Node-Accessible Zirconium MOFs

Defect-Engineered Metal-Organic Framework/Polyimide Mixed Matrix Membrane for CO2 Separation

Defect-engineered metal-organic frameworks (MOFs) with outstanding structural and chemical features have become excellent candidates for specific separation applications. The introduction of structural defects in MOFs as an efficient approach to manipulate their functionality provides excellent opportunities for the preparation of MOF-based mixed matrix membranes (MMMs). However, the use of this strategy to adjust the properties and develop the separation performance of gas separation membranes is still in its early stages. Here, a novel defect-engineered MOF (quasi ZrFum or Q-ZrFum) was synthesized via a controlled thermal deligandation process and incorporated into a CO2-philic 6FDA-durene polyimide (PI) matrix to form Q-ZrFum loaded MMMs. Defect-engineered MOFs and fabricated MMMs were investigated regarding their characteristic properties and separation performance. The incorporation of defects into the MOF structure increases the pore size and provides unsaturated active metal sites that positively affect CO2 molecule transport. The interfacial compatibility between the Q-ZrFum particles and the PI matrix increases via the deligandation process, which improves the mechanical strength of Q-ZrFum loaded membranes. MMM containing 5 wt.% of defect-engineered Q-ZrFum exhibits excellent CO2 permeability of 1308 Barrer, which increased by 99 % compared to the pure PI membrane (656 Barrer) at a feed pressure of 2 bar. CO2/CH4 and CO2/N2 selectivity reached 44 and 26.6 which increased by about 70 and 16 %, respectively. This study emphasizes that defect-engineered MOFs can be promising candidates for use as fillers in the preparation of MMMs for the future development of membrane-based gas separation applications.

Defect-enabling zirconium-based metal-organic frameworks for energy and environmental remediation applications

This comprehensive review explores the diverse applications of defective zirconium-based metal-organic frameworks (Zr-MOFs) in energy and environmental remediation. Zr-MOFs have gained significant attention due to their unique properties, and deliberate introduction of defects further enhances their functionality. The review encompasses several areas where defective Zr-MOFs exhibit promise, including environmental remediation, detoxification of chemical warfare agents, photocatalytic energy conversions, and electrochemical applications. Defects play a pivotal role by creating open sites within the framework, facilitating effective adsorption and remediation of pollutants. They also contribute to the catalytic activity of Zr-MOFs, enabling efficient energy conversion processes such as hydrogen production and CO2 reduction. The review underscores the importance of defect manipulation, including control over their distribution and type, to optimize the performance of Zr-MOFs. Through tailored defect engineering and precise selection of functional groups, researchers can enhance the selectivity and efficiency of Zr-MOFs for specific applications. Additionally, pore size manipulation influences the adsorption capacity and transport properties of Zr-MOFs, further expanding their potential in environmental remediation and energy conversion. Defective Zr-MOFs exhibit remarkable stability and synthetic versatility, making them suitable for diverse environmental conditions and allowing for the introduction of missing linkers, cluster defects, or post-synthetic modifications to precisely tailor their properties. Overall, this review highlights the promising prospects of defective Zr-MOFs in addressing energy and environmental challenges, positioning them as versatile tools for sustainable solutions and paving the way for advancements in various sectors toward a cleaner and more sustainable future.

Analyzing Stabilities of Metal-Organic Frameworks: Correlation of Stability with Node Coordination to Linkers and Degree of Node Metal Hydrolysis

Among the important properties of metal-organic frameworks (MOFs) is stability, which may limit applications, for example, in separations and catalysis. Many MOFs consist of metal oxo cluster nodes connected by carboxylate linkers. Addressing MOF stability, we highlight connections between metal oxo cluster chemistry and MOF node chemistry, including results characterizing Keggin ions and biological clusters. MOF syntheses yield diverse metal oxo cluster node structures, with varying numbers of metal atoms (3-13) and the tendency to form chains. MOF stabilities reflect a balance between the number of node-linker connections and the degree of node hydrolysis. We summarize literature results showing how MOF stability (the temperature of decomposition in air) depends on the degree of hydrolysis/condensation of the node metals, which is correlated to their degree of substitution with linkers. We suggest that this correlation may help guide the discovery of stable new MOFs, and we foresee opportunities for progress in MOF chemistry emerging from progress in metal oxo cluster chemistry.

Two-dimensional metal-organic framework for post-synthetic immobilization of graphene quantum dots for photoluminescent sensing

Immobilization of graphene quantum dots (GQDs) on a solid support is crucial to prevent GQDs from aggregation in the form of solid powder and facilitate the separation and recycling of GQDs after use. Herein, spatially dispersed GQDs are post-synthetically coordinated within a two-dimensional (2D) and water-stable zirconium-based metal-organic framework (MOF). Unlike pristine GQDs, the obtained GQDs immobilized on 2D MOF sheets show photoluminescence in both suspension and dry powder. Chemical and photoluminescent stabilities of MOF-immobilized GQDs in water are investigated, and the use of immobilized GQDs in the photoluminescent detection of copper ions is demonstrated. Findings here shed the light on the use of 2D MOFs as a platform to further immobilize GQDs with various sizes and distinct chemical functionalities for a range of applications.

Multifunctional Metal-Organic Frameworks as Catalysts for Tandem Reactions

Owing to well-defined structure as well as easy synthesis and modification, metal-organic frameworks (MOFs) have emerged as promising catalysts for tandem reactions. In this article, we aim to summarize the development of multifunctional MOFs, including mixed metal MOFs, MOFs that are synergistically catalyzed by metal nodes and organic linkers, MOFs loaded with metal nanoparticles, etc, as heterogenous catalysts for tandem reactions over the past five years. This concept briefly discusses on present challenges, future trends, and prospects of multifunctional MOFs catalysts in tandem reactions.

