Xenon Gas Separation and Storage Using Metal-Organic Frameworks

Please cite this article in press as: Banerjee et al., Xenon Gas Separation and Storage Using Metal-Organic Frameworks, Chem (2018), https:// doi.org/10.1016/j.chempr.2017.12.025

Review

Xenon Gas Separation and Storage Using Metal-Organic Frameworks Debasis Banerjee,1,* Cory M. Simon,2 Sameh K. Elsaidi,1 Maciej Haranczyk,3,4 and Praveen K. Thallapally1,*

The global demand for Xenon (Xe), a noble gas with applications in electronics, lighting, and the medical industry, is expected to increase significantly over the coming decades. However, the low abundance of Xe in the Earth’s atmosphere and the costly cryogenic distillation process that is used to obtain Xe commercially via air separation have limited the scale of applications of Xe. A physisorption-based separation using porous materials could be a viable and cost-effective alternative to cryogenic distillation. In particular, metalorganic frameworks (MOFs) have shown promise as highly Xe-selective porous solids. In this review, we discuss the recent advances of MOFs as adsorbents for noble gas adsorption and separation and the role of computer simulation in finding optimal materials for Xe adsorption.

INTRODUCTION Xenon (Xe) is a noble gas with significant uses in the semiconductor, lighting, space, and medical industries as well as in basic research.1 The main source of Xe is the Earth’s atmosphere. However, with a concentration of 0.087 ppmv, Xe is extremely rare compared with other nonradioactive noble gases such as He (5.2 ppmv), Ar (9,340 ppmv), Kr (1 ppmv), and Ne (18 ppmv).2 Studies have shown that more than 90% of the expected amount of Xe is missing, a finding often referred to as the ‘‘missing Xe paradox.’’3 It is postulated that the missing Xe is present in Earth’s core because it formed complexes with iron and nickel under extremely high pressure (>350 GPa).4 Commercially, all pure Xe is obtained as a by-product of cryogenic fractional distillation of air. In a typical large-scale industrial setup, air is liquefied and subjected to fractional distillation in an array of distillation columns at different temperatures to obtain different gases as different terminals, eventually ending up with an approximately 20/80 v/v mixture of noble gases Xe and Kr, respectively. Given their difference in boiling point and other physical properties (
108.1 C and
153.2 C for Xe and Kr, respectively), efficient fractional distillation can be achieved.5 The price of Xe is high, at approximately $100–$120/100 g, primarily because of its rarity in air and partly because of the number of additional distillation steps that are required to obtain high purity (>99.99%) Xe gas.5 Moreover, as the production of Xe is complementary to the production of O2 and N2, with no dedicated Xe-producing industrial cryogenic unit in operation, the production of Xe is geographically constrained. For small-scale, on-site operation, cryogenic distillation is not an economical technique.5

The Bigger Picture Xenon (Xe) is a noble gas used in electronics, lighting, medicine, and ion propulsion. Because of its rarity, pure Xe is difficult to produce, and the widespread use of Xe is cost prohibitive. Physisorptive separation is an energy-efficient technology for obtaining pure Xe from air to mitigate production costs. The success of an adsorption-based separation is predicated upon a high-performance material. Traditional porous materials such as zeolites and activated carbon leave room for materials with better capacity, kinetics, and separation capabilities. Metalorganic frameworks (MOFs), a class of porous materials synthesized by linking metal nodes or clusters to organic struts, offer a very high surface area and modular, tunable chemistry for targeting the adsorption of Xe. This review highlights several MOFs exhibiting excellent Xe adsorption and separation properties and thus holding promise for application in Xe production to reduce the cost of pure Xe for its myriad applications.

In this regard, physisorption-based separation of gas mixtures and purification using porous materials are considered the most effective methods.6 Although several challenges remain, such as finding a suitable material for a particular gas-mixture

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separation (or purification) and extent of separation, physisorption-based separation and purification of gas mixtures at or near room temperature using pressure-swing or vacuum-swing methods offer distinct cost advantages over traditional cryogenic distillation methods. Such separation is generally carried out by traditional, solidstate, permanently porous materials such as aluminosilicate zeolites, clays, and activated carbon.6 For example, LiX, a faujasite type zeolite, is used industrially for O2 production (or enrichment) from air via pressure-swing adsorption (PSA).7 Similarly, various types of zeolites and activated carbon have been tested and proposed for separation of Xe and Kr from each other as part of a larger air separation unit or exclusively for Xe purification on a small scale.8,9 However, these solid sorbents have low capacity and selectivity (see Traditional Adsorbent Materials: Activated Carbon and Zeolite). In addition, CO2 and water molecules need to be removed up front when these sorbents are used, because CO2 and water compete for the same adsorption sites as Xe. Another drawback of these sorbents (activated carbon) when used in nuclear reprocessing plants for separation of Xe from gas mixtures is the potential for bed fires because of the presence of NOx. Therefore, alternative materials are needed to separate and store Xe from various sources. In this regard, metal-organic frameworks (MOFs), a relatively new class of hybrid, permanently porous, functional materials have been investigated for adsorption and separation of noble gases, particularly Xe, under both static and dynamic conditions.1,10–22 MOFs are potentially useful for a number of applications, including gas storage and separation, catalysis, ion exchange, and photonics.23–29 The introduction of MOF-based solid-state adsorbents has several advantages, including but not limited to high internal surface area and pore volume and functionalized pore surfaces. The reports so far show that MOFs indeed offer advantages over many traditional sorbents.1 So far, Xe adsorption using MOFs has followed two distinct pathways: Xe storage and Xe separation from other gases. The materials for the two applications are different: with their large surface area and high void fraction, functionalized MOFs are good for storage applications,27 whereas MOFs with optimal pore sizes to fit a Xe atom are considered ideal for separation (purification applications).10 In this review, we discuss Xe applications that would benefit from physisorptionbased Xe separation using solid-state adsorbents. We also highlight recent experimental and computational work related to finding the optimal Xe-selective materials for separation.1,10,11,14–18,20–22,27,30,31

APPLICATIONS OF XENON Xe-based devices have a wide range of applications, ranging from lighting, the semiconductor industry, lasers, and space technology to medical devices.1 However, cost concerns related to the expensive production process hamper further widespread use of Xe. For example, the current price range for Xe is between US$30–$40/L; to put this in perspective, it is projected that a NASA (National Aeronautics & Space Administration) deep-space rocket with an ion-thrust engine will use approximately 16 metric tons of Xe, which would cost US$81–$100 million at today’s market price.32 In this section, we discuss briefly why Xe is important for our daily lives and why we need to find cheaper alternatives for Xe production.

1Physical

and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA

2Oregon

State University, School of Chemical, Biological, and Environmental Engineering, Corvallis, OR, USA

3IMDEA

Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid, Spain

4Computational

Xe has been used extensively in nuclear magnetic resonance (NMR) spectroscopy as an inert physical probe of the solution and gas state in addition to studies of chemistry and weak molecular bonding.33,34 For example, dissolved Xe gas (or Xe compounds) has a chemical shift range of 200 ppm, depending on the properties of the liquid.33 It is thus an excellent indicator of the physical properties of a solution.

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Research Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA *Correspondence: [email protected] (D.B.), [email protected] (P.K.T.) https://doi.org/10.1016/j.chempr.2017.12.025

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Figure 1. Xenon Binding Sites within the Protein (A) Crystal structure of maltose-binding protein (MBP) from Escherichia coli. (B) Effects of MBP on 129 Xe chemical shifts. The changes in 129 Xe chemical shift and resonance line width result from increased xenon-protein interactions with concentration. Reprinted from Rubin et al. 36

Similarly, Xe plays a major role as a probe molecule to examine the chemistry of proteins and related biomolecules.35–37 Xe interacts well with the hydrophobic cores of proteins and related biomolecules with ease because of its high polarizability (Figure 1).36,37 As a result, 129Xe has been used to identify protein-active sites and cavities that might be part of the pathways by which substrates reach active sites.36 Further, because of the ‘‘simplistic’’ nature of Xe as a probe, numerous reports exist on the use of Xe as part of theoretical or experimental studies on ligand-protein binding.38 Similar to biomolecules, Xe NMR is used to examine the properties of materials, particularly porous materials.39–41 For example, Kaskel and coworkers used 129Xe NMR spectroscopy to understand activation-induced flexibility in a flexible porous MOF, namely Ni2(2,6-ndc)2(dabco) (DUT-8(Ni): DUT = Dresden University of Technology; 2,6-ndc = 2,6-naphthalenedicarboxylate; dabco = 1,4-diazabicyclo [2.2.2]octane) (Figure 2).40 Similarly, the gate-opening behavior of the MOF upon activation and subsequent gas adsorption was monitored by 129Xe NMR; the linewidth and chemical shift of the 129Xe NMR signal were shown to be very sensitive parameters for the detection of this structural transition from a narrow pore system with low porosity to a wide-pore state. Further work on MOF flexibility involving 129 Xe NMR spectroscopy is reported notably by Ferey and coworkers on the benchmark MIL-53(Al) system.42 MIL-53(Al) exhibits a structural transformation between two possible porous forms: large pore (lp) and narrow pore (np) forms based on the temperature and guest molecules such as Xe. Complementary 129Xe NMR spectra show that the two structures, characterized by two distinct lines, coexist for Xe pressures above 5 3 104 Pa at room temperature, but a complete

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Figure 2. Monitoring the Phase Transformation with 129Xe NMR (A) The paddle wheel unit. (B) View along the crystallographic [001] direction of Ni 2 (2,6-ndc) 2 (dabco). (C) 129 Xe NMR spectra showing the phase change: (top) narrow pore at room temperature, (middle) large pore state at 200 K, and (bottom) large pore state at room temperature after sample cooling and subsequent heating. Reproduced from Klein et al.40 with permission from the Royal Society of Chemistry.

transformation is achieved when the temperature is decreased. The rate of structural transformation was further quantified on the basis of the NMR spectra at room temperature.

