(173c) Computational Study of Heterogenous Propene Metathesis on WOx/SiO2 Catalysts

Low recycling rates and continued increase of plastic production have made plastic recycling at the forefront of engineering challenges. Alkane metathesis, consisting of tandem alkane (de-)hydrogenation and olefin metathesis reactions, provides a potential route for the catalytic deconstruction of polyolefins into fuels, lubricants, and functionalized alkane oligomers. Despite being practiced industrially for more than 50 years, there seem to be remaining gaps in our understanding of heterogeneously catalyzed metathesis reactions. Silica-supported tungsten oxides, WO_x/SiO₂, a widely used industrial metathesis catalyst due to its low costs and robustness, appear to be less studied than ReO_x/SiO₂ and MoO_x/SiO₂. In this work, we investigate the model reactions of propene metathesis on WO_x/SiO₂ catalysts using periodic ab-initio density-functional theory methods. Our calculations confirm that heterogenous metathesis largely follows Chauvin’s homogenous metathesis cycle with an overall energetic barrier of ~140 kJ/mol. Our results further show that the formation of the initial alkylidene active species has a barrier of 198 kJ/mol for the most favorable pseudo-Wittig mechanism, followed by allylic and vinylic hydride transfer mechanisms (235 kJ/mol and >300 kJ/mol, respectively). However, the initial steps of the allylic mechanism can occur relatively easily, leading to acetone formation and some of the tungsten sites trapped in an unproductive state. These results provide plausible explanations for the experimental observations that WO_x/SiO₂ requires a high temperature activation (>300°C), a flow system or adsorbent materials to continuously remove oxygenates, and a low number of tungsten sites (< 10%) being catalytically active.