15 Faujasite-Type Structures: Aluminosilicate
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Framework: Positions of Cations and Molecules: Nomenclature J. V. SMITH Department of the Geophysical Sciences, University of Chicago, Chicago, Ill. 60637 The framework distorts in response to cations and molecules. Type X zeolite has strong Al,Si long-range order, but the order—disorder in faujasite and Type Y zeolite is equivocal. Positions of framework hydroxyls in heated NH -exchanged faujasite were inferred from interatomic distances and infrared data. Exchangeable cations in strictly dehydrated specimens occupy sites offering minimum electrostatic energy; the center of the hexagonal prism is preferred. For incomplete dehydration (typical for most commercial catalytic processes), cations bond to residual molecules in the sodalite units. The location of exchangeable cations and water molecules in hydrated specimens is uncertain. Hydration complexes of cations occur in the supercage. In the Al-rich varieties, cations certainly enter the sodalite unit. In the Al-poor varieties, x-ray diffraction evidence on cation positions is equivocal. 4
T h i s paper reviews data available by January 1970 on the crystal structures of materials containing an aluminosilicate framework with the topology of the mineral faujasite. The nomenclature will be discussed in more detail at the end. Briefly, names will be assigned which specify the treatment applied to the 3 basic starting materials—viz., faujasite, Linde Y, and Linde X zeolites. The latter 2 are synthetic materials prepared in hydrous sodium systems, the former being richer and the latter poorer in Si. The Si,Al content of Y overlaps that of faujasite. Although x-ray diffraction techniques can yield data on atomic positions and occupation frequency which appear highly precise, these data
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1
A
171 Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
172
MOLECULAR SIEVE ZEOLITES
1
are subject to p e r s o n a l i n t e r p r e t a t i o n . X - r a y d i f f r a c t i o n m e t h o d s a c t u a l l y y i e l d m a p s of e l e c t r o n d e n s i t y . A s s i g n m e n t of e l e c t r o n d e n s i t y to p a r t i c u l a r atoms i n v o l v e s c h e m i c a l assumptions. N o one w i l l d i s p u t e assignment of atoms to e l e c t r o n d e n s i t y peaks i n q u a r t z , b u t i n c o m p l e x
zeolites
assignment of peaks c a n b e most u n c e r t a i n , e s p e c i a l l y w h e n m o r e t h a n 1 k i n d of c a t i o n is present a n d w h e n s m a l l m o l e c u l e s m i g h t o c c u p y t h e same p a r t of space as cations. A n o t h e r c h a r a c t e r i s t i c feature of x - r a y d i f f r a c t i o n is t h a t e a c h d i f Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
f r a c t e d w a v e a u t o m a t i c a l l y sums the v e c t o r a m p l i t u d e f r o m a l l u n i t cells of e a c h coherent c r y s t a l u n i t .
Hence, random occupancy
of 1 site b y
A l a n d S i atoms results i n just 1 e l e c t r o n d e n s i t y c o n c e n t r a t i o n . T h e r m a l v i b r a t i o n a n d r a n d o m p o s i t i o n a l d i s p l a c e m e n t s of atoms f r o m 1 u n i t c e l l to another y i e l d a s i m i l a r b l u r r i n g - o u t of the e l e c t r o n d e n s i t y ; f u r t h e r m o r e , i t is difficult e x p e r i m e n t a l l y to d i s t i n g u i s h b e t w e e n r e d u c t i o n of h e i g h t of a n e l e c t r o n d e n s i t y p e a k r e s u l t i n g f r o m t h e a b o v e 2 factors a n d that f r o m a l o w e r o c c u p a n c y X-ray powder
factor.
d a t a y i e l d m u c h m o r e u n c e r t a i n results o n
atomic
positions t h a n d a t a f r o m single crystals. L a r g e s y n t h e t i c crystals h a v e not b e e n a v a i l a b l e u n t i l r e c e n t l y , a n d s i n g l e - c r y s t a l d a t a h a v e b e e n t a k e n m o s t l y o n t r e a t e d faujasite. A l t h o u g h a zeolite is a single c h e m i c a l system w i t h m u t u a l r e l a t i o n ships b e t w e e n a l l parts, it is c o n v e n i e n t to o r g a n i z e this p a p e r i n the sequence a l u m i n o s i l i c a t e f r a m e w o r k , cations, a n d molecules. Aluminosilicate
Framework
T h e o x y g e n atoms l i e at the corners of n e a r - r e g u l a r t e t r a h e d r a w h o s e centers are o c c u p i e d b y either A l or S i atoms. E a c h corner is s h a r e d b y 2 t e t r a h e d r a , a n d the l i n k a g e of the r e s u l t i n g f r a m e w o r k is specified b y p l a c i n g t e t r a h e d r a l centers at the corners of t r u n c a t e d o c t a h e d r a w h o s e centers l i e at the positions of c a r b o n atoms i n the d i a m o n d structure. T h e t r u n c a t e d o c t a h e d r a l i e i n s u c h positions t h a t t h e y are j o i n e d
by
h e x a g o n a l p r i s m s . M o d e l s b u i l t f r o m c a r d b o a r d p o l y h e d r a or f r o m w i r e t e t r a h e d r a l i n k e d b y s p a g h e t t i t u b i n g are p a r t i c u l a r l y easy to T h e c o n s t r u c t i o n k e y is to b u i l d a t r u n c a t e d o c t a h e d r o n
assemble.
first,
assemble
4 h e x a g o n a l p r i s m s onto a n y 4 h e x a g o n a l faces w h i c h are n o n a d j a c e n t , a n d a t t a c h to the opposite face of e a c h h e x a g o n a l p r i s m a t r u n c a t e d o c t a h e d r o n s u c h t h a t its h e x a g o n a l faces are staggered w i t h respect to those of t h e first t r u n c a t e d o c t a h e d r o n . R e p e a t e d a p p l i c a t i o n of this s i m p l e r u l e a u t o m a t i c a l l y generates t h e i n f i n i t e f r a m e w o r k . S u c h abstract m o d e l s
of w i r e or c a r d b o a r d are r a t h e r m i s l e a d i n g
w h e n a t o m i c interactions are c o n s i d e r e d , a n d a m o d e l a s s e m b l e d
from
balls—e.g., c o r k or p o l y s t y r e n e — i s v e r y v a l u a b l e , t h o u g h n o t easy
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
to
15.
Faujasite-Type
SMITH
m a k e . B r e c k (17)
figured
173
Structures
several m o d e l s , a n d O l s o n a n d c o w o r k e r s g a v e
v a l u a b l e stereoscopic d r a w i n g s i n t h e i r papers. W . M . M e i e r a n d D . H . O l s o n (44)
s h o w stereoscopic d r a w i n g s of the entire z e o l i t e g r o u p .
T h e space i n s i d e a t r u n c a t e d o c t a h e d r o n is c o m m o n l y c a l l e d sodalite cage.
the
E a c h l a r g e interstice ( c o m m o n l y c a l l e d a s u p e r c a g e ) is
l i n k e d to 4 other supercages t h r o u g h n e a r - c i r c u l a r 1 2 - m e m b e r e d rings a n d to 4 sodalite units t h r o u g h 6-rings.
E a c h s u p e r c a g e shares 4-rings
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w i t h 6 other sodalite u n i t s .
Figure
1.
Idealized
projection
of sodalite unit with atom clature
nomen
The intersections of the polyhedral edges show the positions of Τ atoms. Oxygen atoms are shown at the mid-points of the edges, but should be dis placed to correspond to a tetrahedral environment for each Τ atom. Some of the positions for the 4 types of oxygens are shown. The center of the truncated octahedron is marked by U. Four axes of inverse 3-fold sym metry pass through this point: 3 are visible, and 1 is hidden because it lies perpendicular to the plane of the diagram. Four hexagonal prisms share a hexagonal face with the truncated octahedron; 1 is hidden at the back side. The cation sites are: I at the center of an hexagonal prism, Γ displaced from a shared hexagonal face into the sodalite cage, ΙΓ displaced from an un shared hexagonal face into the sodalite cage, II slightly displaced into the supercage, and II* displaced considerably into the supercage. For abcdefghij, see Figure 2
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
174
MOLECULAR SIEVE ZEOLITES 1
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I'
Figure Figure cation broken oxygen
2. Section through abcdefghij in 1 showing the relative positions of sites and framework oxygens. The lines show the principal cationbonds in a dehydrated faujasite con taining divalent cations.
Figures 1 a n d 2 show the generally accepted nomenclature for the a t o m sites.
R e a d e r s s h o u l d note that earlier papers use a v a r i e t y of
nomenclatures w h i c h must be checked very carefully.
F i g u r e 1 is a
p r o j e c t i o n of a sodalite u n i t d o w n a t r i a d axis v i e w e d f r o m
infinity.
T h e s i l i c o n a n d a l u m i n u m atoms ( c o l l e c t i v e l y c a l l e d Τ atoms ) l i e at t h e 4 - f o l d intersections of t h e f r a m e w o r k .
F i g u r e 2 is a section t h r o u g h
F i g u r e 1, as e x p l a i n e d i n t h e figure l e g e n d . O x y g e n atoms l i e near t h e m i d - p o i n t s of the edges, b u t are d i s p l a c e d to a t t a i n t h e t e t r a h e d r a l c o n f i g u r a t i o n a r o u n d t h e Τ atoms. I n a d d i t i o n , the a t o m positions i n a c t u a l structures differ i n d e t a i l f r o m this i d e a l i z e d topologic
pattern.
T h e oxygen
atoms f o r m a n i n t e r e s t i n g p o l y h e d r a l
a r r a n g e m e n t i n w h i c h t e t r a h e d r a are c o m b i n e d w i t h t r u n c a t e d h e x a g o n a l p r i s m s a n d p o l y h e d r a f o r m e d b y t r u n c a t i n g t h e t r u n c a t e d o c t a h e d r a of Τ atoms.
T h e i n t e r e s t i n g m a t h e m a t i c a l relations b e t w e e n t h e n e t w o r k s
f o r m e d f r o m Τ atoms a n d those f o r m e d f r o m Ο atoms s h o u l d b e e x p l o r e d . E a c h t r u n c a t e d o c t a h e d r o n d e f i n i n g t h e Τ atoms is c o m p o s e d
of 4
hexagonal rings, each shared w i t h a hexagonal p r i s m (using 0 2 a n d 0 3 o x y g e n a t o m s ) , 4 free h e x a g o n a l rings s h a r e d w i t h a supercage
(using
0 2 a n d 0 4 ) a n d 6 b r i d g i n g 4-rings ( u s i n g 0 3 a n d 0 4 ) . E a c h h e x a g o n a l p r i s m contains 2 6-rings c o m p o s e d of 0 2 a n d 0 3 a n d 6 4-rings c o m p o s e d of Ο Ι , 0 2 , a n d 0 3 . Site U is at the center of the t r u n c a t e d o c t a h e d r o n a n d lies at t h e i n t e r s e c t i o n of 4 axes of inverse 3-fold r o t a t i o n s y m m e t r y . S i t e I , at t h e
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
Faujasite-Type
SMITH
Structures
175
center of e a c h h e x a g o n a l p r i s m , is a p o t e n t i a l site f o r cations.
exchangeable
Site I I projects s l i g h t l y o u t w a r d s f r o m t h e free 6 - r i n g i n t o t h e
supercage.
Sites I ' a n d I F , r e s p e c t i v e l y , project i n t o the sodalite u n i t f r o m
t h e s h a r e d a n d free 6-rings. F i g u r e 2 shows t h e distances b e t w e e n t h e positions m o r e c l e a r l y t h a n F i g u r e 1. Site I I * projects f u r t h e r i n t o t h e supercage a w a y f r o m t h e free 6 - r i n g t h a n w o u l d b e e x p e c t e d f o r a c a t i o n b o n d e d to o x y g e n atoms of t h e 6 - r i n g . Site V is near t h e center of t h e 12-membered
ring between
t h e supercages,
a n d is n o t s h o w n i n t h e
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figures. T h e e a r l y structure d e t e r m i n a t i o n s of t h e f r a m e w o r k t o p o l o g y ( 2 , 14, 15, 16, 19)
of faujasite h a v e b e e n c o n f i r m e d b y a b o u t 30 f u r t h e r
d e t e r m i n a t i o n s . Interest has s h i f t e d to 4 p r i n c i p a l features of the a l u m i n o silicate f r a m e w o r k : shape, S i , A l o r d e r - d i s o r d e r , a t t a c h m e n t of
protons
to y i e l d h y d r o x y l , a n d r e m o v a l of h y d r o x y l , a l u m i n u m , or b o t h f r o m t h e framework. Shape. T h e changes of shape of the t e t r a h e d r a are significant ( T a b l e I).
T h e greatest d i s t o r t i o n occurs i n d e h y d r a t e d forms, a n d t h e d i s t o r
tions c a n b e e x p l a i n e d q u i t e satisfactorily b y s i m p l e b o n d i n g
theory.
T h u s , the m o r e s t r o n g l y a n o x y g e n is b o n d e d to t h e exchangeable cations, the m o r e distant i t tends to l i e f r o m t h e Τ atoms. listed quantitative data.
