Xenon Adsorption Research Articles

Docking of CuI and AgI in Metal–Organic Frameworks for Adsorption and Separation of Xenon

AbstractWe present a metal docking strategy utilizing the precise spatial arrangement of organic struts as metal chelating sites in a MOF. Pairs of uncoordinated N atoms on adjacent pyrazole dicarboxylate linkers distributed along the rod‐shaped Al–O secondary building units in MOF‐303 [Al(OH)(C5H2O4N2)] were used to chelate CuI and AgI with atomic precision and yield the metalated Cu‐ and Ag‐MOF‐303 compounds [(CuCl)0.50Al(OH)(C5H2O4N2) and (AgNO3)0.49Al(OH)(C5H2O4N2)]. The coordination geometries of CuI and AgI were examined using 3D electron diffraction and extended X‐ray absorption fine structure spectroscopy techniques. The resulting metalated MOFs showed pore sizes matching the size of Xe, thus allowing for binding of Xe from Xe/Kr mixtures with high capacity and selectivity. In particular, Ag‐MOF‐303 exhibited Xe uptake of 59 cm3 cm−3 at 298 K and 0.2 bar with a selectivity of 10.4, placing it among the highest performing MOFs.

Enhanced Adsorption and Separation of Xenon over Krypton via an Unsaturated Calcium Center in a Metal-Organic Framework.

Krypton (Kr) and xenon (Xe) are nowadays widely applied in technical and industrial fields. Separating and collecting highly pure Xe from nuclear facilities are necessary and urgent. However, the technology is limited due to the inert nature of Xe and other interferential factors. In this work, a calcium-based metal-organic framework, Ca-SINAP-1, which comprises a three-dimensional microporous framework with a suitable pore width, was researched for xenon and krypton separation through both experimental and theoretical methods. Ca-SINAP-1, synthesized in solvothermal and gamma ray conditions, features accessible open-metal sites, exhibits a high Xe/Kr selectivity of 10.32, and owns a Xe adsorption capacity of 2.87 mmol/g at room temperature (1.0 bar). Particularly, its excellent chemical stability (from pH 2 to 13) and thermal stability (up to 550 °C), as well as radiation-resistance (up to 400 kGy β irradiations), render this material a promising candidate for radioactive inert gases treatment.

Inside Back Cover: Docking of CuI and AgI in Metal–Organic Frameworks for Adsorption and Separation of Xenon (Angew. Chem. Int. Ed. 7/2021)

Xenon separation from krypton by porous adsorbents is a sustainable technology for the production of small-volume but high-value noble gases. MOFs are designer xenon adsorbents whose adsorption separation performance can be enhanced by tuning the pore metrics, functionalities, and counterions. In their Communication on page 3417, Yue-Biao Zhang, Omar M. Yaghi, and co-workers describe a new methodology for postsynthetic metalation of MOFs by harboring extra metal ions and counterions for boosting the adsorption selectivity and uptake capacity simultaneously.

Xenon adsorption isotherms on chabazite. Statistical physics modeling investigation: Adsorption energy and pore size distributions computation

In order to investigate the toxic Xenon removal by an adsorption process on chabazite, the grand canonical formalism of statistical physics has been used. This conducted statistical physics treatment enabled us to draw information about the interactions between Xenon atoms and chabazite surface through the determined physico-chemical parameters. The adsorption energy and pore size distributions have been derived by using an integral expression based on a monolayer model as local isotherm. The fitted values of the steric coefficient showed that one chabazite active site can trap only one Xenon atom. The evaluated densities of receptor sites played an important role in the assess of the adsorption capacity. Ca−CHA sample showed the maximum adsorption capacity (2.36 mmol.g−1). By using the adsorption energy distributions (GD), the physisorbed character of the Xenon adsorption on chabazite samples was brought out. The diversification of the active sites is noticed for all chabazite samples. The highest adsorption energies are observed in the case of the Xenon adsorption on CA−CHA sample. Furthermore, by using the pore size distribution (PSD), the microporous character of all samples (Ca−CHA, Cs−CHA, Li−CHA, K−CHA and Rb−CHA) has been showed. The maximum surface area was found to be 187.23 m2. g−1 for Ca−CHA. The calculated surface tensions are in the range [10.07E-5 N.m−1-13.46E-5 N.m−1]. Finally, by combining the thermodynamical and statistical physics treatments, the chemical potential describing the Xenon adsorption, is calculated at different pressures.

