Potential of metal-organic frameworks for separation of xenon and krypton

Pore Chemistry and Architecture Control in Anionic Functional Ultramicroporous Materials for Record Dense Packing of Xenon

Adsorptive separation of Xe and Kr is an industrially promising but challenging process because of their identical shape and similar physicochemical properties. Herein, we demonstrate a strategy through rationally designing the linkers of anionic functional ultramicroporous materials (FUMs) to finely regulate the pore chemistry and architecture, which creates unique stepped channels incorporating dense polar nanotraps to generate a larger effective pore space and enables dense packing of Xe. A new hydrolytically stable FUM (ZUL-530) was prepared, which for the first time achieves a Xe packing density exceeding the liquid Xe density at atmospheric conditions in metal-organic frameworks (MOFs) (based on experimental data), resulting in both excellent Xe uptake (2.55 mmol g-1 at 0.2 bar) and high IAST selectivity (20.5). GCMC and DFT-D calculations reveal the essential role of the stepped traps in the dense packing of Xe. Breakthrough experiments demonstrate remarkable productivities of both high-purity Kr (6.70 mmol g-1) and Xe (1.78 mmol g-1) for the Xe/Kr (20:80) mixture. In a model nuclear industry exhaust gas, ZUL-530 exhibits a top-class Xe dynamic capacity (28.8 mmol kg-1) for trace Xe, which proves it is one of the best candidates for Xe/Kr separation.

Selective Xenon Recovery Using Aluminum-Based Metal-Organic Frameworks with Conserved Pore Topology

Xenon (Xe) is a commercially valuable element found in trace amounts in the off-gas from used nuclear fuel. Recovering Xe from these streams provides a cost-effective means to increase its supply. However, achieving high-purity Xe recovery is challenging due to the need for separation from nearly identical krypton (Kr). Metal-organic frameworks (MOFs), a class of crystalline porous materials, show potential to separate Xe and Kr by utilizing differences in their kinetic diameters, allowing for selective separation. In this work, we study the impact of pore aperture and volume on selective Xe recovery using four robust aluminum MOFs: Al-PMOF, Al-PyrMOF, Al-BMOF and MIL-120, all with conserved structural topology. The pore topology in each MOF is dictated by the dimensions of the tetracarboxylate ligand employed, with larger ligands leading to MOFs with increased pore size and volume. Our experimental and computational investigations revealed that MIL-120 exhibits the highest affinity (21.94 kH(Xe) = 21.94 mmol g-1 bar-1) for Xe among all MOFs, while Al-BMOF demonstrates the highest Xe/Kr selectivity of 14.34. We evaluated the potential of both MIL-120 and Al-BMOF for Xe recovery through breakthrough analysis using a mixture of 400 ppm Xe:40 ppm Kr. Our results indicate that due to its larger pore volume, Al-BMOF captured more Xe than MIL-120, demonstrating superior Xe/Kr separation efficiency.

PHAHST Potential: Modeling Sorption in a Dispersion-Dominated Environment

PHAHST (potentials with high accuracy, high speed, and transferability) is a recently developed force field that utilizes exponential repulsion, multiple dispersion terms, explicit many-body polarization, and many-body van der Waals interactions. The result is a systematic approach to force field development that is computationally practical. Here, PHAHST is employed in the simulation for rare gas uptake of krypton and xenon in the metal-organic material, HKUST-1. This material has shown promise in use as an adsorptive separating agent and presents a challenge to model due to the presence of heterogeneous interaction sorption surfaces, which include pores with readily accessible, open-metal sites that compete with dispersion-dominated pores. Such environments are difficult to simulate with commonly used empirical force fields, such as the Lennard-Jones (LJ) potential, which perform better when electrostatics are dominant in determining the nature of sorption and alone are incapable of modeling interactions with open-metal sites. The effectiveness of PHAHST is compared to the LJ potential in a series of mixed Kr-Xe gas simulations. It has been demonstrated that PHAHST compares favorably with experimental results, and the LJ potential is inadequate. Overall, we establish that force fields with physically grounded repulsion/dispersion terms are required in order to accurately model sorption, as these interactions are an important component of the energy. Furthermore, it is shown that the simple mixing rules work nearly quantitatively for the true pair potentials, while they are not transferable for effective potentials like LJ.