TRADITIONAL ADSORBENT MATERIALS: ACTIVATED CARBON AND ZEOLITE Activated carbon (or activated charcoal), characterized by its low cost, stability, and high surface area, has been used for Xe adsorption studies.9,21 Although the overall capacity was found to be high (50 wt % at 1 bar of Xe at 298 K), the Xe/Kr uptake ratio was calculated to be 2–3.21 Although activated carbon shows moderate Kr capacity, the fire hazard because of the presence of NOx in the off-gas stream barred any future use in nuclear reprocessing plants.1 Apart from activated carbon, zeolite molecular sieves are also used for applications related to Xe adsorption and separation. For example, the benchmark zeolites NaX and NaA have been tested for Xe/Kr separation, and showed high selectivity for Xe over Kr (4–6); these, however, are hindered by their low Xe capacity at room temperature.43 It was found that the nature of the cation within the zeolite pore determines the adsorption characteristic and preference. For example, protonexchanged mordenite shows higher Xe and Kr adsorption than natural mordenite.44 Silver-exchanged mordenite and protonated mordenite doped in a polyacrylonitrilebased macroporous polymer have also been investigated in term of Xe gas adsorption. It was found that the silver-exchanged form exhibited higher Xe uptake at room temperature than the protonated form, whereas this trend was reversed by lowering the temperature to 191 K.30 It is postulated that the Ag+ ion (or Ag nanoclusters) within the pore of the zeolite leads to favorable interaction of Xe. Similarly, a silvernanoparticle-loaded zeolite, namely Ag-ETS-10, showed remarkable affinity toward Xe as indicated by high Xe isosteric heat of interaction (Qst) (40–90 kJ/mol).8

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Another study on silver-loaded zeolite was reported recently by Farrusseng and coworkers, demonstrating the correlation between the Xe adsorption performance and the Ag loading in five commercially available zeolites.45 This study concluded that increasing the Ag loading in zeolites enhances the Xe adsorption capacities at low pressures. The same group further demonstrated high Xe uptake (and Xe/Kr selectivity) properties of a zeolite-supported silver nanoparticle matrix under simulated nuclear reprocessing conditions (520 ppm Xe, 60 ppm Kr, balanced with air; Table 1).46 We recently reported a chabazite zeolite-based membrane (i.e., SAPO-34) that can effectively separate Kr/Xe mixtures under industrially relevant conditions. The membrane can separate Kr/Xe mixtures with Kr permeance of 1.2 3 10
7 mol/m2 s Pa and separation selectivity of 35 for a molar composition close to the concentration of Xe and Kr in air (9:1 Kr/Xe by mol).47,48 Along these lines, Zhang and coworkers reported the Kr adsorption and separation properties of ETS-10, a supported hollow carbon nanosorbent [C@ETS-10].49 To reduce the explosion hazard of activated carbon with the NOx, only 10 wt % of carbon nanoparticles were introduced into ETS-10. The material showed higher Kr uptake (0.45 mmol/g) at 263 K than commercial activated carbon. In short, zeolite- and activated-carbon-based adsorbent materials have been used for Xe (and Kr) adsorption and separation applications for a long time, but the research to date remains limited. In recent years, several organic molecules that possess intrinsic porosity have shown selective Xe adsorption as a result of their close pore-size-Xe matching. In this regard, the most prominent example of a porous organic adsorbent molecule is CC3, an organic cage molecule.50 In the solid state, CC3 packs such that the internal cage cavities are connected via four cage windows, which form the narrowest point in the resulting diamondoid pore network of internal diameter 3.6 A˚. CC3 shows selective Xe adsorption over other gases, especially Kr, with a saturation uptake of 2.2 mmol/g at 1 bar and 298 K.51 Breakthrough measurements under simulated nuclear reprocessing conditions (400 ppm Xe, 40 ppm Kr, balanced with air) on CC3 recorded a Xe uptake capacity of 11 mmol/kg with Xe/Kr selectivity of 16 (Table 1), as calculated on the basis of the experimental data. Crystallographic experiments revealed that the Xe atom fits almost perfectly within the cage, closely interacting with the side walls. Among other types of intrinsically porous organic molecules, we recently reported Xe adsorption (and Xe/Kr separation) in noria.52 Noria is an organic oligomeric compound with a central molecular cavity of diameter in the range of 5–7 A˚, close to the kinetic diameter of Xe (4.1 A˚).53 Indeed, noria shows a strong preference for Xe adsorption over Kr at a broad range of temperatures (278–298 K).52 At 1 bar and 298 K, noria adsorbs 1.55 mmol/g, whereas Kr adsorbs 0.64 mmol/g. Noria exhibits a Xe Qst value of 24.5–26.9 kJ/mol according to the Clausius-Clapeyron equation using the pure-component Xe isotherms, whereas Qst (Kr) was found to be 20.1–21.4 kJ/mol within the same loading range. Under nuclear reprocessing conditions (400 ppm Xe, 40 ppm Kr, balanced with air), the calculated Xe/Kr selectivity for noria was found to be 9.4 (Table 1). Similarly, several papers on covalent organic frameworks and carbon derived from ZIF materials have reported Xe uptake with improved performance over pristine ZIF and other MOF materials.54–56 Different from these classes of materials, chalcogenide aerogels that can be constructed from pairs of chalcogenide clusters (SnS4
) and metal ions were also demonstrated for Xe/Kr separation. The selectivity in chalcogenides was attributed to the high polarizability of the framework. The ideal adsorbed solution theory (IAST) selectivity was found to be 6 from a 10 to 90 mixture of Xe and Kr at room temperature.57 Although very few organic molecules with intrinsic porosity have been tested for Xe-Kr adsorption, the results are quiet promising. However, for practical

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Table 1. Comparison of Xenon Uptake, Qst, and Xe/Kr Selectivity in MOFs, Zeolites, and Porous Organic Materials, Including Porous Organic Cages and Covalent Organic Frameworks, at Room Temperature MOF

Xe Uptake (mmol g
1a or mmol Kg
1b)

Xe Qst (kJ mol
1)

Xe/Kr Selectivity

CaSDB (SBMOF-1)

1.4a (13.2)b

35

16a

26.4

10c

22

7.3c

28

12d

26.9

8.4e (3)b

a

SBMOF-2

2.06

Ni-MOF-74

4.19a (4.8)b a

Co3(HCOO)6

2

HKUST-1

3.3a a

MOF-5

1.98

15

–

FMOF-Cu

0.8a

15

2f

a

20

4.7e

MFU-4l

1.8

MOF-505

2.2a

–

8g

SIFSIX-3-Ni

2.51

a

18.9

–

SIFSIX-3-Fe

2.45a

27.4

–

37.4

22c

30.5

15.5c

–

3.8g

a

CROFOUR-1-Ni

1.8

CROFOUR-2-Ni

1.6a a

UIO-66

1.58

PCN-14

7.1a

17.9

6.5d

NOTT-100

6.1

a

18.6

6.7d

NOTT-103

4.1a

19.7

5.5d

ZIF-8

–

–

1.9

[Zn(tmz)2]

3a

23

15.5f

–

5.92

–

10.37

–

5.76

MOF-74 Mg

5.58

MOF-74-Co

6.1a

MOF-74-Zn

3.88

a

a

Porous Organic Cages, Carbons, and Covalent-Organic-Framework-like Materials Noria

1.55a

24.5–26.9

9.4a,b

CC3

2.2a (11)b

31.3

20.4a (16)b

COP-4

1.7

a

–

1.5

Activated Carbon

4.2a

–

8

Carbon-Zx

4.42

a

–

–

Carbon-Z

3.17a

–

–

Zeolites and Chalcogenides 65

40h

MoSx

0.40

a

22.8

6.0b

SbS-I

0.18a

18.8

2.8b

Ag@ZSM-5

0.35h

a

At 298 K and 1 bar. Breakthrough experiments at 298 K using a gas mixture of 400 ppm Xe and 40 ppm Kr balanced with air at room temperature. Xenon capacity is in mmol/kg. c From IAST calculation (Xe/Kr 20/80). d From IAST calculation (Xr/Kr 10/90). e Henry’s constant based on a single-component isotherm. f From breakthrough experiment (Xe/Kr 50/50). g From breakthrough experiment (Xe/Kr 20/80). h Breakthrough curves for standard air containing 520 ppm Xe and 60 ppm Kr. b

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Figure 3. Schematic Diagram of the Selected MOFs and Organic Cages Used for Xe Adsorption and Separation Applications

applications, factors such as thermal and chemical stability, recyclability, and cost need to be taken into account.

