Controlling Dissolution and Transformation of Zeolitic Imidazolate Frameworks by using Electron‐Beam‐Induced Amorphization

Amorphous zeolitic imidazolate frameworks (ZIFs) offer promising applications as novel functional materials. Herein, amorphization of ZIF‐L through scanning‐electron‐beam exposure is demonstrated, based on amorphization of individual ZIF‐L crystals. The amorphized ZIF product has drastically increased stability against dissolution in water. An electron dose that allows for complete preservation of amorphous particles after immersion in water is established, resulting in new shapes of amorphous ZIF‐L with spatial control at the sub‐micrometer length scale. Changed water stability as a consequence of scanning‐electron‐beam exposure is demonstrated for three additional metal–organic frameworks (ZIF‐8, Zn(BeIm)OAc, MIL‐101), highlighting the potential use of an electron beam for top‐down MOF patterning. Lastly, recrystallization of ZIF‐L in the presence of linker is studied and shows distinct differences for crystalline and amorphized material.     

Solvent-Free Synthesis of Zeolitic Imidazolate Frameworks and the Catalytic Properties of Their Carbon Materials

Zeolitic imidazolate frameworks (ZIFs) are traditionally synthesized solvothermally by using cost- and waste-incurring organic solvents. Here, a direct synthesis method is reported for ZIF-8, ZIF-67, and their heterometallic versions from solid precursors only. This solvent-free crystallization method not only completely avoids organic solvents, but also provides an effective path for the synthesis of homogeneous mixed-metal ZIFs. Furthermore, under templating by NaCl/ZnCl₂ eutectic salt, carbonization of the ZIF materials gives rise to a series of N-containing high-surface-area carbon materials with impressive catalytic properties for the oxygen reduction reaction.     

Fischer–Tropsch synthesis using zeolitic imidazolate framework (ZIF-7 and ZIF-8)-supported cobalt catalysts

The Fischer–Tropsch synthesis using cobalt catalysts supported on zeolitic imidazolate frameworks (ZIFs), ZIF-7 and ZIF-8, has been investigated. ZIF-7, ZIF-8 and corresponding cobalt-containing catalysts, after preparation, were characterized using various techniques. Thermogravimetric analysis results show that ZIF-7 and ZIF-8 supports have good thermal stability for the Fischer–Tropsch synthesis reaction, and weaker interaction between cobalt and zinc in the ZIF-7 and ZIF-8 supports results in more cobalt reduction. The catalytic performance was evaluated in Fischer–Tropsch synthesis and compared with that of a cobalt catalyst supported on SBA-15 promoted with zinc. The pore structure of the ZIF supports plays an essential role in product selectivity for the prepared catalysts. The carbon number in hydrocarbon products and olefin selectivity depend on cobalt dispersion and support structure owing to the impacts of site density and carrier skeleton on the speed of diffusion-enhanced olefin re-adsorption reactions.     

Hollow Zn/Co Zeolitic Imidazolate Framework (ZIF) and Yolk–Shell Metal@Zn/Co ZIF Nanostructures

Metal–organic frameworks (MOFs) feature a great possibility for a broad spectrum of applications. Hollow MOF structures with tunable porosity and multifunctionality at the nanoscale with beneficial properties are desired as hosts for catalytically active species. Herein, we demonstrate the formation of well‐defined hollow Zn/Co‐based zeolitic imidazolate frameworks (ZIFs) by use of epitaxial growth of Zn‐MOF (ZIF‐8) on preformed Co‐MOF (ZIF‐67) nanocrystals that involve in situ self‐sacrifice/excavation of the Co‐MOF. Moreover, any type of metal nanoparticles can be accommodated in Zn/Co‐ZIF shells to generate yolk–shell metal@ZIF structures. Transmission electron microscopy and tomography studies revealed the inclusion of these nanoparticles within hollow Zn/Co‐ZIF with dominance of the Zn‐MOF as shell. Our findings lead to a generalization of such hollow systems that are working effectively to other types of ZIFs.     

