Metal–organic frameworks in Germany: From synthesis to function

1D Metal Mediated Hydrogen Bonded Rods with Rich Phenyl Groups for Highly Efficient Oil Removal

In this work, a novel 1D metal-mediated hydrogen-bonded framework with rich phenyl groups is first synthesized employing Co2+ and dibenzoylmethane (DBM) as precursors, which are named HOF-Co-DBM and exhibit exceptional thermal stability, excellent chemical durability, and super hydrophobicity. These distinctive properties can be attributed to the high density, robust Co─O coordination bonds, and the presence of strong hydrogen bonds (O─H─O─C) characterized by short bond distances, which contribute to its close-packed structure. Additionally, the benzene rings flanking the framework further enhance its hydrophobicity. The HOF-Co-DBM is subsequently integrated into a polyurethane (PU) sponge, resulting in exceptional oil removal performance. This study demonstrates the potential for preparing ultra-stable and superhydrophobic hydrogen-bonded organic framework materials with a wide range of applications.

Ultralong Room-Temperature Phosphorescence in Ca(II) Metal-Organic Frameworks Based on Nicotinic Acid Ligands

In recent years, metal-organic framework (MOF) materials with long persistent luminescence (LPL) have inspired extensive attention and presented various applications in security systems, information anticounterfeiting, and biological imaging fields. However, obtaining LPL materials with ultralong lifetime remains challenging. Halogen atoms, as nonmetallic elements existing in the frameworks, can not only induce the heavy-atom effect, effectively enhancing spin-orbit coupling and promoting intersystem crossing (ISC) processes, but also suppress non-radiative transition of the triplet states through the intra- and intermolecular interactions. Specifically, fluorine atoms with the strongest electronegativity may form intermolecular aggregate interlockings through halogen-bonding interactions that restrict molecular motions and vibrations, thereby improving phosphorescent lifetime. With the aforementioned considerations, two distinct types of MOFs with/without fluorine atoms (namely, Ca-MOF and 5FCa-MOF) were synthesized. Notably, by introducing fluorine atoms into MOFs, fluorine-induced intermolecular aggregate interlockings effectively enhanced the phosphorescent lifetime of 5FCa-MOF exceeding 264 ms compared to that of Ca-MOF (103.94 ms). Remarkably, both MOFs displayed bright LPL to the naked eye after removal of the irradiation source, especially 5FCa-MOF which can last for about 2 s. By introducing fluorine atoms, 5FCa-MOF exhibits greatly enhanced ISC with a rate constant up to 4.1 × 106 s-1 and suppressed non-radiative decay down to 3.73 s-1, thereby extending the LPL time. The thus obtained LPL provides potential in information encryption, security systems, optical anticounterfeiting, and so on.

A sulfonate ligand-defected Zr-based metal-organic framework for the enhanced selective removal of anionic dyes

In this work, we introduce a novel defective analogue of the representative 6-connected zirconium-based metal-organic framework (MOF-808), by employing 5-sulfoisophthalic acid monosodium salt (H2BTC-SO3Na) as a defect inducer via a mixed-linker approach. The structural integrity and different physicochemical properties were investigated by various characterization techniques, including powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and nitrogen physisorption at 77 K. Additionally, proton nuclear magnetic resonance (1H-NMR), energy-dispersive X-ray (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES) were employed to confirm the presence of 6.9 mol% of the 5-sulfoisophthalate ligand within the highly crystalline MOF-808 structure. The defective material exhibited significant enhancements in the removal efficiency of various organic dyes, including approximately 64% and 77% for quinoline yellow and sunset yellow, and 56% and 13% for rhodamine B and malachite green, compared to its pristine counterpart. Importantly, the defective MOF-808 showed a remarkable selectivity toward anionic species in binary-component dyes comprising both anionic and cationic dyes.

A Zn-modified PCN-224 fluorescent nanoprobe for selective and sensitive turn-on detection of glutathione

Monitoring endogenous glutathione (GSH) levels in living cells is essential for cancer diagnose and treatment. In this work, GSH responsive fluorescent nanoprobe with turn-on property was constructed using Zn-modified porphyrinic metal-organic frameworks (PCN-224-Zn). The introduced Zn2+ could quench the fluorescence of PCN-224 by the metallization of organic ligand (TCPP) and serves as sensing site for GSH. When exposed to GSH, the strong binding affinity of GSH generates the formation of Zn-GSH complex, eliminating the fluorescence quenching effect of Zn2+. Based on the constructed PCN-224-Zn nanoprobe, selective determination of GSH was achieved in the range of 0.01-6 μM with a detection limit of 1.5 nM. Furthermore, the constructed nanoprobe can realize the fluorescence imaging of endogenous GSH in MCF-7 and HeLa cells. Meanwhile, PCN-224-Zn could also monitor GSH in cell lysate with recovery rates from 93.8 % to 102.3 %. The performance of PCN-224-Zn demonstrates its capacities in the application of fluorescence sensing and bio-imaging fields.

