Perspectives in Molecular Sieve Science - American Chemical Society

Chapter

8

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FT-Far IR Spectroscopic Studies of Alkali and Alkaline Earth Linde Type A Zeolites Probing Extraframework Cations in Molecular Sieves Mark D. Baker, John Godber, and Geoffrey A. Ozin Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada -1

The FT-FAR-IR spectra (350-30 cm ) of self-supporting wafers of alkali and alkaline earth Linde Type A zeolite and some of their binary combinations are reported for the first time. The sensitivity of this region of the vibrational spectrum to metal cation type, location, coordination number, geometry and population, provides a convenient diagnostic for probing metal cation site distributions, the sequence of metal cation replacement resulting from ion-exchange and metal cation mobility and reorganization during the removal of intrazeolitic water. Systematic trends in the far-IR spectra of the metal cations in conjunction with cation frequency, intensity, mass, charge, bond-length relationships, enables the establishment of an internally consistent set of vibrational signatures for different cations in specific sites of alkali and alkaline earth Linde Type A zeolites. Studies of this kind are likely to find utility in the identification and manipulation of pore blocking cations in Linde Type A zeolites; a science at the centre of their application for gas separation and purification, (1). P i n p o i n t i n g t h e s i t i n g o f c a t i o n s i n L i n d e Type A z e o l i t e s i s c r u c i a l i n t h e d e s i g n and u n d e r s t a n d i n g o f experiments aimed a t f i n e t u n i n g t h e dimensions o f t h e e n t r a n c e windows o f t h e s u p e r c a g e s . The window s i z e s can be p r e c i s e l y a d j u s t e d by t h e c o n t r o l l e d p l a c e ment o f c a t i o n s a t s t r a t e g i c l o c a t i o n s i n t h e A - z e o l i t e l a t t i c e , t h e r e b y p e r m i t t i n g o n l y m o l e c u l e s w i t h s u i t a b l e dimensions t o penet r a t e the c r y s t a l . T h i s i s e x e m p l i f i e d by K8Na4~A and N a i 2 " which both c o n t a i n monovalent c a t i o n s p a r t i a l l y b l o c k i n g t h e e i g h t - r i n g windows between s u p e r c a g e s , thus r e s t r i c t i n g e n t r a n c e t o m o l e c u l a r d i f f u s a n t s w i t h a maximum k i n e t i c d i a m e t e r o f and 4X (2) r e s p e c t i v e l y ( F i g u r e 1 ) . I n c o n t r a s t t h e c a t i o n s p r e f e r e n t i a l l y occupy s i x - r i n g s i t e s i n Ca4Na4A, removed from t h e w i n d o w , ^ r e s u l t i n g i n an expanded e n t r a n c e , a l l o w i n g m o l e c u l e s w i t h up t o 5 A k i n e t i c diameA

0097-6156/88/0368-0136S06.00/0 © 1988 American Chemical Society

Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Alkali and Alkaline Earth Linde Type A Zeolites

137

t e r to e n t e r the s u p e r c a g e . A j u d i c i o u s s e l e c t i o n of c o - c a t i o n s t h e r e f o r e i s a p r e r e q u i s i t e f o r p r e c i s e adjustment of the b l o c k i n g a c t i o n at the 0.1 A l e v e l . Once t h i s has been a c h i e v e d , t h e s e mat e r i a l s can be f u r t h e r e x p l o i t e d i n gas s e p a r a t i o n and p u r i f i c a t i o n applications (1). The f a r - I R s p e c t r a (350-30 cm" ) of L i n d e Type A z e o l i t e s can be d i v i d e d i n t o two r e g i o n s based on the o r i g i n of the o b s e r v e d v i b r a t i o n a l modes. Framework v i b r a t i o n s o c c u r above about 230 cm"l and are e s s e n t i a l l y d e c o u p l e d from the extra-framework c a t i o n modes. The c a t i o n t r a n s l a t i o n a l v i b r a t i o n s o c c u r below 230 cm~l. This useful d i v i s i o n of the v i b r a t i o n a l s p e c t r a of z e o l i t e s i n t o framework and extra-framework modes has been d i s c u s s e d p r e v i o u s l y ( 3 - 5 ) . In t h i s paper the f a r - I R s p e c t r a l s i g n a t u r e s of a l k a l i and a l k a l i n e e a r t h exchanged L i n d e Type A z e o l i t e s w i l l be examined i n a d e s i g n e d e f f o r t to address the q u e s t i o n of c a t i o n s i t e l o c a t i o n and population. The u n i t c e l l of L i n d e Type A shown i n F i g u r e 1 c o n t a i n s charge b a l ancing c a t i o n s , which are l o c a t e d on the F i g u r e i n b o l d l e t t e r i n g (6). These l i e i n the f o l l o w i n g p o s i t i o n s : a) On the C 3 a x i s of the s i n g l e s i x - r i n g s ( s i t e s A, B and C) b) In the p l a n e of the e i g h t - r i n g s ( s i t e E) c) On the f o u r f o l d a x i s of the f o u r - r i n g s , ( s i t e H, a - c a g e ) , ( s i t e G, B-cage). (23) 1

