Investigating the SO₂ absorption behavior of pyrimidine-based deep eutectic solvents via a dual-site thermodynamic model

Deep eutectic solvents (DESs), composed of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs), are widely used in flue gas desulfurization due to their excellent SO₂ absorption properties. In this work, 14 DESs with pyrimidine derivatives and their isomers (including 2-aminopyrimidine (AmPyr), 2-chloropyrimidine, 2-bromopyrimidine, 4-amino-2-hydroxypyrimidine, 2,4-dihydroxypyrimidine, 4,6-diaminopyrimidene, 2-aminopyrazine, and 3-aminopyridazine, respectively) as HBDs and 1-ethyl-3-methylimidazolium chloride (C₂mimCl) and 1-ethyl-3-methylimidazolium bromide as HBAs have been successfully prepared and used for SO₂ absorption. Among them, C₂mimCl-7 + AmPyr exhibited the highest SO₂ absorption (19.032 mol kg⁻¹, at 298.15 K and 1.0 bar), rapid gas–liquid equilibrium within 40 s, and an exceptional ideal selectivity of 528.7 for SO₂/CO₂. After 30 cycles of absorption–desorption, the SO₂ absorption capacity remained as high as 18.265 mol kg⁻¹. A dual-site reaction equilibrium thermodynamic model (DS-RETM) was established for absorption behavior analysis. Using C₂mimCl-7 + AmPyr as a case study, Henry's constant, equilibrium constants, and other thermodynamic parameters were determined. DS-RETM fitting further enabled visualization of the potential absorption behavior of each DES component, facilitating comparison of HBA and HBD effects on SO₂ absorption. This study offers new insights into the development of high-performance flue gas desulfurization absorbents and introduces a novel model for thermodynamic analysis of SO₂ absorption in DESs.