Balancing the Kinetic and Thermodynamic Synergetic Effect of Doped Carbon Molecular Sieves for Selective Separation of C2H4/C2H6

Selective separation of ethylene and ethane (C2H4/C2H6) is a formidable challenge due to their close molecular size and boiling point. Compared to industry-used cryogenic distillation, adsorption separation would offer a more energy-efficient solution when an efficient adsorbent is available. Herein, a class of C2H4/C2H6 separation adsorbents, doped carbon molecular sieves (d-CMSs) is reported which are prepared from the polymerization and subsequent carbonization of resorcinol, m-phenylenediamine, and formaldehyde in ethanol solution. The study demonstrated that the polymer precursor themselves can be a versatile platform for modifying the pore structure and surface functional groups of their derived d-CMSs. The high proportion of pores centered at 3.5 Å in d-CMSs contributes significantly to achieving a superior kinetic selectivity of 205 for C2H4/C2H6 separation. The generated pyrrolic-N and pyridinic-N functional sites in d-CMSs contribute to a remarkable elevation of Henry selectivity to 135 due to the enhancement of the surface polarity in d-CMSs. By balancing the synergistic effects of kinetics and thermodynamics, d-CMSs achieve efficient separation of C2H4/C2H6. Polymer-grade C2H4 of 99.71% purity can be achieved with 75% recovery using the devised d-CMSs as reflected in a two-bed vacuum swing adsorption simulation.

Thermally Tunable Adsorption of Xenon in Crystalline Molecular Sorbent

The thermostability of encapsulated xenon is investigated in a series of isostructural crystalline sorbents. These sorbents consist of metal-organic capsules, with the general formula of [ConFe4-nL6]4- (n = 1, 2, 3 and 4), where L2- is an organic linker with two sulfonate groups. In the crystalline sorbent, guanidinium cations form H-bond networks with the peripheral sulfonate groups in the solid state and trap xenon in the molecular cavities, which are at least 2.7 times the volume of xenon. When heated, the sorbent retains xenon up to 561 K, i.e., 396 K higher than the boiling point of xenon. Furthermore, the thermostability of trapped xenon can be modulated by varying the ratio of Co:Fe in the crystalline sorbent. Elemental analysis on a single crystal by energy dispersive X-ray spectroscopy confirms the homogeneous distribution of Co and Fe in the sorbent. Structural analyses reveal that the expansion of capsule cavity is proportional to the Co:Fe ratio, with increases of 0.049(1) Å and 6.4(8) Å3 in metal-metal distance and cavity volume, per substitution of Fe by Co center. Steric repulsion between peripheral sulfonate groups is found to render a hypothetical face-centered cubic structure of (C(NH2)3)4[Fe4L6] not accessible, which would have trapped xenon with exceptional thermostability. The stable and tunable trapping of xenon in crystalline sorbents by over-sized molecular cavities suggests a new strategy for separation and storage of xenon, through introduction of kinetic barriers, such as H-bond networks.

4,6-Diamino-2-thiopyrimidine-based Cobalt Metal Organic Framework (Co-DAT-MOF): green, efficient, novel and reusable nanocatalyst for synthesis of multicomponent reactions

In this study, Co-DAT-MOF powder was prepared via the solvothermal method using 4, 6-diamino-2-thiopyrimidine as the organic linker and Co(NO₃)₂·6H₂O. The synthesized catalysts are characterized using XRD, FT-IR, TGA, SEM, BET, NH₃-TPD, and ICP-OES techniques. SEM analysis clearly indicated the formation of nanosheet microspheres. NH₃-TPD-MS was employed as a means of identifying the various strengths of acid sites and their relative abundance in an attempt to explain the effect of the catalyst surface acid sites. We identified a new acidic feature in Co-DAT-MOF catalyst, related to the presence of desorption peaks in the NH₃-TPD profiles. The activity of Co-DAT-MOF catalyst for the synthesis of multicomponent reactions correlates with lewis acidity. In addition, Co-DAT-MOF exhibited excellent performance for the synthesis of pyrroloacridine-1(2H)-one and chromeno [2, 3- d] pyrimidin-8-amines, as well as good reusability and recyclability.

