Restricting Lattice Flexibility in Polycrystalline Metal-Organic Framework Membranes for Carbon Capture

Sub-Minute Fabrication of Metal Organic Framework Membranes via Additive-Accelerated Electrodeposition

Efficient fabrication of metal organic framework (MOF) membranes is important for their broad applications in molecular separations. However, current approaches for MOF membrane fabrication are usually time-consuming due to the slow, random nucleation and crystal growth, particularly the lack of in-situ defect healing ability. Here, we report an additive-accelerated electrodeposition method, which allows ultrafast fabrication of MOF membranes through the synergy of electric field and catecholamine additives. The strong electric field facilitates the directed nucleation of MOF on the substrates while the multifunctional additives accelerate the MOF crystallization, growth and grain-boundary defect healing. Consequently, we fabricate well-intergrown, uniform MOF (ZIF-8) membranes with an ultrathin thickness of ∼180 nm in 30 s, which is the most rapid fabrication of MOF membranes till now. The membranes exhibit superior C3H6/C3H8 separation performance with a C3H6 permeance of 145 GPU and a C3H6/C3H8 separation factor of 151, as well as good stability at high pressure of 7 bar, and the ultrafast membrane fabrication can be achieved on commercial ceramic substrates, exhibiting the potential for practical applications. This work may establish a platform for fast and controllable fabrication of MOF membranes as well as many other membranes based on metal-coordination chemistry.

Directly Gel-Thermal Processing of Linker-Mixed Crystal-Glass Composite Membranes for Sorption-Preferential Gas Separation

Membrane processes are promising for energy-saving industrial applications. However, efficient separation for some valuable gas mixtures with similar characteristics, such as CH4/N2 and O2/N2, remains extremely challenging. Metal-organic framework (MOF) membranes have been attracting intensive attention for gas sieving, but it is difficult to manufacture MOF membranes in scalability and precisely tune their transport property. In this study, Gel-thermal processing of linker-mixed MOF crystal-glass composite membranes is reported directly, with the mechanism of adjusting metal-linker bond strengths and angles to disrupt long-range periodicity of MOFs and promote glass phase formation, for sharply sorption-preferential gas separation. This strategy can be realized by using a simple, solvent/precursor-less, and cost-effective gel-thermal approach with two steps of gel coating and thermal conversion, thereby constructing crystal-glass composite membranes in a controllable, processable, versatile, and environmentally friendly route. Moreover, the mixed linkers enable preferential gas affinities and the ultramicroporous glasses can eliminate any membrane defects. The membranes exhibit outstanding gas separation performance for the challenging systems of CH4/N2 and O2/N2, with mixture selectivities up to 9.3 and 9.6, respectively, far exceeding those of polymer, MOF, and mixed-matrix membranes. The study provides an available route for architecting high-performance membranes for gas separations.

Modification of ZIF-8 Membranes for Gas Separation Using X-ray Radiation

We report an X-ray radiation-induced modification of the structure and gas permeation behavior of ZIF-8 membranes. With 300 min irradiation time, CO2 permeance decreases by only 9 %, while N2 and CH4 permeances reduce by 75 and 65 %, respectively, leading to 3.7- and 2.6-fold enhancements in ideal selectivity for CO2/N2 and CO2/CH4.

High-performance H2/CO2 separation from 4-nm-thick oriented Zn2(benzimidazole)4 films

High-performance membrane-based H2/CO2 separation offers a promising way to reduce the energy costs of precombustion capture. Current membranes, often made from two-dimensional laminates like metal-organic frameworks, have limitations due to complex fabrication methods requiring high temperatures, organic solvents, and long synthesis time. These processes often result in poor H2/CO2 selectivity under pressurized conditions due to defective transport pathways. Here, we introduce a simple, eco-friendly synthesis of ultrathin, intergrown Zn2(benzimidazole)4 films, as thin as 4 nm. These films are prepared at room temperature using water as the solvent, with a synthesis time of just 10 minutes. By using ultradilute precursor solutions, nucleation is delayed, promoting rapid in-plane growth on a smooth graphene substrate and eliminating defects. These membranes exhibit excellent H2 permselectivity under pressurized conditions. The combination of rapid, green synthesis and high-performance separation makes these membranes highly attractive for precombustion applications.

