Zr-oxo Clusters Research Articles

Location-Specific Microenvironment Modulation Around Single-Atom Metal Sites in Metal-Organic Frameworks for Boosting Catalysis.

Despite coordination environment of catalytic metal sites has been recognized to be of great importance in single-atom catalysts (SACs), a significant challenge remains in the understanding how the location-specific microenvironment in the higher coordination sphere influences their catalysis. Herein, a series of Cu-based SACs, namely Cu1/UiO-66-X (X = -NO2, -H, and -NH2), are successfully constructed by anchoring single Cu atoms onto the Zr-oxo clusters of metal-organic frameworks (MOFs), i.e. UiO-66-X. The -X functional groups dangling on the MOF linkers could be regarded as location-specific remote microenvironment to regulate electronic properties of the single Cu atoms. Remarkably, they exhibit significant differences in the catalysis toward the hydroboration of alkynes. The activity follows the order of Cu1/UiO-66-NO2 ˃ Cu1/UiO-66 ˃ Cu1/UiO-66-NH2 under identical reaction conditions, where Cu1/UiO-66-NO2 showcases the phenylacetylene conversion of 92%, ~3.5 times higher efficiency than that of Cu1/UiO-66-NH2. Experimental and calculation results jointly support that the Cu electronic structure is modulated by the location-specific microenvironment, thereby regulating the product desorption and promoting the catalysis.

Cu-UiO-66 Catalyzed Synthesis of Imines via Acceptorless Dehydrogenative Coupling of Alcohols and Amines.

Herein, the Cu-UiO-66 catalyst was developed for acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines. The Cu-UiO-66 catalyst was synthesized by installing Cu2+ onto Zr-oxo clusters in UiO-66, and the catalyst efficiently catalyzes the ADC reaction under mild and environmentally friendly conditions with excellent selectivity. Mechanistic studies reveal that the O2·- radicals and porosity of formed in Cu-UiO-66 participate cooperatively during the catalytic cycle. Meanwhile, the only by-product of the system is environmentally benign water. Cycling tests and hot filtration tests showed that the Cu-UiO-66 catalyst exhibited excellent stability and catalytic activity during the reaction. Importantly, the Cu-UiO-66 catalyst might provide a promising strategy for the ADC reaction between alcohols and amines to produce imines.

Mechanism Insight into Direct Amidation Catalyzed by Zr Salts: Evidence of Zr Oxo Clusters as Active Species.

The capricious reactivity and speciation of earth-abundant metals (EAM) hinder the mechanistic understanding essential to boost their efficiency and versatility in catalysis. Moreover, metal's solution chemistry and reactivity are conventionally controlled using organic ligands, while their fundamental chemistry in operando conditions is often overlooked. However, in this study, we showcase how a better understanding of in operando conditions may result in improved catalytic reactions. By assessing the composition and structure of active species for Zr-catalyzed direct amide bond formations under operating conditions, we discovered zirconium oxo clusters form quickly and are likely active species in the reactions. Formation of these clusters dismisses the use of additional organic ligands, inert atmosphere, anhydrous solvents, or even water scavenging to provide amides in good to excellent yields. More specifically, dodeca- and hexazirconium oxo clusters (Zr12 and Zr6, respectively) rapidly form from commercial, readily available Zr salts under reaction conditions known to afford amides directly from nonactivated carboxylic acid and amine substrates. Extended X-ray absorption fine structure (EXAFS) experiments confirm the presence of oxo clusters in solution throughout the reaction, while their key role in the mechanism is supported by an in-depth computational study employing density functional theory (DFT) and molecular dynamics (MD) methods. These results underline the value of (earth-abundant) metals' intrinsic solution chemistry to transformative mechanistic understanding and to enhance the sustainability of organic transformations.

Reticular Chemistry Directed "One-Pot" Strategy to in situ Construct Organic Linkers and Zirconium-Organic Frameworks.

