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.     

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.     

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.     

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.     

Gel-to-crystal route towards MOF-mixed MOF-matrix membranes

Metal-organic framework (MOF) membranes attract intensive attention for precisely molecular sieving. It is of greatly scientific interest to adjust transport property of MOF membranes. Inspired by formation and separation mechanisms of mixed matrix membranes (MMMs), in this study, we report a kind of MOF-mixed MOF-matrix membranes (M₅), which consist of dispersed MOF fillers and continuous MOF matrixes and combine different features of two MOFs. A simple, controllable, and versatile gel-to-crystal methodology, similar to solvent-evaporation induced phase-inversion of polymer-based membranes and performed by precursor coating and thermally treating crystallization, is proposed to construct M₅. The incorporated MOF fillers can improve membrane crystallization and have good compatibility toward MOF matrixes. As an example, because of UiO-66-NH₂ filler has larger aperture than ZIF-8 matrix, the prepared M₅ shows enhanced permeability while invariable selectivity compared with the pure one.     

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.     

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.     

金属有机骨架材料/聚酰亚胺混合基质膜的制备及气体分离性能

合成了3种不同结构、 粒径和气体吸附性能的金属有机骨架材料(MOFs): 微米级Cu₃(BTC)₂、 亚微米级ZIF-8和S-Cu₃(BTC)₂. 氮气吸附等温线分析结果表明, ZIF-8和Cu₃(BTC)₂具有较大比表面积(1653和1439 m²/g), S-Cu₃(BTC)₂的比表面积为171.4 m²/g. 用共混法将MOFs直接引入聚酰亚胺中制备了MOFs/聚酰亚胺混合基质膜(MMMs). X射线衍射(XRD)和全反射红外光谱(FTIR-ATR)分析结果表明, MOFs在混合基质膜中保持物理和化学稳定. 气体渗透测试结果表明, MOFs的加入使膜的气体渗透分离性能明显提高, S-Cu₃(BTC)₂使渗透系数增加了1.75倍; ZIF-8和Cu₃(BTC)₂使渗透系数增加了3倍左右; 同时, 膜的气体分离系数变化很小.     

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.     

Preparation and characterization of ZIF-8 and ZIF-67 incorporated poly(vinylidene fluoride) membranes for pervaporative separation of methanol/water mixtures

Metal–organic frameworks (MOFs) are made up of metal centers and organic binders with larger surface area and distinct pore structures. Particularly significant advancement in MOF membranes has been achieved in three different directions: preparation of MOF membranes with larger surface area, improving the membrane performance by surface modification, and its usage with added features. However, its significance has not been completely known and concluded yet. MOF membranes are used in a variety of membrane-based separation like gas permeation, nanofiltration, pervaporation, membrane distillation, etc. This research aims to synthesize MOFs (ZIF-8 and ZIF-67) and MOF membranes (ZIF-8/PVDF and ZIF-67/PVDF) and used them in the pervaporative separation of the methanol/water mixture. MOFs and MOF membranes were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and thermogravimetry analysis. Methanol/water mixtures were be used to study the performance of the prepared membranes. A study on the process parameters such as temperature (40, 45, 50, and 55°C), feed pressure (4, 8, 12, and 16 psi), and feed composition (10%, 20%, 30%, and 40% of water) was carried out to examine the effect of each process parameters for pure membrane. In contrast, Taguchi screening design was used to screen the most influential process variable. The optimized conditions based on Taguchi screening method were 55°C, 12 psi, and 40 %vol of water in feed. The obtained total flux of 425 L/m²h was observed for M3 membrane. As feed temperature increased, the total flux of all three membranes was increased.     

Coordination‐Driven In Situ Self‐Assembly Strategy for the Preparation of Metal–Organic Framework Hybrid Membranes

Metal–organic frameworks (MOFs) have emerged as porous solids of a superior type for the fabrication of membranes. However, it is still challenging to prepare a uniformly dispersed robust MOF hybrid membrane. Herein, we propose a simple and powerful strategy, namely, coordination‐driven in situ self‐assembly, for the fabrication of MOF hybrid membranes. On the basis of the coordination interactions between metal ions and ligands and/or the functional groups of the organic polymer, this method was confirmed to be feasible for the production of a stable membrane with greatly improved MOF‐particle dispersion in and compatibility with the polymer, thus providing outstanding separation ability. As an experimental proof of concept, a high‐quality ZIF‐8/PSS membrane was fabricated that showed excellent performance in the nanofiltration and separation of dyes from water.     

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.     

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.     

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.     