Enhanced Propylene/Propane Separation via Aniline-Decorated ZIF-8 Membrane: Lattice Rigidity Adjustment and Adsorption Site Introduction

Metal-organic framework (MOF)-based membranes excel in molecular separation, attracting significant research interest. The crystallographic microstructure and selective adsorption capacity of MOFs closely correlate with their gas separation performance. Here, aniline was added to the ZIF-8 synthesis in varying concentrations. Aniline, encapsulated within ZIF-8 cavities, interacts strongly with the 2-methylimidazole linker, resulting in both a shift in crystallographic phase from I_43 m to Cm in Rietveld refinement of X-ray diffraction (XRD) patterns and the selective adsorption behavior between propylene and propane. Consequently, an aniline decorative ZIF-8 (Anix-ZIF-8) membrane was prepared using a fast current-driven synthesis method, which exhibits good propylene/propane separation selectivity of up to 85. Calculation of the interaction energy between aniline and the various crystallographic phases of ZIF-8 using density functional theory (DFT) further verifies that aniline not only promotes the formation of crystallographic Cm phase, but also enhances the adsorption selectivity of propylene over propane. Aniline modification effectively tunes the crystallographic microstructure of ZIF-8, thereby, improving molecular sieving capabilities.

A general flame aerosol route to kinetically stabilized metal-organic frameworks

Metal-organic frameworks (MOFs) are highly attractive porous materials with applications spanning the fields of chemistry, physics, biology, and engineering. Their exceptional porosity and structural flexibility have led to widespread use in catalysis, separation, biomedicine, and electrochemistry. Currently, most MOFs are synthesized under equilibrium liquid-phase reaction conditions. Here we show a general and versatile non-equilibrium flame aerosol synthesis of MOFs, in which rapid kinetics of MOF formation yields two distinct classes of MOFs, nano-crystalline MOFs and amorphous MOFs. A key advantage of this far-from-equilibrium synthesis is integration of different metal cations within a single MOF phase, even when this is thermodynamically unfavorable. This can, for example, produce single-atom catalysts and bimetallic MOFs of arbitrary metal pairs. Moreover, we demonstrate that dopant metals (e.g., Pt, Pd) can be exsolved from the MOF framework by reduction, forming nanoclusters anchored on the MOF. A prototypical example of such a material exhibited outstanding performance as a CO oxidation catalyst. This general synthesis route opens new opportunities in MOF design and applications across diverse fields and is inherently scalable for continuous production at industrial scales.

Research on carbon dioxide capture materials used for carbon dioxide capture, utilization, and storage technology: a review

In recent years, climate change has increasingly become one of the major challenges facing mankind today, seriously threatening the survival and sustainable development of mankind. Dramatically increasing carbon dioxide concentrations are thought to cause a severe greenhouse effect, leading to severe and sustained global warming, associated climate instability and unwelcome natural disasters, melting glaciers and extreme weather patterns. The treatment of flue gas from thermal power plants uses carbon capture, utilization, and storage (CCUS) technology, one of the most promising current methods to accomplish significant CO2 emission reduction. In order to implement the technological and financial system of CO2 capture, which is the key technology of CCUS technology and accounts for 70-80% of the overall cost of CCUS technology, it is crucial to create more effective adsorbents. Nowadays, with the development and application of various carbon dioxide capture materials, it is necessary to review and summarize carbon dioxide capture materials in time. In this paper, the main technologies of CO2 capture are reviewed, with emphasis on the latest research status of CO2 capture materials, such as amines, zeolites, alkali metals, as well as emerging MOFs and carbon nanomaterials. More and more research on CO2 capture materials has used a variety of improved methods, which have achieved high CO2 capture performance. For example, doping of layered double hydroxides (LDH) with metal atoms significantly increases the active site on the surface of the material, which has a significant impact on improving the CO2 capture capacity and performance stability of LDH. Although many carbon capture materials have been developed, high cost and low technology scale remain major obstacles to CO2 capture. Future research should focus on designing low-cost, high-availability carbon capture materials.

