Investigation of Mn and Ca promoter effects in iron-based catalysts: CO hydrogenation reaction

Abstract

Cu–Fe catalysts incorporated with different amounts of Ca and Mn promoters were prepared using a wet-impregnation method and applied in the Fischer–Tropsch synthesis reaction. The composition of the final iron catalysts in terms of the weight percentage was as follows: 14Fe/3Cu/2Ca/γ-Al₂O₃, 14Fe/3Cu/2Mn/γ-Al₂O₃ and 14Fe/3Cu/1Ca/1Mn/γ-Al₂O₃. The catalysts were characterized by XRD, ICP, BET, FE-SEM, EDX, H₂-TPR and TEM techniques. The performance of the catalysts was also evaluated in a fixed bed reactor under a pressure of 20 atm, a H₂ to CO ratio of 1, a GHSV of 2 L h⁻¹ g cat⁻¹ and a temperature of 285 °C. Finally, the effect of Ca and Mn promoters individually and in combination with each other on catalysts’ performance was investigated. CO conversion and product selectivity were calculated using the results of gas chromatography. The 14Fe/3Cu/2Mn/γ-Al₂O₃ catalyst exhibits a higher selectivity for C₂–C₄ short-chain hydrocarbons, while the 14Fe/3Cu/1Ca/1Mn/γ-Al₂O₃ catalyst displays a much better selectivity for C₅⁺ long-chain hydrocarbons. The C₅⁺ selectivity of the doubly-promoted catalyst was 61.99%, which was higher than those of Fe/Cu catalysts and singly-promoted catalysts, due to the synergistic effect of the two promoters. The doubly-promoted catalyst achieved higher CO conversion (77.6%) and yield (48.03%) than other prepared catalysts. This demonstrates that the amount of Ca and Mn promoters in the synthesized Fe-based catalysts has a great influence on the distribution of hydrocarbon products. The formation of methane and light hydrocarbons was restrained with the combination of calcium and manganese.