Elucidation of Metal Identity Effects on Noble Gas Separations in Zeolitic Imidazolate Frameworks

Pressure-responsive, or breathing, metal-organic frameworks (MOFs) that exhibit step-shaped gas adsorption isotherm behavior are highly promising materials for gas capture and separations. Utilizing porous materials to separate the noble gases Xe and Kr has been recognized as an energy-saving and cost-effective alternative to the traditional separation method of cryogenic distillation. Herein, a series of isoreticular flexible benzimidazolate-based (bim) zeolitic imidazolate frameworks (ZIFs) of the form M(bim)₂ (M = Zn (ZIF-7), Co (ZIF-9), and Cd (CdIF-13)) were examined to elucidate the effects of metal identity on the pressure-responsive-phase changes during Xe and Kr gas adsorption. Phase transition pressure threshold positions, prestep gas uptakes, and step uptake capacities were compared across multiple temperatures. The well studied Zn analog ZIF-7 was used as the baseline material for comparison and determination of metal substitution effects. Compared to ZIF-7, metal substitution to form ZIF-9 and CdIF-13 resulted in substantially improved Xe adsorption performance. Notably, metal identity dramatically influenced prestep adsorption and threshold adsorption pressure, decreasing the prestep adsorption and increasing the threshold pressure needed to induce the phase transition. Remarkably, CdIF-13 showed negligible prestep adsorption of Xe and a Xe capacity of more than 4 mmol/g at 1 bar and 193 K. Comparison of the estimated usable capacities resulted in a ranking of CdIF-13 > ZIF-9 > ZIF-7. Lastly, the Xe adsorption capacity and Xe/Kr selectivity are compared to other promising MOFs for noble gas capture.