Effect of MnO 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 MnO and NaHCO. 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 MnO. We demonstrate through combined computational and experimental studies that HMF oxidation to FDCA is largely dependent on the MnO crystal structure. Density functional theory (DFT) calculations reveal that vacancy formation energies at the planar oxygen sites in α- and γ-MnO are higher than those at the bent oxygen sites. β- and λ-MnO consist of only planar and bent oxygen sites, respectively, with lower vacancy formation energies. Consequently, β- and λ-MnO 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-MnO > λ-MnO > γ-MnO ≈ α-MnO > δ-MnO > ε-MnO; the catalytic activity of β-MnO exceeds that of the previously reported activated MnO 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 H-temperature-programmed reduction measurements for MnO catalysts. The successful synthesis of high-surface-area β-MnO significantly improves the catalytic activity for the aerobic oxidation of HMF to FDCA.