Investigation of Xenon Extraction from ASU Streams by Low-Temperature Adsorption

The article considers a method for extracting xenon from gas concentrates on the basis of batch adsorption separation. An experimental unit for studying xenon adsorption dynamics at cryogenic temperatures was developed. Simulations were carried out in order to investigate the process of xenon adsorption from an air separation unit (ASU) “safety stream” on different sorbent materials (four types of silica gel and active aluminum oxide). A krypton-nitrogen-helium (Kr-N2-He) mixture was selected for the purposes of modeling to comply with fire and explosion safety requirements and to reduce the duration of the experiment. From analyzing the data obtained using the described, specially-developed experimental technique, we selected the sorbent having the maximum dynamic capacity for our application.

On the hysteresis of low temperature adsorption of xenon in graphitic wedge pore

We used Monte Carlo simulation with canonical and grand canonical ensembles to investigate the structure of the adsorbate and the hysteresis in the isotherm for xenon adsorption in a closed-end graphitic wedge pore at temperatures below the simulated bulk triple point of 159 K. The simulation results are compared with those of the corresponding open-end wedge to reveal the interesting behaviours in the presence of the closed end. We found that when the apex angle is less than a critical angle hysteresis occurs because of the ordering of the adsorbate. This is due to the enhancement in the solid-fluid interactions that induces the adsorbate into a sequence of domains of ordered layers, separated by smaller disordered junctions, and each domain has its own characteristic temperature above which it becomes disordered. The difference in the pressure at which the ordered domain forms during adsorption and that at which the evaporation occurs upon desorption is the microscopic reason behind the hysteresis, and we can view the alternate domain/junction as the solid-like structure of the adsorbate in the mesoscopic scale. However, as the adsorbate progresses further away from the apex, it becomes disordered with a clear interface separating the adsorbate and the bulk gas, and it is found that the functional form of the Cohan equation can be used to relate the radius of curvature of the interface to the bulk pressure. Through this equation we have found that the adsorbate is denser than the supercooled liquid, and yet is not as structured as the bulk solid for temperatures below the bulk triple point temperature.

Methyl functionalized Zr-Fum MOF with enhanced Xenon adsorption and separation

Separation of Xe and Kr is of great industrial importance and environmental concern. Adsorptive separation of Xe and Kr based on solid porous materials is a much more economical and energy-efficient alternative compared with the widely used cryogenic distillation process. For practical application, a good absorbent should have both high physicochemical stabilities and excellent performances for noble gases separation at diluted conditions. Herein, we reported a microporous methyl-functionalized Zr-MOF denoted as Zr-Fum-Me isoreticular to UiO-66(Zr) with a high physicochemical stability (thermal stability, water, acid-base and radiation resistant ability) and a good performance for Xe capture and separation. The utramicropores with a narrow size distribution (range from 5.2 to 5.5 Å) and Methyl functional groups on the pore surface remarkably enhanced the framework-Xe interactions. Zr-Fum-Me exhibits significantly higher Xe Henry coefficient and high thermodynamic Xe/Kr selectivity at very low partial pressures in comparison with what was previously observed for UiO-66 type MOF analogous. Single-column breakthrough experiments indicate that the Zr-Fum-Me can effectively capture and separate Xe under dynamic dilute conditions, implying its promising potential to capture and separate Xe in used nuclear-fuel reprocessing facilities.

Adsorption-Induced Expansion of Graphene Oxide Frameworks: Observation by in Situ Neutron Diffraction.

We have investigated adsorption-induced deformation in graphene oxide framework materials (GOFs) using neutron diffraction at sample pressures up to 140 bar. GOFs, made by the solvothermal reaction of graphite oxide and benzene-1,4-diboronic acid, are a suitable candidate for deformation studies due to their narrow (∼1 nm), monodispersed, slit-shaped pores whose width can be measured by diffraction techniques. We have observed, in situ, a monotonic expansion of the slit width with increasing pressure upon adsorption of xenon, methane, and hydrogen under supercritical conditions. The expansion of ∼4% observed for xenon at a pressure of 48 bar is the largest deformation yet reported for supercritical adsorption on a carbonaceous material. We find that the expansion of the three gases can be mapped onto a common curve based solely on their Lennard-Jones parameters, in a manner similar to a law of corresponding states.

