Hydrogen Generation by Steam Reformation of n-Hexane over Zeolite

74 Hydrogen Generation by Steam Reformation of n-Hexane over Zeolite Catalysts C. S. BROOKS Downloaded by UNIV OF PITTSBURGH on January 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch074

United Aircraft Research Laboratories, East Hartford, Conn. 06108

High-activity nickel and cobalt catalysts for hydrogen generation by the steam—hydrocarbon reforming reaction were prepared by ion exchange from synthetic zeolites. The activity of the catalysts for steam-reforming of hydrocarbons was evaluated with n-hexane at 400° to 500°C. The specific reforming activity exceeded by factors of 2 to 30 that obtained for an active commercial nickel-on-alumina zeolite under comparable conditions. The superior activity was attributed to the high state of nickel dispersion in the catalysts prepared by ion exchange. The cobalt zeolites had comparable initial activity but rapidly lost their reforming activity owing to oxidation with reactant steam. Mordenite and faujasite zeolite catalysts with nickel contents less than 4 wt% demonstrated the highest specific reforming activity.

/

i

T h e successful m e t h a n e r e f o r m i n g catalysts (2) A

are p r i m a r i l y n i c k e l -

o n - a l u m i n a w i t h r e l a t i v e l y h e a v y m e t a l l o a d i n g s ( ^ 15 w t % ) .

The

p r i n c i p a l causes o f catalyst d e g r a d a t i o n i n the r e f o r m i n g of l i q u i d h y d r o carbons seem to b e loss o f c a t a l y t i c a l l y active m e t a l surface a n d h e a v y c a r b o n d e p o s i t i o n . It w o u l d appear f r u i t f u l to e x a m i n e the

difference

b e t w e e n m e t h a n e a n d h i g h e r h y d r o c a r b o n r e f o r m i n g a n d the properties d e s i r e d i n a s t e a m - r e f o r m i n g catalyst. T h e o b v i o u s difference i n the ref o r m i n g of l i q u i d h y d r o c a r b o n s a n d of m e t h a n e is that the d e s i r e d catalyst s h o u l d i n c o r p o r a t e efficiency for C — C b o n d r u p t u r e w i t h a m i n i m u m of c o k i n g . T h e zeolites h a v e several u n i q u e s t r u c t u r a l features w h i c h p r o v i d e t h e m w i t h s p e c i a l interest as catalyst substrates. T h e s e features are extensive i n t r a l a t t i c e p o r e v o l u m e , a p o r e size of m o l e c u l a r d i m e n s i o n s , a l a r g e p o p u l a t i o n of c a t i o n exchange sites, a n d the l o c a t i o n of the ex426

In Molecular Sieve Zeolites-II; Flanigen, Edith M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

74.

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427

Reformation

c h a n g e c a t i o n w i t h i n the i n t r a l a t t i c e p o r e v o l u m e . T h e s e s t r u c t u r a l fea tures are of s p e c i a l interest i n v a r i o u s h y d r o c a r b o n reactions,

notably

h y d r o c a r b o n — w a t e r interactions, because of the p o s s i b i l i t i e s f o r

surface

reactions w i t h i n a lattice cage, p r o v i d i n g a n a c i d substrate to f a c i l i t a t e C — C b o n d r u p t u r e , a n d i n t r o d u c i n g c a t a l y t i c metals i n a h i g h l y disperse state o n the c r y s t a l lattice surface a n d w i t h i n the p o r e v o l u m e cages of the zeolites.

