Shaping metal–organic framework (MOF) powder materials for CO2 capture applications—a thermogravimetric study

Journal of Thermal Analysis and Calorimetry - The paper presents the effect of the tabletting pressure and time on the chemical structure and crystallinity of the CuBTC and MIL-53(Al)...     

Sorption of benzene vapors to flexible metal–organic framework [Zn2(bdc)2(dabco)]

The isotherms of benzene sorption by the metal–organic coordination polymer [Zn₂(bdc)₂(dabco)] were studied within the temperature range 25–90 °C at pressures up to 75 torr. The maximal benzene content in [Zn₂(bdc)₂(dabco)] at room temperature was demonstrated to correspond to the composition [Zn₂(bdc)₂(dabco)]·3.8C₆H₆. It was established that the process of benzene desorption from the substance under investigation occurs in three stages. (1) Evaporation of benzene from the phase of variable composition (phase C) with compression and distortion of the unit cell (the composition of the phase C varies from [Zn₂(bdc)₂(dabco)]·3.8C₆H₆ to [Zn₂(bdc)₂(dabco)]·3.2C₆H₆). (2) The transformation of the phase C into phase P. The phase P has the same unit cell geometry as that for the empty framework. The maximal benzene content is [Zn₂(bdc)₂(dabco)]·1.0C₆H₆. (3) Benzene evaporation from the phase P of variable composition. We studied the temperature dependences of the equilibrium vapor pressure of benzene for the samples with compositions [Zn₂(bdc)₂(dabco)]·3.0(3)C₆H₆ and [Zn₂(bdc)₂(dabco)]·2.0(3)C₆H₆ within the temperature range 290–370 K. The thermodynamic parameters of benzene vaporization were determined for the latter compound ( $ \Updelta {\text{H}}_{{{\text{av}} .}}^{o} = 49\left( 1 \right) \,{\text{kJ }}\left( {{\text{moleC}}_{6} {\text{H}}_{6} } \right)^{ - 1} $ ; $ \Updelta {\text{S}}_{{{\text{av}} .}}^{^\circ } = 100\left( 3 \right)\, {\text{J}}\left( {{\text{moleC}}_{6} {\text{H}}_{6} {\text{K}}} \right)^{ - 1} $ ; $ \Updelta {\text{G}}_{298}^{^\circ } = 19.0\left( 2 \right)\, {\text{kJ}}\left( {{\text{moleC}}_{6} {\text{H}}_{6} } \right)^{ - 1} $ ).     

Influence of oxophilic behavior of UiO-66(Ce) metal–organic framework with superior catalytic performance in Friedel-Crafts alkylation reaction

In recent years, one of the analogous metal organic frameworks (MOFs) with UiO-66(Zr) topology receiving wider attention is UiO-66(Ce), which exhibits interesting properties and high thermal and chemical stability. Hence, in the present work, UiO-66(Ce) is synthesized by adopting an earlier procedure and characterized by series of spectroscopic techniques like UV-visible (UV-Vis), Fourier transform infrared (FT-IR), Raman and scanning electron microscope (SEM) to confirm its structural features and crystallinity using powder X-ray diffraction (XRD) and these results are in close agreement with other reports. The catalytic performance of UiO-66(Ce) was evaluated in the Friedel–Crafts alkylation reaction between β-nitrostyrene and indole to obtain heterocyclic compounds with biological activity. A series of control experiments indicate that Ce⁴⁺ located within the framework plays an important role in promoting this reaction and its activity is found to be much superior to that of UiO-66(Zr). This enhanced activity with Ce⁴⁺ compared to Zr⁴⁺ is attributed due to the higher oxophilicity of Ce⁴⁺, which can readily bind with an oxygen-containing substrate such as β-nitrostyrene. The leaching test confirms the heterogeneity of the reaction and the catalyst can be reused three times with identical activity to the fresh solid. UiO-66(Ce) shows wide substrate scope with high yields of the desired product. A proposed mechanism is also discussed.     

