Sulfur dioxide (SO2), mainly emitted from the burning of fossil fuels, brings out severe air pollution such as acid rain. Thus SO2-emitting industries worldwide are under pressure to reduce their sulfur emissions. The technologies for SO2 adsorption have attracted increasing interests, among which flue gas desulfurization is the most efficient way. The traditional flue gas desulfurization technologies include dry scrubbing, wet scrubbing using calcium-based absorbents and absorption by aqueous amines. However, these technologies have some problems such as high cost, large amounts of non-renewable by products, huge water consumption, and secondary pollution, so their application in industry is limited. Compared to the traditional flue gas desulfurization technologies, ionic liquids present distinct advantages, such as recyclability, environmental friendliness, structural tunability, and good solubility. Therefore, in recent years, ionic liquids have been proposed to capture and separate SO2 from flue gas or mixed gas .
The ionic liquids used to adsorb SO2 mainly include hydroxyl ammonium, imidazolium based, guanidinium based and quaternary ammonium based ionic liquids.
Yuan and co-workers  synthesized a series of hydroxyl ammonium ionic liquids (Figure 1), which present a high solubility of SO2. Among these ionic liquids, 2-hydroxyethyl ammonium showed the highest absorption capacity with a value of 1.04 mol SO2/mol ionic liquids. The absorbed SO2 could be released under vacuum or by heating and the ionic liquids can be recycled for 4 times with a recovery larger than 90 %. The absorption capability of these ionic liquids is closely related to the cations and the anions. The effect of cations on the solubilities of SO2 followed the sequence: 2-hydroxyethylammonium > tri-(2-hydroxyethy) ammonium > 2-(2-hydroxyethoxy) ammonium, while the effect of anions is lactate > acetate > formate.
Figure 1. Structure of the hydroxyl ammonium ionic liquids .
Anderson and co-workers  studied the solubility of SO2 in the ionic liquids 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM][Tf2N]) and 1-n-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([HMPY][Tf2N]). The results revealed that the solubility of SO2 in [HMIM][Tf2N] was extremely high and that the absorption capacity can be as high as 1.6 mol SO2/mol ionic liquids at 25 °C and 3.44 atm. Furthermore, in their work, the Henry's law constants were also calculated. There was over an order of magnitude decrease in the Henry's law constants for SO2 compared to that of CO2. This result suggested that these ionic liquids present high selective towards SO2.
Huang and co-workers  synthesized new 1,1,3,3-tetramethylguanidinium-based ionic liquids and investigated their capacity to absorb SO2. The results showed that up to 2 moles of SO2 could be absorbed per mole of ionic liquid at room temperature and almost completely desorbed by heating or lowering of the pressure. In addition, no change in absorbing ability of ionic liquids was observed after several cycles. The absorption of SO2 in the ionic liquids is an exothermic process and standard enthalpy of solution was found to be in the range -21 to -37 kJ mol−1, suggesting a moderate association between gas and liquid.
Guo and co-workers  synthesized a series of caprolactam tetrabutyl ammonium bromide ([CPL][TBAB]) ionic liquids and the solubility of SO2 was determined at atmospheric pressure. The results indicated that the absorption capacity of SO2 in the ionic liquids can be as high as 1.84 mol/mol ionic liquids at 35 °C and 101.325 KPa. The ionic liquids can be reused for 6 times. Therefore, the prepared ionic liquids show great potential as excellent solvents for the adsorption and recovery of SO2 in the industrial process of flue gas desulfurization.
Figure 2. Structure of ionic liquids [CPL][TBAB] .
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