Archetypal Metal-organic Framework Research Articles

Integrating Biocatalysts into Metal-Organic Frameworks: Disentangling the Roles of Affinity, Molecular Weight, and Size.

The integration of biocatalysts within metal-organic frameworks (MOFs) is attracting growing interest due to its potential to both enhance biocatalyst stability and sustain biocatalyst activity in organic solvents. However, the factors that facilitate the post-synthetic infiltration of such large molecules into MOF pores remain unclear. This systematic study enabled the identification of the influence of biocatalyst molecular size, molecular weight and affinity on the uptake by an archetypal MOF, NU-1000. We analyzed a range of six biocatalysts with molecular weights from 1.9 kDa to 44.4 kDa, respectively. By employing a combination of fluorescence tagging and 3D-STED confocal laser scanning microscopy, we distinguished between biocatalysts that were internalized within the MOF pores and those sterically excluded. The catalytic functions of the biocatalysts hosted within the MOF were investigated and found to show strong variations relative to the solvated case, ranging from a two-fold increase to a strong decrease.

Ligand field tuning of d-orbital energies in metal-organic framework clusters

Linker functionalization is a common route used to affect the electronic and catalytic properties of metal-organic frameworks. By either pre- or post-synthetically installing linkages with differing linker moieties the band gap, workfunction, and exciton lifetimes have been shown to be affected. One overlooked aspect of linker functionalization, however, has been the impact on the metal d-orbital energies to which they are bound. The ligand field differences should result in substantial changes in d-splitting. In this study we use density functional theory (DFT) to study the energetics of d-orbital energy tuning as a function of linker chemistry. We offer a general descriptor, linker pKa, as a tool to predict resultant band energies in metal-organic frameworks (MOFs). Our calculations reveal that simple functionalizations can affect the band energies, of primarily metal d lineage, by up to 2 eV and illustrate the significance of this band modularity using four archetypal MOFs: UiO-66, MIL-125, ZIF-8, and MOF-5. Together, we show that linker functionalization dramatically affects d-energies in MOF clusters and highlight that linker functionalization is a useful route for fine-tuning band edges centered on the metals, rather than linkers themselves.

Controlling the Flexibility of MIL-88A(Sc) Through Synthetic Optimisation and Postsynthetic Halogenation.

Breathing behaviour in metal‐organic frameworks (MOFs), the distinctive transformation between a porous phase and a less (or non) porous phase, often controls the uptake of guest molecules, endowing flexible MOFs with highly selective gas adsorptive properties. In highly flexible topologies, breathing can be tuned by linker modification, which is typically achieved pre‐synthetically using functionalised linkers. Herein, it was shown that MIL‐88A(Sc) exhibits the characteristic flexibility of its topology, which can be tuned by 1) modifying synthetic conditions to yield a formate‐buttressed analogue that is rigid and porous; and 2) postsynthetic bromination across the alkene functionality of the fumarate ligand, generating a product that is rigid but non‐porous. In addition to providing different methodologies for tuning the flexibility and breathing behaviour of this archetypal MOF, it was shown that bromination of the formate‐bridged analogue results in an identical material, representing a rare example of two different MOFs being postsynthetically converted to the same end product.

Polymeric Membranes Containing Iodine-Loaded UiO-66 Nanoparticles as Water-Responsive Antibacterial and Antiviral Surfaces

The fight against pathogenic bacteria and viruses represents a challenging task requiring the development of innovative materials. The design of water-responsive disinfecting surfaces constitutes a pertinent endeavor to limit the spread of infectious pathogens that strive in wet environments and are often carried by the droplets or aerosols of biological fluids. In this context, we designed a polymeric nanostructured membrane which, when in contact with water, was able to release elemental iodine, a potent antimicrobial agent. The membrane was based on poly(vinylidene fluoride-co-hexafluoropropylene) electrospun nanofibers containing nanoparticles of an archetypal metal–organic framework (MOF), UiO-66. The gas adsorption capacity of the MOF container was preserved upon incorporation into the polymeric nanofibers, which also exerted a protective effect against the fast structural collapse of UiO-66 in phosphate-buffered saline, a model for biological fluids. The membrane loaded high amounts of iodine via gas diffusion and its release was (mostly) triggered by contact with the aqueous medium. The antibacterial activity of the membrane was tested against the Escherichia coli strain DH5α and revealed prompt and robust disinfecting properties. The membrane also efficiently inhibited the infectivity of viral model vesicular stomatitis virus glycoprotein pseudotyped HIV-1 particles in HEK-293 cells in a short time. The observed synergistic effects between the MOF container and the polymeric support material constitute attractive features for the development of MOF-based materials for biological applications.

