Metal-organic Frameworks Crystals Research Articles

Electrospun hydrogel fibers guide HKUST-1 assembly

Metal organic frameworks (MOFs) have been touted for their large surface areas, with interconnected porosity throughout their crystal framework. Designing MOFs with controlled structure and size and with stable attachment to surfaces are two objectives to enhance practical utilization of MOFs for their many applications. We propose that tuning the composition of electrospun hydrogel nanofibers makes them excellent substrates to promote growth of MOFs (i.e. Cu-based HKUST-1) with controlled size. Hydrogel fiber mats composed of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were produced through electrospinning PAA/PAH solutions followed by a thermal crosslinking. The hydrogel fiber mats were immersed in copper acetate aqueous solutions to uptake copper ions. Upon the immersion of the hydrogel fibers mats with copper ions in a separate linker solution, the released copper ions reacted with linkers, allowing for a controlled assembly of MOF structures on the hydrogel fibers. An increased PAA component in the PAA/PAH hydrogel fibers increased metal ion uptake through the interactions between the carboxylic groups and metal ions. Upon the release of metal ions in the linker solution, MOF crystals of a smaller scale and denser coverage than fibers with a lower proportion of PAA were observed. A similar trend was observed in fibers of varying diameter, with variations in crystal growth attributed to the metal ion flux at the fiber-solution interface. The crystals appeared well-separated and densely coated across the fibers and could undergo rapid partial activation in dry environments.

An unusual magnetic phenomenon in Mg doping [(CH3)2NH2] CoxMg1−x[HCOO]3 frameworks

In this study, Mg doping metal-organic frameworks (MOFs) [(CH3)2NH2] Co1−x Mg x [HCOO]3 single crystals were synthesized by solvothermal method with normal ratios x = 0, 0.1, 0.2, 0.3, 0.4, 0.6. Powder X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) results showed that Mg ion is uniformly doped into Co-based MOFs crystal. With the increase of diamagnetic magnesium ions doping, the antiferromagnetic phase transition temperature of Co-based MOFs for pure Co-MOF decreased from 15 K to 7.5 K for x = 0.4. Abnormal larger magnetic hysteresis was obtained for Mg doping crystals with a large coercive field 3100 Oe (x = 0.1), 3700 Oe (x = 0.2), 2900 Oe (x = 0.3) compared with 600 Oe coercive field for pure Co-MOFs (x = 0). At the same time, step-like behavior was first observed on Mg2+ doping sample with x = 0.1–0.3. This unusual magnetic phenomenon is well interpreted based on the coexistence of long-range canted antiferromagnetic order and isolated single-ion quantum magnets. The diamagnetic magnesium ions doping decreases the strong exchange coupling between neighboring Co ions and step-like behavior stemmed from the quantum tunneling of isolated Co ions.

Facile synthesis of Cu-based metal–organic framework/chitosan composite granules for toluene adsorption

The in situ synthesis of metal–organic framework crystals incorporated into natural polymers is an efficient approach to manufacturing adsorbents of large size, which is convenient in terms of adsorbent handling, operating, and transportation for practical adsorption processes. Herein, CuBTC/CS (BTC = 1,3,5-benzenetricarboxylate; CS = chitosan) composite granules were successfully prepared by dispersing Cu2+ ions into the CS matrix before generating Cu2+/CS composite beads, which subsequently grew into CuBTC crystals incorporated in the CS network. To evaluate their quality, the characteristics of the prepared composites were systematically analyzed, which were observed to be controlled by the amount of Cu2+ ions present in the reaction system. The highest quality material was utilized as an adsorbent for toluene uptake, which was estimated via breakthrough data. The adsorptive interaction was investigated by determining the heat of adsorption, whilst the kinetic behavior of the adsorption process was described by pseudo-first-order and pseudo-second-order models. Additionally, the mechanism of diffusion of toluene molecules into the granular adsorbent was examined in detail. Finally, to predict the adsorption behavior via the mathematical models of Yan and Thomas, the experimental breakthrough data were utilized. The herein-developed route allows granular adsorbents to be manufactured in large quantities and used in the practical volatile organic compound adsorption.

