Designing Open Metal Sites in Metal-Organic Frameworks for Paraffin/Olefin Separations

Comparative study of metal-organic frameworks synthesized via imide condensation and coordination assembly

A series of metal-organic frameworks (1-XDI) have been synthesized by imide condensation reactions between an amine-functionalized pentanuclear zinc cluster, Zn4Cl5(bt-NH2)6, (bt-NH2 = 5-aminobenzotriazolate), and organic dianhydrides (pyromellitic dianhydride (PMDA), naphthalenetetracarboxylic dianhydride (NDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (HFIPA)). The properties of the 1-XDI MOFs have been compared with analogues (2-XDI) prepared using traditional coordination assembly. The resulting materials have been characterized by ATR-IR spectroscopy, acid-digested 1H NMR spectroscopy, elemental analysis, and gas adsorption measurements. N2 adsorption isotherm data reveal modest porosities and BET surface areas (30-552 m2 g-1). All of the new 1-XDI and 2-XDI MOFs show selective adsorption of C2H2 over CO2 while 2-PMDI and 2-BPDI exhibit high selectivity toward C3H6/C3H8 separation. This study establishes imide condensation of preformed metal-organic clusters with organic linkers as a viable route for MOF design.

Engineered Nanoporous Frameworks for Adsorption Cooling Applications

The energy demand for traditional vapor-compressed technology for space cooling continues to soar year after year due to global warming and the increasing human population's need to improve living and working conditions. Thus, there is a growing demand for eco-friendly technologies that use sustainable or waste energy resources. This review discusses the properties of various refrigerants used for adsorption cooling applications followed by a brief discussion on the thermodynamic cycle. Next, sorbents traditionally used for cooling are reviewed to emphasize the need for advanced capture materials with superior properties to improve refrigerant sorption. The remainder of the review focus on studies using engineered nanoporous frameworks (ENFs) with various refrigerants for adsorption cooling applications. The effects of the various factors that play a role in ENF-refrigerant pair selection, including pore structure/dimension/shape, morphology, open-metal sites, pore chemistry and possible presence of defects, are reviewed. Next, in-depth insights into the sorbent-refrigerant interaction, and pore filling mechanism gained through a combination of characterization techniques and computational modeling are discussed. Finally, we outline the challenges and opportunities related to using ENFs for adsorption cooling applications and provide our views on the future of this technology.

Effect of Co-Adsorbed Guest Adsorbates on the Separation of Ethylene/Ethane Mixtures on Metal-Organic Frameworks with Open Metal Sites

Direct determination of the equilibrium adsorption and spectroscopic observation of adsorbent-adsorbate interaction is crucial to evaluate the olefin/paraffin separation performance of porous adsorbents. However, the experimental characterization of competitive adsorption of various adsorbates at atomic-molecular level in the purification of multicomponent gas mixtures is challenging and rarely conducted. Herein, solid-state NMR spectroscopy is employed to examine the effect of co-adsorbed guest adsorbates on the separation of ethylene/ethane mixtures on Mg-MOF-74, Zn-MOF-74 and UTSA-74. 1H MAS NMR facilitates the determination of equilibrium uptake and adsorption selectivity of ethylene/ethane in ternary mixtures. The co-adsorption of H2O and CO2 significantly leads to the degradation of ethylene uptake and ethylene/ethane selectivity. The detailed host-guest and guest-guest interactions are unraveled by 2D 1H-1H spin diffusion homo-nuclear correlation and static 25Mg NMR experiments. The experimental results verify H2O coordinated on open metal sites can supply a new adsorption site for ethylene and ethane. The effects of guest adsorbates on the adsorption capacity and adsorption selectivity of ethylene/ethane mixtures are in the following order: H2O>CO2>O2. This work provides a direct approach for exploring the equilibrium adsorption and detailed separation mechanism of multicomponent gas mixtures using MOFs adsorbents.

