Metal-organic Frameworks Samples Research Articles

Adsorption of Humic Acid from Water Using Metal-Organic Frameworks (MOFs): An Effective Removal Strategy

The escalating presence of humic acid (HA) in aquatic systems poses significant ecological and potable water treatment challenges, altering the color, taste, and odor of water and fueling algal blooms that deplete oxygen levels to the detriment of aquatic life. Traditional techniques like coagulation-flocculation, membrane filtration, oxidation fall short in efficiency due to its small fraction of HA, Humic acids (HAs) can interact with various organic and inorganic substances, affecting their characteristics and behavior in water. These interactions can influence the efficiency of water treatment processes like adsorption and coagulation. Specifically, HAs may compete with other contaminants for treatment sites or alter the chemical conditions of the treatment processes, impacting their effectiveness. This thesis confronts the pervasive issue of HA contamination by leveraging the advanced properties of metal-organic frameworks (MOFs). In this work, four representative MOFs are synthesized and well characterized. Then, their capability to enhance coagulation processes in water for HA and HA-kaolin treatment are detailed evaluated. The findings from this research extend beyond academic curiosity, offering actionable insights for water treatment facilities and environmental management agencies. The synthesis of MOFs which are UiO-66(Zr), MIL-100(Fe), ZIF-8, and Al-fum. Each MOFs have been applied to remove HA by adsorption under different condition, effect of contact time, effect of initial pH, effect of adsorbent dosage. After adsorption experiment, MOF samples prepared to dry and went through characterization. Removal of HA by MOF adsorption showed UiO-66(Zr) and MIL-100(Fe) examined shortest reaching equilibrium state with 10-15 min, however, ZIF-8 and Al-fum gradually reached equilibrium state after 60 min. Among various pH conditions, UiO-66(Zr) and MIL-100(Fe) showed the highest adsorption capacity under acidic conditions, while ZIF-8 excelled at neutral pH, and Al-fum exhibited the highest adsorption capacity under both acidic and basic conditions. ZIF-8 has the highest adsorption capacity among the MOFs with capacity of 754 mg/g. MOF after adsorption samples went through characterizations and their remained characteristics indicating the adsorption not affected the structure of the MOFs.

Application of Raman and chemometrics to the mechanochemical synthesis of 4,4-bipyridine/cobalt based MOF using twin screw extrusion

The process optimization of 4,4-bipyridine based metal organic framework (MOF) synthesis by twin screw extrusion (TSE) is investigated using cobalt nitrate hexahydrate (Co(NO3)2·6H2O) as the metal precursor. The effects of operating parameters including liquid to solid ratio (L/S), screw speed and feeding rate on the product composition are examined. The collected MOF samples were characterized offline by powder X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, and scanning electron microscopy. It was found that the explored feeding and screw rates are insufficient to generate a pure MOF. However the MOF product was obtained using an optimum L/S ratio i.e., 0.5, which improved the wettability and mixing of reactants to allow effective interactions at a molecular level, facilitating the complete reaction inside the extruder. Partial least square based multivariate analysis of the collected Raman spectra was used to obtain predictive models to quantify MOF purity, which reached a maximum of 87.0 wt% using 0.5 L/S, 100 g/h feeding rate and 100 rpm screw speed.

Biodegradable Polymer‐Metal‐Organic Framework (MOF) Composites for Controlled and Sustainable Pesticide Delivery

AbstractDue to high surface area, loading capacity, and selectivity, Metal‐Organic Frameworks (MOFs) have shown much promise recently for potential applications in extraction and delivery of agrochemicals for environmental remediation and sustainable release, respectively. However, application of MOFs for pesticide delivery in wider agricultural context can be restricted by their granular form. Herein, an alternative approach is studied using biodegradable polymer‐MOF composites to address this limitation. The loading and release of a widely used pesticide, 2,4‐dichloropheoxycetic acid (2,4‐D), is studied using two MOFs, UiO‐66 and UiO‐66‐NH2, and the 2,4‐D‐loaded MOFs are incorporated into biodegradable polycaprolactone composites for convenient handling and minimizing runoff. The MOFs are loaded by in‐situ, and post‐synthetic methods, and characterised thoroughly to ensure successful synthesis and loading of 2,4‐D. The pesticide release studies are performed on the MOFs and composites in distilled water, and analysed using UV‐Vis spectroscopy, demonstrating sustained‐release of 2,4‐D over 16 days. The loaded MOF samples show high loading capacity, with up to 45 wt% for the in‐situ loaded UiO‐66. Release kinetics show more sustained release of 2,4‐D from UiO‐66‐NH2 compared to UiO‐66, which can be due to supramolecular interactions between the NH2 group of UiO‐66‐NH2 and 2,4‐D. This is further supported by computational studies.

