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

Abstract

The adsorption of noble gases (Ar, Kr, Xe, and Rn) and N 2 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 (k H ) for nbo-MOFs increase with decreasing pore size, and as pore size decreases, the effect of gas polarizability on k H becomes more profound. This implies that tailored pore size can strongly influence k H , 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.

A stable metal–organic framework with suitable pore sizes and rich uncoordinated nitrogen atoms on the internal surface of micropores for highly efficient CO2 capture

“…[58][59][60][61][62][63][64] The performance of industrial xed bed adsorbers is dictated by a combination of adsorption selectivity and uptake capacity. The proper combination of both of these factors is obtained by the use of breakthrough calculations.…”

Separation of CH4/N2mixtures in metal–organic frameworks with 1D micro-channels

“…10,[17][18][19][20][21][22] For equilibrium separation, the equilibrium effect is related to various structural properties of the adsorbents, such as network topology, surface area, free volume, pore size distribution, pore shape, pore connectivity and polarity of channels. These structural features are thought to have significant influence on diffusion pathways and adsorption potential.…”

Enhanced xenon adsorption and separation with an anionic indium–organic framework by ion exchange with Co2+

“…Another factor currently receiving considerable attention is introducing individual localized binding sites using methods such as (1) deposition of Ag clusters in the porous MOFs and (2) creation of unsaturated metal sites (also called open-metal sites) and more polarizing organic groups (-I, -OH). 40,[45][46][47] Furthermore, the porous solid-Xe molecule interactions can also be enhanced by employing the electric eld across the pore space that is capable of polarizing Xe molecules and increasing the interaction with charged internal pore surface or charged extraframework species. 40 However, this approach is rarely studied because most of MOFs exhibit an electrically neutral framework.…”