Transition-Metal Ion Exchange in Synthetic X and Y Zeolites

20 Transition-Metal Ion Exchange in Synthetic X and Y Zeolites

Downloaded by UNIV QUEENSLAND on October 8, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch020

Stoichiometry and Reversibility A. MAES and A. CREMERS Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, De Croylaan 42, B-3030 Heverlee, Belgium

The ion-exchange reaction of the synthetic zeolites NaX and NaY with cobalt, zinc, and nickel ions is shown to be non-stoichiometric at low bivalent-ion occupancy, the hydrolytic sodium loss being about twice as large for NaX (~5 ions/unit cell) as for NaY. The effect is more pronounced at high temperatures and disappears at high occupancies. Reversibility tests in NaX toward zinc and cobalt ions, as studied by a temperature-variation method, show the temperature history to be an important factor in the irreversibility characteristics. The low-temperature partial irreversibility, induced by a high-temperature treatment (45°C) is interpreted in terms of a temperature-dependent occupancy of the "small-cage" sites by divalent cations, which become irreversibly blocked at low temperature (5°C).

'"P'he

s t u d y of the properties of zeolites, either s y n t h e t i c or n a t u r a l , has

received a great deal of a t t e n t i o n i n recent years. A m o n g t h e s y n t h e t i c zeolites, the faujasites X a n d Y types h a v e been most f r e q u e n t l y a n d t h o r o u g h l y s t u d i e d . A s u m m a r y of the advances i n t h i s area is f o u n d i n a recent r e v i e w b y S h e r r y (1 ). T h e s t r u c t u r e of a n d possible c a t i o n l o c a t i o n i n these m a t e r i a l s is f a i r l y A

w e l l k n o w n (2, 3, 4, 5), a n d t h e i r ion-exchange b e h a v i o r t o w a r d a m u l t i t u d e of p a i r s of ions, m o s t l y i n c l u d i n g s o d i u m , has been measured a n d i n t e r p r e t e d i n t e r m s of basic properties of ions, c r y s t a l structures, a n d pore d i mensions. T h e m a j o r p a r t of these studies is w i t h a l k a l i - a n d a l k a l i n e e a r t h cations, a l k y l a m m o n i u m ions, r a r e - e a r t h cations, a n d s i l v e r a n d t h a l l i u m ions (1). I n contrast, t h e i o n a d s o r p t i o n of t r a n s i t i o n metals i n faujasite has received l i t t l e a t t e n t i o n . 230 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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T h e c a l c u l a t i o n of t h e affinity scale, i n t e r m s of differences i n freeenergy content between the v a r i o u s i o n i c forms of these m a t e r i a l s , i m p l i e s t h a t one is d e a l i n g w i t h e q u i l i b r i u m systems a n d t h a t t h e r e a c t i o n i s b o t h reversible a n d s t o i c h i o m e t r i c , i.e., h y d r o l y s i s p h e n o m e n a are absent i n t h e zeolite. W i t h i n c e r t a i n l i m i t s , these c o n d i t i o n s are generally m e t ; h o w ever, i t is a p p a r e n t t h a t some discrepancies between e x p e r i m e n t a l d a t a h a v e sometimes been a t t r i b u t e d (1) t o a f a i l u r e i n t h e f u l f i l l m e n t of one o r more of these basic prerequisites.


T h i s p a p e r presents some d a t a r e l a t i n g t o these aspects w h i c h h a v e been o b t a i n e d i n the course of a n extensive e x p e r i m e n t a l s t u d y of t h e i o n exchange b e h a v i o r of t r a n s i t i o n - m e t a l ions i n X a n d Y zeolites.

