Acidic Deep Eutectic Solvents As Hydrolytic Media for Cellulose Nanocrystal Production

In this study, a new method to fabricate cellulose nanocrystals (CNCs) based on DES pretreatment of wood cellulose fibers with choline chloride and organic acids are reported. Oxalic acid (anhydrous and dihydrate), <i>p</i>-toluenesulfonic acid monohydrate, and levulinic acid were studied as acid components of DESs. DESs were formed at elevated temperatures (60–100 °C) by combining choline chloride with organic acids and were then used to hydrolyze less ordered amorphous regions of cellulose. All the DES treatments resulted in degradation of wood fibers into microsized fibers and after mechanically disintegrating, CNCs were successfully obtained from choline chloride/oxalic acid dihydrate-treated fibers, whereas no liberation of CNCs was observed with other DESs. The DES-produced CNCs had a width and length of 9–17 and 310–410 nm, respectively. The crystallinity indexes (CrIs) and carboxylic acid content of the CNCs were 66–71% and 0.20–0.28 mmol/g, respectively. CNCs exhibited good thermal stabilities (the onset thermal degradation temperatures ranged from 275–293 °C). The demonstrated acidic DES method exhibits certain advantages over previously reported CNC productions, namely, milder processing conditions and easily obtainable and relatively inexpensive biodegradable solvents with low toxicity (compared, e.g., to ILs).