Highly Water-Permeable Metal-Organic Framework MOF-303 Membranes for Desalination

Oriented Metal-Organic Framework Membranes for Molecular Separations

Metal-organic framework (MOF) membranes, which are recognized as state-of-the-art platforms applied in various separation processes, have attracted widespread attention. Nonetheless, to overcome the trade-off between permeability and selectivity, which is crucial for achieving efficient separation, it is important to rationally design and manipulate MOF membrane structure. Given remarkable advances in the past decade, a timely summary of recent advancement in this field has become indispensable. This review introduces major strategies for fabricating oriented MOF membranes, including in situ growth, contra-diffusion method, interface-assisted approach, and laminated nanosheet assembly. New insights into their updated progress and potential are elucidated. Of particular note, recent development and emerging applications of oriented MOF membranes, illustrating their potential to address environmental and energy challenges, are highlighted. Finally, remaining challenges facing their bath production and practical applications are discussed.

Synergistic Construction of Sub-Nanometer Channel Membranes through MOF-Polymer Composites: Strategies and Nanofiltration Applications

The selective separation of small molecules at the sub-nanometer scale has broad application prospects in the field, such as energy, catalysis, and separation. Conventional polymeric membrane materials (e.g., nanofiltration membranes) for sub-nanometer scale separations face challenges, such as inhomogeneous channel sizes and unstable pore structures. Combining polymers with metal-organic frameworks (MOFs), which possess uniform and intrinsic pore structures, may overcome this limitation. This combination has resulted in three distinct types of membranes: MOF polycrystalline membranes, mixed-matrix membranes (MMMs), and thin-film nanocomposite (TFN) membranes. However, their effectiveness is hindered by the limited regulation of the surface properties and growth of MOFs and their poor interfacial compatibility. The main issues in preparing MOF polycrystalline membranes are the uncontrollable growth of MOFs and the poor adhesion between MOFs and the substrate. Here, polymers could serve as a simple and precise tool for regulating the growth and surface functionalities of MOFs while enhancing their adhesion to the substrate. For MOF mixed-matrix membranes, the primary challenge is the poor interfacial compatibility between polymers and MOFs. Strategies for the mutual modification of MOFs and polymers to enhance their interfacial compatibility are introduced. For TFN membranes, the challenges include the difficulty in controlling the growth of the polymer selective layer and the performance limitations caused by the "trade-off" effect. MOFs can modulate the formation process of the polymer selective layer and establish transport channels within the polymer matrix to overcome the "trade-off" effect limitations. This review focuses on the mechanisms of synergistic construction of polymer-MOF membranes and their structure-nanofiltration performance relationships, which have not been sufficiently addressed in the past.

Mechanochemical Synthesis of MOF-303 and Its CO2 Adsorption at Ambient Conditions

Metal-organic structures have great potential for practical applications in many areas. However, their widespread use is often hindered by time-consuming and expensive synthesis procedures that often involve hazardous solvents and, therefore, generate wastes that need to be remediated and/or recycled. The development of cleaner, safer, and more sustainable synthesis methods is extremely important and is needed in the context of green chemistry. In this work, a facile mechanochemical method involving water-assisted ball milling was used for the synthesis of MOF-303. The obtained MOF-303 exhibited a high specific surface area of 1180 m2/g and showed an excellent CO2 adsorption capacity of 9.5 mmol/g at 0 °C and under 1 bar.

Facilely tuning the coating layers of Fe nanoparticles from iron carbide to iron nitride for different performance in Fenton-like reactions

In this study, a series of Fe-based materials are facilely synthesized using MIL-88A and melamine as precursors. Changing the mass ratio of melamine and MIL-88A could tune the coating layers of generated zero-valent iron (Fe0) particles from Fe3C to Fe3N facilely. Compared to Fe/Fe3N@NC sample, Fe/Fe3C@NC exhibits better catalytic activity and stability to degrade carbamazepine (CBZ) with peroxymonosulfate (PMS) as oxidant. Free radical quenching tests, open-circuit potential (OCP) test and electron paramagnetic resonance spectra (EPR) prove that hydroxyl radicals (OH) and superoxide radical (O2-) are dominant reactive oxygen species (ROSs) with Fe/Fe3C@NC sample. For Fe/Fe3N@NC sample, the main ROSs are changed into sulfate radicals (SO4-) and high valent iron-oxo (Fe (IV)=O) species. In addition, the better conductivity of Fe3C is beneficial for the electron transfer from Fe0 to the Fe3C, thus could keep the activity of the surface sites and obtain better stability. DFT calculation reveals the better adsorption and activation ability of Fe3C than Fe3N. Moreover, PMS can also be adsorbed on the Fe sites of Fe3N with shorter FeO bonds and longer SO bonds than on Fe3C, the Fe (IV)=O is thus present in the Fe/Fe3N@NC/PMS system. This study provides a novel strategy for the development of highly active Fe-based materials for Fenton-like reactions and thus could promote their real application.

