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, θFe3C, 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. © 2024 Science Press. All rights reserved.