Inter- and Intraparticle Diffusion of Ions in Zeolites - ACS Symposium

37 Inter- and Intraparticle Diffusion of Ions in Zeolites G. T. KOKOTAILO, S. L . LAWTON, and S. SAWRUK

Downloaded by UNIV LAVAL on July 9, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch037

Mobil Research and Development Corp., Paulsboro, N.J. 08066

ABSTRACT Ion migration i n and between d i f f e r e n t c a t i o n i c forms of the c r y s t a l l i n e z e o l i t e s A and X has been monitored by x - r a y d i f f r a c t i o n . The m o b i l i t y of the ions i s dependent on t h e i r state of hydration and temperature. The r e l a t i v e order of rates of i n t e r c r y s t a l l i n e ion exchange was determined.

Introduction In t h e d e t e r m i n a t i o n o f z e o l i t e s t r u c t u r e s i t i s d i f f i c u l t i f not impossible t o locate a l l the cations. T h i s i s thought t o be due t o t h e low occupancy o f c e r t a i n s i t e s by these c a t i o n s o r t o t h e i r m o b i l i t y . For example, i n t h e s t r u c t u r e d e t e r m i n a t i o n o f ZK-5 (1), i t was found t h a t a lower R f a c t o r c o u l d be obtained"" f o r x - r a y d i f f r a c t i o n d a t a o b t a i n e d from a sample a t 150°C t h a n f o r t h a t a t room t e m p e r a t u r e . This i s cons i s t e n t with a reduction i n d i f f u s e s c a t t e r i n g by removal o f water and b y a more r i g i d b i n d i n g o f c a t i o n s t o t h e framework. N u c l e a r magnetic s t u d i e s (2) o f h y d r a t e d and o u t gassed NaX and NaY z e o l i t e s have shown t h a t t h e sodium i o n s a r e m o b i l e , w i t h t h e speed o f motion b e i n g a f u n c t i o n o f water c o n t e n t . The temperature dependence o f t h e c a t i o n r e s o n a n c e , and t h e v e r y narrow p r o t o n r e s o n a n c e l i n e w i d t h a t temperatures above -20°C i n d i c a t i n g almost complete freedom o f motion o f water m o l e c u l e s i n t h e z e o l i t e c a v i t i e s , has been found ( 3 ) .

439 Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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H i r s t (£) h a s r e p o r t e d t h a t t h e 23Na NMR s p e c t r a o f water l o a d e d NaA and NaX i n d i c a t e d m o t i o n a l n a r r o w i n g by N a t r a n s l a t i o n a l jumps. The m o b i l i t y o f t h e c a t i o n s i n z e o l i t e s and t h e i r m i g r a t i o n a c r o s s c o n t a c t b o u n d a r i e s between a d j a c e n t c r y s t a l s may be s t u d i e d u s i n g x - r a y d i f f r a c t i o n b y o b s e r v i n g t h e r a t e o f change o f l a t t i c e parameter. In z e o l i t e s t h e c a t i o n jumps w i l l be dominated b y t h e s i z e o f t h e c a v i t i e s , openings t o c a v i t i e s , p e r i o d i c i t y i n t h e c r y s t a l l i n e e l e c t r o s t a t i c f i e l d s and c o n t a c t o f c r y s t a l s w i t h each o t h e r . In t h i s paper we r e p o r t on t h e i n t e r - and i n t r a c r y s t a l l i n e m i g r a t i o n o f c a t i o n s i n LiA-NaA, LiA-CaA, LiA-NH A, LiX-NaX, L1X-NH4X, & LiA-NaX m i x t u r e s as a f u n c t i o n o f h y d r a t i o n and t e m p e r a t u r e . L i X and L i A z e o l i t e s were chosen i n t h i s s t u d y p r i m a r i l y because o f t h e i r l a r g e d i f f e r e n c e s i n l a t t i c e parameters from t h o s e o f o t h e r c a t i o n forms. By making use o f t h i s f a c t , changes i n i o n c o n t e n t may be r e a d i l y m o n i t o r e d b y means o f x - r a y d i f f r a c t ion techniques. The s t u d y o f t h e m o b i l i t y o f i o n s i n z e o l i t e s i s o f i n t e r e s t i n s t r u c t u r e d e t e r m i n a t i o n and i n s o l i d s t a t e i o n exchange. 2 3