A highly porous fiber coating based on a Zn-MOF/COF hybrid material for solid-phase microextraction of PAHs in soil

This study involved the development of a novel adsorbent by combining a Zn-based MOF with a melamine-based COF, resulting in the formation of a hybrid material known as Zn-MOF/COF. The adsorbent was characterized using FT-IR, SEM, XRD, EDX, and BET analysis techniques. The resulting Zn-MOF/COF sorbent was employed to prepare solid-phase microextraction (SPME) fibers for the extraction and enrichment of polycyclic aromatic hydrocarbons (PAHs) in contaminated soil samples, after coupling with GC-FID. A Box-Behnken design (BBD) was used to optimize key variables of SPME conditions. Under optimal conditions of 85 °C for 30 min extraction with 23 μL g-1 sample's moisture level, linear responses of six PAHs were ranging from 1 to 20000 ng g⁻1 with determination coefficients greater than 0.99. Limits of detection (LODs) were over the ranges of 0.1-1 ng g-1. The RSDs for intra-fiber and inter-fiber analyses were obtained 2.2-6.6% and 5.2-11.6%, respectively. Relative recoveries values for real soil samples were found to be 91.1-110.2%. The results showed lower cost and higher extraction efficiency for the Zn-MOF/COF fiber, compared with commercial and homemade adsorbents.

Exceptionally Stable And Super-Efficient Electrocatalysts Derived From Semiconducting Metal Phosphonate Frameworks

Two new isostructural semiconducting metal-phosphonate frameworks are reported. Co2 [1,4-NDPA] and Zn2 [1,4-NDPA] (1,4-NDPA4- is 1,4-naphthalenediphosphonate) have optical bandgaps of 1.7 eV and 2.5 eV, respectively. The electrocatalyst derived from Co2 [1,4-NPDA] as a precatalyst generated a low overpotential of 374 mV in the oxygen evolution reaction (OER) with a Tafel slope of 43 mV dec-1 at a current density of 10 mA cm-2 in alkaline electrolyte (1 mol L-1 KOH), which is indicative of remarkably superior reaction kinetics. Benchmarking of the OER of Co2 [1,4-NPDA] material as a precatalyst coupled with nickel foam (NF) showed exceptional long-term stability at a current density of 50 mA cm-2 for water splitting compared to the state-of-the-art Pt/C/RuO2 @NF after 30 h in 1 mol L-1 KOH. In order to further understand the OER mechanism, the transformation of Co2 [1,4-NPDA] into its electrocatalytically active species was investigated.

An H2 S-BMP6 Dual-Loading System with Regulating Yap/Taz and Jun Pathway for Synergistic Critical Limb Ischemia Salvaging Therapy

Critical limb ischemia, the final course of peripheral artery disease, is characterized by an insufficient supply of blood flow and excessive oxidative stress. H2 S molecular therapy possesses huge potential for accelerating revascularization and scavenging intracellular reactive oxygen species (ROS). Moreover, it is found that BMP6 is the most significantly up-expressed secreted protein-related gene in HUVECs treated with GYY4137, a H2 S donor, based on the transcriptome analysis. Herein, a UIO-66-NH2 @GYY4137@BMP6 co-delivery nanoplatform to strengthen the therapeutic effects of limb ischemia is developed. The established UIO-66-NH2 @GYY4137@BMP6 nanoplatform exerts its proangiogenic and anti-oxidation functions by regulating key pathways. The underlying molecular mechanisms of UIO-66-NH2 @GYY4137@BMP6 dual-loading system lie in the upregulation of phosphorylated YAP/TAZ and Jun to promote HUVECs proliferation and downregulation of phosphorylated p53/p21 to scavenge excessive ROS. Meanwhile, laser-doppler perfusion imaging (LDPI), injury severity evaluation, and histological analysis confirm the excellent therapeutic effects of UIO-66-NH2 @GYY4137@BMP6 in vivo. This work may shed light on the treatment of critical limb ischemia by regulating YAP, Jun, and p53 signaling pathways based on gas-protein synergistic therapy.

Direct Synthesis of Metal-Organic Framework Sols: Advances and Perspectives

The intrinsic lack of processability in the conventional nano/microcrystalline powder form of metal-organic frameworks (MOFs) greatly limits their application in various fields. Synthesis of MOFs with certain flowability make them promising for multitudinous applications. The direct synthesis strategy represents one of the simplest and efficient method for synthesizing solution processable MOF sols/suspensions, compared with other approaches, for instance, the post-synthesis surface modification, the direct dispersion of MOFs in hindered ionic liquids, as well as the calcination method toward a few MOFs with melting behavior. This article reviews the recent direct synthesis strategies of solution processable MOF sols and their typical applications in different fields. The direct synthesis strategies of MOF sols can be classified into two categories: particle size reduction strategy, and selective coordination strategy. The synthesis mechanism of different strategies and the factors affecting the formation of sols are summarized. The application of solution processable MOF sols in different fields are introduced, showing great application potentials. Furthermore, the challenges faced by the direct synthesis of MOF sols and the main methods to deal with the challenges are emphasized, and the future development trend is prospected.