Construction of visible-light-responsive metal–organic framework with pillared structure for dye degradation and Cr(VI) reduction

A water-stable mixed-linker metal–organic framework (MOF) was rationally synthesized using a controllable pillared-layer method. The prepared Co(II)–MOF shows wide-range absorption in the visible light region due to the incorporation of highly conjugated anthracene-based bipyridine ligand. Experiments suggest that the MOF is highly efficient for the photoreduction of toxic Cr(VI) ions in water under visible light. Important issues affecting photocatalytic performance, such as the influence of pH and the control of electron–hole separation by scavenger, were carefully examined. Beyond Cr(VI) ions, we also explored the photocatalytic degradation performance of the MOF using a persistent azo dye as a model substrate, where H₂O₂-involved advanced oxidation process was applied. Control experiments suggest that the introduction of environmentally benign H₂O₂ significantly enhances the degradation performance due to the generation of reactive hydroxyl radicals. The study not only demonstrates the great feasibility of the preparation of a new MOF photocatalyst through a controllable pillared-layer method, but also reveals that rational functionalization of ligand in the MOF is convenient for achieving desirable applications.     

Progress in the study of metal–organic materials applying naphthalene diimide (NDI) ligands

1,4,5,8-naphthalenediimide (NDI) derivatives are versatile in coordination and material chemistry due to their large conjugated planar structure and special electron transfer properties. This review presents an overview of metal–organic materials derived from NDIs with their structural models, analytical techniques and potential applications outlined.     

Adsorption desulfurization of model gasoline by metal–organic framework Ni_3(BTC)_2

This work presented the synthesis of Ni-based metal-organic framework material with a paddle-wheel structure Ni_3(BTC)_2(Ni-BTC) and its application in thiophene(TP) adsorption from gasoline distillate by batch method. Adsorption isotherms of TP, cyclohexene, and toluene in cyclohexane onto Ni-BTC were conducted at 298–308 K to interpret the different effect of cyclohexene and toluene on TP adsorption.The results showed that, compared with cyclohexene, toluene addition in model gasoline led to a more evident decline in sulfur capacity of Ni-BTC, which is opposite to isostructural HKUST-1. The adsorption isotherms of TP, cyclohexene and toluene fit Langmuir model, S-type model and Temkin model well, respectively, indicating that the adsorption mechanisms of TP and the two competitors are different from one another. The adsorption capacities on Ni-BTC followed the order of cyclohexene toluene TP at the same equilibrium concentrations, implying the order of the adsorption affinities, which is in good agreement with the different extent of influence by the two competitors. The enthalpy of TP adsorption on Ni-BTC was estimated to be-80.01 kJ/mol, almost twice that on HKUST-1. The poor reusability of Ni-BTC in batch experiment, which is owing to its sensitivity to the air, can be prevented from regenerating used Ni-BTC in fixed-bed reactor by N_2 flow. The difference between Ni-BTC and HKUST-1 in maximum adsorption capacity(q_0), H of TP adsorption, and stability demonstrates that the central metal in isostructural MOFs plays a key role in adjusting the desulfurization performance, which may open up a potential avenue for the development of MOF-based adsorbents with superior desulfurization performance.     

Thermal transformations of Cu–Mg (Zn)–Al(Fe) hydrotalcite-like materials into metal oxide systems and their catalytic activity in selective oxidation of ammonia to dinitrogen

Layered double hydroxides (LDHs) containing Mg²⁺, Cu²⁺ or Zn²⁺ cations in the Me^II positions and Al³⁺ and Fe³⁺ in the Me^III positions were synthesized by co-precipitation method. Detailed studies of thermal transformation of obtained LDHs into metal oxide systems were performed using high temperature X-ray diffraction in oxidising and reducing atmosphere, thermogravimetry coupled with mass spectrometry and temperature-programmed reduction. The LDH samples calcined at 600 and 900 °C were tested in the role of catalysts for selective oxidation of ammonia into nitrogen and water vapour. It was shown that all copper congaing samples presented high catalytic activity and additionally, for the Cu–Mg–Al and Cu–Mg–Fe hydrotalcite samples calcined at 600 °C relatively high stability and selectivity to dinitrogen was obtained. An increase in calcination temperature to 900 °C resulted in a decrease of their catalytic activity, possibly due to formation of well-crystallised metal oxide phases which are less catalytically active in the process of selective oxidation of ammonia.     

