Investigation into the Effective Chemical Structure of Metal-Containing Ionic Liquids for Oxygen Absorption

To control the properties of Co­(II)­(salen)-based metal-containing ionic liquids (MCILs) for use as O₂ absorbents, the effects of the MCIL chemical structures on viscosity, O₂ absorption amount, and complex-formation reaction rates between O₂ and MCILs were investigated. The chemical structures of the MCILs were systematically changed by coordinating the ligand ILs composed of phosphonium cations with different alkyl-chain lengths and N-methylglycinate or bis­(trifluoromethylsulfonyl)­imide anion to Co­(salen). A series of investigations indicated that intermolecular hydrogen bonding and entanglement of the alkyl chain affected the viscosity of the MCILs. Furthermore, small MCILs afforded large O₂ absorption amounts, high O₂/N₂ absorption selectivities, and fast O₂ absorption reaction. From a numerical calculation of the overall MCIL O₂ absorption rate, it was clarified that the dominant mass-transfer resistance was the diffusion of the O₂–MCIL complex in the MCIL. Based on these results, the design criteria of the MCILs for an efficient O₂ absorbent were revealed.