Progress in SO<sub>2</sub> capture by ionic liquids

Abstract

Minimizing the emissions of SO2 is highly important, because it is a significant source of atmospheric pollution that threatens environment and human health. SO2 can be absorbed by ionic liquids (ILs) due to their unique properties, such as ultra low vapor pressure, wide liquid temperature range, non-flammability, chemical stability, and tunable structure and properties. As a new sorption technology, ILs with useful functional groups are attracting the attention of a growing number of scientists and engineers in recent years. In this article, the recent advances on the studies of ILs in the capture of SO2 were reviewed, where traditional ILs and functionalized ILs were included. Traditional ILs with the anions such as BF4 - , PF6 - , Tf2N - , F - , Cl - , Br - and I - could uptake SO2 at ambient temperature and pressure through physical interaction between IL and SO2, but they could not be used under industrial conditions because of the weak interactions. Industrial condition means high temperature and low SO2 partial pressure, which needs strong interaction between IL and SO2. Functional ILs, including organic acid salt ILs, phenolate ILs, azolate ILs, ether-based ILs, amine-based ILs and multi-functional ILs, showed highly efficient SO2 absorption under ambient conditions. 1,1,3,3-Tetramethylguanidine lactate ([TMG][L]) was the first functional IL used to achieve about equal mole SO2 capacity under 40°C and 8% SO2 through chemical interaction. [SCN]-based ILs were proved to be highly efficient and reversible ILs for SO2 capture under low partial pressure by the single-site interaction mechanism. Although ether-based ILs could achieve much higher SO2 capacity due to their multiple ether groups, they could not be used to capture SO2 under industrial conditions because of the weak physical interaction. Among the functional ILs investigated, novel azolate ILs were found to have the two mole of chemical absorption capacity through two-site cooperative interactions under low partial pressure. Very recently, other kinds of multiple-site based ILs were developed, including sites on the independent cation and anion as well as those on the same anion. [Et2NEmim][Tetz] was found to be a kind of multi-functional IL that could absorb two mole of SO2 under low partial pressure through independent anion···SO2 and amine···SO2 interactions. Among the above mentioned functional ILs, the ILs based on [Tetz] and [SCN] anions were proved to be more efficient and highly reversible for the capture of SO2 under low SO2 partial pressure. The perspective for further researches on the design of functionalized ILs was also provided. Finally, some challenges in the absorption of SO2 from flue gas were indicated, which includes the high cost of ILs, the deep mechanism study of SO2 absorption, the investigations under high temperature and low SO2 partial pressure, as well as the effect of other species (such as CO2, NO x , O2, dust, and moisture) in flue gas. The development of alternative ILs that are able to achieve rapid and reversible SO2 capture in high capacity is always highly desired.