Chapter 25 23
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Characterization of Y Zeolites by Na Magic-Angle-Spinning NMR Spectroscopy L. B. Welsh and S. L. Lambert UOP Research Center, Des Plaines,IL60017
Sodium-23 MASNMR results have been obtained on Na-Y zeolites and a series of partially exchanged (NH ,Na)Y, (Ca,Na)-Y and (La,Na)-Y zeolites which demonstrate that the sodium cations in the supercages can be distinguished from those in the smaller sodalite cages and hexagonal prisms for both hydrated and dehydrated Y zeolites. For the hydrated Y zeolites, spectral simulation with symmetric lines allows the cation distributions to be determined quantitatively. The sodium-23 MASNMR results are consistent with the selective removal of sodium cations from the Y zeolite supercages by the partial cation exchange. The results demonstrate that this technique can be used to monitor how cation distributions in Y zeolites change with various sample treatments. 4
C o n s i d e r a b l e c h a r a c t e r i z a t i o n o f the s i l i c a - a l u m i n a framework o f Y and o t h e r l a r g e pore z e o l i t e s has been r e p o r t e d i n r e c e n t y e a r s (1-2). For example, the d e a l u m i n a t i o n o f Y z e o l i t e has been e x t e n s i v e l y studied using various techniques i n c l u d i n g s i l i c o n - 2 9 NMR, aluminum-27 NMR and STEM/EDX a n a l y s i s ( 3 - 4 ) . On the o t h e r hand, the c h a r a c t e r i z a t i o n o f the s t a t e and 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 has been l e s s w e l l s t u d i e d . The l o c a t i o n s o f the c a t i o n s can p l a y an i m p o r t a n t r o l e i n the i n t r a z e o l i t e d i f f u s i o n o f m o l e c u l e s and i n the shape s e l e c t i v i t y e x h i b i t e d by the z e o l i t e . For example, p o t a s s i u m A z e o l i t e can adsorb o n l y v e r y s m a l l m o l e c u l e s such as hydrogen o r w a t e r , w h i l e a sodium exchanged A z e o l i t e adsorbs m o l e c u l e s as l a r g e as ethane; however, i f A z e o l i t e i s exchanged w i t h c a l c i u m , h a l v i n g the number o f c a t i o n s r e q u i r e d f o r charge compensation, i t i s c a p a b l e o f e x c h a n g i n g l a r g e r n - p a r a f f i n s ( 5 ) . I n any z e o l i t e c o n t a i n i n g more than one 0097-6156/89/0411-0262$06.00/0 © 1989 American Chemical Society
Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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type o f n o n e q u i v a l e n t c a t i o n s i t e , the r e l a t i v e p o p u l a t i o n s o f t h o s e s i t e s may be v e r y i m p o r t a n t i n d e t e r m i n i n g how the m a t e r i a l p e r f o r m s i n a p a r t i c u l a r a p p l i c a t i o n . I n Y z e o l i t e t h e r e are a t l e a s t t h r e e i n e q u i v a l e n t c a t i o n s i t e s (6,7) w h i c h are b e l i e v e d t o have d i f f e r e n t a c c e s s i b i l i t i e s based on c a t i o n exchange s t u d i e s ( 7 ) . For t h i s r e a s o n , and because Y z e o l i t e i s o f c o n s i d e r a b l e commercial i n t e r e s t , i t was chosen f o r t h i s s t u d y o f c a t i o n d i s t r i b u t i o n s b y sodium-23 NMR. A t t h i s t i m e , the l o c a t i o n s o f c a t i o n s i n z e o l i t e s have been d e t e r m i n e d p r i m a r i l y b y X - r a y d i f f r a c t i o n (XRD) techniques. U n f o r t u n a t e l y , t h i s method has the drawback o f b e i n g a b l e t o l o c a t e o n l y the most s t a t i o n a r y c a t i o n s i n z e o l i t e s . I n some s t u d i e s o f h y d r a t e d z e o l i t e s , l e s s t h a n 50% o f the t o t a l c a t i o n p o p u l a t i o n can be a c c o u n t e d f o r . A h i g h e r p e r c e n t a g e o f the c a t i o n s can be l o c a t e d i n d e h y d r a t e d samples, b u t the e f f e c t o f the d e h y d r a t i o n s t e p on the l o c a t i o n o f the c a t i o n s i s g e n e r a l l y not w e l l known. NMR measurements, on the o t h e r hand, are most s e n s i t i v e t o m o b i l e c a t i o n s and c a t i o n s i n h i g h symmetry s i t e s . The XRD d a t a c o l l e c t e d on d e h y d r a t e d Na-Y z e o l i t e s i n d i c a t e t h a t the d e t e c t a b l e sodium c a t i o n s r e s i d e i n t h r e e s i t e s ( 6 ) . S i t e I i s l o c a t e d i n the h e x a g o n a l p r i s m and has a low occupancy, about 10% o f the t o t a l c a t i o n s . S i t e I ' i s l o c a t e d i n the s o d a l i t e cage and about 30% o f the c a t i o n s occupy t h e s e s i t e s . The r e m a i n i n g c a