Experimental M a t e r i a l s . T h e zeolites s t u d i e d a r e t h e c o n v e n t i o n a l s y n t h e t i c X a n d Y zeolites o b t a i n e d f r o m U n i o n C a r b i d e C o r p . , L i n d e D i v i s i o n ( X : l o t # 12.967-38; Y : l o t # 12.805-66). B e f o r e use, t h e s a m p l e s are s a t u r a t e d i n 100 g r a m batches w i t h 1M N a C l a n d d i a l y z e d against d i s t i l l e d w a t e r u n t i l free of c h l o r i d e . T h e y are t h e n d r i e d a t 50° C , g r o u n d , a n d s t o r e d a t r o o m t e m p e r a t u r e over s a t u r a t e d N H C 1 . T h e a n h y d r o u s u n i t cell c o m p o s i t i o n s of the r e s u l t i n g m a t e r i a l s are Na 5Al Siio70384 a n d N a 4 AI54S1138O384, as c a l c u l a t e d f r o m s t a n d a r d c h e m i c a l a n a l y s i s . T h e average v a l u e f o r t h e N a : A l a t o m r a t i o i s 1 =fc 0.015. T h e c o r r e s p o n d i n g c a t i o n exchange capacities are 6.25 a n d 4.23 m e q / g r a m . T h e reagents u s e d were a n a l y t i c a l - g r a d e n i t r a t e s (zinc, n i c k e l , s o d i u m ) o r c h l o r i d e s (cobalt, s o d i u m ) d e p e n d i n g o n t h e p a i r of ions s t u d i e d . T h e isotopes u s e d were ^ C o , N a , Z n , a n d N i , as o b t a i n e d f r o m N . I . R . ( M o l , B e l g i u m ) . R a d i o c h e m i c a l assays were p e r f o r m e d u s i n g either a P a c k a r d T r i c a r b ( m o d e l 2425) s c i n t i l l a t i o n spectrometer o r a P a c k a r d a u t o m a t i c s i n g l e - c h a n n e l g a m m a s c i n t i l l a t i o n spectrometer. T h e V i s k i n g dialysis membranes ( M e d i c e l l I n t . L o n d o n ) were p r o f u s e l y w a s h e d w i t h d i s t i l l e d w a t e r b e fore use i n the d i a l y s i s e q u i l i b r i a . M e t h o d s . T h e h y d r o l y t i c b e h a v i o r of N a X w a s i n v e s t i g a t e d i n a N a C l c o n c e n t r a t i o n range of 10 ~ t o 1 0 ~ i l f , u s i n g a t o m i c a b s o r p t i o n a n d r a d i o c h e m i c a l techniques. A p p r o p r i a t e a m o u n t s of zeolite were w e i g h e d i n t o d i a l y s i s m e m b r a n e s w h i c h were k n o t t e d a t one e n d a n d a i r - d r i e d p r i o r t o use. F i v e m l of N a C l s o l u t i o n of k n o w n c o n c e n t r a t i o n were a d d e d , a n d t h e m e m b r a n e s were closed a n d p l a c e d i n t o p o l y e t h y l e n e t u b e s c o n t a i n i n g k n o w n s o l u t i o n v o l u m e s of the c o r r e s p o n d i n g N a C l c o n c e n t r a t i o n . T h e systems were t h e n s h a k e n for 24 h r at 2 2 ° C i n a n e n d - o v e r - e n d s h a k e r a n d t h e d i a l y s a t e s were a n a l y z e d f o r s o d i u m b y a t o m i c a b s o r p t i o n . T h e h y d r o l y t i c s o d i u m loss w a s o b t a i n e d d i r e c t l y f r o m c o n c e n t r a t i o n differences. T h e r a d i o c h e m i c a l procedure w a s i d e n t i c a l i n e v e r y respect except for the fact t h a t t h e N a C l s o l u t i o n s were l a b e l e d w i t h N a p r i o r t o e q u i l i b r i u m . T h e s o d i u m loss f r o m t h e c r y s t a l s w a s o b t a i n e d d i r e c t l y f r o m t h e r a d i o a c t i v i t i e s of the o r i g i n a l solutions a n d e q u i l i b r i u m d i a l y s a t e s a n d t h e C . E . C . v a l u e s w h i c h were d e t e r m i n e d b y t o t a l a n a l y s i s of t h e samples (6.25 a n d 4.23 m e q / g r a m ) . T h i s i s o t o p i c d i l u t i o n m e t h o d w a s used f o r b o t h X a n d Y samples a t O.OL/lf N a C l . 4