Chemically Reversible CO2 Uptake by Dendrimer-Impregnated Metal-Organic Frameworks

Industrialization over the past two centuries has resulted in a continuous rise in global CO2 emissions. These emissions are changing ecosystems and livelihoods. Therefore, methods are needed to capture these emissions from point sources and possibly from our atmosphere. Though the amount of CO2 is rising, it is challenging to capture directly from air because its concentration in air is extremely low, 0.04%. In this study, amines installed inside metal-organic frameworks (MOFs) are investigated for the adsorption of CO2, including at low concentrations. The amines used are polyamidoamine dendrimers that contain many primary amines. Chemically reversible adsorption of CO2 via carbamate formation was observed, as was enhanced uptake of carbon dioxide, likely via dendrimer-amide-based physisorption. Limiting factors in this initial study are comparatively low dendrimer loadings and slow kinetics for carbon dioxide uptake and release, even at 80 °C.

Design and Advanced Manufacturing of NU-1000 Metal-Organic Frameworks with Future Perspectives for Environmental and Renewable Energy Applications

Metal-organic frameworks (MOFs) represent a relatively new family of materials that attract lots of attention thanks to their unique features such as hierarchical porosity, active metal centers, versatility of linkers/metal nodes, and large surface area. Among the extended list of MOFs, Zr-based-MOFs demonstrate comparably superior chemical and thermal stabilities, making them ideal candidates for energy and environmental applications. As a Zr-MOF, NU-1000 is first synthesized at Northwestern University. A comprehensive review of various approaches to the synthesis of NU-1000 MOFs for obtaining unique surface properties (e.g., diverse surface morphologies, large surface area, and particular pore size distribution) and their applications in the catalysis (electro-, and photo-catalysis), CO2 reduction, batteries, hydrogen storage, gas storage/separation, and other environmental fields are presented. The review further outlines the current challenges in the development of NU-1000 MOFs and their derivatives in practical applications, revealing areas for future investigation.

How Reproducible is the Synthesis of Zr-Porphyrin Metal-Organic Frameworks? An Interlaboratory Study

Metal-organic frameworks (MOFs) are a rapidly growing class of materials that offer great promise in various applications. However, the synthesis remains challenging: for example, a range of crystal structures can often be accessed from the same building blocks, which complicates the phase selectivity. Likewise, the high sensitivity to slight changes in synthesis conditions may cause reproducibility issues. This is crucial, as it hampers the research and commercialization of affected MOFs. Here, it presents the first-ever interlaboratory study of the synthetic reproducibility of two Zr-porphyrin MOFs, PCN-222 and PCN-224, to investigate the scope of this problem. For PCN-222, only one sample out of ten was phase pure and of the correct symmetry, while for PCN-224, three are phase pure, although none of these show the spatial linker order characteristic of PCN-224. Instead, these samples resemble dPCN-224 (disordered PCN-224), which has recently been reported. The variability in thermal behavior, defect content, and surface area of the synthesised samples are also studied. The results have important ramifications for field of metal-organic frameworks and their crystallization, by highlighting the synthetic challenges associated with a multi-variable synthesis space and flat energy landscapes characteristic of MOFs.

Uranyl uptake into metal-organic frameworks: a detailed X-ray structural analysis

Metal-organic frameworks (MOF) are a subclass of porous framework materials that have been used for a wide variety of applications in sensing, catalysis, and remediation. Among these myriad applications is their remarkable ability to capture substances in a variety of environments ranging from benign to extreme. Among the most common and problematic substances found throughout the world's oceans and water supplies is [UO2]2+, a common mobile ion of uranium, which is found both naturally and as a result of anthropogenic activities, leading to problematic environmental contamination. While some MOFs possess high capability for the uptake of [UO2]2+, many more of the thousands of MOFs and their modifications that have been produced over the years have yet to be studied for their ability to uptake [UO2]2+. However, studying the thousands of MOFs and their modifications presents an incredibly difficult task. As such, a way to narrow down the numbers seems imperative. Herein, we evaluate the binding behaviors as well as identify the specific binding sites of [UO2]2+ incorporated into six different Zr MOFs to elucidate specific features that improve [UO2]2+ uptake. In doing so, we also present a method for the determination and verification of these binding sites by Anomalous wide-angle X-ray scattering, X-ray fluorescence, and X-ray absorption spectroscopy. This research not only presents a way for future research into the uptake of [UO2]2+ into MOFs to be conducted but also a means to evaluate MOFs more generally for the uptake of other compounds to be applied for environmental remediation and improvement of ecosystems globally.

Sulfur atom-directed metal-ligand synergistic catalysis in zirconium/hafnium-oxo clusters for highly efficient amine oxidation

The performance applications (e.g., photocatalysis) of zirconium (Zr) and hafnium (Hf) based complexes are greatly hindered by the limited development of their structures and the relatively inert metal reactivity. In this work, we constructed two ultrastable Zr/Hf-based clusters (Zr9-TC4A and Hf9-TC4A) using hydrophobic 4-tert-butylthiacalix[4]arene (H4TC4A) ligands, in which unsaturated coordinated sulfur (S) atoms on the TC4A4- ligand can generate strong metal-ligand synergy with nearby active metal Zr/Hf sites. As a result, these two functionalized H4TC4A ligands modified Zr/Hf-oxo clusters, as catalysts for the amine oxidation reaction, exhibited excellent catalytic activity, achieving very high substrate conversion (>99%) and product selectivity (>90%). Combining comparative experiments and theoretical calculations, we found that these Zr/Hf-based cluster catalysts accomplish efficient amine oxidation reactions through synergistic effect between metals and ligands: (i) The photocatalytic benzylamine (BA) oxidation reaction was achieved by the synergistic effect of the dual active sites, in which, the naked S sites on the TC4A4- ligand oxidize the BA by photogenerated hole and oxygen molecules are reduced by photogenerated electrons on the metal active sites; (ii) in the aniline oxidation reaction, aniline was adsorbed by the bare S sites on ligands to be closer to metal active sites and then oxidized by the oxygen-containing radicals activated by the metal sites, thus completing the catalytic reaction under the synergistic catalytic effect of the proximity metal-ligand. In this work, the Zr/Hf-based complexes applied in the oxidation of organic amines have been realized using active S atom-directed metal-ligand synergistic catalysis and have demonstrated very high reactivity.

