Elucidating the role of active oxygen species and hydroxyl groups in the furfural base-free oxidation to furoic acid over δ-MnO2

Abstract

δ-MnO2 shows potential application in furfural base-free oxidation to furoic acid. While, its catalytic mechanism was unclear, which limited the further improvement of the catalytic activity. Therefore, three δ crystal MnO2 with different hydroxyl and active oxygen species concentrations were prepared and applied in the furfural oxidation reaction. IR, Furfural-IR, O2-TPD, H2O-18O2-TPD, and XPS techniques characterized the hydroxyl and active oxygen species. The results show that the hydroxyl group on δ-MnO2 is important in absorbing and attacking furfural to form geminal diol intermediate. Moreover, two types of oxygen species O-OH and Oads originating from the surface hydroxyl and the strongly adsorbed gas-phase O2, respectively, play an essential role in defhydrogenating geminal diol to furoic acid. Most importantly, the hydroxyl, O-OH, and Oads consumed on δ-MnO2 can be reconstructed under reaction conditions. Among the three δ crystal MnO2, δ-MnO2, with the most abundant hydroxyl and active oxygen species, exhibited the best catalytic performance with 99.53% furfural conversion and nearly 100% furoic acid selectivity. This work identifies the function of hydroxyl and active oxygen species on δ-MnO2 in furfural oxidation reaction, guiding the design of a non-noble metal catalyst for furoic acid synthesis under base-free conditions.