Lignin dissolution in deep eutectic solvents : a molecular dynamics study

Abstract

Deep eutectic solvents (DESs) have shown great potential on lignocellulosic biomass pretreatment. Many DESs are capable of extracting lignin from biomass and dissolving hemicellulose while preserving most of cellulose in lignocellulosic biomass. The objective of this thesis is to understand the interaction between DESs and lignin as well as the role of DES constituents, especially hydrogen bond donor (HBD), in such interactions. DESs with choline chloride as a hydrogen bond acceptor (HBA) and polyol/ carboxylic acid as a HBD were studied for their effects on lignin dissolution behavior. The first part of the thesis focuses on the impact of spatial charge assignment on the simulation of a DES. The results indicate that the spatial charge assignment for DES is a key factor in determining the force/interaction energy, which would further change the microscopic arrangement of HBD/HBA. In the second part of this thesis, solvent structure of DESs with choline chloride as a HBA as well as interaction with lignin were studied via molecular dynamics (MD) simulations. Three HBDs, including ethylene glycol, formic acid, and lactic acid as well as two types of lignin models (GG lignin dimer and Adler lignin) were used to gain comprehensive understanding of the local solvent molecular arrangement, the mechanism of lignin dissolution and the dissociation of lignin from cellulose. The common supramolecular complexes in DES were found to show strong correlation with the solvent hydrogen bond network and lignin dissolution. Specially, both functional groups (hydroxyl group vs. carboxyl group) and oxygen atom number in the HBD of a DES determined its hydrogen bond networks and the strength of its interaction with lignin, which in turn largely defined the structural changes of lignin. Also, the chloride anion as well as HBD would preferentially preposition around hydroxyl groups (e.g., [alpha]-OH and [gamma]-OH) located in the lignin linkages. Such preference implies the potential route for bond cleavage in the lignin depolymerization. Molecular interaction between lignin and a DES played a key role in dissociating lignin from cellulose. We found that the carboxylic acid DESs can detach larger part of lignin from cellulose surface, especially on the hydrophobic surface than the polyol DES. The insights gained in this study would advance the understanding of lignin dissolution at an atomistic level and provide guidance for designing effective DESs for biomass pretreatment as well as lignin extraction and valorization toward sustainable and profitable biorefinery.