METAL-ORGANIC FRAMEWORKS In this section, we discuss Xe adsorption and separation using MOFs (Figure 3). These can be divided into (1) MOFs with high specific surface area (SSA), (2) MOFs with open metal sites, and (3) small-pore MOFs (<8 A˚) without open metal sites. MOFs with High Surface Area High-surface-area MOFs typically possess a permanent SSA > 1,000 m2/g and are envisioned to be used for storage of gases at high pressure.23,24 MOFs with SSA as high as 5,000 m2/g are known and have exhibited high total uptake of gases such as methane.25,58 However, these high-SSA materials tend to be air sensitive and collapse upon solvent removal. Several high-SSA MOFs were tested for Xe adsorption studies, most notably those belonging to the IRMOF (isoreticular) series.

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High-SSA MOFs (>1,000 m2/g) such as IRMOF-1 were among the first to be tested for noble gas adsorption, with Muller and coworkers reporting the very first noble gas adsorption properties on benchmark MOF-5 material, which reaches 100 g/L uptake at room temperature and 2 bar pressure.27 MOF-5 has Zn4O(CO2)6 as a tetra-nuclear-based structural motif (or structural building unit [SBU]), connected by terephthalate linkers in crystal space, forming a three-dimensional framework with permanent porosity (Brunauer-Emmett-Teller [BET] surface area, 3,500 m2/g). It exhibits 4 times enhanced uptake for Xe over Kr under similar experimental conditions, which indicates a separation potential for both gases from a mixture. About 6 years later, we reinvestigated MOF-5 for noble gas adsorption and measured 27 wt % uptake for Xe at 1 bar and room temperature.21 The uptake is about half that registered for commercial activated carbon under similar experimental conditions. Farrussseng and coworkers measured the Qst values of IRMOF-1 and IRMOF-3 (–NH2 functionalized IRMOF-1 analog) for Xe by using a pulseresponse experiment in an ultrahigh vacuum reactor; they obtained values of 16 kJ/mol and 24 kJ/mol, respectively.59 The Qst values for Kr for IRMOF-1 and IRMOF-3 were found to be 10 and 12 kJ/mol respectively, significantly lower than that of Xe. The higher Xe Qst of IRMOF-3 is attributed to the preferential NH2$Xe interaction. Allendorf and coworkers further investigated the effect of functional group polarizability in a series of halo-functionalized (-F, -Cl, -Br, -I) IRMOF-1 variations (also termed IRMOF-2-X, X = F, Cl, Br, I) by using a combination of experimental gas adsorption and molecular simulation.19 The adsorption isotherms were found to be linear in nature at 1 bar and 292 K, which suggests that the nature of the isotherm is governed by the local chemical environment (e.g., linker polarizability) as a result of incomplete saturation. The total uptake, KH (Henry constant), and Xe/Kr selectivity value increases with polarizability of the halogen groups (I > Br > Cl > F). The absolute adsorption uptake and KH values increase with increasing size of the adsorbate (Xe > Kr) and polarizability of the functional groups. UiO-66 is a benchmark MOF, composed of Zr6O4(OH)4 clusters connected by terephthalate linkers, forming a three-dimensional framework with triangular-shaped channels of average pore width of 8–11 A˚ (BET surface area >800 m2/g).60 UiO-66 and related classes of zirconium-based MOFs are particularly interesting because of their exceptional thermal and chemical stability and catalytic properties.61 Bae and coworkers recently reported the Xe adsorption and Xe/Kr separation capability of UiO-66 under dynamic conditions.62 UiO-66 exhibits a Xe saturation capacity of 1.58 mmol/g at 303 K and 1 bar. In comparison, the Kr capacity of UiO-66 is 0.4 mmol/g under similar experimental conditions (Table 1). Moving forward, UiO-66 exhibits facile separation of Xe/Kr (20/80 v/v) under dynamic breakthrough conditions. The Xe/Kr separation selectivity was calculated to be 3.8 under dynamic conditions, lower than the IAST-derived selectivity. The difference is attributed to defects in the experimental sample and a limitation of IAST theory that does not reflect the actual adsorption under dynamic flow conditions. Bae and coworkers further compared the Xe (and Kr) adsorption isotherms with other benchmarks, ultrastable MOFs such as MIL-101(Cr) and MIL-100(Fe) (MIL = Material Institut Lavoisier), which show lower Xe and Kr capacity at 1 bar and room temperature, despite having considerably higher SSA. The higher capacity of UiO-66 was attributed to the comparatively smaller pore of UiO-66 with respect to MILs (8–11 A˚ in UiO-66 compared with 25 A˚ in MIL-101(Cr)), which leads to effective interactions between

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adsorbed Xe and Kr atoms and the pore surface. It was also observed that UiO-66 maintains most of its Xe and Kr uptake capacity, crystallinity, and SSA even after exposure to g radiation for 7 hr. Such radiation stability is crucial for application of materials under nuclear reprocessing conditions. In the early years of noble gas adsorption studies in MOFs, high-surface-areabased materials were chosen for their high gas uptake abilities, although they often lack strong adsorption sites and have low gas selectivity. Recent studies, as discussed in later in this review, show that MOFs with optimal pores and strong binding sites are better suited for noble gas adsorption and separation applications.1,10 MOFs with Open Metal Sites MOFs that possess accessible open metal sites upon activation are often considered ideal candidates for separation-related applications as a result of their enhanced selectivity for one adsorbate over others according to their physiochemical characteristics. As discussed below, several benchmark MOFs with open metal sites have been tested for noble gas adsorption-separation applications.12,16–18,20,21,27,59 In this regard, HKUST-1 (HKUST = Hong Kong University of Science and Technology) has received particular attention because of the presence of both small and large pores within the framework as well as the accessible open copper metal sites.63 The framework is composed of prototypical copper paddle-wheel-based building blocks, connected through an organic (e.g., btc [btc = 1,3,5-benezenetricarboxylate], also known as trimesate) linker to form the overall three-dimensional framework.18,63,64 Each copper metal center of the paddle-wheel building block has an axial water molecule that can be removed to form an open metal site upon activation. Muller and coworkers reported the very first Xe-Kr adsorption-separation experiment in HKUST-1 (termed Cu-BTC-MOF) by using a custom-built breakthrough setup.27 The breakthrough data show that the activated HKUST-1 can selectively adsorb Xe over Kr (Kr/Xe molar ratio, 94/6) at 55 C and 40 bar. The total Xe uptake was found to be 60 wt % under experimental conditions.27 Since the report by Muller and coworkers, several groups, including ours, have reported noble gas adsorption studies on HKUST-1.16,18,59 For example, Farrussseng and coworkers measured the Qst values for Xe and Kr in HKUST-1 at low coverage by using pulse-response experiments in an ultrahigh vacuum reactor system (TAP reactor) and obtained values of approximately 20 kJ/mol and 9 kJ/mol, respectively.59 We subsequently studied the Xe/Kr separation ability of HKUST-1 under selected nuclear reprocessing conditions (400 ppm Xe, 40 ppm Kr, balanced with air) by using a dynamic breakthrough method. The results showed preferential adsorption of Xe over Kr at 298 K and 1 bar pressure.18 The Xe/Kr adsorption selectivity varied with the composition of the gas mixture and was calculated to be 2–3 at 298 K and 1 bar pressure. As expected, a lower separation selectivity was recorded for mixtures with higher Xe content, which is lower than the value calculated (8) by Snurr and coworkers via computer simulation.65 The difference between theoretical and experimental values is postulated to be due to the presence of defects in the activated samples and the contribution of kinetic factors. The molecular simulation studies take into account only the equilibrium-based separation, which has an opposite effect to kinetic separation because the larger Xe molecules have higher polarizability but lower diffusivity with respect to Kr.

