Azine‐Linked Covalent Organic Framework (COF)‐Based Mixed‐Matrix Membranes for CO2/CH4 Separation

Mixed‐matrix membranes (MMMs) comprising Matrimid and a microporous azine‐linked covalent organic frameworks (ACOF‐1) were prepared and tested in the separation of CO₂ from an equimolar CO₂/CH₄ mixture. The COF‐based MMMs show a more than doubling of the CO₂ permeability upon 16 wt % ACOF‐1 loading together with a slight increase in selectivity compared to the bare polymer. These results show the potential of COFs in the preparation of MMMs.     

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.     

Fabrication of a Hydrogen‐Bonded Organic Framework Membrane through Solution Processing for Pressure‐Regulated Gas Separation

Ordered and flexible porous frameworks with solution processability are highly desirable to fabricate continuous and large‐scale membranes for the efficient gas separation. Herein, the first microporous hydrogen‐bonded organic framework (HOF) membrane has been fabricated by an optimized solution‐processing technique. The framework exhibits the superior stability because of the abundant hydrogen bonds and strong π–π interactions. Thanks to the flexible HOF structure, the membrane possesses the unprecedented pressure‐responsive H₂/N₂ separation performance. Furthermore, the scratched membrane can be healed by the treatment of solvent vapor, achieving the recovery of separation performance.     

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.     

Remarkably Enhanced Gas Separation by Partial Self‐Conversion of a Laminated Membrane to Metal–Organic Frameworks

Separation methods based on 2D interlayer galleries are currently gaining widespread attention. The potential of such galleries as high‐performance gas‐separation membranes is however still rarely explored. Besides, it is well recognized that gas permeance and separation factor are often inversely correlated in membrane‐based gas separation. Therefore, breaking this trade‐off becomes highly desirable. Here, the gas‐separation performance of a 2D laminated membrane was improved by its partial self‐conversion to metal–organic frameworks. A ZIF‐8‐ZnAl‐NO₃ layered double hydroxide (LDH) composite membrane was thus successfully prepared in one step by partial conversion of the ZnAl‐NO₃ LDH membrane, ultimately leading to a remarkably enhanced H₂/CH₄ separation factor and H₂ permeance.     

Accelerating Membrane‐based CO2 Separation by Soluble Nanoporous Polymer Networks Produced by Mechanochemical Oxidative Coupling

Achieving homogeneous dispersion of nanoporous fillers within membrane architectures remains a great challenge for mixed‐matrix membrane (MMMs) technology. Imparting solution processability of nanoporous materials would help advance the development of MMMs for membrane‐based gas separations. A mechanochemically assisted oxidative coupling polymerization strategy was used to create a new family of soluble nanoporous polymer networks. The solid‐state ball‐milling method affords inherent molecular weight control over polymer growth and therefore provides unexpected solubility for the resulting nanoporous frameworks. MMM‐based CO₂/CH₄ separation performance was significantly accelerated by these new soluble fillers. We anticipate this facile method will facilitate new possibilities for the rational design and synthesis of soluble nanoporous polymer networks and promote their applications in membrane‐based gas separations.     

In Situ Modification of Metal–Organic Frameworks in Mixed‐Matrix Membranes

Processable films of metal–organic frameworks (MOFs) have been long sought to advance the application of MOFs in various technologies from separations to catalysis. Herein, MOF–polymer mixed‐matrix membranes (MMMs) are described, formed on several substrates using a wide variety of MOF materials. These MMMs can be delaminated from their substrates to create free‐standing MMMs that are mechanically stable and pliable. The MOFs in these MMMs remain highly crystalline, porous, and accessible for further chemical modification through postsynthetic modification (PSM) and postsynthetic exchange (PSE) processes. Overall, the findings here demonstrate a versatile approach to preparing stable functional MMMs that should contribute significantly to the advancement of these materials.     