Trailblazing Kr/Xe Separation: The Birth of the First Kr-Selective Material

Efficient separation of Kr from Kr/Xe mixtures is pivotal in nuclear waste management and dark matter research. Thus far, scientists have encountered a formidable challenge: the absence of a material with the ability to selectively adsorb Kr over Xe at room temperature. This study presents a groundbreaking transformation of the renowned metal-organic framework (MOF) CuBTC, previously acknowledged for its Xe adsorption affinity, into an unparalleled Kr-selective adsorbent. This achievement stems from an innovative densification approach involving systematic compression of the MOF, where the crystal size, interparticle interaction, defects, and evacuation conditions are synergistically modulated. The resultant densified CuBTC phase exhibits exceptional mechanical resilience, radiation tolerance, and notably an unprecedented selectivity for Kr over Xe at room temperature. Simulation and experimental kinetic diffusion studies confirm reduced gas diffusion in the densified MOF, attributed to its small pore window and minimal interparticle voids. The lighter Kr element demonstrates facile surface passage and higher diffusivity within the material, while the heavier Xe encounters increased difficulty entering the material and lower diffusivity. This Kr-selective MOF not only represents a significant breakthrough in Kr separation but also demonstrates remarkable processability and scalability to kilogram levels. The findings presented herein underscore the transformative potential of engineered MOFs in addressing complex challenges, heralding a new era of Kr separation technologies.

Co-adsorption performance of iodine and NOX in iodine exhaust gas by NH2-MIL-125

Ti-based MOFs exhibit ultra-high stability in radioactive waste gases containing nitrogen oxides (NOX) and are effective in capturing radioactive iodine. In this study, NH2-MIL-125 was synthesized via a one-pot solvothermal method and its adsorption performance for iodine was investigated using batch adsorption experiments, the stability of materials was tested by simulating post-processing conditions. The results indicated that NH2-MIL-125 had a maximum iodine adsorption capacity of 1.61 g/g at 75 ℃ and reached adsorption equilibrium within 60 min, and the adsorption capacity of methyl iodine reached 776.9 mg/g. The material also exhibited excellent stability and iodine adsorption performance in the presence of NOX. After soaking in NO2 for 24 h, its structure remained stable and the adsorption capacity for iodine remained at 231.5 mg/g. The excellent co-adsorption performance of NH2-MIL-125 on iodine and NOX was attributed to the synergistic effects of Ti-OH groups and amino functional groups. These findings provide a reference for the capture of radioactive iodine and also demonstrate the potential of NH2-MIL-125 for iodine capture during spent fuel reprocessing.

Biomedical application of metal-organic frameworks (MOFs) in cancer therapy: Stimuli-responsive and biomimetic nanocomposites in targeted delivery, phototherapy and diagnosis

The nanotechnology is an interdisciplinary field that has become a hot topic in cancer therapy. Metal-organic frameworks (MOFs) are porous materials and hybrid composites consisted of organic linkers and metal cations. Despite the wide application of MOFs in other fields, the potential of MOFs for purpose of cancer therapy has been revealed by the recent studies. High surface area and porosity, significant drug loading and encapsulation efficiency are among the benefits of using MOFs in drug delivery. MOFs can deliver genes/drugs with selective targeting of tumor cells that can be achieved through functionalization with ligands. The photosensitizers and photo-responsive nanostructures including carbon dots and gold nanoparticles can be loaded in/on MOFs to cause phototherapy-mediated tumor ablation. The immunogenic cell death induction and increased infiltration of cytotoxic CD8+ and CD4+ T cells can be accelerated by MOF platforms in providing immunotherapy of tumor cells. The stimuli-responsive MOF platforms responsive to pH, redox, enzyme and ion can accelerate release of therapeutics in tumor site. Moreover, MOF nanocomposites can be modified ligands and green polymers to improve their selectivity and biocompatibility for cancer therapy. The application of MOFs for the detection of cancer-related biomarkers can participate in the early diagnosis of patients.

A Microporous Hydrogen-Bonded Organic Framework Based on Hydrogen-Bonding Tetramers for Efficient Xe/Kr Separation

Hydrogen-bonded organic frameworks (HOFs) have been emerging as a new type of very promising microporous materials for gas separation and purification, but few HOFs structures constructed through hydrogen-bonding tetramers have been explored in this field. Herein, we report the first microporous HOF (termed as HOF-FJU-46) afforded by hydrogen-bonding tetramers with 4-fold interpenetrated diamond networks, which shows excellent chemical and thermal stability. What's more, activated HOF-FJU-46 exhibits the highest xenon (Xe) uptake of 2.51 mmol g-1 and xenon/krypton (Kr) selectivity of 19.9 at the ambient condition among the reported HOFs up to date. Dynamic breakthrough tests confirmed the excellent Xe/Kr separation of HOF-FJU-46a, showing high Kr productivity (110 mL g-1 ) and Xe uptake (1.29 mmol g-1 ), as well as good recyclability. The single crystal X-ray diffraction and the molecular simulations revealed that the abundant accessible aromatic and pyrazole rings in the pore channels of HOF-FJU-46a can provide the multiple strong C-H⋅⋅⋅Xe interactions with Xe atoms.

