(411e) Low-Temperature Activity and Initiation in Silica-Supported Mo Olefin Metathesis Catalysts

Olefin metathesis can produce olefins of diverse structures and functionalities, with applications ranging from petrochemicals to pharmaceuticals. Extensive development of transition-metal-based molecular systems has led to homogeneous catalysts with exceptional room temperature activity and selectivity that are widely used in organic and polymer syntheses. Though likely operating via analogous mechanisms, heterogeneous catalysts based on supported Mo or W oxides, used industrially for light olefin conversion, typically require high temperature activation/reaction conditions. The initiation routes of metal oxo sites remain largely unknown, in part because the complex distributions and low quantities of active sites complicate analysis of reaction properties and spectroscopic features. Correspondingly, applications of heterogeneous metathesis catalysts have been limited.

Towards elucidation of structures and initiation pathways of supported olefin metathesis (pre)catalysts, we compare metathesis activity for well-defined molecular and supported Mo-based catalysts, establishing the influences of olefin-surface interactions on reactivity. Tailored metal-organic molecular oxo precursors are grafted onto partially dehydroxylated silica followed by thermal treatment to remove organic ligands, yielding isolated surface metal oxo species identified by X-ray absorption and solid-state NMR spectroscopies. These exhibit low-temperature (<100 °C) metathesis activity for metathesis of linear olefins after activation under reducing conditions, with catalytic properties that depend on synthetic route and corresponding local structures of the precatalytic species.

The supported Mo oxo metathesis catalysts are systematically compared to organometallic catalysts comprised of highly active molecular or supported Mo alkylidenes, which require no pre-activation, as well as catalysts prepared via conventional impregnation approaches. Despite different intrinsic reactivities, the supported catalysts all show reaction rates that depend on linear olefin substrate chain length due to adsorption of reaction products at surface -OH sites, as established by solid-state NMR analyses of post-reaction catalysts and molecular dynamics simulations. The analyses provide generalizable guidelines for design of improved metathesis catalysts by modification of support properties.