In Situ Observation of Successive Crystallizations and Metastable Intermediates in the Formation of Metal–Organic Frameworks

Understanding the driving forces controlling crystallization is essential for the efficient synthesis and design of new materials, particularly metal–organic frameworks (MOFs), where mild solvothermal synthesis often allows access to various phases from the same reagents. Using high‐energy in situ synchrotron X‐ray powder diffraction, we monitor the crystallization of lithium tartrate MOFs, observing the successive crystallization and dissolution of three competing phases in one reaction. By determining rate constants and activation energies, we fully quantify the reaction energy landscape, gaining important predictive power for the choice of reaction conditions. Different reaction rates are explained by the structural relationships between the products and the reactants; larger changes in conformation result in higher activation energies. The methods we demonstrate can easily be applied to other materials, opening the door to a greater understanding of crystallization in general.     

Exchange of Coordinated Solvent During Crystallization of a Metal–Organic Framework Observed by In Situ High‐Energy X‐ray Diffraction

Using time‐resolved monochromatic high energy X‐ray diffraction, we present an in situ study of the solvothermal crystallisation of a new MOF [Yb₂(BDC)₃(DMF)₂]?H₂O (BDC=benzene‐1,4‐dicarboxylate and DMF=N,N‐dimethylformamide) under solvothermal conditions, from mixed water/DMF solvent. Analysis of high resolution powder patterns obtained reveals an evolution of lattice parameters and electron density during the crystallisation process and Rietveld analysis shows that this is due to a gradual topochemical replacement of coordinated solvent molecules. The water initially coordinated to Yb³⁺ is replaced by DMF as the reaction progresses.     

Direct X‐ray Observation of Trapped CO2 in a Predesigned Porphyrinic Metal–Organic Framework

Metal–organic frameworks (MOFs) are emerging microporous materials that are promising for capture and sequestration of CO₂ due to their tailorable binding properties. However, it remains a grand challenge to pre‐design a MOF with a precise, multivalent binding environment at the molecular level to enhance CO₂ capture. Here, we report the design, synthesis, and direct X‐ray crystallographic observation of a porphyrinic MOF, UNLPF‐2, that contains CO₂‐specific single molecular traps. Assembled from an octatopic porphyrin ligand with [Co₂(COO)₄] paddlewheel clusters, UNLPF‐2 provides an appropriate distance between the coordinatively unsaturated metal centers, which serve as the ideal binding sites for in situ generated CO₂. The coordination of Co^II in the porphyrin macrocycle is crucial and responsible for the formation of the required topology to trap CO₂. By repeatedly releasing and recapturing CO₂, UNLPL‐2 also exhibits recyclability.     

pH‐Dependent Proton Conducting Behavior in a Metal–Organic Framework Material

A porous metal–organic framework (MOF), [Ni₂(dobdc)(H₂O)₂]?6 H₂O (Ni₂(dobdc) or Ni‐MOF‐74; dobdc^4?=2,5‐dioxido‐1,4‐benzenedicarboxylate) with hexagonal channels was synthesized using a microwave‐assisted solvothermal reaction. Soaking Ni₂(dobdc) in sulfuric acid solutions at different pH values afforded new proton‐conducting frameworks, H⁺@Ni₂(dobdc). At pH 1.8, the acidified MOF shows proton conductivity of 2.2×10^?2 S cm^?1 at 80 °C and 95 % relative humidity (RH), approaching the highest values reported for MOFs. Proton conduction occurs via the Grotthuss mechanism with a significantly low activation energy as compared to other proton‐conducting MOFs. Protonated water clusters within the pores of H⁺@Ni₂(dobdc) play an important role in the conduction process.     

Coordination‐Driven In Situ Self‐Assembly Strategy for the Preparation of Metal–Organic Framework Hybrid Membranes

Metal–organic frameworks (MOFs) have emerged as porous solids of a superior type for the fabrication of membranes. However, it is still challenging to prepare a uniformly dispersed robust MOF hybrid membrane. Herein, we propose a simple and powerful strategy, namely, coordination‐driven in situ self‐assembly, for the fabrication of MOF hybrid membranes. On the basis of the coordination interactions between metal ions and ligands and/or the functional groups of the organic polymer, this method was confirmed to be feasible for the production of a stable membrane with greatly improved MOF‐particle dispersion in and compatibility with the polymer, thus providing outstanding separation ability. As an experimental proof of concept, a high‐quality ZIF‐8/PSS membrane was fabricated that showed excellent performance in the nanofiltration and separation of dyes from water.     

Smart Approach for In Situ One‐Step Encapsulation and Controlled Delivery of a Chemotherapeutic Drug using Metal–Organic Framework–Drug Composites in Aqueous Media

Controlled release of an anticancer drug, doxorubicin (dox), from metal–organic framework (MOF)–drug composites is demonstrated under different external stimuli. 1,3,5‐Benzenetricarboxylic acid (H₃BTC) is used as an organic ligand, and iron acetate and zinc nitrate are used as metal sources to synthesize Fe–BTC and Zn–BTC MOFs, which are known to be biocompatible. The in situ formation of MOF–drug composites demonstrates high drug loading capacity compared to conventional methods. The present methodology is devoid of any extra steps for loading the drug after synthesis. Moreover, the drug loading is also independent of pore size of the MOF as the drug molecules are embedded inside the MOF during their in situ formation. The drug release was monitored under external stimuli including change to acidic pH and the presence of biocompatible liposomes for a period of more than 72 h. Steady‐state fluorescence spectroscopy is used to monitor the drug release as a function of time and confocal laser scanning microscopy is used to unravel the post‐release fate of doxorubicin in the presence of liposomes. It is found that drug release rate is higher for the Zn–BTC–dox composite than for the Fe–BTC–dox composite. This is attributed to the stronger binding between dox and Fe‐BTC than that between dox and Zn–BTC. This study highlights a novel approach for the preparation of MOF–drug composites in an aqueous medium for future biomedical applications.     

