Prediction of Xe/Kr Separation in Metal-Organic Frameworks by a Precursor-Based Neural Network Synergistic with a Polarizable Adsorbate Model

Adsorption and separation of Xe/Kr are significant for making high-density nuclear energy environmentally friendly and for meeting the requirements of the gas industry. Enhancing the accuracy of the adsorbate model for describing the adsorption behaviors of Xe and Kr in MOFs and the efficiency of the model for predicting the separation potential (SP) value of Xe/Kr separation in MOFs helps in searching for promising MOFs for Xe/Kr adsorption and separation within a short time and at a low cost. In this work, polarizable and transferable models for mimic Xe and Kr adsorption behaviors in MOFs were constructed. Using these models, SP values of 38 MOFs at various temperatures and pressures were calculated. An optimal neural network model called BPNN-SP was designed to predict SP value based on physical parameters of metal center (electronegativity and radius) and organic linker (three-dimensional size and polarizability) combined with temperature and pressure. The regression coefficient value of the BPNN-SP model for each data set is higher than 0.995. MAE, MBE, and RMSE of BPNN-SP are only 0.331, -0.002, and 0.505 mmol/g, respectively. Finally, BPNN-SP was validated by experiment data from six MOFs. The transferable adsorbate model combined with the BPNN-SP model would highly improve the efficiency for designing MOFs with high performance for Xe/Kr adsorption and separation.

Nucleoside Analogs: A Review of Its Source and Separation Processes

Nucleoside analogs play a crucial role in the production of high-value antitumor and antimicrobial drugs. Currently, nucleoside analogs are mainly obtained through nucleic acid degradation, chemical synthesis, and biotransformation. However, these methods face several challenges, such as low concentration of the main product, the presence of complex matrices, and the generation of numerous by-products that significantly limit the development of new drugs and their pharmacological studies. Therefore, this work aims to summarize the universal separation methods of nucleoside analogs, including crystallization, high-performance liquid chromatography (HPLC), column chromatography, solvent extraction, and adsorption. The review also explores the application of molecular imprinting techniques (MITs) in enhancing the identification of the separation process. It compares existing studies reported on adsorbents of molecularly imprinted polymers (MIPs) for the separation of nucleoside analogs. The development of new methods for selective separation and purification of nucleosides is vital to improving the efficiency and quality of nucleoside production. It enables us to obtain nucleoside products that are essential for the development of antitumor and antiviral drugs. Additionally, these methods possess immense potential in the prevention and control of serious diseases, offering significant economic, social, and scientific benefits to the fields of environment, biomedical research, and clinical therapeutics.

Supramolecular Assembly of One-Dimensional Coordination Polymers for Efficient Separation of Xenon and Krypton

Efficient separation and purification of xenon (Xe) from krypton (Kr) represent an industrially crucial but challenging process. While the adsorption-based separation of these atomic gases represents an energy-efficient process, achieving highly selective adsorbents remains a difficult task. Here, we demonstrate a supramolecular assembly of coordination polymers, termed as M(II)-dhbq (M = Mg, Mn, Co, and Zn; dhbq = 2,5-dihydroxy-1,4-benzoquinone), with high-density open metal sites (5.3 nm-3) and optimal pore size (5.5 Å), which are able to selectively capture Xe among other chemically inert gases including Kr, Ar, N2, and O2. Among M(II)-dhbq materials, Mn-dhbq exhibits the highest Xe uptake capacity of 3.1 mmol/g and a Xe/Kr selectivity of 11.2 at 298 K and 1.0 bar, outperforming many state-of-the-art adsorbents reported so far. Remarkably, the adsorption selectivity of Mn-dhbq for Xe/O2, Xe/N2, and Xe/Ar at ambient conditions reaches as high as 70.0, 139.3, and 64.0, respectively. Direct breakthrough experiments further confirm that all M(II)-dhbq materials can efficiently discriminate Xe atoms from other inert gases. It is revealed from the density functional theory calculations that the strong affinity between Xe and the coordination polymer is mainly attributed to the polarization by open metal sites.