Empowering ultrathin polyamide membranes at the water-energy nexus: strategies, limitations, and future perspectives

Membrane-based separation is one of the most energy-efficient methods to meet the growing need for a significant amount of fresh water. It is also well-known for its applications in water treatment, desalination, solvent recycling, and environmental remediation. Most typical membranes used for separation-based applications are thin-film composite membranes created using polymers, featuring a top selective layer generated by employing the interfacial polymerization technique at an aqueous-organic interface. In the last decade, various manufacturing techniques have been developed in order to create high-specification membranes. Among them, the creation of ultrathin polyamide membranes has shown enormous potential for achieving a significant increase in the water permeation rate, translating into major energy savings in various applications. However, this great potential of ultrathin membranes is greatly hindered by undesired transport phenomena such as the geometry-induced "funnel effect" arising from the substrate membrane, severely limiting the actual permeation rate. As a result, the separation capability of ultrathin membranes is still not fully unleashed or understood, and a critical assessment of their limitations and potential solutions for future studies is still lacking. Here, we provide a summary of the latest developments in the design of ultrathin polyamide membranes, which have been achieved by controlling the interfacial polymerization process and utilizing a number of novel manufacturing processes for ionic and molecular separations. Next, an overview of the in-depth assessment of their limitations resulting from the substrate membrane, along with potential solutions and future perspectives will be covered in this review.

Aminal-Linked Covalent Organic Framework Membranes Achieve Superior Ion Selectivity

High-salinity wastewater treatment is perceived as a global water resource recycling challenge that must be addressed to achieve zero discharge. Monovalent/divalent salt separation using membrane technology provides a promising strategy for sulfate removal from chlor-alkali brine. However, existing desalination membranes often show low water permeance and insufficient ion selectivity. Herein, an aminal-linked covalent organic framework (COF) membrane featuring a regular long-range pore size of 7 Å and achieving superior ion selectivity is reported, in which a uniform COF layer with subnanosized channels is assembled by the chemical splicing of 1,4-phthalaldehyde (TPA)-piperazine (PZ) COF through an amidation reaction with trimesoyl chloride (TMC). The chemically spliced TPA-PZ (sTPA-PZ) membrane maintains an inherent pore structure and exhibits a water permeance of 13.1 L m-2 h-1 bar-1, a Na2SO4 rejection of 99.1%, and a Cl-/SO4 2- separation factor of 66 for mixed-salt separation, which outperforms all state-of-the-art COF-based membranes reported. Furthermore, the single-stage treatment of NaCl/Na2SO4 mixed-salt separation achieves a high NaCl purity of above 95% and a recovery rate of ≈60%, offering great potential for industrial application in monovalent/divalent salt separation and wastewater resource utilization. Therefore, the aminal-linked COF membrane developed in this work provides a new research avenue for designing smart/advanced membrane materials for angstrom-scale separations.

Enhanced water permeation through the terahertz-induced phase and diffusion transition in metal-organic framework membranes

Freshwater scarcity is a pressing global concern, and water desalination has emerged as a promising solution. Metal-organic framework (MOF) membranes have demonstrated exceptional potential in this regard. However, previous efforts to improve the permeability of MOFs have primarily focused on chemical modifications and synthesis rather than exploring physical methods. Using molecular dynamics simulations, we propose that the use of terahertz waves at a specific frequency of 7.5 ± 1.0 THz significantly enhances water permeability across MOF membranes, up to 27-fold, while maintaining effective ion rejection capabilities throughout the process. The mechanism behind this enhancement involves the resonance between the terahertz wave and the hydrogen bond vibrations of water within the MOF. This resonance amplifies the rotational kinetic energy of water molecules, disrupting the hydrogen bonds and causing a phase transition from quasi 1D square ice to a gas-like phase. Additionally, the diffusion behavior shifts from Fickian diffusion to sub-diffusion, resulting in improved water permeation across the MOF membrane. This study highlights the potential of terahertz waves as a physical tool to enhance the permeability of MOFs in water desalination, providing new avenues for efficient water treatment and resource sustainability.

Bendable and Chemically Stable Metal-Organic Hybrid Membranes for Molecular Separation

Crystalline porous metal-organic materials are ideal building blocks for separation membranes because of their molecular-sized pores and highly ordered pore structure. However, creating ultrathin, defect-free crystalline membranes is challenging due to inevitable grain boundaries. Herein, we reported an amorphous metal-organic hybrid (MOH) membrane with controlled microporosity. The synthesis of the MOH membrane entails the use of titanium alkoxide and organic linkers containing di/multicarboxyl groups as monomers in the polymerization reaction. The resultant membranes exhibit similar microporosity to existing molecular sieve materials and high chemical stability against harsh chemical environments owing to the formation of stable Ti-O bonds between metal centers and organic linkers. An interfacial polymerization is developed to fabricate an ultrathin MOH membrane (thickness of the membrane down to 80 nm), which exhibits excellent rejections (>98% for dyes with molecular weights larger than 690 Da) and high water permeance (55 L m-2 h-1 bar-1). The membranes also demonstrate good flexibility, which greatly improves the processability of the membrane materials.

