MOF Crystals Research Articles

PH-modulated formation of uniform MOF-5 sheets

We report on a controlled synthesis of uniform MOF-5 sheets. The MOF sheets were solvothermally synthesized in a basic reaction medium in the presence of polyvinylpyrrolidone (PVP). Systematical investigation on the synthetic parameters revealed a pH-modulated mechanism for the formation of sheet-like morphology of MOF-5 crystals and a PVP-promoted shape evolution of the well-defined square sheets with uniform length and thickness.

Electrochemical performance of bifunctional Co/graphitic carbon catalysts prepared from metal–organic frameworks for oxygen reduction and evolution reactions in alkaline solution

Cobalt/graphitic carbon catalysts were prepared from Co-MOF crystals, which were synthesized by a solvothermal method in a Teflon-lined autoclave. The MOF crystals were calcined for 2 h to produce Co-MOF-derived catalysts. X-ray diffraction and X-ray photoelectron spectroscopy analysis showed that calcination of the Co-MOF at 600 °C favors the formation of Co3O4, while calcination at 700 °C produced a catalyst composes of metallic Co embedded in a carbon matrix. Raman spectra showed the carbon layer on the Co particles has graphitic properties. The Co-MOF calcined at 700 °C was observed to have the highest surface area and catalytic activity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. During charge/discharge cycling of the Co-MOF-derived catalyst (N700), the potentials were changed within 10–50 mV from the initial potential after 250 h. This catalyst showed excellent stability for ORR and OER, which were superior to these kinds of catalysts reported in the literatures, because graphitic carbon layers covered on the cobalt nanoparticles.

(Invited) Device Fabrication – Positioning and Alignment of MOF Crystals

Metal organic frameworks (MOFs) are a class of ultra-porous materials with exceptionally high accessible surface area and structure modularity due to the framework made of inorganic nodes coordinated by organic bridging ligands.[ 1 2 , 3 ] A number of increasing studies investigate Metal-Organic Frameworks (MOFs) for a variety of functional applications including sensing, microelectronics, energy production, drug delivery and microfluidics.[ 4 ] However, MOF-based device fabrication requires synthetic protocols for the controlled growth of the ultra-porous crystals and their integration in suitable platforms.[ 4 ] This presentation will summarize the recent progress on MOF crystal engineering aspects from positioning technologies to alignment strategies. [ 5 , 6 , 7 ,8, 9,10,11,12] A particular focus of the presentation will be the current role of ceramics for MOF device fabrication.

Coupling Ru nanoparticles and sulfonic acid moieties on single MIL-101 microcrystals for upgrading methyl levulinate into γ-valerolactone

Heterogeneous catalytic conversion of biomass-derived methyl levulinate (ML) to γ-valerolactone (GVL) is highly attractive as a sustainable route to mitigating the dependence on nonrenewable fossil resources. However, this route is limited by its low selectivity toward GVL and severe reaction conditions. Here, we present a synergistic catalytic process that offers exclusive GVL selectivity with complete ML conversion under mild conditions (80 °C, 0.5 MPa of H2 pressure). This is achieved over a dual-functionalized catalyst of Ru nanoparticles (Ru-NPs) and Brønsted acid SO3H groups in a Cr-based metal-organic framework (MIL-101). The Ru-NPs served as metal sites for the hydrogenation of ML to methyl-3-hydroxyvalerate (MHV) and the acidic SO3H moieties promoted the cyclization of MHV to generate GVL via lactonization. In contrast, GVL production was significantly suppressed over the physically mixed congener (2.0 wt% Ru/MIL-101 + MIL-101-SO3H). This result demonstrates an unrivalled advantage of the dual integration of metal/acid sites in a single MOF crystal, and the relative amount of sulfonic acid groups in the catalyst can exert significant influence on the overall catalytic performances. The activation energies for the hydrogenation and lactonization steps with the developed catalyst were 29.5 and 42.1 kJ/mol, respectively, which are much lower than those observed with the traditional 5.0 wt% Ru/AC catalyst.

