Selective dynamic separation of Xe and Kr in Co-MOF-74 through strong binding strength between Xe atom and unsaturated Co²⁺ site

From pure experimental isotherms and ideal adsorbed solution theory (IAST), we confirmed a recent report that Co-MOF-74 provides the highest Xe uptake as well as the highest Xe/Kr selectivity among the three M-MOF-74 series (M = Co, Mg, and Zn). From breakthrough experiments, we then showed the first demonstration of the potential of Co-MOF-74 for Xe/Kr separations under mixture flow conditions. Remarkably, the experimental breakthrough curves for three consecutive cycles are essentially unchanged even if the column was regenerated under helium flows at room temperature between each cycle. Isosteric heat of adsorption (Q_st) for Xe, adsorbed Xe molecules per metal, and binding strengths and electronic density of states (DOS) analyses from first principles calculations all indicate that unsaturated Co²⁺ sites attract Xe more strongly than do unsaturated Mg²⁺ and Zn²⁺ sites. The DOS analyses show that the d orbital of the Co²⁺ is the main contributors for the strong interaction. These results suggest that Co-MOF-74 is a promising adsorbent for separations of Xe and Kr.