Retarded O2 transport in Co2+-coordinated supramolecular polymer networks for membrane CO2/O2 separations

Abstract

Dissociated Co²⁺ ions in liquids and polymers have been demonstrated to reversibly react with O₂ and increase O₂ permeability. However, we find a series of Co²⁺-coordinated supramolecular polymer networks (SPNs) with enormous O₂ sorption but retarded diffusion, leading to superior CO₂/O₂ separation properties. Specifically, Co(BF₄)₂ is dissolved by cross-linked poly(ethylene oxide) (XLPEO), as validated by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The dissociated Co²⁺ ions increase O₂/CO₂ solubility selectivity but decrease its diffusivity selectivity. For example, adding 6.4 mass% Co(BF₄)₂ in XLPEO increases O₂ solubility by 35 times and O₂/CO₂ solubility selectivity from 0.12 to 5.0, but it decreases O₂/CO₂ diffusivity selectivity from 0.40 to 0.0058, leading to a CO₂/O₂ permeability selectivity of 35, above Robeson's upper bound and superior to that of state-of-the-art polymers. This study unravels an exciting platform of metal ion-coordinated supramolecular networks for various molecular separations by harnessing strong affinity but retarded diffusion despite their stability challenge.