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Energy & Fuels 2007, 21, 640-645
Addition of Acetylene to the Fischer-Tropsch Reaction Yulong Zhang, Li Hou, John W. Tierney, and Irving Wender* Department of Chemical and Petroleum Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261 ReceiVed October 5, 2006. ReVised Manuscript ReceiVed January 30, 2007
Acetylene has been added to the Fischer-Tropsch (F-T) synthesis on cobalt and iron catalysts. The added acetylene initiates the F-T chain growth and is incorporated into the product only once. Acetylene incorporation is three times greater than that of 1-hexyne. With the addition of acetylene, the F-T product distribution follows Anderson-Schulz-Flory (ASF) distribution while the chain growth factor (alpha value) slightly decreases. Only C3 oxygenates are formed via hydroformylation on adding acetylene to cobalt. With iron, C3+ oxygenate production is enhanced and follows the ASF distribution. It is likely that both acetylene and ethylene addition to the synthesis involve the same intermediates but acetylene is more strongly adsorbed on the F-T catalyst, making it a more effective chain initiator at low temperatures.
1. Introduction The Fischer-Tropsch synthesis (F-T) can be briefly defined as the catalytic conversion of carbon monoxide and hydrogen to hydrocarbon products; significant amounts of oxygenates are also produced. A treatise on F-T technology covering commercial applications and reaction pathways was published in 2004.1 Previous work on the incorporation of substituted alkynes such as 1- and 2-hexyne and phenylacetylene to the F-T synthesis has shown that addition of these molecules to cobaltand iron-catalyzed F-T reactions allows the synthesis to proceed at lower-than-normal F-T temperatures.2 Alkynes serve only as chain initiators; they are not incorporated further in the F-T chain. The alkynes are adsorbed on the surface of these F-T catalysts more strongly than carbon monoxide,3 seemingly a reason for their high degree of incorporation. There are numerous studies of the cofeeding of ethylene and higher olefins to the F-T reaction.4-20 This paper investigates * Corresponding author. E-mail:
[email protected]. (1) Studies in Surface Science and Technology, Fischer-Tropsch Technology; Steynberg, A. P., Dry, M. E., Eds.; Springer-Verlag: New York, 2004; Vol. 152. (2) Zhang, Y.; Hou, L.; Tierney, J. W.; Wender, I. Top. Catal. 2005, 32, 123-133. (3) Jackson, S. D.; Hussain, N.; Munro, S. J. Chem. Soc., Faraday Trans. 1998, 94, 955-961. (4) Smith, D. F.; Hawk, C. O.; Golden, P. L. J. Am. Chem. Soc. 1930, 52, pp 3221-3232. (5) Craxford, X. R. Trans. Faraday Soc. 1939, 35, 946-958. (6) Eidus, Y. T.; Zelinskii, N. D.; Ershov, N. I. Dok. Akad. Nauk SSSR 1948, 60, 599-601. (7) Kummer, J. T.; Emmett, P. H. J. Am. Chem. Soc. 1951, 73, 564569 (8) Iglesia, E.; Reyes, S. C.; Madon, R. J.; Soled, S. L. AdV. Catal. 1993, 39, 221-302. (9) Schulz, H.; Claeys, M. Appl. Catal., A 1991, 186, 71-90 (10) Madon, R. J.;Reyes, S. C.; Iglesia, E. J. Phys. Chem. 1991, 95, 7795-7804. (11) Hall, W. K.; Kokes, R. J.; Emmett, P. H. J. Am. Chem. Soc. 1960, 82, 1027-1037. (12) Dwyer, D. H.; Somorjai, G. A. J. Catal. 1979, 56, 249-275. (13) Kibby, C.; Pannell, R.; Kobylinski, T. Prepr. Am. Chem. Soc. DiV. Pet. Chem. 1984, 29, 1113-1119. (14) Adesina, A. A.; Hudgings, R. R.; Silveston, P. L. Appl. Catal. 1990, 62, 295-308.