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Figure 4. Xenon Binding Sites in a Copper Paddle-Wheel MOF View of HKUST-1 showing the strongest binding site for Xe (orange atoms) at the pocket center, two views of the small pocket isolated (top left and bottom left), and a view of one unit cell down the c axis (right). Reprinted with permission from Hulvey et al. 16 Copyright 2013 American Chemical Society.

Forster and coworkers reported a comprehensive theoretical and experimental study on the Xe adsorption mechanism in HKUST-1 (Figure 4).16 The experimental Qst values for Xe and Kr were calculated with data from standard gas adsorption isotherms in a different temperature range (180–260 K for Xe and Kr) and from a gas adsorption calorimetric method /at room temperature. The Qst values were calculated to be 26 G 1 and 18 G 1 kJ/mol for Xe and Kr, respectively, matching well with previously reported experimental and simulated values.18 The Qst values for Xe and Kr were high in the low-pressure region and dropped gradually in the ‘‘mid-loading’’ region before rising again slightly at the maximum loading. Molecular simulation data showed that the initial adsorption of gas molecules happened in the pocket, followed by pocket windows, and there was no adsorption in the vicinity of the open metal site.16 The increase in Qst at higher loading was attributed to interatomic interactions during saturation. The simulation data are also supported by neutron and synchrotron powder X-ray diffraction (XRD) analysis at different gas loading pressures, which showed the same loading sequence for the adsorbate molecules. Among MOFs with accessible open metal sites, M-DOBDC (M = Mg, Fe, Co, Ni, Zn; also known as MOF-74-M and CPO-27-M; DOBDC = 2,5-dihydroxyterephthalate; CPO = coordination polymer of Oslo) analogs have received considerable attention for gas adsorption and separation applications as a result of high CO2 (30–36 wt %) uptake.25,66,67 The significant uptake is attributed to the presence of a large number of accessible metal sites along the microporous hexagonal one-dimensional channel in the activated framework. The framework is topologically quite different from

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Figure 5. Guest Atom Positions inside the Honeycomb Network Structure of Ni-MOF-74 Contour plots of Xe adsorption energy per atom (E ad ) in Ni-MOF-74 on the (001) plane with a loading of (A) one, (B) six, and (C) seven Xe atoms per unit cell. In (C), six Xe atoms are fixed at equilibrium positions at the primary sites, and E ad is taken as that of the seventh Xe atom. The MOF structure was fixed at the optimized geometry, and only regions with exothermic adsorption (E ad % 0) are shown in the contours. Reprinted with permission from Ghose et al. 68 Copyright 2015 American Chemical Society.

HKUST-1, which has distinct pockets rather than uniform channels in its pore structure.63 We performed single-component Xe and Kr adsorption on the Ni analog under ambient conditions.21 Xe adsorption in Ni-DOBDC at 1 bar pressure and 25 C was found to be 55 wt %, comparable with that of activated carbon but almost twice that of the previously described prototypical MOF-5 under similar experimental conditions.21 By comparison, the uptake of Kr in Ni-DOBDC is only 3% under similar experimental conditions. The Xe/Kr uptake ratio was calculated to be 5–6 for most of the pressure range, higher than that of activated carbon under similar experimental conditions. Qst was measured from Xe adsorption isotherms at different temperatures (273, 283, and 323 K) and found to be 22 kJ/mol according to the Clausius-Clapeyron equation.21 The value is consistent for a wide range of loadings, indicating adsorbate interaction with a relatively homogeneous pore system. The calculated adsorption energy (bEo), which signifies the interaction strength between adsorbate and the host surface, is 9.9 kJ/mol, higher than that of commercial activated carbon (Table 1). The high Qst, uptake, and Xe selectivity of Ni-DOBDC is attributed to the preferential interaction between the highly polarizable Xe molecule and the open metal sites along with high SSA and uniform porosity of the activated form. A synchrotron in situ X-ray crystallographic study, carried out at room temperature, showed that, as expected, Xe and Kr adsorb at the open metal sites (Figure 5).68 The adsorption geometries were also confirmed by complementary density functional theory (DFT) calculations. The measured temperature-dependent adsorption capacity of Xe was found to be substantially larger than that of Kr, consistent with the more negative adsorption energy (dominated by van der Waals dispersion interactions) as predicted by DFT. Given the high Xe selectivity of NiMOF-74 with respect to Kr and comparatively high Xe uptake at low pressure (0.35 mmol/g at 30 mbar), we performed breakthrough experiments by using NiMOF-74 under nuclear reprocessing conditions (400 ppm Xe, 40 ppm Kr, balanced with air).18 The results show that NiMOF-74 can selectively adsorb and separate Xe from other gases at nuclear reprocessing off-gas concentration. The Xe capacity was found to be 4.8 mmol/kg (9.3 mmol/kg at 1,000 ppm Xe, balanced air composition). On the other hand, Kr uptake under the breakthrough conditions was calculated to be 0.066 mmol/kg (Figure 6). Subsequently, we reported the silver-nanoparticle-loaded Ni-DOBDC form (termed Ag@MOF-74-Ni) with enhanced Xe uptake (15% higher than parent Ni-DOBDC or MOF-74-Ni).17

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Figure 6. Single-Column Breakthrough Experiments (A and B) 1,000 ppm Xe in air (A) and 1,000 ppm Kr in air (B) breakthrough curves at 298 K for NiMOF-74. (C) 400 ppm Xe and 40 ppm Kr in air breakthrough curves at 298 K for NiMOF-74. The He flow rate is 20 sccm, and the flow rates of Xe in air and Kr in air are also 20 sccm. Reprinted with permission from Liu et al. 18 Copyright 2012 American Chemical Society.

The color of MOF74-Ni (or Ni-DOBDC) changes from yellow to dark brown upon nanoparticle loading because of charge transfer or auto-reduction of the silver ions. The use of a higher concentration silver source leads to surface deposition of silver flakes, as demonstrated by scanning electron microscope images and powder XRD. Silver-nanoparticle-loaded Ni-DOBDC shows a Xe adsorption capacity of 63 wt %, higher adsorption energies (bEo 11 kJ/mol), and higher Xe over Kr (7) selectivity. The improved adsorption properties were attributed to strong dipole-induced dipole interactions between Xe and well-dispersed silver clusters within the framework. Allendorf and coworkers extended the work on the MOF-74-M series by targeting the other metal analogs (M = Co, Ni, Mg, Zn) for noble gas adsorption studies.20 The experimental data show that the gas uptake and interaction energies (i.e., Qst and KH) increase with increase in adsorbate polarizability. However, for a given adsorbate, no clear trend for interaction energies as a function of metal centers was observed and the Qst (or KH) values remain close to each other. This observation is very different from what was observed for CO2 adsorption in the MOF-74-M series, where the Qst changed significantly as a function of metal centers.25 Unlike CO2 adsorption, where the quadrupole moment of the adsorbate plays a crucial role, the noble gas-open metal site interactions are largely pointcharge-induced dipole in nature, and since the charge of the metal centers across the series is essentially the same, the interaction energy for a particular adsorbate remains similar over the entire series. The total adsorption (and associated Qst) relies on the density of the open metal sites and their relative accessibility to the adsorbed atoms instead. The same group further evaluated the Xe adsorption properties of a number of benchmark MOFs with nbo topology to understand the effect of pore size on Xe adsorption when other structural parameters such as metal centers and the coordination environment remain the same.20 The study showed that both the molar uptake and interaction energies for Xe increase with decreasing pore size of the

