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.     

Computational Methods for MOF/Polymer Membranes

Metal–organic framework (MOF)/polymer mixed matrix membranes (MMMs) have received significant interest in the last decade. MOFs are incorporated into polymers to make MMMs that exhibit improved gas permeability and selectivity compared with pure polymer membranes. The fundamental challenge in this area is to choose the appropriate MOF/polymer combinations for a gas separation of interest. Even if a single polymer is considered, there are thousands of MOFs that could potentially be used as fillers in MMMs. As a result, there has been a large demand for computational studies that can accurately predict the gas separation performance of MOF/polymer MMMs prior to experiments. We have developed computational approaches to assess gas separation potentials of MOF/polymer MMMs and used them to identify the most promising MOF/polymer pairs. In this Personal Account, we aim to provide a critical overview of current computational methods for modeling MOF/polymer MMMs. We give our perspective on the background, successes, and failures that led to developments in this area and discuss the opportunities and challenges of using computational methods for MOF/polymer MMMs.     

Microwave‐Assisted Hydrothermal Synthesis of [Al(OH)(1,4‐NDC)] Membranes with Superior Separation Performances

In this study, we report a facile ligand‐assisted in situ hydrothermal approach for preparation of compact [Al(OH)(1,4‐NDC)] (1,4‐NDC=1,4‐naphthalenedicarboxylate) MOF membranes on porous γ‐Al₂O₃ substrates, which also served as the Al³⁺ source of MOF membranes. Simultaneously, it was observed that the heating mode exerted significant influence on the final microstructure and separation performance of [Al(OH)(1,4‐NDC)] membranes. Compared with the conventional hydrothermal method, the employment of microwave heating led to the formation of [Al(OH)(1,4‐NDC)] membranes composed of closely packed nanorods with superior H₂/CH₄ selectivity.     

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.     

The influence of filler type on the separation properties of mixed-matrix membranes

Chitosan-based membranes filled with different metal oxide particles were prepared and their performance in ethanol dehydration process depending on the type of oxide and loading was discussed. For membrane preparation three oxides: TiO₂, Cr₂O₃ or Fe₃O₄ were selected. From experimental data suitable ethanol and water transport coefficients were evaluated. As shown in the results, applied fillers in different ways affect the separation properties. Presence of TiO₂ significantly affects the normalized total flux, increasing its value. On the other hand, addition of Fe₃O₄ influences most of all the separation factor, which is the among all investigated membranes. For membranes containing chromium(III) oxide as a filler, improvement in the separation properties is observed only in the case when the Cr₂O₃ content equals to 5 wt%. Above this concentration significant deterioration of separation properties is observed. The best performance has mixed-matrix membranes (MMMs) with magnetite, where the values of PSI are equal to 16.3 and 296.8 kg/m^?2 h µm for pristine and 15 wt% filler content, respectively.     

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.     

Route to a family of robust, non-interpenetrated metal-organic frameworks with pto-like topology

A combination of topological rules and quantum chemical calculations has facilitated the development of a rational metal–organic framework (MOF) synthetic strategy using the tritopic benzene‐1,3,5‐tribenzoate (btb) linker and a neutral cross‐linker 4,4′‐bipyridine (bipy). A series of new compounds, namely [M₂(bipy)]₃(btb)₄ (DUT‐23(M), M=Zn, Co, Cu, Ni), [Cu₂(bisqui)_0.5]₃(btb)₄ (DUT‐24, bisqui=diethyl (R,S)‐4,4′‐biquinoline‐3,3′‐dicarboxylate), [Cu₂(py)_1.5(H₂O)_0.5]₃(btb)₄ (DUT‐33, py=pyridine), and [Cu₂(H₂O)₂]₃(btb)₄ (DUT‐34), with high specific surface areas and pore volumes (up to 2.03 m³ g^?1 for DUT‐23(Co)) were synthesized. For DUT‐23(Co), excess storage capacities were determined for methane (268 mg g^?1 at 100 bar and 298 K), hydrogen (74 mg g^?1 at 40 bar and 77 K), and n‐butane (99 mg g^?1at 293 K). DUT‐34 is a non‐cross‐linked version of DUT‐23 (non‐interpenetrated pendant to MOF‐14) that possesses open metal sites and can therefore be used as a catalyst. The accessibility of the pores in DUT‐34 to potential substrate molecules was proven by liquid phase adsorption. By exchanging the N,N donor 4,4′‐bipyridine with a substituted racemic biquinoline, DUT‐24 was obtained. This opens a route to the synthesis of a chiral compound, which could be interesting for enantioselective 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.     

