Effective CO₂ and CO Separation Using [M₂(DOBDC)] (M = Mg, Co, Ni) with Unsaturated Metal Sites and Excavation of Their Adsorption Sites

Isostructural [M₂(DOBDC)(EG)₂] (M = Mg, Co, Ni) frameworks are first synthesized by controlling the pH* in the reaction medium. Coordinated ethylene glycols form a hexagonal OH cluster, which works as a template to grow single crystals with high crystallinity. After the liberation of solvated molecules, [M₂(DOBDC)] shows notably higher surface areas than the reported values and completely different CO₂ and CO separation properties depending on the kinds of unsaturated metal. Therefore, breakthrough experiments using a CO₂/CO mixed gas show that Mg-MOF has a longer breakthrough time for CO₂ than for CO, whereas Co/Ni-MOFs have longer breakthrough times for CO than for CO₂. Apart from CO₂ and CO, other gases such as CH₄, H₂, and N₂ were almost not adsorbed at all in these materials at 298 K. To reveal the role of unsaturated metal sites, CO₂ and CO adsorption sites are unequivocally determined by single-crystal X-ray diffraction analysis. One of very interesting discoveries is that there are two CO₂ and CO adsorption positions (sites A and B) in the hexagonal channels. Site A is the unsaturated metal center working as Lewis acidic sites, and site B is the secondary adsorption site located between two A sites. A close inspection of crystal structures reveals that unsaturated Co(II) and Ni(II) sites adsorb both CO₂ and CO, whereas the unsaturated Mg(II) sites strongly capture only CO₂, not CO. Density functional theory calculations elucidate the discrepancy in CO affinity: Co(II) and Ni(II) form strong π-back-donating bonds with CO via electron transfer from the d orbitals of the transition metals to the antibonding molecular orbitals of CO, whereas Mg(II) does not participate in electron transfer or orbital overlap with CO. This observation provides new insight into the synthesis of novel functional materials with high CO₂/CO separation performance.