Facile electrospray fabrication of ultralow biofouling cellulose acetate desalination membrane with nanocellulose/UiO66-NH2 fillers

The inherently high porosity, hydrophilicity, and large surface area of the octahedral UiO66-NH2 nanocrystals were exploited in synergy with cellulose nanocrystals (CNCs) as electrospray coating nanocomposites onto the cellulose acetate (CA) desalination membranes. Optimal loadings of both nanoparticles (0.05% w/v each) enabled the partial embedment of the defective metal-organic framework (MOF) crystals in the dense CA layer, successfully facilitating a faster, low-tortuosity water transport pathway through the active layer. With just one electrospray scan, the CNC/UiO66-NH2-coated CA membrane achieved an almost doubled the water permeability coefficient of 0.24 L m−2 h−1 bar−1 while maintaining a retention efficiency of over 94% in brackish water desalination. Moreover, the electrospray-coated membrane demonstrated healthier flux recovery from continuous E. coli cross-flow filtration exposure, exhibiting great resistance to biofouling. Overall, the CNC/UiO66-NH2-coated CA membrane outperforms many CA-based desalination membranes reported in the literature in terms of water/salt permselectivity and anti-fouling capabilities.

In Situ Synthesis of Hierarchical Porous Zr-MOFs on Columnar Activated Carbon and Application in Toxic Gas Adsorption.

Metal organic frameworks (MOFs) have been explored as adsorption materials owing to their diversity, controllable structure, high specific surface area, and abundant active sites. However, the shaping of MOFs has become a critical issue hindering their commercial application. A binder or high pressure is commonly used in traditional powder shaping, causing pores to be blocked or collapsed and porosity to be decreased, eventually leading to the degradation of adsorption performance. In this paper, Zr-MOFs were in situ grown on a columnar activated carbon (CAC) matrix, and a series of Zr-MOFs/CAC composites were prepared. The adsorption properties for SO2 and NO2 were measured by dynamic adsorption tests, and the Wheel-Jonas model was used to calculate the saturated adsorption capacity. Abundant mesopores can be formed between MOF crystals and activated carbon particles, and the mesoporosity of Zr-MOFs/CAC composites reached over 50%. Owing to the abundant mesoporous, increased activated sites as well as the synergistic effect between MOFs and activated carbon, the as-obtained HP-Zr-MOFs/CAC exhibited the best adsorption performance both for SO2 and NO2, which are 34.2 and 17.4 mg g-1, respectively, while the adsorption capacities of CAC for SO2 and NO2 are 20.9 and 6.6 mg g-1, respectively. The outstanding performance and facile synthesis process of HP-Zr-MOFs/CAC composites could provide ideas to develop other hierarchical porous MOFs/activated carbon composites.

Synthesis of Thorium Dioxide Nanocrystals by Pyrolysis of a Thorium‐based Metal‐organic Framework

AbstractA new method is demonstrated for synthesizing thorium dioxide (ThO2) nanoparticles (NPs) by thermal decomposition of an undiscovered thorium‐based metal‐organic framework (MOF). Upon heating at 520 °C, the thorium‐based MOF crystals, designated Th‐MTB, are converted completely to ThO2 NPs with an average size of 150 nm. The MOF‐derived ThO2 NPs are easily sintered to form ThO2 ceramics which have good thermal conductivity. The crystalline grains of ThO2 grow continuously during sintering.The method for the synthesis of ThO2 NPs could have potential applications in the production of nuclear materials.

(Digital Presentation) Electrochemical Conversion of Cu Nanowires Synthesized By Electrodeposition in Track-Etched Templates to HKUST-1

