Perspectives in Molecular Sieve Science - American Chemical Society

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Determining the Structure of Molecular Sieve Materials Using High-Resolution Powder Data J. M. Bennett Union Carbide Corporation, Old Saw M i l l River Road, Tarrytown, N Y 10591

Molecular sieve compounds are a class of crystalline solids which because of their porous nature have wide uses for catalytic and adsorption processes. The size and shape of these pore openings range from small six ring channels which will only adsorb water to 18-ring channels which are 1.2nm across. The determination of the framework topology which yields both the pore size and shape is critical for understanding and predicting uses for these materials. Single crystal x-ray techniques have been used to determine the topology of over 60 different microporous materials. Unfortunately many molecular sieve materials are synthesized with crystals that are too small to be used with single crystal techniques, and powder techniques have to be used. The framework topologies of the two most widely used molecular sieve materials, type A and type Y zeolites, were first determined from a combination of powder and single crystal techniques in the 1950's by scientists at Union Carbide Corporation. Since then only a relatively few new framework topologies have been determined using powder data. Most are determined from single crystal data using very small crystals. Within the last few years high resolution data collected either from synchrotron or neutron sources has allowed new structures to be solved from powder samples. Powder data from these new sources are superior to that collected from a conventional x-ray source both in resolution and peak to background ratio; these improvements in data quality have the capability of making the determination of new molecular sieve topologies easier but not necessarily routine. Synchrotron radiation has possible future capabilities of being used to collect data from crystals that are only a few microns in size. At that time most newly synthesized molecular sieve materials will be solved by single crystal techniques instead of powder techniques. At the present time an acceptable structure refinement can be obtained from a crystal with a 0097-6156/88/0368-0162$06.00/0 © 1988 American Chemical Society

Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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volume g r e a t e r than 40,000 c u b i c microns u s i n g a s e a l e d x-ray tube. The t o p o l o g y o f s i l i c a l i t e was f i r s t d e t e r m i n e d u s i n g a twinned crystal roughly 20x20x70 microns in size. With a c r y s t a l of t h i s s i z e , data c o l l e c t i o n i s measured in weeks, not hours and b e c a u s e the m a t e r i a l c o n t a i n s o n l y l i g h t a t o m s w h i c h do n o t d i f f r a c t s t r o n g l y , t h e d a t a c o l l e c t i o n s t a t i s t i c s are poor. If the crystals are l a r g e e n o u g h to o b t a i n an adequate s t r u e t u r e refinement , then s i n g l e c r y s t a 1 techniques should s t i l l be used in preference to powder techniques. DISCUSSION. There are many steps in the determination o f an u n k n o w n f r a m e w o r k t o p o l o g y f r o m d i f f r a c t i o n d a t a , and e s p e c i a l l y with p o w d e r d a t a e a c h s t e p has many d i f f i c u l t i e s . The s t e p s c a n be s u m m a r i z e d as follows:

1. 2. 3. 4.

Collection Determinati Determinati Refinement

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the data. of the c e l l dimensions and space group. of the c o r r e c t t r i a l model. the d a t a and s o l u t i o n of the structure.

Usually e a c h s t e p m u s t be s u c c e s s f u l l y a c h i e v e d before t h e n e x t c a n be s t a r t e d . When t h e p o w d e r p a t t e r n o f the new m a t e r i a l matches that of a p r e v i o u s l y s i m u l a t e d h y p o t h e t i c a l m a t e r i a l i t i s p o s s i b l e t o b y p a s s s t a g e 2 a n d go directly to s t a g e 3. T h e s e s t e p s a r e t h e same f o r b o t h s i n g l e c r y s t a l and powder d a t a d e t e r m i n a t i o n of a s t r u c t u r e , but the techniques used with powder data are f a r l e s s f o r m a l i z e d . For example, w i t h s i n g l e c r y s t a l s a s u i t a b l e c r y s t a l i s chosen, mounted on one or more cameras and the u n i t c e l l d i m e n s i o n s and space group determined. With a powder sample t h i s information has to be d e r i v e d e i t h e r from three d i m e n s i o n a l d e c o n v o l u t i o n of t h e one d i m e n s i o n a l i n f o r m a t i o n c o n t a i n e d i n a p o w d e r pattern o r r e c o n s t r u c t e d f r o m i n f o r m a t i o n o b t a i n e d f r o m many d i f f e r e n t crystals using electron diffraction techniques. This simple step with single crystal d a t a has t h e r e f o r e become c o m p l e x w i t h powder d a t a . The d e t e r m i n a t i o n o f the s t r u c t u r e of a molecular sieve material using single crystal techniques i s v e r y i n v o l v e d and t e d i o u s , and i t i s e v e n more so w i t h powder data. However t h e r e s u l t s s u p p l y i n f o r m a t i o n t h a t c a n n o t be o b t a i n e d u s i n g any other technique. C O L L E C T I O N OF D A T A . I t s h o u l d b e e m p h a s i z e d t h a t t h e c o r r e c t c h o i c e and p r e p a r a t i o n of the sample combined with careful collection of d a t a w i l l g r e a t l y a i d i n the t o t a l p r o c e s s . It i s e x t r e m e l y i m p o r t a n t t h a t the sample i s pure, or i f not, that a l l the impurity phases are known. F i g u r e 1 shows a comparison of data collected from a typical 1970's d i f f r a c t o m e t e r , a modern computer c o n t r o l l e d d i f f r a c t o m e t e r and a high resolution diffractometer using synchrotron radiation at the National Synchrotron Light Source. The excellent resolution and high peak to background ratio (typically 1000:1) o b t a i n e d from the s y n c h r o t r o n d a t a enable v e r y w e a k p e a k s t o be e a s i l y observed.


