Molecular Sieves

49 Effect of Zeolite Crystallite Size on the Selectivity Kinetics of the Heterogeneous Catalyzed Isomerization of Xylenes

Downloaded by COLUMBIA UNIV on July 29, 2012 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch049

P. CHUTORANSKY, JR. and F. G. DWYER Mobil Research and Development Corp., Research Department, Paulsboro, N. J. 08066

Using the monomolecular rate theory developed by Wei and Prater, we have analyzed the kinetics of the liquid-phase isom erization of xylene over a zeolitic catalyst. The kinetic anal ysis is presented primarily in terms of the time-independent sele tivity kinetics. With the establishment of the basic kinetics the role of intracrystalline diffusion is demonstrated by analyzing the kinetics for 2to4 μ zeolite catalyst and an essentially diffusion -free 0.2 to 0.4 μ zeolite catalyst. Values for intracrystalline diffusivities are presented, and evidence is given that the isom erization is the simple series reaction o-xylene m-xylene p-xylene. y t d v a n c e s i n k i n e t i c a n a l y s i s (1) a n d t h e h i g h a c t i v i t y zeolite c a t a l y s t s provide powerful tools to investigate catalysis.

T h e zeolite c a t a l y s t s

a l l o w s t u d y of reactions u n d e r c o n d i t i o n s where heterogeneous c a t a l y s i s was p r e v i o u s l y ineffective.

W i t h k i n e t i c a n a l y s i s techniques, results c a n be

interpreted quantitatively. xylene isomerization.

O n e s y s t e m t h a t fits i n t o t h i s c a t e g o r y is

T h i s process is c o m m e r c i a l l y

r e a d i l y c a t a l y z e d i n t h e l i q u i d phase b y

significant a n d is

a zeolite-containing

referred t o as A P or a r o m a t i c s processing c a t a l y s t .

catalyst

P r e v i o u s studies of t h i s

r e a c t i o n c a t a l y z e d b y s i l i c a - a l u m i n a i n the gas phase (2) a n d b y

AP

c a t a l y s t i n t h e l i q u i d phase (3) h a v e s h o w n t h a t t h e k i n e t i c s c a n be r e a d i l y analyzed b y using the monomolecular rate theory.

W i t h t h i s t h e o r y as t h e

basis, c a t a l y s t p a r a m e t e r s s u c h as i n t r a c r y s t a l l i n e diffusion, c a t a l y t i c site d e n s i t y , a n d s t r e n g t h of a c i d sites c a n be i n v e s t i g a t e d q u a n t i t a t i v e l y a t commercial operating conditions. I n t r a p a r t i c l e diffusion c a n affect c a t a l y s t s e l e c t i v i t y a n d a c t i v i t y . S i m i l a r l y , i n t r a c r y s t a l l i n e diffusion c a n affect t h e s e l e c t i v i t y a n d a c t i v i t y of z e o l i t i c c a t a l y s t s w h e r e i n t r a p a r t i c l e diffusion is negligible.

T h e r e f o r e , one

540

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

49.

541

Xylene Isomerization

CHUTORANSKY AND DWYER

of t h e first goals was t o d e t e r m i n e q u a n t i t a t i v e l y t h e effect of i n t r a c r y s t a l l i n e diffusion b y v a r y i n g t h e c r y s t a l l i t e size of t h e z e o l i t i c c o m p o n e n t of t h e A P catalyst. T h i s w o r k presented here covers t h e basic e x p e r i m e n t a l techniques a n d d a t a a n a l y s i s procedures together w i t h t h e a n a l y s i s of t h e c o n t r i b u t i o n of i n t r a c r y s t a l l i n e diffusion t o t h e performance of A P c a t a l y s t s . Theoretical

Background

T h e procedure

u s e d for t h e k i n e t i c a n a l y s i s is t h a t described

by

W e i a n d . P r a t e r (1), a n d i t has been a p p l i e d i n t h e f o l l o w i n g m a n n e r . Downloaded by COLUMBIA UNIV on July 29, 2012 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch049