(48)
Olson and Dempsey
H y d r a t e d specimens h a v e m o r e u n i f o r m
T - O distances a n d O - T - O angles t h a n d e h y d r a t e d ones, a g a i n r e a d i l y Table I. Type
Reference
Τ—Ο Distances T-01
a
T-02
a
Ύ-03
Ύ-ϋ4
α
α
Faujasite Ca-faujasite La-faujasite Ce-faujasite Na-X
(7) (10) (12) (49) (47)
Hydrated Forms 1.643(3) 1.645(3) 1.642(2) 1.640(2) 1.643(3) 1.632(4) 1.64 (1) 1.64 (1) 1.627(7) 1.622(7) 1.738(7) 1.719(7)
1.657(3) 1.655(3) 1.647(4) 1.65 (1) 1.616(7) 1.737(8)
1.642(3) 1.644(3) 1.648(4) 1.65 (1) 1.612(7) 1.723(7)
Ca-faujasite Ni-faujasite L a - f a u j a s i t e , 420°C La-faujasite Ce-faujasite Ba-faujasite K-faujasite* H-faujasite Ca-X Sr-X
Dehydrated Forms 1.633(5) 1.651(5) (8) 1.633(1) 1.641(1) (46) 1.630(4) 1.624(3) (11) 1.632(3) 1.625(2) (9) 1.63 (1) 1.64 (1) (49) 1.626(3) 1.638(3) (52) 1.630(3) 1.649(2) (52) 1.653(2) 1.634(1) (48) 1.653(7) 1.673(6) (48) 1.655(6) 1.682(4) (48)
1.671(5) 1.695(1) 1.683(4) 1.689(3) 1.66 (1) 1.654(3) 1.653(3) 1.662(2) 1.678(8) 1.680(5)
1.620(5) 1.613(1) 1.599(3) 1.602(2) 1.61 (1) 1.637(4) 1.647(3) 1.623(2) 1.650(6) 1.654(4)
6
° Standard errors in brackets to same significance level. Minor changes may occur during further refinement. 6
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
176
MOLECULAR SIEVE ZEOLITES
1
e x p l a i n a b l e b y the m o r e e v e n d i s t r i b u t i o n of the b o n d i n g i n the f o r m e r — see,
for e x a m p l e , q u a n t i t a t i v e d a t a b y B e n n e t t a n d S m i t h (10,
12).
T h e T - O - T angles f a l l m a i n l y i n the range 1 3 0 ° - 1 5 5 ° C w i t h the h y d r a t e d forms y i e l d i n g o n average a s m a l l e r s p r e a d . T h i s r a n g e is t y p i c a l of silicate f r a m e w o r k s
(40).
D e t a i l e d e x p l a n a t i o n of t h e T-O-T
angles w h i c h l i n k t h e t e t r a h e d r a
is not so easy, t h o u g h t h e r e is n o reason to d o u b t that s i m p l e i o n i c b o n d
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ing can yield a plausible model. (42)
M e g a w , Kempster, and Radoslovich
i n t e r p r e t e d the geometry of a n o r t h i t e i n terms of a n e n g i n e e r i n g
m o d e l w i t h struts a n d springs r e p r e s e n t i n g r i g i d S i - O b o n d s a n d c a t i o n c a t i o n r e p u l s i o n s . H o w e v e r , a m o r e s o p h i s t i c a t e d treatment s h o u l d c o n s i d e r covalent b o n d i n g [see
C r u i c k s h a n k for possible effect of ττ-bonding
(21)1 U n f o r t u n a t e l y , there is n o reference s t r u c t u r e free of cations, t h o u g h p e r h a p s a faujasite-type f r a m e w o r k c o n t a i n i n g o n l y S i 0 be
synthesized.
The
dehydrated
H-faujasite
a m m o n i u m - e x c h a n g e d faujasite (48)
2
ultimately w i l l
produced
by
heating
is the closest to a cation-free f a u j a
site, b u t e v e n i t has protons c o n d e n s e d w i t h f r a m e w o r k oxygens to f o r m h y d r o x y Is. I n faujasite, cations of types Γ, I F , a n d I I are p e r m i t t e d b y s y m m e t r y to adjust t h e i r distances f r o m f r a m e w o r k oxygens b y
the
sliding
a l o n g the 3-fold axis. T h e T y p e I cations are fixed b y s y m m e t r y , a n d the 6 0 3 oxygens m u s t adjust p o s i t i o n i n order to a c h i e v e a s u i t a b l e b o n d l e n g t h . C o n s e q u e n t l y , the p r i m a r y factor i n d i s t o r t i o n of the f r a m e w o r k s h o u l d b e the T y p e I cations. T a b l e I I lists the 4 i n t e r t e t r a h e d r a l angles for selected
specimens.
T a b l e I I I shows the distances f r o m site I to the 0 3 a n d 0 2 oxygens a n d t h e diameters of the 2 4 - m e m b e r e d
rings and the 2 6-membered
rings.
T h e r e is n o o b v i o u s p a t t e r n to the angles l i s t e d i n T a b l e I I . F a u j a s i t e a n d other h y d r a t e d forms ( n o t s h o w n ) only a few Ce-faujasite.
h a v e angles w h i c h r a n g e
degrees, b u t this s m a l l r a n g e also occurs
in
over
dehydrated
H y d r a t e d N a - X has a w i d e range of angles, e v e n w h e n
values a v e r a g e d to s y m m e t r y Fd3m
are u s e d .
T a b l e I I I shows significant trends.
I n d e h y d r a t e d H - f a u j a s i t e , the
4-rings are a p p r o x i m a t e l y square. I n d e h y d r a t e d N i - f a u j a s i t e , o c c u p a t i o n of I b y the s m a l l N i i o n causes the I - 0 3 distance to f a l l f r o m 2.68 i n d e h y d r a t e d H - f a u j a s i t e to 2.29A.
The I - 0 2
d i s t a n c e does n o t
significantly, b u t the b r i d g i n g 4 - r i n g ( F i g u r e 1) with 0 3 - 0 3
r i s i n g f r o m 3.58 to 4.27, a n d 0 4 - 0 4
change
distorts t r e m e n d o u s l y f a l l i n g f r o m 3.52
to
3.03A. R e c i p r o c a l l y , o c c u p a t i o n of I b y the large cations Κ or B a causes the I - 0 3 a n d 0 4 - 0 4
distances to increase, a n d the 0 3 - 0 3
distance to
f a l l . I n fact, there is a n excellent c o r r e l a t i o n b e t w e e n the 3 distances for
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type
Table II.
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Type Faujasite Dehydrated H-faujasite Dehydrated Ni-faujasite Dehydrated Ba-faujasite Dehydrated K-faujasite Dehydrated Ce-faujasite Ce-faujasite Na-X α
177
Structures
Intertetrahedral Angles in Selected Specimens Reference
Ί-Ό1-Ύ
α
Ύ-Ό2-Ύ
α
Ύ-03-Τ
Ύ-Ό4-
α
(7)
141
145
141
140
(48)
139
147
140
145
(46)
130
138
125
158
(52)
137
156
147
145
(52)
139
152
146
142
(49) (49) (47)
142 138 132
148 147 141
143 141 136
144 139 147
Angles rounded off to nearest degree.
Standard deviations vary from 0.2° to 1.0°.
a l l structures. Q u a l i t a t i v e l y , t h e k e y effect is that cations i n d u c e d i s t o r tions i n t h e h e x a g o n a l p r i s m s w h i c h t w i s t t h e b r i d g i n g 4-rings. T h e d i s t o r t i o n of t h e 4-rings of t h e h e x a g o n a l p r i s m s is n o t so p r o n o u n c e d , a n d cannot b e c o r r e l a t e d s i m p l y to t h e I cations. T h e diameters of t h e 6-rings also v a r y o n l y a s m a l l a m o u n t .
O c c u p a n c y of I b y s m a l l
d i - a n d t r i - v a l e n t cations seems to increase t h e d i a m e t e r of t h e free 6 - r i n g at t h e expense of t h e d i a m e t e r of t h e p r i s m 6 - r i n g . T h i s is reasonable since t h e I cations p u l l the 0 3 oxygens t o w a r d s t h e t r i a d axis, w h e r e a s the 0 2 oxygens d o n o t m o v e a n e q u a l a m o u n t a w a y f r o m t h e t r i a d . C a r e f u l s t u d y of t h e a b o v e correlations m a y p e r m i t e s t i m a t i o n of a t o m i c positions to a n a c c u r a c y of 0.1 A or better. Si,Al Order-Disorder. E x t e n s i v e d a t a o n a l u m i n o s i l i c a t e f r a m e w o r k structures w i t h S i / A l ^ 1 h a v e s h o w n that n o t m o r e t h a n 1 A l a t o m is b o n d e d to a n o x y g e n a t o m , except i n rare cases. A n o r t h i t e p r o b a b l y has 2 A l atoms a t t a c h e d to some o x y g e n atoms w h e n c r y s t a l l i z e d d r y at h i g h t e m p e r a t u r e (38).
G e h l e n i t e has 2 A l atoms a t t a c h e d to o x y g e n atoms,
b u t i t is a sheet structure stable o n l y at h i g h t e m p e r a t u r e (37).
Nepheline,
w i t h a f r a m e w o r k s t r u c t u r e , appears to h a v e 2 A l atoms s h a r i n g a n o x y g e n b u t t h e c r y s t a l structure s h o u l d b e r e - e x a m i n e d
(30).
S i n c e zeolites
f o r m at l o w t e m p e r a t u r e , t h e a v o i d a n c e r u l e is p r o b a b l y a p p l i c a b l e t o t h e m , except p e r h a p s f o r v e r y f e w atoms. I n faujasite-type structures, o n l y t h e extreme T y p e X w i t h S i / A l == 1 w o u l d b e c o n s t r a i n e d m a t h e m a t i c a l l y b y this a v o i d a n c e
r u l e to h a v e
c o m p l e t e S i , A l order. A l l faujasite-type materials w i t h S i / A l > 1 m a t h e m a t i c a l l y m a y h a v e either long-range or short-range disorder.
However,
f r o m t h e v i e w p o i n t of t h e r m o d y n a m i c s , there s h o u l d b e a d r i v i n g force at
l o w temperature towards
an ordered
pattern w i t h lower
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
internal
178
MOLECULAR SIEVE ZEOLITES
Table III.
1
Dimensions of the
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Reference
Ca-faujasite La-faujasite Dehydrated H-faujasite Dehydrated K-faujasite Dehydrated Ni-faujasite Dehydrated Ca-faujasite Dehydrated La-faujasite Dehydrated Ea-faujasite Na-X e
a b c
energy.
(10) (12) (48) (52) (46) (8) (9) (52) (47)
Diagonals of bridging 4-ring. Diagonals of prism 4-ring. Diagonal of free 6-ring.
A t h i g h t e m p e r a t u r e , the c o n f i g u r a t i o n a l e n t r o p y t e r m m a y b e
m o r e significant t h a n the i n t e r n a l energy factor, t h e r e b y f a v o r i n g d i s order. S i n c e s y n t h e t i c zeolites are g r o w n u n d e r c o n d i t i o n s of h i g h supersaturation, d i s o r d e r is f a v o r e d .
T h e s e considerations h a v e b e e n t r e a t e d
e x h a u s t i v e l y i n papers o n n a t u r a l m i n e r a l s , e s p e c i a l l y feldspars [see,
for
e x a m p l e , Ref. 6 ] . O l s o n (47)
d e m o n s t r a t e d f r o m 3D
s i n g l e - c r y s t a l x - r a y analysis that
h y d r a t e d N a - X has s y m m e t r y Fd3 i n s t e a d of Fd3m framework.
for the i d e a l faujasite
T h e m e a n T - O distances of the 2 i n d e p e n d e n t t e t r a h e d r a
are 1.619(4) a n d 1 . 7 2 9 ( 4 ) A .
T h e latest c o m p i l a t i o n of T - O
distances
suggests reference values of 1.605 a n d 1.757A for S i - O a n d A l - O , respec tively (55).
H e n c e , the O l s o n d a t a suggest o c c u p a n c i e s of 0.91Si, 0.09A1
a n d 0.18Si, 0.82A1 i n the t e t r a h e d r a . T h e c e l l content of 88 A l a n d 104 S i atoms p r o h i b i t s c o m p l e t e order for Fd3 b u t the O l s o n d a t a i n d i c a t e v e r y strong long-range order. P r o b a b l y there is some r e s i d u a l d i s o r d e r , since the first t e t r a h e d r o n contains 0.09A1. O l s o n f o u n d t h a t other c a t i o n forms of X also h a v e s y m m e t r y Fd3, c o n f i r m i n g the d a t a f r o m the N a f o r m . T h e d a t a for the m o r e s i l i c a - r i c h m e m b e r s are c o n t r o v e r s i a l . (61)
a r g u e d t h a t e l e c t r o n d e n s i t y d i s t r i b u t i o n s r e v e a l the
order, whereas c e l l d i m e n s i o n s d e p e n d
Smith
long-range
o n the short-range order.
All
x - r a y d e t e r m i n a t i o n s of e l e c t r o n d e n s i t y for faujasite a n d Y a u t o m a t i c a l l y i n d i c a t e c o m p l e t e S i , A l d i s o r d e r since there is o n l y 1 Τ site w h e n
Fd3m
is used. S e v e r a l authors h a v e discussed v a r i o u s aspects of o r d e r - d i s o r d e r i n these zeolites.
T h e most c o m p l e t e d i s c u s s i o n is b y D e m p s e y
D e m p s e y , K i i h l , a n d O l s o n (25),
(23)
and
w h o w o r k e d o u t m a t h e m a t i c a l l y the
v a r i o u s w a y s of p l a c i n g S i a n d A l atoms w h e n S i / A l ^ 1. U n l e s s there are restrictions o n the c e l l edge a n d the s y m m e t r y , a n infinite n u m b e r of o r d e r e d structures is possible.
D e m p s e y et al. r e s t r i c t e d t h e i r treat-
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type
179
Structures
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Aluminosilicate Framework SI-02
SI-OS
04-04°
03-03'
01-01
02-03
3.52 3.51 3.52 3.45 3.51 3.46 3.34 3.36 3.62
2.76 2.78 2.68 2.86 2.29 2.49 2.54 2.84 2.62
3.73 3.79 3.52 3.97 3.03 3.27 3.41 3.98 3.54
3.41 3.42 3.58 3.36 4.27 4.10 4.02 3.42 3.66
3.70 3.69 3.61 3.73 3.54 3.83 3.86 3.79 3.68
3.75 3.72 3.64 3.63 3.43 3.37 3.25 3.50 3.71
d
e
b
h
02-04°
02-03
5.15 5.15 5.20 5.14 5.30 5.19 5.27 5.19 5.25
5.14 5.12 5.10 5.20 4.88 5.01 4.96 5.14 5.15
d
Diagonal of prism 6-ring. Averaged to pseudosymmetry FdSm.
m e n t b y c o n s i d e r i n g i d e n t i c a l u n i t s h a v i n g zero net charge a n d d i p o l e moment.
