Micropatterned Ultrathin MOF Membranes with Enhanced Molecular Sieving Property

Crosslinking-induced patterning of MOFs by direct photo- and electron-beam lithography

Metal-organic frameworks (MOFs) with diverse chemistry, structures, and properties have emerged as appealing materials for miniaturized solid-state devices. The incorporation of MOF films in these devices, such as the integrated microelectronics and nanophotonics, requires robust patterning methods. However, existing MOF patterning methods suffer from some combinations of limited material adaptability, compromised patterning resolution and scalability, and degraded properties. Here we report a universal, crosslinking-induced patterning approach for various MOFs, termed as CLIP-MOF. Via resist-free, direct photo- and electron-beam (e-beam) lithography, the ligand crosslinking chemistry leads to drastically reduced solubility of colloidal MOFs, permitting selective removal of unexposed MOF films with developer solvents. This enables scalable, micro-/nanoscale (≈70 nm resolution), and multimaterial patterning of MOFs on large-area, rigid or flexible substrates. Patterned MOF films preserve their crystallinity, porosity, and other properties tailored for targeted applications, such as diffractive gas sensors and electrochromic pixels. The combined features of CLIP-MOF create more possibilities in the system-level integration of MOFs in various electronic, photonic, and biomedical devices.

Fabrication of Oriented Polycrystalline MOF Superstructures

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties.

In Situ Growth of Sub-50-nm Zirconium Aminobenzenedicarboxylate Metal-Organic Framework Nanocrystals for Carbon Dioxide Capture

Controlled synthesis of sub-50-nm metal-organic frameworks (MOFs), which are usually called porous coordination polymers, exhibits huge potential applications in gas storage and separation. Herein, surface-confined growth of zirconium aminobenzenedicarboxylate MOF (UIO-66-NH2) nanocrystals on polypyrrole hollow spheres (PPyHSs) is achieved through covalently grafted benzene dicarboxylic acid ligands using bridged molecules. PPyHSs modified with ligand molecules prohibit excessive growth of UIO-66-NH2 nanocrystals on their confined surface, resulting in smaller-sized nanocrystals (<50 nm) and a monolayer UIO-66-NH2 coating. Benefiting from the homogeneous dispersion of UIO-66-NH2 nanocrystals with a smaller size (40 ± 10 nm), the as-prepared PPyHSs@UIO-66-NH2 hybrids with high specific surface area and pore volume exhibit remarkable CO2 capture performance. Moreover, the time required to reach the maximum CO2 adsorption capacity shortens with decreasing UIO-66-NH2 crystals size. As a proof of concept, the proposed covalent grafting strategy can be used for synthesizing sub-50-nm UIO-66-NH2 nanocrystals for CO2 capture.

Locking Patterned Carbon Nanotube Cages by Nanofibrous Mats to Construct Cucurbituril[n]-Based Ultrapermselective Dye/Salt Separation Membranes

Surface patterning is a promising strategy to overcome the trade-off effect of separation membranes. Herein, a bottom-up patterning strategy of locking micron-sized carbon nanotube cages (CNCs) onto a nanofibrous substrate is developed. The strongly enhanced capillary force triggered by the abundant narrow channels in CNCs endows the precisely patterned substrate with excellent wettability and antigravity water transport. Both are crucial for the preloading of cucurbit[n]uril (CB6)-embeded amine solution to form an ultrathin (∼20 nm) polyamide selective layer clinging to CNCs-patterned substrate. The CNCs-patterning and CB6 modification result in a 40.2% increased transmission area, a reduced thickness, and a lowered cross-linking degree of selective layer, leading to a high water permeability of 124.9 L·m^-2 h^-1 bar^-1 and a rejection of 99.9% for Janus Green B (511.07 Da), an order of magnitude higher than that of commercial membranes. The new patterning strategy provides technical and theoretical guidance for designing next-generation dye/salt separation membranes.

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).

