Well‐Defined Metal–Organic Framework Hollow Nanocages

Metal–organic frameworks (MOFs) have demonstrated great potentials in a variety of important applications. To enhance the inherent properties and endow materials with multifunctionality, the rational design and synthesis of MOFs with nanoscale porosity and hollow feature is highly desired and remains a great challenge. In this work, the formation of a series of well‐defined MOF (MOF‐5, Fe^II‐MOF‐5, Fe^III‐MOF‐5) hollow nanocages by a facile solvothermal method, without any additional supporting template is reported. A surface‐energy‐driven mechanism may be responsible for the formation of hollow nanocages. The addition of pre‐synthesized poly(vinylpyrrolidone)‐ (PVP) capped noble‐metal nanoparticles into the synthetic system of MOF hollow nanocages yields the yolk–shell noble metal@MOF nanostructures. The present strategy to fabricate hollow and yolk–shell nanostructures is expected to open up exciting opportunities for developing a novel class of inorganic–organic hybrid functional nanomaterials.     

One‐Step Synthesis of Hybrid Core–Shell Metal–Organic Frameworks

Epitaxial growth of MOF‐on‐MOF composite is an evolving research topic in the quest for multifunctional materials. In previously reported methods, the core–shell MOFs were synthesized via a stepwise strategy that involved growing the shell‐MOFs on top of the preformed core‐MOFs with matched lattice parameters. However, the inconvenient stepwise synthesis and the strict lattice‐matching requirement have limited the preparation of core–shell MOFs. Herein, we demonstrate that hybrid core–shell MOFs with mismatching lattices can be synthesized under the guidance of nucleation kinetic analysis. A series of MOF composites with mesoporous core and microporous shell were constructed and characterized by optical microscopy, powder X‐ray diffraction, gas sorption measurement, and scanning electron microscopy. Isoreticular expansion of microporous shells and orthogonal modification of the core was realized to produce multifunctional MOF composites, which acted as size selective catalysts for olefin epoxidation with high activity and selectivity.     

Synthetic Strategies for Constructing Two‐Dimensional Metal‐Organic Layers (MOLs): A Tutorial Review

Two‐dimensional (2D) metal‐organic layers (MOLs) are the 2D version of metal‐organic frameworks (MOFs) with nanometer thickness in one dimension. MOLs are also known as 2D‐MOFs, 2D coordination polymers, ultrathin MOF nanosheets (UMOFNs) or coordination nanosheets in literature. This new category of 2D materials has attracted a lot of interests because of the opportunity in combining molecular chemistry, surface/interface chemistry and material chemistry of low dimensional materials in these systems. Several synthetic strategies have been developed for the construction of 2D MOLs, but the general synthesis of MOLs still presents a challenge. This tutorial level review summarizes the recent progress in the fabrication of novel 2D MOLs and aims to highlight challenges in this field.     

Spray‐Coating of Catalytically Active MOF–Polythiourea through Postsynthetic Polymerization

A UiO‐66‐NCS MOF was formed by postsynthetic modification of UiO‐66‐NH₂. The UiO‐66‐NCS MOFs displays a circa 20‐fold increase in activity against the chemical warfare agent simulant dimethyl‐4‐nitrophenyl phosphate (DMNP) compared to UiO‐66‐NH₂, making it the most active MOF materials using a validated high‐throughput screening. The ?NCS functional groups provide reactive handles for postsynthetic polymerization of the MOFs into functional materials. These MOFs can be tethered to amine‐terminated polypropylene polymers (Jeffamines) through a facile room‐temperature synthesis with no byproducts. The MOFs are then crosslinked into a MOF–polythiourea (MOF–PTU) composite material, maintaining the catalytic properties of the MOF and the flexibility of the polymer. This MOF–PTU hybrid material was spray‐coated onto Nyco textile fibers, displaying excellent adhesion to the fiber surface. The spray‐coated fibers were screened for the degradation of DMNP and showed durable catalytic reactivity.     

Hydrolytic Transformation of Microporous Metal–Organic Frameworks to Hierarchical Micro‐ and Mesoporous MOFs

A new approach to the synthesis of hierarchical micro‐ and mesoporous MOFs from microporous MOFs involves a simple hydrolytic post‐synthetic procedure. As a proof of concept, a new microporous MOF, POST‐66(Y), was synthesized and its transformation into a hierarchical micro‐ and mesoporous MOF by water treatment was studied. This method produced mesopores in the range of 3 to 20 nm in the MOF while maintaining the original microporous structure, at least in part. The degree of micro‐ and mesoporosity can be controlled by adjusting the time and temperature of hydrolysis. The resulting hierarchical porous MOF, POST‐66(Y)‐wt, can be utilized to encapsulate nanometer‐sized guests such as proteins, and the enhanced stability and recyclability of an encapsulated enzyme is demonstrated.     

