Functional Comparison of Polar Ionic Liquids and Onium Hydroxides for Chitin Dissolution and Deacetylation to Chitosan

A series of ionic liquids (ILs) have been synthesized, and the dissolution ability of chitin has been evaluated under mild conditions. We have focused on tris(2-hydroxyethyl)methylammonium (THEMA) type ILs. These [THEMA]-type ILs partly dissolved chitin, and addition of ethylenediamine (EDA) was confirmed to improve the chitin dissolution. Relatively high hydrogen bond accepting ability of the IL/EDA mixtures was required to dissolve chitin. Among these mixtures, the [THEMA][OAc]/EDA mixture dissolved chitin completely without heating. As other potential solvents, a series of tetraalkylammonium hydroxides/water mixtures was also found to dissolve chitin without heating. Conversion of chitin to chitosan during the dissolution process has been found in such tetraalkylammonium hydroxides/water mixtures in spite of the fact that almost no deacetylation was found in the IL/EDA mixture. The effect of ILs and onium hydroxides on both chitin dissolution and deacetylation ability was discussed.