Solvent-Driven Dynamics: Crafting Tailored Transformations of Cu(II)-Based MOFs

Metal-organic frameworks (MOFs), a sort of crystalline porous coordination polymers composed of metal ions and organic linkers, have been intensively studied for their ability to take up nonpolar gas-phase molecules such as ethane and ethylene. In this context, interpenetrated MOFs, where multiple framework nets are entwined, have been considered promising materials for capturing nonpolar molecules due to their relatively higher stability and smaller micropores. This study explores a solvent-assisted reversible strategy to interpenetrate and deinterpenetrate a Cu(II)-based MOF, namely, MOF-143 (noninterpenetrated form) and MOF-14 (doubly interpenetrated forms). Interpenetration was achieved using protic solvents with small molecular sizes such as water, methanol, and ethanol, while deinterpenetration was accomplished with a Lewis-basic solvent, pyridine. Additionally, this study investigates the adsorptive separation of ethane and ethylene, which is a significant application in the chemical industry. The results showed that interpenetrated MOF-14 exhibited higher ethane and ethylene uptakes compared to the noninterpenetrated MOF-143 due to narrower micropores. Furthermore, we demonstrate that pristine MOF-14 displayed higher ethane selectivity than transformed MOF-14 from MOF-143 by identifying the "fraction of micropore volume" as a key factor influencing ethane uptake. These findings highlight the potential of controlled transformations between interpenetrated and noninterpenetrated MOFs, anticipating that larger MOF crystals with narrower micropores and higher crystallinity will be more suitable for selective gas capture and separation applications.

Selective Adsorption of Oxygen from Humid Air in a Metal-Organic Framework with Trigonal Pyramidal Copper(I) Sites

High or enriched-purity O2 is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O2-selective air separations, including the use of metal-organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O2 over N2 via electron transfer. However, most of these materials exhibit appreciable and/or reversible O2 uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework CuI-MFU-4l (CuxZn5-xCl4-x(btdd)3; H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which binds O2 reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal CuI sites, and we show that this material reversibly captures O2 from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, CuI-MFU-4l retains a constant O2 capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N2, differences in O2 and N2 desorption kinetics allow for the isolation of high-purity O2 (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O2 binds to the copper(I) sites to form copper(II)-superoxide moieties that exhibit temperature-dependent side-on and end-on binding modes. Overall, these results suggest that CuI-MFU-4l is a promising material for the separation of O2 from ambient air, even without dehumidification.

Synergistic Nitrogen Binding Sites in a Metal-Organic Framework for Efficient N2 /O2 Separation

Porous materials with d3 electronic configuration open metal sites have been proved to be effective adsorbents for N2 capture and N2 /O2 separation. However, the reported materials remain challenging to address the trade-off between adsorption capacity and selectivity. Herein, we report a robust MOF, MIL-102Cr, that features two binding sites, can synergistically afford strong interactions for N2 capture. The synergistic adsorption site exhibits a benchmark Qst of 45.0 kJ mol-1 for N2 among the Cr-based MOFs, a record-high volumetric N2 uptake (31.38 cm3  cm-3 ), and highest N2 /O2 selectivity (13.11) at 298 K and 1.0 bar. Breakthrough experiments reveal that MIL-102Cr can efficiently capture N2 from a 79/21 N2 /O2 mixture, providing a record 99.99 % pure O2 productivity of 0.75 mmol g-1 . In situ infrared spectroscopy and computational modelling studies revealed that a synergistic adsorption effect by open Cr(III) and fluorine sites was accountable for the strong interactions between the MOF and N2 .

Introducing of Cu(I) in MOFs by In Situ Reduction with Ni as the Catalyst for Efficient Olefin/Paraffin Separation

Olefins can be cracked to provide more low-carbon olefins than paraffins; therefore, separation of olefin/paraffin mixtures is essential for arranging hydrocarbon molecules for directed conversion. In this article, a simple approach for reducing copper atoms in Cu-BTC has been developed to improve olefin/paraffin adsorption capacity and selectivity. Considering that Cu-BTC shows adsorption benefits, its olefin/paraffin adsorption and separation performance were improved further by in situ reduction of Cu(II) to Cu(I) in Cu-BTC using ethanol as the reducing agent and nickel ions as the catalyst. The results revealed that during the reduction process, Cu ion conversion from tetra-ligand to diligand considerably increased their specific surface area, resulting in more active adsorption sites inside the modified sample. The ratio of Cu(I)/Cu(II) in the modified samples varied from 0.57 to 0.96. When Cu(II) of Cu-BTC was reduced to Cu(I), the adsorption capacities of 1-hexene increased from 145.97 to 243.65 mg/g, whereas n-hexane adsorption increased only slightly from 8.18 to 11.43 mg/g, resulting in an acceptable increase in selectivity from 17.84 to 21.32. Cu-BTC, due to its own Cu atoms, minimizes the substantial requirements for the synthesis process as well as the oxygen avoidance conditions for storage when monovalent copper is introduced, compared to other porous materials. Experimental results found that when Cu(I) was introduced, the Lewis acidic sites of the modified Cu-BTC material were increased, and Cu(I) has an electrical structure that makes it susceptible to both accepting and donating too many d electrons, resulting in a stronger adsorption of olefins containing π-electrons to them. Materials Studio simulation revealed that the isosteric heats of modified Cu-BTC increased by 2.7 kJ/mol, indicating that it has a stronger adsorption capacity for olefins.

