Zeolites: Science and Technology
NATO AS1 Series Advanced Science Institutes Series A Series presenting the results o...
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Zeolites: Science and Technology
NATO AS1 Series Advanced Science Institutes Series A Series presenting the results of activities sponsored by the NATO Science Committee, which aims at the dissemination of advanced scientific and technological knowledge, with a view to strengthening links between scientific communities. The Series is published by an international board of publishers in conjunction with the NATO Scientific Affairs Division A
Life Sciences Physics
Plenum Publishing Corporation London and New York
C
Mathematical and Physical Sciences
D. Reidel Publishing Company Dordrecht and Boston
D
Behavioural and Social Sciences Applied Sciences
Martinus Nijhoff Publishers The HagueiBostonlLancaster
Computer and Systems Sciences Ecological Sciences
Springer-Verlag BerliniHeidelbergiNew York
B
E F
G
Series E: Applied Sciences - No. 80
Zeolites: Science and Technology edited by
F. Ram6a Ribeiro lnstituto Superior Tecnlco Technical University of Lisbon, Portugal
Alirio E. Rodrigues Department of Chemical Engineering University of Porto, Portugal
L. Deane Rollmann Mob11Research and Development Corporation Central Research D~vision Princeton, NJ, USA
Claude Naccache lnstitut de Recherches sur la Catalyse Villeurbanne. France
1984 Martinus Nijhoff Publishers The Hague 1 Boston 1 Lancaster Published in coo~eratlonwlth NATO Scientific Affairs Division
Proceedings of the NATO Advanced Study Institute on Zeolites: Science and Technology, Alcabideche, Portugal, May 1-1 2, 1983
Library of Congress Cataloging in Publication Data NATO Advanced Study Institute on Zeolites--Science and Technology (1983 : Alcabideche, ~ortugal) Zeolites--science and technology. (NATO advanced science institutes series. Series E , Applied sciences ; no. 80) "~roceedin~s of the NATO Advances Study Institute on Zeolites: Science and Technology, Alcabideche, Portugal, May 1-12, 1983"--~.p. verso. "Published in cooperation with NATO Scientific Affairs Division." 1. Zeolites--Congresses. I. Ribeiro, F. Rarnoa. 11. Title. 111. Series. ~ ~ 2 .4~ 55 ~ 3 1983 8 660.2'8423 83-25486
ISBN 90-247-2935-1 (this volume) ISBN 90-247-2689-1 (series)
Distributors for the United States and Canada: Kluwer Boston, Inc., 190 Old Derby Street, Hingham, MA 02043, USA Distributors for all other countries: Kluwer Academic Publishers Group, Distribution Center, P.O. Box 322, 3300 AH Dordrecht, The Netherlands
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, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publishers, Martinus Nijhoff Pubiishers, P.O. Box 566, 2501 CN The Hague, The Netherlands Copyright O 1984 by Martinus Nijhoff Publishers, The Hague Printed in The Netherlands
PREFACE Z e o l i t e s have been t h e f o c u s o f i n t e n s i v e a c t i v i t y and growth i n a p p l i c a t i o n s o v e r t h e p a s t 25 y e a r s i n i o n exchange, i n a d s o r p t i o n and i n c a t a l y t i c p r o c e s s t e c h n o l o g y . Beginning w i t h t h e synt h e t i c z e o l i t e s A , X and Y , c o n t i n u i n g i n t o t h e emerging ZSM s e r i e s , and i n c l u d i n g s e l e c t e d n a t u r a l z e o l i t e s , a p p l i c a t i o n s s p a n t h e r a n g e From l a r g e - s c a l e p u r i f i c a t i o n and s e p a r a t i o n t o such major p e t r o l e u m and p e t r o c h e m i c a l p r o c e s s e s a s c a t a l y t i c c r a c k i n g anFi a r o m a t i c s alkylation. The f u t u r e promises s e v e r a l new a r e a s o f s i g n i c i a n t u s e a s o u r energy r e s o u r c e b a s e i s expanded. A s a r e s u l t , a NATO Advanced Study I n s t i t u t e on Z e o l i t e s was h e l d i n A l c a b i d e c h e , P o r t u g a l , May 1-12, 1983. I t s p u r p o s e was t o summarize t h e s t a t e - o f - t h e - a r t i n z e o l i t e s c i e n c e and t e c h n o l o g y , w i t h p a r t i c u l a r emphasis on r e c e n t developments. T h i s summary i s i n t e n d e d t o complement p r e s e n t a t i o n s of t h e l a t e s t r e s e a r c h r e s u l t s a t t h e 1983 I n t e r n a t i o n a l Z e o l i t e s A s s o c i a t i o n meeting i n Reno, Nevada - USA. Both t h e fundamentals c o n c e p t s and i n d u s t r i a l a p p l i c a t i o n s a r e addressed i n t h e l e c t u r e s of t h e I n s t i t u t e . I n d i v i d u a l c h a p t e r s cover 3 i s t o r i c a l development, s t r u c t u r e , c r y s t a l l o g r a p h y and s y n t h e s i s techniques. B a s i c p r i n c i p l e s of a d s o r p t i o n , d i f f u s i o n , i o n exchange and a c i d i t y a r e reviewed. A s e c t i o n on c a t a l y s i s a d d r e s s e s shape s e l e c t i v i t y , t r a n s i t i o n m e t a l s , b i f u n c t i o n a l c a t a l y s i s and "methanolto-gasoline"
.
.a
I n c l u d e d i n t h e $ k c t i o n on i n d u s t r i a l a p p l i c a t i o n s a r e c h a p t e r s on r e a c t o r and adsorbek d e s i g n , c a t a l y t i c c r a c k i n g , x y l e n e and n- p a r a f f i n s i s o r n e r i z a t i 8 n , a s w e l l a s i o n exchange and a d s o r p t i o n . \! We would l i k e t o Wank t h e members o f t h e Advisory Board f o r t h e s u p p o r t given by them t o t h e o r g a n i z a t i o n of t h i s m e e t i n g ; and a l l l e c t u r e r s a r e t o be thanked f o r t h e i r c l e a r p r e s e n t a t i o n s and w r i t t e n c o n t r i b u t i o n s . We p a r t i c u l a r l y wish t o acknowledge t h e e f f o r t s o f t h e l o c a l committee.
This NATO Advanced Study I n s t i t u t e was made p o s s i b l e t h r o u g h +-he NATO-AS1 Programme. We a r e much i n d e b t e d t o t h i s o r g a n i z a t i o n . May 1983 Alcabideche
F. Ramba R i b e i r o ~ l i r i oE . Rodrigues L . Deane Rollmann Claude Naccache
D i r e c t o r : P r o f . F. ~ a & a R i b e i r o Co-Directors:
D r . C . Naccache, P r o f . A. R o d r i g u e s , D r . D. Rollmann
Advisory Board P r o f . L. A l v e s , I n s t i t u t o S u p e r i o r Tgcnico, P o r t u g a l P r o f . B. Delrnon, U n i v e r s i t e C a t h o l i q u e de Louvain, Belgium
D r . G . M a r t i n o , I n s t i t u t F r a n ~ a i sdu p e t r o l e , F r a n c e P r o f . R. Maurel, ~ n i v e r s i t ed e P o i t i e r s , F r a n c e P r o f . M. F. P o r t e l a , I n s t i t u t o S u p e r i o r Tecnico , P o r t u g a l
Lectures P r o f . R.M. B a r r e r , I m p e r i a l C o l l e g e , London, UK
3r. D . Barthomeuf, Exxon, Linden, USA P r o f . E . Derouane, M o b i l , P r i n c e t o n , USA 3r. Z . G a b e l i c a , ~ n i v e r s i t ed e Namur, Belgium
P r o f . M . G u i s n e t , ~ n i v e r s i t ede P o i t i e r s , F r a n c e P r o f . C . Kenney, U n i v e r s i t y of Cambridge, UK 3 r . P. J a c o b , E l f - A q u i t a i n e ,
France
3 r . G . K o k o t a i l o , U n i v e r s i t y of D r e x e l , USA P r o f . H. de L a s a , U n i v e r s i t y of Western, O n t a r i o , Canada "of.
H. L e c h e r t , U n i v e r s i t y o f Hamburg, FRG
3 r . C . Naccache, I n s t i t u t Recherches C a t a l y s e , F r a n c e 3 r . J . Rabo, Union C a r b i d e , Tarrytown, USA I r o f . F. ~ a m c aR i b e i r o , I n s t i t u t o S u p e r i o r T e c n i c o , P o r t u g a l I r o f . A. R o d r i g u e s , U n i v e r s i t y of P o r t o , P o r t u g a l 2 r . D . Rollmann, Mobil, P r i n c e t o n , USA 3 r . J . Sherman, Union C a r b i d e , Tarrytown, USA
Local Committee
Eng. M. J . P i r e s , I n s t i t u t o S u p e r i o r Tgcnico, P o r t u g a l Eng. J . M. L o u r e i r o , U n i v e r s i t y of P o r t o , P o r t u g a l Eng. F. F r e i r e , I n s t i t u t o S u p e r i o r ~ e c n i c o ,P o r t u g a l Eng. F. Lemos, I n s t i t u t o S u p e r i o r ~ e c n i c o ,P o r t u g a l Eng. M. L. P a l h a , I n s t i t u t o S u p e r i o r Tecnico, P o r t u g a l
TABLE OF CONTENTS Preface
Part I.
History, structure and synthesis E. M. Flanigen Molecular sieve zeolite technology: the first twenty five years
3
R. M. Barrer Zeolite structure G. T. Kokotailo Zeolite crystallography L. D. Rollmann Synthesis of zeolites, an overview
F. Roozeboom, H. Robson, S. Chan Study on the mechanism of crystallization of zeolites A, X and Y Part 11.
127
Physical characterization and sorption fundamentals
H. Lechert The physical characterization of zeolites
151
Z. Gabelica, J. Nagy, Ph. Bodart, G. Debras,
E. Derouane, P. Jacobs Structural characterization of zeolites by high resolution magic-angle-spinning solid state 2 9 ~ i - ~ Mspectroscopy R
193
Ph. Bodart, Z. Gabelica, J. B. Nagy, G. Debras Multinuclear solide-state NMR study of mordenite crystallization
211
R . M. B a r r e r S o r p t i o n by z e o l i t e s :
- E q u i l i b r i a and e n e r g e t i c s I1 - K i n e t i c s and d i f f u s i v i t i e s I
P a r t 111.
Catalysis
J . Rabo Unifying p r i n c i p l e s i n z e o l i t e c h e m i s t r y and catalysis D
291
. Barthomeuf
Acidic C a t a l y s i s w i t h z e o l i t e s E . G. Derouane Molecular s h a p e - s e l e c t i v e c a t a l y s i s by z e o l i t e s
347
C . Naccache, Y . Ben T a a r i t T r a n s i t i o n metal exchanged z e o l i t e s : p h y s i c a l and catalytic properties M. G u i s n e t , G. P e r o t Zeolite bifunctional catalysis
Part IV.
Industrial applications
A . Rodrigues, C . C o s t a , R . F e r r e i r a , J . L o u r e i r o ,
S . Azevedo Design a s p e c t s of c a t a l y t i c r e a c t o r s and adsorbers
H. Lasa E n g i n e e r i n g a s p e c t s of c a t a l y t i c c r a c k i n g
373
E. Derouane, Z. Gabelica Conversion of methanol to gasoline over zeolite catalysts I
Reaction mechanisms
I1
Industrial processes
F. R. Ribeiro Use of platinum HY zeolite and platinum H mordenite in the hydroisomerization of n-hexane
M. Guisnet, N. S. Gnep Zeolites as catalysts in xylene isomerization processes
J. D. Sherman Ion exchange separations with molecular sieve zeolites
P. Jacob Selective adsorption processes: N-Iself C. N. Kenney, N. F. Kirkby Pressure swing adsorption
List o f P a r t i c i p a n t s
545
PART I HISTORY, STRUCTURE AND SYNTHESIS
MOLECULAR SIEVE ZEOLITE TECHNOLOGY: FIVE YEARS*
E d i t h M.
THE F I R S T TWENTY-
Flanigen
Union C a r b i d e C o r p o r a t i o n Tarrytown Technical Center T a r r y t o w n , New Y o r k 1 0 5 9 1 , USA
ABSTRACT I n twenty-five years molecular s i e v e z e o l i t e s have s u b s t a n t i a l l y impacted adsorption and c a t a l y t i c process technology throughout the chemical process industries; p r o v i d e d t i m e l y s o l u t i o n s t o e n e r g y and e n v i r o n m e n t a l p r o b l e m s ; and grown t o o v e r a hundred m i l l i o n d o l l a r industry worldwide. The e v o l u t i o n i n z e o l i t e m a t e r i a l s w i t h improved o r n o v e l p r o p e r t i e s h a s s t r o n g l y i n f l u e n c e d t h e e x p a n s i o n o f t h e i r a p p l i c a t i o n s , a n d p r o v i d e d new f l e x i b i l i t y i n t h e d e s i g n of p r o d u c t s and p r o c e s s e s . INTRODUCTION The y e a r 1979 marked t h e t w e n t y - f i f t h a n n i v e r s a r y of t h e commercial b i r t h of m o l e c u l a r s i e v e z e o l i t e s a s a new c l a s s o f i n d u s t r i a l m a t e r i a l s . They w e r e i n t r o d u c e d i n l a t e 1 9 5 4 a s a d s o r b e n t s f o r i n d u s t r i a l s e p a r a t i o n s and purifications. S i n c e t h a t time t h e f a s c i n a t i o n w i t h and t h e e l e g a n c e o f , t h i s u n i q u e c l a s s of m a t e r i a l s h a s g e n e r a t e d a masss of s c i e n t i f i c l i t e r a t u r e d e s c r i b i n g t h e i r s y n t h e s i s , p r o p e r t i e s , s t r u c t u r e and a p p l i c a t i o n s , w h i c h p r o b a b l y now n u m b e r s w e l l o v e r 1 5 , 0 0 0 s c i e n t i f i c c o n t r i b u t i o n s and over 10,000 issued p a t e n t s . The m o l e c u l a r s i e v e i n d u s t r y h a s been p r o j e c t e d t o h a v e grown i n t o a n e s t i m a t e d q u a r t e r of a b i l l i o n d o l l a r market {I) s e r v i n g a l l o f t h e m a j o r s e g m e n t s o f t h e c h e m i c a l process i n d u s t r i e s including major applications i n t h e p e t r o l e u m r e f i n i n g and p e t r o c h e m i c a l i n d u s t r i e s , and h a s g e n e r a t e d a myriad of o t h e r a d s o r p t i o n , c a t a l y t i c ,
*
This a r t i c l e i s used a s r e f e r e n c e m a t e r i a l f o r J. A. l e c t u r e on " H i s t o r i c a l A s p e c t s of Z e o l i t e s " .
Rabo's
and most r e c e n t l y
i o n exchange a p p l i c a t i o n s .
Milton reviewed t h e beginnings and development of molecular s i e v e z e o l i t e s i n 1967 (21. He t r a c e d t h e e a r l y d i s c o v e r i e s a n d s y n t h e s i s o f t h e new z e o l i t e s A , X , and Y, which l e d t o t h e i r commercial a p p l i c a t i o n s It w i l l be the a s s e l e c t i v e adsorbents and c a t a l y s t s . p u r p o s e of t h i s p a p e r t o r e v i e w t h e e v o l u t i o n o f molecular sieve materials, t h e i r synthesis, properties and a p p l i c a t i o n s , over t h e span of 1954 t o 1979, w i t h emphasis on t h e major m i l e s t o n e s and t r e n d s i n t h e s e areas. There w i l l be no attempt here t o r e p e a t t h e i n d e p t h , c o v e r a g e o f z e o l i t e m o l e c u l a r s i e v e s g i v e n by B r e c k {3} o r B a r r e r 1 4 3 , o r r e c e n t u p - t o - d a t e r e v i e w a r t i c l e s on t h e a p p l i c a t i o n s of m o l e c u l a r s i e v e z e o l i t e s a s a d s o r b e n t s { 6 } , c a t a l y s t s { 7 , 8 ) , and i o n exchangeers ( 9 1 , a n d on n a t u r a l z e o l i t e s and t h e i r a p p l i c a t i o n s {10,11,1,5}. Success of molecular s i e v e z e o l i t e s h a s been due p r i m a r i l y t o t h e d i s c o v e r y o f new m a t e r i a l s w h o s e p r o p e r t i e s h a v e b e e n e n g i n e e r e d i n t o i m p r o v e m e n t s i n known p r o c e s s e s a n d i n t o t h e d e v e l o p m e n t o f new o n e s . This d i s c u s s i o n w i l l t h e r e f o r e emphasize t h e m a t e r i a l s and p r o p e r t i e s a s p e c t s of molecular s i e v e z e o l i t e s a s t h e y e v o l v e d and developed i n t o v a r i o u s a p p l i c a t i o n a r e a s . THE EVOLUTION I N MATERIALS T h e t h e m e i n r e s e a r c h on m o l e c u l a r s i e v e z e o l i t e m a c e r i a l s o v e r t h e twenty-five y e a r period h a s been a q u e s t f o r new s t r u c t u r e s a n d c o m p o s i t i o n s . Because zeol i t e s a r e unique a s c r y s t a l l i n e porous m a t e r i a l s , with s t r u c t u r e as well a s composition controlling properties, t h e r e a r o s e t h e s t r o n g conviction t h a t novel and u s e f u l p r o p e r t i e s w o u l d r e s u l t f r o m t h e d i s c o v e r y o f n e w comp o s i t i o n s and s t r u c t u r e s . L e t u s now t r a c e t h e w e b o f change i n t h o s e d i s c o v e r i e s over twenty-five years. " L o w - S i l i c a " Z e o l i t e s o r Aluminum-Rich Z e o l i t e s . The d i s c o v e r y of z e o l i t e s A and X by M i l t o n {12} a t t h e Union C a r b i d e C o p o r a t i o n L a b o r a t o r j e s r e p r e s e n t e d a f o r t u n a t e optimum i n c o m p o s i t i o n , p o r e volume, a n d c h a n n e l s t r u c t u r e , g u a r a n t e e i n g t h e s e two z e o l i t e s t h e i r l a s t i n g commercial prominence o u t of more t h a n 1 5 0 s y n t h e t i c s p e c i e s known a n d d i s c o v e r e d o v e r t h e l a s t twenty-five years. Both z e o l i t e s a r e n e a r l y " s a t u r a t e d " i n aluminum i n t h e framework c o m p o s i t i o n w i t h a m o l a r r a t i o o f S i I A 1 n e a r o n e , t h e maximum a l u m i n u m c o n t e n t p o s s i b l e i n t e t r a h e d r a l a l u m i n o s i l i c a t e frameworks i f
one a c c e p t s Loewenstein's r u l e . A s a consequence they c o n t a i n t h e maximum n u m b e r o f c a t i o n e x c h a n g e s i t e s b a l a n c i n g t h e framework aluminum, and t h u s t h e h i g h e s t c a t i o n c o n t e n t s and exchange c a p a c i t i e s . T h e s e comp o s i t i o n a l c h a r a c t e r i s t i c s combined g i v e them t h e most h i g h l y h e t e r o g e n e o u s s u r f a c e k n o w n among p o r o u s m a t e r i a l s , due t o exposed c a t i o n i c charges n e s t e d i n a n a l u m i n o s i l i c a t e framework which r e s u l t s i n h i g h f i e l d g r a d i e n t s . Their s u r f a c e i s h i g h l y s e l e c t i v e f o r w a t e r , p o l a r and p o l a r i z a b l e m o l e c u l e s w h i c h s e r v e s a s t h e b a s i s f o r many o f t h e i r a p p l i c a t i o n s p a r t i c u l a r l y i n d r y i n g and p u r i f i c a t i o n . T h e i r p o r e v o l u m e s o f n e a r l y 0 . 5 cm 3 / c m 3 a r e t h e h i g h e s t known f o r z e o l i t e s a n d g i v e t h e m a d i s t i n c t e c o n o m i c a d v a n t a g e i n b u l k s e p a r a t i o n and p u r i f i c a t i o n s where h i g h c a p a c i t y i s e s s e n t i a l t o and economic d e s i g n . Their 3 - d i m e n s i o n a l c h a n n e l s t r u c t u r e s a l l o w t h e maximum i n diffusion characteristics. By a j u d i c i o u s s e l e c t i o n o f c a t i o n c o m p o s i t i o n a c h i e v e d by f a c i l e i o n exchange r e a c t i o n s , n e a r l y t h e e n t i r e s p e c t r u m o f known p o r e s i z e s i n z e o l i t e s can be obtained. The p o r e s i z e s a c h i e v a b l e by c a t i o n e x c h a n g e of t y p e s A and X s p a n t h e e n t i r e r a n g e from t h e s m a l l e s t p o r e - s i z e d z e o l i t e known, C s - A a t 0.2nm i n s i z e ( 1 3 ) t h r o u g h t h e 0.3nm p o t a s s i u m A , t h e 0.4nm sodium A , t h e 0.5nm c a l c i u m A , t o t h e l a r g e s t known w h i c h i s a b o u t 0 . 8 n m i n s o d i u m X. T h i s l a r g e p o r e s i z e o f z e o l i t e X was a key t o i t s introduction a s a catalytic cracking catalyst. "Intermediate Silica" Zeolites. The n e x t e v o l u t i o n i n z e o l i t e m a t e r i a l s was t h e i m p e t u s t o s y n t h e s i z e more s i l i c e o u s z e o l i t e s , p r i m a r i l y t o improve s t a b i l i t y I t was r e c o g n i z e d c h a r a c t e r i s t i c s , b o t h t h e r m a l and a c i d . i n t h e e a r l y 1 9 5 0 ' s by s c i e n t i s t s a t Union C a r b i d e L a b o r a t o r i e s t h a t t h e t e t r a h e d r a l framework aluminum p r o v i d e d a s i t e o f i n s t a b i l i t y f o r a t t a c k by a c i d and water vapor o r steam. A l s o , t h e s i l i c e o u s m i n e r a l zeol i t e m o r d e n i t e w a s known w i t h a S i / A l m o l a r r a t i o o f 5 and p o s s e s s i n g s u p e r i o r s t a b i l i t y c h a r a c t e r i s t i c s . Breck provided t h e f i r s t success i n t h i s quest with t h e d i s c o v e r y of t h e t h i r d c o m m e r c i a l l y i m p o r t a n t m o l e c u l a r s i e v e z e o l i t e t y p e Y ( 1 4 1 , w i t h a n S i / ~ 1r a t i o o f f r o m 1 . 5 t o 3 . 0 , and a framework topology l i k e t h a t of zeol i t e X and t h e r a r e z e o l i t e m i n e r a l f a u j a s i t e . Not o n l y was t h e d e s i r e d improvement i n s t a b i l i t y o v e r t h e more a l u m i n o u s X a c h i e v e d , b u t a l s o t h e d i f f e r e n c e s i n comp o s i t i o n and s t r u c t u r e had a s t r i k i n g , u n p r e d i c t e d e f f e c t on p r o p e r t i e s t h a t h a s l e d t o t h e preeminence of z e o l i t e Y b a s e d c a t a l y s t s i n n e a r l y a l l of t h e i m p o r t a n t c a t a l y t i c applications involving hydrocarbon conversion,
( i . e . , c r a c k i n g , h y d r o c r a c k i n g and i s o m e r i z a t i o n ) i t s i n i t i a l commercial i n t r o d u c t i o n i n 1959.
since
The n e x t commercially i m p o r t a n t s y n t h e t i c z e o l i t e i n t r o d u c e d i n t h e e a r l y 1 9 6 0 ' s was a l a r g e p o r e m o r d e n i t e made b y t h e method o f Sand { 1 5 } a n d m a r k e t e d a s " Z e o l o n " by t h e N o r t o n Co.{16}, which c o n t i n u e d t h e p r o g r e s s i o n toward h i g h e r S i / A l r a t i o , i n t h i s c a s e a v a l u e n e a r 5. Again, t h e r m a l , h y d r o t h e r m a l , and a c i d s t a b i l i t y improvement was e v i d e n t . T h i s improved s t a b i l i t y coupled w i t h i t s s p e c i f i c s t r u c t u r a l and compositional c h a r a c t e r i s t i c s found i t a small but a s i g n i f i c a n t commercial market a s both an adsorbent and hydrocarbon conversion c a t a l y s t . Type L z e o l i t e , d i s c o v e r e d i n t h e e a r l y 5 0 ' s by B r e c k and Acara ( 1 7 ) w i t h a n S i / A l r a t i o of 3.0 and a u n i q u e framework topology, has only r e c e n t l y r e c i e v e d a t t e n t i o n a s a commercial c a t a l y s t i n s e l e c t i v e h y d r o c a r b o n conv e r s i o n r e a c t i o n s {18}. Other z e o l i t e s with "intermediate" Si/Al compositions o f f r o m 2 t o 5 a n d t h e i r own u n i q u e f r a m e w o r k t o p o l o g i e s which have achieved commercial s t a t u s a r e t h e z e o l i t e m i n e r a l s m o r d e n i t e , e r i o n i t e , c h a b a z i t e , and c l i n o p t i l o l i t e , a n d t h e s y n t h e t i c z e o l i t e omega [ 1 9 ] w i t h a t y p i c a l S i / A l of 3 t o 4. T h e i r p r o p e r t i e s e x h i b i t a common c h a r a c t e r i s t i c i n t e r m s of improved s t a b i l i t y o v e r t h e "low" s i l i c a zeolites. However, u n i q u e p r o p e r t i e s a s a d s o r b e n t s , c a t a l y s t s and i o n exchange m a t e r i a l s a r e a l s o e x h i b i t e d which r e f l e c t t h e i r unique s t r u c t u r a l f e a t u r e s . The s u r f a c e of t h e s e i n t e r m e d i a t e s i l i c a z e o l i t e s i s s t i l l h e t e r o g e n e o u s and e x h i b i t s h i g h s e l e c t i v i t y f o r w a t e r and other polar molecules. "High S i l i c a " Z e o l i t e s . The most r e c e n t s t a g e s i n t h e q u e s t f o r more s i l i c e o u s m o l e c u l a r s i e v e c o m p o s i t i o n s was a c h i e v e d i n t h e l a t e 1 9 6 0 ' s and t h e e a r l y 1 9 7 0 ' s w i t h t h e s y n t e h s i s a t t h e Mobil Research and Development Laboratories of t h e "high s i l i c a z e o l i t e s " , compositions e x e m p l i f i e d f i r s t by z e o l i t e b e t a d i s c o v e r e d by W a d l i n g e r , K e r r a n d R o s i n s k i [ 2 0 } , a n d l a t e r ZSM-5 d i s c o v e r e d b y S u b s e q u e n t l y , ZSM-11 { 2 2 } , Argauer and Landolt {21). ZSM-21 { 2 4 } , a n d ZSM-34 { 2 5 } w e r e d e s c r i b e d . T h e s e compositions are molecular sieve zeolites with Si/Al r a t i o s from 1 0 t o 1 0 0 o r h i g h e r , and w i t h u n e x p e c t e d , s t r i k i n g l y different surface characteristics. In contrast to the lllown a n d " i n t e r m e d i a t e " s i l i c a z e o l i t e s , r e p r e s e n t i n g heterogeneous hydrophilic surfaces within a porous c r y s t a l , t h e s u r f a c e of t h e h i g h s i l i c a z e o l i t e s approaches a more h o m o g e n e o u s c h a r a c t e r i s t i c w i t h a n organophilic-hydrophobic selectivity. They more s t r o n g l y a d s o r b t h e l e s s p o l a r
o r g a n i c m o l e c u l e s and o n l y weakly i n t e r a c t w i t h w a t e r and other strongly polar molecules. In addition to this n o v e l s u r f a c e s e l e c t i v i t y , t h e h i g h s i l i c a z e o l i t e comp o s i t i o n s s t i l l c o n t a i n a s m a l l c o n c e n t r a t i o n of aluminum i n t h e framework and t h e accompanying s t o i c h i o m e t r i c c a t i o n exchange s i t e s . Thus, t h e i r c a t i o n exchange prop e r t i e s a l l o w t h e i n t r o d u c t i o n o f a c i d i c OH g r o u p s v i a t h e w e l l known z e o l i t e i o n e x c h a n g e r e a c t i o n s , e s s e n t i a l t o t h e development of acid hydrocarbon c a t a l y s i s properties. S i l i c a Molecular Sieves. The u l t i m a t e i n s i l i c e o u s m o l e c u l a r s i e v e c o m p o s i t i o n s , a n d a much d i s c u s s e d a s p i r a t i o n of e a r l y workers i n z e o l i t e s y n t h e s i s i n t h e 1950%, was a l s o a c h i e v e d i n t h e 1 9 7 0 ' s w i t h t h e s y n t h e s i s of t h e f i r s t pure s i l i c a molecular s i e v e , s i l i c a l i t e (261, c o n t a i n i n g e s s e n t i a l l y no aluminum o r c a t i o n s i t e s . In t h e complete absence of s t r o n g f i e l d g r a d i e n t s due t o framework aluminum and e x c h a n g e a b l e m e t a l c a t i o n s which serve a s hydrophilic s i t e s , s i l i c a l i t e exhibits a high d e g r e e of o r g a n o p h i l i c - h y d r o p h o b i c c h a r a c t e r , c a p a b l e of s e p a r a t i n g o r g a n i c m o l e c u l e s o u t of w a t e r - b e a r i n g s t r e a m s (261. S i l i c a l i t e does however c o n t a i n e x t r a n e o u s o r def e c t hyroxyl groups which c o n t r i b u t e a s m a l l concentrat i o n of h y d r o p h i l i c s i t e s c a p a b l e of i n t e r a c t i n g w i t h A r e l a t e d new c o m p o s i t i o n , water and p o l a r molecules. f l u o r i d e - s i l i c a l i t e (271, completely f r e e of hydroxyl groups, e x h i b i t s t h e u l t i m a t e i n near p e r f e c t hydrop h o b i c i t y , a d s o r b i n g l e s s t h a n 1 w t . % w a t e r a t 20 t o r r the and 25"C, and e v e n e x h i b i t s b u l k h y d r o p h o b i c i t y : c r y s t a l s (d = 1 . 7 g/cm3) a c t u a l l y f l o a t on w a t e r . S i l i c a l i t e r e p o r t e d l y {26a} h a s t h e same framework t o p o l o g y Other s i l i c a molecular sieve a s z e o l i t e ZSM-5 { 2 8 } . compositions have been reported, including s i l i c a l i t e - 2 {29}, and T E A - s i l i c a t e {30). Chemically Modified Z e o l i t e s . An a l t e r n a t e m e t h o d o r producing h i g h l y s i l i c e o u s z e o l i t e c o m p o s i t i o n s had t s b e g i n n i n g s i n t h e mid 1 9 6 0 ' s when t h e r m o c h e m i c a l m o d i f i c a t i o n r e a c t i o n s t h a t l e a d t o framework dealuminat i o n were f i r s t r e p o r t e d ( 3 1 ~ 3 2 ) . These r e a c t i o n s i n c l u d e those described as "stabilization," or "ultrastabilizat i o n " i n v o l v i n g h i g h t e m p e r a t u r e s t e a m i n g o f ammonium e x c h a n g e d o r a c i d f o r m s o f t h e z e o l i t e (311, a n d f r a m e work aluminum e x t r a c t i o n w i t h m i n e r a l a c i d s o r c h e l a t e s . R e p e t i t i v e treatments i n e f f e c t produce z e o l i t e s with framework S i / A l c o m p o s i t i o n s and s t a b i l i t y c h a r a c t e r i s t i c s comparable t o those observed i n t h e synthesized high s i l i c a zeolite compositions. Such h i g h l y s i l i c e o u s v a r i a n t s h a v e b e e n d e s c r i b e d by S c h e r z e r f o r z e o l i t e Y
{ 3 3 } , by Chen { 3 4 } a n d o t h e r s f o r m o r d e n i t e , a n d b y Eberly e t a l . (351 and P a t t o n e t a l . 1361 f o r e r i o n i t e . The s t a b i l i z e d Y z e o l i t e of McDaniel and Maher {31} and t h e h i g h l y s i l i c e o u s m o r d e n i t e p r o d u c t s o f Chen 1 3 4 1 , were reported t o be hydrophobic. Although t h e ultrastabilized and o t h e r d e a l u m i n a t e d forms o f z e o l i t e s emerged a t t h e same t i m e a s t h e s y n t h e s i z e d h i g h s i l i c a z e o l i t e b e t a , focus i n t h e former case i n t h e l a t e 60's was on t h e i r improved s t a b i l i t y c h a r a c t e r i s t i c s and c a t a l y t i c a p p l i c a t i o n s , r a t h e r than on t h e i r s u r f a c e selectivity. O t h e r h i g h l y s i l i c e o u s a n a l o g s o r "pseudomorphs" of c l i n o p t i l o l i t e and mordenite prepared d u r i n g t h e same p e r i o d by a c i d e x t r a c t i o n {37} h a v e S i / A l composit i o n s l i k e t h e h i g h s i l i c a z e o l i t e s and s i l i c a m o l e c u l a r sieves. However, t h e i r c r y s t a l l i n i t y , s t a b i l i t y , and hydrophobicity a r e s u b s t a n t i a l l y less than t h e thermochemically d e r i v e d u l t r a s t a b i l i z e d and dealuminated c o m p o s i t i o n s , presumably due t o t h e p r e s e n c e of h i g h c o n c e n t r a t i o n s of hydroxyl d e f e c t s {37b}, and t h e a b s e n c e of s u b s t a n t i a l s i l i c o n r e i n s e r t i o n i n t o framework t e t r a h e d r a l s i t e s . Natural Zeolites. I n c o n t r a s t t o t h e development of t h e s y n t h e t i c z e o l i t e s which required t h e i r discovery and s u c c e s s f u l s y n t h e s i s i n t h e l a b o r a t o r y , t h e evolut i o n o f t h e n a t u r a l o r m i n e r a l z e o l i t e s d e p e n d e d on t h e i r a v a i l a b i l i t y i n mineable deposits. The d i s c o v e r y i n 1957 of m i n e a b l e d e p o s i t s of r e l a t i v e l y h i g h p u r i t y zeolite minerals i n volcanic tuffs i n the western U n i t e d S t a t e s and i n a number of o t h e r c o u n t r i e s r e p r e s e n t s t h e b e g i n n i n g of t h e c o m m e r c i a l n a t u r a l zeoP r i o r t o t h a t t i m e t h e r e was no r e c o g n i z e d l i t e e r a (101. indication that zeolite minerals with properties useful a s molecular sieve materials occurred i n large deposits. C o m m e r c i a l i z a t i o n of t h e n a t u r a l z e o l i t e s c h a b a z i t e , e r i o n i t e , and mordenite a s molecular s i e v e z e o l i t e s commenced i n 1 9 6 2 w i t h t h e i r i n t r o d u c t i o n a s new a d s o r bent m a t e r i a l s with improved s t a b i l i t y c h a r a c t e r i s t i c s i n various acid natural gas drying applications. Their improved s t a b i l i t y o v e r t h e t h e n p r e v a l e n t s y n t h e t i c z e o l i t e a d s o r b e n t s , t y p e s A and X , a g a i n r e f l e c t s S i / A l r a t i o of 3-5. The their higher intermediate a p p l i c a t i o n s of c l i n o p t i o l i t e i n r a d i o a c t i v e w a s t e r e c o v e r y a n d i n w a s t e w a t e r t r e a t m e n t d u r i n g t h e same p e r i o d o f t h e 6 0 ' s was b a s e d n o t o n l y on s u p e r i o r s t a b i l i t y c h a r a c t e r i s t i c s b u t a l s o a h i g h c a t i o n exchange s e l e c t i v i t y f o r cesium and s t r o n t i u m , o r f o r t h e ammonium i o n .
A summary o f t h e e v o l u t i o n o f m o l e c u l a r s i e v e m a t e r i a l s a s d e v e l o p e d above i s g i v e n i n T a b l e I , w i t h emphasis on t h e framework S i / A l v a r i a t i o n .
TABLE 1 T H E EVOLUTION OF MOLECULAR SIEVE MATERIALS
"Low" S i / A I Z e o l i t e s A, X
(I to 1.5):
" I n t e r m e d i a t e " S i / A l Z e o l i t e s (-2 t o 5 ) : A) Natural Zeolites: erionite, chabazite, clinoptilolite, mordeni t e B) Synthetic Zeolites: Y , L , l a r g e p o r e m o r d e n i t e , omega " H i g h " ~ i / A lZ e o l i t e s ( - 1 0 t o 1 0 0 ) : A) By t h e r m o c h e m i c a l f r a m e w o r k m o d i f i c a t i o n : h i g h l y s i l i c e o u s v a r i a n t s of Y , mordenite, erionite B) By d i r e c t - s y n t h e s i s : ZSM-5 S i l i c a Molecular
Sieves:
Silicalite
During t h i s p e r i o d of t w e n t y - f i v e y e a r s of r e s e a r c h and d e v e l o p m e n t o v e r 1 5 0 s p e c i e s of s y n t h e t i c z e o l i t e s h a v e b e e n s y n t h e s i z e d a n d some 7 m i n e r a l z e o l i t e s h a v e Yet been found i n s u b s t a n t i a l q u a n t i t y and p u r i t y { 3 8 } . commercially only twelve b a s i c types a r e u t i l i z e d . The major Table 2 l i s t s t h e s e twelve types from Ref. 5. l a r g e volume commercial m o l e c u l a r s i e v e z e o l i t e s used i n a d s o r p t i o n and c a t a l y s i s remain a f t e r twenty-five y e a r s , t h e z e o l i t e s A, X and Y .
TABLE -
2
ZEOLITE TYPES I N COMMERCIAL APPLICATIONS { 5 1 Zeolite Minerals
Synthetic Zeolites
Mordeni t e Chabazite Erionite Clinoptilolite
L Omega IIZeolon", Mordenite ZSM-5 F W
N a , K , Ca f o r m s N a , C a , Ba f o r m s N a , C a , NHq, r a r e e a r t h forms K , NH4 forms Na, H f o r m s H , Na f o r m s Various forms K form K form
TRANSITION I N PROPERTIES The t r a n s i t i o n i n p r o p e r t i e s of m o l e c u l a r s i e v e The emphasis m a t e r i a l s i s summarized i n T a b l e 3. chosen i n on t h e framework S i / A l i n c r e a s i n g from aluminum s a t u r a t e d " S i / A l = 1 " t o i n f i n i t y , a s r e p r e s e n t e d by t h e aluminum-free, p u r e s i l i c a m o l e c u l a r s i e v e , silicalite. The p r o p e r t y t r a n s i t i o n s shown a r e somewhat g e n e r a l i z e d and s h o u l d b e c o n s i d e r e d o n l y a s t r e n d s .
TABLE 3 THE TRANSITION I N PROPERTIES Transition in: Si/Al,
from 1 t o
Stability, -1300°C
<700°C t o from -
Surf ace s e l e c t i v i t y , from hydrophilic t o hydrophobic
"Acidity", increasing strength Cation concentration, decreasing S t r u c t u r e , from 4 , 6 , and 8 - r i n g s t o 5 - r i n g s
The s t a b i l i t y c h a r a c t e r i s t i c s v a r y s u b s t a n t i a l l y from a c r y s t a l l i n e d e c o m p o s i t i o n t e m p e r a t u r e n e a r 700°C f o r t h e "low" s i l i c a z e o l i t e s , t o a b o v e 1 3 0 0 ° C f o r s i l i calite. The low s i l i c a z e o l i t e s a r e a t b e s t " f r a g i l e " i n t h e presence of a c i d , whereas t h e high s i l i c a z e o l i t e s a r e completely s t a b l e even i n b o i l i n g , concentrated mineral acids. I n contrast, t h e high s i l i c a z e o l i t e s show d e c r e a s e d b a s e s t a b i l i t y . The s u r f a c e s e l e c t i v i t y c h a n g e s from t h e h i g h l y p o l a r o r h y d r o p h i l i c s u r f a c e e x h i b i t e d by t h e aluminumr i c h z e o l i t e s t o a more homogeneous o r n o n p o l a r organop h i l i c o r hydrophobic p r o p e r t i e s appears t o occur a t an Si/Al near 10. C h a r a c t e r i z a t i o n of t h e s u r f a c e s e l e c t i v i t y of t h e h i g h s i l i c a z e o l i t e s a s compared t o t h e low s i l i c a zeol i t e s h a s been s t u d i e d e x t e n s i v e l y i n t h i s l a b o r a t o r y {39}. I n t h e a d s o r p t i o n of H20, 02 and n - h e x a n e , t h e h y d r o p h i l i c NaX z e o l i t e e x h i b i t s t h e n e a r r e c t i l i n e a r t y p e I i s o t h e r m s h a p e t y p i c a l of z e o l i t e a d s o r p t i o n where t h e mechanism of volume f i l l i n g of m i c r o p o r e s c o n t r o l s i s o t h e r m s h a p e . The h y d r o p h o b i c s i l i c a l i t e s i m i l a r l y p o r e f i l l s w i t h oxygen and n-hexane a t low r e l a t i v e p r e s s u r e , b u t f i l l s o n l y a b o u t 2 5 % of i t s p o r e v o l u m e w i t h w a t e r n e a r s a t u r a t i o n {26a}. T h e h y d r o p h i l i c NaX z e o l i t e r e m o v e s t h e w a t e r , and t h e hydrophobic s i l i c a l i t e removes t h e o r g a n i c , from A s a consequence, separations o r g- a n i c - w a t e r m i x t u r e s . and c a t a l y s i s which u s u a l l y r e q u i r e t h e absence of water w i t h h y d r o p h i l i c z e o l i t e s , c a n now b e c a r r i e d o u t i n t h e presence of water with t h e hydrophobic z e o l i t e s . The t r a n s i t i o n i n h y d r o p h o b i c p r o p e r t i e s w i t h i n one z e o l i t e s t r u c t u r e t y p e i s i l l u s t r a t e d i n t h e c a s e of t h e z e o l i t e m o r d e n i t e { " ~ e o l o n " ) b y C h e n { 3 4 } who s h o w s a q u a n t i t a t i v e l i n e a r r e l a t i o n s h i p of w a t e r c a p a c i t y w i t h i n c r e a s i n g framework S i / A l r a t i o f o r a s e r i e s of v a r i o u s l y s t a b i l i z e d and dealuminated mordenite m a t e r i a l s . There i s only a small l o s s i n c r y s t a l p o r e volume f o r cyclohexane The o n s e t of change i n over t h e S i / A l range of 5 t o 50. s u r f a c e s e l e c t i v i t y from p o l a r t o nonpolar appears t o occur h e r e near an Si/A1 r a t i o of 7.5. The change i n c a t i o n c o n c e n t r a t i o n accompanying t h e change i n Si/Al a f f e c t s c a t i o n s p e c i f i c i n t e r a c t i o n s i n a d s o r p t i o n , c a t a l y s i s and i o n exchange, where t h e e f f e c t of c r y s t a l s t r u c t u r e and r e s u l t i n g c a t i o n s i t i n g a r e a l s o important, C a t i o n exchange s e l e c t i v i t y i s p e r h a p s most strongly affected. I t s i m p o r t a n c e i s o b s e r v e d i n many adsorbent applications involving cation specific inter-
actions such a s a i r separation involving s p e c i f i c i n t e r a c t i o n s of t h e n i t r o g e n q u a d r u p o l e w i t h c a t i o n s (40}, i n i o n e x c h a n g e a p p l i c a t i o n s s u c h a s ammonium r e m o v a l , a n d perhaps e q u a l l y s t r i k i n g l y i n a c i d c a t a l y s i s , where t h e c a t a l y t i c a d v a n t a g e s of z e o l i t e Y v e r s u s z e o l i t e X a r e well established. The c o n c e p t of i n c r e a s i n g a c i d i t y a s w e l l a s s t a b i l i t y i n hydrogen and "decationized" forms of zeol i t e s w i t h i n c r e a s i n g framework S i / A l has p e r s i s t e d s i n c e t h e e a r l y w o r k i n c a t a l y s i s b y Rabo e t a l . C41}. R e c e n t l y s e v e r a l a t t e m p t s t o q u a n t i f y and p r e d i c t t h a t change have been published. Barthomeuf 1 4 2 ) showed a l i n e a r d e c r e a s e i n i n f r a r e d f r e q u e n c y of t h e a c i d h y d r o x y l group w i t h i n c r e a s i n g framework S i / A 1 f o r a l a r g e number of z e o l i t e s t r u c t u r a l t y p e s , and r e l a t e d t h i s t o i n c r e a s e i n a c i d s t r e n g t h of t h e p r o t o n w i t h t h e change i n framework c h a r g e d e n s i t y . Subsequently ( 4 3 ) i n v o k i n g t h e c o n c e p t of a c i d a c t i v i t y c o e f f i c i e n t s i n z e o l i t e s , s h e n o t e d t h a t b o t h a c i d s t r e n g t h and proton a c t i v i t y coefficients increased with decreasing aluminum c o n t e n t i n t h e z e o l i t e framework, w h e r e a s This suggested that acid site concentration decreases. t h e r e s h o u l d b e a maximum i n a c i d s i t e a c t i v i t y a n d i n c a t a l y t i c r e a c t i o n r a t e a t a s p e c i f i c aluminum c o n t e n t . Vedrine e t a l . (441 r e p o r t t h a t t h e acid s i t e s present ( S i / ~ l= 1 9 . 2 ) a r e s i m i l a r t o , b u t s l i g h t l y i n H-ZSM-5 s t r o n g e r than, those p r e s e n t i n hydrogen mordenite ("Zeolon", Si/Al = 5 ) . The c o n c e n t r a t i o n of a c i d s i t e s i s s u b s t a n t i a l l y higher i n the lower Si/A1 mordenite. I n a somewhat d i f f e r e n t a p p r o a c h , M o r t i e r ( 4 5 1 a p p l i e d t h e Sanderson e l e c t r o n e g a t i v i t y model t o zeol i t e s , a n d c o n c l u d e d t h a t t h e a c i d s t r e n g t h of z e o l i t i c protons increases along with the calculated residual hydrogen c h a r g e , w i t h d e c r e a s i n g aluminum c o n t e n t . M o r t i e r , J a c o b s and U y t t e r h o e v e n ( 4 6 ) c o n f i r m e d t h a t t h e o v e r a l l c a t a l y t i c e f f i c i e n c y i n t h e d e h y d r a t i o n of isopropanol increased l i n e a r l y with the residual charge on t h e p r o t o n f o r s e v e r a l z e o l i t e s t r u c t u r e t y p e s . EVOLUTION I N SYNTHESIS The e a r l y e r a of t h e d i s c o v e r y of z e o l i t e s i n t h e l a t e 1 9 4 0 ' s a n d t h e e a r l y 1 9 5 0 ' ~w~h i c h l e d t o a b o u t 2 0 novel s y n t h e t i c z e o l i t e s (471 i n a s h o r t period of time, f l o w e d f r o m t h e d i s c o v e r y o f a new r e g i m e o f c h e m i s t r y (21 involving highly reactive alkaline aluminosilicate g e l s , m e t a s t a b l e c r y s t a l l i z a t i o n , and low t e m p e r a t u r e , low I t was l a t e r emphasized t h a t pressure crystallization.
t h e c a t i o n p l a y e d a dominant r o l e i n d i r e c t i n g t h e formaThese e a r l y z e o l i t e s t i o n of s p e c i f i c s t r u c t u r e s (481. r e s u l t e d from n e a r l y t h e same c h e m i s t r y and t h e u s e of o n l y two a l k a l i c a t i o n s , s o d i u m a n d p o t a s s i u m , o r mixA second important v a r i a b l e i n s y n t h e s i s tures thereof. a s w e l l a s i n p r o p e r t i e s was t h e S i / A l r a t i o . Increase i n t h e Si/A1 i n t h e r e a c t i o n mixture r e s u l t e d i n synthes i s of intermediate o r t r a n s i t i o n Si/Al z e o l i t e s , such a s T ( 4 9 1 a n d L , s t i l l u s i n g t h e same two a l k a l i c a t i o n s . T h e n e x t m a j o r a d v a n c e i n s y n t h e s i s o f new z e o l i t e m a t e r i a l s w a s t h e i n t r o d u c t i o n o f a new c h e m i s t r y i n t o z e o l i t e s y n t h e s i s , t h a t of t h e a d d i t i o n of alkylammonium cations to synthesis gels. Barrer e t a l . I501 f i r s t r e p o r t e d t h e s y n t h e s i s of N-A, a s i l i c e o u s a n a l o g of z e o l i t e A, by a d d i n g tetramethylammonium c a t i o n s t o sodium a l u m i n o s i l i c a t e g e l s , and n o t e d t h e e f f e c t of t h e a l k y l a m m o n i u m i o n o n i n c r e a s i n g f r a m e w o r k S i / ~ lcomposition. Nitrogenous analogs of z e o l i t e s B, X and Y were a l s o synthesized (50b). Thus t h e f i r s t e f f e c t of t h e a d d i t i o n o f t h e alkylammonium c a t i o n s was t o g e n e r a t e m o r e s i l i c e o u s f r a m e w o r k c o m p o s i t i o n s o f p r e v i o u s l y known structure types. Subsequently the addition of alkylammonium c a t i o n s t o s o d i u m a l u m i n o s i l i c a t e g e l s l e d t o t h e c r y s t a l l i z a t i o n o f new z e o l i t e s t r u c t u r e t y p e s , e x e m p l i f i e d b y z e o l i t e ZK-5 ( 5 1 1 , o m e g a { 1 9 , 5 2 ) , a n d I n t h e r e c e n t work by M o b i l R e s e a r c h z e o l i t e N (53). and Development C o r p o r a t i o n s c i e n t i s t s 171 t h e a d d i t i o n o f some n u m b e r s o f alkylammonium a n d o t h e r n i t r o g e n e o u s o r g a n i c m o l e c u l e s ( 5 4 1 , s u c h a s TEA, T P A , TBA, a n d pyrrolidine, t o highly s i l i c e o u s g e l s (Si/A1 = 10 t o 100) r e s u t l e d i n t h e high s i l i c a z e o l i t e m a t e r i a l s . These compositions represent siliceous analogs of previously k n o w n s t r u c t u r e t y p e s , ZSM-21 ( 2 4 1 , a f e r r i e r i t e - t y p e , a n d ZSM-34 { 2 5 ) , a n e r i o n i t e - o f f r e t i t e t y p e , a s w e l l a s n e w s t r u c t u r e t y p e s i n t h e c a s e o f ZSM-5 { 2 1 j , ZSM-11 { 2 2 ) , a n d p r o b a b l y ZSM-12 ( 2 3 ) a n d z e o l i t e b e t a ( 2 0 1 . The a d d i t i o n of alkylammonium c a t i o n s t o p u r e s i l i c a systems u l t i m a t e l y r e s u l t e d i n t h e s i l i c a molecular sieves: s i l i c a l i t e ( 2 6 ) and f l u o r i d e - s i l i c a l i t e (27) w i t h TPA; s i l i c a l i t e - 2 , s t r u c t u r a l l y r e l a t e d t o t h e z e o l i t e ZSM-11 ( 5 5 1 , w i t h TBA ( 2 9 3 ; a n d T E A - s i l i c a l i t e { 3 0 ) , a n a p p a r e n t s t r u c t u r a l a n a l o g o f z e o l i t e ZSM-12, w i t h TEA* Thus, v a r i a t i o n intwo important parameters i n synt h e s i s , c a t i o n and S i / A l r a t i o , has r e s u l t e d i n t h e s p e c t r u m o f s y n t h e t i c z e o l i t e s now k n o w n . T h e i r synthesis still uses the basis reactive gel crystallization method d e v e l o p e d by M i l t o n 1 2 1 i n t h e l a t e 4 0 ' s .
The s y n t h e s i s mechanisms of t h e low s i l i c a and h i g h It is suggested here s i l i c a zeolites appear t o d i f f e r . {56) t h a t t h e n u c l e a t i o n mechanism i n t h e low s i l i c a zeolites involves the formation of s t a b i l i z e d a l k a l i m e t a l c a t i o n a l u m i n o s i l i c a t e complexes and i s p r i m a r i l y c o n t r o l l e d by t h e a l u m i n a t e and a l u m i n o s i l i c a t e s o l u t i o n chemistry. Four and six-membered r i n g s and " c a g e s " of a l u m i n o s i l i c a t e t e t r a h e d r a , s t a b i l i z e d by a l k a l i m e t a l c a t i o n s , dominate t h e s y n t h e s i s chemistry and appear i n the structures. I n t h e c a s e of t h e h i g h l y s i l i c e o u s molecular sieves, a t r u e "templating" o r clathration mechanism p e r v a d e s w h e r e i n t h e alkylammonium c a t i o n forms complexes w i t h s i l i c a v i a hydrogen bonding i n t e r These complexes template o r cause rea c t i o n s (26 a ) . l i c a t i o n of t h e s t r u c t u r e v i a a s t e r e o s p e c i f i c hydrogenb o n d i n g i n t e r a c t i o n o f t h e q u a t e r n a r y ammonium c a t i o n w i t h t h e framework oxygens. Synthesis chemistry and s t r u c t u r e i s now d o m i n a t e d b y s i l i c a , f i v e - r i n g s o f t e t r a h e d r a , and t h e s t e r o s p e c i f i c i n t e r a c t i o n s w i t h alkylammonium i o n . The c o n c e p t of c a t i o n t e m p l a t i n g i n z e o l i t e s y n t h e s i s h a s b e e n d i s c u s s e d by F l a n i g e n 1 4 8 ) and more r e c e n t l y d e v e l o p e d and summarized by Rollman 1 5 7 ) . I t i s i n t e r e s t i n g t o n o t e t h a t r e c e n t t h e o r e t i c a l work on m o l e c u l a r e l e c t r o s t a t i c p o t e n t i a l by M o r t i e r e t a l . 158) shows s t a b i l i z a t i o n of a 6 - r i n g c o n t a i n i n g aluminum due t o t h e p r e s e n c e of m e t a l c a t i o n s n e a r i t s c e n t e r , i n s u p p o r t o f t h e m e c h a n i s t i c c o n c e p t of a s t a b i l i z e d m e t a l aluminosilicate species i n alumina-rich synthesis. Also of n o t e i s t h e o b s e r v a t i o n t h a t t h e c r o s s - o v e r i n z e o l i t e s u r f a c e s e l e c t i v i t y from h y d r o p h i l i c t o hydrophobic a t an S i / A l n e a r 7.5 t o 1 0 , c o r r e s p o n d s t o t h e change i n zeol i t e s t r u c t u r a l f e a t u r e s from 4 , 6 , and 8-ring s t r u c t u r e s t o those c o n t a i n i n g a n i n c r e a s i n g f r a c t i o n of 5-rings This suggests that ( f o r e x a m p l e , Y+R+mordenite+ZSM-5). a s t h e f r a c t i o n o f A 1 d e c r e a s e s below one p e r 6-ring c o r r e s p o n d i n g t o a n S i / A l of 5 , 5 - r i n g f o r m a t i o n i s favored.
Although t h e l e v e l of understanding of t h e s y n t h e s i s chemistry and i t s r e l a t i o n s h i p t o t h e r e s u l t i n g s t r u c t u r a l f e a t u r e s of z e o l i t e s has advanced s u b s t a n t i a l l y over t h e period of twenty-five y e a r s , t h e a b i l i t y t o execute chemical architecture i n the laboratory has unfortunately e l u d e d z e o l i t e s y n t h e s i s s c i e n t i s t s f o r t h e same p e r i o d . T h e s y n t h e s i s o f new z e o l i t e h a s f l o w e d f r o m t h e i n n o v a t i v e a l t e r a t i o n of t h e c h e m i s t r y of t h e s y n t h e s i s system, r a t h e r t h a n from t h e a b i l i t y t o design t h e chemistry t o form a d e s i r e d s t r u c t u r e .
The m a n u f a c t u r e of b a s i c z e o l i t e m a t e r i a l s s t i l l remains s i m i l a r t o t h a t d e v e l s p e d i n t h e i n i t i a l work i n The t e m p e r a t u r e r a n g e t h e l a t e 1 9 4 0 ' s by M i l t o n ( 2 3 . f o r c r y s t a l l i z a t i o n has tended t o increase with increasing S i / A l r a t i o i n t h e z e o l i t e , f r o m 2 5 t o 125OC f o r t h e aluminum-rich z e o l i t e s , t o 100 t o 150°C f o r t h e i n t e r m e d i a t e S i / A l z e o l i t e s , L , omega, and m o r d e n i t e , and t o n e a r 125 t o 200°C f o r t h e h i g h s i l i c a z e o l i t e s a s e x e m p l i f i e d b y ZSM-5. This i s consistent with the s u g g e s t e d r e l a t i o n s h i p o f p o r e v o l u m e a n d s y n t h e s i s temperature 2 , t h a t t h e lower temperatures favor t h e highest p o r e volume m a t e r i a l s ( n e a r 0 . 4 cm3/g) s u c h a s A, X and Y , and t h e h i h e r t e m p e r a t u r e s f a v o r lower p o r e volumes ( 0 . 1 5 - 0 . 2 0 cm / g ) f o r m o r d e n i t e , L , o m e g a a n d ZSM-5.
5
THE EVOLUTION I N APPLICATIONS The m o l e c u l a r s i e v e b e h a v i o r o f c r y s t a l l i n e z e o l i t e s and t h e i r l a r g e p o t e n t i a l i n performing molecular s i e v i n g s e p a r a t i o n s was f i r s t d e m o n s t r a t e d i n t h e p i o n e e r i n g w o r k o f B a r r e r {4} a n d c o l l e a g u e s i n E n g l a n d . With t h e c o m m e r c i a l i z a t i o n of m o l e c u l a r s i e v e z e o l i t e s i n l a t e 1 9 5 4 a new c l a s s o f m a t e r i a l s b e c a m e a v a i l a b l e , c a p a b l e o f b e i n g t a i l o r - m a d e i n t e r m s o f s t r u c t u r e , comp o s i t i o n and p r o p e r t i e s . Y e t many o f t h e i r e a r l y a d s o r b e n t and c a t a l y t i c a p p l i c a t i o n s involved simple replacement of t h e t h e n u s e d a d s o r b e n t and c a t a l y s t m a t e r i a l s i n known a d s o r p t i o n a n d c a t a l y t i c p r o c e s s e s , b a s e d on t h e improved p r o p e r t i e s and p e r f o r m a n c e of t h e m o l e c u l a r sieve zeolites. T h i s i s e x e m p l i f i e d by t h e r e p l a c e m e n t of s i l i c a g e l and a c t i v a t e d a l u m i n a i n d r y i n g and p u r i f i c a t i o n a p p l i c a t i o n s by z e o l i t e s A a n d X , d u e t o t h e improved c a p a c i t y and g r e a t e r s e l e c t i v i t y of t h e z e o l i t e s . T h e i r i n t r o d u c t i o n i n t o c a t a l y t i c c r a c k i n g i n 1 9 6 2 was a s a r e p l a c e m e n t f o r amorphous s i l i c a - a l u m i n a c a t a l y s t s i n e x i s t i n g moving bed and f l u i d bed c a t a l y t i c c r a c k i n g processing. Replacement followed t h e discovery of t h e h i g h e r a c t i v i t y of z e o l i t e X i n t h e c r a c k i n g r e a c t i o n and t h e h i g h e r s e l e c t i v i t y t o g a s o l i n e compared t o silica-alumina. Applications engineered specifically f o r zeolites were developed and c o n t i n u e t o e v o l v e o v e r t h e twentyf i v e year span, especially i n the adsorbent area, i n such processes as isoparaffin/n-paraffin separation, xylene s e p a r a t i o n and o l e f i n s e p a r a t i o n , and p r e s s u r e swing A l l of t h e s e a d s o r b e n t a d s o r p t i o n a i r s e p a r a t i o n {6}. a p p l i c a t i o n s combine t h e u n i q u e a d s o r p t i v e p r o p e r t i e s of a s p e c i f i c t a i l o r - m a d e m o l e c u l a r s i e v e a d s o r b e n t , a n d
a n a d s o r p t i o n p r o c e s s d e s i g n e d a n d engineered to o p t i m i z e t h e product-process c h a r a c t e r i s t i c s . I n t e r e s t i n g l y , t h e o r i g i n a l i n c e n t i v e o f M i l t o n to u s e m o l e c u l a r s i e v e z e o l i t e s w a s to s e p a r a t e o x y g e n f r o m n i t r o g e n as a n e w m e t h o d f o r a i r separation. T h e c o m m e r c i a l i z a t i o n of that a p p l i c a t i o n i n o x y g e n and n i t r o g e n p r o d u c t i o n f r o m a i r v i a t h e n o w used p r e s s u r e swing a d s o r p t i o n p r o c e s s e s (6,593, occurred n e a r l y twenty y e a r s l a t e r w h e n a m a r k e t i n w a s t e w a t e r purific a t i o n and other a p r l i c a t i o n s r e q u i r i n g r e l a t i v e l y l o w tonnage p r o d u c t i o n of o x y g e n allowed it to f i n a l l y compete s u c c e s s f u l l y w i t h t h e established c r y o g e n i c s e p a r a t i o n technology. A d s o r b e n t Applications. T h e u s e o f m o l e c u l a r s i e v e z e o l i t e a b s o r b e n t s t o p e r f o r m a h o s t o f s e p a r a t i o n s and p u r i f i c a t i o n s h a s b e c o m e f i r m l y established i n the chem i c a l p r o c e s s industries. A s u m m a r y l i s t of m a j o r a d s o r b e n t a p p l i c a t i o n s adapted f r o m Anderson's r e v i e w at the l a s t M o l e c u l a r S i e v e Z e o l i t e C o n f e r e n c e i n C h i c a g o (61 i s g i v e n in T a b l e 4. TABLE 4 C OM M E R C I A L -----A D S O R B E N T A P P L I C A T I O N S O F M O L E C U LAR SIEVE ZEOLITES -
A. Purification I.
Drying natural gas (including LNG) cracked gas (ethylene plants) insulated windows refrigerant 11. CO Removal 2 natural gas cryogenic air separation plants 111. Sulfur Compound Removal sweetening of natural gas and liquified pretroleum gas IV. Pollution Abatement removal of Hg, NOx,
B. Bulk Separations
I.
Normal/iso-paraffin separation 11. Xylene separation 111. Olefin separation IV. O2 from air V. Sugar separation (601
It can be seen t h a t p r e s e n t day a p p l i c a t i o n s f a l l i n t o two c a t e g o r i e s , p u r i f i c a t i o n a p p l i c a t i o n s w h i c h i n g e n e r a l depend on s u r f a c e s e l e c t i v i t y f o r p o l a r o r p o l a r i z a b l e m o l e c u l e s s u c h a s w a t e r , C02 o r s u r f a c e c o m p o u n d s ; a n d b u l k s e p a r a t i o n s many o f w h i c h a r e b a s e d on m o l e c u l a r sieving principles. P r e s s u r e swing a d s o r p t i o n i n a i r s e p a r a t i o n , o r i g i n a l l y e n v i s a g e d by M i l t o n a s a m o l e c u l a r s i e v i n g s e p a r a t i o n b a s e d on t h e s l i g h t d i f f e r e n c e i n s i z e of t h e oxygen and n i t r o g e n m o l e c u l e , i s r a t h e r b a s e d on t h e s t r o n g s p e c i f i c i n t e r a c t i o n of t h e n i t r o g e n m o l e c u l a r quadrupole with the cation {40}. Many o f t h e p u r i f i c a tion applications also involve molecular sieving i n that t h e s e l e c t i o n of t h e z e o l i t e adsorbent involves a pore s i z e designed t o exclude p o t e n t i a l l y co-adsorbed molecules, f o r example t h e u s e of t y p e 3A m o l e c u l a r s i e v e i n c r a c k e d g a s d r y i n g t o p r e v e n t t h e c o - a d s o r p t i o n of e t h y l e n e and heavier unsaturated hydrocarbons. Refrigerant drying and p u r i f i c a t i o n (of halogenated hydrocarbons), t h e f i r s t b r o a d l y a p p l i c a b l e commercial u s e of m o l e c u l a r s i e v e s {2), s t i l l remains a major nonregenerative application. Recently molecular sieve z e o l i t e adsorbent separat i o n s have been extended t o l i q u i d phase aqueous systems i n t h e s e p a r a t i o n of f r u c t o s e from f r u c t o s e - d e x t r o s e polysaccharide mixtures (601. Catalyst Applications. Fundamental d i s c o v e r i e s i n t h e use of z e o l i t e s i n hydrocarbon c a t a l y s i s were made i n t h e 5 0 ' s p r i m a r i l y a t t h e l a b o r a t o r i e s o f U n i o n C a r b i d e and Mobil O i l C o r p o r a t i o n s , and Esso R e s e a r c h a n d E n g i n e e r i n g Company {61), w i t h t h e r e c o g n i t i o n o f t h e a c i d i c p r o p e r t i e s of h y d r o g e n , m u l t i v a l e n t m e t a l c a t i o n , and d e c a t i o n i z e d forms of z e o l i t e s X and Y {41,621, and t h e n o v e l s h a p e s e l e c t i v e p r o p e r t i e s of z e o l i t e A ( 6 3 ) . A commercial z e o l i t e Y isomerization c a t a l y s t , i n t r o d u c e d b y U n i o n C a r b i d e i n 1 9 5 9 {2}, w a s t h e f i r s t o f a s e r i e s of molecular s i e v e based c a t a l y s t s f o r t h e petroleum industry. The f i r s t m a j o r c o m m e r c i a l c a t a l y t i c a p p l i c a t i o n r e s u l t e d from t h e i n t r o d u c t i o n of t h e use of z e o l i t e X i n c a t a l y t i c cracking of crude t o produce l i q u i d f u e l s i n 1 9 6 2 , b a s e d on t h e e a r l y work of P l a n k and R o s i n s k i (64). The i n t r o d u c t i o n of z e o l i t e c o n t a i n i n g c a t a l y s t s caused a r e v o l u t i o n i n c a t a l y t i c cracking (651, because of t h e i r i n c r e a s e d c a t a l y t i c a c t i v i t y and improved y i e l d s t o g a s o l i n e compared t o amorphous s i l i c a - a l u m i n a c a t a l y s t s . X e c h a n i s t i c a l l y t h i s h a s b e e n r e l a t e d by Weisz ( 6 6 ) and
o t h e r s t o t h e more e f f i c i e n t h y d r o g e n r e d i s t r i b u t i o n between hydrocarbon molecules over z e o l i t e c a t a l y s t s , r e s u l t i n g i n high r a t e s of intermolecular hydrogen transf e r , coupled with extremely high i n t r i n s i c cracking activity. B e c a u s e of t h e v e r y s t r o n g a d s o r p t i o n f o r c e s w i t h i n z e o l i t e s they a l s o c o n c e n t r a t e hydrocarbon subs t r a t e s t o a much l a r g e r e x t e n t t h a n o t h e r c a t a l y s t s and f a v o r bimolecular r e a c t i o n s ( 7 ) such a s hydrogen transfer. Developments s i n c e 1962 i n z e o l i t e c a t a l y t i c cracki n g h a v e o c c u r r e d b o t h i n m a t e r i a l s and p r o c e s s ( 6 7 , 6 5 1 . Z e o l i t e X h a s b e e n e s s e n t i a l l y r e p l a c e d by t h e more M e t a l s r e s i s t a n t , and s t a b l e and a c t i v e z e o l i t e Y . c o n t r o l l e d combustion z e o l i t e c a t a l y s t s have been developed, t h e f o r m e r t o a l l o w h a n d l i n g of h e a v i e r f e e d s t o c k s , and t h e l a t t e r f o r p o l l u t i o n c o n t r o l t o c o n v e r t CO e m i s s i o n s Z e o l i t e c o n t e n t h a s been i n c r e a s e d from 5-10% t o CO2. Process innovai n 1964, t o a s h i g h a s 40% i n 1979 { 6 8 ) . t i o n s t o u t i l i z e t h e unique p r o p e r t i e s of z e o l i t e s inc l u d e c o n c e p t s b a s e d on s h o r t c o n t a c t r i s e r c r a c k i n g and h a v e l e d t o some number o f p r o p r i e t a r y z e o l i t e e n g i n e e r e d Recent developments d e s i g n s now i n c o m m e r c i a l u s e { 6 7 ) . h a v e b e e n s t r o n g l y i n f l u e n c e d by e n v i r o n m e n t a l r e q u i r e m e n t s i n p o l l u t i o n c o n t r o l and t h e need f o r h i g h e r octane unleaded g a s o l i n e ( e s p e c i a l l y i n the United States). Other e s t a b l i s h e d i n d u s t r i a l processes t h a t u t i l i z e z e o l i t e based c a t a l y s t s i n addtion t o c a t a l y t i c cracking, A l l a r e h y d r o c r a c k i n g , and p a r a f f i n i s o m e r i z a t i o n (71. a r e b a s e d on t h e u n i q u e p r o p e r t i e s o f z e o l i t e c a t a l y s t s w h i c h h a v e i n common, e x t r e m e l y h i g h s t r e n g t h a c i d s i t e s , and s e l e c t i v i t i e s r e l a t e d t o s t r o n g a d s o r p t i v e f o r c e s A l l use hydrothermally s t a b l e acid within the zeolite. forms of l a r g e p o r e z e o l i t e s . Two e x a m p l e s o f s h a p e s e l e c t i v e c a t a l y s i s 163,69J u t i l i z i n g , i n a d d i t i o n t o t h e a b o v e p r o p e r t i e s , a s p e c i f i c p o r e s i z e and s h a p e of Selectoforming z e o l i t e , a r e i n commercial use. The p r o c e s s of Mobil O i l Corp. s e l e c t i v e l y h y d r o c r a c k s t h e normal p a r a f f i n components of c a t a l y t i c r e f o r m a t e u s i n g a n o f f r e t i t e - e r i o n i t e t y p e c a t a l y s t which e x c l u d e s nonnormal p a r a f f i n m o l e c u l e s from a d s o r p t i o n and r e a c t i o n . A second commercial shape s e l e c t i v e hydrocracking p r o c e s s i s c a t a l y t i c dewaxing which t y p i c a l l y employs a l a r g e pore mordenite containing single channels approximately 0.7nm i n d i a m e t e r w h i c h c o n t r i b u t e t o s e l e c t i v i t y 1 7 ) .
The c a t a l y t i c p r o p e r t i e s o f t h e h i g h s i l i c a z e o l i t e The i n i t i a l ZSM-5 h a v e r e c e i v e d much a t t e n t i o n { 7 0 , 7 ) . c o m m e r c i a l o r n e a r c o m m e r c i a l a p p l i c a t i o n s r e p o r t e d C7) f o r ZSM-5 i n c l u d e : a ) t h e i s o m e r i z a t i o n o f Cg a r o m a t i c s t o produce isomerically pure xylenes, e s p e c i a l l y paraxylene f o r polyester manufacture; b) ethylbenzene s y n t h e s i s f o r s t y r e n e p r o d u c t i o n ; and c) c a t a l y t i c de:,?axing ( 7 1 ) . The c o n v e r s i o n of m e t h a n o l t o g a s o l i n e { 7 2 ) a s a new r o u t e f r o m c o a l o r s y n t h e t i c o r n a t u r a l g a s t o a o t o r f u e l i s under development {73}, with the f i r s t p l a n t b a s e d o n c o a l s c h e d u l e d f r o c o n s t r u c t i o n i n New Z e a l a n d < 7 4 ) . More r e c e n t l y M o b i l w o r k e r s h a v e a l s o r e p o r t e d i 7 5 ) a s i m i l a r c o n v e r s i o n o f o x y g e n a t e d h y d r o c a r b o n compounds i n biomass t o g a s o l i n e . T h e s e i n i t i a l c o m m e r c i a l a p p l i c a t i o n s f o r ZSM-5 appear t o b e e l e g a n t examples of s h a p e s e l e c t i v e c a t a l y s i s 3 6 3 , 6 9 1 r e f l e c t i n g i t s u n i q u e c r y s t a l s t r u c t u r e w i t h 0.6nm p o r e s o u t l i n e d b y 10-membered r i n g s o f o x y g e n . In addition t h e y depend upon t h e o t h e r z e o l i t e - s p e c i f i c p r o p e r t i e s o f h i g h l y a c i d i c s i t e s a s i n t h e H-ZSM-5 c a t a l y s t , a n d the substrate or reactant concentration effect. The novel organophilic-hydrophobic s e l e c t i v i t y a l s o appears t o c o n t r i b u t e t o t h e a p p a r e n t l y u n i q u e s e l e c t i v i t y of ZSM-5 f o r t h e c o n v e r s i o n o f o x y g e n a t e d h y d r o c a r b o n s t o s a r a f f i n s and aromatics. The e n t r a n c e i n t o s e v e r a l of t h e s e commercial a p p l i c a t i o n s b y ZSM-5 r e p o r t e d l y ( 7 ) may i n v o l v e r e t r o f i t o f s x i s t i n g p r o c e s s e s w i t h a new c a t a l y s t . I n t h e c a s e of C8 a r o m a t i c i s o m e r i z a t i o n ZSM-5 r e p l a c e s t h e p l a t i n u m / silica-alumina c a t a l y s t s developed f o r t h e Octafining ?recess. I n e t h y l b e n z e n e s y n t h e s i s ZSM-5 c o u l d r e p l a c e t h e c u r r e n t t e c h n o l o g i e s b a s e d o n A 1 C 1 3 a n d BF3 s u p p o r t e d on a l u m i n a . I n t h e c a s e o f c a t a l y t i c d e w a x i n g ZSM-5 may r e p l a c e t h e u s e of m e t a l l o a d e d t u b u l a r z e o l i t e c a t a l y s t s such a s mordenite. E a r l y r e p o r t s of performance advanr a g e s o f ZSM-5 i n t h e p r o c e s s s u g g e s t t h a t t h e H-ZSM-5 z a t a l y s t c a r r i e s out t h e needed hydrogenation f u n c t i o n :
TABLE 5 PRESENT AND PROJECTED APPLICATIONS I N CATALYSIS { 5 } Hydrocarbon conversion Alkylation Cracking Hydrocracking Isomerization Hydrogenation and dehydrogenation Hydrodealkylation Methanation
OF ZEOLITES
Shape-selective reforming Dehydration Methanol t o g a s o l i n e Organic c a t a l y s i s Inorganic reactions H2S O x i d a t i o n NO R e d u c t i o n o f NH3 CO Oxidation H20 + 0 2 + H 2
The f i r s t i n t e r e s t i n g a p p l i c a t i o n of a combined c a t a l y t i c - a d s o r p t i v e i n t e g r a t e d p r o c e s s , named T I P ( t o t a l i s o m e r i z a t i o n p r o c e s s ) f o r g a s o l i n e o c t a n e improvement weds t h e Union C a r b i d e " I s o S i v " m o l e c u l a r s i e v e a d s o r p t i o n p r o c e s s f o r s e p a r a t i n g normal and i s o p a r a f f i n s e m p l o y i n g 5A z e o l i t e , w i t h t h e " H y s o m e r " c a t a l y t i c p r o c e s s o f t h e S h e l l O i l Co. t o i s o m e r i z e n o r m a l p a r a f f i n s t o higher octane branched isomers using a highly acid, l a r g e p o r e z e o l i t e c a t a l y s t b a s e d on a l a r g e p o r e mordenite {7}. Thus, molecular s i e v e z e o l i t e s have found wide use i n both c a t a l y t i c conversion and adsorption s e p a r a t i o n processes over the l a s t twenty-five years. I n both a d s o r b e n t a n d c a t a l y t i c a p p l i c a t i o n s t h e i n i t i a l comm e r c i a l i n t r o d u c t i o n t e n d e d t o i n v o l v e r e p l a c e m e n t of existing non-zeolite adsorbents o r catalysts because of improved z e o l i t e p e r f o r m a n c e . Zeolite engineered p r o c e s s e s e v o l v e d s u b s e q u e n t l y and more f a c i l e l y i n a d s o r p t i o n a p p l i c a t i o n s where t h e c a p i t a l c o s t s a r e g e n e r a l l y lower than those i n major hydrocarbon convers i o n p r o c e s s e s such a s c r a c k i n g and hydrocracking, In t h a l a t t e r c a s e t h e i n t r o d u c t i o n of z e o l i t e s r e s u l t e d i n improved performance and p r o d u c t i o n w i t h o u t e x p e n d i t u r e s of c a p i t a l . A f t e r twenty-five years both kinds of processes are s t i l l i n use. T h e r e t r o f i t p r o c e s s e s h a v e beer. extensively modified o r redesigned t o u t i l i z e unique p r o p e r t i e s of z e o l i t e s , and t h e s o p h i s t i c a t i o n o r tailor-making of the zeolite designed o r engineered p r o c e s s e s h a v e become more i n n o v a t i v e and s y s t e m a t i z e d t o t a k e maximum a d v a n t a g e o f z e o l i t e p r o p e r t i e s .
I o n Exchange A p p l i c a t i o n s . The t h i r d u n i q u e 2 r o p e r t y of z e o l i t e m o l e c u l a r s i e v e s , t h a t of s e l e c t i v e c a t i o n exchange, went through a q u i t e d i f f e r e n t h i s t o r y of development. Among t h e e a r l i e s t a p p l i c a t i o n a r e a s s x p l o r e d i n t h e U n i o n C a r b i d e l a b o r a t o r i e s b y Thomas (76) i n t h e e a r l y 5 0 ' s was t h e u s e of z e o l i t e s A , B and X i n c a t i o n exchange a p p l i c a t i o n s , i n c l u d i n g i n d u s t r i a l and domestic water softening. The s y n t h e t i c p o l y m e r i c i o n zxchange r e s i n s , which themselves were replacements f o r t h e amorphous m e t a l a l u m i n o s i l i c a t e p e r m u t i t e - t y p e > r o d u c t s , had o n l y r e c e n t l y been i n t r o d u c e d t o t h e market?lace. A t t h a t time i t appeared t h a t the organic r e s i n s ?ad t e c h n i c a l performance advantages i n t h e r e g e n e r a t i o n 2 y c l e o v e r t h e s y n t h e t i c z e o l i t e s A a n d X. Thus t h e ?erformance advantages of z e o l i t e s r e a l i z e d i n a d s o r p t i o n and c a t a l y s i s d i d n o t a p p a r e n t l y e x i s t i n i o n e x c h a n g e . C a t i o n exchange of z e o l i t e s i s used r o u t i n e l y i n ~ o d i f y i n gt h e p r o p e r t i e s o f z e o l i t e p r o d u c t s u s e d i n z d s o r p t i o n and c a t a l y s i s , and a l a r g e body of l i t e r a t u r e sn c a t i o n e x c h a n g e s e l e c t i v i t i e s , s t r u c t u r a l c h a r a c t e r i s t i c s , and thermodynamics by B a r r e r , S h e r r y , and o t h e r s -771, evolved very s h o r t l y a f t e r t h e commercial i n t r o S u c t i o n of z e o l i t e s A a n d X i n 1954. Yet, significant 2 s e of t h e z e o l i t e s a s i o n e x c h a n g e r s h a s o c c u r r e d o n l y recently. T h e i r development a s commercial i o n exchangers s t r o n g l y f o l l o w e d t h e e n a c t m e n t o f new e n v i r o n m e n t a l ;ollution standards i n the l a t e 60's. S h e r m a n C91 r e c e n t l y r e v i e w e d t h e s u b j e c t o f i o n ~ s c h a n g es e p a r a t i o n s w i t h m o l e c u l a r s i e v e z e o l i t e s . The s u m m a r y o f p r e s e n t a n d p o t e n t i a l i o n e x c h a n g e a p p l i z a t i o n s r e p o r t e d by Breck [ 5 } and a d a p t e d from Sherman i 9 ) 13 s h o w n i n T a b l e 6 .
TABLE 6 I O N EXCHANGE APPLICATIONS { 9 , 5 1
Present -
Applications
Removal o f CS+ a n d ~ r + + R a d i o i s o t o p e s - LiNDE AW-500, mordenite, c l i n o p t i l o l i t e Removal o f NH4+ f r o m -LINDE F , LINDE W , elinoptilolite Detergent builder Zeolite A, Zeolite X (ZB-100, ZB-300)
Advantage Stable to ionizing radiaticz Low s o l u b i l i t y Dimensional s t a b i l i t y High s e l e c t i v i t y - selective over competing c a t i o n s
wastewater^^^'
P r e s e n t ApplicationsRadioactive waste storage Aquaculture AW-500, c l i n o p t i l o l i t e R e g e n e r a t i o n of a r t i f i a a l Kidney d i a l y s a t e s o l u t i o n F e e d i n g NPN t o r u m i n a n t a n i m a l s M e t a l s removal and r e c o v e r y Ion exchange f e r t i l i z e r s
++
Remove ~ a + +a n d Mg by s e l e c t i v e exchange No e n v i r o n m e n t a l p r o b l e m Advantage Same a s C S + , s r f + r e m o v a l NH4+ - s e l e c t i v e N H ~ +- s e l e c t i v e
R e d u c e s N H ~ +by s e l e c t i v e exchange t o n o n t o x i c l e v e l s High s e l e c t i v i t i e s f o r various metals Exchange w i t h p l a n t n u t r i e n t s s u c h a s NH4+ a n d K+ w i t h s l o w r e l e a s e in soil.
The s i n g l e p o t e n t i a l l y l a r g e s t i o n exchange a p p l i c a t i o n a s b u i l d e r s i n detergents (78) is i r o n i c a l l y i n t h e water s o f t e n i n g a r e a , t h e o r i g i n a l i o n exchange application considered i n the 50's. I t b e c a m e a comm e r c i a l r e a l i t y d u e t o two changed f a c t o r s . First, its use t o soften water as a builder i n detergents i n the 7 0 ' s i s n o n - r e g e n e r a t i v e and t h e r e f o r e t h e main e a r l i e r disadvantage i n regeneration a s evaluated i n the 50's is absent. S e c o n d , t h e r e a r e c u r r e n t l y a number o f a r e a s i n t h e world i n which t h e use of phospate b u i l d e r s i s r e s t r i c t e d f o r environmental reasons. After considerable R and D e f f o r t , a p p a r e n t l y no o t h e r s u i t a b l e s u b s t i t u t e f o r phosphate h a s been found t o d a t e except t h e synt h e t i c z e o l i t e s A and X.
Replacement of p h o s p h a t e s i n d e t e r g e n t s by z e o l i t a i o n e x c h a n g e r s i s a l s o b a s e d on p e r f o r m a n c e and c o s t . The z e o l i t e s i n powder form p r o v i d e t h e same f u n c t i o n a s p h o s p h a t e s , t h e r e m o v a l o f h a r d n e s s i o n s , Ca++ a n d M~+,+, a s a c t i v e i o n s i n t h e wash w a t e r . T h e maximum i o n exchange c a p a c i t y of t h e aluminum s a t u r a t e d z e o l i t e A, t h e o r e t i c a l l y t h e h i g h e s t possible i n z e o l i t e s , coupled with a structurally controlled cation selectivity for ~ a + + ,give z e o l i t e A a unique advantage i n t h i s application. P r o j e c t i o n s show a v e r y l a r g e v o l u m e u s e a n d a b u l k volume c o s t t h a t i s l e s s t h a n t h a t of p h o s p h a t e s {78a}. N a t u r a l z e o l i t e s have played an i m p o r t a n t r o l e i n t h e development of i o n exchange a p p l i c a t i o n s . The u s e o f z e o l i t e m i n e r a l s c h a b a z i t e , m o r d e n i t e and c l i n o p t i l o l i t e f o r t h e removal of recovery of cesium and s t r o n t i u m r a d i o i s o t o p e s i n t h e n u c l e a r i n d u s t r y w a s among t h e e a r l i e s t a p p l i c a t i o n s of z e o l i t e s a s i o n exchangers. Their s u p e r i o r s e l e c t i v i t y and s t a b i l i t y c h a r a c t e r i s t i c s s p u r r e d t h e development of o t h e r z e o l i t e i o n exchange applications. The h i g h s e l e c t i v i t y of c l i n o p t i l o l i t e f o r ammonium i o n i n w a s t e w a t e r t r e a t m e n t a n d o t h e r applications generated i n t e r e s t i n developing synthetic A g a i n t h e s e comz e o l i t e s s u c h a s L i n d e F a n d L i n d e W. mercial applications were responsive t o environmental problems. The s u c c e s s of t h e z e o l i t e s b o t h n a t u r a l and synt h e t i c i n ammonium r e m o v a l a p p l i c a t i o n s r e s t s p r i n c i p a l l y on t h e i r h i g h c a t i o n s e l e c t i v i t y . In t h i s case their performance is f a r superior t o t h e organic r e s i n ion e x c h a n g e r s w h i c h s h o w p o o r s e l e c t i v i t y f o r ammonium i o n s , e s p e c i a l l y i n c o m p e t i t i o n w i t h c a l c i u m and magnesium ion {9>. Natural Zeolites. T h e r e h a v e b e e n a n u m b e r o f comp r e h e n s i v e and e x c e l l e n t r e v i e w s of t h e u s e s o f n a t u r a l A summary zeolites i n the l a s t f i v e years {1,10,11). The a d a p t e d f r o m t h e s e r e f e r e n c e s i s shown i n T a b l e 7 . major use of n a t u r a l z e o l i t e s i s i n bulk mineral applii n Europe i n t h e b u i l d i n g and c o n s t r u c t i o n cations {I): i n d u s t r y , w h e r e p r o x i m i t y t o b u i l d i n g l o c a t i o n makes them c o s t e f f e c t i v e ; a n d i n t h e E a r E a s t a s f i l l e r i n t h e paper i n d u s t r y , l a r g e l y because of t h e u n a v a i l a b i l i t y As discussed previously, of a l t e r n a t e m i n e r a l r e s o u r c e s . a modest market f o r z e o l i t e m i n e r a l s has developed a s a nolecular sieve adsorbent i n acid gas drying i n the n a t u r a l g a s i n d u s t r y , i n NHh r e m o v a l i n w a t e r t r e a t m e n t
s y s t e m s b y i o n e x c h a n g e , and i n the p r o d u c t i o n of oxyg e n and n i t r o g e n v i a a d s o r p t i v e a i r s e p a r a t i o n , e s p e c ia lly i n J a p a n (591. I n g e n e r a l , h o w e v e r , their p e n e t r a t i o n into m o l e c u l a r s i e v e a p p l i c a t i o n s h a s b e e n q u i t e limited. TABLE 7 S U M M A R Y OF USES O F N A T U R A L Z E O L I T E S {1,10,5) Bulk Applications: Filler in Paper Pozzolanic Cements and Concrete Dimension Stone Lightweight Aggregate Fertilizers and Soil Conditioners Dietary Supplement in Animal Nutrition
Molecular Sieve Applications: Separation of Oxygen and Nitrogen from Air Acid-resistant Adsorbents in Drying and Purification Ion Exchangers in Pollution Abatement Processes
E V O L U T I O N IN COMMERCE -T h e synthetic zeolite markets progressed from a r e l a t i v e l y s m a l l a d s o r b e n t m a r k e t of t h e o r d e r of a m i l l i o n d o l l a r s i n the l a t e 50's, through a rapid growth w h i c h w a s influenced by the u s e of z e o l i t e s X and Y i n c a t a l y t i c cracking to a published e s t i m a t e (1) of $ 4 0 M M i n 1 9 7 0 , and a projected $ 2 5 0 M M i n 1979. T h e projected m a r k e t in t h e l a r g e v o l u m e , bulk c o m m o d i t y d e t e r g e n t a r e a f o r z e o l i t e b u i l d e r s i n reported to b e $ 2 5 M M , o r a p p r o x i m m a t e l y l O O M M lbs. of z e o l i t e A i n 1 9 8 0 (791, and a n o p t i m i s t i c p r o j e c t i o n f o r growth t o a 4 0 0 M M lb. m a r k e t i n 1 9 8 2 {78a}. T h e i n t r o d u c t i o n of z e o l i t e s i n t o c r a c k i n g catalysts caused a c o m m e r c i a l as w e l l as a t e c h n i c a l r e v o l u t i o n a i n c a t a l y t i c cracking. I t is r e p o r t e d that z e o l i t e c reking c a t a l y s t s h a v e saved r e f i n e r s over $ 2 5 0 M M per y e a r , and increased g a s o l i n e c a p a c i t y s u b s t a n t i a l l y (80). T o d a y , type Y z e o l i t e h a s 100% of t h e U.S. market, and 75-80% of that o u t s i d e of the United S t a t e s {681. T h e t o t a l w o r l d w i d e c o n s u m p t i o n of z e o l i t e s i n catalytic c r a c k i n g i n 1 9 7 8 is estimated a t b e t w e e n 7 0 - 9 0 M M p o u n d s per y e a r (681, r e p r e s e n t i n g t h e s i n g l e l a r g e s t u s e o f s y n t h e t i c m o l e c u l a r s i e v e z e o l i t e s to date. It is lik e l y t h a t b u l k u s e of s i n t h e t i c z e o l i t e s i n d e t e r g e n t s may s u r p a s s that v o l u m e if p r o j e c t i o n s a r e r e a l i z e d .
I n a r e c e n t e s t i m a t e of t h e n a t u r a l z e o l i t e m a r k e t , Leonard { I } s u g g e s t s worldwide s a l e s beginning i n 1965 o f 24MM l b s . a t a v a l u e o f $ ~ M M 1, 6 0 MM l b s . a t $8MM i n 1 9 7 0 , a n d a 1 9 7 9 p r o j e c t i o n o f 560MM l b s . a t a v a l u e o f $35MM. G r e a t e r t h a n 90% of t h e m a r k e t i s i n b u l k m i n e r a l a p p l i c a t i o n s , and o n l y a b o u t 2% of t h a t i n North America. The m a j o r m a r k e t s a r e i n E u r o p e , R u s s i a , and t h e F a r E a s t , especially Japan The s y n t h e t i c m o l e c u l a r s i e v e z e o l i t e s a s i n d u s t r i a l xaterials are appropriately classified as specialty chemicals, o r preferably a s engineered products. The n o l e c u l a r s i e v e i n d u s t r y i s technology and engineering intensive. The m a j o r i t y of m o l e c u l a r s i e v e z e o l i t e a p p l i c a t i o n s a r e engineered i n a l l r e s p e c t s , from t h e s y n t h e s i s of t h e z e o l i t e m a t e r i a l , t h e m o d i f i c a t i o n of t h e i r p r o p e r t i e s , t h e s e l e c t i o n of a t a i l o r m a d e z e o l i t e ?roduct, t o a process engineering design and execution t h a t i n t e g r a t e s and o p t i m i z e s t h e m a t e r i a l w i t h t h e ?recess. The s u c c e s s f u l g r o w t h of m o l e c u l a r s i e v e zeol i t e s a s a n i n d u s t r i a l c h e m i c a l depended most s t r o n g l y on t h e i r d e v e l o p m e n t a s a n e n g i n e e r e d p r o d u c t . As developed previously, the e f f e c t of outside forces i n t h e changing world surrounding molecular s i e v e s ?ad a profound impact on t h e i r commercial growth and 2evelopment. T h i s i s e s p e c i a l l y true i n t h e c a s e of t h e ? r o b l e m s and c r i s e s i n e n e r g y and environment which svolved i n t h e 6 0 ' s and e r u p t e d i n t h e 7 0 7 s , and which s f f e r e d o p p o r t u n i t i e s f o r t h e i r uniquely s u i t e d pro? e r t i e s i n s e p a r a t i o n s and c a t a l y s i s . This is seen i n t h e n a t u r a l gas p u r i f i c a t i o n a r e a where l a r g e growth 5 a s r e c e n t l y o c c u r r e d i n t h e t r e a t m e n t o f LNG i n g i a n t h s e load f a c i l i t i e s i n t h e Middle East {6}, i n increased FCC p r o d u c t i o n of g a s o l i n e and o c t a n e improvement f o r -nleaded g a s o l i n e , i n t h e f l e d g i n g u s e of z e o l i t e i o n exchangers i n waste water treatment, i n insulated ~ i n d o wa d s o r b e n t s , i n s u b s t i t u t i o n o f z e o l i t e s f o r ;hosphate i n d e t e r g e n t s , and p o s s i b l y i n t h e f u t u r e , =ethanol o r biomass t o g a s o l i a e . The n o r m a l / i s o - p a r a f f i n ~ r o c e s s e su s i n g m o l e c u l a r s i e v e z e o l i t e s w e r e i n f l u e n c e d 5- t h e n e e d f o r b i o d e g r a d a b l e d e t e r g e n t s . The x y l e n e s e p a r a t i o n p r o c e s s e s w e r e i n f l u e n c e d by t h e i n d u s t r i a l :.eed f o r r a w m a t e r i a l f o r p o l y e s t e r f i b e r p r o d u c t i o n .
THE PAST AS INDICATOR OF THE FUTURE: YEARS --
THE NEXT TWENTY-FIVE
The i n i t a l d i s c o v e r i e s of t h e s y n t h e t i c m o l e c u l a r s i e v e z e o l i t e s A, X and Y i n t h e l a t e 4 0 ' s and e a r l y 5 0 ' s spawned a n immense w o r l d w i d e s c i e n c e a n d t e c h n o l o g y , u t i l i z i n g t h e r e s o u r c e s o f many m a j o r i n d u s t r i a l R a n d D organizations throughout t h e world. Zeolites are " r e s e a r c h e d 3 ' and used commercially i n e v e r y major c o u n t r y i n t h e world. (Recently China displayed molecular s i e v e s a s one of i t s i n d u s t r i a l p r o d u c t s a t t h e Shanghai I n d u s t r i z l Exhibit 1811.) W o r l d w i d e t h e r e a r e o v e r a d o z e n manufacturers of zeolites o r zeolite-containing products. A s a n i n d u s t r i a l m a t e r i a l , t h e i r m a r k e t h a s grown t o hundreds of m i l l i o n s of d o l l a r s . Since t h e mid-19601s, t h e l a t e r developing n a t u r a l z e o l i t e s have reached the s t a t u s of a n i m p o r t a n t i n d u s t r i a l m i n e r a l r e s o u r c e 82 w i t h more t h a n 300,000 t o n s mined worldwide, and used principally i n bulk applications. The P a s t . What was n e c e s s a r y f o r t h e s u c c e s s o f t h e molecular sieve z e o l i t e industry' The d e v e l o p m e n t of any new c o m m e r c i a l p r o d u c t a n d p r o c e s s i s u s u a l l y t h e r e s u l t o f c o m p l e x i n t e r a c t i o n s among many c o n t r i b u t i n g f a c t o r s . I w i l l a t t e m p t h e r e t o h i g h l i g h t some k e y f a c t o r s t h a t f a c i l i t a t e d the zeolite "explosion". The commercial u s e o f z e o l i t e s d e p e n d s on: 1) useful p r o p e r t i e s c o n t r o l l e d by t h e i r s t r u c t u r a l c h e m i s t r y ; The p i o n e e r i n g work 2 ) a v a i l a b i l i t y ; a n d 3 ) c o s t {5}. o f B a r r e r i n t h e 4 0 ' s i n o u t l i n i n g t h e l a r g e number o f molecular s i e v e s e p a r a t i o n s p o s s i b l e gave t h e impetus t o t h e i n d u s t r i a l r e s e a r c h e r s , M i l t o n a n d a s s o c i a t e s , who made i n i t i a l key d i s c o v e r i e s i n novel s y n t h e t i c z e o l i t e c o m p o s i t i o n s and a p r a c t i c a l method f o r t h e i r m a n u f a c t u r e . The s c i e n t i f i c and e n g i n e e r i n g r e s o u r c e s were t h e n committed by a l a r g e , major c h e m i c a l c o r p o r a t i o n , Union C a r b i d e C o r p o r a t i o n , w i t h a v a i l a b l e t e c h n i c a l a n d cornmercial resources. A s a r e s u l t t h e y became a v a i l a b l e t o t h e i n d u s t r i a l and s c i e n t i f i c communities i n l a t e 1954. The m a j o r e a r l y s y n t h e s i s e f f o r t s i n t h e l a t e 4 0 ' s and e a r l y 50's could n o t have been successful without t h e development of r a p i d , e f f e c t i v e c h a r a c t e r i z a t i o n t e c h n i q u e s t o e v a l u a t e t h e s y n t h e s i z e d p r o d u c t s {2}. Such t e c h n i q u e s w e r e d e v e l o p e d t o d e t e r m i n e t h e i r s t r u c t u r e , c h e m i c a l compostion, p u r i t y , and a d s o r p t i o n proA s a r e s u l t t i m e e f f i c i e n t a n a l y s i s of a v e r y perties.
l a r g e number o f s y n t h e s i s e x p e r i m e n t s was p o s s i b l e w h i c h f a c i l i t a t e d t h e d i s c o v e r y o f t w e n t y - s o m e new z e o l i t e s p e c i e s and d e l i n e a t e d t h e i r optimum s y n t h e s i s s y s t e m . The g e n e r a l s y n t h e s i s method d e v e l o p e d by M i l t o n provided a simple, cost e f f e c t i v e manufacturing process, involving r e a d i l y a v a i l a b l e , cheap raw m a t e r i a l s such a s hydrated alumina, s o l u b l e a l k a l i s i l i c a t e s , c a u s t i c , and w a t e r , and p r o c e s s c o n d i t i o n s of r e l a t i v e l y low t e m p e r a t u r e and p r e s s u r e a n d s h o r t c r y s t a l l i z a t i o n t i m e s . The u n i t o p e r a t i o n s of b a t c h c r y s t a l l i z a t i o n , f i l t r a t i o n and drying, were w e l l e s t a b l i s h e d i n t h e p r a c t i c e s manufacturing art. Thus, molecular s i e v e z e o l i t e s could be manufactured t o c o m p e t e o n a c o s t p e r f o r m a n c e b a s i s w i t h o t h e r known commercial adsorbent, c a t a l y s t , and i o n exchange m a t e r i a l s . The development of formed o r bonded z e o l i t e p r o d u c t s n e c e s s a r y f o r c o m m e r c i a l a p p l i c a t i o n i n s u p p o r t e d o r movi n g bed s y s t e m s , r e q u i r e d e x t e n s i v e development of f o r m i n g technology. T h e a s - s y n t h e s i z e d 1-5ym z e o l i t e c r y s t a l s a r e formed i n t o b e a d s , p e l l e t s , o r mesh, t y p i c a l l y by u s e of a c l a y b i n d e r . Over t h e p e r i o d o f t w e n t y - f i v e y e a r s , t h e development of an unsung and l i t t l e d i s c u s s e d forming technology has resulted i n the a b i l i t y t o control p a r t i c l e p r o p e r t i e s s u c h a s a s t r e n g t h and a t t r i t i o n r e s i s t a n c e and mass and h e a t t r a n s f e r c h a r a c t e r i s t i c s , and t o optimize t h e formed p r o d u c t s ' p r o p e r t i e s and performance.
A s t h e new m o l e c u l a r s i e v e z e o l i t e s b e c a m e k n o w n and a v a i l a b l e and s u b s e q u e n t l y u s e d , e x t e n s i v e r e s e a r c h e f f o r t i n i n d u s t r i a l and academic c i r c l e s provided a w e a l t h of s c i e n t i f i c i n f o r m a t i o n on t h e p h y s i c a l , chemical, aned s t r u c t u r a l c h a r a c t e r i s t i c s of t h i s unique c l a s s of m a t e r i a l s . The r e s u l t i n g i n - d e p t h u n d e r s t a n d i n g o f p r o p e r t i e s a l l o w e d t h e s e l e c t i o n of z e o l i t e p r o d u c t f o r a s p e c i f i c a p p l i c a t i o n , t h e i d e n t i f i c a t i o n of a p p l i c a t i o n s f o r t h e p r o d u c t , and t h e d e s i g n and e n g i n e e r i n g of t h e a p p l i c a t i o n p r o c e s s . Their application a s adsorbents required a major development i n a d s o r p t i o n p r o c e s s design and e n g i n e e r i n g zechnology. The u n i t o p e r a t i o n of a d s o r p t i o n h a s undergone a m a j o r development i n t h e l a s t t w e n t y y e a r s m a i n l y a s a r e s u l t of t h e i n t r o d u c t i o n o f m o l e c u l a r s i e v e s a s 2ommercial a d s o r b e n t s {6,2}. I t i s now a m a t u r e e n g i n e e r Lng p r a c t i c e t h a t h a s b r o u g h t a d s o r p t i o n t o t h e f o r e f r o n t a s a major t o o l of t h e chemical process industry.
I n d i s p e n s a b l e t o t h e c o m m e r c i a l s u c c e s s of m o l e c u l a r s i e v e z e o l i t e s h a s been t h e d e d i c a t i o n and c o n t r i b u t i o n s o f t h e s c i e n t i s t s a n d e n g i n e e r s who p r o v i d e d t h e k e y t o t h e i r discovery and development, and subsequently unfolded t h e i r e l e g a n t s t r u c t u r a l and chemical a r c h i t e c t u r e and novel properties. The z e a l w i t h which t h e s e m o l e c u l a r sieve "apostles" preached t h e i r gospel t o t h e s c i e n t i f i c w o r l d , t o i n d u s t r i a l t e c h n o l o g y management, and t o t h e hard-to-sell chemical p r o c e s s i n d u s t r i e s , was e s s e n t i a l t o t h e i r success. Hundreds of t h e s e a p o s t l e s and champions became committed, i n a d d i t i o n t o t h e o r i g i n a l p i o n e e r s . The g r o w t h of m o l e c u l a r s i e v e s c i e n c e and t e c h n o l o g y h a s e v o l v e d a community of o u t s t a n d i n g s c i e n t i s t s and e n g i n e e r s . whose c o n t r i b u t i o n s i n c l u d e c r e a t i v e and p r a c t i c a l s c i e n t i f i c work, and t h e s u c c e s s f u l t r a n s l a t i o n of R and D r e s u l t s t o commercial manufacture, s a l e and u t i l i t y . It i s e s t i m a t e d a t t h e o n s e t of t h e i r s e c o n d t w e n t y - f i v e y e a r s , t h a t over f i v e thousand s c i e n t i s t s and engineers d e v o t e a s u b s t a n t i a l p o r t i o n of t h e i r t e c h n i c a l e f f o r t t o molecular sieve zeolites. The F u t u r e . The f u t u r e t r e n d s i n m a t e r i a l s w i l l no d o u b t s e e t h e d e v e l o p m e n t o f new c o m m e r c i a l z e o l i t e s s e l e c t e d from newly d i s c o v e r e d c o m p o s i t i o n s and s t r u c t u r e s , chemical m o d i f i c a t i o n s of p r e s e n t commercial products t o g e n e r a t e new a n d u s e f u l p r o p e r t i e s , a n d a r e e v a l u a t i o n o f t h e h o s t o f k n o w n z e o l i t e s w h i c h n e v e r a c h i e v e d comI t seems l i k e l y t h a t w i t h t h e i n c r e a s mercial success. i n g number o f l a b o r a t o r i e s d e v o t i n g r e s o u r c e s t o t h e s e a r c h f o r new s t r u c t u r e s a n d c o m p o s i t i o n s , new c l a s s e s o f molecular sieve materials w i l l be discovered. The m o d i f i c a t i o n c h e m i s t r y of z e o l i t e s p r a c t i c e d t o d a t e , s u c h a s s t e a m i n g and c h e m i c a l e x t r a c t i o n , l e a v e s a v a s t a r e a of c h e m i c a l and s t r u c t u r a l m o d i f i c a t i o n of s o l i d s a s y e t unexplored with zeolites. A d d i t i o n a l types of n a t u r a l z e o l i t e s w i l l probably n o t achieve commercial prominence s i n c e t h e l a r g e g e o l o g i c a l e x p l o r a t i o n e f f o r t s f o r zeol i t e d e p o s i t s throughout t h e world during t h e l a s t ten t o f i f t e e n y e a r s have probably i d e n t i f i e d a l l of t h e z e o l i t e m i n e r a l s p e c i e s of commercial s i g n i f i c a n c e . The commercialization of " s t o r e d " o r "shelved" z e o l i t e s h a s l a r g e l y b e e n h a m p e r e d by t h e i r l a c k o f g e n e r a l a v a i l a b i l i t y and t h e i r apparent i n a b i l i t y t o compete performance-wise w i t h t h e c u r r e n t commercial products. With t h e worldwide expansion of s c i e n t i f i c z e o l i t e c e n t e r s w i t h t h e c a p a b i l i t y of s y n t h e s i z i n g non-commercial z e o l i t e s and d e t e r m i n i n g t h e i r p r o p e r t i e s and p o t e n t i a l a p p l i c a t i o n s , i t i s l i k e l y t h a t s e v e r a l
"old"
z e o l i t e s w i l l achieve commercial s t a t u s .
It i s l i k e l y t h a t t h e r e w i l l b e more development and change i n z e o l i t e manufacturing p r o c e s s e s during t h e next decade than during t h e l a s t twenty years, due t o t h e c o s t i n c e n t i v e s of t h e b u l k chemical and consumer m a r k e t s , and t h e a v a i l a b i l i t y of t h e n a t u r a l z e o l i t e s .
B r e c k h a s r e c e n t l y r e v i e w e d (5) a l a r g e n u m b e r o f new p o t e n t i a l a p p l i c a t i o n s a r e a s f o r z e o l i t e s . His c o m p i l a t i o n o f p r o p o s e d a p p l i c a t i o n s b a s e d on t h e r e p o r t e d l i t e r a t u r e i s reproduced i n Table 8.
TABLE 8 SOME PROPOSED APPLICATIONS O F ZEOLITES Adsorption New a d s o r b e n t s f o r s i e v i n g Hydrophobic a d s o r b e n t s Gas s t o r a g e s y s t e m s Carriers of chemicals Nuclear
Industry Applications
Environmental Weather m o d i f i c a t i o n S o l a r energy
(51
Agricultural F e r t i l i z e r s and s o i l s Animal c u l t u r e Consumer A p p l i c a t i o n s Beverage carbonation Laundry d e t e r g e n t s Flame e x t i n g u i s h e r s E l e c t r i c a l conductors Ceramics New c a t a l y s t s
The m a j o r t r e n d s i n f u t u r e commercial a p p l i c a t i o n s u i l l p r o b a b l y comprise s u b s t a n t i a l growth i n most of t h e 3 r e s e n t l y e x i s t i n g s e p a r a t i o n s and c a t a l y s i s a r e a s , and t h e d e v e l o p m e n t o f new a p p l i c a t i o n s . The emergence of z e o l i t e i o n exchange a p p l i c a r i o n s could p a r a l l e l t h a t of z e o l i t e adsorption technology over the l a s t twenty-five ::ears. The m a t u r i n g o f t h e d e v e l o p i n g i o n e x c h a n g e 3 r o c e s s d e s i g n and e n g i n e e r i n g t e c h n o l o g y , w i t h t h e z a p a b i l i t y of advanced s y s t e m s d e s i g n and e n g i n e e r i n g c o n c e p t s , should s t i m u l a t e t h e growth and a c c e p t a n c e of z e o l i t e i o n exchange s e p a r a t i o n s i n t h e chemical process F n d u s t r y a l o n g s i d e t h o s e b a s e d on a d s o r p t i o n a n d c a t a l y s i s .
The growth of b u l k c h e m i c a l and consumer a p p l i c a tions for synthetic a s well a s natural zeolites appears to be certain. I n a d d i t i o n t o t h o s e now p r e v a l e n t f o r natural zeolites, Table 8 includes t h e i r use i n agric u l t u r e , b e v e r a g e c a r b o n a t i o n , and raw m a t e r i a l s f o r ceramics
.
N a t u r a l z e o l i t e s s h o u l d c o n t i n u e t o grow a s a n important i n d u s t r i a l mineral resource used p r i n c i p a l l y i n bulk application areas. The " e n g i n e e r i n g 1 ' of t h e mined z e o l i t e s , by b e n e f i c i a t i o n t o u p g r a d e p u r i t y and c h e m i c a l m o d i f i c a t i o n t o t a i l o r p r o p e r t i e s , w i l l no doubt emerge a s t h e l e v e l of t e c h n i c a l l y i n t e n s i v e e f f o r t on n a t u r a l They s h o u l d t h e n e n j o y a l a r g e r z e o l i t e s expands {I}. s h a r e of t h e " e n g i n e e r e d 1 ' m o l e c u l a r s i e v e t y p e a p p l i c a tions. The e x t e n t of s u c h g r o w t h w i l l n o t l i k e l y b e r e l a t e d t o t h e i r lower c o s t but r a t h e r improved p r o p e r t y and performance c h a r a c t e r i s t i c s . The e x p a n s i o n w i l l c o n t i n u e t o b e a r e l a t i v e l y minor p o r t i o n of t h e t o t a l m o l e c u l a r s i e v e a p p l i c a t i o n s m a r k e t , l a r g e l y b e c a u s e of t h e i n c r e a s i n g c a p a b i l i t y i n t h e m a n u f a c t u r e and a v a i l a b i l i t y of a l a r g e number of s y n t h e t i c z e o l i t e s , and t h e a b i l i t y t o c o n t r o l p u r i t y and p r o p e r t i e s d u r i n g manufacture. The t r e n d s i n m o l e c u l a r s i e v e p r o c e s s d e s i g n s h o u l d s e e more compound, m u l t i s t e p p r o c e s s s y s t e m s u t i l i z i n g m u l t i p l e o r composite molecular s i e v e m a t e r i a l s and combined u n i t o p e r a t i o n s s u c h a s i n t e g r a t e d a d s o r p t i o n and c a t a l y t i c systems. The e n e r g y s a v i n g s p o s s i b l e i n adsorption s e p a r a t i o n s has received emphasis only It is l i k e l y t h a t t h e energy e f f i c i e n c y r e c e n t l y (831. i n molecular s i e v e a d s o r p t i o n and c a t a l y t i c processes ' w i l l b e more f u l l y e x p l o i t e d i n t h e p r o c e s s d e s i g n . Over t h e l o n g r a n g e , m o l e c u l a r s i e v e z e o l i t e t e c h n o l o g y s h o u l d c o n t i n u e t o b e s t r o n g l y i n f l u e n c e d by t h e new e m p h a s i s on c l e a r e n v i r o n m e n t , a n d e n e r g y a n d r e n e w able resource technology. Cost e f f e c t i v e and n o v e l s e p a r a t i o n and recovery processes w i l l b e r e q u i r e d t o meet p o l l u t i o n s t a n d a r d s and m a t e r i a l and energy r e Developsource limitations i n thenext several decades. ment of a d e q u a t e e n e r g y r e s o u r c e s , e s p e c i a l l y t h e p r e sently considered alternate synthetic f u e l technologies b a s e d on s y n t h e s i s g a s , o i l s h a l e , c o a l a n d g a s o h o l among o t h e r s , a l l i n v o l v e t e c h n i c a l l y d i f f i c u l t and complex s e p a r a t i o n s and c a t a l y t i c problems.
Molecular s e i v e z e o l i t e s a r e w e l l positioned hist o r i c a l l y and o f f e r a most a p p r o p r i a t e t e c h n o l o g y j e c a u s e of t h e i r u n i q u e p r o p e r t i e s which g i v e them a n e a r - i n f i n i t e f l e x i b i l i t y t o t a i l o r p r o d u c t and p r o c e s s . The r e c e n t e x t e n s i o n of t h e i r s h a p e and s u r f a c e s e l e c t i v i t y c h a r a c t e r i s t i c s w i t h t h e a d v e n t of h i g h s i l i c a z e o l i t e s a n d s i l i c a m o l e c u l a r s i e v e s o f f e r s new o p p o r t u n i t i e s i n d e s i g n p a r a m e t e r s , a s e x e m p l i f i e d by t h e m e t h a n o l t o g a s o l i n e p r o c e s s w i t h ZSM-5. The a v a i l a b i l i t y of hydrophobic molecular s i e v e adsorbents opens u p a new a p p l i c a t i o n a r e a i n r e m o v i n g a n d r e c o v e r i n g organic molecules from aqueous systems. Combined h y d r o s h o b i c and h y d r o p h i l i c a d s o r b e n t s y s t e m s would a l l o w t h e c o n c e n t r a t i o n o r removal of a n o r g a n i c m o l e c u l e from an aqueous s o l u t i o n , and e f f i c i e n t d r y i n g of t h e r e c o v e r e d organic. S i m i l a r s e p a r a t i c n s c h e m e s a r e now u n d e r i n v e s t i g a t i o n i n t h e production of gasohol from g r a i n (84). The z e o l i t e f u t u r e l o o k s b r i g h t .
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18,
206,
ZEOLITE STRUCTURES
R.M.
Barrer
Chemistry Department Imperi a1 College of Science and Technology London SW7 2AY England ABSTRACT An a c c o u n t h a s been given of s i l i c a t e a n i o n s from t o p o l o g i c a l and c o n f i g u r a t i o n a l a s p e c t s , w i t h s p e c i a l r e f e r e n c e t o z e o l i t e s . D i f f e r e n t ways of c o n s t r u c t i n g z e o l i t e a n i o n s a r e d e s c r i b e d which l e a d t o known frameworks and t o a l a r g e number o f n o v e l ones. B r i e f c o n s i d e r a t i o n i s a l s o given t o r e s u l t a n t channel systems and t o A l , S i o r d e r and d i s o r d e r .
1.
INTRODUCTION:
SILICATE ANIONS
The s t r u c t u r a l s i d e of z e o l i t e chemistry i s concerned w i t h t h e i r three-dimensional g i a n t anions and t h e a s s o c i a t e d i n t r a z e o l i t e c a t i o n s and w a t e r m o l e c u l e s . Much t h e most a c c u r a t e s t r u c t u r a l i n f o r m a t i o n r e l a t e s t o t h e a n i o n i c frameworks which Lave g e o m e t r i c a l e l e g a n c e , p r o v i d e numerous honeycomb s t r u c t u r e s and determine i m p o r t a n t z e o l i t e p r o p e r t i e s . However z e o l i t e anions a r e a p a r t only of a remarkable a r r a y of s i l i c a t e anions Ln which many s u b - u n i t s found i n z e o l i t e s a r e a l s o p r e s e n t . I t i s t h e r e f o r e of c o n s i d e r a b l e i n t e r e s t t o g i v e examples of t h e s e znions i n a b u i l d up t o t h e main t o p i c of z e o l i t e a n i o n s . Z e o l i t e s a r e u s u a l l y s y n t h e s i s e d from g e l s o r from o t h e r s i l i c a t e s . 3ecause n u c l e a t i o n can be i n f l u e n c e d by t h e s t a r t i n g m a t e r i a l s sxamples of t h e d i v e r s e s i l i c a t e anions which could be chosen a s s o u r c e s of s i l i c a a c q u i r e an a d d i t i o n a l s i g n i f i c a n c e . Two a s p e c t s of t h e s t r u c t u r e of s i l i c a t e a n i o n s a r e t h e i r z3pology and t h e i r c o n f i g u r a t i o n . By topology we w i l l mean t h e
Table 1:
S i l i c a t e s grouped a c c o r d i n g t o anion t y p e s
Type
Sub-types
F i n i t e anions ("Island" anions)
Ortho- and p y r o - s i l i c a t e s . Short unbranched c h a i n s . S i n g l e r i n g a n i o n s . Branched s i n g l e r i n g s . Double r i n g p r i s m s . S t r u c t u r e s w i t h two o r more i s l a n d a n i o n s .
I n f i n i t e c h a i n anions
Unbranched s i n g l e c h a i n s . Openbranched s i n g l e c h a i n s . Loopbranched s i n g l e c h a i n s ( l i n k e d r i n g s ) . M u l t i p l e c h a i n s (two, t h r e e , four o r f i v e cross-linked Hybrid c h a i n s . Tubular chains) chains.
.
I n f i n i t e sheet anions
Unbranched s i n g l e s h e e t s . Branched s i n g l e s h e e t s . Double s h e e t s . Hybrid t r i p l e s h e e t s .
I n f i n i t e three-dimensional anions
Non-porous and porous t e c t o silicates.
p a t t e r n of t h e Si-0-T bonds (T = A 1 o r S i ) and by c o n f i g u r a t i o n we w i l l mean t h e s p a t i a l d i s p o s i t i o n of t h e bond p a t t e r n . For example, S i 0 4 t e t r a h e d r a can j o i n w i t h o t h e r Si04 t e t r a h e d r a t o g i v e unbranched l i n e a r c h a i n s . These can, a s we s h a l l s e e , be v a r i o u s l y puckered. These a n i o n s a l l have t h e same topology i n terms of t h e bond p a t t e r n , b u t t h e y have d i f f e r e n t c o n f i g u r a t i o n s . Silicates can be grouped a c c o r d i n g t o t h e i r t o p o l o g i e s as i n Table 1. I n t h e f o l l o w i n g s e c t i o n s examples of t h e v a r i o u s c a t e g o r i e s of Table 1 w i l l be g i v e n , l e a d i n g up t o and w i t h emphasis on z e o l i t e s .
2.
FINITE ANIONS Some i s l a n d anions a r e a s f o l l o w s (1) :-
Single tetrahedron
~i02-
Orthosilicates
Two l i n k e d t e t r a h e d r a
si20?-
Disilicates
Chain o f - t h r e e t e t r a h e d r a
S~~O:;
Trisilicates
Ring of t h r e e t e t r a h e d r a ( 3 - r i n g )
si30;-
Benitoite
4-ring
si40?;
Taramellite
6-ring
si60:i-
Beryl
8-ring
sig0i;-
Muirite
9-ring
si90i?-
Eudialite
12-ring
Traski t e
Double 3 - r i n g
( t r i a n g u l a r prism)
si60?;
(Ni (En) 3) 3CSi6015126H20
Double 4 - r i n g
(cubic u n i t )
si80g;
Ekanite
Double 6-ring
(hexagonal prism)
s i l2 0 i i -
Milarite
si50:g-
Zunyite
si60i;-
Eakerite
Branched c h a i n of three tetrahedra
Branched 4-ring 4
Branched 6 - r i n g
S i l 8028- T i e n s h a n i t e
b
The l i n e diagrams g l v e t h e t o p o l o g i e s of t h e l a s t t h r e e a n i o n s . The d o t s r e p r e s e n t t h e Si atoms and t h e l i n e s show t h e l i n k i n g . The oxygen atoms a r e n o t shown: they a r e between t h e p a i r s of Si atoms t h e y l i n k . The non-linking oxygens r e q u i r e d t o complete t h e t e t r a h e d r a l groups a r e , f o r c l a r i t y , o m i t t e d . A d d i t i o n a l l y t o t h e above examples a c r y s t a l l i n e tetra-alkylammonium s i l i c a t e has been r e p o r t e d which c o n t a i n s t h e anion S ~ ~ ~ bOe l i~e v ~e d - t o, be a p e n t a g o n a l p r i s m ( 2 ) .
3.
INFINITE CHAIN ANIONS
Unbranched s i n g l e c h a i n s a r e found i n a v a r i e t y of metas i l i c a t e s . They can have t h e c o n f i g u r a t i o n s i l l u s t r a t e d i n F i g . 1 ( 3 ) , t h e c o n f i g u r a t i o n s b e i n g c h a r a c t e r i s e d by t h e p e r i o d i c i t i e s , i . e . t h e number of l i n k e d t e t r a h e d r a which must be counted a l o n g t h e c h a i n b e f o r e t h e arrangement r e p e a t s i t s e l f . The ~ e r i o d i c i t i e sof F i g . 1 a t o k and s i l i c a t e s t o which they r e f e r a r e a s follows: L e t t e r i n Fig. 1
Mineral Pyroxenes Ba2[Si2o61 Wollastonite Krauskop f i t e
Periodicity
Haradite Rhodoni t e Stokesite Pyroxyferroit e F e r r o s i l i t e I11 Alamosi t e Even-period c h a i n s t e n d t o become l e s s s t r e t c h e d w i t h h i g h e r mean e l e c t r o n e g a t i v i t y and mean v a l e n c e of t h e c h a r g e - b a l a n c i n g c a t i o n s w h i l e f o r o d d - p e r i o d c h a i n s t h e e x t e n t of c h a i n p u c k e r i n g i s s t r o n g l y c o r r e l a t e d w i t h mean e l e c t r o n e g a t i v i t y of t h e c a t i o n s b u t less so with cation radius (4). I n s t a n c e s of open- and loop-branched c h a i n s a r e shown i n F i g . 2 a t o g ( 4 ) . The examples and t h e i r p e r i o d i c i t i e s a r e : Letter i n Fig. 2
Miner a 1
Periodicity
Aeni gmati t e Astrophyllite Deeri t e V l asovit e Lemoyni t e Pellylite Nordi t e ( a ) and ( b ) a r e open-branched s i n g l e c h a i n s ; ( c ) t o ( g ) a r e loopb r a n c h e d s i n g l e c h a i n s . The l a t t e r produce r i n g s l i n k e d t o o t h e r r i n g s v i a Si-0-Si bonds. Some d o u b l e c h a i n s w i t h d i f f e r e n t p e r i o d i c i t i e s a r e shown i n F i g . 3 a t o i (5) f o r t h e m i n e r a l s named below: Letter i n Fig. 3
Mineral
Periodicity
Sillimanite 1 Amphibole 2 S y n t h e t i c Lit, 1 ( ~ i ~ e ~ ) 0 ~ 0 ] 2 Xonotli t e 3 Devi t r i t e 3 S y n t h e t i c Na2Be2HCSi601510H 3 Narsarsukite 4 Inesit e 5 Tuhualite 6 ~ x a m p l e s ( a ) ( c ) ( f ) and ( i ) a r e a l l l a d d e r a n i o n s composed o f 4 - r i n g s l i n k e d by s h a r e d e d g e s , w i t h t h e same t o p o l o g i e s b u t d i f f e r e n t c o n f i g u r a t i o n s . A s y n t h e t i c a l u m i n a t e , Na7iA13081 (6) h a s t h e same t o p o l o g y a s ( e ) ( d e v i t r i t e ) b u t w i t h A 1 r e p l a c i n g S i .
F i g . 1.
Fig. 2.
C o n f i g u r a t i o n s of some s i n g l e c h a i n unbranched anions ( 3 ) . F o r i d e n t i f i c a t i o n of s p e c i e s t o which t h e y r e f e r see text.
Some branched and loop-branched s i n g l e c h a i n a n i o n s ( 4 ) . S p e c i e s t o which t h e y r e f e r a r e i d e n t i f i e d i n t h e t e x t .
F i g . 3.
Some d o u b l e c h a i n a n i o n s w i t h d i f f e r e n t p e r i o d i c i t i e s f o r species identified i n the t e x t (9).
Fig. 4.
S e v e r a l k i n d s of u n b r a n c h e d s h e e t a n i o n f o r s p e c i e s identified i n the text (9).
As f i n a l examples of anions based on i n f i n i t e c h a i n s one may i n c l u d e m u l t i p l e , t u b e and h y b r i d c h a i n anions ( 1 , 7 ) : M u l t i p l e c h a i n anions Three c r o s s - l i n k e d
c h a i n s (8)
Example
Periodicity
Synthetic
2
Four c r o s s - l i n k e d c h a i n s , g i v i n g s t r i p of hexagons, p a t t e r n a s above, t h r e e hexagons wide (8)
S y n t h e t i c Ba5CSi80211
Five c r o s s - l i n k e d c h a i n s , g i v i n g s t r i p of hexagons, p a t t e r n a s above, f o u r hexagons wide (8)
S y n t h e t i c Ba6CSi100261
2
L i t i d i o n i t e NaKCuCSi4Ol01
3
Tube a n i o n s (7)
Hybrid c h a i n anion
Tinaksi t e ~ ~ ~ a C a ~ T i C S i ~ ~ 0 ~ ~3 1 0 H Yodels of complex c h a i n a n i o n s , such a s t u b u l a r a n i o n s , when opened o u t and f l a t t e n e d y i e l d open branched s t r i p a n i o n s ( 7 ) . The n a r s a r s u k i t e a n i o n i n t h i s way opens t o c h a i n of branched 6 - r i n g s ; t h e l i t i d i o n i t e a n i o n g i v e s a s t r i p h y b r i d a n i o n of 4- and 8 - r i n g s ; and t h e m i s e r i t e a n i o n g i v e s a s t r i p h y b r i d anion Various h y p o t h e t i c a l t u b u l a r a n i o n s and of 4-, 6- and 8 - r i n g s . t h e s t r i p a n i o n s t h e y can y i e l d when opened and f l a t t e n e d have a l s o been c o n s i d e r e d ( 7 ) . The s t r i p s can be extended t o make i n f i n i t e s h e e t s composed of v a r i o u s combinations of r i n g s . Some of t h e s e combinations a r e known t o occur i n s h e e t s i l i c a t e s , f o r example 4-, 6- and 12-rings i n manganpyrosmalite, (Mn, Fe) 8 CSi60151iOH, C1) 10. There seems no r e a s o n why complex c h a i n a n i o n s of t h e k i n d s e n v i s a g e d may n o t b e found o r s y n t h e s i s e d i n t h e r i c h f i e l d of s i l i c a t e c h e m i s t r y , and used a s s t a r t i n g materials for zeolite synthesis.
4.
INFINITE SHEET ANIONS
The n e x t s t a g e of p o l y m e r i s a t i o n i n s i l i c a t e a n i o n s y i e l d s i n f i n i t e s h e e t s . Unbranched s h e e t s of s e v e r a l k i n d s a r e shown i n F i g . 4a t o e ( 9 ) . These a r e i d e n t i f i e d below: ( a ) Hexagon b a s e d l a y e r s of t e t r a h e d r a Periodicity found i n mica, where a l l a p i c e s p o i n t i n one d i r e c t i o n .
=
2
(b) Hexagon based l a y e r s i n s e p i o l i t e ; bands w i t h a p i c e s p o i n t i n g up a l t e r n a t e w i t h bands h a v i n g a p i c e s p o i n t i n g down. ( c ) The s i n g l e l a y e r i n d a l y i t e , K2Zr[Si6ol51, (4-, 6- and 8-rings i n l a y e r ) . (d) The s i n g l e l a y e r i n a p o p h y l l i t e , KCa4[Si8020?F.8H20 (4- and 8rings i n layer). ( e ) The manganpyrosmalite, k 6[ S i 1 6 0 3 0 1 ( O H ) ~ C s~h~e e, t , c o n t a i n i n g 4-, 6and 1 2 - r i n g s . Openbranched s i n g l e l a y e r s a r e a l s o found, two such t y p e s b e i n g t h o s e i d e n t i f i a b l e i n p r e h n i t e , Ca2 (Al, Fe) C ( S i 3 , A1)0101 (OH) 2 w i t h p e r i o d i c i t y 2 , and i n z e o p h y l l i t e , Gal 3CSi5OI4l2F8(OH?f i H 2 0 , w i t h p e r i o d i c i t y 4 and a s h e e t of 1 2 - r i n g s . A loop-branched s i n g l e l a y e r of p e r i o d i c i t y 4 o c c u r s i n s y n t h e t i c NaPr[Si6ol41. Double l a y e r s h e e t a n i o n s a r e i l l u s t r a t e d i n F i g . 5 a , b and c , (10) i n which (a) (b) (c)
i s t h e unbranched double l a y e r of h e x a c e l s i a n , Ba[Si2Al2O81 of p e r i o d i c i t y 2; i s t h e loop-branched double l a y e r of d e l h a y e l i t e , Ca4 (Na3, Ca)K7CSi14A120381C12F4 of p e r i o d i c i t y 3 ; and i s t h e loop-branched double l a y e r of c a r l e t o n i t e , K2Na8Ca8[si160361( 0 3 ) 8 (OH,F) 22H20, which h a s a p e r i o d i c i t y of 6 .
Two s u b - l a y e r s of each k i n d a r e l i n k e d v i a t h e oxygen atoms marked w i t h d o t s and l y i n g on m i r r o r o r pseudo-mirror p l a n e s . The bottom p a r t s of t h e diagram show t h e double l a y e r s edge on
5.
SILICATES CONTAINING TWO ANIONS
I n some s i l i c a t e s t h e r e may be more than one k i n d of a n i o n . Examples of s i l i c a t e s of t h i s type i n c l u d e t h e f o l l o w i n g ( 1 ) : Silicate
Anions P r e s e n t
~ o i s i t e ,e p i d o t e , ganomalite, vesuvian, s e r e n d i b i t e , rus tumite
Csi0,14-
[si20716-
Kilchoanite, ardennite
[sio414-
C
S
Meliphanite
[sio414-
C
S
Traskite
C S ~ ~ O ~C I ~S - ~
~
~
O
Miserit e
[si20716-
~
~
O
Baveni t e
C S ~ ~ O [ ~s i 6 ~ 0 I1 6~~ *- -
Eudi a l i t e
~ s i ~ 0 ~[sig 1 ~( O - ,OH)~~I~~*-
Ches t e r i t e
CSi401
Reyerite
~
High p r e s s u r e "garnet1'
[sio4i4-
synthetic S i 5 ( ~ 0 4 ) 6 0
C S ~ O , I ~ - csio618-
High p r e s s u r e K2Si409
[si30916-
~
C
S
~
~
~
O
~
~
~
O
~
~
~
i
~ ~ 0 ~ ~ i ~ ~l
~~
-~
C S ~ O ~ I ~ -
[sio618-
SiO,+ t e t r a h e d r a shar6ng: S i d i s t a n c e (A) Si
corners 3.24
edges 1.87
faces 1.08
SiOb o c t a h e d r a s h a r i n g : Si Si distance
corners 3.56
edges 2.52
faces 2.06
. ..
(2)
~
~
-
~
~
-
~
~
~
~
~
~
~
~
~
0
~
[si601618-
I t i s of i n t e r e s t t h a t under h i g h p r e s s u r e c o n d i t i o n s S i octahedr a l l y co-ordinated w i t h 0 a p p e a r s . I n s t i s h o v i t e , a high pressure form of c r y s t a l l i n e s i l i c a , o c t a h e d r a S i 0 6 appear t o be l i n k e d by edge s h a r i n g a s much a s by c o r n e r s h a r i n g . No example of f a c e s h a r i n g of S i 0 6 o c t a h e d r a i s known. Also, t e t r a h e d r a S i 0 4 ( o r A104) l i n k only by c o r n e r s h a r i n g . The d i s t a n c e s between c e n t r a l S i atoms a r e :
...
~
Edge and f a c e s h a r i n g of S i 0 4 t e t r a h e d r a and f a c e s h a r i n g of o c t a h e d r a draw si4+ c e n t r a l i o n s t o o c l o s e f o r s t a b i l i t y r e l a t i v e t o c o r n e r s h a r i n g f o r S i 0 4 p a i r s and edge and c o m e r s h a r i n g f o r Si06 p a i r s .
1
Fig. 5.
Examples o f unbranched d o u b l e l a y e r a n i o n s i d e n t i f i e d i n the t e x t (10).
PRESSURE, '.01
Fig. 6 .
psi
S o r p t i o n of He i n a- and i n B 1 - t r i d y m i t e
(12)
Among t h e i n f i n i t e s i n g l e c h a i n anions ( F i g . 1) t h e p e r i o d i c i t y a p p e a r s t o be determined o r i n f l u e n c e d by t h e a s s o c i a t e d c a t i o n s . Even-period c h a i n s a r e l e s s s t r e t c h e d t h e h i g h e r t h e mean e l e c t r o n e g a t i v i t y and mean v a l e n c e of t h e c a t i o n s . For oddp e r i o d c h a i n s mean e l e c t r o n e g a t i v i t y more t h a n mean r a d i u s of t h e cations influences the chain puckering (11).
6.
TECTOSILICATES
Three-dimensional frameworks ( c r y s t a l l i n e s i l i c a s , f e l s p a t h o i d s , f e l s p a r s and z e o l i t e s ) have O/(Al + S i ) = 2 , i n d i c a t i n g t h e p r e s e n c e only of t e t r a h e d r a TO4 (T = A 1 o r S i ) where each t e t r a h e d r o n s h a r e s i t s f o u r oxygens w i t h f o u r o t h e r t e t r a h e d r a . I n c l a y m i n e r a l s on t h e o t h e r hand o c t a h e d r a l l a y e r s of A106 o r MgO6 a r e p r e s e n t a t t a c h e d e i t h e r t o one t e t r a h e d r a l l a y e r of 6-rings ( k a n d i t e s ) o r t o two such l a y e r s one one each s i d e of t h e l a y e r of o c t a h e d r a ( s m e c t i t e s , v e r m i c u l i t e s and m i c a s ) . The r e s u l t a n t m u l t i p l e s h e e t s may be uncharged a s w i t h t h e k a n d i t e s o r they may be a n i o n i c a s w i t h most of t h e t h r e e - l a y e r s h e e t s t r u c t u r e s . I n t h e l a t t e r case t h e c h a r g e - n e u t r a l i s i n g c a t i o n s a r e l o c a t e d between s u c c e s s i v e s h e e t s . I n t e c t o s i l i c a t e s t h e r e i s one e q u i v a l e n t of c a t i o n s p e r g. atom of t h e A 1 r e p l a c i n g S i , and t h e i o n s a r e d i s t r i b u t e d i n t h r e e dimensions i n t h e i n t e r s t i c e s of t h e network. The t e c t o s i l i c a t e s may be sub-divided i n t o t h o s e which a r e n o t and t h o s e which a r e porous (Table 1 ) . The f e l s p a r s , t h e d e n s e r c r y s t a l l i n e s i l i c a s such a s q u a r t z and v a r i o u s f e l s p a t h o i d s ( e g s . n e p h e l i n e , k a l i o p h i l i t e , k a l s i l i t e and e u c r y p t i t e ) a r e nonporous; i n them t h e i n t e r s t i c e s a r e n o t l a r g e enough t o c o n t a i n even t h e s m a l l e s t g u e s t m o l e c u l e s . Tridymite and c r i s t o b a l i t e can however take up c o n s i d e r a b l e amounts of He and Ne, a s shown f o r t r i d y m i t e i n F i g . 6 (12) and s o can j u s t be c o n s i d e r e d as porous tectosilicates. Other porous c r y s t a l l i n e s i l i c a s a r e s i l i c a l i t e s 1 and 2(4) dodecasil-3C (15) and melanophlogite ( s e e 5 . 7 ) . Some f e l s p a t h o i d s such a s s o d a l i t e and c a n c r i n i t e ( ~ i g .7 ) a l s o have frameworks w i t h c a v i t i e s o r channels l a r g e enough t o c o n t a i n v a r i o u s s a l t s and/or z e o l i t i c w a t e r . The most i m p o r t a n t porous t e c t o s i l i c a t e s a r e found among t h e l a r g e and growing number of z e o l i t e s , each type h a v i n g i t s i n d i v i d u a l c o n f i g u r a t i o n of windows, c a v i t i e s and c h a n n e l s of m o l e c u l a r dimensions. The pore volumes a c c e s s i b l e t o w a t e r , and o f t e n t o numerous o t h e r g u e s t m o l e c u l e s , range from $0.18 cm3 p e r cm3 of c r y s t a l f o r t h e l e a s t porous (analcime) t o %0.50 cm3 p e r cm3 of c r y s t a l f o r t h e most porous (such a s f a u j a s i t e , c h a b a z i t e and z e o l i t e s A , H-RHO, ZSM-2 and ZSM-3. The open s t r u c t u r e s allow ready m i g r a t i o n of w a t e r molecules and of i o n s i n c a t i o n exchange. The exchange c a p a c i t y of some z e o l i t e s i s given f o r t h e i d e a l i s e d compositions i n Table 2. This c a p a c i t y can of
Table 2:
Exchange c a p a c i t i e s i n meq/lOOg of some z e o l i t e s
Zeolite
I d e a l i s e d composition
Natrolite
Na2 LA1 S t 3 O1 12H20
Analcime
NaCA1Si2O61H20
Levyni t e
CaCA12Si401216H20
Chabazi t e
( ( i ) C a , ~ aC ) A l ~ i ~ 0 ~ 1 3 ~ ~ 0
Gmelini t e
( ( 1 ) C a , ~ a [)A l S i 2 0 6 1 3 ~ 2 0
Edingtoni t e
BaCA12Si301014H20
Fauj a s i t e
(Ca,Na2) CAl2Si5O1t+16.6H20
Harmo tome
(K2,Ba) CAl2Si5Olt+I5H2O
Heulandi t e
Ca[A12Si601615H20
Stilbite
(Na,
Mordeni t e
( ( $ ) C a , ~ a~) A l S i ~ O ~3H20 ~l3.
Exchange c a p a c i t y
( i ) Ca) CAlSi3O8I3H2O
c o u r s e v a r y w i t h t h e S i / A l r a t i o which can u s u a l l y be a l t e r e d according t o s y n t h e s i s conditions. Z e o l i t e s l i k e ZSM-5 o r -11 w i t h S i / A 1 r a t i o s v a r y i n g between a 2 5 and 1000 c a r r y v e r y much l e s s n e g a t i v e charge p e r lOOg and a r e t h u s l e s s p o l a r t h a n t h e most aluminous z e o l i t e s of Table 2 .
7.
ZEOLITE GROUPS
Attempts have been made t o group t o g e t h e r z e o l i t e s which have s t r u c t u r a l elements i n common. For numerous s y n t h e t i c z e o l i t e s t h e s t r u c t u r e s a r e s t i l l unknown, b u t examples of grouping a r e g i v e n i n Table 3. This t a b l e c o n t a i n s i n s t a n c e s of framework anions which a r e v a r i a n t s of a s i n g l e topology, i . e . i s o t y p e s ( e g s . a l l members of t h e analcime group; h e u l a n d i t e and c l i n o p t i l o l i t e i n t h e h e u l a n d i t e group; s t i l b i t e , s t e l l e r i t e and b a r r e r i t e , a l s o i n t h e h e u l a n d i t e group; p h i l l i p s i t e and harmotome i n t h e p h i l l i p s i t e group; gismondine and Na-P a l s o i n t h e p h i l l i p s i t e group; and n a t r o l i t e s c o l e c i t e and m e s o l i t e i n t h e n a t r o l i t e g r o u p ) . V a r i a n t s o f a given topology a r i s e a s a r e s u l t of d i f f e r i n g chemical compos&tions. For examples analcime h a s a c u b i c u n i t c e l l of edge + 13.72 A and u n i t c e l l c o n t e n t Na16CA116Si32096116H20. The ~ a and
T a b l e 3:
1.
C l a s s i f i c a t i o n of some z e o l i t e s and p o r o u s t e c t o s i l i c a t e s
A n a l c i n e Group Analcim Wairaki t e Leucite (felspathoid) Rb-analcime ( " ) Polluci t e Viseite (aluminosilicophosphate) Kehoei t e ( a l u m i n o p h o s p h a t e )
2.
F a u j a s i t e ( z e o l i t e s X and Y) Z e o l i t e ZSM-2 Z e o l i t e ZSM-3 Paulingi t e Zeolite A Z e o l i t e RHO Zeolite 5 5
5.
7.
H e u l a n d i t e Group Heulandi t e Clinoptilolit e Brews t e r i t e
M o r d e n i t e Group Mordeni t e Ferrierite Dachiardit e Epis t i l b i t e Biki t a i t e
8.
N a t r o l i t e Group Natrolite Scolecite Mesoli t e Thomsonite Gonnardi t e Edingtonit e Metanatrolite
Clathrasil-3C
F a u j a s i t e Group
Laumonti t e Group Laumon t i t e ~ u g a w a r a ltie
C l a t h r a t e Group Melanophlogite Z e o l i t e ZSM-39,
4.
6.
C h a b a z i t e Group Chabazite Gmelinite Erioni t e Offretite Levyni t e M a z z i t e ( z e o l i t e a) Zeolite L Sodalite hydrate Cancrinite hydrate
3.
S t i l b it e Stellerite Barrerite
9.
P e n t a s i l Group Z e o l i t e ZSM-5, S i l i c a l i t e I Z e o l i t e ZSM-11, ~ i l i c a l i t e I 1
1 0 . P h i l l i p s i t e Group Phillipsite Harmo tome G i smondine Z e o l i t e Na-P Amici t e Garroni t e Mer l i n o i t e Z e o l i t e Li-ABW -
Fig. 7.
Frameworks of ( i ) s o d a l i t e ( 1 . h . s . ) and ( i t ) c a n c r i n i t e ( r . . I n (i) 14-hedral cages a r e i d e n t i f i a b l e . ( i i ) shows ( a ) 11-hedra t y p i c a l of c a n c r i n i t e and (b) a s e c t i o n normal t o t h e c - d i r e c t i o n , i n d i c a t i n g a wide c h a n n e l c i r c u m s c r i b e d by 1 2 - r i n g s . A1 o r S i a r e c e n t r e d a t e a c h c o r n e r and 0 atoms a r e c e n t r e d n e a r b u t n o t on t h e mid-point of e a c h edge o f F i g . 7 and a n a l o g o u s s u b s e q u e n t framework r e p r e s e n t a t i o n s . The s c a l e s of t h e two d r a w i n g s a r e n o t t h e same.
. .
Radius
Fig
8. W a t e r c o n t e n t s of d i f f e r e n t c a t i o n i c forms of p h i l l i p s i t e plotted against cation r a d i i (16).
Exchange of Na+ by K+ g i v e s t h e H20 occupy d i f f e r e n t s u b - l a t t i c e s . l e u c i t e wihh a t e t r a g o n a l u n i t c e l l h a v i n g a = 1 2 . 9 8 and c = 13.68 A . The c r y s t a l s a r e anhydrous and t h e IZi i o n s a r e i n t h e s u b - l a t t i c e occupied by H20 i n analcime. The topology of t h e analcime anion i s however unchanged. Another way i n which v a r i a n t s of a g i v e n topology may a r i s e i s through changes i n S i / A l r a t i o s i n t h e framework. This changes t h e c a t i o n d e n s i t y , which can a l s o be a l t e r e d by exchanges such a s 2 ~ a ' 2 c a 2 + . These two f a c t o r s o p e r a t e f o r t h e i s o t y p e s h e u l a n d i t e Ca4EAlgSi28872124H20, (momclinic, a = 17.72 b = 17.90 c = 7.43 A , 6 = 116O25') and c l i n o p ~ i l o l i t e ,Na66A16Si30072124H20, (monoclinic, a = 17.64 A , b = 17.90 A , c = 7.40 A , f3 = 1 1 6 0 2 2 ' ) . When one c a t i o n i s r e p l a c e d by a n o t h e r of d i f f e r e n t s i z e o r charge t h e d i s t r i b u t i o n s of t h e i o n s between s u b - l a t t i c e s may change a s w e l l a s t h e w a t e r c o n t e n t under ambient c o n d i t i o n s . Such changes can i n t u r n cause minor o r s i g n i f i c a n t framework d i s t o r t i o n s w i t h o u t a l t e r i n g t h e topology. An example of t h e v a r i a t i o n of w a t e r c o n t e n t w i t h exchange c a t i o n i s shown i n F i g . 8 f o r p h i l l i p s i t e ( 1 6 ) . F o r i o n s of t h e same charge w a t e r c o n t e n t s d e c r e a s e as i o n r a d i u s i n c r e a s e s . A d i f f e r e n t curve i s o b t a i n e d f o r a s e r i e s of d i v a l e n t i o n s than f o r a s e r i e s of u n i v a l e n t ones.
2
i,
2,
Guest s p e c i e s o t h e r than w a t e r can a l s o r e s u l t i n v a r i a n t s of a g i v e n topology. S o d a l i t e ( N a g [A16Si602412NaCl; c u b i c w i t h a = 8.87 2 ) and nosean (Na6[A16Si60241Na2S04;c u b i c w i t h a = 9 . 0 8 ) b o t h have t h e s o d a l i t e framework, s l i g h t l y d i l a t e d i n t h e c a s e of nosean. I n g e n e r a l , t e c t o s i l i c a t e frameworks although s t r o n g and r i g i d can undergo s m a l l a d j u s t m e n t s of c r y s t a l l o g r a p h i c s i g n i f i c a n c e The m a j o r i t y of t h e frameworks i n most z e o l i t e groups a r e n o t v a r i a n t s of a given topology b u t p o s s e s s d i f f e r e n t t o p o l o g i e s , w i t h s t r u c t u r a l s i m i l a r i t i e s which j u s t i f y t h e i r b e i n g grouped t o g e t h e r . Such c l a s s i f i c a t i o n s a r e n o t w i t h o u t a l t e r n a t i v e s because on a b a s i s of t h e p o s s e s s i o n of s t r u c t u r a l s u b - u n i t s i n common c e r t a i n z e o l i t e s could e q u a l l y w e l l appear i n more t h a n one group. Thus i n s o d a l i t e p l a c e d i n t h e c h a b a z i t e group 14-hedral c a v i t i e s e x i s t which a r e a l s o found i n some members of t h e f a u j a s i t e group However s o d a l i t e and c a n c r i n i t e a r e ( f a u j a s i t e and z e o l i t e A) s t r u c t u r a l l y r e l a t e d and c a n c r i n i t e i s w e l l p l a c e d i n t h e c h a b a z i t e group and s o i s s o d a l i t e . The c l a s s i f i c a t i o n of z e o l i t e s i n t o groups i s t h u s somewhat a r b i t r a r y .
.
Z e o l i t e s can a l s o be c o n s i d e r e d i n terms of t h e d e n s i t i e s of Certain bonds Si-0-T ( T = A 1 o r S i ) i n t h e x , y and z d i r e c t i o n s . network a n i o n s have bond d e n s i t i e s comparable i n a l l d i r e c t i o n s ( e . g . groups 1, 2, 3, 4 and 9 of Table 3) ; o t h e r s have bond densi t i e s g r e a t e r i n two d i r e c t i o n s t h a n i n t h e t h i r d (group 5 ) ; and o t h e r s have t h e s e d e n s i t i e s g r e a t e r i n one d i r e c t i o n t h a n i n t h e Bond remaining two ( " f i b r o u s " z e o l i t e s of t h e n a t r o l i t e group) d e n s i t y d i f f e r e n c e s can i n f l u e n c e thermal s t a b i l i t y and r i g i d i t y
.
.
Fig. 9 .
Fig.
10.
Dodecahedra ( b ) , t e t r a d e c a h e d r a (a) and h e x a d e c a h e d r a ( c ) found i n m e l a n o p h l o g i t e ( ( a ) + ( b ) ) and i n dodecasil-3C o r z e o l i t e ZSM-39 ( ( b ) + ( c ) ) ( 1 8 ) .
S t a c k i n g of p e n t a g o n a l dodecahedra i n (a) m e l a n o p h l o g i t e and ( b ) d o d e c a s i l - 3 C o r z e o l i t e ZSM-39 ( 1 8 ) .
and t h e e x t e n t of deformation when undergoing m o d i f i c a t i o n by i o n exchange o r i n changes of t h e Si/A1 r a t i o s and i n d e h y d r a t i o n and rehydration. I n t h e f i b r o u s z e o l i t e s t h e dense-bond c h a i n s a r e c r o s s - l i n k e d t o l i k e c h a i n s by s i n g l e Si-0-T bonds and t h i s appears On t h e o t h e r hand i n t o make t h e frameworks r e a d i l y deformable. z e o l i t e A 14-hedral s o d a l i t e cages a r e each l i n k e d through t h e s i x A-ring f a c e s t o s i x o t h e r s o d a l i t e cages by f o u r Si-0-T bonds c r e a t i n g a c u b i c l i n k a g e u n i t . Cross-connecting s o d a l i t e cage subm i t s by m u l t i p l e Si-0-T bonds produces a more r i g i d l e s s e a s i l y deformable framework t h a n when, a s i n t h e n a t r o l i t e group, c r o s s l i n k i n g i n v o l v e s s i n g l e Si-0-T bonds. S i m i l a r l y i n f a u j a s i t e each s o d a l i t e cage i s l i n k e d t o f o u r o t h e r such cages through f o u r of i t s e i g h t 6 - r i n g f a c e s by s i x Si-0-T bonds, c r e a t i n g a hexagonal ? r i s m l i n k i n g u n i t . R i g i d i t y and thermal s t a b i l i t y i s found f o r such z e o l i t e s a s A and f a u j a s i t e (X and Y) d e s p i t e t h e s m a l l e r S e n s i t y , g r e a t e r i n t r a c r y s t a l l i n e p o r o s i t y , and s m a l l e r number of ( A 1 + S i ) atoms (and hence of Si-0-T bonds) p e r u n i t volume. I n z e o l i t e A, f a u j a s i t e , n a t r o l i t e and thomsoni t e t h e s e numbers a r e as f o l l o w s : Zeolite (A1 + S i ) , p e r 1000
natroli te 14.5
thomsoni t e 14.4
zeolite A 12.9
faujasit e 12.7
I n c l u d e d i n T a b l e 3 a r e examples of porous c r y s t a l l i n e s i l i c a s . I h e s e a r e m e l a n o p h l o g i t e , r a r e i n Nature and only r e c e n t l y synr h e s i s e d ( 1 7 ) , dodecasil-3C (15) and s i l i c a l i t e s I (14) and I1 (15) !lelanophlogite h a s t h e s t r u c t u r e of c l a t h r a t e h y d r a t e t y p e I , i n ;;hi ch t h e p e n t a g o n a l dodecahedra and t e t r a d e c a h e d r a w i t h twelve 3 - r i n g and two 6 - r i n g f a c e s of F i g . 9 (18) b and a r e s p e c t i v e l y (1) 2re s t a c k e d t o a s t o f i l l a l l s p a c e . There areotwo dodecahedra and s i x t e t r a d e c a h e d r a p e r c u b i c u n i t c e l l of 13.4 A edge ( F i g . 10a) 8 ) I n c l a t h r a s i l - 3 C as i n i t s i s o t y p e ZSM-39 (19) p e n t a g o n a l iodecahedra a r e s t a c k e d w i t h hexadecahedra having twelve 5-ring and ?our 6 - r i n g f a c e s t o f i l l a l l space ( F i g . l o b ) . There a r e 6 dodecaIf nedra and 8 hexadecahedra p e r c u b i c u n i t c e l l of edge 19.4 t h e r e was one g u e s t molecule, G, p e r void t h e l i m i t i n g composition 2f m e l a n ~ ~ h l o g i twould e be 23Si02.2G and t h a t of c l a t h r a s i l - 3 C would 3e 13Si02.2G. I f only t h e l a r g e r v o i d s c o n t a i n e d a g u e s t molecule the l i m i t i n g compositions would be 23Si02. 3G and l7SiO2 .G r e s p e c t i v e l y . Because of t h e i r analogy w i t h c l a t h r a t e h y d r a t e s they have been termed t h e c l a t h r a t e group i n Table 3. S i n c e a range of c l a t h r a t e h y d r a t e s i s known s y n t h e s i s may r e v e a l more ?orous s i l i c a s o r z e o l i t e s i n t h i s group s t r u c t u r a l l y l i k e t h e i r : l a t h r a t e h y d r a t e c o u n t e r p a r t s . A l l a r e r i c h i n 5 - r i n g s and indeed t5e c l a t h r a t e , mordenite, h e u l a n d i t e and p e n t a s i l z e o l i t e groups zre a l l related i n t h i s respect.
.
a.
The p e n t a s i l z e o l i t e s ZSM-5 and ZSM-11 and t h e i r r e s p e c t i v e 2nd members s i l i c a l i t e s I a n d I I d i f f e r from m e l a n o p h l o g i t e and
F i g . 11.
Fig.
12.
Frameworks of ZSM-5
(1.h.s.)
and ZSM-11
(r.h.s.).
Formal r e p r e s e n t a t i o n o f c h a n n e l p a t t e r n s i n ( a ) ZSM-11, (c) ZSFI-5 and (b) an i n t ~ l m e d i a t e s t r u c t u r e (20)
.
c l a t h r a s i 1-3C i n h a v i n g r e l a t i v e l y open c o n t i n u o u s channe 1s r a t h e r t h a n s e m i - i s o l a t e d v o i d s p e r m e a t i n g t h e i r s t r u c t u r e s . A view of t h e i r frameworks i s shown i n F i g . l l a and b , and a f o r m a l r e p r e s e n t a t i o n of t h e t h r e e - d i m e n s i o n a l c h a n n e l p a t t e r n s i n F i g . 1 2 c and a t o g e t h e r w i t h t h i s p a t t e r n f o r an i n t e r m e d i a t e s t r u c t u r e , o f which many a r e p o s s i b l e ( F i g . These c h a n n e l s h a v e minimum f r e e d i m e n s i o n s from 5 . 1 12b) t o 5.6 (20B , by c o n t r a s t w i t h t h e l a r g e s t windows i n m e l a n o p h l o g i t e o r d o d e c a s i l - 3 C o f %2.6 2 a t m o s t .
.
T a b l e 3 , w i t h t h e e x c e p t i o n o f kehoeite, r e f e r s o n l y t o s i l i c a and a l u m i n o s i l i c a t e s . A1P04 can c r y s t a l l i s e i n f o m s i s o s t r u c t u r a l w i t h q u a r t z , t r i d y m i t e and c r i s t o b a l i t e and i n a d d i t i o n , u s i n g o r g a n i c b a s e s as t e m p l a t e s , a number o f o t h e r A1P04 s p e c i e s have b e e n made which i n c l u d e a s t r u c t u r a l a n a l o g u e of s o d a l i t e and of e r i o n i t e / o f f r e t i t e ( 2 1 ) . Analogues o f z e o l i t e s a r e known i n which Ga r e p l a c e s Al and Ge r e p l a c e s S i ( 2 2 ) . T a b l e 3 i s n o t i n t e n d e d t o i n c l u d e such m a t e r i a l s .
8.
CONSTRUCTING 3-D FRAMEWORKS
I n what f o l l o w s s e v e r a l ways of c o n s t r u c t i n g z e o l i t e frameworks w i l l b e c o n s i d e r e d . These a r e most i n f o r m a t i v e i n t h a t t h e y show n o t o n l y how known s t r u c t u r e s emerge b u t a l s o many n o v e l ones which p o i n t t h e way f o r f u r t h e r s y n t h e s i s . T h i s a p p r o a c h i s Dreferred t o describing individual crystallogranhic structures. Among t h e ways of v i s u a l i s i n g and c o n s t r u c t i n g z e o l i t e a n i o n s a r e the following: 1. S t a c k i n g of p o l y h e d r a 2. Use of t h e sigma t r a n s f o r m a t i o n 3. Use o f o p e r a t o r s 4 . C r o s s - l i n k i n g c h a i n s of v a r i e d c o m p l e x i t y 5 . C r o s s - l i n k i n g v a r i o u s k i n d s of l a y e r 8 . 1 . Frameworks a s A s s e m b l i e s of P o l y h e d r a l Voids T h i s method of b u i l d i n g p o r o u s frameworks was i l l u s t r a t e d i n Polyhedral voids are v e r y common i n z e o l i t e s t r u c t u r e s , a s i l l u s t r a t e d i n T a b l e 4 ( 2 3 ) , and many frameworks can be b u i l t by s t a c k i n g p o l y h e d r a of one o r n o r e d i f f e r e n t s h a p e s and s i z e s s o as t o occupy a l l s p a c e a v a i l a b l e o r t o c r e a t e open c h a n n e l s s u r r o u n d e d by l i n k e d p o l y h e d r a . The s i m p l e s t example i n v o l v e s t h e s o d a l i t e c a g e ( F i g . 7 ) , i . e . t h e l i - h e d r o n of t y p e I . T h i s i s one of F e d e r o v ' s s p a c e - f i l l i n g ? o l y h e d r a w h i c h , s t a c k e d by f a c e s h a r i n g w i l l f i l l a l l s p a c e w i t h l r s f e l l o w s , i n t h e same o r i e n t a t i o n . The r e s u l t i s t h e s o d a l i t e s r r u c t u r e . The forms o f some of t h e p o l y h e d r a l i s t e d i n T a b l e 4 - r e shown i n F i g . 1 3 (24)
5 . 7 . f o r m e l a n o p h l o g i t e and d o d e c a s i l - 3 C .
.
TABLE 4 Some polyhedra in zeolited23)
Polyhedron
Faces
Vertices"
Approximate free dimension
6-hedron (cube) 8-hedron (hexagonal prism)
2.3 in plane of 6-rings
10-hedron (octagonal prism) 1I-hedron
4.5 in plane of 8-rings 4 . 7 along c axis 3 . 5 normal to c
14-hedron Type I (truncated octahedron) 14-hedron Type 11
6 . 6 for inscribed sphere
17-hedron Type I 17-hedron Type I1 18-hedron (oblate spheroidal form)
26-hedron Type I (truncated cubo-octahedron) 26-hedron Type I1
"If
11
Examples
(-4
6 . 0 along c 7 . 4 normal to c 9 . 0 along c 7 to 7 . 3 normal to C 7.7 along c 6 . 4 normal to c 1 0 . 8 ~ 6 . (6.6 6 is measured between centre planes of opposite 8-rings) I I along c 6 . 5 nornlal to c
zeolite A faujasite, zeolite ZK-5, chabazite, erionite, offretite, levynite paulingite, zeolite
RHO cancrinite, zeolite L, erionite, offretite, zeolite losod sodalite, faujasite, zeolite A gmelinite, offretite, mauite (zeolite Q) levynite zeolite losod paulingite, zeolite ZK-5
chabazite
15 along c 6 . 3 normal to c
erionite
11.4 for inscribed sphere
paulingite, zeolite ZK-5, zeolite A, zeolite RHO faujasite (zeolites X and Y)
11.8 for inscribed sphere
denotes the number of faces, 2 n - 4 gives the n~~rnber of vertices.
Next i n s i m p l i c i t y t o s o d a l i t e a r e frameworks made by s t a c k i n g two k i n d s of polyhedron o n l y , a s e x e m p l i f i e d by m e l a n o p h l o g i t e and dodecasil-3C i n 5 . 7 . Then one may c o n s i d e r s t r u c t u r e s composed of a s s e m b l i e s of t h r e e k i n d s of polyhedron such a s z e o l i t e A ( c u b i c u n i t s , s o d a l i t e cages and 26-hedra of t y p e I) and f a u j a s i t e (hexagonal p r i s m s , s o d a l i t e cages and 26-hedra of t y p e 11). Next come s t r u c t u r e s made by a s s e m b l i n g p o l y h e d r a , of t e n i n columns, which do n o t f i l l a l l s p a c e b u t which l e a v e c o n t i n u o u s c h a n n e l s i n t h e framework. Examples of t h e s e ways of b u i l d i n g z e o l i t e s a r e g i v e n i n T a b l e 5 ( 2 5 ) . Wide p a r a l l e l c h a n n e l s c i r c u m s c r i b e d by 12-rings a r i s e i n u n f a u l t e d , i . e . c r y s t a l l o g r a p h i c a l l y i d e a l , c a n c r i n i t e h y d r a t e , g m e l i n i t e , o f f r e ti t e and mazzi t e Where such c h a n n e l s do n o t o c c u r windows of 8 - r i n g s ( c h a b a z i t e , e r i o n i t e , z e o l i t e A, z e o l i t e ZK-5), of o c t a g o n a l p r i s m s ( z e o l i t e RHO) o r of 12-rings ( f a u j a s i t e ) allow molecules t o migrate, i n t h e s e i n s t a n c e s i n a 1 1 t h r e e dimensions t h r o u g h o u t t h e frameworks
.
.
8 . 2 . The Sigma T r a n s f o r m a t i o n
'
The sigma t r a n s f o r m a t i o n i s a p u r e l y c o n c e p t u a l d e v i c e f o r i n t e r - r e l a t i n g and b u i l d i n g known and hypothe t i c a l z e o l i t e frameworks ( 2 6 ) . A t e t r a h e d r a l l y connected s t r u c t u r e i s expanded by i m a g i n a r y f i s s i o n of T atoms (T = A 1 o r S i ) l y i n g on s p e c i f i e d p l a n e s r u n n i n g through t h e s t r u c t u r e , and c r e a t i n g new oxygen b r i d g e s c o n n e c t i n g p a i r s r e s u l t i n g from t h e f i s s i o n . As a n example F i g . 14, i n t h e t o p h a l f , shows how a s i n g l e t e t r a h e d r o n becomes a p a i r , a 4 - r i n g , a 6 - r i n g and an 8 - r i n g ; and how t h e s e t h r e e r i n g s become r e s p e c t i v e l y cube, hexagonal p r i s m and o c t a g o n a l p r i s m . The bottom h a l f of t h e f i g u r e shows t h e s t a g e s of t r a n s f o r m a t i o n of a s o d a l i t e cage i n t o t h e 26-hedron of t y p e I found i n z e o l i t e A , The b a s i c r e q u i r e m e n t i s t h a t e v e r y T atom l y i n g i n t h e t r a n s f o r m a t i o n p l a n e must have two of i t s l i n k a g e s l y i n g i n t h e p l a n e and t h e o t h e r two emerging from o p p o s i t e s i d e s of t h e p l a n e . An i n v e r s e o f t h e sigma t r a n s f o r m a t i o n may o c c u r w i t h a p l a n e c o n t a i n i n g no T atoms i f e v e r y oxygen b r i d g e c u t by a p l a n e i s a common edge of two 4 - r i n g s . T h i s d e v i c e may n o t i o n a l l y r e d u c e double r i n g s ( p r i s m s ) t o s i n g l e r i n g s o r l a d d e r s t o s i n g l e chains. The t r a n s f o r m a t i o n of s o d a l i t e i n t o z e o l i t e A ( c . f . t h e b o t t o m h a l f of F i g . 14) t a k e s p l a c e v i a two h y p o t h e t i c a l i n t e r m e d i a t e z e o l i t e s . S t a r t i n g w i t h s o d a l i t e numerous o t h e r t r a n s f o r m a t i o n s were e f f e c t e d , t o z e o l i t e RHO w i t h two i n t e r m e d i a t e s t r u c t u r e s , f a u j a s i t e w i t h t h r e e i n t e r m e d i a t e s and c h a b a z i t e w i t h two i n t e r m e d i a t e s . T r a n s f o r m a t i o n s of c a n c r i n i t e y i e l d e d o f f r e t i t e and g m e l i n i t e ; t r i d y m i t e gave p a r a c e l s i a n and p h i l l i p s i t e ; and c r i s t o b a l i t e gave z e o l i t e ABW and gismondine a s w e l l a s two unknown s t r u c t u r e s (26).
Sodal~teunit
FIG.14 Examples of the "sigma transformation". The top half of the figure illustrates the transformation of a single tetrahedron to yield 4-, 6- and 8-rings and 4-4, 6-6- and 8-8- prisms. The bottom half shows stages in transforming a sodalite cage (14-hedron of Type I) to the 26-hedral cage of zeolite ~(16).
-.
.-ig.
15.
Two c o n f i g u r a t i o n s of l a d d e r s of 4 - r i n g s three-fold operator axis (27)
.
relative to a
Fig. 16.
I n ( a ) two connected NFNF l a d d e r s do n o t f a c e e a c h o t h e r d i r e c t l y and g e n e r a t e t h e p r o j e c t i o n ( b ) . I n ( c ) t h e s e connected l a d d e r s f a c e each o t h e r d i r e c t l y and g e n e r a t e t h e p r o j e c t i o n (d) ( 2 7 ) .
8.3.
Use of O p e r a t o r s
I n 5 . 3 . t h e e x i s t e n c e of l a d d e r - l i k e c h a i n s was n o t e d . They c o n s i s t e d of 4-rings each 4 - r i n g l i n k e d t o two o t h e r s by s h a r i n g o p p o s i t e e d g e s . Such l a d d e r s v a r i o u s l y buckled s e r v e t o i l l u s t r a t e t h e u s e of o p e r a t o r s i n producing z e o l i t e frameworks ( 2 7 ) . F i g . 15 shows two c o n f i g u r a t i o n s of 4-ring l a d d e r s r e l a t i v e t o a t h r e e - f o l d o p e r a t o r a x i s . The t e r m i n a l unshared oxygen atoms i n t h e s e l a d d e r s a r e d e s i g n a t e d N and F ("near" and " f a r " a c c o r d i n g t o t h e i r d i s t a n c e from t h e a x i s . Thus on t h e l e f t t h e c h a i n i s i n an c o n f i g u r a t i o n and on t h e r i g h t an NNFF configuration. NFNF
.. .
.. .
The t h r e e - f o l d o p e r a t o r of F i g . 15 s e r v e s t o l i n k t h e l a d d e r s through t h e unshared t e r m i n a l oxygens n e a r t h e a x i s t o produce columns of p o l y h e d r a c h a r a c t e r i s t i c of some z e o l i t e s of t h e c h a b a z i t e group ( s e e Table 5 ) . There a r e t h e n t h r e e ways of e x t e n d i n g t h e column i n t h e p l a n e normal t o t h e o p e r a t o r a x i s :
1. 2.
The c h a i n c o n s i d e r e d i s s h a r e d by two o p e r a t o r s . The c h a i n i s l i n k e d t o a second c h a i n b e l o n g i n g t o a n o t h e r o p e r a t o r i n such a way t h a t ( a ) t h e two connected c h a i n s do n o t f a c e each o t h e r d i r e c t l y ( F i g . 1 6 a ) , and (b) t h e two connected c h a i n s f a c e each o t h e r ( ~ i g .16c)
.. .
The NFNF c h a i n s of F i g . 15 t h e n i n F i g . 16a and c l e a d r e s p e c t i v e l y t o t h e p r o j e c t i o n s normal t o t h e channel a x i s shown i n F i g . 16b and d. These have channels c i r c u m s c r i b e d by puckered 12- and 24-rings. For v a r i o u s c h a i n s h a v i n g p e r i o d i c i t i e s n a l o n g t h e i r lengths (the c d i r e c t i o n ) the structures obtained f o r n < 8 a r e g i v e n i n Table 6 . A l l s t r u c t u r e s i n column 4 have t h e p r o j e c t i o n normal t o t h e c - d i r e c t i o n shown i n F i g . 16b, and a l l s t r u c t u r e s i n column 5 have t h e p r o j e c t i o n s e e n i n F i g . 16d. The l a r g e f r e e d i a m e t e r s o f z e o l i t e s of column 5 and t h e o b s e r v a t i o n t h a t t h e y a r e n o t blocked by s t a c k i n g f a u l t s would make such z e o l i t e s of s p e c i a l i n t e r e s t . The t a b l e i n c l u d e s 33 unknown z e o l i t e frameworks. Other members of t h e chabazi t e group such a s s o d a l i t e , e r i o n i t e , c h a b a z i t e and l e v y n i t e a s w e l l as a d d i t i o n a l unknown s t r u c t u r e s can be g e n e r a t e d by a p p l y i n g 6 3 and 7 axes a s o p e r a t o r s ( 2 7 ) .
8.4.
C r o s s - l i n k i n g of Chains
I n 5 . 8.3 o p e r a t s r s s e r v e d t o c r o s s - l i n k l a d d e r - l i k e c h a i n s and t o form z e o l i t e s . Smith and R i n a l d i (28) c o n s i d e r e d o t h e r a s p e c t s of t h e c r o s s - l i n k i n g o f l a d d e r s based on 4 - r i n g s . Each v e r t e x of a g i v e n r i n g may p o i n t up (U) o r down ( D ) , g i v i n g r i s e t o the f o u r t y p e s of r i n g shown i n F i g . 17 ( 2 8 ) . The l a s t of t h e s e w i t h a n o t h e r of l i k e k i n d can form only t h e c u b i c u n i t shoxm i n t h e
TABLE 5 Exanlples of combinations of polyhedra in zeolites(25) Zeolite
Polyhedra and other features
Cancrinite hydrate
1I-hedra (cancrinite cages). Wide channels 11' to c
Losod
1 1-hedra in columns A 17-hedra of Type I1 in columns B Hexagonal prisms 20-hedra
Chabazite
Gmelinite
Hexagonal prisms. 14-hedra of Type II Wide channels 11' to C
Erionite
Hexagonal prisms I I-hedra 23-hedra
Mazzite
Unknown Faujasite
Hexagonal prisms I I-hedra 14-hedra of Type 11 Wide channels 11' to c 14-hedra of Type I1 Wide channels 11' to c. Narrow channels 11' to c Id-hedra of Type 11. Wide channels 11' to C Hexagonal prisms 14-hedra of Type 1 26-hedra of Type 11
Proportions
Mode of combination 11-hedra in columns 11' t o c. Six linked coiumns surround and create each wide channel. Alternate columns are displaced by c/2. A column of A is surrounded by six of B, 11' to c. B-columns are alternately displaced by c/2. Prisms and 20-hedra alternate in columns 11' t o c. A given column is surrounded by six like columns, three displaced by c/3 and three by 2~13. Prisms and 14-hedra alternate in columns 11' to c. Six linked columns surround and create wide channels. Alternate columns displaced by c/2. Prisms and I I-hedra alternate in columns A , 23-hedra form columns B, both A and B 11' to c. Each column A is surrounded by six of B, where columns B a r e alternately displaced bq cl2. Prisms and I I-hedra alternate in columns A ; 14-hedra form columns B; both A and B are 11' t o c. Three A and three B surround and create each wide channel. Six linked columns of 14-hedra surround and create wide and narrow channels 11' to c. Alternate columns displaced by c?. Six linked columns of 14-hedra surround and create wide channels. All at same height. A given 14-hedron is linked by four prisms arranged tetrahedrally on four of its eight hexagonal faces, to its four nearest 14-hedron neighbours. Arrangement of 14hedra is as are the atoms in diamond. This creates 26-hedral voids, also arranged like atoms in diamond.
TABLE 5-rontin~ted Zeolite Zeolite A
Polyhedra and other features
Proportions
Cubic units 14-hedra of Type 1 26-hedra of Type I
Zeolite RHO Octagonal prisms 26-hedra of Type I Zeolite ZK-5 Hexagonal prisms I 8-hedra 26-hedra of Type I
Mode of combination
A given '14-hedron is linked by cubic units through its six 4-ring faces to a 4-ring face of each of six other 14-hedra. This arrangement creates 26-hedra. A given 26-hedron is linked by octagonal prisms through each of its six 8-ring faces to a n 8-ring face of one of six other 26-hedra. A given 26-hedron is linked by hexagonal prisms through its eight 6-ring faces to a 6-ring face of each of eight other 26-hedra. This creates the 18-hedral voids.
