Stable Metal-Organic Frameworks with Group 4 Metals: Current Status and Trends

∞3[Cu2(mand)2(hmt)]–MOF: A Synergetic Effect between Cu(II) and Hexamethylenetetramine in the Henry Reaction

∞3[Cu2(mand)2(hmt)]·H2O (where mand is totally deprotonated mandelic acid (racemic mixture) and hmt is hexamethylenetetramine) proved to be a stable metal–organic framework (MOF) structure under thermal activation and catalytic conditions, as confirmed by both the in situ PXRD (Powder X-ray diffraction) and ATR–FTIR (Attenuated total reflection-Fourier-transform infrared spectroscopy) haracterization. The non-activated MOF was completely inert as catalyst for the Henry reaction, as the accessibility of the substrates to the channels was completely blocked by H-bonded water to the mand entities and CO2 adsorbed on the Lewis basic sites of the hmt. Heating at 140 °C removed these molecules. Only an insignificant change in the relative ratios of the XRD facets due to the capillary forces associated to the removal of the guest molecules from the network has been observed. This treatment afforded the accessibility of nitromethane and various aldehydes (4-bromobenzaldehyde, 4-nitrobenzaldehyde, and p-tolualdehyde) to the active catalytic sites, leading to conversions up to 48% and selectivities up to 98% for the desired nitroaldol products. The behavior of the catalyst is solvent-sensitive. Protic solvents completely inhibited the reaction due to the above-mentioned strong H-bonds. Accordingly, very good results were obtained only with aprotic solvents such as acetonitrile and 1,4-dioxane. The synthesized MOF is completely recyclable as demonstrated for five successive cycles.

Modulation of crystal growth and structure within cerium-based metal–organic frameworks

Cerium-based metal–organic frameworks' crystal growth and structure dictated using modulating monocarboxylate species.

Green synthesis of a large series of bimetallic MIL-100(Fe,M) MOFs

Here we present a scalable and green methodology to synthesize a large variety of MIL-100(Fe,M), metal-doped iron-based MOFs with high thermal stability and surface areas.

Boosting CO2 adsorption and selectivity in metal–organic frameworks of MIL-96(Al) via second metal Ca coordination

Aluminum trimesate-based MOF (MIL-96-(Al)) has attracted intense attention due to its high chemical stability and strong CO₂ adsorption capacity. In this study, CO₂ capture and selectivity of MIL-96-Al was further improved by the coordination of the second metal Ca. To this end, a series of MIL-96(Al)–Ca were hydrothermally synthesised by a one-pot method, varying the molar ratio of Ca²⁺/Al³⁺. It is shown that the variation of Ca²⁺/Al³⁺ ratio results in significant changes in crystal shape and size. The shape varies from the hexagonal rods capped in the ends by a hexagonal pyramid in MIL-96(Al) without Ca to the thin hexagonal disks in MIL-96(Al)–Ca4 (the highest Ca content). Adsorption studies reveal that the CO₂ adsorption on MIL-96(Al)–Ca1 and MIL-96(Al)–Ca2 at pressures up to 950 kPa is vastly improved due to the enhanced pore volumes compared to MIL-96(Al). The CO₂ uptake on these materials measured in the above sequence is 10.22, 9.38 and 8.09 mmol g⁻¹, respectively. However, the CO₂ uptake reduces to 5.26 mmol g⁻¹ on MIL-96(Al)–Ca4. Compared with MIL-96(Al)–Ca1, the N₂ adsorption in MIL-96(Al)–Ca4 is significantly reduced by 90% at similar operational conditions. At 100 and 28.8 kPa, the selectivity of MIL-96(Al)–Ca4 to CO₂/N₂ reaches up to 67 and 841.42, respectively, which is equivalent to 5 and 26 times the selectivity of MIL-96(Al). The present findings highlight that MIL-96(Al) with second metal Ca coordination is a potential candidate as an alternative CO₂ adsorbent for practical applications.

MIL-96(Al)–Ca1 shows the highest CO₂ adsorption capacity; while MIL-96(Al)–Ca4 displays a distinguished morphology with the highest selectivity of CO₂/N₂.

