High-quality Metal-organic Framework Research Articles

Full potential of microwave-assisted processes: From synthesis of high-quality MIL-101(Fe) catalyst to furfural valorization

Metal-organic frameworks (MOFs) have emerged as promising catalysts in biomass valorization processes, due to mainly their high structural and chemical flexibility when compared to other catalytic materials. However, it is still challenging to achieve perfect control in the synthetic method to obtain high-quality MOF particles containing uniform active sites, which will strongly influence their further catalytic performance. Herein, we have demonstrated that microwaves (MWs) can play a key role not only in the preparation of the MOF catalyst, specifically MIL-101(Fe), but also in performing the MOF-catalyzed oxidative esterification of furfural to methyl-2-furoate under mild conditions and in the absence of a base. MIL-101(Fe) exhibited a high conversion of furfural (>90 %) and excellent selectivity to methyl-2-furoate (>95 %) in just 1 h of reaction and at a moderate temperature (60 °C). A series of control experiments and DFT-mechanistic studies revealed the pivotal role played by the MIL-101(Fe) particles to drive the preferential formation of the target compound methyl-2-furoate, and evidenced that the MOF behaves as a truly heterogeneous catalyst. Overall, this work demonstrates that the potential of MW energy is underexploited, and highlights that there is still a long way to go in the development of new MOF-based catalytic approaches for biomass transformations.

Preparation of UiO-66 membrane through heterogeneous nucleation assisted growth strategy for efficient CO2 capture under humid conditions

Metal-organic frameworks (MOFs) have driven the development of polycrystalline membranes in the field of gas separation owing to the strong adsorption-desorption behavior and stable framework structure. However, the poor heterogeneous bonding ability between crystals and the substrate leads to the unsatisfactory gas separation as well as low stability of MOF membranes, especially in the humid environment. Herein, we prepared a thin and defect-free UiO-66 membrane by a crystal heterogeneous nucleation assisted growth strategy for efficient CO2/N2 separation. The addition of small amount of water in the precursor solution promoted the crystal nucleation process on the substrate surface. Meanwhile, the smooth PDMS interlayer guaranteed the heterogeneous well-intergrown of crystals into thin UiO-66 membranes, and provided a hydrophobic environment. Therefore, the dense UiO-66 membrane with a small thickness exhibited a high CO2 permeance (177.4 × 10−9 mol·m−2·s−1·Pa−1) with moderate CO2/N2 selectivity (24.3) under humid conditions. Furthermore, the prepared membrane demonstrated excellent renewable performance after dehumidification at high temperature, and maintained good hydrothermal stability during long-term operation under the humid environment. This work provides a new insight for the design of high-quality MOF membranes and promotes the development of MOF membranes in practical gas separation process.

Self-condensation-assisted chemical vapour deposition growth of atomically two-dimensional MOF single-crystals

Two-dimensional metal-organic frameworks (MOFs) have a wide variety of applications in molecular separation and other emerging technologies, including atomically thin electronics. However, due to the inherent fragility and strong interlayer interactions, high-quality MOF crystals of atomic thickness, especially isolated MOF crystal monolayers, have not been easy to prepare. Here, we report the self-condensation-assisted chemical vapour deposition growth of atomically thin MOF single-crystals, yielding monolayer single-crystals of poly[Fe(benzimidazole)2] up to 62 μm in grain sizes. By using transmission electron microscopy and high-resolution atomic force microscopy, high crystallinity and atomic-scale single-crystal structure are verified in the atomically MOF flakes. Moreover, integrating such MOFs with MoS2 to construct ultrathin van der Waals heterostructures is achieved by direct growth of atomically MOF single-crystals onto monolayer MoS2, and enables a highly selective ammonia sensing. These demonstrations signify the great potential of the method in facilitating the development of the fabrication and application of atomically thin MOF crystals.

Ultrastrong Electron-Phonon Coupling in Uranium-Organic Frameworks Leading to Inverse Luminescence Temperature Dependence.

