Investigation of the molecular characterization and antioxidant activity of bio-oil produced from high moisture oil palm residues employing hydrothermal liquefaction

Oil palm residues are a promising feedstock for green chemical production; however, they are high in moisture and difficult to use. This study focuses on the conversion of high-moisture biomass, namely empty fruit bunch (EFB), oil palm trunk (OPT), and oil palm frond (OPF), into phenolic-rich bio-oil employing non-catalytic hydrothermal liquefaction (HTL) at 250, 275, and 300 °C for a 30-min holding time. Ethyl acetate was used to extract the HTL product. The results showed that the degree of depolymerization was 83–97%. The weight-average molecular weights (Mw) of the bio-oil produced from EFB at 300 °C (EFB-300 °C), OPT-300 °C, and OPF-275 °C were 130, 599, and 100 g mol−1, respectively. When the HTL temperature was increased, the Mw of the bio-oil products decreased due to depolymerization. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry showed that the molecular weights of the bio-oil products from EFB, OPT, and OPF were m/z 197–271, 208–334, and 203–343, respectively. The ratios of syringyl (S) and guaiacyl (G) units of lignin were in the range of 0.352–0.731. The main lignin components were p-hydroxyphenyl (H) and G units, which favor phenolic resin substitution. The antioxidant activity of the extracted bio-oil from HTL at 300 °C was very strong (IC50: 17.1–39.6 ppm), indicating its potential for future research and applications.