Polymer Composite Membrane with Penetrating ZIF‐7 Sheets Displays High Hydrogen Permselectivity

Hydrogen Separation Membranes: A Material Perspective

The global energy market is shifting toward renewable, sustainable, and low-carbon hydrogen energy due to global environmental issues, such as rising carbon dioxide emissions, climate change, and global warming. Currently, a majority of hydrogen demands are achieved by steam methane reforming and other conventional processes, which, again, are very carbon-intensive methods, and the hydrogen produced by them needs to be purified prior to their application. Hence, researchers are continuously endeavoring to develop sustainable and efficient methods for hydrogen generation and purification. Membrane-based gas-separation technologies were proven to be more efficient than conventional technologies. This review explores the transition from conventional separation techniques, such as pressure swing adsorption and cryogenic distillation, to advanced membrane-based technologies with high selectivity and efficiency for hydrogen purification. Major emphasis is placed on various membrane materials and their corresponding membrane performance. First, we discuss various metal membranes, including dense, alloyed, and amorphous metal membranes, which exhibit high hydrogen solubility and selectivity. Further, various inorganic membranes, such as zeolites, silica, and CMSMs, are also discussed. Major emphasis is placed on the development of polymeric materials and membranes for the selective separation of hydrogen from CH4, CO2, and N2. In addition, cutting-edge mixed-matrix membranes are also delineated, which involve the incorporation of inorganic fillers to improve performance. This review provides a comprehensive overview of advancements in gas-separation membranes and membrane materials in terms of hydrogen selectivity, permeability, and durability in practical applications. By analyzing various conventional and advanced technologies, this review provides a comprehensive material perspective on hydrogen separation membranes, thereby endorsing hydrogen energy for a sustainable future.

Wrinkled metal-organic framework thin films with tunable Turing patterns for pliable integration

Flexible integration spurs diverse applications in metal-organic frameworks (MOFs). However, current configurations suffer from the trade-off between MOF loadings and mechanical compliance. We report a wrinkled configuration of MOF thin films. We established an interfacial synthesis confined and controlled by a polymer topcoat and achieved multiple Turing motifs in the wrinkled thin films. These films have complete MOF surface coverage and exhibit strain tolerance up to 53.2%. The enhanced mechanical properties allow film transfer onto various substrates. We obtained membranes with large H2/CO2 selectivity (41.2) and high H2 permeance (8.46 × 103 gas permeation units), showcasing negligible defects after transfer. We also achieved soft humidity sensors on delicate electrodes by avoiding exposure to harsh MOF synthesis conditions. These results highlight the potential of wrinkled MOF thin films for plug-and-play integration.

Oriented Metal-Organic Framework Membranes for Molecular Separations

Metal-organic framework (MOF) membranes, which are recognized as state-of-the-art platforms applied in various separation processes, have attracted widespread attention. Nonetheless, to overcome the trade-off between permeability and selectivity, which is crucial for achieving efficient separation, it is important to rationally design and manipulate MOF membrane structure. Given remarkable advances in the past decade, a timely summary of recent advancement in this field has become indispensable. This review introduces major strategies for fabricating oriented MOF membranes, including in situ growth, contra-diffusion method, interface-assisted approach, and laminated nanosheet assembly. New insights into their updated progress and potential are elucidated. Of particular note, recent development and emerging applications of oriented MOF membranes, illustrating their potential to address environmental and energy challenges, are highlighted. Finally, remaining challenges facing their bath production and practical applications are discussed.

Recent advances in the interfacial engineering of MOF-based mixed matrix membranes for gas separation

The membrane process stands as a promising and transformative technology for efficient gas separation due to its high energy efficiency, operational simplicity, low environmental impact, and easy up-and-down scaling. Metal-organic framework (MOF)-polymer mixed matrix membranes (MMMs) combine MOFs' superior gas-separation performance with polymers' processing versatility, offering the opportunity to address the limitations of pure polymer or inorganic membranes for large-scale integration. However, the incompatibility between the rigid MOFs and flexible polymer chains poses a challenge in MOF MMM fabrication, which can cause issues such as MOF agglomeration, sedimentation, and interfacial defects, substantially weakening membrane separation efficiency and mechanical properties, particularly gas separation. This review focuses on engineering MMMs' interfaces, detailing recent strategies for reducing interfacial defects, improving MOF dispersion, and enhancing MOF loading. Advanced characterisation techniques for understanding membrane properties, specifically the MOF-polymer interface, are outlined. Lastly, it explores the remaining challenges in MMM research and outlines potential future research directions.

