Tunable Confined Aliphatic Pore Environment in Robust Metal-Organic Frameworks for Efficient Separation of Gases with a Similar Structure

Net-Coded Organic Building Blocks for the Reticular Assembly of High-Connectivity Metal-Organic Frameworks

High-connectivity metal-organic frameworks (MOFs) are of great importance in reticular chemistry because of the designed synthesis by judicious selection of organic and inorganic building blocks. However, it remains a challenge to synthesize well-defined polytopic organic linkers with a connectivity ≥ 12. Here we report the synthesis of (4,12)-connected shp- and (6,12)-connected alb-MOFs─denoted as Cu-HDTB-shp, In-HDTB-shp, Fe-HDTB-alb, and Zr-HDTB-alb─by assembly of a 12-connected dodecacarboxylate ligand and preselected inorganic nodes. This dodecacarboxylate ligand, a feasible net-coded organic building block encoding geometric information on the targeted high-connectivity net, allows the rational synthesis of MOFs based on the shp- and alb-derived nets, including rare node-morphing ceh and key nets. Fe-HDTB-alb and Zr-HDTB-alb, on account of the right pore structures, can potentially be applied to natural gas upgrading. This net-coded strategy presents a significant step toward the rational synthesis of high-connectivity framework structures with molecular-level precision.

Pore space partition on a sulfonate-rich metal-organic framework for purification of methane from natural gas

By implementing the pore space partition (PSP) strategy on a sulfonate-rich acs-type metal-organic framework (MOF), we developed a new pacs structure, which features segmented pores with abundant O/N interacting sites. This structure selectively uptakes C2H6 and C3H8 from the ternary mixture of C2H6/C3H8/CH4 with high selectivity and capacity, enabling effective purification of CH4 from natural gas through a single adsorption process.

Heterometallic Aluminum Metal-Organic Frameworks

From spinel gemstone (MgAl2O4) to layered double hydroxides, nature has long relied on combinations between charge-complementary metal ions such as divalent metal ions (M2+) and Al3+ to create diverse valuable materials. However, for metal-organic frameworks (MOFs), heterometallic combinations such as Mg-Al are conspicuously absent. Here, we report a breakthrough in the synthesis of heterometallic Al-MOFs containing M2+/Al3+ trimeric clusters (M = Mg, Mn, Co, Ni). The synergistic effect between M(II) chlorides and aluminum lactate plays a critical role in the cooperative crystallization of M2+ and Al3+ into pore-space-partitioned MOFs (partitioned acs topology) with fast crystallization kinetics (about 3 h). New M2+/Al3+ MOFs exhibit highly tunable porosity and extraordinarily high uptakes for CO2 and small hydrocarbon molecules (112 cm3/g for CO2, 176 cm3/g for C2H2, 156 cm3/g for C2H4, and 163 cm3/g for C2H6) at 298 K and 1 bar. The high uptake capacity coupled with high selectivity (up to 8.5 for C2H2/CO2, 10.8 for C2H2/C2H4) gives rise to efficient separations of either C2H2/CO2 or C2H2/C2H4 gas mixtures, as confirmed by experimental breakthrough experiments.

C2H2/CO2 Separation by a Carborane Hybrid 2D Metal-Organic Framework

The separation of acetylene (C2H2) from carbon dioxide (CO2) is important in industry but challenging due to their similar physical properties. Herein, a boron-rich 2D metal-organic framework ZNU-14 based on the carborane backbone was readily prepared by the supramolecular assembly of Zn2+, p-C2B10H10-(COOH)2, and di(pyridin-4-yl) amine under mild conditions for C2H2/CO2 separation. ZNU-14 displays a straight 1D channel (7.6 × 12.5 Å2) with an electronegative pore surface. Gas adsorption isotherms show that ZNU-14 has a good C2H2 adsorption capacity of 43.6 cm3 g-1, 181% of the CO2 uptake capacity. The calculated ideal adsorbed solution theory (IAST) selectivity is as high as 6.3-9.7, outperforming many popular materials. The moderate C2H2 adsorption heat of 34.3 kJ mol-1 facilitates the straightforward desorption and regeneration of ZNU-14. Furthermore, the theoretical study confirmed the stronger binding of C2H2 compared to that of CO2. The practical C2H2/CO2 separation performance was fully demonstrated by breakthrough experiments with excellent dynamic selectivity and recyclability under various conditions.

Ionic porous materials: from synthetic strategies to applications in gas separation and catalysis

Ionic porous materials possess a unique combination of tunable pore sizes and task-specific interactions between guest molecules and the charged frameworks, which endow them with versatility across diverse domains in chemistry and materials science. Significant advancements in their applications for gas separation and catalysis have been achieved in recent years due to the incorporation of ionic functionalities and ultra-microporous structures that enable molecular-scale recognition of guest molecules. This review summarizes recent advancements in the synthetic strategies of ionic porous materials, establishing design guidelines for the incorporation of ionic moieties into the backbone to fine-tune pore sizes and chemistry. It highlights the synergistic interplay of task-specific interactions with custom-designed pore structures in key applications, including adsorption separation, membrane separation, and gas conversion. Additionally, it examines structure-property relationships, offering deeper insights into enhancing performance. The report also addresses the current challenges in the practical application of these materials. Finally, the review provides future perspectives on ionic porous materials from both scientific and industrial viewpoints. Overall, this review aims to provide insights into pore structure and chemistry, supporting the precise placement of ionic functionalities.

