Reversed thermo-switchable molecular sieving membranes composed of two-dimensional metal-organic nanosheets for gas separation

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.

Catalysis with Two-Dimensional Metal-Organic Frameworks: Synthesis, Characterization, and Modulation

The demand for the exploration of highly active and durable electro/photocatalysts for renewable energy conversion has experienced a significant surge in recent years. Metal-organic frameworks (MOFs), by virtue of their high porosity, large surface area, and modifiable metal centers and ligands, have gained tremendous attention and demonstrated promising prospects in electro/photocatalytic energy conversion. However, the small pore sizes and limited active sites of 3D bulk MOFs hinder their wide applications. Developing 2D MOFs with tailored thickness and large aspect ratio has emerged as an effective approach to meet these challenges, offering a high density of exposed active sites, better mechanical stability, better assembly flexibility, and shorter charge and photoexcited state transfer distances compared to 3D bulk MOFs. In this review, synthesis methods for the most up-to-date 2D MOFs are first overviewed, highlighting their respective advantages and disadvantages. Subsequently, a systematic analysis is conducted on the identification and electronic structure modulation of catalytic active sites in 2D MOFs and their applications in renewable energy conversion, including electrocatalysis and photocatalysis (electro/photocatalysis). Lastly, the current challenges and future development of 2D MOFs toward highly efficient and practical electro/photocatalysis are proposed.

Exfoliation of Metal-Organic Frameworks to Give 2D MOF Nanosheets for the Electrocatalytic Oxygen Evolution Reaction

The structure and properties of materials are determined by a diverse range of chemical bond formation and breaking mechanisms, which greatly motivates the development of selectively controlling the chemical bonds in order to achieve materials with specific characteristics. Here, an orientational intervening bond-breaking strategy is demonstrated for synthesizing ultrathin metal-organic framework (MOF) nanosheets through balancing the process of thermal decomposition and liquid nitrogen exfoliation. In such approach, proper thermal treatment can weaken the interlayer bond while maintaining the stability of the intralayer bond in the layered MOFs. And the following liquid nitrogen treatment results in significant deformation and stress in the layered MOFs' structure due to the instant temperature drop and drastic expansion of liquid N2, leading to the curling, detachment, and separation of the MOF layers. The produced MOF nanosheets with five cycles of treatment are primarily composed of nanosheets that are less than 10 nm in thickness. The MOF nanosheets exhibit enhanced catalytic performance in oxygen evolution reactions owing to the ultrathin thickness without capping agents which provide improved charge transfer efficiency and dense exposed active sites. This strategy underscores the significance of orientational intervention in chemical bonds to engineer innovative materials.

Bio-Inspired Subnanofluidics: Advanced Fabrication and Functionalization

Biological ion channels can realize high-speed and high-selective ion transport through the protein filter with the sub-1-nanometer channel. Inspired by biological ion channels, various kinds of artificial subnanopores, subnanochannels, and subnanoslits with improved ion selectivity and permeability are recently developed for efficient separation, energy conversion, and biosensing. This review article discusses the advanced fabrication and functionalization methods for constructing subnanofluidic pores, channels, tubes, and slits, which have shown great potential for various applications. Novel fabrication methods for producing subnanofluidics, including top-down techniques such as electron beam etching, ion irradiation, and electrochemical etching, as well as bottom-up approaches starting from advanced microporous frameworks, microporous polymers, lipid bilayer embedded subnanochannels, and stacked 2D materials are well summarized. Meanwhile, the functionalization methods of subnanochannels are discussed based on the introduction of functional groups, which are classified into direct synthesis, covalent bond modifications, and functional molecule fillings. These methods have enabled the construction of subnanochannels with precise control of structure, size, and functionality. The current progress, challenges, and future directions in the field of subnanofluidic are also discussed.

2D MOFs and Zeolites for Composite Membrane and Gas Separation Applications: A Brief Review

Commercial membranes have predominantly been fabricated from polymers due to their economic viability and processability. This choice offers significant advantages in energy efficiency, cost-effectiveness, and operational simplicity compared to conventional separation techniques like distillation. However, polymeric membranes inherently exhibit a trade-off between their permeability and selectivity, which is summarized in the Robeson upper bound. To potentially surpass these limitations, mixed-matrix membranes (MMMs) can be an alternative solution, which can be constructed by combining polymers with inorganic additives such as metal-organic frameworks (MOFs) and zeolites. Incorporating high-aspect-ratio fillers like MOF nanosheets and zeolite nanosheets is of significant importance. This incorporation not only enhances the efficiency of separation processes but also reinforces the mechanical robustness of the membranes. We outline synthesis techniques for producing two-dimensional (2D) crystals (including nanocrystals with high aspect ratio) and provide examples of their integration into membranes to customize separation performances. Moreover, we propose a potential trajectory for research in the area of high-aspect-ratio materials-based MMMs, supported by a mathematical-model-based performance prediction.

