Recent Developments in Pillared Interlayered Clays - American


Clays (PILC) in 1979 (1). much interest has focused on these materials as .... 21.5. 290. 1. (Zr 4 ) y. 21.5 + 25.6. 250. 1. (Ti0 2 ) a. 25А. 300. 26...
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Recent Developments in Pillared Interlayered Clays

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D. E . W. Vaughan Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801

The field of pillared clays has recently expanded by discoveries of new pillaring species, and extensions of the pillaring concept to several new layered materials, including micas, silicic acids, zirconium phosphates and layered metal hydroxides. Pillaring in various sheet materials has been done with clusters of cations, anions and neutral molecules, resulting in a new compositional versatility. Pillared layered materials now represent a rapidly expanding category of catalytic microporous materials having a high degree of chemical and structural diversity. S i n c e t h e f i r s t commercial announcements o f P i l l a r e d I n t e r l a y e r e d C l a y s (PILC) i n 1979 ( 1 ) . much i n t e r e s t has f o c u s e d on these m a t e r i a l s as p o t e n t i a l s o u r c e s o f new m a t e r i a l s i n s o r p t i o n , s e p a r a t i o n s and c a t a l y s i s . I n t h i s r e v i e w I w i l l attempt t o summarise some o f t h e r e c e n t developments t h a t c o n t i n u e t o make PILCs an e x c i t i n g and o p p o r t u n i s t i c r e s e a r c h a r e a f o r t h e p e t r o l e u m oriented industries. These developments c a n c o n v e n i e n t l y be d i v i d e d i n t o the f o l l o w i n g areas:

• • •

New c h e m i c a l u n d e r s t a n d i n g New p i l l a r s New s h e e t s t r u c t u r e s

o f c l a y s and PILCs.

I n t h e sense t h a t much academic PILC r e s e a r c h i s i n d u s t r y - s p o n s o r e d , o n l y t h e f i r s t group may be v i e w e d as n o t b e i n g c l o s e l y i n t e r t w i n e d w i t h a p p l i c a t i o n s - b o t h i n d i r e c t i o n and e x e c u t i o n . Indeed, t h e r e seems t o be l i t t l e d e l a y between t h e d i s c o v e r y o f new PILC m a t e r i a l s and t h e i r e v a l u a t i o n as c a t a l y s t s . The o r i g i n a l o b j e c t i v e s i n d e v e l o p i n g PILCs - t h e c a t a l y t i c p r o c e s s i n g o f heavy r e s i d u a l crude o i l s t o t r a n s p o r t a t i o n f u e l s (2.3) - c o n t i n u e s t o be t h e main f o c u s o f t h e t e c h n o l o g i c a l developments i n t h e f i e l d . This r e f l e c t s not only a dearth o f a l t e r n a t i v e c a t a l y s t s i n t h i s important a p p l i c a t i o n , b u t a l s o a c o n v i c t i o n t h a t they do r e p r e s e n t a major

0097-6156/88/0368-0308$06.00/0 © 1988 American Chemical Society

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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p o s s i b l e s o l u t i o n t o the d i f f i c u l t c a t a l y t i c p r o b l e m o f e f f i c i e n t l y c o n v e r t i n g heavy, m e t a l l a d e n , crude o i l s t o t r a n s p o r t a t i o n f u e l s . PILCs e s s e n t i a l l y f i l l the p o r e s i z e - a c t i v i t y gap between z e o l i t e s and the l a r g e r p o r e amorphous g e l c a t a l y s t s . The s t i m u l u s t o c r e a t e new c o m p o s i t i o n s i s t o meet s e v e r a l major p e r c e i v e d needed improvements t o make PILCs e f f e c t i v e commercial f l u i d c r a c k i n g c a t a l y s t s (FCC):



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• •

L a r g e r s p a c i n g s between t h e c l a y s h e e t s ( t o accommodate l a r g e ' r e s i d ' m o l e c u l e s ) H i g h e r t h e r m a l and steam s t a b i l i t y (so a s t o s u r v i v e FCC r e g e n e r a t i o n ) . B u i l d novel c a t a l y t i c a c t i v i t y into the p i l l a r (to expand p r o c e s s a p p l i c a t i o n s ) .

D e s p i t e t h i s apparent f o c u s , new m a t e r i a l s and new m a t e r i a l s c o n c e p t s a r e d e v e l o p i n g r a p i d l y , and i t i s p r o b a b l y o n l y a m a t t e r o f time b e f o r e b r o a d e r a p p l i c a t i o n s a r e d i s c o v e r e d and d e v e l o p e d f o r PILCs i n b o t h s e p a r a t i o n s and c a t a l y s i s . There a r e many p r o c e s s e s o t h e r t h a n FCC ( a l k y l a t i o n , i s o m e r i s a t i o n e t c . ) t h a t do n o t r e q u i r e any o f the above improvements, and i n such a p p l i c a t i o n s PILCs r e p r e s e n t h i g h s u r f a c e area s o l i d s having unique s t r u c t u r e s . In some c a s e s , however, i t must be n o t e d t h a t the p o r e s i n PILCs may be too l a r g e f o r h i g h s p e c i f i c i t y i n many s m a l l m o l e c u l e p e t r o c h e m i c a l processes. ZSM-5 and r e l a t e d s t r u c t u r e s have become s u c c e s s f u l c a t a l y s t s because they have pore s t r u c t u r e s o f 5 t o 6À - a r e l a t i v e l y r e s t r i c t e d environment which s i g n i f i c a n t l y a f f e c t s the c a t a l y t i c t r a n s i t i o n s t a t e s o r d i f f u s i o n modes, c r e a t i n g n o v e l selectivities. PILCs w i l l be more u s e f u l where the r e a c t a n t s a r e s u b s t i t u t e d m u l t i - r i n g a r o m a t i c s o r n a p t h e n i c m o l e c u l e s , too l a r g e to e f f i c i e n t l y r e a c t i n c o n v e n t i o n a l z e o l i t e s . H y d r o c r a c k i n g and dewaxing a r e the most p r o b a b l e a r e a s o f i n i t i a l a p p l i c a t i o n s g i v e n the p r e s e n t s t a t u s o f PILC s t a b i l i t y . For the p r e s e n t , the c a t a l y s t and p e t r o l e u m p a t e n t l i t e r a t u r e r e m a i n the major s o u r c e s o f new i n f o r m a t i o n and developments. Two r e v i e w s p r o v i d e a background f o r t h i s paper ( 4 . 5 ) . New

