Heterogeneous Catalysis - American Chemical Society

28 Chain Growth and Iron Nitrides in the FischerTropsch Synthesis

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ROBERT B. ANDERSON McMaster University, Department of Chemical Engineering, Institute for Materials Research, Hamilton, Ontario, Canada

Developments in the Fischer-Tropsch synthesis at the Bureau of Mines from 1945 to 1960 include a simple mechanism for chain growth and the use of iron nitrides as catalysts. The chain-growth schene can predict the carbon-number and isomer distribu tions for products from most catalysts with reason able accuracy using only 2 adjustable parameters. Iron nitrides are active, durable catalysts that produce high yields of alcohols and no wax. During the synthesis, the nitrides are converted to carbonitrides. At the end of World War II, widespread fear of petroleum shortages led to the authorization of the U.S. Bureau of Mines to initiate a large research and development program on fossil fuelto-oil processes. A laboratory was established at Bruceton, Pa., under the direction of the late Henry H. Storch (1). Dr. Storch was an authority on catalysis and coal and an outstanding research administrator. This chapter is dedicated to his memory. In research on the Fischer-Tropsch synthesis, FTS, at the Bureau, mechanisms of the growth of the carbon chain and the use of iron nitrides as catalysts were developments that were not anticipated by previous German work. Herington (2) in 1946 was the first to consider chain growth in FTS. He defined a probability $ that the chain will desorb rather than grow at the surface, where

and α is the probability of chain growth and φ. is the moles of product of carbon number i . Equation 1 provides an unambiguous 0097-6156/83/0222-0389$06.00/0 © 1983 American Chemical Society

390

HETEROGENEOUS CATALYSIS

d e s c r i p t i o n o f the growth process and i s s t i l l used o c c a s i o n a l l y (3). F r i e d e l and Anderson (4) showed that i f 3 was constant over a range o f carbon numbers, i n t h i s range

and

=

W * n

α

'

( 2 )

.


(3)

Equation 3 was a l s o derived by a k i n e t i c scheme for adding carbon atoms one at a time to the growing chain when the r a t i o o f the parameters for propagation to termination i s constant. Equation 3 may be summed in various ways; f o r example, Manes ( 5 ) showed how the y i e l d s o f g a s o l i n e , d i e s e l o i l and wax change with a. The d i e s e l - o i l f r a c t i o n , C^-C-g, o f the condensed hydrocarbons was 20 t o 30%, r e l a t i v e l y independent o f the value of a. Anderson, et a l . , (6,7) extended the chain growth process to account for the production o f s t r a i g h t carbon chains and chains with methyl branches; e t h y l - s u b s t i t u t e d species had not yet been found i n synthesis products. Branching was postulated to be a part o f the chain growth as depicted by the network i n Table 1, i n which carbons are added one at a time to the end or penultimate carbons at one end of the chain as i n d i c a t e d by the a s t e r i s k s . Table

1.

Chain Growth Network

28.

391

Fischer-Tropsch Synthesis

ANDERSON

Addition to the penultimate carbon does not occur i f i t i s already attached to 3 carbons. I f the k i n e t i c s constants are arranged as shown i n the network, where f o r s t r a i g h t chains Φ = φ a and f o r branched chains φ ^ = Φ ^ ί , the r a t i o o f the moles o r a monomethyl branched chain to s t r a i g h t chains i n a given carbon number i s f or 2 f , the r a t i o o f dimethyls to s t r a i g h t molecules f or 2f . η + 1

