Molecular Sieves—II

51 Diffusion Effects on the Catalytic Behavior of Ca, Na-Y-Zeolite J. VELEZ, Ε. A. CORNEJO, and Ε. A.

LOMBARDO

*

Downloaded by MONASH UNIV on December 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch051

Facultad de Ingenieria Quimica, Universidad del Litoral, Santa Fe, Argentina

ABSTRACT

The isomerization of both cis-2-butene and 1-butene were studied between 130° and 200°. The data obtained are diagnostic of diffusion limitations. The Wei treatment was used to calculate the effective diffusivities. The pressure dependency was studied in both the chemical and the diffusion limited re gimes. A Langmuir-Hinshelwood model applies in the first regime but not in the second. Introduction I t i s thought t h a t t h e s m a l l openings o f t h e z e o l i t e c r y s t a l s i n t r o d u c e d i f f u s i o n l i m i t a t i o n s when t h e s e s o l i d s a c t as c a t a l y s t s . As e a r l y as i n 1966 Weisz, e t a l . (1) have demonstrated t h e e f f e c t o f 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 z e o l i t e c a t a l y s i s . More o v e r , i n most h y d r o c a r b o n r e a c t i o n s c a t a l y z e d by molec u l a r s i e v e s , " r e s i d u e s " a r e formed (2-5) which f u r t h e r i m p a i r t h e movement o f r e a c t i n g m o l e c u l e s w i t h i n t h e crystal. B u t t , e t a l . (4,5) have r e c e n t l y i n v e s t i g a t e d t h e e f f e c t o f c o k i n g on t h e d i f f u s i v i t y o f r e a c t i n g molecules i n mordenites. Chutoransky and Dwyer (6) have s t u d i e d t h e l i q u i d phase i s o m e r i z a t i o n o f x y l e n e s o v e r a z e o l i t e c o n t a i n i n g c a t a l y s t a v a i l a b l e i n two d i f f e r e n t p a r t i c l e s i z e s . T h i s a l l o w e d them t o s t u d y t h e e f f e c t o f i n t r a c r y s t a l line diffusion. U s i n g t h e k i n e t i c a n a l y s i s o f Wei and P r a t e r (7 8) t h e y have shown t h a t t h e i r system was d i f f u s i o n l i m i t e d when t h e p a r t i c l e s i z e o f t h e zeo l i t e c a t a l y s t was 2 t o 4 μ. The g o a l o f t h e study r e p o r t e d h e r e i n i s t o show the r o l e o f d i f f u s i o n l i m i t a t i o n s on t h e k i n e t i c y

616 Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

51.

VELEZ

ET

AL.

617

Diffusion Effects on Catalytic Behavior

b e h a v i o r o f even a r e l a t i v e l y i n a c t i v e Y - Z e o l i t e c a t a lyst. The i s o m e r i z a t i o n o f the n-butenes p r o v i d e s an adequate model r e a c t i o n f o r t h i s purpose. Furthermore, the r e a c t i o n mechanism f o r t h i s system on s i m i l a r zeol i t e c a t a l y s t s has been t h o r o u g h l y s t u d i e d and r e p o r t e d elsewhere (3,9,10). Experimental


Reactants. The n-butenes were Matheson r e s e a r c h grade and c o n t a i n e d l e s s than 0.2% i m p u r i t y o f the other isomers. C a t a l y s t and P r e t r e a t m e n t . The s t a r t i n g m a t e r i a l was a L i n d e N a - Y - Z e o l i t e ( l o t 1280-133) whose c h e m i c a l a n a l y s i s i s g i v e n elsewhere (11). T h i s m a t e r i a l was p u r i f i e d by s u c c e s s i v e exchange w i t h sodium a c e t a t e f o l l o w e d by washing w i t h s l i g h t l y a l k a l i n e water. The c h e m i c a l a n a l y s i s showed t h a t the p u r i f i e d z e o l i t e cont a i n e d o n l y 0.02% C a . T h i s c o r r e s p o n d s t o 0.3% o f Na r e p l a c e d by C a . An a l i q u o t o f t h i s p a r e n t c a t a l y s t was back exchanged w i t h c a l c i u m a c e t a t e u n t i l 6% of t h e N a was r e p l a c e d by C a + . B e f o r e each run an a l i q u o t o f t h i s m a t e r i a l was t r e a t e d w i t h oxygen a t 1 atm and 400° f o l l o w e d by o v e r n i g h t e v a c u a t i o n t o 10" t o r r a t the same temperature. 2 +

