Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and de Novo Routes

A Novel Modified ZIF-8 Nanoparticle with Enhanced Interfacial Compatibility and Pervaporation Performance in a Mixed Matrix Membrane for De-Alcoholization in Low-Concentration Solutions

This study investigated the enhancement in bioethanol recovery from mixed matrix membranes (MMMs) by functionalizing zeolite framework-8 (ZIF-8) with imidazolate. This study focused on the separation of ethanol from low-concentration ethanol/water mixtures (typical post-fermentation concentrations of 5-10 wt%). Specifically, ZIF-8 was modified by the shell-ligand exchange reaction (SLER) with 5,6-dimethylbenzimidazole (DMBIM), resulting in ZIF-8-DMBIM particles with improved hydrophobicity, organophilicity, larger size, and adjustable pore size. These particles were incorporated into a PEBAX 2533 matrix to produce ZIF-8-DMBIM/PEBAX MMMs using a dilution blending method. The resulting membranes showed significant performance enhancement: 8 wt% ZIF-8-DMBIM loading achieved a total flux of 308 g/m2·h and a separation factor of 16.03, which was a 36.8% increase in flux and 176.4% increase in separation factor compared with the original PEBAX membrane. In addition, performance remained stable during a 130 h cycling test. These improvements are attributed to the enhanced compatibility and dispersion of ZIF-8-DMBIM in the PEBAX matrix. In conclusion, the evaluation of nanofiller content, feed concentration, operating temperature, and membrane stability confirmed that ZIF-8-DMBIM/PEBAX MMM is ideal for ethanol recovery in primary bioethanol concentration processes.

Evaluating Nondestructive Quantification of Composition Gradients in Metal-Organic Frameworks by MeV Ion Microbeam Analysis

We evaluate a method to quantify composition depth gradients in intact metal-organic framework (MOF) single crystals and thereby derive diffusion coefficients of postsynthetically incorporated active sites by nondestructive ion-beam microanalysis. Zr-based UiO-67-bpy (bpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) MOFs were synthesized on Si substrates and then metalated postsynthetically with NiCl2 for 2-48 h, resulting in different Ni depth distributions. Simultaneous micro-Rutherford backscattering spectrometry (μ-RBS) and micro-particle induced X-ray emission (μ-PIXE) analysis were used for the spatially resolved chemical analysis of the MOF single crystals. Qualitative assessment of the μ-RBS spectra indicated the presence of elemental depth gradients and hinted at the governing process of the postsynthetic Ni incorporation, in the present case, molecular diffusion. Quantitative evaluation of the resulting composition depth profiles directly provided the diffusion length and, thereby, the diffusion coefficient of the system. Virtual gradients caused by overhanging tips/edges of the truncated octahedral crystal shape are considered. Furthermore, in the case of insufficient probing depth for μ-RBS, μ-PIXE was still capable of providing qualitative information. In the present system the diffusion coefficient for NiCl2 is found to be (1.72 ± 0.18) × 10-16 m2s-1. The long-term stability of the synthesized and postsynthetically modified MOFs is proved by repeated measurements.

Fluorinated Nanosized Zeolitic-Imidazolate Frameworks as Potential Devices for Mechanical Energy Storage

Fluorination is one of the most efficient and universal strategies to increase the hydrophobicity of materials and consequently their water stability. Zeolitic-imidazolate frameworks (ZIFs), which have limited stability in aqueous media and even lower stability when synthesized on a nanometric scale, can greatly benefit from the incorporation of fluorine atoms, not only to improve their stability but also to provide additional properties. Herein, we report the preparation of two different fluorinated ZIFs through a simple and scalable approach by using mixed ligands [2-methylimidazole, as a common ligand, and 4-(4-fluorophenyl)-1H-imidazole (monofluorinated linker) or 2-methyl-5-(trifluoromethyl)-1H-imidazole (trifluorinated linker) as a dopant], demonstrating the high versatility of the synthetic method developed to incorporate different fluorine-containing imidazole-based ligands. Second, we demonstrate for the first time that these nanoscale fluorinated ZIFs outperform the pristine ZIF-8 for water intrusion/extrusion, i.e., for storing mechanical energy via forced intrusion of nonwetting water due to the improved hydrophobicity and modified framework dynamics. Moreover, we also show that by varying the nature of the F-imidazole ligand, the performance of the resulting ZIFs, including the pressure thresholds and stored/dissipated energy, can be finely tuned, thus opening the path for the design of a library of fluorine-modified ZIFs with unique behavior.

A Method for Determining Incorporation Depth in Core-Shell UiO-66 Nanoparticles Synthesized Via Postsynthetic Exchange

Postsynthetic exchange (PSE) is a key technique for integrating sensitive linkers into metal-organic frameworks (MOFs). Despite its importance, investigations into linker distributions have primarily focused on micrometer-sized crystals due to the analytical limitations, leaving nanoparticles less explored, although they are commonly synthesized and used in applications. In particular, the emergence of core-shell nanostructures via PSE has shown potential for applications in CO2 adsorption and selective catalysis. This study addresses this gap by investigating the formation of core-shell structures on nanoparticles under diffusion-controlled PSE conditions. By analyzing volume-to-surface ratios and conducting time-dependent experiments, we confirmed that these conditions facilitate the development of core-shell architectures. We also developed a straightforward method to calculate the minimum incorporation depth using basic parameters such as particle size and the total amount of incorporated linker. The accuracy of our approach was validated against data obtained from transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy. These findings enhance the understanding of PSE in MOF nanoparticles and open up promising avenues for developing advanced MOF core-shell structures for various applications.

