Strongly visible light-absorbing metal–organic frameworks functionalized by cyclometalated ruthenium(ii) complexes

Four different ruthenium(ii) complexes were incorporated into the metal–organic framework (MOF) UiO-67 using three different synthetic strategies: premade linker synthesis, postsynthetic functionalization, and postsynthetic linker exchange. One of these complexes was of the type (N–N)₃Ru²⁺, and three of the complexes were of the type (N–N)₂(N–C)Ru⁺, where N–N is a bipyridine-type ligand and N–C is a cyclometalated phenylpyridine-type ligand. The resulting materials were characterized by PXRD, SC-XRD (the postsynthetic functionalization MOFs), N₂ sorption, TGA-DSC, SEM, EDS, and UV-Vis spectroscopy, and were digested in base for subsequent ¹H NMR analysis. The absorption profiles of the MOFs that were functionalized with cyclometalated Ru(ii) complexes extend significantly further into the visible region of the spectrum compared to the absorption profiles of the MOFs that were functionalized with the non-cyclometalated reference, (N–N)₃Ru²⁺.

The metal–organic framework (MOF) UiO-67 was functionalized by incorporating different cyclometalated ruthenium(ii) complexes using three different methods: premade linker synthesis, postsynthetic functionalization, and postsynthetic linker exchange.     

Synthesis and adsorption performance of La@ZIF-8 composite metal–organic frameworks

In this study, ZIF-8 with a rhombic dodecahedron structure was prepared by a hydrothermal method. Then La(OH)₃, was successfully loaded onto the ZIF-8 by an immersion deposition method, to form a lanthanide-based metal–organic framework (La@ZIF-8) composites. The structure and properties of La@ZIF-8 were verified by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and zeta potential measurements. The optimum process conditions are discussed within the materials and methods. The effects of initial phosphorus concentration, dosage, pH and contact reaction time on the phosphorus removal performance of the nanomaterial were investigated. The results indicated that La@ZIF-8 exhibited an excellent adsorption capacity (147.63 mg g⁻¹) and its phosphorus removal efficiency could reach as high as 99.7%. Experimental data were interpreted using different adsorption kinetic and isotherm models. The kinetic behavior conformed to the pseudo-second-order kinetic model, which indicated the chemisorption of phosphorus by La@ZIF-8. The adsorption behavior of phosphorus by La@ZIF-8 fitted well to the Langmuir isotherm model, suggesting a monolayer chemical adsorption process. The majority of the adsorbed phosphate could be desorbed by NaOH (2 mol L⁻¹), and the removal efficiency of the recycled La@ZIF-8 reached 90%, even after the fifth cycle. The obtained results demonstrate the great application potential of the prepared La@ZIF-8 as a fascinating adsorbent for the removal of phosphate.

In this study, La(OH)₃ was successfully loaded on ZIF-8 by immersion deposition method, to form lanthanide-based metal–organic frameworks (La@ZIF-8) composites.     

A series of four novel alkaline earth metal–organic frameworks constructed of Ca(ii), Sr(ii), Ba(ii) ions and tetrahedral MTB linker: structural diversity,stability study and low/high-pressure gas adsorption properties

A series of four novel microporous alkaline earth metal–organic frameworks (AE-MOFs) containing methanetetrabenzoate linker (MTB) with composition {[Ca₄(μ₈-MTB)₂]·2DMF·4H₂O}_n (UPJS-6), {[Ca₄(μ₄-O)(μ₈-MTB)_3/2(H₂O)₄]·4DMF·4H₂O}_n (UPJS-7), {[Sr₃(μ₇-MTB)_3/2]·4DMF·7H₂O}_n (UPJS-8) and {[Ba₃(μ₇-MTB)_3/2(H₂O)₆]·2DMF·4H₂O}_n (UPJS-9) (UPJS = University of Pavol Jozef Safarik) have been successfully prepared and characterized. The framework stability and thermal robustness of prepared materials were investigated using thermogravimetric analysis (TGA) and high-energy powder X-ray diffraction (HE-PXRD). MOFs were tested as adsorbents for different gases at various pressures and temperatures. Nitrogen and argon adsorption showed that the activated samples have moderate BET surface areas: 103 m² g⁻¹ (N₂)/126 m² g⁻¹ (Ar) for UPJS-7′′, 320 m² g⁻¹ (N₂)/358 m² g⁻¹ (Ar) for UPJS-9′′ and UPJS-8′′ adsorbs only a limited amount of N₂ and Ar. It should be noted that all prepared compounds adsorb carbon dioxide with storage capacities ranging from 3.9 to 2.4 wt% at 20 °C and 1 atm, and 16.4–13.5 wt% at 30 °C and 20 bar. Methane adsorption isotherms show no adsorption at low pressures and with increasing pressure the storage capacity increases to 4.0–2.9 wt% of CH₄ at 30 °C and 20 bar. Compounds displayed the highest hydrogen uptake of 3.7–1.8 wt% at −196 °C and 800 Torr among MTB containing MOFs.