V

Results:

Na-A

and

Li,

Na-A a

r

e

The f a r - i n f r a r e d s p e c t r a of h y d r a t e d and d e h y d r a t e d Naj^-A shown i n F i g u r e 2,(24). Framework v i b r a t i o n s ot the u n i t c e l l t h a t are r a t h er i n s e n s i t i v e to the n a t u r e of the m e t a l c a t i o n guest o c c u r between 350-250 cm~l. The h y d r a t e d sample e x h i b i t s f o u r w e l l d e f i n e d c a t i o n r e l a t e d a b s o r p t i o n s which s h i f t s l i g h t l y upon d e h y d r a t i o n of the zeolite. The l a r g e s t change o c c u r s f o r the v i b r a t i o n a t 133 cm~l s h i f t i n g to 142 cm~l upon removal of w a t e r . Crystallographic determ i n a t i o n s of s i t e l o c a t i o n s f o r h y d r a t e d and d e h y d r a t e d Na-A are compiled i n T a b l e I . Table I. C r y s t a l l o g r a p h i c a l l y i d e n t i f i e d c a t i o n s i t e s and p o p u l a t i o n s f o r Na-A. The numbers i n t h i s t a b l e r e f e r to the number of c a t i o n s per p s e u d o - u n i t c e l l . Deh. i s dehyd r a t e d , Hyd. i s h y d r a t e d Form

Site

Deh. Deh. Deh. Hyd. Deh. Deh. Deh.

7.8 8.0 8.0 8.0 8.0 8.0 8.0

A

Site E 2.9 4.0 4.0

4.0 3.0 3.0

Site 0.8

1.0 1.0

H

Reference 7 8 9 10 11 12 13

E s s e n t i a l l y , t h e s e l o c a t e 7-8 c a t i o n s i n s i t e A, 3 i n s i t e E and the remainder i n s i t e H. The most i n t e n s e band i n the f a r IR spectrum of Na-A is logic a l l y a s c r i b e d to a N a ( A ) v i b r a t i o n . I t s p o s i t i o n compares to t h a t seen f o r sodium i o n s i n s i t e I I i n z e o l i t e Y ( b l u e - s h i f t e d due t o the h i g h e r l a t t i c e a n i o n i c i t y of A r e l a t i v e to Y ) . Confirmation +


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F i g u r e 1^ Representation of the p s e u d o - c e l l of z e o l i t e A, (Space Group Pm3m) i l l u s t r a t i n g the framework s t r u c t u r e surrounding a large cage. The c a t i o n p o s i t i o n s are shown i n bold face l e t t e r ing using the formalism of M o r t i e r ( 6 ) . The extent of p r o j e c t i o n of c a t i o n s i n s i t e s B,C and G i s exaggerated f o r c l a r i t y .

WAVE

N U M B E R

F i g u r e 2. F a r - i r s p e c t r a of Na-A, recorded at room temperature (A) Hydrated (B) A f t e r a 400°C i n s i t u vacuum dehydration. F denotes a framework band.


8.

