Tuning Ionic Liquids with Functional Anions for SO2 Capture through Simultaneous Cooperation of N and O Chemical Active Sites with SO2

In this contribution, a series of ionic liquids (ILs) with tri-n-butylethyl-phosphonium cation ([P4442]) and functional anions are designed and prepared, including pyridinolate anion ([PyO]), pyridinecarboxylate anion ([PyCOO]), and pyridinesulfonate anion ([PySO3]). Each anion contains both N site and O site, which can react with SO2. It could be found that up to 4.71 mol SO2 mol–1 IL absorption capacity could be received by [P4442][PyO] at 20 °C and 1 bar. Compared to ILs with phenolate anion ([PhO]), benzoate anion ([PhCOO]), and benzenesulfonate anion ([PhSO3]), the pyridine-based counterparts in this work were found to significantly enhance SO2 capture capacity, especially for anions with high basicity. It was demonstrated from the combination of FT-IR and NMR spectroscopic investigations and DFT calculations that the simultaneous cooperative interaction of N···SO2 and O···SO2 between functional anion [PyO] and SO2 was the main reason for high absorption capacity of low-concentration SO2 by [P4442][PyO].