Downloaded by UNIV OF PITTSBURGH on January 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch074

Experimental Selection of Zeolites. T h e selected zeolites consisted of the 2 p r i n c i p a l c r y s t a l l o g r a p h i c types—i.e., m o r d e n i t e a n d f a u j a s i t e — w h i c h p r o v i d e an i n t e r e s t i n g r a n g e of i n t r a c r y s t a l l i n e cage v o l u m e s of 0.29 a n d 0.54 c m p e r c m z e o l i t e , r e s p e c t i v e l y ; net charges p e r surface o x y g e n of 0.083 a n d 0.214 e u n i t s , r e s p e c t i v e l y ; a n d free c a v i t y diameters of a b o u t 9 a n d 11.8 A , respectively ( 3 ) . 3

3

A faujasite z e o l i t e , s u c h as L i n d e 1 3 X ( U n i o n C a r b i d e ) , has the e m p i r i c a l f o r m u l a N a e ( A K > ) ( S i 0 ) ιοβ ' 264 H 0 w h i c h c o r r e s p o n d s to a S i 0 / A 1 0 r a t i o of 1.23 a n d a c a t i o n e x c h a n g e c a p a c i t y ( C E C ) of 4.7 m e q / g r a m . A m o r d e n i t e z e o l i t e , s u c h as N o r t o n s o d i u m Z e o l o n ( N o r t o n C o . ) , has a n e m p i r i c a l f o r m u l a N a ( A 1 0 ) ( S i 0 ) o * 24 H 0 w h i c h corresponds to a S i 0 / A 1 0 r a t i o of 5.0 a n d a C E C of 2.4 m e q / g r a m . C l a y b i n d e r to the extent of 1 7 % was u s e d w i t h L i n d e 13X. T h e N o r t o n Z e o l o n a n d the L i n d e 1 3 X w e r e i n the f o r m of 1 / 1 6 - i n c h c y l i n d r i c a l pellets. T h e L i n d e Y z e o l i t e ( S K 4 0 0 ) is also a faujasite t y p e b u t w i t h a greater S i : A l r a t i o ( > 1.5) t h a n the S i : A l ratio of a b o u t 1.23 c h a r acteristic o f the X z e o l i t e . T h e Y z e o l i t e h a d a C E C of 4.04 m e q / g r a m a n d w a s u s e d i n the f o r m of 3 / 1 6 - i n c h c y l i n d r i c a l pellets. A d d i t i o n a l details o n t h e p r o p e r t i e s of these zeolites are a v a i l a b l e i n the p u b l i s h e d literature (3,4,14). 8

2

2

8 6

2

2

2

8

2

2

8

2

4

2

2

Catalyst Preparation. C a t a l y s t p r e p a r a t i o n consisted of the ex c h a n g e of n i c k e l or c o b a l t nitrate f o r the s o d i u m c a t i o n . R a t i o s of n i c k e l o r c o b a l t to s o d i u m of 20:1 w e r e u s e d f o r m a x i m u m exchange a n d the i o n e x c h a n g e d z e o l i t e pellets w e r e l e a c h e d w i t h d e i o n i z e d w a t e r . Cat alyst p r e p a r a t i o n s w e r e r e d u c e d f o r 16 hours at 4 0 0 ° C i n a stream of h y d r o g e n . S i m i l a r p r o c e d u r e s h a v e b e e n r e p o r t e d (5, 6). Catalyst Characterization. C h e m i c a l analyses, x-ray d i f f r a c t i o n a n alyses, a n d gas a d s o r p t i o n p r o c e d u r e s w e r e u s e d to c h a r a c t e r i z e the c o m p o s i t i o n , c r y s t a l l o g r a p h i c character, a n d surface structure of the n i c k e l a n d c o b a l t z e o l i t e catalyst p r e p a r a t i o n s . T h e c h e m i c a l a n d x - r a y p r o c e d u r e s w e r e s t a n d a r d m e t h o d s w i t h the latter d e s c r i b e d elsewhere (11). C a r b o n m o n o x i d e c h e m i s o r p t i o n measurements p r o v i d e u s e f u l estimates of the surface c o v e r e d b y n i c k e l atoms f r o m the z e o l i t e s u b strate (10). Hydrocarbon Steam Reforming Tests in Microcatalytic Reactor. C a t a l y s t p e r f o r m a n c e f o r steam r e f o r m i n g of n-hexane to p r o d u c e C H , C O , C 0 , a n d H at 4 0 0 ° to 500 ° C w a s e v a l u a t e d i n a stainless steel reactor w h i c h has b a s i c a l l y the same i n s t r u m e n t a t i o n u s e d p r e v i o u s l y f o r o x i d a t i o n catalyst studies (9, 12). T h e p r i n c i p a l m o d i f i c a t i o n s consisted 4