Metal-Organic Framework with Constrained Flexibility for Benchmark Separation of Hexane Isomers

Flexible metal-organic frameworks (MOFs) are promising candidates for adsorptive separations, but achieving a balance among flexibility, adsorption capacity, and selectivity remains challenging. Herein, we report a novel flexible MOF, Ni(bhdc)(ted)0.5 (ZUL-C6), incorporating hybrid three-dimensional alkane-bridged ligands, which realizes high-capacity molecular sieving for hexane isomer separation - a critical process in the petroleum industry. The alkyl-rich, confined pore system within the ZUL-C6 framework facilitated a strong affinity for n-hexane and 3-methylpentane. However, the narrow pore size and the constrained flexibility limited the uptake of 2,2-dimethylbutane (<4.0 mg/g), accompanied by a high gate-opening pressure. The gating behavior was elucidated by guest-loaded single-crystal (SC) X-ray diffraction and density functional theory (DFT) simulations, which revealed a unique SC to SC transformation driven by the non-centrosymmetric rotation of the 3D bhdc linker and distortion of the metal cluster and pillar units, along with a high deformation energy barrier. As a result, ZUL-C6 exhibited not only significantly higher uptake and selectivity than the industrially used 5 A molecular sieve, but also the record-high nHEX/3MP breakthrough uptake (92.8/73.9 mg/g) and unprecedented 22DMB producing time (309.2 min/g, corresponding to the productivity of 770 mmol/kg and yield of 92.8 %) among reported MOFs.

A Microporous Hydrogen-Bonded Organic Framework with Open Pyrene Sites Isolated by Hydrogen-Bonded Helical Chains for Efficient Separation of Xenon and Krypton

Achieving efficient xenon/krypton (Xe/Kr) separation in emerging hydrogen-bonded organic frameworks (HOFs) is highly challenging because of the lack of gas-binding sites on their pore surfaces. Herein, we report the first microporous HOF (HOF-FJU-168) based on hydrogen-bonded helical chains, which prevent self-aggregation of the pyrene core, thereby preserving open pyrene sites on the pore surfaces. Its activated form, HOF-FJU-168a is capable of separating Xe/Kr under ambient conditions while achieving an excellent balance between adsorption capacity and selectivity. At 296 K and 1 bar, the Xe adsorption capacity of HOF-FJU-168a reached 78.31 cm3/g, with an Xe/Kr IAST selectivity of 22.0; both values surpass those of currently known top-performing HOFs. Breakthrough experiments confirmed its superior separation performance with a separation factor of 8.6 and a yield of high-purity Kr (>99.5 %) of 184 mL/g. Furthermore HOF-FJU-168 exhibits excellent thermal and chemical stability, as well as renewability. Single-crystal X-ray diffraction and molecular modeling revealed that the unique electrostatic surface potential around the open pyrene sites creates a micro-electric field, exerting a stronger polarizing effect on Xe than on Kr, thereby enhancing host-Xe interactions.

Superhydrophobic and Self-Healing Porous Organic Macrocycle Crystals for Methane Purification under Humid Conditions

Purifying methane from natural gas using adsorbents not only requires the adsorbents to possess excellent separation performance but also to overcome additional daunting challenges such as humidity interference and durability requirements for sustainable use. Herein, porous organic crystals of a new macrocycle (CaC9) with superhydrophobic and self-healing features are prepared and employed for the purification of methane (>99.99% purity) from ternary methane/ethane/propane mixtures under 97% relative humidity. The high selectivity for methane and water-resistance are attributed to the unique chemical structure of CaC9, possessing an intrinsic 4.2 Å pore along with a pore environment modified with saturated alkyl chains. Besides, CaC9 crystals exhibit a self-healing capacity to realize in situ reconstruction of porosity within 15 min. The transformation of CaC9 crystals from a nonporous state to a porous state can be easily achieved upon treatment with n-hexane vapor, thereby presenting a novel solution to enhance the sustainable separation processes of porous materials. This work introduces a novel molecular-level porous adsorbent for natural gas separation, providing a valuable impetus for designing novel adsorbents with unexpected functions.

Methane Storage and Purification of Natural Gas in Metal-Organic Frameworks

Natural gas, primarily composed of methane (CH4), represent an excellent choice for a potentially sustainable renewable energy transition. However, the process of compressing and liquefying CH4 for transport and storage typically results in significant energy losses. In addition, in order to optimize its efficacy as a fuel, the CH4 content of natural gas needs to be increased to a level of at least 97 % to ensure its quality and efficiency in various applications. Metal-organic frameworks (MOFs) represent a novel category of porous materials that possess exceptional capability in modifying pore size and chemical environment, making them ideally suited for the storage of CH4 and the adsorption of propane (C3H8), ethane (C2H6), carbon dioxide (CO2), nitrogen (N2), and hydrogen sulfide (H2S) to facilitate the purification process of CH4 from natural gas. In this paper, we systematically summarize the mechanism by which MOF materials facilitate the storage of CH4 and the purification of CH4 from natural gas, leveraging the structural characteristics inherent to MOF materials. The focus of further research should also be directed towards the investigation of CH4 storage by flexible MOFs, the resolution of the trade-off dilemma, and the commercial application of MOFs.