8

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6 5

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In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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T h e s t o i c h i o m e t r i c measurements were m a d e u s i n g a s i m i l a r d i a l y s i s t e c h n i q u e . E x a c t l y 50 m g zeolite ( ± 0.1-0.2 mg) was weighed i n t o a d i a l y s i s m e m b r a n e t o w h i c h 5 m l O.OliV N a C l (or N a N 0 ) was p i p e t t e d . T h e m i x t u r e was e q u i l i b r a t e d w i t h 40 m l of a m i x e d e l e c t r o l y t e s o l u t i o n of 0.01 t o t a l n o r m a l i t y , c o n t a i n i n g s o d i u m a n d the b i v a l e n t c a t i o n ( N i , Z n , C o ) i n v a r i o u s p r o p o r t i o n s . E a c h c o m b i n a t i o n requires t w o i d e n t i c a l e x p e r i m e n t s , i n v o l v i n g either a N a or a l a b e l of the t r a n s i t i o n element. E q u i l i b r a t i o n s were m a d e i n a n e n d - o v e r - e n d s h a k e r (5 or 2 5 ° C ) for one t o t w o weeks. T h e i o n d i s t r i b u t i o n s were c a l c u l a t e d f r o m the a m o u n t s of r a d i o a c t i v i t y of the i n i t i a l a n d t h e e q u i l i b r i u m s o l u t i o n s f o r assays of d u p l i c a t e 5 m l samples. F o r p r e c i s i o n , the t o t a l n u m b e r of c o u n t s c o l l e c t e d i n t h e N a - l a b e l e d samples was a l w a y s a b o u t 10 . R e v e r s i b i l i t y tests were made for cobalt a n d z i n c ions i n N a X u s i n g a t e m p e r a t u r e - v a r i a t i o n m e t h o d w h i c h is b a s e d u p o n a c o m p a r i s o n of the l o w t e m p e r a t u r e e q u i l i b r i a of t w o i d e n t i c a l s y s t e m s , one of w h i c h h a d been p r e t r e a t e d a t a h i g h e r t e m p e r a t u r e . S a m p l e s of 50 m g N a X ( + 5 m l N a C l o r O.OliV N a N 0 ) were e q u i l i b r a t e d w i t h 40 m l of m i x e d s o l u t i o n s at 5° a n d 4 5 ° C . A f t e r 24 h r , h a l f of t h e samples at 4 5 ° C were t r a n s f e r r e d t o t h e 5 ° C t h e r m o s t a t a n d v i c e v e r s a . A l l systems were t h e n s h a k e n for a n o t h e r 24 h r , a n d t h e e q u i l i b r i u m s o l u t i o n s were assayed for c o b a l t or z i n c . T h e i n i t i a l e q u i v a l e n t fractions of M i n the s o l u t i o n s are 0.05, 0.1, a n d 1. E a c h i n d i v i d u a l e x p e r i m e n t was p e r f o r m e d i n d u p l i c a t e o r t r i p l i c a t e , a n d a l l assays were made i n d u p l i c a t e . T h e m a x i m u m l o a d i n g s i n X zeolite were o b t a i n e d b y t w o different methods. I n t h e first m e t h o d , k n o w n a m o u n t s of zeolite were e x h a u s t i v e l y s a t u r a t e d w i t h c o b a l t o r z i n c s o l u t i o n s (O.OliV) a t b o t h t e m p e r a t u r e s for 10 d a y s a n d s u b s e q u e n t l y e q u i l i b r a t e d w i t h l a b e l e d s o l u t i o n s of t h e same c o n c e n t r a t i o n d u r i n g three d a y s ; t h e m a x i m u m l o a d i n g was t h e n o b t a i n e d f r o m the changes i n r a d i o a c t i v e c o n t e n t . T h e s e c o n d m e t h o d , w h i c h was l i m i t e d t o 4 5 ° C , differed f r o m t h e first i n t h a t t h e zeolites were e x h a u s t i v e l y s a t u r a t e d for t w o weeks w i t h O.OliV s o l u t i o n s w h i c h c o n t a i n e d a r a d i o a c t i v e l a b e l f r o m the s t a r t ; t h e m a x i m u m l o a d i n g was t h e n o b t a i n e d f r o m i t s r a d i o a c t i v e c o n t e n t assayed after a c i d d i s s o l u t i o n of t h e m a t e r i a l . 5

2 2

2 2


6

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2

Results and Discussion H y d r o l y s i s of N a Z e o l i t e s X a n d Y . T h e h y d r o l y t i c b e h a v i o r of N a X , as o b t a i n e d f r o m a n a l y t i c a l measurements of excess N a i o n i n t h e d i a l y sates, has been s t u d i e d at five N a C l concentrations ( 1 0 , 5.10~ , 1 0 ~ , 5.10~ , 1 0 ~ M ) a n d a t v a r i o u s z e o l i t e : s o l u t i o n r a t i o s v a r y i n g f r o m 0.78 t o n e a r l y 6 g r a m s / l i t e r . T h e p r e c i s i o n a t the highest N a C l concentrations is r e l a t i v e l y poor, b u t i t appears t h a t , w i t h i n e x p e r i m e n t a l error, the excess N a c o n c e n t r a t i o n i n t h e dialysates is p r a c t i c a l l y i n d e p e n d e n t of t h e i n i t i a l N a c o n c e n t r a t i o n a t h i g h zeolite contents. A t l o w zeolite content, there is a slight t e n d e n c y for higher excess N a concentrations t o w a r d t h e l o w c o n c e n t r a t i o n scale. T h e e x p e r i m e n t a l values, expressed i n t e r m s of s o d i u m loss f r o m the c r y s t a l , v a r y f r o m a b o u t 0.7 to n e a r l y 3 i o n s / u n i t cell a t a zeolite content a t a b o u t 0.8 g r a m / l i t e r . T h e results for t h e 1 0 Μ N a C l systems, for w h i c h t h e precision is best, are s h o w n i n T a b l e I ; the +

- 4

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Table L Hydrolysis of N a X in 1 0 ~ M N a C l at Different Zeolite Contents Zeolite Content, gram/liter Sodium Loss, ions/unit cell 4

