8
Energy & Fuels 1989, 3, 8-15
Mechanism of Chain Growth and Product Formation for the Fischer-Tropsch Reaction over Iron Catalysts D. S. Santilli* and D. G. Castner* Chevron Research Company, 576 Standard Avenue, Richmond, California 94802-0627 Received April 25, 1988. Revised Manuscript Received August 25, 1988
Schulz-Flory-Anderson (SFA) product distributions with high percentages of alkenes are often observed for the Fischer-Tropsch (FT)reaction over promoted iron catalysts. This type of product distribution is inconsistent with carbide mechanisms involving alkyl groups as the growing chains. In order to explain these product distributions, the chain growth and product formation steps must be kinetically separated. We propose a straightforward carbide mechanism consistent with the experimentally observed product distributions. In this mechanism, the growing hydrocarbon chains are multiply bonded to the metal surface, the termination step is hydrogenation of the growing chain to a metal-bound alkyl species, and product formation occurs from this alkyl species. This mechanism predicts that alkyl groups are not involved in chain growth, excess methyl groups (compared to a SFA distribution) are present on the catalyst surface, minor amounts of various branched-chain products can be formed, and products with quaternary carbon atoms are not formed.
Introduction The Fischer-Tropsch (FT)reaction is a process in which carbon monoxide (CO) and hydrogen (H,) are catalytically converted into hydrocarbons. It would be useful to know how the reaction proceeds since this knowledge would help one optimize a FT catalyst. For example, iron-potassium catalysts are used to convert a mixture of CO and H2 to methane and alkenes. If the goal is to synthesize only low-molecular-weightalkenes, methane production is undesirable, so knowledge of the mechanism would help narrow the search for an improved catalyst. Although many FT mechanisms have been no published theory involving C,H, intermediate species can accommodate both the yield of alkane and alkene products versus carbon number and the isomer distributions within each carbon number in a straightforward manner. For the typical carbide mechanism discussed in ref 1-8 the alkenes form by @-eliminationof a metal-alkyl species, while methane and higher alkanes form by reductive elimination of the alkyl group. These reactions are in competition with chain growth, which involves the insertion of a methylene group into the growing alkyl chain. Therefore, if this alkyl mechanism is correct and if the reductive elimination step can be suppressed, alkenes can be obtained without making methane. However, for a number of experiments described in the literature (e.g., ref 1,9, and 11-15 and references cited therein), product (1) Anderson, R. B. In Catalysis; Emmett, P. H., Ed.; Reinhold New York, 1956; Vol. 4, Chapters 1-3. (2) Rofer-De Poorter, C. K. Chem. Reu. 1981,81,447-474. (3) Ponec, V. In Catalysis-A Specialist Periodical Report; Bond, G. C., Webb, G., Eds.; The Royal Society of Chemistry: London, 1982, Vol. 5, Chapter 2. (4) Muetterties, E. L.; Stein, J. Chem. Rev. 1979, 79, 479-490. (5) Schulz, H.; Zein el Deen, A. Fuel Process. Technol. 1977,1,45-46. (6) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic: Orlando, FL, 1984. (7) Biloen, P.; Sachtler, W. M. H. In Aduances in Catalysis; Academic: New York, 1981; Vol. 30, pp 165-216. (8) Bell, A. T. Catal. Reo.-Sci. Eng. 1981,23, 203-232. (9) Lee, G. V. D.; Ponec, V. Catal. Reu. 1987,29, 183-218. (10) (a) Ekerdt, J. G.; Bell, A. T. J. Catal. 1979, 58, 170-187. (b) Ekerdt, J. G.; Bell, A. T. J. Catal. 1980, 62, 19-25. (11) Dictor, R. A.; Bell, A. T. Appl. Catal. 1986,20, 145-162. (12) Dictor, R. A.; Bell, A. T. J. Catal. 1986,97, 121-136.
