Many Metal-organic Frameworks Research Articles

Nanoscopic Insights into the Collapse of Metal-Organic Frameworks during Solvent Evacuation: Molecular Simulation Investigation.

Many metal-organic frameworks (MOFs) undergo structural collapse upon solvent evacuation during activation, which is attributed to the capillary force generated by the solvent. However, little effort has been devoted to unveiling the nature of such a force. Herein, we employ molecular dynamics (MD) simulations to investigate the evacuation of different solvents in two MOFs (MOF-5 and UMCM-9). The contractive stress induced by solvent evacuation is quantified and unraveled to positively correlate with the surface tension of the solvent. Moreover, the mechanical strength (or amorphization) of the MOF is calculated using reactive MD simulations. By comparing the contractive stress with the amorphization stress, for the first time, we predict the likelihood of collapse of MOFs during activation by different solvents, which agrees well with the experiments. The methodology developed provides nanoscopic insights into the activation process; it can assist in avoiding structural collapse by judiciously selecting a proper solvent for activation or by modifying a framework.

Chemistry of Formate and Water Ligands on Metal Oxide Cluster Nodes of Metal-Organic Framework hcp Hf-UiO-66: Keys to Understanding Reactivity of Paired μ2-OH and Defect Sites.

Many metal-organic frameworks (MOFs) incorporate nodes that are metal oxide clusters, and ligands that have been observed on these nodes include formates, acetates, water, hydroxyl groups, and others, all of which are potentially important in affecting reactivities for applications in separations, catalysis, and sensing. Formate is a common node ligand, arising from formic acid used as a modulator and from N,N-dimethylformamide used as a solvent in MOF syntheses. Yet only little work has been reported characterizing the reactivities of node formate ligands. Infrared spectra reported here show that formate bonds to two types of sites on the paired Hf6O8 nodes of hcp UiO-66, namely, defect and μ2-OH sites. Quantifying the number of formate ligands by 1H NMR spectroscopy of digested samples showed an almost equal number of formate ligands on the two sites, indicating the likelihood that they neighbor each other. These formate ligands interact with water molecules, reversibly switching their bonding from bidentate to monodentate. The formates on μ2-OH sites of hcp Hf-UiO-66 interact much more strongly with water than those on defect sites of the same node, and both interact more strongly than isolated defect sites of Hf-UiO-66. Correspondingly, the catalytic activities of hcp UiO-66 determined as turnover frequencies on each site are approximately twofold higher than those on UiO-66, bolstering the inference that methanol dehydration is catalyzed by a node defect site and a neighboring node μ2-OH site. The results show how MOFs, with their well-defined node structures, provide unprecedented opportunities to understand details of reactivities and catalysis on metal oxide clusters, in contrast to bulk metal oxide surfaces.

Tunable Low-Pressure Water Adsorption in Stable Multivariate Metal-Organic Frameworks for Boosting Water-Based Ultralow-Temperature-Driven Refrigeration.

The green water-based adsorption refrigeration is considered as a promising strategy to realize near-zero-carbon cooling applications. Although many metal-organic frameworks (MOFs) have been developed as water adsorbents, their cooling performance are commonly limited by the insufficient water uptakes below P/P0 = 0.2. Herein, the development of multivariate MOFs (MTV-MOFs) is reported to highly modulate and boost the low-pressure water uptake for improving coefficient of performance (COP) for refrigeration. Through ligand exchange in the pristine MIL-125-NH2 , a series of MTV-MOFs with bare nitrogen sites are designed and synthesized. The resulting MIL-125-NH2 /MD-5% exhibits the significantly improved water uptake of 0.39g g-1 at 298 K and P/P0 = 0.2, which is three times higher than MIL-125-NH2 (0.12g g-1 ) and comparable to some benchmark materials including KMF-1 (0.4g g-1 ) and MIP-200 (0.36g g-1 ). Combined with its low-temperature regeneration, fast sorption kinetics and high stability, MIL-125-NH2 /MD-5% achieves one of the highest COP values (0.8) and working capacities (0.24g g-1 ) for refrig-2 under an ultralow-driven temperature of 65°C, which are higher than some best-performing MOFs such as MIP-200 (0.74 and 0.11g g-1 ) and KMF-2 (0.62 and 0.16g g-1 ), making it among the best adsorbents for efficient ultralow-temperature-driven refrigeration.

