Room-Temperature Ionic Liquids:  Temperature Dependence of Gas Solubility Selectivity

Abstract

This study focuses on bulk fluid solubility of carbon dioxide (CO 2 ), methane (CH 4 ), hydrogen (H 2 ), and nitrogen (N 2 ) gases in the imidazolium-based RTILs: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf 2 N]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF 4 ]), 1- n -hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf 2 N]), and 1,3-dimethylimidazolium methyl sulfate ([mmim][MeSO 4 ]) as a function of temperature (25, 40, 55, and 70 °C) at near-atmospheric pressures. The experimental behaviors are explained in terms of thermodynamic relationships that account for the negligible vapor pressure of the RTIL as well as the low solubilities of the gases. Results show that, as temperature increases, the solubility of CO 2 decreases in all RTILs, the solubility of CH 4 remains constant in [emim][Tf 2 N] and [hmim][Tf 2 N] but increases in [mmim][MeSO 4 ] and [emim][BF 4 ], and the solubility of N 2 and H 2 increases. Also, the ideal solubility selectivity (ratio of pure-component solubilities) increases as temperature decreases for CO 2 /N 2, CO 2 /CH 4, and CO 2 /H 2 systems. Experimental values for the enthalpy and entropy of solvation are reported.