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 Fe1Zn1.2Ox catalysts such that they selectively converted syngas to linear α-olefins via FTS at 340 °C and 2.0 MPa. The Fe1Zn1.2Ox 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 Fe1Zn1.2Ox but facilitated the formation of Fe5C2. When pure Fe1Zn1.2Ox was calcined at 400 °C, the corresponding catalyst (i.e., Na0.2/Fe1Zn1.2Ox (400)) exhibited higher catalytic activity and stability than the other catalysts for a 50 h reaction. Compared to the other catalysts, Na0.2/Fe1Zn1.2Ox (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 Na0.2/Fe1Zn1.2Ox (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 Na0.2/Fe1Zn1.2Ox catalysts converted α-olefins to branched paraffins (13.9-19.5%) via a series of isomerization, skeletal isomerization, and hydrogenation reactions. The Na0.2/Fe1Zn1.2Ox (400) catalyst had a relatively low consumption rate of internal olefins compared to other catalysts, resulting in the lowest selectivity for branched paraffins. The Na0.2/Fe1Zn1.2Ox (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 Na0.2/Fe1Zn1.2Ox (400) was 37.5% relative to the total α-olefins.