Transformations of tungstacyclopentane species in a WOx/SiO2 catalyst: Different pathways generating olefin metathesis active sites

Surface tungstacyclopentane species can be formed from alkenes and W(IV) oxide species in the industrial WO_x/SiO₂ catalyst for large scale olefin metathesis. In this work, density functional theory (DFT) studies of various experimentally observed transformations of silica-supported unsubstituted tungstacyclopentane species, leading mainly to the formation of olefin metathesis active sites, have been performed. It is shown that the conversion of square pyramidal tungstacyclopentane to W(VI) butylidene species via 1,4-hydrogen transfer is slightly more kinetically favored than the ring contraction to α-methyl-substituted tungstacyclobutane through W(VI) butenyl hydride intermediate. In the case of tungstacyclopentane species with the trigonal bipyramidal geometry, the ring contraction can also occur in a single step. Ethene insertion into the secondary W-C bond of the α-methyl-substituted tungstacyclobutane, giving γ-methyl-substituted tungstacyclohexane species, has also been examined. The activation barriers of the tungstacyclopentane transformations can be reduced by about 50 kJ mol⁻¹ if a surface silanol group adjacent to the metal site takes part. Initially, W(VI) butyl species are formed, which can then convert to W(VI) butylidene species, α-methyl-substituted tungstacyclobutane or 1-butene together with the monooxo W(IV) species. The proposed silanol-assisted transformations of surface tungstacyclopentane species yielding olefin metathesis active sites may compete with other reported activation mechanisms involving a silanol group.