The Research About Functional Ionic Liquid Absorbing SO₂ And Its Mechanism

Sulfur dioxide (SO2) in the atmosphere is mainly coming from the direct combustion of coal. SO2 is not only a main harmful pollutant to the air, but also a kind of important chemical raw material. So it is very necessary to recycle SO2 from flue gas.Ionic liquids are a kind of chemicals that have unique qualities such as low volatile, wide liquid temperature range, high thermal stability, adjustable structure and recyclability, and so on. Scientists found that ionic liquids have high solubility for SO2. Ionic liquids have not been used in industrial flue gas desulfurization directly because these are still a lot of problems that have not been solved. For example, the synthetic process is complex, raw material price is high, viscosity is too high for mass transfer, and the mechanism of absorbing SO2 by ionic liquids is not clear. All these need to be solved for industrial application. In this work, the mechanism of SO2 absorption in functional ionic liquids was studied. Two kinds of functional ionic liquids, protic and aprotic ionic liquids, were chosen to explore the mechanism of SO2 absorption, and an amino functionalized ionic liquid was also chosen for comparison. The main results and conclusions of this work are shown as follows.1. A protic ionic liquid [TETA]L4, an aprotic ionic liquid [N2222]L, and an hydrophobic aprotic ionic liquid [Et2NEmim][PF6]) were synthesized in this work. The effects of temperature and SO2 partial pressure on the absorption of SO2 by the three ionic liquids without water were investigated. The results show that the solubilities of SO2 in the three ionic liquids are decreased with the increase of temperature and increased with the increase of the partial pressure of SO2.2. Through the model and experimental results, it has been obtained that mole chemical stoichiometries of [TETA]L4, [N2222]L and [Et2NEmim][PF6] to SO2 are 2,1 and 1, respectively. It means that one mole of [TETA]L4, [N2222]Land [Et2NEmim][PF6] can absorb 2 moles,1 mole and 1 mole of SO2 through chemical interaction, respectively.3. In order to study the mechanism between ionic liquids and SO2 without water, FT-IR and NMR were used to characterize the structures of ionic liquids before and after SO2 absorption. The results show that whether protic ionic liquid or aprotic ionic liquid, SO2 only interacts with the anion of the ionic liquids, while for the hydrophobic aprotic ionic liquid [Et2NEmim][PF6], SO2 interacts with the N of tertiary amino-group on the cation of [Et2NEmim][PF6].4. The effect of water content in ionic liquids on SO2 absorption has been investigated. The solubilities of SO2 in [TMG]L+H2O and [N2222]L+ H2O binary systems are increased with the increasing of the water content in ionic liquid aqueous solutions.5. Densities and viscosities of ionic liquid and water binary solutions before and after SO2 absorption have been measured. Density and viscosity of the ionic liquid aqueous solutions increase after absorption of SO2, and they decrease when temperature increased. A binary polynomial and Vogel-Tammann-Fulcher equation could be used to correlate the density and viscosity and the deviations is very low.6. Density and viscosity of ionic liquid aqueous solutions during SO2 absorption have been studied. The results show that the density of [TMG]L+ H2O solutions and [N2222]L+H2O solutions has a similar change trendency during SO2 absorption. The density increases quickly at first, then the speed slow down when the solubilitiy of SO2 in ionic liquid reached to about 1 mol SO2/mol IL. And viscosity change is also very obvious, and with the increase of SO2 content, the viscosity is increased first, and then decreased when the solubilitiy of SO2 in ionic liquid reached to about 1 mol SO2/mol IL. This is because the absorption in the beginning is chemical absorption as the main role, when after reaching chemical equilibrium, hydrated ionic liquids can also absorb SO2 through physical interaction. With the turning point it can be concluded that the molar ratio of chemical reaction between aqueous ionic liquid and SO2 is 1:1.7. In order to study the mechanism of SO2 absorption in ionic liquids with water, FT-IR and NMR have been used to characterize the structure of ionic liquids before and after SO2 absorption. The results show that when water exists, SO2 is absorbed chemically by the functional ionic liquids following the principle that a strong acid replaces a weak acid, i.e. sulfurous acid can replace lactic acid during SO2 absorption.