Programmed Polarizability Engineering in a Cyclen-Based Cubic Zr(IV) Metal-Organic Framework to Boost Xe/Kr Separation

Efficient separation of xenon (Xe) and krypton (Kr) mixtures through vacuum swing adsorption (VSA) is considered the most attractive route to reduce energy consumption, but discriminating between these two gases is difficult due to their similar properties. In this work, we report a cubic zirconium-based MOF (Zr-MOF) platform, denoted as NU-1107, capable of achieving selective separation of Xe/Kr by post-synthetically engineering framework polarizability in a programmable manner. Specifically, the tetratopic linkers in NU-1107 feature tetradentate cyclen cores that are capable of chelating a variety of transition-metal ions, affording a sequence of metal-docked cationic isostructural Zr-MOFs. NU-1107-Ag(I), which features the strongest framework polarizability among this series, achieves the best performance for a 20:80 v/v Xe/Kr mixture at 298 K and 1.0 bar with an ideal adsorbed solution theory (IAST) predicted selectivity of 13.4, placing it among the highest performing MOF materials reported to date. Notably, the Xe/Kr separation performance for NU-1107-Ag(I) is significantly better than that of the isoreticular, porphyrin-based MOF-525-Ag(II), highlighting how the cyclen core can generate relatively stronger framework polarizability through the formation of low-valent Ag(I) species and polarizable counteranions. Density functional theory (DFT) calculations corroborate these experimental results and suggest strong interactions between Xe and exposed Ag(I) sites in NU-1107-Ag(I). Finally, we validated this framework polarizability regulation approach by demonstrating the effectiveness of NU-1107-Ag(I) toward C₃H₆/C₃H₈ separation, indicating that this generalizable strategy can facilitate the bespoke synthesis of polarized porous materials for targeted separations.

Shell-like Xenon Nano-Traps within Angular Anion-Pillared Layered Porous Materials for Boosting Xe/Kr Separation

Adsorptive separation of xenon (Xe) and krypton (Kr) is a promising technique but remains a daunting challenge since they are atomic gases without dipole or quadruple moments. Herein we report a strategy for fabricating angular anion-pillared materials featuring shell-like Xe nano-traps, which provide a cooperative effect conferred by the pore confinement and multiple specific interactions. The perfect permanent pore channel (4-5 Å) of Ni(4-DPDS)₂ MO₄ (M=Cr, Mo, W) can host Xe atoms efficiently even at ultra-low concentration (400 ppm Xe), showing the second-highest selectivity of 30.2 in Ni(4-DPDS)₂ WO₄ and excellent Xe adsorption capacity in Ni(4-DPDS)₂ CrO₄ (15.0 mmol kg^-1 ). Crystallography studies and DFT-D calculations revealed the energy favorable binding sites and angular anions enable the synergism between optimal pore size and polar porosity for boosting Xe affinity. Dynamic breakthrough experiments demonstrated three MOFs as efficient adsorbents for Xe/Kr separation.

A Microporous Hydrogen-Bonded Organic Framework for Efficient Xe/Kr Separation

Separation of xenon/krypton gas mixtures is one of the valuable but challenging processes in the gas industries due to their close molecular size and similar physical properties. Here, we report a novel ultramicroporous hydrogen-bonded organic framework (termed as HOF-40) constructed from a cyano-based organic building unit of 1,2,4,5-tetrakis(4-cyanophenyl)benzene (TCPB), exhibiting superior separation performance for Xe/Kr mixtures, as clearly demonstrated by dynamic breakthrough curves. GCMC simulation results indicate that the pore confinement effect and abundant accessible binding sites play a synergistic role in this challenging gas separation. Furthermore, this cyano-based HOF displays excellent chemical stability from 12 M HCl to 20 M NaOH aqueous solutions.

Self-Adjusting Metal-Organic Framework for Efficient Capture of Trace Xenon and Krypton

The capture of the xenon and krypton from nuclear reprocessing off-gas is essential to the treatment of radioactive waste. Although various porous materials have been employed to capture Xe and Kr, the development of high-performance adsorbents capable of trapping Xe/Kr at very low partial pressure as in the nuclear reprocessing off-gas conditions remains challenging. Herein, we report a self-adjusting metal-organic framework based on multiple weak binding interactions to capture trace Xe and Kr from the nuclear reprocessing off-gas. The self-adjusting behavior of ATC-Cu and its mechanism have been visualized by the in-situ single-crystal X-ray diffraction studies and theoretical calculations. The self-adjusting behavior endows ATC-Cu unprecedented uptake capacities of 2.65 and 0.52 mmol g^-1 for Xe and Kr respectively at 0.1 bar and 298 K, as well as the record Xe capture capability from the nuclear reprocessing off-gas. Our work not only provides a benchmark Xe adsorbent but proposes a new route to construct smart materials for efficient separations.

