Immobilization of UiO-67 with photochromic spiropyrans: a quantum chemical study

Post-synthetic modification of MOFs allows tuning the properties according to desired applications. The incorporation of photoactive molecules introduces sensitivity to radiation properties to the matrix of MOFs. We report on the theoretical analysis of possible ways of construction photoactive MOFs from UiO-67 and spiropyran molecules containing different carbonyl substituents. Large-scale computer modeling with the use of density functional theory method allowed us to select the most energy-efficient schemes of design. It was revealed that the most preferred way of immobilization of UiO-67 is the interaction with the carboxylic group in the indoline fragment of spiropyran. These results are promising for the application of MOFs modified in this way as photoactive sensors. Graphical abstract.

Synthesis of ZnO Nanoparticles Doped with Cobalt Using Bimetallic ZIFs as Sacrificial Agents

We report here a simple two-stage synthesis of zinc–cobalt oxide nanoparticles. We used Zn/Co-zeolite imidazolate framework (ZIF)-8 materials as precursors for annealing and optional impregnation with a silicon source for the formation of a protective layer on the surface of oxide nanoparticles. Using bimetallic ZIFs allowed us to trace the phase transition of the obtained oxide nanoparticles from wurtzite ZnO to spinel Co₃O₄ structures. Using (X-ray diffraction) XRD and (X-ray Absorption Near Edge Structure) XANES techniques, we confirmed the incorporation of cobalt ions into the ZnO structure up to 5 mol.% of Co. Simple annealing of Zn/Co-ZIF-8 materials in the air led to the formation of oxide nanoparticles of about 20–30 nm, while additional treatment of ZIFs with silicon source resulted in nanoparticles of about 5–10 nm covered with protective silica layer. We revealed the incorporation of oxygen vacancies in the obtained ZnO nanoparticles using FTIR analysis. All obtained samples were comprehensively characterized, including analysis with a synchrotron radiation source.

In Situ Time-Resolved Decomposition of β-Hydride Phase in Palladium Nanoparticles Coated with Metal-Organic Framework

The formation of palladium hydrides is a well-known phenomenon, observed for both bulk and nanosized samples. The kinetics of hydrogen adsorption/desorption strongly depends on the particle size and shape, as well as the type of support and/or coating of the particles. In addition, the structural properties of hydride phases and their distribution also depend on the particle size. In this work, we report on the in situ characterization of palladium nanocubes coated with HKUST-1 metal-organic framework (Pd@HKUST-1) during desorption of hydrogen by means of synchrotron-based time-resolved X-ray powder diffraction. A slower hydrogen desorption, compared to smaller sized Pd nanoparticles was observed. Rietveld refinement of the time-resolved data revealed the remarkable stability of the lattice parameters of α- and β-hydride phases of palladium during the α- to β- phase transition, denoting the behavior more similar to the bulk materials than nanoparticles. The stability in the crystal sizes for both α- and β-hydride phases during the phase transition indicates that no sub-domains are formed within a single particle during the phase transition.

Graphene Oxide Protected Copper Benzene-1,3,5-Tricarboxylate for Clean Energy Gas Adsorption

Among microporous storage materials copper benzene-1,3,5-tricarboxylate (CuBTC MOF, Cu3(BTC)2 or HKUST-1) holds the greatest potential for clean energy gases. However, its usefulness is challenged by water vapor, either in the gas to be stored or in the environment. To determine the protection potential of graphene oxide (GO) HKUST1@GO composites containing 0-25% GO were synthesized and studied. In the highest concentration, GO was found to strongly affect HKUST-1 crystal growth in solvothermal conditions by increasing the pH of the reaction mixture. Otherwise, the GO content had practically no influence on the H2, CH4 and CO2 storage capacities, which were very similar to those from the findings of other groups. The water vapor resistance of a selected composite was compared to that of HKUST-1. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric (TG/DTG) and N2 adsorption techniques were used to monitor the changes in the crystal and pore structure. It was found that GO saves the copper-carboxyl coordination bonds by sacrificing the ester groups, formed during the solvothermal synthesis, between ethanol and the carboxyl groups on the GO sheets.

