Cation-Anion-CO₂ Interactions in Imidazolium-Based Ionic Liquid Sorbents

A series of functionalized N-alkylimidazolium based ionic liquids (ImILs) were designed, through anion (carboxylates and halogenated) and cation (N-alkyl side chains) structural modifications, and studied as potential sorbents for CO₂ . The sorption capacities of as prepared bare ImILs could be enhanced from 0.20 to 0.60 molar fraction by variation of cation-anion-CO₂ and IL-CO₂ -water interaction. By combining NMR spectroscopy with molecular dynamics simulations, a good description of interactions between ImIL and CO₂ can be obtained. Three types of CO₂ sorption modes have been evidenced depending on the structure of the ImIL ion pair: Physisorption, formation of bicarbonate, and covalent interaction through the nucleophilic addition of CO₂ to the cation or anion. The highest CO₂ sorption capacity was observed with the ImIL containing the 1-n-butyl-3-methylimidazolium cation associated with the carboxylate anions (succinate and malonate). This study provides helpful clues for better understanding the structure-activity relationship of this class of materials and the ion pair influence on CO₂ capture.