F o r faujasite, the smallest u n i t is a d o u b l e sodalite u n i t w i t h
24 p a i r s of Τ atoms o b e y i n g the center of s y m m e t r y at site I of t h e s h a r e d h e x a g o n a l p r i s m . T h e 4 - f o l d m u l t i p l i c i t y of t h e F - l a t t i c e p e r m i t s o r d e r i n g at the f o l l o w i n g contents of A l atoms p e r u n i t c e l l : 96, 88, 80, 72, 64, 56, 48, etc.
F o r 12 A l atoms p e r sodalite u n i t , t h e r e is strict a l t e r n a t i o n of
51 a n d A l atoms.
T h e s y m m e t r y is Fd3 a n d a l l 6-rings h a v e 3 A l a l t e r
n a t i n g i n the 1:3:5 positions. F o r 11 A l atoms, a n y 1 S i c a n b e e x c h a n g e d . Further
exchange
might produce
2 rings w i t h
A l at 1:3
positions.
D e m p s e y et ai. state that a l o w e r electrostatic energy is o b t a i n e d f o r 10 or f e w e r A l atoms i f the S i a n d A l atoms r e d i s t r i b u t e , p r o d u c i n g some rings w i t h 2 A l atoms at 1:4 positions. F u r t h e r d i s c u s s i o n is too d e t a i l e d to abstract here. S i m i l a r considerations o n S i , A l o r d e r i n g w e r e m a d e b y N i g g l i for p l a g i o c l a s e feldspars.
(44)
W h a t e v e r the details of o r d e r i n g , i t seems
l i k e l y t h a t as the S i , A l r a t i o changes i n a n o r d e r e d f r a m e w o r k silicate, there w i l l b e readjustments of t h e t o p o l o g i c a l d i s t r i b u t i o n s . D e m p s e y , K i i h l , a n d O l s o n (25)
suggested t h a t the c e l l d i m e n s i o n s
s u p p o r t e d the c o n c e p t of S i , A l o r d e r i n the faujasite-type zeolites. F i g u r e 3 shows t h e i r d a t a for c e l l d i m e n s i o n s of 21 specimens g r o w n f r o m h y d rous s o d i u m systems p l o t t e d against A l content d e r i v e d f r o m the b u l k c h e m i c a l compositions.
D e m p s e y et al. i n s e r t e d 4 straight lines w i t h d i s
c o n t i n u i t i e s close to 80 a n d 64 A l atoms p e r c e l l , a n d another one n e a r 52 atoms p e r c e l l . T h e f o r m e r 2 d i s c o n t i n u i t i e s w e r e i n t e r p r e t e d i n terms of r e a r r a n g e m e n t of Τ atoms i n a c c o r d w i t h the t h e o r e t i c a l analysis just d e s c r i b e d , w h i l e the latter was a s c r i b e d to possible f o r m a t i o n of a m o r p h o u s s i l i c a i n the m o r e siliceous b u l k compositions. A l s o i n F i g u r e 3 are the f o l l o w i n g d a t a : ( a ) D o t t e d l i n e r e p r e s e n t i n g a least-squares l i n e a r fit to 37 d a t a points w i t h a n e x p e r i m e n t a l error of =b 0.005A i n a a n d ± 0 . 5 w t % i n
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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180
MOLECULAR SIEVE ZEOLITES
24.5
24.6
24.7
24.8 24.9 Cell repeat (&)
25.0
1
25.1
Figure 3. Cell dimensions of faujasite, Y , and X zeolites plotted against the number of Al atoms per unit cell. See text for explanation of symbols. The dotted line was obtained by Breck and Flanigen for zeolites grown from hydrous sodium systems. S i 0 a n d A 1 0 . T h e s e d a t a points o b t a i n e d b y B r e c k a n d F l a n i g e n ( 1 8 ) w e r e o b t a i n e d f o r samples c r y s t a l l i z e d f r o m h y d r o u s s o d i u m systems, as w e r e those of D e m p s e y et ai. 2
(69),
2
3
( b ) F i v e data points obtained b y W r i g h t , Rupert, a n d G r a n q u i s t also for specimens o b t a i n e d f r o m h y d r o u s s o d i u m systems.
( c ) D a s h e d l i n e r e p r e s e n t i n g a subjective i n t e r p r e t a t i o n of 20 d a t a p o i n t s f o r d e h y d r a t e d C a - e x c h a n g e d specimens (18). These might be i n t e r p r e t e d i n terms of d i s c o n t i n u i t i e s n e a r 77 a n d 62 A l atoms p e r c e l l . ( d ) A d a t u m b y W r i g h t et al. (69) a n d one b y B e n n e t t a n d S m i t h ( u n p u b l i s h e d ) f o r N a - e x c h a n g e d faujasite f r o m S a s b a c h . B o t h values are l o w e r t h a n t h e d a t a for s y n t h e t i c m a t e r i a l of s i m i l a r A l content. T h e B e n n e t t - S m i t h s p e c i m e n w a s c h e c k e d b y e l e c t r o n m i c r o p r o b e analysis for absence of a l l cations other t h a n s o d i u m . T h e r e v i e w e r c o u l d not find definitive c r i t e r i a to resolve these c o n flicting
data.
T h e l i k e l i h o o d of a systematic e x p e r i m e n t a l bias b e t w e e n
the t e c h n i q u e s for c e l l - d i m e n s i o n m e a s u r e m e n t c o u l d o n l y b e tested b y exchange of samples b e t w e e n the v a r i o u s w o r k e r s .
It is p o s s i b l e that
there is a n i m p u r i t y i n the zeolite c a u s i n g the b u l k c h e m i c a l analysis to give a false estimate of t h e A l - c o n t e n t of the zeolite. W r i g h t et al. s u g gested t h a t a m o r p h o u s s i l i c a o c c u r r e d i n t h e i r m a t e r i a l , t h e r e b y
indi-
c a t i n g a f a l s e l y l o w A l - c o n t e n t of the zeolite. H o w e v e r , this s h o u l d l e a d to a d e v i a t i o n o p p o s i t e to t h a t g i v e n i n F i g u r e 2 w i t h respect to the d a t a of D e m p s e y et al. N e v e r t h e l e s s , t h e i r i m p l i e d suggestion of some i m p u r i t y c a u s i n g a difference b e t w e e n the b u l k c o m p o s i t i o n a n d t h a t of the zeolite is a possible w a y out of the o b s e r v e d d e v i a t i o n s . N o n e of the authors
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
Faujasite-Type
SMITH
181
Structures
give s t a t i s t i c a l c r i t e r i a u p o n w h i c h t h e v a l i d i t y of the straight lines c a n be judged. T h e c e l l d i m e n s i o n s of zeolites are d e t e r m i n e d b y the t o t a l system, w h i c h i n c l u d e s cations a n d w a t e r molecules.
E v e n i f r e s o l u t i o n of t h e
differences i n e x p e r i m e n t a l d a t a confirms t h e existence of d i s c o n t i n u i t i e s , the p o s s i b i l i t y of effects r e s u l t i n g f r o m r e s i t i n g of cations a n d m o l e c u l e s m u s t b e c o n s i d e r e d as w e l l as A l , S i o r d e r - d i s o r d e r . zeolites s t u d i e d b y O l s o n (47)
C e r t a i n l y the X -
h a v e l o n g - r a n g e Τ order, b u t for faujasite
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a n d Y - z e o l i t e s the e v i d e n c e f r o m c e l l d i m e n s i o n s is e q u i v o c a l .
Ordered
crystals a n d most n a t u r a l m i n e r a l s h a v e u n d e r g o n e m o r e a n n e a l i n g t h a n synthetic specimens.
T h e present d a t a are consistent w i t h t h e p o s s i b i l i t y
that n a t u r a l faujasite is m o r e o r d e r e d t h a n s y n t h e t i c Y z e o l i t e , b u t the e v i d e n c e is i n c o n c l u s i v e . O b v i o u s l y , a m o r e d i r e c t m e t h o d is n e e d e d to test the o r d e r i n g of the s i l i c a - r i c h specimens.
I n feldspars, L a v e s a n d H a f n e r (39)
success
f u l l y e v a l u a t e d the S i , A l o r d e r - d i s o r d e r u s i n g i n f r a r e d a b s o r p t i o n a n d n u c l e a r q u a d r u p o l e resonance t e c h n i q u e s .
T h e latter t e c h n i q u e a p p l i e d
to A l a n d N a n u c l e i u n f o r t u n a t e l y r e q u i r e s single crystals at least several m m across. I n f r a r e d patterns of o r d e r e d feldspars s h o w e d m u l t i p l e peaks for the T - O s t r e t c h i n g a n d b e n d i n g v i b r a t i o n s w h i l e d i s o r d e r e d feldspars s h o w e d a single b r o a d p e a k for each.
W r i g h t et al. (69)
obtained pat
terns w i t h single b r o a d peaks f o r b o t h faujasite a n d T y p e s Y a n d
X
zeolites. T h e peaks c h a n g e d p o s i t i o n l i n e a r l y w i t h the A l content of the s y n t h e t i c zeolites, b u t the n a t u r a l s p e c i m e n d e v i a t e d s i g n i f i c a n t l y f r o m the t r e n d . A t this t i m e i t is not clear w h a t effect m o v e m e n t of
cations
a n d molecules ( e s p e c i a l l y p r o t o n s ) has o n the T - O v i b r a t i o n s , b u t s u c h m o v e m e n t p r o b a b l y y i e l d s so m a n y different c r y s t a l fields t h a t t h e i n f r a r e d peaks are b r o a d e n e d , t h e r e b y i n h i b i t i n g s e p a r a t i o n of b a n d s
from
S i a n d A l atoms. Framework Hydroxyls.
T h e s t r u c t u r a l n a t u r e of f r a m e w o r k
hy-
droxyls is p a r t i c u l a r l y i m p o r t a n t because t h e y p r o b a b l y act as B r o n s t e d a c i d catalysts.
T h e l i t e r a t u r e o n catalysis is m u c h too extensive to
be
r e v i e w e d here, a n d c o m m e n t s w i l l b e r e s t r i c t e d to x - r a y a n d i n f r a r e d e v i d e n c e o n the l o c a t i o n a n d n a t u r e of f r a m e w o r k h y d r o x y l s . R e v i e w s of i n f r a r e d d a t a are g i v e n i n this v o l u m e b y R a b o a n d P o u t s m a a n d b y W a r d (see
also 66,
67).
T h e clearest d a t a are for Η-forms of faujasite-type zeolites. ammonium-exchanged and
t h e r e b y p r o d u c i n g the Η-form.
It w o u l d b e i n c o n c e i v a b l e
c a l l y for the protons not to condense w i t h f r a m e w o r k oxygens. and Dempsey
(48)
The
f o r m is h e a t e d , d r i v i n g off a m m o n i a a n d w a t e r , chemi Olson
d e t e r m i n e d the c r y s t a l s t r u c t u r e of H - f a u j a s i t e h e l d
i n the d e h y d r a t e d f o r m at r o o m t e m p e r a t u r e . U n f o r t u n a t e l y , there w e r e no e l e c t r o n d e n s i t y peaks a s c r i b a b l e to the protons, p r o b a b l y because
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
182
MOLECULAR SIEVE ZEOLITES
1
t h e y do not h a v e the same p o s i t i o n i n a l l u n i t cells. S i n c e S i - O H
bond
lengths are a b o u t 0.08A
bond
longer t h a n S i - O , the o b s e r v e d
T-O
lengths ( T a b l e I ) of 1.653, 1.634, 1.663, a n d 1.623A i n d i c a t e d t h a t most of the protons w e r e a t t a c h e d to 0 3 , a n d a smaller n u m b e r to O l . Olson and Dempsey electrostatic energy
p r e d i c t e d the p o s i t i o n of the protons
considerations.
T h e y postulated
using
(their F i g u r e
1)
that i n e a c h of the s h a r e d 6-rings, a p r o t o n ( d e n o t e d H 2 ) is a t t a c h e d to o n l y 1 of the 3 0 3 atoms, a n d that the O H b o n d is d i r e c t e d t o w a r d s t h e Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
t r i a d axis. A center of s y m m e t r y relates the protons of the s h a r e d 6-rings o n opposite sides of the h e x a g o n a l p r i s m . T h e other k i n d of p r o t o n ( H I ) is a t t a c h e d to a n O l a t o m a n d the O H b o n d is d i r e c t e d a w a y f r o m the t r i a d axis.
O l s o n a n d D e m p s e y r e l a t e d these positions to a p r e f e r r e d
S i , A l o r d e r i n g scheme m e n t i o n e d i n the p r e c e d i n g section. T h e m o d e l is consistent w i t h v a r i o u s i n f r a r e d d a t a . M a n y w o r k e r s (see
references i n Ref. 48) f o u n d that Η - T y p e Y y i e l d s i n f r a r e d s t r e t c h i n g
frequencies at about 3750, 3650, a n d 3550 c m " . T h e h i g h e s t f r e q u e n c y 1
b a n d has b e e n f o u n d for a l l types of zeolites, a n d has b e e n a s c r i b e d t o h y d r o x y l s c o m p l e t i n g the surface of i n d i v i d u a l crystallites or, p e r h a p s m o r e l i k e l y , to S i ( O H )
4
o c c l u d e d i n t h e zeolite ( 1 ) .
