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 (T_max 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.

中文翻译：

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FeCl ₃催化无规柠檬酸水解高效，可持续地制备羧酸和热稳定纤维素纳米晶体

作为一种弱的三羧酸，由于柠檬酸（CA）无毒且易于回收的特性，已被提出用于制备纤维素纳米晶体（CNC）。然而，由于CA的弱酸性，通过CA水解获得的CNC的产率通常低于30％。在这项工作中，FeCl ₃被发现是一种强大的催化剂，可以增强CA水解效率，从而绿色高效地制备羧酸和热稳定的CNCs。在85 wt％CA水溶液和0.02 mmol FeCl ₃的最佳条件下，通过FeCl ₃催化的CA水解可以达到80.28％的最大产率。/ g CA在80°C进行6 h水解。结果发现，所制备的CNC（直径6-12 nm，长度100-250 nm）显示出最大的结晶度指数约为80％，并且具有很高的热稳定性（T _max约为355°C）。而且，获得的被羧基官能化的CNC在水性介质中显示出优异的分散性。此外，CA和FeCl ₃水解后可以很容易地回收，水解后收集的糖可以用作生产平台化学品如乳酸和糠醛的前体。因此，本研究为高效生产具有高度分散性和高热稳定性的羧化CNC提供了一种绿色且可持续的方法。获得的功能性CNC可以在各种高科技领域中找到应用。