Metal-organic frameworks (MOFs) are a novel type of nanoporous materials that have attracted widespread attention over the past two decades [1]. Cu-based metal-organic frameworks such as Cu3(BTC)2 (also known as HKUST-1) are one of the most famous MOF representatives, which exhibit a huge open porosity and thus a remarkably capacity to store and uptake different gases [2, 3]. Recently, increasing efforts are devoted toward finding synthetic routes that enable downsizing MOF crystals to the nanoscale. Achieving control over the size and shape of nanoMOFs and finding ways to assemble them is essential for their exploitation in integrated devices such as sensors, gas separation membranes or photoelectrodes.In this study we explore the conversion of free-standing arrays Cu nanowires with controlled diameter and length synthesized by electrodeposition in etched ion-track membranes into HKUST-1. In a first process step, free-standing Cu wires are produced by dissolving the ion-track polymer template. In a second step, the wires are converted into HKUST-1 structures by electrochemical oxidation. Applying 2.5 V versus a Cu counter electrode, the Cu nanowires are oxidatively dissolved and the MOF is built up as the as-formed Cu2+ ions bind to the BTC3− ligands in the electrolyte solution. The morphology and crystallinity of the samples at different transformation stages is investigated by scanning electron microscopy (Fig. 1) and transmission electron microscopy, respectively. X-ray diffraction spectra measured at different conversion times reveal the appearance of the characteristic reflections of HKUST-1. These results will be compared with previous studies of the transformation of Cu nanowires to HKUST-1 nanowires inside the polymer membrane [4].Figure 1: SEM images of cylindrical Cu nanowires (a) before and (b) during the electrochemical conversion process, and (c) of a representative octahedral particle after complete conversion to HKUST-1.

Microfluidic synthesis of metal-organic framework crystals with surface defects for enhanced molecular loading

Inherent properties of metal–organic frameworks (MOFs) such as porosity and chemical diversity allows utilizing them as drug delivery systems which are not toxic at 25 MOFs per B16-F10 cell. Herein, the MOF developed surface due to intrinsic defects enables exploration of a new form of efficient delivery with remote control. Here we experimentally demonstrate the directed synthesis of MOF microcrystals (HKUST-1) with developed surface defects by a single-step microfluidics method. The synthesis process has been optimized to obtain the desired MOFs that ensure the enhanced loading of molecules (fluorescence dye) on their surface as compared with MOFs synthesized by solvothermal approach. Structural analysis and optical spectroscopy confirmed the defective MOF surface. Dye loading and remote light-induced release has been demonstrated under continuous wave laser irradiation (532 nm). Toxicity of the obtained MOFs and their interaction with murine melanoma cells have been additionally tested. Therefore, the ability to design and synthesize MOFs with the desired physicochemical properties and 2 times higher loading capacities than a solvothermally synthesized one due to the use of microfluidic approach opens up new prospects for the development of improved drug delivery platforms.

Switching from Oxygen Quenching Resistance to Linear Response by Smart Luminescent Iridium(III)-Based Metal-Organic Frameworks.

In this study, we utilize a photo-active Ir-metalloligand, Ir(C^N)2(L) (C^N = 2-(2,4-difluorophenyl) pyridine, L = [2,2'-bipyridine]-5,5'-dicarboxylic acid), to assemble with CdX2 under hydrothermal conditions, yielding highly emissive crystals of two-dimensional metal-organic frameworks (2D MOFs) (named Ir-Cd2X2, X = Cl, Br). The Ir-Cd2X2 MOFs exhibit μs-level phosphorescence lifetimes and more than 55% quantum yield (QY) at room temperature because of the rigid framework connected by Cd2X2 clusters. By immersing Ir-Cd2X2 in water solution for 5 min, a new MOF (Ir-Cd) was obtained, which is given a structure with hydrolyzed Cd-nodes by complete removal of halogen bridges as elucidated by single-crystal diffraction. Although the phosphorescence emission of pristine CdX2 MOFs exhibits oxygen quenching resistance, the converted Ir-Cd MOF possesses sensitively oxygen-responsive 3MLCT properties, showing a KSV value as high as 14.5 with strictly linear relation (R2 = 0.995). This work differs from the traditional method for improving oxygen-sensing metrics by enhancing QY and phosphorescence lifetime in Ir complexes, while also demonstrating that the transformation in the surrounding coordination environment on adjacent metal centers can also constitute key factors for improved photoluminescence stability or responsive properties in Ir-based heteronuclear MOFs, providing clues for the development of either oxygen quenching blockers or sensors suitable for different occasions.

