Chapter 2
Analysis of Cation Position in Ion-Exchanged Y Zeolites by Na N M R Downloaded via TUFTS UNIV on July 16, 2018 at 13:04:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
23
L. B. Welsh and S. L. Lambert Allied-Signal
Engineered Materials Research Center, 50 East Algonquin Road, Box 5016, Des Plaines, IL 60017-5016
The results presented here indicate that sodium ions in different cation sites in Y zeolites can be distinguished by their sodium-23 NMR chemical shift at high magnetic fields. As a result, sodium-23 NMR can be used to monitor the sodium cation distribution extant after partial cation exchange. To illustrate this technique, two cation exchanged series of hydrated Y zeolites, ammonium/sodium and calcium/sodium, have been studied by sodium-23 magic angle spinning NMR (MASNMR). Spectral simulations with symmetric lines were used to determine the chemical shift and relative strength of the constituent lines. These NMR lines were then correlated with the specific cation sites of Y zeolite in the supercages, sodalite cages, and hexagonal prisms using cation site locations and occupancies derived from XRD and IR studies. The sodium-23 MASNMR results are consistent with the selective removal of sodium cations from the Y zeolite supercages by the partial cation exchange, and demonstrate that this technique can be used to monitor how cation distributions in Y zeolites change with various sample treatments. High r e s o l u t i o n s o l i d s N u c l e a r Magnetic Resonance (NMR) t e c h n i q u e s have been used e x t e n s i v e l y i n r e c e n t y e a r s t o c h a r a c t e r i z e z e o l i t i c m a t e r i a l s , by s t u d y i n g the z e o l i t e frameworks. By comparison, r e l a t i v e l y l i t t l e e f f o r t has been devoted t o the study o f c a t i o n s i n z e o l i t e s u s i n g such NMR t e c h n i q u e s . I t i s the i n t e n t o f t h i s paper t o examine the scope o f i n f o r m a t i o n which can be o b t a i n e d v i a the a p p l i c a t i o n o f h i g h r e s o l u t i o n s o l i d s NMR t e c h n i q u e s a t h i g h magnetic f i e l d s t o the study o f c a t i o n s i n z e o l i t e s . Specific a l l y , the use o f sodium-23 magic angle s p i n n i n g NMR (MASNMR) t o probe the c h e m i c a l s h i f t d i f f e r e n c e s o f sodium c a t i o n s i n the s u p e r c a g e s , s o d a l i t e cages and hexagonal prisms o f Y z e o l i t e w i l l be d i s c u s s e d . The l o c a t i o n o f c a t i o n s i n z e o l i t e s i s o f c o n s i d e r a b l e p r a c t i c a l importance s i n c e t h e i r l o c a t i o n s can a f f e c t , among o t h e r s ,
0097-6156/88/0368-0033$06.00/0 © 1988 American Chemical Society
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
34
PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
c a t i o n exchange e f f e c t i v e n e s s , and the s o r p t i o n and d i f f u s i o n p r o p e r t i e s of o r g a n i c m o l e c u l e s i n z e o l i t e s . The c a t i o n p o s i t i o n s i n z e o l i t e s have been probed i n the past u s i n g a number o f d i f f e r e n t techniques. Perhaps the most w i d e l y a p p l i e d i s t h a t of x - r a y d i f f r a c t i o n (XRD) ( 1 - 3 ) . Most o f the c a t i o n l o c a t i o n d a t a i n the l i t e r a t u r e which are r e l e v a n t to t h i s study have been g e n e r a t e d by these XRD methods, i n c l u d i n g what i s known about the l o c a t i o n o f sodium c a t i o n s i n Y z e o l i t e . The work of M o r t i e r et a l . (4) and o