A CuMn2O4 spinel oxide as a superior catalyst for the aerobic oxidation of 5-hydroxymethylfurfural toward 2,5-furandicarboxylic acid in aqueous solvent

Abstract

A CuMn₂O₄ spinel oxide was prepared via a freezing-assisted sol–gel method and used in the aerobic oxidation of 5-hydroxylmethylfurfural (HMF) toward 2,5-furandicarboxylic acid (FDCA) in aqueous solvent. A highest FDCA yield of 92.1% over the CuMn₂O₄ spinel oxide was achieved and the catalyst could be regenerated by calcination in air after the sixth consecutive run, outperforming several other Mn-based spinel and single oxide catalysts. Kinetic studies reveal that HMF → 2,5-diformylfuran → 5-formylfuran-2-carboxylic acid (FFCA) → FDCA is the primary reaction route of the reaction and that the oxidation of FFCA is the rate-determining step over the CuMn₂O₄ spinel. Characterization measurements show that Mn species enrichment and proper Mn⁴⁺/Mn³⁺, Cu²⁺/Cu⁺ and Cu/Mn ratios on the surface of the catalyst led to an appropriate O_latt./O_ads. ratio, which facilitated oxygen mobility between the O_latt. consumption and the O_latt. generation via the refilling of oxygen vacancies. Synergistic effects between Mn and Cu in the CuMn₂O₄ spinel inhibit the secondary reaction and accelerate the rate-determining step rate to enhance FDCA formation.