Mechanism insights for the roles of hydrogen bonds on the dissolution and separation of lignin in LA-ChCl system

Abstract

Deep eutectic solvents (DESs) of lactic acid (LA)-choline chloride (ChCl) currently have attracted increasing attention due to its superior efficiency and performance for lignin dissolution and separation. In this paper, the formation mechanism of hydrogen bonds between LA and ChCl was confirmed with analyses of FTIR, ¹H NMR spectroscopy, and density functional theory (DFT) computational simulations. Subsequently, DFT simulations were performed to calculate all possible binding sites inside lignin dimer and between LA-ChCl and lignin dimer to demonstrate the extensive hydrogen bond network formed between LA-ChCl and lignin, which is significantly stronger than those observed inside lignin to result in the dissolution and separation of lignin from plant fibers. These findings allow the inference of easily formed and broken sites by comparisons of all possible hydrogen bonds, highlight the important role of hydrogen bond interactions between LA-ChCl and lignin molecules to provide guidance for future developments of green DES solvents for the high-value applications of lignin.