Lewis acidic Fe3+-driven catalytic active Ni3+ formation in Fe-free metal-organic framework for enhanced electrochemical glucose sensing

Manipulating metal valence states and porosity in the metal-organic framework (MOF) by alloying has been a unique tool for creating high-valent metal sites and pore environments in a structure that are inaccessible by other methods, favorable for accelerating the catalytic activity towards sensing applications. Herein, we report Fe3+-driven formation of catalytic active Ni3+ species in the amine-crafted benzene-dicarboxylate (BDC-NH2)-based MOF as a high-performance electrocatalyst for glucose sensing. This work took the benefit of different bonding stability between BDC-NH2 ligand, and Fe3+ and Ni2+ metal precursor ions in the heterometallic NixFe(1-x)-BDC-NH2 MOF. The FeCl3 that interacts weakly with ligand, oxidizes the Ni2+ precursor to Ni3+-based MOF owing to its Lewis acidic behavior and was subsequently removed from the structure supported by Ni atoms, during solvothermal synthesis. This enables to create mesopores within a highly stable Ni-MOF structure with optimal feed composition of Ni0.7Fe0.3-BDC-NH2. The Ni3+-based Ni0.7Fe0.3-BDC-NH2 demonstrates superior catalytic properties towards glucose sensing with a high sensitivity of 13,435 µA mM-1 cm-2 compared to the parent Ni2+-based Ni-BDC-NH2 (10897 μA mM-1cm-2), along with low detection limit (0.9 μM), short response time (≤5 s), excellent selectivity, and higher stability. This presented approach for fabricating high-valent nickel species, with a controlled quantity of Fe3+ integrated into the structure allowing pore engineering of MOFs, opens new avenues for designing high-performing MOF catalysts with porous framework for sensing applications.

Construction of Negative Electrostatic Pore Environments in a Scalable, Stable and Low-Cost Metal-organic Framework for One-Step Ethylene Purification from Ternary Mixtures

One-step separation of C2 H4 from ternary C2 mixtures by physisorbents remains a challenge to combine excellent separation performance with high stability, low cost, and easy scalability for industrial applications. Herein, we report a strategy of constructing negative electrostatic pore environments in a stable, low-cost, and easily scaled-up aluminum MOF (MOF-303) for efficient one-step C2 H2 /C2 H6 /C2 H4 separation. This material exhibits not only record high C2 H2 and C2 H6 uptakes, but also top-tier C2 H2 /C2 H4 and C2 H6 /C2 H4 selectivities at ambient conditions. Theoretical calculations combined with in situ infrared spectroscopy indicate that multiple N/O sites on pore channels can build a negative electro-environment to provide stronger interactions with C2 H2 and C2 H6 over C2 H4 . Breakthrough experiments confirm its exceptional separation performance for ternary mixtures, affording one of the highest C2 H4 productivity of 1.35 mmol g-1 . This material is highly stable and can be easily synthesized at kilogram-scale from cheap raw materials using a water-based green synthesis. The benchmark combination of excellent separation properties with high stability and low cost in scalable MOF-303 has unlocked its great potential in this challenging industrial separation.

Incorporation of multiple supramolecular binding sites into a robust MOF for benchmark one-step ethylene purification

One-step adsorption separation of C2H4 from ternary C2 hydrocarbon mixtures remains an important and challenging goal for petrochemical industry. Current physisorbents either suffer from unsatisfied separation performance, poor stability, or are difficult to scale up. Herein, we report a strategy of constructing multiple supramolecular binding sites in a robust and scalable MOF (Al-PyDC) for highly efficient one-step C2H4 purification from ternary mixtures. Owing to suitable pore confinement with multiple supramolecular binding sites, Al-PyDC exhibits one of the highest C2H2 and C2H6 uptakes and selectivities over C2H4 at ambient conditions. The gas binding sites have been visualized by single-crystal X-ray diffraction studies, unveiling that the low-polarity pore surfaces with abundant electronegative N/O sites provide stronger multiple supramolecular interactions with C2H2 and C2H6 over C2H4. Breakthrough experiments showed that polymer-grade C2H4 can be separated from ternary mixtures with a maximum productivity of 1.61 mmol g-1. This material can be prepared from two simple reagents using a green synthesis method with water as the sole solvent, and its synthesis can be easily scaled to multikilogram batches. Al-PyDC achieves an effective combination of benchmark separation performance, high stability/recyclability, green synthesis and easy scalability to address major challenges for industrial one-step C2H4 purification.

