(465g) Mechanism of SO2 Absorption by Ionic Liquids

Nowadays, more and more people have drawn much attention to air pollution. Ionic liquids (ILs) as environmentally benign solvents have been investigated to absorb SO₂ from flue gases efficiently. However, the mechanism is still uncertain. We investigated five types of ILs and measured the physical and chemical prosperities during the absorption to find the mechanism. We found that task-specific ILs, such as 1,1,3,3-tetramethylguanidinium lactate ([TMG]L), could chemically and physically absorb SO₂ and the stoichiometric ratio of SO₂ to ILs is 0.5. There was a chemical equilibrium constant between SO₂ and ILs. But for normal ILs, such as 1,1,3,3-tetramethylguanidinium tetrafluoroborate ([TMG][BF₄]), only physical absorption existed, and it followed Henry’s law.

However, how to divide task-specific and normal ILs is another problem. We studied several kinds of ILs with TMG⁺ as cation and used these ILs to absorb SO₂, as shown in Figure 1.

In order to carry out the experiment, we added little water into some solid ILs, such as [TMG]benzoate and [TMG]methylsulfonate, as water has no effect on the absorption of SO₂ by ILs, which is reported in our former work. We found that the absorption of SO₂ by these ILs was determined by the pKa of the acids, which were the anion of ILs. And the pKa of the acids are shown in Table 1.

When the concentration of SO₂ was very low, there is almost no physical interaction between SO₂ and ILs. From Figure 1 and Table 1, we proposed that task-specific ILs and normal ILs were divided by the pKa of acid. If the acid was stronger than sulfurous acid, there was chemical interaction between SO₂ and IL. And this IL could be regarded as task-specific IL, otherwise it was normal IL.

The authors thank the Natural Science Foundation of China (20746001 and 20776004) and Program for New Century Excellent Talents in University (NCET-08-0710) for financial support. 

Table 1. The pKa of the acids from web of wikipedia.