Investigation of Pretreatment Methods for Improving TEMPO-Mediated Oxidation and Nanofibrillation Efficiency of α-Chitin

Four different pretreatment processes, that is, glycerol swelling, high-pressure steaming, low-temperature freezing, and grinding, were proposed and combined with TEMPO-mediated oxidation in order to prepare chitin nanocrystals. The chemical and crystalline structures as well as the morphological properties of the original, pretreated, and nanofibrillated chitin were evaluated. Accordingly, the effects of the pretreatment on the oxidation and on the subsequent nanofibrillation of chitin were investigated. The results proved that pretreatments effectively improved the oxidation efficiency and reduced the consumption of the oxidants. Oxidized chitin with a high carboxylate content of 1.05-1.70 mmol/g was successfully obtained with a yield in the 40-90% range using different combinations of pretreatments and alkali TEMPO/NaBr/NaClO oxidation. Notably, the carboxylate content reached 0.82 mmol/g after the mechanical-grinding pretreatment and neutral TEMPO/NaClO/NaClO2 oxidation, and the weight ratio of the water-insoluble fraction exceeded 95%. This unique, simple, and high-efficiency pretreatment strategy offers a green and ecologically friendly alternative for the preparation of chitin nanocrystals as part of an integrated biorefinery approach.