16 Zeolite Chemistry in Catalysis JACQUES C. VEDRINE
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch016
Institut de Recherches sur la Catalyse, Albert Einstein, 69626 Villeurbanne, France
The importance of molecular sieves catalysts in industrial catalysis has increased significantly over the past two decades. To date, a l l commercial applications of zeolite catalysts have involved acidic zeolites, particularly the ultra stable rare earth Y-type zeolite used in the catalytic cracking processes. Recent investigations of zeolite chemistry have revealed some particular features of both basic and acidic zeolites and have opened the new field of improved fuel processing. The new chemical evidence has raised the p o s s i b i l i t y that physico-chemical features play a role in catalysis. A large effort has therefore been devoted to the shape-selectivity properties of such materials. Moreover, increasing interest has been focussed recently on the understanding of crystallization of zeolites during synthesis, in the synthesis of new zeolites and in the chemical modifications of zeolites with the objective of expanding the applications of such materials.
The word zeolite stems from the Greek "zeo" (boiling) and "litos" (stone) meaning a material able to eliminate large amounts of water when h e a t e d . More than forty different structures of zeolites have been identified while grea progress has been gained i n s y n t h e t i c z e o l i t e s , p a r t i c u l a r l y by Linde and Mobil . Zeolites are usually alumino s i l i c a t e s with a general formula M D (Al S i , 0 ), mH 0 χ y x+2y n-(x+2y) 2n ' 2 0
N
o
o
where M and D cj^esignajje respectively a mono-(H , N a , K , cjt. .) or a d i v a l e n t (Ba , Mg , Ca , . . . ) cation. They are formed of A10. +
+
+
0097-6156/85/0279-0257$06.00/0 © 1985 American Chemical Society
In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
S O L I D STATE C H E M I S T R Y IN CATALYSIS
258
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch016
and S i O , t e t r a h e d r a b o n d e d t o g e t h e r v i a t h e o x y g e n atoms and assembled i n such such a way to c o n s t i t u t e c a v i t i e s , c a g e s a n d / o r c h a n n e l s l e a d i n g t o a r e g u l a r l a t t i c e . The c a t i o n s compensate the n e g a t i v e charge b o r n by A l due to i t s f o u r f o l d c o o r d i n a t i o n . The d e h y d r a t i o n o f these m a t e r i a l s makes v o i d volume a v a i l a b l e to m o l e c u l e s whose shape and s i z e h a v e t o be c o m p a t i b l e w i t h t h e s i z e o f t h e c a v i t i e s and p o r e s . The d i a m e t e r o f these c a v i t i e s o r pores depends on the type o f z e o l i t e and v a r i e s from O.4 nm ( L i n d e type A) t o O.75 nm ( L i n d e - Y ) which c o r r e s p o n d s t y p i c a l l y to the s i z e o f m o l e c u l e s and i s expressed i n the word " m o l e c u l a r s i e v e " g i v e n t o such m a t e r i a l s . I t f o l l o w s t h a t d i f f u s i v i t i e s o f reagents or products often p l a y a determining r o l e i n c a t a l y s i s . This holds true p a r t i c u l a r l y i n shape s e l e c t i v i t y f e a t u r e s . In F i g u r e 1 examples o f s t r u c t u r e o f some z e o l i t e s a r e g i v e n t o p i c t u r e some t y p i c a l s p a t i a l arrangements of channels, cavities, etc. I n T a b l e s I and I I a r e r e p o r t e d the d i m e n s i o n a l p r o p e r t i e s o f current zeolites with special emphasis on the trior mono-dimensional i n t e r c o n n e c t i o n s between cages, c a v i t i e s or channels which a l l o w more o r l e s s t h e t r a f f i c o f r e a g e n t a n d / o r p r o d u c t m o l e c u l e s t h r o u g h t h e z e o l i t i c n e t w o r k . On the b a s i s o f pore s i z e , z e o l i t e s can be c l a s s i f i e d i n t h r e e g r o u p s : t h e l a r g e p o r e z e o l i t e s , w h i c h show twelve-merabered r i n g s ( s u c h as m o r d e n i t e and f a u j a s i t e ) , t h e i n t e r m e d i a t e p o r e z e o l i t e s w i t h 10-membered r i n g s ( s u c h as Z S M - 5 , Z S M - 1 1 , f e r r i e r i t e ) and t h e s m a l l p o r e z e o l i t e s w i t h 8-membered r i n g s ( s u c h as e r i o n i t e , A , c h a b a z i t e , Z K - 5 ) . The s i z e o f t h e p o r e s o r c a v i t i e s a r e comparable to the c r i t i c a l molecular dimensions f o r some h y d r o c a r b o n s as shown i n Table I I I . A z e o l i t e f r a m e w o r k b e a r s a net n e g a t i v e charge per u n i t c e l l e q u a l t o t h e number o f t e t r a c o o r d i n a t e d A l i t c o n t a i n s . T h i s n e g a t i v e charge i s compensated by c a t i o n s as d i s c u s s e d above. These c a t i o n s are exchangeable w h i c h a l l o w s one t o i n t r o d u c e a d e s i r e d c a t i o n . They a r e n o t l o c a t e d i n t h e f r a m e w o r k at l a t t i c e atom p o s i t i o n but are l o c a t e d i n some d e f i n e d s i t e s w i t h i n t h e c a v i t i e s o r c a g e s . Such l o c a t i o n s c o u l d be c h a r a c t e r i z e d by X - r a y d i f f r a c t i o n s t u d i e s and were d e s i g n a t e d f o r Y type z e o l i t e as S^. ( c e n t r e o f the h e x a g o n a l p r i s m ) , S ^ and S , w i t h i n the s o d a l i t e cage (near the hexagonal p r i s m and near the window t o w a r d t h e s u p e r c a g e , r e s t h e s o d a l i t e cage ( n e a r the hexagonal p r i s m and near the window toward t h e s u p e r c a g e , r e s p e c t i v e l y ) and i n the l a r g e cage ( O . I t was t h u s i n t e r e s t i n g t o determine the l o c a t i o n o f exchanged c a t i o n s and i t was o b s e r v e d t h a t d e h y d r a t i o n o r o u t g a s s i n g r e s u l t s i n a m i g r a t i o n o f the c a t i o n s from S o r S , s i t e s towards i n n e r s i t e s such c a t i o n s as S o r S u n d e r a m b i e n t a t m o s p h e r e s and t e m p e r a t u r e s i s one o f t h e m o s t s t r i k i n g features of z e o l i t e m a t r i c e s . f
J T
T T
In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Zeolite Chemistry in Catalysis
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch016
VEDRINE
Fig.l
: Structures
o f some p e c u l i a r z e o l i t i c
frameworks.
In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
S O L I D STATE C H E M I S T R Y IN CATALYSIS
260
T a b l e I . Pore s t r u c t u r e o f some z e o l i t e s . Interconnection
S i z e o f c h a n n e l s (nm)
Zeolites
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch016
system Chabazite Erionite F a u j a s i t e (X, Y ) Ferrierite ZK-5 Linde A Linde L ZSM-11 ZSM-5 Mordenite Ζ Offretite Rho
8 8 jj2 10 8 8" 12 10 10 12 Î2 V
O.36x0.37 O.36x0.52 O.74 O.43x0.55 O.39 O.41 O.71 O.51x0.55 O.54x0.56 O.67x0.70 O.64 (ID)
3 3 3 1 3 3 1 3 3 1
10 O.51x0.55 «-> 8 O.29x0.57
