Effective delignification of lignocellulosic biomass by microwave assisted deep eutectic solvents

To establish an environmentally friendly and cheaper method to delignify lignocellulosic biomass feedstocks, deep eutectic solvents (DESs) were investigated as a green alternatives to conventional solvents. Six different DESs were facilely prepared and used to delignify miscanthus and birchwood feedstocks, including monocarboxylic acid/choline chloride (ChCl), dicarboxylic acid/ChCl and polyalcohol/ChCl. The enhanced delignification efficiency was evaluated in relation to the nature of the hydrogen bond donors and acid strength of DES. The largest extraction of lignin from the miscanthus and birchwood was achieved using ChCl.formic acid and ChCl.oxalic acid DES, respectively. The TGA and ¹³C NMR characterization results of the extracted lignin samples indicated that the different types of lignin were produced using different DESs. The reaction optimization results showed an increase in lignin extraction with increasing temperature from 60 to 130 °C. However, the optimal reaction time was different, 30 min for miscanthus and 60 min for birchwood. A convenient and reliable method for the quantification of lignin was developed using UV-Vis spectrophotometry.