Yolk-Shell and Hollow Zr/Ce-UiO-66 for Manipulating Selectivity in Tandem Reactions and Photoreactions.

One of the hallmarks of multicomponent metal-organic frameworks (MOFs) is to finely tune their active centers to achieve product selectivity. In particular, obtaining bimetallic MOF hollow structures with precisely tailored redox centers under the same topology is still challenging despite a recent surge of such efforts. Herein, we present an engineering strategy named "cluster labilization" to generate hierarchically porous MOF composites with hollow structures and tunable active centers. By partially replacing zirconium with cerium in the hexanuclear clusters of UiO-66, unevenly distributed yolk-shell structures (YSS) were formed. Through acid treatment or annealing of the YSS precursor, single-shell hollow structures (SSHS) or double-shell hollow structures (DSHS) can be obtained, respectively. The active centers in SSHS and DSHS differ in their species, valence, and spatial locations. More importantly, YSS, SSHS, and DSHS with distinct active centers and microenvironments exhibit tunable catalytic activity, reversed selectivity, and high stability in the tandem reaction and the photoreaction.

Regulating the Topologies of Zirconium-Organic Frameworks for a Crystal Sponge Applicable to Inorganic Matter.

Metal-organic frameworks (MOFs) have been explored as crystal sponges (CSs) to organic substrates, but attention had rarely been paid to inorganic substrates. Herein, hierarchical zirconium-based MOFs exhibiting different topological structures had been fabricated by modulating the functional groups of the V-shaped linkers, including a new 4-fold-interpenetrating one, which displays a great performance as a CS applicable to inorganic matter (I2 and ReO4-) even in the extreme conditions.

Effect of Isomorphic Metal Substitution on the Fenton and Photo-Fenton Degradation of Methylene Blue Using Fe-Based Metal-Organic Frameworks.

The removal of toxic organic compounds (TOCs) using highly porous solids such as metal-organic frameworks (MOFs) has gained significant attention over the past decade. In this study, it has been demonstrated that the efficiency of PCN-250 as a heterogeneous catalyst porous coordination network (PCN) for both Fenton and photo-Fenton reactions can be improved by the isomorphic substitution of Mn and Co for Fe, while it can be inhibited by the substitution of Ni for Fe. Furthermore, the Mn-substituted sample named PCN-250(Fe2Mn) decomposed 100% of methylene blue (MB) in solution in 300 min and displayed good recyclability over three cycles. This work establishes that the highly porous, commercially available, and robust family of MOFs named PCN-250 has the potential to be used as catalysts for Fenton and photo-Fenton reactions as well as broader advanced oxidation processes (AOP) for water purification applications. Overall, this work successfully demonstrates not only the ability to perform isomorphic substitution of various metals within MOFs but also the effect of the substitution on the resulting catalytic performance.

The thermally induced decarboxylation mechanism of a mixed-oxidation state carboxylate-based iron metal-organic framework.

Investigations into a thermally generated decarboxylation mechanism for metal site activation and the generation of mesopores in a carboxylate iron-based MOF, PCN-250, have been conducted. PCN-250 exhibits an interesting oxidation state change during thermal treatment under inert atmospheres or vacuum conditions, transitioning from an Fe(iii)3 cluster to a Fe(ii)Fe(iii)2 cluster. To probe this redox event and discern a mechanism of activation, a combination of thermogravimetric analysis, gas sorption, scanning electron microscopy, 57Fe Mössbauer spectroscopy, gas chromatography-mass spectrometry, and X-ray diffraction studies were conducted. The results suggest that the iron-site activation occurs due to ligand decarboxylation above 200 °C. This is also consistent with the generation of a missing cluster mesoporous defect in the framework. The resulting mesoporous PCN-250 maintains high thermal stability, preserving crystallinity after multiple consecutive high-temperature regeneration cycles. Additionally, the thermally reduced PCN-250 shows improvements in the total uptake capacity of methane and CO2.

Crystallographic Visualization of Postsynthetic Nickel Clusters into Metal-Organic Framework.

