Preparation of Anisotropic MnO2 Nanocatalysts for Selective Oxidation of Benzyl Alcohol and 5-Hydroxymethylfurfural

Abstract

Anisotropic MnO2 nanostructures, including α-phase nanowire, α-phase nanorod, δ-phase nanosheet, α + δ-phase nanowire, and amorphous floccule, were synthesized by a simple hydrothermal method through adjusting the pH of the precursor solution and using different counterions. The catalytic properties of the as-synthesized MnO2 nanomaterials in the selective oxidation of benzyl alcohol (BA) and 5-hydroxymethylfurfural (HMF) were evaluated. The effects of micromorphology, phase structure, and redox state on the catalytic activity of MnO2 nanomaterials were investigated. The results showed that the intrinsic catalytic oxidation activity was mainly influenced by the unique anisotropic structure and surface chemical property of MnO2. With one-dimensional and 2D structures exposing highly active surfaces, unique crystal forms, and high oxidation state of Mn, the intrinsic activities for MnO2 catalysts synthesized in pH 1, 5, and 10 solutions (denoted as MnO2-pH1, MnO2-pH5, and MnO2-pH10, respectively) were twice higher than those of other MnO2 catalysts in oxidation of BA and HMF. With a moderate aspect ratio, the α + δ nanowire of MnO2-pH10 exhibited the highest average oxidation state, most abundant active sites, and the best catalytic oxidation activity.