Fundamental investigation of the gas permeation mechanism of facilitated transport membranes with Co(salen)-containing ionic liquid as O-2 carriers

摘要

Metal-containing ionic liquids (MCILs) composed of a cobalt(II)-salen complex and ionic liquid-based axial ligands (ligand ILs) can chemically absorb O-2 and act as O-2 carriers in facilitated transport membranes. O-2 and N-2 permeation tests using supported MCIL membranes with various MCILs revealed that the effective diffusion coefficients of N-2 and O-2-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 N-2, O-2, and the O-2-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 O-2 permeation in supported MCIL membranes is the use of an O-2-absorbable metal complex without the strong intermolecular interaction.