H2storage in isostructural UiO-67 and UiO-66 MOFs

Ionic Liquid (IL) Laden Metal-Organic Framework (IL-MOF) Electrolyte for Quasi-Solid-State Sodium Batteries

An ionic liquid (IL) laden metal-organic framework (MOF) sodium-ion electrolyte has been developed for ambient-temperature quasi-solid-state sodium batteries. The MOF skeleton is designed according to a UIO-66 (Universitetet i Oslo) structure. A sodium sulfonic (-SO₃Na) group grafted to the UIO-based MOF ligand improves the Na⁺-ion conductivity. Upon lading with a sodium-based ionic liquid (Na-IL), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) in 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Bmpyr-TFSI), the Na-IL laden sulfonated UIO-66 (UIOSNa) quasi-solid electrolyte exhibits a Na⁺-ion conductivity of 3.6 × 10^-4 S cm^-1 at ambient temperature. Quasi-solid-state sodium batteries with the Na-IL/UIOSNa electrolyte are demonstrated with a layered Na₃Ni_1.5TeO₆ cathode and sodium-metal anode. The quasi-solid-state Na∥Na-IL/UIOSNa∥Na₃Ni_1.5TeO₆ cells show remarkable cycling performance.

A hafnium-based metal-organic framework for the entrapment of molybdenum hexacarbonyl and the light-responsive release of the gasotransmitter carbon monoxide

A carbon monoxide-releasing material (CORMA) has been prepared by inclusion of molybdenum hexacarbonyl in a hafnium-based metal-organic framework (MOF) with the UiO-66 architecture. Mo(CO)₆ was adsorbed from solution to give supported materials containing 6.0-6.6 wt% Mo. As confirmed by powder X-ray diffraction (PXRD) and SEM coupled with energy dispersive X-ray spectroscopy, neither the crystallinity nor the morphology of the porous host was affected by the loading process. While the general shape of the N₂ physisorption isotherms (77 K) did not change significantly after encapsulation of Mo(CO)₆, the micropore volume decreased by ca. 20%. Thermogravimetric analysis of the as-prepared materials revealed a weight loss step around 160 °C associated with the decomposition of Mo(CO)₆ to subcarbonyl species. Confirmation for the presence of encapsulated Mo(CO)₆ complexes was provided by FT-IR and ¹³C{¹H} cross-polarization magic-angle spinning NMR spectroscopies. To test the capability of these materials to behave as CORMAs and transfer CO to heme proteins, the standard myoglobin (Mb) assay was used. While stable in the dark, photoactivation with low-power UV light (365 nm) liberated CO from the encapsulated hexacarbonyl molecules in Mo(6.0)/UiO-66(Hf), leading to a maximum amount of 0.26 mmol CO released per gram of material. Under the simulated physiological conditions of the Mb assay (37 °C, pH 7.4 buffer), minimal leaching of molybdenum occurred, PXRD showed only slight amorphization, and FT-IR spectroscopy confirmed the high chemical stability of the MOF host.

Enhanced proton transport properties of sulfonated polyarylene ether nitrile (SPEN) with moniliform nanostructure UiO-66-NH2/CNT

Metal-organic frameworks (MOFs) have been widely investigated for their porosity and functional diversity. Inspired by the flexible designability of MOFs, UiO-66-NH2/CNT with moniliform nanostructure was designed and synthesized successfully. SPEN@UiO-66-NH2/CNT composite proton exchange membranes were prepared by loaded UiO-66-NH2/CNT into sulfonated polyarylene ether nitrile (SPEN). Due to the addition of UiO-66-NH2/CNT, all the properties of composite proton exchange membranes were improved. The composite membranes exhibit excellent thermal stability and dimensional stability. The tensile strength of the composite membranes was improved about twofold compared to that of recast SPEN membrane, which was contributed by the interlaced property and rigid structure of UiO-66-NH2/CNT. Especially, the proton conductivity of the composite membranes was greatly facilitated by the additional proton acceptors and donors provided by the abundant amino groups and carboxyl groups in UiO-66-NH2/CNT. Furthermore, the methanol permeability of SPEN@UiO-66-NH2/CNT reduced consistently (from 6.13 to 0.96 × 10−7 cm2 s−1), which was much lower than that of Nafion membrane (21.36 × 10−7 cm2 s−1). All the results suggest that the design of multifunctional nanofillers based on the skeleton structure of MOFs could provide a new strategy to enhance the performance of PEMs.

