Isomerization of Paraffins - Advances in Chemistry (ACS Publications)

48 Isomerization of Paraffins

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 13, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch048

H. W. KOUWENHOVEN Koninklijke/Shell-Laboratorium, Badhuisweg 3, Amsterdam-N., Netherlands

The application of zeolitic materials as catalysts in paraffin isomerization is discussed. Particular attention is given to cata lyst preparation variables such as sodium removal for zeolite Y and mordenite. Dual function catalysts based on these zeolite are compared with respect to activity. A reaction mechanism for paraffin isomerization over zeolitic dual function catalysts on the basis of literature and own data, is presented. T n c o m m e r c i a l p e t r o l e u m refining, i s o m e r i z a t i o n of l i g h t paraffins has been a p p l i e d for m a n y years. U n t i l r e c e n t l y t h e scope of t h e process w a s l i m i t e d , h o w e v e r , a n d i t s m a i n a p p l i c a t i o n was i s o m e r i z a t i o n of b u t a n e as feed p r e p a r a t i o n for a l k y l a t i o n processes. G e n e r a l l y , except for a few specific cases, n o c o m m e r c i a l j u s t i f i c a t i o n c o u l d be f o u n d for i s o m e r i z a t i o n of p e n t a n e a n d hexane f r a c t i o n s since i n m o s t cases t h e q u a l i t y r e q u i r e m e n t s for m o t o r gasoline c o u l d be m e t b y a l t e r n a t i v e processing routes a n d b y a d d i t i o n of v a r i o u s a d d i t i v e s , s u c h as l e a d t e t r a a l k y l s , t o i m p r o v e f u e l burning characteristics. A

S t r i n g e n t r e g u l a t i o n s r e g a r d i n g emissions of h y d r o c a r b o n s , c a r b o n m o n o x i d e , n i t r o g e n oxide, a n d l e a d c o m p o u n d s f r o m i n t e r n a l c o m b u s t i o n engines, h o w e v e r , w i l l r e q u i r e a change i n gasoline specifications. In p a r t i c u l a r , l e a d a l k y l c o n c e n t r a t i o n s w i l l h a v e t o decrease, r e s u l t i n g i n a n increased need for l i g h t h y d r o c a r b o n s h a v i n g h i g h a n t i - k n o c k q u a l i t y . T o s u p p l y a t least p a r t i a l l y t h e e x p e c t e d d e m a n d for these h i g h q u a l i t y gasoline c o m p o n e n t s , c a t a l y t i c i s o m e r i z a t i o n of p e n t a n e a n d hexane fractions w i l l p r o b a b l y be used m o r e i n f u t u r e refinery operations. Because of i t s h i g h c o n t e n t of τι-paraffins t h e C / C e f r a c t i o n of m o s t c r u d e oils has a n u n l e a d e d research octane n u m b e r ( R O N - O ) of a b o u t 70. I s o m e r i z a t i o n converts t h e l o w octane n o r m a l paraffins i n t o t h e higher octane b r a n c h e d isomers, a n d a c c o r d i n g t o t h e t h e r m o d y n a m i c s of t h e r e a c t i o n t h e highest c o n v e r s i o n i n t o b r a n c h e d isomers is o b t a i n e d a t a l o w r e a c t i o n t e m p e r a t u r e . I n F i g u r e 1 t h e R O N - 0 of t h e e q u i l i b r i u m p e n t a n e a n d hexane m i x t u r e s i s s h o w n as a f u n c t i o n of t e m p e r a t u r e . 5

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

530

MOLECULAR SIEVES

Isomerization Processes and Catalysts L o w t e m p e r a t u r e i s o m e r i z a t i o n c a t a l y s t s are of t h e F r i e d e l C r a f t s t y p e , s u c h as A1C1 a n d A l B r , a c t i v a t e d w i t h H X , a n d d i s s o l v e d i n a s u i t a b l e solvent s u c h as S b C U . A p p l i c a t i o n of these e x t r e m e l y a c i d i c a n d corrosive systems requires special h a n d l i n g a n d disposal of the c a t a l y s t a n d careful p r e t r e a t m e n t of t h e feed-stock t o r e m o v e c o n t a m i n a t i n g m a t e r i a l s . L o w t e m p e r a t u r e i s o m e r i z a t i o n ( < 100° C ) is used m a i n l y f o r i s o m e r i z a t i o n of w-butane, w h i c h is g e n e r a l l y a v a i l a b l e i n sufficient p u r i t y b y n o r m a l refinery processes. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 13, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch048

