Structure-reactivity relationships in the mild reductive depolymerization of technical hydrolysis lignins

This study investigates how the structure of hydrolysis lignins (HLs) impacts both their solubility and behavior in mild reductive catalytic depolymerization (RCD). Thorough characterization of 8 different HLs shows that solubility and depolymerization outcomes depend on intrinsic lignin properties as well as carbohydrate content. A 70/30 vol% ethanol/water mixture is generally the most effective solvent mixture, although the absolute solubility varies substantially among the HLs, with a high carbohydrate content and molecular weight typically lowering the solubility. The β-O-4 bond content is identified as the primary factor dictating both the monomer yield and number of para-substituted side chains formed during RCD, with a linear relationship between β-O-4 bond cleavage and monomer/side chain formation. However, the syringyl/guaiacyl ratio further steers monomer yields, with syringyl-rich lignins generating more monomers per cleaved β-O-4 bond. Despite structural diversity, para-substituted side chain selectivity in the monomer fraction remains consistent for all HLs, with approximately 85% propanol-substituted monomers. Similar trends were observed for other technical lignins, underscoring the broad applicability of the RCD protocol, although minor variations in side-chain selectivity were noted. Carbohydrate content also affects the evolution of aliphatic and carboxylic acid hydroxyl groups during RCD, with higher carbohydrate contents leading to sharp initial decreases in the former and increases in the latter. In all HLs formyl native end groups are reduced to methyl groups, and ethyl ester formation via esterification of carboxylic acid groups is confirmed. Lastly, high-molecular-weight HLs (> 9000 g/mol in this study) may experience initial diffusion limitations, delaying efficient depolymerization.