Synthesis of ordered microporous/macroporous MOF-808 through modulator-induced defect-formation, and surfactant self-assembly strategies

Ordered materials with interconnected porosity allow the diffusion of molecules within their inner porous structure to access the active sites located in the microporous core. As a follow-up of our work on engineering of MOF-808, in this contribution, we study the synthesis of defective MOF-808 using two different strategies: the use of modulators and the surfactant-assisted synthesis to obtain materials with ordered and interconnected pores. The results of the study indicated that (i) the use of modulators of different chain length led to the formation of microporous/mesoporous MOFs through the formation of missing linker defects. However, the use of the acetic acid contributes to the formation of MOFs with larger mesoporous size distributions compared to materials synthesized with formic and propionic acids as modulators, and (ii) the self-assembly of CTAB surfactant produced an ordered microporous/macroporous network which enhanced crystallinity. However, the surface properties of the materials seem to be unaffected by the use of surfactants during synthesis. These results contribute to the development of ordered materials with a broad range of pore size distributions and give rise to new opportunities to extend the applications of MOF-808.

Coordination Polymers Based on Highly Emissive Ligands: Synthesis and Functional Properties

Coordination polymers are constructed from metal ions and bridging ligands, linking them into solid-state structures extending in one (1D), two (2D) or three dimensions (3D). Two- and three-dimensional coordination polymers with potential voids are often referred to as metal-organic frameworks (MOFs) or porous coordination polymers. Luminescence is an important property of coordination polymers, often playing a key role in their applications. Photophysical properties of the coordination polymers can be associated with intraligand, metal-centered, guest-centered, metal-to-ligand and ligand-to-metal electron transitions. In recent years, a rapid growth of publications devoted to luminescent or fluorescent coordination polymers can be observed. In this review the use of fluorescent ligands, namely, 4,4'-stilbenedicarboxylic acid, 1,3,4-oxadiazole, thiazole, 2,1,3-benzothiadiazole, terpyridine and carbazole derivatives, naphthalene diimides, 4,4',4''-nitrilotribenzoic acid, ruthenium(II) and iridium(III) complexes, boron-dipyrromethene (BODIPY) derivatives, porphyrins, for the construction of coordination polymers are surveyed. Applications of such coordination polymers based on their photophysical properties will be discussed. The review covers the literature published before April 2020.

Structural variety of aluminium and gallium coordination polymers based on bis-pyridylethylene: from molecular complexes to ionic networks

Diverse molecular (0D and 1D) as well as ionic (0D, 1D, 2D, mixed 1D–2D) crystal structures of complexes of aluminium and gallium trihalides with bis(4-pyridylethylene) were obtained by solvent-free melt reactions.

A new approach to the mechanism for the acetalization of benzaldehyde over MOF catalysts

The benzaldehyde acetalization catalyzed by UiO-66 and UiO-66F, was carried out in a batch-type reactor at room temperature and atmospheric pressure, and the full kinetic study was performed using the Langmuir–Hinshelwood and Eley–Rideal models.

Recent advances in metallopolymer-based drug delivery systems

Metallopolymers (MPs) or metal-containing polymers have shown great potential as new drug delivery systems (DDSs) due to their unique properties, including universal architectures, composition, properties and surface chemistry. Over the past few decades, the exponential growth of many new classes of MPs that deal with these issues has been demonstrated. This review presents and assesses the recent advances and challenges associated with using MPs as DDSs. Among the most widely used MPs for these purposes, metal complexes based on synthetic and natural polymers, coordination polymers, metal-organic frameworks, and metallodendrimers are distinguished. Particular attention is paid to the stimulus- and multistimuli-responsive metallopolymer-based DDSs. Of considerable interest is the use of MPs for combination therapy and multimodal systems. Finally, the problems and future prospects of using metallopolymer-based DDSs are outlined. The bibliography includes articles published over the past five years.

