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63

Catalytic Activity and the Nature of Active Centers in Zeolites Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch063

J O H N T U R K E V I C H and YOSHIO ΟΝΟ Princeton University, Princeton, N . J.

Catalyst

preparations

with

controlled

number

and

of acid sites were used to study the cracking 2,3-dimethylbutane,

and

these cases, the Bronsted responsible

sites to initiate

required the reaction,

tion of a hydride

activity.

a small

acid sites.

equilibration

Introduction

the use of hydrogen cracking

favorably,

carbon

cracking.

of

hydrocarbon. also.

is catalyzed

by the

as the gas carrier

acid abstrac

was studied

of palladium

The

The

affects the chain

hy Lewis

into the catalyst

but represses the branched

all was

of 2,3-

Lewis

caused by

ion from the saturated

drogen—deuterium

In

OH group

The cracking

number

presumably

of 2,3-dimethylbutenes

cumene,

of xylenes.

acid with its surface

for the catalytic

dimethylbutane

cracking

isomerization

nature

of

and

cumene hydro

* T < h e c a t a l y t i c properties of the zeolites are of u n u s u a l interest.

Tech-

n i c a l l y , t h e y are the most i m p o r t a n t catalyst u s e d i n t h e p e t r o l e u m i n d u s t r y . S c i e n t i f i c a l l y , they are specimens " p a r excellence" f o r s t u d y i n g the n a t u r e of active centers o n a l u m i n a s i l i c a catalysts a n d f o r d e t e r m i n i n g t h e m o d e of a c t i v a t i o n of the various molecules w h o s e reactions t h e y accelerate.

T h e y h a v e the advantage over a l u m i n a s i l i c a g e l catalysts

i n that they are c r y s t a l l i n e . T h e m a i n features of their s t r u c t u r a l arrange m e n t c a n b e d e l i n e a t e d f r o m x-ray i n v e s t i g a t i o n . T h i s p e r m i t s the i d e n t i fication

a n d c h a r a c t e r i z a t i o n of a l i m i t e d n u m b e r of discrete sites of

c a t a l y t i c a c t i v i t y . O n the other h a n d , the structure of the a l u m i n a s i l i c a g e l catalyst c a n o n l y b e s u r m i s e d f r o m g e n e r a l t h e o r e t i c a l considerations. T h e x-ray a n d e l e c t r o n d i f f r a c t i o n patterns are too b r o a d to g i v e a n y u s e f u l s t r u c t u r a l i n t e r p r e t a t i o n . F u r t h e r m o r e , i n d i r e c t e v i d e n c e indicates a b r o a d c o n t i n u o u s s p e c t r u m of active sites reflecting t h e a m o r p h o u s state.

T h i s c o m p l i c a t e s c h a r a c t e r i z a t i o n of t h e a c t i v a t i o n process. A n 315

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

316

MOLECULAR

SIEVE

ZEOLITES

II

other c o m p l i c a t i n g feature of the g e l t y p e catalysts is the v a r i a b l e p o r e structure,

m a k i n g the

preparations

irreproducible and

the

catalytic

activity variable. W e present some r e c e n t results o b t a i n e d i n the P r i n c e t o n U n i v e r s i t y C h e m i s t r y D e p a r t m e n t o n the nature of the active center i n catalysts d e r i v e d f r o m s o d i u m t y p e faujasite catalyst (17, 29, 30, 31).

g i v e n b y one of us a n d b y others (1,4,9,

Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch063

18, 23, 25, 26, 27,

A g e n e r a l s u r v e y of the s t r u c t u r a l characteristics has 25,

28, been

33).

W e s h a l l r e v i e w b r i e f l y those s t r u c t u r a l results w h i c h w i l l b e r e l e v a n t to the d e t e r m i n a t i o n of the n a t u r e of the active site. s o d i u m faujasite

is N a

5 6

( A10 ) 2

5 6

( S i 0 ) i 6 * nH 0. 2

2

3

T h e formula for

T h e water can

r e m o v e d b y heat treatment w i t h o u t d e s t r o y i n g the c r y s t a l structure. n u m b e r of s o d i u m ions is e q u a l to the n u m b e r of a l u m i n u m atoms.

be The In

zeolites, this n u m b e r is at most e q u a l to the n u m b e r of s i l i c o n atoms. T h e a l u m i n u m atoms are n e v e r i n a d j o i n i n g o x y g e n t e t r a h e d r a b u t are separated b y at least 1 s i l i c o n - o x y g e n t e t r a h e d r o n .

The sodium ion can

b e i o n e x c h a n g e d b y m o n o v a l e n t , d i v a l e n t , a n d c e r t a i n t r i v a l e n t ions. I n this i o n exchange, c e r t a i n p r e c a u t i o n s m u s t b e o b s e r v e d .

T h e direct

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

treatment

m a y result i n the d e s t r u c t i o n of the zeolite c r y s t a l structure, p a r t i c u l a r l y w h e n the s i l i c o n - a l u m i n u m ratio is close to 1.