Efficient continuous SF6/N2 separation using low-cost and robust metal-organic frameworks composites

Physisorption presents a promising alternative to cryogenic distillation for capturing the most potent greenhouse gas, SF6, but existing adsorbents face challenges in meeting diverse chemical and engineering concerns. Herein, with insights into in-pore chemistry and industrial process design, we report a systematic investigation that constructed two low-cost composites pellets (Al(fum)@2%HPC and Al(fum)@5%Kaolin) coupled with an innovative two-stage Vacuum Temperature Swing Adsorption (VTSA) process for the ultra-efficient recovery of low-concentration SF6 from N2. Record-high selectivities (> 2×104) and SF6 dynamic capacities (~ 2.7 mmol/g) were achieved, while exceptional SF6 productivities (~ 58.7 L/kg), yields (~ 96.8%), and recyclability (~ 1000 cycles) were demonstrated in fixed-bed adsorption-desorption experiments under mild regeneration conditions. 2D solid-state NMR/in-situ FTIR, DFT-D binding/diffusion simulation analyses revealed the multi-site binding mode and the ultra-fast diffusion of SF6 within the channels. The proposed VTSA processes successfully met the dual stringent requirements of both environmental protection and electricity equipment operation: the SF6 recovery of 99.91% accompanied with a SF6 purity/working capacity of 99.91%/2.1 mmol/g, which significantly outperformed the industrial employed adsorbent zeolite 13X and showed only 18.7% the energy consumption of the cryogenic distillation.

Metal-Organic Framework with Polar Pore Surface Designed for Purification of Both Natural Gas and Ethylene

The advancement of high-value CH4 purification technology within the natural gas industry is paramount for industrial processes. Herein, we constructed ZJNU-402, a new porous material characterized by permanent porosity, as an effective adsorbent for separating C3H8/CH4 and C2H6/CH4 mixtures. The findings reveal an outstanding C3H8 adsorption capacity of 68 cm3 g-1 and a moderate C2H6 adsorption rate of 42 cm3 g-1, with a notably lower CH4 adsorption rate of 11 cm3 g-1. Noteworthy is the exceptional selectivity of ZJNU-402 for C3H8/CH4 and C2H6/CH4, standing at 375 and 31, respectively, surpassing many previously documented high-performance adsorbents and breaking the traditional trade-off between adsorption capacity and separation selectivity. Adsorption heat calculations show that compared with CH4 molecules, C3H8 and C2H6 molecules form stronger bonds with the skeleton, resulting in excellent separation performance. In addition, the breakthrough experiment of ZJNU-402 can also completely separate the ternary component C3H8/C2H6/CH4 mixture to obtain 99.95 % high-purity methane. At the same time, ZJNU-402 also has excellent structural stability, excellent recyclability, and low isosteric adsorption heat. Consequently, ZJNU-402 exhibits substantial potential for augmenting the efficacy of natural gas enrichment processes.

Divergent Adsorption Regulation in Metal-Organic Frameworks for Highly Efficient CF4/C2F6 Separation

The efficient removal of low-concentration components from homologous mixtures is often hampered by the co-directional effect of traditional thermodynamic regulation approaches, typically leading to a trade-off between adsorption capacity and selectivity. Focusing this challenge on the critical task of purifying perfluorocarbons in electronics industry, a divergent regulation strategy is reported that significantly improves the separation efficiency of low-concentration hexafluoroethane (C2F6) from tetrafluoromethane (CF4). This approach involves the selective shielding of open metal sites and the modulation of channel geometry within an electron-deficient ligand-based pore environment, thereby facilitating a C2F6 dense-packing accommodation mode while weakening the CF4 affinity due to the reduced host-guest interactions. Simultaneously enhanced C2F6 adsorption and reduced CF4 adsorption are achieved, resulting in record-high low-pressure C2F6 uptake and C2F6/CF4 selectivity. Comprehensive insights into the unique separation mechanism are illustrated through a combination of solid-state MAS nuclear magnetic resonance (SSNMR), molecular simulations, and meticulously designed comparative experiments. As a result, benchmark C2F6/CF4 separation performance is achieved, as demonstrated by the unprecedented electronic-grade (over 99.999%) CF4 productivity (401 L kg-1) obtained from an industrially relevant C2F6/CF4 (3:97) mixture, as well as the excellent water/air/heat stability and recyclability.

Ultra-High Purity and Productivity Separation of CO2 and C2H2 from CH4 in Rigid Layered Ultramicroporous Material