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adsorbent, most likely as a result of the increasing pore-surface-adsorbate interaction. The increase in uptake and interaction energies for a particular adsorbent was more prominent for highly polarizable gases. Comparison of the two types of adsorbent showed that the members of the MOF-74 series in general have a better overall performance than the nbo-MOFs because of the high density of accessible open metal sites on the pore surface. Along these lines, MOF-505 is a copper paddle-wheel MOF with different pore sizes (4.8, 7.1, and 9.5 A˚) and open metal sites after activation. The material shows a selectivity of 9–10 at 298 K and 1 bar total pressure (Kr:Xe = 80:20) for Xe over Kr. The preferential adsorption of Xe (and thus high selectivity) was attributed to the presence of open metal sites and small pores within the activated material.12 Snurr and coworkers evaluated MOF-505 as a promising material for Xe adsorption and separation by using computer simulation.65 Although MOFs with open metal sites are preferable for separation studies because of the presence of ‘‘highly polarizable’’ open metal sites on the pore surface, which can selectively interact (and thus enhance the uptake in absolute value) with more polarizable noble gas molecules over others in the series, water vapor stability remains a challenge. Water, a highly polarizable molecule by itself, can interact with open metal sites more strongly than noble gas atoms can and can therefore be preferentially adsorbed on the open metal sites, drastically reducing the total uptake as well as adsorption selectivity.69 Small-Pore MOFs Recently, there have been several reports of noble gas adsorption and separation using thermally stable low SSA MOFs with small channels. The interest mainly stems from the fact that several computational studies involving Xe adsorption and separation using microporous materials have indicated that an optimal Xe-selective material should have a pore size similar to the kinetic diameter of Xe.65,70,71 However, MOFs belonging to this class do not show high adsorption capacity because they have lower SSA (<500 m2/g), and the separation potential depends on channel shape and size and the nonbonding interaction between the adsorbate and the pore surface. We reported the first example of noble gas adsorption in this class by using two partially fluorinated three-dimensional MOFs: FMOFCu (also known as Cu(hfipbb) (H2hfipbb) [hfipbb = 4,40 -hexafluoroisopropylidenebisbenzoate]) and FMOFZn, which have different pore sizes and topologies.14,72 The zinc analog possesses tubular cavities of dimensions 4.67 3 4.78 A˚, each of which is connected by a pore-opening diameter of 5.53 A˚.72 The twofold interpenetrated copper analog has a similar channel shape with a pore opening of 3.5 A˚. Both Xe and Kr exhibit typical type-I adsorption profiles without any trace of hysteresis.14 The uptake for both the adsorbates increases with decrease of the temperature, a typical phenomenon for porous materials. The Xe uptake (and the selectivity) is higher at all temperatures and pressure ranges for FMOFZn; however, FMOFCu exhibits completely different adsorption behavior. The uptake of Kr increases at a much faster rate than that of Xe below room temperature. The adsorption amount of Xe begins to decrease below 273 K and reaches sorption values lower than that of Kr at 233 K at 1 bar. The Kr and Xe molar selectivity data as a function of temperature clearly show that the gas selectivity reverts below room temperature. Breakthrough experiments (with 1:1 Kr/Xe mixture) conducted at 278 and 298 K show that the Kr/Xe selectivity increases with a decrease in temperature, reaffirming the results from

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Figure 7. Single-Component Gas Adsorption Isotherms Kr (A) and Xe (B) sorption isotherms for FMOFCu at various temperatures. The Xe uptake decreases below 273 K at all pressures. Reprinted with permission from Fernandez et al. 14 Copyright 2012 American Chemical Society.

single-component isotherms (Figure 7). Such a temperature-dependent selectivity reversal is rare and was reported for the first time in the case of Xe-Kr separation. The observed selectivity reversal was attributed to a temperature-dependent gating effect within the channel of FMOFCu: the decreasing flexibility of the pore windows at lower temperatures restricts the diffusion of the larger Xe atoms through the pore and as a result, the kinetic effect (adsorbate diffusivity) precedes the thermodynamic effect. On the other hand, the adsorption mechanism of the smaller Kr is controlled by thermodynamic factors, leading to stronger adsorbate-adsorbent interaction at lower temperatures. Capillary condensation of comparatively larger Xe atoms within the channel is also suspected, causing a lower adsorption capacity at low temperature. Similarly, metal formates of formula M3(fa)6 (M = Mn, Co, Ni, Mg, Zn fa = formate) possess a diamondoid topology with a pore diameter of 5 A˚.73 Recently, Forster and Li reported the adsorption and separation of Xe from other noble gases by using metal formates.22,74 These materials show a fully reversible type-I adsorption behavior for Xe, with an uptake of 26.5 wt % (2 mmol/g) at room temperature and 1 bar for the cobalt analog.22 In contrast, the adsorption profile for Kr is very different and is essentially almost linear over the entire pressure range. Xe (29 KJ/mol) has a much higher Qst than Kr (22 KJ/mol) at zero loading, indicating a stronger interaction between the framework and Xe atom. The follow-up IAST calculation revealed a Xe/Kr selectivity of 22 at room temperature and 1 bar pressure.22,75 The IAST selectivity was also supported by complementary breakthrough data (for a Xe/Kr 10:90 composition, selectivity is 6 at room temperature).22 Molecular simulation calculations show that each Xe molecule occupies a segment (a zig or zag) of the channel and is fully commensurate with the crystal symmetry of the framework.22,74 The Qst for Xe is high probably because size-wise it is an ideal fit within the adsorption site and thus can interact more effectively with the p cloud of the formate groups of the pore surface.74 SBMOF-1 SBMOFs (SB = Stony Brook) represent a class of calcium-based MOFs made of V-shaped organic linkers that form diamond-shaped channels without the presence

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Figure 8. Xe and Kr Adsorption, Kinetics, and Cylce Performance of SBMOF-1 (A) Experimental Xe and Kr adsorption isotherm. The horizontal line indicates one atom per pore segment. (B) Survey of thermodynamic Xe/Kr separation performance in top-performing MOFs and porous organic cage. (C) Xe adsorption kinetics experiments. (D) Xe adsorption/desorption cycling data. Reprinted from Banerjee et al. 10

of open metal sites.10,11,76 The absence of open metal sites and ionic M–O bonds and the presence of small channels lead to unprecedented thermal and water vapor stability.10,77–80 SBMOF-1 is composed of a calcium metal center and a V-shaped organic linker, 4,40 -sulfonyldibenzoate (SDB), forming a three-dimensional framework with corner-sharing CaO6 octahedral chains along the crystallographic b axes.76 The estimated pore width of the activated material is 4.5 A˚, close to the kinetic diameter of the Xe atom. Computational studies by Smit and coworkers predicted that SBMOF-1 is the best MOF among the experimental MOFs (5,000) and in the 0.01 percentile of the 120,000 hypothetical MOF database, respectively (see Molecular Models and Simulation Methods).71 Inspired by these encouraging computational results, we performed gas adsorption experiments as a function of different activation temperatures (373 and 563 K). Interestingly, although the total Xe uptake at 1 bar remained similar at different activation temperatures, we found that the low-pressure adsorption behavior was very different: SBMOF-1 activated at 373 K adsorbed 2.5 times more Xe at 30 mbar than the SBMOF-1 sample activated at 563 K (Figure 8). We postulate that this is because, at higher temperature, the aromatic rings around the channel rotate slightly, leading to a smaller pore opening. The low temperature

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phase does not have such ring rotation, leading to higher capacity at lower pressure.76,80 In both cases, the Xe uptake capacity (at 30 mbar) surpasses the Xe uptake capacity of benchmark MOFs.21,51 Moreover, Xe Qst was found to be 35 kJ/mol (Kr Qst 26 kJ/mol), one of the highest among other sorbents. SBMOF-1 was found to have a Xe uptake capacity of 13.2 mmol/kg under nuclear reprocessing conditions (400 ppm Xe, 40 ppm Kr, balanced with air; Table 1) and showed excellent Xe/Kr separation performance, as evident from breakthrough experiments (Figure 9). Interestingly, SBMOF-1 retains its Xe uptake capacity at 48% relative humidity under breakthrough conditions. The Xe uptake of SBMOF-1 is higher than benchmark CC3 (11 mmol/kg) and NiMOF-74 (4 mmol/kg) under similar breakthrough conditions. Single-crystal XRD studies revealed that each Xe is adsorbed at a single site, near the midpoint of the channel, interacting with the channel wall (aromatic rings) by mainly van der Waals type interactions (Figure 9). The position of Xe in the pore is consistent with calculated potential energy contours and molecular simulations of Xe adsorption. The saturation loading of Xe approaches two atoms per unit cell, consistent with commensurate Xe adsorption.78 Another microporous variant is SBMOF-2, which is based on a calcium metal center and a tetrahedral organic linker, tcpb (1,2,4,5-tetrakis(4-carboxyphenyl)-benzene), and forms diamond-shaped channels of approximate dimensions 7 3 7 A˚.11 The activated SBMOF-2 adsorbs 2.83 mmol/g of Xe versus 0.92 mmol/g Kr at 298 K and 1 bar (Table 1). The Xe/Kr separation is also confirmed by both experimental and simulated breakthrough, which exhibit preferential adsorption and selectivity toward Xe over Kr. The single-crystal XRD data on Xe- and Kr-loaded SBMOF-2 show that the Xe selectivity may be attributed to the specific geometry of the pores, forming cages built with phenyl rings and enriched with polar –OH groups, both of which serve as strong adsorption sites for polarizable Xe gas. Hybrid Ultra-microporous Materials SIFSIX-M (M = Fe, Co, Ni, Cu, Zn) are a series of benchmark hybrid ultra-microporous materials (HUMMs) with isoreticular structures based on square grid sheets ((M[pyz]2)2+, pyz = pyrazine) connected by pillaring SiF62
anions.81–83 Because of their ultra-microporosity (<5 A˚) and ionic SiF62
pillars, SIFSIX-M analogs are known to have outstanding adsorption properties for polarizable adsorbates such as CO2.81,83 We reasoned that SIFSIX-M will be Xe selective because of their ultramicroporous nature (<5 A˚) and ionic SiF62
pillars. Experimentally, all five materials showed preferential Xe adsorption over Kr at 1 bar and 298 K, and their order of uptake roughly followed their SSA. For example, the -Zn, -Cu, and -Co analogs showed lower Xe overall uptake at 1 bar than the Ni and Fe analogs (Figure 10). The total Xe uptake for Fe and Ni analogs was found to be similar because of their similar surface areas. The Fe analog exhibited the highest Xe Qst (27.4 kJ/mol) at zero loading, higher than all the other analogs.84 Whereas all the other SIFSIX-M (M = Fe, Co, Cu, Zn) analogs showed typical type-I Xe adsorption isotherms, the Ni analog showed a two-step adsorption isotherm for Xe a with temperature-dependent inflection point. The Xe resides in the center of the channel with its CN axis aligned with the C4 axis of the crystal lattice, and both a/b and c axis lattice parameters expand only slightly upon Xe binding. Using molecular modeling, we hypothesize that the inflection point is due to a disordered-to-ordered transition of the rotational configurations for the organic pyrazine rings.

