Engineering halogen ligand in Fe-based ionic liquid for elemental mercury removal from flue gas

The application of Fe-based ionic liquids (ILs) in elemental mercury (Hg0) removal from flue gas is limited by the low oxidative activity of the Fe(III) center. Here, a strategy of engineering halogen ligand to enhance the oxidative activity of Fe-based ILs for mercury removal was proposed. We developed a series of ionic liquids [Bmim][FeCl4-xBrx] (x = 0–4) with varying ratios of Cl− and Br− ligand substitutions. Mercury removal performance evaluations demonstrated a positive correlation between the oxidative activity towards Hg0 and the number of Br ligand substitutions in the ILs. [Bmim][FeBr4] achieved ∼99% removal efficiency in the temperature range of 50–150 °C, with a saturated adsorption capacity reaching 205.4 mg/g (at 150 °C). In-situ Raman spectroscopy, DFT calculations, Hg-TPD, and XPS collectively revealed that the introduction of Br significantly lowered the excitation energy of the ligand–metal p-d orbital electron transitions, which facilitates the generation of active bromine species with unfilled 4p orbital, consequently enhancing the oxidative removal performance of Fe-based ILs for Hg0. This work provides an application of metal-based ILs in the realm of mercury removal and broadens the scope of ligand–metal interaction for IL design.