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The metathesis of ethylene with 2-butene to propene is currently applied for large-scale on-purpose production of this building block of the chemical industry. MoOx-based supported catalysts are promising alternatives to the industrially applied WOx-containing systems. The activity of such catalysts depends on the amount of active Mo–carbene (MoCHR) sites formed in situupon reaction of supported MoOxspecies with olefins. Herein, we demonstrate a way to control the ability of such species on the surface of AlSiOxto be transformed into Mo–carbenes by promoting the support with tiny amounts of alkali or alkaline earth metal (Li, Sr, Ca, Mg, Na, K, Ba, Rb, or Cs) oxides. The electrophilic properties of these additives were established to be relevant for in situtransformation of supported MoOxspecies into Mo–carbenes. The additive electronegativity (Pauling scale) affects olefin adsorption and therefore the formation of carbenes.

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/SiO2can 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/SiO2is required to efficiently use the enhancing effect of the cocatalyst. H/D isotopic exchange tests with 2-C4H8and 2-C4D8suggest 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/SiO2can be regenerated without changing their initial performance as proven in several metathesis/regeneration cycles.