0887-0624/89/2503-0008$01.50/0
distributions containing methane and alkenes closely obey Schulz-Flory-Anderson (SFA) polymerization kinetics. In this paper we describe how this observation provides strong evidence against mechanisms involving a growing alkyl chain and propose a mechanism that is consistent with the observed product distributions. Only the chain growth and product formation steps of the FT reaction will be discussed here. The conversion of CO to the hydrocarbon species utilized in these reaction steps has been discussed in ref 1-8. Figure 1shows typical SFA plots of the log of the rate of product formation versus product carbon number for promoted iron catalysts." In Figure l a the individual alkane and alkene fractions are plotted (both of which are suggested to be primary products of the FT reaction)," while in Figure l b the sum of the alkane plus alkene fractions is plotted. From inspection of Figure 1it is clear that the C1 yield (CH,) falls near the line for the sum of the alkane and alkene produds and significantlyabove the line for the alkane products themselves. There have been many other reports indicating that for the F T reaction over Fe catalysts methane falls closely in line with the C2+ products on a SFA type plot even when the products contain a significant amount of a l k e n e ~ . ~ ~ ~ J ~ - ~ ~ Deviations from SFA product distributions have been commonly observed. However, for reactions at steady state, most substantial deviations appear to be due to experimental artifacts because of difficulties in analyzing heavy hydrocarbons and/or the presence of secondary reaction^.^ (Some deviations may be due to multiple reaction sites with different chain growth parameters.) High methane yields and low yields of some lower olefins (ethene and propene) do occur, but these observations may be due to secondary or parallel reactions such as hydrogenolysis, methanation, and olefin incorporation that have been observed.l0 Therefore, we feel that a mechanism which easily predicts a SFA product distribution is necessary to accommodate the oft observed SFA (and most likely (13) Satterfield, C. N.; Huff, G. A., Jr. J. Catal. 1982, 73, 187-197. (14) Dwyer, D. J.; Hardenbergh, J. H. J. Catal. 1984, 87, 66-76. (15) Krebs, H. J.; Bonzel, H.P.; Schwarting, W.; Gafner, G. J. Catal. 1981, 72, 199-209.
0 1989 American Chemical Society
Mechanism of Chain Growth
Energy & Fuels, Vol. 3, No. 1, 1989 9
0 1-oletina 0 n-paraffins
lo-"
2
4
6
8
IO
12
14
16
lo-ll
18
2
4
6
S
IO
12
14
IS
16
Figure 1. Schulz-Flory-Andersonplots for the (a) alkane and alkene and (b) alkane plus alkene hydrocarbon products formed during CO hydrogenation over a promoted iron catalyst. Reprinted with permission from ref 11. Copyright 1986 Elsevier. Scheme A P1
-o A
k0
k*i Ai
p2
Pn
k21 k1
[MI
m A2
kl
[MI
primary) product distributions over iron catalysts. Finally, the general mechanism we will outline in this paper can be consistent with mechanisms involving oxygenated surface species and even mixtures of triple- and double-bonded hydrocarbons as intermediates. However, oxygenate mechanisms will not be discussed here for the following reasons. First, oxygenated products are generally a minor fraction of the total hydrocarbon product slate for the FT reaction over Fe catalysts. These oxygenated products could be produced at different sites by different mechanisms than the nonoxygenated products or by an occasional CO insertion into a growing nonoxygenated intermediate. Second, much evidence has been presented for CO dissociation and a FT carbide mechanism over Fe Third, SFA product distributions with high alkene contents have been obtained with oxygen-free reactants (diazomethane and Hz; chloromethanes and H2) over Fe ~atalysts.'~J' (Because this paper is directed toward understanding the broader aspects of the primary product distributions observed for FT catalysts producing mixtures of alkenes and alkanes, secondary reactions such as product alkene incorporation, hydrogenation of alkenes to alkanes, and hydrogenolysis of the products are not discussed.)
Discussion In the following sections we will present a review of SFA polymerization kinetics, describe the FT mechanism with alkyl intermediates as the chain growth species, explain how such a mechanism fails to account for the observed product distributions, and propose a mechanism consistent with the observed results. SFA Polymerization. Scheme A represents a reaction in which a series of products (P1-P,+, where the subscript (16)Brady, R. C., 111; Pettit, R. J . Am. Chem. SOC. 1980, 102, 6181-6182. (17)Van Barneveld, W.A. A,; Ponec, V. J. Catal. 1984,88,382-387.
+
k21 An
Pnti
kl
kl
[MI
An+i
I
,
(
[MI
Scheme B CHa-CHz-CH3 CH3-CH3
1
kdH1
indicates the carbon number of the product) is formed by the addition of monomer M to an adsorbed species A,. A, can either grow to A,+, by adding another M or eliminate to form product P,. By application of steady-state (SS) approximations to Scheme A, the relative abundance of products P1to P,+, and absorbed species A, to A,,, can be determined if two assumptions are made: the rate of product formation (A, eliminating to P,) and the rate of chain growth (M adding to A, to produce A,,,) are both independent of the chain The SFA chain growth probability parameter a and product distribution are then given by eq A-1-A-4 for n 1 1.