Nickel-Based Metal-Organic Frameworks for Coal-Bed Methane Purification with Record CH4 /N2 Selectivity

The enrichment and purification of coal-bed methane provides a source of energy and helps offset global warming. In this work, we demonstrate a strategy involving the regulation of the pore size and pore chemistry to promote the separation of CH₄ /N₂ mixtures in four nickel-based coordination networks, named Ni(ina)₂ , Ni(3-ain)₂ , Ni(2-ain)₂ , and Ni(pba)₂ , (where ina=isonicotinic acid, 3-ain=3-aminoisonicotinic acid, 2-ain=2-aminoisonicotinic acid, and pba=4-(4-pyridyl)benzoic acid). Among them, Ni(ina)₂ and Ni(3-ain)₂ can effectively separate CH₄ from N₂ with top-performing performance because of the suitable pore size (≈0.6 and 0.5 nm) and pore environment. Explicitly, Ni(ina)₂ exhibits the highest ever reported CH₄ /N₂ selectivity of 15.8 and excellent CH₄ uptake (40.8 cm³  g^-1 ) at ambient conditions, thus setting new benchmarks for all reported MOFs and traditional adsorbents. The exceptional CH₄ /N₂ separation performance of Ni(ina)₂ is confirmed by dynamic breakthrough experiments. Under different CH₄ /N₂ ratios, Ni(ina)₂ selectively extracts methane from the gaseous blend and produces a high purity of CH₄ (99 %). Theoretical calculations and CH₄ -loading single-crystal structure analysis provide critical insight into the adsorption/separation mechanism. Ni(ina)₂ and Ni(3-ain)₂ can form rich intermolecular interactions with methane, indicating a strong adsorption affinity between pore walls and CH₄ molecules. Importantly, Ni(ina)₂ has good thermal and moisture stability and can easily be scaled up at a low cost ($25 per kilogram), which will be valuable for potential industrial applications. Overall, this work provides a powerful approach for the selective adsorption of CH₄ from coal-bed methane.

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.

Beyond structural motifs: the frontier of actinide-containing metal-organic frameworks

In this perspective, we feature recent advances in the field of actinide-containing metal-organic frameworks (An-MOFs) with a main focus on their electronic, catalytic, photophysical, and sorption properties. This discussion deviates from a strictly crystallographic analysis of An-MOFs, reported in several reviews, or synthesis of novel structural motifs, and instead delves into the remarkable potential of An-MOFs for evolving the nuclear waste administration sector. Currently, the An-MOF field is dominated by thorium- and uranium-containing structures, with only a few reports on transuranic frameworks. However, some of the reported properties in the field of An-MOFs foreshadow potential implementation of these materials and are the main focus of this report. Thus, this perspective intends to provide a glimpse into the challenges, triumphs, and future directions of An-MOFs in sectors ranging from the traditional realm of gas sorption and separation to recently emerging areas such as electronics and photophysics.

Preparation of Ni-microsphere and Cu-MOF using aspartic acid as coordinating ligand and study of their catalytic properties in Stille and sulfoxidation reactions

In this study, the thermal and catalytic behavior of Ni-microsphere and Cu-MOF were investigated with aspartic acid as the coordinating ligand with different morphologies. The Ni-microsphere and Cu-MOF with aspartic acid, as the coordinating ligand, were prepared via a solvothermal method. The morphology and porosity of the obtained Ni microsphere and Cu-MOF were characterized by XRD, FTIR, TGA, DSC, BET and SEM techniques. The catalytic activity of the Ni-microsphere and Cu-MOF was examined in Stille and sulfoxidation reactions. The Ni microsphere and Cu-MOF were easily isolated from the reaction mixtures by simple filtration and then recycled four times without any reduction of catalytic efficiency.

A turn-on fluorescence sensing strategy for rapid detection of flumequine in water environments using covalent-coordination functionalized MOFs

With high output and large use of antibiotics in the process of aquaculture, pollution caused by antibiotics in water environments is becoming a thorny problem, and its ecological risk has aroused widespread concern.

