Thermodynamically Guided Synthesis of Mixed-Linker Zr-MOFs with Enhanced Tunability

A highly augmented, (12,3)-connected Zr-MOF containing hydrated coordination sites for the catalytic transformation of gaseous CO2 to cyclic carbonates

A Zr-MOF, which contains partially hydrated, 12-connected {Zr₆} nodes and extended carboxylate ligands was synthesized, characterised and utilised in CO₂ cycloaddition catalysis.

Zirconium-Porphyrin PCN-222: pH-responsive Controlled Anticancer Drug Oridonin

Drug delivery carriers with a high drug loading capacity and biocompatibility, especially for controlled drug release, are urgently needed due to the side effects and frequent dose in the traditional therapeutic method. Guided by nanomaterials, we have successfully synthesized zirconium-based metal-organic frameworks, Zr-TCPP (TCPP: tetrakis (4-carboxyphenyl) porphyrin), namely, PCN-222, which is synthesized by solvothermal method. And it has been designed as a drug delivery system (DDS) with a high drug loading of 38.77 wt%. In our work, PCN-222 has achieved pH-sensitive drug release and showed comprehensive SEM, TEM, PXRD, DSC, FTIR, and N2 adsorption-desorption. The low cytotoxicity and good biocompatibility of PCN-222 were certificated by the in vitro results from an MTT assay, DAPI staining, and Annexin V/PI double-staining even cultivated L02 cells and HepG2 cells for 48h. Furthermore, Oridonin, a commonly used cancer chemotherapy drug, is adsorbed into PCN-222 via the solvent diffusion technique. Based on an analysis of the Oridonin release profile, results suggest that it can last for more than 7 days in vitro. And cumulative release rate of Ori at the 7 d was about 86.29% and 63.23% in PBS (pH 5.5 and pH 7.2, respectively) at 37°C. HepG2 cells were chosen to research the cytotoxicity of PCN-222@Ori and free Oridonin. The results demonstrated that the PCN-222@Ori nanocarrier shows higher cytotoxicity in HepG2 cells compared to Oridonin.

Interior Decoration of Stable Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a diverse class of hybrid organic/inorganic crystalline materials composed of metal-containing nodes held in place by organic linkers. Through a discerning selection of these components, many properties such as the internal surface area, cavity size and shape, catalytic properties, thermal properties, and mechanical properties may be manipulated. Because of this high level of tunability, MOFs have been heralded as ideal platforms for various applications including gas storage, separation, catalysis, and chemical sensing. (1-8) Regrettably, these theoretical possibilities are limited by the reality of constraining conditions for solvothermal synthesis, which typically include high temperatures (usually over 100 °C), the use of specific solvents, and necessary exposure to acidic or basic conditions. In order to incorporate more delicate functionalities, postsynthesis decoration methods were developed. This feature article focuses on developed interior decoration methods for stable MOFs and the dynamic relationship between such methods and MOF stability. In particular, methods to transform organic, inorganic, and organometallic MOF parts as well as combination techniques, the generation of defects, and the inclusion of enzymes are addressed.

Combining Linker Design and Linker-Exchange Strategies for the Synthesis of a Stable Large-Pore Zr-Based Metal-Organic Framework

A Zr(IV)-based metal-organic framework (MOF), termed reo-MOF-1 [Zr₆O₈(H₂O)₈(SNDC)₄], composed of 4-sulfonaphthalene-2,6-dicarboxylate (HSNDC^2-) linkers and Zr₆O₈(H₂O)₈(CO₂)₈ clusters was synthesized by solvothermal synthesis. Structural analysis revealed that reo-MOF-1 adopts the reo topology highlighted with large cuboctahedral cages (23 Å). This structure is similar to that found in DUT-52 (fcu topology), however, reo-MOF-1 lacks the body-centered packing of the 12-connected Zr₆O₄(OH)₄(CO₂)₁₂ clusters, which is attributed to the subtle, but crucial influence in the bulkiness of functional groups on the linkers. The control experiments, where the ratio of H₃SNDC/naphthalene-2,6-dicarboxylate linkers was varied, also support our finding that the bulky functionalities play a key role for defect-controlled synthesis. The reo-MOF-1_A framework was obtained by linker exchange to yield a chemically and thermally stable material despite its large pores. Remarkably, reo-MOF-1_A exhibits permanent porosity (Brunauer-Emmett-Teller and Langmuir surface areas of 2104 and 2203 m² g^-1, respectively). Owing to these remarkable structural features, reo-MOF-1_A significantly enhances the yield in Brønsted acid-catalyzed reactions.

