Effect of the Zr promoter on precipitated iron-based catalysts for high-temperature Fischer–Tropsch synthesis of light olefins

Abstract

FeMnxZr and FeMnxZr2Na catalysts prepared by coprecipitation and impregnation methods were applied to investigate the promoting effects of Zr on iron-based catalysts for high-temperature Fischer–Tropsch synthesis (HTFT). Ar physisorption and TEM demonstrated that Zr-promoted catalysts have a larger BET surface area and larger pore volume, which are more conducive to the distribution of active phases. Zr promoted the dispersity of iron particles and reduced the grain size of iron particles, resulting in the improvement of the FTS activity. The CO-TPD and MES results revealed that the introduction of Zr promoted the dissociative adsorption of CO, thereby promoting the formation of the active phase χ-Fe₅C₂ while suppressing the formation of ε′-Fe_2.2C and further improving the FTS activity and the selectivity to C₂⁼–C₄⁼ in the hydrocarbon distribution. However, the addition of an excessive amount of Zr caused the excess ZrO₂ to cover the surface of the active site, inhibiting the CO dissociative adsorption and the formation of χ-Fe₅C₂, resulting in a decrease in the CO conversion and the selectivity to C₂⁼–C₄⁼. The XPS and H₂-TPR results proved the interaction between Fe₂O₃ and ZrO₂. And the Fe–ZrO₂ interaction inhibited the coke deposition and aggregation of the Fe species, as confirmed by XRD and TEM, respectively. The FeMn5Zr2Na catalyst exhibited the highest C₂⁼–C₄⁼ selectivity of 34.4% at a CO conversion of 96.3%. And FeMn5Zr2Na displayed the highest production of light olefins (441.6 g h⁻¹ kg_Cat⁻¹), which was significantly higher than FeMn₂Na (162.2 g h⁻¹ kg_Cat⁻¹).