Microwave-assisted extraction of Camellia oleifera shell lignin via dual- acidic deep eutectic solvent and its mechanism

Deep eutectic solvents (DES) are gaining recognition for their utility in the cost-effective and high-solubility pretreatment of lignocellulosic biomass. The unique combination of Brønsted acid and Lewis acid in DES gives it the ability to break hydrogen and ether bonds in lignin-carbohydrate complexes (LCCs) and can be applied to the extraction catalysis of lignin. This study investigates the efficacy of a dual-acidic DES in extracting lignin from Camellia oleifera shells, with the assistance of microwave irradiation. Our findings reveal that a DES composed of ZnCl₂ and lactic acid in a ratio of 1:8(n/n) exhibits optimal performance. Utilizing this DES, we achieve the rapid extraction of lignin with a purity of 91.2%, yield of up to 97.1%, and a low polydispersity index of 1.49, within a remarkably short timeframe of 10 min, under microwave power of 70 W. Characterization by Infrared spectroscopy and NMR spectroscopy indicates that the extracted lignin is of GS type, predominantly preserving its C-C bonds while exhibiting a reduction in ether linkages. Furthermore, the study elucidates that the formation of extensive hydrogen bonding interactions between the dual-acidic DES and lignin is pivotal for the efficient extraction process, while also preserving the structural integrity of cellulose. Additionally, hemicellulose is mainly converted into furfural. The lignin derived from Camellia oleifera shells demonstrates the considerable potential for the production of platform chemicals, liquid fuels, and biomass-derived materials.