Bimetallic MOF-Derived Solar-Triggered Monolithic Adsorbent for Enhanced Atmospheric Water Harvesting

The development of economical, energy-saving, and efficient metal-organic framework (MOF)-based adsorbents for atmospheric water collection is highly imperative for the rapid advancement of renewable freshwater resource exploitation. Herein, a feasible one-step solvothermal formation strategy of bimetallic MOF (BMOF) is proposed and applied to construct a solar-triggered monolithic adsorbent for enhanced atmospheric water collection. Benefiting from the reorganization and adjustment of topology structure by Al atoms and Fe atoms, the resultant BMOF(3) consisting of Al-fumarate and MIL-88A has a higher specific surface area (1202.99 m2  g-1 ) and pore volume (0.51 cm3  g-1 ), thereby outperforming the parental MOFs and other potential MOFs in absorbing water. Expanding upon this finding, the solar-triggered monolithic adsorbent is further developed through a bottom-up assembly of polyaniline/chitosan layers and hybridized BMOF(3) skeletons on a glass fiber support. The resultant monolithic adsorbent exhibits superior sorption-desorption kinetics, leading to directional water transport and rapid solar-assisted vapor diffusion. As a proof-of-concept demonstration, an exquisite water harvester is constructed to emphasize a high water yield of 1.19 g g-1 per day of the designed monolithic adsorbent. Therefore, the design and validation of bimetallic MOF-derived solar-triggered adsorbent in this work are expected to provide a reference for the large-scale applications of MOF-based atmospheric water harvesting.

Does Mixed Linker-Induced Surface Heterogeneity Impact the Accuracy of IAST Predictions in UiO-66-NH2?

To move toward more energy-efficient adsorption-based processes, there is a need for accurate multicomponent data under realistic conditions. While the Ideal Adsorbed Solution Theory (IAST) has been established as the preferred prediction method due to its simplicity, limitations and inaccuracies for less ideal adsorption systems have been reported. Here, we use amine-functionalized derivatives of the UiO-66 structure to change the extent of homogeneity of the internal surface toward the adsorption of the two probe molecules carbon dioxide and ethylene. Although it might seem plausible that more functional groups lead to more heterogeneity and, thus, less accurate predictions by IAST, we find a mixed-linker system with increased heterogeneity in terms of added adsorption sites where IAST predictions and experimental loadings agree exceptionally well. We show that incorporating uncertainty analysis into predictions with IAST is important for assessing the accuracy of these predictions. Energetic investigations combined with Grand Canonical Monte Carlo simulations reveal almost homogeneous carbon dioxide but heterogeneous ethylene adsorption in the mixed-linker material, resulting in local, almost pure phases of the individual components.

Dual-Ligand ZIF-8 Bearing the Cyano Group for Efficient and Selective Uranium Capture from Seawater

Uranium extraction from seawater is a potential technique that will change the world. Adsorption capacity, selectivity, and antibacterial ability for high-performance uranium adsorbents remain the major challenges. In this study, a dual-ligand zeolitic imidazolate framework 8 (ZIF-8) decorated with cyano groups (ZIF-8-CN) is prepared via a facile blend strategy at room temperature. Owing to the abundant mesopores and nitrogen functional groups, ZIF-8-CN shows an extremely high uranium uptake of 1000 mg/g at pH = 6, which is 2.42 times that of pristine ZIF-8. Noteworthily, ZIF-8-CN possesses a 16.2 mg/g uranium adsorption in natural seawater within 28 days, and the distribution coefficient (Kd = 3.25 × 106 mL/g) is far greater than that for other coexisting metal ions, demonstrating a marked preference for uranyl ions. Except for the coordination between uranium and nitrogen in imidazole, the cyano groups provide additional adsorption sites and preferentially bind to uranyl, thereby strengthening the affinity for uranyl. Notably, ZIF-8-CN displays ultrastrong antimicrobial ability against both Escherichia coli and Staphylococcus aureus, which is greatly desired for the scale-up marine tests. Our study demonstrates the high potential of ZIF-8-CN in uranium capture and provides a wide scope for the application of mixed-ligand MOFs.

Bimetallic Zeolitic Imidazole Frameworks for Improved Stability and Performance of Intrusion-Extrusion Energy Applications

Hydrophobic flexible zeolitic imidazole frameworks (ZIFs) represent reference microporous materials in the area of mechanical energy storage, conversion, and dissipation via non-wetting liquid intrusion-extrusion cycle. However, some of them exhibit drawbacks such as lack of stability, high intrusion pressure, or low intrusion volume that make them non-ideal materials to consider as candidates for real applications. In this work, we face these limitations by exploiting the hybrid ZIF concept. Concretely, a bimetallic SOD-like ZIF consisting of Co and Zn ions was synthesized and compared with Co-ZIF (ZIF-67) and Zn-ZIF (ZIF-8) showing for the first time that the hybrid ZIF combines the good stability of ZIF-8 with the higher water intrusion volume of ZIF-67. Moreover, it is shown that the hybrid-ZIF approach can be used to tune the intrusion/extrusion pressure, which is crucial for technological applications.

Drug release and solubility properties of two zeolitic metal-organic frameworks influenced by their hydrophobicity/hydrophilicity

Metal-organic frameworks (MOFs) have shown significant potential for drug delivery applications. However, there remains a scarcity of comprehensive research addressing the influence of surface properties of MOFs on drug release kinetics and drug solubility. This study focuses on examining the influence of MOFs hydrophilicity and hydrophobicity on the controlled release and solubility of drugs. To achieve this, we prepared drug-loaded nanoparticles through in situ synthesis and created a drug-MOF co-amorphous system using the ball milling technique. Under neutral conditions, the hydrophilic MOF-based drug delivery system demonstrated a comparatively slower drug release profile than its hydrophobic counterpart. This observation suggests that the hydrophilic system holds promise in mitigating drug side effects by enabling improved control over drug release. The implementation of hydrophobic MOFs in co-amorphous systems yields a more pronounced effect on enhancing solubility compared to hydrophilic MOFs. This study offers valuable insights for achieving optimal drug release kinetics and solubility by delicately manipulating surface properties of MOFs.