Postsynthetic metalation (PSM) has been employed as a robust method for the postsynthetic modification of metal-organic frameworks (MOFs). However, the lack of relevant information that can be obtained for the postsynthetically introduced metallic ions has hindered the development of PSM applications. Thanks to the advancement in single-crystal X-ray diffraction (SCXRD) technology, there have been a few recent examples in which successful postsynthetic introduction of single metal ions into MOFs occurred at the defined chelating sites. These works have provided useful explanations about the complicated host-guest chemistry involved in PSMs. On the other hand, there are only limited examples with crystallographic snapshots of the postsynthetic installation of metal clusters into the pores of MOFs using an ordinary SCXRD due to the loss of crystallinity of parent matrix during the PSM process. Herein, by the careful selection of starting materials and controlling the reaction conditions, we report the first crystallographic visualization of metal clusters inserted into Zr-based MOFs via PSM. The structural advantages of the parent Zr-MOF, which are inherited from the stable Zr6 cluster and triazole-containing dicarboxylate ligand, ensure both the preservation of high crystallinity and the presence of flexible coordination sites for PSM. Furthermore, PSM of metal clusters in a MOF pore space enhances stability of the final samples while also imparting the functionality of a successful catalyst toward ethylene dimerization reaction. The related construction ideas and structural information detailed in this work can help lay the foundation for further advancements using the postmodification of MOFs as well as open new doors for the utilization of SCXRD technology in the field of MOFs.

Catalytic reactions within the cavity of coordination cages.

Natural enzymes catalyze reactions in their substrate-binding cavities, exhibiting high specificity and efficiency. In an effort to mimic the structure and functionality of enzymes, discrete coordination cages were designed and synthesized. These self-assembled systems have a variety of confined cavities, which have been applied to accelerate conventional reactions, perform substrate-specific reactions, and manipulate regio- and enantio-selectivity. Many coordination cages or cage-catalyst composites have achieved unprecedented results, outperforming their counterparts in different catalytic reactions. This tutorial review summarizes recent developments of coordination cages across three key approaches to coordination cage catalysis: (1) cavity promoted reactions, (2) embedding of active sites in the structure of the cage, and (3) encapsulation of catalysts within the cage. Special emphasis of the review involves (1) introduction of the structure and property of the coordination cage, (2) discussion of the catalytic pathway mediated by the cage, (3) elucidation of the structure-property relationship between the cage and the designated reaction. This work will summarize the recent progress in supramolecular catalysis and attract more researchers to pursue cavity-promoted reactions using discrete coordination cages.

Modulation versus Templating: Fine-Tuning of Hierarchally Porous PCN-250 Using Fatty Acids To Engineer Guest Adsorption.

Modulation and templating are two synthetic techniques that have garnered significant attention over the last several years for the preparation of hierarchically porous metal-organic frameworks (HP-MOFs). In this study, by using fatty acids with different lengths and concentrations as dual-functional modulators/templates, we were able to obtain HP-MOFs with tunable mesopores that exhibit different pore diameters and locations. We found that the length and concentration of the fatty acids can determine if micelle formation occurs, which in turn dictates the porosity of the resulting HP-MOFs. The HP-MOFs with different mesopores differed in their performance in gas uptake and dye adsorption, and the structure-performance relationships were ascribed to the pore diameters and locations. This approach could provide a potentially universal method to efficiently introduce hierarchal mesopores into existing microporous MOF adsorbents with tunable properties.

Unconventional Method for Fabricating Valence Tautomeric Materials: Integrating Redox Center within a Metal-Organic Framework.

Due to the structural advantages displayed by Metal-Organic Frameworks (MOFs), integrating Valence Tautomerism (VT) systems within MOFs could be an effective strategy in order to break through the constraints of the traditional ones. Herein, we report the first successful integration of a VT system into a MOF termed VT-MOF-1. The structural characteristics of VT-MOF-1, such as dinuclear cobalt-catechol clusters and solvent-accessible pores, are both innovative and novel, potentially yielding new vitality within VT field. In addition, VT-MOF-1 exhibits specific behaviors responsive to temperature and different solvent molecules as n-butanol, tert-butanol, and isopropyl alcohol. The entropy values and configurations of the solvent molecules might be responsible for the tunable sensing behaviors.

Bimolecular proximity of a ruthenium complex and methylene blue within an anionic porous coordination cage for enhancing photocatalytic activity.

The charge repulsion between a catalyst and substrate will significantly reduce the contact occurring between them, resulting in low reactivity. Herein, we report an anionic porous coordination cage that is capable of encapsulating both a cationic catalyst and cationic substrate in its cavity at the same time. After encapsulating the [Ru(bpy)3]2+Cl2 (bpy = bipyridine) catalyst, the cage/catalyst composite serves as an active heterogeneous catalyst for the photo-degradation of methylene blue (MB). The highly negatively charged cavity of PCC-2 allows for the sequential encapsulation of the cationic methylene blue substrate and the Ru catalyst, which in turn significantly shortens the distance between them, yielding an increased possibility of MB degradation. Moreover, the encapsulated Ru catalyst dramatically outperformed its homogeneous counterpart in terms of overall degradation performance and recyclability.