Unit-cell-thick zeolitic imidazolate framework films for membrane application

Zeolitic imidazolate frameworks (ZIFs) are a subset of metal-organic frameworks with more than 200 characterized crystalline and amorphous networks made of divalent transition metal centres (for example, Zn2+ and Co2+) linked by imidazolate linkers. ZIF thin films have been intensively pursued, motivated by the desire to prepare membranes for selective gas and liquid separations. To achieve membranes with high throughput, as in ångström-scale biological channels with nanometre-scale path lengths, ZIF films with the minimum possible thickness-down to just one unit cell-are highly desired. However, the state-of-the-art methods yield membranes where ZIF films have thickness exceeding 50 nm. Here we report a crystallization method from ultradilute precursor mixtures, which exploits registry with the underlying crystalline substrate, yielding (within minutes) crystalline ZIF films with thickness down to that of a single structural building unit (2 nm). The film crystallized on graphene has a rigid aperture made of a six-membered zinc imidazolate coordination ring, enabling high-permselective H2 separation performance. The method reported here will probably accelerate the development of two-dimensional metal-organic framework films for efficient membrane separation.

Metal-Organic Framework Membrane Hybridized with Graphitic Materials for Gas Separation

Metal-organic frameworks (MOFs) are an exceptional class of crystalline materials that have been extensively used to fabricate membranes for various applications such as gas separation, ion transport, and desalination due to their well-defined pore structure, chemical features, and simple synthesis process. The incorporation of graphitic carbon materials in MOFs has garnered significant attention as it can provide abundant nucleation sites and modulate gas transport by influencing the orientation or rigidity of MOF crystals without changing their porous structure. This review insights of previous studies utilizing graphene, graphene oxide, carbon nanotubes, and graphene nanoribbons for MOF-based gas separation membranes, particularly focusing on polycrystalline MOF membrane hybridization with graphitic materials. We also briefly discuss the use of carbon/MOF hybrid materials for preparing mixed matrix membranes.

Vacuum-Assisted Self-Healing Amphiphilic Copolymer Membranes for Gas Separation

Membrane gas separation provides a multitude of benefits over alternative separation techniques, especially in terms of energy efficiency and environmental sustainability. While polymeric membranes have been extensively investigated for gas separations, their self-healing capabilities have often been neglected. In this work, we have developed innovative self-healing amphiphilic copolymers by strategically incorporating three functional segments: n-butyl acrylate (BA), N-(hydroxymethyl)acrylamide (NMA), and methacrylic acid (MAA). Utilizing these three functional components, we have synthesized two distinct amphiphilic copolymers, namely, AP_NMA (PBA_x-co-PNMA_y) and AP_MAA (PBA_x-co-PMAA_y). These copolymers have been meticulously designed for gas separation applications. During the creation of these amphiphilic copolymers, BA and NMA segments were selected due to their vital role in the ease of tuning mechanical and self-healing properties. The functional groups (-OH and -NH) present on the NMA segment interact with CO₂ through hydrogen bonding, thereby boosting CO₂/N₂ separation and achieving superior selectivity. We assessed the self-healing potential of these amphiphilic copolymer membranes using two distinct strategies: conventional and vacuum-assisted self-healing. In the vacuum-assisted approach, a robust vacuum pump generates a suction force, leading to the formation of a cone-like shape in the membrane. This formation allows common fracture sites to adhere and trigger the self-healing process. As a result, AP_NMA maintains its high gas permeability and CO₂/N₂ selectivity even after the vacuum-assisted self-healing operation. The ideal CO₂/N₂ selectivity of the AP_NMA membrane aligns closely with the commercially available PEBAX-1657 membrane (17.54 vs 20.09). Notably, the gas selectivity of the AP_NMA membrane can be readily restored after damage, in contrast to the PEBAX-1657 membrane, which loses its selectivity upon damage.

