Linear α-olefin production with Na-promoted Fe–Zn catalysts via Fischer–Tropsch synthesis

Abstract

The production of linear alpha-olefins (α-olefins) is a practical way to increase the economic potential of the Fischer–Tropsch synthesis (FTS) because of their importance as chemical intermediates. Our study aimed to optimize Na-promoted Fe₁Zn_1.2O_x catalysts such that they selectively converted syngas to linear α-olefins via FTS at 340 °C and 2.0 MPa. The Fe₁Zn_1.2O_x catalysts were calcined at different temperatures from 350 to 700 °C before Na anchoring. The increase in the size of the ZnFe₂O₄ crystals comprising the catalyst had a negative effect on the reducibility of Fe oxides and the particle size of Fe₅C₂ during the reaction. The Na species in the catalyst restrained the reduction of Fe₁Zn_1.2O_x but facilitated the formation of Fe₅C₂. When pure Fe₁Zn_1.2O_x was calcined at 400 °C, the corresponding catalyst (i.e., Na_0.2/Fe₁Zn_1.2O_x (400)) exhibited higher catalytic activity and stability than the other catalysts for a 50 h reaction. Compared to the other catalysts, Na_0.2/Fe₁Zn_1.2O_x (400) enabled a higher number of active Fe carbides (Fe₅C₂) to intimately interact with the Na species, even though the catalyst had a lower total surface basicity based on surface area. The Na_0.2/Fe₁Zn_1.2O_x (400) showed a maximum hydrocarbon yield of 49.7% with a maximum olefin selectivity of 61.3% in the C1–C32 range. Examination of the reaction product mixture revealed that the Na_0.2/Fe₁Zn_1.2O_x catalysts converted α-olefins to branched paraffins (13.9–19.5%) via a series of isomerization, skeletal isomerization, and hydrogenation reactions. The Na_0.2/Fe₁Zn_1.2O_x (400) catalyst had a relatively low consumption rate of internal olefins compared to other catalysts, resulting in the lowest selectivity for branched paraffins. The Na_0.2/Fe₁Zn_1.2O_x (400) showed a maximum α-olefin yield (26.6%) in the range C2–C32, which was 27.9–50.0% higher than that of other catalysts. The α-olefin selectivity in the C5–C12 range for the Na_0.2/Fe₁Zn_1.2O_x (400) was 37.5% relative to the total α-olefins.