a c i d e , l e pentane se decompose a u s s i en donnant de l ' i s o b u t e n e . On d i s c u t e l e mecanisme de f o r m a t i o n de l ' i s o b u t a n e p a r l ' i n t e r m e d i a i r e de c a r b o c a t i o n s l i e s a l a s u r f a c e . INTRODUCTION Aluminum o x i d e t r e a t e d w i t h AlC13 vapours i s one o f t h e most a c t i v e a c i d c a t a l y s t s and can be c o n s i d e r e d as s o l i d s u p e r a c i d ( r e f . 1 ) . The i n t r o d u c t i o n o f AlC13 o n t o a l u m i n a s u r f a c e s causes t h e f o r m a t i o n o f new s t r o n g a c c e p t o r s i t e s a b l e t o o x i d i z e p e r y l e n e i n t o c o r r e s p o n d i n g c a t i o n - r a d i c a l w i t h o u t oxygen p r e a d s o r p t i o n ( r e f . 1 ) . These c e n t r e s a r e formed i n t h e f o l l o w i n g way :

A l C13

+

9

A1 -0-A1 -0 -A1

-AA1 c1 -0-A1 -0

A l C l 3 r e a c t s w i t h e l e c t r o n d o n a t i n g exposed oxygen i o n s 02- c a u s i n g t h e e l e c t r o n s h i f t towards AlC13 adsorbed molecule. As a r e s u l t , s u r f a c e aluminum c a t i o n s w i t h pronounced d e f i c i t o f e l e c t r o n s a r e formed. I n o u r p r e v i o u s paper we have proposed t o r e l a t e c a t a l y t i c a c t i v i t y o f t h i s c a t a l y s t w i t h t h e s e s i t e s (ref.

I).

S o l i d s u p e r a c i d s a r e a b l e t o c a t a l y s e n - a l k a n e r e a c t i o n s a t l o w temperatures, even a t 298K ( r e f . 2 ) .

Products o f t h e s e r e a c t i o n s a r e s k e l e t o n isomers and l o w e r

hydrocarbons. Pentane f o r example r e a c t s t o f o r m i s o p e n t a n e and i s o b u t a n e . The mechanisme o f i s o b u t a n e f o r m a t i o n i s s t i l l c o n t r o v e r s i a l . Tanabe e t a l . ( r e f 2) showed t h a t i s o b u t a n e i s a secondary p r o d u c t o f isopentane decomposition w h i l e Gates e t a l . ( r e f . 3 ) c l a i m t h a t i t i s formed f r o m C1o i n t e r m e d i a t e .

214 The aim o f t h i s work was t o v e r i f y t h e h y p o t h e s i s t h a t a c c e p t o r c e n t r e s a r e r e s p o n s i b l e f o r s u p e r a c i d p r o p e r t i e s o f A1203/AlC13 system and t o s t u d y t h e mechanism o f pentane t r a n s f o r m a t i o n i n i t i a t e d by t h i s c a t a l y s t . METHODS Alumina, s i l i c a and s i l i c a - a l u m i n a 187 and 30 % of A12031 were o b t a i n e d by c a l c i n a t i o n a t 823K aluminum and s i l i c o n h y d r o x i d e s o r t h e i r c o p r e c i p i t a t e d m i x t u r e s . The h y d r o x i d e s were prepared by h y d r o l y s i s o f aluminum i s o p r o p o x i d e o r e t h o x y s i l i c o n . S u p e r a c i d c a t a l y s t s were o b t a i n e d by A1C13 s u b l i m a t i o n

x

(T = 573K, p = 1.3Nn-2) t h r o u g h t h e f r e s h l y c a l c i n e d (T = 773K, p = 1.3

1 0 - 2 N K 2 ) s u p p o r t . I R i n v e s t i g a t i o n o f NH3 and p y r i d i n e a d s o r p t i o n were p e r f o r med i n a s p e c i a l IR c e l l ( r e f . 1 ) u s i n g Specord I R 75 spectrophotometer. Onee l e c t r o n a c c e p t o r (0.e.a.)

and o n e - e l e c t r o n donor (0.e.d.)

p r o p e r t i e s were

e v a l u a t e d by p e r y l e n e and t e t r a c y a n o e t h y l e n e (TCNE) a d s o r p t i o n . The q u a n t i t y o f p e r y l e n e and TCNE i o n - r a d i c a l s formed was measured u s i n g J e o l 3X ESR s p e c t r o m e t e r . The number o f s u r f a c e h y d r o x y l s o f o x i d e c a r r i e r s was

e s t i m a t e d by

sodium n a p h t a l e n i d e t i t r a t i o n ( r e f . 4 ) . C a t a l y t i c a c t i v i t y measurements were c a r r i e d o u t u s i n g a 150 cc b a t c h r e a c t o r c o n t a i n i n g l g o f c a t a l y s t . RESULTS I n order t o evaluate the c a t a l y t i c a c t i v i t y o f acceptor s i t e s , the c a t a l y s t s which had been p r e p a r e d f r o m c a r r i e r s possessing d i f f e r e n t q u a n t i t y o f 0.e.d. c e n t r e s were chosen

.

Presence o f these s i t e s i s e s s e n t i a l i n a c c e p t o r c e n t r e s

f o r m a t i o n . I n T a b l e 1 t h e p r o p e r t i e s o f b o t h c a r r i e r s and s u p e r a c i d c a t a l y s t s a r e presented. Obtained r e s u l t s i n d i c a t e t h a t f o r a l l o x i d e s s t u d i e d t h e mechanism o f i n t e r a c t i o n between t h e s u r f a c e and AlC13 vapours i s s i m i l a r . One can observe t h e disappearance o f b o t h s u r f a c e h y d r o x y l s and B r o n s t e d a c i d i t y as w e l l as s u b s t a n t i a l r e d u c t i o n o f 0.e.d. 0.e.a.

c e n t r e s w i t h simultaneous i n c r e a s e o f

sites.

The p r o p e r t i e s o f o b t a i n e d s u p e r a c i d s depend on t h e c a r r i e r composition. The number o f a c c e p t o r s i t e s i n c r e a s e s w i t h S i O z c o n t e n t i n c a t a l y s t s under s t u d y w h i l e Lewis a c i d i t y d i s a p p e a r s f o r s i l i c a r i c h samples (30% A1203-Si02,

Si02).

One can e x p e c t t h a t these two p r o p e r t i e s s h o u l d change s i m i l a r l y because t h e y a r e b o t h connected w i t h t h e presence o f e l e c t r o n d e f i c i e n t aluminum c a t i o n s ( r e f . 5 ) . The observed phenomenon may be e a s i l y expla'ncd i f one assumes t h a t NH3 can be c o o r d i n a t i v e l y bonded o n l y by Lewis s i t e s f r o m A1203 s u b l a t t i c e . X-Ray analysis confirmed t h a t

6 -A1203

phase was p r e s e n t o n l y f o r two c a r r i e r s

s t u d i e d i . e . A1203 and 87% A1203-Si02. F u r t h e r c o n f i r m a t i o n o f above assumption g i v e s I R s p e c t r a o f adsorbed NH3

With the r i s e o f S i O e content i n t h e cata-

l y s t s s t u d i e d one can observe t h e r i s e o f 1550 cm-I band i n t e n s i t y .

215 TABLE 1 Physico-chemical p r o p e r t i e s o f c a t a l y s t s s t u d i e d ~ _ _ _ _ _

Catalyst

Acidity

O n e - e l e c t r o n prop.

L

B

Acceptor T o t a l A1203

aua

0.18

spin/g

0.10

-

0.29

100

0.05

-

0.03

0.17

0.18

-

0.02 0.05

-

100

-

100

126

-

190

-

-

50

-

200

OH

Donor mmole/g

4 min-l

425

0.6b

50 104

0.6

-

52

-

1.5

tr.

0.4

-

-

-

-

1.7

0.6

-

0.4

-

0.3

a I n t e g r a t e d i n t e n s i t y o f adsorbed NH3 IR bands / 1420 cm-l- B r o n s t e d a c i d i t y Lewis a c i d i t y ( L ) / , a f t e r d e s o r p t i o n a t 373K.

(B), 1620 cm-1

-

bone-el e c t r o n a c c e p t o r s i t e s of a1 umina phase. c Absence of IR a b s o r p t i o n bands a t 3700-3600 cm-'. 6 -2 I n i t i a l r e a c t i o n r a t e o f pentane i s o m e r i z a t i o n (T = 473K, p = l x 10 Nm ) . e 87-A1203 means s i l i c a - a l u m i n a composed o f 87% A1203 and 13% S i 0 2 . T h i s band was a s c r i b e d t o t h e NH2 s u r f a c e

groups

(ref.6).

I t seems t h a t

ammonia does n o t adsorb on s u r f a c e s i t e s : Si-0-A1C1 b u t r e a c t s t o f o r m Si-0-A1NH2 s p e c i e s . S i n c e Lewis a c i d s i t e s can r e a c t i n d i f f e r e n t ways w i t h NH3 depending on t h e i r l o c a t i o n (A1203 o r S i O z phase) t h e same phenomenon s h o u l d be observed w i t h 0.e.a.

c e n t r e s . The number o f 0.e.a.

s i t e s of A1203 phase can be

roughly estimated using the f o l l o w i n g formula : A1203 0.e.a.

s i t e s = t o t a l 0.e.a.-x.0.e.a.

s i t e s o f SiO2/AlCl3(x=.i37or.3)

and i s shown i n T a b l e 1. The comparison o f t h e q u a n t i t y o f t h e s e c e n t r e s w i t h t h e number o f 0.e.d.

s i t e s o f untreated supports confirms our hypothesis t h a t

a t l e a s t one k i n d o f a c c e p t o r c e n t r e s i s formed i n t h e r e a c t i o n between A l C 1 3 molecules and 0.e.d.

s i t e s o f surface. Superacid properties o f c a t a l y s t s stu-

d i e d measured by i n i t i a l r e a c t i o n r a t e o f n-pentane i s o m e r i z a t i o n change i n t h e same way as t h e c o n c e n t r a t i o n o f 0.e.a.

c e n t r e s o f A1203 phase.

216 The c l o s e r e l a t i o n s h i p suggests t h a t these s i t e s can be r e s p o n s i b l e f o r catal y t i c a c t i v i t y . To check t h i s hypothesis experiment w i t h c a t a l y s t on which p a r t o f 0.e.a.

c e n t r e s had been blocked w i t h perylene was performed, I n i t i a l reac-

t i o n r a t e of pentane i s o m e r i z a t i o n diminished ca t h r e e times. On the b a s i s o f above f i n d i n g s one can b e l i e v e t h a t 0.e.a.

centres o f A1203 phase a r e respon-

s i b l e f o r pentane i s o m e r i z a t i o n . Since Si02/AlC13 system possesses a small c a t a l y t i c a c t i v i t y one cannot exclude t h a t t h e s i t e s : -0-AlC12 o r (-0-)2AlCl r e s u l t i n g from A1C13 and OH r e a c t i o n have c e r t a i n superacid p r o p e r t i e s . To s t u d y t h e mechanism o f pentane i n t e r a c t i o n w i t h t h e s u r f a c e t h e n a t u r e o f A1203/AlCl3 c a t a l y s t under working c o n d i t i o n s was examined, The working condit i o n s were simulated i n I R experiments by a d s o r p t i o n of CgH12 a t t h e r e a c t i o n temperature. On such p r e t r e a t e d c a t a l y s t p y r i d i n e was adsorbed. The r e s u l t s a r e s u m a r i z e d i n F i g . 1.

t-

It

,3000

2800

I600

1400 t m'

Fig.1. a b s o r p t i o n s p e c t r a o f A1 03/AlC13 ( a ) , ( a ) + 2.7 x 104Nm-2 C5H12 a t 333K ( b ) , (b) + 2.7 x lO3Nm-2 6y a f t e r 1 h r evacuation a t 333K ( c ) .

Pentane a d s o r p t i o n r e s u l t s i n appearance o f new a b s o r p t i o n bands a t 2970, 2930 and-2877 cm-l t y p i c a l f o r s t r e t c h i n g v i b r a t i o n s o f CH3 and CH2 groups ( r e f . 7 ) . P y r i d i n e adsorbs on the s u r f a c e forming b o t h PyH'

(bands a t 1533 cm-l) and Py

c o o r d i n a t i v e l y bonded w i t h Lewis c e n t r e s (bands a t 1460-1440 cm-I). To s t u d y t h e mechanism o f pentane i s o m e r i z a t i o n t h e experiments w i t h d i f f e r e n t i n i t i a l s u b s t r a t e pressure has been performed. The r e s u l t s a r e presented on F i g . 2 . The i n i t i a l r e a c t i o n r a t e o f pentane i s o m e r i z a t i o n does n o t depend on s u b s t r a t e concentration.

W

> 2

&020 g u

I

200

400

600

P E N T A N E PRESSURE

Tr

800

F i g . 2. Dependence o f i n i t i a l r e a c t i o n r a t e o f pentane i s o m e r i z a t i o n (T=333K) on s u b s t r a t e i n i i a l p r e s s u r e ( 1 T r = 133.3 Nm- )

8

20 k0 60

TOTAL CONVERSlO N

F i g . 3. Dependence o f pentane t o i s o pentane ( a ) and i s o b u t a n e ( b ) convers t i o n s on t o t a l c o n v e r s i o n (T = 333K, p = 2.7 x 1 0 4 ~ m - 2 )

DISCUSSION Pentane r e a c t s i n presence o f a l l c a t a l y s t s s t u d i e d . The f o l l o w i n g p r o d u c t s a r e formed : isopentane, isobutane and s m a l l amounts o f isohexanes-less than 1% ( F i g . 3 ) . One can see t h a t isopentane and i s o b u t a n e a r e formed i n p a r a l l e l react i o n s . The i s o m e r i z a t i o n r e a c t i o n stops q u i c k l y w h i l e t h e decomposition proceeds u n d i s t u r b e d . Hence, these two r e a c t i o n s may be considered as independent and c a t a l y s e d by d i f f e r e n t a c t i v e s i t e s . Isopentane f o r m a t i o n The l i n e a r c o r r e l a t i o n between i n i t i a l r a t e o f n-pentane i s o m e r i z a t i o n and t h e number o f 0.e.a.

c e n t r e s o f A1203 phase as w e l l as t h e s e l e c t i v e p o i s o n i n g ex-

p e r i m e n t i n d i c a t e t h a t s u p e r a c i d a c t i v e s i t e s possess a s t r o n g a c c e p t o r n a t u r e . The mechanism of pentane a c t i v a t i o n by t h e s e s i t e s can be e x p l a i n e d by an anal o g y w i t h t h e a c t i o n o f l i q u i d superacids. I n s u p e r a c i d s o l u t i o n p r o t o n a t t a c k s C-H bond forming an u n s t a b l e carbonium c a t i o n ( I ) ( r e f . 8 ) , which decomposes w i t h H2 e v o l u t i o n and c a r b o c a t i o n ( 1 1 ) f o r m a t i o n . One may suppose t h a t i n t h e case o f s o l i d s u p e r a c i d s p r o t o n s w i l l be r e p l a c e d by s t r o n g acceptor cent r e s . Carbonium c a t i o n (111) r e s u l t i n g f r o m an a t t a c k o f a c c e p t o r s i t e C-H bond i n pentane decomposes t o f o r m adsorbed H- and c

~ c a t Hi o n - i s ~o p e n t~a n e p~ r e c u r -

s o r . The d i f f e r e n t decomposition of c a t i o n (111) i s a l s o p o s s i b l e . I n such a case hydrocarbon c h a i n ( I V ) remains on t h e s u r f a c e e x c l u s i v e of H-.