Hierarchical ZIF-8 Materials via Acid Gas-Induced Defect Sites: Synthesis, Characterization, and Functional Properties

Microporous metal-organic frameworks (MOFs) have been widely studied for molecular separation and catalysis. The uniform micropores of MOFs (<2 nm) can introduce diffusion limitations and render the interiors of the crystal inaccessible to target molecules. The introduction of hierarchical porosity (interconnected micro and mesopores) can enhance intra-crystalline diffusion while maintaining the separation/catalytic selectivity. Conventional hierarchical MOF synthesis involves complex strategies such as elongated linkers, soft templating, and sacrificial templates. Here, we demonstrate a more general approach using our controlled acid gas-enabled degradation and reconstruction (Solvent-Assisted Crystal Redemption) strategy. Selective linker labilization of ZIF-8 is shown to generate a hierarchical pore structure with mesoporous cages (∼50 nm) while maintaining microporosity. Detailed structural and spectroscopic characterization of the controlled degradation, linker insertion, and subsequent linker thermolysis is presented to show the clustering of acid gas-induced defects and the generation of mesopores. These findings indicate the generality of controlled degradation and reconstruction as a means for linker insertion in a wider variety of MOFs and creating hierarchical porosity. Enhanced molecular diffusion and catalytic activity in the hierarchical ZIF-8 are demonstrated by the adsorption kinetics of 1-butanol and a Knoevenagel condensation reaction.

Ligand Substitution: An Effective Way for Tuning Structures of ZIF-7 Nanoparticles (NPs) and Improving Energy Recovery Performance of ZIF/PA TFN Membranes

It is an important initiative to reduce the building energy consumption using energy recovering ventilation (ERV) systems. The application of ERV systems is hindered by the low CO₂ barrier performance of commercial total heat exchange membranes (THEMs) that lead to unsatisfactory indoor air refreshing rate, and there is an urgent need for THEMs that have improved CO₂ barrier properties and effective energy recovery efficiencies. Here, we report the formation of novel ZIF/PA TFN THEMs based on ZIF-7-X nanoparticles (NPs) with "core-shell" structures and tunable particle sizes, formed from benzimidazole (BIM) ligands and BIM substituted by -NH₂, -CH₃, -C₂H₅, and -C₃H₇ functional groups. The NPs were mixed with pyr omellitic triformyl chloride (TMC) in the organic phase during the interface polymerization process to form ZIF/PA TFN membranes. The total heat exchange performance of ZIF/PA TFN membranes could be effectively modified by the type and quantity of ZIF-7-X NPs added. The CO₂ barrier properties and water vapor permeability of ZIF/PA TFN membranes could be improved by the addition of optimal levels of ZIF-7-X NPs, showing low CO₂ permeance of 7.76 GPU, high H₂O permeance of 663.8 GPU, and excellent enthalpy exchange efficiency of 72.1%. This work provided an effective strategy for tuning not only the nanostructures of ZIF-7 fillers but also the CO₂ barrier properties of the formed ZIF/PA TFN membranes.

Photo-controllable heterostructured crystals of metal-organic frameworks via reversible photocycloaddition

Metal-organic framework (MOF)-based heterostructures are attractive because they can provide versatile platforms for various applications but are limited by complex liquid epitaxial growth methods. Here, we employ photolithography to fabricate and control MOF-based heterostructured crystals via [4 + 4] photocycloaddition. A layered dysprosium-dianthracene framework, [Dy(NO3)3(depma2)1.5]·(depma2)0.5 (2) [depma2 = pre-photodimerized 9-diethylphosphonomethylanthracene (depma)] underwent a single-crystal-to-single-crystal transition at 140 °C to form [Dy(NO3)3(depma)(depma2)]·(depma2)0.5 (3). The dissociated anthracene moieties are face-to-face π-π interacted allowing a reversible photocycloaddition between 2 and 3. This structural transformation causes a luminescence switch between blue and yellow-green and thus can be used to fabricate erasable 2 + 3 heterostructured crystals for rewritable photonic barcodes. The internal strain at the heterostructure interface leads to photobending and straightening of the crystal, a photomechanical response that is fast, reversible and durable, even operating at 140 °C, making it promising for photoactuation. This work may inspire the development of intelligent MOF-based heterostructures for photonic applications.

Hybrid Benzimidazole-Dichloroimidazole Zeolitic Imidazolate Frameworks Based on ZIF-7 and Their Application in Mixed Matrix Membranes for CO2/N2 Separation