The role of leached Zr in the photocatalytic reduction of CO2 to formate by derivatives of UiO-66 metal organic frameworks

Photoreduction of CO₂ to formate by UiO-66 Zr MOF derivatives is explained by Zr leaching.

Assorted functionality-appended UiO-66-NH2 for highly efficient uranium(vi) sorption at acidic/neutral/basic pH

Eight assorted functionalities were anchored on UiO-66-NH₂via PSM strategy displaying MOFs with similar framework but variable uranyl binding affinities. The excellent sorption capacity of UiO-66-PO-Ph makes it efficient uranium sorbent material.

Enhanced d–d transitions in HKUST/Bi2WO6 nanocomposite mediated visible-light driven selective conversion of benzyl alcohol to benzaldehyde

Graphical representation of the involvement of the d–d transition in the photocatalytic conversion of benzyl alcohol to benzaldehyde.

Intramolecular hydrogen bond-induced high chemical stability of metal–organic frameworks

Intramolecular hydrogen bonds in ligands restrict the rotation of carboxyl groups and consequently enhance the chemical stability of MOFs.

Polyoxometalate adsorbed in a metal–organic framework for electrocatalytic dopamine oxidation

A redox-active polyoxometalate, V₁₀O₂₈, was immobilized into a water-stable zirconium-based metal–organic framework, NU-902. The V₁₀O₂₈@NU-902 can show redox hopping-based charge transport and electrocatalytic activity for dopamine sensing.

Metal-organic framework UiO-66 membranes

Metal-organic frameworks (MOFs) have emerged as a class of promising membrane materials. UiO-66 is a prototypical and stable MOF material with a number of analogues. In this article, we review five approaches for fabricating UiO-66 polycrystalline membranes including in situ synthesis, secondary synthesis, biphase synthesis, gas-phase deposition and electrochemical deposition, as well as their applications in gas separation, pervaporation, nanofiltration and ion separation. On this basis, we propose possible methods for scalable synthesis of UiO-66 membranes and their potential separation applications in the future.

Coordination Engineering in Cobalt-Nitrogen-Functionalized Materials for CO2 Reduction

Cobalt-nitrogen-functionalized materials have been recognized as promising catalysts for the CO₂ reduction reaction because of their superior activity. In order to further improve their activity, we proposed an optimization method through coordination engineering in cobalt-nitrogen-functionalized porphyrin and graphene. By considering a series of derived structures with coordinating nitrogen atoms substituted by carbon or oxygen atoms, a clear activity trend is obtained by constructing a volcano-type plot for activity against adsorption energies of *CO. Detailed electronic structure analysis shows that the enhanced catalytic activity is due to the lacking of π bonding in Co-O bonds compared to Co-C or Co-N bonds in cobalt-centered motifs. This difference allows us to predict the catalytic activity by using the vacancy formation energy of the cobalt atom. Our work provides a general guideline for a rational design of efficient catalysts, which may stimulate further study of coordination engineering for other key energy conversion processes.

Iodosylbenzene Coordination Chemistry Relevant to Metal-Organic Framework Catalysis

Hypervalent iodine compounds formally feature expanded valence shells at iodine. These reagents are broadly used in synthetic chemistry due to the ability to participate in well-defined oxidation-reduction processes and because the ligand-exchange chemistry intrinsic to the hypervalent center allows hypervalent iodine compounds to be applied to a broad array of oxidative substrate functionalization reactions. We recently developed methods to generate these compounds from O₂ that are predicated on diverting reactive intermediates of aldehyde autoxidation toward the oxidation of aryl iodides. Coupling the aerobic oxidation of aryl iodides with catalysts that effect C-H bond oxidation would provide a strategy to achieve aerobic C-H oxidation chemistry. In this Forum Article, we discuss the aspects of hypervalent iodine chemistry and bonding that render this class of reagents attractive lynchpins for aerobic oxidation chemistry. We then discuss the oxidation processes relevant to the aerobic preparation of 2-(tert-butylsulfonyl)iodosylbenzene, which is a popular hypervalent iodine reagent for use with porous metal-organic framework (MOF)-based catalysts because it displays significantly enhanced solubility as compared with unsubstituted iodosylbenzene. We demonstrate that popular synthetic methods to this reagent often provide material that displays unpredictable disproportionation behavior due to the presence of trace impurities. We provide a revised synthetic route that avoids impurities common in the reported methods and provides access to material that displays predictable stability. Finally, we describe the coordination chemistry of hypervalent iodine compounds with metal clusters relevant to MOF chemistry and discuss the potential implications of this coordination chemistry to catalysis in MOF scaffolds.

