PRETREATMENT EFFECT STUDIES WITH A PRECIPITATED IRON FISCHER-TROPSCH CATALYST

A doubly promoted precipitated iron catalyst (100 Fe/0.3 Cu/0.8 K by mass) was characterized after different pretreatment conditions and after Fischer-Tropsch (FT) synthesis in a fixed bed reactor. The BET surface area of the catalyst decreased markedly after pretreatments with hydrogen, carbon monoxide and synthesis gas (H-2/CO 0.67 and 2). Calcined catalyst was in the form of alpha-Fe2O3, which was converted to either metallic iron (alpha-Fe) or a mixture of alpha-Fe and Fe3O4 after hydrogen reductions. During FT synthesis the alpha-Fe was carburized to iron carbides (chi-carbide or epsilon'-carbide) or oxidized to Fe3O4. After carbon monoxide or syngas pretreatments, the chi-carbide was the most dominant phase. During FT synthesis this carbide was partially or completely converted to epsilon'-carbide, Fe3O4, and/or FeCO3. Hydrogen reductions resulted in stable or increasing catalyst activity with time on stream and the reduction conditions had a strong effect on the subsequent catalyst activity. The carbon monoxide and syngas pretreated catalysts deactivated with time. The hydrogen reduced catalysts produced more methane and gaseous hydrocarbons than the carbon monoxide or the syngas pretreated catalysts, and favored secondary hydrogenation and isomerization reactions.