Acid hydrolysis of chitin in calcium chloride solutions

Abstract

Chitin hydrolysis is an important but challenging reaction for shell biorefineries. In this study, we devised a mild aging-hydrolysis integrated approach to selectively produce N-acetyl-D-glucosamine (NAG) from chitin in cheap and abundant CaCl₂-based molten salt hydrate solvents (MSHs). The acid-aged chitin could be directly employed and hydrolyzed in 50 wt% CaCl₂ solution at 393 K for 1 h, producing NAG in 51.0% yield, which was further improved to 66.8% with the addition of ZnBr₂ (10 wt%) as a co-salt. The synergy of aging and the CaCl₂ MSHs was critical for the efficient chitin hydrolysis. A significant decrease of chitin's molecular weight was observed after aging at near room temperature, while the CaCl₂ MSHs enabled chitin swelling and dissolution to facilitate the substantial cleavage of glycosidic bonds during the hydrolysis reaction. The combined contrast experiments, TOF-MS, XPS, and NMR studies revealed that both the Ca²⁺ cation and the Cl⁻ anion have closely interacted with the chitin chains primarily by coordinating with the carbonyl oxygen/amido nitrogen atoms or by forming hydrogen bonds with the –OH groups, respectively, which could alter the electrostatic states of the chitin chains and favor the hydrolysis.