Tailoring Structural Diversity in Dimethyltin Carboxylates by the pH-Controlled Hydrothermal Approach

The study presents a rational synthesis of new dimethyltin carboxylates, Me₂Sn(H₂btec) (1), Me₂Sn(btec)_0.5(2), [Me₂Sn(H₂O)₂(btec)_0.5]·H₂O (3), and [{Me₂SnOSn(OH)Me₂}(Me₂SnOH)(btec)_0.5]·H₂O (4), derived from tetratopic 1,2,4,5-benzenetetracarboxylic acid (H₄btec). The method relies upon hydrothermal reaction (130 °C, 72 h) of dimethyltin dichloride and H₄btec under optimized pH (2 < pH < 8) conditions that allow control over dimethyltin speciation in aqueous medium as well as degree of deprotonation of the tetrafuntional carboxylic acid. The formation of a three-dimensional assembly in 1 is assisted by the bridging bidentate (μ₂) mode of the carboxylate and O-H···O hydrogen bonds involving -COOH groups. The structure represents a unique example of the diorganotin framework derived from a partially deprotonated polyfuntional carboxylic acid. The structure of 2 adopts a three-dimensional motif wherein each pair of μ₂-carboxylate groups (designated by C1 and C4) of the tetraanionic ligand form different spatial arrangements. For 3, the formation of one-dimensional motif with eight-coordinated tin atoms is assisted by the anisobidentate character of the carboxylate groups. The structure of 4 includes linear chains comprised of [Me₂Sn(μ₂-OH)]₂ and the carboxylate ligand which extend to a layered motif with symmetrically substituted ladder-like distannoxanes acting as linkers. The underlying nets of 1, 2, and 4 exhibit sqc11, scu(sqc170), and sql topologies, respectively. Notably, these assemblies are extremely robust and show no sign of degradation upon exposure to neutral as well as weakly acidic/basic aqueous medium for 7 days.

Facile and Rapid Preparation of Ag@ZIF-8 for Carboxylation of Terminal Alkynes with CO2 in Mild Conditions

Metal-organic frameworks (MOFs) are promising hosts for catalytic active sites due to their adjustable porosity and framework chemistry. Strategies to improve synergistic effects between the installed sites and the parent MOF are highly desired. Herein, a facile and rapid method for the preparation of xAg@ZIF-8 materials was reported. The materials were systematically characterized and used as catalysts for carboxylation of terminal alkynes via direct insertion of CO₂ to the C(sp)-H bond (CTA_CO2). It was found that the integrity of the ZIF-8 structure could be retained upon Ag loading, but short-range crystalline ordering was modified. Two types Ag species could be installed, namely, highly dispersed Ag(I) in the backbone (Ag^HD) and aggregated Ag(0) nanoparticles on the outer surface (Ag^NP). The Ag^NP sites are highly effective for the activation of terminal alkynes due to its high accessibility, while the Ag^HD-modified ZIF-8 framework worked as a CO₂ reservoir with enhanced affinity. Combination of these factors translated to high activity in the CTA_CO2 process, the measured turnover frequency and time yield are among the highest among most heterogeneous catalysts.

The Growth of Photoactive Porphyrin-Based MOF Thin Films Using the Liquid-Phase Epitaxy Approach and their Optoelectronic Properties

This study reports on the optoelectronic properties of porphyrin-based metal–organic framework (MOF) thin films fabricated by a facile liquid-phase epitaxy approach. This approach affords the growth of MOF thin films that are free of morphological imperfections, more suitable for optoelectronic applications. Chemical modifications such as the porphyrin ligand metallation have been found to preserve the morphology of the grown films making this approach particularly suitable for molecular alteration of MOF thin film optoelectronic properties without compromising its mesoscale morphology significantly. Particularly, the metallation of the ligand was found to be effective to tune the MOF bandgap. These porphyrin-based MOF thin films were shown to function effectively as donor layers in solar cells based on a Fullerene-C₆₀ acceptor. The ability to fabricate MOF solar cells free of a liquid-phase acceptor greatly simplifies device fabrication and enables pairing of MOFs as light absorbers with a wide range of acceptors including non-fullerene acceptors.

