Solubility, Diffusivity, and Permeability of Gases in Phosphonium-Based Room Temperature Ionic Liquids:  Data and Correlations

Abstract

Reported are the solubility, diffusivity, and permeability data for various gases in five phosphonium-based ionic liquids at 30 °C, as determined with a lag-time technique. The ionic liquids have a viscosity range of 200-3000 cP. The gas solubilities and diffusivities of the phosphonium-based ionic liquids are of the same magnitude as the gas solubilities for the more familiar imidazolium-based liquids. The gas diffusivity appears to be inversely proportional to the viscosity with an average power of 0.35 for the phosphonium-based ionic liquids. This is in contrast to the power of 0.6 for the imidazolium-based ionic liquids, suggesting that the viscosity-diffusivity relationship varies for different classes of ionic liquids. Despite the generally higher viscosities of the phosphonium-based RTILs compared to the imidazolium-based RTILs, the similarity in thermodynamic and transport properties allows the consideration of the phosphonium-based RTILs as lowcost alternatives for reaction media or separation agents. The gases tested were carbon dioxide, ethylene, propylene, butene, and 1,3-butadiene. Also reported are the permeabilities of methane, nitrogen, and oxygen.

Transport Properties of Binary Mixtures of Carbon Dioxide and 1-Butyl-3-methylimidazolium Hexafluorophosphate Studied by Transient Grating Spectroscopy

“…Remarkably faster diffusion of solute molecule has also been reported for RTILs composed of phosphonium cations; 29,40,44 e.g., the diffusion coefficients of CO, DPA, and DPCP did not follow the viscosity dependence obtained for other RTILs, and were much faster than expected from the relation. 29 It has been suggested that the solute molecules may diffuse more rapidly by moving through the void space due to the much bulkier structure of this cation.…”

Study on Gas Permeation and CO2 Separation through Ionic Liquid-Based Membranes with Siloxane-Functionalized Cations

“…Several works have reported an inversely proportional relationship between IL viscosity and gas diffusivity. 23,[45][46][47] For instance, Scovazzo and co-authors proposed a general correlation, 23 which has been used as follows:…”

CO2 separation applying ionic liquid mixtures: the effect of mixing different anions on gas permeation through supported ionic liquid membranes

“…17,18 Consequently, several studies on the permeation properties of gases through SILM systems have explored the effect of the IL structure. Relating to the influence of the cation, a number of groups have investigated the gas permeation properties of different families of ILs (imidazolium, [19][20][21][22][23][24][25][26][27][28][29][30] phosphonium, 31,32 sulphonium, 32 pyridinium, 33 and ammonium 34 ) and improved results were obtained for imidazolium-based SILMs in terms of permeability and selectivity. Other studies, also focused on imidazolium ILs, explored different structural variations of the cation in order to enhance CO 2 solubility and selectivity.…”