Zirconium-based metal-organic frameworks (Zr-MOFs) have emerged as one of the most studied MOFs due to the unlimited numbers of organic linkers and the varying Zr-oxo clusters. However, the synthesis of carboxylic acids, especially multitopic carboxylic acids, is always a great challenge for the discovery of new Zr-MOFs. As an alternative approach, the in situ "one-pot" strategy can address this limitation, where the generation of organic linkers from the corresponding precursors and the sequential construction of MOFs are integrated into one solvothermal condition. Herein, inspired by benzimidazole-contained compounds synthesized via reaction of aldehyde and o-phenylenediamine, tri-, tetra-, penta- and hexa-topic carboxylic acids and a series of corresponding Zr-MOFs can be prepared via the in situ "one-pot" method under the same solvothermal conditions. This strategy can be utilized not only to prepare reported Zr-MOFs constructed using benzimidazole-contained linkers, but also to rationally design, construct and realize functionalities of zirconium-pentacarboxylate frameworks guided by reticular chemistry. More importantly, in situ "one-pot" method can facilitate the discovery of new Zr-MOFs, such as zirconium-hexacarboxylate frameworks. The present study demonstrates the promising potential of benzimidazole-inspired in situ "one-pot" approach in the crystal engineering of structure- and property-specific Zr-MOFs, especially with the guidance of reticular chemistry.

Molecular Insights into Sequence-Specific Protein Hydrolysis by a Soluble Zirconium-Oxo Cluster Catalyst.

The development of catalysts for controlled fragmentation of proteins is a critical undertaking in modern proteomics and biotechnology. {Zr6O8}-based metal-organic frameworks (MOFs) have emerged as promising candidates for catalysis of peptide bond hydrolysis due to their high reactivity, stability, and recyclability. However, emerging evidence suggests that protein hydrolysis mainly occurs on the MOF surface, thereby questioning the need for their highly porous 3D nature. In this work, we show that the discrete and water-soluble [Zr6O4(OH)4(CH3CO2)8(H2O)2Cl3]+ (Zr6) metal-oxo cluster (MOC), which is based on the same hexamer motif found in various {Zr6O8}-based MOFs, shows excellent activity toward selective hydrolysis of equine skeletal muscle myoglobin. Compared to related Zr-MOFs, Zr6 exhibits superior reactivity, with near-complete protein hydrolysis after 24 h of incubation at 60 °C, producing seven selective fragments with a molecular weight in the range of 3-15 kDa, which are of ideal size for middle-down proteomics. The high solubility and molecular nature of Zr6 allow detailed solution-based mechanistic/interaction studies, which revealed that cluster-induced protein unfolding is a key step that facilitates hydrolysis. A combination of multinuclear nuclear magnetic resonance spectroscopy and pair distribution function analysis provided insight into the speciation of Zr6 and the ligand exchange processes occurring on the surface of the cluster, which results in the dimerization of two Zr6 clusters via bridging oxygen atoms. Considering the relevance of discrete Zr-oxo clusters as building blocks of MOFs, the molecular-level understanding reported in this work contributes to the further development of novel catalysts based on Zr-MOFs.

2D zirconium-based metal-organic framework/bismuth(III) oxide nanorods composite for electrocatalytic CO2-to-formate reduction

Electrocatalytic carbon dioxide reduction reaction (eCO2RR) represents one of the most promising technologies for sustainable conversion of CO2 to value-added products. Although metal-organic frameworks (MOFs) can be vastly functionalized to create active sites for CO2RR, low intrinsic electrical conductivity always makes MOFs unfavorable candidates for eCO2RR. Besides, studies on how to regulate eCO2RR activity of MOFs from linkers' functionalities viewpoint lag far behind when compared with the assembly of multinuclear metal-centered clusters. In this work, non-toxic bismuth(III) oxide (Bi2O3) was incorporated into a series of two-dimensional (2D) MOFs (ZrLX) established from Zr-oxo clusters and triazine-centered 3-c linkers with different functionalities (LX = 1–5) to give composites ZrLX/Bi2O3. To investigate how functionalities on linkers distantly tune the eCO2RR performance of MOFs, electron-donating/withdrawing groups were installed at triazine core or benzoate terminals. It is found that ZrL2/Bi2O3 (‒F functionalized on triazine core) exhibits the best eCO2RR performance with the highest Faradaic efficiency (FE) of 96.73% at −1.07 V vs. RHE, the largest electroactive surface (Cdl = 4.23 mF cm−2) and the highest electrical conductivity (5.54 × 10−7 S cm−1), highlighting tuning linker functionalities and hence electronic structure as an alternative way to regulate eCO2RR.

