Improving Water-Treatment Performance of Zirconium Metal-Organic Framework Membranes by Postsynthetic Defect Healing

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.

Nanoarchitectonics of Bimetallic MOF@Lab-Grade Flexible Filter Papers: An Approach Towards Real-Time Water Decontamination and Circular Economy

This study presents a novel approach to decontaminate ferrocyanide-contaminated wastewater. The work effectively demonstrates the use of bimetallic Mo/Zr-UiO-66 as a super-adsorbent for rapid sequestration of Prussian blue, a frequently found iron complex in cyanide-contaminated soils/groundwater. The exceptional performance of Mo/Zr-UiO-66 is attributed to the insertion of secondary metallic sites, which deliver synergistic effects, benefiting the inherent qualities of the framework. Moreover, to extend the industrial applications of metal-organic frameworks (MOFs) in real-world scenarios, an approach is delivered to structure the nanocrystalline powders into MOF-based macrostructures. The work demonstrates an interfacial process to develop continuous MOF nanostructures on ordinary laboratory-grade filter papers. The novelty of the work lies in the development of robust free-standing filtration materials to purify PB dye-contaminated water. Additionally, the work embraces a circular economy concept to address problems related to resource scarcity, excessive waste production, and maintenance of economic benefits. Consequently, the PB dye-loaded adsorbent waste is re-employed for the adsorption of heavy metals (Pb2+ and Cd2+ ). Simultaneously, the study aims to address the problems related to the real-time handling of powdered adsorbents, and the generation of ecologically harmful secondary waste, thereby, progressing toward a more sustainable system.

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.

Pressure-induced postsynthetic cluster anion substitution in a MIL-53 topology scandium metal-organic framework

Postsynthetic modification of metal-organic frameworks (MOFs) has proven to be a hugely powerful tool to tune physical properties and introduce functionality, by exploiting reactive sites on both the MOF linkers and their inorganic secondary building units (SBUs), and so has facilitated a wide range of applications. Studies into the reactivity of MOF SBUs have focussed solely on removal of neutral coordinating solvents, or direct exchange of linkers such as carboxylates, despite the prevalence of ancillary charge-balancing oxide and hydroxide ligands found in many SBUs. Herein, we show that the μ2-OH ligands in the MIL-53 topology Sc MOF, GUF-1, are labile, and can be substituted for μ2-OCH3 units through reaction with pore-bound methanol molecules in a very rare example of pressure-induced postsynthetic modification. Using comprehensive solid-state NMR spectroscopic analysis, we show an order of magnitude increase in this cluster anion substitution process after exposing bulk samples suspended in methanol to a pressure of 0.8 GPa in a large volume press. Additionally, single crystals compressed in diamond anvil cells with methanol as the pressure-transmitting medium have enabled full structural characterisation of the process across a range of pressures, leading to a quantitative single-crystal to single-crystal conversion at 4.98 GPa. This unexpected SBU reactivity - in this case chemisorption of methanol - has implications across a range of MOF chemistry, from activation of small molecules for heterogeneous catalysis to chemical stability, and we expect cluster anion substitution to be developed into a highly convenient novel method for modifying the internal pore surface and chemistry of a range of porous materials.

Waste acrylonitrile butadiene styrene (ABS) incorporated with polyvinylpyrrolidone (PVP) for potential water filtration membrane

Acrylonitrile butadiene styrene (ABS) is one of the most common fused-filament feedstocks for 3D printing. The rapid growth of the 3D printing industry has resulted in huge demand for ABS filaments; however, it generates a large amount of waste. This study developed a novel method using waste ABS to fabricate electrospun nanofiber membranes (ENMs) for water filtration. Polyvinylpyrrolidone (PVP) was employed to modify the properties of waste ABS, and the effect of PVP addition in the range of 0-5 wt% was investigated. The results showed that adding PVP increased the viscosity and surface tension but decreased the conductivity of the precursor solution. After electrospinning, PVP could reduce the number of beads, increase the porosity and fiber diameter, and improve the wettability of the fabricated fibers. Moreover, the bilayer of ABS ENMs achieved a high flux value between 2951 and 48 041 L m^-2 h^-1 and a high rejection rate of 99%. Our study demonstrates a sustainable strategy to convert waste plastics to inexpensive materials for wastewater treatment membranes.