Mixed-linker zeolitic imidazolate frameworks (ZIFs) with the sodalite (sod) topology type and based on ZIF-7 have been prepared by direct synthesis from the mixtures of benzimidazole (BzIm) and 4,5-dichloroimidazole (dcIm). Incorporation of dcIm into the ZIF-7 structure gives ZIF-7/COK-17 hybrids with rhombohedral symmetry that do not show the "open-to-closed form" structural transition upon solvent removal exhibited by ZIF-7. They show Type I isotherms for low molecular weight gases and high affinity for CO₂ even at low partial pressures. Synthesis under mild conditions gives ZIF nanoparticles (250-400 nm) suitable for incorporation into mixed matrix membranes (MMMs): these were prepared with both glassy (Matrimid) and rubbery (PEBAX 1657) polymers. Permeation tests at 298 K and 1.2 bar reveal that the incorporation of Zn(BzIm_0.55dcIm_0.45)₂ nanoparticles at up to ca. 12 wt % gives defect-free membranes with enhanced CO₂ permeability in both polymer matrices, with retention of selectivity (Matrimid) or with an enhancement in selectivity that is most pronounced for the smaller nanoparticles (PEBAX). The membrane with the best performance exhibits a selectivity of ca. 200 for CO₂/N₂ (a 4-fold increase compared to the pure polymer) and a CO₂ permeability of 64 Barrer. At the relatively low loadings investigated, the MMMs' performance obeys the Maxwell model, and the intrinsic property of diffusivity of the ZIFs can be extracted as a result.

MIL-53 and its OH-bonded variants for bio-polyol adsorption from aqueous solution

The adsorption of bio-polyols from dilute aqueous solution is important but faces challenges in the sustainable bio-refinery process. One solution to increase adsorption efficiency is to leverage host–guest interactions between the polyols and materials to grant a preference for polyols. In this study, we synthesized MIL-53 and diverse OH-bonded variants, and studied their adsorption properties towards ethanediol, 1,3-propanediol and glycerol in water. Among the four materials, OH–MIL-53 exhibited fast adsorption kinetics and high capacity, and could be completely regenerated through ethanol elution. Hydrophobic interactions between the alkyl chains of the polyols and the organic linkers of OH–MIL-53 and hydrogen bonding interactions between their OH groups were identified. The synergistic effect of the host–guest interactions is responsible for the unique adsorption performances of OH–MIL-53 towards polyols, and particularly for 1,3-propanediol.

Delicate host–guest interaction drives OH-bonded MOF to capture bio-polyols from diluted aqueous solution, with high capacity, fast kinetics and recyclability.

Efficient capture of radioactive iodine by ZIF-8 derived porous carbon

Due to the rapid diffusion of radioactive iodine, the demand for safe and efficient capture and storage of radioactive iodine is increasing worldwide. The use of porous carbon materials to capture iodine has aroused great interest. This work prepared porous carbon materials derived from polymetallic oxides of the zeolitic imidazolate framework (ZIF) by pyrolysis at 1000 °C. The carbon materials (CZIF-1000) have a high specific surface area of about 1110 m2/g and a total pore volume of 0.92 cm3/g. Adsorption studies have shown that the CZIF-1000 had significant adsorption performance for iodine, and the adsorption capacity can reach 790.8 mg/g at 8h. The potential mechanism of adsorption is that the carbonization causes the charge-transfer interaction and pore size distribution. Compared with the conventional adsorbents, the adsorbents showed faster kinetics and high extraction capacity for iodine. This experiment provides an effective method for designing a highly efficient adsorbent for iodine and broadens the ideas for developing new iodine extraction adsorbents.

Zeolitic Imidazolate Framework Membranes: Novel Synthesis Methods and Progress Toward Industrial Use

In the last decade, zeolitic imidazolate frameworks (ZIFs) have been studied extensively for their potential as selective separation membranes. In this review, we highlight unique structural properties of ZIFs that allow them to achieve certain important separations, like that of propylene from propane, and summarize the state of the art in ZIF thin-film deposition on porous substrates and their modification by postsynthesis treatments. We also review the reported membrane performance for representative membrane synthesis approaches and attempt to rank the synthesis methods with respect to potential for scalability. To compare the dependence of membrane performance on membrane synthesis methods and operating conditions, we map out fluxes and separation factors of selected ZIF-8 membranes for propylene/propane separation. Finally, we provide future directions considering the importance of further improvements in scalability, cost effectiveness, and stable performance under industrially relevant conditions. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Covalent-Linking-Enabled Superior Compatibility of ZIF-8 Hybrid Membrane for Efficient Propylene Separation

The interface is a critical issue for metal-organic-framework hybrid membranes in propylene separation. Here, a covalent-linking strategy is reported for strikingly reinforcing the interfacial compatibility in a ZIF-8-based membrane. A functionalized ZIF-8 material named ZIF-8-CN is synthesized using the mixed-ligand approach. ZIF-8-CN has an identical crystalline structure to ZIF-8, and the 4,5-dicyanoimidazole ligand is available for further functionalization. Covalent linkage of ZIF-8-CN with PIM-1 is driven by the thermal reaction of the cyano groups on both entities, which strengthens the filler-polymer connection in the ZIF-8-CN@tPIM-1 membrane. ZIF-8-CN@tPIM-1 exhibits remarkably enhanced propylene permeation property with C₃ H₆ /C₃ H₈ selectivity of ≈28, which is 350% and 180% higher than those on non-treated ZIF-8-CN/PIM-1 and non-functionalized ZIF-8@tPIM-1, respectively. Additionally, ZIF-8-CN@tPIM-1 shows the highest C₃ H₆ permeability of ≈370 Barrer among all relevant ZIF-8 membranes. This strategy opens an avenue for precise interface engineering in membranes and the resultant high performance is appealing in the propylene separation industry.

An X-ray Photoelectron Spectroscopy Study of Postsynthetic Exchange in UiO-66

Postsynthetic exchange (PSE) is a method that is widely used to change the composition of metal-organic frameworks (MOFs) by replacing connecting linkers or metal nodes after the framework has been synthesized. However, few techniques can probe the nature and distribution of exchanged species following PSE. Herein, we show that X-ray photoelectron spectroscopy can be used to compare the relative concentrations of exchanged ligands at the surface and interior regions of MOF particles. Specifically, PSE of iodobenzene dicarboxylate ligands results in a gradient distribution from surface to bulk in UiO-66 nanoparticles that depends on PSE time. X-ray photoelectron spectroscopy also reveals differences between the surface chemistry of the PSE product and that of the direct synthesis product.

