An efficient electrocatalytic system composed of nickel oxide and nitroxyl radical for the oxidation of bio-platform molecules to dicarboxylic acids

Selective oxidation of biomass and its derivatives to dicarboxylic acids represents a promising route for biomass valorization. However, the co-presence of multiple functional groups in biomass molecules makes the selective oxidation of particular functional a challenging task. Here, we demonstrate an efficient electrocatalytic system consisting of nickel oxide (NiO) and a nitroxyl radical, i.e., 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or 4-acetamido-TEMPO (ACT), for the selective oxidation of key bio-platform molecules including glucose, xylose and 5-hydroxymethylfurfural (HMF) into corresponding dicarboxylic acids, i.e., glucaric acid, xylaric acid, and 2,5-furandicarboxylic acid (FDCA). NiO is clarified as the active catalyst for the oxidation of aldehyde in bio-platform molecules to carboxylic acid, while TEMPO or ACT is responsible for the oxidation of primary alcohol to aldehyde. The combination of NiO and TEMPO or ACT significantly accelerated the tandem oxidation of aldehyde and hydroxyl groups in glucose, xylose and HMF, thus achieving excellent yields (83%–99%) of dicarboxylic acids. Moreover, the combination catalyst enables the selective oxidation of glucose and xylose with high concentrations (e.g., 20 wt%), which offers a promising strategy for biomass valorization.