T h i s is a v o i d e d b y re

p l a c i n g s o d i u m b y a m m o n i u m i o n a n d t h e n d e c o m p o s i n g the a m m o n i u m ion. T h e w a s h i n g of the zeolite m u s t b e c a r r i e d out c a r e f u l l y to a v o i d h y d r o l y s i s a n d r e p l a c e m e n t of the c a t i o n b y the h y d r o g e n i o n . T h i s is p a r t i c u l a r l y true of the d i v a l e n t a n d t r i v a l e n t cations. A n o t h e r c o m p l i c a t i o n m a y arise i n the case of d i v a l e n t a n d t r i v a l e n t cations, i n that o n d e h y d r a t i o n of a h y d r o x y salt c a t i o n m a y b e f o r m e d s u c h as [ M ( O H ) ] , +

[ M ( III ) ( O H ) ] , 2

+

[ M ( III ) ( O H ) ] . 2 +

Since the c a t a l y t i c a c t i v i t y of the

a c i d f o r m of the zeolite is v e r y h i g h , the h y d r o l y s i s processes m a y m a s k the a c t i o n of the m u l t i v a l e n t ions a n d be a s c r i b e d to t h e m .

Sodium and

h y d r o g e n n u c l e a r m a g n e t i c studies i n d i c a t e that the s o d i u m ions

are

h y d r a t e d b y 6 w a t e r m o l e c u l e s i n a n o c t a h e d r a l a r r a y a n d are free to m o v e i n the zeolite c a v i t y w h e n the zeolite is h y d r a t e d . O n d e h y d r a t i o n , w h e n there are less t h a n 6 w a t e r m o l e c u l e s a r o u n d the s o d i u m i o n , it becomes l o c a l i z e d near 1 of the o x y g e n ions next to the a l u m i n u m . I n the faujasite t y p e zeolite, the s i l i c a - a l u m i n a tetrahedra are l i n k e d t h r o u g h s h a r i n g of o x y g e n i n t o 2 p r i m a r y cages: the h e x a g o n a l p r i s m a n d the t r u n c a t e d o c t a h e d r o n ( s o d a l i t e ) .

T h e s e p r i m a r y cages c o m b i n e

to f o r m a superstructure of cages b y h a v i n g 4 of the h e x a g o n a l faces of the sodalite l i n k e d i n a t e t r a h e d r a l m a n n e r via " v i r t u a l " h e x a g o n a l p r i s m s to g i v e the faujasite structure w i t h the i m p o r t a n t s e c o n d a r y cage, the faujasite cage. T h i s is the cage w h e r e i n the c a t a l y t i c a c t i v i t y takes p l a c e .


63.

Catalytic

TURKEVICH AND Ο Ν Ο

Activity

and Active Centers

317

Its 4 ports h a v e a d i a m e t e r 8 - 9 A , p e r m i t t i n g b o t h straight a n d b r a n c h e d h y d r o c a r b o n s to enter the pore. T h e p o r t holes of t h e sodalite cage a n d the h e x a g o n a l p r i s m s are too s m a l l f o r the h y d r o c a r b o n to enter, b u t t h e y d o p l a y a n i m p o r t a n t r o l e i n the a c t i v a t i o n process.

T h e n u m b e r of

a c i d sites c a n b e c o n t r o l l e d b y t h e d e p t h of t h e r e p l a c e m e n t of s o d i u m b y t h e a m m o n i u m i o n . T h e r e are 3 different sites f o r t h e s o d i u m i o n , 16 i n t h e i n t e r i o r of t h e h e x a g o n a l p r i s m s , 32 at t h e h e x a g o n a l faces i n Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch063

the sodalite cages, a n d 8 i n the large cage next to t h e square o x y g e n faces (20,

21).

T h e kinetics of c r a c k i n g of c u m e n e w a s d e t e r m i n e d b y t h e p u l s e technique introduced b y Emmett, Kokes, a n d T o b i n ( S ) , modified b y T u r k e v i c h a n d c o w o r k e r s to d e t e r m i n e t h e n u m b e r of active sites a n d the specific a c t i v i t y (30).

T h e n u m b e r of sites active f o r c u m e n e c r a c k i n g

was e q u a l to t h e n u m b e r of s o d i u m ions r e p l a c e d b y h y d r o g e n ions the a m m o n i u m r o u t e ) replaced.

(via

u n t i l a b o u t h a l f of t h e s o d i u m ions h a d b e e n

F r o m that, the n u m b e r

of active

sites r e m a i n e d

constant.

A p p a r e n t l y , h a l f of the a c i d sites p r o d u c e d b y r e p l a c e m e n t of t h e s o d i u m i o n b y a m m o n i u m i o n a n d subsequent h e a t i n g w e r e n o t a v a i l a b l e f o r c a t a l y t i c a c t i v i t y a n d m u s t b e l o c a t e d i n either the sodalite cage or t h e h e x a g o n a l p r i s m b u t n o t i n t h e faujasite supercage.

Furthermore, the

t e c h n i q u e of a l t e r n a t i n g pulses of r e a c t i o n ( c u m e n e ) a n d p o i s o n ( q u i n o l i n e ) p e r m i t t e d t h e d e t e r m i n a t i o n of a c t i v i t y p e r active site.

This i n

creased w i t h t h e n u m b e r of sites e v e n w h e n t h e sites are n o t a v a i l a b l e to the reactant, i n d i c a t i n g a n i n t e r a c t i o n b e t w e e n the sites via transport of h y d r o g e n i o n o r electrons, b o t h of w h i c h c o u l d pass t h r o u g h t h e s m a l l p o r t holes.

F u r t h e r m o r e , m e a s u r e m e n t of t h e t e m p e r a t u r e coefficient of

the rate of c r a c k i n g of c u m e n e p e r m i t t e d t h e d e t e r m i n a t i o n of b o t h t h e a c t i v a t i o n energy of t h e c r a c k i n g r e a c t i o n a n d t h e e n t r o p y of a c t i v a t i o n . T h e a c t i v a t i o n energy decreased w i t h increase i n n u m b e r of sites b e c a u s e of the i n t e r a c t i o n of sites.