Highlights in Interface of Wastewater Treatment by Utilizing Metal Organic Frameworks: Purification and Adsorption Kinetics

Polluted water has become a concern for the scientific community as it causes many severe threats to living beings. Detection or removal of contaminants present in wastewater and attaining purity of water that can be used for various purposes are a primary responsibility. Different treatment methods have already been used for the purification of sewage. There is a need for low-cost, highly selective, and reusable materials that can efficiently remove pollutants or purify contaminated water. In this regard, MOFs have shown significant potential for applications such as supercapacitors, drug delivery, gas storage, pollutant adsorption, etc. The outstanding structural diversity, substantial surface areas, and adjustable pore sizes of MOFs make them superior candidates for wastewater treatment. This Review provides an overview of the interaction science and engineering (kinetic and thermodynamic aspects with interactions) underpinning MOFs for water purification. First, fundamental strategies for the synthesis methods of MOFs, different categories, and their applicability in wastewater treatment are summarized, followed by a detailed explanation of various interaction mechanisms. Finally, current challenges and future outlooks for research on MOF materials toward the adsorption of hazardous components are discussed. A new avenue for modifying their structural characteristics for the adsorption and separation of hazardous materials, which will undoubtedly direct future work, is also summarized.

Zeolite Imidazolate Frameworks-8@SiO2-ZrO2 Crystal-Amorphous Hybrid Core-Shell Structure as a Building Block for Water Purification Membranes

Metal-organic frameworks (MOFs) are emerging as promising materials for water purification membranes, owing to their uniform microporous structures and chemical functionalities. Here, we report a simple procedure for depositing MOF-based nanofiltration membranes on commercial TiO2 ceramic tubular supports, completely avoiding the use of dispersants or binders. Zeolite imidazolate frameworks-8 (ZIF-8) nanocrystals were synthesized in methanol at room temperature and subsequently coated with an amorphous SiO2-ZrO2 gel to generate a dispersion of ZIF-8@SiO2-ZrO2 core-shell nanoparticles. The amorphous SiO2-ZrO2 gel served as a binding agent for the ZIF-8 nanocrystals, thus forming a defect-free continuous membrane layer. After repeating the coating twice, the active layer had a thickness of 0.96 μm, presenting a rejection rate >90% for the total organic carbon in an aquaculture effluent and in a wastewater treatment plant, while reducing the concentration of trimethoprim, here used as a target pollutant. Moreover, the oxide gel provided the MOF-based active layer with good adhesion to the support and enhanced its hydrophilicity, resulting in a membrane with excellent mechanical stability and resistance to fouling during the crossflow filtration of the real wastewater samples. These results implied the high potential of the MOF-based nanocomposite membrane for effective treatment of actual wastewater streams.

Tunable Low-Pressure Water Adsorption in Stable Multivariate Metal-Organic Frameworks for Boosting Water-Based Ultralow-Temperature-Driven Refrigeration

The green water-based adsorption refrigeration is considered as a promising strategy to realize near-zero-carbon cooling applications. Although many metal-organic frameworks (MOFs) have been developed as water adsorbents, their cooling performance are commonly limited by the insufficient water uptakes below P/P0 = 0.2. Herein, the development of multivariate MOFs (MTV-MOFs) is reported to highly modulate and boost the low-pressure water uptake for improving coefficient of performance (COP) for refrigeration. Through ligand exchange in the pristine MIL-125-NH2 , a series of MTV-MOFs with bare nitrogen sites are designed and synthesized. The resulting MIL-125-NH2 /MD-5% exhibits the significantly improved water uptake of 0.39 g g-1 at 298 K and P/P0 = 0.2, which is three times higher than MIL-125-NH2 (0.12 g g-1 ) and comparable to some benchmark materials including KMF-1 (0.4 g g-1 ) and MIP-200 (0.36 g g-1 ). Combined with its low-temperature regeneration, fast sorption kinetics and high stability, MIL-125-NH2 /MD-5% achieves one of the highest COP values (0.8) and working capacities (0.24 g g-1 ) for refrig-2 under an ultralow-driven temperature of 65 °C, which are higher than some best-performing MOFs such as MIP-200 (0.74 and 0.11 g g-1 ) and KMF-2 (0.62 and 0.16 g g-1 ), making it among the best adsorbents for efficient ultralow-temperature-driven refrigeration.

In Situ Growth of MOF-303 Membranes onto Porous Anodic Aluminum Oxide Substrates for Harvesting Salinity-Gradient Energy

As an emerging metal-organic framework (MOF) material in recent years, the MOF-303 membrane has shown great potential applications in seawater desalination, dehydration, and atmospheric water harvesting. Herein, we report on a dense and uniform MOF-303 membrane fabricated by a facile in situ hydrothermal synthesis approach in the presence of an anodized aluminum oxide (AAO) channel membrane acting as the only Al source and substrate. Interestingly, the MOF-303 isomer can be obtained due to an insufficient amount of organic ligand caused by the less hydrophilic and larger pore size of the AAO substrate. The MOF-based composite membranes possessed surface-charge-governed ionic transport behavior. Moreover, the MOF-303/AAO membrane yielded an output power density of 1.87 W/m2 under a 50-fold KCl concentration gradient. Under a 50-fold gradient of artificial seawater and river water, a maximum power density of 1.46 W/m2 can be obtained. After 30 days of stability testing, the composite membrane still maintained the power output, and the power density was higher than 1.20 W/m2. This work provides a facile and effective strategy for constructing Al-based MOF composite membranes and boosts their applications in harvesting salinity-gradient energy.

