Heterostructured manganese catalysts for the selective oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran

A series of manganese oxide catalysts were synthesised in the presence of various precipitants using hydrothermal approach. The manganese oxide synthesised (MnOx-A-U) using manganese acetate tetrahydrate and urea as precursor and precipitant, respectively, was found to be composed of heterostructures of manganese, that is, MnCO3, epsilon-MnO2 and Mn2O3, and their presence were confirmed by XRD, FTIR and XPS analyses. The catalytic activity of MnOx-A-U towards selective oxidation of HMF to DFF in ethanol afforded 92.0 % conversion along with 88.0 % DFF yield at 120 degrees C, 30 bar O-2, after 4 h of reaction time. Performing HMF oxidation under N-2 atmosphere with MnOx-A-U yielded only 15.0 % DFF, revealing that lattice oxygen played a crucial role in the oxidation process, which was confirmed by subjecting the spent catalyst to XPS analysis. Based on the results obtained from XPS analysis, it was speculated that epsilon-MnO2 and MnCO3 could be the active species which could selectively catalyse the reaction. The MnOx-A-U catalyst was able to recycle for at least three runs with a small loss of activity due to epsilon-MnO2 content decreased.