Polystyrene‐Supported Core–Shell Beads with Aluminium MOF Coating for Extraction of Organic Pollutants

Aluminium‐based metal–organic framework (MOF) coatings on polystyrene bead surfaces were easily synthesized by reacting an intermediate metal hydroxide coating with an organic linker. Several different sizes of polystyrene beads were coated with aluminium metal hydroxide to construct Al@PS core–shell bead materials. The activated Al@PS core–shell beads were involved to make a homogenous MOF‐based layer in the presence of the organic linker. By using different sizes of the PS support the size of MOFs on the PS composites could be fine‐tuned under specific reaction conditions. MOF‐coated core–shell bead materials (Al‐1,4‐NDC@PS and MIL‐53(Al)@PS) were characterized using various analytical techniques. Al‐1,4‐NDC@PS and MIL‐53(Al)@PS were evaluated for solid‐phase microextraction (SPME) of hydrophobic polycyclic aromatic hydrocarbons (PAHs) and hydrophilic non‐steroidal anti‐inflammatory drugs (NSAIDs), respectively. Al‐1,4‐NDC@PS‐1000 displayed high extraction recoveries ranging from 79.2 % to 99.8 % in the SPME of PAHs. Meanwhile, MIL‐53(Al)@PS‐1000 showed 85.9–99.0 % extraction recoveries in the SPME of NSAIDs. These results show that the proposed approach holds potential to extract organic analytes on an industrial scale.     

Facile fabrication of g‐C3N4/MIL‐53(Al) composite with enhanced photocatalytic activities under visible‐light irradiation

A novel visible‐light‐driven g‐C₃N₄/MIL‐53(Al) composite photocatalyst was successfully prepared using a facile stirring method at room temperature. The g‐C₃N₄/MIL‐53(Al) composites were characterized and their effects on the photocatalytic activities for rhodamine B degradation were investigated. The g‐C₃N₄(20 wt%)/MIL‐53(Al) photocatalyst displayed optimal photocatalytic degradation efficiency, which was about five times higher than the photocatalytic activity of pure g‐C₃N₄. The improved photocatalytic performance of the g‐C₃N₄/MIL‐53(Al) photocatalyst was predominantly attributed to the efficient separation of electron–hole pairs and the low charge‐transfer resistance. g‐C₃N₄/MIL‐53(Al) also exhibited excellent stability and reusability. A proposed mechanism for the enhanced photocatalytic activity is also discussed based on the experimental results. Copyright © 2015 John Wiley & Sons, Ltd.     

Monolithic metal–organic framework MIL‐53(Al)‐polymethacrylate composite column for the reversed‐phase capillary liquid chromatography separation of small aromatics

A monolithic capillary column containing a composite of metal–organic framework MIL‐53(Al) incorporated into hexyl methacrylate‐co‐ethylene dimethacrylate was prepared to enhance the separation of mixtures of small aromatic compounds by using capillary liquid chromatography. The addition of 10 mg/mL MIL‐53(Al) microparticles increased the micropore content in the monolithic matrix and increased the Brunauer–Emmett–Teller surface area from 26.92 to 85.12 m²/g. The presence of 1,4‐benzenedicarboxylate moieties within the structure of MIL‐53(Al) as an organic linker greatly influenced the separation of aromatic mixtures through π–π interactions. High‐resolution separation was obtained for a series of alkylbenzenes (with resolution factors in the range 0.96–1.75) in less than 8 min, with 14 710 plates/m efficiency for propylbenzene, using a binary polar mobile phase of water/acetonitrile in isocratic mode. A reversed‐phase separation mechanism was indicated by the increased retention factor and resolution as the water percentage in the mobile phase increased. A stability study on the composite column showed excellent mechanical stability under various conditions. The higher resolution and faster separation observed at increased temperature indicated an exothermic separation, whereas the negative values for the free energy change of transfer indicated a spontaneous process.     

