含铁锰的有机凝胶原位还原和碳化及其费托合成反应性能

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

本研究制备了系列含Fe、Mn的有机凝胶前驱体,在氩气氛围下通过高温热处理,凝胶中铁物种被有机物原位分解进行还原和碳化,制备出了θ-Fe_(3)C含量不同的费托合成催化剂。采用XRD、N_(2)吸附-脱附、Raman、CO-TPD、CO_(2)-TPD、XPS和TEM等手段对催化剂的结构组成、表面性质以及活性物种的电子价态进行了系统的表征和分析。实验结果表明,热处理后获得的催化剂含石墨碳、θ-Fe_(3)C、Fe^(0)和(FeO)_(0.497)(MnO)_(0.503)物相,费托反应后催化剂的结构保持稳定,物相种类不发生变化。考察了反应条件对催化性能的影响,FeMn10催化剂具有较优的催化性能,CO转化率为57.3%,低碳烯烃(C_(2)-C_(4))选择性为37.1%,其中,θ-Fe_(3)C物相作为催化活性位点,催化剂的活性和低碳烯烃的选择性与θ-Fe_(3)C的含量具有正相关性。

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 ofθ-Fe_(3)C species.The structural composition,surface properties and electronic valence states of the active species of the catalysts were systematically characterised and analysed by XRD,N_(2) adsorption,Raman spectroscopy,CO-TPD,CO_(2)-TPD,XPS,and TEM measurements.The resulting catalysts exhibited a composite composition comprising graphitic carbon,θ-Fe_(3)C,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-Fe_(5)C_(2) phase post-reaction.This was attributed to the χ-Fe_(5)C_(2) species had higher intrinsic catalytic activity than θ-Fe_(3)C 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-Fe_(3)C within the catalysts by varying Mn promoter addition enabled a deeper exploration of the correlation between catalytic performance and content ofθ-Fe3C.Fine-tuning the content of θ-Fe_(3)C within the catalysts by varying Mn promoter addition enabled a deeper exploration of the correlation between catalytic performance and content of θ-Fe_(3)C.Quantification ofθ-Fe3C content via XRD revealed that content ofθ-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-Fe_(3)C of the FeMn_(2) 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 ofθ-Fe_(3)C 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 CO_(2) 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 Fe_(3)O_(4) species and exerting a suppressive effect on the water-gas shift(WGS)reaction.θ-Fe_(3)C catalysts exhibited excellent light olefins selectivity and low CO_(2) selectivity in Fischer-Tropsch synthesis,and had potential for industrial applications.