Hydrogen-Bonding Assembly Meets Anion Coordination Chemistry: Framework Shaping and Polarity Tuning for Xenon/Krypton Separation

Hybrid hydrogen-bonded (H-bonded) frameworks built from charged components or metallotectons offer diverse guest-framework interactions for target-specific separations. We present here a study to systematically explore the coordination chemistry of monovalent halide anions, i.e., F- , Cl- , Br- , and I- , with the aim to develop hybrid H-bond synthons that enable the controllable construction of microporous H-bonded frameworks exhibiting fine-tunable surface polarity within the adaptive cavities for realistic xenon/krypton (Xe/Kr) separation. The spherical halide anions, especially Cl- , Br- , and I- , are found to readily participate in the charge-assisted H-bonding assembly with well-defined coordination behaviors, resulting in robust frameworks bearing open halide anions within the distinctive 1D pore channels. The activated frameworks show preferential binding towards Xe (IAST Xe/Kr selectivity ca. 10.5) because of the enhanced polarizability and the pore confinement effect. Specifically, dynamic column Xe/Kr separation with a record-high separation factor (SF=7.0) among H-bonded frameworks was achieved, facilitating an efficient Xe/Kr separation in dilute, CO2 -containing gas streams exactly mimicking the off-gas of spent nuclear fuel (SNF) reprocessing.

Robust Hydrogen-Bonded Organic Framework with Four-Fold Interpenetration for Adsorptive Separation of C2H6/C2H4 and Xe/Kr

Hydrogen-bonded organic frameworks (HOFs) are an emerging class of porous materials that hold promise for the adsorptive separation of industrially relevant gas mixtures. However, developing HOFs with high thermal stability and resistance to water remains a daunting challenge. We report here a microporous HOF (HIAM-103) assembled from a hexacarboxylate linker (2,4,6-trimethylbenzene-1,3,5-triylisophthalic acid, H6TMBTI). The compound crystallizes in the trigonal crystal system, and its structure is a four-fold interpenetrated network. Upon thermal activation, the single crystals remain intact, allowing for precise determination of the activated structure. HIAM-103 exhibits remarkable thermal and hydrothermal stability. Its microporous channels demonstrate selective adsorption of C2H6 over C2H4 and Xe over Kr, and its separation capability toward mixed gases has been validated by column breakthrough experiments under dry and humid conditions. The preferential gas adsorption sites and separation mechanisms have been uncovered through DFT analysis, which suggests that the methyl group decorated 1D channels are the primary reason for the selective adsorption.

Charge Transfer Process of a Solvated Hydrogen-Bonded Organic Network

We show the first example of an organic linker (OL) terminated by carboxylic groups that can form a hydrogen-bonded network/polymer (HBN) in solution under controlled conditions in which the photogenerated charges can hop from a monomer OL to the hydrogen-bonded backbone of OLs, as probed by transient absorption (fs-TA). While fs-TA reveals a slow twisting process in the monomer form of the OL, the formation of a hydrogen-bonded network in solution suppresses such process and favors instead a charge transfer (CT) state along the low-lying hydrogen-bonded backbone. Theoretical calculations show that such solvated HBN in a specific polar solvent is stabilized due to the huge change of the dipole moment from monomer compared to the network, leading to a charge delocalization character due to the symmetry breaking. Our findings will open new avenues for implementing solvated hydrogen-bonded molecules in applications such as sensing and photocatalysis.