+ 1

p


The carbon number d i s t r i b u t i o n i s given by

Φ

η

= Φ

n

2

2

a " (1

+ (n-3) f +

(n-4)(n-5) 2 2 f

(4)

for the range o f η from 4 t o 8. Terms f o r t r i m e t h y l and more h i g h l y s u b s t i t u t e d s p e c i e s must be included f o r l a r g e r carbon numbers. One o f d i s t r i b u t i o n s from the t h e s i s o f Achtsnit (8) f o r products from a Co-Th0 -kieselguhr c a t a l y s t with 2H +1C0 feed a t 187 C and 1 atm was used to i l l u s t r a t e the usefulness o f equation 4, as shown i n Table 2. This product had been hydrogenated to saturate the o l e f i n s to s i m p l i f y the a n a l y s i s , and only data f o r normal and monomethyl branched molecules were reported. The amount o f dimethyl species should be about 10% o f the monomethyls. The agreement between c a l c u l a t e d and experimental values are sastifactory for and above, o f t e n w i t h i n the o v e r a l l u n c e r t a i n t i e s o f the a n a l y s i s . I f the growth parameter f o r is taken as 2a as shown i n Table 1, the value i n parentheses, 6.95, i s obtained. The values for C are widely d i f f e r e n t , and i t i s p o s s i b l e that methane i s produced i n other ways and should not be included i n the c a l c u l a t i o n . 2

2

Equation 2 was rediscovered i n 1976 and perhaps i m p e r t i n e n t l y given the name " S c h u l z - F l o r y equation" honoring the famous polymer chemists (9 to 14). A more appropriate name might have been the "Bureau o f Mines equation". Recently, some have suggested that only s t r a i g h t chains are produced i n the s y n t h e s i s r e a c t i o n s and branched molecules a r i s e from the i n c o r p o r a t i o n i n the synthesis o f o l e f i n s , p a r t i c u l a r l y propylene (12,15). Ethylene and propylene do incorporate on Co c a t a l y s t s , but not on Fe. Lee and Anderson (16) used equation 4 to c a l c u l a t e the moles o f propylene that must be incorporated to produce the branched molecules per mole o f propylene produced. Here equation 4 was used only to represent the d i s t r i b u t i o n data. The values o f propylene incorporated per propylene produced are large numbers approaching or exceeding 1.0; f o r example, for data in Table 2, 0.782, i f the summation i s to η = 14 and 0.914 i f the summation i s to i n f i n i t y . Thus, the i n c o r p o r a t i o n o f propylene cannot be the only source o f branched molecules f o r Co, and with Fe, t h i s mechanism i s d e f i n i t e l y not the s o u r c e o f m e t h y l branches.

392


Table 2.


Carbon number and structure 1

P r e d i c t i o n o f products from a c o b a l t c a t a l y s t a t 187°C, 2H + 1C0 feed, and 1 atm, (1). The parameters are a = 0.742 and f = 0.032.

Exptl. mole %

Calc.

18.74

44.9

2

5.41

3

8.22

13.90(6.95)

7.98 0.500

7.65 0.627

5N 2M

5.89 1.14

5.67 0.932

6N 2M 3M

4.37 0.921 .518

4.21 0.690 .345

7N 2M 3M

3.07 0.595 .691

3.12 0.513 .513

8N 2M 3M 4M

2.35 0.325 .506 .246

2.32 0.380 .380 . 190

a

9N 2+4M 3M

3

a

b

1.72 0.428 .275

E x p t l . , Calc mole %

10N 2M 3M 4+5M

1.31 0.238 .226 .269

1.28 0.210 .210 .315

11N 2M 3M 4M 5M

1.01 0.159 .157 .146 .133

0.948 .155 .155 .155 .155

12N 2M 3M 4M 5+6M

0.788 .110 .117 .108 .149

0.703 .115 .115 .115 .173

13N 2M 3M 4M 5+6M

0.609 .080 .087 .089 .145

0.522 .086 .086 .086 .171

14N 2M 3M 4M 5M 6+7M

0.484 .062 .070 .061 .066 .073

0.387 .063 .063 .063 .063 .095

10.31

4N 2M

κ

Carbon number and s t r u c t u r e

u 1.72 .564 .282 b

9N denotes n-nonane; 2+4M, a composite fraction 1-methyloctane and 4 methyloctane ; and 3M, 3 methyloctane. These values set equal i n the c a l c u l a t i o n

of

28.


ANDERSON

393

In 1948, Jack (17) i^eportée^ the following reaction sequences on iron powders at 300 - 400 C as shown in Table 3. Table 3

Reaction Sequences for Nitrides, Carbonitrides and Carbides of Iron.


