Catalytic Synthesis of Hydrocarbons from Carbon Monoxide and

2 Catalytic Synthesis of Hydrocarbons from Carbon Monoxide and Hydrogen

1

M. A. VANNICE

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Corporate Research Laboratories, Exxon Research and Engineering Co., Linden, N.J. 07036

Different synthesis processes are reviewed briefly before recent research on the catalytic behavior of the Group VIII metals in carbon monoxide hydrogenation, in which spe cific activities have been determined for the first time, is presented. Chemisorption measurements used to define reduced metal surface area indicate that the ordering of specific activities for these supported metals is significantly different from relative activities determined in older studies of unsupported metals in which metal surface areas were not measured. Supported Pt and Pd catalysts have much higher specific activities than unsupported Pt and Pd, and these activity increases are attributed to a crystallite size effect and a metal-support interaction, respectively. Sup ported Ni catalysts show a similar, but less pronounced, activity enhancement compared with unsupported Ni, and in addition exhibit a shift in selectivity to higher molecular weight hydrocarbons.

Hp h e i n c r e a s i n g d e m a n d f o r e n e r g y , c o u p l e d w i t h t h e u n c e r t a i n t y a n d Α

expense of c r u d e o i l i m p o r t s , has r e n e w e d interest i n t h e p r o d u c t i o n

of fuels a n d c h e m i c a l s f r o m h y d r o g e n - d e f i c i e n t m a t e r i a l s . E n e r g y sources s u c h as c o a l , r e s i d u a , o i l shale, a n d t a r sands c a n b e gasified w i t h s t e a m or o x y g e n to p r o d u c e a gas c o n t a i n i n g l a r g e q u a n t i t i e s o f c a r b o n m o n o x i d e a n d h y d r o g e n . O n c e m e t h a n e is r e m o v e d f r o m this C O - H

2

rnixture,

i t is p u r i f i e d to r e m o v e s u l f u r poisons a n d t h e n a l l o w e d t o o v e r a catalyst 1

Current address: Department of Chemical Engineering, The Pennsylvania State University, University Park, Pa. 16802. 15

Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

16

SOLID S T A T E

to p r o d u c e a v a r i e t y of o r g a n i c p r o d u c t s .

CHEMISTRY

T h e synthesis of h y d r o c a r b o n

p r o d u c t s , w i t h t h e e x c e p t i o n of m e t h a n e , is c o m m o n l y r e f e r r e d to as t h e F i s c h e r - T r o p s c h synthesis r e a c t i o n . M a n y o r g a n i c p r o d u c t s c a n b e f o r m e d b y these C O - H reactions.

2

C o n t r o l of the p r o d u c t d i s t r i b u t i o n is of m a j o r

synthesis

importance

since specific p r o d u c t s are r e q u i r e d f o r different e n d uses. F o r i n s t a n c e , m e t h a n e a n d o t h e r l i g h t h y d r o c a r b o n s are necessary f o r t h e p r o d u c t i o n of substitute n a t u r a l gas ( S N G ) . H o w e v e r , i f a s y n t h e t i c n a p h t h a , w h i c h c a n b e u p g r a d e d i n t o h i g h octane m o t o r f u e l , is d e s i r e d , t h e n C4-C10 h y d r o c a r b o n l i q u i d s are p r e f e r r e d . A n o t h e r s i t u a t i o n of interest i n v o l v e s Downloaded by CORNELL UNIV on October 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch002

m e t h a n e w h i c h is n o w b e i n g flared i n the N e a r E a s t . A s a n a l t e r n a t i v e t o b u r n i n g this f u e l , i t c o u l d b e t r a n s p o r t e d to the U . S . as l i q u i d n a t u r a l gas ( L N G ) , m e t h a n o l , or paraffinic l i q u i d s . T h e l a t t e r t w o alternatives i n v o l v e steam r e f o r m i n g of m e t h a n e f o l l o w e d b y different C O - H

2

syn

thesis reactions. F i n a l l y , the p r o d u c t i o n of paraffinic l i q u i d s m a y b e a n important factor i n a combined

coal gasification-electrical generation

p o w e r p l a n t since these paraffinic l i q u i d s ( o r C H O H ) 3

c a n b e easily

s t o r e d d u r i n g off-peak h o u r s f o r use d u r i n g p e a k - l o a d h o u r s . T h e benefits of u n d e r s t a n d i n g a n d c o n t r o l l i n g p r o d u c t s e l e c t i v i t y i n CO-H

2

reactions are a p p a r e n t , a n d k n o w l e d g e of t h e c a t a l y t i c b e h a v i o r

of t h e G r o u p V I I I metals is a n i m p o r t a n t step t o w a r d a c h i e v e m e n t this g o a l .

