Novel Deep Eutectic Solvents with Different Functional Groups towards Highly Efficient Dissolution of Lignin

The development of effective solvents for lignin is of high importance. Herein, two types of novel deep eutectic solvents (DESs) were designed employing lactic acid and N-methylthiourea as hydrogen bond donors and four different quaternary ammonium salts as hydrogen bond acceptors. The designed DESs could efficiently dissolve different types of lignin, including dealkaline lignin (DAL), enzymatic hydrolysis lignin (EHL), sodium lignosulfonate (SL), and organosolv lignin (OL). The investigation of the mechanism revealed that the functional groups in the DESs significantly affected their ability to dissolve lignin, and a positive correlation was found between the solubility of lignin and the subtraction value of α and β (Kamlet–Taft empirical parameters) for the dissolution of EHL, DAL and OL. Additionally, the unique dissolution behaviour of SL was due to its salt-like structure.