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
REFERENCES 1 2
3 4 5 6
7 8 9 10 11 12 13 14 15 16 17
H. Pines and W . M. S t a l i c k , Base-Catalyzed Reactions o f Hydrocarbons and Re1a t e d Compounds, Academic Press, New York , London, 1977. K . Tanabe, T. Yamaguchi and T. T a k e s h i t a , J. Res. I n s t . Catal., Hokkaido Univ., 16 (1968) 425. K. Tanabe, S o l i d Acids and Bases, Academic Press, New York, London, 1970. K. Tanabe, i n J . R. Anderson and M. B o u d a r t (Eds.), C a t a l y s i s : Science and Technology, S p r i n g e r - V e r l a g , B e r l i n , 1981, Vol .2, Chap.5. K. Tanabe, N. Y o s h i i and H . H a t t o r i , J. Chem. SOC. Chem. Commun., (1971) 464. K. Morishige, H . H a t t o r i and K. Tanabe, J . Chem. SOC. Chem. Commun., (1975) 559. H. H a t t o r i , N. Y o s h i i and K . Tanabe, Proc. 5 t h I n t e r n . Congr. C a t a l y s i s , Miami Beach, lO(1972) 233. H . H a t t o r i , K. Maruyama and K . Tanabe, J. Catal., 44 (1976) 50. R. Ohnishi and K . Tanabe, Chem. L e t t . , (1974) 207. K. Shimazu, K. Tanabe and H. H a t t o r i , J . Catal 45 (1977) 302. H . H a t t o r i , K. Tanabe, K . Hayano, H . Shirahama and T. Matsumoto, Chem. L e t t . , (1979) 133. A. H a t t o r i , H . H a t t o r i and K . Tanabe, J . Catal 65 (1980) 245. H . Matsuhashi, H . H a t t o r i and K. Tanabe, Chem. L e t t . , (1981) 341. H. Matsuhashi and H . H a t t o r i , J. Catal., 85 (1984) 457. K . Tanabe and R. N o y o r i , Super Acids and Super Bases, Kodansha S c i e n t i f i c , Tokyo, 1980, Chapt.4. 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 . , 25 (1977) 669. Sumitomo Chemical I n d . , Japan P a t e n t , Sho 50-3274 ( 1 9 7 5 ) .
., .,
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.
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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,
This page intentionally left blank
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
The d e p o s i t i o n o f phosphoric a c i d gives r i s e t o systems, characterized by an a c i d i t y w i t h i n the f o l l o w i n g l i m i t s : -12.7<-H0
<-13.75.
The i n t r o d u c t i o n of
PC13, o r PC15 gives r i s e t o systems o f higher a c i d i t y , whereas the i n t r o d u c t i o n o f P205 produces systems o f a superacid character -Ho<-13.75.
The a d d i t i o n of
phosphoric a c i d r a d i c a l s t o the OH groups takes p l a c e d u r i n g t h e evaporation o f P2O5 on aluminum
g e l . Simultaneously, t h e dehydration o f the surface occurs.
The most l i k e l y course o f the r e a c t i o n i s as f o l l o w s :
OH I
OH
A1
A1
I
A1
lo \o./ /
The A1203-P205 c a t a l y s t s are acids s t r o n g enough t o b r i n g about t h e c h a i n i s o m e r i t a t i o n o f n-alkenes a t t h e temperature o f 150°C (ref.3). The increase o f b a s i c i t y and b a s i c s t r e n g t h o f t h e oxide surface The d e p o s i t i o n o f a l k a l i c a t i o n s such as a l k a l i hydroxides, carbonates o r oxalates d i s s o l v e d i n d i f f e r e n t solvents such as water, alcohols, dioxane, e t c . has been known f o r a long time. This procedure was used e i t h e r f o r poisoning of t h e a c i d i c centres o r producing new b a s i c centres on t h e surface of g r e a t e r strength.The p r o p e r t i e s o f the c a t a l y s t , prepared i n such a manner, depend on t h e k i n d o f a l k a l i i o n , on t h e anion and t h e temperature o f c a l c i n a t i o n o r activation. The impregnation of aluminum gel w i t h an aqueous NaOH s o l u t i o n and c a l c i n a t i o n of the sample, i n i t i a l l y ( d e p o s i t i o n o f up t o 0.25 mmol NaS/g A1203) b r i n g s about small changes i n t h e number o f b a s i c centres and t h e i r b a s i c i t y . The depos i t i o n o f g r e a t e r amounts than 2,.5 mmol Naf/g r e s u l t s i n a decay of a c i d i c cent r e s of g r e t e r a c i d i t y H, from -3 t o -13.1. centres o f low a c i d i t y (H0>4.2)
On t h e o t h e r hand, the number o f
considerably increases. The number o f basic
centres increases s l i g h t l y and t h e i r b a s i c s t r e n g t h does n o t exceed t h e value H,24.6
(ref.4).
Basic s t r e n g t h i s n o t increased by d e p o s i t i n g NaOH from an aqueous s o l u t i o n onto magnesium oxide. The concentration o f b a s i c centres s l i g h t l y increases, depending upon the amount o f NaOH which has been introduced. The h i g h e s t concent r a t i o n i s achieved by i n t r o d u c i n g 0.35 mmol Na+/g HgO. The e l e v a t i o n o f the c a l c i n a t i o n temperature from 550" t o 1000" causes a decrease i n the number as
61
w e l l as the strength o f basic centres ( r e f . 5 ) . TABLE 2 The change i n the number o f basic centres depending on the introduced amount o f Na+
Amount deposited mmol NaOH/g MgO
Concentration o f b s i c centres of the f o l l o w i n g strength i n mmol/m
h
12 Q H - 4'15 0.005 0.01 0.07 0.35 0.82
18 4 H- 4 23
27 4 H- 4 33
0.010 0.008 0.018 0.025 0.020
0.015 0.014 0.011 0.030 0.008
0.022 0.021 0.015 0.030 0.008
The change i n the acid-basic p r o p e r t i e s o f the surface i s dependent on the k i n d o f introduced c a t i o n as w e l l
as the k i n d o f accompanying anion. As an
example, the impregnation o f aluminum oxide w i t h K o r Cs ethoxides r e s u l t s i n the formation o f a more basic surface than i n the case o f impregnation w i t h L i o r Na ethoxides ( r e f . 6 ) . TABLE 3 The concentration o f b a s i c c e n t r e s d i f f e r e n t strength on A1203 a f t e r the depos i t i o n o f a l k a l i metal ethoxides
Amount o f deposited metal i o n i n mmol/g
Pure A1203 Li
A1203
0
1.41
Ethoxide Na K 1.07 0.79
cs 0.58
Concentration o f centres o f d i f f e r e n t basic
H-
strength i n mmol/g
12.2 15.0 18.4 22.3 24.6 26.5 33
1.17 0.99 0.99 0.91 0.91 0.91 0
1.60 1.60 1.31
1.31 1.00 1.00 0
1.82 1.82 1.82 1.19 1.19 1.19 0
2.22 2.03 2.03 2.03 2.03 2.03 0
2.42 2.22 2.02 1.81 1.81 1.81 0
The evaporation o f strong e l e c t r o n donors, such as a l k a l i metals ( w i t h the o x i d a t i o n number o f zero, and t h e r e f o r e i n m e t a l l i c s t a t e ) , on oxides, i n s u l a t o r s and semiconductors causes the greatest changes i n the acid-basic p r o p e r t i e s o f t h e i r surfaces.
62
We have i n v e s t i g a t e d the i n f l u e n c e o f the evaporation of m e t a l l i c sodium on the surface o f A1203
prepared by means o f h y d r o l y s i s o f aluminum isopropoxide
and c a l c i n a t i o n a t 550°C. TABLE 4 The change i n concentration o f a c i d i c and basic centres influenced by the evapor a t i o n o f Na
A c i d i t y -Hd 2.5
-3.0
-5.6
*l2'3 5.5 A1202 + Na"14.0
1.8 14.0
1.5 2.0
H,
-8.2 1.5 1.0
<
-12.7 1.5 0
B a s i c i ty H-
> 24.6
-13.1
17.2
18.4
22.3
26.5
1.5
2.2
2.2
2.2
1.9
1.9
0
2.5
2.5
2.5
2.5
2.5
The evaporation of m e t a l l i c sodium r e s u l t s i n a decay o f h i g h l y a c i d i c c e n t r e s = -12.7. A t the same time weak a c i d i c centres are being formed. Simultane-
ously, t h e r e i s an increase i n t h e number of one-electron donor and b a s i c c e n t r e s
.
( ref. 6) Catalysts o f ve,ry h i g h b a s i c i t y are obtained by evaporating a l k a l i metals on MgO. The b a s i c i t y i s dependent on the temperature o f c a l c i n a t i o n o f MgO c a r r i e d o u t b e f o r e the evaporation. Three groups o f basic centres o f d i f f e r e n t b a s i c i t y have been found on the surface : 18.4
; 25
< H-<33
i s possible t h a t the strength o f basic centres o f H->35
; H->35.
It
i s greater. This i s
i n d i c a t e d by the appearance o f a dark-red c o l o u r a t i o n a f t e r the adsorption o f cumene (pKa = 37) on MgO, which has been p r e v i o u s l y c a l c i n e d a t 550", 650", 750"
+
Nay The adsorption i s n o t accompanied by the formation o f paramagnetic surface
species. The red c o l o u r a t i o n i s most probably due t o carbanion formation (ref.7). Centres o f b a s i c i t y H->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-<
35. On the other hand,
they contain a l a r g e number o f one-electron centres o f h i g h donor power ( r e f . 8 ) . The evaporation of m e t a l l i c sodium on A1203 a l s o r e s u l t s i n the formation o f superbasic centres H-.35.
However the number o f these centres i s much smaller
63 t h a n i n t h e case o f MgO. On t h e o t h e r hand, f o r m a t i o n o f s u p e r b a s i c c e n t r e s does n o t o c c u r when m e t a l l i c sodium i s evaporated on S i 0 2 o r T i 0 2 . The f o r m a t i o n o f s u p e r b a s i c c e n t r e s r e s u l t i n g f r o m t h e d e p o s i t i o n o f a l k a l i m e t a l s on t h e s u r f a c e o f i n s u l a t o r s can be r a t i o n a l i z e d as an e l e c t r o n t r a n s f e r f r o m an a l k a l i metal atom t o an a p p r o p r i a t e a c c e p t o r c e n t r e on t h e s u r f a c e . The a c c e p t o r c e n t r e s can be 0- i o n s l o c a t e d i n a p r o p e r environment, f o r example adj a c e n t t o a c a t i o n vacancy. The c e n t r e s can be c o n s t i t u t e d o f e i t h e r s i n g l e 0i o n s o r c l u s t e r s c o n t a i n i n g one t o t h r e e 0- i o n s o r f i n a l l y s u r f a c e OH groups.
ci
0OH,
t
t Na"
2 OH,
t
-
Nao
Na"
-
02- [
It
Nat
ONa, t 1/2 H2 ONa,
t
H20
The e v a p o r a t i o n o f a l k a l i m e t a l s on t r a n s i t i o n m e t a l o x i d e s , besides analogue r e a c t i o n s as i n t h e case o f MgO, may a l s o b r i n g a b o u t t h e r e d u c t i o n o f t h e metal i o n , c a u s i n g changes i n 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 . The evap o r a t i o n o f m e t a l l i c sodium on t h e s u r f a c e o f T i 0 2 , ZnO, Cr203 o r N i O does n o t r e s u l t i n t h e f o r m a t i o n o f s u p e r b a s i c c e n t r e s on t h e s u r f a c e . However i t does l e a d t o t h e f o r m a t i o n ( o r i n c r e a s e i n t h e number) o f one e l e c t r o n donor c e n t r e s (ref.9). TABLE 5
Basic properties oxide
O n e - e l e c t r o n donor p r o p e r t i e s
o x i d e t Nao
oxide
Oxide t Na"
Z nO
H-<9
Lack o f superbase
0
1
Ti02
H-<9
Lack o f superbase
2
10
Cr203 NiO
H-<9 H- < 9
Lack o f superbase Lack o f superbase
0 0
7 0.2 -
The e v a p o r a t i o n o f m e t a l l i c magnesium vapour on MgO l e a d s t o t h e f o r m a t i o n o f new c a t a l y t i c systems, c o n t a i n i n g c o n s i d e r a b l y less s u p e r b a s i c c e n t r e s t h a n i n thecase o f e v a p o r a t i o n o f a l k a l i m e t a l s . On t h e o t h e r hand a l a r g e number o f o n e - e l e c t r o n donor c e n t r e s o f h i g h donor power i s formed ( r e f . 1 0 ) . The f o r m a t i o n o f c o m p l e t e l y new c a t a l y t i c systems i s a r e s u l t o f t h e evaporat i o n o f m e t a l l i c z i n c on oxides, i n s u l a t o r s and semiconductors. E v a p o r a t i o n o f Znmet on T i 0 2 , N i O , Cr2O3 o r Moo3 does n o t b r i n g a b o u t an i n c r e a s e i n t h e b a s i c i t y o f t h e s u r f a c e . The s u r f a c e o f t h e T i 0 2
-
Znmet system c o n t a i n s c e n t r e s
w h i c h have s t r o n g o n e - e l e c t r o n donor c h a r a c t e r . The s u r f a c e o f a system produced
64 by evaporating m e t a l l i c z i n c on z i n c oxide has a b a s i c i t y o f H-<9.
It consists
o f a l a r g e number of one-electron donor centres ( r e f . 1 1 ) .
The b a s i c i t y o f the surface o f ZnO i s n o t a f f e c t e d during the evaporation of z i n c vapour. Catalysts, prepared by evaporation o f m e t a l l i c z i n c on Ti02, Crp03, Moo3 o r N i O have s i m i l a r p r o p e r t i e s . The b a s i c i t y o f the surface i s almost the same as
i n the i n i t i a l oxide and amounts t o about H-<9.
On the o t h e r hand, the number
o f one-electron donor centres increases i n the cases o f Ti02 and Cr203. This i s
r e f l e c t e d by the c a t a l y t i c a c t i v i t y o f these samples
. The Ti02,
Moo3, NiO+Zn,,t
systems are a c t i v e i n the dehydrogenation o f alcohols, i n the dehydrogenation o f ethylbenzene. They e x h i b i t , however low a c t i v i t y i n the degradation o f cumene t o benzene and propylene. TABLE 6 The change i n the number o f one-electron donor centres a f t e r the evaporation o f Znme t ~
I n conventiona? u n i t s
Pure oxide
Oxide t Znmet
Ti02
80
700
Cr203 MOO
4 13
11 0
0
0
NiO
REFERENCES
1 M. Marczewski, M. Derewinski and S t . Malinowski, Canad. J. Chem. Eng., 6 1 (1983) 93. 2 M. Derewinski and St. Malinowski , Proc. Confern on heterogeneous Catalysis. B u l g a r i a n Acad. Sci., Varna 1979. 325. 3 M.Marczewski, I . Saluda and S t . Malinowski, B u l l . Acad. Polon. Sci., Ser. Sci Chem. 27 (1979) 737. A. Krzywicki , M. Marczewski , R. Modzelewski and K. Pelszik, React. Kinet. Catal. L e t t . 13 (1980) 1. A. Krzywicki and M. Marczewski, J. Chem. SOC. Faraday 1.. 76 (1980) 1311. 4 M. Marczewski , J. K i j e n s k i and S t . Malinowski, Personal communication. 5 J . K i j e n s k i and S t . Malinowski, B u l l . Acad. Polon., Ser. S c i . Chem. 26 11978) 183. 6 7. Wozniewski and S t . Malinowski, B u l l . Acad. Polon. Sci. Ser. Sci. Chem. 26 (1978) 829. R. Hombek, J . K i j e n s k i and S t . Malinowski, Proc. Second I n t r . Sympos. on Scient. Bases. Preparation Heterog. Catalysts. , Louvain l a Neuve (1978) 132. 7 J . K i j e n s k i and S t . Malinowski J . Chem. SOC.. Faraday Trans. I . 74 (1978) 250. 8 J . K i j e n s k i , M. Marczewski and S t . Malinowski, React. Kinet. Catal. L e t t . 7 (1977) 151.
.
65
J . K i j e n s k i and St. Malinowski, B u l l . Acad. Polon. Sci. Ser. Sci. Chem., 25 (1977) 331. J . K i j e n s k i and S t . Malinowski, J. Res. I n s t . Hokkaido Univ., 28 (1980) 97. 9 J . K i l e n s k i , M. Marczewski and St. Malinowski, React. Kinet. Catal. L e t t . , 7 (1977) 157. 10 J . K i l e n s k i , React. K i n e t . Catzl. L e t t . 18 (1981) 317. 11 E. Kaniewska and S t . Malinowski, unpublished r e s u l t s .
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67
B. Imelik e t al. (Editors), Catalysis by Acids and Bases o 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
OF TOLUENE BY METHANOL
B A S I C MOLECULAR S I E V E CATALYSTS//SIDE-CHAIN ALKYLATION
J. M. GARCES,*
G. E. VRIELAND, S . I. BATES,
F. M. SCHEIDT
The Dow Chemical Company, CR-IMC L a b o r a t o r y , Midland, M i c h i g a n 48640
INTRODUCTION The use o f z e o l i t e s as a c i d i c c a t a l y s t s has r e c e i v e d widespread a t t e n t i o n s i n c e t h e i r i n t r o d u c t i o n i n i m p o r t a n t i n d u s t r i a l processes.
I n contrast, only
s c a n t a t t e n t i o n has been p a i d t o t h e i r use as b a s i c c a t a l y s t s ( 1 ) . The base c a t a l y z e d 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 w i t h o l e f i n s r e s u l t s i n s e l e c t i v e a d d i t i o n o f the o l e f i n t o the s i d e chain (2). S i m i l a r l y , basic c a t a l y s t s l e a d t o t h e s i d e c h a i n a l k y l a t i o n o f t o l u e n e w i t h methanol ( 1 ) .
This
r e a c t i o n t a k e s p l a c e o v e r a l k a l i m e t a l exchanged z e o l i t e s ( 3 - l o ) , b a s i c o x i d e s ( l l ) , and b a s i c s a l t s e i t h e r p u r e o r s u p p o r t e d on carbon (12,13). I n z e o l i t e X, t h e l a r g e r a l k a l i m e t a l c a t i o n s l e a d t o more a c t i v e c a t a l y s t s (3,4),
and boron and phosphorous a d d i t i v e s g i v e more a c t i v e and s e l e c t i v e
c a t a l y s t s f o r t h e d i r e c t s y n t h e s i s o f s t y r e n e ( 5 ) . L i t h i u m exchanged z e o l i t e s produce o n l y xylenes, t y p i c a l o f a c i d i c c a t a l v s t s 13,4\.
I R s t u d i e s (14) o f v a r i o u s c a t i o n forms o f z e o l i t e
X
used i n methanol
decomposition showed f o r m a t i o n o f methoxide, f o r m a t e and carbonate s p e c i e s on c a t a l y s t s t r e a t e d w i t h methanol a t 673°K. Laser Raman s t u d i e s ( 1 5 ) , UV d i f f u s e r e f l e c t a n c e (16), and C-13 NMR (17) s t u d i e s o f benzene adsorbed on z e o l i t e v i b r a t i o n a l and electronic,between and Cs'
w i t h i n the zeolite.
X
suggest a s t r o n g i n t e r a c t i o n , b o t h
t h e a r o m a t i c n u c l e u s and t h e c a t i o n s K',
Rb'
t
A p p a r e n t l y r i n g a l k y l a t i o n i s h i n d e r e d b y CS
c a t i o n s w i t h i n t h e supercage. I n t h i s paper we t r y t o p r o v i d e a comprehensive model t o account f o r t h e b e h a v i o r o f t h e c a t a l y s t s s t u d i e d so f a r .
We use chemical o b s e r v a t i o n s , e q u i -
l i b r i u m thermodynamic data, mass spectroscopy, e t c . t o s u p p o r t an e q u i l i b r i u m model which i n v o l v e s metal oxides, metal carbonates, z e r o v a l e n t m e t a l s and COP. The model p r o v i d e s a common ground t o i n t e r p r e t t h e e f f e c t s o f c a t a l y s t composit i o n , r e a c t a n t s t o i c h i o m e t r y , added gases, and temperature on t h e p r o p e r t i e s o f t h e d i f f e r e n t c a t a l y s t s used t o date.
We a l s o g i v e e v i d e n c e t h a t s p a t i a l l i m i -
t a t i o n s p l a y a c r i t i c a l r o l e on t h e a l k y l a t i o n of t o l u e n e by methanol i n nonz e o l i t e catalysts. *To whom a l l i n q u i r i e s r e g a r d i n g t h i s paper s h o u l d be addressed
68
EXPERIMENTAL Materials
-
Reagent grade (99+%) toluene, methanol, styrene, e t c . were used
without further purification. Catalysts
-
Z e o l i t e X c a t a l y s t s were made by ion-exchange o f zeol t e 13X
(Union Carbide Co.) a t s e l e c t e d pH values, using 0.1-0.5 s o l u t i o n s a t 298°K.
m n i t r a t e o r hydroxide
P e l l e t s pressed a t 10,000 p s i g and s i z e d t o 14-20 mesh were
a c t i v a t e d i n s i t u under N2 f l o w w h i l e h e a t i n g from 298°K t o 773°K over a p e r i o d o f 2 hours. X-Ray a n a l y s i s shows t h a t o n l y z e o l i t e X i s present as a c r y s t a l l i n e phase i n 13X.
I t s composition, v i a neutron a c t i v a t i o n a n a l y s i s was Na85(A102),5
The BET surface area (Si02)107. The u n i t c e l l dimension was a0=24.9129 A". a f t e r c a l c i n a t i o n a t 773°K was 686 m2/g. Ion-exchanged d e r i v a t i v e s had the f o l l o w i n g c a t i o n compositions, given as percents f o r samples e q u i l i b r a t e d a t pH 13:
L i ( 2 8 ) , Na(100), K(88), Rb(60), Cs(39).
A c t i v a t e d carbon c a t a l y s t s were prepared by impregnating carbon w i t h aqueous s o l u t i o n s o f a l k a l i metal s a l t s .
A f t e r the s o l u t i o n penetrated t h e pores t h e
r e a c t a n t s were heated t o 338°K f o r about t h r e e hours.
The s o l i d s were separated
by f i l t r a t i o n , d r i e d and used i n the r e a c t o r . The Armak Carbon (Taiyo Koken, Tokyo) support had a surface area of 1212 m2/g w i t h a 7-8 A" pore diameter. weight % B.
The c a t a l y s t contained 13.2 weight % Cs and 0.23
Catalyst Testing C a t a l y s t s were evaluated i n 10 cm3 o r 100 cm3 f i x e d bed u n i t s a t atmospheric pressure.
The c a t a l y s t was loaded between quartz wool plugs i n the s . s t e e l
reactor.
The r e a c t o r and the feed preheating c o i l s were suspended w i t h i n an
e l e c t r i c a l l y heated furnace.
The a i r w i t h i n the furnace was maintained a t
u n i f o r m temperature by r a p i d c i r c u l a t i o n w i t h a fan.
I n a t y p i c a l evaluation,
a toluene/methanol feed w i t h a 5/1 molar r a t i o and LHSV o f 4 h r - ' was passed over t h e bed o f c a t a l y s t a t 698°K a t atmospheric pressure.
The r e a c t o r e f f l u e n t
was condensed using an ice-water bath and t h e condensate was analyzed by gas
A 5% SP-1200/1.75% bentone 34 on 100/120 Supelcoport column, 6 f t x 1 / 8 ss was employed. Samples o f t h e non-condensable products were c o l l e c t e d i n glass p i p e t t e s and submitted f o r GC-mass spec. a n a l y s i s . chromatography.
RESULTS AND DISCUSSION Molecular Sieve Catalysts
A screening e f f o r t t o f i n d c a t a l y s t s f o r the s i d e c h a i n a l k y l a t i o n o f toluene w i t h methanol r e s u l t e d i n t h e s e l e c t i o n o f CsNaX z e o l i t e and a CsB-carbon as t h e most promising c a t a l y s t s . Fig. 1.
T y p i c a l r e s u l t s f o r these c a t a l y s t s are shown i n
D e a c t i v a t i o n o f the c a t a l y s t was associated w i t h pore volume l o s s due
t o coke deposits.
I t i s s i g n i f i c a n t t h a t these two c a t a l y s t s c o n t a i n cesium
69
and have s i m i l a r pore diameters, b u t d i f f e r e n t backbone compositions.
This
suggests t h a t t h e a l k a l i metal i o n and the pore s i z e p l a y important r o l e s i n t h e reaction.
We should add t h a t o t h e r carbon supports w i t h l a r g e r pores were n o t
as e f f e c t i v e (18).
6 .
I
1
I
I
-
-
Fig. 1.
A c t i v i t y change w i t h time f o r
t h e a l k y l a t i o n of toluene w i t h methanol Conditions:
5 tol/MeOH, 698"K, LHSV 4 h r - l , 1 atm.
Catalyst:
CsNaX ( O ) , CsB-Carbon (0)
*
-
Arom. Products % = Arom. Products
+
xloo To1
.
0
0
10
30
20
40
50
PROCESS TIME (HRS.) The CsB-carbon c a t a l y s t produces mainly ethylbenzene as an a l k y l a t i o n product w i t h l i t t l e o r no styrene.
I t e x h i b i t s maximum a l k y l a t i o n a c t i v i t y a t 710+10"K.
I t s l e s s e r i n i t i a l a c t i v i t y w i t h respect t o CsNaX z e o l i t e i s probably due t o lower Cs l o a d i n g i n the carbon support. We found t h a t over CsNaX o r CsB-carbon, styrene i s r e a d i l y hydrogenated by methanol t o ethylbenzene. amounts o f e t h y l benzene.
Under the same c o n d i t i o n s H2 produced o n l y minor Apparently the hydrogenating species i n t h e t o 1 uene-
methanol r e a c t i o n i s d e r i v e d mainly from methanol and n o t from HP. Reaction Temperature The e f f e c t o f temperature on the r e a c t i o n between toluene and methanol on CsNaX z e o l i t e under constant feed composition, feed r a t e , and pressure i s shown i n Fig. 2.
S i m i l a r r e s u l t s were r e p o r t e d by Yashima, e t a l . ( 4 ) f o r KNaX and
RbNaX i n t h e a l k y l a t i o n o f toluene w i t h formaldehyde and methanol, r e s p e c t i v e l y . Likewise, I t o h , e t a l . ( 8 ) found 693°K t o be t h e most s u i t a b l e r e a c t i o n temperat u r e f o r the a l k y l a t i o n o f xylenes w i t h methanol on RbLiNaX z e o l i t e , i n accordance w i t h the a l k y l a t i o n o f toluene w i t h methanol.
The s i d e chain a l k y l a t i o n
o f toluene by ethylene, ethylbenzene by methanol and xylenes w i t h methanol, on RbNaX was s t u d i e d by Yashima, e t a l . ( 4 ) a t 693"K, presumably because i t was t h e optimum temperature.
I t i s s i g n i f i c a n t t h a t the temperatures f o r the onset o f
a l k y l a t i o n and t h e c o n d i t i o n s g i v i n g maximum y i e l d are very close f o r t h e K, Rb, and Cs forms o f the z e o l i t e s and f o r d i f f e r e n t r e a c t a n t s and products.
Likewise
i t i s i n t e r e s t i n g t h a t CsNaX and CsB-carbon g i v e maximum y i e l d a t s i m i l a r
temperatures.
Apparently t h e r e i s a comnon r a t e - l i m i t i n g step i n the a l k y l a t i o n
r e a c t i o n s t a k i n g p l a c e on these c a t a l y s t s .
We suggest t h e a c t i v a t i o n o f t h e
70
alkyl group in the alkylaromatics could be t h e r a t e - l i m i t i n g s t e p . Fig. 2.
50
6 -
Effect of reaction temper-
a t u r e on toluene alkylation with 40 methanol on CsNaX. Conditions: 5 tol/MeOH, - 30 >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.
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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.
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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.
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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
bond, then
the reaction can occui in a concerted mode through an E2-like intermediate. This latter situation obviously applies t o the low-temperature dehydration of alcohols, , while at increasing reaction temperatures the t i m e required t o break the C becomes much shorter than t h a t required to break the C p
-0 bond
-H bond, and consequently
an E l - l i k e intermediate can be formed. In conclusion, even though
protonic acidity
may not be detectable, catalytic reactions may occur via proton-induced mechanisms provided that the relevant time constants adopt the appropriate relative values.
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
D.P. Burke, Chemical Week, 42 (1979). K.H. Schmidt, Chem. Ind. X X X V (1983) 69. B.C. Lippens and J.H. de Boer, Acta Cryst., 1 7 (1964) 1312 B.C. Lippens and J-J. Steggerda, in B.G. Linsen (Ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, London, New ‘fork, 1970, 171 pp. A.J. Leonard, P.N. Semaille, and J.J. Fripiat, Proc. Br. Ceram. Soc. 103 (1969). C.S. John, N.C.M. Alma, and G.R. Hays, Appl. Catal., 6 (1983) 341. J.B. Peri, J. Phys. Chem., 69 (1965) 220. J.E. Dabrowski, J.B. B u t t , and H. Bliss, J. Catal., 1 8 (1970) 297. J.B. B u t t and L.T. Starzec, J. Catal., 32 (1974) 99. S. Soled, J. Catal., 81 (1983) 252. H.P. Boehm, Adv. Catal., 16 (1966) 179. H. Knozinger and P. Ratnasamy, Catal. Rev.-Sci. Eng., 17 (1978) 31. R.L. Burwell, Jr., J. Catal., 86 (1984) 301. H. Knozinger, Adv. Catal., 25 (1976) 184. K. Tanabe, Solid Acids and Bases, Kodansha, Tokyo, Academic Press, New York, London, 1970. H.P. Boehm and H. Knozinger, in J. R. Anderson and M. Boudart (Eds.), Catalysis - Science and Technology, Springer, Berlin, Heidelberg, New York, 1983, Vol. 4, 39pp. M. Deeba and W.K. Hall, J. Catal., submitted. H. Knozinger and H. Stolz, Ber. Bunsenges. Phys. Chem., 75 (1971) 1055. V.B. Kazansky, V.Yu. Borovkov, and L.M. Kustov, Proc. 8th Intern. Congr. Catal., W.-Berlin, 1984, Vol. I l l , 3 pp. H. Knozinger and C.-P. Kaerlein, J. Catal., 25 (1972) 436. H. Pines, J. Catal., 78 (1982) 1. N.E. Tretyakov and V.N. Filimonov. Kinet. Katal., 1 3 (1972) 815. H. Knozinger, in P. Schuster, G. Zundel, and C. Sandorfy (Eds.), The Hydrogen Bond, North-Holland, Amsterdam, New York, Oxford, 1976; Vol. 111, 1263 pp. P.O. Scokart and P.G. Rouxhet, J. Chem. Soc. Faraday I , 76 (1980) 1476. E.A. Paukshits, N.S. Kotsarenko, and L.G. Karakchiev, React. Kinet. Catal. L e t t . , 12 (1979) 315. E.A. Paukshits, P.I. Soltanov, E.N. Yurchenko, and K. JirBtov-5, Collect. Czech. Chem. Commun., 47 (1982) 2044. H. Knozinger, H. Kietenbrink, H.-D. Muller, and W. Schulz, Proc. 6th Intern. Congr. Catal., London, 1976, Vol. 1 (1977) 183. G.J.K. Acres, A.J. Bird, J.W. Jenkins, and F. King, Catalysis, Vol. 4 (1981) 1 pp.
124 29 30 31 32 33
G.D. P a r f i t t , Pure Appl. Chem., 48 (1976) 415. G.A. Parks, Chem. Rev., 65 (1965) 177. L i Wang and W.K. H a l l , J. Catal., 77 (1982) 232. J. Aveston, E.W. Anacker, and J.S. Johnston, Inorg. Chem., 3 (1964) 735. W.S. M i l l m a n , M. Crespin, A.C. Cirillo,Jr., S. Abdo, and W.K. Hall, J. Catal., 60 (1 979) 404. 34 W.K. H a l l , Proc. Climax 4th Intern. Conf. on "Chemistry and Uses o f Molybdenum" (H.F. B a r r y and P.C.H. M i t c h e l l , Eds.). Climax Molybdenum Company, Ann A r b o r , Michigan, 1982, 224 pp. 35 H. Jeziorowski, H. Knozinger, E. Taglauer, and C. Vogdt, J. Catal., 80 (1983) 286. 36 J. C. Summers and S.A. Ausen, J. Catal., 52 (1978) 445. 37 M.J. D ' A g n e l l o , J. Catal., 69 (1981) 9. 38 D.C. Kennedy, Chern. Eng., 87 (1980) 106. 39 Yen-Shin Shyr and W.R. Ernst, J. Catal., 63 (1980) 425. 40 Yu. I. Yerrnakov, Catal. Rev.- Sci. Eng., 13 (1976) 77. 41 Yu.1. Yertnakov, Proc. 7th Intern. Congr. Catal., Tokyo, 1980, Kodansha, Tokyo, Elsevier Publ. Comp., Amsterdam, Oxford, New York, 1981, Part A , 57 pp. 42 Yu.1. Yermakov, B.N. Kuznetsov, and V.A. Zakharov, Catalysis by Supported Cornplexes, Elsevier Publ. Comp., Amsterdam, Oxford, New Y o r k , 1981. 43 Y. Iwasawa, H. Ichinose, and S. Ogasawara, J. Chern. SOC. Faraday I , 77 (1981) 1763. 44 H.E. Evans and W.H. Weinberg, J. Am. Chem. SOC., 102 (1980) 2548. 45 M. Laniecki and R.L. Burwell, Jr., J. Colloid Interface Sci., 75 (1980) 95. 46 A.K. Smith, F. Hugues, A. Theolier, J.M. Basset, R. Ugo, G.M. Zanderighi, J.L. Bilhou, V. Bougnol, and W.F. Graydon, Inorg. Chem., 1 8 (1979) 3104. 47 A.K. Smith, B. Besson, J.M. Basset, R. Psaro, A. Fusi, R. Ugo, J. Organomet. Chem., 192 (1980) C31. 48 M. Deeba and B.C. Gates, J. Catal. 67 (1981) 303. 49 H. Knozinger and Y. Zhao, J. Catal., 71 (1981) 337. 50 B.C. Gates, L. Guczi, and H. Knozinger (Eds.), M e t a l Clusters in Catalysis, Elsevier Publ. Comp., in preparation. 51 F. Hugues, J.M. Basset, Y. Ben Taarit, A. Choplin, M. Prirnet, D. Rojas, and A.K. Smith, J. Am. Chem. SOC., 104 (1982) 7020. 52 C. Tessier-Youngs, F. Correa, D. Pioch, R.L. Burwell, Jr., and D.F. Shriver, Organornetallics, 2 (1983) 898. 53 E.O. Odebunmi, Y. Zhao, H. Knozinger, B. Tesche, W.H. Manogue, B.C. Gates, and J. Hulse, J. Catal., 86 (1984) 95. 54 F.H. van Cauwelaert and W.K. Hall, Trans. Faraday SOC., 66 (1970) 454. 55 H. Pines and J. Ravoire, J. Phys. Chern., 65 (1961) 1859. 56 I. Yasumorl and S. Ohno, Bull. Chem. SOC. Japan, 39 (1966) 1302; ibid., 41 (1968) 2227. 57 Y. Amenomiya, J. Catal., 22 (1971) 109. 58 J.W. Hightower and W.K. Hall, Trans. Faraday SOC., 66 (1970) 477. 59 M.P.Rosynek and J.W. Hightower, Proc. 5 t h Intern. Congr. Catal., Palm Beach, 1972, Vol. 2 (1973) 851. 60 Y. Sakural, T. Onishi, and K , Tamaru, Bull. Chern. SOC. Japan, 45 (1972) 980. 61 T. Kondo and K. Tamaru, in E. Drauglis and R.I. Jaffe (Eds.), The Physical Basis for Heterogeneous Catalysis, Plenum Press, New Y o r k , 1975, 525pp. 62 B.T. Hughes, C. Kernball, and J.K. Tyler, J. Chem. SOC. Faraday I, 71 (1971) 1285. 63 H. Knozinger, A. Corado, Gy. GBti, H. H i e r s t e t t e r , A. Kiss, R. Letterer, and H.D. Muller, Proc. Syrnp. Mechanisms o f Hydrocarbon Reactions, Sibfok, Hungary, 1973, 333PP. 64 J.B. Peri, J. Phys. Chern., 70 (1966) 3168. 65 J. Hightower and W.K. Hall, J. Catal., 13 (1969) 161. 66 H.R. Gerberich and W.K. Hall, J. Catal. 5 (1966) 99. 67 D.S. Mclver, W.H. Wilmot, and J.M. Bridges, J. Catal., 3 (1964) 502. 68 A. Ghorbel, C. Hoang-Van, and S.J. Teichner, J. Catal., 33 (1974) 123. 69 J. Medema, J. Catal., 37 (1975) 91. 70 A. Corado, A. Kiss, H. Knozinger, and H.D. Muller, J. Catal., 37 (1975) 68. 71 Gy. GBti and H. Knijzinger, Proc. 5 t h Intern. Congr. Catal., Palm Beach, 1972, Vol. 1 (1973) 819.
125 72
C.S. John, C.E. Marsden, and R. Dickinson, J. Chem. SOC. Faraday I , 72 (1976)
2923. 73 74 75 76
W.K. Hall, F.E. Lutinski, and H.R. Gerberich, J. Catal., 3 (1964) 512. H. Knozinger and B. Stubner, J. Phys. Chem., 8 2 (1978) 1526. H. Pines and J. Manassen, Adv. Catal., 16 (1966) 49. H. Knozinger, Angew. Chem., 80 (1968) 778; Angew. Chern. Int. Ed. Engl., 7
(1968) 791. 77 78 79 80 81 82 83 84 85 86
H. Knozinger, in S. Patai (Ed.), The Chemistry o f t h e Hydroxyl Group, WileyInterscience Publ., London, 1971, 641pp. H.G. Karge, R.W. Tower, Z. Dudzik, and Z.M. George, Proc. 7 t h Intern. Congr. Catal., Tokyo, 1980, Vol. 1 (1981) 643. H.G. Karge, I.G. D a l l a Lana, S. Trevizan de Suarez, and Yingzhen Zhang, Proc. 8 t h Intern. Congr. Catal., West-Berlin, 1984, Vol. I l l , 453pp. M. A k i m o t o and I.G. Dalla Lana, J. Catal., 62 (1980) 84. R. Fiedorow, R. LBautB, and I.G. Dalla Lana, J. Catal., 8 5 (1984) 339. J.E. Gerrnain, L. Bassery, and M. Blanchard, Bull. Chem. SOC. Fr., (1958) 958. H. Pines and W. Haag, J. Am. Chern. SOC., 1 2 (1964) 2471. Gy. G’ati and H. Knozinger, Z. Phys. Chern. (Frankfurt), 78 (1972) 243. E. Baurngarten, D. Hollenberg, and H. Hoffkes, J. Catal., 80 (1983) 419. S.C. John and M.S. Scurrell, Catalysis, The Chemical Society, London, 1977, Vol.1,
136pp. 87
G.H. Posner, Angew. Chern., 90 (1978) 527 ; Angew. Chem. Int. Ed. Engl., 1 7 (1978) 487. 88 H. Knozinger, H. BGhl, and K. Kochloefl, J. Catal. 24 (1972) 57. 89 J.J. Fripiat, Catal. Rev., 5 (1971) 269.
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B. Imelik et al. (Editors), Catalysis by Acids and Bases B.V.,Amsterdam -Printed in The Netherlands
127
B 1985 Elsevier Science Publishers
REACTIVITY OF ISOPROPANOL ON K- AND CS-EXCHANGED ZSM-5 AND MORDENITE
J . B.NAGY, J.4.LANGEl, A. GOURGUE',
P . BODART and Z. GABELICA
Facul t e s U n i v e r s i t a i r e s de Namur, L a b o r a t o i r e de Catalyse, Rue de B r u x e l l e s , 61,
8-5000
Namur, Belgium
Present address : F r i t z - H a b e r - I n s t i t u t der Max-Planck-Gesell s c h a f t , Faradayweg, 4-6, D-1000
B e r l i n 33/Dahlem, F.R.G.
Present address : Colgate R & D I n c . , Avenue du Parc I n d u s t r i e l , B-4411
M i l m o r t , Belgium.
ABSTRACT The e f f e c t o f a c i d i t y and channel s i z e s o f K- and Cs-exchanged ZSM-5 and mord e n i t e on t h e r e a c t i v i t y o f isopropanol i s t h o r o u g h l y i n v e s t i g a t e d by 13C-NMR i n t h e adsorbed phase. The c a t a l y s t s are c h a r a c t e r i z e d by X - r a y z $ i f f r a h f i o n t e c h n i Si-, A l - and ue, and by magic-angle-spinning h i g h r e s o l u t i o n s o l i d s t a t e j%-NMR, as w e l l as by p r o t o n induced y-ray emission (PIGE) and EDX methods. TG a n a l y s i s o f adsorbed n-pentane leads t o t h e d e t e r m i n a t i o n o f pore volume. For t h e ZSM-5 z e o l i t e s c h a r a c t e r i z e d by narrower channels w i t h r e s p e c t t o t h e corresponding mordenites, t h e s i z e o f t h e c a t i o n has a d e f i n i t e i n f l u e n c e on t h e react i o n products. A t ZOO'C, t h e a c i d c a t a l y z e d dehydration o f isopropanol i s essent i a l l y observed, l e a d i n g t o propylene. A t 3OO0C, t h e h i g h e r b a s i c i t i y o f CsZSM-5 favours t h e f o r m a t i o n o f acetone, t h e dehydrogenation p r o d u c t o f i s o p r o panol
.
RESUME
L ' e t u d e p a r RMN-13C de l a r e a c t i v i t e de 1 'isopropanol adsorbe dans l e s 260l i t h e s ZSM-5 e t mordenite echangeesau K e t au Cs permet de m e t t r e en evidence l ' i n f l u e n c e de l ' a c i d i t e e t de l a dimensiondes canaux. Les t a l y u r s SO carac, '?A1 e t "C, e t t e r i s e s p a r DRX, RMN haute r e s o l u t i o n i l ' e t a t s o l i d e du %i p a r l e s methodes d ' a n a l y s e P I G E e t EDX. L ' a n a l y s e thermique du pentane-n adsorbe renseigne s u r l e volume poreux. Dans l e cas de l a z e o l i t h e ZSM-5 ayant des canaux p l u s e t r o i t s que l a mordenite, l a dimension des c a t i o n s a une i n f l u e n c e c e r t a i n e s u r l a d i s t r i b u t i o n des p r o d u i t s de r e a c t i o n . A 200°C, c ' e s t l a react i o n de d e s h y d r a t a t i o n de l ' i s o p r o p a n o l en propylene q u i e s t observee. A 300°C, l a b a s i c i t @ p l u s @levee de l a Cs-ZSM-5 f a v o r i s e l a f o r m a t i o n de l ' a c e t o n e , prod u i t de deshydrogenation de 1 ' i s o p r o p a n o l . INTRODUCTION While t h e a r i d c a t a l y s i s by z e o l i t e s has been thoroughly i n v e s t i g a t e d l ) , the r o l e o f b a s i c s i t e s i s o n l y p o o r l y understood ( r e f . 2 ) . The t r a n s -
(ref.
formation o f isopropanol i s an e x c e l l e n t c a t a l y t i c probe t o determine t h e r e l a t i v e importance o f c a t a l y s i s by a c i d o r b a s i c s i t e s .
Indeed, t h e a c i d c a t a l y s i s
leads e s s e n t i a l l y t o t h e dehydration product propylene ( r e f . 3 ) , w h i l e acetone
128
i s obtained by dehydrogenation of isopropanol on basic s i t e s ( r e f . 4 ) . In addition, the s i z e of the z e o l i t i c channels can introduce a modification i n the s e l e c t i v i t y of this reaction. For example, small s i z e channels can impede the formation of larger molecules such as ethers ( r e f . 5 ) . On the other h a n d , the conventional h i g h resolution "C-NMR study of the adsorbed molecules revealed to be a powerful tool to monitor the reactions occurring i n situ, in the adsorbed phase ( r e f . 6 ) . The aim of this paper i s t o follow the behaviour as well as the reactivity of 2-13C-isopropanol by I3C-NMR i n a batch-reactor ( r e f . 7 ) . The shape-selectivity of ZSM-5 and mordenite will also be examinedthrough the crac-kirq of the polymer products. Finally, the strongly adsorbed species will be analyzed by high-resolution magic-angle-spinning solid s t a t e 13C-NMR. EXPERIMENTAL The ZSM-5 sample was synthesized following a procedure described in the l i t e rature ( r e f . 8). The TPA-containing precursor was calcined a t 550°C f o r 9 h under nitrogen, followed by an a i r calcination during 14 h a t the same temperature. Thk so-obtained Na-form was acidified three times a t 50°C with portions of 10 ml of HCI 0.5 N per gram of zeolite. The K and Cs-forms were f i n a l l y obtained by exchange w i t h e i t h e r aqueous KN03 0.5 N or CsN03 0.5 N. The synthesis of the mordenite samples was described elsewhere ( r e f . 9 ) . The Na-form was exchanged three times a t 60°C w i t h portions of 10 ml of NH4N03 0.5 N per gram of mordenite. The NH4-form was dealuminated by HC1 a t 80°C. 15 ml of HC1 per gram of zeolite was used with increasing concentrations : 1 N f o r 1 h , 2' N f o r 2 h , 4 N f o r 2 h and f i n a l l y 6 N f o r 16 h . The dealumination of morden i t e was completed by a steaming treatment a t 600°C f o r 10 h . Finally, the dealuminated samples were exchanged four times a t 80°C with portions of 20 ml KN03 o r CsN03 0.5 N per gram of zeolite. The isopropanol used was 13C-labelled a t C-2 (90,O % 1 3 C , Merck). A l l the precursor and f i n a l crystalline products were identified using X-ray powder diffraction patterns (XRD) , recorded on a Philips PW-1349/30 diffractometer(Cu-Ka radiation). Crystal sizes and morphologies were determined by scanning electron microscopy (SEM) using a Jeol JSM 35 instrument. Their S i , A l , Na, K and Cs contents were measured by proton induced y-ray emission (PIGE) and energy dispersive X-ray analysis ( E D X ) ( r e f . 10 and 11). The water content and the amount of adsorbed pentane and diisopropylether were determined by thermal analysis (TG-DTA-DTG) on a Stanton Redcroft STA-780 thermoanalyzer ( r e f . 1 2 ) . Conventional h i g h resolution "C-NMR spectra were recorded on a Bruker CXP200 spectrometer using ca n/6 pulse length under proton broad band decoupling. Solid s t a t e NMR spectra were obtained a t room temperature. The h i g h resolution magic-anqle-spinning cross-polarization 13C-NMR spectra were recorded using
129
The 13C (50.3 MHz) and 'H (200.0 MHz) r f - f i e l d s were 39.0 G and 9.8 G r e s p e c t i v e l y s a t i s f i y n g t h e Hartmann-Hahn condition. A contact time o f 5.0 ms and a r e c y c l e time o f 4.0 s were used. The polymethylmethacryl a t e r o t o r was spun a t 3.1 kHz a t the magic angle. About one thousand scans a s i n g l e contact sequence.
were accumulated p r i o r t o F o u r i e r transformation. spectra were recorded a t 39.7 MHz i n s i m i l a r conditions b u t
The "Si-NMR
w i t h o u t cross-polarization. The 27A1-NMR spectra were recorded a t 52.1 MHz using time i n t e r v a l s o f 0.1 s and ca 5000 f r e e i n d u c t i o n decays were accumulated per sample. A l l the c a t a l y s t s (ca 0.6 g) were progressively dehydrated and a c t i v a t e d f o r 2 h a t 400°C and a t a f i n a l pressure o f 10-4Torr. 50 M I o f 2-13C-labelled i s o propanol were then adsorbed a t room temperature.
The k i n e t i c measurements were
performed by successive heating cycles a t the r e a c t i o n temperature, t h e 13C-NMR spectra being recorded a t room temperature where t h e r e a c t i o n was quenched. RESULTS AND DISCUSSION The physico-chemical c h a r a c t e r i s t i c s o f the f i n a l K- and Cs-exchanged ZSM-5 and mordenite samples are shown i n Table 1. The mordenite samples show s t i c k cross-section. The ZSM-5 l i k e c r y s t a l l i t e s o f ca 10-30 urn length and ca 5 c r y s t a l l i t e s are smaller (1-3 vm) and have a spheroidal shape.
The c r y s t a l mor-
phology and s i z e a l s o remain constant during the preparation o f t h e f i n a l samp l e s . The d i f f e r e n t c a t i o n exchanges and/or the dealumination do n o t change the c h a r a c t e r i s t i c s o f the c r y s t a l s t r u c t u r e , as checked by XRD f o r each sample. The Si/A1 r a t i o s o f mordenites determined by PXGE and EDX methods are much lower than those measured by "Si-NMR.
This l a t t e r technique y i e l d s e s s e n t i a l l y
the Si/A1 r a t i o o f the framework ( r e f . 1 3 ) .
I t i s thus suggested t h a t the t e t r a -
hedral aluminium atoms leave the framework s i t e s t o form an e x t r a z e o l i t i c amorphous phase.
Indeed, the "Si-NMR
spectrum does n o t a l l o w t o d e t e c t any frame-
work t e t r a h e d r a l aluminium w i t h i n the experimental u n c e r t a i n t y (ca. 1 a t . % A l . w i t h respect t o t o t a l (Si
+ A l ) ) , w h i l e the
27Al-NMR spectrum reveals the pre-
sence o f an amorphous aluminic phase (very broad band o f = 7-9 kHz l i n e w i d t h ) . The e v o l u t i o n of the "Si-NMR
spectrum during the dealumination shows the subse-
quent rearrangement o f the mordeni t e s t r u c t u r e . The sodium content i s q u i t e low i n the K-and Cs-mordenite samples (Al/Na %
70-80) (Table l ) , as compared t o t h e i r r e l a t i v e l y high amount o f e i t h e r K o r
Cs, confirming the effectiveness o f the i o n i c exchanges.
The Si/A1 r a t i o o f ZSM-5 z e o l i t e s does n o t change s i g n i f i c a n t l y d u r i n g t h e exchange processes.
However, an aluminium g r a d i e n t i s detected i n t h e f i n a l sam-
ples by the complementary use of P I G E and EDX methods. Indeed the former techniques probes the c r y s t a l l i t e s down t o ca 10 vm depth, w h i l e the l a t t e r analyzes o n l y a depth o f ca 1-2 pin.
A h i g h e r aluminium concentration i s thus detected i n
130
the o u t e r s h e l l o f the c r y s t a l l i t e s ( r e f . 8 ) . 6
Q ,
I n a d d i t i o n , t h e 27A1-NMR l i n e a t
confirms t h e presence o f framework o f t e t r a h e d r a l A1 &toms
55 ppm vs A1 (H20)3t,
The A l / M r a t i o o f 1.2 i s c h a r a c t e r i s t i c of a r a t h e r w e l l exchanged z e o l i t e , t h e r e m a i n i n g sodium c o n t e n t b e i n g low (Al/Na = 17-22).
The w a t e r c o n t e n t i s h i g h e r
f o r t h e K-ZSM-5 sample. The n-pentane a d s o r p t i o n y i e l d s t h e expected channel l e n g t h o f
%
The
8 nm.
d i i s o p r o p y l e t h e r cannot f i l l c o m p l e t e l y t h e a v a i l a b l e l e n g t h , t h e s t e r i c h i n drance b e i n g l a r g e r when l a r g e r Cs' i o n s a r e { r e s e n t a t t h e channel i n t e r s e c t i o n s . TABLE 1 Physi co-chemi c a l c h a r a c t e r i z a t i o n o f K and Cs-exchanged ZSM-5 and mordeni t e Characteristics
Method
K-MOR
CS-MOR
K-ZSM-5
Si/A1
PIGE~ EDX~ 29Si-NMR
8.5 6.6 100
8.1 6.8 100
48 29 (30)
EDX PIGE
8.6
70
8.2 80
1.2 16.8
Amorphous
Amorphous
Yes
Yes
phase
phase
N0
N0
8.2 8.1 6.2
2 8.0 5.2
8
8
A ~ / M ~ Al/Na presence o f -A$
27A1-NMR
-A15 Water c o n t e n t (wt%) Pentane ads. (nm)f Diisoprop 1 ether ads/ (nm)
+
T G ~
13C-NMR
Edif ( i s 0 r0p a n o f j ( k J .moP-1)
-
-
3.5
3.5
CS-ZSM-5 44 28 (30lC 1.2 22.0
:Proton i n d u c e d y - r a y e m i s s i o n (PIGE). Energy d i s p e r s i v e X-ray a n a l y s i s (EDX) , 'These values a r e u n c e r t a i n due t o t h e p r o b a b l e o v e r l a p p i n g o f t h e Si(nA1) NMR l i n e s b y those b e l o n g i n g t o t h e s i l a n o l groups, dM stands f o r K o r Cs. e A l and A l o : r e s p e c t i v e l y t e t r a h e d r a l l y and o c t a h e d r a l l y c o o r d i n a t e d A1 atoms fChlfnnel l e n g t h i n nm/unit c e l l , based on a model i n v o l v i n g an end-to-end f i l l i n g o f t h e pore system. gTG : thermogravi m e t r y . h A c t i v a t i o n energy f o r d i f f u s i o n , computed f r o m t h e v a r i a t i o n o f I n ( l i n e w i d t h ) as a f u n c t i o n o f 1/T. F i n a l l y , t h e a c t i v a t i o n energy f o r t h e d i f f u s i o n o f t h e i s o p r o p a n o l molecul e computed f r o m t h e 13C-NMR l i n e w i d t h v a r i a t i o n w i t h t e m p e r a t u r e i n ZSM-5 i s about t w i c e as h i g h as t h a t determined i n mordenite : t h i s d i f f e r e n c e i s p r o b a b l y due t o t h e s m a l l e r channel s i z e and t h e r e l a t i v e l y h i g h e r aluminium c o n t e n t o f ZSM-5. F i g . 1 and 2 i l l u s t r a t e t h e c o n v e r s i o n a t 200°C o f 2-13C-isopropanol bed on K-ZSM-5,
Cs-ZSM-5,
K-MOR and Cs-MOR r e s p e c t i v e l y .
adsor-
While t h e d e h y d r a t i o n
131
product observed on a l l c a t a l y s t s a t 200°C i s e s s e n t i a l l y propylene, t h e presence of acetone i s found a t 3 O O O C i n each case. A t t h i s l a t t e r temperature, 30 mol. % acetone was detected a t 50 % conversion on Cs-ZSM-5 z e o l i t e , while onl y 10 % acetone was obtained on K-ZSM-5 a t t h e same conversion.
(CS) ZSM-5
(K) ZSM-5 I
1
I
1
I
I
I
,
(K) MOR
(Cs) MOR
l
-0 H
0 nin
0 min
10 nil
I
150
I
I
I
0
L
2I!!?!
h, 30 i i n
150
0
150
0
150
0
-Fig. 1. Evolution of 13C-NMR spectra of 2- 13C-isopropanol and of i t s reaction products i n the adsorbed s t a t e on K-ZSM-5, Cs-ZSM-5, K-MOR and Cs-MOR(Reaction temperature : 200°C ; measuring temperature : 25OC). These r e s u l t s suggest t h a t the acetone formation i s favoured on the more basic z e o l i t e Cs-ZSM-5 a n d a t high temperature (300°C), t h i s being due t o the higher energy of a c t i v a t i o n f o r the dehydrogenation r e a c t i o n . The mordeni t e c a t a l y s t s show i n b o t h cases a higher a c t i vi ty than the corresponding ZSM-5 z e o l i t e s . The subsequent polymerization of propene t o form p a r a f f i n i c compounds i s q u i t e s i g n i f i c a n t on K-ZSM-5, K-MOR a n d Cs-MOR z e o l i t e s , while the Cs-ZSM-5 y i e l ds only about 1 0 %of paraffins a f t e r 9 0 % conversion of isopropanol . A c e r t a i n deactivation of the c a t a l y s t s occurs, due t o probable pore blocking by polymers and/or coke formation. On K-ZSM-5, o l e f i n s d i f f e r e n t from propylene are a l s o detected. I t i s important t o note t h a t no diisopropylether has been detected on any of the catalysts.
132 A f t e r 90-100 %
c o n v e r s i o n , t h e p o l y m e r i z e d p r o d u c t s a r e cracked a t 370'C.
A f t e r c r a c k i n g , t h e I3C-NMR l i n e s become narrower, showing t h e presence o f compounds o f l o w e r m o l e c u l a r w e i g h t .
The s t r o n g l y adsorbed s p e c i e s e n t r a p p e d i n
t h e channels, as a r e s u l t o f p o r e b l o e k i n g , a r e f i n a l l y analyzed by h i g h r e s o l u t i o n magic-angle s p i n n i n g s o l i d s t a t e 13C-NM?
( r e f . 14), (Fig.3). The Cs-ZSM-5
e s s e n t i a l l y y i e l d s propene, c i s - and trans-2-butenes,
and cis-2-pentene,
toge-
t h e r w i t h some a r o m a t i c compounds (35 mol % ) , and butane (65 % ) . The K-ZSM-5 o n l y y i e l d s c i s - and trans-2-butenes
as o l e f i n s and t o l u e n e and ethylbenzene as
a r o m a t i c s ( 1 5 % ) . The d i s t r i b u t i o n o f p a r a f f i n i c compounds i s l a r g e r on t h e KZSM-5 z e o l i t e : butane, i s o b u t a n e , pentane and i s o p e n t a n e ( 8 5 % ) . I n a d d i t i o n , i s o p r o p o x y groups l i n k e d t o t h e s u r f a c e a r e d e t e c t e d a t ca 73 ppm. The Cs-ZSM-5 y i e l d s a s m a l l e r amount o f a r o m a t i c compounds w i t h r e s p e c t t o o l e f i n s .
loo%?
I
I
I
I
100%
1
A
2-propanol
I \
I 50
2-propanol olefins (tpropene) propene
1 E 0100%
p
30 I
60
I
90
I
120
180
150
'""1
I
20
I
0 30
100%
I
I
I
2-propanol
20
40
60
I
I
I
I
15
30
45
Fig. 2. Conversion o f 2-13C-isopropanol i n the adsorbed s t a t e on K-ZSM-5, Cs-ZSM-5, K-MOR and Cs-MOR a t 200°C i n a b a t c h - r e a c t o r .
I0 60
133 The p r o d u c t d i s t r i b u t i o n on b o t h
K- and Cs-MOR a r e q u i t e s i m i l a r : butane and
i s o p e n t a n e ( 7 5 % ) ; c i s - and trans-2-butenes
and a r o m a t i c s ( 2 0 % ) and e s t e r
groups ( 5 % ) .
I
I
I
I
I
I
EtO 1
B2cis
I 100
I
200
I
0 c 6(ppm)
F i g . 3. High r e s o l u t i o n m a g i c - a n g l e - s p i n n i n g s o l i d s t a t e 13C-NMR s p e c t r a o f t h e cracked p r o d u c t s o f t h e d e h y d r a t i o n - p o l y m e r i z a t i o n p r o d u c t s of 2-13Cisopropanol
.
As a c o n c l u s i o n , t h e s i z e of t h e c a t i o n s has a d a f i n i t e i n f l u e n c e on t h e r e a c t i o n p r o d u c t s for t h e ZSM-5 z e o l i t e s c h a r a c t e r i z e d by narrower channels, w i t h respect t o the corresponding mordenites.
I n a d d i t i o n , t h e Cs-ZSM-5 shows
a h i g h e r b a s i c i t y , l e a d i n g t o t h e dehydrogenation p r o d u c t o f i s o p r o p a n o l a t 3OOOC.
134
ACKNOWLEDGMENT
The authors wish t o acknowledge Mr. G . Daelen f o r h i s help i n taking the A . Gourgue and P. Bodart thank IRSIA (IWONL) f o r financial
NMR s p e c t r a .
support. REFERENCES 1 See e.g. P.A. Jacobs, Carboniogenic a c t i v i t y of z e o l i t e s , Elsevier, Amsterdam, 1977, 253 pp. 2 Y . Ono, Stud. Surf. Sci. C a t a l . , 5 C1980) 19-27. 3 K . Kochloefl and H . KnEzinger, i n J.W. Hightower (Ed.), Proc. 5th I n t . Congr. Catal ., Miami Beach, Florida, 1972, North-Holland Amsterdam, 1973, p p . 1171-1179. 4 S . Siddhan and K. Narayanan, J . Catal , 59 (1979) 405-422. 5 P . B . Weisz, W.J. F r i l e t t e , R.W. Maatman and E . B . Mower, J . C a t a l . , 1 (1962) 307- 312. 6 E.G. Derouane and J . B.Nagy, ACS Symposium S e r i e s , No 248, C a t a l y t i c Materials : Relationship Between Structure and Reactivity, T . E . Whyte, R . A . Dalla Batta, E . G . Derouane and R . T . K . Baker, ( E d s . ) , 1984, p p . 101-126. 7 J.-P. Lange, Memoire de Licence, FNDP, Namur, 1983, 66 p p . 8 E . G . Derouane. S . Detremmerie, Z . Gabelica and N . Blom, A.p.p l . C a t a l . , 1 (1981) 201-224. 9 P. Bodart, J . B.Nagy, E . G . Derouane, Z . Gabelica, A . Gourgue and S. Maroie, Appl Catal , i n press. 10 G.Debras, E.G. Derouane, 3.-P. Gilson, Z . Gabelica and G . Demortier, Z e o l i t e s , 3 (1Y83) 37-49. 11 Z. Gabelica, N . Blom and E . G . Derouane, A p p l . C a t a l . , 5 (1983) 227-248. 12 Z. Gabelica, J . B.Nagy, E . G . Derouane and J.-P. Gilson, Clay Minerals,
.
.
.
i n press. 13 J . B.Nagy, Z. Gabelica, G. Debras, P . Bodart and E.G. Derouane, J . Molec Catal , 20 (1983) 327-336. 14 E . G . Derouane, J.-P. Gilson and J . B.Nagy, Z e o l i t e s , 2 (1982) 42-46.
.
135
B. Imelik et 41. (Editors), Catalysis b y Acids and Bases 0 1985 Elsevier Science Publishers B.V..Amsterdam -Printed in The Netherlands
QUANTITATION AND MODIFICATION OF CATALYTIC SITES IN ZSM-5 E . G . Derouanel, L. Baltusis2, R.M. Dessau and K.D. Schmitt
Mobil Research and Development Corporation, Central Research Division, P.O. Box 1025, Princeton, New Jersey 08540 (USA) 1Present address : Laboratoire de Catalyse, Department de Chimie, Facultes Universitaires de Namur, Rue de Bruxelles, 61, B-5000 Namur (Belgium) 2Present address: Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (USA)
ABSTRACT Elemental analysis, temperature-programmed desorption of ammonia (TPDA), and 27Al magic angle sample spinning (MAS) NMR were combined to quantitate the amount of framework tetrahedral aluminum in ZSM-5 zeolites, either pretreated in controlled conditions to avoid alteration of structural aluminum or modified by exposure to boron trichloride to intentionally vary the aluminum content. The cracking of n-hexane at 811 K (alpha test) was used to evaluate the relative acid catalytic activity of these materials. In both cases, activity is governed by the framework tetrahedral aluminum content. Some of the critical variables for 2781 MAS-NMR quantitation are delineated. IIB MAS-NMR was used as a complementary technique to confirm the substitution of aluminum by boron in the zeolite framework. The ion-exchange capacity stays nearly constant as a result of such an atom interchange. A mechanism is proposed. INTRODUCTION There exists ample evidence that the carboniogenic catalytic activity of zeolites depends to a large extent on their framework Si02/A1203 ratio.
The
case of highly siliceous zeolites, such as ZSM-5 and ZSM-11, is particularly demonstrative as these materials can be prepared with widely different activities which are, nevertheless, still proportional to their structural aluminum content (ref.1-4). Related to these findings which indicate that all catalytic sites in ZSM-5 have comparable acid catalytic activity, virtually independent of Si02/A1203 ratio for samples carefully pretreated in controlled conditions as to preserve the integrity of the zeolite framework, are two fundamental questions.
One
concerns the identification and measurement of the aluminum present in tetrahedral (T) sites, which is responsible for the BrBnsted acidic sites and the zeolite carboniogenic activity. We will show that the combination of elemental analysis, temperature programmed desorption of ammonia (TPDA) , and "A1
magic
angle sample spinning (MAS) NMR enables the unambiguous quantitation of such sites.
136 The other question addresses the activity o f such materials following a postsynthesis modification of their A1 content. Until recently, investigations of zeolite dealurnination and of its consequences were essentially limited to zeolite Y, mordenite, erionite, and ZSM-5 (ref.5).
Special attention has been
paid in the recent years to the use of volatile acidic halides such as SiC14, Pcl3, ~ ~ 1 COC12, 3 , etc . . . (ref.6-9).
Applications of such a procedure to ZSM-5
have been disclosed in both the patent (ref.9) and the journal literatures (ref.10).
Dealumination of ZSM-5 by BCl3 is described in the present paper.
We will propose a mechanism for the replacement of structural A1 by B and demonstrate that, here too, the acid catalytic activity is governed by the concentration of tetrahedral framework aluminum. EXPERIMENTAL Samples The ammonium forms of a variety of ZSM-5 samples prepared by a conventional synthesis method (ref.ll), with different particle sizes and broadly varying A1 contents (between 0.03 and 1.2 wt. % Al), were used to investigate the relationship existing between tetrahedral A1 observed by NMR, A1 from elemental analysis, and the number of effective A1 framework sites derived from TPDA. Assynthesized materials were carefully pretreated and ammonium-exchanged to avoid alteration of structural aluminum prior to the NMR, TPDA, and catalytic measurements.
Two ZSM-5 materials were used for the dealumination studies, both originally in the ammonium form. NH4-ZSM-5S has an average Si02/A1203 ratio of 78. The same ratio for NH4-ZSM-5L is about 102. These samples have particle sizes (S, small vs. L, large) which differ by over one order of magnitude. Na-ZSM-5S is the Na-form of corresponding NH4-ZSM-5S. BC13 Treatment The ammonium form of the zeolite (2.5 g) was first heated for 1 hr at 773 K in a dry N2 stream (100 cc min-1). the N2 stream (P = 1 atm).
BC13 was then introduced (20 cc min-l) in
After the desired reaction time, the BCl3 supply was
disconnected and the zeolite flushed with N2 for 1.5 h at 773 K.
After cooling,
it was ammonium-exchanged with 1 M NH4C1 (0.5 1; pH adjusted to about 9) at ambient temperature and under stirring for 12 h.
The final product was
thoroughly washed with distilled water and air-dried. I n a control experiment, a sample of Na-ZSM-5S was exposed to BC13 in the same conditions for 3.5 h.
Elemental Analysis Elemental analyses for the samples used in or resulting from the BC13
137 treatments are given in Table 1.
HF dissolution and carbonate fusion were used
for the A1 and B analyses, respectively. TABLE 1 Properties of BC13-treated NH4-ZSM-5 cata1ysts.a
Sampleb
Time
(h)
f NMR Analysis
Elemental Analysis
TPDA Analysis'
A 1 (wt.%)
B (wt.%)
<548 K
-
1.08 0.59 0.44 0.07
0.14 0.17 0.28
0.01 0.21 0.26 0.32
0.41 0.19
-
0.38
0.17
0.00 0.38
0.32 0.17
s(IV)
3.5
S(Na)d
3.5
0.40
0.24
L(I) L(I1)e
0.0 3.5
0.97 0.21
-
-
-
0.32
0.22
0.19
S(I) s (11)
S(II1)
0.0 0.5 1.o
>548K
B (wt.%)
A1 (wt.%) 1.17 0.56 0.45 0.13
0.26 0.21 0.27
-
0.14 0.06
a773 K; 20 cc min-l BCl3 in I00 cc min-1 N2; P = 1 atm. bS(I-IV): NH4-ZSM-5S, S(Na): Na-ZSM-5S, L(1,II): NH4-ZSM-5L. CNH3 (meq. g-1) released at a heating rate of 20 K min-1. dRelative hexane cracking activity (alpha value) smaller than 1. eRelative hexane cracking activity (alpha value) equal to 4. fAbsolute measurements within 15% for 2781 and 30% for IlB, MAS-NMR Measurements 2781 MAS-NMR spectra were obtained using a JEOL FX-200 Fourier Transform Spectrometer using 90" pulses at 160 millisecond intervals, 3.8-4.2 kHz sample spinning rate, and gated proton high power decoupling.
1lB MAS-NMR spectra
were recorded on a modified 150 MHz Nicolet Fourier Transform spectrometer using a 5 microsecond pulse length and a 2 second recycle time, with a typical sample spinning rate of 4.2 kHz. Proton decoupling was not used in obtaining the 1lB NMR spectra. Temperature Programmed Desorption of Ammonia (TPDA) TPDA studies were performed using a DuPont 951 Thermogravimetric Analyzer coupled to a Metrohm Automatic Titrator as described elsewhere (ref.12). Standard heating rates were 20 K min-1.
TPDA spectra were also obtained at
different heating rates (1-30 K min-1) to estimate the desorption energies characteristic of adsorbed ammonia (ref.13). Catalytic Activity The acid catalytic activity was measured by the alpha test which consists in the determination of the relative rate of n-hexane cracking, following the procedure described in ref. 1.
The reference catalyst (alpha
=
1) is an
138 amorphous silica-alumina for which the reaction rate constant is 0.016 sec-1 (ref. 1 4 , 1 5 ) . RESULTS AND DISCUSSION Quantitation of Solid State 2781 and llB MAS-NMR Quantitation of spin n/2 nuclei in the solid state is not as straightforward as for spin 1/2 nuclei.
For spin 1/2 nuclei attention to spin lattice relaxa-
tion times and nuclear Overhauser enhancement is sufficient for reasonable quantitation, but for spin n/2 nuclei quadrupole coupling complicates matters. Quadrupole coupling is generally, but not always, large enough in the solid state that under the usual MAS (magic angle sample spinning) or VAS (variable angle sample spinning) conditions only the central -1/2 to 1/2 transition is observed (ref.16).
Intensity is lost to higher order transitions. This is
further complicated by Schmidt's calculation (ref.17) that the intensity of the central transition under pulse conditions is not a simple fraction of the intensity observed for solutions as might be expected if the quadrupolar splitting was treated like a scalar coupling.
The central transition intensity
depends on whether or not the pulse is short enough, that is, has a broad enough frequency distribution, to excite equally all transitions. As an example, NaBH4 has negligible quadrupole coupling in both solution and solid state.
Conse-
quently, the intensity found for a given number of moles of B is the same in the solid state and in solution. On the other hand, NaBH3CN shows substantial quadrupolar coupling in its MAS solid spectrum which, in turn, shows only 22% of the solution intensity. This is close to the 20% calculated by Schmidt (ref. 17) for B under conditions of no excitation of higher transitions. The tetrahedral B species discussed in this paper, like NaBH4, have negligible quadrupolar coupling (line widths of 40-60 Hz) whereas the tetrahedral A1 species show a central transition intensity only 9% of that in solution. The effective flip angle experienced for the central transition is also affected by the degree of excitation of the higher transitions as predicted by Schmidt (ref.17).
This is illustrated by Fig. 1 which compares the peak height
response for A1 in solution to A1 under MAS conditions in several zeolites and in alumina (pseudo-boehmite).
The effective 90" pulse is smaller for the
zeolite structural A 1 than in solution and larger than for A1 in alumina, reflecting the zero quadrupole coupling in solution, the small coupling in zeolites, and the somewhat larger coupling in alumina. The effect of such differential response on quantitation was minimized by using Al(N03)3*9H20, which has quadrupole coupling quite similar to the zeolites studied as an A1 quantitation standard (see Fig. 1, Al(H2O)63+
solid).
On the other hand,
differential response can be exploited to separate resonances with similar
139
+ y Y
3 sooo*
a a
t
0
2.5
5 7.5 10 WLSE LENGTH IN MICROSECONDS
12.5
15
Fig. I . Aluminum peak height response to pulse length (at 110 W) for A1 in solution and A1 under MAS conditions in Y and ZSM-5 zeolites and alumina. chemical shifts in a two-dimensional NMR experiment as described by Samoson and Lippmaa (ref.l8,19). All samples were examined in the ammonium form to avoid changes in the distribution of charges around aluminum, which are known to affect the A1 resonance linewidth because of variations in the electric field gradient at the aluminum nucleus (ref.20,21). Good quantitation can be obtained for B and A1 in the solid state, if differential pulse response and spin lattice relaxation times (Ti) are given careful attention.
For the materials in this study, 27Al T i ' s were found to be general-
ly smaller than 10 milliseconds but IIB Ti's were much longer, ranging from 130 to 650 milliseconds. Catalytic Activity vs. Framework Aluminum Content The NMR resonance characteristic of tetrahedral A1 in NH4-ZSM-5 appears near
54 ppm (referenced to a 1 M AlCl3 solution) with a linewidth of about 450 Hz. chemical shift and linewidth vary only to a limited extent with the A 1
The "A1
content of the zeolite, factors which do not affect the quantization provided care is taken of the parameters discussed above. Fig. 2A demonstrates the excellent linear relationship which exists between the atomic T-site fractions of structural A 1 measured by elemental analysis, TPAD, and A1 MAS-NMR, for a variety of NH4-ZSM-5 zeolites with composition comprising from less than 300 ppm up to 1.2 wt.% A l .
In Fig. 2B, the acidic
catalytic activity of the same materials, after deammoniation into their H-form, is shown to correlate linearly with their tetrahedral framework A1 content,
140
Fig. 2. (A) Correlation between the atomic fractions of structural A1 from elemental analysis ( 0 ) or TPAD (0) and same parameter estimated by 2781 MAS-NMR. (B) Relative hexane cracking activity at 811 K (alpha value) as a function of the atomic fraction of structural A 1 measured by 27Al MAS-NMR. also expressed as atomic T-site fraction, over nearly three orders of magnitude. The correlation line represents the ideal dependence of relative hexane cracking activity on framework A 1 content as it has been described and discussed earlier (ref.l,2). These results show that the combination of 27Al MAS-NMR which probes A1 tetrahedral sites, TPAD which counts Brdnsted acidic sites, and elemental analysis which measures the total A 1 content, enables the unambiguous evaluation of the amount of structural aluminum responsible for the catalytic activity of ZSM-5.
Obviously, the same approach could and should be used for other
zeolites. The linear correlation extrapolating to the origin, which is observed between activity and Al-content, confirms that all catalytic sites in ZSM-5 have on the average a similar activity, independent of Si02/A1203 ratio (ref. 1,2,31).
141 Modification of ZSM-5 by Boron Trichloride Acidic halides of trivalent elements, BCl3 in particular, are known to react with strained siloxane, Si-0-Si, bridges such as those produced at the surface of silica upon high temperature degassing (ref.22-24).
In zeolites, they react
with Al-0-Si bridges and recent years have witnessed increasing of activity aimed at modifying the structural A1 content by reaction with metal and non-metal, preferentially volatile, halides (ref.6-10).
In the present study we use BC13
at 773 K to dealuminate ZSM-5, having thereby the opportunity to complement our other results by 1lB MAS-NMR data. Dealumination of mordenite by the same technique was recently observed independently by Fejes et al. (ref.8). Following deammoniation of the precursors at 773 K (samples S(1-IV) and
L(I,II)),
treatment of H-ZSM-5 by BCl3 at the same temperature, leads to pro-
gressive dealumination and incorporation of boron as shown by the data listed in Table 1.
Na-ZSM-5S shows a similar behavior although dealumination appears
somewhat less efficient. It is known that TPDA measurements on zeolite ammonium-forms enable the determination of their total concentration in Brdnsted acidic sites, and thereby of their total ion-exchange capacity, as well as the discrimination between sites of different sorptive strength (ref.12,13).
In contrast to the parent
ZSM-5 zeolites which are characterized by a unique NH3 desorption peak near
673 K, BClytreated materials have, after reammoniation, TPDA spectra of the type shown in Fig. 3. by about 200 K.
Two states of ammonia are observed which are separated
The desorption activation energies corresponding to these
ammonia species can be readily evaluated by performing TPDA measurements at Oo4*
r
TEMPERATURE I K)
Fig. 3. 773 K.
TPAD spectrum (20 K min-1) of NH4-ZSM-5S treated with BCl3 0.5 h at
142 different heating rates (1-30 K) and assuming a first-order desorption process (ref.13,25).
Experimental values are of 62-68 kJ mol-1 for the l o w temperature
state (373-548 K) and of 158-163 kJ mol-l for the high temperature state (548850 K ) indicating a major difference in the acidic strength of the corresponding chemisorption sites. The high temperature ammonia peak is the only one observed in the parent materials, it varies proportionally with A 1 content as seen from Table 1, and it is characterized by an activation energy comparable to that reported in the literature for ammonia desorbing from Brdnsted sites associated to structural A 1 in ZSM-5 (gamma state in ref. 13).
It is straightforwardly assigned to the
desorption of ammonia from Al-O(NH4)-Si
groups.
The low temperature peak has an
intensity which parallels the variation of the boron content of the zeolite measured by elemental analysis or by B MAS-NMR (see Table 1) and it is attributed to ammonia desorption from boron-containing species, including the B-O(NHh)-Si
groups discussed below.
This assignment is consistent with the fact
that B-substitution in the ZSM-5 framework leads to materials possessing very weak acid activity (ref.26,27). Figure 4 shows the variation in the low and high temperature ammonia states and the total amount of ammonia desorbed as a function of the relative hexane cracking activity (alpha-value) of the corresponding materials.
Clearly, the
catalytic activity increases with the number of acidic sites stemming from the presence of structural aluminum (ammonia desorbed above 548 K) whereas it is negatively correlated with the amount o f ammonia desorbed from boron-containing
a -value
Fig. 4 . Ammonia evolution from BC13-treated NH4-ZSM-5S samples as a function of their relative hexane cracking activity (alpha value). 0 : Ammonia released below 548 K; 0 : Ammonia released above 548 K; +: Total amount of ammonia evolved.
143 species.
It demonstrates thereby that aluminum sites are the active entities
in boron-containing aluminosilicates, in agreement with earlier findings and conclusions for related catalysts used in a variety of organic conversions (ref. 29,30).
The total amount of desorbed ammonia stays nearly constant upon treat-
ment with BCl3, which suggests that substitution of structural A 1 by B occurs (one trivalent element is replaced by another trivalent element). 2 7 A l MAS-NMR enables, as above, the quantitation of structural A1 in BCl3-
modified ZSM-5 materials. those of Fig. 2.
Figure 5 (A and B) shows correlations analogous to
The linear relationship of Fig. 5(A) confirms that the high
temperature ammonia state (T > 548 K) is indeed representative of structural A 1 and that non-framework A 1 species are essentially absent following the BCl3 treatment. The dependence of the relative hexane cracking activity on A 1 content fits the expected linear correlation discussed before, as shown in
lOOx[AI/lAt + S i g (NMRI
Fig. 5. (A) Correlation between the atomic fractions of structural A 1 from elemental analysis ( 0 ) or TPAD (0) and same parameter estimated by 2 7 A l MAS-NMR for BClg-treated NH4-ZSM-5S. (B) Relative hexane cracking activity at 811 K (alpha value) as a function of the atomic fraction of structural A 1 measured by 2781 MAS-NMR for BC13 treated NH4-ZSM-5s.
144 Fig. 5 ( B ) , confirming that tetrahedral framework aluminum sites govern the acid catalytic behavior.
llB MAS-NMR provides evidence for the structural replace-
ment of A 1 by B , i.e., the appearance of B tetrahedral sites in the ZSF-5 framework.
These sites are characterized by a chemical shift of about -3.7 ppm
(referenced by BF3 etherate (ref.28))
and a linewidth of 40-60 Hz.
after quantitation the results listed in Table 1. between the “B
They give
The observed parallelism
NMR and the low temperature ammonia peak intensities supports
the assignment of these NMR peaks to structural boron. Direct replacement of structural aluminum by boron hence occurs according to the overall site specific scheme: H
I
o\A,s”\s~o 0’\o
o”0
BCl3
-~1c13
-
H
I
which accounts for the disappearance of catalytically active A 1 sites, but the conservation of the total ion-exchange capacity. A possible mechanism is tentatively proposed in Fig. 6 in which the driving
force is the electrophilic attack of the acid B C l 3 on a base oxygen framework
Fig. 6. Proposed mechanism for the reaction between structural aluminum and BCl3.
146
anion coordinated to Al.
Reaction may preferentially take place at such a site
because of the increased negative charge on the oxygen atoms surrounding aluminum. After elimination of HC1, repeated cleavage of A1-0 bond occurs until an ionic intermediate (such as V) is formed. The latter structure can rearrange to eliminate AlC13 and incorporate boron at T-sites, as boron is more electronegative than aluminum. CONCLUSIONS Provided that some critical variables which we have delineated, are kept under control, it is possible to achieve satisfactory quantization of the MASNMR of quadrupolar nuclei. We have shown for ZSM-5, in particular, that the combination of elemental analysis, TPDA (or ion-exchange capacity), and 27Al MAS-NMR results enables the unambiguous evaluation of the amount of structural
aluminum present in the zeolite. This conclusion holds for zeolites, either carefully pretreated to avoid alteration of their framework aluminum or intentionally modified to vary the latter.
In principle, tetrahedral A1 quantita-
tion in ZSM-5 should be possible up to a Si02/A1203 ratio of 100,000, i.e., down to about 10 ppm aluminum. Treatment of ZSM-5 by BCl3 leads to replacement of A1 by B in T-site positions.
TPDA data indicate that hydroxyl groups attributed to the presence
of framework boron are much less acidic than the Brdnsted sites associated with structural aluminum. The total ion-exchange capacity is maintained nearly constant when this substitution occurs, whilst the acidic catalytic activity is decreased. The carboniogenic activity of H-ZSM-5 is in both cases, i.e., for the nonaltered and the BC13-treated materials, governed by its structural aluminum content which can be evaluated by 2781 NMR.
