title: Role of Tricoordinate Al Sites in CH3ReO3/Al2O3 Olefin Metathesis Catalysts.
abstract: 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/Al2O3, 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 (AlIII) and CH2 bound to a four-coordinated Al (AlIVb) 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 (AlIVa/AlIVa). 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 AlIII-defect sites and adsorbed H2O.
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1-5, 8093 Zurich, Switzerland.
authors.forename: Maxence
authors.initials: M
authors.lastname: Valla
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1-5, 8093 Zurich, Switzerland.
authors.forename: Raphael
authors.initials: R
authors.lastname: Wischert
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1-5, 8093 Zurich, Switzerland.
authors.forename: Aleix
authors.initials: A
authors.lastname: Comas-Vives
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1-5, 8093 Zurich, Switzerland.
authors.forename: Matthew P
authors.initials: MP
authors.lastname: Conley
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1-5, 8093 Zurich, Switzerland.
authors.forename: René
authors.initials: R
authors.lastname: Verel
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1-5, 8093 Zurich, Switzerland.
authors.forename: Christophe
authors.initials: C
authors.lastname: Copéret
authors.affiliation: Univ Lyon, Ens de Lyon, CNRS, Université Lyon 1 , Laboratoire de Chimie UMR 5182, F 69342 Lyon, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Sautet