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Porous Hydrogen-Bonded Frameworks Assembled from Metal-Nucleobase Entities for Xe/Kr Separation

2021; Chinese Chemical Society; Volume: 4; Issue: 1 Linguagem: Inglês

10.31635/ccschem.021.202100824

2096-5745

Ying Liu, Juanjuan Dai, Lidong Guo, Zhiguo Zhang, Yiwen Yang, Qiwei Yang, Qilong Ren, Zongbi Bao,

Chemical Synthesis and Characterization

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Porous Hydrogen-Bonded Frameworks Assembled from Metal-Nucleobase Entities for Xe/Kr Separation Ying Liu, Juanjuan Dai, Lidong Guo, Zhiguo Zhang, Yiwen Yang, Qiwei Yang, Qilong Ren and Zongbi Bao Ying Liu Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 , Juanjuan Dai Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 , Lidong Guo Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 , Zhiguo Zhang Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 Institute of Zhejiang University-Quzhou, Quzhou 324000 , Yiwen Yang Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 Institute of Zhejiang University-Quzhou, Quzhou 324000 , Qiwei Yang Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 , Qilong Ren Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 Institute of Zhejiang University-Quzhou, Quzhou 324000 and Zongbi Bao *Corresponding author: E-mail Address: [email protected] Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027 Institute of Zhejiang University-Quzhou, Quzhou 324000 https://doi.org/10.31635/ccschem.021.202100824 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail It is challenging to obtain high-purity xenon (Xe) and krypton (Kr) from the by-products of the air separation process due to their similar atom size and physical properties. Adsorption using porous materials such as metal–organic frameworks (MOFs) has been considered a promising technology to separate Xe/Kr. Herein, we report two novel isostructural ionic supramolecular MOFs (SMOFs; SMOF-PFSIX-1 and SMOF-AsFSIX-1), in which inorganic anions (PF6− or AsF6−) and cationic metal–organic entities have self-assembled through hydrogen bonds to give three-dimensional pore channels. The two kinds of SMOFs can efficiently separate Xe/Kr with ideal adsorbed solution theory (IAST) selectivity values of 6.9 and 6.7 under 298 K and 1.0 bar, respectively. The breakthrough experiments further confirm their industrial application potential. The grand canonical Monte Carlo (GCMC) and density functional theory (DFT) calculations revealed that there are multiple adsorptive sites to capture the Xe atom, and the affinity between Xe and frameworks can be attributed to the inorganic anions and amino groups on the ligands. To the best of our knowledge, this was the first report of using SMOFs for Xe/Kr separation, and we proposed a new strategy for Xe/Kr separation based on the synergistic effect of amino and inorganic anions. Download figure Download PowerPoint Introduction Noble gases such as xenon (Xe) and krypton (Kr) play an important role in lighting, lasers, medical imaging, and ion propulsion.1–4 In a typical process, a mixture of Xe/Kr (20/80, v/v) can be obtained as a byproduct of air separation.5 It remains a challenge to produce high-purity Xe and Kr because of their similar atomic size and physical properties. The reprocessed off-gases of used nuclear fuel (UNF) containing radioactive noble gases (127Xe and 85Kr) is also worth consideration.6 The separation and storage of 127Xe and 85Kr from UNF off-gases will reduce the harm to the environment and human beings.7 Cryogenic distillation based on differences in boiling points (164.9 K for Xe and 119.8 K for Kr at 1 atm) is used to separate Xe/Kr in most industrial processes but is energy- and capital-intensive, finally leading to high prices