Counterdiffusion of Liquid Hydrocarbons in Type Y Zeolites

55 Counterdiffusion of Liquid Hydrocarbons in

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Type Y Zeolites CHARLES N. SATTERFIELD and JAMES R. KATZER

1

Massachusetts Institute of Technology, Cambridge, Mass.

The nature of the cations present in a zeolite can have a marked effect upon the rate of intracrystalline counterdif fusion, as shown by studies with several selected aromatic hydrocarbons in a series of ion-exchanged forms of the type Y zeolite. For 1-methylnaphthalene diffusing from type Y into bulk cumene, the desorptive diffusion coefficients vary by 2 orders of magnitude over different ion-exchanged forms in the order: NaY < CaY < SK-500 < CeY < HY The results are interpreted in terms of the size of the diffus ing molecule and the effect of the cation upon the pore size of the zeolite. Counterdiffusion of the molecules studied occurs readily in the various forms of type Y zeolite, but molecule-molecule interactions between the counterdiffusing molecules have a pronounced effect upon the diffusion rate. Τ η m o s t a p p l i c a t i o n s of zeolites, i t is necessary for m o l e c u l e s to b e a b l e -*· to diffuse i n t o or out of t h e i r fine p o r e structure, a n d i n m a n y of these a p p l i c a t i o n s , p a r t i c u l a r l y catalysis, the c o u n t e r d i f f u s i o n of at least 2 different k i n d s of m o l e c u l e s o c c u r s .

T h e rates of these d i f f u s i o n p r o c

esses c a n h a v e a p r o f o u n d effect u p o n the a p p a r e n t a c t i v i t y a n d selec t i v i t y of z e o l i t i c catalysts (21 ) a n d u p o n s u c h characteristics as d i s p e r s i o n a n d sharpness of s e p a r a t i o n i n the use of zeolites i n s e p a r a t i o n

and

p u r i f i c a t i o n processes. T h e state of k n o w l e d g e of i n t r a c r y s t a l l i n e d i f f u s i o n i n zeolites is r e v i e w e d b y B a r r e r i n a p a p e r f o r this s y m p o s i u m (4).

Little

is k n o w n a b o u t u n i d i r e c t i o n a l d i f f u s i o n i n zeolites of substances of i n d u s 1

Present address : University of Delaware, Newark, Del.

193

Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

194

M O L E C U L A R SIEVE

t r i a l interest

ZEOLITES

II

s u c h as h y d r o c a r b o n s , a n d n o i n f o r m a t i o n seems t o b e

a v a i l a b l e o n c o u n t e r d i f f u s i o n except f o r t h e s e l f - d i f f u s i o n of w a t e r (4, 5). L i m i t e d i n f o r m a t i o n is a v a i l a b l e o n d i f f u s i o n i n zeolites w h e n c o n t a c t e d w i t h a l i q u i d ( I , 2, 3, 19) a n d a p p a r e n t l y none i n t h e t y p e X a n d Y zeolites, w h i c h are o f i m p o r t a n c e i n catalysis. T h e p o r e structure of t y p e Y is d e s c r i b e d , f o r e x a m p l e , b y B r e c k a n d F l a n i g e n ( 9 ) . I t consists essentially o f a t h r e e - d i m e n s i o n a l a r r a y of large cavities w i t h a d i a m e t e r of a b o u t 12 A i n t e r c o n n e c t e d b y p o r e apertures w i t h diameters of a b o u t 8 A . T h e s e d i m e n s i o n s v a r y s l i g h t l y w i t h t h e S i / A l ratio i n t h e zeolite a n d t h e n u m b e r a n d k i n d s of cations

present.

T h e m a i n objective o f this s t u d y was to d e t e r m i n e t h e c o u n t e r d i f f u s i o n characteristics

of selected l i q u i d h y d r o c a r b o n s — b e n z e n e ,

cumene,

1 - m e t h y l n a p h t h a l e n e , a n d 2 - e t h y l n a p h t h a l e n e — i n several i o n - e x c h a n g e d f o r m s o f the t y p e Y zeolite a n d thus to b e g i n t o p r o v i d e a n u n d e r s t a n d i n g of t h e role w h i c h d i f f u s i o n m a y b e p l a y i n g i n v a r i o u s a p p l i c a t i o n s o f zeolites. T h i s is a p a r t of the larger, t h e o r e t i c a l p r o b l e m of u n d e r s t a n d i n g t h e p h y s i c s o f m o l e c u l a r m o t i o n i n s i d e pores

with

diameters

which

a p p r o a c h t h e d i a m e t e r o f t h e molecules d i f f u s i n g i n t h e m . Experimental

Materials and Methods

M a t e r i a l s U s e d . T h e N a Y zeolite a n d a n i o n - e x c h a n g e d f o r m of i t , SK-500, were supplied b y U n i o n C a r b i d e C o r p . , L i n d e D i v i s i o n , i n the f o r m of uncalcined powder. T h e SK-500

( L o t N u m b e r 12506-39) is a

r a r e e a r t h - a m m o n i u m e x c h a n g e d t y p e Y zeolite a n d h a d n o t b e e n a c t i v a t e d p r e v i o u s l y o r c a l c i n e d i n its p r e p a r a t i o n . T h e c a l c u l a t e d u n i t c e l l f o r m u l a was (Re +) . ( N H ) i . i ( N a + ) . 3

8

8

4

+

2

8

3

[(Α10 ) .7 (Si0 ) 2

5 5

2

1 3 6

.3]

ZH 0 2

A c c o r d i n g to t h e analysis s u p p l i e d b y L i n d e , t h e rare earths w e r e p r e dominantly lanthanum.

A n i n d e p e n d e n t s o d i u m analysis a g r e e d

with

that s u p p l i e d b y L i n d e . T h e N a Y zeolite p o w d e r ( L o t N u m b e r 12, 119-69) h a d a c a l c u l a t e d formula of Na

5 7

[(Α10 )β7 (Si0 ) 2

2

135

]

·

ZH 0 2

T h e N a Y w a s d i v i d e d i n t o 4 e q u a l p o r t i o n s . O n e of these w a s u s e d as s u p p l i e d ; t h e other 3 w e r e i o n - e x c h a n g e d to either t h e c a l c i u m , c e r i u m , or a m m o n i u m f o r m .

I o n exchange w a s c a r r i e d o u t b y a c o m b i n a t i o n

batchwise—continuous p r o c e d u r e at a b o u t 80 ° C w i t h a large excess of e x c h a n g e s o l u t i o n . T h e s e w e r e p r e p a r e d f r o m reagent grade c h l o r i d e s a n d c o m p r i s e d 5 w t % C e C l , 10 w t % C a C l , o r 10 w t % N H C 1 , respec 3

2

4

t i v e l y , i n d i s t i l l e d w a t e r . A f t e r exchange, t h e zeolites w e r e w a s h e d w i t h


55.