Mixed-linker strategy for suppressing structural flexibility of metal-organic framework membranes for gas separation

Structural flexibility is a critical issue that limits the application of metal-organic framework (MOF) membranes for gas separation. Herein we propose a mixed-linker approach to suppress the structural flexibility of the CAU-10-based (CAU = Christian-Albrechts-University) membranes. Specifically, pure CAU-10-PDC membranes display high separation performance but at the same time are highly unstable for the separation of CO₂/CH₄. A partial substitution (30 mol.%) of the linker PDC with BDC significantly improves its stability. Such an approach also allows for decreasing the aperture size of MOFs. The optimized CAU-10-PDC-H (70/30) membrane possesses a high separation performance for CO₂/CH₄ (separation factor of 74.2 and CO₂ permeability of 1,111.1 Barrer under 2 bar of feed pressure at 35°C). A combination of in situ characterization with X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, as well as periodic density functional theory (DFT) calculations, unveils the origin of the mixed-linker approach to enhancing the structural stability of the mixed-linker CAU-10-based membranes during the gas permeation tests.

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.

Challenges in Developing MOF-Based Membranes for Gas Separation

Metal-organic frameworks (MOFs) are promising candidates for membrane gas separation. MOF-based membranes include pure MOF membranes and MOF-based mixed matrix membranes (MMMs). This Perspective discusses the challenges for the next stage of the development of MOF-based membranes based on research conducted in the past decade. We focused on three major issues associated with pure MOF membranes. First, some MOF compounds have been overstudied, despite the availability of numerous MOFs. Second, gas adsorption and diffusion in MOFs are often independently investigated. The correlation between adsorption and diffusion has seldom been discussed. Third, we identify the importance of characterizing the gas distribution in MOFs to understand the structure-property relationships for gas adsorption and diffusion in MOF membranes. For MOF-based MMMs, engineering the MOF-polymer interface is essential for achieving the desired separation performance. Various approaches to modify the MOF surface or polymer molecular structure have been proposed to improve the MOF-polymer interface. Herein, we present defect engineering as a facile and efficient approach for engineering the MOF-polymer interfacial morphology and its extended application for various gas separations.

Structure Regulation of MOF Nanosheet Membrane for Accurate H2 /CO2 Separation

High-purity H₂ production accompanied with a precise decarbonization opens an avenue to approach a carbon-neutral society. Metal-organic framework nanosheet membranes provide great opportunities for an accurate and fast H₂ /CO₂ separation, CO₂ leakage through the membrane interlayer galleries decided the ultimate separation accuracy. Here we introduce low dose amino side groups into the Zn₂ (benzimidazolate)₄ conformation. Physisorbed CO₂ served as interlayer linkers, gently regulated and stabilized the interlayer spacing. These evoked a synergistic effect of CO₂ adsorption-assisted molecular sieving and steric hinderance, whilst exquisitely preserving apertures for high-speed H₂ transport. The optimized amino membranes set a new record for ultrathin nanosheet membranes in H₂ /CO₂ separation (mixture separation factor: 1158, H₂ permeance: 1417 gas permeation unit). This strategy provides an effective way to customize ultrathin nanosheet membranes with desirable molecular sieving ability.

