Environmental concerns have brought attention to the requirement for more efficient and renewable processes for chemicals production. Lignin is the second most abundant natural polymer, and might serve as a sustainable resource for manufacturing fuels and aromatic derivatives for the chemicals industry after being depolymerised. In this work, two main approaches were investigated with the aim of treatment of lignin with a mediator, 2,2’-azinobis(3-ethylbenthiazoline-6-sulfonic acid) diammonium salt (ABTS), in 1-ethyl-3-methylimidazolium ethyl sulfate, ([C2mim][C2SO4]). In the first approach, laccase from Trametes versicolor (LTV) was used to treat organosolv lignin, using [C2mim][C2SO4] as a co-solvent in the presence of ABTS. LTV was shown to possess catalytic activity for the degradation of organolsov lignin in systems containing ionic liquid and syringaldehyde was found to be a major product obtained from the process.

ABTS alone has been evaluated for its reaction with lignin by means of cyclic voltammetry (CV). Here, the non-phenolic lignin model compound veratryl alcohol and three types of lignin (organosolv, Kraft and lignosulfonate) were specifically examined. The presence of either veratryl alcohol or organosolv lignin increased the second oxidation peak of ABTS under select conditions, indicating the ABTS-mediated oxidation of these molecules at high potentials in [C2mim][C2SO4]. Furthermore, CV was applied as a quick and efficient way to explore the impact of water in the ABTS-mediated oxidation of both organosolv and lignosulfonate lignin. Higher catalytic efficiencies of ABTS were observed for lignosulfonate solutions either in sodium acetate buffer, or when [C2mim][C2SO4] (15% v/v) was present in an aqueous solution, whilst there was no change found in the catalytic efficiency of ABTS in neat [C2mim][C2SO4]-lignosulfonate mixtures relative to ABTS alone. In contrast, organosolv showed an initial increase in oxidation, followed by a significant decrease on increasing the water content of a [C2mim][C2SO4] solution.

Despite enhanced lignin solubility in ionic liquids, the yields of small molecules attributed to depolymerisation in ionic liquids are often quite low. Since depolymerisation approaches examined herein are thought to proceed via free-radical mediated mechanisms, two different stable radicals 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ABTS were assessed for the rapid monitoring of radical activity of lignin-related compounds in ionic liquid systems. While these assays are successful in aqueous and organic solvent systems, the presence of the ionic liquids complicates the assay procedure, requiring further developmental work.

Using lignin, a waste product of the paper industry, to make the chemicals we use in daily living (e.g. painkillers, laundry perfumes), has the potential to impact society by making value out of waste and making chemicals production renewable. Lignin is extracted from trees in the paper-making process and it is a long chain molecule. In order to use it to make chemicals, we need to first break it down into small parts. Think of it like a Lego tower that needs to be broken down. The bricks are stuck together well, and you can hammer at the tower until they break apart but you risk damaging the bricks (as happens when you use heat and pressure to break lignin down). Something which will selectively push in the gaps between the bricks and prise the bricks apart is needed, and that is an enzyme (biological catalyst) such as laccase, lignin peroxidase or manganese peroxidase.

I investigated laccase and used models of lignin (donated by Queen's University Belfast and St Andrews University) to understand how laccase breaks lignin down and what laccase's limitations are. The difference between lignin and a Lego tower is that there is only one way in which the Lego bricks can connect to each other, whereas in lignin there are many ways the units can connect. Laccase was able to break the most common connection in the models rapidly, whereas other connections posed a problem because they wouldn't break and they caused the 'tower' to get 'taller'.