Closer Look at Adsorption of Sarin and Simulants on Metal–Organic Frameworks

The development of effective protection against exposure to chemical warfare agents (CWAs), such as sarin, relies on studies of its adsorption on the capturing materials and seeking candidates capable of adsorbing large amounts of sarin gas. Many metal-organic frameworks (MOFs) are promising materials for the effective capture and degradation of sarin and simulant substances. Among the simulants capable of mimicking thermodynamic properties of the agent, not all of them have been investigated on the ability to act similarly in the adsorption process, in particular, whether the agent and a simulant have similar mechanisms of binding to the MOF surface. Molecular simulation studies not only provide a safe way to investigate the aforementioned processes but can also help reveal the mechanisms of interactions between the adsorbents and the adsorbing compounds at the molecular level. We performed Monte Carlo simulations of the adsorption of sarin and three simulants, dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), and diisopropyl fluorophosphate (DIFP), on selected MOFs that have previously shown strong capabilities to adsorb sarin. On the basis of the calculated adsorption isotherms, enthalpy of adsorption, and radial distribution functions, we revealed common mechanisms among the particularly efficient adsorbents as well as the ability of simulants to mimic them. The findings can help in selecting a suitable simulant compound to study CWA adsorption on MOFs and guide further synthesis of efficient MOFs for the capture of organophosphorus compounds.

Insights and Heuristics for Predicting Diffusion Rates of Chemical Warfare Agents in Zirconium Metal-Organic Frameworks.

Designing nanoporous catalysts to destroy chemical warfare agents (CWAs) and environmental contaminants requires consideration of both intrinsic catalytic activity and the mass transfer of molecules in and out of the pores. Polar adsorbates such as CWAs experience a heterogeneous environment in many metal-organic frameworks (MOFs) due to the arrangement of the metal nodes and organic linkers of the MOF. However, quantitative relationships between the pore architecture and the resulting diffusion properties of polar molecules have not been established. We used molecular dynamics simulations to calculate the diffusion coefficients of the CWA simulant dimethyl methyl phosphonate (DMMP) in a diverse set of 776 MOFs with Zr6 nodes. We developed a 4-parameter machine learning model to predict DMMP diffusivities in Zr6 MOFs and found the model to be transferable to the CWA sarin. We then developed a simplified heuristic based on the machine learning model that the node-node distance and accessible surface area should be maximized to find MOFs with rapid CWA diffusion.

Understanding the Structural Collapse during Activation of Metal-Organic Frameworks with Copper Paddlewheels.

Many metal-organic frameworks (MOFs) suffer from stability issues as they can be easily amorphized from various external stimuli. In particular, it is common to observe structural collapse during the activation process of removing the synthesis solvent. In this study, we conduct high-throughput computational analysis that focuses on the activation status of MOFs that possess copper paddlewheel metal nodes. From the analysis, various mechanical properties (e.g., bulk, Young's, and shear moduli) were found to be good predictors for collapse. Furthermore, we have identified anomaly MOFs with good mechanical stability that were previously reported to collapse. Accordingly, the activation process was reattempted with improved techniques, and one of these MOFs was successfully activated.