2

2


428

M O L E C U L A R SIEVE ZEOLITES

II

of the i n t r o d u c t i o n o f a d u a l i n j e c t i o n m e t e r i n g p u m p , a v a p o r i z e r , a v a p o r m i x e r , a n d a w a t e r c o o l e d condensate trap. C a t a l y s t charges r a n g e d f r o m 1-12 grams a n d w e r e interspersed t h r o u g h 1 / 8 - i n c h a l u m i n a pellets ( N o r t o n S u p p o r t S A 5 1 0 1 ) to f o r m the catalyst b e d ( 3 5 c m ) . T h e u n c o n v e r t e d n-hexane was c o l l e c t e d a n d w e i g h e d after the condensate w a s r e t r i e v e d f r o m t h e condensate t r a p . E f f l u e n t gases w e r e a n a l y z e d b y c h r o m a t o g r a p h y a n d mass s p e c t r o m e t r y for a l l v a p o r components u p to C hydrocarbons. 3

6


Discussion Catalyst Composition. C h e m i c a l c o m p o s i t i o n s o f t y p i c a l n i c k e l a n d c o b a l t zeolites are s u m m a r i z e d i n T a b l e I . B a s e d o n the t o t a l C E C de r i v e d f r o m the i n i t i a l s o d i u m c o m p o s i t i o n , 23 t o 3 7 % 8.4%

o f the Z e o l o n a n d

o f the L i n d e S K 4 0 0 exchange sites are o c c u p i e d b y n i c k e l cations.

In Zeolon, 55%

o f the exchange sites are o c c u p i e d b y c o b a l t cations. A

r a t i o o f 1.41:1 for c o b a l t to n i c k e l o n the Z e o l o n exchange sites r e s u l t e d where n i c k e l and cobalt were exchanged under comparable Table I. Catalyst N a Zeolon N i - C o Zeolon N i ( N a ) Zeolon C o ( N a ) Zeolon Linde SK400 a 6

conditions.

Chemical Composition of Zeolites CEC,

Wt%

Cation Na+ Ni +-Co Ni + Co Na+-Ni + 2

2

2 +

2

2 +

5.33° 1.52-2.15* 3.24 3.77 8.7-0.76* δ

δ

Meq/Gram

2.32 0.52 N i + - 0 . 7 3 C o 1.10 1 28 3.78 N a + - 0 . 2 6 N i + 2

2 +

2

Atomic absorption. Wet analysis. Crystallographic Character of Zeolites. I t is essential f o r

adequate

c h a r a c t e r i z a t i o n o f the zeolite catalyst p r e p a r a t i o n s that v e r i f i c a t i o n o f s t r u c t u r a l i n t e g r i t y b e m a d e b y x - r a y d i f f r a c t i o n analysis.