Efficiently obtaining both high-purity gas-phase and adsorbed-phase products in a single physisorption process presents the challenge of simultaneously achieving high selectivity and uptake and rapid diffusion in adsorbents. With a focus on natural gas purification and high-purity acetylene production, we report for the first time that the synergistic ligand/anion binding mode and multiple diffusion pathways in a robust 2D layered ultramicroporous framework (ZUL-100) enable unprecedented carbon dioxide/methane and acetylene/methane separation performance. Taking advantage of its rich anion, functional ligand ,and rigid 3D interpenetrated ultramicroporous channels, ZUL-100 achieved record IAST selectivities for equimolar carbon dioxide/methane (3.2 × 105) and acetylene/methane (1.7 × 1010) mixtures, accompanied by record dynamic uptakes of carbon dioxide (3.10 mmol/g) and acetylene (4.79 mmol/g), respectively. The strong affinity and fast mass transfer of carbon dioxide and acetylene on ZUL-100 were systematically elucidated by a combination of in situ FTIR, single-crystal XRD, kinetic tests, and DFT-D adsorption/diffusion modeling. In particular, high-purity (≥99.999%) methane and carbon dioxide (acetylene) can both be obtained on ZUL-100 through a single adsorption-desorption cycle, with exceptional productivity (2.81-4.22 mmol/g of methane, 2.96 mmol/g of carbon dioxide, and 4.31 mmol/g of acetylene) and high yield (95.5% for carbon dioxide and 90.0% for acetylene).

Metal-organic frameworks with two different-sized aromatic ring-confined nanotraps for benchmark natural gas upgrade

Recovery of light alkanes from natural gas is of great significance in petrochemical production. Herein, a promising strategy utilizing two types of size-complementary aromatic ring-confined nanotraps (called bi-nanotraps here) is proposed to efficiently trap ethane (C2H6) and propane (C3H8) selectively at their respective sites. Two isostructural metal-organic frameworks (MOFs, SNNU-185/186), each containing bi-nanotraps decorated with six aromatic rings, are selected to demonstrate the feasibility of this method. The smaller nanotrap acts as adsorption sites tailored for C2H6 while the larger one is optimized in size for C3H8. The separation is further facilitated by the large channels, which serve as mass transfer pathways. These advanced features give rise to multiple C-H⋯π interactions and size/shape-selective interaction sites, enabling SNNU-185/186 to achieve high C2H6 adsorption enthalpy (43.5/48.8 kJ mol-1) and a very large thermodynamic interaction difference between C2H6 and CH4. Benefiting from the bi-nanotrap effect, SNNU-185/186 exhibits benchmark experimental natural gas upgrade performance with top-level CH4 productivity (6.85/6.10 mmol g-1), ultra-high purity and first-class capture capacity for C2H6 (1.23/0.90 mmol g-1) and C3H8 (2.33/2.15 mmol g-1).

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.

Isoreticular Metal-Organic Frameworks with Aromatic Pores and Dimethylammonium Cations Enable Separation of Light Hydrocarbons and Xenon/Krypton

The separation of C2-C3 hydrocarbons from methane in natural gas and xenon/krypton purification are crucial yet challenging industrial processes. Herein, we report two isoreticular metal-organic frameworks, ZJU-89 and ZJU-90, featuring aromatic pore environments and dimethylammonium cations, that synergistically enhance the separation of these industrially relevant gas mixtures. ZJU-90 exhibits an exceptional separation performance, achieving C3H8/CH4 and C2H6/CH4 ideal adsorbed solution theory (IAST) selectivities of 1065 and 48, respectively, at ambient conditions, outperforming most reported adsorbent materials. Remarkably, ZJU-90 enables the recovery of >99.95% purity methane from a C3H8/C2H6/CH4 mixture in a single adsorption step. The material also demonstrates the efficient separation of xenon from krypton, even at low concentrations. The superior performance stems from the aromatic rings decorating the pore walls and the free dimethylammonium cations in the channels, which provide an ideal chemical environment for the selective binding of C2H6, C3H8, and Xe through multiple C-H···π interactions and van der Waals forces, as elucidated by theoretical calculations. This work highlights the power of reticular chemistry in designing materials with synergistic pore environments for efficient separations.

Topology Prediction of Gas-Separating Metal-Organic Frameworks with Low Symmetry Vertices

Topology serves as a blueprint for the construction of reticular structures such as metal-organic frameworks, especially for those based on building blocks with highly symmetrical shapes. However, it remains a challenge to predict the topology of the frameworks from less symmetrical units, because their corresponding vertex figures are largely deformed from the perfect geometries with no "default" net embedding. Furthermore, vertices involving flexible units may have multiple shape choices, and the competition among their designated topologies makes the structure prediction in large uncertainty. Herein, the deformation index is proposed to characterize the symmetry loss of the vertex figure by comparing it with its ideal geometry. The mathematical index is employed to predict the shapes of two in situ formed Co-based metalloligands (pseudo-tetrahedron and pseudo-square), which further dictate the framework topology (flu and scu) when they are joined with the [Zr6O8]-based cuboid units. The two frameworks with very similar constituents provide an ideal platform to investigate how the pore shapes and interconnectivity influence the gas separation. The net with cylindrical channels outperforms the other with discreate cages in C3H8/C2H6/CH4 separation, benefiting from the facile accessibility of its interaction sites to the guests imposed by the specific framework topology.