Construction of Porphyrin-Based Bimetallic Nanomaterials with Photocatalytic Properties

The efficient synthesis of nanosheets containing two metal ions is currently a formidable challenge. Here, we attempted to dope lanthanide-based bimetals into porphyrin-based metal-organic skeleton materials (MOFs) by microwave-assisted heating. The results of the EDX, ICP, and XPS tests show that we have successfully synthesized porphyrin-based lanthanide bimetallic nanosheets (Tb-Eu-TCPP) using a household microwave oven. In addition, it is tested and experimentally evident that these nanosheets have a thinner thickness, a larger BET surface area, and higher photogenerated carrier separation efficiency than bulk porphyrin-based bimetallic materials, thus exhibiting enhanced photocatalytic activity and n-type semiconductor properties. Furthermore, the prepared Tb-Eu-TCPP nanomaterials are more efficient in generating single-linear state oxygen under visible light irradiation compared to pristine monometallic nanosheets due to the generation of bimetallic nodes. The significant increase in catalytic activity is attributed to the improved separation and transfer efficiency of photogenerated carriers. This study not only deepens our understanding of lanthanide bimetallic nanosheet materials but also introduces an innovative approach to improve the photocatalytic performance of MOFs.

Solution processing of crystalline porous material based membranes for CO2 separation

The carbon emission problem is a significant challenge in today's society, which has led to severe global climate issues. Membrane-based separation technology has gained considerable interest in CO2 separation due to its simplicity, environmental friendliness, and energy efficiency. Crystalline porous materials (CPMs), such as zeolites, metal-organic frameworks, covalent organic frameworks, hydrogen-bonded organic frameworks, and porous organic cages, hold great promise for advanced CO2 separation membranes because of their ordered and customizable pore structures. However, the preparation of defect-free and large-area crystalline porous material (CPM)-based membranes remains challenging, limiting their practical use in CO2 separation. To address this challenge, the solution-processing method, commonly employed in commercial polymer preparation, has been adapted for CPM membranes in recent years. Nanosheets, spheres, molecular cages, and even organic monomers, depending on the CPM type, are dissolved in suitable solvents and processed into continuous membranes for CO2 separation. This feature article provides an overview of the recent advancements in the solution processing of CPM membranes. It summarizes the differences among the solution-processing methods used for forming various CPM membranes, highlighting the key factors for achieving continuous membranes. The article also summarizes and discusses the CO2 separation performance of these membranes. Furthermore, it addresses the current issues and proposes future research directions in this field. Overall, this feature article aims to shed light on the development of solution-processing techniques for CPM membranes, facilitating their practical application in CO2 separation.

Construction and Tribological Performance of Ag Nanowire-Dotted 2D Ni-BDC Nanocomposites in Oil

The collaborative lubrication effect based on different lubricating nanostructures could not only overcome the respective drawbacks of different nanostructures as nanoadditives but also comprehensively improve friction performance. For this, we first developed a novel collaborative lubrication nanostructure based on Ag nanowires (NWs) and two-dimensional (2D) Ni-BDC nanosheets (Ag NWs/2D Ni-BDC) by the in situ chemical reduction strategy. The structural characterizations corroborated that Ag NWs with a width of approximately 20-30 nm grew on the surface of 2D Ni-BDC nanosheets, which presented the chemical interface between both. As the nanoadditive applied in oils, the tribological finding confirmed that the Ag NWs/2D Ni-BDC enabled the friction coefficient and wear scar diameter to reduce by 38 and 37% at 0.04 wt % additive content, respectively. The worn surface was characterized by a series of analytical techniques, which basically revealed the friction-reducing and antiwear mechanism based on the layered structure and self-repair effect. Consequently, the nanocomposite structure incorporating the nanowire and 2D nanosheets should be taken into account as a new potential lubricating material.

Recent Progress in 2D Metal-Organic Framework-Related Materials

2D metal-organic frameworks-based (2D MOF-related) materials benefit from variable topological structures, plentiful open active sites, and high specific surface areas, demonstrating promising applications in gas storage, adsorption and separation, energy conversion, and other domains. In recent years, researchers have innovatively designed multiple strategies to avoid the adverse effects of conventional methods on the synthesis of high-quality 2D MOFs. This review focuses on the latest advances in creative synthesis techniques for 2D MOF-related materials from both the top-down and bottom-up perspectives. Subsequently, the strategies are categorized and summarized for synthesizing 2D MOF-related composites and their derivatives. Finally, the current challenges are highlighted faced by 2D MOF-related materials and some targeted recommendations are put forward to inspire researchers to investigate more effective synthesis methods.

