Benzoic Acid/TEMPO as a Highly Efficient Metal‐Free Catalyst System for Selective Oxidation of 5‐hydroxymethylfurfural into 2, 5‐diformylfuran

The combination of benzoic acid and 2,2,6,6-teramethylpiperidin-1-oxyl (TEMPO) afforded an efficient catalytic system for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF) under certain conditions. The maximum conversion of 86.7% for HMF was obtained with a nearly 77.8% DFF yield at 1mmol benzoic acid and 0.4MPa oxygen pressure in acetonitrile. The aerobic oxidation ability of HMF into DFF depended on the electron density of the organic acid skeletal structure with the carboxyl group. Meanwhile, the catalyst system of benzoic acid/TEMPO also exhibited good performance for aerobic oxidation of other alcohols. Based on experimental results, we propose that the benzoic acid acted as the primary catalyst and TEMPO was the key co-catalyst, oxidized into oxoammonium cation through electron transport and then regenerated by a superoxide radical during the reaction.