Fabrication of Oriented MOF-Based Mixed Matrix Membrane via Ion-Induced Synchronous Synthesis

Developing a facile strategy for constructing oriented mixed matrix membranes (MMMs) with uniformly dispersed and high-loading metal-organic frameworks (MOFs) is a crucial scientific challenge in probing the enhanced capability and potential applications of MOF-polymer MMMs. Herein, a novel synchronous synthetic method for constructing oriented CuBDC/poly(m-phenylenediamine) (CuBDC/PmPD) MMM with uniform MOF dispersion at high loading at the air-solution interface via the dual function of metal ions is reported. The resulting MMM exhibits excellent separation performance in ion sieving and seawater desalination due to the structural integrity of the proposed membrane and the highly interconnected channels created through the oriented distribution of MOF in a polymer matrix. Such a cutting-edge approach may provide promising insights into the development of advanced MMMs with optimized structure and superior performances.

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.

Solid-solvent processing of ultrathin, highly loaded mixed-matrix membrane for gas separation

Mixed-matrix membranes (MMMs) that combine processable polymer with more permeable and selective filler have potential for molecular separation, but it remains difficult to control their interfacial compatibility and achieve ultrathin selective layers during processing, particularly at high filler loading. We present a solid-solvent processing strategy to fabricate an ultrathin MMM (thickness less than 100 nanometers) with filler loading up to 80 volume %. We used polymer as a solid solvent to dissolve metal salts to form an ultrathin precursor layer, which immobilizes the metal salt and regulates its conversion to a metal-organic framework (MOF) and provides adhesion to the MOF in the matrix. The resultant membrane exhibits fast gas-sieving properties, with hydrogen permeance and/or hydrogen-carbon dioxide selectivity one to two orders of magnitude higher than that of state-of-the-art membranes.

Covalent Organic Frameworks: The Rising-Star Platforms for the Design of CO2 Separation Membranes

Membrane-based carbon dioxide (CO₂ ) capture and separation technologies have aroused great interest in industry and academia due to their great potential to combat current global warming, reduce energy consumption in chemical separation of raw materials, and achieve carbon neutrality. The emerging covalent organic frameworks (COFs) composed of organic linkers via reversible covalent bonds are a class of porous crystalline polymers with regular and extended structures. The inherent structure and customizable organic linkers give COFs high and permanent porosity, short transport channel, tunable functionality, and excellent stability, thereby enabling them rising-star alternatives for developing advanced CO₂ separation membranes. Therefore, the promising research areas ranging from development of COF membranes to their separation applications have emerged. Herein, this review first introduces the main advantages of COFs as the state-of-the-art membranes in CO₂ separation, including tunable pore size, modifiable surfaces property, adjustable surface charge, excellent stability. Then, the preparation approaches of COF-based membranes are systematically summarized, including in situ growth, layer-by-layer stacking, blending, and interface engineering. Subsequently, the key advances of COF-based membranes in separating various CO₂ mixed gases, such as CO₂ /CH₄ , CO₂ /H₂ , CO₂ /N₂ , and CO₂ /He, are comprehensively discussed. Finally, the current issues and further research expectations in this field are proposed.

Mixed Matrix Membrane with Penetrating Subnanochannels: A Versatile Nanofluidic Platform for Selective Metal Ion Conduction

Biological ion channels penetrated through cell membrane form unique transport pathways for selective ionic conductance. Replicating the success of ion selectivity with mixed matrix membranes (MMMs) will enable new separation technologies but remains challenging. Herein, we report a soft substrate-assisted solution casting method to develop MMMs with penetrating subnanochannels for selective metal ion conduction. The MMMs are composed of penetrating Prussian white (PW) microcubes with subnanochannels in dense polyimide (PI) matrices, achieving selective monovalent metal ion conduction. The ion selectivity of K⁺ /Mg²⁺ is up to 14.0, and the ion conductance of K⁺ can reach 45.5 μS with the testing diameter of 5 mm, which can be further improved by increasing the testing area. Given the diversity of nanoporous materials and polymer matrices, we expect that the MMMs with penetrating subnanochannels could be developed into a versatile nanofluidic platform for various emerging applications.