Synthesis and Optimization of Zeolitic Imidazolate Frameworks for the Oxygen Evolution Reaction

Metal–organic frameworks/zeolitic imidazolate frameworks (MOFs/ZIFs) and their post-synthesis modified nanostructures, such as oxides, hydroxides, and carbons have generated significant interest for electrocatalytic reactions. In this work, a high and durable oxygen evolution reaction (OER) performance directly from bimetallic Zn_100−xCo_x-ZIF samples is reported, without carrying out high-temperature calcination and/or carbonization. ZIFs can be reproducibly and readily synthesized in large scale at ambient conditions. The bimetallic ZIFs show a systematic and gradually improved OER activity with increasing cobalt concentration. A further increase in OER activity is evidenced in ZIF-67 polyhedrons with controlled particle size of <200 nm among samples of different sizes between 50 nm and 2 μm. Building on this, a significantly enhanced, >50 %, OER activity is obtained with ZIF-67/carbon black, which shows a low overpotential of approximately 320 mV in 1.0 m KOH electrolyte. Such activity is comparable to or better than numerous MOF/ZIF-derived electrocatalysts. The optimized ZIF-67 sample also exhibits increased activity and durability over 24 h, which is attributed to an in situ developed active cobalt oxide/oxyhydroxide related nanophase.     

Facile mechanosynthesis of amorphous zeolitic imidazolate frameworks

A fast and efficient mechanosynthesis (ball-milling) method of preparing amorphous zeolitic imidazolate frameworks (ZIFs) from different starting materials is discussed. Using X-ray total scattering, N(2) sorption analysis, and gas pycnometry, these frameworks are indistinguishable from one another and from temperature-amorphized ZIFs. Gas sorption analysis also confirms that they are nonporous once formed, in contrast to activated ZIF-4, which displays interesting gate-opening behavior. Nanoparticles of a prototypical nanoporous substituted ZIF, ZIF-8, were also prepared and shown to undergo amorphization.     

Carbon Dioxide Sensitivity of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks of zinc, cobalt, and cadmium, including the framework ZIF‐8 commercially sold as Basolite Z1200, exhibit surprising sensitivity to carbon dioxide under mild conditions. The frameworks chemically react with CO₂ in the presence of moisture or liquid water to form carbonates. This effect, which has been previously not reported in metal–organic framework chemistry, provides an explanation for conflicting reports on ZIF‐8 stability to water and is of outstanding significance for evaluating the potential applications of metal–organic frameworks, especially for CO₂ sequestration.     

Post‐Synthetic Anisotropic Wet‐Chemical Etching of Colloidal Sodalite ZIF Crystals

Controlling the shape of metal–organic framework (MOF) crystals is important for understanding their crystallization and useful for myriad applications. However, despite the many advances in shaping of inorganic nanoparticles, post‐synthetic shape control of MOFs and, in general, molecular crystals remains embryonic. Herein, we report using a simple wet‐chemistry process at room temperature to control the anisotropic etching of colloidal ZIF‐8 and ZIF‐67 crystals. Our work enables uniform reshaping of these porous materials into unprecedented morphologies, including cubic and tetrahedral crystals, and even hollow boxes, by an acid–base reaction and subsequent sequestration of leached metal ions. Etching tests on these ZIFs reveal that etching occurs preferentially in the crystallographic directions richer in metal–ligand bonds; that, along these directions, the etching rate tends to be faster on the crystal surfaces of higher dimensionality; and that the etching can be modulated by adjusting the pH of the etchant solution.     

Influence of Substitutional Defects in ZIF-8 Membranes on Reverse Osmosis Desalination: A Molecular Dynamics Study