H+ +H

-

+H ,

CH2C4Hg

>-CH2C4Hg ( I )

3

( II) + C H ~ C ~ H ~ + H - H

L+ +H

- CH2C4Hg

)--CH2C4Hg

( II) + c H ~ c ~ H ~ + H - L (IV)L-CH~C~H~+H+

superacid s o l u t i o n

H+,

s o l i d superacid

(111)

218 The I R e x a m i n a t i o n o f pentane a d s o r p t i o n on A1203/A1C13 c a t a l y s t c o n f i r m e d t h e e x i s t e n c e o f such s u r f a c e s p e c i e s ( F i g . 2 ) . I t seems p l a u s i b l e t h a t t h e s e spec i e s c o u l d be r e s p o n s i b l e f o r a c t i v i t y decay. P e n t y l c a t i o n ( 1 1 ) formed i n t h e a d s o r p t i o n s t e p o f t h e r e a c t i o n can i s o m e r i z e and desorb f r o m t h e s u r f a c e as isopentane. To c o n f i r m such a r e a c t i o n pathway t h e Langmuir-Hinshelwool t r e a t ment has been a p p l i e d . S i n c e t h e i n i t i a l r e a c t i o n r a t e i s independent on pentane p r e s s u r e ( F i g . 2 ) one can assume t h a t e i t h e r s u r f a c e r e a c t i o n - c a t i o n ( 1 1 ) isomerization, o r isopentane desorption i s t h e r a t e determining step. For these two s t e p s t h e independence o f i n i t i a l r e a c t i o n r a t e on s u b s t r a t e c o n c e n t r a t i o n

i s p o s s i b l e i f t h e p r o d u c t o f pentane i n i t i a l c o n c e n t r a t i o n and pentane adsorpt i o n e q u i l i b r i u m c o n s t a n t i s g r e a t e r t h a n 1 ( r e f . 9 ) . S i n c e on t h e b a s i s o f d a t a p r e s e n t e d i n t h e work ( r e f . 10) one can assume t h a t a d s o r p t i o n e q u i l i b r u i m c o n s t a n t s of b o t h pentane (KI)and

i s o p e n t a n e (KIII)

a r e v e r y c l o s e and pentane

t o i s o p e n t a n e c o n v e r s i o n ( x ) i s f o r a l l c a t a l y s t s s t u d i e d l e s s t h a n 13% t h e f o l l o w i n g i n e q u a l i t y : KI(l-x))>KIIIx

seems t o be t r u e . T a k i n g t h i s s i m p l i f i -

c a t i o n two d i f f e r e n t r a t e e x p r e s s i o n s can be reduced t o one : r = AST (BCo)-I where, A, B : c o n s t a n t s , ST : number o f a c t i v e s i t e s , Co : i n i t i a l pentane concentration S i n c e S, has n o t a c o n s t a n t v a l u e b u t d i m i n i s h e s d u r i n g t h e r e a c t i o n c a u s i n g a c t i v i t y decay, t h e r e a c t i o n r a t e can be expressed i n terms o f Time On Stream Theory o f c a t a l y s t d e a c t i v a t i o n i n t h e f o l l o w i n g way ( r e f . 11) : r = A1 ( 1

+

A2 t ) - N

where, A1, A2, N : c o n s t a n t , t : r e a c t i o n t i m e . To s o l v e t h i s e q u a t i o n t h e l e a s t square method was a p p l i e d . The e x p e r i m e n t a l v a l u e s of r e a c t i o n r a t e s were o b t a i n e d b y numerical d i f f e r e n t i a t i o n o f " s p l i n e " f u n c t i o n s ( r e f . 12) which had been used t o approximate t h e changes o f pentane c o n v e r s i o n vs. r e a c t i o n t i m e . The knowledge of A 1 a l l o w e d t o c a l c u l a t e t h e r a t e c o n s t a n t and t h e n t h e a c t i v a t i o n energy o f i s o p e n t a n e f o r m a t i o n . When d e s o r p t t o n was r a t e l i m i t i n g s t e p Ea= 8 k c a l / m o l , w h i l e f o r t h e o t h e r cases

E a = IOkcal/mol.

Isobutane formation Isobutane, t h e main p r o d u c t o f pentane t r a n s f o r m a t i o n ,

i s formed i n presence o f

d i f f e r e n t a c t i v e s i t e s t h a n t h o s e which c a t a l y s e pentane i s o m e r i z a t i o n . S i n c e i s o b u t a n e f o r m a t i o n needs c e r t a i n i n d u c t i o n p e r i o d ( f i g . 3 ) one can suppose t h a t t h e a c t i v e c e n t r e s i n t h i s r e a c t i o n a r e formed on t h e c a t a l y s t s u r f a c e upon s u b s t r a t e a c t i o n . P y r i d i n e a d s o r p t i o n on t h e A1,03/A1C13

catalyst with

219

preadsorbed pentane p r o v e d t h e e x i s t e n c e o f a c i d c e n t r e s of Lewis and B r o n s t e d t y p e s ( F i g . 1 ) . The t i m e of pentane a d s o r p t i o n was as l o n g as t h e t i m e o f r e a c t i o n needed t o t o t a l d e a c t i v a t i o n o f i s o m e r i z i n g s i t e s , s o observed a c i d c e n t r e s a r e d i f f e r e n t f r o m t h o s e t y p i c a l o f f r e s h c a t a l y s t . The a c i d p r o p e r t i e s o f pentane t r e a t e d A1203/AlC13 c a t a l y s t may be connected w i t h t h e presence o f s u r f a c e h y d r o c a r b o n - l i k e species ( I V ) . The u n i q u e e x p l a n a t i o n o f a c i d p r o p e r t i e s o f adsorbed hydrocarbon c h a i n s i s an assumption t h a t t h e y a r e p r e s e n t i n an as s u r f a c e c a t i o n s . Such c a t i o n s may f r o m as a r e s u l t o f

i o n i z e d from i.e.

i n t e r a c t i o n o f adsorbed hydrocarbon ( I V ) w i t h a d j a c e n t a c c e p t o r s i t e s o r w i t h o t h e r c a t i o n s p r e s e n t i n r e a c t i n g system : i - p e n t y l c a t i o n s . To check t h e poss i b i l i t y o f i s o b u t a n e f o r m a t i o n upon t h e a c t i o n o f s u r f a c e bonded c a t i o n s t h e r e a c t i o n o f pentane w i t h e t h y l c h l o r i d e was performed. I t i s b e l i e v e d t h a t i n presence o f s u p e r a c i d s a1 k y l c h l o r i d e s f o r m c o r r e s p o n d i n g a1 k y l c a t i o n s ( r e f . 3 ) C2H5C1 i n f l u e n c e s i s o b u t a n e f o r m a t i o n r i s i n g i t s y i e l d by f o u r t i m e s . One can e x p l a i n such a phenomenon as a r e s u l t o f f o l l o w i n g r e a c t i o n s :

(V) L

-

+

RCH2'

( CH3)3CH

+

-

L

-

R

-

CH2,

H~C-CHZC~H~

L-RCH2CH2'

-

'\+ >--CH2C3H7

(VI)

I '

7

H3C ' 'CHzC3H7

+

L

-

R

-

C2H5

I

TABLE 2 n-Pentane c o n v e r s i o n s i n t o d i f f e r e n t p r o d u c t s f o r t h e r e a c t i o n s o f : pentane

(I),

(I)

+

C2H5C1/30% mol./

( 1 1 ) . R e a c t i o n temperature 473K.

Reaction

CH4

C3H8

nC4H10

I

0.2

0.7

0.3

6.9

10.4

1.4

18.8

0.5

3.5

1.8

20.5

11.9

1.4

41.5

I1

iC4H1 iCgH12 (%moP)

iCgH14

T o t a l conv.

S u r f a c e c a t i o n s (V) o r C2H5+ a t t a c k t h e C-C bond o f pentane m o l e c u l e . The unst a b l e i o n ( V I ) , p r o d u c t o f t h i s r e a c t i o n , e a s i l y decomposes w i t h b u t y l a c t i o n and l o n g e r h y d r o c a r b o n - l i ke species f o r m a t i o n . S i m i l a r r e a c t i o n were p o s t u l a t e d by Olah e t a l . ( r e f . 1 3 ) f o r a l k y l c h l o r i d e s w i t h alkanes. Completing t h e r e a c t i o n s t h e i s o b u t y l c a t i o n a b s t r a c t s H- f r o m s u r f a c e bonded hydrocarbon and desorbs as i s o b u t a n e t o r e s t o r e t h e a c t i v e s i t e .

220

REFERENCES

A. K r z y w i c k i and M. Marczewski, J.C.S. Faraday I , 76 (1980) 1311-1322 0. Takahashi, T. Yamaguchi, T. Sakuhara, H. H a t t o r i and K . Tanabe, B u l l . Chem. SOC. Jpn. 53 (1980) 1807-1812. 3. G.A. Fuentes and B.C. Gates, J . Catal. 76 (1982) 440-449. 4. J. K i j e n s k i and R. Hombek, J. C a t a l . , 50 (1977) 186-189. 5. B.D. F l o c k h a r t , I . R . L e i t h and R.C. Pink, Trans. Faraday SOC., 62 (1966) 730-740. 6. J.B. P e r i , J. Phys. Chem. 69 (1965) 231-239. 7. J. Datka, Z e o l i t e s , 1 (1981) 113-116. 8. G.A. Olah, G. Klopman and R.H. Schlosberg, J . Am. Chem. SOC., 91 (1969) 3261-3268. 9. Z.G. Szabo, D. K a l l o (Eds.) Contact c a t a l y s i s , Akademiai Kiado, Budapest 1976, pp. 480-537. 10. E . Baumgarten, F. Weinstrauch and H. Hoffkes, J . Chrom., 138 (1977) 347-354. 11. B.W. Wojciechowski, Cat. Rev. - S c i . Eng., 9 (1974) 79-113. 12. G. D a h l q u i s t , A. B j o r c k , Numerical Methods, PWN Warszawa 1983, pp. 128130. 13, G.A. Olah and J. Kaspi, J. Org. Chem., 42 (1977) 3046-3050.

1. 2.

221

B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

FACTORS AFFECTING THE DEACTIVATION OF ZEOLITES BY COKING Eric G. Derouane Facultes Universitaires de Namur, Laboratoire de Catalyse, Rue de Bruxelles, 61, B-5000 Namur. Belgium

AB ST M CT Deactivation by coking is observed in all heterogeneous acid-catalyzed reactions, for example, those occurring in zeolites. The formation of heavy hydrogen deficient molecules is catalyzed in competition or subsequently to the desired conversion sequence and affects the catalyst activity and selectivity. Catalyst aging results from the convolution of two distinct factors, site poisoning and pore blockage, which can sometime be identified using a proper combination of techniques provided enough information is available about the intimate structure of the catalyst. Coke deposition in zeolites is constrained by their molecular shape selective properties which hinders the formation of given coke precursors and their microporous volume which limits the amount of intracrystalline coke that can be deposited. The aging characteristics of zeolites are therefore intimate functions of their pore structure and acid site distribution. INTRODUCTION Carbonaceous residues are the inevitable by-products of most heterogeneouslycatalyzed organic conversions. The term "coke" designates such deposits which often encompass a mixture of hydrogen-deficient molecules, for example, heavy polynuclear aromatics. The formation of coke (coking) is most often acidcatalyzed and it is therefore a major concern when using solid acid catalysts, zeolites, f o r example.

Understanding the mechanisms which control coking, and

its effect on catalytic properties such as activity and selectivity, is then essential in terms of catalyst selection and process design. Extensive reviews discuss the deactivation by coking of acid catalysts (ref. 1,2)

or point out to the shortcomings of conventional approaches (ref.3,4).

An

essential principle which must be recognized in empirical studies is that the aging variable is coke and not time, as coke formation itself is acid-catalyzed, competitively or consecutively to the main reaction sequence. Although interrelated, catalyst coking and aging (deactivation) are therefore distinct processes (ref.3-5). This review aims at delineating the respective roles of the catalytic sites and of steric effects in the deactivation of acid catalysts. For this purpose, zeolites are ideal systems because of the variety of pore networks they represent and of the possibility of varying the concentration of acid active sites

222 in their intracrystalline volume where reactions take place.

Mechanisms

responsible for the formation of coke are not discussed in great detail.

In

contrast, the emphasis is set:

1.

on the relative and complementary functions in aging of site poisoning

and pore blockage by coke, and the identification of such contributions in catalyst deactivation, 2.

on the constraints imposed on coke formation by the zeolite structure,

coke formation being a molecular shape selective process (ref.5-11), 3.

and

on the effect of coke deposits on catalytic activity and selectivity.

Although coking is generally detrimental to catalyst operation, beneficial effects may also be observed. INTRACRYSTALLINE ACID CATALYSIS Zeolites are crystalline materials, usually aluminosilicates, which p o s s e s s characteristic pore networks cages

--

--

encompassing channels, intersections, and

of aperture comparable to molecular size.

Figure 1 compares the main

features of the pore systems of some industrially important zeolites.

Small

pore size zeolites (Type A, Erionite, Ferrierite) accept only in their

CAVITY SIZE

(1)

Fig. 1. Pore structure of some industrially important zeolites (with permission from Academic Press (ref.5)).

intracrystalline volume linear aliphatic molecules.

In contrast, large pore

zeolites (Mordenite, Offretite, Type X and Type Y isotypic to Faujasite) can sorb rather bulky compounds such as poly-alkylaromatics. zeolites (ZSM-5,

Intermediate pore size

ZSM-ll), fill the gap between these two classes.

Small and

intermediate pore size zeolites are usually referred to as molecular shape selective materials in view of their ability to discriminate finely between molecules or different size and conformation (ref.5,6).

A s discussed below,

this classification of zeolites bears directly on the understanding of their coking mechanisms and aging behaviors. Acid sites in zeolites can be either hydroxyl groups (Brdnsted sites), the protons compensating for the negative charges associated to the presence of framework aluminum replacing silicon, or Lewis sites produced by elimination of water from these hydroxyls.

The nature and concentration of these acid sites

are other factors which affect coking and aging. KINETICS OF COKE FORMATION The time-on-stream theory has been applied in all the conventional descriptions of the deactivation of acid catalysts by coking (ref.12-15).

The amount

of coke formed and the activity decay are expressed in these cases as functions of the process time.

However, as coke formation is also catalyzed and depends

on the concentration of the reacting species, aging cannot be a simple function of time.

Both the main reaction and the coking reaction(s) can be affected by

coke formation. Catalyst deactivation must hence be expressed by a deactivation function which is related to coke content rather than to catalyst operating time (ref.3,4,16). Recent models (ref.17,18) applicable to porous catalysts consider that coke deposits affect catalytic activity in two different ways, site coverage ( A , poisoning) and pore blockage (B).