Hetero-Polycrystalline Membranes with Narrow and Rigid Pores for Molecular Sieving

Molecular sieving membranes have great potential for energy-saving separations, but they suffer from permeability-selectivity trade-off limitation. In this report, simultaneous hetero-crystallization and hetero-linker coordination of metal-organic framework (MOF) hollow fiber membranes through one-pot synthesis for precise gas separation is reported. It is found that the hetero-polycrystalline membranes consist of 2D and 3D MOF phases and are defect-free and roughly orientated, hetero-linker exchange of 3D phase by larger geometric ones can narrow transport pathway, and framework rigidification occurs and thus fixes MOF channels. The prepared membranes are robust and reproducible, and exhibit substantially improved performance, with H₂ /CO₂ , H₂ /N₂ , and H₂ /CH₄ selectivities up to 361, 482, and 541, respectively, accompanied by high H₂ permeance over 1100 gas permeation units, which can easily outclass trade-off upper bounds of state-of-the-art membranes.

One-Step Synthesis of Ultrathin Zeolitic Imidazole Framework-8 (ZIF-8) Membrane on Unmodified Porous Support via Electrophoretic Deposition

Metal-organic frameworks (MOFs) are regarded as the next-generation, disruptive membrane materials, yet the straightforward fabrication of ultrathin MOF membranes on an unmodified porous support remains a critical challenge. In this work, we proposed a facile, one-step electrophoretic deposition (EPD) method for the growth of ultrathin zeolitic imidazole framework-8 (ZIF-8) membranes on a bare porous support. The crystallinity, morphology and coverage of ZIF-8 particles on support surface can be optimized via regulating EPD parameters, yet it is still difficult to ensure the integrity of a ZIF-8 membrane with the constant voltage mode. In contrast, the constant current mode is more beneficial to the growth of a defect-free ZIF-8 membrane due to the steady migration rate of colloid particles toward the electrode. With a current of 0.65 mA/cm² and deposition time of 60 min, a 300 nm thick ZIF-8 membrane was obtained, which exhibits a CO₂ permeance of 334 GPU and a CO₂/CH₄ separation factor of 8.8, evidencing the defect-free structure.

Advances in metal-organic framework-based membranes

Membrane-based separations have garnered considerable attention owing to their high energy efficiency, low capital cost, small carbon footprint, and continuous operation mode. As a class of highly porous crystalline materials with well-defined pore systems and rich chemical functionalities, metal-organic frameworks (MOFs) have demonstrated great potential as promising membrane materials over the past few years. Different types of MOF-based membranes, including polycrystalline membranes, mixed matrix membranes (MMMs), and nanosheet-based membranes, have been developed for diversified applications with remarkable separation performances. In this comprehensive review, we first discuss the general classification of membranes and outline the historical development of MOF-based membranes. Subsequently, particular attention is devoted to design strategies for MOF-based membranes, along with detailed discussions on the latest advances on these membranes for various gas and liquid separation processes. Finally, challenges and future opportunities for the industrial implementation of these membranes are identified and outlined with the intent of providing insightful guidance on the design and fabrication of high-performance membranes in the future.

Metal-organic frameworks and covalent organic frameworks as disruptive membrane materials for energy-efficient gas separation

In this Review we survey the molecular sieving behaviour of metal-organic framework (MOF) and covalent organic framework (COF) membranes, which is different from that of classical zeolite membranes. The nature of MOFs as inorganic-organic hybrid materials and COFs as purely organic materials is powerful and disruptive for the field of gas separation membranes. The possibility of growing neat MOFs and COFs on membrane supports, while also allowing successful blending into polymer-filler composites, has a huge advantage over classical zeolite molecular sieves. MOFs and COFs allow synthetic access to more than 100,000 different structures and tailor-made molecular gates. Additionally, soft evacuation below 100 °C is often enough to achieve pore activation. Therefore, a huge number of synthetic methods for supported MOF and COF membrane thin films, such as solvothermal synthesis, seed-mediated growth and counterdiffusion, exist. Among them, methods with high scale-up potential, for example, layer-by-layer dip- and spray-coating, chemical and physical vapour deposition, and electrochemical methods. Additionally, physical methods have been developed that involve external stimuli, such as electric fields and light. A particularly important point is their ability to react to stimuli, which has allowed the 'drawbacks' of the non-ideality of the molecular sieving properties to be exploited in a completely novel research direction. Controllable gas transport through membrane films is a next-level property of MOFs and COFs, leading towards adaptive process deviation. MOF and COF particles are highly compatible with polymers, which allows for mixed-matrix membranes. However, these membranes are not simple MOF-polymer blends, as they require improved polymer-filler interactions, such as cross-linking or surface functionalization.