TABLE 7 Schemes for interconnecting chains of Fig. 18( 3 I). Number
Direct
Unitary
Inverse
Rotational symmetry of scheme
T a b l e 6:
A c t u a l and h y p o t h e t i c a l s t r u c t u r e s b a s e d on l a d d e r s of 4 - r i n g s (27)
.
Periodicity n
2 3a 3b 4 5a 5b 6 7a 7b 7c 7d 8a 8b 8c
Type of Chain
Three-fold o p e r a t o r Chain s h a r e d Chain n o t s h a r e d not facing facing
c a n c r i n i Pe
NF NNF 1 FFN NNFF FNNFN NNFNFF NFNFNFN FNFNFNF
offretite gmelini t e unknown
1 1
'ITFFNFF FFNNFNN FNFFNFN NFNFFNFF FNFNNFNN
]
unknown unknown unknown unknown unknown
unknown zeolite L unknown unknown unknown unknown unknown unknown unknown unknown unknown unknown unknown unknown
unknown unknown unknown* unknown* unkn ownj: unknown unknown* unknown unknown unknown* unknown,? unknown* unknown* unknown
0
a(hex) i n A c(hex) i n Free diameter ofo Main c h a n n e l i n A Ring c i r c u m s c r i b i n g main c h a n n e l Stacking f a u l t s
2
1 2 . 8 t o 13.7 n x 2.5 a
6.5
12-ring Block main Channe 1
12-ring Do n o t b l o c k
24-ring Do n o t b l o c k
*FF l e a d s t o h e a v i l y d i s t o r t e d &-membered r i n g s . f i g u r e . From t h e o t h e r t h r e e k i n d s o f 4 - r i n g t h r e e t y p e s of c h a i n can be made, a s shown i n F i g . 1 7 . The UUDD c h a i n r e s e m b l e s t h e NNFF c h a i n of T a b l e 6 and F i g . 15. From i t 17 o t h e r frameworks were made by v a r i o u s l y c r o s s - l i n k i n g t h e c h a i n s , i n which r e p e a t i n t h e p l a n e normal t o t h e c h a i n d i r e c t i o n were l e s s These i n c l u d e d f e l s p a r , p a r a c e l s i a n , p h i l l i p s i t e and t h a n 15 dis a s y n t h e t i c m a t e r i a l termgd phase A (29) &BaCAlSi2O61C1, OE< group 14/mmm; a = 14.194 A and c = 9.934 A ) . O t h e r s were n o v e l s t r u c t u r e s . Chains can be l i n k e d i n two ways:
tancea. 1.
F l e x i b l e mode. T e t r a h e d r a connected t o one a n o t h e r a l o n g t h e c h a i n a r e b o t h c o n n e c t e d t o t h e same a d j a c e n t c h a i n .
These i n c l u d e p a r a c e l s i a n , p h i l l i p s i t e , gismondine and p h a s e A. I n f l e x i b l e mode. T e t r a h e d r a connected t o one a n o t h e r along the chain a r e linked t o d i f f e r e n t adjacent chains. An example o f t h i s i s f e l s p a r .
2.
I n t h e f l e x i b l e mode 4 - r i n g s can r o t a t e c o - o p e r a t i v e l y b u t t h i s i s n o t p o s s i b l e i n t h e i n f l e x i b l e mode. The second k i n d of c h a i n (UDUD) a p p e a r s u n - c r o s s - l i n k e d a s ~ h ea n i o n i n n a r s a r s u k i t e , Na4(Ti0?2CSi80201, ( s e e 5. 3 and F i g . 3 g ) . I h e s e c h a i n s c o u l d be c r o s s - l i n k e d i n f o u r ways, one of which i s found i n t h e n o n - z e o l i t e b a n a l s i t e , Ba,Na2CA14Si4016I . The t h i r d k i n d of c h a i n (UUUD) could a l s o be c r o s s - l i n k e d t o y i e l d frameworks af n o v e l k i n d s (28)
.
The c h a i n s i n F i g . 17 a r e examples o n l y of t h o s e which may l e a d t h r o u g h c r o s s - l i n k i n g t o t e c t o s i l i c a t e s . O t h e r examples of s i n g l e and d o u b l e c h a i n a n i o n s were r e f e r r e d t o i n 5. 3. A d d i t i o n a l c h a i n s from which z e o l i t e a n i o n s may be c o n s t r u c t e d o c c u r i n t h e x a t r o l i t e group and m o r d e n i t e group. I n t h e n a t r o l i t e c h a i n UDUD i - r i n g s a r e l i n k e d t o g e t h e r by s i n g l e t e t r a h e d r a a s shown i n F i g . 1 8 (30) f o r n a t r o l i t e and t h o m s o n i t e . These c h a i n s d i f f e r i n S i / A l r a t i o , which i s 312 f o r n a t r o l i t e and 1 f o r t h o m s o n i t e . The A1,Si i i s t r i b u t i o n s on t e t r a h e d r a l s i t e s a r e o r d e r e d and t h e p e r i o d i c i t y The h e i g h t s of f r e e v e r t i c e s of t e t r a in t h e c - d i r e c t i o n i s 6 . 6 h e d r a a r e m u l t i p l e s of c / 8 . A l b e r t and G o t t a r d i (31) c o n s i d e r e d Yays of c r o s s - l i n k i n g a c h a i n t o f o u r o t h e r c h a i n s t h r o u g h i t s f r e e -:ertices. Each c h a i n can be coded by t h e h e i g h t , n ( n c / 8 ) of t h e z e n t r e of i t s s i n g l e t e t r a h e d r o n . The l i n k a g e of a g i v e n c h a i n zo i t s n e i g h b o u r s i s t h r o u g h t h e p a i r of U and t h e p a i r of D t e t r a xedra. For t h e l a t t e r t h e p o s s i b i l i t i e s a r e :
2.
1.
The c r o s s - l i n k s a r e b o t h t o D t e t r a h e d r a of two o t h e r c h a i n s ( t h e s e c h a i n s b e i n g a t h e i g h t n) . One c r o s s - l i n k i s t o D and t h e o t h e r t o U ( t h e two a d j a c e n t c h a i n s b e i n g a t h e i g h t s n and ( n - 2 ) ) . Both c r o s s - l i n k s a r e t o U ( t h e a d j a c e n t c h a i n s b e i n g a t h e i g h t s (n-2)) .
2.
3.
Three s i m i l a r ~ o s s i b i l i t i e sa r i s e f o r t h e p a i r of U t e t r a h e d r a , namely b o t h c r o s s - l i n k s t o U t e t r a h e d r a ; one t o U and one zo D ; and b o t h t o D . I n U-D bonds t h e h e i g h t of t h e a d j a c e n t :hain i s n + 2. T a b l e 7 (31) g i v e s t h e schemes f o r c r o s s - l i n k i n g . I n i t t h e symbols =, + and + have t h e f o l l o w i n g meanings: n n n
= +-
+
n f o r a c r o s s - l i n k w i t h a c h a i n a t t h e same h e i g h t n + 2 f o r a cross-link with a chain a t height n + 2 n - 2 f o r a cross-link with a chain a t h e i g h t n - 2
Fig. 17.
Four t y p e s of 4 - r i n g w i t h a p i c e s of t e t r a h e d r a p o i n t i n g up (U) o r down (D). Below, t h r e e k i n d s of c h a i n a r e shown b a s e d on UUDD, UDUD and UDUUD r i n g s , t o g e t h e r w i t h t h e c u b i c u n i t made from two 4 - r i n g s of t h e f o u r t h kind (28).
F i g . 18.
The c h a i n s p r e s e n t i n z e o l i t e s of t h e n a t r o l i t e group ( a ) r e f e r s t o n a t r o l i t e , (b) t o t h o m s o n i t e . (30). Shaded t e t r a h e d r a denote A104, unshaded d e n o t e S i 0 4 . There i s o r d e r i n g i n t h e A1,Si d i s t r i b u t i o n s , and t h e p e r i o d i c i t i e s i n view of t h e o r d e r i n g a r e 3 f o r ( a ) and 6 for (b).
Fig.
19.
Fig. 20.
The framework o f e d i n g t o n i t e viewed a l o n g C1101 ( 3 2 ) .
F o u r k i n d s o f h e x a g o n a l s i n g l e s h e e t d i f f e r e n t i a t e d by p a t t e r n s i n which a p i c e s o f t h e l i n k e d t e t r a h e d r a p o i n t up W ) o r down (35).
(v)
I f c o r r e s p o n d i n g i n v e r s e and d i r e c t schemes a r e c o n s i d e r e d a s a s i n g l e scheme t h e n i n e d i f f e r e n t p o s s i b i l i t i e s i n T a b l e 7 a r e reduced t o s i x . I n f i b r o u s z e o l i t e s two k i n d s o f S i / A l o r d e r i n g have b e e n o b s e r v e d , a s a l r e a d y n o t e d f o r n a t r o l i t e and thomsonite i n F i g . 18. I f p o s s i b i l i t i e s of Si/A1 o r d e r i n g a r e combined w i t h t h e s i x schemes t h e n t h e 15 p o s s i b i l i t i e s of Table 8 (31) a r e found. Ten of t h e s e r e f e r t o unknown s t r u c t u r e s . As an example, F i g . 19 (32) shows a p o r t i o n of t h e e d i n g t o n i t e framework, viewed a l o n g E1101. The c h a i n c h a r a c t e r i s t i c of some z e o l i t e s o f t h e m o r d e n i t e group i s b a s e d on 5 - r i n g s (33) . The c h a i n s r u n i n t h e c d i r e c t i o n E i g h t s t r u c t u r e s which c a n b e made by with a p e r i o d i c i t y 7.52 c r o s s - l i n k i n g t h e c h a i n s a r e g i v e n i n Table 9 ( 3 4 ) . Only two of t h e s e r e f e r t o known m i n e r a l s .
g.
8.5. Frameworks made by c r o s s - l i n k i n g
Sheets
Many k i n d s of s h e e t can be made from polygons s h a r i n g edges w i t h l i k e o r u n l i k e p o l y g o n s , a number of which have b e e n i d e n t i f i e d i n s i l i c a t e s (5.4). These s h e e t s were composed o n l y of 6 - r i n g s (mica) ; of 4- and 8 - r i n g s (apophyl li t e ) ; of 4-, 6- and 8 - r i n g s ( d a l y l i t e ) ; and of 4-, 6- and 1 2 - r i n g s (manganpyrosmalite). Each t y p e of s h e e t can b e f u r t h e r s u b - d i v i d e d , a c c o r d i n g t o t h e Four p a t t e r n s o f un-linked v e r t i c e s p o i n t i n g up (U) and down ( D ) . such p a t t e r n s a r e shown f o r hexagonal s h e e t s i n F i g . 20 ( 3 5 ) . There a r e a c c o r d i n g l y many ways of c r o s s - l i n k i n g s h e e t s , which may i n a d d i t i o n be b u c k l e d i n a v a r i e t y of ways. Puckered s h e e t s of type ( i ) a r e f o u n d , p a r a l l e l t o C O O 1 1 i n b i k i t a i t e , and such s h e e t s a l s o o c c u r i n t h e n o n - z e o l i t e s n e p h e l i n e and c a r n e g i e i t e . I n
T a b l e 8:
Space groups o f t h e n a t r o l i t e group of z e o l i t e s (31)
S t r u c t u r a l Type
2 3
4 5 6
D i s o r d e r e d (A1,Si) distribution
Pman Gonnardi t e ( ?) 162 d Te t r a g o n a l natrolite Pmma Imma Pmna
Natrolite type o r d e r
Thomsonite t y p e order
PZ1Z12 Edingtoni t e ~ 2/ b
~ 4 c2
Fdd2 Natrolite P2/a B2/b PZ1/a
P cnn Thomsonite Not p o s s i b l e
Pcca Not p o s s i b l e Not ~ o s s i b l e
r...-,--
0
,
, # ; , ,
I
:......: .
:
/
,
I
.
.
;
.,".. . ... . . . ...
.........:, . .. ..
.
z
. . , 3 ,, ,
.
.
r 1 i
1
I
J
: o : .
> :...... . ':. '
;
... .
:
'., ... : .:........... .
:
: n :
.
a
......b,
.
:......:
:
.
.
.,, ........:,.' : : 0 : ........ .. .. . .
............ ..,
..
F l c . 2 1 The puckered conformations of hexagonal sheets found in some zeolites are shown edge on (full lines). The interconnections between sheets are shown as dashed lines. The zeolites concerned are: (a) rnordenite; (b) dachlsiiite; (c) epistilbite; (d) ferrierite; (e) bikitaite; and (f) zeolite Li-~Bw(36).1.
0
Heulandite
Stilbite
Brewsterite
F i g . 22.
L a y e r s w i t h a common s t r u c t u r a l u n i t f o u n d i n h e u l a n d i t e , s t i l b i t e and b r e w s t e r i t e ( 3 8 ) .
I 0
Fig. 23.
I bsiny
The laumontite l a y e r showing s u b - u n i t s composed of f o u r 6 - r i n g s and two 4-rings
(39).
b i k i t a i t e t h e s h e e t s a r e l i n k e d t o one a n o t h e r through s i n g l e t e t r a h e d r o n c h a i n s p a r a l l e l t o C0101, whereas i n n e p h e l i n e and c a r n e g i e i t e t h e s h e e t s a r e l i n k e d d i r e c t l y t o one a n o t h e r . S h e e t s of type ( i i ) a r e d i r e c t l y connected t o one a n o t h e r i n Those of type ( i i i ) a r e found i n d a c h i a r d i t e , z e o l i t e ABW. e p i s t i l b i t e and f e r r i e r i t e . They a r e l i n k e d by s i n g l e 4 - r i n g s f o r t h e f i r s t two z e o l i t e s and by s i n g l e 6 - r i n g s f o r f e r r i e r i t e . Type ( i v ) s h e e t s connected through s i n g l e 4-rings form t h e mordenite s t r u c t u r e s . F i g . 21 (36) shows t h e puckered s h e e t s edge on , a s t h e b o l d l i n e s , w h i l e t h e c o n n e c t i o n s between s h e e t s a r e i n d i c a t e d as t h e dashed l i n e s , f o r t h e v a r i o u s z e o l i t e s r e f e r r e d t o i n t h i s and t h e p r e c e d i n g p a r a g r a p h , I n t h e c h a b a z i t e group s h e e t s c o n t a i n i n g e i t h e r 6 - r i n g s o r hexagonal prisms (6-6--rings) can be i d e n t i f i e d . These a r e l i n k e d t o s i m i l a r s h e e t s above and below i n t h e v a r i o u s s t a c k i n g sequences of Table 10 ( 3 4 ) , where l a y e r s c o n t a i n i n g s i n g l e 6 - r i n g s a r e denoted by s m a l l l e t t e r s and t h o s e c o n t a i n i n g 6-6-rings by c a p i t a l l e t t e r s . Novel z e o l i t e s i n v o l v i n g sequences of A , B and C l a y e r s analogous t o those of a , b and c l a y e r s i n l o s o d , l i o t t i t e , a f g h a n i t e and f r a n z i n i t e a r e p o s s i b l e , a s w e l l as sequences of double and s i n g l e 6 - r i n g l a y e r s i n a d d i t i o n t o t h o s e g i v i n g o f f r e t i t e , e r i o n i t e and l e v y n i t e . Two of t h e s e p o s s i b i l i t i e s w i t h sequences ABCB and ABCACB have beenoexamined ( 3 7 ) . have hexagonal u n i t c e l l s w i t h a % 13.7 A and c % 20 and 30 respecti v e l y . Each c o n t a i n s e l o n g a t e d 26-hedra of a t h i r d k i n d , w i t h n i n e 8 - r i n g , t o 6 - r i n g and f i f t e e n 4-ring f a c e s b The f r e e l e n g t h approaches 20 and t h e f r e e d i a m e t e r about 6 . 5 A. In addition the ABCB s t r u c t u r e c o n t a i n s g m e l i n i t e type 14-hedra and t h e ABCACB s t r u c t u r e c o n t a i n s b o t h g m e l i n i t e and 20-hedral c h a b a z i t e type cages.
TheH
B
H e u l a n d i t e , s t i l b i t e and brews t e r i t e can be r e p r e s e n t e d i n terms of o t h e r l a y e r s each c o n t a i n i n g t h e same u n i t composed of f o u r 5 - r i n g s and two 4 - r i n g s , a s shown i n F i g . 22 ( 3 8 ) . The bond d e n s i t y i n t h e l a y e r s i s g r e a t e r t h a n t h a t of t h e l i n k s between l a y e r s , r e s u l t i n g i n t h e i r p l a t y c h a r a c t e r . Laumontite c o n t a i n s s h e e t s of t h e type shown i n F i g . 23 ( 3 9 ) i n which s t r u c t u r a l u n i t s composed of f o u r 6 - r i n g s and two 4-rings o c c u r . Z e o l i t e s can be c o n s t r u c t e d from l a y e r s o f , o r c o n t a i n i n g , s o d a l i t e cages. I n s o d a l i t e i t s e l f t h e l a y e r c o n s i s t s of t h e s e cages each l i n k e d t o f o u r o t h e r cages by s h a r i n g 4-ring f a c e s , and t o a cage i n an upper l a y e r and one i n a lower l a y e r a l s o by s h a r i n g i t s remaining two 4-ring f a c e s . I n z e o l i t e A a s o d a l i t e cage i n a l a y e r i s l i n k e d t o f o u r o t h e r cages by double & - r i n g s ( c u b i c u n i t s ) and t o a s o d a l i t e cage i n t h e l a y e r above and one i n t h e l a y e r below, a l s o by c u b i c u n i t s . From a t o p o l o g i c a l v i e w p o i n t
Table 9 :
Some S t r u c t u r e s Obtainable by Cros s - l i n k i n g Mordeni te Chains (34)
Number
Space group
1 2 3
4 5 6 7 8
~2/m B2/m Pmnm Amam Cmcm Imm Bb cm Bmm
a(%
b
(2)
c
6)
Y
107. g o 123O
.
Example
Dachiardite Modified d a c h i a r d i t e Unknown Unknown Mordeni t e Modified mordeni t e Unknown Unknown
Table 10:
S t r u c t u r e s formed by l i n k i n g l a y e r s c o n t a i n i n g s i n g l e 6 - r i n g s ( a , b , c) and l a y e r s c o n t a i n i n g 6-6-rings (A,B, C) ( 3 4 ) .
No. of l a y e r s i n repeat unit
Structural t Y Pe
ab abacac ab cab cbacb Ab Ab Ac aB aC AbCaBc AB AB C
Space Group
a
(2)
c
(2)
N ame
Canc Sodalit e Los od Liottite Afghani t e Franzinite Offretite Erionit e
-
Levyni t e Gme l i n i t e Chabazi t e
s o d a l i t e c a g e s may r e p r e s e n t t h e p o s i t i o n s occupied by s p h e r e s i n c u b i c close-packing ( 4 0 ) . An i n d e f i n i t e number of s t r u c t u r e s can be made from hexagonal l a y e r s of close-packed s p h e r e s i n v a r i o u s corresponds w i t h c u b i c symsequences. The sequence ABCABC metry a s i n f a u j a s i t e , i n which t h e 14-hedra a r e j o i n e d by hexagonal p r i s m u n i t s t o f o u r o t h e r 14-hedra, t h e ABC l a y e r corsequence b e i n g a l o n g C1111. The l a y e r sequence ABAB responds w i t h a hexagonal s t r u c t u r e , t h e s o d a l i t e 14-hedra b e i n g , l i n k e d , a s i n f a u j a s i t e , by hexagonal p r i s m u n i t s t o f o u r o t h e r 14-hedra. The r e s u l t a n t v e r y open s t r u c t u r e h a s cages and c h a n n e l s comparable w i t h t h o s e i n f a u j a s i t e . This sequence i s shown f o r m a l l y a s F i g . 24a. F i g . 24b i s t h a t i n f a u j a s i t e and F i g . 24c i s t h e sequence ABABCA of one of many p o s s i b l e novel s t r u c t u r e s . Z e o l i t e ZSM-3 might, i t was t h o u g h t , be r e l a t e d t o f a u j a s i t e as one of t h e s e s t a c k i n g arrangements of l a y e r s of s o d a l i t e cages ( 4 0 ) . I t had a hexagonal u g i t c e l l w i t h a = 17.5 A and a maximum p o s s i b l e v a l u e of c of 129 A., S i n c e t h e o d i s t a n c e between a d j a c e n t s o d a l i t e cage l a y e r s i s 1 4 . 3 A, c = 129 A r e p r e s e n t s a 9 - l a y e r sequence.
. ..
...
Foore and Smith (41) gave f u r t h e r c o n s i d e r a t i o n t o s t r u c t u r e s based on s o d a l i t e cages a n d / o r on 26-hedra of t y p e I ( a s found i n z e o l i t e A). I n a short-hand d e s c r i p t i o n of p o s s i b i l i t i e s they used t h e symbols H
=
H'
hexagonal f a c e ( 6 - r i n g )
= hexagonal prism (6-6-ring)
S = s q u a r e f a c e (4-ring) S ' = c u b i c u n i t (4-4-ring) 0 = octagonal face (8-ring) 0 ' = o c t a g o n a l prism (8-8-ring)
The type of c o n t a c t between p o l y h e d r a was i n d i c a t e d by t h e a p p r o p r i a t e one of t h e s e l e t t e r s and t h e l e t t e r s f o l l o w i n g ( i n p a r e n t h e s e s ) denoted t h e t y p e s of p o l y o n a l f a c e s opposing each o t h e r a c r o s s t h e c o n t a c t . The r e s u l t s a r e summarised i n Table 11 i n which frameworks of f o u r s o f a r unknown z e o l i t e s a p p e a r . High r e s o l u t i o n e l e c t r o n microscopy (HREM) has r e v e a l e d a tendency t o r e c u r r e n t twinning i n s y n t h e t i c f a u j a s i t e ( z e o l i t e Y) ( 4 2 ) . T h i s can g e n e r a t e a new s t r u c t u r e w i t h i n t h e z e o l i t e w i t h e l l i p t i c a l a p e r t u r e s along Cl101 and an e l o n g a t e d "hypercagel' t h e l e n g t h of which depends on t h e e x t e n t of twinning. I f A denotes a b u i l d i n g u n i t r e p e a t a l o n g C l l l l ( a 1 4 . 3 A r e p e a t d i s t a n c e ) and V d e n o t e s a twin J a m e l l a , t h e n . . .AAA.. . i s t h e normal l a y e r V/A/V/A.. i s a t u n n e l s t r u c t u r e i n which a twin sequence and . l a m e l l a bounded by a p a i r of { I l l ) twin p l a n e s , denoted by s l a s h e s , a l t e r n a t e s e v e r y 14.2 with the regular f a u j a s i t e lamellae. When t h e r e a r e n twin p l a n e s t h e t u n n e l i n t h e s t r u c t u r e i s %14.3 ( n + l ) + 6.95 i n l e n g t h . The sequence . .AA/V/AA/V..
..
.
2
.
.
..
.-I
cnd
rim
CO ri
d d
- m
0 111
Ill A
-
. .
m
ri
rl d
4m
i n d i c a t e s a twin l a m e l l a between f l a n k i n g p a i r s of r e g u l a r r e p e a t long u n i t s and g i v e s a cage sequence i n which each cage i s 4 9 . 6 i n f r e e d i a m e t e r . This p a r t i c u l a r and v a r i e s between 1 3 and 7.4 sequence h a s been observed ( 4 2 ) . R e c u r r e n t twinning a t t h e u n i t c e l l l e v e l could a l s o c o n v e r t o t h e r known z e o l i t e s t r u c t u r e s i n t o n o v e l ones.
1
a
9.
INTRAZEOLITE CHANNELS
Each d i f f e r e n t z e o l i t e topology has a d i f f e r e n t system of channels and c a v i t i e s . N e v e r t h e l e s s a broad c l a s s i f i c a t i o n i s p o s s i b l e , from t h e viewpoint of m i g r a t i o n of g u e s t m o l e c u l e s , i n t o three divisions :
1. I n t r a c r y s t a l l i n e channels a r e p a r a l l e l and a r e n o t i n t e r connected (1-D d i f f u s i o n ) . 2 . Channels a r e i n t e r - c o n n e c t e d t h e t h i r d (2-D d i f f u s i o n ) . 3. Channels a r e i n t e r - c o n n e c t e d diffusion)
.
i n two dimensions b u t n o t i n i n t h r e e dimensions (3-D
The geometry of channel and c a v i t y systems i s d e f i n e d as p r e c i s e l y a s a r e t h e p o s i t i o n s of framework atoms. I n a d d i t i o n t o v a r i e d s p a t i a l arrangements of t h e open pathways, a s i n t h e above t h r e e c a t e g o r i e s , i n d i f f e r e n t t o p o l o g i e s c r o s s - s e c t i o n a l a r e a s and shapes a r e s p e c i f i c t o each topology and c o n f i g u r a t i o n . C a t i o n s may a l s o be l o c a t e d i n t h e d i f f u s i o n pathways and i f t h e y a r e p r e s e n t a t s t r a t e g i c ~ o i n t s ,such a s t h e windows o r n a r r o w e s t p o i n t s a l o n g a pathway t h e y can a c t a s p a r t i a l o r complete b a r r i e r s t o t h e movement of g u e s t molecules. The l o c a t i o n and numbers of t h e c a t i o n s can be modified by exchanges such a s 2 ~ a +? ca2+ and t h e b l o c k i n g e f f e c t s of c a t i o n s can be d r a m a t i c a l l y changed by such means. Some examples of z e o l i t e s w i t h 1-D, systems a r e : 1-D.
2-D.
3-D.
2-D and 3-D channel
C a n c r i n i t e h y d r a t e ( 1 2 , 6 . 2 ) ; l a u m o n t i t e ( 1 0 , 4 . 0 ~ 5- 6 ) ; m a z z i t e , z e o l i t e Q (12, - 7.4) ; mordenite (12, - 6.7x7.0) ; z e o l i t e L (12, 7.1). D a c h i a r d i t e T 1 0 , 3.7x6.7 and 9 , 3.6x4.8) ; f e r r i e r i t e (10, 4.3x5.5 a n d 8, 3.4x4.8) ;-levynite (8 , 3.3x5.3) ; s z l b i t e ( 1 0 , 4.126.2 and 8, 2.7x5.7) . Chabazite 78, - 3.6x3.7) ; e r T o n i t e ( 8 , 3.6x5.3.) ; f a u j a s i t e (12, 7 . 4 ) ; o f f r e t i t e ( 1 2 , 6.4 and 8, 3 . 2 x 5 . 2 ) ; z e o l i t e A (8,4.1) ; z e o l i t e RHO (8-8, - 3.9x5.i) z e o l i t e ZK-5 (8'. - 3.9).
The u n d e r l i n e d f i g u r e s i n t h e b r a c k e t s i n d i c a t e t h e numbers n of l i n k e d t e t r a h e d r a forming t h e n a r r o w e s t a p e r t u r e s a l o n g t h e d i f f u s i o n pathways and t h e o t h e r f i g u r e s a r e t h e f r e e dimensions of t h e s e a p e r t u r e s . They show, f o r example f o r % r i n g s , t h a t t h e r i n g s may have v a r i o u s c o n f i g u r a t i o n s a c c o r d i n g t o t h e framework i n which i t a p p e a r s .
10.
A l , S i ORDERING
According t o Lowenstein's r u l e (43) A1-0-A1 bonds do n o t o c c u r i n t e c t o s i l i c a t e s f o r which ~ i / A 1> 1. Accordingly where S i / A l = 1 S i and A1 must a l t e r n a t e on a l l t e t r a h e d r a l s i t e s . Recent magic a n g l e s p i n n i n g n u c l e a r magnetic resonance (MASNMR) measurements appeared t o c h a l l e n g e t h e v a l i d i t y of t h i s r u l e f o r z e o l i t e A w i t h S i / A l = 1 b u t s p e c t r a o b t a i n e d on z e o l i t e s A w i t h v a l u e s of ~ i / A 1> 1 l e d t o a r e - a p p r a i s a l which confirmed t h e r u l e ( 4 4 , 4 5 ) . It i s now almost c e r t a i n t h a t t h e r e a r e no known e x c e p t i o n s t o Lowenstein's r u l e f o r t e c t o s i l i c a t e s w i t h ~ i / A 1> 1, and hence t h a t t h e r e i s A l , S i o r d e r i n g whenever ~ i / A l= 1. There can a l s o be ~ 1 u n i t y , an A l , S i o r d e r i n g i n t e c t o s i l i c a t e s i n which ~ i / exceeds example r e f e r r e d t o i n 5 . 8 . 4 and F i g . 1 8 b e i n g n a t r o l i t e where Si/A1 = 312. F a u j a s i t e s h a v i n g wide ranges i n ~ i / A 1r a t i o s gave MASNMR s p e c t r a which have been i n t e r p r e t e d i n terms of o r d e r i n g of A 1 and S i a t s p e c i f i c r a t i o s corresponding w i t h i n t e g r a l numbers of A 1 and S i atoms p e r s o d a l i t e cages ( 4 6 , 4 7 ) . There i s c o n s i d e r a b l e i f n o t y e t t o t a l agreement on t h e o r d e r i n g schemes. The s p e c t r a themselves s e r v e t o g i v e t h e r e l a t i v e numbers of S i atoms l i n k e d ( t h r o u g h 0) w i t h 4 , 3, 2, 1 and 0 A 1 atoms, denoted r e s p e c t i v e l y a s S i ( 4 A l ) , Si(3A1), Si(2A1), S i ( l A 1 ) and S i (OA1) . Table 12 g i v e s a s a f u n c t i o n of t h e S i / A l r a t i o t h e r e l a t i v e p e r c e n t a g e p o p u l a t i o n s of t h e s e u n i t s f o r any t e c t o s i l i c a t e s t r u c t u r e , a c c o r d i n g t o a binomial d i s t r i b u t i o n and when Lowenstein's r u l e i s v a l i d . Where A 1 and S i a r e d i s t r i b u t e d on t e t r a h e d r a l s i t e s i n v o l v i n g more t h a n one s u b - l a t t i c e t h e r e may, a s i n t h e f e l s p a r s (48) be o r d e r e d and d i s o r d e r e d forms, s u b j e c t t o t h e o v e r - r i d i n g i n f l u e n c e of t h e Lowenstein r u l e . I n o r d e r e d a l k a l i m e t a l f e l s p a r s a t e q u i l i b r i u m a t low t e m p e r a t u r e s t h e A 1 atoms a r e found on one subl a t t i c e . High temperature e q u i l i b r i u m forms have A1 atoms d i s t r i b u t e d on more t h a n one s u b - l a t t i c e . O r d e r - d i s o r d e r changes of t h i s k i n d a r e s l u g g i s h and may b e s t be approachable by s y n t h e s i s under h i g h and low t e m p e r a t u r e c o n d i t i o n s provided r e a c t i o n i s a l s o slow. Rapid hydrothermal s y n t h e s e s of a l k a l i m e t a l f e l s p a r have y i e l d e d d i s o r d e r e d forms, presumably m e t a s t a b l e ( 4 9 ) . T h i s may be an example of Ostwald's law of s u c c e s s i v e t r a n s f o r m a t i o n s a c c o r d i n g t o which l e s s s t a b l e phases appear b e f o r e t h e more s t a b l e ones.
Table 12:
11.
R e l a t i v e p e r c e n t a g e p o p u l a t i o n s of S i (nAl) s t r u c t u r a l u n i t s , c a l c u l a t e d from t h e binomial formula f o r d i f f e r e n t t e c t o s i l i c a t e compositions ( 4 6 ) . Lowenstein's r u l e i s obeyed.
CATIONS AND WATER MOLECULES
Accurate and q u a n t i t a t i v e l o c a t i o n of c a t i o n s and w a t e r molecules h a s been more d i f f i c u l t t h a n d e f i n i n g t h e p o s i t i o n s of oxygen and T atoms i n t h e anions of z e o l i t e s . Reasons f o r t h i s include: (i)
The t o t a l number of c a t i o n s i s r a t h e r s m a l l compared w i t h t h e number of 0 and T atoms i n t h e a n i o n s . ( i i ) These c a t i o n s a r e u s u a l l y d i s t r i b u t e d o v e r a number of s u b - l a t t i c e s , and o f t e n t h e r e i s only p a r t i a l occupancy of s i t e s on a g i v e n s u b - l a t t i c e . Thus t h e number nn a g i v e n s u b - l a t t i c e may be low. ( i i i ) I n a number of i n v e s t i g a t i o n s powder X-ray photography has of n e c e s s i t y been used, g i v i n g more l i m i t e d information. (iv) I n t r a c r y s t a l l i n e water i s not necessarily a l l t i g h t l y h e l d on s p e c i f i c s i t e s .
A c o m p i l a t i o n of p u b l i s h e d work on c a t i o n and w a t e r l o c a t i o n s i s a v a i l a b l e , which however cannot a s s e s s accuracy i n t h e l o c a t i o n s claimed ( 5 0 ) . Such an assessment w i l l n o t be a t t e m p t e d h e r e , a l t h o u g h t h e r o l e of c a t i o n s a s m o d i f i e r s of molecular s i e v e behaviour i s of major s i g n i f i c a n c e .
F i g . 24.
S t a c k i n g of l a y e r s o f s o d a l i t e c a g e s i n f a u j a s i t e and r e l a t e d s t r u c t u r e s ( 4 0 ) . The c a g e s a r e r e p r e s e n t e d as the l i n e junctions. The view i s p e r p e n d i c u l a r t o t h e c-axis, 110 p r o j e c t i o n . ( a ) Hexagonal AB s e q u e n c e ; ( b ) c u b i c ABC s e q u e n c e ; ( c ) ABABC s e q u e n c e .
Qdb 5-1
Fig. 25.
4-4-1
Secondagy b u i l d i n g u n i t s (SBU) i n z e o l i t e b u i l d i n g . T h e i r c h a r a c t e r i s t i c i s t h a t a s e l e c t e d z e o l i t e framework can be b u i l t e n t i r e l y f r o m an a p p r o p r i a t e l y chosen one o f t h e s e SBU ( 5 1 ) .
12.
CONDLUDING REMARK
T h i s a c c o u n t of z e o l i t e s t r u c t u r e s has c o n c e n t r a t e d upon t h e a n i o n s which a r e t h e most a c c u r a t e l y d e f i n e d p a r t s and p r o v i d e t h e key t o z e o l i t e d i v e r s i t y , When t e c t o s i l i c a t e frameworks a r e cons t r u c t e d by a s e r i e s of d i f f e r e n t p r o c e e d u r e s one o b t a i n s i n a d d i t i o n t o t h e many known s t r u c t u r e s a remarkable number of o t h e r frameworks, porous on t h e s c a l e of m o l e c u l a r d i m e n s i o n s , which r e p r e s e n t s o f a r unknown z e o l i t e s and p o i n t t h e way t o f u r t h e r chemical d i s c o v e r y . Complementary t o t h e methods of 5 5 . 8 . 1 t o 8 . 5 , one may c o n s i d e r t h e s m a l l e s t number of secondary b u i l d i n g u n i t s (SBU) from which a l l s t r u c t u r e s can be made, assuming t h a t t h e e n t i r e framework i s made up of one t y p e of SBU o n l y . Nine a r e r e p o r t e d , a s shown i n F i g . 25 ( 5 1 , 5 2 ) . I t c a n n o t however b e assumed t h a t t h e s e a r e chemical u n i t s a c t u a l l y added t o a z e o l i t e c r y s t a l d u r i n g i t s growth. A l s o , of t h e many h y p o t h e t i c a l t o p o l o g i e s r e f e r r e d t o i n 5 . 8 , some may be i m p o s s i b l e t o make because o f c o n f o r m a t i o n a l r e s t r i c t i o n s ( 5 2 ) , f o r example i n T-0-T bond a n g l e s .
ACKNOWLEDGEMENTS I w i s h t o thank P r o f e s s o r J . M . Thomas f o r Table 12 and e s p e c i a l l y P r o f e s s o r F . Liebau f o r p e r m i s s i o n t o u s e m a t e r i a l from h i s work on s i l i c a t e c l a s s i f i c a t i o n , and t h e Academic P r e s s f o r p e r m i s s i o n t o use c o n s i d e r a b l e m a t e r i a l from my two r e c e n t books ( r e f s . 22 and 2 3 ) .
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203,
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78,
ZEOLITE CRYSTALLOGRAPHY
G. T. Kokotailo Drexel University Physics Department Philadelphia, Pennsylvania
INTRODUCTION The importance of zeolites in science and technology is well established and the indications are that it will increase. The number of new zeolites synthesized and structures determined is increasing. The lack of large crystals suitable for single crystal structure determination has hindered the progress of structure determination of the new zeolites synthesized. The knowledge of zeolite structures is limited; however the topology or the general features of the framework structure of a fairly large number of zeolites are known. Good structure information regarding metal framework atom distribution, the position of cations, water and organic molecules is available for only a few zeolites. The properties of a zeolite are dependent on the topology of its framework, the size of the free channels, the location, charge and size of the cations within the framework, the presence of faults and occluded naterial, and the ordering of T atoms (framework metal atoms). Therefore, structural information is important in understanding the absorptive and catalytic properties of zeolites. There have been a number of reviews of the structure, chemistry and use of zeolites (1-5). This paper is directed to the review of the structures of zeolites ranging from the topology of the frameworks, cation location,T-atom distribution, faults and imperfections to model building. CLASSIFICATION OF KNOWN ZEOLITE STRUCTURES The framework structure of zeolites consists of linked tetrahedra (metal atoms are tetrahedrally coordinated to four oxygen atoms)
Figure 1 Secondary Building Units (2)
Figure 2 Frameworks of (a) analcite (b) laumontite (5)
Figure 3 Chains i n ( a ) n a t r o l i t e , (b) B r e w s t e r i t e ( c ) ZSM-5
Figure 4 O f f r e t i t e and E r i o n i t e Framework a) o f f r e t i t e (b) c - p r o j e c t i o n of of f r e t i t e ( c ) E r i o n i t e (d) c - p r o j e c t i o n of e r i o n i t e
which form three dimensional four connected nets. The corner sharing of tetrahedra requires that there are twice as many framework oxygens as T-atoms (metal atoms). Cations are required for T-atom charge balance. The first classification of zeolites on the basis of common structural units such as parallel 6-rings was made by Smith (6). Meier (2) classified zeolites into seven groups based on secondary building units or polyhedral building blocks which on linking form the framework structures. These units (Figure I ) (single 4-ring, single 6ring, single 8-ring, double 4-ring, double 6-ring, and 4+1, 5 -F1 and 4+4+l combinations) are sufficient to describe a zeolite framework although 3 and 9 rings should be added. This classification into seven groups should be extended to nine with the addition of the melanophlogite group based on the aluminosilicate analogs of the gas hydrates and the lovdarite group based on 3,5 and 9-rings. The nine groups of zeolites identified on the basis of their framework structure are given in Table I with idealized cell contents and crystallographic data and channel systems. The isotypes of the species listed in Table I are listed in Table 11. ANALCITE GROUP The frameworks of the two members of this group, analcite and laumontite, can be derived by interconnecting 4 and 6-rings as shown in Figure 2. NATROLITE GROUP The chains (Figure 3a) characteristic of this group consist of linkei four 4-ring units. There are three different ways to link these chains resulting in the natrolite (lo), edingtonite (12) and thomsonite (10) frameworks (Figure 4). All the structures have a two dimensional 8-ring channel system. CHABAZITE GROUP The chabazite group frameworks consist of parallel 6-rings. The offretite-erionite a and c projections are shown in Figures 4a-4d. The stacking sequence involves single 6-rings A, B or C, double 6-rings AA, BB or CC or a combination of both. The stacking sequence of the members of the group are: Cancrinite Gmelinite Chabazite Offretite Erionite
AB MBB MBBCC AAB AABAAC
(30) (16) (14) (18) (17)
Levynite Afghanite Losod Liottite TMA-E(AB)
AABCCABBC ABABACAC ABAC ABABAC ABBACC
(19) (25) (27) (28) (29)
TABLE I. Classification and Crystallographic Data for Zeolites Typical Unlt Cell Contents Analcite Group Analcite
Framework Density
Crystal Data
Cubic Ia3d a=13.7A Monoclinic Am or A2 a=7.6, b=14.8, c=13.1A, y=112"
Laumontite
17.7
Natrolite -Group Natrolite Thomsonite Edinqtonite
Orthorhombic Fdd2 a=18.3, b=18.6, c=13.2A Orthorhombic Pnn2 a=l3.1, b=13.1, c=13.2A
17.8
Orthorhombic P2,2,2, a=9.6, b=9.7, c=6.5A
16.6
Chabazite Group Trigonal R3m a=13.2, c=15.1
Chabazite
Hexagonal Ph?/mmc a=13.8, c=lO.OA Hexagonal P63/mmc a=13.3, c=lS.lA Hexagonal ~ 6 m 2 a=l3.3, c=7.6P
Erionite Of fretite
Triqonal R3m a=13.3, c=23.OA Hexagonal P63/mmc
Linde L
Hexagonal P 6 / m a=18.4, c=7.5A Hexagonal P63/mmc
Af ghanlte
a=l2.8, c=lO.iA Losod
Hexagonal ~ 6 2 a=12.9, c=10.5A
Liottite
(CaNa2K2),(AlO,)
,,(Si02)
- (CaNa2K 2 ) n (SO4,C03,Cl)8.2H20 TMA-E
(AB)
Cancrinite
(Me N) 2Na7 (A10 ) (Si02)27.26H20 4 2 9
Hexagonal ~ 6 m 2 a=12.8, c=5.1A Hexagonal P63Immc a=13.3, c=15.2A Hexagonal Pb3 a=12.8, c=5. IA
~
17.7
Channel System
Reference
03 03
Typical Unit Cell Contents
Framework Density
Crystal Data
Channel system
Reference
Phillipsite Group Phillipsite
Monoclinic P21 /m
Gismondite
15.4 Monoclinic P21/a a=9.8, b=10.0 c=10.6A, y=90° Monoclinic PC 18.3 a=6.7,b=14.0,c=10DA,B=112°
Yugawaralite
15.8
Orthorhombic Pna2 a=10.3, b=8.2, c=5.OA
Li A(BW)
19.0
Heulandite Group Heulandite
Monoclinic C m 17.0 a=17.7,b=17.9,~=7.4A,B=~16~
Brewsterite
Monoclinic P2l/m
'
17.5
a=6.8,b=17.5,~=7.7A,B=95~
16.9 Monoclinic F 2/m a=13.6,b=18.2,c=17.8A,B=9l0
Stilbite Mordenite Group
Orthorhombic Cmcm a=18.l,b=20.5,~=7.5A
17.2
Ferrierite
Orthorhombic Imnun a=i9.2,b=14.l,c=7.5A
17.7
Dachiardite
Monoclinic C2/m 17.3 a=18.7,b=7.5,~=10.3A,B=108~
Bikitaite
Monoclinic P2 1 20.2 a=7.6,b=8.6,~=5.OA,y=114~
Epistilbite
Monoclinic C2/m 18.0 a=8.9,b=17.7,~=10.2,B=124~ Orthorhombic Pnma 17.9
Mordenite
ZSM-5
Na, (AlO,), (SiO,) 40.24H20
a=20.l,b=19.9,~=13.4A Tetragonal 15m2 a=20.1, c=13.4A
17.7
(32,331
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TABLE 11. Zeolite Species Isotypes Zeolite
Isotype Species
Analcite
CaD (61), Kehoite (62), Leucite,NaB (63), Pollucite, Viseite (64), Wairakite (65)
Laumontite
Leonhardite (66)
Natrolite
Scolecite, Laubanite
Thomsonite
Gonnardite
Edingtonite
K-F (67)
Chabazite
Herschelite, Linde D (68), Linde K (69)
Gmelinit e Offretite-Erionite
s (70) Linde T (68), O (71)
Levynite
ZK-20 (72)
Eazzite
Omega (73), ZSM-4 (74)
Linde L
BaG (75), P-L (76)
Cancrinite
Cancrinite Hydrate (77)
Phillipsite
Harmatome, Wellsite (78), ZK-19 (79)
Gismondite
Linde B (80), Garronite (81), PC (82), Pt (82), P (70)
Yugawaralite
Sr-Q (75)
LiA (BW)
CaAlSiOq (83), RbAlSiO4 (83)
Feulandite
Clinoptilolite (84)
Stilbite
Barrerite (85), Desmine
Mordenite
Na-D (63), Ptilolite, Zeolon
Ferrierite
Sr-D (75)
3achiardite
Svetlozarite (86)
Faujasite
Linde X (87), Linde Y (88)
Linde A
Alpha (89), ZK-4 (go), ZK-21 (91,92), ZK-22 (91,92)
: 2
5
BaP (93), BaQ (93), P(C1)
(931, Q(Br) (93)
Xerlinoite
K-M (67), Linde W (80)
Sodalite
Basic sodalite (63), Danalite, Hydroxysodalit: (94), Nosean, Tetracalcium trialuminate (95), Tugputite (96), Ultramarine, Zh (97)
Figure 5 Framework of m a z z i t e (a) c - p r o j e c t i o n ( b ) l i n k i n g of g m e l i n i t e columns
Figure 6 Three Types of O-ring Chains UUDD, UDUD, and UDUU (117)
Figure 7 Secondary U n i t s i n (a) h e u l a n d i t e group ( b ) mordenite group ( 2 )
Figure 8 Framework P r o j e c t i o n s Along Main Channels of (a) mordenite (b) d a c h i a r d i t e (c) f e r r i e r i t e (d) e p i s t i l b i t e ( e ) b i k i t a i t e
The framework of mazzite (21) consists of columns of gmelinite cages linked through 8-rings with alternate columns staggered by c/2 (Figure 5). If cancrinite cages are connected through double 6rings to form columns and these columns are linked through 6-rings to form 12-ring channels parallel to the C-axis, as in mazzite, the framework of Linde L (24) is formed. PHILLIPSITE GROUP The framework of members of this group are based on 4-rings with variations of U (up) and D (down) linkages. Three of the four such variations can be linked to form chains (Figure 6). Phillipsite (32,33) and gismondite (34,35) consist of cross-linked UUDD chains. Li-A(BV) (37) and yugawaralite (36) consist of linked single &rings. EEULANDITE GROUP
A building block containing four 5-rings and two 4-rings is common to the framework of all the members of this group. If linked throug?. a common edge, chains are formed (Figure 3b) which when linked together yield brewsterite (40). Linking these blocks through common vertices yield chains which are constituents of heulandite (38,39) and stilbite (41,42). This group of structures contain some 5-rings. MORDENITE GROUP The secondary building block consisting of four 5-rings is common to all members of this group except bikitaite. In mordenite and dachiardite they are linked to form complex chains which in turn are linked in different ways (2). Epistilbite and ferrierite are lamellar structures and they also contain the building block in Figure 7b. The lamellae are normal to a in ferrierite and to b in epistilbite. The projections along the main channels are shown in Figures 8a-8e (2). The projection of bikitaite along b is essentially the same as the projection of epistilbite along a and dachiardite along c. ZSM-5 and ZSM-11 are better described by the chain in Figure 3c. The configurational unit in this chain contains eight 5-rings. If these chains are linked so that alternate pairs are related by a reflection, a layer is formed which is the basic layer in the ZSM-5 and ZS11-11 structures. If this layer is linked so that alternate layers are related by a reflection, S, the ZSM-11 framework is formed (57). If the layers are linked such that alternate layers are related by an inversion I, the ZSM-5 framework results (49). If the stacking sequence is varied, a family of structures results with ZSM-5 and ZSM-11 as the end members. With the lattice parameter doubled there are only two possible stacking sequences, SSII and SISI. The framework for SSII is shown in Figure 9. This variation in stacking sequence has been confirmed by Thomas and Millward (98) with lattice imaging using ultra high resolution electron microscopy.
Figure 10 Arrangement of A Cages in (b) Rho ( c ) Paulingite
(a) ZK-5
Figure 1 1 Polyhedral Units in ZSM-39 and Melanophlogite Structures (a) 12-hedron (b) 14-hedron (c) 15-hedron (d) 16-hedron
Figure 12 Melanophlogit e Framework
FAUJASITE GROUP There are three types of cages in this group, sodalite (30), A and ZK-5. Linde A (55) is formed by linking sodalite cages through double 4-rings. The large cages in Linde A will be referred to as the A cage. If the A cages are linked through double 6-rings, the framework of ZK-5 is formed (58). If they are linked through double 8-rings, the rho (51a) structure results (Figure 10). The cage formed by the octagonal faces of the A cages in ZK-5 is analagous to the gmelinite cage and will be referred to as the ZK-5 cage. Columns of ZK-5 cages linked through double 8-rings when connected together form the merlinoite (53a)framework. Feldspar and phillipsite are closely related to merlinoite formed by sliding layers such that the double 8-rings are broken up. Figure 10a shows part of the framework of paulingite (52a), the sequence of cages along the cubic axis A, ZK-5, ZR-5, A each connected to the other through double 8-rings. No one cage can be considered as the building block, the combination of cages is required. The faujasite framework (52) consists of sodalite cages linked through double 6-rings. If layers of these connected sodalite cages are linked in an ABC sequence the framework is that of faujasite with Fd3m symmetry, the same as diamond. If the stacking sequence is changed to AB, the result is a hexagonal structure. There are now five 12-ring openings to the large cavity compared to four in faujasite. The c parameter and the number of stacking sequences is dependent on the number of layers in the identity period. ZSM-3 (59) was found to be hexagonal with a=17.5 and c=129 A indicating a nine layer stacking sequence. Sodalite cages linked through common 6-rings form the sodalite framework (30). Falth and Anderrson (54a) found that Linde N, a cubic structure with a=36.9AY was an intergrowth of ZK-5 and sodalite. YELANOPHLOGITE GROUP The relationship between hydrogen bonds linking oxygens in gas hydrates (clathrates) and oxygens linking T-atoms in zeolites is well known. Hexagonal ice I is isostructural with 6-tridymite (58a). Appleman (57a) found that the rare mineral melanopnlogite is isostructural with the 12A cubic gas hydrate. The melanophlogite framework consists of interwoven layers of 12 and 14-hedra (Figures 11 and 12). It is a dense structure with only 5 and 6-ring openings. ZSK39 (59a), a high silica synthetic zeolite, was found to be isostructural with the 17A cubic gas hydrate. The ZSM-39 framework consists of 12 and 16-hedra as shown in Figure 11 , with layers of face sharing 12-hedra arranged as in Figure 13. These layers are stacked
F i g u r e 13
ZSM-39 Framework
F i g u r e 14 L o v d a r i t e Framework ( a ) a - a x i s p r o j e c t i o n (b) c - a x i s p r o j e c t i o n
in ABC sequence. The openings in the framework are limited to 5 and 6-rings. Zeolite analogs of water clathrate structures which typically consist of cages comprising of 5-rings constitute a new family of zeolites. LOVDARITE GROUP Lovdarite (60), a unique beryllium zeolite, has a framework consisting of 4 and 8-rings linked via corner sharing 3-rings. It has a two dimensional intersecting channel system bounded by 9-rings (Figure 14). The 9-rings are unique as no other known zeolite has a 9ring channel system. The 129' equilibrium angle of the Be-0-Si linkage indicates that the three-membered rings are not strained. T-ATOM DISTRIBUTION It is desirable to have some knowledge regarding T-atom distribution in zeolites. For several zeolites there is unequivocal evidence of Si,Al ordering (natrolite and gismondite). X-ray and other evidence are consistent with random ordering of T-atoms. The x-ray evidence for the occupancy of tetrahedral sites by Si or Al is based on the Si,Al-0 interatomic distances which differ by about .13A (100). This requires accurate atomic coordinates. The available evidence is also in accord with Lowenstein's rule forbidding A1-0-A1 bonds. Olson (101) found that hydrated Linde X crystals had Fd3 rather than Fd3m symmetry. The cell content prohibits complete ordering (88 Al, 104 Si). It was determined (102) that A1,Si alternate in the 4-rings in mordenite. CATION LOCATION Precise information as to cation positions in zeolites is still rather limited as faults,thermal and positional disorder, partial occupancy act as hindrances. The cation sites and their population in dehydrated mordenite are given in Table 111.
TABLE 111. Site I or I'
_
H
_
.6Na
N_a
Cation Population in Mordenite _
K-
-
Rb -
-
Cs
3.1
II
1.7 3.3
3.6
3.8
111 IV
Ca
Ba 0.3Ca 1.9Ba
0.6
0.3Ba
2.6
3.0
3.1
1.9
0.5
1.1Ba
SG
Cmcm
Pbcn
Pbcn
Pbcn
P2lcn?
Cmcm
Pbcn
Ref.
(103)
(104)
(105)
(106)
(107)
(108)
(109)
Site I lies at the end of the side pocket and is too small for large cations. Site I1 is in the side pocket at the center of the 8-ring. Site IV is in the center of another 8-ring at the junction of the side pocket with the main channel. This site is occupied by all types of cations. Site VI is coordinated to oxygens in wallsof the main channel. Lowering of space groups is partially due to displacement of cations. The number and position of cations is a function of temperature and degree of hydration. The cation site on the threefold axis, outside the 6-rings in ZK-5 is fully occupied in the hydrated state, while at 150°C it is empty (58). In calcium exchanged offretite and erionite the calcium displaces the R from the center of the cancrinite cages on dehydration (110,111). In Ce exchanged faujasite the ~e* ions occupy sites in the center of the 12-ring. On dehydration the cerium ions move into the center of the sodalite cage where each metal ion is coordinated to three framework oxygen ions with Ce-0=2.52, f. O ~ Aand up to three water oxygens with Ce-0=2.44 2.08 (112). This sodalite cage complex is highly stable. STACKING FAULTS The occurrence of stacking faults in a number of zeolites is quite prevalent. Stacking faults can be detected by the presence of broad odd R lines in the diffraction pattern of offretite, by the presence of contrast lines in transmission electron micrographs of erionite (113),by lattice images of ZSM-5 using high resolution electron microscopy (98). STRUCTURE DETERMINATION With increasing knowledge of zeolite frameworks there is a considerable understanding of zeolite chemical principles but many zeolites have been synthesized as small or larger but poorerquality crystals. Increased x-ray intensity sources should help in determining the structure of some of these crystals but if it is not possible to use single crystal methods then we have to turn to other methods. Information regarding lattice parameters, symmetry can be obtained from diffraction studies, estimates of channel dimensions from IR and diffusion studies, and ring ellipsisity from diffusion rates. Physical models using tetrahedral stars to represent T-atoms are connected via tubing to depict zeolite frameworks. These models are built to scale for rapid and accurate estimation of unit cell parameters and atomic coordinates and are useful for determining the symmetry of a structure. The basic building units can be readily identified in these open frameworks.
Trial models may be built using all the available information and satisfying all the known conditions. X-ray patterns can be simulated and compared to experimental ones. SIMULATION OF PATTERNS Interatomic distances and bond angles for zeolites of known composition can be predicted within fairly narrow limits. If the lattice paraaeters are known, the atomic coordinates of individual atoms can be adjusted so that the interatomic distances correspond as closely as possible with predicted distances. The atomic positional parameters can be computed from the prescribed interatomic distances, D y , by a least squares procedure which minimizesthe residual function
where wj is the weight ascribed to the interatomic distance of type j. This DLS (Distance Least Squares) method of refining the positional parameters was described by Meier and Villiger (114). This refinement gives idealized framework models using prescribed interatomic distances and unit cell constants for a given space group. The weight wj of each error equation is based on bonding considerations or observed bond length variations (115). In zeolites the Si-O bond length relation to the Si-0-Si bond angle is given by the function,
Convergence of the least squares refinement is usually rapid for chemically reasonable structures. A final "R-factor" is provided which can be used to estimate the "goodness" or chemical reasonableness of the structure. If the positional parameters of some of the atoms in a structure are determined by single crystal methods, the missing atoms may be located by model building as in the case of Linde N (54) and the structure further refined. Simulated structures can be altered in a manner not possible with actual structures. The size of the atoms can be changed by changing the interatomic distances. We can impose restrictions on the lattice parameters. We can put cations in prescribed locations. The DLS refinement will give us the changes in geometry. This should give us valuable insights into chemical principles. EFFECT OF STACKING FAULTS ON DIFFRACTION PATTERN Stacking faults can be determined by contrast lines in trarismt~sioa electron microscopy (113) and by lattice imaging (98) bat his is a tedious process and is applicable to individual crystals and not the bulk. In order to determine the effect of stackfriz faults t h c
Figure 15 Simulated Plots of Ferrierite
II
Ferrierite-Framework C a q o n s
A c t or=1
Mg C o m p l e x in Ferrieritd
,
v
,
.
AL
I
Li
" .-,--.. .-.
Actor=l
*I
Ferrierite
-
with Mg Comp ex R e m o v e d
!i
%I ,I
two structures are superimposed and an occupancy of 1 is assigned to all atoms which coincide and an occupancy of % to split atoms. The application of this method to offretite-erionite (116) resulted in the determination of the concentration of stacking faults and whether they were random or ordered. CONTRIBUTION OF CATIONS AND WATER 70 POWDER PATTERN The structure of ferrierite was determined by Vaughn (44). He found M~(H~o)~* cation complexes in the cavities with 8-rings. Difference in powder pattern intensities for samples from various locations were found to be due to the Mg complexes in the 8-ring cages. Smith plots of the contribution of the cation complexes (116) are shown in Figure 15. By obtaining plots of structures with partial occupancy better agreement of powder data can be obtained. ACKNOWLEDGEMENTS
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J. M. Thomas and G. R. Millward, J. Chem. Soc. Chem. Commun. 1380 (1982). E. Garnier, P. Gravereau and A. Hardy, Acta. Cryst. B38, 1401 (1982) J. V. Smith and S. W. Bailey, Acta. Cryst. 16,801 (1963).
.
74, 2758 (1970). D. H. Olson, J. Phys. Chem. W. M. Meier, R. Meier and V. Gramlich, Z. Kristallogr. 329 (1978).
147,
W. J. Mortier, J. J. Pluth and J. V. Smith, Mater. Res. Bull. 10, 1319 (1975). -
J. L. Schlenker, J. J. Pluth and J. V. Smith, Mat. Res. Bull. 14, 751 (1979). W. J. Mortier, J. J. Pluth and J. V. Smith, "Mineralogy and Geology of Natural Zeolites", F. A. Mumpton, Ed., Mineral Soc. Am. Short Course Notes Vol. 4, p. 53.
J. L. Schlenker, J. J. Pluth and J. V. Smith, Mater. Res. Bull. 13, 77 (1978). -
J. L. Schlenker, J. J. Pluth and J. V. Smith, Mater. Res. Bull. 13, 901 (1978). W. J. Mortier, J. J. Pluth and J. V. Smith, Mater. Res. Bull. 10, 1037 (1975). J. L. Schlenker, J. J. Pluth and J. V. Smith, Mater. Res. Bull.
13, 169 (1978). -
G. T. Kokotailo and S. L. Lawton, US Patent 3,640,680 (1972).
111.
J. L. Schlenker, J. J. Pluth and J. V. Smith, Acta. Cryst. B33, 3265 (1977).
112. D. H. Olson, G. T. Kokotailo, J. F. Charnell, 41st Colloid. Symposium, Buffalo, NY, June 1967. 113.
G. T. Kokotailo, S. Sawruk and S. L. Lawton, Am. Mineral. 57, 439 (1972). -
114. W. M. Meier and H. Villiger, Z. Kristallogr. 129, 411 (1969). 2, 3 (1977). 115. W. H. Baur, Phys. and Chem. Minerals 116. G. T. Kokotailo and J. L. Schlenker, Advances in X-ray Analysis, Vol. 24 (Plenum Publ. Corp) (l981), p. 49.
117.
S. Merlino, Soc. Ital. diMineral. e Petrogr., Rendecorti 31, 513 (1975). -
SYNTHESIS OF ZEOLITES, AN OVERVIEW
L. Deane Kollrnann Mobil Research
& Development Corporation Central Research Division
PO Box 1025 Princeton, NJ 08540
Zeolites have been an important component in petroleum process technology for mare than twenty years -- and there is considerable indication that kheir role will continue and will expand as utilization of our energy xesource base broadens worldwide. Despite the large and growing number of distinctive structures and compositions, only a handful1 of zeolites, nearly all synthetic, have achieved commercial significance. This paper is directed at the preparation of zeolites of this special group. The first section develops general principles and techniques in the synthesis of these zeolites, particularly the concepts of ternplating and compositional control. The second presents examples, to illustrate practical application of these concepts. Composition in zeolites can be controlled during or after synthesis. In the third section, an example of the lakter is discussed, the dealuminization of mordenite.
The synthesis of zeolites is an art better chronicled in the patent literature perhaps than in classical scientific publications, Nevertheless, several books and review articles can be cited which will provide useful background information and supplementary detail (1-10). The discussion below summarizes and updates, where appropriate, those reviews. In the interest of clarity, discussion is restricted to three-dimensional,
crystalline networks whose framework composition is mol percent Si02, the remainder being Al2O3.
at
least
50
PATTERNS
Most commonly, zeolite crystallization is a nucleation-cantrolled process occurring from molecularly inhomogeneous, alkaline, aqueous gels, at temperatures between about 80 and 300°C, The particular framework structure which crystallizes can be strongly dependent on the cations present in the gel. m Reaction mixture composition in a crystallization experiment is best defined by the set of mol ratios given in Table 1 (10).
Table I. Reaction Mixture Composition Mol Ratio sio2/A1203
Primary Influence Framework Composition Hate, crystallization mechanism Silicate molec. w t , OH- conc Structure, cation distribution Framework aluminum content
A large number of silica and alumina source materials can be used in formulating a gel, and the product obtained is often dependent on the sources selected and on their treatment prior to formulation. For example, Na2Si03, silica gel, silica sol, NaA102, aluminum sulfate, aluminum turnings or alumina itself are all routinely considered. Clays, either directly or after varying heat treatment, can be used as well (11). In describing a reaction mixture composition in terms of the above mol ratios, NaZSi03 i s treated as a mixture of Sio2 and NaOH; NaA102, as A1203 and NaOH; aluminum sulfate, as A1203 and H2S0+, each with the appropriate amount of accompanying H20. Hydroxide in the O H - ~ S ~ O ratio ~ is calculated by subtracting equivalents of acid added from those of hydroxide. It is not hydroxide i o n concentration in the resultant mixture. Organics such as m i n e s , for example, are never included i n calculating OH-/Si02 ratios. In careful work, and as OH-/Si02 ratios approach zero, it is important to recognize that A1203 is incorporated into a zeolite framework as ~ 1 0 ~ -i.e., , each rnol acts as two
equivalents of acid and consumes two equivalents of hydroxide: A l 2 G 3 f 20H2A102 + +
To a first approximation, the
mol ratios in Table 1 can be divided according to primary functi.on. S i 0 2 / A 1 2 0 3 , for example, places a constraint on the framework composition of the zeolite produced, and it may define the structural competitors in a nucleation-controlled experiment. With the exception of the aluminum-rich NaA, zeolites normally incorporate all of the aluminum present in a reaction mixture into their framework structure, leaving varying amounts of silica ( silicate ) in solution according to hydroxide ion concentration, reaction conditions, etc . a.nd OH-/Si02 strongly influence the "molecular" or "polymeric" species present in a reaction mixture composition and the rate at which those species interconvert, by hydrolysi,s, to form the ordered, three-dimensional network of the zeolite hydrolysis rates and product. Through their control of polysilicate (or polyaluminosilicate) distribution, they can significantly influence the "winner" among competing, possibly metastable structures in a crystallization experiment. EI20/SiO2
Cations present in a reaction mixture are often the dominant factox determining which zeolite structure is obtained, as will be discussed below. In addition, their incorporation and presence in a product can be important considerations in subsequent use and handling. Anticipating that discussion, literature data (2) for the synthesis of four different zeolites, Y, Omega, L and TMA-O, are given in Table 3 1 . The primary variable, it is asserted, was the cation. (Each preparation was static, I U U ° C , w i t h sio2/A1203 = 16-20, HZO/Si02 = 14-25, o~-/.Sio,= 0.7-0.9. )
Table 11.
Zeolite Y
omega L TMA-0
Product Dependence on Cation
N ~ + s ~ u ~
TMA+/S~O~
K+/SIO~
0.8 0.6 0.0 0.0
As a result of small changes in the cation content of the reaction rn~eure, completely different zeolite products were obtained .
Product integrity is best "authentic" sample.
r
judged
by
comparison with
an
Zeolite identification is made largely on the basis of X-ray diffraction, and powder patterns are the common measure of purity in a crystallization product. If a pattern shows no evidence for crystalli.ne or amorphous contaminants, purity is estimated by comparing intensities of reflections (at d-spacings smaller than about 6 A) with those of an authentic sample o f the same composition and crystal size. Reflections at higher d-spacing are not used in estimating crystallinity since their intensities are dependent on variables such as moisture content. Except for such large scale commercial products as N a A and Nay, "authentic" samples are normally obtained by repeated and varied crystallization experiments, Once the X-ray diffraction pattern of a preparation is defined, elemental analysis becomes useful, to corroborate assertions of purity, to detail cation content and to critically explore the "ternplating" in a particular crystallization question of sequence. In contrast to more conventional chemical synthesis, elemental analysis is generally not a satisfactory or sufficient criterion for purity in zeolites. Almost all zeolite structures have been prepared in a range of framework compositions, as will be shown below. X-ray diffraction, supported by elemental analysis, can often directly probe composition within a particular zeolite framework. Sorptive and cation exchange properties are a third measure of among well-characterized zeolites. For product integrity example, high purity samples of N a A and N a Y will sorb about 25 g H20 per 100 g of dry zeolite ( 2 5 " C , 1-4 torr). HZSM-5 and N a Y samples should respectively sorb about 11 and 19 percent of their dry weight in n-hexane ( 2 5 O C , 10-20 torr). an as-synthesized, high p u x i t y zeolite sample will conventionally contain at least one cation for every aluminum in the framework. Microscopy is the fourth essential ingredient in product characterization. crystal size and crystal morphology are both important, controllable aspects of a synthesis experiment. Moreover, recent electron microbe analysis of aluminum distribution within individual zeolite crystals offers significant insight into actual crystallization processes, as will be discussed below (12).
a
Composition i s a variable for any given framework structure.
When composition is cited in recent patents on zeolites, a range is almost always presented. Although the limits o f , this range may not be accurately known, it is now widely recognized that no elemental composition is unique to any s p e c i f i c framework structure. Table I11 presents examples which span the Si:Al range from L : 1 to infinity.
Table 111. Variable Composition in Zeolites Framework
~ame
sio2/Al2O3
Linde A N-A
2 2.5 to 6 2.5 to 4
ZK-4
Sodalite Sodalite
Faujasite Faujasite ZSM-5
Sodalite TMA Sodalite
2 10 2 to 3 3 to 6
Linde X Linde Y ZSM-5
References
5
to infinity
16
mere is abundant evidence in the literature to show that these ranges indeed reflect differing framework compositions ( 2, 17-22j
.
That composition can be controlled by cation type was originally discovered for the A framework (13,141. It has been emphatically demonstrated with sodalite ( 15 ), where two "model" compositions are obtained, depending on which cations are present during crystallization. When sodium ions are used, the sodalite obtained has a composition Na3Al3Si3OI2; when tetramethylammonium ions are used in place of sodium, (CH3)4NA1Si5012 is produced. Bath cornpositions have the same framework structure, a three-dimensional network built entirely of truncated octahedra. Each octahedron in TMA sodalite contains (and can accomodate) only one tetramethylammonium ion, thereby restricting ( by the requirement of charge balance) the number of negatively charged AL04 tetrahedra. On average, three sodium ions are present in each octahedron in Na sodalite. Framework composition can often be varied by simply changing the SiU2/'A.L203 ratio of the reaction mixture. ZSM-5 and the synthetic faujasites are examples cited in Table 111. With ZSM-5, framework compositions ranging to over 20,000 can be easily prepared by this technique. Very high Si02/A1203 ratios require special effort to
e x c l u d e a d v e n t i t i o u s aluminum. N u c l e a t i o n and c r y s t a l growth may o c c u r w i t h d i f f e r i n g f a c i l i t y as t h e Si02/A12U3 r a t i o o f a r e a c t i o n m i x t u r e i s changed. crystals may grow f o r example i n a Si02/A1203 range where n u c l e a t i o n would b e d i f f i c u l t - and v i c e v e r s a . S e e d i n g , low-temperature a g i n g and the u s e o f r e a c t i v e g e l s o r d i s p e r s i o n s are all p r a c t i c a l t e c h n i q u e s f o r c o n t r o l l i n g z e o l i t e p u r i t y and/or composition. me synthesis of z e o l l t e Y illustrates the low-temperature aging approach. Four r e a c t i o n m i x t u r e s w e r e p r e p a r e d uslng s l l l c a s o l , NaA102 and NaOH, two a t S102/A1203 = 10 and two a t S102/fi1203 = 30. K u l r a t i o s i n both sets were 08-/S1O2 = 0 . 6 - 0 . 7 , Na / S 1 O Z = 0.8 a n d H 2 0 / S ~ 0 2 = 16, One from e a c h p a l r w a s p l a c e d ~ m m e d l a t e l y l n t o a steam chest at 90 - 95'C; the second was aged f o r 2 4 h o u r s a t room t e m p e r a t u r e b e f o r e h e a t l n g . The r e s u l t s are p r e s e n t e d In T a b l e IV. Except where n o t e d , the Y s a m p l e s w e r e a l l 95 - 1001 c r y s t a l l m e .
Table I V .
Initial Si02/A1203
10
S y n t h e s i s of Z e o l i t e Y
Aging ( hours )
24
crystn (days )
5
Product Zeolite
Y
(trace P )
Product Si02/A12CJ3
5.3
I t i s g e n e r a l l y a c c e p t e d that c r y s t a l l i z a t i o n o f Y w i l l c o n t i n u e u n t i l t h e aluminum i n a r e a c t i o n mixture i s e x h a u s t e d . S u p p o r t i n g that a s s e r t i o n was the fact t h a t , in all t h e above cases, y i e l d exceeded 90% based on a l u m i n a . As a r e s u l t , there w e r e large d i f f e r e n c e s i n r e s i d u a l silica (soluble s i l i c a t e ) i n the two cases. At Si02/A1203 = 1 0 , 50 - 60% o f t h e s i l i c a w a s incorporated into t h e zeolite product; at Si02/A1203 = 30, only a b o u t 20%.
Low-temperature a g i n g i s commonly used s i l i c a sols, and i t s p r i m a r y f u n c t i o n "equilibration of t h e h e t e r o g e n e o u s gel It is l i k e l y to be i m p o r t a n t i n t h e w e l l , however ( 23-26 ), which i n t r o d u c e s
to obtain p u r e Y f r o m i s probably p r e - d i g e s t i o n , w i t h t h e solution" ( 2 ) . i n i t i a l n u c l e a t i o n step as t h e n e x t assertion.
Composition is often variable even within crystals.
individual zeolite
NaX seeds have been frequently used to initiate crystallization of zeolite Y (23), and they of course introduce compositional
heterogeneity into the crystals which result. Sil~ceous external shells have been grown onto aluminum-containing ZSM-5 crystals (27). Evidence is i.ncreasing however that non-uniform composition may be a common, intrinsic characteristic of synthetic zeolite crystals. Such a result should not be particularly surprising since both solution and gel composition are continuously changing during the course of a crystallization (i.e., as zeolite product is formed, effectively removing constituents from the reaction mixture ) . In a large-crystal NaX preparation, for example, there is microprobe evidence that SiO2/Al2O3 ratio increases with increasing distance from the crystal core (28 ). X-ray photoelectron spectroscopy ( X P S ) data show aluminum depletion in the surface of NaA, X, Y, and synthetic mordenite czysta1.s t 29 ) . Changes in unit cell dimension during crystallization have been cited as evidence for S i O Z j A 1 2 0 3 gradients in NaY ( 2 4 1 , a conclusion however which relies heavily on X-ray diffraction analysis of partially crystalline materials.
Large-crystal ZSM-5 preparations have provided the most striking examgle of intrinsi.~ cornpositional heterogeneity to date. Aluminum content increased from core to outer crystal rim, the respective concentrations sometimes differing by a factor of 10 or more (12,30). One note of caution is warranted in generalizing from these observations however. Special techniques have been required to obtain zeolite crystals sufficiently large to permit electron microprobe analysis. These techniques may influence is aluminum distribution in the crystals produced. It nevertheless probable that gradients e x i s t in more conventionally prepared, smallex crystals. Modern analytical techniques will soon deflne site occupancy witbxn zeollte crystals on an atomic level and may contribute to reaction mlxture definition as w e l l . Until recently there has been no dixect probe for aluminum or silicon siting in zeolites, and assertions have been made regarding only average (distributed) siting, 1.argelyon the basis of X-ray diffraction data, sorption, ion exchange or catalytic properties. Nuclear magnetic resonance (NMK) promises to provide the desired direct probe.
In very strong magnetic fields, high resolution magic angle spinning N M N spectra have been obtained far both 2 9 ~ i( I = 1/2, 4 . 7 % abundance ) and 2 7 ~ 1 (I = 5 / 2 , 1008) in a variety of zeolites (31-38). In structures based on 4-membered rings, 2 9 ~ ichemical shift differences clearly distinguish SiU4 tetrahedra according to the number of ~ 1 neighbors 0 ~ (0,1, 2, 3...) (34,37). In more complex structures, like ZSM-5, numerous crystallographically distint sites exist for Si atoms in the lattice, and numerous different Si04 types have been detected ( 36 ) .
a direct probe for aluminum within zeolite crystals and has confirmed that Al is tetrahedrally coordinated In the within a multitude of framework structuxes ( 3 5 ) . ultrastabilization and dealuminization of synthetic faujasites, both tetrahedral and octahedral (non-framework) aluminums have been detected (37). Only the tetrahedral type was present in the original Nay. In "silicalite" 2 7 ~ 1showed all the aluminum to be tetrahedrally coordinated, with at least two distinct environments, and the authors concluded that, structurally, silicalite and zsM-5 are essentially indistinguishable (36). NMK further provides
Organic (and inorganic) catsons can framework structures.
"template" particular
a While the ability of organics to alter the course of crystallization process is becoming increasingly apparent, the specific function of those organics is sensitively dependent on the details of a given experiment. In general, additionof organics to a reaction mixture can effect changes of four types: (a) A different zeolite structure is obtained; (b) Crystallization rate is strongly enhanced (or inhibitedl ); (c) The same framework is obtained but with a significantly new chemical composition; and (d) "Microscopic texture", e.g., crystal size, habit, etc. (39), is altered. It is not uncommon i n exploratory crystallization experiments for organics to be superfluous, i,e,, to simply provide cation balance for varying hydroxide. Only the first and the second of the above (and the second only when the rate is enhanced) represent "templating effects" and then only if it can be shown that the change is not due to the virtually inevitable system perturbations which accompany a new reaction mixture component. A striking example of the first type was found in crystallization experiments with cationic polyelectrolytes ( 40 ) , and it demonstrates the detailed analysis which must be performed before "templating" can be suggested. Several relatively low molecular weight polymers of the following type were prepared by reaction of 1,4-diazaBicyclo[2.2.2Joctane (Dabco) with the compounds Br(CX2 ),Br:
where n = 3 , 4 , 5 , 6 a n d 10 a n d x = 10-60. W i t h 1,4-dibromobutane for example, t h e p o l y m e r was d e s i g n a t e d "Dab-4 B r " . When a d d e d t o a r e a c t i o n m i x t u r e t h a t p r o d u c e d zeolite Y ( a n d / o r P ) a t 85-90°C, t h e s e polymers p r o d u c e d dramatic c h a n g e s as shown i n Table v ( s1O2/Al2o3
= 30, B 2 0 / S i D 2
= 20, 0 H / S i 0 2
=
1.2,
Na/Si02
=
3.2,
3-13 days).
Table V .
Polymer
N+/s~o*
o
None Dab-4
Polymer E f f e c t s i n C r y s t a l l i z a t i o n
Br
Dab-4 Br Dab-4 Br Dab-4 Br
0.01 0.14 0.23 0.43
Product Z e o l i t e ( s ) Y + P Gmelinite ( faulted ) G l n e l i n i t e ( faulted ) Pure gmelinite Amorphous
T h e r e s u l t s show clearly t h a t a very small amount of organnc can completely alter t h e course of these nucleation-controlled
reactions. Moreover, with these polyelectrolytes, a large excess, which cannot be accommodated w i t h i n the zeolite product, actually inhibits all crystallizatian. " t e r n p l a t i n g " the gmelinite structure, the r e s u l t s s h o u l d be s e n s i t i v e t o c h a n g e s i n p o l y e l e c t r o l y t e m o l e c u l a r s t r u c t u r e . T a b l e VI shows t h a t t h i s i s indeed the case. A series o f p o l y e l e c t r o I y t e s , t o g e t h e r with t h e monomeric analogs (prepared by t h e r e a c t , i o n of D a b c o w i t h propyl o r b u t y l b r o m i d e or i o d i d e ) , was s u b s t i t u t e d f o r Dab-4 B r . O n l y Dab-4, -5 and -6 p o l y e l e c t r o l y t e s were e f f e c t i v e i n p r o d u c i n g gmelinite. If t h e Dab-4 Br i n T a b l e V i s i n d e e d
Table K t .
Polymer Dependence in Crystallization
Organic
Product Zeolite( s )
Dab-3 Elr
P Pure gmelinite Gmelinite ( faulted ) Gmelinite ( faulted ) Y + P
Dab-4 Br Dab-5 Br Dab+ Br D a b - 1 0 Br
In theory, gmelinite has a large, 12-ring pore system which should readily admit molecules such as cyclohexane. In fact, both natural and synthetic gmelinites behave like small-pore zeolites, a behavior attributable to chabazite stacking faults. chabazite fault planes, often observable by x-ray diffraction, effectively block or restrict access to the large gmelinite channels (41). The "pure gmelinites" described in Tables V and VI are believed to be f a u l t - f r e e , They sorb 7 . 3 9 cyclohexane. For comparison, a natural (faulted) sample soxbed only 1.0%. It is proposed that the plyelectrolyte is present in the pore as the gmelinite framework forms, the polymeric nature of the organic preventing formation of stacking faults across that pore. If the polymer is an integral part of the product structure, located in t.he pores, certain size and charge balance requirements must be fulfilled. The unit cell in gmelinite is traversed by a single 12-ring channel (7-8 A in diameter) for a distance of 10.0 A.
The D a b c o unit is cylindrical with a dlameter of about 6.1 A and can thus fit comfortably within the gmelinite pore. In length, the repeating units of Dab-3, -4, -5, -6 and -10 measure 7.5, 8.7, 9.9, 11.0 and 14.5 A . Comparing these measurements with the results in Table VX, all polymers which effected gmelinite synthesis had repeating units 9-11 A in length, matching the unit cell dimension of the pore. Charge balance is the second constraint. A repeating unit of the Dab-4 p o l y m e r contains two equivalents of cation and extends about 8.7 A. The unit cell in gmelinite contains a single 12-ring channel 10 A in length and could therefore hold little more than two quaternary cations, Seven different gmelinite preparations with Dab-4 Br averaged 2.3 N/unit cell. Furthermore, elemental analysis suggested that the polymer was intact. The C/N atomic ratio in the seven samples averaged 5 . 4 , compared with 5,l in t,he
srlginal polymer. Once encapsulated within a zeollte framework, quaternary ammonium cations are protected from hydrolysis.
It is well known that quaternary ammonium cations decompose readily in hot. caustic. As the number of examples o f their successful use in zeolite synthesis increases however, it is ~nstructive to examlne / ratios in the synthesis products. Table VI I shows remarkably little dl fference between the C/N expected if the organic remained intact and that found in four as-synthesized zeolites:
Table V I I .
Average C/N Ratios in Zeolite Products c/N Atomic Ratio
Zeolite
Organic
Uffretite
TMA
TMA Sodalit-e
TMA
Dab-4 Gmelinite
Dab-4
!rMA
Expected
5.1
Found
( ref. )
5.4 (40)
That these organics can indeed be intact within a product zeollte has now been directly demonstrated by 13c N M H on TPA ZSM-5 ( 4 4 ) .
EXRMPLES
IN SYNTHESIS
Patent literature is the primary information source in zeolite synthesis, but a set of instructional preparations has been assembled which require no specialized equipment and which can serve as an introduction to experimentation in the area. Details of those preparations will be published shortly (43), but an abbreviated version is presented here for easy reference. Three examples will be given, describing synthesis routes to zeolites A , Y and ZSM-5. They thus include framework compositions known to occur with S i 0 2 / A 1 2 0 3 ratios from L:l to essentially infinity; they demonstrate such techniques as low-temperature nucleation, templating and variable reactant sources. [A fourth example, TMA Uffretite, is given in the above reference but need not be included here.)
r
Zeolite A, a preparation for freshman chemistry.
bench-scale Crystals of N a A can be made in 3-4 hours, a preparation requiring only a stirred, heated beaker. A boiling solution of sodium aluminate and NaOB is added'to one of sodium meta-silicate, and the resultant mixture is heated with stirring at about 90°C until the suspension will settle quickly when the stirring is stopped. The suspension is then filtered (hot), washed repeatedly with water and dried at about 110°C to yield an 80-90% yield of N a 2 0 ' A 1 2 0 3 ' 2 S i 0 2 ' - 4 H 2 0 . Product purity is determined by comparing the X-ray diffraction pattern of the solid with that of an authentic sample of NaA. fie product, after dehydration at 350-40U°C, should sorb about 25% of its weight in water. In the
crystallization, reactants per mole of S i 0 2 are one 20 ( NaOH) and 550 ( H2(3), the water being divided between ( A 1 2 0 3 ), (Commercial aluminate and silicate solutions in the ratio 3:2. sodium aluminate analyzes about 40% A 1 2 0 3 , 33% NaZO and 27% water. )
Zeolite Y, low-temperature aging.
solution of sodium aluminate and NaOH is added, with vigorous stirring, to 30% silica sol (a colloidal silica suspension). The resultant mixture is aged at room temperature for 2-3 days and then crystallized in a steam chest (about 9 5 ' ~no~ stirring) for 1-2 weeks. Solid is withdrawn, filtered, and analyzed by X-ray diffraction every 2-3 days until Nay purity (diffraction pattern intensity) reaches a limiting value. After hot filtration, washing and drying, a 5U-6O% yield (based on SiOZ) is obtained of NaZO'A1203'5.3Si02'5~ with an approximate composition Without the aging, the S i 0 2 / A l Z 0 3 ratio of the product will not exceed 5 and P will be a common contaminant. As with NaA, the purity of the NaY preparation is determined by comparing its X-ray diff ractivn pattern with that of a authentic sample. The sample should sorb 25% of its weight in water, after dehydration. A
Moles of zeackants per mo1.e of silica should be 0.1 (ALZ03), 0 . 8 ( N a O H ) and 16 (water), with the water evenly divided between the silica suspension and the sadium aluminate solution.
TPA ZSM-5, probable templating.
A solution of sodium aluminate and NaOH is added simultaneously with one of TPA Br and H2SO4 to 16% silica sol, and the mixture is
lrnmediately mixed, to form a gel. Placed in a steam chest a-t: about 95°C and sampled periodically, the mixture will produce an 80-90s yield of ZSM-5 (based on Si02) in 10-14 days. Its molar cmposition will be approximately as follows : l.B(TPA)20'1.2Na20'1.3A12033100Si02'7H20. Again, the X-ray diffraction pattern should be compared with that of a known sample. A purified, dehydrated sample of ZSM-5 will sorb about 11% n-hexane, Solutions should be prepared such that moles of reactant per mole of silica are 0.012 ( ~ 1 ~ 00.54 ~ ) (N ~aOH), 0.1 (TPA Br), 0 . 2 (H2SU4) and 45 ( B Z u ) . sodium-stabilized 30% silica sol is the starting material in above preparation. Additional water is divided among the various solutions in the ratio, one (aluminate): two ( W A Br): one (silica sol ) . Higher temperatures, for example 140-180'~~will reduce crystallization time. Furthermore, with' appropriate adjustment for acid and base, no aluminum need be added to the reaction mixture. In that case, only the aluminum present as a contaminant in the various other reactants will be found in the product ZSM-5.
is one of a number of framework structures which can be crystallized with essentially no alumina. Several other structures can apparently exist in or near that compositional (45,461, but must first be synthesized in an range aluminum-containing form. Although direct synthesis routes will likely be discovered, this section reviews experimental techniques for dealminizing these structures, with particular emphasis on the very siliceous compositions. ZSM-5
8 Acld extraction dealuminization.
alone
often
achieves
only
partial
The zeolites Y and mordenite are the most commonly targets for dealuminization, and a substantial literature exists on aluminum removal from both. In the case of Y, it is now generally accepted that controlled, direct addition of acid (such as ethylenediamine tetracetic acid (H4EDTA) can remove at least 5 0 % of the aluminum, to a SiO2/Al2O3 ratio of about 12 (47,481, without significant loss in crystallinity. With mordenite, direct acid leaching can remove up to about 80% of the aluminum (SiCJ2fA1203 = "60) without structure collapse ( 49)
.
Beyond this point, i .e., to a c h i e v e SiO2/'AI2O3 yati os 2309 confbined t h e r m a l a n d c h e m i c a l (acid) t r e a t m e n t s are r e q u i r e d , Samples of s y n t h e t i c f a u j a s i t e w i t h S i 0 2 / A 1 2 0 3 = 100-200 have been r e p o r t e d l y p r e p a r e d by a l t e r n a t e acid l e a c h i n g and s t e a m i n g of u l t r a s t a b l e Y' s ( 45 j . M o r d e n i t e s i n t h i s c o m p o s i t i o n r a n g e a r e p r e p a r e d by t h e r m a l a n d / o r h y d r o t h e r m a l t r e a t m e n t a t t e m p e r a t u r e s dbove 5 0 0 ° C , f o l l o w e d by v a r y i n g a c i d e x t r a c t i o n (46,49 ). Examples are g i v e n i n T a b l e V I I I
.
Table V I I Z . I n i t i a l Sample
D e a l u m i n i z a t i o n of Y a n d of Mordenite Treatment
Product
Reference
( Si02/A1203 1
( Si02/A1203 )
Y (5.3) Y (5.1)
H4EUTA,
U S Y ~( 5 . 2 )
2N HCI,
Hord (15) Moxd ( 1 5 )
6 N H C 1 , 1 h, reflux
slow addition Na2H2EDTA + slow HCl 1
h,
9 0 0 ~
Fau jasite ( 10 ) Faujasite (12)
48
Faujasite
45
47
(10s)
Mord ( 1 5 )
Mordenite ( 20 ) Mordenrte (59) Mordenlte ( 60)
49
6Nac1, 16 h, reflux 6N BCI, 24 h, reflux
Mord (15)
6 5 0 * ~ ,3h; 0.5NHU1,
Mordenite
49
( 73 )
49
49
16 h, refllux
a
-
"Ultrastable Y " , prepared by treatment under self-steaming conditions at 7 6 U - B 1 5 * ~ (45)
techniques for dealuminization, which promise to extend the range i n the above s t r u c t u r e s even further, have very recently appeared (50-52). When Nay was treated with SiC14, for example, a h i g h l y c r y s t a l l i n e , e s s e n t i a l l y aluminum-free faujasite r e p o r t e d l y r e s u l t e d ( 52 ) New
Si02/AL2U3
.
a
In many structures, aluminum removal is site-dependent.
Not all aluminum tetrahedra within a given, siliceous ( S i 0 2 / A 1 2 0 3 > 5 ) framework are e q u i v a l e n t . I n m o r d e n i t e , f o r example, f o u r d i f f e r e n t c r y s t a l l o g r a p h i c s i t e s f o r aluminum p o t e n t i a l l y e x i s t ( 49 ) Aluminum t e t r a h e d r a of d i f f e r i n g acidity are r e c o g n i z e d w i t h i n t h e s y n t h e t i c faujasite framework ( 53-55 ) I t is t h e r e f o r e very r e a s o n a b l e t o e x p e c t that aluminum removal, i . e . , ease of h y d r o l y s i s , w i l l be site d e p e n d e n t .
.
.
The s t r o n g e s t e v i d e n c e t o date f o r s u c h a n a s s e r t ~ o ni s the marked n o n - l i n e a r i t y of kh@ l a t t x c e p a r a m e t e r c o n t r a c t i o n s as aluminum
( framework charge) IS removed from mordenite (49). A very plausible correlation can be developed between the d i f f e r i n g a , b and c projections of t h e four p o t e n t i a l Alo4 sites and the respective non-linearities in t h e t h r e e lattice p a r m e t e r d e p e n d e n c i e s on aluminum content i n this orthorhombic unit cell. HOLY? powerful evidence c a n be e x p e c t e d as 2 Y ~ iand 2 7 ~ 1NMR techniques d e v e l o p . I n a d d i t i o n t o p r o v i d i n g a d i r e c t probe i n dealuminization, those t e c h n i q u e s should clarify a more basic and unanswered question, namely, what is the r e l a t i o n s h i p b e t w e e n an as-synthesized and a dealurninized s t r u c t u r e w i t h t h e same o v e r a l l cornposit ion?
T h l s b r i e f summary oE current thlnklng I n zeolite synthesis
draws e x t e s ~ s l v e l y on the numerous new zeolites a n d new preparation technxques d~scovered In Mobll's Prxnceton and Paulsboro Laboratories over the past 20-25 years. Thanks are due the many authors whose names appear i n the references and whose names wlll be found on p a t e n t s d e s c r ~ b l n g t h e s e dlscoverles f o r t h e ~ r valuable lnput, advlce and suggestions as my own experiments progressed.
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STUDY O N THE MECHANISM OF CRYSTALLIZATION O f ZEOLITES A , X AND Y
F r e d Roozeboorn a t * ) , H a r r y E . Robson a ) and S h i r l e y S. Chan b,
a ) Exxon R e s e a r c h and Development L a b o r a t o r i e s , P.O. Box 2226, Baton Rouge, La. 70821, U.S.A. and b ) Exxon R e s e a r c h and E n g i n e e r i n g Company, P . O . Box 4 5 , L i n d e n , h . J . 07036, U.S.A. ABSTRACT Samples o f A , X , and Y s y n t h e s i s g e l s a t 98 O C w e r e withdrawn a f t e r c r y s t a l l i z a t i o n f o r v a r i o u s t i m e s , t h e n c e n t r i f u g e d w h i l e h o t . The s o l i d and l i q u i d p h a s e s w e r e examined by L a s e r Raman s p e c t r o s c o p y {LRS), X-ray d i f f r a c t i o n ( X R D ) and c h e m i c a l a n a l y s i s ( A l , S i , Na). LRS and XRD on s o l i d s a m p l e s d e t e c t z e o l i t e s t r u c t u r e s a t a b o u t t h e same time, i . e . , when c r y s t a l l i t e s a r e a b o u t 500 fi i n s i z e . LRS on t h e l i q u i d s g i v e s a d d i t i o n a l i n f o r m a t i o n . For i n s t a n c e , t h e Al(OH)4- i n t h e l i q u i d s ( b a n d a t 621 cm-1) d i s a p p e a r e d b e f o r e any z e o l i t e structure was d e t e c t e d . Some s o l u t e s p e c i e s a p p e a r e d i n t h e c o u r s e o f X and Y c r y s t a l l i z a t i o n , h a v i n g b r o a d , weak Raman bands ( a r o u n d 448, 6 0 0 , 777, and 936 c m - I ) , d u e t o f r e e , u n r e a c t e d monomeric and d i m e r i c s i l i c a t e i o n s . The above a n d c h e m i c a l a n a l y s i s r e s u l t s i n d i c a t e t h e d i s s o l u t i o n o f
some s i l i c o n c o n t a i n i n g (monomeric and p o l y m e r i c ) i o n s from t h e amorphous a l u m i n o s i l i c a t e g e l s . The h y d r o x y l a t e d i o n s c o n d e n s e t o form a l a r g e v a r i e t y o f complex a g g r e g a t e s . w i t h Al(OH)4' T h e s e c o m p l e x e s may b e s o l u b l e p o l y m e r i c a l u m i n o s i l i c a t e s r e l a t e d t o z e o l i t i c f r a g m e n t s and t h u s form t h e n u c l e i o f c r y s t a l growth. A mechanism f o r t h e f o r m a t i o n o f 4 and 6 r i n g v s . 5 r i n g s y s t e m s is proposed.
* Present address: E s s o Chemie B.V.,
P.O.
Box 7225, 3000 HE R o t t e r d a m , The N e t h e r l a n d s .
INTRODUCTION
D u r i n g t h e p a s t two d e c a d e s a number o f p a p e r s h a s b e e n p u b l i s h e d on t h e k i n e t i c s and mechanism o f z e o l i t e f o r m a t i o n . I n most c a s e s t h e s y n t h e s i s o f z e o l i t e A and f a u j a s i t e z e o l i t e s was s t u d i e d (1-17) s i n c e t h e s e a r e t h e e a s i e s t a n d f a s t e s t t o s y n t h e s i z e . R e c e n t l y a l s o h i g h l y s i l i c e o u s z e o l i t e s , s u c h as t h e ZSM-5 ( 1 0 , 1 7 ) a n d m o r d e n i t e s y s t e m s ( 1 1) h a v e r e c e i v e d a t t e n t i o n . From t h e b e g i n n i n g r e s u l t s h a v e b e e n c o n t r o v e r s i a l . Zhdanov a r g u e d i n a d e t a i l e d r e v i e w ( 1 ) i n f a v o r of a s o l u t i o n t r a n s p o r t mechanism, f i r s t p u t f o r w a r d by B a r r e r e t a l . ( 2 ) and l a t e r by many o t h e r i n v e s t i g a t o r s (3-11). B a r r e r et a l . proposed t h e n u c l e a t i o n t o b e t h e r e s u l t o f t h e p o l y m e r i z a t i o n o f a l u m i n a t e , s i l i c a t e and p o s s i b l y more complex i o n s i n t h e l i q u i d p h a s e , t h e i o n s b e i n g c o n t i n u o u s l y s u p p l i e d by t h e d i s s o l u t i o n o f t h e s o l i d g e l m a t e r i a l . McNicol e t a l . s t u d y i n g A and f a u j a s i t e t y p e z e o l i t e s ( 1 2 , 1 3 ) p r o p o s e d a s o l i d p h a s e t r a n s f o r m a t i o n mechanism, i n v o l v i n g z e o l i t e c r y s t a l l i z a t i o n i n t h e s o l i d g e l p h a s e v i a c o n d e n s a t i o n between h y d r o x y l a t e d Si-A1 t e t r a h e d r a . T h i s mechanism was a l s o f a v o r e d by P o l a k e t a 1 ( 1 4 , 1 5 ) who s t u d i e d t h e mechanism of f o r m a t i o n o f X and Y z e o l i t e s . F l a n i g e n ( 1 6 ) p r o p o s e d a s i m i l a r mechanism, involving a reordering o f t h e hydrogel t o an ordered c r y s t a l l i n e state v i a s u r f a c e d i f f u s i o n i n t h e absence o f l i q u i d phase t r a n s p o r t . Derouane e t a l . (17, 1 8 1 , s t u d y i n g t h e s y n t h e s i s o f z e o l i t e s w i t h ZSM-5 t o p o l o g y , c o n c l u d e d t h a t b o t h t h e l i q u i d p h a s e i o n t r a n s p o r t a t i o n mechanism and s o l i d h y d r o g e l p h a s e t r a n s f o r m a t i o n mechanism are i m p o r t a n t , d e p e n d i n g on t h e s i l i c a s o u r c e and t h e g e l f o r m u l a t i o n u s e d . I n t h e f o r m e r mechanism o n l y a few n u c l e i a r e formed, y i e l d i n g l a r g e c r y s t a l l i t e s , w h e r e a s t h e l a t t e r i n v o l v e s numerous n u c l e i y i e l d i n g p o l y c r y s t a l l i n e a g g r e g a t e s . T h e s e w o r k e r s a l s o p e r f o r m e d i n f r a r e d s p e c t r o s c o p i c tests s h o w i n g t h e e x i s t e n c e o f ZSM-5 which was n o t X-ray d e t e c t a b l e i n i n t e r m e d i a t e p h a s e s ( 1 7 , 1 9 ; . Most i n v e s t i g a t o r s a t t h e p r e s e n t t i m e t e n d t o a g r e e w i t h B a r r e r ' s c o n c e p t (21, e s p e c i a l l y s i n c e some r e c e n t Raman s p e c t r o s c o p i c (20, 21) and 2 9 ~ NMR i s p e c t r o s c o p i c (22-25) d a t a s u g g e s t t h e e x i s t e n c e o f s o l u t e a l u m i n o s i l i c a t e s p e c i e s . S o f a r , Raman i n v e s t i g a t i o n s h a v e b e e n c o n f l i c t i n g . McNicol e t a l . ( 1 2 , 13) o b s e r v e d no s p e c t r a l c h a n g e s i n t h e l i q u i d p h a s e d u r i n g c r y s t a l l i z a t i o n and t h u s p r o p o s e d a s o l i d p h a s e t r a n s f o r m a t i o n . However, similar e x p e r i m e n t s by A n g e l 1 and F l a n k ( 9 ) showed s p e c t r a l c h a n g e s i n t h e l i q u i d , g i v i n g e v i d e n c e f o r a s o l u t i o n t r a n s p o r t mechanism. Guth e t a l . ( 2 0 ) r e p o r t e d i n d i r e c t e v i d e n c e f o r t h e o c c u r r e n c e o f aluminos i l i c a t e c o m p l e x e s i n s o l u t i o n by c o m p a r i n g t h e s p e c t r a o f s i l i c a t e and a l u m i n a t e i o n s s e p a r a t e l y i n NaOH s o l u t i o n w i t h t h e s p e c t r u m o f s i l i c a t e and a l u m i n a t e i o n s p r e s e n t t o g e t h e r i n s o l u t i o n .
The o b j e c t i v e o f t h e p r e s e n t work was t o s e e k e v i d e n c e f o r t h e existence of (soluble o r solid) z e o l i t i c precursor species i n the e a r l i e r s t a g e s o f z e o l i t e f o r m a t i o n and t o add t o o u r unders t a n d i n g o f t h e mechanism o f z e o l i t e f o r m a t i o n . T h u s , we a n a l y z e d b o t h s o l i d and l i q u i d components o f z e o l i t e A , X a n d Y s y n t h e s i s g e l s by l a s e r Raman S p e c t r o s c o p y ( L R S ) f o r i d e n t i f i c a t i o n o f m o l e c u l a r s p e c i e s and c h e m i c a l a n a l y s i s f o r A l , S i and Na c o n t e n t . S o l i d s w e r e a l s o t e s t e d by X-ray d i f f r a c t i o n (XRD) f o r c r y s t a l l i n i t y . The main r e a s o n f o r u s i n g Raman s p e c t r o s c o p y is i t s u n i q u e a b i l i t y t o examine s o l i d s a m p l e s a s well as s o l u t i o n s a m p l e s , and its a b i l i t y t o m e a s u r e t h e i r l o w e r f r e q u e n c y modes as opposed t o i n f r a r e d s p e c t r o s c o p y . U n l i k e t h e o t h e r Raman i n v e s t i g a t o r s ( 9 , 1 2 , 2 0 ) we had a Raman s p e c t r o m e t e r e q u i p p e d w i t h a n o p t i c a l m u l t i c h a n n e l a n a l y z e r f o r s i g n a l a v e r a g i n g , and t h u s h a d t h e p o s s i b i l i t y o f r e s o l v i n g i n t e r m e d i a t e s p e c i e s which o f t e n exist a t r e l a t i v e l y low c o n c e n t r a t i o n s s p e c i e s . EXPERIMENTAL Z e o l i t e s y n t h e s i s . T h r e e s i m p l e z e o l i t e s y n t h e s i s c a s e s were s e l e c t e d i.e. t y p e A , and t y p e X a n d Y , b e c a u s e o f t h e i r f a s t f o r m a t i o n a t r e l a t i v e l y IQW t e m p e r a t u r e s . The g e l s s t u d i e d w e r e formed by m i x i n g a q u e o u s s o l u t i o n s o f sodium a l u m i n a t e , sodium h y d r o x i d e and c o l l o i d a l s i l i c a s o l ( t u d o x HS-40). Z e o l i t e A c r y s t a l l i z a t i o n was from a g e l w i t h c o m p o s i t i o n No a g i n g was c a r r i e d o u t s i n c e 2.1 Na20.A1203.2Si02.80H20. i t h a s b e e n r e p o r t e d t o h a v e no e f f e c t ( 9 ) and no s e e d was added. F o r z e o l i t e s X and Y t h e g e l c o m p o s i t i o n s were 3.5 NazO.Al203. 5Si02.80H20 and 3 . 3 Na20.A1203.9Si02.140H20 r e s p e c t i v e l y . The z e o l i t e s were s y n t h e s i z e d u s i n g p u b l i s h e d methods of s e e d i n g ( 2 6 , 27), t h e seed s l u r r y composition being 13.3 Na20.Al2O3.12.5SiO2.267H20. The amount o f s e e d was 5 mol 7; By s e e d i n g t h e c r y s t a l l i z a t i o n t i m e was r e d u c e d of t h e t o t a l A l . t o 8 h o u r s f o r z e o l i t e X and 1 2 h o u r s f o r z e o l i t e Y. Raman S p e c t r o s c o p y . P o l y e t h y l e n e b o t t l e s o f t h e master s y n t h e s i s q e l s were w i t h d r a w n from a c o n s t a n t t e m p e r a t u r e oven ( 9 8 O ~ ) a f t e r Garious times of s t a t i c c r y s t a l l i z a t i o n , c e n t r i f u g e d while still h o t and f i l t e r e d t o s e p a r a t e t h e l i q u i d phase and s o l i d p h a s e . The s o l i d s a m p l e s w e r e f u r t h e r washed w i t h w a t e r , d r i e d a t llO°C and c a l c i n e d a t 500°C f o r 2 h r i n a i r t h e n c o o l e d t o room t e m p e r a t u r e . An a r g o n i o n l a s e r was t u n e d t o t h e 514.5 nrn l i n e f o r e x c i t a t i o n . L i q u i d s a m p l e s a t room t e m p e r a t u r e were p l a c e d i n 10mm x 10mm q u a r t z c u v e t t e s . The Raman s i g n a l s were c o l l e c t e d u s i n g 90° s c a t t e r i n g geometry w i t h a F1.2 l e n s . The l a s e r power a t t h e The s o l i d s a m p l e s were s a m p l e l o c a t i o n was s e t a t 70-90 mW. p e l l e t i z e d t o 13 mm d i a m e t e r w a f e r s f o r m o u n t i n g i n a s a m p l e h o l d e r .
Fig. 1
Raman s p e c t r a o f s o l i d phase in zeolite A synthesis
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Raman spectra o f l i q u i d phase i n z e o l i t e A synthesis
The c o l l e c t i o n o p t i c s i n t h i s c a s e was a b a c k s c a t t e r i n g geometry a n d t h e l a s e r power was l o w e r e d t o 10-30 mW. The Raman s p e c t r a were a n a l y z e d by a t r i p l e t monochromator, model DL203 w i t h F4 o p t i c s made by I n s t r u m e n t SA, Metuchen, NJ, and an o p t i c a l m u l t i c h a n n e l a n a l y z e r s y s t e m , model OMA2 e q u i p p e d with a n
i n t e n s i f i e d p h o t o d i o d e a r r a y d e t e c t o r , made by P r i n c e t o n A p p l i e d R e s e a r c h , P r i n c e t o n , NJ. T h i s s y s t e m made i t p o s s i b l e t o c o l l e c t a c o m p l e t e s p e c t r u m o v e r a r a n g e o f t h o u s a n d cm-I s i m u l t a n e o u s l y i n t h e m a t t e r o f s e c o n d s o r m i n u t e s . Thus e a c h s a m p l e c o u l d b e examined i m m e d i a t e l y a f t e r i t s p r e p a r a t i o n p r o c e d u r e w i t h o u t any c h a n c e o f a g i n g which m i g h t a f f e c t its s t r u c t u r a l i n t e g r i t y . The t o t a l a c c u m u l a t i o n time n e e d e d f o r e a c h s p e c t r u m r e p o r t e d h e r e was i n t h e r a n g e 20-100 s e c . The q i g i t a l d i s p l a y o f t h e s p e c t r u m was c a l i b r a t e d t o g i v e 1 . 7 cm- / c h p n n e l w h e r e a s t h e o v e r a l l s p e c t r a l r e s o l u t i o n was a b o u t 8 cm which was a d e q u a t e f o r v i b r a t i o n a l band w i d t h s o v e r 2 0 cm-I. X-Ray d i f f r a c t i o n . S o l i d p h a s e s a m p l e s were s c a n n e d w i t h t h e a i d o f a P h i l i p s X-ray d i f f r a c t o m e t e r , u s i n g CuK-alpha r a d i a t i o n . C h e m i c a l a n a l y s i s . C h e m i c a l c o m p o s i t i o n s (Al, S i and Na c o n t e n t ) o f b o t h s o l i d and l i q u i d p h a s e s were d e t e r m i n e d by p l a s m a s p e c t r o s 111 I n d u c t i v e l y Coupled Plasma/ copy u s i n g a ~ a r r e l l - ~ s h Atomic E m i s s i o n S p e c t r o m e t e r which p e r f o r m s a s i m u l t a n e o u s m u l t i e l e m e n t measurement. S o l i d s a m p l e s w e r e f u s e d i n a s a l t m i x t u r e by a Claisse F l u x e r , which a u t o m a t i c a l l y p o u r s t h e m o l t e n f l u x i n t o a d i l u t e a c i d s o l u t i o n f o r f i n a l d i s s o l u t i o n . The i n i t i a l f u s i o n m i x t u r e c o m p r i s e s 0.1 g . of s a m p l e p l u s 1 . 5 g. o f a Li2COj/ T h i s melt was d i s s o l v e d i n 1 0 0 m l 5% HNO3, Li2B407 m i x t u r e . t h e n d i l u t e d t o 250 m l a n d f i n a l l y i n j e c t e d i n t o t h e a r g o n plasma. L i q u i d p h a s e s were i n j e c t e d d i r e c t l y i n t o t h e a r g o n plasma.
tomc corn^
RESULTS AND DISCUSSION Raman S p e c t r o s c o p y . F i g u r e 1 g i v e s t h e r e s u l t i n g s p e c t r a f o r t h e s o l i d p h a s e s i n z e o l i t e A s y n t h e s i s a f t e r 0 , 1 , 2 , 3 , and 4 h o u r s o f c r y s t a l l i z a t i o n . The s p e c t r a o f t h e i n i t i a l g e l ( 0 h o u r s ) and a f t e r 1 h o u r h a v e no i n t e r p r e t a b l e p e a k s . The p e a k s i n d i c a t e d i n t h e o t h e r s p e c t r a are a l l d u e t o z e o l i t e A f o r m a t i o n and a g r e e r e a s o n a b l y well w i t h t h o ~ e ~ r e p o r t ebyd A n g e l 1 ( 2 8 ) . He reported o n e s t r o n g band ~t 490 cm- and f o u r weak b a n d s a t 700, 4 1 0 , 3 4 0 , and 280 cmThe c o r r e s p o n d i n g band f r e q u e n c i e s o$ o u r measurement c e n t r e a r o u n d 4 9 2 , 7 1 4 , 4 0 5 , 347 and 281 cmW e d i d n o t o b s e r v e s i g n i f i c a n t s h i f t s o f t h e s e band p o s i t i o n s , which m i g h t s u g g e s t t h e g r o w t h from a s o l i d p r e c u r s o r .
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F i g u r e 2 shows t h e s p e c t r a o f t h e c o r r e s p o n d i n g l i q u i d p h a s e s . I t i s s e e n t h a t i n t h e e a r l y s t a g e s , i . e . , d u r i n g i n d u c t i o n and e v e n a f t e r 2 h o u r s when r y s t a l l i z a t i o n h a s set i n , a s t r o n g p e a k was -7 whlch . . o b s e r v e d a t 621 cm 1s a s s i g n e d t o t h e A104 s y m m e t r i c s t r e t c h i n g mode o f t h e monomeric Al(OH)4- a l u m i n a t e a n i o n ( 2 9 ) . O t h e r a u t h o r s f o nd t h i s peak a t f r e q u e n c i e s r a n g i n g from 618 crn-I ( 1 2 ) t o 6 2 5 cm ( 2 9 ) . No e v i d e n c e f o r s o l u t e a n i o n i c a l u m i n o s i l i c a t e p r e c u r s o r s p e c i e s was n o t e d i n t h e l i q u i d p h a s e d u r i n g t h e c o u r s e of t h e c r y s t a l l i z a t i o n . I t is n o t l i k e l y t h a t Raman i n a c t i v e p r e c u r s o r s p e c i e s a r e formed s u g g e s t i n g t h e a b s e n -c e o f well d e f i n e d p r e c u r s o r s p e c i e s i n s o l u t i o n o t h e r t h a n A1(OHI4 T h i s means t h a t numerous i l l - d e f i n e d p r e c u r s o r s p e c i e s may be f o r m e d , e a c h h a v i n g i n s u f f i c i e n t c o n c e n t r a t i o n f o r a Raman p e a k t o be r e s a l v e d . The o b s e r v a t i o n s a l s o i n d i c a t e t h a t n o s i g n i f i c a n t n e t d i s s o l u t i o n of t h e s o l i d g e l phase occurs throughout t h e s y n t h e s i s o f z e o l i t e A: o n l y a l u m i n a t e is b e i n g consumed from t h e l i q u i d p h a s e ( d i s a p p e a r i n g Rarnan p e a k ) on i n c o r p o r a t i o n i n t h e s o l i d p h a s e and no s i l i c a t e p e a k s a p p e a r .
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F i g u r e s 3 and 4 show t h e c o r r e s p o n d i n g d a t a f o r z e o l i t e X a n d F i g u r e s 5 and 6 f o r z e o l i t e Y . The p e a k s i n d i c a t d i n F i g u r e 3 a l l -7 a r i s e from s o l i d z e o l i t e X ( 5 1 7 , 3 7 6 , and 291 cm ) a n d t h o s e i n F i g u r e 5 from z e o l i t e Y ( 5 1 1 , 3 6 9 , and 2 9 8 c m - I ) . Literature r e p o r t s t h e p e a k s t o be - ~i 505, 3 7 5 , and 282 crn-' ( p l u s v e r y weak a t 1 1 0 7 5 and 990 c~ ) f o r z e o l i t e X a n d a t 5 0 3 , 3 5 0 , and 3 0 0 cm- ( a n d 1 1 1 0 cm- ) f o r z e o l i t e Y ( 2 8 ) . T h u s , t h e s e measurements a g r e e reasonably w e l l with t h e l i t e r a t u r e d a t a t o w i t h i n e x p e r i m e n t a l limits.
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The p e a k s o r b a n d s i~ F i g u r e s 4 a n d 6 c l e a r l y show t h a t t h e Al(OH)4 i o n ( p e a k a t 621 cm- ) i s p r e s e n t i n t h e i n i t i a l g e l o f b o t h A s i n t h e c a s e of z e o l i t e A , t h i s z e o l i t e X and z e o l i t e Y . p e a k d i s a p p e a r s a f t e r t h e f i r s t 2-3 h o u r s o f c r y s t a l l i z a t i o n . B u t u n l i k e z e o l i t e A , some s o l u b l e s p e c i e s a r e o b s e r v e d i n t h e course o f the crystallization. The i d e n t i f i c a i o n of t h e s e s p e c i e s , h a v i n g b a n d s a r o u n d 448, 600, 7 7 7 , and 936 cm- f o r z e o l i t e X a n d 600 and 777 cm-I f o r z e o l i t e Y , is d i f f i c u l t b e c a u s e o f t h e b r o a d n e s s and weakness of t h e bands. I n g e n e r a l , a b r o a d Raman band p o i n t s t o a v a r i e t y o f s t r u c t u r e s r a t h e r t h a n t o a v e r y well d e f i n e d s t r u c t u r e of one d i s t i n c t i o n ( l i k e A ~ ( O H ) ~ - ) . T h i s v a r i e t y o f s t r u c t u r e s is common f o r s i l i c a t e i o n s , which a r e known t o b e p r e s e n t i n d i f f e r e n t d e g r e e s o f h y d r o x y l a t i o n a n d p o l y m e r i z a t i o n d e p e n d i n g on c o n c e n t r a t i o n , pH, t e m p e r a t u r e and p r e s s u r e ( 3 0 ) .
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Rarnan s p e c t r a o f l i q u i d phase i n z e o l i t e
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FREQUENCY SHIFT ( c m - I ) Fig. 5
Raman s p e c t r a o f solid phase i n z e o l i t e Y synthesis
Fig. 6
Raman s p e c t r a of l i q u i d phase i n z e o l i t e Y s y n t h e s i s
Chemical A n a l y s i s T a b l e s I and I1 summarize t h e a n a l y s i s r e s u l t s , along with t h e other r e s u l t s . Z e o l i t e A . I n a c c o r d a n c e w i t h what was o b s e r v e d w i t h Raman spectroscopy, t h e alumina concentration decreased a f t e r 3 hours o f s y n t h e s i s t i m e . From t h e r e s u l t s on t h e s o l i d p h a s e , we c a n c o n c l u d e t h a t a l u m i n a t e d i s a p p e a r e d from s o l u t i o n and is i n c o r p o r a t e d i n t h e s o l i d phase. A s t o t h e s i l i c a t e c o n c e n t r a t i o n o f t h e l i q u i d p h a s e , t h i s c o n c e n t r a t i o n d e c r e a s e d less d r a s t i c a l l y t o c o m p a r a b l y low l e v e l s . Z e o l i t e s X and Y . From T a b l e s I and I1 we c a n s e e t h a t i n c a s e o f b o t h z e o l i t e X and z e o l i t e Y i n t h e f i r s t 2-3 h o u r s , i . e . , b e f o r e any c r y s t a l l i t e s w e r e d e t e c t e d , a l u m i n a t e was d i s a p p e a r i n g from s o l u t i o n a s o b s e r v e d w i t h Raman s p e c t r o s c o p y and i n c o r p o r a t e d i n t h e amorphous a l u m i n o s i l i c a t e p h a s e . F o r b o t h z e o l i t e s w e o b s e r v e d i n t h e same p e r i o d a n i n c r e a s e i n s i l i c a t e c o n c e n t r a t i o n i n t h e l i q d i d and a d e c r e a s e i n s i l i c o n c o n c e n t r a t i o n o f t h e s o l i d phase. A f t e r t h i s 2-7 h o u r p e r i o d , t h e s i l i c o n c o n t e n t d e c r e a s e d o r r e m a i n e d t h e same f o r t h e l i q u i d p h a s e s and r o u g h l y r e m a i n e d t h e same f o r t h e s o l i d s . From t h e s e d a t a i t is c l e a r t h a t i n t h e f i r s t 2-3 h o u r s o f t h e c r y s t a l l i z a t i o n o r n u c l e a t i o n some s i l i c o n cont a i n i n g i o n s (monomeric o r p o l y m e r i c ) w e r e d i s s o l v e d from t h e amorphous ( a 1 u m i n o ) s i l i c a t-e g e l . T h e s e h y d r o x y l a t e d i o n s a p p a r e n t l y c o n d e n s e d w i t h t h e A1(OH)4 i o n s p r e s e n t t o form a l a r g e v a r i e t y o f a g g r e g a t e s , which may be t h e n u c l e i f o r c r y s t a l growth. Thus, t h e whole mechanism o f z e o l i t e A , X and Y f o r m a t i o n is a s o l u t i o n t r a n s p o r t mechanism. X-Ray D i f f r a c t i o n The X-Ray d i f f r a c t i o n p a t t e r n s f o r t h e v a r i o u s s o l i d p h a s e s o f z e o l i t e s A , X , and Y a r e summarized i n T a b l e 11. I n comparing XRD and Raman r e s u l t s , we c a n c o n c l u d e t h a t Raman s p e c t r o s c o p y d e t e c t s z e o l i t e f o r m a t i o n i n t h e s o l i d p h a s e s a t a b o u t t h e same t h e c h a r a c t e r i s t i c XRD p e a k s a p p e a r a t t h e same s t a g e a s XRD: p o i n t i n t h e c r y s t a l l i z a t i o n as t h e Raman p e a k s , i . e . , when c r y s t a l l i t e s a r e a b o u t 500 8 i n s i z e a s o b s e r v e d w i t h S c a n n i n g E l e c t r o n Microscopy (SEM). A s i n t h e Rarnan s p e c t r a , no o t h e r XRD p e a k s a p p e a r e d t h a n t h o s e o f t h e z e o l i t e we i n t e n d e d t o s y n t h e s i z e Thus, no i n t e r m e d i a t e p h a s e s were d e t e c t e d by t h e two t e c h n i q u e s . Background F l u o r e s c e n c e Raman s p e c t r o s c o p y o f t h e s o l i d s a m p l e s d o e s n o t g i v e a d d i t i o n a l i n f o r m a t i o n ; by t h e t i m e t h a t z e o l i t e c r y s t a l s a r e l a r g e enough t o b e Raman d e t e c t a b l e , t h e s e c r y s t a l s g i v e s t r o n g XRD p a t t e r n s ( > 500 8 ) .
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L i t e r a t u r e d a t a have r e c e n t l y been r e p o r t e d f o r v a r i o u s s i l i c a t e i o n s ( 1 9 ) : 1 0 0 5 ( s h o u d e r ) , 9 3 0 ( i n t e n s e ) , 830 ( s h o u l d e r ) , 778 ( i n t e n s e ) a n d 448 cm- (medium) f o r t h e monomeric s p e c i e s , 2-1 (medium) f o r d i m e r i c s i l i c a t e , and 6 3 2 , S i O p ( ~ ~ ) 2, 6 0 5 cm 5 5 5 , 5 4 2 , and 5 2 0 cm ( a l l weak) f o r p o l y m e r i c , e s p e c i a l l y c y c l i c , t r i m e r i c a n i o n s . Thus o n e may ? s c r i b e t h e band p o s i t i o n s a t 9 3 6 , 7 7 7 , 600, a n d 448 cm- t o monomeric a n d dirneric s i l i c a t e anions. A s discussed l a t e r i n t h i s paper (Tables I a n d 111, t h e c h e m i c a l a n a l y s i s r e s u l t s show t h a t a f t e r some 2-3 hours o f s y n t h e s i s t h e s i l i c o n concentration i n t h e l i q u i d phase of X a n d Y r e m a i n s t h e same o r d e c r e a s e s , w h e r e a s t h e Rarnan b a n d s become more i n t e n s e . T h u s , i t is c l e a r t h a t t h e b a n d s a r e d u e t o t h e d e p o l y r n e r i z a t i o n o f p o l y m e r i c s i l i c a t e s p e c i e s i n t o monomeric and d i m e r i c s i l i c a t e i o n s .
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The Si02/A1203 r a t i o i n b o t h t h e i n i t i a l g e l f o r m u l a t i o n Thus a n d t h e f i n a l p r o d u c t o f z e o l i t e A s y n t h e s i s was 2.0. a l l s i l i c a t e s p e c i e s p r e s e n t were consumed by c o n d e n s a t i o n w i t h aluminate species. In t h e c a s e of t h e f a u j a s i t e s t h e s y n t h e s i s g e l s were r i c h i n s i l i c a : t h e Si02/A1203 r a t i o s i n t h e i n i t i a l g e l a n d f i n a l p r o d u c t were 5.0 a n d 3.0 f o r z e o l i t e X a n d 9.0 a n d 4.8 f o r z e o l i t e Y . Thus, i n c o n t r a s t t o z e o l i t e A , t h e a p p e a r a n c e o f f r e e s i l i c a t e i o n s was o b s e r v e d d u r i n g t h e c r y s t a l lization reaction. A n g e l 1 a n d F l a n k ( 9 ) o v s e r v e d some s i l i c a t e s p e c i e s w i t h b a n d s a r o u n d 450 a n d 800 cm- ( p r o b a b l y m o n o s i l i c a t e ) i n t h e s e p a r a t e d l i q u i d phase of t h e i r i n i t i a l z e o l i t e A s y n t h e s i s g e l , T h i s is p r e s u m a b l y d u e t o t h e s o d i u m s i l i c a t e , w h i c h t h e y u s e d a s a s i l i c a s o u r c e . We u s e d c o l l o i d a l s i l i c a s o l w h i c h d o e s n o t g i v e r i s e t o any s i g n a l i n t h e g e l . A p p a r e n t l y t h e c o n d e n s a t i o n o f ~ l ( 0 I - l ) ~a -n d t h e d e p o l y m e r i z e d , h y d r o x y l a t e d s i l i c a t e i o n s r e q u i r e s t h e same 2-3 h o u r s f o r a l l t h r e e z e o l i t e s . A f t e r t h i s i n d u c t i o n period t h e aluminate d i s a p p e a r s and, i n c a s e o f X and Y , f r e e s i l i c a t e i o n s a p p e a r i n s o l u t i o n . Schwochow a n d H e i n z e (31) a r g u e d t h a t t h e z e o l i t e s p e c i e s c r y s t a l l i z e d from s e p a r a t e l i q u i d p h a s e s d e p e n d s on t h e c o m p o s i t i o n o f t h e l i q u i d p h a s e a n d , a t c o n s t a n t c o m p o s i t i o n , a l s o on t h e s i z e o f t h e p o l y m e r i c s i l i c a t e i o n s . They c o n c l u d e d t h a t p o l y m e r i c i o n s promote t h e f o r m a t i o n o f a p h i l l i p s i t e t y p e phase o v e r a f a u j a s i t e t y p e a n d t h a t a t t h e time o f n u c l e a t i o n t h e d i s s o l v e d s i l i c a must b e p r e d o m i n a n t l y i n a monomeric s t a t e t o c r y s t a l l i z e a s f a u j a s i t e type structures.
A s t o t h e mechanism o f z e o l i t e , f o r m a t i o n , t h e Raman s p e c t r a s u p p o r t t h e s o l u t i o n t r a n s p o r t mechanism ( s e e l a t e r ) .
P a r t o f t h i s " l a t e " Raman d e t e c t a b i l i t y may b e d u e t o t h e l a r g e f l u o r e s c e n t background t h a t a l l s o l i d z e o l i t e s s a m p l e s g i v e rise t o . T h i s background h a s a l s o been o b s e r v e d by o t h e r a u t h o r s (28, 3 2 , 3 3 ) on t h e e x a m i n a t i o n o f m e t a l o x i d e m a t e r i a l s . They a l s o r e p o r t e d , a s we found i n o u r p r e s e n t work, t h a t p r o l o n g e d e x p o s u r e t o t h e l a s e r beam d e c r e a s e d t h e f l u o r e s c e n c e , i n g e n e r a l . However, e v e n t h i s r e d u c e d t h e f l u o r e s c e n c e h a r d l y s u f f i c i e n t l y f o r t h e o b s e r v a t i o n o f Raman s p e c t r a i n s e v e r a l s a m p l e s , e . g . , AO-A2,XO-X4, a n d YO-Y6. It h a s been s u g g e s t e d t h a t t h e c a u s e o f t h e e x c e s s i v e b a c k g r o u n d c o u l d be the f l u o r e s c e n c e d u e t o t r a n s i t i o n m e t a l i m p u r i t i e s , e s p e c i a l l y ~ e ~ cr3+ + , and Mn 2+ (13). Angel1 ( 2 8 ) o b t a i n e d a h i g h p u r i t y z e o l i t e Y which c o n t a i n e d less t h a n 1 7 ppm F e and showed much less f l u o r e s c e n c e . Q t h e r h i g h p u r i t y m a t e r i a l s ( z e o l i t e A and X ) g a v e r i s e t o a r e l a t i v e l y s t r o n g Raman s p e c t r u m . Thus, i t may v e r y well b e t h a t i n o u r c a s e some r e l e v a n t i n f o r m a t i o n was o b s c u r e d by f l u o r e s c e n c e . E g e r t o n e t a 1 ( 3 4 , 35) r e p o r t e d t h a t i n t h e c a s e o f s i l i c a a n d s i l i c a - a l u m i n a m a t e r i a l s t h e background f l u o r e s c e n c e c a n b e minimized by h e a t i n g t h e s a m p l e s i n oxygen a t h i g h e r t e m p e r a t u r e s (500°C). I t i s r e p o r t e d f o r z e o l i t e s , however, t h a t a c t i v a t i o n o f z e o l i t e s under s i m i l a r c o n d i t i o n s always r e s u l t s i n h i g h e r backgrounds ( 2 8 ) .
ON THE RAMAN DETECTABILITY AND IDENTIFICATION OF COMPLEX ALUMINOSILICATE SPECIES IN SOLUTION R e c e n t l y , Guth e t a 1 . ( 2 0 , 21) p u b l i s h e d some p a p e r s on a Raman i n v e s t i g a t i o n o f s t r o n g l y b a s i c (NaOH) s o l u t i o n s o f d i l u t e d sodium-l a l u m i n a t e and s i l i c a t e . They r e c o r d e d s p e c t r a i n t h e 580-680 cm r a n g e onlyLl I n a l u m i n a t e s o l u t i o n s , a n i n t e n s e p o l a r i z e d peak a t 623-625 c m was o b s e r v e d and t h e y a t t r i b u t e d t h i s peak t o t h e free A ~ ( O H ) ~ i- o n . Only t h i s band had s t r o n g enough Raman c r o s s s e c t i o n t o a l l o w e x p l o i t a t i o n w h e r e a s t h o s e o f t h e s i l i c a t e and a l u m i n a t e o r complex a n i o n s w e r e much t y o weak. I n t h e p r e s e n c e o f s i l i c a t e , t h e a l u m i n a t e band a t 6 2 3 cm- ( S i / A l = l ) d e c r e a s e d t o v a r y i n g e x t e n t s as we f o u n d , t h u s d e n o t i n g i n d i r e c t l y t h e f o r m a t i o n o f a l u m i n o s i l i c a t e s p e c i e s , as i n o u r c a s e w i t h z e o l i t e A. F o r s i l i c a t e / a l u m i n a t e m i x t u r e s w i t h Si/A1=6, Guth e t a l . ( 1 9 ) o b s e r v e d , b e s i d e s t h e d i s a p p e a r g n- c e o f t h e Al(OH)4- peak a t 623 cm-' a n d a d e c r e a s e o f t h e Si02(OH)2 peaks, the appearance of sm~ll p e a k s o r s h o u l d e r s . T h e i r most " i n t e n s e " peak was a t 577 cmYet, i n a l l t h e s e c a s e s t h e s p e c t r a a r e t o o p o o r t o c o n d u c t a s t r u c t u r a l s t u d y o f t h e s e complexes ( 2 0 ) . Again, t h i s might i n d i c a t e t h a t a l a r g e v a r i e t y a f p o l y m e r i c a l u m i n o s i l i c a t e complexes is formed, none o f them h a v i n g enough c o n c e n t r a t i o n o r e x i s t i n g f o r l o n g enough time t o be r e a s o n a b l y d e t e c t a b l e .
.
ON THE MECHANISM OF R I N G FORMATION
Meier ( 3 6 , 37) c l a s s i f i e d z e o l i t e s t r u c t u r e s a c c o r d i n g t o t h e s e c o n d a r y b u i l d i n g u n i t s (SBU). T h e s e u n i t s a r e b a s i c a l l y s i n g l e or d o u b l e 4 , 6 o r 8 r i n g s y s t e m s and more complex 4-1, 4-4-1 and 5-1 u n i t s . F o r s i m p l i f i c a t i o n we w i l l s u b d i v i d e them i n t o even-number membered r i n g s y s t e m s [ 4 , 6 , 81 and 5 r i n g s y s t e m s . The l a t t e r g r o u p may b e e x t e n d e d t o odd-number membered r i n g s y s t e m s 15 and 9 1 , i f we i n c l u d e t h e 9 r i n g systems, r e c e n t l y r e p o r t e d f o r l o v d a r i t e by M e r l i n o ( 3 8 ) . F o r s o l u t i o n s w i t h S i / A l > 5 , Guth e t a l . ( 2 0 ) s u g g e s t e d t h a t a r e l a t i v e f 3 - s t a b l e s p e c i e s e x i s t s as an i n t e r m e d i a t e , i.e. t h e A1(OSi03)4 s p e c i e s . Derouane e t a l . ( 1 7 ) a l s o p o s t u l a t e d t h i s s p e c i e s t o b e t h e s p e c i e s t r a n s p ~ r t e dt h r o u g h t h e l i q u i d p h a s e o f silicon-rich gels. We assume t h i s s p e c i e s w i l l b e formed a l r e a d y a t S i / A 1 = 4 i n s o l u t i o n . A t Si/Al > 4 t h e s o l u t e Al-species a r e completely s a t u r a t e d w i t h A1-0-51 b o n d s formed by t h e c o n d e n s a t i o n r e a c t i o n :
We p o s t u l a t e t h e r e m a i n i n g s i l i c a t e a n i o n s i n t h e s i l i c o n - r i c h m i x t u r e ( S i / A 1 > 4 1 , which h a v e n o t been u s e d f o r t h e c o n d e n s a t i o n l a , t o c o n d e n s e t o p o l y s i j f c- a t e s t r u c t u r e s which c a n e v e n t u a l l y r e a c t w i t h t h e A1(OSi03)4 s p e c i e s t o 5 r i n g systems:
-O,Si \
8
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OR, OR, R 1 t o R4 r e p r e s e n t g r o u p s r a n g i n g from h y d r o g e n t o more c o m p l i c a t e d S i - 0 - S i and 51-0-A1 n e t w o r k s .
A t S i / A l < 4 t h e s o l u t e A ~ ( o H ) ~s p- e c i e s a r e n o t c o m p l e t e l y s a t u r a t e d w i t h A1-0-Si b o n d s a n d t h e c o n d e n s a t i o n r e a c t i o n may f o r examplf be: 10 -
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O,Si S i n c e t h e m i x t u r e i s now r e l a t i v e l y r i c h i n A 1 we may h a v e o t h e r ( s o l i d o r s o l u t e ) polymeric aluminosilicate s p e c i e s r e a c t i n g with t h e s o l u t e u n s a t u r a t e d a l u r n i n o s i l i c a t e s p e c i e s from r e a c t i o n I I a , y i e l d i n g e v e n number r i n g s y s t e m s . F o r t h e 4 r i n g s y s t e m a n example may be:
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+ 20H-
F o r t h e 6 and 8 r i n g s y s t e m s o n e c a n d e s i g n s i m i l a r r e a c t i o n s . The S i / A 1 r a t i o s i n t h e l i q u i d p h a s e o f t h e i n i t i a l g e l ( s e e T a b l e I ) w e r e m e a s u r e d t o b e 0 . 4 4 , 1.44 a n d 1 . 0 5 f o r t h e A , X a n d Y s y n t h e s i s g e l s r e s p e c t i v e l y , t h u s a l l < 4 and a l l o w i n g t y p e I I a condensations. R 1 , R2, R3, a n d R4 i n d i c a t e d i n r e a c t i o n I l b c a n r a n g e f r o m h y d r o g e n t o more c o m p l i c a t e d Si-O-Si a n d Si-O-A1 n e t w o r k s . T h i s means t h a t a l a r g e v a r i e t y o f c o m p l e x i o n s i s p r e s e n t i n t h e s o l u t i o n , e a c h s p e c i e s h a v i n g t o o low c o n c e n t r a t i o n d u e t o i t s g r o w t h t o l a r g e r s p e c i e s ( e . g . D-4-Rings f o r z e o l i t e A a n d D-6-Rings f o r z e o l i t e X/Y and p o s s i b l y l a r g e r s p e c i e s ) .
In our opinion t h i s variety of species i n solution prevents the o b s e r v a t i o n o f s p e c i f i c Rarnan p e a k s d u r i n g n u c l e a t i o n . E v e n t u a l l y t h e g r o w i n g s o l u t e s p e c i e s become v i a b l e c r y s t a l l i z a t i o n c e n t e r s , p r e c i p i t a t i n g f r o m s o l u t i o n . The p r e c i p i t a t e c o n s i s t s o f c r y s t a l s l a r g e enough t o b e Rarnan and X-ray d e t e c t a b l e , t h e s i g n a l s s h o w i n g up s i m u l t a n e o u s l y i n t h e c o u r s e o f c r y s t a l l i z a t i o n . ACKNOWLEDGEMENT T h a n k s a r e d u e t o P r o f . P.J. G e l l i n g s ( T w e n t e U n i v e r s i t y of T e c h n o l o g y , The N e t h e r l a n d s ) a n d P r o f . D . P . Shoemaker ( O r e g o n S t a t e U n i v e r s i t y , C o r v a l l i s , O r e g o n ) f o r t h e i r comments on t h e m a n u s c r i p t a n d t o E x x o n R e s e a r c h & E n g i n e e r i n g Co. 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 p a p e r . Mrs. K. G e r t h i s a c k n o w l e d g e d f o r t y p i n g t h e manuscript.
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.
101
Zhdanov, S.P., Advan. Chem. Ser. (1971) 20 B a r r e r , R.M., J.W. Baynham, F.W. B u l t i t u d e and W.M. Meier, J. Chem. Soc. 195 (1959) K e r r , G.T., J . p h y s . Chem. 70 (1966) 1047 K e r r , G.T., J. phys. Chern. 72 (1968) 1385 C u l f a z , A * , and L.B. Sand, Advan. Chem. Ser. 121 (1973) 140 Cournoyer, R.A., W.L, K r a n i c k and L.B. Sand, J . phys. Chem. 75 (1975) 1578 K a c i r e k , H., and H. Lechert, J. phys. Chem. 79 (1975) 1589 Freund, E.F., J. C r y s t a l Growth 34 (1976) 11A n g e l l , C.L., and W.H. F l a n k , i n - ~ o l e c u l a r Sieves 11" (Katzer. J.R., ed.), ACS Symposium s e r i e s 40 (1977) 194 T.C. Tasi, M.S. ~hen,
x.
Barby, D., I . G r i f f i t h s , A . R . J a c q u e s , and D. Pawson, i n "The modern i n o r g a n i c c h e m i c a l s i n d u s t r y " (Thompson, R., e d . ) , Chemical S o c i e t y , London, 1977, p. 320. Schwochow, F . E . , and G.W. H e i n z e , Advan. Chem. S e r . 101 (1971) 102
Hendra, P.J., and E . J . L o a d e r , T r a n s . F a r a d , Soc. 6 7 ( 1 9 7 1 ) 828 Soc. A 1766 Hendra, P.J., J.R. H o r d e r , and E.J. L o a d e r , J. ~ h e K ( 19 7 1 ) E g e r t o n , T.A., A.H. H a r d i n , Y. K o s i r o w s k i , and N. S h e p p a r d , Chem. Cornrn. 888 ( 1 9 7 1 ) E g e r t o n , T.A., and A.H. H a r d i n , C a b a l . Rev. 11 ( 1 9 7 5 ) 71 Meier , W .M., M o l e c u l a r S i e v e s , S o c i e t y of c h e m i c a l I n d u s t r y , London, 1 9 6 8 , p.10 M e i e r , W.M., and D.H. O l s o n , A t l a s o f Z e o l i t e S t r u c t u r e t y p e s , Juris Druck V e r l a g , ~ G r i c h ,S w i t z e r l a n d , 1978 M e r l i n o , S., T w e l f t h I n t . Congr, C r y s t . , L e D r o i t and Le Clerc P r i n t e r s , Ottawa, Canada, 1 9 8 1 , p. C 189
PART I1 PHYSICAL CHARACTERiZATION AND SORPTION FUNDAMENTALS
THE PHYSICAL CHARACTERIZATION OF ZEOLITES
Ei.
Lechert
Institute of Physical Chemistry, University of Hamburg Laufgraben 24, D-2000 Hamburg 13, Germany
1. INTRODUCTION
The physical characterization of a material, such as a zeolite begins in principle by visual inspection. This inspection can be refined by the use of instruments which enlarge small particles. Such instruments may be: an ordinary microscope, an electron microscope, or a raster scan electron microscope. Crystalline materials may often be identified by the shape of a crystal which also nay give information concerning the presence of crystalline impurities. Furthermore, from the shape of the crystals and their size distribution conclusions may even be drawn which render information concerning the conditions of crystallization or later transformations which will take place, and the colour of the crystals reveals the presence of ionic impurities, such as iron which occurs in natural or industrial zeolites. In most cases, the structure of the crystalline lattice of the zeolites may be obtained by X-ray diffraction methods. These methods range from simple powder patterns up to sophisticated single crystal methods which allow the localization of all atoms in a unit cell and the calculation of the electron distribution between these atoms. The literature, especially the patent literature, shows that powder patterns are used as a means of identifying the structure of a specific zeolite. The X-ray methods may be supplemented by neutron diffraction experiments for the localization of light atoms, especially protons. The neutron scattering experiments may also be used to study
the mobility of water inside zeolite cavities. For samples which are made up of small crystals and for the investigation of special problems, such as the distribution of Si- or Al-atoms in the alumosilicate lattice, electron diffraction may be applied.. In addition to the diffraction methods spectroscopic methods are used to study structure problems of zeolites. In this field of study the IR-spectroscopy and recently the 2 9 ~ i - ~ ~ ~ - s p e c t r o s c o p y has been applied. Strictly speaking, the term'physical characterization' is applied in zeolite chemistry mostly in connection with the description of special structural elements, responsible for the properties needed in the various applications, e.g. the acid OH-groups in catalysis. In this field the different methods of spectroscopy are unique.
With the IR-spectroscopy a large number of studies has been carried out on the characterization of the strength and the accessibility of the acid OH-groups in different zeolites. The Lewis sites have been characterized by studying the IR-spectra of complexes with sorbed organic bases. Another application of the IRspectroscopy is the sorption of molecules with different electronic structures on transition metal ions. These types of sorption complexes may also be studied by UV-, visible, or ESR-spectroscopy. Transition metals which are contained in zeolites, incidentally they are used as catalysts in various reactions with hydrogen have been studied extensively by means of Moessbauer-, ESR-, and XPS-spectroscopy. The possibility of investigating acidic groups and their interaction with sorbed molecules is also described by different NMRspectroscopy techniques, which have also found wide application in the study of mechanisms concerning molecular mobility in the cavities of zeolite structures. Furthermore, under the heading of 'physical characterization' sorption measurements of any kind have to be summarized, giving information on the heat and entropies of sorption and thus on the interaction of molecules with various shapes and polarity with special structures of the walls of zeolite cavities. The accessibility of the cavities may be studied by diffusion measurements. Last but not least the technique of thermal analysis shall now be mentioned: by this'technique the force of interaction and the amount of sorbed molecules may be studied. The different mechanisms of thermal degradation may also be investigated conveniently using this method which yields information on the inter-
mediate states which are often identical with the active catalytic species. Furthermore, information regarding the condition under which the zeolite is stable and where its crystalline structure is destroyed, can be obtained. Investigations of this kind are of great importance for the description of the limits of stability of zeolite catalysts in technical processes. In the following sections the problems of the methods mentioned shall be described in more detail with examples of application for different problems of the chemistry and technology of zeolites. Some further supplementary methods shall be discussed in short:
2 . SOME PROBLEMS OF THE INSPECTION OF CRYSTAL SIZE AND CRYSTAL
SHAPE A visual inspection of zeolite crystals and the inspection using a weakly enlarging instrument, such as a magnifying glass is important for geologists out in the field. In most cases, especially when inspecting synthetic zeolites, a microscope is needed. A microscope is usually used as a first means of identification by looking at the crystal shape of some specific zeolites. The shape of the crystals may sometimes be misleading because it depends on the conditions of growth. More detailed information on the crystal classification may be obtained using a polarization microscope. A very impressive example of the use of the optical microscope may be found in the kinetic studies of an X-type zeolite published in a paper by ZHDANOV and SAMULEVICH (1). These authors collected about 20 of the largest crystals, out of a batch of growing NaXcrystals, and observed their further growth under the microscope. From the increase of the crystal size they obtained the linear growth rate. At the end of the crystallization a representative sample of the product was taken and a histogram of the crystal size distribution was obtained from microscopic measurements. Extrapolating this distribution function with the growth rate which has previously been determined, the kinetics of the nucleation and the kinetics of the growth of the zeolite can be obtained.
In the field of electron optical instrumentation a wide range af techniques is available giving information concerning crystal habit and crystal size and other more specific characteristics. The recent literature in this fleld has been summarized by BAIRD (2). The main techniques used in zeolite research are connected to zransmission electron microscopy (TEM), whereby the technique yielding most information is connected with the use of the scanning electron microscope (SEM) accompanied by a scanning micro-probe analysis. With the transmission electron technique magnification up to ?bout lo6 can be obtained corresponding to a point to point reso-
0
lution of about 3 A. Depending on the sample, this resolution can be improved by using special imaging techniques known as brightfield, dark-field, or lattice imaging which shall not be discussed in detail here. In the scanning electron technique a fine beam of electrons is scanned over the surface of the sample using a system of deflection coils. The various signals produced by the interaction of the electron with the surface, such as secondary electrons, backscattered electrons, or X-rays can be used to form an image. Magnifications in the range of 20 - 50,000 are available with a resolution of about 100 A . on-metallic samples are usually covered with a thin film of carbon and gold to ensure a sufficient electrlc conductivity to prevent a surface charge which leads to distorted pictures. Another effect is the protection of heat sensitive material. The output of the secondary electron varies according to the accelerating voltage of the beam (5 - 50 kV) and the structural characteristics of the sample as well as the particular angle of the incident beam with respect to the surface features. The changes in the secondary electron current induced by these features exhibit therefore a 3-dimensional character of the image. The back-scattered electrons give a signal varying with the respective atomic number. Measuring the wave length of the induced characteristic X-ray radiation with special detectives an elementary analysis of the area hit by the beam can be carried out. This technique is known as electron micro-probe analysis. In principle, this scanning technique and the analysis of the characteristic X-ray radiation can also be applied in transmissions. This method is usually called 'Scanning Transmission Electron Microscopy' or STEM. This method has the advantage of better resolution as compared with the TEM, but has the disadvantage of diffi/ cult sample preparation. In the case of zeolites, high resolution transmission studies are very rare. The reason for this is that the preparation of the specimen is very difficult. After the first attempts made by MENTER (3) and, much later, by BURSILL et al. ( 4 ) it was recognized that structural information could be obtained by the use of electron microscopy. To demonstrate the possibilities of this method, a schematic picture of high-resolution image of a (001) p l a n e of a N a A single crystal is shown schematically compared to e model of the structure drawn in the respective scale (fig. 1 ) . Tt can be seen that the tunnel structure of the zeolite is excellently visible in these pictures. Studies of this kind have bee:. extended to NaA samples which were essentially amorphous showing ir
'ig.
1. Left schematic drawing of a high resolution image of a (001) plane of a NaA single crystal (4). Right model of a (001) plane of the A zeolite structure
rhe TEM image the same sub-unit fragments as in the crystal ( 5 ) . With the TEM technique especially SCHMIDT ( 6 ) and also ZXNER et.al. (7) have studied the distribution of metals in zeolite catalysts and their behaviour during the catalytic process. As has been indicated above the transmission technique generally suffers from the relatively great difficulties in the preparation of the specimen. The most powerful method in the investigation of zeolite problems is the scanning electron microscopy ( S E M ) . This method has above all the advantage of a simple preparation of the samples and gives quick information on the shape and the distribution of the size of the crystals and also of the presence of amorphous material. In fig. 2 an example for the use of the SEM in zeolite synthesis is given. The picture shows two examples of A- and X-type zeolites which were grown from batches with very similar composition, varying only in the excess of alkali. A higher excess of alkali leads to a higher rate of nucleation and finally to smaller particles. The A-zeolite crystallizes in well-defined cubes which are sometimes truncated by octahedral planes, depending on the condition of growth. The X-zeolite crystallizes in octahedra. Fig. 2d shows the X-crystal balls of agglomerated smaller crystals which
F i g . 2 . Raster scan m i c r o g r a p h s o f A a n d X z e o l i t e s grown from batches with d i f f e r e n t a l k a l i n i t y causing a d i f f e r e n t r a t e o f n u c l e a t i o n and d i f f e r e n t p a r t i c l e s i z e . 40 H20 A zeolite a . NaAlO 0 . 8 Na 0 SiO 2 2 A zeolite b . NaRlO 0.8 Na 0 S i 0 2 200 H20 2 2 2 c . NaA1O2 1 . 5 SiO 1.37 N ~ O H / S ~ O100 H20 X zeolite X zeolite d. N a A l O 1 . 5 S i 0 2 1 . 3 7 NaOH/Si02 150 H 2 0 , 2 2 2 w l t h 25 % of P The P zeolite c a n be seen i n t h e p i c t u r e d . by t h e b a l l s marked b y t h e a r r o w .
f i m f r o m center of the c r y s taL 'ig.
3. Dependence of the Al-content in different directions in ZSM-5 crystals on the distance from the center of the crystal (9).
are typical for the P-type zeolite which often occurs as an impurity during the crystallization of faujasite-type zeolites especially xith higher Si/~l-ratios. Extensive kinetic studies of the growth of faujasite based aainly on the analysis of the particle size distribution from scanning electron micrographs have been carried out by KACTREK et al. ( 8 ) . A
very interesting study done with the micro-probe analysis
has been published by v. BALLMOOS and MEIER ( 9 ) . The authors have
studied the distribution of silicon and aluminium in ZSM-5 crystals. These results are very important in explaining the mechanism of zrystallization of the zeolites, due to the rate of nucleation as s i e l l as the growth increase with the Si-content. Furthermore, there Is some importance in explaining the catalytic activity of the substances because the number of active centers within the channel
system is a function of the ~i/~l-ratio. v. BALLMOOS has embedded large ZSM-5 crystals and polished them in different crystallographic directions. From scanning experiments it could be seen thar the aluminium content in the center of the crystals was almost zero so that the conclusion could be drawn that obviously a nucleation of a species with a very high Si/~l-ratiooccurs. The consequences for the catalytic activity are not yet quite clear. Fig. 3 shows the dependence of the Al-content determined in different directions. In summary it may be said that by inspecting a zeolite sample with the appropriate instrument, the shape of the crystal, the t y ~ t of zeolite, and the impurities which are present in the sample may be identified. By observing the time dependency of the crystal size kinetic studies are also possible. Furthermore, special techniques such as electron-microscopy yield detailed information on the distribution of single components in the sample. By using high-resolution TEM-techniques structural details of the zeolite lattice may be resolved.
3. PRINCIPLES OF THE CHARACTERIZATION OF ZEOLITES BY DIFFRACTION TECHNIQUES The most frequently used method for identifying and describing a special zeolite structure, primarily used for patent purposes, is the X-ray powder diffraction method. The reason for this i= that this technique is very simple and that zeolites occur mainly as small crystals where single-crystal techniques cannot be applied. An extended collection of structures described in this way csr be found in the book of BRECK (10). In practice, the pattern obtained from Debye-Scherrer-, a Guinier-, or a diffractometer experiment are indexed and the spacings of the obtained ( h k l ) values are calculated from the angle of the respective peak. The intensities of the individual X-ray deflections are giver in a relative scale using the abbreviations vs, s, ms, m, w , vw, and vvw meaning 'very strong', 'strong', 'medium-strong','medium'. 'weak', 'very weak', and 'very very weak'. This is preferentially done when evaluating photographs. When inspecting diffractometer patterns usually the most intense peak is set to be 100 and the others are set relative to this. By this procedure in most cases 2:. identification of the respective zeolite and of the most important impurities can be achieved. If the impurity has at least one stror-; reflection located at an angle sufficiently far away from the reflections of the main component, impurities occurring only at a f ~ - -
percent can be detected. The unarnbiguos identification of a specific zeolite by such a set of powder reflections can sometimes be very difficult, if the structures of several zeolites in question show only slight variaticns. This has been very impressively shown by BRECK (10) comparing the powder diagrams of gismondine, phillipsite, P-zeolite, and some others. in fig. 4 another example is shown which has some importance in the identification of preparations in the ZSM-series for patent applications. Fig. 4 depicts the X-ray powder-diffractograms of the two pentasils ZSM-5 and ZSM-8. The difference claimed by the patent applications (11, 12) are connected in the group of reflections which are marked. In the ZSM-5 sample this group consists of reflections which are not very well resolved, whereas for the ZSM-8 this group is very well resolved. A third member of this family shows a systematic extinction of the h+k+l = 2n+l reflections and a merging of doublets into singlets which give ~ i s eto the interpretation that ZSM-11 has a body-centred tetragonal and 294-5 an orthorhombic unit cell. The channel structures which have been determined by KOKOTAILO et al. (13, 14) are shown schematically in fig. 5. Whether the ZSM-5 and the ZSM-8 pattern actually belong to different structures is not yet quite clear. Generally, by inspecting a powder pattern one can try and analyze the given structure. This has been.done, particularly for synthetic zeolites with some success. The intensities obtained by the powder patterns may be lost in the background or by an overlapping of reflections so that details concerning the structures, e . g . the exact positions of the cations cannot be resolved. There are several methods reported in the literature for analyzing the peak profiles and to avoid possible disadvantages. One advantage cf the powder data is the reduced secondary extinction which disturbs, for single crystals, especially the strong reflexes so that in some cases powder data may b e used as a supplement to single crystal data. For an exact determination of the crystal structure single crystals of at least 20 - 50 w are necessary and single crystal techniques need to be applied using as many X-ray intensities as possible. Due to the fact that in most diffractional processes the phase is lost and crystal structure determination is, in principle, a trial-and-error procedure, whereby the observed intensities are to
F i g . 4 . X-ray powder d i a g r a m s o f Z S M 5 , ZSM 8 a n d Z S M 11 around the region o f 2 8 = 22O-25O. ZSM 8 shows a s p l i t t i n g in t h e h i g h e s t peak which i s n o t o b s e r v e d f o r ZSM 5 and Z S M 11. The p a t t e r n of ZSM 11 h a s d i s t i n c t l y f e w e r lines t h a n t h e o t h e r s a s h a s been e x p l a i n e d in t h e text.
Fig.
Schematic drawing of the channel s y s t e m of
3e matched to the intensities calculated from structural models. The procedures necessary for a detailed structure determination are beyond the scope of this article. A summary of the modern xethods of structure refinements by X-ray methods may be found in =he book of LUGER (15). The problems which occur especially in the case of zeolite have been summarized by FISCHER (16). One of these difficulties is given by the determination of the 2xact space group symmetry. This difficulty has its origin mainly in the scattering factors of the Si- and Al-atoms which are very similar due to the similar electron density surrounding these at o m s . Therefore, t h e distribution of these a t o m s in the alumo-sili=ate framework seems to be random in most cases and the space group appears to have a higher symmetry than is actually the case. If the refinement is done using a space group with an average A1-0- and Si-0-distance which is too symmetric, then ordering effects m a y be averlooked.
F i g . 6 . Schematic drawing of t h e e l e c t r o n d i f f r a c t i o n p a t t e r n of t h e h01 zones of e r i o n i t e ( l e f t ) and o f f r e t i t e ( r i g h t )
Some problems of t h i s o r d e r i n g a r e d i s c u s s e d i n an a r t i c l e of SMITH (17), where f u r t h e r l i t e r a t u r e may be found. A s p e c i a l method o f f i n d i n g a p p r o p r i a t e models has been developed by MEIER ( 18) .
For d i f f i c u l t i e s of this kind the a p p l i c a t i o n of e l e c t r o n d i f f r a c t i o n i s advantageous. A f u r t h e r problem i s i n t h e s t a c k i n g f a u l t s p o s s i b l e i n t h e v a r i o u s f a m i l i e s of z e o l i t e s .
I n t h e e r i o n i t e - o f f r e t i t e system v e r y c a r e f u l e l e c t r o n d i f f r a c t i o n s t u d i e s have been c a r r i e d o u t by GARD ( 1 9 ) . F i g . 6 shows a s c h e m a t i c drawing of t h e h01 zones of e r i o n i t e and o f f r e t i t e . I t can b e d i s t i n c t l y s e e n t h a t e v e r y second row of t h e 1 - r e f l e x e s i s m i s s i n g i n t h e o f f r e t i t e system. I n s y n t h e t i c samples c a l l e d z e o l i t e T s t r e a k s p a r a l l e l t o c can b e o b s e r v e d i n d i c a t i n g d i s o r d e r i n t h e l a y e r s o f sixmembered r i n g s from which b o t h s t r u c t u r e s can be c o n s t r u c t e d . By t h e l o c a l i z a t i o n o f c a t i o n s i n s i d e t h e c a v i t y systems o f v a r i o u s z e o l i t e s t r u c t u r e s f u r t h e r problems o c c u r . The l i t e r a t u r e which has been p u b l i s h e d i n t h i s f i e l d up t o now h a s been summar i z e d w i t h t h e most r e c e n t r e s u l t s i n t h e a t l a s by MORTIER ( 2 0 ) .
1. SPECTROSCOPIC METHODS FOR THE CHARACTERIZATION OF ZEOLITES
According to the two points of view of a characterization of zeolite samples mentioned in the introduction the discussion of the xethods in this chapter shall be separated into procedures giving :nformation on the alumosilicate framework and structural problems zoncerning the cations within the channel systems of the different zeolites and the description of the effects of modifications of the zeolite samples in order to obtain sultable properties for the ~pplication,especially in catalysis.
2.1 The Infrared Spectroscopy and the ~rameworkStructures of Zeolites Beside the diffraction methods the infrared spectroscopy has zained some importance for the structural characterization of zeolites, especially in situations where no unambiguous X-ray patterns =an be obtained. The most important arguments for the application of this xethod are the straightforward sample preparation and an instrumenzation which is readily available in many laboratories. Typical for the spectra of the alumosilicate lattice is the socalled mid-infrared region of 400 - 1300 cm-l. After a series of papers by various authors on zeolites and Zther tectosilicates FLANIGEN, KHATAMI and SZYMANSKI (21) have 5one pioneering work in measuring spectra of a great variety of zeolites and collecting the arguments for their interpretation. An sxcellent review of the most important literature has been given by FLANIGEN (22). According to FLANIGEN, SZYMANSKI and KHATAMI (21) the midLnfrared vibrations can be cLassified into two types, named internal and external vibrations. - - as they The internal vibrations belong to the ~ i / ~ l 5 qor 2re commonly called - the TOq-tetrahedra. These vibrations are present in the spectra of all zeolites with small changes regardless -f the type of the framework of the zeolite. The external vibrations depend on the structure and are assigned to linkages of the TO4-tetrahedra typical for a special framework topology. Groups of this kind are the rings and double rings, present in a great variety of zeolites and also the various ~olyhedralunits. Thus, the different zeolites have typical spectra, which are 2.9. suited for an identification at least of the zeolite family in
asyrnm. symmetric stretch stretch,
T- 0 , ring , bend,pore,
V'" Fig. 7. Assignments of the different infrared wave number regions with a spectrum of zeolite Y. Solid lines: internal tetrahedra, structure insensitive Dotted lines: external linkages, structure sensitive
many cases. The exact assignment of the bands is rather difficult. An assignment can be only done by a careful empirical comparison of the spectra from zeolites with different structural elements known fror X-ray diffraction studies. In a series of papers comparisons with Raman spectra were made to check special assignments. Fig. 7 shows a spectrum of a N a Y zeolite containing the different assignments adopted until now. The frequency regions where the different kinds of vibrations are located are summarized in the Table 1.
-1
Table 1. Zeolite Infrared Assignments, cm Internal Tetrahedra Asym. stretch 1250-950 Sym. stretch 720-650 T-0 bend 420-500
External Linkages Double ring 650-500 Pore opening 300-420 Sym. stretch 750-820 Asym. stretch 1050-1150 sh
From the bands summarized in Table 1 the two most intense bands are at 950-1250 cm-l and at 420-500 cm-l. These are common for all zec-
lites. The first is assigned to an asymmetric stretching mode and rhe second to a bending mode of the T-0 bond. The bands in the reqion of 650-720 cm-I are assigned to a symmetric stretching within -he TO -tetrahedra. 4 A11 other bands are more or less distinctly dependent on the crystal structure. This can be seen from the following figure 8 where samples of the A and the faujasite structure with different Si/Al-ratios are compared. For all zeolites a nearly linear decrease has been observed for the asymmetric stretch stretch band near 980 to 1100 em-I with increasing fraction of A1 in the tetrahedral sites, as can be seen from Fig. 8. In the region of the external bands, a medium strong band near 300-650 cm-1 is present in all structures with double rings as e.g. zeolite A, the faujasites, chabazite, gmelinite and the offretiteerionite system. Zeolites without double rings show only weak intensities in this region. Inconsistencies are observed with the structures of P and Omega, which contain no double rings. For the band of double sixmembered rings an influence of the Si/~l-ratioof the zeolite could be observed. Further, distinct dependencies of the lattice vibrations on =he cations in the zeolite could be established in the symmetric stretching region as well as in the bending vibrations of the TO4 tetrahedra. Especially deKANTER et al. (24) were able to demonstrate linesr relationships of several types of bands from the reciprocal sum 2f the radii of the ion and the respective cation. From investigations of this kind informations on the cation distribution xithin the zeolite lattice can be obtained. An important application of the lattice vibrations in the midinfrared region is the investigation of nucleation mechanisms during the crystallization of zeolites. In the paper of FLANIGEN (22) 2 series of spectra of growing NaX is demonstrated, showing typical sands of the faujasite from the initial gel to the well crystallized sample. Especially mentioned is a band near 575 cm-I which may ?e assigned to double sixmembered rings.
Furthermore, two papers of JACOBS et al. (25) and COUDOURIER ~t al. (26) shall be mentioned using IR-spectroscopy for the idenzlfication of small particles of ZSM-zeolites, showing no X-ray sattern but the typical effects of the catalytic activity of these substances. This activity could be closely related to the presence ~f a band near 550 cm" from which also an analysis of the pantit?
1200 1000 800
wave number[cm-' 1
MX)
400 20:
wave nurnber[cm-I
Fig. 8. I n f r a r e d spectra o f z e o l i t e samples w i t h d i f f e r e n t S i / ~ l ratio. L e f t : A and NA z e o l i t e s R i g h t : X and Y z e o l i t e s
of the z e o l i t e could be c a r r i e d o u t . 4 . 2 I R - S t u d i e s of S u r f a c e S t r u c t u r e s I m p o r t a n t f o r S o r p t i o n and
Catalysis The s t r u c t u r e s which a r e involved i n t h e m o s t applications i n c a t a l y t i c a l r e a c t i o n s are OH-groups i n d i f f e r e n t s i t u a t i o n s with respect t o the l o c a l structure i n the zeolite framework. These groups act as m o r e o r l e s s s t r o n g ~rbnstedtsites and a r e situated a t t h e Si o r A1 atoms of t h e framework o r a t t h e cations. F u r t h e r , i n d e h y d r o x y l a t e d zeolites a t t h e aluminium atoms L e w i s sites may
1
be formed. Special effects of transition metals present in the structure shall not be discussed in this connection. Typical wave numbers of the bands of the OH groups lie between 3000 and 4000 cm-I and demand some care in the preparation of the samples jecause of the radiation loss from scattering. The preferred technique is here the use of thin self-supporting wafers made of small ?articles.
Because of the strong bands of water in the frequency ,reqion mentioned, typical investigations are carried out on the dehydrated samples or with samples with very low water contents. The presence of water is easily detected by the presence of its bending vibration near 1650 cm-1.
The history of the investigations in this field and the various arguments of an interpretation of the observed bands in different zeolites have been summarized by WARD (27). A more r e c e n t S u r TbTeyhas been given by KUSTOV et.al. (28). Dehydrated samples of A, X, and Y zeolites with alkali cations show no structural OH groups if hydrolysis is avoided. The only band whlch is observed in these samples has a wave number near 3740 crn-I and belongs to terminating silanol groups at the edges of the crystals or at amorphous regions within the crystals. Ba containing samples show sometimes the same behaviour. For zeolites containing divalent cations, again the band at 3740 crn-' and additional bands near 3650 cm-I and 3540 cm-I are observed which also appear in decationized zeolites and can be assigned to different kinds of S i O H groups. A common interpretation of these effects is a hydrolysis mezhanism
+
About an assignment of a band at 3570-3600 cm-1 to the Me OH groups no agreement could be obtained. Careful studies have been carried out with rare earth exchangrd Y zeolites. In general, after removal of the physical adsorbed xater at 450'~ hydroxyl groups near 3740, 3640 and 3540 cm-I re-
.;ah. An additional band at 3470-3520 cm-l is sensitive against the radius of the RE-cation and has to be assigned to RE*' (OH) 2rOups originating from 3+
RE
(H20)
5- Zeolite
i
=
RE^^ (OH) + HO-zeolite
but this band vanishes upon heating to 2OO0c. More detailed assignments of the bands can be obtained from studies of the interaction of the acidic hydroxyl groups with organic bases like pyridine or piperidine. In this way the band at 3640 cm-' has been assigned to OH groups in the supercages of the Y zeolites because it is changed under the influence of a sorptior. of the mentioned bases.
The 3470-3520 cm-' band is not influenced under the sorption of piperidine which is consistent with X-ray results, showing that the RE ions are in the S; positions inside the cubooctahedra as long as the band is observed and go into the hexagonal prisms at higher temperatures, which is accompanied by a vanishing of the band. The deammoniation and the formation of NH4X and NH4Y has been carefully studied by (30). The evolution of the ammonia cou,ldbe appearance of the bands of the ammonium ion 1670 cm-I.
the hydrogen forms of UYTTERHOEVEN et al. observed by the disnear 1450 cm-I and
Summarizing this and a series of other papers it could be stated, that the deammoniation starts at 100°C and is finished at about 400°C depending on the conditions of the heat treatment. The decationized Y zeolite has three major bands at 3740, 3640 and 3540 crn-l . X-ray studies by OLSON and DEMPSEY (31) of the T-0 distances in a HY sample lead to the result that the band at 3640 crn-1 belongs to an OH groups at the oxygen atoms of hexagonal prisms connecting the cubooctahedra, usually denoted by 01. The band at 3540 cm-' was assigned to the 04-ions and is directed intc the hexagonal prisms, which agrees with the most sorption experiments. Deviations are explained by a mobility of the protons. Treatment at higher temperatures leads to the formation of Lewis sites by dehydroxylation, which starts at 400-500°C. During this procedure an increase of the 3740 cm-l band can be observed indicating the formation of amorphous silica. As long as the structure is intact the bands can be restored by a readdition of water. A great variety of papers has been published on decationized zeolites containing defined amounts of different cations, ultrastabilized zeolites and zeolites steamed under different conditions which are adopted to the treatment in technical reactors. A good deal of the literature up to 1976 has been summarized in the paper of WARD (27) already mentioned. Generally, the results in these fields are difficult to survey on a common basis, because of the varying conditions of the sample preparation and treatment.
JACOBS ( 3 2 ) has pointed out in a recent summarizing paper t h z r
the wave numbers of the OH stretching vibrations can be correlated with the acidity of the respective groups. Looking at the spectra of a variety of zeolites the wave numbers of a special OH vibratl of the ion d e c r e a s e s almost linearily with increasing ~ i / ~ratio zeolite. This has been related to an increased acid strength because of a decreasing interaction of the proton with the framework. These arguments have been checked by quantum chemical CNDO calculations of OH groups at 0-bridges with a varying S i / ~ lratio of the neighbouring region (33). Another correlation has been found to the average electronegativity defined by SANDERSON ( 3 4 ) . Looking at an arrangement of atoms with the composition XxuyZ, and and electronegativities of the single components SX,Sy and SZ the average electronegativity S of this arrangement is given according to SANDERSON (34) by S
=
(SXsYs X
Y
Z
z
)
l/{x+y+z)
-1 Plotting the exact wave number e.g. of the 3650 crn vibration against this parameter a linear relationship is obtained. Measuring now, the spectrum of an unknown zeolite, at least indications of the acidity of its OH groups can be obtained.
The changes of the wave numbers under the influence of sorbed molecules obey also a linear relationship with the parameter S. The same is true for the reactivities of a number of acid c a t a l y z e d reactions. Finally, it shall be mentioned that IR spectroscopy allows to distinguish Brghstedt and Lewis acidity in a zeolite sample which may be a partly dehydroxylated BY or H mordenite. The pyridineproton complex shows a very strong band at 1545 cm-I and strong bands at 1655, and 1627 cm-1. The respective complex of a pyridine molecule with a Lewis site has a very strong band at 1455-1442 cm-' and a strong band at 1490 cm-1 which is, however, common to both complexes. Similar bands occur with 2,6-dimethylpyridine. The advantage of this molecule is that because of a steric hindrance of the formation of the complex with the Lewis site, the molecules react first with the Br#nstedt sites and after the saturation of these sites with the Lewis sites (35). A more detailed paper on ~ e w i ssites has been published by JACOBS and BEYER (29)
4.3 Nuclear Magnetic Resonance Studies for the Characterization of Zeolites Up to about five years ago in NMR investigations on zeolites almost only protons were used, looking at the adsorbed molecules and its mechanisms of mobility and in some cases on the OH groups
present inside the cavities of acid zeolites.
Only a few papers dealt with the resonance of nuclei of the cations. For these investigations mostly quadrupolar nuclei w e r e studied giving information on the field distribution around the cation. One of the most important events for zeolite research is the development of high resolution spectroscopy of solids. This development has two important reasons. One is the possibility to create high magnetic fields by the use of superconducting magnets. High fields increase both the sensitivity of the detection and the chemical shift of nuclei in different chemical surroundings. The chemical shift is the most important information obtainable from a high resolution spectrum. All other interactions of the nuclei, especially the dipole-dipole interaction, are unaffected by an increase of the magnetic field. The other important technique developed for routine use in the last year is the magic-angle-spinninc (MAS) by which the dipole-dipole interaction of the nuclei can be minimized below the resolution necessary to detect chemical shifts Further importance have the fourier transform and computer averaging techniques, which allow the accumulation of a large number of spectra and an improvement of the signal-to-noise ratio. Finally, the double resonance techniques shall be mentioned which are in the most cases used for the decoupling of the dipole-dipole interaction of the nuclei in question with the protons of the surroundings. The most valuable informations which could be obtained from these techniques lie in the 2 9 ~ iresonance of the alumosilicate framework of th~~zeolites. For the investigation of sorbed molecules 13c- and N-spectra have got some importance.
4.4 The i~''
Resonance and the Distribution of Silicon and Aluminium in the Alumosilicate Framework of Zeolites
The technique of the 2 9 ~ iresonance in zeolite chemistry was first introduced by LIPPMAA et al. (36, 3 7 ) , who showed that well resolved high resolution spectra can be obtained with a chemical shift characteristic for the local environment of the framework silicon. For a sample of zeolite NaX with S i / ~ l= 1.8 a spectrum with five peaks could be obtained belonging to the five possible local structures Si(n~l),where n can adopt the numbers 0 to 4. The chemical shifts could be assigned to the configurations of n A1 surrounding one Si given in the Table 2. A summary of a great variety of structures investigated by 29Si NMR has been given in the book of B A ~ R E R (38).
Table 2. Chemical shift ranges vs. TMS for Four Coordinated Si environments Chemical shift/ppm Si (4Al) Si(3A1 Si) Si(2Al 2Si) Si(lA1 3Si) Si (4Si)
From the data of the Table 2 follows a nearly linear relationship between the number of A1 atoms around a Si and the chemical shift value, taking into account that the shift is slightly dependent on the structure of the individual zeolite. The most detailed structural investigations have been published by LIPPMAA et al. (36, 37) and by MELCHIOR et al. (39, 40) on the distribution of silicon and aluminium in the alumosilicate lattices of X,Y,A and NA zeolites. The first problem which has already been mentioned in the discussion of the diffraction methods, is the validity of the Loewenstein rule. This rule states that two four coordinated A l atoms can never be nearest neighbours in alumosiLicate structures. The validity of this rule demands for the A zeolite which has a S i / ~ lratio of 1 a strictly alternating order of the Si and Al. Actually the spectrum shows only a single line which is located, however, at -89 ppm belonging to the Si(3A1,Si) coordination in many alumosiLicates (41). Therefore, the authors suggested an ordered structure consisting of pairs of A1-0-A1 and Si-0-Si. This structure could be finally ruled out by studying a series of Na zeolites with varying S i / ~ lratio showing that the observed value for the A structure can be very well understood as perfectly ordered according to the Loewenstein rule. For the faujasites MELCHIOR et al. (39) have carried out careful investigations. The authors compared the obtained spectra with models of different stages of refinement. ~t first it could be shown that the Loewenstein rule is strictly valid. From a calculation of the average number of the nearest neighbours from a random distribution and a distribution according to the Loewenstein rule compared with the respective values obtained from the spectra, it can be seen from fig. 9 that the random distribution can be ruled out very clearly.
. distribution
a
,
,
I
I
Fig. 9. Average number of A1 neighbours for a Si atom in faujasit~ structures with different s ~ / A Iratio, calculated for a random distribution and according to the ~oewensteinrule compared with the numbers evaluated from the 2 9 ~ iresonancE ( 3 9 ) , which are represented by the circles.
Further it could be shown that an improved fit to the experimental spectra is possible if the number of the next nearest A1-AI neighbours is minimized in a cubooctahedron for the respective s i / ~ lratio. For the cubooctahedra in which one, two and four A1 are replaced by Si the distributions given in fig. 10 should be favourable. From a suitable mixing of these configurations optima: fits to the experimental spectra can be obtained. Some of the spectra obtained for faujasite at 11.9 Mc are demonstrated in fig. ll. As a final result it can be stated, that the structure with the most sixmembered rings with two A1 in p-po-
?ig. 10. Cubooctahedra w i t h minimal numbers of A1-0-Si-0-A1 linkages for different ~ i / ratios ~ l (39).
sition are the most favourable compositions as far as they fulfill t h e topological constraints demanded by the overall structure.
