Graphene Oxide: A Convenient Metal-Free Carbocatalyst for Facilitating Aerobic Oxidation of 5-Hydroxymethylfurfural into 2, 5-Diformylfuran

By thermal treatment in vacuum, graphite oxide prepared by Hummer's method was exfoliated and partially reduced. This procedure imparts to the graphene oxide (GO) high reactivity with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as cocatalyst for selective oxidation of 5-hydroxymethylfrufural (HMF) to 2,5-diformylfuran (DFF) under certain conditions (100% HMF conversion with HMF selectivity 99.6% at 80 wt % GO loading, 1 atm air pressure). This study found that GO could function as an oxidant for anaerobic oxidation of HMF during which carboxyl groups in GO were reduced. Importantly, the partially reduced GO material could continue to activate molecular oxygen during aerobic oxidation. Further study showed that oxygen functionalities in the GO material had a crucial effect on the catalytic oxidation of HMF. By control experiments and molecular analogues tests, a plausible mechanism was proposed in which the high reactivity was attributed to the synergistic effect of the carboxylic acid groups and unpaired electrons at GO edge defects, with TEMPO as the cocatalyst and oxygen as the terminal oxidant.