Laccase-mediator system for the ionic liquid-assisted treatment of a technical lignin with partial dissolution

The valorisation of lignin is a key aspect in the optimal exploitation of the paradigm of a biorefinery based on lignocellulosic biomass. In this work, the treatment of the technical lignin Indulin AT with a recombinant laccase, or with a combination of this enzyme with the mediator 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), has been carried out at 25 °C in a mildly acidic buffered aqueous medium. The presence of the ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) has also been investigated. Due to the limited solubility of Indulin AT in water, only partial dissolution occurred. In the lignin recovered after the treatments, a preferential diminution of the guaiacyl units was observed. In the analysis of the aqueous phases, a variety of phenolic compounds were identified, deriving from either guaiacyl or p-hydroxyphenyl units, although in moderately low concentrations. The highest assigned concentrations, always achieved when using the laccase-ABTS system (versus the treatment with no mediator), corresponded to different phenolic compounds, depending on whether the ionic liquid was involved or not in the treatment. In general, the overall concentration of phenolic compounds (measured as equivalents of gallic acid) was much greater after treatments involving the ionic liquid.