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139

of t h i s assignment comes from an i n t e n s i t y a n a l y s i s of the asymmetric and symmetric a b s o r p t i o n bands of the s i t e A v i b r a t i o n s (3). This l e a d s to an a n t i c i p a t e d p y r a m i d a l a n g l e of 113° compared to t h a t determined c r y s t a l l o g r a p h i c a l l y of 119.2° ( 7 ) . (For band p o s i t i o n s see F i g u r e 3 ) . F u r t h e r support f o r t h i s assignment comes from the f a r - I R s p e c t r a o f p r o g r e s s i v e l y L i ion-exchanged Na-A. The l o c a t i o n of L i i n the A l a t t i c e i s known from c r y s t a l l o g r a p h y to be r e s t r i c t ed to the s i x - r i n g s i t e s ( 1 4 ) , and does not show any f a r - I R L i - 0 s t r e t c h i n g v i b r a t i o n s , (22). In F i g u r e 4 the s p e c t r a of d e h y d r a t e d Nai2~A, Na()Li3-A and NafcLi^-A are d i s p l a y e d . Here i t i s o b s e r v e d , t h a t upon p r o g r e s s i v e l o a d i n g w i t h L i + i o n s , the Na+(A) v i b r a t i o n f i r s t d e c r e a s e s i n i n t e n s i t y , f o l l o w e d by changes i n the low f r e q u e n c y r e g i o n . T h e r e f o r e the s i t i n g o f L i i o n s i n the LTA s i x - r i n g s i s found to p r o c e e d a t the expense of the N a ( A ) i o n s . In the r e g i o n around 100 cm""l, o t h e r changes are o c c u r r i n g upon p r o g r e s s i v e replacement of Na+ by L i i o n s , s i g n a l l i n g the e x i t of sodium i o n s from a n o t h e r site. Bands i n t h i s r e g i o n are a s c r i b e d to the s i t e H c a t i o n based on two c o n s i d e r a t i o n s : i ) The l o c a l symmetry of N a ( H ) i s r e m i n i s c e n t of s i t e I I I i n Na-Y. In the l a t t e r case an a b s o r p t i o n a t 90 cm~l i s a s c r i b e d to t h i s c a t i o n ( 3 , 4 ) . i i ) The low i n t e n s i t y o f t h i s c a t i o n v i b r a t i o n i s c o n s i s t e n t w i t h the low p o p u l a t i o n of t h i s s i t e . I n Naj^-A, s i t e E i s the second most populous s i t e accommodating about t h r e e i o n s per p s e u d o - u n i t c e l l . Based on t h i s p o p u l a t i o n , one would a n t i c i p a t e the s i t e E a b s o r p t i o n band to be the second most i n t e n s e , p i n p o i n t i n g i t a t 177 cm"^. A c a t i o n bound at s i t e E, as shown i n F i g u r e 5 , i s i n t e r a c t i n g p r i m a r i l y w i t h two 01 and one 02 l a t t i c e oxygens i n l o c a l C 2 symmetry. In t h i s symmetry the f o l l o w i n g c a t i o n - o x y g e n s t r e t c h i n g v i b r a t i o n s are i n f r a r e d a c t i v e . +

+

+

+

+

+

V

r

x

(A )

= 1//2

r

2

(A )

= AR

(B )

= 1//2

T

x

3

x

1

(Ar + A r ) x

2

(v^ (v ) 2

(Ar x

Ar ) 2

(v > 3

An a n a l y s i s o f the two A^ modes u s i n g the sum and p r o d u c t i n t e n s i t y r u l e s (15) shows t h a t the band at 142 cm~l i n the o b s e r v e d spectrum i s due to the V2 mode. The lower i n t e n s i t y mode o c c u r s as a weak, c u r v e r e s o l v a b l e band at 110 cm~l. Potassium-Exchanged