Effect of Chlorine-Containing VOCs on Silver Migration and Sintering in ZSM-5 Used in a TSA Process

Silver nanoparticles are currently one of the most studied nanostructured nanomaterials. Because nanoparticle size and dispersion act together in determining a material’s physical and chemical properties, there is a continuous quest to develop size-controlled synthesis methods. Nonetheless, the instability of the nanometer-sized particles, which is caused by their tendency to aggregate irreversibly into larger particles, remains a recurrent problem. The use of confining scaffolds, such as the regular system of cages in a crystalline zeolite-type material, is often reported in the literature as an efficient solution to overcome particle migration at the surface. Silver nanoparticles encapsulated in ZSM-5 (Ag@ZSM-5) represent a new generation of adsorbent for Xe enrichment from the atmosphere that is currently being developed at the pilot scale in a Temperature Swing Adsorption (TSA) process. In this study, we have found that the presence of Cl-containing compounds in the air (VOCs) leads to a poisoning of the active silver phase by the formation of silver chloride. By a careful study of process parameters, we have found that most of the chlorine can be removed by heat treatment above 573 K so that the adsorption properties of silver are regenerated. That said, when applying 573 K temperature regeneration at the pilot scale, we observe a very minor but observable decay of xenon adsorption capacity that continues cycle after cycle. The mechanism of capacity decay is discussed in terms of (i) the residual presence of Cl at the surface of silver nanoparticles, (ii) the aggregation of silver nanoparticles into larger particles (sintering mechanism), and (iii) the acceleration of silver particle migration to the surface and sintering.

High-Pressure in Situ 129Xe NMR Spectroscopy: Insights into Switching Mechanisms of Flexible Metal-Organic Frameworks Isoreticular to DUT-49.

Flexible metal–organic frameworks (MOFs) are capable of changing their crystal structure as a function of external stimuli such as pressure, temperature, and type of adsorbed guest species. DUT-49 is the first MOF exhibiting structural transitions accompanied by the counterintuitive phenomenon of negative gas adsorption. Here, we present high-pressure in situ 129Xe NMR spectroscopic studies of a novel isoreticular MOF family based on DUT-49. These porous materials differ only in the length of their organic linkers causing changes in pore size and elasticity. The series encompasses both, purely microporous materials as well as materials with both micropores and small mesopores. The chemical shift of the adsorbed xenon depends on xenon–wall interactions and thus on the pore size of the material. The xenon adsorption behavior of different MOFs can be observed over the whole range of relative pressure. Chemical shift adsorption/desorption isotherms closely resembling the conventional, uptake-measurement-based isotherms were obtained at 237 K where all materials are rigid. The comparable chemical environment of the adsorbed xenon in these isoreticular MOFs allows to establish a correlation between the chemical shift at a relative pressure of p/p0 = 1.0 and the mean pore diameter. Furthermore, the xenon adsorption behavior of MOFs is studied also at 200 K. Here, structural flexibility is found for DUT-50, a material with an even longer linker than that of the previously known DUT-49. Its structural transitions are monitored by 129Xe NMR spectroscopy. This compound is the second known MOF showing the phenomenon of negative gas adsorption. Further increase in the linker length results in DUT-151, a material with an interpenetrated network topology. In situ 129Xe NMR spectroscopy proves that this material exhibits another type of flexibility compared to DUT-49 and DUT-50. Further surprising observations are made for DUT-46. Volumetric xenon adsorption measurements show that this nonflexible microporous material does not exhibit any hysteresis. In contrast, the in situ 129Xe NMR spectroscopically detected xenon chemical shift isotherms exhibit a hysteresis even after longer equilibration times than in the volumetric experiments. This indicates kinetically hindered redistribution processes and long-lived metastable states of adsorbed xenon within the MOF persisting at the time scale of hours or longer.