Nitrides

Carbonitrides

Carbides

CO or H + CO

> NH

0

The dashed arrows correspond to these reactions that do not occur readily. These sequences were subsequently shown to proceed more rapidly or at lower temperatures on porous iron catalysts of moderate surface area (18) than on the iron powders. The iron carbonitrides seemed to be the best candidates for the initial tests in the Fischer-Tropsch synthesis (FTS); however, apparatus for preparing them was not available and the nitrides were used in the first tests. The e-Fe^N was rapidly hydrogenated in pure H as shown in Figure 1, but this reaction as well as a variety or hydrogenolysis reactions, are inhibited by CO in the FTS. The nitrogen was removed only slowly during the FTS and the nitrogen lost was largely replaced by carbon to form an ε-iron carbonitride as shown in Figure 2_ (19 to 22). Iron catalysts are oxidized during the FTS, presumably by water produced in the reaction. Figure 3, shows that the nitrided catalysts also oxidized more slowly than reduced catalysts (22). In the experiments described in the present paper two fused magnetite catalysts were used: D3001 containing K 0 and MgO and D3008 containing K 0 and AIJD Both catalysts were nearly completely reduced in H at 450 c and some reduced samples were nitrided in NH at 350°C7 2

2

3

In synthesis tests with D3001 and D3008 with 1H + 1C0 gas at 7.8 atm and an hourly space velocity of 100, the activity of the reduced catalysts decreased rapidly (23), for D3001 a 2-fold decrease in 8 weeks and for D3008 a 3-fold decrease in 4 weeks. Initially the nitrides were about twice as active as the reduced 2


394


F i g u r e 1. Rate of removal of n i t r o g e n with pure H from a sample of D3O01 converted to ε-Fe N. (Reproduced from Ref. 18. Copyright 1953, American Chemical Society.)

28.

ANDERSON



Component phases, used catalyst

Ο

10 DAYS

20 OF

30

40

SYNTHESIS

F i g u r e 2. Composition changes of n i t r i d e d c a t a l y s t D3001 during synthesis with + ICO gas at 21.4 atm. Phases found i n the used c a t a l y s t by x-ray d i f f r a c t i o n are given at the top where ε denotes ε-nitride or c a r b o n i t r i d e , M magnetit and S FeCO or MgCO (22).


396

HETEROGENEOUS

Ο

ΙΟ

20

30

40

CATALYSIS

50

DAYS OF SYNTHESIS

Figure 3. Oxygen content of reduced and reduced-and-nitrided c a t a l y s t D3001 during synthesis with 1H + ICO gas at 7.9 and 21.4 atm (22). 1

28.

ANDERSON


397

catalysts. The a c t i v i t y of D3001 was constant for 10 weeks, and D3008 l o s t about h a l f i t s a c t i v i t y i n 7 weeks. At 21.4 atm the i n i t i a l a c t i v i t y of the n i t r i d e s was more than twice those a t 7.8 atm. After an i n i t i a l decrease the a c t i v i t y o f both n i t r i d e d c a t a l y s t s at 21.4 atm remained constant u n t i l the t e s t s were terminated v o l u n t a r i l y a f t e r 7 or more weeks (23). S e l e c t i v i t i e s are shown i n Figure 4, where the y i e l d s are given in weight % on a (XL- and H 0-free b a s i s . The blocks i n the histogram s t a r t i n g from the top denote gaseous hydrocarbons and on the bottom, f r a c t i o n s from a 1-plate d i s t i l l a t i o n representing g a s o l i n e (464°C). In the gaseous hydrocarbon blocks = denotes the % o l e f i n s (23). The gasoline and d i e s e l o i l f r a c t i o n s were examined by i n f r a r e d , and estimates were made o f o l e f i n content as bromine number, Br, weight % hydroxyl group by OH and weight % carbonyl by CO. For these two c a t a l y s t s n i t r i d i n g sharply decreased the y i e l d of wax without i n c r e a s i n g the gaseous hydrocarbons. The

Heterogeneous Catalysis - American Chemical Society

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