Carboxylated Chitosan Nanocrystals: Novel Synthetic Route and Application as Superior Support for Gold-Catalyzed Reactions

Chitin nanocrystals (ChNCs) were prepared by partial cleavage of glycosidic bonds in chitin with concurrent oxidation of chitin C6 primary alcohols to produce carboxylate groups on the surface of the ChNCs. Following alkaline deacetylation of the ChNCs in the presence of NaBH₄ to inhibit “end-peeling” afforded chitosan nanocrystals (ChsNCs) with a degree of deacetylation (DDA) >80%. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier-transform Infrared (FTIR) spectroscopy were used to determine the morphology and composition of these carboxylated ChNCs and ChsNCs. Subsequently, two methods were used to deposit Au onto the nanocrystals, and the catalytic activities of the resulting biomass-based nanocatalysts were tested for the 4-nitrophenol reduction and the aldehyde-amine-alkyne (A³) coupling reaction. In particular, Au nanoparticles over ChsNCs featured the highest turnover frequency value for the 4-nitrophenol reduction reported to date. Spectroscopic and imaging techniques confirmed the importance of controlling precisely the redox state of Au as it is being deposited to afford highly disperse active site on the bio-nano-support.