Characterization of the New Zeolite ECR-1 - ACS Publications

Chapter 28

Characterization of the New Zeolite ECR-1 1

D. E . W. Vaughan, M . E . Leonowicz , and K . G . Strohmaier

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Exxon Research and Engineering Company, Annandale, N J 08801

Results from diverse experimental methods were combined to arrive at structure proposals for ECR-1, a t y p i c a l example of a microcrystalline zeolite. The zeolite features a 12ring single channel, formed by a regular alternation of connected sheets of mordenite and mazzite. Crucial clues to the structure came from the HREM observation of a mazzite e p i t a x i a l overgrowth on an ECR-1 c r y s t a l . Electron and x-ray d i f f r a c t i o n , infra-red spectroscopy and synthesis phase relationships were essential additional data sources.

K n o w i n g t h e s t r u c t u r a l t o p o l o g y o f a new z e o l i t e a l l o w s one, by analogy w i t h t h e p r o p e r t i e s o f other e s t a b l i s h e d and w e l l r e s e a r c h e d z e o l i t e s , t o r e a d i l y assess potential applications f o r t h e new m a t e r i a l . However, r a r e l y do new s y n t h e t i c z e o l i t e s grow a s c r y s t a l s l a r g e enough t o be r a p i d l y a n a l y z e d b y s i n g l e c r y s t a l d i f f r a c t i o n methods. A l t h o u g h s i n g l e m i c r o - c r y s t a l a n a l y s e s ( 5 ) , i n d i c a t i n g a p o s s i b l e c l o s e r e l a t i o n s h i p between ECR-1 a n d m a z z i t e . However, t h e new z e o l i t e forms l o n g l a t h l i k e c r y s t a l s - a morphology v e r y s i m i l a r t o mordenite i n n a t u r a l and s y n t h e t i c o c c u r r e n c e s . Sorption

Properties

E x p e r i m e n t s t o o p t i m i z e t h e ECR-1 c r y s t a l l i z a t i o n r e s u l t e d i n a p p a r e n t l y p u r e ECR-1 on t h e b a s i s o f PXD a n d s c a n n i n g EM a n a l y s e s , b u t some s a m p l e s h a d v a r i a b l e hexane s o r p t i o n p r o p e r t i e s reminiscent o f large and small port mordenite, o f f r e t i t e o r g m e l i n i t e . V a r i a t i o n s i n hydrocarbon s o r p t i o n c a p a c i t i e s u s u a l l y i n d i c a t e blockage o f the channels i n these s t r u c t u r e s by f a u l t i n g , i n t e r g r o w t h s o f secondary c o e x i s t e n t s t r u c t u r e s (such as c h a b a z i t e i n g m e l i n i t e , o r e r i o n i t e i n o f f r e t i t e ) , d e t r i t a l g e l components o r r e s i d u a l c o k e f r o m t h e t e m p l a t e b u r n o f f p r o c e d u r e . I n optimum material, sorption i s o t h e r m s f o r l i n e a r h y d r o c a r b o n s were s i m i l a r t o t h o s e f o r o f f r e t i t e o r L (FIGURE 1) , h a v i n g a maximum n-hexane c a p a c i t y o f 8wt.%, a n d s i m i l a r b u t s l o w e r s o r p t i o n o f c y c l o - h e x a n e , i n d i c a t i n g a p o s s i b l e 10- o r 12r i n g channel system. T h e r m o g r a v i m e t r i c a n a l y s e s (FIGURE 2) showed t h a t t h e d i m e t h y l d i e t h y l ammonium t e m p l a t e was removed f r o m ECR-1 a t a b o u t 450°C a n d f r o m t h e ECR-4 f a u j a s i t e (11) s u p e r c a g e a t a b o u t 350°C. I n c o n t r a s t , TMA i s r e m o v e d f r o m f r o m t h e m a z z i t e 8 - r i n g g m e l i n i t e c a g e s a t a b o u t 560°C, a n d TEA(12) f r o m t h e L i n d e L 1 2 - r i n g c h a n n e l s a t 460°C, i n d i c a t i n g t h a t ECR-1 p r o b a b l y h a d a 1 0 - o r 1 2 - r i n g p o r e o p e n i n g . A f t e r p u r g i n g o f t h e t e m p l a t e f r o m optimum ECR-1 s a m p l e s , t h e z e o l i t e s o r b e d between 14 and 15% wt. o f w a t e r . Spectroscopic Analyses 2 9