4.5 The Use of Quadrupole Nuclei for the Characterization of Structures i n Zeolites Nuclei w i t h a s p i n I > 1 / 2 have an electric quadrupole moment which experiences a torque in e l e c t r i c field gradients caused by the crystal fields and bond electrons within a solid structure. Whereas from single crystals very detailed informations can be obtained about the field gradient tensor and the field distribution around the nucleus, these informations are for powders and especi-
Fig. 11. 2 9 ~ ispectra of faujasites with different s$/A~ ratios. The measured s p e c t r a are simulated by constructing structures from different amounts of the cubooctahedra shown in Fig. 10 ( 3 9 ) .
ally for complicated structures like zeolites rather poor. The e l e c t r i c field gradient tensor can be described by its value in the direction of the highest field inhomogeneity, called
L3
Fig. 12. Comparisons of two Na spectra of a dehydrated NaX sample with S F / A ~= 1.03 at different amplification and span with a computer simulated spectrum, using the data of model calculations of the electric field gradients at the different sites of the sodium ions.
"field gradient" and by the deviation from rotational symmetry perpendicular to this direction, which is described by a dimensionless parameter between 0 and 1 called "asymmetry parameter". A very careful single crystal experiment of natrolite has been published by PETCH and PENNINGTON (42). The authors showed that the A1 as well as the Na ions in the natrolite structure are in chemical equivalent positions and obtained very exact values for the field gradient tensors.
The absolute values for the field gradient at the sites of the A1 show that the AIOq tetrahedron is only weakly distorted. The field gradient at the Na sites is rather large compared with other sodium containing crystals. In a series of papers published by the author ( 4 3 - 4 6 ) the Na resonance and the resonances of other nuclei of the alkali group in faujasite and A zeolites have been measured. The results have been compared with model calculations of the electric field 23
gradient tensor. The aim of these investigations was to find a model suitable to describe the electric field distribution within the zeolite cavities, which is interesting for the explanation of the phenomena connected with the catalytic activity of these substances. In Fig. 12 two spectra of an X zeolite are compared with a 'calculated spectrum. It can be seen that in the calculated and in the measured spectra two peaks appear at lower fields the position of which agrees well with the position calculated for the S2 and the S ; sites. The strong intensity in the center of the line cannot be assigned exactly, but it must belong to the extra framework ions for which several different sites are reported (see e.g. the atlas of MORTIER ( 2 0 ) ) . The spectra can be explained by the assumption that these sites have varying field gradients and high asymmetry parameters, which seems to be plausible. For the calculation of the field gradients a point multipole model has been used. In this model at first the point charge contribution to the fields and field gradients at the sites of all ion's of a cubooctahedron was evaluated. With the obtained values, the strengths of the induced dipoles and quadrupoles were calculated for different polarizabilities of the oxygen ions and the contributions of these multipoles added to the point charge contribution. A similar model has been used by DEMPSEY (47) who has carried out the calculation self-consistent without the induced quadrupoles
.
The calculations have been done for different positions of the mentioned extra framework cations in the dehydrated zeolite and for different models with adsorbed molecules. In the fully hydrated zeolite NaX a single slightly asymmetric line can be observed which is appreciably narrower than the central line belonging to the transitions m = -1/2 +-) m = + 1 / 2 in the dehydrated zeolite. By measurements of pulsed experiments on Nay samples BASLER (48) has found that this resonance must be attribuked to S2 sites
with an adsorbed water molecule which changes its site rather quickly. The field gradients which can be obtained from this experiment for the S2 sites are in good agreement with the calculated values for the described configuration of a S2 Na ion with an attached water molecule. 27
BOSACEK et al. (49) have studied the A1 resonance in decationated samples of Y zeolite in comparison to the respective Nay, and samples with different degrees of cation exchange. The spectrum obtained in dehydrated Nay can be explained by the presence of sites with a distribution of field gradients and
and asymmetry parameters.
+
d-
When t h e Na i o n s a r e exchanged a g a i n s t NH4 i o n s and t h e samples a r e t r e a t e d a t 4 0 0 ° C OH groups a r e formed n e a r t h e A 1 i o n s and a l a r g e f i e l d g r a d i e n t i s c r e a t e d causing a v e r y broad and weak l i n e which cannot be observed any more. The formation of u l t r a s t a b i l i z e d z e o l i t e i n which a c e r t a i n amount of A 1 i o n s i s i n e x t r a l a t t i c e p o s i t i o n s i s s t u d i e d by complexing t h e s e i o n s by acetylacetone. The complex provides a r a t h e r symmetric surrounding of the A 1 nucleus and shows a narrow l i n e from which t h e number of these i o n s can be e a s i l y e v a l u a t e d . 4.6 Some S p e c i a l Questions Solved by Proton NMR Spectroscopy
I n t h i s s e c t i o n some problems of s p e c i a l s t r u c t u r a l i n v e s t i gations by p r o t o n resonance measurements s h a l l be d i s c u s s e d . The problems of s o r p t i o n and m o b i l i t y i n v e s t i g a t i o n s by p r o t o n NMR spectroscopy a r e beyond t h e scope of t h i s a r t i c l e . I n a paper of BASLER and MAIWALD ( 5 0 ) t h e OH groups i n t h e c a v i t i e s of A z e o l i t e has been s t u d i e d on a v a r i e t y of samples of d i f f e r e n t o r i g i n . These samples have been c a r e f u l l y checked f o r s t r u c t u r a l i n t e g r i t y by X-ray and s o r p t i o n measurements. A s h a s been observed a l s o f o r samples of f a u j a s i t e z e o l i t e s , t h e water molecules i n t h e supercages and t h e cubooctahedra can be q u a n t i t a t i v e l y s e p a r a t e d by NMR p u l s e experiments. The r e s p e c t i v e decay functions a f t e r a 90'-pulse a r e shown f o r a number of z e o l i t e s i n f i g . 13. The s h o r t decay belongs t o t h e water molecules i n s i d e t h e cubooctahedra and t h e long one t o t h e molecules i n t h e supercages. By a c a r e f u l a n a l y s i s of t h e temperature behaviour of both decay f u n c t i o n s it could be shown t h a t i n t h e s h o r t decay a t h i r d decay f u n c t i o n should be hidden w i t h almost t h e same decay t i m e , which could be a s s i g n e d t o A l ( 0 ~ ) o r s i m i l a r e n t i t i e s w i t h i n t h e 3 cubooctahedra. The amount of t h e s e e n t i t i e s i s d i s t i n c t l y dependent on t h e c o n d i t i o n s of t h e growth o f t h e samples. I t could be demonstrated t h a t o u t of 31 samples grown under d i f f e r e n t condit i o n s o r i n d i f f e r e n t l a b o r a t o r i e s no sample was without t h e s e aluminate i n c l u s i o n s .
S i m i l a r i n v e s t i g a t i o n s have been c a r r i e d o u t w i t h t h e z e o l i t e ( 5 1 ) . This s t r u c t u r e i s exc e l l e n t l y s u i t e d for s t u d i e s of h y d r o l y s i s p r o c e s s e s w i t h i n t h e s e large c a v i t i e s .
ZK 5 , c o n t a i n i n g o n l y l a r g e c a v i t i e s
D i r e c t l y a f t e r t h e removal of t h e o r g a n i c c a t i o n about two OH groups could be observed i n one c a v i t y , c a u s i n g an exchange of
protons w i t h t h e water molecules. This exchange e x p r e s s e s i t s e l f i n a minimum o f t h e tempera-
--
Nay
Fig. 13.
4W ppm Fe
k
ih---
Na Y 90 pp m F e
-
I
NaX
~agnetizationdecays after a 90°-pulse of a NMR pulse experiment on protons in different zeolites. The short decay belongs to water molecules inside the cubooctahedra, the long decay to the molecules inside the large cavities.
ture function of the transverse relaxation time as it is shown in fig, 14a. Treating the sample with 0.1 n NaOH the signal of the OH groups falls below the limit of detection and only one kind of protons can be observed in the magnetization decays as well as in the temperature functions of the relaxation times (fig. 14b). The absolute values of the relaxation time in the short component can be calculated assuming an interaction of the protons with the aluminium of the lattice taking the usual distances of an A ~ O Hbond. Finally, the problem of the mobility of the protons of the acid OH groups in the cavities of zeolites shall be discussed in short. This problem has been studied with NMR methods by MESTDAG et al. ( 5 2 ) , by FREUDE et al. ( 5 3 ) and by FREUDE and P F E I F E R ( 5 4 ) . FREUDE and PFEIFER have shown from the temperature dependence of the transverse relaxation times in decationized Y and.mordenite that the correlation times of the motion of the protons are dependent on the temperature of the pretreatment. A strong increase of the mobility with residual ammonia could be observed. A systematic study showed that the pyridinium ions have an increased mobility, too. The number of the mobile protons could be obtained by measuring the proton NMR relaxation after the sorption of deuterated pyridine. The absolute number of pyridinium ions could be also determined by 13c resonance measurements, where characteristic shifts can be observed in the spectra of the protonated and the nonprotonated species. The ratio of the number of the acid sites and the correlation
-- (H,Na)ZK-5 TI t o t a l :- (a) 0000 00 0 0
C
NaZK-5
o 0 0 O
rn
00%'
-
T2 H 2O
w w 7
TI t$o 0
8 0
%a
i (b) .2.0,.0
0
r
--
T2 H20 @a,
w
a
-.
L
I
o
-,
I
L
m
. -
e
E-
A T2OH
--
,r
Fig.
I
a
a
*
A
1
14. Temperature functions of t h e l o n g i t u d i n a l r e l a x a t i o n times T1 and t h e temperature functions of t h e nuclear r e l a x a t i o n times T I and T2.
a.
T2 of water (black c i r c l e s ) , T2 of OH groups (triangles) T of a l l protons (open c i r c l e s ) of (H,Na)ZK 5 with 2 4 8 rng of water/g;
b.
T2 (black c i r c l e s ) and TI (open circles) of water i n NaZK 5 with 317 m g of water/g.
time of t h e mobility of i t s protons which can be regarded a s t h e r a t e of c r e a t i o n of f r e e o r protons a t t a c h e d t o a sorbed molecule i s a good measure f o r t h e a c i d i t y of t h e r e s p e c t i v e OH group.
4.7 The Application of Moessbauer Spectroscopy in Metal Containing Zeolites In zeolite research Moessbauer spectroscopy is applied mostly for the investigations of iron in its different oxidation states and local environments. The oxidation state may be seen from the shift of the observed lines and the environment expresses itself in a characteristic structure of the spectrum because the 5 7 ~ enucleus usually applied for these investigations has an electric quadrupole moment which may interact with gradients of the electric field of its surroundings as it has been discussed in connection with the NMR experiments with quadrupolar nuclei. Furthermore, magnetic ordering in a sample can be detected because the 5 7 ~ enucleus has the spin 3 / 2 . Investigations of this kind have received considerable attention because of their importance in adsorption and catalysis. The different oxidation states of the iron and especially the conditions of the existence of the zerovalent iron inside the cavities has become important for the preparation of catalysts for hydrogenation reactions. The characteristic argumentation shall be shown from results obtained by SCHMIDT ( 5 5 ) . Fig. 15 shows a Moessbauer spectrum of a Y zeolite which has been exchanged under very careful pH conditions under nitrogen with ferrous sulphate to prevent the formation of ferric hydroxide. The samples were partly reduced, by exposing them to Na vapour at 673 K. The central doublet is due to iron clusters, which can be shown to be superparamagnetic by magnetic measurements. This superparamagnetism expresses itself in the weak six peak component in the spectrum. The results obtained from these investigations have been confirmed by electron microscopic experiments. Numerous investigations by Moessbauer spectroscopy have been reported by REES et al. (56, 57) and by DELGASS et al. ( 5 8 ) .
4.8 Further Application of Spectroscopic Methods In this section some further spectroscopic methods shall be mentioned in short which have been applied to solve special problems in zeolite research which are not typical characterization problems.
A special field of research in zeolite chemistry is the investigation of transition metals and transition metal complexes. The transition metal complexes have been proposed since a long time as reactive intermediates in heterogeneous catalysis. Inspite
.
-8.0
-4.8
I. 6
-1.6
4.8
8.0
velocity [ mm/sec] 0
2+
Fig. 15. Moessbauer spectrum of a Fe - Y / F ~ -Y z e o l i t e (a) at 298 K and its computer fit (b).
Fig. 16. ESR powder absorption spectrum for an axially symmetric g-tensor (a) and its derivative (b). of the great importance comparatively few work has been done in this field. Some of the most important papers have been published by LUNSFORD (59). The theoretical background has been summarized by KASAI and BISHOP (60). The shape of an electron spin resonance ( E S R ) spectrum is mainly determined by the g-tensor and the hyperfine coupling tensor. The g-tensor measures the deviation of the g-value from the value ge of the free electron caused by a spinorbit coupling which is dependent on the direction in the crystal. Because zeolites are usually not available in single crystals which are large enough to measure this tensor only the socalled powder patterns are usually observed. Fig. 16 shows a powder pattern of an axially symmetric g-tensor, and its derivative which is measured in the ESR technique. The hyperfine splitting is caused by an interaction with the nuclear spins in the neighbourhood of the electronic spin. In the usual practice the parameters of the g-tensor and the hyperfine splitting are taken approximately
Fig.
17. ESR The 1 4 due
f
spectrum o f NO adsorbed on Cu -Y z e o l i t e . s p l i t t i n g i s due t o t h e h y p e r f i n e c o u p l i n g w i t h t h e s~p i n . The d i s t u r b a n c e i n t h e c e n t r a l p a -r t i s p o s s i b l y t o t h e p r e s e n c e of a s m a l l amount of c u 2 + i o n s .
from t h e measured spectrum and t h e n a computer f i t i s made t o f i n d the e x a c t v a l u e s , and t o d e c i d e whether a d d i t i o n a l i n t e r a c t i o n s have t o be t a k e n i n t o a c c o u n t . A t y p i c a l spectrum i s shown i n f i g . 1 7 w i t h NO sorbed on Cu(1)Y. The spectrum i s s i m i l a r t o t h a t shown by KASAI and B I S H O P ( 6 0 ) . The s p l i t t i n g i s due t o t h e i n t e r a c t i o n w i t h t h e 1 4 n~u c l e u s .
G e n e r a l l y from t h e symmetry o f t h e g - t e n s o r c o n c l u s i o n s on t h e symmetry o f t h e c r y s t a l f i e l d a t t h e s i t e o f t h e c a t i o n o r o f the s o r p t i o n complex can be drawn, which can be r e f i n e d by t h e u s e of t h e d a t a from t h e h y p e r f i n e i n t e r a c t i o n . The t r a n s i t i o n m e t a l c a t i o n s and a l s o t h e s o r p t i o n complexes have been s t u d i e d a l s o by o p t i c a l s p e c t r o s c o p y i n t h e r e f l e c t a n c e technique g i v i n g a l s o i n f o r m a t i o n on t h e c r y s t a l l i n e o r t h e l i g a n d f i e l d a t t h e s i t e of t h e i o n . The b a s i c problems of t h i s f i e l d have been summarized by K L I E R and KELLERMANN ( 6 1 ) . A r e c e n t comp a r i s o n o f a l l methods on t h e rhodium complexes i n z e a l i t e s h a s
been made by LUNSFORD ( 6 2 ) . Some papers on t h e valence s t a t e of t r a n s i t i o n m e t a l s i n zeol i t e s can be found by t h e X P S method ( 6 2 ) .
5. THE CHARACTERIZATION BY ADSORPTION OF SPECIFIC PROBE MOLECULES Sorption p r o c e s s e s w i l l be t r e a t e d i n a s e p a r a t e l e c t u r e of t h i s c o u r s e . Therefore, i n t h i s c h a p t e r o n l y some s o r p t i o n e x p e r i ments s h a l l be mentioned, which may be used a s a means of charact e r i z a t i o n of a s p e c i f i c z e o l i t e . A f i r s t information on t h e pore volume can be o b t a i n e d from t h e s o r p t i o n c a p a c i t y f o r water. The amount of l a r g e r p o r e s may be determined by t h e s o r p t i o n c a p a c i t y f o r l a r g e r molecules which cann o t e n t e r e , g . t h e cubooctahedra of f a u j a s i t e . A molecule o f t e n used f o r t h e s e purposes i s cyclohexane.
Sorption c a p a c i t i e s a r e u s u a l l y determined by balance t e c h n i ques o r f o r gaseous components volumetric. Another p o s s i b i l i t y i s t h e g a s chromatographic method. The accuracy o f such a determinat i o n i s mostly b e t t e r than one p e r c e n t . Thus, t h e s o r p t i o n capacit i e s a r e o f t e n used t o c h a r a c t e r i z e t h e p u r i t y e . g . of a s y n t h e t i c z e o l i t e from i n c l u s i o n s of s i l i c a o r o t h e r d i s t u r b a n c e s of t h e structure. I n e a r l i e r times t h e z e o l i t e s have been c l a s s i f i e d according t o t h e i r a b i l i t y t o adsorb o r t o exclude molecules of a p a r t i c u l a r s i z e . D e t a i l s a r e summarized i n t h e book of BRECK ( 1 0 ) . The s o r p t i o n isotherms c h a r a c t e r i z i n g t h e s o r p t i o n behaviour f o r any s p e c i f i c s u b s t a n c e can be measured i n more d e t a i l by t h e same methods a s t h e s o r p t i o n c a p a c i t i e s . The balance technique can be regarded a s having t h e h i g h e s t a c c u r a c i e s , where e s p e c i a l l y a l s o d i f f u s i o n d a t a can be obtained o b s e r v i n g t h e e s t a b l i s h m e n t of t h e equilibrium. Very conveniently t h e isotherms can be measured by g a s chromatographic s t e p o r p u l s e methods, where t h e c o n c e n t r a t i o n of t h e s o r b a t e i n a g a s stream over t h e z e o l i t e i s changed i n a s t e p o r a p u l s e of t h e substance i s i n j e c t e d i n t o t h e stream of a c a r r i e r g a s . From t h e response f u n c t i o n t h e isotherm can be obtained. For a p u l s e t h e isotherm may be c a l c u l a t e d e.g. by t h e r e l a t i o n s given by HUBER and GERRITSE (63)
.
The g a s chromatographic method i s p r e f e r a b l e i f isotherms o v e r a wide range o f e q u i l i b r i u m p r e s s u r e s a r e needed. E s p e c i a l l y f o r l i q u i d s low p a r t i a l p r e s s u r e s a r e d i f f i c u l t t o m a i n t a i n exactl y o v e r a long time which i s necessary i f t h e isotherms a r e meas-
ured by the balance. From the isotherms at different temperatures the heats and entropies of sorption may be evaluated giving information on the strength of the interaction of the molecules with the cavity system and among themselves. Special effects have to be expected, if reactions inside the cavities take place, as this is the case e.g. at sorption processes of bases in zeolite cavities with acidic groups. An example will be discussed in connection with the thennoanalytical methods.
6. CHARACTERIZATION OF ZEOLITES BY THERMAL ANALYSIS
Thermoanalytical methods are among the most important tools of the characterization of zeolites. Generally, thermal analysis describes a group of methods whereby the dependence of the parameters of any physical property of a substance on temperature is measured. The two techniques measuring the change of heat and the change of weight are the methods used preferably for the characterization of zeolite properties. These methods are called differential thermal analysis (DTA) and thermogravimetric analysis (TGA) . In both methods the sample and possibly a reference sample are heated or cooled at a controlled rate. In the DTA technique the difference in temperature between a substance and a reference material against either time or temperature is recorded. If any heat releasing or heat consuming process takes place in the sample, the temperature of the sample increases or remains behind the temperature of the reference. If the process is finished the temperatures of both specimen become equal again. The peak, obtained in the recorded curve can be evaluated to get the kinetics and the amound of the heat transfer. In the thermogravimetric analysis the weight of the sample is recorded in dependence on the temperature. In modern devices both principles of measurement are often realized in one apparatus. The DTA method has a sensitivity of about lo-* Joule. With the TGA method weight changes of about 10-8 g can be detected. A summary of the most important effects observed in zeolites has been recently published by DIMITROV et.al. (64).
temperature [ K ] Fig. 18, DTA curves in the region of the dehydration for A zeolite with different cations.
In the typical behaviour of a zeolite being heated and subjected to differential thermal analysis three typical regions can be distinguished. The first region begins slightly above room temperature, has its maximum mostly near 500 K and is finished at about 750 K. This region expresses itself as an endotherm in the DTA curves and is caused by the evolution of water and possibly other volatile substances in the zeolite cavities. Between about 900 and 1500 K often two exotherms can be observed which are associated with the collapse of the zeolite lattice and sometimes at much higher tempqratures recrystallization to a new phase.
In the first region often a stepwise evolution of water can be observed. Fig. 18 shows DTA curves for A zeolites containing different cations.
A LiA
900
3000 1100 1200 1300
temperature [ K ] Fig. 19. High temperature effects in the DTA curves of different A zeolites.
A detailed thennoanalytical study has been carried out by DYER and WILSON ( 6 5 ) on NaA, who measured beside the DTA curves thermogravimetric data which were compared with X-ray experiments looking at the sites of the cations and water molecules. According to these studies the first endothermic effect at about 395 K is connected with an evolution of 10 water molecules per unit cell which are bonded very loosely. A second peak at 438 K is connected with a loss of 8 water molecules, which have been sorbed at the sodium ions in the S1 positions. Of the remaining 10 molecules six are released at temperatures between 445 and 625 K. The four molecules inside the sodalite cavities leave the structure at higher temperatures.
Changing the cations characteristic changes in the DTA curves are observed which depend on the degree of the exchange and may often be explained by the typical complexes with these ions. The high temperature effects have been extensively studied by BERGER et al. (66). Some typical exothermic effects of A zeolites with different cations are shown in Fig. 19.
The process of the dehydration of faujasite zeolites is more complicated. This is especially also due to the fact that at higher temperature dehydroxylation occurs. Special attention has been devoted to the deammoniation and the formation of the hydrogen form. In dependence on the composition of the sample for the end of the deamrnoniation of various zeolites temperatures between 570 and 770 K are reported, so that the dea-oniation and the dehydroxylation often cannot be resolved. The process can only be studied exactly by analyzing the released substances simultaneously to the DTA measurement. Furthermore, in the high temperature effects influences of ultrastabilization can be detected. A summarizing article on this topic has been published by McDANIEL and MAHER ( 6 7 ) . For the two methods a large number of variations have been reported giving answer to questions different properties of zeolites and zeolite catalysts. Finally, a special method shall be mentioned with which centers of different acid strength can be characterized. The acid form of the zeolite in question is exposed to ammonia at temperatures near 500 K, where no more physical adsorption occurs and the sorbed ammonia is then desorbed by programmed heating at higher temperatures near 850 K detecting the amount in a TGA experiment. The temperature at which an evolution of ammonia is observed, is then a good measure of the acidity of the respective site. Although thermal analysis gives valuable information on a series of properties of zeolites and zeolite catalysts, it has not successfully provided a reproducible and standard method for measuring thermal properties because too many factors of the particular instrument and the conditions of the experiment influence the measured parameters. ~lthoughit is often quite useful for direct comparisons, it is difficult to compare results reported by different authors.
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62. Lunsford, J.H., The Chemistry of Ruthenium in Zeolites, in Studies in Surface Science and Catalysis, Vol. 12, Metal Microstructures in Zeolites, P.A. Jacobs, N.I. Jaeger, P. Jiru and G. Schulz-Ekloff, eds. (Elsevier Publ. Comp., Amsterdam, Oxford, New York 1982) p. 1 63. Huber, J.F.K. and G. Gerritse, Journal of Chromatography 58 (1971) 137 64. Dimitrov, Ch., Z. Popova, S. Mladenov, K.-H. Steinberg and H. Siegel, Zeitschrift fur Chemie 21 (1981) 387 65. Dyer, A. and M.J. Wilson, Thermochimica Acta 5 (1973) 91 66. Berger, A.S., T.I. Samsonova and L.K. Jakovlev, Izvest. Akad. Nauk USSR, Ser. Chim. (1971) 2129 67. McDaniel, C.V. and P.K. Maher, Zeolite Stability and Ultrastable Zeolites, in J.A. Rabo, Zeolite Chemistry and Catalysis, ACS Monograph 171 (Arner. Chem. Soc. Washington D.C. 1976) p. 285
STRUCTURAL CHARACTERIZATION OF ZEOLITES BY HIGH RESOLUTION MAGIC-ANGLE-SPINNING SOLID STATE 2 9 ~ i SPECTROSCOPY - ~ ~ ~
a a a Z e l i m i r G a b e l i c a , J a n o s 3.Nagy , P h i l i p p e B o d a r t Guy Debras a , E r i c G . Derouane a , C and P e t e r A . J a c o b 's b a - F a c u l t & 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 C a t a l y s e , Rue de B r u x e l l e s , 61 B-5000 Namur, Belgium b-Centrum v o o r O p p e r v l a k t e s c h e i k u n d e e n C o l l o i d a l e Scheikunde, K a t h o l i e k e U n i v e r s i t e i t Leuven, D e C r o y l a a n , 4 2 , B-3030 Leuven ( H e v e r l e e ) Belgium c-Present address : Mobil T e c h n i c a l C e n t e r , C e n t r a l Research D i v i s i o n , P.O. Box 1025, P r i n c e t o n , N J 08540, U.S.A. ABSTRACT The s t r u c t u r e o f v a r i o u s n a t u r a l and s y n t h e t i c z e o l i t e s was i n v e s t i g a t e d by h i g h r e s o l u t i o n magic-angle-spinning (HRMAS) o s c~ o p y~ . The NMR a n a l y s i s i s b a s e d on s o l i d s t a t e 2 9 ~ sip e-c t r~ the g r e a t s e n s i t i v i t y of t h e 2 9 ~ i - c h e m i c a l s h i f t s e i t h e r on t h e chemical environment ( i . e . t h e number o f aluminium atoms i n t h e second c o o r d i n a t i o n s h e l l of s i l i c o n ) , on t h e c r y s t a l symmetry ( i . e . t h e number of c r y s t a l l o g r a p h i c a l l y d i f f e r e n t s i t e s ) o r on local s t r a i n s i n the c r y s t a l . Silicon-aluminium o r d e r i n g s c o u l d b e d e s c r i b e d i n z e o l i t e s such a s f a u j a s i t e s , m o r d e n i t e , f e r r i e r i t e , ZSM-5, ZSM-8, ZSM-11, ZSM-39 and ZSM-48 by examining s y s t e m a t i c a l l y e i t h e r p r o g r e s s i v e ly d e a l u m i n a t e d samples o r s y n t h e t i c z e o l i t e s w i t h d i f f e r e n t S i / A l r a t i o s . The S i / A l r a t i o s were d e t e r m i n e d from t h e r e l a t i v e l i n e i n t e n s i t i e s . It i s concluded t h a t aluminium atoms occupy p r e f e r e n t i a l l y s p e c i f i c s i t e s i n ZSM-5, ZSM-11, m o r d e n i t e , f e r r i e ZSM-8 and ZSM-48 z e o l i rite,ZSM-35 and p r o b a b l y a l s o i n ZSM-39. t e s a r e shown t o c o n t a i n combined 4- and 5- membered r i n g s forming layer sequences s i m i l a r t o those c h a r a c t e r i z i n g p e n t a s i l s t r u c t u r e s . M R a l s o used t o follow t h e d i f f e r e n t F i n a l l y , HRMAS ~ ~ S ~ - N was silicon-aluminium arrangements t h a t occur d u r i n g p r o g r e s s i v e t r a n s f o r m a t i o n s of amorphous g e l s i n t o c r y s t a l l i n e ZSM-5 p h a s e s .
1 . INTRODUCTION The h i g h r e s o l u t i o n magic-angle-spinning (HRMAS) s o l i d s t a t e NMR h a s become a p o w e r f u l t o o l f o r d e t a i l e d s t r u c t u r a l i n v e s t i g a t i o n s of s o l i d z e o l i t i c m a t e r i a l s . S h o r t - r a n g e s i l i c o n - a l u m i n i u m framework o r d e r i n g s w i t h i n c r y s t a l l o g r a p h i c a l l y n o n - e q u i v a l e n t s i t e s of v a r i o u s n a t u r a l and s y n t h e t i c A l - r i c h z e o l i t e s have been r e s o l v e d (1-9). The p o s i t i o n of t h e 2 9 ~ i r-e s~o n a~n c~e e s s e n t i a l l y depends on t h e number of t e t r a h e d r a l A 1 atoms i n t h e second c o o r d i n a t i o n s p h e r e o f S i ( 1 0 ) . An i n t e r v a l of a b o u t 5 ppm u s u a l l y separ a t e s two n e i g h b o u r i n g NMR r e s o n a n c e s b e l o n g i n g t o each Si(nA1) c o n f i g u r a t i o n . The a s s i g n m e n t of t h e l i n e s t o t h e v a r i o u s Si-A1 o r d e r i n g s i s u s u a l l y a c h i e v e d by f o l l o w i n g t h e i r i n t e n s i t y v a r i a t i o n s f o r d i f f e r e n t dealuminated m a t e r i a l s . Such a p r o c e d u r e was s u c c e s s f u l l y a p p l i e d i n t h e c a s e o f f a u j a s i t e (11-13), m o r d e n i t e ( 9 ) and p e n t a s i l m a t e r i a l s ( 1 4 ) . The p e r c e n t a g e of e a c h Si(nA1) u n i t i n t h e s t r u c t u r e c a n b e e v a l u a t e d by measuring t h e corresponding l i n e i n t e n s i t y
.
Quantitative determination of S i / A l r a t i o s i n the z e o l i t e framework c a n a l s o b e d e r i v e d from NMR d a t a ( 4 , 6 ) . Recent s t u d i e s have shown t h a t t h e c h e m i c a l s h i f t s which b e l~ o n g~ ing t o d e f i n e d Si(nA1) c h a r a c t e r i z e t h e 2 9 ~ lii n-e s ~ conf i g u r a t i o n s were a l s o i n f l u e n c e d by t h e a c t u a l geometry of t h e T-0-T l i n k a g e s (T = S i o r A l ) o r by l o n g e r range s t r u c t u r a l f e a t u r e s such as t h e (T-O)p r i n g s i z e s ( 1 5 , 1 6 ) . T h i s was t u r n e d t o account a d v a n t a g e o u s l y f o r t h e e f f e c t of t h e a c t u a l r i n g s t r u c t u r e and s t r a i n s t h a t o c c u r i n h i g h s i l i c e o u s z e o l i t e s (ZSM-5, ZSM-11, silicalite, .) ( 1 4 , 1 7 , 1 8 ) , where Si(OA1) c o n f i g u r a t i o n s a r e predominant.
..
I n a d d i t i o n , 2 9 ~ il i n-e s~b e~ l o n~ g i n g t o s i l a n o l groups (-Si-0-H) i n d e a l u m i n a t e d f a u j a s i t e s ( 1 2 ) and i n ZSM-5 m a t e r i a l s (1 9) w e r e unambiguously i d e n t i f i e d , u s i n g t h e c r o s s - p o l a r i z a t i o n technique. The p u r p o s e of t h e p r e s e n t p a p e r i s f i r s t t o show how t h e i n f o r m a t i o n s o b t a i n e d from t h e HRMAS s o l i d s t a t e 2 9 ~ cia n- be~ ~ ~ used i n r e s o l v i n g framework d i s t r i b u t i o n of Si-A1 atoms i n v a r i o u s z e o l i t i c s t r u c t u r e s . Secondly, t h i s t e c h n i q u e i s a l s o u s e d t o f o l l o w t h e p r o g r e s s i v e s t r u c t u r a l changes of t h e Si-A1 o r d e r i n g s d u r i n g t h e t r a n s f o r m a t i o n of amorphous g e l s i n t o c r y s t a l l i n e ZSM-5 phases.
2 2.1
EXPERIMENTAL Materials
Na-X and Na-Y z e o l i t e s were s y n t h e s i z e d by D r D . Barthomeuf (Lyon, F r a n c e ) . Mordenite, a Na-zeolon s a m p l e , was s u p p l i e d by t h e Norton Company. ZSM-5 and ZSM-11 were s y n t h e s i z e d a c c o r d i n g t o p u b l i s h e d methods ( 2 0 - 2 3 ) , u s i n g d i f f e r e n t S i / A l r a t i o s . The h i g h l y pure ZSM-5 z e o l i t e , p a r e n t m a t e r i a l f o r v a r i o u s c h e m i c a l modificat i o n s , was o b t a i n e d u s i n g d i l u t e d c o n d i t i o n s , a s d e s c r i b e d e a r l i e r ( 2 4 ) . P r o g r e s s i v e c r y s t a l l i z a t i o n of ZSM-5 z e o l i t e s was conducted f o l l o w i n g two d i f f e r e n t p r o c e d u r e s a s r e p o r t e d p r e v i o u s ly ( 2 1 , 2 5 , 2 6 ) . From e a c h p r o c e d u r e , i n t e r m e d i a t e p h a s e s w i t h i n c r e a s i n g d e g r e e s of c r y s t a l l i n i t y were i s o l a t e d and a n a l y z e d . ZSM-5 was o b t a i n e d from b o t h p a t e n t ( 2 7 ) and own-modified ( 2 2 ) recepies ZSM-39 was s y n t h e s i z e d a s d e s c r i b e d by Dwyer e t a1. ( 2 8 ) . The o t h e r z e o l i t e s : f e r r i e r i t e , ZSM-35 and ZSM-48 were o b t a i n e d by own s y n t h e s e s , a c c o r d i n g t o p r o c e d u r e s p u b l i s h e d i n t h e c u r r e n t or p a t e n t l i t e r a t u r e ( 2 9 ) .
.
The chemical c o m p o s i t i o n of each sample was d e t e r m i n e d by Proton Induced y-Ray Emission ( P I G E ) ( 3 5 ) , by Energy D i s p e r s i v e X-Ray A n a l y s i s (EDX) (23) o r by Atomic A b s o r p t i o n S p e c t r o s c o p y Water and o r g a n i c c o n t e n t s o f t h e p e n t a s i l m a t e r i a l s were (AAS) The s t r u c t u r a l i d e n measured by t h e r m a l methods (21-23,31,32). t i t y of a l l t h e z e o l i t e s was a s c e r t a i n e d by t h e c l a s s i c a l powder X-ray d i f f r a c t i o n method.
.
2.2
2 9 ~ measurements i - ~ ~ ~
The HRMAS *'S~-NMR s p e c t r a were o b t a i n e d a t room t e m p e r a t u r e on a Bruker CXP-200 s p e c t r o m e t e r o p e r a t i n g i n t h e F o u r i e r Transform mode, u s i n g a "one c y c l e " t y p e measurement. An r . f . - f i e l d o f 49.3 O e was used f o r t h e ~ / 2p u l s e s of 2 9 ~(uo i = 39.7 MHz) Magic-angle-spinning was a t a rate of 3.1 kHz. u s i n g D e l r i n c o n i c a l r o t o r s . Time i n t e r v a l s between p u l s e sequences were 3.0 s. 2,UUu t o 20,600 f r e e i n d u c t i o n decays w e r e accumulated p e r sample. Chemical s h i f t s ( 6 1 , i n ppm, were measured w i t h r e s y e t t o t e t r a n i e f h y l s i l a n e (TMS), used a s e x t e r n a l r e f e r e n c e .
.
3 3.1
RESULTS AND DISCUSSION Zeolites with crystallographically equivalent tetrahedral sites : faujasite
The s t r u c t u r e of f a u j a s i t e i s b u i l t up of c o r n e r s h a r i n g TO4 t e t r a h e d r a ( T = S i o r A l ) . 24 t e t r a h e d r a are j o i n e d t o form a cuboctahedron o r s o d a l i t e cage. Cages a r e s t r o k e d t e t r a h e d r a l l y
t o form a c u b i c diamond l a t t i c e . A l l t h e 1 ': s i t e s are crystallog r a p h i c a l l y (but n o t c h e m i c a l l y ) e q u i v a l e n t ( 3 3 ) . Five types of g r o u p i n g s can b e distinguished w i t h S i l i n k e d through oxygen t o 0 , 1 , 2 , 3 o r 4 A 1 atoms. I n t h e c a s e of f a u j a s i t e , t h e r e i s no chemical i s h i f t s f o r t h e above groupings and a overlap i n the 2 9 ~ s t r u c t u r a l model could b e developped t h a t s u c c e s s f u l l y e x p l a i n s t h e S i , A l framework d i s t r i b u t i o n , i n r e l a t i o n w i t h t h e NMR d a t a ,over a wide range of c o m p o s i t i o n s . Provided t h e Lowenstein r u l e i s obeyed i n t h e z e o l i t e , i . e . no A1-0-A1 l i n k a g e s a r e p r e s e n t , t h e Si/A1 r a t i o of t h e f a u j a s i t e l a t t i c e can be e s t i m a t e d from t h e r e l a t i v e s i g n a l i n t e n s i t i e s of 29si (4) :
It can be used a s an e x c e l l e n t q u a n t i t a t i v e method of measuring z e o l i t e composition i n d e p e n d e n t l y of t h e e l e m e n t a l chemical anal y s e s . A non-coincidence between NMR Si/A1 r a t i o s and t h o s e obt a i n e d by chemical methods i n d i c a t e s t h a t non-framework A 1 o r S i a r e p r e s e n t (34)
.
D e t a i l e d s t r u c t u r a l models and S i , A l o r d e r i n g s i n f a u j a s i t e have been developped and d i s c u s s e d i n many r e c e n t p a p e r s (1-8, 10-13,16,35-37). F i g u r e 1 shows examples of HRMAS s p e c t r a of t h r e e f a u j a s i t e s , a N a X z e o l i t e w i t h S i / A 1 = 1.25 and two Nay zeol i t e s w i t h Si/A1 = 1.82 and 2.4. The i n t e n s i t i e s of t h e NMR l i n e s c h a r a c t e r i z i n g t h e f i v e Si(nA1) ( n = 0 , 1 , 2 , 3 , 4 ) c o n f i g u r a t i o n s a r e computed i n Table 1 Good agreement i s o b t a i n e d between t h e ( S i / A l ) r a t i o s a s c a l c u l a t e d u s i n g e q u a t i o n [ l ] and t h o s e measured by t h e EDX t e c h n i q u e .
.
TABLE 1 2 9 ~ i c-h a~r a ~ c t e~r i s t i c s of f a u j a s i t e s Si'A1
(EDX)
R e l a t i v e p o p u l a t i o n s of v a r i o u s Si(nA1) con£i g u r a t i o n s , i n % (6 i n pprn from TMS)
Si/A1
(NMR)
Si(4A1) Si(3A1) Si(2A1) S i ( l A 1 ) Si(OA1) (-84.6) (-89.1) (-94.5) (-99.5) (-103.4)
Na-X
1.25
53.8
26.5
12.2
5 .O
2.5
1 .23
Na-Y
1.82
15.9
31.4
32.7
15.5
4 ,5
1.7
Na-Y
2.4
2.9
15.0
39.4
37.3
5.4
2.3
b(oom) I TMS
Figure 1 .
3.2
HRMAS 2 9 ~ si p e-c t~ r a of ~ t~ hree faujasite zeolites.
Z e o l i t e s with c r y s t a l l o g r a p h i c a l l y non-equivalent t e t r a h e d r a l sites
I n z e o l i t e s whose s t r u c t u r e s e x h i b i t d i f f e r e n t framework topol o g i e s , some o v e r l a p i n t h e ~ ~ S ~ - N lM i nRe s c h a r a c t e r i z i n g v a r i o u s Si(nA1) c o n f i g u r a t i o n s c a n o c c u r b e c a u s e t h e c h e m i c a l s h i f t s of a n NMR r e s o n a n c e b e l o n g i n g t o a g i v e n c o n f i g u r a t i o n does a l s o depend on t h e a c t u a l symmetry of t h a t Si(nA1) s i t e a n d / o r on t h e s i z e of t h e (T-0)p r i n g t o which t h a t c o n f i g u r a t i o n b e l o n g s . T y p i c a l examples are e n c o u n t e r e d i n nlordenite ( 9 , 3 7 ) , p e n t a s i l z e o l i t e s ( 1 5 , 1 6 , 1 9 ) and ZSM-39 ( 3 8 ) z e o l i t e . I n s u c h c a s e s , t h e unambiguous a t t r i b u t i o n of a n NMR l i n e w i t h i n the o v e r l a p r e g i o n becomes h i g h l y u n c e r t a i n . T h e r e f o r e , v a r i o u s e x p e r i m e n t a l p r o c e d u r e s a r e developped t o d e r i v e t h e a c t u a l S i , A1 o r d e r i n g s i n z e o l i t i c frameworks from 2 9 ~ i - ~ M R s p e c t r a : p r o g r e s s i v e d e a l u r n i n a t i o n of t h e sample o r s y n t h e s i s of t h e z e o l i t e w i t h v a r i o u s ~ i / A lc o n t e n t s .
3 . 2 . 1 . Mordenite g r p u p . A common f e a t u r e o f t h e z e o l i t e frame...................... works b e l o n g i n g t o t h e m o r d e n i t e group i s t h e e x i s t e n c e of 6 - r i n g s h e e t s . I n mordenite, t h e s e a r e l i n k e d t o each o t h e r through s i n g l e
4-membered rings.
r i n g s and i n f e r r i e r i t e
,
t h r o u g h s i n g l e 6-membered
S i n g l e c r y s t a l X-ray r e f i n e m e n t of m o r d e n i t e (39) h a s suggest e d t h a t t h e A 1 atoms must b e l o c a t e d on t h e 4-membered r i n g s . Using t h a t p r o p o s a l , the t h e o r e t i c a l d i s t r i b u t i o n o f t h e A 1 atoms w i t h i n t h e m o r d e n i t e s t r u c t u r e can b e c a l c u l a t e d f o r v a r i o u s Si/AI r a t i o s . The s o o b t a i n e d t o t a l number of Si(nA1) c o n f i g u r a t i o n s d i s t r i b u t e d w i t h i n one u n i t c e l l c a n b e computed and t h e correspond i n g t h e o r e t i c a l . NMR l i n e i n t e n s i t i e s compared t o t h e e x p e r i m e n t a l data. The H W S 2 9 ~ si p e-c t~ ra ~ of Na-mordenite ~ ( S i / A l = 5 . 5 ) shows t h r e e l i n e s a t -95, -105 and -110 ppm ( F i g . 2 ) , which a r e a t t r i b u t e d r e s p e c t i v e l y t o t e t r a h e d r a l a r r a n g e m e n t s of Si(2A1) , ~ i ( l A 1 ) and Si(OA1). The p a r a l l e l d e c r e a s e o f t h e f i r s t two l i n e s and t h e i n c r e a s e of t h a t a t -1 10 ppm ,with p r o g r e s s i v e d e a l u r n i n a t i o n of the p a r e n t sample ( F i g . 2 ) , c o r r e s p o n d s t o t h e t h e o r e t i c a l p r e d i c t i o n and c o n f i r m s t h e a s s i g n m e n t . Q u a n t i t a t i v e d a t a a r e g i v e n i n T a b l e 2. A more complete NMR s t u d y of m o r d e n i t e and i t s dealumin a t e d forms i s p r e s e n t e d e l s e w h e r e ( 9 ) .
TABLE 2 HRMAS 2 9 ~ cih a-r a ~ cte~ r i z a~ t i o n of v a r i o u s m o r d e n i t e samples
Sample
Si/Al (AAS)
Na-Mor
5.5
Relative l i n e i n t e n s i t i e s (2) (a) C o n f i g u r a t i o n : Si(2A1) 6/ppm : -95 13.2
Si(lA1) -105
Si(OA1) -1 1 0
Si/Al (NMR)
44.7
42.1
5.4
H-MorlCb) 20.5
0
29.8
70.2
21.6
H - M o ~ ~)( ' 3 1 . 2
0
22.4
77.6
30.7
( a ) Normalized t o 100 f o r Na-Mor ( b ) Na-Mor, l e a c h e d w i t h 4 M HNO3 a t 90°c f o r 24h ( c ) Na-Mor, l e a c h e d w i t h 14M HNO3 a t 90°c f o r 24h.
The HRMAS 2 9 ~ NMR i s p e c t r u m o f f e r r i e r i t e and of i t s S i - r i c h e r a n a l o g u e H-ZSM-35 shows t h r e e l i n e s l o c a t e d a t -101, -108 and -1 13 ppm. They a r e a t t r i b u t e d t o t h e ~ i ( 2 A 1 ) , Si(lA1) and S i ( 0 ~ 1 ) configurations respectively The f e r r i e r i t e u n i t c e l l c o n t a i n s two t y p e s o f t e t r a h e d r a l l y coord i n a t e d T atoms. The r e l a t i v e i n t e n s i t i e s o f t h e t h r e e l i n e s , a s e x p e r i m e n t a l l y measured on b o t h f e r r i e r i t e and H-ZSM-35, c o r r e s pond w e l l t o t h o s e e v a l u a t e d t h e o r e t i c a l l y f o r s i m i l a r ~ i / A 1rat i o s . F o r t h e l a t t e r , i t was a s s u m d t h a t A l atoms were s p e c i f i c a l l y l o c a t e d i n 6-membered r i n g s l i n k i n g one d i m e n s i o n a l s h e e t s
.
i n t h e s t r u c t u r e ( s i t e s T I ) ( 4 0 ) , and t h a t a 6-membered r i n g w i t h one s i n g l e Alatom i s more s t a b l e than one c o n t a i n i n g two A 1 a t o m s . A s i m i l a r t h e o r e t i c a l c o m p u t a t i o n , a s s u m i n g t h e A 1 atoms prefer e n t i a l l y l o c a t e d i n 6-membered r i n g s o f t h e s h e e t s , a t a maximum d i s t a n c e from each o t h e r ( s i t e s T 2 ) , d o e s n o t f i t t h e e x p e r i m e n t a l d a t a (Table 3 ) .
-90
-130
- 1 10
b (PP~) Figure 2 .
HRMAS 2 9 ~ of i m o~ r d e~ n i t e~ s : e f f e c t of d e a l u m i n a t i o n .
TABLE 3 H W S 2 9 ~ i of - ~f e r~t i ~e r i t e and H-ZSM-35 z e o l i t e : comparison of t h e o r e t i c a l and e x p e r i m e n t a l l i n e i n t e n s i t i e s i
Sample
S i /AL
R e l a t i v e l i n e i n t e n s i t i e s (%) configuration : Si(2Al) 6/ppm : -101
f i c t i t i o u s (Alon T2) f i c t i t i o l u s (Alon T i ) Ferrierite
S i(lA1) -1 0 8
Si(OA1: -1 13
8.2 8.2 8.2
0 5.9 8
48.6 36.8 39
51.4 57.3 53
f i c t i t i o u s (A1 on T2) 1 0 . 4 f i c t i t i o u s ( A l o n T i ) 10.4 H-ZSM-35 10.4
0 3.7
39 31.7 34
61 64.6
6
60
r a o~ f H-ZSM-5 ~ and 3.2.2 P e n t a g--i 1 ze_~liLes. The HRMAS 2 9 ~ sip e-c t ~ -----------H-ZSM-11 z e o l i t e s h a v i n g d i f f e r e n t S i / A 1 r a t i o s (from 30 t o 1000) have been s t u d i e d i n d e t a i l ( 1 4 , 1 6 ) . A t low r e s o l u t i o n , e s s e n t i a l l y t h r e e r e s o n a n c e s were o b s e r v e d a t -105, -113 and -115 pprn. The d e c r e a s i n g i n t e n s i t y of t h e -1 15 pprn l i n e w i t h t h e i n c r e a s i n g SiIA1 r a t i o i s d i r e c t l y l i n k e d t o t h e p a r a l l e l i n t e n s i t y i n c r e a s e of t h e -1 13 pprn l i n e , w h i l e t h e i n t e n s i t y of t h e -1 15 pwm l i n e remains c o n s t a n t i n b o t h H-ZS11-5 and H-ZSM-I I ( F i g . 3)
.
Figure 3 .
V a r i a t i o n of t h e 2 9 ~ i - ~ lm i n e i n t e n s i t i e s , as a f u n c t i o n of S i / A l r a t i o s inH-ZSM-5 ( w h i t e s p o t s ) and i n H-ZSM-11 (black spots) z e o l i t e s
.
These v a r i a t i o n s a r e e a s i l y e x p l a i n e d i f t h e -105 and -113 ppm l i n e s are a t t r i b u t e d t o S i (1Al) and Si(OA1) c o n f i g u r a t i o n s r e s p ec t i v e l y . The SiIA1 r a t i o s computed u s i n g e q u a t i o n [I] , I t o t a l / 10.25 I (-105) (every A 1 i s s u r r o u n d e d by 4 Si), match e x a c t l y t h e e x p e r i m e n t a l v a l u e s . I n a d d i t i o n , t h e i n t e n s i t y r a t i o I (-115) ZSM-11/I (-1151, ZSM-5, e q u a l t o 2, i s d i r e c t l y p r o p o r t i o n a l t o t h e amount of 4-membered r i n g s p r e s e n t i s one u n i t c e l l of e a c h z e o l i t e : f o u r i n ZSM-5 and e i g h t i n ZSM-11. The -115 pprn l i n e can t h e r e f o r e b e a t t r i b u t e d t o Si(OA1) c o n f i g u r a t i o n s l o c a t e d i n 4-membered r i n g s . T h i s also i m p l i e s t h a t A 1 atoms a r e e x c l u s i v e l y l o c a t e d i n t h e 5-membered r i n g s of b o t h z e o l i t e s and t h u s n o t s t a t i s t i c a l l y d i s t r i b u t e d throughout t h e p e n t a s i l l a t t i c e . In t h e 2 9 ~ sip e-c t r~ um~ of ~ h i g h l y s i l i c e o u s H-ZSM-5 (Si/Al=lOOO), up t o 8 l i n e components c a n b e d i s t i n g u i s h e d ( F i g . 4 ) .
Figure 4 .
High r e s o l u t i o n 2 9 ~ i spectrum - ~ ~ ~ of H-ZSM-5 ( S i / A l = 1000)
.
T h i s m u l t i p l i c i t y a r i s e s from c r y s t a l l o g r a p h i c a l l y non equiUsing t h e i n t e n s i t i e s of t h e w e l l valent S i ( O M ) arrangements resolved s i g n a l s l o c a t e d a t -109.2 ppm and a t -116.3 ppm a s a base u n i t of one, t h e t o t a l i n t e n s i t y i s found t o be approximatel y 2 4 , i n agreement w i t h previous f i n d i n g s ( 1 6 ) . The number of Si atoms i n t h e 24 non-equivalent s i t e s i n t h e r e p e a t - u n i t of t h e ZSM-5 s t r u c t u r e i s given i n p a r e n t h e s e s i n F i g . 4. The unambiguous assignment of a l l t h e s e l i n e s s t i l l needs f u r t h e r i n v e s t i g a t i o n s . N e v e r t h e l e s s , t h e f o l l o w i n g i n f e r e n c e s can be made : ( i ) t h r e e m a g n e t i c a l l y d i f f e r e n t S i atoms i n t h e 4-membered r i n g s c o n t r i b u t e t o t h e t h r e e l i n e components observed i n t h e --,. , . --T h l s r e v e a l s t n a t s t r a l n must e x .l s c. -1 I5 ppm r e g l o n ( 1 4 ) i n t h e s e r i n g s , i n b o t h ZSM-5 and ZSM-I I ; ( i i ) t h e s u b s t i t u t i o n o f S i by A 1 only t a k e s p l a c e i n t h e 5-memb e r e d r i n g s , where t h e energy i s pnobably lower t h a n i n t h e s t r a i n e d 4-membered r i n g s .
.
.
.
The chemical s h i f t s and r e l a t i v e l i n e i n t e n s i t i e s i n 2 9 ~ i NMR s p e c t r a of samples which by XRD were found t o b e ZSM-8 and ZSM-48,appear t o be v e r y s i m i l a r t o t h o s e of ZSM-5 and ZSM-ll ( F i g . 5 ) . T h i s s u g g e s t s t h a t b o t h z e o l i t e s could a l s o belong t o
t h e p e n t a s i l f a m i l y . I n p a r t i c u l a r , t h e r e s o n a n c e l i n e a t -115 ppm shows t h a t ZSM-8 a n d ZSM-48 frameworks a l s o c o n t a i n 4-membered r i n g s . F o r ZSM-8, t h e r e l a t i v e i n t e n s i t y o f t h i s l i n e f a l l s b e t w e e n t h o s e m e a s u r e d f o r ZSM-5 a n d ZSM-11. T h i s s u g g e s t s t h a t ZSM-8 c o u l d be a mixture o f ZSM-5 and ZSM-11 o r a n i n t e r g r o w t h of t h e s e two s t u c t u r e s a s s u g g e s t e d by K o k o t a i l o ( 4 1 ) . -
1-
r
I
I
1
pZ.1
1
I
Si/AI = 5 0
Si/AI= 5 0
1
I
Si/AI = 4 8 0
- 80
I
I
-100
- 120
F i g u r e 5. HRMAS "S~-NMR
-80
1
I
-100
-120
s p e c t r a of four p e n t a s i l z e o l i t e s .
3 . 2 . 3 . ZSM-39 z e o l i t e . T h i s S i - r i c h z e o l i t e b e l o n g s t o t h e " C l a t h ...................... r a t e g r o u p t t . I t s framework c o n s i s t s o f a n a r r a n g-e m e n t of TO4 12-and l b - h e d r a i n w h i c h t h r e e t y p e s of T a t o m s (TI , T z a n d ~ 3 w) e r e i d e n t i f i e d ( 4 2 ) . T h e i r n a t u r e and t h e i r d i s t r i b u t i o n i n t h e u n i t c e l l a r e d e t a i l e d i n T a b l e 4 . The HRMAS ~ ~ S ~ - N M s pRe c t r u m of ZSM-39 shows t h r e e w e l l r e s o l v e d r e s o n a n c e s a t 6 = -109,-115 a n d -120 ppm ( ~ i g . 6 ) ~ A l t h o u g h t h e number of r e l e v a n t S i ( n A 1 ) c o n f i u r a t i o n s i s r e s t r i c t e d t o S i ( J A 1 ) a n d Si(OA1) d u e t o t h e h i g h S i A 1 r a t i o s , t h e i n t e n s i t y r a t i o of t h e t h r e e NMR l i n e s c a n n o t b e e x p l a i n e d w i t h o u t computing a l l t h e p o s s i b l e A 1 p o t e n t i a l s i t i n g s i n t h e l a t t i c e . This has been envisaged i n d e t a i l elsewhere (38).
7
TABLE 4
R e p a r t i t i o n o f T atoms i n t h e u n i t c e l l o f ZSM-39
T atom
Ring s t r u c t u r e
T1
s h a r e d between
T2
in
0
0
Number/u. c .
Number of n e i g h b o u r i n g T atoms of t y p e
T1
*2
T3
8
0
4
0
32
1
0
3
96
0
1
3
s h a r e d be tween
*3
Figure 6 .
0 and
0
HRMAS 2 9 ~ spectrum i - ~ ~o f ~ ZSM-39 z e o l i t e .
T a b l e 5 compares t h e e x p e r i m e n t a l and t h e v a r i o u s t h e o r e t i c a l l i n e i n t e n s i t i e s o b t a i n e d assuming e i t h e r a s p e c i f i c l o c a t i o n of t h e A 1 atoms on Tl,T2 ar T 3 s i t e s o r t h e i r s t a t i s t i c a l d i s t r i b u t i o n between a l l t h e T s i t e s i n t h e l a t t i c e .
'
TABLE 5 Cbmparison b e t w e e n t h e r e l a t i v e t h e o r e t i c a l i n t e n s i t i e s c a l c u l a t e d f r o m v a r i o u s A 1 s i t i n g s i n t h e l a t t i c e and t h e e x p e r i m e n t a l ones m e a s u r e d f o r H-ZSM-39 ( S i / A l = 54.3)
6 (ppm)
R e l a t i v e t h e o r e t i c a l i n t e n s i t i e s (%) assuming A 1 s i t e d i n TI
-104 - 109 -1 15 - 120
0 11.5 16.6 71.9
in
T2
I .8 4.2
27.6 66.4
i n T3 0 7.8 27.7 64.5
statistically
0.5 7.1 27 .O 65.4
Exper. v a l u e s (NMR) O 9
29 62
L
The b e s t a g r e e m e n t between e x p e r i m e n t a l a n d t h e o r e t i c a l v a l u e s i s o b t a i n e d when A 1 c a t i o n s a r e s p e c i f i c a l l y l o c a t e d o n s i t e s T3 i . e . i n t h e 6-membered r i n g s . However, b e c a u s e o f t h e h i g h proport i o n of t h e T s i t e s , a s t a t i s t i c a l d i s t r i b u t i o n a l s o g i v e s a c l o s e 3 agreement. 2 9 ~ i - N k B s p e c t r a of ZSM-39 s a m p l e s w i t h a l o w e r ~ i / r~a tli o a r e needed t o differentiate u n e q u i v o c a l l y b e t w e e n t h e two d i s t r i b u t i o n s .
3.3 C r y s t a l l i z a t i o n o f ZSM-5 from amorphous g e l s I n f o r m a t i o n s on s t r u c t u r a l e v o l u t i o n o f v a r i o u s c o n s t i t u e n t s p e c i e s formed w i t h i n g e l m i x t u r e s o r s o l u t i o n s , p r e c u r s o r s t o crys t a l l i z a t i o n o f m o r d e n i t e (43) o r ZSM-5 ( 2 5 , 2 6 1 , h a v e b e e n o b t a i n e d r e c e n t l y u s i n g HRMAS m u l t i n u c l e a r ( I ~ c2, 7 ~ 12, 9 ~ i )NMR. The h y d r o t h e r m a l s y n t h e s i s of ZSM-5 c a n i n v o l v e a t l e a s t two d i f f e r e n t mechanisms whose v a r i o u s a s p e c t s h a v e b e e n d e v e l o p p e d in d e t a i l e l s e w h e r e ( 2 1 , 2 3 ) . P r o c e d u r e A y i e l d s l a r g e s i n g l e c r y s t a l s o f ZSM-5 which grow s l o w l y i n a n A l - r i c h g e l t h r o u g h a l i q u i d p h a s e i o n t r a n s p o r t a t i o n mechan2sm. P r o c e d u r e B g i v e s r a p i d l y small ZSM-5 p o l y c r y s t a l l i n e a g g r e g a t e s w h i c h a p p e a r v e r y e a r l y w i t h i n the h y d r o g e l , w h e r e t h e y r e m a i n s t a b i l i z e d a s v e r y s m a l l s i z e d "X-ray amorphous" z e o l i t e s ( 2 1 , 2 5 , 4 4 ) . S e v e r a l i n t e r m e d i a t e p h a s e s formed d u r i n g t h e c r y s t a l l i z a t i o n of ZSM-5 c o n d u c t e d u s i n g b o t h p r o c e d u r e s A a n d B were isolated and i n v k s t i g a t e d by HRMAS 29~i-NMR. The s p e c t r a of g e l s e x h i b i t v e r y b r o a d r e s o n a n c e s . T h e i r maxima a r e l o c a t e d b e t w e e n -100 and - 1 1 1 ppm, d e p e n d i n g o n t h e i r a c t u a l S i / A l r a t i o . The l i n e s become n a r r o w e r and a r e s h i f t e d t o w a r d s h i g h e r f i e l d s , a s ZSM-5 i s p r o g r e s s i v e l y formed w i t h i n t h e g e l s . T h i s e v o l u t i o n i s shown i n F i g . 7 f o r 3 p h a s e s f r o m s y n t h e s i s A .
~helinssappearing below - 1 1 1 ppm must essentially belong to Si atoms which have only S i as neighbours, while those located above - 100 ppm should reflect various Si(nA1) configurations, where n > 0. In that case, Si atoms are surrounded either by Al, randomly arranged in amorphous phase, or by silanol groups which should appear in that region in amorphous or crystalline ZSM-5 (1 9). The variation of the corresponding relative intensities, respectively noted by I (6 < - I 1 1) and I ( 6 > -loo), as well as that of the total 2 9 ~ i - ~ ~ ~ intensity, as a function of synthesis time, leads to interesting conclusions.
~ ~ ~ of some intermediate phases Figure 7. HRMAS 2 9 ~ i -spectra obtained using procedure A.
3.3.1. ZSM-5 synthesized according---to procedure ----------------------------------------A . I(&<< - 1 1 1 ) and I (6 >-loo) respectively increase and decrease as the crvstallization proceeds, confirming that the number of Si(nA1) (n > 0) configurations decreases in a parallel way with the decreasing global A1 concentration in the solid phases ( ~ i / ~progressively l increases from 1.8 (0 % crystallinity) to 13.2 (100 X crystallinity)], while more Si(OA1) configurations appear ordered in a crystalline zeolite phase. (Fig; 8,A) d
Figure 8. Variation of the relative 2 9 ~ line i - intensities ~ ~ of the peaks located below - 1 1 1 ppm (6 < - 1 1 1) and above -100 ppm (6 > -100) and of the % crystallinity (XRD) for various A and B-type intermediate phases, as a function of crystallization time.
The progressive Al-depletion of the solid phases is also characterized by the sygmoidal increase of the total intensity, which follows the XRD crystallinity (Fig. 9A).
Synthesis time (h)
-
Figure 9 Variation of the total 2 9 ~ i intensities - ~ and of the % of crystallinity of A and B-type intermediate phases, as a function of synthesis time.
______________
3.3.2, ZSM-5 synthesized _ _ _ _ - _ _ _ _ _ _according _ _ _ _ _ _ _ ____ to procedure __________B. Oppositely to -100) is the synthesis A, in the beginning of the process B, I (6 low, while I (6 < -1 11) is a l r e a d y high (Fig. 8 B ) . Both intensities show little variations as the crystallization proceeds. This only reflects the low amount of Si(nA1) configurations within the Si-rich
-
( g e l + z e o l i t e ) p h a s e s i n which S i / A l 35 and remains c o n s t a n t during the synthesis course. By c o n t r a s t , I ( t o t a 1 ) remains c o n s t a n t o n l y d u r i n g t h e t i m e i n t e r v a l i n which t h e X-ray amporphous ZSM-5 c r y s t a l l i t e s a r e d e t e c t e d ( ~ i 9~ ~ .) . I t shows, however, a s h a r p i n c r e a s e when t h e ZSM-5 cryst a l l i t e s b e g i n t o grow. Because ~ i / remains ~ l constant during the c r y s t a l l i z a t i o n , t h e o r d e r i n g of S i atoms w i t h i n a ZSM-5 l a t t i c e i s t h e o n l y wa t o e x p l a i n t h a t i n c r e a s e . T h i s l a t t e r i s p r o b a b l y due t o s h o r t e r $9 Si-NMR r e l a x a t i o n t i m e s f o r S i l o c a t e d w i t h i n a n o r d e r e d l a t t i c e t h a n f o r S i surrounded by a random environment i n t h e g e l . T h i s phenomenon i s p r e s e n t l y b e i n g i n v e s t i g a t e d by determining t h e T I r e l a x a t i o n t i m e s i n t h e g e l and c r y s t a l l i n e p h a s e s . his inc r e a s e however a p p e a r s t o b e a r e m a r k a b l e q u a l i t a t i v e i l l u s t r a t i o n of t h e o r d e r i n g p r o c e s s which o c c u r s d u r i n g t h e growth of z e o l i t e crystals.
4. CONCLUSIONS HRMAS 2 9 ~ porved i - ~ t o b e a powerful t e c h n i q u e t o determine t h e d i s t r i b u t i o n of t h e A 1 atoms i n v a r i o u s z e o l i t i c frameworks. The chemical s h i f t s on t h e 2 9 ~ lii n-e s ~e s s e n t i a l l y depend on t h e chemical environment of t h e Si atoms. Moreover, f o r a given S i ( n A l ) c o n f i g u r a t i o n , t h e y a r e a l s o s e n s i t i v e t o t h e a c t u a l symm e t r y o f t h e Si-0-T l i n k a g e s a s w e l l a s t o l o c a l framework s t r a i n s . I n such c a s e s , t h e s i l i c o n - a l u m i n i u m o r d e r i n g can be r e s o l v e d by f o l l o w i n g t h e r e l a t i v e i n t e n s i t i e s of e a c h l i - n e component c h a r a c t e r i a i n g a g i v e n c o n f i g u r a t i o n , a s a f u n c t i o n of t h e S i / A l r a t i o i n t h e z e o l i t e . I t i s concluded t h a t most of t h e i n v e s t i g a t e d m a t e r i a l s ( z e o l i t e s b e l o n g i n g t o t h e m o r d e n i t e o r ~ e n t a s i lf a m i l y and ZSM-39) a r e c h a r a c t e r i z e d by referential d i s t r i b u t i o n of A 1 atoms on specific sites. F o r z e o l i t e s whose s t r u c t u r e and Si-A1 o r d e r i n g s a r e known, the measure of t h e i n t e n s i t y of each l i n e component l e a d s t o a s t r a i g h t forward d e t e r m i n a t i o n of t h e Si/A1 r a t i o s . However, f o r i n t e r m e d i a t e a l u m i n o - s i l i c a t e phases obtained during t h e s y n t h e s i s , t h e l i n e int e n s i t i e s were found t o b e p r o p o r t i o n a l t o t h e d e g r e e of o r d e r i n g of t h e p h a s e s . T h i s p r o p e r t y was t h e r e f o r e e x p l o i t e d t o f o l l o w q u a l i t a t i v e l y t h e p r o g r e s s i v e c r y s t a l l i z a t i o n and growth of (ordered) z e o l i t i c frameworks from amorphous ( r a n d o m ) a l u r n i n o - s i l i c a t e g e l s .
5. ACKNOWLEDGEMENTS The a u t h o r s wish t o t h a n k M r . G. Daelen f o r h i s s k i l f u l h e l p i n t a k i n g t h e NMR s p e c t r a . P.A. J a c o b s acknowledges a r e s e a r c h p o s i t i o n a s I 1 S e n i o r R e s e a r c h A s s o c i a t e ' ' from NFWO-FNRS ( ~ e l g i u r n ) and P. Bodart t h a n k s IRSIA (Belgium) f o r f i n a n c i a l s u p p o r t .
6
1. 2. 3.
4. 5. 6. 7.
8.
9. 10.
11. 12.
13.
14. 15. 16. 17.
REFERENCES J . Klinowski, J . M . Thomas, C.A. F y f e and J . S . Hartman, J . Phys. Chem. 8 5 , 2590 (1981). E . ~ i ~ p m a aM ., Mggi, A. Samoson, M . Tarmak and G . E n g e l h a r d t , J. Am. Chem. Soc. 103, 4992 ( 1 9 8 1 ) . G. E n g e l h a r d t , E . Lippmaa and M. MZgi, J . Chem. Soc., Chem. Commun. 19-81, 712. G. E n g e l h a r d t , U . Lohse, E. Lippmaa, M. Tarmak and M . MSgi, Z. Anorg. A l l g . Chem. 482, 49 (1981). S . Ramdas, J . M . Thomas, J . K l i n o w s k i , C . A . F y f e and J . S . Hartman, N a t u r e , 292, 228 ( 1 9 8 1 ) . J . Klinowski, S . Ramdas, J.M. Thomas, C.A. F y f e and J . S . Hartman, J . Chem. Soc. Faraday T r a n s . 11, 7 8 , 1025 (1982). J . M . Thomas, C.A. F y f e , J . Klinowski and GZ. Gobbi, J . Phys. Chem. 8 6 , 3061 ( 1 9 8 2 ) . M.T. ~ a c h i o r ,D.E.W. Vaughan, R.H. Jarman and A . J . J a c o b s o n , Nature, 298, 455 ( 1 982) G . Debras, J . B.Nagy, Z . G a b e l i c a , P. Bodart and P.A. J a c o b s , Chem. L e t t . 1983, 199. E . Lippmaa, M. ~ g g i ,A . Samoson, G . E n g e l h a r d t and A.-R. Grimmer, J . Amer. Chem. Soc. 102, 4889 ( 1 9 8 0 ) . J . Klinowski, J . M . Thomas, M. A u d i e r , S. Vasudevan, C.A. F y f e , and J . S . Hartman, J. Chem. Soc. Chem. Comun. 1981, 570. G . E n g e l h a r d t , U . Lohse, A . Samoson, M. Magi, M . Tarmak and E . Lippmaa, Z e o l i t e s , 2 , 59 ( 1 9 8 2 ) . I . E , Maxwell, W.A. v a n - ~ r ~ G.R. , Hays, T . Couperus, R . Huis Clague, J . Chem. Soc. Chem. Commun. 1982, 523. and A.D.M. J . B.Nagy, Z . G a b e l i c a , E.G. Derouane and P.A. J a c o b s , Chem. L e t t . 1982, 2003. J . B.Nagy, J . P . G i l s o n and E . G . Derouane, J . Chem. Soc. Chem, Commun. 1981, 1129. C.A. F y f e , G . C . Gobbi, J . Klinowski, J . M . Thomas and S . Ramdas Nature, 296, 530 (1 982) P.A. J a c o b s , M. T i e l e n , J. B.Nagy, G. Debras, E.G. Derouane and Z . G a b e l i c a , i n "Proc. S i x t h I n t e r n . Conf. ~ e o l i t e s " , Reno, 1983 ( s u b m i t t e d ) J. B.Nagy, Z . G a b e l i c a , G. Debras, P. B o d a r t , E.G. Derouane and P.A. J a c o b s , J . Molec. C a t a l . , i n p r e s s . J. B.Nagy, Z . G a b e l i c a - a n d E.G. Derouane, Chem. L e t t . 1982,1105. P.A. J a c o b s , J . A . M a r t e n s , J . Weitkamp,and H.K. Beyer, Faraday D i s c u s s . Chem. Soc. 72, 353 ( 1 9 8 1 ) . E . G . Derouane, S . ~ e t r e m m e r i c2 . G a b e l i c a and N .Ellom, Appl. C a t a l . 1 , 201 ( 1 9 8 1 ) . 2 . ~ a b e i i c a ,E.G. Derouane and N . Blom, Appl. C a t a l . 5 , 109 (1983). Z . G a b e l i c a , N. Blom and E.G. Derouane, Appl. C a t a l . 5 , 227 ( 1983) P.A. J a c o b s , a n d R . Von Ballmoos, J . Phys. Chem. 8 6 , 3050 ( 1 9 8 2 ) . Z . G a b e l i c a , J . B.Nagy and G . Debras, J . C a t a l . , s u b m i t t e d .
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26. Z . G a b e l i c a , J . B.Nagy, G . D e b r a s , a n d E.G. Derouane, i n roc. S i x t h I n t e r n . Conf. Z e o l i t e s " Reno, 1983 ( s u b m i t t e d ) 27. N . Y . Chen, U.S. P a t . 3,700,585 ( 1 9 7 2 ) . 28. F.G. Dwyer and E.E. J e n k i n s , U.S. P a t . 4,287,166 (1981). 29. R.M. B a r r e r , " ~ ~ d r o t h e r r n aChemistry l o f z e o l i t e s ' ; Academic P r e s s , London and New York, 1982. 30. G. Debras, E.G. Derouane, J . P . G i l s o n , Z . G a b e l i c a and G. D e m o r t i e r , Z e o l i t e s , 3 , 37 ( 1 9 8 3 ) . 31. Z . G a b e l i c a , J . P . ~ i l s o n ; G. Debras and E.G. Derouane, i n "Thermal A n a l y s i s , P r o c . Seventh I n t e r n . Conf Thermal Anal .", (B. M i l l e r , e d . ) v o l . 11, Wiley-Heyden, New York, 1982, pp. 1203, 32. J . B.Nagy, Z . G a b e l i c a and E.G. Derouane, z e o l i t e s , 3 , 43 (1983) 33. W.H. Meier and H . J . Moeck, J . S o l . S t a t e Chem. 27, 3z9 ( 1 9 7 9 ) . , ature, 34. J . K l i n o w s k i , J . M . Thomas, C . A . Fyfe and G . C . ~ x b i N 296, 533 ( 1 9 8 2 ) . 35. M.T. M e l c h i o r , D.E.W. Vaughan and A . J . J a c o b s o n , J. Amer. Chem. SOC. 104, 4859 (1 9 8 2 ) . 36. M. Mzgi, A . Samoson, M . Tarmak, G. E n g e l h a r d t and E . Lippmaa, Dokl. Akad. Nauk SSSR ( E n g l . T r a n s l ) 261, 1159 ( 1 9 8 1 ) . 37. J . K l i n o w s k i , J . M . Thomas, M . W . Anderson, C . A . Fyfe and G . C . Gobbi, Z e o l i t e s , 3 , 5 ( 1 9 8 3 ) . , Z . G a b e l i c a , E.G. Derouane 38. P. B o d a r t , J . B . N ~ ~G . ~ Debras, and P.A. J a c o b s , S u b m i t t e d f o r p u b l i c a t i o n . 39. V. Gramlich, Ph. D. D i s s . ETH n04633, Ziirich (1971). 40. R.M. B a r r e r " Z e o l i t e s and Clay M i n e r a l s a s S o l v e n t s and M o l e c u l a r Sieves1' Academic P r e s s , London and New York, 1978. 41. G . T . K o k o t a i l o , Eur. P a t . 18,090 (1980) and U.S. P a t . 4,289,607 ( 1 9 8 1 ) . 4 2 . J . L . S c h l e n k e r , F.G. Dwyer, E . E . J e n k i n s , W.J. Rohrbaugh, G . T . K o k o t a i l o and W.M. M e i e r , N a t u r e , 294, 340 (1981). 43. P . B o d a r t , 2. G a b e l i c a , J . B.Nagy and G . D e b r a s , i n " Z e o l i t e s S c i e n c e and Technology", L i s b o n , 1983, ( t h i s m e e t i n g ) 44. P.A. J a c o b s , E.G. Derouane and J. Weitkamp, J . Chem. Soc. Chem. Commun. 1981, 591.
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MULTINUCLEAR SOLID-STATE NMR STUDY OF PIORDENITE CRYSTALLIZATION
P h i l i p p e B o d a r t , Z e l i m i r G a b e l i c a , J g n o s B.Nagy and Guy Debras
a
F a c u l t g s U n i v e r s i t a i r e s d e Namur L a b o r a t o i r e de C a t a l y s e , Rue de B r u x e l l e s , 61 B-5000 Narnur, Belgium a- P r e s e n t a d d r e s s : L a b o f i n a S . A . , Zoning I n d u s t r i e l , B-6520 F e l u y , Belgium.
ABSTRACT S o l i d i n t e r m e d i a t e p h a s e s o b t a i n e d from amorphous a l u m i n o s i l i cate g e l s d u r i n g m o r d e n i t e c r y s t a l l i z a t i o n h a v e b e e n c h a r a c t e r i z e d by m u l t i n u c l e a r s o l i d - s t a t e NMR s p e c t r o s c o p y . The p r o g r e s s i v e r e o r g a n i z a t i o n of t h e amorphous g e l i n t o t h e c r y s t a l l i n e o r d e r e d z e o l i t i c phase h a s b e e n d e t e c t e d by h i g h r e s o l u t i o n magic-an l e - s p i n n i n g (HNIAS) s o l i d s t a t e 2 9 ~ i -Broad ~ ~band ~ . s o l i d s t a t e 2 9A 1 and 2 3 ~ a were - ~ ~ used ~ t o f o l l o w t h e i n c o r p o r a t i o n o f A l - and Na-atoms i n t o t h e m o r d e n i t e l a t t i c e and c h a n n e l s . An e v i d e n c e of aluminium g r a d i e n t i n t h e c r y s t a l l i t e s i s o b t a i n e d from t h e comparison b e t ween s u r f a c e and b u l k a n a l y s i s methods.
1
INTRODUCTION
During t h e l a s t t h i r t y y e a r s , t h e s y n t h e s i s o f z e o l i t e s h a s been e x t e n s i v e l y d e v e l o p e d t o p r o v i d e new o r improved m a t e r i a l s f o r c a t a l y s i s a n d / o r m o l e c u l a r s i e v i n g (1,2) However, t h e s y n t h e s i s p r o c e s s e s , i. e n u c l e a t i o n and c r y s t a l growth from amorphous aqueous aluminosilicate g e l s a r e s t i l l poorly understood (3) R e c e n t l y , mult i n u c l e a r s o l i d s t a t e NMR h a s p r o v e n t o b e a p o w e r f u l t o o l i n t h e T h i s method was a l s o i n v e s t i g a t i o n o f z e o l i t e s t r u c t u r e s (4-14) s u c c e s s f u l l y used t o c h a r a c t e r i z e i n t e r m e d i a t e s o l i d p h a s e s o b t a i ned d u r i n g c r y s t a l l i z a t i o n of ZSM-5 m a t e r i a l s (13-15).
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S i n c e t h e f i r s t s y n t h e s i s o f t h e s i l i c a - r i c h z e o l i t e mordenite by B a r r e r ( I b ) , c o n s i d e r a b l e l i t e r a t u r e d a t a have b e e n p u b l i s h e d on i t s p r e p a r a t i o n (17-23), t h e mechanism of t h i s c r y s t a l l i z a t i o n proc e s s remains however q u a s i unknown. The aim of t h i s p a p e r i s t o c h a r a c t e r i z e by s o l i d s t a t e m u l t i n u c l e a r NMR t h e s t r u c t u r a l r e a r rangements t h a t o c c u r d u r i n g t h e p r o g r e s s i v e t r a n s f o r m a t i o n of amorphous g e l s i n t o c r y s t a l l i n e m o r d e n i t e .
2 2.1
EXPERIMENTAL Mordenite s y n t h e s i s
The s y n t h e s i s of m o r d e n i t e was b a s e d on p u b l i s h e d methods of p r e p a r a t i o n ( 1 9 , 2 2 ) . An aqueous a l u m i n o s i l i c a t e g e l h a v i n g t h e m o l a r c o m p o s i t i o n 2 . 4 Na20 A1203 1 1 1 S i 0 2 220 H20 was p r e p a r e d from N a - s i l i c a t e (Merk, a r t . 5 6 2 1 ) , s i l i c a g e l (Davison g r a d e 950) and Na-aluminate ( R i e d e l de Haen, a r t . 1 3 4 0 4 ) . I t was d i v i d e d i n t o s e v e r a l p o r t i o n s a n d s e a l e d i n i d e n t i c a l 20 m l p y r e x t u b e s . The l a t t e r were h e a t e d a t 1 6 5 ' ~u n d e r a u t o g e n e o u s p r e s s u r e f o r g i v e n p e r i o d s of t i m e and p r o g r e s s i v e l y removed from t h e oven, i n o r d e r t o i s o l a t e m a t e r i a l s w i t h i n c r e a s i n g d e g r e e s of c r y s t a l l i n i t y . A f t e r cool i n g , e a c h sample ( g e l + z e o l i t e ) was f i l t e r e d , washed w i t h c o l d w a t e r and d r i e d a t 1 2 0 ' ~ o v e r n i g h t , b e f o r e c h a r a c t e r i z a t i o n . The p e r c e n t a g e of m o r d e n i t e i n t h e a s - s y n t h e s i z e d p h a s e s was e v a l u a t e d by t h e c o n v e n t i o n a l X-ray d i f f r a c t i o n t e c h n i q u e (XRD) ( P h i l i p s PW 1349/30 d i f f r a c t o m e t e r , Cu Ka r a d i a t i o n ) , u s i n g r e f l e c t i o n s occu100% r i n g a t 28 a n g l e s o f 2 2 . 3 , 2 5 . 6 , 2 6 . 3 and 27.9 d e g r e e s . c r y s t a l l i n i t y was a s s i g n e d t o t h e most c r y s t a l l i n e p h a s e o f t h e s e r i e s , which proved t o be 115% c r y s t a l l i n e w i t h r e s p e c t t o a comm e r c i a l H-Zeolon from t h e Norton Company, g e n e r a l l y u s e d a s s t a n dard reference
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2 . 2 NMR s p B c t r a S o l i d s t a t e NMR s p e c t r a were o b t a i n e d a t room t e m p e r a t u r e , u s i n g a B r u k e r CXP-200 s p e c t r o m e t e r o p e r a t i n g i n t h e F o u r i e r t r a n s form mode. An r . f . f i e l d of 4 9 . 3 Oe was u s e d f o r t h e ~ r / 2p u l s e s of 2 9 ~ (3i 9 . 7 MHz). The D e l r i n c o n i c a l r o t o r was s p u n a t a r a t e of 3 . 1 kHz. Time i n t e r v a l s between p u l s e s e q u e n c e s were 3 . 0 s and o v e r 15000 f r e e i n d u c t i o n decays were accumulated p e r sample. Chemical s h i f t s ( 6 i n ppm) were measured from t e t r a m e t h y l s i l a n e (TMS). 2 7 ~h 1 i g h power NMR s p e c t r a were r e c o r d e d a t 5 2 . 1 MHz. Chemical s h i f t s were measured w i t h r e s p e c t t o AI ( ~ ~63+, 0 ) used a s e x t e r n a l r e f e r e n c e . Time i n t e r v a l s of 0 . 1 s were u s e d between p u l s e s e q u e n c e s and 5000 f r e e i n d u c t i o n d e c a y s were a c c u m u l a t e d p e r sample. 2 3 ~ a - , l i s~p~e c t r a were r e c o r d e d a t 5 2 . 9 MHz, i n s t a t i c c o n d i t i o n s . F!aiting t i m e s between p u l s e s e q u e n c e s were 0 . 2 s and 2000 f r e e i n d u c t i o n d e c a y s were a c c u m u l a t e d p e r s a m p l e .
3 3.1
RESULTS AND DISCUSSION XRD c r y s t a l l i n i t v of t h e i n t e r m e d i a t e p h a s e s
Figure 1 mediate s o l i d sigmoFd c u r v e crystallizing
shows t h e . v a r i a t i o n o f c r y s t a l l i n i t y f o r some i n t e r phases a s a f u n c t i o n of s y n t h e s i s time. A c l a s s i c a l i s obtained a s u s u a l l y observed f o r v a r i o u s z e o l i t e s from non-seeded s y s t e m s (21-23).
Time (h)
Figure 1 .
3.2
V a r i a t i o n of r e l a t i v e c r y s t a l l i n i t y of t h e i n t e r m e d i a t e s o l i d p h a s e s a s a f u n c t i o n of s y n t h e s i s t i m e .
S o l i d s t a t e HmAS " S ~ - N M R
study
u m ~o f ~100% c r y s t a l l i n e The s o l i d s t a t e HRMAS 2 9 ~ sip e-c t r~ m o r d e n i t e i s r e p o r t e d i n f i g . 2 . The t h r e e r e s o n a n c e l i n e s t h a t a p p e a r a t 6 = -1 1 0 , -105 and -96 ppm, c o r r e s p o n d t o Si-atoms hav i n g r e s p e c t i v e l y 0 , 1 and 2 Al-atoms i n t h e i r s e c o n d c o o r d i n a t i o n s h e l l s ( 1 1).
Figure 2.
S o l i d s t a t e HRMAS
29
Si-NMR s p e c t r u m of m o r d e n i t e .
F i g u r e 3 shows HRMAS 2 9 ~ sip e-c t~ r a of ~ i~ n t e r m e d i a t e phases o b t a i n e d a t v a r i o u s c r y s t a l l i z a t i o n t i m e s . The f i r s t s p e c t r u m ( A ) o o n s i s t s 6f a b r o a d r e s o n a n c e l i n e c e n t e r e d a t 6 = -95 ppm, char a c t e r i z i n g A l - r i c h amorphous g e l p h a s e s . With i n c r e a s i n g c r y s t a l l i z a t i o n times ,the amorphous p h a s e becomes r i c h e r i n s i l i c o n and a s a r e s u l t , i t s s t i l l b r o a d r e s o n a n c e l i n e s h i f t s t o h i g h e r f i e l d s (Table I ) ( f i g . 3 , A,B,C).
Figure 3 .
E v o l u t i o n of t h e s o l i d s t a t e HRMAS 2 9 ~ NMR i spectrum of i n t e r m e d i a t e p h a s e s formed d u r i n g m o r d e n i t e c r y s tallization.
Time (h)
F i g- u r e 4.
V a r i a t i o n of 2 9 ~ il i n-e ~ i n t e~n s ~ i t i e s a t 6 > -94 ppm (amorphous p h a s e ) and 6 < - 108 ppm ( m o r d e n i t e ~i(0~ 1 ) r e s o n a n c e l i n e ) a s a f u n c t i o n of s y n t h e s i s time (NMR line intensities relative t o the t o t a l intensity).
The l i n e c o r r e s p o n d i n g t o t h e amorphous phase d e c r e a s e s r a p i d l y and d i s a p p e a r s a t t h e end of t h e c r y s t a l l i z a t i o n s t e p . O p p o s i t e l y , t h e m o r d e n i t e Si(OA1) l i n e i n t e n s i t y i n c r e a s e s w i t h i n c r e a s i n g c r y s t a l l i n i t y . These r e s u l t s i n d i c a t e t h a t 2 9 ~ lii n-e ~ ~ ~ i n t e n s i t i e s can b e v a l i d l y used t o c h a r a c t e r i z e t h e c r i s t a l l i n i t y of t h e i n t e r m e d i a t e p h a s e s formed d u r i n g t h e s y n t h e s i s c o u r s e of mordenite. S i m i l a r r e s u l t s have been o b t a i n e d i n t h e s t u d y of ZSM-5 s y n t h e s i s (15). Broad band s o l i d s t a t e 2 7 ~ 1s t-u d~ v ~ ~ 27 F i g u r e 5 shows A1-NMR s p e c t r a of a s y n t h e t i c Na-mordenite and o f t h e same sample t r e a t e d w i t h aqueous H C 1 0 . 2 N . The Nam o r d e n i t e i s c h a r a c t e r i z e d by a r e s o n a n c e l i n e a t 6 2 53 ppm, c o r r e s p o n d i n g t o Al-atoms s i t e d i n t e t r a h e d r a l p o s i t i o n 9n t h e framework. Another r e s o n a n c e l i n e a p p e a r s n e a r 6 = 0 ppm a f t e r t h e HC1 l e a c h i n g . It i s due t o e x t r a - l a t t i c e o c t a h e d r a l l y coord i n a t e d Al-atoms, which have b e e n e x t r a c t e d o u t of t h e a l u m i n o s i l i c a t e framework and d e p o s i t e d i n t h e z e o l i t e c h a n n e l s o r on i t s e x t e r n a l s u r f ace ( 1 1) 3.3
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TABLE 1 Variations of ~ i / A lr a t i o s and 2 9 ~ chemical i s h i f t s ( 6 ) , of some amorphous i n t e r m e d i a t e phases formed d u r i n g t h e f i r s t s t e p of mordenite c r y s t a l l i z a t i o n . C r y s t a l l i z a t i o n time
Si/A1 (a)
O P ~ ()b )
0.5
3.2
-95.4
8
4.8
-97.8
16
5.2
-102.3
(h)
( a ) determined by energy d i s p e r s i v e X-ray a n a l y s i s (EDX) (24) ( b ) from TMS The l i n e i n t e n s i t y measured above -94 ppm ( 6 > -94 ppm) c o r r e s ponds e s s e n t i a l l y t o t h e amorphous s o l i d , w h i l e t h e one a t 6 <-108 pprn c o n t r i b u t e s e s s e n t i a l l y t o t h e S i ( 0 Al) resonance l i n e of t h e c r y s t a l l i n e mordenite p r e s e n t i n t h e i n t e r m e d i a t e ( z e o l i t e + g e l ) phase. I n between t h e -94 and -108 ppm r e g i o n , t h e resonance l i n e s from t h e amorphous phase a s w e l l a s t h o s e p e r t a i n i n g t o t h e S i ( 2 A l ) and S i ( 1 Al) c o n f i g u r a t i o n s i n c r y s t a l l i n e m o r d e n i t e , w i l l c o n t r i b u t e t o t h e s ectrum. Table 2 and f i g u r e 4 i l l u s t r a t e t h e v a r i a t i o n of t h e 29Si-NMR i n t e n s i t i e s a t 6 r -94 ppm and 6 < -108 ppm with t h e XRD c r y s t a l l i n i t y . TABLE 2
Evolution of t h e 2 9 ~ il i n-e ~ i n t~ e n s~ i t i e s a s a f u n c t i o n of cryst a l l i z a t i o n time T
Crystallization time ( h )
i i n t e n s i t i e s ( Z ) (a) X-Ray d i f f r a c t i o n 2 9 ~ NVR (%) 6 > - 9 4 npm 6 < - 1 0 8 p p m crystallinity
0.5
0
43.6
6.9
8
0
14.1
8.5
16
11
6.9
25.8
24
37
10.1
38.2
48
99
1.9
41.6
68
1 00
1 .8
39.8
120
100
0.7
47.0
100
0.7
47.9
280
-
(a) r e l a t i v e t o t h e t o t a l 2 9 ~ i - l~i nl e~ i n t e n s i t y
I
I
- H
F i g u r e 5.
- MOR
S o l i d s t a t e 2 7 ~ 1s p-e c~t r a~ of~ m o r d e n i t e : a . Na-mordenite ( ~ i / A l= 6.5) b . t h e same, l e a c h e d w i t h HC1 0 . 2 N ( S i / A l = 6.5)
T a b l e 3 sums up t h e 2 7 ~ chemical 1 s h i f t s ( 1 5 ) and l i n e w i d t h s f o r d i f f e r e n t s o l i d phases t h a t appear d u r i n g mordenite c r y s t a l h e m i c a l s h i f t s s u g g e s t s t h a t Al-atoms l i z a t i o n . The v a l u e of 2 7 ~c 1 i n t h e i n t e r m e d i a t e p h a s e s b a s i c a l l y occupy t e t r a h e d r a l p o s i t i o n s d u r i n g a l l t h e c r y s t a l l i z a t i o n p r o c e s s . Furthermore, t h e correson ding l i n e w i d t h s d e c r e a s e a s t h e c r y s t a l l i z a t i o n p r o c e e d s and remain s t a b i l i z e d a t about 1 . 9 kHz a s soon a s 100% c r y s t a l l i n e m o r d e n i t e i s o b t a i n e d (Table 3 and F i g u r e 6 ) .
TABLE 3 2 7 ~ 1 characterization - ~ ~ ~ of the intermediate solid phases formed during mordenite crystallization.
-
100 h
aP
Y
I
-
0-6
I
XRD crystallinity
7
-
-4 -2
AH
100
200
300
Time (h)
figure 6 .
Variation of the *'A~-NMR linewidths (AH) and of the XRD crystallinity of the intermediate phases as a function of synthesis time.
T h i s d e c r e a s e can b e r e l a t e d t o t h e p r o g r e s s i v e o r d e r i n g o f t h e g e l p h a s e d u r i n g t h e growth p r o c e s s . I n d e e d , an 2 7 ~ 1l i n-e w ~i d~ th ~ can b e c o n s i d e r e d as d i r e c t l y r e l a t e d t o t h e homogeneity of A l - s i t e s d i s t r i b u t i o n w i t h i n t h e s o l i d p h a s e . I n amorphous p h a s e s , t h e d i s t r i b u t i o n i s random,while i n o r d e r e d c r y s t a l l i n e p h a s e s a l l t e t r a hedral s i t e s a r e regularly\! arranged. TABLE 4
E v o l u t i o n of 2 7 ~ 1l i -n e~i n~ t e n~s i t i e k w i t h XRD c r y s t a l l i n i t y and t h e A l - c o n c e n t r a t i o n XRDcrystallinity
(W>
CAI x 1 04 (a) (mol g-1)
1 ~ (1a . u . )
e
I
0
33.0
18.5
0
23.0
13.6
11
23,4
14.0
37
22 .O
13.4
99
19.4
12.8
100
18.9
12.2
100
17.6
12.3
10 0
14.4
10.8
( a ) d e t e r m i n e d by EDX (24)
Figure 7.
C o r r e l a t i o n between 2 7 ~ 1l i-n e~i n~t e n ~s i t i e s and A1 c o n c e n t r a t i o n , measured i n some i n t e r m e d i a t e p h a s e s obtained during mordenite c r y s t a l l i z a t i o n . = s m a l l s i z e d p a r t i c l e s e n t i r e l y probed by EDX = l a r g e r p a r t i c l e s n o t e n t i r e l y probed by EDX
---
The s t r a i g h t l i n e ( c a l i b r a t i o n c u r v e ) , shown i n f i g u r e 7 , was e s t a b l i s h e d from a n a l y s e s o f p h a s e s c o n s i s t i n g of s m a l l s i z e d ( l e s s than 2 pm; p a r t i c l e s which a r e e n t i r e l y probed by t h e EDX t e c h n i q u e (Table 4 ) ( 2 4 ) . The more c r y s t a l l i n e and S i - r i c h e r p h a s e s a r e composed of p a r t i c l e s a v e r a g i n g 30 u m i n d i a m e t e r , f o r which t h e EDX ocated i n the outer s h e l l . technique d e t e c t s o n l y A l - and Si-atoms As t h e amount of aluminium d e t e c t e d by 2JA1-NMR (a b u l k a n a l y s i s method) i s d i f f e r e n t from t h a t d e t e r m i n e d by a " s u r f a c e " a n a l y s i s (EDX) ( f i g u r e 7 , d o t t e d l i n e ) , i t i s concluded t h a t an Al-concent r a t i o n g r a d i e n t must e x i s t i n t h e 100 % c r y s t a l l i n e m o r d e n i t e p a r t i c l e s : t h e i r i n n e r c o r e i s r i c h e r i n aluminium t h a n t h e i r o u t e r rim. S i / A l r a t i o s of a 100 % c r y s t a l l i n e m o r d e n i t e , a s measured by d i f f e r e n t a n a l y t i c a l methods, a;e compared i n t a b l e 5 . T h e i r regular d e c r e a s e w i t h t h e d e p t h of t h e a n a l y s i s c o n f i r m s t h e e x i s tence of an A l - g r a d i e n t . TABLE 5
V a r i a t i o n of t h e S i / A l r a t i o of m o r d e n i t e ( c r y s t a l l i z a t i o n t i m e 68 h o u r s , 100 % XRD c r y s t a l l i n i t y ) a s a f u n c t i o n of t h e d e p t h of a n a l y s i s . ' ~ n a l ~ t i c amethod l
Depth of a n a l y s i s
~ i / A l
0
XPS (ESCA) ( a )
EDX
PIGE ( b )
~ ~ A I - N M R( c )
30 A
8.0
2 Pm
6.5
10 pm
5.2
bulk analysis
5.7
(a) X-ray p h o t o e l e c t r o n s u r f a c e a n a l y s i s (25) (b) P r o t o n induced y-ray e m i s s i o n (26) (c) Determined u s i n g t h e c a l i b r a t i o n c u r v e of f i g . 7.
3.4
Broad-band
23Na-~M~ study
The 2 3 ~ a s- p e~c t~r a ~of i n t e r m e d i a t e p h a s e s o b t a i n e d d u r i n g mordenite c r y s t a l l i z a t i o n mainly c o n s i s t of a b r o a d r e s o n a n c e l i n e c e n t e r e d a t 6 = -17 ppm ( v s aqueous NaClO 4 ) ( f i g u r e 8 ) .
-
6 (ppm) 1 aq. NaCIO,
F i g u r e 8.
2 3 ~ a s-p e ~ c t r~a ~ of some i n t e r m e d i a t e p h a s e s o b t a i n e d during mordenite c r y s t a l l i z a t i o n .
During t h e s y n t h e s i s p r o c e s s , t h e 2 3 N a - ~l i~n ~ e w i d t h (AH) r a p i d l y d e c r e a s e s and remains n e a r l y c o n s t a n t as t h e c r y s t a l l i z a t i o n i s complete ( f i g . 8 and 9 ) .
11 10
XRD crystallinity
A
N
I Y
Y
I
a
-
*
AH '
-
--
5
Yo
0
100
200
300
Time (h)
Figure 9 .
V a r i a t i o n of 2 3 ~ a l-i n~e w~i d t~h s and XRD c r y s t a l l i n i t y a s a f u n c t i o n of s y n t h e s i s t i m e .
T h i s i l l u s t r a t e s t h e p r o g r e s s i v e i n c o r p o r a t i o n of sodium i o n s into the z e o l i t e l a t t i c e . I n the intermediate phases, t h e d i s t r i Moreover, t h e e l e c t r i c bution of t h e chemical s h i f t s i s b r o a d . f i e l d g r a d i e n t pn t h e 2 3 ~ a - n u c l e u s must a l s o b e d i f f e r e n t i n an amorphous i n t e r m e d i a t e phase w i t h r e s p e c t t o t h e 100 % c r y s t a l l i f i e ~ h a s e , h e n c ea b z o a d N M R l i n e i s o b s e r v e d . O p p o s i t e l y , i n t h e o r d e r e d c r y s t a l l i n e p h a s e s , t h e chemical s h i f t d i s t r i b u t i o n i n n a r r o w e r and t h e l i n e w i d t h of t h e c o r r e s p o n d i n g NMR r e s o n a n c e d e c r e a s e s . When t h e 100 % c r y s t a l l i n e m o r d e n i t e i s l e f t i n t h e a u t o c l a v e f o r a long t i m e , a new r e s o n a n c e l i n e a p p e a r s a t 6 = - 2 ppm. It can be t e n t a t i v e l y a t t r i b u t e d t o sodium i o n s i n c o r p o r a t e d i n t o a p a r a s i t e s p e c i e s (such a s analcime) ( f i g . 8 ) . F i g u r e 10 and T a b l e 6 compare t h e r a t i o of t h e n o r m a l i z e d i n t e n s i t i e s of 27~1l i n e s w i t h t h e Al/Na a t o m i c r a t i o measured by EDX. A r e l a t i v e l y good c o r r e l a t i o n i s o b s e r v e d . T h i s iques demonstrates t h a t t h e combination of 2 7 ~ 1a- n d 2 3 ~ a -t e~c h~n ~ can be v a l u a b l y used b o t h t o show t h e i n c o r p o r a t i o n of A l - and Naatoms i n t h e z e o l i t e and t o e s t i m a t e t h e ~ 1 / r~a tai o i n t h e s a m p l e , provided a c a l i b r a t i o n c u r v e .
TABLE 6 V a r i a t i o n of t h e NMR i n t e n s i f y r a t i o
N a-NMR
'
with t h e
atomic Na
r a t i o measured by EDX i n t h e i n t e r m e d i a t e p h a s e s . LA1
A1 Na (EDX)
3G
0.83 0. 91 0.98 1.04 1.13 1.14 1.13 1.13
0.58 0.48 0.60 0.59 0.74 0.68 0.77 0.66
F i g u r e 10.
+
27 E v o l u t i o n o f the . A l - and 2 3 ~ a i-n t~e n~s i t~y r a t i o w i t h Al/Na a t o m i c r a t i o as d e t e r m i n e d by EDX
4 CONCLUSIONS M u l t i n u c l e a r s o l i d s t a t e NMR r e v e a l s t o b e a p o w e r f u l t o o l f o r t h e c h a r a c t e r i z a t i o n of i n t e r m e d i a t e s o l i d p h a s e s o c c u r i n g a can ~ be ~ during c r y s t a l l i z a t i o n of zeolites.HRMAS 2 9 ~ sip e-c t r~ v a l i d l y r e l a t e d t o t h e o r d e r i n g of t h e i n i t i a l amorphous s o l i d . The p r o g r e s s i v e i n c o r p o r a t i o n of Al-atoms i n an o r d e r e d z e o l i t i c l a t t i c e i s c h a r a c t e r i z e d by "AI-NMR. This technique i s a l s o able t o d i s c r i m i n a t e between t e t r a - and o c t a - c o o r d i n a t e d Al-atwms It can be used t o d e t e r m i n e t h e Al-content of a s o l i d p h a s e , p r o v i d e d a c a l i b r a t i o n c u r v e . From t h e comparison of t h e Si/A1 r a t i o s determined by d i f f e r e n t s u r f a c e and b u l k a n a l y s i s methods, e v i d e n c e can be o b t a i n e d f o r t h e aluminium g r a d i e n t s i n t h e s m a l l c r y s t a l l i t e s . I n t h e p a r t i c u l a r c a s e of m o r d e n i t e , t h e o u t e r s h e l l of t h e 100 % c r y s t a l l i n e p a r t i c l e s c o n t a i n s l e s s aluminium t h a n t h e i r i n n e r core. The e v o l u t i o n of ~ ~ N ~ - N Ml i Rn e s shows t h e i n c o r p o r a t i o n of sodium i o n s i n t o t h e z e o l i t e l a t t i c e . T h i s t e c h n i q u e can be used t o g e t h e r w i t h 2 7 ~ 1 t-o ~d e~t e r~m i n e t h e Al/Na r a t i o o f t h e s t u d i e d sample.
.
The a u t h o r s wish t o thank M r . G. Daelen f o r h i s a p p r e c i a t e d help i n t a k i n g t h e NMR s p e c t r a and M r . F. V a l l e t t e f o r h i s t e c h n i c a l a s s i s t a n c e . P. Bodart t h a n k s IRSIA (Belgium) 6 o r f i n a n c i a l support.
6
REFERENCES
1 . D.W. B r e c k " Z e o l i t e M o l e c u l a r S i e v e s " , J . W i l e y & S o n s , New York ( 1 9 7 4 ) . 2 . R.M. B a r r e r "Hydrothermal Chernis t r y o f Z e o l i t e s " , Academic P r e s s ,London ( 1 982) 3 . L.B. Sand i n " P r o c e e d i n g s o f t h e 5 t h I n t e r n a t i o n a l C o n f e r e n c e on Z e o l i t e s " , (L.V. R e e s , e d . ) , Heyden, London, p . 1 ( 1 9 8 0 ) . 4. G. E n g e l h a r d t , U . L o h s e , E. Lippmaa, M. Tarmak a n d M. Mggi, Z. Anorg. A l l g . Chem., 482, 49 ( 1 9 8 1 ) . 5 . J . K l i n o w s k i , S . Ramdas, J . M . Thomas, C.A. F y f e a n d J . S . Hartrnan, J . Chem. Soc. , F a r a d a y T r a n s 11, 78, 1025 ( 1982) 6. J.M. Thomas, C.A. F y f e , J . K l i n o w s k i and GZ. Gobbi, J . P h y s . Chem., 8 6 , 3061 ( 1 9 8 2 ) . G.C. , Gobbi, J . S . Hartman, R.E. L e n k i n s k i a n d J . H . 7. C.A. F ~ Z O ' B r i e n , J . Magn. Reson., 47, 168 ( 1 9 8 2 ) . J.M. Thomas a n d S. Ramdas, 8 . C.A. F y f e , G.C. Gobbi, J . = i n o w s k i , 296, 530 (1982). Nature, 9 . J. B.Nagy, J.-P. G i l s o n and E.G. D e r o u a n e , J . Chem. Soc.,Chem. Cornrnun., 1981, 1129. 10. J. B.Nagy, Z . G a b e l i c a , E .G. Derouane and P.A. J a c o b s , Chem. L e t t . , 1982, 2003. 1 1 , G. D e b r a s , J . B.Nagy, Z . ~ a b e l i c a ,P. B o d a r t and P.A. J a c o b s , Chem. L e t t . , 1983, 199. 12. J . B.Nagy, Z . G a b e l i c a , G. D e b r a s , P. B o d a r t , E.G. Derouane a n d P.A. J a c o b s , J . Mol. C a t a l . , i n p r e s s . 13. Z . G a b e l i c a , J . B.Nagy, P. B o d a r t , G. D e b r a s , E . G . Derouane a n d P.A. J a c o b s , i n " Z e o l i t e s S c i e n c e and T e c h n o l o g y v , L i s b o n , 1983 ( t h i s m e e t i n g ) 14. J. B.Nagy, Z . GabelicaandE.G.Derouane,Zeolites, 3 , 4 3 ( 1 9 8 3 ) . Proc. 6th 1 5 , Z . G a b e l i c a , J . B.Nagy, G. D e b r a s and E.G. ~erouan;, I n t . Conf. Z e o l i t e s , Reno, 1983 ( s u b m i t t e d ) . 16. R.M. B a r r e r , J . Chem. S o c . , 1948, 2518. 17. L.L. Ames and L.B. Sand, Amer. M i n e r . , 4 3 , 476 ( 1 9 5 8 ) . 18. L. B. Sand i n " ~ o l e c u l a rs i e v e s " , ~ o c i e t y o fChemical I n d u s t r y , London, p . 7 1 ( 1 968) 19. L.B. Sand, U.S. P a t . 3 , 4 3 6 , 1 7 4 ( 1 9 6 9 ) . 5 7 , 1146 ( 1 9 7 2 ) . 20. O . J . Whitternore J r . , h e r . M i n e r . , 1 2 1 , 140 ( 1 9 7 3 ) . 21. A. C u l f a z and L.B. Sand, Adv. Chem. S e r . , 22. P.K. B a j p a i , M.S. Rao a n d K.V.G.K. G o k h a l e , I n d . Eng. Chem. 223 ( 1 9 7 8 ) . P r o d . R e s . Dev., 23. S. Ueda, H . M u r a t a , M. Koizumi and H . N i s h i m u r a , Amer. M i n e r . , 6 5 , , 1012 ( 1 9 8 0 ) . 5, 227 24. G a b e l i c a , N . Blom and E.G. D e r o u a n e , Appl. C a t a l . ( 1983) 25. E.G. D e r o u a n e , J.-P. G i l s o n Z . G a b e l i c a C . Ilousty-Desbuquoit and J. V e r b i s t , J . C a t a l . , 5 1 , 447 ( 1 9 6 1 j . 26. G. D e b r a s , E.G. D e r o u a n e , J.-P. G i l s o n , Z . ~ a b e l i c aand G. D e m o r t i e r , Z e o l i t e s , 2, 37 ( 1 9 8 3 ) .
.
.
.
.
17,
.
SORPTION BY ZEOLITES PART I. E Q U I L I B R I A AND ENERGETICS
R.M.
Barrer
Chemistry Department I m p e r i a l C o l l e g e of S c i e n c e and Technology London SbJ7 2AY
ABSTMCT S t e r i c f a c t o r s g o v e r n i n g p e n e t r a t i o n of t h e h o s t z e o l i t e by g u e s t s p e c i e s have b e e n c o n s i d e r e d . A f t e r e q u i l i b r i u m i s e s t a b l i s h e d q u a n t i f i c a t i o n of t h e r e s u l t s can b e made i n s e v e r a l ways. S e l e c t i v i t y of s o r ~ t i o nr e q u i r e s i n t e r n r e t a t i o n of h e a t s and e n t r o n i e s of u p t a k e i n terms r e s p e c t i v e l y o f u n i v e r s a l ( o r nons p e c i f i c ) and of s p e c i f i c components of h e a t , and i n t e r n s of t h e ohysical s t a t e o f t h e s o r b e d s p e c i e s . These a s ~ e c t shave a l s o b e e n considered. I s o t h e r m m o d e l l i n p h a s b e e n d i s c u s s e d i n terms o f t h e v i r i a l i s o t h e r m e q u a t i o n , and i n terms OF a s i t e model i n which a s i t e i s i d e n t i f i e d w i t h a c a v i t y and s o i s a b l e t o h o l d a s m a l l c l u s t e r of m o l e c u l e s .
Z e o l i t e s and Dorous c r y s t a l l i n e s i l i c a s v r o v i d e s t a b l e , h i p h capacity, m i c r o p o r e s o r b e n t s w i t h d i v e r s e m o l e c u l e s i e v i n g p r o p e r t i e s . Each framework t o ~ o l o g yp r o v i d e s i t s own u n i q u e s y s t e m of c h a n n e l s and c a v i t i e s . For example, i n z e o l i t e s i n which wide s t r a i p h t channels a r e found t h e d e t a i l of t h e s e c h a n n e l s i s a u i t e d i f f e r e n t , rYlAZ and MER i n F i g . 1 ( 1 ) . One as i l l u s t r a t e d f o r z e o l i t e s LTL, expects and f i n d s d i f f e r e n c e s i n s o r o t i o n b e h a v i o u r f o r e a c h topology. The b e h a v i o u r i s f u r t h e r m o d i f i e d f o r z e o l i t e s by t h e number, l o c a t i o n and s i z e of t h e i n t r a c r y s t a l l i n e c a t i o n s which n e u t r a l i s e t h e n e g a t i v e c h a r g e on t h e framework. Thus, s i n c e c a t i o n s are exchangeable, m o d i f i e d s o r b e n t s c a n b e made from a g i v e n frarnework t o ~ o l o g yby c a t i o n exchange. The u p t a k e of g u e s t m o l e c u l e s b y z e o l i t e h o s t c r y s t a l s h a s e q u i l i b r i u m and e n e r g e t i c p r o p e r t i e s ,
LTL
MAZ
MER
F i g . 1.
F i g . 2.
The c h a n n e l s i n z e o l i t e L, LTL, m a z z i t e , MZA , and m e r l i n o i t e , E R (1)
.
Some p o l y h e d r a l v o i d s found i n z e o l i t e s . The c h a b a z i t e 20-hedron, capped w i t h hexagonal (i) prisms. The g m e l i n i t e 14-hedron of t y p e 11. (ii) ( i i i ) The e r i o n i t e 23-hedron The l e v y n i t e 17-hedron of t y p e I . (iv) The l o s o d 17-hedron of t y p e I1 w i t h a s s o c i a t e d (v) 11-hedral c a n c r i n i t e cage.
considered i n t h i s P a r t , and k i n e t i c a s p e c t s , d i s c u s s e d i n P a r t 11. I n terms of t h e d e f i n e d s t r u c t u r e s of c h a n n e l s and c a v i t i e s z e o l i t e s provide model systems f o r q u a n t i t a t i v e s t u d i e s of s o r p t i o n .
2.
CONDITIONS FOR PENETRATION OF ZEOLITE LATTICES
Provided a z e o l i t e c r y s t a l i s open enough t o admit t h e g u e s t species under c o n s i d e r a t i o n s o r p t i o n c o m ~ l e x e sform i n which t h e amount of g u e s t s o r b e d i n a g i v e n w e i g h t of z e o l i t e depends o n l y upon t h e p r e s s u r e of t h e vapour of t h e g u e s t and t h e t e m ~ e r a t u r e . However, t h e e a s e of p e n e t r a t i o n of t h e o u t g a s s e d z e o l i t e by g u e s t molecules depends upon t h e f o l l o w i n g f a c t o r s :
1.
2. 3.
4. 5.
6.
The s i z e and shape of t h e windows c o n t r o l l i n g e n t r y t o t h e c h a n n e l s and c a v i t i e s i n t h e z e o l i t e . The s i z e and shane of t h e g u e s t m o l e c u l e s . The number, l o c a t i o n s and s i z e of t h e exchangeable c a t i o n s . The p r e s e n c e o r absence of d e f e c t s such a s s t a c k i n g f a u l t s which may narrow d i f f u s i o n pathways a t p l a n e s where such f a u l t s occur. The p r e s e n c e o r absence of d e t r i t a l m a t e r i a l l e f t i n t h e channels during synthesis o r introduced subsequently, f o r e x a m ~ l e ,by chemical means such a s s i l a n a t i o n ( 2 ) . The p r e s e n c e o r absence of o t h e r s t r o n g l y h e l d g u e s t m o l e c u l e s l i k e w a t e r , ammonia ( 3 ) and s a l t s ( 4 , 5 ) , introduced i n t e n t i o n a l l y i n metered amounts.
The s i x t h f a c t o r i s r e f e r r e d t o i n P a r t 11, Here we w i l l refer primarily t o the f i r s t four factors. 2.1.
F r e e Dimensions of Llindows
The windows o r o ~ e n i n g swhich c o n t r o l e n t r y t o t h e i n t r a c r y s t a l l i n e p o r e s and c h a n n e l s a r e r i n g s of l i n k e d (A1,Si) o4 tetrahedra c i r c u m s c r i b i n g c h a n n e l s l i k e t h o s e shown i n F i g . 1, o r allowing a c c e s s t o p o l y h e d r a l c a v i t i e s l i k e t h o s e i l l u s t r a t e d i n Fig. 2 . The i m p o r t a n t o p e n i n g s from t h e p r e s e n t v i e w p o i n t a r e those composed of 8, 1 0 o r 12 l i n k e d t e t r a h e d r a . These 8-, 10and 12-rings a r e l i n e d on t h e i r i n n e r p e r i ~ h e r i e sby oxygen atoms, and i n t h e frameworks of z e o l i t e s h a v i n g d i f f e r e n t t o p o l o g i e s t h e y may b e v a r i o u s l y e l o n g a t e d , o r puckered t o d i f f e r e n t c o n f o r m a t i o n s , such a s crown, b o a t o r c h a i r c o n f i g u r a t i o n s . T h e r e f o r e a g i v e n n-ring may have d i f f e r e n t f r e e dimensions and s o may imnose d i v e r s e molecule s i e v i n g b e h a v i o u r f o r t h e same v a l u e of n, a c c o r d i n g t o the z e o l i t e i n which i t o c c u r s . The v a r i a t i o n s i n f r e e dimensions a r e shown i n T a b l e 1 f o r t y p i c a l z e o l i t e s w i t h 8-, 1C- and 1 2 - r i n p windows. By 11 f r e e dimensions" one means t h a t t h e s p a c e t o which the dimensions r e f e r i s n o t impinged upon by t h e p e r i p h e r a l oxygens l i n i n g t h e i n s i d e of each r i n g .
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2.2.
Molecular Dimensions
The i d e a of u s i n g s i m p l e molecules of known dimensions t o c h a r a c t e r i s e m o l e c u l a r s i e v e z e o l i t e s w a s ' i n t r o d u c e d some t i m e ago (6,7) and h a s proved most u s e f u l i n g r o u p i n g z e o l i t e s i n t o d i f f e r e n t Based on P a u l i n g ' s v a l u e s c a t e g o r i e s of m o l e c u l a r s i e v e ( 6 , 7 , 8 ) . of bond l e n g t h s and van d e r FJaals r a d i i ofoatoms and m o l e c u l e s t h e dimensions of some y a r d s t i c k molecules i n A a r e a s f o l l o w s (8) :
0
For CH4, 4 . 0 A i s a v a l u e assuming i t t o be a smooth s p h e r e . The value i n b r a c k e t s t a k e s i n t o a c c o u n t t h e t e t r a h e d r a l s t r u c t u r e . For t h e o t h e r m o l e c u l e s t h e f i g u r e s I n b r a c k e t s a r e t h e d i a m e t e r s of t h e c i r c u m s c r i b i n g s p h e r e s and t h o s e n o t i n b r a c k e t s a r e t h e h e i g h t s o f t h e m o l e c u l e s s i t t i n g on a t r i a n g u l a r b a s e . One o r other of t h e s e dimensions s h o u l d b e c r i t i c a l i n d e t e r m i n i n g w h e t h e r t h e molecule w i l l e n t e r t h e z e o l i t e l a t t i c e , For s e v e r a l dumbell-shaped m o l e c u l e s t h e c r o s s - s e c t i o n a l diameter and ( i n b r a c k e t s ) t h e l e n g t h s a r e a s f o l l o w s (8) :
Idhere t h e l e n g t h e x c e e d s t h e c r o s s - s e c t i o n a l d i a m e t e r i t w i l l b e the l a t t e r which i s t h e c r i t i c a l dimension f o r p e n e t r a t i n p t h e zeolite. S i t u a t i o n s have been a b u n d a n t l y d e m o n s t r a t e d i n which one molecular s p e c i e s i s t o t a l l y e x c l u d e d w h i l e a n o t h e r i s r e a d i l y The s e n a r a t i o n can be quantsorbed by a g i v e n z e o l i t e (6,7,8) i t a t i v e i n a s i n g l e s t e p and c a n b e most i m p o r t a n t f o r l a r g e s c a l e s e p a r a t i o n s and i n shape s e l e c t i v e c a t a l y s i s . T a b l e 2 i l l u s t r a t e s s i e v e c h a r a c t e r i s t i c s f o r t h r e e z e o l i t e s , Ca-A, ZSM-5 and f a u j a s i t e ( ~ a - X )f o r which t h e windows a r e r e s p e c t i v e l y 8-, 10- and 1 2 - r i n g s . For f u l l y s t r e t c h e d n-para£ f i n s t h e c r o s s - s e c t i o n a l dimension c r i t i c a l f o r p e n e t r a t i o n i s t h a t i n t h e p l a n e o f t h e zig-zap (4.9 2); f o r benzene t h i s dimension i s t h e d i s t a n c e a c r o s s i n t h e p l a n e o f the m o l e c u l e . For non-symmetrical m o l e c u l e s t h e b e s t o r i e n t a t i o n f o r p e n e t r a t i o n can b e seen by ? r e s e n t i n g a s c a l e model of t h e g u e s t t o a s c a l e model of t h e window.
.
The c r i t i c a l dimensions f o r e n t r y a r e s e e n t o be g r e a t e r than t h e f r e e dimensions of t h e openings (Table 2 ) . This happens because the g u e s t molecules and the l a t t i c e atoms (here oxygens) a r e n o t h a r d s p h e r e s b u t a r e deformable and a l s o because included i n l a t t i c e v i b r a t i o n s t h e r e a r e b r e a t h i n g f r e q u e n c i e s f o r t h e openings a s a whole. Accordingly n - p a r a f f i n s d i f f u s e r a t h e r e a s i l y i n t o Ca-A, although s i g n i f i c a n t energy b a r r i e r s a r e involved. The energy b a r r i e r s i n c r e a s e r a p i d l y a s t h e c r i t i c a l dimensions i n c r e a s e , and those f o r t o t a l e x c l u s i o n a r e soon reached, a s i n d i c a t e d i n Table 2. Temperatures a t which t h e uptake occurs can p l a y a n imnortant p a r t i n molecule s i e v i n g . Because of t h e energy b a r r i e r s involved d i f f u s i v i t i e s , D, follow an h r r h e n i u s r e l a t i o n : D = Do ~ X ~ - E / ~ T . Thus, the lower t h e temperature t h e more s e n s i t i v e the molecules i e v i n g p r o c e s s becomes. A t low temperatures (-183OC) i t i s poss i b l e t o s e p a r a t e 0 2 and A r q u a n t i t a t 8 v e l y u s i n Na-A o r l e v y n i t e ( t h e i r c r i t i c a l dimensions being 2 . 8 A and 3 . 8 a l s o t o o b t a i n l a r g e r a t e d i f f e r e n c e s between O2 Such d i f f u s i o n a l a s p e c t s w i l l be r e f e r r e d t o i n P a r t 11.
I
2.3.
Blocking of Windows by Cations
I n Ca-A, ZSM-5 and f a u j a s i t e t h e windows a r e n o t blocked by c a t i o n s and t h e s i e v e c h a r a c t e r could t h e r e f o r e b e a s s e s s e d from t h e f r e e dimensions of t h e windows deduced from the s t r u c t u r e s . However a f t e r exchanges c a 2 + -+ 2 ~ a +o r 2 ~ +the number of c a t i o n s in z e o l i t e A i s doubled and ~ a and + K+ i o n s occupy p o s i t i o n s i n 8 - r i n g windows. Na-A no longer admits n - p a r a f f i n s a t room temperature, but s t i l l s o r b s 02, N2, A r and small p o l a r molecules. Likewise Ca-chabazi t e w i l l s o r b a-paraf f i n s b u t Na-chabazite does n o t sorb even oxygen ( 1 1 ) . Z e o l i t e RHO s o r b s only water and ammonia u n t i l converted i n t o i t s H-form when i t s o r b s permanent gases and n - p a r a f f i n s c o p i o u s l y . The windows a r e octagonal prisms of f r e e diameter 3.9 x 5 . 1 8, b u t t h e s e windows appear t o be s e l e c t i v e c a t i o n t r a p s (12). When i n z e o l i t e Na-A one exchanges 2 ~ a +by ca2+ i n s t a g e s n - p a r a f f i n s b e g i n t o be sorbed f r e e l y when about 30Z of t h e ~ a + i s replaced ( 1 3 ) . This s e n s i t i v e range occurs when enough 8-ring windows have been f r e e d of c a t i o n s t o g i v e a f r a c t i o n of c l e a r pathways through t h e three-dimensional network of channels. S i m i l a r behaviour i s observed i n o t h e r exchanges of mono- by d i v a l e n t c a t i o n s (11,14)
.
2.4.
Control of Access by S t a c k i n g F a u l t s
Stacking f a u l t s can occur i n c a n c r i n i t e and g m e l i n i t e and sequence of l a y e r s sometimes i n of f r e ti t e . I n c a n c r i n i t e an ab..
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c o n t a i n i n g s i n g l e hexagons ( 6 - r i n g s ) c a n be i n t e r r u p t e d by a n abc. sequence t y p i c a l of s o d a l i t e . I n g m e l i n i t e a n A3. t sequence of l a y e r s c o n t a i n i n g hexagonal p r i s m s i s i n t e r r u p t e d by a n ABCT sequence, and i n o f f r e ti t e an Ab sequence i s i n t e r sequence of e r i o n i t e . I n a l l t h e s e s i t u a t i o n s r u p t e d by AbAcwide c h a n n e l s curcumscribed by 1 2 - r i n g s a r e b l o c k e d , by windows no w i d e r t h a n t h e 6 - r i n g s of s o d a l i t e o r t h e 8 - r i n g s of c h a b a z i t e and e r i o n i t e f o r c a n c r i n i t e and f o r g m e l i n i t e and of f r e ti t e respectively, I t i s n o t d i f f i c u l t however t o make o f f r e t i t e s f r e e of such i n t e r growths and s t a c k i n g f a u l t s .
..
.
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.
.-
I n t h e c a s e of c a n c r i n i t e i n p a r t i c u l a r s t a c k i n g f a u l t s may n o t be t h e o n l y cause of b l o c k i n g . Both c a n c r i n i t e and s o d a l i t e are n o t a b l e f o r t r a p p i n g s a l t s and c a u s t i c soda a l o n g w i t h z e o l i t i c w a t e r d u r i n g s y n t h e s i s , and t h e r e i s some a n a l y t i c a l e v i d e n c e t h a t d e t r i t a l s i l i c a t e may b e a f u r t h e r b l o c k i n g f a c t o r i n c a n c r i n i t e
(5) 3.
DISTRIBUTION PATTERNS OF GUEST MOLECULES
The p o r e s p a c e i n z e o l i t e s i s p a r c e l l e d up i n t o c a v i t i e s and/ o r c h a n n e l s of m o l e c u l a r dimensions. The r e s u l t a n t p a t t e r n s of pathways through which g u e s t molecules of t h e r i g h t s i z e and shape can move can be p l a c e d i n t h r e e c a t e g o r i e s :
1.
2.
A l l pathways a r e p a r a l l e l and n o n - i n t e r c o n n e c t e d (l-dimensi o n a l (1-D) channel systems a s i n m o r d e n i t e , m a z z i t e , l a u m o n t i t e o r z e o l i t e L. The pathways whether p a r a l l e l o r n o t a r e i n t e r c o n n e c t e d t o g i v e 2-dimensional (2-D) channel s y s tems Guest molecules may m i g r a t e between l a y e r s b u t cannot move from one l a y e r t o a p a r a l l e l layer i n the c r y s t a l (heulandite, levynite, s t i l b i t e and f e r r i e r i t e ) The pathwavs may be s o i n t e r c o n n e c t e d a s t o a l l o w guest molecules t o m i g r a t e i n 3-dimensions (3-D channel s y s tems a s i n c h a b a z i t e , e r i o n i t e , z e o l i t e A and z e o l i t e s ZSM-5, RHO - and ZK-5)
.
.
3.
.
D e t a i l e d c h a n n e l g e o m e t r i e s a r e however d i f f e r e n t f o r each framework topology, a s was s e e n i n F i g . 1 f o r t h e 1-D channels of z e o l i t e s L (LTL) - , m a z z i t e (MAZ) and m e r l i n o i t e (ME?@). The d i s t r i b u t i o n of t h e g u e s t molecules i n t h e h o s t z e o l i t e s i s determined by t h e 1-D, 2-D o r 3-D n a t u r e of t h e c h a n n e l s i n which t h e y a r e l o c a t e d . Thus i n 1-D systems t h e y a r e p r e s e n t a s p a r a l l e l filaments s u p p o r t e d by t h e c h a n n e l w a l l s . For example i n z e o l i t e L t h e r e are r e s t r i c t i o n s a l o n g each channel of 7 . 1 8 f r e e d i a m e t e r a l t e r n a t i n g w i t h w i d e r p a r t s of % 12 f r e e d i a m e t e r . Gihen t h e channel i s f u l l of s m a l l molecules l i k e w a t e r o r oxygen t h e s e form l i q u i d - l i k e beads o r c l u s t e r s i n t h e wide p a r t connected i n t o f i l a m e n t s through the
-
8
0
7 . 1 A openings t o o t h e r b e a d s . I n o f a u j a s i t e each 26-hedron of type I1 of f r e e d i a m e t e r a b o u t 12 A i s connected through 7.4 b: openings t o f o u r more such t e t r a h e d r a . L i q u i d - l i k e c l u s t e r s i n each 26-hedron a r e connected t o t h o s e i n i t s f o u r n e i g h b o u r s t o give a 3-D p a t t e r n of connected c l u s t e r s a r r a n g e d l i k e t h e bond p a t t e r n i n diamond. The s m a l l e r t h e f r e e d i a m e t e r o f t h e c o n n e c t i n g windows t h e more i s o l a t e d each m o l e c u l a r c l u s t e r becomes from i t s n e i g h b o u r s while t h e s m a l l e r t h e c a v i t y a n d / o r t h e l a r g e r t h e g u e s t molecule
Table 3:
zeolite
Chabazi t e
C l u s t e r s i z e s a t s a t u r a t i o n of c a v i t i e s i n c h a b a z i t e and faujasite (zeolite XI.
Cavities
20-hedra (6 x 8 - r i n g s 2 x 6-rings 12 x 4 - r i n g s )
Guest molecules p e r c a v i t y
12-14 H 2 0 -7.7 NH3 ~6 A r , N 2 , O2 $ 4 . 9 CH3NH2 ~ 4 . 3 CH3C1 $3.1 C P 2 C I 2 -2.0
Faujasite
26-hedra (4 x 1 2 - r i n g s 4 x 6-rings 18 x 4 - r i n g s )
l2
a32 H 2 0 (28 + 4)' 17-19 A r , N2, O2 ~ 7 . 5I 2 %7.8 C F 4 -6.5 SF6 Q5.8 C2F6 a5.6 c y c l o ~ e n t a n e ~ 5 . 4benzene ~ 4 . 6t o l u e n e ~ 4 . 5n-CgH12 % 4 . 1 cyclohexane $4.1 p e r f l u o r o c y c l o b u t a n e $4.1 C2FqC12 Q3.5 n-C7H16 -3.4 C 3 F g
-2.9 n-C4F1o
~ 2 . 8 SO-C8H1 ~ 2 . 3perfluoromethylcyclohexane
-2.1 B
perfluorodimethylcyclohexane
Four of t h e w a t e r molecules a r e t h o u g h t t o be i n t h e s o d a l i t e - t y p e 14-hedra a l s o p r e s e n t i n f a u j a s i t e .
t h e fewer a r e t h e m o l e c u l e s p e r c l u s t e r (Table 3 ) . Thus i n t h e 14-hedral c a v i t i e s o o f s o d a l i t e h y d r a t e each c a v i t y h a s a f r e e d i a m e t e r of % 6.6 A and i s connected by 6 - r i n g windows of Q 2 . 1 A f r e e d i a m e t e r t o e i g h t n e a r e s t neighbour 14-hedra. I t can,accoamodate f o u r w a t e r molecules (van d e r Waals d i a m e t e r % 2.8 A ) , a s a n e a r l y i s o l a t e d c l u s t e r due t o t h e s m a l l f r e e d i a m e t e r of t h e windows. A t h i g h p r e s s u r e and t e m p e r a t u r e i t o c a n accommodate one o n l y Ar o r K r (15) of d i a m e t e r 3 . a 3 and 3.g4 A r e s p e c t i v e l y . This corresponds with i n t e r s t i t i a l s o l u t i o n as t h e l i m i t t o the l a r g e r c l u s t e r s i l l u s t r a t e d i n Table 3 f o r t h e 20-hedra n r e s e n t i n chabazi r e and t h e 26-hedra i n f a u j a s i t e .
4.
TYPES OF ISOTHERM AND GUEST-ZEOLITE COMPLEX
Under a p p r o p r i a t e c o n d i t i o n s z e o l i t e s can s o r b non-polar and p o l a r molecules, s a l t s o r metals. I n a d d i t i o n m e t a l s may b e i n t r o duced by r e d u c i n g c a t i o n i c forms of t h e z e o l i t e o r by s o r b i n g v o l a t i l e m e t a l l i c compounds such a s c a r b o n y l s and t h e n decomposing t h e s e . S a l t s may be i n t r o d u c e d , i n c o m p e t i t i o n w i t h w a t e r , during hydrothermal s y n t h e s i s , o r from aqueous s o l u t i o n i n t o t h e a l r e a d y formed z e o l i t e , a s w e l l a s from s a l t m e l t s o r vapours. Isotherms of non-polar g u e s t molecules a r e a s a r u l e of type T i n B r u n a u e r ' s c l a s s i f i c a t i o n (16) a s shown i n F i g . 3 ( 1 7 ) . The more condensable t h e g u e s t molecule o r t h e lower t h e t e m p e r a t u r e t h e more r e c t a n g u l a r t h e i s o t h e r m s become. On t h e o t h e r hand t h e l e s s condensable t h e s o r b e d molecule o r t h e h i g h e r t h e t e m p e r a t u r e the more n e a r l y does t h e i s o t h e r m approach t h e Henry's law l i m i t (uptake proportional t o pressure). There a r e however s o r p t i o n complexes c h a r a c t e r i s e d by v e r y s t r o n g molecule-molecule i n t e r a c t i o n s between p a i r s o f g u e s t molec u l e s which d r a m a t i c a l l y change t h e i s o t h e r m c o n t o u r s t o t y p e s IV o r V i n Brunauer's c l a s s i f i c a t i o n . These w i l l b e i l l u s t r a t e d f o r a n e l e c t r o n e g a t i v e e l e m e n t , a s a l t and a m e t a l . Thus F i g . 4 (18) shows i s o t h e r m s n e a r l y of type V f o r t h e r e v e r s i b l e u ~ t a k eof phosphorus i n z e o l i t e Na-X. The s t r o n g upward i n f l e x i o n may a r i s e from t h e o n s e t of p o l y m e r i s a t i o n of s m a l l e r phosphorus s p e c i e s such as Pq w i t h i n t h e z e o l i t e . When s a l t s a r e i n c o r p o r a t e d i n t o s o d a l i t e d u r i n g hydrothermal s y n t h e s i s k e e p i n g w a t e r , c a u s t i c soda and m e t a k a o l i n i t e c o n s t a n t i n t h e r e a c t i o n m i x t u r e t h e i s o t h e r m s of s a l t u ~ t a k ea r e a g a i n of type I a s shown i n F i g . 5 ( 1 9 ) . However when s a l t s were t a k e n up from aqueous s o l u t i o n s i n t o pre-formed z e o l i t e s t h e i s o t h e r m s i n F i g . 6 (20) were of type 111, w i t h c u r v a t u r e i n t h e o p p o s i t e s e n s e t o those i n F i g . 5. T h e i r shape i s determined by a Donnan e q u i l i b r i u m whereas t h e shape i n F i g . 5 may a r i s e because t h e s a l t s a r e a c t i n g a s tem!hen p l a t e s d u r i n g a c t u a l c r y s t a l n u c l e a t i o n and growth ( 2 1 ) .
Pressure (cmHg)
Fig. 3 .
Type I isotherms of CFI, i n ~ a - f a u j a s i t e a t v a r i o u s absolute temperatures ( 1 7 ) .
Phosphorus pressure cm Hg
~ i g .4 ,
Isotherms f o r uptake of phosphorus i n Na-X (18). Temperatures a r e i n O C . 0 = a d s o r p t i o n p o i n t s ; = desorption points.
0
1
02
L
06
-
J
10
-
l
1.4
L
U
1.8
Mdarity of salt In synthes~ssolution
Fig. 5.
I s o t h e r m s f o r u p t a k e of NaC1Q4 (0) and NaC103 (a) from aqueous s o l u t i o n s d u r i n g s y n t h e s i s of s o d a l i t e ( 1 9 ) . 32g NaOH; 2g metaThe c o n d i t i o n s of f o r m a t i o n were: k a o l i n ; 200 m l d i s t i l l e d w a t e r , t o which t h e d e s i r e d amounts o f s a l t were added. R e a c t i o n i n p o l y p r o p y l e n e b o t t l e s r o t a t e d a t 80°C f o r 6 d a y s .
Mdes MCI per litre of solution
Fig. 6 .
I s o t h e r m s f o r s a l t s a t 2 5 O ~i n pre-formed z e o l i t e X (20) a = KC1 a = LiCl 0 = NaCl A = CsCl A = CaC1,
Pressure ( mm Hg
Fig. 7 .
I s o t h e r m s f o r u p t a k e s i n z e o l i t e K-L a t 245OC of A V a p o u r i s e d N H 4 C 1 ( i . e . NH3 + H C 1 i n 1:l r a t i o ) B H C 1 gas a l o n e C NH3 a l o n e ( 2 2 ) .
Pressure cm Hc
Fig. 8.
Isotherms f o r uptake of H g curve represents the l i m i t s a t u r a t i o n vapour p r e s s u r e experimental temperature. p o i n t s ; @, A , I and are
+
i n Na-X ( 2 3 ) . The u p p e r t o t h e u p t a k e s e t by t h e of l i q u i d Eg a t each 0, A , 0 and o are a d s o r p t i o n desorption points.
Pressure cm Hg
Fig. 9.
S o r p t i o n of H g i n Ag-X ( 2 3 ) 0 ( a ) Uptakes a t two t e m p e r a t u r e s ( C) 0 (b) A s o r p t i o n - d e s o r p t i o n c y c l e a t 235.2 C 0 (c) Two s u c c e s s i v e s o r p t i o n - d e s o r p t i o n c y c l e s a t 270 C
F i g . 10.
A s o r p t i o n - d e s o r p t i o n c y c l e f o r u p t a k e of p-xylene i n z e o l i t e H-ZSM-5 (sio2/A1203 = 226) a t 70°c ( 2 4 ) .
NH4Cl was sorbed from i t s vapour i n t o z e o l i t e s a t h i r d i s o t h e r m contour was found, of type V , and r a t h e r s i m i l a r t o t h a t i n Fig. 4 . This i s i l l u s t r a t e d i n F i g . 7 ( 2 2 ) . When ammonium c h l o r i d e i s vapourised i t d i s s o c i a t e s v i r t u a l l y completely i n t o a 1:l mixture of HC1 + NH3. Therefore from i t s vapour i t i s t h i s mixture which is sorbed i n t o t h e z e o l i t e . I n Fig. 7 curve C i s the r e v e r s i b l e isotherm of NH3 alone i n z e o l i t e K-L a t 245OC; curve B i s t h i s isotherm f o r pure H C 1 ; and curve A shows what happens when t h e 1:l mixture of NH3 + H C 1 ( i . e . NH4C1 vapour) i s sorbed. There i s clearly a s t r o n g i n t e r a c t i o n between NH3 and H C 1 w i t h i n t h e z e o l i t e .
When metals a r e i n i t i a l l y a t o m i c a l l y d i s p e r s e d i n z e o l i t e s , f o r example by r e d u c t i o n s such a s
and the system is s u b j e c t e d t o f u r t h e r h e a t i n g , t h e metal atoms tend t o n u c l e a t e i n t o c l u s t e r s w i t h i n t h e z e o l i t e o r o u t s i d e i t as small c r y s t a l l i t e s . This tendency can be s t u d i e d f o r i n t r a c r y s t a l l i n e n u c l e a t i o n u s i n g mercury a s t h e g u e s t s p e c i e s . When the c o n c e n t r a t i o n of mercury atoms i s low n u c l e a t i o n does n o t occur and i n t r a z e o l i t e s o r p t i o n follows Henry's law, a s shown i n F i g . 8 + pb2+ a r e d e r i v e d from for uptake i n Na- and Pb-X ( 2 3 ) . ~ a and elements h i g h e r i n t h e e l e c t r o c h e m i c a l s e r i e s than Hg. I n c o n t r a s t with t h i s behaviour when s o r p t i o n occurred i n Hg- o r Ag- z e o l i t e s the isotherms became once more of type I V i n t h e Brunauer c l a s s i f i c a t i o n , a s seen i n Fig. 9 ( 2 3 ) . The i o n s Hg2+ o r Ag+ o r i g i n a l l y in the z e o l i t e a r e d e r i v e d from elements as low a s o r lower i n t h e electrochemical s e r i e s than mercury, s o t h a t r e d u c t i o n s can occur:
Such r e d u c t i o n s , which may r e p r e s e n t t h e f i r s t s t e p i n the isotherms of Fig. 9 , appear t o t r i g g e r o f f c l u s t e r i n g processes:
Ag + xHg
-t
+ xHg Hg2 2+
AgxHg
(2) -t
~
~
x+2
2
+
Isotherms w i t h contours l i k e those of P , N H 4 C l o r Hg a r e r a r e compared with those shown f o r CF4 i n Fig. 3. It i s thus of i n t e r e s t t h a t a type I V i s o t h e r m h a s been found f o r p-xylene i n ZSM-5 i n which a c l e a r s t e p , t h i s time w i t h some h y s t e r e s i s , occurs ( 2 4 ) . The explanation o f f e r e d was n o t i n terms of s t r o n g molecule-molecule interaction, but t h a t a t a c e r t a i n i n t r a c r y s t a l l i n e concentration
t h e y pack i n a new c o n f i g u r a t i o n , more economical of s p a c e ( F i g . 10).
5.
SELECTIVITY I N MIXTURE SEPARATION AND HEATS OF SORPTION
That m i x t u r e s c a n b e s e p a r a t e d , o f t e n q u a n t i t a t i v e l y and i n a s i n g l e s t e p , by m o l e c u l e s i e v i n g h a s b e e n v e r y f u l l y e s t a b l i s h e d . T h i s i s i l l u s t r a t e d i n T a b l e 4 i n which t h e c r y s t a l l i n e z e o l i t e was powdered n a t u r a l c h a b a z i t e (7,s). Ca-chabazite i s one of a c l a s s of z e o l i t e s a b l e t o s e p a r a t e n - p a r a f f i n s from i s o - neo- and c y c l o p a r a f f i n s and a r o m a t i c s . O t h e r s i n t h i s c l a s s a r e Ca-A, e r i o n i t e , z e o l i t e ZK-5 and t h e hydrogen form o f z e o l i t e RHO. S t r o n g s e l e c t i v i t i e s a r e a l s o p o s s i b l e when b o t h components of a b i n a r y m i x t u r e c a n e n t e r t h e z e o l i t e and e q u i l i b r a t e w i t h i t . To u n d e r s t a n d t h e s e s e l e c t i v i t i e s one may c o n s i d e r t h e components of t h e p h y s i c a l bond between g u e s t m o l e c u l e s and h o s t c r y s t a l s . These i n c l u d e D i s p e r s i o n e n e r g y , $D Close-range r e p u l s i o n e n e r g y , $R P o l a r i s a t i o n e n e r g y , +p Field-dipole energy, F i e l d gradient-quadrupole energy, $ FQ Guest-guest self-energy, $ SP ~ l e c t r i cmoments i n t h e g u e s t molecule may b e permanent, o r they may b e i n d u c e d by t h e l o c a l e l e c t r o s t a t i c f i e l d of s t r e n g t h F . Thus p o l a r i s a t i o n e n e r g y can have components o f f i e l d - i n d u c e d dipole o r dipole-induced dipole. The components bD, $R and $p are termed " n o n - s p e c i f i c " because t h e y a r e always i n v o l v e d i n t h e h o s t - g u e s t bond. w i t h t h e advent of high s i l i c a z e o l i t e s , hydrogen z e o l i t e s ~d porous c r y s t a l l i n e s i l i c a s l o c a l f i e l d s F and f i e l d g r a d i e n t s F can be much reduced s o t h a t @p can become s m a l l . The components I $ ~ + , and for m o l e c u l e s w i t h permanent d i p o l e moments p o r m o l e c u l a r quadrupole moments Q are termed " s p e c i f i c " components of t h e h o s t-gues t bond b e c a u s e t h e y do n o t a r i s e w i t h non-polar m o l e c u l e s l i k e t h e r a r e g a s e s . They c a n b e v e r y l a r g e i n aluminous z e o l i t e s and c a n l e a d t o u n u s u a l l y h i g h h e a t s of s o r p t i o n and s e l e c t i v i t i e s (e.g. H 2 0 o r NH3).
$iQ
The g u e s t - g u e s t i n t e r a c t i o n s g i v i n g 4Sp may i n extreme cases be c h e m i c a l a s w e l l a s p h y s i c a l i n n a t u r e , a s f o r P and f o r (NH3 + HC1) o r i n c l u s t e r i n g of Hg atoms ( 5 . 4 ) . U s u a l l y however o n l y p h y s i c a l i n t e r a c t i o n s a r e i n v o l v e d . These a r e u n i v e r s a l l y d i s p e r s i o n and c l o s e - r a n g e r e p u l s i o n , and f o r m o l e c u l e s w i t h permanent e l e c t r i c moments t h e r e may b e terms i n 4 such a s SP
Condit~onsand Comments - .- - - - As l i q u ~ dat -20 C Rapd and quantltntlVC As above As ahove
Componcnt(r 1 Sorbed
Mixture
-
.
CHaOH t (CH.),
-
I C.H OH i CHCI:
cb
As abovc
+
As above
odj
c,k,, C,H,OH
I H,O (C,H,):O C,H,OH I CH,COC,H,
I
i
I
N,O, HzS
a,
I
I CH NH
c,h.od'
As liquid at 112 C . Separation rapid and comnktc AS ii&irl at -20 <.. Slow but comotrte
I
A\ rbove. CO, on,. tially i n wlurion. As above. SO, in,tially in solution. As above. N.Oa In solution. As above. H,S in solution. AS liq.uid at -mc. I a p l d and quantitative. As above As above As above As liquid at 50'. Partial removal in a
HCI I CHCI,
CI, t C.H. Br, t CC'I,
I
l
As liquid at
I
-21Y'c.
Semnlion wmpkte within I 2 days. As above As liquid at I I T C . Separation nearly c o n l ~ hwithin 24
I
hi..
wak.
C,H.Br
Compktc r c ~ v aofl b o t h constltuentb within 16 hours.
C,H.Br -t CH(CH,KI,
As liquid at -20°C.
I
IOO'C. Sepration compkte within two days. As liquid at -20°C. Dirrolrcd HCI removed uickly and compkdy. As above As above. Equilibrium reparation nearly CompkIc within 16 haum. As i&id at -20 C. hrrllrnmoval w~thIn rout days. As liquid at -2WC. N u d y m p k t e In 12 days. As liquid at - W C . Raptd and complnc. A S liquid at 112-C. R a M and c-ompktc. As liquid at llZ,C. R r t ~ aaepntion l in t h e day3. As liquid u -20'C. Sep.nlionrmnpkte. As above
As rbove
so*
CH N H t C H O H N(LH.~.~
As liquid at
As above
CO,
'
II
As above 8
CH.0. H ,O
i
.
-20. Sbw but c o m p k r xpntion. As liquid at ll)ObC. Se n t M n only partiaEn five day.. As liquid at -ZO°C. Rapid and auanaitativc. As above As above As liquid at
Sorpllon vtr vapour In equilibrium with sotulaon Cbrnolrte %enaratlon. Ar I q u ~ dat -ZU C Soparatton blow but complele As above
d~ +
CH I C~(CH,),OH C.H OH C ~ )HOH C,H n -
C o n d i l ~ n sand Cmmcnts
-
I CH ..
C,H.Br* C,H,Br*
AS abour. Sspamtion campkc. As liquid at 97°C. +,mth ~ p * t e wtthtn two days.
which r e p r e s e n t d i p o l e - d i p o l e , dipole-quadropole terms. When t h e amount sorbed i s s m a l l , a s i n t h e Henry's law range, can be n e g l e c t e d .
mSp
mF,,
I t i s p o s s i b l e t o e s t i m a t e t h e s p e c i f i c components, and s e p a r a t e l y from the n o n - s p e c i f i c ones, $*, $R and 4p. 4~ and $p a r e f u n c t i o n s of t h e p o l a r i s a b i l i t y , a , and t h e r e f o r e f o r a s e r i e s of non-polar g u e s t molecules one may p l o t t h e i n i t i a l value (when $sp = 0) of t h e d i f f e r e n t i a l molar h e a t of s o r p t i o n , AE, a g a i n s t t h e p o l a r i s a b i l i t y . The curves obtained a r e i l l u s t r a t e d i n F i g . 11 ( 2 5 ) . Within t h e range of p o l a r i s a b i l i t i e s covered and f o r simple non-polar s p e c i e s they c a l i b r a t e the h o s t c r y s t a l . When the i n i t i a l h e a t f o r a molecule with a permanent e l e c t r i c moment i s p l o t t e d on t h e same diagram the d i f f e r e n c e between t h e p o i n t on t h e r e f e r e n c e curve corresponding with t h e p o l a r i s a b i li t y of the p o l a r molecule and the a c t u a l h e a t measures the c o n t r i b u t i o n of $F,, and/or $$Q t o t h i s h e a t . Such a s e p a r a t i o n of s p e c i f i c and non-specific components i s i l l u s t r a t e d i n Table 5 ( 2 5 ) . The specifi components f o r N 2 , N20, C02, NH3 and H20 a r e always s i g n i f i c a n t and f o r NH3 and Hz0 a r e dominant. The a n a l y s i s makes i t c l e a r why z e o l i t e s a r e e x c e l l e n t d e s i c c a n t s of i n d u s t r i a l gases and l i q u i d s ; and why N 2 (which has a molecular quadrupole moment) i s s e l e c t i v e l y sorbed compared w i t h O2 (which has an almost z e r o moment).
$kq,
To a f i r s t approximation t h e d i s p e r s i o n energy can be cons i d e r e d a d d i t i v e f o r each p a i r of i n t e r a c t i n g atoms, one being i n t h e g u e s t molecule and one i n t h e h o s t z e o l i t e . This means t h a t f o r n - p a r a f f i n s i n z e o l i t e s such a s A, L , o r f a u j a s i t e -AK can inc r e a s e c o n t i n u a l l y with carbon number. I n (Ca,Na)-A a t 50°c the heal i s a l r e a d y about 2 4 k c a l mol-l f o r n-C12H26 ( 2 6 ) . Heats of physical s o r p t i o n can indeed exceed t h e h e a t s of many chemical r e a c t i o n s . Z e o l i t e s o r b e n t s a r e u s u a l l y e n e r g e t i c a l l y heterogeneous. This i s shown by a decrease i n -AH a s t h e amount sorbed i n c r e a s e s , and i s exemplified f o r CH4 i n z e o l i t e H-L i n F i g . 12 ( 2 7 ) . When p o l a r molecules a r e s o r b e d (NH3, H20) t h e decrease i n -AH with uptake i s sometimes very s h a r p , u s u a l l y more s o than f o r non-polar guest molecules. However guest-guest i n t e r a c t i o n s r e s u l t i n g i n the component $sp can o f f s e t t h e decrease i n -AH a s the amount sorbed i n c r e a s e s . This e f f e c t can be observed i n F i g . 12. I t becomes more important the more condensable t h e p a r a f f i n , s o t h a t f o r n-CbH1 0 f o r example i n s t e a d of d e c l i n i n g with uptake i n c r e a s i n g -AH = qst a c t u a l l y i n c r e a s e s . when s a t u r a t i o n of t h e i n t r a c r y s t a l l i n e s o r p t i o n volume i s approached -AE = qst can through the c o n t r i b u t i o n of $sp pass through a maximum and then, a s s o r p t i o n on e x t e r n a l s u r f a c e s culminating i n c a p i l l a r y condensation be tween c r y s t a l l i t e s becomes the dominating p r o c e s s , -AH d e c l i n e s towards the h e a t of condensation.
a x lo2' cm' per molecule
F i g . 11.
I n i t i a l v a l u e s of -AH p l o t t e d a g a i n s t p o l a r i s a b i l i t y of s o r b a t e f o r s m a l l r e l a t i v e l y symmetrical molecules i n s e v e r a l sorbents (25)
.
1
0
I
I
I
1 20
1
I
1
1 40
1
Volume sorbed /cm3 (s.1.p.) g - ' o f outgassed material-
Fig. 1 2 .
P l o t s of qst = -AE a g a i n s t uptake f o r CX4 (bottom) t o n-C4HI0 ( t o p ) . I n s e t : p l o t s of i n i t i a l v a l u e of qSt a g a i n s t carbon number f o r K-L ( t o p ) and H-L (bottom) F i g u r e s i n b r a c k e t s are h e a t s i n kJ mol-l ( 2 7 ) .
.
Table 5:
D i v i s i o n of components of i n i t i a l h e a t s , of g u e s t s i n s e v e r a l z e o l i t e s ( 2 5 ) .
Zeolite
Outgassed a t OC
AH
( c a l mol''),
-AH
Guest Total
Dispersion + Repulsion + Polarisation
Dipole + Quadrupole
Chabazi t e
Fau j as i te (Na-X)
350
6,500 12,200 NH3 1 8 , 0 0 0 H z 0 $34,000 N2
co
Fau j a s i te (Na-Y)
(a) Assuming I mordenite.
$ ~
35 0 + $R
(b) Assuming $D +
6.
C0
8,200
+ $p does n o t d i f f e r between H- and Na+ $p does n o t d i f f e r between Na-X
and Na-Y.
THERMODYNAMIC CONSIDERATIONS
A s w i t h any d i s t r i b u t i o n e q u i l i b r i u m thermodynamic a n a l y s i s can be v e r y i n f o r m a t i v e , and i n d e e d h a s been a n t i c i p a t e d i n 55 i n d i s c u s s i n g t h e d i f f e r e n t i a l h e a t of s o r p t i o n . Heat and E n t r o p y of S o r p t i o n An e q u i l i b r i u m c o n d i t i o n f o r t h e d i s t r i b u t i o n of a m o l e c u l a r s p e c i e s between t h e gas phase i n which i t s c h e m i c a l p o t e n t i a l i s and t h e z e o l i t e i n which t h i s p o t e n t i a l i s us i s l-lg
where
ASi s
t h e d i f f e r e n t i a l entropy of s o r p t i o n p e r mole and H and denote d i f f e r e n t i a l e n t h a l p y and e n t r o p y r e s p e c t i v e l y . For a pure molecular s p e c i e s = Hg and Fg = Sg where Hg and S a r e enthalpy g and energy p e r mole.
%
AH may be determined c a l o r i m e t r i c a l l y , o r from t h e ClapeyronClausius e q u a t i o n
The term on t h e 1.h.s. i s the s l o p e of a p l o t of Rnp a g a i n s t T f o r a c o n s t a n t uptake, n s , of g u e s t s p e c i e s i n a f i x e d weight of zeolite,
6.2.
Equilibrium
The dimensionless e q u i l i b r i u m c o n s t a n t , K , f o r p a r t i t i o n of the g u e s t between the z e o l i t e and the e x t e r n a l phase i s
where a denotes a c t i v i t y , C i s c o n c e n t r a t i o n and y i s the a c t i v i t y c o e f f i c i e n t . For a p e r f e c t gas and i n t h e Henry's law d i l u t e range of uptake where each y + 1 eqn. 5 becomes
Thus i n t h i s d i l u t e range of uptake eqn. 5a s e r v e s t o give t h e dimensionless thermodynamic e q u i l i b r i u m c o n s t a n t . The s t a n d a r d energy and e n t r o p y of s o r p t i o n a r e then
In
experimental s t u d i e s the Henry's law l i m i t t o t h e isotherm i s o f t e n e x p r e s s e d as
Since f o r a p e r f e c t gas p
=
C RT one has f o r t h i s case g
If s o r p t i o n occurs a t c o n s t a n t p r e s s u r e , p , and the change i n volume per mole sorbed i s AV the e n t h a l p y and energy of s o r p t i o n a r e r e l a t e d by AH = AE + pAV. However, pAV 2, -RT and s o AH = AE-RT. I t follows from t h i s r e s u l t and eqn. 9 t h a t
Plots of i n a g a i n s t ( T / K ) - ~ a r e shown f o r some gases i n H-chabazite i n Fig. 13 ( 8 ) . From t h e s l o p e s t h e s t a n d a r d h e a t s AH@ may be found, and a r e i l l u s t r a t e d i n Table 6 . AH' i s i n t h e o r d e r of t h e p o l a r i s a b i l i t i e s of molecules with n e g l i g i b l e e l e c t r i c moments b u t i s augmented f o r N 2 and C02 by t h e quadrupole monents of t h e s e two molecules.
"n
6.3.
E n t r o ~ vof Sorbed Molecules
From = AH/T and AT = T,-s one may, when t h e phase e x t e r n a l t o the z e o l i t e i s a p e r f e c t g a s , o i t a i n the d i f f e r e n t i a l entropy per mole of sorbed g u e s t f o r each amount sorbed. This e n t r o p y i s
a
1 atm) The entropy Sg of many gases a t the s t a n d a r d p r e s s u r e may be i n t e r p o l a t e d from t a b l e s f o r the experiment a t temperature T. Ss i s f i n i t e and has t h e following c h a r a c t e r i s t i c s : 1. I n e n e r g e t i c a l l y n e a r l y uniform s o r b e n t s Ss d e c r e a s e s continually w i t h i n c r e a s i n g i n t r a c r y s t a l l i n e up t a k e s . However a s the r e l a t i v e vapour p r e s s u r e approaches u n i t y and s o r p t i o n upon Table 6:
Gas
Standard h e a t s of s o r p t i o n i n k3 mol-l i n H-chabazi t e (28)
Temperature i n t e r v a l (T/K)
.
-AH'
Gas
f o r some gases
Temperature i n t e r v a l (T/K)
-AH
0
F i g . 13.
Rn Kp p l o t t e d a g a i n s t r e c i p r o c a l a b s o l u t e temgerature f o r some gases i n H-chabazite ( 2 8 ) .
-,
20
-
-
-
-
0
40
80
Volume occluded cm3 (s.t.p)n-1unit cells
F i g . 14.
D i f f e r e n t i a l e n t r o p y p e r mole f o r s o r p t i o n of N 2 i n K-f a u j a s i t e (K-X) a t s e v e r a l a b s o l u t e temperatures ( 2 9 ) .
external s u r f a c e s t a k e s over from i n t r a c r y s t a l l i n e s o r p t i o n rises again.
Ss
2. I n e n e r g e t i c a l l y non-uniform s o r b e n t s ( t r u e of most z e o l i t e s ) Ss may i n i t i a l l y d e c l i n e only a l i t t l e as uptake i n c r e a s e s , o r may i n i t i a l l y i n c r e a s e w i t h uptake. I t t h e n passes through a maximum and f i n a l l y d e c l i n e s a g a i n , as i n c a s e 1 above.
3. A t d i f f e r e n t temperatures the curves a g a i n s t uptake a l l tend t o run p a r a l l e l with each o t h e r . Ss has a w e l l defined p o s i t i v e temperature c o e f f i c i e n t ( F i g . 14 ( 2 9 ) )
.
-
-
Ss may be considered i n terms of a thermal p a r t , STh, and a configurational p a r t , S,. For the i d e a l Langmuir i s o t h e r m (K = O/p(l 0 ) ) t h e thermal p a r t should be independent of t h e degree of f i l l i n g , 0 , of i n t r a c r y s t a l l i n e pore space, while t h e
-
c o n f i g u r a t i o n a l p a r t i s given by
Thus a s O + 0,-S, -t + a and as 0 -t 1, S, -t - a. A t O = 0.5 2, = 0 and thus = ST^. However, t h e Langmuir isotherm i s an i d e a l i s ation from which r e a l systems d i f f e r t o a g r e a t e r o r l e s s e r degree. 6.4.
Heat Capacity of ~ n t r a c r y s t a l l i n eGuest Molecules
The d i f f e r e n t i a l h e a t c a p a c i t y p e r mole of sorbed g u e s t , Cs, i s , f o r a given uptake r e l a t e d t o Fs by
Over a f i n i t e temperature i n t e r v a l , ST, i n which eqn 13 may be r e p l a c e d by
changes by 6%
where the s u b s c r i p t m denotes t h e mean value over the i n t e r v a l BT. When the g u e s t molecules were t h e r a r e gases the values of are exemplified i n Table 7. The value a n t i c i p a t e d f o r an i d e a l E i n s t e i n o s c i l l a t o r i s about 25 JK-I mol-l . For hydrocarbons i n Na-X Fm increases with carbon number, b u t was always l e s s than t h e v a l u e f o r corresponding bulk l i q u i d (30)
.
Table 7:
-c,(JK-~
Zeolite
mol-I.) f o r K r and Xe i n some z e o l i t e s
T /K
m
Uptake (cm3 a t s . t . p .
g-l)
'rn Kr
Xe
N a-Y Ca-X Ca-A
H-mordeni t e Na-rnordeni t e Chabazi t e H-offretite H-erioni te H-L
(a) molecules p e r u n i t c e l l
7.
ISOTHERM FORMULATION
A f u l l t r e a t m e n t of t h e isotherm e q u a t i o n would need t o take account of t h e following experimental f e a t u r e s .
1. A t a given r e l a t i v e p r e s s u r e apparent s a t u r a t i o n capacities f o r a given guest s p e c i e s decrease as the temperature r i s e s . 2 . A t c o n s t a n t s o r p t i o n p o t e n t i a l t h e thermal expansion coe f f i c i e n t s of g u e s t s p e c i e s w i t h i n z e o l i t e s a r e n o t very d i f f e r e n t from those of corresponding l i q u i d s (31).
3 . Fewer l a r g e molecules than s m a l l ones s a t u r a t e the i n t r a c y s t a l l i n e pore space (Table 3 ) . Thus c l a s s i c a l s i t e models with one molecule p e r s i t e a r e inadequate i n t h a t t h e s o c a l l e d s i t e and t h e number of s i t e s would need t o be d i f f e r e n t according t o the molecular volume of each g u e s t . 4 . I n i n t r a c r y s t a l l i n e pores and channels t h e molecules of a given g u e s t s p e c i e s a r e not a l l bound with t h e same energy. The b i n d i n g energy v a r i e s with p o s i t i o n of a molecule r e l a t i v e t o t h e w a l l s of t h e p o r e s and channels and t o t h e c a t i o n s p r e s e n t . 5. Molecule-molecule i n t e r a c t i o n i n the c l u s t e r s o r filaments of g u e s t molecules w i t h i n t h e z e o l i t e cannot normally be ignored (5. 5 ) . The f i r s t two of the above p r o p e r t i e s , and t h e m o b i l i t y of
sorbed molecules, suggest l i q u i d - l i k e p r o p e r t i e s of c l u s t e r s and filaments of g u e s t s p e c i e s i n t h e z e o l i t e . One may then assume that there i s a mean h y d r o s t a t i c s t r e s s i n t e n s i t y , P, i n t h i s f l u i d which i s r e l a t e d t o Cs by a v i r i a l equation:
If the gas phase behaves i d e a 3 l y ( p = CgRT) then a thermodynamic argument g i v e s t h e v i r i a l i s o t h e r m e q u a t i o n a s
o r the corresponding e x p r e s s i o n f o r K = K RT (eqn. 9 ) i n which Cg i s replaced by p. The A i ( i = 1, 2, ) correspond w i t h v i r i a l c o e f f i c i e n t s i n the v i r i a l e q u a t i o n of bulk g u e s t s p e c i e s i n s e n s e , b u t because of the r e s t r i c t e d environment n o t i n numerical v a l u e s . The c o e f f i c i e n t s A; w i l l t h e r e f o r e take c a r e of molecule-molecule i n t e r a c t i o n s . The A; may be f u n c t i o n s of temperature b u t n o t of Cs. The e x p o n e n t i a l term i s t h e a c t i v i t y c o e f f i c i e n t , ys, of the sorbed molecules. A s Cs approaches i t s s a t u r a t i o n value t h e number of c o e f f i c i e n t s A; needed i n eqns. 14 and 15 i n c r e a s e s . However Fig. 15 (28) shows as an example t h a t w i t h not more than t h r e e c o e f f i c i e n t s A; isotherms of some gases i n H-chabazite a r e w e l l represented. When C s becomes small t h e e x p o n e n t i a l term i n eqn. 15 declines t o u n i t y s o t h a t Henry's law i s then o b t a i n e d . Thus t h e isotherm e q u a t i o n lends i t s e l f t o thermodynamic a n a l y s i s . I t s g r e a t g e n e r a l i t y does n o t however g i v e much i n s i g h t about e v e n t s a t molecular leve 1.
5 . ..
The second approach which w i l l be considered i s a s i t e model i n which each c a v i t y i s regarded a s a s i t e - c a p a b l e of accommodating a c l u s t e r of up t o m g u e s t molecules. The i s o t h e r m h a s been developed using d e t a i l e d b a l a n c i n g (32) and by s t a t i s t i c a l mechanics (33) with t h e expected e q u i v a l e n t r e s u l t s . I n terms of d e t a i l e d b a l a n c i n g the isotherm e q u a t i o n i s w r i t t e n a s
In eqn.
16 r+ c
1
F i g . 15.
I s o t h e r m s f o r g a s e s i n H-chabazite a t t h e a b s o l u t e temperatures indicated (28). 0 = experimental points = calculated using the v i r i a l isotherm equation.
+ ki i s t h e r a t e c o n s t a n t f o r c o n d p s a t i o n i n t o t h e c a v i t y ( o r s i t e ) i s the r a t e constant f o r already h o l d i n g i molecules and kCi+l evaporation of a molecule from a c a v i y c a r r y i n g ( i + 1 ) g u e s t molecules. K i + l i s t h e e q u i l i b r i u m c o n s t a n t betwe$n i and ( i + 1) molecules pFr c a v i t y . I n t h e i d e a l c a s e when a l l ki a r e e q u a l and so a r e a l l k one h a s (;+I)
L
and eqn. 18 r e d u c e s t o Langmuir's e q u a t i o n
Langmuir's e q u a t i o n i s a l s o recovered from eqn. 16 when rn = 1, s o that i t becomes i n e i t h e r of t h e s e two ways an i m p o r t a n t l i m i t i n g case. The v a l i d i t y of eqn. 18 would mean n e g l i g i b l e moleculemolecule i n t e r a c t i o n w i t h i n t h e z e o l i t e ; c o n v e r s e l y s u c h i n t e r actions w i l l r e s u l t i n v a l u e s of R; which change w i t h i . Reference t o Table 3 s u g g e s t s v a l u e s f o r m f o r a number of g u e s t m o l e c u l e s m w i l l be t h e n e a r e s t i n c h a b a z i t e and f a u j a s i t e ( z e o l i t e X) whole number f o r t h e c l u s t e r s i z e .
.
I n measurements of i s o t h e r m s a t 6 7 3 K f o r C l O , C12, C 1 4 , C 1 6 , and C1 n-paraf f i n s i n (Mg, Na)-A and (Ca, N a ) - A t h e v a l u e of m was near t o u n i t y f o r e a c h hydrocarbon, i n d e p e n d e n t l y o f carbon number ( 3 4 ) . This s u g g e s t s t h a t e a c h c h a i n i s c o i l e d s o t h a t , e x c e p t i n the a c t of d i f f u s i n g , e a c h molecule i s c o n f i n e d t o one c a v i t y and there i s no room f o r more t h a n one c o i l p e r 26-hedral c a v i t y ( 3 5 ) . Here, where m = 1, i t i s of i n t e r e s t t h a t t h e s t r o n g l y curved i s o therms obeyed Langmuir ' s i s o t h e r m r e a s o n a b l y we 11. The semii s o l a t i o n of one c o i l i n e a c h c a g e would r e d u c e molecule-molecule i n t e r a c t i o n , which i n t u r n would f a v o u r t h e Langmuir i s o t h e r m . When, i n t h e above s t u d y , t h e v a l u e of K g i v e n by e q n . 19 was p l o t t e d a g a i n s t c a r b o n number K a t f i r s t i n c r e a s e d b u t t h e n p a s s e d through a maximum. A r e a s o n f o r t h i s c o u l d be s e e n from
0
0
0
AH and AS a r e e a c h n e g a t i v e , s o that-TAS' i s p o s i t i v e and AH 0 The v a l u e s of -AH and+Cb~: oppose e a c h o t h e r i n d e t e r m i n i n g K . and -AS b o t h i n c r e a s e w i t h carbon number, b u t t h e c o i l e d n - p a r a f f i n s of h i g h e s t c a r b o n nu b e r f i t more t i g h t l y w i t h i n t h e c a v i t y and s o i n c r e m e n t s i n -TAS as c a r b o n number i n c r e a s e s f i n a l l y outweigh i n c r e m e n t s i n AH^ i n i n f l u e n c i n g AGO o r K . The more r e s t r i c t e d t h e c o i l e d hydrocarbon i s w i t h i n i t s c a v i t y t h e g r e a t e r t h e l o s s i n e n t r o p y when s o r p t i o n o c c u r s .
8
t
-
--
203 7
-
215 4
-- -* P
-2
I
I
/I
313 2
237 4
I
- - - --
293 2
-2
' 333 2
I
I
I
1
1
O
( d l n-C,H,
-2
1
0
1
04
I
I
08
I
0
I
0 4
1
1
08
8
F i g . 16.
Log CO/p(l - 0 ) l p l o t t e d a g a i n s t O f o r some hydrocarbons i n H-chabazite, a t v a r i o u s absolute t e m p e r a t u r e s ( 3 6 ) .
A s e n s i t i v e way t o check how n e a r l y a c t u a l i s o t h e r m s approach the i d e a l c a s e i s t o p l o t 0 / ~ ( 1- O ) , o r i t s l o g a r i t h m o r r e c i p r o c a l a g a i n s t O. Langmuir's i s o t h e r m g i v e s a s t r a i g h t l i n e p a r a l l e l t o t h e a x i s of O . Examples of s u c h p l o t s a r e g i v e n i n Fig. 16 (36) f o r C1, C2, C g and n-C4 p a r a f f i n s i n H-chabazite a t each of a s e r i e s of t e m p e r a t u r e s . The b e h a v i o u r i s o f t e n c h a r a c t e r istic i n that 1. 2. 3.
a t low t e m p e r a t u r e s n e g a t i v e s l o p e s a r e o b t a i n e d ; a s t h e t e m p e r a t u r e r i s e s t h e s l o p e s become l e s s n e g a t i v e ; and a t s t i l l h i g h e r t e m p e r a t u r e s h o r i z o n t a l r e g i o n s may be obtained, or regions with p o s i t i v e slopes.
The s l o p e s of t h e l i n e s w i l l be t h e n e t t r e s u l t of t h e i n t e r play of t h e u s u a l e n e r g e t i c h e t e r o g e n e i t y (which on i t s own would make -AH d e c r e a s e w i t h i n c r e a s i n g O and would t h u s r e s u l t i n negative s l o p e s ) ; of molecule-molecule i n t e r a c t i o n (which i s u s u a l l y exothermic and i f s o t e n d s t o g i v e p o s i t i v e s l o p e s ) ; and of v a l u e s o f R ( i + l ) which a s a r e s u l t o f e n e r g e t i c h e t e r o g e n e i t y and moleculemolecule i n t e r a c t i o n would v a r y w i t h i ( s o t h a t t h e u n a b r i d g e d eqn. 1 8 would b e r e q u i r e d r a t h e r t h a n t h e Langmuir l i m i t i n g c a s e ) . Values of m i n c h a b a z i t e would b e a b o u t 6 f o r CH4, 4 f o r C2H6, 3 for C2H8 and 2 f o r n-C4H1 0 .
3.
CONCLUDING REMARK
It i s hoped t h a t t h e n e c e s s a r i l y l i m i t e d examples g i v e n have shown t h e importance of combining a c c u r a t e thermodynamic d a t a w i t h modelling f o r i s o t h e r m s and e n e r g e t i c s i n i n t e r p r e t i n g t h e p h y s i c a l bond i n h o s t - g u e s t complexes, s e l e c t i v i t y i n s o r p t i o n and t h e c h a r a c t e r of t h e i n t r a c r y s t a l l i n e f l u i d . C o n s i d e r a b l e p r o g r e s s h a s been made i n t h i s i m p o r t a n t a r e a b u t much remains t o be done.
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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.
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of
No. 102, 1971) p . 9 7 . 34. F i e d l e r , K . , A . Roethe, K.P. Roethe and D. G e l b i n , Z . Phys. Chem. L e i p z i g , 257, (1978) 979. 35. B a r r e r , R.M., J. Chem. Techn. B i o t e c h n o l . , 31, (1981) 71. 36. Ref. 2 , p . 117.
SORPTION BY ZEOLITES KINETICS AND DIFFUSIVITIES
PART 11.
3.M. Barrer
Chemistry Department Imperial College of Science and Technology London SW7 2AY England
An objective in the study of sorption-desorption kinetics is the accurate determination of differential intrinsic and selfdiffusivities and their concentration dependance. Attention has been drawn to experimental problems in such measurements, to situations where intracrystalline diffusion is not rate-controlling and to ways in which these alternative controls may be recognized and minimised. Some properties of self- and intrinsic diffusivities h a v e been reviewed, including their dependance on molecular dimensions and shape in relation to window apertures and the types of cation present in the zeolite. 1.
