Degradation of Chitosan by Hydrothermal Process in the Presence of Sonication Pre-Treatment with Supercritical CO₂ as Pressurized Fluid

Summary For chito oligomer production, the efficient hydrolysis of chitosan to smaller substances is important. The focus of this study is to explore the formation and changes in the chitosan degradation product and properties after chitosan hydrolysis in subcritical water. The hydrothermal process was pressurized by supercritical CO₂ used as the pressurized fluid and catalyst. Pre-treatment sonication was also used to change the molecular weight of the chitosan. The effect of the reaction time on the formation of various products and chitosan residue was studied. The chitosan was pre-treated by sonication at 60°C for 120 min before subjected to the hydrothermal process at 200°C and pressure of 23 MPa for 3-5 min. The chitosan water slurry of 1 wt % in a batch reactor was rapidly heated to the reaction temperature for a specific time. After the reaction, the product was rapidly cooled in a cooling medium. The total yield reached about 15% based on the initial chitosan at 200°C in 5 min. Upon an increase in the reaction time, the side group of monomers (NH₂ or N-acetyl) tended to be attacked and replaced by OH to produce glucose and also partially degrade into 5-HMF. The hydrothermal process had no significant effect on the chitosan structure except for the changes in the inter- and intramolecular hydrogen bondings of chitosan and the degree of crystallinity of the chitosan residue in the range of 19.2 to 28.9%.