SATTERFiELD

A N D KATZER

Counterdiffusion

of Hydrocarbons

195

d i s t i l l e d w a t e r u n t i l n o trace of the c h l o r i d e i o n a p p e a r e d i n the w a s h w a t e r . T h e degree of exchange b a s e d o n the n u m b e r of s o d i u m cations r e p l a c e d p e r u n i t c e l l was 8 9 % CeY.

for C a Y , 9 7 %

for N H Y , a n d 7 0 % 4

for

T h e l o w degree of exchange for c e r i u m p r o b a b l y r e s u l t e d f r o m the

i n a b i l i t y of the h y d r a t e d c e r i u m i o n to enter the sodalite units t h r o u g h t h e s i x - m e m b e r e d o x y g e n rings a n d replace the 16 s o d i u m cations p e r u n i t c e l l r e s i d i n g there

(22).

A f t e r the s t a n d a r d a c t i v a t i o n p r o c e d u r e ( d e s c r i b e d b e l o w ) , the N a Y and

S K - 5 0 0 r e m a i n e d h i g h l y c r y s t a l l i n e . T h e degree of c r y s t a l l i n i t y as

o b s e r v e d b y J . F . C h a r n e l l at the M o b i l O i l C o . e q u a l l e d the m a x i m u m f o u n d there f o r other samples of e a c h zeolite. N e i t h e r s a m p l e c o n t a i n e d a p p r e c i a b l e a m o r p h o u s matter (26).

T h e b e n z e n e s o r p t i o n c a p a c i t y after

i o n exchange a n d a c t i v a t i o n of e a c h of the t y p e Y zeolites w a s essentially the same a n d e q u a l to the v a l u e r e p o r t e d f o r N a X ( 8 ) , f u r t h e r i n d i c a t i n g that these p r o c e d u r e s d i d not result i n a n y m a r k e d changes i n the c r y s t a l structure.

H o w e v e r , use of a s l i g h t l y larger m o l e c u l e s u c h as t r i e t h y l -

a m i n e w o u l d h a v e b e e n a m o r e c r i t i c a l test. T h e d i f f u s i o n characteristics of a h y d r o c a r b o n c a n be affected m a r k e d l y b y traces of i m p u r i t i e s , so c o n s i d e r a b l e care w a s d e v o t e d t o w a r d s o b t a i n i n g a n d m a i n t a i n i n g materials of highest a v a i l a b l e p u r i t y .

Several

sources of s u p p l y w e r e e x a m i n e d for each m a t e r i a l , a n d the a v a i l a b i l i t y of a p a r t i c u l a r c o m p o u n d i n h i g h p u r i t y w a s a c r i t e r i o n i n the c h o i c e of diffusants. E a c h h y d r o c a r b o n was e x a m i n e d f o r i m p u r i t i e s b y gas c h r o m a t o g r a p h y a n d t h e n stored i n the d a r k over a m i x t u r e of f r e s h l y a c t i v a t e d 4 A a n d 1 3 X zeolite pellets. T h e b e n z e n e , f r o m F i s c h e r , w a s t h i o phene-free

a n d 99.944 m o l e %

p u r e , the o n l y m e a s u r a b l e

impurities

b e i n g m e t h y l c y c l o p e n t a n e a n d a n u n k n o w n c o m p o u n d of h i g h e r m o l e c ular weight. T h e cumene was from Matheson, C o l e m a n a n d B e l l and w a s 9 9 . 9 5 3 % p u r e , w i t h the o n l y m e a s u r a b l e i m p u r i t y b e i n g a trace of benzene.

B o t h s u b s t i t u t e d naphthalenes w e r e f r o m C o l u m b i a O r g a n i c

C h e m i c a l s C o . T h e 2 - e t h y l n a p h t h a l e n e ( 2 - E N ) was 99.92 m o l e %

pure

a n d c o n t a i n e d o n l y 1 i m p u r i t y a p p e a r i n g before the 2 - E N peak a n d a second i m p u r i t y some distance b e y o n d the 2 - E N peak.

The

1-methyl-

n a p h t h a l e n e ( 1 - M N ) was 99.39 m o l e % p u r e a n d c o n t a i n e d 3 i m p u r i t i e s a p p e a r i n g before the 1 - M N p e a k a n d 1 a p p e a r i n g after it. N o a t t e m p t w a s m a d e to i d e n t i f y these i m p u r i t i e s . Apparatus and Procedure. T h e zeolite i n the f o r m of single crystals was a c t i v a t e d to r e m o v e w a t e r , a n d also a m m o n i a i n the case of the S K 500 a n d N H Y , f r o m the p o r e structure. T h e a c t i v a t i o n w a s c a r r i e d o u t b y s l o w l y h e a t i n g the zeolite s p r e a d out i n a l a y e r a b o u t 5 m m t h i c k to 5 0 0 ° C at 0.5 ° C p e r m i n u t e either i n a v a c u u m or i n a stream of p r e d r i e d air ( t o t a l a c t i v a t i o n t i m e = 18 h r ) . T h e S K - 5 0 0 w a s a l w a y s a c t i v a t e d i n a n a i r stream, whereas a l l other zeolites w e r e a c t i v a t e d i n a v a c u u m . T h e N H Y thus a c t i v a t e d is c o n v e r t e d to a n H Y f o r m . A f t e r a c t i v a t i o n , 4

4


196

M O L E C U L A R

SIEVE

ZEOLITES

II

the zeolite w a s saturated f r o m t h e v a p o r phase w i t h t h e h y d r o c a r b o n w h o s e d e s o r p t i v e d i f f u s i o n rate w a s to b e m e a s u r e d , s a t u r a t i o n c o n d i t i o n s b e i n g s u c h that essentially c o m p l e t e s a t u r a t i o n w a s a c h i e v e d . W i t h b e n z e n e a n d c u m e n e , the s a t u r a t i o n w a s c a r r i e d out at r o o m t e m p e r a t u r e , whereas w i t h 1 - m e t h y l a n d 2 - e t h y l n a p h t h a l e n e , the t e m p e r a t u r e w a s 46 ° C because of t h e i r l o w v a p o r pressure. E a c h d i f f u s i o n r u n w a s c a r r i e d out i n a n a p p a r a t u s c o n s i s t i n g of a s t i r r e d , 5 0 0 - m l M o r t o n flask i n a constant t e m p e r a t u r e b a t h ( ± 0 . 2 ° C ) . A t zero t i m e , the zeolite saturated w i t h the d e s i r e d h y d r o c a r b o n w a s p l a c e d i n the s t i r r e d flask w h i c h c o n t a i n e d a k n o w n q u a n t i t y of a s e c o n d , l i q u i d h y d r o c a r b o n . S a m p l e s of a b o u t 1 m l w e r e r e m o v e d f r o m the s t i r r e d flask at p r e d e t e r m i n e d t i m e intervals b y a h y p o d e r m i c needle a n d s y r i n g e , a n d the zeolite w a s i m m e d i a t e l y r e m o v e d f r o m the m i x t u r e b y f o r c i n g it t h r o u g h a M i l l i p o r e filtering u n i t . P r e c a u t i o n s w e r e t a k e n to ensure that the s a m p l e w i t h d r a w n w a s representative of the m i x t u r e i n the flask (14). T h e c o m p o s i t i o n of the h y d r o c a r b o n phase w a s d e t e r m i n e d b y gas chromatography. T h e d e s o r p t i v e d i f f u s i o n process w a s a s s u m e d to f o l l o w F i c k ' s s e c o n d l a w i n s p h e r i c a l geometry. pore

structure,

D i f f u s i o n c a n o c c u r i n 3 d i m e n s i o n s i n this

a n d electron

micrographs

w e r e n e a r l y s p h e r i c a l i n shape.