X-ray Diffraction and Molecular Simulations in the Study of Metal-Organic Frameworks for Membrane Gas Separation

For more than a decade, researchers have been developing metal-organic frameworks (MOFs) in the form of pure MOF membranes as well as MOF-containing mixed-matrix membranes. MOF membranes have been used for H₂/CO₂ or C₃H₆/C₃H₈ separation, but relatively few MOF membranes enable the high-performance separation of CO₂/N₂, CO₂/CH₄, or N₂/CH₄. This article describes the use of in situ XRD analysis and molecular simulation to elucidate gas transport within MOFs and derivative membranes at the molecular level. In a review of recent studies by the authors and other research groups, this article examines the flexibility of MOFs initiated by activation, gas adsorption, and aging effects during gas permeation. This article also discusses the application of XRD analysis in conjunction with computational methods to investigate the CO₂-MOF Coulombic interaction and its effects on CO₂ separation. Note that this combined analysis approach is also useful in studying the effects of linker rotation on N₂/CH₄ separation. This article also examines the use of computational tools in identifying new MOFs for gas separation and, more importantly, in elaborating the relationship between the structure of MOFs and their corresponding gas transport properties.

MOF-COF "Alloy" Membranes for Efficient Propylene/Propane Separation

Molecular-sieving membranes from metal-organic frameworks (MOFs) are promising candidates for separating olefin/paraffin mixtures, a critical demand in sustainable chemical processes and a grand challenge in molecular separation. Currently, the inherent lattice flexibility of MOFs severely compromises their precise sieving ability. Here, a proof-of-concept of "alloy" membranes (AMs), which are fabricated by incorporating quaternary ammonium (QA)-functionalized covalent organic frameworks (COFs) into a zeolitic imidazolate framework-8 (ZIF-8) matrix is demonstrated. The Coulomb force between the COFs and the ZIF-8 restricts the linker rotation of the ZIF-8, generating a distinct alloying effect, by which the lattice rigidity of ZIF-8 can be conveniently tuned through varying the content of the COFs, similar to the flexible-to-rigid transition in aluminum alloy manufacturing. Such an alloying effect confers the AM's superior propylene/propane separation performance, with a propylene/propane separation factor surpassing 200 and a propylene permeance of 168 GPU. Hopefully, the AMs concept and the concomitant alloying effect can update the connotation of mixed matrix membranes and stimulate the re-envisioning about the design paradigm and development of advanced membranes for energy-efficient separations.

In Situ Tracking of Nonthermal Plasma Etching of ZIF-8 Films

Surface characterization is critical for understanding the processes used for preparing catalysts, sorbents, and membranes. Nonthermal plasma (NTP) is a process that achieves high reactivity at low temperatures and is used to tailor the surface properties of materials. In this work, we combine the capabilities of infrared reflection absorption spectroscopy (IRRAS) with NTP for the in situ interrogation of zeolitic imidazolate framework-8 (ZIF-8) thin films to probe modifications in the material induced by oxygen and nitrogen plasmas. The IRRAS measurements in oxygen plasma reveal etching of organic ligands with sequential removal of the methyl group and imidazole ring and with the formation of carbonyl moieties (C═O). In contrast, nitrogen plasma induces mild etching and grafting of nitrile groups (-C≡N). Scanning electron microscopy imaging shows that oxygen plasma, at prolonged times, significantly degrades the ZIF-8 film at the grain boundaries. Treatment of ZIF-8 membranes using mild plasma conditions yields a fivefold enhancement for H₂/N₂ and CO₂/CH₄ ideal selectivities and an eightfold enhancement for CO₂/N₂ ideal selectivity. Additionally, the new tools described here can be used for spectroscopic in situ tracking of plasma-induced chemistry on thin films in general.