Figure 1. Hydrocarbon product distribution of incorporation of 1% acetylene on a cobalt catalyst: 10Co/90Al2O3, 300 psi, 180 °C, H2/ CO ) 1.
the addition of acetylene, the first member of the alkyne series, to the F-T reaction, and is to the best of our knowledge the first report of work on the addition of acetylene to cobalt-based F-T synthesis. Acetylene differs from monosubstituted alkynes in that it has a hydrogen bonded to each carbon of the triple bond, giving it a center of symmetry and hence a zero dipole moment. It is a linear molecule in which two of the atomic orbitals on carbon are sp-hybridized and two form π bonds. Because of the two filled π orbitals of acetylene, there is a greater concentration of electron density between the carbon atoms than is found in ethylene. Acetylene is a chemical readily available commercially so that it can be considered as an additive to the F-T reaction. Previous work with acetylene added to the F-T synthesis has been carried out by Russian scientists who introduced 1% acetylene labeled with 14C to an F-T reaction on a fused iron catalyst at 150-190 °C at a high pressure (1500-1300 psi) in an effort to obtain higher alcohols; they succeeded in obtaining (15) Jordan, D. S.; Bell, A. T. J. Phys. Chem. 1986, 90, 4797-4805. (16) Baker, J. A.; Bell, A. T. J. Catal. 1982, 78, 165-181. (17) Davis, B. H.; Xu, L.; Bao, S. In Natural Gas ConVersion IV; Pontes, M., Espinoza, R. L., Nicolaides, C. P., Scholz, J. H., Scurell, N. W., Eds.; Elsevier: New York, 1997; Vol. 107, pp 175-180. (18) Boelee, J. H.; Custers, J. M. G.; van der Wiele, K. Appl. Catal. 1999, 53, 1-13. (19) Fujimoto, K.; Fan, L.; Yoshii, K. Top. Catal. 1995, 2, 259-266. (20) Hanlon, R. T.; Satterfield, C. N. Energy Fuels 1988, 2, 196-204.
10.1021/ef060497j CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007
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Figure 2. GC analysis of F-T products with incorporation of acetylene on a cobalt catalyst: 10Co/90Al2O3, 300 psi, 180 °C, H2/CO ) 1, 1% acetylene.
a series of alcohols from C1 to C2021-23 They found that acetylene initiated the synthesis but did not appear again in the product. Carter24 passed a mixture of 85% synthesis gas and 15% acetylene over Co(HCN)2-Fe(HCN)2-Co(HCN)2, termed a “string catalyst,” supported on silica gel at 18-20, 100, and 200 °C at 300 psi. Linear hydrocarbons were produced from synthesis gas, whereas branched hydrocarbons were formed when acetylene was added. Although the catalyst was prepared from a mixture of K4(CN)6 and CoCl2, no conversions were found when either compound was used separately. It was concluded that one carbon of acetylene acted as both tether and terminus in chain growth. 2. Experimental Section Supported cobalt catalysts were prepared by incipient wetness impregnation of cobalt nitrate on a spray-dried alumina (70140 mesh) as described elsewhere.2 The catalyst composition is 10 wt % Co on an alumina support (10Co/90Al2O3). XRD shows that the calcined cobalt species appear as Co3O4. After reduction at 350 °C for 10 h in H2, cobalt is reduced to cobalt metal.25 A precipitated iron catalyst with a composition of 100Fe/4.4Si/1.25K was obtained from Dr. B.H. Davis of the University of Kentucky.26,27 A rhodium catalyst was prepared (21) Loktev, S. M. J. Catal. 1982, 17, 255-230. (22) Kliger, G. A.; Glebov, L. S.; Popova, T. P.; Marchevskaya, E. V.; Beryezkin, V. G.; Loktev, S. M. J. Catal. 1988, 111, 418-20. (23) Slivinskii, E. V.; Rumyantsev, V. Y.; Voitsekhovskii, Y. P.; Zvezdkina, L. I.; Loktev, S. M. J. Catal. 1990, 123, 333-340. (24) Carter, M. R. J. Mol. Catal. 2001, 172, 193-206 (25) Zhang, Y. L.; Wei, D. G.; Hammache, S.; Goodwin, J. G. J. Catal. 1999, 188, 281-290. (26) Raje, A. P.; O’Brien, R. J.; Davis, B. H. J. Catal. 1998, 180, 3643.
