Crystalline-phase-dependent catalytic performance of MnO 2 for aerobic oxidation reactions

Knowledge of the catalytic performances of different crystalline phases of a material is vital for the development of superior catalysts. In this study, different phases of MnO 2 (α, β, γ, and δ) have been prepared by the oxidation of Mn 2+ , and their catalytic performances were evaluated using the aerobic oxidation of benzyl alcohol to benzaldehyde as a model reaction. α-MnO 2 promoted the reaction to the highest yield. However, when the yields were normalized by the corresponding surface areas, δ-MnO 2 exhibited the highest specific activity and α-MnO 2 the lowest, indicating that the diverse microstructures resulting from the crystalline phase have a profound effect on catalytic performance. α-MnO 2 showed the highest catalytic stability, resulting from its unchanged composition and morphology after use. Informed by the experimental results, a possible reaction mechanism involving the Mars-van Krevelen process was proposed. This work provides useful information for the development of effective catalysts for aerobic oxidation.