Platinum Catalysts Supported on Zeolite. Hydrogenation of

44 Platinum Catalysts Supported on Zeolite. Hydrogenation of Cyclopropane and Hydrogenolysis of Ethane

Downloaded by TOWSON UNIV on October 19, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch044

C. NACCACHE, N. KAUFHERR, M. DUFAUX, J. BANDIERA, and B. IMELIK Institut de Recherches sur la Catalyse, 79, boulevard du 11 Novembre 1918, 69626, Villeurbanne Cédex, France ABSTRACT Pt-zeolite catalysts were prepared by ion exchange technique. Hydrogen chemisorption and electron microscopy have been employed for measurements of metal dispersion and p a r t i c l e size. 10 Å metal aggregates were found on zeolites calcined i n oxygen at 623°K and 15-20 Åsize metal c r y s t a l l i t e s on zeolites calcined at 773°K. Comparison of the catalytic a c t i v i t y for ethane hydrogenolysis over Pt-zeolite and Pt-SiO2 showed a minor effect of the c a r r i e r . In contrast, the patterns of catalytic a c t i v i t y for cyclopropane hydrogenation indicated a high promoting effect of the zeolite which was interpreted i n terms of the electrostatic f i e l d .

Introduction Since the importance of metal d i s p e r s i o n i n the e f f i c i e n t use of metal c a t a l y s t s has been w e l l e s t a b l i s h e d , extensive r e s e a r c h has been c a r r i e d out to develop methods o f p r e p a r a t i o n that produce f i n e l y dispersed metal c a t a l y s t s . Several r e p o r t s have been pub l i s h e d on the use o f z e o l i t e s as c a r r i e r s . Rabo e t a l . (J_) who prepared supported P t - z e o l i t e s suggested that the metal was almost a t o m i c a l l y d i s p e r s e d . Lewis (2) found that platinum i n z e o l i t e s was present as two s i z e s , p a r t i c l e s of 10 Â s i z e , small enough to f i t i n s i d e the z e o l i t e cages and o f 60 Â s i z e on the e x t e r n a l s u r f a c e . Kubo e t a l (3) concluded from chemisorption and e l e c t r o n microscopy s t u d i e s that the platinum p a r t i c l e s i z e was s t r o n g l y dependent on the c a l c i n a t i o n temperature o f Pt exchanged z e o l i t e before I ^ - r e d u c t i o n . D a l l a B e t t a and Boudart (4) have suggested that small platinum c l u s t e r s which contained l e s s than 6 atoms were formed i n the supercages. More r e c e n t l y , Imelik and coworkers (5) (6) showed that depending on the c a l c i n a t i o n temperature, e i t h e r Pt-agglomerates (10 Â s i z e ) o r P t - c r y s t a l l i t e s (20 Â s i z e ) were present i n Y - z e o l i t e s . Although the behaviour o f these small metal p a r t i c l e s i n hydrogen and oxygen chemisorption has i n t e r e s t e d s e v e r a l authors, l e s s a t t e n t i o n has been paid to t h e i r c a t a l y t i c p r o p e r t i e s . Kubo e t a l . (7) found a good c o r r e l a t i o n between the amount o f hydrogen ad538 In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

44.

Platinum

NACCACHE ET AL.

