Influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins
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摘要: 采用浸渍法制备了不同Li含量的Li/MgO催化剂，并通过TG-DTA、N2吸附及XRD等手段对其进行了表征；对丙烷在Li/MgO催化剂表面的两种吸附状态所占据的空间体积大小进行了计算，研究了Li含量对其丙烷氧化脱氢制烯烃反应催化性能的影响。结果表明，随着Li/MgO催化剂中Li含量的增加，丙烷转化率与乙烯、甲烷、乙烷和COx的选择性均先增加，在Li物质的量分数为3%时达到最高，然后随Li含量增加而降低，而丙烯选择性则出现相反的变化趋势。丙烷在Li/MgO催化剂上的吸附和反应同时受动力学和热力学两个因素的制约；Li负载量不同，反应活性位Li+O-分散状态也不同，导致产物分布发生变化。活性位分散度高时受热力学因素影响，有利于生成丙烯；活性位分散度低时受吸附动力学影响，更趋向于生成乙烯等其他产物。Abstract: A series of Li/MgO catalysts with different Li loadings were prepared by incipient wetness impregnation method and characterized by TG-DTA, N2 sorption and XRD; two modes for propane adsorption on Li/MgO were considered by calculation with Material Studio and the influence of Li loading on the catalytic performance of Li/MgO in the oxidative dehydrogenation of propane to olefins was investigated. The result indicated that with the increase of Li loading, the conversion of propane and the selectivity to C2H4, C2H6, CH4, COx increases at first, reaches the highest values at a Li loading of 3% and then decreases with further increasing the Li loading, whereas the selectivity to propene changes in an opposite trend. The adsorption and dehydrogenation of propane on Li/MgO surface are controlled by both thermodynamic and kinetic factors, whilst the dispersion of the active Li+O- sites is related to the loading of Li. Over the highly-dispersed active Li+O- sites, the dehydrogenation is thermodynamically controlled, which favors the formation of propene, whereas over the poorly-dispersed Li+O- sites, the reaction is dominated by the kinetic factor, leading to a high selectivity to ethene and other by-products.
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Key words:
- propane /
- oxidative dehydrogenation /
- Li/MgO /
- olefins /
- Li loading /
- dispersion
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图 2 未焙烧Li含量催化剂的DTA谱图
Figure 2 DTA profiles of the un-calcined Li/MgO catalysts with different Li loadings
a: m2; b: m8; c: m30; d: m80; e: m120; f: m250; g: m300
图 3 加入LiNO3后表面MgO水解、脱水、脱氮示意图
Figure 3 Sketch of hydrolyzation, dehydration and denitrification of MgO loading with LiNO3
图 4 不同Li含量催化剂的XRD谱图
Figure 4 XRD patterns of the Li/MgO catalysts with different Li loadings
●: MgO; ▽: Li2CO3; ◆: Mg (OH)2
图 5 Li负载量对丙烷脱氢性能的影响
Figure 5 Performance of Li/MgO catalysts with different Li loadings in propane ODH
图 6 空间尺寸示意图
Figure 6 Schematic diagrams of (a) propane molecule, (b) lattice structure of MgO, (c) reaction process over Li/MgO with low Li loading, (d) and (e) reaction process over Li/MgO with high Li loading
表 1 不同Li含量催化剂的比表面积以及Li原子表面密度
Table 1 Specific surface area and superficial density of the Li/MgO catalysts with different Li loadings
Sample Li loading wmol/% ABET /(m2·g-1) Pore volume v/(cm3·g-1) Li superficial density /(mol·nm-2) Atomic Li numbers per crystal lattice M0 0 107 1.05 M2 0.2 71 0.60 7.04×10-25 0.15 M8 0.8 39 0.43 5.16×10-24 1.11 M30 3 16 0.13 4.73×10-23 10.08 M80 8 10 0.09 2.06×10-22 42.87 M120 12 5 0.02 6.60×10-22 134.61 M250 25 2 0.03 2.87×10-21 550.25 M300 30 1 0.01 8.48×10-21 1 591.36 
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