16 Crystal Chemical Relationships in the Analcite Family II. Influence of Temperature and Ρ on Structure Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 27, 2018 | https://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch016
H2O
1
WILLIAM D. BALGORD and RUSTUM ROY The Materials Research Laboratory, The Pennsylvania State University, University Park, Pa.
Synthetically prepared analcites of normal, high, and low Al O : SiO ratio and their cation-exchanged derivatives were investiga by stepwise thermogravimetry and powder x-ray diffraction at elevated temperatures under controlledP (7.9 and740torr). The dehydration curves commonly display second-order disco tinuities, in contrast to the P-type zeolites (Taylor and Roy's classification) and represent true phase-transition type reacti In exceptional cases (e.g., the dehydration of Sr-exchanged ana cite), discontinuities in the TGA curves are a far more sensitive indicator of phase changes than are x-ray methods. Substitutio of small, divalent ions for Na resulted in marked displacements in the dehydration curve upward in temperature and in a higher degree of reversibility during cooling. The Al O :SiO ratio influenced the dehydration characteristics of analcite only to a limited extent Although both high and low alumina analcites dehydrate in a continuous fashion, the high alumina form under goes water loss at a lower temperature and is more nearly re versible. 2
2
H2O
+
2
3
2
A s s h o w n b y T a y l o r a n d R o y (1) t h e b e h a v i o r of s m a l l - p o r e zeolites does n o t necessarily c o n f o r m t o t h e classical d e f i n i t i o n of a " z e o l i t e . " R a t h e r , t h e properties e v i d e n c e d b y the P - z e o l i t e s , a n d perhaps o t h e r classes of s m a l l - p o r e zeolites as w e l l , c o n s t i t u t e a basis for possible f u t u r e t e c h n i c a l i n n o v a t i o n i n selective a d s o r p t i o n a n d heterogeneous c a t a l y s i s . T h e zeolite s t r u c t u r e , a n d hence t h e size a n d shape of i t s c e l l apertures a n d cavities a n d d i s p o s i t i o n of m o b i l e cations m a y differ s u b s t a n t i a l l y a t ele v a t e d t e m p e r a t u r e s f r o m w h a t i t is u n d e r a m b i e n t c o n d i t i o n s . Present address: Research and Development Unit, New York State Department of Environmental Conservation, Albany, Ν. Y. 12201 1
189
Meier and Uytterhoeven; Molecular Sieves Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
3
190
MOLECULAR SIEVES
E a r l i e r studies o n t h e p r o p e r t i e s of a n a l c i t e — a v e r y s m a l l pore z e o l i t e — c o n c e n t r a t e d o n e l u c i d a t i n g the c r y s t a l s t r u c t u r e (2, 3), i o n exchange a n d gaseous diffusion (4, δ, 6) a n d m e t h o d s of synthesis of n o n - s t o i c h i o m e t r i c a n a l c i t e (7).
S t u d i e s p e r t a i n i n g t o t h e d e h y d r a t i o n of a n a l c i t e
d a t e f r o m 1925, w h e n R o t h m u n d (8) r e p o r t e d a n a r r o w t e m p e r a t u r e range of reversible d e h y d r a t i o n .
M i l l i g a n a n d W e i s e r (9) r e p o r t e d a d e h y d r a
t i o n i s o b a r ( P H O = 23.6 t o r r ) for a n a l c i t e w h i c h r e v e a l e d n o steps.
The
2
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w o r k of K o i z u m i {10),
PH O 2
unspecified, t e n d e d t o c o n f i r m t h e results of
M i l l i g a n a n d W e i s e r b u t also suggested a d i s c o n t i n u i t y a t 2 2 5 ° C . A p p a r e n t i n t h e f o r e g o i n g r e v i e w is t h e l a c k of a n y s y s t e m a t i c a t t e m p t to i n v e s t i g a t e h y d r a t i o n states a n d s t r u c t u r a l m o d i f i c a t i o n s of t h e analcite framework temperatures.
