Ultralow Emissions of Low-Concentration SO₂ in Flue Gas Using Azole-Based Ionic Liquids

In recent years, ultralow emissions (ULE) have attracted more attention due to the stricter environmental pollution standards. Developing desulfurizers to meet the needs of ultralow emissions standards is pressingly needed. Herein, various ionic liquids (ILs) were designed using the basicity-tunable and cation-tunable strategies for the capture of 2000 ppm of SO2 in flue gas. Furthermore, NMR spectra, FT-IR spectra, and DFT calculations, including noncovalent interaction (NCI) and reduced density gradient (RDG) analysis, were used to testify the absorption mechanism. Anions of ILs were proved to play a vital role in ultralow emissions, and [Bentriz] was regarded as an ideal anion for the ultralow emission in this work, while cations of ILs had little effect on ultralow emissions performance. [P4442][Bentriz] exhibited excellent ability of ultralow emissions (5 ppm), splendid removal efficiency (>99%), outstanding absorption capacity (1.03 mol SO2/mol IL), and good reversibility (6 cycles).