Regeneration, degradation, and toxicity effect of MOFs: Opportunities and challenges

Metal organic frameworks (MOFs) have been investigated extensively for separation, storage, catalysis, and sensing applications. Nonetheless, problems associated with their toxicity, recycling/reuse/regeneration, and degradation have yet to be addressed as one criterion to satisfy their commercialization. Here, the challenges associated with MOF-based technology have been explored to further expand their practical utility in various applications. We start a brief description of challenges associated with MOF-based technology followed by a critical evaluation of toxicity and need of technical options for regeneration of MOFs. Importantly, diverse techniques/process for reuse and regeneration of MOFs like activation of MOFs by heat, vacuum, solvent exchange, supercritical carbon dioxide (SCCO₂) and other miscellaneous options have been discussed with recent examples. Afterward, we also present an economical aspect and future perspectives of MOFs for real world applications. All in all, we aimed to present opportunities and critical review of the current status of MOF technology with respect to their recycling/reuse/regeneration to consider their environmental impact.

Topological analysis of the layered uranyl compounds bearing slabs with UO2:TO4 ratio of 2:3

Nine new templated uranyl sulfates and selenates, [(H9O4)2(H2O)][(UO2)2(SO4)3(H2O)2] (H9US), [(C5H7 NO)2(H2O)][(UO2)2(SeO4)3(H2O)2](H2O) (OUSe), [C6H6N3][H5O2] [(UO2)2(SeO4)3(H2O)] (BH5USe), [C6H6N3][H7O3][(UO2)2(SO4)3 (H2O)](H2O) (BH7US), [C6H16N][H5O2][(UO2)2(SeO4)3(H2O)] (TeH5USe), [C6H18N2][(UO2)2(SO4)3(H2O)] (TmUS), [H5O2]2 [(UO2)2(SeO4)3(H2O)](H2O) (H5USe-1), [H5O2]2[(UO2)2(SeO4)3 (H2O)2](H2O)9 (H5USe-2), and [C4H14N2][(UO2)2(SeO4)3(H2O)](H2O) (DmUSe) have been prepared by isothermal evaporation of aqueous solutions containing extra sulfuric or selenic acid. Their crystal structures can be considered as organo-inorganic hybrids constructed of alternating [(UO2)2 (TO4)3(H2O) n ]2− slabs (T = Se6+, S6+, n = 1, 2) and layers containing templating organic moieties and/or hydronium ions and water molecules. The organic and inorganic parts of the structures are linked by multiple hydrogen bonds. Besides structure description, we offer topological analysis of the inorganic fragments with UO2:TO4 ratio of 2:3 as modular units resulting from self-assembly of fundamental chains formed by [(UO2)2(TO4)2] tetramers and TO4 tetrahedra.

Water depollution using metal-organic frameworks-catalyzed advanced oxidation processes: A review

This paper presents a review on the environmental applications of metal-organic frameworks (MOFs), which are inorganic-organic hybrid highly porous crystalline materials, prepared from metal ion/clusters and multidentate organic ligands. The emphases are made on the enhancement of the performance of advanced oxidation processes (AOPs) (photocatalysis, Fenton reaction methods, and sulfate radical (SO₄^-)-mediated oxidations) using MOFs materials. MOFs act as adsorption and light absorbers, leading to superior performance of photocatalytic processes. More recent examples of photocatalytic degradation of dyes are presented. Additionally, it is commonly shown that Fe-based MOFs exhibited excellent catalytic performance on the Fenton-based and SO₄^•--mediated oxidations of organic pollutants (e.g., dyes, phenol and pharmaceuticals). The significantly enhanced generation of reactive species such as OH and/or SO₄^- by both homogeneous and heterogeneous catalysis was proposed as the possible mechanism for water depollution. Based on the existing literature, the challenge and future perspectives in MOF-based AOPs are addressed.