5.88 2.45 1.63 1.13 0.77

0.73 1.49 2.10 2.60 2.70


d a t a are averages of d u p l i c a t e measurements for w h i c h t h e agreement is of the order of 5 % . T h e p H values of t h e e q u i l i b r i u m s o l u t i o n , as occasion a l l y measured, were a l w a y s a b o u t 6. A d d i t i o n a l checks were made u s i n g a d i a l y s i s procedure i n w h i c h zeolite samples w h i c h h a d been i s o t o p i c a l l y e q u i l i b r a t e d w i t h a N a 0 . 0 1 M N a C l s o l u t i o n were d i a l y z e d against d i s t i l l e d w a t e r . A f t e r repeated w a s h ings, the s o d i u m loss f r o m N a X reached a steady v a l u e of 2.8 ( ± 0 . 1 ) i o n s / u n i t cell a t a zeolite content of 0.44 g r a m / l i t e r , i.e., a v a l u e w h i c h is n e a r l y i d e n t i c a l t o t h e d a t a i n T a b l e I for a 10 ~ W N a C l c o n c e n t r a t i o n . Under s i m i l a r conditions, t h e s o d i u m loss f r o m N a Y is m u c h less a n d corresponds t o 1.5 ( ± 0 . 1 ) i o n s / u n i t cell. These results were c o n f i r m e d b y e l e c t r i c a l c o n d u c t i v i t y measurements o n the respective d i a l y s a t e s ; t h e c o n d u c t i v i t y for N a X is a b o u t t w i c e as large (7.5 X 1 0 ~ m h o s / c m ) as for N a Y (3.9 X 10~ m h o s / c m ) . 2 2

6

6

T h e h y d r o l y t i c b e h a v i o r of b o t h N a X a n d N a Y as measured a t 1 0 i l f . N a C l b y isotopic d i l u t i o n methods has also been measured as a f u n c t i o n of the zeolite content. T h e results, a l o n g w i t h s t a n d a r d d e v i a t i o n s f r o m t h e m e a n , are s u m m a r i z e d i n T a b l e I I ; t h e figure i n parenthesis indicates t h e - 2

Table II.

Hydrolysis of N a X and N a Y at 1 0 " M N a C l Obtained from Isotopic Dilution Zeolite Content, gram/liter Ν a Loss, ions/unit cell 2

NaX

NaY α

0.82 2.1 7.5 0.82 3.0

5.2 ± 0 . 5 ( 7 ) · 1.8 ± 0 . 2 5 (2) 0.75 ± 0 . 0 9 (3) 2.9 ± 0 . 3 (7) 1.3 ± 0 . 0 4 (3)

Number of duplicates.

n u m b e r of duplicates. I t is a p p a r e n t t h a t t h e isotopic d i l u t i o n m e t h o d leads t o somewhat higher values for t h e s o d i u m loss f r o m t h e zeolite c r y s tals. E v i d e n t l y t h e results o b t a i n e d f r o m changes i n r a d i o a c t i v e c o n t e n t i n d u c e d b y isotopic d i l u t i o n i n t h e zeolite are f a i r l y sensitive t o t h e v a l u e t a k e n for t h e C . E . C . ; a t worst, a 1 % error i n the C . E . C . v a l u e is reflected i n a 1 0 % difference i n t h e s o d i u m loss, a figure w h i c h is t h e order of r e p r o d u c i b i l i t y of t h e measurement. W h a t e v e r t h e reason for t h e d i s c r e p a n c y



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between t h e t w o methods, i t is o b v i o u s t h a t t h e h y d r o l y t i c s o d i u m loss i n N a X is n e a r l y t w i c e t h e v a l u e f o u n d for N a Y . I t i s t e m p t i n g t o correlate this figure w i t h the p r o p o r t i o n of m o n o v a l e n t cations i n t h e " s u p e r c a g e s " for X a n d Y w h i c h is also a b o u t 2. W h e t h e r t h i s is a n y t h i n g m o r e t h a n a coincidence is difficult t o decide. A t a n y rate, t h e difference i n b e h a v i o r between N a X a n d N a Y corroborates t h e finding t h a t the first one is genera l l y m u c h m o r e sensitive t o a c i d b r e a k d o w n t h a n t h e second, as evidenced b y t h e fact t h a t N a X is c o m p l e t e l y dissolved i n O . l i V H C 1 a n d N a Y is n o t . F r o m a p u r e l y p r a c t i c a l p o i n t of v i e w i t m a y be c o n c l u d e d t h a t t h e w a s h i n g process, for t h e purpose of r e m o v i n g excess s o d i u m w i t h d i s t i l l e d w a t e r , is not l i k e l y t o i n d u c e a n y significant s o d i u m loss i n either m a t e r i a l since t h e w a t e r : z e o l i t e p r o p o r t i o n s are u s u a l l y m u c h smaller t h a n t h e values used i n this work. S t o i c h i o m e t r y . T h e effect of b i v a l e n t i o n o c c u p a n c y u p o n t h e s t o i c h i o m e t r y is s h o w n i n F i g u r e 1. T h e s t o i c h i o m e t r y factor / is defined as t h e n u m b e r of N a + ions d e s o r b e d / M ions adsorbed a n d t h e d e v i a t i o n f r o m 2 is a measure of t h e h y d r o l y t i c s o d i u m loss. A s before, i t appears t h a t N a X is m u c h more sensitive t o excess s o d i u m loss t h a n N a Y ; a t l o w occ u p a n c y of M , t h e d a t a are c o m p a r a b l e w i t h t h e results of T a b l e I I : 6-8 i o n s / u n i t cell ( N a X ) a n d 2.5-3 ( N a Y ) , w h i c h a g a i n differ b y a f a c t o r of a b o u t 2. 2 +