Water-stable hydrophilic metal organic framework composite for the recognition of N-glycopeptides during diabetes progression by mass spectrometry

A hydrophilic Al-MOFs composite was prepared using cheap and available reagents in water via a suitable large-scale production, an economical and environment-friendly method for capturing N-glycopeptides. The prepared Al-MOFs composite with high hydrolytically stable and hydrophilic 1D channels exhibits an ultralow detection limit (0.5 fmol/μL), and excellent reusability (at least 10 cycles) in the capture of N-glycopeptides from standard bio-samples. Interestingly, the Al-MOFs composite also shows remarkable performance in practical applications, where 300 N-glycopeptides ascribed to 124 glycoproteins were identified in 1 µL human serum and were successfully applied in profiling the differences of N-glycopeptides during diabetes progression. Moreover, 12 specific glycoproteins used as biomarkers to accurately distinguish the progression of diabetes are identified. The present work provides a potential commercial method for large-scale glycoproteomics research in complex clinical samples while offering new guidance for the precise diagnosis of diabetes progression.

Regulating the thickness of nanofiltration membranes for efficient water purification

Fabrication of an organic polymer nanofiltration membrane with both high water permeability and high salt rejection is still a big challenge. Herein, phytic acid (PhA)-modified graphene oxide (GO) was used as the membrane thickness modifier, which was introduced into the thin-film nanoparticle composite (TFN) membrane via in situ interfacial polymerization (IP) on a porous substrate. The water flux of the optimally tuned TFN-GP-0.2 composite membrane is 48.9 L m-2 h-1, which is 1.3 times that of the pristine thin-film composite (TFC) nanofiltration membrane (37.9 L m-2 h-1) (GP represents the PhA modified GO composite). The rejection rate of 2000 ppm MgSO4 for TFN-GP-0.2 membranes was maintained at 97.5%. The increased water flux of the TFN-GP composite membrane compared to that of the TFN nanofiltration membrane was mainly attributed to enhanced hydrophilicity and reduced thickness of the polyamide (PA) layer. Molecular dynamics (MD) simulations confirm that the diffusion rate of amine monomers is reduced by the presence of a GP complex in the IP process, which facilitates the formation of PA layer with thinner thickness. In addition, the TFN-GP-0.2 composite membrane also showed good long-term stability; after 12 h of continuous operation, the water flux only decreased by 0.1%. This study sheds new light on the development of GO-based nanofiltration for potential implementation, as well as a unique concept for manufacturing high-performance nanofiltration membranes.

A Mini Review of Ceramic-Based MOF Membranes for Water Treatment

Ceramic membranes have been increasingly employed in water treatment owing to their merits such as high-stability, anti-oxidation, long lifespan and environmental friendliness. The application of ceramic membranes mainly focuses on microfiltration and ultrafiltration processes, and some precise separation can be achieved by introducing novel porous materials with superior selectivity. Recently, metal-organic frameworks (MOFs) have developed a wide spectrum of applications in the fields of the environment, energy, water treatment and gas separation due to the diversity and tunable advantages of metal clusters and organic ligands. Although the issue of water stability in MOF materials inhibits the development of MOF membranes in water treatment, researchers still overcome many obstacles to advance the application of MOF membranes in water treatment processes. To the best of our knowledge, there is still a lack of a reviews on the development process and prospects of ceramic-based MOF membranes for water treatment. Therefore, in this review, we mainly summarize the fabrication method for ceramic-based MOF membranes and their application in water treatment, such as water/salt separation, pollutant separation, heavy metal separation, etc. Following this, based on the high structural, thermal and chemical stability of ceramic substrates, and the high controllability of MOF materials, the superiority and insufficient use of ceramic-based MOF membranes in the field of water treatment are critically discussed.

Guanidinium-based covalent organic framework membrane for single-acid recovery

Acids are extensively used in contemporary industries. However, time-consuming and environmentally unfriendly processes hinder single-acid recovery from wastes containing various ionic species. Although membrane technology can overcome these challenges by efficiently extracting analytes of interest, the associated processes typically exhibit inadequate ion-specific selectivity. In this regard, we rationally designed a membrane with uniform angstrom-sized pore channels and built-in charge-assisted hydrogen bond donors that preferentially conducted HCl while exhibiting negligible conductance for other compounds. The selectivity originates from the size-screening ability of angstrom-sized channels between protons and other hydrated cations. The built-in charge-assisted hydrogen bond donor enables the screening of acids by exerting host-guest interactions to varying extents, thus acting as an anion filter. The resulting membrane exhibited exceptional permeation for protons over other cations and for Cl^- over SO₄^2- and H_nPO₄^(3-n)- with selectivities up to 4334 and 183, respectively, demonstrating prospects for HCl extraction from waste streams. These findings will aid in designing advanced multifunctional membranes for sophisticated separation.

Metal organic frameworks-in-nanochannels: A tailorable chromatography membrane for isolation of target protein

Metal-organic frameworks (MOFs) demonstrate strong potential in biosample separation. However, the obtained MOFs powders are unsuitable for recovery techniques in an aqueous solution, especially the challenges of withdrawing MOFs particles and expanding their functions for specific applications. Herein, a general strategy is designed utilizing metal oxide-nanochannel arrays as precursors and templates for in-situ selective growth of MOFs structures. The exemplary MOFs (Ni-bipy) with tailored composition are selectively grown in NiO/TiO₂ nanochannel membrane (NM) using NiO as the sacrificial precursor, which enables one to achieve a ∼262 times concentration of histidine-tagged proteins within 100 min. The significantly improved adsorption efficiency in a wide pH range and the effective enrichment from a complex matrix as a nanofilter illustrate the great potential of MOFs in nanochannels membranes for the high-efficiency recovery of essential proteins in complex biological samples. The porous self-aligned Ni-MOFs/TiO₂ NM exhibits biocompatibility and flexible functionalities, which is desirable for the generation of multifunctional nanofilter devices and developing biomacromolecule delivery vehicles.

