Preparation of layered K/Mg-Fe-Al catalysts and its catalytic performances in CO hydrogenation

A series of K promoted K/MgFeAl-HTLcs catalysts with different Mg/Fe/Al molar ratios were prepared by means of coprecipitation and impregnation method for direct synthesis of light olefins from CO hydrogenation. The samples were characterized by XRD, N2 adsorption-desorption, SEM, TG, H2-TPR and XPS measurements. The results show that MgFeAl-HTLcs catalyst precursors have typical layered structure. MgO, Fe2O3 and small amount of MgFeAlO4 are formed after calcination. MgCO3 and Fe3O4 could be observed after reaction, and a little Fe5C2 iron carbide with broad and weak peaks appears simultaneously. Thermal stability of K/MgFeAl-HTLcs is improved due to recovery of hydrotalcite-like structure after K promotion. With increase of Al content, specific surface area of the precursors decreases monotonically after structure reconstruction. Reduction of Fe2O3 to Fe3O4 is inhibited with addition of Al, compared with K/Mg-Fe sample. Fe enrichment before reaction and K enrichment after reaction are observed on secondary calcination samples. During CO hydrogenation, the prepared samples show high activity and C2=-C4= selectivity with low C5+ weight fraction. C5+ hydrocarbons decrease and olefin selectivity increases with increasing Fe/Al molar ratio. The C5+ decreases from 22.17% to 10.90%, and C2=-C4= weight content increases from 40.98% to 47.28% on K/1.5Mg-0.67Fe-0.33Al sample compared with that of K/1.5Mg-0.67Fe sample.