E. coli@UiO-67 composites as a recyclable adsorbent for bisphenol A removal

E. coli@UiO-67 composites were obtained using an effective and simple self-assembly method. The composites showed unique properties as a remarkable and recyclable adsorbent for the efficient removal of bisphenol A (BPA) from water with a high adsorption capacity (402.930 mg g^-1). The increase in pore size is a key factor why E. coli@UiO-67 composites maintained high capacity. The reason might be due to that the composites with large pore sizes and defects could effectively improve mass transport and active molecular metal sites. The adsorption of BPA is a chemisorption process due to the Zr-OH groups in UiO-67 exhibit affinity toward BPA molecules, π-π interaction, and electrostatic attraction. The adsorption efficiency remained at 82.5% after 15 cycles without any remarkable changes in the PXRD patterns of E. coli@UiO-67. Moreover, the use of microorganism-loading MOFs could reduce the cost to at least 50% and minimize secondary pollution through nanoscale MOFs usage reduction. The developed composites have advantages, including low-cost, high adsorption capacity, easy to be separated and regenerated from aqueous solution, a large number of cycles, short adsorption equilibrium time, and stability, showing excellent application prospects. The presented strategy would be a potentially promising way to produce novel MOFs-based adsorbents with high-performance to control environmental pollution from wastewater.

porE: A code for deterministic and systematic analyses of porosities

Accurate numerical calculations of porosities and related properties are of importance when analyzing metal-organic frameworks (MOFs). We present porE, an open-source, general-purpose implementation to compute such properties and discuss all results regarding their sensitivity to numerical parameters. Our code combines the numerical efficiency of Fortran with the user-friendliness of Python. Three different approaches to calculate porosities are implemented in porE, and their advantages and drawbacks are discussed. In contrast to commonly used implementations, our approaches are entirely deterministic and do not require any stochastic averaging. In addition to the calculation of porosities, porE can calculate pore size distributions and offers the possibility to analyze pore windows. The underlying approaches are outlined, and pore windows are discussed concerning their impact on the analyzed porosities. Comparisons with reference values aim for a clear differentiation between void and accessible porosities, which we provide for a small benchmark set consisting of eight MOFs. In addition, our approaches are used for a bigger benchmark set containing 370 MOFs, where we determine linear relationships within our approaches as well as to reference values. We show how these relationships can be used to derive corrections to a give porosity approach, minimizing its mean error. As a highlight we show how complex workflows can be designed with a few lines of Python code using porE.

Analytical Approach to Screen Semiconducting MOFs Using Bloch Mode Analysis and Spectroscopic Measurements

A rapid and simple analytical approach is developed to screen the semiconducting properties of metal organic frameworks (MOFs) by modeling the band structure and predicting the density of state of isoreticular MOFs (IRMOFs). One can consider the periodic arrangement of metal nodes linked by organic subunits as a 1D periodic array crystal model, which can be aligned with any unit-cell axis included in the IRMOF's primitive cubic lattice. In such a structure, each valence electron of a metal atom feels the potential field of the entire periodic array. We allocate the 1D periodic array in a crystal unit cell to three IRMOFs-n (n = 1, 8, and 10) of the Zn₄O(L)₃ IRMOF series and apply the model to their crystal lattices with unit-cell constants a = 25.66, 30.09, and 34.28 Å, respectively. By solving Schrödinger's equation with a Kronig-Penney periodic potential and fitting the computed energy spectra to IRMOFs' experimental spectroscopic data, we model electronic band structures and obtain densities of state. The band diagram of each IRMOF reveals the nature of its electronic structures and density of state, allowing one to identify its n- or p-type semiconducting behavior. This novel analytical approach serves as a predictive and rapid screening tool to search the MOF database to identify potential semiconducting MOFs.

Raman spectroscopy-based sensitive, fast and reversible vapour phase detection of explosives adsorbed on metal–organic frameworks UiO-67

A sensitive, selective, rapid, and reversible detection of explosive molecules in the vapour phase, adsorbed on metal–organic frameworks (MOFs) under ambient laboratory conditions is demonstrated using Raman spectroscopy.

The in situ fabrication of ZIF-67 on titania-coated magnetic nanoparticles: a new platform for the immobilization of Pd(ii) with enhanced catalytic activity for organic transformations

The synthesis of a magnetic zeolitic-imidazolate-framework-67-supported Pd catalyst was demonstrated, and its catalytic activity for oxidation, reduction, and the oxidative deprotection of oximes was studied.

Fine-tuning the pore structure of metal–organic frameworks by linker substitution for enhanced hydrogen storage and gas separation

The functionalized Zr-MOFs with narrowed cavities and exposed sites exhibit improved H₂ storage and adsorption selectivity towards binary mixtures.

Cyclodextrins: a new and effective class of co-modulators for aqueous zirconium-MOF syntheses

Zr-MOFs exhibiting superior textural properties with BET surface area as high as 1451 m² g⁻¹ were successfully synthesized under hydrothermal conditions using native α-CD and β-CD as macromolecular additives.