Direct Probing of Vibrational Interactions in UiO-66 Polycrystalline Membranes with Femtosecond Two-Dimensional Infrared Spectroscopy

UiO-66 is a benchmark metal-organic framework that holds great promise for the design of new functional materials. In this work, we perform two-dimensional infrared measurements on polycrystalline membranes of UiO-66 grown on c-sapphire substrates. We study the symmetric and antisymmetric stretch vibrations of the carboxylate groups of the terephthalate linker ions and find that these vibrations show a rapid energy exchange and a collective vibrational relaxation with a time constant of 1.3 ps. We also find that the symmetric vibration of the carboxylate group is strongly coupled to a vibration of the aromatic ring of the terephthalate ion. We observe that the antisymmetric carboxylate vibrations of different terephthalate linkers show rapid resonant (Förster) energy transfer with a time constant of ∼1 ps.

Advances in metal-organic framework-based membranes

Membrane-based separations have garnered considerable attention owing to their high energy efficiency, low capital cost, small carbon footprint, and continuous operation mode. As a class of highly porous crystalline materials with well-defined pore systems and rich chemical functionalities, metal-organic frameworks (MOFs) have demonstrated great potential as promising membrane materials over the past few years. Different types of MOF-based membranes, including polycrystalline membranes, mixed matrix membranes (MMMs), and nanosheet-based membranes, have been developed for diversified applications with remarkable separation performances. In this comprehensive review, we first discuss the general classification of membranes and outline the historical development of MOF-based membranes. Subsequently, particular attention is devoted to design strategies for MOF-based membranes, along with detailed discussions on the latest advances on these membranes for various gas and liquid separation processes. Finally, challenges and future opportunities for the industrial implementation of these membranes are identified and outlined with the intent of providing insightful guidance on the design and fabrication of high-performance membranes in the future.

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.

Ultrafast vibrational dynamics of aqueous acetate and terephthalate

We study the vibrational population relaxation and mutual interaction of the symmetric stretch ( ν s) and antisymmetric stretch ( ν as) vibrations of the carboxylate anion groups of acetate and terephthalate ions in aqueous solution by femtosecond two-dimensional infrared spectroscopy. By selectively exciting and probing the ν s and ν as vibrations, we find that the interaction of the two vibrations involves both the anharmonic coupling of the vibrations and energy exchange between the excited states of the vibrations. We find that both the vibrational population relaxation and the energy exchange are faster for terephthalate than for acetate.

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.

Molecular orientation of small carboxylates at the water-air interface

We study the properties of formate (HCOO−) and acetate (CH3COO−) ions at the surface of water using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. For both ions we observe a response...

Recent advances in metal-organic framework membranes for water treatment: A review

Among many separation membranes reported to date, the favorable polymer affinity and unique physio-chemical performances of metal-organic frameworks (MOFs) including ultra-high surface area, regular and highly controlled porosity have drawn widespread attention in industrial and academic communities. In this comprehensive review, the developmental timeline of MOF containing membranes for water treatment were clarified. The removal efficiencies, elimination mechanisms, as well as possible influencing factors of various MOF containing membranes that applied to water treatment were systematically summarized. The excellent removal performances of MOF containing membranes for various pollutants were determined by the size-exclusion, π-π stacking interaction, electrostatic interaction, hydrogen bonding and so on. Since the progress of engineered MOF containing membranes for practical wastewater treatment applications lags, we further analyzed the potential environmental application of MOF containing membranes from four aspects (stability of MOFs, antifouling performance of membranes, compatibility between MOF fillers and polymer matrix, dispersity of MOF nanoparticles in matrix), hoping to provide some meaningful insights.

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

New membrane materials with excellent water permeability and high ion rejection are needed. Metal-organic frameworks (MOFs) are promising candidates by virtue of their diversity in chemistry and topology. In this work, continuous aluminum MOF-303 membranes were prepared on α-Al₂O₃ substrates via an in situ hydrothermal synthesis method. The membranes exhibit satisfying rejection of divalent ions (e.g., 93.5% for MgCl₂ and 96.0% for Na₂SO₄) on the basis of a size-sieving and electrostatic-repulsion mechanism and unprecedented permeability (3.0 L·m^-2·h^-1·bar^-1·μm). The water permeability outperforms typical zirconium MOF, zeolite, and commercial polymeric reverse osmosis and nanofiltration membranes. Additionally, the membrane material exhibits good stability and low production costs. These merits recommend MOF-303 as a next-generation membrane material for water softening.

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.