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.

Paramagnetic Shifts and Guest Exchange Kinetics in ConFe4-n Metal-Organic Capsules

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [Co_nFe_4-nL₆]^4- (n = 1-3), where L^2- = 4,4'-bis[(2-pyridinylmethylene)amino]-[1,1'-biphenyl]-2,2'-disulfonate. H bond networks between capsule sulfonates and guanidinium cations promote the crystallization of [Co_nFe_4-nL₆]^4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH₂)₃)₄[Co_1.8Fe_2.2L₆]·69H₂O (1) and (C(NH₂)₃)₄[Co_2.7Fe_1.3L₆]·73H₂O (2), and two Xe- and CFCl₃-encapsulated forms, (C(NH₂)₃)₄[(Xe)_0.8Co_1.8Fe_2.2L₆]·69H₂O (3) and (C(NH₂)₃)₄[(CFCl₃)Co_2.0Fe_2.0L₆]·73H₂O (4), respectively. Structural analyses reveal that Xe induces negligible structural changes in 3, while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl₃, in 4. These guest-encapsulated [Co_nFe_4-nL₆]^4- molecules reveal ¹²⁹Xe and ¹⁹F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin Fe^II by high-spin Co^II. Likewise, the temperature dependence of the ¹²⁹Xe and ¹⁹F NMR resonances increases by 0.1 and 0.06 ppm/K, respectively, with each additional paramagnetic Co^II center. The optimal temperature for hyperpolarized (hp) ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST) with [Co_nFe_4-nL₆]^4- capsules was found to be inversely proportional to the number of Co^II centers, n, which is consistent with the Xe chemical exchange accelerating as the portals expand. The systematic study was facilitated by the tunability of the [M₄L₆]^4- capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted ¹²⁹Xe resonances.

Bottom-Up Formation of Carbon-Based Magnetic Honeycomb Material from Metal-Organic Framework-Guest Polyhedra for the Capture of Rhodamine B

Three-dimensional carbon-based porous materials have proven to be quite useful for tailoring material properties in the energy conservation and environmental protection applications. In view of the three-dimensional and well-defined structure of metal-organic frameworks (MOFs), a novel carbon-based magnetic porous material (HKUST-Fe₃O₄) has been designed and constructed by MOF-guest interactions of high-temperature pyrolysis. The obtained HKUST-Fe₃O₄ exhibited the unique features of superparamagnetism, a macro/mesoporous structure, environmental protection (inexistence of toxic heavy metal ions), and physicochemical stability and has shown high adsorption capacity and rapid adsorption for carcinogenic organic pollutants (for example, rhodamine B) with an environmentally friendly character and excellent reusability. We demonstrate that the unique/superior advantages of HKUST-Fe₃O₄ could meet the requirements of environment cleaning, especially for removing the targeted organic pollutant from water. Moreover, the specific HKUST-Fe₃O₄ and organic pollutant interaction mechanism has been analyzed in detail via parameter-free calculations. This study proposes a promising strategy for constructing novel carbon-based magnetic nanomaterials for various applications, not limitated to pollutant removal.

Screening metal-organic frameworks for capturing radioactive gas Rn in indoor air

The radioactive gas Rn in indoor air shows severe harm to the human body and even induces the occurence of lung cancer. Finding an excellent candidate that can efficiently and selectively adsorb Rn, is still a great challenge. In this work, we use the grand canonical ensemble Monte Carlo simulation to systematically investigate the selectivities of 23 different kinds of representative metal organic framework (MOFs) for Rn separation from the Rn/N₂ and Rn/O₂ mixtures at 298 K. Results indicate that ZIF-12, HKUST-1, IRMOF-62 and ZIF-11 are four kinds of excellent candidates for capturing Rn. In particular, the selectivities of ZIF-12 for Rn/N₂ and Rn/O₂ mixtures with Rn molar fraction X_Rn = 0.001 reach ∼2500 and 1200 at 1 atm and 298 K, respectively. Moreover, when the Rn concentration becomes smaller, the selectivity of ZIF-12 for Rn would increase significantly. In short, we found that ZIF-12 is indeed a very promising candidate for trapping radioactive Rn in indoor air.

