16 Infrared Structural Studies of Zeolite Frameworks
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EDITH M. FLANIGEN and HASSAN KHATAMI Union Carbide Corp., Linde Division Laboratory, Tarrytown Technical Center, Tarrytown, Ν. Y. 10591 HERMAN A. SZYMANSKI Alliance College, Cambridge Springs, Pa. 16403 Mid-infrared
spectroscopy has been applied to zeolite struc
tural problems. -1
to 1300 cm
The infrared spectrum in the region of 200
is a sensitive tool indicating structural features
of zeolite f r a m e w o r k s . Preliminary
interpretation
suggests
infrared specificity for zeolite structure type and group, and for structural subunits such as double rings and large pore openings.
It is proposed that the major structural
groups
present in zeolites can be detected from their infrared pat tern. This hypothesis is based on correlation of newly deter mined
infrared
spectra
of synthetic
zeolites with
x-ray
structure data for most of the known structural classes of zeolites. Other structural information obtained from infra red studies includes framework Si/Al composition, structural changes during thermal decomposition, ment during dehydration and
and cation move
dehydroxylation.
* " p h e m e t h o d s a p p l i e d to t h e d e t e r m i n a t i o n o f zeolite structures
have
b e e n the c l a s s i c a l c r y s t a l l o g r a p h i c t e c h n i q u e s o f x - r a y d i f f r a c t i o n a n d m o r e r e c e n t l y e l e c t r o n d i f f r a c t i o n . A s a result o f extensive x - r a y structure studies o n zeolites since a b o u t 1955, the f r a m e w o r k structures o f some 40 zeolites are k n o w n . A classification o f zeolite structure types a n d g r o u p s has b e e n p r o p o s e d b y S m i t h (43), F i s c h e r a n d M e i e r (18), a n d M e i e r (28).
T h e structure types a n d classes are b a s e d o n a s i m i l a r i t y i n f r a m e
w o r k t o p o l o g y a n d c o m m o n elements of s e c o n d a r y b u i l d i n g u n i t s w h i c h c o m p r i s e r i n g s o f t e t r a h e d r a , d o u b l e r i n g s , a n d larger s y m m e t r i c a l p o l y 201
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
202
MOLECULAR SIEVE ZEOLITES
1
h e d r a l u n i t s s u c h as the 18-tetraherda " c a n c r i n i t e " u n i t or t h e 24-tetrahedra truncated octahedron "sodalite" unit (28).
I t is difficult to deter-
m i n e a t o m i c positions i n the c r y s t a l structure of n e w zeolite species b y x - r a y t e c h n i q u e s because of the large u n i t cells a n d t h e l a r g e n u m b e r of p o s s i b l e w a y s of l i n k i n g t e t r a h e d r a . T h e p r o b l e m is m a g n i f i e d w i t h s y n t h e t i c zeolites w h e r e the s t r u c t u r a l investigator most f r e q u e n t l y is l i m i t e d to x - r a y p o w d e r d a t a o w i n g to the u n a v a i l a b i l i t y of l a r g e r single crystals. T h e object of this s t u d y was to a p p l y m i d - i n f r a r e d s p e c t r o s c o p y to zeolite s t r u c t u r a l p r o b l e m s w i t h the u l t i m a t e h o p e of u s i n g i n f r a r e d , a r e l a t i v e l y r a p i d a n d r e a d i l y a v a i l a b l e a n a l y t i c a l m e t h o d , as a t o o l
to
c h a r a c t e r i z e t h e f r a m e w o r k structure a n d p e r h a p s to detect the presence of t h e p o l y h e d r a l b u i l d i n g units present i n zeolite f r a m e w o r k s .
The
m i d - i n f r a r e d r e g i o n of t h e s p e c t r u m w a s u s e d ( 1300 to 200 c m " ) since
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1
that r e g i o n contains the f u n d a m e n t a l v i b r a t i o n s of t h e f r a m e w o r k ( S i , A l ) 0
4
t e t r a h e d r a a n d s h o u l d reflect the f r a m e w o r k structure. I n f r a r e d d a t a
i n s i m i l a r s p e c t r a l regions h a v e b e e n p u b l i s h e d for m a n y m i n e r a l zeolites (30)
a n d a f e w s y n t h e t i c zeolites (23,
49, 50).
