Fundamentals and application potential of the synergy effect between ZnO and Mo/SiO2 for propene production in the metathesis of ethylene and trans-2-butene

The metathesis of ethylene with 2-butene to propene is a 100%-carbon-efficient process for on-purpose production of this important building block in the chemical industry. Herein, we demonstrate that the activity of Mo/SiO2 can be increased up to 270-fold when using ZnO as a cocatalyst, which is inert for the target reaction. An intimate physical contact between ZnO and Mo/SiO2 is required to efficiently use the enhancing effect of the cocatalyst. H/D isotopic exchange tests with 2-C4H8 and 2-C4D8 suggest that the enhancing effect depends on the ability of ZnO to activate 2-butene. A gas-phase promoter (not 1-butene but probably butene radical) formed in this reaction is suggested to contribute to the formation of catalytically active Mo-carbenes under reaction conditions and/or hinder their transformation into inactive ones. This fundamental knowledge opens the possibility for further improvements through catalyst or cocatalyst design and reactor operation. The obtained results are also attractive from a practical point of view. Both ZnO and Mo/SiO2 can be regenerated without changing their initial performance as proven in several metathesis/regeneration cycles.