New 5,5-(1,4-Phenylenebis(methyleneoxy)diisophthalic Acid Appended Zn(II) and Cd(II) MOFs as Potent Photocatalysts for Nitrophenols

Metal-organic frameworks (MOFs) are peculiar multimodal materials that find photocatalytic applications for the decomposition of lethal molecules present in the wastewater. In this investigation, two new d10-configuration-based MOFs, [Zn2(L)(H2O)(bbi)] (1) and [Cd2(L)(bbi)] (2) (5,5-(1,4-phenylenebis(methyleneoxy)diisophthalic acid (H2L) and 1,1'-(1,4-butanediyl)bis(imidazole) (bbi)), have been synthesized and characterized. The MOF 1 displayed a (4,6)-connected (3.43.52)(32.44.52.66.7) network topology, while 2 had a (3,10)-connected network with a Schläfli symbol of (410.511.622.72)(43)2. These MOFs have been employed as photocatalysts to photodegrade nitrophenolic compounds, especially p-nitrophenol (PNP). The photocatalysis studies reveal that 1 displayed relatively better photocatalytic performance than 2. Further, the photocatalytic efficacy of 1 has been assessed by altering the initial PNP concentration and photocatalyst dosage, which suggest that at 80 ppm PNP concentration and at its 50 mg concentration the MOF 1 can photo-decompose around 90.01% of PNP in 50 min. Further, radical scavenging experiments reveal that holes present over 1 and ·OH radicals collectively catalyze the photodecomposition of PNP. In addition, utilizing density of states (DOS) calculations and Hirshfeld surface analyses, a plausible photocatalysis mechanism for nitrophenol degradation has been postulated.

Combination of High-Throughput Screening and Assembly to Discover Efficient Metal-Organic Frameworks on Kr/Xe Adsorption Separation

Recycling Kr and Xe from used nuclear fuel (UNF) is conducive to regenerating economy and protecting the environment, and it is urgent to screen or design high-performance cutting-edge metal-organic framework (MOF) materials for Kr/Xe adsorption separation. After grand canonical Monte Carlo (GCMC) simulations of Kr/Xe adsorption separation on 11,000 frameworks in CoRE MOFs (2019), the important structure-adsorption property relationship (SAPR) was induced; that is, the porosity (φ) at 0.30-0.40, LCD/PLD at 1.00-1.49, density (ρ) range between 1.20 and 2.30 g/cm3, and PLD at 2.40-3.38 Å can be utilized to screen for high-performance G-MOFs and hMOFs. In addition, the key "genes" (metal nodes and linkers) of MOFs determining the Kr/Xe adsorption separation were data-mined by a machine learning technique, which were assembled into novel MOFs. After comprehensive consideration of thermal stability and the adsorbent performance score (APS), eight promising MOFs on Kr/Xe separation with the APS more than 1290.89 were screened out and assembled, which are better than most of the reported frameworks. Note that the adsorption isotherms of these MOFs on Kr and Xe belong to type I curve with the thermodynamic equilibrium mechanism on Kr/Xe based on the confinement effect. Furthermore, according to the electronic structure calculations of the independent gradient model based on Hirshfeld partition (IGMH) and energy decomposition analysis, it is found that the interactions between guests and frameworks are vdW forces with dominant induction energy (Eind). In addition, the electrostatic potential gradients of frameworks are generally linearly negative correlated with Kr uptakes. Therefore, both the geometrical and electronic structures dominate the adsorption separation performance on Kr/Xe. Interestingly, these eight MOFs are also suitable for the separation of CH4/H2 with considerable selectivities and CH4 uptakes of up to 2566.67 and 3.04 mmol/g, respectively. Herein, the accurately constructed SAPR and material genomics strategy should be helpful for the experimental discovery of novel MOFs on Kr/Xe separation experimentally.

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.

First-Principles Study of Nitrogen Adsorption and Dissociation on ZrMnFe(110) Surface

The adsorption, dissociation and penetration processes of N₂ on the surface of ZrMnFe(110) were investigated using the first-principles calculation method in this paper. The results indicate that the vacancy Hollow 1 composed of 4Zr1Fe on the surface of ZrMnFe(110) is the best adsorption site for the N₂ molecule and N atom, and the adsorption energies are 10.215 eV and 6.057 eV, respectively. Electron structure analysis indicates that the N₂ molecule and N atoms adsorbed mainly interact with Zr atoms on the surface. The transition state calculation shows that the maximum energy barriers to be overcome for the N₂ molecule and N atom on the ZrMnFe(110) surface were 1.129 eV and 0.766 eV, respectively. This study provides fundamental insight into the nitriding mechanism of nitrogen molecules in ZrMnFe.