A convenient catalytic method for preparation of new tetrahydropyrido[2,3-d]pyrimidines via a cooperative vinylogous anomeric based oxidation

In this study, a novel functionalized metal-organic frameworks MIL-125(Ti)-N(CH₂PO₃H₂)₂ was designed and synthesized via post-modification methodology. Then, MIL-125(Ti)-N(CH₂PO₃H₂)₂ as a mesoporous catalyst was applied for the synthesis of a wide range of novel tetrahydropyrido[2,3-d]pyrimidines as bioactive candidate compounds by one-pot condensation reaction of 3-(1-methyl-1H-pyrrol-2-yl)-3-oxopropanenitrile, 6-amino-1,3-dimethylpyrimidine-2,4(1H,3H)-dione and aromatic aldehydes at 100 °C under solvent-free condition. Interestingly, the preparation of tetrahydropyrido[2,3-d]pyrimidine was achieved via vinylogous anomeric based oxidation mechanism with a high yield and short reaction time.

Metal-organic framework enables ultraselective polyamide membrane for desalination and water reuse

While reverse osmosis (RO) is the leading technology to address the global challenge of water scarcity through desalination and potable reuse of wastewater, current RO membranes fall short in rejecting certain harmful constituents from seawater (e.g., boron) and wastewater [e.g., N-nitrosodimethylamine (NDMA)]. In this study, we develop an ultraselective polyamide (PA) membrane by enhancing interfacial polymerization with amphiphilic metal-organic framework (MOF) nanoflakes. These MOF nanoflakes horizontally align at the water/hexane interface to accelerate the transport of diamine monomers across the interface and retain gas bubbles and heat of the reaction in the interfacial reaction zone. These mechanisms synergistically lead to the formation of a crumpled and ultrathin PA nanofilm with an intrinsic thickness of ~5 nm and a high cross-linking degree of ~98%. The resulting PA membrane delivers exceptional desalination performance that is beyond the existing upper bound of permselectivity and exhibited very high rejection (>90%) of boron and NDMA unmatched by state-of-the-art RO membranes.

Robust ultrathin nanoporous MOF membrane with intra-crystalline defects for fast water transport

Rational design of high-performance stable metal–organic framework (MOF) membranes is challenging, especially for the sustainable treatment of hypersaline waters to address critical global environmental issues. Herein, a molecular-level intra-crystalline defect strategy combined with a selective layer thinning protocol is proposed to fabricate robust ultrathin missing-linker UiO-66 (ML-UiO-66) membrane to enable fast water permeation. Besides almost complete salt rejection, high and stable water flux is achieved even under long-term pervaporation operation in hash environments, which effectively addresses challenging stability issues. Then, detailed structural characterizations are employed to identify the type, chemical functionality, and density of intra-crystalline missing-linker defects. Moreover, molecular dynamics simulations shed light on the positive atomistic role of these defects, which are responsible for substantially enhancing structural hydrophilicity and enlarging pore window, consequently allowing ultra-fast water transport via a lower-energy-barrier pathway across three-dimensional sub-nanochannels during pervaporation. Unlike common unfavorable defect effects, the present positive intra-crystalline defect engineering concept at the molecular level is expected to pave a promising way toward not only rational design of next-generation MOF membranes with enhanced permeation performance, but additional water treatment applications.

The development of highly water-permeable membranes is key for the treatment of high salinity waters. Here the authors enhance the water permeability of a metal-organic framework nanoporous membrane via an intra-crystalline defect engineering strategy.

