Evidence for a chemical clock in oscillatory formation of UiO-66

Chemical clocks are often used as exciting classroom experiments, where an induction time is followed by rapidly changing colours that expose oscillating concentration patterns. This type of reaction belongs to a class of nonlinear chemical kinetics also linked to chaos, wave propagation and Turing patterns. Despite its vastness in occurrence and applicability, the clock reaction is only well understood for liquid-state processes. Here we report a chemical clock reaction, in which a solidifying entity, metal–organic framework UiO-66, displays oscillations in crystal dimension and number, as shown by X-ray scattering. In rationalizing this result, we introduce a computational approach, the metal–organic molecular orbital methodology, to pinpoint interaction between the tectonic building blocks that construct the metal–organic framework material. In this way, we show that hydrochloric acid plays the role of autocatalyst, bridging separate processes of condensation and crystallization.

Reactions with non-linear kinetics, such as chemical clocks, are reasonably common but only well understood in the liquid phase. Here, the authors report and rationalize a chemical clock reaction taking place in a solidifying metal-organic framework.

Encapsulation of redox polysulphides via chemical interaction with nitrogen atoms in the organic linkers of metal-organic framework nanocrystals

Lithium polysulphides generated during discharge in the cathode of a lithium-sulphur redox cell are important, but their dissolution into the electrolyte from the cathode during each redox cycle leads to a shortened cycle life. Herein, we use in situ spectroelectrochemical measurements to demonstrate that sp² nitrogen atoms in the organic linkers of nanocrystalline metal-organic framework-867 (nMOF-867) are able to encapsulate lithium polysulphides inside the microcages of nMOF-867, thus helping to prevent their dissolution into the electrolyte during discharge/charge cycles. This encapsulation mechanism of lithiated/delithiated polysulphides was further confirmed by observations of shifted FTIR spectra for the C = N and C-N bonds, the XPS spectra for the Li-N bonds from nMOF-867, and a visualization method, demonstrating that nMOF-867 prevents lithium polysulphides from being dissolved in the electrolyte. Indeed, a cathode fabricated using nMOF-867 exhibited excellent capacity retention over a long cycle life of 500 discharge/charge cycles, with a capacity loss of approximately 0.027% per cycle from a discharge capacity of 788 mAh/g at a high current rate of 835 mA/g.

Free Energy of Ligand Removal in the Metal-Organic Framework UiO-66

We report an investigation of the "missing-linker phenomenon" in the Zr-based metal-organic framework UiO-66 using atomistic force field and quantum chemical methods. For a vacant benzene dicarboxylate ligand, the lowest energy charge-capping mechanism involves acetic acid or Cl^-/H₂O. The calculated defect free energy of formation is remarkably low, consistent with the high defect concentrations reported experimentally. A dynamic structural instability is identified for certain higher defect concentrations. In addition to the changes in material properties upon defect formation, we assess the formation of molecular aggregates, which provide an additional driving force for ligand loss. These results are expected to be of relevance to a wide range of metal-organic frameworks.

Electronic origins of photocatalytic activity in d0 metal organic frameworks

Metal-organic frameworks (MOFs) containing d⁰ metals such as NH₂-MIL-125(Ti), NH₂-UiO-66(Zr) and NH₂-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH₂-MIL-125(Ti) promote a long lived ligand-to-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π* orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.

Stereoselective Halogenation of Integral Unsaturated C-C Bonds in Chemically and Mechanically Robust Zr and Hf MOFs