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Figure 9. Breakthrough Plots and Xe/Kr Binding Sites Single-column breakthrough experiment under nuclear reprocessing conditions (400 ppm Xe, 40 ppm Kr, balanced with air): (A) under dry conditions and (B) under 42% relative humidity. (C) Xe and Kr positions in SBMOF-1 were determined by single-crystal X-ray diffraction. Reprinted from Banerjee et al. 10

Similarly, CROFOUR-1(2)-Ni belongs to the HUMM family of materials; it is formed by pillaring square grid sheets [Ni(L)2]2+ (L = 1,2-bis(4-pyridyl)ethylene or 4,40 -azopyridine) connected by CrO42
as an angular inorganic pillar with an mmo type

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Figure 10. Xe Adsorption Isotherms in Ultra-microporous MOFs (A) Xe adsorption isotherms collected at 298 K for SIFSIX-Fe and SIFSIX-Ni. (B) The location of Xe from in situ synchrotron diffraction. Reprinted from Elsaidi et al., 84 published by the Royal Society of Chemistry.

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 CrO42
moiety); the second is lined by the functionalized organic linker (N=N from 4,40 -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 Qst of 37.4 kJ/mol at zero loading, the highest reported to date (Table 1).85 For further investigation of the potential of these materials under dynamic conditions, column breakthrough experiments were conducted at 298 K for Xe/Kr gas mixtures on CROFOUR-1-Ni and CROFOUR-2-Ni. The separation times between Xe and Kr gases for the 20:80 Xe:Kr gas mixture were found to be 39 and 32 min/g for CROFOUR-1-Ni and CROFOUR-2-Ni, respectively.75,85 Molecular simulation supported by in situ synchrotron powder XRD studies revealed that the primary adsorption site for Xe in both materials was located in the cage that contained three CrO42
ions in proximity to each other. Xe atoms interacted with six terminal oxygen atoms simultaneously: two each from three different CrO42
moieties. In both cases, the pore size was slightly larger than the kinetic diameter of Xe and provided a favorable fit for the adsorbed Xe atoms. The primary adsorption site for Kr in both materials was the same as that for Xe. However, interactions between Kr and the moieties at this site in both materials were weaker, presumably because of the lower polarizability of Kr. The use of small-pore, permanently porous, chemically and thermally stable MOFs for noble gas separation offers several advantages and disadvantages. Among the advantages, the materials are generally thermally and chemically stable and as a result can be used for ‘‘real-life’’ separation; molecular-sieving-induced separation can also lead to sought-after size-exclusion-based separation for ideal materials. On the other hand, the small surface area (low total adsorption capacity) for the gases presents a significant drawback for these materials. Along these lines, MOFs with flexible linkers show structural flexibilities based upon external stimuli such as external guests, temperature, or pressure.26,86–88 Such MOFs can be particularly advantageous for separation of adsorbate mixtures.89 Wang, Hu, and coworkers reported [Zn(mtz)2] (also named USTA-49: USTA = University of Texas

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Figure 11. Xe/Kr Adsorption in Hybrid Ultra-microporous Materials (A) Single-component gas adsorption isotherms for CROFOUR-1-Ni collected at 298 K. (B) Position of adsorbed Xe in CROFOUR-1-Ni and CROFOUR-2-Ni. Reprinted with permission from Mohamed et al. 85 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

at San Antonio; Hmtz = 5-methyl-1H-tetrazole), which exhibits high capture capacity and selective adsorption selectivity for Xe over other noble gases at room temperature.90,91 Zn(tmz)2 has a three-dimensional framework with dia topology. It possesses a cage of 4 A˚ diameter, accessible through two different types of pore windows of diameter between 2.9 A˚ and 4 A˚.90 Although these pore windows are smaller than the kinetic diameter of Xe (4.1 A˚) or Kr (3.6 A˚), because of the flexible nature of the framework and associated vibrational motion of the adsorbate Xe, [Zn(mtz)2] adsorbs a significant amount of Xe at 1 bar and 298 K (3.0 mmol/g; Table 1).91 [Zn(mtz)2] shows a significantly lower Kr capacity under similar conditions (0.38 mmol/g). The Xe-selective nature of [Zn(mtz)2] was further confirmed by a column breakthrough study. Very few MOFs with flexible linkers have been reported for Xe/Kr adsorption and separation, apart from the above example. ZIF-8 was shown to have a gate-opening mechanism slightly different from breathing.

SEPARATION OF XE FROM ANESTHETIC GAS MIXTURES Xe is considered a better anesthetic than the existing technology in clinical medicine, which uses a mixture of N2O and fluoroethers.92 Xe has a minimum alveolar concentration of 72% at age 40 years, making it 44% more potent than N2O as an anesthetic.93 Thus, it can be used with oxygen in concentrations that have a lower risk of hypoxia. Unlike N2O, Xe is not a greenhouse gas, and as such is environmentally friendly.94 In general, anesthetics work by interacting with receptor targets such as GABAA and the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor.35,94 Such interaction inhibits excitatory neurotransmission and leads to inhibitory neurotransmission. Xe interacts with many different receptors and ion channels, and like many theoretically multimodal inhalation anesthetics, these interactions are likely complementary.95,96 Xe as an anesthetic works by blocking the response of NMDA receptors.97 But the key difference of Xe from other similar anesthetics is that it is not a neurotoxin, and it inhibits the neurotoxicity of ketamine. Unlike these other anesthetics, Xe does not simulate a dopamine efflux in the nucleus. As a result, Xe does not simulate any ‘‘memory’’ effect in the nerve cell, leading to faster post-surgery recovery of the patient.94 Moreover, a clinical trial showed that Xe inhibits nicotinic acetylcholine a4b2 type receptors, which contribute to spinally mediated analgesia.97

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Figure 12. Schematic Representation of Xe Recycling and Recovery Using MOFs at Room Temperature Reprinted with permission from Elsaidi et al. 100 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Xe also induces robust cardioprotection through a variety of mechanisms.93 For example, it was noted that Xe is cardioprotective by activating protein kinase C-epsilon and downstream p38 mitogen-activated protein kinases.98 Detailed discussion on the biological role of Xe can be found in a recent review.35 According to the literature and clinical studies, it is clear that Xe offers a safer anesthetic solution than existing technology.35,93,94 Recently, when Xe was added to the ventilation mix of a gravely ill newborn, the baby survived.94 The treatment was done simultaneously with cooling the body temperature to 33.5 C. So why is Xe not used as a general anesthetic worldwide? The answer boils downs to the cost. A recent cost-analysis report by Morita and coworkers showed that for a 4 hr anesthesia treatment, Xe costs approximately $356, whereas traditional N2O-isoflurane or N2O-sevoflurane type anesthesia costs $52.99 The authors concluded that for the use of Xe-based anesthesia to become widespread, a closed system must be developed with an efficient system of Xe recovery and reuse. Such recovery and recycling of Xe gas within a closed system would likely involve PSA or vacuum-swing adsorption (VSA)-type porous materials as adsorbents (Figure 12). Recently, we demonstrated for the first time the applicability of MOFs for Xe recovery and recycling from an anesthetic gas mixture using two benchmark MOFs (Ni-DOBDC and HKUST-1) and PCN-12.100 PCN-12 exhibited remarkable performance for Xe separation and recovery as exemplified by its superior Xe adsorption capacity and Xe/CO2, Xe/N2, and Xe/O2 selectivity under conditions relevant to anesthetic gas mixture recycling. Similarly, Robson and coworkers showed the use of Zn-based MOFs for inhalation anesthetics.101 A closed system containing MOFs as novel sorbents for portable breathing units would allow the medical industry to recycle and reuse Xe efficiently, which offers a distinct cost advantage for the widespread use of Xe as an anesthetic gas. Given the potential of MOFs and porous organic molecules, we believe an even better adsorbent can be devised to recover and recycle Xe from anesthetic gas mixtures.

