Polymorph-Dependent Oxidation Activity of MnO2: Influence of Surface Water, Morphology, and Surface Area in Benzylic Oxidation

Direct comparisons of MnO2 polymorphs in synthetic benzylic oxidation remain limited, particularly regarding the combined effects of crystallinity, morphology, surface area, and surface-associated water. In this study, α-, β-, and γ-MnO2 were prepared by hydrothermal methods and benchmarked against nano-MnO2 for the oxidation of diphenylmethane to benzophenone. XRD, TGA, FTIR, BET, and SEM analyses confirmed phase-defined crystalline polymorphs, distinct wire- or rod-like morphologies, and marked differences in surface area and water retention. Nano-MnO2 exhibited a porous, poorly crystalline nanoscale structure with the highest surface area and delivered the greatest oxidation efficiency under aerobic, atmospheric, and anaerobic conditions. Among the crystalline phases, α-MnO2 showed the highest activity despite its lower BET surface area than β-MnO2, indicating that surface-associated water is more influential than surface area alone. The loss of activity after drying at 120 °C, prolonged storage, or reuse further supports the critical role of labile surface-bound water. Overall, this work establishes a structure–morphology–water–reactivity relationship for MnO2-mediated arylmethylene oxidation and identifies water-rich nano-MnO2 as the most effective material for converting benzylic substrates to ketones.