Computational Insights into Active Site Formation during Alkene Metathesis over a MoOx/SiO2 Catalyst: The Role of Surface Silanols

A MoOx/SiO2 system is an effective catalyst for alkene metathesis; however, the mechanism of the transformation of the surface metal oxide species into active alkylidene sites is not well recognized. In this work, comprehensive density functional theory studies of the initiation mechanisms for alkene metathesis on the MoOx/SiO2 catalyst have been performed. It is shown that surface silanol groups interacting with Mo species and constituting Brionsted acid sites can play a key role in reduction of the dioxo Mo(VI) species to the mono-oxo Mo(IV) species by alkene, through Mo(VI) alkoxy species, and in subsequent formation of the Mo(VI) alkylidene species. An alternative activation pathway avoiding the reduction step is also possible. The proposed mechanisms of silanol-assisted reduction/initiation with propene are predicted to be more kinetically and thermodynamically accessible than the often assumed pseudo-Wittig mechanism. The silanol-assisted activation of the mono-oxo Mo(IV) species by propene is kinetically preferred over non-silanol-assisted initiation mechanisms, that is, 1,2-hydrogen shift mechanism, ally! mechanism, and oxidative coupling mechanism involving molybdacyclopentane species. The reactivity of the Mo sites is significantly affected by their geometry and the local structure of silica. Our results suggest that only a small fraction of the Mo oxide species with a suitable geometry and neighborhood can be effectively activated by alkenes.