Characterization and catalytic application of highly dispersed manganese oxides supported on activated carbon

Activated carbon supported manganese oxides (Mn/AC) were prepared by a conventional wet impregnation method using manganese nitrate as the precursor. The nature of supported manganese oxides, e.g., dispersion, oxidation state, local coordination, was characterized by X-ray diffraction (XRD), electron spin resonance (ESR), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) spectroscopies, and hydrogen temperature-programmed reduction (H₂-TPR). Manganese loading and pretreatment temperature were found to be vital factors in controlling the dispersion and chemical environment of supported manganese oxides. Highly dispersed manganese oxides can be obtained with a Mn loading up to ca. 5 wt.% under modest pretreatment temperatures, whereas large amount of Mn resulted in aggregated MnO_x crystalline clusters. The highly dispersed manganese oxides, uniformly distributed on activated carbon surface mainly as coexistence of Mn²⁺ and Mn³⁺, have been demonstrated to be catalytically active in the aerobic oxidation of benzyl alcohol using molecular oxygen. Benzyl alcohol conversion as high as 42.5% and over 99% benzaldehyde selectivity can be achieved within 4 h under low reaction temperature (373 K).