Metal-organic frameworks (MOFs) containing Zr(IV) -based secondary building units (SBUs), as in the UiO-66 series, are receiving widespread research interest due to their enhanced chemical and mechanical stabilities. We report the synthesis and extensive characterisation, as both bulk microcrystalline and single crystal forms, of extended UiO-66 (Zr and Hf) series MOFs containing integral unsaturated alkene, alkyne and butadiyne units, which serve as reactive sites for postsynthetic modification (PSM) by halogenation. The water stability of a Zr-stilbene MOF allows the dual insertion of both -OH and -Br groups in a single, aqueous bromohydrination step. Quantitative bromination of alkyne- and butadiyne-containing MOFs is demonstrated to be stereoselective, as a consequence of the linker geometry when bound in the MOFs, while the inherent change in hybridisation and geometry of integral linker atoms is facilitated by the high mechanical stabilities of the MOFs, allowing bromination to be characterised in a single-crystal to single-crystal (SCSC) manner. The facile addition of bromine across the unsaturated C-C bonds in the MOFs in solution is extended to irreversible iodine sequestration in the vapour phase. A large-pore interpenetrated Zr MOF demonstrates an I2 storage capacity of 279 % w/w, through a combination of chemisorption and physisorption, which is comparable to the highest reported capacities of benchmark iodine storage materials for radioactive I2 sequestration. We expect this facile PSM process to not only allow trapping of toxic vapours, but also modulate the mechanical properties of the MOFs.

Zirconium (IV)-based metal organic framework (UIO-67) as efficient sorbent in dispersive solid phase extraction of plant growth regulator from fruits coupled with HPLC fluorescence detection

A stable zirconium (Ⅳ)-based metal organic frameworks (UIO-67) material possessing good chemical, thermal and water stability was synthesized and applied as a sorbent for the dispersive solid phase extraction (DSPE) of 8 plant growth regulators (PGRs) in fruit samples. Fluorescence labeling combined with high performance liquid chromatography fluorescence detection (HPLC-FLD), was used to quantify the target analytes. Characterization of the UIO-67 material was performed by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The experimental parameters, such as amount of UIO-67, type and volume of eluting solvent, adsorption and desorption time, were optimized. Under the optimized conditions, good linearity was observed in the range of 10-1000 pmol/mL with R(2)>0.9989. The limits of detection and limits of quantification were in the range of 0.21-0.57ng/mL and 0.81-1.91ng/mL, respectively. The intra-day and inter-day precisions (based on the relative standard deviation, n=3) of the PGR derivatives were under 3.1% and 5.3% respectively and the accuracies of the method for the PGRs were in the range from 89.3% to 102.3%. The developed method was successfully applied to analyze PGRs residues in fruit samples. The proposed method is proved to be simple, environment-friendly and inexpensive and it is feasible to directly use UIO-67 as sorbent to extract targets by varying conditions.

Selective capture of hexavalent chromium from an anion-exchange column of metal organic resin-alginic acid composite

We report an anion exchange composite material based on a protonated amine-functionalized metal-organic framework, denoted Metal Organic Resin-1 (MOR-1), and alginic acid (HA). MOR-1-HA material shows an exceptional capability to rapidly and selectively sorb Cr(vi) under a variety of conditions and in the presence of several competitive ions. The selectivity of MOR-1-HA for Cr(vi) is shown to be the result of strong O3CrVI···NH2 interactions. The composite sorbent can be successfully utilized in an ion-exchange column, in contrast to pristine MOR-1 which forms fine suspensions in water passing through the column. Remarkably, an ion exchange column with only 1% wt MOR-1-HA and 99% wt sand (an inert and inexpensive material) is capable of reducing moderate and trace Cr(vi) concentrations to well below the acceptable safety limits for water. The relatively low cost of MOR-1-HA/sand column and its high regeneration capability and reusability make it particularly attractive for application in the remediation of Cr(vi)-bearing industrial waste.

Mechanochemical and solvent-free assembly of zirconium-based metal-organic frameworks

We develop the first mechanochemical and solvent-free routes for zirconium metal-organic frameworks, making the frameworks UiO-66 and UiO-66-NH2 accessible on the gram scale without strong acids, high temperatures or excess reactants. The frameworks form either by milling, or spontaneous self-assembly by simply exposing solid mixtures of reactants to organic vapour. The generated frameworks exhibit high porosity and catalytic activity in the hydrolysis of model nerve agents, on par with their solvothermally generated counterparts.

