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.