Catalysts

Supported

on

Zeolite

539


sorbed and the c a t a l y t i c p r o p e r t i e s of the z e o l i t e supported p l a t i num. However they d i d not mention a p a r t i c u l a r behaviour f o r these c a t a l y s t s . D a l l a B e t t a and Boudart (4) reported that small Pt c l u s t e r s possessed c a t a l y t i c a c t i v i t i e s f o r hydrogénation of ethylene or neopentane hydrogenolysis an order of magnitude higher than Pt supported on s i l i c a . Imelik and co-workers (5) found that P t - c r y s t a l l i t e s i n z e o l i t e s have an enhanced c a t a l y t i c a c t i v i t y . In view of these previous r e s u l t s i t was thought of i n t e r e s t to f u r t h e r i n v e s t i g a t e the c a t a l y t i c p r o p e r t i e s of small platinum p a r t i c l e s supported on z e o l i t e s f o r carbon-hydrogen and carbon-carbon bond a c t i v a t i o n i n order to determine the s i g n i f i c a n t i n f l u e n c e of the zeol i t e support. Experimental Sodium and ammonium forms of Linde Y z e o l i t e ( S i / A l = 2.43) were exchanged w i t h ( P t ( N H ^ ) ^ ) s o l u t i o n s . Samples were then f i l tered, thoroughly washed and d r i e d at 313°K. Pt contents as d e t e r mined by chemical a n a l y s i s were i n the range 0.5 - 10 wt %. To form the platinum metal c a t a l y s t s the samples were g i v e n the f o l lowing treatments : about 0.1 g of the exchanged z e o l i t e was i n t r o duced e i t h e r i n t o the c e l l used f o r hydrogen a d s o r p t i o n or i n t o the c a t a l y t i c r e a c t o r . The standard pretreatment was to purge w i t h pure oxygen at 298°K and then to r a i s e the temperature up to 623°K at 0.5°C min~l while oxygen was c o n t i n o u s l y c i r c u l a t e d through the sample. T h i s treatment removed N H 3 l i g a n d s without s i g n i f i c a n t r e d u c t i o n of Pt · i o n s . Furthermore, at 623°K NaY was almost dehydrated and NH^Y decomposed to the HY form. Hydrogen r e d u c t i o n was c a r r i e d out at 623 K f o r a t l e a s t 3 hours. A s e r i e s of samples were c a l c i n e d i n oxygen up to 773 K and then ^ - r e d u c e d at 673°K. D i s p e r s i o n i s d e f i n e d as the percent of platinum atoms at the s u r f a c e . The number of s u r f a c e platinum atoms was determined by the hydrogen a d s o r p t i o n technique i n a v o l u m e t r i c apparatus. The metal s u r f a c e was c a l c u l a t e d from s 6/pd where ρ i s the platinum d e n s i t y and d the mean p a r t i c l e diameter. P a r t i c l e s i z e s were f u r t h e r determined by e l e c t r o n microscopy u s i n g the same procedure as i n (6) ; c a t a l y t i c a c t i v i t i e s were measured i n a g l a s s fixed-bed d i f f e r e n t i a l continuous flow r e a c t o r at atmospheric p r e s s u r e . Ethane hydrogeno l y s i s c a t a l y t i c measurements were made i n the temperature range 573 - 673°K at a flow r a t e of 6 1 h " . 2+

e

P

β

1

The a c t i v i t y of the c a t a l y s t s decreased w i t h time. Hence to o b t a i n r e p r o d u c i b l e data, the samples were regenerated between each gas chromatography a n a l y s i s by the procedure proposed by Yates and S i n f e l t ( 8 ) . Cyclopropane hydrogénation was c a r r i e d out between 293°K and 313"^ at a flow r a t e of 18 1 I T . 1

Results The e l e c t r o n micrographs r e v e a l e d that the platinum p a r t i c l e s supported on z e o l i t e s v a r i e d i n t h e i r s i z e as a f u n c t i o n of the

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

MOLECULAR SIEVES—Π

540

c a l c i n a t i o n temperature i n oxygen. On almost a l l the z e o l i t e sam p l e s oxygen-treated at 623°K and then reduced at the same tempera ture the metal p a r t i c l e s were around 10 A s i z e , w h i l e they were about 15 - 20 A s i z e on the z e o l i t e s that were oxygen-treated at 773°K. F i g u r e 1 shows the s i z e d i s t r i b u t i o n of platinum c r y s t a l l i tes i n NaY and NH^Y z e o l i t e s d e r i v e d from e l e c t r o n micrographs. In t a b l e s I I , I I I , IV, V are c o l l e c t e d p a r t i c l e s i z e s o b t a i ned from e l e c t r o n microscopy, platinum d i s p e r s i o n values c a l c u l a t e d from hydrogen a d s o r p t i o n .