Modulated synthesis of monodisperse MOF-5 crystals with tunable sizes and shapes

We report on a modulated synthesis method that enables the preparation of monodisperse MOF-5 crystals with tunable sizes and shapes. This controlled synthesis involves the base-tuned nucleation and the surfactant-modulated facet growth of MOF-5 during its crystallization. Systematic reactions with triethylamine (TEA) as additional base and polyvinylpyrrolidone (PVP) as capping agent were investigated. Under optimized experimental conditions, monodisperse MOF-5 cubes (with tunable sizes ranging from ~400 nm to ~2500 nm), truncated cubes, truncated octahedrons, and octahedrons could be produced with high reproducibility.

Energy Storage in a Transition Metal Doped MOF Derived Carbon Nanostructure

We illustrate a novel concept in tailoring the architecture of MOF-derived carbons by doping MOF crystals with transition metal ions prior to carbon structure growth via chemical vapor deposition (CVD). MOF-5 was isomorphously doped with various concentrations of Ni2+ during synthesis and was subsequently subjected to CVD to grow a labyrinth of carbon nanofibers (CNF) with high electrical conductivity and higher surface area than that of the parent MOF. TEM of the inner structure of the CNFs revealed a chevron arrangement with ~0.34 nm visible fringe spacing. The CNF diameters increased with increasing Ni concentration. This is attributed to the decreasing distance between Ni dopants with increasing Ni concentration, which may facilitate coalescence of the Ni catalyst during the heat treatment and thereby increase the diameter of the resultant CNF. However, Ni doping concentration greater than 30% led to large agglomerations of Ni particles which eventually ceased CNF growth. The electrical conductivity and the electrochemical double layer capacitance of the CNFs also increased with increasing Ni concentrations. The highest electrochemical double layer capacitance and the lowest resistance was observed at a Ni doping concentration of 30%. An alkaline electrolyte was used to engage the Ni particles inside the CNFs to participate in a reversible redox reaction shuttling between Ni and Ni(OH)2, which resulted in a specific capacitance as high as 349 F/g. The possibility of doping transition metals into a wide variety of MOF structures can provide a new method to synthesize novel carbon structures with high energy storage capabilities.

Creating Hierarchical Pores by Controlled Linker Thermolysis in Multivariate Metal-Organic Frameworks.

Sufficient pore size, appropriate stability, and hierarchical porosity are three prerequisites for open frameworks designed for drug delivery, enzyme immobilization, and catalysis involving large molecules. Herein, we report a powerful and general strategy, linker thermolysis, to construct ultrastable hierarchically porous metal-organic frameworks (HP-MOFs) with tunable pore size distribution. Linker instability, usually an undesirable trait of MOFs, was exploited to create mesopores by generating crystal defects throughout a microporous MOF crystal via thermolysis. The crystallinity and stability of HP-MOFs remain after thermolabile linkers are selectively removed from multivariate metal-organic frameworks (MTV-MOFs) through a decarboxylation process. A domain-based linker spatial distribution was found to be critical for creating hierarchical pores inside MTV-MOFs. Furthermore, linker thermolysis promotes the formation of ultrasmall metal oxide nanoparticles immobilized in an open framework that exhibits high catalytic activity for Lewis acid-catalyzed reactions. Most importantly, this work provides fresh insights into the connection between linker apportionment and vacancy distribution, which may shed light on probing the disordered linker apportionment in multivariate systems, a long-standing challenge in the study of MTV-MOFs.

Locating the binding domains in a highly selective mixed matrix membrane via synchrotron IR microspectroscopy.

The binding domains within a mixed matrix membrane (MMM) that is selective for CO2 comprising MFM-300(Al) and the polymer 6FDA-Durene-DABA have been established via in situ synchrotron IR microspectroscopy. The MOF crystals are fully accessible and play a critical role in the binding of CO2, creating a selective pathway to promote permeation of CO2 within and through the MMM. This study reveals directly the molecular mechanism for the overall enhanced performance of this MMM in terms of permeability, solubility and selectivity for CO2.