Impact of Compression on the Textural and Structural Properties of CPO-27(Ni)

The employment of metal-organic frameworks in powder form is undesirable from an industrial perspective due to process and safety issues. This work is devoted to evaluating the impact of compression on the textural and structural properties of CPO-27(Ni). For this purpose, CPO-27(Ni) was synthesized under hydrosolvothermal conditions and characterized. Then, the resulting powder was compressed into binderless pellets using variable compression forces ranging from 5-90 kN (37-678 MPa) and characterized by means of nitrogen adsorption/desorption, thermogravimetric analysis and powder X-ray diffraction to evaluate textural, thermal and structural changes. Both textural and structural properties decreased with increasing compression force. Thermal stability was impacted in pellets compressed at forces over 70 kN. CPO-27(Ni) pelletized at 5, 8 and 10 kN, and retained more than 94% of its initial textural properties, while a loss of about one-third of the textural property was observed for the two most compressed samples (70 and 90 kN) compared to the starting powder.

Thiazole functionalized covalent triazine frameworks for C2H6/C2H4 separation with remarkable ethane uptake

A highly stable thiazole functionalized covalent triazine framework, namely CTF-BT-500, was developed for C2H6/C2H4 separation, which exhibits a record-high ethane uptake (99.7 cm3 g-1) among all reported COFs at 298 K and 1 bar. This work not only presents an excellent C2H6-selective adsorbent, but also provides guidance for the construction of robust adsorbents for value-added gas purification.

Programmed fluorine binding engineering in anion-pillared metal-organic framework for record trace acetylene capture from ethylene

Porous physisorbents are attractive candidates for selective capture of trace gas or volatile compounds due to their low energy footprints. However, many physisorbents suffer from insufficient sorbate-sorbent interactions, resulting in low uptake or inadequate selectivity when gases are present at trace levels. Here, we report a strategy of programmed fluorine binding engineering in anion-pillared metal-organic frameworks to maximize C2H2 binding affinity for benchmark trace C2H2 capture from C2H4. A robust material (ZJU-300a) was elaborately designed to provide multiple-site fluorine binding model, resulting in an ultrastrong C2H2 binding affinity. ZJU-300a exhibits a record-high C2H2 uptake of 3.23 millimoles per gram (at 0.01 bar and 296 kelvin) and one of the highest C2H2/C2H4 selectivity (1672). The adsorption binding of C2H2 and C2H4 was visualized by gas-loaded ZJU-300a structures. The separation capacity was confirmed by breakthrough experiments for 1/99 C2H2/C2H4 mixtures, affording the maximal dynamic selectivity (264) and C2H4 productivity of 436.7 millimoles per gram.

ZIF-62 glass foam self-supported membranes to address CH4/N2 separations

Membranes with ultrahigh permeance and practical selectivity could greatly decrease the cost of difficult industrial gas separations, such as CH₄/N₂ separation. Advanced membranes made from porous materials, such as metal-organic frameworks, can achieve a good gas separation performance, although they are typically formed on support layers or mixed with polymeric matrices, placing limitations on gas permeance. Here an amorphous glass foam, a_gfZIF-62, wherein a, g and f denote amorphous, glass and foam, respectively, was synthesized by a polymer-thermal-decomposition-assisted melting strategy, starting from a crystalline zeolitic imidazolate framework, ZIF-62. The thermal decomposition of incorporated low-molecular-weight polyethyleneimine evolves CO₂, NH₃ and H₂O gases, creating a large number and variety of pores. This greatly increases pore interconnectivity but maintains the crystalline ZIF-62 ultramicropores, allowing ultrahigh gas permeance and good selectivity. A self-supported circular a_gfZIF-62 with a thickness of 200-330 µm and area of 8.55 cm² was used for membrane separation. The membranes perform well, showing a CH₄ permeance of 30,000-50,000 gas permeance units, approximately two orders of magnitude higher than that of other reported membranes, with good CH₄/N₂ selectivity (4-6).