C a t a l y t i c studies The r a t e equation f o r ethane hydrogenolysis was determined over the pressure ranges 30 - 100 t o r r f o r ethane and 190 - 450 t o r r f o r hydrogen. Table I summarizes the change of the orders of r e a c t i o n w i t h temperature. TABLE I . -

Catalyst

Orders f o r ethane h y d r o g e n o l y s i s ,

T°K reaction

P„ H

torr 2

30 -

Â

30 -

d = 15 A

599 631 643

d = 10

Pt-Si0

2

r C

578 637 663

Pt/NaY

P

100

TT 2 6

β

k p

c

C

p 11 2 6 —H

m

H

2

η

m

450

1 0.9

- 2.5 - 1.1 - 1

450

1 1 0.9

-

torr

H

190 -

tt

tt

II

tt

100

r

190 -

tt

tt

It

tt

1.8 1.2 1.1

From the k i n e t i c data i t appeared that there were l i t t l e changes i n the orders of r e a c t i o n over the temperature range 6 3 3 - 6 6 3 ° K f o r a l l the samples examined i n the present study. Thus the s p e c i f i c a c t i v i t y f o r ethane hydrogenolysis was determined at a standard set of c o n d i t i o n s : P =317 torr, PC2H6 t o r r , Τ = 6 6 3 ° K . The apparent a c t i v a t i o n energies were c a l c u l a t e d i n the temperature range 6 3 3 - 6 6 3 ° K . The r e s u l t s are c o l l e c t e d i n t a b l e I I . The a c t i v i t i e s of Pt-NH/Ï and P t - S i 0 f o r hydrogenolysis of propane were a l s o s t u d i e d under standard c o n d i t i o n s over the temperature range 4 7 3 - 5 3 3 ° K . The r e s u l t s are shown i n t a b l e I I I . Values f o r the apparent a c t i v a t i o n energies shown i n t a b l e I I I are very s i m i l a r f o r the two c a t a l y s t s . The hydrogenolysis of propane over these c a t a l y s t s r e s u l t e d i n the formation of methane and ethane w i t h a CH^/C H^ r a t i o of 1 . The s p e c i f i c a c t i v i t y f o r propane hydrogenolysis was determined at standard c o n d i t i o n s : PR = 317 torr, P C 3 H 3 " torr, Τ - 533 Κ. s

3

3

H 2

2

2

3

3

2


Phtinum

NACCACHE ET A L .

Catalysts

Supported

on

Zeolite

ι 1 wjdi»

_!_ M i *


(b)

•n

dÀ

"A,

2

1 M i I i d Snjdj2

Ad^njdj2

(d)

W

dA 10

ΓΤΐ. 1 • L 10

20

dA,

20

Figure 1. Size distribution of platinum particles, (a), PtNaY 3.8% wt Pt 0 350°C-H 350°C; (b), PtNaY 3.8% wt Pt 0 500°C-H 400°C; (c), PtNHJ 10.8% wt Pt 0 350°C-H 350°C; (d), PtNH>Y 10.8% wt Pt 0 500°C-H 400°C. 2

2

2

2

2

2

2


2


542

TABLE I I . - Hydrogenolysis of ethane ;