Four novel microporous alkaline earth metal–organic frameworks (AE-MOFs) containing methanetetrabenzoate linker (MTB): UPJS-6, UPJS-7, UPJS-8 and UPJS-9 have been successfully prepared, characterized and tested as adsorbents for different gases.     

Modelling drug adsorption in metal–organic frameworks: the role of solvent

Solvent plays a key role in biological functions, catalysis, and drug delivery. Metal–organic frameworks (MOFs) due to their tunable functionalities, porosities and surface areas have been recently used as drug delivery vehicles. To investigate the effect of solvent on drug adsorption in MOFs, we have performed integrated computational and experimental studies in selected biocompatible MOFs, specifically, UiO-AZB, HKUST-1 (or CuBTC) and NH₂-MIL-53(Al). The adsorption of three drugs, namely, 5-fluorouracil (5-FU), ibuprofen (IBU), and hydroxyurea (HU) were performed in the presence and absence of the ethanol. Our computational predictions, at 1 atmospheric pressure, showed a reasonable agreement with experimental studies performed in the presence of ethanol. We find that in the presence of ethanol the drug molecules were adsorbed at the interface of solvent and MOFs. Moreover, the computationally calculated adsorption isotherms suggested that the drug adsorption was driven by electrostatic interactions at lower pressures (<10⁻⁴ Pa). Our computational predictions in the absence of ethanol were higher compared to those in the presence of ethanol. The MOF–adsorbate interaction (U_HA) energy decreased with decrease in the size of a drug molecule in all three MOFs at all simulated pressures. At high pressure the interaction energy increases with increase in the MOFs pore size as the number of molecules adsorbed increases. Thus, our research shows the important role played by solvent in drug adsorption and suggests that it is critical to consider solvent while performing computational studies.

Solvent plays a key role in drug loading in metal–organic frameworks.     

Screening of metal–organic frameworks for water adsorption heat transformation using structure–property relationships

It is of great importance to correlate the water adsorption performance of MOFs to their physicochemical features in order to design and prepare MOFs for applications in adsorption heat transformation. In this work, both data analysis from existing studies and Grand Canonical Monte Carlo molecular simulation investigations were carried out. The results indicated that the highest water adsorption capacity was determined by the pore volume of MOF adsorbents, while there was a linear correlation interrelationship between isosteric heats of adsorption and the water adsorption performance at a low relative pressure. More detailed analysis showed that the charge distribution framework and pore size of MOFs contributed together to the hydrophilicity. Electrostatic interaction between water molecules and the framework atoms played a key role at low relative water pressure. A quantitative structure–property relationship model that can correlate the hydrophilicity of MOFs to their pore size and atomic partial charge was established. Along with some qualitative considerations, the screening methodology is proposed and is used to screen proper MOFs in the CoRE database. Seven MOFs were detected, and four of them were synthesized to validate the screening principle. The results indicated that these four MOFs possessed outstanding water adsorption performance and could be considered as promising candidates in applications for adsorption heating and cooling.

Quantitative structure–property relationship models that correlate the water adsorption performance of MOFs to their physicochemical features have been established.     