A MOF-based carrier for in situ dopamine delivery.

MIL-88A (Fe) MOF crystals were nucleated and grown around a polymer core containing superparamagnetic nanoparticles to assemble a new class of biocompatible particles for magnetophoretic drug delivery of dopamine. The carrier enabled efficient targeted release, dopamine protection from oxidative damage, long-term delivery and improved drug delivery cost-efficiency. After loading, dopamine was stable within the carrier and did not undergo oxidation. Drug release monitoring via spectrofluorimetry revealed a shorter burst effect and higher release efficiency than silica based carriers. The in vitro cytotoxicity at different MOF concentrations and sizes was assessed using PC12 cells as the neuronal cell model. The drug was directly uptaken into the PC12 cells avoiding possible side effects due to oxidation occurring in the extracellular environment.

Facilely controlled synthesis of a core-shell structured MOF composite and its derived N-doped hierarchical porous carbon for CO2 adsorption.

A new strategy for controlled synthesis of a MOF composite with a core–shell structure, ZIF-8@resorcinol–urea–formaldehyde resin (ZIF@RUF), is reported for the first time through in situ growth of RUF on the surface of ZIF-8 nanoparticles via an organic–organic self-assembly process by using hexamethylenetetramine as a formaldehyde-releasing source to effectively control the formation rate of RUF, providing the best opportunity for RUF to selectively grow around the nucleation seeds ZIF-8. Compared with the widely reported method for MOF composite synthesis, our strategy not only avoids the difficulty of incorporating MOF crystals into small pore sized materials because of pore limitation, but also effectively guarantees the formation of a MOF composite with a MOF as the core. After carbonization, a morphology-retaining N-doped hierarchical porous carbon characterized by its highly developed microporosity in conjunction with ordered mesoporosity was obtained. Thanks to this unique microporous core–mesoporous shell structure and significantly enhanced porosity, simultaneous improvements of CO2 adsorption capacity and kinetics were achieved. This strategy not only paves a way to the design of other core–shell structured MOF composites, but also provides a promising method to prepare capacity- and kinetics-increased carbon materials for CO2 capture.

Box-like gel capsules from heterostructures based on a core-shell MOF as a template of crystal crosslinking.

New polymer capsules (PCs) were obtained using a crystal crosslinking (CC) method on core-shell MOF crystals. The latter are based on the epitaxial growth of two isostructural coordination polymers which are then selectively crosslinked. Decomposition of the non-reticulated phase leads to new PCs, possessing a well-defined hollow cubic shape reflecting the heterostructure of the template.

Controllable Modular Growth of Hierarchical MOF-on-MOF Architectures.

Fabrication of hybrid MOF-on-MOF heteroarchitectures can create novel and multifunctional platforms to achieve desired properties. However, only MOFs with similar crystallographic parameters can be hybridized by the classical epitaxial growth method (EGM), which largely suppressed its applications. A general strategy, called internal extended growth method (IEGM), is demonstrated for the feasible assembly of MOFs with distinct crystallographic parameters in an MOF matrix. Various MOFs with diverse functions could be introduced in a modular MOF matrix to form 3D core-satellite pluralistic hybrid system. The number of different MOF crystals interspersed could be varied on demand. More importantly, the different MOF crystals distributed in individual domains could be used to further incorporate functional units or enhance target functions.

Controllable Modular Growth of Hierarchical MOF‐on‐MOF Architectures

AbstractFabrication of hybrid MOF‐on‐MOF heteroarchitectures can create novel and multifunctional platforms to achieve desired properties. However, only MOFs with similar crystallographic parameters can be hybridized by the classical epitaxial growth method (EGM), which largely suppressed its applications. A general strategy, called internal extended growth method (IEGM), is demonstrated for the feasible assembly of MOFs with distinct crystallographic parameters in an MOF matrix. Various MOFs with diverse functions could be introduced in a modular MOF matrix to form 3D core–satellite pluralistic hybrid system. The number of different MOF crystals interspersed could be varied on demand. More importantly, the different MOF crystals distributed in individual domains could be used to further incorporate functional units or enhance target functions.