Chitosan/polyvinyl alcohol/amino functionalized multiwalled carbon nanotube pervaporation membranes: Synthesis,characterization, and performance

In the present study, novel biodegradable nanocomposite membranes were prepared by adding the amino functionalized multiwalled carbon nanotube (NH₂‐MWCNT) to the chitosan/polyvinyl alcohol blend polymers, and the obtained membranes were used for dehydration of isopropyl alcohol through pervaporation process. For this purpose, the membranes were prepared with chitosan/polyvinyl alcohol ratio of 4:1 on the basis of “solution casting” method and then crosslinked using glutaraldehyde, after addition of different amounts of NH₂‐MWCNT. The prepared membranes were characterized using scanning electron microscopy, contact angle, mechanical strength, degree of swelling (DS), and biodegradability. Also, the ability of the prepared membranes in dehydration of isopropyl alcohol was determined using pervaporation experiments. Results indicated that contact angle, mechanical resistance, separation factor (α), and pervaporation separation index were increased with the addition of NH₂‐MWCNT up to 10 wt% (relative to the total amount of polymer) and then decreased in the higher presence of nanotubes (15 wt%). Furthermore, the DS and permeate flux were first decreased and then increased for the same mentioned amounts of additive. In this study, optimized membrane was obtained by the addition of 10 wt% NH₂‐MWCNT. This membrane showed the maximum α (99.5), pervaporation separation index parameter (78.29 kg m⁻² h⁻¹), biodegradability, and mechanical stability as well as minimum DS.     

Enhanced CO2-selective behavior of Pebax-1657: A comparative study between the influence of graphene-based fillers

Mixed matrix membranes (MMMs) containing graphene-based fillers have attracted considerable attention in the field of gas separation. In this study, two types of graphene derivatives (Graphene (G) and Graphene Oxide (GO)) were embedded into the poly-ether-block-amide (Pebax) based MMM to investigate and compare CO₂/N₂ separation at various filler loadings (0.3–1 wt%). The morphologies of the prepared neat Pebax and MMMs were characterized by SEM, XRD, FTIR and DSC. Compared with the neat Pebax, the permeability of all gases was increased by adding filler content in the MMMs due to the crystallinity decrement of the polyamide (PA) segment. The best separation performance of the Pebax/G MMMs occurred at 0.7 wt%, where the CO₂ permeability increased from 26.51 to 44.78 Barrer (~1.7 times). Also, for the Pebax/GO MMMs, the CO₂ permeability was increased up to 58.96 Barrer (~2.2 times) by adding 0.5 wt% filler. This further gas permeation increment for the Pebax/GO sample was attributed to the higher affinity of GO nanosheets to CO₂ sorption, which can facilitate CO₂ gas transition through the membrane matrix. Moreover, the CO₂/N₂ ideal selectivity increased from 74.26 for the neat Pebax to 111.95 (~1.5 times) and 120.72 (~1.62 times) by adding 0.7 wt% G (Pebax/G-0.7) and 1 wt% GO (Pebax/GO-1) into Pebax, respectively. As a consequence, graphene derivatives can be recognized as a promising developer of permselectivity (permeability and selectivity) of the MMMs.     

Pervaporation with chitosan membranes II. Blend membranes of chitosan and polyacrylic acid and comparison of homogeneous and composite membrane based on polyelectrolyte complexes of chitosan and polyacrylic acid for the separation of ethanol-water mixtures

Three different types of blend membranes based on chitosan and polyacrylic acid were prepared from homogeneous polymer solution and their performance on the pervaporation separation of water-ethanol mixtures was investigated. It was found that all membranes are highly water-selective. The temperature dependence of membrane permselectivity for the feed solutions of higher water content (>30 wt%) was unusual in that both permeability and separation factor increased with increase in temperature. This phenomenon might be explained from the aspect of activation energy and suggested that the sorption contribution to activation energy of permeation should not always be ignored when strong interaction occurs in the pervaporation membrane system.A comparison of pervaporation performance between composite and homogeneous membranes was also studied. Typical pervaporation results at 30°C for a 95 wt% ethanol aqueous solution were: for the homogeneous membrane, permeation flux = 33 g/m² h, separation factor = 2216; and for the composite membrane, permeation flux = 132 g/m² h, separation factor = 1008. A transport model consisting of dense layer and porous substrate in series was developed to describe the effect of porous substrate on pervaporation 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.     

PERVAPORATION OF ETHANOL/WATER MIXTURES WITH HIGH FLUX BY ZEOLITE-FILLED PDMS/PVDF COMPOSITE MEMBRANES

Thin-film zeolite-filled silicone/PVDF composite membranes were fabricated by incorporating zeolite particles into PDMS(poly(dimethylsiloxane)) membranes.The morphology of zeolite particles and zeolite filled silicone composite membranes were characterized by SEM.The zeolite-filled PDMS/PVDF composite membranes were applied for the pervaporation of ethanol/water mixtures and showed higher flux compared with that reported in literatures.The effect of zeolite loading and Si/Al ratio of zeolite particles on...     