Chemistry

S t u d i e s o f the s o l u t i o n c h e m i s t r y o f p o t e n t i a l p i l l a r i n g s p e c i e s a r e o f major i n t e r e s t i n PILC s t u d i e s , b u t have o n l y r e c e n t l y a c h i e v e d the l e v e l o f o b s e r v i n g s p e c i f i c s p e c i e s i n s o l u t i o n p r i o r t o , d u r i n g , and a f t e r p i l l a r i n g ( 6 ) . Although s o l u t i o n NMR methods o f f e r the o p p o r t u n i t y f o r A l , S i and Zr polymer d e f i n i t i o n , and have been demonstrated f o r s p e c i f i c A l (7.8) and S i (9-11) c a s e s , t h e s e s t u d i e s a r e l a r g e l y t o p i c s f o r f u t u r e r e s e a r c h . E x a m i n a t i o n o f a comprehensive t e x t on m e t a l s p e c i e s i n s o l u t i o n , such as r e f e r e n c e ( 1 2 ) . and r e c e n t l i t e r a t u r e , c l e a r l y demonstrate l a r g e gaps i n our knowledge o f the d e t a i l e d n a t u r e o f complex m u l t i - a t o m i c polymers i n aqueous s o l u t i o n s i n d i c a t e d by many p o t e n t i o m e t r i c t i t r a t i o n s t u d i e s . Recent PILC developments d e s c r i b e d i n t h i s paper demonstrate t h a t i m p o r t a n t s o l u t i o n s p e c i e s e x i s t , and can be " s i z e d " by PILC s p a c i n g s and p o r e volumes, b u t s p e c i f i c and d i r e c t i n f o r m a t i o n on b o n d i n g , s t r u c t u r e and

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d i m e n s i o n a l i t y i s l a c k i n g i n most c a s e s . However, the t r e n d s have c l e a r l y been demonstrated by s e v e r a l r e c e n t s t u d i e s o f z e o l i t e 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 u s i n g b o t h 2 9 g i s o l u t i o n NMR (13.14) and Raman s p e c t r o s c o p y ( 1 5 ) . I n t h e s e c a s e s s m a l l p o l y h e d r a l c l u s t e r s have been i d e n t i f i e d i n s y n t h e s e s o f z e o l i t e s A and ZSM-5, and t h e s e same t e c h n i q u e s s h o u l d be q u i t e a d a p t a b l e t o s t u d y the c h a r a c t e r o f some p i l l a r p r e c u r s o r s . As w i t h many d i f f i c u l t problems, t h i s a r e a o f s c i e n c e needs the s i m u l t a n e o u s a p p l i c a t i o n o f s e v e r a l complimentary t e c h n i q u e s t o e l u c i d a t e c l u s t e r s p e c i e s i n solution. I f s u c c e s s f u l , such an approach w i l l y i e l d s e v e r a l f a m i l i e s o f new p i l l a r e d m a t e r i a l s . Some o f the most i m p o r t a n t developments have t a k e n p l a c e i n the e l u c i d a t i o n o f new d e t a i l s o f the s t r u c t u r e o f the c l a y s t h e m s e l v e s , t o g e t h e r w i t h some i n i t i a l and p r o v o c a t i v e s t u d i e s o f the e f f e c t s o f p i l l a r i n g - a l l u s i n g v a r i o u s magic a n g l e s p i n n i n g n u c l e a r magnetic r e s o n a n c e t e c h n i q u e s (MASNMR). ^ A 1 and Si-MASNMR s t u d i e s o f c l a y s by H e r r e r o e t a l . (16) have demonstrated t h a t t e t r a h e d r a l l a y e r o r d e r i n g o f S i and A l f o l l o w s the g e n e r a l r u l e s o f A l a v o i d a n c e c h a r a c t e r i s t i c o f z e o l i t e s p r o p o s e d by L o e w e n s t e i n (17) and extended by M e l c h i o r e t a l . ( 1 8 ) . T h i s i m p l i e s t h a t c a t i o n s , i n c l u d i n g p i l l a r i n g c a t i o n s , a r e l i k e l y t o be e v e n l y s p a c e d w i t h i n the i n t e r l a y e r space. Assuming an absence o f p i l l a r a g g l o m e r a t i o n on d e h y d r a t i o n , the o r i g i n a l assumption i s s u p p o r t e d t h a t a PILC may be r e g a r d e d as a m o l e c u l a r s i e v e h a v i n g e v e n l y spaced p i l l a r s p r o p p i n g open the l a y e r s ( 1 9 ) . An a l t e r n a t i v e model i s t h a t i n some c l a y s p i l l a r s may c l u s t e r , and p o s s i b l y i n the i n i t i a l s t a g e s o f c a l c i n a t i o n o r steaming, p i l l a r c l u s t e r i n g may be an i m p o r t a n t mechanism o f s t a b i l i z a t i o n . T h i s p a r t i c u l a r l y may a p p l y i n c l a y s where the amount o f p i l l a r s p e c i e s f i l l s the i n t e r l a y e r i n the hydrated state. As more work on PILCs i s p u b l i s h e d , i t becomes c l e a r t h a t the c l a y i t s e l f i s an i m p o r t a n t v a r i a b l e , and t h a t more t h a n one model may be needed t o e x p l a i n the e x p e r i m e n t a l d a t a . 7