The catalytically active entities
in boron-containing aluminosilicate ZSM-5 are hence associated with framework aluminum. ACKNOWLEDGMENTS Fruitful discussions with Professor E. Oldfield (University of Illinois), Drs. W.O. Haag, R. von Ballmoos, and C.D. Chang are gratefully acknowledged. We are indebted to G.T. Kerr for obtaining the TPAD results and for stimulating suggestions. The authors thank R. Lago, D.H. Olson, C.D. Chang, and G.H. Kuehl for supplying well-characterized ZSM-5 samples used in the NMR quantitation studies. The skillful technical help of F.X. Ryan and S.W. van Etten is also acknowledged.
146 REFERENCES 1 Olson, D.H., Haag, W.O. and Lago, R.M., J. Catal., 61 (1980) 390. 2 Haag, W.O., Proc. Sixth Intern. Zeolite Conf., Reno, Nevada (1983) in press. 3 Post, M.F.M., van Amstel, J. and Kouwenhoven, H.W., Proc. Sixth Intern. Zeolite Conf., Reno, Nevada (1983) in press. 4 Gilson, J.P. and Derouane, E.G., J. Catal., submitted for publication. 5 von Ballmoos, R . , "The I80-Exchange Method in Zeolite Chemistry: Synthesis, Characterization, and Dealumination of High Silica Zeolites" in "Texte zur Chemie und Chemietechnik", Salle and SaueJlander, Frankfurt, 1981; Scherzer, J., Proc. 1 9 t h State-of-the-Art A.C.S. Symp. "Catalytic Materials", A.C.S. Symp. Ser., (1984) in press. 6 Beyer, H.K. and Belenkaja, I., Proc. CNRS Symp. "Catalysis by Zeolites", Stud. Surf. Sci. Catal., 5 (1980) 203. 7 Fejes, P., Kiricsi, I. and Hannus, I., Acta Phys. Chem., 375 1982) 173. 8 Fejes, P., Kiricsi, I., Hannus, I. and Schobel, G., Magy Kem. Foly, 89 (1983) 264. 9 Chang, C.D., US Patent 4,273,753 (1981) assigned to Mobil O i l Corporation. 10 Jacobs, P.A., Tielen, M., B. Nagy, J., Debras, G., Derouane, E.G. and Gabelica, Z., Proc. Sixth Intern. Zeolite Conf., Reno, Nevada (1983) in press. 11 Argauer, R.J. and Landolt, G.R., US Patent 3,702,886 (1972). 12 Kerr, G.T. and Chester, A.W., Thermochim. Acta, 3 (1971) 113. 13 For example: Topsde, N.Y., Pedersen, K. and Derouane, E.G., J. Catal., 70 (1981) 41. 14 Weisz, P.B. and Miale, J.N., J. Catal., 4 (1965) 527. 15 Miale, J.N., Chen, N.Y. and Weisz, P.B., J. Catal., 6 (1966) 278. 16 Ganapathy, S., Schramm, S. and Oldfield, E., J. Chem. Phys., 77 (1982) 4360. 17 Schmidt, V.H., "Pulsed Magnetic and Optical Resonance", Proc. Ampere Intern. Summer School 11, September 1971, R. Blinc (ed.), Ljubljana, Yugoslavia, 1972, pp. 75-83. 18 Samoson, A . and Lippmaa, E., Chem. Phys. Lett., 100 (1983) 205. 19 Samoson, A. and Lippmaa, E., Physical Review B, 28 (1983) 6567. 20 Kentgens, A.P.M., Scholle, K.F.M.G.J. and Veeman, W.S., J. Phys. Chem. 87 (1983) 4357. 21 B. Nagy, J., Gabelica, Z., Debras, G., Derouane, E.G., Gilson, J.P. and Jacobs, P.A., Zeolites, 4 (1984) 133. 22 Morrow, B.A. and Cody, I.A., J. Phys. Chem., 80 (1976) 1995. 23 Morrow, B.A. and Cody, I.A., J. Phys. Chem., 80 (1986) 1998. 24 Bermudez, V.M., J . Phys. Chem., 75 (1971) 3249. 25 Redhead, P.A., Vacuum, 12 (1962) 203. 26 Taramasso, M., Perego, G. and Notari, B., in Proc. 5th Intern. Conf. Zeolites (L.V. Rees, ed.), Heyden (London) 1980 p. 40. 27 Ione, K.G., Vostrikova, L.A., Paukshtis, E.A., Yurchenko, E.N. and Stepanov, V.G., Dokl. Akad. Nauk, (SSSR), 261 (1981) 1160. 28 Et20.BF3 is the usually accepted standard for "B NMR; see Harris, R.K. and Mann, B.E., "NMR and the Periodic Table", Academic Press, N.Y., 1978, p. 91. 29 Lago, R.M. and Haag, W.O., unpublished results. 30 Chu, C.T-W. and Chang, C.D., unpublished results. 31 Haag, W.O., Lago, R.M. and Weisz, P.B., Nature (1984) in press.
B. Imelik e t al. (Editors), Catalysis b y Acids and Buses @ 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
147
CHARACTERIZATION OF ACIDIC PROPERTIES OF HETEROPOLY COMPOUNDS IN RELATION TO HETEROGENEOUS CATALYSIS Makoto MISONO Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113 (Japan
ABSTRACT Acidic properties and acid c a t a l y s i s of Keggin-type heteropoly acids and their s a l t s i n s o l i d s t a t e have been reviewed and compared w i t h t y p i c a l s o l i d acids l i k e silica-alumina. C a t a l y t i c reactions a r e c l a s s i f i e d i n t o "bulk-type" and " surface type" reactions depending on whether the reaction takes place i n t h e bulk o r only on the surface. In t h e case of bulk-type r e a c t i o n s , "pseudo-liquid" behavior provides a high c a t a l y t i c a c t i v i t y and unique s e l e c t i v i t y . A high a c t i v i t y i s often observed i n t h e surface-type, as well. Les propri&t&sc a t a l y t i q u e s e t acides des heteropolyacides d u type Keggin e t comparees d c e l l e s des acides s o l i d e s convenl e u r s s e l s a l ' e t a t s o l i d e ont @t@ t i o n e l s t e l s que l a silice-alumine. Chaque reaction c a t a l y t i q u e peut-0tre c l a s s i f i e e en "type-volume" ou "type-surface", selon q u ' e l l e s e produit dans l e volume ou seulement en surface. En ce qui concerne l e s reactions du type volume, l a propri&t&"pseudol i q u i d e " des heteropolyacides donne une a c t i v i t e c a t a l y t i q u e @levee e t une select i v i t e unique. Une bonne a c t i v i t e e s t aussi souvent observee dans l e type surface. INTRODUCTION Since heteropoly compounds of Mo and IJ ( o r V, Nb, e t c . ) a r e acids and a t the same time oxidizing agents, they a r e used as acid and oxidation c a t a l y s t s (ref.l,2). Many patents have already been published u t i l i z i n g them f o r a v a r i e t y of s y n t h e t i c reactions ( r e f . 3 ) . Heteropoly compounds a r e s u i t a b l e a l s o f o r fundamental s t u d i e s of c a t a l y s i s , because ( i ) t h e s t r u c t u r e can be characterized a t a molecular level of heteropoly anion, ( i i ) t h e a c i d i t y and redox propertiescan be controlled in a systematic manner, and ( i i i ) owing t o the behavior we c a l l e d "pseudo-liquid phase ( r e f . 4 ) " , c a t a l y t i c r e a c t i o n s sometimes proceed not only on t h e surface b u t a l s o in the inner bulk. The 'bseudo-liquid" behavior endows a high c a t a l y t i c a c t i v i t y and unique s e l e c t i v i t y t o these compounds. Moreover, owing t o t h i s behavior, the chemical changes a r e often not limited t o t h e s u r f a c e , b u t they a r e expanded t o the whole bulk. T h i s magnifying e f f e c t makes, for example, spectroscopic s t u d i e s much more f e a s i b l e and r e a l i s t i c .
148
Here, the present status of understanding of the acidic and catalytic properties of Keggin-type heteropoly compounds in the solid state wi 1 1 be reviewed mainly based on our own studik, placing stress on advances after the previous review (ref.2). Some comparison with other solid acids will be made, with reference to the "pseudo-liquid" behavior. STRUCTURAL CHARACTERISTICS PERTINENT TO CATALYSIS Primary and secondary structures Heteropoly compounds in the solid state consist of heteropoly anion(Keggin structure here, XM,2040: X = P, Si, M = Mo, W, Fig. la), cation (proton, metal or onium ion), and water and/or organic molecules. We propose to call this three-dimensional arrangement secondary structure, and the heteropoly anion primary structure (ref.5,7). X-ray diffraction (XRD) gives the information o f the secondary structure and IR data the information of the primary structure. According to XRD and IR studies, the primary structure is rather stable, but the secondary sturcture i s readily variable upon the interactions with water, metal ion, etc.(ref.6,7). It is necessary for the proper understanding of the catalysis of heteropoly compounds in solid state to distinguish between the primary and secondary structure. Thermal stability, surface area, and content of water of crystallization They depend on the constituent elements and structure (ref.6-12). H3PMo12040(PMo12) and H3PW12040(PW12) are thermally stable at least at 300350°C. Salt formation with Cs, K, etc, enhances the thermal stability. PMo12, PWI2 and water-soluble salts of them usually contain a great number o f water molecules. They are mostly removed at 100-150°C and the surface areas become 1-15 m'9-I (ref.10). In the case of water-insoluble salts (Cs, K salt), the surface area is 100-200 m2gq1; the nirmber of water molecules contained is
Keggin anion Fig. '1.
Surface-type
-
BuIk type
Keggin anion (a), surface-type reactions (b), and bulk-type reactions (c). 0 : reactant, @ : product.
149
small and water is removed by evacuation at 25°C. Other structural properties as well as catalytic behavior are also quite different between water-soluble and insoluble salts. "Pseudo-1 i quid phase. Due to the variable nature of the secondary structure, such polar molecules as water, alcohols (oxygen bases), ammonia, and pyridine (nitrogen bases) are easily absorbed into the internal bulk of water-soluble heteropoly compounds, while non-polar molecules like hydrocarbons are adsorbed only on the surface (ref.2,6). As an example, the absorption process of pyridine at 25 " C is shown in Fig. 2. The absorptivity depends primarily on the basicity and secondarily on the size of each molecule. Note that this absorption-desorption process of polar molecules are not diffusion in micropores. BET surface areas of samples which show pseudo-liquid behavior are usually very small and there are no such micropores. The process is accompanied by changes in the distance between polyanions; the molecules are absorbed in the enlarged inter-polyanion space. Increase of volume is visible (swelling). By this process, for example, 0-18 exchange between water vapor and anions of the whole bulk is rapid (ref.11). The rapid diffusion of polar molecules and the easy rearrangement of the secondary structure as well as the fact that certain reactions actually proceed in the internal bulk (see below) may allow us to call this state of heteropoly compounds a "pseudo-liquid phase" (ref.4). I'
Fig. 2. Absorption and desorption of pyridine by H3PMo12040 (25"C, 21 Torr)
150
ACIDIC PROPERTIES Ac,idity of heteropoly acids It was shown by amine titration that heteropoly acids in the solid state are strongly acidic and contain a large number of acid sites (ref.13). The number almost corresponded to the whole protons in the bulk. The reason why the whole protons were measurable, although they were not exposed on the BET surface, was first understood by the concept of the "pseudo-liquid phase". IR study demonstrated that PMo12, SiMo12 and PWI2 are purely protonic and very strong solid acids (ref.5-7). As shown in Fig. 2, pyridine was absorbed rapidly even at 25°C (volume of sample increased visibly). After evacuation at 130°C, homogeneous, stoichiometric pyridinium salts were formed, as confirmed by gravimetric and IR analyses. Higher temperature makes easier the formation o f homogeneous salts. The salts were usually very stable at high temperatures. Typical IR spectra are shown in Fig. 3. It is interesting to note that the 1540 cm-'-band reversibly disappeared by the presence of excess pyridine (ref.6). Thermal desorption of pyridine for some heteropoly compounds and silicaalumina are compared in Fig. 4 (ref.2,12). It may be noted that number of pyridine molecule agreed with the number of protons expected from formula,after evacuation at 130°C. High acid strength may be seen from the temperature of pyridine desorption. Thermal desorption of NH3 gave similar results (ref .14).
1700 1600 1500 W Wave numbeflcm-1
Fig. 3. Infrared spectra of pyridine absorbed by H3PMo,2040. (a) Evacuated for 30 s at 25"C, (b) pyridine absorbed in excess, (c) evacuated at 130°C for 1 h after (b). (Bands due to H20, H30+appear in spectrum (a))
151
I "
100
200
300
Evacuation temp./
4000
OC
Fig. 4. Thermal desorption of pyridine from several heteropoly compounds. (a) H3PW12040, (b) H3PMo12040, (c)Cu3/2PW12040, (d) Na3PW12040, (el Cs3 PW12040, and (f) silica-alumina. Acidity of salts Stoichiometric Na salt shows some weak protonic acidity (Fig. 4), possibly due to partial hydrolysis. The origin of the acidity of Cu2+ salt (Fig. 4) has been controversial; partial hydrolysis, acidic dissociation of water coordinated to Cu2+ (ref.15a) and/or proton formation by Cu2+ + 1/2H2+ Cu' + H+ (ref. 15b). Acidity of water-insoluble salts like Cs salt will be discussed in the next section in relation to catalytic activity. ACID CATALYSIS Bulk-type vs. Surface-type reactions In some cases, catalytic reactions over heteropoly compounds take place not only on the outer surface or the surface in pores, but also in the bulk, owing to the "pseudo-liquid" behavior. In this case, whole protons in the bulk can take part in catalysis, so that very high catalytic activity and unique selectivity are often observed, particularly at low temperatures. Examples are shown in Table 1. A high catalytic activity reported for alcohol dehydration (ref.13) seems to be mainly due to this behavior. Other reasons for high activities may be strong acidity and stabilization of reaction intermediate by complex formation, e.g., an anion-alkyl cation complex (ref.17). It was recently demonstrated by a transient response method that at the stationary state of dehydration of 2-propanol, 2 to 8 molecules of alcohols per Keggin anion were absorbed in the bulk and the rate of absorption-desorption was much faster than the rate of dehydration (effectiveness factor = 1) (ref.16). Therefore, these reactions may be called "bulk-type" reactions. In contrast,
152
TABLE 1 Comparison of catalytic activity of heteropoly acids with silica-alumina Reaction
~ ~ ~ Rat $ i oa! ~Ref.-
Catalyst
2-Propanol 4 propene + H20 Isobutene + CH30H +MTBE Isobutyric acid -c propene t CO + H20 Benzene + CH30H- toluene Toluene -benzene + xylene
150 pw12 PMo12, PW12/Si02 90
b
100 3 00
C
PW12,
240
4
d
pwl 2 PW,
250 250
we me
f
Q
~~
f
~
aThe ratio of catalytic activity of heteropoly compounds to that of silicaba 1 umi na . dref.13, 'A. Igarashi et al., J. Japan Petrol. Inst., 22 (1979) 331, M. Otake and T. Onoda, J. Catal., 38 (1975) 494. eSil ica-alumina showed significant activity above 400°C, fT. Okuhara et al., Shokubai, 22 (1980) 226. molecules which can not be absorbed react only on the surface. Butene isomerization and cumene cracking are the examples and may be called "surfacetype'' reactions. Surface-type and bulk-type reactions are schematically illustrated in Fig. 1. Contrast between the two types was found also in the effect of surface area. Increase of surface area by dispersion on a support has a marked influence in the case of surface-type reactions, while the effect was naturally small in the bulk-type reactions (ref.14).
pyidine/Keggin anion (Wac.at 300°C) Fig. 5. Relationships between catalytic activity and b u l k acidity 0) Dehydration o f 2-propanol, (0)of formic acid, and PW12040. ( (A)of methanol to hydrocarbons.
Of
NaxH3,.x
153
Acidity, absorptivity, and catalysis Acidity vs. catalytic acitvity. In the case of bulk-type reactions, the acidity measured by thermal desorption of pyridine or NH3 showed a good correlation with catalytic activity (Fig. 5 ) (ref.2,12). This is because the reactions are bulk-type and the acidity measured is bulk acidity. On the other hand, in the surface-type reactions, the bulk acidity has sometimes little correlation with catalytic activity. But after the bulk and surface were once homogenized, for example, by treatment with water vapor, a monotonous relationship was obtained (ref.12). Absorptivity vs. selectivity. Changes in absorption properties are reflected in catalytic selectivity, as well. The absorption amount of alcohols decreased markedly with increasing Cs content in C S ~ H ~ - ~ P W . ,while ~ O ~ ~ the , change was much less for NaxH3-xPW12040. Accordingly, ethylenelether ratio from dehydration of ethanol varied markedly with the Cs content (ref.l8), while little change was observed for Na salts. Therefore, the variation is not ascribable to the difference in acidity, but to the difference in absorptivity. We previously presumed that ethylene was mostly formed in the bulk and ether on the surface (ref .12). Olefin/paraffin ratios in the products of dimethyl ether conversion to hydrocarbons were also well correlated with the absorptivity of ether into each catalyst as shown in Fig. 6 (ref.19). The two types of reactions are present also in the case of oxidation catalysis at high temperatures, but by a different mechanism (ref.20,21).
Absorptivity/Surface layer DME
Fig. 6. Alkene/alkane ratios in the products of dimethyl ether conversion to hydrocarbons over various heteropoly compounds at 290°C.
154
Fig. 7. Catalytic activity as a function of Cs content. Dehydration of 2-propanol at 110°C after 110°C-pretreatment (0)and after 300°C-pretreatment ( 0 ) . Conversion of dimethyl ether at 290°C after 300°Cpretreatment (13).
Acidity and catalytic activity of-Cs salts. As a typical water-insoluble salt, CsxH3-xPWq2040 was studied in more detail (ref.22). In Fig. 7, catalytic activities for two dehydration reactions are shown as a function of x, after pretreatments at two different temperatures. For low-temperature treatment, the activity decreased lineraly to zero as x increased from zero to two, and a small maximum appeared at about x = 2.5. For high-temperature treatment, the catalyst became inactive for x = 1.0 and 2.0, but the activity much increased for x = 2.5. The catalyst with x= 2.5 after treated at a high temperature showed a higher activity than the acid form and a much higher selectivity to olefins in the conversion of dimethyl ether to hydrocarbons (ref .9). Based on chemical and XRD analyses, the following hypothesis was proposed to explain the peculiar behavior shown in Fig. 7. (1) In the range of O(x22, precipitates are C S ~ H P W ~ ~and O ~the ~ . catalysts obtained after drying the precipitate and solution are mixtures o f H3PW12040 and C S ~ H P W ~ ~ The O ~ fraction ~. of H3PW12040 which i s active (bulk-type) decreases from unity to zero as x changes from 0 to 2, where C S ~ H P W , is , ~ much ~~~ less active (as x = 2 was s o ) . Heat treatment at 300°C makes possible the + diffusion of Cs and ,'H and probably transforms the mixture (x = 1,2) into more nearly homogeneous acidic salts which are much less active. ( 2 ) In the range of 2 4 x < 3 , the catalyst after drying is a mixture of Cs3PW12040 and C S ~ H P W ~ ~ The O ~ ~heat . treatment at 300°C homogenizes the
155
mixture, resulting in the increase of the proton concentration near the surface. The acidic salt may not be very active, but owing to its greatly increased surface area it shows a high catalytic activity. Acidity in oxidation catalysis. In the case of oxidation of methacrolein t o methacrylic acid, the acidity catalyzes the formation of an intermediate which is transformed to product by a redox mechanism (ref.5). In this case, acid and redox properties function cooperatively. On the other hand, in the oxidative dehydrogenation of isobutyric acid, the two properties compete, since acidity catalyzes a side reaction to form propene and CO (ref. 3). Control of acidic properties. The acidity can be controlled by (a) selection of the constituent elements of the anion (poly and hetero atoms), counter cation (metal or organic cation), and other molecules contained (water of crystallization, etc.), (b) partial neutralization, and (c) dispersion on a support. FUTURE ASPECTS The followings may be suggested for future development as in the previous review (ref.2). i ) Catalyst design for organic synthesis, by controlling (a) the acidity in combination with redox properties and (b) pseudo-liquid behavior which may provide a quite different environment for reacting molecules. i i ) Descripition of catalytic processes at a molecular level. Not only spectroscopic studies but also syntheses of model compounds related to reaction intermediates are useful. i i i ) Application as materials other than catalysts : sensors, solid electrolytes, medicines, materials for chromatographic separation, etc.
Acknowledgment. The author acknowledges the financial support in part of this work by the Grant-in-Aid for Scientific Research by the Ministry of Education, Science and Culture, and by the Asahi Glass Foundation for Industrial Technology. Useful discussion with Prof. Y. Yoneda, Dr. Okuhara and other colleagues is gratefully acknowledged.
156 REFERENCES
1 M. Otake and T. Onoda, Shokubai, 18 (1976) 169-179; M. Misono, Kagaku no R y o i k i , 35 (1981) 43-50; Y. Ono, T, M o r i , and T. K e i i , Proc. 7 t h I n t e r n . Congr. Catal., Tokyo, 1980, Kodansha(Toky0)-Elsevier(Amsterdam), 1981,1414 pp; T. Matsuda, M. Sato, T. Kanno, H. Miura, K. Sugiyama, J. Chem. SOC., Faraday I, 77 (1981) 3107-3117; H. Hayashi and J.B. M o f f a t , J. Catal., 77 (1982) 473-484; M. Ai, Appl. Catal., 4 (1982) 245-256. 2 M. Misono, Proc. t h e Climax F o u r t h I n t e r n a t i o n a l Conference on t h e Chemistry and uses o f Molybdenum, Climax Molybdenum Co., Ann Arbor, 1982, 289pp. 3 M. Misono and T. Okuhara, P e t r o t e c h , 4 (1981) 831-838; Y. Izumi and M. Otake, Kagaku Sosetsu, No.34, Chem. SOC. Japan, 1982, 116 pp. 4 M. Misono, K. Sakata and Y. Yoneda, ACS/CSJ Chemical Congress, Honolulu, A p r i l 1979; 1 s t French-Japanese C a t a l y s i s Seminar, Lyon, J u l y 1979; Shokubai, 21 (1979) 307-309. 5 M. Misono, K. Sakata, Y. Yoneda and W.Y. Lee , Proc. 7 t h I n t e r n . Congr. Catal., Tokyo, 1980, Kodansha(Toky0)-Elsevier(Amsterdam), 1981, 1047 pp. 6 M. Misono, N. Mizuno, Y. K o n i s h i , K. Katamura, A. Kasai, K. Sakata, T. Okuhara and Y. Yoneda, B u l l . Chem. SOC. Jpn., 55 (1982) 400-406. 7 M. F u r u t a , K. Sakata, M. Misono and Y. Yoneda, Chem. L e t t . , (1979) 31-34. 8 G.A. T s i g d i n o s , T o p i c s C u r r . Chem., 76 (1978) 1-64. 9 K. Eguchi, N. Yamazoe and T. Seiyama, Nippon Kagaku K a i s h i , (1981) 336-342; H. Niiyama, Y. S a i t o , S. Yoshida and E. Echigoya, i b i d . , (1982) 569-573. 10 M. Misono, Y. K o n i s h i , M. F u r u t a and Y. Yoneda, Chem. L e t t . , (1978) 709-712. 11 K. Sakata, M. Misono and Y. Yoneda, Chem. L e t t . , (1980) 151-154; N. Mizuno, K. Katamura, Y. Yoneda and M. Misono, J. Catal., 83 (1983) 384-392. 12 T. Okuhara, A. Kasai, N. Hayakawa, Y. Yoneda and M. Misono, J. C a t a l . , 83 (1983) 121-130; Shokubai, 22 (1980) 226-228; Chem. Lett.., (1981) 391-394. 13 M. Otake and T. Onoda, Shokubai, 17 (1975) 13-15. 14 N. Hayakawa, T. Okuhara, M. Misono and Y. Yoneda, Nippon Kagaku K a i s h i , (1982) 356-363. 15a H. Niiyama, Y. S a i t o and E. Echigoya, Proc. 7 t h I n t e r n . Congr. Catal., Tokyo, 1980, Kodansha(Toky0)-Elsevier(Amsterdam), 1981, 1416 pp. b T. Baba, H. Watanabe and Y. Ono, J. Phys. Chem., 87 (1983) 2406-2411. 16 T. Okuhara, T. Hashimoto, M. Misono, Y. Yoneda, H. Niiyama, Y. S a i t o and E. Echigoya, Chem. L e t t . , (1983) 573,576. 17 K. Urabe, K. F u j i t a and Y. Izumi, Shokubai, 22 (1980) 223-225; W.H. Knoth and R.L. Harlow, J. Amer. Chem. SOC., 103 (1981) 4265-4266. 18 T. Okuhara, T. H i b i , S. Tatematsu, T. I c h i k i and M. Misono, Proc. 9 t h I b e r o a m e r i c a n Symp. Catal., Lisbon, 1984, 623 pp. 19 T. Okuhara, T. H i b i , K. Takahashi, S. Tatematsu and M. Misono, J. Chem. SOC., Chem. Commun., (1984) 697-698; Chem. L e t t . , (1981) 449-450: (1982) 1275-1276. 20 M. Misono, N. Mizuno and T. Komaya, - . Proc. 8 t h I n t e r n . Conqr. Catal.. B e r l i n . 1984, Vol; 5, 487 pp. 21 N. Mizuno and M. Misono, Chem. L e t t . , (1984) 669-672. 22 S. Tatematsu, T. H i b i , T. Okuhara and M. Misono, Chem. L e t t . , (1984) 865868.
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B. Imelik e t al. (Editors), Catalysis b y Acids nnd Bases o 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
HETEROPOLY COMPOUNDS:
J.B.
SOLID A C I D S WITH GUARDED PROTONS
MOFFAT
Department o f Chemistry and Guelph-Waterloo
Centre f o r Graduate Work Chemistry, U n i v e r s i t y o f Waterloo, Waterloo, Ontario, Canada N2L 361
in
ABSTRACT Heteropoly acids w i t h anions o f Keggin s t r u c t u r e are i o n i c s o l i d s w i t h protons d i s t r i b u t e d throughout t h e i r structure. These protons are o f t e n found w i t h hydrogen-bonded water which may block access t o incoming sorbates. Although the water can be removed the protons can only be thermally e l i m i nated through the a d d i t i o n o f anionic oxygen atoms t o form water. Semiempirical (EXH) calculations, temperature-programmed desorption, exchange, and reduction, and i n f r a r e d photoacoustic spectroscopy, the l a t t e r w i t h ammonia and p y r i d i n e as probe molecules, have been employed t o y i e l d information on the protons and t h e i r environment i n heteropoly compounds. INTRODUCTION
The surfaces o f s o l i d s are characterized by r e l a t i v e l y high energies, s t r u c t u r e s perturbed from those i n the bulk, and valencies and charges which o f t e n r e q u i r e the a d d i t i o n o f supplementary atoms t o e s t a b l i s h energetic stability.
Frequently t h i s balance i s provided by protons, thereby producing
an a c i d i c surface. With composite c a t a l y s t s such as s i l i c a - a l u m i n a the a c i d i c functions, both Bronsted and Lewis, w i l l p r i m a r i l y reside on the surface, as opposed t o t h e bulk,
although a s i g n i f i c a n t proportion o f t h a t surface may be c o n t r i -
buted by the porous structure.
Although w i t h z e o l i t e s t h e a c i d i c centres are
again located so as t o be accessible t o molecules o f a range o f sizes,
such
locations are not on surfaces i n the sense o f a d i s c o n t i n u i t y , but are an i n t r i n s i c p a r t o f the c r y s t a l structure. Heteropoly compounds are u n l i k e e i t h e r o f solids,
the previously mentioned
p r i n c i p a l l y i n the absence o f a network structure,
p o l y anion i s a high molecular weight,
but the hetero-
cagelike species (Fig.
1).
The
heteropoly compounds o f i n t e r e s t i n the present work are t y p i f i e d by 12tungstophosphoric acid,
H3PW120b0, the anion of which can be considered as
containing a POI, tetrahedron a t i t s centre,
surrounded by and sharing i t s
oxygen atoms w i t h twelve octahedra o f oxygen atoms w i t h a tungsten a t each of
158
t h e i r centres.
Although Keggin f i r s t
h e t e r o p o l y compounds, formed by Brown e t al.
i n v e s t i g a t e d the s t r u c t u r e s o f such
t h e most complete s t r u c t u r a l
a n a l y s i s has been p e r -
on H3PW12040-6H20 w i t h X-ray and neutron d i f f r a c t i o n
(1). These workers have not o n l y shown t h a t the much e a r l i e r work o f Keggin i s s u r p r i s i n g l y accurate,
but a l s o have i d e n t i f i e d the proton as surrounded
by f o u r water molecules.
As a consequence o f a t w o - f o l d thermal d i s o r d e r
o n l y two o f these water molecules are hydrogen-bonded t o t h e proton a t a given time.
The water molecules are themselves hydrogen-bonded t o t h e o u t e r
oxygen atoms o f the anions (Fig. a Keggin Unit(KU).
1). The anion i s f r e q u e n t l y r e f e r r e d t o as While heteropoly compounds o f s t o i chiometry s i m i l a r t o 12-tungstophosphoric a c i d are c o r r e c t l y r e f e r r e d t o as s o l i d acids,
i t i s evident t h a t t h e correspon-
dences w i t h other s o l i d a c i d c a t a l y s t s may o f t e n disappear.
With t h e h e t e r o p o l y com-
pounds the presence o f c a t i o n s not o n l y a t t h e surface but throughout the b u l k would appear t o be necessary i n order t o m a i n t a i n structural integrity.
There are a number
o f questions which can be asked.
What i s
t h e s t a b i l i t y o f t h e anion s t r u c t u r e and t o what e x t e n t i s i t dependent on t h e presence
o f protons o r c a t i o n s i n general? How accessible are t h e protons i n t h e s t o i c h i o m e t r i c heteropoly acids and t o what e x t e n t
Fig. 1. Structural arrangement: protons, water molecules, and anions i n &W120I,0=6H20 (1)
does t h e p e n e t r a b i l i t y a l t e r w i t h e i t h e r removal o f protons o r exchange by o t h e r cations?
methanol on 12-tungstophosphoric(HPW),
Studies o f the conversion o f
12-molybdophosphoric(HPMo)
t u n g s t o s i l i c i c (HSiW) acids have shown t h a t t h e former
and 12-
and t h e l a t t e r a r e
e f f e c t i v e c a t a l y s t s f o r t h e p r o d u c t i o n o f hydrocarbons w h i l e t h e heteropoly a c i d c o n t a i n i n g molybdenun as the p e r i p h e r a l metal element generates p r i m a r i l y o x i d a t i o n products (2).
The c e n t r a l metal atom i n t h e anion s t r u c t u r e
e v i d e n t l y has r e l a t i v e l y l i t t l e d i r e c t i n f l u e n c e on t h e a c i d i c p r o p e r t i e s o f t h e h e t e r o p o l y acid,
w h i l e the p e r i p h e r a l metal atoms i n t h e anion are a
dominant f a c t o r .
METHODS Methods are described i n d e t a i 1 elsewhere (3,4,5). RESULTS Semi-empirical extended Huckel c a l c u l a t i o n s (3) on a simulated h e t e r o -
159
p o l y anion were c a r r i e d o u t by employing a fragment o f t h r e e octahedra and t h e c e n t r a l tetrahedron,
(XM,O,,-n)
consisting
where X r e f e r s t o t h e c e n t r a l
metal atom and M t o t h e p e r i p h e r a l metal atom (Fig.
2).
The n e t atomic
charges on t h e o u t e r oxygen atoms o f t h e anions are n e a r l y i d e n t i c a l f o r t h e PW and S i W fragments b u t are appreciably fragment fragment
(Fig.
3).
l a r g e r i n magnitude f o r t h e PMo
F u r t h e r t h e p a r t i t i o n e d energy f o r t h e M-0
( o u t e r ) bonds i s c o n s i d e r a b l y lower w i t h PMo than f o r PW and SiW.
These
r e s u l t s c o r r o b o r a t e t h e experi-mental r e s u l t s f o r t h e conversion o f methanol and f u r t h e r suggest t h a t t h e a c i d i c s t r e n g t h s o f t h e h e t e r o p o l y a c i d s can be r e l a t e d i n v e r s e l y t o t h e magnitude o f t h e n e g a t i v e charge on t h e o u t e r oxygen atoms o f the anions,
w h i l e t h e o x i d a t i v e c a p a b i l i t i e s may r e q u i r e a r e l a -
t i v e l y weaker b i n d i n g o f these oxygen atoms t o t h e p e r i p h e r a l metal atoms.
(b)
Fig.
2.
(a)
Fra-nt
XM3O16-n of heteropoly anion (b) Heteropoly anion
showing position o f fragment. Fig. 3.
Effect
of Central and Peripheral Atoms o f the Heteropoly Anion on
Charge of Oxygen Atom and Partitioned Energy f o r KO (outer oxygen) (a)
(c) pF103016-9, (b, bridging; 0, outer) F u r t h e r evidence f o r t h i s i n t e r p r e t a t i o n may be seen from t h e r e s u l t s o f
(b)
sibf3016''o,
temperature programmed desorption (TPD) s t u d i e s (4) (Fig. peaks are obtained a t approximately 200'C tively),
4).
With HPW two
and 500°C (peaks 1 and 2,
respecIt w h i l e t h r e e peaks a r e found f o r HPMo and HSiW, a l l due t o water.
appears t h a t peak 1 i n a l l cases i s due t o water h e l d on t h e surface i n molecular form.
Peak 1 f o r
HPW corresponds
i n magnitude t o 6.5
water
molecules and was s u b s t a n t i a l l y reduced a f t e r p r e l i m i n a r y outgassing a t 19O'C and a f t e r 320'C
was eliminated,
w h i l e peak 2 was e s s e n t i a l l y
unchanged.
160
While no TPO peaks were observed a f t e r outgassing a t 450°C, t h e water could be replaced r e v e r s i b l y up t o t h i s temperature. Peaks 1 and 2 f o r HSiW appear a t temperatures s i m i l a r t o those f o r HPW s i m i l a r t o those f o r HPW, while w i t h HPMo peaks 1, 2, and 3 are centred a t lower temperatures 100, 400, and 450"C, respectively.
i
. d
Peak 2
f o r HPW i s equivalent i n magnitude t o 1.4
W v) 0 z
and peaks 2 and 3 f o r HSiW and HPMo t o 1.9
Id u)
and 1.5 water molecules per anion, respect i v e l y . These values correspond c l o s e l y t o
a
[L
8 I-
t h e number o f protons held by these acids.
w U
Approximate energies o f 15-20, 30-50, and 60-100 k c a l h o l e are associated w i t h peaks I
I
773
l
l
573
1,2,
I 373
and 3 respectively.
No peaks above
300°C were observed i n t h e TPD p r o f i l e s o f
TEMP/%
s t o i c h i o m e t r i c salts, f o r example NaPW. With a l l three heteropoly acids, peak
1 i s e v i d e n t l y due t o t h e e v o l u t i o n o f water e x i s t i n g on t h e s o l i d i n molecular form. The magnitudes o f t h e energies associated w i t h t h i s peak are consistent
Fig. 4. TPD p r o f i l e s f o r HPM, HSiW, and HPMo a f t e r p r e t r e a t ment a t 298'K Further,
f o r 16 hrs.
w i t h hydrogen-bonding o f t h e water.
the q u a n t i t y o f water responsible f o r peak 1 i n t h e case o f HPW i s
consistent
w i t h the e l i m i n a t i o n o f
all
i n c l u d i n g t h a t contained w i t h i n i t s bulk.
molecular water
from the s o l i d
Thus i t i s apparently possible t o
remove a1 1 t h e water molecules guarding the protons without d e s t r u c t i o n o f the s o l i d i t s e l f . The q u a n t i t i e s o f water,
the r e l a t i v e l y high temperatures,
and t h e
energies associated w i t h i t s e v o l u t i o n for peaks 2 and 3 are i n d i c a t i v e o f an associative desorption o f water and again one i n which both surface and bulk species are involved.
Since the q u a n t i t i e s o f water evolved correspond t o
the expected numbers o f protons,
the evolved water must r e s u l t from these
protons and anionic oxygen atoms.
The lower temperatures at which both peaks 1 and 2 appear w i t h HPMo suggest t h a t molecular water i s less s t r o n g l y bound i n t h i s s o l i d and t h e anionic oxygen atoms more e a s i l y combine w i t h the a c i d i c protons. The l a t t e r may be i n d i c a t i v e o f a smaller binding energy o f these atoms t o the adjacent p e r i p h e r a l metal atoms, a r e s u l t consistent w i t h the t h e o r e t i c a l predictions.
However, X-ray d i f f r a c t i o n data confirm t h a t
161 t h e s t r u c t u r e remains preserved,
i n s p i t e o f t h i s loss o f anionic oxygen
a t oms. Further information on the a c c e s s i b i l i t y o f the protons i n the heterop o l y acids may be obtained from the use of probe molecules and photoacoustic spectroscopy
Ammonia i s a useful species f o r t h i s purpose.
(5).
After
evacuation w i t h heating t o 2OO"C, approximately 5 molecules/KU were taken up i n 5 min by HPW at room temperature, but a s u b s t a n t i a l amount o f those were removed on heating i n s t a t i c vacuum above 150°C.
A f r e s h a l i q u o t o f t h e HPW
which had been evacuated w i t h heating t o 2OO'C ammonia at 150'C
(Fig. 5).
KU were taken up,
was dosed stepwise w i t h
Since no more than t h r e e molecules o f ammonia p e r
i t appears t h a t the ammonia was able t o penetrate i n t o the
bulk o f the s o l i d and i n t e r a c t w i t h the a c i d i c protons. The PAS spectrum shows the development o f bands at 3200 cm-' and 1420 cnr' c h a r a c t e r i s t i c o f the NHr+ i o n u n t i l w i t h uptake o f 3 molecules NH3/KU the spectrum resembles t h a t o f the amnonium salt. The l a t t i c e parameter a. shrinks from t h e value o f 12.11 A found w i t h HPW t o t h a t o f 11.71 A expected f o r the ammonium s a l t , while the cubic s t r u c t u r e i s retained.
The continuously decreasing back-
ground continuum evident i n the PAS spectrum (Fig.
5) may be ascribed t o
proton m o b i l i t y both on t h e surface and i n the bulk o f the s o l i d (6). f i v e o r s i x bands i n t h e spectrum
below 1100 cm-'
The
may be ascribed t o the
Keggin u n i t (7-9) and thus may be conveniently used t o monitor the r e t e n t i o n o f the anionic structure. When HPW was heated t o 450'C i n vacuo these bands, although broadened, were retained, i n d i c a t i n g r e t e n t i o n o f s t r u c t u r e . P y r i d i n e may also be conveniently employed as a probe molecule, but i t s behaviour w i t h heteropoly compounds i s somewhat d i f f e r e n t from t h a t o f ammonia (10).
250'C,
A f t e r pre-evacuation a t
on exposure t o excess pyridine,
HPW sorbed p y r i d i n e a t 25°C i n two stages, the f i r s t r a p i d and t h e second slow, up t o approximately 6 p y r i d i n e
lb)
* N y ( O 55'KUI
molecules per KU i n one hour. However, t h e PAS spectrum a f t e r evacuation (Fig. 6 ) shows t h a t t h e formation o f the pyridinium ion i s inhibited.
The background continuum
Fig. 5. E f f e c t o f dosing NH3 (mole-
found w i t h HPW i s again observed but a
c u l e s sorbed/KU) stepwise a t 150'C
band a t 1540 cm-' which i s character-
on ~PpW120,,o
(pre-evacuated a t ZOO'C).
i s t i c o f the pyridinium i o n i s b a r e l y
162 detectable.
However,
t h e e x i s t e n c e o f p y r i d i n e i n hydrogec-bonded forms i s
e v i d e n t from t h e observation o f bands a t 1605, 1489, 1443, and 1425 c m - ' ( l l ) . I n contrast,
when HPW was dosed
pyridine/KU was taken up i n 15 min, s t r o n g bands (1640, 1610, 1537, pyridine.
The absence o f
i n measured amounts
a t 25"C,
0.94
and t h e PAS spectrum (Fig. 6c) e x h i b i t s
and 1485 c m ' )
c h a r a c t e r i s t i c o f protonated
bands c h a r a c t e r i s t i c
of
o t h e r types
of
bound
p y r i d i n e (11) and t h e s i m i l a r i t i e s o f r e l a t i y e peak i n t e n s i t i e s w i t h those o f p y r i d i n i u m s a l t s i n t h e l i t e r a t u r e (12) i n d i c a t e t h a t a l l t h e sorbed p y r i d i n e Most o f t h e p y r i d i n e (2.7/KU) o f a
has been converted t o p y r i d i n i u m ion.
s t o i c h i o m e t r i c dosed amount (3/KU) was sorbed a t 25'C The PAS spectrum (Fig.
hours.
i n approximately 2.5
6d) shows t h e p y r i d i n i u m i o n t o be t h e major
species although a band c h a r a c t e r i s t i c o f H-bonded p y r i d i n e i s a l s o p r e s e n t a t 1443 cm-l.
On h e a t i n g t h i s sample t o 100°C i n s t a t i c vacuum,
a t t r i b u t a b l e t o t h e p y r i d i n i u m i o n were enhanced (Fig. 1443 cm-'
bands
6e),
w h i l e both t h e
band and t h e background continuum were attenuated,
apparently as a
r e s u l t o f t h e conversion o f t h e p y r i d i n e remaining i n H-bonded form t o t h e p y r i d i n i u m s a l t (Fig. 6 f ) .
Thus,
p y r i d i n e l e s s than s t o i c h i o m e t r i c ,
when HPW a t 25'C
i s exposed t o amounts o f
t h i s sorbate a p p a r e n t l y s u f f e r s l i t t l e o r
no hindrance t o i t s p e n e t r a t i o n i n t o t h e bulk o f t h e s o l i d t o form t h e pyridinium salt.
However,
exposure o f t h e s o l i d t o q u a n t i t i e s o f p y r i d i n e
equal t o o r g r e a t e r than s t o i c h i o m e t r i c produces a hydrogen-bonded complex, p o s s i b l y i n v o l v i n g two-pyridine molecules and one p r o t o n (13,14). E a r l i e r work from t h i s l a b o r a t o r y has
(11 (WH)jpw12Q10(200.)
~
shown t h a t a number o f m e t a l l i c s a l t s o f
\
conversion o f methanol t o hydrocarbons(l5). O f t h e v a r i o u s s a l t s examined those o f
(ei ascdyt
(dl
~
HPW are a l s o e f f e c t i v e c a t a l y s t s f o r t h e
I,
sodium and aluminum were t h e l e a s t and most active,
rpy(ZVKU1
respectively.
The a c t i v i t y ,
as
measured through t h e p r o d u c t i o n o f C, hydrocarbons, was shown t o increase as t h e estimated magnitude o f t h e n e g a t i v e charge on t h e o u t e r a n i o n i c oxygen atoms decreased (a)
Since t h e conversion i s c a t a l y z e d by
ypw1p~(zcaO)
Bronsted acids, t h i s observation i s consist e n t with the calculational results. Fig. 6.
(a) H9PW12O40 (pre-evacuated a t 250'C);
p y r i d i n e a t 25'C
and evacuation;
pyridine (0.94 py/KU);
(b) a f t e r exposure t o excess
(c) as (a) exposed t o a c o n t r o l l e d dose of
(d) as (a) exposed t o a l a r g e r dose o f pyridine
(2.7 py/KU); (e) as (d) a f t e r h e a t i n g i n s t a t i c (f) p y r i d i n i u m s a l t , (pfl)3PWl2O40, pre-evacuated a t 2oO'C
vacum
at
1OO'C;
( f o r comparison).
163 Subsequent s t u d i e s (5) w i t h PAS o f t h e s o r p t i o n o f ammonia on t h e sodium and aluminum s a l t show t h a t p r o t o n s remain i n t h e s e s a l t s ,
as expected b o t h
f r o m t h e methanol c o n v e r s i o n r e s u l t s and f r o m t h e c a l c u l a t i o n s . o f i n t e r e s t t o n o t e t h a t t h e l a t t i c e parameter a.
It i s also
was found t o have values
o f 11.94 and 12.14 A f o r t h e sodium and aluminium s a l t s ,
respectively.
On temperature programmed d e s o r p t i o n o f t h e sodium and magnesium s a l t s
(4), a f t e r p r e t r e a t m e n t a t 25'C a p p r o x i m a t e l y 200'C above 400'C
f o r two hours,
were observed,
( n o t shown).
peaks
(due t o w a t e r )
at
and no d e s o r p t i o n o f water was e v i d e n t
Two endotherms were e v i d e n t i n t h e 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 o f HPW, HPMo, and HSiW, t h e f i r s t between 110 and 125"K, t h e and 162°C f o r HPMo.
second a t 280'C f o r t h e f o r m e r and t h e l a t t e r , were observed a t 615, 460, and 530'C,
Exotherms
respectively.
When HPW was exposed t o p y r i d i n e a t room temperature f o l l o w e d by h e a t i n g up t o 150°C and evacuation,
t h e s t o i c h i o m e t r i c p y r i d i n i u m s a l t was shown by
PAS t o have been obtained.
I n contrast,
when HPW c o n t a i n i n g 2.0 p y r i d i n i u m
ion/KU was exposed t o excess p y r i d i n e a t 250'C an i n c r e a s e t o 2.2 achieved.
and evacuated a t 150'C,
only
p y r i d i n i u m ion/KU and n o t t h e s t o i c h i o m e t r i c amount was
Since t h e e v a c u a t i o n methods i n t h e s e two cases were i d e n t i c a l , i t
i s u n l i k e l y t h a t d e s o r p t i o n i s r e s p o n s i b l e f o r such d i f f e r e n c e s . would have been a n t i c i p a t e d t h a t t h e use o f
Although i t
a h i g h e r t e m p e r a t u r e would
i n c r e a s e t h e r a t e o f p y r i d i n i u m i o n f o r m a t i o n , t h e s e r e s u l t s suggest e i t h e r a structural or s t e r i c i n h i b i t i o n o f the penetration o f pyridine.
It s h o u l d be
n o t e d t h a t such behaviour was found t o be g e n e r a l and n o t r e s t r i c t e d t o t h e pyridinium salt.
To i l l u s t r a t e t h e d i f f e r e n c e i n s o r p t i v e b e h a v i o u r between
p y r i d i n e and ammonia under t h e s e c o n d i t i o n s ,
HPW was
exposed t o excess
t h e aluminum s a l t ( F i g 7a) o f
p y r i d i n e a t 250'C
f o r 1 hour
(Fig.
subsequently exposed t o ammonia under t h e same c o n d i t i o n s (Fig.
7b)
7c).
and While
1.2 ammonium i o n s were shown t o be p r e s e n t p e r Keggin U n i t , o n l y 0.2 p y r i d i n i u m i o n s p e r Keggin u n i t were formed. However, when t h e aluminum s a l t was exposed t o p y r i d i n e a t 25°C and t h e n heated t o 150°C and evacuated, i n c r e a s e t o 1.2 p y r i d i n i u m i o n s p e r Keggin u n i t was observed, w i t h t h e r e s u l t s f o r ammonia.
an
i n agreement
Q u a l i t i t a t i v e l y s i m i l a r o b s e r v a t i o n s were made
w i t h t h e sodium s a l t o f HPW. It should be n o t e d t h a t a t 25°C p y r i d i n e i s unable t o d i s p l a c e w a t e r
f r o m a sample o f HPW p r e v i o u s l y outgassed a t 25'C t h a t p y r i d i n e c o u l d be desorbed p r e v i o u s l y taken
up more
than
into the one
.
TPD s t u d i e s a l s o show
gas phase f r o m HPW which
pyridine
molecule
per
Keggin
However, when s m a l l e r q u a n t i t i e s o f p y r i d i n e were sorbed a t 25'C, o n l y decomposition products, water,
nitrogen,
and carbon d i o x i d e .
had unit.
TPD y i e l d e d
164
7
Fig.
(a) At3+ s a l t (pre-evacuated a t 350'C);
(b) as (a) a f t e r exposure
t o excess p y r i d i n e a t 250'C and evacuation a t 150'C;
(c) as (b) a f t e r
exposure t o excess NH3 a t 250'C and evacuation a t 150'C. Fig.
8
(a) TPR p r o f i l e f o r HPW (b)-(d) and y(0) d u r i n g TPE o f HPW.
Temperature
Programmed Reduction
Mole f r a c t i o n s Dp(g)-(o), (TPR)
and
Temperature
HD(g)(o)
Programmed
Exchange (TPE) experiments provide f u r t h e r information on t h e protons and p r o t o n i c a c i d i t y i n t h e heteropoly acids (Fig. 8). f o r HPW,
The observed TPR peaks
pretreated i n d r i e d helium f o r 2 hours a t 320°C, are s i m i l a r i n
o v e r a l l shape and p o s i t i o n s t o those found i n the TPD experiments, also due t o water.
and were
Because o f the apparent correspondences, peak 2 i n TPR
was also assigned t o water r e s u l t i n g from protons and anionic oxygen.
Peak 1
i n TPR was much smaller than t h a t 'in TPD, and was unexpected i n t h e former because o f t h e high pretreatment temperature.
The r e p o r t by Misono e t al.
(16,17) o f a low temperature uptake o f hydrogen may be r e l a t e d t o t h i s observation.
I t appears t h a t peak 1 i n the TPR experiments may r e s u l t from t h e
water formed by r e a c t i o n o f hydrogen w i t h extremely l a b i l e anionic oxygen atoms. Above 550°C a continuous reduction i s evident f o r HPW (Fig. 8) (450'C f o r HPMo) and subsequent X-ray d i f f r a c t i o n analysis showed t h a t amorphous materials had formed.
Since such continuous reduction began near the decom-
p o s i t i o n temperatures as measured by TPD and DTA and continued t o higher temperatures, these continuously increasing peaks may be a t t r i b u t e d t o the react i o n of hydrogen w i t h the oxides r e s u l t i n g from decomposition o f the acids.
165
Exchange between Dz(g) and HPW was detected at a temperature as low as A s i m i l a r temperature was found w i t h HSiW but no exchange was
353'C.
observed w i t h HPMo although 9 ( g ) was consumed and the acid was reduced i n the l a t t e r case.
Maximum rates o f exchange were observed between 400 and
f o r both HPW and HSiW.
425'C
Hydrogen deuteride was the predominant product
i n the exchange r e a c t i o n s and approximately complete exchange occurred.
The
exchange was noted (Fig. 8) t o be complete before the major p o s i t i o n o f peak
2 had evolved. DISCUSSION
E v i d e n t l y exchange between H20(g) and D2(g) was minimal.
The r e s u l t s o f the various experiments reported here provide i n f o r m a t i o n on a number o f the f a c t o r s which influence t h e proton, i n t e r a c t i o n s i n the heteropoly acids.
i t s m o b i l i t y , and i t s
As noted e a r l i e r ,
Brown e t al.
(1)
have shown t h a t i n HPW the proton i s surrounded by f o u r water molecules, although only hydrogen-bonded t o two o f these molecules at a given time. Since t h e water i s also hydrogen-bonded t o t h e outer oxygen atoms of the anions,
not o n l y does the water block
increases the separation o f the anions.
access t o the protons,
but a l s o
Although p y r i d i n e would be expected
t o bind more s t r o n g l y t o t h e proton, p a r t i c u l a r l y through formation of t h e p y r i d i n i u m ion, p y r i d i n e i s unable t o displace water from the heteropoly acids.
This may be more o f a consequence o f the i n a b i l i t y o f the p y r i d i n e t o
approach s u f f i c i e n t l y c l o s e l y t o the water,
due t o the b l o c k i n g e f f e c t s o f
the water and other d i f f u s i o n a l b a r r i e r s w i t h i n the s o l i d r a t h e r than t o smaller i n t e r a c t i o n energies between the proton and the p y r i d i n e r e l a t i v e t o those between the former and water. E l i m i n a t i o n o f the water molecules may be achieved by increasing t h e temperature t o appropriate
values.
It i s o f
i n t e r e s t t o note t h a t t h e
hydrogen-bonded water i s more r e a d i l y removed from the molybdenum based acids than from those containing tungsten. The source o f t h i s d i f f e r e n c e i s a t present unclear, but presumably i s r e l a t e d t o differences both i n the atomic charges on the outer anionic oxygen atoms and geometrical f a c t o r s w i t h i n t h e secondary structure. The r e s u l t s of the TPD, TPR, and TPE experiments show t h a t protons can be t h e r m a l l y removed from the anhydrous m a t e r i a l s but only accompanied by loss o f anionic oxygen atoms. The ease o f removal o f protons appears t o r e f l e c t the binding energy of the oxygen i n the anion r e l a t i v e t o t h a t o f t h e proton. Larger negative charges on outer oxygen atoms may be associated w i t h less mobile protons and smaller binding energies f o r the oxygen atoms. P a r t i a l exchange of protons w i t h other cations apparently a l t e r both t h e s t r u c t u r a l features of the s o l i d and the c a t a l y t i c properties, presumably as a r e s u l t o f s h i f t s i n e l e c t r o n density.
Cations which are l a r g e r than t h e
166
p r o t o n may have a s h i e l d i n g e f f e c t on t h e anions which i n t u r n may a l t e r t h e d i s t r i b u t i o n o f a c i d s t r e n g t h s and expansion o f t h e secondary s t r u c t u r e may c o n t r i b u t e t o these f a c t o r s . ACKNOWLEDGEMENT The f i n a n c i a l support o f t h e N a t u r a l Sciences and E n g i n e e r i n g Research C o u n c i l o f Canada i s g r a t e f u l l y acknowledged. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16.
G.M. Brown, M.R. N o e - S p i r l e t , W.R. Busing, and H.A. Levy, Acta., Cryst., 533 (1977) 1038. H. Hayashi and J.B. M o f f a t , J.Cata1. 77 (1982) 473. J.B. M o f f a t , J. M o l e c u l a r C a t a l . ( i n p r e s s ) . 8. K. Hodnett and J.B. M o f f a t , J. C a t a l . ( i n p r e s s ) . J.G. H i g h f i e l d and J.B. M o f f a t , J. Catal. ( i n p r e s s ) . H. Knozinger i n The Hydrogen Bond, P. Schuster, G. Zundel, and C. Sandorfy, Vol. 3, Chapt. 27 and r e f e r e n c e s t h e r e i n . North-Holland, Amsterdam. N.E. Sharpless and J.S. Munday, Anal. Chem. 29 (1957) 1619. D.H. Brown, Spectrochim. A c t a 19 (1963) 585. C. R o c c h i c c i o l i - D e l t c h e f f , R. Thouvenot, and R. Franck, Spectrochim. A c t a 32A, 587 (1976). J.G. H i g h f i e l d and J.B. M o f f a t , J. C a t a l . (accepted f o r p u b l i c a t i o n ) . E.P. P a r r y , J. Catal. 2 (1963) 371. D.Cook, Can. J. Chem. 39 (1961) 2009. R. Clements and J.L. Wood, J. Mol. S t r u c t u r e 17 (1973) 265. M. Misono, N. Mizuno, K. Katamura, A. Kasai, Y. K o n i s h i , K. Sakata, T. Okuhara, and Y. Yoneda, B u l l . Chem. SOC. Japan 55 (1982) 400. H. Hayashi and J.B. M o f f a t , J. Catal. 81 (1983) 61. N. Mizuno, K. Katamura, Y. Yoneda and M. Misono, 3. C a t a l . 83 (1983) 384.
P e r m i s s i o n t o use c e r t a i n f i g u r e s has been k i n d l y g r a n t e d by Academic Press and t h e I n t e r n a t i o n a l Union o f C r y s t a l l o g r a p h y .
B. Imelik et al. (Editors), Catalysis b y Acids and Bases b 1985 Elsevier Science Publishers B.V., Amsterdam -F'rinted in The Netherlands
167
HETEROPOLYACIDS AS SOLID-ACID CATALYSTS Y. ONO, M. TAGUCHI, GERILE, S. SUZUKI, and T. BABA Department of Chemical Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152 (Japan)
ABSTRACT Heteropolyacids and their metal salts are active catalyst for methanol conversion. The activity of the silver salt is enhanced by the presence of gaseous hydrogen. Generation of acid sites in the silver salt by its interaction with hydrogen was discussed. Interaction of hydrogen with group VIII metals induces high solid acidity as evidenced by enhancement o f the catalytic activity for methanol conversion. Bifunctional catalysis in alkane isomerization is also described. INTRODUCTION Heteropolyacids like dodecatungstophosphoric acid (H3p[(12040,~~p) and dodecatungstosilicic acid (H SiW12040, HTS) are highly active for.conversion of 4 methanol into hydrocarbons. It was also found that silver salts of HTP and HTS are more active than the corresponding parent acid. The activity for methanol conversion was greatly enhanced by using heteropolyacid in conjunction with a group VIII metal and gaseous hydrogen. The purpose of the present review is to describe unique feature of heteropolyacid as solid-acid catalyst, which are found in catalysis for methanol conversion. Special emphasis will be placed on the role o f hydrogen in generating acid sites in the silver salts and also in heteropolyacid-metal systems. Isomerization o f alkanes by heteropolyacid-group VIII metal catalyst is also described. CONVERSION OF METHANOL OVER HETEROPOLYACIDS Zeolites, especially ZSM-5, are very effective catalysts for converting methanol into hydrocarbons. Ono and Mori have studied the mechanism of the conversion and have concluded that Bronsted acid sites are sole responsible sites for the conversion (ref.1). This led to the idea that methanol conversion should proceed also over solid substances other than zeolites, if they are highly acidic. In fact, HTP and HTS are found to be very active for the conversion (refs.1-3). The high catalytic activity of HTP seems to be rather unusual, considering the small surface area of HTP (4 rn'g-'). To determine the effective number of active sites of HTP, the effect of pyridine sorption was examined. The reaction
168
was s t a r t e d by feeding methanol and n i t r o g e n a t 573 K. After 2 h, p y r i d i n e was added t o t h e f e e d , which was f i n a l l y changed back t o t h e s t a r t i n g mixture. The c a t a l y t i c a c t i v i t y a f t e r p y r i d i n e s o r p t i o n decreased l i n e a r l y with t h e amount of t h e sorbed pyridine, t h e complete l o s s o f t h e a c t i v i t y being a t t a i n e d when t h e molar r a t i o of t h e sorbed p y r i d i n e and HTP used a s c a t a l y s t was t h r e e . The amount of a c i d thus estimated i n d i c a t e s t h a t a l l t h e protons i n s o l i d HTP a r e a c c e s s i b l e t o p y r i d i n e molecules and a l s o t o methanol molecules. The
s t o i c h i o m e t r i c absorption of p y r i d i n e by heteropolyacids a t 573 K was reported e a r l i e r ( r e f s . 4 , 5 ) . Since methanol and o t h e r p o l a r molecules a r e known t o be adsorbed by HTP even a t room temperature ( r e f s . 6 , 7 ) , it i s presumed t h a t methanol molecules p e n e t r a t e i n t o t h e bulk o f s o l i d HTP a t t h e 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 n t h e bulk. This explains t h e high a c t i v i t y of HTP i r r e s p e c t i v e of i t s small s u r f a c e a r e a . To confirm t h a t t h e phenomenon i s not s p e c i f i c t o t h e conversion of methanol, t h e number o f a c i d s i t e s e f f e c t i v e f o r t h e dehydration o f 1-butanol was d e t e r mined i n a s i m i l a r manner ( r e f . 8 ) . The amount of a c i d s i t e s responsible f o r t h e dehydration i s again t h r e e times a s l a r g e a s t h e amount of HTP used a s c a t a l y s t . Thus, t h e dehydration of 1-butanol i s a l s o presumed t o proceed i n t h e bulk of t h e s o l i d . The “pseudo-licpiid behavior o f heteropolyacids have been well described by Misono and coworkers ( r e f . 9 ) . CONVERSION OF METHANOL OVER METAL SALTS OF HETEROPOLYACIDS Besides heteropolyacids, metal salts o f heteropolyacids have a c t i v i t i e s f o r many r e a c t i o n s , f o r which Bronsted a c i d s i t e s a r e supposed t o be responsible ( r e f s . 5 , l O ) . Therefore, t h e c a t a l y t i c a c t i v i t i e s of various metal s a l t s o f HTP and HTS f o r t h e conversion o f methanol were examined a t 573 K ( r e f s . l l , l 2 ) .
The a c t i v i t i e s o f t h e s e r i e s of metal s a l t s of HTP and HTS a r e l i s t e d a s follows
For s a l t s of HTP Ag(98) > 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<
H, CiS
’CH3
c=c
9 H‘
Reaction mechanisms o f i s o m e r i z a t i o n o f butenes on s-ZrP.
3
trans
190
higher temperatures(700 - 1100 K), the i n i t i a l &/trans r a t i o was 1 ; t h e rea c t i o n proceeds on the residual phosphate groups. I n t h i s c a s e , the reaction intermediates of isomerization of butenes a r e common secondary butyl carbenium ions [I 31. Because o f the presence of P-0-P bonds a f t e r evacuation a t higher temperat u r e s ( s e e Fig. 31, t h e protonic character of the residual phosphate g r o u p s ( ~ 2% of original E - Z r P ) which a r e located on t h e c r y s t a l surfaces must be extremely high as i n the following scheme. Since P-0-P bonds could accumulate t h e e l e c t r o n s from the residual phosphate groups. H
H+ 0-
0
\p/
’
0
0‘
Scheme 1 I
-PI
Evacuated a t 300
-
600 K
I
-PI
Evacuated a t 700
-
1100 K
REFERENCES 1 G. A l b e r t i , Acc. Chem. Res., 11 (1978) 163. 2 A. C l e a r f i e l d and D.S. Thakur, J . Catal., 65 (1980) 185. 3 D.S. Thakur and A. C l e a r f i e l d , J . Catal., 69 (1981) 230. 4 T. H a t t o r i , A. Ishiguro, and Y . Murakami, Nippon Kagaku Kaishi, (1977) 761. 5 A. C l e a r f i e l d and J.A. Stynes, J. Inorg. Nucl. Chem., 26 (1964) 117. 6 A. C l e a r f i e l d , A.L. Landis, A.S. Medina, and J.M. Troup, J . Inorg. Nucl. Chem., 35 (1973) 1099. 7 J.M. Troup and A. C l e a r f i e l d , Inorg. Chem., 16 (1977) 3311. 8 S.E. Horsley and D.V. Nowell, J . Appl. Chem. Biotechnol., 23 (1973) 215. 9 D.E.C. Corbridge and E.J. Lowe, J . Chem. S O C . , (1954) 493. 10 S.E. Horsley, D.V. Nowell, and D.T. Stewart, Spectrochimica Acta, 30A (1974) 535. 11 J.W. Hightower and W.K. H a l l , J . Phys. Chem., 72 (1968) 4555. 12 A. Kayo, T. Yamaguchi, and K. Tanabe, J. C a t a l . , 83 (1983) 99. 13 J.W. Hightower and W . K . H a l l , J . Phys. Chem., 71 (1967) 1014. 14 W.O. Haag and H. Pines, J . Am. Chem. SOC., 82 (1960) 2488. 15 J . Medema, J . Catal., 37 (1975) 91.
B. Imelik et ul. (Editors), Catulysis b y Acids and Bases
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0 1985 Elsevier Science Publishers B.V.,Amsterdam -F'rinted in The Netherlands
CALORIMETRIC STUDY OF ADSORPTION OF AMMONIA AT 420 K ON BISMUTH MOLYBDATE ( 2 : l ) L. STRADELLA
l s t i t u t o di Chimica Generale ed Inorganica, Facoltd di Farmacia Universitd di Torino, Via P. Giuria, 9 - 10125 TORINO ( I t a l y )
ABSTRACT Adsorption-desorption cycles of ammonia on Bi203-Mo03 samples , reduced or almost stoichiometric, have been performed a t 420 K by means of a microcalorimeter. Practically irreversible adsorption of ammonia occurs on the reduced specimen which shows a fraction of highly energetic s i t e s (qdiff 150 kJ/mol). The surface concentration of such s i t e s seems however the same on b o t h samples.
SOMMAIRE On a effectue des mesures calorimetriques concernant l'adsorption e t l a d@s o r p t i o n d'ammoniac 1 420 K sur B i 03 MoO3, reduit ou stoechiometrique. L'ammoniac e s t adsorbe presque irreversi2lement sur 1 '@chantillonr @ d u i t qui presente une fraction des s i t e s d haute energie (qdiff 150 kJ/mol). La concentration de ces s i t e s sur la surface semble neanmoins l a mPme pour les deux echantillons. INTRODUCTION I t is recently t h a t acid-base property of metal oxides or mixed metal oxides surfaces have been measured w i t h accuracy. For some solids, e.g. clay minerals or s i l i c a alumina, several investigations have led t o a satisfactory description of the nature o f the acid s i t e s . For other mixed metal oxides, on the contrary, the surface acidity i s l e s s defined and the participation of the acidic s i t e s t o the c a t a l y t i c process (e.g. an oxidation reaction) i s s t i l l a moot question (ref.1). In the present report we give some preliminary results of a calorimetric investigation of theacidity of a bismuth molybdate ( 2 : l ) sample : the adsorption of ammonia has been used t o show the change of surface acidity resulting from a reduction treatment of the sample. EXPERIMENTAL The sample of Bi203-Mo03, supplied by the I n s t i t u t de Recherches sur l a Catalyse (M. Forissier) was prepared according t o Batist e t a l . ( r e f . 2 ) , i t s X-ray diagram was coincident w i t h that cited i n l i t e r a t u r e (ref.3), i t s specific surface area amounts t o 7.8 m2/g. The oxidized sample (cited here as (Bi203.Mo03)ox was standardized by heating i n va~uum(lO-~Pa) f o r 30 mil a t 473 K and by
192
oxidizing a t 623 K f o r 1 hour under 6650 Pa of oxygen. The reduced sample, f o r which we shall use the symbol (Bi203.M003)red was obtained by s u b m i t t i n g the oxidized sample t o a pressure of 6650 Pa of hydrogen a t 623 K f o r 30 min. The reduction percentage of such a specimen i s very low (0.91%) ( r e f . 4 ) . The adsorption measurements were performed a t 420 K, employinga Tian-Calvet microcalorimeter associated t o a vacuum l i n e f r e e grease. The ammonia employed was spectroscopically pure. RESULTS AND DISCUSSION In f i g . 1 and 2 the calorimetric adsorption-desorption isotherms of ammonia (e.g; the integral heats of adsorption as a function of equilibrium pressures) and (Bi203.Mo03)ox. The reduced samples show more are shown f o r (Bi203.M003)~~d evolved heats (as well as greater adsorbed amount i n the corresponding volumet r i c isotherms n o t given here) a t each equilibrium pressure. From the desorption branch one may note t h a t f o r (Bi203.MO03)red a great p a r t of amnonia is irreversibly adsorbed (95%), while f o r (Bi203.Mo03)ox only a 10% i s not desorbed. A quite similar hysteresis and about the same r a t i o (99%) between reversibly and irreversibly adsorbed ammonia have been obtained w i t h a hexagonal molybdenum oxide, Mo03(H) ( r e f . 5 ) .
Fig. 1.
Calorimetric adsorption ( empty points ) and desorption ( f u l l points) isotherms of amnonia on (Bi203.MO03)red a t 420 K.
193
Q'
I
I
[ I
I
I
Fig.2. Calorimetric adsorption (empty points) and desorption ( f u l l points)
isotherms of amnonia on (Bi203.Mo03)ox. In f i g . 3 the d i f f e r e n t i a l heats of adsorption as a function of adsorbed quantities are given : (Bi203.Mo03)red shows much greater d i f f e r e n t i a l heats of adsorption t h a n (Bi203.Mo03)ox along the whole coverage fraction ; the most energetic s i t e s surface concentration, on the contrary, i s practically the same on both samples.
1
I
1
I
1
I
Fig.3.
Differential heats of NH3 adsorption
194
Further i n s i g h t i n t o the ammonia adsorption process may be obtained from the shape of the heat emission peaks. I t i s well known t h a t t h e heat f l u x curves depend on the k i n e t i c s of t h e process i n a complex manner and several corr e c t i o n s a r e needed t o have the t r u e thermokinetics ( r e f . 1 2 ) b u t here we want only q u a l i t a t i v e information from a comparison of t h e half deviation times of t h e emission peaks as a function of the equilibrium pressure f o r the two specimens ( s e e f i g . 4 ) .
- - - }L ; 1
I
I
20
10
1I
30 p~ ( Pa
lo-*)
Fig. 4. Half deviation times of t h e calorimetric peaks vs. t h e equilibrium pressures: [3 (Ei203.Mo03) red A (Bi203-Mo03)
ox
The heat emission times a r e in both cases g r e a t e r than the time constant of the microcalorimeter (about 300 s e c ) i n d i c a t i n g t h a t a c t i v a t e d processes have occurred: the phenomenon i s p a r t i c u l a r l y evident f o r t h e (Bi203.M003)red, which e x h i b i t a q u i t e d i f f e r e n t k i n e t i c behaviour in comparison with (Bi203.Mo03)ox. The high d i f f e r e n t i a l heats f o r the reduced sample ( a 1 5 0 kJ/mol) should not be due t o an oxidation surface r e a c t i o n , t h a t could imply a mechanism o f ammonia d i s s o c i a t i o n on a cation vacancy ( r e f . 7): in f a c t i t seems well estabiished t h a t such a reaction shows s i g n i f i c a n t r a t e on a Bi-Mo oxide s t a r t i n g from 673 K ( r e f . 6 ) . Several s t u d i e s of adsorption of ammonia on t i t a n i a ( r e f . 8 ) , on alumina ( r e f . 9 ) , on s i l i c a ( r e f . l o ) , have suggested a d i s s o c i a t i v e chemisorption on Lewis acid s i t e s . I n the case of ammonia adsorbed on molybdenum oxide a strong coordination with Mo atoms has been proposed in the i n t e r p r e t a t i o n of TPD a n d
195
I . R . measurements ( r e f . 5 ) . The p r e s e n t r e s u l t s c o u l d be i n t e r p r e t e d assuming an i n t e r a c t i o n o f ammonia w i t h o x i d e s u r f a c e s t r o n g Lewis a c i d s i t e s : t h e f a c t t h a t t h e most e n e r g e t i c f r a c t i o n o f these s i t e s i s p r e s e n t on b o t h o x i d i z e d and reduced samples suggests t h a t Lewis s i t e s p r o b a b l y a r i s e f r o m m e t a l l i c atoms i n d i f f e r e n t s t e r e o c h e m i c a l environments. T h i s p i c t u r e , as p r e v i o u s l y proposed f o r w a t e r a d s o r p t i o n ( r e f . 1 1 ) m i g h t be c o n s i s t e n t w i t h a s u r f a c e s t r u c t u r e e x h i b i t i n g a t h i n l a y e r c h a r a c t e r i z e d by s u r f a c e domains h a v i n g some s t o i c h i o m e t r i c d e f e c t s . REFERENCES
D. Barthneuf, F. F i g u e r a s , i n J.L. P o r t e f a i x , F. F i g u e r a s (Eds), Chemical and P h y s i c a l Aspects o f C a t a l y t i c O x i d a t i o n , CNRS, P a r i s , 1980. 2 Ph. B a t i s t , J.G.H. Bowens and G.C.A. S c h u i t , J . C a t a l . , 25 (1972) 1 . 3 A.F. Van E l z e n and G.D. Rieck, Acta C r i s t a l l . Sec. B, 29 (1973) 2436. 4 L. S t r a d e l l a and G. V e n t u r e l l o , Proc. 7 t h ICTA, Kingston, O n t a r i o , 1982, A. M i l l e r (Ed.), W i l e y Heyden Co, England. 5 N. Sotani, S . Masuda, Y. Iwata, H. Hasegawa, i n H.F. B a r r y , P.Ch. M i t c h e l l (Eds.) Chemistry and Uses o f Molybdenum, Climax Molybdenum C. Ann Arbor, 1970, p. 132. 6 J.E. Germain, R . Perez, B u l l . SOC. Chim. F r . , 3 (1975) 739. 7 A.W. S l e i g h t , i n J.J. B u r t o n , R.L. Garten ( E d s . ) Advanced M a t e r i a l s i n C a t a l y s i s , A.P. 1977. 8 G.D. P a r f i t t , J. Ramsbotham, C.H. Rochester, Trans. Far. SOC., 67 (1965) 231. 9 J.B. P e r i , J . Phys. Chem., 69 (1965) 231. 10 3.8. P e r i , J . P h i s . Chem., 70 (1966) 2937. 11 L. S t r a d e l l a , G.F. V o g l i o l o , Z . F u r Phys. Chemie, N.F., 137 (1983) 99. 12 C . B r i e , J.L. P e t i t , P.C. G r a v e l l e , J . Chim. Phys., 72 (1975) 66. 1
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B. Imelik et al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
197
SKELETAL ISOMERIZATION OF N-BUTENE OVER MODIFIED BOKON PHOSPHATE BJORN PETTER NILSEN', 'Statoil,
P.O.
MICHAEL STOECKER2 and TRYGVE R I I S '
Box 300 Forus, N-4001 Stavanger (Norway)
%epartment o f Petrochemistry, Central I n s t i t u t e f o r I n d u s t r i a l Research, P.O.
Box 350 Blindern, N-0314 Oslo 3 (Norway)
ABSTRACT The 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, catalyzed by boron phosphate, s i l i c a t e d boron phosphate, s i l i c a t e d alumina and boron phosphate supported on both alumina and s i l i c a t e d alumina was studied a t 475°C. The s i l i c a t i o n o f the boron phosphate seems t o s t a b i l i z e t h e surface area and s u b s t a n t i a l l y improves t h e c a t a l y t i c a b i l i t i e s o f t h e system. I n t h e isomerization r e a c t i o n , t h e s i l i c a t e d boron phosphate maintained high s e l e c t i v i t y throughout t h e run, i n opposition t o the n o n - s i l i c a t e d c a t a l y s t . Furthermore, analysis of higher by products ( C + ) i s o f g r e a t importance concerning the s e l e c t i v i t y of the c a t a l y s t s . Z i l i c a t e d boron phosphate produce mainly Cg as by products, w h i l e s i l i c a t e d alumina cracks the higher compounds t o C5, Cgy C7 etc. RESUME L ' i s o m e r i s a t i o n du n - b u t h e en isobutene, catalysee par i e phosphate de bore, l e phosphate de bore s i l i c a t e , A1 0 e t l e phosphate de bore support6 sur A1203 ou A1203 s i l i c a t e e a 6 t e etudiee i75.C. La surface specifique e s t s t a b i l i s e e e t l e s proprietes c a t a l y t i q u e s sont considerablement a m @ l i o r e e s lorsque l e phosphate de bore e s t s i l i c a t e . On observe en p a r t i c u l i e r une s e l e c t i v i t e elevee pour l ' i s o b u t e n e pendant t o u t e l a r e a c t i o n d ' i s o m e r i s a t i o n , contrairement au catalyseur non s i l i c a t e . L'analyse des p r o d u i t s lourds ( C5) met en evidence des d i f f e r e n c e s de s e l e c t i v i t e : l e phosphate de bore s i l i c a t e p r o d u i t p r i n c i p a l e ment des C, comme sous p r o d u i t s e t A1203 s i l i c a t e des Cg, C,; C 7 e t c ... INTRODUCTION The branched chain o l e f i n e s i n general, and e s p e c i a l l y isobutene, studied i n our paper, have a wide v a r i e t y o f u t i l i t i e s . For example, isobutene may be converted t o methyl-tert.-butyl
ether, a h i g h octane gasoline a d d i t i v e , by
methods we1 1 known i n t h e 1iterature. Branched chain o l e f i n s are a l s o useful as s t a r t i n g m a t e r i a l i n numerous chemical processes, e.g.
, as
a l k y l a t i n g agents f o r the a l k y l a t i o n o f aromatic
hydrocarbons, phenols and a1 i p h a c i c nydrocarbons, and as monomers in polymeriz a t i o n and co-polymerization reactions w i t h a wide range o f c a t a l y s t s t o produce various rubber and p l a s t i c m a t e r i a l s [l]. Much work has been c a r r i e d o u t on the s k e l e t a l isomerization o f n-outene by
198
t h e use o f d i f f e r e n t c a t a l y s t s such as metal h a l i d e s , alumina, aluminos i l i c a t e s , phosphoric a c i d c a t a l y s t s , f l u o r i n a t e d aluminas and phosphate c a t a l y s t s t21. Among t h e phosphate catalysts,,boron
phosphate has been o f i n t e r e s t as an
i n d u s t r i a l c a t a l y s t f o r t h e i s o m e r i z a t i o n o f n-butenes. Mcrieil and Reynolds [31 r e p o r t e d t h a t a s u p p o r t e d o r unsupported boron phosphate a t 350-600°C i s o m e r i s e s s t r a i g h t c h a i n t o branched c h a i n o l e f i n s . U n f o r t u n a t e l y , t h e most used method f o r s y n t h e s i s o f boron phosphate ( f r o m a m i x t u r e o f b o r i c and phosphoric a c i d s ) does n o t l e a d t o specimens w i t h a l a r g e s u r f a c e . The s u r f a c e can b e i n c r e a s e d by u s i n g a l k y l d e r i v a t i v e s of b o r i c a c i d as t h e i n i t i a l m a t e r i a l s [4,5]. I n t h e p r e s e n t paper we d e s c r i b e t h e r e s u l t s o f m a x i m i z i n g b o t h s u r f a c e a r e a and c a t a l y t i c a c t i v i t y o f t h e b o r o n phosphate. Furthermore, o u r a t t e m p t s t o s t a b i l i z e t h e surface o f t h e c a t a l y s t by promoting w i t h s i l i c o n a r e reported. The r e s u l t s were d i s c u s s e d on t h e b a s i s o f s u r f a c e p r o p e r t i e s , p r e p a r a t i o n procedures and c a t a l y t i c a c t i v i t i e s .
RESULTS AND DISCUSSION The r e s u l t f s o f t h e i s o m e r i z a t i o n r e a c t i o n s a r e summarized i n Table 1, t o g e t h e r w i t h t h e p r e p a r a t i o n methods and t h e measured s u r f a c e areas f o r t h e
i-2. As
i n t h e Table, t h e BP04 samples were s y n t h e s i z e d i n 3- showed h i g h i n i t i a l a c t i v i t y and s e l e c t i v i t y . B u t t h e s e l e c t i v i t y d e c l i n e d r a p i d l y and t h e c a t a l y s t - and 2 - were found showed q u i c k d e a c t i v a t i o n . The two l o w s u r f a c e a r e a c a t a l y s t s 1 t o have t h e same c a t a l y t i c b e h a v i o u r , t h a t means l o w c o n v e r s i o n and l o w y i e l d o f catalysts
shown
d i f f e r e n t ways, and t h e h i g h s u r f a c e a r e a c a t a l y s t
i s o b u t e n e . Obviously, t h e d i f f e r e n t c o n v e r s i o n s o f t h e systems
1-2- a r e
due t o
d i f f e r e n t s u r f a c e areas o f t h e c a t a l y s t s . The p r e p a r a t i o n c o n d i t i o n s , which a f f e c t e d 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 e 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, were s t u d i e d f o r t h e s i l i c a t e d b o r o n phosp h a t e s (no. 4-g). The c a t a l y s t s were p r e p a r e d under d i f f e r e n t c o n d i t i o n s v a r y i n g t h e c a l c i n a t i o n and s i l i c a t i o n temperature, t h e s i l i c a t i n g reagent, t h e steaming c o n d i t i o n s f o r t h e s i l i c a t e d BP04 and t h e P / B - r a t i o .