+

2 +

+

2

5

Reactor. A 300 c c s t a t i c r e a c t o r , d e s c r i b e d elsewhere (9), c o n t a i n i n g 100 mg o f c a t a l y s t was used. The r e a c t o r temperature was e l e c t r o n i c a l l y c o n t r o l l e d to +0?5. The same a l i q u o t o f c a t a l y s t was used throughout the experiments r e p o r t e d h e r e . No m o d i f i c a t i o n i n a c t i v i t y was o b s e r v e d when the c a t a l y s t was p r e t r e a t e d as d e s c r i b e d above. Treatment o f Data. f i n e d as f o l l o w s .

The

r a t e constants are

de-

k21 1-buteneÇ

cis-2-butene (1)

trans-2-butene The d i f f u s i v i t i e s o f the t h r e e isomers were a s sumed t o be e q u a l . They were c a l c u l a t e d from the k i n e t i c d a t a u s i n g the t r e a t m e n t o f Wei (8) which i s summarily d e s c r i b e d below. A complex system o f f i r s t o r d e r r e a c t i o n s can be

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

618

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described

by t h e m a t r i x

SIEVES—Π

equation

d a / d t = Ka

(2)

where a i s t h e c o m p o s i t i o n v e c t o r and Κ i s t h e r a t e c o n s t a n t s m a t r i x . Wei and P r a t e r (7_) have shown t h a t t h i s h i g h l y c o u p l e d system c a n be c o n v e r t e d i n t o an e q u i v a l e n t u n c o u p l e d system o f s p e c i e s B-^ w i t h r a t e c o n s t a n t s Xj=0, and X and λ3 r e l a t e d t o t h e k ^ j con s t a n t s o f t h e r e a l system. 2


B-L does n o t change

B B

λ

2

λ

3

4o

2

3

(3)

^ 0

Wei has f u r t h e r shown (8) t h a t i n a system w i t h pore d i f f u s i o n l i m i t a t i o n s e q u a t i o n (4) a p p l i e s +

da/dt = K a

(4)

+

where K i s now made up o f t h e d i f f u s i o n - d i s g u i s e d rate constants. T h i s system c a n a l s o be u n c o u p l e d w i t h the new d i s g u i s e d r a t e c o n s t a n t s b e i n g Xj=0, X£, and Xt. I f the d i f f u s i v i t i e s of the r e a c t i n g species are a l l e q u a l , t h e r e i s a s i m p l e r e l a t i o n s h i p between X^ and λ Τ ι λ+ = η λ 1

η

±

1

= 3φ ' (φ ±

2

(5)

1

±

coth

φ -1)

(6)

±

(7)

iKf

where i s the e f f e c t i v e n e s s f a c t o r , φί the T h i e l e modulus, R t h e p a r t i c l e r a d i u s and D the e f f e c t i v e diffusivity. To c a l c u l a t e t h e e f f e c t i v e d i f f u s i v i t y f o r t h e n-butenes a computer program was w r i t t e n . The d a t a i n p u t c o n s i s t e d o f t h e d i f f u s i o n - d i s g u i s e d r a t e con s t a n t s and t h e e x t r a p o l a t e d r e a l r a t e c o n s t a n t s . The p a r t i c l e e q u i v a l e n t s p h e r i c a l r a d i u s was t a k e n t o be 1 m i c r o n , based on average c r y s t a l s i z e s o f s y n t h e t i c Y-Zeolites. G i v e n t h e s e d a t a t h e computer c a l c u l a t e d the v a l u e s o f X^ and X^ and then used e q u a t i o n s (5) e f


51.