Metal–Organic Frameworks Based Electrocatalysts for the Oxygen Reduction Reaction

In view of the clean and sustainable energy, metal–organic frameworks (MOFs) based materials, including pristine MOFs, MOF composites, and their derivatives are emerging as unique electrocatalysts for oxygen reduction reaction (ORR). Thanks to their tunable compositions and diverse structures, efficient MOF‐based materials provide new opportunities to accelerate the sluggish ORR at the cathode in fuel cells and metal–air batteries. This Minireview first provides some introduction of ORR and MOFs, followed by the classification of MOF‐based electrocatalysts towards ORR. Recent breakthroughs in engineering MOF‐based ORR electrocatalysts are highlighted with an emphasis on synthesis strategy, component, morphology, structure, electrocatalytic performance, and reaction mechanism. Finally, some current challenges and future perspectives for MOF‐based ORR electrocatalysts are also discussed.     

Electroactive Metal–Organic Frameworks as Emitters for Self‐Enhanced Electrochemiluminescence in Aqueous Medium

Metal–organic frameworks (MOFs) have limited applications in electrochemistry owing to their poor conductivity. Now, an electroactive MOF (E‐MOF) is designed as a highly crystallized electrochemiluminescence (ECL) emitter in aqueous medium. The E‐MOF contains mixed ligands of hydroquinone and phenanthroline as oxidative and reductive couples, respectively. E‐MOFs demonstrate excellent performance with surface state model in both co‐reactant and annihilation ECL in aqueous medium. Compared with the individual components, E‐MOFs significantly improve the ECL emission due to the framework structure. The self‐enhanced ECL emission with high stability is realized by the accumulation of MOF cation radicals via pre‐reduction electrolysis. The self‐enhanced mechanism is theoretically identified by DFT. The mixed‐ligand E‐MOFs provide a proof of concept using molecular crystalline materials as new ECL emitters for fundamental mechanism studies.     

Multi‐shelled Hollow Metal–Organic Frameworks

Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi‐shelled hollow chromium (III) terephthalate MOFs (MIL‐101) with single‐crystalline shells through step‐by‐step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi‐shelled hollow MIL‐101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi‐shelled hollow structures and the further expansion of their applications.     

Direct and Post‐Synthesis Incorporation of Chiral Metallosalen Catalysts into Metal–Organic Frameworks for Asymmetric Organic Transformations

Two chiral porous metal–organic frameworks (MOFs) were constructed from [VO(salen)]‐derived dicarboxylate and dipyridine bridging ligands. After oxidation of V^IV to V^V, they were found to be highly effective, recyclable, and reusable heterogeneous catalysts for the asymmetric cyanosilylation of aldehydes with up to 95 % ee. Solvent‐assisted linker exchange (SALE) treatment of the pillared‐layer MOF with [Cr(salen)Cl]‐ or [Al(salen)Cl]‐derived dipyridine ligands led to the formation of mixed‐linker metallosalen‐based frameworks and incorporation of [Cr(salen)] enabled its use as a heterogeneous catalyst in the asymmetric epoxide ring‐opening reaction.     

Design and Synthesis of a Water‐Stable Anionic Uranium‐Based Metal–Organic Framework (MOF) with Ultra Large Pores

Ionic metal–organic frameworks (MOFs) are a subclass of porous materials that have the ability to incorporate different charged species in confined nanospace by ion‐exchange. To date, however, very few examples combining mesoporosity and water stability have been realized in ionic MOF chemistry. Herein, we report the rational design and synthesis of a water‐stable anionic mesoporous MOF based on uranium and featuring tbo‐type topology. The resulting tbo MOF exhibits exceptionally large open cavities (3.9 nm) exceeding those of all known anionic MOFs. By supercritical CO₂ activation, a record‐high Brunauer‐Emmett‐Teller (BET) surface area (2100 m² g^?1) for actinide‐based MOFs has been obtained. Most importantly, however, this new uranium‐based MOF is water‐stable and able to absorb positively charged ions selectively over negatively charged ones, enabling the efficient separation of organic dyes and biomolecules.     

