SULFOMETHYLATION REACTIVITY OF ALKALI LIGNIN WITH LACCASE MODIFICATION

The alkali lignin was modified by laccase,followed by sulfomethylation. To study the mechanism of laccase modification on the influence of sulfomethylation reactivity, the control and laccase modified alkali lignins were characterized by chemical, spectroscopic and chromatographic methods. The molecular weight of sulfomethylated lignins was measured by gel permeation chromatography (GPC), and the structural changes in laccase modified lignins were characterized by functional groups measurements,headspace gas chromatography (GC), infrared spectroscopy (IR) and nuclear magnetic resonance (NMR). It is proposed that laccase was able to promote both polymerization and depolymerisation. The molecular weight (M-w) changed a little,but the polydispersity was larger than that of the control. The demethylation by laccase oxidation made the increase of the content of phenolic group,while the content of syringyl decreased. At the same time, laccase could oxidise the phenolic end-groups into stabilized radicals that subsequently undergo radical-radical coupling. The charge density of lignin model compounds was calculated by molecular simulation. It was indicated that why demethylation benefited the sulfomethylation. According to structural changes in lignin, the mechanism of the influence laccase modification on the sulfomethylation reactivity was deduced. The sulfomethylation reactivity, measured by the content of sulfonic groups,was increased by 35% after laccase modification. It was mainly due to the increase of the content of phenolic groups and the decrease of steric hindrance. In addition, the dispersion efficiency of sulfomethlated alkali lignin with laccase modification on the TiO2 slurry improved significantly.