Green Fractionation and Structural Characterization of Lignin Nanoparticles via Carboxylic-Acid-Based Deep Eutectic Solvent (DES) Pretreatment

Green fractionation and a comprehensive overview of lignin molecular structures during the DES (deep eutectic solvent) pretreatment are very important for valorization whole biorefinery process. Herein, intractable woody biomass (poplar wood) was pretreated with five types carboxylic-acid-based DESs (acetamide served as an HBA (hydrogen bond acceptor), propanedioic acid, tartaric malic glutaric succinic acid HBDs donors)) under optimized conditions. Results showed that optimal delignification ratio achieved tartaric-acid-based at 140 °C 20 min microwave-assisted heating. Two-dimensional HSQC NMR data demonstrated isolated poplar consisted mainly β-β, β-O-4 (normal acylated forms), β-5, esterified p-hydroxybenzoates (PBs) in different contents. Especially, contents fractions varied based on temperature chemical compositions DES. The antioxidant activity (2,2-diphenyl-1-picrylhydrazyl, DPPH analysis), microstructure (scanning electron microscope, SEM), weights (gel permeation chromatography, GPC) promoted rapid assembly nanoparticles (LNPs) could yield homogeneous (1.23 < PDI 1.58) controlled nanometer size (30–170 nm) good activity. This study will improve knowledge structural changes pretreatments maximize valorization.