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.

Stable Metal-Organic Frameworks: Design, Synthesis, and Applications

Metal-organic frameworks (MOFs) are an emerging class of porous materials with potential applications in gas storage, separations, catalysis, and chemical sensing. Despite numerous advantages, applications of many MOFs are ultimately limited by their stability under harsh conditions. Herein, the recent advances in the field of stable MOFs, covering the fundamental mechanisms of MOF stability, design, and synthesis of stable MOF architectures, and their latest applications are reviewed. First, key factors that affect MOF stability under certain chemical environments are introduced to guide the design of robust structures. This is followed by a short review of synthetic strategies of stable MOFs including modulated synthesis and postsynthetic modifications. Based on the fundamentals of MOF stability, stable MOFs are classified into two categories: high-valency metal-carboxylate frameworks and low-valency metal-azolate frameworks. Along this line, some representative stable MOFs are introduced, their structures are described, and their properties are briefly discussed. The expanded applications of stable MOFs in Lewis/Brønsted acid catalysis, redox catalysis, photocatalysis, electrocatalysis, gas storage, and sensing are highlighted. Overall, this review is expected to guide the design of stable MOFs by providing insights into existing structures, which could lead to the discovery and development of more advanced functional materials.

MIL-125-NH2@TiO2 Core-Shell Particles Produced by a Post-Solvothermal Route for High-Performance Photocatalytic H2 Production

Metal-organic frameworks (MOFs) have proven to be an interesting class of sacrificial precursors of functional inorganic materials for catalysis, energy storage, and conversion applications. However, the controlled synthesis of MOF-derived materials with desirable compositions, structures, and properties still remains a big challenge. Herein, we propose a post-solvothermal route for the outer-to-inner loss of organic linkers from MOF, which is simple, rapid, and controllable and can be operated at temperature much lower than that of the commonly adopted pyrolysis method. By such a strategy, the MIL-125-NH₂ particles coated by TiO₂ nanosheets were produced, and the thickness of TiO₂ shell can be easily tuned. The MIL-125-NH₂@TiO₂ core-shell particles combine the advantages of highly active TiO₂ nanosheets, MIL-125-NH₂ photosensitizer, plenty of linker defects and oxygen vacancies, and mesoporous structure, which allows them to be utilized as photocatalysts for the visible-light-driven hydrogen production reaction. It is remarkable that the hydrogen evolution rate by MIL-125-NH₂@TiO₂ can be enhanced 70 times compared with the pristine MIL-125-NH₂. Such a route can be easily applied to the synthesis of different kinds of MOF-derived functional materials.

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

Group 4 metal-based metal-organic frameworks (M^IV-MOFs), including Ti-, Zr-, and Hf-based MOFs, are one of the most attractive classes of MOF materials owing to their superior chemical stability and structural tunability. Despite being a relatively new field, M^IV-MOFs have attracted significant research attention in the past few years, leading to exciting advances in syntheses and applications. In this outlook, we start with a brief overview of the history and current status of M^IV-MOFs, emphasizing the challenges encountered in their syntheses. The unique properties of M^IV-MOFs are discussed, including their high chemical stability and strong tolerance toward defects. Particular emphasis is placed on defect engineering in Zr-MOFs which offers additional routes to tailor their functions. Photocatalysis of M^IV-MOF is introduced as a representative example of their emerging applications. Finally, we conclude with the perspective of new opportunities in synthesis and defect engineering.

Retrosynthesis of multi-component metal-organic frameworks

Crystal engineering of metal−organic frameworks (MOFs) has allowed the construction of complex structures at atomic precision, but has yet to reach the same level of sophistication as organic synthesis. The synthesis of complex MOFs with multiple organic and/or inorganic components is ultimately limited by the lack of control over framework assembly in one-pot reactions. Herein, we demonstrate that multi-component MOFs with unprecedented complexity can be constructed in a predictable and stepwise manner under simple kinetic guidance, which conceptually mimics the retrosynthetic approach utilized to construct complicated organic molecules. Four multi-component MOFs were synthesized by the subsequent incorporation of organic linkers and inorganic clusters into the cavity of a mesoporous MOF, each composed of up to three different metals and two different linkers. Furthermore, we demonstrated the utility of such a retrosynthetic design through the construction of a cooperative bimetallic catalytic system with two collaborative metal sites for three-component Strecker reactions.

