Kr Adsorption Research Articles

Kinetic Barriers and Microscopic Mechanisms of Noble Gas Adsorption by Nanoporous γ‐Mg(BH4)2 Obtained by Means of Sub‐Second X‐Ray Diffraction

AbstractGas adsorption by porous frameworks sometimes results in structure “breathing”, “pores opening/closing”, “negative gas adsorption”, and other phenomena. Time‐dependent diffraction can address both kinetics of the guest uptake and structural response of the host framework. Using sub‐second in situ powder X‐ray diffraction, three intracrystalline diffusion scenarios have been evaluated from the isothermal kinetics of Ar, Kr, and Xe adsorption by nanoporous γ‐Mg(BH4)2. These scenarios are dictated by two possible simultaneous transport mechanisms: diffusion through the intra‐ (i) and interchannel apertures (ii) of γ‐Mg(BH4)2 crystal structure. The contribution of (i) and (ii) changes depending on the kinetic diameter of the noble gas molecule and temperature regime. The lowest single activation barrier for the smallest Ar suggests equal diffusion of the atoms trough both pathways. Contrary, for the medium sized Kr we resolve the contributions of two parallel transport mechanisms, which tentatively can be attributed to the smaller barrier of the migration paths via the channel like pores and the higher barrier for the diffusion via narrow aperture between these channels. The largest Xe atoms diffuse only along 1D channels and show the highest single activation barrier.

Kinetic Barriers and Microscopic Mechanisms of Noble Gas Adsorption by Nanoporous γ-Mg(BH4 )2 Obtained by Means of Sub-Second X-Ray Diffraction.

Gas adsorption by porous frameworks sometimes results in structure "breathing", "pores opening/closing", "negative gas adsorption", and other phenomena. Time-dependent diffraction can address both kinetics of the guest uptake and structural response of the host framework. Using sub-second in situ powder X-ray diffraction, three intracrystalline diffusion scenarios have been evaluated from the isothermal kinetics of Ar, Kr, and Xe adsorption by nanoporous γ-Mg(BH4 )2 . These scenarios are dictated by two possible simultaneous transport mechanisms: diffusion through the intra- (i) and interchannel apertures (ii) of γ-Mg(BH4 )2 crystal structure. The contribution of (i) and (ii) changes depending on the kinetic diameter of the noble gas molecule and temperature regime. The lowest single activation barrier for the smallest Ar suggests equal diffusion of the atoms trough both pathways. Contrary, for the medium sized Kr we resolve the contributions of two parallel transport mechanisms, which tentatively can be attributed to the smaller barrier of the migration paths via the channel like pores and the higher barrier for the diffusion via narrow aperture between these channels. The largest Xe atoms diffuse only along 1D channels and show the highest single activation barrier.

Adsorption control of Xe and Kr in SBMOF-2 metal-organic framework by ligand functionalization and different metal atoms

Radioactive Xe and Kr released from nuclear reactors and spent nuclear fuel reprocessing plants can be converted in to useful ones after the radioactive decay. The adsorption techniques employing porous materials are a cost effective solution for the capture of these volatile radionuclides compared to expensive cryogenic distillation process. Among various porous materials, metal-organic frameworks (MOFs) show excellent storage and separation properties. In this paper, we investigate the Xe and Kr adsorption properties of recently reported SBMOF-2 using van der Waals corrected density functional theory. In particular, SBMOF-2 has two types of pores with distinct linker groups along pore channels. Our studies show that Xe is preferred over Kr in both the pore channels, which are in complete agreement with experimental observations. The adsorption energies obtained for SBMOF-2 is comparable to other efficient MOFs reported in the literature for Xe/Kr separation. The two pore channels show slightly different binding energies depending on their linker groups and pore characteristics. The impact of central metal atom on adsorption properties is evaluated. A new SBMOF-2(Mg) with magnesium as central metal atom shows Xe/Kr adsorption behavior similar to the experimentally studied SBMOF-2(Ca). Next, we have investigated the effect of polarizable groups in the pore channels where linker hydrogen atoms are substituted with halogen atoms. There is a significant increase in the adsorption energy of noble gases in the halogenated pore channels. Due to the presence of halogen atoms, electron charge density redistribution takes place in the MOF network leading to permanent dipoles. Accordingly, the adsorption energies increase with the polarizability of halogen atoms. The ab initio calculations performed in this study suggest that Xe/Kr separation properties of MOFs depend upon many competing interactions, which originate from their structural and chemical properties.

