DFT insights into the oxygen-assisted selective oxidation of benzyl alcohol on manganese dioxide catalysts

The reactivity pattern of the MnO₂ catalyst in the selective aerobic oxidation of benzyl alcohol is assessed by density functional theory (DFT) analysis of adsorption energies and activation barriers on a model Mn₄O₈ cluster. DFT calculations predict high reactivity of defective Mn(IV) sites ruling a surface redox mechanism, L-H type, involving gas-phase oxygen. Bare and promoted (i.e., CeO_x and FeO_x) MnO_x materials with high surface exposure of Mn(IV) sites were synthesized to assess kinetic and mechanistic issues of the selective aerobic oxidation of benzyl alcohol on real catalysts (T, 333–363 K). According to DFT predictions, the experimental study shows: i) comparable activity of bare and promoted catalysts due to surface Mn(IV) sites; ii) the catalytic role of oxygen-atoms in the neighboring of active Mn(IV) sites; and iii) a 0th-order dependence on alcohol concentration, diagnostic of remarkable influence of adsorption phenomena on the reactivity pattern. Evidences of catalyst deactivation due to the over-oxidation of benzyl alcohol to benzoic acid, acting as poison of the active sites, are discussed.