showed that

the

particles

T h e e x p e r i m e n t a l results w e r e fitted to

the s o l u t i o n of the d i f f u s i o n e q u a t i o n f o r the p r o p e r i n i t i a l a n d b o u n d a r y c o n d i t i o n s (4, 10, 12, 14, 20),

a n d p o i n t values of D

e f f

, the effective d i f f u

s i o n coefficient, w e r e d e t e r m i n e d for the i n i t i a l p o r t i o n of the d a t a a n d at 25, 60, a n d 7 5 % K a t z e r (14). 75%

of the a p p r o a c h to e q u i l i b r i u m . D e t a i l s are g i v e n b y

T h e ratio of the v a l u e at 2 5 %

of e q u i l i b r i u m to that at

gives a measure of the degree of d e p a r t u r e f r o m F i c k ' s l a w .

The

t y p e Y zeolite particles h a d a rather n a r r o w p a r t i c l e size d i s t r i b u t i o n , w i t h three-fourths of the particles f a l l i n g i n the r a n g e of p a r t i c l e d i a m e t e r of 0.65 to 1.25 m i c r o n s . T h e a r i t h m e t i c m e a n r a d i u s of 0.55 m i c r o n w a s u s e d for the characteristic

d i f f u s i o n l e n g t h i n the d i f f u s i o n e q u a t i o n .

T h e r e w a s n o d i s c e r n i b l e difference b e t w e e n the p a r t i c l e size d i s t r i b u t i o n of the N a Y a n d that of the S K - 5 0 0 . Results

and

Discussion

C o u n t e r d i f f u s i o n of c u m e n e a n d 1 - M N o c c u r r e d r e a d i l y i n t y p e Y zeolite, as s h o w n b y several studies.

T h e 1 - M N is selectively a d s o r b e d

r e l a t i v e to c u m e n e ; thus, w h e n the z e o l i t e w a s i n i t i a l l y saturated c u m e n e a n d p l a c e d i n 1 - M N , essentially 1 0 0 %

with

of the c u m e n e d i f f u s e d

o u t ; b u t w h e n t h e zeolite was saturated w i t h 1 - M N a n d p l a c e d i n c u m e n e , o n l y a b o u t 7 4 % of the 1 - M N d i f f u s e d out. T h e same e n d p o i n t w a s also r e a c h e d w h e n S K - 5 0 0 saturated w i t h c u m e n e w a s p l a c e d i n a m i x t u r e of 1 - M N a n d c u m e n e i n the p r o p e r r a t i o . T h i s selective a d s o r p t i o n e q u i l i b r i u m v a l u e w a s essentially i n d e p e n d e n t of t e m p e r a t u r e a n d , except f o r the c e r i u m f o r m of t y p e Y , w a s i n d e p e n d e n t of the n a t u r e of t h e c a t i o n


55.

SATTERFiELD

A N D KATZER

Figure

w i t h i n the structure.

1.

Counterdiffusion

of Hydrocarbons

Diffusion rates from SK-500 cumene

197

into

T h i s b e h a v i o r is i n d i r e c t contrast to t h e system

of b e n z e n e - c u m e n e i n H - m o r d e n i t e , i n w h i c h c o u n t e r d i f f u s i o n of b e n z e n e a n d c u m e n e does not r e a d i l y o c c u r i n the H - m o r d e n i t e pores

(14).

Diffusion Coefficients. F i g u r e 1 presents d i f f u s i o n rates f r o m S K - 5 0 0 of c u m e n e i n t o b e n z e n e a n d of 1 - M N a n d 2 - E N into c u m e n e . T h e curves s h o w the r e p r o d u c i b i l i t y of the d a t a a n d the effect of t e m p e r a t u r e

and

of the nature o f t h e c o u n t e r d i f f u s i n g species u p o n the d i f f u s i o n rate. T a b l e I s u m m a r i z e s c a l c u l a t e d effective d i f f u s i v i t i e s i n S K - 5 0 0 f r o m these a n d other runs. T h e rate of d i f f u s i o n of b e n z e n e or c u m e n e f r o m S K - 5 0 0 i n t o the o p p o s i t e h y d r o c a r b o n w a s too r a p i d to a l l o w a n accurate surement to b e m a d e , a n d the q u o t e d v a l u e of 1.4 Χ 10" approximation.

11

L a r g e r m o l e c u l e s s u c h as the s u b s t i t u t e d

mea

c m / s e c is a n 2

naphthalenes

diffuse o u t of t y p e Y m u c h m o r e s l o w l y . T h e rate of d i f f u s i o n of 1 - M N at 0 ° or 2 5 ° C ( i n t o c u m e n e ) is of the r i g h t o r d e r of m a g n i t u d e to a l l o w the effects of c a t i o n exchange to be d i s c e r n e d r e a d i l y . T h e e x p e r i m e n t a l results i n d i c a t e d , as d i d also mass transfer c a l c u l a t i o n s (14),

that mass

transfer rates f r o m the p a r t i c l e surface to the b u l k l i q u i d w e r e a n i n s i g n i f i c a n t resistance a n d therefore that d i f f u s i o n w i t h i n the t y p e Y zeolite particles themselves is the r a t e - c o n t r o l l i n g step.

Z e o l i t e crystals t e n d to

a g g l o m e r a t e i n m a n y l i q u i d s , b u t d i f f u s i o n a l resistance t h r o u g h the m a c r o pores b e t w e e n t h e crystals i n a n agglomerate w a s a p p a r e n t l y n e g l i g i b l e .


198

M O L E C U L A R

T a b l e I.

SIEVE

ZEOLITES

II

Desorptive Diffusion

Compound Desorbing

Saturated Zeolite Placed In

Temp.,

Cumene 1-MN 1-MN 1- M N 2- E N 2-EN

Benzene Cumene Cumene Cumene Cumene Cumene

8 25 8 0 8 0

°C

A t t h e s t i r r i n g rate of a b o u t 600 r p m , v i s u a l observations i n c l u d i n g the rate of s e t t l i n g after s t i r r i n g i n d i c a t e d t h a t the agglomerates w e r e less t h a n 0.1 m m i n size, a n d f o r this size essentially c o m p l e t e i n t e r c h a n g e t h r o u g h the m a c r o p o r e s w i l l o c c u r i n a t i m e m u c h less t h a n t h a t r e q u i r e d f o r t h e d i f f u s i o n effects o b s e r v e d here. F u r t h e r m o r e , there seemed to be n o a p p r e c i a b l e effect of t h e n a t u r e of the h y d r o c a r b o n o n degree

of

a g g l o m e r a t i o n , a l t h o u g h i t h a d a m a r k e d effect o n t h e d i f f u s i o n rates. F i g u r e 2 presents d i f f u s i o n rates f r o m N a Y of c u m e n e i n t o b e n z e n e or i n t o 1 - M N . T h e s e a n d other results w e r e u s e d to c a l c u l a t e the d i f f u s i o n coefficients r e p o r t e d i n T a b l e II.

