Characterisation of laccase-mediated lignin polymerisation: implications for molecular weight, thermal stability, and electrical properties

Kraft lignin was oligomerised using a bulk laccase solution in varying amounts of buffer and ethanol solvents. The activity of laccase and kinetic parameters was analysed by adjusting the buffer pH and ethanol:buffer ratio. The qualitative weighted average molecular weight (M-w) distribution showed significant variation when ethanol (40:60) solvent was used. GPC analysis confirmed the presence of fractions with higher molecular weight, where the M-w after 30 min was 7859, compared to 90 min in the buffer medium, where M-w was only 1779. These condensed units exhibited lower antioxidant values, characteristic of a polymerisation reaction. Py-GC/MS analysis demonstrated that the sample with the highest molecular weight displayed increased G-type lignin units and higher levels of polysaccharides residue compared to the original kraft lignin. The addition of ethanol as a green solvent, mixed with buffer, resulted in a 4.4-fold increase in Mw compared to buffer alone. These findings highlight the advantageous role of ethanol as a medium to obtain lignin units with high molecular weight.