Lignin dissolution and lignocellulose pretreatment by carboxylic acid based deep eutectic solvents

Deep eutectic solvents (DESs) composed by carboxylic acids and quaternary ammonium salts are potential candidate for lignocellulose dissolution and pretreatment. To both explore the most efficient DESs for lignocellulose dissolution and pretreatment as well as unveil the process mechanism, DESs composed by carboxylic acids as the hydrogen bond donors (HBDs) and quaternary ammonium salts as the hydrogen bond acceptors (HBAs) were prepared and further applied for kraft lignin (KL) dissolution as well as poplar and pine pretreatment. We found DES composed by lactic acid (LA) and allyltrimethylammonium chloride (ATMAC) with mole ratio of 2:1 is high-efficient for the dissolution of KL. In addition, 70% poplar lignin and 82% pine lignin could be removed at 120 ℃ for 6 h with a solid-to-liquid ratio of 1:10. Furthermore, the -OH and -COOH groups in HBDs of DESs provided active protons to cleave ether bonds and part of carbon-carbon bonds during lignin dissolution and extraction. Additionally, the KL solubility and the Kamlet-Taft (K-T) parameters (α and π * ) of the DESs showed good positive correlation. These findings give a deep insight into the dissolution mechanism of KL and pretreatment of lignocellulose, which is helpful for designing robust DESs for dissolution and pretreatment of lignocellulose.