Highly selective formation of linear α-olefins over layered and hydrophilic Fe3O4/MAG catalysts in CO hydrogenation

Abstract

Controlling the product distribution of Fischer–Tropsch synthesis (FTS) reaction has always been a challenge. Fe-based catalysts can produce linear α-olefins with high selectivity in FTS, but it is significantly affected by the secondary reactions of primary olefins. To enhance the olefin selectivity in CO hydrogenation, layered magadiite (MAG) supported Fe3O4 (Fe3O4/MAG) catalysts with hydrophilic property was prepared by solvent (ethanol and water) dispersion methods. The selectivity of C2–C4 olefins can reach 46% (O/P = 6.15) over Fe3O4/MAG (ethanol) catalyst, which is much higher than that of 14% over unsupported Fe3O4 under the same reaction conditions. The particle size of Fe3O4 in Fe3O4/MAG has a great influence on the product distribution. The Fe3O4 with particle size of about 20 nm affords high olefin selectivity to C2–C4 (52%) with super high O/P value of 15 (olefin selectivity of 93.8% in C2–C3 fraction) and low CH4 selectivity of 7%, while the one with particle size about 400 nm affords extremely high linear α-olefin selectivity of 77.9% in C2+ hydrocarbons. In situ DRIFTS and C2H4/H2-TPSR/MS showed that the layered structure and hydrophilic groups can reduce the possibility of secondary hydrogenation reactions. Also, the complex formed by ethanol and surface hydroxyls induces the chain propagation, and promotes more linear α-olefin formation.