Synthesis of Stable and Low-CO₂Selective Phase-Pure ϵ-Iron Carbide Catalysts in Synthesis Gas Conversion

Abstract

The Fe-catalyzed Fischer-Tropsch (FT) synthesis reaction is the core of the coal-to-liquids (CTL) process, which is an efficient route to convert coal into liquid fuels via synthesis gas (a mixture of CO and H₂). Conventional Fe-based FT catalysts convert typically 30% of the feed CO to undesirable CO₂during the FT step. A decrease of CO₂production in FT units can reduce the profitability of the overall CTL process. In this context, we synthesized phase-pure ϵ-Fe-carbide FT catalysts by careful control of the pretreatment and carburization conditions starting from Raney Fe. The resulting phase-pure Fe-carbide exhibits a low CO₂selectivity during the Fischer-Tropsch reaction. The developed preparation method can be applied to supported Fe catalysts and does not require expensive starting chemicals. The purity and durability of as-synthesized phase-pure ϵ-Fe-carbide catalyst under FT conditions were characterized in detail by in situ characterization techniques. A bulk phase-pure ϵ-Fe-carbide catalyst operated at 23 bar, 235 °C and a H₂/CO ratio of 1.5 shows stable performance with no deactivation during a 150 h pilot test. The catalyst is free of primary CO₂formation and presents reduced secondary CO₂formation compared to conventional Fe-based FT catalysts. These findings contribute to the development of new Fe-based FT catalysts for CTL processes.