Engineering of the Filler/Polymer Interface in Metal–Organic Framework‐Based Mixed‐Matrix Membranes to Enhance Gas Separation

Traditional films cannot fully adapt to industrial applications and to intensified processes. Advanced mixed‐matrix membranes comprising metal–organic frameworks (MOF) embedded in a polymer matrix have been developed with the goal of breaking the trade‐off effect of traditional polymer membranes and achieving separation performance beyond Robeson's upper limit. The key challenges in the fabrication of MOF‐based mixed‐matrix membranes are an enhancement in compatibility between the inorganic filler and the polymer matrix, elimination of the irregular morphology and non‐selective interfacial defects, and further improvement in the gas‐separation performance. This review summarizes the recent advances in protocols and strategies in terms of designing interfacial interactions to enhance the MOF/polymer interface compatibility. This review aims at providing some meaningful insights into preparing MOF‐based mixed‐matrix membranes targeting ideal interfacial morphology and leading to excellent gas‐separation performance.     

Two‐Dimensional Metal–Organic Framework Nanosheets for Membrane‐Based Gas Separation

Metal–organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass‐transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet‐based membranes remain as great challenges. A modified soft‐physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub‐10 nm‐thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H₂/CO₂ separation performance, with a separation factor of up to 166 and H₂ permeance of up to 8×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at elevated testing temperatures owing to a well‐defined size‐exclusion effect. This nanosheet‐based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.     

A Novel Hybrid Metal–Organic Framework–Polymeric Monolith for Solid‐Phase Microextraction

This study describes the fabrication of a novel hybrid metal–organic framework– organic polymer (MOF–polymer) for use as a stationary phase in fritless solid‐phase microextraction (SPME) for validating analytical methods. The MOF–polymer was prepared by using ethylene dimethacrylate (EDMA), butyl methacrylate (BMA), and an imidazolium‐based ionic liquid as porogenic solvent followed by microwave‐assisted polymerization with the addition of 25 % MOF. This novel hybrid MOF–polymer was used to extract penicillin (penicillin G, penicillin V, oxacillin, cloxacillin, nafcillin, dicloxacillin) under different conditions. Quantitative analysis of the extracted penicillin samples using the MOF–organic polymer for SPME was conducted by using capillary electrochromatography (CEC) coupled with UV analysis. The penicillin recovery was 63–96.2 % with high reproducibility, sensitivity, and reusability. The extraction time with the proposed fabricated SPME was only 34 min.     

All‐Nanoporous Hybrid Membranes: Redefining Upper Limits on Molecular Separation Properties

New membrane‐based molecular separation processes are an essential part of the strategy for sustainable chemical production. A large literature on “hybrid” or “mixed‐matrix” membranes exists, in which nanoparticles of a higher‐performance porous material are dispersed in a polymeric matrix to boost performance. We demonstrate that the hybrid membrane concept can be redefined to achieve much higher performance if the membrane matrix and the dispersed phase are both nanoporous crystalline materials, with no polymeric phase. As the first example of such a system, we find that surface‐treated nanoparticles of the zeolite MFI can be incorporated in situ during growth of a polycrystalline membrane of the MOF ZIF‐8. The resulting all‐nanoporous hybrid membrane shows propylene/propane separation characteristics that exceed known upper‐bound performance limits defined for polymers, nanoporous materials, and polymer‐based hybrid membranes. This serves as a starting point for a new generation of chemical separation membranes containing interconnected nanoporous crystalline phases.     

Mixed‐Matrix Membranes

Research into extended porous materials such as metal‐organic frameworks (MOFs) and porous organic frameworks (POFs), as well as the analogous metal‐organic polyhedra (MOPs) and porous organic cages (POCs), has blossomed over the last decade. Given their chemical and structural variability and notable porosity, MOFs have been proposed as adsorbents for industrial gas separations and also as promising filler components for high‐performance mixed‐matrix membranes (MMMs). Research in this area has focused on enhancing the chemical compatibility of the MOF and polymer phases by judiciously functionalizing the organic linkers of the MOF, modifying the MOF surface chemistry, and, more recently, exploring how particle size, morphology, and distribution enhance separation performance. Other filler materials, including POFs, MOPs, and POCs, are also being explored as additives for MMMs and have shown remarkable anti‐aging performance and excellent chemical compatibility with commercially available polymers. This Review briefly outlines the state‐of‐the‐art in MOF‐MMM fabrication, and the more recent use of POFs and molecular additives.     