The crystal engineering of metal–organic frameworks has led to the construction of complex structures, but has yet to reach the same level of sophistication as organic synthesis. Here, Zhou and colleagues use retrosynthetic chemistry to design and produce complex multi-component frameworks.

Boronic Acid Moiety as Functional Defect in UiO-66 and Its Effect on Hydrogen Uptake Capacity and Selective CO2 Adsorption: A Comparative Study

Herein, we use linker fragmentation approach to introduce boronic acid moieties as functional defects into Zr-based metal-organic frameworks (MOFs, UiO-66). Our findings show that the amount of permanently incorporated boronic acid containing ligand is directly dependent on the synthesis method. The accessible boronic acid moieties in the pore surfaces significantly improve the hydrogen uptake values, which are 3.10 and 3.44 wt % at 21 bar, 77 K for dimethylformamide (DMF)/H₂O and DMF/HCI synthesis methods, respectively. Also, CO₂ selectivity of the resulting MOFs over N₂ and CH₄ significantly increases due to the quadrupolar interaction between active surfaces and CO₂ molecules. To the best of our knowledge, both hydrogen storage and selectivity of CO₂ for UiO-66 are the highest reported values in the literature to date. Furthermore, another striking result that emerged from the high-pressure hydrogen uptake isotherms is the direct correlation between the defects and hysteric adsorption behavior, which may result in the shift from rigidity to flexibility of the framework due to the uncoordinated sites.

Effect of Redox "Non-Innocent" Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal-Organic Frameworks

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu(II) precursors triggers the reduction of Cu(II) by the phenyl-catechol groups to Cu(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal-support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports.

Tunability of fluorescent metal–organic frameworks through dynamic spacer installation with multivariate fluorophores

Through dynamic spacer installation, five fluorescent metal–organic frameworks (MOFs) have been constructed based on a proto-MOF LIFM-28 and multivariate fluorophores.

A new transformation of coumarins via direct C–H bond activation utilizing an iron–organic framework as a recyclable catalyst

A new mixed-linker iron-based MOF VNU-20 [Fe₃(BTC)(NDC)₂·6.65H₂O] was solvothermally synthesized, and utilized as catalyst for the coupling transformation of coumarins with N,N-dimethylanilines.

Supramolecular frameworks based on 5,10,15,20-tetra(4-carboxyphenyl)porphyrins

Hydrogen bonding is used to prepare porphyrin-containing supramolecular frameworks.

Construction of unprecedented pillar-layered metal organic frameworks via a dual-ligand strategy for dye degradation

Presented here are two new pillar-layered MOFs based on different metal–carboxylate SBUs employed by mixing D-H₂Cam and OH-bim ligands. The Zn/Co mixed compound 2 exhibits an adjustable Zn/Co ratio, which reversibly results in an adjustable photocatalytical property of dye degradation.

Structures and single crystal to single crystal transformations of cadmium frameworks using a flexible tripodal ligand

Various cadmium coordination polymers were constructed using a flexible tris-imidazole ligand and they showed single crystal to single crystal transformations during anion exchange.

Stable metal–organic frameworks as a host platform for catalysis and biomimetics

Recent years have witnessed the exploration and synthesis of an increasing number of metal–organic frameworks (MOFs). The utilization of stable MOFs as a platform for catalysis and biomimetics is discussed.

Incorporation of an intact dimeric Zr12 oxo cluster from a molecular precursor in a new zirconium metal–organic framework

A dimeric Zr₁₂ oxo cluster was used as new molecular building block in construction of metal–organic frameworks utilizing the precursor approach.

Cross-dehydrogenative coupling of coumarins with Csp3–H bonds using an iron–organic framework as a productive heterogeneous catalyst

The iron–organic framework VNU-20 was utilized as an active heterogeneous catalyst for the cross-dehydrogenative coupling of coumarins with Csp³–H bonds in alkylbenzenes, cyclohexanes, ethers, and formamides.

