Hydrothermal liquefaction of Caragana korshinskii to bio-oil: Effect of liquefaction solvents and catalysts

This study systematically investigated the effects of different organic solvent-water systems, catalysts, and solvent-catalyst synergies on the hydrothermal liquefaction (HTL) process of biomass Caragana korshinskii (CK). Results indicated that all the tested organic solvent-water systems enhanced bio-oil yield, particularly, the acetone-water system yielded the highest amount of bio-oil (up to 60.63%), representing a 1.36-fold increase compared to the pure water system. Among the tested catalysts, none significantly affected the bio-oil yield; however, they enhanced the carbon and hydrogen content while reducing oxygen content, thus resulting in elevated higher heating value. Across the solvent-catalyst systems tested, γ-Al2O3 demonstrated synergy with ethanol/isopropyl alcohol, significantly boosting bio-oil yield, whereas it exhibited inhibition with acetone. Notably, the ethanol-water system combined with γ-Al2O3 not only increased bio-oil yield but also exhibited higher selectivity towards esters and hydrocarbons, with an increased H/C ratio and a reduced O/C ratio, approaching conventional diesel standards. Finally, the potential reaction mechanism of CK HTL was proposed. This study opens a new avenue for the production of high-quality bio-oil, and provides insightful clues for understanding the complicated mechanisms underlying CK HTL conversion.