Figure

2.

T a b l e s I a n d I I s h o w that the

Cumene diffusion rates NaY into benzene

from


55.

SATTERFIELD A N D

Coefficients f r o m D

M /Mœ -> 0 t

199

Hydrocarbons

Cm /Sec

l

Mt/Mœ 0.25

16.6 4.31 3.09 40.3 23.8

of

SK-500

X 10 \

EFF

Counterdiffusion

KATZER

16.6 4.06 2.96 40.3 23.8

2

=

M /M«> = 0.60

M /M = 0.75

t

16.5 3.07 1.72 40.3 22.8

t

~140 14.2 2.66 1.12 39.2 21.1

D .2S/D 0

{

1.17 1.52 2.64 1.03 1.13

d e s o r p t i v e d i f f u s i o n coefficients f o r c u m e n e d i f f u s i n g i n t o b e n z e n e v a r y b y o v e r 3 orders of m a g n i t u d e b e t w e e n t h e f o r m s of t y p e Y r e p r e s e n t e d b y N a Y a n d SK-500.

L i k e w i s e , for 1 - M N diffusing into cumene, the

coefficient varies b y n e a r l y 2 orders of m a g n i t u d e b e t w e e n N a Y a n d H Y . E x c e p t f o r situations i n w h i c h t h e d i f f u s i o n rate w a s v e r y r a p i d , t h e m a x i m u m d e v i a t i o n i n t h e d i f f u s i o n coefficients w a s a b o u t 1 0 % . T h e ratio D =

0

25/^0.75

is the r a t i o o f t h e effective d i f f u s i o n coefficient at Mt/M^

0.25 t o that at 0.75. R u d l o f f (20)

has s h o w n that f o r a p a r t i c l e size

d i s t r i b u t i o n h e s t u d i e d i n w h i c h 8 5 % of t h e p a r t i c l e s f e l l w i t h i n a r a n g e i n w h i c h t h e p a r t i c l e d i a m e t e r v a r i e d b y a f a c t o r of 2 — s l i g h t l y n a r r o w e r t h a n o u r d i s t r i b u t i o n f o r t y p e Y (14)—the

r a t i o o f t h e effective d i f f u s i o n

coefficient ( t h a t o b s e r v e d f o r t h e g r o u p o f p a r t i c l e s ) at Mt/M» to that at Mi/Moo =

=

0.25

0.75 is 1.20. T h e v a l u e o f t h e r a t i o f o r these 2

Mi/Mo, p o i n t s (D0.25/D0.75) f o r S K - 5 0 0 as f o u n d here is close t o this v a l u e (except f o r the low-temperature r u n ) , w h i c h indicates that our desorp t i v e d i f f u s i o n studies w i t h S K - 5 0 0 fit t h e F i c k ' s l a w d i f f u s i o n m o d e l f a i r l y well.

T h e same is true f o r those w i t h H Y a n d C e Y , b u t t h e d e v i a t i o n s

f r o m i d e a l i t y b e c o m e q u i t e l a r g e f o r those w i t h C a Y a n d N a Y ( see T a b l e II).

I t is a p p a r e n t that F i c k ' s l a w is f o l l o w e d m u c h m o r e c l o s e l y w h e n

faster d i f f u s i o n rates are b e i n g m e a s u r e d , w h e t h e r t h e faster rate is c a u s e d b y a higher temperature or b y the particular c o m b i n a t i o n of diffusant a n d f o r m o f z e o l i t e . P l a u s i b l y , w h e n t h e size of t h e d i f f u s i n g m o l e c u l e s is v e r y close to t h e size of t h e passageways, a v e r y slight degree o f i r r e g u l a r i t y o f s t r u c t u r e o r s l i g h t v a r i a t i o n i n t h e l o c a l degree of i n t e r a c t i o n b e t w e e n d i f f u s a n t m o l e c u l e a n d zeolite c a n h a v e a m a r k e d b l o c k i n g effect. F o r those situations i n w h i c h t h e d i f f u s i o n rate is q u i t e rapid—e.g., 2 - E N d e s o r p t i o n f r o m S K - 5 0 0 at 0 ° a n d 8 ° C a n d 1 - M N at 2 5 ° C — t h e v a l u e of Mt/Mao at t h e first d a t a p o i n t w a s q u i t e h i g h , r e s u l t i n g i n f o r c e d i n a c c u racies i n D0.25 a n d therefore i n D0.25/D0.75. E f f e c t o f C a t i o n i n Z e o l i t e . T h e d i f f u s i o n coefficients f o r 1 - M N i n t o c u m e n e f o r t h e i o n - e x c h a n g e d zeolites v a r y b y a b o u t 2 orders o f m a g nitude f r o m N a Y to H Y i n the following order:


200

M O L E C U L A R

N a Y < C a Y < SK-500 < CeY
0 t

0.126 1.25 4.73 0.489 2.95 0.360 1.84 10.7 20.0

X 10 \

Mt/M* 0.25 0.101 0.983 4.68 0.392 2.91 0.242 1.32 10.7 20.0

l

=

Cm /Sec 2

Mt/M0.60

=

0.0553 0.374 3.39 0.075 2.22 0.105 0.718 9.98 17.1

M /M oo 0.75 t

=

0.0406 0.289 2.31 0.016 1.80 0.079 0.608 8.13 10.6

DQ.

2δ/Α)·75

2.49 3.40 1.98 24.2 1.62 3.07 3.04 1.32 1.87


202

M O L E C U L A R

SIEVE

ZEOLITES

II

not a significant v a r i a b l e . D i f f u s i o n coefficients v a r i e d b y o n l y 2 0 % f o r 1 - M N diffusing f r o m SK-500 into cumene w h e n it was activated accord i n g to the 2 different p r o c e d u r e s a n d saturated at 1 0 0 ° rather t h a n 46 ° C . T h e cations i n t h e t y p e Y zeolite d o n o t affect g r e a t l y the effective p o r e d i a m e t e r as d e t e r m i n e d b y a d s o r p t i o n capacities ( 9 ) , b u t d i f f u s i o n rates are p r o b a b l y e x t r e m e l y sensitive to s l i g h t v a r i a t i o n s i n structure w h e n the d i f f u s i n g m o l e c u l e is n e a r l y t h e same size as t h e m i n i m u m p o r e aperture. Effect of N a t u r e of Diffusing Molecule. T h e d a t a i n T a b l e I also a l l o w c o m p a r i s o n to b e m a d e of t h e effect of t h e n a t u r e of the d i f f u s i n g m o l e c u l e o n t h e i n t r a c r y s t a l l i n e d i f f u s i o n coefficient.

Benzene

diffuses

f r o m S K - 5 0 0 i n t o c u m e n e as r a p i d l y as does c u m e n e i n t o benzene.

