Direct pretreatment of raw ramie fibers using an acidic deep eutectic solvent to produce cellulose nanofibrils in high purity

Abstract

Due to biomass recalcitrance, the pulping and bleaching processes are usually needed to break down the resistance of the lignocellulosic matrix for nanocellulose extraction. In this study, an acidic deep eutectic solvent (DES) based on choline chloride-oxalic acid dihydrate was used to directly pretreat raw ramie fibers (RFs) without further bleaching/purification steps for producing cellulose nanomaterials. Raw RFs, DES pretreated RFs, and degummed RFs were comparatively studied as starting materials for the preparation of cellulose nanofibrils (CNFs) by ball milling. There was no significant difference between CNF samples prepared from the DES pretreated RFs after 6 h of ball milling and those from the degummed RFs after 12 h of ball milling. After 4 h DES pretreatment at 100 °C, the obtained CNFs had a high purity of glucan up to 90.31% and an average width of 14.29 nm. They also showed high crystallinity (79.17%) and high thermal stability (> 316.7 °C). The tensile strength and elastic modulus for the CNF films were 98.071 MPa and 2.749 GPa, respectively. Overall, with the choline chloride-oxalic acid dihydrate DES pretreatment, no additional pulping/bleaching processes were required for the preparation of nanocellulose from raw cellulosic fibers while the properties of the as-prepared nanocellulose were preserved.