Sensing mechanism elucidation of a europium(III) metal–organic framework selective to aniline: A theoretical insight by means of multiconfigurational calculations

A theoretical procedure, via quantum chemical computations, to elucidate the detection principle of the turn-off luminescence mechanism of an Eu-based Metal-Organic Framework sensor (Eu-MOF) selective to aniline, is accomplished. The energy transfer channels that take place in the Eu-MOF, as well as understanding the luminescence quenching by aniline, were investigated using the well-known and accurate multiconfigurational ab initio methods along with sTD-DFT. Based on multireference calculations, the sensitization pathway from the ligand (antenna) to the lanthanide was assessed in detail, that is, intersystem crossing (ISC) from the S₁ to the T₁ state of the ligand, with subsequent energy transfer to the ⁵D₀ state of Eu³⁺. Finally, emission from the ⁵D₀ state to the ⁷F_J state is clearly evidenced. Otherwise, the interaction of Eu-MOF with aniline produces a mixture of the electronic states of both systems, where molecular orbitals on aniline now appear in the active space. Consequently, a stabilization of the T₁ state of the antenna is observed, blocking the energy transfer to the ⁵D₀ state of Eu³⁺, leading to a non-emissive deactivation. Finally, in this paper, it was demonstrated that the host-guest interactions, which are not taken frequently into account by previous reports, and the employment of high-level theoretical approaches are imperative to raise new concepts that explain the sensing mechanism associated to chemical sensors.     

NiFe2O4@SiO2@Cu3(BTC)2 nanocomposite as a magnetic metal–organic framework

A new magnetic metal–organic framework (MOF), namely, NiFe₂O₄@SiO₂@Cu₃(BTC)₂, was synthesized via an in situ method using Fe(NO₃)₃, Ni(NO₃)₂, CuN₂O₆, TEOS, (3-aminopropyl)triethoxysilane, and benzene-1,3,5-tricarboxylic acid. Three different samples were fabricated according to a formula; xNiFe₂O₄@(100 − x)SiO₂@Cu₃(BTC)₂, where x = 10, 30, and 50. The integration of the intrinsic characteristic of Cu₃(BTC)₂ as an MOF with strong magnetic properties of NiFe₂O₄ could lead to an exquisite material with specific behaviors. X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), diffuse reflectance spectroscopy (DRS), photoluminescence (PL), vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), and simulated thermal analyzer (STA) were utilized to characterize the mentioned samples. Results approved that the synthesized compounds were composed of SiO₂ and Cu-MOF and NiFe₂O₄ crystalline phases with rod-like morphology. The similarity between the morphology of the synthesized samples and Cu-MOF approved that an appropriate fabrication method has been selected. This fact led to observe mesoporous composites with 38–90 m² g⁻¹ specific surface area. PL spectroscopy confirmed the near bandgap emission, ligand-to-metal charge transfer, and metal-to-ligand charge transfer. Although all the samples had magnetic hysteresis, the highest magnetization was seen in the 50NiFe₂O₄@SiO₂@Cu₃(BTC)₂ sample. This composite compound with a magnetization value of 2 emu g⁻¹ at 8000 Oe and a specific surface area of 90 m² g⁻¹ could be classified as a magnetic MOF (MMOF). STA results suggested that 400°C is the highest operating temperature for this compound.     

Efficient removal of As(V) from simulated arsenic-contaminated wastewater via a novel metal–organic framework material: Synthesis,structure, and response surface methodology