Template‐Induced Diverse Metal–Organic Materials as Catalysts for the Tandem Acylation–Nazarov Cyclization

In our continuing quest to develop a metal–organic framework (MOF)‐catalyzed tandem pyrrole acylation–Nazarov cyclization reaction with α,β‐unsaturated carboxylic acids for the synthesis of cyclopentenone[b]pyrroles, which are key intermediates in the synthesis of natural product (±)‐roseophilin, a series of template‐induced Zn‐based ( 1–3 ) metal‐organic frameworks (MOFs) have been solvothermally synthesized and characterized. Structural conversions from non‐porous MOF 1 to porous MOF 2 , and back to non‐porous MOF 3 arising from the different concentrations of template guest have been observed. The anion–π interactions between the template guests and ligands could affect the configuration of ligands and further tailor the frameworks of 1–3 . Futhermore, MOFs 1–3 have shown to be effective heterogeneous catalysts for the tandem acylation–Nazarov cyclization reaction. In particular, the unique structural features of 2 , including accessible catalytic sites and suitable channel size and shape, endow 2 with all of the desired features for the MOF‐catalyzed tandem acylation–Nazarov cyclization reaction, including heterogeneous catalyst, high catalytic activity, robustness, and excellent selectivity. A plausible mechanism for the catalytic reaction has been proposed and the structure–reactivity relationship has been further clarified. Making use of 2 as a heterogeneous catalyst for the reaction could greatly increase the yield of total synthesis of (±)‐roseophilin.     

Advances of Metal‐Organic Frameworks in Energy and Environmental Applications

Nowadays, energy shortage and environmental pollution issues are increasingly severe and urgent to be solved. The effective storage and use of environmentally friendly fuels and removal of harmful gases from the environment are great challenges and of great importance both for the environment protection and for human health. Porous metal‐organic frameworks (MOFs) are highly ordered crystalline materials formed by the self‐assembly process of metal ions and organic ligands. Their good features such as ultrahigh porosity, large surface area, structural diversity and functionalities make them promising candidates for applications in energy and environmental fields. MOF thin films and MOF composites have also been investigated to further enhance the properties and introduce new functionalities. This review provides an overview of the synthesis methods of pristine MOFs, MOF thin films and MOF composites, and significant advances of MOFs in energy and environment applications such as energy storage (H₂, CH₄), CO₂ capture and separation, adsorption removal and sensing of harmful gases in the environment.     

Toward 3D printed hydrogen storage materials made with ABS‐MOF composites

The push to advance efficient, renewable, and clean energy sources has brought with it an effort to generate materials that are capable of storing hydrogen. Metal–organic framework materials (MOFs) have been the focus of many such studies as they are categorized for their large internal surface areas. We have addressed one of the major shortcomings of MOFs (their processibility) by creating and 3D printing a composite of acrylonitrile butadiene styrene (ABS) and MOF‐5, a prototypical MOF, which is often used to benchmark H₂ uptake capacity of other MOFs. The ABS‐MOF‐5 composites can be printed at MOF‐5 compositions of 10% and below. Other physical and mechanical properties of the polymer (glass transition temperature, stress and strain at the breaking point, and Young's modulus) either remain unchanged or show some degree of hardening due to the interaction between the polymer and the MOF. We do observe some MOF‐5 degradation through the blending process, likely due to the ambient humidity through the purification and solvent casting steps. Even with this degradation, the MOF still retains some of its ability to uptake H₂, seen in the ability of the composite to uptake more H₂ than the pure polymer. The experiments and results described here represent a significant first step toward 3D printing MOF‐5‐based materials for H₂ storage.     

Template‐Directed Synthesis of Porous and Protective Core–Shell Bionanoparticles

Metal–organic frameworks (MOFs) are promising high surface area coordination polymers with tunable pore structures and functionality; however, a lack of good size and morphological control over the as‐prepared MOFs has persisted as an issue in their application. Herein, we show how a robust protein template, tobacco mosaic virus (TMV), can be used to regulate the size and shape of as‐fabricated MOF materials. We were able to obtain discrete rod‐shaped TMV@MOF core–shell hybrids with good uniformity, and their diameters could be tuned by adjusting the synthetic conditions, which can also significantly impact the stability of the core–shell composite. More interestingly, the virus particle underneath the MOF shell can be chemically modified using a standard bioconjugation reaction, showing mass transportation within the MOF shell.     

Mixed‐Matrix Membranes

Research into extended porous materials such as metal‐organic frameworks (MOFs) and porous organic frameworks (POFs), as well as the analogous metal‐organic polyhedra (MOPs) and porous organic cages (POCs), has blossomed over the last decade. Given their chemical and structural variability and notable porosity, MOFs have been proposed as adsorbents for industrial gas separations and also as promising filler components for high‐performance mixed‐matrix membranes (MMMs). Research in this area has focused on enhancing the chemical compatibility of the MOF and polymer phases by judiciously functionalizing the organic linkers of the MOF, modifying the MOF surface chemistry, and, more recently, exploring how particle size, morphology, and distribution enhance separation performance. Other filler materials, including POFs, MOPs, and POCs, are also being explored as additives for MMMs and have shown remarkable anti‐aging performance and excellent chemical compatibility with commercially available polymers. This Review briefly outlines the state‐of‐the‐art in MOF‐MMM fabrication, and the more recent use of POFs and molecular additives.     