T h e r e is a n
extensive
literature o n i n f r a r e d spectra of s i l i c a , silicates, a n d a l u m i n o s i l i c a t e s H o w e v e r , n o systematic s t u d y of the i n f r a r e d characteristics of f r a m e w o r k s as r e l a t e d to t h e i r c r y s t a l structure has a p p e a r e d .
Figure 1. Infrared spectra of zeolites A, X, and Y and hydroxy sodalite (HS); Si/Al in X is 1.2, and in Y , 2.5
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
(17). zeolite
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16.
FLANIGEN E T A L .
Infrared
Structural
Studies
203
Figure 2. Infrared spectra of zeolites B(F1), ΖΚ-5, Ω, and S; right hand por tion of figure represents a higher zeolite concentration in the wafer than in the left portion for Figures 2, 5, and 7 Experimental T h e m a j o r i t y of t h e i n f r a r e d t r a n s m i s s i o n s p e c t r a w e r e o b t a i n e d u s i n g t h e K B r w a f e r t e c h n i q u e ( 3 7 ) . S p e c t r a of at least 2 a n d often m o r e p r e p a r a t i o n s of e a c h z e o l i t e w e r e o b t a i n e d b e f o r e the s p e c t r u m w a s a c c e p t e d as b e i n g c h a r a c t e r i s t i c of t h e z e o l i t e species. S p e c t r a also w e r e o b t a i n e d u s i n g C s l w a f e r s a n d i n the case of s e v e r a l zeolites w i t h m i n e r a l o i l m u l l s a n d p u r e z e o l i t e s e l f - s u p p o r t e d w a f e r s , to e s t a b l i s h a n y m a t r i x effect. O n l y m i n o r s p e c t r a l v a r i a t i o n s w e r e o b s e r v e d a m o n g the s e v e r a l t e c h n i q u e s a n d m a t r i c e s . E x c e p t w h e r e o t h e r w i s e n o t e d , t h e spectra r e p o r t e d here are for h y d r a t e d zeolites i n K B r or C s l w a f e r s . A t y p i c a l w a f e r c o n c e n t r a t i o n w a s 0.5 m g o f zeolite i n 300 m g of K B r or C s l ; h o w e v e r , zeolite c o n c e n t r a t i o n sometimes w a s v a r i e d to o b t a i n t h e d e s i r e d a b s o r b a n c e or to increase the s e n s i t i v i t y to w e a k b a n d s . S p e c t r a w e r e d e t e r m i n e d w i t h a P e r k i n E l m e r M o d e l 621 d o u b l e b e a m g r a t i n g spec trometer. E s s e n t i a l l y i d e n t i c a l spectra w e r e o b t a i n e d u s i n g a P e r k i n E l m e r M o d e l 225 d o u b l e b e a m g r a t i n g spectrometer a n d a B e c k m a n M o d e l I R - 1 2 d o u b l e b e a m g r a t i n g spectrometer as w e r e o b t a i n e d w i t h the P - E 621 f o r zeolites A , X , a n d Y . A f e w spectra w e r e m e a s u r e d for d e h y d r a t e d zeolites b y a c t i v a t i n g the z e o l i t e p o w d e r s i n a i r at 3 5 0 ° C , r a p i d l y q u e n c h i n g i n t o d r y m i n e r a l o i l , a n d r u n n i n g the s p e c t r u m as a m i n e r a l o i l m u l l . D e h y d r a t i o n studies r e p o r t e d f o r C a - e x c h a n g e d Y z e o -
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
204
MOLECULAR SIEVE ZEOLITES
1