Zapping a ZIF amps up gas separation

Exposing a membrane made of the metal-organic framework ZIF-8 to an external electric field transforms the material from one with a modest ability to separate propene from propane to a good separator of these gases. The finding, from a team led by Leibniz University Hannover scientists Jurgen Caro and Alexander Knebel, could be broadly applicable to different classes of framework materials that are similar to ZIF-8 (Science 2017, DOI: 10.1126/science.aal2456). ZIF-8 is a soft MOF crystal with zinc-coordinated imidazolate linker units that move in a manner described as “breathing.” Applying an external electric field of 500 V/mm to ZIF-8 reduces this motion and causes the material’s lattice to polarize, prompting a transition in its crystal structure. The zapped ZIF-8 has a 0.36-nm pore aperture, which is only slightly larger than the 0.34-nm opening in the unzapped material. However, because the linkers move less when electrified, the MOF’s ability to separate

Metal organic framework of MOF-5 with hierarchical nanopores as micro-gravimetric sensing material for aniline detection

Aromatic amine such as aniline is one kind of carcinogenic substances that needs to be detected at low concentration. To sense trace level aniline, high performance sensing material is of great importance. Herein, metal organic framework of Zn4O(1,4-benzenedicarboxylate)3 (commonly abbreviated as MOF-5) with abundant hierarchical nanopores is firstly employed as mass-type sensing material for low concentration aniline detection. By using one typical micro-gravimetric transducer of resonant microcantilever, the aniline-adsorbing induced mass-addition of MOF-5 is transformed to the detectible sensing signal of frequency shift. Based on resonant microcantilever with mass resolution of 1.5pg/Hz, the fabricated sensor exhibits good sensitivity for aniline detection. The limit of detection is better than 1.4ppm, and the selectivity is satisfactory to nine kinds of common interfering gases. The three-cycle repeatability of the sensor is also investigated with a relative standard deviation (RSD) of 4%. After 20days, the sensor only shows a negligible response degradation which represents a good reproducibility. Materials Studio simulation results manifest that 1,4-benzenedicarboxylic ligand in MOF-5 crystals is the preferential site for aniline molecules adsorbing.

Unidirectional compression and expansion of a crosslinked MOF crystal prepared via axis-dependent crosslinking and ligand exchange

The artificial construction of anisotropic deforming materials is one of the great challenges in materials and polymer chemistry. In this paper, we demonstrate a unidirectionally deformable material with reversibility. First, crystal crosslinking of a pillared-layer metal–organic framework (PLMOF) was accomplished, followed by the exchange of the pillar ligand with the monotopic ligand. The obtained crosslinked MOF crystal showed reversible unidirectional compression and expansion during cycles of drying and immersion in good solvents. The macroscopic unidirectional deformation was derived from microscopic variations in the layer distance in the MOF crystal. The polymerization between the organic ligand and the crosslinker effectively reinforced the MOF crystal, which had enough durability for reversible unidirectional deformation. Our strategy is a promising general method for the construction of anisotropic deforming materials, which are often seen in biological systems and mechanical devices. An unidirectionally deformable material with reversibility was achieved. The method relies on crystal crosslinking of pillared-layer metal–organic framework (PLMOF), followed by exchange of the pillar ligand to monotopic ligand. The obtained crosslinked MOF crystal exhibited reversibly unidirectional compression and expansion upon cycles of drying and immersion in good solvents. The preservation of layer structure enabled to confirm the unidirectional deformation not only macroscopically but also microscopically. Our strategy will be a promising general method for construction of anisotropic deforming materials, which can be often seen in biological systems or mechanical devices.