Temperature tuned syntheses of two new d10-based Cd(ii) cluster metal–organic frameworks: luminescence sensing and photocatalytic properties

The solvothermal reactions of 5,5′-(1,4-phenylenebis(methyleneoxy))diisophthalic acid (H₄L) and the N-donor ancillary ligand 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (bpz) with cadmium(ii) salts at two different reaction temperatures yielded two new metal–organic frameworks (MOFs), viz., [Cd(H₂L)(bpz)]_n (1) and [Cd₂(H₄L)(L)(bpz)₂]_n (2), which have been characterized by FTIR and single crystal X-ray diffraction. The single crystal X-ray diffraction studies revealed that 1 displays a 3-periodic network with monometallic SBU, while 2 exhibits a 3-periodic network with a 2-fold interpenetration feature. The effects of variation in reaction temperature on the architecture of MOFs 1 and 2 have been discussed. The luminescence investigation indicates that both 1 and 2 displayed good turn-off luminescence sensing against nitroaromatic compounds (NACs), especially m-nitrophenol (MNP), via a decrease in their luminescence intensities with a K_sv value of 6.43 × 10³ for 1 and 2.03 × 10⁴ for 2 and LOD values of 1.09 and 0.81 ppm for 1 and 2, respectively. The plausible mechanism for the decline in luminescence intensity of the MOFs with NACs has been addressed using theoretical calculations. The photocatalytic properties for both the MOFs demonstrated that they display efficient photocatalytic performances to degrade methyl violet (MV) under UV irradiation. The plausible mechanism through which these MOFs exhibited photocatalytic properties has been suggested using band gap calculations.

Two new d¹⁰-based Cd(ii) MOFs were synthesized and their photoluminescence sensing to selectively detect m-nitrophenol (MNP) and ability to decompose the organic dye methyl violet (MV) were explored.     

Hydrogen adsorption mechanism of MOF-74 metal–organic frameworks: an insight from first principles calculations

The microscopic mechanism of the H₂ adsorption of two Mg-MOF-74 isoreticular frameworks, one with a benzenedicarboxylate (BDC) linker and the other with a dihydroxyfumarate (DHF) linker, were studied on the basis of density functional theory (DFT) method. Possible adsorption sites on the internal surface of the two MOFs were detected using ab initio molecular dynamics (AIMD) annealing simulations. The simulations were able to reproduce all adsorption sites which have been experimentally observed for the BDC-based M-MOF-74 frameworks with M = Ni and Zn. In descending order of binding strengths, they are the adsorption sites primarily induced by the open metal sites P₁, the oxygen atoms of the oxido groups P₂ and the aromatic rings P₃. The H₂–framework binding strengths were properly evaluated by taking into account the vibrational zero-point energy (ZPE) contribution. An additional type of adsorption sites induced by the oxygen atoms of the carboxyl groups P₄ is predicted for the Mg-MOF-74 framework. Two types of adsorption sites primarily induced by the open metal sites P₁ and oxygen atoms of the carboxyl groups P₂ were predicted for the DHF-based Mg-MOF-74 framework. Detailed analysis of the electron density showed that the electrostatic interaction of the H₂ molecule with the charge distribution of the local framework environment within a radius of ∼3.5 Å is a key factor to define adsorption positions and binding strength. The absence of the P₄ sites in the BDC-based Zn-MOF-74 framework is caused by the lower charge density at the oxygen atoms induced by less electro-positive metal. The substitution of the nonaromatic DHF linker for the aromatic BDC linker reduces the binding strength at the metal induced adsorption sites by 1.45 kJ mol⁻¹ due to the absence of the aromatic ring.

The microscopic mechanism of the H₂ adsorption of two Mg-MOF-74 isoreticular frameworks, one with a benzenedicarboxylate linker and the other with a dihydroxyfumarate linker, were studied on the basis of density functional theory (DFT) method.     