Metal-to-Semiconductor Transition in Two-Dimensional Metal-Organic Frameworks: An Ab Initio Dynamics Perspective

Two-dimensional (2D) π-stacked layered metal-organic frameworks (MOFs) are permanently porous and electrically conductive materials with easily tunable crystal structures. Here, we provide an accurate examination of the correlation between structural features and electronic properties of Ni₃(HITP)₂, HITP = 2,3,6,7,10,11-hexaiminotriphenylene, as an archetypical 2D MOF. The main objective of this work is to unravel the responsive nature of the layered architecture to external stimuli such as temperature and show how the layer flexibility translates to different conductive behaviors. To this end, we employ a combination of quantum mechanical tools, ab initio molecular dynamics (AIMD) simulations, and electronic band structure calculations. We compare the band structure and projected density of states of equilibrated system at 293 K to that of the 0 K optimized structure. Effect of interlayer π-π and intralayer d-π interactions on charge mobility is disentangled and studied by increasing the distance between layers of Ni₃(HITP)₂ and comparison to an exemplary case of Zn₃(HITP)₂ 2D MOF. Our findings show how a structural change, which can be deformations along the layers, slipping of layers, or change of the interlayer distance, can induce metal-to-semiconductor or indirect-to-direct semiconductor transition, suggesting a way to adjust or even switch between the intralayer vs interlayer conductive anisotropy in Ni₃(HITP)₂, in particular, and 2D MOFs in general.

High-Silica CHA Zeolite Membrane with Ultra-High Selectivity and Irradiation Stability for Krypton/Xenon Separation

Capture and storage of the long‐lived ⁸⁵Kr is an efficient approach to mitigate the emission of volatile radionuclides from the spent nuclear fuel reprocessing facilities. However, it is challenging to separate krypton (Kr) from xenon (Xe) because of the chemical inertness and similar physical properties. Herein we prepared high‐silica CHA zeolite membranes with ultra‐high selectivity and irradiation stability for Kr/Xe separation. The suitable aperture size and rigid framework endures the membrane a strong size‐exclusion effect. The ultrahigh selectivity of 51–152 together with the Kr permeance of 0.7–1.3×10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ of high‐silica CHA zeolite membranes far surpass the state‐of‐the‐art polymeric membranes. The membrane is among the most stable polycrystalline membranes for separation of humid Kr/Xe mixtures. Together with the excellent irradiation stability, high‐silica CHA zeolite membranes pave the way to separate radioactive Kr from Xe for a notable reduction of the volatile nuclear waste storage volume.

The chemistry and applications of hafnium and cerium(iv) metal-organic frameworks

The coordination connection of organic linkers to the metal clusters leads to the formation of metal-organic frameworks (MOFs), where the metal clusters and ligands are spatially entangled in a periodic manner. The immense availability of tuneable ligands of different length and functionalities gives rise to robust molecular porosity ranging from several angstroms to nanometres. Among the large family of MOFs, hafnium (Hf) based MOFs have been demonstrated to be highly promising for practical applications due to their unique and outstanding characteristics such as chemical, thermal, and mechanical stability, and acidic nature. Since the report of UiO-66(Hf) and DUT-51(Hf) in 2012, less than 200 Hf-MOFs (ca. 50 types of structures) have been reported. Besides, tetravalent cerium [Ce(iv)] has been proven to be capable of forming similar topological MOF structures to Zr and Hf since its first discovery in 2015. So far, ca. 40 Ce(iv) MOFs with 60% having UiO-66-type structure have been reported. This review will offer a holistic summary of the chemistry, uniqueness, synthesis, and applications of Hf/Ce(iv)-MOFs with a focus on presenting the development in the Hf/Ce(iv)-clusters, topologies, ligand structures, synthetic strategies, and practical applications of Hf/Ce(iv)-MOFs. In the end, we will present the research outlook for the development of Hf/Ce(iv)-MOFs in the future, including fundamental design of Hf/Ce(iv)-clusters, defect engineering, and various applications including membrane development, diversified types of catalytic reactions, irradiation absorption in nuclear waste treatment, water production and wastewater treatment, etc. We will also present the emerging computational approaches coupled with machine-learning algorithms that can be applied in screening Hf and Ce(iv) based MOF structures and identifying the best-performing MOFs for tailor-made applications in future practice.

