The oxidative depolymerization of kraft lignin into high-value aromatics such as vanillin represents a crucial pathway for biomass valorization. However, its efficiency is severely constrained by lignin's intrinsic recalcitrance and structural heterogeneity. These characteristics are highly sensitive to the purification strategy, which complicates the rational selection of an optimal lignin feedstock. To unravel the underlying structure-reactivity relationship, three kraft lignins purified via distinct strategies (1,4-dioxane, DL; γ-valerolactone, GL; alkali-tolerant enzymatic treatment, AL) were systematically evaluated for vanillin production using environmentally benign sodium percarbonate oxidation. Comprehensive characterization revealed that DL possessed the highest purity (97.47%), the richest content of cleavable β-O-4 linkages (13.9/100 Ar), and an optimal abundance of guaiacyl-OH (1.483 mmol/g) and carboxyl groups (0.626 mmol/g). The superior vanillin yield from DL (2.46%) over GL (1.85%) and AL (0.95%) was attributed to its highest content of cleavable β-O-4 linkages and an optimal abundance of guaiacyl-OH and carboxyl groups. Process optimization boosted the vanillin yield of DL lignin to 2.75%, equivalent to 88% of the nitrobenzene oxidation (NBO) reference value, along with 86% selectivity under mild conditions. Narrowly dispersed solid residue (PDI = 1.126) and bio-oil (PDI = 1.016) were also obtained. The sodium percarbonate (SPC) selectively cleaves β-O-4 bonds and aliphatic side chains, generating uniform low-molecular-weight products. This study elucidates the precise regulatory mechanism of purification on lignin's depolymerization reactivity and establishes an integrated strategy that couples feedstock selection with process intensification, providing a novel mechanistic framework and a scalable, green protocol for the selective upgrading of lignin.
Liu et al. (Fri,) studied this question.