Catalyst

°K Treatment 02 H2

d

Dispersion H

Â EM

%

2

Ε

rate mole/h m Pt χ 10 2

ap K c a l mole

4

Pt-NaY

623

623

100

10

50

60

3.8

773

673

16

15

13

58

Pt-NH.Y

623

623

62

13

32

53

3.5

773

673

22

20

6

54

Pt-NH,Y

623

623

78

10

60

56

10.8

773

673

30

15

17

54

623

623

72

15

14

44

773

673

40

40-60

3

50

%

%

%

Pt-Si0 2.6

9

%

TABLE I I I . - Hydrogenolysis of ρ ropane

r a t e χ 10

Â

Eapp

EM

K c a l mole

Pt-NH.Y 4 10.8 %

10

36

60

15

36

17

Pt-Si0

15

29

14

Catalyst

2.6

d

2

%

40 - 60

-1

2

mole/h^ ιm

4

Pt

2.8

29

Cyclopropane hydrogénation The r e a c t i o n of cyclopropane with hydrogen was s t u d i e d i n the temperature range 273 - 313 K. The orders of r e a c t i o n i n hydrogen and cyclopropane were r e s p e c t i v e l y 0 and 0.6 f o r a l l the c a t a l y s t s examined. The temperature dependence of the r e a c t i o n r a t e s at constant hydrogen and cyclopropane pressures gave an apparent a c t i v a t i o n energy of about 12 k c a l . m o l e " . The s p e c i f i c a c t i v i t i e s were then determined at standard c o n d i t i o n s : Τ r e a c t i o n • 295°K, PCoH$ t o r r , Ρ ^ « 327 t o r r . Table IV summarizes the r e s u l t s obtained f o r platinum c a t a l y s t s supported on NaY, Ce-Y, A1«0^ and S i 0 c a r r i e r s . The data g i v e n i n t a b l e V have been computed from the r e s u l t s obtained w i t h Pt-NH4Y c a t a l y s t s . In order to provide some evidence that cyclopropane hydrogenae

1

β

1 1

2

2



44.

NACCACHE ET AL.

Platinum

Catalysts

Supported

on

Zeolite

543

t i o n can or cannot proceed by a d u a l - f u n c t i o n a l mechanism on the platinum supported on a c i d i c c a r r i e r s the r e a c t i o n of i s o m e r i z a t i o n of cyclopropane to propene over Pt-HY z e o l i t e s and Pt-NaY was i n v e s t i g a t e d . At room temperature no s i g n i f i c a n t i s o m e r i z a t i o n r e a c t i o n occured ; propylene appeared as a product only at about 473°K. Furthermore the acceptor p r o p e r t i e s of Pt-NH^Y z e o l i t e s a c t i vated at 773°K as a f u n c t i o n of platinum content were s t u d i e d f o l lowing the procedure by Figueras et a l . (9) which c o n s i s t e d of determining the number of r a d i c a l c a t i o n s formed by a d s o r p t i o n of anthracene. The data obtained i n d i c a t e that the i n c r e a s e of p l a t i num content produced a decrease i n the acceptor p r o p e r t i e s of the c a r r i e r . When the platinum content reached a value of about 1.9 wt Z, Pt-NH^Y a c t i v a t e d at 773°K showed the same very low Lewis a c i d i t y as Pt-NaY z e o l i t e . TABLE IV : Hydrogénation of cyclopropane and P t - A l 0 o

°K C a t a l y s t Treatment 02 H 2

2

Â EM

-

34 45

-

154

37

100

-

556

89

673

77

13

2.1

0.32

773

673

27

>40

4.0

0.30

623

623

72

15

9

0.90

85 11

Pt-NaY 3.8 %

623 773

623 773

100 16

10 15 -

Pt-NaY 7.9 %

773

673

18

Pt-CeY 1.1 %

623

623

773

Pt-Al 0. 9

10 %

Pt-SiO. 2.6 %

Z

653 140

623 673

J

1

35 21

623 773

?

Ν h" χ 10

160 13

Pt-NaY 1.1 %

Pt-Al 0~ 1.7 %

3 r a t e χ 10 mole/h.g

d

Dispersion % H

over Pt-NaY, Pt-SiO,

3

20



544

TABLE V : Cyclopropane hydrogénation over Pt-NH.Y

3

e

K Treatment 02 H

Catalyst

9

Dispersion % H


9

d? A EM

r a t e χ 10 mole/h.g.