Si-MASNMR s p e c t r a o f ECR-1 a r e c o m p l e x , a n d a t t e m p t s t o d e a l u m i n a t e ECR-1 were u n s u c c e s s f u l , making such i n f o r m a t i o n d i f f i c u l t t o i n t e r p r e t ( t h e ECR-1 m o d e l s t u r n o u t t o h a v e t e n d i f f e r e n t T atoms) . A t y p i c a l ECR-1 IR s p e c t r u m i s compared w i t h s p e c t r a f o r m a z z i t e a n d m o r d e n i t e i n FIGURE 3, a n d c o n t a i n s t w o s h a r p p e a k s a t 1210 a n d 1240 cm" , a r e g i o n p r e v i o u s l y a s s i g n e d t o 5 - r i n g e l e m e n t s ( 1 3 ) . M o r d e n i t e shows a peak a t 1220 c m " b u t m a z z i t e shows no a b s o r p t i o n i n t h i s r e g i o n , even t h o u g h t h e s t r u c t u r e c o n t a i n s 5 - r i n g s . ECR-1 a l s o h a s a 6 - r i n g peak a t 615 cm"" , i n t h e i d e n t i c a l p o s i t i o n t o mazzite, and mordenite has strong a b s o r p t i o n s a t 580 a n d 625 cm"" . These a n a l y s e s s t r e n g t h e n e d our o r i e n t a t i o n towards 5 - r i n g s t r u c t u r e s o f t h e mordenite t y p e . However, many p r o p o s e d t h e o r e t i c a l p o s s i b i l i t i e s i n t h i s s y s t e m (14, 15) f a i l e d t o g i v e t h e r e q u i r e d u n i t c e l l values. 1

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FIGURE 3: IR s p e c t r a f o r ECR-1, m o r d e n i t e and L i n d e Omega ( m a z z i t e ) , showing common framework v i b r a t i o n s , and a d i s t i n c t i v e 5 - r i n g d o u b l e t a t 1210 and 1240cm.-1 f o r ECR-1.