3

3

RESEARCH OCTANE NUMBER

70 Ο

I 100

I 200

I 300

1

TEMPERATURE, C e

Figure 1. Octane numbers of equilibrium mixtures of pentanes and hexanes A t s l i g h t l y h i g h e r t e m p e r a t u r e s (100°-200°C) c a t a l y s t s c o n s i s t i n g of c h l o r i d e d a l u m i n a i n c o m b i n a t i o n w i t h a noble m e t a l , s u c h as p l a t i n u m , are used. A s a c o c a t a l y s t HC1 or a n organic c h l o r i d e is s u p p l i e d w i t h t h e feedstock. T h e h i g h r e a c t i v i t y of these c a t a l y s t systems requires c a r e f u l feed p r e t r e a t m e n t f o r r e m o v a l of d e a c t i v a t i n g m a t e r i a l s . S e v e r a l p l a n t s (1, 2) u s i n g t h i s t y p e of c a t a l y s t , a n d one v e r s i o n of t h i s process especially developed t o c o n v e r t C /Ce feed, h a v e r e c e n t l y been b u i l t . 5

A t a b o u t 250° C a c a t a l y s t c o n s i s t i n g of a l o w s o d i u m zeolite a n d a noble m e t a l is used i n a r e c e n t l y developed process (3). I t is c l a i m e d t h a t n o extensive feed p r e t r e a t m e n t is r e q u i r e d a n d t h a t t h e s t a b i l i t y of t h e c a t a l y s t is n o t i m p a i r e d b y c o m m o n feed i m p u r i t i e s . A n older process u s i n g a c a t a l y s t c o n s i s t i n g of p l a t i n u m s u p p o r t e d o n a m o r p h o u s s i l i c a a l u m i n a (4) operates a t 400°C. N a t u r a l l y t h e h i g h e r t h e o p e r a t i o n

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

48.

531

Isomerization of Paraffins

K O U W E N H O V E N

t e m p e r a t u r e , t h e l o w e r t h e octane i m p r o v e m e n t w h i c h is o b t a i n e d d u r i n g i s o m e r i z a t i o n of C s / C e streams. N o b l e m e t a l - c o n t a i n i n g c a t a l y s t systems are m o r e stable t h a n F r i e d e l C r a f t s t y p e c a t a l y s t s , a n d u s u a l l y c a t a l y s t s o p e r a t i n g a t h i g h e r t e m p e r a t u r e s are less susceptible t o d e a c t i v a t i o n . W h i c h e v e r t y p e of process is m o s t p r o f i t a b l e i n a p a r t i c u l a r s i t u a t i o n depends o n m a n y factors, a n aspect w h i c h f a l l s outside t h e scope of t h i s review.


Reaction Mechanisms I n t h e presence of h y d r o g e n t h e i s o m e r i z a t i o n of paraffins of five or more c a r b o n a t o m s over d u a l f u n c t i o n c a t a l y s t s , s u c h as a m o r p h o u s s i l i c a a l u m i n a s u p p o r t e d p l a t i n u m , c a n be described b y t h e f o l l o w i n g s c h e m e : + H

nP

±5:

- H +

2

nF

-H2

±5:

+H+

nP

+

±iP iP+

+H+

±Zi

- H

2

i P " ±Hs iP

- H +

+H

2

Olefins are f o r m e d b y d e h y d r o g e n a t i o n of t h e η-paraffin feed o v e r t h e m e t a l l i c h y d r o g e n a t i o n - d e h y d r o g e n a t i o n f u n c t i o n a n d are a d s o r b e d o n t h e a c i d i c surface of t h e c a t a l y s t as c a r b o n i u m ions b y p r o t o n a d d i t i o n . A f t e r s k e l e t a l i s o m e r i z a t i o n t h e y are desorbed as isoolefins a n d s u b s e q u e n t l y h y d r o g e n a t e d t o t h e c o r r e s p o n d i n g isoparaffins. T h e n e t r e s u l t (i.e., the f o r m a t i o n of c a r b o n i u m ions) of t h e a c t i o n of m e t a l a n d a c i d i n d u a l f u n c t i o n c a t a l y s i s i s , o n p u r e F r i e d e l - C r a f t s t y p e c a t a l y s t s , described b y t h e scheme: + H -

riP

±5: —H

_

nP+

±5:

- H -

i?