t h e r s has shown t h a t t h e r e are c a t i o n s l o c a t e d i n the hexagonal p r i s m s , i n the s o d a l i t e cages and i n the supercages o f the f a u j a s i t e structure. The most s i g n i f i c a n t problem encountered i n u s i n g XRD f o r t h i s purpose i s t h a t o n l y the s t a t i o n a r y c a t i o n s can be l o c a t e d . S i n c e some of the c a t i o n s i n z e o l i t e s are v e r y m o b i l e , e s p e c i a l l y i n the p r e s e n c e o f water, o f t e n fewer than 50% of the c a t i o n s can be l o c a t e d f o r h y d r a t e d samples by XRD t e c h n i q u e s . A higher percentage ( n e a r l y 100%) of the c a t i o n s can o f t e n be l o c a t e d i f the sample i s d e h y d r a t e d to reduce c a t i o n m o b i l i t y ; however, the d e h y d r a t i o n step may i t s e l f a l t e r the d i s t r i b u t i o n of c a t i o n s . IR methods have a l s o been used to study c a t i o n l o c a t i o n s i n zeolites. For example, the work o f Roessner et a l . (5) f o l l o w s the m i g r a t i o n of c a l c i u m and r a r e e a r t h c a t i o n s to v a r i o u s s i t e s i n Y zeolite. By l o o k i n g at the d o u b l e - s i x - r i n g v i b r a t i o n band, he shows t h a t the c a t i o n l o c a t i o n s depend on the exchange p r o c e d u r e and t h e r m a l treatment of the samples. Only l i m i t e d i n f o r m a t i o n was o b t a i n e d f o r the c a t i o n s i n the supercage and the s o d a l i t e cage u s i n g IR, the same c a t i o n s which x - r a y t e c h n i q u e s have the most d i f f i c u l t y l o c a t i n g . In c o n t r a s t , as a t e c h n i q u e to study c a t i o n s i n z e o l i t e s , NMR has the advantage that the m o b i l e c a t i o n s are the most e a s i l y o b s e r v e d . S i n c e the e f f e c t s of s e c o n d - o r d e r q u a d r u p o l a r i n t e r a c t i o n s are reduced at h i g h magnetic f i e l d s , the use of a h i g h f i e l d NMR s p e c t r o m e t e r f o r t h i s study s i g n i f i c a n t l y enhances the s p e c t r a l r e s o l u t i o n t h a t can be a c h i e v e d and i n c r e a s e s the p e r c e n t age o f NMR o b s e r v a b l e sodium c a t i o n s . F o r our h i g h r e s o l u t i o n s o l i d s NMR s t u d i e s , two s e r i e s o f samples were p r e p a r e d i n which the l o c a t i o n s of the r e m a i n i n g sodium i o n s were v a r i e d i n a s y s t e m a t i c manner. These s e r i e s were p a r t i a l l y exchanged Y z e o l i t e s w i t h e i t h e r c a l c i u m or ammonium i o n s r e p l a c i n g some of the sodium i o n s . The exchange i s o t h e r m s f o r Y z e o l i t e s , w i t h framework S i / A l mole r a t i o s near 2.7, i n d i c a t e t h a t f o r c a l c i u m and ammonium ions o n l y about 70% of the sodium c a t i o n s can be exchanged at ambient temperatures ( 6 ) . C o n s i d e r i n g the r e l a t i v e p o p u l a t i o n s of the exchange s i t e s i n the h e x a g o n a l p r i s m ( s i t e I ) , i n the s o d a l i t e cage ( s i t e I ) and i n the supercage ( s i t e I I ) as determined by XRD ( 1 - 4 ) , these d a t a i n d i c a t e t h a t o n l y the supercage s i t e s are i n i t i a l l y exchanged. S h e r r y has p o i n t e d out t h a t t h i s l i m i t may be k i n e t i c , s i n c e he a c h i e v e d 95% exchange o f c a l c i u m f o r sodium i n a Y z e o l i t e by u s i n g an e x h a u s t i v e exchange p r o c e d u r e ( 7 ) . H i s r e s u l t s a l s o i n d i c a t e t h a t water s t r i p p i n g of c a t i o n s can p l a y an important r o l e i n e q u i l i b r a t i o n of cation locations in Y zeolites. Thermal t r e a t m e n t s o f p a r t i a l l y exchanged Y z e o l i t e s are a l s o known to a c c e l e r a t e the r e d i s t r i b u t i o n o f the c a t i o n s among a l l of the exchange s i t e s ( 7 ) . These d a t a were used to h e l p i n t e r p r e t the NMR d a t a c o l l e c t e d i n t h i s s t u d y . 1