Electron-phonon interactions, crucial in condensed matter, are rarely seen in Metal-Organic Frameworks (MOFs). Detecting these interactions typically involves analyzing luminescence in lanthanide- or actinide-based compounds. Prior studies on Ln- and Ac-based MOFs at high temperatures revealed additional peaks, but these were too faint for thorough analysis. In our research, we fabricated a high-quality, crystalline uranium-based MOF (KIT-U-1) thin film using a layer-by-layer method. Under UV light, this film showed two distinct "hot bands," indicating a strong electron-phonon interaction. At 77 K, these bands were absent, but at 300 K, a new emission band appeared with half the intensity of the main luminescence. Surprisingly, a second hot band emerged above 320 K, deviating from previous findings in rare-earth compounds. We conducted a detailed ab-initio analysis employing time-dependent density functional theory to understand this unusual behaviour and to identify the lattice vibration responsible for the strong electron-phonon coupling. The KIT-U-1 film's hot-band emission was then utilized to create a highly sensitive, single-compound optical thermometer. This underscores the potential of high-quality MOF thin films in exploiting the unique luminescence of lanthanides and actinides for advanced applications.

Improved optical quality of heteroepitaxially grown metal-organic framework thin films by modulating the crystal growth.

Fabricating high-quality thin films of metal-organic frameworks (MOFs) is important for integrating MOFs in various applications. Specifically, optical/electrical devices require MOF thin films that are crystallographically oriented, with closely packed crystals and smooth surfaces. Although the heteroepitaxial growth approach of MOFs on metal hydroxides has been demonstrated to control the orientation of the three crystallographic axes, the fabrication of MOF thin films with both three-dimensional crystallographic orientation and smooth surfaces remains a challenge. In this study, we report the fabrication of high-quality thin films of MOFs with closely packed MOF crystals, smooth surfaces, optical transparency, and crystal alignment by modulating the crystal growth of MOFs using the heteroepitaxial growth approach. High-quality thin films of Cu-paddlewheel-based pillar-layered MOFs are fabricated on oriented Cu(OH)2 thin films via epitaxial growth using acetate ions as modulators to control the crystal morphology. Increasing the modulator concentration results in a uniform crystal shape with a relatively long one-dimensional pore direction and uniform heterogeneous nucleation over the entire film. The MOF thin films fabricated using the modulator exhibit high optical transparency. High-quality MOF thin films with dense and flat surfaces will pave the way for integrating MOFs into sophisticated optical and electrical devices.

Preparation and Lithium-Ion Separation Property of ZIF-8 Membrane with Excellent Flexibility.

Metal-organic framework (MOF) membranes exhibit immense potential for separation applications due to their regular pore channels and scalable pore sizes. However, structuring a flexible and high-quality MOF membrane remains a challenge due to its brittleness, which severely restricts its practical application. This paper presents a simple and effective method in which continuous, uniform, defect-free ZIF-8 film layers of tunable thickness are constructed on the surface of inert microporous polypropylene membranes (MPPM). To provide heterogeneous nucleation sites for ZIF-8 growth, an extensive amount of hydroxyl and amine groups were introduced on the MPPM surface using the dopamine-assisted co-deposition technique. Subsequently, ZIF-8 crystals were grown in-situ on the MPPM surface using the solvothermal method. The resultant ZIF-8/MPPM exhibited a lithium-ion permeation flux of 0.151 mol m-2 h-1 and a high selectivity of Li+/Na+ = 1.93, Li+/Mg2+ = 11.50. Notably, ZIF-8/MPPM has good flexibility, and the lithium-ion permeation flux and selectivity remain unchanged at a bending curvature of 348 m-1. These excellent mechanical characteristics are crucial for the practical applications of MOF membranes.

Research progress on the substrate for metal–organic framework (MOF) membrane growth for separation

During the last decade, metal–organic frameworks (MOFs) have been applied in various fields due to their unique chemical and functional advantages. One of the widespread research hotspots is MOF-based membranes for separations, specifically continuous defect-free MOF membranes, which are usually grown on porous substrates. The substrate not only serves as the MOF layer support but also has a great influence on the membrane fabrication process and the final separation performance of the resultant membrane. In this review, we mainly introduce the progress focused on the substrates for MOF membranes fabrication. The substrate modifications and seeding methods aimed at synthesizing high-quality MOF membranes are also summarized systematically.

Ionothermal Synthesis of Metal-Organic Frameworks Using Low-Melting Metal Salt Precursors.

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with myriad potential applications in chemical separations, catalysis, and drug delivery. A major barrier to the application of MOFs is their poor scalability, as most frameworks are prepared under highly dilute solvothermal conditions using toxic organic solvents. Herein, we demonstrate that combining a range of linkers with low-melting metal halide (hydrate) salts leads directly to high-quality MOFs without added solvent. Frameworks prepared under these ionothermal conditions possess porosities comparable to those prepared under traditional solvothermal conditions. In addition, we report the ionothermal syntheses of two frameworks that cannot be prepared directly under solvothermal conditions. Overall, the user-friendly method reported herein should be broadly applicable to the discovery and synthesis of stable metal-organic materials.