T h e 3550 b a n d is
not p e r t u r b e d b y s o r p t i o n of most molecules w h i c h cannot pass i n t o t h e sodalite u n i t , a n d h e n c e r e a s o n a b l y w a s a s c r i b e d to H 2 p r o t e c t e d i n s i d e the h e x a g o n a l p r i s m . T h e 3650 b a n d , w h i c h is p e r t u r b e d b y large s o r b e d molecules,
w a s a s c r i b e d to
assignment (48, p p .
HI.
O t h e r considerations
supported
this
230-231).
A l t h o u g h the basic m o d e l looks v e r y g o o d , the a c t u a l s i t u a t i o n m u s t be m o r e c o m p l e x since faujasite a n d Y zeolites cannot h a v e f u l l l o n g range order.
T h e a c t u a l positions of the H atoms m u s t v a r y s o m e w h a t
f r o m 1 u n i t c e l l to another, a n d s u c h v a r i a t i o n s m a y l e a d to p a r t i c u l a r l y a c t i v e c a t a l y t i c sites i n just a f e w of the u n i t cells.
H o p e f u l l y , single-
c r y s t a l i n f r a r e d a b s o r p t i o n studies, s i m i l a r to those c a r r i e d out for m a n y silicates, w i l l y i e l d d a t a o n the o r i e n t a t i o n of the O H groups,
thereby
testing t h e O l s o n - D e m p s e y m o d e l . O l s o n a n d D e m p s e y (48) lower frequency
b r i e f l y e x p l a i n e d the d i s a p p e a r a n c e of the
b a n d u p o n a b s o r p t i o n of l a r g e b a s i c m o l e c u l e s
like
p i p e r i d i n e b y m o b i l i t y of the protons o n the zeolite surface. W a r d ( 66 ) p r e s e n t e d some e v i d e n c e for m o b i l i t y of protons at h i g h t e m p e r a t u r e . X - r a y d a t a are m u c h too insensitive to test for the l o c a t i o n of f r a m e w o r k O H groups i n zeolites c o n t a i n i n g exchangeable cations.
Neverthe
less, the presence of s i m i l a r O H s t r e t c h i n g b a n d s i n m a n y c a t i o n - b e a r i n g zeolites suggests
that the protons
are a t t a c h e d to the same
oxygens.
A n o t h e r w o r d of c a u t i o n is desirable. T h e sequence of frequencies 3750, 3650, a n d 3550 corresponds q u a l i t a t i v e l y to the expected values for the c r y s t a l fields of e n v i r o n m e n t s of o n e - s i d e d outer surface, o n e - s i d e d large
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
Faujasite-Type
SMITH
cage w i t h
diameter
183
Structures
11A, a n d small
enclosed
volume.
Perhaps
any
h y d r o x y l p r o j e c t i n g i n t o the supercage w i l l h a v e a f r e q u e n c y near 3650 i r r e s p e c t i v e of w h i c h o x y g e n it is a t t a c h e d to, a n d s i m i l a r l y for the 3550 f r e q u e n c y a n d the sodalite u n i t . D e t a i l e d i n t e r p r e t a t i o n of h y d r o x y l l o c a t i o n i n c a t i o n - b e a r i n g systems is g i v e n i n Refs. 26, 48, 67, 68.
T h e p a p e r b y U y t t e r h o e v e n et al.
(65)
r e v i e w s m a n y of the p r e s e n t l y a v a i l a b l e d a t a , a n d lists m a n y proposals c o n c e r n i n g O H groups. D i s c u s s i o n of O H groups c o o r d i n a t e d to cations Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
is g i v e n later. Oxygen Removal from the Framework. P r o g r e s s i v e h e a t i n g of a m monium-exchanged
Type Y
first
gives d e h y d r a t e d Η - T y p e Y as just
d e s c r i b e d , a n d t h e n b y r e m o v a l of H 0 y i e l d s " d e c a t i o n a t e d " m a t e r i a l 2
(32,
41, 63, 64, 6 5 ) .
M c D a n i e l a n d M a h e r (41)
produced an "ultra-
s t a b l e " f o r m t h e r m a l l y stable to t e m p e r a t u r e s o v e r 1000 ° C . R e c e n t w o r k b y K e r r a n d c o w o r k e r s (31, 32, 34, 35, 36)
has s h o w n t h a t the reactions
are c o m p l e x , a n d d e p e n d c r i t i c a l l y o n the m e t h o d of
de-ammoniation
and dehydration. T h e simplest e x p l a n a t i o n of d e h y d r a t i o n of H - Y is t h a t 2 h y d r o x y l s f r o m the f r a m e w o r k condense to f o r m w a t e r a n d a n oxide i o n . T h e oxide i o n poses n o p r o b l e m since i t c a n f o r m p a r t of the a l u m i n o s i l i c a t e f r a m e w o r k , b u t the o x y g e n lost i n the w a t e r leaves a v a c a n c y i n the f r a m e w o r k . T h e v a c a n c y c a n b e a c c o u n t e d for i f readjustment takes p l a c e y i e l d i n g 3 - c o o r d i n a t e d cations i n a f r a m e w o r k w h i c h is s t i l l c o n t i n u o u s . a t t a c h e d to either 2 A l or 1 A l +
Oxygen
1 S i s h o u l d b e lost m o r e easily t h a n
one a t t a c h e d to 2 S i . H e n c e , the s i m p l e m o d e l p r e d i c t s t h a t 3 - c o o r d i n a t e d A l a n d S i atoms are f o r m e d , since oxygens a t t a c h e d o r i g i n a l l y to 2 A l s h o u l d b e rare. S t r u c t u r a l l y , there is c o n s i d e r a b l e d i f f i c u l t y i n d e v i s i n g a n a t o m i c p a t t e r n since r e m o v a l of a n o x y g e n leaves 2 h i g h l y c h a r g e d cations f a c i n g e a c h other w i t h o u t a s h i e l d i n g oxygen. the f r a m e w o r k recrystallizes i n t o S i 0
2
A n extreme a l t e r n a t i v e is t h a t w i t h e l i m i n a t i o n of t h e A 1 0 2
3
i n t o a n a m o r p h o u s or p o o r l y - c r y s t a l l i n e phase. 2 HAlSi 0 2
6
H 0 + Al Si O 2
2
4
n
A1 0 + 4 Si0 2
3
2
T h e final p r o d u c t c o u l d b e a s i l i c a f r a m e w o r k w h o s e c e l l d i m e n s i o n s h o u l d be a p p r o x i m a t e l y 2 4 . 0 A w h e n r e h y d r a t e d ( e s t i m a t e d f r o m S i - O . — 1.605, A l - O ~
1.757A, s t a r t i n g m a t e r i a l ~
59 A l p e r u n i t c e l l , a ^
24.7A ).
H o w e v e r , the a c t u a l reactions are not consistent w i t h this s i m p l e scheme. K e r r (32)
f o u n d t h a t h e a t i n g H - Y i n a d r y a t m o s p h e r e l e d to a
m a t e r i a l w i t h p o o r t h e r m a l s t a b i l i t y , p r e s u m a b l y because of m a n y c r y s t a l l i n e defects.
H e a t i n g at 7 0 0 ° - 8 0 0 ° C for 2 - 4 hours i n a n i n e r t static
a t m o s p h e r e w i t h w a t e r r e m a i n i n g near the zeolite p r o d u c e d a substance of u n u s u a l l y h i g h t h e r m a l s t a b i l i t y . A b o u t 2 5 % of the A l is present as
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
184
MOLECULAR SIEVE ZEOLITES
1
exchangeable cations, suggesting t h a t a b o u t 12 A l p e r u n i t c e l l h a v e left the f r a m e w o r k .
K e r r p r o p o s e d a 2-step m e c h a n i s m i n w h i c h w a t e r reacts
w i t h the f r a m e w o r k p r o d u c i n g ( O H )
4
groups (as o c c u r i n h y d r o g a r n e t )
a n d A l ( O H ) . T h e latter reacts w i t h the f r a m e w o r k , r e m o v i n g a h y d r o 3
gen and producing water a n d A l ( O H )
2
+
.
The
(OH)
4
groups
break
d o w n , y i e l d i n g 2 H 0 a n d 2 oxide ions w h i c h r e m a i n i n the f r a m e w o r k . 2
K e r r (33)
r e m o v e d A l f r o m the f r a m e w o r k of N a - Y b y e x t r a c t i o n
w i t h c h e l a t i n g agent H E D T A . 4
H e (34)
f o u n d that the c e l l d i m e n s i o n
of H - Y p r e p a r e d b y c a l c i n a t i o n of N H - e x c h a n g e d Y w a s 24.74A
but
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4
d r o p p e d to 24.51A after e x t r a c t i o n b y E D T A , r e d u c i n g the A l content f r o m 50 to 32.8 atoms p e r c e l l .
T h e extracted m a t e r i a l i n c r e a s e d i n
s t a b i l i t y b o t h for t h e r m a l d e g r a d a t i o n at h i g h t e m p e r a t u r e a n d h y d r o t h e r m a l d e g r a d a t i o n at l o w t e m p e r a t u r e . decomposition
of N H - Y 4
K e r r (31)
found that thermal
at 760 torr a n d 5 0 0 ° C y i e l d e d Η-zeolite i f
a m m o n i a w a s r e m o v e d r a p i d l y , a n d u l t r a s t a b l e zeolite i f a m m o n i a w a s r e m o v e d s l o w l y . K e r r a n d S h i p m a n (36)
d e s c r i b e d the r e a c t i o n of H - Y
w i t h a m m o n i a at h i g h t e m p e r a t u r e p r o d u c i n g " a m i d o - z e o l i t e Y . " T h e d e t a i l e d s t r u c t u r a l m e c h a n i s m s c o n c e r n i n g t h e loss of a m m o n i a a n d w a t e r f r o m N H - e x c h a n g e d zeolite h a v e not b e e n d e t e r m i n e d 4
by
x - r a y d i f f r a c t i o n w o r k . I t seems l i k e l y f r o m a g e n e r a l b o d y of k n o w l e d g e o n n a t u r a l a n d s y n t h e t i c zeolites that protons act as a catalyst i n p r o m o t i n g a r e c r y s t a l l i z a t i o n of the f r a m e w o r k . J . M . B e n n e t t a n d J . V . S m i t h ( u n p u b l i s h e d ) m a d e x-ray s t r u c t u r a l analyses of p o w d e r e d kindly supplied by
samples of d e c a t i o n a t e d
Y
("ultrastable" type)
C . V . M c D a n i e l . A l t h o u g h refinement w a s
quite
satisfactory f r o m t h e t e c h n i c a l v i e w p o i n t , the l o w r e s o l u t i o n p o s e d p r o b lems of i n t e r p r e t a t i o n . D e t a i l s of the p o w d e r d a t a o b t a i n e d d i r e c t l y at temperatures of 2 5 ° , 4 0 0 ° , 7 0 0 ° , a n d 9 0 0 ° C m a y b e o b t a i n e d f r o m the authors. Site Γ w a s o c c u p i e d at a l l temperatures, p o s s i b l y r e s u l t i n g f r o m some species c o n t a i n i n g a l u m i n u m . T h e d a t a for the h y d r a t e d f o r m at 25 ° C i n d i c a t e d o c c u p a n c y of O l a n d 0 4 b y 69 ±
8 a n d 79 ±
9 oxygens
( i n s t e a d of i d e a l 96) w h e n these sites w e r e assigned the same B - v a l u e s as 0 2 a n d 0 3 . A t 700° a n d 900 ° C , the o c c u p a n c i e s w e r e n o r m a l .
These
d a t a m i g h t b e e x p l a i n a b l e i n terms of r e c r y s t a l l i z a t i o n , b u t m u s t b e tested b y m o r e accurate s i n g l e - c r y s t a l data. K e r r et al. (35)
suggested that the t e r m d e c a t i o n a t e d not b e u s e d .
They distinguished between:
a m m o n i u m zeolite Y ; h y d r o g e n z e o l i t e Y
produced by controlled deammination and dehydration; dehydroxylated f o r m p r o d u c e d b y h e a t i n g a b o v e 4 5 0 ° C at 10" torr o r a b o v e 6 0 0 ° C at 6
760 t o r r , loss of 1 w a t e r p e r p a i r of N a - f r e e A 1 0 +
2
g i v i n g negative
4-co-
o r d i n a t e d A l a n d p o s i t i v e 3 - c o o r d i n a t e d S i ; u l t r a s t a b l e zeolite for m a t e r i a l of M c D a n i e l a n d M a h e r ( 4 1 ) ,
w i t h some f r a m e w o r k A l t u r n e d i n t o
cationic A l .
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type Table IV.
Type
Reference
185
Structures
Cation Occupancy and Position IP
I
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Strongly Dehydrated
II
Forms
20.2 N a 0.069
31.2 N a 0.223
12.0 Κ
14.6 Κ 0.075
31.0 Κ 0.250
16 A g
11.0 A g 0.075
29.2 A g 0.228
12.9 Κ 0.072
31.7 Κ 0.250
Na-Y
(29)
K-Y
(29)
Ag-Y
(29)
K-faujasite
(62)
8.6 Κ
Ba-faujasite
(52)
7.3 B a
5.0 B a 0.068
11.3 B a 0.247
Ca-faujasite
(8)
14.2 C a
2.6 C a 0.061
11.4 C a 0.229
Ni-faujasite
(46)
10.6 N i
3.2 N i 0.054 5.8 H 0 0.081
7.8 N a
2
La-faujasite La-faujasite (420°C)
(9) (U)
6.4 N i 0.233
1.9 N i 0.207 1.9 H 0 0.161 2
11.8 L a
2.5 L a 0.067
1.5 L a 0.234
11.7 L a
2.5 L a 0.066
1.4 L a 0.233
Partly Dehydrated
Forms
N a , Ce-faujasite
(49)
3.4 N a
11.5 C e 0.067
La-Y 725°C
(62)
5.2 L a
8.9 L a 0.068
La-X
(49)
Ca-X
(48)
7.5 C a
16 H 0 0.162 2
10.7 N a 0.236 5.5 L a 0.227
30 L a 0.067
32 H 0 0.165
17.3 C a
9.0 C a 10.5 H 0
17.3 C a
4.2 S r 5.4 H 0
19.5 S r
6.4 S r 7.7 H 0
20.3 S r
2
2
Sr-X 16 h r s . 400°C
(48)
Sr-X 16 h r s . 680°C
(48)
La-X 425°C
(IS)
5.0 L a
15.2 L a 0.065
4.9 L a 0.230
La-X 735°C
(IS)
5.2 L a
14.1 L a 0.065
6.3 L a 0.231
11.2 S r
7.0 S r
2
6.1 S r
12.0 S r
2
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
186
MOLECULAR SIEVE ZEOLITES
Exchangeable
1
Cations
T h e a v a i l a b l e d a t a o n the positions of exchangeable cations are v e r y difficult to i n t e r p r e t since several specimens w e r e not f u l l y and
exchanged
most d e h y d r a t e d specimens s t i l l c o n t a i n e d significant a m o u n t s
w a t e r or other o x y g e n species. incomplete.
