Metal-organic Framework Membranes Research Articles

Large‐Area Ultrathin Metal–Organic Framework Membranes Fabricated on Flexible Polymer Supports for Gas Separations

AbstractUltrathin continuous metal–organic framework (MOF) membranes have the potential to achieve high gas permeance and selectivity simultaneously for otherwise difficult gas separations, but with few exceptions for zeolitic‐imidazolate frameworks (ZIF) membranes, current methods cannot conveniently realize practical large‐area fabrication. Here, we propose a ligand back diffusion‐assisted bipolymer‐directed metal ion distribution strategy for preparing large‐area ultrathin MOF membranes on flexible polymeric support layers. The bipolymer directs metal ions to form a cross‐linked two‐dimensional (2D) network with a uniform distribution of metal ions on support layers. Ligand back diffusion controls the feed of ligand molecules available for nuclei formation, resulting in the continuous growth of large‐area ultrathin MOF membranes. We report the practical fabrication of three representative defect‐free MOF membranes with areas larger than 2,400 cm2 and ultrathin selective layers (50–130 nm), including ZIFs and carboxylate‐linker MOFs. Among these, the ZIF‐8 membrane displays high gas permeance of 3,979 GPU for C3H6, with good mixed gas selectivity (43.88 for C3H6/C3H8). To illustrate its scale‐up practicality, MOF membranes were prepared and incorporated into spiral‐wound membrane modules with an active area of 4,800 cm2. The ZIF‐8 membrane module presents high gas permeance (3,930 GPU for C3H6) with acceptable ideal gas selectivity (37.45 for C3H6/C3H8).

Molecular dynamics simulations of Li+/Mg2+ separation using metal-organic frameworks

In this work, we investigate the extraction of Li+ from solutions containing Li+ and Mg2+ using metal-organic framework (MOF) membranes through molecular dynamics simulations (MD). Five MOFs with different pore sizes are studied. It is found that there is a critical pressure for MOFs with pore size smaller than 7.08 Å, below which Li + can be completely separated from Mg2+. This critical pressure increases as the pore size decreases. Under practical pressures (<50 MPa), MOFs with pore size around 6.5–7.0 Å are optimal for complete separation of Li+ and Mg2+. If high pressures can be reached, MOF with pore size of 5.48 Å appears to perform better because it ensures not only perfect separation but also a high flux of Li+. The hydration energy, potential of mean force, and density distribution of Li+ and Mg2+ are computed to explain the separation mechanisms.

Engineering ZIF-67 loaded nanofibrous membrane with thermal stabilization treatment for efficient photocatalytic CO2 reduction

The photocatalytic Metal-organic frameworks (MOFs) membrane addresses challenges faced by MOF powders in practical applications such as aggregation and difficult recovery, but its current application is mainly confined to wastewater treatment. Expanding the application of MOFs membranes to CO2 reduction could unleash their greater potential in energy conversion and storage field. In this study, ZIF-67 was deposited on polyacrylonitrile (PAN) nanofiber membranes (NFMs) using an in-situ growth technique, followed by thermal stabilization to produce SZIF-67/PAN NFMs for photocatalytic CO2 reduction. ZIF-67 was uniformly and densely distributed on the NFMs, with a mass loading of up to 8 wt%. The thermal stabilization process significantly enhanced the photothermal conversion capability and solvent resistance of the NFMs, enabling SZIF-67/PAN NFMs to exhibit outstanding photocatalytic activity even under mild environmental conditions, with a remarkable CO generation rate of 39,250 μmol g−1h−1. Furthermore, the performance remained at 70 % even after five consecutive tests. This study presents a novel and effective method for preparing MOF NFMs with excellent photocatalytic CO2 reduction performance, offering valuable insights for advancing the practical application of MOF materials.

Progressive Insights into Metal-Organic Frameworks and Metal-Organic Framework-Membrane Composite Systems for Wastewater Management.

