58 Kinetics of Sorption Processes as a Basis for Estimating Cation Distribution in Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch058
Unit Cells of Zeolites FRIEDRICH WOLF, FLORIN DANES, and KURT PILCHOWSKI Chair of Chemical Technology, Martin Luther University Halle-Wittenberg, Halle/Saale, German Democratic Republic
Kinetic studies of ion exchange on partially ion-exchanged type A zeolites of Mg , Ca , and Mn revealed that minimums and maximums characterize the differential coefficients of internal diffusion for every exchange of 2 Na ions for one divalent cation per unit cell of the zeolite. On the basis of these observations, assuming definite interactions between the cations and the zeolite lattice, predictions can be made concerning the distribution and arrangement of cations in the unit cells of a type A zeolite. Research on liquid phase adsorption of n-alkanes on partially ion-exchanged type A zeolites indicated that the differential diffusion coefficients for alkane adsorption are influenced likewise by cation distribution in the unit cells of the zeolite. 2+
2+
2+
+
C i n c e t h e n a t u r a l zeolites w e r e d i s c o v e r e d a n d i n t r o d u c e d as adsorbents, ^
n u m e r o u s investigations h a v e b e e n d e v o t e d to the s o r p t i o n b e h a v i o r
a n d i o n exchange properties of zeolites (1, 2, 5, 14).
Sorption behavior
of zeolites c a n b e i n f l u e n c e d p o w e r f u l l y b y i o n exchange, p a r t l y i n t h e u p t a k e c a p a c i t y of the zeolites a n d p a r t l y i n s o r p t i o n rate since these m a y u n d e r g o d e c i s i v e changes.
T h e present s t u d y u n d e r t a k e s , o n t h e
basis of k i n e t i c research i n i o n exchange a n d i n a l k a n e a d s o r p t i o n o n t y p e A zeolites after p a r t i a l i o n exchange,
to s h o w w h a t influence is
exerted b y c a t i o n d i s t r i b u t i o n o n s o r p t i o n b y t y p e A zeolites. Kinetics
of Ion
Exchange
A l t h o u g h the kinetics of i o n exchange o n t y p e A zeolites has b e e n d i s c u s s e d p r e v i o u s l y i n several papers (4, 10, 12, 15, 19),
n o systematic
229
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
230
M O L E C U L A R SIEVE Z E O L I T E S
y %
0
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-9-6 !
-14 0 1 ' 0
1
—
•
25
50
75
100
I—ι—ι
1
1—I—ι
1
I
2
I
I
I
I 4
3
II
I
I
5
I
6
Figure 1. Coefficients of internal diffusion in exchange between divalent cations and Na ions on type A zeolite +
D : Differential diffusion coefficient N : Number of divalent cations per cavity y: Valence fraction of the divalent cation in the zeolite P
r e s e a r c h has b e e n r e p o r t e d o n i o n e x c h a n g e kinetics as i n f l u e n c e d b y c a t i o n u p t a k e of t h e zeolite.
A c c o r d i n g l y , w e s t u d i e d t h e k i n e t i c s of
ion exchange w i t h M g , C a , a n d M n 2 +
2 +
2 +
ions o n p a r t i a l l y i o n - e x c h a n g e d
t y p e A zeolites. O u r s t a r t i n g m a t e r i a l w a s Z e o s o r b 4 A , a synthetic zeo l i t e of V o l k s e i g e n e r B e t r i e b ( S t a t e - O w n e d )
Chemiekombinat Bitterfeld.
T h e i o n e x c h a n g e s t u d y w a s p e r f o r m e d i n a b a t c h process w i t h 0.1 IV aqueous
salt s o l u t i o n .
Internal diffusion (particle
d i f f u s i o n ) w a s the
r a t e - d e t e r m i n i n g e x c h a n g e step u n d e r t h e h y d r o d y n a m i c c o n d i t i o n s e m p l o y e d here ( s h a p e of c o n t a i n e r a n d agitator, agitator speed, concentra t i o n o f t h e s o l u t i o n , quantities of s o l u t i o n a n d o f z e o l i t e ) .
Changes i n
concentration generally were f o l l o w e d b y complexometric titration, a n d r a d i o a c t i v e isotopes of C a
2 +
and M n
2 +
w e r e u s e d f o r v e r y fast e x c h a n g e
processes.
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
58.
W O L F
Kinetics
E T A L .
of Sorption
Processes
231
F o r e a c h c a t i o n , t h e t o t a l u p t a k e r a n g e of t h e z e o l i t e w a s d i v i d e d i n t o 12 zones a n d t h e k i n e t i c s t u d y of i o n exchange each zone.
was individual i n
F o r this p u r p o s e , t h e final u p t a k e ( e q u i l i b r i u m c h a r g e ) of
e a c h z o n e c o r r e s p o n d e d to t h e i n i t i a l u p t a k e of t h e next zone, so that the t o t a l u p t a k e r a n g e w a s c o v e r e d . T h e r e q u i s i t e e q u i l i b r i u m isotherms f o r c a l c u l a t i n g d i f f u s i o n coefficients also w e r e d e t e r m i n e d e x p e r i m e n t a l l y i n t h e b a t c h process.