Metal-Organic Frameworks Constructed from Branched Oligomers

Metal-organic frameworks (MOFs) prepared from oligomeric or polymeric organic ligands have been studied and are termed oligoMOFs and polyMOFs, respectively. Herein, several oligoMOFs are described that have been prepared from branched oligomers with dendritic or star-like architectures. Branched oligomeric ligands with four (4(H2bdc)-b) or eight (8(H2bdc)-b) 1,4-benzene dicarboxylic acid (H2bdc) groups were prepared and used to synthesize isoreticular-type Zn(II)-based MOFs (IRMOF). A branched tetramer (4(H2bdc)-b) produced an oligoIRMOF-1 with improved ambient stability compared with IRMOF-1 or previously described oligoMOFs. To understand the effect of the ligand architecture, oligoIRMOFs were also prepared from a linear tetramer (4(H2bdc)-l). For a branched octamer (8(H2bdc)-b), it was found that the addition of an organic base was required to produce crystalline oligoIRMOFs. Multivariate MOFs (MTV-MOFs) could also be readily prepared with a combination of an octamer (8(H2bdc)-b) and H2bdc.

A novel threefold interpenetrated zirconium metal-organic framework exhibiting separation ability for strong acids

We report on the synthesis and selective adsorption property of a novel threefold interpenetrated Zr-based metal-organic framework (MOF), [Zr12O8(OH)8(HCOO)15(BPT)3] (BPT3- = [1,1'-biphenyl]-3,4',5-tricarboxylate) (abbreviated as Zr-BPT). This MOF shows a high tolerance to acidic conditions and has permanent pores, the size of which (approx. <5.6 Å) is the smallest ever reported among porous Zr-based MOFs with high acid tolerance. Zr-BPT selectively adsorbs aryl acids due to its strong affinity for them and exhibits separation ability, even between strong acid molecules, such as sulfonic and phosphonic acids. This is the first demonstration of a MOF exhibiting selective adsorption and separation ability for strong acids.

A Nanocavitation Approach to Understanding Water Capture, Water Release, and Framework Physical Stability in Hierarchically Porous MOFs

Chemically stable metal-organic frameworks (MOFs) featuring interconnected hierarchical pores have proven to be promising for a remarkable variety of applications. Nevertheless, the framework's susceptibility to capillary-force-induced pore collapse, especially during water evacuation, has often limited practical applications. Methodologies capable of predicting the relative magnitudes of these forces as functions of the pore size, chemical composition of the pore walls, and fluid loading would be valuable for resolution of the pore collapse problem. Here, we report that a molecular simulation approach centered on evacuation-induced nanocavitation within fluids occupying MOF pores can yield the desired physical-force information. The computations can spatially pinpoint evacuation elements responsible for collapse and the chemical basis for mitigation of the collapse of modified pores. Experimental isotherms and difference-electron density measurements of the MOF NU-1000 and four chemical variants validate the computational approach and corroborate predictions regarding relative stability, anomalous sequence of pore-filling, and chemical basis for mitigation of destructive forces.

Geometry and Chemistry: Influence of Pore Functionalization on Molecular Transport and Diffusion in Solvent-Filled Zirconium Metal-Organic Frameworks

Postsynthetic modification (PSM) of metal-organic frameworks (MOFs) enables incorporation of diverse functionalities in pores for chemical separations, drug delivery, and heterogeneous catalysis. However, the effect of PSM on molecular transport, which is essential for most applications of MOFs, has been rarely studied. In this paper, we used perfluoroalkane-functionalized Zr-MOF NU-1008 as a platform to systematically interrogate transport processes and mechanisms in solvated pores. We anchored perfluoroalkanes onto NU-1008 nodes by solvent-assisted ligand incorporation (SALI-n, with n = 3, 5, 7, and 9 denoting the number of fluorinated carbons). Transport of a luminescent molecule, BODIPY, through individual crystallites of four versions of methanol-filled SALI-n was monitored by confocal fluorescence microscopy as a function of time and location. In comparison with the parent NU-1008, the diffusivity of the probe molecules within SALI-n declined by 2- to 7-fold depending on chain length and loading, presumably due to the reduction in pore diameter or adsorptive interactions with perfluoroalkyl chains. Atomistic simulations were performed to uncover the microscopic behavior of the BODIPY diffusion in SALI-n. The perfluoroalkyl chains are observed to stay close to the pore walls, instead of extending toward the pore center. BODIPY molecules, which preferably interact with linkers, were pushed to the interior of the channels as the chain length increased, resulting in solvated diffusion and minor differences in the short-time mobility of BODIPY in SALI-n. This suggested that the observed decline of transport diffusivity in SALI-n mainly stemmed from the reduction in the pore size when these flexible chains are present. We anticipate that this proof of concept will assist in understanding how pore functionalization can physically and chemically affect mass transport in MOFs and will be useful in further guiding the design of PSM to realize the optimal performance of MOFs for various applications.

"Ship-in-a-Bottle" Integration of Ditin(IV) Sites into a Metal-Organic Framework for Boosting Electroreduction of CO2 in Acidic Electrolyte

The electrochemical CO2 reduction reaction (eCO2RR) under acidic conditions has become a promising way to achieve high CO2 utilization because of the inhibition of undesirable carbonate formation that typically occurs under neutral and alkaline conditions. Herein, unprecedented and highly active ditin(IV) sites were integrated into the nanopores of a metal-organic framework, namely NU-1000-Sn, by a "ship-in-a-bottle" strategy. NU-1000-Sn delivers nearly 100% formic acid Faradaic efficiency at an industry current density of 260 mA cm-2 with a high single-pass CO2 utilization of 95% in an acidic solution (pH = 1.67). No obvious degradation was observed over 15 hours of continuous operation at the current density of 260 mA cm-2, representing the remarkable eCO2RR performance in acidic electrolyte to date. The mechanism study shows that both oxygen atoms of the key intermediate *HCOO can coordinate to the two adjacent Sn atoms in a ditin(IV) site simultaneously. Such bridging coordination is conducive to the hydrogenation of CO2, thus leading to high performance.

