Ultrahigh SO2 selective absorption in SO2/CO2 mixtures via reversible solid–liquid phase transition in tunable ionic liquids

The high-purity separation of gases with similar properties (e.g., SO 2 vs. CO 2 ) remains a major challenge for ionic liquid (IL) absorbents. In this work, we report the highly efficient and selective capture of SO 2 via SO 2 -triggered reversible solid–liquid phase transition in a series of tunable alkyl quaternary ammonium hydrogen sulfate ionic liquids (AQA-HSO 4 ). The results show that AQA-HSO 4 achieve ultrahigh IAST SO 2 /CO 2 selectivities (up to 855) and high uptake capacity (7.1 mmol g −1 ) at 293.2 K and 100 kPa. This superior performance is rooted in a rationally designed phase transition mechanism, where the modulation of alkyl chain lengths (from ethyl to hexyl) finely tunes the ionic microenvironment by balancing electron-donating and steric effects. Systematic investigations, including SO 2 absorption and kinetic analyses, in-situ FT-IR, DFT and COSMOtherm calculations, reveal that the [HSO 4 ] − anion serves as the primary active site, forming suitable cooperative hydrogen bonding and Coulombic interactions specifically with SO 2 , while remaining inert to CO 2 /N 2 . Kinetic analysis confirms that the spontaneous phase transition of AQA-HSO 4 drastically lowers the mass transfer energy barrier, enabling [N 4444 ][HSO 4 ] to reach 67% of its saturation capacity within 5 min. Additionally, [N 4444 ][HSO 4 ] exhibits outstanding stability and recyclability, with negligible capacity loss (<0.1%) after 20 absorption–desorption cycles, and can be fully regenerated under mild, energy-efficient conditions (depressurization to ∼20 kPa) without external heating. This work provides a promising strategy for developing next-generation, phase-transition-responsive absorbents for high-purity gas separation processes.