Fischer-Tropsch Synthesis to Lower Olefins over Potassium-Promoted Reduced Graphene Oxide Supported Iron Catalysts

Fischer Tropsch synthesis to lower olefins (FTO) opens up a compact and economical way to the production of lower olefin directly from syngas (CO and H-2) derived from natural gas, coal, or renewable biomass. The present work is dedicated to a systematic study on the effect of K in the reduced graphene oxide (rGO) supported iron catalysts on the catalytic performance in FTO. It is revealed that the activity, expressed as moles of CO converted to hydrocarbons per gram Fe per second (iron time yield to hydrocarbons, termed as FTY), increased first with the content of K, passed through a maximum at 646 mu mol(CO) g(Fe-1) s(-1) over the FeKl/rGO catalyst, and then decreased at higher K contents. Unlike the evolution of the activity, the selectivity to lower olefins increased steadily with K, giving the highest selectivity to lower olefins of 68% and an olefin/paraffin (O/P) ratio of 11 in the C-2-C-4 hydrocarbons over the FeK2/rGO catalyst. The volcanic evolution of the activity is attributed to the interplay among the positive effect of K on the formation of Hagg carbide, the active phase for FTO, and the negative roles of K in increasing the size of Hagg carbide at high content and blocking the active phase by K-induced carbon deposition. The monotonic increase in the selectivity to lower olefins is ascribed to the improved chain-growth ability and surface CO/H-2 ratio in the presence of K, which favorably suppressed the unwanted CH4 production and secondary hydrogenation of lower olefins.