29

The m o l e c u l a r s i e v e s o r p t i o n p r o p e r t i e s o f PILCs (19.20) i n d i c a t e z e o l i t e l i k e b e h a v i o u r as shown i n F i g u r e 1, o f t e n w i t h a sharp c u t - o f f i n m o l e c u l a r s i z e s e p a r a t i o n , b u t t h e s e experiments do n o t a d e q u a t e l y d i f f e r e n t i a t e between l a r g e r e c t a n g u l a r p o r e s (a c l u s t e r e d PILC model) and the s m a l l e r , more u n i f o r m , z e o l i t e - l i k e ( e . g . , as i n h e u l a n d i t e o r s t i l b i t e ) p o r e s thought t o be c h a r a c t e r i s t i c o f PILCs i n the i s o l a t e d p i l l a r model. More s o p h i s t i c a t e d m o l e c u l a r p r o b i n g i s t h e r e f o r e needed, t o g e t h e r w i t h a n a l y s i s o f i n t e r l a y e r c a t i o n o r d e r i n g experiments, to f u r t h e r e l u c i d a t e the p i l l a r i n g models. One i n t e r e s t i n g new method o f pore a n a l y s i s d e v e l o p e d a t Exxon ( 2 1 ) . and r e c e n t l y c o m m e r c i a l i z e d , uses the v e r y low p r e s s u r e p a r t o f the s o r p t i o n i s o t h e r m t o r e s o l v e p o r e s i n the m o l e c u l a r range. A comparison o f such an a n a l y s i s f o r z e o l i t e s 5A, NaX and an A l - P I L C i s shown i n F i g u r e 2, and g i v e s a p o r e d i a m e t e r f o r the PILC l a r g e r t h a n f a u j a s i t e , b u t showing a more d i s p e r s e d pore s i z e d i s t r i b u t i o n t h a n o b s e r v e d f o r the z e o l i t e s , and v e r y c l o s e t o the model p r o p o s e d f o r an A l - P I L C . Note t h a t the z e o l i t e s have pore d i a m e t e r s c l o s e t o t h e i r c r y s t a l l o g r a p h i c window dimensions i n t h i s a n a l y s i s . As the measurements were made a t 87°K and r e p r e s e n t a r e l a t i v e l y r i g i d l a t t i c e , t h e y a r e r e m a r k a b l y c l o s e to a c t u a l i t y . A t o r c l o s e t o room temperature the b r e a t h i n g modes

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Φ

Pressure-torr F i g u r e 1. A b s o r p t i o n i s o t h e r m s a t 0°C, 25°C, 50°C and 75°C f o r n-octane i n an A l - P I L C and a Zr-PILC, u s i n g a b e n t o n i t e - 3 2 5 base c l a y (American C o l l o i d C o . ) . The shape i s t y p i c a l l y " z e o l i t i c " (Type I ) , w i t h some c a p i l l a r y c o n d e n s a t i o n between t h e s m a l l c r y s t a l s a t low t e m p e r a t u r e s .

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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UNCORR. PORE DIA. (ANG.) F i g u r e 2. A comparison o f the n i t r o g e n pore s i z e d i s t r i b u t i o n a n a l y s i s f o r Ca, NaA (5A), NaX and an A l - P I L C o b t a i n e d u s i n g an Omnisorb 360 i n the p a r t i a l p r e s s u r e range o f 10~5 t o 10"3. The r e l a t i v e o r d e r o f the pore s i z e s i s i n good agreement w i t h o t h e r experiments.

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o f f a u j a s i t e i n c r e a s e the a c t u a l window d i a m e t e r t o about 8À, and i f s i m i l a r s t r u c t u r a l v i b r a t i o n a l a m p l i t u d e s a r e assumed f o r the PILC, the room temperature PILC pore d i a m e t e r would be about 9 t o 10Â v e r y c l o s e t o the p r e d i c t e d s i z e b a s e d on a u n i f o r m pore model. E x p l o r a t i o n o f the p i l l a r - c l a y s h e e t r e a c t i v i t y and c o n n e c t i v i t y a l s o i n d i c a t e the i m p o r t a n t r o l e o f the s p e c i f i c c l a y type. 27^1 and 29gi-MASNMR experiments have shown d i s t i n c t i v e d i f f e r e n c e s between p i l l a r i n g mechanisms i n t r i o c t a h e d r a l h e c t o r i t e and d i o c t a h e d r a l m o n t m o r i l l o n i t e . Whereas P l e e e t a l . (22) c o n c l u d e d t h a t c h e m i c a l c r o s s l i n k i n g may o c c u r between the p i l l a r and t e t r a h e d r a l l a y e r i n a b e i d e l l i t e m o n t m o r i l l o n i t e , P i n n a v a i a e t a l . (23) showed t h a t i t d i d n o t o c c u r i n a h e c t o r i t e . These a r e the f i r s t o b s e r v a t i o n s o f a complex p r o c e s s t h a t may depend upon s e v e r a l s t r u c t u r a l and c h e m i c a l f a c t o r s , such as s u b s t i t u t i o n o f A l i n the t e t r a h e d r a l l a y e r , o r the need f o r v a c a n c i e s i n the o c t a h e d r a l l a y e r to a l l o w r o t a t i o n o f s t r u c t u r a l u n i t s or m i g r a t i o n o f r e a c t a n t species to f a c i l i t a t e c r o s s l i n k i n g . Ongoing r e s e a r c h s h o u l d f u r t h e r e l u c i d a t e r e f i n e m e n t s on t h e s e mechanisms, and d i r e c t the t e c h n o l o g y towards more o p t i m i z e d c a t a l y s t s - presumably t h o s e which form c h e m i c a l bonds between the p i l l a r and c l a y l a y e r . New

Pillar

Compositions

The o r i g i n a l PILC c o m p o s i t i o n s p r o p o s e d a f a m i l y o f p i l l a r i n g s p e c i e s t h a t r e s u l t e d i n a b r o a d spectrum o f q u i t e d i f f e r e n t PILC p r o d u c t s (1.5.24.25). b a s e d p r i m a r i l y on (Α1)χ3 o r ( Z r ) 4 polymers, o r such polymers m o d i f i e d p r i m a r i l y w i t h Mg, S i and T i s p e c i e s . New p i l l a r compositions include:

groups

of polymers

of

(Τίθ2)χ·



C r e a t i o n o f new



E x t e n t i o n o f some o f t h e o r i g i n a l p o l y m e r s by s u b s t i t u t i o n of p a r t of the A l i n the Keggin i o n w i t h secondary elements.