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Experimental The z e o l i t e s used were L i n d e 13X (NaX), L o t 187832; L i n d e 10X (CaX), L o t 109; L i n d e 4A (NaA), L o t 4353; and L i n d e 5A (CaA), L o t 5104. Samples o f L i X and L i A were p r e p a r e d b y aqueous i o n exchange w i t h r e a g e n t grade L i C l a t 90°C. The i o n exchanges were r e p e a t e d u n t i l no f u r t h e r change i n l a t t i c e parameter was o b s e r v e d . The samples were washed w i t h h o t water and d r i e d , and 92% o f t h e Na c a t i o n s were r e p l a c e d by L i i n NaA and 95% i n NaX. Samples o f NH4X and NH4A were p r e p a r e d b y exchanging t h e sodium forms w i t h NH4NO3 a t room temperature, washing and drying. In t h e h y d r a t e d m i x t u r e s t u d i e s t h e component z e o l i t e s were f i r s t e q u i l i b r a t e d a t room temperature w i t h an atmosphere a t a r e l a t i v e h u m i d i t y o f 50%. The mixed systems were t h e n p r e p a r e d b y p l a c i n g 0.2 gm o f

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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each component i n a p l a s t i c v i a l , t o g e t h e r w i t h a p l a s t i c sphere, and m i x i n g w i t h a W i g g l e Bug f o r two minutes. The W i g g l e Bug d e s c r i b e s a f i g u r e 8 motion w i t h 100 v i b r a t i o n s p e r second. This a c t i o n gives a h i g h degree o f m i x i n g and p a r t i c l e - t o - p a r t i c l e c o n t a c t w i t h o u t i n t r o d u c i n g any a p p r e c i a b l e g r i n d i n g a c t i o n . X - r a y d i f f r a c t i o n p a t t e r n s were o b t a i n e d immediately a f t e r mixing. In t h e d e h y d r a t e d m i x t u r e s t u d i e s t h e i n d i v i d u a l components o f e q u a l weight were f i r s t c a l c i n e d f o r s i x hours a t 500°C i n ambient a i r , then mixed h o t and c a l c i n e d f u r t h e r . Samples were w i t h drawn a t v a r i o u s t i m e i n t e r v a l s and immediately scanned b y an x - r a y d i f f r a c t o m e t e r . The x - r a y d i f f r a c t i o n p a t t e r n s were o b t a i n e d u s i n g a N o r e l c o x - r a y d i f f r a c t o m e t e r and n i c k e l f i l t e r e d copper r a d i a t i o n . P a t t e r n s were o b t a i n e d w i t h i n t h e a n g u l a r range 2Θ-j3) . i t has 4.1A openings and a l l t h e c a t i o n s occupy s i t e s i n t h e l a r g e c a v i t y ; none have been found i n t h e s o d a l i t e cages. The L i n d e X s t r u c t u r e has a l a r g e c a v i t y 13A i n d i a m e t e r and windows o f about 7.4A i n d i a m e t e r (9,10). The l a r g e c a v i t i e s and openings i n L i n d e X s h o u l d make i o n and w a t e r m i g r a t i o n more f a c i l e t h a n i n L i n d e A. The r e s u l t s o f NMR s t u d i e s ( 3 ) o f c a t i o n s and w a t e r i n z e o l i t e s i n d i c a t e t h a t c a t i o n s form a k i n d o f c a t i o n s o l u t i o n w i t h a d s o r b e d water and t h e a n i o n i c framework. The i n t e r s i t e m o t i o n o f c a t i o n s would t h u s be f a c i l i t a t e d by the presence o f water. A l l the c a t i o n s a r e s i t u a t e d on t h e s u r f a c e o f t h e a l u m i n o - s i l i c a t e framework. T h i s i o n m i g r a t i o n would t h e n be a s u r f a c e r a t h e r t h a n b u l k phenomenon. Barrer(11) reported that s u r f a c e d i f f u s i o n c o e f f i c i e n t s a r e much l a r g e r t h a n bulk. The v e r y f a s t i n t e r - and i n t r a p a r t i c l e m i g r a t i o n o f l i t h i u m , sodium and ammonium i o n s i n t h e h y d r a t e d case confirms t h i s . The h y d r a t e d i o n i c r a d i i , and t h e number o f water m o l e c u l e s a s s o c i a t e d w i t h a f u l l y hy drated i o n i n d i c a t e d i n parentheses, o f L i , Na , K , and N H a r e 3.82 (2.8), 3.58 (1.2), 3.31 (0.9) and 3.31A(0.5), r e s p e c t i v e l y , compared w i t h t h e d e h y d r a t e d i o n i c r a d i i 0.60, 0.95, 1.33 and 1.48A (12). The s m a l l e r i o n s become more h y d r a t e d and t h e i r l a r g e r a d i i reduce t h e r a t e o f m i g r a t i o n . The l a r g e i o n s such as +