Single Crystals Heterogeneity Impacts the Intrinsic and Extrinsic Properties of Metal-Organic Frameworks

At present, an enormous characterization gap exists between the study of the crystal structure of a material and its bulk properties. Individual particles falling within this gap cannot be fully characterized in a correlative manner by current methods. The authors address this problem by exploiting the noninvasive nature of optical microscopy and spectroscopy for the correlative analysis of metal-organic framework particles in situ. They probe the intrinsic as well as extrinsic properties in a correlated manner. The authors show that the crystal shape of MIL-88A strongly impacts its optical absorption. Furthermore, the question of how homogeneously water is distributed and adsorbed within one of the most promising materials for harvesting water from humid air, MOF-801, is addressed. The results demonstrate the considerable importance of the particle level and how it can affect the property of the material.

Anisotropic MOF-on-MOF Growth of Isostructural Multilayer Metal-Organic Framework Heterostructures

Isostructural MOFs with similar crystallographic parameter are easily available for MOF-on-MOF growth and possible to form core–shell structure by isotropic growth. However, due to well-matched cell lattice, selective growth in isostructural MOF heterostructures remains a great challenge for engineering atypical MOF heterostructures. Herein, an anisotropic MOF-on-MOF growth strategy was developed to structure a range of multilayer sandwich-like ZIF-L heterostructures via stacking isostructural ZIF-L-Zn and ZIF-L-Co alternately with three-, five-, seven-, and more layer structures. Moreover, these heterostructures with highly designable feature were fantastic precursors for fabricating derivatives with tunable magnetic and catalytic properties. Such strategy explores a novel way of achieving anisotropic MOF-on-MOF growth between isostructural MOFs and opens up new horizons for regulating the properties by MOF modular assembly in versatile functional nanocomposites.

Elemental Depth Profiling of Intact Metal-Organic Framework Single Crystals by Scanning Nuclear Microprobe

The growing field of MOF–catalyst composites often relies on postsynthetic modifications for the installation of active sites. In the resulting MOFs, the spatial distribution of the inserted catalysts has far-reaching ramifications for the performance of the system and thus needs to be precisely determined. Herein, we report the application of a scanning nuclear microprobe for accurate and nondestructive depth profiling of individual UiO-66 and UiO-67 (UiO = Universitetet i Oslo) single crystals. Initial optimization work using native UiO-66 crystals yielded a microbeam method which avoided beam damage, while subsequent analysis of Zr/Hf mixed-metal UiO-66 crystals demonstrated the potential of the method to obtain high-resolution depth profiles. The microbeam method was further used to analyze the depth distribution of postsynthetically introduced organic moieties, revealing either core–shell or uniform incorporation can be obtained depending on the size of the introduced molecule, as well as the number of carboxylate binding groups. Finally, the spatial distribution of platinum centers that were postsynthetically installed in the bpy binding pockets of UiO-67-bpy (bpy = 5,5′-dicarboxyy-2,2′-bipyridine) was analyzed by microbeam and contextualized. We expect that the method presented herein will be applicable for characterizing a wide variety of MOFs subjected to postsynthetic modifications and provide information crucial for their optimization as functional materials.

Optimized CO2 Capture of the Zeolitic Imidazolate Framework ZIF-8 Modified by Solvent-Assisted Ligand Exchange

Zeolitic imidazolate frameworks, like ZIF-8 and related structures, have shown great potential for the capture of carbon dioxide. Modifying their structure by exchanging part of the constituent organic ligands is a proven method for enhancing the capacity to absorb CO₂. In this work, we performed solvent-assisted ligand exchange (SALE) on nanosized ZIF-8 (nZIF-8) with a series of functionalized imidazole derivatives (exchange percentages, after 24 h): 2-bromoimidazole (19%), 2-chloroimidazole (29%), 2-trifluoromethylbenzimidazole (4%), 2-mercaptobenzimidazole (4%), and 2-nitroimidazole (54%). The sodalite topology and porosity of nZIF-8 were maintained with all SALE modifications. Low-pressure CO₂ adsorption of nZIF-8 (38.5 cm³ g^-1) at STP was appreciably enhanced with all mixed-linker SALE products. Using halogenated (-Cl, -Br, and -CF₃) imidazole derivatives in a 24 h SALE treatment resulted in increases between 11 and 22% in CO₂ adsorption, while the thiol (-SH)- and nitro (-NO₂)-functionalized SALE products led to 32 and 100% increases in CO₂ uptakes, respectively. These CO₂ uptakes were further optimized by varying the SALE treatment time. The SHbIm- and NO₂Im-exchanged SALE products of nZIF-8 show 87 and 98 cm³ g^-1 of CO₂ uptakes after 60 and 120 h of SALE, respectively. These are record high CO₂ adsorptions for all reported ZIF derivatives at low-pressure conditions.