l i t e u s i n g s e l f - s u p p o r t e d w a f e r s w e r e c a r r i e d out w i t h a c e l l a n d t e c h n i q u e essentially t h e same as t h a t d e s c r i b e d b y A n g e l l a n d Schaffer ( J ) . T h e s p e c t r a l r e s o l u t i o n is b e t t e r t h a n 5 c m " a n d the e s t i m a t e d a c c u r a c y ± 5 c m " u s i n g the same t e c h n i q u e a n d i n s t r u m e n t , b u t c a n b e as h i g h as ± 1 0 c m " w i t h other m e a s u r e m e n t v a r i a t i o n s . A l l of t h e s y n t h e t i c zeolites i n v e s t i g a t e d w e r e p r e p a r e d i n this l a b o r a t o r y w i t h the e x c e p t i o n of the Z e o l o n p r o d u c t a n d Z K - 5 , a n d w e r e f u l l y c h a r a c t e r i z e d i n terms of c h e m i c a l c o m p o s i t i o n , x - r a y , a n d a d s o r p t i o n p u r i t y . A l l represent z e o l i t e contents of greater t h a n 9 0 % a n d c o n t a i n e d n o c r y s t a l l i n e i m p u r i t i e s detectable b y x - r a y p o w d e r d i f f r a c t i o n analysis. T h e N a " Z e o l o n " u s e d w a s o b t a i n e d f r o m t h e N o r t o n C o . , a n d the e x p e r i m e n t a l s a m p l e of Z K - 5 w a s p r e p a r e d b y K . R . M u l l e r at t h e U n i o n C a r b i d e E u r o p e a n R e s e a r c h Associates L a b o r a t o r y i n Brussels, B e l g i u m . I n the t h e r m a l d e c o m p o s i t i o n studies of A , X , Y , a n d L zeolites, p o w d e r samples of e a c h z e o l i t e w e r e h e a t e d i n a m b i e n t a i r f o r 16 h o u r s at i n c r e a s i n g temperatures to y i e l d a series of t h e r m a l d e c o m p o s i t i o n p r o d ucts w i t h successively l o w e r r e s i d u a l zeolite x - r a y c r y s t a l l i n i t y . T h e h e a t e d p o w d e r s w e r e h y d r a t e d at r o o m t e m p e r a t u r e a n d r u n as K B r w a f e r s to o b t a i n t h e i r i n f r a r e d patterns. 1
1
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1
Results I n f r a r e d s p e c t r a w e r e o b t a i n e d o n t h e s y n t h e t i c zeolites A , N - A , X , Υ, Β ( P I ) , K Z - 5 , o m e g a ( Ω ) , S, R , G , D , T , L , W , s y n t h e t i c analogues of mordenite ("Zeolon"), a n d analcime ( C ) , a n d for the related synthetic
1200
1000
800
600
400
200 Crrr
Figure 3.
800
600
400
200
1
Infrared spectra of zeolites R, G, and D
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
FLANIGEN E T A L .
Infrared
Structural
Studies
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16.
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
205
206
M O L E C U L A R SIEVE ZEOLITES
Table I.
1
Infrared Spectral
Si0
2
Zeolite
AW*
A Ca A N-A N-A X Y Y La Y Y e x
e x
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BCD Hydroxy sodalite (HS) Ω ZK-5 R G D S Τ Hydroxy cancrinite (HC) L C Zeolon
Asym. Stretch 1050vwsh 1055vwsh
Sym. Stretch 742vwsh 750vwsh 750vwsh 746m 760m 784m 790m 789m 772mwsh
660vw 705vwsh 665vw 675vw 698vw 690wsh 668m 686m 714m 705m 718m 670mw 738mw
729m
701mw
660m
805mw
722mw 730mw 678w 696wsh 711w 722mw
690vwsh
1.88 1.9 3.58 6.01 2.40 3.42 4.87 5.0 5.63 2.8
1090vwsh 1130vwsh 1131vwsh 1151vwsh
2.0
1096vwsh
1030s 1044s 971s 985s 1005s 1006s 1017s 9951000s 986s
7.7 6.0 3.25 5.44 4.62 2.5
1130wsh 1158wsh 1136mwsh 1138mwsh 1184mwsh 1140wsh
1024s 1048s 1007s 1027s 1018s 1020s
7.0 2.0
1156wsh 1095mw
1059s 1035msh
1010s 1000s
771w 755w
718w
6.0 4.0 9.95
1162vwsh 1216w
1012s 1180vwsh
1015s 952s 1046s
3.6
1128msh
767mw 740m . 40m 795\ , 772/ 7861 756/
721mw 686wb 715\ . 690f 691mwb
1060msh 1135msh 1130msh 1135msh 1130msh 1105mw8h
995s
890vwb 738w 720w 755wsh 770vwsh
w b
W
1006s
642vwsh
w b
K
m w b
Figure 6. Infrared spectra of zeolites A and N-A; numbers in parenthesis are Si/Al values
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
16.
