Synergistic effects of formic acid on the supercritical alcoholysis of Caragana korshinskii into valuable bio-oils

The efficient development and clean utilization of biomass energy have garnered increasing attention, with potential solutions being proposed to alleviate global pressures on energy and chemical demands. In this work, a degradation system of Caragana korshinskii ( Ck ) with formic acid (FA) as an additive was constructed, and combinations of FA with five solvents, including cyclohexane, ethyl acetate, methanol, ethanol, and isopropanol, were compared. Among them, FA and ethanol exhibit a positive synergistic effect in Ck degradation. As a proton-type nucleophilic reagent, ethanol can effectively swell Ck by providing a supercritical environment, penetrating into the dense macromolecular skeleton, and participating in alkylation, alkyl/alkoxy transfer, and transesterification. In addition, FA provides H⁺ and in-situ hydrogen, promoting cleavage > C-O- bonds and stabilizing active fragments. Density functional theory calculation results also support the proposed FA-ethanol synergistic effect, because H⁺-assisted phenethoxybenzene cleavage pathways of FA-ethanolysis have lower energy barriers than those of conventional ethanolysis. Results show that FA-alcoholysis increases both oil yield and O content. Under optimal conditions (2 g Ck , 16 mL ethanol, 4 mL FA, 300 °C, and 60 min), bio-oil yields increased from 17.8 wt% for conventional ethanolysis to 49.6 wt% for FA-ethanolysis. By comparing the composition differences among three bio-oils derived from FA-alcoholysis, it was found that abundant aliphatic C_al-O/C=O and arenes/C_ar-O species can be obtained from FA-methanolysis/ethanolysis and FA-isopropanolysis, respectively. Overall, the combination of FA and ethanol is more favorable than other tested systems.