+

±^

iP

+ H -

C a r b o n i u m ions a n d isoparaffins are f o r m e d b y h y d r i d e i o n a b s t r a c t i o n a n d h y d r i d e i o n t r a n s f e r reactions. T h i s m e c h a n i s m has been d e s c r i b e d for HF.SbF (δ). I s o m e r i z a t i o n of η-paraffins o v e r m o n o f u n c t i o n a l a c i d i c c a t a l y s t s has also been c l a i m e d for m o r d e n i t e (6, 7), for sieve Y (8), a n d for the base of t h e c a t a l y s t of undisclosed c o m p o s i t i o n a p p l i e d i n t h e i s o m e r i z a t i o n process u s i n g a noble m e t a l o n a n a c i d i c zeolite base (3). A b i m o l e c u l a r m e c h a n i s m for i s o m e r i z a t i o n of Ce paraffins was p r o posed b y B o l t o n (9). A c c o r d i n g t o h i m , paraffin molecules are 1,3-dia d s o r b e d o n a p a i r of a c t i v e sites close t o each o t h e r . A s a second step d e h y d r o g e n a t i o n t o adsorbed olefins is envisaged, f o l l o w e d b y a b i m o l e c u l a r r i n g closure t o a c y c l o h e x a n e - t y p e r i n g . T h e s i x - r i n g c a n be c r a c k e d i n t o v a r i o u s hexenes w h i c h are a g a i n h y d r o g e n a t e d o v e r t h e d u a l f u n c t i o n c a t a l y s t t o f o r m isohexanes. T h e p r o p o s e d m e c h a n i s m is i l l u s t r a t e d b e l o w for t h e i s o m e r i z a t i o n of n-hexane. 5


532

MOLECULAR SIEVES

|cd

Jab


3MeC

5

2MeC

Jxy

23DiMeC + n-C

6

jab

2MeC

e

4

jab

3MeC

5

5

+

«-C

6

A l t h o u g h i s o m e r i z a t i o n of b u t a n e requires a b i m o l e c u l a r m e c h a n i s m , t h e m e c h a n i s m proposed i n R e f . 9 for hexane does n o t seem t o g i v e a m o r e s t r a i g h t f o r w a r d e x p l a n a t i o n of t h e p h e n o m e n a t h a n t h e classical d u a l function mechanism. I n t h e r e a c t i o n m e c h a n i s m s described a b o v e t h e a c i d i t y of t h e c a t a l y s t p l a y s a n i m p o r t a n t role. Zeolites c a n be c o n v e r t e d i n t o the H f o r m a n d as s u c h are p o w e r f u l c a t a l y s t s for a c i d - c a t a l y z e d reactions. W e discuss below some aspects of i s o m e r i z a t i o n c a t a l y s t p r e p a r a t i o n t o d e m o n s t r a t e factors w h i c h influence t h e a c t i v i t y of c a t a l y s t s based o n zeolites. I n t h i s discussion we are concerned w i t h zeolite Y a n d m o r d e n i t e . D a t a o n paraffin i s o m e r i z a t i o n o v e r d u a l f u n c t i o n c a t a l y s t s besed on o t h e r zeolites are scarce, a n d n o d a t a h a v e been p u b l i s h e d s h o w i n g t h a t m a t e r i a l s l i k e zeolite X , zeolite L , offretite, zeolite omega, or g m e l i n i t e c a n be c o n v e r t e d i n t o c a t a l y s t bases h a v i n g a n i s o m e r i z a t i o n a c t i v i t y c o m p a r a b l e w i t h t h a t of Η-zeolite Y or H - m o r d e n i t e . +

Conversion into Isomerization Catalyst Base Z e o l i t e Y . T h e r e is s t r o n g evidence t h a t B r o n s t e d t y p e a c i d i t y is responsible for c a r b o n i u m i o n t y p e reactions b o t h i n m u l t i v a l e n t i o n ex changed a n d Η-zeolite Y based c a t a l y s t s . A t h o r o u g h discussion of t h i s p r o b l e m w a s presented b y R a b o a n d P o u t s m a (16). A t present t h e h y d r o gen f o r m of zeolite Y seems t o h a v e t h e highest a c t i v i t y p r o v i d e d t h a t i t is prepared so t h a t i t s c r y s t a l l i n i t y is preserved. T h e Η-form is u s u a l l y p r e p a r e d b y c a l c i n a t i o n of N H i o n exchanged m a t e r i a l . D i r e c t exchange w i t h acids c a n n o t be a p p l i e d o w i n g to the i n s t a b i l i t y of zeolite Y i n aqueous acids. NHi-zeolite Y h a v i n g a v e r y l o w s o d i u m content a n d a h i g h c r y s t a l l i n i t y is c o n v e n i e n t l y p r e p a r e d b y e x c h a n g i n g zeolite Y as s y n t h e sized w i t h a m m o n i u m salts u n t i l a s o d i u m c o n t e n t of 2 - 3 w t % is reached. A t t h i s stage t h e m a t e r i a l is c a l c i n e d (deep b e d procedure) a t a t e m p e r a t u r e 4


48.