INTRODUCTION
Sorption rates in beds of zeolite crystals play an important part in separations of mixtures dependent on partial and total molecule sieving. These rates are equally significant in catalysis where reactants must reach intracrystalline catalytic centres against a counter flow.of resultants out of the crystals. Accordingly increasing study is being devoted to this area. The present account will be limited to rates of sorption and desorption of single species. 2.
SORPTION K I N E T I C S If the rates of uptake are sufficiently slow, giving long
h a l f - l i f e times, t h e s e r a t e s can reasonably be a s c r i b e d t o i n t r a c r y s t a l l i n e d i f f u s i v e flow. I h i l e most s t u d i e s have involved beds of z e o l i t e powders o r of bonded p e l l e t s c o n t a i n i n g z e o l i t e crystals, a l i m i t e d number have been made u s i n g one l a r g e c r y s t a l (1,2) o r a s m a l l number of p i e c e s of such a c r y s t a l ( 3 ) . I n t h e s e l a r g e p i e c e s t h e r e i s no d i f f i c u l t y i n studying i n t r a c r y s t a l l i n e d i f f u s i o n , b u t i n f i n e powders o f t e n i n t h e s i z e range %0.1~ t o % l o p o r i n bonded p e l l e t s c o n t a i n i n g t h e s e small c r y s t a l s c o m ~ l i c a t i o n s may a r i s e . P o s s i b l e r a t e - c o n t r o l l i n g s t e p s a r e :
1. 2. 3. 4.
5.
Intracrystalline diffusion. I n t e r c r y s t a l l i n e d i f f u s i o n and flow. Transmission through s u r f a c e s k i n s . Evolution of h e a t on s o r p t i o n and c o o l i n g on desorption with r e s u l t a n t time-dependent d r i f t s of s o r p t i o n equilibria. Combinations of two o r more of t h e above p o s s i b i l i t i e s .
Accordingly i n t e r p r e t a t i o n of s o r p t i o n k i n e t i c s needs c a r e .
2.1.
Evaluation of I n t r a c r y s t a l l i n e D i f f u s i v i t i e s
F o r long h a l f - l i f e times such t h a t i n t r a c r y s t a l l i n e d i f f u s i o n i s normally r a t e - c o n t r o 1l i n g t h e s o r p t i o n r a t e curves a r e available f o r i n t e r p r e t a t i o n i n terms of i n t r a c r y s t a l l i n e d i f f u s i v i t i e s . Those of i n t e r e s t a r e d i f f e r e n t i a l i n t r i n s i c and s e l f - d i f f u s i v i t i e s , D and D*, and, where t h e s e a r e f u n c t i o n s of c o n c e n t r a t i o n , the corresponding i n t e g r a l d i f f u s i v i t i e s D = ( 1 / ~ ) JC DdC and C 0 D* = ( 1 / ~ ) J ~*r',c.Dand 5* a r e r e l a t e d approximately by the Darken r e l a t i & 6 = E* ddlna/d!Ln~( s e e 8 - 5 ) . I n t h e s e r e l a t i o n s C i s t h e c o n c e n t r a t i o n and a t h e a c t i v i t y of sorbed g u e s t .
I n the s i m p l e s t s i t u a t i o n when 5 i s c o n s t a n t 5 = D and t h e d i f f u s i o n e q u a t i o n i s , f o r c o n c e n t r a t i o n C and time t ,
aC/at
=
D d i v grad C
(1)
I f s o r p t i o n occurs i n t o a powder of i s o t r o p i c c r y s t a l s approximated a s s p h e r e s a l l of r a d i u s ro, and i f t h e boundary c o n d i t i o n s a r e C
= Cm
at r
=
ro f o r t > 0
C = C o f o r O < r < r a t t = 0
the solution i s
i i O I! J
or a l t e r n a t i v e l y 00
L M Q,
ro '
ierfc
-
(4)
n=l
o
In t h e s e e x p r e s s i o n s Cm i s t h e e q u i l i b r i u m c o n c e n t r a t i o n o f g u e s t w i t h i n t h e c r y s t a l s and Co i s t h e i n i t i a l c o n c e n t r a t i o n . ", Q, and Q, a r e amounts o f g u e s t i n t h e c r y s t a l s a t t = t , w and 0 r e s p e c t i v e l y , w h i l e i e r f c x d e n o t e s ~ ( l / f i ) e x ~ - x ~ - x ( l - e rxf ) l . The s e r i e s i n eqn. 3 c o n v e r g e s o n l y s l o w l y f o r s m a l l t ; t h a t i n eqn. 4 c o n v e r g e s o n l y s l o w l y f o r l a r g e t . For s m a l l t eqn. 4 g i v e s I 2
Mm
r
(5) 0
6
I
so t h a t It./M_ i s p r o p o r t i o n a l t o t \ w i t h s l o p e then o b t a r n a b l e . For l a r g e t eqn. 3 reduces t o
o
( )
D is
so t h a t a p l o t o f xn(l-Mt/&) a g a i n s t t approaches a s t r a i g h t l i n e of s l o p e - DT'- and t h i s s e r v e s a l s o t o g i v e D. -"
r2
0
The boundary c o n d i t i o n s i n eqn. 2 r e q u i r e t h a t u p t a k e s h o u l d occur a t c o n s t a n t p r e s s u r e of g u e s t i n t h e g a s p h a s e j u s t o u t s i d e t h e s u r f a c e s r = ro o f t h e c r y s t a l s . A l t e r n a t i v e l y s o r p t i o n c a n occur a t c o n s t a n t volume and v a r i a b l e p r e s s u r e . A t t = 0 a d o s e of gas e n t e r s t h e s o r p t i o n volume and i s t a k e n up by t h e h o s t . provided Henry's l a w g o v e r n s t h e e q u i l i b r i u m i s o t h e r m s ( C = k p ) , new e q u a t i o n s c o r r e s p o n d i n g w i t h 3 and 4 d e s c r i b e t h e u p t z k e . F o r l a r g e t t h a t c o r r e s p o n d i n g w i t h eqn. 3 g i v e s
6K(K
+
1) (7)
9 ( K + 1) + a: K~
where K = amount o f g u e s t i n t h e g a s phase/amount i n t h e c r y s t a l s , a t e q u i l i b r i u m , and a1 i s t h e f i r s t r o o t of
A p l o t o f Rn
Q m - 0 aL g a i n s t t a g a i n s e r v e s t o f i n d D.
(urn - so)
For
nin
f
F i g . 1.
"'
S o r p t i o n k i n e t i c s of w a t e r a t c o n s t a n t p r e s s u r e i n t o c r y s t a l s of c h a b a z i t e , g m e l i n i t e and h e u l a n d i t e ( 3 ) . The dashed l i n e s a r e c a l c u l a t e d c u r v e s . The tempera t u r e s of t h e r u n s i n O C are: c h a b a z i t e , @, 75.4; 0 , 3 0 . 8 ; g m e l i n i t e , 0, 6 2 . 5 ; 0 , 31.7; h e u l a n d i t e , 0. 7 7 . 8 ; 0, 3 7 . 4 .
=r t
Fig. 2.
P l o t of 1
0
( ~ ~ / ~ , ) adg ta i n s t t f o r n-C6Hlr, a t 3 3 . 4 ' ~
i n z e o l i t e H-RHO
(4)
.
small t t h e e q u i v a l e n t of eqn. 4 g i v e s
and a p l o t of M ~ / M , a g a i n s t
& again s e r v e s t o g i v e D.
Examples of r a t e curves f o r t h e uptake of H 2 0 by s i z e a b l e pieces of s i n g l e c r y s t a l s of c h a b a z i t e , g m e l i n i t e and h e u l a n d i t e a t each of two temperatures a r e shown i n Fig. 1 ( 3 ) f o r t h e c o n s t a n t pressure boundary c o n d i t i o n s of eqn. 2 . \ h e n t i s small they show the v a l i d i t y of t h e fi eqn. D i f f u s i o n c o e f f i c i e . n t s can be determined i n o t h e r ways. One such involves p l o t t i n g (Mt/M_)dt a g a i n s t t a s shown f o r n-CsH14 a t 3 3 . 4 ' ~ i n z e o P i t e RHO i n F i g . 2 ( 4 ) . This p l o t approaches an asymptote of u n i t s l o p e which makes a n i n t e r c e p t La on the a x i s of t . For s e v e r a l geometries and f o r c o n s t a n t p r e s s u r e , or f o r c o n s t a n t volume combined w i t h ~ e n r y ' slaw, t h e r e s u l t s are (5) :
st
La Constant p r e s s u r e
Constant volume, Henry's law
Spheres, r a d i u s ro
r$/l5~
~;K/~SD(K+~)
Long c y l i n d e r s , r a d i u s r,
rg/8~
r 02 ~ / (8~ +~1 )
Sheet, t h i c k n e s s 22
t2/ 3~
R ~ K /(~~ D +1)
These r e s u l t s allow ready e v a l u a t i o n s of D f o r appropri-ate geome t r i e s and c o n s t a n t d i f f u s i v i t i e s .
D i f f e r e n t i a l s e l f - d i f f u s i v i t i e s can be measured through exchange d i f f u s i o n between t h e z e o l i t e having a given l o a d i n g of sorbate and t h e gas phase of t h e s o r b a t e , E i t h e r t h e z e o l i t e o r the gas phase c o n t a i n s l a b e l l e d molecules f o r time t > 0. L a b e l l i n g may f o r example be w i t h radio-carbon, o r w i t h deuterium. The boundary c o n d i t i o n s of 5. 2.1 a r e convenient and with them t h e mathematical r e l a t i o n s a l r e a d y given a r e a p p l i c a b l e . Another e f f e c t i v e way of measuring s e l f - d i f f u s i o n i s by pulsed f i e l d g r a d i e n t NMR. For a bed of z e o l i t e powder an o v e r - a l l o r e f f e c t i v e d i f f u s i v i t y i s given by
I n t h i s e x p r e s s i o n A i s the time i n t e r v a l between p u l s e s of width <w2) i s t h e mean square displacement of a o and amplitude g. molecule over the i n t e r v a l A ; T i s t h e l i f e t i m e of guest molecules w i t h i n t h e c r y s t a l s ( a l l of r a d i u s r o ) . y i s t h e gyromagnetic r a t i o , p i s t h e f r a c t i o n of the molecules i n t h e bed which a r e i n t h e gas phase and D" i s t h e d i f f e r e n t i a l s e l f - d i f f u s i v i t y of the g u e s t i n t h i s gas p a s e . There a r e two extremes i n t h e above expression:
a
: I
1. 2.
-r >> A and so (w2) r << A and so (w2)
ro. >> r
I n t h i s l i m i t-D e f f Here D e f f IpD8.
+
Dk.
These two extremes can be i l l u s t r a t e d by r e s u l t s f o r cyclohexane i n Na-X, when Rn Deff i s p l o t t e d a g a i n s t K/T ( F i g . 3 ( 6 ) ) . Below -65OC D e f f -+B* while above +lo0 D e f f + P6g, the temperature c o e f f i c i e n t b e i n g p r i m a r i l y t h a t of p . The r a t h e r f l a t intermediate region i s one where t r a n s l a t i o n a l d i f f u s i o n i s r e s t r i c t e d mainly t o t h e i n t r a c r y s t a l l i n e pore space of Na-X because the thermal energy i s i n s u f f i c i e n t f o r most molecules t o evaporate from the c r y s t a l s . Besides g i v i n g and pE* the NMR method, from p l o t s of D e f f against K / T , allows one t o f i n d b e temperature range where gas phase and i n t r a c r y s t a l l i n e d i f f u s i o n a r e both i n f l u e n c i n g D e f f , a s seen i n Fig. 3 .
E*
2.3.
Determining the Enhrrrv of A c t i v a t i o n f o r iffu us ion
I n t r a c r y s t a l l i n e d i f f u s i o n involves m i g r a t i o n of molecules from one s i t e t o a n o t h e r o f t e n with a squeeze p a s t an o b s t r u c t i o n such as a narrow window. Thus u n i t d i f f u s i o n s t e p s r e q u i r e an a c t i v a t i o n energy, E , and D can be expressed i n terms of t h e Arrhenius r e l a t i o n
I n p l o t s of M ~ / Ma, g a i n s t t a t two temperatures T1 and T2 one measures t h e times tl and t 2 f o r M ~ / Mt o~ reach a chosen value. Then, from eqn. 3 f o r example, one must have f o r c o n s t a n t pressure
Accordingly, from eqns. 11 and 1 2 i-
1
Fig.
Fig. 4.
for cyclohexane i n Na-X (10) w i t h 'L 2-molecules p e r l a r g e c a v i t y ( D ) , and comparison w i t h D* (dashed l i n e ) and w i t h corresponding v a l u e s from s o r p t i o n r a t e s using t h e method of moments ( 9 ) .
Deff
Sorption-desorption k i n e t i c s of n-hexane i n z e o l i t e ( 4 ) . Qt i s the amount sorbed o r desorbed a t time t and &D i s the f i n a l amount sorbed o r desorbed. Temperatures of runs i n OC a r e : H-RHO
Sorption
0, 33.4
x, 95.6 A , 141.4
Desorption
0 , 33.4
m,
95.6
m , 14.4
s o t h a t E i s o b t a i n e d . T h i s method i s a p p l i c a b l e f o r any d i s t r i b u t i o n o f c r y s t a l s h a p e s and s i z e s because i t i n v o l v e s o n l y eqn. 10 and e x p e r i m e n t a l c u r v e s of M / M ~a g a i n s t t , t
3.
COMPLICATING FACTORS
The e x p r e s s i o n s i n § 2.1 r e l a t i n g M ~ / M and t i m e r e f e r t o s t r i c t boundary c o n d i t i o n s , c o n s t a n t d i f f u s y v i t y , c r y s t a l s a l l of one s i z e and s h a p e , i s o t h e r m a l c o n d i t i o n s t h r o u g h o u t e a c h bed w i t h i n e a c h c r y s t a l and i n t r a c r y s t a l l i n e d i f f u s i o n a s t h e r a t e c o n t r o l l i n g s t e p . However e q n s . 5 and 9 , i n which M ~ / iEs ~ p r o p o r t i o n a l t o fi, a p p l y t o s m a l l t f o r any d i s t r i b u t i o n of c r y s t a l s i z e and s h a p e . I t i s o n l y n e c e s s a r y t h a t t i s s u c h t h a t e a c h c r y s t a l l i t e i s b e h a v i n g a s a s e m i - i n f i n i t e medium ( i . e . t h e deep i n t e r i o r s a r e f r e e o f d i f f u s a n t ) . Idhen t h e above r e q u i r e m e n t s of t h e r e l a t i o n s h i p s i n 5 2 . 1 are compared w i t h r e a l s i t u a t i o n s t h e f o l l o w i n g d i s t u r b i n g f a c t o r s may a r i s e (cf. § 2):
1. Boundary c o n d i t i o n s a s g i v e n i n 5 2 . 1 a r e n o t m a i n t a i n e d , and i n t h e c a s e of v a r i a b l e p r e s s u r e c o n s t a n t volume r u n s s o r p t i o n i s o t h e r m s do n o t obey H e n r y ' s law. 2. Where i n t r a c r y s t a l l i n e d i f f u s i o n i s f a s t t h e r a t e d e t e r m i n i n g s t e p may c e a s e t o be governed o n l y by i n t r a c r y s t a l l i n e d i f f u s i o n b u t by s u r f a c e b a r r i e r s o r by i n t e r - p a r t i c l e f l o w (5 2 . 2 . , eqn. 1 0 ) .
3. I n t r a - c r y s t a l l i n e d i f f u s i v i t i e s may b e f u n c t i o n s of t h e c o n c e n t r a t i o n of d i f f u s a n t .
4 . Because o f l i b e r a t i o n of t h e h e a t of s o r p t i o n , t h e uptake of g u e s t s d o e s n o t o c c u r u n d e r i s o t h e r m a l c o n d i t i o n s . H e a t i n g and t h e n c o o l i n g of t h e bed and of e a c h c r y s t a l l i t e s u p e r p o s e s d r i f t s in e q u i l i b r i a and r a t e s e x p e c t e d f o r i s o t h e r m a l u p t a k e s . These e f f e c t s a r e f u r t h e r d i s c u s s e d i n o r d e r , below. 3.1. Maintenance of Boundarv c o n d i t i o n s The problem of m a i n t a i n i n g r i g o r o u s boundary c o n d i t i o n s i s i l l u s t r a t e d by a r e c e n t s t u d y of n-hexane u p t a k e i n t h e hydrogen form of z e o l i t e RHO ( F i g . 4 ( 4 ) ) . The boundary c o n d i t i o n s sought f o r s o r p t i o n were t h o s e of eqn. 2, and t h e c o r r e s p o n d i n g ones f o r d e s o r p t i o n were
(i)
C = Co i n e a c h c r y s t a l a t time t = 0
( i i ) C = 0 j u s t w i t h i n e a c h c r y s t a l a t ro f o r t > 0.
The uptakes were measured g r a v i m e t r i c a l l y u s i n g a s i l i c a s p r i n g balance w i t h a bed of 0.3 g of z e o l i t e powder i n a g l a s s bucket. Fig. 4 shows t h a t (a) s o r p t i o n r a t e s measured a s Q ~ / Q , vs t were much l a r g e r than d e s o r p t i o n r a t e s ; and (b) f o r s o r p t i o n Q ~ / Q , a t given t had a n e g a t i v e temperature c o e f f i c i e n t whereas i n d e s o r p t i o n QI/&O had a p o s i t i v e temperature c o e f f i c i e n t . The h i g h e r t h e temperature t h e r e f o r e t h e more n e a r l y did Qt/Om vs t f o r s o r p t i o n and d e s o r p t i o n approach one a n o t h e r . The above behaviour was considered i n t h e following terms. Although t h e c o n s t a n t p r e s s u r e boundary c o n d i t i o n s of eqn. 2 were sought, when the v a l v e from t h e l i q u i d n-hexane r e s e r v o i r t o t h e sorption volume was opened a c o n s i d e r a b l e p e r i o d e l a p s e d b e f o r e the e q u i l i b r i u m vapour p r e s s u r e was e s t a b l i s h e d i n the s o r p t i o n volume. A t low temperature, say T I , a l l p r e s s u r e s i n t h e s o r p t i o n volume correspond w i t h p o i n t s along the f l a t top of t h e very rectangular s o r p t i o n isotherm, and t h e r e f o r e w i t h s a t u r a t i o n uptake. The boundary c o n d i t i o n s of eqn. 2 were accordingly met:
(i)
C = 0 a t t = 0 i n each c r y s t a l
(ii)
C = Cw a t t > 0 j u s t w i t h i n each c r y s t a l a t ro.
As T i n c r e a s e s the isotherm becomes l e s s and l e s s r e c t a n g u l a r s o t h a t the changing p r e s s u r e means t h a t C j u s t w i t h i n a c r y s t a l Thus Qt/qw i s varies with time and only slowly approaches C,. l e s s than i t would have been a t time t i f the second of t h e above boundary c o n d i t i o n s had been v a l i d a t once. I f t h e r a t e of uptake by t h e A second f a c t o r a l s o o p e r a t e s . c r y s t a l s i s comparable with the r a t e of supply of vapour by i n t e r c r y s t a l flow t h e r e w i l l be a p r e s s u r e g r a d i e n t i n t o the bed. A t the low temperature, TI, the r e c t a n g u l a r isotherm s t i l l e n s u r e s the approximate v a l i d i t y of the second o f the above boundary conditions. However, f o r the l e s s r e c t a n g u l a r isotherms a t h i g h e r T t h i s boundary c o n d i t i o n w i l l be l e s s and l e s s c o r r e c t and the deviation w i l l i n c r e a s e with denth i n t h e bed. This behaviour augments t h a t of the previous paragranh i n making Q ~ / \ even l e s s than i t would have been had the boundary c o n d i t i o n s of eqn. 2 been met. I f the two e f f e c t s i n f l u e n c e the temperature dependance of Q ~ / Q more ~ than does t h e d i f f u s i v i t y , with i t s p o s i t i v e temperature dependance , then the observed n e g a t i v e temperature c o e f f i c i e n t i n curves of ~ ~ / &vso t f o r s o r p t i o n w i l l r e s u l t .
We n e x t consider why d e s o r p t i o n i s s o much slower than s o r p t i o n and has a p o s i t i v e temperature c o e f f i c i e n t ( F i g . 4 ) . On reducing the p r e s s u r e of s o r b a t e o u t s i d e the bed t o t h e lowest p o s s i b l e vacuum l e v e l , w i t h i n the bed, a s the c r y s t a l s shed n-hexane, t h e r e
w i l l remain r e s i d u a l p r e s s u r e s which a r e g r e a t e s t a t depth. A t t h e l o w temperature, TI, the r e c t a n g u l a r i s o t h e r m ensures l a r g e d e p a r t u r e s from t h e second boundary c o n d i t i o n f o r d e s o r p t i o n ( C = 0 f o r t > 0 j u s t w i t h i n each c r y s t a l s u r f a c e ) . Therefore ~ ~ i1s much % l e s s a t given t than i t would be had t h i s second boundary c o n d i t i o n been met. I t w i l l t h e r e f o r e be much l e s s a l s o than Q ~ / Qf o~r s o r p t i o n a t time t and temperature TI where the boundary c o n d i t i o n s of eqn. 2 a r e met.
A s temperature i n c r e a s e s and the isotherms become l e s s and l e s s r e c t a n g u l a r the r e s i d u a l p r e s s u r e s a t depth i n the bed w i l l r e s u l t i n s m a l l e r d e p a r t u r e s from t h e c o n d i t i o n C = 0 f o r t > 0 j u s t w i t h i n each c r y s t a l s u r f a c e , and O t / k f o r given t w i l l inc r e a s e a s compared with qt/Qm a t t h e low temperature TI. When t h i s e f f e c t i s combined w i t h the p o s i t i v e temperature c o e f f i c i e n t of i n t r a c r y s t a l l i n e d i f f u s i v i t y t h e p o s i t i v e t e m ~ e r a t u r ecoeff i c i e n t i n curves of Ot/Qm vs t f o r d e s o r p t i o n (Fig. 4 ) i s t o be expected. I t can be concluded t h a t f o r r e c t a n g u l a r isotherms, provided s o r p t i o n i s s u f f i c i e n t l y slow t o minimise h e a t i n g of the bed and t o avoid r a t e c o n t r o l by i n t e r p a r t i c l e flow, s o r p t i o n k i n e t i c s can s e r v e t o give d i f f u s i v i t i e s , D, b u t d e s o r p t i o n may n o t . Also, as isotherm c u r v a t u r e becomes l e s s and l e s s , i n t h e Henry's law l i m i t curves of Q ~ / Q , f o r s o r p t i o n and d e s o r p t i o n should coincide and e i t h e r could, under c o n s t a n t p r e s s u r e o r c o n s t a n t volume v a r i a b l e p r e s s u r e c o n d i t i o n s , s e r v e t o give D.
The major importance of boundary c o n d i t i o n s i s thus c l e a r , through t h e i r i n f l u e n c e on t h e i n t e r p r e t a t i o n of t h e k i n e t i c s . Equations such a s those i n 5 2 . 1 a r e v a l i d f o r t h i s purpose only when the boundary c o n d i t i o n s f o r which they were derived a r e s t r i c t l y maintained. This can experimentally be d i f f i c u l t t o achieve.
3 . 2 . The R a t e - d e t e r m i n i n ~ S t e ~ For i n t r a - c r y s t a l l i n e d i f f u s i o n t o be rate-determining the process must be much slower than t h e r a t e of supply of g u e s t molecules t o t h e s u r f a c e s of t h e c r y s t a l s comprising t h e bed. In some z e o l i t e s , such a s &-A o r f a u j a s i t e ( z e o l i t e X and Y) , t h i s i s f a r from the c a s e f o r many g u e s t molecules. Attempts t o e v a l u a t e s e l f - d i f f u s i v i t i e s f o r i n t r a - c r y s t a l l i n e d i f f u s i o n from s o r p t i o n k i n e t i c s have o f t e n l e d t o gross d i s c r e p a n c i e s when compared w i t h those determined from pulsed f i e l d g r a d i e n t NMIZ, ( 5 2.2) as i l l u s t r a t e d by the f i g u r e s i n Table 1 ( 7 ) . The NMR procedure can give D* f o r i n t r a c r y s t a l l i n e d i f f u s i o n f o r high v a l u e s of D* and i s t h e r e f o r e i n t h e s e s i t u a t i o n s a s u i t a b l e s t a n d a r d f o r comparisons (8, 9 , 10). The t a b l e shows t h a t t h e
Temp. (OC)
Molecules p e r cavity (NMR) 2x10-5 2x104 5x10-~ 5x10-~ 4.5~10'~ 2~ 1 0 ' ~ 2 ~ 1 0 - 5(0% A ~ ) 7 x 1 0 (10% ~ ~ Ag) 2x10-~(20%Ag) 2 ~ 1 0 '(507, ~ A ~ ) 10- ( iooz ~ g )
(NMR)
D*
7
-
5x10-I O 10-10 3x10;-I l 3x10-~ 4x10-~ 10-10 5x10'~ (0% Ag) 10-7(iox A ~ ) 5x10'~ (20% Ag) 2x10'~ (50% Ag) 10-8 (loox Ag)
D* (sorption rates)
Comparison of D* (cm2 s - l ) from pulsed NMR and from sorption Kinetics ( 7 ) ,
System
Table 1:
d i s c r e p a n c i e s can be v e r y g r e a t i n d e e d ; t h e y i n d i c a t e t h a t i n sorption k i n e t i c s the. rate-controlling s t e p i s not j u s t i n t r a c r y s t a l l i n e d i f f u s i o n b u t must i n v o l v e o t h e r f a c t o r s such a s i n t e r c r y s t a l flow and h e a t e f f e c t s . The k i n e t i c d a t a have i n many c a s e s been wrongly i n t e r p r e t e d i n terms of i n t r a - c r y s t a l l i n e d i f f u s i o n alone. I f i n t r a - c r y s t a l l i n e d i f f u s i o n i s rate-controlling then the v a l u e s of t h e d i f f u s i v i t y o b t a i n e d from s o r p t i o n k i n e t i c s must ( a ) b e independent of t h e s i z e of t h e c r y s t a l l i t e s used i n t h e measurements, f o r beds of t h e same form and d e p t h ; and (b) be independent of t h e t h i c k n e s s of t h e bed f o r c r y s t a l l i t e s a l l of t h e same s i z e . These c r i t e r i a have n o t o f t e n been a p p l i e d . A s a t h i r d c r i t e r i o n , of c o u r s e , k i n e t i c s and p u l s e d NMR must g i v e t h e same v a l u e s of s e l f - d i f f u s i v i t y f o r e q u a l l o a d i n g s of t h e c r y s t a l s . Attempts which have been r e a s o n a b l y s u c c e s s f u l have been made t o r e c o n c i l e d i s c r e p a n t v a l u e s o f B* from p u l s e d f i e l d g r a d i e n t NMR and from s o r p t i o n k i n e t i c s by making t h e c r y s t a l l i t e s l a r g e r ( F i g . 5) and making t h e bed of extreme t h i n n e s s (6, l l ) , o p t i m a l l y no more t h a n t h e t h i c k n e s s of a s i n g l e c r y s t a l . Extreme c a r e i n the k i n e t i c measurements i s r e q u i r e d even s o , a s e x e m p l i f i e d by t h e h a l f - l i f e t i m e s , t l = 2 . 1 ~ 1 0 - r~ $ / ~ i,n v o l v e d f o r s y n t h e t i c z e o l i t e s i n t h e u s u a l s i z e range 0 . 1 t o 1 0 p ( i . e . r a d i u s ro from cm t o cm). t l i s given i n Table 2 (12) f o r v a r i o u s values of ro and D. The dashea l i n e i n t h e t a b l e d i v i d e s t h e h a l f - l i v e s from s o r p t i o n r a t e s i n t o t h o s e too s m a l l f o r a c c u r a t e measurements w i t h normal equipment from t h o s e which a r e l o n g enough f o r a c c u r a t e d e t e r m i n a t i o n . I n k i n e t i c measurements c r y s t a l dimensions and d i f f u s i v i t i e s should be such a s t o g i v e v a l u e s of t i below t h e dashed line.
Table 2:
H a l f - l i f e times, t l ( s ) f o r : 2
Measurements of D* by p u l s e d NMR on t h e o t h e r hand a r e normally cm2 s - I . s u i t a b l e only f o r s e l f - d i f f u s i v i t i e s above a b o u t This method would t h e r e f o r e be u n s u i t a b l e f o r s m a l l d i f f u s i v i t i e s f o r example t h o s e of n - p a r a f f i n s i n some z e o l i t e s such a s (Na,Ca)-A, c a - r i c h c h a b a z i t e o r e r i o n i t e . I n (Na,Ca)-A t r a c e r d i f f u s i o n u s i n g hydrocarbons l a b e l l e d w i t h r a d i o c a r b o n has been s u c c e s s f u l i n measurements of E* ( 1 3 ) . T a b l e 2 a l s o shows t h a t by s u i t a b l y inc r e a s i n g t h e c r y s t a l dimensions v a l u e s of d i f f u s i v i t i e s o f any magnitude, from v e r y s n a l l (< 10-l4 cm2 s - l ) t o v e r y l a r g e cm2 S-I) a r e p o s s i b l e , u s i n g s o r p t i o n k i n e t i c s . (> Under c o n d i t i o n s where i n t r a c r y s t a l l i n e d i f f u s i v i t i e s do n o t wholly o r even p a r t i a l l y govern s o r p t i o n k i n e t i c s i t may s t i l l be p o s s i b l e t o d e s c r i b e t h e system i n terms of an e f f e c t i v e d i f Conditions can be made such t h a t i n t h e i n t e r fusivity, Deff. c r y s t a l s p a c e s m o l e c u l e - c r y s t a l c o l l i s i o n s g r e a t l y exceed moleculemolecule c o l l i s i o n s , s o t h a t m o l e c u l a r s t r e a m i n g (Knudsen flow) o c c u r s i n t h e i n t e r - c r y s t a l s p a c e s . We c o n s i d e r flow through a c y l i n d r i c a l bed bounded by p l a n e s x = 0 ( t h e e n t r y f a c e ) and x = R ( t h e e x i t f a c e ) , and w i t h no flow through t h e curved s u r f a c e ( i . e . t h e bed i s i n an open c y l i n d r i c a l c o n t a i n e r ) . The t o t a l flow, J , e n t e r i n g t h e bed a c r o s s t h e p l a n e x = 0 p e r u n i t a r e a of t h i s p l a n e can be w r i t t e n a s
Deff
i s an e f f e c t i v e d i f f u s i v i t y , E i s t h e p o r o s i t y of t h e bed ( e x l u d i n g i n t r a c r y s t a l l i n e p o r o s i t y ) , C i s t h e t o t a l number of molecules p e r u n i t volume of porous medium a t x = 0 , and C; and C: a r e t h e numbers of molecules p e r u n i t volume of gas phase and of c r y s t a l s , averaged a c r o s s t h e p l a n e a t x = 0 . Each g r a d i e n t i s an average of a l l l o c a l g r a d i e n t s a c r o s s t h e p l a n e . Also we may w r i t e
where J g and J i a r e r e s p e c t i v e l y gas phase and i n t r a c r y s t a l l i n e components of J and
where t h e g r a d i e n t s a r e a g a i n averages over t h e p l a n e x = 0 , Dg i s
274 the Knudsen flow d i f f u s i v i t y and D;- the i n t r a c r y s t a l l i n e d i f f u s i v i t y . Combination of eqns. 14 t o 15b gives
Because i n the t r a n s i e n t s t a t e the g r a d i e n t s i n eqn. 1 7 a r e functions of time Deff may have s i g n i f i c a n t t i m e dependance when I n the s t e a d y s t a t e of flow, however, such time dependDg # D;. ances w i l l have disappeared, f o r the boundary c o n d i t i o n s (i.1 (ii) (iii)
C = O f o r O < x < R a t t = O C = C o a t x = O f o r t > O C = C << Co a t x = R f o r t > 0 R
A p a r t i c u l a r i n t e r e s t i s t o show i n a given system whether i n t e r - o r i n t r a c r y s t a l l i n e flow i s t h e dominant component of J through x = 0. This i s p o s s i b l e t o f i n d when Henry's law governs s o r p t i o n e q u i l i b r i u m (C: = kc'), Dg i s independent of C i and i n this g range D; i s independent of Ci. Eqn. 1 7 now reduces t o
s o t h a t Deff i s constant. For uniform packing of the bed dC/dx i n t h e steady s t a t e i s t h e same a t a l l planes x = X and equals - ( c ~ - C ~ ) so / ~ t h a t , from J = D ~ ~ ~ ! c ~ - C ~D )e f/f ! ~i s, found. E and k a r e a v a i l a b l e independently. Dg i s b e s t a s s e s s d using helium as a Re v i t u a l l y non-sorbed r e f e r e n c e gas t o e v a l u a t e Dg from J~~ = J!~. I f M i s t h e molecular weight of the d i f f u s a n t and MH, t h a t of helium, then
Thus D;
i n eqn, 18 can be found and thence the r a t i o
The s t e a d y - s t a t e flow method has been a p p l i e d not t o z e o l i t e Some results compacts alone, b u t t o g r a p h i t e - z e o l i t e A compacts (14) a r e given f o r n i t r o g e n i n Table 3, i n which an extended a n a l y s i s served t o give n o t only Dg and D i b u t a l s o a s u r f a c e d i f f u s i v i t y Ds, a s s o c i a t e d l a r g e l y with the g r a p h i t e . The i n t r a - c r y s t a l l i n e d i f f u s i v i t i e s of N2 i n z e o l i t e A a t the experimental temperatures a r e reasonable i n v a l u e . The method m e r i t s f u r t h e r a t t e n t i o n for large i n t r a c r y s t a l l i n e d i f f u s i v i t i e s .
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F i g . 5.
I n t r a c r y s t a l l i n e s e l f - d i f f u s i v i t y (0) f r o m p u l s e d f i e l d g r a d i e n t NMR and apparent d i f f u s i v i t y from s o r p t i o n rates of CH4 i n c h a b a z i t e s a t OOC, a s functions of c r y s talli t e r a d i i ( 6 ) .
Fig. 6.
p l o t s of MJM, = ( Q ~ - Q/ (~Q) - - Q ~ ) a g a i n s t fi f o r n-C6Hl4 i n 30.55Z~a-exchanged (Ca,Na)-A, a t c o n s t a n t p r e s s u r e and a t 348K ( 1 5 ) . Q_ = 120.05 mg g-l. Curve 1: Curve 2:
Qo = 0 Q, - 48.08 mg g-l
Curve 3: Curve 4:
Qo = 6 3 . 3 7 m g g-l. Qo = 96.99 mg g-l,
277 3 . 3 . Concentration-Dependent D i f f u s i v i t i e s When the d i f f e r e n t i a l i n t r i n s i c d i f f u s i v i t y i n t h e z o l i t e i s a function of concentration C w i t h i n the c r y s t a l s * one may add d i f fusant i n small increments, so t h a t under constant p r e s s u r e boundary -conditions (eqn. 2) (Cm - C,) i s small and, i n t h i s i n t e r v a l i n C , D may be considered c o n s t a n t . Eqns. 3 t o 6 a r e then s t i l l v a l i d and h e a t evolution a s a complicating f a c t o r i s minimised. When isotherms a r e r e c t a n g u l a r i n shape the e q u i l i b r i u m p r e s s u r e
i s often s o small t h a t constant p r e s s u r e s o r p t i o n of increments i s impossible. Constant volume v a r i a b l e p r e s s u r e a d d i t i o n i s a l s o n o t suitable because even though (Cw-Co) i s small t h e p r e s s u r e surge may i n i t i a l l y r a i s e i n t r a c r y s t a l l i n e concentrations w e l l above Coo n e a r the surface of each c r y s t a l , and the assumption t h a t 5 i s c o n s t a n t because (Cw-Co) i s small i s no longer t e n a b l e . The method of small The c r y s t a l s increments has t h e r e f o r e been modified a s follows (15) were e q u i l i b r a t e d t o an i n i t i a l uptake, Qo, and then were brought t o s a t u r a t i o n uptake a t Q, under n e a r l y constant p r e s s u r e of s o r b a t e vapour. The value of Q was p r o g r e s s i v e l y i n c r e a s e d , while Qm was 9 % of course constant. Initially l i n e a r p l o t s of ~ ~ =1 (Qt-Qo)/ (Q -Qo) a g a i n s t Jf were s t i l l obtained (Fig. 6 ) b u t , i n t h e ex, D must be an average o r i n t e g r a l d i f f pression M ~ / M , = 6
.
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0
R
usivity over the i n t e r v a l (Qm-Q,)
.
Assuming t h e r e f o r e t h a t
i s the d i f f e r e n t i a l d i f f u s i v i t y corresponding t o a p a r t i c u l a r where Qt(Q, > Qt > Q ) one may e v a l u a t e i n t e g r a l d i f f u s i v i t i e s from Mt/Mm = j and from p l o t s of D a g a i n s t (&, f o r any value of =o Qo, because eqn. 22 can a l t e r n a t i v e l y be w r i t t e n as
6
This method was used t o e v a l u a t e d i f f u s i v i t i e s of n-Cb, n-C6 and n-Cg p a r a f f i n s i n (Na,Ca)-A. The e x t e n t s of exchange were r e g u l a t e d so t h a t uptake was slow, t o ensure t h a t i n t r a c r y s t - a l l i n e d i f f u s i o n was r a t e c o n t r o l l i n g and p e r t u r b a t i o n of the k i n e t i c s by h e a t *The s u b s c r i p t i of 5 3 . 2 i s omitted h e r e where only i n t r a c r y s t a l l i n e flow need be considered.
278 e v o l u t i o n was minimised. I l l u s t r a t i v e values of 6 f o r n-C4HI0 i n ( ~ a , C a ) - A a r e given i n Table 4. Isotherms of t h e p a r a f f i n s i n z e o l i t e A a r e of type 1 i n Brunauer's c l a s s i f i c a t i o n (16) and often approximate t o Langmuirts isotherm (17). For the i d e a l isotherm based on h i s p o s t u l a t e s t h e i n t r i n s i c d i f f e r e n t i a l d i f f u s i v i t y should be p r o p o r t i o n a l t o (1-0)LdRn p/dRn07 s i n c e (1-0) i s the chance t h a t a vacant s i t e occurs i n t o which a molecule on an a d j a c e n t s i t e may jump and dRnp/dgn0 i s the Darken a c t i v i t y corr e c t i o n . This c o r r e c t i o n f o r Langmuir's isotherm equation (kp = 0/(1-08,is ( 1 - 0 ) so t h a t 5 should be c o n s t a n t , and theref o r e 5 = D. The t r e n d s i n 5 i n Table 4 can be considered a s one measure of d e v i a t i o n s of t h e a c t u a l isotherms from Langmuir's i d e a l i s a t ion. Analysis of the temperature c o e f f i c i e n t s of 5 i n terms of the Arrhenius equation D = Do exp-E/RT e x t r a p o l a t e d t o Qo = 0 a r e given i n Table 5 (15). E tends t o decrease a s the e x t e n t of exchange Table 4:
-
D for n-C4H10 i n 30.55, Z e o l i t e A (15).
32,54 and 34.10% Ca-exchanged
Table 5
% Ca exchange i n (Ca,Na)-A
Diffusant
(b)
Do (m2 s - l )
in
5
= DO exp-E/RT f o r Qo = 0
of ~ a 'by ca2+ i n c r e a s e s , and Do tends t o decrease a s E d e c r e a s e s . The energy b a r r i e r s a r e c o n s i d e r a b l e f o r t h e r a t e - c o n t r o l l i n g u n i t d i f f u s i o n s t e p s because i f t h e r e i s a s i g n i f i c a n t ~ a c+o n t e n t many of these i o n s p a r t i a l l y block t h e 8-ring windows through which d i f f u s a n t must pass. Even i n absence of c a t i o n s t h e windows have free diameters of about 4 . 1 A compared w i t h a c r i t i c a l dimension of a s t r e t c h e d hydrocarbon c h a i n of about 4 . 9 9 .
3.4. Evolution of t h e Heat of Sorption The exothermal h e a t of s o r p t i o n produces both l o c a l and extended temperature changes i n t h e bed of s o r b e n t . That i s , i n each c r y s t a l l i t e t r a n s i e n t temperature g r a d i e n t s appear, t h e h e a t being produced i n i t i a l l y a t t h e s u r f a c e s , w i t h t h e temperature wave then spreading both i n t o t h e body of t h e c r y s t a l s a s w e l l as h e a t i n g the ambient gas phase. A f t e r an i n t e r v a l , t h e temperature of t h e whole bed becomes more uniform b u t i s above t h a t of t h e thermostat i n which i t may be immersed. A t s t i l l l o n g e r times t h e temperature f a l l s a s y m p t o t i c a l l y t o t h a t of t h e t h e r m o s t a t . Attempts have been made t o allow f o r the h e a t e f f e c t s i n t h e second and t h i r d s t a g e s (18) b u t any comprehensive t r e a t m e n t allowing f o r h e a t e v o l u t i o n and heat and m a t t e r flows and t h e i r i n t e r a c t i o n , t o g e t h e r w i t h timedependent d r i f t s i n e q u i l i b r i a d u r i n g h e a t i n g and c o o l i n g of beds must be very d i f f i c u l t . I t i s t h e r e f o r e p r e f e r a b l e t o aim a t conditions a s n e a r i s o t h e r m a l a s p o s s i b l e by:
1.
2.
Making t h e beds a s t h i n a s p o s s i b l e , o p t i m a l l y a l a y e r one ,crystal thick. C i r c u l a t i n g t h e vapour o r l i q u i d g u e s t through t h e bed t o remove h e a t a s q u i c k l y as p o s s i b l e .
2 80 3.
4. 4.
Avoiding very r a p i d s o r p t i o n , by having s u i t a b l y l a r g e c r y s t a l s ( s e e Table 2 ) . Adding only small increments of d i f f u s a n t a t a time.
SOME ADDITIONAL EFFECTS
D i f f u s i o n i n s i d e z e o l i t e c r y s t a l s may be i n f l u e n c e d by f a c t o r s o t h e r than those d e s c r i b e d above. It has been suggested t h a t s u r f a c e s k i n s may sometimes be p r e s e n t which form an added r e s i s t a n c e t o flow. Such s k i n s may e x i s t i n p a t c h e s o n l y , s o t h a t the a r e a through which d i f f u s a n t s e n t e r t h e z e o l i t e c r y s t a l s i s reduced o r t h e s k i n s could cover t h e whole e x t e r n a l s u r f a c e . I n t h e l a t t e r circumstance t h e c r y s t a l s could become non-sorbents i f the s k i n was g l a s s y and impermeable. For t o t a l c o n t r o l of s o r p t i o n r a t e s by s u r f a c e s k i n s (1-Mt/&) should d e c l i n e e x p o n e n t i a l l y w i t h time (19) r a t h e r than following i n i t i a l l y t h e eqn. 5 ( M ~ / M _= 6 - ) However eqn. 5 h a s been w e l l v e r i f i e d i n many
.
0
r a t e s t u d i e s which s u g g e s t s t h a t s u r f a c e s k i n s a r e n o t normally important. Hydrothermal t r e a t m e n t s which diminish s o r p t i o n r a t e s have on t h i s evidence been considered t o r e s u l t i n s k i n s (19 a and b) Pre-treatments may be such t h a t t h e c r y s t a l s s u f f e r chemical damage (e.g. g r i n d i n g , hydrothermal t r e a t m e n t s and even t h e outWith bonded p e l l e t s t h e bond may block some g a s s i n g procedures) a r e a s of t h e c r y s t a l l i t e s u r f aces. Within t h e c r y s t a l l i t e s there may be v a r i a b l e amounts of d e t r i t a l m a t e r i a l occluded d u r i n g s y n t h e s i s ; and l a t t i c e d e f e c t s such as s t a c k i n g f a u l t s may occur p e r i o d i c a l l y o r a t random t o an e x t e n t varying between preparations. Also t h e Si/A1 r a t i o s of d i f f e r e n t p r e p a r a t i o n s of t h e same zeolite may vary according t o t h e methods of p r e p a r a t i o n , and t h e r e f o r e the c a t i o n c o n c e n t r a t i o n s and l o c a t i o n s i n t h e i n t r a c r y s t a l l i n e channels and c a v i t i e s . A l l t h e s e f a c t o r s can r e s u l t i n d i f f e r e n t values of d i f f u s i v i t i e s among d i f f e r e n t samples of the same z e o l i t e . When such r e s u l t s , obtained i n d i f f e r e n t l a b o r a t o r i e s , a r e t o be compared pre-treatment and s y n t h e s i s c o n d i t i o n s should be s t a n d a r d i s e d and/ o r samples from each l a b o r a t o r y 'should b e exchanged and t h e measurements made on both samples i n each l a b o r a t o r y .
.
A purpose of § § 2 t o 4 has been t o draw a t t e n t i o n t o ways i n which t h e study of s o r p t i o n k i n e t i c s can be improved f o r determina t i o n s of i n t r a c r y s t a l l i n e d i f f u s i v i t i e s , by i n d i c a t i n g some complications and ways of avoiding o r minimising t h e s e .
5.
SELF-DIFFUSIVITY AND DARKEN'S
RELATION
Exact r e l a t i o n s have been derived between d i f f e r e n t i a l intrinsic
281 and d i f f e r e n t i a l s e l f - d i f f u s i v i t i e s (20,21,22). Here we w i l l consider only the simpler approximately v a l i d Darken r e l a t i o n which i s
The a c t i v i t y c o r r e c t i o n d~na/dRnCf o r an i d e a l gas phase i s replaceable by dknp/dRn~, which can be found from t h e s l o p e s of isotherms ( p l o t t e d a s RnC a g a i n s t Rnp). The c o r r e c t i o n s o d e t e r mined assumes t h a t everywhere t h e r e i s l o c a l e q u i l i b r i u m between gaseous and i n t r a z e o l i t e s o r b a t e . In the Henry's law range of s o r p t i o n ( C p) d~np/dknCi s unity. Outside t h i s range f o r type I isotherms dQnp/dQnCbecomes greater than u n i t y and f o r r e c t a n g u l a r isotherms may become very large a s the uptake approaches i t s s a t u r a t i o n value. Darken's relation h a s been t e s t e d f o r water d i f f u s i n g i n n a t u r a l c h a b a z i t e , Both i n t r i n s i c and s e l f - d i f f u s i v i t i e s heulandi t e and gmelini t e ( 3 ) were measured, the l a t t e r using H20 t o d i s p l a c e D20. The c r y s t a l s were l a r g e p i e c e s of t h e z e o l i t e , and an i n e r t c a r r i e r gas was charged with water vapour a t constant r e l a t i v e p r e s s u r e s and c i r culated c o n t i n u a l l y through t h e samples suspended from a s i l i c a spring balance i n an open mesh g l a s s - f i b r e bucket. Conditions were such as t o ensure v i t u a l l y complete s a t u r a t i o n of z e o l i t e by e i t h e r D 2 0 or H 2 0 .
.
The r e s u l t s i n Table 6 show t h a t Darken's r e l a t i o n i s , a s expected from the more r e f i n e d theory (20,21,22), only approximately valid. However the dominant c o r r e c t i o n i s c e r t a i n l y t h a t f o r t h e a c t i v i t y (d!Znp/dRnC)
.
Pulsed f i e l d g r a d i e n t NMR has been used t o study s e l f - d i f f u s i o n of n-C4 t o n-CI8 alkanes i n z e o l i t e X over the range -100 t o +200°c (23). This very complete study showed t h e following behaviour:
1. 2,
3. 4.
5.
For comparable degrees of f i l l i n g of the i n t r a c r y s t a l l i n e pore space D* decreased with i n c r e a s i n g carbon number. A t c o n s t a n t temperature D* decreased monotonically f o r each p a r a f f i n as the amount sorbed i n c r e a s e d . It has a l r e a d y been i n d i c a t e d t h at for an i d e a l Langmuir s o r p t i o n one -0 (1-0) where O i s the degree of would expect D* = D * f i l l i n g . For n-para f ~ i n s t h i s i s normally an overi d e a l i s a t i o n , but a decrease i n D* w i t h O i s as expected. For a given temperature and p a r a f f i n D* was w i t h i n an o r d e r of magnitude of D* f o r the corresponding l i q u i d p a r a f f i n . For a c o n s t a n t amount sorbed D* depended e x p o n e n t i a l l y on temperature = Do exp-E/RT) . The h e a t of s o r p t i o n f o r a given degree of f i l l i n g , 0 , of i n t r a c r y s t a l l i n e pore space i n c r e a s e d continuously with i n c r e a s i n g carbon number, b u t a c t i v a t i o n e n e r g i e s , E , f o r
(E*
s e l f - d i f f u s i o n approached an asymptotic l i m i t and were never more than a r a t h e r s m a l l f r a c t i o n of the h e a t of s o r p t i o n ( ~ i g .7 ( 2 3 ) ) . I t has been suggested (24) t h a t t h e behaviour of E means that a s t h e n-paraffin chains i n c r e a s e i n length each u n i t d i f f u s i o n s t e p tends i n c r e a s i n g l y t o involve segmental r o t a t i o n s of p a r t only of t h e molecule round a C-C bond. This r e s u l t s i n changes of p o s i t i o n of t h e c e n t r e of mass of t h e molecule, b u t n o t n e c e s s a r i l y i n t r a n s l a t i o n of a l l p a r t s of t h e molecule simultane o u s l y , The longer t h e chains t h e more probable t h e segmental mechanism. I f f o r longer chains t h i s mechanism involves segments of s i m i l a r s i z e , e v e n t u a l l y n e a r l y independent of chain length, then, a s observed, E would become almost constant. Segmental u n i t d i f f u s i o n s t e p s can a l s o account f o r the decrease i n with i n c r e a s i n g carbon number, because the number of u n i t s t e p s required
E*
Table 6 :
Relation between 6 and % f o r water i n s e v e r a l z e o l i t e s near s a t u r a t i o n of the z e o l i t e ( 3 ) .
Zeolite
T (OC)
10axD*
d Rnp (a) d RnC
(cm2 s - l )
Chabazite B
75 65 55 45 35
Heulandi t e
46.2 31.8 21.4
14.1 9.0
1o=x5* dan~ d RnC (cm2 s-1)
23.0 (b (b 24 "(b) 25.5 27 .O (b 2 8.0 (b
106
5
cn2 s-1
10.6,
11.7
7.60 5.4, 3.80 2.50
8.9 6.6 4.8 3.4
75 65 55 45 35
Gmelini t e
(a) Determined from t h e n e a r l y f l a t top of the isotherm. (b) Determined from isotherms measured i n sample A of chabazite.
Fig. 7 .
Heats of sorption and energies of a c t i v a t i o n f o r s e l f d i f f u s i v i t i e s , f o r n-alkanes i n Na-X z e o l i t e , as functions o f carbon number. The upper curve gives the hear of sorption, the lower curve the a c t i v a t i o n energy ( 2 3 ) . The ordinate i s i n kJ mol-I and the abscissa i s the carbon number.
Fig. 8 .
(a) Sorption r a t e s of 0 2 , N2 and Ar i n l e v y n i t e a t -184OC ( 2 5 ) . For 02, Q = 10.02, for N2 i t is 9 . 7 7 and f o r Ar, 10.01, a l l ?n cm3 a t s . t . p , g-l. (b) Sorption rates of 0 2 ,N2 and A r i n Ca-mordenite a t -78O(26)
.
Fig. 9.
Sorption rates a t 30°c in Na-Y of liquid 1,3,5trimethyl-(El) , 1 , 3 , 5 - t r i e t h ~ l - ( 0 ) and 1,3,5-triisopropyl-benzene (01 (2 7)
.
Fig. 10.
Diffusivities in cm2 s - I of HZ, 02, N p and A r i n mordenite a t -183OC, as functions of the amount of pre-sorbed NH3 ( 3 0 ) .
285 ,for migration of an e n t i r e molecule from one c a v i t y t o t h e n e x t i n zeolite X w i l l i n c r e a s e . with chain l e n g t h .
6.
MOLECULE SIEVING
According t o the f r e e dimensions of t h e meshes through which guest molecules must migrate i n t r a c r y s t a l l i n e d i f f u s i v i t i e s a r e found t o change very g r e a t l y w i t h t h e molecular dimensions of t h e guest. Windows o r meshes c o n t r o l l i n g molecule s i e v i n g a r e 6-, 8-, 10- and 12-rings of v a r i e d conformations, and-octagonal prisms. With some z e o l i t e s t h e r e i s a d d i t i o n a l o b s t r u c t i o n due t o l o c a t i o n of cations i n o r n e a r t h e windows. C r y s t a l s with r e s t r i c t e d windows d i f f e r e n t i a t e between groups of small molecules and exclude larger ones a l t o g e t h e r ; c r y s t a l s w i t h l a r g e r windows d i f f e r e n t i a t e between groups of b i g g e r molecules, and s o on, Even q u i t e small differences i n molecular dimensions can r e s u l t i n d r a m a t i c a l l y d i f f e r e n t r a t e s of s o r p t i o n . This i s i l l u s t r a t e d i n Fig. 8 f o r uptakes of 02, N2 and A r i n l e v y n i t e (25) and Ca-mordenite (26) which function a s fine-mesh s i e v e s . F i g . 9 shows a s i m i l a r behaviour i n a wide-mesh s i e v e , Na-Y, f o r 1,3,5-trimethyl-l,3,5t r i e t h y l - and 1,3,5-triisopropyl-benzene, a l l as l i q u i d s ( 2 7 )
.
The r e l a t i o n between molecular dimensions and d i f f u s i v i t i e s i s i l l u s t r a t e d f o r f i n e mesh s i e v e s K-A and K-mordenite i n Table 7 (28). Them a r e f i v e o r d e r s of magnitude between d i f f u s i v i t i e s of Ne ( 3 . 2 d i a m e t e r ) and Kr (3.9 1) i n K-A a t 20°c; and a l s o between HZ (2.4 x 3.1 2) and K r ( 3 . 9 4 A) i n K-mordenite a t -78OC. There i s a p a r a l l e l i n c r e a s e i n the energy b a r r i e r , E , involved i n each u n i t d i f f u s i o n s t e p .
1
The s t r o n g i n f l u e n c e of t h e exchange c a t i o n upon t h e uptake of A r by v a r i o u s ion-exchanged forms of mordenite a t -78OC i s shown i n changes of B/t$ (26) with c a t i o n : Ca-mordeni t e K-mordeni t e Ba-mordeni t e Na-mordeni t e Li-mordeni te NH4 (H) -mordeni t e From the sequence above d i f f e r i n g c a t i o n p o s i t i o n s as well as c a t i o n s i z e may p l a y a p a r t . Again t h e extreme range i s f i v e o r d e r s of magnitude
.
One may a l s o a l t e r the d i f f u s i v i t i e s of g u e s t molecules by presorbing metered amounts of a s t r o n g l y h e l d guest (e.g. H20 o r N H ~ ) which i s v i r t u a l l y immobile a t t h e temperature a t which l e s s p o l a r guest molecules a r e t o be sorbed. This e f f e c t can be very l a r g e as
Table 7:
Zeolite
R e l a t i o n between d i f f u s i v i t i e s and molecular dimensions (27) Molecule
Equilibrium, Dimensions (A)
-
D o r D-k(cm2 s - l )
(and T°C)
E ( k c a l mol-l)
i n Fig. 10 when W2, 02, N2 and A r were sorbed a t -183OC i n mordenite c o n t a i n i n g metered pre-sorbed amounts of NH3 (30)
.
7.
CONCLUDING REMARK
This account has emphasised some experimental problems and means of overcoming t h e s e , because much previous work on r a t e s of uptake of simple gases i n z e o l i t e A and of t h e s e and hydrocarbons i n zeoli t e s X and Y h a s , as shown by pulsed f i e l d g r a d i e n t NMR, been m i s interpreted, Some p r o p e r t i e s of i n t r a c r y s t a l l i n e d i f f u s i v i t i e s have been d e s c r i b e d , emphasising i n p a r t i c u l a r t h e i r g r e a t s e n s i t i v i t y t o molecular dimensions of d i f f u s i n g s p e c i e s . A s i n d i c a t e d i n § 1 d i f f u s i o n can o e o r f i r s t importance n o t o n l y i n s e p a r a t i o n process e s , b u t p a r t i c u l a r l y i n c a t a l y s i s where r e a c t a n t s must reach active c e n t r e s w i t h i n c r y s t a l s and r e s u l t a n t s must migrate out of the c r y s t a l s . Such counter-current flows a r e much more d i f f i c u l t t o i n t e r p r e t because of cross-coef f i c i e n t s be tween flows. Formulation of t r a n s p o r t i s now b e s t done i n terms of s o - c a l l e d i r r e v e r s i b l e thermodynamics and phenomenological s t r a i g h t and cross-coef f icients. Not much h a s been done s o f a r i n t h e a r e a s of co- and counterd i f f u s i o n of mixtures i n z e o l i t e s (31,32).
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-
T i s e l i u s , A , , Z . Phys. Chem., AL69, (1934), 425. T i s e l i u s , A , , Z . Phys. Chem., A174, (1935), 401. B a r r e r , R.M. a n d B.E.F. Fender, J . Phys. Chem. S o l i d s , 21, (1961), 12. 4, B a r r e r , R.M. and M.A. Rosemblat, Z e o l i t e s , 2 , (1982), 231. Ash, R . , R.M. B a r r e r and R . J . B . Craven, J. Chem. Soc. Faraday 5. Trans. TI, 74, (1978), 40. 6. Karger, J . , and J . Caro, J . Chem. Soc. Faraday T r a n s . I , 73, (1977), 1363. 7. Barrer, R.M., Z e o l i t e s and Clay M i n e r a l s and S o r b e n t s and Molecular S i e v e s (London, Academic P r e s s , 1 9 7 8 ) , p . 317. g 248, , (1971), 27. Karger, J . , Z . Phys. Chem., ~ e i ~ z i 8. 9. Karger, J . , S.P. Shdanov and A. W a l t e r , Z . Phys. Chem. ~ e i p z i g , 256, (1975) , 319. 10. Caro, J . , J. K a r g e r , H. P f e i f e r and R. S c h o l l n e r , 2 . Phys. Chem. Leipzig, 256, (1975), 698. I, 11. Karger, J . , and D.M. Ruthven, J. Chem. Soc. Faraday T r a n s . 77, (1981) , 1485. 12. Ref. 7 , p. 318. 13. Quig, A . , and L.V.C. Rees, Proc. 3rd I n t e r n a t . Conference on Molecular S i e v e s ( Z u r i c h , S e p t . 3-7, 1973) p. 277. 3, (1965), 14. B a r r e r , R.M. , and J . H . P e t r o p o u l o s , S u r f a c e S c i , 126 and 143. 15. B a r r e r , R.M., and D . J . C l a r k e , J . Chem. Soc. Faraday T r a n s . I , 70, (1974), 535. 16. Brunauer, S . , The Adsorption of Gases and Vapours, ( ~ e wYork, Oxford Univ. P r e s s , 1944) p. 150. 17. Ref. 7 pp. 112-114. 18. Ruthven, D . , i n P r o p e r t i e s and A p p l i c a t i o n s o f Z e o l i t e s , E d i t o r , R.P. Townsend (London, The Chemical s o c i e t y , 1 9 8 0 ) , p. 43. 31, ( 1 9 7 4 ) , 19. Karger, J . , and P. Hermann, Ann. Phys. L e i p z i g , 277. 19a. Bulow, M . , P. S t r u v e and S . P i k u s , Z e o l i t e s , 2 , (1982), 267. 19b. Karger, J . , W. Heink, H. P f e i f e r , M. ~ a u s e h e r - a n d J . Hoffmann, z e o l i t e s , 2 , (1982), 275. 20. Ash, R . , azd R.M. B a r r e r , S u r f a c e S c i . , 8 , (1967), 461. 36, (1973), 791. 21. Karger, J . , S u r f a c e S c i . , 22. Karger, J . , S u r f a c e S c i . , 5 7 , (1976), 749. 23. K a r g e r , J . , H. P f e i f e r , ~ . R a u s c h e rand A. W a l t e r , Z . Phys. Chem. , L e i p z i g , 259, (1978), 784. 24. B a r r e r , R.M., Symposium on t h e C h a r a c t e r i s a t i o n of Porous S o l i d s (Neuchatel , 9-13th J u l y , 1978). 25. B a r r e r , R.M., N a t u r e , 159, (1947), 508. 26. B a r r e r , R.M., T r a n s . Faraday Soc., 45, (1949), 358. 2 7 . S a t t e r f i e l d , C . N . , and C.S. Cheng, A m e r . I n s t . Chern. Eng., 6 8 t h N a t i o n a l Mtg., Houston, Feb. 28-Mar. 4 t h (1971) Adsorption P t . 1, Paper 1 6 f .
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31. 32.
R e f . 7 , p . 292. W a l k e r , P . L . Jr., L.G. Austin and S.P. N a n d i , i n C h e m i s t r y and P h y s i c s of Carbon, V o l . 2 , ( D e k k e r , 1966) pp. 257-371. B a r r e r , R.M., and L.V. C . R e e s , T r a n s . F a r a d a y Soc. , 50, (1954), 852 a n d 989. K a r g e r , J . , M. Bulow a n d W. S c h i r m e r , Z . Phys. Chem. ~ e i p z i g , 256, (19751, 144. Moore, R.M. and J . R . K a t z e r , J. Amer. Inst. Chem. Eng., 18, (1972) , 816.
PART Ill CATALYSlS
UNIFYING P R I N C I P L E S I N ZEOLITE C H E M I S T R Y A N D CATALYSIS
JULE A .
RABO
Union C a r b i d e C o r p o r a t i o n Tarrytown, New York 10591
INTRODUCTION I n r e c e n t y e a r s molecular s i e v e c a t a l y s t s have assumed a n i n c r e a s i n g l y i m p o r t a ' n t r o l e i n i n d u s t r i a l catalysis. A p p l i c a t i o n s of z e o l i t e c a t a l y s t s a r e exp a n d i n g f r o m t h e t r a d i t i o n a l p e t r o l e u m r e f i n i n g t o new and i m p r o v e d f u e l p r o c e s s i n g a p p l i c a t i o n s , a n d t o new r o l e s i n b o t h t h e p e t r o c h e m i c a l and chemical i n d u s t r i e s . Up t o t h e p r e s e n t , a l l c o m m e r c i a l a p p l i c a t i o n s o f z e o l i t e c a t a l y s t s have been c a r r i e d out with a c i d i c z e o l i t e s . Recent i n v e s t i g a t i o n s of z e o l i t e c h e m i s t r y r e v e a l e d s e v e r a l i m p o r t a n t f e a t u r e s w h i c h a p p e a r common t o b o t h T h ' i s new c h e m i c a l e v i d e n c e a l k a l i and a c i d i c z e o l i t e s . r a i s e s t h e p o s s i b i l i t y t h a t t h e underlying p'hfrsicochemical f e a t u r e s of b o t h t y p e s of z e o l i t e s p l a y a r o l e i n c a t a l y s i s . Chemistry and C a t a l y s i s With A l k a l i Z e o l i t e s Chemistry The i n t r a c r y s t a l l i n e p o r e - c a v i t y s y s t e m i n z e o l i t e s , o f t e n c a l l e d t h e z e o l i t i c s u r f a c e , i s s u r r o u n d e d by t h e z e o l i t e c r y s t a l l a t t i c e , and consequently i t i s s t r o n g l y i n f l u e n c e d by t h e z e o l i t e c r y s t a l f i e l d . This c r y s t a l f i e l d , pervading t h e i n t r a c r y s t a l l i n e pore-cavity system, r e n d e r s z e o l i t e s s o l i d e l e c t r o l y t e s . Depending on t h e i o n i c c h a r a c t e r of t h e c r y s t a l which i s m a i n l y cont r o l l e d by t h e a l u m i n a c o n t e n t , z e o l i t e s show p r o p e r t i e s A unique characterof w e a k t o v e r y s t r o n g e l e c t r o l y t e s . i s t i c of t h e s e s o l i d e l e c t r o l y t e s i s t h a t b y a d m i t t i n g
v a r i o u s a d s o r b a t e s o r r e a c t a n t s , o n e c a n c h a n g e t h e chemici composition of t h e e l e c t r o l y t e . M o r e o v e r , by e v a l u a t i n g t h e e f f e c t of t h e z e o l i t e upon a p p r o p r i a t e l y c h o s e n o c c l u d e d m o l e c u l e s , one can m o n i t o r t h e s t r e n g t h of zeol i t e electrolyte. I t s e e m s r e a s o n a b l e t o e x p e c t t h a t t h e e f f e c t of t h e z e o l i t e a s e l e c t r o l y t e upon o c c l u d e d m o l e c u l e s , and v i c e v e r s a , i s m a i n l y i n f l u e n c e d by t h e r m o d y n a m i c s , namely by t h e t e n d e n c y t o m i n i m i z e t h e f r e e e n e r g y of t h e z e o l i t e guest m a t e r i a l system. Unfortunately, the l a t t i c e e n e r g i e s o f z e o l i t e c r y s t a l s a r e n o t known. Their calc u l a t i o n w i t h any d e g r e e of r e l i a b i l i t y i s i m p o s s i b l e w i t h o u t k n o w i n g t h e c o v a l e n t a n d i o n i c c h a r a c t e r of t h e l i n k a g e s between l a t t i c e atoms. Several observations, e x p e c i a l l y t h e p r e f e r e n c e of z e o l i t e c a t i o n s f o r w a t e r o v e r f r a m e w o r k o x y g e n , i n d i c a t e t h a t t h e bond b e t w e e n framework c a t i o n s and oxygen i s s t r o n g l y c o v a l e n t . N e v e r t h e l e s s , a c e r t a i n d e g r e e o f i o n i c c h a r a c t e r must e x i s t , and a c r y s t a l f i e l d c o r r e s p o n d i n g t o t h e i o n i c c h a r a c t e r must pervade t h e whole porous c r y s t a l . Occluded molecules, especially the polar or strongly polarizable o n e s , w i l l b e p o l a r i z e d b y t h e z e o l i t e , a n d t h e y thems e l v e s w i l l e x e r t a s i m i l a r e f f e c t on t h e z e o l i t e c r y s t a l as well. T h e o v e r a l l i n t e r a c t i o n b e t w e e n z e o l i t e and a d s o r b a t e must t e n d t o m i n i m i z e t h e f r e e e n e r g y of t h e zeolite-adsorbate system. T h e r e f o r e , t h e s i m p l e s t way t o f o r e c a s t t h e r m o d y n a m i c a l l y f a v o r e d r e a c t i o n s i n zeol i t e s i s t o e v a l u a t e t h e i r e f f e c t on t h e s t a b i l i t y of the zeolite crystal.
I n s p e c t i o n of t h e i o n i c l a t t i c e ene-gy e q u a t i o n shows t h a t w i t h o r d i n a r y i o n i c c r y s t a l s t h e f r e e energy i s r e d u c e d by p l a c i n g m o r e i o n p a i r s o r i o n s o f h i g h e r According t o v a l e n c e w i t h i n t h e same c r y s t a l v o l u m e . t h i s s i m p l e r e l a t i o n s h i p , t h e o c c l u s i o n of added i o n i c m a t e r i a l i n t h e porous z e o l i t e l a t t i c e should generally l o w e r t h e l a t t i c e e n e r g y , and c o n s e q u e n t l y i n c r e a s e the O f course, thermodynamic s t a b i l i t y of t h e o v e r a l l system. t h e o c c l u s i o n o f a d d e d i o n i c m a t e r i a l may a f f e c t t h e z e o l i t e s t r u c t u r e a n d s y m m e t r y , u l t i m a t e l y a f f e c t i n g the Madelung c o n s t a n t o f t h e z e o l i t e c r y s t a l . T h i s may render t h e i n t e r a c t i o n between z e o l i t e and t h e o c c l u d e d i o n i c s p e c i e s c o m p l i c a t e d and d i f f i c u l t t o a n a l y z e . In spite of t h e s e l i m i t a t i o n s , t h e s t u d y o f i n t e r a c t i o n s between z e o l i t e s and o c c l u d e d i o n i c m a t e r i a l i s a c o n v e n i e n t t e c h n i q u e t o a s s e s s t h e r o l e of t h e z e o l i t e e l e c t r o l y t e u p o n z e o l i t e c h e m i s t r y , a n d u l t i m a t e l y on z e o l i t e catalysis.
In order to evaluate the influence of z e o l i t e s as e l e c t r o l y t e s on c a t a l y t i c phenomena, some o f t h e r e l e v a n t d a t a on s a l t o c c l u s i o n a n d r e d o x c h e m i s t r y i n z e o l i t e s w i l l be reviewed. It has been suggested t h a t the porous z e o l i t e l a t t i c e has s t r o n g a f f i n i t y f o r t h e o c c l u s i o n o f a d d e d i o n i c species (1). T h i s o c c l u d e d i o n i c m a t e r i a l may c o n s i s t of i o n i c c o m p o u n d s s u c h a s s a l t s , o r i t may c o n s i s t o f atoms o r m o l e c u l e s which a r e n o t i o n i c o u t s i d e t h e z e o l i t e , b u t become i o n i z e d u p o n o c c l u s i o n i n t o t h e z e o l i t e c r y s t a l (2). T h e r e a r e s e v e r a l e x a m p l e s of b o t h phenomena.
The o c c l u s i o n of s a l t s i n z e o l i t e s h a s been d e s c r i b e d earlier (1). The e x p e r i m e n t a l e v i d e n c e w i t h z e o l i t e s A ( 3 ) and Y c l e a r l y s h o w s t h a t i o n i c c o m p o u n d s , s u c h a s s a l t s , r e a d i l y p e n e t r a t e t h e z e o l i t e c r y s t a l , f i l l i n g up t h e available space i n large i n t r a c r y s t a l l i n e c a v i t i e s . Salts, especially s a l t s of univalent anions, can even p e n e t r a t e This is surprising the s o d a l i t e cages i n Y z e o l i t e ( 1 ) . because t h e s i z e of t h e 06-ring p o r t of t h e s o d a l i t e cage I t was i s o n l y a b o u t - 2 . 4 8 , much s m a l l e r t h a n a n a n i o n . found t h a t t h e o c c l u s i o n of s a l t s i n t o t h e s o d a l i t e c a g e s requires high temperatures i n order t o provide the high a c t i v a t i o n energy needed t o e n l a r g e t h e 06-ring p o r t , p r o b a b l y b y t h e t e m p o r a r y c l e a v a g e o f 0-A1 o r 0 - S i b o n d s . I n t e r e s t i n g l y , b o t h h a l i d e s a l t s as w e l l a s s a l t s of l a r g e c o m p l e x a n i o n s s u c h a s C103- a n d N O 3 - c a n b e o c c l u d e d i n t h e s o d a l i t e cage. From t h e p o i n t o f v i e w o f t h i s d i s c u s s i o n , t h e m o s t important a s p e c t of s a l t o c c l u s i o n i s t h e enhanced s t a b i l i t y of t h e z e o l i t e - s a l t o c c l u s i o n compound. It has been reported t h a t i n every successful s a l t occlusion experiment b o t h t h e Y z e o l i t e a n d t h e o c c l u d e d s a l t w e r e r e n d e r e d t h e r m a l l y more s t a b l e , showing d e c o m p o s i t i o n t e m p e r a t u r e s w e l l beyond t h e d e c o m p o s i t i o n t e m p e r a t u r e of t h e z e o l i t e I t was a l s o f o u n d t h a t t h e o r of t h e g u e s t s a l t i t s e l f . g u e s t s a l t o c c l u d e d i n t h e s o d a l i t e c a g e s c a n n o t b e removed f r o m t h e h o s t Y z e o l i t e c r y s t a l e v e n b y s t e a m i n g Thus t h e z e o l i t e l a t t i c e i s s t a b i l i z e d by a t 700°C ( 1 ) . t h e occluded salt and v i c e versa. The a f f i n i t y o f z e o l i t e s f o r i o n i c s p e c i e s i s b e s t shown b y t h e i o n i z a t i o n o f o c c l u d e d a t o m s a n d m o l e c u l e s . The m o s t f r e q u e n t l y o b s e r v e d i o n i z a t i o n r e a c t i o n i n z e o l i t e s i s t h e i o n i z a t i o n of w a t e r by c a t i o n h y d r o l y s i s . This is a particularly important reaction i n zeolites b e c a u s e t h i s r e a c t i o n g e n e r a t e s a c i d i c OH g r o u p s a t t a c h e d
t o framework c a t i o n s which a r e d i r e c t l y r e s p o n s i b l e f o r acidic behavior ( 4 , l T h e s e a c i d i c OH g r o u p s i n z e o l i t e s have been w e l l c h a r a c t e r i z e d by I R s p e c t r o s c o p y (5) as w e l l a s by o t h e r t e c h n i q u e s ( s e e n e x t s e c t i o n ) . Other i o n i z a t i o n e f f e c t s i n z e o l i t e l a t t i c e s have been observed by t h e o c c l u s i o n of a l k a l i m e t a l s and I t w a s r e p o r t e d t h a t a l k a l i metals certain gas molecules. r e a d i l y reduce b o t h a l k a l i X and a l k a l i Y z e o l i t e s ( 6 ) . The sodium i o n s of t h e h o s t c r y s t a l c a p t u r e t h e e l e c t r n $ from t h e occluded a l k a l i atoms forming symmetrical ~ a i o r Na;' centers in the large cavities: NaY
NaX
NaO Na
b N a ; + ~
0
,N~:+X
It has a l s o been reported t h a t c e r t a i n t r a n s i t i o n m e t a l z e o l i t e s r e a d i l y i o n i z e a d s o r b e d NO r a d i c a l s , forming e l e c t r o n t r a n s f e r complexes w i t h z e o l i t e cations (2) :
A n o t h e r i n t e r e s t i n g i o n i z a t i o n phenomenon i s t h e i n t e r a c t i o n o f N O a n d NO2 r a d i c a l s w i t h i n z e o l i t e s . H e r e , t h e s e r a d i c a l s become i o n i z e d , p r e s u m a b l y forming a s a l t - l i k e complex i n t h e z e o l i t e ( 2 ) :
The e l e c t r o l y t i c s t r e n g t h o f X a n d Y z e o l i t e s i n t h e s e r e d o x r e a c t i o n s i s b e s t d e m o n s t r a t e d by t h e f a c t t h a t these redox r e a c t i o n s a r e a l l highly endothermic in t h e gas phase outside t h e z e o l i t e c r y s t a l . F o r example, t h e f o r m a t i o n o f t h e NO+ a n d N O 2 - i o n s f r o m N O a n d NO2 r a d i c a l s i s e n d o t h e r m i c by a b o u t 5 . 5 e V ("126 k c a l / r n o l ) , n o t c o n s i d e r i n g c o u l o m b i c i n t e r a c t i o n b e t w e e n t h e products, I n s p i t e o f t h i s , t h i s r e a c t i o n r e a d i l y o c c u r s o n Nay a t moderate temperatures. S i m i l a r l y , t h e i n t e r a c t i o n between s o d i u m m e t a l a n d Nay, t h e h i g h l y e n d o t h e r m i c i o n i z a t i o n o f s o d i u m m e t a l ( I . P . = 5 . 1 4 e V , - 1 1 8 k c a l / m o l ) , i s readily a c c o m p l i s h e d a t 3 0 0 ' ~ w i t h a ~ a $ +c e n t e r f o r m e d i n e a c h large cavity.
These examples d e m o n s t r a t e w e l l t h e h i g h a f f i n i t y of zeolites f o r ionic species. They a l s o p r o v i d e i n f o r m a t i o n on t h e l a r g e c o n t r i b u t i . o n o f z e o l i t e s t o t h e i o n i z a t i o n of o c c l u d e d s p e c i e s , d i s p l a y i n g t h e c h a r a c t e r i s t i c s o f a very strong e l e c t r o l y t e . I t s h o u l d a l s o b e n o t e d t h a t seve'ral of t h e i o n i z a tion processes described above r e a d i l y occur both w i t h Na-zeolite and w i t h a v a r i e t y of c a t i o n exchanged z e o l i t e s .
Catalysis with Alkali Zeolites The i m p o r t a n t i n d u s t r i a l a p p l i c a t i o n s o f z e o l i t e c a t a l y s t s a r e based on carbenium i o n c h e m i s t r y . This c h e m i s t r y i s i n d u c e d by s t r o n g - a c i d h y d r o x y l g r o u p s i n A t present, no a c i d - f r e e , zeolites (see next section). a l k a l i c a t i o n z e o l i t e c a t a l y s t i s used i n i n d u s t r i a l l y important processes. N e v e r t h e l e s s , i t seems w o r t h w h i l e to review here t h e i n f l u e n c e of a l k a l i z e o l i t e s a s e l e c t r o l y t e s upon h y d r o c a r b o n r e a c t i o n s b e c a u s e of t h e l a r g e chemical i n f l u e n c e t h e y e x e r t upon o c c l u d e d m o l e c u l e s (see previous section). The important e f f e c t s d i s c u s s e d above i n c l u d e s t r o n g a d s o r p t i o n a n d a v e r y s t r o n g a f f i n i t y f o r i o n i c m a t e r i a l s , even i n c l u d i n g t h e i o n i z a t i o n of occluded molecules. Such a s t r o n g d i s p l a y of c h e m i c a l activity, particularly i n the ionization reactions, s u g g e s t s t h a t t h e s e phenomena a r e p r o b a b l y a l s o r e l e v a n t i n some way t o t h e c a r b e n i u m i o n t y p e c a t a l y s i s c a r r i e d In order t o evaluate the e f f e c t o u t on a c i d i c z e o l i t e s . o f t h e z e o l i t e as e l e c t r o l y t e , w i t h o u t c o n t r i b u t i o n f r o m acidic hydroxyl groups, t h e e f f e c t of a l k a l i z e o l i t e s in hydrocarbon cracking w i l l be b r i e f l y reviewed. T h e ' T n f l u e n c e o f a l k a l i z e o l i t e s (K-Y ) f r e e o f a c i d i c h y d r o x y l s o n t h e c r a c k i n g of h y d r o c a r b o n s was i n v e s t i g a t e d A t a b o u t 5 0 0 ° c t h e K-Y with hexanes a s r e a c t a n t s {7,8}. c a t a l y z e d c r a c k i n g gave c o n v e r s i o n l e v e l s up t o 5 t i m e s h i g h e r r e l a t i v e t o t h e same r e a c t o r f i l l e d w i t h q u a r t z The p r o d u c t c o m p o s i t i o n s c h i p s a t t h e same t e m p e r a t u r e . o b t a i n e d f r o m n - h e x a n e a n d i t s i s o m e r s o v e r K-Y w e r e markedly d i f f e r e n t from t h o s e o b t a i n e d o v e r a c i d i c zeolites. Significantly, they w e r e also quite different from t h e p r o d u c t o b t a i n e d i n a r e a c t o r f i l l e d w i t h q u a r t z c h i p s a t t h e same r e a c t i o n c o n d i t i o n s ( s e e T a b l e 1 ) .
TABLE 1 P r o d u c t s f r o m P y r o l y s i s a n d K-Y c a t a l y z e d a C r a c k i n g at 500'~ (mo1/100 mol converted) n-Hexane Thermal K-Y Hydrogen Me thane Ethylene Ethane Propylene Propane l-Butene 2~Butene Isobutene Isobutane l-Pentene 2-Pentene 2-Methyl-l-butene 3-Methyl-l-butene 2-Methyl-2-butene
a
Rate enhancements:
#5-fold.
2-Methylpentane Thermal K-Y -
{7},
3,-Methylpentane Thermal K-Y -
The a n a l y s i s o f t h e r e a c t i o n m e c h a n i s m b a s e d o n p r o duct d i s t r i b u t i o n s o b t a i n e d f r o m a l l h e x a n e i s o m e r s showed t h a t o v e r K-Y t h e f r e e r a d i c a l t y p e m e c h a n i s m p r e v a i l e d , b u t w i t h s p e c i f i c changes i n t h e r a t e s of c e r t a i n r e a c t i o n steps r e l a t i v e t o t h e "hot tube." A mechanistic study r e v e a l e d t h a t t h e K-Y d o e s n o t c h a n g e t h e s e l e c t i v i t y o f alkyl r a d i c a l s e i t h e r i n t h e c h o i c e between d i f f e r e n t 0-scission o p t i o n s o r between d i f f e r e n t H atom a b s t r a c t i o n s t e p s when p o s i t i o n a l l y d i f f e r e n t c h o i c e s a r e a v a i l able. H o w e v e r , t h e K-Y h a s a v e r y l a r g e e f f e c t u p o n t h e r a t i o of t h e r a t e s of H a t o m a b s t r a c t i o n s t e p s o v e r t h e @-scission steps. Mechanistic a n a l y s i s of t h e cracked products o b t a i n e d from t h e h e x a n e i s o m e r s shows t h a t o v e r K-Y t h e r a t i o o f r a t e o f H - a b s t r a c t i o n / r a t e o f 8 - s c i s s i o n i s i n c r e a s e d f o r f r e e a l k y l r a d i c a l s by a This e ffect accounts for a l l signif a c t o r of 6 t o 9 . f i c a n t d i f f e r e n c e s i n p r o d u c t f o r m a t i o n between t h e nonc a t a l y z e d a n d t h e K-Y " c a t a l y z e d " r e a c t i o n s { 7 - 9 ) . S p e c u l a t i n g o n t h e r e a s o n why z e o l i t e s e n h a n c e t h e r a t e of H - a b s t r a c t i o n o v e r $ - s c i s s i o n , i t was c o n s i d e r e d t h a t H - a b s t r a c t i o n i s a b i m o l e c u l a r r e a c t i o n s t e p whose r a t e i s s t r o n g l y i n f l u e n c e d by t h e c o n c e n t r a t i o n of t h e reactant, whereas $-scission, being a unimolecular proc e s s , i s u n a * f f e c t e d by c h a n g e s i n r e a c t a n t c o n c e n t r a t i o n . A c c o r d i n g l y , t h e " c a t a l y t i c " e f f e c t o f K-Y i s t h e r e s u l t of i n c r e a s e d r e a c t a n t c o n c e n t r a t i o n w i t h i n t h e z e o l i t e cavities r e l a t i v e t o t h e surrounding gas phase (9,lO). The c o n c e n t r a t i o n o r a d s o r p t i o n o f h y d r o c a r b o n w i t h i n z e o l i t e c r y s t a l s i s , of c o u r s e , w e l l documented i n adsorpt i o n s t u d i e s c a r r i e d o u t a t low t e m p e r a t u r e s . For t h e temperature range used i n t h e hexane cracking s t u d y , t h e h e x a n e l o a d i n g o n K-Y a t 5 0 0 ' ~ w a s e s t i m a t e d f r o m t h e r e p o r t e d A r r h e n i u m p l o t of n-hexane l o a d i n g on NaX a t t e m p e r a t u r e s u p t o 3 0 0 ~ ~ T. h e e s t i m a t e d h e x a n e l o a d i n g a t 5 0 0 ' ~ i s s u b s t a n t i a l , and i t i s c o n s i s t e n t w i t h t h e r e a c t a n t c o n c e n t r a t i o n s u g g e s t e d by t h e m e c h a n i s t i c analysis. It i s concluded from t h e hexane c r a c k i n g s t u d y t h a t zeolites concentrate hydrocarbon r e a c t a n t s within t h e zeolite crystal, that t h i s concentration is substantial, and t h a t t h i s e f f e c t i s r e s p o n s i b l e f o r t h e s u b s t a n t i a l changes i n p r o d u c t f o r m a t i o n . Methanistically, the s p e c i f i c r e s u l t of t h i s e f f e c t i s a s u b s t a n t i a l enhancement o f t h e r a t e s o f b i m o l e c u l a r r e a c t i o n s t e p s ( 9 ) . The h i g h h y d r o c a r b o n c o n c e n t r a t i o n w i t h i n Y z e o l i t e c r y s t a l s i s t h e d i r e c t r e s u l t o f t h e h i g h a f f i n i t y of this strong electrolyte fo r ionic or polar matter.
Adsorbed h y d r o c a r b o n s become p o l a r i z e d upon a d s o r p t i o n , and they are consequently s t r o n g l y h e l d i n t h e z e o l i t e crystal. S i n c e t h e c o n c e n t r a t i o n . e f f e c t of z e o l i t e s depends mainly on t h e i o n i c c h a r a c t e r and t h e r e l a t e d e l e c t r o l y t i c s t r e n g t h o f t h e c r y s t a l , i t i s o f i n t e r e s t t o corn p a r e t h e c r a c k i n g of n-hexane o v e r monacidic moleculars i e v e s o f s t r o n g a n d weak e l e c t r o l y t i c c h a r a c t e r : F o r c o n t r a s t , t h e K-Y a n d S i l l i c a l i t e m o l e c u l a r s i e v e s were c h o s e n as c a t a l y s t c a n d i d a t e s . The K-Y r e p r e s e n t s a n aluminum r i c h , v e r y s t r o n g e l e c t r o l y t e . On t h e o t h e r h a n d , S i l i c a l i t e i s s u b s t a n t i a l l y f r e e o f b o t h a l u m i n u m and z e o l i t e c a t i o n s , a n d c o n s e q u e n t l y i t i s a v e r y weak electrolyte. F i g u r e 1 shows t h e r e a c t i o n p a t h o f r a d i c a l - t y p e c r a c k i n g f o r n-hexane and i t s c r a c k e d f r a g m e n t s . The measurement of t h e c o n c e n t r a t i o n e f f e c t r e s t s upon a comparison between Reactions a 3 and a2 because t h e relat i v e r a t e s o f t h e s e r e a c t i o n s t e p s r e v e a l t h e enhancement of t h e H - t r a n s f e r r e a c t i o n s t e p o v e r 8 - s c i s s i o n , respect i v e l y , o n t h e same c r a c k e d f r a g m e n t . E x p e r i m e n t s showi n g t h e r a t e s o f R e a c t i o n s a 2 a n d a 3 f o r K-Y a n d S i l i c a I n a d d i t i o n , t h e t a b l e inl i t e a r e shown i n T a b l e 2 . c l u d e s e x p e r i m e n t a l d a t a s h o w i n g t h e e f f e c t o f a b o u t tenf o l d helium d i l u e n t a p p l i e d t o t h e n-hexane feed during t h e hexane cracking experiments. The S i l i c a l i t e used i n t h e experiment was t r e a t e d w i t h p o t a s s i u m s a l t s o l u t i o n t o remove t r a c e s o f a c i d i c hydrogen i o n s . The d a t a show t h a t t h e a p p l i c a t i o n o f h e l i u m gas d i l u e n t r e s u l t s i n a s h a r p r e d u c t i o n o f t h e r a t e o f Ha b s t r a c t i o n s t e p (a3) r e l a t i v e t o t h e 8-scission step ( a 2 ) b o t h w i t h t h e r e a c t o r f i l l e d w i t h q u a r t z c h i p s as T h i s r e s u l t i s expected, w e l l a s w i t h K-Y a n d S i l i c a l i t e . b e c a u s e t h e a d d i t i o n of d i l u e n t t o t h e r e a c t a n t r e d u c e s t h e n-hexane c o n c e n t r a t i o n , r e s u l t i n g i n l e s s frequent i n t e r a c t i o n between n-hexane molecules r e q u i r e d f o r the H-abstraction step. T h e d i f f e r e n c e b e t w e e n t h e K-Y and S i l i c a l i t e "catalysts" is very substantial. S i l i c a l i t e , i n sharp c o n t r a s t t o K-Y, performs l i k e i n e r t quartz chips. It h a s , w i t h i n e x p e r i m e n t a l e r r o r , no e f f e c t on t h e r a t e
the H-abstraction step. T h i s m o l e c u l a r s i e v e , 2n s p i t e its h i g h s u r f a c e a r e a (-400 m 2 / g ) , h a s no i n f l u e n c e on thermal c r a c k i n g of n-hexane. Considering the strongly covalent c h a r a c t e r of t h e Si-0 l i n k a g e s forming t h e s i l i c a l i t e c r y s t a l , i t i s n o t s u r p r i s i n g t h a t t h e n-hexane c r a c k i n g e x p e r i m e n t s show t h a t S i l i c a l i t e d o e s n o t d i s p l a y the p r o p e r t i e s of z e o l i t e e l e c t r o l y t e s . From t h e a b s e n c e o f s k e l e t a l i s o m e r i z a t i o n i n t h e c r a c k i n g of n-hexane o v e r K-Y, t h e a b s e n c e of i o n i c (carbenium i o n ) r e a c t i o n s can b e i n f e r r e d . Hence t h e K-Y f r e e o f OH g r o u p s i s n o t c a p a b l e o f i o n i z i n g e i t h e r hexane o r t h e r a d i c a l s formed upon i t s f r a g m e n t a t i o n . This i s somewhat s u r p r i s i n g c o n s i d e r i n g t h e f a c i l e i o n i z a t i o n of o c c l u d e d i n o r g a n i c compounds ( s e e f o r m e r s e c t i o n ) . One h a s t o r e c o g n i z e , h o w e v e r , t h a t t h e i o n i z a t i o n o f hydrocarbons r e q u i r e s v e r y high energy. For example, i o n i z a t i o n of a tertiary-C-H bond t o form a t e r t i a r y carbenium i o n a n d h y d r i d e i o n i s e n d o t h e r m i c by 240-250 kcal/mol w h i l e t h e i o n i z a t i o n p o t e n t i a l of a t e r t i a r y r a d i c a l i s -170 kcal/mol. Even i f t h e a b s e n c e of d i r e c t i o n i z a t i o n of h y d r o carbons i s w e l l demonstrated i n t h e s e experiments, it i s reasonable t o expect t h a t strong, zeolite-type e l e c t r o l y t e s e x e r t a s t r o n g i n f l u e n c e upon carbenium i o n r e a c t i o n s . F i r s t , by t h e c o n c e n t r a t i o n o f t h e h y d r o c a r b o n r e a c t a n t s i n t h e c a t a l y s t c r y s t a l , and s e c o n d , b y a i d i n g t h e f o r mation of hydrocarbon c a t i o n s and by s t a b i l i z i n g t h e s e ionic species a f t e r formation.
C h e m i s t r y a n d C a t a l y s i s W i t h H-Acid Formation of
Zeolites
Acid S i t e s
The h y d r o x y l g r o u p s w i t h v e r y a c i d i c hydrogen have been w e l l r e c o g n i z e d a s t h e s o u r c e of a c i d i c a a t a l y t i c activity i n zeolites. P r i o r t o t h e discovery of a c i d i c z e o l i t e c a t a l y s t s , t h e m o s t n o t a b l e a c i d c a t a l y s t was t h e silica-alumina gel. Here t h e s o u r c e o f a c i d i t y i s h y d r o x y l g r o u p s g l i n k e d t o aluminum o r t o s i l i c o n - a l u m i n u m s i t e s . With s i l i c a - a l u m i n a g e l t y p e c a t a l y s t s , a l l m e t a l c a t i o n s These c a t i o n s were r e c o g n i z e d as p o i s o n s f o r a c i d s i t e s . r e p l a c e d a c i d i c h y d r o x y l h y d r o g e n s and f u l l y removed a c i d activity. With t h i s background i n a c i d c a t a l y s t chemistry, i t w a s s u r p r i s i n g t h a t z e o l i t e s N a X a n d N a y , when c a t i o n exchanged w i t h b i - o r m u l t i v a l e n t m e t a l c a t i o n s , gave rise t o a c i d i c c a t a l y t i c a c t i v i t y , w e l l exceeding t h e acid s t r e n g t h of silica-alumina g e l . I t was a l s o found
t h a t N a y , w h e n N H e~ x c h a n g e d a n d a c t i v a t e d a t h i g h t e m p e r a t u r e s t o remove ammonia, f u l l y r e t a i n e d t h e c r y s t a l f r a m e w o r k o f t h e o r i g i n a l Nay, a n d i t g a v e r i s e t o v e r y strong a c i d i c c a t a l y t i c a c t i v i t y , w e l l exceeding i n 1301. s t r e n g t h a l l o x i d e - t y p e a c i d c a t a l y s t s known b e f o r I n c o n t r a s t t o t h i s m a t e r i a l , t h e NaX f o l l o w i n g N H f4 e x c h a n g e a n d h e a t t r e a t m e n t showed c o m p l e t e l o s s of c r y s t a l s t r u c t u r e , and i t d i s p l a y e d no s i g n i f i c a n t a c i d i c catalyt i c activity. These phenomena, d t s c o v e r e d i n t h e l a t e 1950s, were subsequently thoroughly i n v e s t i g a t e d and further characterized. The c a u s e of t h e g r e a t d i f f e r e n c e s i n t h e a p p l i c a t i o n of b i - o r m u l t i - v a l e n t c a t i o n s i n X and Y-type z e o l i t e s v e r s u s s i l i c a - a l u m i n a g e l was a s c r i b e d t o t h e s t r o n g I t was tendency of z e o l i t e s toward c a t i o n h y d r o l y s i s . found t h a t t h e tendency t o c a t i o n hydrolysis increases The cause w i t h t h e Si/AI r a t i o o f t h e z e o l i t e framework. of t h i s r e a c t i o n i s t h e i n c r e a s i n g d i s t a n c e between t h e a n i o n s i t e s (A104)- a t h i g h e r S i / ' A l r a t i o s . The i n c r e a s e d d i s t a n c e betweenthe s i n g l y charged anion sites renders t h e e l e c t r o s t a t i c s h i e l d i n g of h i g h l y charged c a t i o n s ineffective. I t w a s a l s o s u g g e s t e d t h a t Y z e o l i t e i s a very s t r o n g e l e c t r o l y t e , and c o n s e q u e n t l y i t f a v o r s r e a c t i o n s r e s u l t i n g i n t h e f o r m a t i o n of added i o n s i n t h e z e o l i t e Upon c a t i o n h y d r o l y s i s i n Y c r y s t a l (hydrolysis) (93. z e o l i t e , two d i s t i n c t h y d r o x y l g r o u p s a r e formed: one of them i s a t t a c h e d t o t h e z e o l i t e c a t i o n w h i l e t h e other i s l i n k e d t o t h e f r a m e w o r k c a t i o n s { l l ) . I t i s a l s o expected t h a t cation hydrolysis i n Y z e o l i t e s occurs f i r s t I t was w i t h c a t i o n s a t low c o o r d i n a t i o n s i t e s ( S i t e 2 ) . determined t h a t , following hydrolysis, t h e hydroxylated c a t i o n s s h i f t from t h e low c o o r d i n a t i o n s i t e s t o t h e With two c a t i o n s moving i n t o t h e s o d a l i t e cage E l l , 12). s o d a l i t e c a g e , a c a t i o n - o x y g e n / h y d r o x y l c l u s t e r of g r e a t s t a b i l i t y i s f o r m e d , e n h a n c i n g t h e t h e r m a l s t a b i l i t y of The c a t a l y t i c a l l y e f f e c t i v e the Y zeolite crystal. acid s i t e s i n these bi- or multivalent cation Y zeolites a r e t h e a c i d i c h y d r o x y l s l i n k e d t o framework Si-A1 s i t e s . S e v e r a l i m p o r t a n t c h a r a c t e r i s t i c s of b i - and multiv a l e n t c a t i o n Y z e o l i t e s have been i d e n t i f i e d . F o r example, t h e a b s e n c e of s t r o n g a c i d a c t i v i t y w i t h a l k a l i n e e a r t h Y u p t o a b o u t 4 5 % c a t i o n e x c h a n g e was e x p l a i n e d o n t h e b a s i s t h a t bivalent cations corresponding t o t h i s cation e x c h a n g e l e v e l o c c u p y t h e o c t a h y d r a l s i t e s i n t h e hexagonal Extensive r i n g , and t h e y do n o t undergo c a t i o n h y d r o l y s i s . i n f r a r e d s p e c t r o s c o p i c s t u d i e s o f t h e 0-H b o n d i n b i v a l e n t c a t i o n Y reveaLed an i n f l u e n c e of t h e s i z e of c a t i o n s a n d t h e S i / A l r a t i o o f t h e z e o l i t e f r a m e w o r k on
the wave number of t h e 0-H acidic h y d r o x y l s (51.
band
(-3640 c m - I )
representing
The c h e m i c a l and s t r u c t u r a l c h a n g e s o c c u r r i n g i n Y z e o l i t e s u p o n NH4 e x c h a n g e f o l l o w e d b y t h e r m o l y s i s o f the NH4 groups have been e x t e n s i v e l y i n v e s t i g a t e d {13). It i s assumed t h a t c r y s t a l r e t e n t i o n of .the a c t i v a t e d N R 4 - Y z e o l i t e v e r s u s t h e s i m i l a r l y t r e a t e d NH4-X r e s t s on t h e l a r g e r n u m b e r o f s t a b l e S i - 0 - S i l i n k a g e s {30]. The a c i d i t y h e r e , s i m i l a r t o b i l m u l t i v a l e n t c a t i o n z e o l i t e s , i s c e n t e r e d on hydroxyls a t t a c h e d t o framework cations. Infrared spectroscopic investigations revealed several hydroxyl types. One o f t h e s e c a t e g o r i e s i s r e p r e s e n t e d by s p e c i e s i n t h e l a r g e c a v i t y , g i v i n g r i s e t.0 a c i d i c b e h a v i o r i n c a t a l y s i s w h i l e o t h e r s a r e o c c l u d e d in the s o d a l i t e cage o r i n the hexagonal ring. Both t h e r m a l and s t e a m t r e a t m e n t s r e s u l t i n c h e m i c a l A t the mildest thermal changes i n t h e NH4Y z e o l i t e . treatment t h e t e t r a h e d r a l c o o r d i n a t i o n of t h e S i and A 1 a t o m s a s s o c i a t e d w i t h a c i d s i t e s may b e r e t a i n e d I 1 4 ) . In the p r e s e n c e o f nascent moisutre o r added steam, an e x t e n s i v e h y d r o l y s i s o f framework aluminum b e g i n s {15, 17-22). Upon e x t e n s i v e h y d r o l y s i s t h e a l u m i n u m i s l i n k e d with hydroxyl groups, l e a v i n g t h e z e o l i t e framework t o U l t i m a t e l y , h y d r o x y l a t e d aluminums assume c a t i o n s i t e s . form s t a b l e o x i d e / h y d r o x y l c l u s t e r s o c c u p y i n g t h e s o d a l i t e cage. The framework s i t e s v a c a t e d by aluminum a r e o c c u p i e d by s i l i c o n w h i c h i s r e n d e r e d more m o b i l e b y t h e r e m o v a l of a d j a c e n t aluminum. U l t i m a t e l y , a r e h e a l i n g of t h e c r y s t a l s t r u c t u r e occurs w i t h t h e s i l i c o n atoms f i l l i n g up t h e t e t r a h e d r a l framework s i t e s v a c a t e d by aluminum. A r e d u c t i o n o f t h e number o f u n s t a b l e d e f e c t s i t e s ( c a t i o n vacancy) a l s o t a k e s p l a c e i n o r d e r t o r e e s t a b l i s h s h o r t A s a r e s u l t of t h e s e p r o c e s s e s , range c r y s t a l c o n t i n u i t y . It empty pockets" a r e formed throughout t h e z e o l i t e c r y s t a l . The r e c r y s t a l l i z e d p a r t o f t h e Y z e o l i t e f r a m e w o r k i s now enriched i n s i l i c a , and t h e c r y s t a l i s f u r t h e r s t a b i l i z e d by t h e aluminum o x i d e / h y d r o x y l c l u s t e r s o c c u p y i n g s o d a l i t e cages.
The s u c c e s s o f " s t e a m s t a b i l i z a t i o n " d e p e n d s o n a d e l i c a t e b a l a n c e between t h e r a t e s of aluminum h y d r o l y s i s , s i l i c o n m i g r a t i o n , and s i l i c o n r e s u b s t i t u t i o n r e a c t i o n s . I n a p p r o p r i a t e r e a c t i o n c o n d i t i o n s , c a u s i n g r a p i d aluminum hydrolysis without adequate s i l i c o n substitution,readily r e s u l t i n a c o l l a p s e of t h e c r y t a l s t r u c t u r e . Conseouently, temperature, steam p r e s s u r e , and time are a l l important variables.
C h a r a c t e r i z a t i o n o f Acid S i t e s Some o f t h e c h e m i c a l a n d s t r u c t u r a l a s p e c t s o f s t e a m s t a b i l i z a t i o n remain unresolved i n s p i t e of extensive a t t e m p t s a t c h a r a c t e r i z a t i o n u s i n g m a i n l y i n f r a r e d spectroscopy and c r y s t a l l o g r a p h y . Furthermore, characteristics o f s a m p l e s p r e s u m a b l y o f t h e same t y p e p r e p a r e d b y different i n v e s t i g a t o r s , o f t e n s h o w d i f f e r e n t b e h a v i o r b e c a u s e of d i f f e r e n c e s , n o m a t t e r how s m a l l , i n t r e a t m e n t a n d i n crystal source. The m o s t i m p o r t a n t s t r u c t u r a l a s p e c t o f steams t a b i l i z e d Y z e o l i t e i s t h e f u l l r e t e n t i o n of crystallinity i n s p i t e o f t h e e x t e n s i v e i o n i c m i g r a t i o n i n v o l v e d i n the process. Crystallographic studies, particularly the s t u d y of Si(A1)-0 bond l e n g t h s , s u g g e s t t h a t p r o b a b l y t h r e e o u t o f t h e f o u r c r y s t a l l o g r a p h i c a l l y d i s t i n c t oxygen atoms p a r t i c i p a t e i n t h e f o r m a t i o n of a c i d i c hydroxyl The s i l i c o n e n r i c h m e n t i n t h e groups {12, 23, 24). c r y s t a l framework o f s t e a m - s t a b i l i z e d Y i s i n d i c a t e d by s h r i n k i n g o f t h e c r y s t a l u n i t c e l l , c o r r e s p o n d i n g t o the A s m a l l e r l e n g t h o f S i - 0 b o n d s v e r s u s A1-0 l i n k a g e s . s i m i l a r r e l a t i o n s h i p between u n i t c e l l s i z e and increasing S i / A 1 r a t i o h a s b e e n w e l l r e c o g n i z e d b e f o r e , and i t has been used t o estimate t h e Si/A1 r a t i o of t h e Y z e o l i t e With s t e a m - s t a b i l i z e d Y , t h e determinaframework ( 2 5 ) . t i o n o f t h e f r a m e w o r k S i / A 1 r a t i o b a s e d on x - r a y d i f f r a c The u n i t c e l l s i z e i s tion pattern alone i s difficult. a f f e c t e d b o t h by s i l i c o n e n r i c h m e n t o f t h e f r a m e w o r k and by t h e s t u f f i n g o f t h e s o d a l i t e c a g e w i t h aluminum oxide/ h y d r o x i d e c l u s t e r s , and t h e s e two phenomena a r e expected t o c o n t r i b u t e t o changes i n u n i t c e l l s i z e i n opposing T h e r e f o r e , t h e d e t e r m i n a t i o n of d i r e c t i o n s (1, 2 5 1 . c a t i o n e x c h a n g e c a p a c i t y r e m a i n s a more r e l i a b l e technique t o d e t e r m i n e framework aluminum c o n t e n t . The c o m p l e x i t y of t h e c h e m i s t r y and s t r u c t u r e of s t a b i l i z e d Y i s r e f l e c t e d i n t h e i n f r a r e d s p e c t r u m of 0-H b o n d s . Here, depending on t h e d e g r e e o f framework a l u m i n u m h y d r o l y s i s , s e v e r a l i n f r a r e d 0-H b a n d s a r e d e t e c t e d , r e p r e s e n t i n g b o t h " a c c e s s i b l e " and "nonaccessible" hydroxyls. Since the extinction coefficient f o r a l l t h e s e i n f r a r e d b a n d s i s n o t known, i t i s n o t p o s s i b l e t o d e t e r m i n e t h e r e l a t i v e c o n c e n t r a t i o n of h y d r o x y l s o f d i f f e r e n t 0-H i n f r a r e d f r e q u e n c y w i t h p r e cision. N e v e r t h e l e s s , a s s i g n m e n t s o f 0-H b a n d s t o a s s u m e d s t r u c t u r a l h y d r o x y l s h a v e b e e n m a d e , a n d t h e y are used f r e q u e n t l y t o monitor s t e a m s t a b i l i z a t i o n and a c i d i t y (13).
It h a s been d i s c u s s e d above t h a t t h e hydroxyls formed i n a c t i v a t e d NH4Y r e p r e s e n t s e v e r a l d i s t i n c t s p e c i e s differing i n l i n k a g e , c o o r d i n a t i o n , and a c c e s s i b i l i t y . C o r r e s p o n d i n g l y , t h e d e t e r m i n a t i o n of z e o l i t e a c i d The strength i s d i f f i c u l t by any s i n g l e measurement. u s e of c o l o r e d d y e s o f t e n u s e d w i t h o t h e r s o l i d a c i d s i s not p o s s i b l e b e c a u s e o f t h e l a r g e s i z e o f t h e s e m o l e c u l e s . T i t r a t i o n w i t h s m a l l b a s e s s u c h a s NH3 i s u s e f u l . However, one m u s t c o n s i d e r t h a t NH3 i s s t r o n g l y a t t a c h e d t o b o t h Lewis a n d B r o n s t e d a c i d s i t e s . C o n s e q u e n t l y , NH3 s o r p t i o n measures b o t h a c i d i c h y d r o x y l s and a c c e s s i b l e c a t i o n s . One o f t h e m o s t r e l i a b l e m e a s u r e m e n t s o f a c i d c o n c e n t r a t i o n r e s t s on t h e i n s p e c t i o n of t h e i n f r a r e d s p e c t r u m of pyridine adsorbed on Y c a t a l y s t s . Here, the infrared bands r e p r e s e n t i n g d i s t i n c t p r o t o n a t e d o r p o l a r i z e d pyridine s p e c i e s g i v e q u a n t i t a t i v e account of Brons t e d a n d L e w i s a c i d i t y C5, 1 3 ) . Steric hindrance introduced onto t h e p y r i d i n e by m e t h y l - o r e t h y l - s u b s t i t u t i o n s a t a p p r o p r i a t e p o s i t i o n s g i v e s f u r t h e r i n f o x m a t i o n o n OH accessibility i n the zeolite structure.
One o f t h e s h o r t c o m i n g s o f N - b a s e t i t r a t i o n r e s u l t s from t h e h i g h e n e r g y o f i n t e r a c t i o n b e t w e e n s t r o n g b a s e s and z e o l i t e a c i d s i t e s , r e n d e r i n g t h e i n t e r a c t i o n a l m o s t i r r e v e r s i b l e a t low t e m p e r a t u r e s a n d p r e v e n t i n g e q u i l i b r a tion a m o n g . s i t e s of v a r i o u s a c i d s t r e n g t h . A g o o d "compromise" t e c h n i q u e u s e s t h e r m o m e t r i c t i t r a t i o n ( 2 6 - 2 8 ) with a m i n e s a p p l i e d i n a s o l v e n t . Here an aromatic solvenz serves t o e x p e d i t e t h e d e s o r p t i o n and t h e e q u i l i b r a t i o n of t h e amine a p p l i e d . This r e s u l t s i n aiding equilibration, f i r s t n e u t r a l i z i n g t h e s t r o n g e s t and t h e n g r a d u a l l y the weaker a c i d s i t e s . Unfortunately, even t h i s technique has s t e r i c o r d i f f u s i o n - r e l a t e d l i m i t a t i o n s , a n d r e l i a b l e data a r e obtained only w i t h c r y s t a l s w i t h three-dimens i o n a l p o r e s y s t e m s (Y z e o l i t e ) . These t i t r a t i o n s a r e much l e s s s u c c e s s f u l w i t h z e o l i t e s o f l e s s o p e n s t r u c t u r e (mordenite) (29). A s t r o n g enhancement i n c r y s t a l s t a b i l i t y and a c i d activity with increasing Si/A1 r a t i o w a s recognized i n the e a r l y d a y s of z e o l i t e c a t a l y s i s , u s i n g z e o l i t e s X a n d Y (30). I t was shown t h a t b a s e d on e l e c t r o s t a t i c c o n s i d e r a tions, the charge density at a cation s i t e increases with I t was c o n c e i v e d t h a t t h e s e increasing Si/A1 r a t i o . phenomena a r e r e l a t e d t o a r e d u c t i o n o f e l e c t r o s t a t i c i n t e r a c t i o n between framework s i t e s , and p o s s i b l y t o a d i f f e r e n c e i n o r d e r i n g of aluminum i n t h e z e o l i t e c r y s t a l (31). Later, using t i t r a t i o n s with various amines, i t was f o u n d t h a t t h e s l o p e o f t h e t i t r a t i o n c u r v e o f NaH-Y This slope zeolites i s l i n e a r with c a t i o n exchange {32}.
changes w i t h t h e S i / A 1 r a t i o , and i t r e f l e c t s t h e degree Higher o f " e f f i c i e n c y " of t h e a v e r a g e (A1O4)- s i t e { 3 3 ) . a l u m i n u m c o n t e n t s r e s u l t i n " i n t e r f e r e n c e " b e t w e e n alumina s i t e s , and t h u s i n l e s s t h a n a l i n e a r i n c r e a s e of t i t r a t a b l e strong a c i d i t y (33). Conversely, lowering the alumina c o n c e n t r a t i o n i n t h e Y z e o l i t e framework f i r s t r e s u l t s o n l y i n a d e p l e t i o n of t h e w e a k e r a c i d s i t e s w i t h o u t a f f e c t i n g t h e s t r o n g a c i d s i t e s (>90X H2S04) 1 3 4 ) . Only e x t e n s i v e framework aluminum h y d r o l y s i s , r e s u l t i n g i n l e s s t h a n 1 6 A 1 p e r u n i t c e l l , r e s u l t s i n a + d e c l i n e of s t r o n g a c i d s i t e s {13}. The d i r e c t r e l a t i o n s h i p b e t w e e n a c i d s t r e n g t h and S i / A 1 r a t i o s e e m s t o a p p l y i n some cases e v e n f o r zeol i t e of d i f f e r e n t s t r u c t u r e . I t was r e p o r t e d t h a t t h e a c i d s i t e s p r e s e n t i n H-ZSM-5 (Si/Al = 1 9 . 2 ) a r e stronger In a t h a n t h o s e p r e s e n t i n H-zeolon ( S i / A l = 5) {351. d i f f e r e n t a p p r o a c h i t was found t h a t , u s i n g a n e l e c t r o n e g a t i v i t y model f o r c a l c u l a t i n g t h e r e s i d u a l charge for hydrogen atoms i n z e o l i t e s , t h e a c i d s t r e n g t h d e c l i n e s w i t h i n c r e a s i n g aluminum c o n t e n t 1 3 6 ) . Catalytic data c o n s i s t e n t w i t h t h e s e c a l c u l a t i o n s confirm t h e chemical tendency s u g g e s t e d by t h e s e c a l c u l a t i o n s ( 3 7 1 . C a t a l y t i c P r o p e r t i e s of
Acid Z e o l i t e s
The main c a t a l y t i c c h a r a c t e r i s t i c s of a c i d zeol i t e s i n hydrocarbon r e a c t i o n s a r e t h e i r s e l e c t i v i t y a s m o l e c u l a r s i e v e s , t h e i r h i g h a c i d s t r e n g t h , and t h e i r e l e c t r o l y t i c p r o p e r t i e s which i n f l u e n c e reaction kinetics. 1.
Molecular Sieve Effect.
Shape S e l e c t i v i t y
I n many a c i d - c a t a l y z e d r e a c t i o n s t h e r e a c t i o n r a t e depends on t h e r a t e of carbenium i o n f o r m a t i o n . Normally, t e r t i a r y c a r b o n s f o r m c a r b e n i u m i o n s e a s i e r t h a n secondary carbons. Primary carbons do n o t form carbenium ions r e a d i l y under m i l d c o n d i t i o n s , and c o n s e q u e n t l y tend t o be unreactive. The d i f f e r e n c e s i n t h e e a s e o f i o n form a t i o n a r e r e a d i l y e x p l a i n e d by s i m i l a r d i f f e r e n c e s i n t h e e n e r g y r e q u i r e d f o r t h e r e m o v a l o f H- i o n f r o m C-H bonds formed w i t h t e r t i a r y , s e c o n d a r y , p r i m a r y carbon atoms. F o r t h i s r e a s o n , i s o p a r a f f i n s a r e n o r m a l l y much more r e a c t i v e t h a n n - p a r a f f i n s . The o r d e r o f t h i s welle s t a b l i s h e d r e a c t i v i t y f o r p a r a f f i n s i s , h o w e v e r , often r e v e r s e d i n c e r t a i n z e o l i t e s as a r e s u l t o f s t e r i c h i n d e r a n c e o r d i f f u s i o n l i m i t a t i o n i m p o s e d b y t h e zeolite structure. E f f e c t s o f t h i s t y p e a r e o f t e n referred t o as s h a p e s e l e c t i v i t y ( 3 8 1 .
With z e o l i t e s c o n t a i n i n g p o r e s j u s t l a r g e enough t o admit s m a l l , m o n o - s u b s t i t u t e d p a r a f f i n s , t h e r a t e o f d i f f u s i o n f o r t h e i s o p a r a f f i n s i s v e r y s u b s t a n t i a l l y less than t h e r a t e of d i f f u s i o n of n-paraf f i n s . Additionally, the d i f f u s i o n r a t e of n - p a r a f f i n s can d i f f e r by s e v e r a l orders of magnitude between s m a l l p o r e z e o l i t e s and t h e large pore z e o l i t e Y. Changes i n t h e d i f f u s i o n r a t e by several o r d e r s of magnitude can r e a d i l y reverse t h e o r d e r of r e a c t i o n s e l e c t i v i t y , r e s u l t i n g i n h i g h e r r e a c t i o n rates f o r t h e molecule of smaller c r i t i c a l dimension { 3 9 ) . In t h e c a s e of d i f f u s i o n - c o n t r o l l e d s e l e c t i v i t y , t h e s i z e of t h e c r y s t a l a l s o b e c o m e s a n i m p o r t a n t s e l e c t i v i t y parameter. Molecular s i e v e e f f e c t s i n c a t a l y s i s can be d i s t i n guished w i t h r e s p e c t t o whether t h e z e o l i t e p o r e s i z e prevents t h e p a s s a g e of the r e a c t a n t o r t h e p r o d u c t through t h e z e o l i t e p o r e s , o r w h e t h e r t h e f o r m a t i o n of a p a r ticular transition s t a t e required f o r product formation is prevented. T h e r e a r e c o n v i n c i n g e x a m p l e s f o r e a c h of T h e r e a r e many e x a m p l e s o f m o l e c u these c a t e g o r i e s { 3 9 ) . lar s i e v e effects-shape s e l e c t i v i t y - u s i n g mixtures of small and l a r g e r o l e f i n s o r a l c o h o l s , w i t h t h e s m a l l e r molecules r e a c t i n g w h i l e t h e l a r g e r ones remain i n t a c t { 4 0 ) . Molecular s i e v e e f f e c t s showing product-size s e l e c t i v i t y have been demonstrated i n t h e isomerization of n - p a r a f f i n s w i t h small-pore t y p e a c i d i c z e o l i t e s . Here, i n s p i t e of t h e p r e s e n c e of a c i d s i t e s , no i s o p a r a f f i n s are r e l e a s e d i n t h e product, and t h e product c o n t a i n s only a m i x t u r e of methane and e t h a n e i n a d d i t i o n t o t h e n-hexane r e a c t a n t . Here t h e s m a l l z e o l i t e pores prevent t h e d e s o r p t i o n of i s o p a r a f f i n s whkch a r e p r e s u m a b l y f o r m e d i n t h e l a r g e c a v i t i e s of t h e z e o l i t e c r y s t a l . An i n t e r e s t i n g e x a m p l e o f p r o d u c t - s i z e t y p e s e l e c t i v i t y i s shown i n t h e i s o m e r i z a t i b n of n-hexane For t h i s with t h e medium p o r e s i l i c a l i t a ( 4 3 2 . e x p e r i m e n t t h e s i l i c a l i t e was s y n t h e s i z e d w i t h a s m a l l aluminum i m p u r i t y ( c a l c u l a t e d as 0 . 6 w t . % a l u m i n a ) . Following t r e a t m e n t w i t h a n NH4 s a l t s o l u t i o n and t h e r m a l a c t i v a t i o n , t h e silfc a l i t e c o m p o s i t i o n s h o w e d a m o d e s t d e g r e e of a c i d i t y , a s shown i n T a b l e 3 .
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From t h e d i f f e r e n c e i n r e a c t i o n t e m p e r a t u r e r e q u i r e d t o reach s i m i l a r conversions f o r n-hexane, i t i s c l e a r that t h e a c i d i t y of t h e S i l i c a l i t e sample i s v e r y i n f e r i o r Nevertheless, a t higher r e l a t i v e t o s t e a m - s t a b i l i z e d Y. temperatures t h e S i l i c a l i t e r e a d i l y produced a mixture of methylpentanes, c l o s e t o t h e e q u i l i b r i u m composition. However, i n s h a r p c o n t r a s t t o t h e s t e a m - s t a b i l i z e d Y c a t a l y s t , t h e S i l i c a l i t e produced no s i g n i f i c a n t amounts of 2,2-dfmethylbutane. The a b s e n c e o f t h e r e l a t i v e l y large s i z e 2,2-dimethylbutane i n t h e product obtained with S i l i c a l i t e i s c o n s i s t e n t w i t h i n d e p e n d e n t a d s o r p tion experiments, showing t h a t s i l i c a l i t e does n o t adsorb t h e s i m i l a r s i z e n e o p e n t a n e m o l e c u l e . Other examples of molecular s i e v e e f f e c t s are r e In this case, lated t o t r a n s i t i o n s t a t e s e l e c t i v i t y . certain reactions a r e prevented because the transitions t a t e complex i s t o o l a r g e f o r t h e p o r e o r p o r e - c a v i t y system of a z e o l i t e . I n some of t h e s e c a s e s , n e i t h e r the r e a c t a n t s nor t h e products a r e prevented from passing t h r o u g h t h e z e o l i t e p o r e s . Examples of s u c h t r a n s i tion s t a t e s e l e c t i v i t y a r e displayed i n t h e transa l k y l a t i o n and c e r t a i n i s o m e r i z a t i o n r e a c t i o n s of a l k y l benzenes ( 3 9 , 4 1 ) . 2.
The C r a c k i n g of H y d r o c a r b o n s Zeolites
Over A c i d i c Y
T h e c r a c k i n g o f n - h e x a n e o v e r H-Y z e o l i t e s h o w s markedly d i f f e r e n t b e h a v i o r { 4 2 ) from t h a t shown by a l k a l i c a t i o n z e o l i t e s C7). Here c a t a l y t i c a c t i v i t y i s much h i g h e r , s k e l e t a l i s o m e r i z a t i o n o f f r a g m e n t s becomes d o m i n a n t , a n d t h e p r o d u c t d i s t r i b u t i o n i s generally consistent with t h a t occurring with other strong Bronsted a c i d c a t a l y s t s f o r which a carbenium ion r a t h e r t h a n f r e e r a d i c a l mechanism i s g e n e r a l l y accepted. Significantly, the product contains a relat i v e l y low c o n c e n t r a t i o n o f o l e f i n s , and i t a l s o cont a i n s s i g n i f i a a n t amounts of a r o m a t i c s .
The i s o m e r i z a t i o n o f t h e h e x a n e i s o m e r s was s t u d i e d a t low c o n v e r s i o n l e v e l s u s i n g a p a l l a d i u m - l o a d e d zeolite catalyst. H e r e , t h e primary products derived from the i n d i v i d u a l hexane isomers cannot be explained i n terms of an i n t r a m o l e c u l a r rearrangement of a carbenium It w a s observed t h a t isomerization ion intermediate. i s i n v a r i a b l y accompanied by h y d r o c r a c k i n g , b u t t h e hydrocracked porducts a r e n o t c o n s i s t e n t w i t h a simple A b i m o l e c u l a r mechanism cleavage of t h e hexane molecules. was p r o p o s e d w h i c h s a t i s f a c t o r i l y e x p l a i n s b o t h t h e
observed p r o d u c t s from b o t h isornerization and hydrocracking r e a c t i o n s as w e l l as t h e presence of heptanes i n the product { 4 9 ) . In the industrial c a t a l y t i c cracking process, the a c i d i c Y z e o l i t e - b a s e d c a t a l y s t s show c e r t a i n new f e a t u r e s n o t found w i t h t h e e a r l i e r used a c i d c a t a l y s t s ( 4 4 , 45). F i r s t , t h e a c t i v i t y o f Y z e o l i t e - t y p e c a t a l y s t s i s much higher relative to silica-alumina gel. In addition, z e o l i t e c a t a l y s t s produce s u b s t a n t i a l l y l a r g e r gasoline yields. The c o m p o s i t i o n of t h e g a s o l i n e produced over Y z e o l i t e s c o n t a i n s much l e s s o l e f i n s a n d s u b s t a n t i a l l y m o r e a r o m a t i c s r e l a t i v e t o t h e g a s o l i n e made w i t h s i l i c a alumina g e l (see Table 4 ) . M e c h a n i s t i c s u g g e s t i o n s by Weisz ( 4 6 ) r e g a r d i n g t h e s e i n t e r r e l a t e d d i f f e r e n c e s i n g a s o l i n e y i e l d a n d c o m p o s i t i o n h a v e b e e n made t o t h e e f f e c t t h a t t h e r e i s a more e f f i c i e n t hydrogen r e d i s t r i bution between hydrocarbon molecules over t h e z e o l i t e catalyst. A c c o r d i n g t o t h e g a s o l i n e s e l e c t i v i t y m o d e l of Weekman a n d N a c e ( 4 7 1 , t h e p r i m a r y c r a c k i n g r e a c t i o n p r o d u c e s g a s o l i n e a n d g a s , w h e r e a s the s e c o n d a r y c r a c k i n g r e a c t i o n produces o n l y g a s , and t h u s lower g a s o l i n e yield. Gas
oil
>-
primary Gasoline
The d i f f e r e n c e i n product d i s t r i b u t i o n between z e o l i t e and s i l i c a - a l u m i n a can b e e x p l a i n e d q u a n t i t a t i v e l y by t h e g r e a t e r o c c u r r e n c e o f t h e f o l l o w i n g o v e r a l l hydrogen redistribution reaction in the zeolite: Olefins
+
n a p h t h e n e s ~ pj a r a f f i n s
+
aromatics
I t was s u g g e s t e d 1 4 6 ) t h a t t h i s r e a c t i o n e f f e c t i v e l y r e d u c e s t h e s e c o n d a r y c r a c k i n g i n t h e c r a c k i n g o f gas o i l by c o n v e r t i n g t h e i n i t i a l l y produced o l e f i n s and n a p h t h e n e s t o more r e f r a c t o r y p a r a f f i n s and a r o m a t i c s before they crack f u r t h e r t o gas. The h i g h e f f i c i e n c y of t h e c o n v e r s i o n of o l e f i n s and naphthenes t o paraffins a n d a r o m a t i c s may b e e x p l a i n e d b y t h e s u p e r i o r h y d r o g e n r e d i s t r i b u t i o n v i a h y d r i d e i o n s h i f t between carbenium i o n s and n e u t r a l hydrocarbons over t h e z e o l i t e catalyst,
It i s apparent from t h e information presented here t h a t t h e c r a c k i n g o f h y d r o c a r b o n s o v e r K-Y a n d a c i d i c Y z e o l i t e s s h o w a n i m p o r t a n t common f e a t u r e . I n b o t h cases t h e e f f i c i e n c y of t h e hydrogen t r a n s f e r s t e p between r e a c t a n t m o l e c u l e s i s g r e a t l y e n h a n c e d by b o t h t y p e s of I n t h e K-Y c a t a l y z e d c r a c k i n g o f zeolite catalysts. n-hexane, t h i s e f f e c t r e s u l t e d i n a s u b s t a n t i a l increase i n t h e f o r m a t i o n of p r o p a n e and a s i m i l a r l y s u b s t a n t i a l d e c r e a s e i n t h e p r o d u c t i o n o f f r a g m e n t s s m a l l e r t h a n C3. I n t h e c a s e of gas o i l cracking o v e r a c i d i c Y z e o l i t e b a s e d c a t a l y s t s , a n e x t e n s i v e r e d i s t r i b u t i o n of hydrogen i s achieved t o t h e e x t e n t t h a t hydrogen i s t r a n s f e r r e d from naphthenes t o o l e f i n s forming p a r a f f i n s and aromatics. T h i s o v e r a l l r e a c t i o n , c a t a l y z e d by a c i d i c Y z e o l i t e s , f o l l o w s t h e thermodynamics p r e v a i l i n g a t t h e catalytic cracking process conditions. O n e o f t h e new c h e m i c a l f a c t o r s i n t r o d u c e d by z e o l i t e c a t a l y s t s i s t h a t t h e y a r e a b l e t o c a r r y o u t b o t h h y d r o g e n a t i o n a n d dehydrog e n a t i o n r e a c t i o n s e f f i c i e n t l y v i a m u l t i p l e h y d r i d e transfer steps. T h i s phenomenon i s q u i t e s z l m i l a r t o t h e enhancement of t h e hydrogen t r a n s f e r s t e p s found w i t h the n o n a c i d i c K-Y.
S i m i l a r d i f f e r e n c e s i n p r o d u c t f o r m a t i o n were rep o r t e d r e c e n t l y i n t h e d i r e c t s y n t h e s i s of hydrocarbons f r o m s y n g a s o v e r two m o l e c u l a r s i e v e c a t a l y s t s ( 4 8 ) . One o f t h e c a t a l y s t s c o n s i s t e d o f a m i x t u r e o f i r o n m e t a l z e o l i t e w h i l e t h e o t h e r c o n s i s t e d o f a mixture a n d H-ZSM-5 o f a s i m i l a r f r a c t i o n o f i r o n m e t a l a n d S i l i c a l i t e . By comparison w i t h a p u r e i r o n c a t a l y s t , b o t h molecular-sievebased c a t a l y s t s s u b s t a n t i a l l y reduced t h e b o i l i n g range T h e H-ZSM-5 c a t a l y s t d i s of t h e hydrocarbon product. p l a y e d b o t h h i g h e r a c t i v i t y and h i g h e r b o i l i n g - r a n g e selectivity than the silicalite-based catalyst. I n spite o f t h e s i m i l a r i n f l u e n c e o f b o t h ZSM-5 a n d ~ i l i c a l i t eon p r o d u c t b o i l i n g r a n g e , t h e two c a t a l y s t s p r o d u c e d hydroc a r b o n s of e n t i r e l y d i f f e r e n t c o m p o s i t i o n s . The i r o n H-ZSM-5 c a t a l y s t produced hydrocarbons r i c h i n aromatics and v e r y low i n o l e f i n s w h i l e t h e i r o n - s i l i c a l i t e catalyst produced m a i n l y o l e f i n s and o n l y minor amounts of a r o matics. The d e t e r m i n a t i o n as t o w h a t e x t e n t t h e a c i d strength, c o n c e n t r a t i o n , and e l e c t r o l y t i c s t r e n g t h a r e responsible f o r t h e c o n t r a s t i n product compositions is not possible without establishing the i n t r i n s i c acid strength, acid c o n c e n t r a t i o n , and e l e c t r o l y t e s t r e n g t h i n each c a t a l y s t . However, t h e s i m i l a r i t y w i t h t h e c r a c k i n g d a t a discussed above f o r t h e s t r o n g e l e c t r o l y t e Y z e o l i t e v e r s u s s i l i c a l i t e s u g g e s t s t h a t i n t h e h y d r o c a r b o n s y n t h e s i s experiments
t h e H-ZSM-5 p l a y s t h e r o l e o f a s t r o n g a c i d a s w e l l as a relatively stronger electrolyte. The e x p e r i m e n t a l e v i d e n c e s t r o n g l y s u g g e s t s t h a t the cause of t h e hydrogen t r a n s f e r enhancement w i t h both K-Y a n d w i t h a c i d i c Y z e o l i t e s i s common, b o t h r e l a t e d t o t h e h i g h c o n c e n t r a t i o n of r e a c t a n t hydrocarbons i n t h e zeolite c r y s t a l r e l a t i v e t o the surrounding gas phase. The h i g h c o n c e n t r a t i o n o f reactant m o l e c u l e s e n h a n c e s t h e r a t e of b i m o l e c u l a r r e a c t i o n s t e p s - t h e hydrogen t r a n s f e r s t e p - w i t h b o t h K-Y a s w e l l a s w i t h a c i d i c Y , o v e r t h e unimolecular cracking s t e p . The h i g h e r c o n c e n t r a t i o n of r e a c t a n t s , p e r s i s t i n g even a t h i g h r e a c t i o n temperat u r e s , i s t h e r e s u l t of s t r o n g i n t e r a c t i o n between t h e p o l a r i z a b l e hydrocarbons and t h e s t r o n g l y p o l a r i n t r a crystalline zeolite surface. The d e g r e e o f t h i s i n t e r action r e f l e c t s t h e strength of t h e z e o l i t e e l e c t r o l y t e .
CONCLUSIONS
A l a r g e body of e x p e r i m e n t a l e v i d e n c e i n z e o l i t e chemistry and z e o l i t e c a t a l y s i s s u g g e s t s t h a t t h e key distinguishing. f e a t u r e s of z e o l i t e c a t a l y s t s used i n i n d u s t r i a l a p p l i c a t i o n s are:
1.
The c a t a l y t i c s e l e c t i v i t y b a s e d on m o l e c u l a r s i e v e e f f e c t s o r on d i f f u s i o n l i m i t a t i o n s .
2.
The h i g h c o n c e n t r a t i o n of s t r o n g l y i o n i c h y d r o g e n (H+) a t o m s a t t a c h e d t o f r a m e w o r k oxygen atoms.
3.
The l a r g e enhancement of a n d the s t a b i l i z a t i o n of
4.
The h i g h c o n c e n t r a t i o n of hydrocarbon r e a c t a n t s within zeolite crystals, resulting in the enhancement of b i m o l e c u l a r r e a c t i o n s t e p s o v e r unimolecular reaction steps.
ionization reactions carbenium ions.
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1 ~
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(13
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10,
-
18,
A C I D I C CATALYSIS WITH ZEOLITES
Deni se B a r t homeuf Exxon Research and E n g i n e e r i n g Company Linden, New Jersey 07036
1.
Uses o f Z e o l i t e s a s A c i d i c C a t a l y s i s
A f t e r t h e work o f B a r r e r , z e o l i t e s have been used a s s e l e c t i v e adsorbent s. The d i scovery o f t h e i r c a t a l y t i c p r o p e r t i e s i n 1960 s t a r t e d a new e r a i n t h e f i e l d o f c a t a l y s i s . (1,2) Less t h a n t e n y e a r s l a t e r , 90% o f c a t a l y t i c A cracking u n i t s i n t h e U n i t e d S t a t e s used z e o l i t e s . continuous improvement of zeolites properties led t o a simultaneous i n n o v a t i o n and cornpetiton i n u n i t s design. A t t h e present t i m e t h e f o u r main i n d u s t r i a l a p p l i c a t i o n s o f z e o l i t e s The most i m p o r t a n t i s i n v o l ve t hei r a c i d i c p r o p e r t i e s . c a t a l y t i c c r a c k i n g which uses z e o l i t e s t h e r m a l l y s t a b i 1 i z e d by v a r i o u s treatments, The h y d r o c r a c k i ng process superimposes t h e cracking due t o t h e a c i d p r o p e r t i e s o f t h e z e o l i t e w i t h t h e hydrogenation due t o n o b l e metal. The selectoforrning adds a shape s e l e c t i v e e f f e c t o f t h e z e o l i t e cages t o t h e hydrocracking by t h e c h o i c e o f z e o l i t e s ( e r i o n i t e , o f f r e t i t e ) w h i c h s e l e c t i v e l y c o n v e r t o n l y C 5 t o Cg n - p a r a f f i n s from naphthas and reforrnates. ( 3 ) O i 1 dewaxi ng a1 so uses a shape s e l e c t i v e c a t a l y s t f o r t h e hydrocracking o f n-paraffins. Resides t h o s e i n d u s t r i a l uses, z e o l i t e s a r e a1 so very good c a t a l y s t s f o r other r e a c t i o n s i n v o l v i n g a c i d i c c a t a l y s i s such a s isomerization, hydration-dehydration, a l k y l ation, is 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 o f a1k y l benzene, etc. Z e o l i t e s have been a b l e t o r e p l a c e o l d c a t a l y s t s as i n c a t a l y t i c c r a c k i n g o r generate novel a p p l i c a t i o n s a s in s e l e c t o f o r m i n g o r o i l dewaxing m a i n l y because o f t h e i r very high s e l e c t i v i t y . T h i s i s s t r o n g l y r e l a t e d t o t h e i r open s t r u c t u r e which may induce several speci f i c and d i f f e r e n t t y p e s
o f selectivity. The shape s e l e c t i v e c a t a l y s i s known i n ZSM-5 o r m o r d e n i t e i s d i s c u s s e d i n d e t a i l s elsewhere i n t h i s course. A s e l e c t i v i t y r e l a t e d t o chemical e f f e c t s o f t h e cages i s involved i n z e o l i t e s w i t h l a r g e r cavities. Zeolites give a g r e a t e r amount of p a r a f f i n s and a r o m a t i c s t h a n amorphous s i 1 i c a alumina c a t a l y s t s i n i n d u s t r i a l c r a c k i n g . T h i s i s e x p l a i n e d by an o v e r a l l hydrogen r e d i s t r i b u t i on r e a c t i o n i n t h e z e o l i t e . ( 4 ) Scheme 1 ( f r o m Ref. 4) Saturates
+
Saturates
+ 01 e f i n s
>-
Secondary P r o d u c t s
f
nap h t hene s haturates
+
Aromatics
T h i s p r o d u c t d i s t r i b u t i o n l e a d s t o h i g h gas01 i n e y i e l d s and low coke f o r m a t i o n . Usual ly, t o t a k e advantage o f those p r o p e r t i e s , z e o l i t e s a r e mixed w i t h a m a t r i x i n a p r o p o r t i o n c l o s e t o 15%. T h i s o p t i m i z e s t h e p e r c e n t c o n v e r s i o n and t h e g a s o l i n e and coke formations. I t a l so g i v e s t o t h e c a t a l y s t o t h e r i m p o r t a n t p r o p e r t i e s such a s r e s i stance t o a t t r i t i o n , l o w e r c o s t , improved heat t r a n s f e r performances.
..
2.
Main Parameters Which Determine C a t a l y t i c P r o p e r t i e s i n A c i d i c Zeol i t e s
conversion, selectivity) The c a t a l y t i c p r o p e r t i e s ( depend on z e o l it e s t r u c t u r e , chemi c a l c o m p o s i t i o n and v a r i o u s p r e t r e a t m e n t s w h i c h may change l o c a l l y t h e c o m p o s i t i o n o r atom location i n the zeolites.
2.1
A1 Content ( S i / A l
Ratio)
The A1 c o n t e n t o f a z e o l i t e may be expressed a s t h e r a t i o A l / A l + S i (molar f r a c t i o n o f A104 t e t r a h e d r a i n t h e t o t a l number S i ce the o f A104 and SiO t e t r a h e d r a ) o r a s t h e r a t i o S i / A l . atorn a c i d i t y a r i s e s r m t h e replacement o f a t e t r a v a l e n t S i " by a t r i v a l e n t A l g + atom i n t h e a l u m i n o - s i l i c a t e framework, the number o f a c i d s i t e s generated should p a r a l l e l v e r y s t r o n g l y Any change i n S i / A l r a t i o should t h e n be t h e A1 c o n t e n t . reflected i n c a t a l y t i c properties. I n f a c t an i n c r e a s e i n the S i /A1 r a t i o improves several z e o l it e p r o p e r t i e s . I t increa ses
4
the conversion o f hydrocarbons, a c i d i t y strength.
t h e thermal
s t a b i l i t y and t h e
With regards t o c a t a l y t i c a c t i v i t y , i n t h e f a u j a s i t e series t h e X z e o l i t e s (Si/A1 = 1.25, A l / A l + S i = 0.44) a r e l e s s active than Y t y p e s ( S i / A l = 2.4, A l / A l + S i = 0.29) d e s p i t e Similar t h e i r h i g h e r t h e o r e t i c a l number o f a c i d sites.(1,5-7) kinds o f i n c r e a s e w i t h t h e Si/A1 r a t i o a r e o b t a i n e d i n cumene cracking, (5,7-12) o-xylene isomerization, ( 6 ) gas oi1 cracki n g ( l 3 ) o r cyclopentane isomerization. (14) They can be explained by a h i g h e r e l e c t r o s t a t i c f i e l d i n Y than i n X z e o l i t e s ( 1 5 ) o r by an i n c r e a s e i n t h e s o - c a l l e d e f f i c i e n c y o f By c o n t r a s t reverse r e s u l t s a r e acid s i t e s from X t o Y. (16) obtained i n t h e low A1 range: the c a t a l y t i c a c t i v i t y increases, a s one c o u l d expect, w i t h t h e A1 content. Recent r e s u l t s have been p u b l i shed f o r ZSM-5 z e o l i t e i n a very l a r g e range o f composition a t a low A1 l e v e l r a n g i n g from 10 t o 10,ODO ppm. (17,18) A 1 inear re1a t i o n s h i p is observed between the i n c r e a s e i n c a t a l y t i c a c t i v i t y f o r n-hexane c r a c k i n g and the A1 content, For a1 1 t h e t y p e s o f r e a c t i o n mentioned above which a l l need s t r o n g a c i d s i t e s , one can t h e n expect a maximum i n t h e c a t a l y t i c p r o p e r t i e s f o r i n t e r m e d i a t e A1 contents. Values o f Si/A1 between 4 and 8 have been found f o r f a u j a s i t e structure (Figure 1)(7-14) o r between 8 and 25 f o r mordenites. (l9,20) F o r r e a c t i o n s which i n v o l v e o n l y weak a c i d s i t e s such a s isopropanol d e h y d r a t i o n a l i n e a r r e l a t i o n s h i p i s found i n a wide range o f composition(21,22) f o r v e r y d i f f e r e n t z e o l i t e s s t r u c t u r e s when c o n s i d e r i n g t h e t u r n - o v e r number versus t h e Sanderson e l e c t r o n e g a t i v i t y which r e f l e c t s a c i d i t y strength. A s i m i l a r r e l a t i o n s h i p is a1 so observed f o r t h e hydroconversion o f n-decane. (22) The Sander son e l e c t r o n e g a t i v i t y o f zeol it e s discussed ! a t e r i n d e t a i 1 s i s very h e l p f u l t o p r e d i c t c a t a l y t i c p r o p e r t i e s i n such r e a c t i o n s . It i s nevertheless not able t o depict c a t a l y t i c p r o p e r t i e s a t very low A1 c o n t e n t i n t h e range where l o c a t i o n o f s i t e s and m a l l changes i n t h e i r number a r e o f prime importance.
O f very g r e a t i n d u s t r i a l importance a r e t h e u l t r a stab1 e i n which t h e S i / A l r a t i o i s i n c r e a s e d by z e o l i t e s (U.S.) special t r e a t m e n t i n o r d e r t o remove A1 atoms from t h e framework. T h i s can be done e i t h e r by chemical e x t r a c t i o n w i t h c h e l a t i n g agents such a s EDTA o r acetylacetone(8,23-25) o r by procedures invol v i ng steaming o f ammoni um zeol it e s forms. (26) These z e o l i t e m o d i f i c a t i o n s l e a d t o a u n i t c e l l shrinkage and a considerable i n c r e a s e i n s t a b i l i t y ( 2 6 , 2 7 ) which i s o f m a j o r improvement f o r p r a c t i c a l use. No s i g n i f i c a n t improvements i n c a t a l y t i c a c t i v i t y o r s e l e c t i v i t y have been described. A
FIGURE 1: Change in cumene cracking as a function of Si/AI in faujasites (a) ~ a form ~ +(8-12) (b) ca2+ form (8-12) (c) H+ form (7-12)
r e 1a t i v e l y 1ow amount o f fundamental r e s u l t s has been r e p o r t e d z e o l i t e s Y a s opposed t o t h e usual n o t s t a b i l i z e d on those U.S. Y z e o l i t e s o f l e s s p r a c t i c a l value. A good thermal s t a b i l i t y of z e o l i t e s i s needed f o r t h e i r use i n r e a c t o r s and f o r t h e i r r e g e n e r a t i o n u s u a l l y by coke r e s i d u e burning. The a c i d i c forms o b t a i n e d by replacement of exchangeable c a t i o n s by p r o t o n s a r e u s u a l l y l e s s s t a b l e than t h e p a r e n t forms. An i n c r e a s e i n t h e S i / A l r a t i o improves g r e a t l y t h e thermal o r hydrothermal s t a b i l i t y . (27) Thi s has been a s c r i b e d t o t h e lower d e n s i t y o f hydroxyl groups which p a r a l l e l t h a t o f A1 content. A l o n g e r d i stance between h y d r o x y l s decreases t h e p r o b a b i l i t y o f d e h y d r o x y l a t i o n then a1 so t h a t o f d e f e c t generation. (28)
2.2
C a t i o n Content and I d e n t i t y
The a c i d c a t a l y s i s i n z e o l i t e s i s s t r o n g l y dependent on the p r o t o n content and t h e a c i d i t y strength. B o t h parameters vary w i t h remaining c a t i o n s . The c a t a l y t i c a c t i v i t y i n cracking, isomerization etc. r e a c t i o n s i n c r e a s e s a s t h e sodi urn content is decreased. Figure 2 i s t y p i c a l o f r e s u l t s obtained with Y z e o l i t e s i n r e a c t i o n s such a s curnene c r a c k i n g , ( 2 9 ) isooctane cracking(30,31) o r o-xyl ene isomerization (32) which require medium and s t r o n g a c i d s i t e s . S i m i l a r behavior i s observed f o r o t h e r zeol i t e s t r u c t u r e s such as o f f r e t i t e ( 3 3 ) b u t i t shows some exceptions. For i n s t a n c e Mg HY z e o l i t e s show a maximum i n 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 t h e formation o f The L t y p e z e o l i t e a l s o g i v e s a very s t r o n g a c i d s i t e s . ( 3 4 ) maximum a t 50% exchange o f t h e c a t i o n s which cannot be explained by any c r y s t a l 1 i n i t y l o s s o r ul t r a s t a b i 1 i z a t i o n effect.(35) Nevertheless t h e usual e x p l a n a t i o n f o r F i g u r e 2 ' s results correlates the increase i n a c t i v i t y w i t h the higher acid s t r e n g t h o f s i t e s generated i n t h e h i g h l y p r o t o n a t e d zeolites. The exchange o f monovalent i o n s by p o l y v a l e n t c a t i o n s Those h i g h l y charged improves t h e c a t a l y t i c p r o p e r t i e s . ( l , 2 ) a hydrolysi s cations create very acidic centers by phenomenon (36)
Na PER
U.C.
FIGURE 2: Change in acid catalysis activity as a function of ~ a content + in Y zeolite
The new a c i d c e n t e r s formed i n c r e a s e c a t a l y t i c a c t i v i t y i n many r e a c t i o n s such a s cumene c r a c k i ng(7) o r o-xyl ene i somerization.(37) The r a r e e a r t h z e o l i t e s appear t o be very attractive, more t h a n c a l c i um forms f o r instance. They generate a h i g h e r a c t i v i t y , a h i g h e r thermal s t a b i l i t y and a lower s i t e deactivation. 2.3
I n f l u e n c e o f P r e t r e a t m e n t T e m ~ e r a t u r es
For most a c i d c a t a l y z e d r e a c t i o n s t h e a c t i v i t y i n c r e a ses up t o a maximum f o r p r e t r e a t m e n t temperatures compri sed between 700 and 900K. (38-41) A s e l e c t i v i t y change i s observed f o r 2propanol d e h y d r a t i o n over v a r i o u s Y z e o l i t e s a f t e r degassing a t 673K. (42) E t h e r is formed p r e f e r e n t i a l l y a t t h e maximum hydroxyl c o n c e n t r a t i o n c o n f i rrni ng t h e hypothesi s t h a t i t s f o r m a t i o n r e q u i r e s a p a i r o f hydroxyl groups.(43) For p a r a f f i n c r a c k i n g t h e maximum i n a c t i v i t y o c c u r s a f t e r p r e t r e a t m e n t a t temperatures 100 t o 300 degrees h i g h e r t h a n t h e maximum hydroxyl c o n c e n t r a t i o n . F i g u r e 3 g i v e s a s an example t h e change i n C3 f o r m a t i o n from n-hexane a t 623K(40) a d t h e t o t a l z e o l i t e hydroxyl c o n c e n t r a t i o n (3660 and 3550 cm-' bands) a s determined by i n f r a r e d spectrometry(44) a s a f u n c t i o n o f preThe e x p l a n a t i on usual l y t r e a t m e n t temperatures o f NH4Y. accepted r e l i e s on t h e f a c t t h a t only t h e s t r o n g e s t a c i d s i t e s a r e r e q u i r e d f o r c a t a l y s i s and t h e y a r e generated a t h i g h temperatures. (40) A simi 1 a r b e h a v i o r has been observed f o r The temperature f o r t h e d i f f e r e n t zeol it e s t r u c t u r e s . (38) maximum o f c a t a l y t i c p r o p e r t i e s a l s o depends on which a c i d s t r e n g t h i s needed, i.e. which c a t a l y t i c r e a c t i o n i s under study. Jacobs(41) g i v e s a c l a s s i f i c a t i o n of r e a c t i o n s i n i n c r e a s i n g o r d e r o f a c t i v a t i o n temperature which c o r r e l a t e s w i t h a decreasing number o f s i t e s and an i n c r e a s i n g s t r e n g t h o f t h e i r a c i d i t y (Table 1).
3.
C o r r e l a t i o n s Between A c i d i c and C a t a l v t i c P r o ~ e r t i e si n Zeol i t e s
The importance o f a c i d i t y f o r c a t a l y t i c p r o p e r t i e s has been p o i n t e d o u t i n t h e p r e v i o u s paragraphs. Many c o r r e l a t i o n s have been p u b l i s h e d between b o t h o f t h o s e p r o p e r t i e s ( f o r example 45,46). Most.. o f a c i d c a t a l y t i c r e a c t i o n s a r e r e l a t e d t o t h e presence o f Bronsted a c i d i t y . For i n s t a n c e i t has been shown t h a t t h e maximum i n c a t a l y t i c p r o p e r t i e s as a f u n c t i o n o f p r e t r e a t m e n t temperature c o r r e l a t e s r a t h e r we1 1 w i t h Bronsted a c i d i t y which shows a maximum i n t h e same temperature range. There i s no obvious c o r r e l a t i o n w i t h t e w i s a c i d i t y . (44) Nevert h e l e s s i t has n o t been p o s s i b l e up t o now t o f i n d a unique
PRETREATMENT TEMPERATURE (K) FIGURE 3: a: Total OH content (44) and b: n-Hexane cracking (40) as a function of pretreatment temperature
c o r r e l a t i o n between a l l t h e d i f f e r e n t t y p e s o f a c i d c a t a l y z e d r e a c t i o n s and t h e p r o t o n i c zeol i t e a c i d i t y . Moreover t h e p r o t o n i c a c i d i t y i t sel f depends on several parameters. Thi s e x p l a i n s why so much work has been devoted t o t h e study o f A b e t t e r understanding o f t h i s p r o p e r t y zeolite acidity. should be a b a s i s f o r a more v a l u a b l e p r e d i c t i o n of c a t a l y t i c properties.
4.
Zeolite Acidity
The main d i f f e r e n c e between zeol i t e s and o t h e r o x i d e s is t h e i r very open s t r u c t u r e which makes t h e l a r g e r p a r t o f t h e framework A1 0 and S i O4 t e t r a hedra acce s s i h l e t o ad sorbed molecules. t r i d i m e n s i o n a l network c r e a t e s c a v i t i e s and channel s i n which molecules may undergo c a t a l y t i c r e a c t i o n s . Then t h e surface o f a z e o l i t e i s i n f a c t i n s i d e t h e bulk o f t h e c r y s t a l and one may expect t h a t t h e surface p r o p e r t i e s , i.e. t h e c a v i t y wall p r o p e r t i e s , a r e s t r o n g l y dependent upon t h e framework c o n s t i t u t i n g atoms. Thi s dependence should be
fie
m h
m
c
o
e
c
c
m
c
m
a
e f f e c t i v e n o t o n l y on a s h o r t range b u t a l s o on a r a t h e r l o n g range a l l over t h e framework. I n t h e l a s t few y e a r s r e s e a r c h on z e o l i t e a c i d i t y has been moved from t h e c h a r a c t e r i z a t i o n o f the c o n c e n t r a t i o n and s t r e n g t h of isol a t e d s i t e s towards attempts t o present an o v e r a l l view of t h e a c i d p r o p e r t i e s . Before c o n s i d e r i n g those approaches i t i s u s e f u l t o p o i n t o u t the main parameters which c h a r a c t e r i z e a c i d i t y i n zeol i t e s and t h e methods which a r e used t o study these p r o p e r t i e s .
4.1
A c i d i t y Study Methods
The i d e a l method o f a c i d i t y measurement should g i v e i n f o r mation on several parameters: t h e number, nature, s t r e n g t h , l o c a t i o n , envi ronment and mean l i f e t i m e of a c i d s i t e s . Hence i t should be a b l e t o c h a r a c t e r i z e a c i d c e n t e r s p r e c i s e l y enough t o assign one t y p e t o t h e 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 a reactant molecule. I n f a c t each method g i v e s i n f o r m a t i o n b u t none f u l l y d e s c r i b e s t h e a c i d s i t e s . Spectrometric met hods have been w i d e l y used. C e r t a i n l y I R spectrometry i s t h e more usual. It has g i v e n a 1arge number o f r e s u l t s r e l a t e d t o hydroxyl c h a r a c t e r i z a t i o n and t o Bronsted P y r i d i n e and t o a l e s s e r e x t e n t NH3 and Lewis a c i d i t y . (47,48) a r e t h e bases commonly used f o r s e m i - q u a n t i t a t i ve o r q u a n t i t a Table 2 t i v e e v a l u a t i o n s o f s t r e n g t h o r c o n c e n t r a t i o n . (49,50) gives t h e assignments of p y r i d i ne a b s o r p t i o n bands. 2- 6 d i m e t h y l p y r i d n e has been suggested a s a base s p e c i f i c f o r p r o t o n i c c e n t e r s c h a r a c t e r i z a t i o n s i n c e s t e r i c hindrance o f t h e n i t r o g e n atom p.revents i t s c o o r d i n a t i o n t o A1 atoms. (51,52) Recently t h e Bronsted a c i d s t r e n g t h has been evaluated i n zeol i t e s from t h e frequency s h i f t o f hydroxyl groups upon i n t e r a c t i o n w i t h hydrogen bond a c c e p t o r molecules such a s UV spectroscopy was used t o study t h e benzene. (22,53,54) adsorption of v a r i o u s m o l e c u l e s on zeol ites. (55,56) I n the case o f p y r i d i n e on X z e o l i t e w i t h sodium c a t i o n s ( 5 6 ) t h r e e k i n d s o f adsorbed species c h a r a c t e r i z e d r e s p e c t i v e l y t h e I t was i n t e r a c t i o n s w i t h c a t i o n s , p r o t o n s and Lewis s i t e s . noted t h a t UV methods have t h e advantage t o be very s e n s i t i v e but i t i s d i f f i c u l t t o d i s t i n g u i s h t h e p o s i t i o n s o f t h e r e s p e c t i v e peak maxima. O p t i ca1 e l e c t r o n i c spectroscopy was a1 so used to di fferenti ate protonic and non-protoni c sites. ( 5 7 ) Besides t h e study o f redox p r o p e r t i e s o f z e o l i t e s , (58) ESR has been a p p l i e d t o study atomic hydrogen formed on y i r r a d i a t i o n and which i s r e l a t e d t o p r o t o n i c a c i d i t y . (59) R e l a x a t i o n and 1 inewidt h s t u d i e s i n NMR have been employed t o c h a r a c t e r i z e t h e p r o t o n m o b i l i t y and i n t e r a c t i o n s w i t h cations.(60,61) These s t u d i e s a f f o r d a f u r t h e r i n s i g h t It i s proposed t h a t t h e i n t o the nature o f protonic a c i d i t y . h i g h e r i s t h e jump frequency ( i n v e r s e mean l i f e t i m e a t l a t t i c e
.
Table 2 Assignment* I R Band; o f P y r i d i ne (Py ) Adsorbed on B r o n s t e d (H ) o r L e w i s S i t e s ( L )
V i h r a t i o n a l Mode
oxygen atoms), t h e h i g h e r i s t h e s t r e n g t h o f t h e p r o t o n . (60,61) I t i s shown t h a t an adsorbed base such as p y r i d i n e i n c r e a s e s (up t o 60 t i m e s a t 200") t h e p r o t o n m o b i l i t y p r o b a b l y because d u r i n g p y r i d i n i u m i o n f o r m a t i o n and d e c o m p o s i t i o n a h y d r o x y l p r o t o n must be f i r s t a t t a c h e d t o t h e p y r i d i n e m o l e c u l e and t h e n I t is hence suggested t h a t g i v e n back t o a n o t h e r oxygen atom. i n v e s t i g a t i o n s o f p r o t o n m o b i l i t y can l e a d t o c o n c l u s i o n s on a c i d i t y o n l y i f t h e study i s made i n t h e presence o f b a s i c molecules.(61) NMR measurements a l s o p e r m i t s t h e computation o f an elementary " p r o t o n c a p t u r e p r o b a b i l i t y " by t h e a c c e p t o r m o l e c u l e d i f f u s i ng upon t h e surface. T h i s p r o b a b i 1it y decreases f r o m 1 t o 0.06 a f t e r a l o n g o u t g a s s i n g performed between 0' and 300°C(60) ( f o r NH3). B e s i d e s t h e s p e c t r o s c o p i c met hods several approaches have been used t o c h a r a c t e r i z e a c i d i c p r o p e r t i e s . Minachev, Bremer e t a l . performed s e v e r a l works u s i n g Hz-0 exchange t o study Several therma? methods have been t h e p r o t o n mobi 1 ity. (62-64) used t o study t h e i n t e r a c t i o n s o f bases w i t h a c i d s i t e s (DTA, (65) c a l o r i m e t r y and chromatography). (66-68) I t was shown t h a t NH3 o r b u t y l a m i n e g i v e a ma11 heat o f a d s o r p t i o n on cations. The d i s t r i b u t i o n o f t h e s t r e n g t h s o f a c i d s i t e s c o u l d be o b t a i n e d from t h e h e a t o f a d s o r p t i o n o f benzene on p r o g r e s s i v e l y p y r i d i ne p o i soned sample s. ( 6 6 ) A met hod based on t h e d e t e r m i n a t i o n o f t h e amount o f oxygen used f o r t h e o x i d a t i o n o f NH3 i n t h e ammonium forms o f z e o l i t e s has been d e s c r i b e d t o I t d i s t i n g u i s h e s between measure t h e number o f a c i d c e n t e r s . B r o n s t e d and Lewis a c i d i t i e s . ( 6 9 ) A f t e r t h e e a r l y work o f H i r s c h l e r , ( 7 0 ) t i t r a t i o n w i t h b u t y l a m i n e and c o l o r e d i n d i c a t o r s The method a l l o w s an easy d e t e r m i has been used.(25,71-75) n a t i o n o f a c i d s i t e c o n c e n t r a t i o n and s t r e n g t h t o be done b u t i t does n o t g i v e i n f o r m a t i o n on t h e p r e c i s e n a t u r e o f a c i d centers. The q u e s t i o n a r i s e s a l s o a s t o t h e s i z e o f t h e
reactants may modify t h e r e s u l t s . I n fact i n Y zeolites, the number o f a c i d s i t e s determined i n t h i s way i s c l o s e t o t h a t deduced from I R experiments u s i n g p y r i d i n e . ( 4 9 ) The r e s u l t s performed w i t h v a r i o u s bases, i n d i c a t o r s and zeol it e s suggest that w i t h r e g a r d t o t h e a p e r t u r e o f t h e z e o l i t e channels, t h e size o f t h e base molecule i s more c r i t i c a l than t h a t o f t h e indicator. Assuming t h a t a t t h e e q u i l i b r i u m t h e base, small enough t o move i n t h e channel s, n e u t r a l i z e s t h e same f r a c t i o n o f a c i d s i t e s wherever t h e y a r e l o c a t e d ( i n s i d e o r o u t s i d e t h e p a r t i c l e s ) , t h e l a r g e i n d i c a t o r s molecules may d e t e c t t h e end o f n e u t r a l i z a t i o n from t h e r e a c t i o n w i t h t h e o n l y a c c e s s i b l e sites. (74,75) Among a l l t h e s e methods, i n f r a - r e d spectroscopy i s t h e most powerful since i t g i v e s a very l a r g e number of i n f o r mations on t h e a c i d s i t e s . However t h e d i f f i c u l t y t o c o r r e l a t e preci sely i n f r a - r e d r e s u l t s w i t h o t h e r p r o p e r t i e s such a s c a t a l y t i c behavior, p o i n t s o u t t h e f a c t t h a t t h e c h a r a c t e r i zation o f s t a t i c and d e f i n i t e hydroxyl groups i s n o t p r e c i s e enough t o e x p l a i n which p r o t o n s a c t i n t h e dynamic c a t a l y t i c processes.
4.2
Nature and Generation o f A c i d S i t e s
The n e g a t i v e charges i n excess due t o t h e replacement of Si04 t e t r a h e d r a by A104- t e t r a h e d r a i n t h e framework a r e n e u t r a l i z e d by p r o t o n s o r o t h e r c a t i o n s . The p r o t o n i c a c i d centers a r e generated i n v a r i o u s ways.
i. The thermal decomposition o f ammoni um exchanged zeol it e s y i e l d s t h e hydrogen form. Deammi n a t i o n i n anhydrous c o n d i t i o n s o f a1 kylammoni um, p i p e r i d i n i um o r p y r i d i n i u m Y z e o l i t e s produces a s t o i c h i o m e t r i c hydrogen z e o l i t e o n l y i n t h e case o f p r i m a r y alkylammonium ions. With o t h e r c a t i o n s a c o n s i d e r a b l e dehydroxyl a t i o n is observed p r o d u c i n g a s o - c a l l e d dehydroxylated z e o l i t e w i t h Lewi s a c i d s i t e s . (76) Thi s dehydroxyl a t i o n e f f e c t is a1 so observed d u r i n g t h e a d s o r p t i o n o f ami nes, particularly with p y r i d i ne. (77,78) One can wonder whether t h e h i g h p r o t o n m o b i l i t y i n t h e presence o f p y r i d i n e ( 6 1 ) does n o t f a c i l i t a t e t h e d e h y d r o x y l a t i o n phenomena. ii. The ~ r o n s t e da c i d i t y due t o t h e w a t e r i o n i z a t i o n on p o l y v a l e n t c a t i o n s (36) a l r e a d y described (Scheme 2 ) has been s t u d i e d by v a r i o u s methods. NMR was a p p l i e d t o t h e c a l c u l a t i o n o f t h e i n t e r p r o t o n d i stance (dH20) i n water c o o r d i n a t e l y bonded t o t h e c a t i o n s . (79)
iii. The r e d u c t i o n by hydrogen o f t r a n s i t i o n metal c a t i o n s i n z e o l i t e g was supposed t o form a hydrogen zeolite.(80,81) Such Bronsted a c i d i t y has been observed i n an I R study o f a hydrogen reduced CU'+Y zeol i t e. (82)
The r e d u c t i o n o f cu2+ c a t i o n w i t h CO (R2) o r i t s s e l f r e d u c t i o n (83) g i ve s cuprou s i o n s and Lewi s a c i d i t y . The c o n c e n t r a t i o n o f OH groups o f Y z e o l i t e s c o n t a i n i n g N i , Co o r Cu was n o t e d t o i n c r e a s e by r e d u c t i o n w i t h hydrogen a t 250-450" and t o increase w i t h the r i s e o f the r e d u c t i o n temperature. (84) A r e d u c t i o n by hydrocarbon s o f cations t o metals w i t h formation o f protonic a c i d i t y has been shown i n t h e case o f N i , Fe and Co z e o l i t e s d u r i n g t h e cumene cracking. A similar reduction i s p o s t u l a t e d w i t h C r and Cd z e o l i t e s : (85)
iv.
~ r G n s t e d a c i d s i t e s a r e a l s o generated i n b i v a l e n t cation-contai ning Y z e o l i t e s on exposure a t room temperature t o h a l i d e compounds(86) o r a t 150-400°C t o C02* (87)
The v a r i o u s and independent ways o f a c i d s i t e g e n e r a t i o n show t h a t t h e experimental c o n d i t i o n s o f z e o l i t e p r e t r e a t m e n t s o r a c i d i t y mea surement s coul d rnodi f y g r e a t l y the i n t r i n s i c acidity. F u r t h e r t h i s suggests t h a t z e o l i t e a c i d i t y may be changed by t h e presence o f (dehydroxyl a t i on by reactants, catalytic reagents r e d u c t i o n o f r e d u c i b l e c a t i o n s by hydrocarbons, r e a c t i o n w i t h a c i d i c compounds). Hence independently o f a g i ng effects, t h e r e may be l a r g e d i f f e r e n c e s i n a c i d i t y between t h e f r e s h and t h e a c t u a l c a t a l y s t . Scheme ( 5 ) d e p i c t s t h e f o r m a t i o n o f Lewis a c i d i t y from Bron sted s i t e s(88) :
H
The ~ r o n s t e d (OH) and Lewis ( A - s i t e s can be p r e s e n t simultaneously i n t h e s t r u c t u r e a t h i g h temperature. In faujasi tes, t h e d e h y d r a t i o n r e a c t i o n occurs above 873K which decreases t h e number o f p r o t o n s and i n c r e a s e s t h a t o f Lewis sites. (44) Scheme 5 has been c o n f i rmed by I R spect romet rye A t temperatures h i g h e r t h a n 673K t h e sum o f t h e number o f Bronsted sites p l u s t w i c e t h a t o f Lewi s s i t e s is almost constant. (44)
4.3
Hydroxyl Groups i n Zeol i t e s
I n t h e e a r l y works on z e o l i t e a c i d i t y t h e f o r m a t i o n o f two d i f f e r e n t hydroxyl groups has been r e p o r t e d i n f a u j a s i t e s upon ammonium i o n decomposition. (47,88,89) They a r e asigned t o OH v i b r a t i n g i n two d i f f e r e n t cages, t h e supercage and t h e soda1 i t e e For most z e o l i t e s several hydroxyl bands a r e reported corresponding t o OH groups v i b r a t i n g i n d i f f e r e n t c a v i t i e s (Table 3).(90) The hydroxyl s i n very small c a v i t i e s are not a c c e s s i b l e t o hydrocarbons o r base molecules. I f they are a c i d i c , they can n e v e r t h e l e s s i n t e r a c t w i t h those molecules, t h e p r o t o n b e i n g a t t r a c t e d i n t h e l a r g e cages. This occurs f o r t h e 3550 cm-I h y d r o x y l s i n f a u j a s i t e s which move from t h e s o d a l i t e cage t o t h e supercage upon base adsorption. The q u e s t i o n then a r i s e s t o know w h i c h hydroxyl is i n v o l v e d i n the c a t a l y t i c process. I t was shown t h a t i n t h e case o f faujasite the active s i t e s cumene c r a c k i n g a r e t h e hydroxyls v i b r a t i n g a t 3650 cmmfor The low frequency h y d r o x y l s (3550 cm-l) s t a r t t o i n t e r a c t w i t h t h e hydrocarbon o n l y a t higher temperatures (>365"C) when t h e y become s u f f i c i e n t l y activated. (91) Each o f t h e two hydroxyl groups i n f a u j a s i t e s behaves separately. Exchange o f p r o t o n s w i t h i n c r e a s i n g amounts o f cations such a s sodium decreases more r a p i d l y t h e i n t e n s i t y o f the 3540 cm-' hydroxyl band because o f t h e p r e f e r e n t i a l l o c a t i o n o f t h e f i r s t c a t i o n s i n t h e hexagonal p r i s m s and t h e s o d a l i t e cages.(32) Each o f t h e h y d r o x y l s a l s o i n c l u d e s a large range o f p r o t o n i c a c i d strengths. The weaker s i t e s a r e n e u t r a l i z e d f i r s t upon t h e exchange o f p r o t o n s by o t h e r cations. It has been r e p o r t e d t h a t approximately 30% o f t h e s i t e s a r e weak. (71) The o r i g i n o f t h e d i f f e r e n c e i n wavenumber a c c o r d i n g t o t h e hydroxyl group l o c a t i o n has been f o r a l o n g t i m e a m a t t e r o f speculation. The e x i stence o f hydrogen bonding w i t h c l o s e oxygen atoms has been p o s t u l a t e d . (28,47) Recently i t has been shown t h a t t h e h i g h wavenumber band r e l a t e s t o an unperturbed hydroxyl. (90) The s h i f t t o a l o w e r wavenumber f o r t h e hydroxyl s i n small c a v i t i e s a r i s e s from t h e i r d i s t u r b a n c e by e l e c t r o s t a t i c f i e 1 d c r e a t e d by t h e n e a r e s t oxygen s. The
Table 3 H y d r o x y l S t r e t c h i n g F r e q u e n c i e s i n Hydrogen-Zeol it e s and T h e i r Proposed Assignment (from Ref, 9 0 )
Zeol i t e FAU
OH 7:;yygncy
365ga 3584
3578
Assignment 01-H i n supercages 0 -H i n soda1 i t e cages (2-memh. r i n g , s i t e I*) 02(04)-H i n 6-nemb. r i n g s ( s i t e 11)
FAU*
RE-FAU
3560
Crystal termi nating, i n f l uenced by sample compo s i t i on I n 8-memb. r i n g s ( p o r e s )
36 3oa 3540
8-memb. 6-memb,
(3650)
Amorphous phase w i t h ST1 composition
36.20~ 3575
C r y s t a l terrni n a t i n g , i n f 1 uenced by sarnpl e composi t i on 8-memb. r i n g s
ERI
3612~ 3563
8-memb. 6-memb.
MO R
3720
SiOH o f u n i d e n t i f i e d
3650
nature Occluded i m p u r i t i e s
HEU
3620a
ring ring
r ings rings
Zeol i t e
OH F r e q ency (cm-'I
A s s i gnmen t I n pores E x t r a l a t t i c e Si-OH I n pore i n t e r s e c t i o n s E x t r a - 1 a t t i c e S i -OH I n pore i n t e r s e c t i o n s
a
MAZ
I n pores
LTL
I n pores
FER
I n p o r e s (10-memb. rings)
OFF
I n pores (?)
RHO
I n p o r e s ( ? ) (8-memb. r i ngs)
These f r e q u e n c i e s r e p r e s e n t OH groups, v i b r a t i n g i n t h e l a r g e s t p o r e s and/or cages o f t h e r e s p e c t i v e z e o l i t e s For r e f e r e n c e s see (90)
frequency s h i f t from t h e u n p e r t u r b e d h y d r o x y l s f o l l o w s a l i n e a r r e l a t i o n s h i p w i t h t h e i n v e r s e o f t h e squared d i s t a n c e o f t h e proton t o t h e n e a r e s t oxygen f o r a s e r i e s o f z e o l i t e s c o n t a i n i n g t h e p e r t u r b e d h y d r o x y l s i n 6- o r 8-membered r i n g s .
4.4
Number o f S i t e s
The p o t e n t i a l number o f a c i d s i t e s i z e o l i t e s equal s t h a t The number o f o f A1 atoms p e r any r e f e r e n c e u n i t (g, c a c i d s i t e s p r e s e n t i n a sample i s u s u a l l y l o w e r s i n c e i t depends on many p a r a m e t e r s : degree of crystallinity, dehydroxyl a t i o n , p a r t i a l n e u t r a l i z a t i o n w i t h c a t i o n s o r bases etc. The number o f a c i d s i t e s a c t i v e i n a g i v e n c a t a l y t i c r e a c t i o n can be even s m a l l e r due f o r i n s t a n c e t o t h e i n a c c e s s i b i l i t y o f s i t e s ( f o r i n s t a n c e OH groups i n small cages) o r t o t h e r e q u i r e m e n t f o r t h e r i g h t a c i d s t r e n g t h ( T a b l e 1). A quantitative evaluation of both types o f hydroxyls i n f a u j a s i t e s g i v e s a maximum v a l u e o f 16 h y d r o x y l s p e r u n i t c e l l
v i b r a t i n g i n t h e supercage a t 3650 crn-l,(50,92) i.e. one hydroxyl group on average p e r hexagonal prism. Titration with p y r i d i n e g i v e s c l o s e t o 35 OH p e r u n i t c e l l i n t h e f a u j a s i t e supercage(92) which i s c l o s e t o t h e number o f p y r i d i n i u m formed. T h i s i g h t a r i se from a t t r a c t i o n by p y r i d i n e o f some o f t h e 3550 cm-PI h y d r o x y l s i n t h e supercage.
