Simultaneous production of lignin and polysaccharide rich aqueous solutions by microwave-assisted hydrothermal treatment of rapeseed meal

This work addresses a novel and green 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 46–63, 5.8–6.4, 28–42 and 2–6, respectively. Py-GC/MS analysis revealed that the solid decomposed into phenols (1–19%), sugars (0–15%), N-compounds (0–31%), carboxylic acids (37–75%), hydrocarbons (4–20%) and furans (1–8%). The liquid phase comprised oligo- and mono/di-saccharides (33–51 C-wt.%, 0–3 C-wt.% and 0–6 C-wt.%) and carboxylic acids (40–62 C-wt.%). 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. The acid can be recovered from the sugar mixture, which not only improves the efficiency of the process but also allows the production of a pure saccharide (92 C-wt.%) product.