Control of zeolite framework flexibility for ultra-selective carbon dioxide separation

Molecular sieving membranes with uniform pore size are highly desired for carbon dioxide separation. All-silica zeolite membranes feature well-defined micropores, but the size-exclusion effect is significantly compromised by the non-selective macro-pores generated during detemplation. Here we propose a template modulated crystal transition (TMCT) approach to tune the flexibility of Decadodecasil 3 R (DD3R) zeolite to prepare ultra-selective membranes for CO₂/CH₄ separation. An instantaneous overheating is applied to synchronize the template decomposition with the structure relaxation. The organic template molecules are transitionally converted to tight carbon species by the one-minute overheating at 700 °C, which are facilely burnt out by a following moderate thermal treatment. The resulting membranes exhibit CO₂/CH₄ selectivity of 157~1,172 and CO₂ permeance of (890~1,540) × 10⁻¹⁰mol m⁻² s⁻¹ Pa⁻¹. The CO₂ flux and CO₂/CH₄ mixture selectivity reach 3.6 Nm³ m⁻² h⁻¹ and 43 even at feed pressure up to 31 bar. Such strategy could pave the way of all-silica zeolite membranes to practical applications.

All-silica zeolite membranes are highly desired for natural gas upgrading but the size-exclusion effect is compromised by defects generated during high-temperature detemplation. Here, the authors develop a strategy to fabricate ultra-selective DD3R zeolite membranes via tuning the zeolite flexibility under rapid template decomposition.

X-ray diffraction for probing free energy profiles and self-diffusivity of gases in metal–organic frameworks

This paper presents a novel methodology for measuring the free energy profiles and the self-diffusivity of gases in crystalline microporous materials.

Suppressing Defect Formation in Metal-Organic Framework Membranes via Plasma-Assisted Synthesis for Gas Separations

Metal-organic frameworks (MOFs) are considered as promising materials for membrane gas separations. Structural defects within a pure MOF membrane can considerably reduce its selectivity and possibly result in a nonselective separation. This work proposes a solution-phase synthesis with dielectric barrier discharge (DBD) plasma to suppress the formation of defects in the pure MOF membrane of CPO-8-BPY. Through comprehensive solid-state characterization with XRD, SEM, XPS, solid-state NMR, and XAFS, DBD plasma is demonstrated to facilitate deprotonation in the H₂aip linker, which leads to a smaller and more uniform particle size of CPO-8-BPY. The narrow grain size distribution effectively reduces the pinhole-type defects in the pure CPO-8-BPY membrane and endows it with good ideal selectivity for H₂/CH₄ (α_H2/CH4 = 28.2) and N₂/CH₄ (α_N2/CH4 = 5.4). The selectivity for H₂/CH₄ of this membrane from a mixed-gas permeation test is found to be 15.4. Molecular simulations are also performed to gain insights into the gas transport properties of this MOF. The results suggest that ligand rotation plays an important role in CPO-8-BPY when being applied to the membrane separation of N₂/CH₄.

Membrane Materials for Selective Ion Separations at the Water-Energy Nexus

Synthetic polymer membranes are enabling components in key technologies at the water-energy nexus, including desalination and energy conversion, because of their high water/salt selectivity or ionic conductivity. However, many applications at the water-energy nexus require ion selectivity, or separation of specific ionic species from other similar species. Here, the ion selectivity of conventional polymeric membrane materials is assessed and recent progress in enhancing selective transport via tailored free volume elements and ion-membrane interactions is described. In view of the limitations of polymeric membranes, three material classes-porous crystalline materials, 2D materials, and discrete biomimetic channels-are highlighted as possible candidates for ion-selective membranes owing to their molecular-level control over physical and chemical properties. Lastly, research directions and critical challenges for developing bioinspired membranes with molecular recognition are provided.