I t turned o u t t h a t
these f a c t o r s were o f s m a l l i m p o r t a n c e f o r t h e performance o f t h e s i l i c a t e d b o r o n phosphate c a t a l y s t as l o n g as t h e s i l i c o n c o n t e n t was i n t h e range o f 3-15%. Steaming o f t h e s i l i c a t e d b o r o n phosphate (no. l 2 )as w e l l as enhancement o f t h e P / B - r a t i o (no. 1_4) - l e d t o a decrease i n t h e s u r f a c e a r e a . These c a t a l y s t s (no. 12 and l4) showed a decrease i n a c t i v i t y b u t m a i n t a i n e d o r d e c l i n e d o n l y i n a small e x t e n t t h e s e l e c t i v i t y . The e x a m i n a t i o n of o u r r e s u l t s l e d t o t h e c o n c l u s i o n t h a t t h e s i l i c a t i o n o f t h e boron phosphate seems t o s t a b i l i z e t h e s u r f a c e a r e a and s u b s t a n t i a l l y improves t h e c a t a l y t i c a b i l i t i e s of t h e system. C a l c i n a t i o n of t h e s i l i c a t e d
199
boron phosphate a t h i g h temperature gave a c a t a l y s t w i t h much h i g h e r s u r f a c e area t h a n t h e n o n - s i l i c a t e d boron phosphate. I n t h e i s o m e r i z a t i o n r e a c t i o n , t h e s i l i c a t e d boron phosphate m a i n t a i n e d h i g h s e l e c t i v i t y t h r o u g h o u t t h e run, i n o p p o s i t i o n t o t h e n o n - s i l i c a t e d c a t a l y s t . The y i e l d o f i s o b u t e n e was h i g h o n l y f o r t h e s i l i c a t e d boron phosphate and n o t f o r t h e n o n - s i l i c a t e d one. Alumina, a w e l l known s k e l e t a l i s o m e r i z a t i o n c a t a l y s t , was s i l i c a t e d w i t h d i f t e r e n t amounts o f t e t r a e t h o x y s i l a n e . None o f t h e s i l i c a t e d a l u m i n a c a t a l y s t s showed h i g h e r s e l e c t i v i t y t h a n alumina i t s e l f . The c a t a l y s t s w i t h a l o w degree o f s i l i c o n showed much h i g h e r conversions and y i e l d s o f i s o b u t e n e t h a n alumina. I n t h e range 0.75 t o 6.1 % Si02, t h e a c t i v i t y i n c r e a s e d w i t h s u r f a c e s i l i c o n amount, b u t t h e s e l e c t i v i t y t o i s o b u t e n e decreased. I n c r e a s e d c a l c i n a t i o n temperature o f t h e c a r r i e r decreased t h e OH-group c o n t e n t and t h e maximum s i l i c o n c o n c e n t r a t i o n . The e f f e c t s on a c t i v i t y and s e l e c t i v i t y were s m a l l . The s i l i c a t i o n o f a l u m i n a improved t h e d e a c t i v a t i o n p r o f i l e of alumina. The r e s u l t s o f t h e i s o m e r i z a t i o n o f n-butene c a t a l y z e d by b o r o n phosphate s u p p o r t e d on a l u m i n a were depending on t h e p r e p a r a t i o n procedures f o r t h e c a t a l y s t s . The b e s t c a t a l y s t s showed s i m i l a r a c t i v i t y and b e t t e r s e l e c t i v i t y compared t o s i l i c a t e d alumina. The a c t i v i t y p a t t e r n compared t o b o r o n phosphate c a t a l y s t s showed t h a t t h e r e were no a c t i v e B-P s i t e s on t h e s u r f a c e . Most p r o b a b l y boron phosphate m o d i f i e s t h e a l u m i n a s u r f a c e a b o u t t h e same way as s i l i c o n does. Concerning t h e b o r o n phosphate s u p p o r t e d on s i l i c a t e d alumina as a c a t a l y t i c system, two c a t a l y s t s w i t h d i f f e r e n t amounts o f s i l i c o n were s y n t h e s i z e d . The c a t a l y s t w i t h a l o w degree o f s i l i c o n showed an improved d e a c t i v a t i o n p r o f i l e and gave h i g h e r y i e l d s o f i s o b u t e n e and i n c r e a s e d c o n v e r s i o n a f t e r a w h i l e on s t r e a m t h a n t h e system w i t h a h i g h s i l i c o n c o n t e n t . O l i g o m e r i z a t i o n and c r a c k i n g o f o l i g o m e r p r o d u c t s a r e s e v e r e problems i n t h i s system. To o b t a i n t h e most c o r r e c t v a l u e o f s e l e c t i v i t y t o i s o b u t e n e , i t i s necessary t o t a k e compounds up t o Cl0-Cl2
i n t o account. A n a l y z i n g t h e s e h i g h e r
compounds and s e p a r a t i n g butenes a r e a l m o s t i m p o s s i b l e
on one GC column,
SO
a
system w i t h two d i f f e r e n t c a p i l l a r y columns on a d u a l channel GC was e s t a b l i s h e d . The s e l e c t i v i t y dropped w i t h about 10 % w i t h a l l the catalysts, b u t t h e by-product p a t t e r n s showed some i n t e r e s t i n g d i f f e r e n c e s : The s i l i c a t e d b o r o n phosphate produced m a i n l y C8 as,by- p r o d u c t s , whereas t h e s i l i c a t e d a l u m i n a formed b y p r o d u c t s from C5 t o h i g h p r o d u c t s , i n d i c a t i n g t h a t t h e c r a c k i n g a c t i v i t y was l a r g e r .
EXPERIMENTAL The i s o m e r i z a t i o n o f n-butene was c a r r i e d o u t i n a f i x e d bed r e a c t o r a t 475OC w i t h a WHSV o f 2.
TABLE 1 S k e l e t a l i s o m e r i z a t i o n o f n-hutene over m o d i f i e d boron phosphates -
Calcination temperature
No.
Catalyst
Preparation
(OC)
1
BP04
H3P04+H3B03
300
Surface
6
Time on stream (hr)
Yield o f isobutene
Selectivity t o isobutene
(%I
(%I
1
6
2 1 2 1
1
24
6
2
52
4
1 6
32
90
36
92
11
1.5
10 31
95
33
6.25
16
92
17
1
36
81
44
6
10
91
11
1
33
90
37
6.25
10
86
11
1 6.25
32
96
33
10
97
10
1
31
89
35
9
36
10
6
2-
BP04
-
BP04
4-
Si/BP04
commerci a1 (A1 f a )
BP04+(EtO)4Si
300
10.5
500
62
200
105
9.3% Si02
5-
Si/BP04
BP04+( E t O ) 4 S i
300
110
5.7% Si02 a ) -
Si/BP04
BP04+(EtO)4Si
7
Si/BP04
BP04+( E t O ) 4 S i
8
Si/BP04
BP04+( E t O ) 4 S i
400
93
500
80
550
175
8.7% Si02 a ) 2.3% Si02 a) s i l i c a t i o n temp. 12o0, 15.6% Si02
9-
Si/BP04
BP04+(Et0)4Si s i l i c a t i o n temp. 160°, 15.5% S i 0 2
(cant. )
550
183
1
6.25
Conversion (%)
a7
2
60
1
90
2
56
1
92
27
TABLE 1 (cont.) ~
~
No.
Catalyst
10
Si/BP04
11 12 13 14 15 16
1_7 13
Preparation
BP04+( Et) ) C1 14.4% Si02 Si/BP04 BP04+SiC14 10.1% Si02 Si/BP04 BP04+(Et0)4Si steamed, 12.3% Si02 Si/BP04 BP04+(EtO)4Si P/B=0.94, 3.6% Si02 Si/BP04 BP04+(Et0)4Si P/B = 1.25, 14.7% Si02 Si/A1203 A1 203+( EtO)4Si 0.75% Si02 Si/A1203 A1203+(EtO)4Si 2.70% Si02 Si/A1203 A1203+(EtO)4Si 6.10% S i 0 2 BP04 on A1203 H3P04+B(OPr)3/A1203
- BP04 on A1203 19 (cent.)
5% aq. BPO4/Al2o3
Calcination Surface temoerature (OC) (m ae! /!3) 550
146
550
38
550
44
550
119
550
66
400
203
400
173
400
199
550
180
550
179
Time on stream (hr) 1 6 1 6 1 6 1 6 1 6.5 1 6 1 6 1 6 1 5.5 1 6
Yield o f isobutene
Sel ecti vi ty to isobutene
(%I 35 25 .35 19 25 9 34 14
30 8 32 17 35 33 31 34 30 21 29 19
~
~~
Conversion
(%I 77 93 83 91 97 97 82 90 93 39 72 78 72 76 55 70 82 85 84 85
45 27 42 21 26 9 42 16 32
9 44 22 49 44 56 49 36
25 34
22
KI
TABLE 1 ( c o n t . )
No.
Catalyst
0
tu
Preparation
Calcination temperature (OC)
Surface area (m2/!3)
Time on stream (hr)
Yield of isobutene
(%I
~~~~
~
20
' 21
BP04 on H3P04+B(OPr)3/Si/A1203 Si/A1203 0.75% S i 0 2 BP04 on H3PO4+B(0Pr),/Si/Al2O3 Si/A1 203 1.85% S i 0 2
550
164
550
163
a ) The boron phosphate c a r r i e r was heat-treated before s i l i c a t i o n .
1 5.5 1 5.5
34 23
32 27
Sel ec ti vi t y t o isobutene (%)
-
79 83 82
85
Conversion
(%I ~43 34 39 32
203 As an example f o r t h e p r e p a r a t i o n o f t h e c a t a l y s t s , t h e s y n t h e s i s o f s i l i c a t e d boron phosphate i s described as f o l l o w s : BP04 was o b t a i n e d by s t i r r i n g a s o l u t i o n o f H3P04 (85%) and B(OPr)3 a t 12OoC f o r 1 hour. The BP04 was d r i e d under vacuum a t 100°C f o r - 3 hours. Afterwards, BP04 was mixed w i t h Si(OEt)4 under argon atmosphere and k e p t a t room temperature f o r 5 hours. The s u r p l u s Si(OEt)4 was removed and t h e c a t a l y s t was d r i e d under vacuum f i r s t a t room temperature f o r 1 hour and l a t e r a t 7OoC f o r 1 hour. A f t e r treatment a t 2OO0C f o r 16 hours i n n i t r o g e n t h e c a t a l y s t was heated up t o 55OoC f o r 0.75 h i n n i t r o g e n and, f i n a l l y , k e p t a t 55OoC f o r 5 hours i n a i r . The a n a l y s i s o f t h e C4 f r a c t i o n was c a r r i e d o u t on a PLOT c a p i l l a r y column. During a n a l y s i s i n c l u d i n g h i g h e r compounds, t h e t r a n s f e r 1 i n e from r e a c t o r t o GC was heated and samples i n j e c t e d t o a SP 2100 c a p i l l a r y column and a PLOT
column w i t h s h o r t t i m e i n t e r v a l s . ACKNOMLEDGEMENT Our thanks a r e due t o Norsk Agip A/S f o r sponsoring t h i s p r o j e c t and t o D r . B. N o t a r i and coworkers, Assoreni, Milano ( I t a l y ) f o r h e l p f u l discussions. REFERENCES 1 A.J. Reid and K.R. Olson, I i o r l d I n t e l l e c t u a l Property Organization, I n t . Appl. Publ. under t h e Patent Cooperation Treaty, N O 82/03136 (30.09.1932). 2 V.R. Choudhary, Chem. I n d . Devel. I n c o r p . (1974) 32-41. 3 D.McNeil and P.W. Reynolds, US Patent 2,554,202 (22.05.1951). 4 L.E. K i t a e v and A.A. Kubasov, V e s t n i k Moskovskoso - U n i v e r s i t e t a , Khimiva, 32 (1977) 269-276. 5 J.B. Moffat, C a t a l . Res. - Sci. Eng., 18 (1978) 199-258.
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205
B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V..Amsterdam -Printed in The Netherlands
CATALYTIC APPLICATION OF HYDROPHOBIC PROPERTIES OF HIGH-SILICA ZEOLITES
11. ESTERIFICATION OF ACETIC A C I D WITH BUTANOLS S. NAMBA, Y . WAKUSHIMA, T. S H I M I Z U , H. MASUf4OTO and T. YASHIMA Department o f Chemistry, Tokyo I n s t i t u t e o f Technology Ookayama, Meguro-ku, Tokyo 152, Japan
ABSTRACT 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 c i d w i t h n-, i-o r t - b u t a n o l on h i g h - s i l i c a z e o l i t e s was i n v e s t i g a t e d . HZSM-5 and dealuminated HY 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 t o g e t h e r w i t h c a t i o n exchange r e s i n s were used as c a t a l y s t s . A l t h o u g h t h e non-dealuminated HY z e o l i t e was h a r d l y 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 e dealuminated HY and HZSM-5 z e o l i t e s were a c t i v e and t h e i r a c t i v i t i e s changed w i t h t h e i r Si/A1 r a t i o s . The a c t i v i t y o f t h e z e o l i t e s used was much l o w e r 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 . The reasons why t h e z e o l i t e was l e s s a c t i v e were discussed. INTRODUCTION Common s o l i d a c i d c a t a l y s t s which a r e i n s o l u b l e i n water, such as s i l i c a alumina and HY z e o l i t e , a r e generally c o n s i d e r e d t o be i n a c t i v e i n t h e presence o f w a t e r a t r e l a t i v e l y l o w temperatures.
Because t h e s e c a t a l y s t s a r e hydro-
p h i l i c and w a t e r c o v e r s t h e s u r f a c e o f t h e c a t a l y s t s and p r e v e n t s t h e a d s o r p t i o n o f organic materials.
On t h e o t h e r hand, c a t i o n exchange r e s i n s a r e a c t i v e as
s o l i d a c i d c a t a l y s t s i n t h e presence o f w a t e r , p r o b a b l y because t h e y have good a f f i n i t y f o r o r g a n i c m a t e r i a l s even i n aqueous s o l u t i o n s .
However, t h e c a t i o n
exchange r e s i n s do n o t have h i g h thermal and mechanical s t a b i l i t i e s . I n o u r p r e v i o u s paper [l],we f i r s t r e p o r t e d t h a t h i g h - s i l i c a z e o l i t e s , such as dealuminated H-mordenite and HZSM-5, showed t h e h i g h c a t a l y t i c a c t i v i t y f o r h y d r o l y s i s o f e t h y l a c e t a t e i n an aqueous s o l u t i o n and t h a t t h e HZSM-5 had a c i d s i t e s whose s t r e n g t h was -5.6 > 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 .
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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<
will resemble type I aging but the existence of the smaller pore system will be beneficial for catalyst regeneration. This situation is encountered for offretite (ref.20), although somewhat complicated by the fact that its channels also contain cages (see Fig. 1).
For case 111, coking in the S channel will
result in faster aging than coking in the P cage but the respective rates of coke formation may be reversed ( P > S ) because of steric constraints. Cases I-IV are bulk blocking modes.
A recent Monte-Carlo analysis of a model related to
case I1 indicates a rapid decrease in molecular effective diffusivity with increasing bulk blockage, as the tortuosity and length of the diffusion paths increase (ref.21). Case V shows different types of diffusion constraints imposed by coke deposition. V-A corresponds to pore mouth (border) blockage of which the effect is to reduce the rate of entrance of the reactants into the crystallites, to increase the tortuosity of diffusion paths to and from the catalytic sites, and to reduce the intrinsic number of accessible sites (ref.21).
V-B (pore mouth re-
striction) and v-C (bulk restriction) are coking modes which will progressively reduce the diffusivity of reactant and product molecules as a result of effective pore opening reduction. Type V coking will affect the molecular shape properties of the zeolite as a function of time-on-stream (ref.20).
Coking of
type V-A and V-B explains the enhanced para-aromatic selectivity of coked ZSM-5 catalysts in the alkylation of toluene by methanol and the disproportionation of toluene which yield para-xylene as the main and desired product (ref.22-26). Distinguishing between site coverage and pore blockage effects on deactivation is no easy matter. Combining a variety of techniques is an approach that has been recommended and used (ref.20,27), however in only a limited number of cases. Figure 3 lists methods which are available to discriminate between these contributions and to identify the occurrence of either site poisoning or pore blockage. Eventually, however, understanding deactivation will need the relation of the kinetics of the main reaction sequence and that of the coke formation to the amount of coke deposited. The classification we just proposed and the information from the above techniques should help the development of quantitative schemes using an approach identical to that advocated by Froment and co-workers (ref.3-4,16-18).
226
CATALYTIC ACTIVITY NH3 PROGRAMMED DESORPTION S
4,
NH3 ADSORPTION ISOTHERMS MICROCALORIMETRY INFRARED SPECTROSCOPY: ACID SITES DEPOSITS UV-VISIBLE SPECTROSCOPY:
ACID SITES
-,
-
GENERAL MECHANISM OF COKE FORMATION Coke originates mainly from olefinic (ref.28) and aromatic (ref.29)compounds, not all of these having the same efficiency for its formation nor leading to the same type of deposit (ref.19,30).
An operating definition for coke is that of a
carbonaceous residue with C / H ratio of 1.5-2.6 (ref.31), consisting largely of fused-ring hydrocarbons (ref.29). for example (ref.7-11,28-33,39)),
On the basis of the available literature (see we propose the general scheme of Fig. 4 for
the formation of carbonaceous residues and coke. This reaction sequence shows intermediates and/or products with increasing spatial requirements, molecules become progressively bulkier and have increasing molecular weight.
Reactants can of course enter this sequence at any of its
stages. A s recognized many years ago, zeolite aging is analogous to reverse molecular shape selectivity (ref.19,34).
Coke formation occurs when heavy
molecules are formed, which are irreversibly sorbed or adsorbed. Higher coke yields are expected for zeolites which larger pore size as they exert less constraints on the reaction(s) leading to coke deposition.
Supporting this view is
an impressive correlation ( s e e Fig. 5) between the amount of coke formed in paraffinic cracking over a variety of zeolites and their shape selectivity (measured by the ratio of the cracking rates for n-hexane (NC6) and 3-methylpentane (3MP))
(ref.9).
The next section will discuss in detail how molecular
shape selectivity turns coke formation into a reaction directly controlled by the zeolite pore structure. In small and medium pore zeolites, deposition of pseudo-aromatic coke cannot occur internally. Deactivation may however result from site occupancy and pore
227
/PARAFFINSc
x A.1
_____
I v)
Fig. 4 . Schematic representation of the mechanisms responsible for the formation of carbonaceous deposits. A = Alkylation; C = Cracking; D = Dehydrogenation; H = Hydrogen transfer; M = Miscellaneous; 0 = Oligomerization; P = Polymerization; R = Cyclization; T = Transalkylation; W = Dehydration.
n
p
I C
L
>
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
<-
'Huile minerale (200 Neutral)
0,879
6,21
40,79
98
-
A1 kyl benzenes
0,855
6,55
34,41
115
Poly-d-olefi nes*
("C)
60
242
11
224
-30
230
-
Catalyse par BF, support&. La c a t a l y s e par l e s complexes du t r i f l u o r u r e de bore, aussi a t t r a y a n t e s o i t e l l e , pose t o u t e f o i s de serieux problemes en raison de l a nature meme des complexes fluoboriques ( s e n s i b i l i t e a l a temperature, d i f f i c u l t 6 de r6cuperation du complexe apres r e a c t i o n ) TOTAL-CFR a mis au point une nouvelle formule ( r e f . 20) en f i x a n t l e t r i f l u o r u r e de bore sur u n s u p p o r t c o n s t i t u e par de 1 'alumine. Ce type de catalyseur presente 2 avantages d e c i s i f s par rapport aux formules a base de BF3 complex@. I1 permet de t r a v a i l l e r dans u n plus large domaine de temperature,
248
de c a t a l y s e r un p l u s grand nombre de r e a c t i o n s t e l l e s que l ' a l k y l a t i o n du benzene p a r 1 'ethylene ( o b t e n t i o n d'ethylbenzene pour f a b r i q u e r du s t y r e n e ) 1 ' o l i g o m e r i s a t i o n d ' o l e f i n e s legeres -en C3/C4 vers l a p r o d u c t i o n de bases petrochimiques e t p e t r o l i e r e s ( r e f . 13
- 21),
l a trimerisation/tetramerisation
d ' d - o l e f i n e s l i n e a i r e s (en C1o p a r exemple) pour f a b r i q u e r des l u b r i f i a n t s synthetiques ( r e f . 22). Dans l a prC-sentecommunication, nous nous bornons
a
presenter l e s travaux
e f f e c t u e s par TOTAL-CFR en m a t i e r e d ' o l i g o m e r i s a t i o n d ' o l e f i n e s legeres en C3/C4 produi t e s par craquage c a t a l y t i q u e de coupes p e t r o l i e r e s . Les charges que l ' o n peut t r a i t e r dans l e procede sont des coupes o l e f i n i q u e s en C3+C4 ou C4 ou m6me C3.
A t i t r e d'exemple, une charge en C3/C4, u t i l i s e e couramment, comprend 45% en poids d ' o l e f i n e s dont 5 2 10% d'isobutene e t 20% de propene. E l l e c o n t i e n t des p o l l u a n t s , e s s e n t i e l l e m e n t du butadiene avec des t r a c e s de d e r i v e s azotgs. Precisons que toutes l e s charges sont sechees au p r e a l a b l e sur tamis 3A e t que l e s c o n d i t i o n s de r e a c t i o n
sont : temperature 60-1OO0C, pression 10-30 bars
v i t e s s e s p a t i a l e h o r a i r e 3-6. On notera en p a r t i c u l i e r que l e s c o n d i t i o n s de temperature e t de pression sont nettement moins severes que l o r s q u ' o n u t i l i s e l e s systemes
a
base d ' a c i d e phos-
phorique depose s u r kieselguhr; t o u t e f o i s , pour m a i n t e n i r 1 ' a c t i v i t e c a t a l y t i q u e dans l e temps, il e s t necessaire d ' i n j e c t e r en c o n t i n u dans l e m i l i e u r e a c t i o n n e l un f l u x de BF3. Le c a t a l y s e u r BF3/alumine e s t obtenu au s e i n m6me du r e a c t e u r en t r a i t a n t l ' a l u m i n e sous f l u x de BF3. On e f f e c t u e e n s u i t e l e reglage des c o n d i t i o n s de marche (temperature, pression, d e b i t de charge, c o n c e n t r a t i o n en 5F3 gazeux dans l e m i l i e u ) . La f i g u r e 2 represente l e schema de procede s i m p l i f i e . I 1 comprend 3 s e c t i o n s principales : a) S e c t i o n a l i m e n t a t i o n . La coupe C4 ou l e s coupes C3+C4 peuvent 6 t r e p r G t r a i t 6 e s ou non. S i on veut l e s v a l o r i s e r totalement
, on
peut d ' a b o r d e l i m i n e r selectivement
l ' i s o b u t e n e de l a charge, comme on l ' a vu prGc6demment e t en r e t i r e r une gamme de p r o d u i t s (dimeres, t r i m e r e s , tetrameres) v a l o r i s a b l e s s u r l e marche. Sinon, l a charge passe simplement s u r tamis pour e n l e v e r 1 'eau
(de d i s s o l u t i o n )
qu ' el l e c o n t i e n t . Un a p p o i n t de BF3 e s t necessaire pour compenser l e s pertes dans l e r e a c t e u r .
249
OLIGOMERES FRACTIONNEMENT
Figure 2
-
Schema de p r i n c i p e d'une u n i t e d ' o l i g o m e r i s a t i o n s u r c a t a l y s e u r BF3/ alumi ne.
b ) Section o l i g o m e r i s a t i o n . Le c a t a l y s e u r e s t prepare au s e i n meme du r e a c t e u r come i n d i q u e p l u s haut. La r e a c t i o n peut t t r e e f f e c t u e e s o i t dans un r e a c t e u r m u l t i t u b u l a i r e du type echangeur de chaleur, s o i t dans un r e a c t e u r
a
l i t s m u l t i p l e s avec quench
intermedi a i res. c) Section fractionnement des e f f l u e n t s .
A l a s o r t i e du reacteur, l ' e f f l u e n t e s t soumis successivement
un f l a s h
( s e p a r a t i o n en t C t e des hydrocarbures en C3/C4 r e s i d u a i r e s e t du BF3 pour recyclage) e t
a
une s t a b i l i s a t i o n : c e t t e seconde t o u r permet de recup6rer
en t @ t e une coupe "butane-propane" e t , en fond, l e s oligomeres s t a b i l i s e s . La coupe "butane-propane" e s t e n s u i t e envoyee au stock apres lavage
a
la
soude (61i m i n a t i on des traces de BF3 r e s i d u a i r e ) . Avant fractionnement, l e s oligomeres sont t r a i t t s & contre-courant dans une t o u r de lavage
a
l a soude pour e l i m i n e r l e f l u o r (contaminant g t n a n t dans
l e cas des coupes legeres C7/C8). Apres un lavage soumis
a
a
1 'eau, lesoligomeres sont
un fractionnement classique pour i s o l e r l e s d i f f e r e n t e s coupes.
Les performances c a t a l y t i q u e s du c a t a l y s e u r BF3/ alumine sont e s s e n t i e l l e m e n t f o n c t i o n des c o n d i t i o n s de p r e t r a i t e m e n t du support (temperature e t duree). Nous avons f a i t v a r i e r ces parametres dans un vaste domaine, avant d ' e f f e c t u e r 1 'impregnation p a r BF3, comme pr@cedemmentd e c r i t , puis soumis l e s c a t a l y s e u r s a i n s i prepares
a
des t e s t s c a t a l y t i q u e s .
250
La conversion des olefines demeure Glev@e( 80%) mOme avec des catalyseurs prepares d p a r t i r de supports t r a i t & d haute temperature. Les conditions de prgtraitement d u support permettent d'orienter selectivement + l a reaction vers des coupes legeres (C7-C12) ou des coupes plus lourdes (C13 ) . Ainsi, un catalyseur de haute temperature donne avec des conversions de 1 'ordre de 80-85% une s @ l e c t i v i t @ de 70% en coupes legeres. Un catalyseur de basse temperature e s t prefer6 pour obtenir des coupes lourdes ; l a conversion e s t alors voisine de 95%.
Une procedure simple de reactivation du catalyseur apres un cycle, mise au point par TOTAL-CFR consiste principalement d laver l e catalyseur avec u n solvant aromatique, dans des conditions operatoires donnees, purger ensuite l e reacteur e t les lignes, remettre enfin l ' u n i t e en service dans les conditions de marche d@sirees. Ainsi, aprss un cycle au cours duquel l a conversion des olefines C3/C4 passe de86 I 79%, on retrouve l ' a c t i v i t g i n i t i a l e au terme de c e t t e procedure. Le procede TOTAL-CFR, par sa souplesse de marche, pemet de fabriquer une gamme de produits couvrant un large domaine d'application, come l e montre l e Tableau V. Ainsi , l e s COUPES LEGERES donnent des produits directement u t i l i s a b l e s p a r la petrochimie (synthese 0 x 0 , a1 kylation d'aromatiques). Elles peuvent egalement donner des carburants synth6tiques avec u n excellent indice d'octane sans plomb (indice d'octane recherche de l'ordre de 98), en raison mOme de leur structure olefinique t r e s ramifiee. Les COUPES LOURDES en C1; conduisent, apres fractionnement, d des bases pet r o l i e r e s interessantes (gazole d tres f a i b l e point d'ecoulement (-60°C) e t apres hydrogenation I des solvants isoparaffiniques, des liquides hydrauliques e t des huiles isolantes (fluides d basse temperature).
251
TABLEAU V : Les produits
APPLICATIONS
NATURE
Coupes 1egPres : HeptSnes/Octenes Coupes Cg-C12
-
+ Petrochimie alcool 0x0 -Bases pour carburants synthetiques ,P&trochimie -+ alcools 0x0 A1 kyl benzenes
-
Solvants isoparaffiniques
Coupes lourdes : '13-'16
'15-%5
c25-c40
--c
Solvants isoparaffiniques lourds Gazole i bas p o i n t d'ecoulement pour moteur Diesel Fluides hydraul iques
-.
-
Bases pour huiles isolantes f 1 ui des 21 basse temperature
CONCLUSION : La catalyse fluoborique offre, avec les 3 modes de rnise en oeuvre de BF3, decrits dans ce t e x t e , un vaste champ d'application, notamment en matiere d'oligom6risation d'olefines. On peut : - Extraire selectivement 1 'isobutene des coupes (C3tC4) ou C4 olefiniques, 3 l ' a i d e de BF3 injecte sous forme gazeuse. On obtient, dans ces conditions, des polyisobutenes (dimeres, trimeres, tetrameres), produits demand& sur l e rnarche. - Utiliser les complexes (BF3, alcool) pour transformer l e s d-olefines (en C8C1o) en lubrifiants synthetiques, d o n t les performances sont t r e s nettement sup@rieures i celles des huiles minerales raffinees. - Enfin, t r a i t e r s u r catalyseur BF3 support6 (alurnine) les olefines legeres en C3/C4 (proci.de TOTAL-CFR), produites par exemple par craquage catalytique des coupes petrolieres lourdes. Ce procede, p a r sa souplesse, permet de fabriquer une gamme de produits (bases p6trochimiques et/ou bases petrolieres) t r e s vaste, valorisables sur l e marche.
REFERENCES. 1 Olah, Friedel-Crafts and related reactions, lnterscience Publishers, 1963. Evans e t G.W. Meadois, J . of Polymer Science, 4 , 359 (1949). 3 F.S. Dainton e t G.B.M. Sutherland, J . of Polymer Science, 4 , 37 (1949). 2 A.G.
252
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
G . E . Langlois, I . E . C . , 45, n 0 7 , 1470 (1953). BASF, Brevet FranGais 7 800 293. BASF, Brevet FranGais 2 378 049 (1978). R.G. Heiligmann,J. o f Polymer Science, 4, 183 (1949). EXXON Res. & Eng. Co., Brevet U.S.3 932 553 (1976). ESSO Kes. & Eng. Lo., Brevet Fr. 1 500 178 (1Y67). ESSO Res. & Eng. Co., Brevet Brit. 887 590 (196L). J.A. Brennan, 1.E.L.-Prod. Res. Dev. , 1980, 19, p.2. R.L. Shubkin, M.S. Baylerlanet A.R. Maler, flE.C-Prod. Res. Dev. 1981),19, p. 15. U.O.P. Brevet U.S. 1 0 2 5 896 (1962). TOTAL-LFR, Brevet Fr. 82-19009 (1982). MOBIL Res. & Dev. Corp., Brevet U.S.3 382 291 (1968). MOBIL Res. & Dev. Corp., Brevet U.S.3 742 082 (1973). MOBIL Res. & Dev. Corp. , Brevet U.S.3 769 363 (19/3). LTHYL Corp., Brevet U.S.3 763 244 (1973). ETHYL Corp., Brevet U.S.3 780 128 (1973). TOTAL-CFR , Brevet Fr. 81-07555 (1981). TOTAL-CFR, Brevet Fr. 82-20782 (1982). Brevet U.S. 124457 (1983). A.M. Madgavkar e t H.E. Swift, 1.E.C-Prod. Res. Dev. , 1983, 2, p.675.
B. Imelik e t al. (Editors), Catalysis by Acids and Bases o 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
253
UPGRADING OF C4 CRACKING CUTS WITH A C I D CATALYSTS
B. JUGUIN, B. TORCK and G. MARTINO. I n s t i t u t Francais du P e t r o l e , B.P.
311, 92506 Rueil-Malmaison Cedex (France)
RESUME Cet a r t i c l e f a i t l e p o i n t sur l e s d i v e r s procedes de v a l o r i s a t i o n des coupes C4 f a i s a n t appel 1 l a c a t a l y s e acide. Apres un b r e f rappel des u t i l i s a t i o n s act u e l l e s e t p o t e n t i e l l e s de ces coupes, nous presenterons l e s d i v e r s procedes e t l e s catalyseurs u t i l i s e s . Ces procedes de t r a n s f o r m a t i o n peuvent Otre classes en deux categories selon que l a r e a c t i o n s ' e f f e c t u e en phase l i q u i d e ( a l k y l a t i o n a l i p h a t i q u e , h y d r a t a t i o n , polymCrisation c a t i o n i q u e ) , ou 1 l a surface d'un c a t a l y s e u r sol i d e ( e t h e r i f i c a t i o n , 01 igomeri s a t i o n , isomeri sation, aromati sation). ABSTRACT Among t h e numerous processes used f o r t h e upgrading of C4 cuts, many o f them use a c i d c a t a l y s t s . A f t e r a b r i e f l o o k a t t h e present and p o t e n t i a l uses of these c u t s , we w i l l go over t h e d i f f e r e n t processes and t h e c a t a l y s t s used. These t r a n s f o r m a t i o n processes can f i r s t be c l a s s i f i e d according t o whether the react i o n takes place i n a l i q u i d phase ( a l i p h a t i c a l k y l a t i o n , h y d r a t a t i o n , c a t i o n i c polymerization), o r a t t h e surface o f a s o l i d c a t a l y s t ( e t h e r i f i c a t i o n , oligomer i z a t i o n , i s o m e r i z a t i o n , aromatization)
.
INTRODUCTION The C4 c u t s r e p r e s e n t approximately 10 % o f t h e production (1) o f a c a t a l y t i c cracking u n i t (FCC). Among the numerous processes used f o r t h e i r upgrading, many o f them use a c i d c a t a l y s t s . A f t e r a b r i e f l o o k a t the present and p o t e n t i a l uses o f these c u t s , we w i l l go over the d i f f e r e n t processes and t h e c a t a l y s t s used. Up u n t i l a few years ago, the C4 c u t s were used, e s p e c i a l l y i n France, e i t h e r t o a d j u s t the vapor pressure o f g a s o l i n e o r as l i q u e f i e d gas. I n f a c t , these c u t s can be used i n many o t h e r r e a c t i o n s ( 2 , 3) because t h e i r o l e f i n s content i s very h i g h :
-
as f i r s t generation petrochemical i n t e r m e d i a r i e s ( 2 ) : isobutene, butene 1,
butenes 2 (see t a b l e I ) ;
-
as raw m a t e r i a l s o f h i g h octane content such as a l k y l a t e s and polymerisates
which have been used now f o r q u i t e some time, o r e i t h e r such as m e t h y l t e r t i o b u t y l e t h e r (MTBE) which has been introduced more r e c e n t l y ( 4 ) .
2 54
TABLE I The p r e s e n t chemical uses o f butenes. End Uses
Intermediates Butene 1
/ __t
\
Polybutene 1 Copolymer HDPE Butylene oxide
___c
P i p i ng
___*
Anti-corrosives, p o l y o l s
Methyl e t h y l c e t o n e Butene 1 or butenes 2
d
B u t y l i c alcohol
__+
Maleic anhydride
\
-
Heptenes and octenes B u t a d i ene Polyisobutene
-c
I:
___+
-
O i 1 additives Additives Sul f o n a t e s Tyres
___)
I s o p r e n e copolymers S t y r e n e and l a t e x c o p o l y m e r s
T e r t i a r y butylamine Methylmetacrylate
c -
U n t i l r e c e n t l y , t h e r e was a l a r g e excess
Solvents Sol v e n t s Lube o i l a d d i t i v e s Polyesters Plasticizers Lube o i l a d d i t i v e s
Phtalates Sol v e n t s , m e t h a c r o l e r n e Alkylphenolic resins A n t i oxydan t A n t i UV A n t i - r u s t agent Synthetic glass
( 5 ) o f butenes t h r o u g h o u t t h e
w o r l d ( s e e t a b l e 11), b u t a new f a c t o r , t h e l i k e l y l e g i s l a t i o n on t h e l i m i t a t i o n o f t h e l e a d c o n t e n t i n g a s o l i n e , should change t h i s s i t u a t i o n r a d i c a l l y . T h i s s h o u l d e n t a i l a change i n t h e consumption o f i s o b u t e n e f o r t h e p r o d u c t i o n o f t h e most p r o b a b l e o c t a n e enhancer which w i l l a c t as i t s replacement, i n o t h e r words MTBE. TABLE I 1 The butenes market ( n t i s o ) . l O
3 t o n s p e r y e a r : e s t i m a t e s f o r 1985. France
A v a i l a b l e from Steam c r a c k i n g + c a t a l y t i c c r a c k i n g + diverses p y r o l i s e s
World
1100
37 000
Demanded as Chemical p r o d u c t s and g a s o l i n e
250
24 000
S u r p l u s used as LPG and f u e l gas
850
13000
The enforcement o f t h i s l a w w i l l l e a d , t h e r e f o r e , t o t h e f o l l o w i n g s i t u a t i o n :
-
a l a c k o f i s o b u t e n e f o r chemical uses ;
-
an i n c r e a s e i n t h e n butenes r e a d i l y a v a i l a b l e , which c o i n c i d e s w e l l w i t h
255
t h e development i n t h e demand f o r butene 1, which i t s e l f i s used as aco-monomer f o r the production o f low density polyethylene. The processes f o r c o n v e r t i n g butenes c a r r i e d o u t i n t h e presence o f a c i d c a t a l y s t s can f i r s t be c l a s s i f i e d a c c o r d i n g t o whether t h e r e a c t i o n t a k e s p l a c e : i n a l i q u i d phase o r a t t h e s u r f a c e o f a s o l i d c a t a l y s t .
LIQUID
PHASE PROCESSES (6)
A l i p h a t i c a l k y l a t i o n (7, 8, 9 1 T h i s i s an o p e r a t i o n which makes a p a r a f f i n . c o n t a i n i n g a t e r t i a r y hydrogen, such as isobutane, r e a c t on an o l e f i n , t h u s p r o d u c i n g branched p a r a f f i n s which a r e e x c e l l e n t f u e l components thanks t o t h e i r h i g h o c t a n e number : f o r example t h e r e a c t i o n between i s o b u t a n e and butenes 2 .
CH3
CH3
A l k y l a t i o n has been developped e s p e c i a l l y i n t h e USA and o n l y r e c e n t l y i n Europe ( w i t h i n i n t h e l a s t 10-12 y e a r s ) . The w o r l d c a p a c i t y f o r t h e p r o d u c t i o n o f a l k y l a t e s i s e s t i m a t e d a t 40 m i l l i o n t o n s p e r y e a r . T h i s r e a c t i o n , which r e q u i r e s t h e a c t i v a t i a n o f t h e n bond o f t h e o l e f i n and a l s o t h e c a r b o n - h y d r o g e n d b o n d o f t h e p a r a f f i n , i s c a r r i e d o u t i n t h e presence
o f a c i d c a t a l y s t s such as s u l p h u r i c a c i d ( 9 5
-
98 %) o r w a t e r f r e e h y d r o f l u o r i c
a c i d w i t h t h e carbonium i o n s a c t i n g as i n t e r m e d i a r i e s . I n o r d e r t o l i m i t t h e secondary r e a c t i o n s , t h e a l k y l a t i o n process i s c a r r i e d o u t i n a l i q u i d phase a t a l o w temperature (between 0
-
90 "C), a c c o r d i n g t o t h e r e a c t i o n s shown i n t h e
f o l l o w i n g diagram which e x p l a i n t h e f o r m a t i o n o f isomers (see diagram). Diagram o f t h e a l k y l a t i o n o f t h e butenes
CH3 CH3 cy3 1. CH3-C @
I
+ CH3-CH=CH-CH
3
I
CH3
I
I
2. CH3-C-CH2-C-CH3
0
(
3
CH3 1
C8 + CH3-C-H
(C8 H
=
1
0
CH3CH3 I
CH3-C-CH-CH-CH3
0'
@ I
CH3
Y 3 %) C8 H + CH3-C
I
I
CH3
CH3
TM 2,2,4 P, TM 2,3,4 P, TM 2,3,3
1
CH3-C-C-CH2-CH3
CH3
CH3
3.
I
-CH-CH-CH3
CH3
CH3 CH3
1
4 CH3-C
P)
256
A t t h e f i r s t stage, t h e t . b u t y 1 c a t i o n r e a c t s w i t h t h e o l e f i n , thus g i v i n g a secondary carbonium i o n which i s f a r l e s s s t a b l e than t h e t e r t i a r y ions. The r a t e of t h e i n t r a m o l e c u l a r r e o r g a n i z a t i o n (stage 2.)
by means o f the t r a n s f e r
o f h y d r i d e and methyl, which leads t o t e r t i a r y ions, i s much q u i c k e r than t h a t o f t h e i n t e r m o l e c u l a r r e a c t i o n i n t h e t r a n s f e r o f h y d r i d e w i t h isobutane (stage 3 & ) . The i s o m e r i z a t i o n o f t h e carbonium ions helps t o e x p l a i n t h e s t r u c t u r e o f t h e d i f f e r e n t isooctanes obtained, which vary according t o t h e n a t u r e o f t h e butenes (see t a b l e 111). TABLE I11 A l k y l a t i o n o f isobutane on t h e butenes HF
-
T = 25 "C
-
iC4/butenes = 13 '4'lefin
T r i m e t h y l 2,2,3 T r i m e t h y l 2,3,3 T r i m e t h y l 2,3,4 T r i m e t h y l 2,2,4 Dimethylhexanes
pentane pentane pentane pentane
Butenes 2
Isobutene
1.4 13.8 20.3 54.4 10.1
1.3
Butene 1 1.5 13.3 15.6 46.3 23.3
11.7 13.4 62.6 11.0
I n o r d e r t o g i v e p r i o r i t y t o t h e main r e a c t i o n which leads t o a l k y l a t e s o f a h i g h e r octane number, i n s t e a d o f t h e secondary r e a c t i o n s , a c e r t a i n number o f o p e r a t i n g c o n d i t i o n s must be c l o s e l y checked, e s p e c i a l l y t h e temperature and t h e molar i s o b u t a n e / o l e f i n r a t i o (between 8 and 15). H y d r a t a t i o n o f t h e butenes (10) The h y d r a t a t i o n o f isobutene and n butenes which produces t . b u t y l i c and s . b u t y l i c a l c o h o l s i s c a r r i e d o u t i n t h e presence o f a c i d c a t a l y s t s . These a l c o h o l s a r e sought a f t e r as s o l v e n t s and as i n t e r m e d i a r i e s f o r t h e p r o d u c t i o n o f methylethylketone obtained through t h e dehydrogenation of s . b u t y l i c a l c o h o l . Depending on t h e r e a c t i v i t y o f t h e o l e f i n , t h e h y d r a t a t i o n can e i t h e r be done d i r e c t l y o r must be done i n d i r e c t l y . Isobutene.
Because o f i t s g r e a t r e a c t i v i t y , i s o b u t e n e
can be hydrated d i r e c t -
l y i n t h e presence o f s o l i d a c i d c a t a l y s t s such as t h e s u l f o n i c ion-exchanger e s i n , i n a l i q u i d phase and under m i l d temperature c o n d i t i o n s (50
-
100 "C).
By combining several r e a c t o r s , i t i s p o s s i b l e t o c o n v e r t up t o 90 % t h e i s o -
butene contained i n t h e C4 c u t s w i t h a s e l e c t i v i t y o f 99.9 %.The r e a c t i o n i s c a r r i e d o u t by an a c i d i c c a t a l y s t mechanism i n which the t r a n s f e r o f t h e p r o t o n t o one of t h e carbons of t h e double bond i s t h e r a t e determining step (see f o l l o w i n g diagram).
257
+
ti-:-:@ I
H20
4
=Z rapid
1 1
H-C-C-OH
t H
@
N-butenes. Since t h e n-butenes a r e f a r l e s s r e a c t i v e than isobutene, the hydratation i s d o n e i n the presence of sulphuric acid (75 % concentrate) i n which the n butenes a r e absorbed t o give sulphates i n sulphuric s o l u t i o n . These sulphates a r e hydrolyzed a t t h e second stage by d i l u t i n g i n water, t h u s forming b u t y l i c alcohol according t o the following reactions : CH3-CH=CH-CH3
t H2S04
CH3-CH2-CH-CH3 1
O-S03H
+
CH3-CH2-CH-CH3
H20
-+
CH3-CH2-CHOH-CH3
+
H2S04
I
O-S03H
The sulphuric acid which has been d i l u t e d t o 4 5 % must be reconcentrated by d i s t i l 1 a t i o n . The s . b u t y l i c alcohol can a l s o be obtained by d i r e c t hydration i n t h e presence o f s o l i d acid c a t a l y s t s such as phosphoric acid on c a r r i e r , heteropolyacids such as s i l i c o t u n g s t i c acid and mixed oxides and z e o l i t e s . The r e a c t i o n i s c a r r i e d out a t a higher temperature (200 - 250 "C) but t h e r a t e s of convers i o n per pass a r e low. Cationic polymerization of isobutene. Synthetic rubber, a copolymer of isobutene and isoprene i s t h e most import a n t i n d u s t r i a l product obtained by c a t i o n i c polymerization. The polymerization of the isobutene containing between 1 - 5 % isoprene i s c a r r i e d out a t 4 1 0 0 "C i n t h e presence of Lewis a c i d s such a s aluminium c h l o r i d e dissolved i n methylene chloride, by means of carbonium ions r e a c t i n g successively w i t h the isobutene. The polymerization can be stopped e i t h e r by the t r a n s f e r of a proton from the carbonium ion t o isobutene, o r by the t r a n s f e r of hydride from an o l e f i n , which i t s e l f i s transformed i n t o a very s t a b l e a l l y l i c c a t i o n , t o t h e carbonium ion, according t o t h e following r e a c t i o n s : c3; CH3 -CH2-C
@
I
+
CH3 I CH =C T CH3 2 ,
-
"2 C I
cy3 t CH3-C I
a
2 58
-:'
0
CH3 CH3
M.
CH 2
t H-CH-CH=CH2
-+
CH3
r
C-H t CH-CH=CH2
y
~
1
1
I
I
CH3
R
CH3
R
The m o l e c u l a r w e i g h t o f t h e p o l y i s o b u t y l e n e which may be i n t h e o r d e r o f
10
6
depends o n t h e p r e s e n c e o f t h e compounds which s t i m u l a t e t h e p r o t o n t r a n s -
f e r s o r t h e h y d r i d e t r a n s f e r s . The t r a n s f e r r e d p r o t o n agents, such as t h e a l k y l h a l i d e s , reduce t h e m o l e c u l a r
w e i g h t w i t h o u t a f f e c t i n g t h e y i e l d , whereas t h e
t r a n s f e r r e d h y d r i d e agents such as t h e 1 o l e f i n s , l o w e r t h e y i e l d w i t h o u t a l t e r i n g t h e molecular weight. PROCESSES USING SOLID A C I D CATALYSTS. E t h e r i f i c a t i o n processes (11, 12, 13, 14) The m e t h y l t . b u t y l e t h e r (MTBE) which i s used as a component i n g a s o l i n e because o f i t s h i g h o c t a n e number, i s o b t a i n e d by t h e r e a c t i o n o f methanol on i s o butene c o n t a i n e d i n t h e c r a c k i n g c u t s , i n t h e presence o f a s o l i d a c i d c a t a l y s t such as s u l f o n i c r e s i n .
As i n t h e case o f h y d r a t i o n ,
the
r a t e determining step i s the formation o f
t h e t . b u t y 1 c a t i o n , achieved by t h e slow t r a n s f e r o f a p r o t o n f o l l o w e d by t h e r a p i d a d d i t i o n o f methanol. Because of t h e e x o t h e r m i c n a t u r e o f t h e r e a c t i o n , t h e temperature which i s k e p t below 100 "C must be p e r f e c t l y r e g u l a t e d so as t o a v o i d t h e f o r m a t i o n o f secondary p r o d u c t s such as d i m e t h y l e t h e r and i s o b u t e n e o l i g o m e r s . O l i g o m e r i z a t i o n processes The r e a c t i o n s o f o l i g o m e r i z a t i o n , u s i n g S t r o n g B r o n s t e d a c i d s as c a t a l y s t s , proceed w i t h a p o l a r mechanism, t h u s r e q u i r i n g t h e a d d i t i o n o f a p r o t o n t o an olefin,
t o g i v e a carbonium i o n which i s a b l e t o a t t a c k a second o l e f i n mole-
cule (15).
@ R-CH=CH2 + H 0+ R-CH
K-CH=CH2 d
0 R-CH-CH2-CH
1
I
I
CH3
CH3
R
T h i s carbonium i o n can t h e n e i t h e r l o s e a p r o t o n i n o r d e r t o form an o l e f i n , o r a t t a c k a t h i r d o l e f i n molecule. R-CH-CH=CH-R
8 R-CH-CH2-CH I
1
CH3
R
t
H0
I
CHs 3
0 R-CH-CH2-CH-CH2-CH
;
259
The i s o m e r i z a t i o n r e a c t i o n s of a carbonium i o n can a l s o occur t h r o u g h t h e m i g r a t i o n o f a hydrogen o r an a l k y l group ( 4 )
0 R-CH2-CH-CH3
R-C -CH -CH-CH3
8 R-CH-CH2-CH3
R-C-CH
I
I
CH3
CH3
-CH-CH3 1
CH3
The processes o f o l i g o m e r i z a t i o n a p p l i e d t o C4 c u t s can be d i v i d e d i n t o two c a t e g o r i e s : those where t h e maXiIWr!I
p o s s i b l e t r a n s f o r m a t i o n i n t h e butenes i s
sought, and those where a s e l e c t i v e t r a n s f o r m a t i o n o f one o f t h e non s a t u r a t e d isomers i s t h e o b j e c t . Non s e l e c t i v e o l i g o m e r i z a t i o n .
The
t r a n s f o r m a t i o n by p o l y m e r i z a t i o n o f t h e
n butenes and of t h e i s o b u t e n e c o n t a i n e d i n t h e C4 c u t s , l e a d s t o an i s o o l e f i n i c oligorner made up o f dirners, t r i m e r s , t e t r a m e r s
and
pentamers. D i f f e r e n t types
o f c a t a l y s t s a r e p o s s i b l e , t h e most common b e i n g supported p h o s p h o r i c a c i d and c e r t a i n silica-aluminas.
. ZuppgrJei
ph2sphgr-i-c-agid.
T h i s t y p e o f c a t a l y s t made up o f p h o s p h o r i c
a c i d and k i e s e l g u h r o r s i l i c a , was p e r f e c t e d by I p a t i e f f d u r i n g t h e 2nd w o r l d war and i s s t i l l used t o d a y ( 1 6 ) . The o l i g o m e r i z a t i o n o f t h e butenes can be c a r r i e d o u t under d i f f e r e n t temper a t u r e s and p r e s s u r e s . G e n e r a l l y speaking, t h e p r e s s u r e s w i l l range f r o m 35 t o 70 bars, and t h e temperatures f r o m 140
-
210 'C.
Because t h e r e a c t i o n i s h i g h l y
exothermic, t h e i n c r e a s e i n temperature i s r e g u l a t e d by means o f quenching i n j e c t i o n s between t h e c a t a l y s t beds. T h i s quench i s made up o f t h e C4
-
part o f
t h e e f f l u e n t f r o m t h e r e a c t o r (non c o n v e r t e d and i n e r t p a r a f f i n s ) . The r a t e o f t h e t r a n s f o r m a t i o n o f t h e butenes can be up t o 95 %. The p r o d u c t o b t a i n e d , t h a t i s t o say t h e C5+ f r a c t i o n , has an ASTM i n t e r v a l o f d i s t i l l a t i o n between 60
220 "C.
-
T h i s i s a m i x t u r e o f dimers and t r i m e r s and i t s r e s e a r c h c l e a r o c t a n e
number i s i n t h e o r d e r o f 97 t o 99 (16). The w a t e r c o n t e n t on e n t r y i n t o t h e r e a c t o r must be a c c u r a t e l y a d j u s t e d i n o r d e r t o o b t a i n t h e optimum a c t i v i t y and s e l e c t i v i t y o f t h e p h o s p h o r i c c a t a l y s t : t h e l i f e cycles o f t h e c a t a l y s t are q u i t e short.
. -S i l i ~ a ~ a ~ u 111). m i ~ aC~e r t a i n
amorphous s i l i c a - a l u m i n a s w i t h h i g h
silica
c o n t e n t s a r e e s p e c i a l l y w e l l adapted f o r t h e o l i g o m e r i z a t i o n of butenes. They have t h e advantage o f b e i n g s t a b l e and t h u s l e a d t o processes where t h e c a t a l y s t l i f e c y c l e s a r e l o n g . A l s o , t h e y can be r e a c t i v a t e d , p o l a r compounds (H20, NH3, e t c ...)
when
o r r e g e n e r a t e d , when
poisoned
by
there i s a deposit
o f h i g h polymers. The use o f t h i s t y p e o f c a t a l y s t l e a d s t o v e r y f l e x i b l e p r o cesses, and by changing t h e o p e r a t i n g c o n d i t i o n s , p a r t i c u l a r l y t h e r e a c t i o n
260
temperature, one can improve :
-
e i t h e r t h e f o r m a t i o n o f dimers and t r i m e r s , thus o n l y o b t a i n i n g a c a r gaso-
l i n e w i t h an octane number v a r y i n g between 96 and 98 (see t a b l e I V ) ; - o r t h e f o r m a t i o n o f t r i m e r s and tetramers thus o b t a i n i n g a product which, from t h e p o i n t o f view of i t s f r e e z i n g p o i n t and smoke p o i n t , i s a h i g h q u a l i t y base f o r j e t f u e l (see t a b l e I V ) . TABLE I V The c h a r a c t e r i s t i c s o f t h e products obtained by n o n - s e l e c t i v e o l i g o m e r i z a t i o n of butenes i n t h e presence o f amorphous s i l i c a - a l u m i n a s . Variants
-
-
-
-
C h a r a c t e r i s t i c s o f t h e products
CaL-!la?oline D e n s i t y a t 20 "C D i s t i l l a t i o n curve (ASTM) i n i t i a l point 50 % volume end p o i n t Octane number research c l e a r motor c l e a r
0.739
. .
52 "C 124 "C 218 "C
. . .
97 84
J_et-fuel_ D e n s i t y a t 20 "C D i s t i l l a t i o n curve (ASTM) i n i t i a l point 50 % volume end p o i n t
0.798
. .
160 "C 209 "C 285 "C
.
X
<-
Freezing p o i n t Smoke p o i n t
60 "C 33 m i l l i m e t e r s
x a f t e r hydrogenation o f p r o d u c t These operations can be c a r r i e d o u t i n r e l a t i v e l y easy c o n d i t i o n s (temperature
: 70
-
150 "C, o p e r a t i n g pressure
15
- 40 bars)
which can be v a r i e d
according t o t h e o b j e c t i v e : be i t c a r g a s o l i n e o r j e t f u e l . S e l e c t i v e o l i g o m e r i z a t i o n (18).
By s e l e c t i v e p o l y m e r i z a t i o n i n a l i q u i d pha-
se, i t i s p o s s i b l e t o t r a n s f o r m t h e isobutene from t h e C4 c u t i n t o premium grade g a s o l i n e (octane number 102), w i t h o u t consuming any o r by j u s t consuming very few o f t h e butenes 1 and 2. A f t e r t h e separation o f t h e premium grade gasol i n e , i t i s much e a s i e r t o o b t a i n t h e butene 1 o f (99.5 %) p o l y m e r i z a t i o n grade by s u p e r f r a c t i o n a t i o n because t h e C4 c u t which i s t o be d i s t i l l e d i s v e r y poor i n isobutene. I n f a c t , t h e butene 1 and t h e isobutene have v e r y s i m i l a r b o i l i n g p o i n t s ( 2 ) (see t a b l e V ) . This p u r i f y i n g process can t h e r e f o r e be i n t e g r a t e d i n t o a p r o d u c t i o n l i n e o f h i g h p u r i t y butene 1. A t present, t h e demand f o r butene 1 i n Western Europe amounts t o 40000 tons per y e a r and w i l l c e r t a i n l y exceed lOOD00 tons per y e a r
261 i n 1990 (1). TABLE V C4 cuts from c a t a l y t i c cracking
-
Average weight Composition i n % Isobutane Isobutene Butene 1 N butane Butene 2 c i s Butene 2 trans
-
Composition
D i s t i l l a t i o n characteristics.
Boi 1i n g p o i n t i n "C
35 15 13 11 10 16
-
-
+
+
R e l a t i ve v o l a t i 1it y a t 60 "C
11.4 6.9 6.3 0.5 3.7 0.9
1.130 1.019 1.000 0.863 0.865 0.830
The composition o f a C4 c u t before and a f t e r s e l e c t i v e oligomerization, as w e l l as the p r o p e r t i e s o f the oligomerizate are shown i n the t a b l e V I . TABLE V I Composition and c h a r a c t e r i s t i c s o f the product obtained by s e l e c t i v e oligomer i zation.
!elsht-co!eosition-?! Isobutane N butane Butene 1 I sobutene Butenes 2 01 igomers
Feed --_35 11 13 15 26
-
Products 35 11 12.9 0.1 25.9 15.1
1 0.740
Density a t 20 "C ASTM d i s t i 11a t i on i n i t i a l point 20 % volume 50 I volume 70 % volume 90 % volume end p o i n t Octane number research c l e a r motor c l e a r
. . .
45 107 121 162 185 234
.
.
.
.
"C "C "C "C "C "C
102 84
.
The c a t a l y s t used are amorphous silica-aluminas which are i d e n t i c a l t o those used i n t h e non s e l e c t i v e process o f oligomerization. This operation i s c a r r i e d o u t i n m i l d conditions : temperature
50 t o 80 "C,
operating pressure
from 8 t o 16 bars, these being v a r i a b l e according t o t h e aims o f the operation.
262 I s o m e r i z a t i o n o f t h e carbon s k e l e t o n The m a r k e t f o r i s o b u t e n e as an i n t e r m e d i a t e i n t h e p r o d u c t i o n o f MTBE, and f o r i s o b u t a n e as an a l k y l a t i n g a g e n t i s r a p i d l y changing, and so t h e y m i g h t
soon become scarce, whereas l i n e a r s t r u c t u r e s a r e overabundant, and so t h e i s o m e r i z a t i o n o f t h e l a t t e r has been s t u d i e d and improved. I s o m e r i z a t i o n o f normal butane ( 2 2 ) .
The i s o m e r i z a t i o n o f n butane i s q u i t e
a d i f f i c u l t r e a c t i o n , and i t i s achieved w i t h c a t a l y s t s whose bases a r e p l a t i num s u p p o r t e d on h e a v i l y c h l o r i n a t e d alumina o'f l a r g e s p e c i f i c s u r f a c e area. T h i s t y p e o f c a t a l y s t p r e s e n t s a good s e l e c t i v i t y and g r e a t s t a b i l i t y , w i t h t h e c o n t i n u o u s i n j e c t i o n o f s m a l l amounts ( a few ppm a c c o r d i n g t o t h e f e e d ) o f promoters which a r e made up o f halogenated compounds such as CC14. The o u t l i n e o f t h e process i s q u i t e c l a s s i c a l and i n v o l v e s o n l y one i s o t h e r mal r e a c t o r , and a r e c y c l e o f t h e non t r a n s f o r m e d n butane. Because o f t h e s e n s i t i v i t y o f t h e c a t a l y s t t o w a t e r , t h e use o f d r i e r s on t h e feed and on t h e a u x i l i a r y hydrogen i s necessary. The average o p e r a t i n g c o n d i t i o n s o f t h e process a r e : temperature 180 220 "C,
-
hydrogen p a r t i a l p r e s s u r e 15 - 20 b a r s .
Under t h e s e c o n d i t i o n s , t h e r e s u l t s on a C4 c u t o f p r i m a r y d i s t i l l a t i o n a r e the following :
-
C4'
y i e l d from t h e feed
3 98 w e i g h t %
an approach t o thermodynamic e q u i l i b r i u m a 9 5 %, i n o t h e r words a r a t i o
of i s o b u t a n e / i s o b u t a n e
t
n butane around 58 %.
An i s o m e r i z a t i o n u n i t such as t h i s is g e n e r a l l y c o u p l e d t o an i s o b u t a n e -
n butenes a l k y l a t i o n u n i t . I s o m e r i z a t i o n o f t h e normal butenes (19, 20, 2 1 ) .
The development o f t h e
MTBE has been t h e d r i v i n g f o r c e b e h i n d t h e r e s e a r c h i n t o economical methods o f i s o m e r i z i n g n butenes, because a l a c k o f i s o b u t e n e i s l i k e l y t o o c c u r f r o m 1985 onwards. The p r e s e n t methods a v a i l a b l e r e q u i r e amorphous s i l i c a - a l u m i n a c a t a l y s t s , w i t h l o w s i l i c a c o n t e n t , o p e r a t i n g a t h i g h temperature. Under t h e s e c o n d i t i o n s , t h e i r d e a c t i v a t i o n t a k e s o n l y a s h o r t t i m e and t h e processes a r e c y c l i c . I n o r d e r t o l i m i t t h e i r d e a c t i v a t i o n t h r o u g h c o k i n g , and i n o r d e r t o i n c r e a s e t h e s e l e c t i v i t y o f t h e o p e r a t i o n , steam i s used. T h i s o p e r a t i o n i s c a r r i e d o u t i n q u i t e severe c o n d i t i o n s : temperature 480 Pressure 1
-
-
580 "C, o p e r a t i n g
5 b a r s and a steam/C4 c u t molar r a t i o between 0.5 and 3. Under
these c o n d i t i o n s , t h e r a t e of t r a n s f o r m a t i o n o f t h e butenes i s around 30 % and t h e s e l e c t i v i t y t o i s o b u t e n e from 92 t o 96 % ; t h e l e n g t h o f t h e r e a c t i o n c y c l e can v a r y between 30 and 40 hours. Some t y p i c a l r e s u l t s a r e p r e s e n t e d i n t h e t a b l e V I I .
263
TABLE VII Isomerization of the n butenes into isobutene. Weight composition o f product Ethane Ethylene Propane Propene Isobutane N butane Butene 1 I sobutene Butenes 2 01 igomers
Before i somerizati on
After i somerizat i on
0.01 0.36
0.07 0.19 0.01 1.50
7.14 20.29
5.69 19.95
12.34 1.26 58.60
16.08 20.60 34.24
-
c5+
1.58
Butadiene 1-3
0.09
Length of reaction cycle
Several days
The inclusion of an n butenes isomerizing unit into a production line of MTBE, increases the yield significantly. CONCLUSION To get the best out of the C4 cuts, these processes are used in different combination (1, 3) or with other processes like dehydrogenation ( 8 ) or hydroisomerization (23). New possibilities have been brought up recently like dehydrocyclodimerization (24) or aromatization (25, 26).
REFERENCES Derrien, Valorisation des fractions l@g@res du FCC, AFTP, Paris, March 28, 1984. F.M. Heck et al. , Hydrocarbon Processing 59 ( 4 ) 185 (1980). G. Martino, Some new aspects of catalysis in the field of refining, 9th Ibero-American Symposium on Catalysis, Lisbon, july 1984. D.V. Quang, C. Raimbault and M. Hellin, Hydrocarbon Processing 134, (sept. 1981). M. Derrien, Valorisation des coupes olgfiniques, AFTP, Paris, Nov. 6, 1979. 8. Torck, Catalyse acido-basique en phase homogGne, Techniques de 1'Ingenieur, J 1186, (1983). G.H. Dale, PD 8 ( 3 ) , 1 1 t h World Petroleum Congress, London 1983. G.R. Muddaris, M.J. Pettman, Hydrocarbon Precessing, 91, Oct. 1980. J. Weitkamp, S. Maixner, Erdol and Kohle, 2 (11) 523, 1983. F. Vachez, Rev. de Inst. Fr. Petr. 724 (1963). R. Csikos et al., Hydrocarbon Processing 121, july 1976. R. Csikos, I. Pallay and 3 . Laky, 10th World Petroleum Congress, Bucharest 1979. A. Convers, B. Juguin and B. Torck, Hydrocarbon Processing 95, march 1981. A. Gicquel and B. Torck, J. Catal. 9 (1983). M.
5 6
7 8 9 10 11 12 13 14
264
15 16 17 18 19 20 21 22 23 24 25 26
M.J. A s t l e , The Chemistry of Petrochemicals, Reinhold Pub. Cop. 1956. E. Weisang and P.A. Engelhard, Bul. SOC. Chim. F. 1811 (1968). B. Juguin and J. Miquel, I.F.P. - French Patent 84 401 246 (1984). B. Juguin, J. Cosyns and J. Miquel, I.F.P. US Patent 4 423 264 (1983). V . Choudhary, Chem. Ind. Dev. 8 ( 7 ) 32 (1974). B. Juguin and J. Miquel, 1.F.P: US Patent 4 434 315 (1984). G. F r a n t e t al., Huls US Patent 4 367 362 (1983). Making I s o p a r a f f i n s - O i l and Gas J. 66, august 1971. R.M. Heck e t al.., Ind. Eng. Chem. Prod. Res. Dev. 20 474 (1981). F. Janovski, F. Wolf and A. Sophianos, Chem. Techn. 2 33 (1979). J.F.G. E l l i s , B.P. US Patent 4 334 114 (1982). J.A. Johnson and G.K. H i l d e r , NPRA Annual Meeting, San Antonio (Texas), march 1984.
265
B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V..Amsterdam -Printed in The Netherlands
HYDROCRACKING OF n-HEPTANE ON Pt-HZSM-5.
EFFECT OF CALCINATION AND REDUCTION CONDITIONS G. GIANNETTO, G. PEROT and
M. GUISNET
U n i t @Associee au CNRS 350
-
U.E.R.
Catalyse
Organique
Sciences, 40 Avenue du Recteur Pineau
-
-
86022 P o i t i e r s Cedex (France)
ABSTRACT The temperature o f c a l c i n a t i o n under high a i r - f l o w qf a Pt-te ramine-ZSM-5 prepared by competitive i o n exchange o f HZSM-5 w i t h NH /Pt(NH ) !j+ has a s i g n i f i c a n t i n f l u e n c e on the platinum d i s p e r s i o n where& the 't?mperature o f r e duction under H has p r a c t i c a l l y no e f f e c t . The best metal dispersion was obtained, as wai the case w i t h P t - Y o r P t - X c a t a l y s t s , f o r a c a l c i n a t i o n temper a t u r e o f about 300°C. This shows t h a t the nature o f the support does n o t modif y s i g n i f i c a n t l y the decomposition process o f t h e P t - t e t r a m i n e complex. I n agreement w i t h the step-by-step process o f n-a1 kane transformation on b i f u n c t i o n a l c a t a l y s t s , the c a t a l y s t which i s t h e b e s t dispersed i s the most a c t i v e f o r n-heptane transfotmation and the most s e l e c t i v e i n isomerization. INTRODUCTION
The a c t i v i t y , t h e s t a b i l i t y and the s e l e c t i v i t y o f b i f u n c t i o n a l z e o l i t e c a t a l y s t s ( g e n e r a l l y noble metal-loaded a c i d z e o l i t e s ) are governed c h a r a c t e r i s t i c s o f t h e i r a c i d and hydrogenating s i t e s . I n p a r t i c u l a r ,
by the a bifunc-
t i o n a l c a t a l y s t prepared from a z e o l i t e w i t h given s t r u c t u r a l and a c i d propert i e s i s u s u a l l y a l l the more e f f i c i e n t as i t s hydrogenating s i t e s are more numerous and b e t t e r dispersed (1). The high dispersion o f platinum i n z e o l i t e s can be obtained by P t ( N H 3 ) i + exchange followed by c a l c i n a t i o n under a i r - f l o w and reduction under hydrogen. I t i s w e l l established i n the case o f P t - X o r P t - Y c a t a l y s t s t h a t the metal dispersion depends very much on the way the metal was introduced and p a r t i c u l a r l y on the conditions under which the c a l c i n a t i o n o f the platinum tetrammi ne zeol it e was c a r r i e d o u t (2-5)
.
This paper r e p o r t s on the influence o f t h e c a l c i n a t i o n and reduction condit i o n s o f a platinum tetrammine HZSM-5 c a t a l y s t (0.30 w t
I
o f P t ) on i t s a c t i v i t y
and s e l e c t i v i t y f o r n-heptane hydrocracking a t 250°C and 350°C. The c a t a l y s t samples were characterized by t h e i r a c t i v i t i e s f o r benzene hydrogenation and b y t h e i r accessible metal area. Dry a i r c a l c i n a t i o n , hydrogen treatment and react i o n were c a r r i e d o u t under atmospheric pressure i n a f l o w apparatus w i t h a fixed-bed r e a c t o r . For c a l c i n a t i o n
and hydrogen treatment, the h e i g h t of the
bed, the a i r o r hydrogen f l o w r a t e and temperature increases were chosen i n order t o avoid as much as possible the c o n t a c t o f desorbed H20 o r NH3 w i t h the catalyst
.
266
EXPERIMENTAL Catalysts The c a t a l y s t s were prepared from pure HZSM-5 ( s y n t h e t i z e d according t o Mobil p a t e n t s ) i n powder form o r from extrudates (2.5 x 1 mm) o f HZSM-5 ( 3 0 w t % ) and alumina (70 w t % ) . Platinum was introduced by c o m p e t i t i v e i o n exchange (72 hours w i t h 24 hours s t i r r i n g ) w i t h a 1/200 P t (NH3)4C12/NH4N03 s o l u t i o n so as t o o b t a i n a good uptake o f p l a t i n u m by the z e o l i t e and a homogeneous macroscop i c d i s t r i b u t i o n o f p l a t i n u m i n t h e extrudates ( 6 ) . Two s e r i e s o f Pt-HZSM-5 samples were obtained from the platinum tetrammine z e o l i t e by v a r y i n g e i t h e r the c a l c i n a t i o n o r the r e d u c t i o n temperature. I n t h e f i r s t s e r i e s the c a l c i n a t i o n was c a r r i e d o u t a t 350°C and the r e d u c t i o n between 400 and 550°C, i n t h e second the c a l c i n a t i o n between 250 and 500°C and the r e d u c t i o n a t 500°C. C a l c i n a t i o n was c a r r i e d o u t i n a dynamic f l o w r e a c t o r i n the f o l l o w i n g condit i o n s : z e o l i t e : 1 g ; h e i g h t o f bed : 0,5 cm ; a i r f l o w r a t e : 20 1 h - l ;
4 h r s h e a t i n g a t 180°C, l0C/mn temperature increase t i l l f i n a l c a l c i n a t i o n temperature ( t h i s temperature was maintained f o r 6 hours), c o o l i n g down t o room temperature.
P r i o r t o u t i l i z a t i o n , the c a t a l y s t (about 0,l g ) was heated
under H2 f l o w ( 4 1 h-')
f o r 6 hours a t the chosen r e d u c t i o n temperature.
Metal d i s p e r s i o n was determined i n a G.C.
p u l s e system by H 2 chemisorption
and H 2 - 0 2 t i t r a t i o n . The procedure used has already been described ( 7 ) . Reactions Reactions were c a r r i e d o u t i n a dynamic f l o w r e a c t o r under atmospheric pressure w i t h a hydrogenlhydrocarbon molar r a t i o o f 9. Benzene hydrogenation was c a r r i e d o u t a 100°C, n-heptane t r a n s f o r m a t i o n a t 2 5 O O C f o r 2 hours then a t 350°C f o r t h e same time. The r e a c t i o n products were analysed on l i n e u s i n g a 100 m Squalane SCOT c a p i l l a r y column. For benzene hydrogenation, the hydrocarbon f l o w r a t e and the q u a n t i t y o f c a t a l y s t were chosen i n order t o o b t a i n a conversion l e s s than 10% and f o r n-heptane transformation, l e s s than 20% a t 250°C. RESULTS Metal d i s p e r s i o n and a c t i v i t . v f o r benzene hvdroaenation F i g . l a shows the i n f l u e n c e o f the r e d u c t i o n temperature on the a c t i v i t y f o r benzene hydrogenation (AH) o f samples c a l c i n e d
at
350°C. A s l i g h t maximum
f o r AH i s observed a t 500°C. On t h e o t h e r hand, t h e c a l c i n a t i o n temperature has a v e r y s i g n i f i c a n t e f f e c t on A,,
(Fig. l b ) . The most a c t i v e c a t a l y s t c o r r e s -
ponds t o a c a l c i n a t i o n temperature of 300°C. AH being p r o p o r t i o n a l t o the metal surface area ( t u r n o v e r number between 1000 and 1300 h-'),
t h e same type of
curve w i l l r e s u l t i f we r e p l a c e AH i n F i g . 1 by the number o f accessible p l a t i num atoms.
267
a
AH
b
AH
I
40 0
4 50
550
50 0
300
50 0
400
TR(oc'
TCCC)
F i g . 1 : I n f l u e n c e o f t h e r e d u c t i o n temperature ( T ,a) and o f t h e c a l c i n a t i o n temperature (TC>b) on the a c t i v i t y f o r bekzene hydrogenation a t 100°C 1 (AH, 10-3mole h - l g- ) . A1203-Pt-H-ZSM-5 extrudates (+) Pt-H-ZSM-5
.
powder
(0).
I t can be noted t h a t AH i s independent o f the c a t a l y s t t e x t u r e (Pt-HZSM-5
powder o r A1203-Pt-HZSM-5 e x t r u d a t e ) . n-Heptane t r a n s f o r m a t i o n
A t 250°C as w e l l as a t 350"C, i s o m e r j z a t i o n and c r a c k i n g a r e p r a c t i c a l l y t h e o n l y r e a c t i o n s observed. However, toluene t r a c e s can be noted a t 350°C. For a l l the c a t a l y s t s t h e d e a c t i v a t i o n i s very slow (example F i g . 2 : t h e d e a c t i v a t i o n a t 250°C o f t h e c a t a l y s t c a l c i n a t e d a t 350°C then reduced a t 500°C). The t o t a l a c t i v i t y f i r s t increases p r o p o r t i o n a l l y t o the hydrogenating a c t i v i t y , AH,
(and t h e r e f o r e w i t h the number o f accessible platinum atoms) t o
reach then a p l a t e a u ( F i g . 3). The i s o m e r i z a t i o n / c r a c k i n g r a t e r a t i o equal t o zero f o r the ZSM-5 z e o l i t e increases c o n t i n o u s l y w i t h AH ( F i g . 3 ) . On a l l the c a t a l y s t s , methylhexanes are t h e main products o f n-heptane i s o m e r i z a t i o n (between 95 % t o 99 % depending on t h e c a t a l y s t ) . 2,3- and 2,4dimethylpentane a r e p r a c t i c a l l y the o n l y o t h e r products, as shown already ( r e f .
8 ) 2,2- and 3,3- dimethylpentane are n o t observed. The methylhexanes/dimethylpentanes r a t e r a t i o increases w i t h t h e hydrogenating a c t i v i t y : f o r a 20 % conversion r a t e , i t
is
20
for
t h e lowest value o f AH and 45 f o r t h e highest.
268
40-
20
*
-
*
I
-
10
I
I
1
t i m e cmn) F i g . 2 : Influence of working time on the cracking and isomerization a c t i v i t i e s a t 350°C (10-3mole h-’ g - l ) a t 250°C o f a powder Pt-H-ZSM-5 calcined and reduced a t 500°C The 2- methyl/3- methyl hexane r a t e r a t i o i s practically independent of the c a t a l y s t (between 1.60 and 1.70 f o r a 20 % conversion r a t e a t 250°C i . e . a value higher than the equilibrium value : l . l . ( r e f . 9 ) . On a l l the catalysts the main cracking products are butanes and propane (> 99 %) i n equimolar quantities w i t h a very high isobutane/n-butane r a t i o (Table 1 ) .
TABLE 1 Molar cracking product d i s t r i b u t i o n (%). (*) Equilibrium ( r e f . 9 ) .
~~
0
0
50.4
48.5
0.6
0.1
0.2
0.2
80 (1.2f
269
5
10
15
Fig. 3 : Activities of Pt-HZSM-5 catalysts f o r n-heptane transformation a t 250' (AT; lom3 mo1e.h-l.g-l) and isomerization/cracking s e l e c t i v i t i e s (I/C) versus t h e i r a c t i v i t i e s for benzene hydrogenation a t l0OoC (A,,, lom3 mole h -1 g -1 ). DISCUSSION This work confirms the significant e f f e c t the calcination temperature of Pt-(NH3)4-ZSM-5 has on platinum dispersion ( r e f . 2-5). This dispersion has a maximum value f o r a calcination temperature under dry a i r of about 300°C. I t was f o r t h i s same temperature that other authors ( r e f . 2,4,5) found a maximum dispersion value on Pt-Y and on Pt-X c a t a l y s t s , which shows t h a t the nature of the zeolite used as a support does not modify significantly the decomposition process of the tetrammine complex. According t o Reagan e t a1 . ( r e f . 4 ) the maximum dispersion value i s obtained a t the minimal temperature necessary t o a t t a i n complete decomposition of the complex in given experimental calcination conditions. Below t h i s minimal temperature, p a r t o f the complex i s not decomposed. Subsequently i t s reduction by H2 provokes the formation of neutral unstable species ( P t ( N H 3 ) 2 H 2 ) which lead t o P t agglomeration ( r e f . 1 0 ) . I t would also be the formation under a i r of these unstable species which could be respon-
270
s i b l e f o r t h e low p l a t i n u m d i s p e r s i o n d u r i n g t h e c a l c i n a t i o n above t h e minimal temperature. This f o r m a t i o n would i m p l y the f o l l o w i n g r e a c t i o n s ( r e f . 4,5)
-
[Pt(NH3)d 2t + L Pt(NH3)x12t [Pt(NH3)x12t 2Pto-(NH)
P t o t 2NH3
4
+ H2
Pto-(NH)
+
+
(4-x)NH3
2Ht t (x-1)uH3
2Pt0 t N2 t H2
(3)
Pt(NH3),H2
(4)
The treatment temperature under hydrogen has o n l y a very s l i g h t i n f l u e n c e on t h e p l a t i n u m d i s p e r s i o n . This treatment would have as o n l y r o l e t o e l i m i n a t e t h e oxygen adsorbed
on t h e m e t a l l i c p l a t i n u m formed by a u t o r e d u c t i o n of t h e
complex ( r e a c t i o n s 1-3). The change of the Pt-HZSM-5 a c t i v i t y i n n-heptane t r a n s f o r m a t i o n as a funct i o n o f t h e i r hydrogenating a c t i v i t y ( F i g . 3) i s t h a t can be p r e d i c t e d w i t h t h e c l a s s i c a l b i f u n c t i o n a l mechanism : For low hydrogenating a c t i v i t i e s , (AH),
of t h e c a t a l y s t s , t h e m e t a l l i c r e a c t i o n steps (n-alcane dehydrogenation, o l e f i n hydrogenation) a r e slow and t h e n-heptane t r a n s f o r m a t i o n r a t e i s p r o p o r t i o n a l t o AH. Above a c e r t a i n value o f AH t h e a c i d steps (rearrangement o r c r a c k i n g of i n t e r m e d i a t e o l e f i n s ) become l i m i t i n g and t h e n-heptane t r a n s f o r m a t i o n r a t e does no l o n g e r depend on AH b u t o n l y on t h e a c i d i t y o f t h e c a t a l y s t s . T h i s r a t e mole remaining c o n s t a n t on Pt-HZSM-5 c a t a l y s t s w i t h AH g r e a t e r than 5. h-lg-l ( F i g . 3 ) , i t can be concluded t h a t a t l e a s t these c a t a l y s t s p r e s e n t t h e same a c i d i t y . A c t u a l l y we were a b l e t o show t h a t t h e i n t r o d u c t i o n o f p l a t i n u m i n t o t h e ZSM-5 z e o l i t e d i d n o t m o d i f y i t s a c t i v i t y f o r m-xylene i s o m e r i z a t i o n ( r e f . 11) and so d i d
n o t modify i t s a c i d i t y .
The l a r g e amount o f isobutane i n the l i g h t products i n d i c a t e t h a t t h e e n t i r e c r a c k i n g r e a c t i o n f o l l o w s t h e rearranqement o f t h e n-heptane skeleton. This agrees p e r f e c t l y w i t h t h e r e a c t i o n a l schema proposed f o r n-alkane t r a n s f o r m a t i o n on b i f u n c t i o n a l c a t a l y s t s ( r e f . 1 ) .
This schema adapted t o t h e n-heptane t r a n s -
f o r m a t i o n ( F i g . 4) a l s o a l l o w s t o e x p l a i n the increase w i t h AH o f the r a t e r a t i o s o f t h e monobranched/bibranched
isomers and o f i s o m e r i z a t i o n / c r a c k i n g .
E f f e c t i v e l y t h e h i g h e r t h e v a l u e o f AH, t h e g r e a t e r , i n comparison w i t h t h e c a r b o c a t i o n rearrangement o r s c i s s i o n r a t e s , w i l l be t h e r a t e s o f c a r b o c a t i o n
271 nC7
mC6
dm C ,
IC,
+ c,
F i g . 4 : B i f u n c t i o n a l t r a n s f o r m a t i o n o f n-heptane (nC7) mC6 : methylhexane ; dmC5 : dimethylpentanes ; C+ : c a r b o c a t i o n ; 0 : o l e f i n ; 1 : l i g h t products. The r e v e r s i b l e t r a n s f o r m a t i o n o f an alkane i n t o a c a r b o c a t i o n (+---I:comprises ? chemical steps : dehydrogenation and hydrogenation on m e t a l l i c s i t e s , a d s o r p t i o n on Bronsted s i t e s o f t h e i n t e r m e d i a t e o l e f i n s (and d e s o r p t i o n ) and t r a n s p o r t steps from m e t a l l i c t o a c i d s i t e s (and v i c e versa). f o r m a t i o n ; t h e r e f o r e we s h a l l o b t a i n more s e l e c t i v e l y n-heptane i s o m e r i z a t i o n and t h e more s e l e c t i v e i t w i l l be i n monobranched isomers.
ACKNOWLEDGMENT The authors thank
D. Duprez and M. Mendez f o r d i s p e r s i o n measurement.
G. Giannetto thanks Consejo de D e s a r r o l l o C i e n t i f i c o y Humanistico de l a Univer-
s i d a d Central de Venezuela f o r g r a n t .
REFERENCES
1 M. Guisnet and G. Perot, " Z e o l i t e : Science and Technology" NATO AS1 S e r i e s F.R. R i b e i r o e t a l . Eds, Martinus Nyhoff Publishers, The Hague, Boston, Lancaster, 1984, p. 397. 2 T. Kubo, H . A r a i , H. Tominaga and T. Kunugi, B u l l . Chem. SOC. Jpn., 45 (1972) 607. 3 P. Gallezot, Catal. Rev, S c i . Eng., 20 ( 1 ) (1979) 121. 4 W.J. Reagan, A.W. Chester and G.T. K e r r , J. Catal., 69 (1981) 89. 5 a ) D. Exner, N. Jaeger, K . M o l l e r and G. S c h u l z - E k l o f f , J. Chem. SOC., Faraday Trans., 78 ( 1 ) (1982) 3537. b) D. Exner, N. Jaeger, R. Navak, G. S c h u l z - E k l o f f and P. Ryder, Proc. 5th. I n t . Symp. on Heterogeneus C a t a l y s i s , Varna, (1983), P u b l i s h i n g House of t h e B u l g a r i a n Academy of Sciences, 1983, Vol. 2, p. 13.
272
6
J.F. Le Page, "Catalyse de Contacte", p . 141, Technip Editions, Paris, (1978). 7 D. Duprez, A. Miloudi, J. L i t t l e and J. Bousquet, Applied Catalysis, 5 (19831 219. 8 G. Pehot, P. H i l a i r e a u and M. Guisnet, Proc. 6th. I n t . Z e o l i t e Conference, Reno, (1983) , i n press. 9 D.R. S t u l l , E.F, Westrum, Jr., and G.C. Sinke, "The Chemical Thermodynamics o f Organic Compounds", Wiley, New York, Chichester, Brisbane, Toronto, (1969). 10 R.A. D a l l a Betta and M. Boudart, Proc. 5th. I n t . Cong. Catalysis, Palm Beach, (1972), Noth Holland, Amsterdam, 1972, Vol. 2, p. 1329. 1 1 . G. Giannetto, G. B o u r d i l l o n and M. Guisnet, t o be published.