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X-ray and E l e c t r o n D i f f r a c t i o n A n a l y s i s Most i n i t i a l s y n t h e s e s o f ECR-1 were c o n t a m i n a t e d w i t h a n a l c i m e , a n d a c u r s o r y e v a l u a t i o n o f PXD p a t t e r n s showed a s i m i l a r i t y t o t h o s e o f m o r d e n i t e a n d m a z z i t e (FIGURE 4 ) , b u t w i t h d i s t i n c t i v e d i f f e r e n c e s . S c a n n i n g EM c l e a r l y showed o n l y one p h a s e i n a d d i t i o n t o a n a l c i m e , a n d l a t e r s a m p l e s were c l e a r l y monophasic. I n both cases t h e data r e s i s t e d i n d e x i n g by c o n v e n t i o n a l c o m p u t e r t e c h n i q u e s , p r o b a b l y due t o t h e combined e f f e c t o f d i s p a r a t e c e l l l e n g t h s a n d s y s t e m a t i c absences i n t h e p a t t e r n s ( v i d e i n f r a ) . The u n i t cell g e o m e t r y was e l u c i d a t e d w i t h s e l e c t e d area electron d i f f r a c t i o n experiments, although a complete d e s c r i p t i o n o f t h e l a t t i c e f r o m p a t t e r n s t i l t e d a b o u t a common r e c i p r o c a l l a t t i c e v e c t o r c o u l d not be o b t a i n e d because o f i n s t r u m e n t a l c o n s t r a i n t s . Approximate c e l l constants obtained from electron diffraction patterns o f t h ethree principal r e c i p r o c a l l a t t i c e p r o j e c t i o n s , shown i n FIGURE 5 were u s e d t o i n d e x t h e PXD p a t t e r n w i t h a p r i m i t i v e o r t h o r h o m b i c u n i t c e l l having least-squares r e f i n e d values o f a=7.310(4)A; b = 1 8 . 1 4 4 ( 6 ) A ; £=26.31(1)A. F u r t h e r m o r e , t h e hkO p r o j e c t i o n displayed a systematic e x t i n c t i o n c o n d i t i o n h+k=2n+l, i m p l y i n g an n - g l i d e p l a n e n o r m a l t o t h e 26A a x i s . Pmmn i s t h e only c e n t r o - s y m m e t r i c space group c o n s i s t e n t w i t h these d i f f r a c t i o n data. Unit c e l l constants play a c r i t i c a l role i n z e o l i t e s t r u c t u r e d e t e r m i n a t i o n w i t h model b u i l d i n g t e c h n i q u e s . D i f f e r e n t t h r e e d i m e n s i o n a l f r a m e w o r k s o f t e n c o n t a i n common s t r u c t u r a l s u b u n i t s , the presence o f which are i n d i c a t e d by "magic number" l a t t i c e c o n s t a n t s , s u c h as 5A, 7.5A a n d lOA. Such d i v e r s e s t r u c t u r e t y p e s a s o f f r e t i t e , m a z z i t e , L i n d e L, m o r d e n i t e a n d t h e h y p o t h e t i c a l omega s t r u c t u r e a l l c o n t a i n c o r r u g a t e d t h r e e t e t r a h e d r a c h a i n s , r e p e a t i n g e v e r y 7.5A, and i n t e r c o n n e c t e d t o form 8 - r i n g p o r e s normal t o t h e c h a i n . Thus, t h e p r e s e n c e o f a 7.5A u n i t c e l l c o n s t a n t i n an unknown s t r u c t u r e s u g g e s t s t h i s c a n d i d a t e c o n n e c t i v i t y . The l a s t f o u r s t r u c t u r e s l i s t e d above a l s o h a v e a n 18A c e l l c o n s t a n t , b u t long repeat distances a l l o w s o many p e r m u t a t i o n s i n t e t r a h e d r a l c o n n e c t i v i t y that s i m i l a r i t i e s a r e tenuous. However, some a s s u m p t i o n s must b e made a b o u t t h e t o p o l o g y o f c o m p l e x , unknown s t r u c t u r e s t o r e d u c e t h e d e g r e e s o f freedom t o manageable p r o p o r t i o n s i n model b u i l d i n g . S i m i l a r i t i e s i n u n i t c e l l v a l u e s and symmetries b e t w e e n known a n d unknown s t r u c t u r e s may i n d i c a t e a s t a r t i n g p o i n t . F o r ECR-1 t h e combined data d e s c r i b e d i n the p r e v i o u s s e c t i o n s i n d i c a t e d that mordenite, a s t r u c t u r e with a centered orthorhombic l a t t i c e o f d i m e n s i o n s 18A x 20A x 7. 5A, w o u l d b e a g o o d candidate s t r u c t u r e . Unfortunately, e f f o r t s t o interconnect 7.5A x 18A m o r d e n i t e s h e e t s w i t h i n t h e c o n s t r a i n t s o f Pmmn, w h i l s t s t r e t c h i n g t h e r e p e a t d i s t a n c e f r o m 20A t o 26A were u n s u c c e s s f u l . M o d e l s b a s e d on m o d i f i c a t i o n s o f o t h e r r e l a t e d frameworks were e q u a l l y d i s a p p o i n t i n g .


