7
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The Influence of Exchangeable Cations on Zeolite Framework Vibrations I. E . M A X W E L L and A. B A K S Koninklijke/Shell-Laboratorium, Amsterdam, Netherlands (Shell Research B. V.)
The influence of exchangeable monovalent cations on the framework vibrations for the hydrated zeolites Linde A and X has been investigated. An approximately linear relationship is found between the frequency of some absorption bands and the inverse of the sum of the cation and framework oxygen ionic radii. It is proposed that the shift in framework vibrations is largely caused by those cations which are strongly interacting with the zeolite framework. Thus the linear relationship indicates that these monovalent cations are all similarly sited in the zeolite lattice. This is consistent with the presently available x-ray analyses on some of these zeolites. Since Rb and Cs are only partially exchangeable in both Linde A and Linde X, these cations deviate from this linear relationship. +
+
" O ecently Flanigen, Khatami, and Szymanski (1) studied the framework vibrations of a variety of zeolites in the infrared region, 1200-250 cm . Based on a comparison of the framework structures and the i n frared spectra, empirical assignments were proposed for the major vibrational bands. It was shown that framework building units (e.g., double six-membered rings, double four-membered rings) in zeolites could be identified by mid-infrared spectroscopy. In most cases these studies were carried out on the Na+ exchanged form of the zeolite. - 1
However, the influence of the exchangeable cation on the framework vibrations has not been systematically investigated. F r o m x-ray diffraction studies (2) on zeolites it is known that most of the exchangeable cations are firmly bound onto the negatively charged framework. Therefore these cations might have some influence on the lattice vibrational modes. T h e object of the present investigation was to study systematically the effect of monovalent cations on the lattice vibrations in the synthetic zeolites Linde A and Linde X . It was reasoned that mid-infrared spectroscopy might yield information on cation siting in these zeolites. 87 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
88
MOLECULAR SIEVES
Experimental T h e o r i g i n a l zeolite samples were p o w d e r e d L i n d e 4 A a n d 1 3 X ( w i t h o u t binder) i n t h e N a f o r m s , m a n u f a c t u r e d b y L i n d e A i r P r o d u c t s C o . E x c e s s N a O H was r e m o v e d b y w a s h i n g w i t h d i s t i l l e d w a t e r u n t i l the p H of the effluent s o l u t i o n was r e d u c e d t o 8. E x c h a n g i n g solutions (1 M) were m a d e f r o m t h e chloride salts ( a n a l y t i c a l grade) of t h e a p p r o p r i a t e cations (except for A g a n d T l f o r w h i c h t h e n i t r a t e salts were used). A b a t c h c a t i o n exchange m e t h o d was a d o p t e d s t a r t i n g w i t h 5 g r a m s of t h e N a f o r m of t h e zeolite a n d i n v o l v i n g four steps of s t i r r i n g w i t h 200 m l of e x c h a n g i n g s o l u t i o n . E a c h step w a s of a p p r o x i m a t e l y 16-hours d u r a t i o n i n c l u d i n g i n t e r m e d i a t e w a s h i n g w i t h d i s t i l l e d w a t e r . A l l exchange r e a c t i o n s were c a r r i e d o u t a t r o o m t e m p e r a t u r e . T h e degree of c a t i o n exchange w a s d e t e r m i n e d b y d i r e c t a n a l y s i s of t h e exchanged s o l i d m a t e r i a l . T h e cation-exchanged zeolites were d r i e d u n d e r v a c u u m a n d h y d r a t e d b y e q u i l i b r a t i n g over a s a t u r a t e d a m m o n i u m chloride solution. I n f r a r e d s p e c t r a were m e a s u r e d o n a P e r k i n - E l m e r m o d e l 225 spect r o m e t e r i n t h e range 1200-200 c m . T h e c a l i b r a t i o n of t h e i n s t r u m e n t was checked b y m e a s u r i n g t h e frequencies of C 0 v i b r a t i o n a l b a n d s . T h e i n f r a r e d spectra of zeolite samples were m e a s u r e d as pressed pellets c o n t a i n i n g a p p r o x i m a t e l y 3 m g of zeolite i n 300 m g of C s l . +
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+
+
+
- 1
2
Results and Discussion T h e i n f r a r e d s p e c t r a of t h e L i + , N a + , A g + K + , T1+ R b + , C s + , a n d N H exchanged forms of L i n d e A a n d L i n d e X , i n t h e r e g i o n 2 0 0 - 1 2 0 0 c m , are c o m p a r e d i n F i g u r e s 1-4. F r o m t h e s p e c t r a i t is e v i d e n t t h a t 4
+
- 1
TRANSMITTANCE (%) 100, ,
(\
80
I 60
40
\
f
IT X li
\
1
20
r
\\
i 1 i1
£
i i
\
A
a/ J \
N?
K
M V//
X
i I
t,1 •• • -
til 0 1200
>Jr'
r/
i 1000
800
600
400
200
WAVELENGTH (cm" ) 1
Figure 1. Infrared spectra of Li , Na , K , changed forms of Linde A +
+
+
and Tl ex+
In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
7.
Cation Exchange on Zeolite Vibrations
MAXWELL AND BAKS
89
t h e exchangeable c a t i o n influences b o t h t h e r e l a t i v e intensities a n d frequencies of t h e f r a m e w o r k v i b r a t i o n s .
A b s o r p t i o n b a n d frequencies a n d
r e l a t i v e intensities together w i t h cation-exchange d a t a are l i s t e d i n T a b l e s I andII.
T h e v i b r a t i o n a l b a n d assignments are those of F l a n i g e n et al. (1).
