Robust and Radiation-Resistant Hofmann-Type Metal–Organic Frameworks for Record Xenon/Krypton Separation

Abstract

The discovery of high-performance adsorbents for highly efficient separation of xenon from krypton is an important but challenging task in the chemical industry due to their similar size and inert spherical nature. Herein, we report two robust and radiation-resistant Hofmann-type MOFs, Co­(pyz)­[Ni­(CN)4] and Co­(pyz)­[Pd­(CN)4] (termed as ZJU-74a-Ni and ZJU-74a-Pd), featuring oppositely adjacent open metal sites and perfect pore sizes (4.1 and 3.8 Å) comparable to the kinetic diameter of xenon (4.047 Å), affording the benchmark binding affinity for polarizable Xe gas. These materials thus exhibit both record-high Xe uptake capacities (89.3 and 98.4 cm3 cm–3 at 296 K and 0.2 bar) and Xe/Kr selectivities (74.1 and 103.4) at ambient conditions, all of which are the highest among all the state-of-the-art materials reported so far. The locations of Xe molecules within ZJU-74a-Ni have been visualized by single-crystal X-ray diffraction studies, in which two oppositely adjacent metal centers combined with the right aperture size can construct a unique sandwich-like binding site to offer unprecedented and ultrastrong Ni2+–Xe–Ni2+ interactions with xenon, thus leading to the record Xe capture capacity and selectivity. The excellent separation capacity of ZJU-74a-Pd was verified by breakthrough experiments for Xe/Kr gas mixtures, providing both unprecedentedly high xenon uptake capacity (4.63 mmol cm–3) and krypton productivity (214 cm3 g–1).

Recent advances in microporous metal–organic frameworks as promising adsorbents for gas separation

“…237–239 Based on these studies, several MOFs have recently been established for highly efficient Xe/Kr separation. 240–254…”

“…27a). 250 Due to the unique pore confinement, ZJU-74a-Ni and ZJU-74a-Pd exhibit a record-high binding affinity toward xenon, and thus a large Xe uptake of 89.3 and 98.4 cm 3 cm −3 at 296 K and 0.2 bar, respectively (Fig. 27b).…”

A microporous Zr6@Zr-MOF for the separation of Xe and Kr

“…The value is higher than those of most reported MOFs including Co 3 (HCOO) 6 , 21 MOF-505, 19 MOF-74-Ni, 22 SBMOF-2 23 etc., but is lower than those of the recently reported Co 3 (C 4 O 4 ) 2 (OH) 2 , 10 CROFOUR-1-Ni 24 etc. 9,11,20,[25][26][27][28] A trend of decreasing adsorption selectivity as a…”

“…Various porous materials, such as MOFs, organic cages, zeolites, etc., have been explored for the separation of Xe/Kr. [10][11][12][13][14] Among them, MOFs hold great promise as their nearly unlimited structures offer the opportunity for achieving high separation performance through structural regulation.…”

Chemical conversion of metal–organic frameworks into hemi-covalent organic frameworks

“…However, their relatively poor stability associated with the weaker strength of coordination bonds relative to covalent bonds retards their practical applications to quite a large degree. It is worth noting that metal ions in porous MOFs have been found to play an important role in many applications including gas adsorption and separation, [24][25][26][27] catalysis, [28][29][30][31][32] and chemical sensing. 33,34 As a consequence, the integration of MOF and COF comple-mentarity to create metal-containing COFs (MCOFs), through either the introduction of metal complexes as building blocks or the formation of metal-coordinating molecular subunits during the assembly as well as the post-synthetic modification (PSM) processes, should be able to further expand COF functionalities.…”