Prediction of coexistence of anomalous valley Hall and quantum anomalous Hall effects in breathing kagome-honeycomb lattices

To explore the behaviors of valley degrees of freedom, we built a breathing kagome-honeycomb (BKH) lattice composed of a breathing kagome sublattice (BKL) and a breathing honeycomb sublattice (BHL). Many metal-organic frameworks (MOFs) without space-inversion symmetry (SIS) can be modeled as BKH lattices. The electronic states and the evolution of Dirac cones of the constructed BKH lattice under magnetic exchange and spin-orbit coupling interactions were investigated based on a tight-binding model and first-principles calculations. Coexistence states of anomalous valley Hall (AVH) and quantum anomalous Hall (QAH) effects, namely, the multiple Hall effects, were achieved in the BKH lattice with ${p}_{z}$ orbitals located at the BKL and ${p}_{x}$, ${p}_{y}$ (and also ${d}_{xy}$, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$) orbitals located at the BHL. The properties of the coexisting state, including the valley polarized status (at $K$ or ${K}^{\ensuremath{'}}$) and the chirality of the QAH effect, were determined by the overlapping styles of the bands from the two sublattices. An MOF of ${\mathrm{C}}_{6}{\mathrm{N}}_{3}{\mathrm{H}}_{3}\mathrm{Au}$ with a BKH lattice is proposed. Given the coexistence of the AVH and QAH effects in this built MOF, the metallic edge state protected by the topology of the MOF is 100% valley polarized, different from the ordinary QAH effect. Our results provide insights into the electronic states of the MOFs without SIS and a material platform with potential applications in not only valleytronics but also electronics and spintronics.

Efficient elimination of organic contaminants with novel and stable zeolite@MOF layer adsorbents

Many metal–organic frameworks (MOFs) trapped in water exhibit instability and small-particle agglomeration issues, which unquestionably constrain their potential applications, such as the capture of organic contaminants (OCs). In this study, four types of micron-sized MOFs (Zn/Cu-BTC, MOF-5, ZIF-8, and UiO-66) were grown within a zeolite-13X support to form millimeter-sized zeolite-13X@MOF composites for the elimination of benzothiophene, methyl orange, and tetracycline from the liquid phase by dynamic adsorption in a column. We observed that the 13X@Zn/Cu-BTC exhibited extraordinarily high OC capture capacities as a result of the Zn2+ and Cu2+ combinative effects of the acid–base interaction. Remarkably, the 13X@UiO-66 preserved its structural integrity when immersed in water for 15 days, in contact with boiling water for 12h, and in both strong acidic and basic aqueous media. Moreover, the OC capture abilities of the 13X@UiO-66 only underwent a slight change after the fifth round. This work provides new method for the design of desirable millimeter-sized zeolite@MOFs, thereby advancing their practical application for OC capture.

Transient Catenation in a Zirconium-Based Metal-Organic Framework and Its Effect on Mechanical Stability and Sorption Properties.

Interpenetration of two or more sublattices is common among many metal-organic frameworks (MOFs). Herein, we study the evolution of one zirconium cluster-based, 3,8-connected MOF from its non-interpenetrated (NU-1200) to interpenetrated (STA-26) isomer. We observe this transient catenation process indirectly using ensemble methods, such as nitrogen porosimetry and X-ray diffraction, and directly, using high-resolution transmission electron microscopy. The approach detailed here will serve as a template for other researchers to monitor the interpenetration of their MOF samples at the bulk and single-particle limits. We investigate the mechanical stability of both lattices experimentally by pressurized in situ X-ray diffraction and nanoindentation as well as computationally with density functional theory calculations. Both lines of study reveal that STA-26 is considerably more mechanically stable than NU-1200. We conclude this study by demonstrating the potential of these MOFs and their mixed phases for the capture of gaseous n-hexane, used as a structural mimic for the chemical warfare agent sulfur mustard gas.

Dialing in Catalytic Sites on Metal Organic Framework Nodes: MIL-53(Al) and MIL-68(Al) Probed with Methanol Dehydration Catalysis.