N,Zn-Doped Fluorescent Sensor Based on Carbon Dots for the Subnanomolar Detection of Soluble Cr(VI) Ions

The development of a fluorescent sensor has attracted much attention for the detection of various toxic pollutants in the environment. In this work, fluorescent carbon dots (N,Zn-CDs) doped with nitrogen and zinc were synthesized using citric acid monohydrate and 4-pyridinecarboxyaldehyde as carbon and nitrogen sources, respectively. The synthesized N,Zn-CDs served as an "off" fluorescence detector for the rapid and sensitive detection of hexavalent chromium ions (Cr(VI)). The zinc metal integrated into the heteroatomic fluorescent carbon dot played a functional role by creating a coordination site for the hydrogen ions that were displaced after the addition of Cr to the solution matrix. The stepwise addition of Cr(VI) effectively quenched the fluorescence intensity of the N,Zn-CDs, and this phenomenon was attributed to the internal filter effect. A low detection limit of 0.47 nmol/L for Cr(VI) was achieved in the fluorescence experiments. Real water samples were used to evaluate the practical application of N,Zn-CDs for the quantification of Cr(VI). The results show acceptable recoveries and agreement with ion chromatography-ultraviolet spectrometry results. These good recoveries indicate that the fluorescence probe is very well suited for environmental measurements.

Recent applications of organic molecule-based framework porous materials in solid-phase microextraction for pharmaceutical analysis

Sample preparation is an indispensable part of detection of complex samples in pharmaceutical analysis. Solid-phase microextraction (SPME) has obtained a lot of attention due to its advantages of time saving, less solvent and easily automation. A variety of functional materials are used as sorbents in SPME to carry out selective and high extraction. This review centers around the recent applications of organic molecule-based framework porous materials, such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs), as SPME coating materials mainly focus on pharmaceutical analysis in food, environment, and biological samples. Four representative extraction devices are introduced, including on-fiber SPME, in-tube SPME, thin film SPME, stir bar SPME. The application prospect of other organic porous materials as sorbents for pharmaceutical analysis are also discussed, such as hyper crosslinked polymers (HCPs) and conjugated microporous polymers (CMPs). The progresses and discusses are provided to offer references for further research focusing on application and development of organic molecule-based framework porous materials in the field of SPME.

Self-luminescent europium based metal organic frameworks nanorods as a novel electrochemiluminescence chromophore for sensitive ulinastatin detection in biological samples

In this work, we developed a novel electrochemiluminescence (ECL) biosensor for ulinastatin (UTI) detection based on self-luminescent metal-organic framework (L-MOF) nanomaterials. The L-MOFs could be simply prepared by one-pot methods using Eu³⁺ and 4,4',4″-s-triazine-1,3,5-triyltri-m-aminobenzoic acid (H₃TATAB) as the metallic center and organic ligand, respectively. The Eu-TATAB exhibited high efficiency and stable ECL performance when using K₂S₂O₈ as coreactant. For the established biosensor, Eu-TATAB was both used as the ECL chromophore and protein carrier due to its outstanding biocompatibility and large superficial area, which could load sufficient antibodies to link with antigen in the biosensor for subsequent detection. The established sandwich ECL biosensor showed a wide linear range of 0.1 ng mL^-1 - 10⁵ ng mL^-1 and a low limit of detection of 9.7 pg mL^-1 for UTI detection. In addition, the developed ECL biosensor could also be successfully applied to the real UTI sample determination in serum. The reported biosensor strategy could provide a guide for developing more other novel and promising high-performance ECL nanomaterials, and also be used as a potential method for ultrasensitive UTI detection in disease research.

Coordination-Induced Band Gap Reduction in a Metal-Organic Framework

Herein, we report on the synthesis of a microporous, three-dimensional phosphonate metal-organic framework (MOF) with the composition Cu3 (H5 -MTPPA)2  ⋅ 2 NMP (H8 -MTPPA=methane tetra-p-phenylphosphonic acid and NMP=N-methyl-2-pyrrolidone). This MOF, termed TUB1, has a unique one-dimensional inorganic building unit composed of square planar and distorted trigonal bipyramidal copper atoms. It possesses a (calculated) BET surface area of 766.2 m2 /g after removal of the solvents from the voids. The Tauc plot for TUB1 yields indirect and direct band gaps of 2.4 eV and 2.7 eV, respectively. DFT calculations reveal the existence of two spin-dependent gaps of 2.60 eV and 0.48 eV for the alpha and beta spins, respectively, with the lowest unoccupied crystal orbital for both gaps predominantly residing on the square planar copper atoms. The projected density of states suggests that the presence of the square planar copper atoms reduces the overall band gap of TUB1, as the beta-gap for the trigonal bipyramidal copper atoms is 3.72 eV.

