19.4.1 FT catalysts

The main requirement for a good FT catalyst is high hydrogenation activity in order to catalyze the hydrogenation of CO to higher hydrocarbons. The only metals with sufficiently high hydrogenation activity to warrant application in FT synthesis are four transition metals of the VIII group of the periodic table: Fe, Co, Ni and Ru. Although Ru exhibits the highest hydrogenation activity, its extremely high price and low availability render it unsuitable for large-scale applications such as the FT process. Nickel, on the other hand, is essentially a methanation catalyst, its application leading to the undesired production of large amounts of methane. Therefore, Fe and Co are the only industrially relevant catalysts that are currently commercially used in FT. The choice of catalyst depends primarily on the FT operating mode. Fe-based catalysts are suitable for the high temperature Fischer-Tropsch (HTFT) operating mode that takes place in the 300-350°C temperature range and is used for the production of gasoline and linear low molecular mass olefins. Both Fe and Co catalysts can be used for the low temperature Fischer-Tropsch (LTFT) that operates in the 200-240°C range and produces high molecular mass linear waxes (Dry, 2002). Moreover, the choice of metal also depends on the feedstock used for the FT synthesis. As Fe, unlike Co, catalyzes the WGS reaction, it is usually used for hydrogen-poor synthesis gas, most especially that from coal (~0.7 H2/CO molar ratio), to increase via the WGS reaction the hydrogen content of syngas to the optimum 2 H2/CO ratio of the FT reaction. Cobalt is, therefore, the catalyst of choice for GTL processes, using natural gas as feedstock. Whether the catalysts are Fe or Co, FT catalysts are notorious for their sensitivity towards sulphur and their permanent poisoning by sulphur compounds. As aforementioned, syngas requirements for FT synthesis ask for a sulphur content of below 0.05 ppm (Dry, 1990).

An extensive amount of research has been performed on several aspects of the Fe and Co catalysts, including fundamental, basic and applied research. These efforts include investigation of the effect of promoters, supports, additives, pre­treatments, preparation and generally all chemical and physical properties of the materials in order to increase catalyst activity, enhance selectivity to the desired products, inhibit formation of unwanted products, especially methane, and improve resistance to sulphur poisoning. A summary of improved, modified Fe and Co catalysts employed in industry for the FT process is presented in Table 19.2 (Bartholomew, 1990).

Iron catalysts

Iron-based catalysts are used in both LTFT and HTFT process modes. Precipitated iron catalysts, used in fixed bed or slurry reactors for the production of waxes, are prepared by precipitation and have a high surface area. A silica support is commonly used with added alumina to prevent sintering. HTFT catalysts for fluidized bed applications must be more resistant to attrition. Fused iron catalysts, prepared by fusion, satisfy this requirement (Olah and Molnar, 2003). For both types of iron-based catalysts, the basicity of the surface is of vital importance. The probability of chain growth increases with alkali promotion in the order Li, Na, K and Rb (Dry, 2002), as alkalis tend to increase the strength of CO chemisorption and enhance its decomposition to C and O atoms. Due to the high price of Rb, K

is used in practice as a promoter for iron catalysts. Copper is also typically added to enhance the reduction of iron oxide to metallic iron during the catalyst pre­treatment step (Adesina, 1996). Under steady-state FT conditions, the Fe catalyst consists of a mixture of iron carbides and re-oxidized Fe3O4 phase, active for the WGS reaction (Adesina, 1996).