A

The f a r IR s p e c t r a of the d e h y d r a t e d forms of N a ^ A , KsNa4A and Kll.6NaQ.4A a r e shown i n F i g u r e 6. The c r y s t a l l o g r a p h i c o b s e r v a t i o n s of p o t a s s i u m i o n s i n m a t e r i a l s s i m i l a r t o those d e s c r i b e d h e r e a r e l i s t e d i n T a b l e I I . The spectrum o f KgNa4A ( F i g u r e 6B) e x h i b i t s a s t r o n g band a t 213 cm~l which i s c l e a r l y due t o sodium i o n s i n s i t e A. Of the r e m a i n i n g bands, the most i n t e n s e a t 179 cm~l i s a t t r i b u t e d to K+(B) f o r a number of r e a s o n s . i) T h i s i s the most p o p u l a t e d s i t e and w i l l t h e r e f o r e g i v e r i s e to the most i n t e n s e a b s o r p t i o n .


140


Wavenumber Figure 3. observed

Curve resolved spectrum of dehydrated Na-A. (B) simulation (C) curve resolution.


(A)

8.

BAKER ET AL.


300

200

141

100

W A V E N U M B E R

A

A

Figure 4. F a r - i r spectra of dehydrated N a " ( ) » N a L i ~ A (B), and N a L i - A (C). Note that the Na (A) v i b r a t i o n decreases f i r s t . 1 2

9

3

+

6

6


142


e E,

WAVENUMBER Figure 6. F a r - i r spectra of dehydrated: Na K -A; (C) K - A . 4

8

(A) Nai -A; 1

9 2

(B)

1 2


8.

BAKER ET AL.

ii)

Hyd. Deh. Deh. Deh.

143

+

+

The c a l c u l a t i o n (3) of the f r e q u e n c y of K ( B ) u s i n g the N a ( A ) observed f r e q u e n c y l e a d s t o an a n t i c i p a t e d f r e q u e n c y of 173 cm~l. Table I I .

Form


Site A

1.0

4.4Na

C r y s t a l l o g r a p h i c a l l y i d e n t i f i e d c a t i o n s i t e s and p o p u l a t i o n s f o r K-A. The numbers i n t h i s t a b l e r e f e r t o the number o f c a t i o n s per p s e u d o - u n i t c e l l . Deh. i s d e h y d r a t e d , and Hyd. i s h y d r a t e d . The l a s t e n t r y i n the T a b l e i s f o r a K Na - A sample

Site B

Site C

8.0 6.0 6.3 3.6

Site E

-

-

2.0 1.4

3.0 3.0 2.0

—

Site G

-

-

0.1

0.3 0.6

—

Reference

Site H

[16] [16] [17] [18]

The u n r e s o l v e d s h o u l d e r on the low f r e q u e n c y s i d e o f the main a b s o r p t i o n , s i g n a l s the p r e s e n c e o f another s i t e which i s p r o b a b l y K ( C ) . The remainder o f the f a r - I R spectrum c o n t a i n s a p o o r l y r e s o l v e d abs o r p t i o n below 100 cm"l, which i n the c u r v e r e s o l u t i o n (21) i s shown t o p r o b a b l y c o n t a i n two bands, and i s due t o v i b r a t i o n s of the pore b l o c k i n g potassium c a t i o n . The f a r - I R spectrum o f e s s e n t i a l l y f u l l y exchanged KA ( F i g u r e 6C) c l e a r l y i n d i c a t e s t h a t the N a i o n s have been r e p l a c e d s i n c e the Na+(A) band a t 213 cm" seen i n KgNa4-A ( F i g u r e 6B) i s a b s e n t . The dominant f e a t u r e of the K-A spectrum i s a broad i n t e n s e a b s o r p t i o n at 172 cm-1, due t o K+(B) i o n s (6.3K+ i o n s , T a b l e I I ) w i t h a s m a l l c o n t r i b u t i o n from K ( C ) p r o d u c i n g a weak s h o u l d e r a t about 147 cm"" (see F i g u r e 6B). Curve r e s o l u t i o n (21) r e v e a l s t h a t K (B) i s made up o f two components. These a r e the Ai and E symmetry IR a c t i v e v i b r a t i o n s ( 3 ) . I n t e n s i t y a n a l y s e s g i v e a p y r a m i d a l a n g l e of 110.3°, where the c r y s t a l l o g r a p h i c d e t e r m i n a t i o n (17) g i v e s 110.8°. +

