Facile Extraction of Wheat Straw by Deep Eutectic Solvent (DES) to Produce Lignin Nanoparticles

A detailed investigation of choline chloride-lactic acid (ChCl-LA)-based deep eutectic solvent (DES) extraction of lignin nanoparticles from herbaceous biomass (wheat straw) was conducted. It was found that DES can extract high purity lignin (up to 94.8%) with a high yield (up to 81.5% from air-dried samples and 85.9% from oven-dried samples) from wheat straw. The cleavages of ether bonds in lignin, as well as the linkages between lignin and hemicelluloses during DES treatments at different conditions, were analyzed. The effects of reaction time, treatment temperature and water content in wheat straw on lignin yield, purity, and chemical structure were determined. The water content in biomass was found to affect the hydrogen bond interaction between lignin and DES, which was a key factor influencing the lignin extraction yield and chemical properties of separated lignin. The lignin extracted from wheat straw by DES consists of well-dispersed nanoparticles with a narrow size distribution peaking at 70–90 nm. The mechanism of ChCl-LA-based DES depolymerization and extraction of lignin from wheat straw is discussed.