by incipient wetness impregnation of rhodium(III) chloride hydrate on alumina. Reactions with and without acetylene were carried out in a computer-controlled fixed-bed reactor of stainless steel with i.d. 3/8 in. Copper, which can replace a hydrogen atom in acetylene to form explosive acetylides, was not present in the reaction system. The explosive limits for acetylene in air range from 2 to 82% by volume. Acetylene was introduced from a tank of premixed gas containing (mol): 1% acetylene, 10% Ar, 44% CO, and 45% H2 obtained from Praxair. Hydrogen was added to change the H2/CO ratio to 2. A low conversion was carried out so as to avoid accumulation of high-boiling-point products in transfer lines. Cobalt catalysts were activated by hydrogen at a rate of 50 mL/min, with a temperature program ramping from room temperature to 350 °C at 1 °C/min, holding at 350 °C for 10 h. Iron catalysts were activated similarly but kept at 350 °C for 2 h. The F-T reaction was started by gradually increasing the CO/H2 flow rate to avoid a temperature surge due to active sites in the fresh catalyst. A hot trap at 200 ( 5 °C was placed immediately after the outlet of the reactor. A stream of gases controlled by a needle valve kept at 230 ( 5 °C was split out after the hot trap and sent to GC for analysis. The remaining products were passed through a cold trap and then to a backpressure regulator. Transfer lines from the reactor to the GC for product analysis were maintained at 230 ( 5 °C. Products were analyzed every 3-6 h by online GCs controlled by HP Chemstation, equipped with three columns (a HP-5 capillary column with FID for whole hydrocarbon product analysis, a Porapak Q packed column with FID for C1-C5 hydrocarbon analysis, and a Carbonsphere packed column with TCD for permanent gas analysis). Reactions reached a quasi(27) OBrien, R. J.; Xu, L. G.; Spicer, R. L.; Davis, B. H. Energy Fuels 1996, 10, 921-926.
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Table 1. Reactions of Acetylene Compared with Those of 1-Hexyne on a Cobalt Catalyst at 220 °C and 100 psi
Table 2. Reactions of Acetylene without/with Acetylene on a Cobalt Catalyst at Different Temperaturesa
selectivity (%) hydrogenation dimerization hydroformylation incorporation
yield (mg/h)
acetylene
1-hexyne2
14 28 4 54
53 29 1 17
steady state in about 5 h, and results were reported as an average of three analyses. Errors between different analyses were usually less than 5% relatively. F-T reactions at various temperatures in the absence of acetylene were compared with reactions carried out in the presence of acetylene. 3. Results 3.1. Incorporation of Acetylene with Cobalt Catalysts. F-T reactions were carried out with a cobalt catalyst at 180 °C and 300 psi with a CO/H2 ratio of 1. The yields of hydrocarbon products from the reactions run in the absence of acetylene and from the F-T synthesis with added acetylene are shown in Figure 1. The addition of acetylene resulted in an increase in the yield of C2+ hydrocarbons up to C10 with a major peak at C4 that is due mainly to the dimerization of acetylene. The C5 peak includes contributions from the normal F-T reaction, acetylene-initiated chain growth, and chain growth from dimers derived from acetylene. 2-Butene derived from acetylene was observed in the GC-MS spectrum of the liquid products when acetylene was added; it was not found in the products from the reactions run in the absence of acetylene. In both cases, with and without the addition of acetylene, the products follow Anderson-Schulz-Flory (ASF) distribution, although the addition of acetylene causes a decrease in the alpha value from 0.74 to 0.65. Patzlaff et al.28 reported similar results upon adding olefins to the F-T. The difference in alpha values is consistent with acetylene acting only as a chain initiator, not as a chain propagator. It is important to note that the addition of acetylene resulted in the formation of branched hydrocarbons, mostly 2-methylbutane and 3-methylpentane, which were negligible when acetylene was absent (Figure 2). These branched products probably originated from four-carbon molecules, such as 2-butene, derived from acetylene; these molecules served as additional chain initiators to form branched products. Baker and Bell16 added cis-2-butene to syngas over a Ru/SiO2 catalyst and observed that about 4% of 1-butene and less than 0.1% of 2-butene were converted to C1-C3 and C5+ hydrocarbons. Carbon monoxide is adsorbed more strongly than olefins, leading to relatively poor incorporation of added olefins. In situ formed C4 hydrocarbons from acetylene dimerization are already adsorbed on the surface of F-T catalysts so that they are incorporated more easily into the growing chains. The addition of acetylene also results in a small increased production of internal olefins. The following reactions of cofed alkynes may occur under F-T conditions: hydrogenation, dimerization, and incorporation into F-T products. Schulz and Claeys29 found that hydrogenation was the fastest of all reactions of added olefins with cobalt catalysts.9 Under our reaction conditions, incorporation was the most important reaction with added acetylene; as much as 54% of added acetylene was incorporated into F-T products. (28) Patzlaff, J.; Liu, Y.; Graffman, C.; Gaube, J. Appl. Catal. 1999, 186, 109-119. (29) Schulz, H. Top. Catal. 2003, 26, 73-79.