Considerable

i n f o r m a t i o n o n the structure o f zeolites, s u c h as 1 3 X , has b e e n p u b l i s h e d (5, 6). X - r a y d i f f r a c t i o n analyses o f several o f the s o d i u m , n i c k e l , a n d c o b a l t zeolites e s t a b l i s h e d the p r i n c i p a l i n t e r p l a n a r d-spacings i n A n g s t r o m units a n d the r e l a t i v e intensity, l O O I / I o , o f these lines. T h e p r i n c i p a l conclusions w a r r a n t e d b y these x-ray d i f f r a c t i o n d a t a are that the z e o l i t e structure was essentially i n t a c t after e x c h a n g e of n i c k e l f o r s o d i u m i n Z e o l o n b u t n o t i n the case o f 13X, a significant a m o u n t o f n i c k e l crys tallites large e n o u g h f o r d e t e c t i o n b y x-ray w e r e present i n the h y d r o g e n r e d u c e d 1 3 X n i c k e l zeolite b u t not i n the case o f n i c k e l Z e o l o n , a n d there w a s i n n o instance e v i d e n c e o f a significant a m o u n t o f c o b a l t i n crystallites l a r g e e n o u g h f o r d e t e c t i o n b y x-ray d i f f r a c t i o n . State of Dispersion of Metal. C h e m i s o r p t i o n o f c a r b o n m o n o x i d e at 23 ° C a n d x - r a y d i f f r a c t i o n l i n e b r o a d e n i n g h a v e b e e n u s e d t o measure


74.

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Reformation

429

the state of d i s p e r s i o n of the n i c k e l o n 3 of these catalyst p r e p a r a t i o n s . T h e s e results are g i v e n i n d e t a i l i n Ref. 10.

F o r e x a m p l e , the

carbon

m o n o x i d e c h e m i s o r p t i o n estimates of the n i c k e l areas r e s u l t e d i n a spe cific n i c k e l area of 30 m / g r a m of catalyst f o r the Z e o l o n s u p p o r t , c o m 2

p a r e d w i t h 10 m / g r a m of catalyst 2

case of G 5 6 . that 5 8 %

f o r the n i c k e l - o n - a l u m i n a i n

T h e x-ray d i f f r a c t i o n l i n e b r o a d e n i n g measurements

the

show

of the t o t a l n i c k e l surface area i n the case of the G 5 6 a l u m i n a

s u p p o r t e d catalyst is c o n t r i b u t e d b y n i c k e l crystallites greater t h a n 100 A . O n the other h a n d , o n l y 0.4%

of the n i c k e l area f o r n i c k e l Z e o l o n is c o n


t r i b u t e d b y n i c k e l crystallites greater t h a n 100 A . F u r t h e r m o r e , i f the c a r b o n m o n o x i d e c h e m i s o r p t i o n is expressed

as m o l e c u l e s

of

carbon

m o n o x i d e p e r n i c k e l a t o m , a r a t i o of 1.03:1 is o b t a i n e d f o r n i c k e l Z e o l o n ( T a b l e I I I , Ref. 10).

T h i s last result demonstrates that the n i c k e l o n the

f r e s h l y p r e p a r e d n i c k e l Z e o l o n catalyst is present i n an a t o m i c state of dispersion. Steam Reforming of w-Hexane Over Nickel and Cobalt Catalysts. T h e p e r f o r m a n c e of s e v e r a l of the n i c k e l a n d c o b a l t z e o l i t e catalysts f o r steam r e f o r m i n g of n-hexane at 400°—500° C has b e e n e v a l u a t e d b y short test runs w i t h the reactor a n d the p r o c e d u r e s d e s c r i b e d a b o v e

(Table

II ). A G i r d l e r r e f o r m i n g catalyst ( G 5 6 ) was tested u n d e r the same c o n ditions as a c o m p a r a t i v e s t a n d a r d . A l l tests w e r e c o n d u c t e d at a t o t a l pressure of 1 a t m . Plateaus of sustained r e f o r m i n g a c t i v i t y w e r e estab l i s h e d w i t h i n 1 h o u r . T h e c o b a l t catalysts lost essentially a l l r e f o r m i n g a c t i v i t y w i t h i n 3 h o u r s , p r e s u m a b l y because of o x i d a t i o n b y steam.

