Selective adsorption of acidic gases from ternary mixture by acetate- and sulfonate-based ionic liquids at molecular level

Abstract

This paper attempts to elucidate the competitive adsorption mechanism of thin films of ionic liquids (ILs) on the surface of porous materials for acidic gases at a molecular level in order to design a proper material for the diminishment of gas emissions. Thin film 1-butyl-3-methylimidazalium ([BMIM]+) cation-based IL systems composed of four different anions such as $[CH_3 CO_2 ]- and [CF_3 CO_2 ]-$(acetate-based), and $[CH_3 SO_3 ]- and [CF_3 SO_3 ]-$ (sulfonate-based) are created in contact with the gas phase containing ternary $H_2 S/CO_2 /CH_4$ :1/25/74 mixture. To define gas adsorption performance at gas–liquid interface and bulk liquid phase, classical molecular dynamics simulations are carried out. Adsorption of acidic gases is governed by the formation of an adsorbed gas layer on the surface of the ionic liquid based on thermodynamics aspects, and then the partial dissolution of gases in the bulk liquid phase is accompanied by the transport of gases. These behaviors are followed by several analysis methods in simulation approaches such as radial distribution function (RDF) of gases around specific atoms of ILs, lateral displacement of gas molecules, radial distance between gas and ILs, interaction energy between gases and ILs, and average number of hydrogen bonding between ions with and without adsorbed gases. Acetate-based ILs performed twice as good in $CO_2$ adsorption capacity than sulfonate-based ILs. However, the one having $–CF_3$ group in acetate-based ILs has short CO2 retention time and high $CO_4$ adsorption capacity, diminishing the $H_2 S+CO2_ /CH_4$ adsorption selectivity. High $CO_2$ adsorption performance of acetate-based ILs is related to their strong anion–cation interaction and less hydrogenbonding ability between cation tail and anion, which is the source for free space between anion and cation. Those with high adsorption capacities and long retention times are those that can ensure that $CO_2$ molecules are coordinated in these free volumes between cation tails and anions. Therefore, here, the effect of different parameters on the $CO_2 and H_2$ S adsorption over $CO_4$ is revealed via atomistic design, and the importance of selection of suitable anions in IL for identifying potential nanocomposite adsorbent materials for acidic gas removal is highlighted.