MIL‐53(Al) as a Versatile Platform for Ionic‐Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium‐based ionic liquids (ILs) were incorporated into a metal–organic framework (MOF), MIL‐53(Al), to investigate the effect of IL incorporation on the CO₂ separation performance of MIL‐53(Al). CO₂, CH₄, and N₂ adsorption isotherms of the IL/MIL‐53(Al) composites and pristine MIL‐53(Al) were measured to evaluate the effect of the ILs on the CO₂/CH₄ and CO₂/N₂ selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF₆]/MIL‐53(Al) exhibited the largest increase in CO₂/CH₄ selectivity, 2.8‐times higher than that of pristine MIL‐53(Al), whilst [BMIM][MeSO₄]/MIL‐53(Al) exhibited the largest increase in CO₂/N₂ selectivity, 3.3‐times higher than that of pristine MIL‐53(Al). A comparison of the CO₂ separation potentials of the IL/MOF composites showed that the [BMIM][BF₄]‐ and [BMIM][PF₆]‐incorporated MIL‐53(Al) composites both showed enhanced CO₂/N₂ and CO₂/CH₄ selectivities at pressures of 1–5 bar compared to composites of CuBTC and ZIF‐8 with the same ILs. These results demonstrate that MIL‐53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas‐separation applications.     

A Systematic Study on the Stability of Porous Coordination Polymers against Ammonia

To establish a strategy for designing porous coordination polymers (PCPs) for ammonia capture, the first systematic study on the stability of PCPs against ammonia was conducted. Various types of PCPs were investigated by comparing their powder XRD patterns before and after treatment with ammonia. Among the PCPs tested, ZIF‐8, MIL‐53(Al), Al‐BTB, MOF‐76(M) (M=Y or Yb), MIL‐101(Cr), and MOF‐74(Mg) were stable up to 350 °C under an ammonia atmosphere at ambient pressure. The origin of the stability of PCPs is discussed from the viewpoint of their components, metal cations, and organic linkers. Furthermore, adsorption isotherm measurements show that the adsorptive behavior of PCPs is independent of their stability.     

Extra‐Framework Aluminum Species of Zeolite that Surrogate the Growth of Metal Organic Framework from Zeolite Matrix

Constructing a robust hybrid material with a porous inorganic and a porous organic framework is highly intriguing owing to its diverse functionality and porosity. However, the line of synthesis is not straightforward, since their nucleation and crystal growth processes are incompatible. Here, a simple method for the fabrication of hybrid zeolite/metal–organic framework of different framework structures is developed wherein the less‐useful extra‐framework aluminum species present in the zeolite surrogate the growth of metal organic framework (MOF) from the zeolite matrix in the presence of organic linkers of the corresponding MOF. An NMR study confirms that all the octahedral Al species are converted to Al‐MOF. TGA analysis shows that 32 % Al of H‐Beta is converted to Al‐MOF. Furthermore, NH₃ TPD analysis shows that most of the weak acid sites disappear but strong acid sites are preserved suggesting the utilization of weakly bound Al species of H‐Beta in the growth of Al‐MOF. The synthesis strategy is successfully demonstrated using H‐Beta, H‐ZSM‐5, and H‐Y zeolites for the growth of MIL‐53 and MIL‐96 MOFs from the zeolite matrix. This synthesis strategy enables application‐based engineering of the framework structures, functionality, and porosity of the materials.     

The synthesis of metal–organic framework Al‐MIL‐53‐derived Brønsted acid catalyst and its application in the Mannich reaction

A metal–organic framework Al‐MIL‐53‐NH₂‐derived Brønsted acid catalyst (Al‐MIL‐53‐RSO₃H) has been synthesized employing a post‐synthetic modification strategy under mild conditions. The Al‐MIL‐53‐RSO₃H catalyst was successfully utilized in the nitro‐Mannich reaction taking advantage of its strong Brønsted acidity. Good to excellent yields of Mannich adducts were achieved for a variety of acylimine substrates in the presence of 0.1 mol% Al‐MIL‐53‐RSO₃H. Furthermore, the Al‐MIL‐53‐RSO₃H catalyst can be recycled five times without decreasing the yield and selectivity of Mannich adducts.     

Synthesis,Water Adsorption,and Proton Conductivity of Solid‐Solution‐Type Metal–Organic Frameworks Al(OH)(bdc‐OH)x(bdc‐NH2)1−x

Mixed‐ligand metal–organic frameworks Al(bdc‐OH)_x(bdc‐NH₂)_1?x (H₂bdc‐NH₂=aminoterepthalic acid, H₂bdc‐OH=hydroxyterephthalic acid) were synthesized and their water adsorption behavior and proton conductivity were investigated. All obtained compounds were isostructural to MIL‐53 (MIL=Materials of Institut Lavoisier) according to XRD measurements under ambient humidity conditions, and were also found to be single phase across the whole mixing ratio from the XRD measurements under humidified conditions. This result clearly shows that all compounds are a solid‐solution‐type mixture of ligands. MIL‐53‐NH₂ adsorbs one water molecule per formula with humidification whereas MIL‐53‐OH adsorbs five water molecules. The mixing ratio of the ligands in Al(OH)(bdc‐OH)_x(bdc‐NH₂)_1?x affected the gate‐opening pressure for water adsorption and total water uptake. Proton conductivity of these compounds largely depends on the adsorbed amount of water, which indicates that the proton conductivity of these compounds depends strongly on the hydrogen‐bond network of the conducting media.     

