NOx absorption and conversion by ionic liquids.

Ionic liquids (ILs) can be used as absorbents and catalysts for NOx absorption and conversion due to their low toxicity, low energy consumption and excellent reusability. The capacity and absorption mechanism of NOx absorption by ILs are presented in this paper. Generally, NOx are physically absorbed by conventional ILs such as imidazolium-based ILs. The absorption capacity is as follows: NO2>NO>N2O, which is in good agreement with the binding energy between NOx and ILs. Furthermore, low temperature, high pressure and large cation volume are favorable for NOx absorption. The strategies of enhancing NOx capacity through functionalized ILs with metal-containing anions (e.g. [FeCl4]2-), amine groups, sulfonate and carboxylate anions are also concluded. Active N or O sites in functionalized ILs can react with the dimer of NO (N2O2), resulting in high capacity. Moreover, introducing electron-withdrawing substituents such as chlorine and bromine into carboxylate or sulfonate anions reduces desorption residue. Besides NOx absorption, ILs with [NO3]- can activate NO and efficiently catalyze its conversion into HNO3 in the presence of O2 and H2O, and have better performance than ILs with [Cl]-, [Ac]- and [CF3SO3]-, which is attributed to the strong oxidization capability of [NO3]-. In addition, low temperature and high O2 content can further improve NO conversion.