A Metal Chelating Porous Polymeric Support: The Missing Link for a Defect‐Free Metal–Organic Framework Composite Membrane

Since the discovery of size‐selective metal–organic frameworks (MOFs), researchers have tried to incorporate these materials into gas separation membranes. Impressive gas selectivities were found, but these MOF membranes were mostly made on inorganic supports, which are generally too bulky and expensive for industrial gas separation. Forming MOF layers on porous polymer supports is industrially attractive but technically challenging. Two features to overcome these problems are described: 1) a metal chelating support polymer to bind the MOF layer, and 2) control of MOF crystal growth by contra‐diffusion, aiming at a very thin nanocrystalline MOF layer. Using a metal chelating poly‐thiosemicarbazide (PTSC) support and adjusting the metal and organic ligand concentrations carefully, a very compact ZIF‐8 (ZIF=zeolitic imidazolate framework) layer was produced that displayed interference colors because of its smooth surface and extreme thinness—within the range of visible light. High performances were measured in terms of hydrogen/propane (8350) and propylene/propane (150) selectivity.     

Unusual Transformation from a Solvent‐Stabilized 1D Coordination Polymer to a Metal–Organic Framework (MOF)‐Like Cross‐Linked 3D Coordination Polymer

An unusual 1D‐to‐3D transformation of a coordination polymer based on organic linkers containing highly polar push–pull π‐conjugated side chains is reported. The coordination polymers are synthesized from zinc nitrate and an organic linker, namely, 2,5‐bis{4‐[1‐(4‐nitrophenyl)pyrrolidin‐2‐yl]butoxy}terephthalic acid, which possesses highly polar (4‐nitrophenyl)pyrrolidine groups, with high dipole moments of about 7 D. The coordination polymers exhibit an unusual transformation from a soluble, solvent‐stabilized 1D coordination polymer into an insoluble, metal–organic framework (MOF)‐like 3D coordination polymer. The coordination polymer exhibits good film‐forming ability, and the MOF‐like films are insoluble in conventional organic solvents.     

Photoinduced Postsynthetic Polymerization of a Metal–Organic Framework toward a Flexible Stand‐Alone Membrane

Metal–organic frameworks (MOFs) are a promising class of nanoporous polymeric materials. However, the processing of such fragile crystalline powders into desired shapes for further applications is often difficult. A photoinduced postsynthetic polymerization (PSP) strategy was now employed to covalently link MOF crystals by flexible polymer chains, thus endowing the MOF powders with processability and flexibility. Nanosized UiO‐66‐NH₂ was first functionalized with polymerizable functional groups, and its subsequent copolymerization with monomers was easily induced by UV light under solvent‐free and mild conditions. Because of the improved interaction between MOF particles and polymer chains, the resulting stand‐alone and elastic MOF‐based PSP‐derived membranes possess crack‐free and uniform structures and outstanding separation capabilities for Cr^VI ions from water.     

Microporous 3D aluminum MOF doped into chitosan‐based mixed matrix membranes for ethanol/water separation

An aluminum metal–organic framework (Al‐MOF), [Al(OH)(BPDC)] (DUT‐5; BPDC = Biphenyl‐4,4′‐dicarboxylate), was synthesized using solvothermal reactions. The high surface area and micropores (approximately 1.2 nm) of DUT‐5 were characterized using N₂ gas sorption measurements. The thermal stability of DUT‐5 and its phase purity were also investigated. The different amounts of DUT‐5 (0.1, 0.15, and 0.2 wt%) were successfully incorporated into the chitosan (CS) polymer to prepare a mixed matrix membrane (MMM) for the pervaporation of water/ethanol at 25°C. In particular, when 0.15 wt% of DUT‐5 was loaded, the DUT‐5@CS MMMs displayed excellent permeability and selectivity in ethanol/water separation. The results indicated that compared with pristine chitosan membranes, the flux of DUT‐5@CS membranes with 0.15 wt% loading significantly increased from 315 to 378 (g/m² h^?1) and the separation factor from 347 to 3,429. These promising results of the microporous Al‐MOF doped into chitosan MMMs reveal its good application potential for the bio‐ethanol separation processes.     