273
B. Imelik e t al. (Editors), Catalysis by Acids and Bases D 1985 Elsevier Science Publishers B.V..Amsterdam -Printed in The Netherlands
TRANSITION IONS EXCHANGED ZEOLITES AS CRACKING CATALYSTS D . CORNET and A . CHAMBELLAN
" S t r u c t u r e e t R e a c t i v i t e d'Especes Adsorbees", U.A. C.N.R.S.
n o 414 ; I n s t i t u t
des Sciences de l a M a t i e r e e t du Rayonnement, 5 avenue d'Edimbourg, 14032 Caen Cedex (France)
ABSTRACT The c a t a l y t i c and s t r u c t u r a l p r o p e r t i e s o f chroniium-exchanged Y z e o l i t e s a r e compared w i t h those o f t h e p r o t o n forms NaHY and HY. Using t h e c r a c k i n g and i s o m e r i z a t i o n o f 3-methylpentane as a t e s t r e a c t i o n , i t i s found t h a t chromium enhances t h e a c t i v i t y o f NaHY z e o l i t e , b u t n o t t h a t o f HY. Chromium-containing z e o l i t e s a r e a l s o c h a r a c t e r i z e d by a m o d i f i e d s e l e c t i v i t y ( i s o m e r i z a t i o n vs. cracking), a s m a l l e r r a t e o f c o k i n g , a s m a l l number o f t h e r m a l l y s t a b l e OH groups, and a h i g h e r s t a b i l i t y o f t h e l a t t i c e . B u t t h e number and s t r e n g t h o f a c i d i c s i t e s a r e comparable i n t h e C r - and H- forms. RESUME
Les p r o p r i e t e s c a t a l y t i q u e s e t s t r u c t u r a l e s des z @ o li t h e s Y chromees s o n t comparees a c e l l e des formes protonees NaHY e t HY. On u t i l i s e comme t e s t react i o n n e l l e craquage e t 1 ' i s o m e r i s a t i o n du m e t h y l - 3 pentane. L ' a j o u t d ' i o n s chromiques c o n f e r e une a c t i v i t e a l a z e o l i t h e NaHY mais ne m o d i f i e pas c e l l e de l a HY. Les z e o l i t h e s chromees se c a r a c t e r i s e n t egalement p a r une s e l e c t i v i t e d i f f e r e n t e ( i s o m e r i s a t i o n augmentee p a r r a p p o r t au craquage), une v i t e s s e de cokage diminuee, un nombre peu e l e v e d ' h y d r o x y l e s s t a b l e s a h a u t e t e m p e r a t u r e e t une p l u s grande s t a b i l i t e du reseau. Mais l e nombre e t l a f o r c e des s i t e s a c i d e s des z e o l i t h e s chromees ou protonees s o n t comparables. INTRODUCTION Introducing t r a n s i t i o n ions i n t o a z e o l i t e cracking c a t a l y s t i s often i n t e n ded t o improve t h e t h e r m a l and hydrothermal s t a b i l i t i e s . B u t t h e added i o n s may, among o t h e r s , m o d i f y t h e c a t a l y s t a c i d i t y , t h u s a f f e c t i n g t h e a c t i v i t y and s e l e c t i v i t y f o r t h e c r a c k i n g , and a l s o t h e r a t e of coke f o r m a t i o n . S e t t i n g a p a r t t h e m o d i f i c a t i o n s b r o u g h t a b o u t by r a r e - e a r t h i o n s , s t u d i e s have appeared deal i n g w i t h t h e use o f t r a n s i t i o n i o n s - m o d i f i e d z e o l i t e s i n f l u i d i z e d bed p r o c e s ses : i t i s o f t e n c l a i m e d i n d e e d t h a t t h o s e c a t a l y s t s e x h f b i t a b e t t e r s e l e c t i v i t y , a s l o w e r r a t e o f c o k i n g , and improved r e g e n e r a t i o n p r o p e r t i e s ( r e f . 1 ,
2).
I n t h e case o f chromic i o n s , i t has been suggested t h a t t h e r e a c t i o n of w a t e r t h e r m o l y s i s p l a y e d an i m p o r t a n t r o l e i n t h e whole c a t a l y t i c c y c l e . As t h e r e o x i d i z e d c a t a l y s t i s s u b m i t t e d t o d e h y d r a t i o n a t temperatures h i g h e r t h a n 773 K, a p a r t i a l r e d u c t i o n o f t h e C r c a t i o n s occurs, evidenced by oxygen f o r m a t i o n ( r e f 3) :
274
R e h y d r a t i n g t h e c a t a l y s t a t a l o w e r temperature, f o r i n s t a n c e i n t h e steam s t r i p p i n g s e c t i o n , p r o v i d e s f o r t h e second s t e p o f w a t e r t h e r m o l y s i s , w i t h hydrogen e v o l u t i o n :
"(zeal.) 2+ +
H20
-
3+ + -21 H2 + "(zeal .)
OH-
I t i s t h o u g h t t h a t t h e f i r s t r e a c t i o n produces c a t a l y s t - b o u n d hydrogen spe-
c i e s , t h a t l a t e r on a r e t r a n s f e r r e d o n t o t h e cracked p r o d u c t s . I n t h i s way, t h e enhanced r a t e o f c r a c k i n g and t h e m o d i f i e d s e l e c t i v i t y f o r p r o d u c t s a r e b o t h rationalized (ref.1). The case of n i c k e l and vanadium s h o u l d a l s o be mentioned.
Although n i c k e l
i s recommended i n some hydrogen-consuming processes ( r e f . 4 ) i t i s g e n e r a l l y f o u n d t h a t N i and V deposed f r o m t h e charge c o n f e r t o t h e z e o l i t e some dehydrog e n a t i o n f u n c t i o n , t h u s f a v o r i n g coke b u i l d - u p ( r e f . 5 ) . Moreover, vanadium i s d e t r i m e n t a l t o t h e z e o l i t e s t a b i l i t y . Several processes, f o r i n s t a n c e i n c o r p o r a t i o n o f boron,antimony o r z i n c ( r e f . 6 ) have been d e s c r i b e d t o p a s s i v a t e contaminants such as N i ,
V o r Fe and, i n t h e case o f n i c k e l , r e d u c i n g t h e c a t a -
l y s t a f t e r r e g e n e r a t i o n i s u s e f u l i n r e d u c i n g coke f o r m a t i o n ( r e f . 7 ) . Our purpose h e r e w i l l be l i m i t e d t o t h e chromium-containing z e o l i t e s . A s e r i e s of Y z e o l i t e s w i t h d i f f e r e n t sodium c o n t e n t s w i l l be m o d i f i e d by i n t r o d u c i n g Cr(H20);+,
and we s h a l l f o l l o w t h e ensuing changes i n c a t a l y t i c a c t i v i -
ty, s e l e c t i v i t y , t h e a c i d i t y and some s t r u c t u r a l f e a t u r e s .
EFFECT OF CHROMIUM UPON 3-METHYLPENTANE CONVERSION The f o l l o w i n g s e r i e s o f z e o l i t e s was p r e p a r e d s t a r t i n g w i t h NaY ( S t r e m ) . Symbol
Na c o n t e n t w t % ion/u.c
C r content Preparation w t % ion/u.c
NaHY NaCrY NaHCrY HY HCrY
3.26 5.1 1.12 0.20 0.11
0 4.12 5.46 0 3.07
18 28 6 1 0.6
0 10 12.9 0 7
Ion-exchange w i t h NH4N03 Ion-exchange w i t h Cr(N03)3 Successive NH4+, t h e n Cr3+ Exch. w i t h N a t 793 K, exch. NH Exch. w i t h a t 793 K, exch.Crj+
’
B e f o r e t h e c a t a l y t i c t e s t i s performed, z e o l i t e samples a r e a c t i v a t e d f o r 15 h r s a t 633 K, under h e l i u m o r oxygen f l o w . T h e i r r e s i d u a l w a t e r c o n t e n t i s a b o u t 2 % by w e i g h t . The c o n v e r s i o n o f 3-methylpentane i s performed a t 633 K ( o r 593
K), w i t h t h e c a t a l y s t p l a c e d i n f i x e d bed i n a c o n t i n u o u s f l o w r e a c t o r .
The c a r r i e r gas i s helium, and t h e hydrocarbon p a r t i a l p r e s s u r e amounts t o 2.3 x 103 Pa. As t h e c a t a l y s t s s t e a d i l y l o s e a c t i v i t y , t h e i r a c t i v i t i e s were
275
compared a f t e r a c o n s t a n t working time, namely 6 mn. Analysis o f products o b t a i ned i n these standard c o n d i t i o n s over f i v e z e o l i t e samples i s shown i n Table 1. We f i r s t note t h a t t h e o v e r a l l conversion g r e a t l y v a r i e s from sample t o sample. For t h e f u l l y exchanged c a t a l y s t s , t h e e f f i c i e n c y o f z e o l i t e H C r Y i s n o t h i g h e r than t h a t o f HY f o r c o n v e r t i n g 3-methylpentane. For t h e p a r t i a l l y exchanged c a t a l y s t s , t h e doubly exchanged NaHCrY i s d e f i n i t e l y more a c t i v e than t h e single-exchanged forms NaHY and NaCrY, which, however, have a h i g h e r sodium c o n t e n t . TABLE 1 Reaction o f 3-methylpentane a t 633 K Product d i s t r i b u t i o n and s e l e c t i v i t y a f t e r 6 mn working time ( r e f . 1 0 ) ~~
~
Catalyst O v e r a l l conversion %
NaHCrY 3.5
HCrY 25.7
1.7 4.2 23.4 20.8 3.5
-
-
4.2 16.7 25.1 9.0
1.9 28.5 35.8 0.9
1.8 33.5 42.4 1.1
-
-
0.2
0.1
NaHY 1.3
NaCrY 0.8
0.1 2.8 41.2 21.4 5 .O
-
Cracking
C1 C2 c3 c4 c5
2,2-dimethylbutane 2,3-dimethylbutane 2-me thy1 pentane n-hexane
2.1
HY 28.7
2.8
1.o
7.8
5.8
1.o
15.8 0.1
9.4 0.1
28.6 0.6
24.9 1.2
19 .o 0.9
6.2
17.8
6.8
0.4
0.7
4.6
14.6
5.5
0.5
0.2
0.46
0.33
0.20
1.1 0.4 0.3
0.2 0.03 0.03
0.1 0.04 0.05
Dehydrogenation alkenes
I/\
Dehydroisom&i z a t i o n alkenes
h/\
Selectivity ratios Xiso/Xiso
+
xcrack
C4H8/C4H 10 C5H10ICgH12 C6H12/C6H142 2
0.25 1.6 0.8 0.6
0.30 10.5 9.0 3.1
C6H14 amount excludes 3-methyl pentane. The two main r e a c t i o n s observed on these a c i d i c surfaces a r e c r a c k i n g and
s k e l e t a l i s o m e r i z a t i o n . Cracking i s favored on every of t h e c a t a l y s t s , and
276
y i e l d s m a i n l y C3 and C4 hydrocarbons. Thus, c r a c k i n g i s n e i t h e r the r e s u l t o f a s i n g l e s p l i t t i n g o f t h e C6 hydrocarbon i n t o C2 + C4,
n o r C1
+
C5. The d i s t r i b u -
t i o n o f cracked products r a t h e r suggests t h a t b i m o l e c u l a r processes occur. S k e l e t a l i s o m e r i z a t i o n i s r a t h e r s e l e c t i v e , y i e l d i n g m a i n l y 2-methylpentane, w i t h s m a l l e r amounts of 2,3-dimethylbutane.
A t t h e h i g h e r conversions obtained
o v e r HY and HCrY, small amounts of n-hexane and 2,2-dimethylbutane
are also
observed. I n the data r e p o r t e d i n Table 1, hexenes w i t h t h e s k e l e t o n o f 2-methylpentane and 2,3-dimethylbutane t h e s e l e c t i v i t y r a t i o xiso/xiso
have been added t o t h e alkanes i n computing
+
xcrack.
Concerning t h i s s e l e c t i v i t y , d a t a
from Table 1 c l e a r l y show t h a t a d d i t i o n o f chromium t o e i t h e r NaY o r HY f a v o r s s k e l e t a l i s o m e r i z a t i o n versus c r a c k i n g . Other r e a c t i o n products mentionned i n Table 1 i n c l u d e hexenes, m a i n l y isohexenes. Thus, t h e s t a r t i n g a1 kane undergoes some dehydrogenation, b u t t h e prop o r t i o n o f o l e f i n s decreases as t h e conversion l e v e l r i s e s . T h i s r e a c t i o n r e v e a l s t h a t hydrogen t r a n s f e r s d o occur. May these be r e l a t e d t o c a t a l y s t c o k i n g ? The amount o f coke i s n o t shown i n t h e d a t a o f Table 1, as i t s r a t e of d e p o s i t has n o t been monitored. Nevertheless, t h e c a t a l y s t d e a c t i v a t i o n as a r e s u l t o f i r r e v e r s i b l e c o k i n g may be determined by measuring the r e a c t i o n conv e r s i o n as a f u n c t i o n o f c a t a l y s t time-on-stream t. The q u a n t i t i e s measured a r e t h e degrees o f conversion xiso(t)
and xcrack(t).
Assuming the s i m p l i f i e d reac-
t i o n network w i t h f i r s t - o r d e r r e a c t i o n s :
cracked products t h e two r a t e constants may be computed a t any working time, f o r s k e l e t a l isomerization :
and f o r c r a c k i n g :
Thus, under constant f l o w c o n d i t i o n s , we determined t h e e v o l u t i o n of t h e two r a t e s : riso = kl(t)
Co and rcrack = k 2 ( t ) Co,
w i t h time-on-stream.
Results
obtained f o r the f i v e h e l i u m - a c t i v a t e d z e o l i t e s working under a hydrocarbon flow
F, = 5.11)~~ mole h-' a r e w e l l c o r r e l a t e d w i t h t h e two-parameter formula :
r = a t -b
Some o t h e r d e a c t i v a t i o n laws were t r i e d , f o r i n s t a n c e r = (1 t B t ) - b ,
but
t h e y r e s u l t e d i n e i t h e r i m p r e c i s i o n i n d e t e r m i n i n g t h e parameters, o r p o o r c o r r e l a t i o n ( r e f . 8 ) . The s i m p l e r t-bf o r m u l a was t h e n p r e f e r r e d , a l t h o u g h i t f a i l s t o y i e l d a c t i v i t i e s a t z e r o w o r k i n g t i m e . The v a l u e s computed f o r t h e exponent b i n t h e r a t e o f c r a c k i n g a r e g i v e n i n T a b l e 2, t o g e t h e r w i t h t h e v a l u e s o f risoand
rcrack a f t e r t = 6 mn. I t appears t h a t i n t r o d u c i n g chromium i n t o z e o l i t e s NaHY tends t o i n c r e a s e t h e v a l u e o f exponent b ; hence t h e r a t e o f c o k i n g i s i n c r e a sed, a t l e a s t when t h e c a t a l y s t has been d e h y d r a t e d under helium. The e f f e c t on z e o l i t e NaHCrY i s r a t h e r s t r i k i n g . I t i s f e a s i b l e t o l o o k f o r a r e l a t i o n s h i p between c o k i n g tendency ( i l l u s t r a -
t e d by parameter b ) and t h e importance o f processes i n v o l v i n g H - t r a n s f e r .
Such
r e a c t i o n s produce hexenes, b u t a l s o decrease t h e amount o f alkenes i n f r a c t i o n s
C4 and C 5 . From o u r r e s u l t s , such a r e l a t i o n s h i p does n o t appear t o be v a l i d . F o r example, z e o l i t e NaHY d e a c t i v a t e s more s l o w l y t h a n NaCrY a l t h o u g h i t i s more e f f i c i e n t f o r h y d r o g e n - t r a n s f e r processes. TABLE 2 D e a c t i v a t i o n o f c r a c k i n g w i t h time-on-stream a t 633 K ( r e f . l O ) , Catalyst Rates a f t e r 6 mn (mole h - 1 kg-1 x l o 2 ) b c r a c k (He a c t i v a t i o n ) activation) bcrack
(9
riso rcrack
NaHY
NaCrY
NaHCrY
HCrY
HY
3 9.1
2.6 5.8
18.3 21.4
138 235
112 339
0.18
0.23 0
0.37 0.17
0.20 0.21
0.20
-
-
The e f f e c t o f an o x i d i z i n g atmosphere d u r i n g c a t a l y s t p r e t r e a t m e n t was a l s o examined. Such a t r e a t m e n t b r i n g s most o f t h e C r i o n s t o t h e s t a t e o f Cr(V1) ( r e f . 9 ) . The r a t e s o f 3-methylpentane c o n v e r s i o n were a l s o determined a t v a r i o u s times-on-stream o f t h e C r c a t a l y s t s . F o r s h o r t t i m e s , t h e y d i f f e r v e r y l i t t l e f r o m those o b t a i n e d a f t e r a c t i v a t i o n under helium, b u t ( s e e T a b l e 2 ) p r e t r e a t ment w i t h oxygen r e s u l t s i n l o w e r v a l u e s f o r exponent b.
hus t h e 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 i s d e f i n i t e l y lowered, i n agreement w i t h
ref.1).
It was a l s o
v e r i f i e d t h a t t h e c r a c k i n g a c t i v i t y o f t h e coked c a t a l y s t c o u l d be f u l l y r e s t o r e d a f t e r a i r - r e g e n e r a t i o n performed a t 673 K. SURFACE ACIDITY The r e a c t i o n s examined h e r e a r e e v i d e n t l y a c i d - c a t a l y z e d , and we may ask i n which way t h e a c i d i t y o f z e o l i t e s NaHY and HY i s m o d i f i e d upon C r a d d i t i o n . To s o l v e t h i s , a t i t r a t i o n o f t h e number o f a c i d s i t e s was undertaken, t h r o u g h t h e method o f p r o g r e s s i v e p o i s o n i n g by a b a s i c m o l e c u l e . The c o n v e r s i o n o f 3-methylpentane was t h e n r u n a t 593 K, and a l i q u o t s o f b u t y l a m i n e ( i n some cases q u i n o -
278 1 i n e ) were i n j e c t e d upon t h e c a t a l y s t s i n a1 t e r n a n c e w i t h t h e r e a g e n t 3-methylpentane. The r a t e s o f c r a c k i n g ( rcrack) and i s o m e r i z a t i o n ( rise) were determined as above, when t h e c o n v e r s i o n reached a s t a b l e v a l u e a f t e r each a d d i t i o n o f base. T y p i c a l p o i s o n i n g curves showing
r r
0
= f (added base) a r e shown on F i g . l .
HCrY (Butylamine)
\
\"\
I
r u n p o i soned
@ c r a c k = 389 = 407
$iw
I
\
1
1.8
2
*
NaHY (Quinoline) @crack = @is0 = 3
I o Cracking
\ \
\
Isornerization
F i g . 1. P o i s o n i n g o f c a t a l y t i c a c t i v i t y by bases ( r e f . 1 0 ) . The shapes o f these curves f i r s t suggest t h a t t h e y a r e n o t s t r o n g l y a f f e c t e d by i n t r a p a r t i c l e d i f f u s i o n o f t h e hydrocarbon r e a g e n t . However t h e e s t i m a t e d values o f t h e T h i e l e modulus I$ f o r t h e unpoisoned c a t a l y s t a r e r a t h e r h i g h (see F i g . 1) showing t h a t t h e r e a c t i o n s on t h e most a c t i v e c a t a l y s t s a r e d i f f u s i o n - c o n t r o l l e d under o u r c o n d i t i o n s ( g r a i n s i z e 1 urn, l o w hydrocarbon concent r a t i o n ) . B u t a p p a r e n t l y p o i s o n i n g b y t h e base occurs r a t h e r u n i f o r m l y i n each r p l o t a r e n o t v e r y pronoun-
p a r t i c l e , so t h a t d e v i a t i o n s f r o m l i n e a r i t y i n t h e ced ( r e f . 1 1 ) .
r0
It i s t h e n necessary t o e x t r a p o l a t e t h e curves p e r t a i n i n g t o c r a c -
219
k i n g towards t h e p o i n t o f zero a c t i v i t y because, i n t h i s region, small amounts o f butylamine degradation products mix up w i t h t h e products o f 3-methylpentane cracking. The numbers o f a c i d s i t e s determined i n t h i s way f o r t h e s e r i e s o f z e o l i t e s i n v e s t i g a t e d a r e r e p o r t e d i n Table 3. TABLE 3 Numbers o f a c i d i c s i t e s (N meq.g-l) measured by p o i s o n i n g 3-methylpentane conversion a t 593 K . (Po : r a t e f o r unpoisoned c a t a l y s t , as mole h-’.kg-’) Zeol it e
NaHY
NaCrY
NaHCrY
HCrY
HY
Base
Quinoline
Butylamine
Butylamine
Butylamine
Butylamine
I somerization
-
is0
0.06
0.06 0.02
0.8 0.59
1.8 2.68
1.7 1.68
1 0.31
0.04 0.07
0.9 0.40
2.56
1.8 1.93
‘2;:~
Cracking Ncrack ( r o ) crack
From these, we conclude t h a t adding chromium t o e i t h e r z e o l i t e NaHY o r HY does n o t modify t h e number o f a c i d i c s i t e s . The r e a c t i o n r a t e s f o r t h e unpoisoned c a t a l y s t s are also r e p o r t e d i n t h e Table : they a r e n o t p r o p o r t i o n a l t o t h e numbers provided by base t i t r a t i o n . The method o f base p o i s o n i n g measures t h e p o p u l a t i o n o f a s e t o f a c i d s i t e s , a small f r a c t i o n o f them o n l y s e r v i n g as c a t a l y t i c a l l y a c t i v e s i t e s . But t h e p r o p o r t i o n o f these s t r o n g s i t e s considerab l y v a r i e s w i t h t h e e x t e n t o f exchange. The behavior o f z e o l i t e NaHY i s p e c u l i a r : i t decomposes butylamine t o o f a s t , b u t i n j e c t i o n s o f q u i n o l i n e poisons t h e alkane cracking, n o t t h e i s o m e r i z a t i o n . Z e o l i t e HY behaves i n r a t h e r the same way. We i n f e r t h a t these c a t a l y s t s c a r r y a t l e a s t two types o f s i t e s , perhaps s p e c i f i c f o r each process. These r e s u l t s and o t h e r s obtained from
i s o m e r i z a t i o n and c r a c k i n g o f hexenes ( r e f . 1 0 ) suggest
a c l o s e r e l a t i o n s h i p between t h e s t r e n g t h o f t h e a c i d s i t e and t h e n a t u r e o f the c a t a l y t i c transformation. NUMBER OF HYDROXYL GROUPS
Chromium i o n s i n t r o d u c e d i n t o z e o l i t e s a r e l i k e l y t o increase t h e i r number o f hydroxyls. Upon thermal a c t i v a t i o n , t h e C r 3 + c a t i o n f i r t s loses coordinated water molecules, and beyond 573 K , the remaining H20 l i g a n d may be d i s s o c i a t e d according t o
280
0
0
Do t h e s e p r o t o n s combine w i t h framework oxygens t o f o r m s t a b l e OH g r o u p i n g s ? P e r f o r m i n g t h e r m o g r a v i m e t r i c a n a l y s i s under n i t r o g e n f l o w g i v e s i n f o r m a t i o n s a b o u t t h e number o f OH bound t o t h e a c t i v a t e d z e o l i t e . The f i r s t p a r t o f t h e w e i g h t loss, up t o 773 K, corresponds t o t h e e v o l u t i o n o f water, and ofammonia i n t h e NH;
-exchanged c a t a l y s t s . It i s f o u n d t h a t ammonia i s r e l e a s e d a t l o w e r
temperatures f r o m t h e C r - c o n t a i n i n g z e o l i t e s . Then t h e w e i g h t l o s s due t o w a t e r l e a v i n g t h e s o l i d f r o m 773 K up t o t h e l a t t i c e c o l l a p s e temperature p r o v i d e s a measurement o f t h e OH number, a t a r a t e o f two OH p e r r e l e a s e d H20. The c o r r e s ponding f i g u r e s f o r o u r c a t a l y s t s a r e r e p o r t e d i n T a b l e 4. TABLE 4 H y d r o x y l numbers and c r y s t a l l o g r a p h i c parameter a f o r m o d i f i e d Starting material
Cr ions/u.c.
NH4 ions/u.c.
mole/u.c.
NaY NaCrY NaNH4Y NaCrNH4Y NH4HYx HNH4CrY*
0 12.5 0 7 0 7
0 0 44 21 22 4
8 9.5 30 19 36 22
Y
zeolites
Alcat ion/u.c.
Temp. (K) o f collapse
Parameter a (nm) 873 Kxx 300 K
1.5 >5 4.5 6.2 7.2
1125 1223 1210 1253 1253 1268
2.466 2.463 2.469 2.467 2.465 2.462
-
2.464 2.460 2.457 2.461 2.446 2.445
x Z e o l i t e s steamed a t 793 K b e f o r e t h e second i o n exchange xx A c t i v a t i o n i n deep bed. We see t h a t t h e l a r g e s t number o f OH s t a b l e up t o 773 K belong t o t h e z e o l i t e s w h i c h c o n t a i n e d NH;
a t t h e s t a r t o f t h e TGA. These h y d r o x y l s e i t h e r a r e
p r e s e n t i n t h e s t a r t i n g m a t e r i a l , o r appear upon thermal a c t i v a t i o n d u r i n g TGA. B u t Cr-Na c o n t a i n i n g z e o l i t e s possess v e r y few h i g h t e m p e r a t u r e s t a b l e hydroxyls.
As a rough f i g u r e , one C r produces 2/3 OH, which i s n e a r l y t h e v a l u e y i e l d e d by t
a NH4
i o n . Thus t h e amount o f s t a b l e OH has no d i r e c t consequence upon c a t a l y -
t i c a c t i v i t y . The l a t t e r i s m o s t l y a f u n c t i o n o f t h e r e s i d u a l sodium c o n t e n t . MODIFICATIONS OF THE ZEOLITE FRAMEWORK An X-ray s t u d y ( r e f . 1 2 )
has shown t h a t , i n t h e t h r e e t y p e s o f C r - z e o l i t e s ,
NaCrY, NaHCrY and HCrY, t h e C r i o n s e x h i b i t e d a s t r o n g p r e f e r e n c e f o r s i t e s
I'
as l o n g as t h e C r c o n t e n t d i d n o t exceed 8 i o n s p e r u n i t c e l l . I t i s found t h a t C r i o n s i n t h e s o d a l i t e cages a r e bound t o t h r e e oxygensof a p r i s m , w i t h one
e x t r a l i g a n d , p r o b a b l y an OH. T h i s makes p l a u s i b l e t h e above-mentioned c l e a v a g e
281 o f an aquo l i g a n d . Table 4 a l s o r e p o r t s t h e c e l l parameter a, measured f i r s t on t h e s t a r t i n g z e o l i t e , then a f t e r thermal a c t i v a t i o n a t 873 K. I n t r o d u c t i o n o f chromium e n t a i l s a small c o n t r a c t i o n o f t h e c e l l dimensions, w h i l e NH;
has t h e o p p o s i t e e f f e c t .
A f t e r d e h y d r a t i o n performed i n a m u f f l e f u r n a c e a t 873 K , t h e a parameter o f ammonium-containing z e o l i t e s s h a r p l y decreases. T h i s e v o l u t i o n denotes a m o d i f i c a t i o n o f t h e l a t t i c e which, under t h e s e s e l f - s t e a m i n g c o n d i t i o n s , a c q u i r e s a b e t t e r s t a b i l i t y . T h i s b e h a v i o r i s n o t observed upon h e a t i n g NaCrY z e o l i t e s . Besides, t h e e x o t h e r m i c peak a t T > 1000 K i n t h e DTA p a t t e r n i n d i c a t e s t h e l a t t i c e breakdown, and t h u s g i v e s an i d e a upon t h e r e l a t i v e s t a b i l i t i e s o f t h e d i f f e r e n t samples. F i g . 2 shows t h e v a r i a t i o n o f t h e breakdown temperature as a f u n c t i o n o f t h e sodium exchange l e v e l ( r e f . 1 4 ) .
15
1. Na56Y
2. NH444Na12Y 3. N H ~ , ~ ~ N a ~ H y / 7 9 3 '
4 * Cr12.5Na24Y 5. C r 7 .2NH46HY/923x 6. Cr7NH44HY/793x 7. Cr6Na37Y
x These z e o l i t e s were I
I
I
I
10
30
50
steamed a t t h e quoted temperature.
* Na/u .c
F i g . 2. Temperature o f thermal c o l l a p s e v e r s u s Na c o n t e n t . F o r t h e NaCrY samples, t h e s t a b i l i t y i s a p p r e c i a b l y i n c r e a s e d a t t h e h i g h e s t exchange l e v e l s : Tdec moves t o 1223 K f o r 12.5 C r p e r u n i t c e l l ( s e e T a b l e 4). I t i s w o r t h n o t i n g t h a t t h i s i n c r e a s e d s t a b i l i t y i s p r e s e r v e d when t h e DTA i s
performed under oxygen f l o w and t h u s most o f t h e Cr3'
are oxidized.
In t h e s e r i e s i n v e s t i g a t e d here, t h e i n t r o d u c t i o n of chromium n e v e r causes d e s t a b i l i z a t i o n ; and t h e r e l a t i v e i n s t a b i l i t i e s o f NaNH4Y and NaNH4HY (steamed a t 793 K ) a r e a t t e n u a t e d upon replacement o f NH;
by C r 3 + .
I t was t h e n p o s s i b l e
t o o b t a i n an u l t r a s t a b l e H C r Y (Tdec > 1250 K) by steaming an NaNH4Y f o r m a t 923 K, f o l l o w e d by exchange w i t h C r 3 + . Modifications leading t o s t a b i l i z a t i o n o f the z e o l i t e involve i n the f i r s t p l a c e t h e e x t r a c t i o n o f some framework aluminiums, which m i g r a t e towards c a v i -
282
t i e s (ref.15).
I n our s e r i e s , t h e amount o f extra-framework A1 , denoted A l c a t
i n Table 4, was estimated upon e x t r a c t i n g w i t h a 0.1 N sodium hydroxide s o l u t i o n the z e o l i t e s p r e v i o u s l y heated a t 633 K. The r e s u l t s a r e o n l y i n d i c a t i v e , and probably w e l l under t h e r e a l values. Nevertheless, these f i g u r e s show t h a t t h e C r - z e o l i t e s , as w e l l as t h e p r o t o n forms, possess a l a r g e number o f Al-oxo c a t i o n s . As these a r e o f t e n r e p o r t e d t o be a source o f a c i d i c s i t e s , i t appears from o u r base-poisoning experiments, t h a t added chromium a p p r e c i a b l y modifies t h e n a t u r e o f these s i t e s .
REFERENCES Hosheng Tu, U.S. Pat., 4, 348, 272 (1982). A. Corma, A, Lopez-Agudo, I . N e l o t and F. Tomas, J.Catal., 77 (1982) 159. P.H. KasaT and R.J. Bishop, J.Phys.Chem., 81 (1977) 15. K. Becker, K. Steinberg, C . S z k i b i k , M. Klotzsche, H. Neubeurer, W. Lambrecht, K. N e s t l e r , H. Bremer and W. Franck, East German Patent, 121, 331 (1976). 5 T. Masuda, M. Ogata, S. Yoskia and Y. Nishimura, Sekigu GakkaTschi, 26 (1983) 19 and 344. 6 a. S h e l l Intern.Research, Japan Kokai Tokkyo Koho, 79 (1979) 122, 692. b. E.H. Hirschberg, R.J. B e r t o l a c i n i and F.S. Modica, U.S. Pat., 4, 363, 720
1 2 3 4
(1982). 7
8 9 10 11 12
13 14 15
c. A.W. Chester, P r e p r i n t s , Div.Petr.Chem., 26 (1981) 505. R. Bearden and G. Stuntz, U.S. Pate., 4, 409, 093 (1983). A. Corma and 6. Wojciechowski, Catal.Rev.Sci.Eng., 24 (1982) 1. J.M. Goupil, J.F. H@midy and D. Cornet, J.Chim.Phys., 4 (1976) 431. A. Chambellan, E. Thursch and D. Cornet, J.Chim.Phys., 81 (1984) 121. J.Chim.Phys., 81 (1984) 127. J.M. Smith, Chemical Engineering K i n e t i c s , Mc Graw H i l l , London (1970) 459. E. Thursch, A. Chambellan and D. Cornet, J.Chim.Phys., 79 (1982) 479. Z. Tvaruzkova and V . Bozacek, C o l l .Czeck.Chem.Comm., 45 (1980) 2499. J.M. Goupil, J.P. Badault, Th. Chevreau, 0. Saur and D. Cornet, Thermochimica Acta, 75' (1984) no 3, 275. G.T. K e r r , J.Catal., 15 (1969) 200.
283
B. Imelik e t al. (Editors), Cotalysis b y Acids and Bases 0 1985 Elsevier Scierxe Publishers B.V.,Amsterdam - Printed,in The Netherlands
CHARACTERIZATION OF ACID CATALYSTS BY USE OF MODEL REACTIONS M. GUISNET
L a b o r a t o i r e de C a t a l y s e en Chimie Organique (U.A. C.N.R.S.
350), U n i v e r s i t e
de P o i t i e r s , 40 Avenue du Recteur Pineau, 86022 P o i t i e r s (France)
SOMMAI RE
L ' u t i l i s a t i o n de r e a c t i o n s modeles permet de c a r a c t e r i s e r 1 ' a c i d i t e d ' u n s o l i d e e t de v e r i f i e r son interOt comme c a t a l y s e u r . Pour c e l a l e s p r o p r i e t e s des s i t e s a c t i f s - n a t u r e , d i s p o s i t i o n s p a t i a l e , f o r c e de l e u r s composants.. d o i v e n t 6 t r e connues. Les r e a c t i o n s d o i v e n t O t r e simples, l e u r v i t e s s e i n i t i a l e f a c i l e a mesurer avec p r e c i s i o n ( d e s a c t i v a t i o n l e n t e . . .) e t l e s t r a n s f o r m a t i o n s secondai r e s 1 i m i t e e s Par a i 11 eurs ces r e a c t i o n s modeles ne d o i v e n t pas O t r e c a t a l y s e e s p a r p l u s i e u r s t y p e s de c e n t r e s a c t i f s . On montre que de nombreuses r e a c t i o n s d ' h y d r o c a r b u r e s o l e f i n i q u e s , aromatiques ou s a t u r e s s a t i s f o n t ces exigences. On d i s p o s e donc d ' u n e l a r g e g a m e de r e a c t i o n s modeles parmi l e s q u e l l e s on p e u t c h o i s i r l a mieux adaptee aux c a t a l y s e u r s a c a r a c t e r i s e r ( e n p a r t i c u l i e r a l a f o r c e de l e u r s s i t e s a c i d e s ) . S i l a v i t e s s e i n i t i a l e des react i o n s e s t l ' i n f o r m a t i o n e s s e n t i e l l e il e s t a u s s i i m p o r t a n t d ' e x a m i n e r e n d e t a i l l e u r s e l e c t i v i t e . En e f f e t c e r t a i n s p r o d u i t s secondaires meme e n f a i b l e q u a n t i t e peuvent m o d i f i e r de f a c o n i m p o r t a n t e l e s v i t e s s e s de r e a c t i o n . C ' e s t e n p a r t i c u l i e r ce q u i r e n d s i compliquee l a c a r a c t e r i s a t i o n des c a t a l y s e u r s b i f o n c t i o n n e l s metal-acide.
.-
.
ABSTRACT Model r e a c t i o n s a l l o w one n o t o n l y t o v e r i f y r a p i d l y t h e s u i t a b i l i t y o f a s o l i d a c i d as a c a t a l y s t b u t a l s o t o c h a r a c t e r i z e i t s a c i d i t y . F o r t h i s purpose a l l t h e p r o p e r t i e s o f t h e a c t i v e s i t e s must be known. Moreover t h e s e r e a c t i o n s must be s i m p l e and n o t c a t a l y z e d by s e v e r a l d i f f e r e n t t y p e s o f a c t i v e c e n t e r s . T h e i r i n i t i a l r a t e has t o be easy t o measure a c c u r a t e l y ( s l o w d e a c t i v a t i o n . . .) and any s i d e r e a c t i o n must be i n s i g n i f i c a n t . I t i s shown t h a t numerous r e a c t i o n s o f o l e f i n i c , a r o m a t i c and s a t u r a t e d hydrocarbons s a t i s f y these r e q u i r e m e n t s . W i t h i n t h i s range o f model r e a c t i o n s i t w i l l be p o s s i b l e t o choose t h e o n e ( s ) b e s t adapted t o t h e c a t a l y s t t o be c h a r a c t e r i z e d i n p a r t i c u l a r t a k i n g i n t o account i t s a c i d s t r e n g t h . I f pure a c i d c a t a l y s t s can be a c c u r a t e l y c h a r a c t e r i z e d by t h i s s i m p l e method, t h e presence o f h y d r o g e n a t i n g s i t e s c o m p l i c a t e c o n s i d e r a b l y t h e process. INTRODUCTION C a t a l y s t m a n u f a c t u r e r s and users must d i s p o s e o f s i m p l e a c c u r a t e methods f o r c h a r a c t e r i z i n g t h e i r c a t a l y s t s . There e x i s t s f o r a c i d c a t a l y s t s v a r i o u s p h y s i cochemical methods g e n e r a l l y based on c h e m i s o r p t i o n o f b a s i c compounds ( 1 - 3 ) . I f t h e i n t e r e s t o f t h e s e methods i s q u i t e obvious, i t must be n o t e d t h a t t h e y
l e a d t o i n f o r m a t i o n which g e n e r a l l y does n o t p e r m i t t o j u d g e c o m p l e t e l y o f t h e q u a l i t y o f t h e c a t a l y s t . Indeed t h e s u r f a c e c h a r a c t e r i z e d i s o f t e n f a r d i f f e r e n t
284
from t h a t o f the c a t a l y s t i n t h e i n d u s t r i a l r e a c t i o n p a r t i c u l a r l y because t h e o p e r a t i n g c o n d i t i o n s a r e n o t i d e n t i c a l . Moreover a d i s t i n c t i o n must be made between a d s o r p t i o n s i t e s and a c t i v e s i t e s ( 4 ) . Indeed the s u r f a c e o f a c i d catal y s t s comprises a l a r g e v a r i e t y o f species which d i f f e r by t h e i r chemical n a t u r e and which can e x i s t i n many d i f f e r e n t c o n f i g u r a t i o n s ( s u r f a c e s i t e s ) . For these species, alone o r i n combination, t o be a c t i v e i n a g i v e n process ( a d s o r p t i o n o r r e a c t i o n ) they must s a t i s f y numerous geometric, c o n f i g u r a t i o n a l and e n e r g e t i c requirements. This can o n l y be the case f o r a very low percentage o f these s u r f a c e s i t e s . The requirements depending o b v i o u s l y on t h e process, i t f o l l o w s t h a t a d s o r p t i o n s i t e s are n o t n e c e s s a r i l y a c t i v e s i t e s . Thus whereas f o r oxides the d e n s i t y o f the chemisorption s i t e s v a r i e s form 10l2 t o 1013 s i t e s / c m t h a t o f the a c t i v e centers can be as low as 3.10
3
sites/cm
2
(4) 2 ( v a l u e determined
by t h e t r a n s i t i o n s t a t e method f o r t-butylbenzene c r a c k i n g on a s i l i c a - a l u m i n a catalyst (5)). The b e s t way t o c h a r a c t e r i z e a c i d c a t a l y s t s i s t h e r e f o r e through model r e a c t i o n s . The d e n s i t i e s o f a c t i v e s i t e s can be determined, a t l e a s t i n simple cases by means o f a k i n e t i c s method which however r e q u i r e s a g r e a t number o f experiments ( 5 ) . That i s why i t i s o f t e n p r e f e r r e d t o proceed i n a s i m p l e r way and c h a r a c t e r i z e t h e a c i d c a t a l y s t s by t h e i r a c t i v i t y i n v a r i o u s simple reactions.
GENERALITIES Model r e a c t i o n s a l l o w one n o t o n l y t o v e r i f y t h e s u i t a b i l i t y o f a s o l i d a c i d as a c a t a l y s t b u t a l s o t o c h a r a c t e r i z e i t s s u r f a c e a c i d i t y . V e r i f i c a t i o n o f the s u i t a b i l i t y o f catalysts Model r e a c t i o n s c o n s t i t u t e f o r c a t a l y s t manufacturers and users an e f f i c i e n t means f o r v e r i f y i n g the s u i t a b i l i t y o f t h e i r c a t a l y s t s b e f o r e use i n i n d u s t r i a l p l a n t s . A comparison o f a l l the main c h a r a c t e r i s t i c s o f the c a t a l y s t s e l e c t i v i t y and s t a b i l i t y
- with
-
activity,
those o f a r e f e r e n c e can be made i n o p e r a t i o n a l
c o n d i t i o n s s i m i l a r t o those o f t h e i n d u s t r i a l process. I t i s consequently easy t o e l i m i n a t e those c a t a l y s t s which do n o t present the a p p r o p r i a t e c h a r a c t e r i s tics.
I f t h e t r a n s f o r m a t i o n o f a r e a l feed i s g e n e r a l l y p r e f e r r e d , a model compound r e a c t i o n can a l s o be used. This r e a c t i o n must be chosen so as t o reproduce as c l o s e l y as p o s s i b l e t h e t r a n s f o r m a t i o n s ( p r i m a r y and secondary) i n v o l v e d i n t h e i n d u s t r i a l process. For example a g r e a t deal o f i n f o r m a t i o n can be obtained concerning c r a c k i n g c a t a l y s t s f r o m alkane t r a n s f o r m a t i o n : a c t i v i t y o f the catal y s t f o r C-C bond s c i s s i o n b u t a l s o f o r t h e secondary r e a c t i o n s : hydrogen t r a n s f e r , coke formation.
.. and s t a b i l i t y .
285
Characterization o f the active s i t e s The use o f model r e a c t i o n s allows one t o go beyond the simple screening o f c a t a l y s t s . T h e i r s u p e r f i c i a l s i t e s can be d e f i n e d and thus one can d i s c o v e r why such and such a s o l i d i s a b e t t e r c a t a l y s t than another. To o b t a i n t h i s r e s u l t , t h e p r o p e r t i e s of t h e s i t e s components
-
-
nature,' s p a t i a l d i s p o s i t i o n and s t r e n g t h o f t h e
must be known.
However i f t h e model r e a c t i o n s are t o be o f use, i t
is
obvious t h a t i t must
n o t be c a t a l y z e d by several d i f f e r e n t types o f s i t e s . Moreover these r e a c t i o n s must be as simple as p o s s i b l e . T h e i r i n i t i a l r a t e has t o be easy t o measure a c c u r a t e l y (slow d e a c t i v a t i o n , no thermodynamic l i m i t a t i o n s . . .) and any s i d e r e a c t i o n must be i n s i g n i f i c a n t . We s h a l l study various r e a c t i o n s f o r the purpose o f determining t h e i n t e r e s t t h e r e o f f o r the 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 . We have l i m i t e d ourselves t o a c e r t a i n number o f hydrocarbon r e a c t i o n s although o t h e r r e a c t i o n s o f hydrocarbons o r o f f u n c t i o n a l compounds (such as a l c o h o l s ) can be a l s o used. HYDROCARBON REACTIONS FOR A C I D CATALYST CHARACTERIZATION O l e f i n isomerization Double-bond s h i f t and c i s t r a n s i s o m e r i z a t i o n .
Because o f i t s g r e a t s i m p l i -
c i t y , n-butene i s o m e r i z a t i o n was one o f t h e model r e a c t i o n s most used. This r e a c t i o n can occur on metals and bases as w e l l as on acids and t h i s by a l a r g e v a r i e t y of mechanisms. Thus on alumina, by t h e use o f deuterated r e a c t a n t s and poisoning techniques we have been a b l e t o show the existence o f a t l e a s t t h r e e types o f r e a c t i o n s (6,7)
-
:
double-bond s h i f t and c i s t r a n s i s o m e r i z a t i o n o c c u r r i n g w i t h exchange o f
hydrogen between the o l e f i n and t h e c a t a l y s t on p u r e l y a c i d s i t e s (Lewis t Bronsted).
-
c i s t r a n s i s o m e r i z a t i o n w i t h o u t exchange independent o f double-bond s h i f t
catalyzed by weak Lewis acid-base s i t e s .
-
double-bond s h i f t and c i s trans i s o m e r i z a t i o n w i t h o u t exchange c a t a l y z e d
by s t r o n g Lewis acid-base s i t e s . The s e l e c t i v i t y o f nondeuterated butene t r a n s f o r m a t i o n does n o t a l l o w t o d i s t i n g u i s h between these mechanisms and a f o r t i o r i , when several o f them a r e i n v o l v e d t o determine t h e i r r o l e i n the i s o m e r i z a t i o n . For t h i s reason the t r a n s f o r m a t i o n o f nondeuterated butenes w i l l n o t enable us t o c h a r a c t e r i z e d e f i n i t e l y the a c i d i t y o f the c a t a l y s t s . However t h i s w i l l be p o s s i b l e i f s e l e c t i 2 v e l y deuterated butenes such as [1,4 H 6 ] cis-but-2-ene are used. U n f o r t u n a t e l y t h e study o f the t r a n s f o r m a t i o n o f these butenes i s l e n g t h y and complicated. Skeletal isomerization.
C o n t r a r i l y t o double-bond s h i f t i s o m e r i z a t i o n ,
s k e l e t a l i s o m e r i z a t i o n occurs n e i t h e r on metals nor on bases. Moreover i t i s c l e a r l y shown t h a t i t occurs on Brijnsted a c i d s i t e s v i a carbenium i o n i n t e r -
286 mediates (8,9) by a three-step mechanism : a) p r o t o n a t i o n of the r e a c t a n t b ) rearrangement o f the carbenium i o n formed c ) desorption o f the product. Step b i s the r a t e - l i m i t i n g step. Ift h i s step does n o t imply a change i n the degree o f branching, t h e carbenium i o n rearrangement occurs by a1 k y l t r a n s f e r ( t y p e A rearrangement ( 10) )
-
I f i t does i m p l y a change ( t y p e B rearrangement) an a l k y l t r a n s f e r would need the formation o f a h i g h l y u n s t a b l e primary carbenium i o n .
/5.+
c-c-c-c-c-c
+c c-c-c-c-c
By t h e use o f I3C l a b e l l e d molecules i t can be shown t h a t t h i s rearrangement
occurs then through protonated cyclopropane intermediates w i t h o u t primary carbeniurn i o n formation (11,12).
Type A rearrangements a r e d e f i n i t e l y f a s t e r than type
B (12,13)
: thus r e a c t i o n
1 i s about 20 times f a s t e r than t h e i s o m e r i z a t i o n o f n-hexenes i n t o methylpentenes ( r e a c t i o n 2 ) . Type B rearrangements r e q u i r e s t r o n g e r a c i d s i t e s than type A : thus an alumina, a c t i v e i n r e a c t i o n
1 can be
i n t o methylcyclopentenes i s o m e r i z a t i o n ( r e a c t i o n
i n a c t i v e i n cyclohexene
3 type
8) :
-Ht d F
tH+
281
Double-bond s h i f t and c i s t r a n s i s o m e r i z a t i o n b e i n g g e n e r a l l y much f a s t e r than s k e l e t a l i s o m e r i z a t i o n ( 1 4 ) , t h e r e a c t i o n a l m i x t u r e s a r e o f t e n v e r y complex : t h u s n-hexenes i n t o methylpentenes i s o m e r i z a t i o n l e a d s t o a m i x t u r e d i f f i c u l t t o a n a l y z e f o r i t c o n t a i n s t h e 14 o l e f i n i c isomers w i t h n-hexane o r methylpentane s k e l e t o n s . The a n a l y s i s can be s i m p l i f i e d i f t h e m i x t u r e o b t a i n e d i s p r e v i o u s l y hydrogenated. However model r e a c t i o n s l e a d i n g t o a more l i m i t e d number o f p r o d u c t s w i l l be p r e f e r r e d . Such would be t h e case o f 3,3 d i m e t h y l 1-butene ( t y p e A) o r cyclohexene ( t y p e B ) i s o m e r i z a t i o n (15-18). The a p p a r e n t l y v e r y s i m p l e i s o m e r i z a t i o n n-butenes = i s o b u t e n e
i s used v e r y r a r e l y because i t
i s a v e r y s l o w r e a c t i o n and accompanied by f o r m a t i o n o f secondary p r o d u c t s ( 1 9 ) . T h i s slow r a t e i s due t o t h e f a c t t h a t a p r i m a r y carbenium i o n i n t e r m e d i a t e must be formed ( 1 2 ) : t
c-c=c-c
tL -\H+
+
a a
O l e f i n s can undergo v a r i o u s secondary r e a c t i o n s : c r a c k i n g , o l i g o m e r i z a t i o n , p o l y m e r i z a t i o n , c y c l i z a t i o n , hydrogen t r a n s f e r , coke f o r m a t i o n . .
. Moreover,
the
c a t a l y s t s can be r a p i d l y d e a c t i v a t e d by coke. The s i g n i f i c a n c e o f t h e s e reactions
being
greater
t h a n t h e c a t a l y s t i s more a c i d , i t would be
d i f f i c u l t t o use s k e l e t a l i s o m e r i z a t i o n o f o l e f i n s t o c h a r a c t e r i z e h i g h l y a c i d c a t a l y s t s . I t must be n o t e d however t h a t secondary r e a c t i o n s (and d e a c t i v a t i o n ) can be m i n i m i z e d b y w o r k i n g a t l o w o l e f i n p a r t i a l p r e s s u r e s i n c e these r e a c t i o n s i m p l y b i m o l e c u l a r s t e p s whereas on t h e c o n t r a r y i s o m e r i z a t i o n occurs i n t r a m o l e cularly. Aromatic t r a n s f o rma t i ons On a c i d c a t a l y s t s , a l k y l a r o m a t i c s can undergo s e v e r a l r e a c t i o n s namely deal k y l a t i o n , isomeri z a t i o n and d i s p r o p o r t i o n a t i o n . The r e l a t i v e s i g n i f i c a n c e o f these r e a c t i o n s depends on t h e a r o m a t i c , on t h e c a t a l y s t and t h e o p e r a t i n g conditions. Dealkylation.
Cumene d e a l k y l a t i o n i n t o benzene and propene i s t h e most
used r e a c t i o n f o r c h a r a c t e r i z i n g a c i d c a t a l y s t s . T h i s v e r y s i m p l e r e a c t i o n can be c o n s i d e r e d as a r e a c t i o n o f e l e c t r o p h i l e s u b s t i t u t i o n o f an i s o p r o p y l i o n by a p r o t o n :
-H+
&
Q) c-c-c t
t
-H+_
Q) c-c=c
7
t
288
A very complete k i n e t i c study o f t h i s r e a c t i o n was c a r r i e d o u t on s i l i c a alumina by P r a t e r and Lago (20) who show t h a t a l l t h e i r r e s u l t s are compatible w i t h a k i n e t i c r e a c t i o n scheme i n which the cumene adsorption f o l l o w s a Langmuir isotherm and the r a t e o f c r a c k i n g i s p r o p o r t i o n a l t o the number o f cumene molecules chemisorbed on a c t i v e s i t e s . As can be expected from t h i s scheme, the r e a c t i o n e x h i b i t s zero order k i n e t i c s a t h i g h cumene pressure and i t i s thus p o s s i b l e by means o f absolute r a t e theory (21) t o c a l c u l a t e the
number o f a c t i v e s i t e s . Moreover, a h i g h l y s i g n i f i c a n t i n h i b i t i n g e f f e c t o f hydroperoxide
i s found ( r e a c t i o n r a t e d i v i d e d by 2 f o r 0.05 % o f hydroperoxide
i n cumene). This
shows
that
i t i s e s s e n t i a l t h a t cumene be completely
p u r i f i e d b e f o r e use. Various secondary r e a c t i o n s can accompany the t r a n s f o r m a t i o n s o f cumene i n propene and benzene. Thus on alumina between 350 and 550°C i t has been shown t h a t cumene cracks roughly 50 % by an i o n i c mechanism and 50 % by a r a d i c a l type mechanism w i t h formation o f styrene and methylstyrene ( 2 2 ) . This r a d i c a l path f o r c r a c k i n g i s a l s o found a t temperaturesabove 500°C on a l k a l i and a l k a l i n e e a r t h i o n exchanged Y z e o l i t e s ( 2 3 ) . A t low temperature (180°C) on the support o f a hydrocracking c a t a l y s t , d i s p r o p o r t i o n a t i o n becomes t h e p r e v a i l i n g r e a c t i o n . F i n a l l y on a h i g h l y a c t i v e lanthanum exchanged Y type z e o l i t e c a t a l y s t ( 2 4 ) over 60 r e a c t i o n products have been observed ; under i n i t i a l c o n d i t i o n s a t 360°C cumene d e a l k y l a t i o n (based on propene y i e l d ) accounts f o r o n l y 6 4 % o f t h e t o t a l cumene converted ( 2 4 ) . On a l l t h e c a t a l y s t s , coke i s a s i g n i f i c a n t product, which i s e a s i l y understood s i n c e the r e a c t i o n a l m i x t u r e contains aromat i c s and o l e f i n s (propene b u t a l s o styrene and methylstyrene) w e l l known as coke precursors ( 2 5 ) . The Br'6nsted a c i d i t y o f c a t a l y s t s can a l s o be c h a r a c t e r i z e d by using o t h e r alkylbenzenes. Indeed several s t u d i e s o f the i n f l u e n c e o f the a l k y l a r o m a t i c s t r u c t u r e on t h e i r r e a c t i v i t y have shown t h a t the o r d e r o f r e a c t i v i t y i s what i s expected when c r a c k i n g occurs through mechanism
(A) (26-29).
I n a l l the
s e r i e s o f hydrocarbons used : monoalkylbenzenes, s u b s t i t u t e d cumenes...
linear
f r e e energy r e l a t i o n s h i p s were a p p l i e d s u c c e s s f u l l y t o the i n t e r p r e t a t i o n o f the r e s u l t s . For example, gbod l i n e a r r e l a t i o n s h i p s were observed between the r a t e constants logarithms o f monosubstituted alkylbenzenes (@
R ) dealkyla-
t i o n and t h e enthalpy o f formation o f the corresponding carbenium i o n
R+ (26,
27). Isomeri z a t i on and d i s p r o p o r t i o n a t i o n .
D i s p r o p o r t i o n a t i o n and isomeri z a t i on
mechanisms o f aromatics depend on the c a t a l y s t employed, on the o p e r a t i n g c o n d i t i o n s and on the a l k y l s u b s t i t u e n t s o f the aromatic r i n g . Thus the isomer i z a t i o n o f methylbenzenes occurs by an i n t r a m o l e c u l a r mechanism i n v o l v i n g as l i m i t i n g step a 1,2 a l k y l s h i f t i n a benzenium i o n :
289
whereas t - b u t y l benzene i s o m e r i z a t i o n occurs by t r a n s a l k y l a t i o n o r v i a deal k y l a tion-a1 k y l a t i o n ( 1 0 ) . Xylene i s o m e r i z a t i o n i s v e r y much employed t o c h a r a c t e r i z e a c i d c a t a l y s t s .
This very simple r e a c t i o n occurs on Bronsted a c i d c e n t e r s as demonstrated by Ward and Hansford's work ( 3 0 ) . For two series o f s i l i c a - a l u m i n a c a t a l y s t s t h e authors observed a l i n e a r r e l a t i o n s h i p between t h e o-xylene i s o m e r i z a t i o n r a t e and the Bronsted a c i d i t y . This a c i d i t y was measured by the i n t e n s i t y o f t h e a b s o r p t i o n band a t 1545 cm-l ( a r i s i n g from p y r i d i n i u m i o n s ) i n the spectrum o f chemisorbed p y r i d i n e recorded a f t e r evacuation a t 125°C. D i s p r o p o r t i o n a t i o n i n t o toluene and trimethylbenzenes and coke formation a r e the o n l y r e a c t i o n s which accompany xylene i s o e m r i z a t i o n . D i s p r o p o r t i o n a t i o n b e i n g a bimolecular r e a c t i o n , i t i s p o s s i b l e t o reduce i t s s i g n i f i c a n c e by decreasing t h e xylene p a r t i a l pressure. B u t t h i s r e a c t i o n can a l s o give i n f o r m a t i o n concerning the a c i d i t y o f t h e c a t a l y s t . Two types o f mechanisms have been proposed t o e x p l a i n t h i s react i o n (10). The f i r s t , l i k e isomerization, occurs through benzenium i o n intermed i a t e s w i t h two p o s s i b i l i t i e s : i)d e a l k y l a t i o n - a l k y l a t i o n i i ) d i r e c t a l k y l t r a n s f e r from one aromatic molecule t o the o t h e r
t h e second occurs through b e n z y l i c carbenium i o n s which mechanism ( F i g . 1) i s the most l i k e l y i n t h e case o f xylenes ( 3 1 ) . The a c t i v e s i t e s a r e most probably d i f f e r e n t f o r d i s p r o p o r t i o n a t i o n and i s o m e r i z a t i o n since t h e i r r a t e r a t i o ( D / I ) depends t o a g r e a t e x t e n t o f the c a t a l y s t employed. For example a l l t h e t r e a t ments o f a p r o t o n i c mordenite (dealumination, wet a i r treatment, exchange by various c a t i o n s ) modify t h e D/I value (32). f t does n o t seem as i f t h e a c t i v e s i t e s a r e o f a d i f f e r e n t nature : i t has been proved t h a t Bronsted a c i d s i t e s a r e responsible f o r d i s p r o p o r t i o n a t i o n (33-35) as f o r isomerization. Moreover we were a b l e t o show by p y r i d i n e p o i s o n i n g t h a t the d i f f e r e n c e between the a c t i v e s i t e s was n o t the r e s u l t of a d i f f e r e n c e between t h e i r a c i d s t r e n g t h s (31) : a HY z e o l i t e presenting s i t e s w i t h v e r y d i f f e r e n t a c i d s t r e n g t h s was
290
-
L"2 ,.,
F i g . 1. D i s p r o p o r t i o n a t i o n mechanism of xylenes v i a b e n z y l i c c a r b o c a t i o n intermediates. chosen. The r a t i o o f t h e o-xylene t r a n s f o r m a t i o n r a t e s on t h e p y r i d i n e p o i s o n e d c a t a l y s t and on t h e f r e s h c a t a l y s t was determined as a f u n c t i o n o f t h e desorpt i o n temperature o f t h i s poison. Very s l i g h t l y d i f f e r e n t curves were o b t a i n e d f o r i s o m e r i z a t i o n and d i s p r o p o r t i o n a t i o n . I t i s t h e r e f o r e most l i k e l y t h e "demanding" c h a r a c t e r o f t h e d i f f e r e n t r e a c t i o n s which e x p l a i n s t h e change o f t h e D/I v a l u e w i t h t h e a c i d i t y : t h e b i m o l e c u l a r d i s p r o p o r t i o n a t i o n r e a c t i o n would demand a p a i r o f a d j a c e n t B r o n s t e d a c i d s i t e s whereas i s o m e r i z a t i o n would r e q u i r e o n l y one. The D/I v a l u e b r i n g s t h e r e f o r e s i g n i f i c a n t i n f o r m a t i o n regarding the d e n s i t y o f the a c i d s i t e s . Trimethylbenzenes can a l s o be used f o r c h a r a c t e r i z i n g a c i d c a t a l y s t s . They have a f a s t e r
-
roughly twice
-
t r a n s f o r m a t i o n r a t e t h a n xylenes ( 3 6 ) . By
p y r i d i n e p o i s o n i n g i t c a n be shown t h a t t h e a c i d s t r e n g t h r e q u i r e d f o r
1,2,4-
t r i m e t h y l b e n z e n e t r a n s f o r m a t i o n s i s weaker t h a n t h a t r e q u i r e d f o r o - x y l e n e ( 3 7 ) . D i s p r o p o r t i o n a t i o n o f t o l u e n e (38-40) o f e t h y l b e n z e n e ( 3 4 ) and o f cumene ( 4 1 ) have a l s o been proposed f o r c h a r a c t e r i z i ' n g a c i d c a t a l y s t s . I n t h e two l a t t e r cases, t h e c h o i c e o f o p e r a t i n g c o n d i t i o n s and e s p e c i a l l y o f t h e t e m p e r a t u r e i s v e r y i m p o r t a n t f o r 1 i m i t i n g secondary r e a c t i o n s and d e a c t i v a t i o n by coke. A1 kane t r a n s f o r m a t i o n s Over a c i d c a t a l y s t s , alkanes undergo t h r e e main r e a c t i o n s : i s o m e r i z a t i o n , c r a c k i n g and d i s p r o p o r t i o n a t i o n . A l l t h r e e o f them i n v o l v e carbenium i o n s as i n t e r m e d i a t e s and t h e i r r e l a t i v e s i g n i f i c a n c e depends b o t h on t h e c h a r a c t e r i s -
291
t i c s o f t h e alkane and o f t h e c a t a l y s t . F o r heavy alkanes (% C 7 ) c r a c k i n g i s p r a c t i c a l l y t h e o n l y r e a c t i o n , f o r C5-C6 f o r C3-C4
i s o r n e r i z a t i o n accompanies c r a c k i n g and
d i s p r o p o r t i o n a t i o n i s o f t e n t h e main r e a c t i o n . I n a l l t h e cases, t h e
a c t i v e centers are probably Bronsted a c i d s i t e s . Alkane c r a c k i n g o c c u r s through steps 1-4 o f F i g . 2.
c; +
0
(X-Y)
F i g . 2 . I s o m e r i z a t i o n and c r a c k i n g o f p a r a f f i n s on a c i d c a t a l y s t s . P : p a r a f f i n ; 0 : o l e f i n ; C+ : c a r b o c a t i o n ; x,y : number o f carbon atoms. I n s t a t i o n a r y s t a t e , t h e carbenium i o n f o r m a t i o n ( s t e p 1 ) r e s u l t s f r o m h y d r i d e t r a n s f e r between a r e a c t a n t m o l e c u l e and a preadsorbed carbenium i o n . The c r a c k i n g r a t e i s determined by t h e s t a b i l i t y o f t h e
:C and C+ carbenium i o n
Y i n t e r m e d i a t e s . Thus on HY z e o l i t e , a t 400°C ( 4 2 ) t h e i s o o c t a n e c r a c k i n g which involves only t e r t i a r y carbocations
w+ C-E-C-E-C
-
c-
E-c
+
C-i=C
i s 10 t o 15 t i m e s f a s t e r t h a n t h e c r a c k i n g o f 2 and 3-methylpentane,
30 t o 40
t h a n n-hexane and 150 t o 200 t h a n n-pentane c r a c k i n g t h i s 1a t t e r i n v o l v i n g an
-
ethylprimary carbocation :
P+
c-c-c-c-c
+ c-c
+
c=c-c
By p y r i d i n e p o i s o n i n g we have shown ( 3 7 ) t h a t t h e more d i f f i c u l t t h e react i o n t h e g r e a t e r must be t h e a c i d s t r e n g t h o f t h e s i t e s necessary f o r i t s c a t a l y s i s : t h u s n-hexane c r a c k i n g a t 350°C demands s i t e s capable o f k e e p i n g t h e p y r i d i n e adsorbed a t l e a s t a t 520°C whereas i s o o c t a n e c r a c k i n g r e q u i r e s s i t e s capable o f k e e p i n g t h e p y r i d i n e adsorbed a t 350°C. I t can be n o t e d t h a t a c r a c k i n g mechanism i n v o l v i n g a p e n t a c o o r d i n a t e d
carbonium i o n i n t e r m e d i a t e was r e c e n t l y proposed (43) t o e x p l a i n t h e f o r m a t i o n o f C1 and C2 hydrocarbons by c r a c k i n g o f C6 alkanes a t h i g h temperatures and low pressures on z e o l i t e s and on s i l i c a - a l u m i n a . n-Hexane c r a c k i n g i s one o f t h e most employed r e a c t i o n 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 f c a t a l y s t s (a v a l u e ( 4 4 , 4 5 ) ) . I t i s v e r y i n t e r e s t i n g t o n o t e t h a t
292 t h e apparent a c t i v a t i o n energy o f t h i s r e a c t i o n i s t h e same on c a t a l y s t s d i f f e r i n g s t r o n g l y by t h e i r a c i d i t y . This a l l o w s the r e a c t i o n temperature t o be chosen as a f u n c t i o n o f the a c i d i t y o f t h e c a t a l y s t s t o be c h a r a c t e r i z e d from 300°C f o r the more a c i d t o over 500°C f o r the l e s s a c i d thus c o v e r i n g a range o f a c t i v i t i e s o f more than f o u r orders o f mangitude ( 4 5 ) . The constancy of t h e apparent a c t i v a t i o n energy i n s p i t e o f l a r g e v a r i a t i o n s i n s e l e c t i v i t y suggests a u n i f o r m i t y i n the c h a r a c t e r o f t h e r a t e l i m i t i n g s t e p ( i n t h i s case t h e f o r m a t i o n o f the carbenium i o n , s t e p 1 F i g . 2) ; p r o d u c t v a r i a t i o n s r e s u l t from subsequent secondary r e a c t i o n processes which depend on t h e r e a c t i o n temper a t u r e as w e l l as on the a c i d i t y . The s e l e c t i v i t y can t h e r e f o r e g i v e i n t e r e s t i n g i n f o r m a t i o n on the a c i d i t y o f t h e c a t a l y s t s provided i t i s determined a t t h e same temperature. Butane t r a n s f o r m a t i o n which leads t o much l e s s products than t h a t o f n-hexane c o u l d seem s i m p l e r (46-48). However t h e r e a c t i o n a l scheme o f t h i s t r a n s f o r m a t i o n i s g e n e r a l l y complex i n v o l v i n g as a f i r s t s t e p butane d i s p r o p o r t i o n a t i o n i n t o propane and pentanes, then r a p i d c r a c k i n g of pentanes ( 4 9 ) . Butane i s o m e r i z a t i o n occurs a l s o b y d i s p r o p o r t i o n a t i o n (50,51).
F i g . 3. shows
as an example how propane and isopentane can be formed by n-butane d i s p r o p o r tionation.
+ t
2(C-C-C-C t
C-C-C-C
+
c-c-c-c
t
c-c-c t
c-5-c-c L
F i g . 3.
C-C=C-C
t
Ht
-
C C t
RH)
--+
sCH3 ,sH
t
t
t
c-c=c-c
t
c-c-c-c-c-c C+-C-C
t
C-C-C-C
Ht d
, RH ,
RH
c-
t
__$
c-c-c
t
c=c-c-c C
C-Z-C-C
C-C-C
E
-c-c
C- -C-C
c
Rt
t t
Rt
n-Butane d i s p r o p o r t i o n a t i o n . Simp1 if i ed mechani sm.
Butane d i s p r o p o r t i o n a t i o n a t 350°C r e q u i r e s extremely s t r o n g a c i d s i t e s (capable o f keeping p y r i d i n e adsorbed a t 550°C (52)) and t h e r e f o r e c o u l d o n l y be employed f o r c h a r a c t e r i z i n g v e r y a c i d c a t a l y s t s . Moreover i t i s more o r l e s s c e r t a i n t h a t t h i s r e a c t i o n r e q u i r e s , 1 i k e t h e d i s p r o p o r t i o n a t i o n o f aromatics two a d j a c e n t a c i d s i t e s ( 4 9 ) .
293 CATALYST CHARACTERIZATION BY USE OF MODEL REACTIONS Choice o f the r e a c t i o n As has been seen, t h e r e e x i s t s a l a r g e v a r i e t y o f r e a c t i o n s which a l l o w the c h a r a c t e r i z a t i o n o f a c t i v e s i t e s o f a c i d c a t a l y s t s . I n t h e r e a c t i o n s t h a t have been examined here, t h e a c t i v e s i t e s can be c l a s s i f i e d i n two c a t e g o r i e s : acidobasic s i t e s and Bronsted a c i d s i t e s . The Lewis a c i d s i t e s alone do n o t seem t o be r e s p o n s i b l e f o r these r e a c t i o n s b u t probably c o u l d c a t a l y z e o t h e r r e a c t i o n s . Acidobasic c e n t e r s c a t a l y z e f a c i l e r e a c t i o n s such as double-bond s h i f t and c i s t r a n s i s o m e r i z a t i o n o f o l e f i n s whereas Bronsted a c i d s i t e s a r e r e s p o n s i b l e f o r more d i f f i c u l t t r a n s f o r m a t i o n s . The a c i d s t r e n g t h t h a t Brtinsted a c i d s i t e s must have t o be a c t i v e depends a g r e a t deal on t h e r e a c t i o n considered ; thus f o r c a t a l y z i n g 3,3-dimethyl-1
it i s
butene i s o m e r i z a t i o n a t 250"C,
s u f f i c i e n t f o r t h e s i t e s t o be a b l e t o r e t a i n p y r i d i n e adsorbed a t 290°C whereas f o r c a t a l y z i n g isobutane d i s p r o p o r t i o n a t i o n a t 350"C, they must be a b l e t o r e t a i n i t a t 550°C (37,52).
Moreover, b i m o l e c u l a r r e a c t i o n s o f alkane and
aromatic d i s p r o p o r t i o n a t i o n seem t o demand p a i r s o f a d j a c e n t Bronsted s i t e s f o r t h e i r c a t a l y s i s whereas one s i n g l e Bronsted s i t e i s enough f o r c a t a l y z i n g monomolecular r e a c t i o n s . The choice o f t h e r e a c t i o n w i l l depend f i r s t l y on t h e a c i d s t r e n g t h o f t h e c a t a l y s t t o be c h a r a c t e r i z e d . For acidobasic oxides such as alumina t h e o l e f i n i s o m e r i z a t i o n s a r e t h e r e a c t i o n s t h e b e s t adapted ; t h e o t h e r r e a c t i o n s can be used b u t r a d i c a l p a t h t r a n s f o r m a t i o n s can make t h e i n t e r p r e t a t i o n o f t h e r e s u l t s d i f f i c u l t . For h i g h l y a c i d c a t a l y s t s alkane and aromatic transformat i o n s a r e t o be p r e f e r r e d . I s o m e r i z a t i o n o f o l e f i n s must be avoided f o r these compounds can undergo many s i d e r e a c t i o n s m e r i z a t i o n , coke f o r m a t i o n . .
.
-
-
cracking, o l i g o m e r i z a t i o n , p o l y -
which can d i s t u r b t h e c h a r a c t e r i z a t i o n o f t h e
c a t a l y s t s . Moreover as the r e a c t i o n s w i l l be v e r y f a s t , they c o u l d be l i m i t e d by t r a n s p o r t phenomena. I n each o f these two s e r i e s o f c a t a l y s t s t h e samples t o be c h a r a c t e r i z e d can d i f f e r h i g h l y by t h e i r a c i d i t y . I t i s hence v e r y i n t e r e s t i n g t o employ the r e a c t a n t which transforms i t s e l f t o several p a r a l l e l o r successive r e a c t i o n s demanding s i t e s o f d i f f e r e n t s t r e n g t h s . This i s p a r t i c u l a r l y t h e case f o r 3,3-dimethyl-1 t e d t o 2,3-dimethylbutenes
butene whose t h e rearrangement w i l l be l i m i -
on s l i g h t l y a c i d c a t a l y s t s b u t c o u l d l e a d t o
n-hexenes on h i g h l y a c i d c a t a l y s t s ( 1 5 ) . One can a l s o use two o r more r e a c t i o n s belonging t o a same c l a s s i n which t h e r e a c t a n t changes s y s t e m a t i c a l l y i n i t s a c i d s t r e n g t h requirement. Good examples a r e c r a c k i n g o f s u b s t i t u t e d cumenes (29)
-
or o f t-butyl,
s - b u t y l and n - b u t y l
-
benzenes (28), i s o m e r i z a t i o n o f
xylenes and trimethylbenzenes (36), c r a c k i n g o f n-alkanes ( 4 9 )
... L a s t l y
i n the
p a r t i c u l a r case o f shape-selective z e o l i t e s , i t i s obvious t h a t the s i z e o f the r e a c t a n t s and o f t h e products must be such t h a t the r e a c t i o n r a t e w i l l be
294
l i m i t e d n e i t h e r by t r a n s p o r t phenomena nor by s t e r i c c o n s t r a i n t s i n t h e format i o n o f intermediates o r t r a n s i t i o n states. Method f o r c h a r a c t e r i z a t i o n The e s s e n t i a l i n f o r m a t i o n f o r t h e c h a r a c t e r i z a t i o n o f a c t i v e s i t e s i s o b v i o u s l y the r e a c t i o n r a t e which must be a c c u r a t e l y determined ( g e n e r a l l y i n a f l o w r e a c t o r ) b e f o r e t h e d e a c t i v a t i o n o f the c a t a l y s t . As has a l r e a d y been i n d i c a t e d , the r i g h t choice o f t h e r e a c t i o n can o f t e n a v o i d a s i g n i f i c a n t d e a c t i v a t i o n - b y coke. Operating c o n d i t i o n s : temperature, p a r t i a l pressure o f reactant
... a r e
o f g r e a t importance. L a s t l y by use o f p u l s e technique i t i s
sometimes p o s s i b l e t o measure t h e i n i t i a l a c t i v i t y o f c a t a l y s t s h i g h l y s e n s i t i v e t o d e a c t i v a t i o n . However i t i s w e l l known t h a t t h i s technique gives e x a c t values o n l y i n t h e case o f l i n e a r r e a c t i o n s ( 5 3 ) . Moreover t h e r e a c t i o n r a t e must be measured i n absence o f d i f f u s i o n a l l i m i t a t i o n s i n t h e porous c a t a l y s t p a r t i c l e . D i f f u s i o n e f f e c t s can be considered as n e g l i g i b l e (54) f o r isothermal r e a c t i o n s i f
where r i s t h e r e a c t i o n r a t e (moles/cm3.sec),
c t h e c o n c e n t r a t i o n o f t h e reac-
t a n t (moles/cm 3 ), R the r a d i u s o f t h e c a t a l y s t p a r t i c l e s (cm), De the e f f e c t i v e d i f f u s i v i t y o f t h e r e a c t a n t i n t h e porous c a t a l y s t a t t h e r e a c t i o n temperature 2 (cm /sec). U n f o r t u n a t e l y , De i s v e r y o f t e n d i f f i c u l t t o determine. I n o r d e r t o o b t a i n an accurate r a t e value, h i g h conversions t o o c l o s e t o e q u i l i b r i u m must be avoided. I n t h e case o f a r e v e r s i b l e transformation,
it i s
o f i n t e r e s t t o study the r e a c t i o n i n a thermodynamically f a v o r a b l e d i r e c t i o n ; f o r example t h e t r a n s f o r m a t i o n
w i l l be s t u d i e d i n the forward r a t h e r than i n the reverse d i r e c t i o n . Problems c r e a t e d by t h e a c i d i t y c h a r a c t e r i z a t i o n o f b i f u n c t i o n a l c a t a l y s t s These c a t a l y s t s which a s s o c i a t e a hydrogenating component (metal o r s u l f u r ) w i t h an a c i d support are employed i n numerous i n d u s t r i a l processes. T h e i r a c i d i t y p l a y s a v e r y s i g n i f i c a n t r o l e i n t h e i r a c t i v i t y and s e l e c t i v i t y (see f o r example r e f . 56 f o r alkane h y d r o c r a c k i n g ) . I t i s t h e r e f o r e i m p o r t a n t t o be a b l e t o c h a r a c t e r i z e t h e a c i d i t y n o t o n l y b e f o r e t h e i n t r o d u c t i o n o f hydrogenating
295
component b u t a l s o on the b i f u n c t i o n a l c a t a l y s t and i f p o s s i b l e i n the conditions o f i t s u t i l i s a t i o n . Karge e t a1 (57) propose t o c h a r a c t e r i z e b i f u n c t i o n a l c a t a l y s t a c i d i t y by ethylbenzene d i s p r o p o r t i o n a t i o n . However they found f o r t h i s r e a c t i o n an i n h i b i t i n g e f f e c t o f hydrogen on the a c t i v i t y o f b i f u n c t i o n a l z e o l i t i c c a t a l y s t s which e f f e c t was a t t r i b u t e d t o an a c i d i t y decrease. Another i n t e r p r e t a t i o n o f t h i s i n h i b i t i n g e f f e c t however was proposed (58) : thedecrease i n a c t i v i t y would be r e l a t e d t o a decrease i n the c o n c e n t r a t i o n o f b e n z y l i c carbocations ( i n t e r mediates o f the d i s p r o p o r t i o n a t i o n ) due e i t h e r t o hydrogen ( 4 0 )
+f
f
o r t o a low q u a n t i t y o f branched alkanes produced by ethylbenzene hydrogenation
(59).
A t the present
time i t i s d i f f i c u l t t o a t t r i b u t e d e t i n i t e l y t h e i n h i b i t i n g e f f e c t
o f hydrogen t o t h e decrease i n a c i d i t y o r i n b e n z y l i c carbocation concentration. I n n-hexane cracking, the a c t i v i t y o f P t H Y c a t a l y s t s , measured i n the absence o f hydrogen i n o r d e r t o avoid b i f u n c t i o n a l c a t a l y s i s , i s d e f i n i t e l y g r e a t e r than t h a t o f a p l a t i n u m - f r e e HY z e o l i t e (60). This can be connected t o the formation o f a small amount o f hexenes by dehydrogenation o f n-hexane on platinum s i t e s ( 6 1 ) . Indeed i t i s a w e l l known f a c t t h a t o l e f i n s can increase t h e alkane c r a c k i n g r a t e by i n c r e a s i n g t h e number o f c h a i n i n i t i a t o r s . One o f t h e s i g n i f i c a n t c r i t e r i a i n the choice o f an a c i d i t y c h a r a c t e r i z a t i o n r e a c t i o n o f b i f u n c t i o n a l c a t a l y s t s w i l l be t h a t r e a c t a n t s and products undergo t h e l e a s t p o s s i b l e s i d e r e a c t i o n s on hydrogenating s i t e s . Poisoning of these s i t e s can be envisaged b u t then i t must be made sure than the a c i d i t y w i l l n o t be a f f e c t e d ( 6 0 ) . The problems created by b i f u n c t i o n a l c a t a l y s t c h a r a c t e r i z a t i o n are e v i d e n t l y met again f o r a c i d c a t a l y s t s presenting hydrogenating i m p u r i t i e s . This i s p a r t i c u l a r l y the case f o r cracking c a t a l y s t s which a f t e r use c o n t a i n appreciable amounts o f n i c k e l and vanadium. I t i s therefore always
e s s e n t i a l t o c o n t r o l i n d e t a i l the r e a c t i o n s e l e c t i v i t y and i n p a r t i c u l a r t o take i n t o c o n s i d e r a t i o n the formation o f any new secondary product and i t s p o s s i b l e e f f e c t on the r e a c t i o n r a t e .
296 ACKNOWLEDGEMENT The a u t h o r thanks ELF FRANCE (Centre de Recherche de S o l a i z e ) f o r s u p p o r t i n g t h i s research. REFERENCE
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
K. Tanabe, " S o l i d Acids and Bases",Academic Press, New York (1970). H.A. Benesi and B.H.C. Winquist, i n D.D. Eley, H. Pines, P.B. Weisz (Eds), Advances i n C a t a l y s i s , Academic Press, New York, San Francisco, London, Vol. 27, 1978, p. 97. L . F o r n i , i n H. Heineman (Ed.),Catalysis Reviews, M. Deltker, New York, Vol. 8, 1974, p . 65. H. Knozinger, i n D.D. Eley, H. Pines, P.B. Weisz (Eds), Advances i n Catal y s i s , Academic Press, New York, San Francisco, London, Vol. 25, 1976, p. 184. R.W. Maatman, i n H. Heineman (Ed.), C a t a l y s i s Reviews, M. Dekker, New York, Vol 8, 1974, p . 1. M. Guisnet, J.L. Lemberton, G. P e r o t and R . Maurel, J. Catal.,48 (1977) 166. J.L. Lemberton, G. P e r o t and M. Guisnet, J . Chem. Research,S (1979) 94, M (1979) 1290. C.S. John, C . Kemball and R.A. Rajadhyaksha, J . Cata1.,57 (1979) 264. E.A. I r v i n e , C.S. John, C. Kemball, A.J. Pearman, M.A. Day and R.J. Sampson, J . Catal.,61 (1980) 326. M.L. Poutsma, Z e o l i t e Chemistry and C a t a l y s i s , J.A. Rabo Ed. ACS Monograph 171, American Chemical S o c i e t y , Washington D.C., 1976, Ch 8, p. 437. D.M. Brouwer, Rec. Trav. Chim.,87 (1968) 1435. F. Chevalier, M. Guisnet and R. Maurel, Proceedings 6 t h I n t . Congr. Catal., G.C. Bond P.B. Wells F.C. Tompkins Eds , The Chemical Society, London, 1977, Vol. 1, p . 478. J. Weitkamp and H. Farag, Proceedings o f t h e Symposium on Z e o l i t e s , Szeged 1978, p . 327. G. Perot, J.L. Lemberton e t M. Guisnet, B u l l . SOC. Chim.,(1977) 355. H. Pines, J . Cata1.,78 (1982) 1. W.O. Haag and H. Pines, J . h e r . Chem. Soc.,82 (1960) 2488. L. Bassery, B u l l . SOC. Chim. ,(1969) 1077. L. Ponsolle, L. Bassery, J.P. J o l y e t D. V a i l l a n t , B u l l . SOC. Chim., (1969) 1083. J . Dubien, L. de Mourgues e t Y. Trambouze, B u l l . SOC. Chim., (1967) 108. C.D. P r a t e r and R.M. Lago, i n D.D. Eley, H. Pines, P.B. Weisz (Eds), Advances i n C a t a l y s i s , Academic Press, New York, San Francisco, London, V o l . 8, 1956, 293. S. Glasstone, K.J. L a i d l e r and H. Eyring, "The Theory o f r a t e processes", Mac Graw H i l l , New York, Ch 7, 1941. S.E. Tung and E. Mc I n i n c h , J . Catal.,4 (1965) 586. J.T. Richardson, J . Catal.,9 (1967) 182. D. B e s t and B.W. Wojcieckowski, J. Catal.,47 (1977) 11. D.E. Walsh and L.D. Rollmann, J . Cata1.,49 (1977) 369. I. Mochida and Y. Yoneda, J . Catal.,7 (1967) 386 ; 7 (1967) 393 ; 8 (1967) 223. HI-Matsumoto, J . I . Take and Y . Yoneda, J. Catal.,l9 (1970) 113. P. Andreu and M.M. Ramirez, Proceedings 6 t h I n t . Congr. C a t a l . , G.C. Bond P.B. Wells, F.C. Tompkins Eds, The Chemical S o c i e t y , London, 1977, Vol. 1, p. 593. N.S. Gnep, These I n g e n i e u r , P o i t i e r s 1974. J.W. Ward and R . C . Hansford, J . C a t a l . , l 3 (1969) 154. M. Guisnet and N.S. Gnep, " Z e o l i t e s : Science and Technology" NATO A S 1 Series, F.R. R i b e i r o e t a1 Eds, Martinus N i j h o f f Publishers, The Hague, Boston, Lancaster (1984) p. 571.