Metal-organic frameworks decorated wood aerogels for efficient particulate matter removal

The severe indoor/outdoor air contamination caused by hazardous particulate matters (PMs) has a devastating impact on the environment and human health. Herein, air filters fabricated from delignified wood aerogels and metal–organic frameworks (MOFs) were used for PM removal. Two specially designed MOFs (i.e. Zn-based ZIF-L and Zr-based UiO-66-NH2) were deposited into a highly porous wood matrix through in situ growth approach. The MOF crystals are attached tightly to the highly available cellulose surface groups and distributed uniformly. The integration of MOFs can regulate the aerogel morphology and surface property, leading to enhanced PM removal performance. Bare wood aerogels beard removal efficiencies of 70.7% and 75.0% for PM2.5 and PM10, with a low-pressure drop of 15 Pa. By contrast, ZIF-L decorated wood aerogels exhibited PM2.5 and PM10 removal efficiencies of 97.6% and 99.5%, respectively, which are ascribed to the increased interception/impaction of PMs with the positively charged ZIF-L layer. While UiO-66-NH2 functionalized wood aerogels demonstrated PM2.5 and PM10 removal efficiencies of 96.4% and 98.9% because of the hierarchical filter matrix and the presence of polar –NH2 groups. Moreover, MOF modified wood aerogels can be easily regenerated and superior reusability can be achieved. This work provides an effective and economic strategy for the preparation of air filters from renewable materials.

Copper-induced synthesis of zinc imidazolate metal-organic framework (ZIF-8) crystals and coatings

Zeolitic imidazolate framework-8 (ZIF-8) is one of the most stable Metal Organic Frameworks currently available, with promising properties in adsorption, separation and catalysis. Herein, we report a method for the rapid production of ZIF-8 crystals at room-temperature from methanolic solutions containing sodium acetate and metal salts. Of the different metal salts tested, copper nitrate presence prompted formation of cubic shaped ZIF-8 crystals within 5 min. Derived from this finding, an adaption of the synthesis route was implemented over copper foils and copper based metallic/non-metallic substrates, which yielded high quality shape- and size-controlled ZIF-8 crystal coatings. The outcomes also revealed a significant impact of sodium acetate as modulator and of copper ions released in the reaction media as triggering agent/co-modulator. Together these chemical species are understood to be involved in a multitude of coordination complex formation and deprotonation equilibria, thus largely determining the ZIF-8 crystal morphology and growth, especially near the copper releasing sites. Overall, this work presents an important platform for rapid and controlled synthesis of ZIF-8 crystals and coatings over metallic/non-metallic substrates, which can be adapted for advanced applications such as device integration.

Improving the permselectivity of asymmetric isoporous membranes by blending the micro-sized metal-organic frameworks(MOFs) crystals with block copolymer

Nanomaterials are widely used to improve the separation performance for asymmetric isoporous membranes. Different from those prevalent researches, MOFs crystals with particle sizes of 0.52–0.75 μm which were an order magnitude larger than the molecular size of block copolymers (BCPs) were loaded to casting solution, and their tailoring on membrane formation and permselectivity of isoporous membranes prepared via the SNIPS were carefully investigated. As far as we know, 3D particles with such big size have never been added to isoporous membranes before. Results from small angle X-ray scattering indicated that the microphase separation in solutions had been increased obviously although MOFs crystals were too large to co-assemble with BCPs. This interesting finding was assigned to the breathing effect from MOFs crystals that increased the effective concentrations of BCPs. Morphology observation indicated that MOFs crystals located on surface were fully covered by a thin layer of BCPs which also assembled into defect-free, isoporous structures. This result was observed even though the contents of MOFs in membranes reached 40 wt%. It was measured that the permeability and selectivity to protein mixtures improved obviously by the MOFs crystals blended to membranes. Finally, with the removal of MOFs from membranes by acid etching, the permselectivity of membranes improved further. Considering the easy synthesis and good dispersibility, micro-sized MOFs crystals can be used as more effective additives to improve the separation performances for isoporous membranes than that of nanomaterials. This finding will pave the ways for preparing the high-performance isoporous membranes by using MOFs crystals as the additives for block copolymers.