A short r e v i e w of e x i s t i n g synthesis processes w i l l

the state of the art t o d a y i n C O - H

2

of

describe

catalysis. R e c e n t r e s e a r c h u s i n g

w e l l - c h a r a c t e r i z e d , s u p p o r t e d m e t a l catalysts is p r e s e n t e d , a n d the s i g nificance of these results is discussed. Different

Synthesis Processes

T h e gasification of h e a v y , h y d r o g e n - d e f i c i e n t m a t e r i a l s p r o v i d e s o n e r o u t e to p r o d u c e

c l e a n fuels

l a r g e q u a n t i t i e s of C O a n d H of p r o d u c t s . F i g u r e 1.

2

T h i s process

generates

w h i c h c a n react to f o r m a w i d e v a r i e t y

A s i m p l i f i e d s c h e m e of the o v e r a l l process is s h o w n i n

The C O - H

c o n t a i n i n g poisons. the w a t e r - g a s

and chemicals.

2

s t r e a m is p u r i f i e d to r e m o v e

C0

2

and sulfur-

T h e d e s i r e d H / C O r a t i o is t h e n o b t a i n e d b y u s i n g

shift r e a c t i o n .

2

Finally, by

the a p p r o p r i a t e

choice

of

catalyst a n d r e a c t i o n c o n d i t i o n s , the p r o d u c t d i s t r i b u t i o n is a d j u s t e d to m a x i m i z e p r o d u c t i o n of the d e s i r e d

compounds.

T h e t h e r m o d y n a m i c s for the f o r m a t i o n of o r g a n i c c o m p o u n d s

have

b e e n c a l c u l a t e d a n d d i s c u s s e d i n d e t a i l elsewhere ( J ) . W i t h the e x c e p t i o n of a f e w c o m p o u n d s s u c h as f o r m a l d e h y d e a n d acetylene, the A G ° values at 2 9 8 ° Κ are n e g a t i v e ;

therefore,

p r o d u c t i o n of

an

enormous

v a r i e t y of c o m p o u n d s at reasonable r e a c t i o n t e m p e r a t u r e s is t h e o r e t i c a l l y possible. T h e p r o d u c t i o n of different p r o d u c t s , h o w e v e r , c a n b e f a v o r e d b y a j u d i c i o u s c h o i c e of catalyst a n d t h e a p p r o p r i a t e r a n g e of t e m p e r a -


2.

Catalytic

VANNICE

Synthesis

CO + Η·

CH,

WATER-GAS SHIFT


L _

17

Hydrocarbons

PURIFIER

GASIF1ER

SYNTHESIS — t » C H -^C H 2

REACTOR

HEAT

o

1.

Gasification

4

A

—CH3OH —«* GASOLINE

COAL RESIDUA OIL SHALE TAR SANDS

STEAM OR 0

Figure

of

of hydrogen-deficient

materials

provides

one

t u r e a n d pressure as i l l u s t r a t e d i n F i g u r e 2. M e t a l catalysts f a v o r t h e p r o d u c t i o n of n o r m a l paraffins a n d olefins, w h e r e a s m e t a l oxides s u c h as Th0 /Al 0 2

2

3

can produce

branched-chain hydrocarbons.

Metal

oxides

a n d d o p e d m e t a l oxides a r e also r e q u i r e d f o r t h e f o r m a t i o n o f alcohols. A r o m a t i c s , a l d e h y d e s , ketones, a n d acids c a n also b e p r o d u c e d .

A dis

c u s s i o n o f e a c h of these processes i n s o m e w h a t m o r e d e t a i l w i l l p r o v i d e a f a m i l i a r i z a t i o n w i t h t h e catalysts u s e d a n d t h e t y p i c a l p r o d u c t d i s t r i b u tions a t t a i n e d . A n u m b e r of c o m p l i c a t i n g reactions c a n o c c u r c o n c u r r e n t l y w i t h t h e synthesis r e a c t i o n ; s o m e of t h e m o s t i m p o r t a n t reactions a r e s h o w n i n T a b l e I . S i n c e w a t e r is a p r i m a r y p r o d u c t i n m o s t of t h e synthesis r e a c tions, t h e w a t e r - g a s shift r e a c t i o n c a n o c c u r b e t w e e n this w a t e r a n d c a r b o n m o n o x i d e f r o m t h e f e e d s t r e a m . T h i s r e a c t i o n changes t h e o x y g e n containing by-products from H hydrogen a n d carbon monoxide.

