Efficient Extraction of Cellulose Nanocrystals through Hydrochloric Acid Hydrolysis Catalyzed by Inorganic Chlorides under Hydrothermal Conditions

Four inorganic chlorides were introduced into hydrochloric acid hydrolysis to extract cellulose nanocrystals (CNCs) from microcrystalline celluloses (MCC) under hydrothermal conditions. The as-prepared CNCs were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT–IR), and thermogravimetric analysis (TGA). The role of inorganic chlorides including ferric chloride hexahydrate (FeCl3·6H2O), copper chloride dihydrate (CuCl2·2H2O), aluminum chloride (AlCl3), and manganese chloride tetrahydrate (MnCl2·4H2O) in the extraction and properties of high quality CNCs was determined. It is observed that the introduction of inorganic chlorides obviously enhanced the hydrolysis process through faster degradation of the disordered region of cellulose. Compared with those for pure hydrochloric acid hydrolysis, smaller diameter and a larger length to diameter ratio of CNCs could be obtained through salt-catalyzed hydrolysis, which could contribute to greater enhancement on the mechanical properties of polylactic acid (PLA) nanocomposite films. Moreover, it is found that the highest reinforcing effects for the PLA matrix as well as the best transparency among all the nanocomposites were achieved in the presence of ferric chlorides, benifiting from the largest length to diameter ratio and most white of the corresponding CNCs. These results show that the use of salt-catalyzed hydrolysis especially ferric chloride has a significant improvement in achieving the energy-efficient and cost-effective conversion of cellulose starting materials into high quality CNCs.