Electronic Peculiarities of a Self-Assembled M12L24 Nanoball (M = Pd+2, Cr, or Mo)

We use molecular mechanics and DFT calculations to analyze the particular electronic behavior of a giant nanoball. This nanoball is a self-assembled M₁₂L₂₄ nanoball; with M equal to Pd⁺²; Cr; and Mo. These systems present an extraordinarily large cavity; similar to biological giant hollow structures. Consequently, it is possible to use these nanoballs to trap smaller species that may also become activated. Molecular orbitals, molecular hardness, and Molecular Electrostatic Potential enable us to define their potential chemical properties. Their hardness conveys that the Mo system is less reactive than the Cr system. Eigenvalues indicate that electron transfer from the system with Cr to other molecules is more favorable than from the system with Mo. Molecular Electrostatic Potential can be either positive or negative. This means that good electron donor molecules have a high possibility of reacting with positive regions of the nanoball. Each of these nanoballs can trap 12 molecules, such as CO. The nanoball that we are studying has large pores and presents electronic properties that make it an apposite target of study.

Water as a structure-driving agent between the UiO-66 and MIL-140A metal-organic frameworks

We report a careful investigation of a selective phase formation in the zirconium-terephthalic acid system during solvothermal synthesis, which could result in the UiO-66 (Zr6O6(OH)4(BDC)6) or MIL-140A (ZrO(BDC)) metal-organic frameworks (MOFs). The introduction of water varies the phase from MIL-140A to UiO-66 by producing at the nucleation stage tetragonal ZrO2 nanoparticles, where the local arrangement of Zr and O atoms is similar to that in the UiO-66 SBU.

Partial and Complete Substitution of the 1,4-Benzenedicarboxylate Linker in UiO-66 with 1,4-Naphthalenedicarboxylate: Synthesis, Characterization, and H2-Adsorption Properties

We describe the synthesis and corresponding full characterization of the set of UiO-66 metal-organic frameworks (MOFs) with 1,4-benzenedicarboxylate (C₆H₄(COOH)₂, hereafter H₂BDC) and 1,4-naphthalenedicarboxylate (C₁₀H₆(COOH)₂, hereafter H₂NDC) mixed linkers with NDC contents of 0, 25, 50, and 100%. Their structural (powder X-ray diffraction, PXRD), adsorptive (N₂, H₂, and CO₂), vibrational (IR/Raman), and thermal stability (thermogravimetric analysis, TGA) properties quantitatively correlate with the NDC content in the material. The UiO-66 phase topology is conserved at all relative fractions of BDC/NDC. The comparison between the synchrotron radiation PXRD and 77 K N₂-adsorption isotherms obtained on the 50:50 BDC/NDC sample and on a mechanical mixture of the pure BDC and NDC samples univocally proves that in the mixed linkers of the MOFs the BDC and NDC linkers are shared in each MOF crystal, discarding the hypothesis of two independent phases, where each crystal contains only BDC or NDC linkers. The careful tuning of the NDC content opens a way for controlled alteration of the sorption properties of the resulting material as testified by the H₂-adsorption experiments, showing that the relative ranking of the materials in H₂ adsorption is different in different equilibrium-pressure ranges: at low pressures, 100NDC is the most efficient sample, while with increasing pressure, its relative performance progressively declines; at high pressures, the ranking follows the BDC content, reflecting the larger internal pore volume available in the MOFs with a higher fraction of smaller linkers. The H₂-adsorption isotherms normalized by the sample Brunauer-Emmett-Teller specific surface area show, in the whole pressure range, that the surface-area-specific H₂-adsorption capabilities in UiO-66 MOFs increase progressively with increasing NDC content. Density functional theory calculations, using the hybrid B3LYP exchange correlation functional and quadruple-ζ with four polarization functions (QZ4P) basis set, show that the interaction of H₂ with the H₂NDC linker results in an adsorption energy larger by about 15% with respect to that calculated for adsorption on the H₂BDC linker.