(Invited, Digital Presentation) Atomic Layer Deposited Templates for Constructing Functional Metal-Organic Framework Thin Films

Metal-organic frameworks (MOFs) are a class of porous crystalline materials featuring high surface area, ordered pore structures and amenability for post-synthetic modification. Integrating MOF thin films with various substrate materials could enable applications in separations, heterogeneous catalysis, chemical sensors and electronic devices. However, it is difficult to find a universal approach to deposit uniform and high-quality MOF coatings on a variety of substrate materials. Previous attempts often require surface modification methods that may not be generally applicable.In this talk, interfacial engineering using atomic layer deposited (ALD) templates will be discussed for forming MOF thin films on various substrates. We found that ALD metal oxides are effective in promoting the heterogeneous nucleation of MOFs and also readily hydrolyzed into hydroxy double salts (HDSs) for rapid transformation to MOFs. With ALD template layers, conformal and functional MOF coatings were integrated on fibers, membranes and laminated structures. Accordingly, we developed a series of MOF-functionalized fibers for adsorption and catalytic degradation of hazardous compounds. Furthermore, we obtained ultra-thin mixed-metal MOF skin layers on porous alumina supports for membrane separation, and achieved large propylene permeance (up to 4.5×10-8 mol·m-2·s-1·Pa-1) and high propylene/propane selectivity over 250. We anticipate that this platform technology will also lead to potential device structures for chemical sensing, energy storage and many other scenarios.

The Growth Mechanism of a Conductive MOF Thin Film in Spray-based Layer-by-layer Liquid Phase Epitaxy.

The layer-by-layer liquid-phase epitaxy (LBL-LPE) method is widely used in preparing metal-organic framework (MOF) thin films with the merits of controlling thickness and out-of-plane orientation for superior performances in applications. The LBL-LPE growth mechanism related to the grain boundary, structure defect, and orientation is critical but very challenging to study. In this work, a novel "in-plane self-limiting and self-repairing" thin-film growth mechanism is demonstrated by the combination study of the grain boundary, structure defect, and orientation of Cu3 (HHTP)2 -xC thin film via microscopic analysis techniques and electrical measurements. This mechanism results a desired high-quality MOF thin film with preferred in-plane orientations at its bottom for the first time and is very helpful for optimizing the LBL-LPE method, understanding the growth cycle-dependent properties of MOF thin film, and inspiring the investigations of the biomimetic self-repairing materials.

Synthesis of metal-organic framework HKUST-1 via tunable continuous flow supercritical carbon dioxide reactor

Scaling-up synthesis of metal–organic framework (MOF) materials is critical for their practical applications. Due to its unique properties, supercritical carbon dioxide (scCO2) is an attractive media single-step synthesis of MOFs. This paper, for the first time, presents a rapid synthesis of the high-quality copper-based MOF HKUST-1 in a continuous flow scCO2 flow reactor, evaluating the effects of pressure, scCO2 injection temperature, scCO2 mass fraction, and the reactor residence time on HKUST-1 properties over 23 experiments. The morphology, crystallinity, and porosity of MOF are characterized by scanning electron microscopy, X-ray diffraction, and physisorption analysis. Four sets of experiments showed excellent repeatability in particle size and specific surface area yielding high-quality HKUST-1 MOFs. The particle sizes ranged from 55 to 300 nm with an average of 135 nm, and the BET surface -- from 1,374 to 2,389 m2·g−1 with an average of 1,712 m2·g−1. Longer residence time increased particle size and led to MOF shape change from spherical to octahedral; however, these did not significantly correlate with MOF surface area. Pressure variation in the 8–10 MPa range did not impact HKUST-1 properties. Reactant temperature variations between 80 and 120 °C showed a weak correlation with particle size. Increasing reactor section temperature independently of the reactant temperature had a statistically significant negative correlation to particle size, r-value = -0.86, and p-value = 0.04. The scCO2-assisted synthesis yielded high-quality MOFs under all studied conditions with residence time in the range of 70–170 s, demonstrating the robustness and high throughput of the method.

Accelerated HKUST-1 Thin-Film Property Optimization Using Active Learning.

