Noble Gas\nAdsorption in Metal–Organic Frameworks\nContaining Open Metal Sites

Abstract

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