Hydrogen-Bonded Metal-Nucleobase Frameworks for Efficient Separation of Xenon and Krypton

Xe/Kr separation is an industrially important but challenging process owing to their inert properties and low concentrations in the air. Energy-effective adsorption-based separation is a promising technology. Herein, two isostructural hydrogen-bonded metal-nucleobase frameworks (HOF-ZJU-201 and HOF-ZJU-202) are capable of separating Xe/Kr under ambient conditions and strike an excellent balance between capacity and selectivity. The Xe capacity of HOF-ZJU-201a reaches 3.01 mmol g-1 at 298 K and 1.0 bar, while IAST selectivity and Henry's selectivity are 21.0 and 21.6, respectively. Direct breakthrough experiments confirmed the excellent separation performance, affording a Xe capacity of 25.8 mmol kg-1 from a Xe/Kr mixed-gas at dilute concentrations. Density functional theory calculations revealed that the selective binding arises from the enhanced polarization in the confined electric field produced by the electron-rich anions and the electron-deficient purine heterocyclic rings.

Creating Optimal Pockets in a Clathrochelate-Based Metal-Organic Framework for Gas Adsorption and Separation: Experimental and Computational Studies

The rational design and synthesis of robust metal-organic frameworks (MOFs) based on novel organic building blocks are fundamental aspects of reticular chemistry. Beyond simply fabricating new organic linkers, however, it is important to elucidate structure-property relationships at the molecular level to develop high-performing materials. In this work, we successfully targeted a highly porous and robust cage-type MOF (NU-200) with an nbo-derived fof topology through the deliberate assembly of a cyclohexane-functionalized iron(II)-clathrochelate-based meta-benzenedicarboxylate linker with a Cu₂(CO₂)₄ secondary building unit (SBU). NU-200 exhibited an outstanding adsorption capacity of xenon and a high ideal adsorbed solution theory (IAST) predicted selectivity for a 20/80 v/v mixture of xenon (Xe)/krypton (Kr) at 298 K and 1.0 bar. Our extensive computational simulations with grand canonical Monte Carlo (GCMC) and density functional theory (DFT) on NU-200 indicated that the MOF's hierarchical bowl-shaped nanopockets surrounded by custom-designed cyclohexyl groups─instead of the conventionally believed open metal sites (OMSs)─played a crucial role in reinforcing Xe-binding affinity. The optimally sized pockets firmly trapped Xe through numerous supramolecular interactions including Xe···H, Xe···O, and Xe···π. Additionally, we validated the unique pocket confinement effect by experimentally and computationally employing the similarly sized probe, sulfur dioxide (SO₂), which provided significant insights into the molecular underpinnings of the high uptake of SO₂ (11.7 mmol g^-1), especially at a low pressure of 0.1 bar (8.5 mmol g^-1). This work therefore can facilitate the judicious design of organic building blocks, producing MOFs featuring tailor-made pockets to boost gas adsorption and separation performances.

The First Sulfate-Pillared Hybrid Ultramicroporous Material, SOFOUR-1-Zn, and its Acetylene Capture Properties

Hybrid ultramicroporous materials, HUMs, are comprised of metal cations linked by combinations of inorganic and organic ligands. Their modular nature makes them amenable to crystal engineering studies, which have thus far afforded four HUM platforms (as classified by the inorganic linkers). HUMs are of practical interest because of their benchmark gas separation performance for several industrial gas mixtures. We report herein design and gram‐scale synthesis of the prototypal sulfate‐linked HUM, the fsc topology coordination network ([Zn(tepb)(SO₄)]_n), SOFOUR‐1‐Zn, tepb=(tetra(4‐pyridyl)benzene). Alignment of the sulfate anions enables strong binding to C₂H₂ via O⋅⋅⋅HC interactions but weak CO₂ binding, affording a new benchmark for the difference between C₂H₂ and CO₂ heats of sorption at low loading (ΔQ_st=24 kJ mol⁻¹). Dynamic column breakthrough studies afforded fuel‐grade C₂H₂ from trace (1 : 99) or 1 : 1 C₂H₂/CO₂ mixtures, outperforming its SiF₆²⁻ analogue, SIFSIX‐22‐Zn.