Cluster small or c o l l o i d compositions of metal oxides or metal hydroxides.

transition

The s y n t h e s i s o f l a r g e s p a c i n g T1O2 PILCs i s one o f the most i m p o r t a n t r e c e n t developments, i n t h a t m a t e r i a l s h a v i n g a h i g h degree o f o r d e r i n g , w i t h about 30Â (001) b a s a l s p a c i n g s , have been reported. C l a y - a l c o h o l i n t e r c a l a t e s w i t h l a r g e r t h a n 50À b a s a l s p a c i n g s a r e known, b u t i t i s u n l i k e l y t h a t i n o r g a n i c p i l l a r e d c l a y s o f such dimensions would have e i t h e r s t a b i l i t y o r s i g n i f i c a n t i n t e r n a l f r e e volume. The 30Â PILCs o f S t e r t e (26) have h i g h s u r f a c e a r e a s , and h y d r o t h e r m a l s t a b i l i t y comparable t o Z r - P I L C s , and w i t h i n the range u s e f u l f o r FCC c a t a l y s t s . Although lowered s p a c i n g T1O2 PILCs were p r o p o s e d i n the o r i g i n a l PILC p a t e n t s (1.23.24). t o g e t h e r w i t h Z r - T i and A l - T i PILCs, p r e c i p i t a t i o n r e a c t i o n s i n h i b i t e d the development o f s i g n i f i c a n t o r d e r e d p i l l a r i n g , o r major s u r f a c e a r e a s . Sterte observed a h i g h l e v e l of o r d e r , b u t a t t h i s time no hypotheses have been advanced t o e l u c i d a t e the s t r u c t u r e o f the near-20Â t i t a n i a complexes p a r t i c i p a t i n g i n the i n t e r c a l a t i o n r e a c t i o n . I n a d d i t i o n t o 30À

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s p a c i n g PILCs, S t e r t e r e p o r t s 25À l a y e r e d m a t e r i a l s u s i n g a s m a l l e r T1O2 s p e c i e s . T h i s has a s i m i l a r b a s a l s p a c i n g t o the p r e v i o u s l y r e p o r t e d 25.5À Zr02-PILC ( 1 ) . which presumed c r o s s l i n k i n g o f the Zr4 square p l a n a r complex r e p o r t e d by C l e a r f i e l d and Vaughan (27). However, t h e r e are s i g n i f i c a n t d i f f e r e n c e s i n the aqueous s o l u t i o n c h e m i s t r y o f the s m a l l e r t i t a n i u m c a t i o n ( r a d i u s 0.61À) compared to the z i r c o n i u m c a t i o n ( r a d i u s 0 . 7 2 À ) . I t has been r e p o r t e d t h a t the c h a i n l i k e ( T i - 0 ) 2 + c a t i o n s (e.g., as i n T1OSO4) p r e f e r not t o c r o s s l i n k i n the manner o f s i m i l a r Zr c a t i o n s (28). Yamanaka and co-workers (29) have a l s o made 25À l a y e r e d T1O2-PILCs, b u t u s i n g h y d r o l y s e d t e t r a e t h o x y t i t a n i u m complexes. These too have h i g h s t a b i l i t y and h i g h i n t e r n a l pore volume, as shown i n T a b l e I .

TABLE I Polymer S p e c i e s

AI13 (Si ) Zr4 (Zr ) (Zr ) (Ti0 ) (Ti0 )b 8

n

4

x

4

y

2

2

a

COMPARISON OF THE PROPERTIES OF A l . Z r . S i and T i - P I L C s (001) C l a y S p a c i n g Surface Area 17.8Â - 18.5Â 19Â 18.0Â 21.5 21.5 + 25.6 25À 29À

400 300 260 290 250 300 330

Reference 1 34 1 1 1 26 29

The t i t a n i u m a l k o x i d e s are used e x t e n s i v e l y i n s u r f a c e c o a t i n g p r e p a r a t i o n s f o r m e t a l s and p l a s t i c s , and a r e thought t o form s h e e t l i k e t w o - d i m e n s i o n a l polymers (30). b a s e d on the s t r u c t u r a l u n i t shown i n F i g u r e 3, i n c o n t r a s t w i t h o t h e r o n e - d i m e n s i o n a l T i c h a i n polymers, and Zr4 p l a n a r c l u s t e r s . I t i s p o s s i b l e t h a t the 25À T i - P I L C o f S t e r t e , made v i a an i n o r g a n i c r o u t e , may be d i f f e r e n t from the s i m i l a r s i z e d 25À T i - P I L C o f Yamanaka e t a l made v i a the a l k o x i d e , and t h e r e f o r e t h e s e may have d i f f e r e n t p r o p e r t i e s . As f o r Z r p o l y m e r s , the low m o l e c u l a r weight T i s o l u t i o n polymers are d i f f i c u l t t o c h a r a c t e r i z e , and a w a i t the development o f new e x p e r i m e n t a l t e c h n i q u e s b e f o r e t h e i r n a t u r e can be f u l l y e l u c i d a t e d . Zr and T i NMR s h o u l d have major impact h e r e , i f s u i t a b l e t e c h n i q u e s can be d e v e l o p e d . I n c o n t r a s t to Zr02, t i t a n i u m may form a range o f n o n - s t o i c h i o m e t i c o x i d e s i n a d d i t i o n t o the t h r e e s t o i c h i o m e t r i c T1O2 forms o f r u t i l e , a n a t a s e and b r o o k i t e . T h i s r a i s e s the p o s s i b i l i t y of having s e v e r a l " t i t a n i a " p i l l a r types, p a r t i c u l a r l y under r e d u c i n g r e a c t i o n c o n d i t i o n s c h a r a c t e r i s t i c o f many c a t a l y t i c p r o c e s s e s , and p r e s e n t s a major c h a l l e n g e t o p r e s e n t instrumental c a p a b i l i t y to s p e c i f i c a l l y i d e n t i f y the " r e a l p i l l a r " compositions. T h i s w i l l be an i s s u e o f some importance i n the p a t e n t a r e a i f some c o m p o s i t i o n s show n o v e l p r o p e r t i e s . The m o d i f i c a t i o n o f the AI13 s p e c i e s by s u b s t i t u t i o n o f s e c o n d a r y m e t a l i o n s i n t o the complex ( 3 1 ) . r a t h e r t h a n by a d d i t i o n p o l y m e r i z a t i o n ( 2 5 ) . o r p o s t i o n exchange ( 2 4 ) . as o r i g i n a l l y p r o p o s e d , adds a new and wide r a n g i n g v e r s a t i l i t y to the p i l l a r itself. These m a t e r i a l s may be e x p e c t e d to show n o t j u s t a d i f f e r e n t c a t a l y t i c r e a c t i v i t y , b u t changes i n the way the p i l l a r s