Scalable crystalline porous membranes: current state and perspectives

Crystalline porous materials (CPMs) with uniform and regular pore systems show great potential for separation applications using membrane technology. Along with the research on the synthesis of precisely engineered porous structures, significant attention has been paid to the practical application of these materials for preparation of crystalline porous membranes (CPMBs). In this review, the progress made in the preparation of thin, large area and defect-free CPMBs using classical and novel porous materials and processing is presented. The current state-of-the-art of scalable CPMBs with different nodes (inorganic, organic and hybrid) and various linking bonds (covalent, coordination, and hydrogen bonds) is revealed. The advances made in the scalable production of high-performance crystalline porous membranes are categorized according to the strategies adapted from polymer membranes (interfacial assembly, solution-casting, melt extrusion and polymerization of CPMs) and tailored based on CPM properties (seeding-secondary growth, conversion of precursors, electrodeposition and chemical vapor deposition). The strategies are compared and ranked based on their scalability and cost. The potential applications of CPMBs have been concisely summarized. Finally, the performance and challenges in the preparation of scalable CPMBs with emphasis on their sustainability are presented.

Microporous framework membranes for precise molecule/ion separations

Microporous framework membranes such as metal-organic framework (MOF) membranes and covalent organic framework (COF) membranes are constructed by the controlled growth of small building blocks with large porosity and permanent well-defined micropore structures, which can overcome the ubiquitous tradeoff between membrane permeability and selectivity; they hold great promise for the enormous challenging separations in energy and environment fields. Therefore, microporous framework membranes are endowed with great expectations as next-generation membranes, and have evolved into a booming research field. Numerous novel membrane materials, versatile manipulation strategies of membrane structures, and fascinating applications have erupted in the last five years. First, this review summarizes and categorizes the microporous framework membranes with pore sizes lower than 2 nm based on their chemistry: inorganic microporous framework membranes, organic-inorganic microporous framework membranes, and organic microporous framework membranes, where the chemistry, fabrications, and differences among these membranes have been highlighted. Special attention is paid to the membrane structures and their corresponding modifications, including pore architecture, intercrystalline grain boundary, as well as their diverse control strategies. Then, the separation mechanisms of membranes are covered, such as diffusion-selectivity separation, adsorption-selectivity separation, and synergetic adsorption-diffusion-selectivity separation. Meanwhile, intricate membrane design to realize synergistic separation and some emerging mechanisms are highlighted. Finally, the applications of microporous framework membranes for precise gas separation, liquid molecule separation, and ion sieving are summarized. The remaining challenges and future perspectives in this field are discussed. This timely review may provide genuine guidance on the manipulation of membrane structures and inspire creative designs of novel membranes, promoting the sustainable development and steadily increasing prosperity of this field.

Water and Metal-Organic Frameworks: From Interaction toward Utilization

The steep stepwise uptake of water vapor and easy release at low relative pressures and moderate temperatures together with high working capacities make metal-organic frameworks (MOFs) attractive, promising materials for energy efficient applications in adsorption devices for humidity control (evaporation and condensation processes) and heat reallocation (heating and cooling) by utilizing water as benign sorptive and low-grade renewable or waste heat. Emerging MOF-based process applications covered are desiccation, heat pumps/chillers, water harvesting, air conditioning, and desalination. Governing parameters of the intrinsic sorption properties and stability under humid conditions and cyclic operation are identified. Transport of mass and heat in MOF structures, at least as important, is still an underexposed topic. Essential engineering elements of operation and implementation are presented. An update on stability of MOFs in water vapor and liquid systems is provided, and a suite of 18 MOFs are identified for selective use in heat pumps and chillers, while several can be used for air conditioning, water harvesting, and desalination. Most applications with MOFs are still in an exploratory state. An outlook is given for further R&D to realize these applications, providing essential kinetic parameters, performing smart engineering in the design of systems, and conceptual process designs to benchmark them against existing technologies. A concerted effort bridging chemistry, materials science, and engineering is required.

Polycrystalline Advanced Microporous Framework Membranes for Efficient Separation of Small Molecules and Ions

Advanced porous framework membranes with excellent selectivity and high permeability of small molecules and ions are highly desirable for many important industrial separation applications. There has been significant progress in the fabrication of polycrystalline microporous framework membranes (PMFMs) in recent years, such as metal-organic framework and covalent organic framework membranes. These membranes possess small pore sizes, which are comparable to the kinetic diameter of small molecules and ions on the angstrom scale, very low thickness, down to tens to hundreds of nanometers, highly oriented crystalline structures, hybrid membrane structures, and specific functional groups for enhancing membrane selectivity and permeability. Recent advances in the fabrication methods of advanced PMFMs are summarized. Following this, four emerging separation applications of these advanced microporous framework membranes, including gas separation, water desalination, ion separation, and chiral separation, are highlighted and discussed in detail. Finally, a summary and some perspectives of future developments and challenges in this exciting research field are presented.