.
297
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
M.L. M a r t i n de Arrnando, N.S. Gnep and M. Guisnet, J . Chern. Research,S (1981) 8 ; M (1981) 243. P.A. Jacobs, H.E. Leeman and J.B. Uytterhoeven, J. Catal.,33 (1974) 31. H.G. Karge, J. Ladebeck, Z . Sarbak and K . Hatada, Z e o l i t e s , 2 (1982) 94. H.G. Karge, K. Hatada, Y . Zhang and R. Fiedorow, Z e o l i t e s , 3 (1983) 13. N.S. Gnep, J. Tejada and M. Guisnet, B u l l . SOC. Chim., (1984) 5. G. B o u r d i l l o n , C. Gueguen and M. Guisnet, i n p r e p a r a t i o n . H.A. Benesi, J. Catal .,8 (1967) 368. N.S. Gnep, M.L. M a r t i n de Armando, C. M a r c i l l y , B.H. Ha and M. Guisnet, " C a t a l y s t D e a c t i v a t i o n " Studies i n Surface Science and C a t a l y s i s 6, E l s e v i e r , Amsterdam (1980) p. 79. N.S. Gnep and M. Guisnet, A p p l i e d C a t a l y s i s , 1 (1981) 329. R.P.L. A b s i l , J.B. B u t t and J.S. Dranoff, J. Catal.,85 (1984) 415. G. Lopez, These P o i t i e r s , 1977. W.O. Haag, R.M. Dessau, Proceedings 8th I n t . Congr. Catal., Dechema, 1984, Vol. 2, p. 305. P.B. Weisz, J.N. Miale, J. Catal., 4 (1965) 527. J.N. Miale, N.Y. Chen and P.B. Weisz, J. Catal., 6 (1966) 278. H. R a s t e l l i , B.M. Lok, J.A. Duisman, D.E. E a r l s and J.T. Mullkaupt, Canad. J. Chem. Eng., 60 (1982) 44. J. Blanco, A . Ramos and J. Soria, J. Catal ., 54 (1978) 365. G.B. Mc Vicker, G.M. Kramer and J.J. Ziemak, J. Catal., 83 (1983) 286. F . Avendano, These P o i t i e r s , 1984. G.A. Fuentes and B.C. Gates, J. Catal., 76 (1982) 440. C. Bearez, F. C h e v a l i e r and M. Guisnet, React. K i n e t . Catal. L e t t . , 22 (1983) 405. C . Bearez, F . C h e v a l i e r and M. Guisnet, i n p r e p a r a t i o n . T . H a t t o r i and Y . Murakami, J. Catal., 10 (1968) 114. J.M. Smith, "Chemical Engineering K i n e t i c s " , Mac Graw H i l l , Second E d i t i o n (1970). American Petroleum I n s t i t u t e , Research P r o j e c t 44, Selected Values o f P r o p e r t i e s o f hydrocarbons and r e l a t e d compounds. M. Guisnet and G. Perot, " Z e o l i t e s : Science and Technology" NATO AS1 Series ; F.R. R i b e i r o e t a1 Eds, Martinus N i j h o f f Publishers, The Hague, Boston, Lancaster (1984), p . 397. H.G. Karge, Z. Sarbak, K. Hatada, J. Weitkamp, P.A. Jacobs, J. Catal., 82 (1983) 236. M. Guisnet, J. Catal., 88 (1984) 249. N.S. Gnep and M. Guisnet, React. K i n e t . Catal. L e t t . , 22 (1983) 237. G. B o u r d i l l o n , G. Giannetto, C. Gueguen.and M. Guisnet, i n p r e p a r a t i o n . M. Guisnet, G. Giannetto, P . H i l a i r e a u and G. Perot, J. Chem. SOC. Chem. Com.,(1983) 1411.
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299
APPLICATION DE L A RESONANCE MACNETIQUE NUCLEAIRE A L'ETUDE DE
L A DISTRIBUTION ET DE L'ACIDITE DE L'EAU DE CONSTITUTION DES SOLIDES. C.DOREMIEUX-MORIN et J.FRAISSARD Laboratoire de Chimie des Surfaces, associk au C.N.R.S. (ERA 457/02) UniversitC Pierre et Marie Curie, Tour 55, 4 Place Jussieu 75230 Paris Cedex 05, (France)
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ABSTRACT Bronsted acidity of OH groups on solids and nature of superficial constitut i v e water is reviewed through l i t e r a t u r e NMR examples : ( i ) direct study ofthe, solid : "rigid lattice" wide band and high resolution W R ; ( i i ) a f t e r agaseous phase adsorption : conventional MR, relaxation studies and high resolution "R. Aciditd de Brijnsted de groupes OH de solides et nature de l'eau de constitut i o n superficielle dtudides par F3iN 2 travers des exemples de l a l i t t d r a t u r e : ( i ) etude directe du solide : large bande en ttr6seau rigide" et hauterbsolution; ( i i ) a p r h adsorption d ' m p h a s e gazeuse supplhentaire : F3iN conventionnelle, relaxation et haute resolution.
INTRODUCTION L'eau joue un rSle fondamental sur les propriktks superficielles des solides. Parmi les questions qui se posent h ce propos, nous retiendrons les suivantes: Y a-t-il des groupes OH ? A quelle quantitd de protons correspondent-ils ? Quelles sont leurs propriktks acide de Bronsted ? Les interactions solide-adsorbat dkpendent des rkponses i ces questions. Nous montrerons ci-aprks Pint&& de la rksonance. magnktique nuclkaire (RMN) pour dkterminer d'une part la nature et la distribution de I'eau de constitution superficielle de solides, d'autre part la force acide des groupes hydroxyles. DISTRIBUTION DES SPINS NUCLEAIRES IH EN RESEAU RIGIDE. APPLICATION
...
A L'ETUDE DE L'EAU DE CONSTITUTION (OH, H20 etc ) DES SOLIDES.
On utilise la RMN de IH (spin l/2) dans I'hypothkse dite du "rkseau rigidel', c'est dire dans le cas oh les groupes d'atomes oi interviennent ces spins ne sont animks ni de rotations ni de translations. Les spectres sont alors principalement dkterminks par I'interaction dipolaire spin-spin. Leur largeur et leur forme dkpendent htroitement de la gdomktrie des ensembles de spins proches voisins.
300 L e s c o u r b e s d'absorption o n t ktk l e plus souvent c a r a c t k r i s k e s par leur second m o m e n t M 2 h l a s u i t e d e s t r a v a u x d e Van Vleck (ref.l), reliant M
2
thkorique k la
distribution d e s spins. Dans c e r t a i n s c a s la valeur e x p k r i m e n t a l e d e M2 p e r m e t d e c o n c l u r e h I'existence d ' e s p k e s dkterrninkes, en particulier d e OH. C i t o n s les k t u d e s d e Stevenson (ref.2) et d e Dernarquay et al. (ref.3) relatives a la distribution d e s protons, respectivement sur zkolithes HY et sur gel d e silice. L a description d'un s p e c t r e RMN par un s p e c t r e calculk fournit un moyen plus klabork d ' i n t e r p r k t a t i o n d e s rksultats expkrimentaux. I1 f a u t pour c e l a c o n s i d i r e r
les p r o t o n s proches voisins d a n s d e s configurations gkomktriques mod6les pour lesquelles on c a l c u l e les i n t e r a c t i o n s magnktiques.
L e s configurations rnagngtiques
a
2 spins (H 0 ) et a trois spins a u s o m m e t d'un t r i a n g l e kquilatkral (H 0') s o n t 2 3 connues depuis longtemps. L e s s p e c t r e s correspondants sont e n gknkral bien rksolus
et t r &
l a r g e s par rapport h c e u x d e spins plus ou moins isolks (OH). C e p e n d a n t
une m k t h o d e d e simulation d e s s p e c t r e s expkrimentaux par simple addition d e s contributions d e s configurations magnktiques correspondant h OH, H 0 et H O+ n ' e s t 2 3 p a s rCpandue, car e l l e conduit le plus souvent h d e s e r r e u r s grossieres. Nous avons m o n t r k e n effet que, dans un m @ m e composk, les i n t e r a c t i o n s e n t r e les protons d ' u n e mol6cule d ' e a u et celui d'un groupe OH voisin n e s o n t g k n k r a l e m e n t pas nkgligeables. Pour e n rendre c o m p t e nous avons proposk I'utilisation d'une configurat i o n magnktique h t r o i s spins 1/2 s i t u k s a u s o m m e t d'un t r i a n g l e isoc&e. L a b a s e R du t r i a n g l e p e u t e n effet reprksenter l a d i s t a n c e i n t e r p r o t o n s dans une molkcule d'eau. L e s grands c8tks kgaux, R', correspondent h une approximation souvent suffis a n t e d e s plus c o u r t e s d i s t a n c e s e n t r e protons d e H 2 0 et protons d e OH (ref.4). D e s p r o g r a m m e s e n F o r t r a n p e r m e t t e n t d e simuler la plupart d e s s p e c t r e s exp6rimentaux.
a
partir
des
kventuelles
contributions d e s configurations convenables.
L e s p a r a m k t r e s peuvent &re optimisks par moindres carrks. L e s s p e c t r e s calculks d k c r i v a n t l e s s p e c t r e s e x p k r i m e n t a u x sont suffisamrnent sensibles a u x variations d e s d i f f k r e n t s p a r a m 6 t r e s pour q u e la prkcision sur les d i s t a n c e s HH s o i t bonne, e n p a r t i c u l i e r pour l e s c o u r t e s distances. Oxydes Nous rksumons les r6sultats o b t e n u s sur I'oxyde d e t i t a n e
a
titre d'exemple
(ref.5,6,7). L e s kchantillons, d e g r a n d e surface spkcifique, sont t o u s dksorbks sous torr. 11s sont t r a i t & .?I d i f f k r e n t e s t e m p k r a t u r e s e n t r e 20 et 400°C. Dans l e
cas d e s phases cristalliskes, rutile et a n a t a s e , les rksultats numkriques o b t e n u s k p a r t i r d e s s p e c t r e s sirnulks s o n t c o m p a r k s a u x rkpartitions d e protons nkcessaires pour compenser l e s insaturations crkkes par I'existence d e l a s u r f a c e . C e t t e d e r n i g r e
est
g k n k r a l e m e n t f o r m k e d e plans d e g r a n d e densitk atomique. Nous n'avons retenu q u e les plans d o n t l'kvolution d e I'hydratation est c o m p a t i b l e a v e c I'kvolution d e s s p e c t r e s RMN,du double point d e vue d e s c o n c e n t r a t i o n s e n OH et H 0 et d e s d i s t a n c e s H-H. Nous obtenons a l o r s l e s r k s u l t a t s suivants:
2
301
L e s plans les plus probables e n s u r f a c e s o n t le (110) pour le rutile (fig.1) et l e (001) pour I'anatase. Pour les 6chantillons t r a i t & h 20°C sous
torr, c ' e s t h d i r e les plus hydratks
q u e nous ayons gtudigs, le nombre d e groupes OH e s t tel q u e ceux-ci recouvrent la s u r f a c e d'une m a n i i r e continue. 11s sont d e deux types, e n nombres Cgaux; les uns, fix& s u r d e s a t o m e s Ti d e s u r f a c e , sont I 6 g i r e m e n t n6gatifs (fig.1, s i t e C ) ; les a u t r e s Igg6rement positifs (fig.1, s i t e A) sont f o r m & p a r fixation d'un proton sur un a t o m e 0 n'ayant pas s a coordination normale. L e n o m b r e d e ces d e r n i e r s & a n t kgal
a
celui des mol&ules
d'eau, nous a d m e t t o n s q u e celles-ci leur s o n t liges par
...
liaison hydrog6ne (OH OH ) (fig. 1, s i t e B). L a dkshydratation d e c e s &hantillons, 2 particuli6rement hydra&, c o m p o r t e non s e u l e m e n t l e d 6 p a r t d ' e a u rnol6culaire p r g e x i s t a n t e mais aussi celui d ' e a u f o r m g e a p a r t i r d e groupes OH.
I
*O*
l
O
*hl
V
.b. l
0
*O*
0
Fig.1 - R g p a r t i t i o n d e s protons sur le plan ( I 0) du 2 rutile; c h a q u e &chantillon R x c o n t i e n t I'gauivalent d e x m o l i c u l e s d ' e a u Dour 00 A L e s s i t e s A et C correspondent d e s p r o t o n s d e groupes OH r e s p e c t i b e m e n t I6g;rement positifs et nggatif;,les s i t e s B c e u x d ' e a u rnolgculaire.
a
.
a
Une g t u d e moins d i t a i l l k e d e I'enko et al. (ref;8) donne d e s r 6 s u l t a t s e n a c c o r d a v e c les prgcgdents. L a m6thode c o n v i e n t aussi pour d e s s o l i d e s d e f a i b l e s u r f a c e spkcifique. L a 2 2 comparaison d'un s a b l e ( < 1 m /g) et d'un gel d e s i l i c e Davison (200 m /g) a montrk que, a p r i s t r a i t e m e n t sous 10-4 torr h t e m p g r a t u r e a m b i a n t e , la q u a n t i t 6 d e groupes OH est relativernent beaucoup moins i r n p o r t a n t e h la s u r f a c e du s a b l e qu'h c e l l e
du gel d e d i c e (57% c o n t r e 87%) (ref.9).
302 D ' a u t r e s oxydes, par e x e m p l e d e m o l y b d k e e t d e tungst6ne o n t &t>udiks par Pitsyuga et al. (ref.10). ZColithes .Freude et a l ; ( r e f . l l ) o n t 6tudig la f o r m e d e s s p e c t r e s p o u r d e s O H dilu6s. L ' k t u d e d e s z&olithes NaX et NaY par O e h m e et al. (ref.121, utilisant une m b t h o d e e s t i m a t i v e d ' i n t e r p r 6 t a t i o n d e s spectres,. p e r m e t
ces a u t e u r s d e trouver d e 0,6 & 1,2
groupes OH par c a v i t 6 jusqu'i d e s r e c o u v r e m e n t s moyens e n eau. Nous c i t e r o n s aussi les t r a v a u x d e S t a u d t e (ref.13) sur l a mordknite H, Ernst et al. (ref.14) s u r I'hydrosodalite, Basler (ref.15) sur la zkolithe Linde 1 3 X , F r i p i a t et a l . (ref.19) sur
argiles. ACIDITE DES GROUPES OH DE S U R F A C E On vient d e voir que, d a n s l e s solides, I'interaction dipole-dipole d e s spins nucl6aires m a s q u e t o u t e s t r u c t u r e f i n e et n e p e r m e t donc p a s une m e s u r e d i r e c t e d e s d 6 p l a c e m e n t s chimiques par les m&thodes RMN classiques. Or l e dkplacement chimique 6 g H d e p r o t o n s d e groupes OH d e v r a i t c o n s t i t u e r une m e s u r e d e l'acidit6 d e ces protons sur l e solide S-OH (ref.16) puisqu'il t r a d u i t I'environnement d e c t r o n i q u e d e ceux-ci.
I1 f a u t donc, pour a r r i v e r
cette mesure, &miner
I'effet du couplage
dipolaire spin-spin. Deux moyens s o n t utilis&s d a n s ce but, suivant q u e l e s s p e c t r e s observ&s e n RMN s o n t c e u x d ' u n e phase adsorbke jouant l e r61e d e b a s e (dans ce cas la RMN d e t y p e conventionnel p e u t suffire), ou q u e ce sont c e u x d e noyaux a p p a r t e n a n t d i r e c t e m e n t a u cornpos6 &tudi&. R & t r & c i s s e m e n td e s signaux par &change rapide L'adsorption d'une phase gazeuse, AH, pouvant jouer l e t a l e d e base donne lieu
I'
Cquilibre h&t&rog&ne:
s-
OH + A H
e s - 0- +
AH^+
(1)
Quand un &change rapide i n t e r v i e n t e n t r e les protons d e S-OH et c e u x d e la mol6cule AH, les protons a c i d e s a f f e c t e n t le dCplacement chimique d e s protons d e la p h a s e H adsorb&e. L e s p e c t r e ne c o m p o r t e a l o r s q u ' u n e raie, d e d k p l a c e m e n t chimique 6obs
,
r6sultant d e la c o a l e s c e n c e d e s raies c a r a c t 6 r i s t i q u e s d e s groupes AH, AH2+ et OH H d o n t l e s d & p l a c e m e n t s chimiques s o n t r e s p e c t i v e m e n t f j A H , L a raie est a l o r s suffisamrnent &troite pour q u e la m e s u r e puisse @tre rCaIisee sur un appareil conventionnel. H 6obs est exprim& par la relation:
OG
p. e s t la c o n c e n t r a t i o n d e s a t o r n e s H dans les groupes i. L e nornbre d e s p a r a r n h t r e s
d e ( 2 ) n k c e s s i t e une &quation s u p p l 6 m e n t a i r e pour leur d&termination. Pour I'obtenir il e s t n 6 c e s s a i r e que la phase adsorb&e AH contienne au observable e n RMN, par e x e m p l e 15N ou " 0 .
moins, un a u t r e noyau
303
Fig.2 - S c h b m a d e principe relatif a u r6tr6cissernent du s p e c t r e d e s protons d e S-OH ; localisation du signal observ.6 lors d e I'adsorption d e A H et &change rapide d e s protons selon l a reiation (1). N o u s consid6rons l'exernple d'adsorption d'arnrnoniac s u r x6rogel d e s i l i c e pr&
t r a i t C e sous los4 t o r r 5 150, 400 et 600°C (ref.17). H D'aprGs (2) 6obs dkpend d e s c o n c e n t r a t i o n s relatives e n O H et NH3. Mais apr6s H torr h 3 0 T , 60bstend v e r s une valeur l i m i t e d e 4,2; d6sorption d'arnmoniac sous 3,9 et 3,7 pprn par rapport a u TMS pour c h a c u n d e s Cchantillons c o m p o r t a n t respecti-
v e m e n t 5,7; 4,2 et 1,4 O H nrn-2 : on o b s e r v e q u e la dissociation d e s O H et I ' a c i d i t i moyenne par groupe O H croissent I&g&rementa v e c la densit6 d e s OH. n6cessite c e l l e du c o e f f i c i e n t CY d e dissociation d e SIOH. La dgtermination d e A H I PH la R M N d e N fournit
L e recours
0;
P
N H ~N.H 3
+ P N H + = 1, 4
D e s m e s u r e s d e 6," ppm par rapport
'NH~
et P N H + & a n t les c o n c e n t r a t i o n s respectives en N H 3 et 4
= 22,O pprn a p r & d&orption sous 10
i %H3,
il rksulte q u e
-4
N t o r r et d e 6 N H + = 43,5 4
304 N ----A PNH; *obs
1
- N 'NH~
-
22 0 43,5
?,
o,5
et le c o e f f i c i e n t d e dissociation d e OH, c1 = 0,5. En utilisant I'kquation (2) et les H valeurs d e a et d e 6obs limite, on o b t i e n t par exemple, pour l a d i c e t r a i t C e h 400°C 6 E H = 2 ppm C e t t e valeur est indgpendante d e l a phase adsorb6e. Rappelons quelques valeurs t r o u v g e s par cette m 6 t h o d e (ref.18) &than01 silica gel
H ; '
PPm
0,5
2
d i c e -aluming HY z6olithe HCOOH 4-5
8
I1
(rCf6rence : g a z TMS)
Cette technique n e p e u t fournir q u e d e s valeurs moyennes d'aciditg. MobilitC d e s protons d e s groupes OH induite par adsorption d'une phase g a z e u s e basique Ces Ctudes n e conduisent p a s
d e s valeurs d e dCplacements chimiques m a i s
e l l e s renseignent sur la mobilit6 d e s protons acides. C i t o n s d e s r6sultats d e I'Cquipe
PFEIFER e t al. (ref. 20)obtenus
a
partir de mesuresrdes temps, de relaxation &s.prOtOnS.
En l ' a b s e n c e d'adsorbat il n'y a p a s d e relation gCnCrale e n t r e les propriCt6s a c i d e s d e s zColithes HY et l a mobilit6 d e s protons (ref.20). Un rnodile d e la m o b i l i t i d e s protons d e groupes OH d e HY induite par d e s ions pyridinium a CtC proposC (ref.20): un ion pyridinium rompt s a liaison hydrogkne a v e c le rbseau z6olithique et diffuse jusqu'h un a u t r e a t o m e 0 du rkseau oh il dgpose l e proton. I1 diffuse e n s u i t e a I ' g t a t d e molCcule d e pyridine jusqu'h un groupe OH qu'il q u i t t e l e plus souvent h I'Ctat d e molCcule d e pyridine et plus r a r e m e n t sous f o r m e d'ion pyridinium. L e s variations d e s t e m p s d e relaxation T I e t T
d e s protons d e zgolithes Y d&ationis&es,en fonction 2 d e la tempGrature, sont i n t e r p r @ t & e s en t e r m e s d'Cchange e n t r e deux rggions d e mobilitgs d i f f g r e n t e s attribuCes h d e s molCcules d e pyridine adsorb6es et h d e s
ions
pyridinium (ref.21). L a m @ m e Cquipe a u t i l i s i la t e c h n i q u e d e g r a d i e n t d e
c h a m p e n RMN p u l s i e pour Ctudier l'autodiffusion
i n t r a c r i s t a l l i n e d e I'eau dans
la zColithe ZSM-5 (ref.22). Fripiat (ref.23) et Resing (ref.24) o n t aussi CtudiC ces ph&om&nes. RktrCcissement d e s signaux par les techniques d e h a u t e r6solution dans les solides C e s techniques sont d e deux t y p e s : c e r t a i n e s sgquences d'impulsions multiples e t la*rotation h I'angle magique"(MAS); elles sont souvent associCes dans une m @ m e Ctude. L a p a r t i e s e c u l a i r e d e I ' i n t e r a c t i o n dipolaire e n t r e 2 spins A et B d i s t a n t s d e r e s t proportionnelle h des f a c t e u r s d e 2 n a t u r e s diffgrentes, d ' u n e p a r t
305 - I / 4 ( I i I i + I i I i ) ) , f a c t e u r d e spins, oh 1:
($,I:
, :I , 1:
y, z du m o m e n t angulaire I du spin n, et If = 1: .It[ (I-3cos 2 BAB)/r3 , f a c t e u r n d ' e s p a c e , 0;
eAB est
s o n t les projections suivant x,
; d'autre part d e
I'angle e n t r e l e c h a m p magnktique
directeur H
et la d r o i t e joignant les spins. 0 C e r t a i n e s skquences d e pulses t e l l e s que Wahuha 4, MREV-8,
a f i n d e moyenner
a
... o n t
ktk calculkes
0 l e f a c t e u r d e spins d e l ' i n t e r a c t i o n dipolaire. On o b s e r v e a l o r s
l e dkplacernent chimique a n i s o t r o p e d e s spins. Indkpendarnment,
i d k f a u t d e pouvoir aligner l e s spins e n i n t e r a c t i o n dipolaire
d a n s une direction 0 = 54"44' d i t e d e "l'angle magique", t e l l e q u e (I-3cos 2 0,) = 0, 0
o n p e u t obtenir un effet semblable par une rotation d e I'kchantillon a u t o u r d'un a x e inclink d e 0 par rapport 0
Ho. Pour une rotation s u f f i s a m m e n t rapide, I'orientation
moyenne d e AB a la valeur requise. D e plus il est possible d e t r a n s f k r e r une p a r t i e d e I'knergie d e r i s o n a n c e d e noyaux non rares t e l s q u e ' H b d ' a u t r e s noyaux gkomktriquement voisins t e l s q u e I5N ou 29Si par la technique d e polarisation c r o i s k e q u e I'on a s s o c i e souvent a u MAS sous l e nom d e CP/MAS. On o b s e r v e alors la rksonance d e s noyaux rares. L e s ktudes s o n t c o n d u i t e s soit par I'intermkdiaire d'une phase supplkmentaire adsorbke
(i
laquelle a p p a r t i e n n e n t l e s noyaux rksonants) soit d i r e c t e m e n t sur l e
solide. P a r I'intermkdiaire d'une phase adsorbke Des recherches o n t , bien sCr, ktk rkaliskes par RMN du. 13C. C e p e n d a n t nous passerons d i r e c t e m e n t
a
d e s rksultats o b t e n u s par RMN d e 15N (ref.25-28)
dont
R i p m e e s t e r d i t qu'elle p a r a i t mieux a d a p t k e a I'identification d e s i t e s d e s u r f a c e q u e c e l l e d e 13C (ref.27). C e s ktudes sont g6nkralernent conduites par la technique CP/MAS, par adsorption d e pyridine e n r i c h i e e n 15N, soit seule, soit e n cornpktition a v e c d e la n-butylamine non enrichie. L e s &changes & a n t lents, il e s t possible d'une part
de
r6aliser
d e s expkriences sans porte-kchantillon
rigoureusement k t a n c h e ,
d ' a u t r e p a r t d'observer plusieurs signaux d e rksonance. On p e u t ainsi distinguer d e s s i t e s a c i d e s d e Lewis d e s s i t e s a c i d e s d e Bronsted et des molkcules d e pyridine d'ions pyridinium.
Ainsi R i p m e e s t e r (ref.27) n e t r o u v e p a s d'ions pyridinium sur
I'alumine y e n I'absence d'humiditk, alors qu'il e n t r o u v e associks
a
des sites d e
Bronsted sur l a m o r d k n i t e H. Haw et al. (ref.25) observent l e r e m p l a c e m e n t d e la pyridine fixke sous f o r m e d'ions pyridiniurn sur silice-alumine par la n-butylamine. Sindorf e t al. (ref.29) k t u d i e n t la dkshydratation d e la s u r f a c e d e silicagel par RMN d e 29Si e n CP/MAS a p r & adsorption d'hexamkthyldisilisane.
11s e n concluent q u e
I'klimination c o m p l 6 t e d e s groupes silanol n ' e s t approchke q u ' a d e s t e m p k r a t u r e s supkrieures h c e l l e s g k n k r a l e m e n t utiliskes lors d e s t r a i t e m e n t s ( > 1000°C) et qu'il reste d e I'eau rnolkculaire s u r d e s kchantillons "secs".
306 E t u d e d i r e c t e du solide Nous c i t e r o n s d'abord les t r a v a u x ddjh anciens d e Vaughan e t al. (ref.30). G r i m m e r e t al. (ref.31) sernblent avoir, les premiers, indique la valeur du deplacernent chirnique d e protons d e groupes OH, o b t e n u e par les m i t h o d e s d e la h a u t e rdsolution dans les solides. L e cornposd etudid e t a i t un s i l i c a t e d e c a l c i u m C a 2 (OH)(HSi04). Deux equipes s e distinguent actuellernent par leurs d t u d e s d e I'eau d e c o n s t i t u t i o n superficielle d e solides par RMN d e 'H e n h a u t e resolution. C e s o n t c e l l e d e P f e i f e r (ref.32-34) et c e l l e d e Scholle (ref.35-36). T o u t e s deux utilisent la technique du MAS a v e c une t u r b i n e porte-&chantillon doublernent e n t r a i n 6 e par un gaz. P f e i f e r et al. introduisent d a n s la turbine d e s Cchantillons e n t u b e scelle. 11s H cornparent les valeurs d e s d e p l a c e m e n t s chimiques GOHde gel d e s i l i c e , d ' a l u r n i n e y
,
d e d i f f d r e n t e s z6olithes Y et d e mordknite H (ref.32). 11s e t u d i e n t I'influence
d e s conditions d e t r a i t e r n e n t therrnique d e zdolithe NH4Y (ref.33) par I'interrnkdiaire d e I ' i n t e n s i t e d e s d i f f e r e n t s signaux d e resonance d e s groupes OH; la cornparaison d e c e s r6sultats a v e c c e u x d e RMN d e 27Al les conduit a u x conclusions suivantes: (i) il n ' a p p a r a i t pas d'aluminiurn du reseau tricoordonnd pendant la deshydroxylation d e zdolithes HY; (ii) la deshydroxylation a u voisinage d'un a t o r n e A1 du rdseau est toujours a c c o m p a g n d e d e son expulsion d e la c h a r p e n t e . L e s s p e c t r e s d e silice-alurnine arnorphe (ref.34) cornportent un signal d e resonance d e 'H
v e r s 7 ppm, p r e s e n t
d a n s les s p e c t r e s d e z6olithes et a b s e n t d e s s p e c t r e s d e silice et d e c e u x d'alurnine. L'intensitd d e ce signal e s t m a x i m a l e pour la rnCrne t e n e u r d e Al2O3 q u e I ' a c t i v i t e c a t a l y t i q u e d e s echantillons, justifiant l'attribution du signal h d e s groupes hydroxyles acides. Scholle et al. Ctudient la z e o l i t h e H-ZSM-5: d'une p a r t l e p h e n o m b e ddsorptionadsorption d ' e a u , sur l e s groupes silanol et sur les groupes a c i d e
d e Bronsted ou
OH s t r u c t u r a u x (ref.35); d ' a u t r e p a r t I'acidit6 d e s groupes OH d e la charpente,paral-
16lernent
l a ddsorption d e NH3
t e r n p d r a t u r e prograrnrnde (ref.36). L a cornparaison
a v e c l a b o r a l i t e leur perrnet d e c o n c l u r e q u e I'acidite d d c r o i t d e Si-OH-AI, h Si-OH-B,
h Si-OH. L e t a b l e a u ci-dessous rdsume l e s r6sultats o b t e n u s sur zdolithes. 6 E H e n pprn/TMS N a t u r e d e s OH
I
1
*
terrninaux SiOH AlOH
6,8-8
4,2-5,0 I
A
structuraux H
307
Sindorf e t Maciel ont 6tudi6 un gel d e silice ddrat6/rChydrate par RMN d e *?Si en CP/MAS (ref.37). L e u r s r6sultats n e peuvent pas @ t r e i n t e r p r @ t & spar un modele s t r u c t u r a l unique du t y p e d e c e u x prCc6dernrnent proposes pour les s u r f a c e s d e silice. C e p e n d a n t ils s o n t e n a c c o r d a v e c une s u r f a c e d e silice h e t e r o g 6 n e forrnde d e rkgions skparkes, ressemblant aux plans (100) et (111) d e la c r i s t o b a l i t e B
.
CONCLUSION Nous esperons avoir rnontr6 q u e la RMN s'est deveioppee e n techniques diversifiees,
utiles pour la d6terrnination d e la distribution d e I'eau d e constitution superficielle d e s solides e t c e l l e d e I'acidite d e Bronsted d e s groupes OH. D e ces techniques, c e l l e s qui fournissent une valeur nurnerique d e d6placernent chirnique d E H s e r n b l e n t
les plus irnrn6diaternent prornetteuses car e l l e s peuvent conduire h I'etablissernent d ' u n e e c h e l l e d ' a c i d i t e dans les solides. L ' e t u d e r e c e n t e par S c h r o t e r , Rosenberger et Hadzi (ref.38) d e cornpos6s solides h liaisons hydrogene, et, en particulier, d ' a c i d e s organiques va dans ce sens.
308 REFERENCES 1 J.H. Van Vleck, Phys. Rev. fi, 1168 (1948). 2 R.L. Stevenson, J. C a t a l . 3, 113 (1971). 3 J. Dernarquay, J. Fraissard et B. Imelik, C.R. Acad. Sci.=, 1405 (1971). 4 C. DorCrnieux-Morin, J. Magn. Res., 2,419 (1976); 2,505 (1979). 5 M.A. Enriquez, C. DorCmieux-Morin et J. Fraissard, Appl. Surf. Sci. 2, 180 (1980). 6 M.A. Enriquez, C. DorCrnieux-Morin et J. Fraissard, J. Solid S t a t e Chern. 233 (1981). 7 C. DorCmieux-Morin, M.A. Enriquez, J. S a n z et J. Fraissard, J. Colloid Interf. 502 (1983). Sci. 8 V.S. I'enko et A.V. Uvarov, Kolloidnyi Zhurnal )7, 1161 (1975). 9 J.L. Bonardet, 0. Bouloussa, C. DorCmieux-Morin e t J. Fraissard, R a p p o r t d e f i n d'6tude DGRST, "RCcupdration assist6e du pdtrole", D6cision d'aide n'77.7.1464. 10 V.G. Pitsyuga, L.A. Pozharskaya, M.V. Mokhosoev et EH.D. Serdyukova, 2 . neorg. Chirn. 2,891 (1980). I 1 D. F r e u d e , D. Muller et H. Schmiedel, Surf. Sci. 2,289 (1971); D. F r e u d e et H. Schmiedel, Phys. S t a t . Sol. (b) 2, 631 (1972). 12 W. O e h m e , D. Freude, 5. Klepel, H. P f e i f e r et H. Schmiedel, Z. Phys. Chern. 259, 1137 (1978). 13 T S t a u d t e , 2 . Phys. Chern. 258, 805 (1977). 14 H. Ernst, H. P f e i f e r et S.P. Zhdanov, Zeolites, 2, 209 (1983). 15 W.O. Basler, Z. Naturforsch. 2, 1417 (1980). 16 J.L. Bonardet, J.P. Fraissard et L.C. d e Mthorval, Sixth International Congress on Catalysis, Londres B6 (1976). 17 J.L. Bonardet, L.C. d e Mdnorval et J. Fraissard, M a g n e t i c R e s o n a n c e in Colloid and I n t e r f a c e Science, ACS Symposium S e r i e s 3, H.A. Resing and C.G. Wade Editors, San Francisco, p. 248 (1976). 18 J. Fraissard, Second I n t e r n a t i o n a l Symposium in Magnetic Resonance in Colloid a n d I n t e r f a c e Science, N a t o Advanced Study Institute, Menton, p. 269 (1979). 19 J. Hougardy, W.E.E. S t o n e et J.3. Fripiat, J. Chem. Phys., 64,3840 (1976). J.J. F r i p i a t , M. Kadi-Hanifi, J. C o n a r d et W.E.E. Stone, Magnetic R e s o n a n c e in Colloid and I n t e r f a c e Science, N a t o Adv. Inst. Ser. J. Fraissard et H.A. Resing Edit., Reidel Publ. Comp. p. 529 (1980). 20 D. F r e u d e , H. P f e i f e r , W. Ploss et B. S t a u d t e , J. Molec. Catalysis, g, I (1981). 21 H.J. Rauscher, D. Michel et H. P f e i f e r , J. Molec. Catalysis, 12,159 (1981). 22 3. K a r g e r , W. Krause et H. P f e i f e r , Z. Phys. Chem., 838 (1982). 23 J.J. F r i p i a t , Magnetic R e s o n a n c e in Colloid and I n t e r f a c e Science, ACS Symposium Series, 3,H.A. Resing et C.G. Wade Edit., p.261 (1976). 24 H.A. Resing, Magnetic R e s o n a n c e in Colloid and I n t e r f a c e Science, N a t o Adv. Inst. Series, J. F r a i s s a r d et H.A. Resing Edit., Reidel Publ. Cornp., P.219 (1980). 25 D. Michel, A. Gerrnanus et H. P f e i f e r , J. Chern. SOC. F a r a d a y Trans. I, 237 (1982). 26 T. Bernstein, L. Kitaev, D. Michel, H. P f e i f e r et P. Fink, J. Chem. SOC., F a r a d a y 761 (1982). Trans. I, 2, 27 J.A. Riprneester, J. Am. Chem. SOC., 105,2925 (1983). 28 J.F. Haw, I.S. Chuang, B.L. Hawkins et C.E. Maciel, J. Am. C h e m . SOC., 105, 7206 (1983). 29 D.W. Sindorf et G.E. Maciel, J. Phys. C h e m . 87, 5516 (1983). 30 R.W. Vaughan, L.B. Schreiber et J.A. Schwarz, M a g n e t i c Resonance in Colloid a n d I n t e r f a c e Science, ACS Symposium series, 3, H.A. Resing et C.W. Wade Edit., p.275 (1976). 31 A.R. G r i m m e r et H. Rosenberger, 2 . Chern., &, 378 (1978). H. Rosenberger et A.R. G r i m m e r , Z. anorg. allg. C h e m . I 1 (1979). 32 D. F r e u d e , M. Hunger et H. P f e i f e r , C h e m . Phys. L e t t e r s , 307 (1982). 33 D. F r e u d e , T. Frohlich, M. Hunger, H. P f e i f e r et G. Scheler, Chern. Phys. L e t t e r s , 98, 263 (1983).
c,
z,
m,
m,
m,
z,
s,
a,
309 34 D. Freude, M. Hunger et H. P f e i f e r , H. B r e m e r , M. J a n k et K.P. Wendlandt, Chem. Phys. L e t t e r s 100,29 (1983). Scholle, W.S. Veernan, J.G. P o s t et J.H.C. Van Hooff, Zeolites, 35 K.F.M.G.J. 3, 214 (1983). 36 X.F.M.G.J. Scholle, A.P.M. Kentgens, W.S. Veeman, P. F r e n k e n et G.P.M. van d e r Velden, 3. Phys. Chem., 88, 5 (1984). 37 D.W. Sindorf et G.E. Maciel, J. Am. C h e m . SOC., 105,1487 (1983). 38 B. S c h r o t e r , H. R o s e n b e r g e r et D. Hadzi, J. Molec. S t r u c t . 96,301 (1983).
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311
B. Imelik et al. (Editors), Catalysis by Acids and Base6
o 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed
in The Netherlands
MICROCALORIMETRIC CHARACTERIZATION OF ACIDITY AND BASICITY OF VARIOUS METALLIC OXIDES
A l i n e AUROUX and Jacques C. VEDRINE I n s t i t u t de Recherches s u r l a Catalyse, CNRS 2, avenue A l b e r t E i n s t e i n
-
F 69626 V i l l e u r b a n n e
-
France.
ABSTRACT M i c r o c a l o r i m e t r y h a s been u s e d w i t h s e v e r a l m e t a l l i c o x i d e s (MgO, SiOp, S i 0 2 - A l 2 0 3 , Al2O3, Bi3FeMo2 012,Ti02) and z e o l i t e s (Y-and ZSM-5-types i n t h e i r Na and H forms) f o r t h e measurement o f t h e d i f f e r e n t i a l h e a t o f a d s o r p t i o n o f p r o b e m o l e c u l e s , a c i d i c and b a s i c i n n a t u r e . Small probe molecules have been chosen, namely NH3 and CO2, i n o r d e r t o a v o i d d i f f u s i o n l i m i t a t i o n , p a r t i c u l a r l y f o r small pore z e o l i t e s . I t was observed t h a t a c i d i f i c a t i o n o f z e o l i t e n o t o n l y c r e a t e d s t r o n g a c i d i c s i t e s b u t a l s o a f e w s t r o n g b a s i c s i t e s . On A1203 b o t h s t r o n g a c i d i c and b a s i c s i t e s were evidenced, w h e r e a s o n l y s t r o n g b a s i c s i t e s were d e t e c t e d on MgO. Few weak a c i d s i t e s were observed on Bi3FeMo2012 c a t a l y s t s and some a c i d i c and b a s i c s i t e s were d e t e c t e d on s i l i c a . T i 0 2 ( e i t h e r anatase o r r u t i l e ) p r e s e n t s a few and v e r y weak a c i d s i t e s . However, a c t i v a t i o n i n a i r a t 400'C c r e a t e d some s t r o n g a c i d s i t e s . C o r r e l a t i o n w i t h p e c u l i a r c a t a l y t i c p r o p e r t i e s i n v o l v i n g t h e presence o f b o t h a c i d i c and b a s i c s i t e s and w i t h t h e e l e c t r o n donor and 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 o f o x i d e s may be envisaqed. RESUME
La t e c h n i q u e de m i c r o c a l o r i r n 6 t r i e a i t 6 u t i l i s e e pour essayer de c a r a c t C r i s e r l e s p r o p r i 6 t C s a c i d e s ou basiques de c a t a l y s e u r s oxydes m e t a l 1 i q u e s comme MgO, Si02, Si02-Al203, Al2O3, Bi3FeMo2012, T i 0 2 ou de c a t a l y s e u r s z e o l i t h i q u e s comme l e s z 6 o l i t h e s de t y p e Y o u ZSM-5 s o u s f o r m e sodique ou acide. L'ammoniac NH3 (base f o r t e ) e t l e gaz carbonique ( a c i d e f a i b l e ) o n t 6 t 6 c h o i s i s 'a cause de l e u r f a i b l e t a i l l e p o u r 6 v i t e r l e s l i m i t a t i o n s d i f f u s i o n n e l l e s , en p a r t i c u l i e r pour l e s z i o l i t h e s 'a p e t i t s pores. On a o b s e r v i que l ' a c i d i f i c a t i o n d e s z e o l i t h e s crCe non seulement des s i t e s acides f o r t s mais 6galement quelques s i t e s basiques f o r t s . L ' a l u m i n e p r 6 s e n t e 'a l a f o i s des s i t e s a c i d e s e t b a s i q u e s f o r t s t a n d i s que MgO p r 6 s e n t e e s s e n t i e l l e m e n t des s i t e s b a s i q u e s f o r t s e t Bi3FeMo2012 q u e l q u e s s i t e s a c i d e s f a i b l e s . T i 0 2 ( a n a t a s e ou r u t i l e ) p r e s e n t e t r s s peu de s i t e s acides mEme f a i b l e s . L ' a c t i v a t i o n 'a 400'C sous a i r C r e e des s i t e s a c i d e s f o r t s . P a r c o n t r e l a s i l i c e p r 6 s e n t e tr& peu de s i t e s f o r t s s o i t acides s o i t basiques. Des c o r r e l a t i o n s avec l e s p r o p r i 6 t 6 s c a t a l y t i q u e s n i c e s s i t a n t 'a l a f o i s des s i t e s a c i d e s e t b a s i q u e s f o r t s o u avec l e s p r o p r i e t i s accepteur e t donneur d ' 6 l e c t r o n s peuvent E t r e envisag6es.
312 INTRODUCTION The a d s o r p t i o n o f m o l e c u l e s on s u r f a c e s i s o f p r i m a r y importance i n c a t a l y t i c processes. U n d e r s t a n d i n g t h e n a t u r e o f t h e a d s o r b a t e - a d s o r b e n t
interaction
p r o v i d e s u s e f u l i n s i g h t i n t o t h e p r o p e r t i e s o f t h e adsorbent s u r f a c e . While many d i v e r s e t e c h n i q u e s have been used t o s t u d y t h e s u r f a c e r e g i o n o f adsorbents o n l y a few o f them p r o v i d e i n f o r m a t i o n about t h e s t r e n g t h o f c h e m i s o r p t i o n i t s e l f . We r e p o r t h e r e t h e use o f m i c r o c a l o r i m e t r y t o d e t e r m i n e t h e i n t e r a c t i o n b e t w e e n s u r f a c e s i t e s and
a p p r o p r i a t e adsorbates.
By c h o o s i n g b a s i c o r a c i d i c
a d s o r b a t e s i t may t h e n b e p o s s i b l e t o p r o b e t h e s u r f a c e s i t e s o f a g i v e n m a t e r i a l , p a r t i c u l a r l y f o r t h e i r a c i d i c and b a s i c p r o p e r t i e s . The i n t e r a c t i o n b e t w e e n an a d s o r b a t e and a c r y s t a l h a s f o u r o r f i v e c o m p o n e n t s as d i s c u s s e d i n d e t a i l b y B a r r e r and G i b b o n s ( l ) , n a m e l y t h e dispersion
(@F+),
( h ) ,short-range
r e p u l s i o n (@R), p o l a r i z a t i o n ( @p), f i e l d d i p o l e
and t h e f i e l d g r a d i e n t - q u a d r u p o l e (@;-a) A H =@D
+
@R
+
@p t
@
F-u+
energies : @ *
F-Q
The f i r s t two terms depend on t h e s o r b e n t w h i l e t h e o t h e r t h r e e terms depend on b o t h t h e h e t e r o p o l a r i t y o f t h e s o r b e n t and t h e n a t u r e o f t h e s o r b a t e . The l a s t two terms
a r e dependent on t h e o r i e n t a t i o n o f t h e s o r b a t e m o l e c u l e w i t h r e s p e c t
t o t h e s o r p t i o n s i t e . The i n t e r a c t i o n b e t w e e n p a i r s o f s o r b a t e m o l e c u l e s i s n e g l e c t e d when i n i t i a l h e a t s o f a d s o r p t i o n a r e considered. The t o -A/@
adsorbent-absorbate
and
A
Kirkwood-Mu1 l e r f o r m u l a e . p o t e n t i a1 )
D t e r m i s equal
where r i s t h e d i s t a n c e between t h e c e n t e r s o f t h e i n t e r a c t i n g is @R
a
constant
calculated
from
London
or
depends on B / r I 2 f u n c t i o n ( L e n n a r d J o n e s
. @p =
-01
F2/2
( a p o l a r i z a b i l i t y o f t h e adsorbate,
F field
s t r e n g t h ) and @F-Q=-QF ( 3 c o s z e -1)/4 r ( Q quadrupole moment)
The h e a t o f a d s o r p t i o n o f m o l e c u l e s as NH3 a n d / o r C o p has w i d e l y b e e n m e a s u r e d c a l o r i m e t r i c a l l y on many o x i d e s (SiO2, SiOz-Al203, z e o l i t e s m a t r i c e s (1,4-11)).
Si02-MqO (2,3)
or
I t has been shown t h a t an i m p o r t a n t c o n t r i b u t i o n t o
t h e a d s o r b e n t - a d s o r b a t e i n t e r a c t i o n comes f r o m d i p o l e (NH3) o r quadrupole (C02) e n e r g i e s a s s o c i a t e d w i th t h e d i p o l e o r q u a d r u p o l e moment o f t h e m o l e c u l e s . I n t h e c a s e o f z e o l i t e s t h e i n t e r a c t i o n w i t h c a t i o n s a t exchangeable l o c a t i o n and
w i t h l a t t i c e o x y g e n i o n s was shown t o b e d e t e r m i n i n g .
In addition t o the
p h y s i s o r p t i o n as d e s c r i b e d a b o v e a s t r o n g c h e m i c a l r e a c t i o n b e t w e e n t h e a d s o r b a t e and t h e a d s o r p t i o n s i t e s may o c c u r ( i r r e v e r s i b l e a d s o r p t i o n ) p a r t i c u l a r l y when b a s i c (NH3) o r weakly a c i d i c ( C o p ) molecules a r e used, which a l l o w s one t o c o r r e l a t e w i t h a c i d i c and 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 . A c i d i c and b a s i c p r o p e r t i e s o f o x i d e s and z e o l i t e s have a l a r g e i n f l u e n c e on t h e i r c a t a l y t i c p r o p e r t i e s . However, t h e c h a r a c t e r i z a t i o n o f t h e a c i d i c and
313
basic sites is still not satisfactory even if many chemical and physical methods have been developped (12) since none of them is able alone to give all the informations necessary about the amount, the nature, the strenqth and distribution in strengths. The present work has been aimed at brinqinq some contribution to the characterization of acidic and basic properties of catalysts by microcalorimetrically measuring the heat o f adsorption o f some probe molecules. The molecules selected for this work, namely NH3 and C02, are small enough to readily penetrate porous materials, particularly into small pores of zeol i tes. EXPERIMENTAL Materials : The samples were either supplied by commercial firms or synthetized in the laboratory, namely : precipitated silica from Rh6ne Poulenc (2175 MP, 180n1~g-~),fumed silica (200 m2g-1), 7-A1203 (240m29-1), &A1203 (2001n~g-~),silica-alumina from Ketjen (13 wt% A120-3, 580m2q-1), Ti02 anatase ) , (200m2q-I), Bi3Fe1W2012 (2.3 m2g-I), (25 m2g-1) Ti02 rutile (21 n ~ ~ q - ~IinO ZSM-5 zeolite in Na- and H- forms’and Y-type zeolite from llnion Carbide in Hand Na-forms. Microcalorimetric measurements : The adsorption of C02 or NH3 was followed both calorimetrically and volumetrically with a Tian-Calvet type calorimeter. Calibration procedures for the heat-flow microcalorimeter and the ancillary volumetric line have a1read.y been described (10). The qases were dried over Na-wire and further purified by freeze-pump-thaw cycles. The introduction of small pulses of gaseous probe molecules, the measure of the chanqe in pressure over the sample with a Barocel datametrics gauge and the measurement of the heat of adsorption were automatically monitored using an Apple I 1 microcomputer. The samples were outgassed overnight at a given temperature before the probe molecules were introduced onto the sample. RESULTS AND DISCUSSION The adsorption temperature for PIH3 was chosen from previous experiments (10) and was fixed to equal 150°C. For CO2 (and NH3 for some experiments) the adsorption was performed at room temperature (4), involving thus physisorption and chemisorption. The differential heats of gaseous probe adsorption as a function o f amount of gas adsorbed are given in fig.1 to 5 for the different samples. The followina features have to be noted : (i) H-ZSM-5 zeolite presents a curve with a volcanoe shape ( i i ) silica has very weak and very few acidic and basic sites. Precioitated silica is more acidic than fumed silica (fig.3).
314
N H ~ ADSORBED ( c i n 3 . g - 1 )
Fig.2 : V a r i a t i o n s o f t h e d i f f e r e n t i a l h e a t o f NH3 a d s o r D t i o n vs coverage a t 150'C f o r MgO (T) p r e c i p i t a t e d SiO2 ( 0 ) and H-Y ( 0 ) outgassed a t 400°C and f o r B i FeM 012 outgassed a t 150°C an9450'C (0).
Fig.1 : v a r i a t i o n s o f t h e 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 vs coverage a t 150 C f o r H-ZSM-5 (Si/A1=23)(m) and Na-ZSM-5 ( S i / A l = 2 1 ) (+) outgassed a t 400°C, f-Al2O3 outgassed a t 350'C ( 0 ) and 7-A1203 outgassed a t 400'C M. 10
(4
20
l
L
1 2 3 4 N H A~D S O R B E D I c m 3 . g - l )
F i 9 . 3 : V a r i a t i o n s o f t h e d i f f e r e n t i a l h e a t of NH3 a d s o r p t i o n vs coverage a t 23 C f o r p r e c i p i t a t e d SiO2 ( m ) fumed SiO2 ( 0 ) o u t q a s s e d a t 50°C and f o r r u t i l e (a) and anatase (A) c a l c i n e d i n a i r a t 400'C and outqassed a t 350°C.
315
( i i i ) MgO has s t r o n g and numerous b a s i c s i t e s b u t v e r y weak a c i d s i t e s ( i v ) a c i d i f i c a t i o n o f z e o l i t e s creates obviously strong a c i d i c s i t e s b u t a l s o a few b u t s t r o n g b a s i c s i t e s .
( v ) alumina p r e s e n t s b o t h s t r o n g a c i d i c and b a s i c s i t e s . Alumina has more a c i d i c s i t e s t h a n alumina (fig.1). ( v i ) X-phase Bi3FelPio2 o x i d e p r e s e n t s a few a c i d s i t e s o f weak s t r e n g t h ( v i i ) T i 0 2 e i t h e r anatase o r r u t i l e a c t i v a t e d a t room temperature p r e s e n t s few a c i d i c s i t e s . However, a c t i v a t i o n o f s u c h m a t e r i a l s i n a i r a t 400°C p r i o r t o o u t g a s s i n g a t 3 5 0 ° C c r e a t e s a c i d i c s i t e s o f r a t h e r h i g h s t r e n g t h (120-140 k J
mol-1) ( ~ i g . 3 ) .
t
P
I\
1
co,
Fig.4 heat ture (+ ;
2
3
4
ADSORBED (crn3.g-1)
: Variations o f the d i f f e r e n t i a l o f CO2 a d s o r p t i o n a t room tempera vs coverage f o r Na-Y ( 0 ) and H-Y 82% exchanged) outgassed a t 400'C
Fig.5 : V a r i a t i o n s o f t h e d i f f e r e n t i a l h e a t o f COP a d s o r p t i o n a t room t e m p e r a t u r e v s c o v e r a g e f o r p r e c i t a t e d SiO2 (O), SiOz-Al203 (m), ~-A1203%utgassed a t 500 C and MgO (V) Na-ZSM-5 ( 0 ) (Si/A1=21),H-ZSM-5 (+) (Si/A1=23) and H-ZSM-11 ( 0 ) (Si/A1=31) outgassed a t 400'C.
316 The maximum i n t h e Q v s c o v e r a g e c u r v e f o r NH3 a d s o r b e d on H - Z S M - 5 a l r e a d y been d i s c u s s e d p r e v i o u s l y ( 9 ) .
has
I t was shown t h a t i t c o u l d n o t b e
e x p l a i n e d c l a s s i c a l l y f o r i n s t a n c e b y d i p o l e - d i p o l e i n t e r a c t i o n when coveraqe i n c r e a s e s as o b s e r v e d f o r p h y s i s o r p t i o n o f C02 on d i f f e r e n t m o l e c u l a r s i e v e s
(4,6).
I t seems
r a t h e r due t o d i f f u s i o n l i m i t a t i o n o f NH3 molecules i n
narrow pores o f ZSiM-5
the
z e o l i t e and h e t e r o g e n e i t y i n a c i d s t r e n s t h a l o n q t h e
c h a n n e l s . T h i s c o u l d b e p r o v e d e i t h e r by u s i n g v e r y s m a l l p u l s e s o f NH3 o r b y w a i t i n g l o n g e r t i m e f o r t h e r m a l e q u i l i b r i u m b e t w e e n s u c c e s s i v e p u l s e s . The maximum was t h e n e i t h e r s u r D r e s s e d o r a t l e a s t decreased. The maximum i n t h e d i f f e r e n t i a l heat curve i s then r e a s o n a b l y
explained by a quasi e q u i l i b r i u m
model i n w h i c h b o t h h i g h and l o w energy s i t e s a r e l o c a t e d w i t h i n t h e channels, t h e l o w e r energy s i t e s b e i n g p r e s e n t l y c l o s e r t o t h e c r y s t a l l i t e s u r f a c e and t h e h i g h e r e n e r g y s i t e s b e i n g i n s i d e . T h i s i s r e a s o n a b l e s i n c e l a t t i c e aluminum atoms were shown n o t t o be r e g u l a r l y d i s t r i b u t e d i n t h e z e o l i t e c h a n n e l s y s t e m ( 1 4 ) . Such a maximum was n o t o b s e r v e d f o r o t h e r s a m p l e s o f H-ZSM-5, w h i c h s u p p o r t s t h e above c o n c l u s i o n s i n c e t h e a c i d i c
strength d i s t r i b u t i o n alonq t h e
c h a n n e l w i t h i n a z e o l i t e g r a i n i s depending on t h e s y n t h e s i s . Note t h a t such a maximum i n c u r v e was n o t o b s e r v e d f o r C02 a d s o r p t i o n a t v a r i a n c e t o o t h e r f i n d i n g s (4,6).
and 5 f o r C02
A n o t h e r s t r i k i n g f e a t u r e i s seen i n f i g . 4
a d s o r p t i o n s i n c e t h e a c i d i f i c a t i o n o f z e o l i t e s (Y- o r ZSM-5 t y p e s ) r e s u l t s i n a d e c r e a s e i n t h e number o f a d s o r b e d C02 w h i l e few b u t s t r o n g e r b a s i c s i t e s a r e f o r m e d . The 4 0 k J m o l e - 1 f o r N a - z e o l i t e correspond t o C02 adsorbed on Na i o n s
(1,4,6).
The p r e s e n c e o f s t r o n g b a s i c s i t e s u p o n a c i d i f i c a t i o n i s r a t h e r
unexpected.
I t c o u l d be p o s s i b l e t h a t dehydration a f t e r a c i d i f i c a t i o n created
some o c c l u d e d sodium h y d r o x i d e c l u s t e r s f r o m r e m a i n i n g sodium i o n s as p r e v i o u s l y o b s e r v e d on Ca-Y and Mg-Y z e o l i t e s (16). Unless i t corresponds t o some alumina stemming f r o m small l a t t i c e d e a l u m i n a t i o n upon d e h y d r a t i o n a t 400°C. Such an o b s e r v a t i o n i s i n t e r e s t i n g s i n c e i t shows t h a t a c i d i f i c a t i o n may also create strong basic sites.
T h i s c o u l d a l s o e x p l a i n some c a t a l y t i c
p r o p e r t i e s which i n v o l v e t h e presence o f b o t h s i t e s f o r i n s t a n c e i n t h e formation
of
the
first
C-C
bond
in
the
conversion
of
methanol
into
hydrocarbon.Further work u s i n g more a c i d i c probe m o l e c u l e s as a c e t i c a c i d s h o u l d be done t o c o n f i r m such p r e l i m i n a r y r e s u l t s . The c a s e o f A1203 i s a l s o i n t e r e s t i n g s i n c e t h e presence o f b o t h a c i d i c ( r e l a t e d t o A1 i o n s ) and b a s i c ( r e l a t e d t o 02- i o n s ) s i t e s i s c l e a r l y seen i n f i g u r e s 1 and 5. T h i s i s c o n s i s t e n t w i t h e l e c t r o n d o n o r - a c c e p t o r observed 15 y e a r s ago
r o u g h l y t w i c e more s t r o n g a c i d s i t e s t h a n
presents
)(-A1203 ( f i g . 1 ) f o r a s i m i l a r s u r f a c e
area, which i s presumably due t o t h e f a c t t h a t ?-A1203 ions i.e.
properties
b y ESR t o c o - e x i s t on such s u p p o r t s (15) ;'-A1203
e x h i b i t s more s u r f a c e A1
more Lewis s i t e s . Note a l s o t h a t an o x i d e such as MgO
-
w e l l known t o
b e b a s i c - p r e s e n t s s t r o n g and numerous b a s i c s i t e s and no ( o r a t l e a s t v e r y
317 weak) a c i d i c s i t e s . The h i g h e r a c i d i t y observed f o r p r e c i p i t a t e d s i l i c a w i t h r e s p e c t t o fumed s i l i c a i s v e r y p r o b a b l y due t o t r a c e s o f a c i d i m p u r i t y a r i s i n g f r o m t h e p r e p a r a t i o n m e t h o d . Bi3FeMoOlp c a t a l y s t known t o b e an e x c e l l e n t c a t a l y s t f o r p a r t i a l o x i d a t i o n o r ammoxidation o f o l e f i n s p r e s e n t s o n l y v e r y weak and v e r y few a c i d s i t e s i f any. A t l a s t n o t e t h a t a c t i v a t i o n o f T i 0 2 e i t h e r anatase o r r u t i l e a t 400°C i n a i r c r e a t e s new a c i d i c s i t e s . T h i s i s i n aqreement w i t h p r e v i o u s w o r k b y P r i m e t e t a1 ( 1 7 ) which showed t h a t OH groups i n anatase and r u t i l e h a v e
a weak
acid strength(not
acidic
for
NH3,
acidic for
t r i m e t h y l m i n e ) . S t r o n g Lewis s i t e s a r e t h e n c r e a t e d b y t h e removal o f i s o l a t e d OH groups.
I n conclusion,
i t t u r n s o u t t h a t c a l o r i m e t r y t e c h n i q u e may f r u i t f u l l y be
used t o c h a r a c t e r i z e a c i d i t y and b a s i c i t y o f o x i d e and z e o l i t i c c a t a l y s t s . I t a l l o w s one t o g a i n i n f o r m a t i o n about t h e number, t h e s t r e n g t h and d i s t r i b u t i o n i n strengths o f acidic or basic sites.
The m e t h o d i s l i m i t e d s i n c e n o
i n f o r m a t i o n i s o b t a i n e d a b o u t t h e n a t u r e o f t h e s i t e s and t h e r e i s no d i r e c t r e l a t i o n s h i p b u t o n l y a r e a s o n a b l e p r o p o r t i o n a l i t y between t h e h e a t o f a d s o r p t i o n and t h e a c i d i c o r b a s i c s t r e n g t h o f t h e s i t e s . F u r t h e r e x p e r i m e n t a l work and
t h e o r e t i c a l a n a l y s i s o f a d s o r b a t e - a d s o r b e n t i n t e r a c t i o n s a r e needed
f o r a b e t t e r c h a r a c t e r i z a t i o n o f t h e a c i d i c and b a s i c p r o p e r t i e s o f c a t a l y s t s .
REFERENCES
1 R.M. B a r r e r and R.M. Gibbons Trans. Faraday SOC. 59 (1963) 2569-2582 ; 6 1 (1965) 948-961 2 T. Flasuda, H. Taniguchi, K. Tsutsumi and H. Takamashi, B u l l . Chem. SOC. Japan, 51 (1978) 633-634. 3 T. Masuda, H. Taniguchi, K.Tsutsumi and H. Takamashi, B u l l Chem. SOC. Japan , 51 (1978) 1968-1969. 4 P. Cartraud, Thermochim.Acta 16 (1367) 197-211. 5 K. Tsutsumi, H.Q. Koh, S. Hagiwara and H. Takahashi B u l l . Chem. SOC. Japan 48 (1975) 3576-3580. 6 P. Cartraud, B. Chauveau, M. Bernard and A. C o i n t o t J. Thermal A n a l y s i s 11 (1977) 51-60 P. C a r t r a u d , A. C o i n t o t and B. Chauveau, 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 Series, 40 (1977) 367-378. 7 S.S. Khovoshchev, V.E. Skazyvaev, S.P. Zhdanov and I.V.Karetina, I z v e s t i y a Akad. Nauk SSSR, Ser. K h i m i i 1 (1978) 23-28. 8 M.D. S e f c i k and H.K. Yuen Thermochim.Acta 26 (1978) 297-310. 9 A. Auroux, P. Wierzchowski and P.C. G r a v e l l e , Thermochim. A c t a 32 (1979) 165-170. 10 A. Auroux, P.C. G r a v e l l e , J.C. Vedrine and M. Rekas i n L.V. Rees (Ed), Proceed. Vth i n t e r n . Confer. on z e o l i t e s , N a p o l i , j u n e 1980, Heyden, London 1980 pp 433-439. 11 S.S. Khvoshchev, V.E. Skazyvaev and E.A. V a s i l j e v a i n L.V. Rees (Ed),Proceed Vth i n t e r n . Confer. on z e o l i t e s , N a p o l i , j u n e 1980, Heyden, London, 1980 pp 476-482. 12 J.C. VCdrine, A. Auroux and G. Coudurier, i n T.E. Whyte e t a l . ( E d . ) 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 b e t w e e n s t r u c t u r e and r e a c t i v i t y , ACS Symposium S e r i e s 248 (1984) 253-273.
.
318
13 D. Carson, M. F o r i s s i e r and J.C. V i d r i n e J. Chem. SOC., Faraday Trans. I, 80 (1984) 1017-1028. 14 A. Auroux, H.Dexpert, C. L e c l e r c q and J.C. V e d r i n e Appl. C a t a l . 6 (1983) 95-119. 15 C. Naccache, Y. K o d r a t o f f , R.C. P i n k and B. I m e l i k , J. Chim. Phys. 63 (1966) 341-344. Y. K o d r a t o f f , C. Naccache and B. I m e l i k , J. Chim. Phys. 65 (1968) 562-566. 16 A. Aboukai's, C. iilirodatos, J. Massardier, D. Barthomeuf and J.C. Vedrine J. Phys. Chem.,81 (1977) 397. 17 M. P r i m e t , P. P i c h a t and M.V. M a t h i e u J. Phys. Chem. 75 (1971) 1221.
319
B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0
1985 Elsevier Science Puhlishers B.V.,Amsterdam -Printed in The Netherlands
DETERMINATION DE L'ACIDITE DE CATALYSEURS SOLIDES EN MILIEU AQUEUX A L'AIDE
D ' U N MARQUEUR CINETIQUE
R.DURAND,P.GENESTE,C.MOREAU
e t S.MSEDD1
L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimique A p p l i q u e e s
C.N.R.S.,UA
418, E c o l e N a t i o n a l e S u p e r i e u r e de Chimie de M o n t p e l l i e r ,
8 Rue E c o l e Normale
-
34075 M o n t p e l l i e r Cedex,France.
ABSTRACT The h y d r o l y s i s o f 2-phenyl 1,3-dioxolane ( I ) , 2-phenyl 1 , 3 - o x a t h i o l a n e (11) and benzaldehyde d i m e t h y l a c e t a l (111) was i n v e s t i g a t e d i n aqueous s o l u t i o n o v e r v a r i o u s s o l i d a c i d c a t a l y s t s such as z e o l i t e s o r c l a y s . L i n e a r p l o t s o f t h e observed r a t e c o n s t a n t s a g a i n s t t h e amount o f c a t a l y s t a r e o b t a i n e d . The comp a r i s o n w i t h t h e h y d r o l y s i s o f t h e same compounds under homogeneors c o n d i t i o n s , where t h e r e a c t i o n r a t e s a r e known t o obey t h e e q u a t i o n kob =kH[H ] o r kob = kNh depending on t h e s t r e n g t h o f t h e c a t a l y s t , enables t o a e t e r m i n e t h e F o l l o w i n g Hammett a c i d i t y c o n s t a n t s Ho ( f o r 10 grams o f c a t a l y s t p e r l i t r e o f water): 3.6 f o r m o r d e n i t e H-MI (Si/A1=5), 3.5 f o r m o r d e n i t e H-M (Si/A1=6.9), 2.4 f o r K-10 m o n t m o r i l l o n i t e , 2.0 f o r dealuminated m o r d e n i t e 2H-M2 (Si/A1=11 . I ) and 0.5 f o r HC1 (0.274 M). INTRODUCTION Les p r o p r i e t e s a c i d o - b a s i q u e s de nombreux c a t a l y s e u r s s o l i d e s , en p a r t i c u l i e r l e s a r g i l e s e t l e s z e o l i t h e s , s o n t en g e n e r a l a t t r i b u e e s
a
l e u r a c i d i t 6 de
surface, que c e l l e - c i s o i t de B r a n s t e d ou de Lewis. Ceci a c o n d u i t de nombreux auteurs
a
r e c h e r c h e r des moyens de d e t e r m i n a t i o n de l ' a c i d i t e , s o i t p a r des
methodes physiques,
I R (ref.l),
s o i t p a r des methodes a n a l y t i q u e s , t e l l e s que
c e l l e s des i n d i c a t e u r s c o l o r e s ( r e f . 2 e t 3) ; c e t t e d e r n i B e ne permet t o u t e f o i s d ' a c c e d e r q u ' a une f o u r c h e t t e d ' a c i d i t e p o u r un c a t a l y s e u r donne e t p a r consequent
a
une f a i b l e p r e c i s i o n de l a mesure de l a c o n s t a n t e Ho de Hammett.
De p l u s , t r e s peu de t r a v a u x o n t e t e e f f e c t u e s en m i l i e u aqueux 00 l ' a c i d i t e e s t e s s e n t i e l l e r n e n t de B r b n s t e d ( r e f . 2 ) .
I 1 nous a donc paru i n t e r e s s a n t de
developper une rnethode s i m p l e p e r m e t t a n t d ' a c c e d e r m i l i e u aqueux avec une p r e c i s i o n s u p e r i e u r e
a
a
l ' a c i d i t e des s o l i d e s en
c e l l e donnee en p a r t i c u l i e r p a r
l e s indicateurs colores. La rnethode que nous avons essaye de r n e t t r e en oeuvre repose s u r l ' e t u d e de r e a c t i o n s connues p o u r e t r e sournises
a
une c a t a l y s e a c i d e s p e c i f i q u e ou gene-
r a l e en s o l u t i o n ( h y d r o l y s e d ' a c e t a l s , d ' e s t e r s . . .) p e r m e t t a n t a i n s i d ' e t a b l i r des c o r r e l a t i o n s e n t r e l a r e a c t i v i t e d ' u n cornpos6 e t l ' a c i d i t e du m i l i e u . C e t t e
320 mesure d ' a c i d i t e p e u t theoriquement c o n d u i r e 5 d e t e r m i n e r l e c a r a c t e r e a c i d e de n ' i m p o r t e quel s y s t h e c a t a l y t i q u e , donc en p a r t i c u l i e r d ' u n systeme h e t e rogene. Pour e t a b l i r ces c o r r e l a t i o n s e n t r e l a r e a c t i v i t e d ' u n compose e t l ' a c i d i t e du m i l i e u nous avons c h o i s i d ' e t u d i e r l a r e a c t i o n d ' h y d r o l y s e du p h e n y l - 2
( I ) , du p h e n y l - 2 oxathiolanne-1,3
dioxolanne-1,3
(11) e t de 1 ' a c e t a l d i m e t h y l e
du benzaldehyde (111) :
II
I
*
Les r a i s o n s de ce c h o i x s o n t l e s s u i v a n t e s : 1) l ' h y d r o l y s e des a c e t a l s 1,II e t I11 e s t soumise 5 une c a t a l y s e a c i d e specifique, c'est-5-dire,
que l a v i t e s s e d ' h y d r o l y s e e s t p r o p o r t i o n n e l l e 5 l a
c o n c e n t r a t i o n en p r o t o n s en m i l i e u a c i d e d i l u e , kobs=kHIHj, ou a l a f o n c t i o n d ' a c i d i t e de Hammett ho en m i l i e u a c i d e f o r t , kobs=kHho. 2) l a r e a c t i v i t e de ces composes permet de c o u v r i r un domaine d ' a c i d i t e c o m p a t i b l e avec ce que l ' o n c o n n a i t s u r l ' a c i d i t e des c a t a l y s e u r s s o l i d e s .
PARTIE EXPERIMENTALE
-
synthese des p r o d u i t s : l e s a c e t a l s 1 , I I e t I11 o n t
ete
synthetises selon
des methodes d e c r i t e s dans l a l i t t e r a t u r e ( r e f . 4 e t 5 ) .
-
c a t a l y s e u r s : l e s c a t a l y s e u r s u t i l i s d s s o n t s o i t commerciaux,
Chemie), M o r d e n i t e H-M1,
M o r d e n i t e H-M2, Zeolon 100-H, Si/A1=6,9
(Norton) s o i t p r e p a r e s p a r des methodes
d e c r i t e s dans l a l i t t e r a t u r e , H-M2 desaluminee, Si/A1=11,1
-
K 10 (Siid-
A l i t e 180, Si/A1=5 ( S o c i e t e Chimique Grande P a r o i s s e ) , (ref.6).
c i n e t i q u e s : l e s reactions d'hydrolyse sont s u i v i e s par spectrophotometrie
U.V.
( G i l f o r d 250 equip6 d ' u n compartiment t h e r m o s t a t e e t d ' u n passeur automa-
t i q u e de cuves) en s u i v a n t l ' a u g m e n t a t i o n de d e n s i t e o p t i q u e due A l ' a p p a r i t i o n du b e n z a l d e h y l e forme l o r s de l ' h y d r o l y s e 5 280nm. Les c o n d i t i o n s o p e r a t o i r e s en homogene s o n t l e s s u i v a n t e s : l e s cuves en q u a r t z c o n t i e n n e n t 2 m l de s o l u t i o n d ' a c i d e c h l o r h y d r i q u e de c o n c e n t r a t i o n connue, on a j o u t e 20 ~1 d'une s o l u t i o n 0,7 l o - % d ' a c e t a l dans du dioxane. Apr@s a g i t a t i o n ,
la
densite
optique
de l a s o l u t i o n
est enregistree
en
f o n c t i o n du temps. Les c o n d i t i o n s operatoires en phase h@tiSrog@nesont l e s suivantes : un vase t h e r mostate, h o r s de 1 ' e n c e i n t e du spectrophotometre, c o n t i e n t 40 m l d'eau d i s t i l l e e e t une q u a n t i t e connue de c a t a l y s e u r s o l i d e en suspension. C e t t e suspension e s t
x ) v o i r p. 324.
321 a g i t e e magnetiquement e t 10 ~1 d ' a c e t a l p u r s o n t a j o u t e s . Un s y s t b e d ' a n a l y s e
a
en c o n t i n u permet,
l ' a i d e d ' u n e pompe p e r i s t a l t i q u e , de f i l t r e r l e melange
r e a c t i o n n e l , d ' a n a l y s e r l e f i l t r a t dans une cuve
UV a c i r c u l a t i o n de 100 p1 e t
de r e c y c l e r l e f i l t r a t . La d e n s i t e o p t i q u e e s t e n r e g i s t r e e en f o n c t i o n du temps. Dans l e s deux cas, l e s c o n s t a n t e s de v i t e s s e s o n t obtenues en l i s s a n t l ' e x p o n e n t i e l l e correspondant
D=D,
+
(D,-Do)e-kt
a
l a d i s p a r i t i o n pseudo-monomoleculaire de l ' a c e t a l
pa.r l a methode des moindres c a r r e s .
RESULTATS Dans un p r e m i e r temps, nous avons e t u d i e l ' h y d r o l y s e des a c e t a l s 1 , I I e t I11 p o u r d i f f e r e n t e s c o n c e n t r a t i o n s en a c i d e c h l o r h y d r i q u e . En p o r t a n t l e s l o g a r i t h mes des c o n s t a n t e s de v i t e s s e observ6es kobs (exprimees en s - I ) en f o n c t i o n de l a f o n c t i o n d ' a c i d i t e de Hammett Ho ( v o i r F i g . l ) ,
nous obtenons des r e l a t i o n s
l i n e a i r e s avec une p e n t e v o i s i n e de 1 c a r a c t e r i s t i q u e d ' u n e c a t a l y s e a c i d e spec i f i q u e ( r e f . 7). Ceci c o n s t i t u e n o t r e s y s t h e d ' e t a l o n n a g e pour d e t e r m i n e r 1 ' 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 .
I
I
I
I
I
0
1
2
3
4
F i g . 1 Diagramme r e p r e s e n t a n t l a v a r i a t i o n de l o g k o s ( s - 1
,8
,no
en f o n c t i o n de Ho
observee dans 1 ' h y d r o l y s e du p h e n y l - 2 d i o x o l a n n e - I ( O ) , du p h e n y l - 2 oxat h i o l a n n e - I ,3 ( A ) e t du d i m e t h y l a c e t a 1 du benzaldehyde ( m ) dans 1 ' a c i d e c h l orhydrique.
322
Dans un deuxihe temps, l'acide chlorhydrique est remplace par des catalyseurs solides et nous avons mesure les vitesses d'hydrolyse du phenyl-2 dioxolanne-l,3 pour des quantites connues en chaque catalyseur. On constate (voir Fig.2) que la vitesse d'hydrolyse varie lineairement avec la quantite de catalyseurs. Nous avons egalement montre que la vitesse d'hydrolyse etait independante de la vitesse d'agitation du melange reactionnel (de 100 2 1100 tours par minute).
--
/ 0
02
I
-
. I
2
I
I
I
0,s
-
I
I
a
I
, f
masse d e 'catalyseur
(9.1 Fig.2 Diagramme representant la constante de vitesse d'hydrolyse du phenyl-2 dioxolanne-1,3 en fonction de la masse de catalyseur solide dans 40 ml d'eau, mordenite H-M1, Si/A1=5 (x), mordenite H-M Si/A1=6,9 ( o ) , H-montmorillonite ?' K-10 ( m ) et mordenite H-M2 desaluminee, Si/Al=ll,l (4).
323
Les constantes de v i t e s s e du second ordre rapportees dans 1 1 d'eau sont rassemblees dans l e Tableau 1 .
a
l g de c a t a l y s e u r
Tableau 1 . Constantes de v i t e s s e d u second ordre (1 .g-'.s-') du phenyl-2 dioxolanne-l,3 pour d i f f e r e n t s c a t a l y s e u r s Ca talyseur
k2 1.g
-1
.s
d a n s 1 'hydrolyse
-1 H,a
~~
HC1 H-montmorillonite K-10 mordenite H-M, (Si/A1=5) mordenite H-M2 (Si/A1=6,9) mordenite H-M2 (Si/A1=11 , I )
5,12 lo-' 7,OO I O - ~ 5,56 I O - ~ 5,90 I O - ~ 1,98 I O - ~
0,5 234 3,6 325 2,o
a Fonction d ' a c i d i t e de Hammett Ho calculee pour log de catalyseur par l i t r e d'eau A l a l e c t u r e de ce tableau on constate qu'en milieu aqueux l e c a t a l y s e u r l e plus e f f i c a c e r e s t e 1 ' a c i d e chlorhydrique. Parmi l e s catalyseurs sol ides, pour des q u a n t i t e s identiques de catalyseur, l a mordenite ayant l e rapport Si/A1=11,1 e s t environ 35 f o i s plus a c t i v e que l e s mordenites ayant des rapports Si/Al de 5 e t 6,9. Cette observation va dans l e m6me sens que c e l l e f a i t e par Namba et a l . dans l'hydrolyse de ? ' a c e t a t e d ' e t h y l e c a t a l y s e e , e n t r e a u t r e s , par des morden i t e s de rapport Si/A1 variable ; l a surface de l a zeolithe devenant plus hydrophobe a une meilleure a f f i n i t e pour l e compose organique (ref.2). Une a u t r e observation i n t e r e s s a n t e qui r e s s o r t de ce tableau concerne l ' a r g i l e K-10 qui possede une a c i d i t e qui s e rapproche assez de c e l l e de l a morden i t e desaluminee H-M2 ( S i / A l = l l , l ) en milieu aqueux ; ceci ouvre des p o s s i b i l i t e s i n t e r e s s a n t e s quant au choix du c a t a l y s e u r & u t i l i s e r .
CONCLUSIONS e t PERSPECTIVES En conclusion, l e s r e s u l t a t s reportes dans ce t r a v a i l permettent de mieux p r e c i s e r l ' a c i d i t e de catalyseurs s o l i d e s lorsque ceux-ci sont u t i l i s e s en sol u t i o n . Ces r e s u l t a t s pourraient peut-0tre egalement permettre d'acceder & une determination plus precise du caractere hydrophobe de c e r t a i n s catalyseurs. Par a i l l e u r s , une question qui se pose toujours e s t de savoir s i l a reaction s ' e f f e c t u e a l ' i n t e r i e u r , a l ' e n t r e e ou & l ' e x t e r i e u r de l a zeolithe. L ' u t i l i s a t i o n d ' u n second marqueur cinetique t e l que 1 'acbtal dimethyle du benzaldehyde (111) conduit, comparativement au phenyl-2 dioxolanne-1,3 ( I ) , a u n rapport de r e a c t i v i t e d i f f e r e n t selon que l ' o n e s t en milieu homogene ( I I I / I = l O ) ou en milieu heterogene ( 1 1 1 / 1 = 3 & 5 ) . La difference e s t c e r t e s minime mais n ' e s t , pas denuee de t o u t fondement s i l ' o n considere l e s conformations s t a b l e s de ces molecules qui peuvent e n t r a i n e r des modifications sur l a t a i l l e c r i t i q u e de ces
324
composes.
Nos travaux se poursuivent actuellement dans le sens d'une extension des mesures d'acidite i d'autres catalyseurs heterogenes en milieux aqueux et non aqueux et egalement dans le sens d'une meilleure connaissance des relations pouvant exister entre la structure d'un catalyseur heteroghe et la taille critique des molecules. REFERENCES 1 A.K.Ghosh et G.Curthoys,J.Catal. ,82 (1983) 469, et references citees. 2 S.Namba,N.Hosonuma et T.Yashima,J.Catal. (1981) 16. 3 H.A.Benesi,J.Amer.Chem.Soc. ,78 (1956) 5490. 4 T.H.Fife et L.K.Jao,J.Amer.Chem.Soc. ,91 (1969) 421 7. 5 E.C.Taylor et C.S.Chiang,Synthesis, (1977) 467. 6 F.Fajula,R.Ibarra,F.Figueras et C.Gueguen,J.Catal., (1984) sous presse. 7 F.A.Long et M.A.Pau1 ,Chem.Rev., 57 (1957) 935.