Many metal organic frameworks (MOFs) incorporate metal oxide clusters as nodes. Node sites where linkers are missing can be catalytic sites. We now show how to dial in the number and occupancy of such sites in MIL-53 and MIL-68, which incorporate aluminum-oxide-like nodes. The methods involve modulators used in synthesis and postsynthesis reactions to control the modulator-derived groups on these sites. We illustrate the methods using formic acid as a modulator, giving formate ligands on the sites, and these can be removed to leave μ2-OH groups and open Lewis acid sites. Methanol dehydration was used as a catalytic reaction to probe these sites, with infrared spectra giving evidence of methoxide ligands as reaction intermediates. Control of node surface chemistry opens the door for placement of a variety of ligands on a wide range of metal oxide cluster nodes for dialing in reactivity and catalytic properties of a potentially immense class of structurally well-defined materials.

The Surface Chemistry of Metal Oxide Clusters: From Metal-Organic Frameworks to Minerals.

Many metal–organic frameworks (MOFs) incorporate nodes that are small metal oxide clusters. Some of these MOFs are stable at high temperatures, offering good prospects as catalysts—prospects that focus attention on their defect sites and reactivities—all part of a broader subject: the surface chemistry of metal oxide clusters, illustrated here for MOF nodes and for polyoxocations and polyoxoanions. Ligands on MOF defect sites form during synthesis and are central to the understanding and control of MOF reactivity. Reactions of alcohols are illustrative probes of Zr6O8 node defects in UiO-66, characterized by the interconversions of formate, methoxy, hydroxy, and linker carboxylate ligands and by catalysis of alcohol dehydration reactions. We posit that new reactivities of MOF nodes will emerge from incorporation of a wide range of groups on their surfaces and from targeted substitutions of metals within them.

Design and Fabrication of a Novel Water- and Heat-Resistant Cu-Based Coordination Compound for Dibenzothiophene Capture

Many metal–organic frameworks (MOFs) suffer from hydrothermal instability issues, which definitely restricted their practical applications, such as deep desulfurization. In the present contribution...

Solvent-Driven Reversible Phase Transition of a Pillared Metal-Organic Framework.

Over the last decade, the controllable reversible phase transition of functional materials has received growing interest as it shows unique suitability for various technological applications. Although many metal-organic frameworks (MOFs) possess a lamellar structure, the reversible structural transformation of MOFs between their three-dimensional (3D) phase and two-dimensional (2D) phase remains a largely unexplored area. Herein, we report for the first time a europium MOF with unprecedented reversible morphology in different solvents at room temperature. This europium MOF displayed a 3D nanorod morphology in organic solvent and a 2D nanobelt architecture in water. As a proof of concept for potential applications of this reversible-phase-transition MOF, we were able to use a delamination recovery method to load dye molecules that previously could not be loaded into europium MOFs.

Little Thermodynamic Penalty for the Synthesis of Ultraporous Metal Organic Frameworks.

Many metal-organic frameworks (MOFs) of ultrahigh porosity (with molar volumes more than ten times greater than those of the corresponding dense phases) have been synthesized. However, the number of possible structures far exceeds those that have been made. It is logical to ask if there are energetic barriers to the stability of ultraporous MOFs or whether there is little thermodynamic penalty to their formation. Herein, we show that although the molar volumes of MOF-177 and UMCM-1 reach ultrahigh values, their energetic metastability is in the same range (of 7-36 kJ mol(-1)) as that seen previously for other porous materials. These findings suggest that there is little thermodynamic penalty for the synthesis of structures with varying porosity, and hence, ultraporous frameworks are energetically accessible. Therefore, innovative synthesis methods have the possibility to overcome the drawbacks of conventional approaches and greatly extend the number, porosity, and properties of new framework materials.

The direct heat measurement of mechanical energy storage metal-organic frameworks.

In any process, the heat exchanged is an essential property required in its development. Whilst the work related to structural transitions of some flexible metal-organic frameworks (MOFs) has been quantified and linked with potential applications such as molecular springs or shock absorbers, the heat related to such transitions has never been directly measured. This has now been carried out with MIL-53(Al) using specifically devised calorimetry experiments. We project the importance of these heats in devices such as molecular springs or dampers.