Metal–organic framework MIL‐125(Ti) for efficient adsorptive removal of Rhodamine B from aqueous solution

A metal–organic framework material, MIL‐125(Ti), was solvothermally prepared and characterized using X‐ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and surface area measurements. MIL‐125(Ti) was then used as an adsorbent for Rhodamine B (RhB) removal in aqueous solution. The adsorption kinetics, adsorption mechanism, adsorption isotherm, activation energy and various thermodynamic parameters were studied in detail. The maximum adsorption capacity of MIL‐125(Ti) for RhB was 59.92 mg g^?1. MIL‐125(Ti) appears to be a promising material for RhB adsorption from aqueous solutions. Copyright © 2014 John Wiley & Sons, Ltd.     

Multi‐frequency (S,X, Q and W‐band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal–Organic Framework MIL‐53

Doping the well‐known metal–organic framework MIL‐53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as‐synthesised vanadium‐doped MIL‐53 are recorded at S‐, X‐, Q‐ and W‐band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0–2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with ¹H, ²⁷Al and ⁵¹V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework.     

Enhancing the Water Stability of Al‐MIL‐101‐NH2 via Postsynthetic Modification

The resistance of metal–organic frameworks towards water is a very critical issue concerning their practical use. Recently, it was shown for microporous MOFs that the water stability could be increased by introducing hydrophobic pendant groups. Here, we demonstrate a remarkable stabilisation of the mesoporous MOF Al‐MIL‐101‐NH₂ by postsynthetic modification with phenyl isocyanate. In this process 86 % of the amino groups were converted into phenylurea units. As a consequence, the long‐term stability of Al‐MIL‐101‐URPh in liquid water could be extended beyond a week. In water saturated atmospheres Al‐MIL‐101‐URPh decomposed at least 12‐times slower than the unfunctionalised analogue. To study the underlying processes both materials were characterised by Ar, N₂ and H₂O sorption measurements, powder X‐ray diffraction, thermogravimetric and chemical analysis as well as solid‐state NMR and IR spectroscopy. Postsynthetic modification decreased the BET equivalent surface area from 3363 to 1555 m² g^?1 for Al‐MIL‐101‐URPh and reduced the mean diameters of the mesopores by 0.6 nm without degrading the structure significantly and reducing thermal stability. In spite of similar water uptake capacities, the relative humidity‐dependent uptake of Al‐MIL‐101‐URPh is slowed and occurs at higher relative humidity values. In combination with ¹H‐²⁷Al D ‐HMQC NMR spectroscopy experiments this favours a shielding mechanism of the Al clusters by the pendant phenyl groups and rules out pore blocking.     

One‐pot self‐assembly and photoreduction synthesis of silver nanoparticle‐decorated reduced graphene oxide/MIL‐125(Ti) photocatalyst with improved visible light photocatalytic activity

In recent years, tremendous research efforts have been made towards developing metal–organic framework (MOF)‐based composites for photocatalytic applications. In this work, bipyramid‐like MIL‐125(Ti) frustum enwrapped with reduced graphene oxide (rGO) and dispersed silver nanoparticles (Ag NPs) was fabricated using an efficient one‐pot self‐assembly and photoreduction strategy. The as‐obtained materials were characterized using field emission scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, nitrogen adsorption–desorption isotherms, and X‐ray photoelectron, ultraviolet–visible diffuse reflectance and photoluminescence spectroscopies. It is found that the as‐prepared Ag/rGO/MIL‐125(Ti) ternary hybrids have large surface area, microporous structure, enhanced visible light absorption and prolonged lifetime of charge carriers. Compared with pure MIL‐125(Ti) and its binary counterparts, the ternary composite exhibits more efficient photocatalytic performance for Rhodamine B (RhB) degradation from water under visible light irradiation. The photodegradation rate of RhB on Ag/rGO/MIL‐125(Ti) is 0.0644 min^?1, which is 1.62 times higher than that of the pure MIL‐125(Ti). The improved photocatalytic performance is ascribed to the indirect dye photosensitization, the Ag NP localized surface plasmon resonance, the Ti³⁺–Ti⁴⁺ intervalence electron transfer and the synergistic effect among MIL‐125(Ti), Ag NPs and rGO. Ag NPs serve as an efficient ‘electron reservoir’ and rGO as an electron transporter and collector. Therefore, this work provides a new pathway into the design of MOF‐based composites for application in environmental and energy fields. Copyright © 2016 John Wiley & Sons, Ltd.     

