Structural and morphological evolution of mesoporous α-MnO2 and β-MnO2 materials synthesized via different routes through KMnO4/H2C2O4 reaction

• Mesoporous α-MnO2 and β-MnO2 were prepared using KMnO4 and H2C2O4.
• The morphology of the as-prepared α-MnO2 and β-MnO2 was different.
• The mechanism of the morphological and textural change of the materials was proposed.
• The materials possessed good catalytic activity for oxidation of o-xylene.

Mesoporous α-MnO2 and β-MnO2 were prepared using KMnO4 and H2C2O4 (Oxalate) as precursors via aqueous and hydrothermal routes respectively. The morphological and structural evolution of the MnO2 materials were tracked by Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. The preparation of MnO2 materials was found to proceed through four steps: well-crystallized α-MnO2 was formed first in aqueous solution, and then translated into microcrystalline α-MnO2 microspheres consisting of nanoparticles and nanowires via a dissolution-recrystallization process. Subsequently, the microcrystalline α-MnO2 microspheres collapsed, and γ-MnOOH nanowires were formed. Finally, β-MnO2 nanowires were obtained after calcining the γ-MnOOH nanowires in air at 400 °C for 6 h. During the process, oxalate acted as an important reagent and played an important role for the structural and morphological evolution of MnO2. Nitrogen sorption analysis showed that the as-prepared α-MnO2 and β-MnO2 materials all exhibited type IV isotherms, indicating mesoporous characters. In addition, the performance of the MnO2 materials as a catalyst was evaluated in the complete catalytic oxidation of a typical volatile organic compound (VOCs), o-xylene.