Ab InitioStudy of Adsorption of Fission Gas Atoms Xe and Kr on MoS2Monolayer Functionalized with 3d Transition Metals

It is well known that a nuclear reactor generates various fission products including radioactive fission gases made of isotopes of Xe and Kr. The separation of Xe and Kr isotopes and their entrapment are important tasks for efficient operation of nuclear reactors and fuel-reprocessing plants. Two-dimensional materials are known to have a large surface-to-volume ratio; this makes them prospective candidates as gas adsorbent materials. Motivated by this, we carry out ab initio density functional theory-based calculations to explore the reactivity and selectivity of a monolayer of pristine and 3d transition metal (TM)-functionalized MoS2 toward the fission gas atoms Xe and Kr. To this end, we first study and analyze the adsorption of the TM adatoms on the monolayer of MoS2 at different inequivalent crystallographic sites. Further, we calculate partial atomic charges and atom-projected density of states of the functionalized composite systems to understand the bonding mechanism of TM adatoms with the monolayer of MoS2. We predict the coexistence of both ionic and covalent bonding between the TM atoms and the surface atoms. Subsequently, we probe the adsorption of Xe and Kr atoms on both the pristine and the TM-functionalized MoS2 surfaces. Our calculated results indicate that the adsorption energies for Xe (Kr) gas atoms on the functionalized MoS2 are enhanced up to a maximum of 2.77 (2.86) times of adsorption energy found in case of the pristine surface. We find that the adsorption energy of Xe and Kr gas atoms over different TM atom-functionalized MoS2 follows the order Ti > V > Co > Ni > Fe > Sc > Mn > Cr > Cu. Charge density difference analyses indicate that the polarization of Xe/Kr adatoms is enhanced in the case of functionalized surfaces, which leads to the strengthened interaction between the noble gas (NG) atoms and the functionalized surfaces, compared to the adsorption of NG on the pristine MoS2 surface. This polarization has direct one-to-one correspondence with the adsorption energy of these gas atoms on the surface. Moreover, our calculated results suggest that both the pristine and the functionalized MoS2 surfaces show selectivity for Xe atom over Kr atom. The present study predicts that the TM atom-functionalized MoS2 surfaces may have great potential for adsorption and selective separation of Xe and Kr atoms, which may have important implication in the field of spent nuclear fuel management.

Non-Isothermal Kinetics of Kr Adsorption by Nanoporous γ-Mg(BH4)2 from in Situ Synchrotron Powder Diffraction.

Crystalline materials with pore dimensions comparable to the kinetic diameters of the guest molecules are attractive for their potential use in adsorption and separation applications. The nanoporous γ-Mg(BH4)2 features one-dimensional channels matching this criterion for Kr uptake, which has been probed using synchrotron powder diffraction at various pressures and temperatures. It results in two coexisting crystalline phases with the limiting composition Mg(BH4)2·0.66Kr expecting the highest Kr content (50.7 wt % in the crystalline phase) reported for porous materials. Quasi-equilibrium isobars built from Rietveld refinements of Kr site occupancies were rationalized with a noncooperative lattice gas model, yielding the values of the thermodynamic parameters. The latter were independently confirmed from Kr fluorescence. We have also parameterized the pronounced kinetic hysteresis with a modified mean-field model adopted for the Arrhenius kinetics.

Switching Xe/Kr adsorption selectivity in modified SBMOF-1: a theoretical study.

The separation of Xe/Kr mixtures in used nuclear fuel (UNF) has attracted lots of attention, but no report on the adsorption and separation of Kr from mixed Kr/Xe at room temperature can be found. From grand canonical Monte Carlo (GCMC) simulation, it is found that by replacing the metal center Ca of SBMOF-1 with Mg, due to the appropriate pore size, the adsorption selectivity (SKr/Xe) was extremely high (250 000) and the adsorption capacity for Kr on Mg–SBMOF-1 modified with –NH2 was increased by 300% to 1.020 from 0.248 mmol g−1. Based on the calculations of density functional theory (DFT), we found that the stronger electron-donating ability of a functional group will increase the polarizability of the ligand, and thus increase the adsorption capacity to Kr. In addition, the analysis of electronic structures with independent gradient model (IGM) and energy decomposition analysis (EDA) indicates that van der Waals forces will be responsible for the interaction of Mg–SBMOF-1 and Kr gas. Among them, the interaction of Mg–SBMOF-1 and Kr gas is mainly an induction force, while that of modifications with –CH3 and –NH2 is mainly a dispersion force. The present theoretical study represents the first report of the separation of Kr from Xe with MOF adsorption at room temperature. We hope this work may promote the experimental synthesis of Mg–SBMOF-1 for efficient separation of Kr and Xe.