A l a r g e number o f s t u d i e s have been performed u s i n g base They g i v e t h e t o t a l number t i t r a t i o n wi$h c o l o r e d i n d i c a t o r s . o f s i t e s Bronsted and Lewis. A d e t a i l e d study o f a f a u j a s i t e t y p e s e r i e s a s a f u n c t i o n o f c a t i o n c o n t e n t (Na, K , Ca, La) o r S i / A l r a t i o showed t h a t f o r each c a t i o n exchanged by one p r o t o n o n l y one f r a c t i o n o f an a c i d s i t e c o u l d be t i t r a t e d . This f r a c t i o n a. i s small i n h i g h l y alumineous z e o l i t e s such a s X and i t i n c r e a s e s a t lower a1 urninurn c o n t e n t ( F i g u r e 4). ( 2 5 ) Such an e f f i c i e n c y o r s e l f - i n h i b i t i o n c o e f f i c i e n t should r e f l e c t t h e h i g h d e n s i t y of a c i d s i t e s a t h i g h A1 content. It c o u l d work a s an a c t i v i t y c o e f f i c i e n t i n concentrated solutions. (16) T h i s would e x p l a i n t h e lower c a t a l y t i c a c t i v i t y o f X z e o l i t e s compared t o Y mentioned e a r l i e r .
4.5
A c i d i t y Strength
The a c i d s t r e n g t h o f s i t e s i n z e o l i t e s depends on t h e S i / A l ratio. From a l l t h e r e s u l t s p u b l i s h e d two i d e a s emerge emphasizing t h e s u p e r i m p o s i t i o n o f short range and l o n g range e f f e c t s i n d e t e r m i n i n g t h e a c i d strength.
20
40
Y60
80 X
ALUMINUM PER U.C.
FIGURE 4: Dependence of efficiency of acid site ao on the aluminum content in faujasites ( 2 5 )
4.5.1
Short Range I n t e r a c t i o n s
The q u e s t i o n o f whether t h e A1 o r d e r i n g i s t h e same f o r X and Y z e o l i t e s was r a i s e d a l o n g t i m e ago.(93) An a c i d i t y study o f these z e o l i t e s i n which A1 atoms were p r o g r e s s i v e l y removed from t h e framework by dea 1 umi n a t i o n suggested t h a t t he acid s t r e n g t h o f s i t e s was dependent upon t h e S i o r A1 atom environment, d e s p i t e t h e f a c t t h a t a l l t h e T (A1 o r S i ) positions are s t r u c t u r a l l y equivalent. (25,30,71) Seve r a 1 attempts have been made t o account f o r t h e h e t e r o g e n e i t y o f acid strengths. The general i d e a o f these approaches is t h a t the p r o t o n i c a c i d i t y s t r e n g t h a s s o c i a t e d w i t h tetrahedron i s h i g h e s t f o r t h e s m a l l e s t number o f an c l o sAe1 O ~ ; neighbors. Since, except f o r z e o l i t e s w i t h a Si/A1 r a t i o o f 1 (A o r X t y p e ) , t h e A1 d i s t r i b u t i on i s n o t p e r f e c t l y homogeneous a range o f a c i d s t r e n g t h s i s expected t o occur. Dempsey was the f i r s t t o r e l a t e q u a n t i t a t i v e l y t h e a c i d s t r e n g t h o f t h e proton t o t h e geometry o f t h e s t r u c t u r e and t h e environment o f the A1 atoms. (94) I n a more general model t h e s t r e n g t h o f t h e protons was d e r i v e d from t h e s t a t i s t i c a l d i s t r i b u t i o n o f A1 atoms i n t h e fauja s i t e s t r u c t u r e . (95) A f u r t h e r extension considers t h e next n e a r e s t n e i g h b o r s o f each A1 atom and t h e b u f f e r i n g a c t i o n o f t h e c a t i o n s t o e x p l a i n t h e known changes i n proton a c i d s t r e n g t h and t h e thermal s t a b i l i t y of h y d r o x y l groups. (96) I n t h i s model t h e parameters o f importance a r e t h e distance and number o f c l o s e A1 atoms, o f c a t i o n s and o f hydroxyl s. U n t i l r e c e n t l y no i n f o r m a t i o n was a v a i l a b l e on t h e A1 d i s t r i b u t i o n i n t h e framework. The use o f 29% NMR(97) and 2 7 A l NMR(98) has made p o s s i b l e t h e d e t e r m i n a t i o n o f t h e arrangement o f S i and A1 atoms. T h i s i s a very v a l u a b l e source of i n f o r m a t i o n f o r c a l c u l a t i o n s on t h e a c i d s t r e n g t h d i s t r i b u t i o n of protons e x i s t i n g i n a given structure. The e x i s t e n c e o f such d a t a f o r v a r i o u s l y t r e a t e d z e o l i t e s , f o r instance dealuminated Y, (99) and u l t r a s t a b l e ( 9 8 ) may h e l p i n t h e understanding o f t h e i r changes i n a c i d i c p r o p e r t i e s , A1 so t h e presence o f d i f f e r e n t a c i d s t r e n g t h s observed by Jacobs e t al. (54) i n h i g h l y s i 1 iceous zeol it e s o f simi l a r chemical composition (ZSM-5, ZSM-11 and d e a l umi nated f a u j a s i t e ) c o u l d be r e l a t e d t o environmental e f f e c t s d e t e c t a b l e by 2 9 S i NMR o r 2 7 ~ 1 NMR
.
Besides t h e e f f e c t s due t o t h e geometry o f t h e A1 d i s t r i bution, o t h e r e f f e c t s a r e w e l l known t o modify t h e a c i d s t r e n g t h o f protons. (48) The most i m p o r t a n t is t h e exchange o f The s t r o n g e s t s i t e s a r e n e u t r a l i z e d f i r s t protons by c a t i o n s . and t h e a c i d site distribution moves t o weaker a c i d i t y . Polyvalent c a t i o n s generate s t r o n g a c i d s i t e s by w a t e r
h y d r o l y s i s. These a c i d i t y changes a r e due t o l o c a l i z e d e f f e c t s (chemical r e a c t i o n s ) and a r e t h e n a l s o r e l a t e d t o s h o r t range in t e r a c t ions. 4.5.2
Overall I n t e r a c t i o n s
The i d e a o f an i n f l u e n c e o f a l l t h e A1 atoms on t h e p r o p e r t i e s of z e o l i t e s has grown a c i d i c and c a t a l y t ic progressively. A t t e m p t s have been made t o q u a n t i f y it. (14,16,21,22,25,71,90,100) I n c o n t r a s t t o t h e p r e v i o u s models which deduce 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 from t h e n a t u r e o f t h e c l o s e n e i g h b o r s atoms (A1 o r S i ), models t a k i n g i n t o account a very l a r g e number o f atoms o n l y c a l c u l a t e t h e e f f e c t s o f t h e average They g i v e a mean va1 ue o f a c i d e n v i ronment. (21,22,100-102) a given d e n s i t y o f s t r e n g t h f o r a g i v e n A1 c o n t e n t , i.e. charges i n t h e s t r u c t u r e .
A very successful approach uses t h e Sanderson e l e c t r o n e g a t i v i t y equal i z a t i o n concept. (103) It has been a p p l i e d t o t h e c a l c u l a t i o n o f t h e z e o l i t e e l e c t r o n e g a t i v i t y and the charges on v a r i o u s framework atoms and on cations.(21,22,100) T h i s 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 is p o s t u l a t e d t o be the geometric mean o f t h e compound atoms of t h e molecule, i.e. o f a g i v e n p o r t i o n o f a z e o l i t e network. Very i n t e r e s t i n g l y i t has been shown t h a t t h e charge on t h e p r o t o n i n c r e a s e s a s t h e A1 c o n t e n t (A1 /A1 + S i ) decreases f o r f a u j a s i t e s w i t h v a r i o u s A1 c o n t e n t s o r f o r v a r i o u s zeol i t e s t r u c t u r e s (L, mordenite, Thi s f o l 1 ow s t h e experimental order c l i n o p t i lo1i t e . (21) observed f o r t h e i n c r e a s e i n s t r o n g p r o t o n i c a c i d i t y f o r the Correlations w i t h catalytic same z e o l i t e series. (16,41) p r o p e r t i e s g i v e a good r e l a t i o n s h i p o n l y w i t h r e a c t i o n s which r e a c t i o n s c a t a l y z e d by i n v o l v e a l l t h e hydroxyl groups, i.e. t h e p r o t o n i c s i t e s r e g a r d l e s s o f t h e i r s t r e n g t h (i sopropanol d e h y d r a t i o n ) o r r e a c t i o n s which woul d i n v o l v e a constant f r a c t i o n of h y d r o x y l s (n-decane hydroconversi on). (22) For in stance t h e isopropanol d e h y d r a t i o n t u r n - o v e r numbers(21,22) a r e p r o p o r t i o n a l t o t h e c a l c u l a t e d charge on t h e proton. All t h e s e s t u d i e s show t h a t t h e mean charge on t h e p r o t o n i s s h i f t e d r e g u l a r l y towards h i g h e r v a l u e s a s t h e A1 content decreases. Simultaneously t h e t o t a l number of acidic h y d r o x y l s, governed by t h e A1 content, has t o decrease. This s t r o n g l y suggests t h a t t h e e n t i r e a c i d s t r e n g t h d i s t r i b u t i o n (weak, medium, s t r o n g s i t e s ) i s s h i f t e d towards stronger values. The weaker a c i d s i t e s should become s t r o n g e r w i t h the Thi s i s i n f a c t observed. The decrease i n t h e A1 content. i n f r a r e d wavenumber o f a c i d i c hydroxyl s ( h i g h frequency band), f o r a l a r g e number o f z e o l i t e s t r u c t u r e s , has been shown t o
decrease w i t h t h e A1 c o n t e n t . (41,104) The c o r r e s p o n d i n g decrease i n t h e f o r c e c o n s t a n t c h a r a c t e r i z e s an i n c r e a s e i n acid strength. More p r e c i sely, i n f o r m a t i o n may be o b t a i n e d on the weakest a c i d s i t e s . S t a r t i n g from a z e o l i t e i n a f u l l y cationated form, Na f o r i n s t a n c e , t h e exchange o f t h e f i r s t cations c r e a t e s weak a c i d i c OH groups ( h i g h frequency band) detectable by I R spectroscopy. values o f these f i r s t hydroxyls formed decrease w i t h c o n t e n t i n d i c a t i n g an increase i n t h e i r a c i d s t r e n g t h . B o t h CNDO(101-102) and Sander son c a l c u l a t i o n s g i v e o n l y an average p r o t o n i c a c i d s t r e n g t h and l i t t l e i n f o r m a t i o n on t h e strongest s i t e s . I t has been known f o r a l o n g t i m e t h a t t h e strength o f s t r o n g p r o t o n i c s i t e s i n c r e a s e s a s t h e A1 c o n t e n t decreases. F o r i n s t a n c e , t h i s has been shown e x p e r i m e n t a l l y from t h e changes i n t h e s t r e n g t h o f base a d s o r p t i o n ( l 6 , 4 1 and references t h e r e i n ) , t h e wavenumber o f a c i d i c OH(41,104) t h e i n t e g r a t e d e x t i n c t i o n c o e f f i c i e n t o f OH groups(22) o r t h e s h i f t upon t h e i n t e r a c t i o n o f t h e h y d r o x y l s w i t h a hydrogenbond-acceptor mol e c u l e. (22,53)
kH
I n t r y i n g t o understand how a change i n c o m p o s i t i o n may increase t h e a c i d s t r e n g t h two f e a t u r e s have t o be considered. F i r s t l y z e o l i t e s are inorganic acids, secondly t h e y a r e polymeric a c i d s . These two p r o p e r t i e s modify t h e a c i d s t r e n g t h separately b u t always s i m u l t a n e o u s l y . I n an a t t e m p t t o c l a r i fy these p o i n t s , i t i s p o s s i b l e t o d i s t i n g u i s h t h e two c o n t r i b u t i o n s i n an a n a l y t i c a l approach. As oxyacids, z e o l i t e f o r m u l a s may be w r i t t e n a s TO,(OH),, ( T = S i o r A1 ) w i t h m = A1 /Al+Si and n = 2-m.(19) T h i s i s comparable t o t h e way o f w r i t i n g , f o r instance, t h e s e r i e s o f o x y c h l o r o a c i d s C10 (OH),. I t i s well known t h a t t h e i r a c i d s t r e n g t h i n c r e a s e s :ith n i n the series
E x p l a n a t i o n s based e i t h e r on e l e c t r o s t a t i c o r e l e c t r o n d e l o c a l i z a t i o n c o n s i d e r a t i o n s have been proposed. C a l c u l a t i o n s based on t h e Sanderson e l e c t r o n e g a t i v i t y e q u a l i z a t i o n p r i n c i p l e g i v e the p a r t i a l charge on t h e p r o t o n w h i c h p a r a l l e l s t h e i n c r e a s e i n n and t h e pKA o f t h e s e acids. I n z e o l i t e s n v a r i e s from 1.5 (Al/Al+Si = 0.5) t o 2 ( A l / A l + S i = 0). The i n c r e a s e i n n, i.e. increase i n a c i d s t r e n g t h , p a r a l l e l s t h e decrease i n A1 c o n t e n t which is i n l i n e w i t h t h e s t r e n g t h s o f t h e o x y c h l o r o a c i d s . The value o f n, l i m i t e d t o between 1.5 and 2, c l o s e t o t h a t o f s u l f u r i c a c i d S02(OH)2, i s i n agreement w i t h what i s known since z e o l i t e s a r e c o n s i d e r e d a s s t r o n g a s s u l f u r i c a c i d . T h i s analogy w i t h o x y c h l o r o a c i d s p o i n t s o u t t h e importance o f t h e A1 content which d e t e r m i n e s n and t h e n t h e a c i d s t r e n g t h . (16)
This contribution t o the composition c o n t r i b u t i on.
acid
strength
may
be
called
a
As t o t h e second p o i n t , i n p o l y m e r i c acids, t h e charge on t h e p r o t o n depends on t h e i n t e r a c t i o n s between t h e OH groups themselves and/or t h e p r o t o n s and t h e surroundi ng molecules. T h i s e f f e c t i s comparable i n some ways w i t h what happens i n c o n c e n t r a t e d s o l u t i o n s where e m p i r i c a l a c t i v i t y c o e f f i c i e n t s have t o be considered. I n z e o l i t e s , t h e h i g h e r t h e number o f t h e h i g h e r t h e A1 content, t h e a n i o n s i n t h e polymer, i.e. g r e a t e r t h e OH-OH and OH/framework i n t e r a c t i o n s and t h e weaker is t h e acid. The importance o f these i n t e r a c t i o n s suggests the n e c e s s i t y t o c o n s i d e r a c t i v i t y c o e f f i c i e n t s i n z e o l i t e s ; these would reduce t h e e f f i c i e n c y o f p r o t o n s i n c a t a l y s i s . (16) T h i s c o n t r i b u t i o n t o a c i d s t r e n g t h may be c a l l e d t h e c o n c e n t r a t i o n c o n t r i b u t ion. I n c r e a s i n g t h e A1 c o n t e n t i n a z e o l i t e simultaneously and i t s concentram o d i f i e s t h e n a t u r e o f t h e m o i e t y (TO,) tion. Roth e f f e c t s decrease t h e a c i d s t r e n g t h o f t h e m protons and a r e concornittent. The q u e s t i o n t h e n a r i s e s a s t o whether o r n o t t h e charge on t h e p r o t o n , c a l c u l a t e d from t h e Sanderson e l e c t r o n e g a t i v i t y model, r e p r e s e n t s t h e t r u e a c i d s t r e n g t h o f I t o b v i o u s l y t a k e s i n t o account t h e so-called the zeolite. composition c o n t r i b u t i o n b u t i t i s n o t c l e a r i f i t i n c l u d e s the concentration contribution. The c a l c u l a t i o n i n v o l v e s the composition of o n l y one i s o l a t e d molecule. For i n s t a n c e i n z e o l i t e s t h e c a l c u l a t e d p r o t o n charge i s e x a c t l y t h e same f o r one fragment o f s t r u c t u r e c o n t a i n i n g one p r o t o n o r f o r any 1a r g e r domain considered, having a l a r g e number o f i n t e r a c t i n g OH groups. I n s p i t e o f these u n c e r t a i n t i e s , t h e t h e r o e t i c a l calculat i o n s allowed much p r o g r e s s t o be made i n our knowledge of The CND0/2 c a l c u l a t i o n t y p e i s z e o l i t e overall properties. genera1 i n i t s concept since i t c o n s i d e r s t h e Sf, A1 d i s t r i b u t i o n but i t has o n l y been a p p l i e d t o c l u s t e r s s i m u l a t i n g the No r e s u l t s f a u j a s i t e s t r u c t u r e i n i t s X o r Y forms. (101,102) a r e a v a i l a b l e f o r z e o l i t e s i n which t h e Al/Al+Si r a t i o l i e s between 0 and 1/6 s i n c e t h e t o t a l number o f ( A l + S i ) atoms i n t h e c l u s t e r i s 6. Very l a r g e c l u s t e r s should be used t o r e p r e s e n t h i g h l y s i 1 i c e o u s zeol i t e s i n which t h e A1 / A l + S i r a t i o v a r i e s from 0.1 t o 0.01 o r even 0.005 ( f o r i n s t a n c e ZSM-5 w i t h S i / A l r a t i o from 10 t o 200). These l i m i t s r e s t r i c t t h e genera l i z a t i o n t o v a r i o u s s t r u c t u r e s and t o low A1 c o n t e n t s which a r e b o t h of g r e a t i n t e r e s t f o r c a t a l y t i c purposes. The Sander son e l e c t ronegat iv i t y t y p e c a l c u l a t i o n s a r e a1 so general i n t h e sense t h a t t h e y a r e based o n l y on t h e chemical A t low A1 composition, and a r e independent o f t h e s t r u c t u r e .
content t h e r e s u l t s o b t a i n e d a r e l e s s s i g n i f i c a n t since t h e c a l c u l a t i o n does n o t d i s t i ngui sh between two d i f f e r e n t A1 d i s t r i b u t i o n s which may g i v e very d i f f e r e n t a c i d strengths. The Sanderson e l e c t r o n e g a t i v i t y , which appears t o be e x t r e m e l y i n rationalizing overall acid properties h e l p f u l (21,22,90,100) and some c a t a l y t i c p r o p e r t i e s o n l y a l l o w s a comparison o f zeolites w i t h s i m i l a r acid strength d i s t r i b u t i o n . No model i s quite satisfactory t o describe the properties o f the highly s i 1iceou s zeo1 ites. Another p o i n t concerns t h e wavenumber o f acid hydroxyl s ( h i g h frequency band). The observed, a l m o s t constant, value o f FOH f o r A l / A l + S i r a t i o s l o w e r than 0.17-0.15 was proposed t o show t h e absence o f any s i g n i f i c a n t i n t e r a c t i o n s between t h e a c i d s i t e s . The a c t i v i t y c o e f f i c i e n t s I n fact, would be 1 f o r t h e p r o t o n s i n these z e o l i t e s . ( l 0 5 ) the Sander son e l e c t r o n e g a t i v i t y o f these z e o l i t e s c o r r e l a t e s well w i t h t h e 'jOH v a l u e s b u t n o t w i t h t h e c a t a l y t i c p r o p e r t i e s o r w i t h t h e a c i d s t r e n g t h measured i n a d i f f e r e n t way. (22) An attempt was made t o c o r r e l a t e t h e CoH s h i f t t o a f i e l d param e t e r ( l 0 5 ) b u t t h e r e l a t i o n s h i p i s n o t l i n e a r a t low A1 contents. I n view o f these r e c e n t r e s u l t s i t becomes more and more d o u b t f u l t h a t i n t h e range of A l / A l + S i r a t i o s lower t h a n 0.16 o v e r a l l p r o p e r t i e s can be deduced from simple model s. In the f a u j a s i t e s t r u c t u r e t h i s value corresponds t o one A1 p e r six-membered r i n g . For such " d i l u t e " composition, local pa ir s o f envi ronment effects (gradient of compo s i t ion, sites.. ) would p r e v a i 1 over t h e whole chemical composition. I n t h i s range, t h e o v e r a l l p r o p e r t i e s would s t i l l be o f s i g n i f i c a n c e o n l y f o r z e o l i t e s which would show comparable A1 d i s t r i b u t i o n in t h e framework.
.
4.5.3
Connection Between Short Range and Long Range Protonic Acid Strength
From t h e evidence p r o v i d e d by t h e s h o r t range and t h e l o n g range approach i t i s suggested t h a t two k i n d s o f p r o t o n i c strengths have t o be considered. An o v e r a l l a c i d s t r e n g t h is a c h a r a c t e r i s t i c o f t h e z e o l i t e and i s determined by an i n t r i n s i c parameter such as t h e A l / A l + S i r a t i o o r t h e Sanderson e l e c t r o negativity. An e n v i ronmental a c i d s t r e n g t h modi f i e s t h e f i r s t one by t a k i n g i n t o account t h e l o c a l n e i g h b o r i n g e f f e c t s and gives t h e d i s t r i b u t i o n o f s t r e n g t h s around t h e average o v e r a l l value. The f i r s t o r i g i n a t e s from o v e r a l l p r o p e r t i e s and t h e second from s h o r t range i n t e r a c t i o n s .
A t low A1 c o n t e n t s ( A l / A l + S i < 0.15-0.17) small changes i n A1 d i s t r i b u t i o n would g r e a t l y modi fy t h e e n v i ronmental a c i d strength. I n t h e i n t e r m e d i a t e and h i g h A1 c o n t e n t range a decrease i n A l / A l + S i r a t i o s h i f t s t h e whole a c i d s t r e n g t h
towards s t r o n g e s t a c i d i t y . The a b s o l u t e o v e r a l l a c i d s t r e n g t h o f each p r o t o n i s increased. 4.5.4
Lewi s A c i d i t y S t r e n g t h
-
I n t e r a c t i o n Between Close S i t e s
Si nce t h e importa-pce o f Lewi s a c i d i t y i n c a t a l y s i s is much l e s s than t h a t of Bronsted a c i d i t y , no d e t a i l e d s t u d i e s have been performed t o c h a r a c t e r i z e it. It i s u s u a l l y considered as a "by-product" o f p r o t o n i c a c i d i t y . The connections between t h e two t y p e s o f s i t e s make i t i n t e r e s t i n g t o look a t t h e i r changes i n s t r e n g t h upon v a r i o u s treatments. I t has been known f o r a l o n g t i m e t h a t f o r a g i v e n a c i d i c z e o l i t e t h e Lewis a c i d i t y i s always s t r o n g e r t h a n t h e Bronsted a c i d i t y (1013-150" d i fference i n t h e temperature o f complete p y r i d i n e evacuation). (48) Such a para1 l e l ism s t i 11 e x i s t s when t h e p r o t o n i c a c i d i t y s t r e n g t h i s changed upon v a r i o u s t r e a t ments. The s t r e n g t h o f Lewi s s i t e s i n c r e a s e when t h e A1 content decreases. I t v a r i e s as t h e Bronsted s i t e s s t r e n g t h . ( l 0 7 ) A s i m i l a r c o r r e l a t i o n between t h e simultaneous decrease i n B r o n s t e d and Lewi s a c i d i t y s t r e n g t h s upon dehydroxyl a t i o n i n NaHY zeol it e s has been p o i n t e d o u t very r e c e n t l y . (108) A1 1 t h e r e s u l t s p o i n t o u t a very strong interdependence between t h e s t r e n g t h o f t h e two t y p e s o f s i t e a t l e a s t f o r a consta*?t A1 /Al+Si r a t i o . L u n s f o r d proposed t h a t t h e s t r e n g t h o f Bronsted s i t e s may be i n c r e a s e d by e l e c t r o n a t t r a c t i o n by Lewis s i t e s . (109) From h i s new data, D a t k a ( l 0 8 ) suggests t h a t t h e i n v e r s e e f f e c t a1 so e x i s t s ; t h e s t r e n g t h of b o t h t y p e s o f s i t e depends on t h e l a t t i c e p o l a r i z a t i o n . I n contrast t o p r e v i o u s models, t h i s one t a k e s i n t o account t h e i n t e r a c t i o n s between s i t e s o f a very d i f f e r e n t n a t u r e f o r a g i v e n Al/Al+Si ratio. I t i m p l i e s t h a t s i t e s a r e c l o s e enough so t h a t t h e p o l a r i z a t i o n e f f e c t can occur. I t e x p l a i n s t h e presence o f very s t r o n g a c i d s i t e s i n steamed m o r d e n i t e s ( l l 0 ) which a r e supposed t o be formed a s superacids i n s o l u t i o n , (Al0)p d e p o s i t s a c t i n g a s Lewis s i t e s . I n t h i s hypothesis o f a recipr o c a l i n f l u e n c e of s i t e s , t h e Lewi s a c i d s t r e n g t h should a1 so be very high. T h i s i s i n good agreement w i t h a c a l c u l a t i o n o f Lewi s s i t e s t r e n g t h o f e x t r a framework c a t i o n i c aluminum. (11 1) As a r e s u l t o f t h e i d e a o f r e l a t e d s t r e n g t h s , i t may be expected t h a t t h e a d s o r p t i o n o f a molecule on one s i t e w i l l m o d i f y t h e e q u i l i b r i u m between t h e s t r e n g t h o f b o t h t y p e s o f site. The a c i d s t r e n g t h of t h e s i t e which i s adsorbing the molecule will i t s e l f be d i sturbed. T h i s may happen i n a c i d i t y mea surement s and a1 so d u r i n g a c a t a l y t i c reaction.
I n a s i m i l a r s i t u a t i o n i t has been suggested t h a t i n t e r actions between c l o s e o x i d i z i n g and r e d u c i n g s i t e s e x i s t i n zeolites. I t has been shown t h a t z e o l i t e s possess e l e c t r o n acceptor and e l e c t ron-donor c e n t e r s whi ch i n t e r a c t w i t h e l ectron-donors ( p e r y l ene f o r i n s t a n c e ) o r electron-acce+ptors ( t e t r a c y a n o e t h y l ene) t o form paramagneti c p o s i t i v e (Pe ) o r Strong i n t e r a c t i o n negative (TCNE-) ions, r e s p e c t i v e l y . (58) between t h e two t y p e s o f s i t e s i s shown by enhancement (up t o t e n f o l d ) o f t h e r e d u c i n g power o f z e o l i t e samples when c e r t a i n electron-donor molecules a r e adsorbed on t h e surface. Studies o f the e f f e c t o f the s i t e density or strength ( i o n i z a t i o n p o t e n t i a1 ) o f t h e e l ectron-donor mol ecul e show t h a t t h e enhancement requires neighboring s i t e s o f not too high energy. The same e x p l a n a t i o n has been used t o d e s c r i b e t h e enhancement o f t h e zeol it e e l e c t ron-donor p r o p e r t i e s upon interaction of small Pt particles with the oxidizing sites. (112) The metal p a r t i c l e s become e l e c t r o n - d e f i c i e n t . Various p r o o f s have been g i v e n o f t h e e l e c t r o p h i l i c c h a r a c t e r o f these m a l l P t p a r t i c l e s encaged i n t h e z e o l i t e channels. I n summary, b o t h a c i d i t y r e s u l t s and t h e behavior o f reducing and o x i d i z i n g s i t e s p r o v i d e good evidence of a s t r o n g interdependence between s i t e s i n c l ose p r o x i m i t y .
5.
Overal l Concepts o f Z e o l i t e P r o p e r t i e s
Several u n i f y i n g p r i n c i p l e s emerged physicochemical f e a t u r e s o f z e o l i t e s .
from
underlying
D e t a i l e d s t u d i e s on i o n i z i n g p r o p e r t i e s o f zeol it e ( l l 3 115) l e d Rabo t o p r e s e n t an o v e r a l l view o f z e o l i t e s considered as e l e c t r o l y t e s . (116) Strong i n t e r a c t i o n s between t h e p o l a r i z a b l e hydrocarbons and t h e s t r o n g l y p o l a r i n t r a c r y s t a l l i n e surface g i v e r i s e t o a h i g h c o n c e n t r a t i o n o f r e a c t a n t s p e r s i s t i n g even a t h i g h temperatures. As a consequence, t h e r a t e o f b i m o l e c u l a r r e a c t i o n steps is enhanced over t h a t o f unimolecul a r r e a c t i o n steps. An example i s t h e hydrogen t r a n s f e r step (Scheme 1) from c y c l o a l k a n e s t o o l e f i n s g i v i n g t h e h i g h aromatics + p a r a f f i n s y i e l d c h a r a c t e r i s t i c o f c r a c k i n g i n zeol ites. Hydrogen t r a n s f e r r e a c t i o n s a r e observed w i t h . b o t h K Y o r a c i d i c HY. They a r e then n o t r e l a t e d t o t h e Bronsted a c i d i t y only b u t t o a cage e f f e c t . The b e h a v i o r o f z e o l i t e s a s e l e c t r o l y t e s i s a l s o r e f l e c t e d i n a l a r g e enhancement o f i o n i z a t i o n r e a c t i o n s and i n t h e s t a b i 1i z a t i o n o f carbonium ions. S t u d i e s on t r a n s i t i o n metal complexes i n z e o l i t e s ( l l 7 , 1 1 8 ) showed t h a t zeol it e s e x h i b i t some p r o p e r t i e s o f c o n v e n t i o n a l s o l v e n t s and behave a s a s o l i d m a t r i x . Nevertheless t h e cage
geometry i s s t i l l very i m p o r t a n t f o r t h e complex f o r m a t i o n and usual s t a b i l i z a t i o n and i t d i s t i n g u i s h e s z e o l i t e s from so1 vents. C a l c u l a t i o n s , u s i n g t h e Sanderson e q u a l i z a t i o n p r i n c i p l e , on charges on atoms were made i n o r d e r t o r a t i o n a l i z e t h e p r o p e r t i e s o f z e o l i t e s . (21,100) F u r t h e r c a l c u l a t i o n s on a l a r g e v a r i e t y o f z e o l i t e s t r u c t u r e s w i t h d i f f e r e n t chemical composit i o n s gave a s t r o n g b a s i s f o r a u n i f y i n g concept.(22) Some m a j o r zeol i t e p r o p e r t i e s (wavenumber o f OH groups, a c i d i t y s t r e n g t h , t u r n over number i n isopropanol decomposition o r ndecane hydroconversi on) can be r e l a t e d t o t h e z e o l i t e Sanderson e l e c t r o n e g a t i v i t y ( 2 2 ) which can be i d e n t i f i e d t o t h e negative chemical p o t e n t i a l (119) ( F i gure 5). Thi s g i v e s an i m p o r t a n t t o o l f o r t h e p r e d i c t i o n and t h e understanding o f a c i d i c and c a t a l y t i c properties. F u r t h e r improvements o f t h e model a r e needed i n t h e range o f low A1 c o n t e n t since t h e Sanderson e l e c t r o n e g a t i v i t y does n o t t a k e i n t o account t h e great importance o f A1 d i s t r i b u t i o n s , i.e. o f l o c a l geometry. The f i n d i n g t h a t sel f - i n h i b i t i o n c o e f f i c i e n t s i n a c i d i t y changed u n i f o r m l y w i t h t h e zeol it e A l c o n t e n t (25) generated the Activity i d e a t h a t z e o l i t e s behave a s solutions.(l6,120) c o e f f i c i e n t s should e x i s t and would g r e a t l y reduce any c a t a l y t i c r a t e a t h i g h a c i d i t y c o n c e n t r a t i o n i.e. h i g h A1 content. By c o n t r a s t a t low A1 l e v e l , a s i n d i l u t e sol u t i o n s , no i n t e r a c t i o n should decrease t h e r e a c t i o n r a t e and t h e a c i d The s i t e s should behave a s i f t h e y were f u l l y i s o l a t e d . e x i stence o f such a c t i v i t y c o e f f i c i e n t s e x p l a i n s t h e maxima observed i n v a r i o u s r e a c t i o n s ( F i g u r e 1 ) a s a f u n c t i o n o f the S i / A l ratio. The l i n e a r i n c r e a s e i n n-hexane c o n v e r s i o n w i t h t h e ZSM-5 z e o l i t e A l c o n t e n t ( l 7 , 1 8 ) i s a l s o i n l i n e w i t h a c o n s t a n t value o f 1 f o r t h e a c t i v i t y c o e f f i c i e n t i n t h e low A1 l e v e l range. The c a l c u l a t e d p r o b a b i l i t y f o r having no close neighbor i n 4 - r i n g s f a u j a s i t e s t r u c t u r e , i.e. no c l o s e s i t e i n t e r a c t i o n s , f o l l o w s t h e sel f - i n h i b i t i o n c o e f f i c i e n t curves a s a f u n c t i o n o f A1 c o n t e n t a s g i v e n i n F i g u r e 4. (14) The analogy o f z e o l i t e s w i t h s o l u t i o n s may a1 so be extended t o e l e c t r o c h e m i s t r y f o r metal-loaded z e o l i t e s .
6.
Concl u s i on s
Many r e a c t i o n s i n v o l v i n g carboni urn i o n s i n t e r m e d i a t e s are c a t a l y z e d by a c i d i c z e o l i t e s . (121) W i t h respect t o a p u r e l y chemical standpoint t h e r e a c t i o n mechani sms a r e n o t fundament a l l y d i f f e r e n t w i t h z e o l i t e s o r w i t h any o t h e r a c i d i c oxides. What z e o l i t e s add a r e cage e f f e c t s , even i n the absence o f geometrical shape s e l e c t i v i t y , and o v e r a l l properties. The cage e f f e c t s , a r i s i n g from t h e h i g h l y i o n i z i n g power
o f z e o l i t e s , (116) a r e p r o b a b l y r e s p o n s i b l e f o r t h e l a r g e v a r i e t y o f s e l e c t i v i t i e s observed. I t seems r a t h e r d i f f i c u l t t o r a t i o n a l i z e and p r e d i c t those e f f e c t s i n t h e very near future. The o v e r a l l p r o p e r t i e s have been more deeply They a l l o w a b e t t e r understood i n t h e l a s t years. (16,22) understanding and improved p r e d i c t i o n s of t h e e x t e n t o f t r a n s f o r m a t i o n o f any r e a c t a n t . They a1 so a f f o r d a f u r t h e r i n s i g h t i n t o t h e n a t u r e o f a c i d i t y and i t s c o r r e l a t i o n w i t h c a t a l y t i c properties. R e s p i t e a l l t h e p r o g r e s s made, these are s t i 11 o p p o r t u n i t i e s f o r new d i s c o v e r i e s i n t h e fundamental and applied f i e l d s o f the acid c a t a l y s i s w i t h zeolites.
Refe -ence s
1. 2. 3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
15. 16. 17. 18.
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-
-
R. Beaumont and D. Barthomeuf, J. Catal. 1972, 27, 45. K. V. Topchieva and H, S. Thuoang, Zh. F i z . ~ x m . 1973, 47, 2103. 1. Moscou and R. Mone, J. Catal. 1973, 30, 417. W. F. K l a d n i g, J. Phys. Chem. 1979, 76583, and 1976, 80, 262. D. Barthomeuf, J. Phys, Chem. 1979, 83, 766. P. A. Jacobs and J. B. ~ytterhoeven,~. Catal. 1972, 26, 175. P. A. Jacobs, B. K. G. Theng and J. B. Uytterhoeven, J. Catal. 1972, 26, 191. M. F. G u i l l e u y J. F. Tempere and D. Delafosse, J. Chern. Phys. 1974, 6, 963. V. G. ~ v a k a G y a , V. I. K u l i v i d z e and G. V. T s i t s i s h v i l i , 223, 273. Dokl. Akad. Naurk. SSSR 1975, J. A. Rabo, V. Schomaker and P. E. P i c k e r t , Proc. Tnterrn. Cong. Catal., 3 r d Amsterdam, 1964, 2, 1264. D. M. Breck, C. R. C a s t o r and R. F. M i l t o n , U.S. P a t e n t 3,013,990 (1961). C. Naccache and Y. Ben T a a r i t , J. C a t a l . 1971, 22, 171. P. A. Jacobs and H. K. Reyer, J. Phys. ~ h e m . 7 9 7 9 , 83, 1174. K. H. S t e i n b e r g , Kh.M. Minachev, H. Bremer, R. V. D m i t r i e v and A. N. Detyuk, Z. Chem. 1975, 15, 372. K. Tsutsumi, S. F u j i and H. Takahashi, J. Catal. 1972, 24, 8. C. L. Angel1 and M. V. Howell, J. Phys. Chem. 1970, 74, 27 37. C. M i r o d a t o s , P. P i c h a t and D. Barthomeuf, J. Phys. Chem. 1976, 80, 1335. L. G. C h r i s t n e r and W. K. H a l l , J. J. B.'Uytterhoeven, Phys. Chem. 1965, 69, 2117. J. W, Ward, J. ~ a t x . 1967, 9, 225. 2, 266. P. A. Jacobs and W. J. ort tier, Z e o l i t e s 1982, J. W. Ward, J. C a t a l . 1968, 11, 259. P. A. Jacobs, L. J. D e c l e r T L. J. Vandamme and J. B. Uytterhoeven, J.C.S. Faraday Trans. I , , 1975, 71, 1545. P. E. P i c k e r t , J. A. Rabo, E. Dempsey and ~ T ~ c h o m a k e r , Proceed. 3 r d I n t . Cong. C a t a l . Amsterdam, 1964, 714. E. J. Dempsey, J. C a t a l . 1974, 33, 497 and 1975, 39, 155. R. J. Mikowsky and J. F. ~ a r s h a l l , J. C a t a l . 1976, 44, 170. W. Wachter, B r i t i sh Z e o l i t e Ass. Meeting, Chi s l e h u r s t , 1981. G. Engel h a r d t , D. Zeigan, E. Lipmaa and M. Magi, Z. Anorg. A1 1 g. Chem. 1980, 468, 35. J. K l i n o w s k i , J. M. ~ h o m C.i A. F y f e and G. C. Gobbi, N a t u r e 1982, 296, 533.
G. Engelhardt, U. Lohse, A. Samoson, M. Magi, M. Tarmak and E. Lippmaa, Z e o l i t e s 1982, 2, 59. W. J. M o r t i e r , J. C a t a l . 1978, 5 5 , 138. S. Reran and J. Dubsky, J. ~ h y s 7 c h e m . 1979, 83, 2538. W. J. M o r t i e r and P. G e e r l i n g s , J. Phys. hex 1980, 84, 1982. R. T. Sanderson, "Chemical Bonds and Bond Energy", Academic Press, New York, 1976. O. Barthomeuf, J.C.S. Chem. Comm. 1977, 743. D. Barthomeuf i n " C a t a l y s i s by Z e o l i t e s " , Elsevier, Amsterdam, 1980, 55. N. Y. Topsoe, K. Pedersen and E. G. Derouane, J. C a t a l . 1981, 70, 41. R. Beaumont, P. P i c h a t , D. Barthomeuf and Y. Trambouze, " C a t a l y s i s", J. W. H i ghtower, ed., N o r t h Hol l a n d , Amsterdam, 1973, 1, 343. F a r a d . Trans. I, 1981, 77, 2877. J. Datka, J.C.S. J. H. Lunsford, J. Phys. Chem.. 1968, 72T4163. C. M i r o d a t o s and D. Barthomeuf, Chem. z m m . 1981, 39. S. Beran, P. J i r u and B. W i c h t e r l o v a , J. Phys. Chem. 1981, 85, 1581. M. Dufaux, C. Naccache and B. I m e l i k , J. c.-Vedrine, J.C.S. Farad. Trans. I, 1978, 74, 440. P. H. Kasai and R, J, bishop:^. Phys. Chem. 1973, 77, 2308. J. A. Rabo and P. H. Kasai, Prog. S o l i d S t a t e Chem. 1975, 9, 1. J. A. Rabo i n " Z e o l i t e Chemistry and C a t a l y s i s " , J. A. Rabo ed., ACS, Washington, DC, 1976, 332. J. A. Rabo, Catal. Rev. Sci. Eng., 1981, 23, 293. J. H. Lunsford, Catal. Rev. Sci. Eng. 1 9 7 K 12, 137. Y. Ben T a a r i t and M. Che 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 al., ed.) E l s e v i e r , Amsterdam, 1980, 5, 167. R. G. Parr, R. A. Donne1 l y , M. Levey and W. E T ~ a l k e ,J. Chem. Phys. 1978, 68, 3801. D. Barthomeuf, C . C A c a d . Sci, P a r i s , Ser. C, 1978, 181. M. L. Poutsma i n " Z e o l i t e Chemistry and C a t a l y s i s " , (J. A. Rabo ed,) ACS Monograph 1976, 171, 437.
286,
MOLECULAR SHAPE-SELECTIVE CATALYSIS BY ZEOLITES
Eric G. Derouane Mobil Research and Development Corporation Central Research Division P. 0. Box 1025 Princeton, New Jersey 08540 U.S.A.
1.
INTRODUCTION
Zeolites are three-dimensional framework aluminosilicates presenting a large intracrystalline free volume which consists of cavities and/or pores. Building-in catalytically active sites within such structures is the essence of molecular shape-selective catalysis. These active sites can pertain to the zeolite framework itself: Br$nsted acidic sites are associated to the presence of framework aluminum atoms; they can be converted into Lewis acidic sites by dehydroxylation. Catalytic centers can also be metals or their ions which can be introduced into the zeolite framework by a variety of techniques, ion-exchange being most commonly used. Weisz and Frilette (1) were first to report 23 years ago on molecular shape-selective catalysis. It is now recognized that molecular shape-selectivity can be achieved by virtue of diffusional effects, steric constraints, aqd even coulombic field interactions. Since the original article by these authors, more than 300 essential contributions have appeared in the journal and patent literatures, Critical discussions and reviews of molecular shapeselective effects in catalysis have been proposed recently by Csicsery (2), Weisz ( 3 ) , and Derouane (4,5)
.
The concept of molecular shape-selective catalysis (3) is based on the action of catalytically active sites, internal to the z e o l i t i c framework, on molecular structure(s) which can exist and/ or diffuse in the structural environment where such sites have been generated. Clearly, it implies an intimate interaction between the shape, size, and configuration of the molecules taking part in the
reaction, and the dimension, geometry, and tortuosity of the channels and cages of the zeolite used as catalyst or catalytic support. Molecular shape-selective catalysis must therefore be considered as one of the first, and an outstanding, illustration of "molecular engineering" (6). Several types of molecular shape-selective effects exist: (1) REACTANT OR CHARGE SELECTIVITY will take place if the reactants can be divided into two or more classes of molecules, of which one, at least, will not be able to enter or diffuse freely within the intracrystalline volume of the zeolite because of diffusion constraints, selective sorption, or molecular sieving effects. Molecular shape-selective cracking, hydrocracking, and selectoforming are typical processes which take advantage of this property. (2) PRODUCT SELECTIVITY occurs when similar restrictions apply to the product molecules. It plays an important role in the selective production of para-aromatic compounds over ZSM-5 zeolite based catalyst and also affects the deactivation by coking of zeolite catalysts in general. ( 3 ) RESTRICTED TRANSITION-STATE MOLECULAR SHAPE SELECTIVITY is observed when local configuration constraints, acting in the direct environment of the catalytically active sites, will prevent or decrease the occurrence probability of a given (bimolecular, for example) transition state, characteristic of an elementary catalytic step. It can act directly on the reactants as proposed in the cracking of paraffins (7) or by impeding the formation of bimolecular complexes, as claimed to justify the near-absence of transalkylation in the isornerization of the xylenes (8) and the typical hydrocarbon distribution from the methanol conversion (9) over ZSM-5 based catalysts.
The former molecular shape-selective effects are schematized in Fiqure 1. Other manifestations of molecular shape-selectivity are the concentration effect proposed by Rabo (lo), the general concept of molecular traffic control (11-13) which still needs definitive support, and the importance of molecular circulation in the internal free volume of zeolites such as erionite and offretite (14,15). Clearly, the discussion of molecular shape-selective catalysis hence requires (a) a brief description of the relevant zeolite frameworks and pore systems (5,16), (b) the delineation of the essential factors whichgovern intracrystalline diffusion (3,5,17), and (c) a classification and an illustration of typical molecular shape-selective effects and their uses. Table 1 lists some of the major industrial processes based on molecular shape-selective zeolites (3).
roduct sekctivii (Pam-dlrected orornatics reactions)
w.4
0 Restricted transition state selectivity
(Prevention of t rum-alkylation)
Figure 1. Idealized representation of typical molecular shapeselective effects in zeolite catalysis (from reference 4). TABLE 1
Industrial Molecular Shape-Selective Processes (adapted from reference 3) process
Major chemical/ Process Characteristics
Objective
SeZectofoming
Octane number increase in gasoline; LPG production
Selective n-paraffin cracking
M- Forming
Octane number increase in gasoline
Cracking depending on degree of branching; aromatics alkylation by cracked fragments
Light fuel from heavy fuel oil; reduction of lubes pour point
Cracking of high molecular weight n- and mono-methyl paraffins
Xy Zene Isomerization
High yield para-xylene production
Ethy Z Benzene
High yield ethyl benzene production
Toluene Disproportionation
Benzene and xylenes from toluene
MethanoZ-toGas0 line
Methanol conversion to high grade gasoline
1
II
High yield, long cycle life; suppression of side reactions
Synthesis of hydrocarbons restricted to gasoline range, including aromatics
2.
PORE SYSTEM CHARACTERISTICS OF INDUSTRIALLY IMPORTANT ZEOLITES
Zeolites of which the industrial importance has been widely recognized are the A, X I and Y zeolites, ZSM-5, eri~nite~offretite, and mordenite. Figure 2 summarizes the major features of their pore structure (5) and compare their critical dimensions to those of typical hydrocarbon molecules. Zeolites A, X, Y, erionite and offretite have both channels and cages while ZSM-5 and mordenite only have channels. The intersecting channels of ZSM-5 may allow a three-dimensional motion for molecules of the proper size whilst the differentiated pores of mordenite renders the latter structure essentially unidimensional with respect to the diffusion of hydrocarbon molecules. Asevidenced from Figure 2, zeolite ZSM-5 (as well as ZSM-11) has unique channel dimensions (ca. 0 . 5 5 nm) and bridges the two classical zeolite categories, i.e., the large pore mordenite and faujasite-structure (x,Y) materials which accept in their free intracrystalline space most simple organic molecules (linear and branched aliphatics and single ring aromatics) and the small pore structures which only adsorb linear aliphatics. Zeolite ZSM-5 can adsorb, by decreasing order of preference, normal paraffins, isoparaffins, other monomethylsubstituted paraffins, and single ring aromatic hydrocarbons containing up to ca. 10 C-atoms (18). Detailed descriptions of the ZSM-5 zeolite adsorptive properties have been given by Dessau (19), Olson ( 2 0 ) , Gabelica (21), and Jacobs ( 2 2 ) .
-4 -LINEAf?
PARAFFINS
-7 NAPHTHALENE
Figure 2. Pore structure of industrially important zeolites (from reference 5) .
Table 2 compares hydrocarbon sorptions by zeolites ZSM-5 and ZSM-ll ( 2 2 ) . As expected from its structural characteristics, ZSM-5 has a larqer sorption capacity. The comparison of methylnonanes adsorptions over both zeolites also indicates that the substituted carbon atom and its methyl side-chainprefersto sit at the channel intersections. Such preferential.molecular configurations affect the shape-selective behavior of these zeolites.
TABLE 2
Hydrocarbon Adsorptions over Zeolites HZSM-5 and HZSM-11 (adapted from reference 22) Sorption Temperature Sorbate
(K)
Maximum number of molecules per unit cell HZSM-5 HZSM-11
C3* C4* i-C4* C5* i-C5*
C6 *
c7*-* C8* *
C9** C10* * 2MC9 * *
3MC9* * 4MC9* * 5MC9* *
Neopentane* p-Xylene** m-Xylene** o-Xylene* *
*Derived from adsorption isotherms. **Derived from thermogravimetric data at p/po
=
0.5.
3.
DIFFUSION IN ZEOLITES
The classical theory of diffusion considers two regimes: the normal diffusion regime in which the pore size of the host material is greater than the mean free path of the diffusing molecules and the Knudsen regime for which the diffusivity decreases with the pore dimension. As emphasized by Weisz, a new diffusion regime exists in zeolites, i.e., configurational diffusion (23). It implies that molecular migration within the zeolite framework necessitates the matching of size, shape, and configuration of the diffusing species to the corresponding parameters of the zeolite. Figure 3 illustrates the various diffusion processes encountered in porous solids. Barrer (24) has reviewed the major features of diffusion in zeolites. Some of those were also discussed by Derouane (5) who insisted on the distinction to be made between classical non-equilibrium measurements and self-diffusion (at equilibrium) experiments (using NMR pulsed field-gradient techniques for example (25)). Self-diffusion becomes equivalent to counterdiffusion when all the diffusing molecules are identical. Weisz and Prater (26) demonstrated that the observed rate of catalytic reactions in zeolites are moderated by an effectiveness parameter q which is itself a function of a dimensionless variable 4' defined as:
CONFIGURATIONAL
ANGSTOMS
I
/-L
PORE SIZE
Figure 3. Diffusion mechanisms, diffusivity vs. pore size, in porous materials (from reference 23).
D being the diffusivity coefficient in the particle of equivalent radius R, C the concentration of the reactant(s), and dn/dt the observed reaction rate (27). Figure 4 illustrates the former dependence. The observed rate constant, kobs, is then related to the intrinsic rate constant, k, by kobs = q-k The interaction between diffusion and kinetics is then obvious: reactions characterized by a small ? value l (formation or transformation of antiselective species) will be selectively retarded with respect to tSose having a higher effectiveness factor (formation or transformatioc of proselective species) .
.
4. GEOMETRIC VS. ELECTROSTATIC EFFECTS Geometric effects (pore size, shape, and tortuosity) are generally claimed as the major factors affecting the molecular shape0 ~ decreases, selective behavior of zeolites. As the S i O 2 / ~ 1 ~ratio however, the influence of electrostatic fields will become more noticeable as a result of the higher framework charge and the larger concentration in counterions, offering thereby a better discrimination between polar molecules ( 6 , 2 8 ) and leading to selective sorption.
Figure 4. Effectiveness factor, ll, as a function of the dimensionless variable @ (from references 26 and 27).
Electrostatic interactions may take place between the net dipolar moment of the reactant moLecules and the electrostatic field at the pore mouths (windows), orienting the diffusing molecule in a favorable or critical position with respect to the channel (window). The role of electrostatic effects has been demonstrated most convincingly for type A molecular sieves, in particular for the isomerization of 1-butene to cis- and trans-2-butenes (28) (see Figure 5).
5.
MOLECULAR SHAPE-SELECTIVITY MODIFICATIONS
Before discussing specifically molecular shape-selective effects, it is necessary to make a formal distinction between the molecular shape-selective active sites present in the intracrystalline volume of the zeolite and those present on the external surface of the crystallites. Those will also show activity but no shape-selectivity. Typically, the "external" surface area will represent about one percent of the total zeolite surface area for a crystallite size of one micron (29).
But -I-ene
cis-But2-ene
trans-But- 2-ene
Figure 5. Electrostatic effects in molecular shape-selective catalysis. (A) Dipolar moment orientation in the butene isomers; (B) molecular orientation with respect to the pore openings (from reference 28) .
That reactions occur in the intracrystalline volume of zeolites was demonstrated in the very early stages of molecular shape-selective catalysis ( 6 , 3 0 ) . Zeolite Linde type A was found to crack selectively linear paraffins (30). A (Pt,Na)-mordenite hydrogenation catalyst was able to remove selectively ethylene from mixed propylene-ethylene feeds by converting the latter to ethane (6). Small crystallites, with a larger external surface, will have decreased molecular shape-selectivity as it can be illustrated, for example, by the increased production of unwanted durene in the methanol-to-gasoline conversion (MTG) over HZSM-5 type catalysts (see Table 3) . It is then easily conceived that the molecular shape-selective groperties of a zeolite are maximized when it is feasible to deactivate its external surface. Several such improvements have been reported recently for ZSM-5 catalysts. The active ZSM-5 phase can be bound by a preferably alumina-free material (32) or its surface coated by a metacarborane-siloxane polymer ( 3 3 ) . The most elegant way to deactivate the ZSM-5 zeolite external surface is, however, to take advantage of the fact that it can be prepared with nearly infinite SiO2/A12O3 ratio. ZSM-5 crystallites terminated by a virtually aluminum-free outer shell are more selective for the production of para-aromatic compounds: for example, the para/metaxylene ratio in the products is nearly doubled in the conversion of a mixed C6-Cg aliphatic-aromatic feed (315OC, 14 atm., hydrogen/ hydrocarbons = 3.6) (34)
.
As also demonstrated for zeolite ZSM-5, a fine tuning of the molecular shape-selective properties can also be achieved by selective coke deposition which may restrict the pore mouths in addition to deactivating the external surface or by chemical modifications with P ( 3 5 ) , Sb ( 3 6 , 3 7 ) , B ( 3 8 ) , Mg (39) containing compounds. Large cations such as CS+ and ~ a were ~ +claimed to increase the ethylene production selectivity in the methanol conversion (40) while the addition of ~ a + cations or of a group Va element maximizes the ~ ~ + - h ~ d r o c a r byield on in certain operating conditions (41). Most of these observations are understandable in terms of a modification of the catalyst diffusion characteristics, notwithstanding, however, secondary effects on its acidity.
TABLE 3 Durene Production in the MTG Conversion Over HZSM-5 Catalysts of Varying Crystallite Size (31) (Temperature = 371°C; Si02/AZ203 = 185; WHSV fh-1) = 2 ) Crystallite Size (microns) 0.02 2-5
Durene in HC Product (wt.% ) 5.9 2.6
6. REACTANT MOLECULAR SHAPE-SELECTIVITY A major application of molecular shape-selective catalysis is the removal of linear paraffins from liquid reformates to improve their octane number, or from distillates to lower their viscosity, pour point, and freezing point.
In Mobil's selectoforminu, linear paraffins are hydrocracked selectively from a mixture of paraffinic and aromatic hydrocarbons using a low potassium (Ni,H)-erionite zeolite as catalyst (42). Branched and cycloparaffins and aromatic hydrocarbons are not affected. As described by Chen and Garwood (43-45), linear chain hydrocarbons only can enter the erionite framework. Maxima are observed in the catalytic activity pattern for the cracking of C g and C10-11 hydrocarbons. They are attributed to a "cage" effect which is analogous in essence to the "window effect" described by Gorring (46) to justify the product distribution from the cracking of n-tricosane over H-erionite and the variation of the n-paraffins diffusion coefficients in zeolite T.
CAf3BCWl NUMBER OF NORMAL PARAFFIN
Figure 6. Product distribution from the cracking of n-tricosane over H-erionite (A) and diffusion coefficients of linear paraffins in zeolite T (B) (both at 340°C) (from reference 46).
In .the latter case, there is obviously a close relationship between the parameters which govern intracrystalline diffusion and the catalytic selectivity (see Figure 6 ) . Zeolite T is mainly offretite with a minor intergrowth of erionite; the erionite structure being less open than the offretite structure, erionite will limit the diffusion behavior of hydrocarbons in the intracrystalline free space of zeolite T. The erionite structure has a "window of high transmittance" for molecules ( C 3 - 5 and Cg-13) which can either orient themselves quickly with respect to the 8-membered ring window of the erionite cage or retain some orientation because they extend through the window limiting the cage. As a maximum is observed in the product distribution (C6-9) when large pore zeolites or silicaalumina are used as catalysts ( 4 7 1 , this observation demonstrates the superposition of a shape-selective pattern of diffusivities onto an intrinsically continuous reaction product distribution. As demonstrated recently by Haag et al. ( 4 8 ) , diffusion inhibition effects can be dissociated from the action of steric constraints on the transition state complex by considering zeolite crystallites with different sizes and activities. Table 4 lists pertinent data for the cracking of paraffinic and olefinic hydrocarbons over two HZSM-5 catalysts with different particle size. TABLE 4 Observed and Intrinsic Rate Constants for the Cracking of Hydrocarbons over HZSM-5 Catalysts (at 5 3 8 O C ) *
CRYSTAL SIZE, R(pm)
kobs 0.025 1.35 --
k
0 0.025 1.35 -
COMPOUND
Hexane 3-Methylpentane 2,2-Dimethylbutane Octane 2-Methylheptane Nonane 2,2-Dimethylheptane Dodecane
*See text for definition of symbols; from reference 48.
2 R k
1.35
They indicate that mass-transport limitations occur in the cracking of hexenes and of gem-dimethyl-paraffin isomers. Branching of the aliphatic chain is the essential factor which affects the relative effective diffusivities of the reactants at steady-state reaction conditions. The effects of chain length and branching on the relative cracking rates of Cg-7 paraffins have been described in detail by Chen and Garwood (49). As seen from Table 5, the following trends hold in the cracking of these paraffins:
(b) straight chain > 2-methyl > 3-methyl > dimethylor ethyl substituted. In contrast to the "window" or "cage" effect which is observed in erionite ( 4 6 , 4 7 ) , pore size has more importance for ZSM-5 cracking catalysts than the actual channel tortuosity. These unique molecular shape-selective properties of ZSM-5 catalysts constitute the essence of the Mobil distillate dewaxing (MDDW) process (50,511 in which a mixed feed of linear paraffins, isoparaffins, highly branched paraffins, and aromatics (gas-oil distillate) is selectively hydrocracked. Linear and isoparaffins react preferentially, as illustrated in Figure 7 , and the freeze and pour points of the distillate are lowered, thereby enabling one to adapt the properties of the product to climatic or utilization conditions requirements.
TABLE 5
Reactant Molecular Shape-Selective Effects in the Cracking of Paraffins over HZSM-5 (from reference 49) (Temperature = 340°C, Pressure = 35 atm., LHSV (h-1) = 1.41 Paraffin n-Heptane
2-Methylpentane
Relative Cracking Rate 1
0.38
I
50
'
m
~
l
g
.
,
,
I$ 200 260 320(hold) PROGRAMMED TEMPERATURE ("C)
Figure 7. Dewaxing of a midcontinent distillate ( 3 4 0 - 3 9 0 ° C ) on a ZSM-5 based catalyst : (A) before hydrocracking; ( B ) after processing, the n-C16-28 paraffins being selectively removed (from reference 5 0 ) . Reactant molecular shape-selectivity can also play a role in metal-catalyzed hydrogenation and oxidation (52). It has been observed, for example, that a Pt-ZSM-5 catalyst hydrogenates preferentially linear olefins while a Cu-ZSM-5 catalyst oxidizes mostly para-xylene when it is admixed with its ortho-isomer, the latter of course because of the higher diffusivity of para-xylene in ZSM-5.
7.
PRODUCT MOLECULAR SHAPE-SELECTIVITY
The striking analogy of the hydrocarbon product distributions stemming from the methanol and several triglycerides conversions on ZSM-5 illustrates product molecular shape-selectivity (see Figure 8) (3,531The methanol-to-9asoline (MTG) conversion is a large scale application of the concept of product molecular shape-selectivity. It is discussed at length elsewhere in this volume (54). The effect of pore size on the selectivity of the methanol or dimethylether conversion to olefins has been identified by Cormerais et al. (55) who compared the activities and selectivities of H-erionite, HZSM-5, and zeolite H-Y. The smaller the pore opening, the higher is the yield in (22-3 olefins. The best demonstration of product molecular shape-selectivity is found in a variety of reactions which use ZSM-5 based catalysts and aim at the selective preparation of para-aromatic compounds. Such processes are the disproportionation and alkylation (by methanol) of toluene (56-60) and the xylenes isomerization (56).
0 Porgffin
W ~ ~ A ~ O ~ Q Non Wmatics
Com oil Cs7H& 450dC, WHSV 2.4,3O/t caka 0
Methanol 45m,W HSV 0.67
C
O
~
Figure 8. Hydrocarbon product distributions from the conversions of methanol (A) and corn oil triglyceride, C57HIo406 ( B ) on HZSM-5 (from reference 3) .
Yields in para-xylene, exceeding the expected equilibrium values, can be observed because of diffusion/reaction interaction. Factors which increase the diffusion path length (larger crystals) or decrease the effective pore size of the ZSM-5 catalyst (bulky atoms such as P, bulky counterions, presence of inorganic fillers) and lower the activity of its non-selective external surface, favor the formation of the para-isomer ( 5 6 - 6 0 ) . Considering the disproportionation of toluene, Haag and Olson (61) have correlated the para-xylene selectivity of ZSM-5 catalysts of different crystal sizes and pore tortuosities (because of the presence of inorganic salts or coke plugging) with a diffusion residence time for orthoxylene obtained from separate sorption measurements (see Figure 9). Para-xylene selectivity is noticeably enhanced when diffusion/ reaction interactions increase. Young et al. (60) have recently analyzed in detail the reaction paths which lead to the formation of para-xylene in the toluene disproportionation or its alkylation by methanol and the xylenes isomerization. Figure 10 shows the
a
SORPTION TIME FOR 0-XYLENE, ~ , ~ % ( m i n )
Figure 9. Selectivity to para-xylene in the toluene disproportionation reaction vs. ortho-xylene sorption time for various ZSM-5 based catalysts. 0 = different crystal sizes; A = tortuosity increased by inorganic salts; = coked catalysts (from references 3 and 61). reaction paths,for the isomerization of the pure xylene isomers over non-modified and modified ZSM-5 catalysts as well as over nonzeolitic catalysts. Paths A, B, and C correspond to non-shape selective catalysts such as silica-alumina or phosphoric acid on Kieselguhr while paths A', B ' , and C ' are those followed with shapeselective catalysts. These are Mg and P-modified ZSM-5; as readily seen they yield a para-xylene concentration in excess of the expected thermodynamic equilibrium value. The same authors have compared the relative activities for toluene alkylation and xylene isomerization of the same catalysts. Modified shape-selective catalysts are characterized by a toluenemethanol alkylation rate which is about 2.5 to 15 times that of xylene isomerization. In contrast, non-modified HZSM-5 is ten times more active for xylene isomerization than for toluene alkylation. These observations are explained by an increased diffusion resistance in the shape-selective catalysts and by the relative diffusivities of toluene and methanol (high), para-xylene (medium), and orthoand meta-xylenes (low)
.
The selectivity of these reactions to yield para-xylene is favored additionally by restricted transition-state molecular shape selective constraints as discussed in the next section. These constraints apparently prevent the formation of the himolecular transition state which is necessary to transalkylate toluene within the zeolite and explain the high value of the rate ratio xylene isomerization/toluene disportionation, i-e., 100-1000 over nonmodified HZSM-5 ( 6 0 , 6 2 ) .
Figure 10. Reaction paths for the isomerization of the pure xylenes over shape-selective (A',B',C1) and non-shape-selective ( A , B , C ) catalysts (from reference 60).
8.
RESTRICTED TRANSITION STATE MOLECULAR SHAPE S E L E C T I V I T Y
Restricted transition state molecular shape-selectivity is observed when steric constraints in the environment of the catalytic site affect or prevent the formation of intermediate complex structures. The inability to reach a given transition state will affect both monomolecular and bimolecular reactions; it can, of course, also discriminate between mono- and bimolecular complexes which occur along the various possible reaction paths for a given These constraints will act on intrinsic kinetics reaction ( 3 , 4 ) . rather than by diffusion/reaction interaction.
As mentioned in Section 7, this type of molecular shape-selectivity explains the low xylene disproportionation activity of HZSM-5. Haag and Dwyer (62) and Gnep et al. (63) have correlated the activity of vaxious zeolites (ZSM-5, ZSM-4, mordenite, and
Type Y) for the former reaction with their effective pore size. As expected, the transalkylation is dramatically inhibited as the pores become more restrained. Restricted transition state molecular shape-selectivity is essential to account for the high ethylbenzene selectivity which characterizes the formation of ethylbenzene by the Mobil-Badger process (64,65). In contrast to other zeolites such as mordenite or faujasite which rapidly deactivate as a consequence of coking, ZSM-5 based catalysts have a stable activity for cycle lengths of several weeks and yield ethylbenzene almost stoichiometrically. Further alkylation of ethylbenzene is prevented by the complementary actions of restricted transition state and diffusion constraints. The same argumentation and advantages hold to describe and justify the high para-methylstyrene yields obtained in the direct alkylation of toluene by ethylene over ZSM-5 class catalysts ( 6 6 ) . This particular type of selectivity, as discussed elsewhere (4,54,67,68), explains probably partially the selectivity of the formation of aromatic compounds (cut-off at C10) in the methanol conversion using HZSM-5 catalysts. The bimolecular cyclo-addition of an olefin and a carbeniurn ion is only possible at the channel intersections of ZSM-5, the smaller size products (following further dehydrogenation, alkylation, and isomerization) being able to diffuse out through its channels. Constraint index measurements which consist in the evaluation of the ratio of the cracking rates of n-hexane and 3-methylpentane, are recognized as means to characterize zeolites ( 6 9 ) . It has been demonstrated recently (48,70) that shape-selective constraints on the local kinetics govern these reactions. Indeed, the relative cracking rates of these two hydrocarbons are independent of crystal size (as shown for HZSM-5 catalysts) and then, apparently, free of diffusional effects ( 7 0 ) . The cracking mechanism implies hydrogen transfer between the reacting molecule and a carbenium ion as schematized in Figure 11. It is obvious that the larger transition state required for 3-methylpentane will lead to more severe steric inhibitions and lower conversions, in particular over ZSM-5 catalysts with critical pore diameter of ca. 0.6 nm. Constraint index measurements can then be considered as indirect evaluations of the free space available in the direct environment of the catalytic sites, which give support to their use as zeolite characterization means.
9.
MOLECULAR SHAPE-SELECTIVE EFFECTS I N THE COKING AND AGING OF ZEOLITES
Coke deposition in zeolites originates mainly from olefinic (71) and aromatic (72) compounds condensation and dehydrogenation reactions. The formation of coke can be viewed as follows (5,73):
-
n hexane
c. cross -section 4 . 9 6% ~
Fiqure 11. Transition states in the cracking of the hexane isomers (from reference 4 8 ) .
olefins (possibly resulting from paraffins dehydrogenation) are first cyclo-oligomerized to naphthenes which, in turn, can be converted to aromatics by successive dehydrogenation and hydrogen transfer steps; these aromatic compounds can be further alkylated and dehydrogenated to yield fused-ring aromatic compounds. Ultimately, those are progressively dehydrogenated into coke. Rollmann and Walsh (74-77) have investigated in detail the deposition of coke over a variety of zeolite catalysts and proposed an impressive correlation between coking activity and molecular shape-selectivity (77) (measured by the ratio of the cracking rates for n-hexane and 3-methylpentane). The latter plot, which is shown in Figure 12, was obtained by reacting a mixed feed of C6 hydrocarbons over various zeolites (425OC, 15 atm, H ~ / H C = 3). Intracrystalline coking clearly depends on the pore structure of the zeolite. The alkylation of aromatic compounds which can be converted to fused-ring products at a later stage, was found to be the initial and decisive step in the coking of mordenite and zeolite H-Y. For the small pore structures such as ferrierite and erionite, the Low coking activity seems to be related to constraints acting on the formation of cyclic coke precursors (naphthenes and cycloparaffins) from aliphatic reactants. The unusually low coking activity of ZSM-5 is attributed to restricted transition-state shape-selectivity which prohibits secondary reactions of alkylaromatics in its intermediate gore-size channels (74). As discussed by Derouane (5) and Dejaifve et al. (78), zeolite aging because of the deposition of carbonaceous residues is a function of two factors, namely the probability P(t) that an active site is accessible at time t and the corresponding conditional probability S(t) that it is not poisoned at the same time (79,80).
Figure 12. Coke yield vs. molecular shape-selectivity (relative cracking rate of n-hexane vs. 3-methylpentane) in the conversion of hydrocarbons over zeolite catalysts (from reference 77).
?(t) is a function of the channel network geometry while S(t) is
related to the characteristics (activity and molecular shape-selectivity) of the active site and its environment. A comparison of the aging and rejuvenation behaviors of HZSM-5, offretite, and mordenite indicated that they were intimately depending on the actual pore system structure. It also confirmed the proposal (5) that aging was less rapid for zeolites possessing interconnected channels. The initial coking activity was found to be directly related to the availability of the acid catalytic sites (78). When cavities are present, products can be formed which have a size too large to be desorbed through the windows leading to other cages or to the channels (a reversed product molecular shape-selective effect). Such bulky molecules will act as coke precursors and deactivate rapidly the zeolite. This situation is illustrated by
the "Faujasite Trap", described by Venuto et al., which occurs in the isomerization-oligomerization of 1-hexene over rare-earth exchanged zeolite-X (81). 10. MISCELLANEOUS MOLECULAR SHAPE-SELECTIVITY EFFECTS
Acidic Y-zeolite based catalysts show an increased H-abstraction rate compared to that of 6-scission when compared to silica-alumina gels. While the latter reaction is a monomolecular process, the former one is a bimolecular event which involves a hydride shift from a neutral hydrocarbon to a carbenium ion. Rabo et al. (82) rationalized this observation by proposing that "zeolites concentrate hydrocarbon reactants to a large extent...this concentration effect enhancing the rate of bimolecular reaction steps...over unimolecular (fragmentation) stepstf. Consequently, secondary cracking reactions become less important as olefins and naphthenes are more readily converted into more refractory (paraffinic) products, and larger net gasoline yields are observed. Jacobs et al. (83) have used the bifunctionaZ conversion of n-decane over Pt-loaded zeolites to characterize the catalyst molecular shape-selective properties. They demonstrated that: the distribution of the feed isomers, in particular the yield of 2-methylnonane, could be used to discriminate between the ZSM-5 and the ZSM-11 structures. The conversion of n-decane over Pt-ZSM-5 shows a high (ca. 60%) and unusual production of the above-cited decane isomer, far above equilibrium and at the expense of the other methylnonanes. Protonated cyclopropane structures were postulated as reaction intermediates and possibly explain the observed differences between ZSM-5 and ZSM-11 based catalysts. A preliminary evaluation of these results indicate that this type of test could be used to evidence intergrowths in pentasil zeolites and to detect inhomogeneities in the distribution of aluminum through the zeolite crystallites. Recently, the concept of rnoZecuZar traffic controz was proposed (11) on the basis of sorption measurements on zeolite ZSM-5 and
generalized (12) to include transformations in which (some of) the reactants reach the active sites through diffusion pathways less readily accessible to (some of the) products, or vice versa. Principally, molecular traffic control can occur in zeolite structures presenting non-equivalent but intersecting channels and should be effective when diffusion-limited kinetics take place. Although derived from near-equilibrium sorption measurements, the concept of molecular traffic control obviously applies only to dynamic systems (13). The alkylation of para-xylene by methanol over HZSM-5 (linear and zig-zag channels) and HZSM-11 (straight channels only) has been used to test this concept ( 8 4 ) . The higher aromatic-ring alkylation activity of HZSM-11, compared to HZSM-5, can be explained
by the absence of molecular traffic control although differences in these zeolite acid strengths (83) could also play a non-negligible role. The molecular traffic control concept clearly deserves more attention. Support for its existence should eventually be gained in dynamic reaction conditions, at or near stationary state. Mirodatos and Barthomeuf (85,86) have proposed from comparative studies of the cracking of n-heptane and n-octane on offretite and mordenite that moZecuZar circuzation was an important factor affecting the catalytic activity of zeolites. If molecular circulation is impeded, for example, at high cationic content levels or by the presence of coke, molecules that enter cages or channels can undergo more severe transformations. The molecular circulation effect seems apparented to the "window effect" put forward by Gorring (46).
11.
CONCLUSIONS
The matching of size and configuration of diffusing (reacting) species to those of the zeolite channels affect the kinetics of their catalytic conversion. Conversely, the diffusion of product molecules is influenced by the same factors. A more subtle molecular shape-selectivity effect is that of restricted transition-state shape-selectivity which implies local constraints on the active site kinetics, possibly leading to discrimination between unimolecular and bimolecular transition states.
The above factors give zeolites unique catalytic properties: their high activity which results from their ability to operate at high temperature with minimal deactivation is often complemented by an unusual selectivity because of their particular structural peculiarities.
ACKNOWLEDGMENTS
The author thanks the following publishers for having released their copyrights on the following figures and tables: Elsevier Publishing Company (Figure 1); Academic Press (Figures 2, 6, 10, and 12); American Chemical Society (Figure 3); American Association for the Advancement of Science (Figure 4); Petroleum Publishing Company (Figure 7); Pergamon Press (Figures 8 and 9, and Table 1) ; The Royal Society of Chemistry (Figure 11 and Table 4); Butterworth & Co., Publishers (Table 2).
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TRANSITION METAL EXCHANGED ZEOLITES : PHYSICAL AND CATALYTIC
PROPERTIES
Claude Naccache and Younes Ben T a a r i t 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. E i n s t e i n , 69626 V i l l e u r b a n n e , CcSdex, France
INTRODUCTION C a t a l y s i s by t r a n s i t i o n m e t a l compounds i n s o l u t i o n o r s u p p o r t e d on s o l i d s i s an a c t i v e f i e l d o f r e s e a r c h o f c o n s i d e r a b l e i n t e r e s t . I t i s worthwile t o r e c a l l t h a t t r a n s i t i o n m e t a l i o n s were found act i v e and s e l e c t i v e f o r a g r e a t number o f r e a c t i o n s among them, o x i dation of e t h y l e n e t o a c e t a l d e h y d e , hydroformylation o f o l e f i n s t o aldehydes, c a r b o n y l a t i o n and homologation o f a l c o h o l s , hydrogenation and i s o m e r i s a t i o n o f o l e f i n s , o l i g o m e r i s a t i o n and c y c l o d i m e r i s a t i o n of o l e f i n s , w a t e r g a s s h i f t r e a c t i o n e t c . I n recent years a new a r e a o f r e s e a r c h developed which c o n s i s t e d t o anchor o r imrnob i l i z e t o a s o l i d s u p p o r t a s o l u b l e t r a n s i t i o n m e t a l complex t o produce a p o t e n t i a l a c t i v e and s e l e c t i v e new t y p e o f heterogeneous c a t a l y s t . I n a d d i t i o n i t i s thought t h a t r e l a t i o n s h i p between homogeneous and heterogeneous c a t a l y s i s may be found through t h e s t u d i e s o f such h e t e r o g e n e i z e d c a t a l y s t s . S e v e r a l d i s t i n c t ways f o x "heterogeneizing homogeneous c a t a l y s t s " have been proposed. One approach was t o anchor t h e s o l u b l e m e t a l complex t o an o x i d e s u r f a ce e i t h e r through s u r f a c e oxygen bond r e s u l t i n g from t h e r e a c t i o n of t h e metal l i g a n d s w i t h hydroxyl groups, o r by l i g a n d exchange with f u n c t i o n a l i z e d o x i d e s u r f a c e . An a l t e r n a t i v e means f o r convert i n g homogeneous m e t a l complexes i n t o heterogeneous c a t a l y s t s i s t o introduce t h e a c t i v e complex i n t o t h e i n t e r c r y s t a l space o f a l a y e r l a t t i c e s i l i c a t e by exchanging t h e ~ a ' c a t i o n s o f t h e l a y e r s i l i c a t e by c a t i o n i c t r a n s i t i o n m e t a l i o n s . Z e o l i t e s c o n t a i n a l s o exchangeable c a t i o n s , t h u s t h e y can be used f o r anchoring s o l u b l e t r a n s i t i o n metal complexes. The z e o l i t e s t r u c t u r e t h u s behaves a s a " s o l i d solvent" and l e a d s t o a new c l a s s o f c a t a l y s t s when t h e m a t e r i a l i s exchanged w i t h t r a n s i t i o n m e t a l i o n s . E x t e n s i v e s t u d i e s on t r a n s i t i o n metal exchanged z e o l i t e s have been performed d u r i n g t h e l a s t
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two decades. S e v e r a l e x c e l l e n t reviews have been p u b l i s h e d i n recent y e a r s (1-8). I n t h i s review we have n o t a t t e m p t e d t o i n c l u d e every m a t e r i a l s o f i n t e r e s t concerning t r a n s i t i o n m e t a l i o n s i n z e o l i t e s , b u t r a t h e r we have p u t o u r e f f o r t s t o p r o v i d e a comprehensive s u r vey on t h e c h e m i s t r y o f t r a n s i t i o n m e t a l i o n s i n z e o l i t e s and t o show how such c h e m i s t r y h a s opened a v e r y promising new a r e a i n heterogeneous c a t a l y s i s . I t i s o u r hope t h a t t h e few examples assemb l e d i n t h e s e l e c t u r e s w i l l s t i m u l a t e t h e i n t e r e s t o f c a t a l y s t scient i s t s i n t h e s e m a t e r i a l s . I t i s worthwhile t o n o t e t h a t most of the m a t e r i a l s t h a t w i l l be d i s c u s s e d a r e of t h e f a u j a s i t e - l i k e s t r u c t u r e , which among o t h e r t y p e z e o l i t e s o f f e r t h e advantages o f having a t h r e e dimensional p o r e arrangement, and r e l a t i v e l y l a r g e c a v i t i e s .
1
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Procedures f o r t h e p r e p a r a t i o n o f z e o l i t e - s u p p o r t e d t r a n s i t i o n metal ions.
S e v e r a l methods have been used t o p r e p a r e z e o l i t e s c o n t a i n i n g t r a n s i t i o n metal i o n s . The most common method i s t h e w e l l known c o n v e n t i o n a l i o n exchange t e c h n i q u e . Other methods such a s reaction w i t h c h l o r i d e s a l t s , a d s o r p t i o n from t h e vapor phase o r i n s o l u t i o n o f m e t a l c a r b o n y l compounds, r e a c t i o n o f c a t i o n s p r e s e n t i n t h e zeol i t e c a v i t i e s w i t h a n i o n i c metal complexes, impregnation w i t h a sol u t i o n o f t h e m e t a l s a l t s . The d i s p e r s i o n o f t h e c a t i o n s and t h e i r l o c a l i z a t i o n w i t h i n t h e z e o l i t e framework o r / a n d t h e e x t e r n a l surfac e depend s t r o n g l y on t h e method employed. T h i s paragraph w i l l refer t o t h e s e v a r i o u s methods.
1-1- P r e p a r a t i o n by i o n exchange t e c h n i q u e . The procedure which i s c e r t a i n l y t h e most s u i t a b l e t o i n t r o d u c e c a t i o n s i n t o t h e zeolite framework c o n s i s t s o f exchanging t h e Na+ c a t i o n s which e q u i l i b r a t e t h e n e g a t i v e charge b e a r e d by A l O 4 t e t r a h e d r a , w i t h a s o l u t i o n of t h e m e t a l s a l t , through c o n v e n t i o n a l i o n exchange t e c h n i q u e . This p r o c e d u r e h a s been s u c c e s s f u l l y a p p l i e d i n most c a s e s , e s p e c i a l l y w i t h a l k a l i , a l k a l i - e a r t h and r a r e - e a r t h c a t i o n s . I n r e c e n t years s e v e r a l s t u d i e s on t h e k i n e t i c s and t h e e q u i l i b r i u m o f t h e exchange r e a c t i o n have been p u b l i s h e d . The g e n e r a l c o n c l u s i o n s which may be g i v e n a r e t h a t t h e exchange r e a c t i o n e q u i l i b r i u m depends on t h e conc e n t r a t i o n o f t h e exchanging metal s a l t s o l u t i o n , on t h e temperature. The g e n e r a l f e a t u r e s o f t h e e q u i l i b r i u m and k i n e t i c a s p e c t s have been reviewed r e c e n t l y ( 9 ) . I t was shown t h a t many exchange react i o n s i n X and Y z e o l i t e s f a i l t o proceed t o completion and i t was s u g g e s t e d t h a t t h e l a c k o f t o t a l sodium exchange i s due t o t h e diffic u l t y o f d i s p l a c i n g t h e r e s i d u a l Na+ c a t i o n s p r e s e n t i n t h e sodalite c a g e s , i n Nay z e o l i t e a b o u t 16 ~ a a+r e l o c a l i z e d i n t h e s o d a l i t e cages. S i n c e t h e exchangeable c a t i o n s a r e g e n e r a l l y h y d r a t e d , the free d i a m e t e r o f such s o l v a t e d c a t i o n s i s g e n e r a l l y l a r g e r than t h e d i a m e t e r o f the 6-membered r i n g window o f t h e s o d a l i t e cage, about 0 . 2 2 nun, t h u s t h e ~ a i+n t h e s o d a l i t e c a g e s a r e n o t a c c e s s i b l e by t h e exchangeable h y d r a t e d c a t i o n s . I t r e s u l t s t h a t a complete exchange w u l d o c c u r i f t h e r e i s a r e d i s t r i b u t i o n o f t h e c a t i o n s
between t h e s o d a l i t e and t h e supercages. T h i s w i l l n e c e s s i t a t e a p a r t i a l dehydration of t h e s o l v a t e d exchangeable c a t i o n . The r e s u l t s on lanthanum i o n exchange a r e s i g n i f i c a t i v e of t h i s r e s p e c t . Hydra+ ted ~ a c ~ a t i o+n s were found t o r e p l a c e Na p r e s e n t i n t h e supercage on NaY z e o l i t e o n l y . However a t 1 8 0 ° C t h e exchange p r o c e e d s t o comp l e t i o n , ~ a i+n t h e s o d a l i t e cages b e i n g removed by ~ a ~ I+t .was concluded t h a t a t 180°C La-H 0 bond a s weakened t h u s allowing t h e 2 s p l i t t i n g o f H 0 l i g a n d s from t h e L a (H 0 ) complex. T h i s r e s u l t s i n a f a c i l e d i g f u s i o n o f t h e dehydrated ?,a3' c a t i o n s i n t h e s o d a l i t e cages a t 180°C where exchange with ~ a t+a k e s p l a c e (10-11). I n conclusion o f t h e s e d a t a i t i s now w e l l e s t a b l i s h e d t h a t h y d r a t e d cations, hydrated ~ a h~a s ' a diameter of 3 -96 A , cannot p e n e t r a t e t h e s o d a l i t e cages o f X and Y z e o l i t e s . Only a t high temperature t h e hydrated c a t i o n s l o s e t h e i r c o o r d i n a t e d H20 molecules which a l l o w them t o p e n e t r a t e t h e s o d a l i t e cages. However during t h i s p r o c e s s hydroxylation of t h e c a t i o n through water i o n i z a t i o n may occur r e s u l t i n g i n t h e formation of Men+ - OHx e n t i t i e s . A t high temperature OH condensation between Men+ - OHx s p e c i e s would r e s u l t i n t h e f o r mation of Me-0-Me bonds. According t o t h i s OH condensation oxide inside t h e z e o l i t e framework may be formed t h u s l e a d i n g t o c a t i o n collapse d u r i n g t h e subsequent c a l c i n a t i o n of t h e z e o l i t e . I t i s clear t h a t t h i s undesired c a t i o n c l u s b r i n g r e q u i r e s t h e e x i s t e n c e of hydroxometal c a t i o n s , which a r e produced by h y d r o l y s i s o f t h e exchangeable c a t i o n s , i n s u f f i c i e n t number.
Y+
To summarize t h e a b v e d i s c u s s i o n one may conclude t h a t although
it i s r e l a t i v e l y e a s y t o change t h e n a t u r e of t h e c a t i o n s i n zeol i t e s by i o n exchange, t h e d i f f u s i o n o f t h e bulky hydrated c a t i o n s w i l l be l i m i t e d by t h e z e o l i t e pore s i z e . Reduction i n s o l v a t e d cation s i z e would be n e c e s s a r y i n o r d e r f o r t h e i o n s t o p a s s f r e e l y through t h e oxygen membered r i n g s . I t i s a l s o known t h a t c a t i o n exchange i n t o hydrogen form z e o l i t e , i s o f t e n a d i f f i c u l t p r o c e s s , due t o t h e s t r e n g h t o f t h e bonding o f t h e p r o t o n s w i t h t h e l a t t i c e oxygen. To overcome t h e p r o t o n exchange l i m i t a t i o n i t i s recommended t o transform t h e hydrogen form i n t o ammonium form, N H c ~a t i o n s w i l l behave s i m i l a r l y t o ~ a c+a t i o n s . The s t u d y of t r a n s i t i o n metal i o n s exchanged z e o l i t e h a s r e v e a l e d t h a t the s t a t e of t h e t r a n s i t i o n metal c a t i o n s i s s t r o n g l y dependent on t h e method and on t h e experimental c o n d i t i o n s employed f o r the i o n exchange (11). I t was shown t h a t sodium form z e o l i t e s i n s o l u t i o n g i v e a b a s i c r e a c t i o n w i t h t h e subsequent i n c r e a s e o f the pH of t h e exchanging s o l u t i o n . Since t r a n s i t i o n metal i o n s i n b a s i c s o l u t i o n a r e e a s i l y hydrolyzed, t h e i o n exchange can be accompanied with h y d r o l y s i s o f t h e t r a n s i t i o n metal i o n s and t h e subsequent p r e c i p i t a t i o n o f t h e hydroxy anion t r a n s i t i o n m e t a l , Thus t h e t r a n s i t i o n metal i o n s would be adsorbed i n t h e form o f metal hydroxide e i t h e r i n s i d e t h e z e o l i t e c a v i t i e s o r on t h e e x t e r n a l s u r f a c e . Prec i p i t a t i o n of hydroxide s p e c i e s o f t r a n s i t i o n m e t a l i o n s i n , in/on t h e z e o l i t e would be avoided i s t h e pH o f t h e s o l u t i o n i s
maintained low enough such t h a t no c a t i o n h y d r o l y s i s o c c u r s . However a t low pH, s e v e r a l t y p e z e o l i t e s such a s A, X I Y a r e r e l a t i v e l y unst a b l e , t h e i r s t r u c t u r e s c o l l a p s e i n a c i d i c s o l u t i o n , p H < 4. The ext e n t t o which t h e m a t e r i a l l o s e s i t s c r y s t a l l i n i t y i n a c i d i c medium depends on t h e n a t u r e o f t h e z e o l i t e and t h e e x t e n t t o which c a t i o n h y d r d y s is o c c u r s depends on t h e p H o f t h e s o l u t i o n and on t h e n a t u r e o f t h e t r a n s i t i o n m e t a l i o n , t h e second and t h i r d s e r i e s of t r a n s i t i o n metal i o n s b e i n g more e a s i l y hydrolyzed than t h e f i r s t series. The g e n e r a l e q u a t i o n f o r i o n exchange i s
a t high p H hydrolysis occurs following
Copper i o n exchange p r o c e s s on X and Y z e o l i t e s h a s been s t u d i e d ext e n s i v e l y . The i n t e n s i t y o f t h e esr s i g n a l o f cu2+ exchanged Y-zeolit e h a s been followed a s a f u n c t i o n of t h e pH of t h e exchanging solut i o n ( 1 2 ) . The pH was v a r i e d from 3 t o 10. Ammonia s o l u t i o n was used f o r samples p r e p a r e d a t h i g h pH. A s t h e pH i n c r e a s e d t h e e s r signal i n t e n s i t y d e c r e a s e d u n t i l a minimum was reached a t a p H o f 8-9. The d e c r e a s e o f t h e esr s i g n a l i n t e n s i t y was a t t r i b u t e d t o a decrease of copper i o n d i s p e r s i o n which produced a e s r l i n e broadening through d i p o l e - d i p o l e i n t e r a c t i o n . The lower cu2+ d i s p e r s i o n a s t h e pH of t h e s o l u t i o n i n c r e a s e d i s due t o c a t i o n h y d r o l y s i s producing c u 2 + ( 0 ~ ) s p e c i e s w i t h t h e subsequent formation o f Cu-0-Cu b r i d g e s upon dehyd r a t i o n . Thus when t h e exchange was c a r r i e d o u t a t h i g h pH, upon d e h y d r a t i o n o f t h e m a t e r i a l , cu2+ e x i s t s i n t h e z e o l i t e i n t h e form o f a polymeric hydroxy anion copper s p e c i e s which e x h i b i t a r e l a t i v e l y broad e s r s i g n a l . However a t v e r y h i g h pH, h i g h e r t h a n 10, i n NH OH medium t h e e s r s i g n a l i n c r e a s e d a b r u p t l y , which i n d i c a t e d that 4 t h e formation o f Cu-0-Cu b r i d g e s was h i n d e r e d . The i n h i b i t i o n w a s t h e consequence o f t h e formation o f ammonia-copper complexes cu2+ (NH ) 3 4 which allowed c u p r i c i o n s t o remain d i s p e r s e d i n t h e z e o l i t e . Cob a l t and n i c k e l have t h e same c h a r a c t e r i s t i c f e a t u r e s a s those ex i b i t e d by copper i n z e o l i t e , I t i s p o s s i b l e f o r b o t h ~ i and ~ + Co t o be s t a b i l i z e d i n t h e z e o l i t e framework a s i s o l a t e d c a t i o n s by mixing t h e sodium form o f t h e z e o l i t e w i t h a s o l u t i o n o f t h e met a l s a l t , provided t h e p H o f t h e s o l u t i o n be k e p t low enough. By c o n t r a s t t h e exchange c o n d i t i o n s must be more c a r e f u l l y c o n t r o l l e d when p r e p a r i n g chromium, i r o n o r aluminium exchanged z e o l i t e s , part i c u l a r l y w i t h X o r Y-type z e o l i t e s . Indeed t h e s e c a t i o n s form very e a s i l y hydroxy a n i o n s which polymerize t o form c l u s t e r s . Furthermore e x t e n s i v e exchange w i t h t h e s e c a t i o n s produces o f t e n an appreciable l a t t i c e d e s t r u c t i o n . A n i m p o r t a n t l o s s o f l a t t i c e c r y s t a l l i n i t y was observed f o r X I Y z e o l i t e s e x t e n s i v e l y exchanged (13, 1 4 ) while when cr3' exchange l e v e l was low, l e s s t h a n 2 5 % of ~ a being +
4+
exchanged, no framework d e s t r u c t i o n was observed by X-ray analys i s ( 1 5 ) . S i m i l a r l y it h a s been shown t h a t iron-exchanged z e o l i t e s were u n s t a b l e toward high temperature t r e a t m e n t ( 1 6 ) . Group V I I I metal ion-exchanged z e o l i t e s have been widely used a s s t a r t i n g m a t e r i a l s t o p r e p a r e h i g h l y d i s p e r s e d supported noble metal c a t a l y s t s . While i n t h e c a s e of t h e f i r s t s e r i e s o f t r a n s i t i o n metal ions one can use v a r i o u s metal s a l t s , c h l o r i d e , n i t r a t e , s u l f a t e , oxalate, i n aqueous s o l u t i o n f o r exchange, group V I I I metal i o n s were g e n e r a l l y i n t r o d u c e d i n t h e z e o l i t e framework by i o n exchange using metal ammine complexes. Platinum exchanged f a u j a s i t e $Ype z e o l i t e was o b t a i n e d when Nay sample was t r e a t e d with a P t (NH3)4 solution ( 1 7 ) Since p t 2 + (NH3) c a t i o n i s s t a b l e and n o t s u b j e c t e d to h y d r o l y s i s o v e r a wide pH range t h e ion-exchange may be p e r formed over a wide range pH v a l u e s , s o l u t i o n s w i t h p H up t o 9 have been used. The exchange p r o c e s s i s r e p r e s e n t e d by t h e f o l l o w i n g expression P t (NH ) + ? N ~ + z+ p t 2 + ( N H ~ 4) 2 2 + 2Na when 3 4 p t 2 + ( N H 3 ) 4-Z i s h e a t e d i n oxygen t h e complex decomposed with t h e subsequent removal o f NH l i g a n d . However i n t h e temperature range 3 200-600°C no c a t i o n h y d r o l y s i s occurs. I t h a s been shown by X-ray d i f f r a c t i o n a n a l y s i s t h a t p t 2 + i o n s remain d i s p e r s e d i n t h e z e o l i t e framework ( 1 8 ) .
.
+
Palladium exchanged z e o l i t e s were prepared u s i n g t h e same procedure a s f o r p t 2 + - ~ a y , u s i n g a s o l u t i o n o f P ~ ~ + ( N H ~f )o *r t h e exchange (19). I t appeared t h a t pd2+ (NH ) c a t i o n s d i d n o t e x p e r i e n c e h y d r o l y s i s i n the pH range 6-9. Fur$hermore, a s it h a s been observed f o r p l a t i num exchanged z e o l i t e , pd2+ c a t i o n s i n Nay do n o t show a tendency t o h y d r o l y s i s . The palladium ammine complex pd2+ ( N H ~4I, following thermal t r e a t m e n t l o s e s i t NH l i g a n d s and t h u s m i g r a t e s from t h e 3 supercage towards t h e s o d a l i t e cage. The l o c a l i z a t i o n of p t 2 + and pd2' has been determined by X-ray d i f f r a c t i o n a n a l y s i s (18-20). Rhodium, i r i d i u m and ruthenium c a t i o n s have been exchanged i n z e o l i t e s u s i n g t h e i r a m i n e complexes. However i t was a l s o claimed t h a t , a t l e a s t f o r rhodium and ruthenium, aquo complexes could be used. However s i n c e t h e s e R h , I r and R u c a t i o n i c forms a r e e a s i l y hydrolyzed it i s n e c e s s a r y t o c a r r y o u t t h e exchange r e a c t i o n i n w e l l c o n t r o l l e d experimental c o n d i t i o n s . F h 3 + c a t i o n s can be e i t h e r uniformly d i s t r i b u t e d through t h e z e o l i t e framework when proper rhodium s a l t s and experimental c o n d i t i o n s a r e used o r predominantly supported on t h e e x t e r n a l s u r f a c e . RhC13, 3H 0 i n aqueous s o l u t i o n forms seve2 r a l rhodium s p e c i e s such a s RhC1 1 3-, [%(H 6 2 0 )4 C 12 +, [ R ~ ( H ~ o ) ~ c+ ~ ] e t c , t h e i r r e l a t i v e concentrations depend on t h e pH and t h e temperature of t h e s o l u t i o n . The c a t i o n i c c h l o r o aquo rhodium complexes a r e favoured a t 80-90°C, t h u s rhodium exchange w i l l be favoured when t h e r e a c t i o n i s c a r r i e d o u t a t 80-90°C. Rh-Nay sam3H20 sop l e s have been p r e p a r e d by mixing Nay z e o l i t e with a RhCl 3' l u t i o n a t 80-90°C ( 2 1 - 2 2 ) . From t h e d i f f u s e r e f l e c t a n c e spectrum of fi3+and t h e Xps measurement of atomic r a t i o Cl/Rh i t was concluded
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L
1
9i o n s
a r e exchang ?$ a s [ R ~ ( H ~ o ) ~ 1 12+ and complex h a s Eeen used a l s o t o prep a r e &-Nay by i o n exchange. Ton exchange w a s c a r r i e d o u t e i t h e r a t room t e m p e r a t u r e -or a t 8 0 ° C ( 2 3 ) . Pentammine c h l o r o i r i d i u m chloride salt [1r ( N H ~5~ ) C l h a s a l s o been used t o p r e p a r e i r i d i u m ex2 changed z e o l l t e s ( 2 4 ) . Although RuC13, H 2 0 s a l t has been used i n aqueous s o l u t i o n t o p r e p a r e Ru exchanged z e o l i t e it appeared t h a t t h i s s a l t was n o t s u i t a b l e f o r exchange s i n c e r a p i d h y d r o l y s i s occur e d . I n aqueous s o l u t i o n t h e ruthenium s a l t most o f t e n used f o r exchange i s Ru(NH ) C 1 ( 2 5 ) . 3 6 3 t h a t indeed
[m ( H
0)
J
. [a( N H ~ 5) ~ 1 ]
11
1-2- Ion exchange by i n t e r a c t i o n of anhydrous m e t a l s a l t s with t h e z e o l i t e framework. Ion exchange i n l i q u i d phase o c c u r s r e a d i l y when t h e exchanging m e t a l i s i n i t s c a t i o n i c form i n t h e s o l u t i o n . I t was found t h a t i o n exchange occured by i n t e r a c t i o n between anhydrous s a l t s and z e o l i t e s . H e a t i n g a t 300°C NH C 1 w i t h sodium form + 4 + (26). Similarly z e o l i t e r e s u l t e d i n t h e replacement o f N a by NH i t was shown t h a t zirconium phosphate Zr(HPOq)Z 4n t h e d r y s t a t e ex+ changed H w i t h c a t i o n s when h e a t e d i n t h e p r e s e n c e o f metal c h l o r i d e , HC1 g a s evolved d u r i n g t h e r e a c t i o n ( 2 7 ) T r a n s i t i o n metal ions s u c h a s t i t a n i u m , chromium, molybdenum a r e d i f f i c u l t t o exchange into z e o l i t e s because t h e y a r e s t a b l e i n t h e i r c a t i o n i c form a t low pH where i n g e n e r a l z e o l i t e s decompose. E f f e c t i v e exchange i s p o s s i b l e through r e a c t i o n o f t h e H-form z e o l i t e w i t h m e t a l c h l o r i d e . The exchange procedure c o n s i s t s i n : i ) exchange of t h e sodium form with + f o l l o w i n g b y h e a t t r e a t m e n t t o produce H-form z e o l i t e
.
NH4
Na-Zeol
+
+ NH4
+
NH
4
-
Zeol.
ii) r e a c t i o n o f H-Zeol w i t h m e t a l c h l o r i d e :
n H-Zeol
+
Me C l n
-t
Me
n+
Zeol
+
n HCI
Titanium exchanged Y z e o l i t e s were p r e p a r e d by t h i s method. Recently molybdenum c o n t a i n i n g Nay z e o l i t e s were p r e p a r e d by s o l i d - s o l i d react i o n ( 2 8 ) . The NH -Y form was deaminated by h e a t i n g t h e s o l i d a t 4 3 5 0 ° C . MoC15 was ground w i t h H-Y which formed a H-Y z e o l i t e support e d MoOC14. When t h e mixture was h e a t e d a t 4 0 0 ° C exchange occured f o l l o w i n g t h e r e a c t i o n MoOC14 + 4H-0-Zeol -t O=Mo (0-Zeol) + 4HC1. 1-3- Z e o l i t e - s u p p o r t e d m e t a l complexes,
t r a n s i t i o n metal i o n s by r e a c t i o n w i t h
A - Uniform d i s t r i b u t i o n o f t h e metal i o n s on t h e z e o l i t e could be o b t a i n e d by t h e r e a c t i o n between t h e m e t a l exchanged z e o l i t e and a m e t a l - c o n t a i n i n g c o o r d i n a t i o n compound such as a metal cyanide complex ( 2 9 ) . The method c o n s i s t s t o r e a c t t h e metal exchanged zeol i t e , i r o n z e o l i t e f o r example, w i t h a s o l u b l e i r o n cyanide complex i n solution :
The i r o n f e r r o cyanide r e s u l t i n g i s i n s o l u b l e and t h u s p r e c i p i t a t e d within t h e z e o l i t e c a v i t i e s . Subsequently it r e s u l t e d i n a uniform d i s t r i b u t i o n o f t h e complex w i t h i n t h e z e o l i t e framework. I r o n potassium Y z e o l i t e was o b t a i n e d by r e a c t i n g K4 [ F ~ ( c N ) ~ w i t h Fe, NH -Y z e o l i t e , I r o n cobalt-Y was o b t a i n e d through t h e r e a c t i o n o f 4 Co, NH -Y w i t h (NH ) [F~(cN)~] 4
4 4
B - Reaction o f t h e z e o l i t e w i t h m e t a l carbonyl compounds. The a d s o r p t i o n o f m e t a l carbonyl compounds on z e o l i t e h a s been used a s an a l t e r n a t i v e r o u t e t o p r e p a r e z e r o v a l e n t t r a n s i t i o n m e t a l supported on z e o l i t e . The method c o n s i s t e d i n adsorbing z e r o v a l e n t met a l carbonyl compounds w i t h i n t h e z e o l i t e c a v i t i e s followed by a thermal decomposition t o remove t h e c a r b o n y l l i g a n d s ( 3 0 ) . T h i s procedure was f i r s t a p p l i e d w i t h Mo (CO) and Ru3 (CO) 12 ' Fe(C0I5, Fe2(C0I8 Fe3 (CO) 12 supported on Y z e o l i t e samples have been p r e p a r e d ( 3 ) . I t was shown t h a t upon t h e r m a l decomposition o f t h e c a r b o n y l , t h e f i n a l oxidation s t a t e of t h e m e t a l depended on t h e a c i d i t y o f t h e s t a r t i n g z e o l i t e on H-form Y z e o l i t e t h e metal was o x i d i z e d by t h e p r o t o n s with t h e subsequent e v o l u t i o n of hydrogen
The i n t e r a c t i o n o f i r o n carbonyl w i t h HY z e o l i t e h a s been s t u d i e d by I R and g r a v i m e t r i c methods (31, 3 2 ) . I t was shown than i n HY, i n t e r a c t e d s t r o n g l y w i t h t h e OH groups p r e s e n t i n t h e s u p e r )::::: 3 2 ) . During t h e t h e r m a l decomposition t h e i r o n i s p a r t i a l l y oxidized f o l l o w i n g t h e r e a c t i o n
?
1-4- Location and o x i d a t i o n s t a t e o f t r a n s i t i o n m e t a l i o n s i n zeolites . S i n c e t h e c a t a l y t i c behaviour o f z e o l i t e - c o n t a i n e d t r a n s i t i o n metal i o n s i s dependent o f t h e l o c a t i o n and t h e o x i d a t i o n s t a t e of the c a t i o n s , i t i s i m p o r t a n t t o e s t a b l i s h t h e d e g r e e o f d i s p e r s i o n of the t r a n s i t i o n metal i o n s , t h e i r l o c a t i o n w i t h i n t h e z e o l i t e cav i t i e s o r on t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e c r y s t a l , t h e i r oxydation s t a t e . S e v e r a 1 methods have been used f o r t h e s e s t u d i e s such as X-ray d i f f r a c t i o n , W s p e c t r o s c o p y i n f r a r e d , e l e c t r o n s p i n r e s o nance, M6ssbauer s p e c t r o s c o p y , xps. A m o n g t h e s e t e c h n i q u e s X-ray p h o t o e l e c t r o n s p e c t r o s c o p y was s u c c e s s f u l l y employed t o determine both t h e o x i d a t i o n s t a t e and t h e l o c a t i o n o f c a t i o n s . Thus Minachev exe t a 1 (33,34) have shown t h a t upon r e d u c t i o n o f N i 2 + , cu2+, changed z e o l i t e t h e c a t i o n s a r e s t a b i l i z e d t o lower o x i d a t i o n s t a t e and a l s o t h a t t h e y m i g r a t e t o t h e e x t e r n a l s u r f a c e . The xps s p e c t r a a r e c h a r a c t e r i z e d by xps peaks which p o s i t i o n s a r e c h a r a c t e r i s t i c o f t h e t r a n s i t i o n m e t a l i o n and i t s o x i d a t i o n s t a t e ,
and i n t e n s i t i e s c h a r a c t e r i s t i c o f t h e c o n c e n t r a t i o n of t h e c a t i o n s on t h e s u r f a c e ; depending o f t h e element examined t h e xps w i l l rev e a l a s u r f a c e d e p t h o f a b o u t 2-5 nm. The peak i n t e n s i t y i s given by t h e r e l a t i o n :
where n i s t h e c o n c e n t r a t i o n o f t h e element, 0 t h e c r o s s s e c t i o n for p h o t o e l e c t r o n e m i s s i o n from t h e l e v e l , X i s t h e escape d e p t h , F e t K p a r a m e t e r s depending of t h e s p e c t r o m e t e r . Thus t h e r e l a t i v e s u r f a c e c o n c e n t r a t i o n o f two elements w i l l be g i v e n by t h e r e l a t i o n : n1/n2
=
I1 a2 / I2
2+ where 1 e t I2 a r e t h e peak a r e a s [ R ~ ( N H ~~ ) l ] - NaY h a s been 1 s t u d i e d by xps ( 3 5 ) . The b i n d i n g e n e r g i e s 0% Rh 3d3/2 and Rh 3d5/2 were found e q u a l t o 3 10.8 and 3 15.7 eV as e x p e c t e d f o r Rh3+. Furthermore atomic r a t i o s determined by xps and chemical a n a l y s i s a r e very c l o s e as shown i n t a b l e I :
Table I : xps r e s u l t s f o r
Chemical analysis
1
Rh(NH3)+C1
Nay
5
These r e s u l t s i n d i c a t e t h a t t h e rhodium complex i s i n t r o d u c e d without decomposition and homogeneously d i s t r i b u t e d o v e r t h e z e o l i t e framework. F u r t h e r i n v e s t i g a t i o n s o f rhodium exchanged z e o l i t e by xps have been made ( 3 6 ) . The d a t a g i v e n i n ( 3 6 ) confirmed t h a t t h e atomic r a t i o Rh : N : CL found i n rhodium pentarnmine exchange X zeolite i s a s expected f o r t h e c a t i o n [ R ~ ( N H ~ ) ~ c ~ The ] Si/Rh r a t i o determined by xps i s c l o s e t o t h a t p r e d i c t e d on t h e b a s i s o f t h e bulk composition f o r homogeneous d i s t r i b u t i o n of t h e c a t i o n i n t o t h e zeol i t e . A d d i t i o n a l xps evidence o f homogeneous d i s t r i b u t i o n o f Rh cat i o n s i n Nay z e o l i t e , when [R~(NH ) was used f o r i o n exchange w a s g i v e n i n ( 3 7 ) . The atomic &/gi r a t i o s measured from xps exp e r i m e n t s a r e i d e n t i c a l t o t h e t h e o r e t i c a l v a l u e s assuming t r u e ion exchange. I n c o n t r a s t i t was found t h a t when t h e i o n exchange was c a r r i e d o u t i n a s o l u t i o n o f Rh(N03)3, 2W 0 t h e m e t a l i o n s were pre2 dominantly l o c a l i z e d on t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e . Indeed for t h e s e samples t h e xps atomic r a t i o s R.h/Si were one o r d e r of mag n i t u d e h i g h e r t h a n t h e t h e o r e t i c a l v a l u e s . Molybdenum-containing z e o l i t e s p r e p a r e d by s o l i d - s o l i d r e a c t i o n between MoOCl and HY 4 z e o l i t e were examined by xps ( 2 8 ) . The b i n d i n g e n e r g i e s f o r t h e Mo(3d3/2) and ~ o ( 3 d 5 / 2 )w e r e r e s p e c t i v e l y 235.8 and 232.7 eV
.
~11 *'
i n d i c a t i n g that molybdenum i o n s were p r e s e n t as Mo(V1) i n t h e z e o l i t e s . Mo-exchanged z e o l i t e o b t a i n e d by s o l i d - s o l i d r e a c t i o n between MoOC14 and HY e x h i b i t e d a xps Mo/Si r a t i o c l o s e t o t h e t h e o r e t i c a l value, which s u g g e s t s t h a t Mo i o n s were homogeneously d i s t ~ i b u t e di n the z e o l i t e framework, w h i l e t h e ~ o / S ir a t i o i n samples p r e p a r e d by impregnation o f Nay w i t h MoCl s o l u t i o n i s a b o u t s e v e n f o l d g r e a t e r 5 than t h e t h e o r e t i c a l v a l u e , molybdenum b e i n g mainly on t h e z e o l i t e external surface ( 2 8 ) .
-
2
R e a c t i v i t y of t r a n s i t i o n metal i o n s i n z e o l i t e s
The i n t r a c r y s t a l l i n e p o r e and c a v i t y system o f z e o l i t e s and t h e important l a t t i c e i n d u c e unusual r e a c t i v i t y t o t h e e n t r a p p e d t r a n s i t i o n metal i o n s . I n a d d i t i o n t h e r e a c t i v i t y o f t h e s e i o n s i n zeol i t e s appeared t o be v e r y s i m i l a r t o t h e i r homologous i n s o l u t i o n . This paragraph w i l l p r o v i d e some i n t e r e s t i n g r e a c t i v i t i e s o f t r a n s i t i o n metal i o n s exchanged z e o l i t e s . 2-1-
I o n i s a t i o n o f molecules i n z e o l i t e s .
The v e r y s t r o n g i o n i s a t i o n p r o p e r t i e s of z e o l i t e s a r e r e s p o n s i ble f o r t h e i o n i s a t i o n of w a t e r by m u l t i v a l e n t exchanged c a t i o n s . There a r e now l a r g e number o f e x p e r i m e n t a l e v i d e n c e s showing t h e ~ + and i o n i s a t i o n o f H 0 w i t h t h e subsequent f o r m a t i o n of ~ e (OH) 2 a c i d i c OH groups (38) When ~ e i s~ a +t r a n s i t i o n m e t a l i o n , ?henna1 decomposition o f t h e h y d r a t e d sample o f t e n l e d t o t h e r e d u c t i o n o f the c a t i o n . I t h a s been shown t h a t d e h y d r a t i o n o f F ~ ~ + - N ~z Ye o l i t e produced ~ e i o~n s +( 3 9 ) From Mijssbauer s p e c t r o s c o p y s t u d i e s and q u a n t i t a t i v e measurements o f 0 adsorbed i t h a s been assumed t h a t the f o l l o w i n g r e a c t i o n s o c c u r e a on f e r r o u s ion-exchanged z e o l i t e (40)
.
.
These o b s e r v a t i o n s were f u r t h e r extended t o s e v e r a l t r a n s i t i o n metal ions and it was concluded t h a t t h e z e o l i t e s have t h e remarkable p r o p e r t i e s t o decompose w a t e r i n t o oxygen and hydrogen f o l l o w i n g a thermochemical c y c l e ( 4 1 ) . The r e a c t i o n s producing w a t e r s p l i t t i n g are t h e f o l l o w i n g : H 0 ionisation :
2
0
2
2 Cu
2+
+
2 H20
-t
2
+ +
CU-OH
2~'
and R 0 d e s o r p t i o n 2
2 OHReduction
+
2 cu2+
H ~ O+
+
2 e-
1/2 +
o2 +
2 cu
2e
-
f
Rehydration The i o n i s a t i o n p r o p e r t y o f z e o l i t e s appeared t o be r e s p o n s i b l e f o r t h e h y d r o l y s i s o f group V I I I t r a n s i t i o n m e t a l i o n s i n z e o l i t e .
Ru3+(NH ) -exchanged z e o l i t e s have been i n v e s t i g a t e d by e s r and 6 IR ( 4 2 ) . $he white sample R U ~ + ( N H ) -N Y showed a n e s r spectrum w i t h g = 2.20 and was a t t r i b u t e d t o 6Rugf (NH3) Reflectance spectroscopy showed a b a n d a t 38.000 cm-I (43) and IR sPlowed a band a t 1360 cm(42,43) due t o NH l i g a n d s i n R U ~ + (NH?) complex. Thus t h e 3 r e s u l t s p r e s e n t e d confirmed t h a t on a f r e s h y p r e p a r e d ruthenium exchanged z e o l i t e from hexammine ruthenium s o l u t i o n , t h e complex i s i n t h e form of R U ~ + ( N H 3 ) 6 l o c a l i z e d i n t h e supercage. When t h e samp l e was outgassed t h e sample t u r n e d p r o g r e s s i v e l y red-wine along w i t h a d r a s t i c change b o t h of t h e e s r spectrum of R U ~ ' i o n s and the appearance of I R band a t 1460 cml due t o t h e formation of NH (42). 2+ I t was concluded t h a t (Ru ( N H ) OH) , and ruthenium r e d were formed 3 following thermal t r e a t m e n t a ?ow temperature (42,43,25,44)
.
+
Ru
3+
2+
+ H20 -+
(NH316
Ru(NH ) OH
3 5
2+
[RU(NH~) 50~]
+
NH
+ 4
polymerisation of
would lead t o ruthenium r e d :
Thus it appeared t h a t upon o u t g a s s i n g h y d r o l y s i s of t h e ruthenium ammine complex occured. S i m i l a r l y when NH i s adsorbed on a f r e s h l y 3 p r e p a r e d R U ~ (NH + ) -Nay t h e samp e immediatly showed an e s r spec$+ trum a t t r i b u t e d go6 and I R s p e c t r a i n d i c a t e d t h e + (45) Th2 ) 2OH) formation o f NH4 a c t t h a t o u t g a s s i n g Ru(NH adsorbing NH on t h i s sample produced t h e same e f f e c t 3 t i a l h y d r o l y s i s of t h e ruthenium hexammine complex was i n t e r p r e t e d i n t h e following manner : 3+ Ru (NH3) i s r e l a t i v e l y u n s t a b l e i n b a s i c media. However when i n t r o d u c e 3 i n t h e supercage o f t h e z e o l i t e and when t h e c a v i t y i s f u l l y h y d r a t e d , R U ~ + ( N H ) 6 i s p r o t e c t e d from t h e i o n i z i n g power of z e o l i t e by t h e water mo?ecules f i l l i n g t h e c a v i t y . Upon dehydration following o u t g a s s i n g , t h e complex i s s u b j e c t e d t o i o n i s a t i o n with t h e subsequent h y d r o l y s i s . S i m i l a r l y by adsorbing NH NH forms with 3' 3 t h e water molecules p r e s e n t i n t h e c a v i t y NH OH, which i s immediate4 l y i o n i z e d forming high c o n c e n t r a t i o g + o f OH- groups. I n t h e presence (NH3)6 occurs. of t h e s e OH- groups h y d r o l y s i s o f Ru
LRU(NH .
T h i s s t r o n g i o n i z i n g p r o p e r t y of z e o l i t e i s t h u s r e s p o n s i b l e f o r t h e f a c i l e h y d r o l y s i s o f group V I I I t r a n s i t i o n metal i o n s i n z e o l i t e . Upon dehydration a t h i g h temperature t h e hydrolyzed group V I I I t r a n s i t i o n metal i o n s form r a p i d l y metal o x i d e s which migrate on t h e e x t e r n a l surface of t h e z e o l i t e . 2-2-
Oxygen-transition metal i o n s
1t i s w e l l known t h a t tetraphenyl-porphyrin
c o b a l t (11) adsorbs molecular oxygen, t h e e l e c t r o n c o n f i g u r a t i o n i n t h i s oxygen-cobalt adduct approaches C o ( I I 1 ) one e l e c t r o n being t r a n s f e r e d from
05,
the c o b a l t o r b i t a l t o t h e oxygen molecule. Pentaamrnine Co(I1) forms also with oxygen a p-peroxodicobalt amrnine complex ( N H 3 ) 5 CO-0 2 t h e oxygen molecule b r i d g i n g two c o b a l t complexes. IdenCo (NH3),-, t i c a l r e a c t i o n s occured w i t h C o ( I 1 ) exchanged N a y z e o l i t e ( 4 6 ) . The p.-peroxodicobalt complex was formed when 0 r e a c t e d w i t h C o ( I 1 ) ( N H 3 ) 5 2 entrapped complex. I n c o n t r a s t only t h e monomeric oxygen c o b a l t spec i e s was formed when NH l i g a n d s were r e p l a c e d by propylene diarnmine 3 ligand ; t h e s t r u c t u r e 3f t h e oxygen adduct was :
Rhodium I1 porphyrin i n dimethyl-formamide adsorbs H d i s s o c i a t i v e 2 ly ( 4 7 ) following t h e r e a c t i o n :
R~(I)was very s e n s i t i v e t o oxygen and was o x i d i z e d w i t h t h e subsequent formation o f Rh(11) and H20 :
Rhodium exchanged z e o l i t e s behave s i l i l a r l y . E s r s t u d y o f oxygen adsorption i n a c t i v a t e d Rh-Nay r e v e a l e d t h e formation of a y-peroxodirhodium adduct ( 2 3 ) . I t was suggested t h a t f o l l o w i n g thermal act i v a t i o n of R h ( I I 1 )-Nay s e l f r e d u c t i o n of R h ( I I 1 ) t o Rh(I) occured Rh(1) i o n s were bound t o l a t t i c e oxygen i o n s . Upon a d d i t i o n o f O2 molecules t h e u-peroxodirhodium complex Rh (11) - 0;- - Rh(11) was formed one e l e c t r o n from each Rh(1) i o n being t r a n s f e r r e d t o t h e 0 molecule. 2 Z e o l i t e was found t o s t a b i l i z e new type o f oxygen adducts which were not r e v e a l e d i n s o l u t i o n . Example i s given by palladium exchanged z e o l i t e ( 4 7 ) . When P d ( I 1 ) - m o r d e n i t e was a c t i v a t e d i n vacuum e s r s t u d i e s showed t h e formation o f P d ( 1 ) i o n s due t o s e l f r e d u c t i o n of t h e Pd(I1) i o n s . T e a d s o r p t i o n o f O2 a t room temperature was s t u d i e d P7 by e s r , using O e n r i c h e d oxygen gas. I t was suggested t h a t O2 molecule was trapped between t h r e e P d ( 1 ) i o n s each P d ( I ) t r a n s f e r r i n g to t h e oxygen molecule one e l e c t r o n . The r e s u l t i n g oxygen s p e c i e s 3- complex, t h e e l e c t r o n conf i g u r a was d e s c r i b e d a s a [ P ~ ( I I ) ] t i o n o f t h e charged O2 molecu eing si m i l a r t o ~ 1 ion. 2
2-3-
N i t r i c oxide a d d u c t s
T r a n s i t i o n metal i o n exchanged z e o l i t e s showed high a f f i n i t y f o r n i t r i c o x i d e and it was thought t h a t t h e s e m a t e r i a l s could be advantageously used a s c a t a l y s t s i n t h e d i s s o c i a t i o n and r e d u c t i o n of n i t r i c oxide. This h a s prompted s e v e r a l s t u d i e s of t h e formation of n i t r o s y l complexes w i t h i n t h e z e o l i t e c a v i t i e s . E s r and i n f r a r e d techniques were used t o i n v e s t i g a t e t h e e l e c t r o n i c s t r u c t u r e o f t h e metal n i t r o s y l complexes. Although t h e i o n i z a t i o n p o t e n t i a l of NO i s
r e l a t i v e l y h i g h (9.3 eV) i n s e v e r a l c a s e s , due t o t h e i n t r a c r y s t a l l i n e i o n i z i n g power o f the z e o l i t e , t h e formation o f t h e n i t r o s y l complex was accompanied by a t r a n s f e r o f e l e c t r o n from NO t o t h e t r a n s i t i o n m e t a l i o n w i t h i t s subsequent r e d u c t i o n . 2+ Nickel exchanged Y z e o l i t e ( N i -Y) does n o t e x h i b i t an esr s i g n a l . When Ni2+y i s c o n t a c t e d w i t h NO a s t r o n g e s r s i g n a l i s observed. n (3dY e l e c t r o n c o n f i g u r a t i o n ) . T h i s s i g n a l was a t t r i b u t e d t o ~ i i o+ Furthermore I R d a t a i n d i c a t e d t h e p r e s e n c e o f a n i t r o s y l adduct (VNO = 1892 c m - l ) (48-49). The i n t e r e s t i n g a s p e c t o f t h e s e s t u d i e s + i s t h a t NO c o o r d i n a t e s t o ~ i + i n+z e o l i t e t o form N i NO+ complex. + + T h i s complex N i NO was c h e m i c a l l y i n e r t toward oxygen (48-49). 4N i t r i c oxide forms a l s o w i t h cr2+ i n Nay z e l i t e ( C r NO+) complex S+ which was a l s o i n e r t toward oxygen ( 4 9 ) . C r -Nay was o b t a i n e d by Hg-reduction o f cr3+-Nay. N i t r i c oxide r e a c t s w i t h f e r r o u s i o n s i n i r o n exchanged z e o l i t e t o form an i r o n n i t r o s y l complex ( 5 0 ) . Both + were present low s p i n ( s = 1 / 2 ) and h i g h s p i n ( s = 3 L 2 ) Fe(1)NO i n t h e z e o l i t e c a v i t i e s and were i d e n t i f i e d by e s r and i n f r a r e d . The h i g h s p i n i r o n n i t x o s y l complex showed an esr spectrum w i t h g = 4.07 I and g, = 2.003 and an TR band a t 1890 c m - l . The s t u d y o f t h e i n t e r a c t i o n o f NO w i t h t r a n s i t i o n m e t a l i o n exchanged z e o l i t e s h a s reinf o r c e d t h e i d e a concerning t h e h i g h i o n i z i n g p r o p e r t y o f z e o l i t e which f a c i l i t a t e s e l e c t r o n t r a n s f e r r e a c t i o n s . Furthermore metal nit r o s y l complexes e x h i b i t e d an unusuallyhigh s t a b i l i t y toward oxidat i o n by 0 2'
2-4-
R e a c t i v i t y w i t h CO
The r e a c t i v i t y o f 1 s t s e r i e s t r a n s i t i o n m e t a l i o n s exchanged z e o l i t e s h a s been c a r r i e d o u t p a r t l y t o i n v e s t i g a t e t h e l o c a t i o n o f t h e t r a n s i t i o n m e t a l i o n s i n t h e z e o l i t e s t r u c t u r e ( f o r example c a t i o n s w i t h i n t h e s o d a l i t e o r t h e hexagonal p r i s m o f Nay w i l l be hidden from CO i n t e r a c t i o n ) and p a r t l y t o probe t h e o x i d a t i o n s t a t e o f t h e t r a n s i t i o n m e t a l i o n from t h e CO I R f r e q u e n c i e s . I n general CO forms weak bondswith t h e 1 s t s e r i e s t r a n s i t i o n m e t a l i o n s and n+ t h e Me -CO complex i s e a s i l y d e s t r o y e d by o u t g a s s i n g t h e samples. The I R s t u d i e s of carbon monoxide adsorbed on t r a n s i t i o n metal ion exchanged z e o l i t e s have been reviewed i n ( 5 1 ) . More r e c e n t spectrosc o p i c s t u d i e s o f t h e i n t e r a c t i o n o f carbon monoxide w i t h group V I I I t r a n s i t i o n m e t a l ion-exchanged z e o l i t e s have shown t h a t w e l l defined m e t a l carbonyl compounds were formed w i t h i n t h e z e o l i t e c a v i t i e s ( 8 ) . I t a p p e a r s i n t e r e s t i n g t o d e s c r i b e t h e formation and i d e n t i f i c a t i o n o f t h e mononuclear and p o l y n u c l e a r m e t a l c a r b o n y l complexes which were s y n t h e s i z e d i n t h e s u p e r c a g e s of t h e f a u j a s i t e l i k e s t r u c t u r e N a Y z e o l i t e i ) z e o l i t e e n t r a p p e d mononuclear carbonyl compounds Rh(II1) -Nay r e a c t s w i t h CO a t room temperature forming Rh ( I ) dicarbony1 compound which s t r u c t u r e was i d e n t i f i e d by I R spectroscopy. I n a d d i t i o n xps measurements were o b t a i n e d t o f u r t h e r confirm t h e r e d u c t i o n o f R h ( 1 l I ) t o Rh(I) d u r i n g t h e r e a c t i o n w i t h CO. I R spect r a o f t h e carbonyl complex formed w i t h i n t h e z e o l i t e c a v i t i e s showed bands a t 2100-2020 cm-' a s e x p e c t e d f o r Rh(1) ( C O ) * species.
The g e n e r a l e q u a t i o n f o r t h e r e a c t i o n i s ( 3 5 )
Rh(II1) Rh(1)
+
+ 2CO
CO
H2° + +
Rh(I)
+
2 ~ +
+
C02
Rh(1) ( C 0 I 2
S i m i l a r l y Ir ( 111)-Nay r e a c t s w i t h CO t o form monovalent i r i d i u m c a r bony1 s p e c i e s ( 5 2 ) . Q u a n t i t a t i v e measurement o f CO uptake a t 170°C i n d i c a t e d a C O / I r r a t i o o f a b o u t 4. Furthermore I R bands a t 2086 and 2001 cm-l were observed. The d a t a were i n t e r p r e t e d i n t h e same manner a s for Rh-Nay, 1r (111) b e i n g reduced by CO i n t o I r ( I ) w i t h t h e subsequent formation o f Ir ( I )(CO) carbonyl s p e c i e s .
ii) Z e o l i t e - e n t r a p p e d m e t a l carbonyl c l u s t e r s . R e c e n t l y t h e r e h a s been i n t e r e s t i n s u p p o r t e d metal carbonyl c l u s t e r s . The i n v e s t i g a t i o n s have been d i r e c t e d towards : h e t e r o g e n i z i n g s o l u b l e c a t a l y s t s , preparing h i g h l y d i s p e r s e d m e t a l c a t a l y s t s , p r e p a r i n g s u p p o r t e d subcarbonyl metal c l u s t e r s showing unique c a t a l y t i c p r o p e r t i e s . In g e n e r a l , it was found t h a t t h e carbonyl m e t a l c l u s t e r immobilized on s o l i d s u r f a c e s such a s s i l i c a , alumina, m a i n t a i n t h e i r molecular s t r u c t u r e o n l y when t h e s u r f a c e was h i g h l y dehydroxylated o r when the s u r f a c e was f u n c t i o n a l i z e d by phosphine l i g a n d s . The Nay f a u j a site type z e o l i t e appeared an i n t e r e s t i n g m a t e r i a l for s t a b i l i z h g within t h e z e o l i t e c a v i t i e s m e t a l carbonyl c l u s t e r s such a s R h (CO) 6 16 ' Ir4(CO) S i n c e t h e c r i t i c a l dimension o f t h e c a r b o n y l c l u s t e r s a r e g e n e r a l l y l a r g e r t h a n t h e z e o l i t e cavity-windows, i t appeared necessary t o s y n t h e s i z e t h e carbonyl m e t a l c l u s t e r d i r e c t l y w i t h i n t h e cavities (6). Rhodium and i r i d i u m c a r b o n y l c l u s t e r s a r e p o t e n t i a l l y a c t i v e c a t a l y s t s for r e a c t i o n s such a s o x i d a t i o n , hydrogenation and hydroformylation of o l e f i n s . I t i s e x p e c t e d t h a t i m m o b i l i z a t i o n o f t h e c a r b o n y l met a l c l u s t e r i n c r e a s e s i t s s t a b i l i t y toward a g g r e g a t i o n . T h i s was accomplished i n two d i f f e r e n t ways : a ) a d i r e c t s y n t h e s i s of t h e metal c l u s t e r w i t h i n t h e z e o l i t e c a v i ties, b ) an i n s e r t i o n o f t h e m e t a l c l u s t e r w i t h i n t h e c a v i t i e s by sublimation. Our r e s u l t s have shown t h a t t h e f i r s t procedure i s more s u i t a b l e t o obtain h i g h l y d i s p e r s e d m e t a l c a r b o n y l c l u s t e r s * a
e n t r a p p e d w i t h i n t h e Nay z e o l i t e c a v i t i e s was o b t a i n e d Nay w a s exchanged w i t h m3+. A f t e r dehydration o f t h e W a Y sample a t 5 7 3 K , t h e z e o l i t e was t r e a t e d with a mixture of CO : H2 a t a b o u t 15 atmospheres and a t 3 0 0 K f o r few hours. The samples turned c o l o r e d and e x h i b i t e d a w e l l d e f i n e d i n f r a r e d spectrum w i t h two s t r o n g c a r b o n y l bands a t 2095 and 1765 cm-l. By comparison w i t h o t h e r rhodium c a r b o n y l compounds t h i s c l u s t e r formed and stai n f r a r e d spectrum was a s c r i b e d t o Rh6(CO) b i l i z e d w i t h i n t h e z e o l i t e c a v i t i e s . I d e n t g c a l s p e c i e s were a l s o w a s f i r s t supported s t a b i l i z e d w i t h i n t h e z e o l i t e when Rh (CO) 6 on t h e z e o l i t e s u r f a c e by s u b l i m a t i o n a 37163( followed by a d e c a r bonylation and r e c a r b o n y l a t i o n a t 373 K ( 5 4 ) . The i n f r a r e d spectrum Rh6(CO)
i n t h e hgJlowing ways (53, 8)
.
o f t h i s sample a l s o showed two s t r o n g bands a t 2095 and 1765 cm-I a t t r i b u t e d t o Rh6(CO) e n t r a p p e d i n t h e z e o l i t e . The s t a b i l i t y of 1 t h e rhodium carbonyl c f u s t e r w a s f u r t h e r i n v e s t i g a t e d . The i n f r a r e d r e s u l t s i n d i c a t e d t h a t z e o l i t e - e n t r a p p e d Rh (CO) may be decarbo6 1 n y l a t e d , w i t h o u t s i g n i f i c a n t a g g r e g a t i o n , by r e a c t i n g t h e sample a t 373 K w i t h H 2 . 0 r 02. Recarbonylation by CO a t 373 K regenerated t h e i n f r a r e d bands a t 2095 and 1765 cm-l. These s t u d i e s show t h a t t h e r e t e n t i o n o f t h e c l u s t e r i n t e g r i t y i s p o s s i b l e by u s i n g a zeol i t e support. Ir4(CO) I r 6 ( C 0 ) 1 6 have a l s o been s y n t h i z e d and s t a b i l i z e d within z e o l i t e s ( 8 ) . The exchanged I r NaY z e o l i t e was reduced by a mixture o f CO : H2 a t atmospheric p r e s s u r e and a t 443 K. The sample t u r n e d c o l o r e d and showed i n t h e c r b o n y l s t r e t c h i n g region - An i d e n t i c a l i n f r a r e d i n f r a r e d bands a t 2086, 2 0 4 0 and 1813 cm spectrum was observed when Ir (CO) was d e p o s i t e d on t h e z e o l i t e 4 1 from s o l u t i o n . These r e s u l t s show c l e a r l y t h a t t h e z e o l i t e matrix i s p a r t i c u l a r l y s u i t a b l e n o t only f o r t h e preparation of highly disp e r s e d m e t a l c a t a l y s t s b u t a l s o f o r s t a b i l i z i n g w e l l d e f i n e d metal clusters.
f.
2-5-
Reactivity with H
2
: formation o f s m a l l m e t a l p a r t i c l e s
S i n c e t h e importance o f metal d i s p e r s i o n i n t h e e f f i c i e n t use o f m e t a l c a t a l y s t s h a s been w e l l e s t a b l i s h e d e x t e n s i v e work h a s been c a r r i e d o u t t o develop methods o f p r e p a r a t i o n t h a t should produce m e t a l c a t a l y s t s f i n e l y d i s p e r s e d . The h i g h e s t d e g r e e o f metal d i s p e r s i o n was o b t a i n e d by f i x i n g metal c a t i o n s on a c a r r i e r , generally s i l i c a o r alumina, by i o n exchange t e c h n i q u e , f o l l o w i n g by reduction i n hydrogen (55-56). Because o f t h e i r p a r t i c u l a r s t r u c t u r e s and t h e i r ion-exchange p r o p e r t i e s , z e o l i t e s appeared p a r t i c u l a r l y approp r i a t e f o r s t a b i l i z i n g f i n e l y d i s p e r s e d m e t a l p a r t i c l e s . Rabo e t a 1 ( 5 7 ) were among t h e f i r s t t o r e p o r t on z e o l i t e - s u p p o x t e d platinum. From t h e i r r e s u l t s t h e y concluded t h a t p l a t i n u m was almost atomicall y d i s p e r s e d w i t h i n t h e zeolite-framework. F u r t h e x i n v e s t i g a t i o n s on z e o l i t e - s u p p o r t e d p l a t i n u m were performed by s e v e r a l a u t h o r s (58-60). G e n e r a l l y it was concluded t h a t t h e p l a t i n u m p a r t i c l e - s i z e depended s t r o n g l y on t h e p r e t r e a t m e n t c o n d i t i o n s o f t h e m a t e r i a l s b e f o r e H - r e d u c t i o n . G a l l e z o t e t a 1 ( 5 9 ) from t h e i r e l e c t r o n micros2 copy and X-ray d i f f r a c t i o n s t u d i e s conc uded t h a t t h e metal dispers i o n would depend on t h e p o s i t i o n o f P t c a t i o n s i n t h e z e o l i t e cag e s . S i n c e much o f t h e work concerned p l a t i n u m c a t a l y s t s , we have i n v e s t i g a t e d t h e behaviour of o t h e r z e o l i t e - s u p p o r t e d group V I I I m e t a l s i n o r d e r t o p r o v i d e a g e n e r a l t r e n d on t h e p r e p a r a t i o n and t h e p r o p e r t i e s o f z e o l i t e - s u p p o r t e d group V I I I m e t a l s . The p r e p a r a t i o n o f z e o l i t e - s u p p o r t e d noble m e t a l s i n v o l v e d t h e subst i t u t i o n o f ~ a ' c a t i o n s i n Nay, by ion-exchange w i t h group V I I I met a l s i n t h e i r c a i o n i c form,. For i o n e change w e have used a ueous S+ 9+, s o l u t i o n s o f ~t (m3) pd2+ ( N H ~ ) Ru (NH ) 6 , ( ~ (NH h ) ~ 1 ) ( I r ( N H ) C1I2'. A f t e r b e i n g c a r e f u l l y washe2 and dried3$e exchanged 3 5 f a u j a s l t e - t y p e z e o l i t e s were f i r s t c a l c i n e d i n oxygen i n t h e
iJ+
3+
*,
temperature range 473-773 K , followed by H2-reduced i n t h e temperature range 383-773 K. Metal d i s p e r s i o n and p a r t i c u l e s i z e measurements were o b t a i n e d by H a d s o r p t i o n and t r a n s m i s s i o n e l e c t r o n micros2 copy. X-ray d i f f r a c t i o n , e l e c t r o n s p i n resonance, X p s and i n f r a r e d were used t o i n v e s t i g a t e t h e s t a t e o f t h e metal c a t i o n s w i t h i n t h e z e o l i t e framework b e f o r e H - r e d u c t i o n , s i n c e it h a s been a l r e a d y 2 s t a t e d f o r P t t h a t t h i s parameter may s t r o n g l y i n f l u e n c e t h e metal dispersion. A s a l r e a d y observed by o t h e r s (58-60) t h e average p a r t i c l e diameter obtained from e l e c t r o n micrographs and t h a t c a l c u l a t e d from hydrogen adsorption d a t a a r e i n good agreement i n t h e c a s e of Pt-Nay samples calcined i n oxygen a t 573 K before H2-reduced. I n c o n t r a s t Pt-Nay calcined i n oxygen a t 773 K and then H2-reduced a t 773 K showed by H2-adsorption an a p p a r e n t p a r t i c l e s i z e of about 5nm while t h e e l e c tron micrographs i n d i c a t e d t h a t t h e p a r t i c l e s i z e were around 2 nm. This discrepancy was i n t e r p r e t e d i n terms o f t h e e x i s t e n c e of p l a t i num a t o m i c a l l y d i s p e r s e d i n t h e s o d a l i t e cages, which d i d n o t adsorb hydrogen. X-ray d i f f r a c t i o n a n a l y s i s ( 6 1) have i n d i c a t e d t h a t t h e increase o f the a t i v a t i o n temperature p r i o r t o r e d u c t i o n produced a migration o f P t c a t i o n s from t h e supercages t o t h e s o d a l i t e cages. Thus r e d u c t i o n o f p t 2 + c a t i o n s p r e s e n t i n t h e supercages produced platinum a g g r e g a b s of about 1 nm i n d i a m e t e r , while t h e reduction of pt2' c a t i o n s i n t h e s o d a l i t e cages would l e a d t o a t o m i c a l l y dispersed platinum.
5+
Ruthenium, rhodium o r i r i d i u m exchanged z e o l i t e s behaved d i f f e r e n t l y . Samples p r e c a l c i n e d i n oxygen below 510 K p r i o r t o H -reduction d i d 2 form h i g h l y d i s p e r s e d metal c a t a l y s t s . The p a r t i c l e s i z e s a s d e t e r mined by e l e c t r o n microscopy and by H2 a d s o r p t i o n were i n good agreement and i n t h e range 1-1.5 nm. Furthermore t h e e l e c t r o n micrographs i n d i c a t e d t h a t t h e metal p a r t i c l e s were l o c a t e d w i t h i n t h e z e o l i t e c a v i t i e s , probably i n t h e supercages. I n c o n t r a s t very l a r g e metal p a r t i c l e (10-20 nm) were formed f o r t h o s e samples p r e c a l c i n e d i n oxygen a t 7 7 3 K, E l e c t r o n micrographs showed t h a t t h e s e p a r t i c l e s were on t h e e x t e r n a l s u r f a c e of t h e z e o l i t e . These r e s u l t s c l e a r l y i n d i c a t e a d i f f e r e n t behaviour o f R u , Rh, I r exchanged Nay z e o l i t e s compared with Pt-Nay samples. To b e t t e r understand t h e e f f e c t of oxygen p r e t r e a t m e n t on t h e R u , R h , I r d i s p e r s i o n , t h e s t a t e of t h e metal p r e c u r s o r b e f o r e H - r e d u c t i o n , was i n v e s t i g a t e d . 2
It_ was shown ( 3 ) t h a t upon a c t i v a t i o n o f Ru-exchanged Y z e o l i t e t h e (NH~) 3+ complex was p r o g r e s s i v e l y transformed i n t o s p e c i e s , f o l l o w i n g h y d r o l y s i s of t h e hexammine [RU ( N H ~ ) (OH) ruthenium compyex. Hence ruthenium i n i t s c a t i o n i c form i s p r o g r e s s i vely transformed i n an a n i o n i c s p e c i e s which w i l l be no more bound t o the z e o l i t e l a t t i c e by e l e c t r o s t a t i c f i e l d . C a l c i n a t i o n a t a temper a t u r e of 773 K produced a dehydration o f t h e ruthenium hydroxyanions and t h e subsequent formation of Ru 0 Ru02 on t h e e x t e r n a l 2 3' surface o f t h e z e o l i t e . The e f f e c t of t h e oxygen t r e a t m e n t o f Ru(NH ) -Nay z e o l i t e s p r i o r H2-reduction on t h e f i n a l metal p a r t i 3 6 c l e s IS now w e l l understandable : up t o a temperature o f about 523 K ,
LRU
63
1
3+
complexes remained well d i s p e r s e d i n t h e supercages of t h e z e o l i t e . These w e l l d i s p e r s e d s p e c i e s formed upon H -reduction very s m 11 metal p a r t i c l e s . A t higher temperature t h e hy roxyanions of Ru?+ were dehydrated i n t o l a r g e Ru203 c r y s t a l l i t e s , l o c a t e d on the e x t e r n a l s u r f a c e o f t h e z e o l i t e . Obviously t h e H -reduction of these l a r g e oxide c r y s t a l l i t e s generated l a r g e meta? p a r t i c l e s . Ru
5
S i m i l a r s t u d i e s were c a r r i e d with ( I r(NH C 1 ) 2+ Nay. X-ray d i f fract i o n and i n f r a r e d s5ydies i n c i d a t e d h a t upon c a l c i n a t i o n i n c a t i o n s were p r o g r e s s i v e l y t r a n s formed i n t o oxygen ( I r(NH3) 5 C 1 ) hydroxyanions ( I r (NH3)5-x (OH) x C l ) which upon complete o x i d a t i o n a t 773 K formed l a r g e Ir02 c r y s t a l l i t e s . The g e n e r a l conclusion which may be derived from t h i s study i s the following : z e o l i t e s a r e s u i t a b l e m a t e r i a l s t o prepare and t o stabil i z e h i g h l y d i s p e r s e d metal c a t a l y s t s , provided t h a t t h e metal prec u r s o r s remain h i g h l y d i s p e r s e d i n t h e z e o l i t e c a v i t i e s before H 2 reduction. Group V I I I t r a n s i t i o n metals may be c l a s s i f i e d i n t o two groups :
i) those f o r which t h e c a t i o n i c form i s r e l a t i v e l y s t a b l e , such as platinum, palladium ; t h e s e c a t i o n s remain always h i g h l y dispersed i n t h e z e o l i t e c a v i t i e s , t h u s form h i g h l y d i s p e r s e d metal c a t a l y s t s . ii) those f o r which t h e c a t i o n i c form i s u n s t a b l e , rhodium, ruthenium, i r i d i u m , and e a s i l y transformed i n t o an hydroxyanion. A s long a s t h e hydroxyanions remain d i s p e r s e d i n t h e z e o l i t e framework, one o b t a i n s upon H -reduction h i g h l y d i s p e r s e d metal c a t a l y s t s . However 2 s i n c e a t high temperature t h e hydroxyanions a r e dehydrated i n t o t h e oxide form, one should avoid such formation of l a r g e oxide c r y s t a l l i t e i n o r d e r t o form h i g h l y d i s p e r s e d metal c a t a l y s t s . 3
-
C a t a l y t i c p r o p e r t i e s of t r a n s i t i o n metal ion-exchanged z e o l i t e s
I t i s w e l l recognized t h a t z e o l i t e s a r e widely used a s c a t a l y s t s for
a broad range of hydrocarbon transformation. Although t h e major a p p l i c a t i o n s of z e o l i t e s a s c a t a l y s t s , e s p e c i a l l y i n t h e Petroleum I n d u s t r y , a r e based on a c i d form z e o l i t e s s e v e r a l i n v e s t i g a t o r s have discovered t h a t s p e c i f i c c a t a l y t i c p r o p e r t i e s a r e shown by t r a n s i t i o n metal i o n exchanged z e o l i t e s toward r e a c t i o n s generally c a t a l y z e d by t h e p a r e n t metal i o n s ' i n s o l u t i o n . I n t h i s paragraph some r e c e n t a s p e c t s of t h e c a t a l y t i c p r o p e r t i e s of "immobilized t r a n s i t i o n metal i o n s " i n z e o l i t e s w i l l be given. I t i s worthwhile t o r e c a l l t h a t most of t h e work which w i l l be described d e a l s with f a u j a s i t e - t y p e z e o l i t e s , t h e s e m a t e r i a l s appearing t h e most suitable c a r r i e r f o r "Immobilizing" s o l u b l e c a t a l y s t s because of t h e r e l a t i vely l a r g e space of t h e i r c a v i t i e s . 3-1- Oligomerisation, a d d i t i o n , cyclodimerisation of unsatured hydrocarbons The homogeneous c a t a l y t i c formation of irkines by a d d i t i o n of
primary a l i p h a t i c amines t o a c e t y l e n e s i n t h e presence of z i n c acetate needs high p r e s s u r e ( 6 2 ) Z n ( I 1 )-exchanged Y z e o l i t e was found active f o r t h e a d d i t i o n o f methylamine t o methylacetylene a t atmospheric pressure.N-isopropylidene methylamine r e s u l t e d f o l l o w i n g t h e reaction ( 6 3 )
.
CH NH 3 2
+
CH - C r
3
CH
+
(CH ) C = N C H 3 2 3
However N i , Pd, P t and Cu(1) exchanged z e o l i t e s ( i s o e l e c t r o n i c w i t h Z n ( I 1 ) ) were found i n a c t i v e i n t h i s r e a c t i o n . The l a c k of a c t i v i t y was a t t r i b u t e d t o a r a p i d r e d u c t i o n of t h e t r a n s i t i o n metal i o n s . Thus it i s c l e a r t h a t when u s i n g t h e s e m a t e r i a l s f o r r e a c t i o n s catalyzed by i o n s i n s o l u t i o n , i t i s important t o avoid t h e r e d u c t i o n of t h e exchanged t r a n s i t i o n metal i o n s i n t o t h e i r m e t a l l i c forms. Benzene formation from t h e t r i m e r i s a t i o n o f a c e t y l e n e was c a t a l y z e d by N ~ ~ + - Nz ~e oYl i t e . The r e a c t i o n was found t o occur w i t h i n t h e l a r ge c a v i t i e s o f the z e o l i t e ( 6 4 ) . The d i m e r i s a t i o n o f e t h y l e n e over rhodium exchanged z e o l i t e h a s been i n v e s t i g a t e d 5 6 5 ) . I t was found t h a t t h e r a t e of formation of n-butenes depended on the temperature of a c t i v a t i o n of RhY, the maximum r a t e b e i n g reached when RhY was a c t i v a t e d around 400°C. Furthermore it was found t h a t t h e dimerisation a c t i v i t y was lowered by t h e a d d i t i o n o f p y r i d i n e o r CO which are known t o i n t e r a c t w i t h t h e R h c a t i o n s , while an a p p r o p r i a t e amount o f HC1 i n c r e a s e d t h e r e a c t i o n r a t e . Xps measurements i n d i c a ted t h e e x i s t e n c e o f monovalent R . ( I ) on t h e a c t i v e Rh-Y samples. I t was concluded t h a t Rh(1) i n Nay was t h e a c t i v e s i t e s f o r t h e d i merization o f e t h y l e n e . The e f f e c t of HC1 i s s i m i l a r t o t h a t encountered i n homogeneous systems f o r which it h a s been shown t h a t the a d d i t i o n o f H C 1 t o &(I) complexes a c t i v a t e s t h e c a t a l y s t ( 6 6 ) . Copper-exchanged z e o l i t e s have shown i n t e r e s t i n g c a t a l y t i c propert i e s f o r t h e c y c l o d i m e r i s a t i o n o f b u t a d i e n e t o vinylcyclohexene ( 6 7 ) . Nickel ( 0 ) complexes i n s o l u t i o n c a t a l y z e t h e c y c l o d i m e r i s a t i o n of butadiene i n t o 4-vinylcyclohexene ( 6 8 ) . Thus it appeared t h a t it i s Cu(1) which i s i s o e l e c t r o n i c with N i ( 0 ) which i s t h e a c t i v e s i t e f o r the cyclodimensation o f butadiene by Cu(I1)-Y z e o l i t e s Cu(1)-Y were f u r t h e r i n v e s t i g a t e d f o r t h e c y c l o d i m e r i s a t i o n of butadiene ( 6 9 ) . Cu(1)-Y samples were p r e p a r e d e i t h e r by d i r e c t exchange w i t h cuprous s o l u t i o n o r by r e d u c t i o n of Cu(I1)-Y by CO following t h e procedure given i n ( 7 0 ) , monovalent-copper c o n t a i n i n g z e o l i t e s e x h i b i t e d a h i g h s e l e c t i v i t y (90 % ) f o r t h e formation of 4-vinylcyclohexene . However Cu(1.)-Y samples o b t a i n e d from CO r e d u c t i o n showed a r a p i d d e a c t i v a t i o n due t o t h e formation and d e p o s i t i o n o f polymer b u t a diene on t h e z e o l i t e s u r f a c e . The a c t i v e s i t e s f o r p o l y m e r i s a t i o n were Br8nsted/Lewis c e n t r e s generated d u r i n g r e d u c t i o n of Cu(I1)-Y t o Cu(I)-Y. When t h e a c i d s i t e s were n e u t r a l i z e d by ammonia t h e cat a l y s t d e a c t i v a t i o n d u r i n g t h e c y c l o d i m e r i s a t i o n o f butadiene was lowered. Cu(1)-Y p r e p a r e d by d i r e c t exchange with cuprous s o l u t i o n was found s u b s t a n t i a l l y more s t a b l e due t o t h e absence o f a c i d s i t e s .
I n conclusion, i t was s t a t e d t h a t CO-reduced C u ( I 1 ) -Y i s due t o t h e The r o l e o f t h e z e o l i t e framework i o n s , which allowed t h e o x i d a t i v e les.
t h e d i f f e r e n c e between Cu(1)-Y and a c i d i t y g e n e r a t e d by CO-reduction. i s t o s t a b i l i z e monovalent copper coupling of two butadiene molecu-
3-2- Oxidation r e a c t i o n s . T r a n s i t i o n metal i o n s exchanges Y z e o l i t e s were found a c t i v e f o r s e l e c t i v e o x i d a t i o n o f hydrocarbons. I n t h e o x i d a t i o n of e t h y l e n e i n t o acetaldehyde (Wacker p r o c e s s ) which i n t h e l i q u i d phase i s c a t a l y z e d by a s o l u t i o n c o n t a i n i n g C u ( I 1 ) and P d ( I 1 ) i o n s was found t o occur i n t h e g a s - s o l i d r e a c t i o n u s i n g P d ( I I ) , Cu(I1)-exchanged Y z e o l i t e ( 7 1 ) . I t should be noted t h a t t h e y i e l d o f acetaldehyde decreased a t temperatures h i g h e r than 115°C a s t h e r e s u l t of r e d u c t i o n of P d ( 1 I ) and C u ( I 1 ) i n t o metal forms
.
C u ( 1 I ) -Y z e o l i t e was a l s o i n v e s t i g a t e d i n t h e vapor phase oxidation o f benzyl a l c o h o l a t a temperature range o f 300-390°C ( 7 2 ) . The act i v e s i t e s f o r t h e o x i d a t i o n of benzyl a l c o h o l i n t o benzaldehyde were Cu (11) i n t h e z e o l i t e framework. The conversion o f benzylalcohol i n c r e a s e d a b r u p t l y beyong 30 % C u ( I 1 ) i o n exchange, which indicated t h a t C u ( I 1 ) i o n s were, below 30 % exchange l e v e l i n hidden s i t e s . Beyong 30 % exchange C u ( I 1 ) i o n s were l o c a t e d w i t h i n t h e z e o l i t e supercages and t h u s a c c e s s i b l e t o t h e r e a c t a n t s . C o b a l t ( I 1 )-exchanged Nay z e o l i t e was a l s o found a c t i v e and s e l e c t i v e i n t h e oxidation o f benzyl a l c o h o l i n t o benzaldehyde ( 7 3 ) . Co-Nay was found much more s e l e c t i v e t h a n Cu-Nay. A s i n t h e c a s e o f Cu-Nay, t h e y i e l d of benzaldehyde i n c r e a s e d a b r u p t l y beyong about 2 0 & exchange. The e f f e c t o f amine a d d i t i o n on t h e benzaldehyde y i e l d demonstrated t h e sirnil a r i t y o f t h e behevior o f C o ( I 1 ) i n s o l u t i o n and w i t h i n t h e z e o l i t e framework. The a d d i t i o n o f p y r i d i n e o r p i p e r i d i n e would p u l l o u t of t h e s o d a l i t e cages Co (11) and t h u s i n c r e a s e s t h e number of access i b l e C o ( I 1 ) i o n s , which would l e a d t o an i n c r e a s e of t h e benzaldehyde y i e l d . However when e t h y l e n e diamine was adsorbed, t h e y i e l d o f benzaldehyde decreased. T h i s was a t t r i b u t e d t o t h e formation of Co (111)( e n ) 0; complex i n t h e l a r g e c a v i t i e s which appeared rel a t i v e l y s t a b f e . I n c o n t r a s t C o ( 1 I ) i n t h e presence of p y r i d i n e or ammonia forms with 0 a dimeric Cobalt-oxygen adduct such a s 2 which was considered a s t h e p r e c u r s o r for LxCo (11)-02-Co (11)L d i s s o c i a t i o n o f 0 mofecule. The formation of Co-0 i s t h u s f a c i l i 2 t a t e d i n t h e presence of p y r i d i n e r e s u l t i n g i n t h e i n c r e a s e o f the benzaldehyde y i e l d . The o x i d a t i o n o f cyclohexene over molybdenum z e o l i t e s i n t h e Liquidphase was found t o occur with r e l a t i v e l y high s e l e c t i v e l y toward e p o x i d a t i o n ( 7 4 ) a t 50 8 conversion t h e s e l e c t i v i t y f o r cyclohexane oxide was about 50 %. I t was concluded t h a t a t low Mo c o n t e n t the cyclohexene e p o x i d a t i o n w a s i n i t i a t e d by a r a d i c a l mechanism, the formation of cyclohexenyl hydroperoxide being t h e l i m i t i n g s t e p Z e o l i t e s c o n t a i n i n g both molybdenum and c o b a l t e x h i b i t e d a c t i v i t i e s and s e l e c t i v i t i e s i n cyclohexene o x i d a t i o n comparable t o homogeneous
c a t a l y s t s such a s Co ( a c a c ) /MOO ( a c a c )2 . 2 2
3-3-
Carbonylation of methanol
The c a r b o n y l a t i o n of methanol i n t o a c e t i c a c i d o r m e t h y l a c e t a t e was developed by Mmsanto i n t h e l i q u i d phase u s i n g rhodium based c a t a l y s t s , i n t h e p r e s e n c e o f an i o d i d e promotor ( 7 4 ) . Rhodium exchanged z e o l i t e s were found a c t i v e and s e l e c t i v e f o r t h e vapor phase carbonylation o f methanol ( 7 5 ) . S i m i l a r l y i r i d i u m exchanged z e o l i t e showed i n t e r e s t i n g a c t i v i t y i n t h i s r e a c t i o n . The r e a c t i o n was c a r r i e d o u t a t 150-180°C and a t atmospheric p r e s s u r e . The k i n e t i c s t u d i e s r e v e a l e d i d e n t i c a l r a t e e x p r e s s i o n b o t h i n l i q u i d phase and i n vapor phase t h a t i s : f i r s t o r d e r w i t h r e s p e c t t o CH 1 promotor and ( R h ) c o n c e n t r a t i o n and z e r o o r d e r w i t h r e s p e c t t o C$ OH and CO. 3 The v a r i o u s s t e p s of t h e r e a c t i o n were i n v e s t i g a t e d by i n f r a r e d and the r e s u l t s p a r a l l e l t h o s e o b t a i n e d w i t h t h e homogeneous rhodium c a t a l y s t s . I n t h e presence o f CO R .( 1 1 1 ) i o n s were reduced t o Rh ( I ) which c o o r d i n a t e d two CO molecules t o g i v e Rh ( I )( C O ) a c t i v e s i t e s . These s p e c i e s added CH I fhrough a n o x i d a t i v e a d d i t i o n w i t h t h e subsequent formation -Rh (111)( C 0 ) (I)] complexes. T h i s complex was r e l a t i v e l y u n s t a l e and r e a r r a n g e d r a p i d l y i n t o a rhodium a c e t y l complex . Methanol o r w a t e r r e a c CO) Rh (CO) ( 1 ted e a s i l y w i t h t h e acety?. adduct and methyl a c e t a t e o r a c e t i c a c i d were evolved. The r o l e o f t h e z e o l i t e appeared t o be a s a c a r r i e r f o r b e t t e r s t a b i l i z a t i o n and f o r b e t t e r d i s p e r s i o n o f & ( I ) k p e c i e s .
02 LCH 2 L(CH
12
These few examples a r e s u f f i c i e n t t o show t h a t a l a r g e number o f r e a c t i o n s c a t a l y z e d by s o l u b l e m e t a l complexes i n l i q u i d phase can be c a r r i e d o u t i n t h e vapor phase, by z e o l i t e s exchanged w i t h t h e
analogous metal i o n s . I n g e n e r a l t h e z e o l i t e m a t r i x a f f o r d s t h e h i g h e s t metal i o n d i s p e r s i o n i n comparison w i t h o t h e r s u p p o r t s and a f f o r d s h i g h s t a b i l i s a t i o n f o r c a t i o n s i n low o x i d a t i o n s t a t e . I n a d d i t i o n i n s e v e r a l c a s e s , on z e o l i t e s t h e r e a c t i o n can be c a r r i e d out a t much lower p r e s s u r e s t h a n t h o s e r e q u i r e d by o t h e r homogeneous c a t a l y s t s i n l i . q u i d phase. The a p p l i c a b i l i t y o f z e o l i t e s c o n t a i n i n g t r a n s i t i o n m e t a l i o n s i s f a r from b e i n g exhausted, and it i s c l e a r t h a t s e v e r a l new a p p l i c a t i o n s f o r producing chemicals w i l l be found in the near futur.
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ZEOLITE BIFUNCTIONAL CATALYSIS
M. G u i s n e t and G . P e r o t
L a b o r a t o i r e A s s o c i 6 au CNRS - C a t a l y s e Organique Universitg de P o i t i e r s , France
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 metal-loaded a c i d z e o l i t e s a r e employed i n numerous p r o c e s s e s i n p e t r o l e u m r e f i n i n g and i n p e t r o c h e m i c a l i n d u s t r i e s (1,2) : h y d r o c r a c k i n g , s e l e c t o f o r ming, dewaxing, h y d r o i s o m e r i z a t i o n of C5-C6 a l k a n e s , h y d r o i s o m e r i z a t i o n of C8 a r o m a t i c s The c a t a l y s t s used i n t h e s e p r o c e s s e s p r e s e n t two t y p e s o f s i t e s : m e t a l l i c s i t e s whose main f u n c t i o n i s t o h y d r o g e n a t e and t o d e h y d r o g e n a t e and a c i d s i t e s whose main function is t o c r a c k o r t o isomerize.
...
Because o f t h e p r o g r e s s i v e e v o l u t i o n i n t h e s u p p l y towards h e a v i e r c r u d e s and i n t h e demand towards l i g h t p r o d u c t s , hydroc r a c k i n g , t h e most i m p o r t a n t p r o c e s s i n v o l v i n g 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 s t i n e d t o have a n i m p o r t a n t development. Compared t o c a t a l y t i c c r a c k i n g , i t h a s t h e a d v a n t a g e of b e i n g more f l e x i b l e indeed improvements i n p r o c e s s e s and i n c a t a l y s t s have made i t p o s s i b l e t o c o n v e r t a l a r g e v a r i e t y of f e e d s t o c k s (from n a p h t h a t o heavy g a s o i l s ) and t o o b t a i n a l a r g e v a r i e t y of h i g h q u a l i t y prod u c t s . Hydrocracking r e q u i r e s c a t a l y s t s w i t h a h i g h a c i d i t y count e r b a l a n c e d by a h i g h h y d r o g e n a t i o n a c t i v i t y , which i m p l i e s t h e u s e of 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 . The c r a c k i n g f u n c t i o n i s p r o v i d e d by a Y z e o l i t e i n which t h e sodium i o n s h a v e been r e p l a c e d by hydrogen, by r a r e e a r t h o r by d i v a l e n t c a t i o n s and t h e h y d r o g e n a t i n g f u n c t i o n i s p r o v i d e d by n o b l e o r non-noble metals ( 1 , 3 ) . I n marked c o n t r a s t t o amorphous s i l i c a - a l u m i n a c a t a l y s t s , z e o l i t e c a t a l y s t s can o p e r a t e i n t h e p r e s e n c e o f s u b s t a n t i a l amounts of ammonia and o t h e r b a s i c n i t r o g e n compounds. T h i s g r e a t e r a b i l i t y of the z e o l i t e s t o t o l e r a t e b a s i c compounds c a n b e a t t r i b u t e d t o t h e i r g r e a t e r number o f a c i d s i t e s . Moreover t h i s g r e a t e r a c i d i t y enhances the r e s i s t a n c e of t h e h y d r o g e n a t i n g f u n c t i o n t o p o i s o n i n g by s u l f u r
compounds and hydrocracking catalysts show a high stability with regards to sulfur poisoning. Selectoforming and Mobil Dewaxing (MDDW (4)) processes take advantage of the shape-selective properties of zeolites. Selectoforming allows the selective hydrocracking of n-paraffins in a gasoline reformate. In this process, a bifunctional catalyst with a non-noble metal as hydrogenating component and a small pore zeolite (T zeolite) as a cracking component is employed. The MDDW process uses a Pd or Ni exchanged ZSM-5 zeolite (an intermediate pore-size zeolite) which cracks preferentially the n-paraffins particularly those with higher boiling temperatures. Hydroisomerization of light naphtha (C -C6 alkanes) is carried out in the Hysomer process (Shell) on a nob?e metal highly dispersed on a large pore zeolite with a high acidity. Bifunctional catalysts containing mordenite or ZSM-5 zeolite can also be used for the isomerization of C 8 aromatic cuts. Numerous parameters govern the activity and the selectivity of bifunctional zeolite catalysts. This is namely the case for the characteristics of the hydrogenating and acid functions as well as for their "balance". To show the influence of these parameters, examples will be selected from the most typical reactions of the industrial processes that is the isomerization and the cracking of n-alkanes. Naturally, the characteristics of bifunctional zeolite catalysts depend largely on the preparation procedure and particularly on the way the hydrogenating component is introduced. However the preparation of metal-loaded catalysts,, described elsewhere ( 5 , 6 ) will not be discussed here.
1. MECHANISMS OF ALKANE TRANSFORMATION ON BIFUNCTIONAL CATALYSTS 1.1
Bifunctional Catalysis
1.1.1 Generalities. In bifunctional catalysis, reactions occur in successive steps involving two different types of sites (7). As an example, the conventional bifunctional process of isomerization of n-hexane into methylpentanes found on platinum-silica alumina is shown in Fig. I. This catalyst presents two types of sites : i) platinum sites whose function is to dehydrogenate n-hexane into n-hexenes (reaction 1) and to hydrogenate methylpentenes into methylpentanes (reaction 5) ; ii) acid sites whose function is to isomerize hexenes into methylpentenes (reaction 3). Beside these chemical steps, a bifunctional process requires diffusion steps of the intermediate species. In this case, olefin intermediates diffuse from the metallic to the acid sites (step 2) and from the acid to the metallic sites (step 4).
U l C-C-C-C-C-C
C-C-C-C-C-C
-
C:c~C~C-xLC
.I.t Si02-A1 0
C:.%x=c 1
49
Cr'c-X%%%
2P 3
F
+H21T-H2 C
1
C-C-C-C-C
\
!
C,C^X%%
3.f
4
.
C
It
cLT=cz.-x Pt -
2
C-~-xI=cC
5
+H2
it+?
C
I
C-C-C-C-C
I
T I C
I
C-C-C-C-C
Fig. 1 .
C
I
C-C-C-C-C
Bifunctional process of n-hexane isomerization on platinum-silica alumina.
The existence of this bifunctional process is now well established : i) although highly unfavoured thermodynamically under the usual operating conditions,the intermediate olefins were detected by GLC or by mass spectrometry (8,lO). Moreover, the skeletal isomerization of olefins (reaction 3 in Fig. 1 .) is known to occur very readily on acid catalysLs (7,8,11) ; ii) the participation in the reaction of both acid and hydrogenating centers was clearly demonstrated by using physical mixtures
of an a c i d c a t a l y s t and of a metal d e p o s i t e d on a n i n e r t c a r r i e r (7,12-14) : t h e a c t i v i t i e s of t h e m i x t u r e s were d e f i n i t e l y g r e a t e r than t h e sum of t h e a c t i v i t i e s of t h e components ; i i i ) t h e change i n t h e i s o m e r i z a t i o n a c t i v i t i e s of bifunct i o n a l c a t a l y s t s ( d i f f e r i n g by t h e i r platinum c o n t e n t ) a s a function of t h e i r hydrogenation a c t i v i t i e s was t h e change expected from the m u l t i s t e p b i f u n c t i o n a l process (15) : - f o r low hydrogenation a c t i v i t i e s t h e l i m i t i n g s t e p i s t h e n-hexane dehydrogenation ( r e a c t i o n 1 ) o r t h e methylpentene hydrog e n a t i o n ( r e a c t i o n 5) on t h e m e t a l l i c s i t e s ; under t h e s e condit i o n s , t h e isomerization a c t i v i t y increases proportionally t o the hydrogenation a c t i v i t y (Fig. 2 . ) ; - f o r high hydrogenation a c t i v i t i e s , r e a c t i o n s 1 and 5 become very f a s t compared t o r e a c t i o n 3 which i s t h e n t h e r a t e - l i m i t i n g s t e p ; under t h e s e c o n d i t i o n s t h e i s o m e r i z a t i o n a c t i v i t y of t h e b i f u n c t i o n a l c a t a l y s t s no l o n g e r depends on t h e i r hydrogenation a c t i v i t y b u t only on t h e i r a c i d i t y . Thus, t h e i s o m e r i z a t i o n a c t i v i t y of b i f u n c t i o n a l c a t a l y s t s with a given a c i d c a r r i e r remains constant beyond a c e r t a i n v a l u e of t h e hydrogenation a c t i v i t y (Fig. 2 . ) . Moreover, t h e g r e a t e r t h e a c i d i t y of t h e c a r r i e r , t h e h i g h e r t h e maximum v a l u e of t h e i s o m e r i z a t i o n a c t i v i t y of t h e b i f u n c t i o n a l catalvst
.
Fig. 2.
n-Hexane i s o m e r i z a t i o n on p l a t i n u m - s i l i c a alumina ( P t / S A ) . I n f l u e n c e of t h e hydrogenating a c t i v i t y (H) and of t h e a c i d i t y (SAI i s more a c i d t h a n SA2) of t h e c a t a l y s t s on t h e i r i s o m e r i z a t i o n a c t i v i t i e s ( I ) (from r e f e r e n c e 12).
1.2 Mechanisms of t h e r e a c t i o n s o c c u r r i n g on b i f u n c t i o n a l c a t a lysts.
I s o m e r i z a t i o n and c r a c k i n g of a l k a n e s can t a k e p l a c e by t h e b i f u n c t i o n a l p r o c e s s b u t can a l s o b e c a t a l y z e d independently by t h e acid or by t h e m e t a l l i c s i t e s . 1 . 2 . 1 Acid - c a t a l y z e d r e a c t i o n s . The mechanism of a l k a n e isomer i z a t i o n and c r a c k i n g on a c i d s i t e s i s shown i n F i g . 3. Carbocations, involved a s i n t e r m e d i a t e s , a r e formed e i t h e r by a l k a n e a d s o r p t i o n on Br6nsted o r Lewis a c i d s i t e s :
o r by hydride t r a n s f e r from t h e a l k a n e t o a preadsorbed c a r b o c a t i o n :
1.2.2 Metal - c a t a l y z e d r e a c t i o n s . Two mechanisms have been proposed t o account f o r a l k a n e i s o m e r i z a t i o n on m e t a l s , p a r t i c u l a r l y on platinum : t h e b o n d - s h i f t mechanism a t t r i b u t e d t o t h e f o r m a t i o n of a a y - t r i a d s o r b e d i n t e r m e d i a t e s bonded t o two a d j a c e n t metal atoms (16,17) and t h e c y c l i c mechanism which i n v o l v e s t h e f o r m a t i o n of c y c l o p e n t a n i c i n t e r m e d i a t e s and t h e i r s c i s s i o n (18,19). On l a r g e platinum c r y s t a l l i t e s , i s o m e r i z a t i o n o c c u r s mainly by t h e bond-shift mechanism whereas on small platinum c r y s t a l l i t e s , t h e c y c l i c mechanism i s p r i v i l e g e d ( 2 0 ) . The a l k a n e c r a c k i n g on metal s i t e s (hydrogenolysis) i n v o l v e s t h e s c i s s i o n of h i g h l y dehydrogenated s p e c i e s adsorbed by two adjacent carbon atoms on a d j a c e n t metal s i t e s ( 2 1 ) . Contrary t o t h e cracking r e a c t i o n s by a c i d o r b i f u n c t i o n a l c a t a l y s i s , hydrogenolysis produces methane and e t h a n e .
Fig. 3 .
I s o m e r i z a t i o n and c r a c k i n g of p a r a f f i n s on a c i d c a t a l y s t s . : c a r b o c a t i o n ; x,y : numb e r of carbon atoms. P : p a r a f f i n ; 0 : o l e f i n ; C'
1.2.3 Reactions involved i n t h e b i f u n c t i o n a l p r o c e s s . This bifunct i o n a l p r o c e s s of a l k a n e i s o m e r i z a t i o n h a s a l r e a d y been developed i n t h e n-hexane example ( F i g . 1 .) The c r a c k i n g r e a c t i o n s o c c u r by s c i s s i o n o f o l e f i n i n t e r m e d i a t e s . The H o r i u t i P o l a n y i mechanism i s commonly a c c e p t e d . t o e x p l a i n o l e f i n h y d r o g e n a t i o n ( r e a c t i o n 5) o r a l k a n e d e h y d r o g e n a t i o n ( r e a c t i o n I ) . The c r a c k i n g and t h e i s o m e r i z a t i o n of i n t e r m e d i a t e o l e f i n s on a c i d s i t e s i n v o l v e carbenium i o n s formed by o l e f i n a d s o r p t i o n o n p r o t o n i c s i t e s ( F i g . 4 . ) . The r e a r r a n g e m e n t ( s t e p 2) and t h e c r a c k i n g ( s t e p s 3 , 3 ' ) of c a r b o c a t i o n s a r e t h e l i m i t i n g s t e p s of i s o m e r i z a t i o n and c r a c k i n g r e s p e c t i v e l y , s i n c e t h e c a r b o c a t i o n f o r m a t i o n and d e s o r p t i o n a r e v e r y rapid. It i s w i d e l y a d m i t t e d t h a t s k e l e t a l r e a r r a n g e m e n t s o f carbocat i o n s w i t h o u t change i n t h e c h a i n l e n g t h (termed t y p e A ( 2 2 ) ) proceed v i a a l k y l - s h i f t :
.
whereas r e a r r a n g e m e n t s w i t h change i n t h e c h a i n l e n g t h ( t y p e B) proceed v i a p r o t o n a t e d c y c l o p r o p a n e i n t e r m e d i a t e s (23,24) :
+ c-c-c-c-c-c
#%\
4
-
.
/H c-d----t-c-c
C I + c-c-c-c-c
T h i s p r o c e s s a v o i d s t h e f o r m a t i o n of t h e v e r y u n s t a b l e p r i m a r y c a r b o c a t i o n s which would b e i n v o l v e d i n a n a l k y l - s h i f t b r a n c h i n g mechanism :
P+
C-C-C-C-C-C
-P +
C-C-C-C-C
F o r t h e s a k e of c o n c i s e n e s s , t h e p r o t o n a t e d c y c l o p r o p a n e s w i l l be p i c t u r e d a s f a c e - p r o t o n a t e d c y c l o p r o p a n e s ( a ) . Such a n e n t i t y seems however t o have l i t t l e p h y s i c a l r e a l i t y compared t o a l k y l b r i d g e d (b) o r e d g e - p r o t o n a t e d c y c l o p r o p a n e s ( c ) (25) :
Fig. 4.
I s o m e r i z a t i o n and c r a c k i n g of o l e f i n s on a c i d c a t a l y s t s . 0 : o l e f i n ; C+ : c a r b o c a t i o n ; x,y : number of carbon atoms.
