Ionic liquids: Functionalization and absorption of SO 2

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 SO 2 absorption and separation. As a result, a large number of ILs have been synthesized to capture SO 2 from flue gas or simulated gas, but a part of them just have physical interaction with SO 2 and can hardly absorb SO 2 when the content of SO 2 is very low. Hence, functional ILs, which can chemically absorb a large amount of SO 2 with low contents, have been designed and synthesized for SO 2 capture. Up to now, many kinds of functional ILs were investigated for SO 2 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 SO 2 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 SO 2 removal is presented. The present problems and the further studies are also discussed.