Highly absorption separation of SO2 from CO2 by carboxylate-functionalized ionic liquid-based mixed solvents: Experimental and thermodynamic insights

The efficient and selective capture of SO 2 from flue gas remains a critical challenge due to the high viscosity and poor SO 2 /CO 2 selectivity of conventional functionalized ionic liquids (FILs). Herein, a series of carboxylate-functionalized ionic liquids based on tetraethylammonium ([N 2222 ]) with different anions (DMG, NA, ImAc) were synthesized, and high-boiling sulfolane (SUL), ethylene glycol (EG), and N -methylpyrrolidone (NMP) were introduced as co-solvents to reduce viscosity and tune the interaction environment. Among all systems, the [N 2222 ][ImAc]-SUL mixed solvent exhibited the best comprehensive performance, achieving an ultrahigh SO 2 absorption capacity of 15.9 mol/kg at 293.2 K and an outstanding SO 2 /CO 2 selectivity of 36.0. Combined FT-IR and NMR characterizations with DFT calculations and molecular dynamics simulations revealed a multi-site synergistic mechanism: the carboxylate oxygen (−COO − ) serves as the primary chemisorption site (ΔH = −67.9 kJ/mol), the sp 2 -hybridized imidazole nitrogen provides auxiliary coordination, and sulfolane contributes physical solvation via dipole–dipole interactions. Furthermore, the system demonstrated satisfactory cyclic stability over five absorption–desorption cycles at a mild regeneration temperature of 343.2 K. This work provides a highly promising absorbent for industrial SO 2 emission control and offers mechanistic insights for designing selective acid gas capture systems.