Reticular Chemistry in Pore Engineering of a Y-Based Metal-Organic Framework for Xenon/Krypton Separation

The fine-tuning of metal-organic framework (MOF) pore structures is of critical importance in developing energy-efficient xenon/krypton (Xe/Kr) separation techniques. Capitalizing on reticular chemistry, we constructed a robust Y-based MOF (NU-1801) that is isoreticular to NPF-500 with a shortened organic ligand and a larger metal radius while maintaining the 4,8-connected flu topology, giving rise to a narrowed pore structure for the efficient separation of a Xe/Kr mixture. At 298 K and 1 bar, NU-1801 possessed a moderate Xe uptake of 2.79 mmol/g but exhibited a high Xe/Kr selectivity of 8.2 and an exceptional Xe/Kr uptake ratio of about 400%. NU-1801 could efficiently separate a Xe/Kr mixture (20:80, v/v), as validated by breakthrough experiments, due to the outstanding discrimination in van der Waals interactions of Xe and Kr toward the framework confirmed by grand canonical Monte Carlo simulations. This work highlights the importance of reticular chemistry in designing structure-specific MOFs for gas separation.

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.

Identifying porous cage subsets in the Cambridge Structural Database using topological data analysis

As rationally designable materials, the variety and number of synthesised metal-organic cages (MOCs) and organic cages (OCs) are expected to grow in the Cambridge Structural Database (CSD). In this regard, two of the most important questions are, which structures are already present in the CSD and how can they be identified? Here, we present a cage mining methodology based on topological data analysis and a combination of supervised and unsupervised learning that led to the derivation of - to the best of our knowledge - the first and only MOC dataset of 1839 structures and the largest experimental OC dataset of 7736 cages, as of March 2022. We illustrate the use of such datasets with a high-throughput screening of MOCs and OCs for xenon/krypton separation, important gases in multiple industries, including healthcare.

Synthesis of Iron(II) Clathrochelate-Based Poly(vinylene sulfide) with Tetraphenylbenzene Bridging Units and Their Selective Oxidation into Their Corresponding Poly(vinylene sulfone) Copolymers: Promising Materials for Iodine Capture

The development of a simple and efficient synthetic methodology to engineer functional polymer materials for gas adsorption is necessary due to its relevance for various applications. Herein, we report the synthesis of metalorganic poly(vinylene sulfide) copolymers CTP1-3 with iron(II) clathrochelate of various side groups connected by tetraphenylbenzene units. CTP1-3 were subsequently oxidized into their respective poly(vinylene sulfone) copolymers CTP4-6 under green reaction conditions. The target copolymers CTP1-6 were characterized using various instrumental analysis techniques. Examination of the iodine adsorption properties of the copolymers revealed high iodine uptake properties, reaching 2360 mg g−1 for CTP2, and whose reusability tests proved its efficient regeneration, thus proving the importance of iron(II) clathrochelate polymers in iodine capture.

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.

Recent Development in Coordination Compounds as a Sensor for Cyanide Ions in Biological and Environmental Segments

Rapid detection of toxic ions has taken great attention in the last few decades due to its importance in maintaining a greener environment for human beings. The extreme toxicity of cyanide (CN-) ions is a great environmental concern as its continued industrial use generates interest in facile and sensitive methods for CN- ions detection. Since CN- ions act as a ligand in coordination chemistry which rapidly coordinates with suitable metals and forms complexes, this ability was mainly explored in its detection. It also attacks the central metal in coordination compounds and gives a fluorimetric response. Coordination compounds behave as a sensor for the detection of important ions like CN- ions and have gained great attention due to their facile synthesis, multianalyte detection, clear detection and low detection limit. Recently, considerable efforts have been devoted to the detection and quantification of hazardous multianalyte using a single probe. Cu2+ complexes are the main complexes used for CN- ions detection; however, the complexes of many other metals are also used as sensors. Four basic types of interaction have been discussed in coordination compound sensors for CN- detection. The performances of different sensors are compared with one another and the sensors which have the lowest detection limit are highlighted. This review comprises the progress made by coordination compounds as sensors for the detection of CN- ions in the last six years (2015-2021). To the best of our knowledge, there is no review on coordination compounds as a sensor for CN- ions during this period. [Figure: see text].