FLANIGEN E T A L .
Infrared
Structural
207
Studies
D a t a f o r S y n t h e t i c Zeolites Cm~
7a
Dbl. Rings
508vwsh 500wsh 500wsh 504mwsh
464m 460m 474m 475m 458ms 460ms 455ms 450ms 456ms
600m
406w
435ms 461ms
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Pore Opening?
TO Bend
550ms 542ms 572ms 581ms 560m 564m 572m 565m 575m
610mw 572m 625m 632m 631m
508mw 515m 513m 518mb
595sh/ 623mw 624m
606m 615vw 621w 637mw
378ms 376m 385m 393m 365m 372m 380m 382m 383m 380mwsh
260vwb?
250vwb? 260vwb? 315vwsh
282vwb?
432ms
372m
451ms 445m 452m 460m 459m 448m
426m 408m 415m 424ms
467ms 458ms
433ms 429ms
474ms 442ms 448ms
435wsh 410msh
408wsh 370vwsh 378vwsh 376vwsh 370vwsh
mb
575w 567m
498mw
580wsh 571\ 555/ 590wb
512vwsh
483vwsh
410vwsh 390mw
366wsh 353wb
375vwsh 370vwsh
432ns
375vwsh
° s = strong; ms = medium strong; m = medium; mw = medium weak; w = weak; vw = very weak; sh = shoulder; b = broad.
f e l s p a t h o i d phases, h y d r o x y sodalite ( H S ) a n d h y d r o x y c a n c r i n i t e ( H C ) . T h e I R spectra a r e s h o w n i n F i g u r e s 1 - 6 , a n d s p e c t r a l f r e q u e n c i e s l i s t e d i n T a b l e I . C a t i o n a n d f r a m e w o r k c o m p o s i t i o n s a n d references d e s c r i b i n g t h e i r synthesis a n d properties a r e g i v e n f o r a l l of t h e zeolites i n T a b l e I I . S t r u c t u r a l characteristics b a s e d p r i n c i p a l l y o n M e i e r ' s ( 2 8 ) a n d Barrer's ( 2 ) reviews are c o m p i l e d i n T a b l e I I I a n d a summary of s t r u c t u r a l elements a n d b u i l d i n g u n i t s i n z e o l i t e f r a m e w o r k s g i v e n i n T a b l e I V . T h e d e f i n i t i o n of s e c o n d a r y b u i l d i n g u n i t s ( S B U ) a n d b u i l d i n g b l o c k s u s e d h e r e is n o t as precise as that o f M e i e r (28), a n d there is some m i n o r v a r i a t i o n f r o m M e i e r ' s s t r u c t u r a l classification. T h e zeolites w e r e chosen to represent a s p e c t r u m of s t r u c t u r a l types a n d S B U a n d p o l y h e d r a l b u i l d i n g u n i t s i n t h e f r a m e w o r k s as w e l l as a r a n g e o f S i , A l f r a m e w o r k compositions. C o r r e l a t i o n of t h e i n f r a r e d spectra w i t h z e o l i t e s t r u c t u r e has l e d u s to p r o p o s e t h e f o l l o w i n g i n t e r p e r t a t i o n s a n d hypotheses.