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High resolution transmission electron microscopy (HRTEM) l a t t i c e i m a g i n g h a s r e c e n t l y been d e m o n s t r a t e d as one o f t h e most p o w e r f u l methods o f s o l v i n g unknown z e o l i t e s t r u c t u r e s ( 1 6 ) . The f a i l u r e t o s o l v e t h e ECR-1 s t r u c t u r e by a l t e r n a t e m e t h o d s c l e a r l y i n d i c a t e d t h a t HRTEM was t h e p r e f e r r e d approach, p a r t i c u l a r l y as t h e s o r p t i o n data i n d i c a t e d t h e presence o f l a r g e channels and t h e p o s s i b i l i t y o f s t r u c t u r a l f a u l t i n g . U n f o r t u n a t e l y , s a m p l e s t h i n enough f o r i m a g i n g a l o n g t h e l a t h a x i s , t h e d i r e c t i o n o f any 10- o r 12-ring channels i n t h e m a t e r i a l , r e q u i r e d ultramicrotomy o f embedded c r y s t a l l i t e s . M i c r o s c o p y e f f o r t s were a l s o hampered by t h e s e n s i t i v i t y o f t h e z e o l i t e s t o damage b y t h e 120KV beam o f t h e P h i l i p s 420ST TEM t h a t was a v a i l a b l e f o r t h e i m a g i n g e x p e r i m e n t s ; l o w d o s e t e c h n i q u e s were e m p l o y e d t o o b t a i n l a t t i c e images o f ECR-1 w i t h t h i s i n s t r u m e n t . L a t t i c e i m a g e s t a k e n n o r m a l t o t h e l a t h a x i s were s t r u c t u r a l l y uninformative, but r e g u l a r i t y i n the fringe p a t t e r n s s u g g e s t e d t h a t f a u l t i n g was n o t an i m p o r t a n t mechanism i n c h a n n e l b l o c k a g e ( FIGURE 6 ) . The d e f i c i e n c i e s i n s o r p t i o n p r o p e r t i e s t h e r e f o r e p r o b a b l y r e s u l t from a combination of the high aspect r a t i o of t h e l a t h c r y s t a l s ( i e . v e r y l o n g channels) and d e t r i t a l m a t e r i a l o c c l u d e d i n t h e c h a n n e l s . Images a l o n g t h e l a t h a x i s were much more r e v e a l i n g , s h o w i n g a r e g u l a r a r r a y o f c i r c u l a r c h a n n e l s . One s u c h image showed a t w i n mode o f ECR-1 t h a t p r o v i d e d k e y s t r u c t u r a l i n f o r m a t i o n . I n t h e m i c r o g r a p h shown i n FIGURE 7 a h e x a g o n a l p h a s e i s o b s e r v e d t o s y n t a c t i c a l l y o v e r g r o w on an ECR-1 l a t h , s h a r i n g a t w i n p l a n e d e f i n e d b y t h e 7 . 5A x 18A d i r e c t i o n s . D e t a i l e d examination of the contrast patterns i n t h i s r e g i o n o f t h e micrograph r e v e a l s t h a t t h e l a y e r used t o c o n s t r u c t t h e h e x a g o n a l p h a s e i s a l s o p r e s e n t i n t h e ECR-1 region of the c r y s t a l l i t e , but with a d i f f e r e n t layer i n t e r p o s e d t o e x t e n d t h e p e r i o d i c i t y t o 26A. T h i s s u g g e s t s t h a t ECR-1 a n d i t s h e x a g o n a l o v e r g r o w t h phase s h a r e a common s t r u c t u r a l layer. Although naive i n t e r p r e t a t i o n of l a t t i c e images c a n be m i s l e a d i n g b e c a u s e c o n t r a s t l e v e l s a r e s t r o n g l y a f f e c t e d by sample t h i c k n e s s and o r i e n t a t i o n and by microscope defocus ( w e l l i l l u s t r a t e d by t h e examples i n ( 1 7 ) ) ; s u c h a f f e c t s s h o u l d be m i n i m a l i n t h e s h o r t d i s t a n c e s over which t h i s overgrowth extends. Three w e l l known z e o l i t e s t r u c t u r e t y p e s - L i n d e L ( 1 8 ) , m a z z i t e (19) a n d t h e h y p o t h e t i c a l o m e g a - s t r u c t u r e ( 2 0 ) - have e s s e n t i a l l y i d e n t i c a l h e x a g o n a l l a t t i c e c o n s t a n t s w i t h a=18A and £=7 . 5A, b u t o n l y m a z z i t e h a s a 7. 5A x 18A l a y e r t h a t c o n t a i n s an n - g l i d e p l a n e o p e r a t o r ; t h e t h i c k n e s s o f t h i s l a y e r i s -15.5A. I f t h e h e x a g o n a l o v e r g r o w t h i s m a z z i t e , t h e e l e c t r o n micrograph interpretation i m p l i e s t h a t ECR-1 c o m p r i s e s m a z z i t e s h e e t s i n t e r c o n n e c t e d w i t h some o t h e r s u b u n i t t o f o r m t h e o b s e r v e d 2 6A r e p e a t d i s t a n c e . The