O n t h e other h a n d , t h e e n t r o p y of a c t i v a t i o n

was n e g a t i v e , i n d i c a t i n g that i n t h e t r a n s i t i o n state t h e a d s o r b e d m o l e cules are o r d e r e d .

F u r t h e r m o r e , w i t h increase i n n u m b e r of sites, t h e

e n t r o p y of a c t i v a t i o n decreases, i n d i c a t i n g that t h e greater t h e n u m b e r of sites, the greater

the r e q u i r e d o r d e r i n g .

I n t h e c a t a l y t i c c r a c k i n g of

c u m e n e , a large n u m b e r of c u m e n e m o l e c u l e s a d s o r b e d o n a large n u m b e r of i n t e r a c t i n g sites, w i t h 1 of these a d s o r b e d m o l e c u l e s u n d e r g o i n g the r e a c t i o n . T h e process does n o t i n v o l v e 1 c u m e n e m o l e c u l e r e a c t i n g w i t h 1 site. I t is the c r y s t a l l i n e structure of t h e zeolite, w h i c h p e r m i t s inter a c t i o n b e t w e e n a large n u m b e r of a c i d sites, that makes t h e zeolite catalyst s u p e r i o r to the a m o r p h o u s a l u m i n a s i l i c a g e l catalyst. T h e next step i n the d e t e r m i n a t i o n of t h e m e c h a n i s m of the c a t a l y t i c a c t i o n o f t h e a c i d zeolite catalyst is e l u c i d a t i n g the n a t u r e of the a c t i v e


318

M O L E C U L A R SIEVE

ZEOLITES

II

site—is i t a B r o n s t e d a c i d , L e w i s a c i d , o r base site? T h e B r o n s t e d a c i d site is a p r o t o n a d s o r b e d o n a n o x y g e n a t o m next to a n a l u m i n u m a t o m . T h e L e w i s a c i d sites are p r o d u c e d f r o m 2 B r o n s t e d sites b y e l i m i n a t i o n of w a t e r w i t h 2 protons a n d a n o x y g e n f r o m t h e a l u m i n u m — s i l i c o n b o n d . T h e d e h y d r a t i o n process w a s q u a n t i t a t i v e l y s t u d i e d b y T u r k e v i c h a n d C i b o r o w s k i (27),

u s i n g a stream of h e l i u m gas f o r d e h y d r a t i o n a n d a

t h e r m a l c o n d u c t i v i t y f o r d e t e r m i n a t i o n of t h e a m o u n t of w a t e r o r a m Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch063

m o n i a p r o d u c e d at a n y d e s i r e d t e m p e r a t u r e .

A l l the water was removed

f r o m the s o d i u m faujasite b y 2 5 0 ° C a n d n o f u r t h e r w a t e r w a s e v o l v e d e v e n w h e n the s o d i u m faujasite w a s h e a t e d to 9 0 0 ° C .

T h i s w a s t a k e n as

p r o o f that i n a s o d i u m zeolite there are n o free h y d r o x y l groups n o r a n y h y d r o g e n b o n d s h o l d i n g the structure together.

T h e s e findings are c o n

sistent w i t h the structure of t h e faujasite as d e t e r m i n e d b y x - r a y c r y s t a l l o g r a p h y . T h e w a t e r e v o l v e d i n t h e d e h y d r a t i o n of the s o d i u m faujasite was the w a t e r i n the cavities. W h e n t h e same p r o c e d u r e w a s a p p l i e d to a m m o n i u m faujasite, the same a m o u n t o f w a t e r w a s p r o d u c e d b e l o w 2 5 0 ° C as f r o m the p u r e s o d i u m faujasite. c o m p l e t e d b y heat treatment

at 4 0 0 ° C .

T h e a m m o n i a e v o l u t i o n is

T h e p r o d u c t at this stage is

essentially a B r o n s t e d a c i d . F u r t h e r heat treatment i n t h e t e m p e r a t u r e range of 4 5 0 ° - 6 0 0 ° C

p r o d u c e s f u r t h e r d e h y d r a t i o n , w h i c h converts the

B r o n s t e d acids i n t o a L e w i s a c i d a n d a B r o n s t e d base site w i t h r e m o v a l of 2 h y d r o g e n atoms a n d a n o x y g e n b e t w e e n a n a l u m i n u m a n d s i l i c o n a t o m i n the faujasite skeleton. T h u s , t h e c o n d i t i o n s h a v e b e e n established f o r the p r e p a r a t i o n of t h e a c i d zeolite i n t h e B r o n s t e d f o r m — h e l i u m flow gas at a t e m p e r a t u r e u p to 400 ° C — a n d the L e w i s a c i d B r o n s t e d base f o r m b y d e h y d r a t i o n i n a flow of h e l i u m at a t e m p e r a t u r e 550°C.

at least o f

A m i x t u r e of b o t h types of a c i d centers is p r o d u c e d i n t h e t e m

p e r a t u r e r a n g e of 4 5 0 ° - 5 5 0 ° C .

W e are thus i n a p o s i t i o n to establish

w h i c h t y p e of a c i d i t y is r e s p o n s i b l e f o r the various types of c a t a l y t i c a c t i v i t y of the zeolites. O t h e r w o r k e r s h a v e s t u d i e d the t h e r m a l d e c o m p o s i t i o n of s o d i u m a n d a m m o n i u m zeolites b o t h b e f o r e a n d after the T u r k e v i c h a n d C i b o r o w s k i studies. T h e results w e r e at best s e m i q u a n t i t a t i v e , since t h e y w e r e c a r r i e d o u t i n a static gas flow or i n a v a c u u m w h e r e the rate of r e m o v a l of gaseous p r o d u c t s , w a t e r , a n d a m m o n i a w a s u n d o u b t e d l y d i f f u s i o n c o n trolled.

