Study on different synthesis methods of molybdenum-based oxide and sulfide catalyst and its performance in syngas to ethanol
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摘要: 合成气一步制乙醇是利用非石油资源生产乙醇的重要方法,如何提高乙醇选择性、创制高效催化剂是改善过程经济性的重点。本研究从硫化钼前体出发,分别采用传统热法和射频低温等离子体法制备钼基氧硫复合物催化剂并考察其催化合成气制乙醇反应性能。利用XRD、UV-visible、HR-TEM、SEM、HAADF-STEM、XPS、CO-TPD、H2-TPD、CO2-TPD和In-situ DRIFTS等表征手段研究不同制备方法下合成的钼基氧硫复合物催化剂的不同物化性质,进而探究特征差异引发的催化反应性能变化。其中,MOS-P催化剂表现出最佳性能,在6 MPa、320 ℃、空速4500 h−1的反应条件下,CO转化率达到22.5%,总醇选择性可达71.4%,其中,总醇中乙醇占比为29.1%。有关研究将为合成气定向转化提供理论指导并为新型钼基材料的设计与制备提供借鉴。Abstract: Ethanol is a significant chemical feedstock, which can be employed not only as a raw material for chemicals and polymers, but also as an additive to petrol. It is typically produced in industry through either fermentation or ethylene hydration. In light of the growing demand for ethanol, it is imperative to investigate the potential of multi-channel production of ethanol. One-step ethanol production from syngas represents a significant method of ethanol production from non-fossil oil energy sources, and it is also an important means of clean utilization of coal. The cost of direct ethanol production from syngas is relatively low, but the distribution of alcohols with different carbon numbers in the alcohol product is wide, which makes subsequent separation difficult and restricts its large-scale development. Consequently, in the research of direct ethanol production from syngas, the key points to improve the process economics and promote the development of this technology are to improve the selectivity of ethanol and develop the efficient catalysts. Mo-based catalysts can be employed for low-hydrogen syngas. At the same time, it is challenging to deposit carbon, exhibits robust resistance to sulfur poisoning, and demonstrates excellent stability, which also extends the reaction cycle. However, the methanol content of the alcohol product is relatively high. Although the use of Fischer-Tropsch element modification can significantly reduce the methanol selectivity, it will inevitably lead to the problem of broadening the distribution of alcohols. In recent years, there has been a growing attention in the preparation of catalysts using non-thermal plasma technology. Non-thermal plasma comprises not only electrons, ions, molecules and free radicals, but also photons and excited substances. Previous studies have demonstrated that the non-thermal plasma method can induce alterations in the nucleation of the active phase and the crystal growth mode in the preparation of catalysts. Concurrently, for thermodynamically unfavorable reactions, the utilization of non-thermal plasma technology can disrupt the thermodynamic equilibrium limit, thereby facilitating the reaction. In this study, Mo-based oxide and sulfide composite catalysts were prepared from the precursor of molybdenum sulfide by two distinct methods: the conventional thermal method and the RF non-thermal plasma method. The catalytic performance of Mo-based oxide and sulfide composite catalysts for the synthesis of ethanol from syngas was then investigated. A range of analytical techniques were employed to investigate the physical and chemical properties of the molybdenum-based oxygen-sulfur complex catalysts synthesized by different preparation methods. These included XRD, UV-visible, HR-TEM, SEM, HAADF-STEM, XPS, CO-TPD, H2-TPD, CO2-TPD and In-situ DRIFTS. Moreover, the objective was also to ascertain the impact of the physical and chemical properties on the catalytic performance of the different catalysts. Among them, the MOS-P catalyst exhibited the best catalytic performance. Under the reaction conditions of 6 MPa, 320 ℃, and a space velocity of 4500 h−1, the CO conversion reached 22.5%. The selectivity of total alcohols was 71.4%, with ethanol accounting for 29.1% of the total alcohols. This research will provide theoretical guidance for the directional conversion of syngas and serves as a reference for the design and preparation of new molybdenum-based materials.
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Key words:
- syngas /
- CO hydrogenation /
- ethanol /
- molybdenum-based oxide and sulfide /
- RF non-thermal plasma
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图 2 合成气制乙醇反应流程示意图
Figure 2 Schematic diagram of reaction device for alcohols synthesis from syngas
1: Reagent gas; 2: Pre-system pressure setter; 3: Mass flowmeter; 4: Temperature controller; 5: Heating furnace;6: Reactor; 7: Catalytic bed; 8: Heat trap; 9: Cold trap;10: Post-system pressure setter; 11: Gas chromatography;12: Gas outlet.
图 5 (a)MS催化剂的SEM图像;(b)MS催化剂的HR-TEM图像;(c)MOS-T催化剂的SEM图像;(d)MOS-T催化剂的HR-TEM图像;(e)MOS-P催化剂的SEM图像;(f)MOS-P催化剂的HR-TEM图像;(g)MO催化剂的SEM图像;(h)MO催化剂的HR-TEM图像;(i)−(n)各催化剂中MoS2和MoO3的平均粒径
Figure 5 (a) SEM image of the MS catalyst; (b) HR-TEM image of the MS catalyst; (c) SEM image of the MOS-T catalyst; (d) HR-TEM image of the MOS-T catalyst; (e) SEM image of the MOS-P catalyst; (f) HR-TEM image of the MOS-P catalyst; (g) SEM image of the MO catalyst; (h) HR-TEM image of the MO catalyst; (i)−(n) Particle size of MoS2 and MoO3 in each catalyst
表 1 不同催化剂的CO、H2和CO2吸附量
Table 1 Amount of CO, H2 and CO2 on different catalysts
Catalyst Amount of CO/(μmol·g−1) Amount of H2/(μmol·g−1) Amount of CO2/(μmol·g−1) peak i peak ii total MS 39 − 28 28 376 MOS-T 47 116(60%) 78(40%) 194 352 MOS-P 80 90(45%) 111(55%) 201 294 MO 21 − 10 10 − 表 2 不同催化剂的合成气制乙醇催化反应性能a
Table 2 The catalytic performance of different catalysts for the synthesis of ethanol from syngas a
Catalyst CO conv./% STYEtOH
/(mg·mL−1·h−1)Product selectivity s/% b Alcohol distribution w/% Hydrocarbon distribution w/% ROHc CHnd MeOH EtOH C3+OHe C1 C2 C3+ MS 14.1 13.5 68.8 31.2 66.5 22.5 11.0 57.2 26.1 16.7 MOS-T 18.9 18.9 69.6 30.4 64.3 23.6 12.1 55.3 27.7 17.0 MOS-P 22.5 32.0 71.4 28.6 55.2 29.1 15.7 62.5 23.1 14.4 MO 12.3 14.2 55.9 44.1 53.6 28.3 18.1 61.2 24.2 14.6 a: Reactions were carried out at 320 ℃, 6.0 MPa, GHSV=4500 h−1, H2/CO =2. STY is space-time yield; b: CO2 free; c: ROH means total alcohols and d: CHn means total hydrocarbons; e: Alcohols with carbon number above 3 were obtained in the product (propanol, butanol and pentanol). 表 3 本研究与文献中合成乙醇Mo基催化剂的催化性能对比
Table 3 A comparison of the catalytic performance of Mo-based catalysts for ethanol synthesis between this study and the literature
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