A facile and general coating approach to moisture/water-resistant metal-organic frameworks with intact porosity.

The moisture sensitivity of many metal-organic frameworks (MOFs) poses a critical issue for their large-scale real application. One of the most effective methods to solve this problem is to convert the surface of MOFs from hydrophilic to hydrophobic. Herein, we develop a general strategy to modify hydrophobic polydimethysiloxane (PDMS) on the surface of MOF materials to significantly enhance their moisture or water resistance by a facile vapor deposition technique. MOF-5, HKUST-1, and ZnBT as representative vulnerable MOFs were successfully coated by PDMS, and these coated samples well inherited their original crystalline nature and pore characteristics. Strikingly, the surface areas of these MOFs were nearly 100% retained upon PDMS-coating. Such a coating process might render MOFs applicable in the presence of water or humidity in extended fields such as gas sorption and catalysis.

Idiosyncrasies of Co2(dhtp): In situ-annealing by methanol

Structural instability in humid surroundings is one of the major drawbacks of many metal organic frameworks (MOFs) compared to zeolites. In this study we investigate the pronounced sensitivity of MOF Co2(dhtp) (Co-MOF-74 or CPO-27-Co) towards humid air. Guest uptake in big Co2(dhtp) crystals has been studied by IR microscopy. We found that already a short exposure of 30s to moist air causes a surface blockage, completely preventing the adsorption of even small guest molecules. It could be confirmed in complementary PXRD measurements that the interior of the crystals remains intact during such short exposure times. Indications for a loss of crystallinity were evidenced only for notably longer exposure times (4h). Most remarkably, the defects could at least be partially annealed in methanol atmosphere. Reversible adsorption of hydrocarbons in initially blocked crystals was found after exposing the crystals to methanol vapor for about 0.5h.

Dimerization of a Metal Complex through Thermally Induced Single‐Crystal‐to‐Single‐Crystal Transformation or Mechanochemical Reaction

Single-crystal-to-single-crystal (SCSC) transformations, which involve cooperative movement of atoms in the solid state, have received much attention from chemists. Through SCSC transformation, new complexes, which may not be obtained under conventional conditions, can be obtained in high yield. The SCSC transformation was first reported as a photoreaction in organic complexes, which could be considered as a new organic synthetic method. Solid-state photochemical [2+2] cycloadditions of organic molecules have been well studied. Recently, SCSC transformation has been extended to other complexes, such as metal complexes and coordination polymers (or metal–organic frameworks). In the past decades, many metal–organic frameworks (MOFs) that undergo SCSC transformation have been reported. Most of these reports describe the sliding of layers or breathing of 3D porous MOFs through solvent exchange. SCSC transformations involving breaking and forming bonds in the solid state remain quite rare. The MacGillivray and Foxman groups attained considerable success in solid-state photodimerization of organic groups mediated by metals. Converting a finite metal complex into an infinite coordination network was also reported by the groups of MacGillivray and Vittal. The reported topochemical dimerizations of metal complexes involve the organization of two double or triple bonds in the solid state: the geometric criteria are very important for such SCSC reactions. However, the coordination geometry of the metal ion remains unchanged in these topochemical dimerizations. More recently, zur Loye and co-workers described a 3D cobalt–organic framework exhibiting reversible shrinkage and expansion involving a change in the cobalt coordination geometry through SCSC transformation induced by the removal and re-absorption of guest molecules. Studies on the change in the metal coordination geometry caused by the removal and addition of ligands from the framework itself through SCSC transformation remain less common. Herein, we describe a new type of SCSC dimerization of a mononuclear copper complex, specifically, thermally induced head-to-tail dimerization involving the formation of a new Cu Oligand bond and a change in the metal coordination geometry. The transformation induced a color change from blue to green, which was caused by the change in the copper coordination geometry (Scheme 1). Interestingly, the trans-