Isomenzation of Paraffins

KOUWENHOVEN

533

between 400° a n d 8 5 0 ° C a n d is s u b j e c t e d t o f u r t h e r exchange w i t h a m m o n i u m s a l t solutions, a s o d i u m l e v e l 0.2 w t % easily b e i n g reached. M o r e complete s o d i u m r e m o v a l c a n be a c h i e v e d b y r e p e a t i n g c a l c i n a t i o n a n d i o n exchange w i t h aqueous a m m o n i u m salts. U s i n g t h i s m e t h o d s o d i u m levels of 0.02 w t % or l o w e r c a n be reached. T h i s procedure a n d the r e l a t e d u l t r a s t a b i l i z a t i o n process was first a n n o u n c e d b y M c D a n i e l a n d M a h e r (10) a n d was l a t e r discussed b y others (11,12). T h e c a l c i n a t i o n of N H zeolite Y has been i n v e s t i g a t e d t h o r o u g h l y (13, 14, 16) for samples of v a r i o u s s o d i u m levels. A t t e m p e r a t u r e s h i g h e r t h a n 4 5 0 ° C n o N H is r e t a i n e d b y Η-zeolite Y . 4


3

M o r d e n i t e . F o r m a t i o n of low s o d i u m forms of m o r d e n i t e is s i m p l e r t h a n for zeolite Y , p r e s u m a b l y because the s o d i u m ions are l o c a t e d i n t h e m a i n channels a n d are easily exchangeable. T h e m o r d e n i t e s t r u c t u r e is stable o n t r e a t m e n t w i t h acids, a n d the h y d r o g e n exchanged zeolite c a n be o b t a i n e d b y a c i d l e a c h i n g . D u r i n g t h i s t r e a t m e n t some a l u m i n a is dissolved, however, a n d i n some cases i t m a y be advantageous to prepare t h e h y d r o g e n f o r m of mordenite b y c a l c i n i n g t h e N H exchanged m a t e r i a l or b y a c o m b i n a t i o n of b o t h m e t h o d s (23). I n a i r H - m o r d e n i t e does n o t r e t a i n N H a t t e m p e r a t u r e s h i g h e r t h a n 650° C . 4

3

Preparation of Dual Function Catalysts Based on Zeolite Y and Mordenite D u a l f u n c t i o n c a t a l y s t s based o n Η-zeolites are c o n v e n i e n t l y p r e p a r e d b y i o n exchange of i o n i c a m m i n o complexes of t h e n o b l e m e t a l s w i t h either t h e H o r t h e N H zeolite. C a t a l y t i c a l l y a c t i v e samples are o b t a i n e d b y a staged c a l c i n a t i o n i n a i r of t h e n o b l e m e t a l c o m p l e x - c o n t a i n i n g samples. T h e c a l c i n a t i o n is v e r y c r i t i c a l , a n d for zeolite Y (17) presence of a i r is essential for o b t a i n i n g h i g h l y dispersed p l a t i n u m o n sieve Y . The 4

c o m p l e x P t ( N H ) - H - z e o l i t e Y , w h i c h was t h e s t a r t i n g m a t e r i a l i n these e x p e r i m e n t s , is stable i n a i r u p t o 2 0 0 ° C . I R measurements s h o w e d t h a t no N H was adsorbed o n t h e zeolite d u r i n g d e c o m p o s i t i o n of the a m m i n o complex. B e l o w 400° C , Η-zeolite Y adsorbs N H ; we c a n therefore assume t h a t the N H is c a t a l y t i c a l l y o x i d i z e d b y a i r o v e r t h e p l a t i n u m m e t a l a c c o r d i n g t o a n e x o t h e r m i c r e a c t i o n s u c h as : 3

4

3

3

3

4 NH + 3 0 — 2 N + 3 H 0 3

2

2

2

T o prepare noble metal o n H-mordenite catalysts the noble m e t a l a m m i n o c o m p l e x - c o n t a i n i n g m a t e r i a l is n o r m a l l y h e a t e d i n a i r u s i n g staged h e a t i n g (21, 22, 23, 24). I n R e f . 24 t h e c a l c i n a t i o n of P t ( N H ) - N H m o r d e n i t e is discussed i n d e t a i l , a n d i t is s h o w n t h a t d u r i n g c a l c i n a t i o n i n a i r a t a b o u t 300° C a s t r o n g l y e x o t h e r m i c r e a c t i o n occurs, p r e s u m a b l y a r e s u l t of t h e o x i d a t i o n of N H . D a t a are presented o n t h e influence of calcination conditions on p l a t i n u m dispersion. 3

3


4

4

534

MOLECULAR SIEVES

Catalytic Activity Catalysts B a s e d on Zeolite Y .