Ionic liquid accelerates the crystallization of Zr-based metal-organic frameworks

The Zr-based metal–organic frameworks are generally prepared by solvothermal procedure. To overcome the slow kinetics of nucleation and crystallization of Zr-based metal–organic frameworks is of great interest and challenging. Here, we find that an ionic liquid as solvent can significantly accelerate the formation of Zr-based metal–organic frameworks at room temperature. For example, the reaction time is shortened to 0.5 h in 1-hexyl-3-methylimidazolium chloride for Zr-based metal–organic framework formation, while that in the conventional solvent N,N-dimethylformamide needs at least 120 h. The reaction mechanism was investigated in situ by ¹H nuclear magnetic resonance, spectroscopy synchrotron small angle X-ray scattering and X-ray absorption fine structure. This rapid, low-energy, and facile route produces Zr-based metal–organic framework nanoparticles with small particle size, missing-linker defects and large surface area, which can be used as heterogeneous catalysts for Meerwein–Ponndorf–Verley reaction.

Crystallization kinetics of metal-organic frameworks in conventional organic solvents are usually very slow. Here, the authors show that an ionic liquid medium accelerates considerably the formation of Zr-based metal-organic frameworks that are active catalysts in the Meerwein-Ponndorf-Verley reaction.

A Stimuli-Responsive Zirconium Metal-Organic Framework Based on Supermolecular Design

A flexible, yet very stable metal-organic framework (DUT-98, Zr₆ O₄ (OH)₄ (CPCDC)₄ (H₂ O)₄ , CPCDC=9-(4-carboxyphenyl)-9H-carbazole-3,6-dicarboxylate) was synthesized using a rational supermolecular building block approach based on molecular modelling of metal-organic chains and subsequent virtual interlinking into a 3D MOF. Structural characterization via synchrotron single-crystal X-ray diffraction (SCXRD) revealed the one-dimensional pore architecture of DUT-98, envisioned in silico. After supercritical solvent extraction, distinctive responses towards various gases stimulated reversible structural transformations, as detected using coupled synchrotron diffraction and physisorption techniques. DUT-98 shows a surprisingly low water uptake but a high selectivity for pore opening towards specific gases and vapors (N₂ , CO₂ , n-butane, alcohols) at characteristic pressure resulting in multiple steps in the adsorption isotherm and hysteretic behavior upon desorption.

Ultrahigh adsorption and singlet-oxygen mediated degradation for efficient synergetic removal of bisphenol A by a stable zirconium-porphyrin metal-organic framework

Bisphenol A (BPA), one of 23 most important endocrine disrupting chemicals, was efficiently removed and sequentially photodegraded by a zirconium-porphyrin metal–organic framework (MOF) catalyst under visible light for water treatment. Well control of photodegradation allows the kinetic separation of adsorption step and photodegradation step. Ultrahigh adsorption uptake of 487.69 ± 8.37 mg g⁻¹ is observed, while efficient photodegradation could be observed within 20 min at the rate of 0.004 mg min⁻¹. The synergetic effect boosts the photocatalytic efficiency and confirms that the catalysis happens inside the MOF pores other than in the solution phase. Furthermore, the mechanism was elucidated by diverse control experiments, such as in the conditions of ¹O₂ scavenger, in darkness and with the changes of light sensitizing ligands. It confirmed that BPA was oxidized by the ¹O₂ which was generated from porphyrin ligand within MOFs under visible-light. The excellent reusability and wide range of suitable pH range make the Zr-porphyrin MOFs practical for the photocatalytic water treatment processes.

Overcoming the crystallization and designability issues in the ultrastable zirconium phosphonate framework system

Metal-organic frameworks (MOFs) based on zirconium phosphonates exhibit superior chemical stability suitable for applications under harsh conditions. These compounds mostly exist as poorly crystallized precipitates, and precise structural information has therefore remained elusive. Furthermore, a zero-dimensional zirconium phosphonate cluster acting as secondary building unit has been lacking, leading to poor designability in this system. Herein, we overcome these challenges and obtain single crystals of three zirconium phosphonates that are suitable for structural analysis. These compounds are built by previously unknown isolated zirconium phosphonate clusters and exhibit combined high porosity and ultrastability even in fuming acids. SZ-2 possesses the largest void volume recorded in zirconium phosphonates and SZ-3 represents the most porous crystalline zirconium phosphonate and the only porous MOF material reported to survive in aqua regia. SZ-2 and SZ-3 can effectively remove uranyl ions from aqueous solutions over a wide pH range, and we have elucidated the removal mechanism.