In this study, molecular dynamics simulation is used to investigate the effects of water-based substitutional defects in zeolitic imidazolate frameworks (ZIF)-8 membranes on their reverse osmosis (RO) desalination performance. ZIF-8 unit cells containing up to three defect sites are used to construct the membranes. These substitutional defects can either be Zn defects or linker defects. The RO desalination performance of the membranes is assessed in terms of the water flux and ion rejection rate. The effects of defects on the interactions between the ZIF-8 membranes and NaCl are investigated and explained with respect to the radial distribution function (RDF) and ion density distribution. The results show that ion adsorption on the membranes occurs at either the nitrogen atoms or the defect sites. Complete NaCl rejection can be achieved by introducing defects to change the size of the pores. It has also been discovered that the presence of linker defects increases membrane hydrophilicity. Overall, molecular dynamics simulations have been used in this study to show that water-based substitutional defects in a ZIF-8 structure reduce the water flux and influence its hydrophilicity and ion adsorption performance, which is useful in predicting the type and number of defect sites per unit cell required for RO applications. Of the seven ZIF-8 structures tested, pristine ZIF-8 exhibits the best RO desalination performance.     

Facile synthesis of size-tunable ZIF-8 nanocrystals using reverse micelles as nanoreactors

This paper describes a robust method for the synthesis of high-quality ZIF-8 nanocrystals using reverse micelles as discrete nanoscale reactors.The precise size control of ZIF-8 nanocrystals is conveniently achieved by tuning the concentration of precursors,reaction temperatures,the amount of water,and the structure of surfactants.The as-synthesized ZIF-8 nanocrystals are of narrow distribution and tunable size.A size-dependent catalytic activity for Knoevenagel condensation reaction is further demonstrated by using ZIF-8 nanocrystals with different sizes as the catalysts.This facile method opens up a new opportunity in the synthesis of various ZIFs nanocrystals.     

Towards Predicting the Sequential Appearance of Zeolitic Imidazolate Frameworks Synthesized by Mechanochemistry

We use computational materials methods to study the sequential appearance of zinc-based zeolitic imidazolate frameworks (ZIFs) generated in the mechanochemical conversion process. We consider nine ZIF topologies, namely RHO, ANA, QTZ, SOD, KAT, DIA, NEB, CAG and GIS, combined with the two ligands 2-methylimidazolate and 2-ethylimidazolate. Of the 18 combinations obtained, only six (three for each ligand) were actually observed during the mechanosynthesis process. Energy and porosity calculations based on density functional theory, in combination with the Ostwald rule of stages, were found to be insufficient to distinguish the experimentally observed ZIFs. We then show, using classical molecular dynamics, that only ZIFs withstanding quasi-hydrostatic pressure P ≥ 0.3 GPa without being destroyed were observed in the laboratory. This finding, along with the requirement that successive ZIFs be generated with decreasing porosity and/or energy, provides heuristic rules for predicting the sequences of mechanically generated ZIFs for the two ligands considered.     

Selective Capture of Carbon Dioxide under Humid Conditions by Hydrophobic Chabazite‐Type Zeolitic Imidazolate Frameworks

Hydrophobic zeolitic imidazolate frameworks (ZIFs) with the chabazite ( CHA ) topology are synthesized by incorporating two distinct imidazolate links. Zn(2‐mIm)_0.86(bbIm)_1.14 (ZIF‐300), Zn(2‐mIm)_0.94(cbIm)_1.06 (ZIF‐301), and Zn(2‐mIm)_0.67(mbIm)_1.33 (ZIF‐302), where 2‐mIm=2‐methylimidazolate, bbIm=5(6)‐bromobenzimidazolate, cbIm=5(6)‐chlorobenzimidazolate, and mbIm=5(6)‐methylbenzimidazolate, were prepared by reacting zinc nitrate tetrahydrate and 2‐mIm with the respective bIm link in a mixture of N,N‐dimethylformamide (DMF) and water. Their structures were determined by single‐crystal X‐ray diffraction and their permanent porosity shown. All of these structures are hydrophobic as confirmed by water adsorption isotherms. All three ZIFs are equally effective at the dynamic separation of CO₂ from N₂ under both dry and humid conditions without any loss of performance over three cycles and can be regenerated simply by using a N₂ flow at ambient temperature.     

