CH₃-ReO₃ on γ-Al₂O₃: Activity, selectivity, active site and deactivation in olefin metathesis

The active sites (15% of total Re) of CH₃ReO₃ supported on alumina arise solely from the reaction of the C{single bond}H bond of CH₃ReO₃ with reactive Al_S{single bond}O_S sites of a γ-Al₂O₃ surface to yield [Al_SCH₂ReO₃], whereas the major species, inactive, corresponds to MeReO₃ chemisorbed through its oxo ligand(s) on Lewis acid sites of alumina. Monitoring the active sites of CH₃ReO₃ supported on alumina by solid-state NMR spectroscopy shows that no carbenic signals are observed and that the resting states of the catalyst in the metathesis of propene correspond to μ-methylene and μ-ethylidene species, so that the propagating carbenic species are likely formed only in the presence of olefins. Although this system is highly active in the metathesis of propene, with initial rates similar to some of the best catalysts, it undergoes fast deactivation, which is due to the propene metathesis products, most likely ethene.