Defect creation in metal-organic frameworks for rapid and controllable decontamination of roxarsone from aqueous solution

Given the great harm to the human health of organic arsenic compounds (OACs), developing highly efficient adsorbents with both rapid adsorption rate and high saturation capacity is paramount important. Herein, Zr-based metal-organic frameworks (MOFs) of UiO-66 have been successfully exploited for the efficient decontamination of a typical organic arsenic compound of roxarsone (ROX) from aqueous solution. The influences of the most significant parameters such as contact time, adsorbate concentration, pH as well as ionic strength on the adsorption of ROX were investigated. The amount of missing-linker defects in UiO-66 was systematically tuned by changing the concentration of modulator in the reactants. The presence of the defects not only resulted in the dramatically enhanced porosity, but also induced the creation of ZrOH groups which served as the main active adsorption sites for efficient ROX sequestration. As a result, adsorptive capacity of ROX over UiO-66 could be improved to 730 mg/g, which was much higher than those of many reported adsorbents. Meanwhile, the adsorption equilibrium time could be reduced to as short as 30 min. These merits, combined with their excellent stability, prefigure the great potentials of these defect-tunable UiO-66 MOFs as adsorbents for the efficient removal of various OACs from the polluted water.

Rapid, green and inexpensive synthesis of high quality UiO-66 amino-functionalized materials with exceptional capability for removal of hexavalent chromium from industrial waste

UiO-66-NH₂ type materials show excellent capability for removal of Cr(vi) from industrial waste.

Tuning the properties of the metal–organic framework UiO-67-bpy via post-synthetic N-quaternization of pyridine sites

N-Quaternization of pyridine sites in UiO-67-bpy affords a cationic MOF UiO-67-bpy-Me with faster and improved anionic dye and CO₂adsorption, and extended visible-light absorption properties, which make UiO-67-bpy-Me more efficient in photodegrading organic dyes.

Positive effect of the fluorine moiety on the oxygen storage capacity of UiO-66 metal–organic frameworks

The capacity to store oxygen and nitrogen within UiO-66 fluorine-containing MOFs has been tested through high pressure adsorption isotherms.

Ruthenium(ii)-polypyridyl zirconium(iv) metal-organic frameworks as a new class of sensitized solar cells

A series of Ru(ii)L2L' (L = 2,2'-bipyridyl, L' = 2,2'-bipyridine-5,5'-dicarboxylic acid), RuDCBPY, -containing zirconium(iv) coordination polymer thin films have been prepared as sensitizing materials for solar cell applications. These metal-organic framework (MOF) sensitized solar cells, MOFSCs, each are shown to generate photocurrent in response to simulated 1 sun illumination. Emission lifetime measurements indicate the excited state quenching of RuDCBPY at the MOF-TiO2 interface is extremely efficient (>90%), presumably due to electron injection into TiO2. A mechanism is proposed in which RuDCBPY-centers photo-excited within the MOF-bulk undergo isotropic energy migration up to 25 nm from the point of origin. This work represents the first example in which a MOFSC is directly compared to the constituent dye adsorbed on TiO2 (DSC). Importantly, the MOFSCs outperformed their RuDCBPY-TiO2 DSC counterpart under the conditions used here and, thus, are solidified as promising solar cell platforms.

Metal organic frameworks mimicking natural enzymes: a structural and functional analogy

In this review, we have portrayed the structure, synthesis and applications of a variety of biomimetic MOFs from an unprecedented angle.

High MOF loading in mixed-matrix membranes utilizing styrene/butadiene copolymers

Combining metal–organic frameworks with styrene-based polymer binders yields new mixed-matrix membranes with good mechanical and physical properties.

The UiO-66-SO3H metal–organic framework as a green catalyst for the facile synthesis of dihydro-2-oxypyrrole derivatives

The multicomponent domino reaction synthesis of dihydro-2-oxopyrroles has been performed using the sulfonated metal–organic framework, UiO-66-SO₃H (UiO is the University of Oslo), as a highly efficient acid catalyst.

Defect engineering of UiO-66 for CO2 and H2O uptake – a combined experimental and simulation study

Defect concentrations and their compensating groups have been systematically tuned within UiO-66 frameworks and are found to have a pronounced effect on CO₂ and H₂O adsorption at low and high pressure.