+

1

+

1

+

Of the r e m a i n i n g c a t i o n s i n K]^-A the most populous a r e the imp o r t a n t pore b l o c k i n g K ions i n s i t e E. In KgNa^A, v i b r a t i o n s at 98 and 72 cm~l were a s s i g n e d t o the B^ and A\ symmetry v i b r a t i o n s r e s p e c t i v e l y of K ( E ) . I n Ki2~A, a b s o r p t i o n s a t 77 and 61 c m are a t t r i b u t e d t o t h e s e same v i b r a t i o n s of K+(E). The r e d s h i f t of t h e s e bands i s i n d i c a t i v e of e i t h e r a f o r c e c o n s t a n t , o r geometry change of t h i s s i t e i n Ki2~A r e l a t i v e to KgNa4-A. I f the d i f f e r e n c e s a r e p u r e l y a n g u l a r i n o r i g i n , then one can e s t i m a t e the 01-K-01 a n g l e i n K8Na4-A u s i n g the f r e q u e n c i e s o f the B i modes i n K i 2 ~ A and KgNa4"A and the c r y s t a l l o g r a p h i c a l l y determined a n g l e i n K i 2 ~ of 102.5° (17). T h i s c a l c u l a t i o n p r e d i c t s a v a l u e of 110.5° f o r K ( E ) i n KgNa4~A and t h e r e f o r e s u g g e s t s t h a t the p o t a s s i u m i o n i n the f u l l y exchanged m a t e r i a l i s h e l d l e s s s t r o n g l y i n s i t e E, and thus p r o j e c t s f u r t h e r i n t o the e i g h t - r i n g window, i m p l y i n g t h a t the pore s i z e would be s l i g h t l y s m a l l e r . Of the r e m a i n i n g c a t i o n s i n K^2"" , t h e r e a r e 0.3 K+ i o n s per pseudo u n i t c e l l i n s i t e H and 0.1. K+ i o n s per pseudo c e l l i n s i t e G. The v i b r a t i o n a l f r e q u e n c y of Na+(H) c a t i o n s would t r a n s l a t e t o a K+(H) c a t i o n v i b r a t i o n a t 49 cm~l. I t appears from the spectrum t h a t a weak band a t about 117 cm" - i s b e s t a s c r i b e d t o K+(G) , w i t h K+(H) +

+

-1

A

+

A

1


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PERSPECTIVES IN MOLECULAR SIEVE SCIENCE +

e i t h e r o v e r l a p p e d by the K ( E ) bands around 77-60 c m o c c u r s a t s t i l l lower f r e q u e n c i e s ( F i g u r e 6C). Rubidium-Exchanged

-1

or that i t

A

The f a r - I R s p e c t r a of h y d r a t e d and d e h y d r a t e d Rb,Na-A a r e shown i n F i g u r e 7. The spectrum of the l a t t e r can be d i v i d e d i n t o two " gions._j_ The h i g h e r f r e q u e n c y shows c h a r a c t e r i s t i c bands due to Na (A) and Na (E) as seen e a r l i e r a t 217 and 180 cm" r e s p e c t i v e l y (Figure 2). The lower f r e q u e n c y r e g i o n , (110-30 cm" ) shows bands due t o the m o t i o n of Rb+ i o n s . C r y s t a l l o g r a p h i c d e t e r m i n a t i o n s have shown t h a t r u b i d i u m i o n s occupy s i t e s B/C and E and the a b s o r p t i o n bands are a s s i g n e d to t h e s e s i t e s (see F i g u r e 7 ) . I t i s i n t e r e s t i n g to n o t e t h a t the f r e q u e n c i e s of the Na+ and Rb+ modes f a l l i n d i s t i n c t regions. This observation i n d i c a t e s that monitoring i n t e n s i t y changes i n a s e r i e s of Rb, Na-A samples w i t h v a r y i n g u n i t c e l l c a t ion complements would a l l o w f o r a f i r m statement t o be made r e g a r d ing the p o p u l a t i o n s o f the d i f f e r e n t i o n s i n t h e i r extra-framework positions. In c o n j u n c t i o n , a d s o r p t i o n experiments on t h e s e materi a l s , would s e r v e as a u s e f u l method f o r i n v e s t i g a t i n g and o p t i m i z i n g the s i e v i n g p r o p e r t i e s of the samples w i t h r e s p e c t t o a s p e c i f i c separation process. In t h i s c o n t e x t , the f a r IR s p e c t r a suggest t h a t the pore s i z e of the d e h y d r a t e d Rb, Na-A sample i s more c o n s t r a i n e d than i n the p a r e n t Nai2~A z e o l i t e , based on the o b s e r v a t i o n of a subs t a n t i a l f a r - I R a b s o r p t i o n ( s i t e p o p u l a t i o n ) due to R b ( E ) . Further s t a t e m e n t s i n t h i s s p i r i t w i l l s u r f a c e when more d a t a have been c o l l e c t e d c o n c e r n i n g t h e s e samples. r e