Nonlinear 3D Ligand-Based Metal-Organic Framework for Thermodynamic-Kinetic Synergistic Splitting of Mono-/Dibranched Hexane Isomers

The selective splitting of hexane isomers without the use of energy-intensive phase-change processes is essential for the low-carbon production of clean fuels and also very challenging. Here, we demonstrate a strategy to achieve a complete splitting of the high-RON dibranched isomer from the monobranched and linear isomers, by using a nonlinear 3D ligand to form pillar-layered MOFs with delicate pore architecture and chemistry. Compared with its isoreticular MOFs with the same ted pillar but different linear 3D or linear 2D in-layer ligands, the new MOF constructed in this work, Cu(bhdc)(ted)0.5 (ZUL-C5), exhibited an interesting "channel switch" effect which creates pore space with reduced window size and channel dimensionality together with unevenly distributed alkyl-rich adsorption sites, contributing to a greatly enhanced ability to discriminate between mono- and dibranched isomers. Evidenced by a series of studies including adsorption equilibrium/kinetics/breakthrough tests, guest-loaded single-crystal/powder XRD measurement, and DFT-D modeling, a thermodynamic-kinetic synergistic mechanism in the separation was proposed, resulting in a record production time for high-purity 2,2-dimethylbutane along with a high yield.

Enhanced water permeation through the terahertz-induced phase and diffusion transition in metal-organic framework membranes

Freshwater scarcity is a pressing global concern, and water desalination has emerged as a promising solution. Metal-organic framework (MOF) membranes have demonstrated exceptional potential in this regard. However, previous efforts to improve the permeability of MOFs have primarily focused on chemical modifications and synthesis rather than exploring physical methods. Using molecular dynamics simulations, we propose that the use of terahertz waves at a specific frequency of 7.5 ± 1.0 THz significantly enhances water permeability across MOF membranes, up to 27-fold, while maintaining effective ion rejection capabilities throughout the process. The mechanism behind this enhancement involves the resonance between the terahertz wave and the hydrogen bond vibrations of water within the MOF. This resonance amplifies the rotational kinetic energy of water molecules, disrupting the hydrogen bonds and causing a phase transition from quasi 1D square ice to a gas-like phase. Additionally, the diffusion behavior shifts from Fickian diffusion to sub-diffusion, resulting in improved water permeation across the MOF membrane. This study highlights the potential of terahertz waves as a physical tool to enhance the permeability of MOFs in water desalination, providing new avenues for efficient water treatment and resource sustainability.

Defect engineering improves CO2/N2 and CH4/N2 separation performance of MOF-801

A defect engineering modification method is reported to improve the CO2/N2 and CH4/N2 separation performance of MOF-801, owing to skeleton shrinkage caused by defect modification, Zr-FA0.5 shows excellent gas separation performance compared with the prototype MOF.

Metal-Organic Frameworks Possessing Suitable Pores for Xe/Kr Separation

Adsorption separation of the Xe/Kr mixture remains a tough issue since Xe and Kr have an inert nature and similar sizes. Here we present a chlorinated metal-organic framework (MOF) [JXNU-19(Cl)] and its nonchlorinated analogue (JXNU-19) for Xe/Kr separation. The two isostructural MOFs constructed from the heptanuclear cobalt-hydroxyl clusters bridged by organic ligands are three-dimensional structures. Detailed contrast of the Xe/Kr adsorption separation properties of the MOF shows that significantly enhanced Xe uptakes and Xe/Kr adsorption selectivity (17.1) are observed for JXNU-19 as compared to JXNU-19(Cl). The main binding sites for Xe in the MOF revealed by computational simulations are far away from the chlorine sites, suggesting that the introduction of the chlorine groups results in the unfavorable Xe adsorption for JXNU-19(Cl). The optimal pores, high surface area, and multiple strong Xe-framework interactions facilitate the effective Xe/Kr separation for JXNU-19.

Non-CO2 greenhouse gas separation using advanced porous materials

Global warming has become a growing concern over decades, prompting numerous research endeavours to reduce the carbon dioxide (CO2) emission, the major greenhouse gas (GHG). However, the contribution of other non-CO2 GHGs including methane (CH4), nitrous oxide (N2O), fluorocarbons, perfluorinated gases, etc. should not be overlooked, due to their high global warming potential and environmental hazards. In order to reduce the emission of non-CO2 GHGs, advanced separation technologies with high efficiency and low energy consumption such as adsorptive separation or membrane separation are highly desirable. Advanced porous materials (APMs) including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), porous organic polymers (POPs), etc. have been developed to boost the adsorptive and membrane separation, due to their tunable pore structure and surface functionality. This review summarizes the progress of APM adsorbents and membranes for non-CO2 GHG separation. The material design and fabrication strategies, along with the molecular-level separation mechanisms are discussed. Besides, the state-of-the-art separation performance and challenges of various APM materials towards each type of non-CO2 GHG are analyzed, offering insightful guidance for future research. Moreover, practical industrial challenges and opportunities from the aspect of engineering are also discussed, to facilitate the industrial implementation of APMs for non-CO2 GHG separation.

Hydrogen-bonded frameworks containing aliphatic 3D linkers show high-capacity water vapour sorption

Hydrogen-bonded frameworks were prepared from a tetra-amidinium component and three-dimensional cubane and bicyclopentane dicarboxylate linkers. Despite the incorporation of aliphatic components, the frameworks demonstrate strong and reversible uptake of water vapour, with one of the frameworks showing water uptake at very low relative humidity.