for pure products.8 Adsorption has been considered a promising method to separate Xe/Kr for its advantages of lower cost and milder operation conditions.9 Activated charcoal and zeolites with high surface areas have been studied to capture and separate Xe and Kr, but they show poor performance in balancing capacity and selectivity.10,11 In the past few decades, as a new class of porous material, metal–organic frameworks (MOFs) have demonstrated excellent gas separation performance not only for hydrocarbons but also for Xe/Kr due to their high porosities, adjustable pore structures, and functional pore surfaces.12,13 The kinetic diameters of Xe and Kr are close (4.047 Å for Xe and 3.655 Å for Kr), MOFs with appropriate pore size can separate Xe/Kr through the molecular sieving effect. At temperatures below 273 K, FMOFCu adsorbed Kr inversely but excluded larger Xe to achieve a switching Kr/Xe-selective separation.14 A commonly used strategy for Xe/Kr separation focuses on the difference in polarizability (40.44 × 10−25 cm3 for Xe and 24.84 × 10−25 cm3 for Kr). The introduction of open metal sites (OMSs), polar functional groups, and anions can polarize the pore surfaces to selectively capture Xe.15–17 The abundant OMSs in Ni-MOF-74 result in the highest Xe uptake (4.2 mmol/g) with an ideal adsorbed solution theory (IAST) selectivity of 5–6 under 298 K and 1 bar.18 The interaction between Xe and MOFs can be further enhanced by depositing Ag nanoparticles in Ni-MOF-74 with increased Xe capacity by 15.6%.19 A study on Xe/Kr separation performances of a series of functionalized UiO-66 materials synthesized by introducing several polar functional groups (–F, –NH2, and –OMe) revealed that UiO-66-NH2(OMe)2 with the highest electron density exhibited the best Xe/Kr separation performances.20 The selectivity for Xe/Kr separation of SBMOF-2 and [Co3(C4O4)2(OH)2]·3H2O was up to 10 and 69.7, respectively, which benefited from their suitable pore size and pore surfaces decorated with polar hydroxyl groups.17,21 Besides, CROFOUR-1-Ni provided a new strategy to separate Xe/Kr with a selectivity of 22 through synergy between the pore size and strong electrostatics afforded by the CrO42− anions.16 This work proposed a new strategy for efficient Xe/Kr separation through the cooperative interactions attributed by a new class of anions (PF6− and AsF6−) and polar amino groups. The strong affinity between Xe and the ionic supramolecular MOFs (SMOFs) is derived from the inorganic anions (PF6− and AsF6−) and polar amino groups of ligands. Compared with the coordinated SiF62− anions in the SIFSIX series,22 the anions in SMOFs connected by weaker hydrogen bonds possessed more uncoordinated F, thus affording stronger Xe adsorptive sites. To the best of our knowledge, this is the first SMOFs have been used to separate Xe/Kr with excellent performance. Experimental Section Materials and measurements All chemicals were purchased from commercial suppliers without further purification. Powder X-ray diffraction (PXRD) analysis was performed on an Xpert diffractometer (PANalytical, Malvern, United Kingdom) using Cu Kα (λ = 0.1543 nm) radiation at 40 kV from 5–40° (2θ angle range) with a step size of 0.026°. The thermogravimetric analysis (TGA) was carried out on a Pyris 1 TGA instrument from 50 to 400 °C in a nitrogen atmosphere with a constant rate of 10 °C/min. The single-crystal X-ray diffraction data were collected at 296 K on the Gemini A Ultra diffractometer graphite-monochromatic enhanced Mo radiation (λ = 0.71073 nm). The structure was solved by direct methods and refined by a full-matrix least-squares methods with the SHELXTL program package. Crystal data and structure refinement for as-synthesized SMOF-PFSIX-1 and SMOF-AsFSIX-1 are listed in