Air/liquid interfacial formation process of conductive metal-organic framework nanosheets

The air/liquid interface is a superior platform to create nanosheets of materials by promoting spontaneous two-dimensional growth of components. Metal-organic frameworks (MOFs)-intrinsically porous crystals-with π-conjugated triphenylene-based ligands show high electrical conductivities. Forming nanosheets of such conductive MOFs should enable their use in electronic devices. Although highly conductive MOF nanosheets have been created at the air/liquid interface, direct control of their continuity, morphology, thickness, crystallinity, and orientation directly influencing device performance remains as an issue to be addressed. Here, we present detailed insights into the formation process of electrically conductive MOF nanosheets composed of 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and Ni2+ ions (HITP-Ni-NS) at the air/liquid interface. The morphological and structural features of HITP-Ni-NS strongly depend on the standing time-the time without any external actions involved, but leaving the interface undisturbed after setting the ligand solution onto the metal-ion solution. We find that the fundamental features of HITP-Ni-NS are determined by the standing time with conductivity sensitively influenced by such pre-determined HITP-Ni-NS characteristics. These findings will lead towards the establishment of a rational strategy for creating MOF nanosheets at the air/liquid interface with desired properties, thereby accelerating their use in diverse potential applications.

Metallosupramolecular polymers: current status and future prospects

Metallo-supramolecular polymers have gained increasing attention and witnessed continuous development as a vibrant new research interest in the domain of soft materials. These nonconventional polymers have found widespread application in materials and biology owing to their well-defined and diversified topologies and the distinct dynamic nature of the metallosupramolecular interactions against various stimuli. Because of the intriguing redox, photonic, electronic, and magnetic properties, these stimuli-responsive supramolecular structures have attracted considerable interest for optoelectronic device fabrication. However, it still remains challenging to develop stimuli responsive systems with offbeat applications. Furthermore, achieving spatiotemporal control remains elusive with thermoresponsive and sono-responsive metallosupramolecular polymers, which encounter the disadvantage of poor precision control. Additionally, controlling the morphology of these soft materials on the mesoscale, both in solution and on substrates, has many challenges. In this review, we discuss the recent developments and future directions for the construction of stimuli responsive metallosupramolecular systems targeting practical applications. Furthermore, we discuss the synthetic methodologies that have been used to regulate the mesoscale morphology of these materials, such as coordination modulation and pseudomorphic replication. Finally, we briefly cover the burgeoning field of programmed synthesis of metallosupramolecular polymers, emphasizing techniques, such as living polymerization and chemical fuel-driven transiently active systems, which we believe will be the major research directions in the future.

Tuning the Stacking Modes of Ultrathin Two-Dimensional Metal-Organic Framework Nanosheet Membranes for Highly Efficient Hydrogen Separation

Two-dimensional (2D) metal-organic framework (MOF) membranes are considered potential gas separation membranes of the next generation due to their structural diversity and geometrical functionality. However, achieving a rational structure design for a 2D MOF membrane and understanding the impact of MOF nanosheet stacking modes on membrane separation performance remain challenging tasks. Here, we report a novel kind of 2D MOF membrane based on [Cu2 Br(IN)2 ]n (IN=isonicotinato) nanosheets and propose that synergetic stacking modes of nanosheets have a significant influence on gas separation performance. The stacking of the 2D MOF nanosheets is controlled by solvent droplet dynamic behaviors at different temperatures of drop coating. Our 2D MOF nanosheet membranes exhibit high gas separation performances for H2 /CH4 (selectivity >290 with H2 permeance >520 GPU) and H2 /CO2 (selectivity >190 with H2 permeance >590 GPU) surpassing the Robeson upper bounds, paving a potential way for eco-friendly H2 separation.

Exfoliation of 2D Metal-Organic Frameworks: toward Advanced Scalable Materials for Optical Sensing

Two-dimensional metal-organic frameworks (MOFs) occupy a special place among the large family of functional 2D materials. Even at a monolayer level, 2D MOFs exhibit unique sensing, separation, catalytic, electronic, and conductive properties due to the combination of porosity and organo-inorganic nature. However, lab-to-fab transfer for 2D MOF layers faces the challenge of their scalability, limited by weak interactions between the organic and inorganic building blocks. Here, comparing three top-down approaches to fabricate 2D MOF layers (sonication, freeze-thaw, and mechanical exfoliation), The technological criteria have established for creation of the layers of the thickness up to 1 nm with a record aspect ratio up to 2*10^4:1. The freezing-thaw and mechanical exfoliation are the most optimal approaches; wherein the rate and manufacturability of the mechanical exfoliation rivaling the greatest scalability of 2D MOF layers obtained by freezing-thaw (21300:1 vs 1330:1 aspect ratio), leaving the sonication approach behind (with a record 900:1 aspect ratio) have discovered. The high quality 2D MOF layers with a record aspect ratio demonstrate unique optical sensitivity to solvents of a varied polarity, which opens the way to fabricate scalable and freestanding 2D MOF-based atomically thin chemo-optical sensors by industry-oriented approach.

