Atomistic Modeling of the Active Site Formation Mechanism and Olefin Metathesis Kinetics for WOx/SiO2

Abstract

Atomically dispersed WO_x on amorphous silica (WO_x/SiO₂) is known to be an active catalyst for olefin metathesis. There has been longstanding interest in determining the structure of the WO_x/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 WO_x/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.