The

l a r g e variations i n t h e d i f f u s i o n coefficient f o r b e n z e n e , 2 - E N , a n d 1 - M N , e a c h i n t o c u m e n e , seem to b e c a u s e d b y the v a r i a t i o n i n the c r i t i c a l d i a m eter of the d e s o r b i n g m o l e c u l e .

T h e calculated value for benzene a n d

c u m e n e is 6.29 A , that of 2 - E N is 6.96 A , a n d that of 1 - M N is 7.70 A b a s e d o n b o n d lengths, b o n d angles, a n d i o n i c r a d i i .

(14)

O t h e r m e t h o d s of

c a l c u l a t i o n w i l l give s l i g h t l y different values, b u t the o r d e r r e m a i n s the same.

H o w e v e r , some factors other t h a n c r i t i c a l d i a m e t e r seem to b e

involved.

I n S K - 5 0 0 at 8 ° C , c u m e n e diffuses i n t o b e n z e n e m u c h m o r e

r a p i d l y t h a n does 1 - M N i n t o c u m e n e , y e t i n N a Y at 2 5 ° C

(or 8 ° C )

c u m e n e diffuses into b e n z e n e m o r e s l o w l y t h a n does 1 - M N i n t o c u m e n e at 2 5 ° C ( o r e v e n 0 ° C ) (see

T a b l e s I a n d I I ) . S m a l l amounts of i m p u r i

ties i n the 1 - M N m a y interact i n a different f a s h i o n w i t h different forms o f Y sieve to g i v e rise to v a r i o u s b l o c k i n g effects. Activation Energies. T a b l e I I I lists a c t i v a t i o n energies f r o m A r r h e n i u s plots of values of D =

e f f

c a l c u l a t e d f r o m t h e i n i t i a l rate o r at

M /M t

x

0.60. T h e c o n s i d e r a b l e d e v i a t i o n s f r o m F i c k ' s l a w that exist i n some

cases are reflected i n a c o n s i d e r a b l e v a r i a t i o n i n c a l c u l a t e d a c t i v a t i o n e n e r g y f o r different p o r t i o n s of the d e s o r p t i o n process, so o n e m u s t b e cautious i n i n t e r p r e t i n g t h e values o b t a i n e d . Substances s o r b e d i n zeolites e x h i b i t s o m e w h a t l i q u i d - l i k e p r o p e r t i e s , b u t i t is a p p a r e n t that t h e a c t i v a t i o n energies here are m u c h h i g h e r t h a n those f o r b u l k l i q u i d - p h a s e diffusion.

F u r t h e r , the highest a c t i v a t i o n energies are f o u n d w i t h t h e

systems h a v i n g t h e lowest d i f f u s i o n rates, w h i c h w o u l d b e e x p e c t e d w i t h a h i g h l y r e s t r i c t e d f o r m of d i f f u s i o n . T u a n g et al

(24)

have reported

t h e heat of a d s o r p t i o n of c u m e n e v a p o r o n N a Y to b e 19.3 k c a l / m o l e , w h i c h indicates a value of about a c t i v a t i o n energy

11 k c a l / m o l e f o r l i q u i d .

f o r d e s o r p t i v e d i f f u s i o n of c u m e n e

S i n c e the

is c o n s i d e r a b l y

greater t h a n the heat of a d s o r p t i o n , c o r r e c t e d to l i q u i d - p h a s e c o n d i t i o n s , this a g a i n suggests that t h e a c t i v a t i o n e n e r g y o b s e r v e d reflects p r i m a r i l y a d i f f u s i o n process rather t h a n one of d e s o r p t i o n .


55.

SATTERFiELD

A N D KATZER

Table III.

Counterdiffusion

of Hydrocarbons

Activation Energies Activation

Zeolite

Diffusing

SK-500 SK-500 NaY NaY

203

System

1 - M N i n t o cumene 2 - E N i n t o cumene C u m e n e i n t o benzene 1 - M N i n t o cumene

Other Comments. C h e n g ( I I )

Energy,

M\lMoo-Initial 11.3

Κcal/Mole

M /M*> t

1.5

db

=

0.60

15.0 ± 1.5 11.3 19.8 19.4

9.95 17.4 13.8

has r e c e n t l y s h o w n that the rate of

s o r p t i o n of p u r e , l i q u i d c u m e n e i n t o f r e s h l y a c t i v a t e d N a Y is extremely r a p i d e v e n at —15 ° C .

S o r p t i o n e q u i l i b r i u m w a s essentially a c h i e v e d b y

C h e n g i n less t h a n 30 seconds at —15 ° C , w h e r e a s i n this s t u d y m o r e t h a n 1 w e e k w a s r e q u i r e d f o r d e s o r p t i v e c o u n t e r d i f f u s i o n to r e a c h e q u i l i b r i u m ( c u m e n e d e s o r b i n g f r o m N a Y i n t o b e n z e n e at 8 ° C ) w h i c h is e q u i v a l e n t to a ratio of the d i f f u s i o n coefficient f o r s o r p t i o n to that f o r the d e s o r p t i v e c o u n t e r d i f f u s i o n of c u m e n e greater t h a n 2000. T h i s v a l u e is m u c h larger t h a n the r a t i o of the c a l c u l a t e d d i f f u s i o n coefficient f o r the a d s o r p t i o n of w a t e r to that f o r the s e l f - d i f f u s i o n of w a t e r i n c h a b a z i t e , r e p o r t e d b y Barrer and Fender (5)

to be 38, a n d it therefore appears t h a t t h e r e is

no r e l a t i o n b e t w e e n the rate of a d s o r p t i o n a n d that of c o u n t e r d i f f u s i o n . T h i s large r e d u c t i o n i n the rate f r o m p u r e s o r p t i o n to c o u n t e r d i f f u s i o n w a s also f o u n d f o r 1 - M N a n d appears to be the result of large tions b e t w e e n the c o u n t e r d i f f u s i n g molecules.

interac

T h i s h y p o t h e s i s is f u r t h e r

s u p p o r t e d b y t h e f a c t that the rate at w h i c h the p r e a d s o r b e d h y d r o c a r b o n desorbs is affected b y the n a t u r e of the species d i f f u s i n g i n t o the

pore

structure ( c o u n t e r d i f f u s i n g to i t ) . T h i s is c l e a r l y s h o w n b y t h e 2 n d a n d 4 t h entries i n T a b l e I I , i n w h i c h a l l c o n d i t i o n s w e r e t h e same except f o r the

hydrocarbon

placed.

i n t o w h i c h the

z e o l i t e saturated

with

cumene

T h e same effect o c c u r r e d w i t h c u m e n e d i f f u s i n g f r o m

T h i s b e h a v i o r is c o n t r a r y to K n u d s e n d i f f u s i o n , i n w h i c h e a c h

was

SK-500. flux

is

independent.

It is also different f r o m n o r m a l surface d i f f u s i o n , for w h i c h

W h a n g (25)

has s h o w n that the flux i n one d i r e c t i o n is i n d e p e n d e n t of

the flux i n the o p p o s i t e d i r e c t i o n f o r s i m p l e m o l e c u l e s surface d i f f u s i n g on 9 6 %

s i l i c a glass.

