Selective olefin production on silica based iron catalysts in Fischer–Tropsch synthesis

Abstract

We demonstrated two design methods for obtaining Fe catalysts to improve the olefin selectivity in Fischer–Tropsch synthesis (FTS) reactions through the use of a SiO₂ support. Fe/SiO₂ catalysts, synthesized through classical impregnation, and Fe–SiO₂ catalysts, synthesized through impregnation followed by calcination at 1700 °C, with Fe loadings of 1, 5, and 10 wt% were obtained. The Fe–SiO₂ catalysts achieved C2–C4 olefins in C2–C4 hydrocarbons, O/(O + P), of 62.5–73.0%, whereas the O/(O + P) of the Fe/SiO₂ catalysts decreased significantly with high Fe loadings. Using various characterization techniques, the main carbide species was identified to be Fe₅C₂, and based on the crystal size and composition of the Fe species, it was concluded that the interactions of Fe–SiO₂ catalysts with SiO₂ are of moderate strength. The iron time yield of C2–C4 olefins (FTY_{C2–C4 olefins}) as a function of the Fe₅C₂ composition was higher for the Fe–SiO₂ catalysts than for the Fe/SiO₂ catalysts with corresponding Fe loadings; the highest value obtained for 1 wt% Fe–SiO₂ was a factor of 3.5 higher than that of 1 wt% Fe/SiO₂. The reaction results showed that the C2–C4 olefins can be selectively produced using the Fe–SiO₂ catalyst even with a high Fe content. A density functional theory (DFT) study was performed to investigate the effect of SiO₂ promotion on Fe carbides, and it was concluded that SiO₂ promotion resulted in high selectivity for olefins, CH₄, and CO₂. To suppress CH₄ or CO₂ formation and boost CO conversion, Na-added catalysts with various Na ratios were tested, and successfully reduced CH₄ selectivity and enhanced CO conversion.