VÊLEZ

E T

through


AL.

(7) t o c a l c u l a t e t h e e f f e c t i v e

619

diffusivity.

Results Temperature Dependency. Two s e r i e s o f e x p e r i ments were performed u s i n g e i t h e r 1-butene o r c i s - 2 butene as r e a c t a n t s a t temperatures r a n g i n g from 130° to 200°. I n e v e r y case 2H20/cage were added as c o catalyst. A t a l l temperatures s t u d i e d t h e r e a c t i o n s were f i r s t o r d e r . The r a t e c o n s t a n t r a t i o s ^21/^33. and k / 3 2 c a l c u l a t e d from t h e s e d a t a were p l o t t e d a g a i n s t 1000/T i n F i g . 1. These p l o t s obey t h e A r r h e n i u s law up t o a c e r t a i n c r i t i c a l temperature beyond which they show a d e f i n i t e c u r v a t u r e . The same b e h a v i o r f o r d i f f e r e n t Ca, N a - Y - Z e o l i t e c o m p o s i t i o n s was r e p o r t e d elsewhere (3,11,12). Moreover t h e c r i t i c a l temperature d e c r e a s e d as t h e c a t a l y t i c a c t i v i t y increased. S i m i l a r r e s u l t s were o b t a i n e d when npentenes were i s o m e r i z e d on t h e same c a t a l y s t s (13>) . The i n d i v i d u a l r a t e c o n s t a n t s were c a l c u l a t e d from t h e a c t i v i t y and s e l e c t i v i t y d a t a and t h e e q u i librium constants. The A r r h e n i u s p l o t f o r each r a t e c o n s t a n t a l s o showed a d e f i n i t e c u r v a t u r e a t h i g h e r temperatures, d i a g n o s t i c o f pore d i f f u s i o n l i m i t a t i o n s . The k i n e t i c d a t a were p r o c e s s e d as e x p l a i n e d i n the p r e c e e d i n g s e c t i o n t o c a l c u l a t e t h e e f f e c t i v e d i f f u s i v i t i e s a t d i f f e r e n t temperatures. The r e s u l t s a r e given i n Table I. k


1 2

TABLE I . -

E f f e c t i v e D i f f u s i v i t i e s o f n-Butenes i n Ca, N a - Y - Z e o l i t e C a l c u l a t e d Under R e a c t i o n C o n d i t i o n s (1)

D2

D3

D

e f

Temperature

cm2/sec χ Ι Ο

180 190 200

9.1 3.9 1.6

9.7 6.5 2.8

1 1

9.4 5.2 2.2

(1) The d i f f u s i v i t i e s o f t h e n-butenes were assumed t o be e q u a l . D and D^ were g i v e n by t h e computer, D f i s t h e average v a l u e . 2

P r e s s u r e Dependency. Two temperatures were chosen t o study t h e e f f e c t o f p r e s s u r e , one i n t h e c h e m i c a l regime and t h e o t h e r i n t h e i n t e r v a l when


e

620

MOLECULAR

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 e f f e c t s are

SIEVES—Π

limiting.

i ) C h e m i c a l Regime. The runs were performed a t p r e s s u r e s o f c i s - 2 - b u t e n e r a n g i n g from 50 t o 150 mm Hg a t 150°. The r a t e c o n s t a n t v a l u e s ( k i 2 + k ) 9 n i n T a b l e I I . When p l o t t e d a g a i n s t 1/P t h e shape o f the c u r v e i s c o n s i s t e n t w i t h t h e f o l l o w i n g mechanism. a

r

e

i v ç

3 2

(8) (c-2-b) ^.^Tt-2-b)


c-2-butene

t-2-butene

s

Assuming t h a t t h e r a t e d e t e r m i n i n g s t e p i s t h e s u r f a c e r e a c t i o n and t h a t the t h r e e isomers w i l l comp e t e f o r the a d s o r p t i o n s i t e s , t h e f o l l o w i n g e q u a t i o n a p p l i e s f o r the i n i t i a l d i s a p p e a r a n c e o f any o f t h e isomers and p a r t i c u l a r l y f o r the c i s - 2 - b u t e n e r =

