(177b) Understanding Surface Structures and Reactivity of Heterogeneous Olefin Metathesis Catalysts through Sensitivity-Enhanced Solid-State NMR

Olefin metathesis is a key technology for the formation of C=C bonds by rearrangement of alkylidene fragments among olefins. Heterogeneous olefin metathesis catalysts, mostly based on supported Mo or W oxides, are industrially used for the upgrading of light olefins but require high-temperature activation and/or reaction conditions (>150 °C). They are composed of ill-defined surface structures with low (<5%) quantities of active sites that are proposed to require complex initiation processes. These shortcomings have limited their broader adoption. Here, by leveraging the synthetic principles of surface organometallic chemistry (SOMC), monodispersed silica-supported Mo or W oxo sites are generated as well-defined analogues to classical catalysts prepared via impregnation. The SOMC-derived catalysts are amenable to spectroscopic analysis, most notably by solid-state NMR, a powerful tool for establishing the types and distributions of surface sites and their relation to macroscopic reactivity. Recent advances in solid-state NMR methodology and instrumentation enable dramatic sensitivity enhancements and measurement of 2D heteronuclear correlation spectra of surface sites, reaction products, and/or adsorbates, providing direct molecular-level insights into surface structures and interactions. For example, solid-state NMR analyses of supported Mo- and W-based catalysts with probe molecule adsorbates establish the presence of strong Brønsted acid sites associated with the metal oxo centers. These Brønsted acid sites are linked to the initiation of the catalytic centers under propene metathesis conditions by ex-situ 2D ¹³C-¹H heteronuclear correlation analysis of post-reaction catalysts, complemented by in-situ FTIR spectroscopy and first-principles calculations. Extension of these methods to catalysts after liquid phase metathesis reactions establish the role of olefin-surface adsorption phenomena in determining low-temperature (<100 °C) reaction properties. Finally, we explore new NMR-based methods to access the surface structures of (pre)catalytic metal sites. The results elucidate the origins of olefin metathesis catalyst activity and are leveraged to develop improved catalyst materials with desired reaction properties.