t i o n s , a p p r o x i m a t e l y 60%, are found i n S i t e I I , w h i c h i s l o c a t e d i n the supercage o f the Y z e o l i t e ( 6 ) . The ammonium/sodium exchange i s o t h e r m f o r Y z e o l i t e (7) shows t h a t w i t h one exchange a t room temperature a maximum o f 60 to 65% o f the sodium i o n s can be r e p l a c e d by ammonium i o n s . I n l i g h t o f the above X - r a y d i f f r a c t i o n d a t a , these i o n exchange d a t a i n d i c a t e t h a t o n l y t h o s e c a t i o n s i n the supercage s i t e s o f Y z e o l i t e are r e a d i l y exchanged a t room t e m p e r a t u r e . A s i m i l a r i s o t h e r m i s o b s e r v e d f o r the exchange o f sodium by c a l c i u m i n Y z e o l i t e ( 7 ) , and f o r the exchange o f sodium by lanthanum i n Y z e o l i t e ( 7 , 8 ) . The a b i l i t y t o d i s t i n g u i s h a t l e a s t two t y p e s o f sodium c a t i o n s i n h y d r a t e d Y z e o l i t e s , u s i n g h i g h f i e l d sodium-23 magic a n g l e s p i n n i n g NMR (MASNMR), has r e c e n t l y been demonstrated by the a u t h o r s ( 9 ) . S p e c t r a o f h y d r a t e d Na-Y and s e r i e s o f p a r t i a l l y exchanged (NH^,Na)-Y and (Ca,Na)-Y z e o l i t e s were o b t a i n e d w h i c h a l l o w e d the s p e c i f i c sodium-23 MASNMR l i n e s t o be a s s o c i a t e d w i t h sodium c a t i o n s i n e i t h e r the supercage or the s m a l l e r cage s i t e s . A t l e a s t two, and p o s s i b l y t h r e e , l i n e s were i d e n t i f i e d . A resonance n e a r -2 ppm, w h i c h i s i n i t a l l y the most i n t e n s e l i n e , l o s e s i n t e n s i t y r a p i d l y as the sodium i s exchanged out o f the Y z e o l i t e . T h i s resonance has been a s s i g n e d to the sodium c a t i o n s i n the supercage. The second major resonance i s l o c a t e d n e a r -7 ppm and i t s i n t e n s i t y d e c r e a s e s o n l y v e r y s l o w l y , even a f t e r the m a j o r i t y o f the sodium has been exchanged out o f the z e o l i t e . T h i s second l i n e i s a s s i g n e d t o sodium c a t i o n s i n the s o d a l i t e cages and p o s s i b l y the h e x a g o n a l p r i s m s i t e s . There i s a l s o the p o s s i b i l i t y o f a t h i r d , p o o r l y r e s o l v e d l i n e o f low i n t e n s i t y . T h i s l i n e i s b r o a d , making i t s NMR p r o p e r t i e s d i f f i c u l t t o d e t e r m i n e . The o r i g i n o f t h i s t h i r d l i n e may i n c l u d e
Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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c o n t r i b u t i o n s from sodium c a t i o n s i n Type I s i t e s , nonmobile c a t i o n s i n the s u p e r c a g e s , and r e s i d u a l sodium w h i c h i s n o t associated with the cation s i t e s . The work d i s c u s s e d h e r e compares t h e r e l a t i v e s i t e o c c u p a n c i e s f o r t h r e e s e r i e s o f h y d r a t e d Y z e o l i t e s : ammonium, c a l c i u m and lanthanum exchanged. The d a t a f o r t h e s e same t h r e e s e r i e s o f samples i n the d e h y d r a t e d forms a r e a l s o p r e s e n t e d and the assignment o f t h e o b s e r v e d resonances i s d i s c u s s e d . EXPERIMENTAL Sample P r e p a r a t i o n Three s e r i e s o f p a r t i a l l y exchanged Y z e o l i t e s were u s e d f o r t h i s NMR s t u d y : ammonium, c a l c i u m and lanthanum exchanged. These samples were p r e p a r e d u s i n g a s i n g l e , 72 hour, room t e m p e r a t u r e exchange o f LZY-52, a sodium Y z e o l i t e . Ten grams, v o l a t i l e - f r e e , of LZY-52 were used p e r 100 c c o f exchange s o l u t i o n . To a c h i e v e the v a r y i n g degrees o f exchange, the m o l a r i t i e s o f t h e exchange s o l u t i o n s were v a r i e d as i n d i c a t e d i n T a b l e I . Each sample was washed w i t h 250 c c d e i o n i z e d water and t h e n d r i e d a t 40°C. The a n a l y s e s o b t a i n e d on each sample, as w e l l as t h e c a l c u l a t e d p e r c e n t exchange, a r e g i v e n i n T a b l e I . The h y d r a t e d samples were p r e p a r e d b y e q u i l i b r a t i n g t h e samples f o r 24 h o u r s i n a chamber m a i n t a i n e d a t 50% r e l a t i v e h u m i d i t y . The d e h y d r a t e d samples were p r e p a r e d b y d r y i n g the m a t e r i a l s a t 150°C f o r 24 h o u r s . The d e h y d r a t e d samples were s e a l e d i m m e d i a t e l y upon removal from t h e oven and were t r a n s f e r r e d t o the NMR r o t o r under a d r y n i t r o g e n atmosphere.