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.

Core-Shell Carbon-Based Bifunctional Electrocatalysts Derived from COF@MOF Hybrid for Advanced Rechargeable Zn-Air Batteries

The development of highly active carbon-based bifunctional electrocatalysts for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desired, but still full of challenges in rechargeable Zn-air batteries. Metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have gained great attention for various applications due to their attractive features of structural tunability, high surface area and high porosity. Herein, a core-shell structured carbon-based hybrid electrocatalyst (H-NSC@Co/NSC), which contains high density active sites of MOF-derived shell (Co/NSC) and COF-derived hollow core (H-NSC), is successfully fabricated by direct pyrolysis of covalently-connected COF@ZIF-67 hybrid. The core-shell H-NSC@Co/NSC hybrid manifests excellent catalytic properties toward both OER and ORR with a small potential gap (∆E = 0.75 V). The H-NSC@Co/NSC assembled Zn-air battery exhibits a high power-density of 204.3 mW cm^-2 and stable rechargeability, outperforming that of Pt/C+RuO₂ assembled Zn-air battery. Density functional theory calculations reveal that the electronic structure of the carbon frameworks on the Co/NSC shell can be effectively modulated by the embedded Co nanoparticles (NPs), facilitating the adsorption of oxygen intermediates and leading to enhanced catalytic activity. This work will provide a strategy to design highly-efficient electrocatalysts for application in energy conversion and storage.

Rational design of mixed-matrix metal-organic framework membranes for molecular separations

Conventional separation technologies to separate valuable commodities are energy intensive, consuming 15% of the worldwide energy. Mixed-matrix membranes, combining processable polymers and selective adsorbents, offer the potential to deploy adsorbent distinct separation properties into processable matrix. We report the rational design and construction of a highly efficient, mixed-matrix metal-organic framework membrane based on three interlocked criteria: (i) a fluorinated metal-organic framework, AlFFIVE-1-Ni, as a molecular sieve adsorbent that selectively enhances hydrogen sulfide and carbon dioxide diffusion while excluding methane; (ii) tailoring crystal morphology into nanosheets with maximally exposed (001) facets; and (iii) in-plane alignment of (001) nanosheets in polymer matrix and attainment of [001]-oriented membrane. The membrane demonstrated exceptionally high hydrogen sulfide and carbon dioxide separation from natural gas under practical working conditions. This approach offers great potential to translate other key adsorbents into processable matrix.

Phase control of ZIF-7 nanoparticles via mechanochemical synthesis

MOF crystal phase control is made possible through a mechanochemical process.

Elimination of Grain Boundary Defects in Zeolitic Imidazolate Framework ZIF-95 Membrane via Solvent-Free Secondary Growth

Metal-organic framework membranes are usually prepared by in situ or secondary growth in a solution/hydrogel. The use of organic solvents may cause safety and environmental problems and produce solvent-induced defects. Here, highly oriented and permselective ZIF-95 membranes are prepared for the first time via a solvent-free secondary growth method. The solvent-free growth is not only helpful to control the membrane microstructure and thickness, but also to reduce the intercrystalline defects. In case of solvent-free growth, a perfectly oriented structure leads to an outstanding reduction of intercrystalline defects and transport resistances. For the separation of equimolar binary gas mixtures by using the highly oriented ZIF-95 membrane at 25 °C and 1 bar, the mixture separation factors of H₂ /CO₂ and H₂ /CH₄ are 184 and 140, respectively, with H₂ permeance of over 1.9×10^-7  mol m^-2  s^-1  Pa^-1 which are much higher than those of the randomly oriented ZIF-95 membrane.

