MOF‐5‐Polystyrene: Direct Production from Monomer,Improved Hydrolytic Stability,and Unique Guest Adsorption

An unprecedented mode of reactivity of Zn₄O‐based metal–organic frameworks (MOFs) offers a straightforward and powerful approach to polymer‐hybridized porous solids. The concept is illustrated with the production of MOF‐5‐polystyrene wherein polystyrene is grafted and uniformly distributed throughout MOF‐5 crystals after heating in pure styrene for 4–24 h. The surface area and polystyrene content of the material can be fine‐tuned by controlling the duration of heating styrene in the presence of MOF‐5. Polystyrene grafting significantly alters the physical and chemical properties of pristine MOF‐5, which is evident from the unique guest adsorption properties (solvatochromic dye uptake and improved CO₂ capacity) as well as the dramatically improved hydrolytic stability of composite. Based on the fact that MOF‐5 is the best studied member of the structure class, and has been produced at scale by industry, these findings can be directly leveraged for a range of current applications.     

NiII Coordination to an Al‐Based Metal–Organic Framework Made from 2‐Aminoterephthalate for Photocatalytic Overall Water Splitting

The aluminum‐based metal–organic framework (MOF) made from 2‐aminoterephthalate is a photocatalyst for oxygen evolution. This MOF can be modified by incorporating Ni²⁺ cations into the pores through coordination to the amino groups, and the resulting MOF is an efficient photocatalyst for overall water splitting.     

MOF‐Templated Synthesis of Ultrasmall Photoluminescent Carbon‐Nanodot Arrays for Optical Applications

Arrays of ultrasmall and uniform carbon nanodots (CDs) are of pronounced interest for applications in optical devices. Herein, we describe a low‐temperature calcination approach with rather inexpensive reactants. After glucose molecules had been loaded into the pores of metal–organic frameworks (MOFs), well‐defined CD arrays were produced by heating to 200 °C. The size and spacing of the CDs could be controlled by the choice of templating MOF: HKUST‐1, ZIF‐8, or MIL‐101. The sizes of the obtained CDs were approximately 1.5, 2.0, and 3.2 nm, which are close to the corresponding MOF pores sizes. The CD arrays exhibited interesting photophysical properties, including photoluminescence with tunable emission and pronounced nonlinear optical (NLO) effects. The NLO properties of the obtained CD arrays were significantly different from those of a CD suspension, thus indicating the existence of collective phenomena.     

Homochiral MOF–Polymer Mixed Matrix Membranes for Efficient Separation of Chiral Molecules

Homochiral metal–organic framework (MOF) membranes have been recently reported for chiral separations. However, only a few high‐quality homochiral polycrystalline MOF membranes have been fabricated due to the difficulty in crystallization of a chiral MOF layer without defects on porous substrates. Alternatively, mixed matrix membranes (MMMs), which combine potential advantages of MOFs and polymers, have been widely demonstrated for gas separation and water purification. Here we report novel homochiral MOF–polymer MMMs for efficient chiral separation. Homochirality was successfully incorporated into achiral MIL‐53‐NH₂ nanocrystals by post‐synthetic modification with amino acids, such as l ‐histidine (l ‐His) and l ‐glutamic acid (l ‐Glu). The MIL‐53‐NH‐l ‐His and MIL‐53‐NH‐l ‐Glu nanocrystals were then embedded into polyethersulfone (PES) matrix to form homochiral MMMs, which exhibited excellent enantioselectivity for racemic 1‐phenylethanol with the highest enantiomeric excess value up to 100 %. This work, as an example, demonstrates the feasibility of fabricating diverse large‐scale homochiral MOF‐based MMMs for chiral separation.     

Mixed‐Metal MOFs: Unique Opportunities in Metal–Organic Framework (MOF) Functionality and Design

Mixed‐metal metal–organic frameworks (MM‐MOFs) can be considered to be those MOFs having two different metals anywhere in the structure. Herein we summarize the various strategies for the preparation of MM‐MOFs and some of their applications in adsorption, gas separation, and catalysis. It is shown that compared to homometallic MOFs, MM‐MOFs bring about the opportunity to take advantage of the complexity and the synergism derived from the presence of different metal ions in the structure of MOFs. This is reflected in a superior performance and even stability of MM‐MOFs respect to related single‐metal MOFs. Emphasis is made on the use of MM‐MOFs as catalysts for tandem reactions.     

