C h a p t e r 12
Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 10, 2018 at 12:17:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Long-Range Versus Short-Range Effects in the Distribution of Κ Ions in Hydrated K Y Zeolites 1,3
2
W. J . Mortier , D. E . W. Vaughan , and J . M . Newsam
2
1
Laboratorium voor Oppervlaktechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3030 Leuven (Heverlee), Belgium Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801 2
The cation site population is used for monitoring the cation-framework interaction energy in hydrated K-Y-type zeolites with varyinq Si/Al ratio. The K-ion content of a series of Y-type zeolites was modified by (i) changing the framework Al content (by direct synthesis or by dealumination) and by (ii) exchange with NH cations, and their structures determined with X-ray powder methods. The population of site I with Κ ions depends primarily on the framework Al content, and is empty for Si:Al ratios over 2.7. The sites I' and II depopulate much more slowly. The cation distribution pattern seems to be determined by the short-range cation-framework interaction energy only, independent of the details of the population of the other sites (at high cation loading, the site I-site I' repulsion becomes important however). This is in agreement with recent statistical-thermodynamical developments. 4
The c a t i o n d i s t r i b u t i o n i n z e o l i t e s i s t h e r e s u l t o f an e n e r q y minimization process. The s i t e enerqy i s d e t e r m i n e d by t h e i n t e r a c t i o n o f t h e c a t i o n s w i t h t h e framework, w i t h t h e adsorbed m o l e c u l e s and by t h e mutual r e p u l s i o n between them. P r o v i d e d t h a t an e q u i l i b r i u m d i s t r i b u t i o n i s p o s s i b l e , we may e x p e c t t h a t t h e c a t i o n d i s t r i b u t i o n c o n t a i n s i n f o r m a t i o n about the enerqy l e v e l s o f the s i t e s . R e c e n t l y , a s t a t i s t i c a l m e c h a n i c a l model was proposed f o r e x p l a i n i n q t h e c a t i o n d i s t r i b u t i o n i n z e o l i t e s as a f u n c t i o n o f 3
Current address: Exxon Chemical Holland B.V., BCT, P.O. Rotterdam, Holland
Box 7335, 3000 HH
0097-6156/88/0368-0194$06.00/0 © 1988 American Chemical Society
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
Distribution of K Ions in KY Zeolites
12. MORTIER ET AL.
195
the t e m p e r a t u r e (JJ . F o r d e h y d r a t e d f a u j a s i t e - t y p e z e o l i t e s , i t was p o s s i b l e t o w r i t e a n e x a c t e x p r e s s i o n f o r t h e p a r t i t i o n f u n c t i o n , and t h e r e f o r e a l s o f o r a l l d e r i v e d thermodynamic functions. T f we a s s u m e t h a t Nj^ c a t i o n s a r e l o c a t e d a t s i t e i ( e n e r q y l e v e l e^ d e p e n d i n g o n c a t i o n - f r a m e w o r k i n t e r a c t i o n s o n l y ) w i t h a maximum o c c u p a n y o f B^ c a t i o n s a t t h i s s i t e ( t h e s i t e p o p u l a t i o n Ρ ί = Ν ^ / Β ^ ) , we may t h e n e n u m e r a t e t h e t o t a l number o f c o n f i g u r a t i o n s i . e . t h e p r o d u c t o f t h e number o f w a y s we c a n permute c a t i o n s on B ^ - s i t e s as
Ω =
I.
(1)
% i ! («i-Ni) !
