Ionic liquids: Functionalization and absorption of SO2

Abstract

Room-temperature ionic liquids (ILs), which have excellent properties, such as high gas absorption abilities, extremely low volatility and tunable structures, are regarded as environmentally-friendly absorbents and widely used in SO2 absorption and separation. As a result, a large number of ILs have been synthesized to capture SO2 from flue gas or simulated gas, but a part of them just have physical interaction with SO2 and can hardly absorb SO2 when the content of SO2 is very low. Hence, functional ILs, which can chemically absorb a large amount of SO2 with low contents, have been designed and synthesized for SO2 capture. Up to now, many kinds of functional ILs were investigated for SO2 absorption from flue gas. In this review, the functional ILs are classified into guanidinium based ILs, hydroxyl ammonium based ILs, imidazolium/pyridinium based ILs, quaternary ammonium based ILs, phosphonium based ILs, and other kinds of ILs according to their cations. The capacities of SO2 absorption in these ILs, the mechanism of the absorption, and the ways to enhance the absorption are briefly introduced. The prospect of functional ILs for their application in SO2 removal is presented. The present problems and the further studies are also discussed.