Selective ligand removal to improve accessibility of active sites in hierarchical MOFs for heterogeneous photocatalysis

Metal-organic frameworks (MOFs) are commended as photocatalysts for H2 evolution and CO2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities. For dynamic processes in liquid phase, the accessibility of active sites becomes a critical parameter as reactant diffusion is limited by the inherently small micropores. Our strategy is to introduce additional mesopores by selectively removing one ligand in mixed-ligand MOFs via thermolysis. Here we report photoactive MOFs of the MIL-125-Ti family with two distinct mesopore architectures resembling either large cavities or branching fractures. The ligand removal is highly selective and follows a 2-step process tunable by temperature and time. The introduction of mesopores and the associated formation of new active sites have improved the HER rates of the MOFs by up to 500%. We envision that this strategy will allow the purposeful engineering of hierarchical MOFs and advance their applicability in environmental and energy technologies.

Metal-organic framework structures of fused hexagonal motifs with cuprophilic interactions of a triangular Cu(I)3(pyrazolate-benzoate) metallo-linker

The reaction of the N,O-heteroditopic bifunctional ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H2mpba) with Cu(NO3)2·2.5H2O and Zn(NO3)2·4H2O or Zn(CH3COO)2·2H2O in N,N-dimethylformamide (DMF) results in concomitant formation of three bimetallic metal-organic frameworks (MOFs) with...

Hydrated Metal Ions as Weak Bronsted Acids Show the Promoting Effects in Proton Conduction

It is well-known that the hydrated metal ions can act as Bronsted acids, which tend to donate protons increasing the acidic proton concentration in materials, as well as the proton...

Zirconium and Aluminum MOFs for Low-Pressure SO2 Adsorption and Potential Separation: Elucidating the Effect of Small Pores and NH2 Groups

Finding new adsorbents for the desulfurization of flue gases is a challenging task but is of current interest, as even low SO₂ emissions impair the environment and health. Four Zr- and eight Al-MOFs (Zr-Fum, DUT-67(Zr), NU-1000, MOF-808, Al-Fum, MIL-53(Al), NH₂-MIL-53(Al), MIL-53(tdc)(Al), CAU-10-H, MIL-96(Al), MIL-100(Al), NH₂-MIL-101(Al)) were examined toward their SO₂ sorption capability. Pore sizes in the range of about 4-8 Å are optimal for SO₂ uptake in the low-pressure range (up to 0.1 bar). Pore widths that are only slightly larger than the kinetic diameter of 4.1 Å of the SO₂ molecules allow for multi-side-dispersive interactions, which translate into high affinity at low pressure. Frameworks NH₂-MIL-53(Al) and NH₂-MIL-101(Al) with an NH₂-group at the linker tend to show enhanced SO₂ affinity. Moreover, from single-gas adsorption isotherms, ideal adsorbed solution theory (IAST) selectivities toward binary SO₂/CO₂ gas mixtures were determined with selectivity values between 35 and 53 at a molar fraction of 0.01 SO₂ (10.000 ppm) and 1 bar for the frameworks Zr-Fum, MOF-808, NH₂-MIL-53(Al), and Al-Fum. Stability tests with exposure to dry SO₂ during ≤10 h and humid SO₂ during 5 h showed full retention of crystallinity and porosity for Zr-Fum and DUT-67(Zr). However, NU-1000, MOF-808, Al-Fum, MIL-53(tdc), CAU-10-H, and MIL-100(Al) exhibited ≥50-90% retained Brunauer-Emmett-Teller (BET)-surface area and pore volume; while NH₂-MIL-100(Al) and MIL-96(Al) demonstrated a major loss of porosity under dry SO₂ and MIL-53(Al) and NH₂-MIL-53(Al) under humid SO₂. SO₂ binding sites were revealed by density functional theory (DFT) simulation calculations with adsorption energies of -40 to -50 kJ·mol^-1 for Zr-Fum and Al-Fum and even above -50 kJ·mol^-1 for NH₂-MIL-53(Al), in agreement with the isosteric heat of adsorption near zero coverage (ΔH_ads⁰). The predominant, highest binding energy noncovalent binding modes in both Zr-Fum and Al-Fum feature μ-OH^δ+···^δ-OSO hydrogen bonding interactions. The small pores of Al-Fum allow the interaction of two μ-OH bridges from opposite pore walls with the same SO₂ molecule via OH^δ+···^δ-OSO^δ-···^δ+HO hydrogen bonds. For NH₂-MIL-53(Al), the DFT high-energy binding sites involve NH^δ+···^δ-OS together with the also present Al-μ-OH^δ+···^δ-OS hydrogen bonding interactions and C₆-π^δ-···^δ+SO₂, N^δ-···^δ+SO₂ interactions.