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19. VAUGHAN

Developments in Pillared

Interlayered

Clays

315

Possible (Cr) 3 and (Cr) 4 hydroxy complexes M,0

M,0

Cr HO

^ O H

H0

Μ,Ο

H0

t

t

;

"c> 0

Η Ο

Χ

Η

1 I

. O H

/

*P

\

I H,0



ÔH

I H^O

~

.

' \ I/

, " ··.

OM

f* -.

C

s \~

Ci^

OM H,O

^ O

\

A

1

/

OH

is"'

^ C . _

I ^·

Cr M0 ?

H0 7

F i g u r e 3. Models o f v a r i o u s oxy-hydroxy i n o r g a n i c polymers p o s s i b l y used to p i l l a r s e v e r a l l a y e r s t r u c t u r e s , i n c l u d i n g complexes o f T i ( 2 9 ) ; ( S i , A l ) (9,34); (Al,M)13(31); Z r ( 2 7 ) and Cr(39,40).

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

316

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE

may r e a c t w i t h the s h e e t s t r u c t u r e s . Preliminary evaluation of a s i m i l a r C r - A l - P I L C polymer by C a r r a d o e t a l . (32) i n d i c a t e s n o v e l reactivity. I n c o n t r a s t t o the case o f the m i c a - m o n t m o r i l l o n i t e PILC s u b s t i t u t e d w i t h N i i n the o c t a h e d r a l s h e e t ( 3 3 ) . the p i l l a r may now a l s o a c t as a r e s e r v o i r f o r c r e a t i n g c a t a l y t i c m e t a l c l u s t e r s i n the i n t e r l a y e r , l e a d i n g t o n o v e l and enhanced reactivities.

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2 +

A t t e n t i o n r e c e n t l y f o c u s e d on a wide range o f s i l i c a polymers (selsequisiloxanes) p i l l a r i n g montmorillonites ( 3 4 ) . and t h e s e can a l s o be A l ^ + s u b s t i t u t e d i n the manner d e m o n s t r a t e d by Hoebbel e t a l (9). T h i s l a r g e group o f p o t e n t i a l p i l l a r s a l s o o f f e r s the p o s s i b i l i t y o f c r e a t i n g i n t e r l a y e r s p a c i n g s comparable to those o b s e r v e d f o r T1O2. However, as i s w e l l known i n s i l i c a t e c h e m i s t r y , the s t a b i l i t y o f such m a t e r i a l s i n the p r e s e n c e o f water w i l l be r e l a t i v e l y low. T r a n s i t i o n m e t a l o x i d e p i l l a r s , f i r s t i n v e s t i g a t e d as i n t e r c a l a t e s by Yamanaka and B r i n d l e y (35.36). have r e c e i v e d renewed a t t e n t i o n by P i n n a v i a and co-workers (37.38) . who have c r e a t e d l a r g e s p a c i n g PILCs p r o p p e d by new t r a n s i t i o n m e t a l h y d r o x i d e c l u s t e r structures. I n t h e s e c a s e s , t r a n s i t i o n m e t a l s o l u t i o n s were t r e a t e d i n an manner analogous t o t h a t u s e d to make the AI13 polymer from A l s o l u t i o n s , y i e l d i n g s i m i l a r s i z e d 8-10Â s o l u t i o n s p e c i e s which p i l l a r the c l a y t o b a s a l s p a c i n g s o f o v e r 20Â. The s t r u c t u r e s o f the p i l l a r i n g s p e c i e s have n o t y e t been i d e n t i f i e d , b u t s e v e r a l p o l y m e r i c chromium h y d r o x i d e polymers have been p r o p o s e d (39.40). as shown i n F i g u r e 3. These a r e the f i r s t h i g h l y o r d e r e d , l a r g e s p a c i n g PILCs, h a v i n g i n t e r e s t i n g t r a n s i t i o n m e t a l c a t a l y s t p o s s i b i l i t i e s , i n t h a t they have h i g h v o i d volumes and m e t a l loadings. F i g u r e 3 summarizes the s t r u c t u r e s o f p o s s i b l e p i l l a r i n g s p e c i e s d e s c r i b e d above, b a s e d on known s t r u c t u r e s o f s p e c i e s p r e s e n t p r i o r t o a d d i t i o n o f the c l a y component. M u l t i p l e s o f t h e s e b a s i c u n i t s p r o b a b l y c o n s t i t u t e the a c t u a l p i l l a r s i n many c a s e s . C o a l e s c e n c e o f such c l u s t e r s i s p a r t i c u l a r l y e x p e c t e d d u r i n g c a l c i n a t i o n o r s e t t i n g o f the c l u s t e r t o form the p i l l a r , e s p e c i a l l y i n the p r e s e n c e o f low water p a r t i a l p r e s s u r e s . New

