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 O 2 -selective adsorbents is of great significance. Metal-organic frameworks with coordinatively unsaturated transition metal sites have great potential in O 2 -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 ·H 2 O] n ·2nH 2 O with coordinatively unsaturated cobalt(II) and nickel(II) sites, which can preferentially adsorb O 2 versus N 2 , and their IAST O 2 /N 2 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, O 2 adsorption–desorption cycle experiments showed that these adsorbents can be completely regenerated at ambient temperature after adsorbing O 2 , and the breakthrough experiments further confirmed their dynamic O 2 /N 2 separation potential and regeneration ability. The results of theoretical calculations suggested that the interactions between adsorbents and O 2 are stronger than those of N 2 , and they have relatively obvious differences in the N 2 and O 2 interaction energies. This work provides inspirations for searching for metal–organic frameworks that can selectively adsorb O 2 at ambient temperature.