Zeolitic Imidazolate Framework/Graphene Oxide Hybrid Nanosheets as Seeds for the Growth of Ultrathin Molecular Sieving Membranes

A defect‐free zeolitic imidazolate framework‐8 (ZIF‐8)/graphene oxide (GO) membrane with a thickness of 100 nm was prepared using two‐dimensional (2D) ZIF‐8/GO hybrid nanosheets as seeds. Hybrid nanosheets with a suitable amount of ZIF‐8 nanocrystals were essential for producing a uniform seeding layer that facilitates fast crystal intergrowth during membrane formation. Moreover, the seeding layer acts as a barrier between two different synthesis solutions, and self‐limits crystal growth and effectively eliminates defects during the contra‐diffusion process. The resulting ultrathin membranes show excellent molecular sieving gas separation properties, such as with a high CO₂/N₂ selectivity of 7.0. This 2D nano‐hybrid seeding strategy can be readily extended to the fabrication of other defect‐free and ultrathin MOF or zeolite molecular sieving membranes for a wide range of separation applications.     

Salt‐Induced Fabrication of Superhydrophilic and Underwater Superoleophobic PAA‐g‐PVDF Membranes for Effective Separation of Oil‐in‐Water Emulsions

Conventional polymer membranes suffer from low flux and serious fouling when used for treating emulsified oil/water mixtures. Reported herein is the fabrication of a novel superhydrophilic and underwater superoleophobic poly(acrylic acid)‐grafted PVDF filtration membrane using a salt‐induced phase‐inversion approach. A hierarchical micro/nanoscale structure is constructed on the membrane surface and endows it with a superhydrophilic/underwater superoleophobic property. The membrane separates both surfactant‐free and surfactant‐stabilized oil‐in‐water emulsions under either a small applied pressure (<0.3 bar) or gravity, with high separation efficiency and high flux, which is one to two orders of magnitude higher than those of commercial filtration membranes having a similar permeation property. The membrane exhibits an excellent antifouling property and is easily recycled for long‐term use. The outstanding performance of the membrane and the efficient, energy and cost‐effective preparation process highlight its potential for practical applications.     

Simultaneous Spray Self‐Assembly of Highly Loaded ZIF‐8–PDMS Nanohybrid Membranes Exhibiting Exceptionally High Biobutanol‐Permselective Pervaporation

The ability to obtain a maximum loading of inorganic nanoparticles while maintaining uniform dispersion in the polymer is the key to the fabrication of mixed‐matrix membranes with high pervaporation performance in bioalcohol recovery from aqueous solution. Herein, we report the simultaneous spray self‐assembly of a zeolitic imidazolate framework (ZIF)–polymer suspension and a cross‐linker/catalyst solution as a method for the fabrication of a well‐dispersed ZIF‐8–PDMS nanohybrid membrane with an extremely high loading. The ZIF‐8–PDMS membrane showed excellent biobutanol‐permselective pervaporation performance. When the ZIF‐8 loading was increased to 40 wt %, the total flux and separation factor could reach 4846.2 g m⁻² h⁻¹ and 81.6, respectively, in the recovery of n‐butanol from 1.0 wt % aqueous solution (80 °C). This new method is expected to have serious implications for the preparation of defect‐free mixed‐matrix membranes for many applications.     

Rational Tuning of Zirconium Metal–Organic Framework Membranes for Hydrogen Purification

The effect of organic ligands on the separation performance of Zr based metal–organic framework (Zr‐MOF) membranes was investigated. A series of Zr‐MOF membranes with different ligand chemistry and functionality were synthesized by an in situ solvothermal method and a coordination modulation technique. The thin supported MOF layers (ca. 1 μm) showed the crystallographic orientation and pore structure of original MOF structures. The MOF membranes show excellent selectivity towards hydrogen owing to the molecular sieving effect when the bulkier linkers were used. The molecular simulation confirmed that the constricted pore apertures of the Zr‐MOFs which were formed by the additional benzene rings lead to the decrease in the diffusivity of larger penetrants while hydrogen was not remarkably affected. The gas mixture separation factors of the MOF membranes reached to H₂/CO₂=26, H₂/N₂=13, H₂/CH₄=11.     

A MOF Glass Membrane for Gas Separation

Metal–organic framework (MOF) glasses are promising candidates for membrane fabrication due to their significant porosity, the ease of processing, and most notably, the potential to eliminate the grain boundary that is unavoidable for polycrystalline MOF membranes. Herein, we developed a ZIF‐62 MOF glass membrane and exploited its intrinsic gas‐separation properties. The MOF glass membrane was fabricated by melt‐quenching treatment of an in situ solvothermally synthesized polycrystalline ZIF‐62 MOF membrane on a porous ceramic alumina support. The molten ZIF‐62 phase penetrated into the nanopores of the support and eliminated the formation of intercrystalline defects in the resultant glass membrane. The molecular sieving ability of the MOF membrane is remarkably enhanced via vitrification. The separation factors of the MOF glass membrane for H₂/CH₄, CO₂/N₂ and CO₂/CH₄ mixtures are 50.7, 34.5, and 36.6, respectively, far exceeding the Robeson upper bounds.     