The effect of doping on adsorption of Xe and Kr on graphyne and graphdiyne

Sequestration of radioactive noble gases xenon (Xe) and krypton (Kr) from off-gas streams of nuclear reactors and spent nuclear fuel reprocessing plants are essential. The Generally employed cryogenic distillation process for Xe/Kr separation is expensive and adsorption based techniques using porous materials are viable alternative. Recently, carbon allotropes such as graphynes (Gyn) and graphdiynes (Gdyn) attained importance due to their significance in many applications similar to graphene. In both graphyne and graphdiyne, the carbon atoms are sp-sp2 hybridized where benzene rings are linked through acetylenic and diacetylenic linkages respectively. They are good candidates for gas adsorption/separation due to their chemical stability and large surface area. In this regard, we have investigated the noble gas adsorption properties of doped/undoped graphyne and graphdiyne materials using density functional theory calculations. The graphyne and graphdiyne display enhanced adsorption affinity for Xe and Kr when doped with selective heteroatoms.

Confinement-Directed Adsorption of Noble Gases (Xe/Kr) in MFM-300(M)-Based Metal–Organic Framework Materials

Entrapment of radioactive inert gases, Xe and Kr, generated from the spent nuclear fuel reprocessing or nuclear accidents is one of the challenging issues in successful implementation of nuclear energy as an alternative and sustainable energy. Metal–organic frameworks (MOFs) gained immense research interest for adsorption and separation of various important gases because of their fine tunable pore chemistry and topology. Here, we investigated a series of MOFs, MFM-300(M) (M = Al, In, Ga, Sc, V, Cr, and Fe), for selective entrapment of Xe over Kr using both density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulation techniques. The calculated structural parameters of all of the considered MOFs are consistent with the reported experimental results. Different textural properties such as pore-limiting diameter, largest cavity diameter, surface area, void fraction, etc., are measured. From the binding energies calculated through DFT calculations at different loading capacities, adsorption of Xe is found to be stronger as compared to adsorption of Kr, and the binding energy is found to increase with loading. GCMC simulations indicate that the considered MOFs have significantly higher uptake capacities and are selective for Xe over Kr. Energy decomposition analysis indicates a strong adsorbate–adsorbate interaction at higher loading, which is more significant for Xe as compared to Kr. The strong adsorbate–adsorbate interactions are driven by the confinement effects of the one-dimensional cylindrical channels present in MFM-300(M). Among the series of MOFs considered, MFM-300(In) is shown to have the best selectivity for Xe over Kr. Our computational studies can provide valuable inputs for exploring MFM-300(M) MOFs for selective separation and storage of noble gases.

Nanodisperse gold catalysts in oxidation of benzyl alcohol: comparison of various supports under different conditions

Monodisperse gold particles (ca. 2 nm) were prepared and deposited on various supports (SiO2, Al2O3, HAP, MgAl2O4 and MgO). The acid/base properties of supports were characterized by NH3 and CO2 sorption. The size of the gold nanoparticles spans in the 1.7–6.5 nm mean diameter range after calcination as determined from TEM measurements. The amounts of accessible surface sites were estimated by binary concentration pulse chromatography of CO with Kr adsorption. The data are in agreement with the results of CO adsorption obtained by DRIFT spectroscopy. The activities of the catalysts were compared in the oxidation of benzyl alcohol in stirred batch reactors under two different conditions: in xylene solvent with atmospheric oxygen at 60 °C (in presence and in absence of K2CO3), and in a solvent-free mixture at elevated pressure and temperature (5 bar O2, 150 °C, 5 h). The activities of catalysts in benzyl alcohol conversion are described in two variants, namely related to (i) active catalytic sites (ASNA), and (ii) number of Au atoms on the geometric surface of particles (GSNA). The activities of catalysts in xylene solvent at 60 °C were excellent, with 0.28–1.11 s−1 characteristic GSNAini values (initial reaction rates related to surface Au atoms, Ausurf) in presence of K2CO3. The observed order of activities under these conditions is Au/SiO2 < Au/Al2O3 < Au/HAP < Au/MgAl2O4 < Au/MgO. In the experiments performed at 150 °C under solvent-free conditions, the reaction partners are depleted in greater extents (with the exception of Au/Al2O3), thus the obtained average GSNAave (average reaction rate during 5 h reaction related to Ausurf) values are less reliable, however selectivity data provide useful information as well. These estimated average GSNAave values (0.14–0.83 s−1) attest still good activities. For the interpretation of the obtained data, the roles of active sites on gold nanoparticles of various dispersion and the accessibility of their surfaces as well as the acid–base properties and surface hydroxyl concentration of supports, water ad- and desorption phenomena are considered simultaneously.