The sustainable management of wastewater through recycling and utilization stands as a pressing concern in the trajectory of societal advancement. Prioritizing the elimination of diverse organic contaminants is paramount in wastewater treatment, garnering significant attention from researchers worldwide. Emerging metal-organic framework materials (MOFs), bridging organic and inorganic attributes, have surfaced as novel adsorbents, showcasing pivotal potential in wastewater remediation. Nevertheless, challenges like limited water stability, elevated dissolution rates, and inadequate hydrophobicity persist in the context of wastewater treatment. To enhance the performance of MOFs, they can be modified through chemical or physical methods, and combined with membrane materials as additives to create membrane composite materials. These membrane composites, derived from MOFs, exhibit remarkable characteristics including enhanced porosity, adjustable pore dimensions, superior permeability, optimal conductivity, and robust water stability. Their ability to effectively sequester organic compounds has spurred significant research in this field. This paper introduces methods for enhancing the performance of MOFs and explores their potential applications in water treatment. It delves into the detailed design, synthesis strategies, and fabrication of composite membranes using MOFs. Furthermore, it focuses on the application prospects, challenges, and opportunities associated with MOF composite membranes in water treatment.

Structure optimization of ceramic-based metal−organic framework membrane for efficient desalination

To address global water pollution and crisis issue, wastewater recycling, seawater and brackish water desalination have been increasingly utilized to produce clean water. Ceramic membranes have been gaining widespread interest due to their unique physicochemical properties and high stability. Integrating nanoporous MOF (metal organic framework) with ceramic membranes could enable a rational design of ceramic-based MOF membranes with enhance sieving performance. Herein, we report an in-situ method to construct an inter-crystalline defect-free MOF layer on ZrO2 ceramic membranes for efficient desalination application. After optimizing synthesis parameters, an inter-crystalline defect-free and well-intergrown ceramic-based MOF membrane was obtained. The ceramic-based MOF membranes not only demonstrated a near-complete salt rejection, but also exhibited high water flux, which are superior to most of the reported zeolite and MOF membranes. In addition, the MOF membranes exhibited promising performance and structure stability even when exposed to harsh water environments. Our work introduces a protocol for the design of high-performance ceramic-based MOF membrane as a promising candidate for challenging desalination applications under different environments.

In situ electrochemical potential-induced synthesis of metal organic framework membrane on polymer support for H2/CO2 separation

Metal-Organic Framework (MOF) membranes act as selective layers have offered unprecedented opportunities for energy-efficient and cost-effective gas separation. Searching for the green and sustainable synthesis method of dense MOF membrane has received huge attention in both academia and industry. In this work, we demonstrate an in situ electrochemical potential-induced synthesis strategy to aqueously fabricate Metal Azolate Framework-4 (MAF-4) membranes on polypropylene (PP) support. The constant potential can induce the heterogeneous nucleation and growth of MAF-4, resulting an ultrathin membrane with the thickness of only 390 nm. This high-quality membrane exhibits a high H2/CO2 separation performance with the H2 permeance as high as 1565.75 GPU and selectivity of 11.6. The deployment of this environment friendly one-step fabrication method under mild reaction conditions, such as low-cost polymer substrate, water instead of organic solvent, room temperature and ambient pressure shows great promise for the scale-up of MOF membranes.

Ultrastable ceramic-based metal–organic framework membranes with missing linkers for robust desalination

Ultrastable ceramic-based metal–organic framework membranes with missing linkers for robust desalination

Design and Machine Learning Prediction of In Situ Grown PDA-Stabilized MOF (UiO-66-NH2) Membrane for Low-Pressure Separation of Emulsified Oily Wastewater.