Supporting Information Table S1. Preparation of SMOF-PFSIX-1 About 3 mL of 1∶1 acetonitrile/water dissolving adenine (20.27 mg, 0.15 mmol) was layered above 3 mL of the aqueous solution with Cu(NO3)2·3H2O (18.36 mg, 0.076 mmol) and NH4PF6 (24.78 mg, 0.152 mmol). About 1 mL of 1∶1 acetonitrile/water was layered between the top and bottom solutions to slow the diffusion rate. After slow diffusion at room temperature for around 20 days, we obtained dark black, rectangular crystals with about 40% yield. A single crystal of SMOF-PFSIX-1 was obtained by the same methods with diluted solution. The as-synthesized SMOF-PFSIX-1 was then collected and washed by fresh methanol several times to replace the free solvent molecules. The sample was activated at room temperature for 24 h to remove guest solvent molecules before further gas measurements. Preparation of SMOF-AsFSIX-1 About 3 mL of 1∶1 acetonitrile/water dissolving adenine (20.27 mg, 0.15 mmol) was layered above 3 mL of the aqueous solution with Cu(NO3)2·3H2O (18.36 mg, 0.076 mmol) and KAsF6 (34.66 mg, 0.152 mmol). About 1 mL of 1∶1 acetonitrile/water was layered between the top and bottom solutions to slow the diffusion rate. After slow diffusion at room temperature for around 20 days, dark black, rectangular crystals with about 40% yield were obtained. The single crystal of SMOF-AsFSIX-1 was obtained by the same methods with the diluted solution. The as-synthesized SMOF-AsFSIX-1 was collected and washed by fresh methanol several times to replace the free solvent molecules. The sample was activated at room temperature for 24 h to remove guest solvent molecules before further gas measurements. Gas adsorption measurements Gas adsorption isotherms data were collected on the Micromeritics ASAP 2460 adsorption apparatus. Around 100 mg of sample was prepared according to the above methods and used for the gas adsorption measurement. The outgassing was performed under a vacuum at room temperature for 24 h, and the free space was filled with nitrogen gas. The single-component adsorption isotherms were performed at temperatures of 273 and 298 K and pressures from 0 to 1.0 bar. The dead space of the system was determined by ultrahigh purity grade He. The column breakthrough experiments About 0.6215 g of activated SMOF-PFSIX-1a and 0.6442 g of activated SMOF-AsFSIX-1a were packed into the column (4.6 mm ID × 50 mm; ID = inside diameter), respectively. The packing process was performed in a glovebox filled with nitrogen gas, and the particle size of the adsorbent used in the breakthrough experiments was around 5 μm. A helium flow of about 5 mL/min was introduced into the column to purge the adsorbents. The breakthrough experiments of SMOF-PFSIX-1a and SMOF-AsFSIX-1a were conducted at a temperature of 298 K under a flow of Xe/Kr (20/80, v/v) at flow rates of 1.86 and 2.50 mL/min, respectively. The outlet gas was collected and analyzed by a Hiden HPR-20 evolved gas analysis (EGA) mass spectrometer (Hiden Analytical Ltd., Warrington, England). The columns were regenerated by helium gas flow with a flow rate of 20.0 mL/min at room temperature to achieve complete desorption. Computational methods Grand canonical Monte Carlo simulations The charges of SMOFs frameworks were determined by the charge equilibration approach (QEq; an important computational method for the determination of atomic charges) using the Forcite modules in Materials Studios (Accelrys, San Diego, CA) with the universal force field (UFF).23 The grand canonical Monte Carlo (GCMC) calculations were performed using the Materials Studios' Sorption codes to simulate the adsorption in the SMOFs. A 2 × 2 × 2 crystallographic unit cell was used for GCMC simulation and a total of 5,000,000 equilibration steps and 5,000,000 