Controlling Size, Defectiveness, and Fluorescence in Nanoparticle UiO-66 Through Water and Ligand Modulation

UiO-66, a zirconium(IV) metal-organic framework (MOF) comprised of six-metal clusters and terephthalic acid ligands, displays excellent thermal and chemical stability and has functions in gas storage, catalysis, selective adsorption, and drug delivery. Though the stability of UiO-66 is highly advantageous, simultaneous synthetic control over particle size and defectiveness of UiO-66 remains difficult to attain. Using an acid-free solvothermal synthesis, we demonstrate that particle size, defectiveness, and inherent fluorescence of UiO-66 can be precisely tuned using the molar ligand to metal ratio, quantified water content, and reaction time during synthesis. These three synthetic handles allow for reproducible modulation of UiO-66 defectiveness between 0 and 12% and particle size between 20 to 120 nm, while maintaining high crystallinity in the nanoparticles that were formed. We also find that particle defectiveness is linked to common over-estimation of particle size measurements obtained via dynamic light scattering (DLS) and propose a model to correct elevated hydrodynamic diameter measurements. Finally, we report inherent fluorescence of non-functionalized UiO-66, which exhibits peak fluorescence at a wavelength of 390 nm following excitation at 280 nm and is maximized in large, defect-free particles. Overall, this synthetic approach and characterization of defect, size, and fluorescence represent new opportunities to tune the physiochemical properties of UiO-66.

Flexibility in Metal–Organic Frameworks: A Basic Understanding

Much has been written about the fundamental aspects of the metal–organic frameworks (MOFs). Still, details concerning the MOFs with structural flexibility are not comprehensively understood. However, a dramatic increase in research activities concerning rigid MOFs over the years has brought deeper levels of understanding for their properties and applications. Nonetheless, robustness and flexibility of such smart frameworks are intriguing for different research areas such as catalysis, adsorption, etc. This manuscript overviews the different aspects of framework flexibility. The review has touched lightly on several ideas and proposals, which have been demonstrated within the selected examples to provide a logical basis to obtain a fundamental understanding of their synthesis and behavior to external stimuli.

Pore-Surface Engineering by Decorating Metal-Oxo Nodes with Phenylsilane to Give Versatile Super-Hydrophobic Metal-Organic Frameworks (MOFs)

Hydrophobization of metal-organic frameworks (MOFs) is important to push forward their practical use and thus has attracted increasing interest. In contrast to the previous reports, which mainly focused on the modification of organic ligands in MOFs, herein, we reported a novel strategy to decorate the metal-oxo nodes of MOFs with phenylsilane to afford super-hydrophobic NH₂ -UiO-66(Zr), which shows highly improved base resistance and holds great promise in versatile applications, such as organic/water separation, self-cleaning, and liquid-marble fabrication. This work demonstrates the first attempt at metal-oxo node modification for super-hydrophobic MOFs, advancing a new concept in the design of MOFs with controlled wettability for practical applications.

Highly Stable Zr(IV)-Based Metal-Organic Frameworks with Chiral Phosphoric Acids for Catalytic Asymmetric Tandem Reactions

Heterogeneous Brønsted acid catalysts featuring high porosity, crystallinity, and stability have been of great interest for both fundamental studies and practical applications, but synthetically, they still face a formidable challenge. Here, we illustrated a ligand design strategy for directly installing chiral phosphoric acid catalysts into highly stable Zr-MOFs by sterically protecting them from coordinating with metal ions. A pair of chiral porous Zr(IV)-MOFs with the framework formula [Zr₆O₄(OH)₈(H₂O)₄(L)₂] were prepared from enantiopure 4,4',6,6'-tetra(benzoate) and -tetra(2-naphthoate) ligands of 1,1'-spirobiindane-7,7'-phosphoric acid. They share the same topological structure but differ in channel sizes, and both of them demonstrate excellent tolerance toward water, acid and base. Significantly enhanced Brønsted acidity was observed for the phosphoric acids that are uniformly distributed within the frameworks in comparison with the nonimmobilized acids. This not only facilitates the catalysis of asymmetric two-component tandem acetalization, Friedel-Crafts, and iso-Pictet-Spengler reactions but also promotes the catalysis of asymmetric three-component tandem deacetalization-acetalization and Friedel-Crafts reactions benefiting from the synergy with exposed Lewis acidic Zr(IV) sites. The enantioselectivities are comparable or favorable compared to those obtained from the corresponding homogeneous systems. The features of high reactivity, selectivity, stability, and recyclability for Zr(IV)-MOFs make them hold promise as a new type of heterogeneous acid catalyst for the eco-friendly synthesis of fine chemicals.