Metal–organic framework-based mixed-matrix membranes for gas separation: An overview

Mixed-matrix membranes (MMMs) have been studied widely in the field of gas separation due to their potential to overcome performance barriers found in traditional polymeric membranes. Most polymeric membranes exhibit a trade-off between permeation and selectivity, which has limited their development in many challenging separation applications. One solution to this issue utilizes the introduction of fillers into the polymer matrix to produce MMMs. Out of the many different fillers, metal–organic frameworks stand out as a promising candidate due to their highly tunable structure, molecular sieving effect, and superior compatibility with the polymer matrix. This review will provide an in-depth look into the basic mechanisms of MMMs for gas separation and different approaches to model the permeation of gases through the membrane. In addition, challenges facing the field and recent research trends for MMMs will be discussed as well as their many applications for different gas separations. Finally, some insight on the future direction for MMMs will be covered, focusing on many intriguing opportunities and challenges that must be further explored to advance this technology.     

Homochiral MOF–Polymer Mixed Matrix Membranes for Efficient Separation of Chiral Molecules

Homochiral metal–organic framework (MOF) membranes have been recently reported for chiral separations. However, only a few high‐quality homochiral polycrystalline MOF membranes have been fabricated due to the difficulty in crystallization of a chiral MOF layer without defects on porous substrates. Alternatively, mixed matrix membranes (MMMs), which combine potential advantages of MOFs and polymers, have been widely demonstrated for gas separation and water purification. Here we report novel homochiral MOF–polymer MMMs for efficient chiral separation. Homochirality was successfully incorporated into achiral MIL‐53‐NH₂ nanocrystals by post‐synthetic modification with amino acids, such as l ‐histidine (l ‐His) and l ‐glutamic acid (l ‐Glu). The MIL‐53‐NH‐l ‐His and MIL‐53‐NH‐l ‐Glu nanocrystals were then embedded into polyethersulfone (PES) matrix to form homochiral MMMs, which exhibited excellent enantioselectivity for racemic 1‐phenylethanol with the highest enantiomeric excess value up to 100 %. This work, as an example, demonstrates the feasibility of fabricating diverse large‐scale homochiral MOF‐based MMMs for chiral separation.     

Defect‐Free Mixed‐Matrix Membranes with Hydrophilic Metal‐Organic Polyhedra for Efficient Carbon Dioxide Separation

Defect‐free mixed‐matrix membranes (MMMs) were prepared by incorporating hydrophilic metal‐organic polyhedra (MOPs) into cross‐linked polyethylene oxide (XLPEO) for efficient CO₂ separation. Hydrophilic MOPs with triethylene glycol pendant groups, which were assembled by 5‐tri(ethylene glycol) monomethyl ether isophthalic acid and Cu^II ions, were uniformly dispersed in XLPEO without particle agglomeration. Compared to conventional neat XLPEO, the homogenous dispersion of EG₃‐MOPs in XLPEO enhanced CO₂ permeability of MMMs. Upon increasing the amount of EG₃‐MOPs, the membrane performance such as CO₂/N₂ selectivity was steadily improved because of unsaturated Cu^II sites at paddle‐wheel units, which was confirmed by Cu K‐edge XANES and TPD analysis. Therefore, such defect‐free MMMs with unsaturated metal sites would contribute to enhance CO₂ separation performance.     

Microporous Metal–Organic Frameworks for Gas Separation

Microporous metal–organic frameworks (MOFs) are comparatively new porous materials. Because the pores within such MOFs can be readily tuned through the interplay of both metal‐containing clusters and organic linkers to induce their size‐selective sieving effects, while the pore surfaces can be straightforwardly functionalized to enforce their different interactions with gas molecules, MOF materials are very promising for gas separation. Furthermore, the high porosities of such materials can enable microporous MOFs with optimized gas separation selectivity and capacity to be targeted. This Focus Review highlights recent significant advances in microporous MOFs for gas separation.     