A new far-UVC class tricarboxylate-Bi-based metal organic frameworks grown by hydrothermal method

The new class of Tricarboxylate-Bismuth (TB)-based Metal Organic Frameworks (MOFs) was attempted, using the impressive capability of alkaline earth metal Bismuth (Bi), to realize more massive complex structures for real-world applications including supercapacitors, energy storage devices, biomedical imaging, drug delivery, fluorescence sensing and far-UVC applications. The grown structure and band gap of the TB-based MOFs samples were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and Ultraviolet-Visible Absorption Spectroscopy (UV-Vis spectra), respectively. The SEM observation identifies the TB-based MOFs nanosheet layered with orthorhombic structure, which shows strong agglomeration with a grain size of 668 nm. The EDS analysis indicates the presence of Bi-peaks as well as carbon peaks (organic linkers) in the grown samples. Furthermore, the peak of TB-based MOFs structure during the Raman spectroscopy was also confirmed. The optical band gap of the newly synthesized TB-based MOFs was estimated and the optical band gap ([Formula: see text] approximately [Formula: see text][Formula: see text]eV was confirmed. These findings open the possibility of Bi-based MOFs for the applications in far-UVC emission range.

Aqueous Stability of Metal-Organic Frameworks Using ReaxFF-Based Metadynamics Simulations.

Aqueous stability is a critical property for the application of metal-organic framework (MOF) materials in humid conditions. The sampling of the free energy surface for a water reaction is challenging due to a lack of a reactive force field. Here, we developed a ReaxFF force field for simulating the reaction of zeolitic imidazole frameworks (ZIFs) with water. We carried out metadynamics simulations based on ReaxFF to study the reaction of water with a few different types of MOFs. We also conducted an experimental water immersion test and characterized the XRD, TG, and gas adsorption properties of the MOFs before and after the immersion test. By considering the energy barrier for a hydrolysis reaction, the simulation results are in good agreement with the experiments. MOFs with open structures and large pores are found to be unstable in metadynamics simulations, where the water molecule can attack or bond with the metallic node relatively easily. In contrast, it is more difficult for water to attack the Zn atom in the ZnN4 tetrahedral structure of ZIFs. We also found that ZIFs with the -NO2 functional groups have higher water stability. Discrepancies between the metadynamics simulation and gas adsorption experiments have been accounted for from the phase/crystallinity change of the structure reflected in the X-ray diffraction and thermogravimetry analysis of the MOF samples.

Drug Delivery on Mg-MOF-74: The Effect of Drug Solubility on Pharmacokinetics.

Biocompatible metal-organic frameworks (MOFs) have emerged as potential nanocarriers for drug delivery applications owing to their tunable physiochemical properties. Specifically, Mg-MOF-74 with soluble metal centers has been shown to promote rapid pharmacokinetics for some drugs. In this work, we studied how the solubility of drug impacts the pharmacokinetic release rate and delivery efficiency by impregnating various amounts of ibuprofen, 5-fluorouracil, and curcumin onto Mg-MOF-74. The characterization of the drug-loaded samples via X-ray diffraction (XRD), N2 physisorption, and Fourier transform infrared (FTIR) confirmed the successful encapsulation of 30, 50, and 80 wt % of the three drugs within the MOF structure. Assessment of the drug delivery performances of the MOF under its various loadings via HPLC tests revealed that the release rate is a direct function of drug solubility and molecular size. Of the three drugs considered under fixed loading condition, the 5-fluorouracil-loaded MOF samples exhibited the highest release rate constants which was attributed to the highest degree of solubility and smallest molecular size of 5-fluorouracil relative to ibuprofen and curcumin. It was also noted that the release kinetics decreases with drug loading, due to a pharmacokinetic shift in release mechanism from singular to binary modes of compound diffusion. The findings of this study highlight the effects of drug's physical and chemical properties on the pharmacokinetic rates from MOF nanocarriers.

