Delignification Mechanism during High-Boiling Solvent Pulping Part 3. Structural Changes in Lignin Analyzed by 13C-NMR Spectroscopy

Summary

Milled wood lignin as well as wood meal from todo fir (Abies sachalinensis MAST) was treated with 70% aqueous 1,4-butanediol solution at 220°C without acid catalyst in order to elucidate the delignification mechanism during high-boiling solvent (HBS) pulping. HBS lignin was recovered from black liquor by precipitation, and was subjected to gel permeation chromatography and quantitative ¹³C-NMR study. The average molecular weight of MWL decreased. The content of phenolic hydroxyl groups in MWL increased from 0.28 to 0.68/OMe. The decrease in the content of β-O-4 substructures was evident, which may be attributed to the homolytic cleavage of phenolic β-aryl ethers. The changes in the content of β-5 and β-β substructures were rather small. The benzyl ether formation of β-O-4 structures with 1,4-butanediol was also confirmed by ¹³C-NMR. However, the signals corresponding to Hibbert's ketones were not observed, which suggests that acidolysis was not an important degradation pathway during HBS pulping.