Ν h" χ 10

1

Pt-NH.Y 0.5 Ζ

623

623

65

90

56

Pt-NH.Y 0.95 %

623 773

623 673

62 29

187 80

62 57

Pt-NH.Y 1.9 %

623 773

623 673

67 24

342 83

54 36

Pt-NH.Y 3.5 %

623 773

623 673

62 22

486 143

42 37

Pt-NH.Y 10.8 %

773

673

22

396

33

10 15 - 20 20

DISCUSSION The r e s u l t s of t h i s work, i n agreement w i t h p r e v i o u s l y p u b l i shed data, have shown that exchanged platinum ions y i e l d e d on r e d u c t i o n h i g h l y d i s p e r s e d platinum metal. The s a t i s f a c t o r y agreement between the r e s u l t s from the hydrogen chemisorption and e l e c t r o n microscopy i n d i c a t e s that i n z e o l i t e s oxygen-calcined at 623°K be f o r e r e d u c t i o n , the m e t a l l i c phase i s made up of small p a r t i c l e s of 8-10 A s i z e and homogeneously dispersed i n s i d e the supercages. On ce formed the p a r t i c l e s are too l a r g e to escape through the small 7.2 Â port windows. Thus the z e o l i t e framework would i n c r e a s e the s t a b i l i t y of the metal p a r t i c l e s to s i n t e r i n g and allows o b t a i n i n g a r e l a t i v e l y h i g h metal l o a d i n g without s u b s t a n t i a l i n c r e a s e of the metal p a r t i c l e s i z e . The r e s u l t s obtained w i t h platinum loaded z e o l i t e s oxygen t r e a t e d at 773 K showed c o n s i d e r a b l e d i s c r e p a n c i e s among the p a r t i c l e s i z e c a l c u l a t e d from the chemisorption of hydrogen and e l e c t r o n microscopy. In Pt^+Y z e o l i t e s c a l c i n e d a t 773°K a l a r g e number of Pt^+ ions are l o c a t e d i n the s o d a l i t e cages (5, 6) . Upon H2-reduction Pt^" " form Pt (0) atoms, a p a r t of which being s t a b i l i z e d i n s i d e t h e s o d a l i t e cages. Platinum atoms which escape through the 2.2 A cage windows agglomerate i n the supercages and form platinum aggregates (8 - 10 1 s i z e ) or c r y s t a l l i t e s (15 - 20 Â ) . As the e l e c t r o n micrographs have shown the platinum c r y s t a l l i t e s were a l s o embedded w i t h i n the z e o l i t e , one could suggest that during the metal aggregation p a r t of the z e o l i t e framework around the c r y s t a l l i t e was destroyed thus e n l a r g i n g the supercage s i z e ( 6 ) . F o l l o w i n g t h i s a n a l y s i s the apparent d i s c r e p a n c y between chemisorption and e l e c t r o n microscopy can be understood i f one suggest that a t o m i c a l l y dispersed Pt(0) atoms have l i t t l e a b i l i t y to adsorb hydrogen (3, _5). Hence only an "apparent d i s p e r P

1

o


44.

NACCACHE ET

AL.

Platinum

Catalysts

Supported

on

545

Zeolite

s i o n " would be measured by hydrogen chemisorption f o r Pt-loaded z e o l i t e s c a l c i n e d at 773°K. S t r i k i n g e f f e c t s of the z e o l i t e c a r r i e r , c l o s e to those r e p o r ted by e a r l i e r workers ( 4 , 5) have been obtained f o r cyclopropane hydrogénation. Tables IV and V show that z e o l i t e - s u p p o r t e d p l a t i num c a t a l y s t s were more a c t i v e f o r cyclopropane hydrogénation than Pt-Si02 or Pt-Al2(>3 at a l l s t a t e s of platinum d i s p e r s i o n . From t a b l e s IV and V the f o l l o w i n g orders of r e a c t i v i t y f o r cyclopropane hydrogénation were found ( t a b l e V I ) .


TABLE VI.- Orders of s p e c i f i c a c t i v i t y f o r C^H^

Catalyst

Pt-CeY

Electrostatic field Turnover Number Ν h"

strong

89

Pt-HY or LY

Pt-NaY or n e u t r a l i z e d a c i d i c zeolite

medium

low

56

36

hydrogénation

Pt/conventional c a r r i e r A I 2 O 3 or SiO?