Eliminating lattice defects in metal-organic framework molecular-sieving membranes

Metal-organic framework (MOF) membranes are energy-efficient candidates for molecular separations, but it remains a considerable challenge to eliminate defects at the atomic scale. The enlargement of pores due to defects reduces the molecular-sieving performance in separations and hampers the wider application of MOF membranes, especially for liquid separations, owing to insufficient stability. Here we report the elimination of lattice defects in MOF membranes based on a high-probability theoretical coordination strategy that creates sufficient chemical potential to overcome the steric hindrance that occurs when completely connecting ligands to metal clusters. Lattice defect elimination is observed by real-space high-resolution transmission electron microscopy and studied with a mathematical model and density functional theory calculations. This leads to a family of high-connectivity MOF membranes that possess ångström-sized lattice apertures that realize high and stable separation performance for gases, water desalination and an organic solvent azeotrope. Our strategy could enable a platform for the regulation of nanoconfined molecular transport in MOF pores.

Engineering Metal-Organic-Framework (MOF)-Based Membranes for Gas and Liquid Separation

Separation is one of the most energy-intensive processes in the chemical industry, and membrane-based separation technology contributes significantly to energy conservation and emission reduction. Additionally, metal-organic framework (MOF) materials have been widely investigated and have been found to have enormous potential in membrane separation due to their uniform pore size and high designability. Notably, pure MOF films and MOF mixed matrix membranes (MMMs) are the core of the "next generation" MOF materials. However, there are some tough issues with MOF-based membranes that affect separation performance. For pure MOF membranes, problems such as framework flexibility, defects, and grain orientation need to be addressed. Meanwhile, there still exist bottlenecks for MMMs such as MOF aggregation, plasticization and aging of the polymer matrix, poor interface compatibility, etc. Herein, corresponding methods are introduced to solve these problems, including inhibiting framework flexibility, regulating synthesis conditions, and enhancing the interaction between MOF and substrate. A series of high-quality MOF-based membranes have been obtained based on these techniques. Overall, these membranes revealed desired separation performance in both gas separation (e.g., CO₂, H₂, and olefin/paraffin) and liquid separation (e.g., water purification, organic solvent nanofiltration, and chiral separation).

Facile Synthesis of Ultra-microporous Pillar-Layered Metal-Organic Framework Membranes for Highly H2-Selective Separation

Recently, pillar-layered MOF materials have attracted much attention and shown great potential in separation application due to their fine pore size/channel and pore surface chemistry tunability and designability. In this work, we reported an effective and universal synthesis strategy for preparing ultra-microporous Ni-based pillar-layered MOF [Ni₂(L-asp)₂(bpy)] (Ni-LAB) and [Ni₂(L-asp)₂(pz)] (Ni-LAP) (L-asp = L-aspartic acid, bpy = 4,4'-bipyridine, pz = pyrazine) membranes on a porous α-Al₂O₃ substrate with high performance and good stability by secondary growth. Through this strategy, the seed size reduction and screening engineering (SRSE) is proposed to obtain uniform sub-micron size MOF seeds by high-energy ball milling-combined solvent deposition. This strategy not only effectively addresses the issue of obtaining the uniform small seeds being significant for secondary growth but also provides an approach for the preparation of Ni-based pillar-layered MOF membranes where the freedom of synthesizing small crystals is lacking. Based on reticular chemistry, the pore size of Ni-LAB was narrowed by making use of shorter pillar ligands of pz instead of the longer pillar ligand of bpy. The prepared ultra-microporous Ni-LAP membranes exhibited a high H₂/CO₂ separation factor of 40.4 with H₂ permeance of 9.69 × 10^-8 mol m^-2 s^-1 Pa^-1 under ambient conditions and good mechanical and thermal stability. The superiority of the tunable pore structure and the remarkable stability of these MOF materials showed great potential for industrial H₂ purification. More importantly, our synthesis strategy demonstrated the generality for preparation of MOF membranes, enabling the regulation of membrane pore size and surface functional groups by reticular chemistry.

Metal-organic frameworks for high performance desalination through thickness control and structural fine-tuning

Two-dimensional nanoporous membranes hold great promise for the design of state-of-the-art desalination architectures to alleviate the increasing global water scarcity. Herein, by employing molecular dynamics simulations, we demonstrate the great potential of two recently reported metal-organic frameworks (MOF) membranes, namely NiIT and NiAT, as efficient desalination membranes that reach super high water flux and high salt rejection. The desalination performance of the MOF membrane is highly tunable through controlling the membrane thickness from one layer to five layers. Double layer NiIT membrane exhibits excellent salt rejection of 100% for NaCl, and meanwhile achieving high water permeability of ∼45 L/cm²/MPa/day. While for the convertible double-layer NiAT, it effectively rejects ∼96% ions with an improved water permeation of over 70 L/cm²/MPa/day. Quantitative analysis of water distribution reveals a denser water solvation shell around NiAT membrane than NiIT and a higher water velocity through the nanopore of NiAT than that of NiIT, contributing to the enhanced water permeability. Through calculating free energy for water/ions translocating through two membranes, a clear energy barrier is observed for ions to penetrate through the sub-nanosized pores in both membranes, leading to the high salt rejection. The present study suggests that these two MOF membranes can serve as a promising semipermeable membrane for energy-efficient desalination which is highly prospective in industrial applications.