MaximizinaiJ , subject t o the r e s t r i c t i v e condition that o b t a i n t h e most p r o b a b l e e q u i l i b r i u m d i s t r i b u t i o n a s :
N=£N^,
we
Pi = 1 -
θ
χ ρ (*,+
e i
)/kT
(2)
/ P
i
wherey**represents thechemical p o t e n t i a l o f thecations i n the z e o l i t e p h a s e , ft, e n t e r s t h i s expression through t h e Lagrange m u l t i p l i e r * a s s o c i a t e d w i t h f i x i n q N: I (low A l c o n t e n t ) t o I > I I > I' (high A l c o n t e n t ) . T h i s was understood by r e a l i z i n g t h a t the e f f e c t o f the A l s u b s t i t u t i o n w i l l have t w i c e the e f f e c t i n s i t e I (double s i x - r i n g ) than i n the s i n g l e s i x - r i n g ( s i t e s 1 and II) and w a l l s i t e s ( " s i t e III") . e
e
a n
t
n
e
n
1
T
X I
1
'"his paper w i l l be concerned with i n v e s t i q a t i n g the K-ion d i s t r i b u t i o n s i n h y d r a t e d K-exchanged FAU-type z e o l i t e s with v a r y i n g Al content. I t i s an e q u i l i b r i u m s i t u a t i o n (hydrated s t a t e ) so t h a t we can a p p l y the p r i o r model developed f o r the d e h y d r a t e d s t a t e .
Experimental Sample p r e p a r a t i o n h-K
3 4 e 9
FAU
T h i s m a t e r i a l was o b t a i n e d by d i r e c t s y n t h e s i s u s i n g a m o d i f i e d o r g a n i c template p r o c e d u r e p r e v i o u s l y r e p o r t e d (3_) . I t s A l c o n t e n t , and o f a l l o t h e r s , was determined from the v a r i a t i o n o f t h e u n i t c e l l c o n s t a n t with A l c o n t e n t f o r a range o f f u l l y K-exchanged h y d r a t e d z e o l i t e s (5) . The A l c o n t e n t i s then c a l c u l a t e d as ( a - 25.127)/0.011388, which a p p l i e s o n l y t o h-K-FAU-type zeolites. * S i NMR and c h e m i c a l a n a l y s e s c o n f i r m e d t h e s e d a t a . Q
2
3
T h i s sample i s a shallow-bed steamed v e r s i o n (600°c, 3 h) o f t h e LZ 210 p a r e n t m a t e r i a l used f o r h - K 5 F A n . A d i l u t e m i n e r a l room temperature a c i d exchange (pH = 2.5) was used t o remove d e t r i t a l A l . A f t e r a second steam-leach treatment the sample was K-exchanqed as d e s c r i b e d above. 3 2 e
h-K H FAn 1 5
4 0
T h i s i s a r e g u l a r commercial Y-type z e o l i t e , t w i c e exchanged with a 15 wt % KC1 s o l u t i o n ( z e o l i t e / s o l u t i o n = 7 %) a t 60°C, then d o u b l e exchanged with N F 0 1 a t 22°C (10 wt % s o l u t i o n ; 10 % z e o l i t e ) f o l l o w e d by f i l t e r washing w i t h 5 % NH4OH t o c o n v e r t the NHJ t o H O + . 4
3
X-ray d a t a sampling and r e f i n e m e n t . The d i f f r a c t i o n p r o f i l e s were r e c o r d e d i n two p a r t s by u s i n g an automated powder d i f f T a c t o m e t e r (Siemens Π-500) with N i - f i l t e r e d Cu r a d i a t i o n i n s t e p s o f 0.01° 2Θ (0.1° i n c i d e n t beam d i v e r q e n c e s l i t below 35° 2Θ and 1.0° above 35° 2Θ; r e c e i v i n g s l i t 0.D5 mm; t y p i c a l c o u n t i n g times r e s p e c t i v e l y 25s and 6 5 s ) . The X - r a y d a t a sampling ( p e a k - f i t t i n g of t h e i n d i v i d u a l peaks u s i n g an e x p e r i m e n t a l p r o f i l e ) and r e f i n e m e n t p r o c e d u r e s have been d e s c r i b e d e x t e n s i v e l y elsewhere ( 6 - 7 ) . Agreement f a c t o r s a r e d e f i n e d a s : c
\-\« \\ "G
=
—
Γ W R r
=
£