+

1

1

+

Calcium

Exchanged

A

D i v a l e n t i o n s m a i n l y occupy the found i n these p o s i t i o n s i n A

six-ring sites, zeolites.

and

are e x c l u s i v e l y

Ca Na A 4

4

The c r y s t a l l o g r a p h y of Ca4Na -A demonstrates t h a t the c a t i o n s a r e d i s t r i b u t e d o n l y i n the t h r e e - f o l d s i t e s , on the s i x - r i n g s of the s o d a l i t e cages (19,20). The d i v a l e n t Ca2+ i o n s l o c a t e n e a r the p l a n e o f the s i x - r i n g ( s i t e A ) , w h i l e the sodium i o n s a r e d i s p l a c e d i n t o the supercage ( s i t e B ) . A n e u t r o n d i f f r a c t i o n study of Ca Na4-A (20) a l s o showed t h a t the s i x - r i n g s c o n t a i n i n g the C a ^ were s e v e r e l y d i s t o r t e d , and t h a t the Ca2+-03 bond l e n g t h was q u i t e s h o r t a t 2.9 A. The f a r - I R s p e c t r a o f the h y d r a t e d and d e h y d r a t e d Ca4Na4~A sample a r e shown i n F i g u r e 8. In a d d i t i o n t o the framework a b s o r p t i o n a t 260 c m , two bands dominate the spectrum. These a r e l o g i c a l l y a s s o c i a t e d w i t h N a ( B ) i o n s (191 cm" ) and C a ( A ) c a t i o n s (237 c m " ) . The a s s e r t i o n o f t h e s e assignments i s founded on the f o l l o w i n g criteria: i ) S i m i l a r s i n g l e s i x - r i n g f r e q u e n c i e s were o b s e r v e d i n CaX and CaY ( 3 , 4 , 9 ) . i i ) A good f i t of o b s e r v e d (237 cm" ) and c a l c u l a t e d (207 cm" ) frequencies. i i i ) The h i g h f r e q u e n c y of the Ca^+ i o n i s r a t i o n a l i z e d by a l a r g e i n t e r a c t i o n of the 2+ i o n w i t h the a n i o n i c l a t t i c e compared to the sodium i o n . 4

4

+

-1

+

1

2 +

1

1

1


8. BAKER ET AL.


W A V E N U M B E R

Figure 7. F a r - i r spectra of (A) hydrated and (B) dehydrated Na, Rb-A.


145

146


Ca A 6

2+ In the d e h y d r a t e d form 4.4 Ca i o n s per pseudo u n i t c e l l a r e found i n s i t e B, ( C a - 03 = 2.27 A ) , almost i n the p l a n e of the 03 oxygens of the s i x - r i n g , and 1.2 C a + i o n s a r e l o c a t e d i n s i t e C i n the s o d a l i t e cage ( C a + - 03 = 2.32 A ) . The f a r - I R band at 198 cm~l ( F i g u r e 9) i s a s c r i b e d to the C a i o n w i t h the l o n g e r bond l e n g t h , namely s i t e C. U s i n g the C a ( C ) asymmetric c a t i o n f r e q u e n c y and the c r y s t a l l o g r a p h i c d e t a i l s of C a ( B ) one can e s t i m a t e the Ca +(B) asymmetric v i b r a t i o n a l f r e q u e n c y at 250 c m . This f i t s quite well w i t h the assignments above f o r the C a ^ N a ^ A sample, as w e l l as the o b s e r v a t i o n o f a band i n t h i s r e g i o n i n the curve r e s o l v e d spectrum of Ca -A ( F i g u r e 9). 2 +