Situation A is that of a poisoned site in an

open pore whereas situation B corresponds to an inaccessible active site therefore deactivated

--

in a blocked pore.

--

The kinetics of catalyst aging can

then be expressed conveniently as a function of two probabilities, i.e., P(t) the probability for a site to be accessible at time t and S(t) the conditional probability that this particular site is not poisoned (covered) at the same time.

The same analysis must of course hold for zeolites which are microporous

acid catalysts.

In the latter case, P(t) depends on the structure of the

zeolite pore network which controls the access to the active sites. S(t) is essentially related to the zeolite pore size which may impose constraints on the deposition of coke (molecular shape selectivity). Site occupancy during the catalytic process can be reversible (by reactants, products, and reaction intermediates) or irreversible (by extraneous poison molecules, heavy sorbed products, or coke).

The latter case is referred to as

site coverage and is at the origin of deactivation. This paper only considers site coverage and deactivation by coke or heavy residues with low diffusivities in the reaction conditions. In the particular case of zeolite catalysts for which a variety of pore networks are encountered, several mode'l cases exist which should lead to distinct P(t) functions. We propose the classification of pore blockage effects into the five categories schematized in ~ i 2. ~ .Although the mathematical expressions of P(t) have not yet been derived for these cases -- research in this direction should be stimulated and is contemplated by the author -- a qualitative description of aging by pore blockage effects can be proposed for increasing coke content. Zeolites with non-interconnecting uniform channels (I) or noninterconnecting non-uniform channels (111) will age more rapidly, all other variables being identical, than their counterparts with interconnecting channels (11) or cages (IV). Interconnected pore networks indeed offer a much larger number of access paths to active sites through which molecules can diffuse randomly, thereby decreasing the number of situations B (free site in blocked pore) mentioned above. The occurrence of cages in non-interconnecting (111) or interconnecting (IV) networks provides room to accommodate initially some coke without immediately blocking the pores.

These deposits, however, may

grow to a size greater than the pore or window aperture, leading to a situation

I

NON-NTERCONNECTINQ UNIFORM CHANNELS

Ill. NON-INTERCONNECTING

NON-UNIFORM CHANNELS

11. lNTERCONNECTlNG CHANNELS

V.

DIFFUSION CONSTRAINTS

Fig. 2. Classification of pore blockage effects in zeolites

such that the catalyst cannot be regenerated under mild conditions. This particular effect was observed for the isomerization-oligomerization of 1-hexene on a rare-earth-exchanged X zeolite and referred to as the "Faujasite Trap" (ref. 19).

High olefinic oligomers (C6nH12n, n=2-5) were found as intracrystalline

product, although not appearing in the liquid product, and caused rapid deactivation of the catalyst. For cases 11-IV, two pore dimensions may be necessary to define the pore system: P, for the primary (larger) channels (or cages), and S, for the secondary (smaller) channels (or windows).

If S

8 LL z

a

c

D

0

-

L

P

0.1

8

I

z

0.01

I

I

I

10

kNC6lksMp

AT 427T

Fig. 5. Coke yield as a function of shape selectivity for the conversion of paraffins by acidic zeolite catalysts (with permission from Academic Press (ref. 9)).

228

blockage by high molecular weight (nearly) linear oligomers if the reaction temperature is too low to induce cracking (ref.27,35-37,50-51).

When aromatic

coke is formed, in large pore zeolites or at the surface of all zeolites, a variety of reactions may take place and sophisticated mechanisms have been proposed for the formation of coke precursors such as for example those shown in Fig. 6 for coking during reaction of butadiene over zeolite (Na,H)-Y (ref.33).

F

c-c-c-c-c

20

96

and o t h a l u m r s

Fig. 6. Formation of coke precursors from butadiene (with permission from Elsevier Sci. Pub. Co. (ref.33)). Figure 4 shows that coke deposition in hydrocarbon reactions is either consecutive or competitive to the main reaction sequence. Experimental evidence exist for both cases which may also occur simultaneously (ref.11,38).

Coke

formation being catalyzed, carbonaceous residues should be looked at as any other reaction products. Therefore, the instantaneous coke yield at a given time will depend on the process (contact time, space velocity (WHSV),

...)

or

catalyst (concentration in acid sites,...) characteristics. This is illustrated in Fig. 7 which describes possible reaction pathways for consecutive (A) and competitive or parallel (B) coking.

In both cases, coke formation increases

with higher contact time (lower space velocity, higher concentration in active

...) and decreases when shape selective constraints are operative (dashed

sites, area).

When coke formation is parallel to the main reaction sequence, decreas-

ing contact time (B,3) always reduces coke and product yields. In contrast, two different situations are met when coking is consecutive. Reducing the

229

CONTACT

TIME, l/WHSV. SITES

Fig. 7 . Reaction pathways for the formation of products and coke. A . Consecutive coking; B. Competitive coking. R = Reactants; P = Products; I = Internediates; C = Coke. contact time at high space velocity ( A , 1 ) also decreases coke and product yields whereas at low space velocity (A,2) coke is reduced and products are enhanced. Obviously, optimum operating conditions must exist which maximize the product/ coke yield ratio. They depend not only on intrinsic reaction kinetics but also on catalyst characteristics such as the accessibility and concentration of the acid sites. In other words, for a given reaction and predetermined operating conditions with isostructural catalysts (i.e., having the same structure and shape selective properties), the rates of coking and deactivation (therefore also the product selectivity and the catalyst lifetime) are expected to vary with the concentration of structural aluminum, possibly showing an optimum. Ideal operating conditions will vary with time-on-stream to account for the loss of sites deactivated by coking.

Two additional remarks need to be made in relation to the former discussion. Decreasing consecutive coke formation by reducing the contact time at low space velocity will obviously not increase the product yield in any observable way if the coke deposition kinetics is much slower than the rate of product formation. However, it will still manifest itself as a decrease in aging rate.

In addition,

changing the density of acid sites may have other effects than simply varying the effective contact time. For example, it can also modify the relative rate constants for product and coke fornation as discussed in the next section. A remaining mechanistic question concerns the nature of the active sites in-

volved in coke formation. From a comparison of the initial rates of coking of

ZSM-5, offretite, and modenite during the conversion of methanol to hydrocarbons, it was concluded that Broensted sites played an essential role (ref.20). Initial coking rates per acid Broensted site were found identical in all three cases. An EPR study of the activation of benzene over €3-ZSM-5 and H-mordenite indicated that Lewis acid sites were needed for aromatic coupling (producing

230 diphenyl) (ref.54), but we will show below that such reactions are probably not the major contributors to coking. A recent study of coke formation during the cracking of olefins over H-Y zeolites indicated that coke formation was directly proportional to the consumption of hydroxyl groups (ref.39).

Dehydroxylated

catalysts however behaves similarly, suggesting a synergism between Brbensted sites and neighboring Lewis acid sites (dual-site mechanism) in line with earlier postulates proposing inductive effects of Lewis sites on nearby residual hydroxyls (thereby increasing their protonic acid strength) (ref.40,41).

Coke

formation in the conversion of butadiene by (Na,H)-Y was attributed to Lewis site catalyzed Diels-Alder additions and Br#ensted site catalyzed hydrogentransfer reactions (see Fig. 6 (ref.33)).

A recent investigation of the aging

of ZSM-5 in the methanol conversion indicated that the deactivation rate and the aromatic yields increased with A 1 content (ref.42).

It was concluded from these

observations that the formation of carbonaceous residues was proceeding on multipoint adsorption centers, containing more than one A 1 atom.

Unfortunately,

the authors failed to consider two of the critical factors mentioned in the preceding section. Namely, that border coking effect always takes place and gives a finite lifetime even to the best shape selective zeolite, and that coke formation is essentially a consecutive reaction in that conversion process. Hence, it is naturally expected that aging should increase with A 1 content. Finally, external coke produced on solid acid catalysts should not b e considered an inert deposit.

It has been shown, for example, that carbonaceous

residues produced in the conversion of propylene over silica-aluminas of variable compositions had a high radical content and consisted of "living" species reorganizing to non-desorbing products of higher aromatic character, even after stopping the propylene flow (ref.43). sites can also exist on coke itself.

This observation suggests that active Such sites could be carbonium ions and be

responsible for the formation of interparticulate coke in zeolite pellets (ref. 44). To summarize, we propose that both Brdensted and Lewis sites are active in the coking of high A 1 content zeolites such as type X or type Y, that Brdensted sites play the essential role in the coking of high silica zeolites, and that catalytic sites on coke itself can lead to growth of external coke or reorganization of surface coke deposits.

SHAPE SELECTIVE CONSTRAINTS IN ZEOLITE COKING The purpose of this section is to demonstrate that coke formation is controlled by the zeolite molecular shape selective properties, i.e., its pore structure. A s already shown in Fig. 5 , the amount of coke deposited in given operating conditions on a variety of zeolites is negatively correlated to zeolite pore size (ref.9).

The critical observation that coke yield in the

231 molecular shape selective (small and intermediate pore) zeolites is at least an order of magnitude lower than in the large pore materials leads to the suggestion that coking is a spatially demanding reaction (ref.11).

Recent and

extensive work has been directed to the assessment of these views (ref.7-11,20). When using mixed paraffinic-aromatic feeds (see A and B in Table l), several

routes are possible to carbonaceous deposits:

(1) conjunct polymerization of

olefins (ref.52,53), (2) aromatic coupling reactions (ref.54),

(3) polyalkyla-

tion of aromatics, and (4) unimolecular aromatization of alkylaromatics (ref.11). Experimental data militate against the first possibility (low concentration in olefins, little effect of H2/HC ratio).

Bimolecular aromatic coupling and

polyalkylation reactions which have important spatial requirements are likely to be much less important in intermediate pore (10-membered ring) than in large pore zeolites and should not occur in small pore (8-membered ring) catalysts I4C tracer studies using

unless large cages are present (such as in erionite).

large pore (12-membered ring) zeolites indicate that aromatic alkylation is probably the initial and decisive step in coke formation (ref.8,lO).

Cycliza-

tion of polyalkylaromatics results in fused-ring structures which by dehydrogenation are eventually converted to coke. The data of Table 1 are then easily rationalized considering the respective zeolite structures and accepting that the zeolite pore size controls intracrystalline coking. Large pore zeolites (mordenite, offretite, Type Y) coke heavily as aromatic alkylation and polyalkylaromatic aromatization readily take place.

Offretite which possesses large cages in addition to its channels cokes

more than mordenite, but still less than Type Y which has only very large cages connected by windows (ref.11).

Interestingly, the maximum amount of coke that

can be deposited in mordenite and offretite is directly related to their free pore volume (ref. 2 0 ) . Low coke yields are observed for the molecular shape selective zeolites, but a distinction must be made between small pore materials (erionite, ferrierite) which do not accept cyclic structures and intermediate pore zeolites (ZSM-5) in which simple aromatic molecules can be formed and diffuse.

Coke in ferrierite

and erionite derives essentially from the paraffinic component of the feed as seen from labeling studies (ref.11).

The narrow pores in these materials exert

constraints on the formation of cycloparaffins or naphthenes (aromatic precursors) and the main reaction that takes place is paraffin cracking.

Olefins

are formed in this process; they can oligomerize to form higher molecular weight products which do not desorb, i.e., coke (although of observed in the other cases).

a

different nature than

Naturally, the coke yield is higher for erionite

which presents large cavities along its channels than for ferrierite which has "pure" tubular channels.

232

TABLE 1 Structural effects on coke deposition in zeolites. Pore Volumee

Cages

8

0.35

Yes

Ferrierite

10

0.28

ZSM-5

10

0.29

Zeolite Erionite

Sized -

Coke Yieldb

Total CokeC

A Bf

0.14 3.40 (2.3)

-

No

A

0.03

-

No

A B C

0.04 0.22 (2.5)

2.2

-

Offretite

12

0.40

Yes

A C

0.70

16.8

Mordenite

12

0.28

No

A B C

0.30 7-17 (1.7)

-

-

8.7

A B

2.20 37 (1.1)

-

Type-Y

12

0.48

Yes

-

aA: 5-component feed, 13 atm, H2/HC=3, 316OC (ref.7-11). B: Benzene/n-hexane feed, 13 atm, H2/HC=1.4, 360°C (ref.10,ll). C: Methanol feed, 1 atm, WHSV=lO h-1, 377°C (ref.20). bCoke yield in g/lOOg of feed converted. Parentheses indicate (H/C) ratio of carbonaceous residues. 'Maximum amount of coke deposited (wt.%) from thermogravimetric measurements Aref. 20). Pore aperture expressed as the oxygen-membered ring size. epore volume in cm3/cm3 of zeolite (ref.45). fB conditions but temperature = 454OC. TABLE 2 Coke origin vs. zeolite A 1 contenta (ref.10,ll). Zeolite

A1-Densityb

Coke from Benzene (%)

Coke YieldC

ZSM-5

0.7 1.9

48 30

0.2 0.2

Mordenite

0.4 0.7 1.9

47 49 59

7 9 11

3.4 7.4

76 78

33 37

aFeed = benzene:n-hexane = 1:1, 13 atm, H2/HC = 1.4, 360'C. bAl-density = A1 per nm3 estimated from Si02/A1203 ratio and structural data. Woke yield = coke deposited (9) per 100 g of feed converted.

233

ZSM-5, as mordenite, can accept aromatics. However, aromatic alkylation is limited to the formation of methylaromatics or directed towards the formation of para-alkylaromatics (ref.6,26).

In both cases, alkylaromatics aromatization can

not take place in the bulk of the crystals, which explains the unusual resistance of ZSM-5 to coke formation. However, coke deposition can still occur on the external surface and border pore blockage effects need to be considered. They may play a non-negligible role in certain hydrocarbon conversions. These data point out that bulk coking is unlikely to occur in molecular shape selective zeolites unless the presence of cages or large channel intersections offer locations where aromatization can take place. The above analysis is slightly more complicated when the density of acidic (Al) sites available to the reactants is brought into the picture. It is known that the polar character of zeolites, among which ZSM-5, decreases with increasing Si02/A1203 ratio (ref . 4 6 - 4 8 ) ,

hence selective adsorption of aromatics is

likely to be preferred at high A1 site density (ref.11).

Table 2 compares coke

origin and yield for ZSM-5, mordenite, and Type Y, deduced from labeling studies (benzene:n-hexane feed), as a function of their A1 site density. At low A1 content (< or = 0.7), aromatics and paraffins contribute almost identically to coke although coke yields are vastly different for mordenite and ZSM-5. At high A1 content (> or = 1.9), there is an increase in the aromatic contribution to coke and in coke yield for the large pore zeolites. The adsorbed aromatics concentration is increased as well as the probability (because of the large pores) of their further conversion into coke.

For ZSM-5, the coke yield stays

about constant whereas the aromatic contribution to coke decreases.