Three dimensional cyclic trinuclear units based metal-covalent organic frameworks for electrochemical CO2RR

A three-dimensional metal-covalent organic framework (3D-MCOF) based on cyclic trinuclear units was synthesized using organic tetrahedral linkers and copper-based cyclic trinuclear complexes. The novel type of 3D-MCOF, named 3D-CTU-MCOF, with the ctn topology, is reported herein for the first time. Our study demonstrated enhanced electrocatalytic capacity for CO2 reduction reaction of 3D-CTU-MCOF compared to independent cyclic trinuclear units.

Rationally Tailored Mesoporous Hosts for Optimal Protein Encapsulation

Proteins play important roles in the therapeutic, medical diagnostic, and chemical catalysis industries. However, their potential is often limited by their fragile and dynamic nature outside cellular environments. The encapsulation of proteins in solid materials has been widely pursued as a route to enhance their stability and ease of handling. Nevertheless, the experimental investigation of protein interactions with rationally designed synthetic hosts still represents an area in need of improvement. In this work, we leveraged the tunability and crystallinity of metal-organic frameworks (MOFs) and developed a series of crystallographically defined protein hosts with varying chemical properties. Through systematic studies, we identified the dominating mechanisms for protein encapsulation and developed a host material with well-tailored properties to effectively encapsulate the protein ubiquitin. Specifically, in our mesoporous hosts, we found that ubiquitin encapsulation is thermodynamically favored. A more hydrophilic encapsulation environment with favorable electrostatic interactions induces enthalpically favored ubiquitin-MOF interactions, and a higher pH condition reduces the intraparticle diffusion barrier, both leading to a higher protein loading. Our findings provide a fundamental understanding of host-guest interactions between proteins and solid matrices and offer new insights to guide the design of future protein host materials to achieve optimal protein loading. The MOF modification technique used in this work also demonstrates a facile method to develop materials easily customizable for encapsulating proteins with different surface properties.

Synergy of Paired Brønsted-Lewis Acid Sites on Defects of Zr-MIL-140A for Methanol Dehydration

As a common defect-capping ligand in metal-organic frameworks (MOFs), the hydroxyl group normally exhibits Brønsted acidity or basicity, but the presence of inherent hydroxyl groups in the MOF structure makes it a great challenge to identify the exact role of defect-capping hydroxyl groups in catalysis. Herein, we used hydroxyl-free MIL-140A as the platform to generate terminal hydroxyl groups on defect sites via a continuous post-synthetic treatment. The structure and acidity of MIL-140A were properly characterized. The hydroxyl-contained MIL-140A-OH exhibited 4.6-fold higher activity than the pristine MIL-140A in methanol dehydration. Spectroscopic and computational investigations demonstrated that the reaction was initiated by the respective adsorption of two methanol molecules on the terminal-OH and the adjacent Zr vacancy. The dehydration of the adsorbed methanol molecules then occurred in the Brønsted-Lewis acid site co-participated associative pathway with the lowest energy barrier.

Water-Accelerated Transport: Vapor-Phase Nerve Agent Simulant Delivery within a Catalytic Zirconium Metal-Organic Framework as a Function of Relative Humidity

Zirconium-based metal-organic frameworks (MOFs) are candidate materials for effective nerve agent detoxification due to their thermo- and water stability as well as high density of catalytic Zr sites. However, as high-porosity materials, most of the active sites of Zr-MOFs can only be accessed by diffusion into the crystal interior. Therefore, the transport of nerve agents in nanopores is an important factor in the catalytic performance of Zr-MOFs. Here, we investigated the transport process and mechanism of a vapor-phase nerve agent simulant, dimethyl methyl phosphonate (DMMP), through a representative Zr-MOF, NU-1008, under practical conditions of varying humidity. Confocal Raman microscopy was used to monitor the transport of DMMP vapor through individual NU-1008 crystallites, where the relative humidity (RH) of the environment was tuned to understand the impact of water. Counterintuitively, water in the MOF channels, instead of blocking DMMP transport, assists DMMP diffusion; indeed, the transport diffusivity (D_t) of DMMP in NU-1008 is one order of magnitude higher at 70% than 0% RH. To understand the mechanism, magic angle spinning NMR and molecular dynamics simulations were performed and suggested that high water content in the channels prevents DMMP from hydrogen-bonding with the nodes, allowing for faster diffusion of DMMP in the channels. The simulated self-diffusivity (D_s) of DMMP is observed to be concentration-dependent. At low loading of DMMP, D_s is higher at 70% RH than 0% RH, while at high loadings the trend reverses due to the DMMP aggregation in water and the reduction of free volume in channels.

Unveiling Unexpected Modulator-CO2 Dynamics within a Zirconium Metal-Organic Framework

Carbon capture, storage, and utilization (CCSU) represents an opportunity to mitigate carbon emissions that drive global anthropogenic climate change. Promising materials for CCSU through gas adsorption have been developed by leveraging the porosity, stability, and tunability of extended crystalline coordination polymers called metal-organic frameworks (MOFs). While the development of these frameworks has yielded highly effective CO₂ sorbents, an in-depth understanding of the properties of MOF pores that lead to the most efficient uptake during sorption would benefit the rational design of more efficient CCSU materials. Though previous investigations of gas-pore interactions often assumed that the internal pore environment was static, discovery of more dynamic behavior represents an opportunity for precise sorbent engineering. Herein, we report a multifaceted in situ analysis following the adsorption of CO₂ in MOF-808 variants with different capping agents (formate, acetate, and trifluoroacetate: FA, AA, and TFA, respectively). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis paired with multivariate analysis tools and in situ powder X-ray diffraction revealed unexpected CO₂ interactions at the node associated with dynamic behavior of node-capping modulators in the pores of MOF-808, which had previously been assumed to be static. MOF-808-TFA displays two binding modes, resulting in higher binding affinity for CO₂. Computational analyses further support these dynamic observations. The beneficial role of these structural dynamics could play an essential role in building a deeper understanding of CO₂ binding in MOFs.