production steps were set for fixed pressure simulation. The 12-6 Lennard–Jones (LJ) potential was used to describe the adsorbate–framework interactions. For the SMOFs atoms, the van der Waals parameters were taken from the UFF, and the LJ parameters for Xe and Kr were obtained from literature ( Supporting Information Table S2)24. Density functional theory simulations The first-principle density functional theory (DFT) calculations were performed used the CASTEP package in Materials Studios.25 All calculations were measured under a functional of generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE), and the transition state (TS) custom method was used for DFT-D (dispersion correction for DFT). A cutoff energy of 544 eV and the 2 × 2 × 2 k-point were found to be enough for the total energy to converge within 0.02 meV/atom. The frameworks of the as-synthesized materials were calculated to initially complete the geometry optimization to generate a structure with minimal energies. The structures of Xe and Kr atoms were optimized under the same parametric settings to obtain the minimal energy of adsorbate. To calculate the binding energy, the optimized Xe and Kr were introduced to different adsorption sites as the GCMC calculation results showed. The static binding energy (EB) was calculated based on the equation EB = E(adsorbate) + E(adsorbent) − E(adsorbate + adsorbent). Results and Discussion Characterization SMOF-PFSIX-1 and SMOF-AsFSIX-1 were successfully synthesized according to previously reported methods but with NH4PF6 and KAsF6 in place of (NH4)2SiF6, respectively.26,27 The purity and thermal stability of the as-synthesized sample were confirmed by PXRD and TGA ( Supporting Information Figures S1 and S2). The single-crystal X-ray structures of SMOF-PFSIX-1 and SMOF-AsFSIX-1 revealed that they crystallized in the same monoclinic space group C2/m with similar cell parameters ( Supporting Information Table S1). They are isostructural three-dimensional (3D) porous frameworks self-assembled from metal–organic complexes and inorganic anions. As shown in Figure 1a, organic ligands were coordinated to Cu2+ to construct two kinds of paddle-wheel shaped metal–organic entities. One of the paddle-wheels [Cu2(Hade)4(CH3CN)2, Hade= adenines] contained adenines to present the +4 oxidation state, while another [Cu2(ade)4(CH3CN)2, ade = adeninates] contains deprotonated adenines (adeninates), and is neutral. Two different paddle-wheel entities and four inorganic anions construct the minimum periodic unit to make the whole framework electronically neutral. Figure 1 | (a) The formation of SMOF-PFSIX-1 and SMOF-AsFSIX-1. The hydrogen-bonding networks viewing from the c axis of (b) SMOF-PFSIX-1 and (c) SMOF-AsFSIX-1. (d) The local hydrogen-bonding environments of two kinds of PF6− in SMOF-PFSIX-1. (e) The local hydrogen-bonding environments of two kinds of AsF6− in SMOF-AsFSIX-1. The hydrogen bonds are shown as red-dotted lines and C, black; N, blue; H, light grey; F, cyan; Cu, green; P, yellow; As, pink. Download figure Download PowerPoint As can be seen in Figures 1b and 1c, metal–organic entities and inorganic anions are connected through hydrogen bonds. Analysis of the hydrogen-bonding networks of SMOF-PFSIX-1 and SMOF-AsFSIX-1 revealed that there are two different types of anions [PF6−(1) and PF6−(2), AsF6−(1) and AsF6−(2)] participating in the formation of supramolecular frameworks. Figure 1d demonstrates that PF6−(1) in SMOF-PFSIX-1 interacts with four nearest metal–organic entities via four C–H⋯F weak hydrogen bonds with a bond length in the range of 2.36–2.98 Å and four N–H⋯F hydrogen bonds (2.41–2.96 Å), while PF6−(2) interacts with only two metal–organic entities via six C–H⋯F weak hydrogen bonds (2.30–2.90 Å). In