T h e a c t i v i t y for o-xylene i s o m e r i z a t i o n

of m o n o f u n c t i o n a l N a - H - z e o l i t e Y samples measured as the t e m p e r a t u r e for a g i v e n conversion is, i n t h e range of 4 - 1 0 w t % N a , a n e a r l y l i n e a r f u n c t i o n of t h e s o d i u m c o n t e n t of t h e s a m p l e (18).

W i t h increasing sodium removal

a t l o w e r s o d i u m levels ( > 0.2 w t % N a ) , however, c a t a l y t i c a c t i v i t y for t h i s r e a c t i o n increases m o r e r a p i d l y .

D a t a for paraffin i s o m e r i z a t i o n o v e r d u a l

f u n c t i o n c a t a l y s t s i n a c o m p a r a b l e range of s o d i u m c o n c e n t r a t i o n s are n o t


available.

O u r o w n d a t a i n t h e 2 . 5 - 0 . 0 2 w t % N a range s h o w t h a t t h e

7i-pentane i s o m e r i z a t i o n a c t i v i t y of P d - N a - H - z e o l i t e Y samples strongly on sodium concentration (Table I).

depend

T h e large effect of s o d i u m a t

l o w c o n c e n t r a t i o n s c l e a r l y i n d i c a t e s t h a t d u r i n g t h e r m a l a c t i v a t i o n of t h e c a t a l y s t s o d i u m is r e d i s t r i b u t e d o v e r t h e zeolitic surface a n d d e a c t i v a t e s a c i d i c sites w h i c h are e a s i l y accessible t o b o t h n o r m a l a n d isoparaffins. A s i m i l a r r e d i s t r i b u t i o n p h e n o m e n o n is described for zeolites of t h e o f f r e t i t e erionite group

(19).

U s i n g p u b l i s h e d d a t a t h e paraffin i s o m e r i z a t i o n a c t i v i t y of

catalysts

based o n zeolite Y samples p a r t i a l l y exchanged w i t h h i g h e r v a l e n t ions is difficult t o c o m p a r e w i t h t h a t of samples based o n Η-zeolite Y .

T h i s is

p a r t l y t h e r e s u l t of t h e p r o f o u n d influence of c a l c i n a t i o n c o n d i t i o n s o n b o t h the c r y s t a l l i n i t y of t h e s a m p l e a n d t h e n o b l e m e t a l d i s p e r s i o n a n d p a r t l y t h e r e s u l t of t h e s t r o n g influence o n c a t a l y t i c a c t i v i t y of s m a l l c o n c e n t r a t i o n s of s o d i u m .

T h e c o m p a r a t i v e l y h i g h a c t i v i t y of a l k a l i n e e a r t h or

r a r e e a r t h exchanged samples is p r o b a b l y caused b y t h e i r g e n e r a l l y h i g h e r t h e r m a l s t a b i l i t y , r e s u l t i n g i n m o r e c r y s t a l l i n e c a t a l y s t samples. Catalysts B a s e d on Mordenite.

I s o m e r i z a t i o n of paraffins o v e r H -

m o r d e n i t e based c a t a l y s t s has been d e s c r i b e d (6, 7,14) (7) reports t h a t cyclohexane

Minachev

20, 21).

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

Na-H-mordenite

c a t a l y s t s increases l i n e a r l y w i t h H + c o n c e n t r a t i o n i n t h e zeolite for 2 5 - 9 4 % exchange.

H e f u r t h e r o b s e r v e d t h a t H - m o r d e n i t e is d e a c t i v a t e d b y o t h e r

cations s u c h as L i , K , M g , C d , Z n , a n d A l . work

(6);

he reported that, compared

T h i s agrees w i t h B r y a n t ' s

with Pd-H-mordenite,

samples

i n w h i c h h y d r o g e n was p a r t l y r e p l a c e d b y C a or Z n h a d a n a p p r e c i a b l y l o w e r n-pentane i s o m e r i z a t i o n a c t i v i t y . T a b l e I.

H y d r o i s o m e r i z a t i o n of n - P e n t a n e O v e r P d - H - Z e o l i t e Y : I n f l u e n c e of S o d i u m o n C a t a l y t i c A c t i v i t y

Conditions :

H 2 / n - C molar ratio : 2.5; pressure : 30 k g / c m : W H S V : 1 kg/kg/hr Crystallinity, 30% Conversion wt % (x-ray) Na 0, wt% at Different Temp., °C 2

5

2

90 80 80

2.02 0.27 0.02

305 300 250



48.