2

0 to C 0

2

a n d alters t h e usage r a t i o of

T h e B o u d o u a r d r e a c t i o n results i n t h e

d i s p r o p o r t i o n a t i o n o f C O to p r o d u c e c a r b o n o n t h e catalyst surface a n d C0 . 2

C a r b o n d e p o s i t i o n c a n also o c c u r b y t h e d i r e c t r e a c t i o n o f h y d r o

gen a n d carbon monoxide.

T h i s is b a s i c a l l y t h e reverse r e a c t i o n o f c o a l

gasification i f t h e l a t t e r is r e p r e s e n t e d Table I.

i n a very simplified manner.

Complicating Reactions in C O Hydrogénation

W a t e r gas s h i f t

CO + H 0 2

B o u d o u a r d reaction C o k e deposition Carbide formation

C0 + H 2

2 C O -> C + C 0 H

2

2

2

+ C O -> C + H 0 2

xM + C

M*C



Figure

2.

4

Many different

10 Pressure, atm

Fixed bed -CoCarbonyls Paraffins Olefins

-RuHigh-melting paraffins

ZnO Methanol

100

2

3

-

1,000

Carbonyls -

• CH3OCH3

reactions can occur

(6)

Kirk-Othmer Encyclopedia of Chemical Technology

carbon monoxide-hydrogen

Ni — • Carbonyls Paraffins Olefins

CH

Fe Olefins - ParaffinsChemicals I

Fluid bed

ZnO + alkali Methanol and higher alcohols

2

ZnO + Al 0 Branched-chain olefins and — alcohols —

Th0

Aromatic* Alicyelts Branched-chain paraffins


10,000

2.

vANNiCE

Catalytic

Table II.

Synthesis

of

19

Hydrocarbons

Product Distribution from Medium-Pressure Synthesis Processes ( 1 ) Mol%

Catalyst

Paraffins

F e ( 2 2 0 ° C , 10 a t m ) C o (200°C, 7 atm)

46.4 79.0

Olefins 33.1 20.0

Alcohols 20.5 1.0

The Fischer-Tropsch and Related Syntheses

F i n a l l y , w h e n c a r b o n exists o n t h e surface of a m e t a l catalyst, t h e r e is Downloaded by CORNELL UNIV on October 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch002

the p o s s i b i l i t y of m e t a l c a r b i d e f o r m a t i o n . T h e f o r m a t i o n of these c a r b i d e s , s u c h as F e C , C o C , a n d N i C , c a n alter the c a t a l y t i c b e h a v i o r of 2

2

3

t h e m e t a l that w a s i n i t i a l l y present. M e t h a n a t i o n has r e c e i v e d t h e most a t t e n t i o n d u r i n g t h e past

few

years, p r i m a r i l y b e c a u s e i t converts a l o w B t u syn-gas m i x t u r e i n t o a h i g h B t u substitute n a t u r a l gas

(SNG).

T y p i c a l l y , R a n e y n i c k e l or

m a s s i v e n i c k e l / a l u m i n a catalysts are u s e d i n a t e m p e r a t u r e r a n g e 523-673°K.

of

T e m p e r a t u r e s h i g h e r t h a n 700° Κ c a n r e s u l t i n severe c a t a

lyst d e a c t i v a t i o n c a u s e d b y m e t a l s i n t e r i n g . A w i d e r a n g e of pressures c a n b e e m p l o y e d , b u t o p e r a t i o n at 3 6 0 0 - 7 2 0 0 k P a ( 5 0 0 - 1 0 0 0 p s i ) a l l o w s a d i r e c t t i e - i n w i t h p i p e l i n e n a t u r a l gas.

U n d e r these o p e r a t i n g c o n d i

tions m e t h a n e is t h e p r e p o n d e r a n t h y d r o c a r b o n p r o d u c t since o n l y s m a l l a m o u n t s of ethane, p r o p a n e , a n d b u t a n e are f o r m e d .

T h i s r e a c t i o n is

d i s c u s s e d i n d e t a i l b y M i l l s a n d Steffgen ( 2 ) , V l a s e n k o a n d Y u z e f o v i c h (3), and Greyson

(4).