UiO‐66 and hcp UiO‐66 Catalysts Synthesized from Ionic Liquids as Linker Precursors

Using ionic liquids (ILs) as linker precursors, the well‐known metal‐organic framework (MOF) UiO‐66 (Universitetet i Oslo) and the recently reported MOF hcp UiO‐66 (hexagonal closed packed) have been successfully synthesized and characterized. The advantage of the applied novel synthesis approach is an economically and environmentally benign work‐up procedure, due to the better solubility of the IL. Additionally, the reactivity of the terephthalate anions is increased compared to terephthalic acid, resulting in faster MOF formation with an increased amount of defects in the MOF structure. In order to explore to the influence of defects on the catalytic performance, the cyclisation of citronellal to isopulegol was employed as test reaction. The activity of hcp UiO‐66 and fcc UiO‐66 (face centered cubic) is improved compared to other MOF or zeolite based catalysts, while the selectivity is similar.

Ratiometric fluorescent sensing carbendazim in fruits and vegetables via its innate fluorescence coupling with UiO-67

A ratiometric fluorescent sensor was facilely fabricated using innate fluorescence of carbendazim (MBC) and fluorescent UiO-67 to sensitively and selectively detect MBC in food matrixes. The innate fluorescence of MBC provided a signal at 311 nm (F₃₁₁), and the fluorescent UiO-67 at 408 nm (F₄₀₈) could recognize MBC through π-π stacking inducing fluorescent quenching relied on photoelectron transfer (PET). The ratio (F₃₁₁/F₄₀₈) of the fluorescence enhancement of MBC and the quenching of UiO-67 linearly responded to the MBC concentrations of 0-47.6 μmol/L with a low limit of detection (LOD) of 3.0 × 10^-3 μmol/L. The reverse response signals of the sensor enhanced the sensitivity toward MBC and presented remarkable anti-interference capability in complex matrices. The as-prepared sensor was applied to detect MBC residues in apple, cucumber and cabbage, obtaining satisfactory accuracy and precision with the recovery of 90.82-103.45% and RSDs of lower than 3.03%.

MW Synthesis of ZIF-7. The Effect of Solvent on Particle Size and Hydrogen Sorption Properties

We report here fast (15 min) microwave-assisted solvothermal synthesis of zeolitic imidazolate framework material (ZIF-7). We have optimized solvent composition to achieve high porosity and hydrogen capacity and narrow particle size distribution. It was shown that synthesis in N,N-diethylformamide (DEF) results in a layered ZIF-7 III phase, while N,N-dimethylformamide (DMF) as solvent leads to a pure ZIF-7 phase in microwave conditions. A mixture of toluene with DMF allows the production of pure ZIF-7 material only with the triethylamine additive. Obtained materials were comprehensively characterized. We have pointed out that both X-ray diffraction and infrared spectroscopy could be used for the identification of ZIF-7 or ZIF-7 III phases. Although samples obtained in DMF, and in a mixture of DMF, toluene, and triethylamine were assigned to the pure ZIF-7 phase, solvent composition significantly affected the size of particles in the material and nitrogen and hydrogen adsorption process.

Semiliquid electrolytes with anion-adsorbing metal-organic frameworks for high-rate lithium batteries

Adding particles of metal-organic frameworks (MOFs) into liquid electrolytes leads to semiliquid electrolytes, where nanoporous MOFs enclose anions while facilitating lithium-ion conduction. The improved transport efficiency of lithium-ions in semiliquid electrolytes boosts effective reaction kinetics, mitigates polarization, and produces affinitive electrolyte-electrode interfaces, which afford enhanced cycle durability for high-rate lithium batteries.

Influence of Defects and H2O on the Hydrogenation of CO2 to Methanol over Pt Nanoparticles in UiO-67 Metal-Organic Framework