Scalable Green Synthesis of Robust Ultra-Microporous Hofmann Clathrate Material with Record C3 H6 Storage Density for Efficient C3 H6 /C3 H8 Separation

Developing porous materials for C₃ H₆ /C₃ H₈ separation faces the challenge of merging excellent separation performance with high stability and easy scalability of synthesis. Herein, we report a robust Hofmann clathrate material (ZJU-75a), featuring high-density strong binding sites to achieve all the above requirements. ZJU-75a adsorbs large amount of C₃ H₆ with a record high storage density of 0.818 g mL^-1 , and concurrently shows high C₃ H₆ /C₃ H₈ selectivity (54.2) at 296 K and 1 bar. Single-crystal structure analysis unveil that the high-density binding sites in ZJU-75a not only provide much stronger interactions with C₃ H₆ but also enable the dense packing of C₃ H₆ . Breakthrough experiments on gas mixtures afford both high separation factor of 14.7 and large C₃ H₆ uptake (2.79 mmol g^-1 ). This material is highly stable and can be easily produced at kilogram-scale using a green synthesis method, making it as a benchmark material to address major challenges for industrial C₃ H₆ /C₃ H₈ separation.

Stepwise Engineering the Pore Aperture of a Cage-like MOF for the Efficient Separation of Isomeric C4 Paraffins under Humid Conditions

The separation of isomeric C4 paraffins is an important task in the petrochemical industry, while current adsorbents undergo a trade-off relationship between selectivity and adsorption capacity. In this work, the pore aperture of a cage-like Zn-bzc (bzc=pyrazole-4-carboxylic acid) is tuned by the stepwise installation methyl groups on its narrow aperture to achieve both molecular-sieving separation and high n-C₄ H₁₀ uptake. Notably, the resulting Zn-bzc-2CH₃ (bzc-2CH₃ =3,5-dimethylpyrazole-4-carboxylic acid) can sensitively capture n-C₄ H₁₀ and exclude iso-C₄ H₁₀ , affording molecular-sieving for n-C₄ H₁₀ /iso-C₄ H₁₀ separation and high n-C₄ H₁₀ adsorption capacity (54.3 cm³  g^-1 ). Breakthrough tests prove n-C₄ H₁₀ /iso-C₄ H₁₀ can be efficiently separated and high-purity iso-C₄ H₁₀ (99.99 %) can be collected. Importantly, the hydrophobic microenvironment created by the introduced methyl groups greatly improves the stability of Zn-bzc and significantly eliminates the negative effect of water vapor on gas separation under humid conditions, indicating Zn-bzc-2CH₃ is a new benchmark adsorbent for n-C₄ H₁₀ /iso-C₄ H₁₀ separation.

Fabrication of Highly Oriented Ultrathin Zirconium Metal-Organic Framework Membrane from Nanosheets towards Unprecedented Gas Separation

Concurrent regulation of crystallographic orientation and thickness of zirconium metal-organic framework (Zr-MOF) membranes is challenging but promising for their performance enhancement. In this study, we pioneered the fabrication of uniform triangular-shaped, 40 nm thick UiO-66 nanosheet (NS) seeds by employing an anisotropic etching strategy. Through innovating confined counter-diffusion-assisted epitaxial growth, highly (111)-oriented 165 nm-thick UiO-66 membrane was prepared. The significant reduction in thickness and diffusion barrier in the framework endowed the membrane with unprecedented CO₂ permeance (2070 GPU) as well as high CO₂ /N₂ selectivity (35.4), which surpassed the performance limits of state-of-the-art polycrystalline MOF membranes. In addition, highly (111)-oriented 180 nm-thick NH₂ -UiO-66 membrane showing superb H₂ /CO₂ separation performance with H₂ permeance of 1230 GPU and H₂ /CO₂ selectivity of 41.3, was prepared with the above synthetic procedure.

Fermi–Löwdin orbital self-interaction correction of adsorption energies on transition metal ions

Density functional theory (DFT)-based descriptions of the adsorption of small molecules on transition metal ions are prone to self-interaction errors. Here, we show that such errors lead to a large over-estimation of adsorption energies of small molecules on Cu+, Zn+, Zn2+, and Mn+ in local spin density approximation (LSDA) and Perdew, Burke, Ernzerhof (PBE) generalized gradient approximation calculations compared to reference values computed using the coupled-cluster with single, doubles, and perturbative triple excitations method. These errors are significantly reduced by removing self-interaction using the Perdew–Zunger self-interaction correction (PZ-SIC) in the Fermi–Löwdin Orbital (FLO) SIC framework. In the case of FLO-PBE, typical errors are reduced to less than 0.1 eV. Analysis of the results using DFT energies evaluated on self-interaction-corrected densities [DFT(@FLO)] indicates that the density-driven contributions to the FLO-DFT adsorption energy corrections are roughly the same size in DFT = LSDA and PBE, but the total corrections due to removing self-interaction are larger in LSDA.