A Decade of UiO-66 Research: A Historic Review of Dynamic Structure, Synthesis Mechanisms, and Characterization Techniques of an Archetypal Metal–Organic Framework

UiO-66 is an archetypal metal–organic framework (MOF) with a very high surface area as well as high thermal stability. It is found that the stability can be attributed to the metal oxide node being...

Electroactive Ferrocene at or near the Surface of Metal-Organic Framework UiO-66.

Here, we describe the installation of a ferrocene derivative on and within the archetypal metal-organic framework (MOF), UiO-66, by solvent-assisted ligand incorporation. Thin films of the resulting material show a redox peak characteristic of the Fc/Fc+ couple, as measured by cyclic voltammetry. Consistent with restriction of redox reactivity solely to Fc molecules sited at or near the external surfaces of MOF crystallites, chronoamperometry measurements indicate that less than 20% of the installed Fc molecules are electrochemically active. Charge-transport diffusion coefficients, DCT, of 6.1 ± 0.8 × 10-11 and 2.6 ± 0.2 × 10-9 cm2/s were determined from potential step measurements, stepping oxidatively and reductively, respectively. The 40-fold difference in DCT values contrasts with the expectation, for simple systems, of identical values for oxidation-driven versus reduction-driven charge transport. The findings have implications for the design of MOFs suitable for delivery of redox equivalents to framework-immobilized electrocatalysts and/or delivery of charges from a chromophoric MOF film to an underlying electrode, processes that may be central to MOF-facilitated conversion of solar energy to chemical or electrical energy.

Molecular simulations of self-assembly processes in metal-organic frameworks: Model dependence.

Molecular simulation is a powerful tool for investigating microscopic behavior in various chemical systems, where the use of suitable models is critical to successfully reproduce the structural and dynamic properties of the real systems of interest. In this context, molecular dynamics simulation studies of self-assembly processes in metal-organic frameworks (MOFs), a well-known class of porous materials with interesting chemical and physical properties, are relatively challenging, where a reasonably accurate representation of metal-ligand interactions is anticipated to play an important role. In the current study, we both investigate the performance of some existing models and introduce and test new models to help explore the self-assembly in an archetypal Zn-carboxylate MOF system. To this end, the behavior of six different Zn-ion models, three solvent models, and two ligand models was examined and validated against key experimental structural parameters. To explore longer time scale ordering events during MOF self-assembly via explicit solvent simulations, it is necessary to identify a suitable combination of simplified model components representing metal ions, organic ligands, and solvent molecules. It was observed that an extended cationic dummy atom (ECDA) Zn-ion model combined with an all-atom carboxylate ligand model and a simple dipolar solvent model can reproduce characteristic experimental structures for the archetypal MOF system. The successful use of these models in extensive sets of molecular simulations, which provide key insights into the self-assembly mechanism of this archetypal MOF system occurring during the early stages of this process, has been very recently reported.

In Situ Monitoring of the Mechanosynthesis of the Archetypal Metal-Organic Framework HKUST-1: Effect of Liquid Additives on the Milling Reactivity.

We have applied in situ monitoring of mechanochemical reactions by high-energy synchrotron powder X-ray diffraction to study the role of liquid additives on the mechanochemical synthesis of the archetypal metal-organic framework (MOF) HKUST-1, which was one of the first and is still among the most widely investigated MOF materials to be synthesized by solvent-free procedures. It is shown here how the kinetics and mechanisms of the mechanochemical synthesis of HKUST-1 can be influenced by milling conditions and additives, yielding on occasion two new and previously undetected intermediate phases containing a mononuclear copper core, and that finally rearrange to form the HKUST-1 architecture. On the basis of in situ data, we were able to tune and direct the milling reactions toward the formation of these intermediates, which were isolated and characterized by spectroscopic and structural means and their magnetic properties compared to those of HKUST-1. The results have shown that despite the relatively large breadth of analysis available for such widely investigated materials as HKUST-1, in situ monitoring of milling reactions can help in the detection and isolation of new materials and to establish efficient reaction conditions for the mechanochemical synthesis of porous MOFs.

Dihydrolevoglucosenone (Cyrene) As a Green Alternative to N,N-Dimethylformamide (DMF) in MOF Synthesis

Cyrene, a green bioderived solvent from waste cellulose, was applied into the synthesis of metal–organic frameworks (MOFs). The MOF, HKUST-1, exhibited an even better Brunauer–Emmett–Teller surface area than HKUST-1 synthesized with DMF. Four additional archetypal MOFs were also synthesized to verify the universal application of Cyrene in MOF manufacturing. However, the BET surface areas were lower than DMF-made MOFs. It was observed that an aldol-condensation product of two Cyrene molecules in addition to Cyrene trapped within the pores are responsible for the lower than expected surface area. The use of Cyrene has led to a series of design principles that eliminate the need for problematic solvents such as DMF and can be applied to the synthesis of a wide range of MOFs.