Mechanistic Study on Thermally Induced Lattice Stiffening of ZIF-8

The flexibility of the ZIF-8 aperture, which inhibits a molecular cutoff of 3.4 Å, can be reduced by rapid heat treatment to obtain CO₂-selective membranes. However, the early stages of the structural, morphological, and chemical changes responsible for the lattice rigidification remain elusive. Herein, using ex situ and in situ experiments, we determine that a small shrinkage of the unit-cell parameter, ∼0.2%, is mainly responsible for this transformation. Systematic gas permeation studies show that one needs to achieve this shrinkage without a disproportionately large shrinkage in the grain size of the polycrystalline film to avoid the formation of cracks. We show that this condition is uniquely achieved in a short time by exposure of ZIF-8 to a mildly humid environment where lattice parameter shrinkage is accelerated by the incorporation of linker vacancy defects, while the shrinkage in grain size is limited. The water-vapor-led incorporation of linker vacancy defects takes place with an energy barrier of 123 kJ mol^-1, much higher than that for the thermal degradation of ZIF-8, <80 kJ mol^-1. The latter is promoted by heat treatment in a dry environment at a relatively higher temperature; however, this condition does not shrink the lattice parameters at short exposure time.

Hybrid ZIF-8-90 for Selective Solid-Phase Microextraction of Exhaled Breath from Gastric Cancer Patients

Metal-organic frameworks (MOFs) are a new kind of microporous materials whose unique properties make them promising as coatings for solid phase microextraction (SPME). However, previous MOF coatings for SPME exclusively focus on single-linker MOFs, and the selective enrichment of polar or nonpolar targets depends on the polarity of linker on the surface of MOFs, which greatly limits the application of MOF coating for SPME in real samples. Here, we report a hybrid MOF-coated stainless steel fiber for SPME of biomarkers in exhaled breath from gastric cancer patients. Zeolitic imidazolate framework-8-90 (ZIF-8-90) possesses the aldehyde groups and methyl groups in the framework as a model MOF, and eight biomarkers (ethanol, acetone, hexanal, hexanol, nonane, isoprene, heptane, and decane) were used as the target analytes. The ZIF-8-90-coated fiber shows high enrichment efficiency for hydrophilic targets and hydrophobic targets, wide linearity (three orders of magnitude), and low detection limits (0.82-2.64 μg L^-1). The ZIF-8-90-coated fiber exhibited higher enrichment performance for all the investigated analytes as a result of the synergy of methyl and aldehyde groups, the porous structure, and the suitable pore size of ZIF-8-90 (4-5 Å). The relative standard deviation (RSD) of six repetitions for extractions using the same ZIF-8-90-coated fiber ranged from 2.5 to 7.3%. The reproducibility between the three fibers prepared in parallel varied in the range of 4.8-12% (RSD). The fabricated ZIF-8-90-coated fiber lasted for at least 120 cycles of extraction/desorption/conditioning without an obvious reduction in extraction efficiency and precision. Finally, the developed ZIF-8-90-coated SPME fiber has been successfully used for the analysis of exhaled breath samples from gastric patients with satisfied recoveries (88-106%).

Flow Microreactor Synthesis of Zeolitic Imidazolate Framework (ZIF)@ZIF Core-Shell Metal-Organic Framework Particles and Their Adsorption Properties

Metal-organic frameworks (MOFs) with core-shell structures enable to enhance intrinsic properties of constituent MOFs and impart additional functional activities. Although shell thickness is a key factor for regulating the properties of core-shell MOF particles, controlling it has been challenging. The widely used batch reactor synthesis cannot produce core-shell particles with uniform shell thickness because of poor reactant mixing. A microreactor could ensure excellent mixing, and that would allow to control shell thickness. In this study, we synthesized zeolitic imidazolate framework-8 (ZIF-8)@ZIF-67 and ZIF-67@ZIF-8 core-shell particles using a microreactor and investigated the effects of the mixing performance on the shell thickness of the obtained particles. Our results demonstrated that rapid mixing was critical for the uniformity of the synthesized particles. The concentration of core particles is another key factor that can preferentially induce heterogeneous nucleation on the surface of the core particles without inducing self-nucleation in the bulk solution, particularly when the self-nucleation rate of the shell MOF is high. The N₂ adsorption isotherms of the synthesized particles revealed their unique adsorption properties, which were ascribed to the core-shell structures obtained at low shell formation rates. Our simple and versatile synthesis technique not only allowed the preparation of ZIF@ZIF particles with novel functionalities but also can be extended to synthesize core-shell MOF particles with different combinations of core particles and shells.

Effect of Linker Distribution in the Photocatalytic Activity of Multivariate Mesoporous Crystals

The use of Metal-Organic Frameworks as crystalline matrices for the synthesis of multiple component or multivariate solids by the combination of different linkers into a single material has emerged as a versatile route to tailor the properties of single-component phases or even access new functions. This approach is particularly relevant for Zr₆-MOFs due to the synthetic flexibility of this inorganic node. However, the majority of materials are isolated as polycrystalline solids, which are not ideal to decipher the spatial arrangement of parent and exchanged linkers for the formation of homogeneous structures or heterogeneous domains across the solid. Here we use high-throughput methodologies to optimize the synthesis of single crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to produce a family of UiO-68-TZDC_% binary frameworks, which respect the integrity and morphology of the original crystals. Our results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling crystals for a uniform mixture or the formation of core-shell domains. We also demonstrate how the possibility of generating an asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.

Solvent Choice in Metal-Organic Framework Linker Exchange Permits Microstructural Control

Linker exchange is a widely applied, robust technique for elaboration of metal-organic frameworks (MOFs) post-synthesis. The observation of core-shell microstructures under certain conditions was hypothesized to arise from diffusion rates into the MOF that are slower than linker exchange. Here the relative contributions of these processes are manipulated through solvent choice in order to modulate shell thickness and exchange extent. The findings allow tailoring MOF microstructure to application.

