Conversion of highly polymerized lignin into monophenolic products via pyrolysis: A comparative study of acidic and alkaline depolymerization pretreatments using deep eutectic solvents

Converting industrial residual lignin into monophenolic compounds remains a formidable challenge within biorefinery processes, especially when dealing with highly polymerized lignin. This study focuses on the depolymerization of industrial softwood enzyme hydrolysis lignin (SEHL), a by-product of bioethanol production, which possesses a substantial molar mass (M̅_w > 10000 g/mol), The approach involves utilizing acidic or alkaline deep eutectic solvent (DES) for pre-depolymerization, followed by rapid pyrolysis, achieving the selective generation of phenolic monomers. Applying acidic or alkaline DES pretreatments initiates depolymerization by breaking the β–aryl ether bonds (β–O–4) in lignin, but consequently triggers distinct reaction pathways. It is found that this pre-depolymerization leads to higher yields of phenolic monomers dominated by 4–methylguaiacol during the subsequent pyrolysis, moreover, the yields resulted from acidic DES-lignin were much higher than those from alkaline DES-lignin. We have comprehensively elucidated the lignin depolymerization process during DES pretreatments through computational simulations and experimental investigations. These efforts have provided valuable insights into the mechanisms involved in the structural changes of DES-lignin. Furthermore, we have established a more profound comprehension of how both acidic and alkaline DES pretreatments enhance the performance of lignin pyrolysis. This study emphasizes the importance of thoroughly understanding the intricate interplay between lignin structure and its thermochemical properties. Such insights are crucial for efficiently valorizing highly polymerized lignin to produce valuable phenolic compounds using the rapid pyrolysis technique.