New insights into lignin modification during chlorine dioxide bleaching sequences (I): Chlorine dioxide delignification

Chlorine dioxide delignification (D-0) modifies kraft residual lignin by oxidizing phenolic groups to both quinonoid and muconic structures. Results from various in situ analyses (i.e., kappa/Klason lignin, methanol, and methoxyl group analyses) demonstrated that the Do stage preferentially oxidizes phenolic entities to quinones over muconic structures in similar to2:1 ratio (1.7 mmol/g lignin vs. 0.8 mmol/g lignin), respectively. The level of quinones detected by these in situ methods was significantly higher than those determined by the NMR analysis of isolated Do residual lignin. It was discovered that the acidic procedure used to isolate the oxidized lignin for NMR analysis chemically eliminated the majority of quinonoid structures, which resulted in lower amounts of quinones detected. This study clearly indicates that quinones are a significant reaction product formed during the D-0 stage.