Metal-Organic Framework Containing Planar Metal-Binding Sites: Efficiently and Cost-Effectively Enhancing the Kinetic Separation of C2H2/C2H4.

One of the emerging problems plaguing the chemical industry today is the efficient and cost-effective separation of C2 hydrocarbons. In order to help address this problem, we report a new material, NbU-1, constructed by extremely cheap starting materials. The special structural characteristics of NbU-1 such as the planar, mixed-valence copper clusters and Lewis-basic adsorption channels enforce interactions with acetylene molecules that lead to the highest kinetic separation efficiency for C2H2/C2H4. Via DFT-D calculations, it is indicated that C2H2 molecules are adsorbed onto the two adjacent Cu(I) centers, but not the usual single open metal sites of Cu centers. The reported results not only provide information for a deep investigation into the separation mechanism but also offer an alternative strategy for preparing cost-effective materials that can perform highly efficient separations of light hydrocarbons.

Facile Fabrication of a Multifunctional Metal-Organic Framework-based Sensor Exhibiting Exclusive Solvochromic Behaviors toward Ketone Molecules.

To probe the efficient strategy for preparing a multifunctional sensing material, the facile synthesis strategies and successful examples are urgently required. Through the utilization of a hexadentate ligand derived from cyclotriphosphazene, which displays spiral configurations and multiple connection modes, a novel metal-organic framework (MOF) was constructed via one-step synthesis from low-cost raw materials. The presence of multiple interaction sites decorating the helical channels of the reported MOF gives rise to exclusive solvochromic-sensing behavior for small ketone molecules such as acetone, acetophenone, and 2,5-diketohexane. Additionally, the helical structure of a manganese-carboxylate chain allows for the pore volume not only be available for the adsorption of large organic molecules but also enables the enantiopure selective separation of 1-phenylethanol (ee 35.99 %). Furthermore, the structural analysis of the acetophenone-encapsulated sample allowed the solvochromic mechanism to be elucidated, which should be ascribed to the strong hydrogen-bonding interaction between the guest molecules and specific sites on a host matrix. The experimental results have not only clearly manifested the vital role of starting materials of MOFs, including the connection modes and spatial configuration, but also have provided very valuable insight for the future assembly of novel multifunctional sensing materials.

Sophisticated Construction of Electronically Labile Materials: A Neutral, Radical-Rich, Cobalt Valence Tautomeric Triangle.

Herein, we report the construction of a neutral, radical-rich, cobalt valence tautomeric triangle, which consists of two types of radical groups including tetrazine-based bridges and semiquinone anions at high temperature and has traits of high intensity and density of sensing sites. The mechanism of the Valence Tautomerism process within the triangle has been illustrated as one electron transfer, preceding a two electrons transfer along with the phenomenon of spin flipping.

From fundamentals to applications: a toolbox for robust and multifunctional MOF materials.

In recent years, metal-organic frameworks (MOFs) have been regarded as one of the most important classes of materials. The combination of various metal clusters and ligands, arranged in a vast array of geometries has led to an ever-expanding MOF family. Each year, new and novel MOF structures are discovered. The structural diversity present in MOFs has significantly expanded the application of these new materials. MOFs show great potential for a variety of applications, including but not limited to: gas storage and separation, catalysis, biomedicine delivery, and chemical sensing. This review intends to offer a short summary of some of the most important topics and recent development in MOFs. The scope of this review shall cover the fundamental aspects concerning the design and synthesis of MOFs and range to the practical applications regarding their stability and derivative structures. Emerging trends of MOF development will also be discussed. These trends shall include multicomponent MOFs, defect development in MOFs, and MOF composites. The ever important structure-property-application relationship for MOFs will also be investigated. Overall, this review provides insight into both existing structures and emerging aspects of MOFs.

Suspension Processing of Microporous Metal-Organic Frameworks: A Scalable Route to High-Quality Adsorbents.