A collection of forcefield precursors for metal–organic frameworks

A host of important performance properties for metal–organic frameworks (MOFs) and other complex materials can be calculated by modeling statistical ensembles. The principle challenge is to develop accurate and computationally efficient interaction models for these simulations. Two major approaches are (i) ab initio molecular dynamics in which the interaction model is provided by an exchange–correlation theory (e.g., DFT + dispersion functional) and (ii) molecular mechanics in which the interaction model is a parameterized classical force field. The first approach requires further development to improve computational speed. The second approach requires further development to automate accurate forcefield parameterization. Because of the extreme chemical diversity across thousands of MOF structures, this problem is still mostly unsolved today. For example, here we show structures in the 2014 CoRE MOF database contain more than 8 thousand different atom types based on first and second neighbors. Our results showed that atom types based on both first and second neighbors adequately capture the chemical environment, but atom types based on only first neighbors do not. For 3056 MOFs, we used density functional theory (DFT) followed by DDEC6 atomic population analysis to extract a host of important forcefield precursors: partial atomic charges; atom-in-material (AIM) C₆, C₈, and C₁₀ dispersion coefficients; AIM dipole and quadrupole moments; various AIM polarizabilities; quantum Drude oscillator parameters; AIM electron cloud parameters; etc. Electrostatic parameters were validated through comparisons to the DFT-computed electrostatic potential. These forcefield precursors should find widespread applications to developing MOF force fields.

Atom-in-material (AIM) partial charges, dipoles and quadrupoles, dispersion coefficients (C₆, C₈, C₁₀), polarizabilities, electron cloud parameters, radial moments, and atom types were extracted from quantum chemistry calculations for >3000 MOFs.     

Optimization of metal–organic (citric acid) frameworks for controlled release of nutrients

Although polymer-coated controlled-release fertilizers have been under development for decades, their high costs, complex production processes, and potential environmental hazards have limited their application. Therefore, it is necessary to design and develop new materials for controlled nutrient release. In this study, two novel MOFs, compounds I and II, were successfully fabricated and optimized using ferric chloride, phosphoric acid, citric acid, and urea under hydrothermal conditions. The N, P, and Fe contents in compound I were 9.05%, 14.92%, and 14.55%, respectively, while the corresponding values in compound II were 10.78%, 14.10%, and 16.68%. The soil incubation results revealed that both compounds showed good slow-release longevity (more than 100 days). This study provides a new strategy for the fabrication of novel controlled-release fertilizers.

Novel MOFs were fabricated to control nutrient release under hydrothermal conditions, providing new strategies for the development of controlled-release fertilizers with relative low cost, simple production processes and little environmental impact.     

Boosting CO2 adsorption and selectivity in metal–organic frameworks of MIL-96(Al) via second metal Ca coordination

Aluminum trimesate-based MOF (MIL-96-(Al)) has attracted intense attention due to its high chemical stability and strong CO₂ adsorption capacity. In this study, CO₂ capture and selectivity of MIL-96-Al was further improved by the coordination of the second metal Ca. To this end, a series of MIL-96(Al)–Ca were hydrothermally synthesised by a one-pot method, varying the molar ratio of Ca²⁺/Al³⁺. It is shown that the variation of Ca²⁺/Al³⁺ ratio results in significant changes in crystal shape and size. The shape varies from the hexagonal rods capped in the ends by a hexagonal pyramid in MIL-96(Al) without Ca to the thin hexagonal disks in MIL-96(Al)–Ca4 (the highest Ca content). Adsorption studies reveal that the CO₂ adsorption on MIL-96(Al)–Ca1 and MIL-96(Al)–Ca2 at pressures up to 950 kPa is vastly improved due to the enhanced pore volumes compared to MIL-96(Al). The CO₂ uptake on these materials measured in the above sequence is 10.22, 9.38 and 8.09 mmol g⁻¹, respectively. However, the CO₂ uptake reduces to 5.26 mmol g⁻¹ on MIL-96(Al)–Ca4. Compared with MIL-96(Al)–Ca1, the N₂ adsorption in MIL-96(Al)–Ca4 is significantly reduced by 90% at similar operational conditions. At 100 and 28.8 kPa, the selectivity of MIL-96(Al)–Ca4 to CO₂/N₂ reaches up to 67 and 841.42, respectively, which is equivalent to 5 and 26 times the selectivity of MIL-96(Al). The present findings highlight that MIL-96(Al) with second metal Ca coordination is a potential candidate as an alternative CO₂ adsorbent for practical applications.

MIL-96(Al)–Ca1 shows the highest CO₂ adsorption capacity; while MIL-96(Al)–Ca4 displays a distinguished morphology with the highest selectivity of CO₂/N₂.     