T h e i s o m e r i z a t i o n of xylenes is a s s u m e d t o be k i n e t i c a l l y first order a n d c a n be described b y t h e f o l l o w i n g a p p a r e n t r e a c t i o n scheme : Αι

A

+=±-

3

A

2

where A i is o-xylene, A is ra-xylene, a n d A is p - x y l e n e . 2

3

T h e rate equation

for t h i s r e a c t i o n s y s t e m is g i v e n b y

where A is t h e x y l e n e c o m p o s i t i o n v e c t o r a n d Κ is t h e m a t r i x of r e a c t i o n r a t e constants (&21

Κ =

+

&3l)

— &12

fc l

—

(&12 +

2

— hi

—

— &32

&13

— #23

hi) (&23

+

hi)

T h e s y s t e m described is h i g h l y coupled, a n d a l t h o u g h i t c a n be s o l v e d a n a l y t i c a l l y b y c o n v e n t i o n a l m e t h o d s , s u c h a s o l u t i o n requires s u b s t a n t i a l d a t a t o ensure r e l i a b l e e v a l u a t i o n of t h e r a t e constants.

Therefore, this

s y s t e m is t r a n s f o r m e d i n t o a n u n c o u p l e d s y s t e m w h e r e i n Bo does not react λι Βχ

0

λ B -*0 2

2

b y the following transformation


(2)

542

MOLECULAR SIEVES

Β =

X^A

(3)

T h e t r a n s f o r m a t i o n m a t r i x X is d e t e r m i n e d e x p e r i m e n t a l l y a n d is r e l a t e d t o the rate constant matrix, Κ : Κ

(4)

= ΧΑΧ-

1

where Λ is a d i a g o n a l m a t r i x of t h e r a t e constants i n t h e t r a n s f o r m e d s y s t e m 0 A

(5)

=

λ Downloaded by COLUMBIA UNIV on July 29, 2012 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch049

2

I n a d d i t i o n , W e i (4) has s h o w n t h a t i f i n t r a p a r t i c l e diffusional effects are significant a n d i f t h e diffusivities of t h e r e a c t i n g species are e q u a l , t h e Λ m a t r i x becomes 0 A

+

(6)

=

where ηι a n d η are effectiveness factors defined b y T h i e l e (δ). 2

This rela

t i o n s h i p ( E q u a t i o n 6) p e r m i t s a n a l y s i s of diffusion effects b y u s i n g selec t i v i t y data only.

Experimental M a t e r i a l s . T h e r e a c t a n t s were P h i l l i p s p u r e grade xylenes percolated t h r o u g h a c t i v a t e d a l u m i n a a n d s t o r e d u n d e r n i t r o g e n . A l l c a t a l y s t s used were p r e p a r e d b y t h e C a t a l y s t R e s e a r c h a n d D e v e l o p m e n t G r o u p , M o b i l R e s e a r c h a n d D e v e l o p m e n t C o r p . , i n c l u d i n g b o t h the synthesis of zeolite a n d the t r a n s f o r m a t i o n i n t o the c a t a l y t i c f o r m . A p p a r a t u s . K i n e t i c d a t a were o b t a i n e d i n a c o n t i n u o u s u p f l o w m i c r o reactor s y s t e m . T h e r e a c t o r was 4 X / i n c h i d stainless steel w i t h a t h e r m o w e l l , / i n c h o d stainless steel, e x t e n d i n g a x i a l l y t h r o u g h the bed. X y l e n e feed was c o n t r o l l e d b y a m o d e l 196-32 M i l t o n R o y i n s t r u m e n t m i n i p u m p . R e a c t o r t e m p e r a t u r e s were m a i n t a i n e d b y a three-zone L i n d b e r g H e v i - D u t y furnace. P r o d u c t a n a l y s i s w a s p e r f o r m e d b y gas phase c h r o m a t o g r a p h y u s i n g a t e m p e r a t u r e - p r o g r a m m e d F & M M o d e l 5754 c h r o m a t o g r a p h a n d a 24 f t c h r o m a t o g r a p h i c c o l u m n p a c k e d w i t h 4 % d i i s o d e c a p t h a l a t e , 4 % bentone-34 s u p p o r t e d o n 60-80 mesh C h r o m o s o r b W H M D S . Procedure. B e f o r e e v a l u a t i o n , c a l c i n e d c a t a l y s t samples were d r i e d i n a i r (16 h r at 800° F ) . N o r m a l c a t a l y s t charge w a s 12 m l ; s m a l l e r c a t a l y s t charges were a c h i e v e d b y q u a r t z d i l u t i o n m a i n t a i n i n g the 12 m l t o t a l charge. T h e t e c h n i q u e for c h a r g i n g t h e c a t a l y s t a n d the range of flow rates i n v e s t i g a t e d were designed to m i n i m i z e t h e effects of a x i a l diffusion, b y p a s s i n g , s h o r t - c i r c u i t i n g , a n d excessive b a c k m i x i n g . A f t e r c h a r g i n g the reactor w i t h c a t a l y s t , t h e s y s t e m was pressurized w i t h n i t r o g e n a n d 9