,z
*
au p o i n t concernant l'hydrolyse des acetals Chem. Rev., 74 (1974) 581.
mise
:
E.H.Cordes et H.G.Bul1,
325
B. Imelik et ol. (Editors), Catalysis b y Acids and Bases o 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
DEGRADATION MECHANISM OF 3-METHYL-PENTANE ON A SUPPORTED S U P E R A C I O CATALYST
S T U D I E D BY THE I 3 C ISOTOPIC TRACER TECHNIQUE. F . LE NORMAND and F . FAJULA* L a b o r a t o i r e de C a t a l y s e e t Chimie des Surfaces, U . A . 423 du CNRS, U n i v e r s i t e L o u i s Pasteur, I n s t i t u t Le Bel, 4, r u e B l a i s e Pascal, 67000 S t r a s b o u r g (France)
ABSTRACT The d e g r a d a t i o n mechanism o f branched hexanes,i.s.the production o f paraffins o f l o w e r and h i g h e r m o l e c u l a r weight, has been determined on t h e s u p e r a c i d i c cat a l y s t antimony p e n t a f l u o r i d e i n s e r t e d i n t h e g r a p h i t e . The mechanism c o r r e s ponds t o a p o l y m e r i z a t i o n - d e p o l y m e r i z a t i o n process, i n s t e a d o f a d i r e c t p r o t o l y s i s . I t c o n s i s t s i n ( i ) an i n i t i a t i o n by a d i r e c t 6 - s c i s s i o n o f t h e m e t h y l i s o b u t y l i o n i n t o an i s o p r o p y l i o n and a propene molecule, t h i s l a t t e r i n i t i a t i n g ( i i ) t h e p r o p a g a t i o n o f a c h a i n mechanism i n c l u d i n g a l k y l a t i o n s t e p s t o f o r m adsorbed o l i g o m e r s , rearrangements and 6 - s c i s s i o n s o f t h i s o l i g o m e r , and u l t i m a t e l y intermolecular hydride t r a n s f e r . INTRODUCTION I s o m e r i z a t i o n o f p a r a f f i n s i s one o f t h e most i n t e r e s t i n g r e a c t i o n t h a t ac i d c a t a l y s i s p r o v i d e s . Systems such as z e o l i t e s , s i l ica-alumina,
Lewis a c i d s
i n c o m b i n a t i o n w i t h B r d n s t e d a c i d s , w a t e r o r s a l t s have p r o v e d t o be i n i t i a l l y v e r y e f f e c t i v e f o r such a r e a c t i o n ( r e f . 1 , 2 , 3 ) . However t h e y a l l s u f f e r f r o m t h e appearance o f s i d e p r o d u c t s , namely p a r a f f i n s and o l e f i n s o f h i g h e r and l o wer m o l e c u l a r w e i g h t ( r e f . 4 , 5, 6, 7, 8 ) . I n t h i s paper these p r o d u c t s w i l l be named d e g r a d a t i o n p r o d u c t s . Two mechanisms can be proposed t o account f o r t h e p r o d u c t i o n o f such s i d e p r o d u c t s . A c l a s s i c a l one, p u t f o r w a r d as e a r l g a s 1960, develops t h r o u g h carbonium i o n s i n t e r m e d i a t e s ( r e f . 6 ) .
I t supposes t h e presence o f o l e f i n s , even i n
t r a c e amounts, w h i c h r e a c t w i t h c a t i o n s , s u c c e s s i v e l y t o f o r m a complex o l i g o mer, rearrangement, c r a c k i n g by 6 - s c i s s i o n and u l t i m a t e l y i n t e r m o l e c u l a r h y d r i de t r a n s f e r g i v i n g t h e more thermodynamically f a v o u r e d p a r a f f i n s , m a i n l y i s o b u t a n e and isopentane, b u t a1 so branched hexanes and heptanes. A second mechanism has been more r e c e n t l y d i s c o v e r e d b y Olah e t a1 ( r e f . 9 ) ,
e s p e c i a l l y on s u p e r a c i -
media and p o s t u l a t e s a p e n t a - c o o r d i n a t e d carbonium i o n t r a n s i t i o n s t a t e ( r e f . 1 0 ) . ~~
~
*Present address: L a b o r a t o i r e de Chimie Organique Physique Appliquge, E.N.S.C.M. 8, r u e de 1 ' E c o l e Normale, 34075 M o n t p e l l i e r Cedex, France.
326
The main p r o d u c t s a r e t h e n o b t a i n e d i n a d i r e c t s t e p , g i v i n g m a i n l y hydrogen, methane and ethane. The aims o f t h i s work were t o i n v e s t i g a t e t h e c o n t r i b u t i o n o f each mechanism and, i n t h e case o f t h e carbenium i o n mechanism, t o e l u c i d a t e t h e o r i g i n o f o l e -
'v
f i n s . We choose as a s t a r t i n g p a r a f f i n a d i l a b e l l e d hexane, t h e [ l , 5 methylpentane (
-
I3C2]
3
) and as c a t a l y s t t h e antimony p e n t a f l u o r i d e i n s e r t e d
6
i n t h e g r a p h i t e . I n a s e r i e s o f p r e c e d i n g papers ( r e f . 1 1 , 12, 1 3 ) , we showed t h a t such a c a t a l y s t had two d i s t i n c t regimes a c c o r d i n g t o t h e r e a c t i o n temperature : A t -3O"C,
o n l y i s o m e r i z a t i o n o f branched hexanes o c c u r s v i a i n t r a m o l e c u l a r
alkyl shifts. A t +20°C,
d e g r a d a t i o n o c c u r s w i t h i s o m e r i z a t i o n , v e r y r a p i d l y and t o a l a r -
ge e x t e n t . Between t h e s e two regimes, i t becomes t h e r e f o r e p o s s i b l e t o s t u d y t h e degrad a t i o n process a t v e r y l o w c o n v e r s i o n l e v e l s . EXPERIMENTAL Catalyst The i n s e r t i o n o f antimony p e n t a f l u o r i d e was achieved f o l l o w i n g a method desc r i b e d by M e l i n and H e r o l d ( r e f . 1 4 ) . G r a p h i t e was c o n t a c t e d d u r i n g t h r e e days a t 70°C w i t h excess o f antimony p e n t a f l u o r i d e vapor i n a vacuum s e a l e d t u b e . M i c r o a n a l y s i s showed t h a t t h e p r o d u c t corresponded t o a f i r s t s t a g e i n s e r t i o n o f molar formula C
6,5
Sb F5. D u r i n g t h e s t o r a g e and t h e m a n i p u l a t i o n , c o n t a c t
o f t h e c a t a l y s t w i t h m o i s t u r e has been avoided. C a t a l y t i c r u n s and gas chromatographic a n a l y s i s The r e a c t i o n were c a r r i e d o u t i n an a l l g l a s s , grease f r e e , f l o w system, a t atmospheric hydrogen p r e s s u r e w h i c h has been p r e v i o u s l y d e s c r i b e d ( r e f . 1 5 ) . Unl a b e l l e d 3-methylpentane,
F l u k a p u r i s s grade, was used w i t h o u t f u r t h e r p u r i f i -
cation. Samples c o l l e c t e d d i r e c t l y f r o m t h e f l o w l i n e were a n a l y z e d on a GLC apparat u s w i t h a f l a m e i o n i z a t i o n d e t e c t o r equipped w i t h a DC 200 on a f i r e b r i c k column w h i c h a l l o w e d s e p a r a t i o n o f most hydrocarbons i n t h e range C2 t o C8. A few s e p a r a t e analyses were performed on a Porapack column t o check t h e absence o f methane and h i g h
olefins.
L a b e l l e d compound and mass s p e c t r o m e t r i c a n a l y s i s The G r i g n a r d r e a c t i o n o f e t h y l a c e t a t e w i t h e t h y l [ 2 - 13C ] o d i d e was used 13 t o p r e p a r e [ 1,5- C2 ] 3-methyl-3 p e n t a n o l . The p r e p a r a t i o n of [1-5] I 3 C 2 d i l a be1 1ed 3-methyl pentane had been p r e v i o u s l y d e s c r i b e d ( r e f . 1 6 ) The r e s u l t i n g
-
327 p r o d u c t , as analysed by mass spectrometry,
had t h e f o l l o w i n g c o m p o s i t i o n : 78%
o f d i l a b e l l e d i n p o s i t i o n s 1 and 5, 21% o f m o n o l a b e l l e d i n p o s i t i o n 1 and 1%of u n l abel l e d 3-methyl pentane. Mass s p e c t r a were r e c o r d e d on a V a r i a n Mat CH7 apparatus u s i n g 70 eV e l e c t r o n s t o i o n i z e t h e m o l e c u l e s . Mass s p e c t r a o f propane, isobutane, i s o p e n t a n e and 3-methylpentane were o b t a i n e d by u s i n g t h e d i r e c t i n l e t system and a h i g h e r r e s o l u t i o n ( c a 2000) t o r e s o l v e t h e m u l t i p l e t s a t masses m/e = 44. The m i x t u r e o f hexanes i n c l u d i n g 3-methylpentane were analysed by u s i n g a mass spectromet e r - g a s chromatograph c o u p l i n g d e v i c e . The procedure and e x p e r i m e n t a l c o n d i t i o n s o f t h e a n a l y s i s have been d e s c r i b e d elsewhere ( r e f . 1 5 ) . The r e c o r d e d spect r a , c o r r e c t e d f r o m background i f necessary, f o r n a t u r a l l y o c c u r i n g i s o t o p e s u s i n g Beynon's tab1 es ( r e f . 17) and f o r carbon-hydrogen f r a g m e n t a t i o n , i s compar e d w i t h a c a l c u l a t e d spectrum i s s u e d f r o m c o n s i d e r a t i o n s on t h e r e a c t i o n p a t h way. The assumption t h a t no 1 3 C p r i m a r y i s o t o p e e f f e c t may a f f e c t t h e fragment a t i o n p a t t e r n s has been checked f o r a l a r g e p a r t i n t h e case o f f r a g m e n t a t i o n s o f hexanes and pentanes. I n a t y p i c a l a n a l y s i s , we determined f i r s t t h e 1 3 C i s o t o p i c c o n c e n t r a t i o n x p e r carbon atom. F o r i n s t a n c e , f o r a p a r e n t i o n [Cn H2n t2]
!
i=n
c i [ 13ci 12cn-i H ~ ~ + ~ I P i=o where i i s t h e number o f 1 3 C atoms i n t h e p a r e n t i o n . Then we compared t h e r e x
=-
corded and t h e c a l c u l a t e d d i s t r i b u t i o n s f r o m t h e mean square d e v i a t i o n A
A=L
i=n i=o
[(13'i
'"n-i
t
H2n+2). obs
-
(
13
'i
12 'n-i
H2n+2).+ c a l c
l2
RESULTS Experiment w i t h u n l a b e l l e d 3-methylpentane We performed an e x p e r i m e n t w i t h u n l a b e l l e d 3-methylpentane ( p u l s e o f ca 2 0 ~ 1 ) a t 0°C.
I n F i g u r e 1, we r e p o r t t h e v a r i a t i o n w i t h t i m e on stream o f t h e c o n v e r s i o n and o f t h e p r o d u c t d i s t r i b u t i o n f o r i s o m e r i z a t i o n p r o d u c t s o n l y . The i s o m e r i z a t i o n c o n v e r s i o n was always v e r y h i g h (2 60%) and went t h r o u g h a maximum. The e q u i l i b r a t i o n o f methylpentanes was reached immediately, whereas t h e 2,3-dimethy1 butane i n c r e a s e d s l i g h t l y and c o n t i n u o u s l y t o r e a c h i t s thermodynamic e q u i l i b r i u m ( r e f . 1 8 ) a t t h e end o f t h e p u l s e . N-hexane and neohexane r e p r e s e n t e d always l e s s t h a n 2% o f t h e i s o m e r i z a t i o n p r o d u c t s and no o l e f i n s were desorbed. I n F i g u r e 2, we r e p o r t t h e same v a r i a t i o n s f o r m o l a r c o n v e r s i o n and d i s t r i b u t i o n f o r d e g r a d a t i o n p r o d u c t s . The c o n v e r s i o n i n d e g r a d a t i o n p r o d u c t s went t h r o u g h an a c c e n t u a t e d minimum near t h e m i d d l e o f t h e p u l s e , and was always l e s s than 1 5 % . Dramatic changes o c c u r r e d i n t h e d e g r a d a t i o n p r o d u c t d i s t r i b u t i o n .
328
o co nversion
Fig 1 :Conversion and product distribution in the isomeri.zation of 3-methylpentane
o conversion
p1
Time
on stream(mn)
Fir 2 :Molar conversion and product distribution in the decradation of 3-methyl-pentane.
329
Propane was t h e o n l y i n i t i a l p r o d u c t . However i t disappeared r a p i d l y and i s o b u tane, i s o p e n t a n e and branched heptanes, a l l p r o d u c t s w i t h a t l e a s t one t e r t i a r y carbon atom, were formed. We checked t h a t methane, n o t c o n s i d e r e d here, was
o b t a i n e d always i n n e g l i g e a b l e amounts compared t o propane. L a b e l l e d experiment We r e p e a t e d t h e e x p e r i m e n t w i t h a f r e s h c a t a l y s t i n t h e same c o n d i t i o n s as above f o r t h e [1,5
-
I3C2]
3-methylpentane. Three f r a c t i o n s , n o t e d PI,
P2, P 3
and m a t e r i a l i z e d i n F i g u r e s 1 and 2, were c o l l e c t e d . 3-methyl pentane I n Table 1, we g i v e t h e mass s p e c t r o m e t r i c
d i s t r i b u t i o n f o r the parent ion
and f o r t h e demethylated fragment o f 3-methylpentane. TABLE 1 I s o t o p i c d i s t r i b u t i o n o f 3-methyl-pentane samples Ions
13
Sample
‘i Startinga
Parent
pla
p2a
Scrambled P36 Distribution
C2
77,6
77,4
77,2
77,8
77,6
c1
20,9
20,5
21,l
21,o
20,9
13c0
1,5 29,4
2,O 29,3
1,6 29,3
1,2 29,4
1,5 29,4
66,7
39,9
44,l
48,4
51,3
30,3
53,3
50,O
46,7
43,7
3,o 32,8
6,8 26,6
5,9 27,7
4,8 28,7
5,o 29,3
13
X
13 C2 13 Demethyl a t e d C1 13
co
X
a A n a l y s i s by d i r e c t i n l e t d e v i c e . b A n a l y s i s by mass spectrometer-gas chromatograph coup1 i n g d e v i c e . ‘See
text.
The d i s t r i b u t i o n o f t h e p a r e n t i o n was u n a f f e c t e d (no n o t i c e a b l e h e a v i e r i o n s
( 13C3 and 13C4) and same e n r i c h m e n t ) , b u t t h e demethylated fragment was q u i t e d i f f e r e n t . A comparison o f t h e observed d i s t r i b u t i o n w i t h a c a l c u l a t e d one assuming a complete s c r a m b l i n g o f t h e l a b e l s w h i c h i n v o l v e t h e s i x carbon p o s i t i o n s , i n t e r n a l and e x t e r n a l , i n t h e hexane m o l e c u l e shows t h a t t h e observed d i s t r i b u t i o n s t e n d from P1 t o P3 t o t h e s t a t i s t i c a l one. The same t r e n d s were observed f o r t h e i s o t o p i c d i s t r i b u t i o n s o f 2-methylpentane and 2,3-dimethylbutane. From these date, t h r e e main c o n c l u s i o n s can be drawn :
330 F i r s t l y a l l t h e rearrangements,i.e. nes and 2 , 3 - d i m e t h y l b u t a n e ,
t h e i s o m e r i z a t i o n process o f methylpenta-
a r e intramolecular, whatever t h e nature o f these
rearrangements. Secondly, these i n t r a m o l e c u l a r pathways a r e s u f f i c i e n t l y f a s t t o e q u i l i b r a t e t h e t h r e e b r a n c h e d hexane i s o m e r s . L a s t l y , w i t h i n a g i v e n b r a n c h e d hexane t h e s i x c a r b o n p o s i t i o n a r e n e a r l y equilibrated. Propane Among t h e d e g r a d a t i o n p r o d u c t s we examined f i r s t t h e propane, w h i c h was i n i t i a l l y t h e m a i n one. I n T a b l e 2, we r e p o r t t h e d i s t r i b u t i o n o f t h e p a r e n t i o n f o r samples PI,
P2 and P3.
TABLE 2 I s o t o p i c d i s t r i b u t i o n o f p r o p a n e samples Sample
‘i
13c3 13 c2 13c1 13 X
Distribution
B
A
P1
P2
p3
14,4
14,O
14,9
15,6
1.2 17,5
56,6
52,7
50,8
57,O
49,5
29,O
33,3
34,3
27,4
31,8
28,5
26,9
26,9
29,4
29,4
A/A
5
57
88
A/B
69
27
16
These d i s t r i b u t i o n a r e compared w i t h t w o c a l c u l a t e d d i s t r i b u t i o n , namely : D i s t r i b u t i o n A : propane r e s u l t s f r o m t h e B - s c i s s i o n o f t h e secondary methyl i s o b u t y l c a r b e n i u m i o n a c c o r d i n g t o Scheme 1
L
----- +
- w e + -
Scheme 1
I t has been shown i n e f f e c t t h a t p r o p a n e i s a l s o t h e p r i m a r y p r o d u c t i n t h e d e
g r a d a t i o n o f 2 - m e t h y l p e n t a n e ( r e f . 1 2 , 2 1 ) . F o r t h i s c a l c u l a t i o n , we assume a complete p o s i t i o n n a l scrambling o f t h e l a b e l s i n t h e 2-methylpentane. D i s t r i b u t i o n B : p r o p a n e r e s u l t s f r o m t h e 6 - s c i s s i o n o f a complex o l i g o m e r . F o r c a l c u l a t i o n b a s i s , we chose a C12
p o l y m e r f o r m e d b y d i r e c t c o m b i n a t i o n o f two
hexane m o l e c u l e s ( o r hexane p l u s two p r o p a n e m o l e c u l e s ) w i t h a c o m p l e t e scramb l i n g o f t h e l a b e l s . The c o m p l e t e s c r a m b l i n g o f p o s i t i o n and number o f a C8 e n t i t y r e q u i r e s o n l y a l i m i t e d number o f s t e p s ( r e f . 1 9 ) .
331 From Table 2 i t i s c l e a r t h a t t h e d i s t r i b u t i o n o f t h e propane produced i n i t i a l l y ( c o l l e c t e d i n sample P1) agrees f a i r l y w e l l w i t h t h e h y p o t h e s i s A o f a d i r e c t 6 - s c i s s i o n o f a m e t h y l - i s o b u t y l i o n . As t h e r e a c t i o n proceeds t h i s agreement i s no l o n g e r observed, and on t h e c o n t r a r y we g e t a b e t t e r f i t w i t h hypothesis
B. A c t u a l l y , a t t h e end o f t h e r u n most o f t h e propane i s o b t a i n e d by a
p o l y m e r i z a t i o n mechanism. Lower v a l u e s o f x compared t o t h e hexanes i n d i c a t e however some d e v i a t i o n f r o m t h e s t a t i s t i c a l s c r a m b l i n g . i s o b u t a n e an i s o p e n t a n e I s o b u t a n e and i s o p e n t a n e can r e s u l t from t h e d e s o r p t i o n t h r o u g h h y d r i d e t r a n s f e r o f t - b u t y l and t - a m y l
c a t i o n s produced a c c o r d i n g t o t h e mechanism B
d e s c r i b e d above. They may a l s o be t h e r e s u l t o f d i r e c t carbon-carbon bond r u p t u r e s i n t h e branched hexanes. T h i s mechanism i s b a s i c a l l y t h e p r o t o l y s i s r e a c t i o n observed b y Olah e t a1 ( r e f . 9 ) i n s u p e r a c i d medium and proposed by Haag e t a1 i n t h e c r a c k i n g o f hexanes on a ZSM5 c a t a l y s t ( r e f . 2 0 ) . A c c o r d i n g l y two d i s t r i b u t i o n s have been c o n s i d e r e d t o account f o r t h e observed i s o b u t a n e and i s o p e n t a n e : t h e d i s t r i b u t i o n c o r r e s p o n d i n g t o a s t a t i s t i c a l r e d i s t r i b u t i o n f o l l o w i n g mechanism B and t h e d i s t r i b u t i o n C c o r r e s p o n d i n g t o p r o t o l y s i s r e a c t i o n s (mechanism C), i e
Scheme
The r e s u l t s o f such a t r e a t m e n t a r e summarized i n Table 3 . TABLE 3 Mechanism o f f o r m a t i o n o f i s o b u t a n e and i s o p e n t a n e D i s t r i b u t i o n (%) Hydrocarbons
A
L
Sample
p1+p2 p3
p1+p2 p3
a: see t e x t
Ba
Ca
100
-
100 80
20
90
10
2
332
They show t h a t i s o b u t a n e i s formed e s s e n t i a l l y by a depolymerization-polymeriz a t i o n process ( t y p e B mechanism). F o r i s o p e n t a n e a small b u t d e f i n i t e
contri-
b u t i o n o f t h e p r o t o l y s i s r e a c t i o n must be considered.
DISCUSSION Most o f t h e d e g r a d a t i o n p r o d u c t s formed d u r i n g t h e c o n v e r s i o n o f 3-methylpentane o v e r SbF5 i n t e r c a l e d i n t o g r a p h i t e a r e e x p l a i n e d by t h e c l a s s i c a l c a r benium i o n mechanism r e p r e s e n t e d i n t h e scheme I11
Scheme 3
Initiation
1
I;;
1
& + A= ):
Propagation
I Aging
333 The i n i t i a t i o n o f t h e d e g r a d a t i o n i s a 6 - s c i s s i o n o f a m e t h y l - i s o b u t y l i o n which g i v e s a secondary propenium i o n and a p r o p y l e n e molecule. The l a t t e r const i t u t e s t h e o l e f i n source which p e r m i t s t h e development o f a p o l y m e r i z a t i o n - d e p o l y m e r i z a t i o n p r o c e s s . The s o c a l l e d o l i g o m e r c h a i n s a r e c o n t i n u o u s l y broken by B - s c i s s i o n s l e a d i n g t o b u t y l , t - a m y l , t - h e p t y l c a t i o n s and remade by f u r t h e r a d d i t i o n o f t - h e x y l c a t i o n s . The l a t t e r a r e more p r o b a b l y formed, b u t , owing t o t h e low c o n v e r s i o n s used, t h e y a r e d i l u t e d by t h e u n r e a c t e d und i s o m e r i z e d hexanes and cannot be d e t e c t e d . Since 6 - s c i s s i o n s produce a l s o adsorbed o l e f i n s , t h e r e a c t i o n should, i n p r i n c i p l e , c o n t i n u e i n d e f i n i t e l y , w i t h o u t f u r t h e r p r o p y l e n e f o r m a t i o n as seen above. Aging can n e v e r t h e l e s s occur, due t o c o k i n g : however no d e a c t i v a t i o n was n o t i c e d i n o u r s c a l e t i n e . The combined use o f p r o d u c t s d i s t r i b u t i o n a t low d e g r a d a t i o n c o n v e r s i o n s and o f h i g h l y carbon 13 enr i c h e d molecules a l l o w s t o s i g n i f i c a n t l y p r e c i s e two p o i n t s i n t h i s scheme. F i r s t l y t h e hexane isomers a r e i n t h e i r thermodynamic e q u i 1 ib r i u m and t h e i r carbon p o s i t i o n s have l o s t t h e i r uniqueness. T h i s shows t h a t t h e carbon skeleton o f t h e f e e d m o l e c u l e has undergone e x t e n s i v e b r a n c h i n g and non b r a n c h i n g r e a r rangements. The n a t u r e o f these rearrangements has been e x t e n s i v e l y s t u d i e d (ref.13,
2 1 ) . A c c o r d i n g l y we cannot c o n s i d e r a s i n g l e isomer as s t a r t i n g hydro-
carbon b u t i n s t e a d a m i x t u r e o f r e a r r a n g e d h e x y l c a t i o n s . Among these, t h e o n l y one which can g i v e a d i r e c t @ - s c i s s i o n w i t h o u t f o r m a t i o n o f an u n s t a b l e p r i m a r y c a t i o n i s t h e secondary m e t h y l - i s o b u t y l i o n . The s t a b i l i t y o f such an i o n i s f a r less
than t e r t i a r y h e x y l c a t i o n s ( r e f . 2 2 ) . T h i s e x p l a i n s why, whatever
t h e branched hexane used as s t a r t i n g m a t e r i a l , propane i s t h e i n i t i a l c r a c k i n g product. Secondly these t e c h n i q u e s g i v e us an e f f i c i e n t way t o q u a n t i t a t i v e l y d i s t i n g u i s h i n t h e d e g r a d a t i o n process between J polymerization-depolymerization t y p e A mechanism and a p r o t o l y s i s t y p e B mechanism. Thus we can s t a t e t h a t under our c o n d i t i o n s o n l y f o r t h e i s o p e n t a n e f o r m a t i o n a small c o n t r i b u t i o n o f p r o t o l y s i s c o u l d be i n c l u d e d . Indeed t h e c o m p e t i t i o n between t h e s e two mechanisms depends on s e v e r a l parameters such as temperature, hydrocarbon s t r u c t u r e and pressure, s t r e n g t h and n a t u r e o f t h e a c i d s i t e s . T h i r d l y t h e o c c u r r e n c e of t h e polymerization-depolymerization mechanism i s governed by t h e presence of o l e f i n s i n t h e medium. We propose h e r e t h a t these o l e f i n s a r e produced b y d i r e c t B - s c i s s i o n o f an h e x y l c a t i o n . Such a process i s l i k e l y f o r hexanes and h i g h e r hydrocarbons s i n c e t h e 6 - s c i s s i o n o f a p r o p e r cat i o n can l e a d t o a secondary c a t i o n . T h i s i s no l o n g e r t h e case o f pentanes and s m a l l e r hydrocarbons. A c t u a l l y r e a c t i o n r a t e s and d i s t r i b u t i o n s i n t h e degradat i o n o f isopentane and branched hexanes d i f f e r markedly ( r e f . 2 3 ) .
Other ways t o
produce o l e f i n s c a n n o t however be d i s c a r d e d . Amoung these, t h e presence o f o l e f i n s as i m p u r i t i e s i n t h e s t a r t i n g p a r a f f i n s ,
(ref.24) o r a displacement o f the
334
deprotonation-protonation e q u i l i b r i u m o f a carbenium i o n have been p u t f o r w a r d i n t h e l i t e r a t u r e ( r e f . 7 ) . We must keep i n mind however t h a t t h e d e p r o t o n a t i o n c o n s t a n t o f t - b u t y l i o n i n t h e s u p e r a c i d i c medium HF-SbF5 i s as l o w as 10-10 ( r e f . 25)
A + H+
K=10
-10
F u r t h e r d e t a i l s i n t h e s t u d y o f t h e d e g r a d a t i o n process i n a s u p e r a c i d i c medqum
w i l l be p r e s e n t e d 1 a t e r . REFERENCES
1 F.E. Condon i n " C a t a l y s i s " , R h e i n h o l d P . C . , V I , (1958) 43. 2 M.L. Poutsma, Adv. Chem. Ser., 171 (1976) 504. 3 H. P i n e s "The Chemistrv of C a t a l v t i c hvdrocarbon c o n v e r s i o n s " . Academic Press (1958) 1. 4 H. P i n e s and R . C . Wackher, J. Am. Chem. SOC., 68 (1946) 595. 5 D.A. Mac Cauley, J. Am. Chem. SOC., 81 (1959) 6437. 6 A. Schriesheim and S. H. Khoobiar. 2nd I n t . Cona. C a t a l . P a r i s (1960) P 30 7 G . A . Fuentes and B.C. Gates, J. Cata1.,76 (19827 440. 8 G.A. Fuentes, J.V. Boegel and B.C. Gates, J. C a t a l . , 78 1982) 436. 9 G . A . Olah, Y . Halpern, J. Shen and Y . K . Mo, J . Am. Chem SOC., 95 (1973 4960. 10 G.A. Olah, J. Am. Chem. SOC., 94 (1972) 803. 11 F. Le Normand, F . F a j u l a and J . Sommer, Nouv. J. de Chimie, 6 (1982) 291. 12 F. Le Normand, F . F a j u l a , F.G. G a u l t and J . Sommer, Nouv. J . de Chimie, 6 (1982) 411. 13 F. Le Normand, F. F a j u l a , F.G. G a u l t and J . Sommer, Nouv. J. de Chimie, 6 (1982) 417. 14 J. M e l i n and A. Herold, C.R. Acad. Sciences, 280, S e r i e C (1975) 641. 15 F . G a r i n and F.G. G a u l t , J . Am. Chem. SOC., 97 (1975) 4466. 16 M. Daage and F . F a j u l a , J. C a t a l . , 81 (1983) 394. 17 J.H. Beyon, "Mass s p e c t r o m e t r y and i t s A p p l i c a t i o n t o Organic Chemistry", E l s e v i e r (1960). 18 A . Farkas, P h y s i c a l Chemistry o f Hydrocarbons", Academic Press, 9 (1950) 19 F. F a j u l a and F.G. G a u l t , J. C a t a l . , 68 (1981) 312. 20 W.O. Haag and R.M. Dessau, 8 t h I n t . Cong. C a t a l . , B e r l i n (1984) 11, 305. 21 M. Daage and F. F a j u l a , J . Catal., 81 (1983) 405. 22 D.M. Brouwer, Rec. Trav. Chirn. Pays Bas, 87 (1968) 211. 23 F. Le Normand, F. F a j u l a and J . Sommer, i n p r e p a r a t i o n . 24 A. Brenner and P.H. Emmett, J. Catal., 75 (1982) 410. 25 H. Hogeveen, C.J. Gaasbek and A.F. B i c k e l , Rev. Trav. Chim. des Pays Bas, 88 (1969) 703.
B. Imelik e t al. (Editors), Catalysis b y Acids and Bases 0
335
1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
RELATIONSHIP BETWEEN CATALYTIC ACTIVITY AND ACID STRENGTH OF LaHY ZEOLITES IN CUMENE CRACKING AND 0-XYLENE ISOMERIZATION SHE LI-QIN, HUNG SU, LI XUAN-WEN Dept. of Chemistry, Peking University, Beijing (P.R. China)
ABSTRACT The surface acidity of a series of LaHY/LaNaHY, prepared by successive poisoning with NaOH, has been investigated in relation to their catalytic activities in cumene cracking and o-xylene isomerization. The results showed that the acid sites having acid strength Ho5 -3.0 were catalytically active for both reactions. A decrease in apparent activation enerny wi.th increased acid strength was observed. By using "regional analysis" the relationship between activity and acid strength was established and followed the Bronsted catalysis law approximately. In comparison with their activity in o-xylene isomerization, the activity of LaHY/LaNaHY zeolites in cumene cracking was much higher. The rate constants were 1-2 orders of magnitude higher and apparent activation energies much lower. RESUME L'acidit6 de surface d'une s6rie des z6olithes LaHY/LaNaHY, pr6par6es par empoisonnement progressif par NaOH a 6t6 Btudi6e en relation avec l'activit6 catalytique er' rgactions de craquage du cumzne et d'isomerization de l'oxyle'ne. Les rgsultats ont montr6 que les sites acides ayant une force d'acidit6 de Ho< -3.0 sont actifs dans les deux rEactions. Une diminution de l'6nergie d'activation apparente en fonction de l'augmentation de la force d'aciditg a 6t6 observge. La corrglation entre l'activit6 et la force d'aciditg a gt6 e'tablie en utilisant "l'analyse de region".Elle obEit approximativement 1 la loi catalytiquede BrEnsted. Compar6e 5 l'isomerization de 1'0-xylEne, l'activit6 des zgolithes LaHY/LaNaHY dans le craquage du cumgne est bien plus 6lev6e: les constantes catalytiques sont de I0 2 lo2 plus grandes et 1'Energie d'activation apparente est bien plus faible. INTRODUCTION
In homogeneous acid-catalysed reactions, the Brhsted relation and the relationship between the rates and Hammett acidity function have already been well established. As far as catalysis by zeolites is concerned numerous investigations of the correlations between the acidic and catalytic Droperties have been performed. Various reviews appeared, some of them quite recently ( I - & ) . However, because of the limitation of experimental methods used, most of the papers did not go further than to relate the catalytic activity to the total number of acidic sites or to the overall average acidity. I n this investigatian, the improved Benesi method ( 5 ) was used to determine the acid strength distributionln zeolites. By using the "regional analysis" (6) the acid strength of a region and its catalytic activity could be obtained. We attempted to check
336
whether the Brsnsted relation describes the above-mentioned relationship (activity-acidity) for cumene cracking and o-xylene isomerization. Next to the acidity, various other factors (pore size, channel structure, formation of coke
...)
influence the activity of zeolites. To minimize the role of these
factors, Faujasite type LaHY zeolites were chosen, which have open structures with relatively large pores and channels to prevent the perturbation by steric and space restrictions. Acid sites were poisoned with NaOH and the acid strength was varied as well. Calcination temperature was controlled to achieve an
predominantly of the Brsnsted type. The reactions were carried
acidity
out by a pulse technique to obtain the activity characteristic of the fresh surface of catalysts. EXPERIMENTAL Sample preparation The starting material was a commercial NaY with Si02/A1203 ratio of 4 . 9 8 . From this NaY a LaHY zeolite was prepared by ion exchange with La(N0 ) solu3 3 tion and by calcination,followed by exhaustive exchange with 10% NH4N03 solution (exchange levelof La: 80%, Na20<0.3%, w/w) and the calcination for 3 hrs at 673 K to remove ammonia. LaNaHY samples were obtained by adding various amounts of NaOH to aqueous suspensions of LaHY at 363 K and stirring for 1 hr. The products were filtered, dried at 383 K over night, and calcined at 673 K. Titration of surface aciditx The titration consisted of testing portions of catalyst samples with Hammett indicators (anthraquinone, pKa -8.2; benzalacetophenone, pKa -5.6; dicinamalacetone, pKa -3.0; butter yellow pKa +3.3) after they had been equilibrated with varying amounts of n-butylamine in'A.R. petroleum ether(dried by 3A molecule sieve). An ultrasonator was used to reduce the time to reach the adsorption equilibrium. IR spectra of chemisorbed pyridine The IR observations were made with a "SPECORD IR 75" spectrophotometer using 16 mm diameter discs of pure zeolites, formed by pressing 10 mg of powder at 4.5 tons. Discs were mounted in an externally heated vacuum cell equipped with CaF2 windows. Each sample disc was degassed at the required temperature and torr
for 1 hrtkncooled down to 523 K. After adsorption of pyridine, the
samples were evacuated till the pressure was below recorded at room temperature.
torr. Spectra were
331
Determination of catalytic activities The catalytic activities of the samples in the cumene cracking and o-xylene isomerization were characterized by the first order rate constant and apparent activation energies. Kinetic measurements were made using a pulsed catalytic chromatograph. The carrier gas was pure hydrogen. Reaction conditions (temperature,
flm
rata,sample particle size, sample weight) were adjusted to pre-
vent the external and internal diffusion influencing the reaction. Preliminary experiments confirmed that the rate constants of these reactions could be
calculated from Bassett and Habgood equation. (7). RESULTS
Type of acidity and acid strength distribution The Py-infrared spectra of LaHY and LaNaHY exhibited a PyH+ band at 1540 -1
cm
-1
and a Py-La band at 1445 cm
. The
1455 cm-’ band resulting from pyridine
adsorbed on Lewis (dehydroxylated) sites could be considered as negligible (Figure 1 ) .
Figure 1 . IR spectra of chemisorbed pyridine A . LaHY degassed at 6733;
B. LaNaHY-I degassed at 773% C. LaNaHY-2 degassed at S23K.
I
o
1600
,
,
1500
.
.
1400
.
J
( CM-’ )
338 The amounts of NaOH added to LaHY were controlled in order to obtain
the
LaNaHY samples with different acid strengths. These zeolites revealed a monotonous
decrease in acid strength with increasing Na content as shown inTable I.
TABLE 1 Acid strength distribution of the zeolite samples Acid content (meq/g) Sample
Ho ( - 8 . 2
-8.2
-5.6
-3.O5H 0 -<+3.3
LaHY
1
.oo
0.00
0.00
0.50
LaNaHY- 1
0.00
0.40
0.05
0.35
LaNaHY-2
0.00
0.00
0.24
0.19
LaNaHY-3
0.00
0.00
0.00
0.21
It was shown that the sample LaHY exhibited the strongest acid sites, LaNaHY-1 less strong acid sites, LaNaHY-2 and LaNaHY-3 the medium and weak acid sites respectively. As the amount of the NaOH added increased, not only the number of acid sites but also the acid strength decreased. This was likely due to the replacement of protons, which have much higher electrical charge density than Na'ions,
by Na+ ions. Replacement resulted in the decrease of the polari-
zation of neighbouring 0-H bands and hence in the decrease of their strengths (8).
acid
Catalytic activities LaNaHY-3 with Ho
5 +
3.3 acid strength catalyzed neither cumene cracking nor
o-xylene isomerization at temperatures lower than 673 K under our experimental conditions, whereas the other samples were all active. This means that the LaNaHY zeolites with Ho
5
-
3.0 were active in both reactions. The activity of
LaHY decayed gradually as the pulse time increased. The data of the first pulse were used to calculate the rate constants at various temperatures. The activities of LaNaHY zeolites were relatively stable. The same sample could be used repeatedly for the required determinations. Tables 2 and 3 list the observed rate constants and apparent activation energies.
339 TABLE 2
Rate constants and activation energies for cumene cracking sample
LaHY
LaNaHY- 1
LaNaHY-2
reaction temp. (K)
rate constant
* activation energy
502.0 515.5 524.0 534.5 543.0 553.0
**
Kcal/mol
l z
0.898 1.14 1.31 9.1
0.9997
17.3
0.9962
26.8
0.9986
1.76 2.11
602.0 61 1 .O 622.0 632.5 645.0 655.0 659.9
3.79 5.08 3.79 8.20 10.72 13.04 13.64
632.0 635.0 64 1.0 65 1 .I9 661 .O 670.5
2.27 2.62 3.16 4.52 6.1 1 7.78
lo-4
*unit: mol/atm.g.s.
**
r: correlation coefficient
TABLE 3
Rate constants and activation energies for o-xylene isomerization sample
LaHY
LaNaHY- 1
LaNaHY-2
reaction temp. ( K ) 549.0 558.0 568.0 579.0 599.0 608.0
1.31 2.35 2.95
633.5 644.0 659.0 673.0 682.0 700.0
0.92 1.27 2.15 3.30 4. 16 6.53
722.0 733.0 741.0 748.5
0.423 0.757 0.958 1.470 2.15 2.87 4.01
758.0 765.0 776.0
*,**
*
rate constant k
: the same as Table 2
activation energy Kcal/mol
r
**
0.535 0.725 1.00
,o-5
19.1
0.9995
26.3
0.9992
46.9
0.9983
340 DISCUSSION
Relationship between the a c i d s t r e n g t h and a c t i v i t y By using "regional a n a l y s i s " the r e l a t i o n between a c t i v i t y and a c i d s t r e n g t h of the z e o l i t e s studied has been e s t a b l i s h e d .
According to the following equation D
k = k k S ai i where k i s the o v e r a l l r a t e constant on a given z e o l i t e sample, kai i s the "regional" c a t a l y t i c constant ( r a t e constant p e r u n i t a c i d content) of the i t h region of the a c i d s t r e n g t h , S . i s the a c i d content o f t h e same region, P i s the number of regions, the r e g i o n a l c a t a l y t i c constants (Table 4 ) o f v a r i o u s 1
a c i d s t r e n g t h s were c a l c u l a t e d from t h e d a t a of Table 1 and t h e l o g k - r p l o t s . 1
Some of the k values were given by e x t r a p o l a t i o n of the log k-7 p l o t s . I t was assumed t h a t the a c i d s t r e n g t h Ho d i s t r i b u t i o n of Ho equal t o -8.2,
-5.6,
each region peaked near the
-3,O r e s p e c t i v e l y . P l o t s of t h e log kai changed l i n e a r -
l y with t h e a c i d s t r e n g t h H
(Fig.2).
These r e s u l t s show t h a t the r e l a t i o n s h i p
between a c t i v i t y and the a c i d s t r e n g t h on LaHY and LaNaHY z e o l i t e s obeys,for t h e above-mentioned r e a c t i o n s , t h e Brznsted c a t a l y s i s law ka = Ga.KZ Ka-
d i s s o c i a t i o n c o n s t a n t of a c i d s i t e , s i n c e
Ho = -log K
109 ka =
-
(3)
CL Ho+
where CL and G
(4)
log Ga
a r e the c o n s t a n t s of a given r e a c t i o n a t given temperature,
which can be estimated by l i n e a r r e g r e s s i o n a s shown i n Table 5. TABLE 4
Acid s t r e n g t h and c a t a l y t i c constant c a t a l y t i c constant k (mole/atm.meq.s.) acid strength H0S-8. 2
cracking (573k)
isomerization (633k) isomerization (723k)
2 . 8 0 ~1 Ov3
5 :53x1 0-4
3. 6 6 x f 3
-8.2SH0<-5.6
4.83~10~~
2 . 2 7 ~1 0-5
3.06~10-~
-5 .6
1 . 0 8 3 ~ 0-5 1
t .83~10-~
\.92~10-~
-3.0
0
0
0
Because of the l i m i t a t i o n of t h e experimental method, the c o r r e l a t i o n was only checked i n 3 regions and only semi-quantitatively.
However,
of a r e l a t i o n s h i p between a c i d s t r e n g t h and a c t i v i t y i s obvious.
the e x i s t e n c e
34 I TABLE 5
a and G v a l u e s reaction
a
temp.(K)
r
Ga
cumene c r a c k i n g
573
0.47
6 . 0 3 ~ 0-7 1
0.9787
o-xylene
633
0.67
2 . 3 6 ~ O-' 1
0.9929
isomerization
723
0.44
9 . 7 3 ~ 10-7
0.9994
-I(
ka ( K cal/mol) E 501
2 3 4
5 6
7
mion 1 1 9
8
2 7
I . 6
5
-
16
3
6
3
2
9 8 7 6 5 4 3 -%
-Ho
F i g u r e 2. C a t a l y t i c c o n s t a n t v s .
F i g u r e 3.
acid strength
A c t i v a t i o n energy vs. acid strength
a . cumene c r a c k i n g (573K)
a . cumene c r a c k i n g
b. o-xylene i s o m e r i z a t i o n (633K)
b. o-xylene i s o m e r i z a t i o n
c . o-xylene i s o m e r i z a t i o n (723K) R e l a t i o n s h i p between a c t i v a t i o n energy and a c i d s t r e n g t h From t h e a n a l y s i s of a c i d i t y d i s t r i b u t i o n s (Table 1 ) i t can b e s t a t e d t h a t t h e a c i d s i t e s w i t h Ho Ho
21
-3.0
-8.2
i n LaHY, w i t h Ho"
-5.6
i n LaNaHY-1, and w i t h
i n LaNaHY-2 p l a y t h e p r i n c i p a l r o l e i n b o t h t h e above-mentioned
r e a c t i o n s . Using t h e v a l u e s of a c t i v a t i o n energy of t h e s e t h r e e samples we p l o t t e d t h e a c t i v a t i o n energy vs. t h e a c i d s t r e n g t h . F i g u r e 3 shows t h a t a c t i v a t i o n energy changes l i n e a r l y w i t h a c i d s t r e n g t h H
. This
i s not surprising
s i n c e by combining t h e Brznsted Eq.(4) and Arrhenius e q u a t i o n s , we o b t a i n
342
A - log Ga, is a constant, A is the frequency factor, proportiowhere B = log S
nal to the acid content
S.
Comparison between cumene cracking and o-xylene isomerization In comparison with its activity towards o-xylene isomerization the activity of LaHY and LaNaHY in cumene cracking is much higher. The rate constants are 1-2 orders of magnitude higher when measured with the same zeolite sample and at the same temperature while the apparent activation energies are much lower (see Table 2,3). This discrepancy could be explained by the different ratedetermining steps of the reactions (9):
To form the intermediate of the o-xylene isomerization (11), it is necessary to
form a primary carbonium ion. This step must be more difficult than the analogous one in cumene cracking (I), which only requires forming a secondary carbonium ion.
CONCLUSION The acid sites having acid strength H
5:
-
3.0 are catalytically active for
cumene cracking and o-xylene isomerization on LaNaHY zeolites under our experimental conditions. With the same catalyst, the activity in cumene cracking is much higher than that in o-xylene isomerization. The Brksted catalysis law may hold for acid catalysed reaction taking place on the faujasite type zeolites. ACKNOWLEDGEMENT We are indebted to Prof.dr,V,Ponec for his critical reading and Mrs.H.aeEtel for her careful typing of the manuscript. REFERENCES 1 D.Barthomeuf, ACS Symposium series 40, 1977, p.453 p.55 2 D.Barthomeuf in B.Imelik et al(Eds.) ,Catalysis by Zeolites,Elsevier A'dam, 1980 3 P.A.Jacobs, Carboniogenic Activity of Zeolites, Elsevier A'dam 1977 p.253 4 H.A.Benesi and B.H.C.Winquist, Adv.in Catal., Vo1.27 (1978) 97 5 H.A.Benesi, J.Phys.Chem., 61 (1957) 970-973 6 Y.Yoneda, J.Catal., 9 (1967) 51-56 7 D.W.Bassett and H.W.Habgood, J.Phys.Chem., 6 4 (1960) 769-773 8 J.DatkainB.Imeliketal(Eds),Catalysis by Zeolites, Elsevier A'dam,1980, 121 9 H-Matsumoto, J.I.Take, Y.Yoneda, J.Catal., 1 1 (1968) 211-219
B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
343
ACID PROPERTIES OF A BIDIMENSIONAL ZEOLITE 0. PLEE1, A. SCHUTZ',
G. PONCELET' and 3.5. FRIPIAT' 'C.N.R.S. - C.R.S.O.C.I., rue de la F@rollerie, 45045 Orleans (France) 'Groupe de Physico-Chimie Minerale et de Catalyse, Place Croix du Sud 1, 1348 Louvain-la-Neuve (Belgium).
ABSTRACT Montmorillonite and synthetic beidellite have been intercalated with hydroxyaluminum polymers and characterized by everal techniques. The basal 001 reflection of the pillared clays is about 18 and calcination slightly decreases it. From MAS-NMR measurements, it turns out that the pillaring agent is the A113 polymer. Calcination of pillared montmorillonite (with octahedral substitutions) does not bring about modifications in the tetrahedral layer, whereas it does in pillared beidellite (with tetrahedral substitutions), in which coupling between an OH apex of an inverted A1 tetrahedron and an OH of an A1 octahedron from the All3 polymer occurs, resulting in the exposure of the negative charge of the A1 tetrahedron in the open in the interlamellar space. The thermal activation o f pillared beidellite provokes protonation of Si IV0-A1IV linkages (infrared data). These protonated sites are strongly acid.
a
INTRODUCTION Brindley et al. (ref.1) and Lahav et al. (ref.2) were the first to show that montmorillonite and, more generally, dioctahedral phyllosilicates may be expanded in a thermally stable structure by pillaring the bidimensional lattice 0 with aluminum hydroxypolymers. The basal 001 reflection is slightly above 18 A in the air-dried solid. It decreases of about 0.5 A after calcination between 300 and 500°C. The free interlamellar space has thus a thickness of about 7.5 A. The BET surface area of these pillared clays is in the range of 250 - 300 m 2/g. As shown further the acidity measured by the combined pyridine adsorption and infrared technique belongs mainly to the Bronsted type and in pillared montmorillonite the dinsity of acid sites decreases quite rapidly above 200°C. Indeed, the catalytic acitivity in hydrocracking and hydroisomerization is far below that of the ultrastable Y (USY) zeolite. The acid sites in pillared montmorillonite may be of two kinds according to their localization either on the surface of the clay or on that of the pillar. The main source of acidity of the montmorillonite surface is the hydration water (ref.3) and of course it fades away as dehydration progresses. The nature of the acid sites of the pillar is an open question which could be answered only if the nature of this pillar was clearly established. Vaughan et al. (ref.4) and Pinnavaia (ref.5) have suggested that the pillaring cation 0
3 44
i s most 1 ik e l y an A113 polyhydroxypolymer r e l a t e d t o t h e known c a t i o n 7+ A11304(OH)24, t h e schematic s t r u c t u r e o f which i s shown i n F i g . 1. The c h a r a c t e r i s t i c f e a t u r e o f t h i s Al13
polymer is t h e t e t r a h e d r a l aluminum i n t h e h e a r t
o f 3 l a y e r s o f aluminum o c t a h e d r a .
T h i s s t r u c t u r e i s composed f r o m t h e f o u r 0
18 A spacing and i t s
l a y e r s o f oxygen o r h y d r o x y l s r e q u i r e d t o o b t a i n t h e dOOl O 2
s u r f a c e a r e a i s o f t h e o r d e r o f 110 A
c o u n t f o r a r a t h e r l a r g e s u r f a c e area.
.
T h i s p r o v i d e s enough v o i d space t o acT i l l r e c e n t l y , n o t h i n g was known a b o u t
t h e n a t u r e o f t h e t r a n s f o r m a t i o n o f t h e Al13
s p e c i e s a f t e r thermal a c t i v a t i o n
above 300 'C. The f o r m a t i o n o f a s p i n e l - l i k e s t r u c t u r e a s those o b t a i n e d upon c a l c i n i n g b a y e r i t e o r g i b b s i t e c o u l d be s p e c u l a t e d .
I n t h a t case
t h e p i l l a r would c o n t r i b u t e t o t h e a c i d i t y m o s t l y by Lewis s i t e s . P i l l a r i n g b e i d e l l i t e , e.g.
a dioctahedral
s m e c t i t e w i t h S i by A1 s u b s t i t u t i o n s , may c r e a t e a n o t h e r source o f a c i d i t y . the S i - O - A l I "
Indeed,
l i n k a g e i n t h a t s m e c t i t e is
e a s i l y a t t a c k e d by p r o t o n s .
I t has been
shown, f o r i n s t a n c e ( r e f . 6 ) , t h a t by decompo s i n g an ammonium b e i d e l l i t e , S i - O H i n f r a r e d 00 o OH
@HP
s t r e t c h i n g bands appear a t 3500 cm-' and 3420 cm-'. These bands were assigned t o s i l a n o l groups formed upon a d e a m i n a t i o n r e a c t i o n , a s suggested by Uytterhoeven e t a l . f o r
F i g . 1. Exploded r e p r e s e n t a t i o n o f t h e A113 polymer.
X and Y z e o l i t e s ( r e f . 7 ) .
The Al13 polymer
being an a c i d , i t m i g h t be a n t i c i p a t e d t h a t
t h e thermal a c t i v a t i o n o f t h e p i l l a r e d b e i d e l l i t e c o u l d c r e a t e s i m i l a r B r o n s t e d a c i d s i t e s and enhanced c a t a l y t i c p r o p e r t i e s r e l a t e d t o a c i d i t y . The aim o f t h i s paper i s t h u s t o d e s c r i b e t h e s t r u c t u r e o f t h e s o l i d r e s u l t i n g f r o m t h e thermal a c t i v a t i o n o f t h e p i l l a r e d b e i d e l l i t e and i t s a c i d p r o p e r ties.
P i l l a r e d m o n t m o r i l l o n i t e w i l l be used f o r comparison.
STRUCTURE AND TEXTURE OF PILLARED SMECTITES 0
The p r e p a r a t i o n procedure used f o r o b t a i n i n g 18 A p i l l a r e d s m e c t i t e s has been d e s c r i b e d by Schutz e t a l . ( r e f . 8 ) .
Fundamentally,a c l a y s l u r r y o f a 3% ( w e i g h t
by w e i g h t ) o f t h e <2p f r a c t i o n i s c a r e f u l l y di;persed.
S e p a r a t e l y , a 0.4
M
A1(N03)3.9H20 s o l u t i o n is n e u t r a l i z e d by a 0.3 M NaOH s o l u t i o n i n o r d e r t o adj u s t t h e m o l a r OH/A1 r a t i o i n t h e range 1 . 2 t o 1.8.
T h i s s o l u t i o n i s aged a t
50 "C f o r one hour and added t o t h e c l a y s l u r r y such as t o o b t a i n a f i n a l concent r a t i o n between 20 and 30 meq A 1 p e r gram o f c l a y .
A f t e r s t i r r i n g vigorously
345 t h e m i x t u r e f o r 1 hour, t h e suspension i s d i a l y z e d f o r 4 days i n a c e l l u l o s e membrane i n c o n t a c t w i t h d i s t i l l e d water. t h e n c a l c i n e d between 300 and 500 "C.
The suspension i s l y o p h i l i z e d and
F i g . 2.1. shows t h e X-ray d i f f r a c t o g r a m
o f t h e p i l l a r e d m o n t m o r i l l o n i t e a f t e r c a l c i n i n g a t 500 "C.
The average s t r u c t u -
r a l f o r m u l a o b t a i n e d f r o m 6 d i f f e r e n t samples prepared f o l l o w i n g t h i s procedure is : IV VI Na0.064, 1'65 A113' (sig) 9 (A13Fe0.36Mg0.64) > O24.2
a f t e r c a l c i n a t i o n a t 900 "C,
assuming t h a t p i l l a r s a r e made from 13 A1 atoms as shown i n F i g . 1.
AlI3
in
t h i s f o r m u l a stands f o r one p i l l a r aluminum. B e i d e l l i t e i s s y n t h e s i z e d and p i l l a r e d a c c o r d i n g t o t h e procedure d e s c r i b e d I t s average f o r m u l a i s , f r o m 5 d i f f e r e n t samples c a l c i -
by P l e e e t a l . ( r e f . 9 ) . ned a t 900 "C :
9' ('j7. l A ' 0 . 9 ) IV, A1':
022
The abundance o f n a t u r a l b e i d e l l i t e i s so small t h a t a s y s t e m a t i c s t u d y would n o t have been p o s s i b l e .
When p i l l a r e d w i t h t h e same procedure as t h a t d e s c r i b e d
above, t h e s t r u c t u r a l f o r m u l a i s :
a g a i n on t h e b a s i s o f t h e s o l i d c a l c i n e d a t 900 "C. The X-ray d i f f r a c t o g r a m o f t h e p i l l a r e d b e i d e l l i t e i s shown i n F i g . 2.2. i s v e r y s i m i l a r t o t h a t o f p i l l a r e d Wyoming b e n t o n i t e .
It
Because o f t h e small
number o f 001 r e f l e c t i o n s , no q u a n t i t a t i v e i n f o r m a t i o n can be o b t a i n e d f r o m t h e d i f f r a c t o g r a m s e x c e p t t h a t b o t h t y p e s o f s o l i d s have a t u r b o s t r a t i c s t r u c t u r e p o o r l y ordered along the c a x i s .
The most i n t e n s e r e f l e c t i o n s a r e u n d o u b t e d l y
due t o t h e 001 r e f l e c t i o n s o f l a y e r s w i t h s t a c k i n g d e f e c t s and perhaps some mixed l a y e r s c o n t r i b u t i o n .
Thus,as f a r as t h e l o n g range o r d e r i s concerned, p i l l a r e d m o n t m o r i l l o n i t e cann o t be d i s t i n g u i s h e d f r o m p i l lared beidellite. The s p e c i f i c s u r f a c e area, microporous and porous volumes, o b t a i n e d f r o m t h e BET p l o t , t h e D u b i n i n e q u a t i o n ( r e f . 10) and
3.95A
t h e t o t a l porous volume observed f o r t h e a d s o r p t i o n a t -190 " C a r e shown f o r t h e s e two p i l -
35
30
25
20
15
10
5 213
F i g . 2. X-ray d i f f r a c t o g r a m m e o f 1) p i l l a r e d m o n t m o r i l l o n i t e and 2 ) p i l l a r e d b e i d e l l i t e .
l a r e d smectites i n Table l . The c a l c i n e d p i l l a r e d montmor i l l o n i t e (CPM) e x h i b i t s a
s p e c i f i c s u r f a c e a r e a 20 % l o w e r than t h a t o f c a l c i n e d p i l l a r e d b e i d e l l i t e (CPB).
346 However, t h e main d i f f e r e n c e i n between t h e m i c r o p o r o u s volumes i s a b o u t 40 % l e s s f o r CPM.
The m i c r o p o r o u s volume c a l c u l a t e d by assuming an i n t e r l a y e r d i s 0
t a n c e o f 7.5 A and a n hexagonal p a c k i n g o f t h e p i l l a r s i s a l s o shown i n Table 1. The r a t h e r good agreement between t h e c a l c u l a t e d and observed v a l u e s f o r CPB suggests a more o r d e r e d d i s t r i b u t i o n o f t h e p i l l a r s i n CPB, as compared t o CPM. TABLE 1 S u r f a c e a r e a o f c a l c i n e d p i l l a r e d m o n t m o r i l l o n i t e (CPM) and c a l c i n e d p i l l a r e d b e i d e l l i t e (CPB).
T o t a l porous volume, m i c r o p o r o u s volume, r e s i d u a l CEC and c a l -
c u l a t e d m i c r o p o r o u s volume f o r t h e s e 2 samples d r i e d a t 220 "C b e f o r e N2 adsorpt i o n (see r e f . 9).
Sample
Surface area dg-1
T o t a l porous v lume cmj g-1
Microporous volume cm3 g - 1
CPM CPB
Residual CEC meq g - 1
Calculated micro or v o l . cmg g l l
250
0.28
0.11
pli
0.1
0.18
315
0.37
0.17
2,
0.2
0.19
SHORT RANGE ORDERING I N PILLARED SMECTITES I t i s c l e a r t h a t t h e r e s u l t s which a r e sketched above do n o t b r i n g any c l e a r
d i s t i n c t i o n between p i l l a r e d m o n t m o r i l l o n i t e and b e i d e l l i t e .
For t h i s reason
t h e s t u d y o f s h o r t range o r d e r i n g has been u n d e r t a k e n u s i n g h i g h - r e s o l u t i o n s o l i d s t a t e 27Al and *'Si
n u c l e a r magnetic resonance under t h e c o n d i t i o n s of ma-
g i c - a n g l e s p i n n i n g (MAS-NMR).
The "Si
and 27Al s p e c t r a were r e c o r d e d u s i n g two
s p e c t r o m e t e r s o p e r a t i n g a t 8.45 and 11.7 T e s l a and s p i n n i n g f r e q u e n c i e s o f a b o u t 2.6 and 3.5 kHz, r e s p e c t i v e l y .
The r e s u l t s o f t h i s s t u d y a r e r e p o r t e d a t l a r g e
i n a paper by P l e e e t a l . ( r e f . 1 1 ) . Because o f t h e h i g h Fe c o n t e n t i n paramag3+ n e t i c i m p u r i t i e s ( m a i n l y Fe ) i n Wyoming b e n t o n i t e , p i l l a r e d h e c t o r i t e and l a p o n i t e ( t r i o c t a h e d r a l s m e c t i t e s w i t h o u t t e t r a h e d r a l s u b s t i t u t i o n s ) were compar e d w i t h p i l l a r e d b e i d e l l i t e , b e f o r e and a f t e r c a l c i n a t i o n .
The c o n c l u s i o n o f
t h i s s t u d y can be summarized as f o l l o w s : 1 ) The Al13 polymer i s indeed t h e p i l l a r i n g agent f o r a l l t h e i n v e s t i g a t e d smecti t e s . 2) The c a l c i n a t i o n o f t h e p i l l a r e d c l a y s d o e s n ' t l e a d t o t h e t r a n s f o r m a t i o n o f t h e p i l l a r i n t o a pseudo-spinel
structure.
3 ) The c a l c i n a t i o n o f p i l l a r e d s m e c t i t e s w i t h o u t t e t r a h e d r a l s u b s t i t u t i o n s d o e s n ' t l e a d t o a m o d i f i c a t i o n o f t h e t e t r a h e d r a l l a y e r o f t h e sheet s i l i c a t e .
4 ) The c a l c i n a t i o n o f p i l l a r e d s m e c t i t e w i t h t e t r a h e d r a l s u b s t i t u t i o n s ( b e i d e l l i t e ) m o d i f i e s t h e t e t r a h e d r a l l a y e r and l e a d s t o a s t r u c t u r a l m o d i f i c a tion o f the p i l l a r .
The proposed s t r u c t u r e i s shown i n F i g . 3.
The r e a c t i o n o f t h e h y d r o x y l a t e d p i l l a r w i t h t h e t e t r a h e d r a l l a y e r s r e s u l t s
347
f r o m c o u p l i n g t h e OH apex o f an i n v e r t e d aluminum t e t r a h e d r o n and a n OH o f an aluminum octahedron b e l o n g i n g t o Al13.
This leads t o a Si-O-Al"
linkage i n
w h i c h t h e n e g a t i v e charge o f t h e i n v e r t e d t e t r a h e d r o n i s no l o n g e r b u r i e d i n t o a c o n t i n u o u s t e t r a h e d r a l network b u t i s i n t h e open i n t h e i n t e r l a m e l l a r space. P i l l a r i n g b e i d e l l i t e w i t h Al13
and thermal a c t i -
v a t i o n would t h u s r e s u l t i n seeding t h e g r o w t h o f a t r i d i m e n s i o n a l network g r a f t e d on t h e b i d i mensional network o f t h e c l a y .
The r e s u l t i n g
h i g h a r e a s o l i d c o u l d be c o n s i d e r e d as a b i d i mensional z e o l i t e . ACIDITY P r o t o n s f r o m d i f f e r e n t sources may be a t t h e o r i g i n o f t h e a c i d i t y o f p i l l a r e d clays.The water molecules b e l o n g i n g t o t h e h y d r a t i o n s h e l l o f -0
.w o on
charge b a l a n c i n g c a t i o n s a r e s u b m i t t e d t o a s t r o n g e l e c t r i c a l p o l a r i z i n g f i e l d and t h e r e f o r e t h e y have a degree o f d i s s o c i a t i o n s e v e r a l o r d e r s o f magnitude l a r g e r t h a n l i q u i d w a t e r (ref. 3). Water m o l e c u l e s h y d r a t i n g t h e aluminum p i 1 l a r s
F i g . 3. Schematic v i e w of A l l 3 in the space.
a r e f r o m t h a t p o i n t o f v i e w a p o t e n t i a l source o f a c i d i t y whereas i t i s known t h a t t h e h y d r o x y l
groups o f t h e c l a y o c t a h e d r a l l a y e r do n o t c o n t r i b u t e t o a c i d i t y . I n b e i d e l l i t e , p r o t o n s can be c a p t u r e d by t e t r a h e d r a l SilV-O-A1lV
linkages,
y i e l d i n g SiIV-OH.. . A 7 I V groups s i m i l a r t o t h o s e found i n Y z e o l i t e s ( r e f . 6 ) . Except f o r t h e v e r y s m a l l number o f t e t r a h e d r a l s i t e s where S i by A1 s u b s t i t u t i o n o c c u r s i n m o n t m o r i l l o n i t e , t h i s p o s s i b i l i t y does n o t e x i s t i n m o n t m o r i l l o nite. Indeed, as shown b y s p e c t r a ( 1 ) and ( 2 ) i n F i g . 4, upon c a l c i n i n g PB, a r a t h e r s t r o n g OH band appears a t 3440 cm-'
whereas a weak s h o u l d e r i s observed i n CPM.
I t i s n o t p o s s i b l e , by I R spectroscopy ,to d i s t i n g u i s h between SiIV-O-A1 I" groups
used f o r l i n k i n g p i l l a r s t o c l a y t e t r a h e d r a l s h e e t s t h r o u g h i n v e r t e d A1 t e t r a h e d r a w i t h t h o s e w h i c h a r e n o t used f o r t h a t purpose. P y r i d i n e i s a v e r y w e l l documented probe f o r a c i d i t y measurements.
Self-sup-
p o r t i n g w a f e r s o f p i l l a r e d c l a y s were outgassed i n t h e I R c e l l and t h e I R spect r a were r e c o r d e d i n t h e 3000-3800 cm-'
and 1400-1700 cm-'
r e g i o n s b e f o r e and
a f t e r a d s o r p t i o n o f p y r i d i n e and e v a c u a t i o n of excess r e a g e n t a t i n c r e a s i n g tempera t u r e . F o r CPM as w e l l as CPB, more t h a n 90% of t h e a c i d s i t e s a r e BrGnsted s i t e s p r o t o n a t i n g p y r i d i n e i n t o p y r i d i n i u m as i d e n t i f i e d e a s i l y by a c h a r a c t e r i s t i c
348
I R band a t 1540 cm-l. The amount o f chemisorbed p y r i d i n i u m r e t a i n e d by p i l l a r e d c l a y s depends upon t h e p r e t r e a t m e n t t e m p e r a t u r e a t which t h e s o l i d i s a c t i v a t e d and on t h e o u t g a s s i n g temperature. As shown i n F i g . 5 A f o r CPM, i n c r e a s i n g t h e p r e t r e a t m e n t t e m p e r a t u r e decreases t o a l a r g e e x t e n t t h e s u r f a c e d e n s i t y i n chemisorbed p y r i d i n i u m .
Also increa-
s i n g t h e ou t g a s s i ng temperature works a l o n g t h e same way.
For CPB, however,
p r e t r e a t i n g t h e s o l i d between 350 and 500 "C i n c r e a s e s t h e s u r f a c e d e n s i t y i n p y r i -
dinium, and t h e e f f e c t of t h e o u t g a s s i n g temperature i s t o decrease i t t o a l e s s e r e x t e n t t h a n i n CPM ( F i g . 5 8 ) . B e f o r e i n t e r p r e t i n g these observat i o n s , i t i s w o r t h summarizing t h e t h e r mal b e h a v i o r o f t h e two p i l l a r e d c l a y s . The maximum o f t h e endothermic peak r e F i g . 4. I R s p e c t r a . PB c a l c i n e d a t 400 "C (1); PB t p y r i d i n e (2); PM c a l c i n e d a t 400 "C ( 3 ) ; PM t p y r i dine (4).
s u l t i n g from t h e l o s s o f p h y s i c a l l y adsorbed w a t e r i s observed a t a b o u t 100 "C and t h e peak extends up t o about 250 " C .
I t i s f o l l o w e d i n PB by two w e l l - d e f i n e d endothermic peaks a t 330 "C and 540
"C, assigned t o t h e p i l l a r d e h y d r a t i o n ( o r p a r t i a l d e h y d r o x y l a t i o n ) and t o t h e l o s s o f t h e octahedral O H ' S o f t h e c l a y l a t t i c e , r e s p e c t i v e l y . I n PM, a c o n t i n u o u s endothermic e f f e c t extends f r o m a b o u t 250 "C t o 700 "C w i t h maxima a t 330 "C and 630 "C. The c o i n c i d e n c e between t h e temperature a t which p i l l a r s dehydrate and t h e t e m p e r a t u r e a t w h i c h t h e number o f a c i d s i t e s i s r e i n f o r c e d ( F i g . 5 8) suggests s t r o n g l y t h a t some o f t h e p r o t o n s l i b e r a t e d above 350 "C a r e r e s p o n s i b l e f o r t h e f o r m a t i o n o f S i I V - O H . . . A 1 I V groups. T h a t t h e s e groups a r e indeed a c i d i s shown by t h e i r disappearance when CPB has adsorbed p y r i d i n e , as shown i n spectrum 2, F i g . 4. Thus t h e n a t u r e and t h e s t r e n g t h o f t h e a c i d s i t e s i n CPB o f f e r a s t r i k i n g s i m i l i t u d e w i t h t h o s e i n t r i d i m e n s i o n a l z e o l i t e s . T h i s is n o t t h e case f o r CPM. Thus from t h i s p o i n t o f v i e w a l s o CPB deserves t h e a p p e l a t i o n o f b i d i m e n s i o nal zeolite. I t c o u l d be o b j e c t e d t h a t t h e network o f pores i n p i l l a r e d c l a y s i s n o t
349
pretreatement temperature (%I
Fig. 5. Surface d e n s i t y i n chemisorbed p y r i d i n i u m 2 t h e pretreatment temperature o f t h e s o l i d . A: p i l l a r e d montm o r i l l o n i t e ; B: p i l l a r e d b e i d e l l i t e . Outgassing temperat u r e a f t e r p y r i d i n e adsorption: 0, 200 and 300 O C ; n , 400 O C ; 4 , 500 "C. This i s p a r t i a l l y
ordered as i t i s i n a c r y s t a l l i n e t r i d i m e n s i o n a l z e o l i t e .
t r u e since t h e r e i s no o r d e r i n g o f t h e p i l l a r s i n t h e ab plane a t t h e scale o f X-ray d i f f r a c t i o n . However, as shown i n r e f . 9 i n CPB t h e r e must e x i s t a r e l a t i v e l y r e g u l a r d i s 0
t r i b u t i o n o f pores w i t h dimensions o f the order 10/10/7.5 A.
T h i s i s another
d i f f e r e n c e w i t h CPM,as evidenced by the f a c t t h a t an appreciable amount of pyrene i s adsorbed by CPB from a n-hexane s o l u t i o n whereas pyrene i s n o t adsorbed by CPM under s i m i l a r c o n d i t i o n s . REFERENCES
1. G.W.
B r i n d l e y and R.E.
Sempels, Clay Min., 12 (1977) 229-237.
2. N. Lahav, U. Shani and J. Shabtai, Clays and Clay Min., 26 (1978) 107-115. 3. M.M. 4. D.E.W.
Mortland and K.V. Vaughan and R.J.
Z e o l i t e s , E d i t o r L.V. 5. T.J.
Raman, Clays and Clay Min.,
16 (1968) 393-398.
Lussier, Proc. 5 t h I n t e r n a t i o n a l Conference on 1980, pp. 94-101.
Rees, Heyden and Sons U.K.,
Pinnavaia, Science, 220 (1983) 365-371.
6. B. Chourabi and J.J. F r i p i a t , Clays and Clay Min., 29 (1981) 260-268.
7. J.B. Uytterhoeven, L.G. C h r i s t n e r and W.K. H a l l , J . Phys. Chem., 69 (1965) 2117-2126. 8. A. Schutz, G. Poncelet and
P. Jacobs, F r . P a t e n t 81.16 387 (1981).
9. D. Plee, L. Gatineau and 3.5.
F r i p i a t , S u b m i t t e d t o Clays and Clay Min.
350
10. M.M.
Dubinin, 3. Coll. I n t e r f . S c i . , 23 (1967) 489-498.
11. D. Plee, F. Borg, L. Gatineau and J . J . F r i p i a t , Submitted t o J . Am. Chem.Soc.
ACKNOWLEDGMENT The authors acknowledge the Compagnie Francaise de Raffinage f o r f i n a n c i a l support.
B. Imelik et al. (Editors), Catalysis b y Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
THERMAL STABILITY
AND ACIDITY
351
OF A I ~ +CROSS LINKED SMECTITES
Didier TICHIT, Francois F A J U L A , Francois FIGUERAS Laboratoire de C h i m i e Organique Physique et Cinktique chimique AppliquBes - U A 418 du C.N.R.S.
-
E.N.S.C.M.,
8 rue de I'Ecole Normale - 34075 Montpellier Cedex.
Jacques BOUSQUET and Claude GUEGUEN Centre de Recherches ELF-France, B.P. 20 - S t Syrnphorien d'Ozon.
SUMMARY The preparation o f a novel type of wide pore zeolites has been proposed by exchanging dioctaedral srnectites by aluminium macrocations. The thermal s t a b i l i t y o f these solids has been investigated by X-ray d i f f r a c t i o n and measurements o f surface areas and pore size distributions. The evolution o f t e x t u r e w i t h the calcination temper a t u r e corresponds t o t h e sintering o f a f r a c t i o n o f t h e micropores. This evolution is similar for several clays o f different origins and is correlated w i t h t h e degree o f c oss linking. By o p t i m i z a t i o n o f this parameter a residual surface area o f 130 rn'g-' is obtained a f t e r calcination in a i r a t 800OC. The resulting solid has a strong acidity, mainly o f t h e L e w i s type. Pyridine r e t e n t i o n a f t e r evacuation a t 5OO0C shows t h a t the acidic strength is higher than t h a t of Y zeolites.
RESUME L a bibliographie d k c r i t l a prkparation d'un nouveau type de tamis molkculaires B grands pores, utilisant le pontage de smectites dioctaedriques par des macrocations d'alurninium, qui sont introduits par 6change d'ions. L a s t a b i l i t k thermique de ces solides a k t k ktudike e n utilisant l a d i f f r a c t i o n des rayons X e t la mesure des surfaces spkcifiques e t des distributions de rayons de pores. L ' k v o l u t i o n de l a t e x t u r e avec l a tempkrature de calcination correspond principalernent B l a disparition d'une f r a c t i o n des rnicropores; elle est independante de l a nature de I'argile utiliske et sernble l i k e surtout au taux d'kchange, c'est-$-dire au nornbre des ponts entre les feuillets d'argile. Par optimisation de ce taux une surface rksiduelle de 130 rn'9-I est obtenue aprss calcination sous air a 800°C. L'aciditB des solides ainsi obtenus est principalem e n t d u type Lewis; l a retention de pyridine apres evacuation a 500OC revhle une f o r c e acide supkrieure celle des zeolithes Y .
INTRODUCTION Several recent reports describe the preparation of a new class o f catalysts by cross-linking expandable clay minerals of t h e s m e c t i t e type w i t h oligomeric species derived f r o m t h e hydrolysis of polyvalent cations such as A13+ and Zr4+ (1-8). Dehydroxylation o f the oligomeric complexes by thermal treatment leads t o molecular sieve microporous structures in which the layers of t h e clay are maintained separated by p i l lars o f alumina or zirconla. The resulting materials contain acidic sites and are essentially characterized by
352 0
a two dimensional pore system having openings i n the range 7 t o 1 5 A
-
i.e.
larger
than the conventional zeolite catalysts used i n FCC. The advantage of using cross-linked smectites (CLS) instead o f Y-type zeolites for the cracking of bulky molecules has been illustrated by Shabtai et
g
(1, 3, 8).
0
F o r substrates having kinetic diameters larger than 9 P, relative reaction rates were significantly higher with the CLS catalyst than with the zeolites. This was attributed t o sterically unhindered intrasorption of such mdecules in the cross-linked interlamel-
lar space as opposed t o their exclusion from the intracrystalline pore system of the zeolite. The main shortcoming of t h i s type of molecular sieves for application as catalysts i n high-temperature processes i s their insufficient thermal stability. A t the present t i m e most of the researches i n this field are focused on the preparation of thermally stable CLS. The objective of our work has been t o study the mechanisms involved i n the thermal decomposition of such materials and t o characteriz'e their acidity. Three series
of samples have been prepared and the evolutions of their intertamellar spacings, surface areas and nitrogen pore size distributions have been studied after calcination in air between 300 and 800OC. The acidity of uncalcined and calcined samples was
investigated using IR spectroscopy.
EXPERIMENTAL
Clays Three different smectites have been used. An unrefined Wyoming montmorillonite of the Volclay type was obtained f r o m Ward's Natural Science Establishment Inc.
X-ray
analysis showed that this sample
contained quartz and cristobalite. From the intensity of the diffraction peaks the impurity level was estimated t o be i n the range 5-10 wt %. Two smectite samples from Greece ( G ) and Sardaigne ( S ) were furnished by Expansia. These materials were refined by the manufacturer and contained less than 1 % of impurities. The chemical compositions of the three smectites are given i n Table 1. A l l these clays are reported by the manufacturers t o have cation exchange capacities of 100
+ 20 meq/100 9. Cross-linkinq aqent
All hydroxide solutions with OH/AI molar ratios of 2 were prepared as already described by Lahar et
4 (1)
using 0.2
M solutions of NaOH and AICl3. Prior t o use
the solution was aged for at least five days at room temperature.
Preparation o f the cross-linked smectite
5 t o 10 g of clay were dispersed in 2,5 to 5 I of deionised water under vigorous
353 stirring overnight.
The p H of the aged acidic ( p H = 4) Al-hydroxide solution was
adjusted t o 6 w i t h ammonium hydroxide. The desired amount of A1 hydroxide solution, calculated t o obtain Al/clay ratios from 2 to 10 millimoles per gram, was added 1 dropwise ( N 50 m1.h- ) t o the stirred suspension of the clay. The mixture was then heated to 70-8OoC i n order t o polymerise the cross-linking agent (6). A f t e r two hours, the stirring was stopped and the preparation was cooled down t o room temperature. The
supernatant
was
removed and the sediment
collected,
washed several times
w i t h hot deionised water and finally oven dried at 60°C overnight.
Thermal treatment of the CLS 2 g of the freshly prepared CLS w e r e placed in a thin bed configuration, i n an electrically heated quartz tube swept by an air flow of 100 ml.mn-I. was raised at 5O0C.h-'
The temperature
and maintained during 5 hours at the desired level (300 t o
800°C). The oven was then cooled down and the sample recovered for characterization. I n order to avoid cumulative effects a new sample was used for each temperatre investigated.
Characterization studies The thermally
treated samples were characterized by their basal spacings, BET
surface areas and nitrogen pore size distributions. Basals spacings were determined by X-ray diffraction f r o m the position o f the (001) peak. XRD patterns were recorded on a CGR theta 60 instrument using Cu K q radiation. The values of d (001) were 0
obtained with a precision of about 5 0.5 A BET surface area were measured by volumetry
using a home made apparatus
equiped with an integral Barocel pressure transducer.
Prior t o the measurements,
-5
the samples were outgassed for s i x hours at 280OC under 10
torr. The B J H method
was used t o calculate pore size distributions f r o m nitrogen adsorption isotherms. Values of P/Po i n the range 0.19 t o 0.85 were used. The IR spectra were recorded with a Perkin Elmer 397 spectrometer. Self supported wafers ( 2 cm',
10 t o 15 mg) were studied using a glass cell connected t o a grease.
free vacctum system. The design of the cell allowed t o pretreat and load the samples with adsorbates without disconnecting frcm the vacuum line.
RFSULTS AND DISCUSSION General characteristics of the orlqinal clays and cross-linked smectites The clays used in this work belong t o the group of dioctahedral smectites. These 3t or Fe3' ions in tetrahedral coordinaphyUosil1cates consist i n layers of Si4' and A l tion holding a layer of AI3' the charge difference
and MgZt ions in octahedral coordination.
Because o f
between S i and A1 or Fe on one hand and A1 and Mg on the
other hand the l a t t i c e neutrality i s not completed.
The charge deficiency i s then
354 balanced by compensating cations located between t h e s t r u c t u r a l units. Water can also be occluded between t h e layers, bonded either t o the structure itself or t o the cations. Two m a i n properties o f these smectites :
-
t h e possibility o f swelling and
- t h e presence of exchangeable cations are advantageously used i n t h e preparation o f t h e CLS catalysts since the internal microporous
texture
i s generated by ion-exchanging
the original cations by t h e
positively charged Al-hydroxide oligomers and using t h e m as props t o maintain open t h e clay layers. As shown in Table 1 t h e nature o f t h e starting clay and t h e r a t i o Al/clay had l i t t l e e f f e c t on t h e absolute value (19.4 2 0.2) o f t h e interlamellar spacing, d(OOI),
of
Taking i n t o account the w i d t h o f t h e clay layer, this value
t h e cross-linked clay.
0
corresponded t o a f r e e path o f ca 1 0 A
TABLE 1 Chemical compositions and basal spacings o f t h e original clays and cross-linked smectites (samples oven dried at 6OOC).
Clay
Volclav original CLS
Smectite S original CLS
5
Al/clay (mmig)
Smectite G original CLS
5
2
5
10
Cornposit i o n (wt%)
Si02
69.30
52.07
61.80
54.87
61.70
53.48
47.83
47.69
A1203
22.80
29.96
19.10
30.25
20.10
25.79
30.77
31.89
3.80
2.96
5.72
2.48
5.41
2.80
2.68
7.40
CaO
2.80
1.20
0.42
1.98
Na20
0.1 1
0.20
0.1 0
0.20
MgO
1.30
Fez03
KZ0
Ti02
0.06
Si02/Al 0 02 3 d(001)A
3.04 14.7
5.90
3.93
3.90
2.57
0.57
0.1 1
3.60
1.27
0.53 1.74 19.5
Since the pore h e i g h t
3.23 15.4
IS
0.07
0.87 1 .81 19.5
3.07 15.4
2.07 19.3
1.55 19.3
1.49 19.5
controlled by t h e dimensions o f the interlayering complex
the pore size distribution i n the CLS catalysts are more related t o those o f zeolites than of clays.
355 The first column of Table 2 shows
typical
size distributions, obtained
pore
a f t e r decomposition a t 300°C in air of t h e aluminium oligomers.
As expected the
distribution is narrow, w i t h most of t h e surface area in t h e micropores
( L < 20
0
A).
Interpillar pore sizes have been recently evaluated by Shabtai’ e t a(8) using adsorpt i o n of
f l a t condensed aromatic molecules of varying size.
They
found values in
0
the range of 11 t o 19 A , in line w i t h our data.
TABLE 2 Pore size distributions of A l - m o n t m o r i l l o n i t e ( A I / M = 5) i n function of t h e calcination temperature; origin of t h e clay : Greece.
Pore diameter
Temperature 300
600
(“C) 700
800
z 80
3.45
80-60
0.67
1.43
60-40
10.08
6.03
9.35
24.25
40-30
7.64
19.88
17.85
26.57
0
30-20
4.1 5
5.26
4.97
< 20
78.1 3
73.69
63.48
42.77
340
263.7
213.5
125.6
18.10
17.70
17.70
17.70
Specific area mZ/g d
i
As a result of t h e ion-exchange were
process t h e calcium, sodium and potassium levels
decreased and t h e SiO2/AI2O3 r a t i o s were
increased (Table 1). Comparison
o f t h e data obtained on the smectite G sample reveals t h a t f o r A I / M r a t i o s greater than 5, Aluminium is no longer introduced by exchange. The existence of this optimum relates more probably t o the exchange capacity o f t h e clay.
Thermal stability o f t h e cross-linked smectites The t h e r m a l s t a b i l i t y o f t h e cross-linked smectites was investigated by following t h e evolution of t h e basal spacings, surface areas and nitrogen pore size distributions (Fig. 1-2, Table 2). After
treatment
at
300T,
the CLS prepared f r o m t h e t w o r e f i n e d smectites
w i t h A I / M ratios o f 5 or 10 had surface areas o f 250-350 m2.g-l,
70 t o 90 % of
which being in t h e micropores. CLS prepared w i t h A I / M r a t i o s o f 2 or w i t h t h e unrefined m o n t m o r i l l o n i t e exhibited significantly smaller surface areas ( i n t h e range 100 t o 200 m2.g”.
See Fig. 2).
356 0
Basal spacings in t h e CLS samples calcined a t 300°C were by 1 t o 2 A smaller than in t h e freshly prepared material. This
shrinkage
may
arise
from
the
dehydration
o f t h e interlayered clay and from t h e dehydroxylation
of
t h e aluminium hydroxide
agent
into alumina.
It
is thought ( 1 ) t h a t during this l a t t e r s t e p t h e bonding
between
the
oligomer
species and
t h e s m e c t i t e is
gradually shifted from ionic t o near covalent, which results in a stabilization of t h e porous network. When t h e calcination t e m p e r a t u r e was increased u p t o 8 O O O C t h e value of t h e interlamellar spacing was only barely modified.
However, a s illustrated by Figure 1 , t h e
intensity of t h e first order diffraction decreased sharply
after
heating
at
high
t h a t t h e regularity of generated
3 00°C
upon
temperature.
