(176e) Influence of Olefin-Surface Interactions on Liquid-Phase Activity of Silica-Supported Mo-Based Olefin Metathesis Catalysts

Olefin metathesis is a key technology for the formation of C=C bonds by rearrangement of alkylidene fragments among olefins. Decades of research have yielded homogeneous olefin metathesis catalysts based on Mo-, W-, and Ru-alkylidenes that are highly active and selective at mild temperatures in the liquid phase, enabling broad applications in organic and polymer syntheses. By comparison, 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 (150-400 °C). They are composed of ill-defined surface structures with low (<5%) quantities of active sites. These shortcomings have limited their broader adoption for liquid-phase catalysis.

In order to understand and expand the applications of supported metathesis catalysts, we investigate the low-temperature (<100 °C) liquid phase activities of a series of supported and molecular Mo-based olefin metathesis catalysts towards long-chain linear α-olefins (C₈-C₂₀). Different catalysts prepared using surface organometallic chemistry approaches, including silica-supported reduced Mo oxides and well-defined organometallic Mo alkylidenes in silica-supported and molecular forms, display distinct and unexpected reactivity patterns. Mo oxide-based catalysts exhibit slower reaction rates as a function of substrate chain length, while molecular and supported Mo alkylidenes are highly active without such dramatic dependence on substrate structure. State-of-the-art sensitivity enhanced solid-state NMR analyses of post-metathesis catalysts establish the strong adsorption of internal olefin metathesis products on the catalyst surface near surface Si-OH groups, limiting catalyst efficiency for the Mo oxide-based systems. These insights are corroborated by FTIR spectroscopy and molecular dynamics calculations. The results show that in addition to the nature and number of active sites, the metathesis rates and overall catalytic performance in the liquid phase depend on olefin-surface interactions, highlighting the role of support and active site composition, dynamics, and adsorption properties on activity.