In situ X-ray diffraction monitoring of a mechanochemical reaction reveals a unique topology metal-organic framework.

Chemical and physical transformations by milling are attracting enormous interest for their ability to access new materials and clean reactivity, and are central to a number of core industries, from mineral processing to pharmaceutical manufacturing. While continuous mechanical stress during milling is thought to create an environment supporting nonconventional reactivity and exotic intermediates, such speculations have remained without proof. Here we use in situ, real-time powder X-ray diffraction monitoring to discover and capture a metastable, novel-topology intermediate of a mechanochemical transformation. Monitoring the mechanochemical synthesis of an archetypal metal-organic framework ZIF-8 by in situ powder X-ray diffraction reveals unexpected amorphization, and on further milling recrystallization into a non-porous material via a metastable intermediate based on a previously unreported topology, herein named katsenite (kat). The discovery of this phase and topology provides direct evidence that milling transformations can involve short-lived, structurally unusual phases not yet accessed by conventional chemistry.

Surface Termination of the Metal-Organic Framework HKUST-1: A Theoretical Investigation.

The surface morphology and termination of metal-organic frameworks (MOF) is of critical importance in many applications, but the surface properties of these soft materials are conceptually different from those of other materials like metal or oxide surfaces. Up to now, experimental investigations are scarce and theoretical simulations have focused on the bulk properties. The possible surface structure of the archetypal MOF HKUST-1 is investigated by a first-principles derived force field in combination with DFT calculations of model systems. The computed surface energies correctly predict the [111] surface to be most stable and allow us to obtain an unprecedented atomistic picture of the surface termination. Entropic factors are identified to determine the preferred surface termination and to be the driving force for the MOF growth. On the basis of this, reported strategies like employing "modulators" during the synthesis to tailor the crystal morphology are discussed.

On the Electrochemical Deposition Mechanism of Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs) are one of the three most rapidly growing fields in materials science, booming from a few papers per year in 2001 to more than 2000 in 2011 [1]. These coordination polymers are normally synthesized solvothermally mixing a metal salt with the chosen linker and dwelling the mixture at high temperatures; but a new technique patented in 2007 [2] opened the possibility to synthesize them electrochemically in milder conditions. Afterwards it was discovered that this technique can also yield to adherent MOF layers if the right parameters are chosen [3], opening many new possibilities. In fact, in response to that article, a proliferation of others followed, both on the application of MOF layers electrochemically synthesized and on the synthesis of new MOF layers [4].Unfortunately up to now, even if the literature is growing, only a few parameters of the electrosynthesis have been studied, and the understanding of how this anodic deposition process works and what is the cause of the adhesion or lack of adhesion of the MOF, are still largely unknown. Needless to say, this understanding is indispensable to synthesize layers with higher quality and made by the most interesting MOFs. In our study we took HKUST-1 as an archetypal MOF and we made use of several techniques: Quartz Crystal Microbalance, Scanning Electron Microscopy, Electric Impedance and other electrochemical techniques, to explain how the synthesis of MOF layers works and what are the main reasons for the deposition of these crystals on the electrode surface. Based on new experimental data and on the experience gained in earlier works, we propose a growth mechanism for electrochemically synthesized copper(II) MOFs layers divided in four phases, which are summarized in the selected SEM pictures of Figure 1.Phase I: Nucleation. At the anode, when the potential is applied, copper(II) ions are released in solution where the linker is present. When the critical concentration of reagents is reached, the nucleation starts in solution and on the electrode surface. Similarly to what has been observed also for phosphatation [5], this phase is characterized by a well-defined inversion of trend in the QCM plots due to the initial copper loss, followed by the gain in mass due to the precipitation of HKUST-1 (Figure 1d). We observed that the nucleated crystals act as nucleation sites for new crystals and, even if the deposition was stopped for several minutes and the copper(II) ions diffused from the surface, it was not necessary to reach the critical concentration again, but the deposition started directly.Phase II: Growth of islands. After the first crystals are nucleated, new ones grow attached to the surface of the first, either next and on top of them. This phase yields therefore to zones completely covered with crystals and others where almost no crystals can be found.Phase III: Intergrowth. With time, the nucleation and growth of crystals continues covering all the available surface. This is maybe the most important phase since it can yield to a closed layer made by intergrown crystals. Being a porous material, the growth of MOF crystals does not stop the release of copper, but it just hampers it, and this leads to a steadily increase in the potential (see Figure 1d), and to a jump in the impedance response of the layer.Phase IV: Detachment. This phase corresponds to the detachment of some crystals from the synthesized layer caused by their undercut. The dissolution of the substrate, especially at the grain boundaries of the metal, removes the anchoring sites of the MOF crystals that are therefore released from the surface, leaving empty sites behind. During this phase, the impedance signal fluctuates between the high values characteristic of the close layers and the lower values due to the partially MOF-free surface.The previously described phases and other insights have been found which answer the scientific curiosity for a previously not well understood mechanism, and can be helpful to solve technological problems connected with this process applied to the studied MOFs and to new ones.[1] J. Adams and D. Pendlebury, global research report materials science and technology, June 2011, Reuters.[2] U. Mueller, US Patent 2007/0227898 A1 (2007)[3] R. Ameloot et al., Chem. Mater.,21 (2009) 2580[4] N. Campagnol et al., J. Mater. Chem. A, 1 (2013) 5827[5] K. M. Ogle et al., J. Electrochem. Soc.,141 (1994) 2655Figure 1 a, b, and c: SEM pictures taken at 75° angle of the four phases: Nucleation, Growth of island, and Intergrowth. Cu wafer substrate, 2V vs counter, after 10 s, 10 min, 60 min and 125 min. Figure 1 d: QCM plots of HKUST-1 deposition at different current densities (in the inset the voltage relative to each deposition).