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Figure 1: A comparison o f data c o l l e c t e d from typical diffTactometers: ( a ) a 1970's d i f f r a c t o m e t e r , ( b ) a m o d e r n computer c o n t r o l l e d d i f f r a c t o m e t e r and (c) a high r e s o l u t i o n diffTactometer using synchrotron radiation. Note: T r a c e c has been s c a l e d to s i m u l a t e data c o l l e c t e d w i t h copper radiati o n .


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If the m a t e r i a l under examination has an unknown f r a m e w o r k t o p o l o g y a n d t h e r e f o r e u n k n o w n c e l l d i m e n s i o n s , weak peaks in a d i f f r a c t i o n pattern could come e i t h e r f r o m an i m p u r i t y phase or from the sample. The p r e s e n c e o f one peak from an i m p u r i t y p h a s e c a n p r e v e n t t h e c r u c i a l d e t e r m i n a t i o n of the c e l l d i m e n s i o n s i f computer i n d e x i n g programs a r e used routinely without sequential testing for possible stray diffractions. The state of the sample must also be considered. S i n c e most molecular sieve m a t e r i a l s are also adsorbents, the atmosphere around the sample must be controlled to prevent changes i n sample hydration with humidity changes. This s o r p t i o n / d e s o r p t i o n process results i n a c o m p o s i t i o n a l change of the sample d u r i n g d a t a c o l l e c t i o n t h a t w i l l h i n d e r the m o d e l l i n g of the n o n - f r a m e w o r k d e n s i t y . It i s p r e f e r a b l e to d e t e r m i n e t h e t o p o l o g y o f a new phase u s i n g an a n h y d r o u s a n d , i f possible, a cation free sample. The collection of high resolution data is partially machine dependent, however c e r t a i n g e n e r a l i z a t i o n s can be made. I t i s a d v a n t a g e o u s t o u s e t h e f o c u s s i n g p r o p e r t i e s o f a monochrornater such t h a t f o r use with computer autoindexing programs, the c o n d i t i o n s are s e t to y i e l d the b e s t a n g u l a r r e s o l u t i o n a t l o w two t h e t a v a l u e s . F o r d a t a collection the c o n d i t i o n s a r e c h a n g e d so t h a t the b e s t a n g u l a r r e s o l u t i o n i s o b t a i n e d a t h i g h two t h e t a v a l u e s w h e r e o v e r l a p o f the peaks will be the greatest. I t must be r e m e m b e r e d t h a t p a r t i c l e s i z e a f f e c t s the r e s o l u t i o n , and i t is advisable to have crystallites larger than one m i c r o n . Data are c o l l e c t e d i n the usual way by step-scanning at appropriate i n t e r v a l s ; however, b e c a u s e of the e x t r e m e l y s m a l l d i v e r g e n c e o f t h e i n c i d e n t beam, t h e s a m p l e i s u s u a l l y r o t a t e d i f i t i s mounted in a capillary or o s c i l l a t e d i f i t i s a f l a t plate mount to correctly average over a large number of crystallites. The g e o m e t r y o f the d i f f r a c t o m e t e r , t h e t y p e o f d e t e c t o r a n d t h e o p e r a t i n g c o n d i t i o n s f o r t h e i r r a d i a t i n g beam affect t h e maximum r e s o l u t i o n , t h e p e a k t o b a c k g r o u n d r a t i o s a n d t h e maximum i n t e n s i t y t h a t c a n be obtained. A l l these h a v e t o be c o n s i d e r e d i n d e t e r m i n i n g t h e m o s t e f f e c t i v e way t o c o l l e c t the d a t a . I f c a r e i s taken both w i t h the choice and preparation of the sample and with the data collection t e c h n i q u e , t h e n many h o u r s o f f r u s t r a t i o n c a n be s a v e d i n the f i n a l s t a g e s of data p r o c e s s i n g . D E T E R M I N A T I O N OF THE C E L L D I M E N S I O N S AND THE S P A C E GROUP. The best means for determining the cell dimensions and the p o s s i b l e space group uses s e l e c t e d area e l e c t r o n d i f f r a c t i o n techniques; this is equivalent to u s i n g single crystal techniques on a powder s a m p l e . The o t h e r p o s s i b i l i t y i s to use e i t h e r manual or computer indexing processes. Manual indexing techniques a r e u s u a l l y l i m i t e d to m a t e r i a l s h a v i n g cubic or hexagonal symmetry even though lower symmetry materials were o f t e n c o r r e c t l y i n d e x e d u s i n g Bunn c h a r t s i n the e a r l y days of s t r u c t u r a l studies. One difficulty with using computer i n d e x i n g programs is obtaining sufficient a c c u r a c y w i t h t h e low a n g l e peak p o s i t i o n s . Typically with synchrotron data these p p a k p o s i t i o n s c a n be d e t e r m i n e d to b e t t e r t h a n 0 . 0 0 5 ° 29. C o m p u t e r i n d e x i n g p r o g r a m s c a n be u s e d