Efficient Xe/Kr Separation Based on a Lanthanide-Organic Framework with One-Dimensional Local Positively Charged Rhomboid Channels

Efficient xenon/krypton (Xe/Kr) separation has played an important role in industry due to the wide application of high-purity Xe and with regard to the safe disposal of radioactive noble gases (⁸⁵Kr and ¹³³Xe). A less energy-demanding separation technology, adsorptive separation using porous solid materials, has been proposed to replace the traditional cryogenic distillation with intensive energy consumption. As a cutting-edge class of porous materials, metal-organic frameworks (MOFs) featuring permanent porosity, designable chemical functionalities, and tunable pore sizes hold great promise for Xe/Kr separation. Here, we report a two-dimensional (2D) lanthanide-organic framework (termed LPC-MOF, [Eu(Ccbp)(NO₃)(HCOO)]·DMF_0.3(H₂O)_2.5) with one-dimensional (1D) local positively charged rhomboid channels whose size matches well with the kinetic diameter of Xe, leading to its superior Xe/Kr separation performance. Column breakthrough experiments demonstrate that LPC-MOF exhibits a high Xe/Kr selectivity of 12.4 and an Xe adsorption amount of 3.39 mmol kg^-1 under simulated conditions for real used nuclear fuel (UNF)-reprocessing plants. Furthermore, density functional theory (DFT) calculations elucidate not only the intrinsic mechanisms of Xe/Kr separation at the molecular level but also the detailed influence of the local positive charge (N⁺) on the performance of Xe/Kr separation in the MOF system.

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.

Rapid Generation of Metal-Organic Framework Phase Diagrams by High-Throughput Transmission Electron Microscopy

Metal-organic frameworks (MOFs) constructed from Zr₆ nodes and tetratopic carboxylate linkers display high structural diversity and complexity in which various crystal topologies can result from identical building units. To determine correlations between MOF topologies and experimental parameters, such as solvent choice or modulator identity and concentration, we demonstrate the rapid generation of phase diagrams for Zr₆-MOFs with 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)benzene linkers under a variety of conditions. We have developed a full set of methods for high-throughput transmission electron microscopy (TEM), including automated sample preparation and data acquisition, to accelerate MOF characterization. The use of acetic acid as a modulator yields amorphous, NU-906, NU-600, and mixed-phase structures depending on the ratio of N,N-dimethylformamide to N,N-diethylformamide solvent and the quantity of the modulator. Notably, the use of formic acid as a modulator enables direct control of crystal growth along the c direction through variation of the modulator quantity, thus realizing aspect ratio control of NU-1008 crystals with different catalytic hydrolysis performance toward a nerve agent simulant.

Dinuclear Nickel-Oxygen Cluster-Based Metal-Organic Frameworks with Octahedral Cages for Efficient Xe/Kr Separation

Xe/Kr separation is industrially important but remains a daunting issue in chemical separations. Herein, a fluorinated metal-organic framework (MOF), [Ni₂(μ₂-O)(TFBPDC)(tpt)₂]_n (named JXNU-13-F), built from 3,3',5,5'-tetrakis(fluoro)biphenyl-4,4'-dicarboxylic (TFBPDC^2-) and 2,4,6-tri(4-pyridinyl)-1,3,5-triazine (tpt) ligands is provided. JXNU-13-F displays a three-dimensional (3D) framework constructed from distorted octahedral cages and an impressive Xe capacity of 144 cm³ g^-1 at 273 K and 1 bar, ranking among top MOFs. The high Xe uptake and moderate Xe/Kr adsorption selectivity endow JXNU-13-F with efficient Xe/Kr separation demonstrated by experimental column breakthrough tests. The comparative studies of gas adsorption between isostructural JXNU-13-F and JXNU-13 (the nonfluorinated analogue ([Ni₂(μ₂-O)(BPDC))(tpt)₂]_n with biphenyl-4,4'-dicarboxylic (BPDC^2-)) revealed that the F groups serve as the innocent groups during the Xe and Kr adsorption in JXNU-13-F. Thus, a combination of highly hydrophobic and π-electron-rich pore surfaces made of aromatic rings with strong interactions with the Xe atom possessing large polarizability and appropriate pore sizes that match well Xe having a large atom diameter has resulted in high Xe uptake and effective Xe/Kr separation characteristics of JXNU-13-F.

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.

