SO2 absorption in pure ionic liquids: Solubility and functionalization.

The SO2 solubility in ionic liquids and absorption mechanisms with different functionalities, including ether, halide, carboxylate, dicarboxylate, thiocynate, phenol, amino, azole groups, etc., are presented in this review. Strategies of improving SO2 capture with low binding energy and the separation performance from CO2 are also concluded. Generally, moderate basicity is favourable for enhancing SO2 capacity and the water (below 6 wt%) effect on absorption is indefinite but generally slight. Introducing electron-withdrawing substituents such as nitrile, halogen, aldehyde and carboxylic groups are proposed to decrease the chemical absorption enthalpy between ionic liquid and SO2 in order to reduce regeneration power consumption. Although it is promising, the absorption enthalpy is still much higher than the physisorption performance especially of the ether-functionalized ones. The biocompatible choline-based, betaine-based, and amino acid ionic liquids have clear trends to be applied in SO2 capture due to their biodegradability, nontoxicity and easy accessibility. Generally, comparing to the traditional solvents, ionic liquids have made great improvement in SO2 capacity, however, the high viscosity and desorption energy are two main obstacles for SO2 absorption and separation. Molecular simulations have been applied to reveal the absorption regimes involving the roles of basic functionalities and physical interactions especially the hydrogen bonds, which could be referred for structure designing of the available ionic liquids with readily fluid characteristics and absorption ability.