Activation Studies with a Precipitated Iron Catalyst for Fischer-Tropsch Synthesis: II. Reaction Studies

Effects of pretreatment conditions on catalyst performance (activity, selectivity, and stability with time) during Fischer-Tropsch (FT) synthesis were studied in a fixed-bed reactor using a commercial precipitated iron catalyst (100 Fe/5 Cu/4.2 K/25 SiO2 on a mass basis). The catalyst activity increased slightly with time-on-stream after hydrogen reductions, which was accompanied with conversion of metallic iron and part of iron oxides to ϵ′-carbide (ϵ′-Fe2.2C). Initial activity of the H2-reduced catalyst at 280°C for 8 or 24 h was markedly lower than that obtained in other tests. This is attributed to slow carburization of large oxide particles and partial poisoning of catalyst sites by migration of sulfur from the bulk to the surface of the catalyst during the reduction. Pretreatments with carbon monoxide and syngas resulted in partial conversion of Fe2O3 to χ-carbide (χ-Fe5C2). During FT synthesis the CO- and the syngas-pretreated catalyst deactivated slowly with time-on-stream, due to partial conversion of χ-carbide to less active iron oxide phases and buildup of carbonaceous deposits which block the active sites. The hydrogen-reduced catalyst at 280°C, for 1-24 h, produced more methane and gaseous hydrocarbons than the CO- or the syngas-pretreated catalyst and favored secondary reactions (1-olefin hydrogenation, isomerization, and readsorption).