Noble Gas Adsorption in Metal–Organic Frameworks Containing Open Metal Sites

The adsorption of noble gases (Ar, Kr, Xe, and Rn) and N2 by a diverse range of Metal–Organic Frameworks (MOFs) containing open metal sites (OMS) was systematically investigated using volumetric gas porosimetry and grand canonical Monte Carlo simulation. The ten MOFs considered are grouped into two series. The first, MOF-74-x, is based upon MOF-74/CPO-27, which has divalent metal ions Mg, Co, Ni, and Zn, and was chosen to explore the effect of metal identity within a constant topology. The second series, nbo-MOFs, consists of NOTT-100, NOTT-101, NOTT-102, NOTT-103, and PCN-14 and probes the effect of pore size while maintaining constant OMS identity and similar coordination environment. Comparisons of the nbo-MOFs with HKUST-1 are also made, since this MOF has a similar chemical structure, although the topology is different from the nbo structures. Gas uptake and Henry's constant (kH) for nbo-MOFs increase with decreasing pore size, and as pore size decreases, the effect of gas polarizability on kH becomes more profound. This implies that tailored pore size can strongly influence kH, especially for the more polarizable gases Kr and Xe. A detailed analysis of the void space in the nbo-series reveals large cages connected by narrow pores, resulting in tortuous, zigzag diffusion pathways that likely increase the strength of the MOF–gas interaction. Data for the MOF-74-x series show that both the density of OMS and their accessibility in this topology enhance the framework interaction with the noble gases. However, because the interaction is largely due to (point charge)−(induced-dipole) interactions and the charge on the metal ion varies little, the specific identity of the metal ion is relatively unimportant. Consequently, new MOFs tailored to maximize noble gas uptake from air should target structures with both a high density of OMS and pores that are approaching the size of the gas of interest.