Zirconium phosphonate based metal-organic frameworks often exhibit superior chemical stabilities, but typically exist as poorly crystalline or amorphous materials. Here the authors exploit an ionothermal method to obtain highly porous and remarkably stable single crystalline zirconium phosphonate frameworks that can efficiently remove uranyl ions from aqueous solutions.

Construction of hierarchically porous metal-organic frameworks through linker labilization

A major goal of metal–organic framework (MOF) research is the expansion of pore size and volume. Although many approaches have been attempted to increase the pore size of MOF materials, it is still a challenge to construct MOFs with precisely customized pore apertures for specific applications. Herein, we present a new method, namely linker labilization, to increase the MOF porosity and pore size, giving rise to hierarchical-pore architectures. Microporous MOFs with robust metal nodes and pro-labile linkers were initially synthesized. The mesopores were subsequently created as crystal defects through the splitting of a pro-labile-linker and the removal of the linker fragments by acid treatment. We demonstrate that linker labilization method can create controllable hierarchical porous structures in stable MOFs, which facilitates the diffusion and adsorption process of guest molecules to improve the performances of MOFs in adsorption and catalysis.

Expanding pore sizes and volumes in metal-organic frameworks is challenging, but crucial for the encapsulation of larger guest molecules. Here, Zhou and colleagues report a linker labilization strategy to construct MOFs containing hierarchical pore architectures with dimensions ranging from 1.5 to 18 nm.

Hydrolytically Stable Luminescent Cationic Metal Organic Framework for Highly Sensitive and Selective Sensing of Chromate Anions in Natural Water Systems

Effective detection of chromate anions in aqueous solution is highly desirable because of their high solubility, environmental mobility, carcinogenicity, and bioaccumulation effect. A new strategy for precise detection of chromate anions in the presence of a large excess of other anions, such as Cl^-, NO₃^-, and HCO₃^-, in drinking water and natural water systems remains a challenge. Herein, a hydrolytically stable cationic luminescent europium(III)-based metal organic framework (MOF), 1, was successfully synthesized and investigated as a luminescent sensor that exhibits instant and selective luminescence quenching properties toward chromate ions in aqueous solutions. Moreover, 1 can be introduced into high-ionic-strength water system (e.g., seawater) for chromate detection as a consequence of the excellent sensing selectivity. The real environmental application of 1 as a chromate probe is studied in deionized water, lake water, and seawater. The detection limits in these aqueous media are calculated to be 0.56, 2.88, and 1.75 ppb, respectively. All of these values are far below the maximum contamination standard of Cr(VI) in drinking water of 100 ppb, defined by the U.S. Environmental Protection Agency. This excellent chromate sensing capability originates from the fast enrichment of chromate ions in solids of 1 from solutions, followed by efficient energy transfer from the MOF skeleton to the chromate anion, as demonstrated by solution absorption spectroscopy, X-ray diffraction, and chromate uptake kinetics and isotherm investigations. To the best of our knowledge, 1 possesses the lowest chromate detection limit among all reported MOFs up to date and is the only MOF material reported for chromate sensing application under environmentally relevant conditions with high ionic strengths.

Epitaxial Growth of Oriented Metalloporphyrin Network Thin Film for Improved Selectivity of Volatile Organic Compounds

This study reports an oriented and homogenous cobalt-metalloporphyrin network (PIZA-1) thin film prepared by liquid phase epitaxial (LPE) method. The thickness of the obtained thin films can be well controlled, and their photocurrent properties can also be tuned by LPE cycles or the introduction of conductive guest molecules (tetracyanoquinodimethane and C₆₀ ) into the PIZA-1 pores. The study of quartz crystal microbalance adsorption confirms that the PIZA-1 thin film with [110]-orientation presents much higher selectivity of benzene over toluene and p-xylene than that of the PIZA-1 powder with mixed orientations. These results reveal that the selective adsorption of volatile organic compounds highly depends on the growth orientations of porphyrin-based metal-organic framework thin films. Furthermore, the work will provide a new perspective for developing important semiconductive sensing materials with improved selectivity of guest compounds.

Enhancement of photocatalytic performance in two zinc-based metal–organic frameworks by solvent assisted linker exchange

Solvent-assisted linker exchange (SALE) was performed on two pillared metal–organic frameworks (MOFs), [Zn₂(oba)₂(4-bpdb)]_n·(DMF)₂ (TMU-4) and [Zn(oba)(4-bpmb)_0.5]_n·(DMF)_1.5 (TMU-6), to tune their photocatalytic properties.