A flow-coating method termed solution shearing has been shown to grow large-area thin films with no void spaces. Attaining full coverage is one of the key prerequisites for the adoption of any metal-organic framework (MOF) thin film for a variety of practical applications, including separation, membranes and sensors. However, the solution-shearing process has multiple discrete and continuous parameters that can be varied, including the metal ion and linker concentrations, solvents, substrate temperature, coating speed, and the number of coating passes. Optimization of these parameters for full coverage is a time-consuming and daunting process due to vast parameter space. Here, we incorporate an active learning approach into the solution-sheared HKUST-1 thin-film-processing parameters to control the coverage and extend the approach to gain control over the thickness. The understanding of high-quality MOF thin-film formation using solution shearing is improved by correlating the processing parameter sets and their corresponding film coverage. A large area and fully covered HKUST-1 thin film with a minimized thickness of 2.2 μm is fabricated by using guidance from active learning. To confirm full coverage, a redox-active molecule, called 7,7,8,8-tetracyanoquinodimethane (TCNQ), is incorporated along with the HKUST-1 thin film. The TCNQ@HKUST-1 thin film with a minimized thickness has the same order of magnitude of electrical conductivity as that of the TCNQ@HKUST-1 thin film created previously while reducing the film thickness by 60%. We show that active learning has the potential to rapidly navigate the vast processing space in multicomponent systems, especially when experiments are expensive and traditional computational models are not readily available for process optimization.

Electrosynthesis of HKUST-1 with Flow-Reactor Post-Processing

Electrochemical synthesis has been proposed as an efficient method for cost-effective and large-scale production of metal-organic frameworks (MOFs). This work investigates the combined electrochemical synthesis with flow synthesis post-treatment for the production of high surface area HKUST-1. The electrochemical synthesis process used in the experimental work did not require additional electrolytes or washing of the synthesis product. Batch electrosynthesis and electrosynthesis with flow synthesis were compared for the quality of the product using Brunauer–Emmett–Teller (BET) surface area, X-ray diffraction (XRD), and scanning electron microscopy (EIS). Batch electrosynthesis in 0.01 M benzene-1,3,5-tricarboxylic acid (H3BTC) solution produced HKUST-1 with BET surface area of 1550 m2/g which was increased further to 1716 m2/g with post-flow-synthesis treatment. The greatest change in surface area after flow processing was observed when using 0.78 M H3BTC, with corresponding surface areas of 481 m2/g and 1531 m2/g. According to SEM and BET results, the product purity improved during the post-flow-synthesis treatment. The proposed method enables continuous flow synthesis of high-quality MOFs with minimal purification steps.

MXene Derived Metal-Organic Frameworks.

Synthesis of nanoscale metal-organic frameworks (MOFs) is a highly challenging task because conventional soluble metal salt precursors are not easy to manipulate spatially, thus normally leading to bulk MOFs. In the present work, V2CTx MXene is demonstrated for the first time as a metal precursor to fabricate two-dimensional (2D) MOF nanosheets, whose thickness (6 to 18 nm) can be tuned by varying the reaction temperature. The highly electronegative surface atoms of MXene and sufficient accessible attacking sites for ligands are responsible for the evolution of 2D MOF nanosheets. Moreover, highly oriented and smooth MOF thin films have been grown based on these nanosheets using a convenient spin coating process. With the impregnation of nonvolatile H3PO4, the MOF thin film exhibits a proton-conducting property. This study demonstrates that high-quality 2D MOF sheets and thin films are enabled by 2D MXene precursors. We believe that the high-quality MOF films prepared in this study pave the way for many device applications.

Dong-Pyo Kim.

"My biggest motivation is the happiness that comes from scientific discussions. For me, science is a tool of good communication in friendship, irrespective of age, language, or culture. Guaranteed to make me laugh is finding similarities in human nature and science …" Find out more about Dong-Pyo Kim in his Author Profile.

Meniscus-Guided Control of Supersaturation for the Crystallization of High Quality Metal Organic Framework Thin Films

Herein, meniscus-guided crystallization is introduced as a means to grow high quality metal organic framework (MOF) thin films. A meniscus was formed between a target substrate and a coating blade,...

Benign by Design: Green and Scalable Synthesis of Zirconium UiO-Metal–Organic Frameworks by Water-Assisted Mechanochemistry

We present a solvent-free, green, and rapid mechanochemical route for the synthesis of a series of zirconium metal–organic frameworks (MOFs) composed of Zr6 cluster nodes, UiO-66, UiO-66-NH2, MOF-801, and MOF-804, both on a laboratory scale and by scalable and flow mechanochemical processing. The methodology, based on the use of a nonconventional zirconium dodecanuclear acetate cluster and a minute amount of water as an additive, affords high-quality MOFs in less than 1 h of milling, with minimal requirements for workup processing and eliminating the need for conventional hazardous solvents, such as dimethylformamide. Moreover, the use of a dodecanuclear zirconium acetate precursor circumvents the need for modulators resulting in acetic acid as the only byproduct of the reaction, which does not harm these acid-resistant materials. The porosity, thermal and chemical stability, as well as catalytic activity of mechanochemically prepared Zr-based MOFs are similar to those of solvothermally synthesized counte...

