Mechanistic study of heterogeneous propene metathesis on WOx/SiO2 catalysts

• Published in: Journal of catalysis 2023-11, Vol.427, p.115117, Article 115117
• Main Authors: Le, Anne V., Rajbanshi, Biplab, Lobo, Raul F., Bai, Peng
• Format: Article
• Language: English
• Subjects: Heterogeneous catalysis; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Olefin metathesis; Silica-supported tungsten oxides
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2023.115117

[Display omitted] •Olefin metathesis catalyzed by supported WOx/SiO2 largely follows the Chauvin cycle.•Alkylidenes function as the active species that interconvert with metallacyclobutanes.•Activation barriers are higher for active-site formation than for the metathesis cycle.•The pseudo-Wittig mechanism is most favorable for generating the alkylidene active sites. Silica-supported tungsten oxides are widely used industrial catalysts for olefin metathesis due to their low cost and robustness, yet the mechanisms for heterogeneously catalyzed metathesis reactions remain comparatively less understood. In this work, density-functional theory (DFT) calculations were used to study the model reactions of propene metathesis. Our calculations confirm that the metathesis reactions catalyzed by WOx/SiO2 largely follow the Chauvin cycle, with an overall energetic barrier of 142 kJ/mol. To understand how the initial alkylidene active sites are generated, three mechanisms were examined: The pseudo-Wittig mechanism was found to be most favorable and proceeds with a metallacycle intermediate, while the allylic and vinylic CH activations are much more difficult and require the reduction of surface sites to W(+4). Relative to the adsorbed reactant state, the overall intrinsic barriers for three mechanisms were computed to be 193, 261, and 355 kJ/mol, respectively. The higher barriers for active-site formation than for the metathesis cycle are consistent with the difficult, high-temperature pretreatment required in experiments to activate WOx/SiO2 catalysts.