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 has 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 appear 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 C_2-4⁼ 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 C_2-4⁼ 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.