0.03,

0.1

1

The enhanced hydrogenating p r o p e r t i e s of small platinum c l u s ters supported on z e o l i t e was p r e v i o u s l y i n t e r p r e t e d i n terms of e l e c t r o n t r a n s f e r between the metals c l u s t e r s and the z e o l i t e , thus the platinum c l u s t e r s behaving more l i k e i r i d i u m ( 4 ) . However, t h i s i n t e r p r e t a t i o n cannot account f o r the r e s u l t s on cyclopropane hydrogénation f o r the f o l l o w i n g reasons : ( i ) i t i s reasonable to expect that the m e t a l - c a r r i e r i n t e r a c t i o n would have a higher e f f e c t on the e l e c t r o n i c p r o p e r t i e s of small c l u s t e r s than on l a r g e c r y s t a l l i t e s , thus the s p e c i f i c a c t i v i t y f o r hydrogénation should be higher f o r small platinum c l u s t e r s . In c o n t r a s t the observed s p e c i f i c a c t i v i t i e s f o r 10 Â aggregate and 20 Â c r y s t a l l i t e s supported on N H 4 Y z e o l i t e were about the same ( t a b l e V ) . ( i i ) In a study of cyclopropane hydrogénation on group V I I I metals S i n f e l t and co-workers (12) observed that the s p e c i f i c a c t i v i t y of p l a t i num was about 2 orders of magnitude higher than that of i r i d i u m . Thus i f e l e c t r o n t r a n s f e r between small Pt aggregates and the zeol i t e c a r r i e r had occurred, one would have observed a decrease of the s p e c i f i c a c t i v i t y f o r cyclopropane hydrogénation from Pt-Si02 to P t - z e o l i t e c a t a l y s t s . In sharp c o n t r a s t our r e s u l t s i n d i c a t e that z e o l i t e - s u p p o r t e d platinum c a t a l y s t s are 100 times more a c t i v e f o r cyclopropane hydrogénation than Pt-Si02 or Pt-Al203.0n the bas i s of the above a n a l y s i s the major r o l e of the z e o l i t e c a r r i e r i n the enhancement of the hydrogenating p r o p e r t i e s P t - z e o l i t e c a t a l y s t s would be b e t t e r d e s c r i b e d i n terms of the e f f e c t of the e l e c t r o s t a t i c f i e l d , present i n s i d e the z e o l i t e c a v i t i e s , on the r e a c t a n t s



