Sustainable and non-conventional protocols for the three-way valorisation of lignin from grape stalks

The main goal of this work is the exploitation of the lignin fraction present in grape stalks (GS) to produce high-value-added chemicals via non-conventional and sustainable means in a three-way valorisation process. Firstly, delignification using NaOH and Natural Deep Eutectic Solvents (NaDESs), via microwave-assisted extraction (MAE), was performed using a bicomponent (choline chloride + lactic acid) and a tricomponent (choline chloride + lactic acid + glycerol) NaDES and giving engaging MAE results in terms of delignification (around 80%). The liquid fractions obtained from MAE, using NaDESs and NaOH, were analysed, with all of them displaying promising antioxidant activity compared to the standards. Subsequently, oxidation was carried out via microwave (MW) heating, with the lignins that were isolated after the delignification step mainly yielding long-chain fatty acids (LCFA), which are valuable for bio-fuel production. However, when GS was used as the starting material, 16% wt. of bioaromatics (mainly vanillin and syringaldehyde) was produced. Oxidation therefore provides two further pathways for the exploitation of the lignin fraction from this lignocellulosic waste.