Electron-beaming of cellulose solutions in 3-alkyl-1-methylimidazolium acetate ionic liquids: side reactions of the acetate anions

A combination of swelling in imidazolium-type ionic liquids (ILs) and beta-irradiation, used in biomass pretreatments, is not an inert procedure but involves some unexpected side reactions. Beta-irradiation of cellulose dissolved in the ionic liquids 3-ethyl-1-methylimidazolium acetate (EMIm-OAc) or 3-butyl-1-methylimidazolium acetate (BMIm-OAc) caused a "lack" of acetate upon reprecipitation of the cellulose in water, which was mainly replaced by hydroxide anions to maintain charge equivalence. This study addressed the chemical fate of the missing acetate, identification of the byproducts, and discussion of the underlying reaction mechanisms. The acetate anions were consumed by the formation of a mixture of dehydroacetic acid and isodehydroacetic acid, identified after purification through NMR spectroscopy and comparison to authentic samples. Both compounds consist of four condensed C2-units from acetic acid (acetyl groups Department of Natural Sciences and Sustainable Resources). The formation proceeds via aldol-type condensation of two molecules of acetoacetic acid, reacting with each other in two different ways. The acetoacetic acid intermediate, in turn, is formed from acetic acid (acetate) by catalysis through the N-heterocyclic carbene which is generated from the IL cation upon beta-irradiation. Formation mechanism and kinetics are discussed. The study points out that even seemingly "innocuous" IL anions like acetate can well undergo side reactions when extreme conditions, such as beta-irradiation, are in play. It provides yet another example of the fact that combining cellulose solutions in imidazolium ionic liquids with the "physical" process of beta-irradiation actually involves a lot of chemistry and side reactions.