(A-2)
[A,] = [A1]a"-l (A-3) [Pn+11 --a (A-4) [Pnl Alkyl Mechanism. The FT alkyl mechanism is shown in Scheme B. If Scheme A is compared to Scheme B, M is the adsorbed methylene group, A,, is an adsorbed alkyl chain, and P, is a product alkane containing n carbon
Santilli and Castner
10 Energy & Fuels, Vol. 3, No. 1, 1989
Scheme C CH3-CH2-CH3
atoms. Chain growth occurs by insertion of the methylene group into an alkyl chain. A,, and ko from Scheme A do not enter directly into the description of the FT product distribution, but represent the conversion of some C1 species into the adsorbed species, Al in Scheme A and CH, in Scheme B. If all of the FT products are alkanes formed by reductive elimination with rate constant k2[H], then eq B-1-B-6 can be derived. By the use of a steady-state approximation (B-1) [C2H51k2[HI + ~l[CH21[CZH5I = [CH,lkl[CH21
[CzHsl -[CHd Therefore -[CzHd [CHd Similarly
-
kl[CH21 = a k2[H1 + k1[CH21
[czHslkz[Hl =--[CzHsl [CH3IkdH] [CHd
- CY
CH2=CH-CH3
Cn+lto Cnproducts are expressed in eq C-14-6. Note also the slightly different expression for the chain growth probability parameter a' compared to a (eq C-2 versus eq B-2). By applying steady-state approximations to Scheme C, we can derive the following expressions:
(B-2)
(B-3)
[ C ~ + I H ~ ( ~ + I ) ++ ~ Ik1[CH21 ~ ~ [ H I[Cn+lH~(n+l)+ll = a ' ( k2y; k 3 ) (C-5) [CnH~n+1lkl[CH~l (B-4) [Cn+lH2(n+l)+llkl[CH21 = LY (B-5) C [Cn+, products] [CnH~n+ll b [ H l + kl[CH21 C [C, products] Therefore k2[HI [Cn+1H2(n+l)+ll+ ~~[Cn+lH2(n+l)+ll = CY' (C-6) [Cn+1H~(n+1)+21 [ C ~ + I H Z ( ~ + I ) + ~ I ~ Z [ H I k2[HI [CnH~n+ll+ k3[CnH2n+ll = (Y (B-6) [CnH~n+21 [Cn&n+ 11 [HI Since the Cl+ products of a FT reaction producing SOfor all products with n L 1, [ C ? + , H ~ ( ~ + ~ ) + Z I / [ C ~ Hmethane ~ ~ + ~ I and alkenes obey SFA polymerization kinetics = a. Therefore, a SFA product distribution results that (Figure 1 and ref 1, 9, and 11-15), [Pn+l]/[Pn] = a'. In generates a straight line with a slope of log a on a plot of order for the C1and C2products to follow this requirement, log (mol % of C,) versus n . k2[H] + k3 must equal k2[H] in eq C-5. Therefore, k2[H] However, when a significant number of alkenes are >> k,. Furthermore, when olefins dominate the C2+ FT produced by @-eliminationin the FT reaction, the elimiproducts, k3 >> k2[H]. How can this bepossible? This can nation steps of CH3 and C2+ alkyl chains differ as shown only happen if we differentiate between k2 for C2+ alkyl in Scheme C. groups and kz for the methyl group, but this leads to The C2+ alkyl chains have an additional route to further complications. products by @-elimination ( k 3 ) , which yields olefins. First, when many olefins are produced (k3 > k2[H]) in Therefore, the ratio of C2to C1 products and the ratio of order for Cl+ products to fall in line on a SFA plot, the
Mechanism of Chain Growth
-
rate for @-eliminationof C2+ chains must be similar to the rate for reductive elimination of the C1species (k3 k2[H] for Cl). Second, therefore, the rate for reductive elimination of the methyl group is much faster than reductive elimination of the C2+ alkyl chains. These would be two fortuitous circumstances. Therefore, provided that the FT products are formed directly from the alkyl groups, the above requirements argue strongly against Scheme C as the mechanism for FT reactions producing a SF product distribution containing mostly methane and alkenes. If a basic tenet of the alkyl mechanism is true, that is, if the methyl group eliminates by reductive elimination to form methane at the same rate as the C2+ alkyl species eliminate by reductive elimination to form alkanes, methane would fall on the line for the C2+ alkanes in Figure la. And for alkane-producing FT catalysts, this is indeed observed. However, the methane yield is 4-5 times higher than the amount expected from the C2+ alkane distribution and is close to the amount expected from the sum of the C2+ alkane and C2+ alkene distribution. Proposed Mechanism. In order to explain how Cl+ products obey SFA polymerization kinetics when alkenes are formed, the chain growth step should be separated from the product formation step. A general mechanism that includes this separation of steps is presented in Scheme D. The general definition and relationship of A,,, A,,, M, ko, kl, k2, k3, and n are the same as described for Schemes A and C. However, we now introduce two new rate constants (k4 and k4) and a new species as the growing chain (B,). Also, to be general, we use a superscript with k2 and k3, (e.g., ik2)to denote rates for the reactions of the individual Ai's to form products. An's are intermediates for product formation but not for chain growth. Chain growth occurs only by addition of M to the B, species. Chain termination occurs when B,, is converted to A,,. The rate constant of this reaction equals Bnk4[H]. The alkane products (P,,-) and alkene products (P,,=)are formed from A,, with rate constants k2[H] and k3 as before. The total hydrocarbon product at any carbon number n is P,, which is the sum of P,' and P,,-. Although we will use Scheme D to describe alkene and alkane production with Ai = alkyl groups, Bi = alkylidene groups, and M = methylene, it is straightforward to extend it to include other products and intermediates such as oxygenates and alkylidyne intermediates. Using steady-state approximations, we can derive equations for [Pn+ll/[P,,lwhere n 1 2 and for [P21/[P11. Also the relative amounts of various A, species and various Bi species can be described. These are gven in eq D-1-D-7.