Conformal coating of ultrathin metal-organic framework on semiconductor electrode for boosted photoelectrochemical water oxidation

Exploring efficient photocatalysts for water oxidation is highly demanded because of their significant role in artificial photosynthesis. The research of metal-organic frameworks (MOFs) based photoelectrochemical (PEC) water splitting is promising but still in its infancy due to the challenge to fabricate high-quality MOFs photoelectrode. Here an Fe2O3/Fe-based MOF core/shell nanorod is prepared for efficient PEC water oxidation via a facile surfactant-assisted solvothermal method. With polyvinylpyrrolidone (PVP) molecules as intermedia, an ultrathin MOF shell of several nanometers thick can be controllably grown on the surface of Fe2O3 nanorod. Charge dynamical behaviors study by ultrafast transient absorption spectroscopy (TAS) and photoelectrochemical impedance spectroscopy (PEIS), reveal that the ultrathin MOF shell is crucial to promote the charge separation by providing a cascade band level, but also accelerate the hole injection efficiency via exposing more accessible Fe-oxo cluster active sites. Accompanied with good visible-light response, the Ti-doped Fe2O3/NH2-MIL-101(Fe) photoanode delivers a boosted photocurrent density of 2.27 mA cm−2 at 1.23 V vs. RHE, which is about 2.3 folds of that of pristine Fe2O3. This study provides new insights into the rational design of semiconductor/MOFs hybrid materials for photo/photoelectrochemical catalysis.

A Highly Solvent-Stable Metal-Organic Framework Nanosheet: Morphology Control, Exfoliation, and Luminescent Property.

Compared to bulk metal-organic framework (MOF), 2D MOF nanosheets have gained intensive research attention due to their ultrathin thickness and large surface area with highly accessible active sites. However, structural deterioration and morphological damage have impeded producing high-quality MOF nanosheets during exfoliation. Here, first a new layered bulk MOF ZSB-1 is synthesized and several solvents such as isopropanol, methanol, n-hexyl alcohol, and N,N-dimethylformamide are surveyed to examine their performance for the exfoliation of layered ZSB-1. As a result, a highly solvent-stable metal-organic framework rectangular nanosheet retaining undamaged morphology is obtained by the soft-physical method in n-hexyl alcohol. Theoretical simulations reveal that the strong interaction energy between n-hexyl alcohol and MOF layers is responsible for the best exfoliation performance of making the bulk MOF into nanosheets. In addition, ZSB-1 shows a tunable fluorescence peak position, fluorescent lifetime, and quantum yield by simply changing the solvent and morphology. Besides, the ZSB-1 was selected as a fluorescence sensor to detect metal ions, and ZSB-1 nanosheet exhibits excellent sensing ability for Fe3+ . It is worth noting that the ZSB-1 nanosheet has better detection limit performance of 0.054 × 10-6 m than that of its bulk counterpart.

Recent Progress on Metal-Organic Framework Membranes for Gas Separations: Conventional Synthesis vs. Microwave-Assisted Synthesis

Metal-organic frameworks (MOFs) are nanoporous materials that consist of organic and inorganic moieties, with well-defined crystalline lattices and pore structures. With a judicious choice of organic linkers present in the MOFs with different sizes and chemical groups, MOFs exhibit a wide variety of pore sizes and chemical/physical properties. This makes MOFs extremely attractive as novel membrane materials for gas separation applications. However, the synthesis of high-quality MOF thin films and membranes is quite challenging due to difficulties in controlling the heterogeneous nucleation/ growth and achieving strong attachment of films on porous supports. Microwave-based synthesis technology has made tremendous progress in the last two decades and has been utilized to overcome some of these challenges associated with MOF membrane fabrication. The advantages of microwaves as opposed to conventional synthesis techniques for MOFs include shorter synthesis times, ability to achieve unique and complex structures and crystal size reductions. Here, we review the recent progress on the synthesis of MOF thin films and membranes with an emphasis on how microwaves have been utilized in the synthesis, improved properties achieved and gas separation performance of these films and membranes.