T h e analysis w a s m a d e u s i n g moist l i t m u s p a p e r or i n f r a r e d

spectral bands, both methods qualitative or semiquantitative. titative r e l a t i o n s h i p w a s established b e t w e e n e v o l v e d i n the first stage a n d the w a t e r

N o quan

the a m o u n t of a m m o n i a

e v o l v e d i n t h e process

of

d e h y d r o x y l a t i o n i n t h e s e c o n d stage (2, 3, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16,19,22,24,32,

34,35,36).


63.

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319


Experimental T h e catalyst w a s p r e p a r e d b y 70-hour treatment of 100 grams of L i n d e s o d i u m Y z e o l i t e w i t h 1700 m l of 22.1 w t % of a m m o n i u m nitrate. T h e s l u r r y w a s o c c a s i o n a l l y s t i r r e d . A f t e r filtration, t h e s o l i d w a s w a s h e d s e v e r a l times w i t h d i s t i l l e d w a t e r a n d d r i e d at r o o m t e m p e r a t u r e . T h e a m o u n t of s o d i u m r e p l a c e d b y a m m o n i u m i o n w a s 5 2 % b y analysis o f the filtrate f o r s o d i u m u s i n g t h e m a g n e s i u m u r a n y l acetate reagent. T h i s compares f a v o r a b l y w i t h the result o b t a i n e d b y T u r k e v i c h a n d C i b o r o w ski. T h e p a l l a d i u m catalyst w a s p r e p a r e d b y c o n t a c t i n g a s a m p l e of s o d i u m Y z e o l i t e w i t h P d ( N H ) C l to g i v e P d 3 . 5 % , N a 9 6 . 4 % Y z e o l i t e a n d s i m i l a r l y t r e a t i n g a s a m p l e of N H 5 4 % , N a 4 6 % Y zeolite to g i v e Pd 2.6%, N H 5 4 % , N a 4 6 % Y zeolite. E x p e r i m e n t s s t u d y i n g the c h a n g e i n r e t e n t i o n t i m e of b r a n c h e d h y d r o c a r b o n s s h o w e d that there w a s n o c h a n g e i n p o r e size, a n d conse q u e n t l y c r y s t a l l i n e structure, o n t h e heat p r e t r e a t m e n t i n t h e t e m p e r a t u r e studied. A conventional pulse catalytic microreactor was used w i t h 15-65 m g of t h e catalyst f o r the c u m e n e runs a n d 65 m g f o r the 2 , 3 - d i m e t h y l b u t a n e runs. T h e catalyst w a s h e l d b e t w e e n 2 s m a l l p l u g s of b o r o s i l i c a t e glass w o o l i n a 5 - m m I D d i a m e t e r b o r o s i l i c a t e reactor. I n some experiments, the catalyst w a s d i l u t e d w i t h 9 6 % s i l i c a p o r o u s glass p o w d e r . T h e h e l i u m gas w a s p u r i f i e d b y passage t h r o u g h a l u m i n a k e p t at l i q u i d n i t r o g e n temperature. T h e reaction temperature was measured b y a thermocouple l o c a t e d adjacent to the reactor. T h e catalyst was p r e t r e a t e d at t h e d e s i r e d t e m p e r a t u r e f o r 16 hours i n a stream of h e l i u m . T h e p r o d u c t s w e r e a n a l y z e d w i t h a d i o c t y l p h t h a l a t e gas c h r o m a t o g r a p h y c o l u m n at 1 1 0 ° C . 3

4

2

4

4

Cumene

+

+

Cracking

T h e c u m e n e c r a c k i n g r e a c t i o n w a s s t u d i e d at 3 2 5 ° C . kinetic equation F k =

W r

W

T h e simple

1 In 1

— χ

( w h e r e F is the flow rate of the c a r r i e r gas, W t h e w e i g h t of t h e catalyst, k the r e a c t i o n constant, a n d χ t h e i n i t i a l f r a c t i o n c o n v e r t e d )

could not

be u s e d because t h e active catalysts (those p r e p a r e d b y heat treatment b e l o w 5 0 0 ° C ) a l w a y s gave c o n v e r s i o n a b o v e the e q u i l i b r i u m c o n v e r s i o n . T h i s w a s c a u s e d b y t h e s e p a r a t i o n of the p r o d u c t s f r o m e a c h other a n d f r o m the c u m e n e i n the reactor.

T h e r e is a sharp d r o p i n t h e a c t i v i t y of

the catalyst o n pretreatment of the catalyst at temperatures above 5 0 0 ° C . T h e a c t i v a t i o n e n e r g y of the r e a c t i o n i n the t e m p e r a t u r e range of 2 8 5 ° — 325 ° C f o r a n active catalyst w a s e s t i m a t e d to b e 24 k c a l . T h e q u i n o l i n e t i t r a t i o n of the samples o b t a i n e d b y heat treatment at v a r i o u s t e m p e r a tures s h o w e d a s i m i l a r decrease i n the n u m b e r of active centers w i t h increase

i n the t e m p e r a t u r e

of p r e t r e a t m e n t

( F i g u r e 1 ) , a n d this is

s i m i l a r to the c u r v e o b t a i n e d b y d e h y d r a t i o n of the B r o n s t e d acids into



320

M O L E C U L A R SIEVE ZEOLITES

350

400

450

500

550

Π

600

TEMPERATURE OF PRETREATMENT (°C)

Figuré 1. Effect of pretreatment temperature on catalytic cracking of cumene and on hydrogendeuterium exchange L e w i s a c i d a n d B r o n s t e d bases. T h u s , B r o n s t e d a c i d sites are r e s p o n s i b l e for the catalytic cracking of cumene.