Fast screening of porous materials for noble gas adsorption and separation: a classical density functional approach

The design and screening of porous materials for noble gas adsorption and separation are an important issue in the production and utilization of gases. The conventional method to do this is via molecular simulation. In this work, we introduced a classical density functional theory (CDFT) to replace molecular simulation because CDFT is more efficient. A molecular dynamics (MD)/CDFT combined method was proposed to consider the flexibility of the adsorbent. The theory was first examined by comparing it to reported experiments and simulations. Then, the theory was applied to determine the most favorable adsorbents for noble gas adsorption/separation from 4764 real adsorbents and 1200 hypothetical adsorbents. A series of favorable adsorbents was identified, and some of them seemed promising. The macroscopic adsorption isotherms and microscopic density profiles of the most favorable adsorbents were examined, and the adsorption mechanisms were revealed. The specific separation of Kr/Xe was examined, and two of the adsorbents showed higher adsorption efficiency than shown in previously reported data.

Adsorption and Detection of Hazardous Trace Gases by Metal-Organic Frameworks

The quest for advanced designer adsorbents for air filtration and monitoring hazardous trace gases has recently been more and more driven by the need to ensure clean air in indoor, outdoor, and industrial environments. How to increase safety with regard to personal protection in the event of hazardous gas exposure is a critical question for an ever-growing population spending most of their lifetime indoors, but is also crucial for the chemical industry in order to protect future generations of employees from potential hazards. Metal-organic frameworks (MOFs) are already quite advanced and promising in terms of capacity and specific affinity to overcome limitations of current adsorbent materials for trace and toxic gas adsorption. Due to their advantageous features (e.g., high specific surface area, catalytic activity, tailorable pore sizes, structural diversity, and range of chemical and physical properties), MOFs offer a high potential as adsorbents for air filtration and monitoring of hazardous trace gases. Three advanced topics are considered here, in applying MOFs for selective adsorption: (i) toxic gas adsorption toward filtration for respiratory protection as well as indoor and cabin air, (ii) enrichment of hazardous gases using MOFs, and (iii) MOFs as sensors for toxic trace gases and explosives.

Recovery of xenon from air over ZIF-8 membranes

ZIF-8 membranes effectively separated air/Xe gas mixtures via molecular sieving, preferential adsorption, and diffusivity differences.

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.

Ab initio molecular dynamics determination of competitive O₂ vs. N₂ adsorption at open metal sites of M₂(dobdc)

The separation of oxygen from nitrogen using metal-organic frameworks (MOFs) is of great interest for potential pressure-swing adsorption processes for the generation of purified O2 on industrial scales. This study uses ab initio molecular dynamics (AIMD) simulations to examine for the first time the pure-gas and competitive gas adsorption of O2 and N2 in the M2(dobdc) (M = Cr, Mn, Fe) MOF series with coordinatively unsaturated metal centers. Effects of metal, temperature, and gas composition are explored. This unique application of AIMD allows us to study in detail the adsorption/desorption processes and to visualize the process of multiple guests competitively binding to coordinatively unsaturated metal sites of a MOF.

Enhanced xenon adsorption and separation with an anionic indium–organic framework by ion exchange with Co2+

Ion-exchanged Co²⁺-CPM-6 exhibits a distinctly higher Xe/Kr separating ability than organic cation analogues, suggesting a promising candidate material for Xe/Kr separation.