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F I G U R E 6: L a t t i c e i m a g e o f t h e c r y s t a l s i d e v i e w a l o n g t h e ECR-1 l a t h s ( O i l ) , showing a high degree o f r e g u l a r i t y , a n d no evidence o f d e f e c t s a s s o c i a t e d w i t h channel blockage.


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FIGURE 7: L a t t i c e image o f t h e s e c t i o n n o r m a l t o t h e l a t h l e n g t h , showing c i r c u l a r f e a t u r e s e q u a t e d w i t h 12r i n g c h a n n e l s i n E C R - 1 ( a ) , a t r a n s i t i o n zone c o r r e s p o n d i n g t o a m a z z i t e t w i n p l a n e ( c ) , and a s u r f a c e o v e r g r o w t h o f m a z z i t e ( b ) showing t h e c h a r a c t e r i s t i c hexagonal arrangements o f c h a n n e l s .


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s t r u c t u r e model m a t e r i a l i z e d w i t h t h e r e c o g n i t i o n that m o r d e n i t e h a s a -10.5A t h i c k ( 26A - 15.5A ) , 7 . 5A x 18A sheet c o n t a i n i n g a n n _ - g l i d e p l a n e , a n d t h a t i t c a n be interposed with t h e mazzite sheet while simultaneously fulfilling t h e s y m m e t r y r e q u i r e m e n t s o f Pmmn a n d t h e c o n s t r a i n t s o f three d i m e n s i o n a l t e t r a h e d r a l network bonding; the i n t e r c o n n e c t i o n o f mazzite and mordenite sheets i s i l l u s t r a t e d i n FIGURE 8. Even w i t h i n t h e u n i t c e l l a n d symmetry c o n s t r a i n t s o f t h i s s y s t e m , t h e r e a r e two ways t o i n t e r c o n n e c t m a z z i t e a n d m o r d e n i t e s h e e t s i n t h r e e d i m e n s i o n s - one r e l a t e d t o t h e o t h e r b y a s h i f t o f aV2. D i f f e r e n t i a t i o n o f t h e two models w i l l be b e s t r e s o l v e d b y f u l l R i e t v e l d r e f i n e m e n t o f t h e observed data. This s i t u a t i o n o f several r e l a t e d structures h a v i n g t h e same t w o d i m e n s i o n a l p r o j e c t i o n s b u t d i f f e r e n t three dimensional connectivity i s common i n z e o l i t e s t r u c t u r a l c h e m i s t r y ( e g . m a z z i t e - omega; s e v e r a l members o f t h e ABC-6 f a m i l y o f s t r u c t u r e s ) . Structural