The

space velocities r e p o r t e d are c a l c u l a t e d i n terms of t h e o r e t i c a l h y d r o g e n p r o d u c t i o n b a s e d o n the n-hexane i n j e c t i o n rate a n d extent of c o n v e r s i o n ( E q u a t i o n 2, T a b l e I I ). T h e e q u a t i o n for the steam r e f o r m i n g of n-hexane w i t h c o m p l e t e c o n v e r s i o n to c a r b o n d i o x i d e is C H 6

1 4

+

12 H 0 = 2

6 C0

2

+

19 H

(1)

2

S t e a m r e f o r m i n g was the p r i m a r y r e a c t i o n o v e r these n i c k e l catalysts. T h e presence of h y d r o c a r b o n s ( C

2

to C ) w h i c h w o u l d i n d i c a t e c r a c k i n g 5

reactions o c c u r r e d to the extent of less t h a n 1 0 % i n the r e a c t i o n p r o d u c t s . T h e presence of methane, w h i c h w o u l d i n d i c a t e p a r t i a l r e f o r m i n g , d i d n o t exceed 5 %

i n the r e a c t i o n p r o d u c t s . T h e r e does not a p p e a r to b e a n y

significant difference i n p r o d u c t s e l e c t i v i t y f o r the n-hexane steam

re

f o r m i n g r e a c t i o n over n i c k e l o n the 2 q u i t e different s u p p o r t s — z e o l i t e vs. a l u m i n a . C a r b o n a c e o u s residues a c c u m u l a t e d i n the case of a l l the n i c k e l catalysts w h e r e r e f o r m i n g a c t i v i t y was s u s t a i n e d a n d the c a r b o n d e p o s i t i o n o n the zeolite catalysts c o m p a r e d f a v o r a b l y w i t h G 5 6 . sv

=

OLFW (2.24

Χ 10 ) 4

V

(19)

(3600)

=

cm /ideal H 3

2

per c m c a t a l y s t 3


(2)

430

M O L E C U L A R SIEVE ZEOLITES

Table II.

Catalyst


G56 N i Zeolon C o Zeolon G56 N i Zeolon Nickel Y

II

Reforming of w-Hexane

Κ

μ moles

Temp.

Test Period, Min.

500 500 500 500 430 430 430

40-455 73-475 75 200 40-340 60-385 55-390

Hr(Eq. 2)

G Cat Sec {Eq. 3)

1880 3660 920 71 244 556 560

0.28 0.54 0.42 0.015 0.26 0.60 0.60

1

0.36 0.54 0.20 0.006 0.104 0.114 0.115

w h e r e F is n-hexane i n j e c t i o n rate, moles p e r g r a m catalyst p e r sec, V is catalyst b e d v o l u m e , c m , W is w e i g h t of c a t a l y s t / g r a m , a n d a is f r a c t i o n 3

n-hexane r e f o r m e d t o c a r b o n m o n o x i d e , c a r b o n d i o x i d e , o r m e t h a n e . T h e r e f o r m i n g rate constant, K, d e f i n e d as moles n-hexane r e f o r m e d p e r g r a m catalyst p e r second, is g i v e n b y t h e r e l a t i o n

Κ =

F i n [1/(1 -

(3)

a)]

T h i s assumes a first-order r e a c t i o n w i t h respect to t h e n-hexane.

K' is t h e

r e f o r m i n g r a t e constant d e f i n e d as moles o f n-hexane r e f o r m e d p e r g r a m of n i c k e l p e r s e c o n d , a n d F represents moles of n-hexane i n j e c t e d p e r g r a m o f m e t a l p e r second. T h e h y d r o g e n p r o d u c t i o n efficiency, H , w h i c h is t h e ratio o f t h e p

a c t u a l to t h e i d e a l h y d r o g e n p r o d u c t i o n rate, is g i v e n b y H

where

H

2

p

=

H /19 2

(4)

FWOL

represents t h e h y d r o g e n p r o d u c t i o n rate as

moles/second

b a s e d o n t h e effluent gas c o m p o s i t i o n a n d FW represents t h e n-hexane i n j e c t i o n r a t e as ^ m o l e s / s e c o n d

( E q u a t i o n 1 ).

