High-purity lignin from selective biomass fractionation with ternary deep eutectic solvents

Conventional caustic soda kraft pulping is not a sustainable pretreatment as it can degrade useful resources from the biomass. Here we design reusable acidic ternary deep eutectic solvents (TDES) composed of alanine, lactic acid, and ethylene glycol for fractionation of high-purity lignin and augmentation of lignocellulosic biomass. The selective fractionation yielded lignin free of sulfur and silica, while ensuring that lignin condensation was avoided by the desired stabilization with diols. Advanced nuclear magnetic resonance (NMR) spectroscopic analysis provided a mechanistic understanding of the delignification process and lignin structural transformation. Besides the predominant demethoxylation and cleavage of beta-O-4 linkage, lactic acid induced both acylation and oxidation of the substructures, resulting in lignin with desirable molecular weight and monomeric yield (S/G ratio = 3.39), which are beneficial for applications in reinforcement materials and bio-oils. Overall, the green TDES extraction method could shed new light on alternative pretreatments for sustainable biomass valorization.