Polyvinyl alcohol/starch-based film incorporated with grape skin anthocyanins and metal-organic framework crystals for colorimetric monitoring of pork freshness

An intelligent film for the visual monitoring of pork freshness was developed using degradable polyvinyl alcohol (PVA)/starch (PS) to immobilize the chromogenic agent of anthocyanins and the volatile amine collector of metal–organic frameworks (MOFs). The indicative property of grape skin anthocyanins (GSAs) was verified using the UV–vis spectra, corresponding to multi-color changing in a pH range of 2–12. Interestingly, the introduction of MIL-101 crystals in the PS/GSAs film significantly increased the specific surface area (approximately 10 times) of the film, the superior volatile amine enrichment capability of MIL-101 enabling the film to detect freshness with a high degree of sensitivity. Moreover, the as-prepared film exhibited good antibacterial properties attributed to MIL-101, which help maintain the freshness of the pork. Owing to these advantages, the PS-GSAs/MIL-101 film was tested to real-timely monitor pork freshness in package, the results were further confirmed basis the total volatile basic nitrogen values.

Unraveling the molecular mechanism of MIL-53(Al) crystallization

The vast structural and chemical diversity of metal−organic frameworks (MOFs) provides the exciting possibility of material’s design with tailored properties for gas separation, storage and catalysis. However, after more than twenty years after first reports introducing MOFs, the discovery and control of their synthesis remains extremely challenging due to the lack of understanding of mechanisms of their nucleation and growth. Progress in deciphering crystallization pathways depends on the possibility to follow conversion of initial reagents to products at the molecular level, which is a particular challenge under solvothermal conditions. The present work introduces a detailed molecular-level mechanism of the formation of MIL-53(Al), unraveled by combining in situ time-resolved high-resolution mass-spectrometry, magic angle spinning nuclear magnetic resonance spectroscopy and X-ray diffraction. In contrast to the general belief, the crystallization of MIL-53 occurs via a solid-solid transformation mechanism, associated with the spontaneous release of monomeric aluminum. The role of DMF hydrolysis products, formate and dimethylamine, is established. Our study emphasizes the complexity of MOF crystallization chemistry, which requires case-by-case investigation using a combination of advanced in situ methods for following the induction period, the nucleation and growth across the time domain.

Molybdenum-functionalized metal–organic framework crystals interconnected by carbon nanotubes as negative electrodes for supercapacitors

Molybdenum-functionalized metal–organic framework crystals interconnected by carbon nanotubes as negative electrodes for supercapacitors

Reversible and Irreversible Laser Interference Patterning of MOF Thin Films

Laser interference patterning on top of a thin film and inside a crystal is a powerful tool today to create the desired patterns for optical data processing. Here, we demonstrate reversible and irreversible laser interference patterning on a metal-organic framework (MOF) thin film through the water desorption and thermal decomposition processes, respectively. The irreversible interference pattern with a period of the strips of up to 5 µm has been realized, and its morphology has been characterized using confocal Raman and reflection spectroscopy as well as atomic force microscopy. We revealed that reducing the distance between the interference maxima from 10.5 to a record of 5 µm for MOFs yields a 10-fold increase in the surface roughness of the irreversible pattern; on the other hand, the reversible laser pattern provides a completely non-destructive effect of variable optical contrast. The experimental results obtained open up prospects for the use of MOF crystals as photosensitive materials in the template drawing of the desired patterns for different application scopes.

General Synthesis and Solution Processing of Metal-Organic Framework Nanofibers.

By virtue of their extraordinarily high surface areas, ordered pore structures, various compositions, and rich functionality, metal-organic frameworks (MOFs) are of great interest in diverse fields such as gas separation, sensing, catalysis, energy, environment science, and biomedicine. However, the difficulty in processing MOF crystals and controlling the MOF superstructure is emerging as a critical issue in their application. Herein, it is reported that a robust template, i.e., nanofibrillated cellulose (NFC), can be used for the synthesis of MOF materials with 1D nanofiber morphology. NFC@MOF core-shell nanofibers with a uniform network structure and high aspect ratios can be prepared by use of this template. The small crystal size, flexibility, and good dispersity of the NFC@MOF nanofibers make it convenient for the macroscale assembly and solution processing of MOF materials. A proof-of-concept study is demonstrated wherein freestanding MOF nanofiber membranes represent good performance in applications of water treatment and heterogeneous catalysis reaction. This general synthesis and solution-processing strategy may herald a new era in promoting the industrial application of MOFs.