Single-site metal-organic framework catalysts for the oxidative coupling of arenes via C-H/C-H activation

C-H activation reactions are generally associated with relatively low turnover numbers (TONs) and high catalyst concentrations due to a combination of low catalyst stability and activity, highlighting the need for recyclable heterogeneous catalysts with stable single-atom active sites. In this work, several palladium loaded metal-organic frameworks (MOFs) were tested as single-site catalysts for the oxidative coupling of arenes (e.g. o-xylene) via C-H/C-H activation. Isolation of the palladium active sites on the MOF supports reduced Pd(0) aggregate formation and thus catalyst deactivation, resulting in higher turnover numbers (TONs) compared to the homogeneous benchmark reaction. Notably, a threefold higher TON could be achieved for palladium loaded MOF-808 due to increased catalyst stability and the heterogeneous catalyst could efficiently be reused, resulting in a cumulative TON of 1218 after three runs. Additionally, the palladium single-atom active sites on MOF-808 were successfully identified by Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopy.

Enhancing Efficiency and Stability of Photovoltaic Cells by Using Perovskite/Zr-MOF Heterojunction Including Bilayer and Hybrid Structures

In this study, the effectiveness of using a perovskite/Zr-metal-organic frameworks (MOFs) heterojunction in realizing efficient and stable inverted p-i-n perovskite solar cells (PVSCs) is demonstrated. Two types of Zr-MOFs, UiO-66 and MOF-808, are investigated owing to their respectable moisture and chemical stabilities. The MOFs while serving as an interlayer in conjunction with the perovskite film are shown to possess the advantages of UV-filtering capability and enhancing perovskite crystallinity. Consequently, the UiO-66/MOF-808-modified PVSCs yield enhanced power conversion efficiencies (PCEs) of 17.01% and 16.55%, outperforming the control device (15.79%). While further utilizing a perovskite/Zr-MOF hybrid heterojunction to fabricate the devices, the hybrid MOFs are found to possibly distribute over the perovskite grain boundary providing a grain-locking effect to simultaneously passivate the defects and to reinforce the film's robustness against moisture invasion. As a result, the PCEs of the UiO-66/MOF-808-hybrid PVSCs are further enhanced to 18.01% and 17.81%, respectively. Besides, over 70% of the initial PCE is retained after being stored in air (25 °C and relative humidity of 60 ± 5%) for over 2 weeks, in contrast to the quick degradation observed for the control device. This study demonstrates the promising potential of using perovskite/MOF heterojunctions to fabricate efficient and stable PVSCs.

Cobalt-bridged secondary building units in a titanium metal-organic framework catalyze cascade reduction of N-heteroarenes

We report here a novel Ti3-BPDC metal-organic framework (MOF) constructed from biphenyl-4,4'-dicarboxylate (BPDC) linkers and Ti3(OH)2 secondary building units (SBUs) with permanent porosity and large 1D channels. Ti-OH groups from neighboring SBUs point toward each other with an O-O distance of 2 Å, and upon deprotonation, act as the first bidentate SBU-based ligands to support CoII-hydride species for effective cascade reduction of N-heteroarenes (such as pyridines and quinolines) via sequential dearomative hydroboration and hydrogenation, affording piperidine and 1,2,3,4-tetrahydroquinoline derivatives with excellent activity (turnover number ∼ 1980) and chemoselectivity.

