NOx absorption and conversion by ionic liquids

Abstract

Ionic liquids (ILs) can be used as absorbents and catalysts for NO_x absorption and conversion due to their low toxicity, low energy consumption and excellent reusability. The capacity and absorption mechanism of NO_x absorption by ILs are presented in this paper. Generally, NO_x 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 NO_x and ILs. Furthermore, low temperature, high pressure and large cation volume are favorable for NO_x absorption. The strategies of enhancing NO_x 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 NO_x 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.

Keywords: Absorption; Conversion; Ionic liquids (ILs); NO(x).