Energy-efficient extraction of linear alkanes from various isomers using structured metal-organic framework membrane

Extraction of low concentration linear alkanes (C5-C7) from various isomers is critical for the petrochemical industry. At present, the separation of alkane isomers is mainly accomplished by distillation, which results in substantial energy expenditure. Metal-organic frameworks (MOFs) with well-tailored nanopores have been demonstrated to be capable of realizing molecule-level separation. In this study, oriented HKUST-1 membranes are formulated according to the morphology-biased principle and finally realized with a low dose synthesis method for terminating undesired crystal nucleation and growth. The fully exposed triangular sieving pore array of the membrane induces configuration entropic diffusion to split linear alkanes from mono-branched and di-branched isomers as well as their cyclical counterparts. Typically, the current separation technique consumes 91% less energy than vacuum distillation. Furthermore, our membranes can realize one-step extraction of normal-pentane, normal-hexane and normal-heptane from a ten-component alkane isomer solution that mimics light naphtha.

Ultrathin membranes to sieve gases

Mixed-matrix membranes could enable gas separation for carbon capture.

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.

Heat-driven molecule gatekeepers in MOF membrane for record-high H2 selectivity

Hydrogen/carbon dioxide (H₂/CO₂) separation for sustainable energy is in desperate need of reliable membranes at high temperatures. Molecular sieve membranes take their nanopores to differentiate sizes between H₂ and CO₂ but have compromised at a marked loss of selectivity at high temperatures owing to diffusion activation of CO₂. We used molecule gatekeepers that were locked in the cavities of the metal-organic framework membrane to meet this challenge. Ab initio calculations and in situ characterizations demonstrate that the molecule gatekeepers make a notable move at high temperatures to dynamically reshape the sieving apertures as being extremely tight for CO₂ and restitute with cool conditions. The H₂/CO₂ selectivity was improved by an order of magnitude at 513 kelvin (K) relative to that at the ambient temperature.

Assembling Triphenylene-Based Metal-Organic Framework Nanosheets at the Air/Liquid Interface: Modification by Tuning the Spread Solution Concentration

Metal-organic frameworks (MOFs)─crystalline coordination polymers─with unique characteristics such as structural designability accompanied by tunable electronic properties and intrinsic uniform nanopores have become the platform for applications in diverse scientific areas ranging from nanotechnology to energy/environmental sciences. To utilize the superior features of MOF in potential applications, the fabrication and integration of thin films are of importance and have been actively sought. Especially, downsized MOFs into nanosheets can act as ultimately thin functional components in nanodevices and potentially display unique chemical/physical properties rarely seen in bulk MOFs. Assembling nanosheets by aligning amphiphilic molecules at the air/liquid interface has been known as the Langmuir technique. By utilizing the air/liquid interface as a reaction field between metal ions and organic ligands, MOFs are readily formed into the nanosheet state. The expected features in MOF nanosheets including electrical conduction largely depend on the nanosheet characteristics such as lateral size, thickness, morphology, crystallinity, and orientation. However, their control has not been achieved as yet. Here, we demonstrate how changing the concentration of a ligand spread solution can modify the assembly of MOF nanosheets, composed of 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and Ni²⁺ ions (HITP-Ni-NS), at the air/liquid interface. A systematic increase in the concentration of the ligand spread solution leads to the enlargement of both the lateral size and the thickness of the nanosheets while retaining their perfect alignment and preferred orientation. On the other hand, at much higher concentrations, we find that unreacted ligand molecules are included in HITP-Ni-NS, introducing disorder in HITP-Ni-NS. These findings can develop further sophisticated control of MOF nanosheet features, accelerating fundamental and applied studies on MOFs.

Facile Synthesis of Ultra-microporous Pillar-Layered Metal-Organic Framework Membranes for Highly H2-Selective Separation

Recently, pillar-layered MOF materials have attracted much attention and shown great potential in separation application due to their fine pore size/channel and pore surface chemistry tunability and designability. In this work, we reported an effective and universal synthesis strategy for preparing ultra-microporous Ni-based pillar-layered MOF [Ni₂(L-asp)₂(bpy)] (Ni-LAB) and [Ni₂(L-asp)₂(pz)] (Ni-LAP) (L-asp = L-aspartic acid, bpy = 4,4'-bipyridine, pz = pyrazine) membranes on a porous α-Al₂O₃ substrate with high performance and good stability by secondary growth. Through this strategy, the seed size reduction and screening engineering (SRSE) is proposed to obtain uniform sub-micron size MOF seeds by high-energy ball milling-combined solvent deposition. This strategy not only effectively addresses the issue of obtaining the uniform small seeds being significant for secondary growth but also provides an approach for the preparation of Ni-based pillar-layered MOF membranes where the freedom of synthesizing small crystals is lacking. Based on reticular chemistry, the pore size of Ni-LAB was narrowed by making use of shorter pillar ligands of pz instead of the longer pillar ligand of bpy. The prepared ultra-microporous Ni-LAP membranes exhibited a high H₂/CO₂ separation factor of 40.4 with H₂ permeance of 9.69 × 10^-8 mol m^-2 s^-1 Pa^-1 under ambient conditions and good mechanical and thermal stability. The superiority of the tunable pore structure and the remarkable stability of these MOF materials showed great potential for industrial H₂ purification. More importantly, our synthesis strategy demonstrated the generality for preparation of MOF membranes, enabling the regulation of membrane pore size and surface functional groups by reticular chemistry.

