Green and Scalable Synthesis of Chitin Nanocrystals via Microwave-Assisted Hydrolysis Using Recyclable Ferric Chloride Hexahydrate

Chitin nanocrystals (ChNCs) are a green and sustainable material, but their conventional acid hydrolysis process is not considered economically or environmentally friendly. Here, an eco-friendly ferric chloride hexahydrate (FeCl3·6H2O) solution combined with microwave-assisted hydrolysis was employed for ChNCs production. Compared with hydrochloric acid (HCl) hydrolysis, trivalent ferric ion (Fe3+) induced the disruption of intermolecular hydrogen bonds while preventing severe glycosidic bond degradation, thereby promoting the formation of high-aspect-ratio ChNCs rather than small particles. Additionally, an accelerated deacetylation rate was unexpectedly observed, as confirmed by the ζ potential at pH 6 (+25.0 mV), FTIR, XPS, and elemental analysis. The ability to induce molecular polarization enables microwave irradiation to significantly enhance the hydrolysis rate and uniformity, improving the selective removal of the amorphous region. Within 0.75 h, highly crystalline (89.73%), high-yield (83.8%), and well-defined ChNCs (aspect ratio 14, length 224.77 ± 23.76 nm, diameter 15.98 ± 3.52 nm) were obtained. The combined treatment strategy improved the dispersion uniformity and compatibility of ChNCs in poly(vinyl alcohol) (PVOH), resulting in enhanced network formation and viscosity in the suspension. The resulting nanocomposite films exhibited smooth surfaces, combined high strength and elongation at break, and low swelling behavior. Simple crystallization allowed the recovery of FeCl3·6H2O from the hydrolysate without affecting the hydrolysis efficiency in subsequent reuse. A comprehensive sustainability assessment based on green chemistry metrics confirmed a significant reduction in resource consumption compared to conventional acid hydrolysis methods. This green and scalable approach aligns well with the principles of green chemistry and offers an efficient, sustainable route for ChNC production.