Mixed-ligand metal–organic frameworks with coordinatively un-saturated Co(II) and Ni(II) sites for regenerable O2-selective ad-sorption over N2

For air separation, the separation method relying on adsorbents has attracted wide attention as a low energy consumption process, and the development of O2-selective adsorbents is of great significance. Metal-organic frameworks with coordinatively unsaturated transition metal sites have great potential in O2-selective adsorption, but most of them cannot be completely regenerated at ambient temperature or can only be effective at very low temperatures. Herein, we report two mixed-ligand metal–organic framework adsorbents [M(AIP)(BPY)0.5·H2O]n·2nH2O with coordinatively unsaturated cobalt(II) and nickel(II) sites, which can preferentially adsorb O2 versus N2, and their IAST O2/N2 selectivities are both significantly greater than 1 at 25 °C. These materials exhibited excellent stability and had no loss of adsorption capacities even after immersion in water for 7 days. More importantly, O2 adsorption–desorption cycle experiments showed that these adsorbents can be completely regenerated at ambient temperature after adsorbing O2, and the breakthrough experiments further confirmed their dynamic O2/N2 separation potential and regeneration ability. The results of theoretical calculations suggested that the interactions between adsorbents and O2 are stronger than those of N2, and they have relatively obvious differences in the N2 and O2 interaction energies. This work provides inspirations for searching for metal–organic frameworks that can selectively adsorb O2 at ambient temperature.