Modelling

The o b j e c t i v e o f s t r u c t u r e m o d e l l i n g i s t h e i n v e n t i o n of a z e o l i t e s t r u c t u r e having a t h e o r e t i c a l x-ray d i f f r a c t i o n p a t t e r n (21) t h a t matches t h e PXD s p e c t r u m f o r t h e unknown experimental m a t e r i a l . A short-cut i s t o use a p r e v i o u s l y g e n e r a t e d s o u r c e o f PXD p a t t e r n s f o r t h e many t h e o r e t i c a l s t r u c t u r e s a l r e a d y d e s c r i b e d i n t h e l i t e r a t u r e (eg. s e e 2 2 ) . A l t h o u g h s u c h a v a l u a b l e d a t a bank h a s b e e n p r o p o s e d b y Smith (23), i t i s n o t y e t funded o r a v a i l a b l e , except i n p a r t i a l f o r m w i t h i n a few i n d i v i d u a l companies a n d u n i v e r s i t y departments. To s u b s t a n t i a t e a p r o p o s e d model structure, the e x p e r i m e n t a l PXD p a t t e r n must compare f a v o r a b l y w i t h t h e computer g e n e r a t e d p a t t e r n f o r t h e model. R e g a r d l e s s o f t h e s o u r c e o f t h e model, a c c u r a t e atom p o s i t i o n s a r e r e q u i r e d i n order t o c a l c u l a t e accurate u n i t c e l l values, which a r e t h e s t a r t i n g p o i n t f o r g e n e r a t i n g t h e PXD p a t t e r n . I n t h e c a s e o f z e o l i t e f r a m e w o r k a t o m s , s u c h d a t a c a n be o b t a i n e d from distance l e a s t squares (DLS) r e f i n e m e n t o f t h e m o d e l s t r u c t u r e ( 2 4 ) . The c o m p a t i b i l i t y o f t h e u n i t c e l l v a l u e s o f t h e p r o p o s e d framework w i t h t h e o b s e r v e d e x p e r i m e n t a l v a l u e s , i s a good i n d i c a t o r o f model v a l i d i t y . W e i g h t e d mean r e s i d u a l v a l u e s l e s s t h a n 2% a r e c o n s i d e r e d i n d i c a t i o n s o f good a g r e e m e n t s w i t h u n i t c e l l a n d symmetry c o n s t r a i n t s ( 2 5 ) - a minimum r e q u i r e m e n t f o r a v a l i d s t r u c t u r e m o d e l . F i g u r e - o f m e r i t f o r ECR-1 models a r e l o w e r t h a n t h i s t h r e s h o l d . Once t h e r e f i n e d atom c o o r d i n a t e s are available, c a l c u l a t i o n o f t h e powder p a t t e r n i s s t r a i g h t - f o r e w o r d using POW-10 (21) . However, a p a t t e r n s o c a l c u l a t e d o n l y contains c o n t r i b u t i o n s f r o m framework atoms - u s u a l l y t h e S i 0 2 f o r m and does n o t i n c l u d e c o n t r i b u t i o n s f r o m c a t i o n s a n d w a t e r


28.

VAUGHAN ETAL.


315

FIGURE 8: M o d e l s h o w i n g t h e ECR-1 c o n n e c t i v i t y between 10.5A s h e e t s o f m o r d e n i t e (MOR) and 15. 5A s h e e t s o f mazz i t e (MAZ). The two p o s s i b l e s t r u c t u r e s a r e r e l a t e d by a s h i f t o f a/2 i n t h e marked p l a n e s n o r m a l t o t h e page.


316


7-j

DEGREES

FIGURE 9: A comparison o f t h e experimental x-ray d i f f r a c t i o n p a t t e r n f o r c a l c i n e d ammonium E C R - 1 ( a ) , w i t h t h e c a l c u l a t e d p a t t e r n s f o r Model 1 (b) ( 5 - r i n g c o n n e c t i v i t y ) and Model 2 ( c ) ( 4 & 6 - r i n g c o n n e c t i v i t y ) i n t h e r a n g e 12° t o 14° 20.


28.

VAUGHAN ET AL.