Ultramicroporous Lonsdaleite Topology MOF with High Propane Uptake and Propane/Methane Selectivity for Propane Capture from Simulated Natural Gas

Propane (C3H8) is a widely used fuel gas. Metal-organic framework (MOF) physisorbents that are C3H8 selective offer the potential to significantly reduce the energy footprint for capturing C3H8 from natural gas, where C3H8 is typically present as a minor component. Here we report the C3H8 recovery performance of a previously unreported lonsdaleite, lon, topology MOF, a chiral metal-organic material, [Ni(S-IEDC)(bipy)(SCN)]n, CMOM-7. CMOM-7 was prepared from three low-cost precursors: Ni(SCN)2, S-indoline-2-carboxylic acid (S-IDECH), and 4,4'-bipyridine (bipy), and its structure was determined by single crystal X-ray crystallography. Pure gas adsorption isotherms revealed that CMOM-7 exhibited high C3H8 uptake (2.71 mmol g-1) at 0.05 bar, an indication of a higher affinity for C3H8 than both C2H6 and CH4. Dynamic column breakthrough experiments afforded high purity C3H8 capture from a gas mixture comprising C3H8/C2H6/CH4 (v/v/v = 5/10/85). Despite the dilute C3H8 stream, CMOM-7 registered a high dynamic uptake of C3H8 and a breakthrough time difference between C3H8 and C2H6 of 79.5 min g-1, superior to those of previous MOF physisorbents studied under the same flow rate. Analysis of crystallographic data and Grand Canonical Monte Carlo simulations provides insight into the two C3H8 binding sites in CMOM-7, both of which are driven by C-H···π and hydrogen bonding interactions.

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.

Step-Nucleation In Situ Self-Repair to Prepare Rollable Large-Area Ultrathin MOF Membranes

Ultrathin membranes with ultrahigh permeance and good gas selectivity have the potential to greatly decrease separation process costs, but it requires the practical preparation of large area membranes for implementation. Metal-organic frameworks (MOFs) are very attractive for membrane gas separation applications. However, to date, the largest MOF membrane area reported in the literature is only about 100 cm2 . In the present study, a new step-nucleation in situ self-repair strategy is proposed that enables the preparation of large-area (2400 cm2 ) ultrathin and rollable MOF membranes deposited on an inexpensive flexible polymer membrane support layer for the first time, combining a polyvinyl alcohol (PVA)-metal-ion layer and a pure metal-ion layer. The main role of the pure metal-ion layer is to act as the main nucleation sites for MOF membrane growth, while the PVA-metal-ion layer acts as a slow-release metal-ion source, which supplements MOF crystal nucleation to repair any defects occurring. Membrane modules are necessary components for membrane applications, and spiral-wound modules are among the most common module formats that are widely applied in gas separation. A 4800 cm2 spiral-wound membrane module was successfully prepared, demonstrating the practical implementation of large-area MOF membranes.

Halogen-modified metal-organic frameworks for efficient separation of alkane from natural gas

As a rich green energy source, natural gas is widely used in many fields such as the chemical industry, automobile energy, and daily life. However, it is very challenging to separate and recover C2H6 and C3H8 from natural gas. Metal-organic frameworks (MOFs) as an emerging type of multi-pore porous materials show huge potential in gas adsorption separation. Herein, we report pillar-layered MOFs, Ni (BDC)(DABCO)0.5 (DMOF-X), modified by halogen atoms (F, Cl, Br), and investigate their CH4/C2H6/C3H8 separation performance. The experimental results show that DMOF-Cl exhibited a extremely high adsorption capacity for C3H8 and C2H6. Under the conditions of 298 K and 100 kPa, the adsorption capacities for C3H8 and C2H6 on DMOF-Cl are as high as 6.23 and 4.94 mmol g-1, which are superior to the values for most of the porous materials that have been reported. In addition, DMOF-Cl also shows high C3H8/CH4 (5: 85, V/V) and C2H6/CH4 (10: 85, V/V) separation selectivities, with values of 130.9 and 12.5, respectively. Finally, DMOF-Cl also demonstrated great potential as an adsorbent for separating C3H8/C2H6/CH4.

A roadmap to enhance gas permselectivity in metal-organic framework-based mixed-matrix membranes

Performing gas separation at high efficiency with minimum energy input and reduced carbon footprint is a major challenge. While several separation methods exist at various technology readiness levels, porous membrane-based separation is considered as a disruptive technology. To attain sustainability and required efficiency, different approaches of membrane design have been explored. However, the selectivity-permeation trade-off and membrane aging have restricted further advancement. In this regard, a new generation composite made of organic polymers and metal-organic framework (MOF) fillers shows substantial promise. Organic polymer matrix allows easy processibility, but it has poor permselectivity for gas molecules. Metal-organic frameworks are excellent sieving materials; however, they suffer from poor processibility issues. A combination of these two components makes an ideal sieving membrane, which can potentially outnumber the existing energy intensive distillation strategies. In this perspective, we have discussed key indices that regulate gas permselectivity by a careful selection of the existing literature. While the target gas flux and selectivity values have been a part of many previous reviews and articles, we have presented a concise discussion on the interface design of the MOF-polymer membrane, morphology, and orientation control of MOF fillers in the matrix. Following this, a future roadmap to overcome challenges related to MOF-polymer interfacial defects is outlined.