High-Throughput Screening of Metal–Organic Frameworks Assisted by Machine Learning: Propane/Propylene Separation

The separation of a propane (C3H8)/propylene(C3H6) mixture is of paramount importance in the petrochemical industry. Metal–organic frameworks (MOFs), as a class of promising alternative to the traditional adsorbents, have garnered extensive interest. This study proposes a machine learning-assisted high-throughput screening strategy for the identification of suitable MOFs for C3H8/C3H6 separation, striving to accelerate the discovery of high-performance MOF candidates for this particular application. First, a chemical/geometric analysis-based prescreening is applied to a data set of 146 203 MOFs composed of an experimentally synthesized MOF database and a hypothetical MOF database, and MOFs with undesirable chemical/geometric features were excluded. Six structural and nine chemical descriptors were calculated for the remaining MOFs. Random Forest regression algorithm was applied to “learn” the relationship correlations between the features (chemical and/or structural) of MOFs and their C3H8/C3H6 separation capacity. Grand Canonical Monte Carlo (GCMC) simulations were applied to evaluate the C3H8/C3H6 separation performances of the randomly selected training and testing MOF samples. A performance prediction model based on chemical and structural descriptors was obtained with R2 equal to 0.96, which was employed for a separation performance prediction of the remaining MOFs. 2500 MOFs with potential to possess high C3H8/C3H6 separation performance were shortlisted by the prediction model. GCMC simulations were applied to calibrate the prediction results and validate of the machine learning model. MOFs with competitively high C3H8/C3H6 separation potential and regenerability were identified, and a comparison with MOFs reported in the literature was made, indicating that the proposed machine learning-assisted high-throughput screening approach is efficient and effective. Furthermore, structure–property correlation analysis was conducted.

Preparation of cerium-based UiO-66 metal-organic framework (MOF) without addition of solvent for developing its sustainable synthesis

UiO-66(Ce) metal-organic framework (MOF) has been a promising material for broad applications, including gas adsorption, photocatalysis, water treatment and reaction catalysis. For its usual preparation, a large amount of solvent is required and added during its fabrication process. This solvent is harmful to the environment most of the time. It is neither recyclable nor reusable, indicating the difficulty in waste disposal after MOF fabrication and the challenges for its scaling-up production. Given this, a solvent-free way of synthesizing the UiO-66 MOF to develop its sustainable synthesis was reported here. The MOF can be obtained by mechanically grinding the cerium (IV) ammonium nitrate (CAN) and terephthalic acid (BDC) using mortar and pestle without the addition of any solvent. Then the mixture was transferred into an autoclave and heated at 100 °C for one day. In this work, UiO-66(Ce) MOF samples with varying Ce to BDC molar ratios were successfully fabricated using the solvent-free method, supported by the XRD, SEM, EDX and FT-IR analysis results. By looking at their XRD patterns, two characteristic peaks for UiO-66(Ce) MOF were observed at 8.4° and 9.1°. The SEM images revealed the spherical crystalline morphology of UiO-66 MOF. At the same time, the EDX analysis result found the elemental composition of UiO-66 MOF was coherent with the reactants used, suggesting the excellent purity of MOF product formed in this work. As verified by the thermal stability test, the UiO-66 MOF produced here is thermally stable up to 250 °C in the air atmosphere.

Efficient separation of propane and propylene by nanocrystals of a metal–organic framework-based splitter

The separation of propane/propylene mixtures to produce polymer-grade propylene represents an important yet challenging task in the petrochemical industry. Metal-organic frameworks (MOFs) hold particular promise for the energy-efficient adsorptive separation of propane/propylene, in light of their structural diversity as well as highly tunable pore dimensions. MOFs exhibiting selective molecular exclusion for propylene over propane render exceptionally high adsorption selectivity. However, the relatively low adsorption capacity and/or slow diffusion kinetics have largely limited their separation efficiency. Here we report the fabrication of nano-sized crystals of a MOF-based splitter for propane and propylene. Compared to the micro-sized MOF samples, the nanocrystals display enhanced adsorption kinetics as well as notably improved adsorption capacity, making a new record with the highest efficiency for the separation of propane/propylene among all reported MOF-based splitters. Our study may shed light on the development of MOF-based adsorbents with optimal efficiency for industrially challenging gas separations.