A novel metal–organic framework material {[N(C₂H₅)₃][Zn₂(ptmda)₂(μ₂-H₂O)]·(H₂O)_0.5}_n { GUT-3 ; H₂ptmda is 4,4′-([p-tolylazanediyl]bis [methylene])dibenzoic acid} was successfully synthesized using the hydrothermal method and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. GUT-3 has a two-dimensional network based on dinuclear [Zn₂(ptmda)₂(μ₂-H₂O)]⁻ building units which formed an eightfold interpenetration network in GUT-3 molecules. Hirshfeld surface analysis revealed that H–H, C–H, and O–H bonds accounted for the majority of intermolecular interactions. Moreover, the interactions between GUT-3 and As(V) – the form of As(V) is AsO₄³⁻ – were analyzed in aqueous solutions in a batch system to study the effect of pH, concentration, adsorbent dose, adsorption time, adsorption temperature, and shaking speed. The kinetic and isotherm data of arsenic adsorption on GUT-3 were accurately modeled by pseudo-second-order, Langmuir (q_m = 33.91 mg/g), and Freundlich models. The Box–Behnken response surface method was used to optimize the adsorption conditions of As(V) from the simulated arsenic-contaminated wastewater. The effect of various experimental parameters and optimal experimental conditions was ascertained using the quadratic model.     

Conversion of carbon dioxide to propionaldehyde over cobalt and rhodium nanoparticles supported on MIL-53 (Al) metal–organic framework

The two-step conversion of carbon dioxide to propionic acid and propionaldehyde has been studied in the presence of novel catalysts, cobalt and rhodium nanoparticles supported on MIL-53(Al) microporous metal–organic framework. The first step is hydrogenation of carbon dioxide with formation of synthesis gas over cobalt-containing catalyst Co/MIL-53(Al) (500°C, 1 atm), and the second step is continuous (without separation) Rh/MIL-53 (Al)-catalyzed hydroformylation of ethylene with the synthesis gas formed in the first step.     

Determination of urinary methylhippuric acids using MIL-53-NH2 (Al) metal–organic framework in microextraction by packed sorbent followed by HPLC–UV analysis

For the analysis of methylhippuric acids (MHAs) in human urine samples, in this study, a new method based on the metal–organic framework (MOF) of MIL-53-NH₂ (Al) in microextraction by packed sorbent (MEPS) was developed. The synthesis of MIL-53-NH₂ (Al) was characterized by Fourier transform infrared spectra, field emission-scanning electron microscopy and X-ray diffraction. Response surface methodology was used to investigate the influences of several parameters including type and volume of elution, type of conditional solvent, sample volume and extraction cycle on MEPS efficiency. The results showed good recoveries (>94%) and excellent extraction efficiencies (>96%) at three different concentrations of 50, 500 and 1500 μg ml⁻¹ (as low, mid and high concentrations, respectively) of MHA isomers. Calibration curves of MHAs were linear over the concentration range of 1–1500 μg ml⁻¹, with high correlation coefficients (r ≥ 0.998). The reproducibility of the proposed MIL-53-MEPS for determination of three isomers of MHA was found to be in the range of 3.5–11.1%. After optimization of the proposed technique, it was used to analyze MHAs in urine samples of workers exposed to xylenes in a petrochemical plant in Asalouyah, Iran. The results indicated that the MOF–MEPS method was selective, sensitive, rapid and efficient for the extraction of urinary MHAs. The technique is also environmentally friendly and inexpensive, and the MOF sorbent is reusable.     

Thermal decomposition of inclusion compounds on the base of the metal–organic framework [Zn4(dmf)(ur)2(ndc)4]

Inclusion compounds based on metal–organic frameworks (MOFs) have promising practical applications in gas storage, the separation, and fine purification of substances, and also in catalysis. These MOFs are crystalline compounds consisting of metal ions coordinated by bridging organic ligands with the formation of porous structures. We study the kinetic stability of two inclusion compounds on the base of the new framework: [Zn₄(dmf)(ur)₂(ndc)₄]·6C₆H₆ and [Zn₄(dmf)(ur)₂(ndc)₄]·5C₆H₅CH₃ (ndc^2? = 2,6-naphtalenedicarboxylate, ur = hexamethylentetramin, dmf = N,N′-dimethylformamide). The inclusion compound with benzene is more stable than the compound with toluene. The reduced stability of the toluene compound may be connected with the toluene molecule’s shape: the C₆H₅CH₃ molecule is more bulky and asymmetric than the C₆H₆ molecule; the MOF matrix structure must be greatly distorted to include the toluene molecules and the compound stability decreases.