Spectral analysis of the successive degradation process of kraft lignin by alkaline oxygen treatment

Abstract

Softwood kraft lignin was subjected to alkaline oxygen treatment in a fundamental study of lignin degradation. Two different spectral changes were observed in the time course of ultraviolet-visible spectra along with the progress of the treatment. These spectral changes could be recognized as proceeding along certain temporal functions that were based on second-order decays with different halflives. The spectral changes were defined as “fast change” and “slow change.” The fitting studies on the amount of total protons on the unsaturated and aromatic systems, the amount of unconjugated phenolic substructure determined by differential ionization spectra, and the amount of methoxyl group with temporal functions showed that two reaction types (formation of muconate derivatives and ortho-quinone derivatives) can be expected as the major modification types occurring during fast change. The fitting study of the time course of infrared attenuated total reflectance (ATR) spectra gave corresponding infrared ATR spectral features of fast and slow changes. The occurrence of the formation of muconate derivatives by fast change was strongly supported by the spectral feature of fast change. On the other hand, it is suggested that the aromatic structure of lignin was further degraded during slow change. In addition, formation of resistant phenolic substructures is suggested as another possible modification type occurring by fast change.