Significant progress has been made in designing advanced membranes; however, persistent challenges remain due to their reduced permeation rates and a propensity for substantial fouling. These factors continue to pose significant barriers to the effective utilization of membranes in the separation of oil-in-water emulsions. Metal-organic frameworks (MOFs) are considered promising materials for such applications; however, they encounter three key challenges when applied to the separation of oil from water: (a) lack of water stability; (b) difficulty in producing defect-free membranes; and (c) unresolved issue of stabilizing the MOF separating layer on the ceramic membrane (CM) support. In this study, a defect-free hydrolytically stable zirconium-based MOF separating layer was formed through a two-step method: first, by in situ growth of UiO-66-NH2 MOF into the voids of polydopamine (PDA)-functionalized CM during the solvothermal process, and then by facilitating the self-assembly of UiO-66-NH2 with PDA using a pressurized dead-end assembly. A stable MOF separating layer was attained by enriching the ceramic support with amines and hydroxyl groups using PDA, which assisted in the assembly and stabilization of UiO-66-NH2. The PDA-s-UiO-66-NH2-CM membrane displayed air superhydrophilicity and underwater superoleophobicity, demonstrating its oil resistance and high antifouling behavior. The PDA-s-UiO-66-NH2-CM membrane has shown exceptionally high permeability and separation capacity for challenging oil-in-water emulsions. This is attributed to numerous nanochannels from the membrane and its high resistance to oil adhesion. The membranes showed excellent stability over 15 continuous test cycles, which indicates that the developed MOFs separating layers have a low tendency to be clogged by oil droplets during separation. Machine learning-based Gaussian process regression (GPR) models as nonparametric kernel-based probabilistic models were employed to predict the performance efficiency of the PDA-s-UiO-66-NH2-CM membrane in oil-in-water separation. The outcomes were compared with the support vector machine (SVM) and decision tree (DT) algorithm. This efficiency includes various metrics related to its separation accuracy, and the models were developed through feature engineering to identify and utilize the most significant factors affecting the membrane's performance. The results proved the reliability of GPR optimization with the highest prediction accuracy in the validation phase. The average percentage increase of the GPR model compared to the SVM and DT model was 6.11 and 42.94%, respectively.

Hydrogen-Bonded Metal-Organic Framework Nanosheet as a Proton Conducting Membrane for an H2/O2 Fuel Cell.

Proton-conducting metal-organic frameworks (MOFs) have attracted attention as potential electrolytes for fuel cells. However, research progress in utilizing MOFs as electrolytes for fuel cells has been limited, mainly due to challenges associated with issues such as the fabrication of MOF membranes, and hydrogen crossover through the MOF's pores. Here, proton conductivity and fuel cell performance of a self-standing membrane prepared from of a bismuth subgallateMOF nanosheets with non-porous structure are reported. The fabricated MOF nanosheet membrane with no binding agent exhibits structural anisotropy. The proton conductivity in the membrane thickness direction (4.4×10-3Scm-1) at 90°C and RH 100% is observed to be higher than that in the in-plane direction of the membrane (3.3×10-5Scm-1). The open circuit voltage (OCV) of a fuel cell with ≈120µm proton conducting membrane is 1.0V. The non-porous nature of the MOF nanosheets contributes to the relatively high OCV. A fuel cell using ≈40µm membrane as proton conducting electrolyte records a maximum of 25mWcm-2 power density and a maximum of 109mAcm-2 current density with 0.91V OCV at 80°C in humid conditions.

Improving performance prediction of metal-organic framework membranes for reverse osmosis via genetic algorithm optimized artificial neural networks

The pollution of water resources and the rapid development of society are leading to the decrease of available fresh water resources in the world, which intensifies the research on seawater desalination. Metal-organic frameworks (MOFs), as promising membrane materials with diversified sizes, highly porous structures, and tunable chemical functionalities, have been extensively studied for desalination in recent years. In this paper, genetic algorithm (GA) optimized back propagation (BP) artificial neural network was first constructed, which made the prediction model of MOF membranes performances more accurate. Before the GA optimization, the structure of BP model was tuned through cross-validation and hyperparameter adjustment to achieve a better prediction performance and robustness. Then the GA was introduced attaining an enhanced R2 of 0.98 and 0.99 compared to the BP model of 0.90 and 0.97 for water permeability and salt rejection, respectively, which was also superior to the random forest, support vector machine, and multiple linear regression models. In addition, the grey relational analysis was used to screen the feature variables in order to clarify the most influential factors that affect the membrane performances. It is found that the MOF size and the thickness of polyamide layer were the key influential factors. This work will provide insights for the design of high-performance MOF based membranes and further render implications for other membrane developments.