In catalysts for CO₂ hydrogenation, the interface between metal nanoparticles (NPs) and the support material is of high importance for the activity and reaction selectivity. In Pt NP-containing UiO Zr-metal–organic frameworks (MOFs), key intermediates in methanol formation are adsorbed at open Zr-sites at the Pt–MOF interface. In this study, we investigate the dynamic role of the Zr-node and the influence of H₂O on the CO₂ hydrogenation reaction at 170 °C, through steady state and transient isotope exchange experiments, H₂O cofeed measurements, and density functional theory (DFT) calculations. The study revealed that an increased number of Zr-node defects increase the formation rates to both methanol and methane. Transient experiments linked the increase to a higher number of surface intermediates for both products. Experiments involving either dehydrated or prehydrated Zr-nodes showed higher methanol and methane formation rates over the dehydrated Zr-node. Transient experiments suggested that the difference is related to competitive adsorption between methanol and water. DFT calculations and microkinetic modeling support this conclusion and give further insight into the equilibria involved in the competitive adsorption process. The calculations revealed weaker adsorption of methanol in defective or dehydrated nodes, in agreement with the larger gas phase concentration of methanol observed experimentally. The microkinetic model shows that [Zr₂(μ-O)₂]⁴⁺ and [Zr₂(μ–OH)(μ-O)(OH)(H₂O)]⁴⁺ are the main surface species when the concentration of water is lower than the number of defect sites. Lastly, although addition of water was found to promote methanol desorption, water does not change the methanol steady state reaction rate, while it has a substantial inhibiting effect on CH₄ formation. These results indicate that water can be used to increase the reaction selectivity to methanol and encourages further detailed investigations of the catalyst system.

Towards complete elucidation of structural factors controlling thermal stability of IL/MOF composites: effects of ligand functionalization on MOFs

In this work, we incorporated an ionic liquid (IL), 1-n-butyl-3-methylimidazolium methyl sulfate ([BMIM][MeSO4]) into two different metal organic frameworks (MOFs), UiO-66, and its amino-functionalized counterpart, NH2-UiO-66, to investigate the effects of ligand-functionalization on the thermal stability limits of IL/MOF composites. The as-synthesized IL/MOF composites were characterized in detail by combining x-ray diffraction, scanning electron microscopy, Brunauer-Emmett-Teller analysis, x-ray fluorescence, infrared spectroscopies (FTIR), and their thermal stability limits were determined by thermogravimetric analysis (TGA). Characterization data confirmed the successful incorporation of the IL into each MOF and indicated the presence of direct interactions between them. A comparison of the interactions in [BMIM][MeSO4]-incorporated UiO-66 and NH2-UiO-66 with those in their 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])-incorporated counterparts showed that the hydrophilic IL, [BMIM][MeSO4], interacts with the 1,4-benzenedicarboxylate (BDC) ligand of the UiO-66, while the hydrophobic IL, [BMIM][PF6], is interacting with the joints where zirconium metal cluster coordinates with BDC ligand. The TGA data demonstrated that the composite with the ligand-functionalized MOF, NH2-UiO-66, exhibited a lower percentage decrease in the maximum tolerable temperature compared to those of IL/UiO-66 composites. Moreover, it is discovered that when the IL is hydrophilic, its hydrogen bonding ability can be utilized to designate an interaction site on MOF's ligand structure, leads to a lower reduction in thermal stability limits. These results provide insights for the rational design of IL/MOF composites and contribute towards the complete elucidation of structural factors controlling the thermal stability.

Immobilization of UiO-67 with photochromic spiropyrans: a quantum chemical study

Post-synthetic modification of MOFs allows tuning the properties according to desired applications. The incorporation of photoactive molecules introduces sensitivity to radiation properties to the matrix of MOFs. We report on the theoretical analysis of possible ways of construction photoactive MOFs from UiO-67 and spiropyran molecules containing different carbonyl substituents. Large-scale computer modeling with the use of density functional theory method allowed us to select the most energy-efficient schemes of design. It was revealed that the most preferred way of immobilization of UiO-67 is the interaction with the carboxylic group in the indoline fragment of spiropyran. These results are promising for the application of MOFs modified in this way as photoactive sensors. Graphical abstract.

MOF-Based Polymeric Nanocomposite Films as Potential Materials for Drug Delivery Devices in Ocular Therapeutics

Novel MOF-based polymer nanocomposite films were successfully prepared using Zr-based UiO-67 as a metal-organic framework (MOF) and polyurethane (PU) as a polymeric matrix. Synchrotron X-ray powder diffraction (SXRPD) analysis confirms the improved stability of the UiO-67 embedded nanocrystals, and scanning electron microscopy images confirm their homogeneous distribution (average crystal size ∼100-200 nm) within the 50 μm thick film. Accessibility to the inner porous structure of the embedded MOFs was completely suppressed for N₂ at cryogenic temperatures. However, ethylene adsorption measurements at 25 °C confirm that at least 45% of the MOF crystals are fully accessible for gas-phase adsorption of nonpolar molecules. Although this partial blockage limits the adsorption performance of the embedded MOFs for ocular drugs (e.g., brimonidine tartrate) compared to the pure MOF, an almost 60-fold improvement in the adsorption capacity was observed for the PU matrix after incorporation of the UiO-67 nanocrystals. The UiO-67@PU nanocomposite exhibits a prolonged release of brimonidine (up to 14 days were quantified). Finally, the combined use of SXRPD, thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) analyses confirmed the presence of the drug in the nanocomposite film, the stability of the MOF framework and the drug upon loading, and the presence of brimonidine in an amorphous phase once adsorbed. These results open the gate toward the application of these polymeric nanocomposite films for drug delivery in ocular therapeutics, either as a component of a contact lens, in the composition of lacrimal stoppers (e.g., punctal plugs), or in subtenon inserts.

