Characterization of autohydrolysis aspen (P. tremuloides) lignins. Part 3. Infrared and ultraviolet studies of extracted autohydrolysis lignin

Abstract

Infrared and ultraviolet studies of the lignins extracted from extractive-free aspen wood meal after autohydrolysis at 195 °C for periods varying from 5 min to 2 h indicated that these lignins were functionally modified and different from aspen milled wood lignin. The extracted lignins changed from a guaiacyl–syringyl type lignin to a syringyl-deficient type lignin with increasing autohydrolysis time. Extracted lignins were also observed to contain unconjugated β-ketone groups and conjugated carboxylic acid groups. It is proposed that the unconjugated β-ketone groups resulted from the depolymerization reactions of the lignin macromolecule under acidic conditions which formed monomeric, dimeric, and oligomeric/polymeric lignin fragments with Hibbert's ketone side chains. The conjugated carboxylic acid group observed to be present was attributed to p-hydroxybenzoic acid. p-Hydroxybenzoic acid has been postulated as contributing to the extractability of aspen lignin by acting as a blocking agent in the repolymerization of lignin fragments to form insoluble lignin.