Regulating ion affinity and dehydration of metal-organic framework sub-nanochannels for high-precision ion separation

Membrane consisting of ordered sub-nanochannels has been pursued in ion separation technology to achieve applications including desalination, environment management, and energy conversion. However, high-precision ion separation has not yet been achieved owing to the lack of deep understanding of ion transport mechanism in confined environments. Biological ion channels can conduct ions with ultrahigh permeability and selectivity, which is inseparable from the important role of channel size and “ion-channel” interaction. Here, inspired by the biological systems, we report the high-precision separation of monovalent and divalent cations in functionalized metal-organic framework (MOF) membranes (UiO-66-(X)2, X = NH2, SH, OH and OCH3). We find that the functional group (X) and size of the MOF sub-nanochannel synergistically regulate the ion binding affinity and dehydration process, which is the key in enlarging the transport activation energy difference between target and interference ions to improve the separation performance. The K+/Mg2+ selectivity of the UiO-66-(OCH3)2 membrane reaches as high as 1567.8. This work provides a gateway to the understanding of ion transport mechanism and development of high-precision ion separation membranes.

Light-responsive and ultrapermeable two-dimensional metal-organic framework membrane for efficient ionic energy harvesting

Nanofluidic membranes offer exceptional promise for osmotic energy conversion, but the challenge of balancing ionic selectivity and permeability persists. Here, we present a bionic nanofluidic system based on two-dimensional (2D) copper tetra-(4-carboxyphenyl) porphyrin framework (Cu-TCPP). The inherent nanoporous structure and horizontal interlayer channels endow the Cu-TCPP membrane with ultrahigh ion permeability and allow for a power density of 16.64 W m−2, surpassing state of-the-art nanochannel membranes. Moreover, leveraging the photo-thermal property of Cu-TCPP, light-controlled ion active transport is realized even under natural sunlight. By combining solar energy with salinity gradient, the driving force for ion transport is reinforced, leading to further improvements in energy conversion performance. Notably, light could even eliminate the need for salinity gradient, achieving a power density of 0.82 W m−2 in a symmetric solution system. Our work introduces a new perspective on developing advanced membranes for solar/ionic energy conversion and extends the concept of salinity energy to a notion of ionic energy.

MOF-Based Membranes for Remediated Application of Water Pollution.

Membrane separation plays a crucial role in the current increasingly complex energy environment. Membranes prepared by metal-organic framework (MOF) materials usually possess unique advantages in common, such as uniform pore size, ultra-high porosity, enhanced selectivity and throughput, and excellent adsorption property, which have been contributed to the separation fields. In this comprehensive review, we summarize various designs and synthesized strategies of free-standing MOF and composite MOF-based membranes for water treatment. Special emphases are given not only on the effects of MOF on membrane performance, removal efficiencies, and elimination mechanisms, but also on the importance of MOF-based membranes for the applications of oily and micro-pollutant removal, adsorption, separation, and catalysis. The challenges and opportunities in the future for the industrial implementation of MOF-based membranes are also discussed.

