Highly Efficient and Sustainable Preparation of Carboxylic and Thermostable Cellulose Nanocrystals via FeCl3-Catalyzed Innocuous Citric Acid Hydrolysis

As a weak tricarboxylic acid, citric acid (CA) has been proposed for the preparation of cellulose nanocrystals (CNCs) due to its innocuous and easily recyclable nature. However, the yield of the obtained CNCs is normally below 30% by CA hydrolysis due to the weak acidity of CA. In this work, FeCl3 was discovered as a powerful catalyst to enhance the CA hydrolysis efficiency for the green and highly efficient preparation of carboxylic and thermostable CNCs. The maximum yield of 80.28% could be achieved via FeCl3-catalyzed CA hydrolysis under the optimal condition of 85 wt % CA aqueous solution and 0.02 mmol FeCl3/g CA at 80 °C for 6 h hydrolysis. It was found that the as-prepared CNCs (6–12 nm in diameter, 100–250 nm in length) showed a maximum crystallinity index of around 80% and high thermal stability (Tmax is around 355 °C). Also, the obtained CNCs functionalized with carboxylic groups exhibited superior dispersibility in an aqueous medium. In addition, both the CA and FeCl3 could be easily recovered after hydrolysis, and the collected sugars after hydrolysis can be used as precursors for producing platform chemicals such as lactic acid and furfural. Therefore, this study presents a green and sustainable approach for the highly efficient production of carboxylic CNCs with great dispersibility and high thermal stability. The obtained functional CNCs may find applications in a variety of high-tech fields.