Microwave-Assisted Hydrothermal Valorisation of Rapeseed Meal for the Co-Production of High Purity Lignin and Saccharide-Rich Aqueous Solutions

Abstract

This work addresses a novel process for the co-production of lignin and oligosaccharides from rapeseed meal, examining the effects of the temperature (150–210 °C), reaction time (0–60 min) and catalyst amount (1–4 mol/L, CH₃COOH) on the process. The yields to gas, liquid and solid varied by 0–18%, 22–64% and 34–74%, respectively. The solid consisted of high purity lignin (26–88 wt.%) together with unreacted cellulose (0–28 wt.%), hemicellulose (0–28 wt.%) and proteins (11–28 wt.%). Increasing the temperature and/or reaction time produced an increase in the liquid yield and a decrease in the solid yield due to the solubilisation of the cellulosic and hemicellulosic contents of the feedstock. Acetic acid exerted a positive catalytic effect, promoting the solubilisation of cellulose and hemicellulose and preventing humins formation. The relative amounts (wt.%) of C, H, O and N in the solid fraction shifted between 46 and 63, 5.8 and 6.4, 28 and 42, and 2 and 6, respectively. The progressive solubilisation of cellulose and hemicellulose produced an increase in the proportion of C together with a decrease in the amounts of H and O in the solid product, which also accounted for the increase and decrease observed in the proportions of phenols and sugars, respectively. An optimum was found at 186 °C using an acid concentration of 1 mol/L and a total reaction time of 2 min. These conditions maximise the solubilisation of cellulose and hemicellulose without altering the lignin content of the solid, thus allowing the selective and simultaneous production of high purity (85 wt.%) lignin together with a rich oligossacharide (51 °C-wt.%) solution.