Synthesis of 2D Metal-Organic Nanosheets (MONs) by Liquid Phase Exfoliation: Applications in Effective Delivery of Antiulcer Drugs and Selective Adsorption and Removal of Cationic Dyes

Nowadays, the fabrication of 2D metal-organic nanosheets (2D MONs) has entered the research arena fascinating researchers worldwide. However, a lack of efficient and facile methods has remained a bottleneck for the manufacturing of these 2D MONs. Herein, a 2D metal-organic framework (MOF), i.e., 2D Cu-MOF, was synthesized using a facile and convenient stirring method by using 4,4'-trimethylenedipyridine (TMDP) as an organic linker. The as-prepared MOF was characterized in detail and based on single crystal X-ray diffraction analysis, it was established that tangled layers in the 2D Cu-MOF are interconnected to produce thick strands. These tangled layers could be easily separated via ultrasonication-induced liquid phase exfoliation (UILPE) to give the 2D Cu-MON as illustrated through Tyndall light scattering and exhaustive microscopic exploration such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The application of this 2D Cu-MON was assessed in the field of drug delivery revealing exceptional drug loading for the drug lansoprazole (LPZ) by 2D Cu-MONs as well as drug release in the acidic and neutral medium demonstrating that the 2D Cu-MON is an excellent carrier for antiulcer drug delivery. For environmental protection, the application of 2D Cu-MON was also examined toward the removal of various cationic and anionic dyes with excellent selectivity toward cationic dye removal. The plausible mechanism for dye removal indicated the involvement of cation-π and π-π interactions, for the effective adsorption of cationic dyes as well as a increase in the surface area of 2D Cu-MON by UILPE. Remarkably, the high drug loading and dye removal are imputed to the increase in surface area by UILPE. In a nutshell, the developed 2D Cu-MON will prove to be beneficial for application in the field of drug delivery as well as for wastewater treatment.

The recent advance of precisely designed membranes for sieving

Developing new membranes with both high selectivity and permeability is critical in membrane science since conventional membranes are often limited by the trade-off between selectivity and permeability. In recent years, the emergence of advanced materials with accurate structures at atomic or molecular scale, such as metal organic framework, covalent organic framework, graphene, has accelerated the development of membranes, which benefits the precision of membrane structures. In this review, current state-of-the-art membranes are first reviewed and classified into three different types according to the structures of their building blocks, including laminar structured membranes, framework structured membranes and channel structured membranes, followed by the performance and applications for representative separations (liquid separation and gas separation) of these precisely designed membranes. Last, the challenges and opportunities of these advanced membranes are also discussed.

Two-dimensional nanomaterial MXenes for efficient gas separation: a review

Transition metal carbides/nitrides (MXenes) are emerging two-dimensional (2D) materials that have been widely investigated in recent years. In general, these materials can be obtained from MAX phase ceramics after intercalation, etching, and exfoliation to obtain multilayer MXene nanosheet structures; moreover, they have abundant end-group functional groups on their surface. In recent years, the excellent high permeability, fine sieving ability and diverse processability of MXene series materials make the membranes prepared using them particularly suitable for membrane-based separation processes in the field of gas separation. 2D membranes enhance the diversity of the pristine membrane transport channels by regulating the gas transport channels through in-plane pores (intrinsic defects), in-plane slit-like pores, and planar to planar interlayer channels, endowing the membrane with the ability to effectively sieve gas energy efficiently. Herein, we review MXenes, a class of 2D nanomaterials, in terms of their unique structure, synthesis method, functionalization method, and the structure-property relationship of MXene-based gas separation membranes and list examples of MXene-based membranes used in the field of gas separation. By summarizing and analyzing the basic properties of MXenes and demonstrating their unique advantages compared to other 2D nanomaterials, we lay a foundation for the discussion of MXene-based membranes with outstanding carbon dioxide (CO₂) capture performance and outline and exemplify the excellent separation performances of MXene-based gas separation membranes. Finally, the challenges associated with MXenes are briefly discussed and an outlook on the promising future of MXene-based membranes is presented. It is expected that this review will provide new insights and important guidance for future research on MXene materials in the field of gas separation.

2D Organic Materials: Status and Challenges

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

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.

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.

Comparative study of sonication-assisted liquid phase exfoliation of six layered coordination polymers

Sonication-assisted liquid phase exfoliation was applied to six different layered coordination polymers (CPs) in aqueous surfactant solution. The resulting nanosheets were investigated for structural and compositional integrity and microscopic analysis gives insights into the relationship between the crystal structure of the materials and their exfoliability. Larger open pores seem to favour the production of nanosheets with higher aspect ratio of lateral size to thickness.