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 S p e c t r a . 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 . F o r the h y d r a t e d Y z e o l i t e 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 s c a n s , a r e c y c l e time o f 0.5 s e c . 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 , w h i c h i s l e s s t h a n 1/4 o f a 90° p u l s e w i d t h of sodium i n s o l u t i o n . Under t h e s e c o n d i t i o n s , t h e i n t e g r a t e d i n t e n s i t y o f t h e d i f f e r e n t sodium NMR l i n e s i n t h e spectrum c l o s e l y approximates t h e c o n c e n t r a t i o n o f d i f f e r e n t 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 . For the dehydrated Y z e o l i t e s , samples, s p i n n i n g r a t e s o f 6 t o 9 KHz were used. A t y p i c a l spectrum was a c q u i r e d u s i n g 5000 s c a n s , a 2 s e c . r e c y c l e time and a p u l s e w i d t h l e s s t h a n t h a t o f a s o l i d s 45° p u l s e (about 2.5 microsec).
RESULTS To determine t h e o r i g i n o f t h e sodium-23 l i n e s o b s e r v e d i n Na-Y z e o l i t e s , t h e y were p a r t i a l l y exchanged w i t h e i t h e r NH^ o r Ca
Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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Characterization ofYZeolites
TABLE
I
CATION EXCHANGED SAMPLES OF Y ZEOLITE
Sample Type
LZY-52
% Exchange
% Na 0
(of S i t e s )
(VF
(VF)
14.02
61.17
0
2
%S10
2
% A1 0 2
3
(VF)
M o l a r i t y of Exchange Solution
20.67
(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
11 21 35 45 50 52 56 58 62
11..11 9..95 8,.20 6,.92 6..32 5,.99 5,.51 5,.19 4,.81
0.,028 0.,050 0.,082 0.,123 0.,163 0.,245 0.,299 0.,367 0.,463
M M M M M M M M M
(NH ,Na)-Y (NH ,Na)-Y (NH ,Na)-Y (NH ,Na)-Y (NH Na)-Y (NH ,Na)-Y (NH ,Na)-Y (NH ,Na)-Y
9 26 32 41 47 53 55 56
11.,40 9.,32 8,.56 7,.37 6,.59 5,.95 5 .64 5 .48
0.,080 0.,161 0..241 0..361 0,.482 0,.723 0,.883 1..084
M M M M M M N M
(La.Na)-Y (La.Na)-Y (La,Na)-Y (La.Na)-Y (La,Na)-Y (La.Na)-Y (La.Na)-Y (La,Na)-Y
2 9 22 41 52 60 68 66
12.,31 11.,43 9.,81 7.,45 6.,03 5,.01 4,.08 4,.32
0..014 0,.027 0,.041 0,.054 0,.068 0,.095 0 .122 0 .149
M M M M M M M M
4
4
4
4
4>
4
4
4
Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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c a t i o n s ( 9 ) . S i n c e n e i t h e r o f t h e s e c a t i o n s i s s m a l l enough t o e n t e r t h e s o d a l i t e cages o r h e x a g o n a l p r i s m s i n the h y d r a t e d form, the s p e c t r a l changes o b s e r v e d f o r the p a r t i a l l y exchanged z e o l i t e s can be i d e n t i f i e d w i t h the replacement o f the Na c a t i o n s i n t h e supercages b y the NH^ o r Ca c a t i o n s . S i m u l a t i o n s o f such s p e c t r a 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 character q u a n t i t a t i v e l y e s t a b l i s h the amount o f Na c a t i o n s removed from t h e supercage s i t e s b y the c a t i o n exchange ( 9 ) . T h i s t y p e o f a n a l y s i s was extended t o t h e (La,Na)-Y z e o l i t e system f o r two r e a s o n s . F i r s t , t o e s t a b l i s h i f the b e h a v i o r o f a t r i p l y c h a r g e d c a t i o n can be a n a l y z e d by sodium-23 NMR i n a f a s h i o n s i m i l a r t o t h a t o f the s i n g l y c h a r g e d NH^ c a t i o n s and the d o u b l y c h a r g e d Ca c a t i o n s . Second, i o n exchange e x p e r i m e n t s have e s t a b l i s h e d t h a t h y d r a t e d L a c a t i o n s can n o t p e n e t r a t e t h e s m a l l e r cages, b u t t h a t l o w temperature c a l c i n a t i o n o f the z e o l i t e (