Experimental investigation of the dehumidification and decarburization performance of metal–organic frameworks in solid adsorption air conditioning

Solid adsorption air conditioning systems use solid adsorption materials to co-adsorb water vapor and carbon dioxide, allowing the humidity and carbon dioxide concentration in the air-conditioned room to be controlled. Exploring the co-adsorption mechanism of H₂O and CO₂ is essential for the screening of adsorbent materials, system design, and system optimization in solid adsorption air conditioning systems. A fixed-bed adsorption–desorption device was built, and the dynamic adsorption properties of three MIL adsorbent materials MIL-101(Cr), MIL-101(Fe), and MIL-100(Fe) for co-adsorption of H₂O and CO₂ were studied. The results showed that all three MIL adsorbent materials are capable of performing co-adsorption of H₂O and CO₂ and meet the requirements of solid adsorption air conditioning systems. MIL-101(Cr) is recommended for solid adsorption air conditioners where dehumidification is the main focus, while MIL-100(Fe) is recommended for solid adsorption air conditioners where carbon removal is the main focus.

Solid adsorption air conditioning systems use solid adsorption materials to co-adsorb water vapor and carbon dioxide, allowing the humidity and carbon dioxide concentration in the air-conditioned room to be controlled.     

Synthesis of luminescent thorium-based metal–organic frameworks with 1,2,4,5-tetrakis(4-carboxyphenyl)benzene

Three new thorium-based MOFs based on 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (H4TCPB) were obtained under a similar reaction system (metal salt, ligand, solvent, and acid are the same). Th(iv) forms the central unit of the MOFs in mononuclear and binuclear clusters, respectively. All the MOFs show blue ligand-based luminescence under an ultraviolet environment. This is the first time that multiple thorium-based MOFs with luminescence have been found with the same ligand.

Three new thorium-based MOFs based on 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (H4TCPB) were obtained under a similar reaction system (metal salt, ligand, solvent, and acid are the same).     

Highly sensitive detection of nitrobenzene by a series of fluorescent 2D zinc(ii) metal–organic frameworks with a flexible triangular ligand

Four fluorescent zinc(ii) metal–organic frameworks, namely [Zn(HCIA)(4,4′-bipy)] (1), [Zn₂(CIA)(OH)(1,4-bibz)_1.5]·H₂O (2), [Zn(CIA)(OH) (4,4′-bbpy)] (3), and [Zn₂(HCIA) (4,4′-bimp)]·H₂O (4), were prepared hydrothermally with a flexible triangular ligand (H₃CIA) and a series of linear N-donor ligands (H₃CIA = 5-(2-carboxybenzyloxy) isophthalic acid, 4,4′-bipy = 4,4′-bipydine, 1,4-bibz = 1,4-bis(1-imidazoly)benzene; 4,4′-bbpy = 4,4′-bis (imidazolyl) biphenyl; 4,4′-bimp = 4,4′-bis (imidazole-1-ylethyl) biphenyl). Structural analyses revealed that complex 1 exhibited a 2D brick-like network structure based on the basic bimetallic ring, 2 was also a 2D interspersed structure from the 1D tubular structure, compound 3 possessed a 2D (4,4) network with 4,4′-bbpy occupying the holes, and complex 4 displayed a 2D network from the 1D ladder-like chain. The thermal stabilities and fluorescent properties of these complexes were investigated in the solid state. The fluorescent sensing experiments revealed that all Zn-MOFs could highly sensitively detect nitrobenzene in aqueous solution, which indicated that these materials can be used as fluorescent probes for the detection of nitrobenzene.

Four fluorescent 2D Zn-MOFs based on a flexible triangular ligand and linear N-donor ligands are hydrothermally prepared and used to detect nitrobenzene in aqueous solution with high sensitivity, demonstrating their potential as fluorescent sensors.     

