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

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.