Tuning cationic structures in ionic liquids for enhanced sulfur dioxide absorption-desorption and mass transfer efficiency

The mass transfer and diffusion limitations of ionic liquids (ILs) have hindered their application in SO 2 separation. In this work, five diethyl sulfate (DiEtSO 4 ) based ionic liquids (DiEtSO 4 -based ILs) with different viscosities ( μ , 3∼285 cP) were synthesized by tuning the cationic structures. The SO 2 absorption capacities and kinetics of DiEtSO 4 -based ILs were evaluated at 298-313 K. Meanwhile, liquid-side volumetric mass transfer coefficients ( k L a ) and liquid-side mass transfer coefficients ( k L ) were determined using the pressure drop method to assess SO 2 mass transfer in DiEtSO 4 -based ILs quantitatively. The results indicated a physical absorption mechanism for SO 2 , consistent with Henry's law (Δ H < ∼21 kJ/mol). Specifically, both 1-methylimidazole diethyl sulfate ([MIM][DiEtSO 4 ]) and 1,8-diazabicyclo[5.4.0]undec-7-ene diethyl sulfate ([DBU][DiEtSO 4 ] demonstrated high capacities (∼9.6 mmol/g), rapid kinetics (equilibrium within < 2.5 min), and excellent cyclic stability with minimal capacity loss (∼0.1%). Moreover, density functional theory (DFT), conductor-like screening model for real solvent thermodynamics (COSMOtherm), and independent gradient model (IGM) analyses confirmed that Van der Waals and weak electrostatic interactions predominate over chemical bonding in the absorption process. Mass transfer analysis confirmed an inverse relationship between μ and k L ( k L ∝ μ −0.5 ) in DiEtSO 4 -based ILs, which is consistent with Danckwerts' theory. Based on its superior overall performance in SO 2 absorption capacity, mass transfer rate, and cyclic stability, [MIM][DiEtSO 4 ] emerged as the most promising candidate for SO 2 separation and recovery.