Engineering a hierarchical carbon supported magnetite nanoparticles composite from metal organic framework and graphene oxide for lithium-ion storage

Fe based metal organic framework (MOF) materials are being extensively investigated as a precursor sample for engineering carbon supported iron containing nanoparticles composites. Rational design and engineering Fe-containing MOFs with optimized structures using economic and eco-friendly methods is a challenging task. In this work, 1,3,5-benzenetricarboxylic acid (C9H6O6, trimesic acid, H3BTC) and metal Fe are employed to synthesize a MOF sample Fe-BTC in a mild hydrothermal condition. Moreover, with the addition of a small quantity of graphene oxide (GO) as dispersant, a redox coprecipitation reaction has taken place where small Fe-BTC domains well dispersed by reduced graphene oxide (RGO). The Fe-BTC/RGO intermediate sample is finally converted to the hierarchical Fe3O4@C/RGO composite, which delivers an ultrahigh specific capacity of 1262.61 mAh·g−1 at 200 mA·g−1 after 150 cycles and a superior reversible capacity of 910.65 mAh·g−1 at 1000 mA·g−1 after 300 cycles in half cells. The full cell performance for the Fe3O4@C/RGO composite have been studied. It is also revealed that the improved structural stability, high pseudocapacitive contribution and enhanced lithium-ion and electron transportation conditions jointly guarantee the outstanding lithium-ion storage performances for the Fe3O4@C/RGO composite over long-time cycling. The synthesized samples have good potential for wider application.

Light Harvesting Antenna Properties of Framework Solids

ConspectusEnergy transport within organic solids typically takes place via the movement of localized excitation or excitons, i.e., tightly bound electron–hole pairs. Exciton diffusion dynamics are crucial to applications like solar fuels production through photocatalysis and organic optoelectronics. Covalent organic frameworks (COFs) and metal–organic frameworks (MOFs) are classes of framework solids that integrate ordered assemblies of chromophores. These materials possess properties desirable for the aforementioned applications. Their absorption cross sections across the ultraviolet–visible spectrum can be tuned via linker design. They are often crystalline and provide control over the framework topology in terms of spatial separation and relative orientation of chromophores. These properties allow for detailed understanding and tuning of structure–activity relationships.In this Account, we highlight our recent studies on photophysics and photonics of MOFs and COFs. Three materials are discussed with the focus on structural factors that govern the exciton properties in terms of diffusion, delocalization, and transport. The first is COF-5, a layered, two-dimensional COF with boronate ester linkages. We show that the molecular excitons in COF-5 are delocalized over multiple constituent building blocks with optical signatures of mixed (e.g., H- and J-type) excitonic interactions. The exciton of the S1 state decays and evolves into an excimer trap state in a few picoseconds. The excitons in COF-5 possess a diffusivity of about 0.02 cm2/s and a diffusion radius of about 5 nm based on exciton–exciton annihilation studies. To investigate the range of exciton diffusion, parallel exciton dynamics measurements were performed on COF-5 colloids with varying crystallite size, revealing that exciton diffusion is limited by crystallite size. The second system studied is an imine-linked two-dimensional COF with a hexagonal structure, the TAPB-DiOMe COF. To understand the photophysics of this material, we extended the study toward isomeric materials with a lower dimensionality: cofacially stacked nanotubes and free macrocycles assembled from the same primary building blocks. In-plane exciton delocalization and movement cannot occur in these lower dimensional species. We find that the exciton lifetime is prolonged significantly in the π-stacked nanotubes where the macrocycles undergo planarization that restricts atomic movements of the imine bonds in the peripheral groups, which may reduce the rate of internal conversion. In addition, orientation-dependent anisotropic energy transfer through the nanotubes was detected via optical transient absorption anisotropy. The third system discussed is the MOF NDC-NU-1000 decorated with single-atom metal-oxy (i.e., mix of oxo, hydroxo, and/or aqua) TiIV, CoIII, and NiII species mimicking single-atom catalysts (SACs). These composite materials display photoinduced charge separation. We show the formation of charge-separated states in these M-oxy decorated MOF samples upon photoexcitation with an electron transferred from the MOF linker to the M-oxy moiety. We compare and rationalize the differences in photoinduced charge separation kinetics among these M-oxy decorated MOFs and hope to identify energetic and structural factors that make the material an effective light harvesting platform and facilitate formation of the charge-separated state. Finally, we review our progress in context of the research published previously in the field and provide our perspective on the future of crystalline porous assemblies serving as light-harvesting antennae.