Final report of CCQM-K153 Measurement of Specific Adsorption A [mol/kg] of N2 and Kr on nonporous SiO2 at LN temperature (to enable a traceable determination of the Specific Surface Area (BET) following ISO 9277)

CCQM key comparison K-153 Measurement of Specific Adsorption A [mol/kg] of N2 and Kr on nonporous SiO2 at LN temperature (to enable a traceable determination of the Specific Surface Area (BET) following ISO 9277) has been performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee for Amount of Substance (CCQM). The objective of this key comparison is to compare the equivalency of the National Metrology Institutes (NMIs) and Designated Institutes (DIs) for the measurement of specific adsorption, BET specific surface area) of nonporous substances (sorbents, ceramics, catalytic agents, etc) used in advanced technology.In this key comparison, a commercial nonporous silicon dioxide was supplied as a sample. Eight NMIs participated in this key comparison, but only five NMI's have reported in time. All participants used a gas adsorption method, here nitrogen and (or) krypton adsorption at 77.3 K, for analysis according to the international standards ISO 15901-2 and 9277.In this key comparison, the degrees of equivalence uncertainties for specific adsorption nitrogen and krypton, BET specific surface area were established.KEY WORDS FOR SEARCHnonporous SiO2, Specific Adsorption of N2 and Kr, BET specific surface areaMain textTo reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Monolayer adsorption of noble gases on graphene

We report our results of simulations of the adsorption of noble gases (Kr, Ar, Xe) on graphene. For Kr, we consider two configurations: supported and free-standing graphene, where atoms are adsorbed only on one or two sides of the graphene. For Ar and Xe, we studied only the case of supported graphene. For the single-side adsorption, we calculated the two-dimensional gas-liquid critical temperature for each adsorbate. We determined the different phases of the monolayers and constructed the phase diagrams. We found two-dimensional incommensurate solid phases for krypton, argon and xenon, and a two-dimensional commensurate solid phase for krypton. For double side adsorption of Kr, we do not see evidence of an ordering transition driven by the interlayer forces.

Matrix-Assisted Pulsed Laser Evaporation: A novel approach to design mesoporous carbon films

We report on a fast synthesis method of porous carbon films on solid substrates. Thin carbon micro-structures tailored by processing conditions were synthesized for the first time by Matrix-Assisted Pulsed Laser Evaporation. The procedure consists of pulsed laser irradiation of a cryogenic target composed of phloroglucinol/glyoxylic acid organic precursors dissolved in different mixtures of solvents. An excimer UV KrF* pulsed laser was employed inside a vacuum chamber for material expulsion from the target and immobilization on a solid facing collector. By modifying laser energy or target solvents, thin polymeric coatings of hundreds of nanometers with various cross-linking degrees were obtained at room temperature in 10 min only. No drying or thermo-polymerization step is required even for high boiling point solvents such as dymethyl sulfoxide (DMSO). After thermal treatment, mesoporous carbon films exhibiting diverse nano-morphologies and surface areas up to 705 m2 g−1 determined by Kr adsorption were then directly obtained onto various collectors. Processing and assembling mechanisms of phenolic resins were investigated and two competing mechanisms, e.g., target absorption of laser wavelength and subsequent molecules transfer/interactions are accounted for the growth of high quality films as highlighted by several techniques.

The noble gases adsorption on boron-rich boron nitride nanotubes: A theoretical investigation

In this work, using density functional theory (DFT) calculations, we have systematically explored the noble gases (Ng = He, Ne, Ar, Kr) adsorption on boron-rich boron nitride nanotubes (BNNTs) surface with antisite boron atom. One or two nitrogen atoms of BNNTs are replaced by boron atoms, which are considered as boron-rich BNNTs for Ng adsorption. It is found that the boron-rich BNNTs can adsorb Ng in exothermic process, and the adsorption energies increase in order from He to Kr. The quantum theory of atoms in molecules (QTAIM) and noncovalent interactions (NCIs) calculations show that the interactions between boron-rich BNNTs and Ng are noncovalent, and the interactions for Ar and Kr are obviously larger than those for He and Ne. The charge transfer from Ng to boron-rich BNNTs and the changes of energy gap caused by Ng adsorption demonstrate that the boron-rich BNNTs are expected to become the Ng adsorption and sensing materials. Moreover, the 2B-BNNTs do not decrease the Ng adsorption interactions on boron-rich BNNTs, compared with 1B-BNNTs. It is expected that the present results will provide a useful guide to develop novel boron nitride nanomaterials for storage and application of Ng.