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
2. WELSH & LAMBERT
Cation Position in Ion-Exchanged Y Zeolites
35
EXPERIMENTAL Sample D e s c r i p t i o n and P r e p a r a t i o n . A l l o f the samples d i s c u s s e d i n t h i s paper were prepared from a h i g h l y c r y s t a l l i n e sample o f LZY-52, a Na-Y z e o l i t e from L i n d e . Two s e r i e s o f p a r t i a l l y exchanged samples were p r e p a r e d . The same exchange p r o c e d u r e was used i n the p r e p a r a t i o n o f a l l the samples o f a g i v e n s e r i e s . Only the amount o f c a l c i u m o r ammonium s a l t added was v a r i e d . T h i s p r o c e d u r e c o n s i s t e d o f d i s s o l v i n g the c a l c i u m o r ammonium s a l t i n 100 c c d e i o n i z e d water. The s o l u t i o n c o n c e n t r a t i o n s ranged from 0.03M t o 0.5M f o r c a l c i u m and 0.08M t o 1.0M f o r ammonium s a l t s . The Na-Y z e o l i t e (10 g, v o l a t i l e - f r e e ) was s l u r r i e d i n the exchange s o l u t i o n f o r 72 hours a t room temperature. The exchanged Y z e o l i t e was washed and the samples were d r i e d i n a f o r c e d a i r oven a t 40°C o v e r n i g h t t o remove the i n t e r p a r t i c u l a t e water. In the course o f t h i s study we have observed t h a t the exact d i s t r i b u t i o n o f c a t i o n s i n a Y z e o l i t e i s v e r y dependent on both the sample p r e p a r a t i o n and the h a n d l i n g p r o c e d u r e s . Comparison o f these d a t a w i t h d a t a generated from samples whose h i s t o r i e s a r e d i f f e r e n t o r unknown must proceed w i t h c a u t i o n . A nine-sample s e r i e s o f c a l c i u m exchanged Y z e o l i t e s and an e i g h t - s a m p l e s e r i e s o f ammonium exchanged Y z e o l i t e s were p r e p a r e d . The c h e m i c a l a n a l y s e s o b t a i n e d on these samples a r e g i v e n i n T a b l e I a l o n g w i t h the c a l c u l a t e d p e r c e n t exchange. The p e r c e n t exchange was c a l c u l a t e d as the p e r c e n t a g e o f c a t i o n s i t e s which a r e not o c c u p i e d by sodium i o n s d i v i d e d by the t o t a l number o f c a t i o n exchange s i t e s . The ammonium exchanged s e r i e s extended over the range o f 0 t o 56% s u b s t i t u t i o n f o r sodium w h i l e the c a l c i u m exchanged s e r i e s covered the range o f 0 t o 62% s u b s t i t u t i o n . The samples d i s c u s s e d i n t h i s paper were s t u d i e d i n the h y d r a t e d s t a t e . A l l samples were e q u i l i b r a t e d a t 50% r e l a t i v e h u m i d i t y f o r 18 hours b e f o r e the NMR d a t a were o b t a i n e d . T h i s was done t o produce samples w i t h r e p r o d u c i b l e l e v e l s o f h y d r a t i o n . The l o s s on i g n i t i o n (L0I) a t 900°C o f the e q u i l i b r a t e d samples was about 25%.. P o r t i o n s o f each p r e p a r a t i o n were d r i e d a t 150°C, which reduced the L 0 I a t 900°C t o 2%. NMR s p e c t r a o f these dehydrated samples were a l s o o b t a i n e d . The