Demethylation and other modifications of industrial softwood kraft lignin by different laccase-mediator systems

An industrial softwood kraft lignin was treated by different laccase-mediator systems (LMS) followed by comprehensive structural analysis. Analysis revealed that demethylation was a common reaction for all the LMS combinations investigated. After reaction, methanol was always released into the solutions. A reduction in the methoxyl content of the lignins was observed in the obtained products. The treatment reduced the quantities of different types of phenolic aromatic rings, with G units being the most attacked structures. Decarboxylation was also detected in two of the LMS, namely L1-TEMPO andL2-HBT. The highest demethylation rate came from the L1-ABTS combination and the most severe overall structural modification. Lignin polymerisation was observed for all treatments, with the L1-ABTS combination demonstrating the highest Mp, Ma and Mw values. An increased polydispersity was also observed for all the treatments, with the highest increase in polydispersity found for the L1-ABTS treatment. In addition, lignin depolymerisation took place. It has been concluded that the most efficient demethylation was achieved by N51002-ABTS