CoNi Alloy Nanoparticles Embedded in Metal-Organic Framework-Derived Carbon for the Highly Efficient Separation of Xenon and Krypton via a Charge-Transfer Effect

Separation of Xe and Kr is one of the greatest challenges in the gas industries owing to their close molecular structure and similar properties. Energy-effective adsorption-based separation using chemically stable carbon adsorbents is a promising technology. We propose a strategy for Xe/Kr separation using MOF-derived metallic carbon adsorbents. M-Gallate (M=Ni, Co) were used as precursors to fabricate CoNi alloy nanoparticles embedded carbon adsorbents by one-step auto-reduction pyrolysis. The optimal NiCo@C-700 exhibits record-high IAST selectivity (24.1) and Henry's selectivity (20.1) of Xe/Kr among reported carbon adsorbents. DFT calculations, local density of states calculation, charge density difference, and Bader charge analysis reveal the great affinity with Xe benefits from the presence of Ni or CoNi nanoparticles as a result of more charge transfer from Xe than Kr to metal, thus providing higher binding energy. Breakthrough experiments further verify NiCo@C-700 a promising candidate for Xe/Kr separation.

An Ultra-Microporous Metal-Organic Framework with Exceptional Xe Capacity

Molecular confinement plays a significant effect on trapped gas and solvent molecules. A fundamental understanding of gas adsorption within the porous confinement provides information necessary to design a material with improved selectivity. In this regard, metal-organic framework (MOF) adsorbents are ideal candidate materials to study confinement effects for weakly interacting gas molecules, such as noble gases. Among the noble gases, xenon (Xe) has practical applications in the medical, automotive and aerospace industries. In this Communication, we report an ultra-microporous nickel-isonicotinate MOF with exceptional Xe uptake and selectivity compared to all benchmark MOF and porous organic cage materials. The selectivity arises because of the near perfect fit of the atomic Xe inside the porous confinement. Notably, at low partial pressure, the Ni-MOF interacts very strongly with Xe compared to the closely related Krypton gas (Kr) and more polarizable CO₂ . Further ¹²⁹ Xe NMR suggests a broad isotropic chemical shift due to the reduced motion as a result of confinement.

Separation of Xe from Kr with Record Selectivity and Productivity in Anion-Pillared Ultramicroporous Materials by Inverse Size-Sieving

The separation of xenon/krypton (Xe/Kr) mixture is of great importance to industry, but the available porous materials allow the adsorption of both, Xe and Kr only with limited selectivity. Herein we report an anion-pillared ultramicroporous material NbOFFIVE-2-Cu-i (ZU-62) with finely tuned pore aperture size and structure flexibility, which for the first time enables an inverse size-sieving effect in separation along with record Xe/Kr selectivity and ultrahigh Xe capacity. Evidenced by single-crystal X-ray diffraction, the rotation of anions and pyridine rings upon contact of larger-size Xe atoms adapts cavities to the shape/size of Xe and allows strong host-Xe interaction, while the smaller-size Kr is excluded. Breakthrough experiments confirmed that ZU-62 has a real practical potential for producing high-purity Kr and Xe from air-separation byproducts, showing record Kr productivity (206 mL g^-1 ) and Xe productivity (42 mL g^-1 , in desorption) as well as good recyclability.

Adsorptive Separation of Acetylene from Ethylene in Isostructural Gallate-Based Metal-Organic Frameworks

The separation of acetylene from ethylene is of paramount importance in the purification of chemical feedstocks for industrial manufacturing. Herein, an isostructural series of gallate-based metal-organic frameworks (MOFs), M-gallate (M=Ni, Mg, Co), featuring three-dimensionally interconnected zigzag channels, the aperture size of which can be finely tuned within 0.3 Å by metal replacement. Controlling the aperture size of M-gallate materials slightly from 3.69 down to 3.47 Å could result in a dramatic enhancement of C₂ H₂ /C₂ H₄ separation performance. As the smallest radius among the studied metal ions, Ni-gallate exhibits the best C₂ H₂ /C₂ H₄ adsorption separation performance owing to the strongest confinement effect, ranking after the state-of-the-art UTSA-200a with a C₂ H₄ productivity of 85.6 mol L^-1 from 1:99 C₂ H₂ /C₂ H₄ mixture. The isostructural gallate-based MOFs, readily synthesized from inexpensive gallic acid, are demonstrated to be a new top-performing porous material for highly efficient adsorption of C₂ H₂ from C₂ H₄ .