SMOF-AsFSIX-1, AsF6−(1) interacts with four metal–organic entities through five C–H⋯F hydrogen bonds (2.54–2.88 Å) and four N–H⋯F hydrogen bonds (2.51–2.69 Å), while AsF6−(2) interacts through C–H⋯F hydrogen bonds between 2.29 and 2.96 Å (Figure 1e). Differing from SMOF-SiFSIX-1 in which the axial positions of the paddle-wheel are occupied by water, the coordinated acetonitrile molecules in SMOF-PFSIX-1 and SMOF-AsFSIX-1 play a vital role in the construction of hydrogen-bonding networks. It has been reported that the structure of hydrogen-bonded organic frameworks (HOFs) can be influenced by the involvement of solvent molecules in hydrogen-bonded frameworks, and these results also confirm that the solvent molecules deserve consideration in the design of SMOFs.28 The accessible Connolly surface calculation revealed that SMOF-PFSIX-1 and SMOF-AsFSIX-1 possess similar 3D cross-branched pore channels with the narrowest pore size of 3.0 Å for SMOF-PFSIX-1 (2.8 Å for SMOF-AsFSIX-1) and the widest pore size of 7.0 Å ( Supporting Information Figures S3 and S4). The occupancy of two acetonitrile molecules elongated the paddle-wheel in the axial direction, resulting in interconnected pore structures. The as-synthesized sample was collected and washed with fresh methanol several times, and after being vacuumed at room temperature, we obtained the activated samples denoted as SMOF-PFSIX-1a and SMOF-AsFSIX-1a. The porosity of the activated sample was determined by CO2 sorption at 195 K ( Supporting Information Figures S5 and S6). The CO2 sorption isotherms displayed a typical type I sorption behavior for microporous structures, and the mean pore size was estimated to be 4.9 Å for both SMOFs. Besides, the calculated BET surface areas of SMOF-PFSIX-1a and SMOF-AsFSIX-1a are 389 and 422 m2/g, respectively, which are moderate among HOFs and SMOFs with permanent porosity. Gas adsorption analysis The permanent porosity of SMOFs encouraged us to explore the gas adsorption properties of SMOF-PFSIX-1a and SMOF-AsFSIX-1a. The single-component adsorption isotherms of Xe and Kr on the SMOFs at temperatures of 298 and 273 K were collected on the Micromeritics ASAP 2460 adsorption apparatus (Figure 2a and Supporting Information Figures S7 and S8). Under a pressure of 1.0 bar, the Xe adsorption capacity of SMOF-PFSIX-1a was 2.29 mmol/g at 298 K and 3.22 mmol/g at 273 K. Besides, the Xe adsorption capacity of SMOF-AsFSIX-1a was a little lower as 2.18 and 3.08 mmol/g at 298 and 273 K under 1.0 bar, respectively. As shown in Figure 2b, the Xe uptakes of SMOF-PFSIX-1 and SMOF-AsFSIX-1 under 1.0 bar were comparable with HOF-BTB (2.0) and some MOFs with OMSs such as Co3(HCOO)6 (2.2),29,30 and surpassed some reported materials including CROFOUR-1-Ni (1.8), CROFOUR-2-Ni (1.6), and SBMOF-1 (1.4).16,31 The Xe adsorption curves exhibited a steep increase when compared with the Kr adsorption isotherms, indicating different interactions between frameworks and guest molecules with different polarities as well as the potential Xe/Kr separation selectivity. Figure 2 | (a) Single-component adsorption isotherms of Xe, Kr on SMOF-PFSIX-1a and SMOF-AsFSIX-1a at a temperature of 298 K. (b) Comparison of Xe uptake and IAST selectivity for Xe/Kr (20/80, v/v) of various materials. Breakthrough experiments of SMOF-PFSIX-1a (c) and SMOF-AsFSIX-1a (d) for Xe/Kr (20/80, v/v) at 298 K. Download figure Download PowerPoint It is worth noting that the isotherms of Xe under 273 K for the SMOFs showed a certain desorption hysteresis ( Supporting Information Figures S7 and S8). Considering the pore structures of SMOFs were constructed by the hydrogen bonds between inorganic anions and metal–organic entities, the partial breakage of weak interactions might result in minor framework