The promotional effect of Mn on Fe-based Fischer-Tropsch catalysts for the synthesis of C5+ hydrocarbons

Fe-Mn oxide catalysts with different morphologies were synthesized by a one-pot hydrothermal method and applied to the Fischer-Tropsch synthesis reaction. Unpromoted Fe2O3 presents a spindle-like shape. The increase of the Mn/Fe atomic ratio in the catalysts from 5/100 to 10/100 and then to 20/100 causes the morphology transformation of the Fe-Mn oxides from spindles (FeMn5) to cubes (FeMn10) and then to nanoparticles (FeMn20). Although the Mn has a negative influence on the catalytic stability, the introduction of a Mn promoter in the Fe-based catalysts can effectively suppress the CH4 formation and facilitate the carbon chain growth. The FeMn5 spindle catalyst with a low Mn content exhibits a higher selectivity for C-2-C-4 short-chain hydrocarbons, while the FeMn10 cube catalyst with a relatively high Mn content displays a much better selectivity for C5+ long-chain hydrocarbons. This demonstrates that the amount of Mn promoter in the synthesized Fe-Mn oxide catalysts has a great influence on the distribution of hydrocarbon products.