A Review on the Synthesis and Characterization of Metal Organic Frameworks for Photocatalytic Water Purification

This review analyzes the preparation and characterization of metal organic frameworks (MOFs) and their application as photocatalysts for water purification. The study begins by highlighting the problem of water scarcity and the different solutions for purification, including photocatalysis with semiconductors, such as MOFs. It also describes the different methodologies that can be used for the synthesis of MOFs, paying attention to the purification and activation steps. The characterization of MOFs and the different approaches that can be followed to learn the photocatalytic processes are also detailed. Finally, the work reviews literature focused on the degradation of contaminants from water using MOF-based photocatalysts under light irradiation.

Tuning Pt and Cu sites population inside functionalized UiO-67 MOF by controlling activation conditions

The exceptional thermal and chemical stability of the UiO-66, -67 and -68 classes of isostructural MOFs [J. Am. Chem. Soc., 2008, 130, 13850] makes them ideal materials for functionalization purposes aimed at introducing active centres for potential application in heterogeneous catalysis. We previously demonstrated that a small fraction (up to 10%) of the linkers in the UiO-67 MOF can be replaced by bipyridine-dicarboxylate (bpydc) moieties exhibiting metal-chelating ability and enabling the grafting of Pt(ii) and Pt(iv) ions in the MOF framework [Chem. Mater., 2015, 27, 1042] upon interaction with PtCl₂ or PtCl₄ precursors. Herein we extend this functionalization approach in two directions. First, we show that by controlling the activation of the UiO-67-Pt we can move from a material hosting isolated Pt(ii) sites anchored to the MOF framework with Pt(ii) exhibiting two coordination vacancies (potentially interesting for C-H bond activation) to the formation of very small Pt nanoparticles hosted inside the MOF cavities (potentially interesting for hydrogenation reactions). The second direction consists of the extension of the approach to the insertion of Cu(ii), obtained via interaction with CuCl₂, and exhibiting interesting redox properties. All materials have been characterized by in situ X-ray absorption spectroscopy at the Pt L₃- and Cu K-edges.

The nature of the chemical bond in oxyanionic crystals based on QTAIM topological analysis of electron densities

The QTAIM topological analysis of the calculated electron densities in oxyanionic crystals revealed the covalency criteria for metal–oxygen and hydrogen bonds.

Ti-Based Catalysts and Photocatalysts: Characterization and Modeling

This perspective article aims to underline how cutting-edge synchrotron radiation spectroscopies such as extended X-ray absorption spectroscopy (EXAFS), X-ray absorption near edge structure (XANES), high resolution fluorescence detected (HRFD) XANES, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) have played a key role in the structural and electronic characterization of Ti-based catalysts and photocatalysts, representing an important additional value to the outcomes of conventional laboratory spectroscopies (UV-Vis, IR, Raman, EPR, NMR etc.). Selected examples are taken from the authors research activity in the last two decades, covering both band-gap and shape engineered TiO₂ materials and microporous titanosilicates (ETS-10, TS-1 and Ti-AlPO-5). The relevance of the state of the art simulation techniques as a support for experiments interpretation is underlined for all the reported examples.

Autoluminescent Metal-Organic Frameworks (MOFs): Self-Photoemission of a Highly Stable Thorium MOF

A novel thorium(IV) metal-organic framework (MOF), Th(2,6-naphtalenedicarboxylate)₂, has been synthesized via solvothermal reaction of thorium nitrate and 2,6-naphtalendicarboxilyc acid. This compound shows a new structural arrangement with an interesting topology and an excellent thermal resistance, as the framework is stable in air up to 450 °C. Most notably, this MOF, combining the radioactivity of its metal center and the scintillation property of the ligand, has been proven capable of spontaneous photon emission.