E a c h zeolite
species has a t y p i c a l i n f r a r e d p a t t e r n . I n a d d i t i o n , there a r e often g e n e r a l
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
208
MOLECULAR SIEVE ZEOLITES
1
s i m i l a r i t i e s a m o n g t h e s p e c t r a of zeolites w i t h t h e same s t r u c t u r a l t y p e a n d i n t h e same s t r u c t u r a l g r o u p . T h e i n f r a r e d s p e c t r a of zeolites i n the 1 3 0 0 - 2 0 0 c m " r e g i o n a p p e a r to consist o f 2 classes of v i b r a t i o n s : those 1
c a u s e d b y i n t e r n a l v i b r a t i o n s of t h e f r a m e w o r k T 0
4
t e t r a h e d r o n , the
p r i m a r y b u i l d i n g u n i t i n a l l zeolite f r a m e w o r k s , w h i c h t e n d to b e i n s e n s i t i v e to v a r i a t i o n s i n f r a m e w o r k s t r u c t u r e , a n d v i b r a t i o n s r e l a t e d to exter n a l l i n k a g e s b e t w e e n t e t r a h e d r a w h i c h are sensitive to t h e f r a m e w o r k structure a n d to the presence of some S B U a n d b u i l d i n g b l o c k p o l y h e d r a s u c h as d o u b l e r i n g s a n d the large p o r e openings. to A 1 0 T0
4
N o v i b r a t i o n s specific
t e t r a h e d r a o r A l - O b o n d s are assigned b u t r a t h e r v i b r a t i o n s of
groups a n d T - O b o n d s w h e r e t h e v i b r a t i o n a l f r e q u e n c i e s represent
4
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t h e average S i , A l c o m p o s i t i o n a n d b o n d characteristics of t h e c e n t r a l Τ cation.
T h e p r o p o s e d i n f r a r e d assignments are p r e s e n t e d i n d e t a i l i n
T a b l e V a n d i l l u s t r a t e d w i t h the i n f r a r e d s p e c t r u m of zeolite Y i n F i g u r e 7. T h e b a n d s assigned to i n t e r n a l t e t r a h e d r a l v i b r a t i o n s are s h o w n i n t h e Table II.
Compositions of Synthetic Zeolites Ref.
Zeolite
Cation*
A N-A X Y HS ( H y d r o x y Sodalite) ZK-5 B(P1) Omega (Ω) S R G D Τ L HC ( H y d r o x y cancrinite) W Zeolon C
Na TMA , Na Na Na Na
2 2.5-6.0 2.0-3.0 >3-6 2-3
{15, 31) {5) {16, 32) {11, 16) U, 8)
Na, D D O Na Na, T M A Na Na Κ K, Na K, Na Κ; K , N a Na
4-6 2-5 5-12 4.6-5.9 3.5-3.7 2-6 4.6-5.0 6.4-7.4 5.2-6.9 2.0
{22) {33) {6, 19) {10) {35) {3) {12) {13)
Κ Na Na
3.3-4.9 10-11 2-6
6
{14, {8)
{34)
{41)
{38,
39)
° Cation composition as synthesized. T M A = tetramethyl ammonium ion, ( C H ^ N * . D D O = ( C H i N ) = [l,4-dimethyl-l,4-diazoniacyclo (2.2.2.) octane] *. 6
c
8
8
2
16)
2 +
2
2
In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
16.
Infrared
FLANIGEN E T A L .
Structural
Studies
209
figures w i t h a f u l l l i n e d r a w i n g , a n d those to e x t e r n a l l i n k a g e m o d e s w i t h a broken line. T h e first class of v i b r a t i o n s c o m m o n to a l l zeolites a n d assigned to i n t e r n a l t e t r a h e d r o n v i b r a t i o n s i n c l u d e s the 2 most intense b a n d s i n the s p e c t r u m , the strongest at 9 5 0 - 1 2 5 0 c m " a n d the other of m e d i u m i n t e n 1
sity at 420--500 c m " . W e p r o p o s e as a m o d e l f o r c o m p a r i s o n a n d a s s i g n 1
m e n t of i n t e r n a l t e t r a h e d r a l m o d e s the c o m p r e h e n s i v e w o r k of L i p p i n c o t t et al. (26)
o n the i n f r a r e d spectra of the p o l y m o r p h s of s i l i c a . T h e p r i
m a r y b u i l d i n g u n i t of a S i 0
4
tetrahedron linked i n a three-dimensional
t e t r a h e d r a l f r a m e w o r k i n the silicas is analogous to zeolite f r a m e w o r k s , a n d serves as a p o i n t of reference.
Lippincott's S i - O vibrations become
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T - O v i b r a t i o n s i n o u r assignments. A f t e r L i p p i n c o t t , the strongest v i b r a tion i n the 950-1250 c m "
1
r e g i o n is assigned to a T - O stretch i n v o l v i n g
m o t i o n p r i m a r i l y associated w i t h o x y g e n atoms, or a l t e r n a t e l y d e s c r i b e d as a n a s y m m e t r i c s t r e t c h i n g m o d e