Inês C. M. VAZ 1, Samuel MCCALMONT 2, Hanne OORTS1, Zheng GONG 1, Leila MOURA2, Agilio PÁDUA 1, Margarida COSTA GOMES 1
1École Normale Supérieure de Lyon and CNRS, Laboratoire de Chimie, Lyon, France
2QUILL Research Centre, Queen’s University Belfast, School of Chemistry and Chemical Engineering, United Kingdom
effective, implies high energy consumption and high operating costs [1]. Ionic liquids (ILs) have been pointed as potential solvents for more sustainable gaseous separation processes [1]. Due to an unusual combination of properties, as having capacity to dissolve both polar and non-polar solutes and almost negligible vapour pressures, ILs have been growing in popularity [2]. Moreover, the IL properties may be finely tuned by combining different anions and cations, not to mention the possibility of combining different ILs or combining them with other compounds.
To develop a rational design of ILs is essential to understand their solvent behaviour, which is determined by three main characteristics: their free volume (space available to accommodate solute molecules), their cohesive energy (related to the energy penalty associated to the creation of extra space in the solvent) and their potential for interacting with different solutes [3]. Regarding the application of ILs in gas separation processes, phosphinate based ILs stand out for their particularly high capacity to dissolve low molecular weight hydrocarbons [1][4].
In this work, the solubility of ethane, ethylene, propane and propylene at different temperatures was measured in two phosphorous-containing ionic liquids, 1-butyl-3-methylimidazolium dimethylphosphate, [C4C1Im][DMP], and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate, [P6,6,6,14][DiOP], using an isochoric saturation method. From these results, the thermodynamic properties of solvation, as well as, the selectivity of the alkane/alkene gas absorption, were derived. The ionic liquid [C4C1Im][DMP] absorbed between 1 and 20 molecules of gas per each 1000 ion pairs, at 313 K and 0.1 MPa, while [P6,6,6,14][DiOP] absorbed up to 169 molecules of propane per each 1000 ion pairs in the same conditions. [C4C1Im][DMP] showed higher capacity to absorb olefins than paraffins, while the opposite was true for [P6,6,6,14][DiOP]. For all the studied gases and in both ionic liquids, the solvation is ruled by the entropy. Density and viscosity measurements, 2D NMR studies and self-diffusion coefficients, together with the solubility results, suggest that the gases’ solubility is ruled mostly by non-specific interactions with the ionic liquids and that the looser ion packing in [P6,6,6,14][DiOP] makes it easier to accommodate the studied gases.
[1] Oil & Gas Science and Technology, (2016), 2, p.23.
[2] Biophysical Reviews (2018), 10, p.691; “Commercial Applications of Ionic Liquids”, Series: Green Chemistry and Sustainable Technology. Springer Nature. (2020). ISBN: 978-3-030-35245-5.
[3] Chemical Science, (2013), 4, p.2953; Chemical Reviews (2013) 113, p.6536; Physical Chemistry Chemical Physics (2016), 18, p.19267.
[4] J. Phys. Chem. B, (2013), 36, p.10534.