120 °C C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C2-OH C3-OH C4-OH C5-OH C6-OH a
180 °C
220 °C
without
with
without
with
without
with
0.06 0.02 0.04 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.59 17.00 3.63 15.63 3.39 1.10 0.68 0.45 0.26 0.05 0.00 0.00 1.39 0.00 0.00 0.00
3.36 1.21 1.52 1.82 0.88 0.64 0.54 0.46 0.43 0.37 0.33 0.34 0.11 0.00 0.00 0.00
3.36 5.07 4.77 24.06 9.19 4.39 2.44 1.85 1.56 1.23 1.02 0.24 1.82 0.01 0.47 0.06
18.33 2.02 5.80 7.19 7.03 6.13 4.32 3.42 2.21 1.35 1.00 0.65 0.11 0.00 0.00 0.00
22.01 8.49 9.54 24.62 14.34 11.43 6.41 5.26 3.43 2.15 1.13 1.69 1.97 0.21 0.00 0.00
10Co/90Al2O3, P ) 300 psi, H2/CO ) 1, 1% acetylene
Table 3. Selectivity to Reactions of 1% Acetylene with CO/H2 at Three Temperaturesa
hydrogenation dimerization hydroformylation incorporation a
120 °C
180 °C
220 °C
40 28 3 29
8 36 4 52
14 28 4 54
10Co/90Al2O3, P ) 300 psi, H2/CO ) 1.
Selectivities to different reactions of acetylene and of 1-hexyne with a cobalt catalyst at 220 °C are listed in Table 1. Acetylene is more readily incorporated into F-T products than are longchain alkynes such as 1-hexyne. The addition of acetylene also greatly increased the yield of 1-propanol, very likely by hydroformylation of the olefin derived by hydrogenation of acetylene.9,29 C5 oxygenates are similarly formed by hydroformylation of acetylene dimers. With cobalt catalysts, the amount and fate of acetylene added to the F-T is greatly influenced by the temperature of the reaction. Reactions were carried out at three temperatures: 120, 180, and 220 °C. The F-T reaction, when carried out at 120 °C in the absence of acetylene, is essentially negligible; products are formed when the temperature is raised (Table 2). However, when acetylene is added to the F-T synthesis, chain growth occurs even at 120 °C. Incorporation of acetylene increased from 29% at 120 °C to 52% at 180 °C and 54% at 220 °C (Table 3). The amount of methane at 220 °C is consistent with cobaltcatalyzed F-T reactions at higher temperatures. The effect of pressure on the F-T reaction at 100, 300, and 700 psi, with and without the addition of 1% acetylene, was studied at 180 °C with H2/CO ) 1 (Table 4). In the absence of acetylene, the F-T reaction rate increased significantly with pressure because of an increase in the concentration of the reactant gas. Hydrocarbon products in the presence of added acetylene also increase with operating pressure, although the percentage increase is smaller. There was also a very small effect of pressure on selectivity to the different reactions when acetylene was added to the F-T; about 52% of the acetylene was incorporated, 4% hydroformylated to propanol, 36% dimerized, and 8% hydrogenated at all three pressures. 