Separation of Xenon and Krypton in the Metal–Organic Frameworks M₂(m‐dobdc) (M=Co, Ni)

Abstract

The separation of Xe and Kr from air is challenging owing both to the very low atmospheric concentrations of these gases and the need for their distillation at cryogenic temperatures. Alternatively, separation processes based on adsorption could provide a less energy-intensive route to the isolation of these gases. Here, we demonstrate that the metal-organic frameworks M 2 (m-dobdc) (M=Co, Ni; mdobdc 4À = 4,6-dioxido-1,3-benzenedicarboxylate) effectively separate Xe and Kr at ambient temperatures based on the different adsorption enthalpies of each gas at the coordinatively-unsaturated M 2 + sites in each material. In situ Xe-and Kr-dosed powder X-ray diffraction studies further reveal key differences in the binding of Xe and Kr within the materials. In particular, while both gases adsorb near the framework open metal sites at 200 K, much higher Xe occupancies are observed at these sites relative to Kr, corroborating a stronger interaction of the polarizing M 2 + cations with Xe. Further, while krypton is only found located above the open metal sites, two additional adsorption sites are observed for xenon, correlating with the stronger adsorption of Xe over Kr in these materials. These results suggest the possible utility of employing M 2 (m-dobdc) materials in the adsorptive separation of Xe and Kr.

Balancing uptake and selectivity in a copper-based metal–organic framework for xenon and krypton separation

“…The high Xe/Kr Henry selectivity indicates the potential of MOF-11 as a sorbent material for industrial separation of Xe and Kr under extremely dilute conditions. The ideal adsorbed solution theory (IAST) was also applied to calculate the selectivity of a binary gas mixture of 20:80 (v/v) Xe/Kr at 298 K. As demonstrated in , the IAST selectivity of MOF-11 was calculated to be 16.1 at 298 K and 1 bar, which is close to the Henry selectivity and significantly higher than some MOFs with OMSs, such as UTSA-74 (8.4) [18], MOF-74-Ni (9) [34], MOF-505 (9) [35], Ni 2 (m-dobdc) (10.1) [3], Co 2 (m-dobdc) (11.8) [3], and comparable to the ultramicroporous MOFs including Zn(ox) 0.5 (trz) (10.2) [36], SB-MOF-1 (16.5) [8], ECUT-60 (11.4) [30], MOF-Cu-H (16.7) [29], and IISERP-MOF2 (18.5) [32]. In addition, the high IAST selectivity places MOF-11 among a group of benchmark MOFs such as ultra-microporous CROFOUR-1-Ni (22) and CROFOUR-2-Ni (15.5) which exhibit synergetic effect between the pore size and CrO 2 with the highest selectivity of 69.7 [22].…”

“…The channel I is constructed by four oppositely adjacent Cu 2 (CO 2 ) 4 paddle-wheel units and the Cu-Cu distance between neighboring paddle-wheel units is only 4.43 Å, possibly providing a strong binging site for Xe nano-trap due to the optimal pore size that is comparable to the kinetic diameter of Xe (4.047 Å) and highly dense OMSs around the inner walls, thereby affording considerable strong interactions due to the synergetic effect of optimal pore size and OMSs in this region. The channel Ⅱ is assembled by aliphatic hydrocarbon groups of -CH [20], SBMOF-1 (1.40 mmol/g) [8], ECUT-60 (4.30 mmol/g) [30], ZU-62 (3.30 mmol/g) [5], Ag-MOF-303 (3.35 mmol/g) [24], Ag@MOF-74-Ni (4.80 mmol/g) [19], and is only lower than that of Co 2 (m-dobdc) (5.99 mmol/g) and Ni 2 (m-dobdc) (5.58 mmol/g) [3]. In contrast, different sorption behavior was observed for Kr.…”

“…both gases in air are extremely low (Xe, 0.087 ppm; Kr, 1.14 ppm) [3,4]. Commercially, a 20/80 (v/v) mixture of Xe and Kr is obtained as a byproduct from cryogenic air separation, which then undergoes a second cryogenic distillation to obtain high purity Xe and Kr [5].…”

Dense packing of xenon in an ultra-microporous metal–organic framework for benchmark xenon capture and separation

“…(2.44 mmol g -1 ) 53 , SBMOF-1 (1.4 mmol g -1 ) 31 , and Co-squarate (1.34 mmol g -1 ) 32 . At 0.2 bar, which is an indicator of the Xe capture ability of adsorbents from a 20/80 Xe/Kr mixture, MOF-11 exhibits a record high Xe uptake of 4.0 mmol g -1 at 298 K, significantly higher than all the reported benchmark materials including MOF-74-Co (2.57 mmol g -1 ) 42 , Ag-MOF-303 (2.06 mmol g -1 ) 37 , SBMOF-1 (1.27 mmol g -1 ) 31 and Co-squarate (1.18 mmol g -1 ) 32 , setting a new benchmark for Xe capture uptake at 0.2 bar. Considering the relatively low BET surface areas (602.8 m 2 g -1 ) and pore volume (0.23 cm 3 g -1 ), the density of adsorbed Xe inside MOF-11 was calculated to be ultrahigh of 2826 g L -1 at 298K and 1 bar, which is 480 times larger than the density of gaseous Xe (5.89 g L -1 at 1 bar and 273K) and just slightly lower than the liquid Xe density (3058 g L -1 at 1 bar and 165 K).…”

Metal–Organic Framework–Polyacrylonitrile Composite Beads for Xenon Capture

“…Based on our experimental observations, several factors influence the selectivity of weakly interacting gases within MOFs. For example, MOFs with unsaturated open metal sites offer stronger adsorbate–MOF interactions; however, these adsorbents are not selective for noble gases when other competing gases such as H 2 O and CO 2 are present. − Similarly, other MOF attributes studied include ligand polarizability, hydrophilic and hydrophobic surface functionalities, surface area, pore size, presence of metal nanoparticles, and temperature. − Overall, all of these factors play a significant role in the ability of these materials to separate noble gases from each other and from air. − …”