Effect of Structural Promoters on Fe-Based Fischer-Tropsch Synthesis of Biomass Derived Syngas

Biomass gasification and subsequent conversion of this syngas to liquid hydrocarbons using Fischer-Tropsch (F-T) synthesis is a promising source of hydrocarbon fuels. However, biomass-derived syngas is different from syngas obtained from other sources such as steam reforming of methane. Specifically the H-2/CO ratio is less than 1/1 and the CO2 concentrations are somewhat higher. Here, we report the use of Fe-based F-T catalysts for the conversion of syngas produced by the air-blown, atmospheric pressure gasification of southern pine wood chips. The syngas from the gasification step is compressed and cleaned in a series of sorbents to produce the following feed to the F-T step: 2.78 % CH4, 11 % CO2, 15.4 % H-2, 21.3 % CO, and balance N-2. The relatively high level of CO2 suggests the need to use catalysts that are active for CO2 hydrogenation as well is resistant to oxidation in presence of high levels of CO2. The work reported here focuses on the effect of these different structural promoters on iron-based F-T catalysts with the general formulas 100Fe/5Cu/4K/15Si, 100Fe/5Cu/4K/15Al and 100Fe/5Cu/4K/15Zn. Although the effect of Si, Al or Zn on iron-based F-T catalysts has been examined previously for CO+CO2 hydrogenation, we have found no direct comparison of these three structural promoters, nor any studies of these promoters for a syngas produced from biomass. Results show that catalysts promoted with Zn and Al have a higher extent of reduction and carburization in CO and higher amount of carbides and CO adsorption as compared to Fe/Cu/K/Si. This resulted in higher activity and selectivity to C-5+ hydrocarbons than the catalyst promoted with silica.