Efficient Removal of Chromium(VI) Anionic Species and Dye Anions from Water Using MOF-808 Materials Synthesized with the Assistance of Formic Acid

This study presents a simple approach to prepare MOF-808, an ultra-stable Zr-MOF constructed from 6-connected zirconium clusters and 1,3,5-benzene tricarboxylic acid, with tailored particle sizes. Varying the amount of formic acid as a modulator in the range of 200-500 equivalents results in MOF-808 materials with a crystal size from 40 nm to approximately 1000 nm. Apart from the high specific surface area, a combination of a fraction of mesopore and plenty of acidic centers on the Zr-clusters induces a better interaction with the ionic pollutants such as K2Cr2O7 and anionic dyes. MOF-808 shows uptakes of up to 141.2, 642.0, and 731.0 mg/g for K2Cr2O7, sunset yellow, and quinoline yellow, respectively, in aqueous solutions at ambient conditions. The uptakes for the ionic dyes are significantly higher than those of other MOFs reported from the literature. Moreover, the adsorption capacity of MOF-808 remains stable after four cycles. Our results demonstrate that MOF-808 is a promising ideal platform for removing oxometallates and anionic dyes from water.

Metal-organic frameworks as catalytic selectivity regulators for organic transformations

Selective organic transformations using metal-organic frameworks (MOFs) and MOF-based heterogeneous catalysts have been an intriguing but challenging research topic in both the chemistry and materials communities. Analogous to the reaction specificity achieved in enzyme pockets, MOFs are also powerful platforms for regulating the catalytic selectivity via engineering their catalytic microenvironments, such as metal node alternation, ligand functionalization, pore decoration, topology variation and others. In this review, we provide a comprehensive introduction and discussion about the role of MOFs played in regulating and even boosting the size-, shape-, chemo-, regio- and more appealing stereo-selectivity in organic transformations. We hope that it will be instructive for researchers in this field to rationally design, conveniently prepare and elaborately functionalize MOFs or MOF-based composites for the synthesis of high value-added organic chemicals with significantly improved selectivity.

ROS-generating rare-earth coordination networks for photodynamic inactivation of Candida albicans

Water-ethanol suspensions of 2D coordination network (CN) based on rare earth elements and mixed ligands were evaluated as reactive oxygen species (ROS) generators under UV light irradiation, in contact with a biomimetic substrate (tryptophan) or an O2(1Δg) quencher (1,3-diphenylisobenzofuran; 1,3-DPBF). A combination of bottom-up and top-down strategies was implemented in order to obtain nano-sized CN particles and the subsequent colloidal suspensions were also tested towards photodynamic inactivation of Candida albicans (C. albicans). SEM, TEM, FTIR, and XRD techniques were applied to characterize the solids and ICP-AES was employed to determine the metal content of the colloidal suspensions. Promising results were found indicating that the presence of Tb3+ allows an intersystem crossing suitable for singlet oxygen generation, resulting in the antifungal activity of C. albicans culture upon UV-irradiation.

Water-assisted spin-flop antiferromagnetic behaviour of hydrophobic Cu-based metal-organic frameworks

Solvent-dependent magnetism in Cu-based metal-organic frameworks (MOFs) is reported. Spin-flop magnetic behaviour occurs at different dehydrated states of MOFs. The oxygens of guest and coordinated water molecules are responsible as water removal tunes the coordination geometry around the Cu centre and the electronic structure of the framework.

In Situ Tracking of Wetting-Front Transient Heat Release on a Surface-Mounted Metal-Organic Framework

Transient heat generation during guest adsorption and host-guest interactions is a natural phenomenon in metal-organic framework (MOF) chemistry. However, in situ tracking of such MOF released heat is an insufficiently researched field due to the fast heat dissipation to the surroundings. Herein, a facile capillary-driven liquid-imbibition approach is developed for in situ tracking of transient heat release at the wetting front of surface-mounted MOFs (SURMOFs) on cellulosic fiber substrates. Spatiotemporal temperature distributions are obtained with infrared thermal imaging for a range of MOF-based substrates and imbibed liquids. Temperature rises at the wetting front of water and binary mixtures with organic solvents are found to be over 10 K with an ultrafast and distinguishable thermal signal response (<1 s) with a detectable concentration limit ≤1 wt%. As an advancement to the state-of-the-art in trace-solvent detection technologies, this study shows great prospects for the integration of SURMOFs in future sensor devices. Inspired by this prototypal study, SURMOF-based transient heat signal transduction is likely to be extended to an ever-expanding library of SURMOFs and other classes of surface-grafted porous materials, translating into a wide range of convenient, portable, and ubiquitous sensor devices.

An ultra-stable hafnium phosphonate MOF platform for comparing the proton conductivity of various guest molecules/ions

A porous hafnium-phosphonate MOF was synthesized using imidazole ionic liquids (ILs), namely PHOS-100(Hf), which has exceptional chemical stability in aqueous environments, even fuming acids. Its rigid framework with permanent porosity makes PHOS-100 an ideal candidate as a platform to fill with different functional guests such as acidic HCl, H2SO4, or H3PO4. The as-synthesized ILs@PHOS-100 exhibits significant humidity-dependent proton conductivities, increasing by four orders of magnitude from 45% RH to 95% RH at 25 °C. After post-treatment with strong acids, the acids@PHOS-100 show enhanced proton conduction at low relative humidities.