Sheet C o m p o s i t i o n s

H i t h e r t o the most s u c c e s s f u l p i l l a r i n g has been c a r r i e d out on s m e c t i t e type c l a y s . The o r i g i n a l r e s e a r c h on p i l l a r i n g i n c l u d e d s e v e r a l s e r i e s o f u n s u c c e s s f u l e x p e r i m e n t s on mica, v e r m i c u l i t e and the s h e e t s i l i c i c a c i d m i n e r a l s m a g a d i i t e , s i l h y d r i t e and kenyaite. R e c e n t l y renewed a t t e n t i o n has f o c u s e d on t h e s e and o t h e r a l t e r n a t i v e sheet s t r u c t u r e s . They i n c l u d e r e c t o r i t e , z i r c o n i u m p h o s p h a t e s , t e t r a s i l i c i c micas, h y d r o t a l c i t e s and s i l i c i c a c i d s , the g e n e r a l s t r u c t u r a l f e a t u r e s o f which a r e shown i n F i g u r e 4. R e c t o r i t e i s one o f an a l m o s t i n f i n i t e number o f randomly mixed l a y e r c l a y s (41). c o l l e c t i v e l y c a l l e d ' i l l i t e s ' , which i n c l u d e several other ordered i n t e r s t r a t i f i e d v a r i e t i e s , i n c l u d i n g c h l o r i t e , c o r r e n s i t e and a l l e v a r d i t e . R e c t o r i t e has the advantages o f a m i c a and a s m e c t i t e , i n t h a t a l t e r n a t e i n t e r l a y e r s are e x p a n d i n g and non-expanding. I t may be v i e w e d as an o r d e r e d s y n t h e t i c m i c a - m o n t m o r i l l o n i t e (SMM) i n the n o m e n c l a t u r e f a m i l i a r t o c a t a l y t i c

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

F i g u r e 4. Most p i l l a r i n g has been done w i t h s m e c t i t e c l a y s . T h i s f i g u r e shows a v a r i e t y o f new s h e e t s t r u c t u r e s r e c e n t l y p i l l a r e d w i t h v a r i o u s s p e c i e s shown i n F i g u r e 3, o r r e l a t e d polymers.

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chemists. The r e s u l t i n g double s h e e t between the p i l l a r e d expanded l a y e r s i n the PILC form, g i v e s the p i l l a r e d s t r u c t u r e enhanced s t a b i l i t y , which shows up as s u p e r i o r a c t i v i t y i n c a t a l y t i c r e a c t i o n s c a r r i e d out a f t e r c a t a l y s t d e a c t i v a t i o n ( 4 2 ) . Even though the t o t a l i n t e r l a y e r v o i d volume i s t h e o r e t i c a l l y o n l y h a l f t h a t e x p e c t e d o f a f u l l y expanded and p i l l a r e d s m e c t i t e , the enhanced d e a c t i v a t i o n s u r v i v a b i l i t y o f t h a t i n t e r l a y e r compensates f o r the s m a l l e r i n i t i a l i n t e r l a y e r volume. Comparative c a t a l y t i c a c t i v i t i e s , a f t e r s e v e r e d e a c t i v a t i o n , o f PILC and c o n v e n t i o n a l FCC c a t a l y s t s are shown i n F i g u r e 5, and d r a m a t i c a l l y demonstrate the improvements due t o the enhanced s h e e t s t a b i l i t y , and the p o t e n t i a l f o r t h e s e m a t e r i a l s i n FCC. The a d d i t i o n o f z e o l i t e t o such PILC c a t a l y s t s s h o u l d g i v e an even g r e a t e r enhancement i n a c t i v i t y , and added s e l e c t i v i t y c o n t r o l . These d a t a c l e a r l y demonstrate t h a t the p i l l a r may have v e r y h i g h s t a b i l i t y , and t h i s i s p r o b a b l y a n o t h e r case o f s t r o n g s h e e t - p i l l a r c o n n e c t i v i t y . I t seems h i g h l y improbable t h a t a n o n - " c r o s s l i n k e d " pillar would s u r v i v e the s t a t e d d e a c t i v a t i o n c o n d i t i o n s , and re-enforces the p r o p o s a l o f P l e e , e t a l . (22) t h a t c h e m i c a l r e a c t i o n between s h e e t and p i l l a r p l a y s a major r o l e i n PILC s t a b i l i z a t i o n and catalyst survivability. P l e e e t a l . f o c u s e d t h e i r a t t e n t i o n on b e i d e l l i t e , h a v i n g a h i g h t e t r a h e d r a l Al^+ component, which has not y e t been found i n major m i n e r a l a c c u m u l a t i o n s . R e c t o r i t e i s more abundant, b u t a g a i n o c c u r s i n s m a l l d e p o s i t s . The advantages shown by p i l l a r e d r e c t o r i t e w a r r a n t a r e - e v a l u a t i o n o f some o f the p i l l a r e d s y n t h e t i c m i c a - m o n t m o r i l l o n i t e s made by G r a n q u i s t and co-workers (e.g. 43), p r e v i o u s l y i n v e s t i g a t e d f o r h y d r o c r a c k i n g (44) and h y d r o i s o m e r i z a t i o n (45). and h a v i n g the advantage o f no i r o n o x i d e admixed w i t h the c l a y - a major problem w i t h most m i n e r a l clays. A l t h o u g h e a r l y attempts to p i l l a r m a g a d i i t e , s i l h y d r i t e and k e n y a i t e w i t h Α1χ3 and Zr^ polymers f a i l e d t o demonstrate i n t e r c a l a t i o n , r e c e n t advances i n the s t r u c t u r a l u n d e r s t a n d i n g o f t h e s e s h e e t s i l i c a s have shown t h a t some can be i n t e r c a l l a t e d ( 4 6 ) . These m a t e r i a l s r e p r e s e n t a l a r g e group o f r e a c t i v e hydroxylated s i l i c a s (47) o f p o t e n t i a l i n t e r e s t i n p i l l a r i n g r e a c t i o n s . I n many c a s e s the d e n s i t y o f r e a c t i v e " s u r f a c e Ό Η ' " groups i s such t h a t the d e n s i t y o f c r o s s l i n k i n g w i t h s p e c i e s such as s e l s e s q u i s i l o x a n e s w i l l produce f i l l e d i n t e r l a y e r s , and poor s o r p t i o n p r o p e r t i e s (48). P i l l a r i n g the s h e e t s i l i c i c a c i d s to produce open s t r u c t u r e s w i l l t h e r e f o r e depend on v e r y s e l e c t i v e p a r t i a l p i l l a r i n g , o r the c a p a b i l i t y o f p i l l a r i n g o n l y a few s t r u c t u r e s . One r e c e n t example using a synthetic s i l i c i c acid, of undisclosed structure, indicates s u r f a c e a r e a s i n the range o f 580 m^/gm, and p o r e s l a r g e enough to s o r b c y c l o h e x a n e i n c o p i o u s amounts ( 4 9 ) . The t e t r a s i l i c i c micas (TSM) are s y n t h e t i c expandable f l u o r o m i c a s (50) r e c e n t l y made a v a i l a b l e by Topy I n d u s t r i e s (Japan) (51). They can be e f f e c t i v e l y p i l l a r e d ( 5 2 ) . and show i n t e r e s t i n g c a t a l y t i c a c t i v i t y and enhanced s t a b i l i t y , a l t h o u g h n o t a t the l e v e l o f the r e c t o r i t e - P I L C . The c h e m i c a l and p h y s i c a l p r o p e r t i e s o f t h i s group o f m a t e r i a l s do not y e t seem to have been o p t i m i z e d , and p r o b a b l y r e q u i r e more e x t e n s i v e s y n t h e s i s work b e f o r e the f u l l p o t e n t i a l o f TSM can be r e a l i z e d i n p i l l a r e d systems.

