Effect of MnO2 Crystal Structure on Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

Aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastics monomer is efficiently promoted by a simple system based on a nonprecious-metal catalyst of MnO₂ and NaHCO₃. Kinetic studies indicate that the oxidation of 5-formyl-2-furancarboxylic acid (FFCA) to FDCA is the slowest step for the aerobic oxidation of HMF to FDCA over activated MnO₂. We demonstrate through combined computational and experimental studies that HMF oxidation to FDCA is largely dependent on the MnO₂ crystal structure. Density functional theory (DFT) calculations reveal that vacancy formation energies at the planar oxygen sites in α- and γ-MnO₂ are higher than those at the bent oxygen sites. β- and λ-MnO₂ consist of only planar and bent oxygen sites, respectively, with lower vacancy formation energies. Consequently, β- and λ-MnO₂ are likely to be good candidates as oxidation catalysts. On the other hand, experimental studies reveal that the reaction rates per surface area for the slowest step (FFCA oxidation to FDCA) decrease in the order of β-MnO₂ > λ-MnO₂ > γ-MnO₂ ≈ α-MnO₂ > δ-MnO₂ > ε-MnO₂; the catalytic activity of β-MnO₂ exceeds that of the previously reported activated MnO₂ by three times. The order is in good agreement not only with the DFT calculation results, but also with the reduction rates per surface area determined by the H₂-temperature-programmed reduction measurements for MnO₂ catalysts. The successful synthesis of high-surface-area β-MnO₂ significantly improves the catalytic activity for the aerobic oxidation of HMF to FDCA.