In Situ One-Step Synthesis of Platinum Nanoparticles Supported on Metal-Organic Frameworks as an Effective and Stable Catalyst for Selective Hydrogenation of 5-Hydroxymethylfurfural

A facile in situ one-step route for the preparation of platinum nanoparticles supported on metal-organic frameworks (MOFs) without adding stabilizing agents was developed. The obtained 10% Pt@MOF-T3 material possessed a large surface area and high crystallinity. Meanwhile, uniform and well-dispersed platinum nanoparticles were formed inside the cavities of MOFs, which could be attributed to the efficient complexation and stabilization effect derived from the dipyridyl groups. The as-synthesized 10% Pt@MOF-T3 sample showed high activity and selectivity in the hydrogenation of 5-hydroxymethylfurfural (HMF). This excellent catalytic performance could be attributed to the synergistic effects of well-dispersed platinum nanoparticles and electron donation offered by MOFs. Meanwhile, the presence of bipyridine ligands in the MOF framework avoided the irreversible adsorption of the hydrocarbon compounds, leading to the enhanced catalytic efficiency. Besides, it was easily recycled and reused at least five times, showing good recyclability.

Zr(OH)4/GO Nanocomposite for the Degradation of Nerve Agent Soman (GD) in High-Humidity Environments

Zirconium hydroxide, Zr(OH)₄ is known to be highly effective for the degradation of chemical nerve agents. Due to the strong interaction force between Zr(OH)₄ and the adsorbed water, however, Zr(OH)₄ rapidly loses its activity for nerve agents under high-humidity environments, limiting real-world applications. Here, we report a nanocomposite material of Zr(OH)₄ and graphene oxide (GO) which showed enhanced stability in humid environments. Zr(OH)₄/GO nanocomposite was prepared via a dropwise method, resulting in a well-dispersed and embedded GO in Zr(OH)₄ nanocomposite. The nitrogen (N₂) isotherm analysis showed that the pore structure of Zr(OH)₄/GO nanocomposite is heterogeneous, and its meso-porosity increased from 0.050 to 0.251 cm³/g, compared with pristine Zr(OH)₄ prepared. Notably, the composite material showed a better performance for nerve agent soman (GD) degradation hydrolysis under high-humidity air conditions (80% relative humidity) and even in aqueous solution. The soman (GD) degradation by the nanocomposite follows the catalytic reaction with a first-order half-life of 60 min. Water adsorption isotherm analysis and diffuse reflectance infrared Fourier transform (DRIFT) spectra provide direct evidence that the interaction between Zr(OH)₄ and the adsorbed water is reduced in Zr(OH)₄/GO nanocomposite, indicating that the active sites of Zr(OH)₄ for the soman (GD) degradation, such as surface hydroxyl groups are almost available even in high-humidity environments.

Synthesis of ZnO Nanoparticles Doped with Cobalt Using Bimetallic ZIFs as Sacrificial Agents

We report here a simple two-stage synthesis of zinc–cobalt oxide nanoparticles. We used Zn/Co-zeolite imidazolate framework (ZIF)-8 materials as precursors for annealing and optional impregnation with a silicon source for the formation of a protective layer on the surface of oxide nanoparticles. Using bimetallic ZIFs allowed us to trace the phase transition of the obtained oxide nanoparticles from wurtzite ZnO to spinel Co₃O₄ structures. Using (X-ray diffraction) XRD and (X-ray Absorption Near Edge Structure) XANES techniques, we confirmed the incorporation of cobalt ions into the ZnO structure up to 5 mol.% of Co. Simple annealing of Zn/Co-ZIF-8 materials in the air led to the formation of oxide nanoparticles of about 20–30 nm, while additional treatment of ZIFs with silicon source resulted in nanoparticles of about 5–10 nm covered with protective silica layer. We revealed the incorporation of oxygen vacancies in the obtained ZnO nanoparticles using FTIR analysis. All obtained samples were comprehensively characterized, including analysis with a synchrotron radiation source.