Soft Hydrogen-Bonded Organic Frameworks Constructed Using a Flexible Organic Cage Hinge

Soft porous crystals combine flexibility and porosity, allowing them to respond structurally to external physical and chemical environments. However, striking the right balance between flexibility and sufficient rigidity for porosity is challenging, particularly for molecular crystals formed by using weak intermolecular interactions. Here, we report a flexible oxygen-bridged prismatic organic cage molecule, Cage-6-COOH, which has three pillars that exhibit "hinge-like" rotational motion in the solid state. Cage-6-COOH can form a range of hydrogen-bonded organic frameworks (HOFs) where the "hinge" can accommodate a remarkable 67° dihedral angle range between neighboring units. This stems both from flexibility in the noncovalent hydrogen-bonding motifs in the HOFs and the molecular flexibility in the oxygen-linked cage hinge itself. The range of structures for Cage-6-COOH includes two topologically complex interpenetrated HOFs, CageHOF-2α and CageHOF-2β. CageHOF-2α is nonporous, while CageHOF-2β has permanent porosity and a surface area of 458 m2 g-1. The flexibility of Cage-6-COOH allows this molecule to rapidly transform from a low-crystallinity solid into the two crystalline interpenetrated HOFs, CageHOF-2α and CageHOF-2β, under mild conditions simply by using acetonitrile or ethanol vapor, respectively. This self-healing behavior was selective, with the CageHOF-2β structure exhibiting structural memory behavior.

Switchable Xe/Kr Selectivity in a Hofmann-Type Metal-Organic Framework via Temperature-Responsive Rotational Dynamics

The development of adsorbents for Kr and Xe separation is essential to meet industrial demands and for energy conservation. Although a number of previous studies have focused on Xe-selective adsorbents, stimuli-responsive Xe/Kr-selective adsorbents still remain underdeveloped. Herein, a Hofmann-type framework Co(DABCO)[Ni(CN)4 ] (referred to as CoNi-DAB; DABCO = 1,4-diazabicyclo[2,2,2]octane) that provides a temperature-dependent switchable Xe/Kr separation performance is reported. CoNi-DAB showed high Kr/Xe (0.8/0.2) selectivity with significant Kr adsorption at 195 K as well as high Xe/Kr (0.2/0.8) selectivity with superior Xe adsorption at 298 K. Such adsorption features are associated with the temperature-dependent rotational configuration of the DABCO ligand, which affects the kinetic gate-opening temperature of Xe and Kr. The packing densities of Xe (2.886 g cm-3 at 298 K) and Kr (2.399 g  cm-3 at 195 K) inside the framework are remarkable and comparable with those of liquid Xe (3.057 g cm-3 ) and liquid Kr (2.413 g cm-3 ), respectively. Breakthrough experiments confirm the temperature-dependent reverse separation performance of CoNi-DAB at 298 K under dry and wet (88% relative humidity) conditions and at 195 K under dry conditions. The unique adsorption behavior is also verified through van der Waals (vdW)-corrected density functional theory (DFT) calculations and nudged elastic band (NEB) simulations.

Properties of Aliphatic Ligand-Based Metal-Organic Frameworks

Ligands with a purely aliphatic backbone are receiving rising attention in the chemistry of coordination polymers and metal-organic frameworks. Such unique features inherent to the aliphatic bridges as increased conformational freedom, non-polarizable core, and low light absorption provide rare and valuable properties for their derived MOFs. Applications of such compounds in stimuli-responsive materials, gas, and vapor adsorbents with high and unusual selectivity, light-emitting, and optical materials have extensively emerged in recent years. These properties, as well as other specific features of aliphatic-based metal-organic frameworks are summarized and analyzed in this short critical review. Advanced characterization techniques, which have been applied in the reported works to obtain important data on the crystal and molecular structures, dynamics, and functionalities, are also reviewed within a general discussion. In total, 132 references are included.

Pore Environmental Modification by Alkoxy Groups in Pore-Space-Partitioned Metal-Organic Frameworks to Achieve Gas Uptake-Selectivity Balance

Due to the trade-off barrier between high storage capacity and high selectivity, the controllable and systematic design of metal-organic frameworks (MOFs) aiming at performance optimization is still challenging. Herein, considering the effectiveness of alkoxy group functionalization and a pore-space partition strategy, a series of rigid Mg-pacs-MOFs (SNNU-10-n, n = 1-6) with flexible side chains are built for the first time, realizing systematic pore environmental modification. The steric hindrance effects, electron-donating ability, and the flexibility of alkoxy groups are considered as key factors, which lead to a regular change of gas adsorption capacity and selectivity. Notably, methoxy-modified SNNU-10-1 with moderately high storage capacities of C₂H₂ (139.4 cm³ g^-1), C₂H₄ (100.4 cm³ g^-1), CO₂ (105.0 cm³ g^-1), and high selectivity values for equimolar C₂H₂/CH₄ (431.8), C₂H₄/CH₄ (164.2), and CO₂/CH₄ (16.1) mixture separation at 273 K and 100 kPa achieves an ideal gas uptake-selectivity balance. Breakthrough experiments verified that it could effectively separate the above-mentioned mixtures under ambient conditions, and GCMC simulation provides a deep understanding of methoxy group functionalization. Undoubtedly, this work not only realizes controllable regulation of gas adsorption behavior but also proves the validity of improving selectivity by alkoxy groups in those platforms with high gas-uptake potential to overcome the trade-off barrier.

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.