The 8 - s c i s s i o n of a c a r b o c a t i o n l e a d s t o an o l e f i n and a carbocation :
A primary c a r b o c a t i o n i s o b t a i n e d by @ - s c i s s i o n of a l i n e a r carbocation :
+Or\
C-C-C-C-C-C-C
+
--+C-C=C
4
C-C-C-C
Consequently, t h i s t y p e of s c i s s i o n ( s t e p 3 , F i g . 4 . i f 0, i s a l i n e a r o l e f i n ) w i l l be v e r y slow ; a l i n e a r c a r b o c a t i o n w l l l o n l y be isornerized ( t y p e B rearrangement) and t h e c r a c k i n g w i l l occur by s c i s s i o n of c a r b o c a t i o n s w i t h mono and multibranched s k e l e t o n s ( s t e p 3 ' ) , l e a d i n g t o secondary o r t e r t i a r y c a r b o c a t i o n s :
fl+
C-C-C-C-C-C I
C I C-C-C-C-C
D+
I
j
----t
+ c-c-C
+
C-C-C
1
+
C=C-C-C
+ C=C-C
This i s the r e a s o n why t h e c r a c k i n g of n-alkanes f o l l o w s t h e i r isomerizat ion.
2. ACTIVITY OF BIFUNCTIONAL ZEOLITE CATALYSTS
2.1
I n f l u e n c e of t h e hydrogenating a c t i v i t y
On pure a c i d z e o l i t e s , pentanes and hexanes isomerize and c r a c k whereas long-chain a l k a n e s undergo only c r a c k i n g . The deact i v a t i o n i s r a p i d a t low hydrogen p r e s s u r e b u t becomes very slow a t high p r e s s u r e (26). The i s o m e r i z a t i o n t o c r a c k i n g r a t e r a t i o depends n o t only on t h e z e o l i t e but a l s o on o p e r a t i n g c o n d i t i o n s (27) : t h u s , f o r n-hexane t r a n s f o r m a t i o n on H mordenite a t atmosp h e r i c p r e s s u r e , t h i s r a t i o i s p r a c t i c a l l y nu1 a t 4 0 0 " ~but i s equal t o 0.2 a t 2 5 0 " ~; i t i n c r e a s e s with c a t a l y s t d e a c t i v a t i o n by coke d e p o s i t (27,28) and with hydrogen p r e s s u r e (26). However the i n i t i a l i s o m e r i z a t i o n r a t e d e c r e a s e s when hydrogen p r e s s u r e increases (29). When a hydrogenating component i s added t o t h e z e o l i t e , i t s a c t i v i t y , i t s s t a b i l i t y and i t s i s o m e r i z a t ion s e l e c t i v i t y generally i n c r e a s e . Fig. 5. shows, f o r n-hexane i s o m e r i z a t i o n and cracking t h e changes of t h e a c t i v i t i e s of Pt-HY c a t a l y s t s (measured a f t e r an aging p e r i o d ) a s a f u n c t i o n of t h e i r m e t a l l i c s u r f a c e a r e a s (30). These c a t a l y s t s , with a platinum c o n t e n t ranging from 0 t o 17.7 wt%, were prepared by exchange of a s t a b i l i z e d Y z e o l i t e : t h e platinum c r y s t a l l i t e s were between 2 t o 5 nm depending on t h e samples. The i s o m e r i z a t i o n a c t i v i t y i n c r e a s e s very r a p i d l y a t low m e t a l l i c s u r f a c e a r e a s , then remains p r a c t i c a l l y c o n s t a n t above 0.5 m 2 g- 1 (Fig. 5 . ) . This type of curve, expected i n t h e c a s e of a bifunct i o n a l process ( s e e F i g . 2 . ) , was found f o r t h e same r e a c t i o n w i t h o t h e r c a t a l y s t s :Pt-Lay (31), Pt-H mordenite ( 3 2 ) . I t i s a l s o the c a s e f o r t h e i s o m e r i z a t i o n and t h e c r a c k i n g of long-chain n-alkanes (up t o nCI6) w i t h p h y s i c a l mixtures of mordenite and platinum depos i t e d on i n e r t alumina, provided t h a t t h e a c t i v i t i e s a f t e r aging be taken i n t o c o n s i d e r a t i o n ( 1 3) Both i s o m e r i z a t i o n and crackinga c t i v i t i e s per gram of mordenite f i r s t i n c r e a s e p r o p o r t i o n a l l y t o t h e hydrogenation a c t i v i t y and then r e a c h a p l a t e a u ( s e e a s an example F i g . 6 . f o r n-octane t r a n s f o r m a t i o n ) ; f o r a l l t h e a l k a n e s , aHI, t h e v a l u e of t h e hydrogenation a c t i v i t y required t o which i s o b t a i n t h e p l a t e a u f o r i s o m e r i z a t i o n i s g r e a t e r than a t h e value required t o obtain t h e f o r cracking 6.). I f one c o n s i d e r s t h e a c t i v i t i e s b e f o r e d e a c t i v a t i o n , t h e plateau i s reached f o r c r a c k i n g but n o t f o r i s o m e r i z a t i o n (27). The same o b s e r v a t i o n can be made f o r n-heptane t r a n s f o r m a t i o n s on mixtures w i t h Y z e o l i t e . On platinum-alumina, ZSM-5 m i x t u r e s , t h e plateau i s n o t o b t a i n e d e i t h e r f o r i s o m e r i z a t i o n nor f o r c r a c k i n g of n-heptane. This means t h a t t h e hydrogenation a c t i v i t y i s n o t suffic i e n t t o "balance" t h e high i n i t i a l a c i d i t y of t h e s e z e o l i t e s and ~ a r t i c u l a r l yt h a t of ZSM-5 z e o l i t e . The same c o n c l u s i o n was reached by J a c o b s e t a l . f o r n-decane t r a n s f o r m a t i o n on 0.5 w t W Pt-ZSM-5 ( 3 3 ) . P h y s i c a l mixtures of mordenite and NiMo s u l f i d e s deposited on
.
in
?gig.
250°C : 24 b a r PH2 p n-hexane : 6 b a r
*
Fig. 5. R a t e s of n-hexane i s o m e r i z a t i o n (I) and c r a c k i n g (C) a g a i n s t t h e metal s u r f a c e a r e a of t h e samples (Spt) (from r e f e r e n c e 30). alumina were a l s o used. I n n-decane t r a n s f o r m a t i o n a t 400°c under 30 bar hydrogen p r e s s u r e , a s y n e r g i s t i c e f f e c t was observed, which accounts f o r t h e b i f u n c t i o n a l c a t a l y s i s . However t h e poor hydrogenation a c t i v i t y of NiMo s u l f i d e s was n o t s u f f i c i e n t t o "balance" the high a c i d i t y of t h e mordenite and t h u s t h e i s o m e r i z a t i o n and cracking a c t i v i t i e s p e r gram of mordenite i n c r e a s e d w i t h t h e NiMo s u l f i d e c o n t e n t without r e a c h i n g a p l a t e a u (34) It can be noted t h a t on a l l t h e s e m i x t u r e s of a z e o l i t e and a hydrogenating compon e n t , the b i f u n c t i o n a l c h a r a c t e r o f i s o m e r i z a t i o n and c r a c k i n g i s c l e a r l y shown by t h e f a c t t h a t t h e i r a c t i v i t i e s ( c o n s i d e r e d b e f o r e o r a f t e r d e a c t i v a t i o n ) are much g r e a t e r t h a n t h e sum o f t h e a c t i v i t i e s of b o t h components.
.
F o r t h e i s o m e r i z a t i o n of C5-C6 a l k a n e s on n o b l e metal-loaded acid z e o l i t e s , v a r i o u s a u t h o r s c o n t e s t t h e e x i s t e n c e of a bifunct i o n a l p r o c e s s , They s u g g e s t t h a t t h e i n c r e a s e i n i s o m e r i z a t i o n a c t i v i t y caused by the i n t r o d u c t i o n of a hydrogenating component i n t h e mordenite would o n l y be due t o t h e i n c r e a s e i n t h e
A c t i v i t y per gram of mordenite -1 - 1 mo1e.h .g 350°C, 1 atm. H2 : hydrocarbon : 9.0
10,
.. n
-
I
U
I
I I
I I I
I I
C
I
a
I I
I I
I
a
HC
5
I
a~~
1
10
Benzene hydrogenation a c t i v i t y -1 - 1 mo1e.h .g Fig. 6.
n-Octane t r a n s f o r m a t i o n . E f f e c t of platinum-alumina c o n t e n t (numbers i n p a r e n t h e s e s ) o r hydrogenation a c t i v i t y on t h e hydroisomerizat ion ( I ) and hydrocracking (C) a c t i v i t i e s of p h y s i c a l l y mixed c a t a l y s t s (from r e f e r e n c e 13).
c a t a l y s t ' s s t a b i l i t y ; t h e o n l y r o l e of t h e hydrogenating component would be t o keep t h e s u r f a c e of the rnordenite f r e e of coke deposit. A c t u a l l y , when working under c o n d i t i o n s where t h e pure mordenite doqs n o t d e a c t i v a t e , i . e . under high hydrogen p r e s s u r e , t h e addition of t h e hydrogenating component has only a s l i g h t p o s i t i v e e f f e c t on t h e i s o m e r i z a t i o n a c t i v i t y . I n c e r t a i n c a s e s , even a decrease in t h i s a c t i v i t y h a s been observed (35). The p o s i t i v e e f f e c t i s also v e r y weak a t low hydrogen p r e s s u r e , when t h e i n i t i a l a c t i v i t i e s are taken i n t o c o n s i d e r a t i o n : Chicks e t a l . (28) found t h a t t h e i n c r e a s e i n i s o m e r i z a t i o n a c t i v i t y o c c u r s e s s e n t i a l l y a t t h e expense
of t h e c r a c k i n g a c t i v i t y ; they e x p l a i n t h e a c t i o n of t h e hydrogenating component by a m o d i f i c a t i o n i n t h e a c i d p r o p e r t i e s of t h e mordenite. For a l l t h e above a u t h o r s , i s o m e r i z a t i o n on b i f u n c t i o n a l mordenite c a t a l y s t s t h e r e f o r e occurs by t h e a c i d mechanism d e s c r i b e d in Fig. 3. However, a combination of a c i d and b i f u n c t i o n a l mechanism has a l s o been proposed (26). Again, i f the b i f u n c t i o n a l mechanism a l l o w s one t o e x p l a i n t h e cracking of long-chain a l k a n e s (with more than 6 carbon atoms), i t i s not t h e c a s e f o r t h e c r a c k i n g of l i g h t a l k a n e s (32,36). The cracking s t e p s of l i g h t carbenium i o n s involving u n s t a b l e carbocations a r e very slow. Thus t h e 6-cracking of a secondary c a r b o c a t i o n n ~ +always l e a d s t o a primary c a r b o c a t i o n : 6
+q
C-C-C-C-C-C
.-*
C-C=C
+ + C-C-C +
In the same way, t h e c r a c k i n g of t h e c a r b o c a t i o n s i C 6 w i t h a monobranched o r a bibranched s k e l e t o n i n v o l v e s , i n t h e most favorable c a s e , two secondary c a r b o c a t i o n s :
P+ >-
C-C-C-C-C
4-
C-C-C
+
C=C-C
and could be slow compared t o t h e isomer d e s o r p t i o n and t o t h e alkane s c i s s i o n on m e t a l l i c s i t e s (hydrogenolysis). It i s e f f e c t i vely what i s observed i n n-hexane c r a c k i n g on t h e s e r i e s of Pt-HY and Pt-H mordenite c a t a l y s t s ( 3 2 ) : t h e d i s t r i b u t i o n of t h e c r a c k i n g products i s t h e one expected from a simple t y p e hydrogenolysis r e a c t i o n , namely a n important formation of methane and ethane a s well a s a (C1 + C2)/(C,4 + C5) molar r a t i o of about 1 . Moreover, the g r e a t e r t h e metal a r e a t h e g r e a t e r t h e a c t i v i t y ( F i g . 5 . ) .
2.2
I n f l u e n c e of t h e z e o l i t e c h a r a c t e r i s t i c s
A s expected from a b i f u n c t i o n a l p r o c e s s ( a s well a s from a n acid-catalyzed r e a c t i o n ) , t h e i s o m e r i z a t i o n and c r a c k i n g r a t e s depend s t r o n g l y on t h e z e o l i t e a c i d i t y . Thus, t h e s m a l l e r t h e sodium c o n t e n t of t h e Y z e o l i t e , t h e g r e a t e r t h e a c t i v i t y f o r n-pentane i s o m e r i z a t i o n of Pd-HY c a t a l y s t s w i l l be ; t h e e f f e c t i s e s p e c i a l l y pronounced a t low sodium c o n t e n t s . Thus a d e c r e a s e from 0.27 t o 0.02 w t % Na20 enables a r e d u c t i o n of 50°c i n r e a c t i o n temp e r a t u r e f o r a 30 % n-pentane conversion (26).
The p a r t i c i p a t i o n of BrBnsted a c i d c e n t e r s i n n-decane hydrocracking i s demonstrated by t h e p o s i t i v e e f f e c t of water on t h e a c t i v i t y of a Pd-Re X c a t a l y s t . Indeed, t h i s p o s i t i v e e f f e c t can be a t t r i b u t e d t o t h e g e n e r a t i o n of p r o t o n i c s i t e s v i a r a r e e a r t h c a t i o n h y d r o l y s i s . On t h e o t h e r hand, w i t h platinum on a nons t a b i l i z e d HY z e o l i t e a n e g a t i v e e f f e c t due t o a d e c r e a s e of t h e
a c i d s t r e n g t h by h y d r a t i n g . protons i s observed (37). The e f f e c t of t h e Si02/A1203 r a t i o h a s a l s o been determined i n n-pentane i s o m e r i z a t i o n on b ~ f u n c t i o n a lmordenite c a t a l y s t s : a maximum a c t i v i t y was observed f o r a r a t i o of about 16 (26) ; t h i s optimum v a l u e would be t h e r e s u l t of two a n t a g o n i s t i c e f f e c t s of t h e dealumination : one p o s i t i v e , due t o t h e e l i m i n a t i o n of obstruct i o n s i n t h e mordenite channels, t h e o t h e r n e g a t i v e , due t o t h e d e c r e a s e i n t h e number of a c i d c e n t e r s ( 1 ) . Because t h e i s o m e r i z a t i o n a c t i v i t y depends so h i g h l y on Na removal, comparing z e o l i t e s with d i f f e r e n t s t r u c t u r e s i s d i f f i c u l t . However, t h e comparison between low sodium Pd-H mordenite and low sodium Pd-HY shows t h a t both m a t e r i a l s have about t h e same a c t i v i t y f o r n-hexane i s o m e r i z a t i o n (26)
.
2.3
I n f l u e n c e of poisons
The e f f e c t of s u l f u r poisoning was s t u d i e d on a Pd-Ca Y catal y s t by Rabo e t a l . (38) u s i n g an n-pentane feed i n which s u l f u r c o n c e n t r a t i o n s ( a s n-bu t y l mercaptan) were v a r i e d from zero t o 3000 ppm. S u l f u r c o n c e n t r a t i o n s of 6 pprn had no e f f e c t on c a t a l y s t a c t i v i t y ; t h i s o b s e r v a t i o n can be r e a d i l y explained by t h e bifunct i o n a l mechanism i f t h e hydrogenating a c t i v i t y of t h e c a t a l y s t remains high enough t o maintain a pseudo-equilibrium between p a r a f f i n s and o l e f i n s and t o keep a s l i m i t i n g s t e p t h e s k e l e t a l o l e f i n i s o m e r i z a t i o n on a c i d s i t e s . Above t h i s c o n c e n t r a t i o n , sulfur a c t e d a s a temporary poison : it decreased t h e i s o m e r i z a t i o n a c t i v i t y but t h i s a c t i v i t y was r e s t o r e d on r e v e r s i o n t o a c l e a n feed. M o d i f i c a t i o n s i n a c t i v i t y and i n s e l e c t i v i t y caused by poisoning with dimethyl d i s u l f i d e (220 ppm) and n-butylamine (800 ppm) of a Pt-HY z e o l i t e w i t h 6 w t X of platinum were a l s o those expected from t h e b i f u n c t i o n a l mechanism (30). Since s u l f u r poisons reduce t h e a c t i v e platinum a r e a , t h e poisoned c a t a l y s t w i l l a c t l i k e a c a t a l y s t ~ i t ah s m a l l e r platinum c o n t e n t . T h i s was a c t u a l l y observed : t h e poisoned c a t a l y s t had t h e i s o m e r i z a t i o n s e l e c t i v i t y and t h e a c t i v i t y of a c a t a l y s t with a 0.09 t o 0.5 w t % platinum c o n t e n t . A b a s i c poison reduces t h e number of t h e a c i d s i t e s ; s i n c e with t h e c a t a l y s t used, t h e l i m i t i n g s t e p was t h e s k e l e t a l o l e f i n i s o m e r i z a t i o n on a c i d s i t e s , poisoning by n-butylamine must provoke a decrease i n a c t i v i t y with no m o d i f i c a t i o n i n s e l e c t i v i t y . This i s e f f e c t i v e l y what was observed. The a d d i t i o n of d i m e t h y l d i s u l f i d e (1 %) t o n-decane caused a s i g n i f i c a n t decrease i n both t h e i s o m e r i z a t i o n ( I ) and cracking (C) a c t i v i t i e s of a mixture of H mordenite and platinum-alumina. As expected from t h e well known I / C decrease w i t h d e c r e a s i n g hydrog e n a t i o n a c t i v i t y , I w a s much more a f f e c t e d than C. By r e v e r s i o n t o pure n-decane, t h e c a t a l y s t recovered h a l f of i t s o r i g i n a l isomer i z a t i o n a c t i v i t y b u t o n l y 10 % of i t s o r i g i n a l c r a c k i n g a c t i v i t y and I / C was 4 . 5 times g r e a t e r than on t h e f r e s h c a t a l y s t . Since a poisoning of t h e metal should r e s u l t i n a lowering of I/C, i t would
appear t h a t t h e i r r e v e r s i b l e poisoning e f f e c t i s due t o a modification of t h e mordenite r a t h e r than t o a poisoning of t h e metal (39). C a t a l y s t d e a c t i v a t i o n and coke formation were examined on mixtures of platinum-alumina and z e o l i t e . Nearly a l l t h e coke was found t o be d e p o s i t e d on t h e z e o l i t e (14) ; t h e r e f o r e t h e d e a c t i vation can be a t t r i b u t e d t o a d e c r e a s e i n t h e a c i d i t y of t h e bifunct i o n a l c a t a l y s t . The d e c r e a s e i n c r a c k i n g a c t i v i t y a s a f u n c t i o n of time on stream was always g r e a t e r than t h e d e c r e a s e i n isomerization a c t i v i t y . To e x p l a i n t h i s phenomenon one could propose t h a t isomerization and c r a c k i n g occur on d i f f e r e n t c a t a l y t i c c e n t e r s , the cracking s i t e s being more r e a d i l y poisoned by coke than t h e isomerization s i t e s ( 14) However, t h i s phenomenon can a l s o be explained by t h e f a c t t h a t c r a c k i n g i s c o n s e c u t i v e t o i s o m e r i z a t i o n . Indeed t h e comparison of d e a c t i v a t e d c a t a l y s t s with f r e s h c a t a l y s t s shows t h a t poisoning by coke r e s u l t s i n a d e c r e a s e i n z e o l i t e concentration without any change i n t h e i s o m e r i z a t i o n t o c r a c k i n g r a t i o . The f a c t t h a t poisoning by coke does n o t cause any segregation among t h e c a t a l y t i c s i t e s i s i n f a v o r of a s i n g l e type of s i t e s for i s o m e r i z a t i o n and c r a c k i n g (40). Moreover, i t was found t h a t t h e amount of alkane transformed into coke during one experiment was p r o p o r t i o n a l t o t h e t o t a l amount of cracked a l k a n e whereas no c l e a r r e l a t i o n s h i p e x i s t e d between coking and i s o m e r i z a t i o n a c t i v i t i e s . This i n d i c a t e s t h a t coke formation probably r e s u l t s from a secondary t r a n s f o r m a t i o n of o l e f i n s produced by t h e c r a c k i n g r e a c t i o n (40,41).
.
3. SELECTIVITY OF BIFUNCTIONAL ZEOLITE CATALYSTS
3.1
n-Hexane i s o m e r i z a t i o n
3.1.1 I n f l u e n c e of hydrogenating a c t i v i t y . The s e l e c t i v i t y of n-hexane t r a n s f o r m a t i o n on pure z e o l i t e s has been d e s c r i b e d by numerous a u t h o r s . The k i n e t i c model r e p r e s e n t e d i n Fig. 7. allows u s t o account f o r t h e n-hexane i s d m e r i z a t i o n on H mordenite o r on HY z e o l i t e ( 3 2 , 3 5 ) : n-hexane (nC6) l e a d s d i r e c t l y t o a thermodynamic e q u i l i b r i u m mixture of methylpentanes (MP) and 2,3-dimethylv e r y slow "'6
7
2,2-DMB
.low\\
2-MP
F i g . 7.
.A .
K i n e t i c model of n-hexane
3-MP
i s o m e r i z a t i o n an a c i d z e o l i t e s .
b u t a n e (2,3-DMB) and t o a s m a l l q u a n t i t y o f 2 , 2 - d i m e t h y l b u t a n e (2,2-DMB). T h i s i s w e l l e x p l a i n e d by t h e mechanism i n F i g . 3 . i n which t h e l i m i t i n g s t e p i s s t e p 1 t h a t i s t h e f o r m a t i o n of second a r y nc6* c a r b o c a t i o n s . T h i s s t e p i s s l o w e r t h a n t h e c a r b o c a t i o n i s o m e r i z a t i o n ( s t e p . 2 ) and s l o w e r t h a n t h e d e s o r p t i o n of MP o r 2,3-DMB ( s t e p 4 ) which i n v o l v e s t e r t i a r y c a r b o c a t i o n s . The 2,2-DMB f o r m a t i o n i s slow b e c a u s e t h e c a r b o c a t i o n s c o r r e s p o n d i n g t o t h i s a l k a n e are e i t h e r p r i m a r y o r s e c o n d a r y . 2,3-DMB f o r m a t i o n r a t e d e c r e a s e s when t h e m e t a l l i c s u r f a c e area o f Pt-HY o r Pt-H m o r d e n i t e c a t a l y s t s i n c r e a s e s (30,32) : t h u s , t h e 2,3-DMB c o n t e n t i n t h e m i x t u r e of MP and 2,3-DMB which i s c l o s e t o i t s thermodynamic v a l u e on p u r e m o r d e n i t e , d e c r e a s e s down t o 40 % of t h i s v a l u e f o r t h e h i g h p l a t i n u m c o n t e n t m o r d e n i t e c a t a l y s t s . The d e c r e a s e i s more pronounced i n t h e c a s e of Pt-HY c a t a l y s t s : t h e 2,3-DMB c o n t e n t d r o p s t o 10 Z of i t s thermodynamic v a l u e when t h e p l a t i n u m c o n t e n t i s e q u a l t o 0 . 5 %. T h i s s e l e c t i v i t y i s between t h e s e l e c t i v i t y e x p e c t e d o f a n a c i d mechanism and t h a t of a bifunct i o n a l p r o c e s s i n which t h e l i m i t i n g s t e p i s t h e i s o m e r i z a t i o n of t h e i n t e r m e d i a t e o l e f i n s ( s t e p 3 i n Fig. 1 . ) . Indeed, w i t h t h i s b i f u n c t i o n a l p r o c e s s , t h e m e t h y l p e n t a n e s must b e t h e o n l y primary p r o d u c t s o f n-hexane i s o m e r i z a t i o n : t h e m e t h y l p e n t e n e s formed by n-hexene r e a r r a n g e m e n t s on t h e a c i d s i t e s a r e immediately hydrogen a t e d and c a n n o t i s o m e r i z e i n t o d i m e t h y l b u t e n a s . To e x p l a i n t h e d i r e c t f o r m a t i o n o f 2,3-DMB, a t l e a s t two p r o p o s a l s c a n be made (32) : - a n a c i d and a b i f u n c t i o n a l mechanism would b o t h p a r t i c i p a t e s i m u l t a n e o u s l y i n t h e i s o m e r i z a t i o n . On t h e p o r t i o n o f t h e c a t a l y s t c a r r y i n g p l a t i n u m c r y s t a l l i t e s , nC6 would i s o m e r i z e i n t o MP t h r o u g h t h e b i f u n c t i o n a l mechanism. MP would r a p i d l y i s o m e r i z e i n t o 2,3-DMB o n a c i d s i t e s d i s t a n t from p l a t i n u m c r y s t a l l i t e s . T h i s would imply t h a t t h e p a r t of c a t a l y s t w i t h p l a t i n u m and t h a t w i t h o u t p l a t i n u m work i n d e p e n d e n t l y . - a more p r o b a b l e e x p l a n a t i o n i s t h a t t h e m i g r a t i o n o f t h e i n t e r m e d i a t e o l e f i n s from one m e t a l s i t e t o a n o t h e r m e t a l s i t e i s s l o w e r t h a n t h e i r i s o m e r i z a t i o n on a c i d s i t e s . I n t h i s c a s e , n-hexenes have t h e p o s s i b i l i t y of r e a c t i n g s u c c e s s i v e l y on s e v e r a l a c i d s i t e s b e f o r e b e i n g hydrogenated and t h e r e f o r e 2,3-DMB w i l l a p p e a r as a p r i m a r y p r o d u c t o f nC6 i s o m e r i z a t i o n . The l a r g e s t direct 2,3-DMB f o r m a t i o n on Pt-H m o r d e n i t e would be d u e i) t o t h e g r e a t e r r e a c t i v i t y o f o l e f i n s on t h e s t r o n g e r a c i d s i t e s o f m o r d e n i t e i i ) a n d / o r t o t h e 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 one-dimensional p o r o u s s t r u c t u r e of m o r d e n i t e which a r e g r e a t e r t h a n i n t h e threed i m e n s i o n a l s t r u c t u r e of Y z e o l i t e ( 3 2 ) . 3.1.2 I n f l u e n c e of t h e z e o l i t e p o r o u s s t r u c t u r e . On ZSM-5 bifunct i o n a l c a t a l y s t s , t h e d i r e c t f o r m a t i o n o f 2,3-DMB i s much l e s s pronounced t h a n on Y b i f u n c t i o n a l c a t a l y s t s and t h e f o r m a t i o n of 2,2-DMB d o e s n o t o c c u r ( 2 7 ) . T h i s c a n b e e x p l a i n e d by l i m i t a t i o n s t o t h e d i f f u s i o n of t h e o l e f i n s w i t h a 2 , 3 - o r e s p e c i a l l y w i t h a 2,2-DMB s k e l e t o n . Moreover, t h e f o r m a t i o n of 2-MP i s more favoured
with ZSM-5 b i f u n c t i o n a l c a t a l y s t s than w i t h Y c a t a l y s t s . A p o s s i b l e explanation f o r t h i s i s t h a t t h e e.nvironment of t h e ZSM-5 a c i d s i t e s i s such t h a t it p r i v i l e g e s s c i s s i o n a of t h e protonated cyclopropane i n t e r m e d i a t e i n comparison t o s c i s s i o n b :
C
b
L
a
I +
C-C-C-C-C
c-6--2-c-c
c-c-c-c-c On b i f u n c t i o n a l c a t a l y s t s with a small pore s i z e z e o l i t e a s acid component ( e r i o n i t e ...), n-alkanes o n l y undergo c r a c k i n g ; the formation of branched a l k a n e s does n o t o c c u r . Indeed, o n l y l i n e a r compounds can r e a c h t h e a c i d s i t e s and be desorbed from t h e porous s t r u c t u r e . This c h a r a c t e r i s t i c i s used t o e l i m i n a t e l i n e a r alkanes (which have a low o c t a n e number) from naphthas without transforming t h e o t h e r components, e s p e c i a l l y t h e branched a l k a n e s (selec t o £orming)
.
3.1.3 I n f l u e n c e of r e a c t i o n temperature. On a 0 . 6 w t % Pt-H mordenite i t has been shown t h a t temperature i s a determining f a c t o r i n t h e i s o m e r i z a t i o n s e l e c t i v i t y (42) : - a t 250°c, 2,3-DMB c o n t e n t i n t h e mixture of MP and 2,3-DMB formed by n-hexane i s o m e r i z a t i o n i s v e r y c l o s e t o i t s thermodynamic value a s was t h e c a s e w i t h pure z e o l i t e s . - a t 400°c, 2,3-DMB formation i s v e r y slow and t h e s e l e c t i v i t y i s p r a c t i c a l l y t h a t expected from t h e conventional b i f u n c t i o n a l mechanism. I n conclusion, t h e conventional b i f u n c t i o n a l mechanism o r t h e acid mechanism o r t h e i r combination a l l o w one t o e x p l a i n t h e s e l e c t i v i t y of n-hexane i s o m e r i z a t i o n on 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 . However, it can be noted t h a t a n o t h e r mechanism, invoking cyclohexane-type bimolecular i n t e r m e d i a t e s , was proposed by Bolton and Lanewala t o e x p l a i n t h e p a r t i c u l a r s e l e c t i v i t y of a Pt-Re NH4 Y c a t a l y s t (43).
3.2
Long-chain a l k a n e i s o m e r i z a t i o n and c r a c k i n g
3.2.1 " 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 . A d e t a i l e d a n a l y s i s of t h e hydroisomerization and hydrocracking of long-chain a l k a n e s has been reported by Weitkamp (36,44-49). The r e a c t i o n s were c a r r i e d o u t on a 0.5 w t Z Pt-Ca Y c a t a l y s t a t temperatures ranging from 200 t o 300°c, w i t h a t o t a l p r e s s u r e of about 40 b a r and an H2/alkane r a t i o of about 20. I n t h e s e c o n d i t i o n s , i s o m e r i z a t i o n and cracking occur by t h e conventional b i f u n c t i o n a l process i n which t h e l i m i t i n g steps a r e t h e rearrangement o r t h e c r a c k i n g of carbenium i o n
intermediates ; according to Weitkamp's terminology the bifunctional catalysis is then "ideal" (44). Various conversion rates were obtained by varying the contact time or more generally the reaction temperature. As an example, the change of the isomerization and cracking conversion of n-tridecane versus the reaction temperature is shown in Fig. 8. With all alkanes the hydroisomerization is the only reaction observed at low conversion (up to 40 % for n-tridecane, Fig. 8.). The hydroisomerization conversion passes through a maximum which is due to the consumption of branched isomers by hydrocracking. The isomer distribution is markedly dependent on the conversion rate. The branching occurs by consecutive reactions ( 4 9 ) : at low conversion monobranched alkanes (methyl and also ethyl, propyl ...) are formed almost exclusively (at least 95 %) ; by increasing the conversion rate, the content of bibranched alkanes increases. The distribution of monobranched isomers was examined in detail ( 4 9 ) . At low conversion the 2-methyl isomers were generally formed at a lower rate than the 3-methyl isomers. The branching mechanism via
0
200
220
240
260
React ion temperature Fig. 8.
(OC)
Hydroisomerization (I) and hydrocracking (C) of n-tridecane on Pt-Ca Y zeolite (from reference 4 9 ) .
protonated cyclopropanes (type A rearrangement) can explain this observation. However, it does not explain the formation, observed even at low conversion rate, of ethyl, propyl monobranched isomers. To account for their formation, type A rearrangements of methyl branched carbocations, which are known to occur faster than type B rearrangements ( 2 4 , 4 7 ) were proposed (49). Thus the formation of ethylpentane from n-heptane should involve the following carbocation rearrangements :
...
+
CH3-CHCH2-CH CH CH CH
2SW
very fast
-
+ CH3 I CH3-CH-CH-CH2-CH2-CH3
slow
/CH3
CH CH+ 13 I CH~-CH~-CH-~H-CH~-CH~ --,CH~-CH~-CH-CH~-CH~
U
Even at high conversion rates (up to 70-90 2 ) the cracking product distribution was fully symmetrical and the sum of fragments amounted to 200 moles per 100 moles of alkane cracked, indicating pure primary cracking. Methane and ethane were not formed, which rules out hydrogenolysis on platinum. There was a significant amount of branched alkanes (essentially monobranched) in the cracking product ( 3 6 ) . The iso to normal alkane ratio was generally higher than its thermodynamic value which demonstrates that the branched alkanes are primary products of the cracking reaction and do not result from a secondary isomerization of normal alkanes (50). The cracking product distribution is actually the one expected from the cracking of monomethylbranched tertiary carbocations provided that one assumes that these cations are equally reactive and that their relative concentrations are represented by the relative concentrations of methyl isomers ( 3 6 ) . However this cracking reaction requires the highly endothermic (55-60 kcal/mole) 6-scission of a tertiary cation to form a primary cation (22).
ex. :
Such a r e a c t i o n i s most u n l i k e l y . Yet, 6 - s c i s s i o n could be concert e d with hydride s h i f t s o t h a t a secondary c a r b o c a t i o n would be produced d i r e c t l y (22) :
I n any c a s e , t h i s mechanism cannot e x p l a i n t h e high i s o t o normal r a t i o i n t h e cracked p r o d u c t s , p a r t i c u l a r l y i n butanes ( 5 0 ) , and t h e absence of c r a c k i n g up t o a 40 % i s o m e r i z a t i o n of methylnonanes ( 4 7 ) . To e x p l a i n t h i s , c r a c k i n g of c a r b o c a t i o n s w i t h a bibranched s k e l e t o n was proposed (50,51). This r o u t e i n v o l v i n g secondary and t e r t i a r y carbocations i s e n e r g e t i c a l l y favorable :
I n c o n c l u s i o n , it can be s a i d t h a t n-alkane t r a n s f o r m a t i o n 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 occurs v i a t h e s u c c e s s i v e r e a c t i o n s shown i n t h e rake-type scheme of F i g . 9 . The t r a n s f o r m a t i o n s of adsorbed c a r b o c a t i o n s a r e t h e l i m i t i n g s t e p s , t h e i r d e s o r p t i o n into alkanes v i a t h e o l e f i n s being very r a p i d . Consequently, monobranched isomers can be o b t a i n e d with a high s e l e c t i v i t y , p r a c t i c a l l y i n
~rh
IA
II
1 1 fast
I I I t fast
f!
br
,
I
IA
I
1fast
*!
e mb~+
n ~ + s 1ow
In
1A I I t I fast I I VI
I1
l
vi
fast
1~++110
I C + + ~ 1O'
dbc*
s 1ow "'OW
slow Y
O
W
coke
-Fig. 9. "Ideal" 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 of an n-alkane. P: a l k a n e ; 0 : o l e f i n ; c+: c a r b o c a t i o n ; mb: monobranched; db: dibranched ; 1 and 11: l i g h t product from primary and secondary c r a c k i n g .
thermodynamic e q u i l i b r i u m w i t h t h e n-alkane ; w i t h a l o n g e r c o n t a c t time, t h e thermodynamic mixture of mono and bibranched a l k a n e s could be s e l e c t i v e l y formed ; with an even longer c o n t a c t time primary cracking products would be o b t a i n e d . Secondary c r a c k i n g and coking r e a c t i o n s w i l l occur only f o r extremely long c o n t a c t time. To obtain " 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 , t h e b i f u n c t i o n a l c a t a l y s t must be such t h a t a l l t h e s t e p s involved i n t h e a l k a n e d e s o r p t i o n from c a r b o c a t i o n s be v e r y r a p i d i . e . a) t h e d e s o r p t i o n of o l e f i n s from c a r b o c a t i o n s , b ) t h e d i f f u s i o n of o l e f i n s from t h e a c i d t o t h e m e t a l l i c s i t e s , C ) t h e o l e f i n hydrogenation on t h e m e t a l l i c s i t e s . 3,2.2 Deviations from t h e " 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 . S t e p a i s commonly considered t o be more r a p i d than t h e c a r b o c a t i o n rearrangement o r i t s c l e a v a g e , b u t s t e p s b and c can become t h e l i m i t i n g s t e p s of t h e a l k a n e t r a n s f o r m a t i o n . This i s t h e c a s e f o r step b when t h e d i s t a n c e between hydrogenating s i t e s , i . e . t h e d i f f u s i o n a l p a t h of i n t e r m e d i a t e o l e f i n s , i s t o o long and consequently comprises too many a c t i v e a c i d s i t e s ; t h e r e f o r e , o l e f i n s with a monobranched s k e l e t o n formed on an a c t i v e a c i d s i t e , can adsorb on o t h e r a c i d s i t e s and s u c c e s s i v e l y l e a d t o o l e f i n s w i t h a dibranched s k e l e t o n , c r a c k i n g products and coke. Under t h e s e conditions, dibranched a l k a n e isomers and c r a c k i n g p r o d u c t s w i l l b e primary products of n-alkane t r a n s f o r m a t i o n . The same behaviour w i l l be observed i f t h e o l e f i n hydrogenation i s t h e l i m i t i n g s t e p . This occurs when t h e hydrogenating a c t i v i t y i s too small i n r e l a t i o n t o the a c t i v i t y of t h e a c i d s i t e s . Then b i f u n c t i o n a l c a t a l y s i s w i l l be "ideal" only i f t h e c a t a l y s t s have many h i g h l y a c t i v e hydrogenating s i t e s w e l l d i s p e r s e d among t h e a c i d s i t e s . I n t h e l i m i t c a s e t h e d i f f u s i o n a l p a t h of t h e i n t e r m e d i a t e o l e f i n (between two hydrogenating a c t i v e s i t e s ) w i l l comprise o n l y one a c i d s i t e of such s t r e n g t h t h a t it w i l l allow o n l y one o l e f i n t r a n s f o r m a t i o n d u r i n g one sojourn.
As a l r e a d y noted, dibranched isomers a r e formed a s primary products of n-hexane i s o m e r i z a t i o n on Pt-HY and above a l l on Pt-H mordenite c a t a l y s t s . 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 o l e f i n migrat i o n have been invoked t o e x p l a i n t h e non-"ideal" behaviour of these c a t a l y s t s . I n t h e same way dibranched isomers b u t a l s o cracking products a r e found a s primary products of t h e n-heptane transformation on p h y s i c a l mixtures of platinum-alumina and Y o r ZSM-5 z e o l i t e s ( 4 0 ) . 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 (I/C) increases and t h e percentage of bibranched alkanes d e c r e a s e s when the platinum-alumina c o n t e n t i n c r e a s e s . However, even with a platinum-alumina c o n t e n t equal t o 90 Z, t h e d i s t a n c e between hydrogenating s i t e s i s n o t small enough and t h e hydrogenating a c t i v i t y high enough t o o b t a i n a s e l e c t i v e t r a n s f o r m a t i o n of n-heptane i n t o i t s monobranched isomers. 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 narrow porous s t r u c t u r e of t h e ZSM-5 z e o l i t e account f o r t h e v e r y low value of I / C found w i t h t h e platinum-alumina, ZSM-5 mixtures ( 4 0 ) .
On 0.5 w t 2 Pt-ZSM-5 c a t a l y s t a l m o s t no h y d r o i s o m e r i z a t i o n of n-decane i s found (52) ; moreover a s e c o n d a r y c r a c k i n g i s observed a t r e l a t i v e l y low c o n v e r s i o n r a t e s (E 2 0 %). T h i s c a n b e due t o an "imbalance" of t h e a c i d and t h e h y d r o g e n a t i n g f u n c t i o n s ( t h e hydrog e n a t i n g a c t i v i t y would b e t o o s m a l l t o c o u n t e r b a l a n c e t h e h i g h a c t i v i t y of t h e v e r y s t r o n g ZSM-5 a c i d s i t e s ) a s w e l l as t o c o n f i g u r a t i o n a l l i m i t a t i o n s . Another e x p l a n a t i o n would b e t h a t t h e s t r e n g t h o f ZSM-5 a c i d s i t e s would b e s u c h t h a t s e v e r a l c o n s e c u t i v e t r a n s f o r m a t i o n s c o u l d o c c u r d u r i n g t h e same s o j o u r n of o l e f i n s on a z e o l i t e site. 3.2.3 I n f l u e n c e o f t h e z e o l i t e porous s t r u c t u r e . The i s o m e r i z a t i o n and c r a c k i n g s e l e c t i v i t i e s 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 t h Y o r with ZSM-5 z e o l i t e were s i g n i f i c a n t l y d i f f e r e n t ( 4 0 , 5 2 , 5 3 ) . Thus i n n-heptane i s o m e r i z a t i o n , t h e 2-methylhexane/3-methylhexane molar r a t i o i s h i g h e r w i t h ZSM-5 t h a n w i t h Y c a t a l y s t s and 2,3- and 2,4d i m e t h y l p e n t a n e s a r e formed i n d e f i n i t e l y s m a l l e r q u a n t i t i e s ; t h e 2,2- and 3 , 3 - d i m e t h y l p e n t a n e s were formed w i t h Y b u t n o t w i t h ZSM-5 c a t a l y s t s . These s e l e c ? i v i t i e s a r e v e r y similar t o t h o s e observed i n n-hexane i s o m e r i z a t i o n and t h e r e f o r e c a n b e e x p l a i n e d i n t h e same way. On a l l t h e c a t a l y s t s , c r a c k i n g g i v e s m a i n l y propane and b u t a n e i n a p p r o x i m a t e l y e q u i m o l a r amounts. C u r i o u s l y , t h e i s o / n b u t a n e m o l a r r a t i o i s h i g h e r w i t h ZSM-5 t h a n w i t h Y c a t a l y s t s whereas i t was much smaller i n n-octane and n-decane c r a c k i n g . S i n c e t h e f o r m a t i o n of b i b r a n c h e d h e p t a n e s i s v e r y u n f a v o u r e d on ZSM-5 c a t a l y s t s , t h e f o l l o w i n g c r a c k i n g r e a c t i o n i s proposed t o explain t h e high iso/n-butane r a t i o :
However t h e a u t h o r s (40) do n o t e x c l u d e a n o t h e r p o s s i b i l i t y : t h e p r e f e r e n t i a l c r a c k i n g of carbenium i o n s w i t h a 2,4- o r e s p e c i a l l y w i t h a 2 , 2 - d i m e t h y l p e n t a n e s k e l e t o n , whose d e s o r p t i o n s from t h e ZSM-5 p o r o u s s t r u c t u r e a r e i n h i b i t e d . 6 - s c i s s i o n of t e r t i a r y monobranched d e c y l c a t i o n s and even of l i n e a r s e c o n d a r y d e c y l c a t i o n s w e r e f i r s t proposed by J a c o b s e t a l . (52) t o e x p l a i n t h e p r o d u c t s of n-decane c r a c k i n g on Pt-ZSM-5. However, i n a more r e c e n t p u b l i c a t i o n (53) a n o t h e r p r o p o s a l was p r e f e r r e d : c r a c k i n g would imply two r e a c t i o n s : i ) b i b r a n c h e d carbenium i o n c r a c k i n g i i ) monobranched s e c o n d a r y carbenium i o n c r a c k i n g producing s e c o n d a r y carbenium i o n s s u c h as
/L+ --+C-C-C +
C-C-C-C-R
I
+
C=C-R
The d i s t r i b u t i o n of methylnonanes formed from n-decane i s o m e r i z a t i o n on 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 depends v e r y much on t h e porous s t r u c t u r e o f t h e z e o l i t e (53). With Y z e o l i t e , t h e d i s t r i bution a p p r o a c h e s thermodynamic e q u i l i b r i u m a t h i g h c o n v e r s i o n whereas w i t h ZSM-5 z e o l i t e , 2-methylnonane i s much f a v o u r e d . Curiously, w i t h ZSM-11, a n o t h e r p e n t a s i l z e o l i t e , a d i f f e r e n t rnethylnonane d i s t r i b u t i o n i s o b t a i n e d . These d i f f e r e n c e s between ZSM-5 and ZSM-11 would b e mainly t h e r e s u l t s of t r a n s i t i o n - s t a t e shape s e l e c t i v i t y (53).
CONCLUSION The a c t i v i t y , t h e s t a b i l i t y and t h e s e l e c t i v i t y of 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 are c l e a r l y governed by t h e 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 h y d r o g e n a t i n g s i t e s . A s was shown f o r a l k a n e isomer i z a t i o n and c r a c k i n g , t h e h i g h e s t a c t i v i t y would b e o b t a i n e d i f a l l t h e a c i d s i t e s were v e r y a c t i v e and working a t t h e i r maximum. For t h i s t o o c c u r , t h e a c i d s i t e s must be s u f f i c i e n t l y s u p p l i e d with o l e f i n i c i n t e r m e d i a t e s which r e q u i r e s numerous and w e l l d i s t r i buted a c t i v e h y d r o g e n a t i n g s i r e s . I n t h e " i d e a l " c a s e , t h e d i f f u s i o n a l p a t h o f o l e f i n s (between two h y d r o g e n a t i n g s i t e s ) w i l l c o n t a i n o n l y o n e a c t i v e a c i d s i t e . Here t h e c a t a l y s t w i l l a l s o b e the most s e l e c t i v e one i n t h e s e r i e s of c o n s e c u t i v e r e a c t i o n s : thus, when t r a n s f o r m i n g n - a l k a n e s , i t w i l l g i v e t h e b e s t y i e l d of monobranched i s o m e r s and t h e most s e l e c t i v e i s o m e r i z a t i o n . On s u c h a c a t a l y s t , c o k e f o r m a t i o n w i l l be v e r y slow and c o n s e q u e n t l y t h e s t a b i l i t y very g r e a t . Z e o l i t e s a r e p e r f e c t l y adapted t o t h e prep a r a t i o n of " i d e a l " b i f u n c t i o n a l c a t a l y s t s : t h e i r a c i d s i t e s a r e numerous and h i g h l y a c t i v e ; moreover, a h i g h d i s p e r s i o n and a h i g h a c t i v i t y of h y d r o g e n a t i n g s i t e s c a n b e o b t a i n e d when i n t r o d u c i n g t h e m e t a l by a n ion-exchange p r o c e s s .
ACKNOWLEDGMENTS The a u t h o r s t h a n k t h e f o l l o w i n g p u b l i s h e r s f o r h a v i n g r e l e a s e d t h e i r c o p y r i g h t s on t h e f o l l o w i n g f i g u r e s . Academic P r e s s ( F i g . 5 . ) , Heyden ( F i g . 6 . ) , American Chemical S o c i e t y ( F i g . 8. )
.
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PART IV
INDUSTRIAL APPLICATIONS
DESIGN ASPECTS OF CATALYTIC REACTORS AND ADSORBERS
A.Rodri~ues,C.Costa,R.Ferreira,J.Loureiro
and S.Azevedo
Department of Chemical Engineering University of Porto 4099 Porto Codex, Portuaal
This l e c t u r e i s intended t o provide the fundamentals f o r the desion of c a t a l y t i c r e a c t o r s and adsorbers , t h a t i s , the equi pment necessary f o r the "mise-en-oeuvre" of z e o l i t e s as c a t a l y s t s and adsorbents. Industrial applications of z e o l i t e s have been l i s t e d by several authors (Breck I 1 ,Lee I2 1 ,Hei neman 13 J ,Meuqel ) 4 l , e t c . ) ; Tab1 e s I and I1 summarize c a t a l y t i c and adsorption processes , r e s p e c t i v e l y , which use zeol i t e s . A r e a c t o r ( o r adsorber) i s an ensemble of p a r t i c l e s ( c a t a l y s t or adsorbent); i t i s then worthwhile t o look f i r s t a t the s t r u c t u r e of the p a r t i c l e and the mechanisms of mass ( h e a t ) t r a n s f e r a t the p a r t i c l e 1eve1 .
A z e o l i t e p e l l e t can be considered 181 as havinq two pore systems : mi cropore o r adsorbi na pores of zeol i t e c r y s t a l s and transport p o r e s , i . e . , i n t e r s t i c e s between contactinq c r y s t a l s . This biporous s t r u c t u r e i s sketched in Fiaure 1' and i s s i m i l a r , f o r mathematical purposes , t o o t h e r bidisperse materials ,such as macror e t i c u l a r r e s i n s ,a1 though in zeol i t e s micropores have a very uniform size in the ranqe of 3 t o 10 anqstroms.
For pel l e t s of diameter dp=0.16 cm i n packed beds with porosity 0.32 some typical values f o r desion a r e : i n t r a c r y s t a l voids i n t e r c r y s t a l voids
0.191 0.227
TABLE I - Some c a t a l y t i c processes u s i n g z e o l i t e s (3,41
- Cracking (10-40% r a r e e a r t h exchanged H-Y Z e o l i t e dispersed i n a m a t r i x o f s i l i c a alumina o r c l a y )
- Hydrocracking ( l a r g e pore Y z e o l i t e s w i t h 0.5% P t )
-
I s o m e r i z a t i o n o f Cg and C6 p a r a f f i n i c hydrocarbons ( l a r g e pore mordenite w i t h Pd)
- Shape s e l e c t i v e h y d r o c r a c k i ng
-
I s o m e r i z a t i o n o f aromatics Methanol t o g a s o l i n e (ZSM-5)
TABLE II - Some a d s o r p t i o n processes u s i n g zeol it e s 14 Separations o f
-
n - p a r a f f i n s from i s o - p a r a f f i n s (Flolex 15 ; N - I s E l f
aromatics ,e.g. ,p-xylene from a m i x t u r e o f xylene isomers and e t h y l benzene (Parex ( 5 1 )
- o l e f i n s from p a r a f f i n s
-
a i r 17)
Epuration o f
-
s y n t h e s i s gas streams c o n t a i n i n g HC1 ,SOx ,NOx
(Pura-Siv-N)
steam-cracker e f f l u e n t c o n t a i n i n g C02
- LPG (sweetening) Dehydration o f
-
c r a c k i n g gases f o r e t h y l e n e p r o d u c t i o n olefins
- hydrogen
-
16))
solvents
micro sphere pore particle
Figure 1
-
Bidisperse s t r u c t u r e of a z e o l i t e p e l l e t
i n t e r p e l l e t voids 0.32 s o l i d portion of c r y s t a l s 0.199 sol i d i or ti on of binder 0.063 pa ( p e l l e t ) 1.12 g/cm3 Pb 0.76 g/cm30 iZp (40-260 O C ) 0.22 c a l / g C c r y s t a l s i z e of the order of urn Either mass t r a n s f e r through t r a n s p o r t pores o r d i f f u s i o n i n the zeol it e c r y s t a l s can be the control 1ing r a t e process. Garg and Ruthven 191 s t a t e t h a t the r e l a t i v e importance of macropore and micropore r e s i s t a n c e s can be measured by the parameter R ,
where w i s the volume f r a c t i o n of the p e l l e t occupied by z e o l i t e crystals i s the p e l l e t porosity,Dc and D a r e the d i f f u s i v i t i e s '&P P in the zeol i t e c r y s t a l s and i n macropores , r e ~ ~ e c t i v e and l ~ ,rc~ ~ are the r a d i i of p e l l e t and c r y s t a l ,respectively and dq/dc i s the slope of the adsorption equilibrium isotherm. If R < 1 micropore diffusion i s the c o n t r o l l i n p s t e p while f o r R > 100 macropore diffusion i s the l i m i t i n g one. Macropore d i f f u s i o n has been found ( l o ! t o be the control1 ing
mechanism f o r 1 i q u i d s i n m o l e c u l a r s i e v e p e l l e t s (Davison 525) ; f o r a t r a c e r C6H12 i n a f e e d c o n t a i n i n ? C6H6,Lee and Ruthven f o u n d D = 0 . 2 9 ~ 1 0 - ~cm2/s. P D i f f u s i o n measurements i n z e o l i t e s have been made by numerous workers; Ma and Lee I11 I r e p o r t e d v a l u e s f o r d i f f u s i o n of n-butane i n X z e o l i t e p e l l e t s o f R = 0.23 cm and r c = 1 . 0 8 ~ 1 O - ~ c mequal t o D
D = 0.046 cm2/s and Dc= 5: 1x10-" cm2/s. Ruthven and Doetsch 112 1 P d e a l i n g w i t h n-C7H16,C6H12 and C6H5CH3 i n 13X z e o l i t e found d i f f u s i v i t i e s i n t h e range 1 0 - ~ - 1 0 - ~ c m ~ w / sh i l e Doe1 l e and R i e k e r t 1131 f o r t h e system n-butane/NaX z e o l i t e o b t a i n e d a d i f f u s i v i t y o f 2x10-'cm2/s a t 300 K ( l a r g e c r y s t a l z e o l i t e w i t h dc= 80 um so ~ ~ ~ 8 s ) . Kumar e t a1 ( 1 4 1 u s i n g chromatographic t e c h n i q u e s s t u d i e d d i f f u s i o n o f i-C4H10 i n 4A z e o l i t e o b t a i n i n g D = 0.021 cm2/s ( m o l e c u l e t o o
P
l a r g e t o p e n e t r a t e t h e z e o l i t e c r y s t a l ) w h i l e f o r He i n N2 c a r r i e r t h e y o b t a i n e d Dc= 1 .7x1 0-" cm2/s. F o r 4A z e o l it e p e l 1e t s (R = 0.39cm; P r c = 1.74 urn) t h e y a l s o r e p o r t e d r e c i p r o c a l v a l u e s o f t h e d i f f u s i o n t i m e c o n s t a n t s ( l / ~ ~a t ) 306 K f o r CH4,Ar and C 0 , r e s p e c t i v e l y 4 ~ 1 0 - ~ s ,10-2s-1and -l 1.7~10-~s-'. The s u b j e c t o f d i f f u s i o n i n z e o l i t e s i s a v e r y e x c i t i n g one since,as p o i n t e d o u t b y Weisz 1151 , " t h e f i e l d o f shape s e l e c t i v e c a t a l y s i s r e 1 i e s on t h e d i f f e r e n t d i f f u s i v i t i e s o f molecules through spaces o f n e a r - m o l e c u l a r dimensions". C l a s s i c d i f f u s i o n regimes are we1 1 known : - Knudsen d i f f u s i o n ,when p o r e s i z e < m o l e c u l a r mean f r e e path and t h u s d i f f u s i v i t y a ( p o r e d i a m e t e r ) - '
-
o r d i n a r y d i f f u s i o n , w h e n p o r e s i z e > m o l e c u l a r mean f r e e path and t h e n d i f f u s i v i t y = ( c o n s t r i c t i o n f a c t o r ) x ( o r d i n ary diffusion coefficient)
Now a new regime appears c a l l e d t h e " c o n f i g u r a t i o n a l regime" where t h e d i f f u s i o n i s a f f e c t e d b y t h e s i z e and c o n f i g u r a t i o n o f t h e molecules ( F i g u r e 2 ) . I n view o f t h e b i d i s p e r s e n a t u r e o f t h e s e c a t a l y s t / a d s o r b e n t s , mass c o n s e r v a t i o n e q u a t i o n s f o r a volume element o f t h e p e l l e t should be c o r r e c t l y f o r m u l a t e d and as a r e s u l t t h e concept o f c a t a l y s t e f f e c t i v e n e s s f a c t o r s h o u l d be extended. L e t us r e c a l l t h e d e f i n i t i o n o f c a t a l y s t e f f e c t i v e n e s s f a c t o r I f (cs,TS) a r e t h e concenf o r a homogeneous p a r t i c l e o f volume V P' t r a t i o n and temperature a t t h e s u r f a c e t h e e f f e c t i v e n e s s factor,^,
I
0
I
CONFIGURATIONAL
A
(pore
Firlure 2
-
SIZL)
D i f f u s i o n a i regimes ( a f t e r Weisz 115 1 )
i s d e f i n e d as t h e r a t i o between t h e observed r a t e , r o b s = and t h e i n t r i n s i c r a t e ,rint(cs
1 /I/
r(V)dV V~ VP ,TS) c a l c u l a t e d a t t h e s u r f a c e c o n d i -
t i o n s . F o r i r r e v e r s i b l e nth o r d e r r e a c t i o n t h e s t e a d y s t a t e mass balance i n t h e p a r t i c l e volume element,in i s o t h e r m a l o p e r a t i o n , i s :
where f = c / c s i s t h e reduced r e a c t a n t c o n c e n t r a t i o n i n s i d e t h e c a t a l y s t , x = z/R i s t h e reduced space c o o r d i n a t e f o r t h e c a t a l y s t , f o r c y l i n d e r , 3 f o r sphere) and
a i s t h e shape f a c t o r ( 1 f o r slab,2
4 = L!
i s t h e T h i e l e modulus ( k
- characteristic
dimension:
h a l f - t h i c k e n e s s o f t h e s l a b o r p a r t i c l e r a d i u s ; De- e f f e c t i v e d i f f u s i v i ty; k - k i n e t i c c o n s t a n t ) . The c o n c e n t r a t i o n p r o f i l e i s e a s i l y o b t a i n e d f o r f i r s t o r d e r r e a c t i o n s i f we t a k e i n t o account t h e boundary c o n d i t i o n s ,x = 1 ,f = 1 and x = 0 , d f / d x = 0;the e f f e c t i v e n e s s f a c t o r IT i s then:
slab
'1 = 7 th$
c y l in d e r
q=Tm
; diffusional
regime ($ h i g h ) q
1
=
2 ' 11 ( 4 )
3
(rn
3
sphere
1
u=T
- 1
;
w i t h 11 , I o m o d i f i e d Bessel f u n c t i o n s o f f i r s t k i n d , f i r s t orders respectively.
and z e r o
F o r z e r o o r d e r r e a c t i o n s c o n c e n t r a t i o n p r o f i 1es w i l l r e a c h , i n some cases,zero v a l u e s a t a p o i n t x* i n s i d e t h e c a t a l y s t where a l s o d f / d x = 0. S o l u t i o n o f t h e model e q u a t i o n s w i l l l e a d t o t h e e f f e c t i v e n e s s f a c t o r 11 = 1- x * ~shown i n F i g u r e 3. T h i e l e modulus v a l u e s below which t h e c a t a l y s t i s o p e r a t i n g i n t h e chemical regime a r e d 2 a w h i l e t h e a s y m p t o t i c e x p r e s s i o n o f t h e effectiveness f a c t o r f o r high $ , i s simply a /TI$. The i m p o r t a n c e o f t h e e f f e c t i v e n e s s determines t h e r e a l q u a n t i t y o f c a t a l y s t a c e r t a i n degree o f c o n v e r s i o n ;moreover c a t a l y s t can be c r u c i a l when l o o k i n g f o r
f a c t o r i s obvious s i n c e i t t o be used i n o r d e r t o reach t h e w o r k i n g regime o f t h e hiqh s e l e c t i v i t i e s .
F o r b i d i s p e r s e c a t a l y s t s Ihm e t a1 1161 proposed an o v e r a l l e f f e c t i v e n e s s f a c t o r which i s c a l c u l a t e d f r o m t h e knowledge o f m i c r o and m a c r o e f f e c t i v e n e s s f a c t o r s . T h e s e a u t h o r s c o n s i d e r e d a p e l l e t as made o f microspheres w i t h pores among them ; moreover a f r a c t i o n y o f t h e a c t i v e s i t e s i s a t t h e m i c r o s p h e r e s u r f a c e o r pore walls.
inlinite cylinder
I
Figure 3
-
I
I
Effectiveness factor reactions
.
T-I
versus $ f o r z e r o o r d e r
~
The governing steady s t a t e equations a r e then: pore space
R J ~ C : - ~ / ( V ~ D , ~ ,) fa= c a / c s P a pellet.
w i t h 4-,
, xa= R /
R
P
and Va- volume o f
mi crosphere
with $i=r c
kc,n-1 /(nlViDi)
, f.=ci/cS, 1
xi=r/rC,
Vi
-
volume
o f a microsphere and n ' - number o f microspheres i n t h e p e l l e t . I t i s obvious t h a t t h e e f f e c t i v e n e s s f a c t o r f o r a microsphere
is d i f f u s i o n a l f l u x a t the m i c r o p a r t i c l e surface 'i' i n t r i n s i c r a t e i n t h e microsphere a t t h e s u r f a c e c o n d i t i o n s
The average m i c r o e f f e c t i v e n e s s f a c t o r Ti i s d e f i n e d as t h e r a t i o between the d i f f u s i o n f l u x i n t o a l l t h e microspheres o f t h e p e l l e t and t h e i n t r i n s i c r a t e i n a l l t h e microspheres a t t h e surface concentration, c s ; then
On t h e o t h e r hand t h e o v e r a l l e f f e c t i v e n e s s f a c t o r , no,,
i s the
r a t i o between t h e d i f f u s i o n a l f l u x a t t h e p e l l e t s u r f a c e , x a = l , and t h e i n t r i n s i c r a t e i n t h e whole p e l l e t , i . e . , pore space and microcs; then spheres , supposed a t t h e s u r f a c e c o n d i t i o n s ~,
I f we i n t r o d u c e na d e f i n e d a s :
'a=
diffusional f l u x a t the p e l l e t surface [intrinsic rate i n \ (diffusional f l u x i n a l l ) t h e pores a t t h e ( + microspheres w i t h s u r concentration cs j face c o n c e n t r a t i o n cs
I
1
we can show t h a t
and f i n a l l y
Equation ( 9 ( c o n t a i n s some 1i m i t i n ? cases: a) Reaction takes p l a c e o n l y i n t h e mi crospheres ;y = 0
-
1 9a I
vov= nina b ) Reaction o n l y i n t h e pore w a l l s ; y = 1
I n t h e case o f zero-order r e a c t i o n t h e treatment becomes more complex s i n c e t h e c o n c e n t r a t i o n p r o f i1es , f o r t h e macrosphere and/or t h e microsphere can reach zero values a t c e r t a i n r a d i a l p o s i t i o n s . I n F i g u r e 4 e f f e c t i v e n e s s f a c t o r s nayqov and qi are p l o t t e d as f u n c t i o n s o f @i
f o r second o r d e r r e a c t i o n a t g i v e n values o f 4,
and y
2. FUNDAMENTALS OF CATALYTIC REACTION ENGINEERING
I n t h i s s e c t i o n we w i l l discuss some aspects o f design and o p e r a t i o n o f c a t a l y t i c r e a c t o r s F i r s t o f a l l i t should be stressed
.
Figure 4 - nov, n a , T~ versus @ i .
that the choice of the type of r e a c t o r i s the f i r s t s t e p t o be done; then preliminary estimates of concentration and temperature g r a d i e n t s , b o t h i n the film and i n s i d e the p a r t i c l e , a r e needed in order t o guide the use of a s u i t a b l e model. For homogeneous c a t a l y s t s we have ( 17 1 :
-
external concentration gradient , Ace=
C ~ b - -C ~ ~robsR
-Ca=
C ~ b
f Ab
internal concentration gradient , E i = - = 1 - Ca C ~ b
maximum internal temperature r i s e ,AT;=
-
maximum external temperature r i s e , AT:= -
(-AH)DecAb Bi,
-
'eTb
Bib
Tmax-T~= B= ( -AH 1DecAs s 'eTs Ts , r n a ~ - ~=b gb = Tb
These c a l c u l a t i o n s a r e u s e f u l p a r t i c u l a r l y when we a r e i n v o l v e d w i t h t h e d e t e r m i n a t i o n o f r e a c t i o n k i n e t i c s i n t h e l a b o r a t o r y and we t r y t o a v o i d f a l s i f i c a t i o n o f k i n e t i c s due t o d i f f u s i o n i n t r u s i o n . 2.1
-
T u b u l a r F i x e d Bed C a t a l y t i c Reactors (TFBCR)
H i s t o r i c a l l y t h i s i s one o f t h e o l d e s t arrangements f o r conduct i n g gas-sol i d o r 1 i q u i d - s o l i d chemical r e a c t i o n s on an i n d u s t r i a l scale
.
A TFBCR i s s i m p l y an assembly o f u n i f o r m l y s i z e d p a r t i c l e s ( v i z . t h e c a t a l y s t ) whtch a r e randomly a r r a n g e d and which a r e h e l d f i r m l y i n p o s i t i o n w i t h i n a tube o r pipe ( v i z . t h e r e a c t o r ) . Intimate c o n t a c t i s achieved between t h e p a r t i c l e s and t h e r e a c t a n t f l u i d as t h e 1a t t e r f l o w s i n a random manner between, around and, i n t h e case o f porous c a t a l y s t s , i n t o t h e p a r t i c l e s .
Examples o f f i x e d bed c a t a l y t i c processes employed i n b a s i c chemical i n d u s t r i e s a r e steam r e f o r m i n g , carbon monoxide o x i d a t i o n and methanation, t h e s y n t h e s i s o f ammonia, s u l p h u r i c a c i d and methanol C a t a l y t i c r e f o r m i n g , is o m e r i z a t i o n and p o l y m e r i z a t i o n processes a r e u t i l ised i n t h e p e t r o l e u m r e f i n i n g i n d u s t r y . The o x i d a t i o n o f o l e f i n s and a r o m a t i c s , t h e s y n t h e s i s o f v i n y l a c e t a t e , t h e p r o d u c t i o n o f s t y r e n e and o t h e r dehydrogenation processes a r e examples i n t h e p e t r o c h e m i c a l i n d u s t r y .
.
2.1.1 - T h e o r e t i c a l Aspects on M o d e l l i n g . The g e n e r a l problem may be p l a c e d i n p e r s p e c t i v e b y v i e w i n g t h e r e a c t o r i n terms o f l o n g and s h o r t range g r a d i e n t s a l o n g t h e s p a c i a l c o o r d i n a t e s . G r a d i e n t s of s p e c i e s c o n c e n t r a t i o n s and f l u i d - s o l i d t e m p e r a t u r e s which p r e v a i 1 t h r o u g h o u t t h e g e o m e t r i c c o n f i n e s o f t h e r e a c t o r a r e termed i n t e r p a r t i c u l a t e . Those which may p e r s i s t i n t h e f l u i d i m m e d i a t e l y s u r r o u n d i n g t h e c a t a l y s t p a r t i c l e s a r e d e f i n e d as i n t e r p h a s e g r a d i e n t s w h i l s t t h o s e w i t h i n t h e s o l i d porous c a t a l y t i c phase a r e defined as i n t r a p h a s e g r a d i e n t s . The t r a n s p o r t c h a r a c t e r i s t i c s i n s i d e t h e p a r t i c l e a r e d i s t i n c t f r o m t h o s e o u t s i d e and c o n s e q u e n t l y a r e a l i s t i c model f o r a TFBCR s h o u l d c o n s i d e r t h e s i n g l e p a r t i c l e case and t h e n c o n s t r u c t f r o m t h i s t h e o v e r a l l r e a c t o r model i n which t h e c o n s e r v a t i o n e q u a t i o n s i n c l u d e t r a n s f e r between t h e phases i n c o n t a c t .
A model r e s u l t i n g f r o m t h i s approach i s c l e a r l y a complex l e a r n i n g mode1 w i t h such c o m p u t a t i o n a l c o s t s as t o p r e c l u d e i t s use f o r o p t i m i s a t i o n and c o n t r o l purposes. Hence p r e d i c t i v e s i m p l i f i e d models a r e c o n t i n u a l l y sought b o t h a t p a r t i c l e and o v e r a l l r e a c t o r levels. The mathematical t r e a t m e n t o f t h e b e h a v i o r of porous c a t a l y s t
p e l l e t s r e s t s on r e l a t i o n s which l i n k the fluxes of the reacting species t o gradients i n composition, pressure and temperature. A variety of f l u x r e l a t i o n s of varying degrees of complexity i s available in the l i t e r a t u r e 1181. I t i s q u i t e apparent t h a t the point of i n t e r e s t l i e s i n the calculation of the global r a t e f o r a c a t a l y s t p a r t i c l e and where t h i s leads t o a r a t e of reaction per u n i t volume of r e a c t o r bed when the analysis i s extended t o the whole TFBCR. The method of expressing t h i s global r a t e i n a c a t a l y s t p e l l e t as i t s i n t r i n s i c r a t e under surface conditions multiplied by an effectiveness f a c t o r i s now well established. In the developments which follow i t i s assumed t h a t such global rates can be obtained in t h a t way. The focus of a t t e n t i o n will then be on t h e form of conservation models f o r the integral r e a c t o r , i . e . , on how they account f o r i n t e r p a r t i c l e gradients of species concentrations in t h e f l u i d , i n t e r p a r t i c l e gradients of f l u i d and s o l i d temperatures and a l s o f o r the interphase t r a n s p o r t of mass and heat. The more complex s t r u c t u r e s recognize the existence of both phases and form the group of the heterogeneous models. Resulting from the simp1 i f i c a t i o n of these l a t t e r an a1 t e r n a t i v e group of e s s e n t i a l l y predictive s t r u c t u r e s has emerged, v i z . the pseudo-homogeneous models. In these the assumptions a r e made t h a t the r e s i s t a n c e t o interphase t r a n s p o r t of mass and heat i s n e g l i g i b l e , t h a t the physical properties of the f l u i d vary only s l i g h t l y across a p a r t i c l e diameter and t h a t the f l u i d film around a c a t a l y s t part i c l e i s small - such t h a t each p a r t i c l e with i t s surrouding boundary i s regarded as a point source within a homogeneous f i e l d . The two most common forms of d e t e r m i n i s t i c description of the physical and chemical processes occurring in a TFBCR a r e the Fickian models ( a convenient general name f o r those models based on Fick's and F o u r i e r ' s laws f o r mass and thermal dispersion, r e s p e c t i v e l y ) and the c e l l models of Deans and Lapidus 1191 where i n t e r s t i c e s between packing elements a r e idealized as p e r f e c t l y s t i r r e d mixers which give r i s e t o dispersion-type behavior. The c l a s s i f i c a t i o n suggested by Froment 120 1 f o r Fickian models has been extended by Gros and Bugarel 121 1 t o include a l l models and i s presented in Table 111. One of the reasons f o r the wider use of Fickian models i s possibly the f a c t t h a t previous mass and heat t r a n s f e r experiments have been analysed almost exclusively on the basis of such models
Sr W
TABLE I 1 1 - C l a s s i f i c a t i o n o f F i c k i a n and C e l l Models f o r T u b u l a r F i x e d Bed C a t a l y t i c Reactors
-
F i c k i a n Models
Pseudo-homogeneous
C e l l Models
Heterogeneous
Pseudo-homogeneous
Characteristic
Code
Characteristic
Code
Plug f l o w
PHI
PHl +i nterphase transport Hl+intrapartic l e transport Hl+axial dispersion HIltaxial dispersion
HI
Hl + r a d i a1 dispersion
H3
HIl+radial dispersion
HI3
H2+radi a1 dispersion
H4
HI2+radial dispersion
HI4
Onedimensional PHl + a x i a l dispersion
PHl + r a d i a1 dispersion
pH2
PH3
Twodimensional PH2+radi a1 dispersion
PH4
10.
Heterogeneous
Characteristic
Code
Characteristic
Code
Cells i n series
C2
C2+i nterphase transport
CH2
Two-dimensional C4 arrays o f c e l l s
C4+interphase transport
CH4
HI1 H2 HI2
and consequently a 1arge amount o f compatible parameter values a r e available. The o t h e r reason i s concerned w i t h numerical procedure. The c e l l model was o r i g i n a l l y i n t r o d u c e d when i t appeared t h a t t h e s o l u t i o n o f l a r g e number o f a l g e b r a i c equations f o r t h e s t e a d y - s t a t e o r o r d i n a r y d i f f e r e n t i a l equations f o r t h e unsteady-state was simpl e r than t h e s o l u t i o n o f t h e p a r t i a l d i f f e r e n t i a l equations r e q u i r e d by the d i s p e r s i o n model s. Indeed, even today, t h e simpl e imp1 ementation o f methods o f s o l u t i o n o f non-1 i n e a r a l g e b r a i c equations a r e g e n e r a l l y more cumbersome. During t h e p a s t t h r e e years some works have been p u b l i s h e d which q u e s t i o n t h e v a l i d i t y o f t h e F i c k i a n approach 122,23 1 . I t i s indeed apparent t h a t a t p r e s e n t no second-order continuous model s a t i s f i e s a l l t h e requirements which t h e experimental and t h e o r e t i c a l a n a l y s i s i n packed beds suggest.
A t t h e p r e s e n t s t a t e o f t h e a r t , however, t h e F i c k i a n models are a f e a s i b l e and g e n e r a l l y s a t i s f a c t o r y approach f o r design purposes.
-
F i c k i a n Model A n a l y s i s . I n t h i s s e c t i o n a heterogeneous, a 2.1.2 h y bsr i d- a models f o r t h e s t e a d y - s t a t e o f TFBCRs w i l l be examined. The aim i s t o develop e s s e n t i a l l y p r e d i c t i v e s t r u c t u r e s f o r design purposes. I t i s obvious from t h e l i t e r a t u r e t h a t r a d i a l d i s p e r s i o n o f heat and mass i n c o o l e d - w a l l r e a c t o r s i s o f c o n s i d e r a b l e s i g n i f i c a n c e and t h e r e f o r e has t o be i n c l u d e d throughout.
C r i t e r i a which may be a p p l i e d t o determine t h e e x t e n t o f t h e i n f l u e n c e o f thermal and mass a x i a l m i x i n g (Young and F i n l a y s o n 1241, Mears 125) ) show t h a t i n general such i n f l u e n c e i s minimal f o r c o n d i t i o n s o f i n d u s t r i a l p r a c t i c e ( f l o w r e l a t i o n s and l e n g t h / p a r t i c l e diameter r a t i o ) . B u t f o r a d i a b a t i c regimes such a x i a l mixing phenomena a r e u s u a l l y n e g l e c t e d as t h e survey o f t h i r t y - t w o experimental works presented by Feyo de Azevedo 126 1 c o n f irms
.
F o l l o w i n g t h e c l a s s i f i c a t i o n o f Table 111 t h e models t o be developed a r e then o f t h e types H3 o r HI3 and PH3. The Heterogeneous Model The c o n t i n u i t y equations f o r t h e key r e a c t i n g component A and the energy equations a r e seen t o c o n s t i t u t e a s e t o f p a r a b o l i c p a r t i a l d i f f e r e n t i a l equations coup1 ed w i t h a non-1 i n e a r a1 g e b r a i c equation, v i z .
F l u i d Phase (C,T w i t h o u t s u b s c r i p t )
S o l i d Phase ( s u b s c r i p t p f o r p e l l e t v a r i a b l e )
The i n i t i a l and boundary c o n d i t i o n s a r e f o r zl=O ;
C= Co and T = T o ( r l ) 3T
f o r O
?,
f o r O
and
P = 3 T =O
3-7 - 2 7
l10f
I
1109 1
=o
h ( T -T ) = krp WP w P
IlOel
T~ y
where t h e prime denotes v a r i a b l e s w i t h dimensions and av represents t h e i n t e r p h a s e t r a n s f e r area p e r u n i t r e a c t o r volume. Some f u r t h e r remarks a r e due:
i - The c o n t i n u i t y o f thermal t r a n s p o r t t h r o u g h t h e s o l i d phase i s taken i n t o account as suggested by De Wasch and Froment 1271 by i n c l u d i n g a d i s p e r s i o n term i n e q u a t i o n 11Odl. ii - A m a j o r d i f f e r e n c e between t h i s heterogeneous mode1 and t h e h y b r i d and pseudo-homogeneous models i s i n t h e concept of t r a n s p o r t c o e f f i c i e n t s involved - i n the former c o e f f i c i e n t s for each phase (krfykrpyhwp and hwf) a r e c o n s i d e r e d w h i l s t i n t h e l a t t e r e f f e c t i v e parameters (kr,hw)
are involved.
iii - The meaning o f t h e i n t e r p h a s e c o e f f i c i e n t h should also be c l a r i f i e d : The in t e r p a r t i c u l a r r a d i a t i on and c o n d u c t i o n i n t h e heterogeneous
model a r e accounted f o r w i t h i n t h e e f f e c t i v e c o n d u c t i v i t y o f t h e s o l i d and i n p r i n c i p l e o n l y t h e c o n v e c t i v e c o n t r i b u t i o n hc should be considered f o r t h e f i l m c o e f f i c i e n t h
fp'
i.e.,
I f t h e assumption o f p e l l e t i s o t h e r m a l it y ( o r near isothermal i t y ) i s made then t h e c o e f f i c i e n t h i s a q u a n t i t y i n t e r p o l a t e d between t h e f i l m c o e f f i c i e n t hfp and t h e p a r t i c l e c o n d u c t i v i t y as
given by e q u a t i o n 1 1 0 ~ 1 :
The above s e t o f equations 1 lOa 1 t o ( 1 0 i 1 may be w r i t t e n i n dimensionless ;)form u s i n g t h e t r a n s f o r m a t i o n s :
thus g i v i n g t h e f o l l o w i n g r e l a t i o n s h i p s :
w i t h t h e i n i t i a l and boundary c o n d i t i o n s as: for z=O
; X = 1 and 8 = go(') 3 8
f o r O < z < l and r = O ; 0 for 0 < r< 1 and r = 1 ;
and =
3 0 = 2- 0 3 r ar
- Bif(e-8,)
and
sr!= o 3
where avhL a 1- - upc P
krf b~=q$=
d L
dpL
fPehr F
The H v b r i d Model T h i s model d i s t i n g u i s h e s between c o n d i t i o n s i n gas and s o l i d phases b u t makes use o f t h e e f f e c t i v e t r a n s p o r t concept f o r a pseudo-homogeneous medium. A s i m i l a r t r e a t m e n t t o t h a t c a r r i e d o u t i n t h e previous section a1 lows t h e f o l l o w i n g equations t o be e s t a b l ished:
w i t h i n i t i a l and boundary c o n d i t i o n s :
for z= 0
; X= 1 and 0 = e O ( r )
f o r O < z
ax 3 8 =o and r = 0 ; - = 0 and '3 r 3 r
f o r O < z
3 x =o and r = 1 ., 3 r and
and t h e f o l l o w i n g q u a n t i t i e s w r i t t e n as:
and
-
B i = hwR kr
The h e a t t r a n s f e r c o e f f i c i e n t h c o n t a i n e d w i t h i n t h e parameter e i s again a form i n t e r p o l a t e d between t h e t r u e f i l m c o e f f i c i e n t aAd t h e p a r t i c l e e f f e c t i v e c o n d u c t i v i t y . However, i n t h i s model t h e p e l l e t temperature i s o b t a i n e d by a h e a t balance where t h e h e a t removal i s d e s c r i b e d s o l e l y by t h e s u r f a c e h e a t t r a n s f e r c o e f f i c i e n t h. Since t h e t o t a l h e a t removal from each p e l l e t must be accounted f o r i n t h i s balance, i t i s necessary t o i n c l u d e t h e i n t e r p h a s e r a d i a t i v e and c o n d u c t i v e modes o f t r a n s f e r . Thus t h e f i l m c o e f f i c i e n t i s now:
where hr and hD r e p r e s e n t t h e r a d i a t i v e and t h e c o n d u c t i v e c o n t r i b u t i o n s , r e s p e ~ t i v e l y yand a g a i n :
The reader i s r e f e r r e d t o Table I V f o r t h e d e t a i l s o f t h e calculation o f h fp'
The Pseudo-Homogeneous Model I n t h i s more s i m p l i f i e d approach t h e i n t e r p h a s e g r a d i e n t s a r e c o n s i d e r e d n e g l i g i b l e. It i s f u r t h e r assumed t h a t p h y s i c a l p r o p e r t i e s o f t h e f l u i d v a r y o n l y s l i g h t l y across t h e p a r t i c l e d i a m e t e r and t h a t t h e f l u i d f i l m around a c a t a l y s t p a r t i c l e i s s m a l l compared t o t h e dimensions o f t h e p a r t i c l e and c o n s e q u e n t l y each p a r t i c l e w i t h i t s s u r r o u n d i n g gas l a y e r can be regarded as a p o i n t source i n a homogeneous f l u i d . The model e q u a t i o n s a r e :
w i t h i n i t i a l and boundary c o n d i t i o n s : for z= 0
; X = 1 and € I = €Io(')
3X 3 8 =o f o r O < z< 1 and r = O ; - = 0 and 3 r '3 r 3 X =o f o r O < z< 1 and r = l ; 3 r
and
€I - - Bi 3r
( 0 - Ow)
where
and B i =
hwR
k,
2.1.3 - D e t a i l s o f t h e s o l u t i o n t o t h e models. A c l o s e i n s p e c t i o n o f each model d e s c r i b e d above r e v e a l s t h e k i n d o f problems t h a t i t i s necessary t o i n v e s t i g a t e i f a m e a n i n g f u l d e s c r i p t i o n o f a p a r t i c u l a r r e a c t o r i s t o be o b t a i n e d . One p a r t i c u l a r group o f problems can be r e l a t e d t o t h e mode o f o p e r a t i o n o f t h e r e a c t o r . Two d e t a i l s t o o o f t e n u n d u l y n e g l e c t e d
i n v o l v e t h e r a d i a l temperature p r o f i l e o f t h e f e e d a t t h e r e a c t o r i n l e t and t h e mathematical d e s c r i p t i o n o f t h e a x i a l v a r i a t i o n o f t h e r e a c t o r w a l l temperature. The o t h e r group concerns m a i n l y t h e fundamental problems o f the t r a n s p o r t o f heat i n t h e packed bed and t h e k i n e t i c s which occur i n t h e c a t a l y s t p e l l e t . I n s p i t e o f t h e f a c t t h a t most fundamental o b j e c t i o n s t o continuum d i s p e r s i o n models concern t h e mass t r a n s p o r t mechanisms chosen, t h e major problems o f a n a l y s i s l i e i n t h e d e t e r m i n a t i o n o f the heat t r a n s p o r t parameters ( l e a v i n g here a s i d e t h e k i n e t i c problem which i s v e r y much system-dependent) The development o f s t r u c t u r e s r e l a t i n q t h e " e f f e c t i v e parameters" o f t h e h y b r i d and pseudo-homogeneous models t o t h e more elementary parameters o f t h e heterogeneous model has been considered a c o r r e c t procedure f o r some t i m e - n o t o n l y as a form o f improving t h e understanding o f those elementary mechanisms b u t a l s o as a way o f developing more meaningful p r e d i c t i v e r e l a t i o n s h i p s 1281.
.
The r e p r e s e n t a t i o n o f t h e pure thermal b e h a v i o r o f packed beds by t h e s t e a d y - s t a t e models g i v e n b e f o r e has been e x t e n s i v e l y examined by Feyo de Azevedo 126,291 who has e s t a b l i s h e d s u f f i c i e n t c o n d i t i o n s f o r equivalence i n h i g h temperature regimes where r a d i a t i v e heat t r a n s f e r can n o t be neglected. Such c o n d i t i o n s a r e presented i n terms o f c o r r e l a t i o n s between t h e ' e f f e c t i v e ' and t h e 'phase' parameters. A resume o f such c o r r e l a t i o n s i s presented i n Table I V . Some o f t h e parameters which c h a r a c t e r i z e t h e b a s i c t r a n s p o r t processes have been i d e n t i f i e d as o n l y being p o o r l y known and consequently. t h e i r estimates from t h e 1 it e r a t u r e l a c k r e 1 i a b i l i t y , v i z .
i
-
The t u r b u l e n t 1 i m i t P e c l e t group, Per(-).
ii - The f l u i d / w a l l heat t r a n s f e r c o e f f i c i e n t (hwf)o, g i v e n by equation 114bl, p a r t i c u l a r l y f o r t h e range o f p a r t i c l e Reynolds number below 100.
iii
-
The s o l i d / w a l l heat t r a n s f e r c o e f f i c i e n t ( h ) expressed WP 0 i n t h e corresponding B i o t group ( B i ) P 0' T h i s l a c k o f r e l i a b i l i t y and consequent s c a t t e r i n such parameters appears as r e l a t e d t o t h e s c a t t e r which i s a l s o g e n e r a l l y observed i n t h e l i t e r a t u r e concerning t h e e f f e c t i v e t r a n s p o r t parameters (kr)o , (hw)o. I t i s c l e a r t h a t no d e f i n i t e t h e o r y has y e t been advanced t o
e x p l a i n t h e t r a n s p o r t o f h e a t i n packed beds. The b e s t example o f t h i s i s p o s s i b l y t h e t r a n s p o r t o f h e a t through t h e r e a c t o r w a l l s so f a r c h a r a c t e r i z e d by t h e ' c o n v e n i e n t ' w a l l h e a t t r a n s f e r c o e f f i c i e n t s , By c o r r e l a t i n g experimental l y determined values o f t h e P e c l e t and B i o t groups (Pehr)o and ( B i ) o employing t h e approaches presented i n Table I V i t i s p o s s i b l e t o o b t a i n e s t i m a t e s f o r those p o o r l y known parameters which a r e b o t h p h y s i c a l l y and s t a t i s t i c a l l y acceptable. By employinp such e s t i m a t e d values an equivalence i s found between t h e pseudo-homogeneous, h y b r i d and heterogeneous models (as g i v e n i n s e c t i o n 2.3.2) i n t h e d e s c r i p t i o n o f a packed bed thermal b e h a v i o r i n absence o f r e a c t i o n under c o n d i t i o n s where t h e conduct i v e , t h e c o n v e c t i v e and t h e r a d i a t i v e c o n t r i b u t i o n s a r e a l l s i g n i ficant.
TABLE I V
Note
-
-
Synthesis o f C o r r e l a t i o n s f o r t h e E s t i m a t i o n o f T r a n s p o r t Parameters i n Packed Beds 126,29 1
Parameters o r dimensionless groups which a r e presented cont a i n e d w i t h i n b r a c k e t s and w i t h t h e s u b s c r i p t o c h a r a c t e r i z e t h e t r a n s p o r t phenomena i n absence o f r a d i a t i o n , i.e., i n c l u d i n g t h e c o n d u c t i v e and c o n v e c t i v e modes onTy. A l l correlations are i n S I units.
1. General Form f o r Heat T r a n s p o r t R e l a t i o n s h i p s
i - Fluid-phase P e c l e t Group f o r Heat T r a n s f e r ( i n absence o f r a d i a t i o n ) , (Pehr)o
w i t h rm= 1.29 For Per(")
ii
-
(Gunn 130 1 )
.
see s e c t i o n s 2 and 3 o f t h e Table.
F l u i d l w a l l Nussel t Group ( i n absence o f r a d i a t i o n )
For a,b see s e c t i o n 3 o f t h e Table.
N NU,^)^
-
TABLE I V
iii
-
(continuation)
Sol i d / w a l l B i o t Group, B i
P
Bi = (Bi ) P P 0 For ( B i )
P
iv
-
114~1 see s e c t i o n 3 o f t h e Table.
0
The F l u i d / s o l i d Heat T r a n s f e r C o e f f i c i e n t , h
where B = 1 0 , 8 , 6 respectively.
f o r spheres, c y l i n d e r s and s l a b s ,
The t o t a l f i l m c o e f f i c i e n t i s g i v e n by: = hc+ hr+ h fP P a) Convective c o n t r i b u t i o n (Gunn 131 1 )
h
for
0.35 <
E
< 1
, Re
< 10'
P -
b ) Conductive and r a d i a t i v e c o n t r i b u t i o n s bl ) - Argo and Smith (321
with
l o g k * = - 1 . 5 2 + 7 . 4 9 x 1 0 - Pl ~ P
hr= krad(2kp+ hfpdp) d ~ k ~ with
krad
defined i n v i i .
TABLE I V
-
(continuation) Which g i v e s , combining equations 114el t o ) 1 4 h l :
o r , i n absence o f r a d i a t i o n :
b 2 ) - Adderley 133 1 h = k /(2dp) r rad h = k*/(2d ) P P P and use e q u a t i o n 114eJ f o r
h
fp'
I n t h e absence o f r a d i a t i o n :
) = hpthc fp 0 usually hp<< hc (h
v
-
E f f e c t i v e B i o t Group ( i n absence o f r a d i a t i o n ) , ( B i ) o (Dixon and Cresswell 1281 )
vi
-
E f f e c t i v e Radial P e c l e t Group f o r Heat T r a n s f e r ( i n absence of r a d i a t i o n ) , (Pehr)o
,
1281
TABLE IV
-
(continuation) (NS)o= avR2h/(krp)o
where
with ( k r p ) o obtained from the Zehner and Schlunder equation, (341 :
where B
=
c (-)1 -E
E
1019
and c = 1.25 f o r spheres or 1.4 f o r crushed p a r t i c l e s . The c o e f f i c i e n t h i s calculated from equation 114dI using vii
-
(hfpIo.
Effective Radial Conductivity, k r
with
krad= 48, a d T3 P
f o r the emissivity viii
-
e
see section 3 of the Table.
Apparent Wall Heat Transfer Coefficient, hw
TABLE I V ix
-
-
(continuation)
R a d i a t i v e C o n t r i b u t i o n t o t h e Heterogeneous Model Parameters
a ) Fluid-phase thermal c o n d u c t i v i t y , krf
k
rf
= (k
)
rf o
+E
krad
1144
b ) F l u i d l w a l l h e a t t r a n s f e r c o e f f i c i e n t , hwf h = ( h )+€brad wf wf 0
114~1
c ) E f f e c t i v e r a d i a l s o l i d thermal c o n d u c t i v i t y , krp (Zehner and Schlunder 134 1 )
2. Radial Mass D i s p e r s i o n (Gunn 1301)
with
Per(..)
= A
f o r the value o f
I dl 1+19.4 ($)'
Per(..)
see s e c t i o n 3 o f t h i s Table.
3. Values Proposed f o r t h e Parameters (Feyo de Azevedo 126 1 ) 3.1 - Experimental c o n d i t i o n s
i - Packing: Commercial V205 c a t a l y s t f o r s u l p h u r t r i o x i d e s y n t h e s i s Average p a r t i c l e diameter R a t i o t u b e l p a r t i c l e diameter Bed p o r o s i t y
d = 0.0022 m P D/d = 21.6 P E = 0.31
TABLE I V
-
(continuation)
-
iii
-
Ratio
iv
-
Ratio
v
-
Range o f e x p e r i m e n t a l work
k /k P g
7.5
< Re < 76.2 26.9 Temperature up t o
-
3.2
702 K
E s t i m a t e d v a l u e s f o r t h e parameters:
i - Per(..)
= 12
ii - ( B i ) = 11.3 P 0 iii - Constants a,b i n t h e f l u i d / w a l l N u s s e l t group c o r r e l a t i o n ( e q u a t i o n 114b 1 ) a = 0.280 b = 0.617 iv
2.2
-
-
Emissivity
e = 0.675
R a d i a l Flow F i x e d Bed Reactors (RFBR)
C y l i n d r i c a l r a d i a l f i x e d bed r e a c t o r s a r e found i n a number o f i n d u s t r i a l s i t u a t i o n s and t h e i r s t u d y has been t a c k l e d b y s e v e r a l a u t h o r s 135,36,37 1
.
A t y p i c a l RFBR c o n s i s t s o f a h o l l o w c y l i n d r i c a l v e s s e l packed w i t h c a t a l y s t and i s sketched i n F i g u r e 5; i t i s c l a i m e d t h a t an advantage o f t h e RFBR o v e r a x i a l f l o w u n i t s i s t h e h i g h f l o w r a t e s u r f a c e a r e a p e r volume o f c a t a l y s t l e a d i n g t o narrow, l o w p r e s s u r e drop beds. Ponzi and Kaye 1361 c o n s i d e r e d a p l u g f l o w model and r e a c t i o n s w i t h no change i n t h e number o f moles and analysed t h e i n f l u e n c e of gas m a l d i s t r i b u t i o n on t h e c o n v e r s i o n ( f o r f i r s t and second o r d e r r e a c t i o n s ) and s e l e c t i v i t y ( f o r s e r i e s and p a r a l l e l schemes) i n t h e case o f b o t h i s o t h e r m a l and a d i a b a t i c o p e r a t i o n . They c o n s i d e r e d t h a t t h e . s u p e r f i c i a 1 v e l o c i t y a t t h e i n n e r
Figure 5
-
Radial f l o w f i x e d bed r e a c t o r (outward flow)
basket Vr(y) i s r e l a t e d t o t h e a x i a l v e l o c i t y i n t h e c e n t r a l pipe, Wo by
V&Y) =
-
-r where y = r / L RIWO
dU
and
u = W/Wo.
I f t h e r e i s no
du flow m a l d i s t r i b u t i o n arj = - 1 ; otherwise t h e degree of m a l d i s t r i b u t i o n can be simulated by t h e residence time d i s t r i b u t i o n of t h e i n d i v i d u a l elements. The f r a c t i o n o f gas e n t e r i n g t h e bed between d" dy = E ( t r ) d t r , t h e residence a x i a l p o s i t i o n s y and y + dy i s codV -$ t i m e being t r ( y ) = w i t h dV = r(RB- Ri)tlL and F t h e feed molar
flow; t h e mean residence time i s
Tr= {(R;-
R;)
L/(R; No))
.
Balakotaiah and Luss (371 e x t e n s i v e l y s t u d i e d t h e influence of t h e f l o w d i r e c t i o n on conversion f o r isothermal s i n g l e r e a c t i o n s , A + ( l t v ) B t a k i n g i n t o account d i s p e r s i o n phenomenon. For outward f l o w t h e mass balance equation f o r r e a c t a n t A and associated boundary c o n d i t i o n s are:
I n t h i s e q u a t i o n s u i s t h e m o l a r average v e l o c i t y a t r a d i a l d c ~ N(Rl)(l+v)tvaD a , w i t h N - t o t a l molar f l o w p o s i t i o n r ,i.e., u = a (cAf+vcA) r a t e , y -mole f r a c t i o n of A and a - a r e a normal t o f l o w d i r e c t i o n . Re > 1 , we have
For
B
ou d= -~2~ and t h e n :
1 Da x K ~ l ' -- ~ lf ( y ) ( 1 t v y - v y ' / P ~ ) ~ = o
where Da=
R; g ( c A f ) U1
1
Af
F o r i n w a r d f l o w we g e t : I W + w - -Da* f ( w )x I
Pe
w'=O
+v
, x=xl
( I + v w + - )v
WI
Pe
2
=O
with
Da*=
9(cAf)
R;
'zR2'Af At
-
Re < 1
, D=
c o n s t . and then:
outward f l o w "Y)
(xY')'
-'( y ' ) '
-
( 1 t v ) y l - Dax f ( y ) ( l + ~ y - 0) 118aI ~
Pe*
with
Pe*=
U1 R1
- inward f l o w
The c o n c l u s i o n s reached by t h e a u t h o r s a r e : i ) i n absence o f d i s p e r s i o n , Pe-tw, t h e conversion i s independ e n t on t h e f l o w d i r e c t i o n i i ) f o r zero o r d e r r e a c t i o n t h e conversion i s a l s o independent on t h e f l o w d i r e c t i o n i i i ) f o r c o n s t a n t Bo o r c o n s t a n t d i s p e r s i o n and f i r s t o r d e r r e a c t i o n w i t h no change i n t h e number o f moles, conversion i s independent on f l o w d i r e c t i o n . i v ) f o r v = 0 and h i g h Pe, outward f l o w g i v e s a h i g h e r conversion f o r a1 1 convex k i n e t i c laws ( n > 1 ) and l o w e r conversions f o r 0 < n < 1. I n f a t t when Bo o r D a r e c o n s t a n t s t h e d i s p e r s i o n term a t each point i s p r o p o r t i o n a l t o x/Pe and increases m o n o t o n i c a l l y w i t h x; thus i n t h e outward f l o w m i x i n g occurs l a t e r than i n t h e inward f l o w and, f o r a convex r a t e e q u a t i o n , a d e l a y i n t h e m i x i n g improves t h e conversion.
v ) f o r v # 0 the conversion depends on flow d i r e c t i o n except f o r zero order reactions. An i n d u s t r i a l example of RFBR i s found i n the production of p-xylene from xylene isomerization with simultaneous disproportionation reactions ; a1 so c a t a l y s t deactivation by coke occurs. An excel l e n t study of determination of k i n e t i c and d i f f u s i v i t y parameters f o r t h i s system, using an amorphous silica-alumina c a t a l y s t , has been recently pub1 ished by Orr, Cresswell and Edwards 1381. The c a t a l y s t has a surface area (BET) of 264 m2/g with internal porosity of E = 0.5 , apparent d e n s i t y pa= 1.085 g/cm3 and P n a mean pore radius of 40 The authors suggest a reaction scheme o-xylene + m-xylene + p-xylene and f i n a l parameters ( a t 723 K ) were estimated:
a.
Isomerization
k l = 8.92 x k2= 1.21 x E 1 = E 2 = 111.4
Coking
kc= 7.93 x lo-'
Ec= -34.1
Di sproportionation (0-xyl ene + + m-xylene kg= 1.29 x lo-' k7= 2 . 3 7 ~lo-' E6= E7 = 97.6
mol/(Kq c a t x s ) (kPa) mol/(Kq c a t x s ) (kPa) KJ/mole (k~a)-~s" KJ/mol e o-xyl ene -+ products ; m-xyl ene + products) mole/(Kg c a t x s ) (kpa)' mole/(Kg c a t x s ) ( k ~ a ) ~ KJ/mol e +
Some i n d u s t r i a l u n i t s use zeoliges as c a t a l y s t . Typical operating conditions a r e : T = 400-450 C , product equil i brium compos i t i ons : m-xyl ene/xyl enes = 52%, p-xyl enelxyl enes = 24.5%, o-xyl enel xylenes = 23.5%, ethyl benzene ( E B ) / C 8 aromatics = 8% The reactions taking place a r e : m-xylene e o-xylene G== p-xylene and EB e m-xylene. The second procedes as EB + 3 H 2 + ECH (ethylcyclohexane) , ECH DMCH (dimethylcyclohexane) and DMCH * m-xylene + 3 H 2 .
.
+
3. DESIGN OF, FIXED BED ADSORBERS
Fixed bed a d s o r p t i o n i s an unsteady s t a t e process and then i t s behavior i s governed by t h e propagation o f h e a t and c o n c e n t r a t i o n waves through t h e column. The f a c t o r s which i n f l u e n c e wave t r a v e l l i n g can be grouped i n e q u i l i b r i u m , hydrodynamic and k i n e t i c f a c t o r s . A general p i c t u r e o f a d s o r p t i o n i n f i x e d beds i s provided by F i g u r e 6, where i t i s shown t h e progress o f a f r o n t (which keeps i t s form - s t a t i o n a r y f r o n t ) through t h e bed. Under c e r t a i n i d e a l c o n d i t i o n s ( i s o t h e r m a l operation, no d i f f u s i o n a l r e s i s t a n c e s , p l u g f l o w o f the f l u i d phase and f a v o r a b l e e q u i l i b r i u m ) a s t e p change i n c o n c e n t r a t i o n a t t h e i n p u t w i l l propagate as a shock o r d i s c o n t i n u i t y as shown i n F i g u r e 7. These f i g u r e s h e l p us i n i n t r o d u c i n g some u s e f u l terminology: U i s t h e f l o w r a t e , co and ce the i n l e t and o u t l e t concentrations, t h e breakthrough time, tst i s t h e stoechiometric BP t i m e and tf t h e time needed t o completely s a t u r a t e t h e bed. respectively; t
I t i s obvious t h a t i n t h e i d e a l s i t u a t i o n described i n Figure 7
F i g u r e 6 - Propagation o f a s t a t i o n a r y f r o n t through a f i x e d bed adsorber and breakthrough curve ce vs. time.
Figure 7
-
Propagation o f a shock i n a packed bed adsorber
we i n t r o d u c e d i n t h e column d u r i n g a t i m e tst a c e r t a i n amount o f s o l u t e ( = U c o t s t ) which was r e t a i n e d i n t h e v o i d s o f t h e bed ( = E C ~ V ) and sorbed i n t h e adsorbent ( = ( l - E ) Q v) , where v i s t h e bed volume, E t h e p o r o s i t y and Q t h e adsorbed s o l u t e c o n c e n t r a t i o n , i n equil i b r i u m w i t h c o y r e f e r r e d t o t h e s o l i d volume. T h i s e q u a l i t y leads t o tst=
T
( 1 + Cm) where .r=fv/tU i s t h e space t i m e and Sm=(l- E ) Q / ( E C ~ )
i s t h e mass c a p a c i t y f a c t o r . The adsorbent volume needed t o operate t h e bed d u r i n g a t i m e tst i s then v S = ( 1 - ~ ) U t s t / { ~ ( 1 + S m ) 1 . However s i n c e d i s p e r s i v e e f f e c t s a r e always p r e s e n t some designers j u s t sf being a s a f e t y suggest t h e use o f a h i g h e r volume v;=sfvs, f a c t o r 1391. T h i s e m p i r i c a l approach f o r designing f i x e d bed adsorbers can be r e p l a c e d by a semi-empirical approach which uses t h e concept of mass t r a n s f e r zone (MTZ). Coming back t o F i g u r e 6 we can see t h a t , a t a g i v e n t i m e t, t h e column has t h r e e d i f f e r e n t r e g i o n s : near t h e e n t r y i t i s s a t u r a t e d , near t h e o u t l e t no s o l u t e i s y e t adsorbed and i n between those r e g i o n s mass t r a n s f e r i s o c c u r r i n g between f l u i d and s o l i d . We can then d e f i n e , more o r l e s s a r b i t r a r i l y , t h e MTZ as t h e d i s t a n c e between p o s i t i o n s i n t h e column where c2=0.99co and cl=O.O1 co.
t~~
Considering the s t a t e of the column (of length L ) a t t h e time in Figure 6 , the unused bed length ( L U B ) i s simply:
and
L = LUB + LES where LES i s the length of the equilibrium s e c t i o n . This leads t o : LUB = L(l -tBp/tSt) From an experimental breakthrough curve we a r e able t o calculate LUB by measuring t and t s t ; d i f f e r e n t runs a t various operating BP conditions ( f l o w r a t e , p a r t i c l e diameter, e t c . ) will show how LUB depends on those f a c t o r s and then improve design.
B u t a s a t i s f a c t o r y design will r e l y on understanding wave propagation which will enable us t o develop models f o r fixed bed adsorbers. In the next section p r i n c i p l e s of isothermal fixed bed behavior and models of such adsorbers a r e reviewed. 3.4
-
Isothermal Fixed Bed Adsorption
3.1.1 - Single component case. The prediction of breakthrough curves in t h i s case i s q u i t e simple a t l e a s t i f we assume the equilibrium model 1381. Assuming pluq flow of the f l u i d phase and combining the mass balance equation with t h e adsorption equilibrium isotherm, q * = f ( c * ) , we g e t :
Thus, the v e l o c i t y of propagation of a given concentration c i s inversely proportional t o the slope of the isotherm. For unfavor a b l e isotherms, f " ( c ) > O , the wave i s dispersive in nature; in the p a r t i c u l a r case of an isotherm with constant separation f a t t o r K <1 ,
where y and x a r e reduced s o l i d and f l u i d concentrations, respect i v e l y , we have:
co(Vwith
EV)
T=-
(throughput parameter) and
V - volume passed
through the column. For favorable isotherms, f " ( c ) <0 , we will g e t a compressive wave which implies t h a t a shock o r d i s c o n t i n u i t y i s formed t r a v e l l i n g with a velocity u s :
the changes in concentration being calculated between the feed s t a t e (co,qo) and the presaturation s t a t e of the bed Cfor a clean bed: ( 0 , O ) ) . I t can be shown t h a t even when axial dispersion i s s i g n i f i cant the s t a t i o n a r y f r o n t will move a t a velocity equal t o u s . 3.1.2 - Mu1 ticomponent adsorption - Equil i brium Model . Let us consider adsorption of n components contained in a feed a t concent r a t i o n s c i , i n ( i = 1 , 2 , . . , n ) the bed being presaturated with sol id concentrations q i o (i= 1 2 , . . n ) . We assume a f f i n i t i e s ordered as a 1 > a2 > > a . > . . . > a n and Langmuir equilibrium r e l a t i o n s h i p s : 1
.
...
The c a l c u l a t i o n of concentration p r o f i l e s in the bed can be made according t o the theory developed, f o r instance, by He1 f f e r i c h and Klein 1401 f o r stoechiometric ion exchange a s well as f o r a1 141 ( i l l u s t r a t e d t h i s adsorption processes. Recently Tien e t theory using an example. We wi 11 b r i e f l y describe the s t e p s t o design concentration prof i 1es : i)conversion of an n-component adsorption system i n t o the equivalent n t l component, stoechiometric ion exchange system, by the introduction of a f i c t i t i o u s component ( n + 1 ) with mole f r a c t i o n :
and n
with
where
R
i s the transformation factor.
i i ) The h - t r a n s f o r m a t i o n l e a d s t o : 0
1 i=l If
Cll i
xi
or
1 xi n ( h - a ) = O i = l .j=1 1j
a r e known, t h e h v a l u e s a r e f o u n d f r o m t h e r o o t s o f
t h e above e q u a t i o n . n (V-EV) C Ci ,in 1 i=l i i i ) C a l c u l a t i n g h - p r o f i l e s versus where T = n
there are
n+l
p l a t e a u s where t h e hi a r e c o n s t a n t s ; changes i n hi
t a k e p l a c e between p l a t e a u s ( g r a d u a l o r a b r u p t t r a n s i t i o n s ) Let we have :
.
hkR r e p r e s e n t t h e h v a l u e f o r s p e c i e s k a t t h e R plateau;
There i s only one change in value in the h-profile f o r any species; thus knowing the feed condition and the presaturation of the bed, the f i r s t plateau of h i vs. 1/T corresponds t o the feed condition and t h e l a s t t o t h e presaturation condition, i . e . ,
The values of qi and c i ,n+l a r e calculated through equation 1271. The nature of change of the value h k i s determined by
i f Rk,k+l < 1 the t r a n s i t i o n i s abrupt and i f The changes occur a t
6k,kt1 >1
i t i s gradual.
n ntl P ) - I with P k = TI h i k / h l i h Tk,k+l = ( h t , k t l kl k i =I i=l
-
.
i v ) conversion of h-profi l e s i n t o concentration p r o f i l e s using:
'jk=
"hik-alj) i =l ntl "yi - "1 j ) i =l
j = 1,2,..-,ntl
k = 1,2,.-.,ntl
,
#j n Yjk'
i =l ntl
.
I
-
ajl)
j = 1 ,2,.--,nt7
k = 1,2,...,ntl
From these mole f r a c t i o n s we obtain:
T i e n e t a1 141 1 c l a i m e d t h a t d i f f e r e n t c h o i c e s o f R l e a d t o d i f f e r e n t t r a n s i t i o n bounds a l t h o u g h t h e h v a l u e s a r e n o t changed; t h e y t h e n t r y t o p r o v e which R v a l u e i s c o r r e c t . T h i s i s - i r r e l e v a n t a R s i n c e as has been p o i n t e d o u t by H e l f f e r i c h and K l e i n l/Tk,k+l and i n f a c t whatever R i s , t h e c o n c e n t r a t i o n versus d i s t a n c e p r o f i l e i s unique (and t h i s i s t h e f i n a l g o a l ) . Model 1 i n g o f mu1 ticomponent f i x e d bed adsorbers i n v o l v i n g d i f f u s i o n a l r e s i s t a n c e s has been done by s e v e r a l r e s e a r c h e r s 142,43, 44 1 a1 though u s i n g simp1 i f i e d k i n e t i c 1 aws. Numerical t e c h n i q u e s used i n s o l v i n g model e q u a t i o n s a r e e i t h e r f i n i t e d i f f e r e n c e s o r c o l l o c a t i o n methods. R e c e n t l y , Sereno 145 1 s o l v e d a s i n g l e p o r e d i f f u s i o n model u s i n g moving f i n i t e elements. An i m p o r t a n t i n d u s t r i a l a p p l i c a t i o n o f s e l e c t i v e a d s o r p t i o n i s t h e Parex process f o r s e p a r a t i o n o f p - x y l e n e f r o m a m i x t u r e c o n t a i n i n g x y l e n e isomers and ethylbenzene. I n r e c e n t p u b l i c a t i o n s 146,47,481 an i t a l i a n r e s e a r c h group g o t some i n f o r m a t i o n on e q u i l i b r i u m a d s o r p t i o n o f xylenes i n a Y - z e o l i t e adsorbent as w e l l as b r e a k t h r o u g h curves. I n F i g u r e s 8 and 9 we p r e s e n t two b r e a k t h r o u g h curves f r o m the r e f e r e n c e s mentioned above and t h e responses p r e d i c t e d by t h e e q u i 1 ib r i um model
.
The e x p e r i m e n t a l c o n d i t i o n s were: F i x e d bed c h a r a c t e r i s t i c s :
Initial conditions of the bed: Case 1 Case 2
-
Figure 8 - 100% vol Fi~ure9 - 5% vol 5% vol 90% vol
n-octane m-xylene p-xylene n-octane
Feed compositions ( %vol ) : Case 1 - Figure 8 - 5% m-xylene 5% p-xylene 90% n-octane Case 2 - Figure 9 - 12.5% toluene 87.5% n-octane The histories predicted by the equilibrium theory are derived below taking into account the equilibrium data for this system:
m-xyl ene o-xylene p-xyl ene ethyl benzene to1 uene Note:
6 8 36 21 25
11.76 15.68 70.56 41.16 49
1.96 1.96 1.96 1.96 1.96
4.2 4.7 24.0 12.0 8.0
8.232 9.212 47.04 23.52 15.68
1.96 1.96 1.96 1.96 1.96
b, in
liter of solution/mole of solute a in liter of solution/liter of solid j Qj in mole of solute/liter of solid J
Detailed calculations of theoretical histories from mu1 t i component equi 1 i bri um theory (57 OC) Case 1 Feed composition (%vol) : 5% p-xylene 5% m-xylene 90% n-octane
(component 1 ) (component 2) (solvent)
Then:
c1 = 0.4029 mole/R solution cZ1 = 0.4156 mole/R solution
= =
0.1692 mole/R of bed 0.1746 mole/Q of bed
Equi 1 i briurn data: 4.2 a solution/mole = 10 R of bedhole mole/g solid = 1.96 mole/Q solid = Q = 1.75~ = 1 .I368 mol e/a of bed a1 = 64.96 (dimensionless) a2 = 11.368 (dimensionless) b2=
Then the solid composition in equilibrium with feed conditions is:
alCll 411 = +
0.8853 mole/R of bed
=
0.1599 mole/R of bed
blC1 1 b2C21 +
a2C21
q91 =
=
Choosing R = 1, the selectivity coefficients ai a l l= 1
a12=5.7143
=
ai/aj are:
a13= 64.96
and the composition (corresponding to the feed state) of the ntl component equivalent stoechiometric system is:
The composition of the initial state of the bed is: xl3= 0 Y13=0
xz3 = 0 y23= O
X33 =
1
Y33 = I
Using Eq. 1351, we obtain:
and h1
=
3.0427
hZ1
=
hZ2
=
23.31 93
Also for the third plateau we get:
(lSt plateau)
h
13
= h12= 1
h23 = 5.7143
(3rd p l a t e a u )
-
Since R1 = h
13
/h
11
<1
E23= h23/h21
and
<1
the transitions
are b o t h abrupt. To c a l c u l a t e t h e p o s i t i o n o f t h e t r a n s i t i o n s we f i r s t need t h e values o f
Tk,ktl
Then
-
= (hkYntl
-
T1 .2 = 5.2316
hkl
Pk)-l
leads t o :
T2.3 = 2.7857
The v e l o c i t y o f t h e t r a n s i t i o n i s g i v e n by: 'transition Since u1
---
i
rR
ui = 2.841 cm/min we g e t
= 0.4559 cm/min
'2.3=
0.7505 crn/rnin
and f i n a l l y : t1.2 = 85.545 m i n
t Z e 3 =51.968 m i n
The c o n c e n t r a t i o n s a t t h e second p l a t e a u can be now obtained:
Xnn
or
=
4 2 - a 1 2 ) ( h 2 2 - "12)
c~~ = xZ2
L
= 0.3081
2
= 0.2972
mole/Q o f bed = 0.7335 mole/Q s o l u t i o n =
-
Figure 8
Breakthrough curves f o r c a s e 1 . Experimental p o i n t s 147 1 : ( 0 ) m-xylene; ( m ) p-xylene. C a l c u l a t e d curves: (...) from 1471 ; (-) equi1 ibrium t h e o r y ( t h i s work).
Case 2 In t h i s c a s e we have: component
j
1 2 3
i n i t i a l condition % vol .
p-xyl ene to1 uene m-xyl ene n-octane
-~
- -
~
f e e d composition % vol .
5%
-
-
12.5%
5% 90%
87.5%
~
Feed s t a t e c l l = 0 , c2,
=
1 . I 7 7 mole/& s o l u t i o n
=
0.4943 mole/& bed, c31=0
I n i t i a l s t a t e o f t h e bed c14 = 0.4029 mole/& s o l u t i o n
=
0.1692 mole/& bed , c Z 4 = 0
c 3 4 = 0.4156 mole/& s o l u t i o n
=
0.1746 mole/& bed
From equi 1 i brium d a t a bl = 24 R s o l u t i o n / m o l e = 57.143 R b e d h o l e b2 = 8 R s o l u t i o n / m o l e = 19.048 R bed/mole b 3 = 4.2 R s o l u t i o n / m o l e = 10 R bed/mole
Q = 1.1368 molela bed Then
a, = b l Q = 64.96 a 2 = 21.654 , a 3 = 11.368
S o l i d phase c o n c e n t r a t i o n s i n e q u i l i b r i u m with f e e d s t a t e and i n i t i a l s t a t e of t h e bed a r e , r e s p e c t i v e l y : q l l = a l c l l / ( l + b l c l l + b2c21 + b3c31) qZ1 = 1.0277 mol e/R bed , q3] = 0
=
O
and q14 = 0.8853 mole/R bed , q24 = 0 , q34= 0.1599 mole/R bed
I f we t a k e R = 1 , then
The composition f o r t h e f e e d and p r e s a t u r a t i o n s t a t e s o f t h e e q u i v a l e n t (n+l )-component s t o e c h i o r n e t r i c system a r e , r e s p e c t i v e l y : xi1 = 0 yll = 0
~ 3 =1 0 , x4i = 0.5441 yZ1 = 0.9478 , y31 = 0 , ~4~ = 0.0522
xZ1 = 0.4559
and
x 1 4 = 0.1512 , x~~ = 0 , x~~ Y14= 0.7909 , Y24= 0 , Yg4
=
0.1684 , x~~ = 0.6804
=
0.1542 , Y~~ = 0.0548
Calculation of h's:
-
lSt plateau O+ 0.4559(h-all ) ( h - a 1 3 ) ( h - a 1 4 ) t 0 + 0.5441 ( h - a l l ) ( h - a 1 2 ) ( h - a 1 3 ) t h e r o o t s being
hl
=
1
hZ1 = h Z 2 = 5.7143
h31 = h32 = h33 = 31.2476
=
0
- qth p l a t e a u 0.1512(h-a12) (h-a13) (h-a14)
+
0 . 6 8 0 4 ( h - ~)(h-a12)(h-a13) ~~ the r o o t s being
+ 0 + 0.1684(h-a1 =o
)(h-a12) (h-a14)
+
h14= h13= h12= 3 h24 = hZ3 = 3.0427 h34 = 23.3193
Nature o f t h e t r a n s i t i o n s :
-
-
Since R12 = h14/h1 > 1 , t h e f i r s t t r a n s i t i o n i s gradual w h i l s t
< 1 and E34= h34/h31 1 1 a r e a b r u p t . R23" h 24/ h 21
Location o f the t r a n s i t i o n s :
c ) D i f f u s e boundary 1.2:
- For t h e s i d e o f zone ( 1 )
-
For t h e s i d e o f zone ( 2 )
Times a t which t r a n s i t i o n s a r e l o c a t e d ( o u t l e t o f t h e bed): a ) t r a n s i t i o n 1.2 side (1)
ul.l'=
side (2)
0.393 cm/min
t1
.2 = 1.678 cm/min
b ) t r a n s i t i o n 2.3
u
c ) t r a n s i t i o n 3.4
u
2.3 3.4
I
t 2 ,s 2 = 23.24 min
=1.688cm/min
t2.3=23.11 min
= 2.434 cmlmin
t 3 . 4 16.03 = min
C a l c u l a t i o n o f compositions (mole f r a c t i o n s
-
znd
= 99.34 m i n
-
Eq. 1361)
plateau
(h12-a11)(h22-a11)(h32-a11) X12 =
("12-
= 0.4729
all)
and then ( f r o m Eq. 1381 ) : c12 = 0.5293 moel /!?, f o r components 2 and 3:
bed = 1.2603 mole/!?, s o l u t i o n xZ2 = 0 ; cZ2 = 0 X32 = 0 ; C 3 2 = 0
T h i s i s a v e r y small p l a t e a u s i n c e we a r e near a watershed p o i n t ( h 2 4 = 3 . 0 4 2 7 ~h 1 4 = a 1 2 = 3 ) .
- srd p l a t e a u x1 = 0.2049
c13 = 0.2294 mole/!?, bed = 0.5461 m o l e l a s o l u t i o n
0 x33 = 0.2442
cz3 = 0 = 0.2532 mole/!?, bed= 0.6028 mole/!?, s o l u t i o n C33
X23 =
For t h e d i f f u s e boundary, we have f o r components 1 and 2:
when xkk = y k k = 0 and hkl = alk t h e species k appears i n t h e d i f f u s e boundary; then:
Xk
=
( u k / ~ k ) 1 / 2- 9 k
X
hk,k+l - al k Yk =
k ,k + l
( ~ ~ / u - ~Clkl) ~ / ~ Yk ,k+l l/hk,k+l - "kl
t h i s i s t h e case f o r s p e c i e s 1 i n o u r case; so we use Eq. lB1 1 :
TI= l / u l -
x1 = 0.0276
t = 82.37 m i n
T1 = 3.5
x1
0.0792
t =61.77 m i n
T1 = 2
x1 = 0.1811
t = 41.18 m i n
T =1
x1 = 0.3540
t = 27.46 m i n
Tl = .6962
x1 = 0.4729
t = 23.24 m i n
= 5
-
-1
=
F o r s p e c i e s 2 we use Eq. lA1 ( ; i n t h e same p o i n t s we o b t a i n : -
TI = 5
x2 = 0.4293
t = 82.37 m i n
3.5
0.3796
61.77
2
0.281 3
41.18
1
0.1146
27.46
0.6926
0
23.24
I n F i g u r e 9 we compare t h e s e p r e d i c t i o n s w i t h t h e experimental and c a l c u l a t e d r e s u l t s o b t a i n e d by S a n t a c e s a r i a e t a1 1471. 3.2
-
Non I s o t h e r m a l A d s o r ~ t i o n
The b a s i c t h e o r e t i c a l framework f o r t h e u n d e r s t a n d i n g o f non i s o t h e r m a l , and p a r t i c u l a r l y , a d i a b a t i c a d s o r p t i o n has been developed b y Rhee e t a1 149,50,51,521, Pan and Basmadjian 153,54,551 and Basmadjian 3 a1 156,57,58,59 1 f o r s i n g l e and mu1 ticomponent cases. A r e v i e w o f t h e a r e a has been p r e s e n t e d by Sweed 1601. P r a c t i c a l systems have been c o n s i d e r e d i n d r y i n g 161,62,63 1 and f o r t h e s e p a r a t i o n o f gas m i x t u r e s o f n-pentane f r o m iso-pentane b y m o d u l a t i o n o f f e e d temperature 164,651. Methods f o r t h e obtention o f a d s o r p t i o n e q u i l i b r i u m d a t a 166,67.( and k i n e t i c laws 168,69,70( have been implemented, s p e c i a l l y u s i n g s i n g l e p a r t i c l e technique and m i c r o b a l a n c e apparatus (71,72,73,741 ; we s h o u l d emphasize t h e work done by Ma and h i s qroup on ZSM-5 z e o l i t e and s i l i c a l i t e . M o d e l l i n g o f such processes i s o b j e c t o f a number o f papers 175,76, 771 u s i n g a staged approach and s i m p l i f i e d h y p o t h e s i s . A mechanistic
Figure 9
-
Breakthrough curves f o r case 2, Experimental p o i n t s ( 4 7 ( : ( 0 ) m-xylene; ( A ) p-xylene; ( 0 ) toluene. equilibrium C a l c u l a t e d curves: ( . . ) from 1471 ; (-) t h e o r y ( t h i s work).
.
explanation of nonisothermal s o r p t i o n has been r e c e n t l y p u b l i s h e d by H e l f f e r i c h 1781. 3.2.1 - Non i s o t h e r m a l s o r p t i o n i n a p e r f e c t l y mixed sorber. Let us consider t h e system sketched i n F i g u r e 10, where T i s t h e temperature, F t h e t o t a l mass f l u x (Fi = U p . ; i f o r a c t i v e species,
I f o r i n e r t s ) , V t h e m i x t u r e volume and
1-
4 = UA
(To - T) t h e h e a t f l u x
removed by t h e c o o l i n g medium. A f t e r i n t r o d u c i n g dimensionless q u a n t i t i e s , mass and h e a t balances t a k e t h e form 1791 :
F i g u r e 10 with
X i =
pi/"
-
, yi
The nonisothermal p e r f e c t l y mixed sorber
=
qi/Q
-
, T = T/To , e = t/tst.
The model parameters are: 5,
, mass
B
,
capacity f a c t o r
thermicity factor
5 = - 1-E Q m E PO
( - A H ) Po
B=
a
, heat
t r a n s f e r number
a = Uv A
;,
When developing t h e model equations one should note t h a t : a ) s p e c i f i c e n t h a l p y a t temperature T f o r species i hi (T) = hio
+ cpi (T - To)
b ) s p e c i f i c e n t h a l p y o f t h e adsorbent
c ) s p e c i f i c e n t h a l p y o f adsorbed species i h?(T) = hio
+
cpi
( T - To)
+
AH
where AH < 0 (-AH i s t h e h e a t o f a d s o r p t i o n i n c a l / g ) The assumptions made were: (rp i n cal K - ~ )
(f,, i i i ) vscpi i v ) cpi
i n cal s-l
1
K-I
qi << r
P ( T - To) << (-AH)
v ) Tc = T
0
(Tc
-
coo1 i n g medium t e m p e r a t u r e )
F o r a d i a b a t i c systems a = 0. Assuming no mass and h e a t t r a n s f e r r e s i s t a n c e s between f l u3d and s o l i d phases, t h e a d s o r p t i o n e q u i 1 ibrium a t t h e i n t e r f a c e i s governed by
where K(T) = K(To) exp{--y(l
/i - 1) )
w i t h y = -AHI(R To)
The dynamic b e h a v i o r of t h e s t i r r e d c e l l was t h e n s t u d i e d through computer s i m u l a t i o n ; we j u s t p r e s e n t i n F i g u r e s 11,12 and 6 and a, r e s p e c t i v e l y , on t h e h i s t o r i e s o f 13 t h e i n f l u e n c e of 5 c o n c e n t r a t i o n and t e m B e r a ~ u r e .
F i g u r e 11
-
E f f e c t o f t h e t h e r m a l c a p a c i t y f a c t o r , Shy on t h e response o f t h e p e r f e c t l y mixed s o r b e r . K(To) = 100, B = 1, Em= 10, a = 0.
Figure 12
-
Effect of the mass capacity factor,Sm, on the response of the p e r f e c t l y mixed sorber. K(To) = 100, B = 0.1, S h = 10, a = 0.
As can be seen from Figure 11 , o r from Figure 1 2 ) , the removal of a component by adsorption i s negativelly affected by decreasing Sh ( o r 5,) f o r a d i a b a t i c systems. 3.2.2 - Non isothermal sorption in a cascade of s t i r r e d c e l l s . Sorption columns a r e often simulated by a s e r i e s of J s t i r r e d c e l l s , J being then a measure of the mass axial dispersion. In Figures 14 and 15, travel1 ing waves of concentration and temperature a r e shown f o r J = 30; we can see the h i s t o r i e s a t the e x i t of t h e 5th, l o t h , 2oth and 3oth sorbers.
Figure 13
-
Influence of the heat t r a n s f e r number, a , on the response of the perfectly mixed sorber. T o = 3 0 0 K , B = 0 . 1 , K(To)= 100, 5 , = S h = 1 .
-
F i g u r e 14
Response o f a cascade o f a d i a b a t i c s o r b e r s . Sh= f3= 1, a = 0, T = 300 K, K(T ) = 100, 5=, (GAH) = 200 c a l
Yg .
For a d i a b a t i c systems ( F i o u r e 14) and To = 300 K, K(To) = 100,
Em = Sh = 6 = 1 , (-AH)
= 200 c a l /g
,
t h e c o n c e n t r a t i o n h i s t o r y shows a
p l a t e a u between two t r a n s i t i o n s ; s i m i l a r l y t h e temperature h i s t o r y has a p l a t e a u a t t h e same p l a c e . I f h e a t i s removed from t h e system, t h e p l a t e a u i s deformed and i t w i l l disappear f o r i s o t h e r m a l systems ( F i g u r e 1 5 ) . 3.2.3 - S i m p l i f i e d a n a l y s i s o f nonisothermal s o r p t i o n . Some understanding o f t h e b e h a v i o r o f t h e nonisothermal f i x e d bed adsorber can be o b t a i n e d through a simp1 i f i e d a n a l y s i s i n v o l v i n g 1 i n e a r i z a t i o n o f t h e model equations and use o f Laplace t r a n s f o r m technique.
F i g u r e 15
-
Response o f a cascade o f n o n a d i a b a t i c s o r b e r s . To = 300 K, K(To) = 100, (-AH) = 200 c a l / g , 5 =< =B=a=l. m h
F o r a d i a b a t i c s t i r r e d t a n k s and l i n e a r i s o t h e r m s q . = K(T) ci = KO e x p ( - -)AH c 1 RT i
1431
1i n e a r i z a t i o n around(cio ,To) l e a d s t o t h e model e q u a t i o n s , i n t h e Laplace domain and m a t r i x form:
where
i= ( T - To)/To. The system behaves as a second o r d e r one w i t h :
For 5 > 1
The maximum temperature i s :
and occurs a t :
For isothermal systems, 6 = y = 0, and we get:
and
x ( 0 ) = 1 - e-e
For a d i a b a t i c fixed bed adsorbers with plug flow f o r t h e f l u i d phase we have the model equations in the Laplace domain:
or, using
T*
and 5:
d2X t 2 5 T*
dz z = 0: X =
S
I/S,
d? + T*' s 2 X dz-
=
0
1531
T= o
F i n a l l y , the h i s t o r i e s of concentration and temperature a r e :
-,/TI
~ ( , e1) = T*(G+e HIl O - T *(I ] 2 -r*Jy2-I
+
+
1541
J
Several s i t u a t i o n s can then occur as shown in Figure 16, depending on the r e l a t i v e values of Sh and 5, and/or yB. Some of these cases have been experimentally confirmed, as reported in recent work 180,81 ,82 1 , concerning adsorption, on 5A-zeolite, of ethane + helium and n-butane + helium mixtures. 4 . NONLINEAR ADSORPTION COUPLED WITH CHEMICAL REACTION
In a number of situations,from which chromatographic reactor i s an example, nonlinear adsorption i s coupled with reaction. Analysis of the t r a n s i e n t behavior of the column becomes then very complex; f o r i r r e v e r s i b l e and reversible reactions coup1 ed with multicomponent Langmuir adsorption, r e s u l t s have been presented by a1 1831. Rodrigues e t -
General case
e
Fiqure 16 - Possible concentration and temperature h i s t o r i e s in nonisotherrnal sorption.
The p o i n t we want t o b r i e f l y d i s c u s s h e r e i s t h e v a l i d i t y o f usual methods f o r g e t t i n g e q u i l i b r i u m and k i n e t i c parameters f r o m experiments. O f t e n chromatographic t e c h n i q u e s a r e used and,from t h e r e t e n t i o n t i m e o f t h e peak and t h e r a t i o between t h e a r e a under t h e peak and t h e i n j e c t e d q u a n t i t y , t h e k i n e t i c c o n s t a n t i s o b t a i n e d , p r o v i d e d t h e r e a c t i o n i s i r r e v e r s i b l e , f i r s t o r d e r and t h e i s o t h e r m i s linear. I f t h e a d s o r p t i o n i s o t h e r m i s n o n l i n e a r , t h i s method f a i l s . I n o r d e r t o have some i n s i g h t on how n o n l i n e a r a d s o r p t i o n and r e a c t i o n a r e coupled, we s t u d i e d t h i s phenomenon i n a p e r f e c t l y mixed r e a c t o r .
-
k F o r a nth o r d e r i r r e v e r s i b l e r e a c t i o n A B coupled w i t h non l i n e a r a d s o r p t i o n o f component A alone, we have mass balance e q u a t i o n s f o r A and B:
f o r a f e e d c o n t a i n i n g o n l y A. Assuming a Langmuir a d s o r p t i o n isotherm
qA=
QK' c A
----
l+K1cA
9 = t/r, yA= qA/qO
and
r = k c>nd
introducing
x = c/c 0 ,
( c o i s t h e maximum s o l u t e c o n c e n t r a t i o n i n t h e
f l u i d phase, f o r a s t e p change a t t h e i n l e t i n t h e case o f a d s o r p t i o n alone, o r f o r an impulse i n t h e case o f r e a c t i o n a l o n e ) we g e t 1841 :
with
K = 1 + K' c0 Em = ( 1 -E)~'/(EC') N r = ( 1 - ~ ) k - r ( 6c ) n-1 /E
For a s t e p change i n c o n c e n t r a t i o n we o b t a i n : i ) zero order reaction
i i ) f i r s t order reaction
For a Dirac impulse we o b t a i n : i ) zero order reaction
i i ) f i r s t order reaction
with x A ( o t ) defined by equation 1621 :
and shown in Figure 17. In Fiqure 18 we show the response of a p e r f e c t l y mixed tank t o s t e p and Dirac changes i n concentration f o r zero order reaction and Langmuir adsorption. Figure 19 shows t h e response of a p e r f e c t l y mixed adsorptive reactor f o r f i r s t order reaction. The determination of parameters can be e a s i l y made f o r f i r s t order reactions: a ) From a t r a c e r experiment we g e t E . b ) By comparing impulsional responses without and with reaction t h e corresponding we see t h a t a t a given o u t l e t concentration, cA1, .. times a r e , r e s p e c t i v e l y , el and (l+Nr)B1 from which Nr can be obtained; a l s o from the area under the curve in the presence of reaction, we o e t area = c O / ( l + ~ , ) . c ) From the impulsional response and taking i n t o account t h a t
Figure 17
- xA(ot)
versus 6 and K.
F i g u r e 18
-
Response o f a p e r f e c t l y mixed a d s o r p t i v e r e a c t o r t o : a ) s t e p chanae i n i n l e t c o n c e n t r a t i o n ; b ) D i r a c impulse. ( z e r o o r d e r r e a c t i o n , 5=2, K=5)
F i g u r e 19
-
Response of a p e r f e c t l y mixed a d s o r p t i v e r e a c t o r t o : a ) s t e p change i n i n l e t c o n c e n t r a t i o n ; b ) D i r a c impulse. ( f i r s t o r d e r r e a c t i o n , 5=2, K=10)
we g e t f o r each c A isotherm.
t h e c o r r e s p o n d i n g f ( c A ) and t h e n t h e e q u i l i b r i u m
I t i s a l s o i n t e r e s t i n g t o n o t i c e t h a t , i n t h e case o f D i r a c response, t h e ratios(Rln) between t h e areas under t h e responses i n
t h e case o f r e a c t i o n a l o n e and a d s o r p t i o n a l o n e a r e : area under t h e response f o r r e a c t i o n (order n) area un e r t e response f o r a d s o r p t i o n
zero order reaction, n = 0
f i r s t order reaction, n = 1
I
R1l = l-Tq
second o r d e r r e a c t i o n , n = 2 On t h e o t h e r hand, t h e r a t i o s (R2n) between t h e areas under t h e D i r a c responses f o r r e a c t i o n coupled w i t h l i n e a r a d s o r p t i o n and a d s o r p t i o n a1 one a r e : area under t h e D i r a c response f o r r e a c t i o n + t l i n e a r adsorption
1
area under t h e D i r a c response f o r a d s o r p t i o n
f i r s t order reaction, n = 1
second o r d e r r e a c t i o n , n = 2
I
R21 =
Giq
1+E Nr R22 = - l n ( 1 t-)1tc r
I t can be seen t h a t we o b t a i n R2n f r o m Rnl
g i v i n g Rln
we r e p l a c e Nr by
The r a t i o s
i f i n t h e expressions
Nr(l+E)l-n.
i2n f o r t h e case
o f Langmuir t y p e a d s o r p t i o n
isotherms are:
T h i s k i n d o f a n a l y s i s can be u s e f u l when c a r r y i n g o u t experiments i n m i c r o r e a c t o r s i n view o f t h e d e t e r m i n a t i o n o f k i n e t i c parameters.
NOMENCLATURE e x t e r n a l p a r t i c l e s u r f a c e area p e r u n i t r e a c t o r volume
a, A
r e a c t i o n ( a d s o r p t i o n ) component
b
r a t i o between t h e mass and t h e heat B i o t numbers
Bi
B i o t number
Bif
f l u i d / w a l l B i o t number
Bi
s o l i d / w a l l B i o t number
P
Bih
h e a t B i o t number
Bim
mass B i o t number
Bo
Bodenstein number
c
specificheat
P
ci Ci Cpi cs dp
c o n c e n t r a t i o n o f component i i n t h e f l u i d phase c o n c e n t r a t i o n o f component i c o n c e n t r a t i o n o f component i i n s i d e t h e p e l l e t reactant concentration a t the surface o f the p e l l e t p a r t i c l e diameter
D
r e a c t o r diameter, d i f f u s i v i t y
Da
Damkdhler number
Dc
d i f f u s i v i t y i n zeolite crystlas
De
effective d i f f u s i v ity
D
d i f f u s i v i t y i n macropores
P Dr e
emissivity
E
a c t i v a t i o n energy
f
reduced r e a c t a n t c o n c e n t r a t i o n i n s i d e t h e c a t a l y s t ( c / c s )
F
massflux
h
i n t e r p o l a t e d h e a t t r a n s f e r c o f f i c i e n t f o r f i l m surrounding a particle
hc
convection h e a t t r a n s f e r c o e f f i c i e n t f o r f i l m surrounding a particle
r a d i a l e f f e c t i v e mass d i f f u s i v i t y
hc,hr,hp hr
c o n v e c t i v e , r a d i a t i v e and c o n d u c t i v e c o n t r i b u t i o n s f o r e a t t r a n s f e r c o e f f i c i e n t f o r f i l m surrounding a p a r t i c l e
r a d i a t i o n c o n t r i b u t i o n f o r the f i l m heat t r a n s f e r c o e f f i c i e n t
r a d i a t i v e contribution t o the apparent wall heat t r a n s f e r coeffi c i en t apparent wall heat t r a n s f e r c o e f f i c i e n t hw hwf wall heat transfer c o e f f i c i e n t f o r the f l u i d wall heat t r a n s f e r c o e f f i c i e n t f o r the s o l i d phase hwD ( - A H ) heat of reaction (adsorption) I o , I 1 modified Bessel functions of the f i r s t kind, zero and f i r s t orders respectively k k i n e t i c constant gas phase mass t r a n s p o r t c o e f f i c i e n t r e f e r r e d t o u n i t i n t e r kc f a c i a l area kf film mass t r a n s f e r c o e f f i c i e n t k molecular thermal conductivity of the f l u i d 9 pel l e t e f f e c t i v e thermal conductivity P kr radial e f f e c t i v e thermal conductivity krad r a d i a t i v e contribution t o the s t a t i c radial thermal conductivity
brad
radial thermal conductivity of the f l u i d defined by equation 114rl radial thermal conductivity of the sol i d k r ~ K constant separation f a c t o r f o r adsorption isotherm L r e a c t o r (column) length n' number of microspheres in the pel l e t Nr reaction number Ns interphase heat t r a n s f e r group Nu f l uid/sol i d Nussel t number fp NUwff l uidlwall Nussel t number P pressure f Peha f l u i d phase axial Peclet number Peh, radial Peclet number f o r heat t r a n s f e r peLr radial f l u i d Peclet number f o r heat t r a n s f e r Pemr radial Peclet number f o r mass t r a n s f e r Pehz,Pema-axial Peclet number f o r mass t r a n s f e r Per ( m ) tlirbul e n t 1imi t Pecl e t number q adsorbed sol i d concentration krf k0r
adsorbed s o l i d concentration in equilibrium with co r dimension1 e s s radial coordinate r' radial coordinate i n t r i n s i c r a t e of disappearance of component A per u n i t r A vol ume of pel 1e t rc zeol i t e c r y s t a l radius R reactor radius Re p a r t i c l e Reynolds number c a t a l y s t pel l e t radius R~ s Laplace transform parameter Sc Schmidt number t time breakthrough time t~~ tst stoechiometric time T temperature To reference temperature T temperature inside the p e l l e t P temperature a t the surface of the p e l l e t Ts T wall temperature -wT , T reduced temperature u s u p e r f i c i a l gas velocity u 1. i n t e r s t i c i a l velocity U flowrate U overall heat t r a n s f e r c o e f f i c i e n t v r e a c t o r (adsorber) volume V V p e l l e t volume P' a Vi volume of a microsphere x reduced f l u i d phase concentration X r a d i a l l y averaged r a t i o , C/Co
Q
Greek l e t t e r s a a
6
-
B y
y E
E
n
P
shape f a c t o r heat t r a n s f e r number Prater thermici t y f a c t o r P r a t e r thermicity f a c t o r in the bulk conditions f r a c t i o n of a c t i v e s i t e s a t the microsphere surface o r a t pore wall s Arrehnius number (-AH/RTo) r e a c t o r (adsorber) voidage p e l l e t porosity effectiveness f a c t o r
e f f e c t i v e n e s s f a c t o r , defined by equation 181 effectiveness f a c t o r f o r a microsphere T I average microeffectiveness f a c t o r qOv overall effectiveness f a c t o r 8 dimensionless time A, e f f e c t i v e thermal conductivity a' ,Ps apparent density of support bulk density (density of the c a t a l y s t bed) pb
n n -
pt T
r*
Sh 5, w
R
i
t r u e density space time time constant (2" order dynamic system) heat capacity f a c t o r mass capacity f a c t o r volume f r a c t i o n of the p e l l e t occupied by z e o l i t e c r y s t a l s parameter, defined by equation 11 1 . Measures the r e l a t i v e importance between macropore and micropore mass t r a n s f e r resistances damping f a c t o r (2nd order dynamic system)
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ENGINEERING ASPECTS OF CATALYTIC CRACKING
H . de LASA
Chemical Engineering Department Faculty of Engi neeri ng Science The University of Western Ontario London, Ontario, Canada N6A 5B9 The t e c h n o l o a .
S t a t e of the a r t and expected progress.
The technology of f l u i d i z e d bed c a t a l y t i c cracking ( F C C ) has shown a remarkable change i n the l a s t 30 y e a r s , The conventional FCC process, i ntensi vely appl i ed durinq the 5 0 ' s , bei ng basi cal l y the combination of two dense f l u i d i z e d beds ( t h e r e a c t o r and the regenerator) and two t r a n s p o r t l i n e s , may be considered as the 6-innA: genenation of c a t a l y t i c crackers. The heat required f o r the endothermic cracking reactions was supplied by the exothermal coke combustion. From an overall view point the i n d u s t r i a l process was operated under conditions close t o the thermal equilibrium where the silica-alumina c a t a l y s t was t r a n s f e r r i n g the heat from t h e hot regions (regenerator) t o the col d regi ons ( r e a c t o r ) and v i ce-versa. ( ~ i g .1 ) .
Fig. 1
Schematic Description of the F i r s t Generation of Crude Oil C a t a l y t i c Cracki ng Process
However, t h e commercial introduction of t h e microcrystalline s i l ica-alumina c a t a l y s t s (zeol i t e s ) a t t h e beginning of t h e 6 0 ' s , indicated t h e need of handling t h e overall process of t h e c a t a l y t i c cracking i n a new system where t h e remarkable a c t i v i t y and select i v i t y of z e o l i t e s (1 ) were f u l l y used. This s i t u a t i o n generated t h e i n t e r e s t i n a new FCC concept, the necond genaaLLon of catal y t i c crackers, where t h e c a t a l y t i c reaction was conducted in a s h o r t e r period of time ( 2 ) , ( 3 ) , ( 4 ) . In f a c t , t h e cracking react i o n s were s t a r t e d i n t h e t r a n s p o r t l i n e , conveying t h e cata1,yst t o the r e a c t o r (3-4s) and they were proceeded in a shallow dense f l u i d i z e d bed (3-4s) reducing in t h i s way the undesirable overcracking phenomena ( 3 ) , (Fig. 2 ) . T h i s overcracking, r e s u l t i n g from t h e secondary reactions and normally generating excessive amounts of coke and gases, was too dominant in deep f l u i d i z e d beds c o n s t i t u t e d by t h e so a c t i v e z e o l i t e s ( 5 ) .
.
feed
Fig, 2 Schematic Description of the Second Generation of Crude Oil C a t a l y t i c Cracking Process
Consequently, t h e second generation of transported-shallow bed FCCs represent an important e f f o r t t o take f u l l advantage of zeol i t i c c a t a l y s t s minimizing a t t h e same time these undesirable cracking transformations. A typical example of t h i s process i s t h e design patented by Texaco ( 6 ) . However, during t h e l a t e 60's i t was s t r e s s e d the importance of a f u l l use of t h e s p e c i f i c a c t i v i t y of t h e new microcrystalline s i l ica-a1 umina c a t a l y s t s ( z e o l i t e s ) r e c e n t l y introduced in the market ( 6 ) , ( 7 ) . Some d i f f i c u l t i e s were nevertheless forecasted ( 2 ) f o r t h e use of these materials having a dominant microporous
structure (2-7A), (9), much smaller than the amorphous silicaalumina (30-70A) (10). In favour of zeol i tes a 100-70000 times improved activity, in relation with the amorphous silica-alumina, was claimed (1 ) . This fact coupled with an improved hydrogen transfer capability, presumably a consequence of hydrogen mobility between cracked molecules, was indicating an important potential of zeolitic materials for increasing conversions to gasoline with low coke yields (9),(11 ), (12). Besides that, the growing refinery problem resulting from the necessity of processing crudes of' different source and quality, showed that an accurate control of reaction times between the catalyst and the hydrocarbons was a must for FCC crackers. This approach was a very promising concept for the FCC units. In this way was born the Zkitrd genehatLon of crackers based on a reactor which was, in fact, a transport line unit (21, (13 J, (8),(14). This concept was developed in several patents, case of Exxon in the Flexicracking unit, Gulf in Gulf FCC unit, Kellogg in the Orthoflow and the Heavy oil processes, Standard Oil in the Ultracat technology, UOP in the riser cracker units (2), (~~),(16)y(~7)y(18)y(19)y(~O). The peculiar characteristics of FCC transported reactors (21 ) provided better control of gasoline yields (60-65%) improved feedstock conversions (75-80%) and smaller coke yields (coke/feed:3-6%). For instance, the capabil ity of zeol itic catalysts for cracking naphtenic and parafinic hydrocarbons avoiding at the same time the slow decomposition of the aromatic molecules, normally generating high coke,yields, was a significative progress for the FCC technology (41). It must be stressed at the same time that the introduction of the riser reactors in the Z k i n d genehatLon of FCC units was still combined with dense fluidized bed regenerators. However, the high thermal resistance of the zeolites allowing high regeneration temperatures, 740°C instead of 630°C without a noticeable particle deactivation or sintering (16),(22) gave fast coke combustion rates and low CRC (coke on regenerated catalyst) levels. In this way, CRC of the order of 0.05 - 0.2% were obtained (23),(24), (25),(4), (26). These low CRC levels seem to be an important condition for reactivating the zeolitic sites on the catalysts and for achieving a maximum use of zeolite surface (27). Other advantages like a better overall plant thermal balance (14), a reduced recycled oil streams (8) were also claimed for the riser cracking reactors. Finally, more recent developments in the manufacturing of the cracking catalyst have been oriented to increase the zeolite performance. For instance, it has been analysed in which way the type of zeolite affects the conversion and the selectivity (28), (29), how the resistance to the attribution or the R.O.N. yields on gasoline could be improved (30), (2), how the hydrocarbon molecule size affects the zeol i te performance (1 ), (9).
I n s p i t e o f a l l t h i s progress, d u r i n g t h e mid 70s t h e FCC u n i t s were challenged again. The energy c r i s i s r e s u l t i n g from t h e f i r s t Arab embargo showed t h a t new m o d i f i c a t i o n s o r changes were r e q u i r e d t o face t h e new s i t u a t i o n . I n f a c t , new problems needed t o be considered f o r e f f i c i e n t l y processing t h e heavy crudes (14). The bottom o f t h e b a r r e l o r heavy crudes, f r a c t i o n s t h a t c o u l d n o t have been economically cracked b e f o r e t h e 1975 c r i s i s , were more and more considered f o r t h i s purpose (31 ). T h i s s i t u a t i o n cont i n u e s t o be t h e present challenge f o r t h e r e f i n e r i e s . However, another problem appeared t o g e t h e r w i t h t h e ones a l r e a d y described. P a r t i c u l a r l y t h e growing tendency t o mix a b i g g e r r e c y c l e d o r heavy crude f r a c t i o n i n t h e c r a c k e r feed (26), increased t h e c a t a l y s t contamination w i t h n i c k e l and vanadium metals (33). These metals once deposited on t h e z e o l i t i c m a t r i x f a v o u r t h e dehydrogenation r e a c t i o n s g i v i n g more hydrogen and more coke (33), (14), (34), (35), (36), (37),(38). A t t h e same t i m e metals such as n i c k e l and vanadium reduce t h e a c t i v e s i t e s and t h e s p e c i f i c a c t i v e s u r f a c e per u n i t weight decreasing t h e o v e r a l l c a t a l y t i c a c t i v i t y and select i v i t y t o produce gas01 i n e . Some authors claimed t h a t n i c k e l i s around f o u r times as bad as vanadium a t t h e same c o n c e n t r a t i o n s (39), (36), (38). As a consequence and f o r having a standardized b a s i s o f r e f e r e n c e an " e f f e c t i v e metal c o n c e n t r a t i o n " on t h e catal y s t i s d e f i n e d . T h i s e f f e c t i v e parameter i s f o u r times t h e n i c k e l It was a l s o l e v e l p l u s t h e vanadium c o n c e n t r a t i o n (36),(32). p o i n t e d out, however, t h a t n i c k e l and vanadium a c t i v i t y would proba b l y be r e l a t e d t o o t h e r f a c t o r s 1 i k e thermal a c t i v i t y h i s t o r y (40), (38), (41 ). The r e f i n e r i e s would have then, w i t h t h e heavy crudes, t h e p o t e n t i a l problems r e s u l t i n g from a d d i t i o n a l coke f o r m a t i o n and s m a l l e r c a t a l y s t a c t i v i t y (14). I n f a c t , t y p i c a l coke y i e l d s f o r heavy crudes have g i v e n coke c o n c e n t r a t i o n s values as h i g h as 8-16%. (The b a s i s f o r these A p o s s i b l e way o f t a c k l i n g y i e l d s i s t h e f r e s h feed), (31),(14). t h e problem i s e i t h e r t h e use o f antimony p a s s i v a t o r s o f n i c k e l and vanadium as suggested by P h i l 1 i p s Petroleum (42), (39) o r t h e employment o r t h e new f a m i l i e s o f z e o l i t i c c a t a l y s t , t h e Redsicat, t h e GRZ and t h e F i l t r o l (24), (34). These c a t a l y s t s would a l l o w t h e c r a c k i n g process t o perform w i t h s i g n i f i c a n t n i c k e l and vanadium l e v e l s , p e r m i t t i n g a t t h e same t i m e adequate o p e r a t i o n o f t h e FCC u n i t s w i t h r e l a t i v e l y low coke and gas y i e l d s . It i s expected t h a t these c a t a l y s t s would p r o v i d e h i g h crude o i l conversions, w i t h eff e c t i v e (4Ni + V ) c o n c e n t r a t i o n s as h i g h as 5000 ppm, c o n d i t i o n s where a standard c r a c k i n g c a t a l y s t would have shown a n o t i c e a b l e a c t i v i t y decay (24). We may v i s u a l i z e then t h a t we a s s i s t today t o t h e development o f a s i g n i f i c a n t t e c h n i c a l e f f o r t towards t h e p r o d u c t i o n o f new FCC c a t a l y s t s r e q u i r e d f o r t h e processing of heavy crudes. A t t h e same time, t h e o p e r a t i o n o f t h e FCC u n i t s has been cont e s t e d again by t h e new environmental r e g u l a t i o n s . The problem o f
how to reduce the CO emissions, transforming CO in C02, and gaining in the same operation the CO heat of combustion has been one of the major refinery concerns during the 70s, (44),(45), (43). .4 first approach involved the use of CO burners, located in the plant after the regenerator. These burners transformed the CO gases producing at the same time steam for the cracking plant (44). However, a more advanced concept seeks a direct CO to C02 conversion in the regenerator itself. With this purpose catalytic materials named comb unto^ were developed (23), (45). Three basic types of combustors have been suggested: - a platinum group metal (Pt, Pd, Ir) deposited in small concentrations on the cracking catalyst, - a solid promotor mixed to a cracking catalyst without additives (0.1-1%) - a 1 iquid additive injected into the regenerator, (42). Unfortunately, the specific references about the chemicals promoting these effects are very incomplete. For the case of the platinum group metal it would seem that the metal is incorporated to the zeolitic matrix in such a way that would be only contacted by CO and 02 but not by the bigger hydrocarbon molecules. In this manner the CO transformation would be promoted and the adverse dehydrogenation reactions catalyzed by the same metal control 1 ed (45). These combustors, which normally 1 imi t the CO emissions to levels below 500 ppm (maximum allowed CO environmental emission concentration) (46) have shown to be a more appropriate technology than the CO external boilers (23), (45). In fact, through the advanced CO transformation method significant capital gains have been claimed as well as important reductions of CO concentrations in the dilute regenerator phase (250-500 ppm) (43), (35), (14). This low CO concentration in the entrainment phase region is possibly a very important factor for eliminating the troublesome CO oxidation in the 1 ean regenerator phase (postburning) (45). In this way, the uncontrolled temperature increase in the upper part of the regenerator, consequence of CO postburning and frequent cause of catalyst sintering and cyclone damages, is avoided. However, in spite of this progress no technical information is, at least to our knowledge, available to predict the behaviour of the catalytic combustors in front of heavy oils with high nickel and vanadium contents. This is, in our opinion, a research area where significant technological improvement must be achieved in the next few years to really make of the FCC a technological breakthrough. KINETICS OF CATALYTIC CRACKING When a gas oil is catalytically cracked, an ample diversity of chemicals, going from hydrogen and methane to coke forming polymeric materials deposited on the catalyst, are generated. The most popular kinetic models used for the process description are
basically based on a simp1 ified representation involving three characteristic groups of reactants and products: t
A1 : gas oil ; A2: gasoline(C5 - 210°C);
A3: butanes, light, gases and coke
This certainly leads to the conception of the catalytic cracking of the gas oil as follows (48),(49),(50), (51 ), (52), (53).
A1 (gas oil)
1
*
A2 (gasoline)
(1 1
(butanes, light gases and coke) It is important to point out that the catalytic process may be interfered to some extent by the thermal cracking reactions which simultaneously occur. Corrections have been suggested to the gas oil conversion to discount the noncatalytic effect (54), (55), (56). These corrections are around 2% at 490°C (51). Consequently, the scheme presented above with equation [I ] may be a usual approach to predict both the overall gas oil conversion and the selective transformation of gas oil in gasoline. Certainly the second aspect is a key factor for an efficient operation of the FCC units. These two parameters have been estimated by several kinetic models. The most frequently adopted kinetic representation (521,(571,(58), (50), (51 ), (59), considers that the overall gas oil transformation is a second order decomposition and the further gasoline cracking is a first order reaction. r1
=
kO+l(tc)Y12
(gas oil cracking)
[21
r2
=
k2@2(tc)y2
(gas01 ine cracking)
[31
This difference between the gas oil and the gasoline cracking reaction orders may be explained as follows:
- The gasoline is a mixture of hydrocarbons with boiling points
varying in a 1 imited range. In this sense it is normal to expect that the gas01 ine will closely behave like a pure hydrocarbon showing a reaction order equal to one (51 ).
- Conversely, t h e gas o i l i s a f a r more complex m i x t u r e w i t h an ample d i v e r s i t y o f c r a c k i n g r e a c t i o n r a t e s . I n t h i s r e s p e c t t h e gas o i l c r a c k i n g i s t h e summation o f a l a r g e number o f r e a c t i o n s a c t i n g i n p a r a l l e l w i t h v e r y d i f f e r e n t k i n e t i c c o n s t a n t s . The consequence i s an o v e r a l l r e a c t i o n o r d e r l a r a e r t h a n one (60). These f a c t s have been j u s t i f i e d b o t h i n t u i t i v e l y and t h e o r e t i c a l l y ( 5 7 ) , (61 ), (62). A n o t h e r k i n e t i c scheme (63), (49) based on a s i m i l a r i d e a t o e q u a t i o n 121, t h a t use i n s t e a d o f w e i g h t f r a c t i o n s t h e more approp r i a t e m o l a r f r a c t i o n s , i n t r o d u c e s a parameter named c r u d e o i l r e f r a c t o r i n e s s . T h i s parameter lumps i n a s i n g l e f a c t o r t h e c r a c k i n g f e e d s t o c k and c a t a l y s t c h a r a c t e r i s t i c s : (cracking o f crude o i l ) [ 4 ] The parameter W i s c e r t a i n l y d i f f e r e n t f r o m z e r o f o r c o n v e n t i o n a l crudes which n o r m a l l y have b o i l i n g p o i n t s v a r y i n g i n a wide t e m p e r a t u r e range (around 280-550°C). The i n t e r e s t i n g aspect o f t h i s model i s t h e c o n d i t i o n o f a W parameter o s c i l l a t i n g between 0 and 3.3 and showing some t e m p e r a t u r e dependence ( 6 4 ) , (55), ( 6 5 ) , ( 6 6 ) , (49), ( 6 7 ) , ( 6 2 ) . The l o w e r W v a l u e s corresponded t o experiments w i t h a v a r i e t y o f c r a c k i n g c a t a l y s t s where some d i f f u s i o n a l l i m i t a t i o n s were suspected. The h i g h e r W were c o n s i d e r e d f o r La'{ z e o l it i c c a t a l y s t s where d i f f u s i o n a l 1 i m i t a t i o n s c o u l d p r o b a b l y be n e g l e c t e d (28). I n any case i t would seem t h a t f o r z e o l i t e s c r a c k i n g heavy f e e d s t o c k s t h e c a t a l y s t o p e r a t i o n w i t h i m p o r t a n t d i f f u s i o n a l 1i m i t a t i o n s i s h i g h l y p o s s i b l e ( 9 ) .
A t t h e same t i m e on e q u a t i o n s ( 2 ) , ( 3 ) and ( 4 ) i t may be n o t i c e d t h a t two timedependent parameters a r e i n c l uded. Both $l ( t c ) and $ 2 ( t ) i n t r o d u c e i n t h e s e e x p r e s s i o n s t h e c a t a l y t i c a c t i v i t y decay f a k o r s . I t may a l s o be observed t h a t b o t h @ ( t ) and 4 ( t ) a r e f u n c t i o n s o f tc, t h e fieaction lime o r t h & c & n & h ~ on a.$xeh .time. T h i s t i m e i s a key v a r i a b l e when m o d e l i n g a h e t e r o geneous r e a c t i v e system l i k e t h e FCC process. The cause o f t h i s a c t i v i t y decay i s m a i n l y r e l a t e d t o t h e d e p o s i t i o n o r a d s o r p t i o n o f t h e p o l y a r o m a t i c compounds which p o l y m e r i z e on t h e c a t a l y s t s u r f a c e f o r m i n g t h e coke. ( 5 9 ) , (61 ), ( 6 8 ) , ( 2 9 ) . T h i s p o l y m e r i z a t i o n appears a c t i v a t e d by t h e o l e f i n s c a p a b i l i t y o f a b s t r a c t i n g h y b r i d e i o n s f r o m n a p h t e n i c and a r o m a t i c hydrocarbons ( 9 ) , ( 5 6 ) , ( 6 9 ) which produces a h i g h l y condensed a r o m a t i c s o l i d r e s i d u e ( 7 0 ) . I n s p i t e of t h e g e n e r a l a c c e p t a t i o n o f t h e s e f a c t s some arguments were p r e s e n t e d ( 5 6 ) , (71 ) i n d i c a t i n g t h e d i f f i c u l t i e s o f f o r m u l a t i n g a s i m p l e r e l a t i o n s h i p between c a t a l y s t a c t i v i t y and coke c o n c e n t r a tion. Moreover and because i t i s n o r m a l l y assumed t h a t t h e same t y p e o f a c t i v e s i t e s w i l l c r a c k b o t h gas o i l and g a s o l i n e molecules,
an i d e n t i c a l s e t o f @ ( t ) and @ 2 ( t G ) have been proposed ( 5 8 ) . Under t h e s e c o n d i t i o n 1 eGuations [ 2 j and [4] may be w r i t t e n as f o l l ows :
D i f f e r e n t t y p e s o f ? ( t c ) f u n c t i o n s have been proposed t o r e p r e s e n t b o t h t h e c a t a l y s t a c t i v i t y decay and t h e coke f o r m a t i o n . I n p a r t i c u l a r , two o f t h e s e mathematical forms a r e t h e most p o p u l a r ones. One o f them c o n s i d e r s t h a t t h e c a t a l y s t a c t i v i t y decay may be r e p r e s e n t e d w i t h a power law, ? ( t c ) = a t - n ( 7 2 ) , ( 6 0 ) , ( 5 1 ) , ( 7 3 ) . The o t h e r one approaches t h e @ ( t c ) f u n c t i g n u s i n g a d e c r e a s i n g e x p o n e n t i a l @ ( t c ) = b e x p ( - a t c ) , ( 5 9 ) , ( 5 2 ) . Some arguements l i k e a f i n i t e c a t a l y s t a c t i v i t y when t+O ( 5 9 ) and t h e p o s s i b l e r e l a t i o n s h i p o f coke f o r m a t i o n w i t h an i r r e v e r s i b l e a d s o r n t i o n mechanism would s u p p o r t t h e e x p o n e n t i a l decay l a w ( 7 4 ) . I t must however, be i n d i c a t e d t h a t some e f f o r t s were o r i e n t e d t o m o d i f y t h e power l a w c o r r e c t i n g e s s e n t i a l l y t h e anomaly when tc+O, ? ( t ) = a ( 1 + G t c ) - n w i t h G = ( m - l ) K and n = l / ( l - m ) (61), (75), (645, ( 6 3 ) , (65), (76), ( 7 7 ) , ( 7 8 ) . Moreover, t h i s e q u a t i o n has t h e a d d i t i o n a l advantage o f a l l o w i n g t h e d e r i v a t i o n o f an e x p o n e n t i a l l a w when m=l. I n o t h e r words t h e e x p o n e n t i a l l a w i s a p a r t i c u l a r case o f t h e m o d i f i e d power l a w ( 6 3 ) . T h i s m o d i f i e d power l a w e x p r e s s i o n i s a l s o supported by e x p e r i m e n t a l evidence which showed n and m v a l u e s changing between 0.6-30 and 1.03-2.57, r e s p e c t i v e l y (64), (66). Then t h e observed m v a l u e s , depending on t h e coup1 e c a t a l y s t f e e d s t o c k and on t h e o p e r a t i n g c o n d i t i o n s , match t h e e x p o n e n t i a l behaviour, m= 1 o n l y i n a fewcases which c e r t a i n l y showed t h e convenience o f a more q e n e r a l 4 = a ( l + G t c ) - n expression.
I t i s i m p o r t a n t t o mention, however, t h a t b o t h e x p o n e n t i a l o r m o d i f i e d power l a w s c o n t a i n a m a j o r simp1 if i c a t i o n , t h e dependence o f hydrocarbon c o n c e n t r a t i o n on @ ( t c ) i s i g n o r e d . T h i s hypothesis does n o t always seem a p p r o p r i a t e f o r f i x e d bed c a t a l y t i c c r a c k i n g l a b o r a t o r y u n i t s ( 7 9 ) , ( 2 0 ) . A m o d i f i e d e x p o n e n t i a l l a w has been proposed (81 ) as a more s u i t a b l e @ ( t c ) r e l a t i o n s h i p .
A t t h e same t i m e o t h e r e f f e c t s , h a v i n g consequences on c a t a l y s t a c t i v i t y , were d e s c r i b e d . F o r i n s t a n c e t h e a c t i o n o f n i t r o g e n a t e d bases on s i l i c a - a l u m i n a and z e o l i t i c c a t a l y s t s r e c e i v e d p a r t i c u l a r a t t e n t i o n . These compounds, coming a l o n g w i t h t h e f e e d s t o c k stream, would decrease t h e a c i d i t y o f t h e c a t a l y s t m a t r i x r e d u c i n g i n t h i s way i t s a c t i v i t y and s e l e c t i v i t y (82). N e v e r t h e l e s s , t h e appropr ~ i a t eway o f i n c l u d i n g t h i s e f f e c t i n t h e k i n e t i c model i s , a t the present time, q u i t e unclear.
We w i l l c o n s i d e r now t h e i n t e r e s t o f equations [5] and [6] f o r d e f i n i n g t h e gas o i l c o n v e r s i o n and g a s o l i n e f o r m a t i o n r a t e s . I n f a c t , once those expressions a r e d e r i v e d , i t i s p o s s i b l e t o e s t i m a t e t h e 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 gas o i l i n g a s o l i n e . A t t h e same t i m e because gas o i l c o n v e r s i o n t e s t s were f r e q u e n t l y performed i n 1a b o r a t o r y s c a l e f i x e d bed r e a c t o r s , some i n t e r e s t i n g o b s e r v a t i o n s were developed f o r those systems. I t has been r e a l i z e d , f o r i n s t a n c e , t h a t i n l a b o r a t o r y s c a l e u n i t s tv, t h e residence t i m e o f t h e hydrocarbon m i x t u r e , i s n o r m a l l y much s m a l l e r than t h e c a t a l y s t on stream time ( t v << t c ) ( 5 8 ) . I n f a c t tc i s i n t h e o r d e r o f a few minutes when i n o p p o s i t e t v i s o n l y i n t h e o r d e r o f 1-2 seconds. When t v << tc, t h e gas o i l molecules t r a v e l through t h e r e a c t o r w i t h such a h i g h speed, i n r e l a t i o n w i t h t h e r a t e o f a c t i v i t y decay, t h a t t h e bed may appear t o them as having a u n i f o r m a c t i v i t y (50), ( 5 7 ) . Under these c o n d i t i o n s t h e f o l l o w i n g equations may be proposed t o d e s c r i b e t h e behaviour o f t h e c r a c k i n g r e a c t o r .
I f t h e e q u a t i o n [8] i s d i v i d e d change o f y 2 w i t h r e s p e c t t o y l may mentioned however, t h a t e q u a t i o n [9] pressure and n e g l e c t s t h e change o f mixture.
by e q u a t i o n [7] t h e d i f f e r e n t i a l be d e r i v e d . I t must be considers constant t o t a l the molecular weight o f the
An i n t e g r a t i o n o f e q u a t i o n [ 9 ] a l l o w s us t o d e r i v e t h e s e l e c t i v e c o n v e r s i o n o f gas o i l i n g a s o l i n e (58), ( 5 0 ) .
.
Y1 k o e x p ( k 2 - ) + E (k2. - ) w i t h E(x) =
-m
2 ::
Y1
-
1
E@) 0
= exponential i n t e g r a l
[lo1
I t i s important t o s t r e s s t h a t the integrated form of equation [9] provides. an expression f o r estimating the in4;tarz;taneow gas
oi 1 conversion in gas01 ine. This expression, a1 lowing the prediction of a maximum gasoline yield f o r a given s e t of kinetic parameters, i s certainly different from the measured ~ Q U M s e l e c t i v i t y values ($/(l-~Yl ) obtained when fixed bed runs are employgd for-testi ng the adequacy of cracking kinetic models. These y /(l-ly ) s e l e c t i v i t y values are avarages performed during the tc gotal lime on stream period. Another interesting question i s given by the f a c t t h a t equation
191 o r i t s integrated form allows to visualize that the instantan-
eous s e l e c t i v i t y i s not dependent on (p(tc), catalyst a c t i v i t y decay function, none of the following f a c t o r s : t c , tv or catalyst to gas oi 1 r a t i o (48), (54). I t must nevertheless be stressed t h a t Yp # f ( $ ( t c ) behaviour r e s u l t s of (p ( t c )= @ 2 ( t c postulate, ) a common hypothesis of di f f i c u l t veri bi cation. Conversely, i t has been shown t h a t the average s e l e c t i v i t y , a frequent r e s u l t of fixed bed 1 aboratory units , depends on (p(tc), tc, $ and catalystlgas oil relationship (48), (50). This has stimulated researchers to design c a t a l y t i c cracking experiments in such a way t h a t the key instantaneous s e l e c t i v i t y parameter could be estimated. The importance of t h i s instantaneous parameter may also be understood in the l i g h t of i t s i n t e r e s t to define the performance of industri a1 FCC units (48). A most valuable way of finding t h i s instantaneous s e l e c t i v i t y function, y2 = f(yl). i s through the determination of an evolvent curve which 1imi t s a1 1 the possible average selectivi ty functions (52). This instantaneous y curve i s , a t the same time, an optimum s e l e c t i v i t y curve, giving t e maximum possible yield for a certain couple crude feedstock-catalyst type. For t h a t reason i t has been named the opl2mum pen~omanceenvelope. (OPE) (49), ( 6 3 ) , (76).
t
Another aspect of high i n t e r e s t f o r the process of crude oil cracking i s related to the way the different crude o i l types, avai 1 able as. potential refinery feedstocks, a f f e c t the kinetic scheme already described. I t has been verified t h a t , in general terms, t h i s scheme i s s t i l l valid. Only changes in the kinetic constants, k , kl , k2 and the a activi ty decay constant may be expected (503. This analysis was performed considering three typical groups or classes of gas oi 1 consti tuents :-aromatic compounds, a , parafi ni c compounds, p, naphtheni c compounds, n, (51 ) . I t was possible to observe through these studies t h a t crude oi 1s with a high parafi ni c and naphteni c hydrocarbon content present higher ko and kl and smaller k2 and or values (83). A generalization of t h i s concept, including a factor weighting the aromatic t o naphtenic fraction i n Lo and k l definition was proposed (84).
These f a c t s showed t h e h i g h c r a c k a b i l i t y o f t h e p and n compounds and t h e i r s m a l l tendency t o f o r m coke ( 1 5 ) . The r e v e r s e i s a l s o r i g h t . The hydrocarbon m i x t u r e s w i t h a h i g h a r o m a t i c f r a c t i o n I n a word t h e showed s m a l l e r ko and k l and h i g h e r a and k 2 values. crudes w i t h a s i g n i f i c a n t f r a c t i o n o f a compounds w i l l have a h i g h e r r e f r a c t i v i t y t o be c r a c k e d and a s t r o n g e r tendency t o f o r m coke ( 5 1 ) , ( 5 0 ) , (61), ( 1 5 ) . F i n a l l y , more advanced models have been proposed a d d i n g more s p e c i f i c a t i o n s t o t h e p a r a f i n i c , n a p h t e n i c and a r o m a t i c b a s i c d i s t i n c t i o n . F o r i n s t a n c e , t h e hydrocarbons were c l a s s e d i n l i g h t and heavy, p u r e a r o m a t i c compunds o r branced t o p a r a f i n i c o r n a p h t e n i c groups (61 ) . T h i s l a s t s p e c i f i c a t i o n p r o v i d e s a more a c c u r a t e d e s c r i p t i o n o f f e e d s t o c k c r a c k i n g b e h a v i o u r . F o r example, t h i s k i n d o f model i s c o m p a t i b l e w i t h o b s e r v a t i o n s which i n d i c a t e t h a t a r o m a t i c r i n g s have a s p e c i a l c a p a b i l i t y t o a c t i v a t e c r a c k i n g on s i d e branches ( 8 5 ) . However, i n c o u n t e r p a r t w i t h these approaches, t h e s i m p l i c i t y o f k i n e t i c r e p r e s e n t a t i o n s , as t h e ones g i v e n by e q u a t i o n s [ 2 ] and [ 3 ] i s p a r t i a l l y l o s t . To complete t h e m o d e l i n g o f c a t a l y t i c c r a c k i n g t h e dependence o f r e a c t i o n r a t e s w i t h t e m p e r a t u r e was a l s o e x t e n s i v e l y analysed. T y p i c a l e n e r g i e s o f a c t i v a t i o n recommended f o r t h e 480-540°C temperature range (61 ) may be c l a s s i f i e d as f o l l o w s : - f o r t h e r e a c t i o n s o f g a s o l i n e and coke f o r m a t i o n f r o m p and n hydrocarbons, E = 5-9 K c a l l m o l ( 5 1 ) - f o r t h e r e a c t i o n s o f g a s o l i n e and coke f o r m a t i o n f r o m a hydrocarbons, E = 14=18 K c a l l m o l - f o r t h e These v a l u e s t r a n s f o r m a t i o n o f g a s o l i n e and coke, E = 20 Kcal/mol. a r e s u b s t a n t i a l l y d i f f e r e n t f r o m t h e 50 Kcal/mol l e v e l s sugqested f o r t h e gas o i l c r a c k i n g (66), ( 4 9 ) , (67), ( 6 9 ) and f o r t h e gas01 i n e f o r m a t i o n r e a c t i o n ( 6 9 ) . Again t h e p o s s i b l e i n f l u e n c e o f i n t e r n a l d i f f u s i o n a l c o n t r o l s on t h e r e a c t i o n r a t e s may e x p l a i n t h i s d i f f e r e n c e . T h i s view i s c o n f i r m e d by t h e f a c t t h a t t h e d i f f e r e n c e o f t h e enerqy o f a c t i v a t i o n appears c o u p l e d w i t h a v a r i a t i o n on t h e o v e r a l l c r a c k i n g r e a c t i o n orders (49), (67), (69). KINETIC OF REGENERATION The combustion o f t h e coke d e p o s i t e d on t h e m a t r i x o f a c r a c k i n g c a t a l y s t p a r t i c l e i s a process r e s e m b l i n g t h e b u r n i n g o f g r a p h i t e ( 8 6 ) . Some arguments 1 ike t h e presence i n t h e coke of t u r b o s t r a t i c l a y e r s s i m i l a r t o t h e g r a p h i t e s t r u c t u r e s were advanced ( 8 7 ) . I t i s a l s o i m p o r t a n t t o p o i n t o u t t h a t some s i m i l a r i t i e s on t h e o v e r a l l k i n e t i c b e h a v i o u r o f coke b u r n i n g has f u k t h e r supported t h i s v i e w ( 8 6 ) .