Metal-Organic Frameworks (MOFs) have been intensively studied for applications such as gas storage, gas separation, catalysis, drug delivery, and more. Typically, the development of MOFs involves a post-synthetic solvent exchange process, which usually requires a significant investment of time, energy, labor, and resources. Herein, we propose a novel post-synthetic processing methodology for commercial and laboratory-scale MOFs called "Suspension Processing." Suspension processing is a non-destructive, agitation-based technique that provides efficient solvent exchange, pore cleaning, and surface defect removal in MOFs. Suspension processing has shown the capability to significantly improve the surface area and gas uptake properties of microporous MOFs, including PCN-250, UiO-66, and HKUST-1. Suspension processing displays improved time, energy, and labor efficiency, as well as considerably enhanced product quality. These findings confirm suspension processing as a straightforward methodology with applicability as a universal technique for the production of high-quality microporous materials.

Magnetic Metal-Organic Framework Exhibiting Quick and Selective Solvatochromic Behavior along with Reversible Crystal-to-Amorphous-to-Crystal Transformation.

By utilizing a flexible tetrapyridinate ligand, tetrakis(4-pyridyloxymethylene) methane(L), a novel multifunctional soft porous framework, [Co(NCS)2(L)]·2H2O·CH3OH (1), was constructed. This framework exhibits quick and selective solvatochromic and vapochromic behavior during a reversible crystal-to-amorphous-to-crystal (CAC) transformation. Importantly, the rapid CAC transition can selectively be triggered by methanol molecules, even at low concentrations of liquid or gaseous methanol. This reproducible transition can be monitored by single-crystal and power X-ray analysis, IR, and UV-vis, which all powerfully illustrate the selectivity and sensitivity of this system. In addition, the typical magnetic behavior of single ion magnets (SIMs) has been successfully introduced into this 3D framework, and the modified dynamic relaxations have been investigated via experimental and theoretical analysis. The consistent observations of both experimental and theoretical results support that the distortions of the metal coordination environments should be responsible for the finely tuned SIM behavior.

Creating Hierarchical Pores by Controlled Linker Thermolysis in Multivariate Metal-Organic Frameworks.

Sufficient pore size, appropriate stability, and hierarchical porosity are three prerequisites for open frameworks designed for drug delivery, enzyme immobilization, and catalysis involving large molecules. Herein, we report a powerful and general strategy, linker thermolysis, to construct ultrastable hierarchically porous metal-organic frameworks (HP-MOFs) with tunable pore size distribution. Linker instability, usually an undesirable trait of MOFs, was exploited to create mesopores by generating crystal defects throughout a microporous MOF crystal via thermolysis. The crystallinity and stability of HP-MOFs remain after thermolabile linkers are selectively removed from multivariate metal-organic frameworks (MTV-MOFs) through a decarboxylation process. A domain-based linker spatial distribution was found to be critical for creating hierarchical pores inside MTV-MOFs. Furthermore, linker thermolysis promotes the formation of ultrasmall metal oxide nanoparticles immobilized in an open framework that exhibits high catalytic activity for Lewis acid-catalyzed reactions. Most importantly, this work provides fresh insights into the connection between linker apportionment and vacancy distribution, which may shed light on probing the disordered linker apportionment in multivariate systems, a long-standing challenge in the study of MTV-MOFs.

An Amino-Functionalized Metal-Organic Framework, Based on a Rare Ba12 (COO)18 (NO3 )2 Cluster, for Efficient C3 /C2 /C1 Separation and Preferential Catalytic Performance.

A barium(II) metal-organic framework (MOF) based on a predesigned amino-functionalized ligand, namely [Ba2 (L)(DMF)(H2 O)(NO3 )1/3 ]⋅DMF⋅EtOH⋅2 H2 O (UPC-33) [H3 L=4,4'-((2-amino-5-carboxy-1,3-phenylene)bis(ethyne-2,1-diyl))dibenzoic acid] has been synthesized. UPC-33 is a 3-dimensional 3,18-connected network with fcu topology with a rare twelve-nuclear Ba12 (COO)18 (NO3 )2 cluster. UPC-33 shows permanent porosity and a high adsorption heat of CO2 (49.92 kJ mol-1 ), which can be used as a platform for selective adsorption of CO2 /CH4 (8.09). In addition, UPC-33 exhibits high separation selectivity for C3 light hydrocarbons with respect to CH4 (228.34, 151.40 for C3 H6 /CH4 , C3 H8 /CH4 at 273k and 1 bar), as shown by single component gas sorption and selectivity calculations. Due to the existence of -NH2 groups in the channels, UPC-33 can effectively catalyze Knoevenagel condensation reactions with high yield, and substrate size and electron dependency.