ABSTRACT
We performed classical Molecular Dynamics computer simulations to analyze solutions of the gases CO2, N2, and CH4 in four 1-n-butyl-3-methylimidazolium-based ionic liquids (1-n-butyl-3-methylimidazolium acetate, 1-n-butyl-3-methylimidazolium prolinate, 1-n-butyl-3-methylimidazolium bromide, and 1-n-butyl-3-methylimidazolium tetrafluoroborate). Typical experimental conditions (10 bar gas pressure and room temperature) have been chosen to study mixtures of the ionic liquids with the gases at a single gas molar fraction of 0.25. Structural aspects are discussed to judge the absorption capacities of the ionic liquids. We observed that CO2 coordinates preferentially within the polar domain of the ionic liquids with the bromide and tetrafluoroborate anions presenting the best performances. The other gases, N2 and CH4, remain in the less polar domains of the ionic liquids. Cluster size analysis indicates phase separation for these two gases. Considering both, the absorption tendency and gas separation capacity of the ionic liquids, the anion is desired to be small and possessing multiple coordination sites. In this aspect, the tetrafluoroborate anion accomplished the best results.
Subject(s)
Gases/chemistry , Imidazoles/chemistry , Ionic Liquids/chemistry , Molecular Dynamics Simulation , Absorption, Physicochemical , Molecular Conformation , Pressure , TemperatureABSTRACT
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.
