Hydrothermal depolymerization of different lignins: Insights into structures and reactivities

Hydrothermal depolymerization techniques such as hydrothermal liquefaction (HTL) are promising methods for converting biomass into fuel and valuable chemicals. While the HTL of lignin has been extensively studied, its fundamental chemistry remains underexplored, particularly regarding the reactivity differences among various major technical lignins under HTL conditions. A deeper understanding of these variations is essential for optimizing HTL processes. In this report, four major types of technical lignins—dioxane lignin, Kraft lignin, ethanosolv lignin, and soda lignin—extracted from the same pine sawdust were thoroughly characterized and subjected to neutral or base-catalyzed HTL at 330 °C for 1 h. The bio-oils derived from these lignins were analyzed for their physical and chemical properties. The data indicate that, while the structural differences between the lignins influenced the HTL outcomes, their impact was significantly smaller compared to the effect of the presence of a catalyst. To better understand the relationship between lignin structure and the resulting monomeric products, model compounds representing key structural motifs in technical lignins (namely β-O-4, styryl ether, and phenyl glycerol) were synthesized and tested under HTL conditions to simulate the lignin depolymerization process. Additionally, computational methods were employed to elucidate its reaction pathways.