i n t e r p r e t a t i o n o f the dehydrated sample s p e c t r a i s c o m p l i c a t e d by the p r e s e n c e o f l a r g e r q u a d r u p o l a r i n t e r a c t i o n s and broader l i n e w i d t h s . R e s u l t s on the dehydrated z e o l i t e s w i l l be d i s c u s s e d i n a l a t e r p u b l i c a t i o n . A c q u i s i t i o n , P r o c e s s i n g and S i m u l a t i o n o f Sodium-23 NMR Spectra. Sodium-23 MASNMR s p e c t r a were o b t a i n e d on 6.3, 8.45 and 11.7 T e s l a m u l t i n u c l e a r s o l i d s NMR s p e c t r o m e t e r s a t S p e c t r a l Data S e r v i c e s and the U n i v e r s i t y o f I l l i n o i s i n Champaign, I l l i n o i s . Sample s p i n n i n g r a t e s o f 3 t o 5 KHz were used i n most c a s e s . A t y p i c a l spectrum was a c q u i r e d u s i n g 1000 scans, a r e c y c l e time o f 0.5 sec. and an r f e x c i t a t i o n p u l s e w i d t h o f 2.0 m i c r o s e c . Since the sodium-23 n u c l e u s has a q u a d r u p o l a r moment, the p u l s e w i d t h was chosen t o be l e s s than 1/4 o f a 90 degree p u l s e o f sodium i n solution. The i n t e g r a t e d i n t e n s i t y o f the sodium NMR l i n e s i n the spectrum then c l o s e l y approximates the c o n c e n t r a t i o n o f the sodium s p e c i e s g i v i n g r i s e t o the NMR l i n e s . S p e c t r a were t y p i c a l l y p r o c e s s e d u s i n g a 25 Hz e x p o n e n t i a l b r o a d e n i n g .
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
Table I CaCion Exchanged Samples o f Y Z e o l i t e
% Exchange Sample
Type
(of Sites)
% Na 0 2
(VF)
7 Si0 o
2
(VF)
% A l ^ (VF)
Molarity of Exchange
Solution LZY-52
(Ca,Na)-Y (Ca,Na)-Y (Ca,Na)-Y (Ca,Na)-Y (Ca,Na)-Y (Ca,Na)-Y (Ca,Na)-Y (Ca,Na)-Y (Ca,Na)-Y (NH Na)-Y (NH ,Na)-Y (NHV,Na)-Y (NRV,Na)-Y (NHV ,Na)-Y (Nh\,Na)-Y (NHV,Na)-Y (NH^,Na)-Y
0
14..02
11 21 35 45 50 52 56 58 62 9 26 32 41 47 53 55 56
11..11 9..95 8..20 6..92 6..32 5..99 5,.51 5..19 4..81 11..40 9..32 8..56 7..37 6..59 5..95 5,.64 5..48
61.17
20.67
--0..028 0..050 0..082 0..123 0..163 0..245 0..299 0..367 0..463 0..080 0..161 0..241 0..361 0..482 0..723 0..883 1..084
M M M M M M M M M M M M M M M M M
RESULTS The sodium-23 MASNMR spectrum o f a Na-Y z e o l i t e a t 132 MHz (11.7 T e s l a ) i s shown i n F i g u r e l a . As d i s c u s s e d below, t h e f e a t u r e s o f t h i s spectrum a r i s e from the p r e s e n c e o f a t l e a s t two s e p a r a t e NMR lines. A s i m u l a t i o n o f t h i s spectrum, w i t h symmetric l i n e s o f mixed G a u s s i a n / L o r e n t z i a n c h a r a c t e r , i s a l s o shown i n F i g u r e l a , a l o n g w i t h t h e component l i n e s o f the s i m u l a t i o n . Such a spectrum i s d i f f i c u l t t o s i m u l a t e u n i q u e l y because o f the o v e r l a p o f t h e l i n e s and the e r r o r s i n t r o d u c e d i n t o the spectrum as a r e s u l t o f s p e c t r a l p h a s i n g and b a s e l i n e c o r r e c t i o n . To d e t e r m i n e i f the f e a t u r e s o f the spectrum o f F i g u r e l a i n d i c a t e the e x i s t e n c e o f two o r more l i n e s , as s i m u l a t e d , o r a r i s e from a s i n g l e NMR l i n e w i t h a l i n e s h a p e due t o second o r d e r quadrupole i n t e r a c t i o n s , sodium-23 MASNMR s p e c t r a o f a Na-Y z e o l i t e
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
2. WELSH & LAMBERT
Cation Position in Ion-Exchanged
Y Zeolites
e ο •Η U
cd
ΓΗ β •Η CO
· Ν »
eu C ο •H
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>—ι