Liquid-Phase Quasi-Epitaxial Growth of Highly Stable, Monolithic UiO-66-NH2 MOF thin Films on Solid Substrates

High quality, monolithic UiO-66-NH2 thin films on diverse solid substrates have been prepared via a low temperature liquid phase epitaxy method. The achievement of continuous films with low defect densities and great stability against high temperatures and hot water is proven, clearly outperforming other reported types of MOF thin films.

Screen printing directed synthesis of covalent organic framework membranes with water sieving property

An effective approach of screen printing has been developed to direct the synthesis of continuous COF membranes. The prepared membranes exhibit fast and selective water permeation properties.

Hierarchically Assembled Graphene Oxide Composite Membrane with Self-Healing and High-Efficiency Water Purification Performance

Graphene oxide (GO) with a two-dimensional lamellar structure and single-atom thickness has exhibited advantages in water purification by stacking to a continuous membrane. However, a proper method to further increase the separation property of the GO membrane is still urgently needed. Besides, damage to the membrane during the full-scale application processes and the resulted consequential loss are prevalent problems need to be solved. Here, a hierarchically assembled GO composite membrane was developed that can achieve high-efficiency water purification performance and self-healing property via the synergistic effect of the metal-organic framework (MOF) and the coated hydrophilic layer of chitosan. The intercalated MOF effectively expanded the channel space of GO and endowed the channels with molecular-sieving property. Meanwhile, the coated chitosan layer can selectively adsorb water and achieve self-healing through the cross-linking reaction. The prepared GO composite membrane shows largely improved water flux (14.62 L m^-2 h^-1 bar^-1), increased 344% than the water flux of the GO membrane, high rejection ratio (>99% for dyes), and good antifouling performance. In addition, the damaged GO composite membrane can recover its water flux (95%) and rejection ratio (96%) through a facile self-healing process.

Continuous UiO-66-Type Metal-Organic Framework Thin Film on Polymeric Support for Organic Solvent Nanofiltration

For the first time, continuous polycrystalline UiO-66-NH₂ thin film supported by a cross-linked Matrimid substrate was successfully fabricated via in situ solvothermal synthesis at room temperature for organic solvent nanofiltration. The integrated structure of the formed UiO-66-NH₂ selective layer was inferred by various characterizations including X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. We have demonstrated that pretreatment of the substrate by an organic ligand, the number of solvothermal synthesis cycles, and the reaction time play important roles in MOF film formation. The newly developed UiO-66-NH₂ membrane possesses high surface hydrophobicity and mean pore size of 0.89 nm in diameter. It shows an exceptional rejection of 96.33% to Rose Bengal with moderate ethanol permeance of 0.88 L m^-2 h^-1 bar^-1. Benefiting from the extraordinary chemical stability of Zr-MOF crystals, the UiO-66-NH₂ membrane shows excellent stability in different solvents, implying their great potential for real applications. This work provides useful insights into the fabrication of continuous UiO-66-type MOF membranes on polymeric substrates, which are very promising in practical separations involving organic solvents.

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.

MOF Decomposition and Introduction of Repairable Defects Using a Photodegradable Strut

Photoswitchable components can modulate the properties of metal organic frameworks (MOFs); however, photolabile building blocks remain underexplored. A new strut NPDAC (2-nitro-1,4-phenylenediacetic acid) that undergoes photodecarboxylation has been prepared and incorporated into a MOF, using post-synthetic linker exchange (PSLE) from the structural analogue containing PDAC (p-phenylenediacetic acid). Irradiation of NPDAC-MOF leads to MOF decomposition and concomitant formation of amorphous material. In addition to complete linker exchange, MOFs containing a mixture of PDAC and NPDAC can be obtained through partial linker exchange. In NPDAC30-MOF, which contains approximately 30 % NPDAC, the MOF retains crystallinity after irradiation, but the MOF contains defect sites consistent with loss of decarboxylated NPDAC linkers. The defect sites can be repaired by exposure to additional PDAC or NPDAC linkers at a much faster rate than the initial exchange process. The photoremoval and replacement process may lead to a more general approach to customizable MOF structures.