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T h e effective d i f f u s i o n coefficients w e r e c a l c u l a t e d f r o m t h e e x p e r i m e n t a l l y o b s e r v e d d a t a (time, a m o u n t of c a t i o n e x c h a n g e d ,
temperature),
u s i n g Paterson's s o l u t i o n of F i c k ' s s e c o n d l a w , o r p u b l i s h e d a p p r o x i m a t e solutions ( 8 , 16).
T a k i n g i n t o c o n s i d e r a t i o n p a r t i c l e shape a n d p a r t i c l e
size d i s t r i b u t i o n , t h e d i f f e r e n t i a l coefficients of i n t e r n a l d i f f u s i o n i n i o n e x c h a n g e c a n b e ascertained b y a m e t h o d p r e v i o u s l y d e s c r i b e d ( 9 ) . F i g u r e 1 shows t h e d i f f e r e n t i a l d i f f u s i o n coefficients i n i o n exchange as a f u n c t i o n o f zeolite u p t a k e f o r the 3 i n v e s t i g a t e d i o n exchange p r o -
y % — •
0
25
50
75
100
Np—v Figure 2. Activation energies and activa tion entropies of internal diffusion in cation exchange on type A zeolite E : Activation energy N : Number of divalent cations per cavity SA: Activation entropy y: Valence fraction of the divalent cation in the zeolite 1: Ε for the interchange Mg /NaA 2: Ε for the interchange Ca /NaA 3: Ε for the interchange Mn /NaA 4: SA for the interchange Mg /NaA 5: S for the interchange Ca /NaA 6: S for the interchange Ca /NaA P
2+
2+
2+
2+
A
A
2+
2+
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
232
MOLECULAR
SIEVE
ZEOLITES
II
cesses. T h e s u b s t a n t i a l differences a m o n g the i n v e s t i g a t e d i o n exchange systems are c a u s e d p r i m a r i l y b y differences i n a c t i v a t i o n energy.
Figure
2 shows a c t i v a t i o n energies as c a l c u l a t e d f r o m t h e A r r h e n i u s e q u a t i o n a n d t h e a c t i v a t i o n entropies o f i o n exchange, c a l c u l a t e d b y t h e t h e o r y of absolute r e a c t i o n rates, as a f u n c t i o n o f zeolite u p t a k e . T h e o b s e r v e d d o w n w a r d t r e n d o f a c t i v a t i o n energies
a n d a c t i v a t i o n entropies
s u m a b l y is a t t r i b u t a b l e to p a r t i a l s t r i p p i n g a w a y o f the aqueous
pre
sheath
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o n cations e n t e r i n g the zeolite. T h e influence o f c a t i o n u p t a k e o f the zeolite (see F i g u r e s 1 a n d 2 ) o n d i f f u s i o n coefficients, a c t i v a t i o n energies, a n d a c t i v a t i o n entropies i n i o n exchange is m u c h m o r e p r o n o u n c e d t h a n that o f changes i n d i f f u s i o n m a g n i t u d e s f r o m one c a t i o n t o another.
W i t h i n c r e a s i n g zeolite u p t a k e ,
the d i f f e r e n t i a l d i f f u s i o n coefficients t e n d t o d r o p s h a r p l y , the c u r v e b e i n g i n t e r r u p t e d b y m a x i m u m s a n d m i n i m u m s . W h i l e there h a v e b e e n p r e v i ous reports o f f a l l i n g d i f f u s i o n coefficients w i t h r i s i n g zeolite u p t a k e (3, 11, 17, 20), t h e curves w h i c h w e o b t a i n e d s h o w extremes i n d i f f u s i o n coefficients for zeolite uptakes w h i c h are m u l t i p l e s o f 1 / 6 , n a m e l y 1 / 6 , 2/6, 3/6, 4/6, 5/6, and 6/6. E x i s t e n c e o f at least 2 d i f f u s i o n m e c h a n i s m s m u s t b e assumed, tak i n g i n t o c o n s i d e r a t i o n the s t r u c t u r a l p e c u l i a r i t i e s o f t y p e A zeolites, f o r i n t e r p r e t i n g these results.
I t is a f a m i l i a r f a c t that d i f f u s i o n o f cations
o r m o l e c u l e s i n t o zeolites is i n f l u e n c e d b y t h e n a r r o w channels. o f d i f f u s i o n d e p e n d s o n t h e r a t i o d /d , p
cations o r m o l e c u l e s a n d d
k
k
where d
p
is c h a n n e l d i a m e t e r .
d i f f u s i o n is not i n f l u e n c e d b y p o r e size; w h e n d /d p
increase i n d /d p
k
k
W h e n d /d p
~
Rate
is d i a m e t e r o f t h e k
<