Probing the correlations between the defects in metal–organic frameworks and their catalytic activity by an epoxide ring-opening reaction

The number of defects in a series of Zr/Hf-MOFs was found to correlate quantitatively with their catalytic activity.

Controlling embedment and surface chemistry of nanoclusters in metal–organic frameworks

A combined theoretical and experimental approach demonstrates that nanocluster embedment into the pores of metal–organic frameworks (MOF) may be influenced by the chemical functionalisation of the MOF.

Drug delivery and controlled release from biocompatible metal–organic frameworks using mechanical amorphization

We have used a family of Zr-based metal–organic frameworks (MOFs) with different functionalized (bromo, nitro and amino) and extended linkers for drug delivery.

Metal–organic frameworks constructed from crown ether-based 1,4-benzenedicarboxylic acid derivatives

A series of crown ether- and thiacrown ether-derivatized benzene dicarboxylic acid (H₂bdc) ligands has been synthesized and incorporated into the prototypical isoreticular metal–organic framework (IRMOF) and UiO-66 materials.

Aging of the reaction mixture as a tool to modulate the crystallite size of UiO-66 into the low nanometer range

Nanosized UiO-66 was synthesized in high yield by exploiting controlled aging of the reaction mixture.

Template mediated and solvent-free route to a variety of UiO-66 metal–organic frameworks

A green and cost-effective process for UiO-66 synthesis with a facile and efficient solvent-free template oriented route.

A spray-drying continuous-flow method for simultaneous synthesis and shaping of microspherical high nuclearity MOF beads

A novel spray-drying continuous-flow method allows the synthesis of high-nuclearity MOFs as well as multivariate MOFs in the form of compact microspherical superstructures (beads) in good yields and high porosity.

A visually detectable pH responsive zirconium metal–organic framework

A functionalized MOF, NU-1000–CNF, shows simultaneous hydrolysis of nerve agent simulants while visually sensing the acid byproducts produced.

The preparation of an ultrastable mesoporous Cr(iii)-MOF via reductive labilization

Kinetic labilization of the Fe(iii)-O coordination bond in a mesoporous metal-organic framework, PCN-333-Fe(iii), is realized by the reduction of Fe(iii) by Cr(ii). The Fe(ii)-Cr(ii) metathesis is thus completely achieved followed by this reductive labilization process that generates PCN-333-Cr(iii). The kinetic inertness of Cr(iii) provides PCN-333-Cr(iii) with enhanced chemical stability as well as a broader range of applications compared to those of PCN-333-Fe(iii). For instance, alkylamine incorporated PCN-333-Cr(iii) demonstrated significant carbon dioxide uptake while PCN-333-Fe(iii) lost its crystallinity after alkylamine treatment.

Planar Heterojunction Perovskite Solar Cells Incorporating Metal-Organic Framework Nanocrystals

Zr-based porphyrin metal-organic framework (MOF-525) nanocrystals with a crystal size of about 140 nm are synthesized and incorporated into perovskite solar cells. The morphology and crystallinity of the perovskite thin film are enhanced since the micropores of MOF-525 allow the crystallization of perovskite to occur inside; this observation results in a higher cell efficiency of the obtained MOF/perovskite solar cell.

Metal-Organic Frameworks in Adsorption-Driven Heat Pumps: The Potential of Alcohols as Working Fluids

A large fraction of global energy is consumed for heating and cooling. Adsorption-driven heat pumps and chillers could be employed to reduce this consumption. MOFs are often considered to be ideal adsorbents for heat pumps and chillers. While most published works to date on this topic have focused on the use of water as a working fluid, the instability of many MOFs to water and the fact that water cannot be used at subzero temperatures pose certain drawbacks. The potential of using alcohol-MOF pairs in adsorption-driven heat pumps and chillers is investigated. To this end, 18 different selected MOF structures in combination with either methanol or ethanol as a working fluid are considered, and their potential is assessed on the basis of adsorption measurements and thermodynamic efficiencies. If alcohols are used instead of water, then (1) adsorption occurs at lower relative pressures for methanol and even lower pressure for ethanol, (2) larger pores can be utilized efficiently, as hysteresis is absent for pores smaller than 3.4 nm (2 nm for water), (3) larger pore sizes need to be employed to ensure the desired stepwise adsorption, (4) the effect of (polar/apolar) functional groups in the MOF is far less pronounced, (5) the energy released or taken up per cycle is lower, but heat and mass transfer may be enhanced, (6) stability of MOFs seems to be less of an issue, and (7) cryogenic applications (e.g., ice making) become feasible. From a thermodynamic perspective, UiO-67, CAU-3, and ZIF-8 seem to be the most promising MOFs for both methanol and ethanol as working fluids. Although UiO-67 might not be completely stable, both CAU-3 and ZIF-8 have the potential to be applied, especially in subzero-temperature adsorption chillers (AC).

