High solubilities of carbon dioxide in tetraalkyl phosphonium-based ionic liquids and the effect of diluents on viscosity and solubility

Experimental solubilities are reported for carbon dioxide in four ionic liquids: trihexyl tetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate [P(14)666][TMPP], tetrabutylphosphonium bis(2,4,4-trimethylpentyl) phosphinate [P4444][TMPP], trimethyloctylphosphonium bis(2,4,4-trimethylpentyl) phosphinate [P8111][TMPP], and trihexyl tetradecylphosphonium dicyanamide [P(14)666][DCA] from 298 K to 334 K up to 4 MPa. Tetraalkyl phosphonium bis(2,4,4- trimethylpentyl) phosphinate ionic liquids show solubilities for carbon dioxide higher than those in other ionic liquids. In these phosphonium-based ionic liquids, the anion appears to have a larger influence on carbon-dioxide solubility than the cation. Water or 1-butyl-3-H-imidazolium acetate ([BHMIM][AC]) are used as diluents to reduce the large viscosity of [P(14)666][TMPP]. Viscosities are reported for mixtures of [P(14)666][TMPP] with varying content of water or [BHMIM][AC] from 298 K to 343 K at atmospheric pressure. Solubilities for carbon dioxide are reported in mixtures of [P(14)666][TMPP] with varying content of water or [BHMIM][AC] from 298 K to 323 K up to 4 MPa. These diluents very much reduce the viscosity. The addition of [BHMIM][AC] does not seriously lower the solubility of carbon dioxide.