(kf +k| )_KçPç_ Ι+ΣΚ^ 2

(9)

2

where k | j a r e t h e r a t e c o n s t a n t s f o r the s u r f a c e r e a c tion, and P. a r e the a d s o r p t i o n e q u i l i b r i u m c o n s t a n t and the p a r t i a l p r e s s u r e o f t h e i isomer, r e s p e c t i v e l y . I f the K^'s a r e assumed t o be e q u a l , t h e n r =

(kf +kf ) 2

2

K

P

c ; Ρ =ΣΡ 1+KP τ

ί

(10)

T

therefore,

k

12

+ k

32

=

k

+ k

( 12 32) — 1+KP

T

Based on t h e same model i t i s p r e d i c t e d t h a t the s e l e c t i v i t y r a t i o s s h o u l d be p r e s s u r e independent. The s e l e c t i v i t y r a t i o s shown i n T a b l e I I a r e i n f a c t f a i r l y constant.

i i ) D i f f u s i o n L i m i t e d Regime. E s s e n t i a l l y the same experiments were r e p e a t e d now a t 200°. The d a t a i s no l o n g e r c o n s i s t e n t w i t h the Langmuir-Hinshelwood model g i v e n above. T a b l e I I shows t h a t the c a t a l y t i c a c t i v i t y i n c r e a s e s with pressure, while the s e l e c t i v i t y decreases with t h i s v a r i a b l e .


51.

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ET

TABLE I I . -

Chemical


Ρ (mm

Diffusion Effects on Catalytic Behaviot

AL.

P r e s s u r e E f f e c t s on A c t i v i t y and S e l e c t i v i t y o f C i s - 2 - B u t e n e I s o m e r i z a t i o n Over Ca, N a - Y - Z e o l i t e

Regime

Hg)

(k

1 2

T=150°+0.5 +k

49 62 77 152

3 2

)xlO

3

^i

3 1

1

1

" ^" )

k

k

12/ 32

1.4 1.2 1.2 0.7

2.5 2.5 2.6 2.4

D i f f u s i o n L i m i t e d Regime Ρ (mm

Hg)

60 81 160

621

k

( i2

+ k

32^

x l

T=200°+0.5 2

1

° (min^g" )

1.3 2.0 2.3

k

k

l2/ 32 2.2 1.7 1.3

To check t h e s e r e s u l t s a s i m i l a r s e t o f e x p e r i ments was performed w i t h the p a r e n t c a t a l y s t . Due t o i t s lower a c t i v i t y h i g h e r temperatures were r e q u i r e d . Otherwise i t s b e h a v i o r was the same as r e p o r t e d here f o r t h e Ca, N a - Y - Z e o l i t e . Discussion The p l o t s o f F i g . 1 a r e d i a g n o s t i c o f d i f f u s i o n limitations. The c a l c u l a t e d e f f e c t i v e d i f f u s i v i t i e s a r e low i f compared t o the s c a r c e d a t a a v a i l a b l e f o r the d i f f u s i o n o f gases i n z e o l i t e s . No d a t a have been found f o r n-butenes d i f f u s i n g i n t o Y - Z e o l i t e s . T h e r e f o r e i n T a b l e I I I d a t a a r e shown f o r comparison o f n-butenes, p r o p y l e n e and e t h y l e n e d i f f u s i n g i n t o s m a l l er pore z e o l i t e s . I t i s e x p e c t e d t h a t n-butenes d i f f u s i n g i n Y - Z e o l i t e s w i t h l a r g e r pore openings w i l l have a d i f f u s i v i t y two o r t h r e e o r d e r s o f magnitude larger. However, the c a l c u l a t e d v a l u e o f D £ i n c l u d e d f o r comparison i n T a b l e I I I shows t h a t t h i s i s n o t the case. e



622

MOLECULAR

ΙΟΟΟ/Τ

Figure 1.