k4+ %,[HI
+ ,,k3 k-4 + n+lkZIH] + ,+'k3 (D-7) Now we make one assumption. We assume that k4 0; that is, the back reaction of the alkyl group (A,) to form the alkylidene group (B,) is slow compared to the other reactions. Applying this assumption, eq D-5 and D-7 become eq D-8 and D-9, respectively. N
n+1k2[H]+ n+lk "k,[H]
+ "k3 (D-10)
Bn+11
(D-11)
(D-12)
(D-13) Therefore
(D-14) So, regardless of the values for 'k2and ik3,all products, beginning at P1, will fall on a line on a SFA type plot. (Note how the expression for a" in eq D-12 is similar to that of a in eq B-2-only k4 replaces k2.) All we have done, kinetically, is remove the immediate precursor of the products from the chain growth step. The Bi species (alkylidene groups) all obey SFA kinetics, but the Ai species (alkyl groups) do not, as shown in eq D-15 which is derived from eq D-4 by assuming k4 0. N
)(
[A21 [All
- = a"( 2k2:T[il
(D-5)
)
'k,[Hl 2k3+ 2k2[H]
(D-15)
i!n-t"2(n-l)+1
CnHZn+l
I
7%
CH2
&,3
7mZ
-
Cn-l+h(n-l)+l
-"kg
/"\"
/"\"'
kl[CH21
-I
"/
I
/"\"
ki[CH~l
-.
ki[CH21
"'
Cnh+l
I
?$ '/// 4 -"'
-2
Physically, this occurs just because A2 has two ways to make products (@-eliminationand reductive elimination) and Al has only one mode (reductive elimination). If the 'kis are all equal, the [A2]/[Al]ratio can be predicted from the initial [olefin]/[paraffii] ratio of P2([ethene]/[ethane]) and the ratio of C2 to C1 products. So, if ethene/ethane = k3/k2[H]
In Scheme E, we present an example of the chain growth and product formation steps of an FT-carbide mechanism consistent with the general mechanism given in Scheme D. The alkylidene chain 2 (B, in Scheme D) grows by addition of the methylene monomer 1 (M and B1 in Scheme D) and terminates by addition of H to form an alkyl group 3 (A, in Scheme D). The alkene and alkane products are then formed by @-eliminationand reductive elimination of the alkyl group. As long as k1 and k4 are both independent of chain length and k4 0, the alkylidene mechanism of Scheme E predicts that a SFA product distribution will be observed for FT reactions in which any mixture of alkenes and alkanes is produced. No similar steps are required to proceed at different rates and no different steps are required to proceed at similar rates in order to explain the product distributions. To our knowledge, the first explicit description of an alkylidene group as the growing chain was published by Ekerdt and Bell.lo However, in their description there was no intermediate step to products such as the formation of alkyl groups (3 in Scheme E and A, in Scheme D) that do not grow. If we replace the alkyl groups of Scheme C with alkylidene groups, the same critique we presented for the alkyl mechanism applies, and it is difficult to reconcile SFA product distributions for olefin-producing FT catalysts. For alkene-producing FT reactions in which the Cl+ product distribution obeys SFA polymerization kinetics, the back-reaction of species Ai to species Bi in Scheme D must be slow. (In Scheme E this is the dehydrogenation of the alkyl species 3 to the alkylidene species 2 with rate constant k+) However, if a catalyst system exists in which k4is fast relative to 'k2[H], the methane yield will fall below the value expected by extrapolating the line for the C2+products in a SFA plot. This would be true regardless of the relationship between k-4 and k3 as shown in eq D-17-D-20.
-
We will look at two cases:
k4>> 2k3, eq D-5 becomes
-(
[Pll = [Bll [P2I
k-4
[BzI k-,
)(
+ 2k2[H1
+ 'Iz,[H]
>> 2k3and k4