MOF‐on‐MOF: Oriented Growth of Multiple Layered Thin Films of Metal–Organic Frameworks

The precise alignment of multiple layers of metal–organic framework (MOF) thin films, or MOF‐on‐MOF films, over macroscopic length scales is presented. The MOF‐on‐MOF films are fabricated by epitaxially matching the interface. The first MOF layer (Cu₂(BPDC)₂, BPDC=biphenyl‐4,4′‐dicarboxylate) is grown on an oriented Cu(OH)₂ film by a “one‐pot” approach. Aligned second (Cu₂(BDC)₂, BDC=benzene 1,4‐dicarboxylate, or Cu₂(BPYDC)₂, BPYDC=2,2′‐bipyridine‐5,5′‐dicarboxylate) MOF layers can be deposited using liquid‐phase epitaxy. The co‐orientation of the MOF films is confirmed by X‐ray diffraction. Importantly, our strategy allows for the synthesis of aligned MOF films, for example, Cu₂(BPYDC)₂, that cannot be grown on a Cu(OH)₂ surface. We show that aligned MOF films furnished with Ag nanoparticles show a unique anisotropic plasmon resonance. Our MOF‐on‐MOF approach expands the chemistry of heteroepitaxially oriented MOF films and provides a new toolbox for multifunctional porous coatings.     

Solvent‐Induced Control over Breathing Behavior in Flexible Metal–Organic Frameworks for Natural‐Gas Delivery

Finding appropriate stimuli for controlling the breathing behavior of flexible metal–organic frameworks (MOFs) is highly challenging. Herein, we report the solvent‐induced changes in the particle size and stability of different breathing phases of the MIL‐53 series, a group of flexible MOFs. A water/dimethylformamide (DMF) ratio is tuned to synthesize members of the MIL‐53 series which have different behaviors. The breathing is explored by high‐pressure methane sorption tests. Increasing DMF concentration decreases MOF particle size and increases the stability of the porous phases, boosting the 5.8–65 bar sorption difference of methane, which is required for natural‐gas delivery.     

Step‐Growth Copolymerization Between an Immobilized Monomer and a Mobile Monomer in Metal–Organic Frameworks

The A–A/B–B step‐growth copolymerization between a monomer immobilized in the crystalline state and a monomer mobile in the solution state is demonstrated. One of the two monomers was immobilized as organic ligands of the metal–organic framework (MOF) and polymerized with the mobile guest monomer, resulting in the formation of linear polymers. The polymerization behavior was completely different from that of the solution polymerizations. In particular, the degrees of polymerization (DP) converged to a specific value depending on the MOF structures. The inevitable termination is caused not by imperfectness of the polymerization reaction, but by the selection of the two polymerization partners among the several adjacent immobilized monomers. This is fully supported by the Monte Carlo simulation on the basis of the polymerization mechanism. Precise immobilization of monomers in the supramolecular assemblies is a promising way for the controlled A–A/B–B step‐growth polymerization.     

Bi‐Microporous Metal–Organic Frameworks with Cubane [M4(OH)4] (M=Ni,Co) Clusters and Pore‐Space Partition for Electrocatalytic Methanol Oxidation Reaction

Embedding cubane [M₄(OH)₄] (M=Ni, Co) clusters within the matrix of metal–organic frameworks (MOFs) is a strategy to develop materials with unprecedented synergistic properties. Herein, a new material type based on the pore‐space partition of the cubic primitive minimal‐surface net (MOF‐14‐type) has been realized. CTGU‐15 made from the [Ni₄(OH)₄] cluster not only has very high BET surface area (3537 m² g^?1), but also exhibits bi‐microporous features with well‐defined micropores at 0.86 nm and 1.51 nm. Furthermore, CTGU‐15 is stable even under high pH (0.1 m KOH), making it well suited for methanol oxidation in basic medium. The optimal hybrid catalyst KB&CTGU‐15 (1:2) made from ketjen black (KB) and CTGU‐15 exhibits an outstanding performance with a high mass specific peak current of 527 mA mg^?1 and excellent peak current density (29.8 mA cm^?2) at low potential (0.6 V). The isostructural cobalt structure (CTGU‐16) has also been synthesized, further expanding the application potential of this material type.     

An Ultrastable Metal Azolate Framework with Binding Pockets for Optimal Carbon Dioxide Capture

In the evolution of metal–organic frameworks (MOFs) for carbon capture, a lasting challenge is to strike a balance between high uptake capacity/selectivity and low energy cost for regeneration. Meanwhile, these man‐made materials have to survive from practical demands such as stability under harsh conditions and feasibility of scale‐up synthesis. Reported here is a new MOF, Zn(imPim) (aka. MAF‐stu‐1), with an imidazole derivative ligand, featuring binding pockets that can accommodate CO₂ molecules in a fit‐like‐a‐glove manner. Such a high degree of shape complementarity allows direct observation of the loaded CO₂ in the pockets, and warrants its optimal carbon capture performances exceeding the best‐performing MOFs nowadays. Coupled with the record thermal (up to 680 °C) and chemical stability, as well as rapid large‐scale production, both encoded in the material design, Zn(imPim) represents a most competitive candidate to tackle the immediate problems of carbon dioxide capture.     