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Developments in Pillared Interlayered Clays

Deactivation (hours @ 8 0 0 ° C , 1 0 0 % s t e a m , 1 atm.)

F i g u r e 5. A comparison o f c a t a l y t i c a c t i v i t i e s u s i n g a l i g h t c y c l e o i l o v e r d e a c t i v a t e d c a t a l y s t s b a s e d on p i l l a r e d m o n t m o r i l l o n i t e , p i l l a r e d r e c t o r i t e , NaY and an amorphous s i l i c a - a l u m i n a FCC.

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

319

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Z i r c o n i u m phosphates 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 as i n t e r c a l a t i o n h o s t s ( 5 3 ) . and the c a t a l y t i c p r o p e r t i e s o f some p i l l a r e d z i r c o n i u m phosphates has been r e v i e w e d by C l e a r f i e l d ( 5 4 ) . A r e c e n t S h e l l p a t e n t i n d i c a t e s t h a t t h e s e m a t e r i a l s can be p i l l a r e d w i t h s e l s e s q u i s i l o x a n e s ( 5 5 ) . and t h a t they o f f e r p o t e n t i a l as c a t a l y s t systems; opening up y e t another c l a s s o f i n o r g a n i c m i c r o p o r o u s m a t e r i a l s to p e t r o c h e m i c a l and p e t r o l e u m c a t a l y s i s . The s e v e r a l new c h e m i c a l and s t r u c t u r a l systems p r o p o s e d i n r e c e n t d i s c o v e r i e s a r e summarized i n Table I I , and g r e a t l y expand on the a v a i l a b l e v e r s a t i l i t y o f PILCs.

TABLE I I Pillar (Al(13-x) x) Ali3,(All3)x A l i 3 , Zr4 (Si0 ) 15À Spaces 26,29 Active P i l l a r 35,36,37 38 N o v e l Sheet 55 Novel Sheet/chain 49 ;

Our e x p e r i e n c e w i t h s e v e r a l k i n d s o f s h e e t s t r u c t u r e s i s t h a t the s m e c t i t e s a r e the most v e r s a t i l e h o s t m a t e r i a l s , and t h a t most o t h e r c l a s s e s a r e v e r y s p e c i f i c f o r the k i n d s o f s p e c i e s t h a t w i l l p i l l a r . Colloid-Clav

Systems

C o l l o i d - c l a y agglomerates have been r e c o g n i z e d f o r many y e a r s as o f f e r i n g n o v e l p r o p e r t i e s t h a t n e i t h e r a l o n e p o s s e s s . The s e m i - s y n t h e t i c f l u i d c r a c k i n g c a t a l y s t s o f the e a r l y 1950's were such m a t e r i a l s , c o m p r i s i n g v a r i o u s c l a y s i n a s i l i c a o r s i l i c a - a l u m i n a g e l or c o l l o i d matrix. S i m i l a r m a t e r i a l s a r e used as a m a t r i x f o r the c u r r e n t f a m i l i e s o f z e o l i t e promoted f l u i d c r a c k i n g catalysts. A l t h o u g h s e v e r a l such m a t e r i a l s have been 'lumped' w i t h PILCs, they i n f a c t r e p r e s e n t a s e p a r a t e f a m i l y o f m a t e r i a l s which a r e c h a r a c t e r i z e d by c r y s t a l l o g r a p h i c d i s o r d e r , a l t h o u g h they may i n some c a s e s show d i s t i n c t i v e sharp pore s i z e d i s t r i b u t i o n s . The l a t t e r r e s u l t from u n i f o r m i t y i n the c o l l o i d o r c l a y s i z e d i s t r i b u t i o n s used i n t h e i r p r e p a r a t i o n , r a t h e r t h a n from the s p e c i f i c o r d e r e d i n t e r l a y e r i n g o f each c l a y l a y e r w i t h a u n i f o r m molecular c o l l o i d e n t i t y . The t r a n s i t i o n from f u l l o r d e r t y p i c a l o f z e o l i t e s , to f u l l d i s o r d e r as i n the c o l l o i d c l a y agglomerate system, p a s s e s through the PILC which i n v a r i a b l y has some c h a r a c t e r i s t i c s o f b o t h - a m a j o r i t y o f m i c r o - p o r e s o f the former type t o g e t h e r w i t h a m i n o r i t y o f meso-pores c h a r a c t e r i s t i c o f the latter. S e v e r a l r e c e n t p a t e n t s f o c u s on the use o f l a r g e r c l u s t e r - c o l l o i d s p e c i e s o f the t r a s i t i o n m e t a l s to form m a t e r i a l s o f t h i s g e n e r a l form (e.g. 56,57). They c o n s t i t u t e an i m p o r t a n t c l a s s o f c a t a l y s t s , b u t do not f a l l w i t h i n the scope o f t h i s r e v i e w .