Post-Synthetic Anisotropic Wet-Chemical Etching of Colloidal Sodalite ZIF Crystals

Controlling the shape of metal-organic framework (MOF) crystals is important for understanding their crystallization and useful for myriad applications. However, despite the many advances in shaping of inorganic nanoparticles, post-synthetic shape control of MOFs and, in general, molecular crystals remains embryonic. Herein, we report using a simple wet-chemistry process at room temperature to control the anisotropic etching of colloidal ZIF-8 and ZIF-67 crystals. Our work enables uniform reshaping of these porous materials into unprecedented morphologies, including cubic and tetrahedral crystals, and even hollow boxes, by an acid-base reaction and subsequent sequestration of leached metal ions. Etching tests on these ZIFs reveal that etching occurs preferentially in the crystallographic directions richer in metal-ligand bonds; that, along these directions, the etching rate tends to be faster on the crystal surfaces of higher dimensionality; and that the etching can be modulated by adjusting the pH of the etchant solution.

Combination of Optimization and Metalated-Ligand Exchange: An Effective Approach to Functionalize UiO-66(Zr) MOFs for CO2 Separation

The strategy to functionalize water-stable metal-organic frameworks (MOFs) in order to improve their CO2 uptake capacities for efficient CO2 separation remains limited and challenging. We herein present an effective approach to functionalize a prominent water-stable MOF, UiO-66(Zr), by a combination of optimization and metalated-ligand exchange. In particular, by systematic optimization, we have successfully obtained UiO-66(Zr) of the highest BET surface area reported so far (1730 m(2)  g(-1) ). Moreover, it shows a hybrid Type I/IV N2 isotherm at 77 K and a mesopore size of 3.9 nm for the first time. The UiO-66 MOF underwent a metalated-ligand-exchange (MLE) process to yield a series of new UiO-66-type MOFs, among which UiO-66-(COONa)2 -EX and UiO-66-(COOLi)4 -EX MOFs have both enhanced CO2 working capacity and IAST CO2 /N2 selectivity. Our approach has thus suggested an alternative design to achieve water-stable MOFs with high crystallinity and gas uptake for efficient CO2 separation.

Stabilization of volatile Ti(BH4)3 by nano-confinement in a metal-organic framework

Volatile Ti(BH₄)₃ molecules stabilized on the surface of a MOF.

Liquid complex hydrides are a new class of hydrogen storage materials with several advantages over solid hydrides, e.g. they are flexible in shape, they are a flowing fluid and their convective properties facilitate heat transport. The physical and chemical properties of a gaseous hydride change when the molecules are adsorbed on a material with a large specific surface area, due to the interaction of the adsorbate with the surface of the host material and the reduced number of collisions between the hydride molecules. In this paper we report the synthesis and stabilization of gaseous Ti(BH₄)₃. The compound was successfully stabilized through adsorption in nanocavities. Ti(BH₄)₃, upon synthesis in its pure form, spontaneously and rapidly decomposes into diborane and titanium hydride at room temperature in an inert gas, e.g. argon. Ti(BH₄)₃ adsorbed in the cavities of a metal organic framework is stable for several months at ambient temperature and remains stable up to 350 K under vacuum. The adsorbed Ti(BH₄)₃ reaches approximately twice the density of the gas phase. The specific surface area (BET, N₂ adsorption) of the MOF decreased from 1200 m² g^–1 to 770 m² g^–1 upon Ti(BH₄)₃ adsorption.