Fabrication of porous boron-doped diamond on SiO2 fiber templates

Boron-doped diamond (BDD) has become a well-established material in electrochemistry. Yet, many applications of BDD, especially thin films, would greatly benefit from an increase in specific surface area. This paper presents a simple and reproducible method for producing a stable BDD porous surface, which results in an increase in specific surface area. To fabricate porous films, BDD coated electrospun SiO2 fibers are prepared on top of planar BDD films using doctor blade and spin coating techniques. Deposition of BDD is performed using a microwave plasma enhanced chemical vapor technique. Grown porous BDD layers are characterized by scanning electron microscopy and Raman spectroscopy. In addition, specific surface area is determined using cyclic voltammetry and from Kr adsorption isotherms evaluated by the Brunauer–Emmett–Teller method. The highest capacitance value of ca. 7 mF/cm2 is found for the porous BDD prepared by doctor blade coating after removal of SiO2 fibers template by hydrofluoric acid. Electrochemical cycle stability is determined by galvanostatic charge/discharge. The charge retention is 85% after 3000 cycles.

Krypton‐xenon separation properties of SAPO‐34 zeolite materials and membranes

Separation of the radioisotope 85Kr from 136Xe is an important target during used nuclear fuel recycling. We report a detailed study on the Kr and Xe adsorption, diffusion, and membrane permeation properties of the silicoaluminophosphate zeolite SAPO‐34. Adsorption and diffusion measurements on SAPO‐34 crystals indicate their potential for use in Kr‐Xe separation membranes, but also highlight competing effects of adsorption and diffusion selectivity. SAPO‐34 membranes are synthesized on α−alumina disk and tubular substrates via steam assisted conversion seeding and hydrothermal growth, and are characterized in detail. Membrane transport measurements reveal that SAPO‐34 membranes can separate Kr from Xe by molecular sieving, with Kr permeabilities around 50 Barrer and mixture selectivity of 25–30 for Kr at ambient or slight sub‐ambient conditions. The membrane transport characteristics are modeled by the Maxwell‐Stefan equations, whose predictions are in very good agreement with experiment and confirm the minimal competing effects of adsorption and diffusion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 761–769, 2017

Computational Modeling of Adsorption of Xe and Kr in M-MOF-74 Metal Organic Frame Works with Different Metal Atoms

Management of radioactive noble gases, i.e., Xe and Kr, needs special attention in the context of nuclear fuel reprocessing and postulated severe accident scenarios. Entrapment of these species in suitable matrix is generally difficult since Xe and Kr do not form bonds with substrates (physisorbed by van der Waals interactions) due to their chemical inertness. In recent years metal–organic frameworks (MOFs) are being actively discussed for adsorption/separation of various gases including noble gases. MOFs are considered superior in such applications owing to their high intake capacity, better selectivity and also the possibility of tuning the chemical properties by varying organic linkers or metal atoms. Theoretical modeling of MOFs and tailoring their properties appropriately for noble gas adsorption/separation are potentially useful to identify suitable candidates. In this regard, we investigate the binding energies of Xe and Kr in the well-known M-MOF-74 employing computer simulations. To understand th...

Commensurate Phases of Kr Adsorbed on Single-Walled Carbon Nanotubes

In this paper, we show that Krypton atoms form a commensurate solid (CS) phase with a fractional coverage of one krypton atom per every four carbons on zigzag carbon nanotubes. This is a unique phase, different from the $$\sqrt{3} \times \sqrt{3}$$ R30 $$^\circ $$ CS monolayer formed on graphite, which has a lower coverage of one krypton atom per every six carbons. Our prediction disagrees with experiments that observe in nanotubes the same solid structure found on graphite. In order to address this discrepancy, we simulated adsorption of Kr on zigzag and armchair single-walled carbon nanotubes with radii ranging from 4.7 to 28.83 A. Our simulations confirm that the CS of coverage 1/4 forms on medium-sized zigzag nanotubes. We also found the 1/6-coverage solid on graphene, which represents the infinite-radius limit of a nanotube. Our findings are key to experiments of adsorption on nanotubes where the interpretation and justification of the results are based on the monolayer coverage, such as mass or conductance isotherms measurements.