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

19. VAUGHAN

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Pillared

Sheet

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Interlayered Clays

321

Hydroxides

B r u c i t e , Mg(0H>2, c o m p r i s e s a s i n g l e s h e e t o f b r i d g e d o c t a h e d r a analogous t o t h e o c t a h e d r a l l a y e r i n t a l c . A l t h o u g h t h i s s h e e t has no n e t l a y e r c h a r g e , s u b s t i t u t i o n o f t r i v a l e n t m e t a l c a t i o n s f o r Mg2+ c r e a t e s a n e t p o s i t i v e charge on t h e s h e e t , and t h e r e s u l t a n t m a t e r i a l s exchange a n i o n s between t h e s h e e t s . Al^+ r e a d i l y s u b s t i t u t e s i n t o t h i s magnesium h y d r o x i d e s h e e t t o form a m i n e r a l c a l l e d h y d r o t a l c i t e ( 5 8 ) . and a l a r g e number o f t h e s e mixed hydroxides are r e a d i l y synthesized. S i m i l a r s u b s t i t u t e d materials o c c u r i n n a t u r e i n g r e a t d i v e r s i t y , i n some c a s e s c o m p r i s i n g important mineral ores (eg. t a k o v i t e , N 1 6 A I 2 ( O H ) I 6 C O 3 - a N i equivalent of hydrotalcite). Many s u c h m a t e r i a l s have been i n v e s t i g a t e d as i o n exchangers, and shown t o have s p e c i f i c s o r p t i o n p r o p e r t i e s i n forms expanded w i t h C O 3 - , C l " , CN" e t c . (60.61). Recent work by Woltermann (62) has shown t h a t i n some c a s e s v a r i o u s homopoly and h e t e r o p o l y - a c i d a n i o n s , i n c l u d i n g t h o s e h a v i n g t h e K e g g i n i o n s t r u c t u r e i d e n t i c a l t o t h e A I 1 3 polymer, c a n e f f e c t i v e l y p i l l a r t h e s e m a t e r i a l s . The t h e r m a l and h y d r o t h e r m a l p r o p e r t i e s o f t h e s e p r o d u c t s a r e poor compared w i t h c o n v e n t i o n a l c a t i o n p i l l a r e d s m e c t i t e s , and t h e y c o l l a p s e on d e h y d r o x y l a t i o n - u s u a l l y below about 400°C. However, t h e c o m p o s i t i o n a l d i v e r s i t y o f t h e p i l l a r s and t h e l a y e r s make them a c h e m i c a l l y v e r s a t i l e group o f p o t e n t i a l catalysts. The p i l l a r i n g r e a c t i o n s seem t o be f a r more r e s t r i c t i v e t h a n i n i t i a l l y p r o p o s e d by Woltermann, and i n many c a s e s may be p r i m a r i l y o f c o l l o i d - c l a y s h e e t random p a c k e d t y p e , p a r t i c u l a r l y as many o f t h e s y n t h e t i c h y d r o x i d e s form c r y s t a l s o f o n l y s e v e r a l h u n d r e d angstroms d i a m e t e r . However, t h e y comprise a v e r y l a r g e group o f m i c r o p o r o u s m a t e r i a l s , b o t h o r d e r e d and d i s o r d e r e d , and o f f e r many c a t a l y s t o p p o r t u n i t i e s . The c a t a l y s t l i t e r a t u r e c o n t a i n s many examples o f p r e c i p i t a t e d mixed m e t a l o x i d e s and h y d r o x i d e s o f "amorphous" s t r u c t u r e , and p r o b a b l y i n c l u d e s many m a t e r i a l s o f t h i s type. 2

Conclusions P i l l a r e d s h e e t s t r u c t u r e s c o n t i n u e t o expand t h e i r i n t e r e s t and d i v e r s i t y , w i t h major developments t a k i n g p l a c e i n i n c r e a s i n g t h e number o f b o t h p i l l a r and s h e e t c o m p o s i t i o n s . U n f o r t u n a t e l y the s p e c i f i c u n d e r s t a n d i n g and c h a r a c t e r i z a t i o n o f p i l l a r p r e c u r s o r s and PILC p r o d u c t s g r e a t l y l a g s t h e t e c h n o l o g i c a l " p u l l " f o r new m a t e r i a l s - a c l a s s i c c a t a l y s t s c e n a r i o . W i t h major i n t e r e s t now t u r n i n g t o r e l a t i v e l y r a r e c l a y s ( o r ones n o t a v a i l a b l e i n q u a n t i t y ) s u c h as b e i d e l l i t e and r e c t o r i t e , t h e time may be r i p e f o r a r e - e x a m i n a t i o n o f s p e c i f i c c l a y s y n t h e s i s methods, and d e d i c a t e d p r o j e c t s t o s y n t h e s i z e these k i n d s o f m a t e r i a l s . T e t r a s i l i c i c mica i s a r e c e n t s u c c e s s i n t h e s y n t h e s i s a r e a , w h i c h c o u l d be f o l l o w e d by o t h e r s . M a j o r o p p o r t u n i t i e s e x i s t f o r t h e a p p l i c a t i o n o f v a r i o u s s p e c t r o s c o p i c t e c h n i q u e s i n b o t h t h e s o l u t i o n and s o l i d s t a t e s , and m o d e l i n g s t u d i e s on t h e p r o d u c t s would f u r t h e r promote u n d e r s t a n d i n g and improvment i n PILCs h a v i n g a p p l i c a t i o n s as s o r b e n t s and catalysts.

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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