Superspreading‐Confined Assembly of Oriented 2D MOF Membranes for Biomimetic Cation‐Regulated Ion Transport

AbstractInspired by biological channels, metal‐organic framework (MOF) membranes with sub‐nanometer sized channels offer an ideal platform to construct high‐performance artificial nanofluidic system. However, fabricating long‐range ordered MOF membranes is still a significant challenge. Herein, a superspreading‐confined assembly method is developed to prepare large‐scale oriented 2D MOF (Zn‐TCPP (TCPP = meso‐tetra(4‐carboxyphenyl) porphine)) membranes. The superspreading of reactant liquid under oil phase to form liquid layer plays a critical role in membrane formation, which provides a dynamic confined space for layered assembly of Zn‐TCPP nanosheets. Accompanied with solvent diffusion, such liquid layer is completely converted into &lt;001&gt;‐oriented MOF membrane with controlled thickness. The prepared membrane exhibits distinct divalent cation (Mx2+)‐regulated ion transport behavior as that demonstrated in biological channels: K+ flux can be precisely adjusted by anchoring diverse Mx2+ ions to binding sites, which is appealing in designing high‐performance nanofluidic iontronics. Moreover, similar strategies can be employed to fabricate other MOF membranes (such as Hf‐BTB (BTB = 1,3,5‐tris(4‐carboxyphenyl)benzene) and Ni‐TCPE (TCPE = tetrakis(4‐carboxyphenyl)ethylene)), indicating the broad applicability of this method in 2D membrane fabrication.

Two-Dimensional Sulfonate-Functionalized Metal-Organic Framework Membranes for Efficient Lithium-Ion Sieving.

Two-dimensional (2D) membranes have shown promising potential for ion-selective separation but often suffer from the trade-off between permeability and selectivity. Herein, we report an ultrathin 2D sulfonate-functionalized metal-organic framework (MOF) membrane for efficient lithium-ion sieving. The narrow pores with angstrom precision in the MOF assist hydrated ions to partially remove the hydration shell, according to different hydration energies. The abundant sulfonate groups in the MOF channels serve as hopping sites for fast lithium-ion transport, contributing to a high Li-ion permeability. Then, the difference in affinity of the Li+, Na+, K+, and Mg2+ ions to the terminal sulfonate groups further enhances the Li-ion selectivity. The reported ultrathin MOF membrane overcomes the trade-off between permeability and selectivity and opens up a new avenue for highly permselective membranes.

Designing Metal-Organic Framework (MOF) Membranes for Isomer Separation.

Membrane-based separation has the merit of low carbon footprint. In this study, the pore size of metal-organic framework (MOF) membranes is rationally designed for discriminating various pairs of hydrocarbon isomers. Specifically, Zr-MOF UiO-66 (UiO stands for University of Oslo) membranes are developed for separating p/o-xylene due to their proper pore size. For n-hexane/2-methylpentane separation, the functional groups and proportion of the ligands in UiO-66 are gradually adjusted to effectively regulate the pore size, and UiO-66-33Br membranes are constructed. In addition, relying on the utilization of ligands with shorter length, MOF-801 membranes with smaller pore size are fabricated for n/i-butane separation.

Small Channels Assembled by Multilayer ZIF-8 in Nanocomposite Membranes for Filtration of Ofloxacin in Water.

Metal-organic framework (MOF)-based hybrid membranes still face many unsolved difficulties in the field of liquid separation, with a reliable production technique standing out, in particular, for the water-stable MOF membranes. In this study, zeolitic imidazolate framework-8 (ZIF-8) with acceptable water stability, favorable polymer affinity, and high selectivity was meticulously grafted on commercial poly(vinylidene fluoride) (PVDF) via substrate carboxylation-assisted etching and then overlaid onto PVDF to fabricate a novel hybrid membrane by a layer-by-layer self-assembly method. The optimal membrane manufacturing conditions, including assembly time (10 min), Hmim/Zn2+ molar ratio (8:1), and optimal layer number (three layers), were thoroughly investigated for cutting-off ofloxacin in water filtration. Under low pressure, a nanofiltration scale permeability of about 199.2 L m-2 h-1 MPa-1 and 97.9% rejection of ofloxacin were obtained in bench-scale tests based on the synergistic effect of the Donnan effect and steric hindrance. More significantly, the resulting hybrid membrane demonstrated excellent hydrophilicity, high antifouling, and mechanical and repeatability performances, suggesting promising application possibilities in real-world wastewater filtering settings.