T h e q u i n o l i n e does n o t react w i t h

the L e w i s a c i d sites at 3 2 5 ° C b u t o n l y w i t h t h e B r o n s t e d a c i d sites. F u r t h e r m o r e , f o r samples w h o s e p r e t r e a t m e n t t e m p e r a t u r e is e q u a l o r less t h a n 400 ° C , t h e n u m b e r o f active sites is 8.3 χ 1 0 . T h i s v a l u e is 20

to b e c o m p a r e d w i t h 9.9 X 1 0 / g r a m 20

calculated f r o m stoichiometry,

a s s u m i n g 25 w t % o f w a t e r a n d a m m o n i a as d e t e r m i n e d b y T u r k e v i c h and Ciborowski. T h e role of p a l l a d i u m i n zeolite was investigated.

T h e N a 100%

z e o l i t e has n o a c t i v i t y f o r c u m e n e c r a c k i n g at 450 ° C . O n t h e other h a n d , P d 3 5 % N a 9 6 . 4 % Y zeolite cracks c u m e n e i n a stream o f h y d r o g e n w i t h rate constants o f 16.7 m l / m i n / g r a m at 400 ° C a n d 63.8 m l / m i n / g r a m at 443 ° C .

T h e chromatographic column d i d not distinguish between

propylene a n d propane.

T h e p a l l a d i u m u n d o u b t e d l y is r e d u c e d t o t h e

m e t a l , p r o d u c i n g protons o n t h e surface w h i c h act as B r o n s t e d a c i d . T h e i n t r o d u c t i o n o f d e c a t i o n a t e d sites enhances t h e f a v o r a b l e effect o f p a l -


63.

TURKEVICH AND ΟΝΟ

Catalytic

Activity

and Active Centers

l a d i u m , the rate constant at 3 2 5 ° C for P d 2 . 6 %

N H

4

54%

321

N a 43%

Y

zeolite b e i n g 1850 after the catalyst is p l a c e d i n a stream of h y d r o g e n o v e r n i g h t at 4 4 0 ° C .

T h e p a l l a d i u m c a n b e v i s u a l i z e d as a c t i v a t i n g the

h y d r o g e n to r e m o v e carbonaceous at these h i g h temperatures.

m a t e r i a l w h i c h f o r m s o n the surface

H o w e v e r , it is also possible that the p a l

l a d i u m h e t e r o l y t i c a l l y dissociates the h y d r o g e n m o l e c u l e i n t o H " a n d H , +

a n d these c o n v e r t a n y s m a l l n u m b e r of L e w i s a c i d - B r o n s t e d base o n the Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch063

surface i n t o a l u m i n u m h y d r i d e a n d B r o n s t e d a c i d sites. c a r r i e d out at h i g h e r temperatures

Experiments

c o n f i r m this p o i n t of v i e w .

With

p a l l a d i u m o n d e c a t i o n a t e d zeolite, c r a c k i n g of c u m e n e c a n take

place

when

catalyst

is treated

at m u c h h i g h e r temperatures

than without

p a l l a d i u m — e . g . , the rate constant (at 3 2 5 ° C ) for the catalyst t r e a t e d at 4 9 0 ° C is 1850, at 5 4 0 ° C is 1190, at 4 5 4 ° C is 1070, at 5 5 6 ° C is 440, a n d at 6 0 0 ° C is 300.

A t these temperatures, the B r o n s t e d acids w o u l d

be

c o n v e r t e d to L e w i s a c i d - B r o n s t e d base sites. T h e s e w o u l d b e ineffective for c u m e n e c r a c k i n g a n d w o u l d f a v o r c a r b o n f o r m a t i o n . Cracking

of 2,3

-Dimethylbutanè

T h e c r a c k i n g of this b r a n c h e d c h a i n p a r a f f i n w a s c a r r i e d o u t 400°-450°C.

at

T h e p r o d u c t s of the r e a c t i o n w e r e n o t s i m p l y p r o p y l e n e

a n d p r o p a n e , as w o u l d be e x p e c t e d f r o m s i m p l e scission, b u t 38.4 m o l e % C H , 24.8% 3

8

0.8%

C H , 13.8% 3

C H , 3.7%

6

of a n o n i d e n t i f i a b l e C

2

4

4

C H , 4.2% 2

C H

6

4

1 0

, 1.3%

and

hydrocarbon. T h e conversion was propor

t i o n a l to the w e i g h t of the catalyst u s e d a n d i n v e r s e l y p r o p o r t i o n a l to the flow rate of the carrier gas.

A n e x a m i n a t i o n of the degree of de-

c a t i o n i z a t i o n o n the r e a c t i o n s h o w e d that the 1 2 % for s o d i u m d i d not p r o d u c e active centers,

ammonia

exchange

b u t f r o m that p o i n t

the

a c t i v i t y of the catalyst was p r o p o r t i o n a l to the degree of d e c a t i o n a t i o n . T h e a c t i v a t i o n e n e r g y is 27 k c a l / m o l e . T h e effect of the t e m p e r a t u r e of p r e t r e a t m e n t o n the a c t i v i t y of the catalyst shows a m a x i m u m of a c t i v i t y after pretreatment at 4 5 0 ° C a n d t h e n a m a r k e d d r o p i n the

500°-530°C

r e g i o n ( F i g u r e 2 ). T h e s e results suggest that L e w i s a c i d sites are neces sary i n a d d i t i o n to B r o n s t e d a c i d sites to c r a c k b r a n c h e d c h a i n h y d r o carbons.