546


r a t h e r than on the metal. Cyclopropane undergoes hydrogénation r e a c t i o n s which are c h a r a c t e r i s t i c of molecules possessing a c a r bon-carbon double bond, thus suggesting that the molecule has a pseudo o l e f i n i c c h a r a c t e r . However i n c o n t r a s t with ethylene, r e cent molecular o r b i t a l c a l c u l a t i o n s (11) have shown that the lowest unoccupied molecular o r b i t a l i n cyclopropane has a σ-type s t r u c t u r e which favors more l i n e a r geometries. The a d d i t i o n of a negative charge i n t h i s σ-orbital i n c r e a s e s the e q u i l i b r i u m angle of the cyclopropane thus f a v o r i n g the r i n g opening. Therefore one might suggest that the e l e c t r o s t a t i c f i e l d i n the z e o l i t e acts as a ne g a t i v e charge which removes the s t a b i l i z a t i o n of the s t r a i n e d cy clopropane r i n g thus i n c r e a s i n g i t s r e a c t i v i t y . The 3 - f o l d i n c r e a se of the turnover number from Pt-NaY to Pt-CeY c a t a l y s t s i s p r o bably due to the higher e l e c t r o s t a t i c f i e l d i n r a r e e a r t h exchan ged z e o l i t e . Recently i t has been shown that the heat of adsorp t i o n of hydrogen on CaY i s l a r g e r than on NaY, the i n c r e a s e was a t t r i b u t e d to the higher e l e c t r o s t a t i c f i e l d i n CaY z e o l i t e (13). F i n a l l y a dual f u n c t i o n mechanism f o r hydrogénation of c y c l o p r o p a ne over P t - z e o l i t e c a t a l y s t s i s l e s s l i k e l y to occur as i t has been shown that no cyclopropane i s o m e r i z a t i o n occured on these c a t a l y s t s up to about 473°K. Our r e s u l t s on ethane hydrogenolysis provide f u r t h e r arguments f o r r e j e c t i n g the d i r e c t e f f e c t s of p l a t i n u m - z e o l i t e i n t e r a c t i o n on the c a t a l y t i c p r o p e r t i e s of the metal. The c a t a l y t i c hydrogenol y s i s of ethane has been e x t e n s i v e l y used to i n v e s t i g a t e the c a t a l y t i c p r o p e r t i e s of metals (10). In c o n s i d e r i n g the p a t t e r n of the c a t a l y t i c a c t i v i t i e s f o r the hydrogenolysis of ethane i t has been shown that both the e l e c t r o n i c s t r u c t u r e of the metal (10) and to a lower extent, the metal p a r t i c l e s i z e (8) played an important r o l e . The r e s u l t s of the present study on platinum are s i m i l a r to those found by Yates and S i n f e l t on rhodium ( 8 ) . The s p e c i f i c a c t i v i t y f o r ethane hydrogenolysis decreases with the i n crease of the platinum p a r t i c l e s i z e , the c a t a l y s t s c o n t a i n i n g p a r t i c l e s of 10 A s i z e having the highest s p e c i f i c a c t i v i t y . F u r thermore, t a b l e I I shows that the s p e c i f i c a c t i v i t y i s almost i n d e pendent of the nature of the c a r r i e r . For platinum supported on non a c i d i c and a c i d i c z e o l i t e s i n which the metal p a r t i c l e s are 10 Â s i z e , approximatively i d e n t i c a l s p e c i f i c a c t i v i t i e s were found (50-60-10~4 mole/h m^). The s p e c i f i c a c t i v i t i e s i n the hydrog e n o l y s i s r e a c t i o n on 15 Â s i z e platinum c r y s t a l l i t e s supported on NaY, N H 4 Y or SiOo, although lower, were a l s o n e a r l y constant (13, shows that the apparent a c t i v a t i o n energy remains approximately the same f o r a l l the c a t a l y s t s s t u d i e d , the average value of 54 kcal/mole i s i n good agreement with p r e v i o u s l y reported values f o r platinum c a t a l y s t s (10). The enhancement of the c a t a l y t i c a c t i v i t y f o r neopentane hydrogenolysis over z e o l i t e supported small platinum c l u s t e r s was i n t e r p r e t e d i n terms of p a r t i a l e l e c t r o n t r a n s f e r from the Pt c l u s ter behaving more l i k e i r i d i u m ( 4 ) . Our r e s u l t s , at l e a s t f o r ethane and propane hydrogenolysis, are not c o n s i s t e n t w i t h these


44.

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suggestions. In f a c t i t has been shown (10) that s p e c i f i c c a t a l y t i c a c t i v i t y , at standard c o n d i t i o n s , f o r hydrogenolysis of ethane over i r i d i u m was about 5 orders of magnitude higher than over platinum. Furthermore the apparent a c t i v a t i o n energy decreased from 54 kcal/mole f o r Pt to 36 kcal/mole f o r I r (10). More r e c e n t l y Yermakov et a l . (14) p o i n t e d out, from t h e i r ESCA r e s u l t s on (W + Pt) S1O2, a decrease i n the e l e c t r o n i c d e n s i t y of the p l a t i num. Simultaneously the apparent a c t i v a t i o n energy f o r ethane hyd r o g e n o l y s i s decreased from 54 kcal/mole f o r pure platinum c a t a l y s t to 28 kcal/mole f o r (W + P t ) / S i 0 . Hence the constant v a l u e of the apparent a c t i v a t i o n energy f o r ethane hydrogenolysis on P t - z e o l i t e and Pt-Si02 provided f u r t h e r evidence that the z e o l i t e c a r r i e r has a r e l a t i v e l y minor e f f e c t upon the e l e c t r o n i c s t r u c t u r e of the supported encaged platinum aggregate or Pt c r y s t a l l i t e s . Indeed i f the 10 Â s i z e platinum aggregates were more l i k e i r i d i u m on should have observed a s i g n i f i c a n t decrease i n the apparent a c t i v a t i o n energy and a l s o a much higher value of t h e i r s p e c i f i c a c t i v i t y , i n c o n t r a s t w i t h our r e s u l t s .