Increasing

the number of acid sites enhances the paraffin cracking reaction, producing a larger number of olefins and carbonium ions which can eventually contribute to coke. This effect of A1 density adds itself to the effect of A1 concentration on conversion severity (effective contact time) discussed in the former section. A remaining question concerns the chemical nature of these coke deposits. Coke H/C ratios, listed in Table 1, show that the (initial) carbonaceous deposits become more refractory as A1 content and pore size increase (ZSM-5, mordenite, Type Y), indicating that hydrogen transfer reactions become more efficient. More light gas is produced as well. Both observations are a sign that reactions occur consecutively to the initial deposition of carbonaceous residues (ref.7,lO-11).

In other investigations directed at the characterization

of carbonaceous deposits produced during the conversion of methanol to hydrocarbons, EPR indicated that “external” coke on H-ZSM-5 was more polyaromatic in nature than “internal” coke in offretite or modendite (ref .20) whereas l3C F”ASNMR was able to identify a variety of residues (ref.27). Figure 8 compares the NMR spectra of used H-ZSM-5 and H-mordenite catalysts. Three main features are identified. Resonances in the 50-60 ppm region correspond to alkoxide

234

groups which occupy but do not poison the catalytic sites. further when the reaction conditions are restored.

These entities react

The occupied acid sites how-

ever cannot be probed by basic molecules such as ammonia, for example.

120-180 ppm region, 2%-5

In the

present distinct resonances corresponding to methyl-

aromatic compounds whereas mordenite gives a broad resonance, possibly characteristic of a mixture of alkylaromatics and of polyaromatic structures. Resonances below 40 ppm characterize aliphatic carbon chains from either nondesorbed aliphatic molecules or alkylaromatics. Isoparaffins are more abundant than linear chains in ZSM-5, in agreement with classical methanol conversion data (ref.49).

In-situ l 3 C MASNMR hence appears as an attractive method to gain

insight into the nature of coke deposits, the nature of which can be correlated to the known structure and chemistry of the zeolite catalysts (ref.27). To conclude, the formation of carbonaceous residues in zeolites is controlled by the dimension of their pores, channels or cages.

The irreversible adsorption

of aromatics is the initial step of a sequence of reactions leading to more refractory deposits.

t

ti-MOROENITE

R;

Fig. 8. 13C MASNMR spectra of carbonaceous deposits from the catalyzed methanol conversion to hydrocarbons in used H-ZSM-5 and H-mordenite catalysts (with permission from Butterworth Sci. Pub. (ref. 2 7 ) ) .

235 SITE COVERAGE VS. PORE BLOCKAGE EFFECTS: DEACTIVATION OF ZSM-5, MORDENITE, AND OFFRETITE DURING TBE CONVERSION OF METHANOL To illustrate the interaction of site coverage and pore blockage effects on aging, we will now compare the deactivation of H-ZSM-5

(type 11, intermediate

pore size), F-mordenite (type I, large pore size), and H-offretite (pseudo-type 111, large pore size) during the conversion of methanol to hydrocarbons (ref.

20). Figure 9A shows the rates of coke deposition for the methanol conversion at 377°C using H-ZSM-5 (Si/Al (Si/Al

=

4 . 0 ) catalysts.

gravimetry.

=

34.6), H-mordenite (Si/Al

8 . 1 ) , and H-offretite

=

Weight gains vs. time were obtained by isothermal

Figure 9B describes the corresponding oxygenates (methanol and di-

methylether) conversion to hydrocarbon and coke products

VS.

nearly identical conditions (1 atm, MSV = 10 h-1, 377°C).

time-on-stream in A correlation

obviously exists between both variations including an apparent crossover between the observations for modenite and offretite after about 20 min on stream. The qualitative order 02 coking and deactivation rates for these three materials is readily explained using the principles enounced earlier in this review. Low coke formation is observed on ZSM-5, the slightly lower initial yield in hydrocarbons being due to rapid formation of coke on its external surface.

Both mordenite and offretite coke rapidly as expected for large pore

zeolites.

Coke formation and deactivation is less catastrophic for offretite

with a type I11 pore network than for mordenite which has type I channels.

r H-ZSM-5

'

40

'

I20

'

IAO ' -----TIME

'$ 720 id ON STREAM (MINUTES-

Fig. 9. Coke formation and aging of H-ZSM-5, F1-mordenite, and H-offretite during the conversion of methanol to hydrocarbons (1 atm, 377OC, WIISV=lO h-1). (A) Percentage weight gain from carbon residues vs. time from isothermal gravimetry. (B) Percentage of oxygenates converted as a function of time-on-stream. (With permission from Academic Press (ref .20)).

236 To delineate the incidence of the coke formation rate on the desired reaction

sequence

--

or product yield

--

one needs to consider the true coking variable,

i.e., the amount of coke deposited. This is shown in Fig. 10 which plots the yield of hydrocarbon product as a function of the relative amount of coke for the same conditions and catalysts as above. The normalized deactivation sequence is H-offretite > H-mordenice > H-ZSM-5.

Coke formation is more rapid

and more abundant in offretite than in mordenite as offretite has cages along its channels: less constraints are exerted on coke formation and a larger amount of coke can be deposited. Coking effects are minimal for shape-selective ZSM-5 after an induction period during which some of the products rapidly form external coke.

A l l these observations are hence readily rationalized consider-

ing the pore network structure of these catalysts. H-ZSM-5

I-_-

--C-oL=->

/

B",

Offratite

0

I

I

I

10

20

-/.w,=-

x)

40

4k.lalI"~o-

Fig. 10. Methanol conversion to hydrocarbons as a function of the relative amount of coke deposited (same catalysts and conditions as in Fig. 9 ) . p = ratio of hydrocarbon yield at time 'It" to that extrapolated to t = 0. (With permission from Academic Press (ref.20)).

Table 3 lists additional data which enable to discriminate between site coverage and pore blockage effects.

Pore blockage in ZSM-5 is clearly limited

to border effects as a TG/DTA analysis study of n-hexane sorption does indicate changes in sorption rate but not in sorption capacity (ref.55).

Type V-A and

V-B coking effects explain the enhancement of the para-aromatic selectivity with operating time.

Most of the acid sites are still accessible in used catalysts,

some being covered by intermediates which can react further upon restoration of the reaction conditions. The covered sites cannot be probed by ammonia adsorption but yield surface species which can be detected by 13C MASNMR (ref. 27).

237

TABLE 3 Characterization of deactivated H-ZSM-5, H-offretite, and H-mordenite catalyst& Technique

H-ZSM-5

H-mordenite

H-offretite

Ammonia TPD (ref.56)

Strong acid sites partially poisoned

Not available

Not available

Ammonia adsorption is0therms

Strong and medium sites are covered

All types of acid sites are covered

Strong and medium sites are covered

Ammonia adsorption kinetics

Not affected by coking

Decreased in coked catalysts

Not affected by coking

Microcalorimetry

Some of the strongest sites still present

All types of sites are affected

Strong and medium sites affected

p /o-Xylene selectivity

Increases with time

Nearly constant

Nearly constant

Regeneration in oxygen

Rapid

Slow

Rapid

EPR

Pseudo-aromatic coke

Coke less-aromatic than in ZSM-5

Coke less-aromatic than in ZSM-5

13c-NMR (ref.2 7 )

Alkoxide groups Isoaliphatics Methylaromatics

Alkoxide groups Alkylaromatics Polylaromatics

Not available

n-Hexane sorption (ref. 5 5 )

Border blockage

Bulk blockage

Not available

*(ref .20)

Offretite has a three-dimensional pore structure, as ZSM-5, but two of its channels are narrower than the third.

It also has cages. Consequently, intra-

crystalline coke formation occurs and acidic sites, mostly of medium and high strength, are covered by coke and/or other carbonaceous residues.

The weak

acid sites which are less prone to coking and the other non-covered sites can be probed by ammonia adsorption as they stay readily accessible through the smaller channels which are rather free from coke.

It is also believed that the rapid

and easy regeneration of offretite upon air calcination is due to the existence of this secondary channel system which allows a better access of oxygen to the coke deposits.

238 Finally, mordenite is a zeolite with large and unidimensional pores (type

I).

Site coverage (by coke, carbonaceous residues, and intermediates) and pore

blockage (by coke, alkyl- and polyaromatics) will occur simultaneously during coking (ref.27).

Hence, all types of acidic sites are affected by coking as

indicated by ammonia adsorption.

Bulk coking and pore blockage are also

evidenced by the inhibition of ammonia sorption in coked samples: the adsorption rate and capacity are dramatically reduced, obstructions exist to the access of strong acidic sites. No other access pathways are available. These examples illustrate the interplay between site coverage and pore blockage effects and demonstrate that they can be discriminated by combining appropriate techniques. CONCLUSIONS The deactivation of zeolites by coking depends on two factors which are, respectively, the availability of catalytic sites and spatial constraints acting on carbonaceous residue forming reactions. The effective concentration of acid sites catalyzing the main reaction sequence and coke deposition at any time during operation is a function of two independent parameters.

The first one is site coverage which occurs when acid

sites are poisoned by coke, coke precursors, or heavy reaction intermediates or products which do not desorb under reaction conditions.

The second one is pore

blockage which can prevent the access of reactants to active sites.

We have

proposed a classification of pore blockage effects based on the various pore networks which can be found in zeolites. The concentration of acid sites itself has two effects.

The polar character

of zeolites increases with aluminum content and therefore also their selective (ad)sorptive properties for aromatics which are precursors for the formation of coke.

In addition, higher aluminum content also means higher conversion

severity with the effect that more coke is produced.

When consecutive coke

formation is prominent, optimal conditions should exist for which coke deposition is minimized and product yield is enhanced.

If the coke selectivity is

low, this may not be observed although the aging rate will still be decreased. Coke formation is a reaction which is controlled by the zeolite molecular shape selective properties.

In large pore zeolites, the initial steps leading

to coke precursors are aromatic alkylation and alkylaromatic aromatization. Bulk coke deposition does not occur in molecular shape selective (small and intermediate pore) zeolites, unless their pore network also present large cages or channel intersections in which the above reactions can take place.

Coke

aging effects are essentially due in the latter case to border pore blockage. This paper has stressed a number of factors which affect zeolite deactivation by coking.

Coke must be considered as the important variable governing

239 both the desired reaction and coke formation.

Several of the qualitative conclusions which have been proposed deserve a more rigorous and quantitative

approach.

It is our hope that this review will stimulate such investigations.

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 28 29 30 31 32 33 34 35 36 37 38 39

J.B. Butt, Adv. Chem. Ser., 109 (1972) 259. J.B. Butt, A.C.S. Symp. Ser., 72 (1978) 288. G.F. Froment, Proc. Sixth Intern. Congr. Catal., 1 (1976) 10. G.F. Froment, Stud. Surf. Sci. Catal., 6 (1980) 1. E.G. Derouane, in "Intercalation Chemistry", M.S. Whittingham and A.J. Jackson (eds), Academic Press, New York, 1982, pp. 101. E.G. Derouane, in "Catalysis by Zeolites", B. Imelik et al., (eds), Elsevier, Amsterdam, 1980; Stud. Surf. Sci. Catal., 4 (1980) 5. L.D. Rollmann, J. Catal., 47 (1977) 113. D.E. Walsh and L.D. Rollmann, 3 . Catal., 49 (1977) 369. L.D. Rollmann and D.E. Walsh, J . Catal., 56 (1979) 139. D.E. Walsh and L.D. Rollmann, J . Catal., 56 (1979) 195. L.D. Rollmann and D.E. Walsh, "Progress in Catalyst Deactivation", Nijhoff, The Hague, 1982, p. 81. A. Voorhies, Ind. Eng. Chem., 37 (1945) 318. P.E. Eberly, C.N. Kimberlin, W.H. Miller and H.V. Drushel, Ind. Eng. Chem. Process Des. Dev., 5 (1966) 193. S.E. Voltz, D.M. Nace and V.W. Weekman, Ind. Eng. Chem. Process Des. Dev., 10 (1971) 538. D.A. Best and B.W. Wojciechowski, J. Catal., 31 (1973) 74. G.F. Froment and K.B. Bischoff, "Chemical Reactor Analysis and Design", Wiley, New York, 1979. J.W. Beekman and G.F. Froment, Ind. Eng. Chem. Fund., 18 (1979) 245. J.W. Beekman and G.F. Froment, Chem. Eng. Sci., 35 (1980) 805. P.B. Venuto, in "Molecular Sieve Zeolites. II", E.M. Flanigen and L.B. Sand (eds), Adv. Chem. Ser., 102 (1971) 260. P. Dejaifve, A . Auroux, P.C. Gravelle, J.C. Vedrine, 2. Gabelica and E.G. Derouane, J. Catal., 70 (1981) 123. D. Theodorou and J. Wei, J . Catal., 8 3 (1983) 205. N . Y . Chen, W.W. Kaeding and F.G. h y e r , J. Amer. Chem. SOC., l O l ( 1 9 7 9 ) 6783. W.W. Kaeding, C. Chu, L.B. Young and S . A . Butter, 3. Catal., 69 (1981) 392. L.B. Young, S.A. Butter and W.W. Kaeding, J. Catal., 76 (1982) 418. W.W. Kaeding, C. Chu, L.B. Young, B. Winstein and S.A. Butter, J. Catal., 67 (1981) 159. D.H. Olson and W.O. Haag, ACS Symp. Ser., 248 (1984) 275. E.G. Derouane, J.P. Gilson and J. B.Nagy, Zeolites, 2 (1982) 42. P.B. Venuto, in "Catalysis in Organic Synthesis", G.V. Smith (ed), Academic Press, New York, 1977, p. 67. W.G. Appleby, J.W. Gibson and G.M. Good, Ind. Eng. Chem. Prod. Res. Dev., 1 (1962) 102. P.B. Venuto and E.T. Habib, Catal. Rev.-Sci. Eng., 18 (1979) 1. R.C. Haldeman and M.C. Botty, 3. Phys. Chem., 63 (1959) 489. D. Eisenbach and E. Gallei, J. Catal., 56 (1979) 377. B.E. Langner and S. Meyer, Stud. Surf. Sci. Catal., 6 (1980) 91. P.B. Venuto and L.A. Yamilton, Ind. Eng. Chem. Prod. Res. Dev., 6 ( 1 9 6 7 ) 1 9 0 . E . G . Derouane, J.P. Gilson and J. B.Nagy, 3. Molec. Catal., 10 (1981) 331. J.P. van den Berg, J.P. Wolthuizen, A.D.H. Clague, G.R. Hays, R. Huis and J.H.C. van Hooff, 3. Catal., 80 (1983) 130. J.P. van den Berg, J.P. Wolthuizen and J.H.C. van Hooff, J. Catal., 80 (1983) 139. C.C. Lin, S.W. Park and W.J. Hatcher, Jr., Ind. Eng. Chem. Process Des. Dev. 22 (1983) 609. D.G. Blackmond, J.G. Goodwin, Jr. and J.E. Lester, J. Catal., 78 (1982) 34.

240 40 41 42 43 44

J.B. Uytterhoeven, L.G. Christner and W.K. Hall, J. Phys. Chem., 69 ( 1 9 6 5 ) 2117. J . H . Lunsford, J. Phys. Chem., 72 ( 1 9 6 8 ) 4163.

K.G. Ione, G.V. Echevskii and G.N. Nosyreva, J. Catal., 85 ( 1 9 8 4 ) 287. E.G. Derouane, 2. Gabelica and C. Mortier, unpublished results. F.J. Shiring, R. Venkatadri and J.G. Goodwin, Jr., Can. J. Chem. Eng., 61 ( 1 9 8 3 ) 218.

