Oxidative Depolymerization of Hardwood Lignin to High-Value Aromatics: Optimization by Response Surface Methodology and DFT Study of the Monomers

Abstract Beech-wood lignin, derived from the wood of beech trees (genus Fagus ), is a lignin-rich biomass with significant potential for valorization. Beech forests are prevalent in temperate regions worldwide, and beech wood possesses desirable properties for various applications, including the construction industry, furniture-making, and pulp production. The oxidative depolymerization of hardwood lignin is a sustainable approach to converting complex lignin structures into high-value aromatic compounds. In this study, the depolymerization of hardwood lignin was optimized using a Box-Behnken design (BBD) quadratic regression model to maximize monomer yields. The model demonstrated high predictive accuracy, with significant alignment between predicted and actual results, indicating its reliability in optimizing reaction conditions. Key operating parameters – temperature, pressure, and reaction time—were systematically varied, and the optimal conditions for monomer yield were found to be 191 °C, 5 bar O 2 pressure, and 25 min. Under these conditions, the primary monomers obtained included syringaldehyde (5.78 wt%) and vanillin (2.31 wt%), along with smaller quantities of acetovanillone, acetosyringone, syringol, and guaiacol. The reducing ability of the resulting monomers was investigated using density functional theory calculations, where electron, hydrogen-atom, and hydride donation ability were determined. Finally, size-exclusion chromatography confirmed the effective breakdown of lignin, showing distinct differences between blank and oxidized samples. This study highlights the effectiveness of oxidative depolymerization under controlled conditions for converting hardwood lignin into valuable aromatic compounds, with the BBD model playing a crucial role in optimizing the process for efficient lignin valorization.