Ferrocene-functionalized zirconium-oxo clusters for achieving high-performance thermocatalytic redox reactions

The Zr(IV) ions are easily hydrolyzed to form oxides, which severely limits the discovery of new structures and applications of Zr-based compounds. In this work, three ferrocene (Fc)-functionalized Zr-oxo clusters (ZrOCs), Zr9Fc6, Zr10Fc6 and Zr12Fc8 were synthesized through inhibiting the hydrolysis of Zr(IV) ions, which show increased nuclearity and regular structural variation. More importantly, these Fc-functionalized ZrOCs were used as heterogeneous catalysts for the transfer hydrogenation of levulinic acid (LA) and phenol oxidation reactions for the first time, and displayed outstanding catalytic activity. In particular, Zr12Fc8 with the largest number of Zr active sites and Fc groups can achieve > 95% yield for LA-to-γ-valerolactone within 4 h (130 °C) and > 98% yield for 2,3,6-trimethylphenol-to-2,3,5-trimethyl-p-benzoquinone within 30 min (80 °C), showing the best catalytic performance. Catalytic characterization combined with theory calculations reveal that in the Fc-functionalized ZrOCs, the Zr active sites could serve as substrate adsorption sites, while the Fc groups could act as hydrogen transfer reagent or Fenton reagent, and thus achieve effectively intramolecular metal–ligand synergistic catalysis. This work develops functionalized ZrOCs as catalysts for thermal-triggered redox reactions.

Exploring cluster formation in Zr-MOF synthesis in situ using X-ray absorption spectroscopy.

Metal-organic frameworks (MOFs) with a Zr-oxo cluster [(Zr6O4(OH)4)]12- are exceptionally stable and offer vast potential for a wide range of applications. Synthesis parameters strongly affect the quality, stability, morphology, etc., of the MOFs. In this study, we present an experimental set-up that allows following in situ element-specific chemical transformations during synthesis and the effect of various reaction parameters on the reaction products. Zr-fumarate formation was monitored using X-ray absorption spectroscopy (XAS) in ZrCl4-DMF-based solutions. We have studied (i) the local (∼2-5 Å) environment around Zr4+ ions in the early stages of the Zr-oxo cluster formation and MOF-synthesis reaction, (ii) kinetics of the synthesis reaction and its dependence on water and modulator concentrations, and (iii) the effect of the reaction parameters on MOF product quality. XAS data have provided direct evidence that the increased amounts of water and modulator accelerate the reaction. It has also confirmed that water is essential for MOF formation in DMF. Zr-cluster formation and the synthesis reaction were not observed in the absence of water in ZrCl4-DMF and ZrCl4-DMF-linker-modulator solutions. According to the EXAFS data, Zr4+ ions are octahedrally coordinated by chlorine atoms in anhydrous solutions with and without the linker. In contrast, the Zr-oxo clusters and MOFs were formed only in the presence of water. The results of the in situ experiments were correlated with post-synthetic characterization of the resultant MOF products.

High-selectivity turn-on fluorescence-based fluoride ion detection using histidine-functionalized UiO-66-NH2.

Hisditine-functionalized UiO-66-NH2 (UiO-66-NH2@Hst) as a novel "turn-on" fluorescent probe to detect fluoride ions has been successfully synthesized using the solvothermal method. The results showed that fluoride ion detection gave the best fluorescence response in water media and no response was observed in non-polar solvents. Detection at pH 3-10 produces a relatively similar fluorescence response and decreases quite significantly at pH 11-13. UiO-66-NH2@Hst is very selective and stable towards fluoride ions as evidenced by the appearance of blue luminescence under UV light (λ 365 nm) compared to other ions. A possible mechanism for detecting fluoride ions is through the formation of hydrogen bonds, which results in incremental electron transfer from organic ligands to Zr-oxo clusters. The LoD value obtained in this study was 0.013 ppm, which is smaller than the maximum concentration of fluoride ions in drinking water samples (1.5 ppm) set by the World Health Organization (WHO). Therefore, UiO-66-NH2@Hst can be a candidate fluorescence-based sensor for fluoride ions.

Postsynthetic Modification of the Nonanuclear Node in a Zirconium Metal-Organic Framework for Photocatalytic Oxidation of Hydrocarbons.