T h e r e f o r m i n g rate constants, K , r e f e r r e d to t h e t o t a l catalyst w e i g h t f o r a l l t h e n i c k e l catalysts, f a l l w i t h i n a c o m p a r a t i v e l y n a r r o w r a n g e of 0.26 to 0.60 /rnioles p e r g r a m of catalyst p e r second.

Cobalt Zeolon h a d

a n i n i t i a l r e f o r m i n g rate constant, K, c o m p a r a b l e t o that of t h e n i c k e l Z e o l o n b u t this d e c l i n e d r a p i d l y to a m u c h smaller v a l u e . T h e r e f o r m i n g rate constants, K', r e f e r r e d to t h e m e t a l content, s h o w that there is a s i g n i f i c a n t l y greater specific r e f o r m i n g a c t i v i t y b y factors o f 8 t o 30 f o r the n i c k e l zeolites, c o m p a r e d w i t h t h e n i c k e l - o n - a l u m i n a catalyst ( G 5 6 ). T h i s demonstrates

that t h e h i g h state of n i c k e l d i s p e r s i o n o n t h e z e o l i t e

catalysts does p r o v i d e a h i g h e r specific a c t i v i t y n i c k e l t h a n o b t a i n e d w i t h the m u c h h i g h e r n i c k e l c o n t e n t of t h e n i c k e l - o n - a l u m i n a catalysts.


This

74.

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Hydrogen

Generation


Over N i c k e l and Cobalt μ moles G Met Sec (Eq. S)

H (Eq. 4)

2.0 17.0 11.0 0.4 1.79 18.4 60.2

0.86 0.93 0.84 0.67 0.86 0.89 0.81

by Steam

Reformation

431

Catalysts

Mol

c,

p

H0 2

wt. %

C atom

1.4 3.95

2.3 2.5

1.08 3.57 1.78 0.95

2.6 2.4 2.4 2.4

-

-

r e s u l t is consistent w i t h t h e greater n i c k e l specific area d e t e r m i n e d b y carbon monoxide chemisorption. contradict

earlier conclusions

T h i s c o n c l u s i o n , h o w e v e r , appears to

reached

b y Selwood

b a s e d o n m a g n e t i c s u s c e p t i b i l i t y measurements

(13)

that a

which

finite

were

crystallite

size l a r g e e n o u g h to f o r m a f e r r o m a g n e t i c d o m a i n a n d s i g n i f i c a n t l y l a r g e r t h a n a n i c k e l a t o m is r e q u i r e d f o r c a t a l y t i c a c t i v i t y . T h e H

p

values f o r

a l l t h e n i c k e l catalysts w e r e h i g h a n d f e l l w i t h i n t h e r e l a t i v e l y n a r r o w r a n g e o f 0.81 to 0.93, i n d i c a t i n g that the r e f o r m i n g r e a c t i o n proceeds w e l l beyond methane

to c a r b o n

d i o x i d e , a n d that

side reactions,

s u c h as

c r a c k i n g , o c c u r to a m i n i m a l extent.

Conclusions Both mordenite

(nickel Zeolon)

a n d faujasite

(Linde Y SK400)

zeolites w i t h l o w n i c k e l l o a d i n g s , 3.24 a n d 0.76 w t % , r e s p e c t i v e l y , h a v e d e m o n s t r a t e d a c t i v i t y f o r s t e a m - r e f o r m i n g of n-hexane s u p e r i o r t o a c o m m e r c i a l n i c k e l - a l u m i n a catalyst of m u c h greater n i c k e l l o a d i n g . F u r t h e r m o r e , t h e r e f o r m i n g rates r e f e r r e d t o t h e c a t a l y t i c m e t a l e x c e e d that of the n i c k e l - a l u m i n a catalyst b y factors o f 8 to 30. Tests of 4 - 8 h o u r s ' duration resulted i n carbon accumulations

