Influence of extraction solvents on the recovery yields and properties of bio-oils from woody biomass liquefaction in sub-critical water, ethanol or water–ethanol mixed solvent

In this work, the effects of extraction solvents: acetone, ethanol, dichloromethane (DCM) and ethyl acetate (EA), on the yields and characteristics of bio-oil products obtained from liquefaction of rubberwood sawdust (RS) at 300 ℃ for 30 min in water, ethanol or water–ethanol mixed solvent (50/50, v/v) were comparatively studied. When pure water or ethanol was used as the reaction medium, the highest bio-oil yield was obtained by acetone (30.41 wt%) or EA (31.69 wt%), respectively. Besides, significantly higher bio-oil yields were observed in RS liquefaction in water–ethanol mixed solvent with the maximum bio-oil yield of 56.67 wt% obtained by acetone. But ethanol was always the least efficient in recovering bio-oil in all cases. Besides, the elemental analysis showed that DCM was conducive to recover the bio-oil products with lower oxygen contents and the higher HHVs, while the acetone-extracted bio-oils were worse in quality. However, the highest carbon recovery (CR) and energy recovery (ER) were obtained by acetone due to its higher bio-oil yield produced in water–ethanol mixed solvent. As suggested by GC–MS, GPC and TGA analysis, the extraction solvents and reaction mediums both greatly affected the chemical compositions of the bio-oil products. Phenols were dominant in bio-oil products, followed by hydrocarbons, ketones, esters, etc. Acetone tended to extract the bio-oil with larger carbon numbers and molecular weights, which might explain its higher bio-oil recovery yield in water or water–ethanol mixed solvent. DCM was beneficial to recover the bio-oil was more light compounds with smaller molecular weights and lower boiling point distribution. But for the liquefaction in pure ethanol, more similar chemical compositions were observed among different samples when compared to the cases in other reaction mediums.