H i g h - t e m p e r a t u r e exchange
mono-cationic form.
of
R o o m - t e m p e r a t u r e i o n exchange o f t e n is often is n e e d e d
T h e exchange process
to o b t a i n a
m a y result i n entrance
of
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protons i n s t e a d of the chosen c a t i o n . N o t a l l single crystals u s e d for x-ray w o r k h a v e b e e n c h e c k e d b y e l e c t r o n m i c r o p r o b e analysis or other t e c h n i q u e s for the c a t i o n content.
F o r c o n v e n i e n c e , the results w i l l b e d i s
cussed u n d e r 3 s u b h e a d i n g s : d e h y d r a t e d specimens, h y d r a t e d specimens, a n d samples c o n t a i n i n g s o r b e d m o l e c u l e s .
T a b l e s I V a n d V c o n t a i n the
i n d i c a t e d n u m b e r a n d t y p e of cations i n the v a r i o u s sites together w i t h the p o s i t i o n a l c o o r d i n a t e
(x)
s p e c i f y i n g t h e l o c a t i o n of t y p e (xxx)
on
the 3-fold axis; see Refs. 20, 28 for a d d i t i o n a l d a t a not l i s t e d i n t h e tables. Dehydrated Specimens.
I d e a l l y , the w a t e r molecules i n a zeolite
exist as discrete m o l e c u l e s a n d c a n b e r e m o v e d e a s i l y b y h e a t i n g . Table V . Type Faujasite
Reference
Site Occupancy
I
Γ 16 ( N a , C a ) 0.070
(7)
Ca-faujasite
(10)
9.7 C a 0.069
La-faujasite
(12)
3.3 L a 0.069
Ce, N a , Ca-faujasite
(49)
Na-X
(47)
18 N a 0.070 9Na
8 Na 0.060 ll.H 0 0.074 2
Sr
4 2
Sr,
0
Na-X
(51)
Na
(51)
2 4
-X
La-X
(49)
La-X
(49)
In
2.1 S r 12 N a
11.1 S r 0.063 7.3 S r 0.065 12 L a 0.067
5 La?
13.8 L a 0.062
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type
187
Structures
p r a c t i c e , the w a t e r m o l e c u l e s f o r m h y d r a t i o n complexes
w i t h the
ex-
c h a n g e a b l e cations a n d i n t e r a c t e l e c t r o s t a t i c a l l y w i t h f r a m e w o r k oxygens. F o r m o n o v a l e n t cations, the h y d r a t i o n complexes are w e a k l y b o n d e d , a n d d e h y d r a t i o n is r e l a t i v e l y easy.
F o r p o l y v a l e n t cations, t h e h y d r a t i o n
complexes are b o n d e d m o r e strongly. I n a d d i t i o n , t h e c a t i o n m a y p o l a r i z e the w a t e r m o l e c u l e sufficiently t h a t i t splits i n t o a h y d r o x y l g r o u p ( w h i c h b o n d s to the cations ) a n d a p r o t o n w h i c h attaches itself to a f r a m e w o r k oxygen.
U n d e r c o n d i t i o n s of severe h y d r a t i o n at h i g h t e m p e r a t u r e , i t is
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possible for oxide ions to b e p r o d u c e d f r o m the h y d r o x y l s a t t a c h e d to the cations
(53).
T a b l e I V is s p l i t a r b i t r a r i l y i n t o sections c o v e r i n g s t r o n g l y d e h y d r a t e d a n d p a r t l y d e h y d r a t e d forms.
D e h y d r a t i o n of m o n o v a l e n t forms s h o u l d
b e most c o m p l e t e . T h e d a t a for K - Y ( p o w d e r t e c h n i q u e ) a n d K - f a u j a s i t e (single-crystal technique)
are consistent w i t h i n t h e i r respective errors
s h o w i n g o c c u p a n c y of the I, I ' , a n d I I sites. T h e s e sites are o c c u p i e d also i n N a - a n d A g - Y ( p o w d e r d a t a ) b u t w i t h different o c c u p a n c y levels. F o r a l l these forms, there s h o u l d b e a b o u t 58 cations p e r c e l l to b a l a n c e t h e in Hydrated Forms II*
IP
V
32 H 0 0.167
11 H 0 0.272
11.5 C a 0.167
23 H 0 0.274
2.2 C a 0.484
28 H 0 0.167
14 H 0 0.272
10.3 L a 0.491
32 H 0 0.165
26 H 0 0.264
5.8 C e 0.491
2
2
2
2
2
2
26 H 0 0.166 2
2
12 N a 0.230 12 N a 0.238 8H 0 0.245 2
32 H 0 0.173
15.0 S r 0.246
26 H 0 0.170
11.5 S r 0.246
32 H 0 0.165
17 L a 0.235
4 La 0.500
24 H 0 0.166
13.2 L a 0.233
3.4 L a 0.493
2
2
2
2
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
188
MOLECULAR SIEVE ZEOLITES
1
A l atoms, a n d the t o t a l o c c u p a n c i e s e s t i m a t e d f r o m the x - r a y analyses (54, 57, 59, 60) agree f a i r l y w e l l . M o s t t h i n k i n g o n c a t i o n d i s t r i b u t i o n i n silicates is b a s e d o n P a u l i n g ' s rules w h i c h are b a s e d o n a n electrostatic m o d e l .
I n a p p l y i n g t h e m to
zeolites, i t s h o u l d be n o t e d that c o v a l e n t b o n d i n g m a y occur, the A l d i s t r i b u t i o n i n the f r a m e w o r k m a y be i m p o r t a n t i n c a u s i n g
exchangeable
cations to associate w i t h p a r t i c u l a r 6-rings c o n t a i n i n g h i g h amounts
of
A l , a n d P a u l i n g ' s rules w e r e d e v e l o p e d for crystals near c h e m i c a l e q u i Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
l i b r i u m , whereas d e h y d r a t e d zeolites h a v e b e e n b r o u g h t d e l i b e r a t e l y i n t o a state u n s t a b l e w i t h respect to w a t e r . T h e r e are m a n y c o m m e n t s i n the literature o n w h i c h t h e a b o v e r e m a r k s are based. T h e papers of D e m p s e y (23, 24)
are p a r t i c u l a r l y v a l u a b l e since t h e y p r o v i d e d e t a i l e d c a l c u l a t i o n s
of M a d e l u n g potentials. Site I is the o n l y site p e r m i t t i n g a c a t i o n to be enclosed b y f r a m e w o r k oxygens.
Sites Γ, I I ' , a n d I I p e r m i t o n l y 1-sided or a w a i s t e d co
o r d i n a t i o n to oxygens of the adjacent 6 - r i n g . H e n c e , I is the p r e f e r r e d site unless other considerations s u c h as electrostatic r e p u l s i o n or c a t i o n molecule attraction become important. F o r m o n o v a l e n t cations, there are space p r o b l e m s .
Site I c a n h o l d
16 cations w h i l e the other sites c a n h o l d 32 each. C a t i o n s s i m u l t a n e o u s l y o c c u p y i n g I a n d Γ are s h a r i n g the face of a c o o r d i n a t i o n
polyhedron,
w h i c h is u n f a v o r e d electrostatically. T h e s h a r i n g p r o b l e m is p a r t i c u l a r l y severe for sites I I a n d Ι Γ .
E l e c t r o s t a t i c a l l y , the best w a y to d i s t r i b u t e 58
m o n o v a l e n t cations i n faujasite a n d Y is to p u t 32 i n I I , 6 i n I, a n d 20 i n Γ. T h i s d i s t r i b u t i o n p e r m i t s the I a n d F cations to be d i s t r i b u t e d s u c h that n o I a n d I ' cations share a 6 - r i n g . T h e o b s e r v e d o c c u p a n c i e s of I I agree w e l l w i t h the i d e a l v a l u e of 32, b u t the o c c u p a n c i e s of the I a n d Γ sites n e e d d e t a i l e d discussion.
The
s i n g l e - c r y s t a l d a t a for K - f a u j a s i t e fit the p r e d i c t i o n w e l l , b u t the
less
accurate p o w d e r d a t a for N a - , K - , a n d A g - Y do not agree. T h e d e v i a t i o n for N a - Y is c e r t a i n l y w i t h i n e x p e r i m e n t a l error, a n d that for K - Y is p r o b a b l y w i t h i n the error.
C e r t a i n l y , t h e g o o d fit for the K - f a u j a s i t e
suggests t h a t the K - Y d a t a m a y be i n error. T h e d e v i a t i o n for the A g - Y f o r m is r a t h e r h i g h . Is i t possible that i t is m e a n i n g f u l a n d results f r o m a stronger t e n d e n c y for c o v a l e n t b o n d i n g of A g i n site I t h a n for the a l k a l i m e t a l forms?
A n accurate s i n g l e - c r y s t a l analysis is desirable.
C u r r e n t l y there are n o p u b l i s h e d d a t a o n the l o c a t i o n of m o n o v a l e n t cations i n d e h y d r a t e d X zeolite.
T h e r e are c o n s i d e r a b l e
p l a c i n g 86 cations p e r u n i t c e l l . B r e c k (17)
problems
in
p l a c e d 16 i n I , 32 i n I I , a n d
38 i n site I I I . I n site I I I , the c a t i o n projects i n t o the supercage a n d is b o n d e d to the 4 oxygens of b r i d g i n g 4-rings. P e r h a p s there is s i m u l t a n e ous o c c u p a n c y of I a n d Γ, as suggested b y the d a t a for K - a n d A g - Y .
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type
189
Structures
I n Y a n d faujasite, there s h o u l d b e a b o u t 29 d i v a l e n t cations w h i c h could be entirely accommodated
i n sites I a n d I I .
T h e c r y s t a l of
de
h y d r a t e d C a - e x c h a n g e d faujasite ( 8 ) w a s s h o w n free of other cations b y m i c r o p r o b e analysis a n d was severely d e h y d r a t e d at 475 ° C before b e i n g sealed i n its c a p i l l a r y . T h e c a t i o n d i s t r i b u t i o n is close to t h e t h e o r e t i c a l suggestion, b u t there are 2.6 C a atoms i n site Γ.
Bennett and Smith
p o i n t e d out that the 14.2 C a i n I a n d 2.6 C a i n F are consistent w i t h n o s h a r i n g of a p o l y h e d r a l face, since 14.2 - f 2 . 6 / 2 y i e l d s 15.5, w h i c h is less Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
t h a n 16.
D e m p s e y a n d O l s o n (27)
suggested that presence
of
water
m o l e c u l e s d r a w s cations f r o m I a n d I I i n t o Γ s u c h that η ( I ) + 0.5 η ( Γ ) =
16. T h e r e are insufficient d a t a to test r i g o r o u s l y the d e t a i l e d a c c u r a c y
of this e q u a t i o n . T h e t o t a l of B a atoms i n B a - f a u j a s i t e (23.6) is c o n s i d e r a b l y l o w e r t h a n the e x p e c t e d v a l u e . P e r h a p s ion-exchange w a s i n c o m p l e t e , or p e r haps protons w e r e i n c o r p o r a t e d . T h e d a t a for N i - f a u j a s i t e are c o m p l i c a t e d b y i n c o m p l e t e i o n - e x c h a n g e (27 N i a n d 4 C a i n d i c a t e d b y c h e m i c a l a n a l y s i s ) a n d p o s s i b l e of r e s i d u a l w a t e r r e s u l t i n g f r o m d e h y d r a t i o n at 4 0 0 ° C .
presence
Olson
(46)
o b s e r v e d d o u b l e peaks at Γ a n d Ι Γ a s c r i b e d to r e s i d u a l w a t e r m o l e c u l e s adjacent to the N i ions.
T h e s i t u a t i o n is u n c l e a r b u t O l s o n
b o n d i n g b e t w e e n N i ions a n d w a t e r m o l e c u l e s .
suggested
T h e a p p a r e n t deficiency
of N i ions ( e s t i m a t e d total, 22.1) m a y result f r o m neglect of other ions i n c l u d i n g C a , or entrance of protons. T h e d a t a o b t a i n e d b y B e n n e t t a n d S m i t h for L a - f a u j a s i t e d e h y d r a t e d at 475 ° C s h o w that the t r i v a l e n t L a ions p r e f e r site I. A f e w ions p r e f e r I ' a n d I I b u t these m a y be b o n d e d to r e s i d u a l molecules.
