Insights into the catalytic promotion of propylene self-metathesis over silica-supported molybdenum oxide using substituted olefins

Olefin metathesis is a versatile strategy for functional group interconversion around C=C bonds. Here, we investigate in detail a recently discovered promotional effect, where co-feeding 2,3-dimethyl-2-butene isomers (4MEs) increases propylene self-metathesis rates on silica-supported Mo and W oxides by orders of magnitude. Through detailed kinetic measurements on MoOx/SiO2, we validate a dynamic site renewal and decay cycle, analogous to WOx/SiO2, which operates in tandem with the Chauvin cycle and can be effectively modulated by co-feeding 4MEs. Active site titrations indicate that the promotional effect results from an increased density of active sites rather than enhanced per-site catalytic activity. Spectroscopic analyses reveal that the renewal and decay of Mo alkylidene active sites involve proton-transfer mediated by proximal acidic Si-OH groups. Additionally, the co-fed promoters not only reduce Mo (VI) to Mo (IV), thereby increasing the number of pre-active sites, but also act as proton relays, enhancing proton-transfer steps. This dual functionality elucidates the mechanism underlying the enhanced metathesis activity observed with promoter addition.