Purification of EMIMOAc Used in the Acetylation of Lignocellulose

Abstract

Up to now, several methods of purifying ionic liquids (ILs), such as the extraction with supercritical carbon dioxide, crystallization, column chromatography, and so forth were reported. The IL that was used in the acetylation of lignocellulose with the help of acetic anhydride contains an elevated amount of acetic acid. In this paper our investigations on the separation of acetic acid from synthetic mixtures with 1-ethyl-3-methylimidazolium acetate (EMIMOAc) are described. The separation was performed by evaporation, extraction, and esterification. While impurities like ethyl acetate, n-propyl acetate, isopropyl acetate, and tetrahydrofuran (THF) can easily be evaporated from EMIMOAc, it is difficult to remove acetic acid from EMIMOAC or EMIMCl by evaporation below certain concentration levels. In extraction tests acetic acid could be separated from EMIMOAc to some degree, especially with extractants immiscible with EMIMOAc having a high value of ET(30) and a dielectric constant near that of acetic acid. The most successful removal of acetic acid was found to be an esterification of acetic acid at a large excess of alcohol, a long reaction time, and an intensive contact of the educts in the liquid phase at elevated temperature and pressure with subsequent evaporation of the produced acetic acid ester.