Ultrathin metal-organic framework nanosheets as building blocks of lamellar nanofilms for ultrafast molecular sieving

Metal-organic framework (MOF) nanosheets have significant potential applications including separation, catalysis, and sensors. However, the on-demand design with tunable thickness and morphology remains a great challenge, leading to difficulties in modulating their hierarchical assembly for the preparation of macroscopic films. Herein, we report the successful synthesis of smooth and ultrathin MOF (Cu-TCPP (TCPP = 4,4,4,4-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid))) nanosheets used in lamellar nanofilms for the rejection of organic molecules from water. Dopamine hydrochloride (DA-HCl) is used as an adjuvant in the synthesis. Facilitated by a HCl acid environment and DA competitive coordination, the normal and lateral growths of Cu-TCPP nanosheets are modulated to achieve the desired thickness and morphology. DA-HCl can be also easily removed from the nanosheets without affecting their physicochemical properties. The as-synthesized nanosheets are utilized as nanofilm building blocks in which they are stacked into ordered bricks. The obtained membrane displays an ultrahigh water permeance of 2540 L m^-2 h^-1 bar^-1, which is two orders of magnitude higher than the currently reported polymer membranes, while it does not sacrifice the solute rejection as completely determined by the intrinsic pore size of the nanosheets (i.e., 98.8% for molecules larger than 1.3 nm). This work provides a novel and facile strategy to tailor the morphology of the MOF nanosheets for maximizing their functionalities and structure superiority in many engineering applications.

Multidimensional Building Blocks for Molecular Sieve Membranes

Chemical separations aiming for high-purity commodities are critical to modern society. Compared to distillation, chemical absorption, and adsorption, membrane separation is attractive for its energy efficiency, ease of operation, and compact footprint. Molecular sieve membranes (MSMs) are broadly defined as membranes that are constructed from intrinsically and artificially porous materials. On the basis of our recent studies, this Account will first summarize the evolution of MSMs from the viewpoint of dimensionality of building blocks, which fundamentally determines the stacking architectures, intercrystalline gaps, and mass transfer channels of MSMs. Intergrowth of three-dimensional (3D) crystals as primary building blocks gives rise to classical MSMs. However, the poor connection between crystals inherent to those membranes results in intercrystalline gaps that are catastrophic for separation selectivity. We adopted a variety of strategies to close the crystal boundary gaps, including microwave synthesis, electrochemical-ionothermal synthesis, and modular integration. These efforts make us better understand the structure-performance relationship in membranes and create solutions for industrial processes. Excitingly, we first scaled-up the microwave synthesis of a Linde type A (LTA) zeolite membrane and built the world's largest ethanol dehydration membrane unit with an annual capacity of 100,000 tons. MSMs can also be made of two-dimensional (2D) nanosheets as primary building blocks. Those strike a balance between permeation rate and selectivity because the nanometer thickness ensures the minimization of the mass-transfer resistance of the membrane and the layer-by-layer stacking mode can significantly reduce the intercrystalline gaps. By publishing our first report on metal-organic framework (MOF) nanosheet membranes in Science, we committed to establishing top-down and bottom-up methods for assembly of laminae. Once the stacking, orientation, and connection between the layers are meticulously controlled, nanosheet building blocks with diversity open the door for ultrapermeable and selective MSMs. We recently proposed a supramolecule array membrane (SAM) with zero-dimensional (0D) molecules as primary building blocks, which has great potential to absolutely eliminate intercrystalline gaps in membranes. In contrast to the classical transport through nanopores of membranes, selective transport through the intermolecular spacing of supramolecules is creatively realized within the SAM, which marks a new breakthrough in ultraprecise sieving of molecules with tiny differences in size and revolutionizes MSMs in regard to stacking modes, intercrystalline gaps, and transport channels. MSMs have proven to be successful in diverse applications and have triggered wide interest. A unique perspective on the dimensionality evolution of building blocks will accelerate the progress of MSMs. The synergy of multidimensional MSMs will be a positive response to fundamental bottlenecks and industrial questions of membranes and will unlock the potential of membranes to displace the existing separation technologies in the future.

Metal Confined in 2D Membranes for Molecular Recognition and Sieving towards Ethylene/Ethane Separation

Membranes with nanochannels have exhibited great potential in molecular separations, while it remains a great challenge to separate molecules with very close physical properties and kinetic diameters (e.g., ethylene/ethane) owing to the lack of size-sieving property and specific affinity. Herein, a metal confined 2D sub-nanometer channel is reported to successfully discriminate ethylene over ethane via molecular recognition and sieving. Transition metal cations are paired with polyelectrolyte anions to achieve high dissociation activity, forming reversible complexation with ethylene. Aberration-corrected transmission electron microscopy observes that the metals with size of ≈2 nm are uniformly confined in graphene oxide (GO) interlayer channels with average height of ≈0.44 nm, thereby cooperating the size-sieving effect with a molecular recognition ability toward ethylene and stimulating its selective transport over ethane. The resulting ultrathin (≈60 nm) membrane exhibits superior ethylene/ethane separation performance far beyond the polymeric upper-bound. Density functional theory (DFT) and molecular dynamic simulations reveal that the metal@2D interlayer channel provides a molecular recognition pathway for selective gas transport. The proposed metal confined in 2D channel with molecular recognition and sieving properties would have broad application in other related fields such as single-atom catalysis, sensor and energy conversion.

