Active Site Descriptors from 95Mo NMR Signatures of Silica-supported Mo-based Olefin Metathesis Catalysts

The catalytic activity of silica-supported molybdenum oxides for olefin metathesis depends strongly on the metal loading and preparation conditions indicating that the nature and/or amounts of the active sites vary across catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation with different metal loadings (2.5-15.6 wt% Mo) and a well-defined model material (2.3 wt% Mo) prepared via a surface organometallic chemistry (SOMC) synthetic approach. Analyses of FTIR, UV-vis, and Mo K-edge X-ray absorption spectra provide strong evidence that all the (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites; however, they exhibit very different proportions of reducible surface sites and catalytic reaction properties. Specifically, the SOMC-derived catalyst is more active for liquid and gas-phase olefin metathesis conditions than a classical catalyst of similar Mo loading by a factor of 1.5-1.9, depending on precise reaction conditions. Most notably, solid-state 95Mo NMR spectra of these catalysts show distinct features, particularly evident under state-of-the-art high-field (28.2 T) measurement conditions where at least four distinct types of surface Mo dioxo sites are resolved, the distribution of which depends on the preparation methods. In particular, the presence of Mo sites with a specific deshielded 95Mo NMR signal correlates with the catalysts reducibility and metathesis activity; such sites are most prominent in the SOMC-derived catalyst. First-principles calculations show that the 95Mo NMR parameters, specifically the isotropic chemical shift and quadrupolar coupling constant, are good descriptors for local strain and coordination environment: acute (SiO-Mo(O)2-OSi) angles and low coordination numbers at the Mo sites leads to highly deshielded iso chemical shifts and small CQ values, respectively. Orbital and natural chemical shift analyses indicate that the deshielded 95Mo iso values of strained species are directly related to low LUMO energies, consistent with their higher reducibility and corresponding reactivity. Overall, this study shows that solid-state NMR is particularly powerful for the identification of distinct supported Mo dioxo species and that their 95Mo chemical shifts are related to their specific local electronic structures, providing a powerful descriptor for the propensity of Mo sites towards reduction and the formation of active sites.