Heterogeneous catalysis plays an indispensable role in chemical production and energy conversion. Incorporation of transition metals into metal oxides and zeolites is a common strategy to fine-tune the activity and selectivity of the resulting solid catalysts, as either the active center or promotor. Studying the underlying mechanism is however challenging. Decorating the metal-oxo clusters with transition metals in metal-organic frameworks (MOFs) via postsynthetic modification offers a rational approach to construct well-defined structural models for better understanding of the reaction mechanism. Therefore, it is important to expand the materials scope beyond the currently widely studied zirconium MOFs consisting of Zr6 nodes. In this work, we report the design and synthesis of a new (4,12)-connected Zr-MOF with ith topology that consists of rare Zr9 nodes. FeIII was further incorporated onto the Zr9 nodes of the framework, and the resulting MOF material exhibits significantly enhanced activity and selectivity toward the photocatalytic oxidation of toluene. This work demonstrates a delicate ligand design strategy to control the nuclearity of Zr-oxo clusters, which further dictates the number and binding sites of transition metals and the overall photocatalytic activity toward C-H activation. Our work paves the way for future exploration of the structure-activity study of catalysts using MOFs as the model system.

Selectivity Control in the Direct CO Esterification over Pd@UiO-66: The Pd Location Matters.

The selectivity control of Pd nanoparticles (NPs) in the direct CO esterification with methyl nitrite toward dimethyl oxalate (DMO) or dimethyl carbonate (DMC) remains a grand challenge. Herein, Pd NPs are incorporated into isoreticular metal-organic frameworks (MOFs), namely UiO-66-X (X=-H, -NO2 , -NH2 ), affording Pd@UiO-66-X, which unexpectedly exhibit high selectivity (up to 99 %) to DMC and regulated activity in the direct CO esterification. In sharp contrast, the Pd NPs supported on the MOF, yielding Pd/UiO-66, displays high selectivity (89 %) to DMO as always reported with Pd NPs. Both experimental and DFT calculation results prove that the Pd location relative to UiO-66 gives rise to discriminated microenvironment of different amounts of interface between Zr-oxo clusters and Pd NPs in Pd@UiO-66 and Pd/UiO-66, resulting in their distinctly different selectivity. This is an unprecedented finding on the production of DMC by Pd NPs, which was previously achieved by Pd(II) only, in the direct CO esterification.

Turning on Asymmetric Catalysis of Achiral Metal-Organic Frameworks by Imparting Chiral Microenvironment.

The development of heterogeneous asymmetric catalysts has attracted increasing interest in synthetic chemistry but mostly relies on the immobilization of homogeneous chiral catalysts. Herein, a series of chiral metal-organic frameworks (MOFs) have been fabricated by anchoring similar chiral hydroxylated molecules (catalytically inactive) with different lengths onto Zr-oxo clusters in achiral PCN-222(Cu). The resulting chiral MOFs exhibit regulated enantioselectivity up to 83 % ee in the asymmetric ring-opening of cyclohexene oxide. The chiral molecules furnished onto the catalytic Lewis sites in the MOF create multilevel microenvironment, including the hydrogen interaction between the substrate and the chiral -OH group, the steric hindrance endowed by the benzene ring on the chiral molecules, and the proximity between the catalytic sites and chiral molecules confined in the MOF pores, which play crucial roles and synergistically promote chiral catalysis. This work nicely achieves heterogeneous enantioselective catalysis by chiral microenvironment modulation around Lewis acid sites.

One-pot solvothermal synthesis of In-doped amino-functionalized UiO-66 Zr-MOFs with enhanced ligand-to-metal charge transfer for efficient visible-light-driven CO2 reduction

Metal organic frameworks (MOFs) with high porosity and highly tunable physical/chemical properties can serve as heterogeneous catalysts for CO2 photoreduction, but the application is hindered by the large band gap (Eg) and insufficient ligand-to-metal charge transfer (LMCT). In this study, a simple one-pot solvothermal strategy is proposed to prepare an amino-functionalized MOF (aU(Zr/In)) featuring an amino-functionalizing ligand linker and In-doped Zr-oxo clusters, which enables efficient CO2 reduction driven with visible light. The amino functionalization leads to a significant reduction of Eg as well as a charge redistribution of the framework, allowing the absorption of visible light and the efficient separation of photogenerated carriers. Furthermore, the incorporation of In not only promotes the LMCT process by creating oxygen vacancies in Zr-oxo clusters, but also greatly lowers the energy barrier of the intermediates for CO2-to-CO conversion. With the synergistic effects of the amino groups and the In dopants, the optimized aU(Zr/In) exhibits a CO production rate of 37.58 ± 1.06 μmol g-1 h−1, outperforming the isostructural University of Oslo-66- and Material of Institute Lavoisier-125-based photocatalysts. Our work demonstrates the potential of modifying MOFs with ligands and heteroatom dopants in metal-oxo clusters for solar energy conversion.

