Effect of MnO₂ Crystal Type on the Oxidation of Furfural to Furoic Acid

The base-free oxidation of furfural by non-noble metal systems has been challenging. Although MnO2 emerges as a potential catalyst application in base-free conditions, its catalytic efficiency still needs to be improved. The crystalline form of MnO2 is an important factor affecting the oxidation ability of furfural. For this reason, four crystalline forms of MnO2 (alpha, beta, gamma, and delta-MnO2) were selected. Their oxidation performance and surface functional groups were analyzed and compared in detail. Only delta-MnO2 exhibited excellent activity, achieving 99.04% furfural conversion and 100% Propo.(FA) (Only furoic acid was detected by HPLC in the product) under base-free conditions, while the furfural conversion of alpha, beta, and gamma-MnO2 was below 10%. Characterization by XPS, IR, O-2-TPD and other means revealed that delta-MnO2 has the most abundant active oxygen species and surface hydroxyl groups, which are responsible for the best performance of delta-MnO2. This work achieves the green and efficient oxidation of furfural to furoic acid over non-noble metal catalysts.