Oriented self-assembly of metal–organic frameworks driven by photoinitiated monomer polymerization

The self-assembly of metal–organic frameworks (MOFs) is crucial for the functional design of materials, including energy storage materials, catalysts, selective separation materials and optical crystals. However, oriented self-assembly of MOFs is still a challenge. Herein, we propose a novel strategy to drive oriented self-assembly of MOF polyhedral particles at the water–liquid interface by photoinitiated monomer polymerization. The MOF polyhedral particles self-assemble into ordered close-packed structures with obvious orientation in the polymer film, and the orientation is determined by the casting solvent on the water surface. The prepared large-area MOF polymer films show a Janus structure, containing a MOF monolayer and a polymer layer, and can be easily transferred to a variety of substrates. In addition, mixed MOF particles with different sizes and morphologies can also be assembled by this method. This novel method can be foreseen to provide a powerful driving force for the development of MOF self-assembly and to create more possibilities for utilizing the anisotropic properties of MOFs.

The self-assembly of metal–organic frameworks (MOFs) is crucial for the functional design of materials, including energy storage materials, catalysts, selective separation materials and optical crystals.     

Integration of fluorescent probes into metal–organic frameworks for improved performances

Recent years have witnessed a rapid development of fluorescent probes in both analytical sensing and optical imaging. Enormous efforts have been devoted to the regulation of fluorescent probes during their development, such as improving accuracy, sensitivity, selectivity, recyclability and overcoming the aggregation-caused quenching effect. Metal–organic frameworks (MOFs) as a new class of crystalline porous materials possess abundant host–guest chemistry, based on which they display a great application potential in regulating fluorescent probes. This review summarized the research works on the regulation of fluorescent probes using MOFs, with emphasis on the methods of integrating fluorescent probes into MOFs, the regulation effects of MOFs on fluorescent probes, the superiorities of MOFs in regulating fluorescent probes, and the outlook of this subject. It is desirably hoped that this review can provide a useful reference for the researchers interested in this field.

This review surveyed the research works for the regulation of fluorescent probes with metal–organic frameworks based on host–guest chemistry.     

Strategies for the application of metal–organic frameworks in catalytic reactions

Efficient catalysts play crucial roles in various organic reactions and polymerization. Metal–organic frameworks (MOFs) have the merits of ultrahigh porosity, large surface area, dispersed polymetallic sites and modifiable linkers, which make them promising candidates for catalyzation. This review primarily summarizes the recent research progress on diverse strategies for tailoring MOFs that are endowed with excellent catalytic behavior. These strategies include utilizing MOFs as nanosized reaction channels, metal nodes decorated as catalytic active sites and the modification of ligands or linkers. All these make them highly attractive to various applications, especially in catalyzing organic reactions or polymerizations and they have proven to be effective catalysts for a wide variety of reactions. MOFs are still an evolving field with tremendous prospects; therefore, through the research and development of more modification and regulation strategies, MOFs will realize their wider practical application in the future.

Metal–organic frameworks (MOFs) are promising candidates for catalyzation. This review primarily summarized the recent research progress in diverse strategies for tailoring MOFs which are endowed with more excellent catalytic behavior.     

Chemisorption of hydrogen sulfide over copper-based metal–organic frameworks: methanol and UV-assisted regeneration

Three copper-based metal–organic frameworks (MOFs) with different organic linkers were synthesized for the removal of H₂S gas at room temperature. The synthesized MOFs were characterized by microscopic and spectroscopic techniques to understand their structural, functional, and optical properties. The H₂S adsorption capacity of MOFs calculated by column studies followed the trend: 105.6 mg g⁻¹ (CuBDC) > 27.1 mg g⁻¹ (CuBTC) > 1.3 mg g⁻¹ (CuBDC-N) in dry conditions. The adsorption capacity increased in moist conditions due to an easy dissolution and dissociation of H₂S in a film of water. X-ray photoelectron spectroscopy confirmed the presence of sulfur bound to Cu-sites and sulfate ions. The spent MOFs were regenerated by the successive effect of methanol and low power UV-C radiation. The regenerated CuBTC showed an exceptionally high adsorption capacity of 95.6 mg g⁻¹ in the second cycle, which was linked to the reactivation of Cu-sites and improved surface area and porosity. The regeneration process developed in this study is a cost-effective method to recycle chemisorbed MOFs without compromising with their structural and functional integrity.

H₂S adsorption and regeneration of Cu-based MOFs.     