2

2

2 +

2 +

2 +

2

-1

6

A

—6 The f a r - I R s p e c t r a of h y d r a t e d and d e h y d r a t e d SrA a r e d i s p l a y e d i n F i g u r e 10. The form o f the s p e c t r a a s i d e from r e d s h i f t s are r e markably s i m i l a r t o those of the Ca-A d e s c r i b e d above. T h i s i s not s u r p r i s i n g as the s i t i n g and p o p u l a t i o n s of S r + i o n s a r e i d e n t i c a l to C a + i n Ca6~A (19b). The S r + i o n s a r e l o c a t e d i n the s i x - r i n g s , w i t h 4.5 i o n s per p s e u d o - c e l l i n s i t e B, and 1.2 i o n s per p s e u d o - c e l l i n s i t e C. T o g e t h e r the C a and S r -A zeolites yield a consist e n t p i c t u r e o f the d i v a l e n t c a t i o n f a r - I R v i b r a t i o n a l f r e q u e n c i e s , and a r e u s e f u l models f o r the i n v e s t i g a t i o n of the t r a n s i t i o n m e t a l (2+) -A systems. S t r o n t i u m c a t i o n v i b r a t i o n s a n t i c i p a t e d to o c c u r at 175 cm~l ( s i t e B) and at 145 c m ( s i t e C) compare q u i t e w e l l w i t h the o b s e r v e d bands a t 182 and 142 cm~l r e s p e c t i v e l y . 2

2

2

2 +

2 +

-1

Conclusion In t h i s paper the f a r - I R s p e c t r a of a s e r i e s of a l k a l i and a l k a l i n e e a r t h exchanged -A z e o l i t e s have been d e s c r i b e d and assignments of c a t i o n r e l a t e d bands d i a g n o s t i c of s p e c i f i c b i n d i n g s i t e s have been p r o p o s e d . The r e q u i r e m e n t of a r e l i a b l e b a s i s from which to make comparisons i s c r u c i a l i n t h i s type of study. In t h i s r e g a r d , the Na-A f a r - I R spectrum and u n i t c e l l c a t i o n dynamics have been s t u d i e d and d i s c u s s e d i n d e t a i l ( 5 ) . I t i s s t i l l d i f f i c u l t a t t h i s s t a g e however, to s e p a r a t e or gauge the e f f e c t of both geometry and f o r c e c o n s t a n t changes-at a c a t i o n b i n d i n g s i t e on the f a r - I R s p e c t r a . F u r t h e r p r o g r e s s i n t h i s r e g a r d i s a n t i c i p a t e d from complementary Raman d a t a . Acknowledgment s The generous f i n a n c i a l a s s i s t a n c e of the N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l of Canada's O p e r a t i n g and S t r a t e g i c G r a n t s Programmes and the Connaught F o u n d a t i o n of the U n i v e r s i t y of Toronto i s g r e a t l y appreciated. We are a l s o i n d e b t e d to Dr. E d i t h F l a n i g e n (Union C a r b i d e ) f o r s u p p l y i n g v a r i o u s h i g h c r y s t a l l i n i t y and u l t r a h i g h p u r i t y A - z e o l i t e s , as w e l l as i n v a l u a b l e t e c h n i c a l discussions. We would a l s o l i k e to acknowledge Dr. M i c h a e l O l k e n and Mr. A l e x Kuperman f o r i n v a l u a b l e d i s c u s s i o n s , and H e l e n S a r k i s s i a n f o r typing t h i s manuscript.


