Sustainable and General Production of Carboxylated Cellulose Nanocrystals via Synergistic Lewis-Acid/Organic-Acid Hydrolysis under Ambient Conditions

Cellulose nanocrystals (CNCs) stand out as promising biobased nanomaterials owing to their nanoscale dimensions, abundant surface groups, and excellent reinforcement capabilities in polymer composites. However, traditional preparation methods rely on harsh mineral acids, raising environmental and thermal stability concerns. In this work, a chloride/citric acid system of ZnCl₂ or FeCl₃ with citric acid (CA) was employed as a comparative platform to investigate the distinct roles of acid-catalyzed and oxidative hydrolysis in the preparation of carboxylated CNCs under mild conditions. The contrasting Lewis acidity and oxidative behavior of ZnCl₂ and FeCl₃ enabled mechanistic insights into their hydrolytic pathways, while alkaline pretreatment of cellulose further amplified these differences. Carboxylated CNCs produced with ZnCl₂ exhibited markedly better thermal stability. Mild alkaline pretreatment was further introduced to improve molecular accessibility, significantly enhancing surface carboxylation. Carboxylated CNCs prepared via the ZnCl₂/CA system with alkaline pretreatment demonstrated excellent thermal stability (T_max = 375.0 °C), substantial carboxyl content (1.3 mmol/g), and exceptional yield (89.2%). The generality of this method was validated across multiple cellulose sources, highlighting its potential as a sustainable and efficient alternative to conventional acid hydrolysis for CNCs preparation.