However, coke presents a hydrogen f r a c t i o n i n t i m a t e l y associa t e d t o t h e carbon t h a t cannot be neglected. T h i s hydrogen f r a c t i o n l a r g e l y depends on t h e e f f e c t i v e n e s s o f t h e hydrocarbon s t r i p p i n g o p e r a t i o n a t t h e end o f t h e c r a c k i n g r e a c t o r . I n fact, t h e c a t a l y s t i s s t r i p p e d w i t h steam a t t h e e x i t o f t h e c r a c k e r t o remove t h e hydrocarbons remaining i n t h e porous m a t r i x . T h i s e x p l a i n s t h e coke formula r e p o r t e d i n t h e l i t e r a t u r e , CHn w i t h n r a n g i n g between 0.4 t o 1 (88), (89), (86). Even f o r t h e f i r s t coke f r a c t i o n s burned some authors propose n = 2(88). I t i s important t o s t r e s s t h a t t h e u n c e r t a i n t y i n t h e hydrogen content d e f i n i t i o n has concrete consequences on t h e p r e d i c t i o n o f t h e heat of coke combustion, key parameter f o r modeling an i n d u s t r i a l regenerator (47). A d i r e c t e s t i m a t i o n o f hydrogen i n coke i s then h i g h l y adv i sabl e f o r an a p p r o p r i a t e regenerator model ing (90). I n any case, a reasonable average o f coke composition seems t o be CH0.7 (90). Under those c o n d i t i o n s t h e couple o f s t o i c h i o m e t r i c equations d e s c r i b i n g CO and C02 f o r m a t i o n may be w r i t t e n as f o l 1ows (90).
A p r e v a i l i n g approach t o represent t h e coke combustion i s based on t h e assumption t h a t t h e r a t e o f coke disappearance i s a f i r s t o r d e r law w i t h r e s p e c t t o both oxygen and coke concentrations (861, (91).
I n t h i s sense t h i s k i n e t i c model considers t h a t t h e coke burni n g r a t e may be seen as a g r a p h i t e combustion r a t e , g i v i n g no s i g n i f i c a n t weight t o t h e coke h e t e r o g e n e i t i e s . T h i s equation has proved t o be adequate f o r 60 pm p a r t i c l e s and coke concentrations i n t h e o r d e r o f 6% o r s m a l l e r than 6% f o r both amorphous s i l i c a alumina (86) and z e o l i t i c c a t a l y s t s (91 ). I t i s important t o mention, however, t h a t t h i s equation i s o n l y a p p r o p r i a t e t o desc r i b e t h e r a t e o f combustion o f carbonaceous d e p o s i t s i f t h e coke has been we1 1 s t r i p p e d o f v o l a t i 1e hydrocarbons. Otherwise equat i o n [13] may be questionable (92). A second o r d e r equation desc r i b i n g a combination o f v a p o u r i z a t i o n and o x i d a t i o n d u r i n g t h e e a r l y stages o f c a t a l y s t r e g e n e r a t i o n may be more a p p r o p r i a t e . It has a l s o been shown t h a t 60 pm c r a c k i n g p a r t i c l e s w i t h a coke c o n c e n t r a t i o n above 6% l e v e l experienced some d i f f u s i o n a l l i m i t a t i o n s d e r i v e d o f a non-uniform coke d i s t r i b u t i o n i n t h e i n t e r n a l s u r f a c e o f t h e c a t a l y s t (86). It seems then, h i g h l y a d v i s a b l e t o l i m i t t h e coke d e p o s i t below.6% i n o r d e r to.have a coke combustion process dominated by t h e i n t r i n s e c r e a c t i o n r a t e .
T h i s a l s o may c o n t r i b u t e t o an a p p r o p r i a t e m o d e l i n g o f t h e r e g e n e r a t i o n process (go), (93). Another i n t e r e s t i n g p r o p e r t y o f e q u a t i o n [13] i s g i v e n by t h e f a c t t h a t t h i s r e l a t i o n s h i p i s s t i l l v a l i d f o r cokes formed w i t h d i f f e r e n t t y p e s o f f e e d s t o c k s on d i f f e r e n c t c r a c k i n g c a t a l y s t s ( 8 6 ) , (91 ) . Some s i g n i f i c a n t v a r i a t i o n s were, however, observed on t h e s p e c i f i c v a l u e s a s s i g n e d t o t h e k i n e t i c parameters f o r d i f f e r e n t c a t a l y s t s . F o r i n s t a n c e , e n e r g i e s o f a c t i v a t i o n o f 35-41 Kcal/mol were r e p o r t e d f o r coke b u r n i n g on s i l i c a - a l u m i n a (86), (89), ( 9 4 ) ( 8 8 ) . These v a l u e s a r e v e r y c l o s e t o t h e t y p i c a l e n e r g i e s o f a c t i v a t i o n o b t a i n e d by b u r n i n g g r a p h i t e s u r f a c e s ( 8 6 ) . Neverthel e s s , coke combustion r u n s i n a s i m i l a r t e m p e r a t u r e range (420-630°C) b u t u s i n g a s e r i e s o f z e o l it i c c a t a l y s t s , gave e n e r g i e s of a c t i v a t i o n i n t h e o r d e r o f 26 Kcal/mol (91). T h i s m i g h t suggest d i f f u s i o n a l c o n t r o l s o f t h e o v e r a l l combustioncoke process i n those z e o l it i c m a t e r i a l s . Another i m p o r t a n t q u e s t i o n when d e s c r i b i n g t h e coke t r a n s f o r mation i s g i v e n by t h e r e l a t i v e e x t e n s i o n o f C02 and CO f o r m a t i o n reactions. T h i s r e l a t i v e extension i s weighted i n equation [ I 3 1 by t h e k i n e t i c parameters k4 and kg which a r e f u n c t i o n s o f a = C02/CO. The C02/CO r a t i o i s a l s o a key parameter f o r s i m u l a t i n g an i n d u s t r i a l s c a l e r e g e n e r a t o r . To e s t i m a t e t h e C02/CO r e l a t i o n s h i p an e x p o n e n t i a l f u n c t i o n , t e m p e r a t u r e dependent o n l y , which r e s u l t s f r o m t h e pecul i a r c h a r a c t e r i s t i c s o f e q u a t i n g two r a t e s i n v o l v i n g t h e same r e a c t i o n o r d e r s was d e r i v e d ( 9 5 ) . T h i s C02/C0 e q u a t i o n p r e d i c t s a C02/CO r a t i o d e c l in d i n g w i t h temperature. However, p r a c t i c a l e x p e r i e n c e shows t h a t t h i s r e l a t i o n s h i p , b e i n g c o r r e c t below 600-61 O°C, s t a r t s b e i n g d e f i c i e n t above t h e 600-61 0°C l e v e l ( 9 6 ) . Then t h e C02/CO r e l a t i o n s h i p grows w i t h t e m p e r a t u r e and becomes dependent on t h e c r a c k i n g c a t a l y s t c h a r a c t e r i s t i c s . To overcome t h e d i f f i c u l t i e s o f C02/CO p r e d i c t i o n d i f f e r e n t a u t h o r s a d v i s e t o develop a c o r r e l a t i o n f o r t h e p a r t i c u l a r c a t a l y s t b e i n g used (96), ( g o ) , ( 9 3 ) . N e v e r t h e l e s s , t h e complex t a s k o f e s t i m a t i n g C02/CO r a t i o s r e c e n t l y e x p e r i e n c e d a c o n s i d e r a b l e s i m p l i f i c a t i o n w i t h t h e i n t r o d u c t i o n o f combustors which s h i f t t h e coke t r a n s f o r mation t o an a l m o s t complet CO c o n v e r s i o n i n C02. Under t h o s e c o n d i t i o n s (a.+ m ) t h e k i n e t i c m o d e l i n g o f coke r e g e n e r a t i o n depends m a i n l y on t h e adequate c h o i c e o f t h e k k i n e t i c parameter i n v o l v e d i n e q u a t i o n 1131 and on t h e a p p r o p r i a t e s p e c i f i c a t i o n o f t h e hydrogen c o n t e n t i n t h e coke f o r m u l a . SIMULATION AND MODELLING OF THE INDUSTRIAL UNITS When c o n s i d e r i n g t h i s t o p i c a b a s i c d i s t i n c t i o n between t h e d i f f e r e n t FCC r e p r e s e n t a t i o n s may observed: - mu1 t i v a r i a b l e search approaches i n v o l v i n g n o r m a l l y power s e r i e s w i t h f i t t i n g parameters w i t h o u t a p a r t i c u l a r p h y s i c a l meaning - phenomenological models i n c l u d i n g d i f f e r e n t i a l and a l g e b r a i c e q u a t i o n s w i t h
w i t h parameters having c l e a r p h y s i c a l sense. The mu1t i v a r i a b l e models were proposed i n few c o n t r i b u t i o n s (97), (98), (99) f o r searching t h e optimum FCC o p e r a t i n g c o n d i t i o n s . I n t h i s t y p e o f r e p r e s e n t a t i o n a major q u e s t i o n i s t h e number o f key o p e r a t i n g v a r i a b l e s and t h e i r i n t e r a c t i o n s t o be considered (97). For i n s t a n c e i t was claimed (98) t h a t a two v a r i a b l e p o l y nomial model i n v o l v i n g r e a c t o r temperature and mass f l o w v e l o c i t y o f t h e feedstock may be a u s e f u l t o o l f o r p r e d i c t i n g t h e coke f o r m a t i o n and optimum g a s o l i n e y i e l d s . Nevertheless, f u r t h e r cont r i b u t i o n s i n d i c a t e d t h a t a t l e a s t a four v a r i a b l e polynomial may be r e q u i r e d (99). The phenomenological approaches a r e c e r t a i n l y f a r more popular t h a t t h e mu1t i v a r i a b l e models. It seems t h a t broad agreement e x i s t s concerning t h e s t r a t e g y t o adopt f o r f u r t h e r progress i n t h i s f i e l d . More advances c o u l d be expected f o l l o w i n g t h e phenome n o l o g i c a l r o u t e f o r b o t h design and s i m u l a t i o n o f t h e FCC u n i t s . I n t h i s respect d i f f e r e n t c o n t r i b u t i o n s consider t h e r e a c t o r and t h e regenerator as i n d i v i d u a l e n t i t i e s . Those s t u d i e s search app r o p r i a t e d e s c r i p t i o n s o f each u n i t through a c a r e f u l c o n s i d e r a t i o n o f t h e k i n e t i c s , t h e f l u i d dynamic, t h e mass t r a n s f e r and t h e heat t r a n s f e r phenomena. For instance, t h e researchers concerned w i t h t h e c r a c k i n g r e a c t o r proposed phenomenological models f o r dense f l u i d i z e d bed crackers (1 OO), (57), (101 ) and l a t e r on considered r e p r e s e n t a t i o n s f o r t h e more advanced r i s e r u n i t s . Then t h e quick t r a n s i t i o n from dense phase f l u i d i z e d beds t o t r a n s p o r t e d beds was a c l e a r t r e n d on t h e process o f m o d e l l i n g t h e c r a c k i n g r e a c t o r . I n o p p o s i t e t h e regenerators models has been m a i n l y centered on dense f l u i d i z e d beds. The advances on t h i s s e c t i o n o f t h e FCC p l a n t t r i e d t o increase t h e understanding about t h e f a c t o r s c o n t r o l l i n g regenerator performance such as: j e t r e g i o n and entrainment phase (104),(93),(90),(105),(106),(107),(108),(44). When t h e c r a c k i n g r e a c t o r i s designed u s i n g t h e more advanced c r a c k i n g r i s e r techno1 ogy i n a d d i t i o n t o t h e b a s i c d e s c r i p t i o n of t h e c r a c k i n g process g i v e n by equation [I] t h e f o l l o w i n g hypotheses a r e adopted: - a d i a b a t i c r e a c t o r , acceptable simp1 i f i c a t i o n conside r i n g t h e heat losses, below 5%, measured i n a commercial u n i t (102), - p a r t i c l e and f l u i d c i r c u l a t i n g a t t h e same v e l o c i t y , q u i t e reasonable approach s t i l l i n curved r i s e r u n i t s (109) ,(110) t a k i n g i n t o account t h e 60pm t y p i c a l average p a r t i c l e s i z e , - temperature and hydrocarbon c o n c e n t r a t i o n s constant i n t h e r a d i a l d i r e c t i o n , due t o t h e h i g h degree o f turbulence i n t h e suspension, - p i s t o n f l o w model, adequate hypothesis (111) i f D/L r a t i o i s small enough. T h i s k i n d o f r i s e r r e a c t o r r e p r e s e n t a t i o n seems t o p r e d i c t f a i r l y we1 1 b o t h conversions and s e l e c t i v i t i e s o f a 30m l e n g t h commercial u n i t (1 02). However, an i m p o r t a n t assumption, used f r e q u e n t l y i n t h e r i s e r
c r a c k e r models (112), 1 umping t h e s i g n i f i c a n t volume expansion and t h e changing q u a l i t y o f t h e uncoverted f e e i n a second r e a c t i o n o r d e r ( 5 7 ) need f u r t h e r v e r i f i c a t i o n . A more s u i t a b l e approach c o r r e c t i n g t h e f l u i d - p a r t i c l e v e l o c i t y i n r i s e r r e a c t o r s was r e c e n t l y proposed by us (113).
F i g . 3 Schematic o f t h e f l u i d i z e d dense bed i n c l u d i n g t h e j e t r e g i o n , t h e bubbles and t h e emulsion phase.
F i g . 4 Schematic diagram o f t h e f l u i d i z e d bed i n c l u d i n g t h e freeboard region.
!ifhen t h e r e g e n e r a t o r i s i n t u r n s i m u l a t e d u s i n g t h e more advanced f l u i d i z e d bed approaches ( F i g . 3 and Fig. 4 ) t h e e q u a t i o n [ I 3 1 i s coupled t o a model i n c l u d i n g t h e f o l l o w i n g h y p o t h e s i s : a two phase model i s a p p r o p r i a t e f o r d e s c r i b i n g t h e gas c i r c u l a t i o n i n t h e bed - bubbles a r e r e p r e s e n t e d as DSTR, - t h e emulsion phase i s d e s c r i b e d as a CSTR, c o n s i d e r i n g t h e i n t e n s e s o l i d m i x i n g i n t h e bed (93) - t h e p i s t o n f l o w model i s adopted f o r t h e e n t r a i n m e n t phase (104) - t h e p a r t i c l e s e j e c t e d a t t h e bed s u r f a c e a r e r e l a t e d t o t h e bubble wake c h a r a c t e r i s t i c s (114) - t h e p a r t i c l e t r a j e c t o r i e s i n t h e f r e e b o a r d r e g i o n may be p r e d i c t e d n e g l e c t i n g p a r t i c l e - p a r t i c l e i n t e r a c t i o n s ( 1 15), ( 1 1 4 ) . T h i s t y p e o f model r e q u i r e s t h e e v a l u a t i o n o f a s e r i e s o f f l u i d dynamic parameters such as bubble size, j e t p e n e t r a t i o n , s p e c i f i c p a r t i c l e t r a j e c t o r i e s , j e t v e l o c i t i e s and bubble v e l o c i t i e s . Care must be t a k e n t o a p p r o p r i a t e l y e v a l u a t e these parameters i n o r d e r t o reduce t h e u n c e r t a i n t y of coke c o n v e r s i o n e s t i m a t i o n s . I t i s i m p o r t a n t t o mention t h a t these r e p r e s e n t a t i o n s have s t r o n g l y c o n t r i b u t e d t o e s t a b l i s h t h e r e l a t i v e i n f l u e n c e o f d i f f e r e n t regions i n t h e regenerator.
-
F i n a l l y some e f f o r t s have been addressed t o combine t h e r e -
a c t o r and the regenerator models i n t h e same way they a r e linked in t h e plant (1 16) , ( 1 1 7 ) , (118), (112). This kind of a n a l y s i s a l lows: - t o take i n t o account t h e important i n t e r a c t i o n s between t h e two u n i t s - t o design the a u x i l i a r y equipment required in the plant (117) - t o p r e d i c t important e f f e c t s such a s m u l t i p l i c i t y of steady s t a t e operating conditions (1 1 9 ) . We be1 ieve, however, t h a t much more progress i s required a t t h i s s t a g e in t h e combined reactor-regenerator simulation i n order t o achieve a re1 iabl e model of the overall FCC process. Nomenclature a A1 'A2 ,A3
constant r e l a t e d t o the power law c a t a l y s t decay function symbols representing gas oi 1 , gasoline and butane1i g h t gases-coke molecules, respectively b constant re1 ated t o exponen t i a1 c a t a l y s t decay function coke concentration (g coke/g c a t a l y s t ) Cc D diameter of the r i s e r r e a c t o r (cm) E energy of a c t i v a t i o n (Kcal/mol ) G aging parameter (1/ s ) (function of temperature) k overall k i n e t i c regeneration constant (Kmol gas/ Kmol 02.atm.s) k k k k k i n e t i c constants involved i n cracking reactions O' 2' k / ( l t o ) and o k ( 1 t o ) k i n e t i c constants f o r the regenerak43 t i v e reaction (Kmol O2 .atm.s) K deactivation constant (function of temperature) L t o t a l length of the r i s e r (cm) constants of the a c t i v i t y decay function n ,m P t o t a l pressure (atm) r ' r "r reaction r a t e s f o r the gas o i l consumption, gasoline production and coke consumption respectively (g converted/g c a t - s ) S number of moles of gasoline formed per mol of gas oi 1 converted c a t a l y s t on stream time and vapour residence time ( s ) tcytv W refractoriness weight f r a c t i o n s of gas oi 1 , gas01 i ne and oxygen y1 ,y2 ,y3 respectively Y time averaged weight f r a c t i o n Greek L e t t e r s a (J
4
a c t i v i t y decay constant (1/ s ) ( C O )/(CO) a c t i v i t y decay function
Superscri pts
'
molar property
References
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104. de Lasa, H.I., and J . R . Grace, "The I n f l u e n c e o f t h e Freeboard Region i n a F l u i d i z e d Bed C a t a l y t i c Cracking R e g e n e r a t o r " , AIChE J o u r n a l , 25; 984 (1979). 105. de Lasa, H.I. and A.F. Errazu, " I g n i t i o n o f a F l u i d i z e d Bed C a t a l y t i c Cracking Regenerator. Freeboard Region I n f l u e n c e " , Proceedings o f 1980 I n t e r n a t i o n a l F l u i d i z a t i o n Conference, e d . J .R. Grace and J.M. Matsen, Plenum Pub1 i s h i n g C o r p o r a t i o n , New York (1980). 106. de Lasa, H . I . , E r r a z u , A . , B a r r e i r o , E . and S. S o l i o z , "Analysis o f F l u i d i z e d Bed C a t a l y t i c Cracking Regenerator Models i n a Revamped U n i t " , American Chemical S o c i e t y Meeting, Las Vegas (1980) 107. de Lasa, H . I . , "Simulation o f and I n d u s t r i a l S c a l e Regenerator. I n f l uence o f t h e D i f f e r e n t C o n s t i t u t i v e F l u i d i z e d Bed Regions" . Proceedings World Chem. Engng. 54; 549 (1981). 108. Grace, J . R . , and H.I. de Lasa, "Reaction Near t h e Grid i n F l u i d i z e d Beds". AIChE J o u r n a l , 24; 364 ( 1 9 7 8 ) . 109. de Lasa, H . I . , Errazu, A . , P o r r a s , J . , and E . B a r r e i r o , " I n f l u e n c e o f t h e pneumatic t r a n s p o r t l i n e i n t h e s i m u l a t i o n o f a F l u i d i z e d Bed C a t a l y t i c Cracking Regenerator". Lat. J . o f Chem. Engng. 1 1 , 139 ( 1 9 8 1 ) . 110. Errazu, A. F., Porras , J . A . , and H. I . de Lasa, "Model 1 i n g a C a t a l y t i c Cracking Regenerator. I n f l u e n c e o f t h e Pneumatic T r a n s p o r t e d R i s e r " , X J o r n a d a s de I n g e n i e r i a Quimi c a . S a n t a Fe, Argentina (1978). 111. de Lasa, H.I. and G. Gau, " I n f l u e n c e des Agrgats s u r l e Rendement d,un r e a c t e u r a T r a n s p o r t Pneumatique", Chem. Engng. S c i . 28; 1875 ( 1 9 7 3 ) . 112. Wollaston, E . G . , H a f l i n , W.J., Ford, W . D . , and G.J. DISouza, "FCC Model Valuable Operating Tool", Oil and Gas J o u r n a l Sep., 87 (1975). 113. de Lasa, H . I . and L.K. Mok, " E n t r a i n e d Coal G a s i f i e r s ; Model i n g t h e P a r t i c l e A c c e l e r a t i o n " . AIChE Meeting, P h i l a d e l p h i a ( 1 9 8 0 ) . Can. J . Chem. Engng. 56; 658 ( 1 9 8 1 ) . 114. George, S.E., and J.R. Grace, "Entrainment o f P a r t i c l e s from Aggregative F l u i d i z e d Beds", AIChE Symp. S e r . , 74, 176, 6 7 (15173) 115. Do, H.T., Grace, J . R . , and R . C l i f t , " P a r t i c l e E j e c t i o n and Entrainment from F l u i d i z e d Beds", Powder Techno1 ogy , 6 ; 195 (1972) 116. Seko, H . , Tone, S . , and T. Otake, " C o n s i d e r a t i o n o f t h e Treatment o f coke d i s t r i b . u t i o n i n a f l u i d c a t a l y t i c c r a c k e r " , J . Chem. Engng., J a p a n , 1 0 ; 493 (1977). 117. Ewell, R . B . , and G. Gadner, "Design c a t c r a c k e r s by computer", Hydrocarbon P r o c e s s i n g , Apri 1 , 125 ( 1978) . 118. C o r e l l a , J . , B i l b a o , R . and J . Delgado Puche "Modelo Macrocinetico del Process de Craqueo Catal i t c o del Gas Oil en Lecho Fl ui dizado (FCC) en Estado E s t a c i o n a r i o " I n g e n i e r i a Quimi ca , October ( 1980).
119. Elnashaie, S.S.E.H. and I . M . El-Hennawi, " M u l t i p l i c i t y o f t h e Steady S t a t e i n F l u i d i z e d Bed Reactors - 4. F l u i d C a t a l y t i c C r a c k i n g (FCC) Chem. Engng. S c i . 34; 1113 (1979)
CONVERSION OF METHANOL TO GASOLINE OVER ZEOLITE CATALYSTS I. REACTION MECHANISMS
Eric G. Derouane Mobil Research and Development Corporation Central Research Division P. 0. Box 1025 Princeton, New Jersey 08540 U.S.A.
1.
INTRODUCTION
The conversion of methanol into hydrocarbons occurs over a large variety of acidic catalysts. These can be non-zeolitic compounds such as phosphoric acid, heteropolyacids, and silica-aluminas (1-5) or more commonly zeolites (6-11). The use of zeolite ZSM-5 as acid catalyst (8,12-25) is particularly attractive as it offers a new and viable route for the direct production of high-grade gasoline from methanol synthesized from coal or natural gas resources. Zeolite-based ZSM-5 catalysts are the key to Mobil's Methanol-toGasoline (MTG) process for which a commercial plant is presently being built in New Zealand. The MTG process has unique advantages: (a) Hydrocarbons are produced in a rather narrow compositional range; little methane and no hydrocarbons larger than C11 are formed. (b) A hioh conversion of methanol can be combined with a high selectivitv for aromatics and isoparaffins of high octane value. (c) ZSM-5 based catalysts are characterized by a very low aging rate. This paper reviews and discusses the major mechanistic aspects of the MTG reaction and delineates some of the factors which confer unusual catalytic properties to zeolite ZSM-5. An accepted overall reaction scheme consists in the r a ~ i dequilibration of methanol with dimethylether (DME), the production of light olefins from these reactants, and the conversion of the light olefins into higher molecular weight products by repeated alkylation with methanol or dimethylether (DME), followed by crackinq and classical conjunct polymerization (8,17,18,26). The following discussion will be
focused essentially on the still controversial mechanism by which the first C-C bond is formed, the autocatalytic nature of the methanol conversion, and the molecular shape-selective effects which explain the yield of a high octane product.
2.
METHANOL CONVERSION MECHANISM - THE ROUTE TO LIGHT OLEFINS
The most intriguing question associated with the MTG conversion is the reaction pathway which accounts for the first C-C bond formation from methanol or DME. Figures 1 and 2 summarize the essential steps which characterize some of the mechanisms proposed at this date. The hypothesis of carbene intermediacy (Figure 1A) was proposed by Chang and Silvestri (8), carbene intermediates being produced by a-elimination of water from methanol due to the cooperative actions of acidic and basic sites or methylene transfer occurring between two R-0-CH3 species. The latter two processes have, however, a low probability because of the low active site density in ZSM-5 or the constraints imposed on the bimolecular transition complex explaining the methylene transfer. It appears nevertheless that carbene-type C1 intermediates might play a role as suggested by 13c-isotope labeling studies of the conversion of methanol in the presence of added propane (27). Abnormally low iso/n-butane ratios are explained by a carbene-like species insertion into a s-p3 C-H bond (of propane), this suggestion being supported by the 13c isotope distribution in the Cq-products (27) and the observation of shape-selectivity effects which moderate the isomerization of isobutane into n-butane over HZSM-5 (28). Kaeding et al. (16) suggested an intermolecular reaction between a surface dimethyloxonium species and the (slightly) negatively charged methyl group of an incoming DME molecule, aided by a basic site on the catalyst surface (see Figure 1B). However, the basic properties of ZSM-5 have not been recognized at this time and basic sites, if present, would be very weak as a result of the strong catalyst acidity. Van den Berg et al. (29,30) have suggested a mechanism which transfers the known chemistry of tetramethylammonium cations to trimethyloxonium cations. As shown in Figure lC, the key step is the intramolecular, Stevens-like (31), rearrangement of a trimethyloxonium ion formed by the bimolecular addition of a methanol molecule to a dimethyloxonium cation. Questions about this reaction pathway relate again to the availability and role of basic sites on the zeolite surface. In addition, this type of rearrangement has not been observed, at least at present, in more classical reaction conditions such as homogeneous transformations in the presence of a strong base. Nevertheless, this mechanism appears most attractive
A. CARBENE INTERMEDIACY
B. INTERMOLECULAR REACTION OF DME WITH DIMETHYLOXONIUM SPECIES
C. INTRAMOLECULAR REARRANGEMENT OF TRIMETHYLOXONIUMIONS
-
Figure 1.
First C-C bond formation from methanol or DME.
D. METHYLCARBONIUM ION ATTACK ON METHANOL OR DME
8
8
ZO CH3
+
W30R2
-
[:>----CH2GR.]
@
e
ZO
-
CH3CH20R2+ ZOH
E. OXONIUM ATTACK ON METHANOL OR DM€
F. CARBENOID INTERMEDIATES
GAS PHASE CH;+
H$ i CH2: + H30f
ZEOLITE
Figure 2.
First C-C bond formation from methanol or DME (continued)
and deserves additional attention as it explains satisfactorily changes in the light olefinic products distribution as a function of reaction conditions (temperature, methanol/H20 ratio, etc.). C H ~ +species appear as unlikely intermediates as one would then logically expect the formation of large amounts of methane by hydride abstraction. This led Zatorski et al. (9) to postulate the occurrence of a radical mechanism (which has received no support so far) and Ono et al. (22) to envisage the attack of methylcarbonium ions on the C-H bond of methanol or DME via the formation of pentacoordinated carbon-species of Olah's type (32) (see Figure 2D), in a scheme resembling that originally proposed by Pearson (1). Such a mechanism implies that HZSM-5 possesses superacidic properties for which no evidence is presently available; it also ignores the recent observation (33) that the acid strength of the sites active for the methanol or DME conversion is intermediate between those required to isomerize ortho-xylene and disproportionate toluene.
In an effort to ascertain the inter- vs. intramolecular nature of the DME conversion, Van den Berg (30) compared the trialkylammonium vs. the carboxonium route to ethylene. These pathways are schematized in Figure 3. The initial step in route A is the protonation of DME while in route B a hydride is initially abstracted. Kinetic experiments, thermodynamic, and theoretical considerations favor route A and indicate that the formation of the oxonium ylide (reaction A2) is probably rate determining. This scheme has recently obtained some support from labeling experiments which confirm that the conversion of DME occurs via an intermolecular process and point out the important role played by ether intermediates in the methanol conversion (23). Consideration of the former mechanisms indicate that two essential points remain controversial, i.e., (i) the possible role played by carbenoid species and (ii) the nature of the initial oleSemi-empirical quantum mechanical calculations by finic product(s). Beran and Jiru (36) have suggested that methanol molecules, in the presence of a strong electric field such as it may exist in zeolites, can be transformed into CH20 and CH2 species. Hence, it is not unrealistic to consider the scheme shown in Figure 2F which suggests that an equilibrium could exist between a carbonium-type and a carbenoid species, that is between a surface methoxy group and a surface ylide. The internal energy variation corresponding to the deprotonation of CH3+ to CH2 (carbene) in the gas phase is of ca. 39 kcal mol-I (38); it is likely to be smaller for the same transformation occurring on the zeolite surface because of the electrophilic character of both the carbene and hydroxonium entities. It is conceivable that such an equilibrium could be shifted in one or the other direction depending on the actual consumption of the
ROUTE A
ROUTE B
Figure 3. Trialkyloxonium (A) vs. carboxonium route (B) for C-C bond formation (from reference 30).
C1-species, methyl carbonium ions being effective in alkylation reactions and carbenoid-moieties acting in carbene insertion processes or the formation of light olefins (21). Reacting methanol at low conversion can, in principle, give a hint into the nature of the primary olefinic products. Propylene has most often been suggested as the primary olefinic product (3, 23,29,30,33,37,39). Recently, Haag et al. (24) indicated that this conclusion could be affected by diffusion disguise, i.e., when the precursor species to ethylene (sometimes referred to as a surface C2 entity (17)) leads preferentially to sequential reaction products (intermediates) rather than to desorbed ethylene. This analysis meets the suggestion that the conversion of methanol occurs via a rake-type mechanism as depicted in Figure 4 in which ether intermediates play an essential role (23,401. It is now felt that ethylene is a major olefinic product when diffusion effects are reduced (24).
GAS PHASE MeZO
MeOEt
M20
MeOEt
MeOR
ADSORBED
PHASE
4 CZH;
-CI
MeOPr
C3H6
+&H,+ PROPAGATION
Figure 4. Rake-type mechanism for the conversion of methanol and dimethylether into hydrocarbons (adapted from reference 40).
Studies on the conversion of 1 3 labeled ~ methanol by HZSM-5 in the presence of propylene or 1-hexene by Dessau and LaPierre (41) have, however, indicated that ethylene was essentially produced by the cracking of higher olefins. It should, therefore, be considered as a primary methanol reaction product only during the initial initiation phases leading to C3+ olefins. The selectivity to the formation of light olefins is also determined by the structure of the zeolite used as catalyst, in particular its pore size. Cormerais et al. (33) have investigated the conversion of DME over H-erionite, HZSM-5 and H-Y zeolites. They found that smaller pore sizes led to higher yields in ethylene and propylene while the formation of isobutane and Cg+ hydrocarbons was strongly reduced as illustrated in Figure 5.
3.
THE AUTOCATALYTIC CONVERSION OF METHANOL
In the MTG conversion, the consumption of methanol rapidly increases as the hydrocarbon product concentration (olefins and aromatics) increases. The autocatalytic nature of the methanol conversion has been discussed by various authors (42-441, the analysis proposed by Chanq (44) predicting, in particular, conversion and selectivities over a wide range of pressures. Most of the results were obtained for HZSM-5 catalysts although their interpretation is probably not restricted to this particular zeolite.
7
-
I
1
0.51 I 1
%? %-ZSMJ
0.7 1
-
-
-
-
O
I
I , . ,
09
"
-
, S-A
PORE DIAMETER (nm)
Figure 5. Influence of pore aperture on the distribution of hydrocarbon products in the zeolite-catalyzed methanol/DME conversion: v, ethylene + propylene;X, Cg+ hydrocarbons; V , isobutane (from reference 33, with permission from Butterworth & Co., Pub.)
Early in-situ 13c-~MRstudies by Derouane et al. (15) suggested the occurrence of preferential reaction pathways by which methanol was consumed in alkylation reactions of olefinic and aromatic products. They were confirmed recently by complementary investigations of the reaction of methanol in the presence of ethylene using 1 3 ~ labeled reactants (45). Figure 6 shows I~c-NMRspectra of such reaction mixtures at various temperatures and conversions with either labeled methanol or ethylene. Clearly, methanol is essentially used up in aliphatic chain growth or aromatic ring alkylation. The ethylene conversion, which is inhibited by the presence of methanol at temperatures below 200°C, yields ultimately higher olefinic and aromatic products. In support of the autocatalytic conversion of methanol or DME, Van den Berg (30) observed that two routes are operative in the conversion of DME. At low temperature, trialkyloxonium ions are mostly formed via condensation reactions of methanol and DME while above 280°C reaction of dimethylether or methanol with alkyl cations, i.e., alkylation processes, become predominant thereby offering a more direct and efficient route for the conversion of the oxygenated
reactants. Perot et al. (23) have compared the reactivities of ethylene and propylene, both in the presence and absence of DME. At 3 5 0 ° C , propylene alone is about 20 times more reactive than ethylene as reported qualitatively earlier (46). A mixed ethyleneDME feed has essentially the same reactivity as DME, as also concluded by Gilson (45), while that of a mixed propylene feed exceeds the sum of the individual reactant reactivities by nearly 5 0 % , an observation which also explains in part the autocatalytic methanolDME conversion.
8 I3
Q
CH30H+C2H4
CH,OH
0 +13~,~4
c ~ H ~ + OH ~ ~ c H ~
Treatment 5 MIN. AT 25OC
5''"AL JL 123,s
ETHERS
ETHERS
5 MIN. AT 350°C
OLEFINS 1238 AROMATICS 145,6
59*7
125.7~
-CH~-,-CH)
ETHERS
Figure 6. In-situ l3~-NMRspectra of labeled methanol-ethylene mixtures reacted over HZSM-5 (from reference 45).
4.
THE FORMATION OF AROMATIC COMPOUNDS
It has been well established that C2-5 olefins could act as precursors of aromatic compounds (17,18). Reaction pathways are essentially analogous to those describing conjunct polymerization (47,481, that is, the conversion of olefins (by oligomerization and/or cyclization) into naphthenes and the dehydrogenation of naphthenes into aromatics by hydrogen transfer reactions of the type: 1 Naphthene ( c ~ H + ~ ~ 3 ) Olefins (CnH2,)
1 Aromatic (Cn~2n-6)+ 3 Paraffins ( c ~ H ~ ~ + ~ ) This scheme predicts a stoichiometric paraffinic/aromatic ratio of 3 which is effectively observed in a normally run MTG conversion. Figure 7 schematizes the aromatization pathway of olefins, using propylene as a model reactant. UV-spectroscopy allows the identification of the carbocations intermediates and shows that cyclopentenyl carbocations are first formed. Cyclohexenyl species, aromatics, and (po1y)alkylaromatics then appear successively (18).
+R' 8 =CH-CH3~CH3-C=CH=CH-CH3+C~3-$=c~-c~=c~, I Cn3 HYORIOE TRANSFER C H ~ CHS Cg OLIGOMER Ll NEAR * - A U K D E
-He +CH3-CH-CH
F;"2
%cn2;: -RH
c
~ , d 7= c n ; s ~ ~ = ~ty'9':~~+ 8
CYCLIZATION
CH3 PENTADlENYL CATION
CYCLOHEXADIENE
H2C-CH CYCLOPENTENYL CATION
PROTONATED BENZENE
CH' CYCLOHEXENYL CATION
BENZENE
Figure 7 . Typical intermediates in the conversion of olefins to naphthenes and aromatics (from reference 18, with permission from Elsevier Pub. Co. ) .
Restricted transition-state shape-selectivity has been claimed as an essential factor to explain the high iso/normal paraffins ratio in the aliphatic product and the cut-off at C10-~l in the aromatics distribution (49,50). C4-6 olefins are obtained by oligomerization and/or alkylation of the primary olefins and by cracking of higher olefinic products. Oligomerization (C3-5) and dehydrocyclization (Cg-10) of the olefins occur at the channel intersections of HZSM-5 where molecular shape-selectivity constraints act on the formation of intermediate complex structures (50). Steric inhibition, limiting the accommodation of isoaliphatics at these intersections (51) and precluding the accommodation of bimolecular transition complexes of more than ten carbon atoms (49), appears as a decisive parameter. Haag et al. (24) have shown that the distribution of aromatic compounds resulted from the combination of two factors: the methanol alkylation of lower aromatics which is very effective at low methanol conversion and diffusion limitations (product selectivity) which stem from the comparable sizes of the zeolite pores and of the methylaromatics. Evidence for such diffusion constraints was gained from the multiple labeling of the polymethylbenzenes produced in the unlabeled tol~ene-~3~-methanol reaction (25). Molecular traffic control (52,53,54) which directs the flows of (small) reactant and (larger) product molecules in the zeolite channels was mentioned as a likely reason for the absence of major counterdiffusion limitations. Some support seems to have been gained for its existence in comparative studies of the methanol conversion and of the para-xylene alkylation by methanol over HZSM-5 and HZSM-11 catalysts (20). These data need, however, to be further assessed. As discussed elsewhere (50,52), alkylaromatics cannot be converted further in ZSM-5 (by dehydrocyclization and subsequent alkylation). It explains its low coking activity and its unusually high stability as a methanol conversion catalyst.
5.
CONCLUSIONS
The conversion of methanol-to-gasoline-type hydrocarbons occurs via a well-established network of sequential and parallel reaction steps. Among those, the formation of the primary olefins, i-e., of the first C-C bond, still remains controversial. An attractive, usually well-accepted, mechanism postulates trimethyloxonium ions as intermediates while new evidence was recently presented to support the hypothesis of carbenoid intermediacy. In any event, this reaction step is slow and it is followed by competitive and very fast subsequent reactions. Ether-type species are found to play an important role in the autocatalytic conversion of methanol.
The latter is also readily consumed in aromatic alkylation reacti ons. The formation of aromatics occurs via classical reaction pa1 thways which have received ample support. Restricted transition-state shape-selectivity is found to play a role in the oligomerization/dehydrocyclization of olefins into naphthenes and aromatics while diffusion/desorption disguises clearly affect the distribution of aromatic compounds. REFERENCES D.E. Pearson, J. Chem. Soc. Chem. Corm., (1974) 397. L. Kim, M.M. Wald, and S. G. Brandenberger, J. Org. Chem., 43 (1978) 3432. B.J. Ahn, J. Armando, G. Perot, and M. Guisnet, C.R. Acad. Sci. Paris, 288 C (1979) 245. T. Baba, J. Sakai, and Y. Ono, Bull. Chem. Soc. Jpn., 55 (1982) 2657. T. Baba, J. Sakai, H. Watanabe, and Y. Ono, Bull. Chem. Soc. Jpn., 55 (1982) 2555. P.B. Venuto and P.S. Landis, Advan. Cata1.-Relat. Subj., 18 (1968) 259. K.V. Topchieva, A.A. Kubasov, and T.V. Dao, Vestn. Mosk. Univ. Khim., 37 (1972) 620; Khimyia, 27 (1972) 628. C.D. Chang and A.J. Silvestri, J. Catal., 47 (1977) 249. W. Zatorsky and S. Krzyzanowski, Acta Phys. Chem., 24 (1978) 347. M.S. Spencer and T.V. Whittam, Acta Phys. Chem., 24 (1978) 307. M.S. Spencer and T.V. Whittam, J. Molec. Catal., 17 (1982) 271. S.L. Meisel, J.P. McCullough, C.P. Lechthaler, and P.B. Weisz, Chem. Tech., 6 (1976) 86. S.E. Voltz and J.J. Wise, "Development Studies on Conversion of Methanol and Related Oxygenates to-&solinen, Final Report, DOE Contract No. E (49-18)-1773 (1976). A.Y. Kam and W. Lee, "Fluid-Bed Process Studies on Selective Conversion of Methanol and Related Oxygenates to Gasoline", Final Report, DOE Contract No. Ex-76-C-01-2490 (1978). E.G. Derouane, J.B.Nagy, P. Dejaifve, J.H.C. Van Hooff, B.P. Spekman, J.C. Vedrine, and C. Naccache, J. Catal., 53 (1978) 40. W.W. Kaeding and S .A. Butter, J. Catal., 61 (1980) 155. P. Dejaifve, J.C. Vedrine, V. Bolis, and E.G. Derouane, J. Catal., 63 (1980) 331.
18. J.C. Vedrine, P. Dejaifve, E.D. Garbowski, and E.G. Derouane, in "Catalysis by Zeolites", Stud. Surf. Sci. Catal., Vol. 4, B. Imelik et al., eds. (Elsevier Sci. Pub. Co., Amsterdam, The Netherlands, 1980), p. 29. 19. P. Dejaifve, A. Auroux, P.C. Gravelle, J.C. Vedrine, Z. Gabelica, and E.G. Derouane, J. Catal., 70 (1981) 123. 20. E.G. Derouane, P. Dejaifve, Z. Gabelica, and J.C. Vedrine, Faraday Disc. Chem. Soc., 72 (1981) 331. 21. E.G. Derouane, P. Dejaifve, J.P. Gilson, J.C. Vedrine, and V. Ducarme, in "Abstracts 7th North American Meeting of the Catalysis Society", Boston, Massachussetts, October 11-15 (1981). 22. Y. Ono and T. Mori, J. Chem. Soc. Faraday Trans. I, 77 (1981) 2209. 23. G. Perot, F.X. Cormerais, and M.Guisnet, J. Molec. Catal., 17 (1982) 255. 24. W.O. Haag, R.M. Lago, and P.G. Rodewald, J. Molec. Catal., 17 (1982) 161. 25. R.M. Dessau and R.B. Lapierre, J. Catal., 78 (1982) 136. 26. V.N. Ipatieff and H. Pines, Ind. Eng. Chem., 28 (1936) 684. 27. C.D. Chang and C.T. Chu, J. Catal., 74 (1982) 203. 28. P. Hilaireau, C. Bearez, F. Chevalier, G. Perot, and M. Guisnet, Zeolites, 2 (1982) 69. 29. J.P. Van den Berg, J.P. Wolthuizen, and J.H.C. Van Hooff, in "Proc. 5th Int. Conf. Zeolites", L.V.C. Rees, ed. (Heyden and Sons, London, 1980) ; p . 649. 30. J.P. Van den Berg, Ph. D. Thesis, Technical University of Leiden, 1981. 31. T.S. Stevens and W.E. Watts, in "Selected Molecular Rearrangements" (Van Nos trand Reinhold Pub. Co , London, 1973). 32. G.A. Olah, G. Klopman, and R.H. Schlosberg, J. Amer. Chem. Soc., 91 (1969) 3261. 33. F.X. Cormerais, Y.S. Chen, M. Kern, N.S. Gnep, G. Perot, and M. Guisnet, J. Chem. Research (S) , (1981) 290. 34. D. Kagi, J. Catal., 69 (1981) 242. 35. C.D. Chang, J. Catal., 69 (1981) 244. 36. S. Beran and P. Jiru, React. Kinet. Catal. Lett., 9 (1978)401. 37. S. Ceckiewicz, J. Coll. Interface Sci., 90 (1982) 183. 38. J.G. Fripiat and E.G. Derouane, unpublished results. 39. C.D. Chang, W.H. Lang, and R.L. Smith, J. Catal., 56 (1979) 169. 40. F.X. Cormerais, G. Perot, F. Chevalier, and M. Guisnet, J. Chem. Research (S) , (1980) 362. 41. R.M. Dessau and R.B. Lapierre, J. Catal., 78 (1982) 136. 42. N.Y. Chen and W.J. Reagan, J. Catal., 59 (1979) 123. 43. y. Ono, E. Imai, and T. Mori, Zeit. Phys. Chem., N.F.,115 (1979) 99.
.
Chang, Chem. Eng. S c i . , 35 (1980) 619. G i l s o n , Ph. D. T h e s i s , U n i v e r s i t y o f Namur, 1982. J . C . V e d r i n e , P. D e j a i f v e , C . Naccache, and E.G. Derouane, P r o c e e d i n g s V I I t h I n t e r n . Congr. C a t a l . , Tokyo (1980). V.N. I p a t i e f f and H. P i n e s , I n d . Eng. Chem., 28 (1936) 684. M.L. Poutsma, i n " z e o l i t e Chemistry and C a t a l y s i s 1 ' , J . A . Rabo, ed. (American Chemical S o c i e t y , Washington, D . C . , (1976), p. 488. E.G. Derouane and J . C . Vedrine, J. Molec. C a t a l . , 8 (1980) 479. E.G. Derouane, i n " C a t a l y s i s by Z e o l i t e s " , S t u d . S u r f . S c i . C a t a l . , Vol. 4 , B. I m e l i k e t a l . , e d s . ( E l s e v i e r S c i . Pub. Co., Amsterdam, The N e t h e r l a n d s , 1 9 8 0 ) , p. 5 . J. Valyon, J. M i h a l y f i , H.K. Beyer, and P.A. J a c o b s , i n "Proceedings Workshop o n A d s o r p t i o n l ' , B e r l i n , 1979 ; 1 (1979) 134. See l e c t u r e by E.G. Derouane on "Molecular S h a p e - C a t a l y s i s by Z e o l i t e s 1 ' , t h i s volume. E.G. Derouane and Z . G a b e l i c a , J. C a t a l . , 65 (1980) 486. E.G. Derouane, Z . G a b e l i c a , and P.A. J a c o b s , J . C a t a l . , 70 (1981) 238. C.D. J.P.
CONVERSION OF METHANOL OVER ZEOLITE CATALYSTS I1 INDUSTRIAL PROCESSESS
Zelimir Gabelica Facultgs Universitaires de Namur Dgpartement de Chimie, Laboratoire de Catalyse Rue de Bruxelles, 61, B-5000 Namur, Belgium 1
.
INTRODUCTION
In view of the latest critical energy situation in the Western world, extensive research is being conducted to reconsider the potentialities of other non-petroleum materials to become important sources of fuels. In particular, considerable efforts are expanded to develop and improve the various existing technologies for the conversion of coal, heavy petroleum residues, tar sands, shale and biomass, to gasoline. The Bergius ( 1 ) and Fischer-Tropsch (2) processes are among the best known that are currently used to produce gasoline from coal. In the Bergius process (coal liquefaction),. a synthetic crude is produced by slurrying finely divided coal with recycle oil, in the presence of iron catalyst, then by hydrogenating the mixture at high pressure and temperature: '320, 02
-1
H2
gasification)
Coal
+
lliquefact ion1
-
refiner
The product mixture resulting from this liquefaction process requires extensive and costly hydrogenative upgrading to provide high quality fuels. Such processes were used extensively in Germany during world war I1 but no more Bergius plants exist today. Other coal liquefaction-derived processes, such as Synthoil (a variation of the Fischer-Tropsch process, producing a mixture of
gasoline-diesel) are now being developped and large pilot plants are in preparation or under construction. However, these processes still yield hydrogen-deficient products which are not desirable for a high grade gasoline. The classical Fischer-Tropsch approach consists in pro-ilucing motor fuel from coal. The latter is firstly gasified to synthesis gas (CO + H2), which is catalytically converted into products. These consist of a wide spectrum of hydrocarbons (C1 to C40, or higher) and oxygenates covering a broad range of alcohols, ketones, acids and esters: Coal
I hydrocarbon]-* SNG -LPG +Gasoline -'Fuel Oil
Again, products need to be adequately separated and upgraded to produce a high quality gasoline. Such a processing includes costly hydrogenation, reforming, (hydro)cracking or dewaxing procedures. Nevertheless, these technologies are now well established and the by-products are used as raw materials in a variety of industrial, extremely important, secondary processes (3). Plants producing gasoline from coal using the Fischer-Tropsch principle are being working currently in South-Africa. Methanol, a potential motor fuel, is one among the products readily formed in large amounts, from coal or synthesis gas, by existing technologies (4,5). It can be either used directly as a fuel in automotive engines (6),or be blended with gasoline (7,8). Although methanol presents some minor advantages over gasoline, such as reduced emissions, significant difficulties are encountered when it is used alone as fuel. For example, low temperature starting is difficult. Methanol also remains highly corrosive to many materials used in classical engines so that modified systems need to be developped. Other safety and toxicity problems specific to methanol, used either alone or as a component to gasoline, are encountered, some of them being quite costly to solve adequately. ~ethanol's high sensitivity to water, its corrosiveness, its toxicity, its low volumetric energy content and its uriusual volatility, are some of these disadvantages. A methanol-gasoline mixture enhances the fueloverall octane quality but significant problems related to performance and reliability must be solved. The formation of hydrocarbons from methanol was first reported over classical NaX zeolites (10,ll) and, since then, over a large variety of acidic zeolitic or non-zeolitic catalysts (12).
A novel process for the straightforward conversion of methanol to gasoline over ZSM-5 zeolite catalyst has been recently developped by Mobil Oil Co. (9,13). It is commonly referred to as
"
~ethanol-to-~asoline " (MTG) process.
Crude methanol, such as produced from synthesis gas using the current commercially available methanol technology (5), is rapidly and quantitatively converted into olefines, (cyc1o)paraffins and (substituted)aromatics boiling in the gasoline range: Coal Oxygen s t e a m < m gasified reforming +Steam
-
p
Synthesis L
a
'
-1
Steam
Idehydration,
/
1
1-~uel gas
Crude Methanol (at 17XH20)
The process is highly selective and can be schematized by the following overall reactions: -H20 -H20 iso (paraffins) eCH30H C H ~--&HC O C2 - C5 cycloparaffins +H20 ole£ins aromatics The MTG process is distinct from the former conventional discoveries in that: a) Essentially isoparaffins and monocyclic methyl-substituted aromatic hydrocarbons are produced b) A sharp cutoff at C10 is observed in the hydrocarbon product distribution c) Deactivation by coke deposition is slow. The major mechanistic considerations about MTG reactions are reviewed and discussed in the part I of this work (12). The present part I1 summarizes some of the most important technological informations on general industrial processes concerning the MTG conversions.For more details, the reader is referred to technical reparts (20,21,23,28), reviews (1-9,13,14,16,22,27,41, 42) or related ~ublications in current (1 1,12,15,18,29,30,34-39) or patent (10,17,24-26,31-33,40,43) literature.
2. METHANOL CONVERSION OVER H-ZSM-5 : THE MOBIL PROCESS Crude methanol (or selected oxygenates) obtained from coal or natural gas by the above sketched classical routes may be converted to gasoline on various (shape selective) zeolitic catalysts, among which those of H-ZSM-5 type are currently preferred. The active form of ZSM-5 cayalyst is obtained by heating the initially synthesized precursor in inert atmosphere at 550-600" C, followed by exchange with diluted mineral acids or ammonium salts and recalcination in air at 570" C for 24 h (15). Theconversionproceeds by the schematic route depicted in the introduction part. In a single pass, methanol is converted at 99 % to a mixture of hydrocarbons, the composition of which is shown in table 1. Table 1 Typical hydrocarbon distribution in the MTG process over H-ZSM-5 (fixed-heat reactor, V H S V = O . ~ ~ -p=205 ~, psig and inlet temperature=700e F (from ref 9). Products Methane, ethane, ethene Propane Isobutane n-butane Propylene and butenes Cg'non-aromat ics Aromatics
wt % 1.5 5.6 9.0 2.9 4.7 49.0 27.3
Essentially no hydrocarbons are produced above C11, which corresponds to the end point of conventional gasoline. Some 25 % of the hydrocarbons are gases, among which less than 2 % is methane or ethane. This unique narrow range of product molecular weights is consistant with the constrained structure of the zeolite. Hydrocarbons higher than C 1 1 cannot escape from the catalyst; only subsequent isomerization and rearrangements reactions allow them to escape with a lower melecular weight. This is a convenient advantage for gasoline manufacture, since no further distillation step to remove heavy ends is required. About 75 % of the hydrocarbons produced are in the Cg+ gasoline fraction. A significant amount of additional gasoline can be produced by alkylating C3 and C4 olefins with isobutane. The resul-
ting mixture is rich in isoparaffins and aromatics, with an unleaded Research Octane Nomber (RON) of 90 to 100. This quality is far superior to that stemming from a classical Fischer-Tropsch procedure (table 2). Table 2 Comparison of MTG conversion on ZSM-5 with Fischer-Tropsch technology (from ref 16) Product Light gases (Cl+C2) LPG (C3+C4) Gasoline (C5-C 1 l) Fuel Oil (CI Oxygenates Octane of gasoline (clear)
Fischer-Tropsch Fixed bed ~luidizedbed
MTG
23 29 34 5 9 100
2 22 76
11 11 25 51 2 100
-
-
75
100
95
The highest molecular weight member of the alkyl aromatics series which can escape from the catalyst, is durene (1,2,4,5-tetramethylbenzene). This compound, while having a high octane blending number, melts at 80" C and may potentially give carburator problems at high concentrations. Although its appearence in the product mixture is marginal, its production rate must be controlled by using various parameters. For example, lower durene yields (as well as a higher resistance of the catalyst to coke deposition) are achieved by embedding the active phase in a binder that is preferably an Al-free material (17). This improvement proved particularly useful for the MTG conversion conducted at superatmospheric pressures, which enhance the formation of polymethylbenzenes, among which durene (1 8) (see 4.2, table 6). A comparison of ZSM-5 with other zeolites for the MTG conversion is shown in table 3.
Table 3 Comparison of ZSM-5 with other catalysts in the MTG conversion (from ref 16) Montmorillonite % conversion of oxygenates
Narrow page ZSM;5 Large pore Zeolite(5Aj ( 5.6A) ~eolite(9-10~:
0.4
1 1 .O
1 00
100
-
-
100
18.8
HC distribution (wt %) Cl-c4 C5 aliphatics Aromatics
23.7 48.9 27.4
-
93.8 3.9 2.3
3. PROCESS DEVELOPMENT
A major problem in running the methanol conversion is the disposal and/or removal of the heat of reaction (about 406 kcal (13,2 kkal) released per kg (mole) of converted CH30H), which could lead to an adiabatic temperature rise as high as 600' C. Various types of reactors have been described which enable the best use of the exothermic heat. 3.1. Two stage fixed bed reactor (14.19-23)
The schematic Mobil fixed-bed ~ i l o tplant is shown in fig. 1. Crude methanol (17% H201
t
b Gas product
__t
Hydrocarbon liquid product
Water
Figure 1. Schematic of Mobil fixed-bed pilot plant
In the first dehydration reactor, methanol is converted on a conventional dehydration catalyst such as y-A1203 to an equilibrium mixture Hz0 - CH30CH3 - CH30H. These products, which leave the reactor at 360' C, are diluted with recycle gas and pass through the second reactor containing the ZSM-5 based catalyst, where it is converted into hydrocarbons. The reactor effluent is condensed and the water separated from the liquid hydrocarbon. The purification of the latter can be achieved by conventional separation methods. A rather high recycle ratio is necessary to reduce the adiabatic temperature rise to 50 - 110' C under typical operatory conditions. Methanol is converted to hydrocarbons (44 wt %) and water (56 wt W) in nearly stoechiometrical amounts. Small amounts of CO , C02 and coke are formed as by-products. Only the catalyst in the second reactor requires ~eriodicoxidative regeneration to maintain catalyst activity (21,23). Fixed-bed pilot plant studies were completed and their test results reported with respect to long-term catalyst aging (23).
A variant of this system consists in controlling the highly tlexothermic olefin formingreaction stepphich accounts for about 40 % of the reaction heat. In such a process, the reactor tubes in the conversion reactor are adjusted in number, size and length as to perform the adequate exchange capacity. A facility is provided to upgrade the recycle gas (C3 - Cg olefins) together with the product stream coming from the dehydration reactor. The complete scheme of such a "Tube Heat Exchanger Reactor" is detailed in the patent literature (24). Typical process conditions and yields are shown in table 4. Table 4 Yields from methanol in fixed-bed reactor system (28) Temperature, Inlet Outlet
OC
Pressure, kPa Recycle Ratio (mole) Space Velocity (WHSV) Yields, Wt 2 of Methanol Charged Methanol + Ether Hydrocarbons Water co, C02 Coke, Other
Table 4 (continued) Hydrocarbon P r o d u c t s , W t % L i g h t Gas Propane Propylene i-Butane n-Bu t a n e Butenes C5 + G a s o l i n e Gasoline LPG F u e l Gas
It a p p e a r s t h a t t h e hydrocarbon p r o d u c t i s p r i r h a r i l y g a s o l i n e . I t s y i e l d , on t h e b a s i s of t h e t o t a l hydrocarbons produced, i s 85 % W t . Butenes and propene a r e a l k y l a t e d w i t h i s o b u t a n e , t h e amount of t h e l a t t e r b e i n g l i m i t e d by t h e y i e l d of t h e former. Excess i s o b u t a n e i s p u t i n t o LPG ( y i e l d = 1 3 . 6 % Wt). The remaining 1 . 4 % W t i s l i g h t g a s which c o u l d b e used a s f u e l g a s .
3.2. F l u i d bed r e a c t o r s Methanol may a l s o b e c o n v e r t e d i n f l u i d - b e d r e a c t o r s i n a s i n g l e s t e p (21,23,25,26). T h i s p r o v i d e s means f o r c o n t i n u o u s regen e r a t i o n of t h e c a t a l y s t , s o t h a t s t e a d y s t a t e o p e r a t i o n a t n e a r l y c o n s t a n t c a t a l y t i c a c t i v i t y can b e r e a c h e d (compared t o t h e c y c l i c o p e r a t i o n of a f i x e d - b e d ) . With a f l u i d bed, t h e h e a t of r e a c t i o n i s removed by u s i n g h e a t exchange c o i l s w i t h i n t h e bed. Heat t r a n s f e r i s s o f a s t t h a t d a n g e r of l o c a l o v e r h e a t i n g i s minimal. By oper a t i n g a t n e a r l y i s o t h e r m a l c o n d i t i o n s , w i t h s i g n i f i c a n t l y lower r e c y c l e t h a n t h e f i x e d bed system, h i g h e r g a s o l i n e y i e l d s a r e obtained. The scheme of a s m a l l s c a l e f l u i d bed r e a c t o r i s shown i n f i g . 2. The r e a c t o r i s pre-heated by a n e l e c t r i c a l r e s i s t a n c e f u r n a c e . Dur i n g o p e r a t i o n , c h a r g e s t o c k and N2 ( c a r r i e r g a s ) a r e pumped through a p r e h e a t e r c o i l where t h e c h a r g e i s v a p o r i z e d . The v a p o r t h e n pass e s t h r o u g h t h e d i s t r i b u t o r and c o n v e r s i o n o c c u r s w i t h i n t h e d e n s e f l u i d bed c o n t a i n i n g f o u r b a f f l e s t o minimize by-passing. Gaseous p r o d u c t s a r e c a r r i e d o u t of t h e r e a c t o r and t h e mixed H20-hydrocarbon p r o d u c t c o l l e c t e d a f t e r c o n d e n s a t i o n . The g a s e s a r e a n a l y z e d by g a s chromatography a s d e s c r i b e d e l s e w h e r e ( 1 4 , 2 9 ) . The 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 from a f l u i d bed p i l o t u n i t i s g i v e n i n t a b l e 5.
SMALL SCALE FLUID-BED REACTOR
,
Catalyst Fill
Reheater
-
Disengager
\ Reactor
Gas
Liquid Product
DlstributoCatalyst Draln
F i g u r e 2 S c h e m a t i c of f l u i d i z e d d e n s e bed p i l o t p l a n t
Table 5 Y i e l d s from Methanol i n 4 B/D Fluid-Bed Average Bed T e m p e r a t u r e , P r e s s u r e , kPa Space V e l o c i t y (WHSV)
O
C
Y i e l d s . W t % o f Methanol Charged Methanol + E t h e r Hydrocarbons Water CO, C02 Coke, O t h e r Hydrocarbon P r o d u c t , W t % L i g h t Gas Propane Propylene i-Butane n-Butane Butenes
P i l o t U n i t (28)
53 8 Table 5 (continued) C5 + Gasoline
-- - -- - - - - --- --- - - - - - -
60.0
- - - -l_oO*-0--- - -
Gasoline (including Alkylate)
LPG Fuel Gas
-
88.0 6.4 5.6
Fluid bed produces more light olefins and less Cg'gasoline, with respect to the fixed bed system. However, when propylene and butenes are alkylated with isobutane, the final gasoline yield is superior. The principal reasons for the preferred choice of a fluid bed process are the following : a) The superior heat transfer characteristics of the latter which would simplify the heat removal. b) The fluid-bed, when coupled with alkylations, gives a higher gasoline selectivity. c) Catalyst activity in a fluid-bed reactor can be controlled at the optimum gasoline selectivity level by continuous regeneration. An improved fluidized bed was recently described (32). The catalyst, stored in the upper region of the bed at elevated temperature, is transferred to a heat exchanger. The latter is advantageously used to preheat the reactors. That means for example that liquid methanol is charged and vaporized before being fed in the mixing zone.
4.
EFFECT OF PROCESS VARIABLES (14,27)
4.1. Temperature The effect of temperature on the methanol conversion was discussed by Chang et al. (30). The yields of Cg* aliphatics and aromatics show a maximum in the 370-440 C temperature range, at atmospheric pressure. Above 350' C, the reaction rate increases by a factor of 1.5 for each 28' C (21,23) and the production of durene decreases at higher temperatures. Finally the steady state operation of the process is greatly improved by feeding methanol preheated to a selected temperature close to its boiling point, so that the heat needed for its vaporization and rise to reaction temperature is nearly equal to the isothermic heat of conversion (31). O
4.2. Pressure Increasing pressure results in higher methanol conversion, increased gasoline yield and durene formation (9,18,21,23,33)(table6).
Table 6 Effect of pressure on the MTG conversion (700' F, VHSV=1.5h-I, conversion 98-99 % Fixed bed). (9).
.
Pressure C/R~ % durene in HC
0 59 0.375 0.415 3.2 4.4
169 0.450 9.4
353 0.463 9.5
720 0.561 20.5
-
( a ) =(C ~ ~ in ~ aromatic side chainsY(C in aromatic rings) In addition, the catalyst activity is shown to be inhibited at higher pressures by either the adsorption of reactants or products on its surface or by mass transport(diffusion)limitations. Chang et al. also noted an increase in secondary alkylation reaction at higher pressure (9,33). The effect of varying pressure is, as a whole, to change the selective rates of the dehydration and conjunct polymerization steps in the reaction sequences. Increasing pressure increases the overlap of the two reactions and promotes the formation of higher (polymethyl) aromatics, such as the unwanted durene.These effects are shown in fig. 3. More recently, Chang (34) has proposed a kinetic model which describes the methanol conversion path and predicts conversion and selectivities over a wide pressure range (0.04-50 atm)
.
4.3. S ~ a c evelocitv Equilibrium between water, dimethylether and methanol is reached rapidly (fig. 3 - pressure 1 atm). Light olefins appear next and aromatics are formed together with aliphatics,as they are related to hydrogen transfer reactions. These effects are discussed in more detail elsewhere (12). 4.4. Water partial pressure The nature of the product distribution can also be altered by varying the water partial pressure at the inlet of the conversion reactor. In agreement with proposed mechanisms (12), an increase in Hz0 partial pressure essentially decreases the selectivity to aromatics (33) and leads to more light olefins by levelling the acidity of the catalyst and by competing for the strong acid sites,which are responsible for hydrogen transfer reactions. Intracrystalline steam distillation effects could also accelerate the desorption of the less volatile products from the zeolite particles (35). Durene yields can also be minimized by recycling it to the inlet of the conversion reactor,where isomerizations and dealkylations are favoured (23).
LHSV-', h r
Figure 3.Pressure and space velocity effects o n the MTG reactions (18).
5.
AGING AND REGENERATION OF THE REACTOR SYSTEMS
Short -~e-qpa g i n g - -tests (2 1,23) have indicated that the methanol conversion decreases during each cycle (fixed bed). The fraction of light hydrocarbons and the olefins to paraffins ratio decrease while the aromatics concentration increases in parallpel. Si~ g gshown g<e~f that f ~ the durene and coke milarly, ~ g ? g _ t _ e _ e ~ m ~ - ~ g ihave formation is reduced. These effects may be understood by considering that aging (deactivation by deposition of carbonaceous residues) will eventually enhance the apparent shape selective properties of the zeolite. For H-ZSM-5, coking essentially occurs at the external surface (36). Pore mouth restrictions will partially occur (37) and aromatics such as p-xylene will be produced with an increasing yield (36,38,39). Although ZSM-5-based catalysts show an unusually high resistance to coking (36), they nevertheless have to be regenerated either periodically (fixed bed) or continuously (fluid bed). Fixed bed regeneration can be achieved by controlled treatment with air or steam at 340-480' C (21,23). The heat due to the exothermic reaction can be more homogeneously redistributed by mixing the active zeolite with an inert phase, the mixture moving downwards to the regeneration zone (26). Fluid bed conversion-regeneration units require the use of special technologies which are described in detail in the patent literature (25). A simple, fast and very effective way to regenerate an aged catalyst has been disclosed (40). It is called "selictivation" and may be considered as a pre-coking process of a fresh catalyst. The latter is activated in a hydrogen-toluene atmosphere. The resulting coking induces pore mouth restrictions in ZSM-5,which maintain an increased molecular shape selective activity of the zeolite and avoids the need for further controlled cokeredeposition pretreatment.
6.
FUTURE DEVELOPMENTS OF THE MTG PROCESS
The actual use of the Mobil process to make gasoline will depend on competitive economies with other synthetic fuel processes. Technical and economical comparisons have been made at the end of the last decade (41), on the basis of the Mobil pilot plants information, with the commercially available SASOL-type-FischerTropsch (F-T) technology. Table 2 shows that the Mobil process produces 70 % more gasoline than the F-T route. It has a higher thermal efficiency, and also requires less investment so that the gasoline cost is cheaper by 14-31 %. A recent modification of the
MTG process may launch a new era in the F-T technology (42). This Mobil variant is labelled MFT (Mobil-Fischer-Tropsch). The basic idea is to pass the total effluent from a F-T reactor into a second reactor containing ZSM-5. The products include 70 % gasoline with an octane number of 92 and about 20 % LPG and light gas. The MFT process eliminates the heavy conventinnal products by cracking. Another modification of the basic MTG process consists in maximizing olefin production. A modification will yield up to 60 % of C2 - C4 olefins in a pilot plant and even greater yields have been produced in the laboratory. One version of this process is being piloted in South Africa. Another variant is the MOGD process, which produces diesel fuel (80 %) with gasoline (20 %) as a side product, from mixed propylene and butylene as feedstock (42). It thus seems that the Mobil process, either as the basic MTG conversion or in the form of its various modifications, becomes economically attractive, mainly in view of recent price increases of crude oil, in areas where coal and/or natural gas are readily available. The first commercial installation of the MTG process is e.xpected to come on stream in the mid-1980s in New Zealand. It will convert natural gas (instead of coal) to synthesis gas, from which the methanol will be made. This circumstance is peculiar to New Zealand which has a huge amount of natural gas available in its offshore gasfields. The plant will use a fixed-bed reactor system but Mobil is now being developping a fluid-bed variant for the same purpose. The fluid-bed reactor is considered more appropriate for the very huge installations and is being scaled up for commercial operation. By 1985, the New Zealand MTG plant will begin to draw 140 million cu.ft. of gas per day with, as the net result expected, about 14,000 bbl per day of high-quality motor fuel.
7. REFERENCES 1. "Encyclopedia of Chemical Technology" $irk Othmer, Ed.) Vols 4 and 13, Wiley, New York, 1972 2. H.H. Storch, N. Golumbic and R.B. Anderson, "The Fischer-Tropsch and Related Synthesis", Wiley, New York, 1951 3. For example: H. Perry, Scientific American 230, 19 (1 974) 4. "Methanol Technology and ~conomics" (C.A. G n e r , Ed.), Chem. Eng. Progr. Symp. Ser.~' 98, 66 (1970) i n Fuels" (J.K. Paul, 5. ethanol Technology and ~ ~ ~ l i c a t i o nMotor Ed.), Noyer Data Corp., Park Ridge, (N.J.), 1978, Chap2,pp107-165 6. For examp1e:G.A. Mills and B.A. Harney, Chemtech. 4, 26 (1974) ~lends",chap 5,pp 241-315 (ref-5) 7. "~ethanol/~asoline 8. T.B. Reed and R.M. Lerner, Science 1982, 1299 (1973) 9. C.D. Chang,J.C.W. Kuo, W.H.Lang, S.fiacob, J.J.Wise and A.J. Silvestai, Ind. Eng. Chem. Process Des. Dev. vol 17, 255 (1978) 10.W.~. Mattox, U.S. Pat. 3,036,134 (1982) 1l.E.I. Heiba and P.S. Landis J. Catal 3,471 (1964) 12.E.G. Derouane, inl'zeolites, science and Technology1', Lisbon, 1983 (this meeting), part I of the present work, refs (1) to (11) 13.S.L. Meisel, J.P. McCullough, C.H. Lechthaler and P.B. Weisz Chemtech. 6, 86 (1976) 14.see ref (57, p 375-420 15.D.~. Olson, W.O. Haag and R.M. Lago J. Catal. 61, 390 (1980) 16.H. ~einemann:~roc. Vth Ibero-American Symp. ~a=l~sis''(~. Farinha Portela and C.M. Pulido, Eds.) Lisboa, Portugal, 1978, Vol I,p 10 17.C.D. Chang and W.H. Lang, U.S. Pat. 4,013,732 (1977) 18.C.D. Chang, W.H. Lang and R.L. Smith, J. Catal 56, 169 (1979) lg.S.Yurchak, G.E. Voltz and J.P. Warner, Ind. ~ n ~ T ~ h e m . ~ r o c e s s Des. Dev. 18,527 (1979) 20.N. Daviduk, J. Mazuik and J.J. Wise,'~roc 11th Intersociety Energy conversion: vol. I (1976) 21.J.E. Voltz and J.J. Wise "Development Studies on Conversion of Methanol and Related Oxides to Gasoline" Final Report, ERDA Contract no E(49-18) -1773,Nov. 1976 22.5.5. Wise and A.J. Silvestri, Oil and Gas J. (1976) 23.5.5. Wise and J.E. Woltz, Synth. Fuels Proc. Res. ~ i g .ORNL-FE-1, pp 51-63, Nov. 1977 24.C.D. Chang and J.J. Grover, U.S. Pat 4,058,576 (1977) 25.H. Owen and P.B. Venuto, U.S. Pat 4,046,825 (1977);ibid 4,071,573 (1978) 26.N.Y. Chen, U.S. Pat 4,118,431 (1978) 27.D. Liederman, S.M. Jacob, J.E. Voltz and J.J. Wise, Ind. Eng. Chem. Process Des. Dev. 17, 340 (1978) Lee and A.J. ~ilvestri"Alcohol 28.J.E. Penick, S.L. ~eisel,~. Fuels Conference", Sydney, Australia, Aug. 1978 29.J.M. Stockinger, J. Chromatogr. Sci. 15, 198 (1977) 30.C.D. Chang and A.J. Silvestri J. catair 47, 249 (1977) 31 .C.D. Chang,A. J. Silvestri and J.C. zahnec U. S. Pat. 4,044,061 (1 977) ,
32.W. Lee and S. YufohAk, U.S. Pat. 4,197,418 (1980) 33.C.D. Chang, A.J. Silvestri and R.L. Smith, U.S. Pat. 3,894,103 (1975); ibid, 3,928,483 (1975) 34.C.D. Chang, Chern. Eng. Sci. 35, 619 (1980) 35.E.G. Derouane and J.C. VedriK J.Molec. Catal. 8, 479 (1980) 36.P. Dejaifve, A. Auroux, P.C. Gravelle, J.C. VedTine, Z. Gabelica and E.G. Derouane, J. Catal. 70, 123 (1981) 37.2. Gabelica, J.P. Gilson, G. =bras and E.G. Derouane, in "Thermal Analysis, Proc. 7th Int. Conf. Thermal Anal.", (B. Miller, Ed.), vol 11, Wiley-Heyden, New York, 1982, pp 1203 38.N.Y. Chen, W.W. Keading and F.G. Dwyer, J. Amer. Chem. Soc. 101, 6783 (1979) 3 9 . u . Derouane, P. Dejaifve, Z. ~abelicaand J.C. Vedrine Faraday Disc. Chem. Soc., 72,331 (1981) 40.W.0. Haag, W.W. Keading,D.H. Olson and P.D. Rodenwald, Eur. Pat. Appl. 9,894 (1479) 41.W. Lee, J. Mazuik and C. ~ortail,*~nformations Chimie No 188, April 1979, pp 165-73. 42.5. Maggin, Chern. Eng. News, Dec. 1982, pp 9-15 43.ref (5), "Gasoline from Methanol Patent Technology", pp 421-464. 1
USE OF PLATINUM H Y ZEOLITE AND PLATINUM H MORDENITE IN THE HYDROISOMERIZATION OF N-HEXANE
F. ~ a m o aRibeiro Grupo de Estudos de ~atiilise Heterogenea Instituto Superior ~ e c n i c o Av.Rovisco Pais, 1096 Lisboa Codex, Portugal 1. INTRODUCTION Petroleum cuts containing C5/C6 paraffins have a research octane number of about 7 0 due to the high content of monobranched paraffins. To upgrade the octane number of the light gasoline fractions, consisting predominantly of Cg/Cg paraffins, antiknock additives,e.g. tetraethyllead and tetramethyllead were used, but a t present the new environmental regulations defines very low permissible concentration levels for automative p o l lutants, particulary for the lead compounds (1). As a consequence, both industrial and academic research efforts h a v e been made to increase the octane number of the fractions C5/C6 paraffins, in order to compensate for the removal of the lead antiknock a d d i t i ves, through the isomerization of normal paraffins into branched paraffins with higher octane-level. Thermodynamically, the isomerization of C5/Cg pa raffins is reversible and slightly exothermic (1-5 ~ c a i / /mole) and consequently low temperatures are suitable for obtaining maximum yield of branched paraffins, a s shown in Figure 1 (2). Liquid acid catalysts, such as those of the 0 Friedel Crafts type, present a good activity at 85 C , HF (3) at 20°C, but they are and the superacid Sb F 5 extremely corrosive and easy to contaminate.
-
100
100 80
60 a W A
2,2D M BUTANE (91.8R. 0 . N.)
-
I s 0 PENTANE (92.3R.O.N.)
40 -
0
I
20
-
0 J
100
I
I
200
1
I
300
I
I
400
OL 100
1
200
I
I
300
1
I
400
T e m p e r a t u r e (OC)
Figure 1. Equilibrium distributions and the research octane number for the pentanes and hexanes (ref. 2) During World War 1 1 , due to the great need of high-octane aviation f u e l , some isomerization plants w e re built, using aluminum chloride as a catalyst. The process was expensive and the disposal of the sludge formed was a severe problem. I n the 1950's these processes were replaced by the use of bifunctional catalysts composed of a dispersed noble metal o n a classic support such as chloride alumina or silica-alumina ( 4 , 5 ) and several plants (6, 7 , 8 ) using this type of catalyst were built. The process using chloride alumina operates at lower temperature but requires careful feed pretreatment for removal of deactivating substances such as water, sulphur and benzene. The process using silica-alumina as support operates at 400°c, too high a temperature, from the thermo dynamical point of view, which limits the conversion and favours the hydrocarbon side reactions (cracking). Consequently, the need to discover other catalysts, active at low temperatures and not easily conta minable, was very urgent. I t was also important to operate at a temperature sufficiently high to obtain good conversions, close to equilibrium, with highyields of dimethylbutanes.
Since the 1960's many research workers (9, 1 0 , 1 1 , 12) have been reporting the high activity for isomerization of zeolitic catalysts, mainly zeolite Y , and mordenite, loaded with a noble metal to stabilize the activity. The development of highly active zeolitic catalysts has resulted in a hydroisomerization process, known as the Hysomer Process (13) launched by the Shell Oil Company. The first comercial unit was built at the La Spezia refinary in Italy and at present there are about ten units in operation throughout the world. The catalyst, manufactured by Union Carbide, is a platinum exchanged acid mordenite of very low sodium content, presenting a much better resistance to water, sulphur and aromatic compounds over amorphous catalysts. In fact the catalyst remains active with sulphur and water levels in the feedstock of 10 and 50 p.p.m. respectively, and concentrations of aromatics up to 2 wt%. The design unit is shown schematically in Figu4)
RECYCLE 6AS F EFF EXCH
I
FEED
I
MAKE-UP PRODUCT HYDROGEN Figure 2. Shell Hysomer process (ref. 14) This process converts low octane pentane and h e xane streams (RON - 70) to higher octane products (RON - 83) containing isopentane, methylpentanes and dimethylbutanes. It is not used o n C or higher paraf7 fins because undesired reactions (cracking) occur o n these acid zeolitic catalysts.
The process operates at temperatures of 2 5 0 ~ ~ and hydrogen pressure of 10-25 bars. Table I (14) gives the typical properties of C / 5 /C6 feed and isomerized products. TABLE I (ref. 14) Products from hydroisomerization of C /C6 tops ex. 5
~ M x d d l eEast Crude at 260'~
R.O.N.
of C5
+
Feed 67.5
Isomerizate 79.2
composition (wt.7) M e thane Ethane Propane Isobutane n-Butane Isopentane n-Pentane 2,2 - Dimethylbutane 2 , 3 - Dimethylbutane 2 - Methylpentane 3 - Methylpentane n-Hexane Cyclopentane Methylcyclopentane Cyclohexane Benzene
These data show that the hydroisomerization of the light gasoline fractions C5/C6 substantially increa se the research octane number. For the full understanding o f the hydroisomerization of n-paraffins o n Pt-zeolites there is a need to clarify several important aspects such as the preparation conditions, the influence of the reduction temperature of platinum o n the activity, the mechanism and kinetics of the reaction. W e will present a comparison between the P t H Y and Pt H mordenite using as test reaction the hydroiso-
m e r i z a t i o n of
2.
n-hexane.
CATALYSTS
The h y d r o i s o m e r i z a t i o n o f n - p a r a f f i n s u s e b i f u n c t i o n a l c a t a l y s t s - p l a t i n u m a c i d z e o l i t e s - w i t h a hyd r o g e n a t i n g and an a c i d i c f u n c t i o n . 2.1 Acidic Function A c i d i c f u n c t i o n a r i s e s from t h e hydrogen form of zeolite. Rabo a n d form of z e o l i t e i n t h e n-hexane decationization t a l y s t could be
h i s a s s o c i a t e s (9) found t h e sodium Y a s support f o r the platinum, i n a c t i v e i s o m e r i z a t i o n and h a v e shown t h a t t h e increased t h e a c t i v i t y so t h a t t h e caused a t lower temperatures.
Kouwenhoven ( 2 0 ) h a s shown t h a t r e d u c i n g t h e s o d i u m c o n t e n t t o v e r y l o w v a l u e s , i n a c a t a l y s t o f P t HY f o r t h e hydroisomerization of n-pentane, t h e a c t i v i t y i s increased considerably. H o w e v e r , a c o m p l e t e l y s o d i u m f r e e z e o l i t e HY (19) p r e s e n t s lower s t a b i l i t y and, i n o r d e r t o o b t a i n high s t a b i l i t y a n d c o n s e q u e n t l y h i g h a c t i v i t y , some v e r y l o w c o n c e n t r a t i o n s of sodium a r e r e q u i r e d . F i g u r e 3 (15) compares t h e e x t e n t o f sodium r e moved b y ammonium i o n e x c h a n g e f o r t h e z e o l i t e Y ( U n i o n C a r b i d e , 9 . 9 w t . % Na) a n d t h e m o r d e n i t e ( N o r t o n Z e o l o n 1 0 0 , 6 . 0 w t . % N a ) , and i t i s c l e a r t h a t i t i s more d i f f i c u l t t o remove t h e r e s i d u a l sodium of t h e t y p e Y , bec a u s e i t p r e s e n t s some c a t i o n s l o c a t e d i n s m a l l 8 - c a g e s ( 1 6 ) . H o w e v e r , i n a g r e e m e n t w i t h t h e l i t e r a t u r e ( 1 7 , 18, 1 9 ) we f o u n d t h a t a f u r t h e r e x c h a n g e w i t h ammonium i o n s a f t e r steam c a l c i n a t i o n of z e o l i t e Y reduced the sodium c o n t e n t t o 0 . 3 w t . % . 0
A f t e r t h r e e e x c h a n g e s w i t h ammonium i o n s a t 20 C, t h e m o r d e n i t e p r e s e n t e d a v e r y low sodium c o n t e n t (0.016 w t . % ) , b e c a u s e t h e c a t i o n s a r e l o c a t e d i n t h e main c h a n n e l s and t h e y a r e e a s i l y removed. The s t u d y o f t h e a c i d i t y by i n f r a r e d s p e c t r o s c o py ( p y r i d i n e d e s o r p t i o n a t i n c r e a s i n g t e m p e r a t u r e s ) h a s shown a s t r o n g B r B n s t e d a c i d i t y more marked f o r t h e H m o r d e n i t e t h a n f o r HY ( 5 4 ) .
Number of exchanges
Figure 3.
2.2
Comparison of t h e e x t e n t of d e c a t i o n i z a t i o n on Y z e o l i t e and m o r d e n i t e
Hydrogenating function
The h y d r o g e n a t i n g f u n c t i o n i s p r o v i d e d by n o b l e metals, particulary platinum. It i s very important t o employ t e c h n i q u e s o f m e t a l i m p r e g n a t i o n i n o r d e r t o o b t a i n good m e t a l d i s p e r s i o n a n d a homogeneous macroscopic d i s t r i b u t i o n of metal i n t o t h e z e o l i t e . To a c h i e v e t h i s l a s t p u r p o s e we d e v e l o p e d a t e chnique, c a l l e d c o m p e t i t i v e c a t i o n exchange (21,22). I t i s w e l l known t h a t i n o r d e r t o o b t a i n a g o o d d i s p e r s i o n o f t h e m e t a l we m u s t p r o m o t e a s t r o n g i n t e r a c t i o n between t h e m e t a l and t h e s u p p o r t , b u t t h i s condition is not sufficient.
A c l a s s i c a l i o n exchange of m e t a l l i c i o n s i n a s m a l l q u a n t i t y , i n c o m p a r i s o n w i t h t h e number o f e x c h a l geable centers of the support, leads t o a heterogeneous macroscopic d i s t r i b u t i o n of t h e metal over t h e s u r f a c e , due t o t h e exchange r a t e b e i n g h i g h e r t h a n t h e d i f f u s i o n r a t e and t o a n i m p o r t a n t a f f i n i t y o f t h e p l a t i n u m i o n s f o r t h e z e o l i t e Y , a s shown i n F i g u r e 4 ( 1 5 ) .
Pts(equivalent Pt fraction in solution phase)
Figure 4 .
NH4Y /
+I
exchange isotherm a t
2 5 O ~a n d 0 . 1 N Under s u c h c o n d i t i o n s t h e m e t a l l i c i o n s f i x thems e l v e s a t t h e exchange c e n t e r s on t h e s u r f a c e a n d , because the concentration of those ions i n the s o l u t i o n i s l o w ,t h e r e i s n o d i f f u s i o n o f t h e p l a t i n u m i n s i d e t h e g r a i n of c a t a l y s t , and they s t a y c o n f i n e d t o a t h i n layer over the periphery.
+
F o r a z e o l i t e u n d e r t h e NH form t h e i o n exchange r e a c t i o n , f o r t h e p l a t i n u m un8er t h e c a t i o n i c form i s
S Z
-
Solution Zeolite
The t e c h n i q u e o f i o n e x c h a n g e w i t h c o m p e t i t i o n ( 2 3 , 2 4 ) e n a b l e s a homogeneous m a c r o s c o p i c d i s t r i b u t i o n t o be o b t a i n e d . It c o n s i s t s of i n t r o d u c i n g a n e x c e s s of i o n s f o r c o m p e t i t i o n ( N H q + i o n s ) w h i c h d i s p l a c e t h e a bo v e e q u i l i b r i u m t o w a r d s t h e l e f t t h u s i n c r e a s i n g t h e c on c e n t r a t i o n of m e t a l l i c i o n s i n t h e s o l u t i o n and conseq u e n t l y i n c r e a s i n g t h e r a t e of d i f f u s i o n and m i g r a t i o n of t h e metal towards t h e i n s i d e region of the support.
This effect, referred t o as competition, r e f l e c t s i t s e l f on an experimental curve, which r e p r e s e n t s t h e f r a c t i o n of metal i n equilibrium i n t h e s o l u t i o n versus the competition.
where
No - number o f c o m p e t i n g gram i o n e q u i v a l e n t s added t o t h e s o l u t i o n phase
NZ
-
number o f c o m p e t i n g gram i o n e q u i v a l e n t s i n i t i a l l y present i n s o l i d phase n+
Mo
- number o f m e t a l e q u i v a l e n t s p r e s e n t solid-solution system
in
n+ *s
tion
- number o f m e t a l e q u i v a l e n t s i n s o l u t i o n phase
Upon r e a r r a n g i n g t h e e q u i l i b r i u m c o n s t a n t e q u a ( 2 4 ) o f i o n e x c h a n g e r e a c t i o n we o b t a i n
where
Ka
- equilibrium constant f o r t h e i o n exchange reaction
E
- q u a n t i t y w h i c h groups the activity coefficients together
VS VZ
-
volume of
the solution
volume of
the zeolite
The c o m p e t i t i o n c u r v e s f o r t h e z e o l i t e s NH Y a n d 4 MH M a n d p l a t i n u m a r e s h o w n i n F i g u r e 5 .
4
Platinum
1
10
- 100
1000
10000
x la (competition)
Figure 5.
C o m p e t i t i o n c u r v e s f o r t h e z e o l i t e s N H 4Y a n d NH M ( r e f . 2 4 ) ( C o n d i t i o n s : T = 20°C, v Solution = pH = 7 , 4 0 cm 3 g -1 , P NH,Y 3
Y X he c u r v e s w e r e o b t a i n e d u s i n n r e l a t i o n = f (-1 a a Ka ( 2 4 ) a n d t h e mean v a l u e s o f - c a l c u l a t e d f r o m t h e e x E
in
p e r i m e n t a l d a t a , and i t i s found t h a t t h e e x p e r i m e n t a l f o r m a t i o n s h o w s g o o d a g r e e m e n t w i t h t h e o r e t i c a l p r e d i c t-i ons. The i m p o r t a n c e a n d p r a t i c a l u s e o f t h i s t e c h n i q u e of competition l i e s i n the f a c t t h a t the competition c u r v e e n a b l e s o n e t o know w h i c h i s t h e m i n i m u m q u a n t i t y of competing i o n s t o be added t o t h e s o l u t i o n , i n o r d e r t o have metal uniformly d i s t r i b u t e d over the support with simultaneously t h e l e a s t p o s s i b l e f r a c t i o n of t h e m e t a l i n t h e s o l u t i o n . F o r t h e z e o l i t e s NH Y a n d NH M 4 4 t h e optimum v a l u e o f t h e c o m p e t i t i o n from t h e i n d u s t r i a l p o i n t of v i e w s h o u l d l i e b e t w e e n 200 and 300. The P t
-
zeolites,
prepared using the competitive
i o n e x c h a n g e , p r e s e n t a g r e a t e r s t a b i l i t y and activity, a s c o m p a r e d w i t h i d e n t i c a l c a t a l y s t s , i n w h i c h the platinum was deposited using a classical ion exchange. S t u d i e s d e s c r i b e d e l s e w h e r e (25) c o n f i r m e d the a b o ve conclusion. Two PtHY zeolites w i t h the same platinum c o n t e n t ( a b o u t 0.5 wt.% Pt) h a v e b e e n prepared i n d i f f e r e n t c o n d i t i o n s : i n o n e o f them ( c a t a l y s t I) t h e platiz num w a s deposited using the competitive ion exchange and i n t h e o t h e r ( c a t a l y s t 11) u s i n g a c l a s s i c a l i o n e -x. change. Catalyst I 1 presents an heterogeneous platinum distribution, w i t h a n important content of metal o n the external surface. T h e performances of the two PtHY catalysts w e r e s t u d i e d u s i n g t h e h y d r o i s o m e r i z a t i o n o f n-hexane u n d e r a t o t a l p r e s s u r e o f 3 0 b a r s and H 2 / n C6 = 4 (mole r a tio), a s t h e t e s t r e a c t i o n . A s s h o w n i n T a b l e 1 1 , t h e a c t i v i t y o b t a i n e d at 230-280°C u s i n g s e m i d i f f e r e n t i a l c o n d i t i o n s , i s 2.5 t i m e s h i g h e r f o r c a t a l y s t I , a s corn pared w i t h t h a t o f c a t a l y s t 1 1 , and b o t h p r e s e n t v e r y low values of selectivity for the dimethylbutanes. Table I 1 C o m p a r i s o n b e t w e e n two P t H Y c a t a l y s t s ( - --0.5 wt% Pt) one being prepared by competitive ion exchange
.. .
A -
(cat.I),
and t h e o t h e r b y c l a s s i c a l i o n e x c h a --n g e
(cat. 11) 10
4
aI ( m o l e h -1 g -1 of P ~ H Y )
I
T h e d i f f e r e n c e o f a c t i v i t y for c a t a l y s t s I and I 1 m a y b e i n t e r p r e t e d a s f o l l o w s : f o r c a t a l y s t I (homogen e o u s p l a t i n u m d i s t r i b u t i o n ) all t h e s u r f a c e a c t u a t e s acarding to a classical bifunctional metal-acid mechanism, whereas for catalyst I 1 only the external surfac e a c t u a t e s i n t h i s w a y and i t s i n t e r n a l s u r f a c e w i t h o u t p l a t i n u m a c t u a t e s t h r o u g h a n acid m e c h a n i s m w i t h l o w
activity due to a faster deactivation.
2.2.2
Thermal ---treatments
The production of PtHY and P t H M zeolites includes several thermal treatments, whose experimental conditions have a great influence o n their stability and catalytic properties. 2.2.2.1
Preparationof the HYand -HM
The calcination of NH Y and NH4M not only removes 4 the water vapour and ammonia, leading to the HY and HM forms, but also tends to produce important structural changes depending on temperature, time and steam partial pressure conditions. A study described in references 26 and 27 shows the influence of the temperature and composition of the calcination atmosphere on the structure of a very low sodium zeolite. The structure of the HY zeolite quickly collapses at 4500C when calcined under dry air, but the presence of very little water vapour delays the destruc tion, as shown in Figure 6
v/ ;
t
i
c
:
.
i
_
rm quartz wool
I
1
2
3
4
5
6
7
8
NUMBER OF THE BED SECTION
Figure 6. Evolution of the surface area as a f u n c t i on of the number of the powder bed section. The presence of steam at high temperatures during calcination contributes to stabilize the structure, de-
creasing the value of the unit cell parameter down to 24.4 A' (24.65 A? for Na Y) and increasing tho Si02/A1203 ratio. This thermal treatment also modifies the acidlc properties, decreasing the Brsnsted acidity (28).
A literature (17, 18, 29-35) survey o n this subject indicates several techniques to obtain the more or less stabilized structures. John Ward (35) describes, in a U.S. patent, the sequence of operations to prepare a "stabilized" zeolite Y , involving three steps: partial ion exchange of s o dium with ammonium, calcination in the presence of steam above 5000C, followed by an ammonium ion exchange, which reduces soda levels down to 1 wt.%. The stabilization of the zeolite Y structure in the presence of steam is the consequence of two simulta neous processes: extraction of A1 atoms and their repla cement by Si atoms in positions that are not "strategic". The calcination under very dry air accelerates the first process and leads to the destruction of the structure (28). The presence of steam at high temperature ( > 400'~) during calcination favours the second process, which must have a higher activation energy, and leads to the stabilization of the H Y structure. The ammonium form of mordenite in the presence of steam at high temperatures, presents (as the NH Y does) 4 a greater stability as compared with that of mordenite calcinated under dry air. Thus the values of the surface areas of two H mordenites calcinated under wet a'r 2! and dry air, after recalcination at 950'~ are 235 m / g and 9 m2lg respectively (15). These results show that the steam calcination of NH M leads to a stabilization 4 of the structure. However,the high stability of this other mordenite does not require such particular conditions for calcination. 2.2.2.2 Calcination of platinum amino complex-
- containing NHq zeolites The calcination conditions of the [Pt (NH3)4 NH4 zeolite] must be well chosen in order to obtain the best metallic phase dispersion, without damaging the zeolitic structure. For zeolite Y , Boudart (36) has shown that the re duction of [Pt (NH3)4 Ca Y] with hydrogen at 350°C caul
ses the formation of a very labile [Pt (NHg)4 H z ] complex and consequently leads to the metal sintering. I n order to obtain highly dispersed platinum in zeolite Y , it is essential to carry out a staged calcination in dry air (37, 38). Our studies (39) have shown that the presence of high contents of platinum contributes to the damaging of the structure during the calcination when the zeo'lite Y is not previously stabilized. The calcination of [Pt (NH3!4 NH4 mordenite] must be made under a stream of air, uslng staged heating (40, 41). Contrary to what happens with the zeolite Y , the structure of the mordenite (39), either stabilized or not not, always remains well organized after calcination of the [Pt (NH3)4 NH4~]. 2.3.3
Influence of the reduction temperature on the
----P t H mordenite catalytic behaviour of The reduction conditions of the bifunctional catalyst, platinum H zeolite, have a marked influence o n their activity and selectivity. Studies reported in a recent publication (42) have shown the influence of the reduction temperature of a series of platinum H mordenite (PtHM) catalysts with platinum contents varying from 0 to 10.2 wt % , on the n-hexane isomerization and cracking under 24 bars hydro gen pressure. Figures 7 and 8 (42) compare the isomerization and cracking rates of n-hexane for two series of PtHM cata lysts reduced at 4 5 0 and 5000C. It is clear that the r e duction at 5000C increases their isomerization activity and decreases their hydrogenolysis activity. The reduction temperature has no influence on the selectivities for the isomerization products and does not affect the metallic phase dispersion. We have shown (43) that for the zeolitic catalysts, with high content of platinum, the formation of light cracking products results mainly from the hydrogenolysis of n-hexane on metal sites a n d , for low content of platinum, results from the hydrocracking (bifunctional reaction). The reduction treatment at 500°C, having a greater influence on the catalysts with higher content of platinum, affects, therefore, essentially the hydrogenolysis sites.
Figure 7. Influence of catalyst reduction temperature o n the isomerization rate of n-hexane at 2600C on a series of PtHM with several platinum contents
Figure 8. Influence of catalyst reduction temperature on the cracking rate of n-hexane at 260°C o n a series of P t H M with several platinum contents The decrease of the hydrogenolysis activity after the pretreatment under hydrogen at high temperatures, as proposed by Froment et al. (44, 45), seems to b e cag sed by a strong chemisorption of H 2 on platinum.
The increase of isomerization activity can be explained by the fact a decrease of coke formation o n the zeolitic catalysts reduced at 5000C has been shown to be associate with a weaker hydrogenolysis activity than when the reduction w a s carried out at 450'~. Our earlier studies (46) o n PtHM catalysts with high platinum content have shown a simultaneous increase in hydrogenolysis activity and in coke formation. An important conclusion to be drawn is that the PtHM catalysts reduced at 500°C are much more selective for the isomerization of n-hexane than those reduced at 450OC. 3. -Kinetics and -- mechanism ----- - of n-hexane hydroisomerizatio n o n Pt H zeolites The hydroisomerization of n-hexane on metal-loaded zeolites has been the subject of numerous studies (9, 10, 20, 4 7 , 53),particulary its kinetics and mecha nism. The main products of the reaction are 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methyl-pentane and 3-methyl-pentane. Side reactions, such as hydrocracking, occur on those bifunctional catalysts and lead to light alkanes from C1 to C5, which w e must minimize to increa se the selectivities for the products of isomerization. 3.1 Effect of platinum content on the stability and ~
activity of P t H zeolite u p
The study of the aging period for Pt H Y zeolites (Figure 9) at 300°C, #yder a total pressure of 30 bars and a molar ratio ----- - 4 , shows clearly the effect n C 6 of platinum content on the isomerizing activity and st? bility of zeolitic catalysts. The stability of the conversion level during the operating time increases with the increasing of platinum content. The incorporation of platinum onto the hydrogen Y zeolite increases the isomerization, the maximum occuring for low platinum contents. We can assume that the platinum prevents the catalyst deactivation by promoting the hydrogenation of coke precursors. For higher platinum contents there is a stability of conversion level during the operating working time but the isomerization decreases due to an increase of hydrocracking and/or hydrogenolysis of n-hexane.
-
75 -/
-
- ---- - -- = --
1
1
wt % Pt
m
rn
0.5 2.95
a w
I rn
-
a
I
6.0 0.09
rn
I
%
a
w
0 -
w
a
-
m
0.03
w
a
a A
17.7
Conditions: temperature
3 0 0 "C
space velocity
8.6
pressure
30b
oo H2/,,C6 (molar ratio) 4
0 0
5
15
10
time (h) Figure 9. Aging period of Pt H Y catalysts for the hvdroisomerization of n-hexane ( T = 3 0 00c Pt
=
2 3 0 b, - -- 4 ) n C 6
The behaviour of Pt H mordenite o n the hydroisomerization of n-hexane is very similar to that of P t H Y, concerning the aging period. It would be interesting to compare the performances of Pt H Y and P t H mordenite at 260°c, temperature, for which conversions are less than lo%, in order to select the best catalyst and also to determine the opti ma1 platinum content. As shown in Figure 10, at 260'~ under a total pressure of 30 bars, we observe a sharp increase in the
isomerization rate of n-hexane at low platinum contents. The maximum is reached for both catalysts somewhere in the range 2 to 3% Pt. By comparing two curves we conclude that the P t H mordenite presents a higher isomerizing activity, certainly due to its higher acidity (54).
Figure 10. Comparison of the isomerizing activity as a function of the platinum content for Pt H Y and Pt H mordenite
The decrease in the isomerizing activity at higher platinum contents can be explained (46) by the co ke poisoning of the acid function. The higher formati1 on of coke at high platinum contents is caused by the hydrogenolysis of n-hexane on the platinum, which becomes more important than hydrocracking (56, 57). Other authors (55) have compared the performances of H mordenite and H Y loaded with a noble metal, and their results lead to the same conclusion, that the bifunctional catalysts based on H mordenite are more acti ve for paraffin isomerization.
3.2
-Reaction m e c h a n i s m s o f n-hexane hydroisomeri---zation
H a v i n g discussed the influence of the platinum content o n the selectivity and activpty of P t H z e o l i tes w e should n e x t clarify the m e c h a n i s m of h y d r o i s o m e r i z a t i o n o f n-hexane o n metal acid b i f u n c t i o n a l cata lysts. W e i s z (58) proposed a mechanism a s described below
bifunctional mechanism
If the mechanism is purely acid the f o r m a t i o n of t h e c a r b o n i u m i o n i s the rate-determining step.
+
is formed through the h y d r o g e n transfer The C 6 b e t w e e n the n - p a r a f f i n and a c a r b o n i u m ion previously present o r by a r e a c t i o n o n the BrGnsted or L e w i s sites of t h e zeolite (60). F o r low metal c o n t e n t s , the h y d r o g e n a t i o n and dehydrogenation r e a c t i o n s a r e the limiting s t e p , the isome r i z a t i o n activity being proportional to the metal content. If the hydrogenating activity is h i g h , w h i c h h a p p e n s for higher metal c o n t e n t , the limiting step w i l l b e the skeletal isomerization of olefins o n acid sites and the a c t i v i t y d e p e n d s o n l y o n the acidity of the zeolitic c a t a l y s t s (59). H o w e v e r , for the highest platinum contents w e observed a d e c r e a s e i n the isomer i z i n g a c t i v i t y , already considered above. At l o w c o n v e r s i o n , and n e g l e c t i n g the external and internal d i f f u s i o n a l l i m i t a t i o n s , the isomerization rate of n-hexane c a n be e x p r e s s e d , according to the bif u n c t i o n a l m e c h a n i s m , i n w h i c h the limiting step is the acid isomerization of o l e f i n s , as follows:
where k 3 is the isomerization rate constant of carbonium ions, Cm the concentration of Brznsted sites of the zeolite and K1 and K 2 the equilibrium constants of the dehydrogenation of n-hexane and of the carbonium ion formation. The validity of this equation, and consequently of the bifunctional mechanismwas tested (43) for the hydroisomerization of n-hexane o n P t H Y (6.0 wt % ) under a total pressure of 4 0 bars, and a good agreement was found with the experimental values. Kinetics studies of the n-hexane isomerization carried out by Braun and co-workers (61) have shown that no significant mass transfer limitations o n the external and internal surface of pellets of P t H mordenite (0.5% wt % Pt) occur. The experimental values obtained for effectiveness factors corresponding to pellet diameters in the range of 0.5 to 4 mm and at a temperature of 2700C, lie between 0.9 and 1.0. 3.3
Activation energies of isomerization and selectivities as functions of the platinum content for Pt H Y and Pt H mordenite
As shown in Figure 1 1 , the activation energies of isomerization for Pt H Y and Pt H M obtained in a range of 230 to 3000C under a total pressure of 30 bars increase with the increase in the platinum content, reaching a plateau of 37 K cal/mole for low platinum contents (about 0.5 wt % ) . This behaviour can be explained through the bifunctional mechanism described above. For the H Y and H M the limiting steps is the for mation of the carbonium ion whose activation energy islow (60) As the platinum content increases there is a chan ge towards the bifunctional mechanism where the limiting step is the skeletal isomerization of intermediate olefins.
-
Braun et al. (61) obtained similar values as a c t i vation energies of isomerization for Pt H mordenite.
- 30
-
A
Q)
Q
.-
u I
. CU
C3
Y
I
5-
0
0
-
P t HM
1
I
2.5
5
I
7.5 w t % Pt
I
10
10
W
0
F i gur e 11. Activation energy of n-hexane hydroisomerization and selectivity of 2,3 - dimethylbutane on two series of PtHY and PtHM catalysts, as a function of the platinum 0 H2 content (T = 260 C, Pt = 30 bars, ----- - 4 ) . n C6 Plot A, activation energy; plot B, selectivity of 2,3-dimethylbutane.
The c h a n g e o f t h e s e l e c t i v i t i e s o f 2 , 3 - d i m e t h yl butane a s a function of platinum content follows an opposite trend to that obtained for the activation ener g i e s of isomerization. In fact, for Pt H Y there i s a strong decrease of t h e s e l e c t i v i t y f o r 2 , 3 - d i m e t h y l b u t a n e o v e r t h e i n i t i a l p a r t o f t h e c u r v e , which t h e n r e a c h e s a p l a t q a u for platinum contents > 0 . 5 w t % .As r e g a r d s t h e o t h e r i s o m e r s , t h e 2- a n d 3- m e t h y l p e n t a n e a r e o b t a i n e d a s a n e q u i l i b r i u m m i x t u r e and a l s o v e r y low p e r c e n t a g e s of 2 , 2 - d i m e t h y l b u t a n e which h a s a low r a t e of f o r m a t i o n (62). For P t H M, t h e s e l e c t i v i t y decrease f o r 2 , 3 - d i m e t h y l b u t a n e i s much s l o w e r , a n d e v e n f o r h i g h p l a t i n u m c o n t e n t s i t s s e l e c t i v i t y i s a b o u t 7 % . We c a n a s s u m e ( 6 3 ) t h a t the migration from the metal t o the acid s i t e s i s s l o w e r t h a n t h e i s o m e r i z a t i o n and c o n s e q u e n t l y t h e o l e f i n s have s u f f i c i e n t time t o a t t a i n e q u i l i b r i u m on acid s i t e s , thus obtaining the dimethylbutanes as primary p r o d u c t s . Conclusions The a n a l y s i s o f t h e b e h a v i o u r o f t h e c a t a l y s t s P t H m o r d e n i t e and P t H Y i n t h e h y d r o i s o m e r i z a t i o n of n - h e x a n e , l e d u s t o some i m p o r t a n t c o n c l u s i o n s , n a m e l y , the fact that Pt H M has a higher isomerizing activity and s e l e c t i v i t y f o r t h e d i m e t h y l b u t a n e s of h i g h o c t a n e number. T h e s e p r o p e r t i e s may e x p l a i n t h e p r e f e r e n c e t h a t i s g i v e n t o t h e u s e o f P t H m o r d e n i t e i n t h e Hysomer Shell process. The d e v e l o p m e n t o f more a c t i v e c a t a l y s t s w i l l allow the decrease of t h e hydroisomerization temperatur e a n d c o n s e q u e n t l y t o o b t a i n a maximum y i e l d o f b r a n ched p a r a f f i n s w i t h h i g h o c t a n e l e v e l . Acknowledgments Much o f t h e w o r k r e p o r t e d h e r e , w a s c a r r i e d o u t b y F . Ram6a R i b e i r o d u r i n g h i s s t a y a t t h e F r e n c h P e t r g l e u m I n s t i t u t e . S p e c i a l t h a n k s a r e d u e t o D r . Ch. Marc i l l y (French P e t r o l e u m I n s t i t u t e ) and P r o f . D r . M. Guisnet (University of P o i t i e r s ) f o r the h e l p f u l d i s c u s s i o n s on s e v e r a l a s p e c t s of t h i s work.
References -1. J. McEvoy in "Catalyst for the Control of Automati ve Pollutants" Ed. Robert Gould, ACS Monograph 143, 1975, American Chemical Society, Washington 2. A.P.Bolton, in "Zeolite Chemistry and Catalysis" Ed. J.A.Rabo p.750, ACS Monograph 171, 1975, American Chemical Society, Washington 3. J.Oelderick, E.L.Mackor, J.C.Platteeuw, A.Van der Wiel, USP 3 201 494, Shell, International Research Maatschappij Fr. 2 005 043 4. W.N.Lyster, J.L.Hubbs, H.W.Prengle, AIChE J. 10, 1964, 907 5 , 1964, 37 5. J.H.Sinfelt, Advan. Chem. Eng. 6. Oil Gas J. (Aug. 16, 1971),
67
7. A.H.Richardson, M.F.Olive, 68th National Meeting, AIChE, paper 53d, 1971 4 5 , 1953, 8. F.G.Ciapetta, J.B.Hunter, Ind. Eng. Chem. 147 9. J.A.Rabo, P.E.Pickert, D.N.Stamires, J.Boyle, Int. Congr. Catal., 2nd, Paris, 1960, N9104 10. J.A.Rabo, P.E.Pickert, R.L.Mays, Ind. Eng. Chem. 53, 1961, 147 11. V.Schomaker, Proc. Third Intern. Congr. Catalysis, 2 , 1964, 1264 12. A.Voorhies, Ph.Bryant, AIChE J. -14, 1968, 852 53 (9), 1974, 212 13. Hydrocarbon Process, 14. H.W.Kouwenhoven. W.C.Van Zijll Langhout, Chem. Eng. Prog., 67, 1971, 6 5 15. F.Ribeiro, Ph. D. Thesis, Poitiers, France, 1980 16. D.W.Breck, "Zeolite Molecular Sieves", Wiley - Interscience, New York, 1974 17. C.V.McDanie1, P.K.Maher, Proc. Int. Congr. Mol. Sieves, l S t , London, 1967, 186-195 18. G.T.Kerr, J. Catal., 13, 1969, 114 19. U.S.
4.036. 739, 1971
20. H.W.Kouwenhoven,
Adv. Chem. Series, 121, 1973, 529
21. U.S. 3.527.835, 1970 22. J.F. Le Page et al. in "Catalyse de Contact", Ed. Technip, Paris, 1978
F.Ribeiro, Ch.Marcilly, G.Thomas, C.R. Acad. Sc. Paris, 287 C , 1978, 431 F.Ribeiro, Ch.Marcilly, Rev. Inst. Fr. Pet., 34 (3), 1979, 405 F.Ribeiro, Ch.Marcilly, M.Guisnet, M.Portela, Proc. 3rd Int. Chem. Eng. Conference, Portugal, 1981, 2.106 F.Ribeiro, Ch.Marcilly, i n "Recent Progress Reports, 5 t h Int. Conference on Zeolites (Napoli 1980)" (R.Sersale, C.Colella, R.Aiello, Eds.) p.135, Giannini, Napoli, 1981 F.Ribeiro, Ch.Marcilly, M.Guisnet, Rev.Port. QuIm., in press F.Ribeiro, Ch.Marcilly, M.Guisnet, Proc. 7 t h Iberoam. Symp. Catal.,Argentina, 1, 1980, 274 U.S. 3.354.077, 1967 U.S. 3.506.400, 1970 U.S. 3.591.488, 1971 C.V.McDanie1, P.K.Maher, in "Zeolite Chemistry and Catalysis", Ed. J.A.Rabo, p.285, ACS Monograph 171, 1975, American Chemical Society, Washington J.Scherzer, J.L.Bass, J. Catal. 28, 1973, 101 P.Jacobs, J.B.Uytterhoeven, J. Catal. 22, 1971, 193 U.S. 3.897.327, 1975 R.A.Dalla Betta, M.Boudart, Proc. 5 t h Int. Congr., Miami Beach, 2 , 1972, 1329 P.Gallezot, A.Alarcon Diaz, J.A.Delmon, A.J. 39, 1975, 334 Renouprez, B.Imelik, J. Catal., -T.Kubo, H.Arai, H.Tominaga, T.Kunugi, Bull. Chem. Soc. Jap., 45, 1972, 6 0 7 F.Ribeiro, Ch.Marcilly, Proc. V t h Int. S y m ~ .Heterog. Catal., Varna, Bulgaria, 1983 (submitted) P.E.Eberly Jr., C.N.Kimberlin Jr., J. Catal., 22, 1971, 419 Shell International Research Mij., B.P. 1.189.850, (April 29, 1970) F.Ribeiro, Ch.Marcilly, M.Guis.net, Proc. 8 t h Iberoam. Symp. Catal., Spain (Huelva), 1 , 1982, 219
P.G.Menon, G.F.Froment, Applied Catal., 1 , 1981,31 F.Ribeiro, Ch.Marcilly, M.Guisnet, E.Freund, H. Dexpert, "Catalysis by Zeolites", ed. B. Imelik, p.319, 1980, Elsevier Scientific Publishing Company, Amsterdam R.Beecher, A.Voorhies, Ind. Eng. Chem., Prod. Res. 8, 1969, 366 Develop. N.L.Cul1, 833
H.H.Brenner, Ind. Eng. Chem., 53, 1961,
4 , 1964, 8 5 0 Kh. M. Minachev, Neftekhimia, Kh. M. Minachev, Ya. I. Isakov, in "Zeolite Chemistry and Catalysis" Ed. J.A.Rabo, p.592, ACS Monograph 171, 1976, American Chemical Society, Washington F.Chevalier, M.Guisnet, R.Maure1, 6 t h Proc. Intern. Congr. Catal., London, 2 , 1976, 478 M.L.Poutsma, in "Zeolite Chemistry and Catalysis" Ed. J.A.Rabo, p.437, ACS Monograph 171, 1976, American Chemical Society, Washington B.W.Burbridge, I.M.Keen, M.K.Eyles, Advan. Chem. Ser. 102, 1971, 400
P.B.Weisz, in "Advances in Catalysis" ( ~ . D . E l e y , H.Pines and P.B.Weisz Eds.) 13, 1963, 137, Academic Press, New York F.Chevalier, Ph. D. Thesis, Poitiers, France, 1979
61. G.Braun, F . F e l t i n g , H.Schoenberger, i n " M o l e c u l a r S i e v e s II", E d . J . R . K a t z e r , p. 5 0 4 , A C S S y m p o s i u m S e r i e s 40, 1 9 7 7 , A m e r i c a n C h e m i c a l S o c i e t y , washington 62. M.Guisnet, J.J.Garcia, F.Chevalier, Bull. Soc. Chim., 1 9 7 6 , 1 6 5 7
R.Maure1,
ZEOLITES AS CATALYSTS I N XYLENE ISOMERIZATION PROCESSES
M. Guisnet and N.S. Gnep L a b o r a t o i r e AssociB a u CNRS - C a t a l y s e Organique U n i v e r s i t s d e P o i t i e r s , France
The o-xylene and above a l l p-xylene a r e b a s i c s u b s t a n c e s e s s e n t i a l i n t h e o r g a n i c chemical i n d u s t r y e s p e c i a l l y f o r t h e manuf a c t u r e of p l a s t i f i a n t s , r e s i n s , f i b r e s and p o l y e s t e r f i l m s ; mx y l e n e , t h e isomer produced i n t h e g r e a t e s t q u a n t i t y does n o t o f f e r a v e r y g r e a t i n t e r e s t and i s most o f t e n c o n v e r t e d i n t o o- and px y l e n e s ( 1 , Z ) . Xylenes a r e produced by c a t a l y t i c r e f o r m i n g ; t h e C8 a r o m a t i c c u t s coming from t h e d i s t i l l a t i o n of r e f o r m a t e s c o n t a i n 25 %, m-xylene x y l e n e s i n t h e i r e q u i l i b r i u m m i x t u r e (0-xylene 50 % and p-xylene 25 %) and e t h y l b e n z e n e i n h i g h e r q u a n t i t y t h a n a t e q u i l i b r i u m (10 t o 40 Z i n s t e a d of 7 2 ) . According t o t h e economic c o n t e x t and t h e importance of t h e a r o m a t i c i n d u s t r i a l p l a n t , e t h y l b e n z e n e can e i t h e r be r e c u p e r a t e d o r transformed i n t o x y l e n e s . The aim of an i s o m e r i z a t i o n u n i t w i l l be t h e r e f o r e t o o b t a i n a maximum y i e l d of o-xylene and above a l l of p-xylene from C 8 t i c c u t s c o n t a i n i n g o r n o t e t h y l b e n z e n e . Taking i n t o account t h e v e r y h i g h m-xylene c o n t e n t i n t h e e q u i l i b r i u m m i x t u r e of x y l e n e s , t h e c o n v e r s i o n , a t each p a s s a g e , of m-xylene i s v e r y reduced, t h u s t h e recycled q u a n t i t y important ( t y p i c a l value recycled/fresh charge = 3 ) .
-
1.
-
-
MAIN INDUSTRIAL PROCESSES OF C8 AROMATIC ISOMERIZATION
I f x y l e n e i s o m e r i z a t i o n o c c u r s on a c i d c a t a l y s t s , e t h y l b e n z e n e i s o m e r i z a t i o n however i s n o t observed on t h e s e c a t a l y s t s . This r e a c t i o n i n f a c t demands t h e s i m u l t a n e o u s p r e s e n c e of b o t h a n a c i d and a h y d r o g e n a t i n g f u n c t i o n ( b i f u n c t i o n a l c a t a l y s i s ) Theref o r e t h e r e a r e two t y p e s of i s o m e r i z a t i o n p r o c e s s e s , o n e a l l o w i n g t h e isomer i z a t i o n o f the t h r e e x y l e n e s and t h e o t h e r t h e whole C8 a r o m a t i c c u t (2)
.
1.1
Processes of xylene isomerization
The reaction temperature depends directly on the acidity of the catalyst : - < 150°C for very strong acids of Friedel Crafts type, - from 250 to 450°C for zeolitic catalysts, - from 380 to 500°C for medium strength acids such as halogenated alumina or silica-alumina. According to whether the reaction temperature is or is not lower than the critical xylene temperature (about 345"C), the process can be operated - either in liquid phase : e.g. Mitsubishi Gas Chemical Process using HF-BF ; Low Tem3 perature Isomerization from Mobil using ZSM-5 zeolite, - or in gas phase : e.g. Isoforming from Esso Research and Engineering ; Isolene I from Toray. The stronger the catalyst activity and above all the greater its life span (especially its resistance to coking) and its selectivity (limited secondary reaction of disproportionation, preferential orientation towards p-xylene) the better will be the process (2). 1.2
C8 aromatic isomerization process (xylenes + ethylbenzene)
In order to allow ethylbenzene isomerization, all the processes use bifunctional noble metal acid catalysts on fixed beds, operating under hydrogen pressure (1 to 2 MPa) and at a temperature ranging from 380 to 460°C. In these conditions, C8 naphtenes are formed ; they are recycled with unconverted C8 aromatics (2). Different acid supports can be used (1,2) : - Silica-alumina (Octafining from Arco-Engelhard), - Chlorinated alumina (Isomar from UOP), - Fluorinated alumina (Isarom from IFP), - Zeolitic catalysts in mordenite base (Isolene I1 from Toray, Octafining I1 from Arco-Engelhard, Aris from Veb Leuna Werke) or in ZSM-5 zeolite base (Mobil process). Here again the aim is to obtain a very high degree of activity, a good stability and a high selectivity.
2. XYLENE ISOMERIZATION ON ACIDIC ZEOLITE CATALYSTS Over acid catalysts, xylenes undergo several reactions : isomerization but also disproportionation into toluene and trimethylbenzenes and finally coke formation responsible for catalyst deactivation. The importance of these two latter reactions must evidently be limited ; moreover it would be interesting to orient xylene isomerization towards p-xylene formation. It will be shown here, on the basis of the mechanism of these various reactions,
what must b e t h e physicochemical c h a r a c t e r i s t i c s of z e o l i t e c a t a l y s t s i n order t o s a t i s f y these requirements. 2.1
.
I s o m e r i z a t i o n mechanisms
Three mechanisms have been proposed t o a c c o u n t f o r a r o m a t i c hydrocarbon i s o m e r i z a t i o n ( 3 ) : an i n t r a m o l e c u l a r mechanism, an i n t e r m o l e c u l a r mechanism v i a t r a n s a l k y l a t i o n p r o d u c t s and a dealkyl a t i o n - a l k y l a t i o n mechanism. I n t h e c a s e of x y l e n e i s o m e r i z a t i o n , t h e i n t r a m o l e c u l a r mechanism i s t h e most p r o b a b l e , a l t h o u g h on z e o l i t e v a r i o u s a u t h o r s have proposed mechanisms i n v o l v i n g t r a n s a l k y l a t i o n p r o d u c t s ; t h e d e a l k y l a t i o n - a l k y l a t i o n mechanism i s v e r y u n l i k e l y b e c a u s e i t i n v o l v e s a v e r y u n s t a b l e methyl c a r b o c a t i o n (4,5). The i n t r a m o l e c u l a r mechanism i s d e s c r i b e d i n f i g u r e 1 ; t h e s e l e c t i v i t y of m-xylene i s o m e r i z a t i o n ( p a r a l o r t h o r a t i o ) should n o t depend on t h e c h a r a c t e r i s t i c s of a c i d c e n t e r s which a r e p r e s e n t on t h e c a t a l y s t s i n c e t h e f o r m a t i o n s of o r t h o and p a r a isomers proceed t h r o u g h t h e same s t e p s . T h i s e f f e c t i v e l y i s t h e c a s e ( f i g u r e 2 ) : t h e p a r a l o r t h o r a t i o i s t h e same ( e q u a l t o 1 . 1 a t 350°C) on s i l i c a - a l u m i n a , f l u o r i n a t e d alumina and a s e r i e s of s t a b i l i z e d Y z e o l i t e s which d i f f e r by t h e i r a c i d i t y . However, t h e f o r m a t i o n of p a r a isomer which h a s a s m a l l e r mol e c u l a r s i z e t h a n t h e o r t h o isomer c a n be s t r o n g l y f a v o u r e d i f t h e z e o l i t e h a s a p o r e s i z e c l o s e t o t h a t of t h e x y l e n e s . F i g u r e 2 shows t h a t ZSM-5 z e o l i t e i n p a r t i c u l a r i s w e l l a d a p t e d t o o b t a i n s e l e c t i v e l y p a r a i s o m e r . Moreover i t must b e n o t e d t h a t n o t a b l y h i g h e r v a l u e s of t h i s r a t i o c a n b e o b t a i n e d on m o d i f i e d ZSM-5 zeolite (6).
F i g . 1 . I n t r a m o l e c u l a r mechanism of x y l e n e i s o m e r i z a t i o n
A
% p-xylene
15-
0
5
10
15
b
% o-xylene >
F i g . 2. m-Xylene i s o m e r i z a t i o n on s i l i c a - a l u m i n a alumina ( A ) , Y z e o l i t e ( + ) , m o r d e n i t e zeolite ( + )
.
2.2
(
( o ) , fluorinated 0 ) and ZSM-5
D i s p r o p o r t i o n a t i o n mechanisms
B e s i d e s t h e mechanism of d e a l k y l a t i o n - a l k y l a t i o n which would be v e r y u n l i k e l y i n t h e c a s e of x y l e n e s ( c f . i s o m e r i z a t i o n ) two o t h e r mechanisms have been proposed ( 3 ) : - t h e f i r s t i s very s i m i l a r t o t h e dealkylation-alkylation mechanism. The methyl group of a benzenium i o n undergoes t h e nuc l e o p h i l i c a t t a c k of a n a r o m a t i c m o l e c u l e and i s t r a n s f e r r e d w i t h o u t a p p e a r i n g i n c a r b o c a t i o n form ; - t h e second i n v o l v e s b e n z y l i c c a r b o c a t i o n s and d i a r y l methane i n t e r m e d i a t e s ( f i g u r e 3 ) . I t may be n o t e d t h a t t h e f i r s t mechanism invokes t h e same benzenium i o n i n t e r m e d i a t e s a s t h e i n t r a m o l e c u l a r i s o m e r i z a t i o n mechanism, w h i l e t h e second mechanism invokes d i f f e r e n t i n t e r m e d i a t e s . S e v e r a l observations r e c e n t l y published favour t h e d i s p r o p o r t i o n a t i o n mechanism by t h e i n t e r m e d i a t e of b e n z y l i c c a r b o c a t i o n s : - t h e a d d i t i o n t o o-xylene of a l k a n e s c a p a b l e of g i v i n g a ) causes t e r t i a r y c a r b o c a t i o n (methyl-2 p e n t a n e , methylcyclohexane an i m p o r t a n t d e c r e a s e i n t h e d i s p r o p o r t i o n a t i o n a c t i v i t y w i t h o u t
...
Fig. 3.
Disproportionation mechanism of xylenes via benzylic carbocation intermediates.
affecting in a significant manner the isomerization activity (figure 4). In order to explain this result, it is proposed that the disproportionation intermediates but not those of isomerization are consumed by reacting with the alkanes (7). This implies the existence of different intermediates in the two reactions : the disproportionation intermediate cannot therefore be the benzenium ion intermediate of isomerization ; no reaction moreover is possible between this latter intermediate and alkanes. However, benzylic carbocations can react with branched alkanes with hydride transfer from the alkanes to the benzylic carbocations :
0,50-
2
0
Fig. 4.
1
b 10 15 a l k a n e ( p e r c e n t a g e added)
5
T r a n s f o r m a t i o n of o-xylene on Y z e o l i t e a t 350°C : I n f l u e n c e of a l k a n e c o n t e n t on t h e d i s p r o p o r t i o n a t i o n (D) t o i s o m e r i z a t i o n (I) r a t e r a t i o .
- hydrogen under p r e s s u r e i n h i b i t s t o l u e n e d i s p r o p o r t i o n a t i o n on m o r d e n i t e s (8). T h i s i n h i b i t i n g e f f e c t i s e x p l a i n e d by a d e c r e a s e of t h e c o n c e n t r a t i o n s of b e n z y l i c c a r b o c a t i o n i n t e r m e d i a t e s caused by t h e f o l l o w i n g r e a c t i o n ( r e v e r s e r e a c t i o n of b e n z y l i c c a r b o c a t i o n f o r m a t i o n by a n a r o m a t i c a d s o r p t i o n on a BrGnsted a c i d s i t e ) :
Here a g a i n no p o s s i b l e r e a c t i o n e x i s t s between benzenium i o n s and hydrogen. 2.3
~ispro~ortionation/Isomerization rate ratio
The r a t i o between t h e d i s p r o p o r t i o n a t i o n (D) and t h e isomeriz a t i o n r a t e s ( I ) depends a g r e a t d e a l on t h e a c i d i t y and on t h e p o r o s i t y of t h e z e o l i t e s Most a u t h o r s a g r e e t o r e c o g n i z e t h a t t h e s e two r e a c t i o n s i n v o l v e d i f f e r e n t a c i d s i t e s . However, some p o i s o n i n g experiments by p y r i d i n e c a r r i e d o u t on Y z e o l i t e show t h a t a c t i v e s i t e s i n 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 of x y l e n e ( f i g u r e 5) d i f f e r v e r y s l i g h t l y i n a c i d s t r e n g t h ( 9 ) . Yet t h e s e s i t e s a r e c e r t a i n l y d i f f e r e n t i n n a t u r e : t h e i s o m e r i z a t i o n r e q u i r e s o n l y one BrGnsted a c i d c e n t e r ( 1 0 , l l ) w h i l 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 i s more
.
Fig. 5.
I s o m e r i z a t i o n ( I ) and d i s p r o p o r t i o n a t i o n (D) of o-xylene on a p y r i d i n e poisoned Y z e o l i t e : R a t i o of t h e a c t i v i t i e s of poisoned (A ) and unpoisoned (Ao) c a t a l y s t s f o r d i f f e r e n t d e s o P p t i o n t e m p e r a t u r e s (TD)
.
demanding : t h e a c i d c e n t e r s a r e p r o b a b l y c o n s t i t u t e d by a p a i r of a c i d s i t e s ( 1 2 ) . I n agreement w i t h t h i s h y p o t h e s i s , i t can be n o t e d t h a t d i s p r o p o r t i o n a t i o n i s always more s e n s i t i v e t h a n i s o m e r i z a t i o n t o z e o l i t e t r e a t m e n t s which modify t h e a c i d c e n t e r c o n c e n t r a t i o n s (exchange of p r o t o n s by a l k a l i n e i o n s , chemical d e a l u m i n a t i o n , c a l c i n a t i o n under wet a i r ) ( 1 3 ) . Thus t h e exchange of p r o t o n i c morden i t e by sodium i o n s which d e c r e a s e s t h e a c i d c e n t e r c o n c e n t r a t i o n s d e f i n i t e l y i n c r e a s e s the isomerization s e l e c t i v i t y (14,15). The D / I r a t i o depends on z e o l i t e p o r e s i z e and e s p e c i a l l y on the space a v a i l a b l e i n t h e v i c i n i t y of a c t i v e c e n t e r s . Actually, t h e b i m o l e c u l a r i n t e r m e d i a t e s and t r a n s i t i o n s t a t e s of d i s p r o p o r t i o n a t i o n , c o n t r a r i l y t o t h o s e of i n t r a m o l e c u l a r i s o m e r i z a t i o n a r e v e r y b u l k y and t h e i r f o r m a t i o n can be i n h i b i t e d a s a r e s u l t of s t e r i c c o n s t r a i n t s . This e x p l a i n s the D / I decrease with t h e i n t r a c r y s t a l l i n e c a v i t y d i a m e t e r (16) : i t i s n o t a b l y s m a l l e r on morden i t e t h a n on Y z e o l i t e and p r a c t i c a l l y e q u a l t o z e r o on ZSM-5 z e o l i t e . However, a n o t h e r e x p l a n a t i o n must be found f o r t h e low v a l u e of t h e D / I r a t i o i n t h e c a s e of o f f r e t i t e . I n t h i s c a s e , d i f f u s i o n a l l i m i t a t i o n s r e s t r i c t i n g t h e d e s o r p t i o n of t r i m e t h y l benzene t h e s i z e of which i s g r e a t e r t h a n t h a t of p-xylene a r e r e s p o n s i b l e f o r t h e low d i s p r o p o r t i o n a t i o n a c t i v i t y (1 5 ) .
2.4
Activity and stability of catalysts
Figure 6 shows that zeolites are more active than amorphous catalysts. Among the zeolites, mordenite presents a very high initial activity but it is very rapidly deactivated at least under atmospheric pressure. This deactivation is due to the formation of very heavy polyaromatic compounds ("coke"). On the contrary, ZSM-5 zeolite is practically not deactivated. On this zeolite, steric constraints inhibit the formation of highly bulky transition states necessarily involved in the numerous bimolecular steps of coke formation. Coke however can only be formed on external surface of the zeolite (17,18). Mordenite stability can notably be improved (19,20). It is particularly the case if they have been calcined under wet air since this treatment eliminates preferentially the acid sites of great coking activity (21). Mordenite stability moreover is actually improved by operating under high hydrogen pressure ( 2 0 , 2 2 ) . Finally the association of a hydrogenating function with an acid function allow us to limit coke formation and mordenite deactivation ; however the hydrogenating power of catalysts must not be very important so as to avoid secondary reactions of hydrogenolysis and hydrocracking.
SM-5 zeolite
Fig. 6.
.ene isomerization on mordenite, Y zeolite, ZSM-5 zeolite and silica-alumina.
3. ISOMERIZATION OF C8 AROMATIC HYDROCARBONS ON BIFUNCTIONAL ZEOLITE CATALYSTS Ethylbenzene requires for its isomerization bifunctional metalacid catalysts : thus P.B. Weisz (23) shows that platinum and silica-alumina used separately have no activity while their mixture is very active. On these catalysts, xylenes can be isomerized on acid centers or again by the successive intervention of hydrogenating and acid centers as is the case for ethylbenzene. Various secondary reactions moreover can be observed : - xylenes and ethylbenzene disproportionation on acid centers, - dealkylation on metallic or acid centers, - hydrogenation into naphtenes on metallic centers, - hydrocracking through a bifunctional mechanism. 3.1
Ethylbenzene isomerization mechanism
A kinetic study of ethylbenzene isomerization (24) carried out on platinum fluorinated alumina catalysts and on mixtures of platinum-inert alumina and fluorinated alumina has allowed us to propose the reactional scheme shown in figure 7 :
.
EB 1
(ZEOLITE)
(p) 112H2
ECHE
-ECH+ -DMCH' -DMCHE
EB
+
5
-H
+H+
L
2
Fig. 7.
-2H2]l
X
3
L
4
Ethylbenzene isomerization reactionnal path.
ECHE
ECH+ DMCHE
: olefins and carbocations with : ethylbenzsne, and and : olefins and carboan ethylcyclohexane skeleton, cations with a dimethylcyclohexane skeleton, X : xylenes.
DMCH+
Ethylbenzene is hydrogenated on metallic sites into ethylcyclohexenes and ethylcyclohexane ; the highly active ethylcyclohexenes are isomerized on acid centers into dimethylcyclohexenes which are dehydrogenated into xylenes on metallic sites. Over industrial catalysts, the hydrogenation activity is sufficient for the hydrogenation and dehydrogenation reactions to be more rapid than the isomerization of the intermediate olefins ; the slow step of the reactional process is then the rearrangement of ethylcyclohexylic
c a r b o c a t i o n s i n t o d i m e t h y l c y c l o h e x y l i c ones ( s t e p 3) e . g . e t h y l b e n z e n e i s o m e r i z a t i o n i n t o o-xylene.
i n the
According t o N i t t a and J a c o b s (25) t h e mechanism of f i g u r e 7 a c c o u n t s e q u a l l y f o r e t h y l b e n z e n e i s o m e r i z a t i o n on P t - z e o l i t e . 3.2
S e l e c t i v i t y of 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
T h i s s e l e c t i v i t y depends a g r e a t d e a l on t h e r e a c t i o n temper a t u r e (24.25) , - A t low t e m p e r a t u r e , a r o m a t i c h y d r o g e n a t i o n i n t o n a p h t e n e s i s thermodynamically f a v o u r e d . Taking i n t o a c c o u n t t h e h i g h hydrogenat i n g a c t i v i t y o f t h e c a t a l y s t s t h e n a p h t e n e f o r m a t i o n c a n become i m p o r t a n t ; t h e r e f o r e t h e o p e r a t i o n t e m p e r a t u r e w i l l be chosen s o a s t o m a i n t a i n t h e n a p h t e n e c o n t e n t below 10 % ( 2 ) . - A t h i g h t e m p e r a t u r e , d i s p r o p o r t i o n a t i o n and d e a l k y l a t i o n r e a c t i o n s become p r e p o n d e r a n t . Furthermore, d e a c t i v a t i o n becomes rapid.
.
The importance of t h e secondary r e a c t i o n s of d i s p r o p o r t i o n a t i o n and d e a l k y l a t i o n i s a l l t h e g r e a t e r t h a n t h e a c i d c e n t e r s of t h e c a t a l y s t a r e s t r o n g e r ; t h e r e f o r e t h e most s e l e c t i v e c a t a l y s t s f o r e t h y l b e n z e n e i s o m e r i z a t i o n must be of medium a c i d i t y ( 2 4 , 2 5 ) . T h i s i s n o t s u r p r i s i n g s i n c e t h e a c i d s t r e n g t h required f o r o l e f i n skelet a l isomerization ( l i m i t i n g r e a c t i o n i n ethylbenzene isomerization) i s not a s g r e a t a s t h a t required f o r ethylbenzene disproportionat i o n ( 2 6 ) . Moreover, d i s p r o p o r t i o n a t i o n b e i n g b i m o l e c u l a r p r o b a b l y r e q u i r e s a p a i r of a c i d c e n t e r s w h i l e o l e f i n i s o m e r i z a t i o n b e i n g i n t r a m o l e c u l a r r e q u i r e s o n l y one BrGnsted a c i d s i t e . The c a t a l y s t s s h o u l d b e a l l t h e more s e l e c t i v e t h a n t h e d e n s i t y of t h e i r a c i d c e n t e r s i s lower. F i n a l l y , l i k e f o r x y l e n e i s o m e r i z a t i o n , z e o l i t e s which a r e n o t v e r y a c t i v e i n d i s p r o p o r t i o n a t i o n a s a r e s u l t of s t e r i c c o n s t r a i n t s ( m o r d e n i t e and above a l l ZSM-5) w i l l p r e s e n t a g r e a t i n t e r e s t . T h i s i n t e r e s t w i l l be even g r e a t e r i f p-xylene isomer c a n b e p r i v i l e g e d f o r c o n f i g u r a t i o n a l r e a s o n s a s i s t h e c a s e w i t h ZSM-5 z e o l i t e s . Conclusion Numerous p r o c e s s e s of C 8 a r o m a t i c i s o m e r i z a t i o n u s i n g z e o l i t e s a s c a t a l y s t s have been r e c e n t l y developped. Z e o l i t e s such a s m o r d e n i t e and ZSM-5, w h i l e b e i n g v e r y s t a b l e , p r e s e n t t h e advantage of b e i n g more a c t i v e t h a n amorphous c a t a l y s t s . Moreover t h e i r porous s t r u c t u r e g i v e them a h i g h s e l e c t i v i t y : t h e secondary
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 i s v e r y l i m i t e d and t h e i s o m e r i z a t i o n , a t l e a s t i n t h e c a s e of ZSM-5 c a t a l y s t s , i s o r i e n t e d towards t h e p-xylene f o r m a t i o n .
References
1.
2. 3.
4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
17. 18. 19. 20.
21.
Chauvel A . , L e f e b v r e G. and Raimbault C . , P r o d u c t i o n d ' o l 6 f i n e s e t d ' a r o m a t i q u e s ( P u b l i c a t i o n s d e 1 ' I n s t i t u t F r a n ~ a i sdu P g t r o l e , Technip 1980), p. 95. M a r c i l l y C . , Techniquesde 1 1 1 n g 6 n i e u r (1983), i n p r e s s . Poutsma M.L., 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, 1971) pp. 431-528. Lanewala M.A. and Bolton A.P., J . Org. Chem. 34 (1969) 3107. G u i s n e t M . , Gnep N.S., Bearez C . and C h e v a l i e r F . , 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 1 E d s . ) , S t u d i e s i n S u r f a c e S c i e n c e and C a t a l y s i s , Vol. 5 ( E l s e v i e r S c i e n t i f i c P u b l i s h i n g Company, Amsterdam, 1980) pp. 77-83. Young L.B., B u t t e r S.A. and Kaeding W . W . , J . C a t a l . 76 (1982) 418. Gnep N.S. and G u i s n e t M . , R e a c t . K i n e t . C a t a l . L e t t . (1983) i n press. Gnep N.S. and G u i s n e t M . , Applied C a t a l y s i s 1 (1981) 329. G u i s n e t M. and P e r o t G . , Symposium on Shape S e l e c t i v e C a t a l y s i s , 185th ACS N a t i o n a l Meeting, S e a t t l e , March 1983. Ward J . W . and Hansford R.C., J . C a t a l . 13 (1969) 154. Hansford R.C. and Ward J . W . , J . C a t a l . 13 (1969) 316. Ward J . W . , J . C a t a l . 13 (1969) 321 ; I b i d . 14 (1969) 365 ; I b i d . 17 (1970) 355 ; I b i d . 26 (1972) 451 ; I b i d . 26 (1972) 470. C s i c s e r y S.M. and Hickson D . A . , J . C a t a l . 19 (1970) 386. M a r t i n d e Armando M.L., Gnep N.S. and G u i s n e t M . , J . Chem. Research (S) (1981) 8 . Ratnasamy P . , S i v a s a n k a r S. and V i s h n o i S . , J . C a t a l . 69 (1981) 428. T e j a d a G . , Gnep N.S. and G u i s n e t M . , t o b e p u b l i s h e d . Weisz P.B., P r o c e e d i n g s of t h e 7 t h I n t e r n a t i o n a l Congress on C a t a l y s i s , P a r t A (Seiyama T. and Tanabe K. Eds.,Kodansha LTD, Tokyo, 1980) pp. 3-20. B u t t J . B . , Delgado-Diaz S. and Muno W.E., J . C a t a l . 37 (1975) 158. B u t t J . B . , J . C a t a l . 41 (1976) 190. M a r c i l l y C . , French P a t e n t s (1976) Gnep N.S., M a r t i n d e Armando M.L., M a r c i l l y C . , Ha B.H. and G u i s n e t M . , C a t a l y s t D e a c t i v a t i o n (Delmon B. and Froment G.F. E d s . ) , S t u d i e s i n S u r f a c e S c i e n c e and C a t a l y s i s 6 ( E l s e v i e r S c i e n t i f i c P u b l i s h i n g Company, Amsterdam, 1980) pp. 79-89. Mirodatos C . and Barthomeuf D . , J. Chem. Research (1981) 3 9 .
.
22. Gnep N.S., M a r t i n d e Armando M.L. and G u i s n e t M . , R e a c t . K i n e t . C a t a l , L e t t . , 13 (1980) 183. 23. Weisz P.B., Advances i n C a t a l y s i s 13 (Academic P r e s s , London, 1962) p. 137. 24. Gnep N.S. and G u i s n e t M . , B u l l . Soc. Chim. d e F r a n c e , (1977) 429. 25. N i t t a M . and J a c o b s P . A . , C a t a l y s i s by Z e o l i t e s ( ~ m e l i k8 . e t a 1 Eds.) S t u d i e s i n S u r f a c e S c i e n c e and C a t a l y s i s , Vol. 5 ( E l s e v i e r S c i e n t i f i c P u b l i s h i n g Company, Amsterdam, 1980) pp. 251-259. 26. M a r s i c o b s t r e D . , Gnep N.S., G u i s n e t M. and Maurel R . , Rev. P o r t . Quim. 18 (1976) 316.
ION EXCHANGE SEPARATIONS WlTH MOLECULAR SIEVE ZEOLITES
JOHN D. SHERMAN MOLECULAR STEVE DEPARTMENT, UNION CARBIDE CORPORATION, TARRYTOWN TECHNLCAL CENTER, TARRYTOWN, NEW YORK 10591, USA @ 1983 UNION CARBIDE CORPORATION, all rights reserved. ABSTRACT Molecular sieve zeolite cation exchangers provide unique combinations of selectivity, capacity and stability not available in other ion exchangers. Commercial applications include separations of radioisotopes, waste water ammonia removal and as detergent builders. New zeolite products with still higher ammonium ion selectivities and capacities and products with improved calcium and magnesium exchange capabilities for detergent applications have also been developed. New applications in artificial kidney devices, radioactive waste separations and disposal, heavy metals removal and animal feeding are under development. Potential uses in metals recovery and in separations and purifications of non-ferrous metals are being explored. Applications have grown rapidly with increasing awareness of zeolite properties and the imaginative consideration of their potential uses in ion exchange separations. 1 BACKGROUND
Molecular sieve zeolites are crystalline, hydrated aluminosilicates of (most commonly) Na+, K+, Hg++, ~ r + +and ~ a + +cations. The aluminosilicate portion of the structure is a 3-dimensional open framework consisting of a network of A104 and Si04 tetrahedra linked to each other by sharing all the oxygens. Zeolites may be represented by the empirical formula:
In this oxide formula x is normally > 2 since A104 tetrahedra are joined only to Si04 tetrahedra; n is the cation valance. The framework contains channels and interconnected voids occupied by the cations and water molecules. The cations are quite mobile and can usually be exchanged, to varying degrees, by other cations. In 1858, Eichorn (70) showed zeolites are capable of reversible exchange of cations. This is a characteristic property of the zeolites. Extensive studies have also shown exchange selectivities in zeolites do not follow the typical rules and patterns exhibited by other organic and inorganic ion exchangers. Also, many zeolites provide combinations of selectivity, capacity and stability superior to the more common organic and inorganic cation exchangers. The term "zeolite" properly refers to the crystalline molecular sieve ion exchangers. Unfortunately, some confusion has been created by improper use of the term. The accepted term for a synthetic amorphous aluminosilicate is "permutite_". The zeolite cation exchangers discussed herein are of the crystalline framework aluminosilicate type with the scientifically correct designation "zeolite." Union Carbide Corporation owns the rights to the trademark LINDE in the U.S.A., Canada and Mexico. Our references in this paper to LINDE are to the trademark owned by Union Carbide Corporation. We want to avoid any confusion with the LINDE trademark as owned by others elsewhere in the world. Development of zeolites was preceded by development of the permutites which supplemented the use of natural microcrystalline aluminosilicates ("green sands" or glauconites), and led to significant expansion of the use of ion exchange, especially for water softening. A disadvantage of the permutites was their solubility, at extremes of pH, rendering it nearly impossible to regenerate them by H+ exchange. Although the process of ion exchange was discovered in 1850 (185, 192). it was not applied as an industrial separation process until 1905, when Gans (83) demonstrated it could be used as a unit process for both water softening and removal of certain metal ions, especially iron and manganese (56). From 1905 to 1935, the only ion exchangers available were the inorganics, usually aluminosilicates either green sands or permutites, operated only in the neutral pH regions, using salt as the regenerant. In 1935, sulfonated coal ion exchangers which could be regenerated with acid were commercially developed. Condensation polymers with sulfonic and amino groups were developed shortly thereafter. In 1946, sulfonated and aminated copolymers of styrene and divinyl benzene became available. The development of these new organic ion exchangers with superior stability and ease of regeneration enabled a rapid expansion in the range of industrial applications of the ion exchange process (56).
Why, then, has there been a renewed interest in inorganic ion exchangers and particularly, zeolites, for ion exchange applications? The greater stability of some zeolites (vs. organic resin ion exchangers) under certain conditions and their high selectivity towards particular ions have combined to allow the development of new applications for the ion exchange process. This has @ generally involved the displacement by zeolites of other exchangers in existing applications, but rather the development of entirely new applications for which the existing ion exchangers were not well suited. It is reasonable to expect that this trend will continue. Zeolites did not find significant use commercially as ion exchangers until the early 1960's. due to lack of availability and lack of knowledge of their properties. Both barriers disappeared during the 1950's and 1960's. The discovery by R. M. Milton and co-workers at Union Carbide that zeolites could be synthesized at convenient conditions led to the discovery of dozens of new zeolite structures and assured their availability in commercial quantities. Extensive exploration by Union Carbide in the late 1950's also resulted in discovery in the Western U.S. of many deposits of natural zeolites of sufficient quantity and purity for commercial use. Knowledge of the desirable ion exchange properties of zeolites also became available due to the reports of Ames (1-151, Amphlett (161, Barrer (26, 29, 30), Thomas (184) and others. The first commercial uses were developed in the early 1960's by Ames et al. for processing radioactive wastes. Their stability in ionizing radiation and at elevated temperatures and pH levels, and excellent capacities and selectivities, caused certain zeolites to be uniquely suited for recovery and concentration of radioisotopes for long-term storage. Later discovery (also by Ames) that some zeolites had excellent NH4+ ion selectivity led to the development in the late 1960's / early 1970's of the second significant commercial application: the removal of N H ~ +from municipal wastewater. Other applications are being developed. Some are discussed in the present paper. Growing knowledge of zeolites and growing needs (in pollution abatement, energy production, agriculture, aquaculture, animal nutrition, metals processing and biomedical applications) promise many exciting new applications for these unique materials. Back~roundReferences: The structure, chemistry and use of zeolites was broadly reviewed by D. W. Breck (51). including a 64page chapter on ion exchange suggested for background reading. Sherry (172, 174) reviewed ion-sieving and ion selectivity phenomena. Cremers (66) reviewed more recent studies. Others (16, 27-30, 38, 39, 96, 139) reviewed earlier work. Mercer and Ames reviewed radwaste and NH4+ removal applications (127). Zeolite ion exchange applications more broadly were reviewed by Sherman (166).
Barrer (40) reviewed equilibrium and kinetic aspects of zeolite ion exchange at the 5th International Conference on Zeolites. Kees will discuss binary and ternary cation exchange on zeolites at the coming 6th International Conference (141). 2
ION EXCHANGE PROPERTIES OF MOLECULAR SIEVE ZEOLITES
The above papers extensively review the basic ion exchange phenomena observed on zeolites. The present paper includes only a few examples. Typical properties are summarized in Table l(166). TABLE 1. TYPICAL PROPERTIES OF THE MOST COMMON MOLECULAR SIEVE ZEOLITES Typical maximum Typical theoretical cation Pore openings Si02/A1203 exch. capacity Zeolite type (hydrated form) mole ratio (Na+ form, anhydrous) Analcime
2.6 A
4
Chabazite
3.7 X 4.2 A, and 2.6 A
4
Clinoptilolite
4.0 X 5.5 A, and 4.4 X 7.2 A, and 4.1 X 4.7 A
10
Erionite
3.6 X 5.2 A
6
Ferrierite
4.3 X 5.5 A 3.4 X 4.8 A
11
Mordenite'
6.7 X 7.0 A 2.9 X 5.7 A
10
Phillipsite
4.2 X 4.4 A 2.8 X 4.8 A 3.3 A
4.4
4.7
LlNDE A
4.2 A into a cage 2.2 A into P cage
2
7.0
2
7O .
6
3.8
LlNDE
F
Zeolite HS (Hydroxysodalite)
-3.7 A 2.2 A
LINDE L
7.1 A
Large-port Mordenite
6.7 X 7.0 A 2.9 X 5.7 A
LlNDE Omega
7.5 A
Zeolite P (LINDE B l
3.1 X 4.4 A 2.8 X 4.9 A
10 7
3.4
LINDE T
3.6 X 5.2 A
LlNDE W
4.2 X 4.4 A
3.6
5.3
LlNDE X
7.4 A into a cages 2.2 A into P cages
2.5
6.4
LlNDE Y
7.4 A into a cages 2.2 A i n t o 0 cages
4.8
4.4
'The large pores are partially blocked i n natural mordenite.
TABLE 2. ION SIEVING EFFECTS IN MOLECULAR SIEVE ZEOLITE ION EXCHANGE Exchange proceeds Zeolite
Cation
Diameter* (A)
Exchange negligible Cation Diameter* (A)
Size of largest pore openings"
Analcime
~ b +
2.96 (XI 6.58 (H)
Cs+
3.38 (X) 6.58 (H)
2.6 A
Chabazite
Cs+
3.38 (X) 6.58 (HI
N(CHJ)~+
6.94 (X) 7.34 (HI
3.7 X 4.2 A
Clinoptilolite
Cs+
3.38 (X) 6.58 (H)
N(cH~)~+
6.94 ( X ) 7.34 (H)
4.4 X 7.2 A
LlNDE X
N ( c H ~ ) ~ 6.94 (X) 7.34 (H)
N ( C ~ H S ) ~ +8.0 ( x ) 8.0 (H)
7.4 A
'Data from Nightingale '( 1 3 0 ) (X) = {Crystal radius) X (2) (H) = (Hydrated radius) X (2) **From crystallographic data.
Ion Sievin~Effects: Ion sieving effects are observed with the zeolites having the smallest pore openings and with the largest cations, as illustrated in Table 2 (166). Some ions are exchanged even though their hydrated diameters are much larger than the pore openings; this requires an exchange of solvent molecules. Ions with too large anhydrous diameters are totally excluded. Exchange rates vary due to differences in energies required to exchange solvent molecules and may be very slow at lower temperatures if the water of hydration is strongly held: e.g., ~ a + +enters the Type A zeolite rapidly at 25 C, whereas I4gtk enters very slowly (see Figure 1)(167).
Figure
NaA AT ROOM TEMPERATURE 0.60 g NaA/liter
There are also ion sieving effects inside the zeolite cages in which larger ions are excluded from entering some of the inner cavities of the zeolite due to the small size of the entrances to these cavities. For example, at 25 C large ions do not replace all Na+ ions in LINDE Type X or Y zeolites; 16 ~ a +ions per unit cell can not be replaced by La+++ or alkaline earth exchange of zeolite Y (170) or by exchange of Mn++, Cot+ and ~ i + 'on zeolites X and Y (82). These 16 sodium ions are present in side cavities with small entrances which prevent the larger ions from entering at low temperature. The exchangeable ions are those present in the supercages. Effect of Ion Volume: The size of the entering cation can also appreciably affect both the rate and extent of ion exchange. For example, in exchange of various alkyl ammonium ions in clinoptilolite at 60 C, complete replacement of all the ~ a +ions was accomplished by each of these cations, except for trimethyl-, i-propyl-, and n-propylammonium ions, for which only partial replacement was accomplished. The water content of the zeolite decreased with increasing loading and with increased volume of the organic cation. The larger ions above could not enter channel systems controlled by diffusion through 8-rings, whereas they could enter those channels controlled by diffusion through 10-rings. Still larger ions such as tetramethyland t-butyl-ammonium ions were completely excluded from even the largest pores (32). Ion Exchange Capacity: The ion exchange capacity of a zeolite on exchanger is a function of its Si02/A1203 mole ratio, and also of ts cation form. The maximum exchange capacities of common zeolites are given in Table 1. Not all the capacity is available to all ions due to the ion sieving effects discussed above. Comparison of LINDE Type A zeolite and a common strong acid type organic resin ion-exchanger is given in Table 3. As may be seen, the zeolite compares well in capacity on either a weight or volume basis. Table 3.
Comparison of LINDE Type A Molecular Sieve Zeolite with Common (Polystyrene Divinylbenzene Sulfonic Acid Type) Organic Resin Ion Exchangers (166). LINDE A zeolite Form Mesh or Beads Cat ion Na+ Bulk Density (g/ml) 0.65 Total Capacity (max.) 5.6 * g mEq 3.6 mEq / ml
*
Organic Resin Beads Na+ 0.38 4.8 1.8
Based on anhydrous weight of 80% LINDE Type A zeolite / 20% bond.
Ion Exchange Selectivities: Zeolites commonly exhibit high selectivities for exchange among cations which will easily enter the zeolite pores. For example, LINDE Type A zeolite provides a striking selectivity for Ca++ over Na+ compared with common organic resin strong acid type cation exchangers. Each zeolite provides a different pattern of ion exchange selectivity. Selectivity series of increasing preference for exchange of different cations for the most common zeolites are summarized elsewhere (166). In general, common organic resin cation exchangers prefer ions of higher charge. This is also true of many zeolites (e.g., LINDE Type A and X zeolites). However, some zeolites show marked selectivity for some monovalent cations over common divalent cations. For example, LINDE W exchanges N H ~ +in marked preference to ~ a + +and prefers Na+ over Cat'. Host zeolites exhibit selectivities for Ag, T1 and Pb exchange and many also are selective for exchange of other non-ferrous metal cations (e.g., Cd, Zn and Cu). Theoretical analyses of ion exchange in glasses led Eisenman (71) to conclude that selectivity was controlled by the anionic field strength of the exchange site, and to predict ion exchange selectivity series. Sherry (172) showed ion exchange behavior of zeolites is in general agreement with these predictions. The zeolite ion exchange affinity sequence is often found to be in accord with the hydrated ionic radius, so the affinity sequence ~ i +< Na+ < K+ < ~b~ < CS+, and for divalent ions: is: Hg++ Ca++ < Sr++ < ~ a + + . Often a zeolite favors the least hydrated ion, while the solution phase favors the most highly hydrated ion. In other words, water molecules in solution compete with the zeolite for attraction of the cations.
<
In his theoretical analysis of ion exchange selectivity in glasses, Eisenman (71) separated the free energy of the ion exchange reaction into two parts. As applied to zeolites, this may be written as follows (173):
where Z = zeolite and s = solution phase. The first term on the right is the difference between the free energies of ions A and B in the zeolite and the second term is their difference in solution, which equals the differences in the free energies of hydration of the ions. If the electrostatic fields in the zeolite are strong, then the first term dominates and small ions are preferred by the zeolite even though such ions have large free energies of hydration. On the other hand, large, wholly hydrated ions are preferred in "weak" field zeolites, i.e., those with lower aluminum content and, thus, lower framework charge density. Also, if the pore volume of
the zeolite is smaller, then the degree of hydration,of ions inside the zeolite will be less, also favoring exchange of ions with lower hydration energies. For example, LINDE Type A zeolite with a high aluminum content and high void volume exhibits the selectivity series (51): CS+ < Li+ < ~ b +< NH,I,+ < K+ < NaS. On the other hand, chabazite (35,172). erionite (7, 13, 172). and phillipsite (6, 33, 161). which each have lower aluminum content and lower pore volume (VS Type A zeolite), exhibit the "weak field" selectivity series: ~ i +< Na+ < K+ < ~ b +< Csi. Since most zeolites have lower aluminum contents and lower pore volumes, the weak field selectivity series is often observed. However, even among these zeolites the relative selectivities for the cations in the series differ greatly. Decreasing the aluminum content of the zeolite also increases the average distance between the adjacent anionic (A1021 sites on the zeolite framework. This, in effect, increases the difficulty of a single divalent cation in "satisfyingw the fields of two adjacent anionic sites. As a result, as expected, the preference of the zeolite for divalent cations decreases, and that for univalent cations increases, as the aluminum content decreases (173). Exchange Equilibrium Isotherms: It is convenient to express -Ion the results of selectivity measurements as ion exchange isotherms. For a reaction in which the exchanging ions A and B, have charges ZA+ and ZB+, respectively, the ion exchange isotherms are typically expressed as plots of the equivalent cation fraction, As, of the ion A in solution against the fraction Ac of the same ion in the ion exchanging zeolite. Typical isotherms for ion exchange on zeolite A are shown in Figure 2. An excellent discussion of Isotherms for Exchange of Ca++ for Na+ at Room Temperature on LlNDE Type A Molecular Sieve Zeolite(l67).
0.1
2
U -4
B P a! r
gz
:8eE
SOLUTION
-
-
-
g s o
Eg
N
l
O
l
"
"
l
0.5
l
'
L
1 .O mEq Ca*' REMAINING IN SOLUTION/ mEq Cat+ IN INITIAL SOLUTION
procedures for measurement and interpretation of ion exchange data on zeolites has been presented by Dyer, Enamy and Townsend (69). Other variables Affectincation Exchange in Zeolites: The observed ion exchange selectivities and loadings on zeolites are dependent upon pH (HS is a competing cation), temperature and aqueous solution chemistry. The types and concentrations of anions, competing cations, solvent and complexing agents can each alter the quality of the ion exchange separation which can be achieved (via the effects of these variables upon the activities of the cations in solution). Fortunately, because of the rigidity of zeolite frameworks the effects of these variables upon the overall ion exchange performance of the zeolite are generally less complex and somewhat more predictable than with some organic resion ion exchangers in which complex sorption and swelling phenomena can occur, accompanied by changes in their ion exchange characteristics ( 9 6 ) . Complexing of the cation can markedly alter its exchange properties. For example, Ag+ exchange is very much favored over Na+, but ~ a +is preferred over the A ~ ( N H ~ ) ~complexed + ion on LINDE X zeolite. Thus, regeneration of the exchanged zeolites may be accomplished using complexing agents in the regenerant solution. Also, addition of complexing agents may allow zeolite ion exchange se,parations (e.g., for selective non-ferrous metal exchanges) not otherwise obtainable. The effective ion exchange capacity of the zeolite may be greatest at elevated temperatures as in the admission of large ions at elevated temperatures (e.g., see 121). Also, in some cases, the ion exchange which has been carried out at higher temperatures may be partially irreversible at lower temperatures at which the large ions are not capable of diffusing through the smaller pores. Unusual ion exchange behavior Ion Exchange Immiscibility Ga& was observed by Olson and Sherry for ~r++/~a'exchange of Type X zeolite (132) caused by a region of limited mutual solubility of the end members NaX and SrX. The ion exchange loading curves show a sudden jump when the ~ r exchange + ~ levels on the zeolite reach 70%. and this is accompanied by an abrupt change in the unit cell dimensions of the zeolite crystals. Similar immiscibility gaps have been observed in other zeolites; e.g., see Barrer and Munday (34). Exchange of Metal Ions: Exchange of multivalent metal ions is complicated by the need to maintain the pH levels in solution low enough to avoid the solubility limits of the metals and high enough to minimize proton exchange and hydrolysis of the zeolite. Since lower pH levels are required to provide adequate solubility of trivalent ions in aqueous solutions, there are few reports of such primarily on exchanges (principally of Y+++, ~ e + + + ,La'++), more acid-stable zeolites (LINDE X and Y , mordenite, clinoptilolite).
Exchange of LINDE Type A with multivalent ions presents the most difficult task experimentally, because of the limited stability of this zeolite at low pH levels. Breck et al. (48) reported successful exchange of NaA with Ba++, Hg++, Cd++, Zn++, Co++, and Ni++, but that CU++ and Fe+++ exchange destroyed the structure and that no exchange could be observed with ce+++. Mercer and Ames (125) later reported the ce+++ exchange of LINDE A zeolite, but without structural studies confirming retention of crystal structure. Most recently, Wiers et al. (194) reported reversible c a + + / ~ b + + and Ca++/cdt+ exchange, and successful but irreversible CU++ exchange of c ~ + + A ,but observed complete crystal structure loss upon attempts at Hg++ exchange of c ~ + + A . Even in methanol solution, ~ e + + +nitrate and Al+++ nitrate exchange caused complete loss of crystallinity of N ~ + A . On the other hand, for LINDE Type X and Type Y zeolites, Na+/ce+++ (8, 10, 1251, N ~ + / Y + + + (146), and reversible ~ a + / ~ a + + + (170) exchanges have been reported. The above has emphasized some pitfalls which may be encountered if zeolites are employed to separate metal ions. On the other hand, many zeolites are quite selective for exchange of particular metal ions and their use for pollution abatement, metals recovery and other unique separations offer promise for future applications. Hydrogen Ion Exchange of Zeolites: When zeolites are contacted with solutions of lower pH values, proton exchange will occur. When ion exchange reactions are studied with solutions comprising salts which generate low pH solutions, the intended binary ion exchange may, in actuality, involve a ternary ion exchange with the hydronium ion H30+ as a third party to the exchange (e.g., 119-122, 131). The extent of proton exchange can be determined by computation from ion balances in solution or by chemical analyses of the zeolite. Thermodynamic Treatment of Ion Exchange Equilibria and Selectivity Numerous approaches to characterizing the equilibrium between ion exchange materials and aqueous solutions have been described in the literature. Gaines and Thomas (81) in 1953 proposed a rigorous thermodynamic treatment based upon the Law of Mass Action and treating the solvent as an independent variable. This approach appears to have the greatest value for analysis of the exchange behavior of zeolites. Effect of Solution Normality on Ion Exchange Selectivity: Barrer and Klinowski (37) extended the results of Gaines and Thomas to evaluate quantitatively the changes in the shape and position of the ion exchange isotherms with variations in the total normality of the
external electrolyte solution in contact with the zeolite. Their studies have shown that if ZA is not equal to ZB then, with increasing dilution, the isotherms become more and more rectangular and selective to the ion of higher valence (the "concentration / valency" effect), so that ion selectivity can arise universally from high dilution of the electrolyte solution, independently of the exchanger. They also demonstrated the use of this model to estimate ion exchange isotherms at different total solution normalities from a single isotherm at a single total solution normality (37). Thermodvnamic Treatment When Not All Ions are Exchangeable: Sometimes not all the cations BLB+ in the starting zeolite can be replaced by entering cations AZA+, due to steric limitations. This can occur if the entering cations are too large to penetrate into smaller cavities or too voluminous to allow entry of sufficient numbers for complete exchange -as in the case of N ( c H ~ ) ~ + exchange of NaX zeolite In this case, the thermodynamic treatment must be modified to account for the incomplete exchange.
.
The proper calculational procedures for the case of incompete exchange, e.g., as employed by Barrer et al. (31) and Sherry (169, 1711, involves the use of the equivalent cation fractions of the exchangeable cations only. An alternative approach proposed by Vansant and Uytterhoeven (189, 190) and discussed briefly by Breck (52) is incorrect, as has been discussed in great detail by Barrer & al. (36) and, more recently, by Dyer et al.. (69). Ternary Ion Exchange in Zeolites: Except for some early efforts by Mercer and Ames (15, 1251, there has been very little published concerning the modelling of greater-than-binary ion exchange on zeolites. Recently, however, Rees et al. have described the ternary Na++/Ca++/Mgt+ exchange on LINDE Type A zeolite (41, 140, 141). Fletcher, Townsend et al. have derived expressions for thermodynamic equilibrium constants for ternary ion exchange in zeolites (74-78, 187). Their most recent papers (78, 187) demonstrate their success with the ~ a + /NH&+/ K+ -synthetic mordenite system.
3
ZEOLITE STABILITY
The molecular sieve zeolites have rigid, strong frameworks stable to high temperatures, oxidation/reduction, ionizing radiation, and not subject (as are many organic resin ion exchangers) to physical attrition due to osmotic shock. For the same reaons, the ion exchanger properties of the zeolites are relatively more constant and predictable over wide ranges of temperature, ionic strength, etc., than is often the case with other ion exchangers. Similarly, zeolite ion exchangers should not tend to adsorb organic molecules or ions and become "fouled" as readily as other ion exchangers.
Zeolites are also relatively stable at elevated pH levels (a.g., pH 7-11) at which other inorganic ion exchangers (e.g., zirconium phosphates, etc.) tend to lose functional groups due to slow hydrolysis (114). Zeolites are synthesized at elevated pH levels (e.g, pH 12--13+) and temperatures (e.g., 100-300°C) and are relatively stable at conditions only slightly less severe. Practical Limits in the Use of Zeolites in Ion Exchangc Separations: The chief limitation in the use of zeol.ites is due to their solubility in aqueous solutions. This limitation is not severe, except at extremes of pH. At the pH levels of natural surface waters (pH 6-10), most zeolites are relatively stable and will dissolve only very slowly. Zeolite ion exchangers should not be employed below about pH 5-6 except for very brief exposures. Operation above pHrJ6 is preferred. Proton exchange followed by slow hydrolysis of framework aluminum, leading to gradual loss of ion exchange capacity will occur at low pH levels. These reactions occur more readily on zeolites with low Si02/A1203 mole ratios and more rapidly at higher temperatures. If operation at low pH levels is required, actual experimental tests of zeolite stability should be made at conditions of intended use. Zeolite Solubility: Zeolites are relatively stable over very broad ranges of conditions of interest for potential ion exchange applications, as evidenced by the presence of vast quantities of natural zeolites which were formed millions of years ago, as well as the current formation and persistence of vast quantities of certain zeolites (especially phillipsite and clinoptilolite) present in shallow sediments on the floors of the oceans. It is, therefore, worthwhile to consider the "equilibrium" solubility of zeolites, and to evaluate theoretically the effects of solution composition and pH on Lhe solubility of the zeolite. The basic approach involves the modelling of zeolite solubility, as indicated in the following equation for the case of a sodium-. potassium-phillipsite. Based upon the free energy of formation of
this zeolite estimated (188) from literature data (160, ZOO), and assuming the modelling approach developed by natural water chemists, it is possible to calculate various combinations of pH, temperature, and concentrations (and activities) of silicon, aluminum and cation species in solution, which would exist in equilibrium with the bascd upon the WATEQ zeolite. The results of such calculations
-------1)
--------------Sherman, J. D.. Unpublished Studies, Union carbide Corp. (1981)
Figure 3.
50 Si
Conc . (mg / L)
WATEQ2 Calculation
of Si (mg / L) Concentrations in Eguilibrium with Phillipsite and Solutions Containing 0.066 N Na+, 0.066 N K+ and l O O r g A 1 / L at 25 C.
computer model (25, 188) are shown in Figure 3. As may be seen, such calculations predict that zeolites should exhibit low solubilities over a broad range of moderate pH levels (-pH 5.5-10.5). In addition, higher concentrations of cations, silicates or aluminates in solution would be expected to decrease the rate of dissolution of the zeolite. Since the range of concentrations of these ions most commonly observed in natural water are 1 - 15 mg/l for Si, 0.03-0,05 mg/l for Al, with (typically) 0.5-2.0 mEq/L concentrations of cations, and pH levels of 5.0-10.5, it is expected that zeolites in contact with such waters will not be far from equilibrium, and will dissolve rather slowly. Some applications can involve contacting very large quantities of water with rather small quantities of zeolite. For example, in wastewater treatment for N H ~ +removal, simple calculations show that, if the effluent contained only -10 ppm of Si dissolved from the zeolite, then the entire zeolite bed would dissolve within about one year. In fact, actual experience has demonstrated loss rates much lower than this, even with regeneration a t d p H 12. Similarly, in batch experiments, many weeks or months of contact may be required to reach "equilibrium" levels of dissolved Si in solution. Therefore, it is clear that, not only is the equilibrium solubility of zeolites very low but, in addition, the gaa at which this equilibrium is approached is very low over broad ranges of pH. (However, at extremes of pH, e.g., pH 1-2 or pH 13-14, massive dissolution of most zeolites will occur in hours or days.) Caullet, Guth and Way (59-62, 92) measured the solubility of 1 - 2 p m crystals of NaA and NaX for two months in H_ NaOH solutions at 25, 60, and 8 0 C. They observed congruent dissolution at these high pH levels (pH from 11.93 to 13.46). The solubilities increased with increasing pH and temperature, and the NaA was more soluble than the NaX zeolite. The concentration of Si in solution stabilized relatively rapidly (within ~2 weeks), but the concentrations of A1 in solution continued to rise even after 30-60 days in some cases. They also demonstrated that their data could be described by the following dissolution "equilibrium" equations:
Cilley and Wiers (63) measured the acid uptake of NaA and CaA at pH 3-7 in the presence and absence of complexing agents over time periods ranging from a few hours to 21 days. The dissolution of the NaA zeolite was slow at pH 7, but very rapid at pH 5 and below. The CaA zeolite was more stable than the NaA form, in agreement wih the strong preference of LINDE Type A zeolite for ~ a + +over NaS ions. Some limited studies of the stability of 20 X 50 mesh particles
of clinoptilolite were reported by Koon and Kaufman (112, 113). The weight loss of the zeolite was measured after cycling the zeolite between caustic and distilled water solutions at 2 hour intervals. If iL is assumed the attrition occurred only during the high pH portion of the cycle, then the loss rates at pH levels of 11.5, 12.0 and 12.5 were 1.8, 3.0 and 4.8 %/day. Extrapolation suggests loss rates at pH 11, 10 and 9 of 1.1, 0.4 and 0.15 %/day are to be expected. Taken together, these various published data suggest that zeolites differ substantially in their relative stability in aqueous solutions but that, in general, dissolution should be slight at pH levels of approximately 6.0 to 10.5, and will increase rapidly as the pH is increased or decreased outside of this range. Higher temperature levels will, of course, also speed the dissolution. Data from actual commercial applications of zeolites for ion exchange separations is very limited. In addition, the solutions being treated and regenerant solutions (which are often recycled) may contain silicate or aluminate ions, as well as exchangeable cations, and the presence of these will tend to slow down the rate of degradation of the zeolite or allow the degradation to proceed to "equilibrium" levels, and then proceed only slowly towards further degradation. Therefore, the lifetime of zeolite ion exchangers in commercial applications is often measured in years, rather than the days or months suggested by laboratory studies at more severe conditions, as outlined above. On the other hand, the presence of complexing agents or other mechanisms which will remove aluminate and silicate ions from solution as they are formed will, of course, encourage the more rapid degradation of the zeolite. For example, Cilley and Wiers (63) reported that sodium tripolyphosphate, glycine, and citrate at pH 5 all accelerated the degradation of the NaA zeolite by a mechanism of aluminum extraction. Other published literature on zeolite solubility is sparse. However, several conclusions ma be reached from the published literature and our own studies 1 ) , with regard to the patterns of degradakion of zeolites at various pH levels. At very high pH levels, all zeolites tested dissolved congruently. At lower pH levels, e.g., 11-12, the rate of removal of silicon was much more rapid than that of aluminum, i.e., the zeolites dissolved incongruently, leaving behind a solid product containing a lower Si02/A1203 molar ratio than existed in the zeolite initially. At pH levels from 5-10, the rates of loss were very low. Loss rates gradually increased as the pH was increased or decreased beyond this range. ---- ---. --------.---- -" -- - -- ---- .---- ---------- -- --- --- 2) Unpublished studies of C. C. Chao, A. C. Frost, C. H. Nuermberger, R. J. Ross, and J.D. Sherman, assisted by J. Dubaniewicz and P. E. West, Union Carbide Corporation.