45 46 47 48. 49 50 51 52 53 54

W.E. Garwood, P.D. Caesar and J.A. Brennan, U.S. Patent 4,150,062 assigned to Mobil Oil Corporation ( 1 9 7 9 ) . D.H. Olson, W.O. Haag and R.M. Lago, J. Catal., 61 ( 1 9 8 0 ) 390. R.M. Dessau, ACS Symp. Ser., 135 (1980) 123. R. Le van Mao, React. Kinet. Catal. Lett., 12 ( 1 9 7 9 ) 69. C.D. Chang and A.J. Silvestri, J. Catal., 47 ( 1 9 7 7 ) 249. V. Bolis, J.C. Vedrine, J.P. van den Berg, J.P. Wolthuizen and E.G. Derouane, J.C.S. Faraday Trans. I, 7 6 (1980) 1606. J. Haber, J. Komorek-Hlodzik and T. Romotowski, Zeolites, 2 (1982) 179. V.N. Ipatieff and H. Pines, Ind. Eng. Chem., 28 ( 1 9 3 6 ) 684. J.C. Vedrine, P. Dejaifve, E.D. Garbowski and E.G. Derouane, in "Catalysis by Zeolites", B. Imelik et al. (eds), Elsevier, Amsterdam, 1980; Stud. Surf. Sci. Catal., 4 ( 1 9 8 0 ) 29. P. Wierzchowski, E.D. Garbowski and J.C. Vedrine, J. Chim. Phys., 78 ( 1 9 8 1 ) 41.

55 56

Z. Gabelica, J.P. Gilson, G. Debras and E.G. Derouane, in "Thermal Analysis", B. Miller (ed), Wiley, New York, 1982; Vol. 2 , p. 1203. N.Y. Topsoe, K. Pedersen and E.G. Derouane, J. Catal., 7 0 ( 1 9 8 1 ) 41.

241

VALORISATION DES OLEFINES : O.LIGOMERISATION CATALYSEE PAR LE TRIFLUORURE DE BORE C. M4RTY e t Ph. ENGELHARD

TOTAL, Compagnie FranGaise de Raffinage, B.P.

27

-

76700 HARFLEUR, France

RESUME

Les a p p l i c a t i o n s du t r i f l u o r u r e de bore 3 1 ' o l i g o m @ r i s a t i o nc a t a l y t i q u e des o l e f i n e s sont d e c r i t e s a i n s i que sa mise en oeuvre sous forme gazeuse, complexee ou supportee. SUMWRY The c a t a l y t i c o l i g o m e r i z a t i o n o f o l e f i n s , i n the presence o f boron t r i f l u o r i d e o r i n i t s complex o r supported forms i s given.The main conclusions are : - Under i t s gaseous form, i t r e s u l t s t o t he s e l e c t i v e removal o f isobutene from t h e o l e f i n i c C3 t C4 o r C4 cut s produced by c a t a l y t i c cracking o f petroleum fra c t ions. Petrochemicals and petroleum bases (dimers, t r i m e r s and tetramers) a re obtained by t h i s process. The o l i g o meri za ti on o f c8 - C10 l i n e a r - o l e f i n s w i t h BF -alcohol complexes g iv e s a trimers-tetramers mixture. These products a1 low ta o b t a i n wide-temper a t u r e range s y n t h e t i c l u b r i c a n t s . F i n a l l y , BF3 can be supported. TOTAL-CFR has perfected a BF3-alumina c a t a l y s t and designed a process i n order t o v a l o r i z e C3/C4 cuts o f c a t a l y t i c cracking. The f l e x i b i l i t y o f t he process a l l o w t o produce a l a r g e range o f products, from petrochemicals t o petroleum bases.

-

-

INTRODUCTION Les halogenures de bore sont des acides de Lewis. S i l ' o n effectue une comparaison e n t r e BF3 e t l e s autres halogenures de bore, on o t t i e n t 1 'ordre s u iv a n t de f o r c e aci de decroissante :

>

>

BI3 B Br3 BCl3 > BF3 Le t r i f l u o r u r e de bore e s t l e moins acide des 4 en r a i s o n de l ' e f f e t de "back coordination" des atomes de f l u o r p l u s prononce que dans l e cas des autres molecules. Le t r i f l u o r u r e de bore cat alyse de nombreuses reactions ( r e f . 1) t e l l e s que l e s a l k y l a t i o n s e t l e s isomerisations, d'alkylaromatiques,

l a dism ut ation e t l a t r a n s a l k y l a t i o n

l e s rearrangements de Beckmann e t de Fries, l e s a c y l a t i o n s

de composes aromatiques e t l e s polymerisations e t copolymerisations. Bienentendu, l ' o l i g o m e r i s a t i o n des olef ines, cas p a r t i c u l i e r de l a polymer i s a t i o n , e n t r e dans l e cadre general des r e a c t i o n s catalysees par BF3. Le t r i f l u o r u r e de bore peut e t r e u t i l i s e sous 3 formes :

-

La p l u s simple c o n s i s t e

a

t r a i t e r l a charge par un f l u x de BF3 gazeux

242

( r e f . 2. 3. 4.7.8.). On a ainsi polymeris@, par example, de l'isobutene ( r e f . 5. 6 . )

- Le deuxieme mode de mise en oeuvre e s t l ' u t i l i s a t i o n de BF3 complex@avec u n

-

compose oxygen& (eau, alcool) ( r e f . 3. 9. 10. 11. 12.); cependant de t e l s complexes sont sensibles i l a temperature. La troisieme technique consiste 1 f i x e r BF3 sur u n support, de l'aluniine p a r exemple ( r e f . 13. 2 2 ) . O n peut alors t r a v a i l l e r dans u n large domaine de temperature, ce q u i e s t un avantage p a r rapport auxcas precedents.

Mode d'action en matiere de polym@risation d'olefines. I1 e s t bien connu que l e trifluorure de bore a l ' e t a t p u r n'exerce pratiquement aucun e f f e t sur l a polymerisation de l'isobutene ( r e f . 1. 2. 4 ) . I1 e s t donc necessaire de lui adjoindre u n cocatalyseur, generateur de protons,comme par exemple l ' e a u ou u n alcool ; on forme alors des complexes du type :

Lorsqu'on u t i l i s e du t r i f l u o r u r e de bore gazeux 1 l ' @ t a t p u r , c e t t e reaction e s t possible car l e s olefines, issues de coupes i n d u s t r i e l l e s , contiennent dans l a plupart des cas des polluants soufres e t oxygen&s (eau, H2S, mercaptans, a l dehydes, e t c . . . ) . Enfin, la fixation de BF3 sur un support alumine conduit 1 une interaction BF3 - support du type :

\

A1 - OH / 0 )A1 - OH

'A1

BF3

*

/

0 '

\

-0 \ A1 - 0

/B-

'

2HF

/ g@neratricede protons. Cwnpte tenu des donnees precgdentes, l e mecanisme d 'action du t r i f l u o r u r e de bore ( r e f . 1. 4 ) se presente comme u n mecanisme classique par ion carbonium :

243

- phase 1:initiation t

Olefine R- CH = CHR' monomPre BF3

+ cocatalyseur

BF3

t

ou

impuretes

CHR'

AH+ -

R-

ct/ 3 Ct R

- phase RCH2-

I 1 : propagation,.

+CHR'

+

-

RCH =CHRL

RCH2

- CH I

-CH

k

R'

CH3 R -C

I

+

+

RCH-

CHR'

I

-

R'

- phase R-

I11 : terminaison

+

CH - CH - CHR'

CH2-

R'

CH3

I

C-CH

I

I

- CH

I

R' R-

+ -CHR'

1

-H

-+CHR'

I

I R'

1

R-

a3 R-C

R

+

R

- CH2-

CH - C = CHR'

I

R'

R

-H

-+CHR'

I R

+

L

R--C-

I

C=CHR'

1

R' R

OLIGOMERISATION FLUOBORIQUE DES OLEFINES Globalement, l e marche des olefines legeres (Ctylene , propPne, butenes) se caracterise p a r une production importante e t une valorisation insuffisante. De plus, avec l e developpement actuel des unites de conversion petrolieres (craqueurs catalytiques , viscoreducteurs) , ce phenomene va nettement s 'accentuer. En raison de c e t t e s i t u a t i o n , nous cherchons a valoriser les olefines par oligomerisation, avec, comme objectifs principaux, la production d'une game de produits couvrant les domaines suivants : - bases petrochimiques (olefines en solvants fluides 2 basse temperature (Cl3-CI6), exempts des compos&s aromatiques, coupes plus lourdes, u t i l i s a b l e s comme fluides hydrauliques, huiles isolantes, ou come lubrifiants de synthese haute performance (aprPs hydrogenati o n ) . (Ct-C12),

244

Catalyse par BF,< gazeux : obtention selective d'oligomeres d'isobutene, 2 parti-r de coupes C3 +Cd olefiniques (ref. 14). Ce mode de mise en oeuvre est extr6mement simple (Fig.1). I1 consiste a melanger la charge, constituee par une coupe C3 + C4 olefinique, a du trifluorure de bore gazeux sur masse de contact inerte (cailloux, billes de verre). L'effluent reactionnel est ensuite fractionne de faGon classique, afin d'isoler les differents oligomeres.

APRES

ELMNATION

POLYIOEUTENES VERS

Figure 1 :

-

FRACTIONNEMENT

Production selective d'oligomeres d'isobutPne 2 partir de coupes C3 + C4 (ou C4). (Tr = 20°C, P = 30 bars, VVH = 3 , P F 3 3 = 2OOOppm)

L'examen du Tableau I montre que l'on obtient une elimination selective de l'isobutene. Le bilan matiere donne un taux d'elimination de l'ordre de 95%. Les autres olefines ne sont pratiquement pas transformees. TABLEAU I

-

Oligamerisation selective de l'isobutene contenu dans les coupes C3 + C4 olefiniques. Coupes C3+C4 avant traitement

Satur@s 01efi nes dont Propene 1- Butene I sobutene 2- Butene (trans) 2- Butene (cis)

Coupes C3tC4 apres traitement

% vol.

% vol.

54,l 45,9

61,3 38,7

245

Les polyisobutenes obtenus selon c e t t e technique o n t ete i d e n t i f i e s par chromatographie en phase vapeur e t spectrometrie de masse. On i s o l e par fractionnement 4 coupes principales : - 13%(pds) de coupe 100-120°C : e l l e contient 75% en poids de diisobutene et d'homologues en C8. Elle constitue une excellente base pour l e s carburants e t une bonne charge pour la synthese 0x0 (alcools en C9). - 24% de coupe 12O-22O0C, assimilable a une essence lourde, riche en triisobutene e t homologues en C12. Ces composes peuvent s e r v i r come agents d'alkylation du benzene. Par a i l l e u r s , i l s donnent, apres hydrogenation, des isoparaffines (solvants). - 30% de coupe 220-320°C : e l l e contient 80% (pds) de tetraisobutgne e t homologues en c16 u t i l i s a b l e s comme bases pour fluide hydraulique, en raison de leur p o i n t d'ecoulement t r e s bas ( i n f e r i e u r a -60°C) associe a un indice de viscosi t C conforme (80-90). - Enfin 33% d ' u n residu, de point d ' e b u l l i t i o n superieur a 320"C, q u i donne, apres hydrogenation , une h u i l e lourde i soparaffinique , uti 1isable come h u i l e isolante ( r i g i d i t e dielectrique > 60 K V , p o i n t d'ecoulement f a i b l e , de l ' o r d r e de -30 a -40°C). Catalyse par BF2 complex6 : production de poly-+olefines

p a r t i r d'ethylene.

Les poly-M-olefines presentent u n grand interet sur l e marche des lubrifiants synthetiques. En e f f e t , i l s ont des caracteristiques physicochimiques nettement superieures I celles presentees par les huiles minerales raffinees. On prepare l e s poly-d-olefines en 2 @tapes : a ) Par oligomerisation de l'ethylene sur catalyseur type Ziegler on obtient u n melange d ' d - o l e f i n e s en c6-c18 que 1 'on fractionne coupes e t r o i t e s pour i s o l e r les differentes olefines. b ) On oligcinerise ensuite les coupes e t r o i t e s (en C8-c10 notanunent) pour produi re essentiel lement des melanges "trimeres-tetramPres", u t i 1isables , apres hydrogenation, comme huiles synthetiques a haute performance. Cette deuxieme @tapea donne lieu a de nombreux travaux (Ref. 11, 12, 1 5 , 16, 17, 18, 19) en raison de son importance pratique. Plusieurs types d e catalyseur ont ete etudies dans c e t t e deuxieme etape, des aci des de Lewis ( A 1 Cl3, BF3), des i ni t i ateurs de polymerisati on radical ai re (peroxydes) e t des catalyseurs complexes de type Ziegler. Parmi ces catalyseurs, l e t r i f l u o r u r e de bore associe sous forme de complexe a u n promoteur protonique (acide carboxylique, alcool, eau) s ' e s t aver@ comme l ' u n des plus performants.

246

Ces complexes (Tableau 11) sont formes a basse temperature ( r e a c t i o n exothermique ). Dans c e r t a i n s c a s , on peut l e s i s o l e r par d i s t i l l a t i o n sous vide. TABLEAU I1

- Caracteristiques physicochimiques des complexes BF3,HA

Nature d u compl exe BFQ, H20

BF3, BF3, BF3, BF3,

Temperature d'ebullition ( C O )

Mode d ' i oni s a t i on

-

Ht [BF30H]-

*

2H20 CH30H 2CH30H CH3C02H

H30+ [BFQOH] H+ [BF30CH3]CH30H2+[BF30CH3]Ht [BF$H$02]-

* stable jusqu'a

140°C

Temperature Plasse vol ude fusion ("C) mique(a20"C)

58.6 " C / l , P m m 58.6 "C/4m 62 " C / l l m

5 99

1,785

6,2

1,632 1,408 1,212 1,495

- 18,6 - 58,l 37,5

a l a pression atmospherique

Les plus courants sont disponibles sur l e marche. Ce s o n t des acides de f o r c e rnoderee : BF3, HA Ht [BFQA] Pour maintenir une force acide (protonique) s u f f i s a n t e , i l e s t necessaire de sat u r e r l e milieu en BF3 e t de maintenir, en continu, une pression p a r t i e l l e de BF3 au-dessus du milieu reactionnel. Compare aux catalyseurs de type Ziegler ou aux i n i t i a t e u r s r a d i c a l a i r e s , l e complexe "BF3-alcool" (Tableau 111) donne, sur une charge constituee par du 1-decPne, une conversion tres elevee (96%) a basse temperature (3OoC), avec l e minimum de sous-produit. On peut, en p a r t i c u l i e r , noter que l a s e l e c t i v i t e en trimeres e s t nettement plus elevee que dans l e cas des 2 a u t r e s catalyseurs. On s a i t , en e f f e t , que l e s trimeres de d6cPne sont trPs recherches c a r i l s c o n s t i t u e n t une excellente base pour l u b r i f i a n t s synthetiques. Ces produits presentent des s t r u c t u r e s "en e t o i l e " . Les performances comparees ti c e l l e s d'une h u i l e minerale r a f f i n e e e t d ' a l kylbenzenes de synthese sont regroupees dans l e Tableau IV. Les conclusions sont sans ambiguite, l e s performances des poly-d-olefines sont superieures ( i n d i c e de v i s c o s i t e plus eleve avec une v i s c o s i t e plus f a i b l e , point d'ecoulement remarquablement bas < -6O"C, point e c l a i r l e plus e l e v e . )

247

TABLEAU I11 : Oligomerisation c a t a l y t i q u e du 1-decene.