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w i t h almost any symmetry, however a l l programs w i l l u s u a l l y s u p p l y s e v e r a l d i f f e r e n t a n s w e r s . T e s t s can be c a r r i e d out to check the v a l i d i t y of each s o l u t i o n ; u n f o r t u n a t e l y often several solutions appear equally probable. Thus the crystallographer is left with a choice between several p o s s i b l e u n i t c e l l d i m e n s i o n s and i s o f t e n f o r c e d to decide between them u s i n g i n t u i t i o n a n d / o r l u c k . Even when o n l y one u n i t c e l l a p p e a r s p r o b a b l e i t may not be r e c o g n i z e d t h a t the c e l l has h i g h e r symmetry. A l g o r i t h m s are a v a i l a b l e t h a t can a i d i n r e c o g n i z i n g that the c e i l has h i g h e r symmetry, and s h o u l d be used to augment the p r o c e e d u r e s p r e s e n t i n the a u t o i n d e x i n g p r o g r a m s . Most m o l e c u l a r s i e v e m a t e r i a l s have h i g h symmetry; i n f a c t when the t o p o l o g i e s a r e i d e a l i z e d more than 50% have e i t h e r h e x a g o n a l or c u b i c symmetry, w h i l e none have t r i c l i n i c symmetry. SOX of m o l e c u l a r s i e v e s t r u c t u r e s have o r t h o r h o m b i c or h i g h e r symmetry and i t i s r e a s o n a b l e to e x p e c t that a new t o p o l o g y c o u l d a l s o have h i g h symmetry. T h i s r e q u i r e s a check of a l l u n i t c e l l s d e t e r m i n e d by computer i n d e x i n g programs f o r p o s s i b l e h i g h e r symmetry. I f computer i n d e x i n g programs have to be used as the s o l e method for d e t e r m i n i n g the u n i t c e l l d i m e n s i o n s , then i t i s b e s t to employ more than one program u s i n g d i f f e r e n t t e c h n i q u e s and to compare the r e s u l t s . I f i d e n t i c a l r e s u l t s a r e o b t a i n e d from two d i f f e r e n t p r o g r a m s , then i t improves the c o n f i d e n c e l e v e l . C u r r e n t l y the two most w i d e l y used a u t o i n d e x i n g programs a r e V i s s e r s ( l ) and T r e o r ( 2 ) . The t e c h n i q u e s r e q u i r e d to use e i t h e r program are g i v e n i n the program w r i t e u p s . S e l e c t e d a r e a e l e c t r o n d i f f r a c t i o n t e c h n i q u e s have to be used to o b t a i n the s i n g l e c r y s t a l patterns from powder samples. u n i t c e l l d i m e n s i o n s can be measured d i r e c t l y . In the best s i t u a t i o n e l e c t r o n d i f f r a c t i o n t e c h n i q u e s can s u p p l y the correct, cell dimensions and a l l of the systematic a b s e n c e s . With h i g h symmetry space g r o u p s , knowing a l l the systematic a b s e n c e s does not mean that the s p a c e group can be d e t e r m i n e d u n a m b i g u o u s l y . For example, i f the c e l l d i m e n s i o n s i n d i c a t e a g e o m e t r i c a l l y h e x a g o n a l c e l l w i t h no s y s t e m a t i c a b s e n c e s , t h e r e a r e 21 p o s s i b l e space g r o u p s . In the worst situation, electron diffraction techniques can u s u a l l y d e t e r m i n e s e v e r a l of the c e l l d i m e n s i o n s and some of the major systematic absences. Using this p a r t i a l unit c e l l dimension i n f o r m a t i o n i n c o n j u n c t i o n w i t h the r e s u l t s of the computer i n d e x i n g i s p r o b a b l y the best t e c h n i q u e f o r d e t e r m i n i n g the c e l l d i m e n s i o n s and the symmetry of an unknown m o l e c u l a r s i e v e m a t e r i a l . Even p a r t i a l e l e c t r o n d i f f r a c t i o n d a t a w i l l g r e a t l y r e d u c e the number of p o s s i b l e ' c o r r e c t s o l u t i o n s ' d e r i v e d by any computer i n d e x i n g p r o g r a m . Once the u n i t c e l l d i m e n s i o n s have been d e t e r m i n e d , i t i s necessary to d e t e r m i n e the space g r o u p . This involves completely i n d e x i n g the powder p a t t e r n and e x a m i n i n g a l l p o s s i b l e s e t s of h k l v a l u e s f o r each peak to see i f they are c o n s i s t e n t w i t h the chosen space g r o u p . In p a t t e r n s w i t h h i g h symmetry or ones that have two d i m e n s i o n s t h a t a r e r e l a t e d by a constant, this w i l l u s u a l l y r e s u l t in m u l t i p l e hkl values f o r most p e a k s . T h i s m u l t i p l e i n d e x i n g of peaks w i l l result i n s e v e r a l e q u a l l y p o s s i b l e space g r o u p s . I f i t i s n e c e s s a r y