Perhaps h i g h
A l contents of a f e w 6-rings also m a y b e a f a c t o r i n the s i t i n g of these L a atoms. T h e details of the c a t i o n - o x y g e n b o n d i n g are o b s c u r e d b y the d i s o r d e r e d c a t i o n d i s t r i b u t i o n since a c a t i o n i n site I m a y cause
oxygen
atoms i n 0 3 to l i e at different positions t h a n w h e n the c a t i o n is at site I ' . S i n c e x-rays average a l l atoms of a g i v e n t y p e , the a p p a r e n t 0 3 c a n n o t b e r e l a t e d d i r e c t l y to that of t h e cations.
p o s i t i o n of
T a k i n g this f a c t o r
into a c c o u n t , the o b s e r v e d c a t i o n - o x y g e n distances i n these d e h y d r a t e d zeolites seem reasonable i n r e l a t i o n to those of other silicates. I n T a b l e I V , the effect of c a t i o n r a d i u s is s h o w n i n d i r e c t l y b y the ^-coordinates ( e.g., note the large ^-coordinates for sites I I of the B a - a n d K-faujasites ). A s u r p r i s i n g result of the s t u d y of d e h y d r a t e d L a - f a u j a s i t e ( 9 ) the l a r g e d i s p l a c e m e n t of the L a a t o m at S ( I )
was
a l o n g the t r i a d axis.
T h e r m a l v i b r a t i o n seemed u n l i k e l y as a n e x p l a n a t i o n , e s p e c i a l l y as d a t a o b t a i n e d d i r e c t l y at 420 ° C s h o w e d the same effect ( I I ) . S m i t h suggested that the L a a t o m m i g h t be b o n d e d
Bennett a n d
p r e f e r e n t i a l l y to
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
190
MOLECULAR SIEVE ZEOLITES
1
o n l y 3 of the 0 3 atoms, i n d i c a t i n g c o v a l e n t b o n d i n g . S u c h 1-sided b o n d i n g has b e e n o b s e r v e d i n other l a n t h a n u m - o x y g e n c o m p o u n d s
(9).
T u r n i n g n o w to the i n c o m p l e t e l y - d e h y d r a t e d varieties, the d a t a are consistent w i t h strong b o n d i n g b e t w e e n p o l y v a l e n t cations a n d r e s i d u a l w a t e r molecules
(45).
P a r t i c u l a r a t t e n t i o n w a s p a i d to t r i v a l e n t cations
because of t h e i r i m p o r t a n c e i n zeolite catalysts. O l s o n , K o k o t a i l o , a n d C h a r n e l l (49, F l a n i g e n (62)
50)
and Smith, Bennett, and
i n d e p e n d e n t l y f o u n d that t r i v a l e n t ions w e r e p r e f e r e n t i a l l y
e n t e r i n g Γ i n s t e a d of the e x p e c t e d I. T h e latter authors h e a t e d N a i L a i - Y Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
3
in a powder
diffractometer
w i t h the f u r n a c e
h e l i u m . A t 725 ° C , 9 L a atoms o c c u p i e d Γ. n e l l (49,
50)
containing
6
flowing
dry
Olson, Kokotailo, and Char
e x a m i n e d a c r y s t a l of N a ( C a , M g ) C e i - f a u j a s i t e at r o o m 8
8
2
t e m p e r a t u r e after c a l c i n a t i o n at 350 ° C . T h e x-ray d a t a i n d i c a t e d t h a t a l l the C e atoms o c c u p i e d F , the other cations l y i n g i n I a n d I I . E l e c t r o n d e n s i t y i n Ι Γ w a s a s c r i b e d to 16 w a t e r molecules, a n d the C e atoms i n Γ w e r e n i c e l y i n t e r p r e t e d as b o n d i n g to 3 0 3 atoms of the adjacent 6 - r i n g a n d to 1 o r 2 w a t e r molecules i n I I ' . T h e distances of 2 . 4 - 2 . 5 A are q u i t e consistent w i t h this m o d e l .
S m i t h et al.
(62)
suggested
that r e s i d u a l
w a t e r m i g h t o c c u p y site U at the center of the sodalite u n i t ( a g a i n 2 . 5 A f r o m site Γ ) b u t t h e i r e v i d e n c e f r o m p o w d e r d a t a is of m a r g i n a l a c c u r a c y . F o l l o w i n g the a b o v e studies, B e n n e t t a n d S m i t h (9, 11) s h o w e d t h a t stricter d e h y d r a t i o n c a u s e d L a atoms i n L a - e x c h a n g e d faujasite to m o v e i n t o I.
F u r t h e r m o r e , the same d e h y d r a t e d c r y s t a l of L a - f a u j a s i t e h a d
essentially e q u a l o c c u p a n c y factors at 25° a n d 4 2 0 ° C , t h e r e b y r u l i n g out a n y c o n t r o l of c a t i o n d i s t r i b u t i o n f r o m a p u r e t e m p e r a t u r e v a r i a t i o n . A l t h o u g h not s t r i c t l y p r o v e n f r o m x - r a y d a t a , c o r r e l a t i o n of the a b o v e results w i t h those o b t a i n e d b y m a n y authors—see p a r t i c u l a r l y R a b o et al. (53)—from
i n f r a r e d m e t h o d s shows b e y o n d reasonable d o u b t t h a t t h e
positions of cations d e p e n d strongly o n e v e n s m a l l quantities of r e s i d u a l molecules.
It is possible t h a t 1 w a t e r m o l e c u l e is sufficient to
bridge
b e t w e e n 2 or m o r e L a atoms i n P . H e n c e , for 19 L a atoms p e r u n i t c e l l of f u l l y - e x c h a n g e d Y - z e o l i t e , o n l y a b o u t 10 w a t e r m o l e c u l e s are n e e d e d , c o m p a r e d w i t h 260 i n the h y d r a t e d s p e c i m e n . S i n c e most f e e d m a t e r i a l for c a t a l y t i c processes u s i n g zeolites
con
tains a f e w parts per m i l l i o n of w a t e r , it is reasonable to expect that p o l y v a l e n t ions i n the zeolites w i l l b e o c c u p y i n g positions i n the sodalite u n i t r a t h e r t h a n i n site I , a n d t h a t the cations w i l l b e b o n d e d to r e s i d u a l w a t e r or other o x y g e n species.
S u c h b o n d i n g w i l l r e d u c e s t r o n g l y the
electrostatic field generated b y the c a t i o n . So far, w a t e r m o l e c u l e s h a v e b e e n a s s u m e d as the o x y g e n
species.
T h e x - r a y d a t a are not a b l e to d i s c r i m i n a t e w a t e r m o l e c u l e s f r o m h y d r o x y l or f r o m oxide.
S e v e r a l authors i n t e r p r e t e d i n f r a r e d d a t a i n terms
m e t a l - h y d r o x y l b o n d i n g as s u m m a r i z e d i n Ref. 64.
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
of
15.
SMITH
Faujasite-Type Structures
191
T a b l e I V shows the c a t i o n d i s t r i b u t i o n s e s t i m a t e d f o r s e v e r a l forms of X d e h y d r a t e d u n d e r v a r i o u s c o n d i t i o n s .
F o r t r i v a l e n t cations
(.—30
p e r u n i t c e l l ) , sites I a n d I I are most s u i t a b l e e l e c t r o s t a t i c a l l y , b u t t h e s a m p l e of L a - X c a l c i n e d at u n s p e c i f i e d t e m p e r a t u r e ( p r o b a b l y
350°C)
s h o w e d a l l the L a atoms o n F , w h i l e site I F w a s o c c u p i e d b y e l e c t r o n d e n s i t y e x p l a i n a b l e b y 32 w a t e r m o l e c u l e s .
This remarkable distribution
y i e l d s a v e r y stable c h e m i c a l c o m p l e x w i t h e a c h L a b o n d e d to 3 0 3 a n d 3 H o O at 2.5A a n d w i t h e a c h H 0 b o n d e d to either 2 or 3 L a atoms. 2
T h e H 0 also is b o n d e d w e a k l y to 3 0 2 atoms of a free 6 - r i n g . Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
2
X-ray powder
s t u d y of L a - X i n d r y
flowing
h e l i u m at 425°
and
735 ° C s h o w e d a b o u t 15 L a atoms i n F , w i t h the others e q u a l l y s p l i t b e t w e e n sites I a n d I I . T h e simplest e x p l a n a t i o n is t h a t p a r t i a l d e h y d r a t i o n has o c c u r r e d , c a u s i n g some L a atoms to m o v e i n t o sites I a n d I I . T h e d a t a for C a - a n d Sr-varieties of X ( T a b l e I V ) are interprétable i n terms of p a r t i a l d e h y d r a t i o n . Hydrated Species. a n d water molecules e x c h a n g e d forms.
T a b l e V s u m m a r i z e s the assignments of cations m a d e b y v a r i o u s authors for a v a r i e t y of
ion-
T h e d a t a are h i g h l y unsatisfactory since n o one has
b e e n a b l e to find e n o u g h e l e c t r o n d e n s i t y peaks to a c c o u n t for a l l the ions a n d molecules or to find r e a l l y r e l i a b l e c r i t e r i a to p e r m i t assignment to a p a r t i c u l a r species of every peak. T h e first 4 structures of T a b l e V
s h o w a r e m a r k a b l e i d e n t i t y of
e l e c t r o n d e n s i t y peaks i n the sodalite u n i t of p o l y v a l e n t c a t i o n forms of faujasite. T h e authors h a v e assigned the e l e c t r o n d e n s i t y to v a r i o u s atoms or w a t e r m o l e c u l e s , b u t i f the d a t a are t r a n s f o r m e d b a c k i n t o the n u m b e r of electrons, t h e y are consistent w i t h 32 H 0 i n I F a n d a b o u t 2 0 - 2 5 H 0 2
2
i n F , i r r e s p e c t i v e of the t y p e of exchangeable
polyvalent cation.
the strict v i e w p o i n t of t h e c r y s t a l structure, the o n l y o b v i o u s
From
argument
against s u c h a n assignment of w a t e r m o l e c u l e s is t h a t the F - 0 3 distances of 2.5A are r a t h e r short.
H o w e v e r , i t is possible that e x t r e m e l y strong
electrostatic interactions m i g h t p e r m i t s u c h short distances. S h e r r y (56,
57)
s h o w e d that the ion-exchange
p r o p e r t i e s of N a - Y
a n d N a - X zeolite w e r e consistent w i t h 16 N a atoms i n the sodalite cage. S i n c e N a scatters x-rays r a t h e r s i m i l a r l y to H 0 m o l e c u l e s , the presence 2
of 16 N a atoms is consistent w i t h the e l e c t r o n d e n s i t y measurements. T h e p r o b l e m arises w i t h the atoms w h i c h scatter x-rays m o r e s t r o n g l y t h a n w a t e r molecules.
If L a atoms account for the e l e c t r o n d e n s i t y p e a k
at F i n L a - f a u j a s i t e , there are o n l y 3.3 atoms i n a 3 2 - f o l d site. S u c h l o w occupancy
is h a r d to b e l i e v e for a c r y s t a l s t r u c t u r e at l o w t e m p e r a t u r e
i n the presence of w a t e r molecules.
Is it possible that i n faujasite a n d Y ,
p o l y v a l e n t cations p r e f e r to b e h y d r a t e d a n d o c c u p y the supercage, w h i l e N a cations o c c u p y b o t h the sodalite u n i t a n d t h e supercage?
Detailed
x-ray studies of other c a t i o n forms t h a n those of T a b l e V w o u l d b e v a l u -
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
192
MOLECULAR SIEVE ZEOLITES
a b l e , e s p e c i a l l y i n association w i t h a p p r o p r i a t e ion-exchange studies.
1
In
z e o l i t e X , the d a t a c l e a r l y i n d i c a t e d entrance of cations i n t o the sodalite u n i t , b u t this is to b e e x p e c t e d f r o m the h i g h e r f r a m e w o r k charge. A t least some cations i n the supercage w a t e r molecules.
are s u r r o u n d e d e n t i r e l y b y
O l s o n , K o k o t a i l o , a n d C h a r n e l l ( 49 ) l o c a t e d 6 C e atoms
of C e - e x c h a n g e d faujasite i n site V d i s p l a c e d s l i g h t l y f r o m the center of a 12-membered ring.
B e n n e t t a n d S m i t h (12)
l o c a t e d 10 L a atoms at
this site i n a c o m p l e t e l y e x c h a n g e d faujasite, a n d f o u n d d e n s i t y e q u i v a Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
lent to 2 C a atoms at the same site of a f u l l y - e x c h a n g e d C a - f a u j a s i t e
(10).
T h e 1 2 - m e m b e r e d r i n g has a free d i a m e t e r of a b o u t 7.5A, w h i c h is c a p a b l e of a c c o m m o d a t i n g
a c a t i o n h y d r a t e d w i t h about
6 water
molecules.
