Promoting active site renewal in heterogeneous olefin metathesis catalysts

Abstract

As an atom-efficient strategy for the large-scale interconversion of olefins, heterogeneously catalysed olefin metathesis sees commercial applications in the petrochemical, polymer and speciality chemical industries(1). Notably, the thermoneutral and highly selective cross-metathesis of ethylene and 2-butenes(1) offers an appealing route for the on-purpose production of propylene to address the C-3 shortfall caused by using shale gas as a feedstock in steam crackers(2,3). However, key mechanistic details have remained ambiguous for decades, hindering process development and adversely affecting economic viability(4) relative to other propylene production technologies(2,5). Here, from rigorous kinetic measurements and spectroscopic studies of propylene metathesis over model and industrial WOx/SiO2 catalysts, we identify a hitherto unknown dynamic site renewal and decay cycle, mediated by proton transfers involving proximal BrOnsted acidic OH groups, which operates concurrently with the classical Chauvin cycle. We show how this cycle can be manipulated using small quantities of promoter olefins to drastically increase steady-state propylene metathesis rates by up to 30-fold at 250 degrees C with negligible promoter consumption. The increase in activity and considerable reduction of operating temperature requirements were also observed on MoOx/SiO2 catalysts, showing that this strategy is possibly applicable to other reactions and can address major roadblocks associated with industrial metathesis processes. The authors identify a dynamic site renewal and decay cycle, mediated by proton transfers involving proximal BrOnsted acidic OH groups, which operates concurrently with the Chauvin cycle and could address roadblocks associated with industrial metathesis processes.