u n d e r a b r o a d range of c o m p o s i t i o n s
Accordingly,
this study
was
done t o
at
elevated
determine
com
p o s i t i o n a l l i m i t s , d e h y d r a t i o n b e h a v i o r , a n d associated s t r u c t u r a l changes i n the analcite group. Experimental S y n t h e t i c analcites a n d i o n - e x c h a n g e d d e r i v a t i v e s described p r e v i o u s l y b y B a l g o r d a n d R o y (11) were s t u d i e d b y stepwise t h e r m o g r a v i m e t r y a t constant P H O a n d b y p o w d e r x - r a y d i f f r a c t i o n u n d e r s i m i l a r l y c o n t r o l l e d P oT h e a n a l c i t e samples were p r e c o n d i t i o n e d for at least t h r e e weeks i n a sealed atmosphere o v e r s a t u r a t e d M g C l - 6 H 0 ( P H o = 7.9 t o r r ) . E x c e p t i o n s t o t h i s procedure were m a d e for c e r t a i n p o t a s s i u m exchanged f o r m s w h i c h t e n d e d t o u n d e r g o w a t e r loss a n d s t r u c t u r a l changes e v e n at a m b i e n t t e m p e r a t u r e . T h e s e l a b i l e samples were h e l d at 7 ° - 8 ° C o v e r H 0 ( P H O = 8 t o r r ) for storage. T h e r m o g r a v i m e t r i c analyses were c a r r i e d o u t i n 1 0 ° - 3 0 ° t e m p e r a t u r e i n c r e m e n t s w i t h 2 0 0 - m g samples u s i n g a c o n v e n t i o n a l ( M a u e r ) T G A s y s tem. A u t o m a t i c r e c o r d i n g of w e i g h t change was used t o f o l l o w r e a c t i o n to e q u i l i b r i u m , b u t a c t u a l weighings were r e c o r d e d o n l y b y m a n u a l o p e r a t i o n . T h e s a m p l e was b a t h e d c o n t i n u o u s l y i n a i r of c o n t r o l l e d h u m i d i t y ( P H O = 7.9 t o r r ) flowing at 180 c c / m i n . P r e c a u t i o n s were t a k e n t o m i n i m i z e drafts a n d c o n v e c t i v e c u r r e n t s , a n d b u o y a n c y c o r r e c t i o n c u r v e was m a d e t o 9 5 0 ° C . F u r t h e r details o n e x p e r i m e n t a l m e t h o d s are a v a i l a b l e (12). H i g h t e m p e r a t u r e x - r a y d i f f r a c t i o n was done w i t h selected samples u s i n g a T e m - P r e s m o d e l S X d i f f r a c t o m e t e r furnace o n a P i c k e r m o d e l 3 4 8 8 E d i f f r a c t o m e t e r u s i n g filtered C u r a d i a t i o n . H i g h t e m p e r a t u r e r u n s were c o n d u c t e d w i t h samples i n a c o n t i n u o u s l y r e p l e n i s h e d a i r a t m o s p h e r e of 7.9 a n d 740 t o r r P . 2
H 2
2
2
2
2
2
2
H 2 0
Results Parent Analcites. of n o r m a l c o m p o s i t i o n
T h e d e h y d r a t i o n of a s y n t h e t i c a l l y p r e p a r e d a n a l c i t e (Nai Ali Si3 0 6- 1 6 H 0 ) 6
6
2
9
2
designated
114
analcite
(the i n d e x represents t h e m o l a r r a t i o of t h e oxides—e.g., N a 0 - A l 0 2
4 S i 0 , of t h e p a r e n t analcite) proceeds a t 7.9 t o r r 2
PH O 2
with
d i s c o n t i n u i t i e s a t 100° a n d 200° C as s h o w n i n F i g u r e 1.
2
3
second-order
O t h e r w i s e , de-
Meier and Uytterhoeven; Molecular Sieves Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
16.
191
The Analcite Family
BALGORD A N D ROY
h y d r a t i o n progresses s m o o t h l y t o c o m p l e t i o n a t 400° C . A t o t a l w e i g h t loss of 8 . 4 5 % corresponds t o 16.4 H 0 molecules p e r u n i t c e l l . Repeated c y c l i n g between 200° a n d 4 0 0 ° C d e m o n s t r a t e d t h a t r e v e r s i b l e r e h y d r a t i o n on cooling is achieved to about 225°C. B e l o w 200° C , despite closely spaced t e m p e r a t u r e h a l t s a n d l o n g h o l d t i m e s (four m o n t h s a l l o t t e d t o o b t a i n i n g the isobar), a c o n s t a n t three H 0 p e r u n i t c e l l difference persisted between t h e d e h y d r a t i o n c u r v e of t h e v i r g i n zeolite s a m p l e a n d t h e r e h y d r a t i o n c u r v e of t h e m a t e r i a l w h i c h h a d been h e a t e d t o 6 8 0 ° C . 2
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 27, 2018 | https://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch016
2
X - r a y d i f f r a c t i o n of 114 a n a l c i t e a t 25 ° C r e v e a l e d t w o w e a k reflections a t d = 3.80 (at 23.4° 20) a n d 3.24 A (at 27.5 20) i n d e x e d as (320) a n d (411), r e s p e c t i v e l y . T h e s e f o r b i d d e n , w e a k reflections d i s a p p e a r f r o m t h e p a t t e r n o n h e a t i n g t h e zeolite a b o v e 2 0 0 ° C . T h e i r disappearance c o r r e l a t e d closely w i t h t h e p r o n o u n c e d second-order b r e a k i n t h e d e h y d r a t i o n c u r v e a t 200° C a n d suggests a r a n d o m i z a t i o n of t h e p a r t i a l l y ordered d i s t r i b u t i o n of 1 6 N a ions over t h e 2 4 N a sites ( V , 0, y ) i n t h e u n i t c e l l . A t t h e d e c o m p o s i t i o n t e m p e r a t u r e , 9 1 6 ° C , t h e a p p a r e n t c e l l edge i s 13.69 A . +
+
8
4
T h e h i g h - s i l i c a a n a l c i t e (Nai Al Si 6096- 1 8 H 0 ) , d e s i g n a t e d 116 a n a l c i t e , c o n t a i n e d a h i g h e r percentage of w a t e r , 8 . 7 0 % , r e l a t i v e t o 114 a n a l c i t e . D e h y d r a t i o n a n d r e h y d r a t i o n (not shown) were s m o o t h , w i t h o u t 2
1
25 C e
250
12
114 ANALCITEH 0 « 7.9TORRJ
P
ι
1
ι
2
e
".I
300° 916··
2
3
1
•SAMPLE DECOMPOSES •
1
1
1
1
L
-A-
DEGREES 2 9(CUK-
