(705h) Low-Temperature Activity and Initiation in Reduced Supported Mo and W Olefin Metathesis Catalysts

Olefin metathesis is an atom-efficient reaction that can produce olefins of diverse structures and functionalities, with applications ranging from petrochemicals to pharmaceuticals. Extensive studies of molecular catalysts (M = Mo, W, Ru) have enabled development of homogeneous systems with high activity and selectivity at low temperatures (<100 °C) that are widely used in organic and polymer syntheses. Though they likely operate via the same reaction mechanisms, heterogeneous catalysts (Mo or W based), used industrially for light olefin conversion, typically require high temperature reaction conditions (>400 °C). The routes by which high-oxidation-state metal oxo sites form catalytically-active species remain largely unknown. This is due in part to the low quantities and complicated distributions of metal species (isolated sites, oxo oligomers, and/or oxide particles), which yield different reaction properties and spectroscopic signatures that are challenging to interpret. Correspondingly, the application of heterogeneous metathesis catalysts has been limited outside of light olefins.

To elucidate the initiation mechanisms of supported olefin metathesis catalysts, we synthesize well-dispersed W or Mo oxo sites via surface organometallic chemistry. Tailored molecular metal-organic oxo precursors are grafted onto partially dehydroxylated mesoporous supports (amorphous silica or alumina), followed by thermal treatment to remove organic ligands. This yields isolated surface metal oxo species, identified by X-ray adsorption spectroscopy, that are active for low-temperature (<100 °C) metathesis of diverse olefin substrates upon activation under reducing conditions, exhibiting improved activities compared to catalysts synthesized by conventional methods. M(IV) sites are precatalytic species, generating catalytically-active M(VI) alkylidenes in-situ, as demonstrated by comparisons to molecular model systems. Solid-state 1D and 2D ¹H and ¹³C NMR analyses of post-reaction catalysts identify substrate-dependent surface reaction intermediates that provide evidence for initiation of catalytic centers via allylic C-H activation. The results and analyses are generalizable to industrial systems, enabling new applications for heterogeneous olefin metathesis catalysts.