Temperature dependency of selectivity ratios


SIEVES—Π

51.

VELEZ

ET

AL.

TABLE I I I . -

Adsorbant

C H 2

C

4

H

3 6 1-C H t-2-C H c-2-C H


4

8

4

8

4

8

n-butenes

623

Diffusion Effects on Catalytic Behavior c

D i f f u s i v i t i e s of C2~ fl Mono-Olefins i n Z e o l i t e s (1)

Adsorbent

5A 5A 5A 5A 5A

Ε kcal/mol

D.10 11 200°

References

2.75 3.46 3.44 3.46 9.2

10.7 6.3 4.6 6.5 8.5

Ruthven, e t a l . (14)

Na-Y-Z

D f=

:

McGrath, B.Sc. Thesis c i t e d i n (14) Table I

e

(1) L i m i t i n g d i f f u s i v i t i e s c a l c u l a t e d from d a t a i n the l i t e r a t u r e .

given

A p o s s i b l e e x p l a n a t i o n f o r t h i s d i s c r e p a n c y might be the f o r m a t i o n o f r e s i d u e s w h i c h f u r t h e r r e d u c e the d i f f u s i o n a b i l i t y o f the n-butenes. I t i s w e l l docu mented t h a t r e s i d u e s form d u r i n g most h y d r o c a r b o n reactions. P a r t i c u l a r l y , Lombardo and H a l l (_3) have shown t h a t s m a l l amounts o f r e s i d u e s form even i n a l e s s a c t i v e c a t a l y s t ( 5 . 2 x l 0 ~ mmole/g o f N a - Y - Z e o l i t ^ . Another f a c t t h a t i s c o n s i s t e n t with t h i s p i c t u r e i s the d e c r e a s e i n D f as temperature i n c r e a s e s as shown i n T a b l e I I . B u t t , e t a l . (4,5) have r e c e n t l y r e p o r t ed the e f f e c t on D^ o f coke d e p o s i t i o n i n m o r d e n i t e s . H i s l e v e l s o f r e s i d u e s , however, were much h i g h e r than ours. A n o t h e r p o s s i b l e i n t e r f e r e n c e w i t h the movement o f the n-butene m o l e c u l e s i s the p r e s e n c e o f water added as c o - c a t a l y s t s i n c e water i s known t o m o d i f y the d i f f u s i v i t i e s o f s e v e r a l g a s e s . Tempère, e t a l . (14) have r e p o r t e d s i m i l a r r e s u l t s when s t u d y i n g 1-butene i s o m e r i z a t i o n u s i n g a s e r i e s o f d i f f e r e n t z e o l i t e c a t a l y s t s i n a f l o w system. They r e p o r t e d a wide range o f s e l e c t i v i t y v a l u e s i n g o i n g from l a r g e p o r e z e o l i t e s (X o r Y) t o the A t y p e h a v i n g s m a l l e r windows. They i n t e r p r e t e d the t e n - f o l d i n c r e a s e i n c i s / t r a n s r a t i o i n terms o f the much s m a l l e r d i f f u s i v i t y o f the c i s - 2 - b u t e n e i n the AZ e o l i t e (15). Tempère, e t a l . (14) a l s o show i n F i g . 3 o f t h e i r paper A r r h e n i u s p l o t s f o r Na-X and Y - Z e o l i t e s which became c u r v e d a t h i g h e r t e m p e r a t u r e s , as r e p o r t e d here. 2

