(83f) Solubilities, Densities and Viscosities of Two Low-Density Ionic Liquids Containing Small Hydrocarbons

Experimentally, we find that low-density ionic liquid tetrabutylphosphonium bis(2,4,4-trimethylpentyl)phosphinate [P4444][TMPP] shows large solubilities for small paraffins and olefins (methane, ethane, ethylene, propane and propylene). Solubilities of paraffins in [P4444][TMPP] are larger than those of corresponding olefins.These results may make [P4444][TMPP] a useful solvent for separation of olefin and paraffin because the vapor pressure of an olefin is higher than that of its corresponding paraffin at the same temperature. However, the viscosity of [P4444][TMPP] is 1740 mPa·s at 25°C, too large for a separation process.

To reduce the viscosity, we use low-density ionic liquid 1-butyl-3-H-imidazolium acetate [BHMIM][AC] as diluent for [P4444]][TMPP]. We choose [BHMIM][AC] because it has low viscosity, about 7 mPa·s at 25°C. Low-density ionic liquids have higher solubility of gases. We present viscosities and densities of mixtures of [P4444]][TMPP] with 20 or 50 mass % [BHMIM][AC] from 20 to 70°C at atmospheric pressure. We present the solubilities of gaseous methane, ethane, ethylene, and propane in a mixture of [P4444]][TMPP] with 20 or 50 mass % [BHMIM][AC] from 25 to 50°C up to 5 MPa. We find that at 25°C, addition of 20 mass % [BHMIM][AC] to [P4444][TMPP] lowers the viscosity to 183 mPa·s, addition of 50 mass % [BHMIM][AC] to [P4444][TMPP] lowers the viscosity to 77 mPa·s. The solubilities of light olefins and paraffins are reduced by about 10 % when the solvent contains of 20 mass % [BHMIM][AC] and about 45 % when the solvent contains 50 mass %.