P r o b a b l y the w a t e r m o l e c u l e s d o not o c c u p y the same p o s i t i o n f r o m 1 u n i t c e l l to the next, since n o e l e c t r o n d e n s i t y peaks h a v e b e e n r i g o r o u s l y l o c a t e d a n d a s c r i b e d to t h e m . T h e site I I * w i t h χ ,— 0.27 is o c c u p i e d i n a l l h y d r a t e d forms b u t its occupants
are u n c e r t a i n . B a u r ( 7 )
assigned a w a t e r m o l e c u l e to this
site b u t the distances to t h e oxygens of t h e 6 - r i n g — 3 . 3 A to 3 0 2 a n d 3.5A to 3 0 4 — a r e r a t h e r large. B e n n e t t a n d S m i t h (10)
p o i n t e d out t h a t the
e l e c t r o n d e n s i t y p e a k is v e r y b r o a d , a n d suggested
t h a t a m i x t u r e of
w a t e r m o l e c u l e s a n d h y d r a t e d cations o c c u p i e s this site. D e t a i l e d e x a m i n a t i o n of the e l e c t r o n d e n s i t y i n the supercage C a - and La-exchanged
faujasites
showed
a nonzero
electron
of
density
w i t h o u t a n y r e a l l y significant peaks. B a u r ( 7 ) c o r r e l a t e d the absence of definite peaks w i t h a v a r i e t y of p h y s i c a l d a t a , suggesting that the w a t e r m o l e c u l e s a n d cations act as a m o b i l e electrolyte s o l u t i o n . O l s o n s s t u d y of h y d r a t e d N a - X (47) the p o s i t i o n i n g of ions a n d molecules.
has i m p o r t a n t i m p l i c a t i o n s for
A l t h o u g h h e w a s not a b l e to
e n o u g h e l e c t r o n d e n s i t y to satisfy a l l the cations a n d w a t e r
find
molecules,
h e f o u n d t h a t 7 sites i n the supercage w e r e o c c u p i e d at a l o w l e v e l , as w e l l as the s t r o n g l y - o c c u p i e d sites i n the sodalite u n i t l i s t e d i n T a b l e V . T h e X z e o l i t e has s t r o n g l y o r d e r e d S i a n d A l atoms s u c h that the c r y s t a l field i n e a c h supercage w i l l b e f a i r l y s i m i l a r . I n Y zeolite, the S i a n d A l atoms cannot h a v e c o m p l e t e
long-range
order, a n d there m u s t b e
a
c o n s i d e r a b l e v a r i e t y of c r y s t a l fields. C o n s e q u e n t l y , i t is possible that i n the s i l i c a - r i c h varieties, the cations a n d w a t e r m o l e c u l e s of a n y single supercage
t e n d to o c c u p y specific sites b u t that the
i n t e g r a t i o n of the x-ray b e a m fluid.
phase-amplitude
gives the i m p r e s s i o n of a
O f course, there m u s t b e m o v e m e n t
smeared-out
of ions a n d m o l e c u l e s
to
e x p l a i n v a r i o u s p h y s i c a l d a t a s u c h as the h i g h v a l u e of the self-diffusion coefficients, b u t a m a j o r i t y of the ions a n d molecules m a y b e
fixed
for
a n y chosen instant of t i m e . O l s o n (47)
assigned 9 N a atoms to site I , 8 to F , a n d 24 to I I .
The
first assignment s h o u l d b e correct because the i n t e r a t o m i c distances are
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type
Structures
too s m a l l for a w a t e r m o l e c u l e .
193
T h e latter 2 assignments are consistent
w i t h the ion-exchange d a t a of S h e r r y (56, 5 7 ) a n d w i t h i n t e r a t o m i c d i s tances; h o w e v e r , the assignment is not c o m p l e t e l y u n e q u i v o c a l . Olson made
the i m p o r t a n t suggestion
that i n hydrated
systems,
cations o n l y o c c u p y site I I w h e n there are 3 A l atoms i n the 6 - r i n g to provide a favorable
electrostatic e n v i r o n m e n t .
S i n c e faujasite a n d
Y
zeolites p r o b a b l y d o not h a v e m o r e t h a n 2 A l atoms i n almost a l l 6-rings, ions s h o u l d o n l y o c c u p y site I I i n X zeolite. T h e a v a i l a b l e d a t a are c o n Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
sistent w i t h this suggestion. T a b l e V contains 2 sets of d a t a for h y d r a t e d L a - X zeolite, a n d d a t a for X e x c h a n g e d p a r t l y a n d t h e n almost c o m p l e t e d w i t h Sr. T h e e l e c t r o n d e n s i t y of L a is so h i g h that at least 7 a n d p e r h a p s as m a n y as 14 L a ions o c c u p y Γ. Site I F surely is o c c u p i e d b y w a t e r m o l e c u l e s w h i c h b o n d to L a ions i n F .
Site I I m u s t b e o c c u p i e d s t r o n g l y b y L a , a n d site V
appears to be b o n d e d w e a k l y . T h e h i g h e r A l content of the 6-rings m u s t f a v o r o c c u p a n c y of sites F a n d I I , t h e r e b y d e p l e t i n g site V w h i c h w a s strongly o c c u p i e d i n faujasite a n d Y zeolites. T h e assignments of N a a n d Sr atoms g i v e n b y O l s o n a n d S h e r r y seem reasonable. Space p r e c l u d e s extensive discussion, b u t it is o b v i o u s that the s t r u c t u r a l basis of ion-exchange is v e r y c o m p l e x , i n v o l v i n g a n a c t i v a t i o n energy for f o r m a t i o n of a h y d r a t i o n c o m p l e x , a n d i n some instances i n v o l v i n g m o v e m e n t of cations t h r o u g h the 6 - m e m b e r e d rings of the sodalite units. Interested readers are r e f e r r e d to papers b y O l s o n a n d S h e r r y S h e r r y (56, 5 7 ) , a n d to 3 papers b y B a r r e r a n d c o w o r k e r s (3,4,5)
(51), which
give d a t a o n v a r i o u s t h e r m o d y n a m i c functions i n v o l v e d i n i o n exchange and cation hydration.
Sorption Complexes S i m p s o n a n d S t e i n f i n k (58, 59) the 2 complexes
d e t e r m i n e d the c r y s t a l structures of
ra-dichlorobenzene-nickel
faujasite a n d 1 - c h l o r o b u t a n e -
manganese faujasite. T h e y w e r e u n a b l e to locate the s o r b e d
molecules
f r o m i n d i v i d u a l e l e c t r o n d e n s i t y peaks, b u t u s i n g l i q u i d scattering f u n c tions (60),
t h e y w e r e able to estimate the extent of o c c u p a n c y of the
supercage b y the s o r b e d molecules.
I n b o t h structures, the cations p r i n
c i p a l l y o c c u p y F w h i l e electron d e n s i t y i n I F was a s c r i b e d to w a t e r molecules. T h i s suggests that the crystals w e r e o n l y p a r t i a l l y d e h y d r a t e d . A f e w cations w e r e a s c r i b e d to site I I . I n the N i v a r i e t y , some of the N i ions w e r e u n i f o r m l y d i s t r i b u t e d t h r o u g h the sodalite u n i t for purposes of calculation.
S i m p s o n a n d Steinfink c o n c l u d e d that the s o r b e d
m o l e c u l e s exist as a l i q u i d i n the supercages.
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
organic
194
M O L E C U L A R
SIEVE
ZEOLITES
1
Nomenclature N o w that so m a n y d a t a h a v e b e e n a c c u m u l a t e d o n zeolites of t h e faujasite t y p e , names m u s t b e u s e d c a r e f u l l y to r e d u c e c o n f u s i o n . h i g h l y d e s i r a b l e that the n o m e n c l a t u r e b e r e l a t e d to o b s e r v a b l e
I t is
proper-
ties. T h e r e m u s t b e a loose b r o a d n a m e to encompass a g r o u p of m a t e r i a l s possessing a n i m p o r t a n t d i s t i n g u i s h i n g q u a l i t y , a n d there m u s t b e specific subnames to c h a r a c t e r i z e s u b s i d i a r y q u a l i t i e s .
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I n the g e n e r a l class of zeolites, the faujasite t y p e is d i s t i n g u i s h e d b y the s p e c i a l t o p o l o g y of its a l u m i n o s i l i c a t e f r a m e w o r k .
T o a mineralogist,
the p r o t o t y p e faujasite is the m i n e r a l d e s c r i b e d b y D a m o u r (22)
i n 1842
f r o m S a s b a c h , K a i s e r s t u h l , G e r m a n y . T h e s i n g l e - c r y s t a l x - r a y studies of B a u r a n d coworkers
(7)
defined the t o p o l o g y of the a l u m i n o s i l i c a t e
f r a m e w o r k , b u t d i d not d i s t i n g u i s h b e t w e e n t h e locations of S i a n d A l atoms, a n d d i d not f u l l y define the positions of cations a n d molecules. S y n t h e t i c zeolites p r o d u c e d b y scientists at U n i o n C a r b i d e C o r p . f r o m s o d i u m - b e a r i n g systems w e r e s h o w n b y p o w d e r x-ray m e t h o d s
to
h a v e the same f r a m e w o r k t o p o l o g y as t h a t of the m i n e r a l faujasite
(2,
19),
a n d f u r t h e r m o r e , the s o r p t i o n characteristics w e r e consistent w i t h
t h e geometry of the f r a m e w o r k . T y p e s Y a n d X zeolite w e r e d i s t i n g u i s h e d o n the basis of v a r i o u s c h e m i c a l a n d p h y s i c a l properties (18)
such that
the f o r m e r ranges f r o m 48 to 76 A l atoms p e r c e l l a n d the latter f r o m 77 to 96. M o s t measurements h a v e b e e n m a d e o n Y w i t h about 58 atoms p e r c e l l a n d X w i t h a b o u t 88 A l atoms p e r c e l l .
T h e range for Y e n -
compasses the A l - c o n t e n t for n a t u r a l faujasite w h i c h is near 59
atoms
p e r c e l l , t h o u g h some v a r i a t i o n m a y o c c u r i n faujasite specimens. So far, so g o o d ; the p r o b l e m s
arise w i t h the cations a n d
sorbed
molecules ( e s p e c i a l l y w a t e r ) , b o t h i n a m o u n t a n d i n s t r u c t u r a l p o s i t i o n . F u r t h e r m o r e , the constituents of the f r a m e w o r k cause d i f f i c u l t i e s — o r d e r d i s o r d e r of A l , S i atoms, a t t a c h m e n t of protons to f r a m e w o r k a n d v a r i o u s k i n d s of
oxygens,
defects.
T o m e , the most r e l i a b l e n o m e n c l a t u r e is b a s e d o n a c t u a l p h y s i c a l and
c h e m i c a l operations.
T h i s is often c l u m s y , b u t s h o u l d b e r e p l a c e d
b y a c o n c e p t u a l k i n d of n o m e n c l a t u r e o n l y w h e n there is u n i v e r s a l agreem e n t that the c o n c e p t is f u l l y consistent w i t h p h y s i c a l a n d c h e m i c a l d a t a . T h e t o p o l o g y of the faujasite f r a m e w o r k is u n i v e r s a l l y r e c o g n i z e d ,
but
the s t r u c t u r a l n a t u r e of c h e m i c a l a n d p h y s i c a l v a r i a n t s is h i g h l y d e b a t a b l e . T h e r e is no e v i d e n c e to p r o v e that the m i n e r a l faujasite has the same S i , A l a r r a n g e m e n t as synthetic Y w i t h the same S i / A l r a t i o .
Fur-
t h e r m o r e , the m i n e r a l contains v a r i a b l e amounts of N a , C a , M g , a n d other cations. H e n c e , i t s h o u l d not b e c a s u a l l y e q u a t e d w i t h synthetic Y g r o w n f r o m a p u r e N a - b e a r i n g system. exchanged
E v e n w h e n the m i n e r a l has b e e n i o n -
to the f u l l y N a - e x c h a n g e d f o r m i t s h o u l d not b e
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
equated
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15.
SMITH
Faujasite-Type
Structures
195
with Y. Thus, I disagree with the nomenclature proposed by Wright, Rupert, and Granquist (69, 70), especially as their x-ray and infrared data indicate significant differences between the 2 types. Dempsey et al. (25) restricted the use of the terms Y and X zeolite to the composition ranges 53-64 and 80-96 A l atoms per cell. The inter mediate range was denoted the Transition type ( Figure 3 ). The nomen clature proposed by Kerr et al. (35) for "decationated" zeolites was described earlier. Hopefully, further experimental data will reduce the uncertainty in the experimental data on the cell dimensions and the crys tal structures of the zeolites, and provide a firmer basis for nomenclature. At present, the most suitable nomenclature is one which specifies the treatment applied to a starting material—e.g., dehydrated, Ca-exchanged Type Y zeolite. For brevity in this paper, terms such as La-Y have been used: such brevity should cause no confusion since La-exchanged Type Y zeolite is automatically implied. Readers are urged to read the earlier literature carefully since the term faujasite has sometimes been casually applied to Type Y zeolite. Acknowledgment R. M . Barrer, W. M . Meier, D. H . Olson, V. Schomaker, and H. Stein fink kindly provided material incorporated into the manuscript. Michael Bennett, Donald W. Breck, E. Dempsey, David Olson, Jules Rabo, and Hugo Steinfink kindly commented on the manuscript. I thank both the Petroleum Research Fund, administered by the American Chemical Society, and Union Carbide Corp. for grants-in-aid. Literature Cited (1) Angell, C. L., Schaffer, P. C.,J.Phys. Chem. 1965, 69, 3463. (2) Barrer, R. M., Bultitude, F. W., Sutherland, J. W., Trans. Faraday Soc. 1957, 53, 1111. (3) Barrer, R. M., Davies, J. Α.,J.Phys. Chem. Solids 1969, 30, 1921. (4) Barrer, R. M., Davies, J. Α., Rees, L. V. C.,J.Inorg. Nucl. Chem. 1968, 30, 3333. (5) Ibid.,1969,31, 2599. (6) Barth, T. F. W., "Feldspars," Wiley, New York, 1969. (7) Baur, W. H., Am. Mineralogist 1964, 49, 697. (8) Bennett, J. M., Smith, J. V., Mater. Res. Bull. 1968, 3, 633. (9) Ibid., 1968, 3, 865. (10) Ibid., 1968, 3, 933. (11) Ibid., 1969, 4, 7. (12) Ibid., 1969, 4, 343. (13) Bennett, J. M., Smith, J. V., Angell, C. L., Mater. Res. Bull. 1969, 4, 77. (14) Bergerhoff, G., Baur, W. H., Grutter, W. F., Helv. Chim. Acta 1956, 39, 518. (15) Bergerhoff, G., Baur, W. H., Nowacki, W., Neues Jahrb. Mineral. Monatsh, 1958, 193.
