Template-Free Synthesis of Mesoporous β-MnO2 Nanoparticles: Structure, Formation Mechanism, and Catalytic Properties.

Mesoporous β-MnO2 nanoparticles were synthesized by a template-free low-temperature crystallization of Mn4+ precursors (low-crystallinity layer-type Mn4+ oxide, c-distorted H+-birnessite) produced by the reaction of MnO4- and Mn2+. The Mn starting materials, pH of the reaction solution, and calcination temperatures significantly affect the crystal structure, surface area, porous structure, and morphology of the manganese oxides formed. The pH conditions during the precipitation of Mn4+ precursors are important for controlling the morphology and porous structure of β-MnO2. Nonrigid aggregates of platelike particles with slitlike pores (β-MnO2-1 and -2) were obtained from the combinations of NaMnO4/MnSO4 and NaMnO4/Mn(NO3)2, respectively. On the other hand, spherelike particles with ink-bottle shaped pores (β-MnO2-3) were formed in NaMnO4/Mn(OAc)2 with pH adjustment (pH 0.8). The specific surface areas for β-MnO2-1, -2, and -3 were much higher than those for nonporous β-MnO2 nanorods synthesized using a typical hydrothermal method (β-MnO2-HT). On the other hand, c-distorted H+-birnessite precursors with a high interlayer metal cation (Na+ and K+) content led to the formation of α-MnO2 with a 2 × 2 tunnel structure. These mesoporous β-MnO2 materials acted as effective heterogeneous catalysts for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastic monomer and for the transformation of aromatic alcohols to the corresponding aldehydes, where the catalytic activities of β-MnO2-1, -2, and -3 were approximately 1 order of magnitude higher than that of β-MnO2-HT. β-MnO2-3 exhibited higher catalytic activity (especially for larger molecules) than the other β-MnO2 materials, and this is likely attributed to the nanometer-sized spaces.