Integrating Self-Partitioned Pore Space and Amine Functionality into an Aromatic-Rich Coordination Framework with Ph Stability for Effective Purification of C2 Hydrocarbons

A great demand for high-purity C₂ hydrocarbons calls for the development of chemically stable porous materials for the effective isolation of C₂ hydrocarbons from CH₄ and CO₂. However, such separations are challenged by their similar physiochemical parameters and have not been systematically studied to date. In this work, we reported a cadmium-based rod-packing coordination framework compound ZJNU-140 of a new 5,6,7-c topology built up from a custom-designed tricarboxylate ligand. The metal-organic framework (MOF) features an aromatic-abundant pore surface, uncoordinated amine functionality, and self-partitioned pore space of suitable size. These structural characteristics act synergistically to provide the MOF with both selective recognition ability and the confinement effect toward C₂ hydrocarbons. As a result, the MOF displays promising potential for adsorptive separation of C₂-CH₄ and C₂-CO₂ mixtures. The IAST-predicted C₂/CH₄ and C₂/CO₂ adsorption selectivities, respectively, fall in the ranges of 7.3-10.2 and 2.1-2.9 at 298 K and 109 kPa. The real separation performance was also confirmed by dynamic breakthrough experiments. In addition, the MOF can maintain skeleton intactness in aqueous solutions with a wide pH range of 3-11, as confirmed by powder X-ray diffraction (PXRD) and isotherm measurements, showing no loss of framework integrity and porosity. The excellent hydrostability, considerable uptake capacity, impressive adsorption selectivity, and mild regeneration make ZJNU-140 a promising adsorbent material applied for the separation and purification of C₂ hydrocarbons.

Engineering Metal-Organic Frameworks for Selective Separation of Hexane Isomers Using 3-Dimensional Linkers

Metal-organic frameworks (MOFs) are highly tunable materials with potential for use as porous media in non-thermal adsorption or membrane-based separations. However, many separations target molecules with sub-angstrom differences in size, requiring precise control over the pore size. Herein, we demonstrate that this precise control can be achieved by installing a three-dimensional linker in an MOF with one-dimensional channels. Specifically, we synthesized single crystals and bulk powder of NU-2002, an isostructural framework to MIL-53 with bicyclo[1.1.1]pentane-1,3-dicarboxylic acid as the organic linker component. Using variable-temperature X-ray diffraction studies, we show that increasing linker dimensionality limits structural breathing relative to MIL-53. Furthermore, single-component adsorption isotherms demonstrate the efficacy of this material for separating hexane isomers based on the different sizes and shapes of these isomers.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

Combination of Low-Polar and Polar Binding Sites in Aliphatic MOFs for the Efficient C2H6/C2H4 Separation

The selective capture of C₂H₆ from C₂H₆/C₂H₄ mixtures is of critical importance to realize the efficient one-step purification of C₂H₄ but remains challenging due to their similar properties and smaller quadrupole moment of C₂H₆ that usually result in C₂H₄-preferring adsorption. Herein, we reported two isostructural pillared-layer metal-organic frameworks, ZUL-C3 and ZUL-C4, which were constructed by mixed polycycloalkane-type ligands. Their low-polar pore environment along with more accessible low-polar C-H binding sites on the pore surface are conducive to generate more van der Waals interactions with C₂H₆ while the carboxylic groups distributed at four corners of pores form stronger and more dipolar interactions with C₂H₆, cooperatively resulting in a good C₂H₆/C₂H₄ uptake ratio of 1.50 for ZUL-C3 and 1.72 for ZUL-C4 in static adsorption experiments and a high C₂H₄ (>99.99% purity) productivity of 10.1 L/kg for ZUL-C3 and 14.6 L/kg for ZUL-C4 from an equimolar C₂H₆/C₂H₄ mixture in breakthrough experiments.

Synthesis of porphyrin-based 2D ytterbium metal organic frameworks for efficient photodynamic therapy

Photodynamic therapy (PDT), which relies on the photo-induced reactive oxygen species (ROS) to trigger tumor cells apoptosis, has attracted intense focus over the decades due to the minimum invasion, high-precision and controllable therapeutic processes. Tetra(4-carboxyphenyl) porphin (TCPP), as an effective PDT photosensitizer, can harness photons and generate singlet oxygen species (¹O₂) upon illumination; however, poor solubility and low loading rate greatly limit its further use. Although TCPP-based metal-organic-frameworks (MOFs) has been proposed to address these concerns, the relatively large size still limits their biomedical applications. Therefore, in this study, TCPP molecules are coordinated with Yb³⁺, growing into 2D Yb-TCPP MOFs by a wet chemical method; the as-prepared Yb-TCPP MOFs are around 200 nm in size and possess high ¹O₂ generation efficiency with low cytotoxicity. Due to TCPP is appeared as the organic frameworks of Yb-TCPP MOFs, the low loading rate problem is largely addressed; in addition, the absorbance of Yb-TCPP MOFs has been greatly expanded compared with free TCPP molecules due to the coordination with Yb³⁺, allowing the illumination at longer wavelength range, e.g. 655 nm, that possesses high penetration depth and low phototoxicity. Overall, we have prepared 2D Yb-TCPP MOFs suitable for the in vitro anticancer effect, revealing the potential of Yb-TCPP MOFs as the future anticancer agent.