T h e first c o m m e r c i a l F i s c h e r - T r o p s c h catalysts d e v e l o p e d w e r e precipitated C o / K i e s e l g u h r a n d reduced, promoted i r o n oxide. are s u b s t a n t i a l differences

co-

There

i n the product distribution obtained i n the

m e d i u m - p r e s s u r e synthesis process,

w h i c h is n o r m a l l y r u n b e t w e e n

5

a n d 20 a t m . C o b a l t p r o d u c e s paraffins p r i m a r i l y , w h i l e p r o m o t e d i r o n gives a l a r g e r p e r c e n t a g e

of olefins a n d o x y g e n a t e d m a t e r i a l w h i c h is

m o s t l y alcohols, as i n d i c a t e d i n T a b l e I I . A l o o k at just the h y d r o c a r b o n p o r t i o n of the p r o d u c t reveals f u r t h e r differences

i n the c a t a l y t i c b e

h a v i o r of C o a n d F e . A n e x a m i n a t i o n of t h e m o l e c u l a r w e i g h t d i s t r i b u tions s h o w n i n F i g u r e 3 i n d i c a t e s that w i t h F e , a m a x i m u m occurs the C

3

species o n a w t %

for

basis, w h e r e a s m e t h a n e is t h e p r e d o m i n a n t

p r o d u c t over C o w i t h a s e c o n d m a x i m u m o c c u r r i n g for the C 5 f r a c t i o n . T h e enormous a m o u n t of d a t a d e s c r i b i n g these synthesis processes has b e e n d i s c u s s e d b e f o r e ( 1 ). I n the late 1930s P i c h l e r d i s c o v e r e d that r u t h e n i u m p r o d u c e s

very

h i g h m o l e c u l a r w e i g h t paraffinic waxes at l o w temperatures a n d v e r y h i g h pressures. T h i s b e h a v i o r is r e p r e s e n t e d i n F i g u r e 4 w h e r e i t c a n b e seen t h a t over 30 w t % of the p r o d u c t has a n average m o l e c u l a r w e i g h t



The Fischer-Tropsch and Related Syntheses

Figure

3.

Molecular

weight distribution of hydrocarbon medium-pressure synthesis process (1)

products

in

the

> 200,000. T h i s b e h a v i o r has b e e n d i s c u s s e d i n greater d e t a i l b y P i c h l e r a n d co-workers (5,6). T h e m e t a l oxides seem to b e less a c t i v e synthesis catalysts t h a n the metals, thereby necessitating more d e m a n d i n g reaction conditions. T h e y d o p r o d u c e a m u c h different p r o d u c t s p e c t r u m , h o w e v e r . M e t h a n o l c a n b e s y n t h e s i z e d q u i t e s e l e c t i v e l y w i t h Z n O catalysts, a n d z i n c o x i d e c h r o m i a catalysts w e r e first c o m m e r c i a l i z e d i n t h e e a r l y 1920s. T h e l o w


2.

VANNICE

Catalytic

Synthesis

0

of Hydrocarbons

21

200000 LOO 000 600003 800 000 ' WOO 000

MOLECULAR WT.


Chemie Ingénieur Tech ni k

Figure 4. Ruthenium is unique in its production of high molecular weight paraffinic waxes (90°C, 2000 atm) (5).

a c t i v i t y o f these catalysts r e q u i r e d h i g h o p e r a t i n g t e m p e r a t u r e s of 5 5 0 ° 7 0 0 ° K , r e s u l t i n g i n v e r y u n f a v o r a b l e e q u i l i b r i u m conversions since A G ° f o r t h e m e t h a n o l synthesis r e a c t i o n b e c o m e s p o s i t i v e at c a . 4 2 5 ° K . V e r y h i g h r e a c t o r pressures of

100-600

atm were

therefore

necessary

to

a c h i e v e reasonable conversions b e c a u s e of this t h e r m o d y n a m i c l i m i t a t i o n . C a t a l y s t s d e v e l o p e d r e c e n t l y w h i c h c o n t a i n c o p p e r are m o r e a c t i v e t h a n t h e z i n c o x i d e - c h r o m i a catalysts a n d c a n b e u s e d at l o w e r t e m p e r a t u r e s , t h e r e b y r e d u c i n g t h e pressure r e q u i r e d to a c h i e v e t h e same conversions. T h i s c h a n g e results i n a m o r e e c o n o m i c a l process since pressures o f o n l y 50-100 a t m are n o w needed.

