Absorption and Conversion of SO₂ in Functional Ionic Liquids: Effect of Water on the Claus Reaction

The absorption of SO₂ from flue gas and its conversion to chemicals is important in the industry. Functional ionic liquids (ILs) have been broadly used to absorb SO₂ in flue gas, but seldom convert it to chemicals. As we know, water is inevitable in a desulfurization process. In this work, three functional ILs (monoethanolaminium lactate-[MEA][Lac], 1,1,3,3-tetramethylguanidinium lactate-[TMG][Lac], tetraethylammonium lactate-[N₂₂₂₂][Lac]) with or without water were used as absorbents to absorb SO₂ in flue gas, and then the absorbed SO₂ in the absorbents was converted to sulfur via a Claus reaction. The result shows that the three ILs can efficiently absorb SO₂ and convert it to sulfur. But the addition of water in the ILs can reduce the conversion of absorbed SO₂, and the conversion increases with increasing the acidity of absorbents. To explain this phenomenon, we studied the Claus reaction in H₂SO₃, NaHSO₃ and Na₂SO₃ aqueous solutions. It turns out that the conversion of the Claus reaction is related to the species of S (IV) in the order of the oxidability: H₂SO₃ > HSO₃ ^- > SO₃ ^2-, and their proportions dependent on the pH of solutions. On the basis of the absorption mechanism of SO₂ in functional ILs aqueous solution, H₂S reacts with HSO₃ ^- and SO₃ ^2- with weaker oxidability, resulting in the lower conversion. Importantly, we found that the addition of lactic acid could increase the conversion of SO₂ via the Claus reaction.