317

m o l e c u l e s ( i n a n y c a s e t h e p o s i t i o n s o f t h e s e w o u l d n o t be known a t t h i s s t a g e o f t h e m o d e l l i n g ) . The a f f e c t o f t h e s e l a t t e r c o m p o n e n t s may s u b s t a n t i a l l y a l t e r t h e r e l a t i v e i n t e n s i t y v a l u e s o f the peaks i n t h e spectrum. I d e a l l y , t h e agreement between model and e x p e r i m e n t a l PXD p a t t e r n s c a n be o p t i m i z e d by u s i n g h i g h l y d e a l u m i n a t e d z e o l i t e samples t h a t are l a r g e l y c a t i o n and water f r e e . U n f o r t u n a t e l y , attempts t o h i g h l y d e a l u m i n a t e ECR-1 u s i n g c o n v e n t i o n a l t e c h n i q u e s were n o t s u c c e s s f u l a n d t h e b e s t a v a i l a b l e m a t e r i a l s were hydrogen e x c h a n g e d s a m p l e s . ( I n f a c t few l o w S i / A l r a t i o z e o l i t e s c a n be a s e f f e c t i v e l y d e a l u m i n a t e d a s c a n t h e open f a u j a s i t e s t r u c t u r e . ) A c o m p a r i s o n o f PXD p a t t e r n s f o r h y d r a t e d a n d d e h y d r a t e d ECR-1 show m a j o r i n t e n s i t y d i f f e r e n c e s , r e f l e c t i n g t h e p r o n o u n c e d e f f e c t o f w a t e r on t h e peak i n t e n s i t i e s . I n v i e w o f t h i s , o n e d o e s n o t e x p e c t good m a t c h i n g o f peak i n t e n s i t i e s , b u t t h e peak p o s i t i o n s do match i n t h e ECR-1 system. Despite these problems we h a v e sought t o d i f f e r e n t i a t e t h e t w o p o s s i b l e ECR-1 m o d e l s . DLS r e f i n e d c o o r d i n a t e s were u s e d t o c a l c u l a t e X - r a y powder p a t t e r n s f o r t h e t w o ECR-1 m o d e l s . T h e s e a r e v e r y s i m i l a r , b u t a d i f f e r e n c e i n t h e peak i n t e n s i t i e s i n t h e r e g i o n 12°-14° 20 i s a b a s i s f o r d i f f e r e n t i a t i n g t h e t w o s t r u c t u r e m o d e l s . As shown i n FIGURE 9, t h e e x p e r i m e n t a l p a t t e r n f o r a c a l c i n e d ammonium e x c h a n g e d ECR-1 i n t h i s r e g i o n matches t h e 4 - 6 - r i n g s h e e t i n t e r c o n n e c t i v i t y model p a t t e r n (Model 2) b e t t e r t h a n t h e 5 - r i n g c o n n e c t i v i t y model p a t t e r n (Model 1) a n d i s t h e b a s i s f o r f a v o r i n g M o d e l 2 a t t h i s t i m e (a m i x t u r e o f b o t h s t r u c t u r e s w o u l d p r o b a b l y g i v e an optimum m a t c h ) . O n l y a f u l l r e f i n e m e n t o f t h e s t r u c t u r e , i n c l u d i n g c o n t r i b u t i o n s from framework a n d non-framework atoms, w o u l d be e x p e c t e d t o g i v e good agreement i n b o t h l i n e p o s i t i o n a n d i n t e n s i t y d a t a , and d e f i n i t i v e l y r e s o l v e t h e s p e c i f i c ECR-1 model. Conclusions D e s p i t e many a d v a n c e s i n a n a l y t i c a l methods i n r e c e n t years, the s t r u c t u r a l c h a r a c t e r i z a t i o n o f materials that only o c c u r a s m i c r o c r y s t a l s l e s s t h a n a b o u t 30|l i n d i a m e t e r remains d i f f i c u l t and l a b o r i o u s . High r e s o l u t i o n e l e c t r o n m i c r o s c o p y i n t h e l a t t i c e i m a g i n g mode i s b y f a r t h e most powerful t o o l i n g i v i n g t h e d i r e c t evidence o f s t r u c t u r a l details essential f o rmodelling clues, a s h a s been demonstrated i n t h e cases o f recent z e o l i t e structure s o l u t i o n s o f t h e t a - l / Z S M - 2 3 (26) a n d b e t a ( 2 7 ) , i n a d d i t i o n t o ECR-1. X - r a y d i f f r a c t i o n methods p r o v i d e t h e e s s e n t i a l c o n f i r m a t o r y d a t a , and s o r p t i o n m o l e c u l a r p r o b i n g and v a r i o u s w e l l e s t a b l i s h e d s p e c t r o s c o p i c methods a r e u s e f u l a n c i l l a r y tools.

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Received July 1, 1989


Characterization of the New Zeolite ECR-1 - ACS Publications

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