2

+

A t h i g h M + o c c u p a n c y , / becomes 2.15 i n N a X w h i c h corresponds t o a N a loss of a b o u t 3 i o n s / u n i t cell, whereas i n N a Y , / is e x a c t l y 2. T h e effect o n / for t h e t h r e e cations s t u d i e d is q u i t e s i m i l a r a l t h o u g h there is a slight t e n d e n c y for h i g h e r / v a l u e s for n i c k e l . 2

Figure 1. Stoichiometry factor vs. bivalent ion occupancy in NaX (upper curve) and NaY (lower curve) at 25°C for cobalt (squares), nickel (circles), and zinc (triangles); ( ) confidence interval at 95% level


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T h e effect of t e m p e r a t u r e o n h y d r o l y s i s is s h o w n i n F i g u r e 2 for n i c k e l . A t e m p e r a t u r e decrease f r o m 22 t o 5 ° C leads t o a n a p p a r e n t l y significant decrease of / i n N a X , whereas t h e N a Y b e h a v i o r is b a r e l y different. At h i g h N i o c c u p a n c y , the t e m p e r a t u r e has no effect o n either X or Y . The extent of r e p r o d u c i b i l i t y is perhaps best i l l u s t r a t e d b y t h e C . E . C . v a l u e s a t h i g h M + o c c u p a n c y : for N a Y a t 12 < M + < 18 i o n s / u n i t cell, the C . E . C . v a l u e as o b t a i n e d f r o m a d d i t i o n of t h e separately d e t e r m i n e d ions is 4.19 ± 0.04 m e q / g r a m (the average of 20 d e t e r m i n a t i o n s ) , a v a l u e w h i c h is n o t s i g n i f i c a n t l y different f r o m the N a content of the m a t e r i a l as f o u n d b y t o t a l a n a l y s i s . F o r N a X a t 2 0 < M + < 28 i o n s / u n i t cell, t h e C . E . C . v a l u e is 6.01 ± 0.06, a figure w h i c h corresponds t o the a f o r e m e n t i o n e d s o d i u m loss of 3 i o n s / u n i t cell. A t loadings a p p r o a c h i n g s a t u r a t i o n , t h e s t o i c h i o m e t r y f a c t o r i n N a X becomes 2 as e v i d e n c e d b y t h e f a c t t h a t t h e m a x i m u m o c c u p a n c y of cobalt ions o b t a i n e d b y e x h a u s t i v e s a t u r a t i o n a t 4 5 ° C is i d e n t i c a l to the C . E . C . v a l u e : 6.27 ± 0 . 1 1 . 2

2


2

5

10

15

20

25 M**ions/uc

Figure 2. Effect of temperature upon stoichiometry factor for adsorption of nickel ions in NaX (circles) and NaY (triangles): · , A : 25°C; Ο, Δ : 5°C R e v e r s i b i l i t y . A p p a r e n t i r r e v e r s i b i l i t y p h e n o m e n a of i o n exchange i n N a X were s t u d i e d w i t h z i n c a n d c o b a l t ions u s i n g a t e m p e r a t u r e - v a r i a t i o n m e t h o d d e s c r i b e d i n the e x p e r i m e n t a l section. I n v i e w of t h e h i g h selec t i v i t y of N a X for b i v a l e n t cations a t l o w zeolite l o a d i n g , t h e c o n c e n t r a t i o n of b i v a l e n t ions i n the e q u i l i b r i u m s o l u t i o n is q u i t e sensitive t o s m a l l changes i n the surface c o m p o s i t i o n . I n fact, the a d s o r p t i o n r e m o v a l of b i v a l e n t cations a t l o w l o a d i n g , b e l o w 0.2, is q u a n t i t a t i v e or n e a r l y so ( 9 9 . 5 % or better). C o n s e q u e n t l y the v a l u e of the e q u i l i b r i u m c o n c e n t r a t i o n is a n i d e a l c r i t e r i o n for assessing either r e v e r s i b i l i t y or e q u i l i b r i u m conditions.



236

MOLECULAR SIEVES

T a b l e I I I shows t h e results a t t w o loadings i n t e r m s of t h e e q u i l i b r i u m concentrations of c o b a l t a n d z i n c a t t w o temperatures (5° a n d 4 5 ° C ) a n d t w o t e m p e r a t u r e c o m b i n a t i o n s : 24 h r a t 5 ° C followed b y 24 h r a t 4 5 ° C a n d v i c e v e r s a . T h e loadings are expressed w i t h respect t o a 6.25 m e q / g r a m C . E . C . v a l u e a n d the e q u i l i b r i u m concentrations are the averages of three or t w o measurements as i n d i c a t e d i n parenthesis. F o r b o t h ions, i t is a p p a r e n t t h a t the e q u i l i b r i u m concentrations ( w h i c h are i n v e r s e l y p r o p o r t i o n a l t o the s e l e c t i v i t y coefficient) a t l o w t e m p e r a t u r e are t w o t o three times larger t h a n the h i g h - t e m p e r a t u r e values, w h i c h i n d i c a t e s t h e u s u a l a n d r a t h e r i m p o r t a n t e n d o t h e r m i c effect. I t is f u r t h e r more a p p a r e n t t h a t there is a significant preference for zinc as c o m p a r e d w i t h cobalt ions, c o r r e s p o n d i n g to a free energy of a b o u t 0.2 k c a l / e q u i v a lent. Table ILL Trace Region Equilibrium Concentrations of Cobalt and Zinc Ions in N a X at Two Temperatures and Two Temperature Combinations ; Total Normality is O.OliV Cobalt Eq. Cone. Zinc Eq. Cone. Temp., °C