1

1 6

8


49.



543

r a i s e d to t h e r e a c t i o n t e m p e r a t u r e . T h e s y s t e m w a s t h e n flooded w i t h x y l e n e at m a x i m u m p u m p rate. W h e n t h e first l i q u i d was o b s e r v e d at t h e s y s t e m o u t l e t , t h e feed r a t e was reset to t h e desired r a t e . T h i s was c o n sidered t h e s t a r t of t h e r u n . B y t h i s s t a r t - u p procedure a n y g r a d i e n t i n c a t a l y s t a g i n g across t h e b e d was e s s e n t i a l l y e l i m i n a t e d . W h i l e p r o d u c t was c o n t i n u a l l y collected t h r o u g h o u t t h e r u n , o n l y t h a t p o r t i o n collected o v e r t h e final 5 m i n of t h e p e r i o d w a s a n a l y z e d .


Data Interpretation T h e techniques of m o n o m o l e c u l a r r a t e t h e o r y easily t r a n s f o r m measured r e a c t i o n d a t a i n t o a f o r m where we c a n a n a l y z e a p p a r e n t k i n e t i c s a n d t h e effects of i n t r a c r y s t a l l i n e diffusion b y t h e use of s e l e c t i v i t y d a t a . T i m e d e p e n d e n c y has b e e n e l i m i n a t e d . Since s e l e c t i v i t y is e x t r e m e l y r e p r o d u c i b l e a n d is i n d e p e n d e n t of s h o r t - t e r m a g i n g effects, t h e n u m b e r of e x p e r i m e n t a l r u n s is r e d u c e d w h i l e d a t a r e l i a b i l i t y is m a i n t a i n e d . F o r c a t a l y s t e v a l u a t i o n a t a n y t e m p e r a t u r e , i t is necessary t o d e t e r m i n e t h e e q u i l i b r i u m composition a n d the straight-line reaction p a t h . W i t h this i n f o r m a t i o n a n y c a t a l y s t c a n be e v a l u a t e d a t t h i s t e m p e r a t u r e w i t h s i m p l y the additional information from a curved-line reaction p a t h . T h e a p p r o a c h used i n t h e a p p l i c a t i o n of m o n o m o l e c u l a r r a t e t h e o r y t o t h e x y l e n e i s o m e r i z a t i o n s e l e c t i v i t y k i n e t i c s is as follows. R e f e r e n c e is m a d e t o t h e c o m p o s i t i o n d i a g r a m , F i g u r e 1.

O-XYLENE

Figure 1.

Composition diagram


544

MOLECULAR SIEVES

Xylene Equilibrium Composition.