Robust and Radiation-Resistant Hofmann-Type Metal-Organic Frameworks for Record Xenon/Krypton Separation

The discovery of high-performance adsorbents for highly efficient separation of xenon from krypton is an important but challenging task in the chemical industry due to their similar size and inert spherical nature. Herein, we report two robust and radiation-resistant Hofmann-type MOFs, Co(pyz)[Ni(CN)₄] and Co(pyz)[Pd(CN)₄] (termed as ZJU-74a-Ni and ZJU-74a-Pd), featuring oppositely adjacent open metal sites and perfect pore sizes (4.1 and 3.8 Å) comparable to the kinetic diameter of xenon (4.047 Å), affording the benchmark binding affinity for polarizable Xe gas. These materials thus exhibit both record-high Xe uptake capacities (89.3 and 98.4 cm³ cm^-3 at 296 K and 0.2 bar) and Xe/Kr selectivities (74.1 and 103.4) at ambient conditions, all of which are the highest among all the state-of-the-art materials reported so far. The locations of Xe molecules within ZJU-74a-Ni have been visualized by single-crystal X-ray diffraction studies, in which two oppositely adjacent metal centers combined with the right aperture size can construct a unique sandwich-like binding site to offer unprecedented and ultrastrong Ni²⁺-Xe-Ni²⁺ interactions with xenon, thus leading to the record Xe capture capacity and selectivity. The excellent separation capacity of ZJU-74a-Pd was verified by breakthrough experiments for Xe/Kr gas mixtures, providing both unprecedentedly high xenon uptake capacity (4.63 mmol cm^-3) and krypton productivity (214 cm³ g^-1).

Two isomeric metal-organic frameworks bearing stilbene moieties for high volatile iodine uptake

The efficient, green, and economical removal of radioactive iodine (I2) has drawn worldwide attention in the safe development of nuclear energy. Metal-organic frameworks (MOFs) have been demonstrated to be a...

Parametrization of Nonbonded Force Field Terms for Metal-Organic Frameworks Using Machine Learning Approach

The enormous structural and chemical diversity of metal-organic frameworks (MOFs) forces researchers to actively use simulation techniques as often as experiments. MOFs are widely known for their outstanding adsorption properties, so a precise description of the host-guest interactions is essential for high-throughput screening aimed at ranking the most promising candidates. However, highly accurate ab initio calculations cannot be routinely applied to model thousands of structures due to the demanding computational costs. Furthermore, methods based on force field (FF) parametrization suffer from low transferability. To resolve this accuracy-efficiency dilemma, we applied a machine learning (ML) approach: extreme gradient boosting. The trained models reproduced the atom-in-material quantities, including partial charges, polarizabilities, dispersion coefficients, quantum Drude oscillator, and electron cloud parameters, with accuracy similar to the reference data set. The aforementioned FF precursors make it possible to thoroughly describe noncovalent interactions typical for MOF-adsorbate systems: electrostatic, dispersion, polarization, and short-range repulsion. The presented approach can also readily facilitate hybrid atomistic simulation/ML workflows.

Thermodynamic exploration of xenon/krypton separation based on a high-throughput screening

Nanoporous framework materials are a promising class of materials for energy-efficient technology of xenon/krypton separation by physisorption. Many studies on Xe/Kr separation by adsorption have focused on the determination of structure/property relationships, the description of theoretical limits of performance, and the identification of top-performing materials. Here, we provide a study based on a high-throughput screening of the adsorption of Xe, Kr, and Xe/Kr mixtures in 12 020 experimental MOF materials, to provide a better comprehension of the thermodynamics behind Xe/Kr separation in nanoporous materials and the microscopic origins of Xe/Kr selectivity at both low and ambient pressure.

Metal-organic frameworks as superior porous adsorbents for radionuclide sequestration: Current status and perspectives

Efficient separation of hazardous radionuclides from radioactive waste remains a challenge to the global acceptance of nuclear power due to complex nature of the waste, high radiotoxicities and presence of large number of interfering elements. Sorption of radioactive elements from liquid phase, gas phase or their solid particulates on various synthetic organic, inorganic or biological sorbents is looked as one of the options for their remediation. In this context, highly porous materials, termed as metal-organic frameworks (MOFs), have shown promise for efficient capturing of various types of radioactive elements. Major advantages that have been advocated for the application of MOFs in radionuclide sorption are their excellent chemical stability, and their large surface area due to abundant functional groups, and porosity. In this review, recent developments on the application of MOFs for radionuclide sequestration are briefly discussed. Focus has been devoted to address the separation of few crucial radioactive elements such as Th, U, Tc, Re, Se, Sr and Cs from aqueous solutions, which are important for liquid radioactive waste management. Apart from these radioactive metal ions, removal of radionuclide bearing gases such as I₂, Xe, and Kr are also discussed. Aspects related to the interaction of MOFs with the radionuclides are also discussed. Finally, a perspective for comprehensive investigation of MOFs for their applications in radioactive waste management has been outlined.