Highly flexible and superhydrophobic MOF nanosheet membrane for ultrafast alcohol-water separation

High-performance pervaporation membranes have potential in industrial separation applications, but overcoming the permeability-selectivity trade-off is a challenge. We report a strategy to create highly flexible metal-organic framework nanosheet (MOF-NS) membranes with a faveolate structure on polymer substrates for alcohol-water separation. The controlled growth followed by a surface-coating method effectively produced flexible and defect-free superhydrophobic MOF-NS membranes. The reversible deformation of the flexible MOF-NS and the vertical interlamellar pathways were captured with electron microscopy. Molecular simulations confirmed the structure and revealed transport mechanism. The ultrafast transport channels in MOF-NS exhibited an ultrahigh flux and a separation factor of 8.9 in the pervaporation of 5 weight % ethanol-water at 40°C, which can be used for biofuel recovery. MOF-NS and polydimethylsiloxane synergistically contribute to the separation performance.

Fast hydrogen purification through graphitic carbon nitride nanosheet membranes

Two-dimensional graphitic carbon nitride (g-C₃N₄) nanosheets are ideal candidates for membranes because of their intrinsic in-plane nanopores. However, non-selective defects formed by traditional top-down preparation and the unfavorable re-stacking hinder the application of these nanosheets in gas separation. Herein, we report lamellar g-C₃N₄ nanosheets as gas separation membranes with a disordered layer-stacking structure based on high quality g-C₃N₄ nanosheets through bottom-up synthesis. Thanks to fast and highly selective transport through the high-density sieving channels and the interlayer paths, the membranes, superior to state-of-the-art ones, exhibit high H₂ permeance of 1.3 × 10⁻⁶mol m⁻² s⁻¹ Pa⁻¹ with excellent selectivity for multiple gas mixtures. Notably, these membranes show excellent stability under harsh practice-relevant environments, such as temperature swings, wet atmosphere and long-term operation of more than 200 days. Therefore, such lamellar membranes with high quality g-C₃N₄ nanosheets hold great promise for gas separation applications.

In this work, lamellar graphitic carbon nitride nanosheet membranes are constructed for gas separation. Benefiting from their high-density intrinsic in-plane nanopores and broader permeable interlayer channels, the proposed membranes exhibit high H₂ permeance with good selectivity of multiple gas mixtures.

Molecular insight into CO2/N2 separation using a 2D-COF supported ionic liquid membrane

The covalent organic framework (COF) shows great potential for use in gas separation because of its uniform and high-density sub-nanometer sized pores. However, most of the COF pore sizes are large, and there are mismatches with the gas pairs (3-6 Å), and the steric hindrance cannot work in gas selectivity. In this work, one type of COF (NUS-2) supported ionic liquid membrane (COF-SILM) was prepared for use in CO₂/N₂ separation. The separation performance was investigated using molecular dynamics simulation. There was an ultrahigh CO₂ permeability up to 2.317 × 10⁶ GPU, and a better CO₂ selectivity was obtained when compared to that of N₂. The physical mechanism of ultrahigh permeability and CO₂ selectivity are discussed in detail. The ultrathin membrane, high-density pores and high transmembrane driving force are responsible for the ultrahigh permeability of CO₂. The different adsorption capabilities of ionic liquid (IL) for CO₂ and N₂, as well as a gating effect, which allows CO₂ passage and inhibits N₂ passage, contribute to the better CO₂ selectivity over N₂. Moreover, the effects of the COF layer number and IL thickness on gas separation performance are also discussed. This work provides a molecular level understanding of the gas separation mechanism of COF-SILM, and the simulation results show one potential outstanding CO₂ separation membrane for future applications.

Dimensionality Mediated Highly Repeatable and Fast Transformation of Coordination Polymer Single Crystals for All-Optical Data Processing

A family of coordination polymers (CPs) based on dynamic structural elements are of great fundamental and commercial interest addressing modern problems in controlled molecular separation, catalysis, and even data processing. Herein, the endurance and fast structural dynamics of such materials at ambient conditions are still a fundamental challenge. Here, we report on the design of a series of Cu-based CPs [Cu(bImB)Cl₂] and [Cu(bImB)₂Cl₂] with flexible ligand bImB (1,4-bis(imidazol-1-yl)butane) packed into one- and two-dimensional (1D, 2D) structures demonstrating dimensionality mediated flexibility and reversible structural transformations. Using the laser pulses as a fast source of activation energy, we initiate CP heating followed by anisotropic thermal expansion and 0.2-0.8% volume changes with the record transformation rates from 2220 to 1640 s^-1 for 1D and 2D CPs, respectively. The endurance over 10³ cycles of structural transformations, achieved for the CPs at ambient conditions, allows demonstrating optical fiber integrated all-optical data processing.