THE ROLE OF MOLECULAR MODELS AND SIMULATIONS Computer simulations and materials informatics are core components of contemporary materials research.102,103 Here, we review the role of molecular models and simulations of adsorption in the pursuit of MOFs for noble gas separations.

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Molecular Models and Simulation Methods Modeling studies typically approximate the MOF structure as rigid, with noble gas adsorbates as guest molecules. As MOFs exhibit varying degrees of flexibility, the error induced by this assumption depends on the MOF in question. At both low and high temperature, Greathouse and coworkers found that the simulated Xe adsorption isotherm in a rigid IRMOF-1 model does not significantly differ from a flexible model.15 In a more recent, larger-scale study, Witman et al.104 illustrated that modeling the framework’s intrinsic flexibility could be critical to correctly predicting its Xe/Kr selectivity. Although intra-host force fields for modeling flexibility in MOFs are under development,105–107 (1) the computational cost of treating MOF flexibility in molecular simulations can be prohibitive, and (2) more force-field development is needed to cover the diverse coordination environments encountered in MOFs.105,107 The energetics of the interaction of MOF atoms with Xe/Kr are typically classically modeled as pairwise and with Lennard-Jones potentials. Lennard-Jones parameters are identified from experimental data, e.g., by enforcing consistency with vaporliquid equilibria data108 or by fitting to reproduce crystal structures and sublimation energies in a database of compounds.109 As Xe and Kr atoms are uncharged, the influence of electrostatic potential energy is typically neglected. However, underprediction of Xe adsorption by simulations in MOFs with open metal sites may call for polarizable models of Xe for interactions with these open metal sites.20,110 DFT calculations that model electronic structure are generally more accurate than classic force fields, but come at a considerable computational cost so as to make them infeasible for routine Monte Carlo simulations of adsorption. An alternative is to tune a classic force field for a particular MOF by using DFT calculations as training data.111 As Xe adsorption dominantly involves London dispersion interactions, it is important to choose a DFT functional that adequately describes dispersion interactions.112,113 Given an atomistic MOF model and a mathematical description of the energetics of molecular interactions (the force field), Monte Carlo simulations of the grand canonical statistical ensemble (GCMC simulations) mimic experimental equilibrium adsorption measurements of gas in a rigid MOF at a given temperature and chemical potential (coupled to pressure through an equation of state for the gas), and molecular dynamics techniques can simulate the diffusion of gas molecules at a given temperature and loading.114 Comparisons between simulated and experimentally measured Xe and Kr adsorption isotherms are scattered about the literature; the agreement varies from good12,19 to reasonable74 to poor20 in the case of MOFs with open metal sites. For a more comprehensive and statistically sound assessment of the predictive capability of molecular simulations for ranking materials for Xe/Kr separations, we compiled from the literature experimental pure-component Xe and Kr adsorption isotherms at around room temperature in 11 benchmark MOFs and identified the Henry coefficient KH of Xe and Kr in each MOF.10 The Xe/Kr selectivity at dilute conditions follows as the ratio of the Henry coefficients. These data can be found at https://github.com/CorySimon/XeKrMOFAdsorptionSurvey. We then computed the Xe and Kr KH in each of these MOFs by using a rigid MOF model, the UFF115 for MOF atoms, and Boato et al.108 for Xe and Kr. Figure 13A shows the comparison between simulated and experimental Xe/Kr selectivities at dilute conditions. Although simulations generally overestimate measured Xe/Kr selectivity in MOFs,

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Figure 13. In Silico Screening of MOFs for Xe and Kr Separation (A) Benchmarking the ability of molecular simulations to rank MOFs for Xe/Kr separations at dilute conditions relevant to UNF reprocessing. The correlation between experimentally measured (y axis) and simulated (x axis) Xe/Kr selectivity is good. Reprinted from Banerjee et al.10 (B) Hulvey et al. 16 analyzed the spatial probability density of Xe during grand canonical Monte Carlo simulations to elucidate that the primary Xe binding site in HKUST-1 is the small pore and not the open Cu site. Reprinted with permission from Hulvey et al. 16 Copyright 2013 American Chemical Society. (C) Meek et al. 19 measured Xe isotherms in IRMOF-1 with a halogenated linker (IRMOF-2-X, X = F, Cl, Br, I), and molecular simulations properly ranked the Xe adsorption, hinting that simulations can be used for ranking and judiciously choosing linkers for MOFs. Reprinted with permission from Meek et al. 19 Copyright 2012 American Chemical Society. (D) In a high-throughput screening of hypothetical MOFs, Sikora et al. 70 found that the most Xe-selective MOFs have a largest cavity diameter slightly larger than that of a Xe atom. Each point on this plot represents a MOF. Reproduced from Sikora et al. 70 with permission from the Royal Society of Chemistry.

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the correlation is reasonable. Despite the room for improvement of these force fields, the correlation justifies the use of UFF for the purpose of crudely ranking materials for Xe/Kr separations (under dilute conditions, at least) by using off-theshelf molecular models. Although the UFF is known to overestimate dispersion and neglect polarization,116 other sources of discrepancy between experimental and simulated isotherms may emanate from (1) the rigid MOF assumption, (2) the infinite crystal assumption (edge effects, defects in MOF sample), (3) errors in the synthesis and activation of the MOF, and (4) errors in gas adsorption measurements. Revealing Adsorption Mechanisms in a Single MOF We now highlight how molecular simulations can elucidate the mechanisms of adsorption in a given MOF and aid in interpreting experimental data. By examining snapshots of adsorbate positions during molecular simulations of adsorption, one can elucidate binding-site preferences of noble gases in a given MOF. For example, the heterogeneous structure of HKUST-1 offers four different binding sites for methane, where the open Cu site and small cage window sites are the primary (strongest) binding sites.117 By inspecting the spatial probability density of Xe (rather than methane) during GCMC simulations, Hulvey et al.16 showed that the open Cu sites are not the dominant Xe adsorption sites; rather, the center of the small octahedral pocket and the four windows to the small pockets are. These positions observed from simulations agree with synchrotron X-ray and neutron powder diffraction data (see Figure 13B). Molecular simulations also play a role in interpreting experimental data. As an example, we recently found that the Xe adsorption isotherm in SIFSIX-3-Ni exhibits a pronounced inflection point, but the Kr adsorption isotherm is Langmuirian shaped.84 DFT calculations suggested that the pyrazine ligands in SIFSIX-3-Ni undergo a rotational conformation change upon Xe adsorption. We then conducted GCMC simulations where the pyrazine ligands were allowed to freely flip between their two observed rotational configurations. Remarkably, the simulated isotherms revealed an inflection in the Xe adsorption isotherm in SIFSIX-3-Ni but not the Kr adsorption isotherm, consistent with the experiment. Here, molecular modeling supported the hypothesis that adsorption of Xe causes the rotational configurations of the pyrazine rings in SIFSIX-3-Ni to organize to achieve a greater Xe-host interaction. Computational Screening of Known MOF Structures Tens of thousands of MOFs have been reported in the literature.24 There are insufficient resources to take adsorption isotherm and/or column breakthrough measurements to evaluate all of these MOFs for Xe/Kr separations. With predictive molecular models, computer simulations can rapidly and cost-effectively predict Xe/Kr separation performance in a large number of MOFs. These predicted rankings are valuable in focusing experimental efforts in testing MOFs for Xe/Kr separations on the most promising candidates. Furthermore, these so-called high-throughput screenings yield data-driven insights into what structural features endow an MOF with a high Xe/Kr separation performance, revealing design strategies for MOFs tailored for Xe/Kr separations. For example, Snurr and coworkers used binary GCMC simulations to predict Xe/Kr selectivity in a set of eight MOFs for a 20/80 mol % Xe/Kr mixture at 273 K and at 0.1, 1, and 10 bar.65 Pd-MOF was predicted to have the highest Xe/Kr selectivity across all pressure ranges (18–19), with MOF-505 having the second highest