2

In c o n c l u s i o n t h i s work has shown that hydrogenolysis of ethane over supported platinum i s s t r o n g l y dependent on p a r t i c l e s i z e , the smallest p a r t i c l e s being the most a c t i v e . Although other works have i n d i c a t e d that small metal p a r t i c l e s are e l e c t r o n d e f i c i e n t (which means more a c i d i c , thus g i v i n g stronger metal-carbon bonds w i t h dehydrogenated ethane), i t appears from t h i s work that t h i s property does not vary s i g n i f i c a n t l y from one c a r r i e r to another but i s mainly an i n t r i n s i c property of small metal c l u s t e r s . In c o n t r a s t cyclopropane hydrogénation, which i s a "nondemanding" r e a c t i o n , was found to be h i g h l y s e n s i t i v e to the p a r t i c u l a r c a r r i e r employed. The p a t t e r n s of v a r i a t i o n of c a t a l y t i c a c t i v i t y from z e o l i t e s to conventional c a r r i e r s seems to be a consequence of the important p o l a r i z a b i l i t y p r o p e r t i e s of the z e o l i t e . ACKNOWLEDGMENTS The authors are g r a t e f u l to Dr. Dalmai-Imelik G. f o r a s s i s tance with the e l e c t r o n microscopy study and thank Mr. U r b a i n who performed the chemical a n a l y s i s . Literature Cited 1. Rabo, J.Α., Schomaker, V. and P i c k e r t , P.E., Proc. I n t . Con gress C a t a l y s i s 3d, 1964, North Holland Pub. Company, (1965), V o l . 2, 1264. 2. Lewis, P.H., J . C a t a l y s i s , (1968), 11, 162. 3. Kubo, T., A r a i , H., Tominaga, H., Kunugi, T., B u l l . Chem. Soc. Japan (1972), 45, 607. 4. D a l l a B e t t a , R.A. and Boudart Μ., Proc. I n t . Congress C a t a l y s i s 5th, North Holland Pub. Company (1973), 2, 1329. 5. G a l l e z o t , P., Datka, J . , Massardier, J . , Primet, M., Imelik, B. Proc. I n t . Congress C a t a l y s i s 6th, 6. G a l l e z o t , P., Mutin, I . , Dalmai-Imelik, G., Imelik, B., J . M i c r o s c . Spectrosc. E l e c t r o n (1976), 1, 1.



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7. Kubo, T., Arai, H., Tominaga, H., Kunugi, T., Bull. Chem. Soc. Japan, (1972), 45, 613. 8. Yates, D.J.C. and Sinfelt, J.H., J. Catalysis (1967), 8, 348. 9. Figueras, F., Mencier, B., De Mourgues, L., Naccache, C., and Trambouze, Y., J. Catalysis, (1970), 19, 315. 10. Sinfelt, J.H. and Yates, D.C., J. Catalysis, (1967), 8, 82. 11. Buenker, R.J. and Peyrimmhoff, S.D., J. Phys. Chem., (1969), 73, 1299. 12. Dalla Betta, R.A., Cusumano, J.A. and Sinfelt, J.H., J. Cataly sis, (1970), 19, 343. 13. Benson, J.E. and Boudart, Μ., J. Catalysis, (1967), 8, 93. 14. Loffe, M.S., Kuznetsov, B.H., Ryndin, Yu. A. and Yermakov, Yu. I., Proc. 6th. Internat. Cong. Catalysis, London (1976), paper A5.


Platinum Catalysts Supported on Zeolite. Hydrogenation of

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