A potential ionic liquid for CO₂-separating gas membranes: selection and gas solubility studies

The production of hydrogen from fossil fuels by steam reforming/water gas shift can be enhanced by separating the reaction byproduct, CO₂, within the reactor as it is produced. Such a separation-enhanced reaction not only has higher conversion efficiency, but can also be considered a greener process which produces high-purity hydrogen with little CO₂ contamination. Supported ionic liquid membranes may be able to achieve this separation task since they are known to have high CO₂ and low H₂ solubilities. In this study, the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide family of ionic liquids has been selected for this purpose, based on limited literature data. The solubilities of major reaction gases, namely CO₂, H₂, CO, and CH₄, in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared to one another. In addition, the solubilities of CO₂ and H₂ in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared. The results indicate, from a thermodynamic point of view, the possibility of using this family of ionic liquids as separation membranes with practical CO₂/H₂ selectivities.