A g o o d r e v i e w of t h e m e t h a n o l synthesis

r e a c t i o n is p r o v i d e d b y N a t t a ( 7 ) . T h e a d d i t i o n of a l k a l i metals to z i n c o x i d e i m p a r t s t h e c a p a b i l i t y to p r o d u c e l o n g e r - c h a i n alcohols, b u t these catalysts m u s t b e o p e r a t e d at h i g h t e m p e r a t u r e s a n d h i g h pressures. A t these c o n d i t i o n s 2 - b u t a n o l is t h e p r i m a r y p r o d u c t , exclusive of m e t h a n o l , a n d comprises o v e r 50 w t % of t h e p r o d u c t s h o w n i n F i g u r e 5. T h e synthesis of h i g h e r m o l e c u l a r w e i g h t alcohols is discussed i n greater d e t a i l b y N a t t a , C o l o m b o , a n d Pasquon (8). A t these h i g h pressures a n d e v e n h i g h e r t e m p e r a t u r e s , t h e use of Th0 -Al 0 2

products

2

3

catalysts results i n t h e isosynthesis r e a c t i o n i n w h i c h t h e

are p r i m a r i l y b r a n c h e d paraffins r a t h e r t h a n s t r a i g h t - c h a i n

hydrocarbons.

A t t h e c o n d i t i o n s g i v e n i n F i g u r e 6, i s o b u t a n e is t h e

p r i n c i p a l p r o d u c t o n a w t % basis. T h e p r o d u c t i o n of aromatics also has b e e n o b s e r v e d i n this system. A d e t a i l e d d e s c r i p t i o n of t h e isosynthesis r e a c t i o n is g i v e n b y C o h n ( 9 ) . T h e newest development i n C O - H

2

catalysis w a s d i s c l o s e d r e c e n t l y

b y U n i o n C a r b i d e ( 1 0 ) . B y u s i n g a h o m o g e n e o u s R h catalyst at m o d erate t e m p e r a t u r e s

(525°K)

a n d e x t r e m e l y h i g h pressures o f 2 0 , 0 0 0 -


22

SOLID STATE

( 4 1 0 C , 375


e

CHEMISTRY

ATM)

π

i-C

4

ALCOHOL CARBON NUMBER

2-METHYL HIGHER ALC. Catalysis

Figure

5.

Product

distribution for the alcohol synthesis reaction. 14% K 0/ZnO(8).

Catalyst:

9

(450 C, e

300

ATM)

run

C

4

i-C

4

C + 5

(ALC)

CARBON NUMBER Catalysis

Figure

6.

The isosynthesis

reaction favors isobutane ThO /AkO (9). t

formation.

Catalyst:

s


2.

Catalytic

VANNiCE

Synthesis

of

23

Hydrocarbons

50,000 p s i , ethylene g l y c o l c a n b e s y n t h e s i z e d d i r e c t l y f r o m C O a n d H . 2

A b o u t three-quarters of the p o l y h y d r o x y p r o d u c t is c o m p o s e d of e t h y l e n e glycol, w i t h the

balance

c o m p o s e d of p r o p y l e n e g l y c o l a n d g l y c e r i n e .

M e t h a n o l is also p r o d u c e d i n this process.

T h i s result s t r o n g l y suggests

that n e w h o m o g e n e o u s catalysts m a y p l a y a n i m p o r t a n t f u t u r e r o l e i n t h e catalysis of C O - H

2

synthesis reactions.

A l t h o u g h research c o n c e r n i n g C O - H

2

synthesis reactions spans t h r e e -

quarters of a c e n t u r y , a n u m b e r of p r o b l e m s s t i l l r e m a i n to b e

solved.

A s t y p i f i e d b y the figures p r e s e n t e d , there is s t i l l a l a c k of p r o d u c t selec t i v i t y , e s p e c i a l l y i n the synthesis of h y d r o c a r b o n s o t h e r t h a n m e t h a n e . Downloaded by CORNELL UNIV on October 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch002

T h e c a p a b i l i t y of selectively f o r m i n g a specified c o m p o u n d , i.e., e t h y l e n e , w o u l d b e h i g h l y desirable.

I n a d d i t i o n to this p r o b l e m , a l l the G r o u p

V I I I metals are v e r y sensitive to s u l f u r poisons s u c h as H S , a n d a s u l f u r 2

tolerant catalyst w o u l d b e a m a j o r i m p r o v e m e n t . F o r e x a m p l e , the m e t h a n a t i o n processes u s i n g n i c k e l catalysts r e q u i r e that the H S l e v e l i n the 2

f e e d s t r e a m b e r e d u c e d to 0.01-0.1 p p m to a c h i e v e satisfactory catalyst l i f e t i m e s . B e c a u s e of this r e q u i r e m e n t , a d d i t i o n a l p u r i f i c a t i o n steps m u s t b e a d d e d to a t t a i n these v e r y l o w s u l f u r levels. A n o t h e r p r o b l e m is catalyst d e a c t i v a t i o n , w h i c h c a n o c c u r b e c a u s e of s i n t e r i n g of m e t a l p a r t i c l e s , c o k e d e p o s i t i o n , a n d m e t a l c a r b i d e f o r m a tion.

