Synthesis and Aerobic Oxidation Catalysis of Mesoporous Todorokite-Type Manganese Oxide Nanoparticles by Crystallization of Precursors

The pursuit of a high surface area while maintaining high catalytic performance remains a challenge due to a trade-off relationship between these two features in some cases. In this study, mesoporous todorokite-type manganese oxide (OMS-1) nanoparticles with high specific surface areas were synthesized in one step by a new synthesis approach involving crystallization (i.e., solid-state transformation) of a precursor produced by a redox reaction between MnO4– and Mn2+ reagents. The use of a low-crystallinity precursor with small particles is essential to achieve this solid-state transformation into OMS-1 nanoparticles. The specific surface area reached up to ca. 250 m2 g–1, which is much larger than those (13–185 m2 g–1) for Mg-OMS-1 synthesized by previously reported methods including multistep synthesis or dissolution/precipitation processes. Despite ultrasmall nanoparticles, a linear correlation between the catalytic reaction rates of OMS-1 and the surface areas was observed without a trade-off relationship between particle size and catalytic performance. These OMS-1 nanoparticles exhibited the highest catalytic activity among the Mn-based catalysts tested for the oxidation of benzyl alcohol and thioanisole with molecular oxygen (O2) as the sole oxidant, including highly active β-MnO2 nanoparticles. The present OMS-1 nanomaterial could also act as a recyclable heterogeneous catalyst for the aerobic oxidation of various aromatic alcohols and sulfides under mild reaction conditions. The mechanistic studies showed that alcohol oxidation proceeds with oxygen species caused by the solid, and the high surface area of OMS-1 significantly contributes to an enhancement of the catalytic activity for aerobic oxidation.