Effect of alkalis (Li, Na, and K) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis

Alkalis (Li, Na, and K) decorated FeMnZr catalysts prepared by coprecipitation and impregnation methods were used to investigate the effects of three alkali metal promoters and promoter loading (for Na and K) on the iron-based catalyst for high-temperature Fischer-Tropsch synthesis (HTFT). The results showed that alkalis donated electrons to iron species, which promoted CO dissociative adsorption while suppressing hydrogen adsorption, then boosted the selectivity to light olefins and the improvement was enhanced with the increase of the atomic number of alkali metals and the content of alkali metals. Moreover, compared to alkalis-free catalysts, alkalis-promoted catalysts facilitated the formation of χ-Fe5C2 and ɛ'-Fe2.2C. In alkalis decorated catalysts, the χ-Fe5C2 content decreased and the ɛ'-Fe2.2C content increased with increasing basicity of the catalyst surface. In addition, compared with χ-Fe5C2, ɛ'-Fe2.2C has a stronger CC coupling ability and promotes chain growth more. Compared to Li and Na promoters, K was easier to facilitate the formation of ɛ'-Fe2.2C due to its strongest basicity and CO dissociative adsorption ability. The lowest CH4 and highest C5+ selectivity were accompanied by the highest content of ɛ'-Fe2.2C on K-promoted catalysts, which suggested that ɛ'-Fe2.2C had outstanding CC coupling ability.