Post consumer PET waste as potential feedstock for metal organic frameworks

The purity requirement of terephthalic acid (TPA) for synthesis of metal organic frameworks (MOFs) is very stringent. In this paper, we propose a simple and economically viable microwave-assisted process to convert poly(ethylene terephthalate) (PET) waste into high purity TPA which in turn has been used for the synthesis of two archetypal MOFs. The efficiency of this process was compared with conventional heating route, and characteristic parameters like conversion and TPA purity were quantified. The time required for alkalolytic depolymerization could be significantly reduced (~5min) by using microwave as an energy source, as compared to the conventional thermal process, which requires a minimum of 3h to reach the same level. The reaction of the purified TPA with zinc acetate and copper nitrate led to formation of MOF 5 and copper-terephthalate respectively, which were characterized by PXRD, FTIR, TGA and SEM analysis.

Mail-Order Metal–Organic Frameworks (MOFs): Designing Isoreticular MOF-5 Analogues Comprising Commercially Available Organic Molecules

Metal–organic frameworks (MOFs), a class of porous materials, are of particular interest in gas storage and separation applications due largely to their high internal surface areas and tunable structures. MOF-5 is perhaps the archetypal MOF; in particular, many isoreticular analogues of MOF-5 have been synthesized, comprising alternative dicarboxylic acid ligands. In this contribution we introduce a new set of hypothesized MOF-5 analogues, constructed from commercially available organic molecules. We describe our automated procedure for hypothetical MOF design, comprising selection of appropriate ligands, construction of 3D structure models, and structure relaxation methods. 116 MOF-5 analogues were designed and characterized in terms of geometric properties and simulated methane uptake at conditions relevant to vehicular storage applications. A strength of the presented approach is that all of the hypothesized MOFs are designed to be synthesizable utilizing ligands purchasable online.

Correction to “Photoresponse Characteristics of Archetypal Metal–Organic Frameworks”

Correction to “Photoresponse Characteristics of Archetypal Metal–Organic Frameworks”

Photoresponse Characteristics of Archetypal Metal–Organic Frameworks

The photoelectrochemical responses of two archetypal metal–organic frameworks (MOFs), MOF-5 and MOF-177, have been assessed. Films of MOF-5 and MOF-177 were grown on carboxylic-acid-terminated conductive fluorine-doped tin oxide substrates. Separate analyses by powder X-ray diffraction, Raman spectroscopy, and fluorescence spectroscopy collectively indicated these films prepared via a solvothermal method in diethylformamide were free of residual impurities such as ZnO clusters and residual organics. Exposure of these films to white light illumination while immersed in acetonitrile electrolytes elicited measurable photocurrents. Wavelength-dependent analysis of the photoresponses showed that the measured photocurrents were induced by ultraviolet light and that the spectral response profiles followed closely the light absorption profiles of each respective material. Attenuation of the induced photocurrents was noted after prolonged ultraviolet light illumination and/or exposure of the films to H2O(l), indicating that the observed photoresponse properties are directly related to the structural integrity of these MOFs. The cumulative data illustrate that such MOFs have innately light-sensitive properties that are atypical in high surface area materials.

Fast Synthesis of MOF-5 Microcrystals Using Sol−Gel SiO2 Nanoparticles

The work reports on the synthesis of an archetypal metal organic framework (MOF-5) microcrystals with a narrow size distribution using SiO2 nanoparticles with tailored surface chemistry as nucleating agents. The nanoparticles boost the reaction rate by up to an order of magnitude compared to the conventional MOF-5 solvothermal synthesis and can be successfully used as nucleation seeds for the selective growth of MOF-5 on specific substrates. These results are important fundamental advances toward the controlled scale-up of MOF synthesis and directed MOF growth on suitable supports.