45 (2d) 5 (Id) + 45 45 (Id) + 5 5 (2d) 45 (2d) 5 (Id) + 45 45 (Id) + 5 5 (2d)

Z

M

2

0.087 (Id) (Id) 0.173 (Id) (Id)

(Ν Χ

+

0.66 0.71 1.14 1.83 2.14 2.49 4.01 5.17

10«)

± 0.06 ± 0.07 ± 0.13 ±0.21 ± 0.02 ± 0.32 ± 0.05 ±0.21

(3)» (3) (3) (3) (2) (2) (2) (2)

(Ν X

0.49 0.57 0.72 0.95 0.86 0.86 1.53 2.92

± ± =fc ± ± ± ± ±

10")

0.05 0.05 0.08 0.08 0.04 0.02 0.03 0 03

(3) (3) (3) (3) (2) (2) (2) (2)

* Number of measurements in average. T h e m o s t i m p o r t a n t p o i n t is t h e effect of t h e r m a l h i s t o r y u p o n t h e e q u i l i b r i u m l e v e l of c o b a l t a n d z i n c ions i n s o l u t i o n . W i t h i n e x p e r i m e n t a l error, the results o b t a i n e d w i t h the " 4 5 ° C — t w o d a y " systems are i d e n t i c a l t o t h e 45° C systems w h i c h h a d received a p r i o r o n e - d a y t r e a t m e n t at 5 ° C . T h e d u r a t i o n of the experiments has v e r y l i t t l e effect u p o n the e q u i l i b r i u m d i s t r i b u t i o n , as evidenced b y t h e f a c t t h a t the results o b t a i n e d b y l o n g t e r m e q u i l i b r a t i o n s a t b o t h temperatures a n d for b o t h ions were n e a r l y i d e n t i c a l t o those s h o w n i n T a b l e I I I . M o s t i m p o r t a n t h o w e v e r is t h e finding t h a t t h e e q u i l i b r i u m levels of c o b a l t a n d z i n c a t 5 ° C are signifi c a n t l y higher t h a n these w h i c h are o b t a i n e d after a 45° C t r e a t m e n t . T h i s indicates t h a t t h e 5 ° C d i s t r i b u t i o n over t h e v a r i o u s possible sites, as i n d u c e d b y a 4 5 ° C p r e t r e a t m e n t , differs f r o m the " n o r m a l " l o w - t e m p e r a t u r e d i s t r i b u t i o n i n t h a t a significant p o r t i o n of t h e adsorbed b i v a l e n t ions w h i c h p a r t i c i p a t e i n t h e 4 5 ° C e q u i l i b r i u m n o longer do so a t 5 ° C . In other words, w h e n r e t u r n e d t o 5 ° C , p a r t of the solid-phase m e t a l ions appear i r r e v e r s i b l y sequestered i n sites where t h e y are " o u t of r e a c h " a t low temperature.


20.

237


MAES AND CREMERS

T h e t e m p e r a t u r e - d e p e n d e n t m a x i m u m l o a d i n g s for c o b a l t a n d z i n c are p e r t i n e n t .

4.33

=t

0.04 (Co) a n d 5.18 ± 0.05 (Zn) m e q / g r a m a t 5 ° C ; 5.10 (Co) a n d 5.60

T h e results o b t a i n e d b y t h e first m e t h o d a r e :

±

0.05 (Zn) at 4 5 ° C .

The

figures

are t h e averages of t w o experiments.

T h e second m e t h o d leads t o complete s a t u r a t i o n of the zeolites for b o t h ions, c o r r e s p o n d i n g t o a C . E . C . v a l u e of 6.25 m e q / g r a m w i t h i n 1 % .

W e use

t h e results o b t a i n e d b y t h e first m e t h o d as a basis for o u r subsequent analysis since the procedure resembles m o r e closely t h e one u s e d i n t h e temperature-variation method.

T h e d a t a p e r m i t a d i s t i n c t i o n between

a or easily accessible sites, c o r r e s p o n d i n g t o t h e m a x i m u m l o a d i n g a t 5 ° C , Downloaded by UNIV QUEENSLAND on October 8, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch020

a n d β or d i f f i c u l t l y accessible sites w h i c h we t a k e as t h e difference between the h i g h a n d l o w t e m p e r a t u r e C . E . C . v a l u e . is 0.77 (Co) a n d 0.42 (Zn) m e q / g r a m .