Bottom-Up Synthesis of 8-Connected Three-Dimensional Covalent Organic Frameworks for Highly Efficient Ethylene/Ethane Separation

Developing cost-/energy-efficient separation techniques for purifying ethylene from an ethylene/ethane mixture is highly important but very challenging in the industrial process. Herein, using a bottom-up [8 + 2] construction approach, we rationally designed and synthesized three three-dimensional covalent organic frameworks (COFs) with 8-connected bcu networks, which can selectively remove ethane from an ethylene/ethane mixture with high efficiency. These COF materials, which are fabricated by the condensation reaction of a customer-designed octatopic aldehyde monomer with linear diamino linkers, possess high crystallinity, good structural robustness, and high porosity. Attributed to the well-organized micro-sized pores with a nonpolar/inert pore environment, these COFs display high ethane adsorption capacity and good selectivity over ethylene, making them among the best ethane-selective adsorbents for ethylene purification. Their excellent ethylene/ethane separation performance is validated by dynamic breakthrough experiments with high-purity ethylene (>99.99%) produced through a single adsorption process. The separation performance surpasses all reported C₂H₆-selective COFs and even some benchmark metal-organic frameworks. This work provides important guidance for the design of new adsorbents for value-added gas purification.

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).

Immobilization of Lewis Basic Sites into a Stable Ethane-Selective MOF Enabling One-Step Separation of Ethylene from a Ternary Mixture

Purification of C₂H₄ from a ternary C₂H₂/C₂H₆/C₂H₄ mixture by one-step adsorption separation is of prime importance but challenging in the petrochemical industry; however, effective strategies to design high-performance adsorbents are lacking. We herein report for the first time the incorporation of Lewis basic sites into a C₂H₆-selective MOF, enabling efficient one-step production of polymer-grade C₂H₄ from ternary mixtures. Introduction of amino groups into highly stable C₂H₆-selective UiO-67 can not only partition large pores into smaller cagelike pockets to provide suitable pore confinement but also offer additional binding sites to simultaneously enhance C₂H₂ and C₂H₆ adsorption capacities over C₂H₄. The amino-functionalized UiO-67-(NH₂)₂ thus exhibits exceptionally high C₂H₂ and C₂H₆ uptakes as well as benchmark C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivities, surpassing all of the C₂H₂/C₂H₆-selective materials reported so far. Theoretical calculations combined with in situ infrared spectroscopy indicate that the synergetic effect of suitable pore confinement and functional surfaces decorated with amino groups provides overall stronger multipoint van der Waals interactions with C₂H₂ and C₂H₆ over C₂H₄. The exceptional performance of UiO-67-(NH₂)₂ was evidenced by breakthrough experiments for C₂H₂/C₂H₆/C₂H₄ mixtures under dry and wet conditions, providing a remarkable C₂H₄ productivity of 0.55 mmol g^-1 at ambient conditions.

Ethylene/ethane separation in a stable hydrogen-bonded organic framework through a gating mechanism

Porous materials are very promising for the development of cost- and energy-efficient separation processes, such as for the purification of ethylene from ethylene/ethane mixture-an important but currently challenging industrial process. Here we report a microporous hydrogen-bonded organic framework that takes up ethylene with very good selectivity over ethane through a gating mechanism. The material consists of tetracyano-bicarbazole building blocks held together through intermolecular CN···H-C hydrogen bonding interactions, and forms as a threefold-interpenetrated framework with pores of suitable size for the selective capture of ethylene. The hydrogen-bonded organic framework exhibits a gating mechanism in which the threshold pressure required for guest uptake varies with the temperature. Ethylene/ethane separation is validated by breakthrough experiments with high purity of ethylene (99.1%) at 333 K. Hydrogen-bonded organic frameworks are usually not robust, yet this material was stable under harsh conditions, including exposure to strong acidity, basicity and a variety of highly polar solvents.