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to c h o o s e b e t w e e n s e v e r a l p o s s i b l e space groups, then an examination of r e p o r t e d space groups of i d e a l i z e d m o l e c u l a r sieve topologies suggests that s e v e r a l space groups are p r e f e r r e d . T h i s i s a r e s u l t of the r e s t r i c t i o n s p l a c e d upon the f r a m e w o r k a t o m p o s i t i o n s by t h e n e e d t o m a i n t a i n a t e t r a h e d r a l framework. I f one of the s e v e r a l p o s s i b l e s p a c e g r o u p s has a l r e a d y been o b s e r v e d f o r a m o l e c u l a r s i e v e m a t e r i a l i t s h o u l d be t r i e d first. E a c h c h o i c e o f a s p a c e g r o u p and a s e t o f c e l l dimensions a t t e m p t e d r e q u i r e s an e f f o r t e q u i v a l e n t t o t h a t n e c e s s a r y to s o l v e the s t r u c t u r e . I f the s t r u c t u r e d e t e r m i n a t i o n e f f o r t i s u n s u c c e s s f u l , i t i s n o t known w h e t h e r t h e c o m b i n a t i o n o f s p a c e group and cell dimensions is wrong or whether the c r y s t a l l o g r a p h e r has f a i l e d in his task, therefore i t is desirable to have the minimum number of combinations. Fortunately, in the initial stages of a structure d e t e r m i n a t i o n i t i s not a l w a y s n e c e s s a r y to use the correct space group. With a completely ordered molecular sieve m a t e r i a l i t i s o f t e n e a s i e r to d e t e r m i n e the topology making the assumption that a l l t e t r a h e d r a l a t o m s a r e o f t h e same t y p e ; t h e u s e o f an i d e a l i z e d space group will produce an idealized topology. However, the determination of the t o p o l o g y i s t h e l i m i t i n g s t e p i n d e t e r m i n i n g a new structure from powder d a t a . D E T E R M I N A T I O N OF THE CORRECT T R I A L MODEL. B e f o r e t h i s s t a g e c a n be s t a r t e d , a l l n o n c r y s t a l l o g r a p h i c s t r u c t u r a l i n f o r m a t i o n which can aid in producing t h e c o r r e c t t o p o l o g y s h o u l d be o b t a i n e d . For example, t h i s i n f o r m a t i o n s h o u l d include data from s o r p t i o n s t u d i e s which can help to p r e d i c t b o t h the maximum p o r e o p e n i n g a n d t h e f r a m e w o r k d e n s i t y a n d s o l i d s t a t e NMR s t u d i e s w h i c h c a n show t h e n u m b e r o f c r y s t a l l o g r a p h i c a l l y u n i q u e atoms or the c o o r d i n a t i o n s t a t e of the framework atoms. There are two general s u c c e s s f u l l y determine a t r i a l and modelling techniques. At of the two i s most effecti exclusively.

techniques t h a t c a n be u s e d t o m o d e l : ab i n i t i o calculations the p r e s e n t time a c o m b i n a t i o n ve, as neither can be used

Ab Initio Techniques. The use of ab i n i t i o t e c h n i q u e s i s possible with high r e s o l u t i o n neutron and synchrotron data b e c a u s e m o r e n o n - o v e r l a p p i n g r e f l e c t i o n d a t a c a n be o b t a i n e d than i s p o s s i b l e from a standard diffractometer. As the r e s o l v i n g power of t h e s e d i f f r a c t o m e t e r s f u r t h e r i n c r e a s e s , so will t h e s u c c e s s f u l u s e o f ab i n i t i o calculations. Ab i n i t i o t e c h n i q u e s imply the use of e i t h e r a P a t t e r s o n f u n c t i o n or d i r e c t methods t e c h n i q u e s . R e c e n t l y both of t h e s e techniques have been used to s o l v e s t r u c t u r e s ( 3 , 4 ) u s i n g h i g h r e s o l u t i o n powder d a t a ; t h e s e i n i t i a l s u c c e s s e s do not mean that either technique can now be used routinely. The p o s s i b i l i t y of s u c c e s s w i t h e i t h e r t e c h n i q u e i s i n c r e a s e d i f 1) t h e u n i t c e l l i s s m a l l , 2) t h e s p a c e g r o u p i s c e n t e r e d a n d 3) the unit cell has relatively low symmetry. These


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requirements greatly reduce the possibility of o v e r l a p of reflections. U n f o r t u n a t e l y when u s e d with molecular sieve materials, d i r e c t method techniques may f a i l to p r o v i d e a u s e f u l s o l u t i o n s i n c e many m a t e r i a l s h a v e one or more large cell dimensions and a l s o h a v e h i g h s y m m e t r y . To s o l v e these s t r u c t u r e s i t has been f o u n d to be useful to include the correct number of T0« units (as T atoms surrounded t e t r a h e d r a l l y by f o u r h a l f w e i g h t o x y g e n a t o m s ) a s a fragment of the s true t u r e . Modelling techniques. There are s e v e r a l modelling techniques t h a t c a n be u s e d . The f i r s t t e c h n i q u e i s recognition that the new material has a known topology. This is not always t r i v i a l , because the powder patterns may appear to be different due to e i t h e r c o m p o s i t i o n a l d i f f e r e n c e s or c h a n g e s in symmetry r e s u l t i n g from framework d i s t o r t i o n s . T h i s can be i l l u s t r a t e d by c o m p a r i n g t h e f o l l o w i n g s e t s o f p o w d e r p a t t e r n s i n w h i c h the m a t e r i a l s i n each set have the same i d e a l i z e d framework topology: 1_) 1)

Zeolite F(Linde) and Amicite, Gismondine,

Ed i ng t on i t e . ( F i gu r e 2) Gobbinsite and Na-Pl(see

Likewise, i n c a s e s w h e r e t h e two m a t e r i a l s h a v e u n i t c e l l d i m e n s i o n s and s y m m e t r y , s u c h as Zeol a n d M A P O - 4 6 , t h e t o p o l o g i e s c a n n o t be a s s u m e d t o u n l e s s p r o v e n by c a r r y i n g o u t a f u l l s t r u c t u r e d

5).

have s i m ite Q(Li be ident eterminat

ilar nde) ical ion.

