Side Reactions in the System Cellulose/1-Alkyl-3-methyl-imidazolium Ionic Liquid

摘要

In solutions of cellulose in 1-alkyl-3-methyl-imidazolium ILs, both the ionic liquid and the cellulose are evidently not inert. Side reactions can be distinguished according to whether they involve the cation or the anion of the ionic liquid or just autodegradation products of the IL. Ionic liquids with 1-alkyl-3-methyl-imidazolium cations react at C-2 with cellulose at its reducing end (and, if present, other aldehyde functionalities along the chain), forming a carbon-carbon bond. The reaction is strongly catalyzed by bases, so that commonly present impurities in ILs, such as their thermal degradation products, promote this reaction. These thermal degradation products are mainly imidazole, N-alkylimidazole, N-methylimidazole and methylene-bridged dimers. Side reactions of ILs anions are more divers and depend on the respective reaction system. Acetate as the IL anion is generally unsuitable for esterifications by acylating agents (halides, anhydrides), both organic and inorganic ones, for methylations (alkyl halides, dialkyl sulfate), or for silylations (trimethylsilyl halides), since the reagents coreact with the IL anion rather than with the cellulosic hydroxyls. Chloride anions in the IL cannot be used for alkylations (alkyl halides, dialkyl sulfate) or oxidations (NMMO, periodate).