T h e s e L e w i s a c i d sites abstract

a hydride ion, producing a

c a r b o n i u m i o n w h i c h t h e n undergoes v a r i o u s c a r b o n i u m i o n reactions w i t h the h e l p of B r o n s t e d a c i d a n d base sites. T h e necessity of a c e r t a i n n u m b e r of L e w i s sites for the c r a c k i n g of paraffins finds f u r t h e r s u p p o r t i n the results o b t a i n e d w i t h p a l l a d i u m .

Replacement

of 3.4%

of

the

s o d i u m ions b y p a l l a d i u m does not enhance the c r a c k i n g of 2 , 3 - d i m e t h y l butane.

T h e i n t r o d u c t i o n of p a l l a d i u m i n the d e c a t i o n a t e d Y z e o l i t e

( P d 2 . 6 % , N H 5 4 % , N a 43.4% ) a n d treatment w i t h h y d r o g e n at 3 6 0 ° 4

or 495 ° C o v e r n i g h t p r o d u c e d a catalyst w h i c h s h o w e d n o a c t i v i t y f o r


322


Π


10,

ot=-=J 400

1 450

1 500

1— 550

Figure 2. Effect of pretreatment tem perature on catalytic cracking of 2,3-dimeihylbutane, using a decationated zeo lite and a paUadium-decationated zeolite the c r a c k i n g of 2 , 3 - d i m e t h y l b u t a n e at 4 0 0 ° C , w h i l e i t c r a c k e d r e a d i l y at 3 2 5 ° C .

cumene

T h e f u n c t i o n of p a l l a d i u m a n d p r o b a b l y other m e t a l

h y d r o g e n a t i n g a d d i t i o n s i n s u p p o r t e d o x i d e systems is to c a t a l y z e t h e h e t e r o l y t i c s p l i t t i n g of h y d r o g e n gas i n t o n e g a t i v e h y d r i d e i o n a n d a proton.

T h e n e g a t i v e h y d r i d e i o n serves as a h y d r o g e n b r i d g e b e t w e e n

the a l u m i n u m a n d s i l i c o n atoms of the L e w i s a c i d site, a n d t h e p r o t o n n e u t r a l i z e s t h e B r o n s t e d base site to m a k e a B r o n s t e d a c i d . T h i s a c i d m a y b e a l l the stronger b e c a u s e of t h e close p r o x i m i t y of the A l - H - S i b o n d . T h u s , b y using p a l l a d i u m a n d a decationated zeolite, the c r a c k i n g of i n d u s t r i a l l y v a l u a b l e b r a n c h e d c h a i n h y d r o c a r b o n s is m i n i m i z e d w h i l e the " c l i p p i n g off" of a l k y l side chains f r o m aromatics c a n b e c a r r i e d o u t r e a d i l y e v e n at h i g h temperatures. Cracking

of Branched Chain Olefins

T h e c r a c k i n g of the 2 , 3 - d i m e t h y l - 2 - b u t e n e a n d 2 , 3 - d i m e t h y l - l - b u t e n e s h o w e d that t h e rates of t h e v a r i o u s reactions t h e y u n d e r g o w e r e i n d e p e n d e n t of t h e p o s i t i o n of the d o u b l e b o n d or the t e m p e r a t u r e of p r e t r e a t m e n t of t h e catalyst.

B o t h B r o n s t e d a n d L e w i s acids w e r e effective.

A t 200 ° C , the d o u b l e b o n d m i g r a t i o n r e a c t i o n w a s d o m i n a n t a n d a significant c a r b o n skeleton i s o m e r i z a t i o n to 3 , 3 - d i m e t h y l - l - b u t e n e w a s observed.

A s l i g h t a m o u n t of c r a c k i n g w a s n o t e d .

A t 3 0 0 ° C , t h e 2,3-

d i m e t h y l b u t e n e s h a d i s o m e r i z e d extensively i n t o t h e 3 , 3 - d i m e t h y l b u t e n e , a n d this c o m p o u n d u n d e r w e n t c r a c k i n g to p r o d u c e 1 3 % C ' s a n d 2 6 % 5


63.

TURKEVICH AND ΟΝΟ

C 's.

Catalytic

Activity

and Active Centers

323

A t 4 0 0 ° C , a n appreciable amount of 3,3-dimethylbutene still per

4

sists, the C s a n d C s r e m a i n at t h e same l e v e l as at 3 0 0 ° C , b u t the y i e l d 4

5

of p r o p y l e n e increases to 1 0 % o f t h e p r o d u c t .

T h e s e results i n d i c a t e

that t h e c r a c k i n g o f a n olefin first i n v o l v e s a skeleton i s o m e r i z a t i o n to a n olefin w h o s e subsequent c r a c k i n g reactions are n o t e x p l a i n e d r e a d i l y


by a simple carbonium ion mechanism.

Xylene

Isomerization

T h e c o n v e r s i o n o f ortho- t o meta- a n d p a r a - x y l e n e w a s c a r r i e d o u t o n a series o f d e c a t i o n a t e d catalysts w h i c h w e r e subjected t o t h e r m a l treatment at v a r i o u s temperatures.