Design and Application of Materials for Sequestration and Immobilization of 99Tc

⁹⁹Technetium (⁹⁹Tc) is a hazardous radionuclide that poses a serious environmental threat. The wide variation and complex chemistries of liquid nuclear waste streams containing ⁹⁹Tc often create unique, site specific challenges when sequestering and immobilizing the waste in a matrix suitable for long-term storage and disposal. Therefore, an effective management plan for ⁹⁹Tc containing liquid radioactive wastes (such as storage (tanks) and decommissioned wastes) will likely require a variety of suitable materials/matrixes capable of adapting to and addressing these challenges. In this review, we discuss and highlight the key developments for effective removal and immobilization of ⁹⁹Tc liquid waste in inorganic waste forms. Specifically, we review the synthesis, characterization, and application of materials for the targeted removal of ⁹⁹Tc from (simulated) waste solutions under various experimental conditions. These materials include (i) layered double hydroxides (LDHs), (ii) metal-organic frameworks (MOFs), (iii) ion-exchange resins (IERs) as well as cationic organic polymers (COPs), (iv) surface modified natural clay materials (SMCMs), and (v) graphene-based materials (GBMs). Second, we discuss some of the major and recent developments toward ⁹⁹Tc immobilization in (i) glass, (ii) cement, and (iii) iron mineral waste forms. Finally, we present future challenges that need to be addressed for the design, synthesis, and selection of suitable matrixes for the efficient sequestration and immobilization of ⁹⁹Tc from targeted wastes. The purpose of this review is to inspire research on the design and application of various suitable materials/matrixes for selective removal of ⁹⁹Tc present globally in different radioactive wastes and its immobilization in stable/durable waste forms.

A sensing platform for on-site detection of glutathione S-transferase using oxidized Pi@Ce-doped Zr-based metal-organic frameworks(MOFs)

The development of point-of-care testing (POCT) for glutathione S-transferase (GST) is an effective way to establish the mechanism of targeted monitoring of cancer chemotherapy drug metabolism. Assays for GST with high sensitivity as well as on-site screening have been urgently required to monitor this process. Herein, we synthesized oxidized Pi@Ce-doped Zr-based metal-organic frameworks (MOFs) by electrostatic self-assembly between phosphate and oxidized Ce-doped Zr-based MOFs. It was found that the oxidase-like activity of oxidized Pi@Ce-doped Zr-based MOFs was substantially increased after phosphate ion (Pi) assembly. And a stimulus-responsive hydrogel-based kit was constructed by embedding oxidized Pi@Ce-doped Zr-based MOFs into a PVA (polyvinyl alcohol) hydrogel system, we integrated a portable hydrogel kit with a smartphone for real-time monitoring of GST for quantitative and accurate analysis. The color reaction was triggered based on oxidized Pi@Ce-doped Zr-based MOFs with 3,3',5,5'-tetramethylbenzidine (TMB). However, in the presence of glutathione (GSH), the above color reaction was hindered due to the reducibility of GSH. Catalyzed by GST, GSH can react with 1-chloro-2,4-dinitrobenzo (CDNB) to form an adduct, which caused the color reaction to occur again, resulting in the color response of the kit. In combination with ImageJ software, the kit image information acquired by smartphone could be converted into hue intensity, providing a direct quantitative tool for the detection of GST with a detection limit of 0.19mU·L^-1. Based on the advantages of simple operation and cost-effectiveness, the introduction of the POCT miniaturized biosensor platform will meet the requirements of on-site quantitative analysis of GST.

Synthetic Access to a Framework-Stabilized and Fully Sulfided Analogue of an Anderson Polyoxometalate that is Catalytically Competent for Reduction Reactions

Polyoxometalates (POMs) featuring 7, 12, 18, or more redox-accessible transition metal ions are ubiquitous as selective catalysts, especially for oxidation reactions. The corresponding synthetic and catalytic chemistry of stable, discrete, capping-ligand-free polythiometalates (PTMs), which could be especially attractive for reduction reactions, is much less well developed. Among the challenges are the propensity of PTMs to agglomerate and the tendency for agglomeration to block reactant access of catalyst active sites. Nevertheless, the pervasive presence of transition metal sulfur clusters metalloenzymes or cofactors that catalyze reduction reactions and the justifiable proliferation of studies of two-dimensional (2D) metal-chalcogenides as reduction catalysts point to the promise of well-defined and controllable PTMs as reduction catalysts. Here, we report the fabrication of agglomeration-immune, reactant-accessible, capping-ligand-free Co^IIMo₆^IVS₂₄^n- clusters as periodic arrays in a water-stable, hierarchically porous Zr-metal-organic framework (MOF; NU1K) by first installing a disk-like Anderson polyoxometalate, Co^IIIMo₆^VIO₂₄^m-, in size-matched micropores where the siting is established via difference electron density (DED) X-ray diffraction (XRD) experiments. Flowing H₂S, while heating, reduces molybdenum(VI) ions to Mo(IV) and quantitatively replaces oxygen anions with sulfur anions (S^2-, HS^-, S₂^2-). DED maps show that MOF-templated POM-to-PTM conversion leaves clusters individually isolated in open-channel-connected micropores. The structure of the immobilized cluster as determined, in part, by X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) analysis, and pair distribution function (PDF) analysis of total X-ray scattering agrees well with the theoretically simulated structure. PTM@MOF displays both electrocatalytic and photocatalytic competency for hydrogen evolution. Nevertheless, the initially installed PTM appears to be a precatalyst, gaining competency only after the loss of ∼3 to 6 sulfurs and exposure to hydride-forming metal ions.