comparing quite

favorably

w i t h t h a t of t h e n i c k e l - a l u m i n a catalyst. H o w e v e r , t h e l o n g - t e r m s t a b i l i t y a n d r e f o r m i n g a c t i v i t y of these catalysts m u s t b e e s t a b l i s h e d b e f o r e t h e y c a n b e c o n s i d e r e d successful catalysts

competitors

with

alkali-promoted nickel

( J , 2, 7, 8) c u r r e n t l y u n d e r c o n s i d e r a t i o n f o r steam r e f o r m i n g

of l i q u i d h y d r o c a r b o n s .

T h e s t r u c t u r a l i n t e g r i t y of catalysts e x p o s e d f o r

e x t e n d e d p e r i o d s t o r e f o r m i n g process c o n d i t i o n s s h o u l d b e e s t a b l i s h e d b y x-ray d i f f r a c t i o n d e t e r m i n a t i o n of t h e zeolite c r y s t a l l o g r a p h y a n d m e t a l c r y s t a l l i t e size, a n d b y gas c h e m i s o r p t i o n measurements f o r d e t e r m i n a t i o n of t h e persistence o f c a t a l y t i c m e t a l surface area.


432

MOLECULAR

SIEVE

ZEOLITES

Π

Acknowledgment T h e a u t h o r expresses a p p r e c i a t i o n to the U n i t e d A i r c r a f t C o r p . f o r s u p p o r t of this w o r k a n d p e r m i s s i o n to p u b l i s h .


Literature Cited (1) Andrew, S. P. S., Ind. Eng. Chem. Prod. Res. Develop. 1969, 8, 321. (2) Arnold, M. R., Atwood, K., Baugh, H . M., Smyser, H . D., Ind. Eng. Chem. 1952, 44, 999. (3) Barrer, R. M., Ber. Bunsengesellschaft 1965, 69, 786. (4) Breck, D. W., J. Chem. Educ. 1964, 41, 678. (5) Breck, D. W., Castor, C. R., Milton, R. M., U. S. Patent 3,013,990 (1961). (6) Breck, D. W., Milton, R. M., U. S. Patent 3,013,982 (1961). (7) Bridger, G. W., Wyrwar, W., Chem. Process. Eng. 1967, 48, 101. (8) Brogars, D. J., Ind. Chemist 1963, 39, 177. (9) Brooks, C. S., J. Catalysis 1965, 4, 535. (10) Brooks, C. S., Christopher, G. L. M., J. Catalysis 1968, 10, 211. (11) Klug, H. P., Alexander, L. E., "X-ray Diffraction Procedures," Wiley, New York, 1954. (12) Kokes, R. J., Emmett, P. H., J. Am. Chem. Soc. 1960, 82, 1037. (13) Selwood, P. W., Chem. Rev. 1946, 38, 41. (14) Turkevich, J., Catalysis Rev. 1967, 1, 1. RECEIVED February 4,

1970.

Discussion D . J . C . Y a t e s ( E s s o R e s e a r c h C o . , L i n d e n , N . J. 0 7 0 3 6 ) : It is inter e s t i n g that y o u find the same m i g r a t i o n effect of n i c k e l o n N a X zeolites that I f o u n d some t i m e ago o n samples r e d u c e d u n d e r m i l d e r c o n d i t i o n s t h a n y o u u s e d . D i d y o u find less m i g r a t i o n w i t h n i c k e l o n Z e o l o n ? I a m also c o n c e r n e d w i t h the use of C O f o r area m e a s u r e m e n t w i t h n i c k e l , as i t is easy to m a k e n i c k e l c a r b o n y l w i t h r e d u c e d n i c k e l even at q u i t e l o w C O pressures.

D i d y o u c h e c k y o u r results w i t h h y d r o g e n c h e m i s o r p t i o n ?

