Molecular Sieve Zeolites-I - American Chemical Society

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

/

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


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


174

MOLECULAR SIEVE ZEOLITES 1


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


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


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


176


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


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


15.

SMITH

Faujasite-Type

Table II.


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


internal

178


Table III.

1

Dimensions of the


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-


15.

SMITH

Faujasite-Type

179

Structures


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



180


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


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


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


182


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


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


184


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


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 .


15.

SMITH

Faujasite-Type Table IV.

Type

Reference

185

Structures

Cation Occupancy and Position IP

I


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


186


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


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


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


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


188


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 .


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


190


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.


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 -


192


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


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.


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 .


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


equated


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.



196


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.


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 Â scale. Obviously, our limited data are insufficient to confirm or deny the break points found by D K O . 56


198


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


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


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


200


1

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,


the d a t a w i l l be p u b l i s h e d shortly.


Molecular Sieve Zeolites-I - American Chemical Society

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