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Superimposed p a t t e r n ; * denote L i A r e f l e c t i o n s


Two minute W i g g l e Bug m i x t u r e o f 1:1 h y d r a t e d components

P r e c a l c i n e d 6 h r s a t 500°C, mixed h o t and c a l c i n e d 18 h r s a t 500°C



Figure 4.

X-ray diffraction patterns of the LiA-NaX

system

American Chemical Society Library 1155 18th St.. N.W. Washington, D £ 20036


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K and NH4 " a r e e s s e n t i a l l y b a r e o r unhydrated and t h e i r m o b i l i t y i s g r e a t e r because o f t h e i r s m a l l e r hy d r a t e d r a d i i (12). The i n c r e a s e d r a t e o f i n t e r - m i g r a t i o n o f i o n s i n the L i - N H over the L i - N a system i s c o n s i s t e n t with the higher m o b i l i t y of ions with smaller hydrated radii. Of c o u r s e , t h e c a t i o n s i n t h e z e o l i t e systems s t u d i e d may not be f u l l y h y d r a t e d and may have s m a l l e r h y d r a t e d r a d i i w i t h lower m o b i l i t y . The l a c k o f i n t r a p a r t i c l e d i f f u s i o n o f i o n s i n the hydrated L i - C a z e o l i t e system i s p r o b a b l y due t o the d i f f e r e n c e i n v a l e n c e s t a t e o f t h e c a t i o n s . The h y d r a t e d a n d d e h y d r a t e d i o n i c r a d i i o f C a are 4.12 and .99A, i n d i c a t i n g a h i g h d e g r e e o f h y d r a t i o n . T h i s s h o u l d r e d u c e i t s m o b i l i t y somewhat b u t not com pletely. The l a r g e d e c r e a s e i n t h i s r a t e o f m i g r a t i o n a t e l e v a t e d t e m p e r a t u r e s (500°C) c o n f i r m s t h e p a r t t h a t water p l a y s . Of c o u r s e , t h e r e was c o n s i d e r a b l y g r e a t e r c o n t a c t o f z e o l i t e c r y s t a l s i n t h e room temper ature case. P a r t i a l d e h y d r a t i o n o f NaY and f a u j a s i t e z e o l i t e s , which have e s s e n t i a l l y t h e same s t r u c t u r e as NaX b u t d i f f e r i n g i n t h e S i / A l r a t i o and c a t i o n c o n t e n t , cause some o f t h e c a t i o n s t o move i n t o s i t e 1 (the d o u b l e six-membered r i n g ) . Bennett and Smith (13) have r e p o r t e d t h a t i n d e h y d r a t e d calcium-exchanged f a u j a s i t e , t h i s s i t e was f u l l y o c c u p i e d . T h i s was a l s o r e p o r t e d by Baur (14). C a l c i u m i o n s show a l a r g e p r e f e r e n c e f o r water m o l e c u l e s and i n t h e h y d r a t e d z e o l i t e s t r u c t u r e a r e l o o s e l y bound t o the a n i o n i c framework. When t h e c a l c i u m i o n l o s e s i t s water o f h y d r a t i o n , i t wants t o surround i t s e l f w i t h oxygen i o n s and t h e r e f o r e b u r i e s i t s e l f i n t o t h e a n i o n i c framework w i t h a p r e f e r e n c e f o r s m a l l cages, such as the d o u b l e six-membered r i n g s . I t now becomes more t i g h t l y bound and i t s ease o f migration to other s i t e s i s considerably retarded. Re h y d r a t i o n w i l l a l l o w i t t o m i g r a t e from s i t e 1 t o o t h e r s i t e s i n the s t r u c t u r e . The v e r y slow i n t e r and i n t r a p a r t i c l e m i g r a t i o n o f c a l c i u m i o n s i s , i n p a r t , due t o t h i s t i g h t e r b i n d i n g o f c a l c i u m i o n s on dehydration. I t i s a l s o p a r t i a l l y due t o t h e r e p l a c e ment o f one c a l c i u m i o n by two l i t h i u m i o n s i n t h i s +