Isomer of NU-1000 with a Blocking c-pore Exhibits High Water-Vapor Uptake Capacity and Greatly Enhanced Cycle Stability

Chemically and hydrolytically stable metal-organic frameworks (MOFs) have shown great potential for many water-adsorption-related applications. However, MOFs with large pores that show high water-uptake capacity and high hydrolytic and mechanical cycle stability are rare. Through a deliberate adjustment of the linker of a typical zirconium-based MOF (Zr-MOF) (NU-1000), a new isomer of NU-1000 with blocked c-pores, but large mesopores was successfully synthesized. This new isomer, ISO-NU-1000, exhibits excellent water stability, one of the highest water vapor uptake capacities, and excellent cycle stability, making it a promising candidate for water-vapor-sorption-based applications such as water-adsorption-driven heat transfer. We find that the high water-cycling stability of ISO-NU-1000 is traceable to its blocking c-pore that hinders the hydrolysis of node-coordinating formate in the c-pore area and thereby prevents the introduction of node aqua and terminal hydroxo ligands. With the absence of these ligands and their ability to hydrogen-bond to channel-located water molecules, the strength of guest (water)/host (MOF) interactions is diminished and the absolute magnitude of the capillary force exerted by water during its evacuation from MOF channels is attenuated. The attenuation leaves the MOF capable of resisting pore collapse, capacity loss, and crystallinity loss during repetitive evaporative removal (and re-introduction) of water from pores.

Photocatalytic CO2 -to-Syngas Evolution with Molecular Catalyst Metal-Organic Framework Nanozymes

Syngas, a mixture of CO and H₂ , is a high-priority intermediate for producing several commodity chemicals, e.g., ammonia, methanol, and synthetic hydrocarbon fuels. Accordingly, parallel sunlight-driven catalytic conversion of CO₂ and protons to syngas is a key step toward a sustainable energy cycle. State-of-the-art catalytic systems and materials often fall short as application-oriented concurrent CO and H₂ evolution requires challenging reaction conditions which can hamper stability, selectivity, and efficiency. Here a light-harvesting metal-organic framework hosting two molecular catalysts is engineered to yield colloidal, water-stable, versatile nanoreactors for photocatalytic syngas generation with highly controllable product ratios. In-depth fluorescence, X-ray, and microscopic studies paired with kinetic analysis show that the host delivers energy efficiently to active sites, conceptually yielding nanozymes. This unlocked sustained CO₂ reduction and H₂ evolution with benchmark turnover numbers and record incident photon conversions up to 36%, showcasing a highly active and durable all-in-one material toward application in solar energy-driven syngas generation.

Node Distortion as a Tunable Mechanism for Negative Thermal Expansion in Metal-Organic Frameworks

Chemically functionalized series of metal-organic frameworks (MOFs), with subtle differences in local structure but divergent properties, provide a valuable opportunity to explore how local chemistry can be coupled to long-range structure and functionality. Using in situ synchrotron X-ray total scattering, with powder diffraction and pair distribution function (PDF) analysis, we investigate the temperature dependence of the local- and long-range structure of MOFs based on NU-1000, in which Zr₆O₈ nodes are coordinated by different capping ligands (H₂O/OH, Cl^- ions, formate, acetylacetonate, and hexafluoroacetylacetonate). We show that the local distortion of the Zr₆ nodes depends on the lability of the ligand and contributes to a negative thermal expansion (NTE) of the extended framework. Using multivariate data analyses, involving non-negative matrix factorization (NMF), we demonstrate a new mechanism for NTE: progressive increase in the population of a smaller, distorted node state with increasing temperature leads to global contraction of the framework. The transformation between discrete node states is noncooperative and not ordered within the lattice, i.e., a solid solution of regular and distorted nodes. Density functional theory calculations show that removal of ligands from the node can lead to distortions consistent with the Zr···Zr distances observed in the experiment PDF data. Control of the node distortion imparted by the nonlinker ligand in turn controls the NTE behavior. These results reveal a mechanism to control the dynamic structure of MOFs based on local chemistry.

Metal-Organic Framework-Based Photocatalysis for Solar Fuel Production

Metal-organic frameworks (MOFs) represent a novel class of crystalline inorganic-organic hybrid materials with tunable semiconducting behavior. MOFs have potential for application in photocatalysis to produce sustainable solar fuels, owing to their unique structural advantages (such as clarity and modifiability) that can facilitate a deeper understanding of the structure-activity relationship in photocatalysis. This review takes the photocatalytic active sites as a particular perspective, summarizing the progress of MOF-based photocatalysis for solar fuel production; mainly including three categories of solar-chemical conversions, photocatalytic water splitting to hydrogen fuel, photocatalytic carbon dioxide reduction to hydrocarbon fuels, and photocatalytic nitrogen fixation to high-energy fuel carriers such as ammonia. This review focuses on the types of active sites in MOF-based photocatalysts and discusses their enhanced activity based on the well-defined structure of MOFs, offering deep insights into MOF-based photocatalysis.

