In-depth 2D-NMR characterization of lignin and hemicellulose degradation products in recovered acidic deep eutectic solvent

Deep eutectic solvents (DESs) have gained popularity for their ability to selectively fractionate lignin and hemicelluloses from lignocellulosic biomass. Recent work has focused on the recovery and reuse of DESs for multiple iterations, in some cases implicating accumulated impurities as the reason for declining pulping performance. This study reports in-depth 2D-NMR characterization of recovered DESs (RDESs) for advanced impurity identification. Choline chloride/lactic acid was deployed in aspen wood fractionation followed by two recycling iterations. Pulping performance revealed a continuous decrease in delignification with each successive cycle, falling from 84.8% to 71.6%. Mass flow estimations predicted a corresponding decrease in RDES purity due primarily to the uptake of hemicellulose-derived compounds. Combined 2D-HSQC and HMBC NMR analysis revealed the presence of lignin, xylo-oligosaccharides, monomeric sugars, and furans, alongside the formation of complex, crosslinked impurities. Crucially, NMR analysis provided evidence for the formation of complex humin structures characterized by the incorporation of lactic acid and the development of HMF-p-hydroxybenzoate humin-lignin hybrids (HLHs). GPC analysis further demonstrated a shift toward higher-molecular weight, THF-insoluble species with each cycle, suggesting that small degradation fragments are progressively crosslinked into recalcitrant hybrid structures. These results indicate that the accumulation of these specific lignin-humin hybrids interferes with the DES hydrogen bonding network, thereby reducing pulping efficiency. Understanding these chemical transformations is critical for developing mitigation strategies, such as RDES purification or make-up chemical addition.