Metal- and alkali-free aerobic oxidation of 5-hydroxymethylfurfural to 5-formyl-2-furancarboxylic acid facilitated by the multiple hydrogen bond interactions in the deep eutectic solvent

The oxidation of 5-hydroxymethylfurfural (HMF) to produce high value-added 5-formyl-2-furancarboxylic acid (FFCA) represents a promising biomass conversion reaction that holds potential for biomass-based material for the drug synthesis and polyester industry. The present study presents a metal-free and alkali-free catalytic system consisting of N-hydroxyphthalimide (NHPI) as an organic molecule and a deep eutectic solvent (DES). This system was successfully employed for the aerobic oxidation process of HMF to FFCA, demonstrating exceptional catalytic activity with 100% conversion and 96% FFCA yield in the absence of any alkali. Multiple hydrogen bond interactions were found to be indispensable and play a crucial role in promoting the catalytic process. These H-bond interactions serve three essential purposes: facilitating the formation of deep eutectic solvents (DESs) between polyethylene glycol and tetrabutylammonium chloride, accelerating the dissolution and activation of HMF, and aiding in the generation of active free radicals for NHPI. This work demonstrated the predominant oxidation ability of the NHPI-DESs system for the oxidation of HMF to FFCA through hydrogen bond interaction, thereby offering invaluable insights into the aerobic oxidation processes involving hydrocarbons or alcohols.