flexibility when adsorbing gases. Isosteric heats of adsorption and separation selectivity To compare the binding energy between the host frameworks and guest molecules under low coverage, the isosteric heats of adsorption (Qst) of Xe and Kr in SMOF-PFSIX-1a and SMOF-AsFSIX-1a were calculated using the Virial method ( Supporting Information Figure S9), where the fitting parameters are listed in Supporting Information Table S3. The Qst values of Xe in SMOF-PFSIX-1a and SMOF-AsFSIX-1a at zero-loading were 38.4 and 35.0 kJ/mol, as the binding energy for Kr was only 10.5 and 8.5 kJ/mol, respectively. The Qst of Xe surpasses some advanced materials that provide affinity to Xe atoms such as SBMOF-2 (26.4) and MOF-Cu-H (33.4),21,32 reflecting that there also exists stronger interactions between the supramolecular frameworks and Xe atoms. The single-component adsorption isotherms data were fitted with dual-sites Langmuir–Freundlich (DSLF) and Langmuir–Freundlich (LF) models ( Supporting Information Tables S4 and S5). The selectivity for a binary mixture of Xe/Kr (20/80, v/v) on SMOF-PFSIX-1a and SMOF-AsFSIX-1a was calculated using IAST. As indicated in Supporting Information Figures S10 and S11, for a gas mixture of Xe/Kr (20/80, v/v), the IAST selectivities of SMOF-PFSIX-1a and SMOF-AsFSIX-1a were calculated to be 6.9 and 6.7 at 298 K under 100 kPa pressure. As Figure 2b concludes, the Xe/Kr selectivity is higher than that of Ni-MOF-74 (5–6) and HOF-BTB (6),18,30 but lower than the majority MOFs.16,17,21 Furthermore, Henry's constant was calculated to reflect their separation performance under low concentrations (fitting parameters of Langmuir model can be seen in Supporting Information Table S6), and the results were 4.1 and 5.1 for SMOF-PFSIX-1a and SMOF-AsFSIX-1a, respectively. The list of Xe adsorption capacity, Qst of Xe, and Xe/Kr selectivity for various materials can be found in Supporting Information Table S7. Column breakthrough experiments The column breakthrough experiments of the Xe/Kr (20/80, v/v) gas mixtures were performed to examine the potential industrial applications for SMOF-PFSIX-1a and SMOF-AsFSIX-1a (Figures 2c and 2d). For SMOF-PFSIX-1a, the Kr signal was quickly detected as the feed gas fed in under a rate of 1.86 mL/min, while Xe eluted until 22 min with a 0.79 mmol/g adsorption during t0-tbreak. As for SMOF-AsFSIX-1a, the Kr quickly eluted at the beginning when feed gas fed under a rate of 2.5 mL/min, and Xe eluted in 16 min later with an adsorption amount of 0.64 mmol/g. The captured Xe amounts of SMOF-PFSIX-1 and SMOF-AsFSIX-1 were calculated to be 0.79 and 0.62 mmol/g. Besides, the productivity of high-purity Kr (<99.9%) of these two SMOFs was also estimated to be 1.70 and 1.82 mmol/g, respectively. These values were much higher than the amino-functionalized UiO-66-NH2(OMe)2 (0.30 mmol/g) but significantly lower than the state-of-the-art CROFOUR-2-Ni (4.51 mmol/g) and ZU-62 (9.20 mmol/g).33 Adsorption mechanism GCMC simulations The efficient Xe/Kr separation performance of SMOFs encouraged us to explore their adsorption mechanism. The GCMC simulations were used to decide the adsorption sites of Xe and Kr for the reason that SMOF-PFSIX-1 and SMOF-AsFSIX-1 possess complicated 3D pore channels. From the density distribution color maps in Figures 3a and 3b, it was obvious that Xe and Kr were adsorbed in similar positions and there were eight high-density adsorption sites in the minimal cells of SMOF-PFSIX-1. Considering the symmetry of the structure, there are four different adsorption sites in the cell. Since the SMOFs are isostructural, the adsorption sites of Xe and Kr in SMOF-AsFSIX-1 are similar to those in SMOF-PFSIX-1 but more concentrated (Figures 3c and 3d) Figure 3 | GCMC simulations of adsorbed