3.2. Incorporation of Acetylene with Iron Catalysts. Experiments were carried out with precipitated iron catalysts in F-T reactions at temperatures ranging from 120 to 260 °C;
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Energy & Fuels, Vol. 21, No. 2, 2007 643
Table 4. F-T Products without/with Acetylene on a Cobalt Catalyst at Different Pressuresa yield (mg/h) 100 psi C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C 2OH C 3OH C 4OH C 5OH a
300 psi
700 psi
without acetylene
with acetylene
without acetylene
with acetylene
without acetylene
with acetylene
1.69 0.52 0.63 0.61 0.43 0.36 0.33 0.33 0.31 0.29 0.20 0.13 0.21 0 0 0
2.84 3.44 2.76 20.27 8.51 4.09 2.28 1.74 1.24 1.16 0.94 0.57 0.10 1.80 0 0.42
3.36 1.21 1.52 1.82 0.88 0.64 0.54 0.46 0.43 0.37 0.33 0.20 0.34 0.11 0.00 0.00
3.36 5.07 4.77 24.06 9.19 4.39 2.44 1.85 1.56 1.23 1.02 0.78 0.24 1.82 0.01 0.47
3.31 0.76 3.46 3.51 2.98 2.77 2.31 2.00 1.71 1.36 1.14 0.80 0.29 0.06 0.08 0.08
4.06 4.83 8.04 24.74 10.53 6.84 3.34 2.81 2.47 1.53 1.17 1.04 0.17 1.91 0.09 0.87
10Co/90Al2O3, 180 °C, H2/CO ) 1, 1% acetylene.
Figure 3. Hydrocarbon distribution from incorporation of acetylene on an iron catalyst: 100Fe/4.4Si/1.25K, T ) 180 °C, P ) 300psi, H2/ CO ) 1, 1% acetylene.
Figure 4. Chain length distribution of oxygenated products without/ with 1% acetylene addition: 100Fe/4.4Si/1.25K, T ) 180 °C, P ) 300 psi, H2/CO ) 1.
pressures of 100, 300, and 700 psi; a H2/CO ratio of 1; and a flow rate of 4.2 L/h. Yields of hydrocarbon products obtained at 180 °C, with and without added acetylene, are shown in Figure 3. As found with cobalt catalysts, the addition of acetylene with iron catalysts increased the rate of formation of C2+ hydrocarbons. Unlike cobalt-catalyzed F-T reactions, which actively catalyze acetylene dimerization, hydrogenation to ethylene and ethane is the main reaction of acetylene with iron; about 30% of the added acetylene is converted to these two hydrocarbons. Because smaller amounts of C4 products such as 2-butene are formed with iron, fewer branched hydrocarbons are produced compared to those with cobalt catalysts.