Catalyseur

Complexe "BF3-a1 cool " BF3/ROH

d i -t-butyl

peroxyde

30 3

emperature ( " C ) uree de reaction (heure) r e s s i on

Peroxyde

Type Ziegler A 1 (C2H5 ) 3/Ti C14/CHC 13

155

77

4 $3

5,3

atmospherique

atmospherique

atmospheri que 96

87

41

imere

12 54

5 15

13

rimere &tramere + entamere

34

80

78

onversion ( % p d s ) e l e c t i v i t e (% p d s ) :

1

9

TABLEAU IV : Performances des poly-P(-olefines

b

Densite a 15°C

Nature

Viscosite I n d i c e de (mm2.s-') Viscosite 100°C 40°C

Point Point eclair d'ecoulement Cleveland ("C)

0,830

6,04

31,45

138

98

Lenormand et a1 (10) SbF5/CgS5, - 3 O T

2
c4

n-C4 0.5

8

61

25

6

a

2

68

0

30

anodic oxidation

4

77

0

18

SbF5 ( 1-1 )

a 0.03 mole o f

NH; p e r mole o f n-butane.

b 0.03 Faraday p e r mole o f n-butane. c

i n weiqht

i -C4 99.5

+ NHi +

%

0

( pure )

CF3S03H

.

i n c l u d i n g 0.5% o f 2,2,3.3-tetrarnethvbutane

0

379

The r e a c t i v i t y of t h e p u r e a c i d i s v e r y weak. We observe o n l y t h e f o r m a t i o n o f t r a c e s o f n-butane

.

The a d d i t i o n o f SbF5, t h a t l a r g e l y i n c r e a s e s t h e a c i d i t y , i n c r e a s e s t h e r e a c t i v i t y as w e l l . The p r o d u c t i s e ' s s e n t i a l l y n-butane,

i n a p r o p o r t i o n near

t o t h e thermodynamic e q u i 1ibrium. I f the r a d i c a l a c t i v a t i o n also increases t h e r e a c t i v i t y , t h e products are t o t a l l y d i f f e r e n t . No n-butane i s o b t a i n e d alkanes t h a n butane a r e formed

. We

, but

o n l y lowe?

and m a i n l y h i g h e r

-

can n o t i c e s m a l l q u a n t i t i e s o f 2,2,3,3

t e t r a m e t h y l b u t a n e t h a t i s t h e dimer o f t e r t - b u t y l r a d i c a l

.

These r e s u l t s show t h a t t h e r a d i c a l - i n i t i a t e d i s o m e r i z a t i o n o f butane i s n o t r e v e r s i b l e u n l i k e t h a t o c c u r i n g i n s u p e r a c i d i c media. So, t h i s proves w i t h o u t doubt t h a t r a d i c a l a c t i v a t i o n i n i t i a t e s a d i f f e r e n t r e a c t i o n f r o m t h e one due t o t h e a c i d i t y

.

By h y d r i d e a b s t r a c t i o n , t h e p r o t o n induces t h e a c i d - c a t a l y z e d e q u i l i b r i u m

between i s o b u t a n e and n-butane t h a t proceeds v i a a p r i m a r y c a r b o c a t i o n . On t h e contrary, i f t h e r e a c t i o n a c t i v a t i o n runs case o f n-butane,

a9

we p r e v i o u s l y

assumed i n t h e

i t s a c t i o n on i s o b u t a n e i s a l s o t h e f o r m a t i o n o f a n o c t y l

cation. I n f a c t , t h e d i m e r i z a t i o n o f t h e t e r t i o b u t y l r a d i c a l s leads o n l y t o t h e

2,2,3,3-tetrarnethylbutane t h a t i s s t a b l e i n p e r f l u o r o a l k a n e s u l f o n i c a c i d s as we a l r e a d y n o t i c e d . I t i s known t h a t t e r t i a r y a l k y l r a d i c a l s a r e f a v o u r a b l e t o undergo a d i s -

p r o p o r t i o n a t i o n r a t h e r t h a n d i m e r i z a t i o n [13]

.

So, t h e e v o l u t i o n o f t h e r a d i c a l

i s t h e f o r m a t i o n o f butene t h a t can l e a d t o a t e r t i a r y o c t y l c a t i o n by a l k y l a t i o n and i s o m e r i z a t i o n .

I t i s obviou;

t h a t i t i s d i f f i c u l t t o o b t a i n n-butane by

8 - s c i s s i o n o f such a c a t i o n . So, i n t h i s case, t h e r a d i c a l e f f e c t i s t h e format i o n o f lower

and p a r t i c u l a r l y

h i g h e r a l k a n e s t h a n butane. These a r e t h e by-

p r o d u c t s a l r e a d y observed when n-butane i s a c t i v a t e d by r a d i c a l s o r b y a d d i t i o n o f butene o r o c t a n e

.

I n o r d e r t o s p e c i f y t h e a c t i o n o f r a d i c a l a c t i v a t i o n , we a l s o s t u d i e d neopentane. I t does n o t r e a c t i n p u r e trifluoromethanesulfonic a c i d . The r a d i c a l a c t i v a -

t i o n induces i t s t r a n s f o r m a t i o n which i s n o t c a t a l y t i c b u t s t o e c h i o m e t r i c However, t h e p r o d u c t s

.

a r e dependent on t h e n a t u r e o f t h e a c t i v a t i o n . On t h e

one hand, t h e a d d i t i o n o f aminyl r a d i c a l s l e a d s t o i s o b u t a n e , i s o p e n t a n e and isoheptane. On t h e o t h e r hand, no heavy compound i s produced by a n o d i c o x i d a t i o n b u t o n l y isobutane, propane and ethane

.

380 I n t h e f i r s t case, t h e r e s u l t s a r e c o m p a t i b l e w i t h r a d i c a l d i m e r i z a t i o n f o l lowed by i s o m e r i z a t i o n and B - s c i s s i o n .

I n t h e second one, t h e n e o p e n t y l r a d i c a l

c a t i o n l o s e s a methyl r a d i c a l i n s t e a d o f a p r o t o n [ 1 4 1

.

i

C7H16 i -C5H12 i-C4H10

\

-e

t

1

i i-C4H10

The l a s t r e a c t i o n c o n f i r m s t h e occurence o f E.C.E.

CH3

C2H6 * C3H8

mechanism f o r t h e e l e c -

t r o a c t i v a t i o n o f a1 kanes. The r e s u l t s as a whole i n d i c a t e t h a t r a d i c a l a c t i v a t i o n can o c c u r i n s u p e r a c i d i c media. It can induce a c a t a l y t i c i o n i c process t h a t i s d i f f e r e n t from t h e one due o n l y t o t h e p r o t o n . summarized

The case o f t h e i s o m e r i z a t i o n o f butane, t h a t i s

i n t h e f i g u r e 5, i l l u s t r a t e s t h i s phenomenom.

The r a d i c a l - i n i t i a t e d mechanism i n v o l v e s o n l y secondary and t e r t i a r y carboc a t i o n s . So, i t i s e n e r g e t i c a l l y more f a v o u r a b l e t h a n t h e c o n v e n t i o n a l i s o m e r i z a t i o n which i n v o l v e s p r i m a r y i s o b u t y l c a t i o n . We a l s o observed t h a t a d d i t i o n o f o c t a n e o r butene produces t h e same e f f e c t s as t h e r a d i c a l a c t i v a t i o n . The a c t i o n o f t h e a d d i t i o n o f a l k e n e s upon t h e i s o m e r i z a t i o n o f a l k a n e was a l r e a d y known [ 1 5 ] . that

olefins

However t h e i n t e r p r e t a t i o n was d i f f e r e n t : i t was assumed

f u n c t i o n as a s o u r c e o f carbonium i o n by p r o t o n a t i o n . I n o u r

o p i n i o n , we a s s i g n t h e i n c r e a s e i n r e a c t i v i t y t o a new mechanism which o c c u r s by a l k y l a t i o n o f t h e o l e f i n . I n t h i s r e s p e c t , we must n o t i c e t h a t i n s t r o n g s u p e r a c i d i c media l i k e RFS03H

-

SbF5

, the

r a d i c a l a c t i v a t i o n i s n o t s i g n i f i c a n t . This f a c t i s i n

accordance w i t h o u r i n t e r p r e t a t i o n because i n t o o h i g h a c i d i c media, o l e f i n s are completely protonated

.

L i k e w i s e , i n t h i s case, t h e p r o t o l y s i s competes

with the 6-scission. I n t h e same way, Fabre e t a l . [ 1 6 ] determined t h a t radical reactions of alkanes i n superacids can occur only i f t h e a c i d i t y level i s not too high .

abstraction

4 CH I 3

CH3-:H-CH2

i someri z a t i on

+ B-scission

I

+ Fig. 5

. Radical-initiated

isomerization o f n-butane

.

According t o t h e s e considerations, the perfluoroal kanesulfonic acids a r e valuable media f o r the study o f the synergy o f radical and a c i d i c a c t i v a t i o n . We develop this i n the synthesis of perfluorinated organic compounds [ 1 7 1 . I n a mechanistic point of view, t h i s concept could be extended t o the heterogeneous a c i d i c c a t a l y s i s . I n this f i e l d , only c a t i o n i c reactions were considered b u t i t i s not impossible t o consider a l s o a radical p a r t i c i p a t i o n . The case of t h e methanol conversion i n t o hydrocarbons [18 1can provide a good example. Indeed the mechanism of t h e formation of the f i r s t C-C bond is s t i l l not well established. The formation of the cation radical intermediate 'CH20H; , t h a t i s c e r t a i n l y more s t a b l e than CH30H" [19], by t h e connection of a c i d i c and radical actions , could explain t h i s reaction. This work i s in

382 progress. Although no

conclusive r e s u l t concerning t h e conversion o f methanol

was y e t obtained, we can f e e l t h a t the concept presented here open a new f i e l d o f investigation

.

REFERENCES

1 2 3 4 5

6a b

7 8 9 10 11 12

J.J. Solomon and F.H. F i e l d , 3. Amer. Chem. SOC., 97 (1975) 2625. G.J. C o l l i n and J.A. Herman, Canad. J, Chem., 55 (1977) 1939. F. Cacace and P. Giacomello, J. Amer. Chem. SOC., 95 (1973) 5851. W.T. Dixon and R.O.C. Norman, J. Chem. SOC., (1963) 3119. N.C. Deno and D.G. Pohl, J. Amer. Chem. SOC. ,96 (1974) 6680. D. Brunel, J. I t i e r , A. Commeyras, R. Phan Tan Luu and D. Mathieu, B u l l . SOL Chim. Fr. , (1979) 249 and 257 A.Germain, P. Ortega and A. Commeyras, Nouv. J. Chim. , 3 (1979) 415. R.J. G i l l e s D i e . Endeavour, 32 (1973) 3. G.A. Olah, Chem. i n B r i t . , 8 (i972)'281. H.C. Brown, The Non C l a s s i c a l I o n Problem , Plenum Press, New York (1977). J . Grondin, R. Sagnes and A. Commeyras, B u l l . SOC. Chim. Fr.,(1976) 1779. S. P i t t i , M. Herlem and J. Jordan, Tetrahedron Lett., (1976) 3221. G.A. Olah, G. Klopman and R.H. Schlosberg, J. Amer. Chem. Soc.,91 (1969)

3261. 13 14 15 16 17a

.

M.J. G i b i a n and R.C. Corley, Chem. Rev., 73 (1973) 441 D.S. Urch, J . Chem. SOC., (1963) 3460 H. Pines and R.C. Wackher,'J. Amer. Chem. SOC., 68 (1946) 595 P.L. Fabre, J. Devynck and B. T r e m i l l o n , Chem. Rev., 82 (1982) 591 A. Germain and A.Commeyras , J.C.S. Chem. Comm., (1978) 118 . b A. Germain and A.Comeyras , Tetrahedron, 37 (1981) 487 18 Belgium Pat. (1975) 818 709 . 19 W.J. Bouma, R.H. Nobes and L. Random, J. h e r . Chem. SOC., 104 (1982) 2929

.

.

.

.

B. Imelik e t al. (Editors), Catalysis b y Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

ALKYLATION OF BENZENE WITH PROPENE O N BENZYL SULFONIC ACID SILOXANE CATALYSTS A. SAUS*, B. LIMBXCKER, R. BRULLS and R. KUNKEL University Duisburg, Applied Chemistry, Lotharstr. 65, D-4100 Duisburg

ABSTRACT Benzyl sulfonic acid siloxane can be used a s a c a t a l y s t f o r t h e alkylation o f benzene with propene a t high temperatures (-200°C). The c a t a l y t i c a l a c t i -

v i t y can be s i g n i f i c a n t l y increased when t h e c a t a l y s t support c o n s i s t s of binary metal oxides: R-(SiO,/MgO) R-(Si02/Ti0, 1, R-(SiO,/ZrO, 1, R-(SiO,/ A1,0,) with R = > Si(OH)-CH,-CbH..-SOaH. Highest a c t i v i t i e s a r e obtained with t h e s o l i d SiO,/A1,0, a s a c a t a l y s t support. For deeper understanding o f t h e increasing e f f e c t of t h e binary oxides upon t h e c a t a l y t i c a l a c t i v i t y f u r t h e r investigations must be employed.

RESUME L'acide benzylsulfonique siloxane e s t u t i l i s e comme catalyseur d ' a l k y l a t i o n , d 2OO0C, du benzene par l e propene. L ' a c t i v i t e du catalyseur e s t augmentee quand i l e s t support& par des oxydes b i n a i r e s : R-(Si02/MgO), R-(Si32/TiO?) R-(Si02/Zr02), R-(Si02/A1203) avec R = Si (OH)-CH2-C6H4-S03H. La meil leure a c t i v i t e e s t obtenue avec l e support Si02/A1203.

INTRODUCTION

The alkylation of benzene with propene [I-31 on benzyl s u l f o n i c acid s i l o xane ( I ) as a c a t a l y s t has not k e n published previously. Zimmermann [41 who synthesized I f o r t h e f i r s t time by t h e reaction o f benzyl sulfonic acid s i l a n e t r i o l and s i l i c a gel got products which were not reproducible in t h e i r physical and chemical properties. Good r e s u l t s a r e obtained, when trimethoxy

*

To whom correspondence should be addressed.