Pseudomorphic Replacement in the Transformation between Metal–Organic Frameworks toward Three-Dimensional Hierarchical Nanostructures

The construction of three-dimensional hierarchical materials from nano building blocks has received tremendous attention because of the intriguing architecture and promising application potential of these materials. However, the use of traditional assembly methods to construct hierarchical nanostructures with complicated 3D shapes across multiple length scales remains challenging. Herein, we developed a new strategy to fabricate hierarchical nanostructures by borrowing the concept of pseudomorphic replacement from mineralogy; this concept refers to a specific conversion between minerals with the preservation of the pristine shape through an interface-coupled dissolution–precipitation mechanism. By employing pseudomorphic replacement to the transformation between metal–organic frameworks (MOFs), a series of hierarchical MOF materials with similar nanostructures but various shapes and dimensions were successfully fabricated. The experiments verified that these transformations not only preserve the shape of the pristine MOF crystals but also proceed via an interface-coupled dissolution–recrystallization process. To our knowledge, our investigation is the first explicit demonstration of pseudomorphic replacement in the transformation between MOFs, offering a new route to hierarchical materials with enhanced control ability and extended control scope.

Single‐Crystal Capacitive Sensors with Micropatterned Electrodes via Space‐Confined Growth of the Metal–Organic Framework HKUST‐1

AbstractThe polyhedral shape and fragile nature of single crystals of metal–organic frameworks (MOFs) hinder their processing using standard microfabrication techniques. As a result, the fabrication of surface patterns and microelectrodes on MOF single crystals is challenging, which limits both the direct electrical interrogation of crystals in research and their integration into optical and electrical devices. Herein, a new strategy to readily fabricate surface‐embossed micropatterns and surface‐embedded gold electrodes on MOF single crystals using a space‐confined, one‐step solvothermal synthesis is demonstrated. This method is based on the confined growth of the MOF crystal around prefabricated metal patterns with different adhesion strengths to the growth surface, producing features as small as 5 µm. As a proof of concept, micropatterns are created on flat single crystals of the MOF HKUST‐1, exhibiting the function as a diffraction grating. Additionally, a single‐crystal HKUST‐1 capacitive gas sensor with surface‐embedded electrodes that serves as a stable platform for the investigation of ethanol diffusion in the MOF pores is fabricated. The space‐confined fabrication strategy is promising for the integration of fragile MOF single crystals in various optical and electrical devices.

3D Printing of Metal-Organic Framework-Based Ionogels: Wearable Sensors with Colorimetric and Mechanical Responses.

Soft ionotronics are emerging materials as wearable sensors for monitoring physiological signals, sensing environmental hazards, and bridging the human-machine interface. However, the next generation of wearable sensors requires multiple sensing capabilities, mechanical toughness, and 3D printability. In this study, a metal-organic framework (MOF) and three-dimensional (3D) printing were integrated for the synthesis of a tough MOF-based ionogel (MIG) for colorimetric and mechanical sensing. The ink for 3D printing contained deep eutectic solvents (DESs), cellulose nanocrystals (CNCs), MOF crystals, and acrylamide. After printing, further photopolymerization resulted in a second covalently cross-linked poly(acrylamide) network and solidification of MIG. As a porphyrinic Zr-based MOF, MOF-525 served as a functional filler to provide sharp color changes when exposed to acidic compounds. Notably, MOF-525 crystals also provided another design space to tune the printability and mechanical strength of MIG. In addition, the printed MIG exhibited high stability in the air because of the low volatility of DESs. Thereafter, wearable auxetic materials comprising MIG with negative Poisson's ratios were prepared by 3D printing for the detection of mechanical deformation. The resulting auxetic sensor exhibited high sensitivity via the change in resistance upon mechanical deformation and a conformal contact with skins to monitor various human body movements. These results demonstrate a facile strategy for the construction of multifunctional sensors and the shaping of MOF-based composite materials.