Selective adsorption mechanisms of pharmaceuticals on benzene-1,4-dicarboxylic acid-based MOFs: Effects of a flexible framework, adsorptive interactions and the DFT study

The synergetic effects of benzene-1,4-dicarboxylic acid (BDC) linker structure and the metal cluster of MOFs on adsorption mechanisms of carbamazepine, ciprofloxacin and mefenamic acid were investigated in single and mixed solutions. A 1D flexible framework MIL-53(Al), 3D rigid framework UiO-66(Zr) and 3D flexible framework MIL-88B(Fe) were applied as adsorbents. The breathing effect of MIL-53(Al) caused by its flexible structure can enhance intraparticle diffusion for all pharmaceuticals and perform a critical role in excellent adsorption performances. The 3D rigid BDC structure of UiO-66(Zr) caused a steric effect that reflected low or negligible adsorption. Unless concerning accessibility through the internal structure of the MOFs, the binding strengths calculated by the DFT study were in the following order: MIL-88B(Fe) > MIL-53(Al) > UiO-66(Zr). The Fe cluster in MIL-88B(Fe) seems to have the highest affinity for the carboxylic group of pharmaceuticals compared with Al and Zr; however, the lower porosity of MIL-88B(Fe) might limit the adsorption capacity. Moreover, in mixed solutions, the higher acidity of mefenamic acid can enhance competitive performance in interactions with the metal cation cluster of each MOF. Together with the breathing effect, H-bonding and π-π interaction were shown to be the alternative interactions of synergetic adsorption mechanisms.

Experimental Demonstration of Dynamic Temperature-Dependent Behavior of UiO-66 Metal-Organic Framework: Compaction of Hydroxylated and Dehydroxylated Forms of UiO-66 for High-Pressure Hydrogen Storage

High-pressure (700 MPa or ∼100 000 psi) compaction of dehydroxylated and hydroxylated UiO-66 for H₂ storage applications is reported. The dehydroxylation reaction was found to occur between 150 and 300 °C. The H₂ uptake capacity of powdered hydroxylated UiO-66 reaches 4.6 wt % at 77 K and 100 bar, which is 21% higher than that of dehydroxylated UiO-66 (3.8 wt %). On compaction, the H₂ uptake capacity of dehydroxylated UiO-66 pellets reduces by 66% from 3.8 to 1.3 wt %, while for hydroxylated UiO-66 the pellets show only a 9% reduction in capacity from 4.6 to 4.2 wt %. This implies that the H₂ uptake capacity of compacted hydroxylated UiO-66 is at least three times higher than that of dehydroxylated UiO-66, and therefore, hydroxylated UiO-66 is more promising for hydrogen storage applications. The H₂ uptake capacity is closely related to compaction-induced changes in the porosity of UiO-66. The effect of compaction is greatest in partially dehydroxylated UiO-66 samples that are thermally treated at 200 and 290 °C. These compacted samples exhibit XRD patterns indicative of an amorphous material, low porosity (surface area reduces from between 700 and 1300 m²/g to ca. 200 m²/g and pore volume from between 0.4 and 0.6 cm³/g to 0.1 and 0.15 cm³/g), and very low hydrogen uptake (0.7-0.9 wt % at 77 K and 100 bar). The observed activation-temperature-induced dynamic behavior of UiO-66 is unusual for metal-organic frameworks (MOFs) and has previously only been reported in computational studies. After compaction at 700 MPa, the structural properties and H₂ uptake of hydroxylated UiO-66 remain relatively unchanged but are extremely compromised upon compaction of dehydroxylated UiO-66. Therefore, UiO-66 responds in a dynamic manner to changes in activation temperature within the range in which it has hitherto been considered stable.

POM@MOF Hybrids: Synthesis and Applications

The hybrid materials that are created by supporting or incorporating polyoxometalates (POMs) into/onto metal–organic frameworks (MOFs) have a unique set of properties. They combine the strong acidity, oxygen-rich surface, and redox capability of POMs, while overcoming their drawbacks, such as difficult handling, a low surface area, and a high solubility. MOFs are ideal hosts because of their high surface area, long-range ordered structure, and high tunability in terms of the pore size and channels. In some cases, MOFs add an extra dimension to the functionality of hybrids. This review summarizes the recent developments in the field of POM@MOF hybrids. The most common applied synthesis strategies are discussed, together with major applications, such as their use in catalysis (organocatalysis, electrocatalysis, and photocatalysis). The more than 100 papers on this topic have been systematically summarized in a handy table, which covers almost all of the work conducted in this field up to now.