e

f



624

MOLECULAR

SIEVES—Π

I t i s e x p e c t e d t h a t the p r e s s u r e dependency o f the r e a c t i o n c a n be a c c o u n t e d f o r by the LangmuirHinshelwood mechanism shown above s i n c e t h e same model a p p l i e s f o r t h e i s o m e r i z a t i o n o f n-butenes on o t h e r a c i d i c c a t a l y s t s (16,17). A l t h o u g h a d i f f e r e n t b e h a v i o r i s e x p e c t e d when d i f f u s i o n l i m i t a t i o n s e x i s t , the a c t i v i t y and s e l e c t i v i t y b e h a v i o r seem t o be c o n tradictory. The i n c r e a s e i n a c t i v i t y w i t h p r e s s u r e can be i n t e r p r e t e d i n terms o f the r e s u l t s r e p o r t e d by Ruthven, e t a l . (18). They have found t h a t t h e d i f f u s i v i t y increased with increasing pressure for d i f f e r e n t h y d r o c a r b o n s i n s m a l l e r pore z e o l i t e s . Based on t h e s e r e s u l t s Ruthven (19) has shown t h a t t h e e f fectiveness f a c t o r s should increase with increasing pressure. What i s a t v a r i a n c e w i t h t h i s i n t e r p r e t a t i o n , however, i s the v a r i a t i o n i n s e l e c t i v i t y w i t h pressure. I f the d i f f u s i v i t y goes up w i t h p r e s s u r e , so s h o u l d t o o t h e s e l e c t i v i t y r a t i o , b u t t h i s i s n o t the c a s e as shown i n T a b l e I I . In summary, i t has been shown t h a t d i f f u s i o n e f f e c t s are p r e s e n t even i n a system i n v o l v i n g a r e l a t i v e l y i n a c t i v e z e o l i t e and s m a l l h y d r o c a r b o n molecules. The r e a s o n f o r t h i s might be t h e f o r m a t i o n of r e s i d u e s . Perhaps the p r e s e n c e o f water c o u l d a l s o play a role. Note t h a t the most common t e s t t o d e t e c t p o r e d i f f u s i o n l i m i t a t i o n s , namely the u s e o f d i f f e r e n t p a r t i c l e s i z e s , m i g h t not be r e a d i l y a v a i l a b l e f o r some z e o l i t e s . Therefore, other a l t e r n a t i v e diagnostic t e s t s c a n be used when t r y i n g t o d e t e c t d i f f u s i o n e f f e c t s on z e o l i t e s . Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9.

Miale, J . Ν., Chen, Ν. Y. and Weisz, P. B., J. Catal. (1966), 6, 278. Venuto, P. B., Advan. Chem. Ser. (1971), 102, 260. Lombardo, Ε. Α., Hall, W. Κ., Proc. Int. Congr. Catalysis, 5th (1973), 2, 1365. Butt, J . B., Delgado-Diaz, S., Muno. W. E . , J. Catal. (1975), 37, 158. Butt, J . B., J. Catal. (1976), 41, 190. Chutoransky, P., J r . , Dwyer, F. G., Advan. Chem. Ser. (1973), 121, 540. Wei, J., Prater, C. D., Advances in Catalysis (1962), 13, 203. Wei, J., J. Catal. (1962), 1, 526. Lombardo, Ε. Α., Hall, W. Κ., AIChE J . (1971), 17, 1229.


51. VELEZ ET AL.

10. 11. 12. 13. 14. 15.


16. 17. 18. 19.


625

Lombardo, Ε. Α., Velez, J . Advan. Chem. Ser. (1973), 121, 553. Lombardo, Ε. Α., Sill, G. Α., Hall, W. K., J . Catal (1971), 22, 54. Velez, J., Lombardo, Ε. Α., Unpublished results. Lombardo, Ε. Α., Velez, J., Cornejo, E., Acta Cientifica Venezolana (1973), 2±, 160. Tempère, J . F . , Kermarec, J., Imelik, B., Bull. Soc. Chim. de France (1970), 11, 3808. Tempère, J . F . , Imelik, B., Bull. Soc. Chim. de France (1970), 12, 4227. Forni, L., Zanderighi, L., Carra, S., J . Catal. (1968), 12, 298. Ballivet, D., Barthomeuf, D., Trambouze, I . , J . Catal. (1974), 34, 423. Ruthven, D. M., Loughlin, K. F., Derrah, R. I . , Advan. Chem. Ser. (1973), 121, 330. Ruthven, D. M., J. Catal. (1972), 25, 259.


Molecular Sieves—II

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