Novel insights into CO2 inhibition with additives in catalytic aerobic oxidation of biomass-derived carbohydrates to formic acid

Formic acid (FA) is considered an ideal energy carrier for hydrogen. Catalytic aerobic oxidation of biomass -derived carbohydrates is a promising method to produce renewable FA. Interestingly, the addition of some low-carbon alcohols or dimethyl sulfoxide remarkably improved FA selectivity in the popular vanadium-based catalytic system. This work unveils an advanced mechanism of the product improvement with additives. Con-trol experiments of vanadium-substituted heteropolyacids (HPA-n) in different redox states and ESR spectroscopy verified that hydroxyl radicals were formed in the re-oxidation of reduced HPA-n. Different-type oxidations and model reactions revealed that the hydroxyl radicals caused overoxidation by oxidizing aldehydic groups to carboxylic groups as the precursors of CO2. The additives scavenged hydroxyl radicals so glucose was oxidized into FA with high selectivity. Furthermore, the hydroxyl radicals were quantified and the effect of isopropanol dosage on glucose oxidation was investigated. The CO2-inhibition effect of isopropanol was also exhibited for various carbohydrates and vicinal diols. Based on the mechanism, catalytic co-oxidation of glucose and methanol was proposed, showing an extremely high FA yield from glucose and high reactivity of methanol. The findings shed light into the mechanism of the product improvement with additives and pave an efficient way to produce a renewable energy carrier.