Host-Guest Interaction in Ethylene and Ethane Separation on Zeolitic Imidazolate Frameworks as Revealed by Solid-State NMR Spectroscopy

The separation of ethane/ethylene mixture by using metal-organic frameworks (MOFs) as adsorbents is strongly associated with the pore size-sieving effect and the adsorbent-adsorbate interaction. Herein, solid-state NMR spectroscopy is utilized to explore the host-guest interaction and ethane/ethylene separation mechanism on zeolitic imidazolate frameworks (ZIFs). Preferential access to the ZIF-8 and ZIF-8-90 frameworks by ethane compared to ethylene is directly visualized from two-dimensional ¹H-¹H spin diffusion MAS NMR spectroscopy and further verified by computational density distributions. The ¹H MAS NMR spectroscopy provides an alternative for straightforwardly extracting the adsorption selectivity of ethane/ethylene mixture at 1.1∼9.6 bar in ZIFs, which is consistent with the IAST predictions.     

Post-Synthetic Modification of ZIF-8 Crystals and Films through UV Light Photoirradiation: Impact on the Physicochemical Behavior of the MOF

The physicochemical modification of Metal-Organic Frameworks (MOFs) is a current challenge in the search to improve their performance in different technological applications. In this work we analyze the post-synthetic modification of ZIF-8 crystals and films through a simple and clean treatment that involves the exposure to a UV lamp under environmental conditions. It is demonstrated that a short treatment alters the MOF structure and chemistry, providing a modified ZIF-8 due to partial disconnections of its structure which increase the amount of terminal surface species such as Zn−OH and −C=N-H, but without compromising the overall MOF structure, specific surface area or thermal stability. Additionally, it leads to changes in several properties of the ZIF-8, such as its capacity to accumulate charge through pseudocapacitive processes, its interaction with nitric oxide and its light absorption behavior. This strategy of modifying ZIF-8 without the use of chemicals through a gentle disconnection of its own structure could open new perspectives of post-functionalization of crystals and films of ZIF-8 to be used in a wide range of applications.     

Zeolitic Imidazolate framework-8 as efficient pH-sensitive drug delivery vehicle

Zeolitic Imidazolate Framework-8 (ZIF-8), for the first time for ZIFs, exhibits a remarkable capacity for the anticancer drug 5-fluorouracil (5-FU), around 660 mg of 5-FU/g of ZIF-8, and presents a pH-triggered controlled drug release property. These prove ZIF-8 to be a valuable candidate for delivery of anticancer agents and reveal its potential applications in the treatment of cancer.     

Selective synthesis of ZIFs from zinc and nickel nitrate solution for photocatalytic H2O2 production

Hydrogen peroxide (H₂O₂) is one of the most promising, green, and effective oxidants that can be used in different applications. In this study, zeolitic imidazolate frameworks (ZIFs), consisting of organic ligands and metal sites, were selectively prepared from zinc or nickel nitrate solutions for use in photocatalytic H₂O₂ production. High concentrations of zinc nitrate solution provided more metal sites to coordinate with 2-methylimidazole, producing ZIF-8 with larger particle size, whereas low zinc nitrate concentrations resulted in more interconnected N–H⋯N hydrogen bonds, forming 2D-layered ZIF-L, with smaller particle size. Various concentrations of zinc and nickel nitrate solutions produced ZIFs that exhibited ZIF-8 or ZIF-L topology, with bandgap energies of 5.45 and 4.85 eV, respectively. These samples could serve as promising photocatalyst for the successful production of H₂O₂ under Xenon lamp irradiation.     

Zinc hydroxide nitrate nanosheets conversion into hierarchical zeolitic imidazolate frameworks nanocomposite and their application for CO2 sorption

Hierarchical porous zeolitic imidazolate frameworks (HZIFs) are promising materials for several applications, including adsorption, separation, and nanomedicine. Herein, the conversion of zinc hydroxide nitrate nanosheets into HZIF-8 nanocomposite with graphene oxide (GO) and magnetic nanoparticles (MNPs) is reported. The conversion takes place at room temperature in water. This approach has been successfully applied for the formation of leaf-like ZIF(ZIF-L), and their nanocomposites with nanoparticles, such as GO and MNPs. This method offers a simple approach for the synthesis of tunable pore structure using nanoparticles and fast room temperature conversion (30 min) without any visible residual impurities of zinc hydroxide nitrates. The applications of HZIF-8, ZIF-L, and their nanocomposites, for CO₂ sorption, exhibit excellent adsorption properties. The synthesized composites exhibit enhanced CO₂ adsorption capacity due to the synergistic effect between nanoparticles (GO, or MNPs), and ZIF-8. The materials have good potential for further applications.     