Microporous framework membranes for precise molecule/ion separations

Microporous framework membranes such as metal-organic framework (MOF) membranes and covalent organic framework (COF) membranes are constructed by the controlled growth of small building blocks with large porosity and permanent well-defined micropore structures, which can overcome the ubiquitous tradeoff between membrane permeability and selectivity; they hold great promise for the enormous challenging separations in energy and environment fields. Therefore, microporous framework membranes are endowed with great expectations as next-generation membranes, and have evolved into a booming research field. Numerous novel membrane materials, versatile manipulation strategies of membrane structures, and fascinating applications have erupted in the last five years. First, this review summarizes and categorizes the microporous framework membranes with pore sizes lower than 2 nm based on their chemistry: inorganic microporous framework membranes, organic-inorganic microporous framework membranes, and organic microporous framework membranes, where the chemistry, fabrications, and differences among these membranes have been highlighted. Special attention is paid to the membrane structures and their corresponding modifications, including pore architecture, intercrystalline grain boundary, as well as their diverse control strategies. Then, the separation mechanisms of membranes are covered, such as diffusion-selectivity separation, adsorption-selectivity separation, and synergetic adsorption-diffusion-selectivity separation. Meanwhile, intricate membrane design to realize synergistic separation and some emerging mechanisms are highlighted. Finally, the applications of microporous framework membranes for precise gas separation, liquid molecule separation, and ion sieving are summarized. The remaining challenges and future perspectives in this field are discussed. This timely review may provide genuine guidance on the manipulation of membrane structures and inspire creative designs of novel membranes, promoting the sustainable development and steadily increasing prosperity of this field.

Recent Advances of Solid-State NMR Spectroscopy for Microporous Materials

Microporous materials have attracted a rapid growth of research interest in materials science and the multidisciplinary area because of their wide applications in catalysis, separation, ion exchange, gas storage, drug release, and sensing. A fundamental understanding of their diverse structures and properties is crucial for rational design of high-performance materials and technological applications in industry. Solid-state NMR (SSNMR), capable of providing atomic-level information on both structure and dynamics, is a powerful tool in the scientific exploration of solid materials. Here, advanced SSNMR instruments and methods for characterization of microporous materials are briefly described. The recent progress of the application of SSNMR for the investigation of microporous materials including zeolites, metal-organic frameworks, covalent organic frameworks, porous aromatic frameworks, and layered materials is discussed with representative work. The versatile SSNMR techniques provide detailed information on the local structure, dynamics, and chemical processes in the confined space of porous materials. The challenges and prospects in SSNMR study of microporous and related materials are discussed.

Controllable broadband multicolour single-mode polarized laser in a dye-assembled homoepitaxial MOF microcrystal

Multicolour single-mode polarized microlasers with visible to near-infrared output have very important applications in photonic integration and multimodal biochemical sensing/imaging but are very difficult to realize. Here, we demonstrate a single crystal with multiple segments based on the host-guest metal-organic framework ZJU-68 hierarchically hybridized with different dye molecules generating controllable single-mode green, red, and near-infrared lasing, with the lasing mode mechanism revealed by computational simulation. The segmented and oriented assembly of different dye molecules within the ZJU-68 microcrystal causes it to act as a shortened resonator, enabling us to achieve dynamically controllable multicolour single-mode lasing with a low three-colour-lasing threshold of ~1.72 mJ/cm2 (approximately seven times lower than that of state-of-the-art designed heterostructure alloys, as reported by Fan F et al. (Nat. Nanotechnol. 10:796-803, 2015) considering the single pulse energy density) and degree of polarization >99.9%. Furthermore, the resulting three-colour single-mode lasing possesses the largest wavelength coverage of ~186 nm (ranging from ~534 to ~720 nm) ever reported. These findings may open a new route to the exploitation of multicolour single-mode micro/nanolasers constructed by MOF engineering for photonic and biochemical applications.

Metal-Organic Framework Nanoparticles Induce Pyroptosis in Cells Controlled by the Extracellular pH

Ion homeostasis is essential for cellular survival, and elevated concentrations of specific ions are used to start distinct forms of programmed cell death. However, investigating the influence of certain ions on cells in a controlled way has been hampered due to the tight regulation of ion import by cells. Here, it is shown that lipid-coated iron-based metal-organic framework nanoparticles are able to deliver and release high amounts of iron ions into cells. While high concentrations of iron often trigger ferroptosis, here, the released iron induces pyroptosis, a form of cell death involving the immune system. The iron release occurs only in slightly acidic extracellular environments restricting cell death to cells in acidic microenvironments and allowing for external control. The release mechanism is based on endocytosis facilitated by the lipid-coating followed by degradation of the nanoparticle in the lysosome via cysteine-mediated reduction, which is enhanced in slightly acidic extracellular environment. Thus, a new functionality of hybrid nanoparticles is demonstrated, which uses their nanoarchitecture to facilitate controlled ion delivery into cells. Based on the selectivity for acidic microenvironments, the described nanoparticles may also be used for immunotherapy: the nanoparticles may directly affect the primary tumor and the induced pyroptosis activates the immune system.

Vapor-phase linker exchange of metal-organic frameworks

Metal-organic frameworks (MOFs) have been attracting intensive attention because of their commendable potential in many applications. Postsynthetic modification for redesigning chemical characteristics and pore structures can greatly improve performance and expand functionality of MOF materials. Here, we develop a versatile vapor-phase linker exchange (VPLE) methodology for MOF modification. Through solvent-free and environment-friendly VPLE processing, various linker analogs with functional groups but not for straightforward MOF crystallization are inserted into frameworks as daughter building blocks. Besides single exchange for preparing MOFs with dual linkers, VPLE can further be performed by multistage operations to obtain MOF materials with multiple linkers and functional groups. The halogen-incorporated ZIFs exhibit good porosity, tunable molecular affinity, and impressive CO₂/N₂ and CH₄/N₂ adsorption selectivities up to 31.1 and 10.8, respectively, which are two to six times higher than those of conventional adsorbents. Moreover, VPLE can substantially enhance the compatibility of MOFs and polymers.