Conclusions T y p e Y zeolite has a m u c h m o r e o p e n p o r e structure t h a n most other zeolites, b u t c o u n t e r d i f f u s i o n t h r o u g h its p o r e structure of s m a l l a r o m a t i c hydrocarbons can still be quite slow a n d depends strongly u p o n the na ture of cations present.

C o u n t e r d i f f u s i o n i n the t y p e Y z e o l i t e

appears

to b e m o d e l e d best as d i f f u s i o n o v e r p e r i o d i c h i g h energy barriers w h i c h are the p o r e apertures j o i n i n g t h e supercages.

T h e a c t i v a t i o n energy is


204

MOLECULAR SIEVE ZEOLITES

II

t h a t r e q u i r e d f o r a m o l e c u l e to j u m p t h r o u g h the p o r e a p e r t u r e f r o m one supercage

to

another.

The

presence

of cations

n e a r these

pore

apertures increases the resistance t o d i f f u s i o n t h r o u g h t h e m , i n c r e a s i n g t h e h e i g h t of t h e e n e r g y b a r r i e r a n d r e s u l t i n g i n s l o w e r c o u n t e r d i f f u s i o n rates a n d h i g h e r a c t i v a t i o n energies.

T h e n a t u r e of the species d i f f u s i n g

i n one d i r e c t i o n g r e a t l y affects the flux of the species c o u n t e r d i f f u s i n g to it.

T h e i n i t i a l presence

of m o l e c u l e s w i t h i n the z e o l i t e p o r e

structure

i m p e d e s the d i f f u s i o n r a t e of a s e c o n d substance b y orders of m a g n i t u d e b e l o w those o b s e r v e d w h e n t h e p o r e s t r u c t u r e is i n i t i a l l y e m p t y . T h e r a m i f i c a t i o n s of these

findings

are m a n y .

Unlike Knudsen di-

f u s i o n , t h e rate of d i f f u s i o n i n one d i r e c t i o n is affected m a r k e d l y b y the opposite

flux.

A d s o r p t i o n or d e s o r p t i o n m e a s u r e m e n t s

cannot be

used

to a p p r o x i m a t e c o u n t e r d i f f u s i o n rates; these m u s t b e d e t e r m i n e d i n d e pendently.

T h e y are a f u n c t i o n of t h e n a t u r e of the z e o l i t e , t h e t y p e

o f c a t i o n w i t h i n t h e p o r e s t r u c t u r e , a n d the n a t u r e of the c o u n t e r d i f f u s i n g species. Acknowledgment The

authors

acknowledge

the

financial

support

of the N a t i o n a l

S c i e n c e F o u n d a t i o n u n d e r G r a n t G K - 1 7 0 7 a n d t h a t of the H e r t z F o u n d a t i o n to J R K . L i n d e D i v i s i o n of the U n i o n C a r b i d e C o r p . s u p p l i e d the z e o l i t e , a n d J . C h a r n e l l of M o b i l O i l C o r p . p e r f o r m e d the x - r a y d i f f r a c t i o n studies r e p o r t e d . Nomenclature d i f f u s i o n coefficient at M /M t

Deff 2-EN

x

= effective d i f f u s i o n coefficient, = 2-ethylnaphthalene

=

A

cm /sec 2

= g r a m - m o l e s of m a t e r i a l d i f f u s e d o u t of z e o l i t e at t i m e t

M

t

Moo = g r a m - m o l e s of m a t e r i a l d i f f u s e d o u t of z e o l i t e after i n f i n i t e t i m e 1-MN

= 1-methylnaphthalene

t

= t i m e , sec

Literature (1) (2) (3)

Cited

Aleksandrov, G. G., Larionov, O . G , Chmutov, Κ. V., Russ. J. Phys. Chem. 1967, 41, 805. Aleksandrov, G . G., Larionov, O . G., Chmutov, Κ. V., Yudilevich, M . D., Russ. J. Phys. Chem. 1967, 41, 807. Aleksandrov, G. G., Larionov, O . G., Chmutov, Κ. V., Russ. J. Phys. Chem. 1967, 41, 1107.


55. SATTERFIELD AND KATZER Counterdiffusiou of Hydrocarbons 205 (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) RECEIVED

Barrer, R. M . , ADVAN. CHEM. SER. 1971, 102, 1. Barrer, R. M . , Fender, B. E . F . , J. Phys. Chem. Solids 1961, 21 (112), 1. Barry, T. I., Lay, L . Α., J. Phys. Chem. Solids 1968, 29, 1395. Barry, T. I., Lay, L. Α., Nature 1965, 208, 312. Breck, D . W., J. Chem. Educ. 1964, 41, 678. Breck, D. W., Flanigen, Ε . M . , "Molecular Sieves," p. 47, Society of Chemical Industry, London, 1968. Carman, P. C., Haul, R. Α., Proc. Roy. Soc., Ser. A 1954, 222, 109. Cheng, C . S., M.I.T., Cambridge, Mass., personal communication, 1969. Crank, J., "The Mathematics of Diffusion," Oxford University Press, Lon don, 1956. Eulenberger, G. R., Shoemaker, G. P., Keil, J. G., J. Phys. Chem. 1967, 71, 1812. Katzer, J. R., Ph.D. Thesis, M.I.T., Cambridge, Mass., 1969. Olson, D. H . , J. Phys. Chem. 1968, 72, 4366. Olson, D. H . , Dempsey, E., J. Catalysis 1969, 13, 221. Olson, D . H . , Kokotailo, G. T., Charnell, J. F . , J. Colloid Interface Sci. 1968, 28, 305. Pickert, P. E . , Rabo, J. Α., Dempsey, E . , Shoemaker, V., Proc. Intern. Congr. Catalysis, 3rd, Amsterdam, 1964 1965, 1, 714. Roberts, P. V., York, R., Ind. Eng. Chem., Process Design Develop. 1967, 6, 516. Rudloff, W . K., Ph.D. Thesis, I.I.T., Chicago, Ill., 1965. Satterfield, C . N . , "Mass Transfer in Heterogeneous Catalysis," M.I.T. Press, Cambridge, Mass., 1970. Sherry, H . S., J. Colloid Interface Sci. 1968, 28, 288. Smith, J. V., Bennett, J. M . , Flanigen, Ε. M., Nature 1967, 215, 241. Tuang, Ho Chi, Romanovskii, Β. V., Topchieva, Κ. V., Piguzova, L . I., Kinetics Catalysis 1967, 8, 594. Whang, Η. Y., Sc.D. Thesis, M.I.T., Cambridge, Mass., 1961. Wise, J. J., Mobil Oil Corp., Paulsboro, N . J., personal communication, 1968.

February 13, 1970.

Discussion L . V . C . Rees ( I m p e r i a l C o l l e g e , L o n d o n ) : H a v e y o u s t u d i e d c o u n t e r d i f f u s i o n i n X w i t h exchange of N a b y C a ? +

2 +

If not, c a n y o u give a n

e x p l a n a t i o n f o r the r e d u c t i o n i n t h e effective w i n d o w size i n C a X c o m p a r e d w i t h t h a t of N a X ? J . R. Katzer: N o , w e h a v e n o t s t u d i e d c o u n t e r d i f f u s i o n i n t h e t y p e X zeolite, b u t I a m n o w i n the process of d o i n g so. T h e effect appears to b e p r o d u c e d b y t h e cations w i t h i n the p o r e structure a n d resting n e a r the p o r e apertures.