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TRANSMITTANCE (%) 100
1200
1000
800
600
400
200
WAVELENGTH (cm" ) 1
Figure 2. Infrared spectra of NHt+, Ag , Rb , and Cs exchanged forms of Linde A +
+
+
TRANSMITTANCE (%) 100 \ \
/
80
\
\
| r/!/ * u
w »v a
60
I 1
ij i I•1 ii/ t'
\\ •
A. \
A /
ml
A \\
"V/
\
\ i \i V
ii
/'
v
/
/
/
WW?
rJ
\ * /
t I s /' V
J 0 1200
in
t
i
20
>xiT \ ? ' /
L
1000
800
600
400
200
WAVELENGTH (cm" ) 1
Figure 3. Infrared spectra of Li , Na , K , and Tl exchanged forms of Linde X +
+
+
+
In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
90
MOLECULAR SIEVES
TRANSMITTANCE (%) 100 •f A a /
/
+
80 Rb
+
Aa ! +
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60
40
m
' i
/
J l i
\
'// '/f
20
0 1200
^
\
\
u
\
\
A \ NrV!
rv v / V
800
W
1
'
+
hi/l
n r/
\ / V \\ X
A
M
/
if
It i V
1000
i
600
400 WAVELENGTH (cm )
Figure 4-
Infrared spectra of NHt , Ag , Rb , and Cs exchanged forms of Linde X +
+
+
+
F o r b o t h L i n d e A a n d X t h e c a t i o n has the most m a r k e d influence o n a b s o r p t i o n b a n d frequencies i n t h e regions 4 4 0 - 4 9 0 a n d 5 4 0 - 5 9 0 c m . T h e s e b a n d s h a v e been a t t r i b u t e d t o t h e S i ( A l ) - 0 b e n d i n g m o d e a n d d o u b l e r i n g (double 4 - r i n g f o r L i n d e A , d o u b l e 6 - r i n g f o r L i n d e X ) v i b r a t i o n s , r e s p e c t i v e l y (1). T h e L i n d e A double 4 - r i n g v i b r a t i o n was f o u n d t o consist of t w o components. T h e m o r e intense l o w e r frequency c o m p o n e n t , however, was more sensitive t o the exchangeable c a t i o n . - 1
P l o t s of v i b r a t i o n a l f r e q u e n c y , v> against the r e c i p r o c a l of t h e s u m of t h e c a t i o n a n d o x y g e n i o n i c r a d i i , l / ( r t i o n + r 2-), f o r t h e S i ( A l ) - 0 b e n d i n g m o d e a n d t h e d o u b l e r i n g v i b r a t i o n s ( F i g u r e 5) are a p p r o x i m a t e l y l i n e a r f o r the L i + , N a + , A g + , K + , a n d T1+ cations ( w i t h the e x c e p t i o n of t h e d o u b l e 4 - r i n g v i b r a t i o n a l m o d e f o r K + exchanged L i n d e A ) . T h e freq u e n c y increases w i t h t h e r e c i p r o c a l of t h e c a t i o n i c r a d i u s . Since t h e electrostatic p o t e n t i a l f r o m t h e c a t i o n a t t h e f r a m e w o r k o x y g e n a t o m s i s i n v e r s e l y p r o p o r t i o n a l t o t h e c a t i o n - f r a m e w o r k distance, a s i m p l e elect r o s t a t i c m o d e l p r o v i d e s a p l a u s i b l e e x p l a n a t i o n for t h e observed frequency shifts. A s discussed i n t h e n e x t section these results c a n be i n t e r p r e t e d i n t e r m s of c a t i o n s i t i n g i n t h e zeolite f r a m e w o r k s . ca
0
C a t i o n S i t i n g i n L i n d e A . A t the t i m e t h i s w o r k was completed, x - r a y studies o n h y d r a t e d N a A (3, 4) a n d h y d r a t e d K A (5) h a d s h o w n t h a t 8 of t h e 12 exchangeable cations p e r u n i t cell are f i r m l y b o u n d t o t h e zeolite f r a m e w o r k a n d w o u l d therefore b e expected t o h a v e t h e m a j o r influence o n t h e l a t t i c e v i b r a t i o n s . T h e s e cations are s i t e d i n f r o n t of the sodalite
In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
7.
MAXWELL AND BAKS
Table I.
91
Cation Exchange on Zeolite Vibrations
Infrared Frequencies ( c m ) for Monovalent Cation Exchanged -1
Hydrated Zeolite A
a
% Exchange (Relar (ExtlVe -//(/"cation changed to Na + ro ~) Cation) Form) A"
Pore opening
2
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1
94
Li
+
Si(Al)-0 Bend
0.500 251 (w)
361 (ms) 384 (msh)
484 (mb)
Na
100*
0.426 272 (wb)
378 (ms)
466 (m)
Ag
100
0.376
377 (ms)
457 (m)
381 (s)
455 (m)
376 (ms)
454 (m)
382 (ms)
460 (m)
379 (ms)
462 (m)
378 (s)
465 (m)
+
+
K
+
90
Tl
99
NHS
99
Rb+
69
Cs
51
+
+
*™f > 281(vwsh) w
0.366 278 (w) 0.357
272 (w)
0.353 272 (w) 0.347
276 (mw)
Si(Al)-0 Sym Stretch
Double 4-Rings
573 (mw) 602 (m) 627 (wsh) 557 (ms) 572 (msh) 542 (s) 563 (msh) 552 (ms) 572 (ms) 537 (s) 564 (msh) 546 (ms) 564 (ms) 568 S^t)
704 (s) 670 (wb) 665 (wb) 691 (wb) 668 (wb) 688 (wbsh) 678 (wb) 700 (wbsh) 6
6
9
6
0
9
(
m
b
)
^
S f o S