Iodine Capture Using Zr-Based Metal-Organic Frameworks (Zr-MOFs): Adsorption Performance and Mechanism

The effective capture of radioiodine, produced or released from nuclear-related activities, is of paramount importance for the sustainable development of nuclear energy. Here, a series of zirconium-based metal-organic frameworks (Zr-MOFs), with a Zr₆(μ₃-O)₄(μ₃-OH)₄ cluster and various carboxylate linkers, were investigated for the capture of volatile iodine. Their adsorption kinetics and recyclability were investigated in dry and humid environments. The structural change of Zr-MOFs during iodine trapping was studied using powder X-ray diffraction and pore structure measurements. Experimental spectra (Raman and X-ray photoelectron spectroscopy) and density functional theory (DFT) calculations for the linkers and Zr clusters were performed to understand the trapping mechanism of the framework. When interacting with iodine molecules, MOF-808, NU-1000, and UiO-66, with highly connected and/or rigid linkers, have better structural stability than UiO-67 and MOF-867, which have flexible linkers with less connectivity. Particularly, MOF-808, with a rigid and tritopic benzenetricarboxylate linker, has the highest iodine adsorption capacity (2.18 g/g, 80 °C), as well as the largest pore volume after iodine elution. In contrast, UiO-67, with long linear ditopic linkers, exhibits the weakest stability and lowest adsorption capacity (0.53 g/g, 80 °C) because of its most serious collapse of pore structures. After incorporating with strong electron-donating imidazole/pyridine ligands, both the stability and adsorption capacity of MOF-808/NU-1000 decrease. DFT calculations verify that the N-heterocycle groups could enhance the affinity toward iodine by strong charge transfer. DFT calculations also suggest that the terminal -OH in MOF-808 has a strong affinity toward iodine (-54 kJ/mol I₂) and water (-63 kJ/mol H₂O) and a weak affinity toward NO₂ (-27 kJ/mol NO₂). With high adsorption capacity and excellent stability, MOF-808 shows great potential for the sustainable removal of radioiodine.

UiO-67 metal-organic gel material deposited on photonic crystal matrix for photoelectrocatalytic hydrogen production

Robust UiO-67 metal-organic framework nanoparticles have been precisely and uniformly anchored on the surface of a photonic crystal via metal-organic gelation, resulting in a nanoscale UiO-67 composite. Mott-Schottky measurements indicate that UiO-67/B is an n-type semiconductor with electron conduction, and the band gap significantly decreases with the assistance of the photonic crystal matrix with a band gap of 0.75 eV. Benefiting from the abundant photoelectrons trapped from the photonic crystal, good hydrogen evolution reaction performance is achieved under light irradiation. The current density increases from 3.2 to 7.0 mA cm^-2 at -0.6 V (vs. RHE) for UiO-67/B. The optimized carrier density obtained from UiO-67/B is apparently increased 2.15 times under light irradiation for 30 min. This work provides a rational strategy to address the photo-capture and energy transfer issues of metal-organic frameworks under visible light irradiation for H₂ production in artificial photosynthesis.

Amino group dependent sensing properties of metal–organic frameworks: selective turn-on fluorescence detection of lysine and arginine

Recently, metal–organic frameworks (MOFs) have been extensively investigated as fluorescence chemsensors due to their tunable porosity, framework structure and photoluminescence properties. In this paper, a well-known Zr(iv)-based MOF, UiO-66-NH₂ was demonstrated to have capability for detection of l-lysine (Lys) and l-arginine (Arg) selectively from common essential amino acids in aqueous media via a fluorescence turn-on mechanism. Further investigation reveals its high sensitivity and strong anti-interference properties. Moreover, the possible mechanism for sensing Lys and Arg was explored by FT-IR and ¹H-NMR, and the results indicate that the enhancement of the fluorescence could be ascribed to the adsorption of Lys/Arg and the hydrogen bonding interactions between Lys/Arg and the amino group of UiO-66-NH₂. The difference of the sensing capacity and sensitivity between UiO-66 and UiO-66-NH₂ revealed that the amino group plays an essential role in the sensing performance. This work presents a unique example of the functional group dependent sensing properties of MOFs.

The amino group of UiO-66-NH₂ was demonstrated to play an important role in selective fluorescence turn-on sensing of lysine and arginine.

Fluorescence Turn-On Response Amplified by Space Confinement in Metal-Organic Frameworks

Sensitive fluorescence turn-on response to specific substances is highly desired for development of chemical sensors and switches. Here we utilized a "two-in-one" strategy to prepare ionic metal-organic frameworks (MOFs) functionalized with the cationic bipyridinium receptors at the frameworks and anionic fluorescent indicators in the pores. The MOFs are rendered a fluorescence-resting state because the indicator's fluorescence is efficiently quenched by the ground-state charge-transfer (CT) complexation between the indicator and receptor. Addition of an alkylamine efficiently turns on the fluorescence because the indicator is displaced by the CT complexation between alkylamine with receptor. The turn-on response is highly specific to alkylamines. The MOFs can be used as recyclable sensors for selective and sensitive detection of alkylamines, with ultralow detection limits (0.5 nM). The fluorescence in solid state can be reversibly switched on and off with high contrast. The sensitive and high-contrast response can be attributed to the space confinement effects of the porous frameworks. The confined space can significantly enhance indicator-receptor and analyte-receptor interactions, and thereby both the quenching efficiency in the off state and the displacement efficiency in the on state are amplified.

