In-situ reduction and carbonation of organogel containing Fe and Mn and their catalytic performance in Fischer-Tropsch synthesis

Light olefins constitute crucial chemical commodities primarily obtained from petroleum through naphtha cracking processes. Given China's energy landscape, characterized by a scarcity of oil, limited natural gas resources, and substantial coal reserves, leveraging coal for synthesizing light olefins emerges as a strategic pathway. This approach not only reduces reliance on petroleum resources but also enhances the value proposition of coal reservoirs. Coal-to-olefin conversion pathways encompass both direct (FTO) and indirect (MTO) methodologies. Notably, the FTO route stands out as a more efficiently and economically viable strategy for coal resource utilization. Fischer-Tropsch synthesis relies on iron carbides as active sites, posing a challenge in elucidating the distinct roles of single-phase iron carbide species within catalysts derived from CO or syngas. To address this challenge, we synthesized a range of organogel precursors incorporating Fe and Mn species. Subsequent in-situ reduction and carbonization of Fe species within the gel matrix under high-temperature conditions in an argon environment yielded Fischer-Tropsch catalysts featuring varying contents of θ-Fe₃C species. The structural composition, surface properties and electronic valence states of the active species of the catalysts were systematically characterised and analysed by XRD, N₂ adsorption, Raman spectroscopy, CO-TPD, CO₂-TPD, XPS, and TEM measurements. The resulting catalysts exhibited a composite composition comprising graphitic carbon, θ-Fe₃C, Fe⁰, and (FeO)_0.497(MnO)_0.503 phases. Catalysts lacking Mn promoter demonstrated superior catalytic activity (91.4%) but lower selectivity towards light olefins (16.0%), with the emergence of the χ-Fe₅C₂ phase post-reaction. This was attributed to the χ-Fe₅C₂ species had higher intrinsic catalytic activity than θ-Fe₃C species. For the catalysts with Mn promoter, the structure of the catalysts and the species of the physical phase remained stable after the Fischer-Tropsch reaction. We believed that Mn promoter played the role of structural promoter and displayed a stabilizing role in the phase structure of the catalysts. Fine-tuning the content of θ-Fe₃C within the catalysts by varying Mn promoter addition enabled a deeper exploration of the correlation between catalytic performance and content of θ-Fe₃C. Fine-tuning the content of θ-Fe₃C within the catalysts by varying Mn promoter addition enabled a deeper exploration of the correlation between catalytic performance and content of θ-Fe₃C. Quantification of θ-Fe₃C content via XRD revealed that content of θ-Fe₃C of the FeMn10 catalysts exhibited approximately 54.5%, resulting in a CO conversion rate of 57.3% and light olefins selectivity of 37.1%. In contrast, content of θ-Fe₃C of the FeMn2 catalysts displayed roughly 19.3%, yielding a CO conversion rate of 10.7% and light olefins selectivity of 24.1%. These findings underscored the pivotal role of θ-Fe₃C as the catalytic core in Fischer-Tropsch reactions, positively correlating with both CO conversion and light olefins selectivity. In addition, the FeMn catalysts exhibited low CO₂ selectivity attributed to the hydrophobic nature of carbon material generated from organic gel pyrolysis. This phenomenon curbed iron carbide oxidation by water, thereby reducing the formation of Fe₃O₄ species and exerting a suppressive effect on the water-gas shift (WGS) reaction. θ-Fe₃C catalysts exhibited excellent light olefins selectivity and low CO₂ selectivity in Fischer-Tropsch synthesis, and had potential for industrial applications.