X y l e n e e q u i l i b r i u m compositions

were d e t e r m i n e d b y i n t e r p o l a t i o n a n d n o r m a l i z a t i o n of p u b l i s h e d t h e r m o d y n a m i c d a t a (6) ( T a b l e I) a n d are l a b e l e d A * i n a l l figures. T a b l e I. Temp., °F


300 350 400 450 500 600

Xylene Equilibrium Compositions, wt % ip-Xylene m-Xylene o-Xylene .194 .200 .207 .214 .219 .228

.564 .558 .552 .546 .541 .534

.242 .242 .241 .240 .240 .238

C u r v e d - L i n e R e a c t i o n P a t h s . A n i n f i n i t e n u m b e r of c u r v e d - l i n e r e a c t i o n p a t h s exist i n t h e r e a c t i o n s i m p l e x at a n y one t e m p e r a t u r e . For a c c u r a c y i n o u r k i n e t i c a n a l y s i s , we prefer a h i g h l y sensitive or c u r v e d p a t h , b u t slow enough t o a c q u i r e accurate r e a c t i o n d a t a . T h e c u r v e d - l i n e r e a c t i o n p a t h t h a t best s u i t s these c o n d i t i o n s is o b t a i n e d w i t h a s t a r t i n g c o m p o s i t i o n of p u r e o-xylene. T h e c u r v e d - l i n e r e a c t i o n p a t h s are generated f r o m i n d i v i d u a l analyses of p r o d u c t d i s t r i b u t i o n s w h i c h c a n be o b t a i n e d either b y c h a n g i n g c a t a l y s t v o l u m e or x y l e n e flow r a t e or m e r e l y b y s a m p l i n g the p r o d u c t as t h e c a t a l y s t d e a c t i v a t e s w i t h t i m e . S e l e c t i v i t y r e m a i n s essentially c o n s t a n t during aging. Straight-Line R e a c t i o n P a t h s . F o r a three-component reversible monomolecular system only two straight-line reaction paths exist; both c a n be observed e x p e r i m e n t a l l y . N o r m a l l y , t h e " s l o w " s t r a i g h t - l i n e r e a c t i o n p a t h is e s t i m a t e d as t h e t a n g e n t to a n y c u r v e d - l i n e r e a c t i o n p a t h at the e q u i l i b r i u m c o m p o s i t i o n . T h i s p a t h is s u b s e q u e n t l y d e t e r m i n e d more p r e c i s e l y i n t h e l a b o r a t o r y . T h e locus of the second, or " f a s t , " s t r a i g h t - l i n e r e a c t i o n p a t h is t h e n c a l c u l a t e d (1 ). T h e t e c h n i q u e for d e t e r m i n i n g s t r a i g h t - l i n e r e a c t i o n p a t h s i n t h i s w o r k differed f r o m t h e u s u a l e x p e r i m e n t a l a p p r o a c h . O u r a p p r o a c h also d e t e r m i n e d the s t r a i g h t - l i n e r e a c t i o n p a t h b y m i n i m i z i n g t h e d e v i a t i o n b e t w e e n t h e e x p e r i m e n t a l d a t a a n d those p r e d i c t e d . E x p e r i m e n t a l l y d e t e r m i n e d v a l u e s of t h e s t r a i g h t - l i n e i n t e r c e p t s are i n excellent agreement w i t h those c a l c u l a t e d i n t h e t e m p e r a t u r e r a n g e 3 0 0 ° 4 5 0 ° F . B e c a u s e of t h e r e d u c e d c u r v a t u r e of t h e r e a c t i o n p a t h s a t 500° a n d 6 5 0 ° F , i t is difficult t o d i s t i n g u i s h b e t w e e n s t r a i g h t - a n d c u r v e d - l i n e r e a c t i o n p a t h s . T h e r e f o r e , e x p e r i m e n t a l s t r a i g h t - l i n e r e a c t i o n p a t h s were n o t d e t e r m i n e d . A t 6 0 0 ° F i t was necessary t o a n a l y z e o-, m - , a n d p - x y l e n e r e a c t i o n p a t h s s i m u l t a n e o u s l y before a j u d i c i o u s choice of s t r a i g h t - l i n e r e a c t i o n p a t h s c o u l d be m a d e . T h e p r e d i c t e d a n d e x p e r i m e n t a l l y d e t e r m i n e d v a l u e s of s t r a i g h t - l i n e i n t e r c e p t a l o n g t h e o r t h o - m e t a x y l e n e l e g of t h e c o m p o s i t i o n d i a g r a m are s h o w n b e l o w .