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selectivity at 1 bar (11.2). Later, column breakthrough measurements in MOF-505 using a 20/80 Xe/Kr mixture at 298 K affirmed the prediction that MOF-505 exhibits a high Xe/Kr selectivity;12 despite kinetic effects embedded into a breakthrough selectivity, the agreement of Xe/Kr adsorption with simulation was good. Interestingly, HKUST-1 was observed to be highly selective at low pressure because Xe adsorbs dominantly in the small pocket, but at higher pressures, the selectivity decreases as the less selective, larger cavities in HKUST-1 come into play. Consequently, for the rational design of new MOFs for Xe/Kr separations, Snurr and coworkers argued that the most selective MOFs will have uniformly small cavities without any additional larger cavities.65 Van Heest et al.118 computationally screened a set of 3,432 already-synthesized MOFs deposited in the Cambridge Structural Database (CSD) for separating a 20/80 mol % Xe/Kr mixture at 298 K. In addition to performing GCMC simulations to predict equilibrium adsorption, they used transition state theory to estimate self-diffusion coefficients relevant to a kinetic-based separation using membranes. The authors list the most selective MOFs to inspire experimental measurements of Xe/Kr adsorption. Designing MOFs on a Computer We next highlight how we can explore the chemical space of MOFs in silico to direct synthesis efforts toward the most promising envisioned or hypothetical materials. Molecular simulations can aid in judiciously choosing linkers to yield a MOF optimal for Xe/Kr separations. For example, the 1,4-benzenedicarboxylate (BDC) linker used for constructing IRMOF-1 can be halogenated and used as a building block to form the isoreticular IRMOF-2-X series (X = F, Cl, Br, I, ordered by increasing polarizability). Intriguingly, Meek et al.19 showed that GCMC simulations properly rank the Xe adsorption isotherms in this series of MOFs with halogenated linkers as I > Br > Cl > no halogen > F (Figure 13C). This underscores the potential of using GCMC simulations to evaluate for Xe/Kr separations in different envisioned or hypothetical MOFs that have not yet been synthesized. With the aim of rapidly exploring the chemical space of MOFs on a computer, Wilmer et al.119 constructed a database of 137,953 hypothetical MOFs from a library of 102 building blocks. The building blocks were chosen and combined according to rules derived from MOFs that had already been synthesized, much like snapping together tinker toys. The vision is that molecular simulations can rank these hypothetical MOFs for a specific application to focus efforts in synthesizing new MOFs on the most promising materials, analogous to the halogenated linker study by Allendorf and coworkers.19 Furthermore, with a broad coverage of chemical space, simulations in such a large number of MOFs lead to a more global picture of (1) the relationship between structural properties and performance and (2) the expectations of separation performance. To this end, Snurr and coworkers performed binary Xe/Kr GCMC simulations in each of these hypothetical MOFs to predict the Xe/Kr selectivity in the presence of a 20/80 mol % Xe/Kr mixture at 273 K and 1, 5, and 10 bar.70 Most hypothetical MOFs exhibited Xe/Kr selectivity of 2–10 at 5 bar, but thousands of hypothetical MOFs were predicted to exhibit higher Xe/Kr selectivity than MOF-505 (11.2 at 1 bar), a top-ranking material in the smaller-scale screening study.65 The relationship between simulated selectivity and largest cavity diameter reinforced earlier findings120 that the most selective MOFs for Xe have a pore size slightly larger the kinetic

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diameter of a Xe atom. See Figure 13D. Later, we rationalized this with simulations of adsorption in a spherical shell model of varying radii that serves as a caricature of MOFs with differing pore sizes.71 By comparing the ratio of the largest cavity diameter to the pore-limiting diameter, Snurr and coworkers found that the most selective MOFs had tube-like morphologies, whereas large cavities connected by narrow channels yielded a lesser selectivity.70 Later, we used a combination of GCMC simulations and a machine-learning algorithm to screen over half a million nanoporous material structures for separating a 20/80 mol % Xe/Kr mixture at 1 bar and 298 K,71 including the dataset of hypothetical MOFs and a set of 5,000 experimentally reported MOFs.121 Concordant with previous studies, the most selective materials possessed a largest cavity diameter slightly larger than a Xe atom. The binding sites in the most selective materials exhibited a striking diversity of geometries: tubes, cages, pockets, or rings of atoms; this underscores the difficulty of rational design and the importance of high-throughput computational screenings to highlight the best candidates for Xe/Kr separations. SBMOF-1 (CSD: KAXQIL), a MOF reported for high CO2/N2 selectivity,71,76 was found to be the most Xe-selective MOF in our study. Subsequently, we rescreened the databases of MOFs at conditions relevant to nuclear reprocessing by calculating the Henry coefficients KH of Xe and Kr.10 Still, SBMOF-1 ranked as the most selective material among the experimentally reported MOFs. The pure-Xe and pure-Kr adsorption isotherms of SBMOF-1 were experimentally measured, affirming that SBMOF-1 is an excellent candidate for Xe/Kr separations: SBMOF-1 exhibits the highest selectivity and Xe Henry coefficient among all MOFs in which Xe and Kr adsorption has been measured so far. Column breakthrough experiments that mimic a process for removing Xe from used nuclear fuel reprocessing off-gas show that SBMOF-1 has the highest breakthrough capacity reported (13.2 mmol/kg) and an impressive Xe selectivity, only slightly lower than the benchmark Xe-selective material, CC3.10,51 Superb thermal and chemical stability and good column breakthrough performance even in the presence of humidity render SBMOF-1 a practical, near-term MOF for the capture of Xe from nuclear reprocessing off-gas. The discovery of SBMOF-1 for Xe/Kr separations is a rare case of a computationally inspired materials discovery. The discovery also highlights the importance of hypothesis-free science and the Materials Genome Initiative: the discovery of SBMOF-1 as a high-performing material for Xe/ Kr separations was enabled by the tools and datasets developed by researchers of the Nanoporous Materials Genome Center.121 Recently, Kaija and Wilmer122 developed a new method to elucidate design rules for Xe adsorption by more thoroughly exploring the chemical space of MOFs via designing pseudo-materials.

CONCLUSION AND PERSPECTIVE The production and purification of Xe is important from an industrial standpoint because it has a number of applications. The rarity of Xe and its high production costs hinder its widespread use for many of these applications. Although Xe production and recycling depend mostly on energy-intensive cryogenic distillation, research directed toward cost-effective, physisorption-based capture and recovery of Xe remains a current research interest. Indeed, different types of traditional adsorbents have been tested for Xe capture applications. Recently, porous MOF and organic cage-based adsorbents have been tested for Xe gas adsorption and separation applications because of their favorable adsorbent properties such as high SSA and functionalized pore surfaces.

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The introduction of high-throughput computer simulations for studying the adsorption mechanism and predicting the geometric requirements of ideal adsorbents has further led to advancement in this direction. Experimental adsorption data on different types of prototypical MOFs have shown that the adsorption uptake and selectivity depend on a number of structural factors such as pore diameter and pore functionalization. For example, the presence of open metal sites in the pore surface was shown to enhance the uptake and selectivity of highly polarizable Xe over Kr. On the other hand, molecular sieving has been found to be the dominant mechanism of separation for small-pore MOFs. Simulation data predict the ideal adsorbent to have strong binding sites and a pore diameter similar to the size of the adsorbate for optimal separation. However, as in the case of other adsorptionbased separations, an optimal balance between uptake and selectivity is important for real-world application. Finally, the study of MOF-based adsorbents for noble gas separation represents a relatively new research area and is still in its infancy, given that only a handful of MOFs have been evaluated so far. The evaluation of more MOFs with diverse topologies, chemical and physical properties, metal centers, and pore-surface functionalization, guided by computer simulations, will further enhance the abilities of MOF adsorbents for noble gas separations in the near future.

SUPPLEMENTAL INFORMATION A video abstract is available at https://doi.org/10.1016/j.chempr.2017.12.025#mmc1.

ACKNOWLEDGMENTS We thank the editor and reviewers for the excellent comments. We thank the US Department of Energy (DOE) Office of Nuclear Energy for financial support. We also thank Prof. Berend Smit for encouragement. P.K.T. thanks Terry Todd (Idaho National Laboratory), Bob Jubin (Oak Ridge National Laboratory), John Vienna (Pacific Northwest National Laboratory), Kimberly Grey (DOE Headquarters), and Patricia Paviet (DOE Headquarters) for programmatic support and guidance. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the US Department of Energy by Battelle Memorial Institute under contract DE-AC05-76RL01830.

AUTHOR CONTRIBUTIONS P.K.T. proposed the topic of the review. D.B. investigated the literature and wrote the manuscript with input from S.K.E. and P.K.T. C.M.S. and M.H. discussed and wrote the modeling part. All authors discussed and revised the manuscript.

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