Radical-based Oxidative Pretreatment Enhances Biofuel Production From Lignocellulosic Biomass Via Hydrothermal Liquefaction

The sustainable production of biofuels from lignocellulosic biomass is a central goal in the transition to low-carbon energy systems. However, hydrothermal liquefaction (HTL), a promising thermochemical conversion pathway, is constrained by the high oxygen content and complex aromatic structure of lignin, which lowers bio-oil quality. Here, we used a model system of brown-rot-degraded white oak (Quercus alba) to test whether radical-based oxidative pretreatment could enhance HTL performance by converting lignin into more aliphatic intermediates. Oxidation was performed under simulated Fenton conditions using fixed Fe(II) (60 ppm) and two hydrogen peroxide concentrations (3 and 8 M), resulting in extensive lignin depolymerization and structural transformation. Solid-state and gel-phase NMR analyses showed a pronounced loss of aromaticity and an increase in aliphatic carbon, including the formation of lipid-like structures. Elemental analysis revealed decreasing carbon content with increasing oxidative severity, accompanied by a higher hydrogen content and elevated H/C atomic ratios. These changes significantly improved the quality of the resulting bio-oils. Oils derived from oxidized samples exhibited high H/C ratios (1.92–2.00), low O/C ratios (0.05–0.07), and larger higher heating values (HHV) of up to 43 MJ/kg, approaching those of petroleum fuels. Gas chromatography demonstrated progressive enrichment in straight-chain n-alkanes (C10–C27), comprising up to 46% of the chromatographic area in the most oxidized sample (8M_HTL). Complementary NMR analysis confirmed the dominance of saturated hydrocarbons, highlighted by strong alkyl signals, with minor contributions from functionalized aliphatics, olefins, and aromatic species. Simulated distillation further indicated favorable boiling point distributions, centered in the kerosene and diesel ranges. Although this study used a simplified lignocellulosic substrate, it provides mechanistic insight into how radical oxidation transforms lignin and enhances HTL performance. These findings establish proof of concept for oxidative pretreatment as a strategy to selectively depolymerize lignin into aliphatic-rich intermediates, thereby improving bio-oil energy quality. This approach may be particularly valuable for valorizing lignin-rich waste streams such as kraft lignin and black liquor from the pulp and paper industry, which remain underutilized despite their potential as advanced biofuel feedstocks.