Unlocking New Topologies in Zr-Based Metal-Organic Frameworks by Combining Linker Flexibility and Building Block Disorder.

The outstanding diversity of Zr-based frameworks is inherently linked to the variable coordination geometry of Zr-oxo clusters and the conformational flexibility of the linker, both of which allow for different framework topologies based on the same linker-cluster combination. In addition, intrinsic structural disorder provides a largely unexplored handle to further expand the accessibility of novel metal-organic framework (MOF) structures that can be formed. In this work, we report the concomitant synthesis of three topologically different MOFs based on the same M6O4(OH)4 clusters (M = Zr or Hf) and methane-tetrakis(p-biphenyl-carboxylate) (MTBC) linkers. Two novel structural models are presented based on single-crystal diffraction analysis, namely, cubic c-(4,12)MTBC-M6 and trigonal tr-(4,12)MTBC-M6, which comprise 12-coordinated clusters and 4-coordinated tetrahedral linkers. Notably, the cubic phase features a new architecture based on orientational cluster disorder, which is essential for its formation and has been analyzed by a combination of average structure refinements and diffuse scattering analysis from both powder and single-crystal X-ray diffraction data. The trigonal phase also features structure disorder, although involving both linkers and secondary building units. In both phases, remarkable geometrical distortion of the MTBC linkers illustrates how linker flexibility is also essential for their formation and expands the range of achievable topologies in Zr-based MOFs and its analogues.

Utilizing Metal-Thiocatecholate Functionalized UiO-66 Framework for Photocatalytic Hydrogen Evolution Reaction.

Exploiting clean energy is essential for sustainable development and sunlight-driven photocatalytic water splitting represents one of the most promising approaches toward this goal. Metal-organic frameworks (MOFs) are competent photocatalysts owing to their tailorable functionality, well-defined structure, and high porosity. Yet, the introduction of the unambiguous metal-centered active site into MOFs is still challenging since framework motifs capable of anchoring metal ions firmly are lacking. Herein, the assembly using 1,4-dicarboxylbenzene-2,3-dithiol (H2 dcbdt) and Zr-Oxo clusters to give a thiol-functionalized UiO-66 type framework,UiO-66-dcbdt, is reported. The thiocatechols on the struts are allowed to capture transition metal (TM) ions to generateUiO-66-dcbdt-M(M = Fe, Ni, Cu) withunambiguous metal-thiocatecholate moieties for photocatalytic hydrogen evolution reaction (HER).UiO-66-dcbdt-Cuis found the best catalyst exhibiting an HER rate of 4.18mmol g-1 h-1 upon irradiation with photosensitizing Ru-polypyridyl complex. To skip the use of the external sensitizer,UiO-66-dcbdt-Cuis heterojunctioned with titanium dioxide (TiO2 ) and achieves an HER rate of 12.63mmol g-1 h-1 (32.3 times that of primitive TiO2 ). This work represents the first example of MOF assembly employing H2 dcbdtas the mere linker followed by chelation with TM ions and undoubtedly fuels the rational design of MOF photocatalystsbearing well-defined active sites.

Tuning the Hydrophobicity and Lewis Acidity of UiO-66-NO2 with Decanoic Acid as Modulator to Optimise Conversion of Glucose to 5-Hydroxymethylfurfural

Glucose conversion to 5-hydroxymethylfurfural (HMF) is important to the success of a biorefinery. Herein, metal–organic frameworks (MOFs) with the UiO-66 structure were synthesised with decanoic acid as the modulator and used as the catalyst to optimise HMF yield. PXRD, FTIR, and TGA/DSC techniques were applied to characterise the materials. The analysis results show that the materials assembled from the ligand 2-nitroterephthalic acid and hexameric Zr-oxo clusters contain decanoic acid chemically bound in the framework that influences porosity, Lewis acidity, and hydrophobicity. The materials exhibit excellent catalytic performance for HMF production from glucose in DMSO as solvent, attributed to their abundant defects and high hydrophobicity due to the addition of the decanoic acid modulator. Influences of catalyst dosages, reaction duration, and temperature were comprehensively investigated, leading to 98.1% conversion of glucose and 54.5% HMF yield under optimised reaction conditions. The catalytic conversion shows some deterioration after four cycles, yet the reaction selectivity displays no significant decline.