T h e c a p a c i t y of t h e β sites

A c o m b i n a t i o n of these d a t a w i t h

t h e results of T a b l e I I I afford a n i n d i r e c t estimate of the 45° C d i s t r i b u t i o n of d i v a l e n t cations between a a n d β sites.

F i r s t l y , the " 5 ° C — t w o d a y "

results are used for c a l c u l a t i n g the 5 ° C s e l e c t i v i t y coefficient K ' c

(un

c o r r e c t e d for solution-phase a c t i v i t y coefficients) :

in which Z represents t h e l o a d i n g of M + n o r m a l i z e d t o t h e 5 ° C m a x i m u m o c c u p a n c y . T h e K ' v a l u e so o b t a i n e d is t h e n u s e d for c a l c u l a t i n g t h e n e w ( n o r m a l i z e d t o t h e 5 ° C m a x i m u m ) l o a d i n g w h i c h is i n e q u i l i b r i u m w i t h t h e e q u i l i b r i u m s o l u t i o n a t 5 ° C , subsequent t o the 45° C t r e a t m e n t . T h e difference between these t w o figures, expressed i n absolute t e r m s , corresponds t o the n u m b e r of m i l l i e q u i v a l e n t s of M w h i c h were o n β sites a t 45° C a n d became " s h u t off" b y decreasing t h e t e m p e r a t u r e . A t 2

M

c

2

+

the l o w e r l o a d i n g (0.54 m e q / g r a m ) we o b t a i n 0.11 (Zn) a n d 0.15 m e q / g r a m (Co) o n t h e β sites whereas a t t h e h i g h e r l o a d i n g (1.08 m e q / g r a m we find 0.4 (Zn) a n d 0.45 (Co) m e q / g r a m . E x p r e s s i n g these n u m b e r s r e l a t i v e t o the β c a p a c i t y , i t becomes i m m e d i a t e l y o b v i o u s t h a t t h e M + l o a d i n g o n β sites is several times l a r g e r t h a n o n the a sites. T h i s i n d i c a t e s , c o n t r a r y t o some conclusions based o n c a l c i u m a n d b a r i u m a d s o r p t i o n i n N a X (6), a higher a f f i n i t y of t h e M + ions f o r t h e sites i n t h e d i f f i c u l t l y accessible regions. I n other w o r d s , t h e l a c k of p e n e t r a t i o n of M i n t h e β sites is n o t of t h e r m o d y n a m i c o r i g i n . T h i s reasoning rests o n t w o a s s u m p t i o n s : firstly t h a t a t the g i v e n h i g h t e m p e r a t u r e , there is i n fact t h e r m o d y n a m i c e q u i l i b r i u m between a a n d β sites, a n d secondly t h a t t h e l o w - t e m p e r a t u r e s e l e c t i v i t y coefficient for t h e a sites is u n c h a n g e d after " l i f t i n g " p a r t of t h e solid-phase M ions i n t o n o r m a l l y inaccessible regions a t t h a t t e m p e r a t u r e . T h e second a s s u m p t i o n is q u i t e realistic i n v i e w of t h e l o w - l o a d i n g values. 2

2

2

2

+

+

T h e results o b t a i n e d a t h i g h loadings, as s h o w n i n T a b l e I V are e n t i r e l y analogous a n d p o i n t t o a s i m i l a r effect. F o r purposes of c o m p a r i s o n , the results o b t a i n e d after one (Co) a n d t w o weeks (Zn) u n d e r otherwise


238

MOLECULAR SIEVES

i d e n t i c a l c o n d i t i o n s a r e also g i v e n . U n l i k e t h e l o w - l o a d i n g c o n d i t i o n s however, t h e e q u i l i b r i u m s o l u t i o n c o m p o s i t i o n becomes r e l a t i v e l y i n s e n s i t i v e t o s m a l l changes i n t h e s o l i d phase configuration.


Table IV. Equilibrium Distribution of Cobalt and Zinc at H i g h Loading at Two Temperatures and Two Temperature Combinations Temp., °C Szn Ζ Zn &Co Zco 45 (2d) 5(ld) + 45 (Id) 45 (Id) + 5 (Id) 5 (2d) 45 1-2 weeks 5 1-2 weeks β

0.491 ± 0 . 0 0 1 (2)· 0.776 ± 0 . 0 0 7 0.549 ± 0.002 (2) 0.665 ± 0 . 0 0 4 0.505 ± 0 . 0 0 5 (2)

0.751 ± 0 . 0 0 4 0.556 ± 0.001 (2) 0.652 ± 0 . 0 0 2

0.522 ± 0 . 0 0 3 (2) 0.534 ± 0 . 0 0 2 (2)

0.712 ± 0.006 0.567 ± 0.003 (2) 0.632 ± 0 . 0 0 5 0.694 ± 0.005 0.581 ± 0.001 (2) 0 . 6 0 4 ± 0 . 0 0 2

0.500

0.760

0.546

0.646

0.540

0.682

0.569

0.593

Number of duplicates.