While the effect of pH on the stability of the zeolites is in good gualitative agreement with the theoretical solubility predictions discussed above, the zeolites appear to be more acid-stable than would be predicted by Figure 3. However, more detailed characterization of the surviving solid product is required to determine what fraction of the remaining solid retains its ion exchange capacity. Nevertheless, our studies have indicated that the zeolites do retain both their structure and their ion exchange capacity over broader ranges of pH than the equilibrium solubility model would suggest. The likely reason is that the kinetics of dissolution are extremely slow, except at extremes of pH. Our own and other studies indicate that, generally speaking, the aqueous solution stability of a zeolite increases as its silica content increases. The higher si1.ica content zeolites may generally be employed at process conditions involving more severe pH levels with lower rates of loss. Unlike most organic resin cation exchangers, zeolites cannot be regenerated with strong mineral acids for use in demineralization applications. Instead, zeolites will be used in "ion interchange", as opposed to "demineralization" applications. So long as pH levels are not extreme, zeolites offer very attractive combinations of capacity, stability, and often extraordinary selectivities, as compared to the conventional organic resin or other inorganic ion exchangers. 4
PRESENT APPLICATIONS OF MO1,ECULAR SIEVE ZEOLlTES IN ION EXCHANGE
The following sections discuss uses of zeolites in ion exchange by application type, with emphasis on the unique properties of the zeolites employed in each. Recent advances in the development of improved zeolites for use in ammonium ion exchange are described, as is also the newest application of zeolites in ion exchange: as builders in household detergents. Later sections briefly describe opportunities for other new ion exchange separation and purification applications based upon the unique properties of zeolites. 4.1
Ion Exchange of Cesium and Strontium Radioisotopes
Due to their stability in the presence of ionizing radiation, and in aqueous solutions at high temperatures, inorganic ion exchangers offer significant advantages in separation and purification of radioisotopes. Exploratory studies of the ion exchange properties of these materials by Ames (1-13) revealed the high selectivities and capacities of several zeolites for cesium and strontium radioisotopes. SubsequenLly, several new processes were developed using these zeolites, as outlined in Table 4 (166).
Table 4. Cs and Sr Radioisotope Recovery and Purification Service -
Zeolite
137~s/~igh Level Wastes
LINDE AW-500 (IONSIV TE-95) (300 ft3 bed)
Cs/Rb,K,Na Purification of product from above
Large--pore Mordenite
gost?,137~s/ Low Level Waste Water in Fuel Storage Basin
Clinopt iloli te
l3 7 ~/ s Evaporator Overheads & Misc. Wastes
LINDE AW-500 (IONSIV IE-95)
13'cs/ Process Condensate Wastewater
Large pore Mordeni te
Remarks and References First charge treated 3 million gal. then pressure drop increased (almost plugged) due to A1 salt precipitation from the feed solution, 2nd charge treated several million gallons with no significant capacity loss (54, 127). Pilot plant; full-scale facility later operated at Hanford (47, 127).
(1 1. bed)
(4 beds, 5.3 ft3 each)
Nonregenerat ive use. Capacity: 12,000 gal wastewater treated / ft3 of zeolite (90, 127, 195).
Nonregenerative use. Capacity: 23-76,000 gal. wastewater treated ft3 of zeolite (90, 99, 127).
/
(9.2 ft3 bed) 1.8 m deep layer of mordenite plus 0.6 m layer of organic resin ion exchanger. Regenerated with NaN03 solution (94, 127).
4.2 Zeolite Use at the Three Mile Island (TMI) Nuc1.r Power Plant Zeolites have also been employed for the removal of radioactive cesium and strontium from contaminated waters from the accident at the TMI-2 nuclear power plant in Harrisburg, Pennsylvania. This led to increased interest in the radiation stability properties of zeolites. Brief early studies (79) had been noted by Mercer and Ames (127). but there had not been broad dissemination of such information. Zeolite Stability in Ionizing Radiation: Roddy (145) reviewed the use of zeolites in radioactivity removal from liquid waste streams. He reported no deleterious effects on the structure or ion exchange properties of zeolites tested (Union Carbide's IONSIV IE-95 (AW-500), A-51 (4A), X-61 (13X) and Y-71 (Type Y) zeolites; Norton synthetic mordenite; and natural clinoptilolitel have been noted for cumulative exposures from 1 X lo4 to 1 X lo8 Grays (1 X lo6 to ~1 X 10l0 rads). [Note: organic resin exchangers generally
-
show appreciable dqradatio; when accumulated doses exceed 105-106 rad, and massive damage at doses of lo9 rad (176). ] More recently, Bibler, Wallace, and Ebra (43) reported that doses up to 3 X 10l0 rads had no effect on the crystal structure of Union Carbide's IONSIV IE-95 zeolite or on its ability to retain Cs-137. They also studied the generation of gas due to radiolytic decomposition of water contained in the zeolite. Pillay (136, 1371, reported studies of the effects 4.4 X lo6 Gy doses on IONSlV IE-95 zeolite and observed that gas generation increased as the zeolite water content increased. Bibler et al. (43) concluded that, if the water is removed from the zeolite, Cs-137 can safely be stored on the zeolite. Also zeolite incorporation into a glass (melting point greater than 1000 C) should prevent hydrogen production because of the low amount of water remaining in the zeolite in the glass. Decisions were made, in fact, to employ Union Carbide's IONSIV A-51 and IE-96 (a special Na+ form of IONSlV IE-95) zeolite ion exchangers for the clean-up of the contaminated water at Three Mile Island and to ship the spent, radioisotope-loaded zeolite beds to Hanford to be incorporated into glass waste forms for ultimate disposal. Zeolite Ion Exchange System at Three Mile Island: The TMl-2 accident began on Harch 28, 1979. The zeolite ion exchange system for decontamination of the high activity level water in the Reactor Containment Building (RCB) sump and reactor coolant system (RCS) at TMI-2 was developed during early summer 1979. It is known as the SDS (Submerger Demineralizer System)(l42), since it is installed under water in a spent fuel storage basin. Figure 4 (124) shows the SDS flowsheet. Performance tests and design considerations for this system are described (53, 57, 58, 65, 67, 98, 102, 107, 124, 175). The compositions of the contaminated waters are shown in Table 5 (57). Bench scale tests of the IONSIV IE-95 zeolite using RCB water from TMI showed that cesium could be removed from the RCB water by a factor of lo4 for at least 1000 bed volumes (57) (42,000 Ci/liner) without cesium breakthrough, but with significant breakthrough of strontium, which could be removed by additional downstream liners. Table 5.
Composition of Contaminated Waters at Three Mile Island (57) Reactor Coolant System
Vo 1ume Sodium Boron Cesium Strontium
N
90,000 gallons 1350 ppm 3870 ppm 1.5 ppm 0.05 ppm
Reactor Containment Building -700,000 1200 2000 0.8 0.1
gallons ppm ppm ppm ppm
601 Figure 4. Three Mile Island (TMI-2) SDS Radwaste System(l24).
Each "liner" contains approximately 60 gal. of zeolite. Extrapolation indicated that increasing the loadings to 60,000 Ci, or more, of cesium per liner should be readily achievable and desirable to concentrate the wastes onto fewer liners. The cs+/Na+ and srC+/Na' ion exchange selectivities of the zeolites control their performance in this application. The IONSIV IE-96 zeolite employed in this application has excellent CS+ selectivity, but lower ~ r + +selectivity. Therefore, although the Cs+ radioisotopes would be held strongly by the IONSIV IE-96 zeolite, the ~ r + +radioisotopes would be less strongly held and would break through the column first. Therefore, the procedure to load the zeolite columns to 60,000 Ci/liner would generate a number of additional liners loaded with strontium only. During this same time period, we recommended (and later learned others also recommended) that Union Carbide's IONSIV A-51 zeolite ion exchanger be employed for removal of the strontium because of its excellent ~r+'/~a+ ion exchange selectivity and capacity and its excellent stability in ionizing radiaton. Tests using synthetic TML water confirmed the excellent strontium selectivity of the IONSIV A-51, and laboratory tests at ORNL using actual TMI water confirmed that a mixture of IONSIV IE-96 to remove cesium and IONSIV A-51 to remove strontium would provide more efficient decontamination, and generate a much smaller volume of spent zeolite for subsequent
Figure 5.
Comparison of Performance of Single Zeolite and Mixed Zeolite(65).
RESIOENCE TIME 8.4 min
I00
200
400 so0
COLUMN
loo0
2000
5000
10,000
THROUGHPUT (BED VOLUMES)
disposal. These tests and a computerized mathematical model used to evaluate the data and to predict the performance of t.he mixed bed SDS system will be described (102). It was concluded thaL a mixture of 5.5 ft3 IONSIV IE-96 and 2.5 ft3 IONSIV A-51 placed in each column "liner" should provide optimum performance. A test with TMI-2 RCB water and a bed containing equal parts of IONSIV IE-96 and IONSIV A-51 zeolites provided the breakthrough curves in Figure 5 compared with those obtained when using only IONSIV IE-96 (65). The mixed zeolite bed capacity for strontium sorption is increased by a factor of about 10, even though strontium exchange is slower, as indicated by the lower slope of the breakthough curve. The capacity for cesium is adequate for a throughput of up to about 2000 B V , representing a factor of 10 increase over the original design. The TMI-2 SDS system employing the mixture of IONSIV IE-96 and IONSIV A-51 zeolites was operated from June 1981 to February 1982, to successfully process the -650,000 gallons of RCB sump water with the results summarized in Table 6 (98). By use of the mixture of zeolites, the quantities of spent zeolite material loaded with Table 6.
SDS Effectiveness in ProcessinReactor Building Sump Water
Radionuclide
1 nf luent
( p C i/ml) CS-134 CS-137 Sr-90
Effluent (pCi/ml)
Decontamination Factor
320,000 Curies of radioackivity removed from the sump was reduced to only 1550 kg of spent zeolites, corresponding to a volume reduction of 1460 (vs. the contaminated water volume). The spent zeolite beds will be stored for a period at TMI and then shipped to the Pacific Northwest Laboratory for tests to demonstrate the vitrification of the zeolites, on a production scale, using the 'in-can' melting process (175). In light of the excellent performance of the mixed zeolite beds, the SDS system has subsequently been employed for decontamination of the Reactor Coolant System water. This will be continued to reduce radiation hazards during removal of the damaged fuel elements from the reactor to complete the TM1 recovery project. 4.3
Ammonium Removal from Municipal Wastewater
In early studies of the use of ion exchange for wastewater ammonia removal, a permutite type exchanger and various organic resin ion exchangers were found to have poor selectivity for ammonium ions, resulting in unacceptably low ammonium loadings and low regeneration efficiency, corresponding high costs, and problems of brine disposal. This situation was altered dramatically by a report by Ames (15) which presented data showing the superior ammonium ion selectivity of several zeolite ion exchangers tested. Clinoptilolite and Union Carbide's AW-400 were most promising. Subsequent pilot plant tests using clinoptilolite demonstrated ammonium removal greater than 95%. Regeneration was accomplished using a lime salt solution which was effectively reused after ammonia was removed from the regenerant solution by air stripping and exhausted to the atmosphere. An improved process (the ARRP Process) for the rejuvenation of the regenerant solution at elevated pH levels, followed by the removal of the NH3 from the air by acid scrubbing, and recycling the air to the stripper was developed later. All of these developments were reviewed in detail elsewhere (166). Improved Ammonium Exchangers: Although clinoptilolite performs quite well in this service, exchangers with higher capacity should provide significantly improved overall process performance. Studies at Union Carbide Corporation led to the discovery that the synthetic LINDE 1ONSlV F zeolite is more effective than clinoptilolite in removing NH4+ from wastewater (50, 51). Laboratory tests confirmed the higher cycled NH4+ loadings of the LZNDE F zeolite (166). Further exploratory tests led to the discovery that zeolites of the Type W-merlinoite-phillipsite--gismondine-Nap group provide superior N H ~ +exchange characteristics (162, 163, 168). Equilibrium isotherms, shown in Figures 6 and 7 , confirmed the superior N H ~ +exchange capacities of the I.INDE F and W zeolites.
Figure 6.
Figure 7.
0 mSENT
%
?I'
7
-
NU.' ENTERING ZEOLITE NU.' LEAVINGZEOLITE D A T A W AMES Ill
-
Z
STUDIES
s
---
I
.
-
%
2
'
[I
I'
-
Z
.
.
.
.
,
.
.
.
.
PRESENT STUDLES NU,' ENTERING ZEOLITE
00
N U r ' LEAVING ZEOLITE DATA OF AWES Ill
- -
Z
=6b
:5 -
-a-----HECTOR CLlNOPTlLOLlTE
'
I' o
0
-
1
'
'
'
1
"
0 .5. E W W N E N T FRUnKm NH.'
IlNU.'IIlNHI'+
1
6
10
0
IN SOLUllON
Me)\
lo
05 E W I V N E N T FRICTION NU,'
[ lNH.'IIINU.'
IN SOLUTION
+ K'I)
Figure 6.
NH4+ Capacities in Competition with ~ a + in 0.1 N Solutions at Room Temperature(l68).
Figure 7.
NH4+ Capacities in Competition with K+ in 0.1 N Solutions at Room ~emperature(l68).
Cyclic Column exchange Tests: Column tests compared the performance of LINDE F, LINDE W and clinoptilolite at conditions simulating removal of N H ~ +from municipal wastewater (168). Earlier studies (166) had also shown attractive N H ~ +ion exchange performance by other members of the Type W-phillipsite-gismondine family of zeolites. Therefore, a sample of natural phillipsite was also tested. The clinoptilolite and the 1,INDE F samples lost r y 50% and 55%. respectively, of their initial dynamic NH4+ capacities in just three cycles, believed due to the gradual buildup on these zeolites of more strongly held Kf and Ca++ ions which are not efficiently removed in subsequent regenerations. The following steady-state, cycled, dynamic NH4+ (10% breakthrough) ion exchange capacities were estimated. As may be seen, the LINDE W mesh provides .cr 3 1/2 times greater dynamic N H ~ +ion exchange capacity as compared to clinoptilolite (168): IONSIV W Mesh Developmental Sample Pine Valley Phillipsite IONSIV F Mesh Developmental Sample Hector Clinoptilolite
N650 2550 4350 ~ 1 9
BV BV BV BV 0
Other studies indicate that applications of IONSlV Y and W zeolites for wastewater N H ~ + removal should provide substantial reductions in overall process costs. Recent pilot scale tests for actual wastewater NH4+ removal using samples of lONSIV W mesh product prepared at semiworks scale confirmed the above results. Commercial Wastewater NH4+ Removal Processes Employing Zeolites: The first large--scaleinstallations began operation in California in 1978 and in Virginia in 1979. Both installations employ clinoptilolite in 20 X 50 mesh form and both employ the Ammonia Removal and Regeneration Process (ARRP Process) developed by the CH2M-Hill consulting firm, as reviewed elsewhere (143, 166, 181). The Tahoe-Truckee plant encountered CaC03 "scale" formation problems in the ARRP system due to inadequate pH control. These problems were surmounted by modest alterations of equipment and operating procedures (138). Despite initial difficulties, the overall plant performance for the last 30 days of operation covered by the report (55) was quite satisfactory. The total nitrogen concentrations were reduced from 39.0 mg/l in the feed down to only 0.89 mg/l in the final effluent, for a 98% overall nitrogen removal. Recent reports include mathematical modelling of column NH4+ removal (159, 160) and of biological regeneration (154-158) of clinoptilolite. Tests comparing clinoptilolite, erionite, mordenite and phillipsite (106) confirmed, in agreement with our studies, that phillipsite provided the best N H ~ +capacity ( d70% greater than the best clinoptilolite), but it was too friable for commercial use. Liberti et al. have also disclosed an interesting combination of selective phosphate exchange on a weak anion resin exchanger and selective ammonium ion exchange on clinoptilolite in a process which recovers a slow release fertilizer, MgNH4P04.6H20, from the concentrated regenerant stream (44, 117, 118, 134). The value of the fertilizer should pay most of the cost to remove both ammonia and phospate, for an estimated overall cost only ~ 3 0 %of the cost of biological wastewater treatment for ammonia removal (44. 134). Other recent studies include use of clinoptilolite for removal of NH~' ions from drinking water (84). Tests will also be made of the treatment of municipal wastewater for potable water reuse in Denver, Colorado (20, 147). 4.4
Molecular Sieve Zeolite Builders in Detergents
The prime function of phosphates in detergents is to reduce the activity of the "hardnessw ions, ~ a + +and ~ g + + ,in the wash water by complexing. Zeolite ion exchangers in powder form can also provide this service by removing Catt and ~ g from + ~the solution and replacing them with "soft" ions such as ~ a +(42, 152, 182).
Heavy duty powder detergents employing the Na form of the LINDB zeolite in low or zero-phosphate formulations are already being sold in several areas of the United States. Europe, and elsewhere. This use has grown rapidly and now consumes hundreds of millions of pounds of zeolites annually worldwide.
A
This application has been developed primarily by scientists at Henkel (42, 152, 182) in Germany and Procter and Gamble in the United States (151, 194). The literature on this application, especial.1~ the patent literature, is voluminous and specialized. It will not be reviewed in detail here. Instead the discussion will be limited to the related ion exchange behavior of zeolites. Detailed studies of the ion exchange equilibria of Ca++ or ~ g + +with Na+ on the LINDE Type A zeolite have been reported by Barri and Rees (41, 140), as shown in Figure 8. Thermodynamic modelling generated the smoothed curves fitting this data, providing elegant proof of the "concentration/valency" effect discussed earlier. As may be seen, the LlNDE Type A zeolite is very effect.ive in removing calcium from solution. High calcium loadings on the zeolite may be achieved in equilibrium with solutions of very low residual calcium conlent, as desired for efficient soil removal with a laundry detergent. Figure 8. Na-Ca and Na-.MgBinary Exchange Isotherms: X , 0.1 N; , 0.05 N; 0 , 0.01 N;
.
0 , 0.2 N ;
, 0.005 N
In order to provide maximum effectiveness, the zeolite should also perform its function very rapidly. The rate of ~ g + +removal From a water of "average" hardness by LINDE A zeolite is much lower than that of Catk removal, and this difference becomes more pronounced at lower temperatures. The results of ion exchange rate tests with NaA zeolite are shown in Figure 3 (167). As may be seen, the calcium is removed from solution extremely rapidly, but the magnesium is not; even after five hours magnesium exchange equilibrium has still not been attained. Figure 9 (167) displays effects of temperature. The calcium removal is rapid at 20 C but it is even faster at the higher temperatures. At all temperatures the magnesium removal is relatively slow and equilibrium is not achieved at 10 minutes, even at 120 F. Therefore, the Type A zeolite is not well suited for magnesium removal for detergent applications, particularly at the cooler water washing conditions often encountered in the United States. Exploratory studies led to the development of a product, called LlNDE 28300 Zeolite Builder, which removes not only the calcium but also magnesium, rapidly and efficiently. Figure 10 (167) shows results of room temperat.ure t.ests For times of interest in actual laundry detergent uses, but with only magnesium present in the solution. As may be seen, the results for 30 seconds and 10 minutes on the X zeolite are very close together, Figure 10.
Figure 9. g NaA/L mg/L (as CaCOJ C a f t Mgt' = 2 1
ZEOLITE DOSE (E/L)
Figure
9. Simultaneous ~ a + +and Mg++ Exchange by LINDE Type A Zeolite
Figure 10. ~ g + +Exchange by LINDE Type at Room Temperature.
A
and Type X Zeolites
indicating that equilibrium is very rapidly achieved. Therefore, the X zeolite can be considered as an alternative to the A zeolite if magnesium removal as well as calcium removal is desired. With this in mind, the use of mixtures of the two zeolites was tested. One would expect such mixtures would provide performance intermediate between that of the two zeolites separately. However, the performance of the mixtures was found to be substantially superior to that of the zeolites individually, revealing a synergistic effect, shown in Figure 11 (68, 167). The expected performance of the mixtures would lie on straight lines connecting the points on the right-hand and left-hand axes for the two pure zeolites. As may be seen, the mixtures performed far better than expected based upon a linear mixing of the two end-members. This improved performance is particularly noticeable at lower temperatures, but it is also observed at higher temperatures. Such mixtures of detergent grade LINDE Type A and X zeolites are now sold commercially under the designation Union Carbide ZB300 Zeolite Builder. The performance of the ZB300 zeolite product compared with Type A zeolite is shown in Figure 12. As shown, substantially improved magnesium removal is obtained. Figure 12.
Figure 11. 35 Ca*
t ME*
REMOVAL
BvwJ/w.xnOUtE66T IW LAB TEST MIXER INlTUL SOLUTION COYPOSITION
h** ; 1 7 mEq/L
+ MI** ' + 0 8 mEqlL C.** + MI*+ s 2 5 mEq/L
ZEOLITE MIXTURE DOSAGE = 0 6 g/L
-
SIMULTANEOUS+'o~ AND MS* REMOVAL FROM SOLUTION AT ROOM EMRRATURE CONDITIONS INITIAL SOLUTION { ZEOLITE DOSAGE
~31,,,t.r
~ h.rdry.r
~
0 6 g m (anhvdrnurl1l1t.r
-----____ --__ N P E A ZEOLITE
LINDE
10
0'
Ib
Qo
20 TIME (MINUTES1
ZB I*)
3.0
40
sb
io io i o o; 7'0 boiw PERCENT NIX IN ZEOLITE MIXTURE
Figure 11. Ca++ and ~ g + +Removal by NaX / NaA Zeolite Hixtures(l67). Figure 12. Simultaneous Ca++ and ~ g + +Removal from Solution(l67).
~
"Terg-0-Tometer" tests conducted using a variety of soil types, cloth types and skeletal detergent formulations showed that the 28300 mixed zeolite product often provides detergency performance nearly identical to the ZBlOO product (Union Carbide's detergent grade LINDE Type A zeolite). However, in a substantial number of other examples, the 28300 product provides modest but significant improvements in soil removal performance, as reported earlier (167). 5 5.1
FUTURE APPLlCATIONS Radioactive Waste Stor*
As discussed earlier, zeolites are employed in separations of long-lived Cs and Sr radioisotopes. These radioisotopes can also be retained on zeolites for long-term storage by ion exchange onto the zeolite, drying the zeolite to prevent excessive pressure after the container is sealed, and sealing the containers by welding (126). Since zeolites contain alkali metal or alkaline earth oxides, alumina and silica (major constituents of many common glasses), heating to temperatures sufficient to cause destructon of the zeolite crystal structure can convert the zeolite to a glass. Addition of suitable flux calcining agents can allow this to be accomplished at lower temperatures. Leach rates for alkali and alkaline-earth elements from aluminosilicate glasses are extremely low (e.g., l ~ - ~ ~ m / c r n ~ - d aThe ~ ) . chemical durability, low leach rates, and high thermal conductivity of glass combine to make this an ideal form for immobilizing radioactive wastes. One process employs a hydrous metal oxide type cation exchanger (Na Ti205H) to trap 90Sr and other radioisotopes from liquid wastes from fuel reprocessing, followed by a zeolite bed to trap the 134Cs and 137Cs. The ion exchangers are removed, blended, dried and hot-press- sintered to yield stable ceramic discs with low leach rates (18). The process has been tested at Hanford (19). Similarly, radioactive Cs, Sr and Pu were sorbed on LINDE AW-500 zeolite for final soldification in concrete or glass (101, 196). 5.2
Zeolite Enhanced Biological Nitrification
The removal of NH4+ from municipal waste water by use of zeolite ion exchangers in a physical-chemical treatment process involving alternating cycles of loading and regeneration was discussed earlier. In such cycles the zeolite is regenerated by mass-action or chemical driving forces. The NH4+ loaded on the zeolite may also be removed by biological nitrification: bacteria NH4+ + 202
NO3 + 2~'
+ H20
In effect, the addition of a zeolite to the activated sludge will impart selective NH&+ exchange capabilities to the sludge, thus improving its ability to remove N H ~ + from the waste water. The conversion of NH4+ by nitrifying bacteria will regenerate the zeolite. Such "zeolite enhanced biological nitrification" has been demonstrated by Sims (177) and Sims and Little (178). The ability of the zeolite to pick up NH4+ during peak load periods and subsequently gradually release it may provide higher overall nitrification rates and improved ability to handle shock loads. 5.3
NH4+-from Industrial and Agricultu~alWaste Water
In addition to treatment of municipal waste water, it is anticipated that zeolites will find use in removal of NH4+ from indust-rial and agricultural waste water streams. Here the availability of several different zeolites (each having NH~' ion exchange selectivity) with different properties offers the possibility of selecting the optimum exchanger for a particular service on the basis of its ability to selectively remove NH4+ in the presence of different competing cations. 5.4
Regeneration of Artificial Kidney Dialysate Solutions
Hemodialysis treatment in artificial kidney systems involves the transfer of uremic wastes through suitable membranes by dialysis to a dialysate fluid while the small pores of the membranes prevent loss of desirable blood components. Rather large volumes (100-300 liters) of dialysate solution are required for a single treatment. Interests in reducing the size of the equipment required and in achieving portability have led to the development of a process to remove the waste products from the spent dialysate solution, so the dialysate solution can be continuously reused. Then, as little as 1-2 liters of dialysate can be adequate for one hemodialysis treatment. Of all the uremic waste substances, urea is the most abundantly generated. Improved urea-binding sorbents have been sought for many years for dialysate regeneration in hemodialysis or peritoneal dialysis, to reduce the quantities of dialysate required and, thereby, reduce the size and weight of such systems. Unfortunately, although activated carbon is an effective sorbent for many waste metabolites and drugs, it possesses insufficient capacity to remove urea effectively from dilute solutions. Other sorbents (resin ion exchangers, oxystarch, etc.) have also been studied, but a non-soluble, selective urea-binding sorbent with high urea sorption capacity at physiological pH levels has not been found. In an alternative method urea is hydrolyzed to form ammonium ions which are then removed by ion exchange. Dialysis systems employing such a process were developed (88, 89, 123) and sold by CCI Life Systems, Inc., based upon use of an immobilized urease enzyme
catalyst to hydrolyze urea to NH4+, followed by zirconium phosphate to remove the NH4+ cations, and hydrous zirconium , and oxide to remove phosphate and fluoride anions. ~ a + ' l4gM K+ removed by the zirconium phosphate must be added back in the required, controlled concentrations to the regenerated dialysate. The LINDE F and W zeolites discussed earlier provide unique selectivities for NH4+ in the presence of other common alkali and alkaline earth cations and should, therefore, provide improved performance over zirconium phosphate in dialysate regeneration. Andersson et al. (17) disclosed the use of a zeolite ion exchanger for this purpose. Their data confirm our results (see above) showing that several zeolite ion exchangers provide higher selectivities for NH4+ over Na+ and for Na+ over Ca++ and Mg+I-,compared to a zirconium phosphate exchangers. The highest N H ~ +selectivities were provided by phillipsite and LlNDE F zeolite ion exchangers. Both showed higher exchange capacities compared to clinoptilolite. NJ4+ Exchange for Urea Removal: Extension of the use of these new IONSlV F and W ion exchangers to remove urea nitrogen is quite straightforward. The NH4' generated by urea hydrolysis is exchanged onto the zeolite. The C03 simultaneously generated must be neutralized by acid addition (directly or via use of a suitable buffer) to control pH near physiological pH levels ( 4 7 . 4 pH) and at suitable levels (-pH 7-8) to maintain the urease enzyme activity. The system chemistry is shown below (164): Urea Hydrolysis: ( N H Z ) ~ C O+ 2 H20
urease enzyme
-
2 NH4+
+
CO5-
Neutralizat.ion of Ammonium Carbonate: 2 NH4+ + CO3 c 2 NH~' + HC05NH4+ Ion Exchange: (Rq),) zeolite + 2 NH4+
(NH4+)2 zeolite
+ 2/n R"+
The effective NH4+ binding capacities of IONSIV F-80 and IONSIV W-85 zeolites in contact with blood or dialysate solutions were estimated (164) to be quite high, even in the presence of competing K+, ~ a + +and ~ g + +cations. For example, the desired removal of 12-30 gms urea/day for treatment of chronic renal disease would require only 0.9-2.2 lbs/day of lONSIV NH4+ exchanger. Later studies by Klein (103-105) and Gregonis and Walker (90) confirmed the high NH4+ capacities of the IONSIV F and W zeolites at conditions simulating their use in artificial kidney devices (104). The efectrolyte balances required for dialysate regeneration are much more complex than can be provided by simple N H ~ +exchange. As noted above, it is necessary to neutralize the C03 generated by
urea hydrolysis and to maintain suitable pH levels. In addition, K+ and phosphate removal in controlled amounts is desired and blood pH and electrolyte ( ~ a + K', , cab+, ~ g + + ,phosphate, bicarbonate, etc.) balances must be carefully maintained for patient wellbeing. One approach to achieving these desired balances involves the use of these NH4+ selective zeolites in the partially cat+ exchanged form (22, 23, 164, 165). The ~ a + +from the zeolite can then serve as a "buffer" to remove the excess Cog-.-and also, with proper adjustment, provide a means of phosphate removal by formation of calcium phosphate adsorption complexes and/or precipitates, as shown schematically (and over-simplified) below: CaNa zeolite
+ (NH4)2C03 +
X
-
NaH2POq NH4 zeolite + CaCOg
+ Ca phosphate
Animal tests using a novel sorbent--slurrry-reciprocatingdialyzer (SSRD) developed by S. R. Ash (24) have demonstrated the effectiveness of this approach, and have shown that the use of zeolite ion exchangers can make truly portable artificial kidney devices a reality (22). These studies demonstrated the feasibility of a small dialysis device with urea conversion to (NH4)2C03 and the use of ( ~ a + + , ~ a +form) IONSIV F and W zeolite ion exchangers for selective N H ~ +and K+ removal, and Ca++ release, to provide, in actual dog tests, ion fluxes appropriate for the uremic patient (22). 5.5
Aquaculture
Ammonia is extremely toxic to aquatic animals. In closed systems (e.g., aquariums) and when extensive water reuse is practiced in high density fish culture (hatcheries, fish farming), the ammonia released directly by the fish, from their other nitrogenous wastes such as urea, and from bacterial deamination of protein in feed and wastes will quickly reach toxic concentrations if not removed. Microbiological filters may be used for this purpose. However, nitrifying bacteria are easily inhibited or killed by various stresses (low temperatures, sulfides, methanol, heavy metals, antibiotics used to control disease outbreaks, shock loads, etc.). Toxic levels of ammonia may be quickly reached before the biological filter operation can be reestablished to required levels. For these reasons, a number of investigators have studied NH4+ removal by ion exchange on zeolites as an independent standby backup system (for emergency use when upsets occur, or during treatment of diseased cultures), and/or for polishing treatment to remove ammonia which escapes the bioligical filter, or for reliable anunonia removal in lieu of biological filters.
Braico (46) compared ion exchange using a zeolite with other methods of ammonia removal for reuse of fish hatchery waters. He concluded that zeolite ion exchange offers advantages of lower cost, higher ammonia removals, a chemical process which is more controllable than existing biological processes, and lower land area requirements. Konikoff (111) compared the performance of clinoptilolite ion exchange vs. biofilters for ammonia removal. Substantial biological nitrification occurred in the clinoptilolite system at the conditions studied and consequent nitrite toxicity problems were observed in both the biofilter and ion exchange systems. (This can likely he avoided )
.
Johnson and Sieburth (100) tested LINDE AW-500 and clinoptilolite zeolites for ammonia removal from from an active. closed system for Chinook Salmon and from artificial sea water. They concluded that, in high salinity waters, a zeolite ion exchange column is desirable as a secondary or backup system to biological filters for use in low density closed aquaculture systems. However, in fresh water systems, it is feasible to use ion exchange alone (instead of biological nitrification) with the advantage of being ready at any time to remove considerable quantities of toxic ammonia. Total removal of the ammonia using zeolites also avoids the further complication of nitrate buildup resulting from the biological oxidation of ammonia. Slone, Turner, and Jester (180) demonstrated the utility of zeolite removal of ammonia escaping from a biofilter in a closed "silo" type fish culture system. Liao and Hayo (115, 116) have examined broadly the requirements and options in water reconditioning - reuse in intensified fish culture. Peters and Bose (135) also reported studies of the use of zeolite ion exchange for ammonia removal in hatchery or aquaculture water reuse systems. Numerous other studies have also been reported in the literature (129, 166, 179). These have included demonstrations of the utility of zeolites in reducing levels of toxic ammonia in the transport of ornamental freshwater fish (45). Torii (186) reviewed uses of natural zeolites (clinoptilolite and mordenite) in Japan and estimated that 5-10 million pounds per year of clinoptilolite are employed in removal of ammonia from aquaculture (e.g., eels, carps or sweetfish) ponds or tanks. Preliminary tests have shown that the LINDE NH4+ ion exchangers discussed above also provide significant improvements in performance in aquacultural and related applications.
5.6
Yeedinp of Ruminant Animals
The digestive systems of ruminant animals (cattle, sheep, goats, deer, buffalo, etc.) include a bacterial fermentation "vat" (the rumen) in which plant and other feed materials may be broken down into smaller nlolecules and in which amino acids and certain vitamins may be synthesized. These rumen bacteria can even be fed sources of inorganic nitrogen, such as ammonia or urea, and employ these non--proteinnitrogen (NPN) feeds to produce amino acids, which are ultimately converted to animal protein. Suhs1.itution of N P N compounds for a portion (or all) of the natural protein in the animals diet offers major economics in the cost of feed. However, the quantities of NPN which may be fed are limited by the need to keep the ammonia concentrations in the rumen below toxic levels. J f large quantities of urea are fed, toxic levels of ammonia can be reached, since urea fed to the animal is quickly hydrolyzed (by urease enzyme), releasing ammonium ions:
White and Ohlrogge (193) have disclosed Lhat ion exchangers may be introduced into the rumen prior to feeding of NPN compounds so that the NH4+ ions are partially exchanged onto the ion exchanger (to reduce the NH4' concentration in the ruminal fluid) and thereafter slowly released by the regenerant action of saliva (including Na+ and K+ ions) entering the rumen. They note that zeolite ion exchangers, especially the IJNDE P zeolite, provide outstanding NH4+ ion exchange performance for this application. Watanabe (191) has reported studies in Japan of actual feeding of zeolites to cattle as a dietary supplement. Feeding of the zeolite to the animals provided improved feeding efficiency. Kondo et al. (110) have reported the benefits of zeolite feed supplen1ent.s on calf growth. 5.7
Other Uses in Agriculture, Horticulture and Animal Feedins
Mumpton and Fishman (128) and Torii (186) reviewed agricultural uses of natural zeolites including extensive studies in Japan, and some also in the United States, of the feeding of zeolites to animals, use in odor control, use of zeolites as soil conditioners, in fertilizers, as carriers of fungicides and pesticides, and as N H ~ +exchangers to prolong the life of cut flowers. There have been many studies (21, 7 2 , 108, 109, 1.33) of feeding of zeolites to swine and poultry. Reported benefits have included: increased weight gain, increased feeding efficiency, reduced incidence of intestinal and other diseases and reduced death rates; and lower odor of animal excrement.
Although the exact mechanisms by which the benefits of the zeolites are achieved are not known in all of the agricultural applications, ion exchange properties are likely of great importance. The natural zeolites tested, principally mordenite and clinoptilolite (which are known to have selectivity for N H ~ +exchange), provided reduced odor of animal droppings, improved retention of nitrogen fertilizer in the soil, and removal of NH4+ from water. 5.8
Metal Removal, Recovery and Separations
Many zeolites exhibit high selectivities for various heavy metals and are under consideration for use in recovery of precious and semi-precious metals and for removal of heavy metals from industrial and metals processing waste waters. Because of their availability (especially in Japan), the zeolites clinoptilolite and mordenite have been studied for heavy metals (especially Cd, Cu, Pb, and Zn) removal from waste waters (64, 80, 91, 93, 148-.150,153, 183, 197-197). The high selectivities of also suggests their use for several zeolite ion exchangers for ~ g + the recovery of silver from waste waters. Separations and purifications of non-ferrous metals may also be accomplished by zeolite ion exchange. For example, Breck (49) reported the unique separation of Co+' and ~ i + + on LSNDE Type A zeolite. Many other separations of non-ferrous metals are also possible (2_64). Separations of both free and complexed ions may be accomplished (e.g., 49, 51), suggesting that zeolite ion exchangers may provide unique new separations and purifications in the processing of non-ferrous metals. Acknowledgements Mr. R. J. Ross and also Messrs. Y. J . Doerr, J. Dubaniewicz, C. H. Nuermberger, and R. E. West and Ms. C. S. Roth collaborated on the NH4+ exchange work and Dr. J. M. Bennett and Ms. J . P. Cohen performed related crystallographic studies. Dr. A. F. Denny and Messrs. A. J. Gioffre, Jr., and G. M. Straehle shared in various aspects of the detergent product development studies. Dr. A. C. Frost and Messrs. R. J. Ross, C. H. Barkhausen, P. E. West and Ms. J. E. Stern assisted in numerous other ways, all greatly appreciated. I also wish to acknowledge the many contributions to our ion exchange studies made by the late Dr. Donald W. Breck; we miss Don's warm friendship and stimulating discussions. Many thanks also to Ms. B. Bjorkman and Ms. G. Solano for their fine secretarial assistance. Finally, and most of all, I wish to thank Carol, my wife, for her constant good cheer, patience and encouragement during this and many other such projects.
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P.B. 179667 (1969). Gregonis, D. and J. Walker, A Membrane System to Remove Urea from the Dialyzing Fluid of the Artificial Kidney, Proc.1lt.h Annual Contractors Conf., AK-CUP, NLAMMD, Natl. Inst. Health, U.S. Dept. H. E. W., Washington, D.C. (1979) p. 162. Gushima, Y., Japan. Patent 74 05,889 (Jan.19, 1974). Guth, J.A., P. Caullet and R. Wey, Proc.5th Int. Conf. Zeolites, L.V.C. Rees, ed., (Philadelphia, Heyden, 1980) 30-39. Hagiwara, Z. and M. Uchida, in L.B. Sand and F.A. Mumpton, eds., Natural Zeolites, Occurrence, Properties, Use (New York, Pergamon Press, 1978) 463-470. Hanson, G.L., Proc. Natl. Con£. Complete Water Reuse, Washington, D.C., A.1.Ch.E.-E.P.A.. 360-366(1973). Hawkins, D.B. and J.H. Horton, U.S.Atomic Energy Comm. Doc. NO. PP-1245 (1971). Helfferich, F., lon Exchange (New York, McGraw-Hill, 1962). 10-13, 122-123, 133, 145-146, 160-161, 185-193. Hertzenberg, E.P. and H.S. Sherry, in Adsorption and Ion Exchange with Synth. Zeolites, W.H.Flank, ed. (Washington, D.C., Am. Chem. Soc., 1980) 187. Hofstetter, K.J., C.G. Hitz, T.D. Lookabill and S.J. Eichfeld, Submerged Demineralizer System Design, Operation and Results, Presented at Am. Nucl. Soc. Decontamination Conf., Niagara Falls (Sept., 1982). lnternational Atomic Energy Agency, Vienna, Technical Report Series No.136 (1972) 61 and 64; and No.78 (1967) 73. Johnson, P.W. and J.McN. Sieburth, Aquaculture 4 (1974) 61. Kelley, J.A., W.H. Hale, J.A. Stone and J.R. Wiley, A.l.Ch.E. Symp. Series No.154, 72 (1976) 128. King, L.J., D.O. Campbell, E.D. Collins, J.B. Knauer and R.M. Wallace, Evaluation of Zeolite Mixtures for Decontamination of High-Activity--LevelWater in the Submerged Demineralizer System (SDS) Flowsheet at the Three Mile Island Nuclear Power Station, Unit 2, to be presented at 6th Int. Zeolite Conf., Reno, July 10-15, 1983. Klein, E., F.F. Holland and K. Eberle, Trans. Am. Soc. Artif. Organs 24, (1978) 127. Klein, E. and F.F. Holland. Hemoperfusion Kidney Liver Support Detoxif. Proc. lnt. Symp. (1980) 63-79. Klein, E., P.F. Holland, R.P. Wendt, H. Gidden and K. Eberle, Report PB 81-122681, Publ.by NTZS, U.S. Dept. Commerce (Nov.7, 1980). Klieve, J.R. and M.J. Semmens. Water Res. 14 (1980) 161. Knauer, J.B., D.O. Campbell, E.D. Collins and L.J. King, Oak Ridge Natl. Lab. Rept. ORNL/TM-8333 (1982). Komakine, U.S.Patent 3,836,676 (1974). Kondo, T. and B. Wagai. Experiment~lUse of ClinoptilolitetufF as Dietary Supplement for Pigs, Yotonkai (May, 1968) 1-4. Kondo, K., et al. Effect of Zeolite on Calf Growth, Chikusan
No Kenikyu 23 (1969) 987. Konikoff, M., Comparison of Clinoptilolite and Biofilters for Nitrogen Removal in Recirculating Fish Culture Systems, Ph.D. Thesis. Southern Illinois Univ. (1973). Koon, J.H. and W.J. Kaufman, U.S. Environ. Protection Agency Report No.17080 DAR (Sept.,1971). Koon, J.H. and W.J. Kaufman, J. Water Pollu. Cont. Fed. 47 (1975) 448. Larsen, E.M. and D.R. Vissers, J. Phys. Chem. 64 (1960) 1732. Liao, P.B. and R.D. Mayo, Aquaculture 1 (1972) 317. Liao, P.B. and R.D. Mayo, Aquaculture 3 (1974) 61. Liberti, L., Water Res. 13 (1979) 65. Liberti, L., N. Limoni, R. Passino and D. Petruzelli, in Pawlowski, Ed., Physicochemical Methods for Water and Wastewater Treatment (Elmsford, New York, Pergamon Press, (1980) pp. 73-85. Maes, A. and A. Cremers, in W.M.Meier, J.B. Uytterhoeven, eds., Molecular Sieves, 3rd Int. Conf., Adv.Chem.Series 121 (Washington, D.C., Am.Chem.Soc., 1973) p. 230. Maes, A. and A. Cremers, in J.B.Uytterhoeven, ed., Proc.3d Int. Conf. on Molecular Sieves (Leuven Univ. Press, 1973) 192. Maes, A. and A. Cremers, J. Chem. Soc., Far. Trans. 1 71 (1975) 265. Maes. A., J. Verlinden and A. Cremers, in R.P. Townsend, ed., The Properties and Applications of Zeolites, Special Publ. No.33 (London. The Chemical Society, 1979) p. 269. Marantz, L.B., M. Greenbaum and M.J. McArthur, Ger.Offen. 2,100,961 (July 15, 1971). Mc Goey, R.J. and C. Hitz, Processing Three Mile Island Unit 2 Accident Radioactive Waste Waters, Presented at Am. Inst. Chem. Engrs. Meeting, Detroit, Mich. (Aug.19, 1981). Mercer, B.W. and L.L. Ames, Unclassified Hanford Laboratories Report HW-78461,(1963). Mercer, B.W. and W.C. Schmidt, U.S. Atomic Energy Conun. Accession No. 14466, Report No. RL-SA-58 (1965). Mercer, B.W. and L.L. Ames, in L.B. Sand and F.A. Mumpton, eds., Natural Zeolites, Occurrence, Properties, Use (New York, Pergamon Press, 1978) pp. 451-462. Mumpton, F.A. and P. Fishman, J. Animal Sci. 45(5) (1977) 1188. Mumpton, F.A., in L.B. Sand and F.A. Mumpton, eds., Natural Zeolites. Occurrence, Properties, Use (New York, Pergamon Press, 1978) pp. 3-27. Nightingale, E.R., J. Phys. Chem. 63 (1959) 1381. Nikashina, V.A., J. Chrom. 120 (1976) 155. Olson, D.H. and H.S. Sherry, J. Phys. Chem. 72 (1968) 4095. Onogi, T., Experimental Use of Zeolite Tuffs as Dietary Supplements for Chickens, Report Kamagata Stock Raising Institute (1966) 7-18. Passino, R. and L. Liberti. Resource Recov. Conserv. 6 (1981)
263. Peters, M.D. and R.J. Bose, Tech. Rep. Fish Hari. Serv. (Canada) (1975) 535-546. Pillay, K.K.K., A.1.Ch.E. Symp. Series No. 213, 78 (1982) 33 Pillay, K.K.K., Report NE/RWH-80-3, Penn.State Univ.. University Park, Pa. (Oct., 1980). Prettyman, R.F. and R.F. Woods, Ammonia Removal at TahoeTruckee - First Year in Review. 21st Northern Regional Conf., Calif. Water Pollu. Control Assoc., San Francisco, Oct.18, 1979. Rees, L.V.C., Ann. Rept. Progr. Chem. Sect. A, 67 (1970) 191. Rees, L.V.C., in R.P. Townsend, ed., The Properties and Applications of Zeolites, Special Publ. No.33 (London, The Chemical Society, 1980) 218. Rees, L.V.C., Binary and Ternary Cation Exchange in Zeolite A, Paper to be presented at the 6th 1nternatl.Conf.on Zeolites, Reno, Nevada, July 10-15, 1983. Reust, R.R., CNSI/TMI Water Cleanup System Process Description, AGNS Draft Report, Allied General Nuclear Services(Aug., 1979). Robbins, M.H., Jr. and G.A. Gunn, Proc.Water Reuse Symp.; Water Reuse - from Research to Applicn., Vo1.2 (A.W.W.A. Res.Foundn. (1979) 1311. Robie, R.A. and D.R. Waldbaum, Thermodynamic Properties of Minerals and Related Substances at 298.15 K(25.0 C) and One Atmosphere (1.013 Bars) Pressure and at Higher Temperatures, U.S. Geol.Survey Bull. (1968) 1259. Roddy, J.W., Oak Ridge Natl. Lab. Rept. ORNL/TM-7782 (Aug., 1981) . Rosolovskaya,E.N., K.V. Topchievka and S.P. Dorozhko, Russ. J.Phys.Chem. 51 (1977) 861. Rothberg, M.R., S.W. Work, K. Linstedt and E.R. Bennett, Proc. Water Reuse Symp.; Water Reuse - from Research to To Applicn., Vol.1 (1979) A.W.W.A.Res.Foundn., pp.105-138. Sanga, S., A. Kurata and Takahushi, Mizu Shori Gijutsu 14(11)(1973)1151. Sato, I.. Chicka Shigen Chosajo Hokoku (Hokkaido) 47 (1975) 63. Sato, E., Senti Kako 27(12) (1975) 714. Savitsky, A.C., Soap Cosmet.Chem.Spec. 53(3) (March, 1977) 29. Schwuger, M . J . and H.G. Smolka. Physicochemical Aspects of Phosphate Substitution by Heterogeneous Ion-exchangers in Detergency, Presented at 49th Natl.Colloid Symp., Clarkson College, Potsdam, N.Y. (June, 1975). Semmens, M.J. and M. Seyfarth, in L.B. Sand and F.A. Mumpton, eds., Natural Zeolites, Occurrence, Properties, Use (New York, Pergamon Press, 1978) pp. 517-526. Semmens, H.J., J . T . Wang and A.C. Booth, J.Water Pollu. Control Fed. 49 (1977) 2431. Semmens, M.J., The Feasibility of Using Nitrifying Bacteria to
Assist the Regeneration of Clinoptilolite, 32nd, 1ndus.Waste Conf., Purdue Univ., 1977 (pub1.1978). Semmens, M.J. and P.S. Porter, J. Water Pollu. Control Fed. 51(1979) 2928. Semmens, M.J. and R.H. Goodrich, Environ. Sci. Technol. 11 (1977) 255. Semmens, M.J. and R.R. Goodrich, Ibid., 11 (1917) 260. Semmens, M.J., A.C. Booth and G.W. Tauxe, J. Environ. Eng. Div., Proc. Am. Soc. Civil Engrs. 104, No.EE2, 231 (1978). Semmens, M.J., J.R. Klieve, D. Sonabrich and G.M. Tauxe, Water Res. 15 (1981) 655. Sherman, J.D. and R.J. Hoss, Union Carbide, Unpublished Results (1971, 1973) Sherman, J.D. and R.J. Ross, Separation of Ammonium lons from Aqueous Solutions, Ger.Offen. 2,531,338 (Feb.12,1976); Brit.Pat. 1,510,018 (May 10, 1978); U.S.Patent 4,344,851 (Aug. 10, 1982). Sherman, J.D. and Ross, R.J., I.1NDE IONSIV W 85 - Phillipsite - Gismondine Molecular Sieve Zeolites for Ammonium Ion Exchange, Bulletin F-4094a. Union Carbide Corp., New York (1977). Sherman, J.D., LINDE IONSIV Zeolite NH4+ Exchangers for Artificial Kidney Applications, in Artificial Kidneys, Artificial Liver, Artificial Cells, T.M.S.Chang, E d . , Plenum Press, New York (Feb., 1978).pp.267-274. Sherman, J.D., Removal of Uremic Substances wiLh Zeolite Ion Exchangers, European Patent App1.81106583.8, (Aug.25, 1981). Sherman, J.D., lon Exchange Separations with Molecular Sieve Zeolites, in Adsorption and Ion Exchange Separations, J.D.Sherman, ed., A.I.Ch.E.Symp.Series , 74, No.179 (1978).pp.98-116. Sherman, J.D., Denny, A.F. and Gioffre, A.J., Soap Cosmet. Chem. Spec., 33-40 (Dec., 1978) 64-68 . Sherman, J.D. and Ross, H.J., in L.V.C.Rees, ed., Proc. 5th Intl. Conf. Zeolites, (Philadelphia, Heyden, 1980) 823. Sherry, H.S., J. Phys. Chem. 70 (1966) 1158. Sherry, H.S., J. Coll. lnterface Sci 28 (1968) 288. Sherry, H.S., J. Phys. Chem. 72 (1968) 4086. Sherry, H.S., lon Exchange (ed.Marinsky) Vol 2, Chapt.3 (New York, Marcel Dekker, 1969) 89. Sherry, H.S., Ion Exch. Process Ind., Intl. Conf., July, 1969, Soc. Chem Ind. (19691, p.329. Sherry, H.S., in E.M. Flanigen, L.B. Sand, eds., Molecular Sieve Zeolites 1, (Washington, D.C., Am. Chern. Soc., 1971) 89 Siemens, D.H., D.E. Knowlton and M.W. Shupe, A.1.Ch.E. Syrnp. Series No.213, Vol 78 (19821, pp. 41-44. Simon, G.P., Stability of Ion Exchangers in Ionizing Radiation, in Ion Exchange for Pollution Control, C. Calmon and H.Gold, eds., Vol. 1, (Boca Raton, Florida, CRC Press, 1979) 55-70.
(177) Sims, R.C., Environ. Sci. Notes, 9 , 2 (1972). (178) Sims, R.C. and L.W. Little, Environ. Letters 4(1) (1973) 27. (179) Sims, R.C. and E. Hindin, in Chemistry of Wastewater Technology, A.J.Rubin, ed. (Ann Arbor, Mich., Ann Arbor Science, 1978) 305. (180) Slone, W.J., in Proc. Bio-Engrg. Symp. for Fish Culture, L.J. Allen and E.C.Kinney, eds. (Bethesda, Md., Fish Culture Sect., Am. Fisheries Soc., 1981) 104-115. (181) Smith, S.A., R.L. Chapman, O.R. Butterfield, Proc. Water Reuse Symp.; Water Reuse - from Research to Application, Vo1.2 (1979). A.W.W.A. Res. Foundn., pp. 1435-1445. (182) Smolka, H.G. and M.J. Schwuger, in L.B. Sand and F.A. Mumpton, eds., Natural Zeolites, Occurrence, Properties, Use (New York, Pergamon Press. 1978) 487-493. (183) Tanaka, Y . and S. Koide, Bosei Kanri 18(10) (1974) 11-17 . (184) Thomas, T.L., Process for Cation Separation Using Zeolite Materials, U.S.Patent 3,033,641 (May 8 , 1962). (185) Thompson, H.S., J. R. Agric. Soc. Engl., 11, 68 (1850). (186) Torii, K., in Natural Zeolites, Occurrence, Properties, Use, L.B.Sand, F.A.Mumpton, eds., (Pergamon Press, New York, 19781, p.441. (187) Townsend, R.P., P. Fletcher and M. Loisidou, Prediction of Multicomponent lon Exchange Equilibria in Zeolites, to be presented at 6th Internatl. Conf. on Zeolites, Reno, Nevada, July 10-15, 1983. (188) Truesdell, A.H. and B.F. Jones, U.S. Geol. Survey J . Res., 2 ( 2 ) (1974) 233. (189) Vansant, E.F. and J.B. Uytterhoeven, Trans. Far. Soc. 67 (1971) 586. (190) Vansant, E.F. and J.B. Uytterhoeven, Ibid., 67 (1971) 2961. (191) Watanabe, Report on Use of Zeolite Tuff as Dietary Supplement for Cattle, Report of Okayarna Pref. Fed. Agric. Coop. Assoc. (Apr., 1971). (192) Way, J.T., J. R. Agric. Soc. Engl., 11, 313 (1850); 13, 123 (1852). (193) White, J.L. and A.J. Ohlrogge, Canadian Patent 939,186 (Jan.1, 1974) (194) Wiers, B.H., R.J. Grosse and W.A. Cilley, Environ. Sci. Technol. 16 (1982) 617. (195) Wilding, M.W. and D.W. Rhodes, U.S. Atomic Energy Comm. Doc. No. IDO-14624 (1963). (196) Wiley, J.R. and R.M. Wallace, Savannah River Lab. Rept. DP-1388. (197) Yokota, F., Y. Tanaka and H. Fukaya, Aichi-ken Kogyo Shidesho Hokoku 10 (1974) 66-70. (198) Yokota, F. and T. Kato, Kagaku Kojo 18(10) (1974) 75-79. (199) Yoshida, H., A. Kurata and S. Sanga, Mizu Shori Gijutsu 17(3) (1976) 219-226. (200) Zen, E-an., Am. Mineral. 57 (1972) 524.
SELECTIVE ADSORPTION PROCESSES : N-ISELF
Philippe JACOB Process Engineer - FEYZIN Refinery. ELF -AQUITAINE
Part I : A SURVEY OF THE N-ISELF PROCESS I -
GENESIS OF THE N-ISELF PROCESS : the ELF-SOLAIZE Research Center and the preparatory chromatography. 1.1. Presentation of the Elf-Solaize Research Center. 1.2. The stress put by ELF on preparatory chromatography.
I1
-
WHAT IS INTERESTING ABOUT N-ISELF PROCESS : 11.1. Gasoline manufacturing in a refinery. 11.2. How to fractionate light gasoline
.
I11 - PROCESS OPERATION 111.1. 111.2. 111.3. 111.4. 111.5.
:
Principle of operation. Characterization of the separation. Yielding of the process. Adsorbant regeneration. Energy consumption.
I -
GENESIS OF THE N-ISELF PROCESS
1.1. Presentation of the Elf-Solaize Research Center (CRES) : In the Elf-Aquitaine organization, the ELF-SOLAIZE Research Center is linked to the Refining-Marketing Direction of operations, which finances most of its researches, the other ones being paid out directly by SNEA, according to the own scopes of the CRES. It regroups more than 350 people distributed among six sections, whose main activity areas are :
. . . .
. .
processes lubes and additives environment analysis methods chemistry and catalysis energy.
Born 13 years ago, an age that for a research center as for a man is the age of discretion, the CRES may already be credited with a lot of realizations :
. all ELF and ANTAR trade-marks lubes . the polymer-asphalt STYRELF . the oleophile drums for treating oily water . the cloud point additives, the combustion additives
. the anti-fouling SPIRELF exchanger pipes. . the various processes of .
spring for protection of
preparatory chromatography and of course the N-ISELF process.
1.2. The stress put by ELF on preparatory chromatography : Soon after its founding, the Elf Solaize research center developped a lot of preparatory chromatography processes. The principle of these processes is very plain : it's that of chromatography used as an analysis method : the mixture that one wants to separate is periodically introduced into an adsorbant packed column, which selectively retains the components to separate. In this area, the stress was put on improvement of those keypoints that simultaneously allow a good separation, a high percentage of recovery, and a high efficiency, i.e. :
.
the periodic injection system of the feed to be separated, when the feed must be vaporized before entering the separation column, in order to prevent preferential departing.
.
the product trapping and recovery system that prevents the pollution of a product by another one and allows to recycle the intermediate fractions. All these abilities that met in CRES for the stress on preparatory chromatography partly explain the genesis of the N-ISELF process. I1 - WHAT IS INTERESTING ABOUT THE N-ISELF PROCESS II.1.Gasoline manufacturing in a refinery The manufacturing of motor-spirits uses some rather different basis :
.
the reformat, proceeded from hydrotreated heavy naphta, is the main basis, its octane number being very high.
.
the alkylat resulting of the combination of isobutane and isobutene.
.
the fluid catalytic cracker gasoline.
. the steam-cracker pyrolisis gasoline, the light aromatic compounds of which having been previously extracted. In addition to these natural basis come some fatal basis, the octane number of which is much worst : these are light gasolines coming either from topping or from catalytic cracking, the octane number of which is much lower (particularly the motor octane number). Let us notice that this light gasoline is the natural feedstock of the steam cracker, too. We know why light gasoline is neither quite satisfactory as a gasoline basis nor quite proper to be transformated into ethy lene. That is due to the nature of the mixed hydrocarbons, those being either N-paraffins with a linear structure and which are a good feedstock for a steam cracker or I-paraffins or naphtenes having a good octane number but leading in a steam cracker to a poor ethylene yielding. Here appears the interest of light gasoline fractionating : one would obtain a N-paraffin rich cut that could be used according to its degree of purity:
- as a steam-cracker feedstock (cf. fig. 1)
O/o
ETHYLENE YIELD
Oio
PROPYLENE BUTADlENE PYROLYSIS GASOLINE YIELDS
20
Figure 1 STEAM CRACKING YIELDS AND N-PARAFFINE CONTENT. [pressure: 1.7 bar1 [residence time :0.6 s l Itemperature:830°C I [light naphta C5-70°C1 10
100 normal paraffins content in light naphtha.
- as an isomerisation feedstock in order to give back a gasoline basis, nC6, nC
7
- as it appears or re-fractionated into nC 5' as a solvent.
the resulting cut, being iso-paraffin rich, may very easily be integrated to gasolines (cf. fig. 2).
Figure 2
VARIATION OF CLEAR RESEARCH OCTANE NUMBER W I T H CONTENT OF NORMAL PARAFFINS.
% n- paraff ins
II.2.How to fractionate light gasoline ? Isomers have very closed boiling points (for example 36OC for N-pentane and 27.g°C for iso-pentane) what leads for an atmospheric distillation to a considerable number of trays. Moreover it would be necessary to design an unit for each wanted sepa), all this ration deisopentaniser, deisohexaniser, etc bringing about a high investment and a very important energy consumption.
...
In order to reduce all these drawbacks in the light gasoline distillation, the ELF-AQUITAINE research teams met a quite different approach to the problem. So was the N-ISELF process born.
I11 - PROCESS OPERATION The N-ISELF process makes use of the principle of gas-solid chromatography. The solid is a molecular sieve used as an adsorbent. The solid, because of its crystaline structure, only absorbs molecules of equivalent diameter less than 5 A. Normal paraffins are thus adsorbed, while iso-paraffins, naphtenes and aromatics are not. These molecular sieves were designed specially for the N-ISELF process. 1II.l.Principle of separation (cf. fig. 3) Most of the separation processes by adsorption makes use of adsorption-desorption cycles which are determined by variation of a physical parameter such as total pressure or temperature. On the contrary the N-ISELF process is isobar and rather isotherm. The active parameter is the partial pressure of the n-paraffins. periodically some vaporized hydrocarbons, carried by the hydrogen stream, comes into contact with the sieve-bed. Parts of adsordables molecules (n-~araffins)are retained. The other ones then flow out of the bed, followed by those adsorbed molecules which are desorbed by hydrogen.
FIG. 3 : N-ISELF PROCESS FLOWSHEET :
n-paraffinic product
At each injection process, one iso-paraffin rich mixture and one n-paraffin-rich mixture then leave the bed. The products are then cooled and separated from hydrogen gas and then, recycled by a compressor. The industrial process has several beds in parallel. The number of beds is determined by the ratio : cycle timelinjection time of the load on a given bed. Switching between columns is done by a valve system managed in real time by a program clock so as the process becomes pseudo-continuous. The process includes many operating variables such as : total pressure, temperature, cycle time, timing structure of the different sievings. This confers on the N-ISELF unit a large operating flexibility allowing it to produce two cuts more or less enriched with varying yields. III.2.Characterization of the separation : Separation may be characterized by a factor varying from 0 to m , which is the product of two ratios : - the ratio : iso-content in the iso-cut/iso-content
in the N-cut,
-
the ratio : N-content in the N-cut/N-content in the iso-cut.
For a given cut, we may represent on the same figure the percentage of iso-paraffin in the iso-cut and its RON in terms of the yield of the iso-cut. For instance a a RON of about to reach 50 % and containing
feedstock containing 50 % of iso-paraffins has 69. A separation factor of about 30 allows us of iso-cut with a clear octane number of 78, about 82 % of iso-paraffins (cf. fig. 4).
RON of iso paraffinic product
content of isoparaffins in iso-paraffinic pro.duct
80
70 1.0
0
0.5
Iso paraffinic product yield FIG. 4 :CHARACTERIZATION OF THE SEPARATION
1
111.3. Yielding of the process : Operation is characterized by the type of load, the properties of both the N- and iso-cuts, operating variables, and treatment costs. For any given feedstock, the yields in iso-cut (and consequently in N-cut) may be selected between 25 % and 75 %, according to the designed purity. The N-ISELF process may for instance yield high purity n-paraffins. These are indeed always present in effluents, because they desorb slowly, whereas the iso-paraffin peak may cross the bed practically without being distorded. It is then possible, by varying the cut points, to easily modify the product yieldings and purities. This flexibility is all the more operational that, as soon as the process was designed, a high level automatization was achevied to pilot feedstock injection and products recovery. 111.4. Adsorbent regeneration : The sieve efficiency decreases over time. This deactivation may be taken up by modifying the cycle time and cut points, but if one wants to maintain the whole separation range, an "in situ" burn off regeneration must be achieved every 1600 hours for each adsorbent bed. The useful life of adsorbers regenerated in this way is about three years. 111.5. Energy consumption : As the feedstock, recycle gas and product flows are almost stationary, the thermal integration in this process is analogous to that of feed/effluent processes like hydrotreating for example : the feedstock is preheated by the hot streams coming from the separation columns. The fuel consumption is strongly reduced : about 1 TEP for 100 Tons of feed. The N-ISELF process being isobar, the recycle compressor has only to compensate for pressure drop in the streams. Its consumption is about 20 KWH/TO~. At last, one must stabilize the products flowing from the separators, what means a 3 tons consumption of steam for 100 tons of feed.
Part 2 : MODELIZATION AND OPTIMIZATION * I I1
MATHEMATICAL MODEL OF AN ADSORPTION COLUMN
-
PROCESS SIMULATION
I11 - COMPUTER SIMULATION PROGRAMM CAPABILITIES IV -
USE FQR DESIGNING A NEW UNIT OR OPTIMIZING AN EXISTLKG UNIT ON A NEW FEEDSTOCK.
I
-
MATHEMATICAL MODEL OF AN ADSORPTION COLUm The model used here belongs to the MCE type (Mixing cells in Cascade with mass exchange). Adsorption of each n-paraffin is represented by the empirical isotherm of PETERSON & REDLICH taking into account the component coupling :
The hypothesis for this model are the following :
- negligible pressure drop, - constant temperature, - internal adsorption kinetics with a linear potential. When the material balance equations for each component are written, they are linearized around an operational point selected to represent the average conditions of the adsorbent saturation during one cycle. This linearization is justified by the facts that isotherm curvature is slight at high temperature and that recycled gas contains n-paraffins-too, so that there is still partial pressure of n-paraffins beyond which isotherm is practically linear. The linearized equations are solved in the Laplace space in order to obtain a transfer matrix :
One then computes the amount of each component present in a given cut leaving the unit between the and 0 instants. On 2 that purpose a trapping function is defined, such as :
if not
The k component flow in the considered cut is given by :
In that equation, hk is an impulsional response, em(t) being the input signal of ?he m component and @ the convolution product. These functions being periodic, with a T period (the cycle time), they can be decomposed into Fourrier series and the wanted quantity :
= G.
1
sk (t) dt
(G = recycle gas flow)
0
is nothing but the constant term S (0) of the Fourrier transformation equation, multiplie8 by G :
The N-ISELF process is simulated from the above described model by equalizing the ratio : cycle timelinjection time with the number of parallel columns. Each component amount in the N-cut and the Iso-cut is computed, as well as the composition and flow of the hydrogen gas coming from the separators. The recycle flow is adjusted by an hydrogen make-up. Computing is iterative, transfer functions are modified at each step until convergence is achieved (cf. fig. 1).
I2 fUW n .n . r . d I s 0 MU (slhr) 150 c m c z n n 1 ~ ~ 0 1 1 IS0 R E W m X KUN ~ m u n m
327.0 80.4
z
-
- - - - I.0
~UU-L~~V* P.k
-6
48.7
3Y.2 88.0 47.3
2. I
"~Lii
PIG. 1
:
c-t.d
: N-IS-
:
mSSQU
m
sn
: :
INJLRIOll OI : 150-TUP OI Y
----~11.tfi*. PROCESS IOOELIZARMI
m
l2M F t l ~ 1.1 250.0 .C 13.2 MI 2 u . o SIC 67 SIC
ss sac
I11 - COMPUTER SIMULATION PROGRAMM CAPABILITIES The programm is designed for simulating adsorption of nparaffins from C3 to C10. It can handle feeds including as many as 50 components. Each adsorbent is characterized by 3 parameters taking into account its particularities and correcting the empirical isotherm and the kinetics laws :
- an adsorption factor, - a kinetics factor, - an aging factor. These adjusting factors are determined for each adsorbent from the results obtained on a pilot in order to adjust computing to achieved measures. Besides it, the N-ISELF simulation modulus can be integrated to a general program of chemical engineering simulation, so allowing its upstream or downstream integration into a refining schema, specially when the N-cut that it produces is refractionated into nC5, nC6, nC7, or when its feed needs to be pretreated. IV -
USE FOR DESIGNING A NEW UNIT OR OPTIMIZING AN EXISTING UNIT ON A NEW FEEDSTOCK : This option allows us to maximize a criterion (a technical or economical one) with respect to a constraint on product specification (n-paraffin content in the N-cut for instance) : The variable may be : 1. cycle time, 2. pressure, 3. recycle flow, 4. column volume (adsorbent mass), 5. column number. The data are cations of a fications is criteria may
: feedstock composition, temperature and specifikey-product (N-cut or iso-cut). Respect of speciobtained by computing optimal cut-points. Selection be :
1. Operating cost by ton of feedstock, 2. Operating cost by ton of key-product, 3. Actualized profit, 4. Key-product yield, 5. Profitability rate,
6. Pay-out time,
7. Capital investment per ton of key-product. These various options allow us to quickly determine the capital investment and operating costs of a new unit, as well as its main characteristics (column number and sizing) and operating variables. The program gives the intermediate stream characteristics (colums effluents, recycle gas) and so allows us an exact sizing of the equipments (heat-exchangers, furnaces, compressor).
On an existing unit it allows for example to quickly determine the best cycle time, recycle flow and cut points for a given feed, in order to maximize the key-product yield.
Part I11 : INTEGRATION OF THE N-ISELF PROCESS IN THE REFINING AND PETROCHEMICAL INDUSTRY : CASE STUD'IES. INTRODUCTION I -
INTEGRATION OF THE N-ISELF PROCESS IN A REFINERY. 1.1. 1.2. 1.3. 1.4. 1.5.
I1 -
Material Balance. Incidence on the production of premium grade gasoline. Characteristics of the feedstock. Profitability. Case in which the N-cut is sold as a solvent base.
A 100 000 TONS/YEAR N-ISELJ? UNIT FOR N-HEXANE PRODUCTION. 11.1. 11.2. 11.3. 11.4. 11.5.
Feed characteristics. Product characteristics. Description of the unit. Material balance (kg/hr). Profitability.
111.1. Naphta processing. 111.2. Material Balance. 111.3. Profitability.
INTEGRATION OF THEN-ISELF PROCESS IN THE REFINING AND PETROCHEMICAL INDUSTRY : CASE STUDIES : INTRODUCTION The cost of processing is low for a refining process, above all when consideration is given to the economies with respect to refining and petrochemicals. This type of unit may be located in a refinery or in a chemical plant. The main points which are sources of profit in refining are as follows :
. with
an n-iself and a fixed production rate, the pool NOR improves and the reformer, a unit which converts 5 to 10 % of its feed into gas, may be taken partially out of action.
.
the iso-paraffin produced by the n-iself process improve the susceptability of gasoline to lead.
. in
some refineries, the n-iself process increases the motor octane number which is lowered by the F.C.C. gasolines.
. n-iself
provides "light" NOR and increases the volumetric fuel yield of the crude oil processed.
In many cases which have been studied, utilisation is justified by these advantages themselves from the point of view of gasoline pool and is amplified if the advantages from the point of view of petrochemicals are taken into account.
. improvement of ethylene yield, . improvement of propylene yield, . improvement of butadiene yield, . reduction of coking, . reduction
of fuel consumption.
I - INTEGRATION OF THE N-ISELF PROCESS IN A REFINERY
1.1.Material balance : Using two feedstocks of light and heavy Arab origin, the material balance of the N-ISELF unit is the following :
from light Arab
Feedstock ( ~ 5- 70')
Iso-cut
N -cut
density RON (0.4) flow (t/h) yield from heavy Arab density RON (0.4) flow (t/h) yield Gain in octanes : iso-cut/feedstock : from light Arab from heavy Arab
+ 13 points + 9.6 points.
Taking into account the potential of the C5-70' cut contained in each of the two crude oils (3.4 % on light Arab, 3.8 % on heavy Arab) and a treatment of 60 % light Arab and 40 % heavy Arab, the following is retained : iso-cut (density = 0.663 (RON (0.4) = 91.6
feedstock (density = 0.663 (RON (0.4) = 80
1.2.Incidence on the production of premium grade gasoline Without N-ISELF, N and B quantities of naphta and butane enter into the composition of the premium grade gasoline S. With N-ISELF,
the supplementary iso-cut which is going to enter into the composition of the premium grade gasoline will give rise to a supplementary quantity of A S premium grade to the detriment of the AN and AB quantities of naphta and butane.
We will therefore have the folowing equivalence :
The characteristics of the previous components are :
density iso-cut naphta butane premium g r a d e g a s o l i n e
0.663 0.663 0.580 (summer (winter
TV ( b a r )
RON ( 0 . 4 )
0.72 0.68 4.4 0.65 0.8
91.6 80 99 98 98
3 50 k t l y e a r of i s o - c u t a r e a v a i l a b l e (75.4 k m ). L e t u s suppose t h a t h a l f of t h i s c u t w i l l e n t e r i n t o premium g r a d e g a s o l i n e of t h e w i n t e r and summer q u a l i t y . F o r a y e a r , t h e f o l l o w i n g i s found :
(AN =
- 26.8 k
m3 of l i g h t n a p h t a i n t h e pool k m3 of i s o - c u t i n t h e pool 0.2 k m3 of butane i n t h e pool.
(AS =
+ 75.4
(LB
-
=
This leads t o :
. the
usage of 48,380 m3 of n a p h t g t o make 48,380 m g r a d e g a s o l i n e , r e l e a s i n g 200 m of b u t a n e .
3
of premium
Taking i n t o a c c o u n t t h e p a r i t a r y d e n s i t i e s , t h e u s a g e of 32,100 t o n s of l i g h t n a p h t a l e a d s t o producing 36,200 t o n s of premium g r a d e g a s o l i n e and r e l e a s i n g 100 t o n s of b u t a n e .
-
N Paraff inic product (petrochemistry
*
C4 t
F
HDT
REFORMEUR
REFORMAT rn
lsoparaffinic product
Straight run light gasoline F i g u r e 1.
G A S 0 L I N E
1.3. Characteristics of the feedstock : The study on the unit has been carried out with two reference feedstock and a capacity of 100,000 tonslyear for an operation of 7700 hours without regeneration. Feedstock characteristics (hydrotreated light gasolines direc straight run from atmospheric distillation) : Feedstock I C5 - 70°C From heavy Arab
Feedstock I1 C5 - 70°C From light Arab
Density at 15°C TVR % distilled at 70°C
RON clear RON
+ 0.4 g
~b/l
% n-paraffins % Naphta
+
aromatics
% Iso-paraffins
S contents
C 1 PPm
Water contents MON clear MON
.
+ 0.4
g Pb/l
Composition (mole Z ) : N-butane Isopentane N-pentane Cyclo C5
3 methyl C5 N -hexane Cyclohexane Benzene Isoheptane N-heptane
1( PPm 20 to 30 ppm
68
62
83.3
76
1.4. Profitability
Total investments
the project considered requires :
:
43 MF (May 1982) investment Variable costs (for 13 t/h) :
. electricity . fuel gas : . steam : . water :
KW 1400 thermies 1 t/h5 300 m /hr
: 435
426,16 ~ / h r Regeneration (4 per year) :
. electricity
. fuel gas . water : . air : . nitrogen . sieve
: 59 600
:
:
36 57 8 13
:
KW
000 t ermies 600 m3 000 m 200 m3 101 kf/year 3 kf/ year
t3
193,432 Flyear Variable annual costs for 7,692 hr/year Variable costs : Regeneration costs :
3.27 MF 0.19 MF
Annual standing charges 1 man on duty : Staff insurance
1.00 MF 1.00 MF 2.00 ME'
Costs per year :
5.46 MF
Annually the usage of 32,100 t/year of naphta leads to producing 30,200 t / ~ e a rof paritary density premium grade gasoline and to releasing 100 t/year of butane.
Due to unwanted coastal traffic, differentials must b c introduced with respects to the long term prices : - 9.7 F/t for premium grade gasoline
+
230 F/t for butane
- 38.5 ~ / for t naphta.
This means : supplementary premium grade gasoline naphta consumed : butane released : SUPPLEMENTARY VALORIZATION Annual costs : Pay out
=
:
:
+
77.02
- 61,06 t 10.18
20 MF
5.46 MF
43 = 14.54
3.0 years
sensitivity :
.
. .
investments : + 20 % pay out : 3.3 yrs octane gain : 10 % lower hoped for : pay out : 3.5 yrs naphta price (gap between naphta and premium pay out : 3.3 yrs grade gasoline reduced by 10 %)
I.5.Case
in which the n-cut is sold as a solvent base
40,000 ~ / y rof base sold at the price of premium grade gasoline (cf. West ~ e r m a n ~ )
A between naphta and premium grade -
40 ~ / for t transport
Supplementary annual valorization :
Pay out =
43 = 21
2 years.
:
+
225.7 T/t
-I1
- A 100 000 TONS~YEARN-ISELF UNIT FOR N-HEXANE PRODUCTION
I1 - 1. Feed characteristics :
#
- composition -----------
.
:
Butane
. Isopentane . N-pentane . Isohexane
design feed "/,weight :
fluctuation 7, weight :
1
0 ,15 - 2.. 5
4
2.2
- 4,7
. Octanes . Aromatics .
Sulphur ppm
1 - 2
- Bromine - - - - - - - -number ------ Glycol - - - - - - (ethylen glycol)
maximum : 50 ppm
- Pressure - -- --- - -
minimum : 2 eff. bar
- Temperature -----------
25OC minimum
0.05 - 0.10 g ~ r / 1 0 0g
- Water - - - - - - satured - - - - - - - (no free water)
Hydrogen gas :
- composition - - - - - - - - - - - ( X volume)
. Hz . Methane
9 6 % minimum
4 % maximum 5 ppm maximum 30 ppm maximum 19 eff. bar minimum 21 eff. bar maximum
- temperature -----------
ambient
UNIT CAPACITY Rated capacity of the unit is 100 000 T/yr feed, with an operation factor of 7800 hr/yr. 11.2. Product characteristics N-Hexane cut
-
composition ----------- :
. N-hexane content . Aromatics content
85 % weight 200 ppm
. Sulphur content . Density at 20°C
0.660
. Color
1
(H~so~)
Alcohol (OH)
. non-volatiles . Bromine number . Ether . Colors
2 PPm
25 PPm 5 mg/100 ml 0.015 ~ r ~ / 1 0g 0 100 ppm
(hazene)
. aspect
20 limpid, transparent
11.3. Description of the unit The N-hexane cut production unit includes the following sections :
- a section for separating the iso-cut and the N-cut on a molecular sieve (N-ISELF process).
- a hydrogenation section of the N-cut. - a fractionating section.
Separation of the iso-cut and the n-cut : ....................................... The raffinate is fed to the N-ISEW unit by the feed pump, and then mixed to a little part of the recycle hydrogen rich recycle gas. This mixture is preheated in the feedleffluent heat exchanger, then it is heated up to the separation temperature in the furnace. The remaining recycle gas is preheated in the recycle/iso
effluent heat exchanger, and then heated up to a temperature higher than the separation column temperature in the furnace. Indeed, the furnace insuring both the duties : feed and recycle preheating, is controlled by the feed outlet temperature, whereas the recycle temperature at the separation column inlet is controlled by mixing some too hot gas coming out of the furnace and some cold gas taken from the compressor outlet. The recycle gas is used as a carrier gas and is permanently injected, with a flow control, into the five separation columns. The feed is intermittently, cyclically, injected into each of the five separation colums (during 115th of cycle-time into each). The hydrocarbons contained in the feed are adsorbed on the molecular sieve and selectively eluated by hydrogen. First, the iso-paraffins are desorbed and directed to the iso stream (during 1/3rd of cycle-time). During the end of the cycle, N-paraffins come out of the column and are directed to the N-stream, thanks to a timing system on the output valve. The use of five columns allows the continuous injection of the feed, one column after the other. The iso-paraffins are directed (with 1/3rd of the recycle gas) to the separation drum, after cooling down with the recycle gas, and then in a water cooling system. The N-cut, mixed with 2131-d of the recycle gas, is directed to the hydrogenation reactor. N-cut hydrogenation ------------------In order to meet the aromatic content in the N-hexane cut specifications, the N-cut is hydrogenated. The N-cut is cooled down by heat exchange with the feed, then in a cooling system with water ; the gas and liquid are then separated in a separation-drum. The vapors coming from both separators and the make-up gas are recovered by a compressor and form the recycle gas.
Fractionating ------------The N-cut feeds a first fractionating column, called the depentanizer, where three products are withdrawn : on top a vapor distillate which is sent to the fuel-gas network, in the side-stream the n-C5 cut, and at the bottom the n-C6 cut. This column is reboiled with eff. 7 bar steam. The n-C5, n-C6, n-C7 cuts are cooled down in water heatexchangers and sent to the storage capacities. The Iso-cut, after preheating with the stabilized iso-cut, feeds a stabilization column where a vapor distillates is drawn on top, and then sent to the fuel-gas gathering system, and where the stabilized iso-cut is drawn at the bottom. This column is reboiled with eff. 7 bar vapor. A condenser, integrated in the column top, insures the reflux. After heat-exchange with the feed of the stabilization column, the iso-cut is recovered at the bottom by a pump and then cooled down in a water heat-exchanger and sent to the storage facilities. FUEL G A S
ISO-CUT
FEED a
1 i i i i G J FUEL GAS
FIGURE 2 : F l o w sheet
Balance -------
- Feed
A
fuel-gas
Hypothesis ----------
. Iso-cut : Introduction of the iso-cut into the motor-fuel stock brings about the production of 47,512 t/yr of paritary density premium grade gasoline and 130 t/yr of C4 by use of 42,497 t/year of iso-cut.
. N-cut
: the N-cuts, i.e. 32,578 t/yr, are sold on the solvent market at premium grade gasoline price.
CAPITAL INVESTMENT -----------------(including hydrotreatment and additional offsites facilities)
. Variable
charges
Consumption Electricity
339 KW
Steam
2.9 t/hr
Fuel-gas
0.17t/hr 3 285m /hr
Water
Price -
Time/yr
Total -
0.24 F/KWh
7800 hrs
0.635 MF
97 ~ / t
7800 hrs
2.194 MF
7800 hrs
1.986 MF
1498 F/t 0.1 ~
/
TOTAL
. . .
m7800 ~ hrs 7800 hrs
0.222 5.037 MF/YY
4 regenerations each year
0.2
MF/yr
Standing charges
2
MF/yr
20.5
MF/yr
7.4
MF/yr
2.6
MF/yr
Iso-cut valorization
. N-cut valorization . ~osses . Gross Profit : 20.5 + 7.4 - 2. . Pay-out : 40118.1 -v2.2 years
- 0.2
- 5. - 2.5
=
18.1
111.1. Naphta processing (cf. fig. 3) In a first stage, long cut naphta is splitted into heavy naphta and light naphta which is then desulfurized by hydrotreating. This light hydrotreated naphta is then fed to the N-ISELF process which produces two cuts :
-
a N-cut used as a steam-cracking feed. an Iso-cut used in the motor-fuel stock.
The naphta preparation is needful because :
. N-ISELF
is specially adaptated to light naphta.
. Light naphta must be desulfurize in order to remove sulphur containing components and olefins traces, which are impurities harmful to N-ISELF. Of any way, the iso-cut must be desulfurized.
. enrichment with heavier n-paraffins and with C7,
C 8 , doesn't bring about any noticeable increase in the olefin yield during steam-cracking of the so processed naphta. This conclusion arises from extended studies carried in our Company for the operation of our steam-crackers.
So we set up a process in which only light hydrotreated naphta is processed by N-ISELF. III.2.Material balance The long cut naphta feed is 1.36 mt/yr. Preparation of this feed leads to the following quantities :
.
long cut naphta feed :
. light desulfurized naphta . heavy naphta
:
. light iso-naphta : . light N-naphta :
1.36 mt/yr :
0 . 5 7 mt/yr 0 . 7 9 mt/yr
0.31 mt/yr 0 . 2 6 mt/yr
The N-ISELF intended for this project is then 0 . 5 7 mt/~r.
1
light naphta
hydrodesul furation
__c
N-ISELF
-
I long-cut naphta
N-cut to steamcracking
S P L I T T E R
heavy naphta FIG. 3 : REFINING FLOWSHEET
+
111.3. Profitability Capital - - - - - - - - -investment --------- :
. . .
N-ISELF
5 70 kt/yr
Hydrotreating
570 kt/yr
Splitter
Costs -----
1,360 kt/yr
:
Standing yearly costs : N-ISELF : 3 menIyr Hydrotreating
:
1 madyear
2.8 MF
Splitter : 114th madyear
Variable yearly costs : N-ISELF :
16.47 MF
Hydrotreating : Splitter :
Profitability ------------- :
.
Iso-cut : 310 kt of iso-cut will give an increasing and a lightening in the motor-fuel stock, this being done by the process described in the case 1. Valorization will then be of 99.2 MF.
.
.
N-cut : enrichment with N-paraffins brings about better olefin-yields, hence a better naphta valorization. This leads to a 44.5 MF additional valorization of products coming from the 260 kt naphta steam-cracking (cf. detailed economical study in the appendix). Summing-up : the additional valorization is then :
99
+
39 = 138 MF
for a capital investment of 225.3 MF and variable and standing costs of 39.7 MF, i. e. a pay-out of 2.28 years.
A P P E N D I X PROCESSING COSTS Hydrotreatinn :
.
Capacity : 0.57 M ~ / y r investment
.
Standing costs : 1 man/~ear Maintenance (3 % of capital investment) Overhead expenses
. Variable
1.0 MF/yr 1.2 0.6
costs :
Fuel : Electricity : Steam t4t : Water m /hr :
0.0105 at 1.100 ~ / t 11.55 8 K W H / ~at 0.26 F/KW 2.08 0.03 to 97 ~ / t 2.91 0.5 to 0.01
i. e. yearly : Splitter
. Capacity
: 1.36 mt/yr
investment
. Standing costs
:
0.25 man/yr Maintenance (3 % I) Overhead expenses
. Variable
costs :
Steam 0.1 t/t at 92 F/t, i. e. 9.76 F/t i. e. yearly
10.5 MF/ year
PRESSURE SWING ADSORPTION
C.N.
Kenney and N.F.
Kirkby
Department of Chemical Engineering, Cambridge, U.K.
1. : INTRODUCTION
1.1 P r e s s u r e swing a d s o r p t i o n (PSA) i s a process f o r s e p a r a t i n g g a s mixtures based upon r e c e n t l y developed adsorbent s o l i d s capable of s e l e c t i v e l y r e t a i n i n g s p e c i f j c gases. The major o p e r a t i n g parameter i n a PSA system i s p r e s s u r e ; most commercial u n i t s o p e r a t e a t , o r n e a r , ambient temperature. A t y p i c a l PSA c y c l e i s shown i n Fig. 1, t h e b a s i c s t e p s of which may be summarised a s follows : ( 1 ) Feed gas i s compressed i n t o an adsorbent f i x e d bed, t h e ads o r b a b l e components a r e adsorbed and a d s o r b a t e - f r e e g a s accumul a t e s a t t h e c l o s e d end of t h e bed. ( 2 ) A t t h e upper o p e r a t i n g p r e s s u r e , a d s o r b e n t - f r e e gas ( u s u a l l y t h e "product") i s withdrawn from t h e f a r end of t h e bed w h i l s t f e e d flow i s maintained. Three d i s t i n c t r e g i o n s form i n t h e bed: a ) Near t h e f e e d e n t r y t h e bed i s s a t u r a t e d w i t h a d s o r b a t e and t h e g a s phase i s of f e e d composition. b ) Near t h e e x i t t h e bed i s s t i l l a d s o r b a t e - f r e e , and t h e gas phase has product composition. c ) The r e g i o n between t h e above two i s c a l l e d t h e mass t r a n s f e r zone o r a d s o r p t i o n f r o n t . This i s w h ~ ~ a des o r p t i o n i s o c c u r r i n g and g a s phase composition changes r a p i d l y with a x i a l p o s i t i o n . The a d s o r p t i o n f r o n t moves alsong t h e bed a s more feed i s i n t r o duced. Eventually t h e bed would be completely s a t u r a t e d , t h e mass t r a n s f e r zone having reached t h e end of t h e bed, a t which p o i n t f e e d i s s a i d t o "breakthrough" i n t o t h e product stream. ( 3 ) To r e g e n e r a t e t h e s a t u r a t e d bed t h e next s t e p involves reducing t h e o p e r a t i n g p r e s s u r e (Blowdown). The a d s o r b a t e i s l a r g e l y
1.
Pressurisation
2.
Dynamic product release
3.
Blowdown
4.
Purge
-
Feed
Fig. 1 :
Basic steps of a PSA cycle
desorbed i n t o t h e gas phase and r e l e a s e d a s a waste stream. ( 4 ) To complete t h e r e g e n e r a t i o n of t h e bed t h e adsorbent i s purged with product q u a l i t y g a s , u s u a l l y c o u n t e r - c u r r e n t l y from t h e product end. Again d i s t i n c t r e g i o n s form and e v e n t u a l l y purge gas would breakthrough i n t o t h e waste and f u r t h e r purging would n e e d l e s s l y d i s c h a r g e p o t e n t i a l product. Thus PSA has s i m i l a r i t i e s t o o t h e r f i x e d bed , and chromat o g r a p h i c s e p a r a t i o n processes. C a l c u l a t i n g t h e balance between feed flows and p a r t - c y c l e times i s t h e c r u c i a l problem and no comprehensive s o l u t i o n e x i s t s . Most e x i s t i n g p l a n t s have been designed from e m p i r i c a l d a t a , o r g r o s s l y s i m p l i f i e d mathematical models. 1.2 Uses of PSA/Competing Processes There i) ii) iii) iv)
a r e f o u r major a r e a s i n which PSA i s a p p l i e d : A i r drying Hydrogen recovery A i r separation Exotic s e p a r a t i o n s
1 . 2 . 1 A i r drying. This i s t h e o l d e s t use of PSA and probably t h e most widely s t u d i e d . Competing methods f o r removing water from ambient a i r a r e c o m p r e s s o r / c h i l l e r systems, which a r e l e s s energy e f f i c i e n t than PSA, o r thermal swing a d s o r b e r s (where r e g e n e r a t i o n i s achieved by h e a t i n g and purging a t c o n s t a n t p r e s s u r e ) which have t h e disadvantage t h a t t h e i r thermal i n e r t i a n e c e s s i t a t e s long time c y c l e s and t h e r e f o r e low throughput p e r u n i t bed volume. 1.2.2 Hydrogen recovery a p p l i c a t i o n s . I n commercial terms t h i s is t h e l a r g e s t group of a p p l i c a t i o n s ; s i n c e 1966 Union Carbide alone have b u i l t over 40 major u n i t s . Hydrogen i s an expensive feedstock and recovery systems have a t t r a c t e d much a t t e n t i o n . The processes competing with PSA a r e based on palladium d i f f u s i o n o r a r e cryogenic p r o c e s s e s . I n c o n t r a s t t o t h e s e , PSA has proved i t s e l f t o have higher r e l i a b i l i t y and more f l e x i b i l i t y . I t g e n e r a l l y r e q u i r e s l e s s maintenance and can produce hydrogen a t very high p u r i t y . PSA technology has r e c e n t l y been a p p l i e d most s u c c e s s f u l l y t o H 2 recovery from ammonia s y n t h e s i s loop purge gas streams producing up t o 10% r e d u c t i o n s i n f e e d s t o c k / f u e l c o s t s . 1.2.3. A i r s e p a r a t i o n . PSA can be used t o produce e i t h e s oxygen o r n i t r o g e n from a i r . Low volume requirements ( < 200 Nm / h r ) o r oxygen include : B i o l o g i c a l t r e a t m e n t of municipal and i n d u s t r i a l wastewater. i) Feed t o ozone g e n e r a t o r s f o r wastewater t r e a t m e n t . ii)
i i i ) Oxygen f o r r i v e r s and r e s e r v o i r s , e s p e c i a l l y f o r f i s h farming. i v ) Bleaching o f chemical p u l p and t r e a t m e n t o f b l a c k l i q u o r i n t h e paper industry. v) Nonferrous m e t a l s m e l t i n g . v i ) Medical a p p l i c a t i o n s , d o m i c i l i a r y O g e n e r a t o r s e t c . 2 v i i ) Chemical o x i d a t i o n p r o c e s s e s . v i i i ) E n r i c h e d 0 combustion atmosperes t o enhance f u e l economy. 2
Nitrogen a p p l i c a t i o n s c e n t r e around i n e r t g a s r e q u i r e m e n t s ; i n e r t f l u s h i n g g a s f o r petroleum t a n k s , p l a n t f l u s h i n g p r i o r t o s t a r t - u p e t c . For low volume a p p l i c a t i o n s , PSA h a s s p e c i f i c a d v a n t a g e s o v e r c r y o g e n i c methods a n d / o r r o a d t a n k e r d e l i v e r y systems: f a s t and s i m p l e s t a r t - u p and shut-down; i) ii) s i m p l e , r e l i a b l e and maintenance-free o p e r a t i o n ; i i i ) low o p e r a t i n g c o s t s ; iv) low c a p i t a l c o s t s , e s p e c i a l l y a s s p e c i a l m a t e r i a l s o f cons t r u c t i o n a r e not required; h i g h l y f l e x i b l e p r o d u c t i o n c a p a b i l i t y minimises s t o r a g e v) c a p a c i t y a n d / o r a l l o w s e f f i c i e n t o p e r a t i o n even w i t h l a r g e demand f l u c t u a t i o n s ; vi) b e i n g f r e q u e n t l y s k i d mounted, c o n s t r u c t i o n and i n s t a l l a t i o n c o s t s a r e minimised. 1 . 2 . 4 Other s e p a r a t i o n s . ( 1 ) Gaseous i s o t o p e s e p a r a t i o n s i n t h e n u c l e a r r e p r o c e s s i n g industry. ( 2 ) Helium r e c o v e r y from b l a s t f u r n a c e g a s e s . ( 3 ) Hydrocarbon s e p a r a t i o n s , eg. p a r a f f i n s from a r o m a t i c hydrocarbons. 1 . 2 . 5 . PSA d i s a d v a n t a g e s . PSA technology i s n o t u s u a l l y c o m p e t i t i v e a t h i g h t h r o u g h p u t s . Hydrogen p r o c e s s a d s o r b e n t s have been most s u c c e s s f u l l y developed, having extremely low a f f i n i t y f o r hydrogen i n comparison w i t h t h e common i m p u r i t i e s encountered. However, i n a i r s e p a r a t i o n p a r t i c u l a r l y , p r o d u c t t o w a s t e r a t i o s a r e of t h e o r d e r o f 1 : 1 0 o r worse; a t h i g h p r o d u c t f l o w s v e r y l a r g e q u a n t i t i e s of w a s t e g a s have t o be pumped, i n c r e a s i n g c o s t s dramatically Optimal c y c l e s t u d i e s s h o a l d p f f s e t t h e s e a d s o r b e n t l i m i t a t i o n s b u t s u c h s t u d i e s a r e complicated b o t h e x p e r i m e n t a l l y and t h e o r e t i c a l l y e s p e c i a l l y w i t h t h e models a v a i l a b l e .
.
2 . : PSA PROCESSES 2 . 1 H i s t o r i c a l Background The mention o f t h e a d s o r p t i o n of g a s e s and vapours on s o l i d s d e r i v e s from d a t e s back t o B i b l i c a l t i m e s . Langmuir (1)d e r i v e d a t h e o r y of a d s o r p t i o n phenomena based on assuming t h a t o n l y a monolayer
forms, t h a t a d s o r p t i o n i s l o c a l i s e d and t h e h e a t evolved i s independent of s u r f a c e coverage. T h i s l e d t o t h e Langmuir isotherm, s t i l l widely used t o q u a n t i f y t h e amount adsorbed a s a f u n c t i o n of p r e s s u r e a t c o n s t a n t temperature. McBain ( 2 ) coined t h e term "molecular s i e v e w t o d e f i n e porous s o l i d s which e x h i b i t t h e p r o p e r t y of a c t i n g a s s i e v e s on a molecular s c a l e . B a r r e r ( 3 ) published d a t a showing t h e ss.!.ective a d s o r p t i v e p r o p e r t i e s of c h a b a z i t e f o r a i r and c o r r e c t l y a t t r i b u t e d t h e h i g h e r quadrupole moment of n i t r o g e n a s being t h e o r i g i n of i t s high a f f i n i t y . The a d s o r p t i v e p r o p e r t i e s of v a r i o u s forms of carbon and coke became a p p a r e n t , p a r t i c u l a r l y a s t h e r e s u l t of r e s e a r c h a t t h e German Mining Research I n s t i t u t e . A s a r e s u l t o f c o l l a b o r a t i o n D r . Ing H. Kahle a t Linde obtained a German P a t e n t i n 1942 e n t i t l e d "Adsorption Process" which l e d t o t h e SORBOGENl p r o c e s s f o r H20 and CO removal from a i r , by 1954, u s i n g a p r e s s u r e swing c y c l e , Kahle However, most a u t h o r s a t t r i b u t e t h e i n v e n t i o n of PSA t o D r . C.W. Skarstrom, working a t Esso Research and Engineering, t h e first major p a t e n t being given i n 1960 ( s e e Skarstrom ( 5 ) ) . I n t h e s e two decades t h e r e were s e v e r a l important developments; s y n t h e t i c z e o l i t e could be manufactured, and n a t u r a l z e o l i t e doctored by i o n exchange, much improving s e l e c t i v i t y and t o t a l c a p a c i t y . I n Germany e s p e c i a l l y , c o n t r o l o f pore shapes, and s i z e d i s t r i b u t i o n s l e d t o some h i g h l y e f f i c i e n t carbon molecular s i e v e s . The Skarstrom p r o c e s s was probably t h e f i r s t .to e x p l o i t t h e s e developments. These i d e a s spread r a p i d l y and t h e r e a r e now over 400 p a t e n t s h e l d world-wide by some seven o r e i g h t major companies.
(8)).
2.2 Adsorbents
The two major t y p e s of a d s o r b e n t s used, a r e both broadly r e f e r r e d t o a s molecular s i e v e s : molecular s i e v i n g carbon (MSC) and z e o l i t e s . MSC manufacturers have d e c l i n e d r e q u e s t s t o provide adsorbent f o r t h i s r e s e a r c h , s o only a b r i e f review of MSC p r o p e r t i e s i s given. 2.2.1 Z e o l i t e s t r u c t u r e and p r o p e r t i e s . The n a t u r a l and s y n t h e t i c z e o l i t e s a r e c h a r a c t e r i s e d by being a l u m i n o - s i l i c a t e c r y s t a l l i t e s o f complex molecular s t r u c t u r e s . Breck ( 6 ) g i v e s an account o f a l l major v a r i e t i e s , s t r u c t u r e s , p r o p e r t i e s , e t c . Types A and X a r e predominantly used f o r PSA. The c u b i c c r y s t a l l i t e s have roughly s p h e r i c a l c a v i t i e s (a-cages) i n t e r c o n n e c t e d two windows p e r c a v i t y , t i c a l dimensions diameter being 11,4 8 i n t e r n a l diameter f o r a-cages and 4.5 windows (5A Z e o l i t e ) . I n t h e p r e p a r a t i o n , m e t a l i o n s can be exchanged with t h e w a l l s of t h e cages o r windows (4A Z e o l i t e ) , thereby r e g u l a t i n g t h e dimensions and c r e a t i n g e l e c t r o s t a t i c f i e l d s w i t h i n t h e c a v i t i e s . S e l e c t i v e a d s o r p t i v e p r o p e r t i e s r e s u l t from
'B
two f a c t o r s : l a r g e molecules a r e n o t c a p a b l e o f p a s s i n g i n t o t h e a - c a g e s , t h e c u t - o f f b e i n g determined by t h e window dimensions; s e c o n d l y , molecules c a p a b l e o f e n t e r i n g t h e a-cages w i l l be s e l e c t i v e l y r e t a i n e d , p r e f e r e n c e being g i v e n t o molecules w i t h t h e h i g h e s t quadrupole moment. The normal method o f e x p r e s s i n g t h e a d s o r p t i v e c a p a c i t y i s by r e l a t i n g amount adsorbed p e r u n i t mass of a d s o r b e n t t o p r e s s u r e , f o r pure gases a t constant temperature. Typical adsorption i s o therms f o r n i t r o g e n and oxygen on a 5 A z e o l i t e a r e shown i n F i g . 2 . Pure n i t r o g e n i s more s t r o n g l y adsorbed t h a n oxygen a t b o t h t e m p e r a t u r e s shown, b u t t h i s i n f o r m a t i o n does n o t show how m i x t u r e s o f t h e s e g a s e s w i l l be adsorbed. Ruthven ( 7 ) p u b l i s h e d an i s o t h e r m e q u a t i o n , d e r i v e d from s t a t i s t i c a l thermodynamics s p e c i f i c a l l y f o r 5 A z e o l i t e s and g e n e r a l i s e d t h i s i n t o a m u l t i component t h e o r y (Ruthven, Loughlin and Holborow ( 8 ) ) which h a s been shown t o work w e l l f o r b i n a r y m i x t u r e s of g a s e s (Ruthven ( 9 )
.
For m o l e c u l e s t h a t cannot e n t e r t h e a - c a g e s , modelling t h e d i f f u s i o n mechanisms h a s been s t u d i e d and much u s e f u l d a t a , c o r r e l a t i o n s and models p u b l i s h e d (Ruthven, Derrah and Loughlin ( 1 0 ) ) f o r hydrocarbon d i f f u s i o n and Ruthven and Derrah ( 1 1 ) f o r mono- and d i a t o m i c g a s e s o n t o 4 A and 5 A z e o l i t e
.
2.2.2 Molecular s i e v i n g carbons. Commercially a v a i l a b l e MSC a r e manufactured from coke by p r o c e s s e s which narrow t h e p o r e s i z e d i s t r i b u t i o n . J u n t g e n e t a 1 ( 1 2 ) and ( 1 3 ) of Bergbau-Forschung GmbH have developed a c t i v a t e d c a r b o n s t h a t a r e remarkably oxygen s p e c i f i c , and a r e t h e b a s i s o f a n i t r o g e n p r o c e s s (Knoblauch ( 1 4 ) ) . The a c t i v a t e d c a r b o n s used c o n t a i n b o t t l e and s l i t shaped p o r e s . S e p a r a t i o n o f m i x t u r e s o c c u r s n o t because o f e q u i l i b r i u m c a p a c i t y d i f f e r e n c e s b u t because o f wide d i f f e r e n c e s i n t h e r a t e o f upt a k e . (See F i g . 3 ). Reyhing ( 1 5 ) and L e i t g e b ( 1 6 ) d i s c u s s t h i s d i f f e r e n c e i n some d e t a i l , and t h e i m p l i c a t i o n s f o r f u t u r e r e s e a r c h on m o l e c u l a r s i e v e s and p r o c e s s d e s i g n . Walker Lamond and M e t c a l f e ( 1 7 ) , h a v e m a d e MSC's by c a r b o n i s i n g and p o r e - b l o c k i n g t h e r m o s e t t i n g o r g a n i c polymers. Subsequently Nandi and Walker ( 1 8 ) compared t h e s e a d s o r b e n t s w i t h coke based MSC and c a r b o n s o b t a i n e d from coconut s h e l l s . Nandi and Walker ( 1 9 ) d e s c r i b e a n oxygen s e p a r a t i o n p r o c e s s b u t u n l i k e t h e BergbauForschung p r o c e s s t h e s e seem t o be e q u i l i b r i u m i s o t h e r m dominated. They do p o i n t o u t , however, t h a t carbon h a s a major advantage over z e o l i t e because z e o l i t e s a r e h i g h l y h y d r o p h i l i c and f e e d a i r must o f t e n be d r i e d b e f o r e b e i n g f e d t o a i r s e p a r a t i o n b e d s ; t h i s i s n o t n e c e s s a r y w i t h MSC.
Fig. 2 : Zeolite
P r e s s u r c : ( n u n fig )
Fig. 3 : M.S.C.
0
5
10
Time ( r n i n )
Z e o l i t e and m o l e c u l a r s i e v e c a r b o n s e l e c t i v i t y f o r oxygen - n i t r o g e n .
2.3 Industrial Processes
In view of the very large number of processes patented (over 400) it would be a major undertaking to review a fully representative cross-section. Wagner and Stewart (18 ) and Anon (19 )
publish a broad survey of PSA systems applied to hydrogen separations, the latter showing some rare economic data. For oxygen production an introduction to the patents is given by Lee and Stahl (20 ) in which processes were reviewed in six groups: Esso Research and Engineering, Union Carbide, LIAir Liquide, Bayer/Mahler, Nippon Steel and W.R. Grace processes. To up-date this list the BOC and Air Products processes should be added (see Smith and Armond (21 ) and Sircar and Zondlo (22 )). All these processes use zeolite to preferentially adsorb nitrogen from air to produce up to 95% oxygen. A major distinction that should be drawn between these cycles is that several (Batta Union-Carbide,and LIAir Liquide) use a product release that is not accompanied by simultaneous feed. This type of process follows product release. the Kahle system and is called l'non-dynamicll Where feed is introduced and product withdrawn simultaneously the cycles are said to use dynamic product release. To illustrate the common part cycles and multibed arrangements an Esso patent is discussed whilst ,Table 1 is used to summarise the remainder. 2.3.1Air production. Skarstrom ( 5 ) patented a process shown schematically in Fig.4 The process is superatmospheric in that it operates above atmospheric pressure, blowdown being to atmospheric pressure. Pressurisation is followed by product release, and then depressurisation to atmosphere. Bed B is operated a half cycle out of phase, releasing product while Bed A is depressurised, and repressurising. This is the simplest scheme that will give a virtually constant flow of product. A purge stream is added at the end of depressurisation which reduces the product flow but does not take it all. At atmosperic pressure after purging, each bed is opened direct to the high pressure feed to repressurise the bed.
.
Berlin (23 ) introduced "bed pressure equalisation" (BPE) to repressurise partially the purged bed from the exhausted bed. This stage has the advantage of reducing the rather violent repressurisation of the bed coming on stream, and consequently reduces problems of adsorbent attrition, and fatigue of the mechanical structure. However, this step further disrupts the product flow rate and to compensate Marsh ( 24) introduced a storage vessel to conserved compressed gas for the purge step.
PRIMARY PRODUCT
4
Fig. 4
:
Process schematic for E.R.&E. U.S. Patent 2,944,627 (C.W. Skarstrom, July 12, 1960)
Table 1 i s a h i g h l y compressed summary of t h e o t h e r seven major company processes. The following p o i n t s should a l s o be made : ( 1 ) The mid-bed l i n e on t h e L ' A i r Liquide processes makes them remarkably v e r s a t i l e s o t h a t c y c l e rearrangements can be employed t o g i v e l a r g e turndown r a t i o s . ( 2 ) The Bayer s i e v e s , a t l e a s t u n t i l r e c e n t l y , were t h e most s e l e c t i v e f o r a i r s e p a r a t i o n , and do not need c l a y b i n d e r s t o hold t h e z e o l i t e c r y s t a l l i t e s i n p e l l e t form ( s e e Heinze e t a 1 (25 ) ) . ( 3 ) The BOC c y c l e s do n o t involve a f e e d compressor, t h e feed i s sucked i n t o t h e beds by t h e product compressor. Waste i s withdrawn t y p i c a l l y with a water-ring pump and a s a r e s u l t energy consumption i s kept low. This subambient c y c l e works on t h e s t e e p e s t p a r t s of t h e isotherms and t h i s enhances t h e e f f i c i e n c y further ( 4 ) The A i r Products system i s t h e only one which produces both oxygen and n i t r o g e n . A hybrid 5 A z e o l i t e i s used c a l l e d Norton Zeolon 900-Na, i n v e r y small p e l l e t s (1/16 i n c h ) which ensures t h a t very narrow mass t r a n s f e r zones develop w i t h i n t h e beds. 13X z e o l i t e i s used i n t h e pre-treatment columns. ( 5 ) The Bendix Corporation ( s e e Bendix (26 1) hrve adapted t h e s e two bed c y c l e s , p a r t i c u l a r l y t h e B a t t a (27 ) v e r s i o n , f o r r e s p i r a t o r y support systems f o r medical u s e . Other companies have a l s o shown i n t e r e s t i n t h i s p o t e n t i a l l y l a r g e and l u c r a t i v e market (Armond (28 ) ) .
.
I n a paper published i n 1977 (29) t h e same a u t h o r s made p l a n t design c o n s i d e r a t i o n s on an oxygen from a i r process using a 5A molecular s i e v e i n two adsorbent beds a s an i l l u s t r a t i o n . They considered t h e removal of moisture from t h e f e e d , c y c l e time, bed f l u i d i s a t i o n , choice of a d s o r b e n t , power consumption, e f f e c t of ambient temperature and p r o c e s s equipment. The i n f l u e n c e of each of t h e s e f a c t o r s on t h e c o s t of t h e p l a n t and i t s o p e r a t i o n a l c o s t s and e f f e c t i v e n e s s a r e d i s c u s s e d . 2.4 Nitrogen Production n using a coke Following on from t h e work done by ~ c n t ~ e(13) adsorben K. Knoblauch (14) r e c e n t l y published an a r t i c l e advocating t h e use of such an adsorbent coke f o r t h e production of N2 with a p u r i t y of between 97 and 99.9% by volume. The p l a n t i s t h e c l a s s i c a l p r e s s u r e swing process f o r continuous o p e r a t i o n with one r e a c t o r being loaded while t h e o t h e r i s being r e g e n e r a t e d . The optimum p e r i o d f o r loading and r e g e n e r a t i o n i s 60 seconds with d e s o r p t i o n a t a p r e s s u r e of 70 Torr. Higher N2 p u r i t y i s achieved a t t h e expense of gas product q u a n t i t y though t h i s may be compensated f o r by i n c r e a s i n g t h e o p e r a t i n g p r e s s u r e . The u n i t i s v e r s a t i l e enough t o produce 02 a s a main product by d i v i d i n g t h e d e s o r p t i o n s t e p i n t o 3 s t a g e s . The system i s aimed a t small volume
u s e r s and is a v a i l a b l e i n s t a n d a r d s i z e s from 1 0 t o 1000 m3/h. The Linde BF p r o c e s s i s d e s c r i b e d by D r . or^ Reyhing ( 3 0 ) and g i v e s a b r i e f h i s t o r y o f PSA i n v e s t i g a t i o n s a t t h e Linde D i v i s i o n of Union Carbide. The p a p e r i s mainly concerned w i t h a p r o c e s s f o r t h e p r o d u c t i o n of about 99% N2 from a i r a t a p r e s s u r e o f between 2 and 8 b a r u s i n g two 0 . 5 m3 r e a c t o r v e s s e l s . Operation i s i n t h e c o n v e n t i o n a l manner and v a r i o u s r e l a t i o n s h i p s between N2 p r o d u c t q u a n t i t y , a i r p r o d u c t q u a n t i t y and s p e c i f i c energy requirement w i t h f e e d 02 c o n t e n t and p r o c e s s p r e s s u r e a s p a r a m e t e r s a r e g i v e n . I n a s h o r t a r t i c l e by J . C . Davis ( 3 1 ) a summary i s g i v e n of t h e s o - c a l l e d Lindox r o u t e f o r waste-water t r e a t m e n t and a d i s c u s s i o n of ?-bed and 4-bed PSA systems. 3 . : HYDROGEN PRODUCTION AND OTHER GAS SEPARATIONS USING PSA 3 . 1 Hydrogen P r o d u c t i o n The p u r i f i c a t i o n of i n d u s t r i a l hydrogen c o n t a i n i n g g a s e s was h i s t o r i c a l l y t h e n e x t commercial a p p l i c a t i o n o f PSA t o f o l l o w t h e p u r i f i c a t i o n o f a i r f e e d t o c r y o g e n i c p l a n t s , and " h e a t l e s s d r y i n g " . The f i r s t s u c c e s s f u l PSA hydrogen u n i t was s t a r t e d up i n 1966; s i n c e t h a t t i m e o v e r 40 have been d e s i g n e d by Union Carbide a l o n e . Pure hydrogen i s i n growing demand on t h e i n d u s t r i a l market i n t h e f o o d , m e t a l l u r g i c a l , g l a s s , e l e c t r o n i c s and chemical i n d u s t r i e s . S i n c e t h e t r a d i t i o n a l p r o d u c t i o n methods by e l e c t r o l y s i s o r c r a c k i n g i n v o l v e some d i s a d v a n t a g e s i n t e r m s o f c o s t p r i c e and o u t p u t , t h e p u r i f i c a t i o n o f hydrogenated m i x t u r e s a v a i l a b l e a t a r e l a t i v e l y low p r i c e is becoming more i m p o r t a n t , e s p e c i a l l y s i n c e it i s n e a r l y always cheaper t o r e c o v e r a v a i l a b l e H2 t h a n t o manufacture i t . A l e x i s ( 3 2 ) p i o n e e r e d t h e f i r s t work on p u r i f y i n g low g r a d e hydrogen s t r e a m s which i n d i c a t e d t h a t t h e method c o u l d c o m p e t i t i v e l y produce 95% t o 98% H2 when compared t o t h e p a l l a d i u m d i f f u s i o n o r c r y o g e n i c p r o c e s s e s . A t y p i c a l H2 p l a n t w i t h a PSA p u r i f i c a t i o n system i s d i s c r i b e d by Raghuraman and Johansen ( 3 3 ) and f e a t u r e s t h e f o l l o w i n g p r o c e s s s t e p s : f e e d d e s u l p h u r i z a t i o n , steam r e f o r m i n g , h i g h t e m p e r a t u r e s h i f t c o n v e r s i o n , p u r i f i c a t i o n i n a PSA system. The first 3 s t e p s a r e t h e same a s f o r a c o n v e n t i o n a l p l a n t , whereas low t e m p e r a t u r e s h i f t , C02 removal, and methanation a r e r e p l a c e d by one p r o c e s s u n i t - t h e PSA system. I n such a system, hydrogen p u r i t i e s o f up t o 99.999 mole % a r e r e p o r t e d by Heck and Johansen ( 34) u s i n g a naphtha f e e d . A c t i v a t e d carbon and z e o l i t e m o l e c u l a r s i e v e s a r e commonly employed a s a d s o r b e n t s and hydrogen i s v e r y weakly adsorbed a t ambient t e m p e r a t u r e s , t h u s l e n d i n g i t s e l f p a r t i c u l a r l y w e l l t o p r o d u c t i o n a t h i g h p u r i t y by t h e PSA r o u t e . K a t i r a e t a l . ( 3 5 ) p a t e n t e d a system c o n t a i n i n g f o u r beds i n p a r a l l e l which h a s two i n t e r n a l r e c o v e r y s t e p s . The i n l e t s t r e a m c o n t a i n i n g 98 mole % H2 and 2 mole % methane was f e d t o t h e system a t p r e s s u r e s r a n g i n g from 150 t o 400 p . S . i . g . and produced 99.999
mole % hydrogen. A c t i v a t e d carbon was found t o be t h e most economic a d s o r b e n t f o r t h i s s e p a r a t i o n and economic o p t i m i z a t i o n performed showed t h a t t h e r a t i o o f t h e amount o f purge g a s t o amount o f f e e d g a s s h o u l d be k e p t a s low a s p o s s i b l e w i t h o u t d e c r e a s i n g t h e p r o d u c t p u r i t y . W. Wolf, working i n t h e Linde D i v i s i o n , p u b l i s h e d a r e v i e w of Union C a r b i d e ' s PSA hydrogen ( 3 6 ) p u r i f i c a t i o n p r o c e s s c o v e r i n g work done by S t e w a r t and ~eckC37h'his i s b a s i c a l l y t h e same system p a t e n t e d by K a t i r a e t al(38)Wolf p o i n t s o u t t h a t PSA compared w i t h t h e c o n v e n t i o n a l scheme p r o v i d e s : ( 1 ) 10% r e d u c t i o n i n f e e d s t o c k p l u s f u e l c o s t s ( 2 ) 5% t o 7% lower t o t a l p r o d u c t i o n c o s t s ( 3 ) higher r e l i a b i l i t y ( 4 ) l e s s maintenance ( 5 ) more f l e x i b i l i t y ( 6 ) higher product p u r i t y ( 7 ) lower c a p i t a l c b s t s f o r s m a l l H2 p r o d u c i n g u n i t s b u t s l i g h t l y higher c o s t s f o r l a r g e r plants. K a t i r a and S t e w a r t have reviewed t h e performance o f t h e Union C a r b i d e ' s u n i t s b o t h from a mechanical and p r o c e s s s t a n d p o i n t and t h i s shows PSA hydrogen systems t o be g e n e r a l l y proven i n commerc i a l p r a c t i c e w i t h t h e e x p e c t a t i o n o f f u r t h e r and wider u s e i n l a r g e r a p p l i c a t i o n s e.g. p i p e l i n e H2. D e t a i l s have been g i v e n by K. Knoblauch of Bergbau-Forschung and S. Dunlop o f Petrocarbon o f a PSA-based p r o c e s s d e s i g n e d t o r e c o v e r H2 from ammonia purge streams(39)This PSA u n i t was i n s t a l l e d a f t e r t h e ammonia a b s o r b e r which washes NH3-containing purge g a s from t h e s y n t h e s i s loop. Feed t o t h e PSA u n i t t h e r e f o r e c o n s i s t s o f ammonia, w a t e r , methane, argon and n i t r o g e n , which i s produced a s a p u r e p r o d u c t . In the z e o l i t e bed r e a c t o r s t h e optimum a d s o r p t i o n p r e s s u r e was found t o b e between 20 and 25 b a r w i t h a r e g e n e r a t i o n p r e s s u r e o f 3 b a r . The system h a s t h e a b i l i t y t o change t h e o p e r a t i n g c o n d i t i o n s , by removing more p r o d u c t , t o produce lower p u r i t y hydrogen f o r r e c y c l e r a t h e r t h a n 99.9% H2 f o r s a l e . Hence t h e p l a n t can a d a p t t o a f l u c t u a t i n g market. T h i s method of hydrogen r e c o v e r y from an ammonia p l a n t purge s t r e a m w i t h subsequent s a l e o r r e c y c l e back t o t h e s y n t h e s i s l o o p f o r c o n v e r s i o n t o more ammonia h a s been shown by P e t r o c a r b o n Developments ( 4 0 ) and I . C . I . t o be h i g h l y p r o f i t a b l e u s i n g a c r y o g e n i c r e c o v e r y p r o c e s s . The a p p l i c a t i o n of PSA t o t h i s o p e r a t i o n o r s i m i l a r l y t o methanol s y n t h e s i s p u r g i n g g a s i s of g r e a t i n t e r e s t a t t h e moment. The l a t t e r h a s been s t u d i e d by E l u a r d and Simonet ( 4 1 ) u s i n g an " A i r Liquide" m o l e c u l a r s i e v e "Alite". They showed t h e e f f i c i e n c y o f H2 e x t r a c t i o n t o r a n g e between 60% and 93% depending upon t h e t y p e of c y c l e and t h e g a s s t r e a m t r e a t e d . An a v e r a g e e f f i c i e n c y o f about 80% .was a c h i e v e d w i t h c o r r e s p o n d i n g h i g h d e g r e e s o f p u r i t y (above 99.5% i n e v e r y c a s e ) . I n t h e systems u s i n g vacuum d e s o r p t i o n t h e p u r i t i e s o b t a i n e d were t h e h i g h e s t , a t 99.99%+. The a u t h o r s commented upon t h e h i g h d e g r e e of s u c c e s s *hat L ' A i r L i q u i d e h a s a t t a i n e d u s i n g
PSA (1) (2) (3) (4)
i n t h e i r f o u r u n i t s which s e p a r a t e : 99.95% pure H2 from cracked ammonia 99.95% pure H2 from a mixture of 85% H 2 , 15% N2 99.95% pure helium from a mixture of He, 10% a i r e x t r a pure H2 from reforming gas.
3.2 Other Gaseous Mixture S e p a r a t i o n s Pressure swing a d s o r p t i o n a s a method of gas s e p a r a t i o n has not been confined t o t h e g a s mixtures air-:dy mentioned. I n 1974 Bird and G r a n v i l l e (42) published a paper on t h e s e p a r a t i o n of n i t r o g e n from helium using PSA which produced 98.0 t o 99.99 v o l . % helium using a c t i v a t e d carbon beds and a s h o r t c y c l e regime a t ambient temperature. This o p e r a t i o n was e f f e c t i v e i n i t s s e p a r a t i o n , but no economic c o n s i d e r a t i o n s have been. Other PSA s e p a r a t i o n s which have been l e s s widely i n v e s t i g a t e d include p a r a f f i n s from c y c l i c hydrocarbons, n i t r o g e n and methane, n-tetradecane from iso-octane/n-tetradecane mixtures, and hydrocarbon mixtures. 4. : THEORY The c e n t r a l d i f f i c u l t y of a q u a n t a t i v e d e s c r i p t i o n of PSA i s i t s discontinuous n a t u r e , and w h i l s t i n p r i n c i p l e t h i s should not a l t e r t h e fundamental e q u a t i o n s , it does mean t h a t s e p a r a t i o n s o l u t i o n s must be sought f o r each new s e t of i n i t i a l and boundary c o n d i t i o n s t h a t a r i s e from one p r o c e s s s t e p t o a n o t h e r . We s h a l l c o n s i d e r only two t h e o r e t i c a l approaches t o modelling PSA. I n t h e f i r s t a r e l a t i v e l y s i m p l i f i e d model i s adopted f o r t h e a d s o r p t i o n of a s i n g l e g a s i n an i n e r t c a r r i e r which emphasise t h e movement of zones i n p r e s s u r i s a t i o n and purging. Since one column i s i n p r a c t i c e purged with enriched gas from a n o t h e r column t h e r e i s an optimum purge: t o o much purge wastes enriched gas and t o o l i t t l e f a i l s t o r e g e n e r a t e t h e bed. I n t h e second a n a l y s i s d e t a i l e d ( e q u i l i b r i u m ) modelling of a s i n g l e bed i s given. This shows how t h e shape of t h e mass t r a n s f e r zone v a r i e s w i t h o p e r a t i n g parameters and i n p a r t i c u l a r g i v e s a q u a n t i t a t i v e d e s c r i p t i o n of t h e s e p a r a t i o n which occurs i n t h e p r e s s u r i s a t i o n s t e p 4 . 1 S i n g l e Adsorbed Gas
Shendalman and M i t c h e l l ( 4 3 ) worked on t h e i d e a l i s e d system of 1.09% C02 i n He on s i l i c a g e l where only t h e C02 a d s o r p t i o n need be considered. The two bed process involved t h e o r i g i n a l Skarstrom c y c l e of p r e s s u r i s a t i o n t o 59 p s i a , dynamic product r e l e a s e , d e p r e s s u r i s a t i o n t o 21 p s i a and purging from product counter c u r r e n t l y . This is shown i n Fig. ( 5 ) .
rl
PI
a,
a, Fc G 7 MU
-rl
co
xa, cn Fc a 4J
They made t h e following assumptions: 1 ) Instantaneous equilibrium with c o n s t a n t p a r t i t i o n c o e f f i c i e n t k . ( L i n e a r C02 isotherm with no co-adsorption o r h y s t e r i s i s . ) 2 ) Non-adsorbing c a r r i e r gas p r e s e n t i n l a r g e excess. 3 ) Isothermal o p e r a t i o n . 4 ) One-dimensional system i n p l u g flow with no a x i a l d i f f u s i o n . 5) N e g l i g i b l e s p a t i a l p r e s s u r e g r a d i e n t s . 6 ) I d e a l compressible gas behaviour. These assumptions r e s u l t i n a component mass balance of t h e form:
and an o v e r a l l c o n t i n u i t y equation:
Employing t h e e q u i l i b r i u m r e l a t i o n s h i p , N = K C , and assuming i d e a l gas behaviour r e s u l t i n t h e s i n g l e equation
which i s q u a s i - l i n e a r and by t h e method of c h a r a c t e r i s t i c s y i e l d s t h e following p a i r of e q u a t i o n s .
From t h e s e equations Shendalman and M i t c h e l l t r a c e t h e t r a j e c t o r i e s of c o n s t a n t composition i n t h e d i s t a n c e - t i m e p l a n e . During t h e c o n s t a n t p r e s s u r e s t e p s t h e s e t r a j e c t o r i e s have c o n s t a n t v e l o c i t y , whereas during r e p r e s s u r i s a t i o n and d e p r e s s u r i s a t i o n , s i n c e t h e p r e s s u r e drop a c r o s s t h e bed i s n e g l e c t e d , t h e equation ( 2 ) i m p l i e s l i n e a r v e l o c i t y g r a d i e n t s , equation ( 5 )
I t was t h e n observed t h a t because blowdown and r e p r e s s u r i s a t i o n occupy small f r a c t i o n s of t h e t o t a l c y c l e t i m e , and depend only on i n i t i a l and f i n a l s t a t e s t h e s e s t e p s can be considered
instantaneous. By t h e n c o n s i d e r i n g t h e r e l a t i v e p e n e t r a t i o n depths ( t h e d i s t a n c e a c o n c e n t r a t i o n f r o n t t r a v e l s during a process s t e p ) o f t h e product and purge s t e p s , t h e c r i t i c a l purge r a t e , necessary t o prevent feed breakthrough was c a l c u l a t e d . A d d i t i o n a l l y , performance p r e d i c t i o n s f o r t h e s t e a d y - s t a t e (performance a f t e r a l a r g e number of c y c l e s ) could be made a t any purge t o f e e d r a t i o . Their r e s u l t s f o r t h e comparison of t h e o r y and experiment were d i s a p p o i n t i n g ; t h e theory c o n s i s t e n t l y p r e d i c t e d 30% lower c o n c e n t r a t i o n s i n t h e product t h a n shown by experiments. F u r t h e r , t h e p r e d i c t e d purge t o f e e d r a t i o s were not confirmed. E r r o r s were p r i m a r i l y a t t r i b u t e d t o t h e l i n e a r isotherm assumption, although r a t e and d i s p e r s i o n processes were a l s o acknowledged a s being important. Chan e t a 1 ( 45 ) extended t h i s a n a l y s i s by c o n s i d e r i n g t h e blowdown and r e p r e s s u r i s a t i o n s t e p s ; following on from Shendalman and M i t c h e l l ' s a n a l y s i s t h e y d e f i n e a parameter,B, such t h a t ,
where B r e p r e s e n t s t h e r a t i o of t h e s u p e r f i c i a l t o c o n c e n t r a t i o n f r o n t v e l o c i t y . A s a r e s u l t t h e c h a r a c t e r i s t i c equations become :
The p e n e t r a t i o n d i s t a n c e s of t h e f r o n t s d u r i n g t h e c o n s t a n t p r e s s u r e came d i r e c t l y from equation ( 7 )
where vh and vl a r e c o n s t a n t l i n e a r v e l o c i t i e s and t h e A t , h a l f c y c l e tlmes. For t h e s t e p s where p r e s s u r e i s changing, t h e change i n composition and p o s i t i o n of t h e c h a r a c t e r i s t i c s a r e given by
Shendalman's a n a l y s i s can now be r e v i s e d , t h e n e t displacements of f r o n t s i n t h e high and low p r e s s u r e beds being given by
The c r i t i c a l purge t o f e e d r a t i o , y , being when AL1 whence,
=
AL
h = 0,
and t h e r e i s no n e t movement of t h e c o n c e n t r a t i o n f r o n t from one c y c l e t o t h e n e x t . When L1/Lh > Y and L1
N*
Pr YN
=
0
Chan (45)analysed t h e s e e x p r e s s i o n s i n some d e t a i l and compared them w i t h t h e Shendalman and M i t c h e l l ' s experimental r e s u l t s . The conclusions a r e t h a t by t r e a t i n g p r e s s u r i s a t i o n and blowdown i n t h e above manner an e x a c t mass balance i s o b t a i n e d , and t h a t gas composition f o r t h e s e two s t e p s , both i n t h e columns and
i n t h a t r e l e a s e d , a r e f u n c t i o n s o f t h e p o s i t i o n of t h e c h a r a c t e r i s t i c s w i t h i n t h e column. 4.2.
Binary Mixture
Flores-Fernancdez (1978) a p p l i e d t h e g e n e r a l method o u t l i n e d above t o model an a i r s e p a r a t i o n process. A s i n g l e bed was considered and a s a r e s u l t t h e t h e o r y was not extended t o c o n s i d e r p e n e t r a t i o n depths. Equations were d e r i v e d i n which t h e oxygen isotherm was considered l i n e a r and t h e n i t r o g e n was given by t h e Langmuir equation. S u b s c r i p t A r e f e r s t o oxygen and B t o nitrogen. The a n a l y s i s r e c o g n i s e s t h a t s u b s t a n t i a l q u a n t i t i e s of both g a s e s a r e absorbed, u n l i k e s i m p l i f i e d t r e a t m e n t s of chromatography where c o n s t a n t mass flows a r e assumed;
Equation (17) i s i d e n t i c a l with t h e l i n e a r isotherm when K B Like t h e previous a n a l y s i s two mass balance equations a r i s e f o r t h e f l u i d and s o l i d phases.
=
0.
Consider experimental r e s u l t s f o r p r e s s u r i s a t i o n , shown i n Fig. ( 9 ) i n which Y , t h e oxygen gas c o n c e n t r a t i o n i s given f o r a bed c l o s e d a t t i e upper ( e x i t ) end p l o t t e d a g a i n s t p o s i t i o n i n t h e bed, Z = Z / L ; t h e f i n a l p r e s s u r e i n s i d e t h e column ( P 2 ) a s t h e parameter. The s o l i d l i n e s r e p r e s e n t t h e b e s t curves drawn through t h e experimental d a t a ; t h e broken l i n e s g i v e t h e c a l c u l a t e d behaviour. I t can be seen t h a t a s t h e p r e s s u r e i n s i d e t h e column i s i n c r e a s e d t h e g a s phase i s enriched i n oxygen; t h e oxygen enrichment i n t h e upper p a r t of t h e bed, where a p l a t e a u is formed, i s g r e a t e r t h a n i n t h e lower p a r t of t h e bed. The p l a t e a u p r o g r e s s i v e l y moves upwards and i s f u r t h e r enriched i n oxygen by i n c r e a s i n g t h e p r e s s u r e i n t h e system. The r e g i o n b e f o r e t h e p l a t e a u i n d i c a t e s t h e p e n e t r a t i o n r o f t h e feed g a s ; t h e p l a t e a u i t s e l f r e p r e s e n t s t h e i n i t i a l g a s i n t h e bed which has been f o r c e d upwards by t h e f e e d g a s and has been enriched i n oxygen. The c o n c e n t r a t i o n i n t h e lower p a r t of t h e bed i s s l i g h t l y dependent on t h e feed p e n e t r a t i o n depth ( i . e . , f i n a l p r e s s u r e i n t h e bed). Also t h e c o n c e n t r a t i o n p r o f i l e s a r e b a s i c a l l y independent of t h e p r e s s u r i s a t i o n time. This behaviour can be understood using a l i n e a r equilibrium model f o r which t h e a d s o r p t i o n isotherm assumes t h e e q u i l i b r i u m
- I -
I
1
I
I
I
I
I
I
-
-
k
L
5.7
r
7
-
-
7
Feed penetration
Fig. 9 : Experimental concentration profiles for t h e air pressurisation runs.
NX
NX = f ( c ) = K. ( p . ) with t h e e q u i l i b r i u m number of moles of a d s o r b a t e ( 0 o$ N') p e r u n i t mass of s o l i d ; P . i s t h e p a r t i a l 2 2 1 p r e s s u r e of t h e s p e c i e s i i n t h e g a s phase and K . i s a Henry type a d s o r p t i o n c o n s t a n t . I n t h i s work, a t 2 9 ~ ~ ~ : K (0 ) A 2
kmol/(kg a d s o r b e n t ) N/m 2
= 0.129 x
and KB(N2)= 0.301 x
lo-*
2 kmol/(kg a d s o r b e n t ) (N/m 1.
The r a t e s of a d s o r p t i o n of oxygen ( r ) and n i t r o g e n ( r ) A B a r e given by t h e e q u i l i b r i u m r e l a t i o n s expressed by Equations (18) and (19):
( 2 0 ) i s a t o t a l m a t e r i a l balance on a s e c t i o n of bed of t h i c k n e s s AZ, when t h e t o t a l p r e s s u r e i n t h e bed v a r i e s a s a f u n c t i o n of time. F o r component ( A )
with
E
t h e f r a c t i o n a l void space i n t h e packed bed
By s u b s t i t u t i n g Equation (18 ) i n t o Equations (20 ) and ( 2 1 ) t h e following e x p r e s s i o n s r e s u l t
and
where al
EA/RT
+
w KA ; a 2
= EA/RT + w Kg
; a3
= w(KA - Kg).
The oxygen bed c a p a c i t y c o e f f i c i e n t s , a and a 2 , r e p r e s e n t t h e maximum oxygen c a p a c i t i e s r e s p e c t i v e l y o& t h e bed i n t h e gas phase and on t h e s o l i d phase p e r u n i t bed l e n g t h p e r u n i t p r e s s u r e . The boundary c o n d i t i o n s a r e :q(t, Z y,(t y,(t,
L)
= 0.0
= 0 , Z) = y;; Z
P = p(t)
= 0 ) = y Af
(no flow a t t h e c l o s e d end) (uniform i n i t i a l c o n c e n t r a t i o n ) (24) ( c o n s t a n t feed g a s composition) ( t o t a l p r e s s u r e a s a f u n c t i o n of time)
Equations ( 2 2 ) and ( 2 3 ) may be combined t o e l i m i n a t e t h e p a r t i a l derivative (aPA/at); thus
Since t h e c o e f f i c i e n t s ( a a and a 3 ) a r e c o n s t a n t , and t h e t o t a l p r e s s u r e d e r i v a t i v e t e ~ / a t )is only a f u n c t i o n of time ( i . e. , no p r e s s u r e drop along t h e bed) ; Equation ( 2 5 ) can be i n t e g r a t e d i n t h e Z d i r e c t i o n , from any p o s i t i o n i n s i d e t h e bed, Z = Z ' , t o t h e c l o s e d end, Z = L , t o g i v e
where a i s conveniently defined a s a
= a2/al and (1-a) = a3/al.
The above equation s a t i s f i e s t h e flow boundary c o n d i t i o n f o r a closed end a s expressed by Equation ( 24 ). Therefore t h e
independent time f u n c t i o n , f ( t ) , which i s obtained from t h e i n t e g r a t i o n of t h e p a r t i a l d i f f e r e n t i a l e q u a t i o n , Equation (25 1, i s not included because it is e q u a l t o z e r o . Maximum flow i s a t t a i n e d a t t h e bed e n t r a n c e ; where Z = 0 and y = yAf. A The m a t e r i a l balance on A , Equation (23 1, can be r e w r i t t e n as
where t h e c o e f f i c i e n t s a r e r e l a t e d t o t h e parameter 'a' by
a = a 2/a 1 and ( 1 - a ) = a3/al. Equation ( 2 6 ) is a q u a s i l i n e a r p a r t i a l d i f f e r e n t i a l equation of t h e f i r s t o r d e r and can be solved e i t h e r by t h e method of c h a r a c t e r i s t i c s o r by an e x p l i c i t method of f i n i t e d i f f e r e n c e approximations. The c h a r a c t e r i s t i c e q u a t i o n s a r e : -
where q is given by Equation (25 ) and a i s t h e bed f r a c t i o n a t i o n f a c t o r ( a = a / a ). Note t h a t t h e c h a r a c t e r i s t i c s a r e independent o? t k e p a t t e r n s e l e c t e d t o p r e s s u r i s e t h e column. Equation ( 27) and (28 ) f o r t h e l i n e a r e q u i l i b r i u m a d s o r p t i o n of a b i n a r y g a s mixture a r e s i m i l a r t o t h e equations presented by Shendalman and M i t c h e l l (1972) f o r t h e l i n e a r e q u i l i b r i u m a d s o r p t i o n of a s i n g l e component from a d i l u t e s o l u t i o n , but h e r e t h e s u p e r f i c i a l v e l o c i t y g r a d i e n t i n t h e bed i s non-linear and i s a f u n c t i o n of t h e c o n c e n t r a t i o n i n t h e g a s phase. Equation ( 28) can be i n t e g r a t e d from yA1 ( t h e c o n c e n t r a t i o n i n t h e bed a t 2 = 2 and P = P ) t o YA2 ( t h e concentration -1 i n t h e p o s i t i o n Z=Z a t P = p2f when t h e p r e s s u r e changes from P t o P2 ( i . e . , t h e time v a r i e s from tl t o t 2 ) ; where
z
= z/L.
The r e s u l t i s :
Equation ( 27) can be combined with Equation ( 2 8 ) t o o b t a i n a r e l a t i o n between y and 2. The r e s u l r i n g o r d i n a r y A d i f f e r e n t i a l equation can be i n t e g r a t e d from Z1 t o Z 2 , and from yAl t o yA2. The r e s u l t i s given by
where t h e d i s t a n c e Z i s measured from t h e bed i n l e t . For t h e s p e c i a l cases 'y = 0' and'y l ' , t h e c o n c e n t r a t i o n remains A1 A1 constant and t h e characteristics a r e d i s p l a c e d according t o t h e relations
and
(1
- z21
/ (1 -
= (P1/P2) l / a
,
fory
A1
= 1
(32)
which a r e obtained by d i r e c t i n t e g r a t i o n of Equation ( 2 7 ) . I f t h e c h a r a c t e r i s t i c s c r o s s , t h e n t h e r e i s t h e formation of a shock wave. I t can be shown from t h e m a t e r i a l balances a c r o s s t h e shock wave t h a t t h e v e l o c i t y o f t r a n s i t i o n of t h e shock i s given by
where yAa and yAP a r e t h e c o n c e n t r a t i o n s ahead and b e f o r e t h e shock wave. The low r a t e s ahead and b e f o r e t h e shock wave a r e r e l a t e d by:
FEED
68 0
PURGE
CHARACTER ISTIC 'LINES
Characteristic lines - single gas
Fig. 6 :
I
+feed
-
r--r1 - 3. -/
feed ptnelroiion
, I
- 2.2 -. 2
/-------
1
1
1
1
1
1
1
1
1
, , , , , , * y~~ 3
L
...................._.. I
I
I
1
I
,
,
I
I
0
5: 1 P L Characteric lines and concentration profiles during pressurisation. Uniform displacement of initial gas : yg = ygf = 0.21
Fig. 7
:
.
The i n t e g r a t i o n of Equation (33 ) can be performed numerically with t h e a i d of Equations ( 29) t o ( 32). Note t h a t t h r e e c a s e s may a r i s e i n t h e s o l u t i o n of t h e above e q u a t i o n s ; f o r t h e p r e s s u r i s a t i o n s t e p t h e s e depend on t h e r e l a t i v e v a l u e of t h e i n i t i a l c o n c e n t r a t i o n i n t h e bed r e l a t i v e t o t h e feed concentration ) , a s d e p i c t e d i n $igure 7 The c h a r a c t e r i s t i c s a r e uniformly diseofrted by p r e s s u r e i n c r e a s e when t h e i n i t i a l and f e e d c o n c e n t r a t i o n s a r e e q u a l ; f i g u r e where - 0.21 and a = 2715. The c h a r a c t e r i s t i c s c r o s s A :Y A ~ and t h e d l s c o n t l n u i t y o r i g i n a t e d a t = 0 and t = 0 ( o r P-PO) 0 i s propagated a s a shock wave when yA > yAf; f i g u r e 8 where = 0.5, yAf = 0 . 2 1 and a = 2.15. ~ % characteristics e emerging from t h e o r i g i n d i v e r g e and t h e 0 d i s c o n t i n u i t y is gropagated a s a simple wave when y A f i g u r e 8 where y = 0.1, y = 0.21 and a = 2.15. TheYAf; c o n c e n t r a t i o n pro*iles a r e i f s o shown i n f i g u r e 8 .
.
:
'
An 'approximate1 s o l u t i o n t o t h e e q u a t i o n s ( 25) and ( 26) can be more simply obtained numerically by c o n s i d e r i n g t h e bed a s made up of a s e r i e s ( n ) of w e l l mixed c e l l s . The p a r t i a l d i f f e r e n t i a l equation (26 ) l e a d s t o a s e t of o r d i n a r y d i f f e r e n t i a l equations s o l u b l e by Runge-Kutta methods. A refinement i s t o allow f o r t h e n o n - l i n e a r i t y of t h e isotherm f o r n i t r o g e n w r i t i n g t h e isotherm
-8
with KB = 0 . 3 0 1 ~1 0 Kb
2 kgmole/(kg ad.)(N/m ) and
= 7.5 x 10- 7 (N/m2)-I.
The problem now becomes more complicated s i n c e t h e c o e f f i c i e n t s a 2 and a 3 of ( 22) now become a2L = a 1 - a 3L with a 3L given by
Fig. 8 :
Characteristic lines and concentration Profiles during pressurisation.
and t h e modified equation (24 )
i s coupled t o ( 2 3 ) with non-linear c o e f f i c i e n t s a2L and a
3L'
Corresponding c h a r a c t e r i s t i c e q u a t i o n s can be derived, although t h e d e t a i l s a r e not given h e r e . 4 . 2 . 2 P r e s s u r e c y c l i n g . Consider now a s i n g l e bed i n which t h e t h r e e s t e p s of p r e s s u r i s a t i o n , product r e l e a s e and d e p r e s s u r i s a t i o n occur, each f o r a time A. The dimensionless c y c l i n g t i m e , O c , is d e f i n e d by
where A i s t h e s t e p time and n i s t h e number of c y c l e s ; d i s C C t h e r e f o r e t i e range 0 t o 3. I n t h e t h ~ e e s t e pprocess examined t h e e q u a t i o n s and boundary conditions a r e a ) For P r e s s u r i s a t i o n ; 0 < Oc < 1 and equation ( 24) holds
but f o r t h e first c y c l e y (Z) = 0.21 A1
where t h e p o s i t i v e s i g n ( + ) i n t h e l a s t equation i n d i c a t e s t h a t t h e p r e s s u r e i n t h e system i n c r e a s e s with time.
b ) Product Release ( s t e p no. 2 ) , t h e p r e s s u r e i n t h e system i s considered c o n s t a n t , P = Ph, and t h e above e q u a t i o n s reduce t o
and
y A ( e C = 1, Z) yA(eC, z
= 0) =
q ( e c , Z = L) P
= Y ~ (Z) 2 YAf
= 0.21
= qP = c o n s t a n t
= Ph = c o n s t a n t
c ) Depressurisation
0.35
0.25
0.15 0
1
2 3 c Experimental concentration profiles @
Fig.10 :
Fig.11:
Predicted concentration profiles for pressurisation, product release and depressurisation. Numbers 1-5 indicate sample points
where t h e n e g a t i v e s i g n ( - ) i n Equation ( 4 2 ) i n d i c a t e s t h a t t h e p r e s s u r e i n t h e system d e c r e a s e s with time. The s o l u t i o n of t h e above equations has again been obtained numerically by using a f i r s t o r d e r d i f f e r e n c e formulae t o r e p r e s e n t t h e p a r t i a l derivatives. In contrast t o the pressurisation study t h e p r e s e n t system i s not r e a l l y amenable t o s o l u t i o n by t h e method of c h a r a c t e r i s t i c s because of t h e c o n c e n t r a t i o n changes from c y c l e t o c y c l e b e f o r e t h e ' s t e a d y - s t a t e ' i s a t t a i n e d . The c h a r a c t e r i s t i c l i n e s during t h e p r e s s u r e changes a r e not s t r a i g h t l i n e s and t h e r e f o r e t h e t a s k involved i n t r a c k i n g t h e shock and simple waves becomes cumbersome o r i m p r a c t i c a l a f t e r o b t a i n i n g t h e s o l u t i o n f o r t h e f i r s t s t e p of t h e f i r s t c y c l e . Equations ( 2 4 ) t o (30) were programmed on CSMP3 (Continuous System Modelling Program) following t h e g u i d e l i n e s described above and were solved on a 370 IBM computer. From f i g s . l O , l l i t i s evident t h a t t h e non-linear model provides an a c c u r a t e r e p r e s e n t a t i o n of t h e experimental oxygen c o n t e n t of t h e product i n t h e c a s e of t h e l o n g e s t c y c l i n g time (A = 42.25) and t h e f i g u r e shows t h a t t h e d e t a i l e d c o n c e n t r a t i o n i n t h e column can a l s o be modelled e f f e c t i v e l y . I n summary, it appears t h a t t h e nitrogen-oxygen s e p a r a t i o n p r o c e s s can probably be adequately r e p r e s e n t e d by an 'approximate' s o l u t i o n of an e q u i l i b r i u m model t a k i n g i n t o account t h e nonl i n e a r i t y of t h e n i t r o g e n a d s o r p t i o n isotherm. This a n a l y s i s p o i n t s t h e way t o allowing f o r non-equilibrium e f f e c t s and modelling a multibed process. 4.3 Other Modelling Methods L i m i t a t i o n s of space make it impossible t o g i v e an account of o t h e r methods of s o l u t i o n . They have been summarised i n Table 2. 5 . : THE ECONOMICS OF PSA
There a r e many d i f f e r e n t t y p e s of cryogenic process and, a l s o s e v e r a l d i f f e r e n t t y p e s of molecular s i e v e PSA schemes. The most economical p r o c e s s t o o b t a i n a given product must be s e l e c t e d c o n s i d e r i n g f a c t o r s such a s : Q u a n t i t y of products Consumption of energy Investment c o s t s P u r i t y of p r o d u c t s S t a t e of p r o d u c t s Number of products F l e x i b i l i t y of t h e p l a n t Cost of maintenance Admissable dimensions of t h e p l a n t . The consumption of energy f o r t h e production of oxy en, n i t r o g e n and argon i s of g r e a t i n d u s t r i a l i n t e r e s t compares with cryogenic
.
methods t h e i n c r e a s e of energy consumption a t d e c r e a s i n g p l a n t s i z e (< 3,000 nm3/hour) i s mainly due t o . t h e i n c r e a s i n g cold l o s s e s . The decrease i n energy a t i n c r e a s i n g p l a n t s i z e ( > 3,000 nm3/hour) i s mainly due t o t h e i n c r e a s i n g isothermal e f f i c i e n c y of t h e a i r compressors. I n g e n e r a l t h e energy consumption f o r producing oxygen by t h e adsorption p r o c e s s e i t h e r with z e o l i t e molecular s i e v e s o r c o a l molecular s i e v e s i s r e l a t i v e l y high and it seems t h a t it cannot be decreased by i n c r e a s i n g t h e s i z e o f t h e a d s o r p t i o n p l a n t s and it i s apparent t h a t it i s b e t t e r s u i t e d t o small-volume u s e r s . This works t o t h e advantage of PSA s i n c e cryogenic p l a n t s do not s c a l e down w e l l below 30 tons/day. Usually though p l a n t s with a lower s p e c i f i c power consumption (cryogenic and carbon s i e v e ) w i l l be more expensive t o b u i l d . The economics of p u r i f y i n g hydrogen by PSA a r e determined by t h e c o s t of t h e equipment and t h e c o s t of t h e f e e d , s i n c e no v i r t u a l l y u t i l i t i e s a r e r e q u i r e d . The equipment c o s t i s a f u n c t i o n o f t h e throughput, type and q u a l i t y of t h e i m p u r i t i e s t o be removed and adsorbent p r o p e r t i e s . Feed c o s t s may be from zero f o r a gas being f l a r e d but a p p r e c i a b l e i f t h e f e e d has f u e l value o r an a l t e r n a t i v e use. Nontheless, t h e PSA H p r o c e s s i s a simple p l a n t 2 with no r o t a r y equipment, l e a d i n g t o low c a p i t a l expenditure and more r e l i a b l e p l a n t o p e r a t i o n . PSA has advantages i n t h a t it can handle broad ranges of i m p u r i t i e s , low H content i n t h e f e e d gas and i s capable of producing a higher2% p u r i t y H t h a n cryogenics. Bergbau-Forschung and Petrocarbon s e e a major advan?age of t h e PSA u n i t s over cryogenic systems i n t h e f a c t t h a t c a p i t a l c o s t s f a l l s t e a d i l y a s t h e y a r e s c a l e d down, because of t h i s economic advantage of s c a l e PSA purge stream p u r i f i c a t i o n u n i t s a r e of p a r t i c u l a r i n t e r e s t t o o p e r a t o r s o f p l a n t s producing a s m a l l s c a l e purge stream. A summary of t h e e x i s t i n g a p p l i c a t i o n s of PSA and cryogenic systems was included i n Wolf's paper i n 1976. This i n d i c a t e s main a p p l i c a t i o n s of PSA s t i l l l i e w i t h a i r s e p a r a t i o n and t h e p u r i f i c a t i o n of hydrogen c o n t a i n i n g streams. F u r t h e r improvements and a p p l i c a t i o n s a r e f o r e s e e a b l e i n t h e f u t u r e of PSA and w i l l be due t o t h e use of b e t t e r adsorbents and newer c y c l e s . B e t t e r c o n t r o l components could l e a d t o f u r t h e r improvements. The r e s u l t w i l l be t h a t t h e economics of PSA w i l l become more a t t r a c t i v e and more i n d u s t r i a l gases w i l l be s e p a r a b l e by PSA due t o c a p i t a l c o s t r e d u c t i o n and h i g h e r product y i e l d .
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38,
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J u n t g e n , H . , Knoblauch, K . , S c h r o t e r , H . J . , B e r i c h t e d e r B u n s e n - G e s e l l s c h a f t f u r p h y s i k a l i s c h e Chemie, 79 ( 1 9 7 5 ) 9 , 8240826.
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J u n t g e n , H . , K n o b l a u c h , K . , Munzner, H . , S c h i l l i n g , H . D . , 5 t h 1 n t . C o n f . on Magnetohydrodynamic E l e c t r i c a l Power G e n e r a t i o n , Munich, 1 9 - 2 3 A p r i l 1971.'
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17.
W a l k e r , P.L., Lamond, T.G., M e t c a l f , J . E . , P r o c . 2 n d I n t . Carbon a n d G r a p h i t e C o n f e r e n c e ( 1 9 6 6 ) .
18.
Nandi, S.P.,
W a l k e r , P.L.,
Fuel,
19.
Nandi, S.P.,
W a l k e r , P.L.,
Separ.Sci.,
20.
Lee, H.,
21.
S m i t h , K . C . , Armond, J . W . , 1 0 1 (1974).
Cryotech ' 7 3 Proc.
22.
S i r c a r , S.,
Z o n d l o , J.W.,
US P a t e n t 4 , 0 1 3 , 4 2 9
23.
B e r l i n , N.H.,
24.
Marsh, W.D.,
221 (1916)
Proc.Roy.Soc.(London)
, Chem. Eng. Techn , 75
A167,
392 ( 1 9 3 8 ) .
(1954).
US P a t e n t 2 , 9 4 u , 6 2 7
(1960).
Nature, Phys.Sci. 232 ( 2 9 ) :
AIChEJ 22 ( 4 ) :
70 (1971).
Holborow, K . A . ,
Chem.Eng.Sci.
753 ( 1 9 7 6 ) . Canad.J.Chem.
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Chem-Eng. (NY) 85 ( 2 5 ) :
Trans. I ,
87 ( 1 9 7 8 ) .
L i n d e AG Werksgruppe TVT Munchen, O c t . ( 1 9 7 5 ) .
S t a h l , D.E.,
54
AIChE Symp.Ser.
(1975). 11: -
69(134): -
US P a t e n t 3 , 2 8 0 , 5 3 6 ( 1 9 6 9 ) . US P a t e n t 3 , 1 4 2 , 5 4 7
441 (1976).
(1964).
1 (1973). (pub. 1974): (1979).
H e i n z e , G . , Mengel, M . , R e i s s , G . , 2,329,210 ( 1 9 7 6 ) .
German P a t e n t
Bendix C o r p o r a t i o n , UK P a t e n t 1 , 4 6 7 , 2 8 8 ( 1 9 7 7 ) . B a t t a , L.B.,
US P a t e n t 3,717,974
Armond, J . W . ,
US P a t e n t 4,065,272
S m i t h , K.C. a n d Armond, J . W . , (Aberdeen) , (1977). Reyhing, J. David, J . C . ,
(1973).
, Linde
(1976).
UK Chem.Soc.
Aktiengesellschaft
Chem.Engng,
79,
88-89,
Autumn M e e t i n g
, October
(1975 ) Munchen.
October, (1972).
A l e x i s , R.W., Chemical E n g i n e e r i n g P r o g r e s s , 6 3 , ( 5 ) , 69-71, (1967). Raghuraman, K.S. a n d J o h a n s e n , T., October, (1978).
P r o c e s s i n g , 10-11,
Heck, J . L . a n d J o h a n s e n , T . , Hydrocarbon P r o c e s s i n g , 5 7 , (1) 175-7, J a n u a r y , ( 1 9 7 8 ) . K a t i r a , C . H . , D o s h i , K . J . a n d S t e w a r t , H . A . , P a p e r 38a p r e s e n t e d a t 6 8 t h N a t i o n a l M e e t i n g o f t h e AIChE, Houston ( 1 9 7 1 ) . Wolf, W . ,
The O i l a n d Gas J o u r n a l , 7 4 , ( 8 1 , 88 ( 1 9 7 6 ) .
S t e w a r t , H.A. a n d Heck, J . L . , 78, September, (1969). K a t i r a , C.H. 78-84,
and S t e w a r t , H.A., (1974).
Chemical E n g i n e e r i n g P r o g r e s s
Cryotech 73 Proceedings,
P e t r o c a r b o n Developments s e m i n a r , London, J u n e 1 9 7 9 , i n The Chemical E n g i n e e r , 395 ( 1 9 7 9 ) . Banks, R.
, Chemical
Engineering, October 1 0 , 1977.
E l u a r d , R. a n d S i m o n e t , G . , Chimie e t I n d u s t r i e - Genie 15, (1970). Chimique,
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B i r d , G . a n d G r a n c i l l e , W . H . , Advances i n C r y o g e n i c Engineering, 1 9 , 463-73, ( 1 9 7 4 ) . Shendalman, L.H. (1972 ) .
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27,
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F l o r e s - F e r n a n d e z , G . , Ph. D. d i s s e r t a t i o n , Cambridge U n i v e r s i t y , (1978). Chan, Y . N . I . , H i l l , F.B. a n d Wong, Y.W., R e p o r t BNL-25398, Brookhaven N a t i o n a l L a b o r a t o r y , Upton, N Y , USA ( 1 9 7 8 ) . S z o l c s a n y i P., Horvath, G . , K o t s i s , L., Szanya, T . , Proc. Chem.Equip.Des.Automn, S - J , 5 t h Congr.CHISA, P r a g u e , Czechoslovakia (1975).
47.
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-
and K a d l e c , R.H., AIChE J o u r n a l , 1 7 , 335 (1971). and K a d l e c , R . H . ,
AIChE J o u r n a l , 1 8 , 1208 ( 1 9 7 2 ) . AIChE S y m p . S e r i e s , 69 ( 1 3 4 ) :
-
NOMENCLATURE a
1
a a
oxygen bed c a p a c i t y c o e f f i c i e n t , eqn. ( 2 3 ) :
kmol/(m)(~/rn').
n i t r o g e n bed c a p a c i t y c o e f f i c i e n t , eqn. (23) : 3
c
= al - a 2 = w(KA - Kg).
coefficient defined a s a
c o n c e n t r a t i o n i n t h e g a s phase :
kmol/m3. kmol/( k g a d s o r b e n t ) (N/m2 1.
K ,K a d s o r p t i o n e q u i l i b r i u m c o n s t a n t s : A B L b e d l e n g t h : m. AL n
n e t f r o n t displacement:
kmol/(m) (N/m2).
m.
number o f c e l l s . number o f c y c l e s i n p r e s s u r e s w i n g p r o c e s s . m o l a r d e n s i t y o f a d s o r b e d p h a s e p e r u n i t volume o f b e d : kmol m-3 moles o f g a s a d s o r b e d p e r u n i t mass s o l i d :
N/m2
t o t a l pressure:
o r bar.
p a r t i a l p r e s s u r e o f component ' L ' : molar flow r a t e s :
kmol/kg a d s o r b e n t
N/m2.
kmol/s.
r a t e of adsorption :
kmol/( k g a d s o r b e n t ) ( s )
R
gas l a w constant.
t
time:
T
temperature :
v
i n t e r s t i t i a l o r front velocity:
w
weight of adsorbent p e r u n i t l e n g t h of bed:
Y Z Z
mole f r a c t i o n i n g a s p h a s e .
.
s. 0
K.
a x i a l p o s i t i o n from i n l e t :
ms
.
-1
m.
d i m e n s i o n l e s s p o s i t i o n i n bed ( Z / L ) .
kg/m.
bed f r a c t i o n a t i o n f a c t o r ( a / a 1. 2 1 r a t i o of i n t e r s t i t i a l velocity t o f r o n t velocity. e x t e r n a l p o r o s i t y o f bed. d i m e n s i o n l e s s c y c l i n g time. duration of s t e p i n cycle:
s.
c o n d i t i o n s ahead and b e f o r e shock wave. oxygen :
B nitrogen.
p e r t a i n s t o feed. high pressure in cycle. i n i t i a l condition.
TABLE 1 COMPANY
BEDS
OPERATING PRESSURE: RANGE
CYCLE FEATURES
4
Superambient
P r e s s u r i s e - Dynamic p r o d u c t r e l e a s e BPE - P r o v i d e p u r g e - c o m p l e t e blowdown - r e c e i v e p u r g e - BPE - e t c .
>2
Superambient
S i m u l t a n e o u s p r e s s u r i s a t i o n from f e e d a n d p r o d u c t e n d s , non-dynamic p r o d u c t release.
L'air Liquide
1 to 10
From Superto Subambient
1) Each b e d h a s a t h i r d p i p e c o n n e c t e d h a l f w a y up b e d 2 ) Non-dynamic p r o d u c t r e l e a s e t h r o u g h a second bed a t lower p r e s s u r e 3 ) Vacuum a p p l i e d a t mid-point o f bed 4 ) A i r feed drying s e c t i o n of s i l i c a g e l , t o product z e o l i t e .
BayerMahler
3
From Superto Subambient
1 ) Used s p e c i a l 4A s i e v e s ( c a 2 + and sr2+exchanged) 2 ) Each bed h a s d r y i n g s e c t i o n 3 ) Vacuum r e g e n e r a t i o n i n c l u d i n g drying
Union Carbide
.
Nippon Steel
3
From Superto Subambient
1 ) C y c l e s s i m i l a r t o Bayer-Mahler above 2 ) N a t u r a l z e o l i t e used: Calcareous Tufa.
W.R.Grace & Co.
2 t 1 tank
Superambient and Supersubambient
Tuned up E s s o P r o c e s s 1 ) Dynamic p r o d u c t r e l e a s e 2 ) Bed p r e s s u r e e q u a l i s a t i o n 3 ) Vent t o t a n k 4 ) Blowdown t o a t m o s p h e r e o r vacuum pumped.
BOC Techsep
2
Subambient
1 ) Composite b e d s : s i l i c a g e l d r y i n g , 5A z e o l i t e 2 ) Dynamic p r o d u c t r e l e a s e - f e e d s u c k e d t h r o u g h by p r o d u c t compressor 3 ) Vacuum r e g e n e r a t i o n , r e p r e s s u r i s a t i o n from p r o d u c t .
-
TABLE 1 c o n t i n u e d
COMPANY
BEDS
BOC Techsep
3
OPERATING PRESSURE RANGE
CYCLE FEATURES
Subambient
A s above, w i t h 4 ) Breakthrough from p r o d u c t r e l e a s e f e d t o r e g e n e r a t e d bed and r e l e a s e d a s second-out p r o d u c t .
Air 2 + SuperProducts 2 suband t a n k s ambient Chemicals
Produces b o t h oxygen and n i t r o g e n Offspec. 02 p r o d u c t and N2 p r o d u c t c o l l e c t e d s e p a r a t e l y , and used t o c r o s s purge beds i n a 4 - p a r t c y c l e . S e p a r a t e d r y e r s r e q u i r e d u s i n g 13x zeolite. 4 pumps used, f o r recomp r e s s i o n o f purge g a s e s .
TABLE 2
Summary o f Numerical S o l u t i o n s
AUTHOR
TYPE OF SOLUTION AND ASSUMPTIONS
Szolcsanyi
E x p l i c i t f i n i t e d i f f e r e n c e scheme. 1 ) One dimensional flow - p l u g flow no r a d i a l dispersion 2 ) No a x i a l d i f f u s i o n 3 ) Hydrodynamics dominated by v a l v e formulae a t i n l e t and o u t l e t , and Darcy's law w i t h i n t h e bed 4 ) E q u i l i b r i u m a d s o r p t i o n i s maintained 5 ) Linear isotherms 6 ) I d e a l gas b e h a v i o u r 7) Isothermal operation 8 ) Simple i n i t i a l and boundary c o n d i t i o n s .
e t a2 (1975) (46)
Sebasti a n (1978) (47
Garg & Ruthven ( 1 9 7 3 ) & (1974) (48)
Kowler & Kadlec (1972) and Turnock & Kadlec (1971) (491, ( 5 0 ) .
E x p l i c i t f i n i t e d i f f e r e n c e scheme, convergence c o n t a i n e d by monotonicity c o n d i t i o n s . Vacuum d e s o r p t i o n s t e p o n l y . A mathematical paper. 1 ) P l u g flow w i t h a x i a l d i f f u s i o n 2 ) Ergun e q u a t i o n f o r p r e s s u r e drop a c r o s s bed 3 ) I s o t h e r m a l o p e r a t i o n j i d e a l gas b e h a v i o u r 4 ) Unspecified r a t e of adsorption functions 5 ) Complex boundary c o n d i t i o n s . I m p l i c i t f i n i t e d i f f e r e n c e scheme o f t h e CrankNicholson t y p e . Constant p r e s s u r e a d s o r p t i o n process considered only. 1 ) P l u g flow w i t h a x i a l d i f f u s i o n 2 ) No s p a t i a l p r e s s u r e g r a d i e n t s 3 ) I s o t h e r m a l o p e r a t i o n / i d e a l gas b e h a v i o u r 4 ) Non-linear mass t r a n s f e r model s e p a r a t e l y c o n s i d e r i n g micro and macro p o r e d i f f u s i o n . P r o c e s s o p t i m i s a t i o n on a c e l l model. 1) I s o t h e r m a l o p e r a t i o n / i d e a l gas b e h a v i o u r 2 ) Darcy's Law p r e s s u r e drop a c r o s s bed 3 ) V i s c o s i t y o f t h e gas i s composition independent 4 ) P l u g flow 5 ) Instantaneous equilibrium 6 ) Freundlich isotherm a p p l i e s 7) R e l a t i v e v o l a t i l i t y r e l a t e s a d s o r p t i v e c a p a c i t y and i s composition i n v a r i a n t . 8 ) The e q u i l i b r i u m amount adsorbed i s independent o f composition 9 ) Each c e l l i s i d e a l l y mixed and a t c o n s t a n t pressure
LIST OF PARTICIPANTS
BELGIUM
-
Ph. B o d a r t
F a c u l t g s Univ.
d e Namur
-
B 5000 Namur
- U n i v . ~ i s g e- B 4000 ~ i g g e
J. C o s t a
J. Martens
-
K a t h o l i e k e Univ.
-
Univ. N e w B r u n s w i c k
Leuven
-
B 3030 L e u v e n
CANADA D.
Ruthven
F.
Smith
-
Univ.
-
Fredericton
P r i n c e Edward I s l a n d
-
Charlotte
DENMARK N.
Blom - H a l d o r T o p s e A/S
-
DK 2800 Lyngby
T e c h n o l o g i c a l I n s t i t u t - Denmark
J. -sonFRANCE F.
Fajula
N.
Gnep
-
J. L u c i e n
-
Univ. M o n t p e l l i e r
Univ.
-
T. L a b o u r e l
Poitiers
-
-
34075 M o n t p e l l i e r
86022 P o i t i e r s
S h e l l Recherche
-
76530 G r a n d C o u r o n n e
-
-
69360 S t . S y m p h o r i e n
E l f Recherche d ' Ozon
GERMANY D.
A r n t z - Degussa
M.
Baacke
-
-
Degussa
D 6450 Hanau 1
-
D 6450 Hanau 1
GREECE D.
Z a m b o u l i s - Univ.
Thessaloniki - Thessaloniki
ICELAND G. Einarson
-
Technological Institute
-
Reykjavik
ITALY F. Ciambelli - Univ. Napoli - 80134 Napoli P. Corbo - Univ. Napoli - 80134 Napoli I. Ferino - Univ. Cagliari - 09100 Cagliari A. La Ginestra - Univ. Roma - 00185 Roma G. Gubitosa - Donegani Research Institute - 28100 Novara R. Maggiore - Univ. Catania - 95125 Catania R. Monacci - Univ. Cagliari - 09100 Cagliari P. Porta - Univ. Roma - 00185 Roma A. Villanti - Anic/Chisec - 20097 S. Donato Milanese ISRAEL M. Steinberg
-
Univ. Hebrew - Jerusalem
NETHERLANDS C. W. Engelen - Univ. Eindhoven - 5600 MB Eindhoven E. Groenen - Koninklijke-Shell Laboratorium - 1031 CM Amsterdam H. Okkersen - Dow Chemical - Terneuzen F. Roozeboom - Esso Chemie - 3000 HE Rotterdam W. Van Erp - Koninklijke-Shell Laboratorium - 1003 Amsterdam NORWAY G. Boe - ELKEM - N 4620 Vagsbygd G. Haegh - Central Institute Industrial Research - N Oslo 3 K. Kinnari - Statoil - N 4001 Stavanger S. Kolboe - Univ. Oslo - N Oslo 3 0 . Onsager - Univ. Trondheim - N 7034 Trondheim
-
J.Raeder
-
SPAIN A. Corma
-
J. A. C. A. J. A.
Univ. Oslo
-
N Oslo 3
Instituto Catalisis Petroleoquimica - Madrid 6 Juan Aguera - ENPETROL - Cartagena Lopez Agudo - Instituto Catalisis Petroleoquimica - Madrid 6 Ballesteros Martin - Univ. Madrid - Madrid 3 Lucas Martinez - Univ. Madrid - Madrid 3 Pajares - Instituto Catalisis Petroleoguimica - Madrid 6 Villarroya Palomar - Industrias Quimicas del Ebro - Zaragoza
TURKEY E. Alper
-
A. U. Fen Fakultesi
SWITZERLAND G. Gut - ETH-Zentrum
-
-
Ankara
CH 8092 Zurich
UNITED KINGDOM S. Fegan - Univ. Edinburgh - Edinburgh EH93JJ A. Hope - Univ. College London - London WCIH OAJ D. Rawlence - Joseph Crosfield Sons - Warrington WA5 1AB M. Sanders - Univ. College London - London WCIH OAJ D. Swindells - Univ. Aberdeen - Old Aberdeen AB92UE D. Whan - Univ. Edinburgh - Edinburgh EH93JJ U.S.A. L. Sand
-
Worcester Polytechnic Institute
-
Worcester
-
-
PORTUGAL M.
-
J . Bordado
M.
-
Brotas
J. C a e i r o
C. C o s t a M.
-
J. Afonso
C.
-
Inst.
-
Quimigal
1000 L i s b o a
Barreiro
F a c u l d a d e d e ~ i g n c i a s- L i s b o a
-
Petrogal
Lisboa
Faculdade de Engenharia
-
Dias
-
S u p e r i o r TGcnico
Inst.
-
J. F i g u e i r e d o
-
Superior Tgcnico
4099 P o r t o Codex
-
1000 L i s b o a
-
Faculdade de Engenharia
4099 P o r t o
Codex
-
I n s t . S u p e r i o r TGcnico - 1000 L i s b o a
F.
Freire
C.
Henriques
J. J u s t i n 0 F . Lemos
-
-
-
Inst.
-
S u p e r i o r TGcnico
1000 L i s b o a
I n s t . S u p e r i o r ~ G c n i c o- 1 0 0 0 L i s b o a
I n s t . S u p e r i o r ~ G c n i c o- 1 0 0 0 L i s b o a
Lemos - I n s t .
M.
A.
L.
S o u s a Lobo
-
S u p e r i o r ~ G c n i c o- 1 0 0 0 L i s b o a
-
U n i v . Nova d e L i s b o a
2825 .Vonte d a
Caparica J . M.
-
Loureiro
Faculdade de Engenharia
-
4099 P o r t o
Codex
-
M . A . Mendes
M.
Petrogal
-
F. M a r t i n s
-
Lisboa
I n s t . S u p e r i o r ~ G c n i c o- 1 0 0 0 L i s b o a
-
L. P a l h a
Inst.
-
S u p e r i o r Tgcnico
1000 L i s -
boa
-
M.
J. P i r e s
M.
F. P o r t e l a
M.
F.
Ribeiro
M.
C.
Rodrigues
A. N .
Santos
-
I n s t . Superior Tgcnico
-
1000 L i s b o a
Superior Tgcnico
-
1000 L i s b o a
I n s t . S u p e r i o r TGcnico
-
1000 L i s b o a
Inst.
-
Inst.
S u p e r i o r TGcnico
U n i v . Nova d e L i s b o a Caparica
-
-
1000 L i s b o a
2825 - Monte d a
... ,
p:
-
:.
b
e is
I
.
1:
,"X.*l,
: a.
ei.
L
:
afid
.
.
Included in the section on industrial applications are chaptersbn reactor and adsorber design, catalytic uacking, xylme and n-paraffins ipmerization, as well as ioriexchange and adsqrhtion. Whilg'pjmariiy intended for sienti8ts and engihers concarned with catalysis, ad$orbtion and ion-exchange, or reaction.engineering, this boo& wit1 also ,b.suitable graduate dunes incatalysis.
9
.,
,"
2
. , i, .
'
i s a sumpary of the pmceediqof a NATO A&@pxd study I&&&; he& i i b ~..Pbrtug&& %lay 1083, whe,n,par$cula~emphasiswas p l a ' & ~ ~ & ~ ~ e y 6 l o p ~ k in the fietd.of zsol.ite science technology.Lndividual chgpisrs.$f the book]@& hiitorical development, stwmre. crystallqgraphy snd bynmarjis'!fbhn&6sIIII~asic principles @@wrbtion diffusion, ion.excfisr@e and M i t y are reviawed,an$i q soction on datalyrie add&s@s shape-electivity, tramition mewls, bifme-tionalCat* lysis and 'methanol-to-sasolind.
i$is
I
.
Prerented in this volu'& is an updated and integrated picture of the prarenf kboyiidi;; ledae - of the fund&entd$af zeolites a aubiect of growin@impoitme to iWuStVJafw!'A and academic cwnmunitles.