Mechanochemical Amorphization of α-Chitin and Conversion into Oligomers of N-Acetyl-D-glucosamine

Mechanochemical treatment offers great potential for environmentally sustainable processing of chitin within the context of biomass valorization. Using powder X-ray diffraction, we show that crystallinity can be reduced by 50% in 2 h in a controlled way using a ball mill. We correlate this crystallinity reduction with a decrease in interchain hydrogen bonding using infrared spectroscopy as a structural probe. Furthermore, our quantitative interpretation of the spectra reveal a decrease in glycosidic linkage content and retention of N-acetyl groups. The addition of a natural clay, kaolinite, in the ball mill leads to a significant increase in the solubility of the milled materials (up to 75.8% water-soluble products in 6 h, cf. 35.0% without kaolinite). The products of this process were characterized as oligomers of N-acetyl-D-glucosamine (chitin oligomers) with degrees of polymerization (DP) between 1 and 5 using a new quantitative matrix-assisted laser desorption ionization (MALDI-ToF) mass spectrometric method. These data were complemented by a colorimetric assay of reducing ends and size-exclusion chromatography (SEC). N-Acetyl-D-glucosamine (the monomer) and N,N'-diacetylchitobiose (the dimer) were obtained in yields of 5.1 and 3.9 wt %, respectively, within 6 h, which is comparable with yields of glucose and cellobiose from cellulose ball milling.