Oxygen Vacancy-Rich Amorphous Manganese Oxide for the Selective Oxidation of 5-Hydroxymethylfurfural

Metal oxides have been extensively pursued in many applications for their robustness, ease of preparation, and low cost. Within them, amorphous metal oxides exhibit characteristics that make them highly active in many catalytic applications due to their higher number of active sites, oxygen vacancies, and specific surface area compared to their crystalline counterparts. Herein, the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) at ambient pressure and an air atmosphere is carried out by amorphous manganese oxide (A-MnOX). We demonstrate the superior catalytic activity of A-MnOX compared to other MnO2 polymorphs and attribute the increase in conversion to the higher oxygen vacancy concentration and higher specific surface area. The A-MnOX was prepared via a facile inverse micelle, surfactant-based procedure, and was thoroughly characterized using powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), and N2-sorption. Catalytic activity increased in the order of β-MnO2 < α-MnO2 < γ-MnO2 < A-MnOX, correlating to an increase in the specific surface area, as well as Mn3+ presence arising from the lack of oxygen atoms (oxygen vacancies) requiring a charge balance. Thus, the catalytic activity can be linked to the presence of Mn3+.