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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196
MOLECULAR SIEVE ZEOLITES
1
(16) Bergerhoff, G., Koyama, H., Nowacki, W., Experientia 1956, 12, 418. (17) Breck, D. W., J. Chem. Educ. 1964, 41, 678. (18) Breck, D. W., Flanigen, Ε. M., "Molecular Sieves," p. 47, Society of the Chemical Industry (London), 1968. (19) Breck, D. W., Flanigen, Ε. M., Milton, R. W., Reed, T. B., Natl. Meeting, ACS, 134th, Chicago, 1958. (20) Broussard, L., Shoemaker, D. P.,J.Am. Chem. Soc. 1960, 82, 1041. (21) Cruickshank, D. W. J.,J.Chem. Soc. (London) 1961, 5486. (22) Damour, Α., Ann. Mines 1842, 1, 395. (23) Dempsey, E., "Molecular Sieves," p. 293, Society of the Chemical Indus try (London), 1968. (24) Dempsey, E.,J.Phys. Chem. 1969, 73, 3660. (25) Dempsey, E., Kuhl, G. H., Olson, D. H., J. Phys. Chem. 1969, 73, 387. (26) Dempsey, E., Olson, D. H.,J.Catalysis 1969, 15, 309. (27) Dempsey, E., Olson, D. H.,J.Phys. Chem. 1970, 74, 305. (28) Dodge, R. P., Union Carbide Corp., unpublished data. (29) Eulenberger, G. R., Keil, J. G., Shoemaker, D. P., J. Phys. Chem. 1967, 71, 1812. (30) Hahn, T., Buerger, M. J., Z. Krist. 1955, 106, 308. (31) Kerr, G. T.,J.Catalysis 1969, 15, 200. (32) Kerr, G. T.,J.Phys. Chem. 1967, 71, 4155. (33) Ibid., 1968, 72, 2594. (34) Ibid., 1969, 73, 2780. (35) Kerr, G. T., Cattanach, J., Wu, E. L.,J.Catalysis 1969, 13, 114. (36) Kerr, G. T., Shipman, G. F.,J.Phys. Chem. 1968, 72, 3071. (37) Korczak, P., Raaz, F., Ostterr. Akad. Wiss. Math. Naturw. Kl. 1967, 383. (38) Laves, F., Goldsmith, J. R., Z. Krist. 1955, 106, 227. (39) Laves, F., Hafner, S., Norsk. Geol. Tidsskr. 1962, 42, 57. (40) Liebau, F., Acta Cryst. 1961, 14, 1103. (41) McDaniel, C. V., Maher, P. K., "Molecular Sieves," p. 186, Society of the Chemical Industry (London), 1968. (42) Megaw, H. D., Kempster, C. J. E., Radoslovich, E. W., Acta Cryst. 1962, 15, 1017. (43) Meier, W. M., Olson, D. H., ADVAN. CHEM. SER. 1971, 101, 155. (44) Niggli, Α., Schweiz. Mineral. Petrog. Mitt. 1967, 47, 279. (45) Olson, D. H.,J.Phys. Chem. 1968, 72, 1400. (46) Ibid., 1968, 72, 4366. (47) Ibid., 1970, in press. (48) Olson, D. H., Dempsey, E.,J.Catalysis 1969, 13, 221. (49) Olson, D. H., Kokotailo, G. T., Charnell, J. F., J. Colloid Interface Sci. 1968, 28, 305. (50) Olson, D. H., Kokotailo, G. T., Charnell, J. F., Nature 1967, 215, 270. (51) Olson, D. H., Sherry, H. S.,J.Phys. Chem. 1968, 72, 4095. (52) Pluth, J., Schomaker, V., in preparation. (53) Rabo, J. Α., Angell, C. L., Schomaker, V., Intern. Congr. Catalysis, 4th, Moscow, 1968, Preprint 54. (54) Rabo, J. Α., Pickert, P. E., Boyle, J. E., U. S. Patent 3,130,006 ( 1964). (55) Ribbe, P. H., Gibbs, G. V., Am. Mineralogist 1969, 54, 85. (56) Sherry, H. S.,J.Phys. Chem. 1966, 70, 1158. (57) Ibid., 1968,72, 4086. (58) Simpson, H. D., Steinfink, H.,J.Am. Chem. Soc. 1969, 91, 6225. (59) Ibid., 1969, 91, 6229. (60) Simpson, H. D., Steinfink, H., Acta Cryst. 1970, in press. (61) Smith, J. V., Lithos 1970, 3, 145. (62) Smith, J. V., Bennett, J. M., Flanigen, Ε. M., Nature 1967, 215, 241. (63) Stamires, D. N., Turkevich, J., J. Am. Chem. Soc. 1964, 86, 749.
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type
Structures
197
(64) Uytterhoeven, J. B., Christner, L. G., Hall, W. K., J. Phys. Chem. 1965, 69, 2117. (65) Uytterhoeven, J. B., Schoonheydt, R., Liengme, Β. V., Hall, W. K., J. Catalysis 1969, 13, 425. (66) Ward, J. W., J. Phys. Chem. 1969, 73, 2086. (67) Ward, J. W., Hansford, R. C., J. Catalysis 1969, 13, 364. (68) Ibid., 1969, 15, 311. (69) Wright, A. C., Rupert, J. P., Granquist, W. T., Am. Mineralogist 1968, 53, 1293. (70) Ibid., 1969, 54,1484. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
RECEIVED
February 13, 1970.
Discussion D. H. Olson (Mobil Research & Development Corp., Princeton, N. J. ) : In view of the fact that the B F data for dehydrated Ca-exchanged faujasites show breaks agreeing with the position of the breaks shown by D K O for hydrated sodium faujasites, I think the suggestion that resiting of cations is responsible for the breaks can be ruled out. J. V. Smith: The breaks in the D K O data of Figure 3 may result from a resiting of cations as a function of S i / A l ratio. J. Paul Rupert (Carnegie-Mellon University, Pittsburgh, Pa. 15213): Our differing opinions with respect to designation may stem from differ ing points of view between mineralogists and physical chemists. We have suggested that when workers describe measurements made on faujasite structures they designate the material as "synthetic faujasite," specifying the number and type of exchange cations. Thus, "Zeolite Y " would be "synthetic faujasite, N a , " clearly specifying the topology and composi tion. The physical property in question may be a function of both of these. Even though a synthetic faujasite may differ from the naturallyoccurring material with respect to Si,Al ordering in the crystal, there is no assurance that different degrees of ordering do not occur in synthetic materials. Obviously, any nomenclature scheme must be a compromise between descriptiveness and convenience. Alan C. Wright (Baroid Div., National Lead, Houston, Tex. 77001): With reference to your Figure 3, probably the apparent discrepancy be tween the D K O and W R G data sets can be ascribed to experimental technique. It is my understanding that the D K O x-ray measurements utilized only absolute 2-theta values since a relative scale was all that was sought. Our measurements, however, involved the use of an internal standard. Within error limits, the D K O and W R G data may be super imposed simply by shifting the  scale. Obviously, our limited data are insufficient to confirm or deny the break points found by D K O . 56
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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MOLECULAR SIEVE ZEOLITES
1
H a r r y Robson ( E s s o R e s e a r c h L a b o r a t o r y , B a t o n R o u g e , L a . ) : S i n c e a l l the c a t i o n p o s s i b i l i t i e s w h i c h w e c a n locate are o n the d i a g o n a l of the u n i t c e l l , I b e l i e v e w e c o u l d s i m p l i f y the n o m e n c l a t u r e b y d e s i g n a t i n g these sites b y t h e i r p o s i t i o n a l o n g this d i a g o n a l ( n u m b e r s 00 to 5 0 ) . T h i s d e s i g n a t i o n w o u l d be m e a n i n g f u l to crystallographers w i t h o u t reference to p r i o r conventions. N.
G . Parsonage
( I m p e r i a l C o l l e g e , L o n d o n ) : W i t h reference
to
T a b l e V , I a m s u r p r i s e d t h a t some sites are g o o d for b o t h cations a n d Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch015
w a t e r m o l e c u l e s ( e.g., Site I ). A f t e r a l l , the cations s h o u l d seek out p o s i tions of m i n i m u m c o u l o m b i c p o t e n t i a l energy
( a n d consequently
zero
e l e c t r i c field ). If, for e x a m p l e , w e a s s u m e d that H 0 molecules w e r e p r i 2
marily held by
field-induced
d i p o l e forces, t h e y w o u l d seek positions of
m a x i m u m field. T h e y w o u l d not, t h e n , go to the same sites as t h e cations. J . V . Smith: I t h i n k one m u s t b e cautious of i n t e r p r e t a t i o n i n terms of just one p a r a m e t e r s u c h as c o u l o m b i c p o t e n t i a l energy.
If c o u l o m b i c
e n e r g y w e r e the sole t e r m , Site I w o u l d a l w a y s be o c c u p i e d , whereas w e k n o w for sure that i n some varieties of faujasite-type zeolites i t is u n o c c u p i e d . W e m u s t t r y to c o m b i n e a v a r i e t y of concepts i n o r d e r to p r e d i c t the positions of cations a n d m o l e c u l e s .
I see n o reason w h y one t y p e o f
site c a n n o t be o c c u p i e d b y cations as w e l l as m o l e c u l e s .
F o r example, if
there are t e n cations for a 16-fold site, the other six sites m i g h t
be
occupied b y water molecules. J. Turkevich (Princeton University, Princeton, N . J . 08540): I n con n e c t i o n w i t h F i g u r e 3, I w o u l d l i k e to k n o w h o w the s i l i c o n - a l u m i n u m r a t i o is d e t e r m i n e d . T h e synthetic zeolites a n d m a y b e n a t u r a l ones are o b t a i n e d f r o m s i l i c a - r i c h gels w h i c h increase i n s i h c a - a l u m i n a r a t i o i n t h e m o t h e r l i q u i d as the s i l i c o n - a l u m i n u m r a t i o increases i n the crystals. S i l i c a m a y b e o c c l u d e d a n d a d s o r b e d as a c o l l o i d a l g e l o n the z e o l i t e a n d difficult to w a s h off.
T h e s i l i c a r a t i o as m e a s u r e d w i l l b e h i g h e r a n d the
fines a s y m e t r i c a l l y l o w e r . J . V . Smith: I n m y p a p e r , I p o i n t e d out that there w e r e difficulties i n the i n t e r p r e t a t i o n of a b u l k c h e m i c a l analysis of a zeolite. T u r k e v i c h ' s r e m a r k supports m y suggestion that the c l a i m o f D K O for breaks i n the r e l a t i o n b e t w e e n c e l l d i m e n s i o n a n d A l content of synthetic faujasite-type z e o l i t e m u s t b e treated c a u t i o u s l y . Ε. M . Flanigen ( U n i o n C a r b i d e C o r p . , T a r r y t o w n , Ν. Y . 1 0 5 9 1 ) : I n response to the q u e s t i o n p r e s e n t e d b y T u r k e v i c h a s k i n g h o w the A1 0 2
3
Si0 / 2
r a t i o w a s d e t e r m i n e d as p r e s e n t e d b y S m i t h i n F i g u r e 3.
K i i h l r e s p o n d e d that the d a t a of D K O w e r e o b t a i n e d b y w e t c h e m i c a l analysis. T h e d a t a of B r e c k a n d F l a n i g e n w e r e also d e t e r m i n e d b y c h e m i c a l analysis. I w o u l d l i k e to p o i n t out w i t h respect to the B F curves i n F i g u r e 3 t h a t the z e o l i t e samples u s e d w e r e v e r y c a r e f u l l y selected w i t h respect to p u r i t y a n d h o m o g e n e i t y , as d e t e r m i n e d b y x - r a y analysis
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
15.
SMITH
Faujasite-Type
199
Structures
and oxygen adsorption characterization.
O n l y preparations containing
greater t h a n 9 5 % zeolite as d e t e r m i n e d f r o m o x y g e n c a p a c i t y w e r e u s e d to d e t e r m i n e t h e curves.
I t w a s h o p e d t h a t this p r o c e d u r e w o u l d m i n i -
m i z e errors i n b u l k c h e m i c a l analysis r e s u l t i n g f r o m extraneous m a t e r i a l . G . H . K i i h l ( M o b i l Research & Development Corp., Paulsboro, N . J . 08066):
R e f e r r i n g to y o u r F i g u r e 3, s t a t i s t i c a l considerations s h o w t h a t
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there a r e three lines, a l t h o u g h t h e center l i n e is p r o b a b l y p a r a l l e l to t h e
NUMBER OF Go ATOMS PER UNIT CELL AND S i 0 / G a 0 , VS LATTICE PARAMETER a 2
9
n
H 2.80
H 3.05
H 3.33
H3.65
H 4.00
H 4.40
H 4.85
5.37
6.00
other t w o . T h e steps are e x p e c t e d to b e m o r e d i s t i n c t i n gallosilicate faujasite because o f t h e l a r g e r g a l l i u m . O u r measurements o n a v a i l a b l e gallosilicate faujasite samples gave t h e e x p e c t e d results, p r o v i n g t h a t t h e breaks a r e r e a l . J . V . S m i t h : Statistical considerations a r e o n l y as g o o d as t h e ass u m p t i o n s . I f o n e separates t h e d a t a into three p o p u l a t i o n s separated b y the s u p p o s e d d i s c o n t i n u i t i e s , t h e s t a t i s t i c a l r e l i a b i l i t y w i l l a p p e a r m u c h better t h a n i f one treats t h e d a t a as a single p o p u l a t i o n a n d searches f o r d i s c o n t i n u i t i e s . U n d o u b t e d l y , D K O use t h e first a p p r o a c h ; i.e., t h e y are
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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MOLECULAR SIEVE ZEOLITES
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c h e c k i n g a n a s s u m e d m o d e l r a t h e r t h a n l o o k i n g for breaks at a n y A l c o n tent. T h e d a t a for the gallosilicate faujasite analogs c e r t a i n l y s h o w m u c h better e v i d e n c e for breaks t h a n d o the a l u m i n o s i l i c a t e analogs.
Hopefully,
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the d a t a w i l l be p u b l i s h e d shortly.
Flanigen and Sand; Molecular Sieve Zeolites-I Advances in Chemistry; American Chemical Society: Washington, DC, 1974.