Investigating the effects of mixing ionic liquids on their density, decomposition temperature, and gas absorption

Abstract

In order to counter the detrimental effect of chemicals on the environment, the academic and industry research interest have shifted towards “green” solvents. Ionic liquids (ILs) are salt-like compounds that, with few exceptions, are known to have negligible vapor pressure and are widely proclaimed as green solvents. Their physical properties can be tailored for the desired purpose by the careful selection of cation and anion or by mixing with other solvents. In this work, we tested the gas absorption of ILs and measured the thermo-physical properties of individual ILs and their binary mixtures. Density and thermogravimetric measurements were performed on ILs choline octanoate ([Ch][Oct]), 1-methyl-3-octylimidazolium trifluoromethanesulfonate ([Omim][Otf]), and 1-methyl-3-octylimidazolium chloride ([Omim][Cl]), and their equal molar (1:1) mixtures. The effect of different anions [Otf−, Cl−] and different cation groups [Omim, Ch] on the gas absorption potential of ILs have been observed. The density and thermal stability tests have been conducted to understand the changes in the physical properties of ILs when they are mixed together in equal ratios. All samples of ILs ([Ch][Oct], [Omim][Otf], [Omim][Cl], [Ch][Oct] + [Omim][Otf], [Ch][Oct] + [Omim][Cl], and [Omim][Cl] + [Omim][Otf]) were tested as CO2 absorption media at temperatures 298.15, 305.15, and 308.15 K and pressure values ranging from 0 to 50 bar. In addition to that, the CO2 sorption results at 298.15 K were compared with experimentally measured methane (CH4) and nitrogen (N2) data. It was observed that the IL [Ch][Oct] is significantly better for CO2 absorption than [Omim][Cl] ([Ch][Oct] ≥ [Omim][Cl] > [Omim][Otf]). On comparing the CO2 absorption profiles with those of CH4 and N2, the similar trends for IL and IL mixtures were obtained: CO2 > CH4 > N2 and CO2 > CH4 > N2 respectively.