Abstract

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.

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