Current Development and Challenges of Mixed Matrix Membranes for CO2/CH4 Separation

In the early stage of membrane technology development in gas separation, utilization of polymeric membranes has gained attention due to their robustness and ease of fabrication. However, the performance of polymeric membranes is limited by the trade-off between permeability and selectivity. Meanwhile, inorganic membrane is capable to exhibit great enhancement in separation performance but unfortunately its fabrication process is hard and costly. Thus, development of mixed matrix membranes (MMMs) by incorporating inorganic fillers into the polymer matrix has become a potential alternative to overcome the limitations of polymeric and inorganic membranes in gas separation. Nevertheless, fabrication of defect-free MMMs with improved separation performance and without compromising the mechanical and thermal stability is extremely difficult and challenging. In the current review paper, various types of inorganic fillers for MMMs fabrication and recent reported efforts to tailor the underlying problems on MMMs fabrication were discussed. The future outlook to advance the performance of MMMs in gas separation especially for CO₂/CH₄ separation was highlighted.     

二维纳米片用于快速高效膜法气体分离

膜法气体分离作为一类低能耗先进分离技术, 在化工分离中具有广阔的应用前景. 然而商业气体分离膜在实际应用过程中存在选择性和渗透性此消彼长的问题. 以二维纳米片材料为膜构筑基元, 有望突破这一瓶颈. 最具代表性的二维纳米片膜材料当属石墨烯及其衍生物、 二维沸石分子筛、 层状双金属氢氧化物、 二维过渡金属硫化物、 Mxene、 二维共价有机骨架和金属有机骨架材料. 本文对这些二维材料在超薄气体分离膜领域的成果与进展进行介绍, 展现了各类材料在实际分离应用过程中的优势及弊端, 探讨了二维纳米片膜材料在气体分离领域的挑战与发展前景.     

Two‐Dimensional Membranes: New Paradigms for High‐Performance Separation Membranes

Two‐dimensional (2D) materials with atomic thicknesses have aroused great interest as promising building blocks for the preparation of ultrathin 2D membranes. These 2D membranes can exhibit unprecedentedly high separation permeance owing to their ultrasmall membrane thicknesses and superior selectivity because of their size‐selective nanopores and/or nanochannels. Until now, a large number of 2D membranes with good performance have been reported, highlighting the potential of these novel membranes for efficient liquid and gas separations. Summarized in this review are the latest advances in 2D membranes, with a special focus on industrially attractive separation processes, fabrication methods of laminar membranes, choices of membrane materials, designs of membrane structures, and unique membrane transport properties. Opportunities and challenges of 2D membranes for commercial applications are also briefly discussed.     

Creation of Well‐Defined “Mid‐Sized” Micropores in Carbon Molecular Sieve Membranes

Carbon molecular sieve (CMS) membranes are candidates for the separation of organic molecules due to their stability, ability to be scaled at practical form factors, and the avoidance of expensive supports or complex multi‐step fabrication processes. A critical challenge is the creation of “mid‐range” (e.g., 5–9 Å) microstructures that allow for facile permeation of organic solvents and selection between similarly‐sized guest molecules. Here, we create these microstructures via the pyrolysis of a microporous polymer (PIM‐1) under low concentrations of hydrogen gas. The introduction of H₂ inhibits aromatization of the decomposing polymer and ultimately results in the creation of a well‐defined bimodal pore network that exhibits an ultramicropore size of 5.1 Å. The H₂ assisted CMS dense membranes show a dramatic increase in p‐xylene ideal permeability (≈15 times), with little loss in p‐xylene/o‐xylene selectivity (18.8 vs. 25.0) when compared to PIM‐1 membranes pyrolyzed under a pure argon atmosphere. This approach is successfully extended to hollow fiber membranes operating in organic solvent reverse osmosis mode, highlighting the potential of this approach to be translated from the laboratory to the field.     