An ultralight charged MOF as fluoro-switchable monitor for assorted organo-toxins: size-exclusive dye scrubbing and anticounterfeiting applications via Tb3+ sensitization

The trifunctional Li(i)-MOF acts as fluoro-switchable sensor for two organo-toxins, invisible-ink based data encrypter and size–specific dye scavenger, where DFT calculations support sensing and adsorption mechanisms considering extended structure.

Core-Shell MOFs@MOFs: Diverse Designability and Enhanced Selectivity

Metal-Organic frameworks (MOFs), especially MOF-based composites, performed an irreplaceable role in the material fields. By virtue of the tailorability of MOFs, core-shell MOFs@MOFs composites with diverse designability and enhanced selectivity have inspired infinite scientific interest. This review will highlight an up-to-date overview of the designability and enhanced selectivity of core-shell MOFs@MOFs composites, covering the synthetic strategies of an epitaxial growth method, postsynthetic modification, and one-pot synthesis as well as the synergistic selective performance of the synthesized MOFs@MOFs in catalysis, adsorption and separation, and molecular recognition. Finally, the potential development trend and challenges toward core-shell MOFs@MOFs are addressed.

CFA-18: a homochiral metal-organic framework (MOF) constructed from rigid enantiopure bistriazolate linker molecules

In this work, we introduce the first enantiopure bistriazolate-based metal-organic framework, CFA-18 (Coordination Framework Augsburg-18), built from the R-enantiomer of 7,7,7',7'-tetramethyl-6,6',7,7'-tetrahydro-3H,3'H-5,5'-spirobi[indeno[5,6-d]-[1,2,3]triazole] (H2-spirta). The enantiopurity and absolute configuration of the new linker were confirmed by several chiroselective methods. Reacting H2-spirta in hot N,N-dimethylformamide (DMF) with manganese(ii) chloride gave CFA-18 as colorless crystals. The crystal structure with the composition [Mn2Cl2(spirta)(DMF)2] was solved using synchrotron single-crystal X-ray diffraction. CFA-18 shows a framework topology that is closely related to previously reported metal-azolate framework (MAF) structures in which the octahedrally coordinated manganese(ii) ions are triazolate moieties, and the chloride anions form crosslinked one-dimensional helical chains, giving rise to hexagonal channels. In contrast to MAFs crystallizing in the centrosymmetric space group R3[combining macron], the handedness of the helices found in CFA-18 is strictly uniform, leading to a homochiral framework that crystallizes in the trigonal crystal system within the chiral space group P3121 (no. 152).

High Throughput Methods in the Synthesis, Characterization, and Optimization of Porous Materials

Porous materials are widely employed in a large range of applications, in particular, for storage, separation, and catalysis of fine chemicals. Synthesis, characterization, and pre- and post-synthetic computer simulations are mostly carried out in a piecemeal and ad hoc manner. Whilst high throughput approaches have been used for more than 30 years in the porous material fields, routine integration of experimental and computational processes is only now becoming more established. Herein, important developments are highlighted and emerging challenges for the community identified, including the need to work toward more integrated workflows.

Constructing Strategies and Applications of Nitrogen-Rich Energetic Metal–Organic Framework Materials

The synthesis of energetic metal–organic frameworks (EMOFs) with one-dimensional, two-dimensional and three-dimensional structures is an effective strategy for developing new-generation high-energy-density and insensitive materials. The basic properties, models, synthetic strategies and applications of EMOF materials with nitrogen-rich energetic groups as ligands are reviewed. In contrast with traditional energetic materials, EMOFs exhibit some interesting characteristics, like tunable structure, diverse pores, high-density, high-detonation heat and so on. The traditional strategies to design EMOF materials with ideal properties are just to change the types and the size of energetic ligands and to select different metal ions. Recently, some new design concepts have come forth to produce more EMOFs materials with excellent properties, by modifying the energetic groups on the ligands and introducing highly energetic anion into skeleton, encapsulating metastable anions, introducing templates and so on. The paper points out that appropriate constructing strategy should be adopted according to the inherent characteristics of different EMOFs, by combining with functional requirements and considering the difficulties and the cost of production. To promote the development and application of EMOF materials, the more accurate and comprehensive synthesis, systematic performance measurement methods, theoretical calculation and structure simulation should be reinforced.