Microscopic characterization of xenon adsorption in wedge pores

We explored the possibility of using xenon as a molecular probe to characterize graphitic non-uniform pores in activated carbon (AC), and studied its isotherms with a Grand Canonical Monte Carlo (GCMC) simulation. To model non-uniform pores, we used a wedge pore model, a better description of the pore space in AC, and studied the effects of the pore length, the wedge angle and the temperature on the behaviour of the adsorption and desorption isotherms. For temperatures below the triple point, the isotherm exhibits two distinct regions: in the region of low loadings it shows a step-wise behaviour as the adsorption progresses from the tip of the wedge, followed by the second region of gradual increase in the adsorbed density with pressure. A distinct feature is the possible existence of multiple hysteresis loop in the first region and the reversibility of the isotherm in the second region. The hysteresis is due to the commensurate packing of molecular layers across the pore to form domains along the axial direction of the pore and the number of layers is incremented by one as we move from one domain to the next. We have found that the hysteresis disappears when either the wedge angle is greater than a critical value or the temperature is greater than the so-called bifurcation temperature. The second region of reversibility is due to the formation of the condensate separated from the gas phase with a concave interface whose radius of curvature is found to satisfy the Kelvin equation.

Modeling Gas Adsorption in Flexible Metal–Organic Frameworks via Hybrid Monte Carlo/Molecular Dynamics Schemes

AbstractHerein, a hybrid Monte Carlo (MC)/molecular dynamics (MD) simulation protocol that properly accounts for the extraordinary structural flexibility of metal–organic frameworks (MOFs) is developed and validated. This is vital to accurately predict gas adsorption isotherms and guest‐induced flexibility of these materials. First, the performance of three recent models to predict adsorption isotherms and flexibility in MOFs is critically investigated. While these methods succeed in providing qualitative insight in the gas adsorption process in MOFs, their accuracy remains limited as the intrinsic flexibility of these materials is very hard to account for. To overcome this challenge, a hybrid MC/MD simulation protocol that is specifically designed to handle the flexibility of the adsorbent, including the shape flexibility, is introduced, thereby unifying the strengths of the previous models. It is demonstrated that the application of this new protocol to the adsorption of neon, argon, xenon, methane, and carbon dioxide in MIL‐53(Al), a prototypical flexible MOF, substantially decreases the inaccuracy of the obtained adsorption isotherms and predicted guest‐induced flexibility. As a result, this method is ideally suited to rationalize the adsorption performance of flexible nanoporous materials at the molecular level, paving the way for the conscious design of MOFs as industrial adsorbents.

Radon removal trap design and coefficient testing for the development of an effective radioxenon sampling, separation and measurement system

To monitor low-level radioxenon isotopes activity concentrations in the bulk gases, a radioxenon sampling, separation and measurement system has been developed. The xenon enrichment factor of this system is more than 105 after the separation of impurities, including N2, O2, CO2and H2O, as well as radon and its progenies, such as 214Pb and 214Bi. Since radon and its progenies interfere with radioxenon measurement, they have to be removed before radioxenon counting. To separate radon from xenon, different dynamic adsorption coefficients of xenon and radon are used to design small radon removal trap to retain radon after eluting xenon, and the ratio between radon and xenon dynamic adsorption coefficient gives the adsorbent weight relationship between the xenon adsorption trap and its related radon removal trap. To test the effectiveness of radon removal, the relative measuring method is used by measuring γ-rays energies of radon progenies in canister filling with either the measuring sample prepared by the system or the original gas. The results show that the radon removal coefficient and the stable xenon recovery, which are two important parameters in the radioxenon system, are at the order of 10−6 and >70% respectively. These meet the specifications proposed by the Comprehensive Nuclear-Test-Ban Treaty Organization.

Direct probing of heterogeneity for adsorption and diffusion within a SAPO-34 crystal.

The inhomogeneous phenomenon with a gradient distribution in adsorption and diffusion behaviors of xenon within a large SAPO-34 crystal was revealed by 129Xe NMR, 2D EXSY NMR and 129Xe PFG NMR techniques at the micro-scale. A multi-layer adsorption and diffusion model for xenon in a single crystal was proposed.

Design and development of radioactive xenon gas purification and analysis system based on molecular sieves

The dynamic adsorption of xenon on molecular sieve packed columns was investigated. The modified Wheeler-Jonas equation was used to describe adsorption parameters such as adsorption capacity and adsorption rate coefficient. Different experimental conditions were accomplished to study their effects and to touch appropriate adsorbing circumstances. Respectable consistency was reached between experimental and modeled values. A purification and analysis setup was developed for radioactive xenon gas determination. Standard sample analysis results approved acceptable quantification accuracy.

