The bulk of the products that were synthesized from Fischer–Tropsch synthesis (FTS) is a wide range (C1–C70+) of hydrocarbons, primarily straight-chained paraffins. Additional hydrocarbon products, which can also be a majority, are linear olefins, specifically: 1-olefin, trans-2-olefin, and cis-2-olefin. Minor hydrocarbon products can include isomerized hydrocarbons, predominantly methyl-branched paraffin, cyclic hydrocarbons mainly derived from high-temperature FTS and internal olefins. Combined, these products provide 80–95% of the total products (excluding CO2) generated from syngas. A vast number of different oxygenated species, such as aldehydes, ketones, acids, and alcohols, are also embedded in this product range. These materials can be used to probe the FTS mechanism or to produce alternative chemicals. The purpose of this article is to compare the product selectivity over several FTS catalysts. Discussions center on typical product selectivity of commonly used catalysts, as well as some uncommon formulations that display selectivity anomalies. Reaction tests were conducted while using an isothermal continuously stirred tank reactor. Carbon mole percentages of CO that are converted to specific materials for Co, Fe, and Ru catalysts vary, but they depend on support type (especially with cobalt and ruthenium) and promoters (especially with iron). All three active metals produced linear alcohols as the major oxygenated product. In addition, only iron produced significant selectivities to acids, aldehydes, and ketones. Iron catalysts consistently produced the most isomerized products of the catalysts that were tested. Not only does product selectivity provide a fingerprint of the catalyst formulation, but it also points to a viable proposed mechanistic route.

The kinetics and the effect of indigenous and externally added water on methane formation during Fischer–Tropsch synthesis (FTS) was studied over Co based catalysts using a 1 L continuously stirred tank reactor (CSTR). The water cofeeding study (10% water) was conducted over a 0.27%Ru–25%Co/Al2O3 catalyst at a low CO conversion level of 19% at 220 °C in order to lessen the effect of catalyst aging during the addition of water, while the kinetic experiment was conducted over 25%Co/γ-Al2O3 at the conditions of 205–230 °C, 1.4–2.5 MPa, H2/CO = 1.0–2.5, and 3–16 (NL/gcat)/h (X CO < 60%). Indigenous and externally added water decreases methane formation by a kinetic effect. The addition of 10% water led to a decrease in the CH4 rate by 12% (3.5 → 3.0 (mmol/gcat)/h), while little catalyst deactivation was observed during water addition. Increases in indigenous water partial pressure also lowered the CH4 rate and its selectivity. Kinetic analysis was performed using a group of 220 °C data collected between 365 and 918 h when the deactivation rate was very low. An empirical CH4 kinetic model, with a water effect term (P H2O/P H2 ), (r CH4 = kP CO a P H2 b /(1 + mP H2O/P H2 )) was used to fit kinetic data. The CH4 kinetic results suggest a negative water effect on CH4 formation during FTS on the unpromoted cobalt catalyst, consistent with the water effect results. The final methane kinetics (r CH4 ) equation obtained at 220 °C over 25%Co/γ-Al2O3 is as follows: r CH4 /[(mol/gcat)/h] = 0.001053{P CO –0.86 P H2 1.32/[1 + 0.46(P H2O/P H2 )]}. Meanwhile, a methane selectivity model at 220 °C for the 25%Co/Al2O3 catalyst was also developed: S CH4 = 0.0792P CO –0.55 P H2 0.44[(1 – 0.24P H2O/P H2 )/(1 + 0.46P H2O/P H2 )]. The CH4 selectivity model provided a good prediction of CH4 selectivities under the experimental conditions used. Furthermore, our empirical CH4 kinetic results on the cobalt catalyst are consistent with literature kinetic models that were derived from carbide mechanisms; high CH4 selectivity from the cobalt catalyst is found to be mainly due to a high CH4 reaction rate constant.

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