Chiral Recognition and Separation by Chirality‐Enriched Metal–Organic Frameworks

Endowed with chiral channels and pores, chiral metal–organic frameworks (MOFs) are highly useful; however, their synthesis remains a challenge given that most chiral building blocks are expensive. Although MOFs with induced chirality have been reported to avoid this shortcoming, no study providing evidence for the ee value of such MOFs has yet been reported. We herein describe the first study on the efficiency of chiral induction in MOFs using inexpensive achiral building blocks and fully recoverable chiral dopants to control the handedness of racemic MOFs. This method yielded chirality‐enriched MOFs with accessible pores. The ability of the materials to form host–guest complexes was probed with enantiomers of varying size and coordination and in solvents with varying polarity. Furthermore, mixed‐matrix membranes (MMMs) composed of chirality‐enriched MOF particles dispersed in a polymer matrix demonstrated a new route for chiral separation.     

Modular Customization and Regulation of Metal–Organic Frameworks for Efficient Membrane Separations

Metal–organic frameworks (MOFs) are considered ideal membrane candidates for energy-efficient separations. However, the MOF membrane amount to date is only a drop in the bucket compared to the material collections. The fabrication of an arbitrary MOF membrane exhibiting inherent separation capacity of the material remains a long-standing challenge. Herein, we report a MOF modular customization strategy by employing four MOFs with diverse structures and physicochemical properties and achieving innovative defect-free membranes for efficient separation validation. Each membrane fully displays the separation potential according to the MOF pore/channel microenvironment, and consequently, an intriguing H₂/CO₂ separation performance sequence is achieved (separation factor of 1656–5.4, H₂ permeance of 964–2745 gas permeation unit). Taking advantage of this strategy, separation performance can be manipulated by a non-destructive modification separately towards the MOF module. This work establishes a universal full-chain demonstration for membrane fabrication-separation validation-microstructure modification and opens an avenue for exclusive customization of membranes for important separations.     

Balancing the Crystallinity and Film Formation of Metal–Organic Framework Membranes through In Situ Modulation for Efficient Gas Separation

Polycrystalline metal–organic framework (MOF) layers hold great promise as molecular sieve membranes for efficient gas separation. Nevertheless, the high crystallinity tends to cause inter-crystalline defects/cracks in the nearby crystals, which makes crystalline porous materials face a great challenge in the fabrication of defect-free membranes. Herein, for the first time, we demonstrate the balance between crystallinity and film formation of MOF membrane through a facile in situ modulation strategy. Monocarboxylic acid was introduced as a modulator to regulate the crystallinity via competitive complexation and thus concomitantly control the film-forming state during membrane growth. Through adjusting the ratio of modulator acid/linker acid, an appropriate balance between this structural “trade-off” was achieved. The resulting MOF membrane with moderate crystallinity and coherent morphology exhibits molecular sieving for H₂/CO₂ separation with selectivity up to 82.5.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10^?7 mol m^?2 s^?1 Pa^?1 and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

Post‐Synthetic Polymerization of UiO‐66‐NH2 Nanoparticles and Polyurethane Oligomer toward Stand‐Alone Membranes for Dye Removal and Separation

Metal–organic frameworks (MOFs) are widely used as porous materials in the fields of adsorption and separation. However, their practical application is largely hindered by limitations to their processability. Herein, new UiO‐66‐Urea‐based flexible membranes with MOF loadings of 50 ( 1 ), 60 ( 2 ), and 70 wt % ( 3 ) were designed and prepared by post‐synthetic polymerization of UiO‐66‐NH₂ nanoparticles and a polyurethane oligomer under mild conditions. The adsorption behavior of membrane 3 towards four hydrophilic dyes, namely, eosin Y (EY), rhodamine B (RB), malachite green (MG), and methylene blue (MB), in aqueous solution was studied in detail. It exhibits strong adsorption of EY and RB but weak adsorption of MG and MB in aqueous solution. Owing to the selective adsorption of these hydrophilic dyes, membrane 3 can remove EY and RB from aqueous solution and completely separate EY/MB, RB/MG, and RB/MB mixtures in aqueous solution. In addition, the membrane is uniformly textured, easily handled, and can be reused for dye adsorption and separation.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.