KOUWENHOVEN

535


E a r l i e r i t was s t a t e d t h a t d u r i n g exchange of N a - m o r d e n i t e w i t h acids some a l u m i n a is also r e m o v e d , a n d t h e effects of t h e s e v e r i t y of a c i d l e a c h i n g a n d t h e s i l i c a - a l u m i n a m o l a r r a t i o o n n-pentane i s o m e r i z a t i o n a c t i v i t y of d u a l f u n c t i o n c a t a l y s t s h a v e been r e p o r t e d (21,22,23). Hopper (21) p u b l i s h e d d a t a o n t h i s r e a c t i o n over 0.5 w t % P d - H - m o r d e n i t e c a t a l y s t s v a r y i n g i n s i l i c a - a l u m i n a m o l a r r a t i o between 9 a n d 52, a n d l a t e r E b e r l e y et al. (22) extended t h i s range t o a m o l a r r a t i o of 97. T h e d a t a were o b t a i n e d u n d e r c o m p a r a b l e r e a c t i o n c o n d i t i o n s , a n d i n b o t h studies procedures were p r o b a b l y v e r y s i m i l a r . D a t a i n T a b l e II s h o w t h a t t h e i s o m e r i z a t i o n a c t i v i t y m a y be o p t i m a l a t a s i l i c a - a l u m i n a m o l a r r a t i o b e t w e e n 10 a n d 25. U p o n i n c r e a s i n g t h e silica a l u m i n a r a t i o f u r t h e r , n pentane i s o m e r i z a t i o n a c t i v i t y decreases s h a r p l y because of t h e l o w e r n u m b e r of a c i d sites (protons) i n t h e samples of h i g h e r s i l i c a c o n t e n t . O u r o w n measurements u s i n g P t - H - m o r d e n i t e samples p r e p a r e d i n a w a y analogous t o t h a t described i n R e f . 21 a n d 22 show t h a t t h e o p t i m u m i s o m e r i z a t i o n a c t i v i t y is a c h i e v e d near a s i l i c a a l u m i n a r a t i o of 16 ( T a b l e III). T a b l e II shows t h a t t h e presence of a s m a l l a m o u n t of s o d i u m i n m o r d e n i t e poisons t h e m o s t a c t i v e a c i d sites, w h i c h agrees w i t h t h e o b s e r v a t i o n of B e n e s i (14) o n t h e p o i s o n i n g of H - m o r d e n i t e b y s m a l l a m o u n t s of N H . 3

Table II.

Hydroisomerization of w-Pentane : Influence of Silica-Alumina Molar Ratio of Activity of P d - H - M o r d e n i t e Catalysts

Conditions: temperature: 288°C; pressure: 32 k g / c m ; H / C molar ratio = 3.2 mole/mole Relative SiOi/AW* Reference Activity Molar Ratio Na, wt % 2

Nil 0.9 0.03 Nil 0.03

12 14 25 77 93

100 50 78 47 23

0.10 0.05 0.09 0.03

9 10 26 52

60 100 76 3.4

Table III.

2

B

(22)

(21)

Hydroisomerization of n-Pentane : Influence of SiUca-Alumina Molar Ratio on Activity of P t - H - M o r d e n i t e

Conditions: temperature:

Ν a, wt %

250°C; pressure: 30 kg/cm ; mole/mole Si0 /A Wz Molar Ratio

0.03 0.02 0.02

10 17 25

2

2

H2/C5

molar ratio: 2.5

Relative Activity 100 135 84


536

MOLECULAR SIEVES

Reaction Mechanism for Paraffin Isomerization T h e m e c h a n i s m a n d k i n e t i c s of p e n t a n e , hexane, a n d

cyclohexane

isomerization over P d - H - m o r d e n i t e have been extensively investigated b y B r y a n t (6), ventional

Hopper

(21),

dual function

a n d Beecher

mechanism

as

(20).

T h e y assume a

described

earlier.

Bryant

con(6)

p o i n t e d o u t t h a t H - m o r d e n i t e itself has a h i g h a c t i v i t y for p e n t a n e i s o m e r i z a t i o n a n d t h a t i m p r e g n a t i o n of a n o b l e m e t a l does n o t change t h e r a t e of t h e isomerization reaction.

