Reversing the Selectivity of Alkanes and Alkenes in Iron-Based Fischer–Tropsch Synthesis: The Precise Control and Fundamental Role of Sodium Promotor

Abstract

Alkali metals have been extensively used as an industrial promotor with a significant impact on the iron-based Fischer–Tropsch Synthesis (FTS), while their specific roles are still ambiguous due to the difficulty in discriminating the active species, especially in a dynamic evolution under reaction conditions. In this contribution, the dependent FTS performance on alkali metal content over iron-based catalysts by precise regulation of sodium (Na) doping was scrutinized. It is found that a trace addition of Na (0.5% mass fraction) dramatically switches the hydrocarbon selectivity from alkane to alkene, giving rise to total olefin selectivity from 27 to 76%. With the help of probe experiments and operando techniques, we demonstrate that the addition of Na can hardly affect the final state of iron carbide active species, but rather changes the formation rate of iron carbide, especially at the initial reaction stage. The presence of Na plays an exclusive role in regulating the electronic properties of iron carbide and adsorption behaviors of guest molecules, thus tailoring the reaction pathway by promoting the coupling of *CH₂ species while suppressing the excessive hydrogenation, which is deemed to be the intrinsic mechanism behind the high alkene selectivity. This work provides a clear-cut insight into the effect of alkali metal promotors for iron-based F–T catalytic systems as well as a theoretical basis for designing high-performance F–T catalysts.