Effective removal of 2,4,6‐trinitrophenol over hexagonal metal–organic framework NH2‐MIL‐88B(Fe)

A nanostructured organic–inorganic framework, hexagonal NH₂‐MIL‐88B, has been prepared through a facile one‐pot reflux reaction and then it was characterized using various techniques. The as‐prepared sample with high specific surface area (414 m² g^?1) showed excellent adsorption for 2,4,6‐trinitrophenol (TNP) in the liquid phase. Detailed studies of the adsorption kinetics, adsorption mechanism, adsorption isotherm, activation energy and various thermodynamic parameters were conducted. The adsorption mechanism of NH₂‐MIL‐88B for TNP may be ascribed to hydrogen bond interaction, and the complexation between ─OH in TNP and unsaturated Fe(III) on the surface of NH₂‐MIL‐88B. The maximum adsorption capacity of NH₂‐MIL‐88B for TNP based on the Langmuir isotherm was 163.66 mg g^?1. The as‐prepared NH₂‐MIL‐88B adsorbent seems to be a promising material in practice for TNP removal from aqueous solution.     

Nano palladium supported on high‐surface‐area metal–organic framework MIL‐101: an efficient catalyst for Sonogashira coupling of aryl and heteroaryl bromides with alkynes

Palladium nanoparticle‐incorporated metal–organic framework MIL‐101 (Pd/MIL‐101) was successfully synthesized and characterized using X‐ray diffraction, nitrogen physisorption, X‐ray photoelectron, UV–visible and infrared spectroscopies, and transmission electron microscopy. The characterization techniques confirmed high porosity and high surface area of MIL‐101 and high stability of nano‐size palladium particles. Pd/MIL‐101 nanocomposite was investigated for the Sonogashira cross‐coupling reaction of aryl and heteroaryl bromides with various alkynes under copper‐free conditions. The reusability of the catalyst was tested for up to four cycles without any significant loss in catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Turning Redundant Ligands into Treasure: A New Strategy for Constructing MIL‐53(Al)@Nanoscale TiO2 Layers

A strategy for in situ fabrication of nanoscale‐thin layers of anatase TiO₂ coated on the metal–organic framework (MOF) material, MIL‐53(Al), is developed. The preparation conditions for crystallized TiO₂ are normally incompatible with the thermal and chemical stability of MOFs. Based on our strategy, we found that the redundant organic ligands (1,4‐benzenedicarboxylic acid, H₂BDC) within the pores of the as‐synthesized MOF play a key function in the protection and support of the framework during hydrothermal loading of the TiO₂ precursor, as well as in preventing the infiltration of the precursor into the pores. After annealing, a nanoscale‐thin layer of highly crystalline anatase TiO₂, with a thickness of 6–10 nm, was successfully attached to the external surface of the MIL‐53(Al) crystals, while the porous framework remains intact. The core–shell structure of the MOF@TiO₂ nanocomposite endows the resulting materials with additional optical response and enhanced moisture and chemical stability.     

Mechanochemical Immobilisation of Metathesis Catalysts in a Metal–Organic Framework

A simple, one‐step mechanochemical procedure for immobilisation of homogeneous metathesis catalysts in metal–organic frameworks was developed. Grinding MIL‐101‐NH₂(Al) with a Hoveyda–Grubbs second‐generation catalyst resulted in a heterogeneous catalyst that is active for metathesis and one of the most stable immobilised metathesis catalysts. During the mechanochemical immobilisation the MIL‐101‐NH₂(Al) structure was partially converted to MIL‐53‐NH₂(Al). The Hoveyda–Grubbs catalyst entrapped in MIL‐101‐NH₂(Al) is responsible for the observed catalytic activity. The developed synthetic procedure was also successful for the immobilisation of a Zhan catalyst.     

