Atomistic Modeling of the Active Site Formation Mechanism and Olefin Metathesis Kinetics for WO_<i>x</i>/SiO₂

Atomically dispersed WOx on amorphous silica (WOx/SiO2) is known to be an active catalyst for olefin metathesis. There has been longstanding interest in determining the structure of the WOx/SiO2 active sites, the mechanism of their activation from their precursor state, and characterizing their activity toward olefin metathesis. Atomistic knowledge is lacking about the formation mechanism and the identity of the kinetically active W-alkylidene site for ethene/trans-2-butene (C2/C4) metathesis to produce propene (C3). Herein, we analyze the active site formation mechanism and C2/C4 metathesis kinetics for WOx/SiO2 using density functional theory calculations and mean-field microkinetic modeling. We predict that the pseudo-Wittig reaction is the predominant mechanism for W-alkylidene active site formation from the most-abundant W6+ digrafted dioxo precursor (S0) using either C2 or C4 as the activating reagent. Although the W-ethylidene site could be formed by C2 or C4 reacting with S0, the W-methylidene and W-butylidene sites are formed only through S0 activation with C2 and C4, respectively. Microkinetic analysis predicts that the W-methylidene site exhibits an order of magnitude higher rate constant for metathesis reaction to C3 compared to the W-ethylidene site.