From template-assisted mesoporous titania powders to thin films: Differences and similarities

A huge variety of template-assisted mesoporous powder syntheses are known in literature, but the conditions are not directly transferable to thin films. The present study discusses the differences and similarities between the titania powder and thin film synthesis for the preparation of mesoporous titania materials by means of evaporation induced self-assembly. Kr adsorption at −186°C is a sufficient method to analyze the porosity of thin films and is traceable to the N2 adsorption porosity data obtained at −196°C. Next, problems concerning the macroscopic homogeneity of the thin film are discussed as well as the deposition of the porous layer. Finally, the thermal stabilization and calcination process are compared. In contrast to the powder sample, no nanocrystal formation is observed during the thermal stabilization of the thin film. And thus the average pore size of the thin film is independent of the thermal stabilization in the here investigated range and is in all cases 3.4nm. Furthermore, the thin film synthesis leads to a better organized and more uniform pore structure as compared to the powder synthesis. Similar to the powder synthesis, uncontrolled crystallization during calcination may lead to a structural collapse of the mesostructure. Although the crystal size after calcination is much smaller for thin films, it seems that the critical crystal size for a structural collapse is smaller for the thin films as compared to the powders. The present study facilitates the transition from optimal powder to optimal thin film synthesis conditions, which could be useful for example in the synthesis of membranes with uniform pores starting from much more extended literature on solid powders certainly with respect to synthesis-structural property correlation.

On the 2D-transition, hysteresis and thermodynamic equilibrium of Kr adsorption on a graphite surface

The adsorption and desorption of Kr on graphite at temperatures in the range 60–88K, was systematically investigated using a combination of several simulation techniques including: Grand Canonical Monte Carlo (GCMC), Canonical kinetic-Monte Carlo (C-kMC) and the Mid-Density Scheme (MDS). Particular emphasis was placed on the gas–solid, gas–liquid and liquid–solid 2D phase transitions. For temperatures below the bulk triple point, the transition from a 2D-liquid-like monolayer to a 2D-solid-like state is manifested as a sub-step in the isotherm. A further increase in the chemical potential leads to another rearrangement of the 2D-solid-like state from a disordered structure to an ordered structure that is signalled by (1) another sub-step in the monolayer region and (2) a spike in the plot of the isosteric heat versus density at loadings close to the dense monolayer coverage concentration. Whenever a 2D transition occurs in a grand canonical isotherm it is always associated with a hysteresis, a feature that is not widely recognised in the literature. We studied in details this hysteresis with the analysis of the canonical isotherm, obtained with C-kMC, which exhibits a van der Waals (vdW) type loop with a vertical segment in the middle. We complemented the hysteresis loop and the vdW curve with the analysis of the equilibrium transition obtained with the MDS, and found that the equilibrium transition coincides exactly with the vertical segment of the C-kMC isotherm, indicating the co-existence of two phases at equilibrium. We also analysed adsorption at higher layers and found that the 2D-coexistence is also observed, provided that the temperature is well below the triple point. Finally the 2D-critical temperatures were obtained for the first three layers and they are in good agreement with the experimental data in the literature.

Direct Observation of Xe and Kr Adsorption in a Xe-Selective Microporous Metal-Organic Framework.

The cryogenic separation of noble gases is energy-intensive and expensive, especially when low concentrations are involved. Metal-organic frameworks (MOFs) containing polarizing groups within their pore spaces are predicted to be efficient Xe/Kr solid-state adsorbents, but no experimental insights into the nature of the Xe-network interaction are available to date. Here we report a new microporous MOF (designated SBMOF-2) that is selective toward Xe over Kr under ambient conditions, with a Xe/Kr selectivity of about 10 and a Xe capacity of 27.07 wt % at 298 K. Single-crystal diffraction results 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. The Xe/Kr separation in SBMOF-2 was investigated with experimental and computational breakthrough methods. These experiments showed that Kr broke through the column first, followed by Xe, which confirmed that SBMOF-2 has a real practical potential for separating Xe from Kr. Calculations showed that the capacity and adsorption selectivity of SBMOF-2 are comparable to those of the best-performing unmodified MOFs such as NiMOF-74 or Co formate.