This
means
t h e two dimensionnal l a t t i c e
intercalation
was progressively
des-
troyed. The structure collapse is more apparent when t h e evolutions of
t h e s u r f a c e areas a r e considered.
These a r e represented in Figure
2. Above 600°C a
loss of surface a r e a is noticed a t each t e m p e r a t u r e
I
studied. As evidenced by Table 2, this loss is accompanied by a redistribution of t h e porosity : t h e percentage of
s u r f a c e in
t h e micropores decreases while t h a t 0
in pores of 30-60 A in diameter increases. It should be noted however t h a t a f t e r calcination at
800°C
the
cross-linked
significant surface
s m e c t i t e s still retain
a r e a (120-140 mZ.g-’)
mostly
a in
micropores. The relative variations o f surface a r e a of t h e CLS prepared from t h e t h r e e differents clays a r e plotted against
the
calcination
temperature
on
Figure
3.
For a s a m e AI/M ratio, all t h e data fit a unique curve. The t e m p e r a t u r e a t which t h e loss o f surface a r e a s t a r t s is independent of t h e nature of the clay, but
4
3
2
20
1
s e e m s directly connected with t h e number of pillars. A
higher
density
of
pillars
increases t h e stability
of t h e structure. Fig. 1. (001) line of t h e CLS a f t e r calcination a t different temperatures.
357
Fig. 2. Variations of surface area as a function of the calcination temperature. CLS prepared from sample G.
5
Fig. 3. Relative variation of surface area of the various CLS prepared from different clays.
358 A c i d i t y o f t h e cross-linked smectites The acidity o f t h e samples was studied by i n f r a r e d spectroscopy using pyridine as probe molecule. Figure 4 shows t h e spectra obtained on samples outgassed in 3 vacuum (10- ) a t 200OC (Figure 4a) and 500OC (Fig. 4b) a f t e r pyridine adsorption and subsequent evacuation at room temperature. A f t e r degassing
at 200°C,
the intercalated smectite contained both Lewis and 1 Bronsted sites characterized by absorption bands at 1450 c m - I (LPy) and 1545 c m (BPy) (Fig. 4a). The peak a t 1490 cm-’
is due t o both L P y and BPy. When the same
w a f e r was evacuated at 50OOC and a f u r t h e r dose o f pyridine introduced the BPy band a t 1545 cm-’
was n o longer detected whereas the intensity o f t h e L P y one,
a t 1450 c m - I , was not modified. This result shows, in agreement w i t h previous studies (7,8),
t h a t under the conditions o f t h e measurements ( i n vacuum) t h e acid sites in
t h e C L S heated a t high temperatures are mostly o f t h e L e w i s type. Two explanations may b e put forward t o account f o r t h e disappearance o f t h e Bronsted acidity between 200°C and 5OO0C. A dehydroxylation of t h e surface, similar t o t h a t occuring in zeolites for example, which would t r a n s f o r m Bronsted centers i n t o L e w i s sites. It should be noted however t h a t i f t h i s transformation occurred, it would not induce appreciable changes in t h e intensity o f t h e L P y band. The other explanation could be a m i g r a t i o n o f t h e protons.
It has been shown
indeed
(9,lO)
t h a t during the calcination o f
H-montmorillonites the protons m i g r a t e f r o m t h e interlayer space i n t o the octahedral layer. These would not be accessible t o t h e pyridine molecule because of steric hindrance.
In order t o evaluate the strength o f t h e acidic sites, the intensity o f t h e pyridine bands was determined a f t e r evacuation a t increasing temperatures. F o r this purpose a cross-linked s m e c t i t e previously mZ.g-’)
calcined in dry a i r a t 6 8 0 T (surface area 260
was rehydrated and finally outgassed in vacuum a t 500OC. U p t o a desorption
temperature
o f 4 0 0 T (Fig.
5) strong L.Py
also detected in the region 1560-1550 cm-’ t e d acidity.
peaks were observed.
A shoulder was
which could be indicative o f some Brons-
A t 50OOC this l a t t e r band disappeared,
and t h e L P y band diminished
in intensity but was not t o t a l l y eliminated. It is interesting t o compare t h e acid strength o f t h e intercalated clays w i t h t h a t o f Y zeolites, which has been thoroughly investigated
using t h e same procedure ( 1 1).
On Y
zeolites,
t h e disappearance o f
t h e pyridine bands in t h e IR spectrum is generally observed a f t e r evacuation around 350-4OO0C. The L e w i s acidity o f these cross linked smectites is therefore comparable,
or even stronger, than t h a t of conventional zeolitic catalysts. In conclusion, the preparation of cross-linked s m e c t i t e yields solids w i t h reasonable t h e r m a l stability. The loss of surface area i n t h e temperature range 50O-70O0C seems t o be independent o f t h e chemical composition o f t h e clay and seems more related t o t h e density o f t h e pillars. This decrease in surface area corresponds t o the collapse
369
Fig. 4. IR spectra of pyridine adsorbed on AI-CLS previously heated a t 20OoC ( a ) or 5 0 0 T (b).
Fig. 5. Desorption of pyridine from a sample previously calcined a t 6 8 D T in air. Temperat u r e o f desorption : a) 2OO0C, b) 4OO0C, c ) 5OO0C.
lh
I
1600
I
1500
do0
wavenumber (c m-'1
360 o f a fraction of the micropores. The progressive degradation o f the structure has some similarities with the collapse of the clay : intercalation simply displaces the phenomenon towards higher temperatu-
res. Above 800°C, an irreversible chemical transformation of the layers takes place, and this temperature i s probably the l i m i t for thermal stability. These materials exhibit a high acidity, mainly of the Lewis type and the acidic strength o f the sites i s comparable t o that o f Y zeolithes.
I t i s then possible t o
imagine that these intercalated clays may be useful as catalysts for the cracking o f large molecules.
REFERENCES
1
N. Lahav, V. Shani and J. Shabtai, Clays and clay mineral, vol. 26, no2, pp. 107-1 15,
1978.
2
D.E.W. Vaughan, R.J. Lussier, J.S. Magee, US. Patent, 4, 176, 090. 3 J. Shabtai, R. Lazar and A.G. Oblad, 7th International Congress on Catalysis, Tokyo 1980 paper 68. 4 J. Shabtai and N. Lahari, U.S. Patent, 4, 216, 188. 5 R.J. Lussier, J.S. Magee and D.E.W. Vaughan, 7th Canadian Symposium on Catalysis, 1979, p. 88. 6 D.E.W. Vaughan, R.J. Lussier, J.S. Magee, U.S. Patent, 4, 248, 739. 7 M.L. Ocelli and R.M. Tindwa, Clays and clay minerals, 1983, 31 (I), pp. 22-28. 8 J. Shabtai, F.E. Massoth, M. Tokarz, G.M. Tsai and 3. Mc Cauley, 8th International Congress on Catalysis, Berlin 1984, vol. IV, p. 735. 9 J.D. Russel and A.R. Fraser, Clays and clay minerals, 1971, 13, pp. 55-59. 10 S. Yariv and L. Heller-Kallai, Clays and clay minerals, 1973, 21, pp. 199-200. 11 For a review see : J.W. Ward, Zeolite chemistry and catalysis, chap. 3, A.C.S. Monograph 171, J.A. Rabo, Ed., 1976.
361
B. Imelik et ul. (Editors), Cufulysis by Acids and Buses 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
MECHANISMS OF THE A C I D - C A T A L Y Z E D ISOMERIZATION OF PARAFFINS
F. F A J U L A Laboratoire de C h i m i e Organique Physique e t Cinetique Chimique Appliquees U A 418 E.N.S.C.M.,
8, rue de I'Ecole Normale - 34075 Montpellier Cedex
-
France.
ABSTRACT Most o f t h e r e a c t i o n mechanisms involved in the skeletal isomerization o f alkanes over acidic catalysts are w e l l described using t h e general concepts o f t h e chemistry o f carbocations. The commonest pathways include intramolecular branching and non branching rearrangements and multirnolecular processes. Results obtained during the isomerization o f labelled molecules over solid catalysts are used t o illustrate these mechanisms.
RESUME L a plupart des mecanismes reactionnels mis e n jeu lors de I'isomerisation squelettale des alcanes sur des catalyseurs acides sont bien expliquks a p a r t i r des concepts de l a chimie des carbocations. Ces rnkcanismes peuvent 6 t r e monomoleculaires comme les migrations d'alcoyle et les reactions d'allongement et de raccourcissement de c h a h e ou multimolkculaires. Ces differents processus sont illustr6s i c i a p a r t i r de resultats obtenus dans I'isomkrisation de molecules marquees sur des catalyseurs solides.
INTRODUCTION Since t h e early work o f Pines and coworkers (refs.l,Z) in the fourties, which established that the acid-catalyzed isomerization o f paraffins proceeds via a chain-carrying carbenium-ion (ref.3)
intermediate,
a large number o f fundamental studies on the
mechanism o f this r e a c t i o n have been performed w i t h homogeneous and heterogeneous systems. The three basic steps o f t h e process are I) t h e activation o f t h e alkane t o produce a carbocation, iii)
ii)
t h e skeletal rearrangement o f the carbocation intermediate and
an intermolecular hydride transfer
which generates a new carbenium i o n and
t h e isomerized molecule. The source
of t h e i n i t i a l carbocations is controversial.
The easiness o f olefins
in forming carbenium ions by simple protonation l e d t o t h e hypothesis t h a t t r a c e amounts of olefins introduced as impurities ( r e f s . 2 , 4 ) or formed i n situ by thermal cracking or dehydrogenation of t h e p a r a f f i n (refs.5,6)
could act as initiators. A second
view is t h a t carbocations could be generated by a direct a t t a c k of t h e p a r a f f i n by an acidic site in three different ways : an hydride abstraction by a Lewis site, a carbonhydrogen bond a t t a c k by the proton o f a Brdnsted site, and a carbon-carbon bond
362 a t t a c k by a Bronsted site (protolysis). Although had been suggested as soon as 1946 (ref.1)
direct
activation
of
the
paraffin
t h e occurence o f protolysis reactions
has been d e f i n i t i v e l y proved i n a l i m i t e d number of cases, essentially in low nucleophil i c superacid medium (ref.7).
In most studies, t h e answer is not clear, particularly
w i t h solid acid catalysts on which both L e w i s and Bronsted sites may coexist. The N M R studies of stable carbocations in superacidic solutions and the use o f labelled molecules have contributed most t o t h e understanding of the mechanisms
o f t h e rearrangement
of
t h e alkylcarbenium
ions. The f i r s t
technique allows the
d i r e c t observation and the accurate measurement o f the kinetics of rapid hydride agd a l k y l shifts and eventually t o determine t h e non classical nature o f some intermediates whereas the second permits t o differenciate between mono and multimolecul a r processes and t o l i f t t h e degeneracy of apparently undistinguishable pathways. This a r t i c l e presents a b r i e f summary of t h e mechanisms involved during t h e skelet a l rearrangement step. D a t a obtained w i t h labelled alkanes on solid acid catalysts under k i n e t i c control are used t o i l l u s t r a t e these mechanisms.
THE B R A N C H I N G A N D N O N B R A N C H I N G REARRANGEMENTS OF ALKANES The kinetics o f t h e isomerization of 2-rnethylpentane (2MP) in HF-SbF5 at O°C have been studied by Brouwer e t r e a c t i o n was
&
(ref.8)
using N M R spectroscopy. The fastest
t h e mterconversion between 2-methyl and 3-methyl-pentanes
(3MP).
This r e a c t i o n is exemplary o f t h e so-called type A rearrangements between t e r t i a r y cations which do not change t h e degree o f chain branching. Type A rearrangements were found t o have a r a t e constant and activation energy o f t h e order o f k,
= 10 s-’
( a t OOC) and E A = 14-15 kcal/mole. The next fastest reaction was t h e isornerization o f methylpentanes i n t o 2,3
dimethylbutane (2,3DMB).
This is a type B rearrangement
in which t h e degree o f chain branching changes. I t s a c t i v a t i o n energy was i n the range 17-1 8 kcal/mole. Lastly, a t higher conversion, n-hexane (nH) and 2,2 dimethylbutane (2,ZDMB)
are produced in approximately their thermodynamic equilibrium. This
sequence of events is common t o a l l acidic systems. A
more complete prcture of the process
is
obtained when labelled hexanes are
used as reactants. The isomerization o f
I3C
labelled methylpentanes and 2,3
dimethylbutanes has
been investigated at low converslon levels on H mordenite at 1 7 0 T (ref.9) and SbF5 intercalated i n t o graphite (SbF5/CgS5) at - 3 O T t o +20°C (ref.10). isomers and t h e (apparently) unreacted feeds,
13 content.
A l l t h e products,
have been analyzed f o r their carbon
363 TABLE 1 Nonbranching rearrangement of methylpentanes studied w i t h
3 C labelled molecules.
D i s t r i b u t i o n of t h e isotopic isomers.
Total Conversion
E n t r y Reactant
2-methylpent anes Total
%
n/'
L a
2.2
97.8
0.6
2
h a
0.75
99.25
0.2
,
yb
/Lh
1
4 rr t R.
3-methylpentanes Total
,t\ jA
%
96.4
3.6
1.57 0.62
T 'Y
6.2
71.8
22
1.6
98.2
2.5
78.8
18.7
0.9
0.77
2
68.3
29.7
99.1
0.8
5.5
93.7
4.2
4.19
-
85.6
14.4
95.8
-
10.6
89.4
Catalysts : a: H-mordenite, 170°C; b: SbF5/Cge5, -3OOC.
In the reactions o f ZMP, t h e most abundant species formed was an isotopic isomer (2-methylpentane-4-I 3 C
and
2-methylpentane-2-I 3c
runs
in
1 and
2 respectively)
of t h e starting 2-methylpentane resulting f r o m a 1,3 m e t h y l shift (1,3MS). 1,3 m e t h y l s h i f t
Similarly,
in t h e reactions o f 3MP, t h e major reaction product was an isotopic
variety of 3-methylpentane resulting f r o m a 1,2 e t h y l shift (1,2 ES).
r"=?
1,2 e t h y l shift
The occurence o f direct 1,3 m e t h y l shifts and 1,2 e t h y l shifts i s not l i m i t e d t o t h e above systems. D i r e c t 1,3MS have been observed during t h e dehydration o f 4,4 dimethyl-3
pentanol (ref.1 I), t h e reactions o f 2-cyclopropylcarbiny1 cation 2-CL -H-70-aminopinane (ref.13) w i t h nitrous acid and t h e isomerization
ethyl-2
(ref.12) and of
o f m e t h y l pentenes over supported p-toluenesulfonic has been noticed i n superacid solutions by Olah et ions using spin saturation transfer and by Brouwer e t
acid (ref.14).
&
(ref.15)
The ethyl shift
in t h e methylpentyl
(ref.7b) studying t h e 3-ethyl-
pentyl-2-methylhexyl cations equilibrium, and is also described in ref.14. The r e l a t i v e rates of t h e three nonbranching rearrangements o f the methylpentanes
on H-mordenite were estimated (ref.9) by p l o t t i n g t h e 1,3MS and 1,2ES/1,2MS r a t i o s versus isomerization conversion (Fig. 1). The rates of t h e m i g r a t i o n can be classified in t h e sequence 1,ZES > 1,3MS > 1,2MS w i t h values of 10,4 and 1 respectively. The conclusion that the 1,2ES IS t h e fastest reaction in t h e rearrangement
o f t h e branched hexanes has been also reached on
supported (ref.10) and liquid (ref.16) SbF5.
364 10
-
A
t
r1:1.2ES/1.2MS r2: 1,3MS 11.2MS
8 A
0’
1
2
3
4
5
% lsornerization
Fig. 1. Variations of the ratios 1,2ES/1,2MS zation conversion on H-mordenite.
and 1,3MS/1,2MS
as a function of isomeri-
The nonbranching rearrangements of tertiary alkylcarbenium ions can be visualized t o proceed simply by a succession of hydride and alkyl shifts via secondary cations as unstable intermediates. The transition state of the alkyl migration i s represented by a 1.n-C carbonium ion ( w h e r e n = 2 for a 1,2 shift and 3 f o r a 1,3 shift) formed by transfer
of one electron o f
the sigma C-C
bond bearing the migrating group
into the empty p orbital of the cation (Fig. 2).
Fig. 2. The three nonbranching rearrangements o f the hexyl cations.
365 Since the slow step o f these rearrangements i s t h e a l k y l shift, t h e differences observed between t h e rates of t h e three migrations have been t e n t a t i v e l y assigned t o t h e greater symetry o f t h e transition states I1 and I11 as compared t o I (refs.10,16). Typical distributions
of the 2,3
dimethylbutanes produced f r o m 2-methylpentane
on SbF5/CgS5 are given in Table 2. TABLE 2 Branching
rearrangement
of
labelled
2-methylpentanes
on SbF5/Cgq5. Distribution
o f t h e 2,3 dimethylbutanes.
Entry
Reaction temperature OC
Reactant
Total conversion
Total %
&
5.4
0.2
2.3
97.7
8.7
0.33
1.o
99.0
3.7
98.1
1.9
33.8
66.2
(Oh)
AA/-
2
-
3
- 17
AP
25.8
4
+ 20
A-
42
1
30 30
2,3 dimethylbutanes
21
The f a c t t h a t t h e Z-rnethylper1tanes-2-’~C and -4-
13 C l e d t o t h e quasi exclusive
f o r m a t i o n o f 2,3 dimethylbutane-2-I 3C showed conclusively t h a t t h e branching rearrangement isomerizing 2MP i n t o 2,3DMB
corresponded t o a f o r m a l 1,2 isopropyl shift.
1,2 isopropyl shift
A t +20°C t h e results were in agreement w i t h an isopropyl shift taking place on a m i x t u r e o f ZMP, previously isotopically scrambled on the internal positions through l a p i d m e t h y l and e t h y l shifts :
The proposed pathway of this rearrangement is schematically shown below :
366 It involves a series of interconversions between edge and corner protonated cyclo-
propanes and avoids the intermediate participation of p r i m a r y carbenium ions. The concept
o f protonated cyclopropane has been introduced following Roberts'
demonstration of t h e 1,3 hydride shift in deamination reactions i n acid solution (ref.17) and
IS
widely used t o r a t i o n a l i z e t h e branchlng rearrangements. The m a i n argument
i n f a v o r o f this mechanism is t h e so called b u t y l (and propyl) rearrangement. I n e f f e c t , n-butane does not at a l l isomerize t o i-butane under conditions where
n-pent am and n-hexane react rapidly but
n-but anel-' 'C
i s isomerized i n t o n-buta-
n e - 2 - I 3 C a t about the same r a t e as n-pentane t o isopentane. These facts are f u l l y consistent w i t h the mechanism shown in Figure 3.
R
H Fig. 3. The b u t y l rearrangement.
The large difference in r a t e o f
isornerization between n-butane and n-pentane
is due t o the branched cation f o r m e d upon r i n g opening : i t would be p r i m a r y ( R = H)
in t h e case o f butane and secondary (R = C H ) i n t h e case of pentane. By contrast 3 t h e carbon scrambling in t h e n-butane interconverts secondary carbenium ions. Essentially
the same r e a c t i o n accounts
for
the exchange
of
carbons between internal
and external positions in t h e hexyl (entries 1-3, Table 1) and propyl (ref.15) cations.
The c e n t r a l question about this mechanism is whether non classical species such as protonated cyclopropanes or
bridged carbonium ions are only transition states
in rearrangements or whether they can be intermediates in some or a l l cases. B o t h views have t h e i r supporters. o p t i m i z a t i o n on t h e C3H;
Ab i n i t i o molecular o r b i t a l calculations w i t h geometry cations by Radom et
&
(ref.18)
show that,
though t h e
Fig. 4. Representations of t h e stable f o r m of t h e I - p r o p y l cation ( I V ) and o f t h e trans i t i o n states of t h e 1,2 m e t h y l shift ( V ) and 1,3 hydride shift ( V I ) (ref.18).
367 energy differences between various structures
-
edge and corner protonated cyclopro-
panes and primary carbenium ions - are r e l a t i v e l y small, a d e f i n i t e p o t e n t i a l minimum is found f o r the m e t h y l eclipsed f o r m o f t h e I - p r o p y l carbenium i o n ( I V in Fig. 4) i n which a p a r t i a l bonding exists between C1 and C 3 and also C1 ar.d H7 (represent e d by dotted lines in t h e Figure). This cation may rearrange by 1,2 m e t h y l shift or 1,3 hydride shift through alkylbridged ( V ) or edge-protonated cyclopropane ( V I ) transition states and could be therefore considered as an intermediate in reactions involving equilibration o f protonated cyclopropanes.
On t h e other
hand theoretical (ref.19)
and spectroscopic (ref.15)
on t h e 2-norbornyl c a t i o n proved the a l k y l bridged, non classical,
work
f o r m t o be more
stable than the classical, tricoordinated, one. It is l i k e l y therefore t h a t a given struct u r e w i l l change f r o m an unstable transition state t o a relatively stable intermediate depending on the number and positions of i t s alkyl substituents.
M U L T I M O L E C U L A R PROCESSES We have discussed so f a r t h e mechanism o f t h e acid catalyzed skeletal isomeriza-
t i o n o f paraffins in t e r m s o f intramolecular rearrangements o f alkylcarbenium ions. Another r o u t e t o isomerization is a multimolecular pathway. The isomerization o f n-pentane-3-I 3C has been investigated by B o l t o n e t and Daage (ref.21)
4 (ref.20)
using zeolites as catalysts. In both cases t h e isopentane product
contained multilabelled species indicating t h e participation o f a multimolecular process. The f i r s t step o f t h i s mechanism is t h e alkylation o f an o l e f i n b y a carbenium i o n t o f o r m an adsorbed complex on the surface. c r a c k by
This complex m s y rearrange and
6 -fission leading t o a more thermodynamically favoured cation - a t - a m y l ca-
t i o n in this case. The isomer is then desorbed by an intermolecular hydrogen transfer. Since
the
protonation-deprotonation equilibirum
of
carbenium
ions
is strongly
dependent on the acidity o f t h e media the contribution of t h e multimolecular process i n isomerization is mainly determined by the acid strength o f t h e catalyst.
In strong acids the equilibrium i s displaced t o the left. The concentration o f o l e f i n
w i l l be l o w and t h e intramolecular mechanism w i l l prevail.
The contrary w i l l be
t r u e in weaker acid. The structure of t h e reactant may also play a role. Thus, on H-mordenite
isohexanes
are
isomerized
essentially
via
a
rnonomolecular
whereas a multimolecular mechanism is favoured w i t h pentanes (Table 3).
process
368 TABLE 3 Intramolecular versus multirnolecular pathways in t h e isomerization o f labelled pentanes and hexanes. Influence of t h e catalyst and o f t h e reactant. i
5 3M 4-
Catalyst
%
X;
Isomerization pathway intramolecular
multirnolecular
Karabatsos e t a1 (22)
AlX3, O°C
JJ.
9
91
B o l t o n et a1 (20)
Y zeolite, 3 2 5 T
M
66
34
Daage (21)
H-rnordenite, 260°C
M
42
58
Daage e t a1 ( 9 )
H-mordenite, 17OOC
96
4
k
> 98
Lenormand et a1 (10) SbF5/CgS5, - 3 O T
2<
OTHER P A T H W A Y S The l i t t e r a t u r e provides some examples where experimental data cannot be explained on t h e basis of simple carbenium ion chemistry.
In t h e
isomerization o f
acid (ref.14)
2 - m e t h y l h e ~ e n e - Z - ' ~ C over
supported p-toluenesulfonic 13 C are obtained. To
equal amounts of 3-1nethylhexenes-3-~~C and 5-
rationalize this distribution a mechanism involving edge and corner protonated cyclopentanes in equilibrium w i t h 1-4-C methonium ions has been postulated.
A
cyclopentyl intermediate has also been proposed by Bolton et
fi
(ref.20)
to
explain the loss o f uniqueness o f t h e carbon a t o m during the intramolecular isomerizat i o n of n-pentane t o isopentane. Similarly cyclobutanic (ref.14)
and cyclohexanic (ref.23)
intermediates have been
put f o r w a r d during t h e squeletal isomerization o f hexenes and hexanes.
In these examples, a particular However,
the appearance o f abnormal rearrangements i s a t t r i b u t e d t o
distribution of
protonation of
Olah and Lukas (ref.24)
t h e acid centers o f t h e heterogeneous systems used.
cyclopentane i n superacid solution has been observed by resulting In r i n g cleavage and f o r m a t i o n o f a t e r - a m y l ion.
This reaction could be obviously explained by t h e intervention o f protonated cyclopentane.
369 REFERENCES
1 H.S. Bloch, H. Pines and L. Schmerling, J. Am. Chern. SOC., 67 (1945) 914, 6 8 (1946) 153. 2 H. Pines and R.C. Wackher, J. Am. Chern. SOC., 68 (1946) 595, 1642, 2518. 3 I n this paper we shall follow Olah's notation for the naming of carbocations (J. Am. Chem. SOC., 9 4 (1972) 808). Carbeniurn ions will refer t o planar tricoordinated carbocations whereas carboniurn ions t o penta or tetracoordinated ones. 4 D.A. Mc Caulay, J. Am. Chem. SOC., 81 (1959) 6437. 5 A. Brenner and P.H. Emmett, J. Catal., 75 (1982) 410. 6 D.M. Anufriev, P.N. Kuznetsov and K.G. Ione, J. Catal., 6 5 (1980) 221. 7 For a review : a) G.A. Olah, S. Prakash and J. Sommer, Science, 206 (1979) 13. b) D.M. Brouwer and H. Hogeveen, Prog. Phys. Chem., 9 (1972) 179. 8 a) D.M. Brouwer, Rec. Trav. Chim. 8 7 (1968) 210. b) D.M. Brouwer and J.M. Oelderik, Rec. Trav. Chim. 8 7 (1968) 721. 9 M. Daage and F. Fajula, J. Catal., 81 (1983) 405. 1 0 F. L e Normand, F. Fajula, F.G. Gault and J. Sommer, Nouv. J. Chim., 6 (1982) 417. 11 W.A. Mosher and J.C. Cox, Jr, J. Am. Chern. SOC., 72 (1950) 3701. 1 2 G.E. Cartieve and S.C. Bunce, J. Am. Chem. SOC., 8 5 (1963) 932. 1 3 J. Meikle and D. Whittaker, J. Chem. SOC., Perkin Trans. 2, (1974) 318. 1 4 F. Fajula and F.G. Gault, J. Am. Chem. SOC., 98 (1976) 7690 15 G.A. Olah and A.M. White, J. Am. Chem. SOC., 91 (1969) 5801. 1 6 R. Jost, K. Laali and J. Sommer, Nouv. J. Chim., 7 (1983) 79. 1 7 J.D. Roberts and M. Halmann, J. Am. Chern. SOC., 7 5 (1953) 5729. 1 8 L. Radom, J.A. Paple, V. Buss and P.v-R Schleyer, J. Am. Chem. SOC., 94 (1972) 31 1. 1 9 G. Klopman, J. Am. Chern. SOC., 91 (1969) 89. 20 A.P. Bolton, I.R. Ladd and T.J. Weeks Jr., Proc. Sixth Int. Congress Catalysis, London 1976, G.C. Bond, P.B. Wells, F.C. Tompkins, Eds, Vol. l ( 1 9 7 7 ) 316. 21 M. Daage, These SpBcialitB, Strasbourg, 1977. 22 G.J. Karabatsos, F.M. Vane and S. Meyerson, J. Am. Chem. SOC., 8 5 (1963) 729. 23 A.P. Bolton and M.A. Lanewala, J. Catal., 1 8 (1970) 1. 24 G.A. Olah and J. Lukas, J. Am. Chem. SOC., 9 0 (1968) 933.
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371
B. Imelik et al. (Editors), Catalysis b y Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
A C I D I C CATALYSIS AND RADICAL ASSISTANCE D. BRUNEL, H. CHOUKROUN, A. GERMAIN and A. COMMEYRAS
555 CNRS, U n i v e r s i t e des Sciences e t Techniques du Languedoc
E.R.A. 34060
-
MONTPELLIER CEDEX
(FRANCE)
ABSTRACT Evidences a r e p r e s e n t e d about a s y n e r g e t i c e f f e c t when r a d i c a l a s s i s t a n c e i s I t i s demonstrated t h a t a s s o c i a t e d t o t h e 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 alkanes r a d i c a l s i n i t i a t e c a t i o n i c c h a i n mechanisms which a r e d i f f e r e n t f r o m t h o s e which o c c u r i n p u r e s u p e r a c i d s . The r o l e o f the acid s t r e n g t h o n t h e e f f i c i e n c y o f t h e r a d i c a l a s s i s t a n c e i s discussed. F u r t h e r e x t e n s i o n o f t h e concept o f synergy of t h e r a d i c a l and a c i d a c t i o n s t o o r g a n i c s y n t h e s i s and c a t a l y s i s i s considered.
.
RESUME En u t i l i s a n t comme exemple l a c o n v e r s i o n des alcanes, nous demontrons q u ' u n e f f e t de s y n e r g i e p e u t se p r o d u i r e e n t r e l a c a t a l y s e s u p e r a c i d e e t l ' a c t i v a t i o n radicala i re A i n s i , 1 ' i s o m e r i s a t i o n du n-butane, q u i e s t d i f f i c i l e en presence des a c i d e s perfluoroalcanesulfoniques (RFSOQH) s e u l s , e s t a c t i v e e o a r o x y d a t i o n anodique ou p a r a d d i t i o n de r a d i c a u x N H 2 . Dans l e s m6mes c o n d i t i o n s , c o n t r a i r e m e n t 8 ce q u i se p r o d u i t en m i l i e u p l u s a c i d e , l ' i s o b u t a n e n ' e s t pas i s o m e r i s 6 en n-butane mais t r a n s f o r m e en a l c a n e s p l u s l e g e r s e t p l u s l o u r d s . Nous demontrons que l ' a s s i s t a n c e r a d i c a l a i r e i n i t i e , p a r d i m e r i s a t i o n ou d i s m u t a t i o n de r a d i c a u x a l k y l e s , un mecanisme i o n i q u e en c h a i n e d i f f e r e n t de c e l u i provoque uniquement p a r l e s superacides. L ' e t u d e de 1 ' e f f e t de 1 ' a d d i t i o n d ' i s o b u t e n e e t de t r i m e t h y l - 2 , 2 , 4 pentane s u r l ' i s o m e r i s a t i o n du n-butane, a i n s i que l ' e t u d e de l a r e a c t i v i t e du t r i m e t h y l 2,2,4 pentane e t du t&tram@thyl-2,2,3,3 butane dans l e s a c i d e s R F S O ~ H p u r s p e r m e t t e n t de p r e c i s e r l e processus c a t a l y t i q u e . En c e q u i concerne l ' i s o m e r i s a t i o n du butane, il s ' a g i t du c y c l e s u i v a n t : a l k y l a t i o n d ' i s o b u t e n e p a r un c a t i o n b u t y l e , 6 - s c i s s i o n en c a t i o n b u t y l e t e r t i a i r e e t i s o b u t e n e q u i e s t a i n s i r e g & n&r&. Ce mecanisme ne f a i t i n t e r v e n i r que des c a r b o c a t i o n s secondaires e t t e r t i a i r e s . I 1 e s t energetiquement p l u s f a v o r a b l e que l e mecanisme d ' i s o m e r i s a t i o n h a b i t u e l l e m e n t observe en m i l i e u s u p e r a c i d e q u i , dans l e cas du butane, d o i t necessairement Dasser p a r un c a t i o n i s o b u t y l e D r i m a i r e . L ' g t u d e du neopentane permet de a r e c i s e r l e mode d ' a c t i o n de l ' o x y d a t i o n anod i q u e e t de l ' a d d i t i o n de r a d i c a u x L ' i n f l u e n c e de 1 ' a c i d i t e s u r 1 ' e f f i c a c i t e de 1 ' a c t i o n r a d i c a l a i r e e s t d i s c u t e e . Les a c i d e s perfluoroalcanesulfoniques s ' a v e r e n t P t r e des m i l i e u x convenables pour a s s o c i e r l a c a t a l y s e a c i d e e t l ' a s s i s t a n c e r a d i c a l a i r e . L e developpement du concept de " s y n e r g i e des a c t i v a t i o n s a c i d e e t r a d i c a l a i r e en c a t a l y s e e t synt h e s e organique" e s t envisage.
.
.
372
I n o r g a n i c synthesis and c a t a l y s i s , r a d i c a l c h e m i s t r y i s u s u a l l y opposed t o i o n i c chemistry
. This
o p p o s i t i o n o r i g i n a t e s from t h e f a c t t h a t t h e b e s t media
f o r i o n i c r e a c t i o n s a r e p o l a r whereas r a d i c a l r e a c t i o n s a r e m a i n l y r u n i n t h e vapor phase o r non p o l a r media. Now, t h i s d i f f e r e n c e i s n o t so obvious. I o n i c r e a c t i o n s can be s t u d i e d i n vapor phase, f o r i n s t a n c e :
-
-
h y d r i d e t r a n s f e r between gaseous i o n s [l]
m e r i z a t i o n o r a l k y l a t i o n o f carbenium i o n s [2,3].
iso-
On t h e o t h e r hand, r a d i c a l
r e a c t i o n s can be c a r r i e d o u t i n a c i d i c s o l u t i o n , f o r i n s t a n c e : hydrogen atom a b s t r a c t i o n by hydroxyl r a d i c a l [4] o r aminyl r a d i c a l [5]. It was i n t e r e s t i n g t o i n v e s t i g a t e whether t h e a s s o c i a t i o n o f r a d i c a l and a c i -
d i c systems leads t o s y n e r g e t i c e f f e c t s l i k e increases i n r e a c t i v i t y , changes i n mechanism o r new r e a c t i o n s . Such e f f e c t s can be s t u d i e d by two p o s s i b l e ways : e i t h e r a d d i t i o n o f r a d i c a l s i n c a t i o n i c processes, o r use o f a c i d i c media f o r radical reactions. We s h a l l develop t h e f i r s t p o i n t i n t h i s paper. The i o n i c r e a c t i o n t h a t we chose i s t h e i s o m e r i z a t i o n o f alkanes t h a t we have a l r e a d y s t u d i e d i n l i q u i d s u p e r a c i d i c media [ 6 ] . F i r s t o f a l l , l e t us r e c a l l what a superacid i s . Superacids a r e g e n e r a l l y made by d i s s o l v i n g s t r o n g Lewis a c i d s such as SbF5, BF3, AsF5 o r TaF5 i n s t r o n g Bronsted a c i d s such as hydrogen f l u o r i d e , f l u o r o s u l f u r i c a c i d o r 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 . According t o G i l l e s p i e
[71,
t h e a c i d i t y o f superacid i s h i g h e r
than s u l f u r i c a c i d , whose Hammett a c i d i t y f u n c t i o n i s -11.9. One o f t h e most i m p o r t a n t a p p l i c a t i o n s o f superacids i s t h e s t a b i l i z a t i o n o f o r g a n i c c a t i o n s and t h e i n v e s t i g a t i o n o f t h e i r physicochemical p r o p e r t i e s . T h i s f a c t allowed the d e t e r m i n a t i o n o f the s t r u c t u r e o f many carbocations [ 8 ] .
The
c a r a c t e r i z a t i o n o f " n o n - c l a s s i c a l " c a r b o c a t i o n i c s t r u c t u r e s o f which 2-Norbornyl c a t i o n i s t h e most remarkable example can be p a r t i c u l a r l y n o t i c e d . Although t h e concept o f "non c l a s s i c a l " c a r b o c a t i o n i s s t i l l a cause o f debate [9],
it i s
u s e f u l f o r t h e d e s c r i p t i o n o f numerous mechanisms. Another i n t e r e s t o f s u p e r a c i d i c media i s t h e i r use as a c a t a l y s t f o r t h e conversion o f alkanes ( i s o m e r i z a t i o n , a l k y l a t i o n , c r a c k i n g ) . I n t h e case of t h e i s o m e r i z a t i o n , t h e well-known c a t a l y t i c mechanism i s t h e f o l l o w i n g : initiation n-RH
+ H+
sec-R+
+
H2
isomerization sec-R+
tert-Rt
propagation tert-Rt
+
n-RH
i-RH
+
sec-R
t
373
The i s o m e r i z a t i o n s t e p r e q u i r e s a p r o t o n a t e d cyclopropane r i n g i n t e r m e d i a t e hydrogen s h i f t
t h a t g e n e r a l l y l e a d s t o a secondary branched c a t i o n . Then, a gives the t e r t i a r y cation.
The case of butane i s p e c u l i a r because t h e o n l y branched c a t i o n t h a t can be o b t a i n e d by c y c l o p r o p a n e r i n g opening, i s p r i m a r y , as i t i s shown i n t h e f o l 1owing scheme.
+ y 2
+
tCH3-CHz-CH-CH3
CH3-CH-CH3
,=
hydrogen shift *
+
11
CH3-CH2-CH-CH3
CH3-$-CH3
'C-scrambl i n g
isomeriz a t i o n
Obviously, such a process i s e n e r g e t i c a l l y u n f a v o u r a b l e , t h e r e f o r e t h e a c i d i c i s o m e r i z a t i o n o f n-butane i s more d i f f i c u l t and s l o w e r t h a n t h a t o f o t h e r alkanes. So, i t may be more s e n s i t i v e t o an a d d i t i o n a l a c t i v a t i o n . Thus, we chose t h e i s o m e r i z a t i o n o f n-butane t o t e s t 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 d u r i n g an i o n i c r e a c t i o n .
In o r d e r t o s t u d y t h i s problem, we needed some media c o m p a t i b l e w i t h t h e s u b s t r a t e s and r a d i c a l s , b u t a c i d i c enough t o f o r m c a t i o n s . The 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 o f f e r t h e expected
required
T h e i r h i g h e r a c i d i t y 1103 when
compared t o s u l f u r i c a c i d a l l o w e d us t o c l a s s i f y them as " s u p e r a c i d s " . Moreover, contrary
to
f l u o r o s u l f u r i c a c i d , t h e y do n o t possess any s u l f a t i n g power
.
T a b l e 1 shows t h e i n i t i a l r a t e s o f i s o m e r i z a t i o n o f n-butane and n-pentane by v a r i o u s 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 . Table 1 :
I n i t i a l r a t e s o f i s o m e r i z a t i o n o f n-alkanes.
Acid
v. 104 secT1 n-butane
H,
n -oe nt a n e
CqFgS03H
-13.2
0
C2F5S03H
-14.0
0.8
5.6
CFQSOQH
-14.1
2.2
7.5
< -18.4
2.8
CF3S03H-SbF5 (1-0.5)
e
= 25°C ; n-alkane:
20 cm3;
a c i d : 7.5 cm3
.
374
On t h e one hand, was foreseen.
obviously
butane i s o m e r i z e s f a s t e r t h a n pentane as i t
On t h e o t h e r hand, t h e r a t e i s enhanced when t h e a c i d i t y i n c r e a s e s
.
which i s i n agreement w i t h a c a r b o c a t i o n i c mechanism
Concerning t h e r a d i c a l a c t i v a t i o n , t h e f i r s t method t h a t we used was electrolysis. I t i s a l r e a d y known t h a t anodic o x i d a t i o n o f alkanes i n f l u o r o s u l f u r i c a c i d
i s c a r r i e d o u t a c c o r d i n g t o an E.C.E. mechanism [ l l ] :
- --e
RH
't
-e
-H+
R'
RH
E
C
9'
E
The f i r s t s t e p i s an e l e c t r o n i c t r a n s f e r f r o m t h e s u b s t r a t e t o t h e anode. Then t h e r a d i c a l c a t i o n pooses a p r o t o n d u r i n g a chemical s t e p . F i n a l l y , a second e l e c t r o n i c t r a n s f e r from t h e r a d i c a l l e a d s t o t h e c a t i o n . N e v e r t h e l e s s , t h e r a d i c a l i n t e r m e d i a t e may r e a c t b e f o r e t h e second e l e c t r o n i c t r a n s f e r
.
F i g u r e 1 shows t h e e f f e c t o f t h e e l e c t r o l y s i s upon t h e i s o m e r i z a t i o n o f n-butane. We observe t h a t t h e anodic o x i d a t i o n g r e a t l y enhances t h e i s o m e r i z a t i o n . D u r i n g t h e e l e c t r o l y s i s , t h r e e t o f i v e moles o f i s o b u t a n e a r e o b t a i n e d f o r one Faraday. So, t h i s e f f e c t i s c a t a l y t i c . We n o t i c e a l s o t h e f o r m a t i o n o f h i g h e r
a1 kanes t h a n butane.
% I
%
'"1
20
15
15
10
10
5
5
10
15
20 h
Fig. 1 : Effect of anodic oxidation (3.6mFJduring the conversion of n-butane(0.17mole) with 25°C. CF3S03H(O.O85mole)
.
5
10
M
15
h
Fie.2: Effect of N H ~ ( ~ O ~ = 3 x l O - % o l e ; TiX3=3xlO-3mole)Jn the conversion of butane(0.17mole)with CF3S03H (0.085mole ) . 25°C
.
0
X=C1 ;
X=CF3S03
.
375
In order t o v e r i f y t h a t r a d i c a l s a r e involved i n t h e e l e c t r o l y s i s , we studied t h e a c t i o n of added r a d i c a l s . T h u s , aminyl r a d i c a l s a r e generated i n -
situ by reduction of hydroxylamine by titanium I11 NH20H
t
Ht
+ Ti3+
MH;
H20
t
t
.
Ti4+
I n a c i d i c media, the aminyl r a d i c a l s a r e protonated without any doubt , which increases t h e i r e l e c t r o p h i l i c i t y b u t does not change their a b i l i t y t o a b s t r a c t a hydrogen atom. The e f f e c t of radical addition upon the isomerization of n-butane i s shown Figure 2.
on
The aminyl r a d i c a l s a r e introduced a t the beginning of t h e
reaction because o f p r a c t i c a l n e c e s s i t i e s . We observe again a c a t a l y t i c e f f e c t because nine moles of isobutane per mole o f inserted r a d i c a l s a r e obtained. Likewise, In'ghet- alkanes than butane a r e produced. So, t h e addition of r a d i c a l s gives t h e same r e s u l t s a s the e l e c t r o oxidation. Alkyl r a d i c a l s may be generated by a hydrogen a b s t r a c t i o n i n a f i r s t step:
NH;
t
CH3-CH2-CH2-CH3
___c
NH3
CH3-CH-CH2-CH
t
3
Two subsequent r e a c t i o n s can be considered :
-
2
2
a dimerization which leads t o a n octane :
CH3-CH-CH2-CH3
-
CH3-CH-CH2-CH3
I
CH3-CH-CH2-CH3
o r a disproportionation which leads t o CH3-CH-CH2-CH3
-
n-butene and s t a r t i n g n-butane :
CH3-CH=CH-CH3 + CH -CH -CH2-CH3 3 2
Both reactions may be a source of t e r t i a r y octyl cation :
- e i t h e r by ionization of t h e octane followed by isomerization of the octyl cation : C8HI8
+-.
H+
4 2
sec-C8H17
t
tert-C8HI7 t
376
- or C4H8
by a l k y l a t i o n o f butene by a b u t y l c a t i o n p r e s e n t i n t h e a c i d i c phase : t
C4Hi
y-
tert-C8H17f
Amongst t h e o c t y l c a t i o n s , t h e d i m e t h y l n e o p e n t y l carbenium i o n i s t h e most f a v o u r a b l e f o r B - s c i s s i o n . T h i s l e a d s t o t e r t - b u t y l c a t i o n and i s o b u t e n e :
A h y d r i d e exchange between t e r t - b u t y l c a t i o n and t h e s t a r t i n g n-butane accounts f o r t h e f o r m a t i o n o f i s o b u t a n e :
An a l k y l a t i o n o f i s o b u t e n e by secondary b u t y l c a t i o n l e a d s t o a new o c t y l c a t i o n t h a t a l l o w s t h e p r o p a g a t i o n o f t h e c a t a l y t i c process :
The f o r m a t i o n of
higher alkanes can be e x p l a i n e d by s i m i l a r p a t t e r n s , f r o m
unsymmetrical f3-scission o f o t h e r t e r t - o c t y l c a t i o n s . F o r i n s t a n c e , t h e m e t h y l e t h y l i s o b u t y l carbenium i o n g i v e s i s o p r o p y l c a t i o n and pentene :
y 3
CH3
I
CH3-CH-CH2-C-CH2-CH3
CH3
\
CH2-CH3
CH+
/
CH3’
+
CH2
=C
/
\
CH3
T h i s B - s c i s s i o n i s l e s s f a v o u r a b l e t h a n t h e f o r m e r owing t o t h e f o r m a t i o n o f a secondary c a t i o n .
The a l k y l a t i o n o f t h e pentene by b u t y l c a t i o n l e a d s t o nonyl c a t i o n whose
371
8 - s c i s s i o n can g i v e p e n t y l c a t i o n and butene. Then, pentane i s o b t a i n e d from t h e p e n t y l c a t i o n b y h y d r i d e t r a n s f e r and t h e c a t a l y s i s i s propagated by o l e f i n .
The absence o f propane can r e s u l t f r o m t h e a l k y l a t i n g power o f t h e p r o p y l cation t h a t i s c e r t a i n l y higher
than i t s a b i l i t y t o a b s t r a c t a
hydride.
I n o r d e r t o t e s t t h e assumption c o n c e r n i n g t h e r o l e o f o c t y l c a t i o n s a n d m t octane and
o f o l e f i n s , we s t u d i e d t h e e f f e c t s o f t h e a d d i t i o n o f d u r i n g t h e i s o m e r i z a t i o n o f n-butane
isobutene
.
The two curves p r e s e n t e d i n f i g u r e s 3 and 4. ,show t h a t t h e a d d i t i o n o f 2,2,4t r i m e t h y l p e n t a n e and o f i s o b u t e n e p r o v i d e s s i m i l a r e f f e c t s as i n t h e e l e c t r o lysis
.
-
These r e s u l t s p r o v e t h a t b o t h o c t y l c a t i o n s and i s o b u t e n e can e f f e c t i v e l y p a r t i c i p a t e t o t h e new process o f t h e r e a c t i o n
.
I n order t o v e r i f y t h a t B-scission occurs i n perfluoroalkanesulfonic acids, we s t u d i e d t h e r e a c t i o n o f 2,2,4-trimethylpentane
alone.
25 20
15
10 5
10
5
15
20
25
Fig.3: Effect of addition of 2,2,4trimethylpentane ( 2 . 5 ~10-3mole) on the conversion of n-butane (0.17mole)with CF3S03H(0.085 mole) 25°C
.
.
h
5
10
15
20
h
Fig.4: Effect of addition ofisobutene (5.3x10-3mole) on the conversion o f n-bu tane (a 17110 16) w i th CF 3 S 03H 25°C (0.085mole)
.
.
378
T h i s a l k a n e i s e a s i l y i o n i z e d because i t c o n t a i n s a t e r t i a r y carbon and t h e so-formed dimethylneopentylcarbenium i o n i s t h e most a b l e t o undergo t h e B - s c i s s i o n as we a l r e a d y n o t i c e d . When t r e a t e d w i t h trifluoromethanesulfonic a c i d , t h i s o c t a n e i s q u i c k l y c o n v e r t e d t o i t s isomers and
l o w e r a1 kanes.
The absence o f n-butane t h a t cannot be produced by 8 - s c i s s i o n i s a s i g n i f i c a n t result. On t h e contrary., t h e 2,2,3,3-tetramethylbutane,
which i s a p a r t i c u l a r l y
s u i t a b l e octane t o undergo p r o t o l y s i s does n o t r e a c t i n t h e same c o n d i t i o n s .
c
d
We can conclude t h a t p u r e p e r f l u o r i n a t e d s u l f o n i c a c i d s a r e u n a b l e t o cause t h e p r o t o l y s i s o f alkanes media l i k e HF
-
SbF5 o r HS03F
-
i n contrary with other SbF5 [12]
more s u p e r a c i d i c
.
On t h e o t h e r hand, t h e 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 i n d u c e t h e B - s c i s s i o n o f alkanes, which agrees w i t h t h e proposed mechanism f o r t h e r a d i c a l - a s s i s t e d i s o m e r i z a t i o n o f n-butane. Now, l e t us examine t h e r e a c t i o n o f i s o b u t a n e
.
T a b l e 2 g i v e s t h e r e s u l t s o b t a i n e d w i t h trifluoromethanesulfonic a c i d under d i f f e r e n t c o n d i t i o n s i n t h e same way as t h o s e used w i t h n-butane Table 2 :
Conversion o f isobutane(0.17 mole) w i t h CF3S03H(0.085 mole) a t 25°C d u r i n g 20h
.
Conditions
CF3S03H CF3S03H
cF so 3 3
-=c4
-
> 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<
%%
Ch,'cy 0-
0Z
2
I n o u r view t h e i n t e r m e d i a t e M30+ n o r m a l l y r e a c t s t o hydrocarbons ( r e f . 1 1 ) b u t i n t h e presence o f p l a t i n u m i t can be hydrogenated t o methane. The f i r s t r e a c t i o n presumably r e q u i r e s more energy and t h u s a t l o w temperatures no h i g h e r hydrocarbons a r e formed; methane i s t h e s o l e p r o d u c t .
Conversjon_wjth-other_acldIl"ee"rtl
To s t u d y t h e i n f l u e n c e o f a c i d s t r e n g t h which i s known t h a t i t has more b u t we i n c o r p o r a t e d p l a t i n u m i n H-Y z e o l i t e weaker a c i d s i t e s ( r e f . 1 2 ) . We compared t h e c o n v e r s i o n o f MOM/H2 o v e r a 0.5 w t . % P t / H - Y w i t h t h e c o n v e r s i o n o v e r 0.5 w t . % Pt/H-ZSM5 ( s e e F i g . 4 ) . A s can be seen
t h e c o n v e r s i o n versus t e m p e r a t u r e c u r v e i s s h i f t e d t o h i g h e r temperatures. Over Pt/H-Y
a h i g h e r temperature i s r e q u i r e d f o r o b t a i n i n g t h e same methane p r o d u c t -
i o n . A p p a r e n t l y t h e presence o f a l a r g e r number o f a c t i v e c l u s t e r s does n o t r e s u l t i n h i g h e r a c t i v i t y . Also t h e f a c t t h a t d i f f u s i o n a l c o n s t r a i n t i s s m a l l e r f o r H-Y due t o i t s
l a r g e r p o r e volume and p o r e d i a m e t e r , seems t o be o f no
importance s i n c e t h e n an o p p o s i t e r e s u l t would be expected. I t f o l l o w s t h a t t h e d i f f e r e n c e i n a c i d s t r e n g t h i s r e s p o n s i b l e f o r t h e observed temperature s h i f t . Thus a change i n a c i d i t y i n f l u e n c e s t h e methane f o r m a t i o n ; i . e .
the a c i d s i t e
c a t a l y z e s t h e r a t e d e t e r m i n i n g s t e p . T h e r e f o n t h e r a t e o f methane f o r m a t i o n i s
l5
r
F i g . 4 The i n f l u e n c e o f t h e a c i d s i t e s t r e n g t h on t h e c o n v e r s i o n ol' MOM t o methane. a ) H-ZSM5. b ) H-Y; b o t h l o a d ed w i t h 0.5 w t . % p l a t i n u m .
F i g . 5 Methane f o r m a t i o n on two a c i d s u p p o r t s . a ) H-Mordenite. b)8Al2O3; b o t h w i t h 0.5 w t . % p l a t i n u m .
397
equal t o t h e r a t e o f M30t formation: n rCH4 = rM30t= keMOM*Pb,OM
, where
8 i s t h e coverage o f chemisorbed
MOM.
The amount o f chemisorbed MOM appeared, t h e r m o g r a v i m e t r i c a l l y measured, t o b e c o n s t a n t a t t h e e x p e r i m e n t a l c o n d i t i o n s used. F o r o b t a i n i n g t h e r e a c t i o n o r d e r i n MOM, we performed c o n v e r s i o n experiments o v e r 0.5 w t . X Pt/H-ZSM5 a t c o n s t a n t temperatures. As can be seen i n t a b l e 1 t h e o r d e r n has n e a r l y t h e expected v a l u e o f 1.
R e a c t i o n o r d e r i n MOM determined a t
A c t i v a t i o n energy f o r t h e methane f o r -
two temperatures f o r t h e f o r m a t i o n
m a t i o n on 0.5 w t . % P t l a c i d - c a t a l y s t .
o f methane on 0.5 wt.%/H-ZSM5. T ("C)
n
support
E a c t (kJ/mol)
150
1.2
H-ZSM5
42
190
1.1
H-Mordeni t e
57
H-Y
74
Y - A ~ ~ O ~ 79
The f o r m a t i o n of m e t h a n e i s a Simple r e a c t i o n ; no Volume change o c c u r s and t h e hydrocarbon formed i s a s m a l l m o l e c u l e w i t h h a r d l y no i n t e r a c t i o n w i t h t h e z e o l i t e . T h e r e f o R t h e amount o f methane measured i s l i n e a r i l y r e -
. By p l o t t i n g t h i s v a l u e l o g a r i t h m i c a l l y a g a i n s t t h e r e c i p r o c a l o f CH4 t h e a p p l i e d temperature, eMOM *PMOM b e i n g c o n s t a n t , g i v e s a s t r a i g h t l i n e . From
lated t o r
t h e s l o p e t h e a c t i v a t i o n energy f o r t h e M30t f o r m a t i o n can be c a l c u l a t e d ( s e e t a b l e 2). A p p a r e n t l y t h e f o r m a t i o n o f t h e i n t e r m e d i a t e on t h e l e s s s t r o n g a c i d s i t e s o f H-Y demands more energy. T h i s i n d i c a t e s t h a t t h e r e i s a r e l a t i o n between t h e z e o l i t e a c i d s t r e n g t h and t h e a c t i v a t i o n energy o f t h e methane f o r m a t i o n ; t h e h i g h e r t h i s energy t h e weaker t h e s i t e s . I t i s g e n e r a l l y assumed t h a t t h e a c i d i t y o f H-Mordenite l i e s between t h o s e o f H-ZSM5 and H-Y ( r e f . 1 3 ) .
Due t o i t s a c i d i t y t h i s z e o l i t e can, l o a d e d w i t h
p l a t i n u m , c o n v e r t MOM t o methane; t h e c o n v e r s i o n v e r s u s temperature c u r v e i s shown i n F i g . 5 . The c o r r e s p o n d i n g a c t i v a t i o n energy g i v e n i n t a b l e 2, indeed s t r e n g t h e n s t h e assumption o f an i n t e r m e d i a t c a c i d i t y . The a c i d s u p p o r t s used s o f a r where z e o l i t e s . A n o t h e r f r e q u e n t l y used s u p p o r t f o r m e t a l s i s y-A1203. I t i s known t h a t t h i s s u p p o r t c o n t a i n s a c i d s i t e s w i t h enough s t r e n g t h f o r i s o m e r i s a t i o n o r even c r a c k i n g ( r e f . 14,15). I n c o m b i n a t i o n w i t h p l a t i n u m y-A1203 i s a b l e t o c o n v e r t MOM t o methane; t h e c o n v e r s i o n t o methane as f u n c t i o n o f temperature i s shown i n F i g . 4 . The a c t i v a t i o n energy i s
398 g i v e n i n t a b l e 2; t h e s m a l l d i f f e r e n c e w i t h t h e v a l u e o f H-Y i n d i c a t e s t h a t t h e a c i d s t r e n g t h o f y-A1203 i s b u t s l i g h t l y l o w e r . We may conclude t h a t a l r e a d y a t l o w temperatures M30t i s formed, w h i l e t h e subsequent f o r m a t i o n o f hydrocarbons does n o t t a k e p l a c e . Thus when p l a t i n u m i s near t h e a c i d s i t e s t h e i n t e r m e d i a t e i s prone t o h y d r o g e n a t i o n t o methane. I n c o n t r a s t w i t h t h e h y d r o i s o m e r i s a t i o n r e a c t i o n o v e r Pt/H-ZSM5 t h e a c i d s i t e c a t a l y z e s t h e r a t e d e t e r m i n i n g s t e p , t h e f o r m a t i o n o f methane proceeds by i d e a l b i f u n c t i o n a l c a t a l y s i s (ref.12,16).
As a consequence t h e a c t i v a t i o n energy i s
d i r e c t l y related w i t h the strength o f the acid sites. ACKNOWLEDGEMENT T h i s work was supported by t h e N e t h e r l a n d s F o u n d a t i o n o f Chemical Research (SON) w i t h f i n a n c i a l a i d f r o m t h e N e t h e r l a n d s F o u n d a t i o n f o r Pure and S c i e n t i f i c Research (ZWO). C.W.R.
Engelen wishes t o thank f o r t h i s s u p p o r t .
REFERENCES
1 S.L. M e i s e l , J.P. Mccullough, C.H. L e c h t h a l e r and P.B. Weisz, ChemTech., 6 (1376), 86. 2 C.D. Chang, C.T-W. Chu and R.F. Socha, J. C a t a l . , 86 (1984) 289 and 297. 3 W.O. Haag, R.M. Lago and P.G. Rodewald, J. Mol. C a t a l . , 17 (1982) 161. 4 R.M. Dessau and R.B. L a P i e r r e , J. C a t a l . , 78 (1982) 136. 5 J.P. van den Berg, J.P. W o l t h u i z e n and J.H.C. van H o o f f i n Proceedings, 5 t h I n t . Conf. on Z e o l i t e s (L.V. Rees, Ed.) (1980), 649. 6 J.P. van den Berg, J.P. W o l t h u i z e n and J.H.C.
van H o o f f , J. C a t a l . , 80 (1983)
139. 7 R.J. Argauer and G.R. 8 3.M. Bibby, N.B.
L a n d o l t , U.S.
P a t e n t (1972) 3.702.886.
M i l e s t o n e and L.P. A l d r i d g e , N a t u r e , 285 (1980) 30.
9 R. M. Dessau, J. C a t a l . , 77 (1982) 304. P o s t and J.H.C. van H o o f f , Z e o l i t e s 4 (1984) 9. 11 J.P. van den Berg, T h e s i s Eindhoven U n i v e r s i t y o f Technology (1980). 12 P.A. Jacobs, J.B. Uytterhoeuen, M. Steyns, G. Froment and J. Weitkamp i n proceedings, 5 t h I n t . Conf. on Z e o l i t e s (L.V. Rees, Ed.) (1980), 607. 13 P . A . Jacobs, C a t a l . Rev. S c i . Eng., (24) (1982), 413. 10 J.G.
14 H. P i n e s and W.O. Haag, 3 . Am. Chem. SOC. 82 (1960) 2471. 15 W.K. H a l l , F.E. L u t i n s k y and H.R. G e r b e r i c h , J . C a t a l . , 3 (1964) 512. 16 H. P i c h l e r , H. Schulz, H.D. Petrochem. 25 (1972) 494.
Reitemeyer and J. Weitkamp, E r d i i l , Kohl-Erdgas
399
B. Imelik et ol. (Editors), Cotolyris b y Acids ond Bases 1985 Elsevier Science Publishers B.V.,Amsterdam -hinted in The Netherlands
ACID-CATALYZED CONVERSION OF N-DECANE OVER HIGH-SILICA FAUJASITES b y PETER A. JACOBS, JOHAN A. MARTENS and HERMANN K. BEYER* L a b o r a t o r i u m v o o r Oppervlaktechemie, K a r d i n a a l M e r c i e r l a a n 92, 8-3030
*
K.U.
Leuven
Leuven ( H e v e r l e e )
I n s t i t u t e o f Chemistry, Hungarian Academy o f Sciences, P u s z t a s z e r i ut, Budapest, Hungary
SUMMARY Z e o l i t e Y was dealuminated p r o g r e s s i v e l y w i t h S i c 1 and t h e samples t h u s o b t a i n e d were l o a d e d w i t h 1 % o f P t and t e s t e d as c a t a l j s t i n t h e b i f u n c t i o n a l c o n v e r s i o n o f n-decane. Gradual changes i n t h e n a t u r e and d i s t r i b u t i o n o f t h e f e e d isomers as w e l l as o f t h e cracked p r o d u c t s a r e observed w i t h t h e degree o f d e a l u m i n a t i o n . They can b e i n t e r p r e t e d m e c h a n i s t i c a l l y i n terms o f a reduced r a t e o f i s o m e r i z a t i o n v i a a l k y l s h i f t s , compared t o t h e r a t e o f c h a i n b r a n c h i n g v i a p r o t o n a t e d cyclopropane i n t e r m e d i a t e s a t i n c r e a s i n g degree of d e a l u m i n a t i o n . C a t a l y t i c a l l y t h i s i s a t t r i b u t e d t o an enchanced average a c i d strength per s i t e . RESUME Une z e o l i t e Y a i.t@desaluminee de f a c o n p r o g r e s s i v e p a r t r a i t e m e n t avec Sic1 Ces echantillons ont alors ete transformes en catalyseurs b i f o t c t i o n n e l s c o n t e n a n t 1 % de P t e t t e s t e s dans l a c o n v e r s i o n b i f o n c t i o n n e l l e du n-decane. La n a t u r e e t l a q u a n t i t e des i s o d r e s e t p r o d u i t s hydrocraques change g r a d u e l l e m e n t avec l e degre de d e s a l u m i n a t i o n . L ' i n t e r p r e t a t i o n de ces phenomenes a &t@ f a i t e en termes de mecsnisrne de l a r e a c t i o n e t de f o r c e a c i d e de ces c a t a l y s e u r s .
.
INTRODUCTION H i g h - s i l i c a z e o l i t e s a r e known f o r t h e i r e x t r e m e l y h i g h thermal s t a b i l i t y as w e l l as f o r t h e i r h y d r o p h o b i c i t y . The l a s t decade a w e a l t h o f i n f o r m a t i o n appeared
on
properties.
the
properties
On t h e c o n t r a r y ,
of
such
zeolites
exhibiting
shape
selective
much l e s s i s known on t h e p r o p e r t i e s o f v e r y
h i g h - s i l i c a z e o l i t e s w i t h an open s t r u c t u r e as t h e f a u j a s i t e s . I t i s common knowledge t h a t h i g h - s i l i c a f a u j a s i t e s w i t h Si02/A1203 r a t i o s
o v e r 100 can be p r e p a r e d by repeated ammonium exchange, a c i d t r e a t m e n t (1,Z).
steaming and m i n e r a l
Such a procedure i s n o t o n l y l a b o r - and time-consuming
b u t a l s o seems t o damage t h e pore s t r u c t u r e t o a c e r t a i n e x t e n t ( 3 ) .
In
c o n t r a s t t o t h i s , t h e SiC14 method a l l o w s t o dealuminate Y z e o l i t e s o v e r t h e whole range o f c o m p o s i t i o n s w i t h o u t a l t e r a t i o n o f t h e p o r e system ( 4 ) . The SiC14-method was s e l e c t e d i n t h i s work t o prepare h i g h l y dealuminated
400 samples and t o study the e f f e c t o f u l t r a d i l u t i o n o f Brbnsted a c i d s i t e s and increasing hydrophobicity
o f the m a t r i x i n open z e o l i t e s on t h e primary
isomerization and cracking o f long-chain p a r a f f i n s . I t was r e c e n t l y shown t h a t the n-decane hydroconversion r e a c t i o n i s a s u i t a b l e t e s t r e a c t i o n ( 5 ) since i t allows t o compare c a t a l y s t s i n non-deactivating conditions and t o obtain steady state
catalytic
data
in
catalysts
devoid
of
coke.
Furthermore,
this
b i f u n c t i o n a l r e a c t i o n mode a1 lows t o determine products a f t e r a s i n g l e cracking step (6). EXPERIMENTAL The o r i g i n o f the Y-type z e o l i t e and the dealumination procedure have been described e a r l i e r (4).
To vary
t h e degree o f dealumination
the r e a c t i o n
temperature w i t h SiC14 was increased from 520 t o 620 K. The reaction time was u s u a l l y 40 minutes. These samples w i l l be
denoted as Y followed by the number
of Al atoms per u n i t c e l l i n brackets. The USY z e o l i t e ( u l t r a s t a b l e Y) obtained by steaming NH4Y a t 1073 K was used as reference m a t e r i a l . These samples were washed,
i o n excJanged
w i t h NH;
( 4 ) , and exchanged w i t h Pt(NH3)i+ so as t o
obtain a 1 % by weight loading w i t h platinum. The a i r - d r y samples were f u r t h e r a c t i v a t e d i n s i t u i n the continuous f l o w reactor, f i r s t i n f l o w i n g oxygen and then i n hydrogen a t 673 K. The n-decane isomerization and cracking was done a t Fo/W o f 518 kg s mol (Fo, i s the f l o w a t the r e a c t o r entrance and W i s the amount o f c a t a l y s t ) i n the conditions described e a r l i e r ( 7 ) . RESULTS AND DISCUSSION 1. Feed isomerization
The d i f f e r e n t i a l
r e a c t i o n r a t e o f n-decane conversion determined a t low
o v e r a l l conversions represents the r a t e o f branching o f n-decane since under these
conditions
Mechanistically
methyl nonanes
and
ethyloctanes
are
the
t h i s occurs v i a protonated cylopropane (PCP)
only
products.
intermediates
(68) :
I n Fig.
1 i s shown the v a r i a t i o n of
t h i s PCP-isomerization r a t e both on a
sample weight basis and on a s i t e basis (turnover number, TON).
401
Al /UC
Fig. 1. Change in relative reaction rate of n-decane hydroisomerization with the A1 content of the faujasite unit cell for Y zeolite treated with SiC14. It is seen that the overall reaction rate decreases with increasing degree of dealumination. Nevertheless, the decrease in the number of Brfinsted sites, which is considered to be proportional to the A1 content of the samples, is more than compensated by the increasing catalytic efficiency of the remaining ones. As a result the apparent TON increases slightly with increasing degree of dealumination. This indicates either that all OH groups associated with lattice aluminum atoms are not active in the isomerization reaction or that the average strength of the sites increases with increasing degree of dealumination. In fact both explanations may also complement each other. These data do not allow to decide whether at very low A1 contents (less than 3 A1 per unit cell) the reaction rate is still linear in composition as observed for HZSM-5 zeolites in n-hexane cracking (9,lO). Anyway, the results are in qualitative agreement with the work of Barthomeuf and Beaumont (11) in which it is concluded that dealumination by EDTA initially removes the weaker Brfinsted acid sites. Rate of PCP-isomerization against alkyl shifts Methyl-branching of n-decane via PCP-isomerization is expected to give the following isomer distribution, provided all possible PCP structures are present at equal concentrations and the distribution is not disturbed by fast subsequent methyl-shifts : 2-methylnonane = 16.7 % 3-methylnonane = 33.3 % 5-methylnonane = 16.7 % 4-methylnonane = 33.3 %.
2.
402
Fig. 2 compares the r a t i o o f 3- t o 4- methylnonane f o r an USY z e o l i t e and a h i g h l y dealuminated Y a f t e r SiC14 treatment.
120-
g x 01
V
I
4*
901
0
I
I
25
I
75
50 %conversion
Fig. 2. R a t i o o f the 3- t o 4- methylnonane isomers a t increasing n-decane conversion = a, t h e o r e t i c a l r a t i o when only PCP branching occurs; b, observed y i e l d on Y w i t h 3 AL/UC and c, USY; d, thermodynamic r a t i o i n the temperature range o f i n t e r e s t . On
USY
zeolite,
methyl-shift.
initially
PCP-branching
occurs
followed
by
a
rapid
On Y(3) t h e major isomerization parthway i s PCP-isomerization,
even a t high degrees o f conversion, and e q u i l i b r a t i o n v i a a l k y l s h i f t s doesnot occur. When the r a t i o o f the 2- t o 5- methylnonanes i s considered, t h i s behavior is confirmed. Moreover, t h e data o f Fig. 3 show t h a t USY anhY(3) are m a t e r i a l s w i t h extreme properties. A l l other samples w i t h intermediate degree o f dealumination show intermediate behavior. I n other words, the e q u i l i b r a t i o n
of methylnonanes form n-decane v i a f a s t m e t h y l - s h i f t s compared t o PCP-branching becomes slower f o r higher degrees o f dealumination.
403
/
2 0-
m 0
0
0 .t-
0
f 10
I
I
20 isorn. conv.
30
/ %
Fig. 3. Ratio of 2- to 5- methylnonane at increasing isomerization conversion of n-decane for USY and Y, dealuminated with SiC14. The figures on the curves represent the A1 atoms per unit cell, Ethyloctanes on CaY zeolite are shown to be formed via an alkyl-shift from methylnonanes (12). Given the decreased rate of isomerization via methyl-shift it is therefore expected that the relative contribution of the ethyloctanes to the yield o f the monobranched isomers should decrease with increasing degree of dealurnination. This is confirmed experimentally with the data of Fig. 4.
404
'10 isomers
conv.
Fig. 4. Contribution of ethyloctanes to the monobranched feed isomers at low degrees of isomerization conversion for Y zeolites dealuminated to different extent with SiC14. The actual A1 content per unit cell is shown on the experimental curves. Preferred isomerization of n-decane via PCP or type B isomerization (12) also occurs on Pentasil-zeolites (8,13). Moreover, as a result of transition state shape selectivity at the level of the formation of PCP-structures, branching at the end of the hydrocarbon chain is preferred (8). It is hardly conceivable that these effects also play in the more open faujasite structure . It is more probable that these selectivity changes reflect differences in acidity. Indeed, since on USY initial PCP-branching is followed by a more rapid equilibration via alkyl shifts, it follows that the latter reaction can be catalyzed by the weaker Brensted sites. If with increasing degree of dealumination the weaker OH groups are preferentially removed, a gradual depression of isomerization rate via alkyl shifts relative to PCP branching can be understood. 3. Peculiarities in the distribution of the cracked products When the metal- and Brbnsted-acid function of a bifunctional catalyst are in balance, only primary hydrocracking occurs which is reflected by the symmetric distribution of the numbers of the cracked products among the central carbon number (6,8,13). For all catalysts investigated this remains true up to at least 60 % cracking conversion. Since isomerization and hydrocracking are consecutive reacions (6,8,13), it also is expected that changes in the relative rates of isomerization should be reflected in the nature and absolute amount of cracked products formed. Moreover, PCP isomerization dominates in the high-silica faujasites and
405
methyl-shifts are relatively slow, and therefore it is expected that only few feed isomers with aay degree of branching will be formed on these samples. As a result the preferred cracking mode o f Type A (see scheme) will not be dominant. Since Type A is energetically a much preferred cracking mode, its decreased contribution should be reflected in the distribution of the cracked products on the dealuminated samples.
scc +KC
N+AA/ Q
Scheme. Modes of @-scission of decylcarbenium ions (after ref. 13).
In the cracking of n-decane isomers over open pore zeolites of the Y type it is improbable that effects occur which are the result o f desguised kinetics as is observed in ZSM-5 (14). The relatively high yield of 2,3-dimethylbutane (2,3DMC4) formed at low cracking conversion (Fig. 5) indicates that in absence of severe diffusional limitations this product is of true primary nature.
-
10
00 cracking /
%
Fig. 5. Yield of 2,3-dimethylbutane at increasing cracking conversion of n-decane over Y(3) dealuminated zeolite.
406 As i s shown i n Fig. 6 the y i e l d of 2,3DMC4 a t low cracking conversion s t e a d i l y increases f o r higher degrees o f dealumination.
10
0
20
30
40
Al/UC
Fig. 6. Y i e l d o f 2,3-dimethylbutane and 2-methylbutane a t 5 % cracking conversion o f n-decane against the A1 content o f Y z e o l i t e s dealuminated w i t h SiC14. I n a p a r a l l e l way, the y i e l d of 2-methylpentane decreases. Q u a l i t a t i v e l y , t h i s can be understood i n terms of a decreased r a t e o f a l k y l s h i f t s when the degree of dealumination increases. The change i n r e a c t i o n pathway f o r one o f t h e t r i p l y branched feed isomers i s expected t o occur as f o l l o w s :
If
the
explanation
for
the
decreased methyl
shifts
compared
to
PCP
isomerization i s indeed a gradual increase o f the average a c i d strength, i t may be expected t h a t the c o n t r i b u t i o n o f the e n e r g e t i c a l l y l e s s favorable cracking routes (B1, B2 and C ) w i l l increase f o r the more dealuminated samples. The o v e r a l l e f f e c t of products w i l l
t h i s (scheme)
i s t h a t the amount o f unbranched cracked
increase. The experimental confirmation f o r t h i s i s shown i n
A s i m i l a r b u t more pronounced e f f e c t i s observed f o r p e n t a s i l z e o l i t e s (8,13) and was ascribed t o an increased c o n t r i b u t i o n o f t h e C Fig. 7 f o r n-hexane.
cracking mechanism. I n the l a t t e r z e o l i t e s however, t h i s i s n o t a mere r e s u l t
407
o f increased a c i d strength, b u t shape s e l e c t i v e e f f e c t s c o n t r i b u t e as w e l l .
Al /UC
Fig. 7. Content o f n-hexane i n the C6 f r a c t i o n f o r Y samples dealuminated w i t h SiC14. The degree o f dealumination i s i n A1 atoms per u n i t c e l l . CONCLUSIONS
Y z e o l i t e s which are progressively f u r t h e r dealuminated w i t h SiC14 show a decreased number o f Brdnsted s i t e s which on t h e average have an increased a c i d strength.
This
i s derived from t h e r a t e o f branching o f n-decane i n the
b i f u n c t i o n a l mode as w e l l as from the d i s t r i b u t i o n o f the feed isomers and cracked products. Hydroisomerization v i a PCP-chain branching becomes r e l a t i v e l y f a s t e r than a l k y l - s h i f t
isomerization.
This r e s u l t s i n a decreased r a t e o f
formation o f s t r u c t u r e s which are p r e f e r e n t i a l l y cracked. The e f f e c t s o f t h i s on the d i s t r i b u t i o n o f the cracked products i s p r e d i c t e d i n a q u a l i t a t i v e way and confirmed experimentally
.
ACKNOWLEDGMENTS P.A.J.
and J.A.M.
acknowledge the NFWO (Belgium) f o r a research p o s i t i o n and
a research fellowship, respectively. H.K.B.
an P.A.J.
acknowledge sponsoring o f t h i s research by a g r a n t obtained i n
the frame o f an agreement between t h e Hungarian C u l t u r a l I n s t i t u t e and the Belgian National Fund of S c i e n t i f i c Research.
408
REFERENCES 1 3. Scherzer, J. Catalysis, 54 (1978) 285. 2 U. Lohse, E. Alsdorf and H. Stach, Z. Anorg. A l l g . Chem., 447 (1978) 64. 3 11. Lohse, H. Stach, H. Thamm, W. Schirmer, A.A. I s i r i k j a n , N.I. Regent and M.M. Dubinin, Z. Anorg. Allg. Chem., 460 (1980) 179. 4 H.K. Beyer and I.Belenykaya, Studies Surf. Sci. C a t a l y s i s 5 (1980) 203. 5 J.A. Martens, M. Tielen, P.A. Jacobs and Y. Weitkamp, Z e o l i t e s 4 (1984) 98. 6 J. Weitkamp, Erdijl Kohle Erdgas Petrochem., 31 (1978) 13. 7 P.A. Jacobs, J.B. Uytterhoeven, M. Steyns, G. Froment and J. Weitkamp, Proceed. 5 th. I n t . Conf. Zeolites, L.V. Rees, ed., Heyden, 1980, p. 607. 8 P.A. Jacobs, J.A. Martens, J.A. Weitkamp and H.K. Beyer, Disc. Faraday SOC. 72 (1981) 353. 9 J. Dwyer, F.R. F i t c h and E.E. Nkang, J. Phys. Chem, 87 (1983) 5402. 10 D.H. Olson, W.O. Haag and R.M. Lago, J. Catalysis, 61 (1180) 390. 11 R. Beaumont and D. Barthomeuf, J. Catalysis, 26 (1972) 218 and 27 (1972) 45. 12 J. Weitkamp and H. Farag, Acta Phys. Chem. Hung. 24 (1978) 327. 13 J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catalysis, 8 (1983) 123. 14 W.O. Haag, R.M. Lago and P.G. Rodewald, J. Molec. Catalysis, i7 (1982) 161.
B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
409
A NEW APPROACH TO THE CRACKING OF ALKANES AS A TEST REACTION FOR SOLID ACID CATALYSTS. A. Corma
1
and V. Fornds
L
'Instituto de Catdlisis y Petroleoqulmica, C.S.I.C. Serrano, 119. 28006-Madrid. Spain.
.
'Instituto de Flsico-Qulmica Mineral. C . S I .C. Serrano I 115 dup. 28056-Madrid. Spain.
ABSTRACT The relative importance of protolytic and @-scission mechanisms for the initiation of cracking of n-heptane (P/B) has been established for a series of faujasite Y type zeolite catalysts. A direct relation between the (P/B) values and Br$nsted/Lewis ratio of the zeolite catalysts has been found. RESUME
L'importance relative des mechanisms protolytique e t p -scission pwrl'initiation du cracking de n-heptane @/B), a Btd etablie pour une sdrie de catalyseurs t$pe zeolite Y. On a t r o w 6 une relation directe entre les valeurs (P/B) et la proportion de centres Br$nsted/Lewis des catalyseurs.
ItQPDDlEllICN
The most important use of zeolites as catalysts is perhaps the catalytic cracking of hydrocarbons. Commercial catalysts are constituted of 15-25% activated Y zeolite in a clay or amorphous silica-alumina matrix (1,2). The activation of the zeolite takes place by ion exchange of the original sodium by rare earths and/ or ammonium ions followed by a thermal treatment in order to obtain an ultrastable form of Y zeolite, which is more resistant to the hydrothermal conditions to which the catalyst will be subjected in a commercial unit during the regeneration step. It is well known that the activity and selectivity of the cracking catalyst is mainly a function of the content and particular type of zeolite. Several chemical test may be used to compare the activity of different zeolites ( 3 ) , but perhaps the most widely used are the cumene ( 4 ) and n-hexane cracking reactions(5). With any of these test reactions, the common method used to eva-
410
luate the activity and selectivity of different cracking catalysts, has been the comparison of conversion and yields under a given set of experimental conditions. It is obvious that such a method can be quite misleading due to the fact that during catalytic cracking the catalyst is affected by decay, and both, the activity and selectivity are going to be a function of the time on stream at which the measurement has been done. When one assumes that the cracking activity and selectivity have been properly measured, the next step in the evaluation of a zeolite cracking catalyst consists in relating these values with the concentration and nature of the acid sites of the zeolite, in order to use this information for designing a new catalyst with adequate balance of acidity which can give an optimum in activity and selectivity.
y i e l d s and t o t a l conversion
catalyst ch r a c ter l zation (B/L)
modification of preparation rn e t h o d
curves
t initial selectivity P /I3
-
initial selectivity primary products
Fig. 1. A methodology for hydrocarbon cracking studies. Here we propose a methodology (fig. 1) forstudyir7 cracking reactions which avoids the difficulties due to catalyst decay and secondary reactions. It also shows that both types of acid sites, Brphsted and Lewis, are active in alkane cracking through different pathways. It is then possible, by carrying out a cracking reaction, to compare the ratio of Brphsted to Lewis acid sites on different catalysts and to predict differenees in selectivity.
411
EXPERIMENTAL METHODS Materia1s A series of exchanged Y zeolites namely HY, HYS, REY, REYS and REHY80 with unit cell constants of 24.39, 24.32, 24.76, 24.74 0 and 24.59 A, respectively, have been prepared from a SK-40 (Union 0 Carbide, Si/A1=2.4, unit cell constant 24.69 A) in the following way: the HY sample was obtained by r'epeated cycles (eight) of NH; exchange at room temperature, washing, drying and calcination at 5OO0C, until the sodium content was less than 2 % of the original. The same procedure was followed to prepare the REY sample but in + this case La3+ instead of NH4 was used as the exchanged cation. The REY80 was prepared by exchanging 8 0 % of the original sodium content by La3+ following the procedure described above, and then + + the rest of Na was exchanged by NH4. The HYS and REYS correspond to samples of HY and REY which have steamed at 75OOC and 15 Kg/ cm2 of steam partial pressure. The cracking experiments have been carried out in a continuous flow fixed-bed, glass-tubular reactor at atmospheric pressure and 450°C. A fixed amount of n-heptane (8.62 9) was always fed by means of a positive displacement pump. The catalyst to oil ratio (C/O)is defined as the weight of catalyst divided by the weight of n-heptane fed, and has ranged in this work from 0.0116 to 0.348. The time on stream has been changed by feeding the n-heptane at different velocities and in this way experiments from 75 to 1500 seconds of time on stream have been carried out. Total conversion has been calculated as the total number of carbon atoms of outlet hydrocarbons, other than the feed. The yield of a product is defined as moles of product divided by moles of nheptane fed. Infrared spectroscopic measurements have been carried out in a conventional greaseless i.r. cell. The samples were pretreated overnight at 450°C and torr of dynamicvacuum; then 5 torr of pyridine were introduced into the cell at room temperature and after equilibration the samples were outgassed at 350°C under vacuum and the spectra recorded at room temperature in a Perkin Elmer580B spectrophotometer equipped with Data Station. RESULTS AND DISCUSSION In a previous paper (6) it was presented that in the catalytic
412
crackingof paraffins the selectivity to the different products is a function of time on stream for any but for primary and stable products. Indeed, for a class I and I1 decaying catalyst, to which cracking zeolites can be aproximated, it is possible to obtain different selectivity values working at the same or at different levels of conversion by doing the experiment at different times on stream (fig. 2A,B). This effect is even more notorious in class I11 decaying catalysts to which gas-oil cracking on zeolite has been found to apply ( 7 , 8 ) . Therefore we can conclude that if one
T O T A L CONVERSION Fig. 2. Selectivity curves for reaction products on a Class I1 decaying. A, primary unstable product; B, primary plus secondary product. Continuous lines are OPE's and dashed lines represent different C/O loops. wants to avoid the problems derived from catalyst decay one should analyze selectivity data obtained at very short time on stream, or what is equivalent, use as selectivity curves the Optimum Performance Envelopes (OPE'S) (9). On the other hand, it is well known that, specially in the case of zeolites, the products formed in the first cracking events can react very rapidly giving secondary products even at low levels of conversion. If we want to avoid the interference of such consecutive reactions we must work at very low levels of conversion or, even better, use initial selectivities in order to discuss mechanistic aspects of cracking and to be able to establish a carbon number balance of the cracked fragments. Working in this way, the OPE's for the primary products during craking of n-heptane on the different catalysts used in this work have been obtained at -150'C and, as an example, the paraffins and olefins of different carbon chain length are given. (Fig. 3 ) .