Metalloporphyrinic metal-organic frameworks: Controlled synthesis for catalytic applications in environmental and biological media

Recently, as a new sub-family of porous coordination polymers (PCPs), porphyrinic-MOFs (Porph-MOFs) with biomimetic features have been developed using porphyrin macrocycles as ligands and/or pillared linkers. The control over the coordination of the porphyrin ligand and its derivatives however remains a challenge for engineering new tunable Porph-MOF frameworks by self-assembly methods. The key challenges exist in the following respects: (i) collapse of the large open pores of Porph-MOFs during synthesis, (ii) deactivation of unsaturated metal-sites (UMCs) by axial coordination, and (iii) the tendency of both coordinated moieties (at peripheral meso- and beta-carbon sites) and the N₄-pyridine core to coordinate with metal cations. In this respect, this review covers the advances in the design of Porph-MOFs relative to their counterpart covalent organic frameworks (Porph-COFs). The potential utility of custom-designed porphyrin/metalloporphyrins ligands is highlighted. Synthesis strategies of Porph-MOFs are also illustrated with modular design of hybrid guest@host composites (either Porph@MOFs or guest@Porph-MOFs) with exceptional topologies and stability. This review summarizes the synergistic benefits of coordinated porphyrin ligands and functional guest molecules in Porph-MOF composites for enhanced catalytic performance in various redox applications. This review shed lights on the engineering of new tunable hetero-metals open active sites within (metallo)porphyrin-MOFs as out-of-the-box platforms for enhanced catalytic processes in chemical and biological media.

Postsynthetic Modification of ZIF-8 Membranes via Membrane Surface Ligand Exchange for Light Hydrocarbon Gas Separation Enhancement

The ability to tailor the pore structure of metal-organic framework (MOF) membranes enables synthesis of new or modified MOF membranes with enhanced separation characteristics. This work employs a modified version of solvent-assisted ligand exchange, termed membrane surface ligand exchange (MSLE), to modify the pore structure of zeolitic imidazolate framework-8 (ZIF-8) membranes. This paper is the first to perform a time-based, ex situ characterization and gas permeation study of ZIF-8 MSLE with 5,6-DBIM (DBIM, dimethylbenzimidazole) to effectively narrow the ZIF-8 pores, enhance light hydrocarbon gas-phase separations, and give insight into the exchange mechanism with respect to time and temperature. The results show that relatively fast exchange kinetics occur mainly at the outer surface of the ZIF-8 membrane during the initial 30 min of exchange and enables significant (40-70%) increases in propylene/propane selectivity with minimal (10-20%) propylene permeance losses for the modified ZIF-8 membranes. We postulate as the reaction time proceeds, the ligand-exchange rate slows as the DBIM linker diffuses into the ZIF-8 membrane beyond the external surface, exchanges with the original linker, disrupts the original framework's crystallinity, and then increases long-range order/crystallinity as the reaction proceeds. The H₂/C₂ separation factor increases with increased 5,6-DBIM content in the ZIF-8 framework which is facilitated by increased MSLE time and reaction temperature.

Copper nanowires and copper foam multifunctional bridges in zeolitic imidazolate framework-derived anode material for superior lithium storage

Herein, a synthetic strategy for growing trimetallic zeolitic imidazolate framework (ZIF) polyhedrons on copper foam (CF) and interweaving with copper nanowires (CNWs) is proposed. Subsequently, in situ annealing under N₂ atmosphere leads to the formation of multi-doped CNWs/Cu_0.39Zn_0.14Co_2.47O₄-ZnO/CF (CNWs/CZCOZ/CF). The unique structural characteristics of CNWs/CZCOZ/CF allow it to be directly assembled as a working electrode, without additional conductive additives or binders. When it's used as the lithium-ion battery (LIB) anode, this electrode exhibits a significantly high capacity of 2305 mAh g^-1 at 0.1 A g^-1 after 500 cycles. More importantly, kinetic analysis on the basis of cyclic voltammograms (CVs) indicates that the pseudocapacitive effect is the primary contributor to the high lithium storage capacity and also accounts for the exceptionally high rate capacity of 713 mAh g^-1 even if the current density is at a maximum of 10 A g^-1. Moreover, the superior battery performance originates from their advantageous structural diversity and unique compositional features, including synergistic effects among polymetallic components and two highly conductive substrates (CNWs and CF), forming unhindered paths for fast charge transfer.

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

A comprehensive overview of characterization tools for the analysis of well-known metal–organic frameworks and physico-chemical phenomena associated to their applications.

Self-Assembly of Metal-Organic Framework (MOF) Nanoparticle Monolayers and Free-Standing Multilayers

We report the first self-assembled porous monolayer and free-standing multilayer films composed of metal-organic framework (MOF) nanoparticles. Self-assembled MOF monolayers (SAMMs) were assembled at a liquid-air interface to produce films that are 87 wt % (89 vol %) MOF. Monolayer self-assembly was aided by growing a layer of poly(methyl methacrylate) (PMMA) on the particle surface using a histamine anchor. SAMMs could be stacked to obtain MOF multilayers, including alternating MOF/polymer heterostructures. SAMMs were coated on silicon microparticles, and a MOF film constructed of only five stacked layers could be manipulated as a free-standing, opalescent film. These monolayers are a significant advancement for obtaining highly functional porous membranes and coatings.