Linker-exchanged zeolitic imidazolate framework membranes for efficient CO2 separation

Metal organic framework (MOF) membranes have exhibited great potential for efficient gas separation owing to their excellent chemical and structural tunability. To realize CO2 separation from flue gas, linker modification or exchange is generally required to functionalize MOF membranes. However, the reported linker-exchanged polycrystalline MOF membranes have not yet achieved attractive CO2 separation performance due to the sacrificed permeance of the overly reduced pore apertures when employing bulky linkers. Herein, we report a 3-amino-1,2,4-triazole (Atz)-exchanged polycrystalline ZIF-8 membrane (Atz-ZIF-8) with Atz molar ratios ranging from 3% to 29% which shows an excellent CO2/N2 separation performance that significantly surpasses that of other linker-exchanged ZIF-8 membranes. The as-synthesized Atz-ZIF-8 membranes possess additional uncoordinated amines for CO2-preferential adsorption, resulting in an excellent CO2/N2 selectivity of 29. What's more, the size and configuration of Atz is similar to 2-methylimidazole (2-mIm), so the window size of Atz-ZIF-8 membranes is not significantly reduced, and thus high CO2 permeance (690 GPU) is achieved. Considering the superior performance and accessible manufacturing process, we conclude that our work could provide guidelines on designing advanced linker-exchanged ZIF-8 based polycrystalline membranes towards post-combustion carbon capture.

Precise selection and separation of ribavirin by nanoconfined imprinted MOFs membrane

Metal-organic frameworks (MOFs) are highly tunable materials with potential as porous media in membrane separations. However, traditional MOFs are usually not specific and selective. Here, we demonstrate a nanocomposite molecularly imprinted MOFs membrane material that can selectively adsorb and separate specific molecules. We demonstrated this adsorption-separation mechanism by introducing ribavirin molecules into multicomponent MOFs by molecular imprinting strategy and performing permeation and dynamic permeation tests. Due to the previously formed nanocages with a specific structure inside the MOFs, combined with the synergistic effect of the subsequent secondary imprinting, excellent selectivity (more than 6) and effective adsorption (46.46 mg g−1) were displayed by the obtained imprinted membrane. The stability of the membrane was measured by the process of dynamic osmosis, and the results showed that the membrane always had a large factor of separation (greater than 7). The experimental results and conclusions of this paper provide new theoretical and practical support for solving the key scientific issues of the “trade-off” effect between the permeation flux and permeation selectivity of template molecules in the traditional molecularly imprinted membranes (MIMs) osmotic separation process.

Coupling atomic layer deposition with supercritical fluid processing to provide a highly C3H6‐selective ZIF‐8 membrane

AbstractZeolitic Imidazolate Framework‐8 (ZIF‐8) membrane is well recognized for its great capability in efficient C3H6/C3H8 separation, and their sustainable fabrication is extremely advantageous for batch production and commercial application. In this study, a solvent‐free protocol was developed for sustainable production of ZIF‐8 membranes with exceptional C3H6/C3H8 selectivity through combining atomic layer deposition (ALD) technique with in situ supercritical fluid (SCF) processing. Experimental results indicated that maintaining appropriate deposition number and conducting SCF processing in gas‐like supercritical carbon dioxide region were crucial for obtaining ZIF‐8 membranes with excellent C3H6/C3H8 selectivity (~185), one of the highest among ZIF‐8 membranes reported in the literature. Noteworthy, not only residual 2‐methylimidazole ligands along with vented gaseous CO2 could be fully recycled and repetitively utilized, but also obsoleting ZIF‐8 membranes could be effectively patched upon SCF‐based treatment. The above‐mentioned solvent‐free protocol holds great application prospects for sustainable production of diverse high‐performance metal–organic framework membranes.