Stable Zr-Based Metal-Organic Framework Nanoporous Membrane for Efficient Desalination of Hypersaline Water

Treatment of hypersaline waters is a critical environmental challenge. Pervaporation (PV) desalination is a promising technique to address this challenge, but current PV membranes still suffer from challenging issues such as low flux and insufficient stability. Herein, we propose in situ nanoseeding followed by a secondary growth strategy to fabricate a high-quality stable metal-organic framework (MOF) thin membrane (UiO-66) for high-performance pervaporation desalination of hypersaline waters. To address the issue of membrane quality, a TiO₂ nano-interlayer was introduced on coarse mullite substrates to favor the growth of a UiO-66 nanoseed layer, on which a well-intergrown UiO-66 selective membrane layer with thickness as low as 1 μm was finally produced via subsequent secondary growth. The PV separation performance for hypersaline waters was systematically investigated at different salt concentrations, feed temperatures, and long-term operation in different extreme chemical environments. Besides having nearly complete rejection (99.9%), the UiO-66 membrane exhibited high flux (37.4 L·m^-2·h^-1) for hypersaline waters, outperforming current existing zeolite and MOF membranes. The membrane also demonstrated superior long-term operational stability under various harsh environments (hypersaline, hot, and acidic/alkaline feed water) and mild fouling behavior. The rational design proposed in this study is not only applicable for the development of a high-quality UiO-66 membrane enabling harsh hypersaline water treatment but can also be potentially extended to other next-generation nanoporous MOF membranes for more environmental applications.

3D MOF Nanoarchitecture Membrane via Ultrafast Laser Nanoforging

Metal-organic framework (MOF) crystals are useful in a vast area of applications because of their unique chemical and physical properties. Manufacturing of an integrated MOF membrane with 3D nanoarchitectures on the surface is especially important for their applications. However, as MOF crystals usually exist as powdery crystals, fabrication of their large area, monolithic, and high-resolution patterns is challenging. Here, it is found that isolated MOF nanocrystals could be directly converted to a monolithic MOF film with designed 3D nanoarchitectures/patterns via an ultrafast laser induced nanoforging without binders. During the nanosecond laser shock, the voids among MOF nanocrystals are eliminated due to the surface amorphization effect, which allows the fusing of the MOF nanocrystals on the grain boundaries, leading to the formation of a dense film while preserving the nature of the pristine MOF. The high strain rate by laser enhances formability of MOFs and overcomes their brittleness to generate arbitrary 3D nanoarchitectures with feature sizes down to 100 nm and high productivity up to 80 cm² min^-1 . These 3D MOF nanoarchitectures also exhibit boosted mechanical strength up to 100% compared with their powdery particles. This method is facile and low-cost and could potentially be used in various fields, such as devices, separation, and biochemical applications.

Anodic electrosynthesis of MIL-53(Al)-N(CH2PO3H2)2 as a mesoporous catalyst for synthesis of novel (N-methyl-pyrrol)-pyrazolo[3,4-b]pyridines via a cooperative vinylogous anomeric based oxidation

In this paper, the MIL-53(Al)-NH2 metal-organic frameworks (MOFs) was prepared based on the anodic electrosynthesis under green conditions. The anodic electrosynthesis as an environmentally friendly procedure was performed in the aqueous solution, room temperature, atmospheric pressure, and in the short reaction time (30 min). Also, the employed procedure was accomplished without the need for the ex-situ salt and base/probase additives as cation source and ligand activating agent at the constant current mode (10.0 mA cm-2). The electrosynthesized MOFs was functionalized with phosphorus acid tags as a novel mesoporous catalyst. This mesoporous catalyst was successfully employed for synthesis of new series (N-methyl-pyrrol)-pyrazolo[3,4-b]pyridines by one-pot condensation reaction of 3-methyl-1-phenyl-1H-pyrazol-5-amine, 3-(1-methyl-1H-pyrrol-2-yl)-3-oxopropanenitrile and various aromatic aldehydes (mono, bis and tripodal). This catalyst proceeded the organic synthetic reaction via a cooperative vinylogous anomeric based oxidation mechanism with a marginal decreasing its catalytic activity after recycling and reusability.