Multipulsed Millisecond Ozone Gasification for Predictable Tuning of Nucleation and Nucleation-Decoupled Nanopore Expansion in Graphene for Carbon Capture

Predictable and tunable etching of angstrom-scale nanopores in single-layer graphene (SLG) can allow one to realize high-performance gas separation even from similar-sized molecules. We advance toward this goal by developing two etching regimes for SLG where the incorporation of angstrom-scale vacancy defects can be controlled. We screen several exposure profiles for the etchant, controlled by a multipulse millisecond treatment, using a mathematical model predicting the nucleation and pore expansion rates. The screened profiles yield a narrow pore-size-distribution (PSD) with a majority of defects smaller than missing 16 carbon atoms, suitable for CO₂/N₂ separation, attributing to the reduced pore expansion rate at a high pore density. Resulting nanoporous SLG (N-SLG) membranes yield attractive CO₂ permeance of 4400 ± 2070 GPU and CO₂/N₂ selectivity of 33.4 ± 7.9. In the second etching regime, by limiting the supply of the etchant, the nanopores are allowed to expand while suppressing the nucleation events. Extremely attractive carbon capture performance marked with CO₂ permeance of 8730 GPU, and CO₂/N₂ selectivity of 33.4 is obtained when CO₂-selective polymeric chains are functionalized on the expanded nanopores. We show that the etching strategy is uniform and scalable by successfully fabricating high-performance centimeter-scale membrane.

Mechanistic Study on Thermally Induced Lattice Stiffening of ZIF-8

The flexibility of the ZIF-8 aperture, which inhibits a molecular cutoff of 3.4 Å, can be reduced by rapid heat treatment to obtain CO₂-selective membranes. However, the early stages of the structural, morphological, and chemical changes responsible for the lattice rigidification remain elusive. Herein, using ex situ and in situ experiments, we determine that a small shrinkage of the unit-cell parameter, ∼0.2%, is mainly responsible for this transformation. Systematic gas permeation studies show that one needs to achieve this shrinkage without a disproportionately large shrinkage in the grain size of the polycrystalline film to avoid the formation of cracks. We show that this condition is uniquely achieved in a short time by exposure of ZIF-8 to a mildly humid environment where lattice parameter shrinkage is accelerated by the incorporation of linker vacancy defects, while the shrinkage in grain size is limited. The water-vapor-led incorporation of linker vacancy defects takes place with an energy barrier of 123 kJ mol^-1, much higher than that for the thermal degradation of ZIF-8, <80 kJ mol^-1. The latter is promoted by heat treatment in a dry environment at a relatively higher temperature; however, this condition does not shrink the lattice parameters at short exposure time.

Pressure-Responsive Two-Dimensional Metal-Organic Framework Composite Membranes for CO2 Separation

The regulation of permeance and selectivity in membrane systems may allow effective relief of conventional energy-intensive separations. Here, pressure-responsive ultrathin membranes (≈100 nm) fabricated by compositing flexible two-dimensional metal-organic framework nanosheets (MONs) with graphene oxide nanosheets for CO₂ separation are reported. By controlling the gas permeation direction to leverage the pressure-responsive phase transition of the MONs, CO₂ -induced gate opening and closing behaviors are observed in the resultant membranes, which are accompanied with the sharp increase of CO₂ permeance (from 173.8 to 1144 gas permeation units) as well as CO₂ /N₂ and CO₂ /CH₄ selectivities (from 4.1 to 22.8 and from 4 to 19.6, respectively). The flexible behaviors and separation mechanism are further elucidated by molecular dynamics simulations. This work establishes the relevance of structural transformation-based framework dynamics chemistry in smart membrane systems.