Another modelling technique is to compare the cell d i m e n s i o n s , maximum s y m m e t r y and maximum p o r e o p e n i n g with hypothetical t e t r a h e d r a l f r a m e w o r k s t r u c t u r e s s u c h as t h o s e d e t a i l e d b y J . V. S m i t h e t . a l . ( 6 ) a n d o t h e r s ( 7 ) . It should be n o t e d t h a t i f t h e c e l l d i m e n s i o n s h a v e b e e n d e t e r m i n e d o n l y from a model of the h y p o t h e t i c a l structure and not refined using t h e DLS technique, then they can e a s i l y d i f f e r from the idealized values by more than lA. For example, the c dimension of AlP0,-5(8) i s 8.48A as c o m p a r e d to t h e m o d e l value of 10A. A better technique i s to have a v a i l a b l e the simulated powder p a t t e r n s of a l l the p o s s i b l e t e t r a h e d r a l s t r u c t u r e s so that they can be u s e d i n t h e s a m e way as t h e J C P D S f i l e i s used to i d e n t i f y unknown m a t e r i a l s . I f these s i m u l a t e d powder patterns were a v a i l a b l e , i t m i g h t n o t e v e n be n e c e s s a r y t o d e t e r m i n e t h e c e l l d i m e n s i o n s and s p a c e g r o u p of an unknown molecular sieve material. For example, i f the simulated powder p a t t e r n of JVS81-1 had been a v a i l a b l e , and i f i t matches the o b s e r v e d powder p a t t e r n of d e h y d r a t e d VPI-5 then t h e r e w o u l d be no n e e d t o d e t e r m i n e t h e c o r r e c t i d e a l i z e d c e l l dimensions and symmetry. This simple comparison would have y i e l d e d the c o r r e c t t o p o l o g y and shown t h a t the material was an 1 8 - r i n g structure. Unfortunately t h e r e a r e o v e r 500 h y p o t h e t i c a l f r a m e w o r k t o p o l o g i e s p r e s e n t l y e n u m e r a t e d , and the effort required to simulate a l l o f t h e s e p o w d e r p a t t e r n s and m a t c h them w i t h the 50 u n k n o w n f r a m e w o r k s w o u l d be e x t e n s i v e . It is necessary to find a method that would determine the p r o b a b i l i t y of


10. BENNETT

Determining

the Structure of Molecular Sieve Materials 169

2a

8

12

16

20

24

28

32

D E G R E E S

36

40

44

48

52

56

26

F i g u r e 2: A comparison of the d i f f r a c t o m e t e r z e o l i t e F ( L i n d e ) and E d i n g t o n i t e .

pattern


of

170


s y n t h e s i z i n g a h y p o t h e t i c a l t o p o l o g y . As an e x a m p l e , if the ABC-6 r i n g family of h y p o t h e t i c a l s t r u c t u r e s i s c o n s i d e r e d , a l l c a n be s a t i s f a c t o r i l y i d e a l i z e d u s i n g DLS; t h e r e f o r e , a l l are theoretically p o s s i b l e . However, the number of o b s e r v e d m e m b e r s o f t h i s f a m i l y i s o n l y a b o u t 12, and a l l o f t h e s e h a v e both high c r y s t a l l o g r a p h i c symmetry and high geometrical s y m m e t r y . I f we d e s c r i b e s t r u c t u r e s h a v i n g these symmetries as being 'elegant', t h e n i t may be a l a c k o f e l e g a n c e i n many of the h y p o t h e s i z e d t o p o l o g i e s that suggests that they may never be synthesized. T a b l e I c o n t a i n s a l i s t o f a l l ABC-6 r i n g h y p o t h e t i c a l s t r u c t u r e s t h a t c a n be p o s t u l a t e d o u t to a repeat sequence of 12. Table

of

I:

List

number layers 2 3 4 5 6 7 8 9 10 11 12

of

a l l possible

number o f possibilities 1 2 3 5 10 20 45 96 230 529 1303

hypothetical

number of topologies

ABC-6

nets

observed structures

1 2 2

CAN SOD,OFF GME,LOS

4

CHA,EAB,ERI

1 1 1

AFG LEV Franzinit'

(?)

I f we a p p l y t h e e l e g a n c e t e s t t o t h e 12 l a y e r r e p e a t sequence, we f i n d t h a t t h e r e a r e 20 t o p o l o g i e s that have the highest possible symmetry (P6^/mmc) and one o f t h e s e i s b u i l t only from d o u b l e s i x r i n g s . By d e f i n i t i o n t h i s w o u l d be the most elegant o f t h e 1303 p o s s i b l e s e q u e n c e s . As an e x a m p l e F i g u r e 3 shows the o b s e r v e d powder p a t t e r n f o r a c a l c i n e d dehydrated AIPO^ material and the DLS s i m u l a t e d p a t t e r n f o r t h i s m o s t elegant sequence. The close similarity between the two patterns is sufficient justification to a t t e m p t a s o l u t i o n u s i n g as a s t a r t i n g m o d e l the postulated sequence. It may a].so indicate t h a t i t i s p o s s i b l e to r e d u c e the l a r g e number of postulated hypothetical structures down to a more m a n a g e a b l e number of p r a c t i c a l possibilities. The final t e c h n i q u e u s e d a t U n i o n C a r b i d e was developed i n c o n j u n c t i o n w i t h V. S c h o m a k e r ( U . o f W a s h i n g t o n , S e a t t l e ) . The B e n n e t t / S c h o m a k e r method e x t e n d s the c o n c e p t o f the D L S ( 9 ) method which is currently restricted to refining atom parameters and/or c e l l d i m e n s i o n s f r o m a known t o p o l o g y , t o use w i t h a powder p a t t e r n simulation program(lO). The DLS method is successful because, when correctly s e t up, the number o f i n t e r a t o m i c d i s t a n c e s i s always larger than the number of unknown atom parameters. The B e n n e t t / S c h o m a k e r method i s r e s t r i c t e d to use with molecular sieve materials having t e t r a h e d r a l f r a m e w o r k s . The method r e q u i r e s t h a t t h e


