Aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid by modification of the crystalline structure of MnO2 for catalytic purposes

Abstract

Investigating the various polymorphs of MnO₂ for the catalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is motivated by the pressing demand to engineer sustainable and proficient catalytic pathways. These pathways are essential for transforming renewable biomass into valuable chemicals, thereby facilitating the shift towards a circular economy and diminishing our dependence on fossil fuel-derived resources. Herein, three MnO₂ catalysts with different crystal structures were prepared for the efficient catalytic oxidation of HMF to FDCA. The results showed that α-MnO₂ exhibited higher oxidizing activity per unit specific surface area and an appreciably large specific surface area, however, its overall catalytic activity was found to be less than that of β-MnO₂. XRD, SEM, and other characterizations suggest that the presence of smaller nanorods or amorphous particles with reduced crystallinity on the surface of the α-MnO₂ catalysts could be responsible for a decreased lattice oxygen content, thereby diminishing their catalytic activity. To validate this conjecture, this paper cites for the first time the application of a dilute nitric acid immersion treatment in the chemical industry to eliminate amorphous structures from catalysts. The α-MnO₂-H⁺ catalyst, prepared using this innovative method demonstrated exceptional activity in facilitating the conversion of HMF to FDCA under more stringent reaction conditions than any monometallic catalysts previously reported. According to the kinetic analysis of the reaction, it was established that the oxidation of 5-formyl-2-furancarboxylic acid (FFCA) to FDCA constitutes the rate-limiting step within the entire oxidation process. On this basis, FT-IR studies showed that the oxidation initiates at the -CHO terminus of HMF, leading to the formation of intermediates 5-hydroxymethylfurfurylacetic acid and 5-formylfurfurylacetic acid within the Mn⁴⁺/Mn³⁺ redox cycle. In conclusion, the enhancement of catalytic activity by diluting nitric acid immersion treatment presents a promising avenue for further research and application in the field of green chemistry.