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random walk p r o c e s s . Sodium a l s o shows a tendency t o m i g r a t e t o s i t e 1 on d e h y d r a t i o n , as seen i n t h e NaY c a s e (15). The occupancy i s much lower and i t r e h y d r a t e s much more r e a d i l y . The m i g r a t i o n o f i o n s from one z e o l i t e c r y s t a l t o a n o t h e r and t h e i r r e d i s t r i b u t i o n w i t h i n t h e c r y s t a l i s a t l e a s t a two s t e p p r o c e s s . The i o n must f i r s t m i g r a t e a c r o s s a c o n t a c t boundary between two c r y s t a l s * then an i n t e r s i t e m i g r a t i o n w i t h i n t h e c r y s t a l . Inters i t e m i g r a t i o n w i l l depend t o a l a r g e e x t e n t on t h e degree o f h y d r a t i o n . In t h e c a s e o f z e o l i t e X, c a t i o n s i n t h e s m a l l cages w i l l be more t i g h t l y bound than those i n t h e l a r g e cages and i o n m i g r a t i o n i n and o u t w i l l be r e t a r d e d n o t o n l y due t o t h i s t i g h t e r b i n d i n g b u t b y t h e s m a l l windows t o t h e s m a l l c a g e s . I t i s d i f f i c u l t t o s e p a r a t e t h e i n t e r - and i n t r a c r y s t a l l i n e r a t e s o f d i f f u s i o n as i n t h e h y d r a t e d c a s e t h e r a t e i s v e r y r a p i d and i n t h e h i g h tempera t u r e c a s e t h e i n t r a c r y s t a l l i n e m i g r a t i o n becomes more complex. The c r y s t a l s i n t h e z e o l i t e A and X samples used were 1 - 5μ. There a r e 4.9 χ 1 0 c a t i o n s i n 2μ L i A and NH A c r y s t a l s . In t h e m i x i n g p r o c e s s u s i n g a Wiggle Bug, each c r y s t a l w i l l c o n t a c t s i x o t h e r c r y s t a l s , i f t h e r e i s c l o s e p a c k i n g , 100 times p e r second s i n c e t h e p r o b a b i l i t y o f a L i A c r y s t a l c o n t a c t i n g a n o t h e r L i A c r y s t a l i s .5. In t h e h y d r a t e d L i A NH4A system, where a s i n g l e phase LÏNH4A system i s formed a f t e r a two minute m i x i n g i n a W i g g l e Bug, 4.1 χ 1 0 L i and 4.1 χ Ι Ο NH4 i o n s m i g r a t e a c r o s s a c r y s t a l f a c e i n t h i s time i n t e r v a l . The c a t i o n c o n t e n t o f t h e u n i t c e l l o u t e r l a y e r o f a 2μ z e o l i t e A c r y s t a l i s 3.7 χ 10**. The average r a t e o f c a t i o n migration across a c r y s t a l face contact i s 3.4 χ 1 0 p e r second o v e r a two minute p e r i o d . This i s a q u a l i t a t i v e p i c t u r e o f the high rate o f i n t r a p a r t i c l e d i f f u s i o n o f c a t i o n s i n mixed c a t i o n z e o l i t e systems. The r a t e o f i n t r a p a r t i c l e d i f f u s i o n i s much greater than that o f i n t e r p a r t i c l e d i f f u s i o n . I f more t h a n two c a t i o n s a r e i n v o l v e d i n a m i x t u r e o f two z e o l i t e s , t h e number o f phases i n c r e a s e s and t h e c o m p l e x i t y o f t h e d i s t r i b u t i o n o f i o n s increases. 1 0

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In t h e L i n d e A c a s e , t h e i n t r a p a r t i c l e d i f f u s i o n i s f a i r l y straightforward since the cations are a l l i n the l a r g e cages ( t r u n c a t e d c u b o - o c t a h e d r a ) and t h e windows t o t h e s e l a r g e cages a r e a l l eight-membered r i n g s (4.1A). D e h y d r a t i o n r e s u l t s i n a s t r o n g b i n d i n g o f t h e c a t i o n s t o t h e a n i o n i c framework t h e r e by r e d u c i n g t h e r a t e o f i n t e r s i t e m i g r a t i o n . o

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Inter- and Intraparticle Diffusion of Ions in Zeolites - ACS Symposium

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