Degradation characteristics of the lignocellulosic biomass during alkaline oxidative fractionation using molecular oxygen under mild conditions

Alkaline oxidative fractionation of lignocellulosic biomass using O₂ to obtain lignin fraction and cellulose-rich substrate is one of the most promising strategies. This study systematically investigated the removal process and degradation products of carbohydrates and lignin at different stages through four intermittent alkaline oxidative treatments of sawdust, particularly revealing the molecular structural characteristics of extracted lignin through GPC, FTIR, and 2D-HSQC. After 4 treatments, 96.3 % of lignin, 93.5 % of xylose, 97.6 % of galactose, and 93.4 % of mannose were gradually extracted, resulting in a fiber solid containing 84.1 % glucose. The sawdust underwent four stages of structure looseness, crosslink weakening, structure disintegration, and fiber fragmentation during alkaline oxidative fractionation. The removal intensity of xylose, galactose, and mannose first increased and then decreased, reaching a peak of 53.7, 69.6, and 61.0 % respectively in the 2nd treatment. The glucose removal intensity followed a trend of first decreasing and then significantly increasing, having a low point of 7.2 % in the 3rd treatment. The 1st stage has the highest lignin removal and degradation rate, while the 3rd stage has the highest lignin removal intensity. The extracted lignin was oxidized and depolymerized into acid-insoluble lignin fragments (PL, 300-100,000 Da) and acid-soluble lignin fragments (ASL, 150–600 Da), where ASL contains more β-aryl ether and resinol linkages than PL. Part of the carbohydrates and lignin suffer severe oxidative degradation, generating large amounts of organic acids. These results contribute to the quantitative and qualitative understanding of lignin and carbohydrate extraction processes during alkaline oxidative treatment.