C a t a l y s t s that are m o r e sinter-resistant are p a r t i c u l a r l y d e s i r a b l e .

A l s o , some of the G r o u p V I I I metals f o r m v o l a t i l e c a r b o n y l s . T h i s creates a p r o b l e m n o t o n l y because of the t o x i c i t y of these c a r b o n y l s b u t also b e c a u s e i m p r o p e r catalyst t r e a t m e n t c a n result i n m e t a l t r a n s p o r t w i t h i n t h e catalyst b e d a n d e v e n out of t h e reactor. A l t h o u g h m a n y studies i n v o l v i n g heterogeneous catalysts for

CO-H

2

reactions h a v e b e e n c o n d u c t e d , most of the d a t a w e r e o b t a i n e d b e f o r e the a d v e n t of the sensitive a n a l y t i c a l t e c h n i q u e s a v a i l a b l e t o d a y .

There

fore, l a r g e p r o d u c t y i e l d s w e r e r e q u i r e d f o r p r o d u c t analyses, w h i c h w e r e f r e q u e n t l y r e p r e s e n t e d i n terms of d i s t i l l a t i o n fractions. product

distributions were

not commonplace.

Reactors

Detailed

were usually

o p e r a t e d at h i g h conversions, t h e r e b y p r o v i d i n g k i n e t i c d a t a f r o m i n t e g r a l reactors.

T h e s e d a t a are n o t so easily i n t e r p r e t e d as d a t a

from

d i f f e r e n t i a l reactors since heat a n d mass transfer effects, s e c o n d a r y

reac

tions, a n d p r o d u c t i n h i b i t i o n c a n c o m p l i c a t e k i n e t i c analysis. I n a d d i t i o n , n o specific a c t i v i t y rate d a t a h a d b e e n d e t e r m i n e d i n these e a r l i e r studies b e c a u s e c h e m i s o r p t i o n t e c h n i q u e s h a d n o t b e e n d e v e l o p e d to t h e p o i n t where cedure.

they were

u t i l i z e d as a r o u t i n e catalyst c h a r a c t e r i z a t i o n

pro

T h e c o m p a r i s o n of rates o n the basis of u n i t m e t a l surface area

or p e r m e t a l surface site, i.e. t u r n o v e r n u m b e r s , is the o n l y m e a n i n g f u l w a y to c o m p a r e t h e i n t r i n s i c a c t i v i t y of different m e t a l catalysts. B e c a u s e of these l i m i t a t i o n s s t i l l e x i s t i n g i n the 1970s, a s t u d y w a s i n i t i a t e d to p r o v i d e this necessary i n f o r m a t i o n for t h e first t i m e .


24

SOLDO S T A T E

Table III.

Dispersion Variations for Different Metal Catalysts (11) Catalyst

% Metal

15% F e / A l 0 5% R u / A l 0 2% C o / A l 0 5%Ni/Al 0 2% P d / A l 0 1% R h / A l 0 1.75% P t / A l 0 2% I r / A l 0 2

3

2

3

2

3

2

3

2

3

2


2

Dispersion 2* 6* 8' 13' 22* 48' 88* 90'

3

2

3

3

* Assuming bridged bonding of CO. * Assuming linear bonding of CO. Supported Metal

CHEMISTRY

Journal of Catalysis

Catalysts

T h e b e h a v i o r o f G r o u p V I I I metals d i s p e r s e d o n a v a r i e t y o f t y p i c a l m e t a l o x i d e s u p p o r t s has b e e n i n v e s t i g a t e d i n t h e C O — H

2

synthesis r e a c

t i o n ( I I ) . D i s p e r s i n g a m e t a l o n a s u p p o r t i s advantageous n o t o n l y b e c a u s e i t results i n t h e f o r m a t i o n o f v e r y s m a l l m e t a l crystallites, t h e r e b y i n c r e a s i n g t h e s u r f a c e a r e a p e r g r a m o f t h e m e t a l c o m p o n e n t , b u t also b e c a u s e t h e m e t a l surface i s s t a b i l i z e d u n d e r r e a c t i o n c o n d i t i o n s . I t w a s necessary t o s t u d y these w e l l - c h a r a c t e r i z e d G r o u p V I I I m e t a l catalysts i n o r d e r t o m a k e m e a n i n g f u l k i n e t i c c o m p a r i s o n s . tion of H

