High Production Yield and More Thermally Stable Lignin-Containing Cellulose Nanocrystals Isolated Using a Ternary Acidic Deep Eutectic Solvent

From the aspects of green chemistry and sustainability, the use of green and sustainable materials and reagents for nanocellulose production is highly desirable. In this study, an acidic deep eutectic solvent (DES) pretreatment process was developed to fabricate lignin-containing cellulose nanocrystals (LCNCs) from undervalued thermomechanical pulp (TMP). LCNCs were successfully obtained using both binary DES (choline chloride:oxalic acid, 1:1 molar ratio) and ternary DES (choline chloride:oxalic acid:p-toluenesulfonic acid, 2:1:1 molar ratio) followed by a mild mechanical disintegration process. The LCNCs with a width of around 6 nm, thickness of 3.3 nm, retained cellulose I crystallinity of 57.4%, high lignin content of 47.8%, and high yield of 66% were obtained under the optimum conditions using ternary DES at 80 °C for 3 h pretreatment. Meanwhile, the LCNCs obtained from this process showed a high thermal stability (Tmax of 358 °C), which exhibited promising potential for further applications. The results demonstrate that the environmentally friendly DES is a promising solvent, which can provide a prospective future for both lignocellulosic material utilization and LCNCs isolation.