Bottom-Up Formation of Carbon-Based Magnetic Honeycomb Material from Metal-Organic Framework-Guest Polyhedra for the Capture of Rhodamine B

Three-dimensional carbon-based porous materials have proven to be quite useful for tailoring material properties in the energy conservation and environmental protection applications. In view of the three-dimensional and well-defined structure of metal-organic frameworks (MOFs), a novel carbon-based magnetic porous material (HKUST-Fe₃O₄) has been designed and constructed by MOF-guest interactions of high-temperature pyrolysis. The obtained HKUST-Fe₃O₄ exhibited the unique features of superparamagnetism, a macro/mesoporous structure, environmental protection (inexistence of toxic heavy metal ions), and physicochemical stability and has shown high adsorption capacity and rapid adsorption for carcinogenic organic pollutants (for example, rhodamine B) with an environmentally friendly character and excellent reusability. We demonstrate that the unique/superior advantages of HKUST-Fe₃O₄ could meet the requirements of environment cleaning, especially for removing the targeted organic pollutant from water. Moreover, the specific HKUST-Fe₃O₄ and organic pollutant interaction mechanism has been analyzed in detail via parameter-free calculations. This study proposes a promising strategy for constructing novel carbon-based magnetic nanomaterials for various applications, not limitated to pollutant removal.

Zr-Metal-Organic Frameworks Featuring TEMPO Radicals: Synergistic Effect between TEMPO and Hydrophilic Zr-Node Defects Boosting Aerobic Oxidation of Alcohols

Metal-organic frameworks (MOFs) featuring multiple catalytic units are excellent platforms for heterogeneous catalysis. However, the synergism between multiple catalytic units for catalysis is far from being well understood. Herein, we reported the synthesis of a robust 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) radical-functionalized Zr-MOF (UiO-68-TEMPO) in the form of single-crystalline and microsized crystals with varied missing linker defects. Detailed catalytic studies and theoretical calculations reveal that the synergistic effect between the TEMPO radicals and hydrophilic and defective Zr-nodes endows UiO-68-TEMPO with superior catalytic activity toward aerobic oxidation of alcohols. Our work not only offers a new route to design and synthesize highly effective MOF catalysts but also provides insights into the synergism between multiple catalytic sites.

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.

Design of a Semi-Continuous Selective Layer Based on Deposition of UiO-66 Nanoparticles for Nanofiltration

Deposition of UiO-66 metal⁻organic framework nanoparticles onto a porous polymer support is a promising approach to designing highly-permeable, size-selective, flexible, and stable membranes for water filtration. In this article, a series of UiO-66 nanoparticles having different particle sizes were synthesized and employed to prepare UiO-66-deposited composite membranes. It was found that the size of the UiO-66 nanoparticles had great influences on the performance of the composite membranes for the filtration of a methylene blue aqueous solution. The deposition of smaller nanoparticles afforded a selective layer having a greater external surface area and narrower interparticle voids. These features made the deposition of smaller nanoparticles more advantageous in terms of the flux and rejection, while the deposition of greater nanoparticles afforded a selective layer more tolerant for fouling. Bimodal composite membranes were prepared by depositing mixed UiO-66 nanoparticles of smaller and bigger sizes. These membranes successfully combined the advantages of nanoparticles of a distinct size.

Advanced Porous Materials in Mixed Matrix Membranes

Membrane technology has gained great interest in industrial separation processing over the past few decades owing to its high energy efficiency, small capital investment, environmentally benign characteristics, and the continuous operation process. Among various types of membranes, mixed matrix membranes (MMMs) combining the merits of the polymer matrix and inorganic/organic fillers have been extensively investigated. With the rapid development of chemistry and materials science, recent studies have shifted toward the design and application of advanced porous materials as promising fillers to boost the separation performance of MMMs. Here, first a comprehensive overview is provided on the choices of advanced porous materials recently adopted in MMMs, including metal-organic frameworks, porous organic frameworks, and porous molecular compounds. Novel trends in MMMs induced by these advanced porous fillers are discussed in detail, followed by a summary of applying these MMMs for gas and liquid separations. Finally, a concise conclusion and current challenges toward the industrial implementation of MMMs are outlined, hoping to provide guidance for the design of high-performance membranes to meet the urgent needs of clean energy and environmental sustainability.