Oxidation of 5-hydroxylmethylfurfural to 2, 5-furandicarboxylic acid catalyzed by magnetic MnO₂-Fe₃O₄ composite oxides

MnO₂ with different crystal structures was used to catalyze the oxidation reaction of 5-hydroxylmethylfurfural (HMF), and α-MnO₂ exhibited the highest catalytic activity. Magnetic MnO₂-Fe₃O₄ oxides were prepared by α-MnO₂ and Fe₃O₄ and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption of NH₃/CO₂ (NH₃/CO₂-TPD) and Fourier transform infrared reflection spectra of pyridine adsorption (Py-FTIR). The results showed that the composite catalyst still maintained the basic structure of α-MnO₂ and Fe₃O₄, whereas the number of active center Mn⁴⁺·O²⁻ ion pair increased compared with α-MnO₂ and Fe₃O₄, which significantly improved the catalytic activity on HMF oxidation reaction. The reaction conditions of HMF oxidation to 2,5-furandicarboxylic acid (FDCA) were optimized. The composite oxide Mn₈Fe₃O_x showed the best catalytic performance for HMF oxidation. HMF could be completely converted, with 76.9% of FDCA yield under the optimal conditions.