Role of Tricoordinate Al Sites in CH3ReO3/Al2O3 Olefin Metathesis Catalysts

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• 作者：Maxence Valla ; Raphael Wischert ; Aleix Comas-Vives ; Matthew P. Conley ; René Verel ; Christophe Copéret ; Philippe Sautet
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：June 1, 2016
• 年：2016
• 卷：138
• 期：21
• 页码：6774-6785
• 全文大小：721K
• 年卷期：0
• ISSN：1520-5126

文摘

Re₂O₇ supported on γ-alumina is an alkene metathesis catalyst active at room temperature, compatible with functional groups, but the exact structures of the active sites are unknown. Using CH₃ReO₃/Al₂O₃ as a model for Re₂O₇/Al₂O_3, we show through a combination of reactivity studies, in situ solid-state NMR, and an extensive series of DFT calculations, that μ-methylene structures (Al-CH₂–ReO₃–Al) containing a Re═O bound to a tricoordinated Al (Al_III) and CH₂ bound to a four-coordinated Al (Al_IVb) are the precursors of the most active sites for olefin metathesis. The resting state of CH₃ReO₃/Al₂O₃ is a distribution of μ-methylene species formed by the activation of the C–H bond of CH₃ReO₃ on different surface Al–O sites. In situ reaction with ethylene results in the formation of Re metallacycle intermediates, which were studied in detail through a combination of solid-state NMR experiments, using labeled ethylene, and DFT calculations. In particular, we were able to distinguish between metallacycles in TBP (trigonal-bipyramidal) and SP (square-pyramidal) geometry, the latter being inactive and detrimental to catalytic activity. The SP sites are more likely to be formed on other Al sites (Al_IVa/Al_IVa). Experimentally, the activity of CH₃ReO₃/Al₂O₃ depends on the activation temperature of alumina; catalysts activated at or above 500 °C contain more active sites than those activated at 300 °C. We show that the dependence of catalytic activity on the Al₂O₃ activation temperature is related to the quantity of available Al_III-defect sites and adsorbed H₂O.