This paper explores the arguments for using solubility parameters and the regular solution theory for modeling gas solubilities in five different room-temperature ionic liquids (RTILs) at low partial pressures (<1 atm) and low mole fractions (<0.1). The experimentally measured and reported carbon dioxide (CO2) solubilities at low mole fractions (<0.05) suggest positive deviations from Raoult's law for CO2/RTIL solutions. These CO2 solubility deviations from Raoult's law indicate that CO2/RTIL complexations are not the sole controlling factor in relative CO2 solubilities. The RTILs' energies of vaporization and molar volumes appear to be factors in determining relative CO2 solubilities between RTILs. The energies of vaporization for the RTILs were empirically estimated from their melting points.

Low energy separations for <1% CO2 gases would benefit gas treatment and CO2 sequestration. Theoretically, electrochemical pumping can separate and concentrate CO2 from the atmosphere or other gases with <1% CO2 at significantly lower energy cost than current systems. Principles of electrochemical pumping for CO2 separations are discussed and results for both organic solvent and ionic liquid working fluid systems are presented. Due to the large quantities of gases requiring processing during the separation/concentration of <1% CO2 gases, this work looked at solvents with negligible vapor pressures, specifically propylene carbonate and the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Other important parameters, as illustrated by the data and models presented, are low CO2 solubility in the solvent, high CO2 carrier solubility, CO2 binding constants, and the CO2 carrier’s electrochemistry. Reported is the electrochemical pumping of CO2 from 0.5% (in nitrogen) to 100%, a 200-fold increase in partial pressure, using the CO2 carrier 2,6-di-tert-butyl-1,4-benzoquinone in a propylene carbonate solution. The ratio of CO2 moles pumped per electron mole was 0.43. The models determined the optimal CO2 solubility in the solvent and the required redox swing in the CO2 binding constants of the carrier. © 2003 The Electrochemical Society. All rights reserved.

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