There is a marked difference between iron and cobalt catalysts in the distribution of oxygenates in the F-T synthesis.30 With cobalt, the addition of substituted acetylenes such as 1-hexyne or phenylacetylene yields oxygenates formed by the addition of a single carbon; e.g., 1-hexyne yields only heptaldehyde or 1-heptanol, evidently by the hydroformylation of 1-hexene.2 With the addition of acetylene to a cobalt-catalyzed F-T reaction, only C3 and C5 oxygenates increase because of the hydroformylation of C2 and C4 unsaturated hydrocarbons on the catalyst. The formation of oxygenates with iron catalysts is not so limited; large amounts of C3+ oxygenates are formed. The distribution of oxygenated products with iron in the absence or presence of acetylene is plotted as log(Wn/n) against n, where Wn is the weight fraction and n is the carbon number (Figure 4). With iron catalysts, the production of oxygenates and of hydrocarbons each follows the ASF distribution, indicating that they undergo similar chain initiation and propagation steps. Essentially no reaction products were obtained at 120 °C with an iron catalyst in the absence of acetylene, but F-T products were produced at this temperature when acetylene was added to the feed. Incorporation of acetylene was 15% at 120 °C, about 80% of which was hydrogenated to ethylene. On adding acetylene, the yield of hydrocarbon and oxygenated products increased as the temperature was raised from 180 to 260 °C (Table 5). The oxygenated product distribution with added acetylene at 180, 220, and 260 °C is shown in Figure 5; there is a maximum yield at 220 °C. The fate of the added acetylene did not vary significantly with an increase in temperature. Acetylene incorporation was about 61%; 6.1% was dimerized and 32% hydrogenated. The ratio of oxygenated products to hydrocarbons was about 17% at both 180 and 220 °C, falling to 11% at 260 °C. The temperature dependence of the reaction rate on the iron catalyst was obtained from an Arrhenius plot of CO conversion at temperatures ranging from 120 to 260 °C, a H2/CO ratio of 1 and a flow rate of 4.2 L/h (Figure 6). The calculated apparent activation energy is 11 kcal/mol. Dry31 reported values of 13.1 to 14.9 kcal/mol for precipitated iron catalysts depending on whether or not wax was produced. To obtain information on the effect of total pressure on acetylene addition to an iron catalyst, we added it at 100, 300, (30) Shi, B.; Davis, B. H. Top. Catal. 2000, 26, 131-161. (31) Dry, M. E. In Catalysis, Science and Technology; Anderson J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1981; pp 159-255.
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Table 5. F-T Products without/with Acetylene Using an Iron Catalyst at Four Temperaturesa yield (mg/h) 120 °C C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C2-OH C3-OH C4-OH C5-OH C6-OH C7-OH C8-OH C9-OH a
180 °C
220 °C
260 °C
without
with
without
with
without
with
without
with
0.03 0.04 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.32 32.83 1.84 4.20 1.60 0.56 0.17 0.00 0.00 0.00 0.00 0.00 0.00 0.32 0.19 0.00 0.00 0.00 0.00 0.00
1.40 0.93 0.94 0.89 0.56 0.33 0.25 0.14 0.09 0.05 0.03 0.02 0.46 0.25 0.22 0.16 0.14 0.11 0.09 0.05
1.93 16.68 6.92 8.13 5.61 2.74 2.3 1.5 1.0 0.8 0.5 0.3 0.48 2.48 1.50 1.20 0.85 0.65 0.54 0.34
4.79 4.29 5.63 6.16 4.11 3.13 2.88 2.40 2.03 1.84 1.80 1.76 1.64 0.89 0.85 0.80 0.77 0.74 0.70 0.50
5.81 23.28 10.94 12.97 9.11 5.52 4.00 3.33 2.80 2.33 2.00 1.33 2.11 3.58 2.27 2.35 1.47 1.18 0.88 0.59
3.99 5.49 12.58 10.00 8.07 6.15 5.11 3.88 3.15 2.91 2.89 2.84 1.70 1.48 1.04 1.03 0.70 0.59 0.53 0.47
7.41 25.21 17.20 15.53 10.71 7.65 6.36 4.40 3.33 2.70 2.20 1.98 2.35 3.49 1.76 1.71 0.95 0.67 0.40 0.27
100Fe/4.4Si/1.25K, P ) 300 psi, H2/CO ) 1, 1% acetylene.
Figure 5. Oxygenated product distributions at three temperatures with addition of 1% acetylene: 100Fe/4.4Si/1.25K, P ) 300 psi, H2/CO ) 1, 1% acetylene.
Figure 6. Arrhenius plot of CO conversion over an iron catalyst: 100Fe/4.4Si/1.25K, P ) 300 psi, H2/CO ) 1.