I t is t e m p t i n g t o i d e n t i f y t h e easily accessible regions i n t h e s o l i d phase w i t h t h e supercages a n d the d i f f i c u l t l y accessible sites w i t h t h e c u b o o c t a h e d r a a n d hexagonal p r i s m s , p a r t i c u l a r l y f o r z i n c where t h e m a x i m u m l o a d i n g corresponds t o a " m a g i c " 8 2 % v a l u e . S u c h is however n o longer t h e case for c o b a l t for w h i c h , s t r a n g e l y enough, t h e 8 2 % figure is reached at 45°C. T h e trace-region a d s o r p t i o n d a t a show however t h a t , e v e n a t r e l a t i v e l y l o w l o a d i n g of either cobalt o r z i n c , a significant p o r t i o n of these ions, corresponding t o n e a r l y three i o n s / u n i t cell, moves t o regions w h i c h are inaccessible a t l o w t e m p e r a t u r e ( p r o b a b l y t h e " s m a l l - c a g e s i t e s " ) . T h e a p p a r e n t i r r e v e r s i b i l i t y observed a t l o w t e m p e r a t u r e m a y be u n d e r s t o o d i n t e r m s of a t e m p e r a t u r e - i n d u c e d w a t e r - s t r i p p i n g effect as suggested b y S h e r r y (6). T h e m o s t l i k e l y i n t e r p r e t a t i o n is t h e f o l l o w i n g . A t m o d e r a t e ( a n d perhaps also a t low) temperatures, the i n i t i a l a d s o r p t i o n of b i v a l e n t cations induces a r e d i s t r i b u t i o n of t h e N a ions i n t h e c r y s t a l , as was suggested b y T h e n g , V a n s a n t , a n d U y t t e r h o e v e n (7) i n a s t u d y of a l k y l a m m o n i u m ions i n zeolite, i.e., ions w h i c h a t a n y r a t e a r e u n a b l e t o penetrate t h e s m a l l cages. T h i s h y p o t h e s i s w a s c o n f i r m e d f o r zeolite Y i n a n x - r a y a n a l y s i s b y M o r t i e r , Costenoble, a n d U y t t e r h o e v e n (8) w h o f o u n d a significant increase i n N a o c c u p a n c y of t h e " s m a l l cage s i t e s " u p o n a d s o r p t i o n of a l k y l a m m o n i u m ions of i n c r e a s i n g c h a i n l e n g t h . W h e n u s i n g b i v a l e n t cations f o r w h i c h there are n o a priori steric reasons t o p r e v e n t t h e m f r o m entering the cubooctahedra, temporarily vacated "small-cage anionic s i t e s " c a n be t a k e n b y t h e b i v a l e n t cations b u t o n l y a t m o d e r a t e a n d h i g h temperatures.


20. MAES AND CREMERS


239

Conclusions The temperature-dependent irreversibility demonstrates that the ion-exchange behavior of N a X towards bivalent cations depends strongly upon the thermal history of the sample. The rather pronounced differences in behavior of transition-metal ions, also observed in synthetic zeolite 4 A (9), is in very sharp contrast with the nearly identical, either hydrated or crystallographic, dimensions of these ions (10). Ob viously, this observation raises important questions as to the value of the current interpretation (nearly) exclusively in terms of physical dimensions of ions and pore width. In contrast, the similarity of behavior in mont morillonite is remarkably close: the Δ(? value for the replacement of Na by either Ni, Co, Cu, or Zn is —175 cal ( ± l l ) / e q u i v a l e n t , irrespective of the nature of the cation (11). Therefore, the understanding of their difference in behavior in zeolites must take other effects into consideration.


0

Acknowledgment We acknowledge the support by the Belgian Government (Programmatie van het Wetenschapsbeleid). A. Maes is indebted to the "I. W. O.N. L . " for a research fellowship. Literature Cited 1. 2.

Sherry, H. S., ADVAN. CHEM. SER. (1971) 101, 350. Smith, J. V., ADVAN. CHEM. SER. (1971) 101, 171.

3. 4. 5. 6. 7.

Bennett, J. M., Smith, J. V., Mat. Res. Bull. (1968) 3, 933. Olson, D. H., J. Phys. Chem. (1970) 74, 2758. Mortier, W. J., Bosmans, H. J., J. Phys. Chem. (1971) 75, 3327. Sherry, H. S., J. Phys. Chem. (1968) 72, 4086. Theng, B. K. G., Vansant, E., Uytterhoeven, J. B., Trans. Faraday Soc. (1968) 64, 3370 8. Mortier, W. J., Costenoble, M., Uytterhoeven, J. B., to be published. 9. Gal, I. J., Jankovic, O., Malcic, S., Radovanov, P., Todorovic, M., Trans. Faraday Soc. (1971) 67, 999. 10. Nightingale, E. R., J. Phys. Chem. (1959) 63, 1386. 11. Maes, Α., Peigneur, P., Cremers, Α., to be published. RECEIVED November 29, 1972


Transition-Metal Ion Exchange in Synthetic X and Y Zeolites

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