The ideal treatment for heart failer

The acid-base characteristics of the catalyst material are frequently linked to catalytic activity. Metal-organic frameworks (MOFs) have been tested as catalysts for a variety of processes. Based on the observed performance in diverse catalytic processes, Cu-BTC (with a well-defined crystalline structure) and Fe-BTC (with a gel-like porous structure) have both shown the presence of Lewis surface acid sites. In this study, researchers used Inverse Gas Chromatography (IGC) and n-butylamine thermal desorption analysis to assess the acid-base characteristics of both materials. Using acidic, basic, and amphoteric molecules, IGC experiments can access the acid-base characteristics of the material surface. The n-butylamine thermal desorption measurements, on the other hand, allow quantification of those acid and basic sites from the n-butylamine desorption curves of a sample previously saturated with that probe molecule. The Lewis acid properties obtained by both methodologies showed to be more pronounced for the Fe-BTC MOF sample. The IGC measurements exhibited values of acidity constants (KA) of 0.81 and 0.60 for Fe-BTC and Cu-BTC, respectively. Furthermore the n-butylamine thermo desorption experiments indicate total acidities of 2.06 and 0.49 mmol g−1 for Fe-BTC and Cu-BTC, respectively.

A novel 2‐D accordion like Al‐BPED MOF as reusable and selective catalyst for N‐alkylation of amines with dialkylcarbonates

A novel 2‐D flakes like Al‐BPED ((N1,N2‐bis [pyridine‐2‐yl]methyl)ethane‐1,2‐diamine) metal–organic framework (MOF) have been explored for its catalytic potential in the N‐alkylation of amines with dialkyl carbonates. The Al‐BPED MOF catalyst has been synthesized by hydrothermal route. The X‐ray diffraction, Fourier Transform Infrared Spectroscope (FTIR), and High Resolution Transmission Electron Microscopy (HRTEM) images were used to analyze the morphology of the synthesized Al‐BPED MOF samples. With the use of Al‐BPED MOF, the primary and secondary aromatic amines were selectively and efficiently methylated with dimethyl carbonate. The effect of process conditions has been investigated toward N‐methylation, N‐ethylation, and N‐benzylation of aromatic amines. Under the optimal conditions with 10 mol% of the MOF catalyst, a maximum aniline conversion of 88% and a selectivity >90% toward N‐alkylation was achieved. The kinetic studies revealed the N‐alkylation reaction in presence of Al‐BPED MOF followed pseudo‐first order rate kinetics.

How to get maximum structure information from anisotropic displacement parameters obtained by three-dimensional electron diffraction: an experimental study on metal–organic frameworks

Three-dimensional electron diffraction (3D ED) has been used for ab initio structure determination of various types of nanocrystals, such as metal-organic frameworks (MOFs), zeolites, metal oxides and organic crystals. These crystals are often obtained as polycrystalline powders, which are too small for single-crystal X-ray diffraction (SCXRD). While it is now possible to obtain accurate atomic positions of nanocrystals by adopting kinematical refinement against 3D ED data, most new structures are refined with isotropic displacement parameters (U eq), which limits the detection of possible structure disorders and atomic motions. Anisotropic displacement parameters (ADPs, Uij ) obtained by anisotropic structure refinement, on the other hand, provide information about the average displacements of atoms from their mean positions in a crystal, which can provide insights with respect to displacive disorder and flexibility. Although ADPs have been obtained from some 3D ED studies of MOFs, they are seldom mentioned or discussed in detail. We report here a detailed study and interpretation of structure models refined anisotropically against 3D ED data. Three MOF samples with different structural complexity and symmetry, namely ZIF-EC1, MIL-140C and Ga(OH)(1,4-ndc) (1,4-ndcH2 is naphthalene-1,4-dicarboxylic acid), were chosen for the studies. We compare the ADPs refined against individual data sets and how they are affected by different data-merging strategies. Based on our results and analysis, we propose strategies for obtaining accurate structure models with interpretable ADPs based on kinematical refinement against 3D ED data. The ADPs of the obtained structure models provide clear and unambiguous information about linker motions in the MOFs.

Synthesis of MIL-101(Cr) Metal Organic Framework by Green Synthesis for CO2 Gas Adsorption

Global warming issue due to the excessive carbon dioxide gas emission have raised strong interest in capturing or reducing the CO2 from flue gas or atmosphere. Physisorption-based CO2 capture applying the metal organic framework (MOF) provides a promising alternative for capturing CO2 due to the simplicity, low operating cost, and low energy requirement of the adsorption approach combined with the high CO2 adsorption capacity MOF material. In this study, a series of Chromium based MIL MOF with a variety molar ratio of chromium metal to 1,4-Benzene Dicarboxylate (BDC) organic linker were prepared via the solvent-free method (mechanochemical) to develop a clean and efficient way of synthesising MOF samples as promising CO2 adsorbents. The XRD results and FTIR spectra have confirmed the successful fabrication of MIL-101(Cr) MOF using the solvent-free method. The SEM images illustrated fine growth of irregular shaped coarse particles for Cr MOF with equal mole ratio of Cr to BDC. The MIL-101(Cr) samples were also tested on their CO2 adsorption capacity to understand the influence of molar ratio of the starting materials on the CO2 adsorption capacity. It was found that the MIL-101(Cr)1 led to the formation of a product with the highest CO2 uptake capacity of 18.78 mmol/g. In contrast, the EDS analysis result revealed that all the samples synthesised in this work were well incorporated with the Chromium element. It is therefore suggested that the molar ratio of Cr to BDC plays a critical role in determining the CO2 gas adsorption capacity.