10.

BENNETT

Determining

the Structure of Molecular Sieve Materials

3a

1

I 4

r

l

' 1

J

r-

uli

J 1 8

1 12

r

1

1—

I

I

I

!

3b

1 16

1

T1 20

1

1—* 24

1 28

1 D E G R E E S 29

1 36

F i g u r e 3: A c o m p a r i s o n o f t h e d i f f r a c t o m e t e r p a t t e r n o f c a l c i n e d A1P0, m a t e r i a l and t h e s i m u l a t e d p a t t e r n o f a l a y e r ABC-6 n e t topology.


172


c e l l d i m e n s i o n s , s y m m e t r y and c e l l c o n t e n t s a r e known and can sucessfully p r e d i c t a new f r a m e w o r k t o p o l o g y by m a k i n g t h e following assumptions: 1) The method i s r e s t r i c t e d to t e t r a h e d r a l f r a m e w o r k s s u c h t h a t e a c h T atom has o n l y f o u r n e a r T neighbours bonded by oxygen atoms. 2 ) The positions of these framework oxygen atoms can be ignored. 3 ) A l l t h e T-T i n t e r a t o m i c d i s t a n c e s a r e a s s u m e d t o be equal. 4 ) a n d f i n a l l y we h a v e o n l y t o p l a c e t h e T a t o m s i n t h e c h o s e n a s y m m e t r i c u n i t , s i n c e symmetry r e l a t i o n s h i p s can be derived such t h a t atoms i n the a s y m m e t r i c u n i t c o r r e c t l y bond to the T atoms i n the neighbouring asymmetric units. This allows i n t e r a t o m i c d i s t a n c e e q n a t i o n s t o be s e t up t h a t c a n be s o l v e d to y i e l d t h e c o r r e c t p o s i t i o n s o f a l l o f t h e T atoms in the asymetric u n i t c e l l and u l t i m a t e l y , the c o m p l e t e topology. The B e n n e t t / S c h o m a k e r method i s not easy to d e s c r i b e and c a n be i l l u s t r a t e d by t h e f i r s t example (see the Appendix) which derives the idealized faujasite topology. The method d e r i v e s s t a r t i n g x,y,z p a r a m e t e r s f o r the t e t r a h e d r a l atom of (.125,-.050,.038), which compare w i t h r e f i n e d p a r a m e t e r s of ( . 1 2 5 , - . 0 5 4 , . 0 3 7 ) . The s e c o n d e x a m p l e d e r i v e s p a r a m e t e r s for the RH0 topology of ('A, . 1 0 2 , . 3 9 8 ) as compared to r e f i n e d p a r a m e t e r s of (%, . 1 0 1 4 , . 3 9 8 6 ) . The third example derives idealized parameters f o r A1P0.-16 of (.114,.114,.114) as compared to r e f i n e d p a r a m e t e r s of (.1139,.1139,.1139) and (.1156,.1156,.1156). In these three examples the d e r i v e d p a r a m e t e r s a r e more than a d e q u a t e to c o m p l e t e l y describe the idealized framework topology and can even be used as a s t a r t i n g set i n a s t r u c t u r e refinement. Miscellaneous techniques. The s t r u c t u r e o f T h e t a o n e ( l l ) was s o l v e d w i t h n e i t h e r ab i n i t i o c a l c u l a t i o n s n o r m o d e l l i n g , but by permuting a l l p o s s i b l e a s s i g n m e n t s of p h a s e s f o r a s m a l l number o f r e f l e c t i o n s and t h e n examining the Fourier maps. T h i s may s e e m a t e d i o u s w a y to s o l v e an u n k n o w n s t r u c t u r e , but i t was successful. The technique may indeed be a way of solving molecular sieve s t r u c t u r e s with c e n t r i c space groups should a l l else f a i l . REFINEMENT OF THE D A T A AND S O L U T I O N OF T H E STRUCTURE. Once the c o r r e c t f r a m e w o r k t o p o l o g y has been d e t e r m i n e d , the data must be r e f i n e d t o g e t t h e c o m p l e t e s o l u t i o n . Data c o l l e c t e d using high resolution synchrotron r a d i a t i o n are easier to process and r e f i n e t h a n s t a n d a r d x - r a y d a t a 1) b e c a u s e i t i s h i g h l y m o n o c h r o m a t i c a n d t h e p e a k s h a p e s c a n be w e l l d e s c r i b e d by t h e c o n v o l u t i o n o f G a u s s i a n a n d L o r e n t z i a n f u n c t i o n s a n d 2) b e c a u s e i t has e x c e l l e n t r e s o l u t i o n and t h e r e i s l e s s overlap of reflections. The major d i f f i c u l t y with refining powder data m o l e c u l a r s i e v e m a t e r i a l s i s d e t e r m i n i n g the p o s i t i o n s of of the non-framework atoms. In hydrated or as-synthes s a m p l e s , the non-framework atoms are u s u a l l y d i s o r d e r e d that they occupy a v a r i e t y of possible positions orientations. Therefore, t h e y m u s t be m o d e l e d to make


from a l l ized such and both

10.

