Modified Fischer-Tropsch Type Catalysts
Fischer-Tropsch synthesis is a catalysis process that converts syngas into liquid hydrocarbons. The technology was originally developed to produce synthetic fuel and synthetic lubrication oil from coal-derived syngas.
A variety of catalysts can be used for the Fischer-Tropsch process, but the most common are the cobalt-, iron-, and ruthenium-based catalyst systems. Cobalt-based catalysts are highly active, although iron may be more suitable for low hydrogen content synthesis gases such as those derived from coal due to its promotion of the water — gas-shift reaction. In addition to the active metal, the catalysts typically contain a number of "promoters," including potassium and copper. Group I alkali metals, including potassium, are poison for cobalt catalysts but are promoters for iron catalysts. Catalysts are supported on supports such as silica, alumina and zeolites. Cobalt catalysts are more active for Fischer-Tropsch synthesis when the feedstock is natural gas. Natural gas has high hydrogen to carbon ratio, so the water-gas-shift is not needed for cobalt catalysts. Iron catalysts are preferred for lower quality feedstocks such as coal or biomass.
It has been suggested that the mechanism for alcohol formation on modified Fisher-Tropsch catalysts is essentially the same as the one described for Rh catalysts [54]. As shown in Figure 13.2, the reaction starts with CO dissociation and hydrogenation of the adsorbed carbon into CHx surface species, followed by CO insertion into the CHx species which leads to the alcohol formation.
Although the original Fischer-Tropsch catalysts were designed to produce hydrocarbons, ethanol and other higher alcohols have been seen as byproducts in the earlier CO2 hydrogenation experiments using these types of multifunctional catalysts [55-57], where mixtures of Rh, Fe and Cu were used. The proposed mechanism suggested a partial reduction of CO2 to CO followed by a propagation of the chain growth, and insertion of the OH group. Fe-Cu — Al-K-type Fischer-Tropsch catalysts are also known to produce ethanol and small increases in CO concentrations are known to increase the ethanol yield. However, a CO-rich gas reduced ethanol selectivity because CO2 was formed (rather than ethanol) via the shift reaction. The performance of this catalyst is said to be dependent on the oxidation-reduction state of the Fe catalyst during reac — tion—the active phase for CO2 hydrogenation to ethanol is Fe3O4 and is a function of the reduction temperature.
Khadzhiev and coworkers recently reported a very high ethanol yield of 78 wt% using a nanosized Fe-Al-K catalyst under Fischer — Tropsch synthesis conditions [58]. In their experiments, alcohol formation in a three-phase system in the presence of the nanosized 100Fe : 8Al2O3 : 3K2O (parts by weight) iron catalyst under the Fischer-Tropsch synthesis condition has been determined, and it has been found that the molecular weight distribution of alcohols does not follow the Anderson-Schulz-Flory law. However, the principal product was ethanol. A mechanism involving the CO insertion in the metal-carbon bond has been proposed for this catalytic activity. The highest ethanol yield (78wt%) was obtained using 20 atm, at 300°C, and H2/CO = 2.5 (mol/mol), 2wt% catalyst loading [58]. This is an encouraging result in the area providing a lead for the development of efficient Fischer-Tropsch catalysis systems for syngas to ethanol conversion.