CsCu2I3 Nanoparticles Incorporated within a Mesoporous Metal-Organic Porphyrin Framework as a Catalyst for One-Pot Click Cycloaddition and Oxidation/Knoevenagel Tandem Reaction

Metal-organic frameworks (MOFs) and metal halide perovskites are currently under much investigation due to their unique properties and applications. Herein, an innovative strategy has been developed combining an iron-porphyrin MOF, PCN-222(Fe), and an in situ-grown CsCu₂I₃ nontoxic lead-free halide perovskite based on an earth-abundant metal that becomes incorporated within the MOF channels [CsCu₂I₃@PCN-222(Fe)]. Encapsulation was designed to decrease and control the particle size and increase the stability of CsCu₂I₃. The hybrid materials were characterized by various techniques including FE-SEM, elemental mapping and line scanning EDX, TEM, PXRD, UV-Vis DRS, BET surface area, XPS, and photoemission measurements. Hybrid CsCu₂I₃@PCN-222(Fe) materials were examined as heterogeneous multifunctional (photo)catalysts for copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) and one-pot selective photo-oxidation/Knoevenagel condensation cascade reaction. Interestingly, CsCu₂I₃@PCN-222(Fe) outperforms not only its individual components CsCu₂I₃ and PCN-222(Fe) but also other reported (photo)catalysts for these transformations. This is attributed to cooperation and synergistic effects of the PCN-222(Fe) host and CsCu₂I₃ nanocrystals. To understand the catalytic and photocatalytic mechanisms, control and inhibition experiments, electron paramagnetic resonance (EPR) measurements, and time-resolved phosphorescence were performed, revealing the main role of active species of Cu(I) in the click reaction and the superoxide ion (O₂^•-) and singlet oxygen (¹O₂) in the photocatalytic reaction.

Simple Approximation for the Ideal Reference State of Gases Adsorbed on Solid-State Surfaces

Reference states are useful as models for facilitating calculations of equilibrium constants, and they may also serve as standard states that are convenient for organizing and tabulating thermodynamic data; however, standard state conventions and appropriate reference states for adsorbed species have received less attention than those for pure substances and solutes. Here, we compare seven choices of reference states for calculations of equilibrium constants and transition state theory rate constants for flat surfaces, in particular (1) an ideal 2D harmonic oscillator, (2) an ideal rigid-molecule harmonic oscillator, (3) an ideal 2D harmonic oscillator with separable surface modes, (4) a 2D ideal gas, (5) an ideal 2D hindered translator, (6) an ideal 2D hindered translator with lowest-order barriers, and (7) a simple ideal 2D hindered translator proposed in this work. The advantage of models 5-7 is that they can treat both mobile and localized adsorbates in a consistent way, whereas models 1-3 are only appropriate for localized adsorbates, and model 4 is only appropriate for a freely translating adsorbate. Furthermore, models 6 and 7 reduce the computational cost without the user having to calculate barrier heights for diffusion. An advantage of the simple ideal 2D hindered translator is that it has a physical high-temperature limit. We also propose a reference state for nonflat surfaces. The user is encouraged to choose a reference state based on the appropriateness of the model and the practicality of the calculations.

Fabricating defect-rich metal-organic frameworks via mixed linker-induced crystal transformation

Defect-rich hcp UiO-66-NO₂ was synthesized via mixed linker-induced crystal transformation from fcu UiO-66-NO₂/NH₂. The defect concentration and porosity of hcp UiO-66-NO₂ can be fine-tuned by varying the BDC-NH₂/BDC-NO₂ ratio, which in turn endowed hcp UiO-66-NO₂ with superior catalytic performance in the ring-opening reaction of epoxides with alcohols.

Probing the electronic and ionic transport in topologically distinct redox-active metal-organic frameworks in aqueous electrolytes

Three topologically distinct zirconium-based metal-organic frameworks (Zr-MOFs) constructed from redox-innocent linkers, MOF-808, defective UiO-66, and CAU-24, are synthesized, and the spatially dispersed redox-active manganese sites are post-synthetically immobilized on the hexa-zirconium nodes of these Zr-MOFs. The crystallinity, morphology, porosity, manganese loading, and bulk electrical conductivity of each material are studied. The redox-hopping-based electrochemical reaction between the installed Mn(III) and Mn(IV) occurring within the thin films of these MOFs in aqueous electrolytes is investigated, in the presence of various concentrations of Na₂SO₄ in the electrolytes. Cyclic voltammetry is used to qualitatively study the redox-hopping process, and chronoamperometry is used to quantify the electrochemically active fractions of manganese sites within the MOF thin film as well as the values of apparent diffusivity for the redox-hopping process. By adjusting the concentration of Na₂SO₄ in the electrolyte, the rate-determining step for the redox-hopping process can be tuned from ionic transport to electronic transport, and the Mn-decorated MOF-808, which possesses the largest pore size, can achieve the highest value of apparent diffusivity. Findings here shed light on the selection of Zr-MOF as well as the choice of electrolyte concentration for the applications of MOFs in supercapacitors and electrocatalysis relying on such redox-hopping processes in aqueous electrolytes.

Fumarate Based Metal–Organic Framework: An Effective Catalyst for the Transesterification of Used Vegetable Oil

Advancement of technology for the sustainable production of biodiesel is of significant importance in fighting against rising fuel costs due to the fast depletion of fossil fuels. In this regard, the application of highly efficient MOFs (metal–organic frameworks)-based materials as acidic, basic, or supported heterogeneous catalysts plays a crucial role in enhancing the efficiency of biodiesel production processes. In this report, we demonstrate the synthesis and catalytic application of Zr-fumarate-MOF (also known as MOF-801) as a heterogeneous catalyst for the transesterification reaction of used vegetable oil (UVO) for the production of biodiesel. The formation of MOF-801 and its structural stability is confirmed by a variety of characterization techniques including XRD, SEM, EDX, FT-IR, BET, and TGA analyses. The results revealed the formations of highly crystalline, cubic MOF-801 possessing thermal stability below 500 °C. The MOF-801 catalyst demonstrated moderate catalytic activity during transesterification of UVO (~60%) at 50 wt.% of methanol: oil, 10 wt.% catalyst loading, 180 °C reaction temperature, and 8 h of reaction time. Furthermore, the catalyst has exhibited adequate reusability with a slight reduction in the reaction yield of up to ~10% after three cycles.