Observation of an Intermediate to H2 Binding in a Metal-Organic Framework

Coordinatively unsaturated metal sites within certain zeolites and metal-organic frameworks can strongly adsorb a wide array of substrates. While many classical examples involve electron-poor metal cations that interact with adsorbates largely through physical interactions, unsaturated electron-rich metal centers housed within porous frameworks can often chemisorb guests amenable to redox activity or covalent bond formation. Despite the promise that materials bearing such sites hold in addressing myriad challenges in gas separations and storage, very few studies have directly interrogated mechanisms of chemisorption at open metal sites within porous frameworks. Here, we show that nondissociative chemisorption of H₂ at the trigonal pyramidal Cu⁺ sites in the metal-organic framework Cu^I-MFU-4l occurs via the intermediacy of a metastable physisorbed precursor species. In situ powder neutron diffraction experiments enable crystallographic characterization of this intermediate, the first time that this has been accomplished for any material. Evidence for a precursor intermediate is also afforded from temperature-programmed desorption and density functional theory calculations. The activation barrier separating the precursor species from the chemisorbed state is shown to correlate with a change in the Cu⁺ coordination environment that enhances π-backbonding with H₂. Ultimately, these findings demonstrate that adsorption at framework metal sites does not always follow a concerted pathway and underscore the importance of probing kinetics in the design of next-generation adsorbents.

Separation and Purification of Hydrocarbons with Porous Materials

This Minireview focuses on the developments of the adsorptive separation of methane/nitrogen, ethene/ethane, propene/propane mixtures as well as on the separation of C8 aromatics (i.e. xylene isomers) with a wide variety of materials, including carbonaceous materials, zeolites, metal-organic frameworks, and porous organic frameworks. Some recent important developments for these adsorptive separations are also highlighted. The advantages and disadvantages of each material category are discussed and guidelines for the design of improved materials are proposed. Furthermore, challenges and future developments of each material type and separation processes are discussed.

Single-Crystal-to-Single-Crystal Transformations of Metal-Organic-Framework-Supported, Site-Isolated Trigonal-Planar Cu(I) Complexes with Labile Ligands

Transition-metal complexes bearing labile ligands can be difficult to isolate and study in solution because of unwanted dinucleation or ligand substitution reactions. Metal-organic frameworks (MOFs) provide a unique matrix that allows site isolation and stabilization of well-defined transition-metal complexes that may be of importance as moieties for gas adsorption or catalysis. Herein we report the development of an in situ anion metathesis strategy that facilitates the postsynthetic modification of Cu(I) complexes appended to a porous, crystalline MOF. By exchange of coordinated chloride for weakly coordinating anions in the presence of carbon monoxide (CO) or ethylene, a series of labile MOF-appended Cu(I) complexes featuring CO or ethylene ligands are prepared and structurally characterized using X-ray crystallography. These complexes have an uncommon trigonal planar geometry because of the absence of coordinating solvents. The porous host framework allows small and moderately sized molecules to access the isolated Cu(I) sites and displace the "place-holder" CO ligand, mirroring the ligand-exchange processes involved in Cu-centered catalysis.

Using Postsynthetic X-Type Ligand Exchange to Enhance CO2 Adsorption in Metal-Organic Frameworks with Kuratowski-Type Building Units

Postsynthetic modification methods have emerged as indispensable tools for tuning the properties and reactivity of metal-organic frameworks (MOFs). In particular, postsynthetic X-type ligand exchange (PXLE) at metal building units has gained increasing attention as a means of immobilizing guest species, modulating the reactivity of framework metal ions, and introducing new functional groups. The reaction of a Zn-OH functionalized analogue of CFA-1 (1-OH, Zn(ZnOH)₄(bibta)₃, where bibta^2- = 5,5'-bibenzotriazolate) with organic substrates containing mildly acidic E-H groups (E = C, O, N) results in the formation of Zn-E species and water as a byproduct. This Brønsted acid-base PXLE reaction is compatible with substrates with pK_a(DMSO) values as high as 30 and offers a rapid and convenient means of introducing new functional groups at Kuratwoski-type metal nodes. Gas adsorption and diffuse reflectance infrared Fourier transform spectroscopy experiments reveal that the anilide-exchanged MOFs 1-NHPh_0.9 and 1-NHPh_2.5 exhibit enhanced low-pressure CO₂ adsorption compared to 1-OH as a result of a Zn-NHPh + CO₂ ⇌ Zn-O₂CNHPh chemisorption mechanism. The MFU-4l analogue 2-NHPh ([Zn₅(OH)_2.1(NHPh)_1.9(btdd)₃], where btdd^2- = bis(1,2,3-triazolo)dibenzodioxin), shows a similar improvement in CO₂ adsorption in comparison to the parent MOF containing only Zn-OH groups.