C . S. B r o o k s : R e g a r d i n g the q u e s t i o n of the m o b i l i t y of r e d u c e d n i c k e l m e t a l o n oxide supports, I t h i n k there is no q u e s t i o n that n i c k e l does h a v e a h i g h m o b i l i t y , a n d at r e l a t i v e l y l o w temperatures.

In order

to h a v e h i g h d i s p e r s i o n , the most f a v o r a b l e c o n d i t i o n s can be p r o v i d e d b y starting w i t h the highest possible state of d i s p e r s i o n , s u c h as is pos sible b y m o u n t i n g the m e t a l o n a zeolite b y c a t i o n exchange a n d b y u s i n g a relatively l o w metal loading, preferably below 1 w t % .

O f the zeolites

e x a m i n e d here, the n i c k e l m o r d e n i t e w i t h 3.2 w t % n i c k e l a n d n i c k e l Y faujasite w i t h 0.76 w t % n i c k e l d e m o n s t r a t e d the highest state of m e t a l d i s p e r s i o n a n d highest specific r e f o r m i n g a c t i v i t y . I n the case of some of the n i c k e l m o r d e n i t e p r e p a r a t i o n s , n i c k e l m e t a l crystallites w e r e de tected b y x-ray d i f f r a c t i o n analysis.


74.

BROOKS

Hydrogen

Generation

by Steam

Reformation

433

I n r e g a r d to the second q u e s t i o n r a i s e d a b o u t the p o s s i b l e m o b i l i t y of n i c k e l i n d u c e d b y the f o r m a t i o n of n i c k e l c a r b o n y l d u r i n g the C O c h e m i s o r p t i o n measurements, it is c o n s i d e r e d w e l l e s t a b l i s h e d that

the

i n i t i a l contact w i t h C O after h y d r o g e n r e d u c t i o n p r o v i d e s a r e l i a b l e measure of the i n i t i a l state of d i s p e r s i o n of the

reduced nickel metal.

T h e p r o c e d u r e u s e d was e v a l u a t e d i n c o n s i d e r a b l e d e t a i l ( R e f . 10).

It

was p o i n t e d out i n d i s c u s s i o n of this e v a l u a t i o n that n i c k e l d i s p l a c e m e n t b y c a r b o n y l f o r m a t i o n m i g h t o c c u r u p o n h e a t i n g a n d d e s o r p t i o n of the c h e m i s o r b e d c a r b o n m o n o x i d e u p o n c o m p l e t i o n of the i n i t i a l C O c h e m i Downloaded by UNIV OF PITTSBURGH on January 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch074

s o r p t i o n measurement. I n the a c c o u n t of this c h e m i s o r p t i o n w o r k , h y d r o g e n c h e m i s o r p t i o n was also d e s c r i b e d . H y d r o g e n c h e m i s o r p t i o n was consistently less t h a n the C O ( a f t e r c o r r e c t i o n for substrate c o n t r i b u t i o n ) a n d i t is suggested that this difference is o w i n g to the n e e d for n i c k e l crystallites large e n o u g h to p r o v i d e adjacent sites for dissociative a d s o r p t i o n of h y d r o g e n atoms, whereas the C O c a n a d s o r b as an u n d i s s o c i a t e d m o l e c u l e o n i n d i v i d u a l surface n i c k e l atoms. C h e m i s o r p t i o n measurements of either C O or h y d r o g e n o n t r a n s i t i o n metals m o u n t e d o n zeolite substrates r e q u i r e c o n s i d e r a b l e care f o r ade q u a t e i n t e r p r e t a t i o n . T h e z e o l i t e lattices after h i g h - t e m p e r a t u r e e v a c u a t i o n c a n a d s o r b large amounts of h y d r o g e n , a n d d i v a l e n t cations s u c h as c a l c i u m c a n adsorb l a r g e amounts of C O .


Hydrogen Generation by Steam Reformation of n-Hexane over Zeolite

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