Fundamental investigation of the gas permeation mechanism of facilitated transport membranes with Co(salen)-containing ionic liquid as O2 carriers

Abstract

Metal-containing ionic liquids (MCILs) composed of a cobalt(II)–salen complex and ionic liquid-based axial ligands (ligand ILs) can chemically absorb O2 and act as O2 carriers in facilitated transport membranes. O2 and N2 permeation tests using supported MCIL membranes with various MCILs revealed that the effective diffusion coefficients of N2 and O2–MCIL complexes did not follow to the trend expected by power law theories against viscosity (e.g. the Stokes–Einstein equation), because the MCILs are not in normal liquid state due to the strong intermolecular interaction, but instead showed a tendency predicted by the free volume theory. This means the MCILs behave as a solid rather than a liquid, and N2, O2, and the O2–MCIL complexes would diffuse through the free volume of the MCILs. The solubility selectivity and the diffusion selectivity of the supported MCIL membrane decreased and increased, respectively, with the increase on MCIL molecular wight (free volume). Thus, there was trade-off relationship between the solution selectivity and the diffusion selectivity. It was suggested that the most important criteria for designing MCILs for highly selective O2 permeation in supported MCIL membranes is the use of an O2-absorbable metal complex without the strong intermolecular interaction.