Homochiral metal–organic framework used as a stationary phase for high‐performance liquid chromatography

Metal–organic frameworks are promising porous materials. Chiral metal–organic frameworks have attracted considerable attention in controlling enantioselectivity. In this study, a homochiral metal–organic framework [Co₂(D‐cam)₂(TMDPy)] (D‐cam = d ‐camphorates, TMDPy = 4,4′‐trimethylenedipyridine) with a non‐interpenetrating primitive cubic net has been used as a chiral stationary phase in high‐performance liquid chromatography. It has allowed the successful separation of six positional isomers and six chiral compounds. The good selectivity and baseline separation, or at least 60% valley separation, confirmed its excellent molecular recognition characteristics. The relative standard deviations for the retention time of run‐to‐run and column‐to‐column were less than 1.8 and 3.1%, respectively. These results demonstrate that [Co₂(D‐cam)₂(TMDPy)] may represent a promising chiral stationary phase for use in high‐performance liquid chromatography.     

Selective CO2 capture through microporous Tb(BTC)(H2O).(DMF)1.1 MOF as an additive in novel MMMs fabricated from Matrimid® 5218

Mixed matrix membranes (MMMs) fabricated with porous metal organic frame works have enhanced the separation performance of polymer membranes. In this context microporous 3D Tb(BTC)(H₂O).(DMF)_1.1 MOF was incorporated into pristine Matrimid® with loadings of 10, 20 and 30 weight percentages. SEM micrographs indicated proper distribution of filler in the Matrimid and no interfacial voids were observed. Gas permeation studies evidenced the CO₂ permeability to be 13.2 (82.32%) and 18.34 (153.31%) and 25.86 Barrer for 10, 20 and 30 wt% MMMs respectively. The 257.18% increase in CO₂ permeability of 30 wt% MMM than methane was attributed to polar nature of CO₂, its smaller kinetic diameter, condensability, and larger solubility within the Matrimid matrix than non – polar and larger CH₄ molecules.Addition of filler influenced the pure gas selectivity of all MMMs positively. So, 30 wt% MMM exhibited the highest 58.04% increase in selectivity that was attributed to the molecular sieving property of the filler and the size exclusion phenomena as followed by CH₄ and CO₂. The high values of mixed and pure gas selectivity were obtained upon increasing filler concentration. The commercial applicability of these MMMs was tested by checking their selectivity under increased feed concentrations of CO₂ and checking permeability and selectivities at high temperatures. The study depicted that, competitive sorption of gases, prevalence of size exclusion phenomena and polymer chains relaxation at higher temperature were responsible for low gas selectivity. MMM with 30 wt% of MOF lied close to Robson’s Upper bound 2008 that indicated its good separation potential.     

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.     

Microporous Molecular Materials from Dipeptides Containing Non‐proteinogenic Residues

Dipeptides with two hydrophobic side chains have proved to be an exceptional source of microporous organic materials, but since previous structures were limited to the incorporation of only proteinogenic residues, their full potential as adsorbents has remained unexplored. Single‐crystal XRD data for ten new compounds with non‐proteinogenic L ‐2‐aminobutanoic acid and/or L ‐2‐amino‐pentanoic acid are presented. The gas‐phase accessibility of their crystal pores, with cross‐sections of 2.3 to 5.1 Å, was monitored by CO₂ and CH₄ adsorption isotherms. Included CO₂ was also detected spectroscopically by 2D MAS NMR. An extensive conformational analysis reveals that the use of linear rather than branched side chains (such as L ‐valine and L ‐isoleucine) affords peptides with a greater degree of conformational freedom and yields more‐flexible channel surfaces that may easily adapt to a series of potential guest molecules.     

Two‐Dimensional Covalent Organic Frameworks for Carbon Dioxide Capture through Channel‐Wall Functionalization

Ordered open channels found in two‐dimensional covalent organic frameworks (2D COFs) could enable them to adsorb carbon dioxide. However, the frameworks’ dense layer architecture results in low porosity that has thus far restricted their potential for carbon dioxide adsorption. Here we report a strategy for converting a conventional 2D COF into an outstanding platform for carbon dioxide capture through channel‐wall functionalization. The dense layer structure enables the dense integration of functional groups on the channel walls, creating a new version of COFs with high capacity, reusability, selectivity, and separation productivity for flue gas. These results suggest that channel‐wall functional engineering could be a facile and powerful strategy to develop 2D COFs for high‐performance gas storage and separation.