T h i s e x c e p t i o n a l a c t i v i t y of m o r d e n i t e


s i n c e b e e n r e p o r t e d b y B e n e s i (14) a n d M i n a c h e v (7) as w e l l . c h e v ' s p a p e r t h e r e a c t i o n m e c h a n i s m of n - p e n t a n e H - m o r d e n i t e is discussed i n some d e t a i l .

has

In Mina-

isomerization over

T h e r a t e of r e a c t i o n is i n v e r s e l y

p r o p o r t i o n a l t o t h e h y d r o g e n pressure, a n d i t is c o n c l u d e d t h a t t h e r e a c t i o n proceeds a c c o r d i n g t o t h e f o l l o w i n g scheme : nC Hi2 + H-mordenite ±+ 7iC Hn-M + H wC H -M ±> tCsHii-M ICBHU-M + H Î=5 * C H + H-M 6

5

5

2

u

2

T h i s scheme is e q u i v a l e n t t o

5

12

t h e r e a c t i o n m e c h a n i s m w h i c h is g e n e r a l l y

a c c e p t e d for F r i e d e l C r a f t s c a t a l y s t s a n d explains t h e influence of

hy-

drogen

ob-

p a r t i a l pressure s a t i s f a c t o r i l y .

Figure 2.

A d d i t i o n a l d a t a w h i c h we

n-Pentane isomerization activity of mordenite. Platinum stabilizes conversion and increases selectivity.



48.

K O U W E N H O V E N


537

t a i n e d ( F i g u r e 2) s h o w t h a t i n i t i a l n-pentane c o n v e r s i o n o v e r H - m o r d e n i t e decreases w i t h i n c r e a s i n g h y d r o g e n p a r t i a l pressure, w h i c h is i n agreement w i t h t h e d a t a of M i n a c h e v . T h e s e l e c t i v i t y for isopentane f o r m a t i o n a n d t h e s t a b i l i t y of t h e c o n v e r s i o n l e v e l increase w i t h i n c r e a s i n g h y d r o g e n p a r t i a l pressure. T h e s t a b i l i t y a n d s e l e c t i v i t y effect we o b s e r v e d c a n be e x p l a i n e d b y a s s u m i n g t h a t t h e c a r b o n i u m ions, w h i c h were f o r m e d o n t h e surface b y h y d r i d e i o n a b s t r a c t i o n , are i n e q u i l i b r i u m w i t h olefins i n t h e gas phase. U p o n r e a d s o r p t i o n these olefins c a n react w i t h c a r b o n i u m ions present o n t h e surface w i t h f o r m a t i o n of l a r g e r species. These p o l y m e r i c ions w i l l y i e l d s a t u r a t e d c r a c k e d p r o d u c t s a n d surface residues w h i c h are m o r e u n s a t u r a t e d . A c c u m u l a t i o n of t h e residues e v e n t u a l l y d e a c t i v a t e s t h e c a t a l y s t . B r y a n t (6) r e p o r t e d t h a t for i s o m e r i z a t i o n of n - p e n t a n e u n d e r i d e n t i c a l r e a c t i o n c o n d i t i o n s P t - H - m o r d e n i t e is a m o r e selective c a t a l y s t t h a n H - m o r d e n i t e , a n d he suggested t h a t t h e P t - c o n t a i n i n g c a t a l y s t is m o r e stable. O u r d a t a i n F i g u r e 2 c o n f i r m t h i s s u g gestion, a n d t h e results c a n be e x p l a i n e d b y t h e f o l l o w i n g r e a c t i o n m e c h a n i s m ; t h i s is a c o m b i n a t i o n of t h e c o n v e n t i o n a l i s o m e r i z a t i o n m e c h a n i s m s for F r i e d e l C r a f t s t y p e c a t a l y s t s (steps 1, 2, 3) a n d d u a l f u n c t i o n c a t a l y s t s (steps 6 , 4 , 2 , 5 , 7 ) .

(2)

T h e scheme i m p l i e s t h a t i n t h e presence of a m e t a l w h i c h establishes t h e o l e f i n - p a r a f f i n e q u i l i b r i u m , the c a r b o n i u m i o n c o n c e n t r a t i o n o n t h e surface depends o n t h e h y d r o g e n p a r t i a l pressure. T h e s t a b i l i z i n g effect of a g i v e n m e t a l l o a d w i l l d e p e n d o n i t s dispersion a n d d i s t r i b u t i o n a n d o n t h e p r e v a i l i n g h y d r o g e n pressure. S i m i l a r e x p e r i m e n t s s h o w t h a t for zeolite Y based c a t a l y s t s t h e r e a c t i o n m e c h a n i s m is i d e n t i c a l w i t h t h a t discussed a b o v e for mo rdenite. I t has b e e n c l a i m e d t h a t n o b l e m e t a l d u a l f u n c t i o n c a t a l y s t s b a s e d o n H - m o r d e n i t e are m o r e a c t i v e for paraffin i s o m e r i z a t i o n t h a n t h e i r c o u n t e r p a r t s based o n Η-zeolite Υ (2δ). F o r b o t h zeolites t h e i s o m e r i z a t i o n a c t i v i t y depends s t r o n g l y o n t h e degree of s o d i u m r e m o v a l a n d c o m p a r i s o n of l o w s o d i u m P d - H - m o r d e n i t e a n d l o w s o d i u m P d - H - z e o l i t e Y for i s o m e r i z a t i o n of w-hexane a t 250° C shows t h a t b o t h m a t e r i a l s h a v e a b o u t t h e same a c t i v i t y ( T a b l e I V ) , t h e Y sieve b a s e d m a t e r i a l b e i n g s l i g h t l y m o r e active.