Overcoming the Trade-off between Water Permeation and Mechanical Strength of Ceramic Membrane Supports by Interfacial Engineering

Porous ceramic membrane supports with high mechanical strength and permeation are required for highly permeable ceramic membranes. The water permeation of a ceramic membrane support is largely dependent on its level of open porosity, which can be however generally detrimental to the mechanical strength. In this work, low-cost kaolin nanoflakes were rationally composited with coarse alumina particles, and multichannel flat-sheet ceramic supports were successfully fabricated by extrusion and subsequent partial sintering. The macroscopic properties, microstructure characteristics, permeability, and mechanical strength of the ceramic membrane supports were systematically established and comprehensively studied. The incorporation of kaolin nanoflakes effectively reduced the sintering temperature to about 1200 °C. An interesting evolution of the pore structure was evidenced with the increase in sintering temperature. Interestingly, the porous ceramic supports prepared at 1400 °C with a nominal pore size of 1.47 μm showed the highest water permeability of 9911.9 ± 357.5 LMHB, and at the same time the flexural strength reached 109.6 ± 4.6 MPa. The much improved permeability was attributed to the unique multilevel pore structures, and the enhanced flexural strength mainly originated from the strongly interfacial bonding, as evidenced by the trans-granular fracture behavior. Also, the ceramic membrane supports exhibited excellent chemical resistance and good removal efficiency for oily wastewater. This work highlights the significant role of interfacial engineering in simultaneously improving the water permeation and mechanical strength, thereby overcoming their trade-off in porous ceramic membrane supports.

Metal organic framework top-down and bottom-up patterning techniques

Metal organic frameworks (MOFs) have recently attracted considerable research interest in several fields from coordination chemistry and materials science to engineering and medicine not only due to energy and environmental issues but also due to the need for new paradigms of efficiency and sustainability according to the requirements of the 21st century global society. Because of their crystalline and organic-inorganic nature, they are able to crystallize constituting intergrown architectures ductile enough to be patterned, with the use of appropriate techniques, as nano- and micro-devices with multiple applications. This perspective comprehensively summarizes the recent state of the art in the use of top-down and bottom-up methodologies to create MOF structures with a defined pattern at the nano- and micro-scale.

Melamine-Based Microporous Organic Framework Thin Films on an Alumina Membrane for High-Flux Organic Solvent Nanofiltration

Microporous polymer frameworks have attracted considerable attention to make novel separation layers owing to their highly porous structure, high permeability, and excellent molecular separation. This study concerns the fabrication and properties of thin melamine-based microporous polymer networks with a layer thickness of around 400 nm, supported on an α-alumina support and their potential use in organic solvent nanofiltration. The modified membranes show excellent solvent purification performances, such as n-heptane permeability as high as 9.2 L m-2  h-1  bar -1 in combination with a very high rejection of approximately 99 % for organic dyes with molecular weight of ≥457 Da. These values are higher than for the majority of the state-of-the-art membranes. The membranes further exhibit outstanding long-term operation stability. This work significantly expands the possibilities of using ceramic membranes in organic solvent nanofiltration.

Metal-organic framework UiO-66 membranes

Metal-organic frameworks (MOFs) have emerged as a class of promising membrane materials. UiO-66 is a prototypical and stable MOF material with a number of analogues. In this article, we review five approaches for fabricating UiO-66 polycrystalline membranes including in situ synthesis, secondary synthesis, biphase synthesis, gas-phase deposition and electrochemical deposition, as well as their applications in gas separation, pervaporation, nanofiltration and ion separation. On this basis, we propose possible methods for scalable synthesis of UiO-66 membranes and their potential separation applications in the future.

Convergent and Divergent Paired Electrodeposition of Metal-Organic Framework Thin Films