BENNETT


173

c h e m i c a l s e n s e ( i . e . v a l i d i n t e r a t o m i c d i s t a n c e s and a n g l e s ) and to a l s o d u p l i c a t e the f i l l i n g of the voids within the structure. This is difficult w i t h s i n g l e c r y s t a l d a t a and e v e n more so w i t h powder d a t a ; t h e r e f i n e m e n t o f A l P O ^ - 1 6 d a t a illustrates this. The as-synthesized material contains q u i n u c l i d i n e a s t h e t e m p l a t e ; i t was o r i g i n a l l y p l a c e d i n the center of the cavity and aligned along a 3-fold axis. However, the best refinement was obtained when the quinuclidine was r o t a t e d a few d e g r e e s o f f t h e 3 - f o l d a x i s . The difference between the two orientations of the quinuclidine resulted i n the s e c o n d o r i e n t a t i o n c r e a t i n g a s p h e r i c a l l y d i s o r d e r e d r e g i o n which b e t t e r m o d e l l e d the a c t u a l disorder. In many cases i t i s not possible to g e t as s o p h i s t i c a t e d a model f o r the non-framework atoms. Often i t is sufficient f o r t h e f i r s t s t r u c t u r e d e t e r m i n a t i o n o f a new f r a m e w o r k t o p o l o g y u s i n g powder d a t a to model this disorder w i t h a s e r i e s of c a r b o n atoms w i t h l a r g e t e m p e r a t u r e factors, s i n c e the c o r r e c t d e t e r m i n a t i o n of the non-framework atoms i s not n e c e s s a r y . Once the f r a m e w o r k t o p o l o g y has been c o r r e c t l y d e t e r m i n e d from a anhydrous and/or c a l c i n e d sample, then the position of t h e n o n - f r a m e w o r k a t o m s c a n be d e t e r m i n e d using t h e a s - s y n t h e s i z e d m a t e r i a l and t h e u s e o f a n o t h e r d a t a s e t . SUMMARY. M o s t new m o l e c u l a r s i e v e p h a s e s a r e s y n t h e s i z e d w i t h c r y s t a l s o n l y a few m i c r o n s i n s i z e . I t i s not p o s s i b l e to determine the framework topology of such a phase u s i n g p r e s e n t s i n g l e crystal techniques, and powder techniques are becoming increasingly important. With new high resolution d i f f r a c t o m e t e r s the t e c h n i q u e to c o l l e c t excellent data are available. The i n c r e a s e i n r e s o l u t i o n makes i t p r a c t i c a l t o use a u t o i n d e x i n g p r o g r a m s to d e t e r m i n e the c e l l d i m e n s i o n s and t o b e t t e r d e t e r m i n e t h e s y m m e t r y . T h e i m p r o v e m e n t s b e i n g made to R i e t v e l d programs g i v e the c a p a b i l i t y of r e l a t i v e l y easy processing o f the d a t a . The m a j o r s t u m b l i n g b l o c k to t h e use of powder t e c h n i q u e s i s the d i f f i c u l t y of d e t e r m i n i n g a new t o p o l o g y . A t t h e p r e s e n t t i m e ab i n i t i o t e c h n i q u e s a s a p p l i e d to powder d a t a f o r m o l e c u l a r s i e v e m a t e r i a l s have not been used w i t h enough s t r u c t u r e s to d e m o n s t r a t e the c a p a b i l i t y o f the t e c h n i q u e s . M o d e l l i n g t e c h n i q u e s r e q u i r e a wide knowledge of many m o l e c u l a r s i e v e t o p o l o g i e s a n d few m o l e c u l a r s i e v e c r y s t a l l o g r a p h e r s have t h i s i n f o r m a t i o n r e a d i l y a v a i l a b l e . I t would be very helpful i f simulated powder p a t t e r n s were a v a i l a b l e f o r a l l h y p o t h e t i c a l t o p o l o g i e s , but t h i s i s a l a r g e undertaking. The most productive tool may be the B e n n e t t / S c h o m a k e r method d e s c r i b e d h e r e i n , e s p e c i a l l y i f i t can be automated. It may not be possible to t e a c h the c o m p u t e r to a p p l y symmetry o p e r a t i o n s to d e r i v e the correct interatomic relationships, so i t w i l l be n e c e s s a r y f o r t h e c o m p u t e r to permute a l l the p o s s i b l e symmetry operations to s e e i f a s o l u t i o n c a n be o b t a i n e d . T h e n u m b e r o f c a l c u l a t i o n s may be l a r g e , b u t a w e e k o r even a month of a micro VAX computer t i m e t o do t h e t a s k w i l l be c o n s i d e r e d by many t o be a timesaver for s t r u c t u r e determinations.