Effect of Central Metal Ions of Analogous Metal–Organic Frameworks on Adsorption of Organoarsenic Compounds from Water: Plausible Mechanism of Adsorption and Water Purification

The adsorptive removal of organoarsenic compounds such as p‐arsanilic acid (ASA) and roxarsone (ROX) from water using metal–organic frameworks (MOFs) has been investigated for the first time. A MOF, iron benzenetricarboxylate (also called MIL‐100‐Fe) exhibits a much higher adsorption capacity for ASA and ROX than activated carbon, zeolite (HY), goethite, and other MOFs. The adsorption of ASA and ROX over MIL‐100‐Fe is also much more rapid than that over activated carbon. Moreover, the used MIL‐100‐Fe can be recycled by simply washing with acidic ethanol. Therefore, it is determined that a MOF such as MIL‐100‐Fe can be used to remove organoarsenic compounds from contaminated water because of its high adsorption capacity, rapid adsorption, and ready regeneration. Moreover, only one of three analogous MIL‐100 species (MIL‐100‐Fe, rather than MIL‐100‐Al or MIL‐100‐Cr) can effectively remove the organoarsenic compounds. This selective and high adsorption over MIL‐100‐Fe, different from other analogous MIL‐100 species, can be explained (through calculations) by the facile desorption of water from MIL‐100‐Fe as well as the large (absolute value) replacement energy (difference between the adsorption energies of the organoarsenic compounds and water) exhibited by MIL‐100‐Fe. A plausible adsorption/desorption mechanism is proposed based on the surface charge of the MOFs, FTIR results, calculations, and the reactivation results with respect to the solvents used in the experiments.     

Syntheses,Characterizations and Adsorption Properties of MIL‐101/Graphene Oxide Composites

Composites of the Cr³⁺‐based metal‐organic framework (MIL‐101) and graphene oxide (GO) have been synthesized with different ratios of MIL‐101 and GO. The composites and the parent material MIL‐101 were characterized by X‐ray diffraction, scanning electron microscopy and nitrogen adsorption. The results indicated that the incorporation of large amounts of GO (10 and 20 wt%) almost did not prevent the formation of MIL‐101 units, but had an obvious impact on the size of MIL‐101 crystals. On the contrary, small amounts of GO added (2 and 5 wt%) prevented significantly the proper assembly of MIL‐101 units, thus resulting in a pronounced decrease in the porosities of composites.     

Formation of a Single‐Crystal Aluminum‐Based MOF Nanowire with Graphene Oxide Nanoscrolls as Structure‐Directing Agents

An innovative strategy is proposed to synthesize single‐crystal nanowires (NWs) of the Al³⁺ dicarboxylate MIL‐69(Al) MOF by using graphene oxide nanoscrolls as structure‐directing agents. MIL‐69(Al) NWs with an average diameter of 70±20 nm and lengths up to 2 μm were found to preferentially grow along the [001] crystallographic direction. Advanced characterization methods (electron diffraction, TEM, STEM‐HAADF, SEM, XPS) and molecular modeling revealed the mechanism of formation of MIL‐69(Al) NWs involving size‐confinement and templating effects. The formation of MIL‐69(Al) seeds and the self‐scroll of GO sheets followed by the anisotropic growth of MIL‐69(Al) crystals are mediated by specific GO sheets/MOF interactions. This study delivers an unprecedented approach to control the design of 1D MOF nanostructures and superstructures.     

Magnetic porous carbon derived from a metal–organic framework as a magnetic solid‐phase extraction adsorbent for the extraction of sex hormones from water and human urine

An iron‐embedded porous carbon material (MIL‐53‐C) was fabricated by the direct carbonization of MIL‐53. The MIL‐53‐C possesses a high surface area and good magnetic behavior. The structure, morphology, magnetic property, and porosity of the MIL‐53‐C were studied by scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometry, and N₂ adsorption. With the use of MIL‐53‐C as the magnetic solid‐phase extraction adsorbent, a simple and efficient method was developed for the magnetic solid‐phase extraction of three hormones from water and human urine samples before high‐performance liquid chromatography with UV detection. The developed method exhibits a good linear response in the range of 0.02–100 ng/mL for water and 0.5–100 ng/mL for human urine samples , respectively. The limit of detection (S/N = 3) for the analytes was 0.005–0.01 ng/mL for water sample and 0.1–0.3 ng/mL for human urine sample. The limit of quantification (S/N = 10) of the analytes were in the range of 0.015–0.030 and 0.3–0.9 ng/mL, respectively.