Employing the environmentally friendly methods for synthesis of the metal-organic frameworks (MOFs) is an urgent need and sustainable development in the synthesis of these compounds is essential. In this way, ignoring the counter electrode reaction is a potentially negative point from green chemistry standpoint which increases some issues like energy consumption and reaction time. We wish to introduce the "paired electrodeposition" (PED) technique as a new method for the simultaneous synthesis and deposition of the MOF thin films (MOFTFs). This protocol implements the uniform pattern of two MOFTF modified substrates by "convergent (CPED: Zn_a/Zn_c-MOFTFs) and divergent (DPED: Cu_a/Zn_c-MOFTFs) paired electrodeposition" via a one-step synthesis. With the rule of thumb, enhanced energy efficiency and atom economy, increasing electrochemical yield, time-saving along with a variety of products are advantages of this technique. Besides, the "Electrode Modification Efficiency" has introduced for the evaluation of functionality and modification efficiency of electrochemical heterogeneous systems, especially MOFTFs. To investigate this concept, we synthesized Zn₃(BTC)₂ and Cu₃(BTC)₂ as MOF models under constant current electrolysis in water and at room temperature. This work can make a breakthrough in the green synthesis of metal-organic frameworks.

Progress and Perspectives on Ceramic Membranes for Solvent Recovery

With the increase in demand for commodities in the world, it is advisable to conserve resources. In the case of liquid wastes generated from pharmaceutical and petroleum industries, an unconventional solution is provided for the regeneration of solvents. However, this solvent recovery can be carried out using various efficient methods. Recently, Mixed Matrix Membranes (MMM) obtained by the addition of nanoparticles into a polymer matrix as reinforcements, or using a material with a well-defined inorganic network as a membrane like zeolite, silica based, Zeolite imidazolate frameworks (ZIFs) and Metal organic frameworks (MOFs), were explored for a solvent recovery process. These membranes possess characteristics such as high selectivity, flux and stability at various environmental conditions for the solvent recovery process. In this review, we have covered the polymer, nanocomposites, and ceramic membranes for solvent recovery through the pervaporation and organic solvent nanofiltration processes. The key challenges faced by the materials such as MOFs, zeolite, silica, zeolite and ZIFs when they are fabricated (through in situ synthesis or secondary growth process) as membranes and separation of solvents to explore for the solvent recovery process are reviewed.

Ultrathin metal/covalent-organic framework membranes towards ultimate separation

Metal/covalent-organic framework (MOF/COF) membranes have attracted increasing research interest and have been considered as state-of-the-art platforms applied in various environment- and energy-related separation/transportation processes. To break the trade-off between permeability and selectivity to achieve ultimate separation, recent studies have been oriented towards how to design and exploit ultrathin MOF/COF membranes (i.e. sub-1 μm-thick). Given great advances made in the past five years, it is valuable to timely and systematically summarize the recent development and shed light on the future trend in this multidisciplinary field. In this review, we first present the advanced strategies in fabricating ultrathin defect-free MOF/COF membranes such as in situ growth, contra-diffusion method, layer-by-layer (LBL) assembly, metal-based precursor as the pre-functionalized layer, interface-assisted strategy, and laminated assembly of MOF/COF nanosheets. Then, the recent progress in some emerging applications of ultrathin MOF/COF membranes beyond gas separation is highlighted, including water treatment and seawater desalination, organic solvent nanofiltration, and energy-related separation/transportation (i.e. lithium ion separation and proton conductivity). Finally, some unsolved scientific and technical challenges associated with future perspectives in this field are discussed, inspiring the development of next-generation separation membranes.

Micropatterned Ultrathin MOF Membranes with Enhanced Molecular Sieving Property

Metal-organic frameworks (MOFs) are attractive crystalline materials for membranes due to their diverse crystalline pore structures and molecular separation properties. However, the fabrication cost is relatively high compared to conventional polymeric membranes. The concern of the cost could be eased if they are part of a value-added device, for example, as the key separation unit in a lab-on-a-chip device. This study demonstrates the feasibility of miniaturization of MOF membranes by patterning the membrane surface, a necessary step for MOF membranes to be used in compact devices. Water-stable ultrathin UiO-66 membranes with a thickness down to 250 nm on a substrate with a complex pattern were grown. The patterned membranes showed a 100 % improvement in the apparent permeation flux over conventional flat-UiO-66 membranes without compromising the molecular separation property, indicating the complexity of a surface would not be a formidable obstacle to the MOF membrane fabrication.