2

Chemisorp-

a n d C O m e a s u r e d t h e surface a r e a o f t h e r e d u c e d m e t a l ,

t h e r e b y a l l o w i n g t h e c a l c u l a t i o n o f b o t h specific a c t i v i t y , expressed as turnover numbers, a n d m e t a l dispersion, w h i c h is the ratio of surface m e t a l atoms t o t h e t o t a l n u m b e r o f m e t a l atoms i n t h e catalyst. T a b l e I I I s h o w s t h e d i s p e r s i o n d a t a f o r a series o f a l u m i n a - s u p p o r t e d m e t a l s . T h e m o r e n o b l e G r o u p V I I I metals a r e t y p i c a l l y b e t t e r d i s p e r s e d . T h e w i d e Table I V .

(11)

N e w vs. O l d D a t a for Methanation A c t i v i t y Vannice

Fischer et al. (1925) CH Formation/'g Metal t

Ru Ir Rh Ni Co Os Pt Fe Pd

N

CHi

Metal Ru Fe Ni Co Rh Pd Pt Ir

@ 275°C (sec' ) 1

0.181 0.057 0.032 0.020 0.013 0.012 0.0027 0.0018 Journal of Catalysis


2.

VANNICE

Catalytic

Synthesis

of

25

Hydrocarbons

v a r i a t i o n i n d i s p e r s i o n v a l u e s illustrates t h e i m p o r t a n c e of c h a r a c t e r i z i n g catalysts i n this m a n n e r . A d i f f e r e n t i a l , flow m i c r o r e a c t o r w a s o p e r a t e d at steady-state c o n d i tions, a n d c o n v e r s i o n d a t a free f r o m heat a n d mass transfer l i m i t a t i o n s w e r e o b t a i n e d (11).

E v e n at these l o w conversions, t y p i c a l l y less t h a n

5 % , a c c u r a t e p r o d u c t d i s t r i b u t i o n s c o u l d b e d e t e r m i n e d b y gas c h r o m a tography using sub-ambient temperature programming.

Specific activi

ties for t h e G r o u p V I I I metals are l i s t e d i n T a b l e I V . T h e s e results f o r t h e m e t h a n a t i o n r e a c t i o n are c o m p a r e d w i t h those f r o m t h e o n l y o t h e r kinetic study w h i c h encompassed Downloaded by CORNELL UNIV on October 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch002

F i s c h e r i n 1925 ( 1 2 ) .

a l l the G r o u p V I I I m e t a l s — t h a t of

T e c h n i q u e s to m e a s u r e m e t a l s u r f a c e areas w e r e

n o t yet a v a i l a b l e , a n d these a c t i v i t i e s w e r e c o m p a r e d o n a g r a m - m e t a l basis w i t h n o c o r r e c t i o n for surface area differences. W h e n this c o r r e c t i o n is m a d e , significant changes o c c u r i n t h e o r d e r i n g of a c t i v i t y , e s p e c i a l l y f o r F e a n d I r . A n o t h e r i m p o r t a n t feature is t h a t a f a c t o r of o n l y 100 i n specific a c t i v i t y separates t h e least a c t i v e a n d t h e m o s t a c t i v e m e t a l s i n t h e m e t h a n a t i o n r e a c t i o n . T h e same o r d e r i n g of a c t i v i t y exists f o r t o t a l C O conversion

(11).

E v e n at these l o w conversions, h i g h e r m o l e c u l a r w e i g h t p r o d u c t s are easily d e t e c t e d w h e n present, a n d these metals e x h i b i t s e l e c t i v i t y trends r e p r e s e n t a t i v e of t h e i r b e h a v i o r u n d e r t y p i c a l p r o c e s s i n g c o n d i t i o n s .

As

e x p e c t e d , a n increase i n the H / C O r a t i o i n t h e f e e d stream increases 2

t h e r e l a t i v e f o r m a t i o n of m e t h a n e . T h e k i n e t i c parameters o b t a i n e d for this series of a l u m i n a - s u p p o r t e d metals are l i s t e d i n T a b l e V . B y fitting d a t a to a p o w e r rate l a w , a c t i v a t i o n energies a n d p a r t i a l pressure d e p e n d e n c i e s w e r e o b t a i n e d . W i t h the e x c e p t i o n of R u a n d C o , the m e t h a n a t i o n r e a c t i o n is a b o u t first o r d e r i n h y d r o g e n a n d n e a r zero o r d e r i n c a r b o n m o n o x i d e . Table V .