8. BAKER ET AL.


r 300

200

100

W A V E N U M B E R

Figure 8. Ca Na -A. 4

F a r - i r spectra of (A) hydrated and (B) dehydrated,

4

200 WAV E

N UM

B E R

Figure 9. F a r - i r spectra of : (A) hydrated and (B) dehydrated Ca^-A. (C) observed spectrum (D) simulation (E) curve resolution. Note that the A,(B) mode probably l i e s within the framework v i b r a t i o n .

American Chemical Society library 1155 16th N.W.Sieve Science Flank and Whyte; Perspectives in St, Molecular ACS Symposium Series; American Chemical Society: Washington, DC, 1988. Washington, D.C 20036

147

148


Figure 10. F a r - i r spectra of: Sr A.

(A) hydrated and (B) dehydrated

6


8. BAKER ET AL.


Literature Cited 1.

(a) Dwyer, J.; Dyer, A., Chemistry & Industry, p.237 (1984). (b) D.W. Breck, "Zeolite Molecular Sieves", J. Wiley and Sons, New York, (1974). 2. The kinetic diameter is defined as the intermolecular distance of closest approach for two molecules colliding with zero initial kinetic energy (see Ref. 1(b) and is calculated from a Lennard - Jones potential. The difference between the apparent pore diameter (from crystallography) and the kinetic diameter is explained on the basis of activated diffusion of adsorbates into the zeolite pores. 3. Ozin, G.A., Baker, M.D. and Godber, J., J . Am. Chem. Soc., 107, 3033 (1985). 4. Baker, M.D., Ozin, G.A. and Godber, J., Catal. Rev. Sci. Eng., 27, 591 (1985). 5. Ozin, G.A. and Godber, J., J. Phys. Chem. (submitted). 6. Mortier, W.J., "Compilation of Extra Framework Sites in Zeolites"; Butterworths, Surrey, 1982. 7. Pluth, J . J . and Smith, J.V., J. Am. Chem. Soc., 102, 4704 (1980). 8. Reed, T.B. and Breck, D.W., J. Am. Chem. Soc., 78, 5972 (1956). 9. Howell, P.A. Acta, Cryst., 13, 737 (1956). 10. Broussard, L. and Shoemaker, D.P., J. Am. Chem.Soc., 82, 1041 (1960). 11. Smith, J.V. and Dowell, L.G., Kristallogr, Z., 126, 1 (1968). 12. Yanagida, R.Y., Amaro, A.A. and Seff, K., J. Phys. Chem., 77, 805 (1973). 13. Subramanian, Y. and Seff, K. J. Phys. Chem., 81, 2249 (1977). 14. Jirak, Z.; Nosacek, V.; Vratislav, S., Herden H.,; Schollner, R.; Mortier, W.J.; Gellens, L. and Uytterhoeven, J.B., Zeolites, 3, 255 (1983). 15. Wilson, E.B., Decius, J.C. and Cross, P.C., "Molecular Vibrations", McGraw-Hill, New York (1955). 16. Leung, P.C.W.; Kunz, K.B.; Seff, K. and Maxwell, I.E., J . Phys. Chem., 79, 2157 (1975). 17. Pluth, J . J . and Smith, J.V., J. Phys. Chem., 83, 741 (1979). 18. Adams, J.M. and Haselden, D.A., J . Solid State Chem., 47, 123 (1983). 19. a) Pluth, J . J . and Smith, J.V., J. Am. Chem. Soc., 105, 1192 (1983). b) Pluth, J . J . and Smith J.V., J. Am. Chem. Soc., 104, 6977 (1982). 20. Adams. J.M. and Haselden, D.A., J . Solid State Chem., 51, 83 (1984). 21. Baker, M.D. , Ozin, G.A. and Godber, J., J. Phys. Chem., submitted. 22. Godber, J., PhD Thesis 1987, U of Toronto. 23. The spectra described in this paper were recorded using a Nicolet 200SXV Ft far IR spectrometer. The vacuum infrared cell, and sample handling techniques used have been described in detail in a recent review of this subject.(4) 24. The spectra of the former will not be discussed here, but are addressed in more depth elsewhere. (21). RECEIVED January 25, 1988


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