383

384

benzyl s i l a n e , which can be prepared by known methods [5], is reacted w i t h activated s i l i c a gel a t 80-90°C and then sulfonated with 96 percent H,SO, a t 100°C. By t h i s method a final product i s obtained which i s reproducible in the following properties: Ion exchange capacity, surface area, concentration of OHgroups, thermal s t a b i l i t y , density, ' b u l k density and moisture content. From elementary analysis, i r and nmr spectra, concentration of OH-groups in the intermediates and in the final product respectively i t must be concluded, t h a t 45 % of the OH-groups i n the s i l i c a gel a r e condensed w i t h trimethoxy benzyl silane, g i v i n g an organic s i l i c a gel derivative with every benzyl silane aroup attached to two neighbouring siloxane groups of the s i l i c a gel (eq.1-3). Binary metal oxides show a c i d i t i e s larger than the sum of the a c i d i t i e s of the component oxides, as was prooved by numerous examples [6]. According to a theory of Tanabe [71 predictions can be made whether acidic s i t e s and type of acidic s i t e s (Bronsted-Lowry o r Lewis acid type) could be expected from binary metal oxides. The experimental r e s u l t s are i n a good agreement with the theoret i c a l calculations. I t should be of i n t e r e s t , whether the acidity of benzyl sulfonic acid siloxane ( I ) and the c a t a l y t i c a c t i v i t y could be influenced by using mixed metal oxides -Si-0-M-0-Siinstead of pure s i l i c a gel as a catal y s t support. Mixed metal oxides can be prepared by precipitation of sodium s i l i c a t e and metal hydroxides. The attachment of benzyl sulfonic acid siloxane t o the mixed supports can be performed in the same procedure as described.

Ex pe r i men ta 1 8,6 g (0,2 mol) of technical pure propene from a pressure storage vessel were a 75 ml magnetically s t i r r e d autoclave (teflon coated s t i r r e r ) passed into with glass-insert, which was previously f i l l e d w i t h 17,4 g (0,22 mol) of benzene and known amounts of the c a t a l y s t . The autoclave was heated t o the reaction temperature by a programmed heating device, During the heating period the pressure raised t o a maximum of 50 bar a t 200°C. After the reaction period the vessel was cooled t o room-temperature within a constant time (60 min) and the pressure was carefully reduced. Unreacted benzene and a1 kylation compounds were quantitatively analyzed by gaschromtography. Gaschromatography was performed on two different columns f o r each sample a f t e r supporting a known amount of toluene(200-300 mg) as an inner standard. The products were identified by authentic samples. (g.c. conditions: 1 ) 30 m column WG 1 1 ; 105°C isothermal; 0,4 kg/cm' preliminary pressure; 0,8 kp/cm2 H 2 ; 1,5 kg/ cm2 a i r ; evaporator 250°C; FID. 2 ) 45 m column Ucon; 135°C isothermal; 1 , 0 kg/ cm' preliminary pressure; Carlo Erba). For quantitative analysis of the catalysts, elementary analysis, ion exchange capacity (voltametric t i t r a t i o n with 0 , l m NaOH t o pH = 6,8), differential ther-

385

benzyl groups/g :

Calc.: a 1,40 m o l / g ; found: 0,68 mnOl/g b 0,68 mmol/g

c I 2,4 mol OH/g

I,

3,O mnol OH&

1 Dropene vessel 2 level indicator 3 benzene vessel 4 r e c i D r o c a t i n q o r o o o r t i o n i n s oump

5 pressure c o n t r o l 6 preheater

7 r e a c t o r w i t h thermocouDle

3

8 filter 9 a i r cooler

10 pressure r e n u l a t i n o u n i t 11 separator

a valves b manometer

F i g . 1. Flow sheet of apparatus for a l k y l a t i o n

Table 1. Alkylation o f benzene ( 1 7 , 4 g ) w i t h propene ( 8 , 6 9) i n p r e s e n t s o f 1,0 g c a t a l y s t a t 200°C and 1 h r e a c t i o n

time ( d i s t r i b u t i o n o f a l k y l a t i o n products). ca t a l ys t a )

I

I1

I11

1'1

V

SPC 118b)

A 15')

23,7

23,8

25,4

25,6

25,7

34,4

32,5

conversion weight % t o : cumene d i -i sopropylbenzene tri-isopropylbenzene t o t a l conversion %:

7Y7

8,8

993

13,4

10,6

11,l

99 4

191 32,5

0,4 33,O

1,8 35,6

4,O 43,O

2,3 38,5

0,3 45,8

0,9 42,8

selectivity: cumene

65,8

62,O

61,2

50,6

58,3

68,9

69,5

1,4-di-iso-propylbenzene

16,9

17,9

17,8

20,6

17,9

16,7

15,2

897

9,6

9,6

11,5

10,3

10,l

333

3,6

3,6

3Y7

333

3,3

3 2

593

6,8

6,8

11,3

797

0,8

2Y7

-

-

0,8

292

193

0,2

0,6

1,3-di-iso-propylbenzene 1,2-di-iso-propylbenzene 1,2,4-tri-iso-propylbenzene 1,3,5-tri-iso-propylbenzene

a ) For s t r u c t u r e and p r o p e r t i e s o f t h e c a t a l y s t s s e e t a b l e 2 b , Cation exchange r e s i n (Bayer AG)

Amberlyst 15

387

ma1 g r a v i m e t r y , nmr-, i r - s p e c t r a ,

s u r f a c e a r e a (BET), p o r e space, OH-groups,

m o i s t u r e c o n t e n t ( K a r l - F i s c h e r - t i t r a t i o n method), d e n s i t y , a c i d s t r e n g t h ( i n d i c a t o r method w i t h A l d r i c h Hammett I n d . S e t ) , see [8]. Continuous experiments were perfo-rmed i n a p r e s s u r e r e a c t o r ( f i g . 1 ) . R e a c t i o n c o n d i t i o n s f o r t h e c o n t i n u o u s experiments: C a t a l y s t : 5 g; f l o w r a t e : propene 14,4 m l / h (0,178 mol/h); benzene 40,O m l / h (0,45 mol/h); m o l a r r a t i o benzene: propene = 2,5; whsv (benzene) 7,O

h-’

.

whsv (propene) 1,5 h-’;

Results The r e s u l t s c o n c e r n i n g c o n v e r s i o n f a c t o r s and p r o d u c t s e l e c t i v i t i e s o b t a i n e d w i t h 1,0 g o f c a t a l y s t a t 200°C and

h o u r r e a c t i o n t i m e a r e l i s t e d i n t a b l e 1.

I n t h e s e experiments a t o t a l convers on o f propene was r e g i s t e r e d . The optimum c a t a l y s t a c t i v i t i e s were t a k e n f r o m a s e r i e s o f b a t c h experiments w i t h v a r i a t i o n o f c a t a l y s t c o n c e n t r a t i o n , r e a c t i o n t e m p e r a t u r e and r e a c t i o n time. The r e s u l t s o b t a i n e d w i t h 50 mg o f c a t a l y s t a t 200°C under o t h e r w i s e c o n s t a n t r e a c t i o n c o n d i t i o n s a r e summarized i n t a b l e 2. The r e s u l t s a r e average v a l u e s f r o m a t l e a s t t h r e e s e p a r a t e b a t c h experiments. Furthermore, t a b l e 2 i n c l u d e s d a t a about t h e c a t a l y s t s , i . e .

s u r f a c e area, i o n exchange c a p a c i t y , p o r e space,

a t o m i c r a t i o o f S i : M and a c i d s t r e n g t h and s e l e c t i v i t y o f p r o d u c t s . With t h e e x c e p t i o n o f SiO,/Al,O,-binary o f 153 mol/kg.h,

oxide, which y i e l d s a r e l a t i v e l y h i g h a c t i v i t y

t h e r e m a i n i n g b i n a r y o x i d e s show no c a t a l y t i c a l a c t i v i t i e s .

Conversion f a c t o r s and p r o d u c t s e l e c t i v i t i e s i n dependence o f p r e s s u r e and temp e r a t u r e i n c o n t i n u o u s experiments a r e g i v e n i n t a b l e 3. Discussion Benzyl s u l f o n i c a c i d s i l o x a n e c a t a l y s e s t h e a l k y l a t i o n o f benzene w i t h p r o pene a t h i g h temperature. The c a t a l y t i c a l a c t i v i t y can be s i g n i f i c a n t l y i n creased by u s i n g b i n a r y metal o x i d e s as a c a t a l y s t s u p p o r t c o n s i s t i n g o f s i l i c a g e l as t h e dominant compound and metal o x i d e s o f T i , Mg, Z r and A l . S p e c i a l l y

w i t h A l Z 0 3 as t h e m i n o r o x i d e h i g h a c t i v i t i e s a r e o b t a i n e d . F u r t h e r i n v e s t i g a t i o n s must be employed p a r t i c u l a r l y w i t h v a r y i n g amounts o f A1,0,

i n the

s u p p o r t i n o r d e r t o f i n d c o r r e l a t i o n s between t h e c a t a l y t i c a l a c t i v i t i e s and p h y s i c a l and chemical p r o p e r t i e s o f t h e c a t a l y s t s . The r e s u l t s o b t a i n e d h e r e do n o t a l l o w any c o r r e l a t i o n between c a t a l y t i c a l a c t i v i t i e s and t y p i c a l p r o p e r t i e s o f t h e c a t a l y s t s e.g.

s u r f a c e area, a c i d i t y , i o n exchange c a p a c i t y and

a t o m i c r a t i o o f t h e m e t a l s . N e v e r t h e l e s s t h e c a t a l y t i c a l a c t i v i t i e s seem t o be s i g n i f i c a n t l y i n f l u e n c e d by t h e minor metal o x i d e o f t h e c a t a l y s t s u p p o r t i n c o n n e c t i o n w i t h t h e a c i d i c f u n c t i o n a l group, i . e .

benzyl s u l f o n i c a c i d s i l o x a n e .

Table 2. A l k y l a t i o n o f benzene ( 1 7 , 4 g) w i t h propene ( 8 , 6 g) a t 200°C i n presents o f 50 mg c a t a l y s t : C a t a l y s t a c t i v i t i e s (mol/kg.h) catalysta) R- [SiO,/MO,]

a t 200°C; activity

mol

kg-h

SiO,/SiO,

(I)

98

SiO,/TiO,

(11)

147

00

p h y s i c a l and chemical p r o p e r t i e s o f t h e c a t a l y s t s . amountb, o f MOx weight %

BETC) surface m'/g

capacity H+ medg

H,O-po e spaced! ml/ g

0

540

0,78

0,93

2,3

342

0,67

1,I9

atomic ratioe) S i :M

a c i d strength PKa

Select.g)

-

-6,2 t o -6,6

Sc = 100

54,3

-6,2 t o -6,6

Sc = 95,4;

%

Sdi= SiO,/MgO

(111)

165

1,o

334

0,72

1,64

82,5

-6,2 t o -6,6

SiO,/ZrO,

(IV)

446

537

310

0,78

1,60

31,4

-6,2 t o -6,6

Sc = 100

Sc = 95,3; Sdi'

(V)

SiO,/Al,O,

982

6,s

500

0,85

1 ,DO

12,4

-6,6 t o -6,6

,

S i 0, /A1 O,f)

a ) R = HO-Si-CH,-CgHk-S03H;

f,

6,5

153

500

0

1 ,oo

12,3

( c a t a l y s t number see t a b l e 1 )

b 9 c' d y e ) Calculated f o r t h e b i n a r y o x i d e

With exception o f SiO,/Al,O,

t h e remaining b i n a r y metal oxides were c a t a l y t i c a l l y i n a c t i v e

g, S e l e c t i v i t i e s o f Sc = cumene, Sdi

= di-isopropylbenzene,

Stri

= tri-isopropylbenzene

+1,5 t o +1,1

4,7

Sc = 83,3; Sdi=

-----_-__-_______

4,6

16,7

Table 3. Continuous afkylation o f benzene with propene under pressure (molar r a t i o o f benzene : propene whsv (propene) = 1,5 ) catalyst

temp. OC

pressure bar

reaction time h

I

175

15

33 5 20

I I

I

v

200

175

200

15 20

20

30 54 6 18 26 7 27 57 100

1 2 6,s

175

SPC118 175

20

20

53 77 100 3 5 6,5

average conversion mol % a l k y l a t e

937 497 1,7 0 39 63 9 134 039 8,6 493 0 s 0,5

434 3,6 0 9 2

1932 11,8 10,l 7,2

z

selectivities $-a) %'

3i

82 94

16 6

-

84 99 100 91 95 100 100 95 95

14

2

76 86 88 92 100 94

21 13 12 8

100 100

100

-

1

-

9 5

-

5 5

-

-

6

c a t a l y s t completely destroyed

a ) See t a b l e 2; b, C a t a l y s t a c t i v i t y : mol alkylate/kg c a t a l y s t

-h

--

average ca ac t ai vl yi tsyt b )

2,l 134 199 1,5

3

1

-

9,8

=

2,5;

390 Acknowledgement F i n a n c i a l support by t h e Bundesministerium f u r Forschung und Technologie, FRG, i s g r a t e f u l l y acknowledged.

REFERENCES

1 2

3 4 5

6 7 8

F. Asinger, D i e Petrolchemische I n d u s t r i e , 11, Akademie-Verlag, B e r l i n , 1971, 1235 pp. Winnacker-Kuchler, Chemische Technologie, Carl -Hanser, Munchen, 1971, Bd. 111, 237 pp., Bd. I V , 39 pp. G. S t e f a n i d a k i s and J.E. Gwyn, Encylopedia o f Chemical Processing and Design, C. McKetta, W.A. Dekker (Eds.) Cunningham, New York 1977, 2, 357 pp. W. Zimmerrnann, D i s s e r t . H a l l e 1952 W. N o l l , Chemie und Technologie d e r S i l i c o n e , Verlag Chemie, Weinheim/BergstraBe, 1968, r e f . c i t . K. Tanabe, C a t a l y s i s , Science and Technology, R. Anderson and M. Boudart (Eds.), Springer Verlag, Heidelberg 1981, Vol. 2, 231 pp. See r e f . 6 and l i t . c i t . E. Schmidl, D i s s e r t . Duisburg 1984

391

B. Imelik et al. (Editors), Catalysis by Acids ond Bases 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands

0

THE CONVERSION OF DIMETHYLETHER OVER Pt/H-ZSM5. A BIFUNCTIONAL CATALYZED REACTION. C.W.R.

Engelen, J.P. Wolthuizen and J.H.C.

van H o o f f

Eindhoven U n i v e r s i t y o f Technology, L a b o r a t o r y f o r I n o r g a n i c Chemistry and C a t a l y s i s , P.O. Box 513, 5600 143, Eindhoven, The N e t h e r l a n d s .

SUMMARY A t l o w t e m p e r a t u r e s d i m e t h y l e t h e r mixed w i t h hydrogen r e a c t s o v e r a p l a t i n u m l o a d e d H-ZSM5 c a t a l y s t s e l e c t i v i t y t o methane. Two s u c c e s s i v e s t e p s can be distinguished; f i r s t t h e acid-catalyzed formation o f a trimethyloxoniumion, f o l l o w e d by a m e t a l - c a t a l y z e d h y d r o g e n a t i o n t o methane. Experiments w i t h o t h e r z e o l i t e s show t h a t t h e f i r s t s t e p i s r a t e d e t e r m i n i n g ; on s t r m g e r a c i d s i t e s t h e a c t i v a t i o n energy i s l o w e r t h u s t h e r e a c t i o n i s f a s t e r . R e v e r s e l y t h e e x p e r i m e n t a l determined r a t e o f methane f o r m a t i o n can be used t o c h a r a c t e r i z e t h e a c i d strenqth.