413
0 4 TOTAL
8 1 2 CONVERSION
Fig. 3. OPE'S for corresponding groups of primary products in the cracking of n-heptane on REHY (80) (-) and HYS (---I zeolite catalysts. From these curves initial selectivities can be calculated, and then the discussion of ;he mechanism subsequently made (Table I). TABLE I Initial selectivities to propane and butenes obtained during the cracking of n-heptane at 4 5 0 O C . ,Catalyst
IS Propane
IS Butenes
1
REHY-80
6.42
0.29
HY REY
0.36
0.29
0.42 0.38
0.33
0.35
0.33
IHYS
1 REYS
0.34
It is generally accepted that Brdnsted sites present in acid zeolites are the active centers for the cracking of olefins and dealkylation of alkylaromatics via intermediate carbenium ions.
I
414
In the case of alkanes, the mechanism of cracking and therefore the type of acid sites is a subject of controversy. Some authors believe that strong Brghsted sites are able to form a carbenium ion by abstracting a hydride ion from a paraffin or by protonation of the olefins present in the feed (9-11). Others, relate the formation of carbenium ions to the presence of the Lewis sites (trif coordinated aluminium and/or A10 species).Which are capable of abstracting a hydride ion (12,13). Finally, there is a third group which claims a synergistic effect between the two types of sites perhaps by induction of a high protonic strength through Lewis sites or formation of superactive protons at high activation temperature (3). In any case, a conclusive answer has not been found and there are still some puzzling experimental results. For instance, if only Brghsted sites are active it could not be easy to explain why there is not a correspondence between the maximum of activity and the maximum intensity of the hydroxyl bands. Indeed, zeolite catalysts show maximum activity only when considerable dehydroxylation has already occurred (14). Furthermore, if the initiation of cracking would take place by a hydride extraction brought about by strong Br4nsted sites, H2 should be a primary product and this has not be found during our experiments on faujasite zeolites. In any case, if cracking occurs only through intermediate carbenium ions, the paraffin-to-olefin (P/O) ratio calculated from initial selectivity values should be equal to one. This is not true for cracking of n-heptane (Table 11), but with all the cataTABLE I1 Protolytic -to-$-scission ratio ( P / $ ) at 45O0C reaction temperature, and Brghsted to Lewis ratio (B/L) 11550/11450 from pyridine desorbed at 350OC. Catalyst
P/
REHY- 80 HY REY HYS REY s
B
B/L
P/O
0.44
1.00
0.24 0.23
0.48 0.45 0.37 0.26
1.43 1.41 1.23 1.13 1.12
0.15 0.10
415
lysts the P/O ratio is higher than one. These values can not be explained on the basis of olefin saturation and aromatics and coke formation since, as one may see in figure 3c in our system, both compounds appear as secondary products. Recently , we have proposed (15) a dual mechanism which can explain the product distribution observed during cracking of nheptane on faujasite type zeolites. This mechanism assumes that both Br#nsted and Lewis sites are active for paraffin cracking via protolytic (carbonium ion) and $-cracking (carbenium ions) respectively (fig. 4). If this is so, the Brghsted-to-Lewis ratio present on each catalyst which are active for cracking could
n-hep t a n e carbenium ion
branched carbenium i o n o p y l e n e + n- a n d C 4 H i C H++n-and 3 7 i-butene
Fig. 4 , A reaction mechanism for the initiation of paraffin cracking. be calculated from the corresponding initial selectivities of paraffins and olefins in the products. For instance, during the cracking of C 7 to C3+C4 the initial selectivity to propane should include two terms: the initial selectivity of propane formed by protolytic cracking, and that formed by B-scission should be equal to the amount of butenes formed. Taking this into account, the following equation can be written: IS(propane protolytic) = IS (total propane)
-
IS (butenes)
With this equation and the initial selectivity values for propane and butenes given in Table I, it is possible to calculate the protolytic-to-$-cracking (P/B) activity ratios for the five catalysts studied and the resultant values are given in Table 11. From this Table, we can see that the protolytic-to-6-cracking ratio is lower for the steamed than for the unsteamed samples.
416
This result is in agreement with other authors (16) who have shown that steaming decreases the number of the most acidic Br4nstea sites (responsible for the protolytic cracking), while extract ing framework and-aluminiumwhich remains in the zeolite,increases the amount of true Lewis sites. By taking the intensity of the 1450 cm-' and 1550 cm-' i.r. bands of adsorbed pyridine as a measure of the amount of Lewis and Brdnsted acid sites respectively, then the Brpinsted-to-Lewis ratio for each catalyst can be calculated (Table 11). From those values we can see that a linear correlation between P/B and B/L values may be established (fig. 5).
BIL Fig. 5. Relation between P/B and the B/L on different zeolite catalysts. Therefore it is then possible to obtain, following the methodology described in figure 1, the ratio B/L "which are active" for cracking on a given zeolite catalyst, or viceversa i.e. knowing the relative amounts of Br$nsted/Lewis sites, measured by pyridine adsorption, one can estimate the relative importance of the two types of cracking and other selectivity features such as the paraffin/olefin ratio in the products.
417
REFERENCES 1
2 3 4 5 6 7 8 9
10 11 12 13 14
15
16
J.S. Magee, J.J. Blazek, "Zeolite Chemistry and Catalysis". J.A. Rabo Ed., ACSM N o 171 (1976) P.B. Venuto, E.T. Habib, in Fluid Catalytic Cracking with Zeolite Catalysts. Marcel Dekker Inc. (1979) P.A. Jacobs in "CarboniogenicActivity of Zeolites". Elsevier Scientific Publishing Co. (1977) A. Corma, B.W. Wojciechowski, Catal. Rev. Sci. En., 2 (1982)l J.N. Miale, N.Y. Chen, P.B. Weisz, J. Catal., 5 (1966) 278 A. Corma, B.W. Wojciechowski, ACS Div. Pet. Chem., 28 (1983) 861 A.N. KO, B.W. Wojciechowski, Progress in Kinetics (1984) in press A. Corma, J. Juan, J. Martos, J. Molina, 8 Int. Congr. Catal., 11, (1984) 293 M.L. Pontsma, "Zeolite Chemistry and Catalysis". J.A. Rabo Ed., ACS NO171 (1976) P.B. Weisz, Ann. Rev. Phys. Chem., 21 (1970) 175 A. Brenner, P.H. Emme , J. Catal., 75 (1982) 410 H. Hattori, 0. Takahashi, M. Takagi, K. Tanabe, J. Catal., 3 (1981) 132 S.E. Tung, E. McIninch, J. Catal., lo (1968) 166 D. Barthomeuf in "Catalysis by Zeolites". Studies in Surface Science and Catalysis, V5 (1980) 55 A. Corma, J.H. Planelles, J. Sanchez, F. Tomas, J. Catal. (1984) in press L. MOSCOU, R. Mone, J. Catal., 30 (1973) 417
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419
B. Imelik e t al. (Editors), Catalysis by Acids and Bases 0
1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
COMPARISON OF THE REACTIONS OF ETHYLCYCLOHEXANE AND 2-METHYLHEPTANE ON PdlLaY ZEOLITE
J . WEITKAMP and S. ERNST Engler-Bunte-Institute,
U n i v e r s i t y o f K a r l sruhe, Richard-Wi 11s t a t t e r - A 1 l e e 5,
D-7500 K a r l s r u h e ( F e d e r a l R e p u b l i c o f Germany)
SOMMAI RE On a compare l e s r e a c t i o n s de l ' e t h y l c y c l o h e x a n e e t du 2-methylheptane s u r une z e o l i t h e Pd/LaY b i f o n c t i o n e l l e . Pour l e s deux hydrocarbures l ' i s o m & i s a t i o n e t 1 'hydrocraquage s o n t l e s r e a c t i o n s p r i n c i p a l e s . Q u a n t aux d e t a i l s de l e u r s mecanismes on t r o u v e des d i f f P r e n c e s c o n s i d e r a b l e s . ABSTRACT
A comparative i n v e s t i g a t i o n o f 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-methylheptane on a b i f u n c t i o n a l Pd/LaY z e o l i t e c a t a l y s t was undertaken. Both t h e naphthene and t h e a l k a n e undergo i s o m e r i z a t i o n and hydrocracking. Subs t a n t i a l d i f f e r e n c e s a r e encountered c o n c e r n i n g t h e d e t a i l s o f t h e i r mechanisms. INTRODUCTION I t i s now w e l l e s t a b l i s h e d t h a t t h e c o n v e r s i o n o f model hydrocarbons o v e r
s u i t a b l y s e l e c t e d b i d u n c t i o ~ dc a t a l y s t s i s a v a l u a b l e t e c h n i q u e f o r s t u d y i n g t h e i o n i c r e a c t i o n s on t h e acidic s i t e s . Whereas much work a l o n g t h i s l i n e has been done w i t h a l k a n e s (e.g.,
r e f . 1,Z)
r e l a t i v e l y l i t t l e i s known on t h e
mechanisms o f i s o m e r i z a t i o n and h y d r o c r a c k i n g o f napkthenen v i a c a r b o c a t i o n s . T h i s paper d e s c r i b e s t h e i o n i c 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 (E-CHx) on a b i f u n c t i o n a l z e o l i t e c a t a l y s t , v i z . Pd/LaY.
I t i s most l i k e l y t h a t , i n s i d e t h i s
l a r g e - p o r e z e o l i t e , shape s e l e c t i v i t y e f f e c t s a r e absent. The model naphthene was chosen f o r t h e f o l l o w i n g reasons: E-CHx possesses a s u f f i c i e n t l y l a r g e numb e r o f isomers (>20), hence t h e observed pathways o f s k e l e t a l rearrangements can be expected t o be o f more general v a l i d i t y . Yet, t h e number o f p r o d u c t s i s n o t u n d u l y h i g h , hence an i n d i v i d u a l a n a l y s i s o f t h e p r o d u c t s i s f e a s i b l e . The r e s u l t s w i l l be d i s c u s s e d t o g e t h e r w i t h t h o s e o b t a i n e d w i t h a comparable alkane, v i z . 2-methylheptane
(2-M-Hp),
i n order t o p i n p o i n t the differences i n t h e
r e a c t i o n s of a l i c y c l i c and a l i p h a t i c c a r b o c a t i o n s . EXPERIMENTAL E-CHx ( p u r i t y >99.9 w t . - % ) o r 2-M-Hp (no i m p u r i t i e s d e t e c t a b l e by GLC) were c o n v e r t e d under hydrogen p r e s s u r e on Pd/LaY. I t s Pd c o n t e n t , Si/A1 r a t i o , and
420
degree of La3+ exchange were 0.27 w t . - % , 2 . 4 6 , and 72 equiv.-%, respectively. The mass of c a t a l y s t ( W ) amounted t o 500 mg a n d i t s p a r t i c l e s i z e was 0.2 t o 0.3 nun. The preparation a n d i n - s i t u pretreatment of the z e o l i t e as well a s t h e flow type apparatus with a fixed bed r e a c t o r have been described elsewhere ( r e f . 3,4). The p a r t i a l pressures a t the r e a c t o r i n l e t were PHc = 19.4 kPa and P H* = 2.0 MPa. The reaction temperature and the yodified residence time (W/FHc) were varied from 200 t o 300 "C and 70 t o 1200 g.h/mol, respectively. The products were analyzed by temperature programmed c a p i l l a r y GLC using a t l e a s t two d i f f e r e n t s t a t i o n a r y phases ( r e f . 3 ) . After each run t h e c a t a l y s t was purged in pure H, a t 300 "C and 2.0 MPa. RESULTS AND DISCUSSION Fig. 1 shows t h a t the r e a c t i v i t i e s of t h e naphthene and t h e alkane a r e v i r t u a l l y i d e n t i c a l . Up t o ca. 50 % conversion X and Y I s o . coincide, i . e . , s k e l e t a l isomerization i s t h e s o l e reaction. A t elevated conversions carboncarbon bond rupture occurs. Ring opening (Ro.) and hydrocracking (Cr.) a r e defined as the conversion of E-CHx i n t o octanes and t h e formation of hydrocarbons with l e s s than 8 carbon atoms, respectively. I t i s evident from Fig. 1 t h a t the alkane i s cleaved much more e a s i l y than the naphthene. In a l l p r o b a b i l i t y , both hydrocarbons r e a c t according t o a c l a s s i c a l bifunctional mechanism ( r e f . 5 ) . I t was found in additional isomerization experiments t h a t the orders with respect t o E-CHx and H, a r e s l i g h t l y above zero and - 1 , respectively. These values a r e c o n s i s t e n t with t h e bifunctional mechanism under 100
FEED : E - C H X 2 - M - H p 80
-
-0
0
60
>
Y
40
20
0 200
220
240
REACTION
260
280
300
TEMPERATURE. *C
Fig. 1 . Conversions a n d y i e l d s (W/FHc
=
135 t o 150 g.h/mol).
0 -t
w>
421
the assumption that rearrangements of carbocations at the acidic sites are rate controlling. Moreover, essentially the same selectivities were observed on a Pt/CaY zeolite (ref. 3) proving that isomerization does not occur on the noble metal. Isomerization It is useful (ref. 6,7) to distinguish between type A and type B isomerizations. In type A isomerizations the number of ramifications remains constant (e.g., E-CHx -+ propylcyclopentane or methylcycloheptane) whereas in type B isomerizations it increases (e.g., E-CHx ---+ethylmethylcyclopentanes or dirnethylcyclohexanes) or decreases. In general, type A isomerizations are faster. It is, therefore, not surprising that type A isomerizations predominate at low conversions (Fig. Z ) , regardless of the nature of the feed. With E-CHx, however, the contribution of type B isomerizations increases much faster with conversion. No cyclooctane is formed from E-CHx while n-octane is a typical type B product from 2-M-Hp. It is particularly noteworthy that tribranched i octanes do not occur in the products from 2-M-Hp while considerable amounts of trimethylcyclopentanes are formed from E-CHx at elevated conversions. This i s one of the repercussions of the sluggish cleavage of the naphthenic ring: Tribranched carbenium ions, once formed by skeletal rearrangements, either desorb from the acidic sites or crack, preferentially according to the favorable type A 8-scission (as defined in ref. 8). The latter step is the faster one if the tribranched carbenium ion is aliphatic. In alicyclic carbenium ions the
onobranched
(type A )
60 -
F E E D : E-CHx
-
FEED
:
2-M-Hp
4ot dibranched
o
20
40
60
ao
100 0 CONVER SlON , ' l o
Fig. 2. Selectivities of isomerization.
20
40
60
80
100
422
rate of 6-scission is diminished (possible reasons are discussed later), hence the competing desorption is favored. Selectivities of individual isomers from E-CHx are given in Table 1. P-CPn is the predominant type A isomer but small amounts of M-CHp are also formed. The formation of these isomers is readily interpreted in terms of classical hydride and alkyl shifts which bring about ring contraction or enlargement. Indeed, these are rapid ionic reactions as demonstrated by Pines and Shaw (ref. 9). At low conversions the preferred type B isomer from E-CHx is l-ethyl-trans2-methylcyclopentane. It has been shown (ref. 1,lO) that, on the level of the carbocations, type B rearrangements proceed via protonated cyclopropanes (PCPs). To account for the absence of I-E-1-M-CPn and 1-E-3-M-CPn at low conversion the mechanism of branching is best described by the sequence above the dotted line in Fig. 3 which starts from the t a t i m y ethylcyclohexyl cation (I). Such a route might have been anticipated since the tertiary ion (I) is probably much TABLE 1 Selectivity of isomerization of ethylcyclohexane (moles of corresponding isomer formed per moles of E-CHx isomerized in %).
T, "C W/FE-CHx, g-h/mol 'E-CHx'
%
200 70 1 .o
200 280 4.9
220 135 10.1
~
240 273 39.3 ~
~
300 139 94.8 ~
~~
P-CPn M-CHp
61 8
58.8 4.0
53.7 2.2
22.6 0.4
2.2 0
(1-M-E)-CPn 1-E-1-M-CPn 1-E-c-2-M-CPn 1-E-t-2-M-CPn 1-E-c-3-M-CPn 1-E-t-3-M-CPn 1,l-DM-CHx 1~2-DM-CHX It2-DM-CHx Ic3-DM-CHx 1 t3-DM-CHx Ic4-DM-CHx It4-DM-CHx
0 0 3 27 0 0 2 0 4 5 0 0 0
0 1.7 1.9 13.8 2.1 1.8 1.9 1.8 5.4 5.4
0.4 1.5 1.9 11.5 2.9 3.0 1.9
0.6 1.7 1.0 6.7 3.5 4.5 4.4 2.5 10.0 16.4 7.7 10.4
0.8 1.3 0.8
..............................................................................
2.1
0
6.7 6.7 3.4 2.1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
1.4
4.6
4.3 4.2 4.6 1.6 7.8 17.5 9.3 10.3
.............................................................................. 1,1,2-TM-CPn 1,1,3-TM-CPn IcEc3-TM-CPn Ic2t3-TM-CPn ItZc3-TM-CPn Ic2c4-TM-CPn Ic2t4-TM-CPn ItZc4:TM-CPn
0 0 0 0 0 0 0 0
0.9 2.0 0 0.6 1.8 0 0 2.3
3.6 8.2 0.2 2.3 5.8 1.8 8.8
423
Fig. 3. Type B rearrangements of ethylcyclohexyl cations. A, B and C represent protonated cyclopropanes (PCPs). more abundant than the secondary ions (11, 111, IV). However, it must be recalled at this point that the primary products of type B isomerization of a monobranched d k a w , viz. 2-methyloctane, could only be rationalized if branching via PCPs was assumed to start from the he,t 0 6 necondatry carbenium ions (ref. 6,7; the catalyst was Pt/CaY zeolite). This earlier conclusion is now nicely confirmed by the results obtained with 2-methylheptane on Pd/LaY: Assuming that type B rearrangements via PCPs start from the set of secondary 2-methylheptyl cations a theoretical treatment (cf. ref. 1,7) leads to the following predicted distribution of type B isomers: 9 % n-Oc; 9 % 2,2-DM-Hx; 27 % 2,3-DM-Hx; 37 % 2,4-DM-Hx; 18 % 2,5-DM-Hx. It is in rather good agreement with the experimental distribution at X = 13 % : 18 % n-Oc; 13 % 2,2-DM-Hx; 23 % 2,3-DM-Hx; 31 % 2,4-DM-Hx; 15 % 2,5-DM-Hx. More work is needed to arrive at a deeper understanding of this fundamental difference between isomerization of alkanes and naphthenes. To account for the formation o f dimethylcyclohexanes there are two alternative pathways: Starting from ethylmethylcyclopentyl cations ring enlargements Via type A rearrangement lead to dimethylcyclohexyl cations. On the other hand, these dibranched cations might be formed by type B rearrangements starting from met hy 1 cycl o heptyl cat i ons . Ring opening The yield of octanes from E-CHx was small even when W/FE-CHx was increased up to 1200 g.h/mol at 300 "C. Ring opening was never free from hydrocracking (cf. Fig. 1). A typical distribution of the octanes formed from E-CHx ( T = 300 "C, W/FE-CHX = 135 g.h/mol) was: 8 % n-Oc; 17 % 2-M-Hp; 18 % 3-M-Hp;
424 7 % 4-M-Hp;
5 % 2,Z-DM-Hx;
3 % 3,3-DM-Hx.
6 % 2,3-DM-Hx;
17 % 2,4-DM-Hx;
19 % 2,5-DM-Hx;
I t i s d o u b t f u l whether t h e s e d a t a a l l o w c o n c l u s i o n s c o n c e r n i n g
t h e n a t u r e o f t h e r i n g opening s t e p s i n c e secondary rearrangement a ( ~ t e hr i n g opening may have obscured t h e p i c t u r e . I t has been shown above t h a t i o n i c r u p t u r e o f a carbon-carbon bond which
forms p a r t o f t h e naphthenic r i n g proceeds s l u g g i s h l y . The mechanism o f such an
endacycfic cleavage i s a m a t t e r o f debate. I t i s o b v i o u s t h a t c l a s s i c a l 0s c i s s i o n o f a c y c l o a l k y l carbenium i o n r e s u l t s i n an a l k e n y l c a t i o n . I t has been argued ( r e f . 11) t h a t t h e l a t t e r i s s t r o n g l y bound t o t h e a c i d i c s i t e and has a h i g h tendency t o r e c y c l i z e . I n essence, t h i s i s a thermodynamic argument.
A k i n e t i c e x p l a n a t i o n f o r t h e l o w r a t e o f 8 - s c i s s i o n o f e n d o c y c l i c bonds was advanced by Brouwer and Hogeveen ( r e f . 12). A c c o r d i n g t o these a u t h o r s 8s c i s s i o n i n t h e c y c l i c carbenium i o n i s s l o w because o f t h e v e r y u n f a v o r a b l e o r i e n t a t i o n ( p e r p e n d i c u l a r o r n e a r - p e r p e n d i c u l a r i n s t e a d o f c o p l a n a r as i n a l k y l c a r b e n i u m i o n s ) o f t h e vacant p - o r b i t a l a t t h e e l e c t r o n d e f i c i e n t carbon atom and t h e 8-bond which i s t o be broken. As an a l t e r n a t i v e mechanism o f r i n g opening d i r e c t a t t a c k o f t h e a c i d i c p r o t o n a t an e n d o c y c l i c bond o f t h e c y c l o a l k a n e has been envisaged ( r e f . 1 3 ) . I n t h i s p i c t u r e , r i n g opening proceeds v i a a n o n - c l a s s i c a l carbonium ion. I t m i g h t be t h a t t h i s i s , indeed, an i m p o r t a n t mechanism o f r i n g o p e n i n g i n naphthenes s i n c e , under c e r t a i n circumstances, i t even c o n t r i b u t e s t o c r a c k i n g o f a1 kanes i n a c i d i c z e o l i t e s , as demonstrated r e c e n t l y by Haag and Dessau ( r e f . 14). Hydrocrac k i n g D i s t r i b u t i o n s o f t h e c r a c k e d p r o d u c t s formed f r o m b o t h hydrocarbons on Pd/LaY a r e d i s p l a y e d i n F i g . 4. S t a r t i n g f o r m 2-M-Hp b o t h t h e carbon number and t h e isomer d i s t r i b u t i o n s a r e a l m o s t i d e n t i c a l w i t h t h o s e r e p o r t e d e a r l i e r f o r h y d r o c r a c k i n g o f n-octane on Pt/CaY ( r e f . 2). T h i s i n d i c a t e s t h a t ( i ) t h e same mechanism i s o p e r a t i v e on b o t h z e o l i t e s and ( i i ) 8 - s c i s s i o n s t a r t s from t h e same p r e c u r s o r s , r e g a r d l e s s o f whether t h e a l k a n e f e e d i s unbranched o r monobrancned. Note t h a t C1 and C2 as w e l l as C 7 and C6 a r e a b s e n t which proves t h a t t h e r e i s no h y d r o g e n o l y s i s on t h e n o b l e metal. W i t h E-CHx a much h i g h e r r e s i d e n c e t i m e was necessary t o a r r i v e a t a s i m i l a r y i e l d o f c r a c k e d p r o d u c t s . Under t h e s e more severe c o n d i t i o n s m i n o r amounts o f methane and ethane do form, presumably v i a h y d r o g e n o l y s i s . Note, however, t h a t much more C7 and C6 occur and s l i g h t l y more C5 t h a n C3 i s found. A l l t h e s e f i n d i n g s a r e c o n s i s t e n t w i t h some c o n t r i b u t i o n o f an i o n i c d i s p r o p o r t i o n a t i o n t y p e of r e a c t i o n , i.e.,
a b i m o l e c u l a r mechanism, by which two C8 s p e c i e s com-
b i n e ; t h e r e s u l t i n g heavy m o l e c u l e r e a r r a n g e s and c r a c k s , e.g., C
4
into
+ C 5 + C, o r 2 C5 + C6. There i s no i n d i c a t i o n a t a l l f o r a s i m i l a r d i s p r o -
425 FEED: 2-M-Hp
FEED : E - CHx 120-
,
I
,
I
1
1
T
I
= 300 *C
100-830 g . h / m o l
n
u .C
1
2
3
1
CARBON
5
6
7
NUMBER Cp
1
Fig. 4. Hydrocracking o f E-CHx and 2-M-Hp. t h e cracked products. p o r t i o n a t i o n o f 2-M-Hp.
2
3
4
5
6
7
OF CRACKED PRODUCTS Distributions o f
The enhanced tendency o f naphthenes t o undergo d i s p r o -
p o r t i o n a t i o n on b i f u n c t i o n a l c a t a l y s t s has been demonstrated e a r l i e r w i t h methylcyclopentane and cyclohexane on Pt/CaY ( r e f . 15). CONCLUSIONS
A naphthene and an alkane w i t h t h e same carbon number and t h e same degree o f branching were converted on a t y p i c a l b i f u n c t i o n a l c a t a l y s t . A coarse l o o k a t the r e a c t i o n s i n d i c a t e s t h a t b o t h model hydrocarbons undergo i s o m e r i z a t i o n and hydrocracking v i a carbocations. A more s u b t l e examination, however, r e v e a l s considerable d i f f e r e n c e s between t h e i o n i c mechanisms o f t h e a l i c y c l i c and t h e a l i p h a t i c substrate. C l a s s i c a l 6 - s c i s s i o n o f e n d o c y c l i c bonds i n t h e naphthene i s slow which has a v a r i e t y o f repercussions b o t h on s k e l e t a l i s o m e r i z a t i o n and r i n g opening o f t h e naphthene. Apparently, type B rearrangement of the naphthene s t a r t s form t h e t e r t i a r y carbenium i o n whereas w i t h t h e alkane i t s t a r t s from t h e s e t of secondary carbenium ions. D i s p r o p o r t i o n a t i o n i s an important s i d e r e a c t i o n i n hydrocracking of t h e naphthene. ACKNOWLEDGEMENTS We thank Mr. W. Stober f o r s k i l l f u l a s s i s t a n c e w i t h t h e experiments. F i n a n c i a l support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen I n d u s t r i e i s g r a t e f u l l y acknowledged.
426
ABBREVIATIONS
M
methyl
c
E
ethyl
t
cis trans
p
ProPY1
c
cyclo
Pn
pentane
HC
hydrocarbon
.
hydro c r a c k in g
Hx
hexane
Cr
Hp
heptane
Iso. i s o m e r i z a t i o n
Oc
octane
Ro.
r i n g opening
REFERENCES
1 2 3 4
5 6 7
8 9 10 11 12 13 14 15
J. Weitkamp, Ind. Eng. Chem., Prod. Res. Dev., 21 (1982) 550-558. J. Weitkamp, Am. Chem. SOC. Symp. Ser., 20 (1975) 1-27. J. Weitkamp, P.A. Jacobs and S. E r n s t ; S t u d i e s i n Surface Science and C a t a l y s i s , Vol. 18, E l s e v i e r Science Publishers, Amsterdam, 1984, pp. 279-290. H.F. Schulz and J. Weitkamp, Ind. Eng. Chem., Prod. Res. Dev., 11 (1972) 46-53. P.B. Weisz, Adv. Catal., 13 (1962) 137-190. J. Weitkamp and H. Farag, Acta U n i v e r s i t a t i s Szegediensis, Acta Physica e t Chemica, 24 (1978) 327-333. J. Weitkamp and P.A. Jacobs, P r e p r i n t s , Div. Petr. Chem., Am. Chem. SOC., 26 (1981) 9-13. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal., 8 (1983) 123-141. H. Pines and A.W. Shaw, 3. Am. Chem. SOC., 79 (1957) 1474-1482. D.M. Brouwer and J.M. O e l d e r i k , Rec. Trav. Chem., 87 (1968) 721-736. C.J. Egan, G.E. L a n g l o i s and R.J. White, J. Am. Chem. SOC., 84 (1962) 1204-1212. D.M. Bruuwer and H. Hogeveen, Rec. Trav. Chim., 89 (1970) 211-224. S.G. Brandenberger, W.L. C a l l e n d e r and W.K. Meerbott, J. Catal., 42 ( 1976) 282-287. W.O. Haag and R.M. Dessau, Proc. 8 t h I n t e r n . Congr. Catal., Vol. 2, Verlag Chemie, Weinheim, D e e r f i e l d Beach, Basel, 1984, pp. 305-316. H. Schulz, 3. Weitkamp and H. Eberth, Proc. 5 t h I n t e r n . Congr. Catal., Vol. 2, North-Holland P u b l i s h i n g Co., Amsterdam, 1973, pp. 1229-1239.
427
B. Imelik e f ol. (Editors), Cafalysis by Acids and Bases 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
PRIMARY CRALKlNG MODES OF LONG CHAIN PARAFFINIC HYDROCARBONS I N OPEN ACID ZEOLITES by Johan A. Martens',
Jens Weitkamp
2
and P e t e r A. Jacobs
1. Centrum v o o r Oppervlaktechemie, K.U.
1
Leuven
K a r d i n a a l M e r c i e r l a a n 92, B-3030 Leuven ( H e v e r l e e ) 2. E n g l e r - B u n t e - I n s t i t u t e ,
U n i v e r s i t y o f Karlsruhe,
D-7500 K a r l sruhe, FRL;
SUMMARY Cracked p r o d u c t d i s t r i b u t i o n s f r o m octane, nonane, decane, un- , do-, pentaand heptadecane o v e r P t on USY z e o l i t e a r e r e p o r t e d . The d e t a i l e d c o m p o s i t i o n o t t y p i c a l carbon number f r a c t i o n s i s d e t e r m i n e d o v e r t h e whole c r a c k i n g c o n v e r s i o n range. Secandary i s o m e r i z a t i o n and c r a c k i n g r e a c t i o n s d i s t u r b e a t y p i c a l cracked p r o d u c t d i s t r i b u t i o n o b t a i n e d a t s m a l l c r a c k i n g c o n v e r s i o n s and which i s presumably t h e unchanged p i c t u r e o f t h e tragrnents f r o m t h e s c i s s i o n r e a c t i o n . T h i s i n i t i a l c o m p o s i t i o n o f a g i v e n carbon number f r a c t i o n i s shown t o be independent o f c h a i n l e n g t h o f t h e t e e d p a r a f f i n . RESUHE Les p r o d u i t s d'hydrocraquage de l ' o c t a n e , du nonane, du decane, des un-, do-, penta-, heptadecane s u r P t s u r z e o l i t e u l t r a s t a b l e o n t 6 t @ analyses. L ' i n f l u ence du t a u x de c o n v e r s i o n s u r l a c o m p o s i t i o n de quelques coupes t y p i q u e s a e t @ &tucii&e. A f a i b l e t a u x d e c o n v e r s i o n l a c o m p o s i t i o n des p r o d u i t s de craquage, independante de l a l o n g u e u r de l a p a r a f f i n e c o n v e r t i e , e s t c a r a c t e r i s t i q u e du craquage p r i m a i r e . L ' a u g m e n t a t i o n du t a u x de c o n v e r s i o n i n d u i t des r e a c t i o n s d ' i s o m e r i s a t i o n e t de craquage secondai r e .
INTRODUCTION Ultrastable Y zeolite, excellent bifunctional
loaded w i t h 0.5 and 1 % b y w e i g h t o f P t metal i s an
catalyst
(1). Up t o medium conversions,
decane (1,3,4) and dodecane (4) a r e i s o m e r i z e d w i t h o u t c r a c k i n g .
octane(2)
,
A t higher
conversions, t h e f e e d isomers undergo " p r i m a r y " c r a c k i n g r e s u l t i n g i n o n l y two fragments o u t o f one f e e d molecule. Due t o t h e l a r g e number o f c r a c k e d p r o d u c t s o b t a i n e d o u t o f l o n g c h a i n p a r a f f i n s , d i s t r i b u t i o n s o f cracked tragments a r e commonly r e p o r t e d i n terms o f carbon numbers (CN) and o f i s 0 t o normal r a t i o ' s i n t h e CN t r a c t i o n s . present
paper
however
focusses
on
fragments f r o m l o n g c h a i n n - p a r a f f i n s .
the
production
of
individual
The
cracked
The e f f e c t o f t h e c h a i n l e n g t h o f t h e
f e e d p a r a f f i n and t h e i n f l u e n c e o f t h e c r a c k i n g c o n v e r s i o n on t h e c o m p o s i t i o n o f t y p i c a l CN f r a c t i o n s
i s i n v e s t i g a t e d i n t h e r e a c t i o n of nonane, decane,
dodecane, present
pentadecane and heptadecane. typical
featares
shape-sel e c t i v e c o n s t r a i n t s
.
of
The data are selected so t h a t they
bifunctional
hydrocracking
in
absence
of
EXPERIMENIAL 1. M a t e r i a l s
U l t r a s t a b l e Y (USY) z e o l i t e was prepared P t as reported e a r l i e r (4). c a t a l y t i c experiments.
and loaded w i t h 1 % by weight o f
Z e o l i t e pellet;
o f 0.3-0.5
mm were used i n the
The p a r a f f i n s had a p u r i t y o f a t l e a s t 99 %,
the
i m p u r i t i e s being mostly isomers w i t h the same carbon number as the feed. 2. Methods
Hydrocracking experiments were done i n a continuous f l o w t u b u l a r r e a c t o r operating a t atmospheric pressure. The c a t a l y s t was reduced i n s i t u i n f l o w i n g hydrogen a t 673 K a f t e r a c a l c i n a t i o n i n oxygen a t t h e same temperature. The hydrogen/hydrocarbon molar r a t i o was v a r i e d between 60 and 250 and the WHSV between 0.2
and 4.5
h-I.
The r e a c t o r o u t l e t was analysed o n - l i n e by gas
chromatography using a 50 m fused s i l i c a c a p i l l a r y column coated w i t h O V l O l as s t a t i o n a r y phase. The whole cracking conversion range was covered by varying t h e r e a c t i o n temperature between 400 and 500 K a t constant space v e l o c i t y . RESULTS AND DISCUSSION 1. Preference f o r c e n t r a l primary c r a c k i n q Primary cracking o f l o n g chain p a r a f f i n s i n terms o f carbon numbers occurs as f o l l o w s : (CN),
+
(CN)n-x
+
(CN),
w i t h 3 ( x g n-3. Since no primary carbenium ions intervene i n t h i s type o f c a t a l y s i s no methane nor ethane can be formed.
Table 1 shows t h a t a t low
cracking conversions, pure primary cracking i s operative f o r 8 ( n
,<
12. The
hydrocarbon chain i s p r e f e r e n t i a l l y s p l i t i n the center so t h a t w i t h increasing o t the chain length o f the feed p a r a f f i n propane a b s t r a c t i o n becomes l e s s and 1ess probable.
429
TAbLt 1
Distribution
by
carbon
number
of
the
cracked
products
from long
chain
n - p a r a f f i n s over 1 P t on USY z e o l i t e .
feed
octane nonane decane undecane dodecane
y i e l d o f cracked products per carbon number (CN) (mo1/100 mol cracked)
cracking conversion %
7 6 3 30 8
CN3
CN4
CN5
CN6
CN7
CN8
CN9
41 18 10
118 82
-
-
44
49 42
44 41
-
7 5
41 82 66 49 41
-
33
5
57 33
18 57
10
7
-
SUM 200 200 200 200
200
Cracked product d i s t r i b u t i o n s very s i m i l a r t o those o f Table 1 were a l s o obtained over CaY z e o l i t e loaded w i t h 0.5 % by weight o t P t (5,6,7).
It f o l l o w s
t h a t over b i f u n c t i o n a l c a t a l y s t s w i t h o u t shape-selective c o n s t r a i n t t h e same primary c r a c k i n g mechanisms a r e presumably operative. A gradual decrease o f t h e p r e f e r e d c e n t r a l s c i s s i o n o f n-decane was observed f o r z e o l i t e s w i t h lower pore and/or cage diameters ( 3 ) .
2. S u s c e p t i b i l i t y
to
secondary c r a c k i n g o f fragments
o b t a i n t e d by primary
hydrocracki n 9 A t conversions above
60 I f o r penta- and heptadecane, t h e symmetry i n t h e
d i s t r i b u t i o n of t h e carbon numbers (CN) o f t h e cracked products i s l o s t since a d r a s t i c a l increase i n t h e production o f l i g h t fragments,
together w i t h a drop
i n t h e y i e l d of heavier products i s observed. ( F i g . 1, F i g . 2).
430 Y)
1
CARBON NUMBER OF CRACKED PRODUCTS CARBON NUMBER OF CRACKED PRODUCTS
F i g . 1. Carbon number d i s t r i b u t i o n s o f t h e c r a c k e d p r o d u c t s f r o m heptadecane o v e r Pt/USY a t d i f f e r e n t c r a c k i n g c o n v e r s i o n s * ( a ) 70 %, ( b ) 83 %, ( c ) 93 % and '(d) 97 %.
F i g . 2. Carbon number d i s t r i b u t i o n s o f t h e cracked p r o d u c t s f r o m pentadecane o v e r Pt/USY a t v a r y i n g c r a c k i n g c o n v e r s i o n s * ( a ) 64 %, ( b ) 85 %, ( c ) 93 %, ( d j 97 %.
The CN o f t h e f r a c t i o n w h i c h i s most a f f e c t e d b y "secondary"
cracking
decreases w i t h i n c r e a s i n g c r a c k i n g conversion. F o r i n s t a n c e , i n t h e r e a c t i o n o f heptadecane ( F i g .
l ) , t h e f r a c t i o n w i t h CN12 shows t h e s h a r p e s t decrease
(-6,8 m o l ) when t h e h y d r o c r a c k i n g s e v e r i t y goes f r o m 10 % t o 83 % w h i l e between 83 % and 93 % c r a c k i n g i t i s t h e f r a c t i o n w i t h C N l l which decreases most (-6,8 m o l ) . Between 93 % and 97 % c r a c k i n g t h e CNlO and C N l l f r a c t i o n s a r e most a f f e c t e d (-2,8
m o l ) . On a r e l a t i v e b a s i s however,
t h e l o n g e s t fragments a r e
more a f f e c t e d t h a n s h o r t e r ones. A t 83 % c r a c k i n g o f heptadecane,
t h e CN13
f r a c t i o n i s reduced by 60 % compared t o i t s v a l u e a t 70 % c r a c k i n g . On t h e c o n t r a r y t h e f r a c t i o n w i t h CN12 decreases o n l y by 40 % w h i l e t h e C N l l f r a c t i o n o n l y s h r i n k s b y 20 % i n t h e same c o n v e r s i o n range.
It follows
that the
s u s c e p t i b i l i t y o f t h e c r a c k e d p r o d u c t s t o secondary c r a c k i n g decreases w i t h d e c r e a s i n g carbon number.
T h i s can be e a s i l y u n d e r s t o o d i n terms
of
the
c l a s s i c a l b i f u n c t i o n a l mechanism. The f r e e o l e f i n s generated on t h e metal phase compete f o r t h e a v a i l a b l e B r a n s t e d s i t e s so t h a t t h e o l e f i n s w i t h t h e l o n g e r c h a i n a r e adsorbed p r e f e r e n t i a l l y . yield of
the f r a c t i o n with
F o r heptadecane h y d r o c r a c k i n g , t h e absol Ute
CN9 does n o t change w i t h i n c r e a s i n g c r a c k i n g
c o n v e r s i o n . ( t i g . 1). T h i s r e f l e c t s t h a t t h e o v e r p r o d u c t i o n o f CN9 fragments by secundary c r a c k i n g o f CN13 and CN12 i s compensated by secondary c r a c k i n g o f t h e
CN9 f r a c t i o n
itself.
In
the
reaction
of
pentadecane
(Fig.
2)
the
CN8
431
f r a c t i o n shows t h e same behaviour. O u t of a l l short chain products with CN from 3 t o 7 f o r hydrocracking of penta- and heptadecane, only the propane production remains unchanged a t increasing secondary cracking conversions (Fig. 1, Fig. 2 ) . l h i s indicates t h a t a l s o t o r secandary hydrocracking central s c i s s i o n i s preferred over propane abtraction. 3. Mechanism of secondary cracking
The question a r i s e s now whether secondary cracking of a mixture of long fragments obtained from primary hydrocracking of longer hydrocarbon chains gives products i d e n t i c a l t o those from primary hydrocracking of the same hydrocarbon chains. In the reaction of pentadecane the y i e l d of CN12, CN11, C N l O a n d CN9 decreases with 0,8; 4,9; 2,6 and 0,8 mol respectively between 64 and 85 % cracking (Fig. 2). Using the model d i s t r i b u t i o n s given in Table 1, the cracked products formed from the fragments which disappear in t h e 64-85 % cracking range can be calculated and a r e given in Table 2. Since t h e y i e l d of TABLE 2 Generation of secondary cracked products in the hydrocracking of pentadecane assuming primary cracking patterns of the l a r g e s t fragments.
mo I cracked
CN
fraction
d i s t r i b u t i o n of secondary cracked productsa (mo1/100 mol) CN3
CN4
CN5
CN6
CN7
CN8
CN9
0.33 2.16 0.26
0.26 0.34
0.04
12 11 10 9 8
0,8 4.9 2.6 0.8 0.6
0.04 0.34 0.26 0.15 0.25
0.~6 2.16 1.48 0.65 0.70
0.33 2.40 1.72 0.65 0.25
0.34 2.40 1.48 0.15
SUM
9.1
1.04
5.25
5.35
1.u
4.8
5.1
experimenta1
b
-
-
-
-
-
-0.60
-
4.37
2.75
-
0.04
4.2
2.6
-
-
~~
a , calculated using t h e model d i s t r i b u t i o n s of Table 1. b y amount of product formed when the pentadecane cracking increases from 64 t o 85 % ( s e e Fig. 2). the CN8 f r a c t i o n i s constant (Fig. 2 ) , t h e production of CN8 fragments i n t h e secondary cracking of the C N l Z and C N l l f r a c t i o n s has t o be compensated by cracking of an equal amount of CN8, namely 0.6 mol. Table 2 shows t h a t t h e products formed by secondary cracking when the overall cracking of pentadecane i s increased from 64 t o 85 % can be reasonably well predicted when i t i s
432 assumed t h a t secondary c r a c k i n g g i v e s t h e same p r o d u c t s i n i d e n t i c a l amounts as p r i m a r y h y d r o c r a c k i n g . I t f o l l o w s t h a t p r i m a r y and secondary c r a c k i n g p r o b a b l y o c c u r v i a t h e same c r a c k i n g mechanisms and t h a t secondary c r a c k i n g can be modeled by t h e use o f c r a c k i n g p a t t e r n s o f pure compounds. 4. Secondary i s o m e r i z a t i o n o f c r a c k e d fragments
P r i m a r y and secondary tragments.
CN f r a c t i o n s were s e l e c t e d t o
Typical
i l l u s t r a t e t h e e f f e c t o f i n c r e a s i n g c r a c k i n g c o n v e r s i o n on t h e i r composition.
65
x
55 60: 50
.
rC15 c12 0 c10
-
-
2
-
0
$ K
F i g . 3. Composition o f t h e CN6 f r a c t i o n o b t a i n e d f r o m t h e hydroc r a c k i n g o f decane, dodecane, pentadecane and heptadecane as a f u n c t i o n o f t h e c r a c k i n g conv e r s i o n (%).
T
-I
25-
V T G 3MC5
U
0 20
CRACKING CONVERSION ( X )
From
Fig.
3
2,3-dimethylbutane 2,2-dimethylbutane
it
is
clear
that
2-
and
are primary products o f
hnethylpentane,
the cracking
hexane
reaction,
and
whereas
i s o n l y formed as a secondary p r o d u c t a t h i g h c o n v e r s i o n . A t
small c r a c k i n g c o n v e r s i o n s t h e c o m p o s i t i o n o f t h i s CN6 f r a c t i o n i s v e r y s i m i l a r f o r a l l f e e d p a r a t f i n s used (nClo,
-
3-methylpentane
60 64 % 22.5 26.5 %
hexane
9.5
2,3-dimethylbutane
2
2-methylpentane
-
nC12, nC15 and nC17) and i s as f o l l o w s :
-
-
11.5 %
4 %.
A t h i g h e r c o n v e r s i o n s , t h e amount o f 2-methylpentane decreases a t t h e p r o f i t o f
433
3-methylpentane,
hexane,
2,3-
and
2,2-dimethylbutane.
The
amounts
of
2,2-dimethylbutane formed a t h i g h c r a c k i n g c o n v e r s i o n s decrease w i t h i n c r e a s i n g chain
length
of
the
feed
paraffin.
In
the
reaction o f
heptadecane
and
pentadecane, t h e c o m p o s i t i o n o f t h e CN6 f r a c t i o n remains unchanged up t o 80 % c r a c k i n g c o n v e r s i o n , whereas i n t h e r e a c t i o n o f dodecane and e s p e c i a l l y decane, t h e c o m p o s i t i o n changes f r o m l o w c o n v e r s i o n s on. Thus,
secondary i s o m e r i z a t i o n
o f t h e p r o d u c t s formed by p r i m a r y h y d r o c r a c k i n g , j u s t 1 i k e secondary c r a c k i n g becomes g r a d u a l l y more i m p o r t a n t f o r f r a c t i o n s w i t h a CN c l o s e t o t h a t o f t h e feed.
T h i s a l l o w s t o g e n e r a l i z e an e a r l i e r c o n c l u s i o n ( 8 ) on t h e secondary
n a t u r e o f cracked p r o d u c t s w i t h q u a t e r n a r y C-atoms u s i n g dodecane as f e e d and Pt/CaY as c a t a l y t i c system.
F i g . 4. Composition o f t h e CN8 f r a c t i o n o b t a i n e d f r o m t h e h y d r o c r a c k i n g o f dodecane, pentadecane and heptadecane as a f u n c t i o n o f t h e c r a c k i n g convers i o n (%)
.
cracking conversion ( % )
The e f f e c t o f t h e c r a c k i n g c o n v e r s i o n on t h e c o m p o s i t i o n o f t h e CN8 f r a c t i o n (Fig.
4) i s v e r y s i m i l a r t o t h a t o f t h e CN6 f r a c t i o n .
From a l l CN8 p r o d u c t s
o n l y t h o s e w i t h a q u a t e r n a r y C-atom a r e formed i n a c o n s e c u t i v e manner. A t l o w c r a c k i n g c o n v e r s i o n s f o r t h e f e e d p a r a f f i n s used t h e c o m p o s i t i o n o f t h e CN8 fragments i s s i m i l a r :
13 -
methylheptanes + 3-ethylhexane : 78
: : 5 -
d imethy 1hexa ne s octane
81 % 15 % 8%.
The p r o d u c t i o n o f methyl heptanes decreases i n f a v o u r o f t h e o t h e r CN8 p r o d u c t s
w i t h i n c r e a s i n g conversion.
T h i s e f f e c t i s p r e s e n t o v e r t h e whole c o n v e r s i o n
range i n t h e r e a c t i o n o f dodecane, whereas i t occurs o n l y a t h i g h c r a c k i n g conversions
of
hepta-
and
pentadecane.
The
amount
of
2,2-
and
434
3,3-dimethylhexane, produced at high conversions, decreases with the chain length of the feed paraffin.
Differences between primary and secondary isomerization. The C N l O fraction of the cracked products is composed of methylnonanes, dimethyloctanes, ethyloctanes and decane (Fl'g. 5). Over the whole conversion range the yield of methylnonanes decreases while the production of dimethyloctanes increases. The amounts o f decane and ethyloctanes formed increase only slightly. In the reaction o t heptadecane at 40 % cracking, the relative amount of methylnonanes and dimethyloctanes formed is the same as for the reaction of pentadecane at 20 % cracking.
55
Fig. 5. Composition of the CNlO fraction obtained from the hydrocracking of pentadecane and heptadecane as a function of the cracking conversion ( % ) .
4
1
1
20
1
40
1
~
60
"
"
80
CRACKING CONVERSION ( % )
~
435 TOTAL CONVERSION ( 9 6 ) 10
30
50
70
90
Y
CRACKING CONVERSION ( % )
Fig. 6. Composition of the methylnonanes obtained from the hydrocracking of pentadecane and heptadecane as a function of the cracking conversion (%) (full lines) and from the hydroisomerization of decane as a function of the total conversion of decane ( X ) (dashed 1 ines).
Figure 6 specifies the composition of the methylnonanes formed out of pentaand heptadecane by hydrocracking : at increasing cracking conversions the level of 2-methylnonane decreases at the expense of the other isomers. The same preference for the 2-methyl isomer is observed in the CN6 fraction (Fig. 3). For comparison, the composition of the methylnonanes in the hydroisomerization reaction of decane is also shown in Fig. 6. In contrast to the production of methylnonanes via hydrocracking of large paraffins, direct hydroisomerization of n-decane favours 3-and 4-methylnonane. This is evidence that the monobranched cracked products are not formed by the isomerization of normal fragments but by direct hydrocracking. A t high conversions of decane, pentaand heptadecane compositions of the methyl nonanes close to thermodynamic equilibrium are obtained. CONCLUSIONS Over the "open" bifunctional Pt on USY zeolite catalyst a primary hydrocracking mechanism is observed which prefers central scission of the hydrocarbon chain. For hydrocracking at small cracking conversions, the composition of a CN fraction is independent of the chain length of the feed paraffin. This composition deviates strongly from the thermodynamic equilibrium and is characterised by a high 2-methyl isomer content, a low amount of dibranched isomers and the absence of molecules with quanternary C-atoms. This distribution shows therefore the picture of the primary fragmentation.
436
A t i n c r e a s i n g conversion, gradually changes towards
t h i s primary thermodynamic
composition o f a equilibrium via
CN f r a c t i o n "secondary
isomerization reactions. I n the same way, secondary cracking gradually a f f e c t s the
composition
hydrocracking
of
of
the
a CN
higher
CN
fractions.
fraction
is
similar
to
Mechanistically, primary
secondary
cracking of
the
corresponding p a r a f f i n . REFERENCES 1 P.A. Jacobs, J.B. Uytterhoeven, M. S t e i j n s , G. Froment and J. Weitkamp, Proc. 5 t h I n t . Conf. Zeolites, L.V.C. Rees (ed.), Heyden, London, (1980) 607. 2 H. Vasina, M.A. Baltanas and G. Froment, Ind. Eng. Chem. Prod. Res. Dev., 22 (1983) , 526. 3 J.A. Martens, M. Tielen, P.A. Jacobs and J. Weitkamp, Zeolites, vol 4 (1984), 98. 4 M. S t e i j n s , G. Froment, P. Jacobs, J. Uytterhoeven and J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) b54. 5 J. Weitkamp, Erdol , Kohle-Erdgas-Petrochem. , 31 (1918) 13. 6 H.F. Schulz and J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev., 11 (1972) 46. 7 J. Weitkamp i n "Hydrocracking and Hydrotreating", J.W. Ward and S.A. Quader (eds.), ACS Symp. Sec., American Chemical Society, Washington, D.C., Vol 20, (1975) p.l. 8 Prepr. Div. Petr. Chem., Am. Chem. SOC., 17, no. 4 (1972) 6-84.
437
B. Imelik et el. (Editors), Catalysis b y Acids and Bases 1985 Elsevier Science Publishers B.V.,Amsterdam -Printed in The Netherlands
CATALYSEURS ISOLANTS ET ACIDITE
-
LES ACIDES PARADOXAUX
Yves TRAMBOUZE C e n t r e N a t i o n a l de l a Recherche S c i e n t i f i q u e , catalyse
-
69626
-
V i l l e u r b a n n e C6dex
-
I n s t i t u t de Recherches s u r l a
FRANCE
ABSTRACT A f t e r a s h o r t h i s t o r i c a l survey o f t h e e v o l u t i o n o f t h e concept o f a c i d i t y , we i n d i c a t e how i n s u l a t o r s o l i d s , p r e v i o u s l y c o n s i d e r e d as n e u t r a l , c a n e x h i b i t a B r o n s t e d o r L e w i s acido-basic c h a r a c t e r which accounts f o r t h e c a t a l y t i c a c t i v i t y o f "paradoxal acids". SOMMAIRE Apr'es un b r e f h i s t o r i q u e s u r l ' e v o l u t i o n d e l a n o t i o n d ' a c i d i t e , nous m o n t r o n s comment des s o l i d e s i s 0 1 a n t s , c o n s i d e r g s comme n e u t r e s , p e u v e n t p r e s e n t e r un c a r a c t k r e a c i d o - b a s i q u e de B r o n s t e d e t L e w i s c e q u i p e r m e t d ' e x p l i q u e r l ' a c t i v i t e c a t a l y t i q u e de ces acides "paradoxaux". En 1786, Guyton d e Morveau, dans son "Encyclopedie de l a Chimie" 6 c r i t : "Rien de p l u s embarassant que de former une j u s t e idee de l a v e r t u d e f i n i t i o n de l ' a c i d e e s t , 'a v r a i d i r e ,
acide. La
l a c l e de l a chimie, non seulement
parce que ce sont l e s acides q u i produisent l e s p l u s beaux p h e n o m h e s , e t en p l u s g r a n d nombre, m a i s e n c o r e p a r c e que c e s o n t des a s e n t s v i s i b l e s e t palpables, dont l e s e f f e t s frappent nos sens, de s o r t e que l ' o n p e u t d i r e que c ' e s t l a c h i m i e des a c i d e s q u i a ebauch6 l a science, e t q u i s e r t de base au syst'eme g e n e r a l de n o s c o n n a i s s a n c e s dans c e t t e p a r t i e de 1 ' C t u d e de l a nature". Deux cents ans p l u s t a r d , nous ne pouvons que s o u s c r i r e 'a c e p o i n t de vue. Dans un domaine aussi complexe que c e l u i - c i , d'histoire,
il e s t bon de f a i r e un peu
s i n o n d'Cpist6mologieY pour c l a r i f i e r l e probl'eme en l e r e p l a c a n t
dans son c o n t e x t e q u i e s t c e l u i de l a chimie e t de son e v o l u t i o n . La n o t i o n d ' a c i d i t e remonte 'a l a p l u s haute a n t i q u i t e o'u l ' a c i d e ac6tique, maladie du v i n , e t a i t connu
e t donna par "acidus-aiqre"
f a m i l l e des composgs s e m b l a b l e s .
l a r a c i n e l a t i n e 'a l a
P l u s p r k s d e nous e t p a r a n a l o q i e ,
a l c h i m i s t e s consid6r'erent comme acides l e v i t r i o l
.
l ' e s p r i t d e s e l etc...
les Au
XVI'eme s i k c l e , Van Helmont d e f i n i t l e s a l c a l i s ( d e l ' a r a b e a l - k a l i = cendres) en rapprochant l'ammoniaque des bases, souvent confondues avec l e s c a r b o n a t e s . Vers l a mOme epoque, Tachenius f i t une importante c o n s t a t a t i o n : il observa que
438
l e s a c i d e s e t l e s bases connus 'a 1'6poque, r 6 a g i s s a i e n t pour former des s e l s , &action sur l a q u e l l e , t r o i s s i ' e c l e s p l u s t a r d , Arrhenius a l l a i t f o n d e r s a thgorie. Mais 1 a premi'ere t e n t a t i v e d'explication s c i e n t i f i q u e de l ' a c i d i t 6 ne f u t f a i t e q u ' b l a f i n d u X V I I I h e s i ' e c l e par l e p'ere de l a chimie moderne : L a v o i s i e r . I 1 o b s e r v a que l a plupart des composCs q u ' i l obtenait l o r s de ses Ctudes sur l a combustion p r k e n t a i e n t un c a r a c t e r e acide, d'ob i l c o n c l u t aue 1 'oxyg'ene 6 t a i t 'a 1 ' o r i q i n e de c e t t e propri6t6. Mais s e s proDres exp6rierIces u l t 6 r i e u r e s , l a d6couverte que l ' a c i d e m u r i a t i q u e ne c o n t i e n t pas d ' o x y d n e l ' a m e n b r e n t 'a nuancer s a c o n c l u s i o n e t Davy, en 1814, i l y a donc 170 ans, formula une nouvelle hypoth'ese que, d'une c e r t a i n e m a n i s r e , Lewis r e p r i t 110 ans plus t a r d : l e caract'ere acide n ' e s t pas d^u 'a un 616ment p a r t i c u l i e r mais 'a une organisation particuli'ere des g l b e n t s de l a mol6cule. Cependant, t o u s l e s acides connus 'a 1 '6poque contenant de l'hydrog'ene, Gerhardt r e v i n t aux id6es de Lavoisier mais en p r i v i l g g i a n t c e t t e f o i s l'hydrog'ene, e t c e t t e id6e d e v a i t s e perp6tuer de nombreuses ann6es. A r r h g n i u s , 'a l a f i n du si'ecle dernier, qui s ' i n t 6 r e s s a i t d ' a i l l e u r s 'a l a c a t a l y s e acido-basique en solution, f u t amen6, pour expliauer ses r e s u l t a t s , 'a f o r m u l e r s a c6l'ebre th6orie des e l e c t r o l y t e s d'ob i l d6duisit ses d 6 f i n i t i o n s bien connues des a c i d e s e t d e s b a s e s i m p l i q u a n t l a pr6sence de p r o t o n s e t d ' h y d r o x y l e s en s o l u t i o n aqueuse, e t r e p r i t l ' a n c i e n n e a f f i r m a t i o n que acide + base = s e l + eau. E n 1923, Bronsted, au c o w s de s e s travaux sur l a c a t a l y s e acido-basique en s o l u t i o n , f u t amen6 'a 6 l a r g i r l a notion d ' a c i d i t 6 propos6e 3 0 ans p l u s t 6 t p a r A r r h e n i u s , pour j u s t i f i e r l a th6orie d u a l i s t i q u e selon l a q u e l l e l e proton e t l ' i o n hydroxyde ne sont pas l e s s e u l s catalyseurs. I1 proposa, simultan6ment avec Lowry, l e s d C f i n i t i o n s maintenant bien connues : un acide est une substance s u s c e p t i b l e de c6der un proton, sans pr6juger de l a forme sous l a q u e l l e + + i l s e r e t r o u v e , H30 , C2H50H2 ou NH4+ par exemple : une base devient une substance capable de capter un proton quelle que s o i t s a forme. Ces d 6 f i n i t i o n s 6 l a r g i s s e n t considerablement l e nombre des composk qui peuvent pr6senter un caract'ere acido-basique, un sel n ' e s t plus n k e s s a i r e m e n t l e r g s u l t a t d'une n e u t r a l i s a t i o n , l e solvant peut Gtre a u t r e que l ' e a u e t mGme i l peut Gtre absent comme dans l a &action 'a l ' 6 t a t gazeux e n t r e NH3 e t HC1. I1 est mgme possible de considgrer, mais c e l a ne f u t pas f a i t 'a 1'6poque, comme acide ou base des solides perdant ou captant des protons. Mais c e s d g f i n i t i o n s n ' e x p l i q u a i e n t pas d e s r 6 a c t i o n s manifestement a c i d o - b a s i q u e s , bien que l e p r o t o n n ' i n t e r v i e n n e pas, comme SiOp + CaO. 11 f a l l u t donc e n c o r e g 6 n 6 r a l i s e r l a n o t i o n d ' a c i d i t 6 s a n s , c e t t e f o i s , p r i v i l g g i e r un element p a r t i c u l i e r , comme l ' a v a i t d6j'a Dr6conis6 Davy en 1814.
439
C ' e s t c e que f i t Lewis en 1923, 'a l a s u i t e de s e s r 6 f l e x i o n s sur l e s d i f f 6 r e n t e s l i a i s o n s chimiques. Cherchant l e c a r a c t e r e commun 'a t o u s l e s compos6s a c i d e s s e l o n Arrhenius e t Bronsted, i l constata q u ' i l s pr6sentaient tous un d 6 f i c i t 6lectronique dans l e u r couche externe, d'o'u l a d 6 f i n i t i o n : u n a c i d e e s t capable de capter une p a i r e d ' 6 l e c t r o n s pour compl6ter son o c t e t , e t une base e s t capable de f o u r n i r c e t t e p a i r e d'6lectrons pour donner u n comoos6 de c o o r d i n a t i o n e t non p l u s u n s e l , s e l o n A r r h e n i u s , ou un nouveau systgme acido-basique selon Bronsted. Parmi l e s cas l e s en solution aqueuse solution-gaz solide-gaz
plus typiques c i t o n s : Ht+O:H- " , H:O:H t + Ag +2:NH3 A q ( :NH3)2 C13A1+:NH3 C13 A1 :NH3
+
+
Bronstedien non Bronstedien non Bronstedien
avec, dans tous l e s cas, formation d'une l i a i s o n covalente de coordination. C e t t e g6n6ral i s a t i o n a l l a i t permettre d'expl iquer bien des ph6nomGnes, en p a r t i c u l i e r , celui qui nous int6resse : l a c a t a l y s e h6t6roqene par l e s compos6s i sol ants
.
Ce n ' e s t cependant que dix ans apres l a parution d u c6l'ebre l i v r e de Lewis sur l e s l i a i s o n s dans lequel i l exposait incidemment ses i d 6 e s s u r l ' a c i d i t 6 , que Gayer ( I ) , i l y a donc 'a p e i n e 50 ans, f i t une remarque c a p i t a l e en d6couvrant, par des i n d i c a t e u r s c o l o r 6 s , l e c a r a c t e r e a c i d e de c a t a l y s e u r s c o n s t i t u 6 s p a r d e s t e r r e s g e n r e b e n t o n i t e ( r a p p e l o n s q u ' B l a mtme Cpooue, Houdry inventa l e crackinq c a t a l y t i q u e sur l e mtme type de masse de c o n t a c t ) . Mais s ' i l pensa q u ' i l d e v a i t y a v o i r u n r a p p o r t e n t r e a c i d i t 6 e t a c t i v i t 6 c a t a l y t i q u e , s ' i l o b s e r v a u n empoisonnement par l e s b a s e s a l c a l i n e s ,
il
n ' a p p r o f o n d i t pas s e s r 6 s u l t a t s p o u r t a n t f o r t p r o m e t t e u r s , mais n'apportaient r i e n 'a l'int6re"t i n d u s t r i e l de s m t r a v a i l .
clui
I 1 f a l l u t a t t e n d r e l e s ann6es 50 pour que, chronoloqiquement, Thomas, Tarnele, Oblad e t a l , de Mourques e t a l . , Danforth, P e r i e t G e i t e r i c h e t a1 - 1 2 ) , 6 t u d i e n t syst6matiauement l ' a c i d i t 6 d e l'alumine e t des oxydes mixtes s i l i c e - a l u m i n e , son i n f l u e n c e s u r l e u r a c t i v i t 6 c a t a l y t i q u e e t f o r m u l e n t l ' h y p o t h k s e de l a formation d ' i o n s carbonium, t r e s rGactifs, aussi bien p a r l e proton que par l e s s i t e s de Lewis, ces deux types d ' a c i d i t 6 6 t a n t pr6sents dans l e s g e l s m i x t e s . Ces t r a v a u x ne s e r o n t pas d6velopp6s i c i car i l s sont bien connus e t o n t f a i t l ' o b j e t d ' a u t r e s communications. S i q n a l o n s cependant que l ' a c i d i t 6 de ces g e l s mixtes a 6 t 6 reconnue par tous l e s auteurs pr6cit6s comme 6 t a n t due 'a une s u b s t i t u t i o n isomorphe d ' i o n s Si4' par des ions A13'. Cette s u b s t i t u t i o n cr6e un excedent d'6lectrons ce qui s e t r a d u i t par l ' a p p a r i t i o n de c h a r g e s n 6 g a t i v e s s u p e r f i c i e l l e s n e u t r a l i s 6 e s , l o r s de l a preparation p a r des i o n s Na
t
provenant de l ' a q e n t copr6cipitant. Ces cations sont 6chanqeables e t
un simple lavage par une s o l u t i o n a c i d e , ou une s o l u t i o n de s e l d'ammonium s u i v i d ' u n e c a l c i n a t i o n , l e s remplace par des protons,conferant a i n s i , au qel
440 mixte, une a c i d i t 6 de Bronsted. Par r h a u f f a g e , l e s p r o t o n s e t l e s OH l i e s 'a l a s i l i c e e t 'a l'alumine, s ' 6 l i m i n e n t sous forme d'eau,
ce q u i forme des p o n t s oxyqene e t l i b k r e des
s i t e s d ' a l u m i n i u m t r i c o o r d o n n 6 qui, commme dans A1C13, se comportent en acide de Lewis. Le &me
sch6ma a 6 t 6 6tendu aux oxydes rnixtes s i l i c e - r n a q n C s i e ,
s i l ice-oxydes alcalino-terreux, ions Si4+
puis
dans lesquels l a s u b s t i t u t i o n des ions M2+ aux
c r 6 e des c h a r g e s n 6 s a t i v e s e t
des
cations
tetracoordonngs
responsables des a c i d i t 6 s de Bronsted e t de Lewis (10). L ' a n a l o g i e e n t r e s i l i c i u m e t phosphore a c o n d u i t 'a r e c h e r c h e r l e s p r o p r i 6 t 6 s acides des phosphates, en p a r t i c u l i e r d'aluminium. C e u x - c i o n t e f f e c t i v e m e n t r e v 6 1 6 des p r o p r i e t e s acides e t o n t pu t t r e u t i l i s 6 s dans des & a c t i o n s
de c a t a l y s e a c i d e t e l l e s que l ' o l i q o m 6 r i s a t i o n
d ' o l g f i n e s (11). Devant l e succPs de c e t t e t h C o r i e , on essaya de l ' a p p l i q u e r 'a d ' a u t r e s c a t a l y s e u r s e t 'a d ' a u t r e s r 6 a c t i o n s que c e l l e s impliquant l a transformation des hydrocarbures catalys6e par l e s q e l s mixtes,
1' a1 k y l a t ion, 1a d k u l f uration, etc..
.
p a r exemple,
l'oxydation,
M a i s des o b s t a c l e s s ' o p p o s ' e r e n t , 'a l ' g p o q u e , % 1 ' 6 l a r s i s s e r n e n t de l a t h d o r i e de l ' a c i d i t 6 aux c a t a l y s e u r s i s o l a n t s a u t r e s que ceux c o n t e n a n t de 1 a1 umine ou a u t r e s oxydes t e l MgO dont l e s p r o p r i C t 6 s acido-basiques e t a i e n t b i e n connues. En ce q u i concerne l e s oxydes m C t a l l i q u e s , ne c o n t e n a n t pas de proton,
il 6 t a i t b i e n ancr6 dans l e s e s p r i t s q u ' i l s ne pouvaient E t r e que des
bases, d ' a i l l e u r s ,
une d i s t i n c t i o n e n t r e m6taux e t m6talloYdes 6 t a i t fond6e sur
c e t t e a f f i r m a t i o n . P l u s d i f f i c i l e e n c o r e 6 t a i t l e cas des sels, simples ou complexes qui, c o m e on l ' a pens6 Jongtemps, 6 t a i e n t l e p r o d u i t de l a n e u t r a l i s a t i o n d ' u n a c i d e p a r une base e t donc, i l s ne pouvaient &.re
n i l'un, n i
1' a u t r e . C'est a l o r s que, cur ieusement mai s 6 p i stgmologiquement compr6hensibl e, une t h 6 o r i e des ann6es 40 qui a v a i t s u s c i t g de grands espoirs, apporta l a r6ponse 'a l ' a c i d i t 6 des i s o l a n t s . La d e c o u v e r t e de l a n o n - s t o e c h i o m 6 t r i e des oxydes m e t a l l i q u e s q u i l e u r conf'ere des p r o p r i 6 t 6 s 6 l e c t r i q u e s e t magnetiques p a r t i c u 1 i G r e s , a l l a i t p e r m e t t r e , c r o y a i t - o n , de rapprocher l e s i s o l a n t s des c a t a l y seurs m 6 t a l l i q u e s conducteurs. Cette hypothese, 'a l a q u e l l e e s t a t t a c h 6 l e nom de V o l k e n s t e i n , se r 6 v e l e malheureusement n ' t t r e v 6 r i f i 6 e que dans un t r e s p e t i t nombre de cas. Mais l ' i d 6 e de non stoechiom6trie a l l a i t f a i r e son chemin e t p e r m e t t r e d ' e x p l i q u e r l e c a r a c t ' e r e a c i d e de nombreux i s o l a n t s considergs j u s q u ' a l o r s c o m e neutres, v o i r e basiques, d'o'u l e nom d'acides paradoxaux que j e propose. S i l e c a r a c t e r e a c i d e e s t maintenant b i e n mis en 6vidence par de nombreuses methodes q u i s e r o n t d6velopp6es p a r a i l l e u r s , r e s t a i t en e f f e t d ' e x p l i q u e r l ' o r i g i n e de c e t t e p r o p r i 6 t 6 ne s e r a i t - c e que pour l ' 6 t e n d r e 'a d ' a u t r e s acides paradoxaux.
441
O r l e s d e f a u t s s t r u c t u r a u x qui sont 'a l ' o r i g i n e de l a non-stoechiometrie se
traduisent
par
l a pr6senrp
de
lacunes
anioniques
ou
d'electrons
excedentaires e t dans l a mesure OB ces s o l i d e s deviennent capables de c a p t e r ou ceder une p a i r e d ' e l e c t r o n s , Lewis.
De p l u s ,
i l s se comportent comme des acides ou des bases de
l ' e a u t o u j o u r s pratiquement presente peut,
g r z c e 'a c e s
s t r u c t u r e s p a r t i c u l i e r e s , s ' i o n i s e r en donnant des protons e t des
OH- donc
c o n f e r e r au s o l i d e une a c i d i t 6 e t une b a s i c i t e s e l o n Bronsted. M a i s comme p o u r l a p r e p a r a t i o n des semi-conducteurs proprement d i t s , l e dopage, l e s t r a i t e m e n t s thermiques, l e m i l i e u gazeux, j o u e n t un r 6 l e p r i m o r d i a l dans l ' e l a b o r a t i o n d e c e s s o l i d e s a c i d e s , c e q u i r e n d l e u r p r e p a r a t i o n s i d e l i c a t e . Des etudes physiques de p l u s en p l u s complexes d o i v e n t g t r e m i s e s en o e u v r e p o u r s u i v r e 1 ' C v o l u t i o n des s t r u c t u r e s a f i n d ' o b t e n i r l a q u a n t i t e e t l a forme d ' a c i d i t e cherchees. C ' e s t a i n s i que l ' o x y d e de t h o r i u m soumis 'a d i f f e r e n t s t r a i t e m e n t s t h e r m i q u e s sous d i v e r s e s atmosphgres,
en prCsence ou non de dopes,
peut
a c q u C r i r des p r o p r i e t e s a c i d o - b a s i q u e s s e l o n B r o n s t e d e t L e w i s (Tanabe, Blanchard, Claudel) ( 3 ) . Selon ce d e r n i e r , l a s u r f a c e de l a t h o r i n e p o u r r a i t se schematiser a i n s i ( F i g . 1) :
F i g . 1. Modele de l a s u r f a c e de Tho2 Les vacances d'oxygene se comporteraient comme des s i t e s a c i d e s de L e w i s e t l e s OH comme des bases de Bronsted. De mgme, l a d e s h y d r a t a t i o n de T i 0 2 ( P a r f i t t ) ( 4 ) cr6e des s i t e s acides de Lewis que l ' o n peut r e l i e r 'a l a presence de Ti4+ dans d i f f e r e n t s environnements sterCochimiques(Primet e t al.) (5), l e s OH j o u a n t l e u r r 6 l e habitue1 d ' a c i d e ou de base de Bronsted. L ' e f f e t st6reochimique e s t i c i b i e n mis en Cvidence c a r s i l e s OH de l ' a n a t a s e sont amphotgres, ceux du r u t i l e s o n t uniquement b a s i q u e s .
442
OH
OH
H
H
'W
0 I
I
Fig. 2. Deshydratation de Ti02
L'acidite des oxydes mixtes ou des melanges d'oxydes, utilises en particul ier comme catalyseurs d'oxydatfon, a &galanent 6te &tudiee et expliquee par leur structure (Germain, Ai, Barthomeuf-Figueras) (6). L'a encore, elle depend des traitements thermiques et de la nature et des proportions des deux oxydes, donc, de 1 'environnement. Tanabe (7) a propose une hypoth'ese pour expliquer l'acidite paradoxale des oxydes mixtes selon laquelle l'acidite de Bronsted ou de Lewis serait due 'a un exc'es de charges positives ou negatives dans la structure des oxydes binaires en postulant que le nombre de coordination de chaque el&ment positif des deux oxydes est conserve mtme apr'es melange, alors que celui de 1'616ment negatif, l'oxyg'ene, de l'oxyde majoritaire est impose 'a tous les oxygknes de l'oxyde mixte. (fig. 3).
Fig. 3. Structure de Ti02-Si02 a) Ti02 majoritaire b) Si02 majoritaire
443
L ' a c i d i t e de Lewis s e r a i t due 'a l'exc'es de charqes p o s i t i v e s e t c e l l e de Bronsted 'a l ' a s s o c i a t i o n de d e u x p r o t o n s 'a s i x oxygenes pour m a i n t e n i r l a neutral i t 6 Clectrique. P l u s c u r i e u x e s t l e cas des s e l s , qui sont vraiment des acides paradoxaux p u i s q u ' i l s ont longtemps 6t6 considergs comme r e s u l t a n t de l a n e u t r a l i s a t i o n d ' u n acide par une base, donc, neutres. Cependant, Takeshita e t a1 proposent une hypothese pour expliquer comment un s e l aussi simple que l e s u l f a t e de nickel peut pr6senter un c a r a c t e r e a c i d e de Bronsted e t de Lewis. I1 suppose q u ' a u c o u r s de l a d 6 s h y d r a t a t i o n d u monohydrate i l s e forme une phase i n t e r m g d i a i r e m k t a s t a b l e dans l a q u e l l e l e n i c k e l a une o r b i t a l e ( s p3 d 2 ) v i d e qui j o u e r a i t l e r 6 l e d ' a c i d e de Lewis. L ' a c i d i t e de Bronsted s e r a i t l i 6 e 'a l a presence d'eau coordonnee 'a l ' i o n nickel (Fig. 4 ) .
Fig. 4. Structure d u s u l f a t e de nickel deshydrat6 I1 e s t evident, que 1% encore, l e s proprietes acides dependent Gtroitement des traitements thermiques. E n c e qui concerne l e s h e t e r o s e l s , c a t a l y s e u r s d ' u n c e r t a i n nombre de reactions e t bien Ctudies par des Cquipes Japonaises e t Sovietiques, 1 ' a c i d i t 6 p r o t o n i q u e e s t e v i d e n t e lorsque l e proton e s t initialernent preseht c o m e dans M2.5H0.5PM~12040 p a r exemple, mais c e l l e des s e l s neutres e s t paradoxale. Ai (9) estime que, bien que de s t r u c t u r e trPs d i f f e r e n t e , l e s h e t e r o s e l s d o i v e n t l e u r s p r o p r i e t e s a c i d o - b a s i q u e s aux m h e s s i t e s que l e s oxydes m i x t e s , c'est-'a-dire aux cations e t 'a l e u r G l e c t r o n e q a t i v i t e , 1 ' a c i d i t ; de Bronsted &ant l i e e , l'a encore, 'a l a coordination des molecules d'eau. Pour conclure, i l semble que l a p l u p a r t d e s s o l i d e s i s o l a n t s u t i l i s e s comme c a t a l y s e u r s dans d e s r e a c t i o n s a u s s i v a r i 6 e s que l e c r a c k i n q e t l a polym6ri s a t ion, 1a desul f uration e t 1 'oxydation, 1a deshydratation e t 1 ' a1 kyl at i o n , s o n t d e s s y s t h e s a c i d o - b a s i q u e s b i e n que c e l a ne s o i t pas toujours evident 'a p r i o r i d'o'u l e q u a l i f i c a t i f de paradoxal qu'on peut l e u r a t t r i b u e r .
444
I 1 e n e s t d e msme des s u p p o r t s
et la
n o t i o n n o u v e l l e de " F o r t e
que l ' o n p o u r r a i t g 6 n 6 r a l i s e r 'a " F o r t e
I n t e r a c t i o n Metal Support" (F.I.M.S.)
I n t e r a c t i o n Substance c a t a l y t i q u e m e n t a c t i v e Support",
l a substance a c t i v e
p o u v a n t O t r e un o x y d e m e t a l l i q u e , n ' e s t p e u t O t r e q u ' u n a s p e c t d e c e que Trambouze ( 1 2 ) ,
Le B o u f f a n t ( 1 3 ) o n t m i s e n g v i d e n c e il y a une t r e n t a i n e
d ' annees s u r des c a t a l y s e u r s F i s c h e r - T r o p s c h Ni-A1203-kieselquhr
: 1' influence
du s u p p o r t s u r l a c o m p o s i t i o n s u p e r f i c i e l l e de l a masse de c o n t a c t e t l a p l u s o u m o i n s g r a n d e r 6 d u c t i b i l i t e de l a substance a c t i v e supportke. Les s u p p o r t s e t a n t g 6 n G r a l e m e n t des a c i d e s paradoxaux t e l s que Ti02, Tho2, oxydes m i x t e s , s e l s de metaux l o u r d s e t c
..., on
p e u t a t t r i b u e r en qrande p a r t i e c e phenomkne 'a
une v 6 r it a b l e r 6 a c t i o n acido- b a s i que Les d i f f i c u l t & g&ral,
.
r e n c o n t r e e s dans l ' e t u d e des r a p p o r t s a c t i v i t 6 - a c i d i t 6 en
v i e n n e n t de c e que l e s s p e c i a l i s t e s d e l a c a t a l y s e a c i d o - b a s i q u e
t r a v a i l l e n t avec une o p t i q u e e t dans des m i l i e u x d i f f e r e n t s , de l ' o r q a n i c i e n au p h y s i c o c h i m i s t e de l a c a t a l y s e hGterog'ene, du m i n e r a l i s t e au b i o l o q i s t e . S i c e d e r n i e r s e s a t i s f a i t du pH e t de l a t h e o r i e d ' A r r h e n i u s , d ' a u t r e s o n t d^u f a i r e appel aux d i v e r s e s a u t r e s d e f i n i t i o n s expos6es ci-dessus, s o l v a n t s d e F r a n k l i n , ou 'a c e l l e d'Usanovitch, ciens, l a n o t i o n d'acides
o u 'a l a t h e o r i e d e s
o u i n v e n t e r , comme l e s o r q a n i -
d u r s e t mous. Quant 'a nous, nous sommes b i e n o b l i g 6 s
de c o n v e n i r q u ' i l y a au m o i n s u n e d i f f e r e n c e q u a l i t a t i v e q u ' i l nous f a u d r a d G f i n i r , e n t r e l ' a g r e s s i v i t e d'une s o l u t i o n d ' a c i d e s u l f u r i q u e 'a 90 % e t c e l l e d ' u n o x y d e m i x t e s i l i c e - a l u m i n e amorphe o u c r i s t a l l i s e p a r exemple, d o n t l'acidite,
s e l o n l e s c r i t k r e s de Hammett e s t e q u i v a l e n t e !
REFERENCES
1. F.H. Gayer, I n d . Eng. Chem., 25 (1933) 1122. 2. G.L. Thomas, Ind. Eng. Chem., 4 1 (1949) 2564. M.W. Tamele, Discuss. Faraday SOC., 8 (1950) 270. A.G. Oblad, T.H. M i l l i k e n e t G.A. M i l l s , Revue I.F.P. 10 e t 34 (1951). L. d e Mourgues, M. P e r r i n e t Y. Trambouze, B u l l . SOC. Chim. Fr., 19 (1952) 245. J.D. D a n f o r t h , Actes Congr'es C a t a l y s e 11, 1 (1969) 1272 e t 1330, Technip., Paris. J.B. P e r i , J. Chem. Phys., 69, (1965) 220. M.R. G e i b e r i c h , J.W. Hightower e t W.K. H a l l , J. C a t a l y s i s , 8 (1967) 391. 3. Y. I m i z u , T. Yamaguchi, H. H a t t o r i , K. Tanabe, B u l l . ChemT SOC. Japan, 50 (1977) 1040. C. Bouchoule, M. Blanchard, B u l l . SOC. Chim. Fr., (1974) 1455. M. Breysse, B. Claudel, L. Faure, M. Guenin, J. C o l l o i d and I n t e r f a c e Sc., 70 (1979) 201. 4. G.D. P a r f i t t , J. Rambsbothom, C.M. R o c h e s t e r , T r a n s . F a r a d a y SOC., 67 (1971) 841. 5. M. P r i m e t , P. P i c h a t , M.V. Mathieu, J. Phys. Chem., 75 (1971) 1221. 6. J.E. Germain, I n t r a Science Chem., Rep., 6 (1973) 101. M. A i , J. Catal., 54 (1978) 426.
445
7. 8. 9. 10. 11.
12. 13.
0. Barthomeuf e t F. F i q u e r a s , Chemical and Physical aspects o f c a t a l y t i c o x i d a t i o n , C.N.R.S., Lyon, 1980. K. Tanabe, B u l l . Chem. SOC. Japon, 47 (1974) 1064. T. Takeshita, R. O h i n i s h i e t K. Tanabe, Catal. Rev., 8 (1973) 29. M. A i , J. Catal., 71 (1981) 88. H. Niiyama e t E. Echigoya, Shokubui (Tokyo) 10 (1968)129. A. Tada, Y. Yamamoto, M. I t o e t A. Suzuki, Am. Meetinq Chem. S O C . , Japon, Tokyo, 06424 (1969). Y . Trambouze, C.R. Acad. Sc., 227 (1948) 971 e t B u l l . SOC. Chim. Fr., 16 (1949) 200. L. Le Bouffant, These, Lyon, 1951.
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