Metal-Organic Frameworks as Platform for Lewis-Acid-Catalyzed Organic Transformations

Metal-organic frameworks (MOFs) are highly promising Lewis acid catalysts; they either inherently possess Lewis acid sites (LASs) on it or the LASs can be generated through various post-synthetic methods, the later can be performed in MOFs in a trivial fashion. MOFs are suitable platform for catalysis because of its highly crystalline and porous nature. Moreover, with recent advancements, thermal and chemical stability is not a problem with many MOFs. In this Minireview, an enormous versatility of MOFs, in terms of their microporosity/mesoporosity, size/shape selectivity, chirality, pore size, etc., has been highlighted. These are advantageous for designing and performing various targeted organic transformations. Although, many organic transformations catalyzed by MOFs with LASs have been reported in the recent past. In this Minireview, we have restricted ourselves to four important organic reactions: (i) cyanosilylation, (ii) Diels-Alder reaction, (iii) C-H activation, and (iv) CO₂ -addition. The discussion focuses mostly on the recent reports (42 examples).

Metal-Organic Framework-Based Composite for Photocatalytic Detection of Prevalent Pollutant

Photocatalytic properties of 2,5-furandicarboxylic acid (FDCA), a model organic molecule used for biopolymer production, are reported for the first time. Further integration of FDCA into metal-organic framework (MOF) structures and subsequent silver-based photoactivation leads to the next generation of hybrids with controlled morphologies, capable of forming sensorial platforms for prevalent phenol contaminant detection. The mechanisms that allow photocatalytic functionality are driven by the charge carrier generation in the organic molecule (either in its alone or integrated form) and depend on sample's physical and chemical properties as confirmed by scanning and transmission electron microscopy, Fourier transform infrared and X-ray photoelectron spectroscopy, and X-ray diffraction, respectively. Electrochemical analysis using cyclic voltammetry confirmed high sensitivity for p-nitrophenol (p-NP) detection as dictated by the selective electron migration at a user-controlled electrode interface. Considering the wide usage of p-NP and its increased discharge shown to lead to harmful effects on both the environment and biosystems, this new detection method is envisioned to allow effective control and regulation of such compound release, all under low-cost and environmentally friendly conditions.

Crystallographic Visualization of Postsynthetic Nickel Clusters into Metal-Organic Framework

Postsynthetic metalation (PSM) has been employed as a robust method for the postsynthetic modification of metal-organic frameworks (MOFs). However, the lack of relevant information that can be obtained for the postsynthetically introduced metallic ions has hindered the development of PSM applications. Thanks to the advancement in single-crystal X-ray diffraction (SCXRD) technology, there have been a few recent examples in which successful postsynthetic introduction of single metal ions into MOFs occurred at the defined chelating sites. These works have provided useful explanations about the complicated host-guest chemistry involved in PSMs. On the other hand, there are only limited examples with crystallographic snapshots of the postsynthetic installation of metal clusters into the pores of MOFs using an ordinary SCXRD due to the loss of crystallinity of parent matrix during the PSM process. Herein, by the careful selection of starting materials and controlling the reaction conditions, we report the first crystallographic visualization of metal clusters inserted into Zr-based MOFs via PSM. The structural advantages of the parent Zr-MOF, which are inherited from the stable Zr₆ cluster and triazole-containing dicarboxylate ligand, ensure both the preservation of high crystallinity and the presence of flexible coordination sites for PSM. Furthermore, PSM of metal clusters in a MOF pore space enhances stability of the final samples while also imparting the functionality of a successful catalyst toward ethylene dimerization reaction. The related construction ideas and structural information detailed in this work can help lay the foundation for further advancements using the postmodification of MOFs as well as open new doors for the utilization of SCXRD technology in the field of MOFs.

Torsion Angle Effect on the Activation of UiO Metal-Organic Frameworks

We present a systematic investigation of the factors influencing the surface area of zirconium-based UiO-type metal-organic frameworks (MOFs), revealing an important relationship between factors including the conformation of the organic linker in the MOF, surface tension of the guest molecules (solvent), and the stability of MOFs toward activation (removal of guest molecules). The results obtained demonstrate how the structure of the linkers forming the isostructural series of UiO MOFs with fcu topology could alter the resistance and stability of the MOF frameworks toward capillary force-driven structural degradation governed by the solvent during activation.