Propagation of Olefin Metathesis to Propene on WO₃ Catalysts: A Mechanistic and Kinetic Study

The production of propene (H 2 C═CH–CH 3 ) has attracted widespread interest due to industrial demand, and an intriguing route involves the metathesis of ethene (H 2 C═CH 2 ) and trans -2-butene (H 3 C–CH═CH–CH 3 ). We recently investigated the initiation of olefin metathesis on tungsten trioxide (WO 3 ) catalysts, by showing that trans -2-butene more readily forms W-carbene active sites on the surface, compared to ethene. We now present our analysis of the propagation steps at these active sites. We have computed optimized structures and energetics of the reaction intermediates and products of propagation. We have also computed the rate constants for the elementary steps comprising the reaction mechanism. These results are used to elucidate a complete mechanistic description of the metathesis of ethene and trans -2-butene. We find that W═CH 2 sites are less active in the propagation process compared to W═CHCH 3 . Furthermore, [2 + 2] cycloaddition of trans -2-butene to form the six-membered (oxa)metallacycle ring is the rate-determining step; the rate constant is more sensitive to temperature and the reaction becomes faster than (oxa)metallacycle decomposition when the temperature exceeds 750 K. By revealing the relationship between the WO 3 surface structure and catalytic activity for olefin metathesis at different temperatures, we aim to use this information to predictively guide the development of more active and selective catalysts for propene production.