Role of Surface Silanols in Active Site Formation during Olefin Metathesis over a WO _x /SiO₂ Catalyst: A Computational Perspective

A WOx/SiO2 system is the major industrial catalyst for olefin metathesis, but the mechanism of the active site formation from the surface tungsten oxide species is not yet fully understood. In this work, detailed density functional theory (DFT) studies of the initiation mechanisms for olefin metathesis over the WOx/SiO2 catalyst have been carried out. It is shown that dioxo W(VI) species can be reduced to monooxo W(IV) species by alkenes or activated without reduction to form W(VI) alkylidene species. A surface silanol group interacting with a W species plays a key role in these transformations. The first step, protonation of the alkene to generate W(VI) alkoxy species, is common to both pathways. The monooxo W(IV) species can be oxidized to form finally a W(VI) alkylidene site through an alkene complex or W(VI) hydride species. All of these mechanisms are competitive with each other, and the kinetic preferences depend on the geometry of the W sites and the local structure of the silica support. The structure-reactivity relationship is complex. The proposed routes of silanol-assisted reduction and activation are kinetically preferred over the non-silanol-assisted formation of the active sites, i.e., the pseudo-Wittig mechanism, vinylic C-H activation, allylic C-H activation, and oxidative coupling-1,4-hydrogen shift mechanism.