High-yield, green and scalable methods for producing MOF-303 for water harvesting from desert air

Metal-organic frameworks (MOFs) are excellent candidates for water harvesting from desert air. MOF-303 (Al(OH)(PZDC), where PZDC is 1-H-pyrazole-3,5-dicarboxylate), a robust and water-stable MOF, is a particularly promising water-harvesting sorbent that can take up water at low relative humidity and release it under mild heating. Accordingly, development of a facile, high-yield synthesis method for its production at scale is highly desirable. Here we report detailed protocols for the green, water-based preparation of MOF-303 on both gram and kilogram scales. Specifically, four synthetic methods (solvothermal, reflux, vessel and microwave), involving different equipment requirements, are presented to guarantee general accessibility. Typically, the solvothermal method takes ~24 h to synthesize MOF-303, while the reflux and vessel methods can reduce the time to 4-8 h. With the microwave-assisted method, the reaction time can be further reduced to just 5 min. In addition, we provide guidance on the characterization of MOF-303, as well as water-harvesting MOFs in general, to ensure high quality of the product in terms of its purity, crystallinity, porosity and water uptake. Furthermore, to address the need for future commercialization of this material, we demonstrate that our protocol can be employed to produce 3.5 kg per batch with a yield of 91%. MOF-303 synthesized at this large scale shows similar crystallinity and water uptake capacity compared to the respective material produced at a small scale. Our synthetic procedure is green and water-based, and can produce the MOF within hours.

Molecular modeling of thin-film nanocomposite membranes for reverse osmosis water desalination

The scarcity of freshwater resources is a major global challenge causedby population and economic growth. Water desalination using a reverse osmosis (RO) membrane is a promising technology to supply potable water from seawater and brackish water. The advancement of RO desalination highly depends on new membrane materials. Currently, the RO technology mainly relies on polyamide thin-film composite (TFC) membranes, which suffer from several drawbacks (e.g., low water permeability, permeability-selectivity tradeoff, and low fouling resistance) that hamper their real-world applications. Nanoscale fillers with specific characteristics can be used to improve the properties of TFC membranes. Embedding nanofillers into TFC membranes using interfacial polymerization allows the creation of thin-film nanocomposite (TFNC) membranes, and has become an emerging strategy in the fabrication of high-performance membranes for advanced RO water desalination. To achieve optimal design, it is indispensable to search for reliable methods that can provide fast and accurate predictions of the structural and transport properties of the TFNC membranes. However, molecular understanding of permeability-selectivity characteristics of nanofillers remains limited, partially due to the challenges in experimentally exploring microscopic behaviors of water and salt ions in confinement. Molecular modeling and simulations can fill this gap by generating molecular-level insights into the effects of nanofillers' characteristics (e.g., shape, size, surface chemistry, and density) on water permeability and ion selectivity. In this review, we summarize molecular simulations of a diverse range of nanofillers including nanotubes (carbon nanotubes, boron nitride nanotubes, and aquaporin-mimicking nanochannels) and nanosheets (graphene, graphene oxide, boron nitride sheets, molybdenum disulfide, metal and covalent organic frameworks) for water desalination applications. These simulations reveal that water permeability and salt rejection, as the major factors determining the desalination performance of TFNC membranes, significantly depend on the size, topology, density, and chemical modifications of the nanofillers. Identifying their influences and the physicochemical processes behind, via molecular modeling, is expected to yield important insights for the fabrication and optimization of the next generation high-performance TFNC membranes for RO water desalination.

Constructing Janus Microsphere Membranes for Particulate Matter Filtration, Directional Water Vapor Transfer, and High-Efficiency Broad-Spectrum Sterilization

Commercial masks have significant drawbacks, including low water vapor transmission efficiency and limited ability to inhibit harmful microorganisms, whereas in this contribution, a series of Janus microsphere membranes are developed with hierarchical structures by quenching and crystallizing 12-hydroxystearic acid and halicin layer-by-layer on a polypropylene non-woven fabric, laminating them with hydrophilic cotton fibers in a one-pot process, and further demonstrate the potential of this composite system as masks. Through further optimization, excellent superhydrophobic/superhydrophilic properties (contact angle 157.1°/0°), superior filtering effects (93.54% for PM_2.5 and 98.35% for PM₁₀ ), with a low-filtration resistance (57 Pa) and a quality factor of up to 0.072 Pa^-1 are achieved, all better than that of commercial N95 masks. In addition, the membrane allows for the directional transport of water vapor from the inside out, increasing the water vapor transmission rate by more than 20% compared with the monolayer hydrophobic microsphere membrane. It also has a bactericidal capacity of over 99.9999% against Escherichia coli and is tested for robustness and stability in various extreme environments. This work may shed light on designing novel filter media with versatile functions, meanwhile, the materials can also be used in protective equipment against the new coronavirus.

