(391b) Descriptor Analysis for Organic Reductants Capable of Enhancing Active Site Formation in Mo-Based Heterogenous Olefin Metathesis Catalysts

Heterogeneous olefin metathesis has gained significant recent interest due to a series of novel methods for upcycling plastics, helping solve the propylene gap, and production of renewable alkenes from biomass via ethenolysis. Even with this much industrial interest, surprising debate remains in basic attributes of this fascinating reaction chemistry, including the active site formation mechanism for traditional heterogeneous systems. Recent work has highlighted how promoters, often acting like reducing agents, but not always presented as such, like methanol, methane, ethane, ethene, propene, and even organosilicons can be used to drastically increase the population of active sites, while common reductants like H₂ and CO do not increase populations to equivalent levels. This highlights an important question, what descriptors govern supported Mo oxide sites transitioning into olefin metathesis active sites when supported on silica? This work experimentally compared a series of olefins, alkanes, and alcohols of varying structure used as organic reducing agents, seeking to identify the characteristics required to form heterogeneous olefin metathesis active sites. Using gas-phase reactions, temperature programmed reductions coupled with Kissinger analyses, traditional thermochemical calculations, in-situ DRIFTS, and ambient pressure X-ray photoelectron spectroscopy our group identified the free energy of reduction for single sites of MoO₃ supported on silica and kinetic differences as a function reducing agent chemical structure. Hydrocarbon-based soft reductants show two temperature regimes in site creation, a low temperature regime, which is governed by soft reductant structure, and a high temperature regime that is structure agnostic, resulting in H₂ and aromatic formation. We identify thermodynamic and kinetic differences between individual or synergistic soft reductants to control olefin metathesis active site populations. Our results, combined with literature results, support the hypothesis that understanding the descriptors that govern metal oxide single site reduction can facilitate increases in the population of traditional heterogeneous olefin metathesis active sites.