Defective Site Modulation Strategy for Preparing Single Atom-Dispersed Catalysts as Superior Chemiluminescent Signal Probes

Single atom-dispersed catalysts (SADCs) with highly exposed active sites can be used as sensitive signal probes because of their superior catalytic efficiency. However, the dispersed atoms tend to aggregate, restricting the loading capacity of metal atoms. Herein, the defective sites on Zr-oxo clusters of metal-organic frameworks (MOFs) UiO-66-NH2 were modulated by excessive acetic acid and utilized for confining metal atoms with high loading capacity. To verify the feasibility of the designed strategy, the Co element was loaded onto MOFs UiO-66-NH2 to prepare SADCs with desirable Fenton-like activity. The prepared Co SADCs at a low concentration of 1.0 μg mL-1 are found to boost chemiluminescent (CL) emission for 3700 times due to the significantly improved Co content of 5.55 wt %. The superior CL enhancement efficiency is ascribed to reactive oxygen species generated by the accelerated decay of H2O2. To verify the application potential in CL assay, they were used as signal probes to establish an immunoassay method for carbendazim with a dynamic range of 1.0 pg mL-1 to 25 ng mL-1 and a limit of detection of 0.33 pg mL-1. This defective site modulation strategy paves an avenue for preparing SADCs with a high CL response by improving the loading capacity of metal atoms.

Mixed-linker strategy toward enhanced photoreduction-assisted uranium recovery from wastewater and seawater

Integrating multi-functionality into metal–organic frameworks (MOFs) has attracted growing attention as it plays a critical role in realizing the potential of MOFs for a wide variety of applications. Owing to the ultrahigh surface areas and porosity, MOFs have been recognized as potential adsorbents for uranium capture through chemical chelation of uranyl ions (UO22+). As a new type of photocatalyst, however, these MOFs are rarely applied in uranium recovery by photocatalytic reduction of soluble U(VI) to the less soluble U(IV), which is largely due to the limited visible light harvesting as well as the poor separation and transfer of photo-generated carriers. Herein, we present a facile one pot synthetic route to incorporate photo-responsive Ni(II)-centered porphyrin (TCPP(Ni)) ligands into UIO-66-NH2 (TCPP(Ni) ⊂ MOF) while maintaining the crystal structure and ultrahigh chemical stability. TCPP(Ni) not only provides strong visible light harvesting but also improves the charge transfer from excited ligands to Zr-oxo clusters. Consequently, the mixed-linker strategy boosts the photoreduction of soluble U(VI) to the less soluble U(IV) for uranium immobilization. After optimization, TCPP(Ni) ⊂ MOF-3 presents excellent selective uranium capture from wastewater and natural seawater through combined complexation and photoreduction. Strikingly, after immersing in natural seawater for 25 days, a high uranium sorption capacity of 8.95 ± 0.39 mg-U/g-Ads can be achieved together with good antibacterial ability. This work provides a promising strategy to improve the photoreduction potential of MOF adsorbents for uranium capture.

Aquatic arsenic removal with a Zr-MOF constructed via in situ nitroso coupling

Assembly of hexanuclear Zr-oxo clusters via in situ covalent coupling of nitroso groups afforded a metal–organic framework (MOF) with fcu topology. The MOF, namely SUM-8, emerged as an excellent adsorbent for aquatic arsenic removal in terms of adsorptive efficiency, capacity, selectivity, and cyclability, on accounts of its chemical stability and structural parameters suitable for arsenate diffusion/accommodation.

A microporous Zr6@Zr-MOF for the separation of Xe and Kr.

We report here the self-assembly of a she-type zirconium-based metal-organic framework with discrete hexanuclear Zr-oxo clusters residing inside its pore windows. The overall structure features microporosity showing preferential adsorption of Xe over Kr.