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity vs. flexibility, and other features and properties. Collectively and individually, these properties help define overall catalyst functional behaviour. This review focuses on how porous, catalyst-containing MOFs capitalize on molecular-scale confinement, containment, isolation, environment modulation, energy delivery, and mobility to accomplish desired chemical transformations with potentially superior selectivity or other efficacy, especially in comparison to catalysts in homogeneous solution environments.

Ratiometric FRET Encoded Hierarchical ZrMOF @ Au Cluster for Ultrasensitive Quantifying MicroRNA In Vivo

Here, Zirconium metal-organic frameworks @ gold (ZrMOF @ Au) cluster architectures have been fabricated and then functionalized with two fluorescent dyes (Quasar [QS] and Cyanine5.5 [Cy5.5]) through deoxyribonucleic acid hybridization, to form a fluorescence resonance energy transfer (FRET) encoded ZrMOF  @  Au-QS/Cy5.5 complex. In the presence of the target intracellular microRNA (miR)-21, the fluorescence of Cy5.5 at 705 nm (F₇₀₅ ) decreases and the fluorescence of QS at 665 nm (F₆₆₅ ) increases when Cy5.5 is released from the surface of ZrMOF  @  Au-QS/Cy5.5. The change in the fluorescence ratio (F₇₀₅ /F₆₆₅ ) shows an outstanding linear range of 0.006-67.9 amol/ng_RNA , and the limit of detection is 4.51 zmol/ng_RNA in living cells. The high ratio loading of nucleic acid on surface of ZrMOF @ Au cluster and two fluorescence encoded signal enables better sensitivity and reliability. Zeptomolar sensitivity and good linearity against target affords distinct imaging-based monitoring of the cancer marker miR-21 both in living cells and in vivo. At the same time, the architecture displays remarkable photothermal conversion efficiency (53.7%) and gives rise to outstanding therapy ability in vivo. This strategy offers new avenues for the intelligent quantification of miRNAs for simultaneous diagnoses and treatments of early-stage cancers.

Metal–organic framework (MOF)-, covalent-organic framework (COF)-, and porous-organic polymers (POP)-catalyzed selective C–H bond activation and functionalization reactions

The review summarizes the state-of-the-art of C–H active transformations over crystalline and amorphous porous materials as new emerging heterogeneous (photo)catalysts.

The Molecular Path Approaching the Active Site in Catalytic Metal-Organic Frameworks

How molecules approach, bind at, and release from catalytic sites is key to heterogeneous catalysis, including for emerging metal-organic framework (MOF)-based catalysts. We use in situ synchrotron X-ray scattering analysis to evaluate the dominant binding sites for reagent and product molecules in the vicinity of catalytic Ni-oxo clusters in NU-1000 with different surface functionalization under conditions approaching those used in catalysis. The locations of the reagent and product molecules within the pores can be linked to the activity for ethylene hydrogenation. For the most active catalyst, ethylene reagent molecules bind close to the catalytic clusters, but only at temperatures approaching experimentally observed onset of catalysis. The ethane product molecules favor a different binding location suggesting that the product is readily released from the active site. An unusual guest-dependence of the framework negative thermal expansion is documented. We hypothesize that reagent and product binding sites reflect the pathway through the MOF to the active site and can be used to identify key factors that impact the catalytic activity.

Enhancing the Catalytic Activity of MOF-808 Towards Peptide Bond Hydrolysis through Synthetic Modulations

The performance of MOFs in catalysis is largely derived from structural features, and much work has focused on introducing structural changes such as defects or ligand functionalisation to boost the reactivity of the MOF. However, the effects of different parameters chosen for the synthesis on the catalytic reactivity of the resulting MOF remains poorly understood. Here, we evaluate the role of metal precursor on the reactivity of Zr-based MOF-808 towards hydrolysis of the peptide bond in the glycylglycine model substrate. In addition, the effect of synthesis temperature and duration has been investigated. Surprisingly, the metal precursor was found to have a large influence on the reactivity of the MOF, surpassing the effect of particle size or number of defects. Additionally, we show that by careful selection of the Zr-salt precursor and temperature used in MOF syntheses, equally active MOF catalysts could be obtained after a 20 minute synthesis compared to 24 h synthesis.

Active mechanisorption driven by pumping cassettes

[Figure: see text].

Impact of Zr6 Node in a Metal-Organic Framework for Adsorptive Removal of Antibiotics from Water

Quinolone-based antibiotics commonly detected in surface, ground, and drinking water are difficult to remove and therefore pose a threat as organic contaminants of aqueous environment. We performed adsorptive removal of quinolone antibiotics, nalidixic acid and ofloxacin, using a zirconium-porphyrin-based metal-organic framework (MOF), PCN-224. PCN-224 exhibits the highest adsorption capacities for both nalidixic acid and ofloxacin among those reported for MOFs to date. The accessible metal sites of Zr metal nodes are responsible for efficient adsorptive removal. This study offers a pragmatic approach to design MOFs optimized for adsorptive removal of antibiotics.

Double-Walled Zn36@Zn104 Multicomponent Senary Metal-Organic Polyhedral Framework and Its Isoreticular Evolution

Metal-organic polyhedral frameworks are attractive in gas storage and separation due to large voids with windows that can serve as traps for guest molecules. Introducing multivariant/multicomponent functionalities in them are ways of improving performances for certain targets. The high compatibility of organic linkers can generate multivariant MOFs, but by far, the diversity of secondary building units (SBUs) in a single metal-organic framework is still limited (no more than two in most cases). Here we report a new double-walled Zn₃₆@Zn₁₀₄ metal-organic polyhedral framework (HHU-8) with five types of topologically distinct SBUs and its isoreticular evolution to the Zn₃₆@Zn₁₃₆ counterpart (HHU-8s). Both MOFs are the first to be constructed with such high numbers of topologically distinct SBUs as well as topologically distinct nodes, and their formation and evolution provide new insight into SBU's controllability.