Microwave-assisted synthesis of porphyrin conjugated microporous polymers for microextraction of volatile organic acids in tobaccos

Conjugated microporous polymers (CMPs) with permanent microporosity and extended p-conjugated skeletons have recently shown the fascinating application in separation and enrichment. In this report, porphyrin CMP that possessed microporous structure and nitrogen-rich pyrrole building blocks was successfully synthesized by microwave-assisted method. Then, a novel coating based on porphyrin CMP was fabricated on silica fiber for efficient enrichment volatile organic acids (VOAs). The simulation showed the coating exhibited strong interaction with volatile organic acids based on charge transfer interaction, hydrogen bond and size effect. Hence, we proposed a method for determination of volatile organic acids in tobaccos by headspace solid-phase microextraction (HS-SPME) with porphyrin CMP coating by gas chromatography-mass spectrometry. The results showed that the coating provided high enrichment factors for VOAs ranging from 66,657 to 133,970 and low limits of detection from 4.6 to 22 ng/L. A good linearity was observed for propionic acid and crotonic acid ranging from 0.050 to 8.0 μg/L, 2-methylheptanoic acid ranging from 0.063 to 1.5 μg/L, others ranging from 0.025 to 3.0 μg/L with the determination coefficient (R2) between 0.9900 and 0.9980. The strategy for determination of volatile compounds in complex solid samples was successfully applied to the analysis of volatile organic acids in tobacco leaves. The results showed that the method was accurate and reliable.

Employing an Unsaturated Th4+ Site in a Porous Thorium-Organic Framework for Kr/Xe Uptake and Separation

Actinide based metal-organic frameworks (MOFs) are unique not only because compared to the transition-metal and lanthanide systems they are substantially less explored, but also owing to the uniqueness of actinide ions in bonding and coordination. Now a 3D thorium-organic framework (SCU-11) contains a series of cages with an effective size of ca. 21×24 Å. Th⁴⁺ in SCU-11 is 10-coordinate with a bicapped square prism coordination geometry, which has never been documented for any metal cation complexes. The bicapped position is occupied by two coordinated water molecules that can be removed to afford a very unique open Th⁴⁺ site, confirmed by X-ray diffraction, color change, thermogravimetry, and spectroscopy. The degassed phase (SCU-11-A) exhibits a Brunauer-Emmett-Teller surface area of 1272 m²  g^-1 , one of the highest values among reported actinide materials, enabling it to sufficiently retain water vapor, Kr, and Xe with uptake capacities of 234 cm³  g^-1 , 0.77 mmol g^-1 , 3.17 mmol g^-1 , respectively, and a Xe/Kr selectivity of 5.7.

Xenon Recovery at Room Temperature using Metal-Organic Frameworks

Xenon is known to be a very efficient anesthetic gas, but its cost prohibits the wider use in medical industry and other potential applications. It has been shown that Xe recovery and recycling from anesthetic gas mixtures can significantly reduce its cost as anesthetic. The current technology uses series of adsorbent columns followed by low-temperature distillation to recover Xe; this method is expensive to use in medical facilities. Herein, we propose a much simpler and more efficient system to recover and recycle Xe from exhaled anesthetic gas mixtures at room temperature using metal-organic frameworks (MOFs). Among the MOFs tested, PCN-12 exhibits unprecedented performance with high Xe capacity and Xe/O₂ , Xe/N₂ and Xe/CO₂ selectivity at room temperature. The in situ synchrotron measurements suggest that Xe is occupies the small pockets of PCN-12 compared to unsaturated metal centers (UMCs). Computational modeling of adsorption further supports our experimental observation of Xe binding sites in PCN-12.

Tunable Porous Coordination Polymers for the Capture, Recovery and Storage of Inhalation Anesthetics

The uptake of inhalation anesthetics by three topologically identical frameworks is described. The 3D network materials, which possess square channels of different dimensions, are formed from the relatively simple combination of Zn^II centres and dianionic ligands that contain a phenolate and a carboxylate group at opposite ends. All three framework materials are able to adsorb N₂ O, Xe and isoflurane. Whereas the framework with the widest channels is able to adsorb large quantities of the various guests from the gas phase, the frameworks with the narrower channels have superior binding enthalpies and exhibit higher levels of retention. The use of ligands in which substituents are bound to the aromatic rings of the bridging ligands offers great scope for tuning the adsorption properties of the framework materials.