10-yl pulses of o-xylene w e r e u s e d ,

the catalyst a m o u n t w a s 3 0 0 - 3 5 0 m g , a n d t h e flow rate o f t h e h e l i u m c a r r i e r gas w a s 5 0 - 1 0 0 m l / m i n .

T h e p r o d u c t s w e r e a n a l y z e d o n 7.8-

benzoquinoline o n Chromosorb W gas-chromatographic column. T h e ra-xylene

p r e d o m i n a t e d over the p a r a i s o m e r . A s m a l l toluene p r o d u c t i o n

s e e m e d to p a r a l l e l that o f i s o m e r i z a t i o n . T h e c o n v e r s i o n o f x y l e n e

400

440

480

520

560

TEMPERATURE OF PRETREATMENT (°C)

Figure 3. Effect of pretreatment temperature on catalytic isomerization of ortho-xylene


χ

324


( r a t i o of meta-

a n d para-xylenes to t o t a l x y l e n e s )

followed

II

first-order

kinetics

where x

eq

w a s t h e e q u i l i b r i u m v a l u e of 0.782. T h e p l o t o f the l o g a r i t h m

t e r m against W/F Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch063

0

gave a straight l i n e w h o s e slope gave t h e

first-order

r e a c t i o n constant, k. A p l o t of k against t h e t e m p e r a t u r e of the pretreat m e n t of t h e catalyst s h o w e d a m a r k e d d r o p i n t h e c u r v e at 5 0 0 ° - 5 2 0 ° C , a g a i n i n d i c a t i n g that B r o n s t e d

a c i d sites are r e s p o n s i b l e

f o r xylene

i s o m e r i z a t i o n ( F i g u r e 3 ) (15, 36). N o t a l l reactions take p l a c e o n t h e B r o n s t e d a c i d sites. T h e h y d r o gen-deuterium

equilibration

takes

place

on the L e w i s

acid

sites

(Figure 1). T h e c h a r a c t e r i z a t i o n o f these L e w i s a c i d sites b y m a g n e t i c resonance techniques has b e e n extensively p u r s u e d i n t h e l a b o r a t o r y .

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)

Barrer, R. M., Endeavour 1964, 23, 122. Benesi, Η. Α., J. Catalysis 1967, 8, 368. Bertsch, L., Hapgood, H. W.,J.Phys. Chem. 1963, 67, 1621. Breck, D. W.,J.Chem. Educ. 1964, 41, 678. Carter, J. L., Lucchesi, P. J., Yates, D. J.C.,J.Phys. Chem. 1961, 68, 1385. Cattanach, J., Wu, E. L., Venuto, P. B., J. Catalysis 1968, 11, 342. Eberly, P.K.,J.Phys. Chem. 1968, 72, 1042. Emmett, P.H.,Kokes, R. V., Tobin, H. P.,J.Am. Chem. Soc. 1955, 77, 5860. Fischer, L. F., Meier, W. M., Fortschr. Mineral. 1965, 42, 50-86. Hughes, T. R., White, H. M.,J.Phys. Chem. 1967, 71, 2192. Kerr, G. T.,J.Catalysis 1969, 15, 200. Kerr, G. T.,J.Phys. Chem. 1967, 71, 4155. Ibid., 1969, 73, 2780. Kerr, G. T., Shipman, G. F.,J.Phys. Chem. 1968, 72, 3071. Hansford, R. C., Ward, J. W.,J.Catalysis 1969, 13, 316. Liengme, Β. V., Hall, W. K., Trans. Faraday Soc. 1966, 62, 3229. Mackey, J., Thomas, W. H., Turkevich, J., Actes Congr. Intern. Catalyse, 2nd, Paris, 1960, 1961, 1815. Nicula, Α., Stamires, D., Turkevich, J.,J.Chem. Phys. 1965, 42, 3684. Peri, J. B., Actes Congr. Intern. Catalyse, 2nd, Paris, 1960, 1961, 1, 1333. Pickert, P. E., Rabo, J. Α., Dempsey, E., Schomaker, V., Proc. Intern. Congr. Catalysis, 3rd, Amsterdam, 1964, 1965, 714. Rabo, J. Α., Angell, C. L., Kasai, P. H., Shomaker, V., Disc. Faraday Soc. 1966, 41, 328. Smith, J. V., Bennett, J. M., Flanigen, Ε. M., Nature 1967, 215, 241. Stamires, D. N., Turkevich, J.,J.Am. Chem. Soc. 1964, 86, 749, 757. Szymanski, Η. Α., Stamires, D. N., Lynch, G. R.,J.Apt. Soc. Am. 1960, 50, 1323. Turkevich, J., Catalysis Rev. 1967, 1, 1-35.



63. TURKEVICH AND ΟΝΟ Catalytic Activity and Active Centers 325

(26) Turkevich, J., Preprints of Papers from USA for Japan USA Seminar on Catalytic Science, A-3-1-A-3-27, Tokyo and Kyoto, Japan, May 6, 1968. (27) Turkevich, J., Ciborowski, S., J. Phys. Chem. 1967, 71, 3208. (28) Turkevich, J., Ichikawa, Α., Ikawa, T., 140th Meeting, ACS, Chicago, 1961. (29) Turkevich, J., Murakami, Y., Nozaki, F., Ciborowski, S., Chem. Eng. Progr. 1967, 63, 75. (30) Turkevich, J., Nozaki, F., Stamires, D. N., Proc. Intern. Congr. Catalysis, 3rd, Amsterdam, 1964, 1965, 586. (31) Turkevich, J., Ono, Y., Bull. Polytech. Inst. Jassy (Roumania), in press. (32) Uytterhoeven, J. B., Christner, L. G., Hall, W. K., J. Phys. Chem. 1965, 69, 2117. (33) Venuto, S. B., Landis, P. S., Advan. Catalysis 1968, 18, 259. (34) Ward, J. W., J. Catalysis 1967, 9, 225. (35) Ibid., 1968, 10, 34. (36) Ibid., 1969, 13, 321. RECEIVED January 30, 1970.

