Ternary deep eutectic solvents as multifunctional green solvents for lignocellulosic biomass pretreatment

Ternary deep eutectic solvents (tDES) are tunable, multifunctional green solvents that offer clearer advantages over binary deep eutectic solvents (bDES) for selective lignocellulosic biomass fractionation. By adding a third component, tDES allow better control of polarity, acidity, and viscosity, which improves delignification, hemicellulose removal, and lignin stabilization. Key factors include reducing lignin condensation using polyols and Lewis acids, which stabilize reactive lignin intermediates, undesired CC bond formation and repolymerization during fractionation. This helps preserve β-O-4 linkages and yields lighter, lower-molecular-weight lignin that is more suitable for downstream depolymerization and valorization. Moreover, downstream routes to lignin nanoparticles and catalytic depolymerization are outlined, along with emerging "polymerizable" tDES or eutectogels that couple pretreatment with the fabrication of ion-conducting, 3D-printable soft materials. Finally, sustainability is assessed through recyclability, solvent recovery, toxicity, and life-cycle metrics, although challenges remain in high-viscosity mass transfer, component screening and standards, potential metal salt ecotoxicity, and scale-up and techno-economics. This review will inspire more investigation into the use of tDESs towards a sustainable future.