538

MOLECULAR SIEVES

Table IV. Isomerization of n-Hexane Over Pd-H-Zeolite Catalysts: Comparison of Mordenite and Zeolite Y Conditions: WHSV: 1 g/g/h; temperature: 250°C; pressure: 30 kg/cm ; H / C molar ratio: 2.5 mole/mole Mordenite; Zeolite Y; 0.02 wt % Si0 /AkOz Na; 80 wt % crystalline Product = 71; 0.8 wt % Pd (x-ray); 0.8 wt % Pd 2

2

6


2

Conversion, wt % Selectivity Iso/normal ratio 22 DMB, wt %

72 97 2.5 9.5

76 95 2.9 11.5

Conclusion For optimal performance of dual function isomerization catalysts based on zeolite Y or mordenite, extensive removal of sodium is necessary. The finished catalyst must be highly crystalline, and the finely dispersed metallic hydrogenation function should be well distributed throughout the catalyst particles. The proposed mechanism explains the stabilizing influence on conversion and the suppression of cracking reactions by addi tion of the metallic hydrogenation function to the active acidic catalyst base. Literature Cited 1. Oil Gas J. (Aug. 16, 1971), 67. 2. Richardson, A. H., Olive, M . F., 68th National Meeting, AIChE, 1971, paper 53d. 3. Kouwenhoven, H. W., Van Zijll Langhout, W. C., Chem. Eng. Progr. (1971) 67 (4), 65, 1971. 4. Ciapetta, F. G., Hunter, J. B., Ind. Eng. Chem. (1953) 45, 147. 5. Bickel, A. F. et al., Chem. Commun. (1967) 634. 6. Bryant, P. Α., Ph.D. dissertation, Louisiana State University (1966). 7.

Minachev, Kh. et al., ADVAN. CHEM. SER. (1971) 102, 441.

8. Lanewala, M . A. et al., J. Catalysis (1967) 9, 95. 9. Bolton, A. P., Lanewala, Μ. Α., J. Catalysis (1970) 18, 1. 10. McDaniel, C. V., Maher, P. K., Conf. Mol. Sieves, Soc. of Chem. Ind., London (1967). 11. Kerr, G. T. et al., J. Catalysis (1969) 13, 114. 12. Maher, P. K., Hunter, F. D., Scherzer, J., ADVAN. CHEM. SER. (1971) 101, 266. 13. Uytterhoeven, J., Christner, L. G., Hall, W. K., J. Phys. Chem. (1965) 69, 2117. 14. Benesi, Η. Α., J. Catalysis (1967) 8, 368. 15. Cattanach, J., Wu, E . L., Venuto, P. B., J. Catalysis (1968) 11, 342. 16. Rabo, J. Α., Poutsma, M . L., ADVAN. CHEM. SER. (1971) 102, 284. 17. Dalla Betta, R. Α., Boudart, M., Preprints, Intern. Congr. Catalysis, 5th, Palm Beach, 1972, Paper 100. 18. Ward, J. W., Hansford, R. C., J. Catalysis (1969) 13, 364.


48.

KOUWENHOVEN


539

19. Chen, Ν. Y., Preprints, Intern. Congr. Catalysis, 5th, Palm Beach, 1972, Paper 101. 20. Beecher, R. G . , Ph.D. dissertation, Louisiana State University (1967). 21. Hopper, J. R., Ph.D. dissertation, Louisiana State University (1969). 22. Eberly Jr., P. E., Kimberlin, Jr., C. N., J. Catalysis (1971) 22, 419. 23. B. P. Co. Ltd., Dutch Published Application 7003228. 24. Shell Internationale Research Mij, B.P. 1,189,850 (April 29, 1970). 25.

Burbridge, B. W . , Keen, I. M., Eyles, M . K., ADVAN. CHEM. SER. (1971) 102,

400.


RECEIVED February 13, 1972.


Isomerization of Paraffins - Advances in Chemistry (ACS Publications)

Recommend Documents