Pb2+-Containing Metal-Organic Rotaxane Frameworks (MORFs)

The metal-organic rotaxane framework (MORF) structures with the advantage of mechanically interlocking molecules (MIMs) have attracted intense interest from the chemical community. In this study, a set of MORFs (i.e., MORF-Pb-1 and MORF-Pb-2) are constructed using Pb2+, a tetraimidazolium macrocycle (Texas-sized molecular box; 14+), and aromatic dicarboxylate (p-phthalate dianions (PTADAs; 2) or 2,6-naphthalene dicarboxylate dianions (3)) via a one-pot three-layer diffusion protocol. In particular, an unusual Pb…Pb weak interaction was shown in MORF-Pb-1 (charactered with distance of 3.656 Å).

How Reliable Is the Ideal Adsorbed Solution Theory for the Estimation of Mixture Separation Selectivities in Microporous Crystalline Adsorbents?

Microporous crystalline adsorbents such as zeolites and metal-organic frameworks (MOFs) have potential use in a wide variety of separation applications. The adsorption selectivity S _ads is a key metric that quantifies the efficacy of any microporous adsorbent in mixture separations. The Ideal Adsorbed Solution Theory (IAST) is commonly used for estimating the value of S _ads, with unary isotherms of the constituent guests as data inputs. There are two basic tenets underlying the development of the IAST. The first tenet mandates a homogeneous distribution of adsorbates within the pore landscape. The second tenet requires the surface area occupied by a guest molecule in the mixture to be the same as that for the corresponding pure component. Configurational-bias Monte Carlo (CBMC) simulations are employed in this article to highlight several scenarios in which the IAST fails to provide a quantitatively correct description of mixture adsorption equilibrium due to a failure to conform to either of the two tenets underpinning the IAST. For CO₂ capture with cation-exchanged zeolites and MOFs with open metal sites, there is congregation of CO₂ around the cations and unsaturated metal atoms, resulting in failure of the IAST due to an inhomogeneous distribution of adsorbates in the pore space. Thermodynamic non-idealities also arise due to the preferential location of CO₂ molecules at the window regions of 8-ring zeolites such as DDR and CHA or within pockets of MOR and AFX zeolites. Thermodynamic non-idealities are evidenced for water/alcohol mixtures due to molecular clustering engendered by hydrogen bonding. It is also demonstrated that thermodynamic non-idealities can be strong enough to cause selectivity reversals, which are not anticipated by the IAST.

Robust metal-organic framework with multiple traps for trace Xe/Kr separation

Direct separation of Xe and Kr from air or used nuclear fuel (UNF) off-gas by means of porous adsorbents is of industrial importance but is a very challenging task. In this work, we show a robust metal-organic framework (MOF), namely ECUT-60, which renders not only high chemical stability, but also unique structure with multiple traps. This leads to the ultrahigh Xe adsorption capacity, exceeding most reported porous materials. Impressively, this MOF also enables high selectivity of Xe over Kr, CO₂, O₂, and N₂, leading to the high-performance separation for trace quantitites of Xe/Kr from a simulated UNF reprocessing off-gas. The separation capability has been demonstrated by using dynamic breakthrough experiments, giving the record Xe uptake up to 70.4 mmol/kg and the production of 19.7 mmol/kg pure Xe. Consequently, ECUT-60 has promising potential in direct production of Xe from UNF off-gas or air. The separation mechanism, as unveiled by theoretical calculation, is attributed to the multiple traps in ECUT-60 that affords rigid restrict for Xe atom via van der Waals force.

A novel sodium-fluorescent crystal

In this work, a novel sodium-fluorescent crystal (Na-FS) was synthesized from 4-dimethylaminobenzoic acid and sodium hydroxide by one-pot hydrothermal method. The structure and conformation of Na-FS were confirmed by single-crystal X-ray diffraction and scanning electron microscope, and the optical properties were studied by fluorescence spectrometer. The results showed that: Na-FS was a triclinic crystal, space group was P-1, cell parameters a, b and c were 10.5113(3), 15.9198(5) and 15.9560(5) Å, respectively, and the number of independent atoms Z in a structure cell was two. Additionally, Na-FS has a blue fluorescence emission (around 360 nm under excited at the range of 230-300 nm) with great photostability and photobleaching resistance, and the quantum yield of Na-FS is 30.58%.