Promoting active site renewal in heterogeneous olefin metathesis catalysts.

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 industries1. Notably, the thermoneutral and highly selective cross-metathesis of ethylene and 2-butenes1 offers an appealing route for the on-purpose production of propylene to address the C3 shortfall caused by using shale gas as a feedstock in steam crackers2,3. However, key mechanistic details have remained ambiguous for decades, hindering process development and adversely affecting economic viability4 relative to other propylene production technologies2,5. 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 Brønsted 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 °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.

Activation of Methyltrioxorhenium for Olefin Metathesis in a Zirconium-Based Metal-Organic Framework.

The zirconium nodes of the metal-organic framework (MOF) known as NU-1000 serve as competent supports for the activation of methyltrioxorhenium (MTO) toward olefin metathesis. Itself inactive for olefin metathesis, MTO becomes an active catalyst only when immobilized on the strongly acidic Lewis acid sites of dehydrated NU-1000. Uptake of MTO at the dehydrated secondary building units (SBUs) occurs rapidly and quantitatively to produce a catalyst active in both gas- and liquid-phase processes. These results demonstrate for the first time the utility of MOF SBUs for olefin metathesis, an academically and industrially relevant transformation.