Insights into the Absorption of Hydrocarbon Gases in Phosphorus-Containing Ionic Liquids

The solubility of ethane, ethylene, propane, and propylene was measured in two phosphorus-containing ionic liquids, trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate, [P_6,6,6,14][DiOP], and 1-butyl-3-methylimidazolium dimethylphosphate, [C₄C₁Im][DMP], using an isochoric saturation method. The ionic liquid [C₄C₁Im][DMP] absorbed between 1 and 20 molecules of gas per 1000 ion pairs, at 313 K and 0.1 MPa, while [P_6,6,6,14][DiOP] absorbed up to 169 molecules of propane per 1000 ion pairs under the same conditions. [C₄C₁Im][DMP] had a higher capacity to absorb olefins than paraffins, while the opposite was true for [P_6,6,6,14][DiOP], with the former being slightly more selective than the later. From the analysis of the thermodynamic properties of solvation, we concluded that in both ionic liquids and for all of the studied gases the solvation is ruled by the entropy, even if its contribution is unfavorable. These results, together with density measurements, 2D NMR studies, and self-diffusion coefficients suggest that the gases' solubility is ruled mostly by nonspecific interactions with the ionic liquids and that the looser ion packing in [P_6,6,6,14][DiOP] makes it easier to accommodate the gases compared to [C₄C₁Im][DMP].