Methanation Kinetics over Alumina-Supported Metals as Determined by a Power Rate L a w ( 1 1 ) N

Catalyst Ru Fe Ni Co Rh Pd Pt Ir

These data reveal

(

sec ) 1

0.181 0.057 0.032 0.020 0.013 0.012 0.0027 0.0018

C

H

4

=

Ae »' E

R

A ( sec' )

-P /-Pco H

E (kcal/mol)

y

m

1

5.7 χ 2.2 Χ 2.3X 9.0 Χ 5.2 Χ 1.2 X 1.6 Χ 1.4 Χ

T

10 10 10 10 10 10 10 10

8 7

8 7 7 e 4 4

24.2 21.3 25.0 27.0 24.0 19.7 16.7 16.9

X 1.6 1.1 0.8 1.2 1.0 1.0 0.8 1.0

Y -0.6 -0.1 -0.3 -0.5 -0.2 0 0 0.1 Journal of Catalysis


26

SOLID S T A T E C H E M I S T R Y

30 μ


20

10

15

20

25 E

30

(kcal/mole) m

Journal of Catalysis

Figure 7.

The compensation effect for the methanation

reaction

(11)

t h a t a c o m p e n s a t i o n effect exists f o r the m e t h a n a t i o n r e a c t i o n as s h o w n i n F i g u r e 7. S e c o n d l y , w i t h a d s o r p t i o n d a t a f r o m t h e o p e n l i t e r a t u r e , a s t r o n g c o r r e l a t i o n w a s f o u n d to o c c u r b e t w e e n specific a c t i v i t y a n d t h e h e a t of a d s o r p t i o n of C O (11).

T h i s is r e p r e s e n t e d i n F i g u r e 8.

This

c o r r e l a t i o n appears to b e t h e r i g h t - h a n d p o r t i o n of t h e w e l l - k n o w n v o l cano plot.

T h i s r e l a t i o n s h i p is a n i m p o r t a n t r e s u l t since i t n o t

correlates a c a t a l y t i c p r o p e r t y w i t h a p h y s i c a l l y m e a s u r a b l e

only

property,

b u t i t also tells us that w e a k e n i n g the m e t a l - C O b o n d appears t o r e s u l t i n higher activity. Metal Crystallite

Size Effects and Metal-Support

Interactions

M e t a l s w e r e d i s p e r s e d o n a v a r i e t y of m a t e r i a l s t h a t are t y p i c a l l y u s e d as s u p p o r t s , s u c h as A 1 0 , S i 0 , zeolites, a n d c a r b o n . I t w a s f o u n d t h a t t h e s u p p o r t c a n p l a y a v e r y n o t i c e a b l e r o l e i n the catalysis of C O - H reactions b y i n f l u e n c i n g the b e h a v i o r of the m e t a l c o m p o n e n t (IS). For 2

3

2

2



2.

VANNICE

Catalytic

Synthesis

-30

of

27

Hydrocarbons

-40

-50

CO Heat of Adsorption (kcal/aole) Figure 8. Conelation between methanation activity and CO heat of adsorption for alumina-supported metals. Both values for Ni/ηAl O from Table VIII are included. 2

s

i n s t a n c e , s u p p o r t e d P t catalysts h a v e a specific a c t i v i t y t w o orders

of

m a g n i t u d e h i g h e r t h a n u n s u p p o r t e d P t , as s h o w n i n T a b l e V I . T h e m a j o r r o l e of the s u p p o r t i n this case appears to b e the f o r m a t i o n a n d s t a b i l i z a t i o n of v e r y s m a l l P t crystallites since o n l y s m a l l differences i n t u r n o v e r n u m b e r exist b e t w e e n h i g h l y d i s p e r s e d P t o n different s u p p o r t s . Table V I .

Effect of Platinum Crystallite Size on Methanation A c t i v i t y (14) H / C O — 3, Ρ = 2

1 atm

Ncm @ 275°C (sec^XlO )

Catalyst

3

1.75% P t / A l 0 1.75% P t / A l 0 (sint.) 2.0% P t / S i 0 Pt Black 25% Pt Black/Al 0 (Physical mixture) 2

2

This

3

3

2

2

3

2.7 2.2 1.6

Catalytic Synthesis of Hydrocarbons from Carbon Monoxide and

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