MOF‐on‐MOF: Oriented Growth of Multiple Layered Thin Films of Metal–Organic Frameworks

The precise alignment of multiple layers of metal–organic framework (MOF) thin films, or MOF‐on‐MOF films, over macroscopic length scales is presented. The MOF‐on‐MOF films are fabricated by epitaxially matching the interface. The first MOF layer (Cu₂(BPDC)₂, BPDC=biphenyl‐4,4′‐dicarboxylate) is grown on an oriented Cu(OH)₂ film by a “one‐pot” approach. Aligned second (Cu₂(BDC)₂, BDC=benzene 1,4‐dicarboxylate, or Cu₂(BPYDC)₂, BPYDC=2,2′‐bipyridine‐5,5′‐dicarboxylate) MOF layers can be deposited using liquid‐phase epitaxy. The co‐orientation of the MOF films is confirmed by X‐ray diffraction. Importantly, our strategy allows for the synthesis of aligned MOF films, for example, Cu₂(BPYDC)₂, that cannot be grown on a Cu(OH)₂ surface. We show that aligned MOF films furnished with Ag nanoparticles show a unique anisotropic plasmon resonance. Our MOF‐on‐MOF approach expands the chemistry of heteroepitaxially oriented MOF films and provides a new toolbox for multifunctional porous coatings.     

Effect of carbon molecular sieve sizing with poly(vinyl pyrrolidone) K-15 on carbon molecular sieve–polysulfone mixed matrix membrane

Mixed matrix membranes (MMMs) comprising polysulfone (PSF) Udel^® P-1700 and 30 wt% carbon molecular sieve (CMS) particles (<25 μm) have been fabricated and characterized. CMS particles were treated in poly(vinyl pyrrolidone) kollidone 15 (PVP K-15) sizing bath solution (1–10 wt% in isopropanol) prior to embedment into the matrix solution to improve matrix–sieve interfacial adhesion. This study investigated the effects of CMS sizing with PVP K-15 on the morphology and the gas separation performance of PSF–CMS MMMs. The fabricated MMMs were characterized using TGA, DSC, FESEM, ATR-FTIR and single gas permeation test using high purity O₂ and N₂. A dramatic improvement in CMS–PSF adhesion was observed using FESEM micrographs upon incorporation of PVP K-15-sized CMS particles. ATR-FTIR results suggest the occurrence of intermolecular interaction between PVP K-15 sizing layer on the outer surface of CMS particles and PSF matrix. A substantial recovery of separation performance was achieved whereby the PSF–PVP-sized CMS MMM exhibited 1.7 times higher O₂/N₂ selectivity compared to that of unmodified MMM.     

MIL‐53(Al) as a Versatile Platform for Ionic‐Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium‐based ionic liquids (ILs) were incorporated into a metal–organic framework (MOF), MIL‐53(Al), to investigate the effect of IL incorporation on the CO₂ separation performance of MIL‐53(Al). CO₂, CH₄, and N₂ adsorption isotherms of the IL/MIL‐53(Al) composites and pristine MIL‐53(Al) were measured to evaluate the effect of the ILs on the CO₂/CH₄ and CO₂/N₂ selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF₆]/MIL‐53(Al) exhibited the largest increase in CO₂/CH₄ selectivity, 2.8‐times higher than that of pristine MIL‐53(Al), whilst [BMIM][MeSO₄]/MIL‐53(Al) exhibited the largest increase in CO₂/N₂ selectivity, 3.3‐times higher than that of pristine MIL‐53(Al). A comparison of the CO₂ separation potentials of the IL/MOF composites showed that the [BMIM][BF₄]‐ and [BMIM][PF₆]‐incorporated MIL‐53(Al) composites both showed enhanced CO₂/N₂ and CO₂/CH₄ selectivities at pressures of 1–5 bar compared to composites of CuBTC and ZIF‐8 with the same ILs. These results demonstrate that MIL‐53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas‐separation applications.     

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.     

MOF Scaffold for a High‐Performance Mixed‐Matrix Membrane

A novel composite membrane consisting of an interconnected MOF scaffold coated with cross‐linked poly(ethylene glycol) (PEG) has been developed. As a result of its unique structure, the membrane shows an exceptional 18‐fold permeability enhancement as compared to pristine PEG membranes, without compromising the selectivity. This performance is unattainable with current mixed‐matrix membranes (MMMs). Our optimized membrane has a permeability of 2700 Barrer with a CO₂/N₂ selectivity of 35, which surpasses the latest Robeson upper bound.     

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.     

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.