(735e) Revisiting the Role of Solvent for Catalytic Conversion of Methane to Methanol Using H2O2 As an Oxidant

The direct conversion of methane to methanol is an economically beneficial alternative for synthesizing methanol. Recently, attention has been on the aqueous phase partial oxidation of methane. But the role of solvent and its effect on methanol selectivity remains unexplored. Here we used classical molecular dynamics (MD) and density functional theory (DFT) simulations with a few explicit solvent molecules to elucidate the reaction mechanism and the role of solvent on single atom Rh/ZrO₂ and Fe/ZSM-5 catalysts with H₂O₂ as the oxidant. The free energy of solvation of methanol was similar in pure water, sulfolane, and their mixtures, suggesting a similar capability of formation of solvated methanol from the catalytic surface. Preferential formation of Rh-O-O and Fe-OOH active oxygen species from H₂O₂ on Rh/ZrO₂ and Fe-ZSM-5, respectively, was observed in water. Rh-O and Fe-OH were the preferred active oxygen species formed from H₂O₂ in sulfolane. On Rh/ZrO₂ in water, methanol formed when O of Rh-O-O was inserted into a C-H bond of CH₄ (ΔE_r= -1.67 eV) while CH₃OOH formed via a water-mediated reaction as a side product. In sulfolane, methanol formed on the Rh-O as the primary product. The formation of methoxy species on the Zr site and subsequent oxidation towards CO₂ is energetically favorable in water and to a lesser extent in sulfolane, subject to the local hydrogen coverage and dynamics on the surface lattice oxygen atoms. These results suggest that solvents control the active oxygen site generation, directly participate in the reaction, and alter surface coverage of species, thereby tuning the product selectivity.