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.