SMF-PFSIX-1 (a) Xe and (b) Kr. The results of GCMC simulations of adsorbed SMOF-AsFSIX-1 (c) Xe and (d) Kr. The DFT simulation results and calculated static binding energy (ΔE) of (e) adsorbed SMOF-PFSIX-1 Xe and (g) adsorbed SMOF-AsFSIX-1 Xe, the unit of ΔE is kJ/mol and has been omitted. Side view of the Xe atoms in the pore channels of (f) SMOF-PFSIX-1 and (h) SMOF-AsFSIX-1 with framework omitted. Download figure Download PowerPoint DFT simulations To understand the interactions between the gases and the supramolecular frameworks, the first-principal DFT simulations were used to calculate the binding energies. As Figures 3e and 3g showed, the calculated static binding energy of Xe on SMOF-PFSIX-1 (32.0–42.3 kJ/mol) and SMOF-AsFSIX-1 (30.6–42.7 kJ/mol) indicated that the affinity between SMOF-PFSIX-1 and Xe was stronger than that of SMOF-AsFSIX-1. The strongest adsorption site for both SMOFs was site 3. The side view of the Xe atoms in the pore channels of SMOF-PFSIX-1 and SMOF-AsFSIX-1 were demonstrated in Figures 3f and 3h. The DFT simulation results of these four adsorption sites revealed that the strong interactions between adsorbents and the supramolecular frameworks were provided by the amino group of ligand and F of inorganic anions (PF6− and AsF6−), such mechanism was initially reported to separate Xe/Kr. As Supporting Information Figure S12 showed, the adsorption of Kr on SMOFs was similar to that of Xe. The static binding energy of Kr on SMOF-PFSIX-1 (24.9–30.8 kJ/mol) and SMOF-AsFSIX-1 (24.4–30.9 kJ/mol) was lower than Xe, corresponding to the weaker interactions between Kr and frameworks for the reasons that Kr possesses weaker polarizability. Conclusions The amino-functional groups and a new class of inorganic anions synthetically afford the affinity between the supramolecular frameworks and Xe atoms, which provides a strategy for the design of porous materials for Xe/Kr separation. It is the first time that SMOFs are used to separate Xe/Kr with moderate Xe uptake and IAST selectivity. Though SMOFs can be regarded as a promising candidate for noble gases separation, it remains a challenge to design novel SMOFs with proper pore size and stronger host–guest interactions for selectively capturing Xe atoms. Supporting Information Supporting Information is available and includes the results of PXRD, TGA, adsorption isotherms, crystallographic data, and table of fitting parameters. The CCDC deposition number of SMOF-PFSIX-1 and SMOF-AsFSIX-1 is 1980887 and 1980599, respectively. Conflict of Interest There is no conflict of interest to report. Acknowledgments This work was supported by the Zhejiang Provincial Natural Science Foundation of China (no. LR17B060001) and the National Natural Science Foundation of China (nos. 21722609 and 21878260). References 1. Bridges W. B.; Chester A. N.Visible and UV Laser Oscillation at 118 Wavelengths in Ionized Neon, Argon, Krypton, Xenon, Oxygen, and Other Gases.Appl. Opt.1965, 4, 573–580. Google Scholar 2. Beattie J. R.; Matossian J. N.; Poeschel R. L.; Rogers W. P.; Martinelli R. M.Xenon Ion Propulsion Subsystem.J. Propul. Power1989, 5, 438–444. Google Scholar 3. Albert M. S.; Cates G. D.; Driehuys B.; Happer W.; Saam B.; Springer C. 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Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentVolume 4Issue 1Page: 381-388Supporting Information Copyright & Permissions© 2021 Chinese Chemical SocietyKeywordsxenonkryptonsupramolecular metal–organic frameworksgas separationadsorptionAcknowledgmentsThis work was supported by the Zhejiang Provincial Natural Science Foundation of China (no. LR17B060001) and the National Natural Science Foundation of China (nos. 21722609 and 21878260). Downloaded 932 times PDF DownloadLoading ...