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.     

Thermoplastic Membranes Incorporating Semiconductive Metal–Organic Frameworks: An Advance on Flexible X‐ray Detectors

Semiconductive metal–organic frameworks (MOFs) have emerged in applications such as chemical sensors, electrocatalysts, energy storage materials, and electronic devices. However, examples of semiconductive MOFs within flexible electronics have not been reported. We present flexible X‐ray detectors prepared by thermoplastic dispersal of a semiconductive MOF ( SCU‐13 ) through a commercially available polymer, poly(vinylidene fluoride). The flexible detectors exhibit efficient X‐ray‐to‐electric current conversion with enhanced charge‐carrier mobility and low trap density compared to pelleted devices. A high X‐ray detection sensitivity of 65.86 μCGy_air^?1 cm^?2 was achieved, which outperforms other pelleted devices and commercial flexible X‐ray detectors. We demonstrate that the MOF‐based flexible detectors can be operated at multiple bending angles without a deterioration in detection performance. As a proof‐of‐concept, an X‐ray phase contrast under bending conditions was constructed using a 5×5 pixelated MOF‐based imager.     

Dynamic Calcium Metal–Organic Framework Acts as a Selective Organic Solvent Sponge

Herein, we present a Ca‐based metal–organic framework named AEPF‐1, which is an active and selective catalyst in olefin hydrogenation reactions. AEPF‐1 exhibits a phase transition upon desorption of guest molecules. This structural transformation takes place by a crystal to crystal transformation accompanied by the loss of single‐crystal integrity. Powder diffraction methods and computational studies were applied to determine the structure of the guest‐free phase. This work also presents data on the exceptional adsorption behavior of this material, which is shown to be capable of separating polar from nonpolar organic solvents, and is a good candidate for selective solvent adsorption under mild conditions.     

An Ultrahydrophobic Fluorous Metal–Organic Framework Derived Recyclable Composite as a Promising Platform to Tackle Marine Oil Spills

Derived from a strategically chosen hexafluorinated dicarboxylate linker aimed at the designed synthesis of a superhydrophobic metal–organic framework (MOF), the fluorine‐rich nanospace of a water‐stable MOF ( UHMOF‐100 ) exhibits excellent water‐repellent features. It registered the highest water contact angle (≈176°) in the MOF domain, marking the first example of an ultrahydrophobic MOF. Various experimental and theoretical studies reinforce its distinctive water‐repellent characteristics, and the conjugation of superoleophilicity and unparalleled hydrophobicity of a MOF material has been coherently exploited to achieve real‐time oil/water separation in recyclable membrane form, with significant absorption capacity performance. This is also the first report of an oil/water separating fluorinated ultrahydrophobic MOF‐based membrane material, with potential promise for tackling marine oil spillages.     

Hollow Metal–Organic‐Framework‐Mediated In Situ Architecture of Copper Dendrites for Enhanced CO2 Electroreduction

Electrocatalytic reduction of CO₂ to a single product at high current densities and efficiencies remains a challenge. However, the conventional electrode preparation methods, such as drop‐casting, usually suffer from low intrinsic activity. Herein, we report a synthesis strategy for preparing heterogeneous electrocatalyst composed of 3D hierarchical Cu dendrites that derived from an in situ electrosynthesized hollow copper metal–organic framework (MOF), for which the preparation of the Cu‐MOF film took only 5 min. The synthesis strategy preferentially exposes active sites, which favor's the reduction of CO₂ to formate. The current density could be as high as 102.1 mA cm^?2 with a selectivity of 98.2 % in ionic‐liquid‐based electrolyte and a commonly used H‐type cell.     

Three‐Dimensional Metal–Fullerene Frameworks

Hexakis‐substituted [60]fullerene adducts with icosahedral symmetry provide an unprecedented scaffold for the spatial arrangement of twelve functional groups with high geometric precision. This unique molecular symmetry identifies such polyfunctional organic building blocks as potential highly connective linkers for coordination polymer and metal–organic framework synthesis. Hereby, the linker exhibits a higher connectivity than the metal ions and with the main connectivity based on the ligand, this can create a new type of inversely cross‐linked framework. Two hexakis adducts bearing either twelve glycolic acid or 3‐hydroxypropionic acid side chains attached to its malonate units were incorporated as organic connectivity centers in the first fullerene‐containing three‐dimensional frameworks by coordination with Zn²⁺.     

Crystal‐Growth‐Dominated Fabrication of Metal–Organic Frameworks with Orderly Distributed Hierarchical Porosity

Constructing architectures with hierarchical porosity has been widely considered as the most efficient way to bypass the problems related to slow mass transfer and inaccessibility of internal space in MOFs. Now, a crystal‐growth‐dominated strategy is proposed to fabricate hierarchically porous MOFs (HP‐MOFs). When the crystal growth is dominated by the monomer attachment, the aggregation of nonionic surfactant or polymer can be easily captured and released during the crystal growth process, resulting in the formation and ordering hierarchical pores along the radial direction. Owing to the accelerated mass diffusion and more exposed active sites of this design, HP‐MOFs exhibited an enhanced catalytic efficiency in styrene oxidation.