PVP-assisted synthesis of monodisperse UiO-66 crystals with tunable sizes

A polyvinylpyrrolidone (PVP)-assisted synthesis method was reported to prepare monodisperse UiO-66 octahedral crystals with tunable sizes. In this acetic acid/PVP co-modulated synthesis system, the relatively weak coordination of PVP to Zr(IV) played an important role in manipulating the sizes of UiO-66 crystals instead of their shapes. With the optimized PVP concentrations, sizes of the monodisperse UiO-66 crystals could be effectively tuned from ~1900nm to ~700nm. These uniform MOF crystals could be assembled into monolayers on water surface and further transferred onto a solid substrate to yield MOF films with preferential orientation and controlled thicknesses.

Surface modification of PCC filled cellulose paper by MOF-5 (Zn3(BDC)2) metal–organic frameworks for use as soft gas adsorption composite materials

In this paper, composite materials made of cellulose paper and MOF-5 (Zn3(BDC)2) Metal organic frameworks (paper@MOF-5) materials, were prepared by an in situ synthesis method using precipitated calcium carbonate (PCC) filled cellulose papers. PCC fillers in the cellulose paper led to decreased inter-fiber bonds, thus facilitating the formation of small-sized MOF-5 crystals. The resultant cellulose paper@MOF-5 composites exhibited zeolite-like frameworks with high specific surface areas. The composite materials were characterized by XRD, ATR-FTIR, SEM and nitrogen adsorption analysis. The results supported the conclusion that the prepared cellulose paper@MOF-5 composite materials have great potential in gas (N2, CH4, H2, CO2, etc.) adsorption.

Continuous synthesis of UiO-66 in microreactor: Pursuing the optimum between intensified production and structural properties

The increasing demand for MOFs can only be satisfied by establishing robust and reproducible synthesis routes which allow a profitable scale-up of the requested materials while preserving their textural properties and stability. A scalable MOF synthesis may provide a technology platform for the further development and modification of the product, from both a chemical and an engineering perspective. And, above all, MOFs could be delivered to the market at a substantially reduced price.In this work UiO-66 was synthesized continuously by employing micro reactors and the concept of flow chemistry. The microreaction technology (MRT) process turned out to be a viable alternative to the time and energy consuming conventional synthesis processes, as significantly faster reaction rates could be achieved compared to the conventional batch processes (Zhou et al., 2014; Frameworks for Commercial Success, 2016). Microreactors are characterized in particular by high surface-to-volume ratios and channel dimensions in the submillimeter range, which offer a significant enhancement of heat and mass transfer within the reactor, additionally the chemical reactions can be accelerated by orders of magnitude and can thus be processed close to their kinetic optimum conditions.Different reaction parameters were studied in order to elucidate their influence on the product properties. All synthesized MOF crystals were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis and nitrogen adsorption measurements.The MRT approach allowed the continuous fabrication of MOF crystals with distinct morphological characteristics in a time-efficient manner. The obtained results represent a faster and energy-efficient route to continuously produce UiO-66, compared to the standard synthetic protocols. The materials prepared in this work have a lower coordination of the Zr sites in comparison to the UiO-66 that can be produced in batch (Cavka et al. 2008); nevertheless the presence of coordinative unsaturated sites is a preferred feature of MOFs for some specific application such as catalysis (Shearer et al., 2014).

Anisotropically Swelling Gels Attained through Axis‐Dependent Crosslinking of MOF Crystals

AbstractAnisotropically deforming objects have attracted considerable interest for use in molecular machines and artificial muscles. Herein, we focus on a new approach based on the crystal crosslinking of organic ligands in a pillared‐layer metal–organic framework (PLMOF). The approach involves the transformation from crosslinked PLMOF to polymer gels through hydrolysis of the coordination bonds between the organic ligands and metal ions, giving a network polymer that exhibits anisotropic swelling. The anisotropic monomer arrangement in the PLMOF underwent axis‐dependent crosslinking to yield anisotropically swelling gels. Therefore, the crystal crosslinking of MOFs should be a useful method for creating actuators with designable deformation properties.