Direct Visualization of Pyrrole Reactivity upon Confinement within a Cyclodextrin Metal–Organic Framework

Metal–organic frameworks can be used as porous templates to exert control over polymerization reactions. Shown here are the possibilities offered by these crystalline, porous nanoreactors to capture highly‐reactive intermediates for a better understanding of the mechanism of polymerization reactions. By using a cyclodextrin framework the polymerization of pyrrole is restricted, capturing the formation of terpyrrole cationic intermediates. Single‐crystal X‐ray diffraction is used to provide definite information on the supramolecular interactions that induce the formation and stabilization of a conductive array of cationic complexes.     

On‐Chip Tailorability of Capacitive Gas Sensors Integrated with Metal–Organic Framework Films

Gas sensing technologies for smart cities require miniaturization, cost‐effectiveness, low power consumption, and outstanding sensitivity and selectivity. On‐chip, tailorable capacitive sensors integrated with metal–organic framework (MOF) films are presented, in which abundant coordinatively unsaturated metal sites are available for gas detection. The in situ growth of homogeneous Mg‐MOF‐74 films is realized with an appropriate metal‐to‐ligand ratio. The resultant sensors exhibit selective detection for benzene vapor and carbon dioxide (CO₂) at room temperature. Postsynthetic modification of Mg‐MOF‐74 films with ethylenediamine decreases sensitivity toward benzene but increases selectivity to CO₂. The reduced porosity and blocked open metal sites caused by amine coordination account for a deterioration in the sensing performance for benzene (by ca. 60 %). The enhanced sensitivity for CO₂ (by ca. 25 %) stems from a tailored amine–CO₂ interaction. This study demonstrates the feasibility of tuning gas sensing properties by adjusting MOF–analyte interactions, thereby offering new perspectives for the development of MOF‐based sensors.     

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.     

A Surfactant‐Free and Scalable General Strategy for Synthesizing Ultrathin Two‐Dimensional Metal–Organic Framework Nanosheets for the Oxygen Evolution Reaction

Metal–organic framework (MOFs) two‐dimensional (2D) nanosheets have many coordinatively unsaturated metal sites that act as active centres for catalysis. To date, limited numbers of 2D MOFs nanosheets can be obtained through top‐down or bottom‐up synthesis strategies. Herein, we report a 2D oxide sacrifice approach (2dOSA) to facilely synthesize ultrathin MOF‐74 and BTC MOF nanosheets with a flexible combination of metal sites, which cannot be obtained through the delamination of their bulk counterparts (top‐down) or the conventional solvothermal method (bottom‐up). The ultrathin iron–cobalt MOF‐74 nanosheets prepared are only 2.6 nm thick. The sample enriched with surface coordinatively unsaturated metal sites, exhibits a significantly higher oxygen evolution reaction reactivity than bulk FeCo MOF‐74 particles and the state‐of‐the‐art MOF catalyst. It is believed that this 2dOSA could provide a new and simple way to synthesize various ultrathin MOF nanosheets for wide applications.     

Hydrogen Storage and Selective,Reversible O2 Adsorption in a Metal–Organic Framework with Open Chromium(II) Sites

A chromium(II)‐based metal–organic framework Cr₃[(Cr₄Cl)₃(BTT)₈]₂ (Cr‐BTT; BTT³⁻=1,3,5‐benzenetristetrazolate), featuring coordinatively unsaturated, redox‐active Cr²⁺ cation sites, was synthesized and investigated for potential applications in H₂ storage and O₂ production. Low‐pressure H₂ adsorption and neutron powder diffraction experiments reveal moderately strong Cr–H₂ interactions, in line with results from previously reported M‐BTT frameworks. Notably, gas adsorption measurements also reveal excellent O₂/N₂ selectivity with substantial O₂ reversibility at room temperature, based on selective electron transfer to form Cr^III superoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O₂ binding.     

Evaporation‐Directed Crack‐Patterning of Metal–Organic Framework Colloidal Films and Their Application as Photonic Sensors

A straightforward crack‐patterning method is reported allowing the direct formation of periodic cracks in metal–organic framework (MOF) nanoparticle films during dip‐coating deposition. The crack propagation and periodicity can be easily tailored by controlling the evaporation front and the withdrawal speed. Several MOF‐patterned films can be fabricated on large surfaces and on several substrates (flat, curved or flexible) including the inner surface of a tube, not achievable by other lithographic techniques. We demonstrate that the periodic cracked arrays diffract light and, due to the MOF sorption properties, photonic vapor sensors are fabricated. A new concept of “in‐tube”, MOF‐based diffraction grating sensors is proposed with outstanding sensitivity that can be easily tuned “on‐demand” as function of the desired detection range.