Hybrid Ultra‐Microporous Materials for Selective Xenon Adsorption and Separation

Abstract

The demand for Xe/Kr separation continues to grow due to the industrial significance of high-purity Xe gas.Current separation processes rely on energy intensive cryogenic distillation. Therefore,l ess energy intensive alternatives,s uch as physisorptive separation, using porous materials,a re required. Herein we showt hat an underexplored class of porous materials called hybrid ultra-microporous materials (HUMs) affords new benchmark selectivity for Xe separation from Xe/Kr mixtures.The isostructural materials,CROFOUR-1-Ni and CROFOUR-2-Ni, are coordination networks that have coordinatively saturated metal centers and two distinct types of micropores,o ne of which is lined by CrO 4 2À (CROFOUR) anions and the other is decorated by the functionalizedo rganic linker.T hese nets offer unprecedented selectivity towards Xe.M odelling indicates that the selectivity of these nets is tailored by synergy between the pore sizea nd the strong electrostatics afforded by the CrO 4 2À anions.Xenon (Xe) and Krypton (Kr) separation is of industrial interest because these gases present either commercial value as acommodity or an environmental threat. [1] High-purity Xe gas is valuable because of its applications in imaging, lighting, lasers,and medical science.The existing technology for Xe/Kr separation uses cryogenic distillation, which is energetically demanding and laborious.T his was reflected in our recent economic analysis,w hich suggests that metal-organic framework (MOF) based separation at room temperature could be more cost effective than cryogenic distillation in the context of nuclear reprocessing plants.The radioactive 85 Kr and 133 Xe can be introduced to the atmosphere during nuclear reprocessing operations,which means that they need to be captured and sequestered safely. [2] Although Xe is generated as fission product, by the time the fuel is reprocessed, all the radioactive isotopes of Xe have decayed to very low concentrations. Radioactive 85 Kr has al ong half-life (t 1/2 = 10.8 years) and therefore must be captured and removed to prevent its uncontrolled release into the atmosphere.Further presence of any trace amounts of radioactive Kr in Xe rich stream as ab yproduct is not suitable for practical use.T hese factors incentivize the development of an alternative technology for aless energy-intensive,more cost-effective,and safer process to capture Kr and Xe from reprocessing operations.I nt his regard, Zeolites and activated carbon have been evaluated for Xe/Kr separation at ambient conditions,h owever their low Xe selectivity and adsorption capacity renders them impractical. [3] Metal-organic materials (MOMs) [4] have also been widely investigated in terms of this gas separation due to their promising features including their inherent structural modularity,e xtra-large surface area, good thermal stability and recyclability. [5] However, MOMs that rely on unsaturated metal centers have many drawbacks,s uch as their strong tendency to interact with water and the high energy costs associated with their activati...

Xenon Gas Separation and Storage Using Metal-Organic Frameworks

“…topology. 85 CROFOUR-1-Ni and CROFOUR-2-Ni possess two distinct types of micropores: one is decorated by six oxygen atoms from the inorganic linkers (two from each CrO 4 2À moiety); the second is lined by the functionalized organic linker (N=N from 4,4 0 -azopyridine or C=C from 1,2-bis(4-pyridyl)ethylene). Both materials in their activated conditions show more affinity toward Xe than Kr (Figure 11), and CROFOUR-1-Ni has a Q st of 37.4 kJ/mol at zero loading, the highest reported to date (Table 1).…”

Effect of ring rotation upon gas adsorption in SIFSIX-3-M (M = Fe, Ni) pillared square grid networks

“…However, Q st of Xe is lower than in mmo topology nets ( Q st = –37.4 and –30.5 kJ mol –1 for CROFOUR-1-Ni and CROFOUR-2-Ni, respectively, at zero loading). 49 SIFSIX-3-Ni exhibits a BET surface area (368 m 2 g –1 ) and pore size (3.66 Å) similar to its Fe analogue. However, the single component adsorption isotherm of Xe in SIFSIX-3-Ni is qualitatively different from that of SIFSIX-3-Fe.…”

Radiation-resistant metal-organic framework enables efficient separation of krypton fission gas from spent nuclear fuel

“…MOFs are known for their versatile architecture and functionalized pore surface and have shown promise for gas sorption and separation 1,[6][7][8][9][10][11][12][13][14][15][16] . However, as Xe is considerably more polarizable than Kr, porous materials such as MOFs are generally more selective toward Xe over Kr due to stronger van der Waals interactions, which causes further engineering challenges as the Xe will lead to reduced Kr adsorption 2,7,15,[17][18][19] . To avoid this, we recently reported a proof-of-concept study where a dual-bed system, fitted in series, was utilized to separate and store Kr 20 .…”

“…In our continuous search for materials with high Kr adsorption capacity and selectivity, we synthesized, measured and analyzed the SIFSIX-3-M series (M = Zn, Cu, Ni, Co, Fe) of MOFs 7,[23][24][25] . SIFSIX-3-M is a class of isoreticular hybrid ultra-microporous materials (HUMs) based on saturated metal centers (SMCs) and SiF 6 −2 pillars 26,27 .…”