气体分离用MOF基混合基质膜的界面构建策略

混合基质膜结合多孔填料优异的气体分离性能和聚合物材料良好的加工性能,被认为是最具有应用前景的一种气体分离膜材料。金属-有机框架材料(MOF)由于具有高比表面积和孔隙率、可调节的孔径以及可修饰的表面性能,成为制备混合基质膜的重要多孔材料。本文针对MOF基混合基质膜制备中所面临的主要挑战,聚焦于MOF和聚合物界面缺陷问题,分析了界面缺陷的产生原因及其对性能的影响,重点阐述了改善MOF填料和聚合物基质界面相容性的策略,以期为制备具有良好的界面形态和优异的气体分离性能的混合基质膜提供借鉴思路。     

Recent advances in metal‐organic frameworks and covalent organic frameworks for sample preparation and chromatographic analysis

In the field of analytical chemistry, sample preparation and chromatographic separation are two core procedures. The means by which to improve the sensitivity, selectivity and detection limit of a method have become a topic of great interest. Recently, porous organic frameworks, such as metal‐organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely used in this research area because of their special features, and different methods have been developed. This review summarizes the applications of MOFs and COFs in sample preparation and chromatographic stationary phases. The MOF‐ or COF‐based solid‐phase extraction (SPE), solid‐phase microextraction (SPME), gas chromatography (GC), high‐performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) methods are described. The excellent properties of MOFs and COFs have resulted in intense interest in exploring their performance and mechanisms for sample preparation and chromatographic separation.     

The effects of polymer chain rigidification, zeolite pore size and pore blockage on polyethersulfone (PES)-zeolite A mixed matrix membranes

The polyethersulfone (PES)-zeolite 3A, 4A and 5A mixed matrix membranes (MMMs) were fabricated with a modified solution-casting procedure at high temperatures close to the glass transition temperatures (T_g) of polymer materials. The effects of membrane preparation methodology, zeolite loading and pore size of zeolite on the gas separation performance of these mixed matrix membranes were studied. SEM results show the interface between polymer and zeolite in MMMs experiencing natural cooling is better (i.e., less defective) than that in MMMs experiencing immediate quenching. The increment of glass transition temperature (T_g) of MMMs with zeolite loading confirms the polymer chain rigidification induced by zeolite. The experimental results indicate that a higher zeolite loading results in a decrease in gas permeability and an increase in gas pair selectivity. The unmodified Maxwell model fails to correctly predict the permeability decrease induced by polymer chain rigidification near the zeolite surface and the partial pore blockage of zeolites by the polymer chains. A new modified Maxwell model is therefore proposed. It takes the combined effects of chain rigidification and partial pore blockage of zeolites into calculation. The new model shows much consistent permeability and selectivity predication with experimental data. Surprisingly, an increase in zeolite pore size from 3 to 5 Å generally not only increase gas permeability, but also gas pair selectivity. The O₂/N₂ selectivity of PES-zeolite 3A and PES-zeolite 4A membranes is very similar, while the O₂/N₂ selectivity of PES-zeolite 5A membranes is much higher. This implies the blockage may narrow a part of zeolite 5A pores to approximately 4 Å, which can discriminate the gas pair of O₂ and N₂, and narrow a part of zeolites 3A and 4A pores to smaller sizes. It is concluded that the partial pore blockage of zeolites by the polymer chains has equivalent or more influence on the separation properties of mixed matrix membranes compared with that of the polymer chain rigidification.     

Site characteristics in metal organic frameworks for gas adsorption

Metal organic frameworks (MOFs) are a new class of nanoporous materials that have many potential advantages over traditional nanoporous materials for several chemical technologies including gas adsorption, catalysis, membrane-based gas separation, sensing, and biomedical devices. Knowledge on the interaction of guest molecules with the MOF surface is required to design and develop these MOF-based processes. In this review, we examine the importance of identification of gas adsorption sites in MOFs using the current state-of-the-art in experiments and computational modeling. This review provides guidelines to design new MOFs with useful surface properties that exhibit desired performances, such as high gas storage capacity, and high gas selectivity.     

Expanding Linker Dimensionality in Metal-organic Frameworks for sub-Ångstrom Pore Control for Separation Applications

Metal-organic frameworks (MOFs) are a class of porous materials with high surface areas, which are acquiring rapid attention on an exponential basis. A significant characteristic of MOFs is their ability to act as adsorbents to selectively separate component mixtures of similar size, thereby addressing the technological need for an alternative approach to conventional distillation methods. Recently, MOFs comprising a 3-Dimensional (3D) linker have shown outstanding capabilities for difficult separations compared to the parent 2-Dimensional (2D) analogue. 3D-linkers with a polycyclic core are underrepresented in the MOF database due to the widespread preferred use of 2D-linkers and the misconceived high-cost of 3D linkers. We summarize the recent research of 3D-linker MOFs and highlight their beneficial employment for selective gas and hydrocarbon adsorption and separation. Furthermore, we outline forecasts in this area to create a platform for widespread adoption of 3D-linkers in MOF synthesis.     

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.