Hierarchical Pore Development by Plasma Etching of Zr‐Based Metal–Organic Frameworks

The typically stable Zr‐based metal–organic frameworks (MOFs) UiO‐66 and UiO‐66‐NH₂ were treated with tetrafluoromethane (CF₄) and hexafluoroethane (C₂F₆) plasmas. Through interactions between fluoride radicals from the perfluoroalkane plasma and the zirconium–oxygen bonds of the MOF, the resulting materials showed the development of mesoporosity, creating a hierarchical pore structure. It is anticipated that this strategy can be used as a post‐synthetic technique for developing hierarchical networks in a variety of MOFs.     

In Situ Modification of Metal–Organic Frameworks in Mixed‐Matrix Membranes

Processable films of metal–organic frameworks (MOFs) have been long sought to advance the application of MOFs in various technologies from separations to catalysis. Herein, MOF–polymer mixed‐matrix membranes (MMMs) are described, formed on several substrates using a wide variety of MOF materials. These MMMs can be delaminated from their substrates to create free‐standing MMMs that are mechanically stable and pliable. The MOFs in these MMMs remain highly crystalline, porous, and accessible for further chemical modification through postsynthetic modification (PSM) and postsynthetic exchange (PSE) processes. Overall, the findings here demonstrate a versatile approach to preparing stable functional MMMs that should contribute significantly to the advancement of these materials.     

Nylon–MOF Composites through Postsynthetic Polymerization

Hybridization of metal–organic frameworks (MOFs) and polymers into composites yields materials that display the exceptional properties of MOFs with the robustness of polymers. However, the realization of MOF–polymer composites requires efficient dispersion and interactions of MOF particles with polymer matrices, which remains a significant challenge. Herein, we report a simple, scalable, bench‐top approach to covalently tethered nylon–MOF polymer composite materials through an interfacial polymerization technique. The copolymerization of a modified UiO‐66‐NH₂ MOF with a growing polyamide fiber (PA‐66) during an interfacial polymerization gave hybrid materials with up to around 29 weight percent MOF. The covalent hybrid material demonstrated nearly an order of magnitude higher catalytic activity for the breakdown of a chemical warfare simulant (dimethyl‐4‐nitrophenyl phosphate, DMNP) compared to MOFs that are non‐covalently, physically entrapped in nylon, thus highlighting the importance of MOF–polymer hybridization.     

Mineralization‐Inspired Synthesis of Magnetic Zeolitic Imidazole Framework Composites

Metal–organic frameworks (MOFs) capable of mobility and manipulation are attractive materials for potential applications in targeted drug delivery, catalysis, and small‐scale machines. One way of rendering MOFs navigable is incorporating magnetically responsive nanostructures, which usually involve at least two preparation steps: the growth of the magnetic nanomaterial and its incorporation during the synthesis of the MOF crystals. Now, by using optimal combinations of salts and ligands, zeolitic imidazolate framework composite structures with ferrimagnetic behavior can be readily obtained via a one‐step synthetic procedure, that is, without the incorporation of extrinsic magnetic components. The ferrimagnetism of the composite originates from binary oxides of iron and transition metals such as cobalt. This approach exhibits similarities to the natural mineralization of iron oxide species, as is observed in ores and in biomineralization.     

Surface‐Specific Functionalization of Nanoscale Metal–Organic Frameworks

A method for modifying the external surfaces of a series of nanoscale metal–organic frameworks (MOFs) with 1,2‐dioleoyl‐sn‐glycero‐3‐phosphate (DOPA) is presented. A series of zirconium‐based nanoMOFs of the same topology (UiO‐66, UiO‐67, and BUT‐30) were synthesized, isolated as aggregates, and then conjugated with DOPA to create stably dispersed colloids. BET surface area analysis revealed that these structures maintain their porosity after surface functionalization, providing evidence that DOPA functionalization only occurs on the external surface. Additionally, dye‐labeled ligand loading studies revealed that the density of DOPA on the surface of the nanoscale MOF correlates to the density of metal nodes on the surface of each MOF. Importantly, the surface modification strategy described will allow for the general and divergent synthesis and study of a wide variety of nanoscale MOFs as stable colloidal materials.