The role of adsorbate size on adsorption of Ne and Xe on graphite

We have carried out an extensive grand canonical Monte Carlo simulation to investigate the adsorption of neon and xenon on graphite. The adsorbate collision diameters of neon and xenon are smaller and greater respectively, than the commensurate graphite lattice spacing λ=3×3R300 of 0.426 nm. Simulated isotherms and isosteric heats were obtained using a graphite model that has been shown to describe successfully the adsorbate transitions for krypton, methane and nitrogen by Prasetyo et al. (2017), which have collision diameters close to λ. Neon does not exhibit commensurate (C) packing because the gain in the intermolecular potential interactions in the incommensurate (IC) packing when molecules move away from carbon hexagon centres, does not compensate for the increase in the solid-fluid potential energy. Xenon, on the other hand, exhibits IC packing because its molecular size is greater than λ. Nevertheless, at a sufficiently high chemical potential, the first layer of xenon changes from the IC to C packing (in contrast to what is observed for krypton, nitrogen and methane). This transition occurs because the decrease in the xenon intermolecular interactions is sufficiently compensated by the increase in the solid-fluid interaction, and the increase in the fluid-fluid interactions between molecules in the first layer and those in the second layer. This finding is supported by the X-ray diffraction study by Mowforth et al. (1986) and Morishige et al. (1990).

Theoretical study of xenon adsorption on UO2 surfaces

During in-pile operation, fission gases such as xenon are produced and precipitates in nuclear fuel as nano-bubbles, modifying the thermomechanical properties of UO2 fuel matrix. This work aims at precising the nature and the scope of the xenon—UO2 matrix interaction. To this end, we present a theoretical study of xenon adsorption on UO2 surfaces, based on Monte Carlo simulation and semi-empirical interatomic potential. We determine xenon adsorption isotherms on previously relaxed (111) and (100) UO2 surfaces, and we compare them to the incorporation ones inside an empty box in order to isolate interface effects. A specific attention is given to the microstructure of xenon in these systems. Finally, an analysis of the mechanical properties (pressure and stress profiles near by the surface) is achieved in order to investigate the sensitivity to surface orientation and to get pertinent quantities that will supply micromechanical models at higher scale.

Xenon adsorption on geological media and implications for radionuclide signatures

The detection of radioactive noble gases is a primary technology for verifying compliance with the pending Comprehensive Nuclear-Test-Ban Treaty. A fundamental challenge in applying this technology for detecting underground nuclear explosions is estimating the timing and magnitude of the radionuclide signatures. While the primary mechanism for transport is advective transport, either through barometric pumping or thermally driven advection, diffusive transport in the surrounding matrix also plays a secondary role. From the study of primordial noble gas signatures, it is known that xenon has a strong physical adsorption affinity in shale formations. Given the unselective nature of physical adsorption, isotherm measurements reported here show that non-trivial amounts of xenon adsorb on a variety of media, in addition to shale. A dual-porosity model is then discussed demonstrating that sorption amplifies the diffusive uptake of an adsorbing matrix from a fracture. This effect may reduce the radioxenon signature down to approximately one-tenth, similar to primordial xenon isotopic signatures.

Metal–organic framework derived nanoporous carbons with highly selective adsorption and separation of xenon

ZIF-11 derived nanoporous carbons exhibit significantly high Xe adsorption capacity and Xe/Kr selectivity under dilute condition for removal of Xe from nuclear fuel reprocessing plants.

Improved methanol-to-olefin reaction selectivity and catalyst life by CeO2 coating of ferrierite zeolite

The catalytic performance of the methanol-to-olefin (MTO) reaction over ferrierite zeolite was improved greatly by introduction of a CeO2 coating that selectively masked the acid sites on the external surface of the catalyst. In the absence of this coating, significant deterioration of the catalytic performance was observed owing to coke formation on the external surface and resultant blockage of the portals. Selective coating of CeO2 on the external surface was investigated using transmission electron microscopy, temperature-programmed desorption of NH3, N2 adsorption–desorption isotherms, and xenon adsorption measurements to reveal the structure–activity relationship. The resulting catalytic performance of CeO2-coated ferrierite zeolite in the MTO reaction was similar to that of MFI or high-silica CHA zeolite. The observed preferential formation of C4,= and C6,=, ∼90% free of aromatics was obtained because of the negligible deposition of heavy coke. This work shows that selective CeO2 coating on ferrierite zeolite was effective during catalyst regeneration and hydrothermal aging, leading to extended catalyst life and improved selectivity for catalytic applications.