In situ reduction and carbonation of organogel containing Fe and Mn and their catalytic performance in Fischer-Tropsch synthesis
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摘要: 论文制备了系列含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-Fe3C含量不同的费托合成催化剂。采用XRD、N2吸附、Raman、CO-TPD、CO2-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-Fe3C、Fe0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。系统地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（C2-C4）选择性为37.1%，其中θ-Fe3C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-Fe3C的含量具有正相关性。Abstract: 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.
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Key words:
- Fischer-Tropsch synthesis /
- light olefins /
- θ-Fe3C /
- Mn
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图 2 FeMn催化剂的N2吸附-脱附等温线
Figure 2 N2 adsorption–desorption isotherms of the FeMn catalysts
图 7 FeMn催化剂的TEM照片：FeMn20（a,b）, FeMn10（c,d）, FeMn4（e,f）, FeMn2（g,h）
Figure 7 TEM images of the FeMn catalysts: FeMn20(a,b), FeMn10(c,d), FeMn4(e,f), FeMn2(g,h)
图 9 CO转化率随FeMn催化剂中θ-Fe3C含量关系
Figure 9 Relationship between CO conversion with content of θ-Fe3C in the FeMn catalysts
图 11 反应条件对FeMn10催化剂催化性能的影响
Figure 11 Effect of reaction conditions on the catalytic performance of the FeMn10 catalysts
表 1 FeMn催化剂的物理化学性质
Table 1 Physico-chemical properties of the FeMn catalysts
Samples SBET/(m2·g−1) Pore volume/(cm3·g−1) Pore size/nm n(Fe)∶n(Mn)* FeMn0 46.8 0.17 14.7 − FeMn20 26.4 0.10 15.2 15.0 FeMn10 25.6 0.10 17.1 7.8 FeMn4 32.1 0.13 16.1 3.4 FeMn2 35.3 0.15 15.8 1.9 *: Results were obtained by ICP-OES. 
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表 2 FeMn催化剂的CO2脱附量
Table 2 CO2 desorption amount of the FeMn catalysts
Samples CO2 desorption amount (μmol gcat−1) T<200 ℃ T>200 ℃ FeMn0 49.0 20.0 FeMn20 62.0 31.4 FeMn10 71.4 45.1 FeMn4 88.0 32.0 FeMn2 71.8 17.5
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表 3 不同Mn含量的催化剂的催化性能
Table 3 3 Catalytic performance of catalysts with different Mn contents
Samples CO conv.
(%)CO2 sel.
(%)Selectivity (%) O/P CH4 C2=-C4= C20-C40 C5 + FeMn0 91.4 28.6 33.0 16.0 33.2 17.8 0.48 FeMn20 22.0 12.2 29.9 27.5 27.1 15.5 1.02 FeMn10 57.3 26.4 22.0 37.1 13.5 27.4 2.75 FeMn4 34.3 19.6 29.5 30.2 27.9 12.4 1.08 FeMn2 10.7 11.1 33.1 24.1 32.9 9.9 0.73 Reaction conditions:H2/CO = 2，GHSV = 24000 mL·h−1·g−1，t = 300 ℃，p = 2 MPa，TOS = 20 h；O/P = C2=-C4=/C20-C40。
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