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

Abstract

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 MnOx 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). © 2008 Elsevier B.V. All rights reserved.