Olson (Olson, D . H . , /.

Phys. Chem.

1968, 72, 4366)

i n d i c a t e d t h a t s t r u c t u r a l variations o c c u r r e d i n the C a X f o r m w h i c h care f u l e x a m i n a t i o n shows c o u l d r e d u c e the p o r e aperture.

H o w e v e r , h e has

r e c e n t l y i n d i c a t e d ( O l s o n , D . H . , p e r s o n a l c o m m u n i c a t i o n , 1970)

that

the p o r e aperture remains essentially the same u p o n e x c h a n g e f r o m the


206

M O L E C U L A R

SIEVE

ZEOLITES

II

s o d i u m to the c a l c i u m f o r m . T h e effect m u s t therefore be the result of the cations w i t h i n the structure b l o c k i n g the p o r e .

N o t e that there are

m o r e c a l c i u m cations i n the X t h a n i n the Y zeolite because of the l o w e r S i / A l ratio. N i v i k o v a et al. (Kolloidn.

Zh. 1967,

29, 573) have s h o w n that

d i f f u s i o n i n C a X is s l o w e r t h a n i n N a X . Ο. M . Fuller ( M c G i l l U n i v e r s i t y , M o n t r e a l , Q u e b e c , C a n a d a ) : W h a t w e r e the m e t h o d s y o u u s e d to d e t e r m i n e that the resistance to mass transport b e t w e e n the c r y s t a l a n d the fluid w a s n e g l i g i b l e ? J. R. Katzer: F i r s t v a r i a t i o n s i n s t i r r i n g s p e e d h a d no effect u p o n d e s o r p t i o n rate, b u t this w o u l d p r o b a b l y not h a v e a n effect f o r particles this s m a l l unless t h e y a g g l o m e r a t e d

seriously.

Observation

indicated

that t h e y d i d not d o so. T h e l a r g e s p a n of rates o b s e r v e d , the v e r y h i g h a c t i v a t i o n energies

(e.g., 1 0 - 2 0

k c a l / m o l e ) , a n d the fact that

system

changes s u c h as c h a n g i n g the zeolite c a t i o n f o r m or t h e h y d r o c a r b o n t y p e p r o d u c e d m a r k e d changes i n the d i f f u s i o n rate o b s e r v e d a l l i n d i c a t e d that m a s s - t r a n s f e r l i m i t a t i o n s b e t w e e n the p a r t i c l e surfaces a n d the b u l k l i q u i d w e r e not r a t e - l i m i t i n g .

Furthermore, mass-transfer

calculations

a s s u m i n g that the particles f o l l o w e d the streamlines of flow s h o w e d mass transfer

f r o m the p a r t i c l e surfaces

to the b u l k l i q u i d w a s n o t

rate-

l i m i t i n g , the c a l c u l a t e d rate of mass transfer b e i n g greater t h a n 1000 times the fastest rate o b s e r v e d . P. E . Eberly, Jr.

(Esso Research

Laboratories, Baton Rouge, L a .

70821 ) : I n e v a l u a t i n g d i f f u s i v i t i e s , one f r e q u e n t l y finds t h a t the p a r a m e ters D a n d M

interact.

x

of D.

T h u s , b y c h a n g i n g M » , one c a n affect the v a l u e

I n y o u r results c o m p a r i n g the v a r i o u s f o r m s of Y zeolite, c o u l d

changes i n M

œ

h a v e affected the d i f f u s i v i t y values?

J. R. Katzer: T h e a n s w e r is no. T h e a m o u n t of p r e a d s o r b e d h y d r o c a r b o n w h i c h d e s o r b e d f r o m the p r e s a t u r a t e d z e o l i t e at e q u i l i b r i u m , M » , w a s constant at a v a l u e of 73 ±

2%

of the i n i t i a l l y p r e a d s o r b e d h y d r o

c a r b o n i n a l l cases except f o r the c e r i u m f o r m , f o r w h i c h the v a l u e w a s 6 4 % of the i n i t i a l l y p r e a d s o r b e d m a t e r i a l . T h u s , the changes i n not large e n o u g h to p r o d u c e significant effects i n D the M

x

were

value. Furthermore,

eti

u s e d w a s the true e q u i l i b r i u m v a l u e as 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 a l l cases, a n d thus the v a l u e of D

ett

f o u n d s h o u l d b e the same as that

w h i c h s h o u l d also b e f o u n d i f the r u n c o n d i t i o n s w e r e c h a n g e d f o r a g i v e n zeolite s u c h that at e q u i l i b r i u m the v a l u e of M

x

was indeed changed

f r o m its p r e v i o u s v a l u e . R. M . Barrer ( I m p e r i a l C o l l e g e , L o n d o n ) : I n c o u n t e r d i f f u s i o n sys tems s u c h as y o u h a v e e x a m i n e d , the v o l u m e flow of C o m p o n e n t 1 i n t o the crystals is not necessarily e q u a l to the v o l u m e flow of t h e d i s p l a c e d C o m p o n e n t 2 i n t o the e x t e r n a l s o l u t i o n , because the degree of the crystals m a y change w i t h c o m p o s i t i o n of the s o r b e d

fluid.

filling

of

Thus, for


55.

SATTERFiELD

A N D KATZER

Counterdiffusion

of Hydrocarbons

207

p a r t i a l l y filled crystals, b o t h flows s h o u l d b e m e a s u r e d , a n d t w o d i f f u s i o n coefficients are i n v o l v e d . J. R. Katzer: W e w e r e c o n c e r n e d w i t h the q u e s t i o n of e q u a l v o l u m e rates of flow d u r i n g c o u n t e r d i f f u s i o n b u t d i d not attempt to measure

the

t w o flow rates; this w o u l d b e e x t r e m e l y d i f f i c u l t i n a f r e e l y a n d q u i t e r a p i d l y c o u n t e r d i f f u s i n g system as was t h e case here.