Application of Nanofiltration Membrane Based on Metal-Organic Frameworks (MOFs) in the Separation of Magnesium and Lithium from Salt Lakes

With the increasing demand for lithium, the shortage of resources has become increasingly apparent. In order to conserve resources and to improve recovery, the extraction of lithium from salt lakes has become mandatory for sustainable development. Porous metal-organic framework (MOF) materials have attracted extensive attention due to their high/tunable porosity, pore function, multiple pore structures/compositions, and open metal sites. Moreover, MOFs combine the advantages of other porous materials and have a wide range of applications, which have received significant interest from the scientific community. Therefore, the selection of MOFs materials, the optimization of preparation methods, and the research of lithium separators are key directions to improve the total yield of lithium resources in salt lakes in China. This study aims to improve the comprehensive utilization of resources after lithium extraction and strengthen the engineering technology research of lithium extraction from salt lakes. This study can help to achieve the goal of efficient, integrated, and sustainable utilization of salt lake resources. An attempt has been made to summarize the types and preparation methods of MOFs materials, as well as the separation mechanism of MOFs nanofiltration membranes, with reference to its application in lithium extraction from salt lake brine. Finally, the future development of MOFs nanofiltration membranes for lithium extraction from salt lakes is also proposed.

Metal-Organic Frameworks and Their Composites for Environmental Applications

From the point of view of the ecological environment, contaminants such as heavy metal ions or toxic gases have caused harmful impacts on the environment and human health, and overcoming these adverse effects remains a serious and important task. Very recent, highly crystalline porous metal-organic frameworks (MOFs), with tailorable chemistry and excellent chemical stability, have shown promising properties in the field of removing various hazardous pollutants. This review concentrates on the recent progress of MOFs and MOF-based materials and their exploit in environmental applications, mainly including water treatment and gas storage and separation. Finally, challenges and trends of MOFs and MOF-based materials for future developments are discussed and explored.

Forward Osmosis Membranes: The Significant Roles of Selective Layer

Forward osmosis (FO) is a promising separation technology to overcome the challenges of pressure-driven membrane processes. The FO process has demonstrated profound advantages in treating feeds with high salinity and viscosity in applications such as brine treatment and food processing. This review discusses the advancement of FO membranes and the key membrane properties that are important in real applications. The membrane substrates have been the focus of the majority of FO membrane studies to reduce internal concentration polarization. However, the separation layer is critical in selecting the suitable FO membranes as the feed solute rejection and draw solute back diffusion are important considerations in designing large-scale FO processes. In this review, emphasis is placed on developing FO membrane selective layers with a high selectivity. The effects of porous FO substrates in synthesizing high-performance polyamide selective layer and strategies to overcome the substrate constraints are discussed. The role of interlayer in selective layer synthesis and the benefits of nanomaterial incorporation will also be reviewed.

Ultrafast Interfacial Self-Assembly toward Supramolecular Metal-Organic Films for Water Desalination

Supramolecular metal-organic materials are considered as the ideal candidates for membrane fabrication due to their excellent film forming characteristics, diverse metal centers and ligand sources, and designable structure and function. However, it remains challenging to rapidly construct highly permeable supramolecular metal-organic membranes with high salt rejection. Herein, a novel ultrafast interfacial self-assembly strategy to prepare supramolecular metal-organic films through the strong coordination interaction between highly active 1,3,5-triformylphloroglucinol (TFP) ligands and Fe³⁺ , Sc³⁺ , or Cu²⁺ at the organic-aqueous interface is reported. Benefiting from the self-completing and self-limiting characteristics of this interfacial self-assembly, the new kind of supramolecular membrane with optimized composition can be assembled within 3.5 min and exhibits ultrathin, dense, defect-free features, and thus shows an excellent water permeance (21.5 L m^-2  h^-1  bar^-1 ) with a high Na₂ SO₄ rejection above 95%, which outperforms almost all of the non-polyamide membranes and commercially available nanofiltration membranes. This strong-coordination interfacial self-assembly method will open up a new way for the development of functional metal-organic supramolecular films for high-performance membrane separation and beyond.

Preparation of Ni@UiO-66 incorporated polyethersulfone (PES) membrane by magnetic field assisted strategy to improve permeability and photocatalytic self-cleaning ability

Metal-organic frameworks (MOFs) have been considered as promising nanofillers to fabricate mixed matrix membranes for water treatment. However, manipulating distribution of MOFs nanoparticles in the membrane matrix remains a great challenge. In this study, UiO-66 was firstly coated by magnetic Ni via an in-situ reduction reaction, and then incorporated into polyethersulfone (PES) membrane matrix to prepare PES-Ni@UiO-66 membrane. The magnetic Ni allowed to manipulate the distribution of magnetic Ni@UiO-66 in the phase-inversion process by an external magnetic field. The hydrophilic Ni@UiO-66 can be pulled onto membrane surface by the magnetic force, endowing the prepared membrane with rather higher hydrophilicity. The prepared membrane exhibited superior water permeability with a pure water flux of 611.5 ± 19.8 L·m^-2·h^-1 and improved antifouling performance. Moreover, benifiting from photocatalytic activity of the exposed Ni@UiO-66 on membrane surface, the obtained PES-Ni@UiO-66 membrane demonstrated excellent photocatalytic self-cleaning ability with a flux recovery rate (FRR) higher than 95% under UV irradiation. Analyzing by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory indicated that the improved antifouling performance could be attributed to less attractive or even repulsive interaction between the prepared membrane and pollutants. This work provided valuable guidance for structural regulation and development of high-performance MOFs-based membranes for water treatment.