Densification-Induced Structure Changes in Basolite MOFs: Effect on Low-Pressure CH4 Adsorption

Metal-organic frameworks' (MOFs) adsorption potential is significantly reduced by turning the original powder into pellets or granules, a mandatory step for their use at industrial scale. Pelletization is commonly performed by mechanical compression, which often induces the amorphization or pressure-induced phase transformations. The objective of this work is the rigorous study of the impact of mechanical pressure (55.9, 111.8 and 186.3 MPa) onto three commercial materials (Basolite C300, F300 and A100). Phase transformations were determined by powder X-ray diffraction analysis, whereas morphological changes were followed by nitrogen physisorption. Methane adsorption was studied in an atmospheric fixed bed. Significant crystallinity losses were observed, even at low applied pressures (up to 69.9% for Basolite C300), whereas a structural change occurred to Basolite A100 from orthorhombic to monoclinic phases, with a high cell volume reduction (13.7%). Consequently, adsorption capacities for both methane and nitrogen were largely reduced (up to 53.6% for Basolite C300), being related to morphological changes (surface area losses). Likewise, the high concentration of metallic active centers (Basolite C300), the structural breathing (Basolite A100) and the mesopore-induced formation (Basolite F300) smooth the dramatic loss of capacity of these materials.

Adjustment of the performance and stability of isostructural zeolitic tetrazolate-imidazolate frameworks

Presented here are two isostructural SOD-type zeolitic tetrazolate-imidazolate frameworks (ZTIFs), Zn(etz)0.86(mim)1.14 (ZTIF-9, Hetz = 5-ethyltetrazole, Hmim = 2-methylimidazole) and Zn(vtz)0.63(mim)1.37 (ZTIF-10, Hvtz = 5-vinyltetrazole). The adjustment of the ligand ratios within these ZTIFs was realized through changing the substituent groups of tetrazole ligands. Remarkably, ZTIF-9 with a suitable ligand ratio perfectly balances gas uptake and stability, exhibiting 6-fold improvement of C2H2 uptake compared to the prototype ZIF-8 (Zn(mim)2).

Coupling Structural and Adsorption Properties of Metal-Organic Frameworks: From Pore Size Distribution to Pore Type Distribution

Metal-organic frameworks (MOFs) attract a rapidly growing attention across the disciplines due to their multifarious pore structures and unique ability to selectively adsorb, store, and release various guest molecules. Pore structure characterization and coupling of adsorption and structural properties are imperative for rational design of advanced MOF materials and their applications. The pore structure of MOFs represents a three-dimensional network comprised of several types of pore compartments: interconnected cages and channels distinguished by their size, shape, and chemistry. Here, we propose a novel methodology for pore structure characterization of MOF materials based on matching of the experimental adsorption isotherms to in silico-generated fingerprint isotherms of adsorption in individual pore compartments of the ideal crystal. The proposed approach couples structural and adsorption properties, determines the contributions of different types of pores into the total adsorption, and estimates to what extent the pore structure of the sample under investigation is different from the ideal crystal. The MOF pore structure is characterized by the pore type distribution (PTD), which is more informative than the traditional pore size distribution that is based on oversimplistic pore models. The method is illustrated on the example of Ar adsorption at 87 K on hydrated and dehydrated structures of Cu-BTC, one of the most well-known MOF materials. The PTD determined from the experimental isotherm provides an estimate of the crystal fraction in the sample and the accessibility and degree of hydration of different types of pore compartments. In addition, the PTD determined from the experimental adsorption isotherm is used to predict the isosteric heat of adsorption that provides important information on the specifics of adsorption interactions. The results are found to be in excellent agreement with experimental data. Such detailed information about the pore structure and adsorption properties of practical MOF samples cannot be obtained with currently available methods of adsorption characterization.

Two Co-based MOFs assembled from an amine-functionalized pyridinecarboxylate ligand: inorganic acid-directed structural variety and gas adsorption properties

A pair of Co-based MOFs exhibited inorganic acid-driven structural diversities, and one of them displayed the C₂H₂ separation and purification potential.

A highly crystalline anthracene-based MOF-74 series featuring electrical conductivity and luminescence

Recently, a small group of metal-organic frameworks (MOFs) has been discovered featuring substantial charge transport properties and electrical conductivity, hence promising to broaden the scope of potential MOF applications in fields such as batteries, fuel cells and supercapacitors. In combination with light emission, electroactive MOFs are intriguing candidates for chemical sensing and optoelectronic applications. Here, we incorporated anthracene-based building blocks into the MOF-74 topology with five different divalent metal ions, that is, Zn²⁺, Mg²⁺, Ni²⁺, Co²⁺ and Mn²⁺, resulting in a series of highly crystalline MOFs, coined ANMOF-74(M). This series of MOFs features substantial photoluminescence, with ANMOF-74(Zn) emitting across the whole visible spectrum. The materials moreover combine this photoluminescence with high surface areas and electrical conductivity. Compared to the original MOF-74 materials constructed from 2,5-dihydroxy terephthalic acid and the same metal ions Zn²⁺, Mg²⁺, Ni²⁺, Co²⁺ and Mn²⁺, we observed a conductivity enhancement of up to six orders of magnitude. Our results point towards the importance of building block design and the careful choice of the embedded MOF topology for obtaining materials with desired properties such as photoluminescence and electrical conductivity.