Interactive Thermal Effects on Metal–Organic Framework Polymer Composite Membranes

Polymeric membranes are important tools for intensifying separation processes in chemical industries, concerning strategic tasks such as CO₂ sequestration, H₂ production, and water supply and disposal. Mixed‐matrix and supported membranes have been widely developed; recently many of them have been based on metal–organic frameworks (MOFs). However, most of the impacts MOFs have within the polymer matrix have yet to be determined. The effects related to thermal behavior arising from the combination of MOF ZIF‐8 and polysulfone have now been quantified. The catalyzed oxidation of the polymer is strongly affected by the MOF crystal size and distribution inside the membrane. A 16 wt % 140 nm‐sized ZIF‐8 loading causes a 40 % decrease in the observed activation energy of the polysulfone oxidation that takes place at a temperature (545 °C) 80 °C lower than in the raw polymer (625 °C).     

Preparation of a zeolite imidazole skeleton–silica hybrid monolithic column for amino acid analysis via capillary electrochromatography

We developed a novel, convenient and low-cost one-pot strategy for preparing a zeolitic imidazolate framework-8 (ZIF-8)–silica hybrid monolithic column by adding ZIF-8 directly to a polymer solution of the silica matrix. The simulated stationary phase and monolithic column prepared under optimal conditions were characterized using X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis nitrogen physisorption and zeta potential. The results obtained confirmed the successful introduction of ZIF-8 into the silica monolithic column, and the prepared monolithic column exhibited good permeability and physicochemical stability. A capillary electrochromatography method was developed based on a ZIF-8–silica hybrid monolithic column through which 15 mixed amino acids, 4 neutral compounds, 4 nipagin esters and 2 chlorinated fungicides were separated in 14, 5, 7 and 6 min, respectively, under optimal conditions. The relative standard deviations retention times and column efficiencies in run-to-run, day-to-day and column-to-column varied in the ranges of 1.90%–2.21%, 2.13%–2.51% and 3.08%–6.65%, respectively, which demonstrated that ZIF-8–silica hybrid monolithic column exhibited satisfactory reproducibility and stability. The incorporation of ZIF-8 into a silica monolithic column is a promising method for preparing novel monolithic columns composed of a metal–organic framework.     

Electrochemical Synthesis and Catalytic Properties of Encapsulated Metal Clusters within Zeolitic Imidazolate Frameworks

It is very interesting and also a big challenge to encapsulate metal clusters within microporous solids to expand their application diversity. For this target, herein, we present an electrochemical synthesis strategy for the encapsulation of noble metals (Au, Pd, Pt) within ZIF‐8 cavities. In this method, metal precursors of AuCl₄^2?, PtCl₆^2?, and PdCl₄^2? are introduced into ZIF‐8 crystals during the concurrent crystallization of ZIF‐8 at the anode. As a consequence, very small metal clusters with sizes around 1.2 nm are obtained within ZIF‐8 crystals after hydrogen reduction; these clusters exhibit high thermal stability, as evident from the good maintenance of their original sizes after a high‐temperature test. The catalytic properties of the encapsulated metal clusters within ZIF‐8 are evaluated for CO oxidations. Because of the small pore window of ZIF‐8 (0.34 nm) and the confinement effect of small pores, about 80 % of the metal clusters (fractions of 0.74, 0.77, and 0.75 for Au, Pt, and Pd in ZIF‐8, respectively) retain their catalytic activity after exposure to the organosulfur poison thiophene (0.46 nm), which is in contrast to their counterparts (fractions of 0.22, 0.25, and 0.20 for Au, Pt, and Pd on the SiO₂ support). The excellent performances of metal clusters encapsulated within ZIF‐8 crystals give new opportunities for catalytic reactions.