Oriented self-assembly of metal-organic frameworks driven by photoinitiated monomer polymerization

The self-assembly of metal–organic frameworks (MOFs) is crucial for the functional design of materials, including energy storage materials, catalysts, selective separation materials and optical crystals. However, oriented self-assembly of MOFs is still a challenge. Herein, we propose a novel strategy to drive oriented self-assembly of MOF polyhedral particles at the water–liquid interface by photoinitiated monomer polymerization. The MOF polyhedral particles self-assemble into ordered close-packed structures with obvious orientation in the polymer film, and the orientation is determined by the casting solvent on the water surface. The prepared large-area MOF polymer films show a Janus structure, containing a MOF monolayer and a polymer layer, and can be easily transferred to a variety of substrates. In addition, mixed MOF particles with different sizes and morphologies can also be assembled by this method. This novel method can be foreseen to provide a powerful driving force for the development of MOF self-assembly and to create more possibilities for utilizing the anisotropic properties of MOFs.

The self-assembly of metal–organic frameworks (MOFs) is crucial for the functional design of materials, including energy storage materials, catalysts, selective separation materials and optical crystals.

A Review of Advancing Two-Dimensional Material Membranes for Ultrafast and Highly Selective Liquid Separation

Membrane-based nanotechnology possesses high separation efficiency, low economic and energy consumption, continuous operation modes and environmental benefits, and has been utilized in various separation fields. Two-dimensional nanomaterials (2DNMs) with unique atomic thickness have rapidly emerged as ideal building blocks to develop high-performance separation membranes. By rationally tailoring and precisely controlling the nanochannels and/or nanoporous apertures of 2DNMs, 2DNM-based membranes are capable of exhibiting unprecedentedly high permeation and selectivity properties. In this review, the latest breakthroughs in using 2DNM-based membranes as nanosheets and laminar membranes are summarized, including their fabrication, structure design, transport behavior, separation mechanisms, and applications in liquid separations. Examples of advanced 2D material (graphene family, 2D TMDs, MXenes, metal–organic frameworks, and covalent organic framework nanosheets) membrane designs with remarkably perm-selective properties are highlighted. Additionally, the development of strategies used to functionalize membranes with 2DNMs are discussed. Finally, current technical challenges and emerging research directions of advancing 2DNM membranes for liquid separation are shared.

Molecular-Level Insights into Unique Behavior of Water Molecules Confined in the Heterojunction between One- and Two-Dimensional Nanochannels

With the increasing importance of nanoconfined water in various heterostructures, it is quite essential to clarify the influence of nanoconfinement on the unique properties of water molecules in the pivotal heterojunction. In this work, we reported a series of classical molecular dynamics (MD) simulations to explore nanoconfined water in the subnanometer-sized and nanometer-sized heterostructures by adjusting one-dimensional (1-D) carbon nanotubes with different diameters and two-dimensional (2-D) graphene sheets with different interlayer distances. Our simulation results demonstrated that water molecules in the 1-D/2-D heterojunction show an obvious structural rearrangement associated with the remarkable breaking and formation of hydrogen bonds (HBs), and such rearrangements in the subnanometer-sized systems are much more pronounced than those in the nanometer-sized ones. When water molecules in the 1-D/2-D heterojunctions migrate from 2-D to 1-D confinements, the ordered multi-layer structure in the 2-D confinement are completely destroyed and then transform into different circular HB networks near the nanotube orifice for better connecting to the single-file or helical HB network in the 1-D nanotubes. Furthermore, water molecules in the 1-D/2-D heterojunctions can form stronger HBs with those water molecules further away from the 1-D confinement, leading to an asymmetrical orientational distribution near the orifice. More importantly, our comparison results revealed that the 1-D confinement plays a more important role than the 2-D confinement in determining both the structures and dynamics of water molecules in the 1-D/2-D heterojunction.

Molecular Self-Assembly Enables Tuning of Nanopores in Atomically Thin Graphene Membranes for Highly Selective Transport

Atomically thin membranes comprising nanopores in a 2D material promise to surpass the performance of polymeric membranes in several critical applications, including water purification, chemical and gas separations, and energy harvesting. However, fabrication of membranes with precise pore size distributions that provide exceptionally high selectivity and permeance in a scalable framework remains an outstanding challenge. Circumventing these constraints, here, a platform technology is developed that harnesses the ability of oppositely charged polyelectrolytes to self-assemble preferentially across larger, relatively leaky atomically thin nanopores by exploiting the lower steric hindrance of such larger pores to molecular interactions across the pores. By selectively tightening the pore size distribution in this manner, self-assembly of oppositely charged polyelectrolytes simultaneously introduced on opposite sides of nanoporous graphene membranes is demonstrated to discriminate between nanopores to seal non-selective transport channels, while minimally compromising smaller, water-selective pores, thereby remarkably attenuating solute leakage. This improved membrane selectivity enables desalination across centimeter-scale nanoporous graphene with 99.7% and >90% rejection of MgSO₄ and NaCl, respectively, under forward osmosis. These findings provide a versatile strategy to augment the performance of nanoporous atomically thin membranes and present intriguing possibilities of controlling reactions across 2D materials via exclusive exploitation of pore size-dependent intermolecular interactions.