Isomerization of alkanes and alkenes on molybdenum oxides
Isomerization of alkanes and alkenes on molybdenum oxides
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DOI：
10.1016/s0926-860x(02)00216-8
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发表时间：
2003-01
影响因子：
5.5
通讯作者：
P. Wehrer;S. Libs;L. Hilaire
中科院分区：
文献类型：
--
作者：
P. Wehrer;S. Libs;L. Hilaire
研究了在350°C、1atm氢气快速流动下还原的纯MoO 3粉末上低碳烷烃和烯烃的异构化反应。不完全还原的MoO 3样品容易异构化烷烃的顺序nC 4 = nC 5 <nC 6 = nC 7 = nC 8。该速率至少与在铂金属上测量的速率一样高。烯烃的异构化总是比相应的烷烃快。由于水蒸气对钼氧化物还原的抑制作用，非常小的MoO 3样品表现出短暂的异构化性能，而较大的样品，其还原引起更高的水蒸气水平更长的活性。所有的碳氢化合物都有明显的中毒效应，这是由碳氢化合物沉积物引起的。在−195°C下对氮气和氩气的物理吸附表明，样品在还原时变成微孔。反应性样品的X射线分析总是揭示两种钼氧化物的存在，即MoO 2和迄今为止尚未确定的氧化物MoOx，已知其仅在低温下形成;此外，总是存在大量的非晶材料。XPS分析和催化结果表明，含Mo 6+和Mo 5+离子的低价氧化物不参与异构化反应，异构化位点的形成是由于MoO 3样品较深的还原状态。该反应的许多特征有利于双官能酸类型的机理。
Isomerization of light alkanes and alkenes was investigated on pure MoO3powder which was gradually reduced under a rapid flow of hydrogen (1atm) at 350°C. Incompletely reduced MoO3samples readily isomerize alkanes in the order nC4⪡nC5<nC6=nC7=nC8. The rates are at least as high as those measured on platinum metal. Isomerization of olefins is always faster than those of the corresponding alkanes. As a consequence of the inhibiting effect of water vapor on the reduction of the molybdenum oxides, very small MoO3samples exhibit short-lived isomerization properties whereas larger samples whose reduction gives rise to higher water vapor level are more longer active. With all hydrocarbons marked poisoning effects by hydrocarbonaceous deposits take place. Physical adsorption of nitrogen and argon at −195°C shows that the samples become microporous on reduction. X-ray analysis of reactive samples always reveal the presence of two molybdenum oxides, i.e. MoO2and an up-to-now undetermined oxide MoOx, which is known to form only at low temperatures; in addition large amounts of amorphous materials are always present. XPS analysis and the catalytic results suggest that the suboxides containing only Mo6+and Mo5+ions are not responsible for the isomerization; then the formation of the isomerizing sites results from a deeper reduction state of the MoO3sample. Many features of the reaction are in favor of a mechanism of the bifunctional acid type.