Composition of the bio-oil from the hydrothermal liquefaction of duckweed and the influence of the extraction solvents

The influence of the extraction solvents on the yields of the product fractions and on the composition of the bio-oils obtained from the hydrothermal processing of duckweed at 350 °C for 30 min was investigated. Ten solvents were employed including polar solvents (isopropanol, ethyl acetate, dichloromethane, diethyl ether, dichloroethane, benzene, carbon disulfide) and nonpolar solvents (cyclohexane, n-hexane and petroleum ether). The extraction solvents with high relative polarity values tended to produce higher yields of the bio-oils. The highest bio-oil yield of 26 ± 1 wt.% was obtained using isopropanol, followed by dichloromethane (24 ± 1 wt.%). Nonpolar solvents including cyclohexane, n-hexane, and petroleum ether produced the yields of the bio-oils ranging from 3 ± 0.2 to 9 ± 0.4 wt.%. The bio-oils always had a significantly higher C and H contents and a substantially lower O and S contents than those of the biomass material. The C and H contents of the bio-oil from nonpolar solvents, which averaged 78 ± 0.8 wt.% and 10 ± 0.5 wt.%, respectively, were slightly higher than the values from the polar solvents, which averaged 75 wt.% and 9 wt.%, respectively. In contrast, the N and O contents of the bio-oils from nonpolar solvents was lower than that from the polar solvents. The energy recovery (ER) obtained from the polar solvents varied from 42 ± 2 to 60 ± 3%, which is much higher than the ER obtained from the nonpolar solvent (24 ± 1% for cyclohexane) and lowest (10 ± 0.5% for n-hexane). Significant differences in molecular composition were observed in the bio-oils when varying the solvent, and these differences were attributed to the combined effects of the polarity and the molecular structure of each solvent.