Adsorptive separation of xenon/krypton mixtures using a zirconium-based metal-organic framework with high hydrothermal and radioactive stabilities

The separation of xenon/krypton mixtures is important for both environmental and industrial purposes. The potential of three hydrothermally stable MOFs (MIL-100(Fe), MIL-101(Cr), and UiO-66(Zr)) for use in Xe/Kr separation has been experimentally investigated. From the observed single-component Xe and Kr isotherms, isosteric heat of adsorption (Q_st^o), and IAST-predicted Xe/Kr selectivities, we observed that UiO-66(Zr) has the most potential as an adsorbent among the three candidate MOFs. We performed dynamic breakthrough experiments with an adsorption bed filled with UiO-66(Zr) to evaluate further the potential of UiO-66(Zr) for Xe/Kr separation under mixture flow conditions. Remarkably, the experimental breakthrough curves show that UiO-66(Zr) can efficiently separate the Xe/Kr mixture. Furthermore, UiO-66(Zr) maintains most of its Xe and Kr uptake capacity, as well as its crystallinity and internal surface area, even after exposure to gamma radiation (2kGy) for 7h and aging for 16 months under ambient conditions. This result indicates that UiO-66(Zr) can be considered to be a potential adsorbent for Xe/Kr mixtures under both ambient and radioactive conditions.

ZIF-Derived Nitrogen-Doped Porous Carbons for Xe Adsorption and Separation

Currently, finding high capacity adsorbents with large selectivity to capture Xe is still a great challenge. In this work, nitrogen-doped porous carbons were prepared by programmable temperature carbonization of zeolitic imidazolate framework-8 (ZIF-8) and ZIF-8/xylitol composite precursors and the resultant samples are marked as Carbon-Z and Carbon-ZX, respectively. Further adsorption measurements indicate that ZIF-derived nitrogen-doped Carbon-ZX exhibits extremely high Xe capacity of 4.42 mmol g⁻¹ at 298 K and 1 bar, which is higher than almost all other pristine MOFs such as CuBTC, Ni/DOBDC, MOF-5 and Al-MIL-53, and even more than three times of the matrix ZIF-8 at similar conditions. Moreover, Carbon-ZX also shows the highest Xe/N₂ selectivity about ~120, which is much larger than all other reported MOFs. These remarkable features illustrate that ZIF-derived nitrogen-doped porous carbon is an excellent adsorbent for Xe adsorption and separation at room temperature.

A general forcefield for accurate phonon properties of metal–organic frameworks

We report the development of a forcefield capable of reproducing accurate lattice dynamics of metal–organic frameworks.

Tuning the adsorption and separation properties of noble gases and N2 in CuBTC by ligand functionalization

The Grand Canonical Monte Carlo method was used to investigate adsorption and separation properties of noble gas and N₂ mixtures on CuBTCs, functionalized with different groups, including amino, hydroxyl and fluorine groups.

Formation of a quasi-solid structure by intercalated noble gas atoms in pores of CuI-MFU-4l metal–organic framework

Ten crystallographically different positions for Xe and eight positions for Kr form a quasi-solid structures within the large-pore metal–organic framework Cu^I-MFU-4l.

Merging open metal sites and Lewis basic sites in a NbO-type metal–organic framework for improved C2H2/CH4 and CO2/CH4 separation

A new NbO-type metal–organic framework ZJNU-47a incorporating Lewis acidic copper sites and Lewis basic nitrogen donor sites exhibits better performance than the isostructural NOTT-101a in the separation of C₂H₂/CH₄ and CO₂/CH₄ gas mixtures at room temperature.

Separation of rare gases and chiral molecules by selective binding in porous organic cages

The separation of molecules with similar size and shape is an important technological challenge. For example, rare gases can pose either an economic opportunity or an environmental hazard and there is a need to separate these spherical molecules selectively at low concentrations in air. Likewise, chiral molecules are important building blocks for pharmaceuticals, but chiral enantiomers, by definition, have identical size and shape, and their separation can be challenging. Here we show that a porous organic cage molecule has unprecedented performance in the solid state for the separation of rare gases, such as krypton and xenon. The selectivity arises from a precise size match between the rare gas and the organic cage cavity, as predicted by molecular simulations. Breakthrough experiments demonstrate real practical potential for the separation of krypton, xenon and radon from air at concentrations of only a few parts per million. We also demonstrate selective binding of chiral organic molecules such as 1-phenylethanol, suggesting applications in enantioselective separation.