Note, however,

that the t o t a l v o l u m e flow i n e a c h d i r e c t i o n s h o u l d h a v e b e e n essentially the same unless t h e r e w e r e some strange p h e n o m e n a w h i c h o c c u r r e d at i n t e r m e d i a t e c o m p o s i t i o n because it w a s f o u n d that the v o l u m e of p u r e c o m p o u n d s o r b e d at s a t u r a t i o n b a s e d o n the l i q u i d d e n s i t y at s o r p t i o n t e m p e r a t u r e was essentially the same for a l l c o m p o n e n t s s t u d i e d here. R. B. Anderson ( M c M a s t e r U n i v e r s i t y , H a m i l t o n , O n t a r i o , C a n a d a ) : D o e s the d i f f u s i v i t y v a r y w i t h " d r i v i n g f o r c e ? "

F o r e x a m p l e , suppose a

sieve filled w i t h a n a r o m a t i c is p l a c e d i n a large a m o u n t of n o n a r o m a t i c hydrocarbon? J. R. Katzer: I d o not b e l i e v e that the d i f f u s i v i t y w o u l d v a r y w i t h " d r i v i n g f o r c e " to a n y l a r g e r extent t h a n the v a r i a t i o n w i t h Mt/M^ w e o b s e r v e d i n this w o r k a n d n o t e d i n the p a p e r .

which

T h i s c o u l d easily b e

c h e c k e d b y starting w i t h the zeolite p o r e structure e q u i l i b r a t e d w i t h a m i x t u r e of the t w o h y d r o c a r b o n s i n v o l v e d a n d t h e n m e a s u r i n g the a p p r o a c h to e q u i l i b r i u m f o r this system as w a s d o n e i n the a b o v e w o r k . It w o u l d not be possible to c h a n g e the h y d r o c a r b o n t y p e o n the outside a n d effect o n l y a c h a n g e i n the d r i v i n g force because, as w e h a v e c l e a r l y s h o w n i n this p a p e r , the rate of d i f f u s i o n o u t of the structure is h i g h l y d e p e n d e n t u p o n the nature of the h y d r o c a r b o n d i f f u s i n g i n t o the zeolite. P l a c i n g the saturated zeolite i n t o a n o n a r o m a t i c h y d r o c a r b o n changes the n a t u r e of the h y d r o c a r b o n d i f f u s i n g i n t o the structure i n a d d i t i o n to p o s s i b l y a l t e r i n g the d r i v i n g force. D . M . Ruthven a n d K . F. Loughlin ( U n i v e r s i t y of N e w B r u n s w i c k , F r e d e r i c t o n , Ν. B . , C a n a d a ) : W e w o u l d l i k e to r e - e m p h a s i z e the signifi cance of the effects of c r y s t a l size d i s t r i b u t i o n i n the analysis of s o r p t i o n curves.

T h e v a r i a t i o n s i n d i f f u s i v i t y w h i c h are r e p o r t e d i n this p a p e r

s h o w p r e c i s e l y the t r e n d w h i c h is to be expected i f the effect of c r y s t a l size d i s t r i b u t i o n is c o n s i d e r e d . W e feel sure that i f c r y s t a l size d i s t r i b u t i o n measurements are a v a i l a b l e , it s h o u l d b e possible t o o b t a i n the cor rect values of d i f f u s i v i t y u s i n g the t y p e of analysis w h i c h w e p r e s e n t e d i n o u r c o m m e n t o n t h e p a p e r of K o n d i s a n d D r a n o f f . J. R. Katzer: Y o u are correct i n y o u r statement; as w e i n d i c a t e d i n o u r p a p e r , the crystallite size d i s t r i b u t i o n is r e s p o n s i b l e f o r the m a j o r i t y of the v a r i a t i o n f o u n d f o r S K - 5 0 0 u n d e r most c o n d i t i o n s . H o w e v e r , y o u r a p p r o a c h w o u l d not e l i m i n a t e a l l of the v a r i a t i o n f o u n d i n the

systems

s t u d i e d here a n d is a s m a l l c o r r e c t i o n r e l a t i v e to the f a r m o r e significant v a r i a t i o n s f o u n d i n the systems s t u d i e d .


208

M O L E C U L A R

SIEVE

ZEOLITES

II

J. D . Sherman ( 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 ) : T h e heat of a d s o r p t i o n ( A i /

a d s

) of interest is f o r h i g h degrees of filling a n d is

for the difference b e t w e e n the AH's of different m o l e c u l e s at h i g h degrees of

filling.

I n t e r p r e t a t i o n c a n b e f a c i l i t a t e d b y c o m p a r i s o n of E i f f a n d d

Af/ads—i e., the a c t i v a t i o n energies f o r d i f f u s i o n a n d heats of a d s o r p t i o n for a series of p a i r s of c o u n t e r d i f f u s i n g species. J. R. Katzer: T h i s c o m m e n t is i n response to a c o m m e n t b y P a u l E b e r l y n o t i n g that the rates o b s e r v e d increase i n zeolites e x h i b i t i n g a h i g h e r heat of a d s o r p t i o n . H o w e v e r , K h u d i e r et al. (Bull. Acad. Set. Div.

Chem.

of C H 6

6

Set.

1968,

4, 694)

USSR,

f o u n d the d i f f e r e n t i a l heat of a d s o r p t i o n

o n H Y to be l o w e r t h a n that o n N a Y over the entire range.

In

response to the first p o i n t of t h e a b o v e c o m m e n t s , I d o t h i n k that heats of a d s o r p t i o n f o r l o w degrees of filling are p r o b a b l y also of i m p o r t a n c e here because a f e w m o l e c u l e s a d s o r b e d o n sites e x h i b i t i n g a h i g h heat a d s o r p t i o n ( s t r o n g a d s o r p t i o n ) c o u l d g r e a t l y affect t h e d i f f u s i o n

of

rate.

Y o u r s e c o n d c o m m e n t , h o w e v e r , is w e l l t a k e n . D . P. Roelofsen ( T e c h n o l o g i c a l U n i v e r s i t y , D e l f t , N e t h e r l a n d s ) : T h e sequence of the i n v e s t i g a t e d m o l e c u l a r sieves r e g a r d i n g i n c r e a s i n g d e s o r p t i o n velocities is r o u g h l y s i m i l a r to t h e e x p e c t e d sequence of d e c r e a s i n g a d s o r p t i o n affinities of the sieves f o r these large a r o m a t i c c o m p o u n d s . For

instance, N a Y , w i t h its exposed cations i n S

I H

sites, w o u l d b e ex

p e c t e d to a d s o r b n a p h t h a l e n e s stronger t h a n C a Y w i t h its s h i e l d e d cations i n S η positions. T h e r e f o r e , I w o u l d l i k e to suggest that the differences i n d e s o r p t i o n velocities m a y be p a r t l y e x p l a i n e d b y t h e differences i n affinities of the sieves f o r the d i f f u s i n g m o l e c u l e s . T h e s e differences c o u l d b e s t u d i e d , f o r instance, b y m e a s u r i n g the a d s o r p t i o n e q u i l i b r i a of a d i l u t e s o l u t i o n of a s u b s t i t u t e d n a p h t h a l e n e i n a saturated h y d r o c a r b o n solvent a n d the different sieves. J. R. Katzer: T h e r e s h o u l d b e n o cations i n the t y p e I I I sites y o u refer to i n the d e h y d r a t e d f o r m of either t h e N a Y or C a Y zeolite. Y o u r c o m m e n t s o n d i f f u s i o n rates b e i n g affected b y a d s o r p t i o n affinity is a v e r y v a l i d one, h o w e v e r , a n d B a r r e r has a l r e a d y n o t e d i n this conference that the stickiness of w a t e r is i m p o r t a n t to its d i f f u s i o n rate. I t h i n k that there is n o q u e s t i o n b u t w h a t s o r b a t e - z e o l i t e interactions of the n a t u r e y o u p o i n t o u t a n d others h a v e a v e r y large effect o n the d i f f u s i o n rate a n d s h o u l d be s t u d i e d i n m o r e d e t a i l .


Counterdiffusion of Liquid Hydrocarbons in Type Y Zeolites

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