Benchmark C2 H2 /CO2 Separation in an Ultra-Microporous Metal-Organic Framework via Copper(I)-Alkynyl Chemistry

Separation of acetylene from carbon dioxide remains a daunting challenge because of their very similar molecular sizes and physical properties. We herein report the first example of using copper(I)-alkynyl chemistry within an ultra-microporous MOF (Cu^I @UiO-66-(COOH)₂ ) to achieve ultrahigh C₂ H₂ /CO₂ separation selectivity. The anchored Cu^I ions on the pore surfaces can specifically and strongly interact with C₂ H₂ molecule through copper(I)-alkynyl π-complexation and thus rapidly adsorb large amount of C₂ H₂ at low-pressure region, while effectively reduce CO₂ uptake due to the small pore sizes. This material thus exhibits the record high C₂ H₂ /CO₂ selectivity of 185 at ambient conditions, significantly higher than the previous benchmark ZJU-74a (36.5) and ATC-Cu (53.6). Theoretical calculations reveal that the unique π-complexation between Cu^I and C₂ H₂ mainly contributes to the ultra-strong C₂ H₂ binding affinity and record selectivity. The exceptional separation performance was evidenced by breakthrough experiments for C₂ H₂ /CO₂ gas mixtures. This work suggests a new perspective to functionalizing MOFs with copper(I)-alkynyl chemistry for highly selective separation of C₂ H₂ over CO₂ .

Porous Covalent Organic Polymers for Efficient Fluorocarbon-Based Adsorption Cooling

Adsorption-based cooling is an energy-efficient renewable-energy technology that can be driven using low-grade industrial waste heat and/or solar heat. Here, we report the first exploration of fluorocarbon adsorption using porous covalent organic polymers (COPs) for this cooling application. High fluorocarbon R134a equilibrium capacities and unique overall linear-shaped isotherms are revealed for the materials, namely COP-2 and COP-3. The key role of mesoporous defects on this unusual adsorption behavior was demonstrated by molecular simulations based on atomistic defect-containing models built for both porous COPs. Analysis of simulated R134a adsorption isotherms for various defect-containing atomistic models of the COPs shows a direct correlation between higher fluorocarbon adsorption capacities and increasing pore volumes induced by defects. Combined with their high porosities, excellent reversibility, fast kinetics, and large operating window, these defect-containing porous COPs are promising for adsorption-based cooling applications.

2D Porphyrinic Metal-Organic Frameworks Featuring Rod-Shaped Secondary Building Units

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called 'rod MOFs', which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2L-CuII, where H2 refers to the ligand's carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D MnII- or CoII-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

Indium metal-organic frameworks based on pyridylcarboxylate ligands and their potential applications

Indium metal-organic frameworks (In-MOFs) based on pyridylcarboxylate ligands represent a subclass of MOFs featuring diverse structures, a high stability, and various properties. This review discusses the different aspects of In-MOFs including their design, synthesis and structures as well as their typical potential applications in adsorption and separation, catalysis, and chemical sensors. Importantly, the effect of pyridine on the properties and stability of frameworks has been carefully studied. The introduction of a pyridine group not only significantly enriches clusters of In3+ ions, but also enables flexible, controllably synthesized ionic or neutral frameworks to be fabricated. Based on this, we suggest that this type of In-metal organic framework (MOF) should receive more attention in the field of MOF design.

Chemically Stable Hafnium-Based Metal-Organic Framework for Highly Efficient C2H6/C2H4 Separation under Humid Conditions

Realization of ethane-selective porous materials for efficient ethane/ethylene (C₂H₆/C₂H₄) separation is an important task in the petrochemical industry. Although a number of C₂H₆-selective adsorbents have been realized, their adsorption capacity and selectivity might be mostly dampened under humid conditions due to structure decomposition or co-adsorption of water vapor. A desired material should have simultaneously high C₂H₆ uptake and selectivity, excellent water stability, and ultralow water adsorption uptake for industrial applications, but such a material is elusive. Herein, we report a chemically stable hafnium-based material (Hf)DUT-52a, featuring the suitable pore apertures and less hydrophilicity for highly efficient C₂H₆/C₂H₄ separation under humid conditions. Gas sorption results reveal that (Hf)DUT-52a exhibits both high ethane adsorption capacity (4.02 mmol g^-1) and C₂H₆/C₂H₄ selectivity (1.9) at 296 K and 1 bar, which are comparable to the majority of the top-performing materials. Most importantly, the less pore hydrophilicity enables (Hf)DUT-52a to exhibit a negligible water uptake of 0.036 g g^-1 before 40% relative humidity (RH), effectively minimizing the impact of humidity on separation capacity. This material thus shows excellent separation capacity even under 40% RH with a high polymer-grade ethylene production capacity up to 8.43 L kg^-1 at ambient conditions, as evidenced by the breakthrough experiments.