The role of cation and anion structural modifications for the enhanced CO2 solubility of hydroxyl ammonium- and pyridinium-based ionic liquids

Abstract

This study investigates the CO₂ capture efficiency of a series of ionic liquids (ILs) formed through the methodical modifications of cation and anion configurations. ILs based on tris-(2-hydroxyethyl) ammonium, containing acetate [Ac], butyrate [Bu], lactate [La], and ascorbate [As] anions, along with allyl [Ay]- and benzyl [Bz]-substituted ammonium and pyridinium chloride [Cl] salts, were synthesized and characterized using ¹H and ¹³C NMR spectroscopies, elemental analysis, and Karl Fischer titration. Their CO₂ solubility was investigated under pressures ranging from 1 to 20 bar and temperatures between 298.15 and 358.15 K. The solubility results indicated a distinct relationship with pressure: CO₂ solubility rises as pressure increases for all ILs, while elevated temperatures diminish solubility and increase Henry's law constants. Among the anions examined, the [As]-based ILs had the greatest CO₂ affinity, followed by [La]- and [Bu]-based ILs, suggesting that the anion structure and accessible free volume significantly affected gas absorption. Among the [Cl]-based ILs, [Bz]-based ILs exhibited superior CO₂ solubility compared to their [Ay] counterparts, attributable to their increased van der Waals contacts and improved structural accommodation of CO₂. The thermodynamic study utilizing Henry's law constants produced negative values for ΔH⁰ and ΔS⁰ and positive values for ΔG⁰ over the examined temperature range. The results demonstrate exothermic yet non-spontaneous dissolution under the examined circumstances. The values of ΔH⁰ and ΔS⁰ indicate that CO₂ absorption is influenced by a balance of advantageous ion–gas interactions and solvent structuring effects. The results indicate that the targeted alteration of both cation and anion frameworks is a viable method for optimizing IL performance in CO₂ capture applications.