and 700 psi at 180 °C. Pressure had little effect on hydrocarbon production. Although total C2 hydrocarbons were not affected, the ratio of ethylene to ethane was reduced from 5.3 to 2.6 when the pressure was increased from 100 to 300 psi. Acetylene incorporation was about 62%, hydrogenation about 33%, and dimerization about 5% at all three pressures. With cobalt at 300 psi and 220 °C, 54% of acetylene was incorporated into hydrocarbons, 14% hydrogenated, 28% dimerized, and 4% hydroformylated. There was, however, a significant effect of pressure on the formation of oxygenated products with iron. About 11% of oxygenated products were produced on iron
Figure 7. Structure of ethylidyne tricobaltnonacarbonyl. A similar structure is postulated for attachment of ethylidyne to a solid catalyst.41-42
catalysts at 100 psi and 180 °C, 17% were formed at 300 psi, and 23% were formed at 700 psi. The increase in oxygenate production with pressure is consistent with results obtained by Russian scientists. They carried out F-T synthesis with 1% acetylene over a fused iron catalyst at high pressures (1500-3000 psi), obtaining more than 80% of their products as alcohols. The major components of our oxygenated products with iron catalysts were alcohols and aldehydes; a small amount of ketones was produced as well. Variation in space velocity from 1050 to 2100 to 4200 h-1 did not significantly affect the incorporation of acetylene. It is well-known32 that CO can be adsorbed dissociatively and non-dissociatively at temperatures used in the F-T reaction. It was therefore of interest to carry out an experiment by adding acetylene to a rhodium catalyst at F-T conditions resembling those used for cobalt, namely 220 °C and 100 psi. The extent of incorporation of acetylene was low; most of the acetylene dimerized. Dimerization of acetylene to C4 hydrocarbons is lowest with iron catalysts, on which CO dissociates most easily, is somewhat greater with cobalt catalysts, and is the main reaction of acetylene with rhodium. Adsorption of CO is weaker on rhodium so that active sites are occupied by acetylene, which in the absence of strong CO dissociation has fewer C1 monomers with which to form longer chains. 4. Discussion The mode of initiation of the F-T is unclear although the postulate is made later in the paper that two-carbon entities may be involved. It is not possible to directly ascertain that added acetylene initiates F-T reactions, because the two carbon atoms in acetylene are not distinguishable from other carbon atoms in the chain. Work with 14C labeled acetylene showed that acetylene is incorporated only once in the growing chain.21 Our previous work with phenyl-substituted acetylenes showed that alkynes initiate chain growth, forming the first two carbons of the growing chain.2 When acetylene is added to the F-T reaction at 220 °C, two products are formed: about half of the chains are initiated by acetylene; the other half do not contain acetylene (the normal F-T reaction takes place). As the temperature is lowered, initiation by acetylene becomes the main reaction; at 120 °C, essentially all chains are initiated by acetylene. It is known that cofed olefins can initiate the F-T synthesis under normal (commercial) conditions.9 The reactivity of ethylene is 10-40 times higher than that of other olefins. At low temperatures (120 °C) used in the present work, ethylene does not initiate the synthesis because the surface is covered by strongly adsorbed CO. When a mixture of acetylene and ethylene is hydrogenated over Pd or Ni, essentially no ethylene is hydrogenated until all the acetylene is converted to (32) Broden, G.; Rhodin, T. N.; Brucker, C.; Benbow, R.; Hurych, Z. Surf. Sci. 1976, 59, 593-601.
Addition of Acetylene to the Fischer-Tropsch Reaction
Figure 8. Possible surface entities when acetylene and ethylene are adsorbed on a catalyst surface, adapted from ref 37.
