Effect of alkaline-earth metals (Mg, Ca, Sr, and Ba) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis of light olefins

For the high-temperature Fischer-Tropsch synthesis (HTFT) of light olefins (C2=-C4=), alkaline-earth metals (Mg, Ca, Sr, and Ba) modified FeMn catalysts were prepared using precipitation and impregnation methods. It was demonstrated that the electron-giving interaction of Ca, Sr, and Ba decreased the adsorption of H species while improving the dissociative adsorption of CO. This interaction resulted in the production of χ-Fe5C2, which increased CO conversion and C2=-C4= selectivity. Compared with Ca and Ba, the FeMnSr catalyst had the strongest CO dissociative adsorption capacity and the highest χ-Fe5C2 content with the best FTS performance. When Sr and Na were introduced simultaneously, the synergistic effect of Sr and Na inhibited the formation of θ-Fe3C and the aggregation of catalyst particles, and also increased the χ-Fe5C2 content. This synergistic effect resulted in the highest surface basicity of FeMnSrNa catalyst, the highest electron cloud density on the surface of Fe atoms, the weakest H species adsorption, the strongest CO dissociative adsorption, and the highest total amount of active iron carbide, which improved the FTS performance. The light olefins yield of FeMnSrNa catalyst (427.0 g/(h·kgCat)) was significantly higher than that of FeMnSr catalyst (341.4 g/(h·kgCat)) and FeMnNa catalyst (80.6 g/(h·kgCat)).