Oxidative Dehydrogenation of Propane by Monomeric Vanadium Oxide Sites on Silica Support

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• 作者：Xavier Rozanska ; Ré ; my Fortrie ; Joachim Sauer
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：April 26, 2007
• 年：2007
• 卷：111
• 期：16
• 页码：6041 - 6050
• 全文大小：254K
• 年卷期：v.111,no.16(April 26, 2007)
• ISSN：1932-7455

文摘

We investigate possible mechanisms of oxidative dehydrogenation of propane using density functional theory.Monomeric vanadium oxide species supported on silica are modeled by vanadyl-substituted silsesquioxane.Similarly to other catalysts with transition metal oxo bonds, the initial C-H bond activation step is hydrogenabstraction by the vanadyl (O=V^V) group yielding a diradical intermediate in which a propyl radical is boundto a HO-V^IV site. This is followed by a propyl rebound mechanism yielding alkoxide or alcohol attached toa V^III(OSi)₃ surface site from which propene can be formed. Propene is also directly obtained by a secondhydrogen abstraction from the diradical intermediate. Desorption of propyl radicals leads to a stationaryconcentration of propyl in the gas phase and leaves reduced HO-V^IV sites on the surface. Due to fast reoxidationtheir concentration is much smaller than the concentration of O=V^V sites. Therefore the rate of propeneformation after readsorption on O=V^V sites is much larger than the rate of isopropyl alcohol (or propene)formation after readsorption on HO-V^IV sites. Generation of surface propyl radicals by the first hydrogenabstraction becomes rate limiting. We predict that at 750 K the apparent activation energy is 123 ± 5 kJ/moland the rate constant is about 0.26 s^-1, in close agreement with experiments. The first hydrogen abstractionoccurs exclusively on O=V^V sites, while the second hydrogen abstraction can also occur on V-O-Si bridgingoxygen sites.