174


Acknowledgments The author would like to thank Dr. David Cox of the National Synchrotron Light Source at the BrookhavenNational Laboratory, Long Island, New York for his invaluable help. Literature cited 1. Visser, J. W. J. Appl. Cryst. 1969, 2, 89-94 2. Werner, P. E. Z. Krst. 1964, 375-387 3. Lightfoot, P.; Cheetham, A. K.; Sleight, A. W. Inorg. Chem, 1987, 26, 3544-3547 4. Rudolf, P. R.; Saldarriage-Molina, C.; Clearfield, A. J. Phys. Chem. 1986, 90, 6122-6125 5. von Ballmoos, R. Collection of simulated XRD powder patterns for zeolites, Butterworth Ltd., 1984 6. Smith, J. V.; Rinaldi, F. Min. Mag. 1962, 33, 202-212 Smith, J. V. Min. Mag. 1968, 36, 640-642 Smith, J. V. Am. Min. 1977, 62, 703-709 Smith, J. V. Am. Min. 1978, 63, 960-969 Smith, J. V. Am. Min. 1979, 64, 551-562 Smith, J. V.; Bennett, J. M. Am. Min. 1981, 66, 777-788 Smith, J. V. Z. Krst. 1983, 165, 191-198 Smith, J. V.; Bennett, J. M. Am. Min. 1984, 69, 104-111 Smith, J. V.; Dytrych, W. J. Nature 1984, 309, 607-608 Bennett, J. M.; Smith, J. V. Z. Krst. 1985, 171,65-68 Hawthorne, F. C.; Smith, J. V. Z. Krst.1986, 175, 15-30 Smith, J. V.; Dytrych, W. J. Z. Krst. 1986, 175, 31-36 7. Kerr, I. S. Nature 1963, 197, 1194-1196 Sherman, J. D.; Bennett, J. M. Molecular Sieves Adv. in Chem. 121, 1973, p.52-65 Rechsteiner, H. Diplomarbeit thesis, ETH, Zurich, Switzerland, 1979 Gramlich-Meier, R. Z. Krst. 1986 177, 237-245 McCusker, L. B.; Meier, W. M.; Rechsteiner, H. Mat. Res. Bul. 1987, 22 1203-1207 8. Bennett, J. M.; Cohen, J. P.; Flanigen, E. M.; Pluth, J. J.; Smith, J. V. Intrazeolite Chemistry ACS Symp. Ser. 218, 1983, p.109-118 9. Baerlocher, Ch.; Heep, A.; Meier, W. M. DLS-76, A program for the simulation of crystal structures by geometric refinement. ETH, Zurich, p.124 (1978) 10 Smith, D. K.; Nicols, M. C.; Zolensky, M. E. A Fortran Program for Calculating X-ray Powder Diffraction Patterns - Version 10 11. Highcock, R. M.; Smith, G. W.; Wood, D. Acta. Cryst. 1985, C41, 1391 APPENDIX THE IDEALISED FAUJASITE TOPOLOGY. The following assumptions are made 1) the space group is Fd3m (second setting) and 2) all 192 T atoms are in general positions. The cell dimensions are known and the T-T interatomic distance is assumed to be 3 • 1A . For convenience the atom coordinates (x,y,z) are expressed in Angstroms, and not in fractions of the unit cell dimensions.


10.


BENNETT

175

Description of the asymmetric unit. The s p a c e g r o u p has a n a t u r a l d o u b l e a s y m m e t r i c u n i t , a t e t r a h e d r o n b o u n d e d by f o u r mirror planes. I f one such tetrahedron i s chosen with v e r t i c e s , i n 1/8's , o f ( 0 , 0 , 0 ) , ( 4 , 0 , 0 ) , ( 2 , 2 , 2 ) and ( 2 , - 2 , 2 ) , the t e t r a h e d r o n has a t w o - f o l d a x i s r u n n i n g through the p o i n t s (1,1,1) and ( 3 , - 1 , 1 ) . This twofold axis divides the double asymmetric unit i n t o t h e two a s y m m e t r i c units. Derivation of topology. C o n s i d e r t h e p l a n e w i t h ζ = 1, a n d t h e part of the t e t r a h e d r o n with ζ g r e a t e r than 1, f i n a l l y f o r m a t h e m a t i c a l convenience d u r i n g the c a l c u l a t i o n , the o r i g i n i s shifted by ( 1 , 1 , 1 ) , t h e n we find: 1. The x-prοjection. The c h o i c e o f a s y m m e t r i c u n i t causes y t o be z e r o . The Τ atom ( x , y , z ) i n the chosen asymmetric u n i t cell bonds a c r o s s a m i r r o r plane to another Τ atom ( x , z , y ) i n the next asymmetric unit c e l l : d

=

^/(2z).

2. The y p r o j e c t i o n . The Τ atom ( x , y , z ) bonds a c r o s s a mirror plane to another Τ atom (z,y,x) i n the next asymmetric unit c e l l , s i n c e y=0, d'

=

3. Equating χ = 2z.

d

ζ ι

s/[2(z-x)J. and

d',

we

find

that

4. The ζ p r o j e c t i o n . The Τ atom ( x , y , z ) bonds t o t h e Τ atom i n t h e t w i n asymétrie unit w h i c h i s a t (2-y,2-χ,2 -ζ ) . d"

= ^ / [ ( 2 - x - y ) 2

substituting

2z

+

( 2 - x - y )

f o r x,

2 +

and

( 2 - 2 z ) 0

2

] .

f o r y. -

d" 5.

Equating ^/

Perspectives in Molecular Sieve Science - American Chemical Society

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