11 Aluminum Distributions in Zeolites ALAN W. PETERS
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W. R. Grace & Company, Davison Chemical Division, Columbia, M D 21044
Previous work has described a statistical approach to the structure of faujasite. Distributions of local structures were obtained that permit comparisons with the results of Si NMR experiments. The results of these calculations for higher ratio materials, Si/Al ~2.0 to 3.0, are not entirely consistent with the results of NMR experiments. In current work we have used the Si NMR data to determine a distribution of prisms that give a best fit to the NMR data for faujasite at a variety of silicon/aluminum ratios. These results show clearly that structures with four membered rings containing two diagonal aluminum atoms or six membered rings containing meta aluminum atoms are avoided. The calculation permits a transformation of the Si NMR intensities to give distributions of aluminum atoms in faujasite with implications for the numbers of strong and weak acid sites available. 29
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There have been s e v e r a l recent suggestions that s y n t h e t i c f a u j a s i t e may have a short range ordering of S i and A l atoms at S i / A l r a t i o s between 1.4 and 3 . 0 . The r e s u l t s are based on the p o s s i b i l i t y of matching experimental S i MASNMR (magic angle spinning NMR) s p e c t r a with a s p e c t r a c a l c u l a t e d from a s e l e c t i o n of proposed s p e c i f i c l o c a l s t r u c t u r e s ( 1 - 2 ) . The suggestion that the S i and A l may be ordered i n f a u j a s i t e derives from e a r l y work by Dempsey, Kuhl and Olson (3) and by Breck and Flanigen (4). These studies showed that d i s c o n t i n u i t i e s occur i n the c o r r e l a t i o n of S i / A l r a t i o with u n i t c e l l s i z e . This work was subsequently confirmed (5.). 2 9
0097-6156/84/0248-0201$06.00/0 © 1984 American Chemical Society
Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
202
CATALYTIC MATERIALS
In the present work we have developed a procedure f o r using S i MASNMR i n t e n s i t y data to provide s t r u c t u r a l information. An exhaustive l i s t of a l l 19 s t r u c t u r a l l y d i s t i n c t hexagonal prism b u i l d i n g blocks was prepared, Figure 1. Of the 19, only 16 give d i s t i n c t s i l i c o n / a l u m i n u m neighbor relationships. Given a p a r t i c u l a r combination of s t r u c t u r e s , i t i s not d i f f i c u l t to compute the expected S i MASNMR intensities. What i s d e s i r e d , however, i s to be able to go the other way, to estimate the r e l a t i v e number of each of the various s t r u c t u r a l u n i t s from the i n t e n s i t y d a t a . A non l i n e a r o p t i m i z a t i o n procedure, the Box complex algorithm (6), can be used to s e l e c t from the 16 p o s s i b l e s t r u c t u r e s the combination that best f i t s the i n t e n s i t y d a t a . S t r u c t u r a l information acquired i n t h i s way i s e m p i r i c a l i n the sense that i t i s implied by the f i t t i n g procedure plus the d a t a . There i s some ambiguity i n the r e s u l t to the extent that d i f f e r e n t combinations of s t r u c t u r e s can give s i m i l a r i n t e n s i t y r e s u l t s . Several ambiguities of t h i s nature are discussed i n the present case. 2 9
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D i s t r i b u t i o n s of s t r u c t u r e s obtained by f i t t i n g the i n t e n s i t y data can be compared to a most probable d i s t r i b u t i o n of the s i x t e e n s t r u c t u r e s assumming equal a p r i o r i p r o b a b i l i t i e s subject to the c o n s t r a i n t that the c o r r e c t S i / A l r a t i o must be g i v e n . A method f o r c a l c u l a t i n g the most probable d i s t r i b u t i o n of these s t r u c t u r e s has been p r e v i o u s l y reported (7.). Comparisons between the most probable and the best f i t d i s t r i b u t i o n s of s t r u c t u r a l u n i t s show that there appear to be systematic preferences f o r c e r t a i n u n i t s . These preferences are e s p e c i a l l y pronounced at higher S i / A l r a t i o s .
EXPERIMENTAL Four samples of f a u j a s i t e were synthesized at S i / A l r a t i o s of 2.61, 2.80, 2.97 and 3.03 using published methods from seeded s l u r r i e s (8-9) and using p r o p r i e t a r y methods. One a d d i t i o n a l sample of S i / A l r a t i o 2.58 was purchased from Union Carbide. The samples were c h a r a c t e r i z e d by X - r a y powder d i f f r a c t i o n , by surface area measurements, and by wet chemical a n a l y s i s . The r e s u l t s of these measurements are contained i n Table I . The S i MASNMR experiments were performed using a Bruker WH-400 spectrometer at the U n i v e r s i t y of Guelph. The d e t a i l e d experimental procedures and the methods of measuring peak 2 9
Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
PETERS
Aluminum Distributions in Zeolites
PRISM TYPE
SI(OAL) PROBABILITY
Ν - 0
1
2
3
1
12
-
-
-
12
8
3
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24
4
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12
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1
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4
3
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24
2
5
2
-
24
3
3
3
-
12
4
2
2
1
0
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Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch011
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