Syntheses of four porous 3-D Cd(ii) metal–organic frameworks from a new multidentate ligand 5-(imidazol-1-yl)-N′-(pyridin-4-ylmethylene) nicotinohydrazide and their characterization and adsorption properties

Herein, a new multidentate ligand, 5-(imidazol-1-yl)-N′-(pyridin-4-ylmethylene) nicotinohydrazide (L), with an acylhydrazone group was synthesized and characterized. Subsequently, four porous Cd(ii)-MOFs, i.e. [Cd(L)(NO₃)]_n (1), [Cd(L)Cl]_n (2), [Cd(L)Br]_n (3), and [Cd(L)I]_n (4), were assembled using the ligand L by a solvothermal method and characterized by single-crystal X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and powder X-ray diffraction. Structural analysis shows that the coordination environments around Cd(ii) in all the four compounds are different due to the different coordinated anions. Among them, the coordination geometries and the arrangement of five-coordinated groups of the compound 1 containing the coordinated NO₃⁻ anions are significantly different from those of the other three compounds containing halides. However, all the four MOFs have similar one-dimensional rhombic channels. In these channels, both the nitrate ions and the halide ions are attached to the inner walls of the pores. The CO₂ adsorption properties of 1–4 were studied at 273 K, and the results showed that these compounds exhibit different adsorption capacities for CO₂ due to the presence of different ions in their pores.

Herein, four stable 3D porous Cd(ii)-MOFs were constructed from the multidentate acylhydrazone ligand, and their CO₂ adsorption properties revealed that the anions have an obvious effect on the adsorption capacities of these MOFs.     

One-pot synthesis of enhanced fluorescent copper nanoclusters encapsulated in metal–organic frameworks

The encapsulation of Cu nanoclusters (Cu NCs) in metal–organic frameworks (MOFs) would improve the properties of Cu NCs. So far, these composites were reported by a two-step synthesis process. In this work, a facile one-pot synthesis of hybridization of glutathione (GSH) protected Cu NCs (Cu NCs@GSH) and MOF-5 (Cu NCs@GSH/MOFs) composites was reported for the first time. The results of UV-vis, TEM, XPS and SEM proved Cu NCs@GSH were distributed homogeneously over the entire MOF structure. The fluorescence intensity of Cu NCs encapsulated in MOF-5 was enhanced about 35-fold owing to the confining scaffold of the MOF and the stability was extended from 3 days to 3 months. Cu NCs@GSH/MOFs composites exhibited strong orange fluorescence and the emissions could change between blue, orange and red as they were partially reversible in different pH environments. This one-pot synthetic strategy could be extended for the encapsulation of fluorescent Ag NCs in MOFs as well. As-prepared Cu NCs@GSH/MOF-5 composites had high stability, and were easily recycled by centrifugation in aqueous solution, therefore, it would be utilized to develop a reusable sensor for detection of metal ions in the future.

The encapsulation of Cu nanoclusters (Cu NCs) in metal–organic frameworks (MOFs) would improve the properties of Cu NCs.     

Enhanced transformation of CO2 over microporous Ce-doped Zr metal–organic frameworks

Metal–organic frameworks (MOF) have been studied extensively for the adsorption and catalytic conversion of CO₂. However, previous studies mainly focused on the adsorption capabilities of partially or totally Ce substituted UiO-66, there are few studies focusing on transformation of the structure and catalytic activity of these materials. In this work, a series of Zr/Ce-based MOFs with UiO-66 architecture catalysts were prepared for the conversion of CO₂ into value-added dimethyl carbonate (DMC). Owing to the different addition order of the two metals, significantly varied shapes and sizes were observed. Accordingly, the catalytic activity is greatly varied by adding a second metal. The different catalytic activities may arise from the different acid–base properties after Ce doping as well as the morphology and shape changes. Besides, the formation of terminal methoxy (t-OCH₃) was found to be the rate limiting step. Finally, the reaction mechanism of CO₂ transformation in the presence of a dehydrating agent was proposed.

Different doping order of Ce/Zr have a significant effect on the morphologies, acid properties as well as on the activities for CO₂ conversion of the MOF materials.