RESUME A basse temperature, 1 ' e t h e r d i m e t h y l i q u e e t 1 'hydrogene r e a g i s s e n t s u r un c a t a l y s e u r H-2SM5 au p l a t i n e en donnant s e l e c t i v e m e n t du methane. Deux @tapes c o n s e c u t i v e s s o n t c o n s i d e r e e s : l a f o r m a t i o n de l ' i o n t r i m e t h y l o x o n i u m s u r des s i t e s acides, s u i v i e de 1 ' h y d r o g e n a t i o n en methane s u r des s i t e s m e t a l l i q u e s . Des e x p e r i e n c e s avec d ' a u t r e s z e o l i t h e s m o n t r e n t que l a p r e m i e r e e t a p e d e t e r m i n e l a v i t e s s e de r e a c t i o n . Sur l e s s i t e s p l u s a c i d e s , l ' e n e r g i e d ' a c t i v a t i o n e s t p l u s f a i b l e e t l a r e a c t i o n p l u s r a p i d e . Inversement, l a d e t e r m i n a t i o n e x p e r i m e n t a l e de l a v i t e s s e de f o r m a t i o n du methane p e u t - 6 t r e u t i l i s e e p o u r c a r a c t & r i s e r l a f o r c e acide.

IHTROOUCTION A l t h o u g h t h e c o n v e r s i o n o f methanol t o hydrocarbons o v e r t h e a c i d i c z e o l i t e H-ZSMS is a l r e a d y known f o r about t e n y e a r s ( r e f - l ) , t h e r e i s s t i l l no complete understanding o f a l l t h e reactions t h a t take place during t h i s process. E s p e c i a l l y t h e q u e s t i o n a b o u t t h e f i r s t o l e f i n ( s ) formed has generated some c o n t r o v e r s y ( r e f . 2,3,4). Based upon t h e p r o d u c t d i s t r i b u t i o n s o b t a i n e d a t v e r y s h o r t c o n t a c t - t i m e s ( r e f . 2 ) o r experiments w i t h 13C l a b e l l e d methanol ( r e f , 4 ) ,

Mobil researchers

concluded t h a t ethene i s t h e f i r s t p r o d u c t formed, w h i l e propene as w e l l as h i g h e r hydrocarbons a r e p r o d u c t s of c o n s e c u t i v e r e a c t i o n s . Our f o r m e r i n v e s t i g a t i o n s (ref.5)

however, i n d i c a t e t h a t ethene and propene a r e formed para1 l e l

r a t h e r than sequential. The e l u c i d a t i o n of t h i s problem i s hampered b y t h e f a c t t h a t t h e p r i m a r y

392

o l e f i n s a r e i n v o l v e d i n a c o m p l i c a t e d network o f r e a c t i o n s ( p o l y m e r i s a t i o n , d e p o l y m e r i s a t i o n , m e t h y l a t i o n e t c . ) , moreover t h e d e t e c t i o n i s h i n d e r e d by an a d s o r p t i o n / r e a c t i o n i n t h e z e o l i t e pores ( r e f . 6 ) . To c i r c u m v e n t t h e s e problems we t r i e d t o t e r m i n a t e t h e r e a c t i o n sequence d i r e c t l y a f t e r the formation o f the f i r s t o l e f i n ( s ) by a hydrogenation t o t h e p r a c t i c a l l y i n e r t p a r a f f i n s . F o r t h i s purpose we performed t h e r e a c t i o n i n t h e presence o f H2 and p l a t i n u m i n c o r p o r a t e d i n t h e z e o l i t e . The f i r s t experiments showed t h a t a t temperatures below 250°C t h e methanol c o n v e r s i o n i s d r a s t i c a l l y i n f l u e n c e d by t h e presence o f p l a t i n u m . Beyond e x p e c t a t i o n t h e s i n g l e p r o d u c t was methane, a s p e c i e s n o r m a l l y h a r d l y formed. I n t r i g u e d by t h i s r e s u l t we d e c i d e d t o i n v e s t i g a t e t h e o r i g i n e o f t h e observed methane. EXPERIMENTAL

catalyst:.

Z e o l i t e H-ZSM5 was prepared as p r e v i o u s l y d e s c r i b e d ( r e f . 7 ) and

had a Si/A1 r a t i o o f 30 (0.51 mmol A l / g ) . The c r y s t a l l i n i t y was c o n f i r m e d w i t h X-ray d i f f r a c t i o n and porevolume measurement (0.17 m l / g ) . The s i l i c a l i t e was s y n t h e s i z e d analogous t o H-ZSM5 b u t i n absence o f a aluminium source ( r e f . 8 ) . P l a t i n u m was i n c o r p o r a t e d by means o f t h e i n c i p i e n t wetness technique; a f t e r d r y i n g , t h e pores were f i l l e d w i t h a s o l u t i o n 4 w t . % s o l u t i o n o f Pt(NH3)4 (OH)2 t i l l t h e d e s i r e d amount o f p l a t i n u m was p r e s e n t . The impregnated samples were d r i e d o v e r n i g h t i n a i r a t 100°C and a f t e r c a l c i n a t i o n i n an a r t i f i c i a l a i r f l o w a t 300°C, reduced a t t h e same t e m p e r a t u r e i n p u r i f i e d H2. The H-Y and H-Morden i t e were o b t a i n e d f r o m Union C a r b i d e ( L i n d e d i v i s i o n ) and N o r t o n r e s p e c t i v e l y .

f$tgrixalz. A 4 w t . % s o l u t i o n of Pt(NH3)4(0H)2 was o b t a i n e d f r o m Johnson ivlatthey Chemicals L t d . Pure (CH3),0BF4

was purchased b y F l u k a (no.92605).

D i m e t h y l e t h e r was a h i g h p u r i t y r e a g e n t (99,99%) o f Matheson.

Aeparatus-and-eroceduyel.

The c o n v e r s i o n o f d i m e t h y l e t h e r (MOM) was s t u d i e d

u s i n g a f i x e d - b e d c o n t i n u o u s f l o w r e a c t o r c o n t a i n i n g l g o f c a t a l y s t (FIOM/H=0.5, W~SV(M0t~)= 2.2/hr).

The a c t i v e m a t e r i a l was d i l u t e d (1:l) w i t h S i 0 2 0 f t h e same

p a r t i c l e size. A t steady-state ( a f t e r

+

-

15 m i n . ) t h e p r o d u c t s were analyzed on-

l i n e by gaschromatography. The t h e r m o g r a v i m e t r i c experiments were performed w i t h a Cahn-RG-Elektrobalance. RESULTS AND D I S C U S S I O N

converslon-exeerlments

*

To i n v e s t i g a t e t h e i n f l u e n c e o f p l a t i n u m i n c o m b i n a t i o n w i t h H2 on t h e conv e r s i o n o f MOM, we used s e v e r a l c a t a l y s t s ; pure H-ZSM5, H-ZSM5 mixed w i t h P t / S i 0 2

and Pt/H-ZSM5. The main r e s u l t s a r e shown i n f i g u r e 1. As can be seen

393

H-ZSM5+PtlSi02

PtIH-ZSM5

200%

"1

1 2 50°C

20

-

P,

l n o b 1 2 3 4 5 5 + 1 2 3 4 5 5+

1

2

3

4

5 5 +

CARBON NUMBER

F i g . 1 The i n f l u e n c e o f p l a t i n u m on t h e c o n v e r s i o n o f d i m e t h y l e t h e r i n t h e presence o f hydrogen. H-ZSM5: 2.8 w t . % P t / S i 0 2 = l : l , H-ZSM5, b o t h p u r e and mixed w i t h P t - S i O p ,

Pt/H-ZSM5:

5 wt.% P t .

i s n o t a c t i v e a t 200°C, w h i l e s u r p r i s -

i n g l y t h e Pt/H-ZSM5 c a t a l y s t produces s e l e c t i v e l y methane. A t 250°C H-ZSMS g i v e s a normal p r o d u c t d i s t r i b u t i o n ; t h e amount o f methane formed i s b u t s m a l l , i n d i c a t i n g t h a t t h e H2 has l i t t l e i n f l u e n c e . The main e f f e c t o f m i x i n g H-ZSM5 w i t h P t / S i 0 2 i s a h y d r o g e n a t i o n o f t h e alkenes t o alkanes; a g a i n t h e s e l e t i v i t y t o wards methane i s s m a l l . The d i s t a n c e between t h e a c i d and t h e h y d r o g e n a t i o n f u n c t i o n seems t o o l a r g e f o r an e a r l i e r i n t e r v e n t i o n . When t h e p l a t i n u m i s i n t h e v i c i n i t y o f t h e a c i d s i t e s i n t h e z e o l i t e pores t h e i n f l u e n c e on t h e p r o d u c t d i s t r i b u t i o n i s s t r o n g e r . The f o r m a t i o n o f hydrocarbons w i t h more t h a n 1 C atom i s almost c o m p l e t e l y i n h i b i t e d ; i n s t e a d , a pronounced f o r m a t i o n o f methane t a k e s p l a c e even a t 200°C and l o w e r temperatures where n o r m a l l y MOM does n o t r e a c t .

I n o r d e r t o i n v e s t i g a t e t h e importance o f t h e a c i d - s i t e s d u r i n g t h e methane f o r m a t i o n we used t h e n o n - a c i d s i l i c a l i t e as s u p p o r t f o r t h e p l a t i n u m . The conv e r s i o n o f MOM t o methane o v e r P t / s i l i c a l i t e i s n e g l i g i b l e a t temperatureswhere Pt/H-ZSM5 produces s o l e l y methane (see F i g . 2 ) . T h i s s t r e n g t h e n s t h e

394

F i g . 2 The i n f l u e n c e o f a c i d s i t e s on t h e f o r m a t i o n o f methane. a ) 0.5 w t . % Pt/H-ZSM5. b ) s a m e c a t a l y s t poisonned w i t h N H 3 c ) 0.5 w t . % P t / S i l i c a l i t e . O = a c t i v i t y o f b ) a f t e r c a l c i n a t i o n f o r 2 hours a t 420°C.

assumption t h a t t h e r e a c t i o n i s d u a l - f u n c t i o n a l . Moreover, when che a c i d a c t i v i t y o f Pt/H-ZSM5 i s suppressed by exposing t h e z e o l i t e t o ammonia a t 200°C

(ref.9),

t h e methane p r o d u c t i o n s h a r p l y decreases. Only t h e weaker a c i d s i t e s ,

n o t covered by ammonia ( r e f . l O ) , a r e a t a h i g h e r temperature a c t i v e i n t h e methane f o r m a t i o n . The a c t i v i t y o f t h e c a t a l y s t s can be recovered by h e a t i n g t h e poisoned

sample a t temperatures above 4 0 0 O C ; t h e s t r o n g e r a c i d s i t e s a r e r e -

l e a s e d o f ammonia and t a k e p a r t a g a i n i n t h e methane f o r m a t i o n ( s e e F i g . 2 ) . The c o n c l u s i o n t h a t t h e r e a c t i o n i s b i f u n c t i o n a l c a t a l y z e d r e s u l t s i n t h e p o s t u l a t i o n o f t h r e e p o s s i b l e mechanisms:

2

MOM-0

z'+

Pt

MOM-0

Z

- -

H +MOM

H

3

CH4

/

CH2

- M p

M

o+ M M

Pt

CH4

Mechanisms i n which p l a t i n u m o p e r a t e s a f t e r t h e f o r m a t i o n o f

t h e primary

o l e f i n s a r e u n l i k e l y s i n c e t h e r a t e o f h y d r o g e n o l y s i s o f t h e subsequent formed p a r a f i n s (ethane/propane) t o methane i s n e g l i g i b l e ( s e e F i g . 1 ) . The h y d r o g e n a t i o n o f t r i m e t h y l o x o n i u m i o n (M30t), t h e i n t e r m e d i a t e o f t h e l a s t mechanism has never been observed; t h e r e f o r we examined t h e f e a s i b i l i t y o f t h i s r e a c t i o n . F o r t h i s purpose we heated a p h y s i c a l m i x t u r e o f M30BF4 and P t / S i 0 2 a t

395

temperatures below 100°C in H2. The f a c t t h a t t h e m a j o r hydrocarbon formed was methane proves t h a t t h e i n t e r m e d i a t y o f M30t i s n o t u n l i k e l y . By t h e f o r m a t i o n o f t h i s s p e c i e s a second MOM m o l e c u l e r e a c t s w i t h a chemisorbed MOM molecule. This i s

i n c o n t r a s t w i t h f i r s t two mechanisms, where chemisorbed MOM i s e i t h e r

d i r e c t l y c o n v e r t e d t o methane o r r e a c t s m o n o m o l e c u l a r l y t o an i n t e r m e d i a t e t h a t i s hydrogenated.

K ~ s p % ~ m m t sBy means o f t h e r m o g r a v i m e t r y we i n v e s t i g a t e d t h e r e a c t i v i t y o f chemisorbed MOM towards hydrogenation. A t y p i c a l experiment i s shown i n f i g u r e 3; d e p i c t e d i s t h e change i n w e i g h t o f a d r i e d and reduced Pt/H-ZSM5 sample d u r i n g e x p o s i t i o n t o gases i n d i c a t e d . A f t e r s a t u r a t i o n o f t h e z e o l i t e w i t h MOM,

He

+

F i g . 3 The r e a c t i v i t y o f MOM chemisorbed on 5 w t . % Pt/H-ZSM5 t e s t e d w i t h thermog r a v i m e t r y . *= p a r t o f H e f l o w r e p l a c e d by H z ( t o t a 1 f l o w r e mains c o n s t a n t : 1 5 0 ml/Flin)

MOM

I

3

,I lo

1

t /min

I

20

t h e p h y s i s o r b e d p a r t i s removed by a h e l i u m f l o w t i . . c o n s t a n t w e i g h t .

he

d i f f e r e n c e w i t h t h e i n i t i a l w e i g h t e q u a l s t h e w e i g h t o f chemisorbed MOM. A p a r t -

H2, w i t h o u t change i n t h e t o t a l g a s f l o w , has no on t h e c a t a l y s t s ' w e i g h t . I t can be concluded t h a t cheinisorbed MOM cannot be hydrogenated by p l a t i n u m . In a d d i t i o n we analyzed t h e p r o d u c t s l e a v i n g t h e i a l replacement o f t h e He by

effect

sample d u r i n g f l u s h i n g i n H2 a f t e r MOFl a d s o r p t i o n . Only

i n the

presence o f

b o t h weakly and s t r o n g l y bound MOM we c o u l d d e t e c t methane. I n accordance w i t h t h e r e s u l t s p r e s e n t e d i s a mechanism i n which b o t h t h e p l a t i n u m p a r t i c l e s as t h e a c i d s i t e s p a r t i c i p a t e ; t h e d e t a i l s a r e shown i n t h e f o l l o w i n g reaction-scheme:

396

+ CH30H

7%

0

Y3-7 O +- H . +

y 3

O+ CH3 CH3H 0Z

-c

/ \ t \

Pt

r

CH4 + ,O

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