Ionic liquid strategy for chitosan production from chitin and molecular insights

The production of chitosan is an interesting research topic. Chitosan is an important polysaccharide in terms of its various applications in industries and is produced from chitin, an abundant biopolymer in crustacean shell biomass wastes. Traditional processes for chitosan manufacture are commonly based on highly concentrated alkaline or acid solutions, which are, however, severely corrosive and harmful to the environment. In this study, we have described a ‘greener’ method using 1-ethyl-3-methylimidazolium acetate, [Emim][OAc] ionic liquid (IL), for decrystallization of shrimp crystalline chitin flakes followed by a microwave-mediated NaOH or tetrabutylammonium hydroxide, [TBA][OH], solution-based deacetylation step for chitosan production. The decrease in crystallinity in IL-pretreated chitin was confirmed by XRD and SEM analysis, which subsequently benefited chitosan production with up to 85% degree of deacetylation (% DDA) in shorter time periods (1-2 hours) and lower alkaline concentrations (20-40 wt%). The % DDA in chitin/chitosan was estimated using FT-IR and NMR spectroscopies. Notably, we could regenerate the ionic liquids: in the case of [Emim][OAc] 97 wt% and in the case of [TBA][OH] 83 wt% could be reused. Roles of ionic liquids in the process were discussed. Molecular dynamics (MD) simulations showed the roles of [TBA]⁺ cations in the molecular driving forces of the [TBA][OH]-induced deacetylation mechanism. The strategy promises a sustainable and milder approach compared to the existing highly corrosive alkaline- or acid-involved processes for chitosan production.