49.

300 350 400 450 500 600

Results and

Experimental Intercept, mole fraction o-Xylene

Predicted Intercept, mole fraction o-Xylene

Temp.,

545



0.320 0.3150 0.395 0.425

0.317 0.362 0.395 0.427 0.45 0.49

Discussion


Applicability of Monomolecular Rate Theory to Xylene Isomerization T h e k i n e t i c s of l i q u i d - p h a s e

Selectivity Kinetics over Fresh A P Catalyst.

x y l e n e i s o m e r i z a t i o n o v e r fresh zeolite c o n t a i n i n g A P c a t a l y s t a r e effec t i v e l y interpreted b y pseudomonomolecular

rate theory.

T h e agreement

between t h e experimental d a t a (data points) a n d predicted reaction paths (solid lines) f o r o p e r a t i o n a t 400° a n d 600° F i s s h o w n i n F i g u r e 2.

The

c a t a l y s t used w a s i n t h e f o r m of e x t r u d a t e s c o m p r i s e d of t h e zeolite c o m ponent a n d an A 1 0 binder. 2

3

S i n c e x y l e n e d i s p r o p o r t i o n a t i o n t o toluene

a n d t r i m e t h y l b e n z e n e s w a s l o w , s e l e c t i v i t y d a t a were o b t a i n e d b y mere n o r m a l i z a t i o n of t h e x y l e n e c o m p o s i t i o n s (Σ a i n e s = 1.0). xy

Crystallite

Size

Effects

upon

e

A P Catalyst

Selectivity.

Previous

studies h a v e s h o w n t h a t w i t h t h e pellet sizes i n v e s t i g a t e d , gross p a r t i c l e size does n o t affect a c t i v i t y or s e l e c t i v i t y .

I f there are d i f f u s i o n a l l i m i t a

tions, t h e y m u s t b e i n t r a c r y s t a l l i n e a n d therefore a f u n c t i o n of t h e c r y s t a l l i t e size of t h e zeolite c o m p o n e n t . In

this study significant xylene

i s o m e r i z a t i o n s e l e c t i v i t y changes

because of v a r i a t i o n of zeolite c r y s t a l l i t e size were observed.

Qualitatively,

t h e l a r g e r t h e i n d i v i d u a l c r y s t a l l i t e s , t h e more d i f f u s i o n - c o n t r o l l e d t h e c o n v e r s i o n process a n d t h e greater t h e s e l e c t i v i t y f o r p - x y l e n e ( F i g u r e 3 ) . Scanning

electron

micrographs

of t h e zeolite

crystallites used

reveal

a p p a r e n t c r y s t a l l i t e sizes of 2 - 4 a n d 0 . 2 - 0 . 4 μ ( F i g u r e 4 ) . Quantitative

Interpretation

of Intracrystalline

Diffusional

Effects.

Since a q u a l i t a t i v e effect of c r y s t a l l i t e size u p o n s e l e c t i v i t y w a s o b s e r v e d , the n e x t step was t o e x t r a c t some q u a n t i t a t i v e v a l u e s f o r t h e i n t r a c r y s t a l l i n e diffusional p a r a m e t e r s .

T o do t h i s , w e m u s t e i t h e r k n o w t h e i n t r i n s i c o r

diffusion-free k i n e t i c s o r b e able t o m a k e a s i m p l i f y i n g a s s u m p t i o n so t h a t t h e d i f f u s i o n a l p a r a m e t e r s c a n be e x t r a c t e d f r o m t h e a v a i l a b l e d a t a . B a s e d u p o n t h e organic c h e m i s t r y of t h e i s o m e r i z a t i o n of x y l e n e s , t h e p r o b a b i l i t y of a d i r e c t i s o m e r i z a t i o n f r o m o-xylene t o p - x y l e n e is q u i t e small. F u r t h e r m o r e , i n homogeneously catalyzed liquid-phase reaction the s i m p l e series r e a c t i o n scheme has b e e n d e m o n s t r a t e d

(7,8,9,10,11,12).

o-xylene *=>ra-xylene

Molecular Sieves

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