ethylene.33-35 When an alkyne and CO are adsorbed together on a metal surface, no CO is detected on the surface by IR, although the peaks due to the triple bond are modified by the presence of CO. In general, the strength of adsorption is alkynes > CO > olefins. The present work furnishes evidence that a two-carbon entity can initiate the growing chain. Acetylene is adsorbed more strongly than CO in the F-T reaction. In the presence of synthesis gas, acetylene is converted to ethylene, which, already on the catalyst surface (in contrast to cofed ethylene), initiates the F-T reaction at temperatures lower than those achievable in the normal F-T synthesis. The acetylenic molecule must undergo conversion to a surface species that enables CO and H2 to react and form hydrocarbon chains. There is evidence from surface science that the adsorbed acetylene adds a hydrogen atom to form ethylidyne, CH3C≡, a species well-known on metal surfaces.33-39 The carbon atom attached to the surface is sp-hybridized and forms bonds to three metal atoms via hybridized molecular orbitals. A model for the nature of the ethylidyne species is furnished by the structure of the organometallic complex, ethylidyne tricobaltnonacarbonyl, CH3C(Co)3(CO)9 (Figure 7).40-42 There appears to be a close relation between organic ligands in organometallic compounds and their counterparts on solid surfaces. It has been shown that (33) Gland, J. L.; Zaera, F.; Fischer, D. A.; Carr, R. G.; Kollen, E. B. Chem. Phys. Lett. 1988, 151, 227-229. (34) Cheng, C.; Apeloig, Y.; Hoffman, R. J. Am Chem. Soc. 1988, 110, 749-774 (35) Zaera, J. J. Am Chem. Soc. 1989, 111, 4240-4244. (36) Ormerod, R. M.; Lambert, R. M.; Hoffman, H.; Zaera, F.; Wang, L. P.; Bennett, D. W.; Tysoe, W. T. J. Phys. Chem. 1994, 98, 2134-2138. (37) Zaera, F. Chem. ReV. 1995, 95, 2651-2693. (38) Zaera, F.; French, C. R. J. Am. Chem. Soc. 1999, 121, 2336-2343. (39) Zaera, F.; Chrysostoman, D. Surf. Sci. 2000, 457, 71-78. (40) Markby, R.; Wender, I.; Friedel, R. A.; Cotton, F. A.; Sternberg, H. W. J. Am. Chem. Soc. 1958, 80, 6329-63. (41) Sutton, P. W.; Dahl, L. F. J. Am. Chem. Soc. 1967, 89, 261-268 (42) Parker, S. F.; Marsh, N. A.; Camus, L. M.; Whittsey, M. K.; Jayasoviya, U. A.; Keardey, G. F. J. Phys. Chem. 2002, 105, 5797-5802.
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alkylidynes are involved in hydrocarbon reforming and catalytic reactions.38 It is reasonable to postulate that alkylidyne species are involved in the reactions of alkynes and alkenes in the F-T reaction. It is of interest to note that the most common way of preparing ethylidynes on solid surfaces is simply by depositing ethylene at about room temperature on metals such as Pt, Ru, or Ni. Ethyl, vinyl, and alkylidene species may be involved in the conversion of these molecules to alkylidynes. For reactions run in the presence of acetylene at 220 °C, both acetylenic and olefinic molecules form ethylidynes and their possible interconversion is shown in Figure 8. The F-T catalyst surface undergoes a reconstruction during reaction. Activity is initially slow, only increasing with time, indicating a slow process of solid-state transformation to generate the real F-T catalyst. Metal atoms move to restructure the catalyst surface as the ethylidyne moves, opening up active sites. As molecules adsorb on active sites, they move by siteto-site hopping, allowing the chemisorbed molecules to reach a partner or an active site.43 The ethylidyne remodels the surface, allowing CO to bind briefly, and then adds H2 to form growing chains.44 Conclusions The addition of acetylene to the F-T qualitatively resembles the addition of monosubstituted alkynes but acetylene incorporation is much greater, with the extent of incorporation increasing with temperature. The low-temperature initiation of the F-T with acetylene is related to the strength of adsorption of acetylene, which is greater than that of carbon monoxide or olefins. It is likely that acetylene is converted to ethylene, which initiates the F-T reaction, forming the first two carbons of the growing chain. Addition of acetylene to the F-T allows the reaction to proceed at temperatures lower than that at the normal F-T condition. Although there is a slight increase in oxygenate formation with a cobalt catalyst, acetylene addition greatly increases oxygenate formation with an iron catalyst. The mechanism of addition of acetylene to the F-T catalyst may involve the formation of an ethylidyne species having three points of attachment to the catalyst. Acknowledgment. We thank the U.S. Department of Energy for financial support (Grant DE-FC26-99FT40540) and Dr. B. H. Davis of CAER, University of Kentucky, for providing the iron catalyst. EF060497J (43) Veba, V.; Wolf, M. Science 2005, 310, 1774-1775. (44) Somorjai, G. A. Nature 2004, 1774, 730.