Cluster nuclearity control and modulated hydrothermal synthesis of functionalized Zr12 metal–organic frameworks

Cluster nuclearity control over the SBUs of zirconium MOFs to target Zr₆-based and Zr₂-based phases is demonstrated for the Zr terephthalate system (Zr-BDC) using a modulated hydrothermal synthesis method.

Metal–organic framework-based materials for the recovery of uranium from aqueous solutions

This review focuses on the recent progress in MOFs and MOF-based materials as superior adsorbents for the efficient removal of uranium from aqueous solutions.

[Zr6O4(OH)4(benzene-1,4-dicarboxylato)6]n: a hexagonal polymorph of UiO-66

The first polymorph of the paradigmatic UiO-66 and the key conditions that led to its formation are reported.

A superstable 3p-block metal–organic framework platform towards prominent CO2 and C1/C2-hydrocarbon uptake and separation performance and strong Lewis acid catalysis for CO2 fixation

Superstable 3p-block MOF platforms exhibit excellent gas uptake and separation performance, and prominent Lewis acid catalysis for CO₂ fixation.

Multimetal organic frameworks as drug carriers: aceclofenac as a drug candidate

BACKGROUND

Multimetal organic frameworks (M-MOFs) were synthesized by including a second metal ion with the main base metal in the synthesis process to enhance their applications for drug delivery. Aceclofenac (ACF), a nonsteroidal anti-inflammatory analgesic drug of low aqueous solubility, was selected as a candidate for the drug delivery system.

PURPOSE

This study aimed to evaluate the loading capacity (LC) and entrapment efficiency (EE) percentages of multi-Material of Institute Lavoisier (MIL)-100(Fe) (M-MIL-100(Fe)) for ACF.

MATERIALS AND METHODS

Hydrothermal synthesis procedure was used to prepare multi-MIL-100(Fe) samples (Zn I-MIL-100(Fe), Zn II-MIL-100(Fe), Ca I-MIL-100(Fe), Ca II-MIL-100-(Fe), Mg I-MIL-100(Fe), Mg II-MIL-100(Fe), Mn I-MIL-100(Fe), and Mn II-MIL-100(Fe)). The characterization of M-MIL-100(Fe) samples was evaluated by X-ray powder diffraction (XRD), Fourier transform infrared spectra, scanning electron microscope (SEM), TGA, and N2 adsorption isotherms. The LC of M-MIL-100(Fe) and EE of ACF were determined. Nuclear magnetic resonance (NMR) and zeta-potential analyses were employed to confirm qualitatively the drug loading within M-MIL-100(Fe).

RESULTS

The ACF LC of MIL-100(Fe) was 27%, whereas the LC of M-MIL-100(Fe) was significantly increased and ranged from 37% in Ca I-MIL-100(Fe) to about 57% and 59% in Mn II-MIL-100(Fe) and Zn II-MIL-100(Fe), respectively. The ACF@M-MOFs release profiles showed slow release rates in phosphate buffer solutions at pH 6.8 and 7.4 as compared to the ACF@MIL-100(Fe).

CONCLUSION

Therefore, M-MOFs showed a significant potential as a carrier for drug delivery systems.

Sequential Transformation of Zirconium(IV)-MOFs into Heterobimetallic MOFs Bearing Magnetic Anisotropic Cobalt(II) Centers

Heterometallic metal-organic frameworks (MOFs) allow the precise placement of various metals at atomic precision within a porous framework. This new level of control by MOFs promises fascinating advances in basic science and application. However, the rational design and synthesis of heterometallic MOFs remains a challenge due to the complexity of the heterometallic systems. Herein, we show that bimetallic MOFs with MX₂ (INA)₄ moieties (INA=isonicotinate; M=Co²⁺ or Fe²⁺ ; X=OH^- , Cl^- , Br^- , I^- , NCS^- , or NCSe^- ) can be generated by the sequential modification of a Zr-based MOF. This multi-step modification not only replaced the linear organic linker with a square planar MX₂ (INA)₄ unit, but also altered the symmetry, unit cell, and topology of the parent structure. Single-crystal to single-crystal transformation is realized so that snapshots for transition process were captured by successive single-crystal X-ray diffraction. Furthermore, the installation of Co(NCS)₂ (INA)₄ endows field-induced slow magnetic relaxation property to the diamagnetic Zr-MOF.