Formation of Anhydrosugars from Polysaccharides in Ionic Liquids by Microwave Irradiation

Abstract

It is known that some ionic liquids can dissolve cellulose, and that ionic liquids can be heated very rapidly using microwave irradiation. These characteristics make it possible to use ionic liquids as a reaction medium to form anhydrosugars from polysaccharides by microwave irradiation. In the present study, we examined formation of anhydroglucose from glucose, cellobiose, and cellulose using microwave irradiation in several ionic liquids. A single-mode microwave was used to carry out the reactions. The yield of anhydroglucose from cellobiose was quite high: a 60 mol% yield at 270 °C after 4 min of heating time. It has also been found that in the ionic liquids, diethyl-methyl-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide and butyl-methylimidazolium bis(trifluoromethanesulfonyl)imide, a few products were obtained from depolymerization of cellulose and cellobiose. The main products were identified as 1,6-anhydro-β-D-glucopyranose and 1,6-anhydro-β-D-glucofuranose. In contrast, in the ionic liquid butyl-methylimidazolium chloride after a few minutes of microwave heating, the samples turned dark brown; no anhydrosugars were produced. These results suggest the importance of proper selection of the anion in the ionic liquids to produce anhydrosugars.