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.

One pot synthesis of UiO-66@IL composite for fabrication of CO2 selective mixed matrix membranes

In this study, a facile and extensible one pot approach was utilized to synthesize ionic liquid inside a porous metal organic framework (UiO-66). Different characterization techniques were used to confirm the successful synthesis of UiO-66@IL composite. The MMMs were characterized and tested for CO₂ separation from CH₄ or N₂ at ambient and elevated temperatures. SEM images exhibited well dispersion of the filler particles with no notable defect even at high loadings. Single and mixed gas permeation results indicated significant performance (CO₂ permeability: 143 Barrer and CO₂/CH₄, CO₂/N₂ selectivity: 28.32, 61.11 respectively) by enhancing the permeability of CO₂ by 74% and selectivity to 31% and 26% for CO₂/CH₄ and CO₂/N₂ compared with neat Pebax®1657 membrane.

Absorption-Dominant, Low-Reflection Multifunctional Electromagnetic Shielding Material Derived from Hydrolysate of Waste Leather Scraps

High-performance flexible conductive films are highly promising for the development of wearable devices, artificial intelligence, medical care, etc. Herein, a three-step procedure was developed to produce electromagnetic interference (EMI) shielding, Joule heating, and a hydrophobic nanofiber film based on hydrolysate of waste leather scraps (HWLS): (i) electrospinning preparation of the HWLS/polyacrylonitrile (PAN)/zeolitic imidazolate framework-67 (ZIF-67) nanofiber film, (ii) carbonization of the HWLS/PAN/ZIF-67 nanofiber film, and (iii) coating of the carbon nanofiber@cobalt (Co@CNF) nanofiber film with perfluorooctyltriethoxysilane (POTS). The X-ray diffraction results showed that metal nanoparticles and amorphous carbon had obvious peaks. The micromorphology results showed that metal nanoparticles were coated with carbon nanofibers. The conductivity and shielding efficiency of the carbon nanofiber film with 250 μm thickness could reach 45 S/m and 49 dB, respectively, and absorption values (A > 0.5) were higher than reflection (R) values for the Co@CNF nanofiber film, which indicated that the contribution of absorption loss was more significant than that of reflection loss. Ultrafast electrothermal response performances were also achieved, which could guarantee the normal functioning of films in cold conditions. The water contact angle of the Co@CNF@POTS nanofiber film was ∼151.3°, which displayed a self-cleaning property with water-proofing and antifouling. Absorption-dominant and low-reflection EMI shielding and electrothermal films not only showed broad application potential in flexible wearable electronic devices but also provided new avenues for the utilization of leather solid waste.

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.

Super-Assembled Chiral Mesostructured Heteromembranes for Smart and Sensitive Couple-Accelerated Enantioseparation

In the context of sustainable development, chirality, especially chiral drugs, has attracted great interest in the pharmaceutical industry, yet the smart and sensitive separation of enantiomers still presents a major scientific challenge. Herein, inspired by supramolecular templating via chiral transcription nanoparticles, an artificial chiral nanochannel membrane with asymmetric structure, porosity, and abundant chiral surface is fabricated for smart and sensitive enantiomer recognition and separation. Constructed from chiral transcript mesoporous silica (CMS) super-assembled on a porous anode alumina oxide (AAO) support, the obtained heterostructured chiral membrane (CMS/AAO) exhibits enhanced enantioseparation (approximately 170% compared to the supramolecular-templated nanoparticles) among a series of amino acids with various isoelectric points (PIs). Especially for amino acids with a PI greater than 7, the couple-accelerated enantioseparation (CAE) can be achieved for the first time. Further analysis using an osmotic energy conversion test and simulations based on the Poisson-Nernst-Planck (PNP) equations confirm that the heterostructure and charge polarity are the key to achieve chiral amino acids and ion separation. We expect this work will inspire the development of multifunctional membrane systems for more sustainable and energy-efficient enantioseparation.

Two-dimensional metal-organic layers constructed from Hf6/Hf12-oxo clusters and a trigonal pyramidal phosphine oxide ligand

Metal-organic layers (MOLs), a category of two-dimensional materials, have attracted wide interest due to their molecular tunability and the ease of surface modification. Herein, we reported the synthesis and structural determination of a free-standing MOL, {[Hf₆O₈H₄(HCOO)₂(H₂O·OH)₄]₃[Hf₁₂O₁₆H₈(HCOO)_6.8(H₂O·OH)_11.2](TPO)₈}_n, constructed from Hf₆-oxo and Hf₁₂-oxo clusters as secondary building units (SBUs) and the tris(4-carboxylphenyl)phosphine oxide (TPO) ligand. We establish a structure model of this new MOL based on the combined information from different characterization methods.