Steering electrophilic N center in ionic liquid for efficient oxidative capture of elemental mercury

Abstract

Ionic liquids (ILs) show promise for flue gas purification, but suffer from the insufficient oxidation ability for elemental mercury (Hg0) removal. In light of the critical role of electrophilic structure in the oxidation process, we proposed a novel strategy to enhance Hg0 oxidative capture by steering electrophilicity of positively charged N center in the cation of IL. We screened ILs containing cations with N centers featuring varied hybridization and localization, and thioacetate anions that aid in immobilizing the resulting mercury ions. The electrophilicity of ILs, as determined by quantum chemical calculations, adheres to the following hierarchy: [Bpy][ThAc] (highly delocalized sp2-hybridized N) > [Bmim][ThAc] (low delocalized sp2-hybridized N) > [Bpyrr][ThAc] (sp3-hybridized N) ≈ [Bpip][ThAc] (sp3-hybridized N). This hierarchy correlated positively with experimental oxidation activity and Hg0 removal performance. Specifically, the Hg0 adsorption capacity of [Bpy][ThAc] (23.0 mg/g) was 3.6 times larger than that of [Bmim][ThAc]. Hg-TPD, XPS, and theoretical calculations corroborated that the highly delocalized sp2-hybridized N with a stable π system in [Bpy][ThAc] constitutes a strong electrophilic center, boosting electron transfer from Hg0 to the cation. Subsequently, the oxidized Hg2+ was immobilized through coordination with the anion. Furthermore, beyond the specific mercury removal process, the integration of Hg0 capture with desulfurization was successfully achieved by introducing [Bpy][ThAc] into the IL employed for desulfurization, highlighting its adaptability and practicability. This work not only develops an effective remediator for Hg0 removal from the flue gas but also offers design guidelines for ILs used in the oxidative degradation of other contaminants.