Sulfonated Sub-Nanochannels in a Robust MOF Membrane: Harvesting Salinity Gradient Power

We developed a robust, crack-free ultrathin zeolite- imidazole framework (ZIF-8) membrane in-built with a sulfonate-ion-containing polymer (ZIF_Hep) via a vapor-assisted in situ conversion process. The sulfonated sub-nanochannels of the ZIF_Hep membrane afforded a rapid and selective transport of Li⁺ over counteranions and other alkali ions due to electrostatic repulsion and optimal transport kinetics of cation-sulfonate ion pairs. A salinity gradient power generator (SGPG) was built by using the ZIF_Hep membrane as a cell separator coupled with a pair of Ag/AgCl porous membrane electrodes. At a salinity gradient of 10⁵, such a power generator presented a significantly decreased internal resistance (25.6 Ω), three-order of magnitude lower than that reported previously, and an output power as high as 9.03 μW.

Reticular Chemistry of Uranyl Phosphonates: Sterically Hindered Phosphonate Ligand Method is Significant for Constructing Zero-Dimensional Secondary Building Units

Designability is an attractive feature for metal-organic frameworks (MOFs) and essential for reticular chemistry, and many ideas are significantly useful in the carboxylate system. Bi-, tri-, and tetra-topic phosphonate ligands are used to achieve framework structures. However, an efficient method for designing phosphonate MOFs is still on the way, especially for uranyl phosphonates, owing to the complicated coordination modes of the phosphonate group. Uranyl phosphonates prefer layer or pillar-layered structures as the topology extension for uranyl units occurs in the plane perpendicular to the linear uranium-oxo bonds and phosphonate ligands favor the formation of compact structures. Therefore, an approach that can construct three-dimensional (3D) uranyl phosphonate MOFs is desired. In this paper, a sterically hindered phosphonate ligand method (SHPL) is described and is successfully used to achieve 3D framework structures of uranyl phosphonates. Four MOF compounds ([AMIM]₂ (UO₂ )(TppmH₄ )⋅H₂ O (UPF-101), [BMMIM]₂ (UO₂ )₃ (TppmH₄ )₂ ⋅H₂ O (UPF-102), [Py14]₂ (UO₂ )₃ (TppmH₄ )₂ ⋅3 H₂ O (UPF-103), and [BMIM](UO₂ )₃ (TppmH₃ )F₂ ⋅2 H₂ O (UPF-104); [AMIM]=1-allyl-3-methylimidazolium, [BMMIM]=1-butyl-2,3-dimethylimidazolium, [Py14]=N-butyl-N-methylpyrrolidinium, and [BMIM]=1-butyl-3-methylimidazolium) are obtained by ionothermal synthesis, with zero-dimensional nodes of uranyl phosphonates linked by steric tetra-topic ligands, namely tetrakis[4-(dihyroxyphosphoryl)phenyl]methane (TppmH₈ ), to give 3D framework structures. Characterization by PXRD, UV/Vis, IR, Raman spectroscopy, and thermogravimetry (TG) were also performed.

Increasing Alkyl Chain Length in a Series of Layered Metal-Organic Frameworks Aids Ultrasonic Exfoliation to Form Nanosheets

Metal–organic framework nanosheets (MONs) are attracting increasing attention as a diverse class of two-dimensional materials derived from metal–organic frameworks (MOFs). The principles behind the design of layered MOFs that can readily be exfoliated to form nanosheets, however, remain poorly understood. Here we systematically investigate an isoreticular series of layered MOFs functionalized with alkoxy substituents in order to understand the effect of substituent alkyl chain length on the structure and properties of the resulting nanosheets. A series of 2,5-alkoxybenzene-1,4-dicarboxylate ligands (O₂CC₆H₂(OR)₂CO₂, R = methyl–pentyl, 1–5, respectively) was used to synthesize copper paddle-wheel MOFs. Rietveld and Pawley fitting of powder diffraction patterns for compounds Cu(3–5)(DMF) showed they adopt an isoreticular series with two-dimensional connectivity in which the interlayer distance increases from 8.68 Å (R = propyl) to 10.03 Å (R = pentyl). Adsorption of CO₂ by the MOFs was found to increase from 27.2 to 40.2 cm³ g^–1 with increasing chain length, which we attribute to the increasing accessible volume associated with increasing unit-cell volume. Ultrasound was used to exfoliate the layered MOFs to form MONs, with shorter alkyl chains resulting in higher concentrations of exfoliated material in suspension. The average height of MONs was investigated by AFM and found to decrease from 35 ± 26 to 20 ± 12 nm with increasing chain length, with the thinnest MONs observed being only 5 nm, corresponding to five framework layers. These results indicate that careful choice of ligand functionalities can be used to tune nanosheet structure and properties, enabling optimization for a variety of applications.

A series of layered copper metal−organic frameworks (MOFs) were synthesized, utilizing benzene-1,4-dicarboxylates difunctionalized in the 2,5-positions with alkoxy pendent chains (methoxy-pentoxy) as the organic ligands. Increasing the chain length from propoxy to pentoxy increased the interlayer distance, resulted in increased CO₂ uptake, and aided the ultrasonic exfoliation of these MOFs to form nanosheets, producing MONs down to just 5 nm thick.