Efficient dissolution and extraction of the lignin from plant-based agricultural and forestry wastes by diazabicyclo solvent system

Efficient separation and extraction of lignin components is one of the key technical challenges in the utilization of plant-based biomass resources. Among them, the diazabicyclo solvents share the great potential for the dissolution and extraction of lignin from plant-based materials, due to their unique and adjustable molecular structures. This study aims to dissolve and extract the lignin from plant-based agricultural and forestry wastes. A diazabicyclo-based switchable solvent (SS) was also used with 1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU), hexanol, and water. Exceptional potential of the SS was observed in various lignin dissolution. Its model compounds were efficiently obtained even at room temperature. Systematic evaluation revealed that there was an impressive dissolution capacity of 453.2 mg/g for the milled wood lignin (MWL) at 25 °C, thus far exceeding that of conventional solvents, such as methanol, ethanol, and deep eutectic solvents. Kamlet-Taft solvent parameters indicated that the high hydrogen bond basicity (β) and net basicity of the SS substantially contributed to lignin dissolution. Lignin and its model compounds served primarily as hydrogen bond donors, interacting with the strong hydrogen bond acceptor regions within the SS. The formation of strong hydrogen bonds and non-polar interactions (including π-π stacking) synergistically promoted the dissolution of lignin in SS. Once applied to biomass, the SS system achieved high dissolution rates of 90%~93% for the lignin extraction from wheat straw and poplar sawdust. Crucially, the extracted lignin retained its structural integrity. Fourier Transform Infrared Spectrometer (FTIR) and Two-Dimensional Heteronuclear Single Quantum Coherence Nuclear Magnetic Resonance (2D HSQC NMR) analysis revealed that the cleavage of carbonyl (C=O) groups and the formation of α-O-R linkages occurred during the SS treatment. These α-O-R linkages were further hydrolyzed into α-OH, which facilitated the solubility in solvents. The crucial β-O-4 linkages remained intact, thus preserving the native aromatic framework. Gel Permeation Chromatography (GPC) further showed that the regenerated lignin shared a more uniform molecular weight distribution (PDI of 1.56 for poplar and 1.52 for wheat straw), compared with the conventionally extracted lignin. Consequently, the lignin significantly enhanced the solubility in both aqueous and organic solvents. For instance, the poplar lignin solubility in water increased from nearly to 205.68 mg/g, and its solubility in diethyl ether surged from 30.25 to 592.56 mg/g. Wheat straw lignin solubility in diethyl ether similarly increased from 45.21 to 492.05 mg/g. The structural modifications were induced by the SS treatment. In summary, the diazabicyclo-based SS served as a sustainable and effective solvent for lignin extraction. The lignin was efficiently dissolved under mild conditions. The core structural integrity of lignin was then retained for the β-O-4 linkages—downstream functionalization. Mechanistic analysis showed that only the α-carbonyl (C=O) and α-ether bonds were selectively disrupted to produce the lignin stream with a uniform molecular weight distribution, indicating the high solubility in polar solvents. This finding can provide the technical pathway for the separation, extraction, and high-value utilization of lignin from agricultural and forestry residues. Future research can explore the catalytic conversion and high-value utilization of lignin, according to the structural features of the SS-lignin. The carbohydrate components can be expected to be fully utilized in the large-scale recovery of SS solvents.