Discussion R. C . Pink ( Q u e e n s U n i v e r s i t y , Belfast, N o r t h e r n I r e l a n d ) : T u r k e v i c h has s a i d that o n l y the D - H exchange r e a c t i o n c a n , w i t h certainty, b e a t t r i b u t e d to the L e w i s a c i d a c t i v i t y . T h e c y c l o p r o p a n e i s o m e r i z a t i o n r e a c t i o n , h o w e v e r , seems to r e s p o n d to b o t h the B r o n s t e d a n d the L e w i s activity.

NH

4

+

Y zeolites a c t i v a t e d at different temperatures s h o w t w o

t e m p e r a t u r e regions of a c t i v i t y , o n e c o r r e s p o n d i n g closely to the B r o n s t e d a c t i v i t y of the catalyst, the other at a m u c h h i g h e r t e m p e r a t u r e ( — 6 5 0 ° ) c o r r e s p o n d i n g to t h e t e m p e r a t u r e at w h i c h the e l e c t r o n d o n o r - a c c e p t o r p r o p e r t i e s are at a m a x i m u m . J. T u r k e v i c h :

I was i n error i n the statement that h y d r o g e n - d e u

t e r i u m exchange r e a c t i o n is the o n l y r e a c t i o n that takes p l a c e w i t h the h e l p of a c i d sites, f o r w e h a v e f o u n d that butene-1 to butene-2

trans

f o r m a t i o n is c a t a l y z e d b y b o t h B r o n s t e d a n d L e w i s a c i d sites. T h e B r o n sted sites, h o w e v e r , g i v e a m a r k e d stereospecificity i n p r o d u c i n g n o n e q u i l i b r i u m m i x t u r e s of cis a n d trans, w h i l e the L e w i s a c i d sites g i v e a n e q u i l i b r i u m m i x t u r e of the g e o m e t r i c isomers. M . S. Goldstein ( A m e r i c a n C y a n a m i d , S t a m f o r d , C o n n . 0 6 9 0 2 ) : t h i n k that the results f r o m y o u r p u l s e m e t h o d a n d o u r c o n t i n u o u s

I

flow

m e t h o d f o r p o i s o n i n g w i t h q u i n o l i n e ( G o l d s t e i n , M . S., M o r g a n , T . R . , /. Catalysis

1970, 16, 232) are i n g e n e r a l agreement.

However, we found

a correspondence between quinoline adsorption a n d quinoline poisoning. W e also f o u n d that N a Y a d s o r b e d q u i n o l i n e as w e l l as H Y . W e inter p r e t e d the q u i n o l i n e p o i s o n i n g as c a u s e d b y supercage b l o c k a g e .


Would

326


II

y o u agree that o u r results i n d i c a t e that q u i n o l i n e is n o t a s p e c i f i c p o i s o n f o r B r o n s t e d sites? J . T u r k e v i c h : N o , w e are c o n v i n c e d m o r e t h a n ever f r o m the q u a n t i t a t i v e results that w e h a v e p u b l i s h e d that q u i n o l i n e i n the p o i s o n i n g experiments b l o c k s specific sites i n supercages.

T o t a l a d s o r p t i o n of q u i n

o l i n e c a n take p l a c e b e y o n d this specific p o i s o n i n g a n d , b e i n g n o n s p e c i f i c to sites, has n o r e l e v a n c e to catalysis. Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch063

P . B . V e n u t o ( M o b i l O i l C o r p . , Paulsboro, N . J. 08034): I n your s e c o n d s l i d e , y o u s h o w e d a p l o t of c a t a l y t i c a c t i v i t y vs. n u m b e r of c a t a l y t i c sites f o r a n a m m o n i u m - e x c h a n g e d Y system.

I n this s l i d e , c a t a l y t i c ac

t i v i t y d i d n o t increase i n a s i m p l e l i n e a r r e l a t i o n s h i p ; rather, it i n c r e a s e d g r e a t l y at the h i g h e r site concentrations.

D o y o u a t t r i b u t e this to the

a p p e a r a n c e of sites w i t h h i g h e r e n e r g y ( d i f f e r e n t t y p e sites) or to the " c o l l e c t i v e " a c t i o n of sites of s i m i l a r types? J . T u r k e v i c h : T h e s l i d e w a s f r o m a n earlier p u b l i s h e d w o r k p r e sented at the A m s t e r d a m I n t e r n a t i o n a l C o n g r e s s o n C a t a l y s i s . It indicates first that the c a t a l y t i c a c t i v i t y p e r site is not constant as one increases the n u m b e r of sites, b u t m o r e d r a m a t i c a l l y that it increases w i t h increases i n site n u m b e r e v e n w h e n these n e w sites are n o t a v a i l a b l e to the substrate (cumene)

or p o i s o n q u i n o l i n e . T h i s w e i n t e r p r e t to b e o w i n g to inter

a c t i o n a n d m i g r a t i o n of protons ( a n d / o r electrons)

a m o n g the v a r i o u s

sites.


Catalytic Activity and the Nature of Active ... - ACS Publications

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