Recent Progress in Mixed-Matrix Membranes for Hydrogen Separation

Membrane separation is a compelling technology for hydrogen separation. Among the different types of membranes used to date, the mixed-matrix membranes (MMMs) are one of the most widely used approaches for enhancing separation performances and surpassing the Robeson upper bound limits for polymeric membranes. In this review, we focus on the recent progress in MMMs for hydrogen separation. The discussion first starts with a background introduction of the current hydrogen generation technologies, followed by a comparison between the membrane technology and other hydrogen purification technologies. Thereafter, state-of-the-art MMMs, comprising emerging filler materials that include zeolites, metal-organic frameworks, covalent organic frameworks, and graphene-based materials, are highlighted. The binary filler strategy, which uses two filler materials to create synergistic enhancements in MMMs, is also described. A critical evaluation on the performances of the MMMs is then considered in context, before we conclude with our perspectives on how MMMs for hydrogen separation can advance moving forward.

Facile fabrication of a highly (110)-oriented ZIF-7 film with rod-shaped seeds

In this study, we report a novel synthetic strategy to prepare a highly (110)-oriented ZIF-7 film possessing superior anti-corrosion properties via oriented epitaxial growth. Our work provides insights into facile preparation of oriented uniform MOF single seed layers and films with rod-shaped MOF seeds as building blocks.

Carbon hollow fiber membranes for a molecular sieve with precise-cutoff ultramicropores for superior hydrogen separation

Carbon molecular sieve (CMS) membranes with rigid and uniform pore structures are ideal candidates for high temperature- and pressure-demanded separations, such as hydrogen purification from the steam methane reforming process. Here, we report a facile and scalable method for the fabrication of cellulose-based asymmetric carbon hollow fiber membranes (CHFMs) with ultramicropores of 3–4 Å for superior H₂ separation. The membrane fabrication process does not require complex pretreatments to avoid pore collapse before the carbonization of cellulose precursors. A H₂/CO₂ selectivity of 83.9 at 130 °C (H₂/N₂ selectivity of >800, H₂/CH₄ selectivity of >5700) demonstrates that the membrane provides a precise cutoff to discriminate between small gas molecules (H₂) and larger gas molecules. In addition, the membrane exhibits superior mixed gas separation performances combined with water vapor- and high pressure-resistant stability. The present approach for the fabrication of high-performance CMS membranes derived from cellulose precursors opens a new avenue for H₂-related separations.

Energy-efficient hydrogen purification technologies are needed for the hydrogen economy. Here the authors report facile and scalable fabrication of asymmetric carbon molecular sieve membranes for the separation of hydrogen and carbon dioxide.

Anisotropic Gas Separation in Oriented ZIF-95 Membranes Prepared by Vapor-Assisted In-Plane Epitaxial Growth

Control of the microstructure grain orientation, grain boundaries and thickness are crucial for MOF membranes. We report a novel synthesis strategy to prepare highly c-oriented ZIF-95 membranes through vapor-assisted in-plane epitaxial growth. In a mixed DMF/water vapor atmosphere, in-plane epitaxial growth of a ZIF-95 seeds layer was achieved to obtain an oriented and well-intergrown ZIF-95 membrane with a thickness of only 600 nm. Demonstrated by both experimental and simulation studies, the c-oriented ZIF-95 membrane displayed superior separation performance because a perfectly oriented structure resulted in a notable reduction of intercrystalline defects and transport pathways. For the separation of equimolar binary mixtures at 100 °C and 1 bar, the mixture separation factors of H₂ /CO₂ and H₂ /CH₄ were 32.2 and 53.7, respectively, with an H₂ permeance of over 7.9×10^-7  mol m^-2  s^-1  Pa^-1 , which was 4.6 times higher than that of a randomly oriented ZIF-95 membrane.

Unobstructed Ultrathin Gas Transport Channels in Composite Membranes by Interfacial Self-Assembly

Ultrathin unobstructed gas transport channels through the membrane selective layer are constructed in mixed matrix membranes (MMMs) by using gravity-induced interface self-assembly of poly(vinylamine) and polymer-modified MIL-101(Cr). For CO₂ /N₂ (15/85 by volume) mixed gas, the MMMs achieve a high CO₂ permeance of 823 gas permeation units and CO₂ /N₂ selectivity of 242 at 0.5 MPa. Based on economic analyses, a two-stage membrane process can achieve gas separation and economic targets.