The recent strategies in preparation of cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) were described. CNCs and CNFs are two types of nanocelluloses (NCs), and they possess various superior properties, such as large specific surface area, high tensile strength and stiffness, low density, and low thermal expansion coefficient. Due to various applications in biomedical engineering, food, sensor, packaging, and so on, there are many studies conducted on CNCs and CNFs. In this review, various methods of preparation of CNCs and CNFs are summarized, including mechanical, chemical, and biological methods. The methods of pretreatment of cellulose are described in view of the benefits to fibrillation.

In the current work, a green and recyclable FeCl3-catalyzed deep eutectic solvent system (F-DES) was invented to fabricate cellulose nanocrystals (CNCs) with a high yield and excellent thermal stability. It was found that the optimum composition of the FeCl3-catalyzed deep eutectic solvent was composed of oxalic acid dihydrate (Oxd), choline chloride (ChCl), and FeCl3·6H2O in a mass ratio of 4:1:0.2 (corresponding to the molar ratio of 4.43:1:0.1). Results showed that CNCs with a diameter range of 5–20 nm and length of 50–300 nm could be isolated from bleached eucalyptus kraft pulp (BEKP) at a high yield (over 90% based on the cellulose content in BEKP) by a one-step F-DES treatment under mild conditions (80 °C, 6 h). The resultant CNCs showed a much higher thermal stability (onset thermal degradation temperature was over 310 °C) than the traditional sulfuric acid hydrolyzed ones and also exhibited superior dispersion stability in water due to the introduction of carboxyl groups on the surface of CNCs by esterification. In addition, the separated F-DES could be directly reused to produce CNCs at least three times. Intriguingly, all the components of the reused F-DES could be separated by a simple separation process with few pollutants releasing into the environment. Therefore, the F-DES process could be a green and economically feasible method for the preparation of thermally stable and dispersible CNCs.

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