Syngas-derived olefins over iron-based catalysts: Effects of basic properties of MgO nanocrystals
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摘要: 本研究采用等量浸渍法、化学沉淀法和超声浸渍法合成了一系列具有良好外露晶面的Fe/MgO催化剂。采用X射线粉末衍射、高分辨透射电子显微镜、CO2程序升温脱附、H2程序升温还原、X射线光谱学和N2物理吸附等物理化学方法对催化剂进行了表征。MgO纳米晶载体的碱性会影响费-托合成产物的选择性。在超声浸渍过程中，MgO纳米晶载体的碱性得到了保持。研究结果显示，Fe/MgO催化剂的碱性会提高CO解离速率和产物中烯烃的选择性。此外，相比于MgO（100）晶面，MgO（111）晶面负载铁基催化剂具有更高的活性（TOF）和烯烃选择性。MgO（111）晶面上更有利于CO的吸附，抑制二次加氢反应，提高产物中烯烃的收率。Abstract: A series of Fe/MgO catalysts with well-defined exposed crystal planes were synthesized by impregnation, deposition-precipitation and ultrasonic impregnation methods. The catalysts were characterized by X-ray powder diffraction, high-resolution transmission electron microscopy, CO2 temperature-programmed desorption, H2 temperature-programmed reduction, X-ray spectroscopy and N2 adsorption-desorption isotherms. The characterization results indicate that the basicity of MgO supports strongly affect the catalytic performance of iron-based catalysts for Fischer-Tropsch synthesis. It is found that the strong basicity sites of MgO supports remain during the ultrasonic impregnation process. The intrinsic basicity of Fe/MgO catalysts enhances dissociative CO adsorption and promotes the olefin selectivity. In addition, the catalyst of iron particles on the (111) crystal planes of MgO nanosheets presents higher TOF value and olefins selectivity than that of the catalyst using the (100) crystal planes of MgO nanocubes as a support. The effect of basic properties of MgO nanocrystals facilitates CO chemisorption, suppressing H2 adsorption and olefin desorption on the corresponding Fe/MgO catalysts.
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Key words:
- Fischer-Tropsch synthesis /
- iron /
- MgO /
- preparation method
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Figure 2 (a) TEM image of MgO (111) nanosheets, inset is HRTEM image of one MgO (111) nanosheet; (b) SEM image of MgO(111) nanosheets; (c) TEM image of MgO (100) cubes, inset is HRTEM image of one MgO (100) cube; (d) SEM image of MgO(100) cubes
Figure 3 TEM images of (a) as-synthesized Fe2O3 particles, inset is particle distribution of Fe2O3 particles; (b) as-synthesized Fe/MgO-ns-UI sample; (c) as-synthesized Fe/MgO-c-UI sample
Figure 5 XPS spectra of Fe 2p regions for the reduced Fe/MgO-ns catalysts (conditions: the samples were pretreated with hydrogen at 420 ℃ for 3 h)
Figure 7 Schematic diagram of the atomic planes of MgO
(a): MgO (111); (b): MgO (100); red: O; green: Mg
Figure 8 Hydrocarbon product distributions over (a) Fe/MgO-ns-IM catalyst; (b) Fe/MgO-ns-DP catalyst; (c) Fe/MgO-ns-UI catalyst; and (d) Fe/MgO-c-UI catalyst
Figure 9 Time-on-steam evolution of C2-4 olefins selectivity for as-synthesized Fe/MgO catalysts
Table 1 Physicochemical property of the as-synthesized samples
Samples Surface area A/(m2·g-1) Total pore volume v/(cm3·g-1) Average Pore size d/nm MgO-ns 42.4 0.33 25.0 Fe/MgO-ns-UI 38.8 0.29 26.2 Fe/MgO-ns-IM 50.4 0.46 21.8 Fe/MgO-ns-DP 64.9 0.47 21.3 
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Table 2 XPS and ICP analysis results of the reduced Fe/MgO catalysts
Catalyst Binding energy of Fe 2p3/2 Iron content on the surfacea /% Bulk iron contentb/% Fe3O4 Fe0 Fe/MgO-ns-UI 710.4 706.2 1.6 4.9 Fe/MgO-ns-DP 710.8 - 2.6 6.5 Fe/MgO-ns-IM 710.8 706.3 0.7 5.3 a: obtained by XPS measurement and the samples were pretreated with hydrogen at 420 ℃ for 3 h; b: obtained by ICP measurement
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Table 3 Catalytic performance of iron-based catalysts with MgO supports for Fischer-Tropsch synthesis a
Catalyst CO conv.x/% TOF b/
(×10-2 s-1)Hydrocarbon selectivity s/% CH4 C2-40 C2-4= C5+ olefins O/P(C2-4) Fe/MgO-ns-IM 55.6 1.26 14.7 11.2 14.6 59.5 50.0 1.3 Fe/MgO-ns-DP 38.0 2.74 15.8 10.2 15.5 58.5 49.9 1.5 Fe/MgO-ns-UI 35.5 5.73 12.1 7.3 29.6 51.0 60.6 4.1 Fe/MgO-c-UI 35.7 4.34 13.6 9.6 21.5 55.3 53.1 2.2 a: reaction condition: 1.0 MPa, 300 ℃, H2/CO=1, 8 L/(h·g); b: TOF was based on the amount of the total H2 uptake
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