Kinetically Controlled Linker Binding in Rare Earth-2,5-Dihydroxyterepthalic Acid Metal-Organic Frameworks and Its Predicted Effects on Acid Gas Adsorption.

In the pursuit of highly stable and selective metal-organic frameworks (MOFs) for the adsorption of caustic acid gas species, an entire series of rare earth MOFs have been explored. Each of the MOFs in this series (RE-DOBDC; RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; DOBDC = 2,5-dihydroxyterepthalic acid) was synthesized in the tetragonal space group I4/m. Crystallized MOF samples, specifically Eu-DOBDC, were seen to have a combination of monodentate and bidentate binding when synthesized under typical reaction conditions, resulting in a contortion of the structure. However, extended crystallization times determined that this binding is kinetically controlled and that the monodentate binding option was crystallographically eliminated by extended reaction times at higher temperatures. Furthermore, this series allows for the direct study of the effect of the metal center on the structure of the of the MOF; herein, the lanthanide metal ionic radii contraction across the periodic table results in a reduction of the MOF pore size and lattice parameters. Scanning electron microscopy-energy-dispersive spectroscopy was used to investigate the stages of crystal growth for these RE-DOBDC MOFs. All MOFs, except Er-DOBDC had a minimum of two stages of growth. These analogues were demonstrated by analysis of neutron diffraction (PND) to exhibit a cooperative rotational distortion of the secondary building unit, resulting in two crystallographically distinct linker sublattices. Computational modeling efforts were used to show distinct differences on acid gas (NO2 and SO2) binding energies for RE-DOBDC MOFs when comparing the monodentate/bidentate combined linker with the bidentate-only linker crystal structures.

A Two Step Postsynthetic Modification Strategy: Appending Short Chain Polyamines to Zn-NH2-BDC MOF for Enhanced CO2 Adsorption.

Functionalizing metal-organic frameworks (MOFs) with amines is a commonly used strategy to enhance their performance in CO2 capture applications. As such, in this work, a two-step strategy to covalently functionalize NH2-containing MOFs with short chain polyamines was developed. In the first step, the parent MOF, Zn4O(NH2-BDC)3, was exposed to bromoacetyl bromide (BrAcBr), which readily reacts with pendant -NH2 groups on the 2-amino-1,4-benzenedicarboxylate (NH2-BDC2-) ligand. 1H NMR of the digested MOF sample revealed that as much as 90% of the MOF ligands could be functionalized in the first step. Next, the MOF samples 60% of the ligands functionalized with acetyl bromide, Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, was exposed to several short chain amines including ethylenediamine (ED), diethylenetriamine (DETA), and tris(2-aminoethyl)amine (TAEA). Subsequent digested 1H NMR analysis indicated that a total of 30%, 28%, and 19% of the MOF ligands were successfully grafted to ED, DETA, and TAEA, respectively. Next, the CO2 adsorption properties of the amine grafted MOFs were studied. The best performing material, TAEA-appended-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, exhibits a zero-coverage isosteric heat of CO2 adsorption of -62.5 kJ/mol, a value that is considerably higher than the one observed for the parent framework, -21 kJ/mol. Although the boosted CO2 affinity only leads to a slight increase in the CO2 adsorption capacity in the low-pressure regime (0.15 bar), which is of interest in postcombustion carbon dioxide capture, the CO2/N2 (15/85) selectivity at 313 K is 143, a value that is ∼35 times higher than the one observed for Zn4O(NH2-BDC)3, 4.1. Such enhancements are attributed to accessible primary amines, which were grafted to the MOF ligand. This hypothesis was further supported via in situ DRIFTS measurements of TAEA-Ac-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8 after exposure to CO2, which revealed the chemisorption of CO2 via the formation of hydrogen bonded carbamates/carbamic acid and CO2δ- species; the latter are adducts formed between CO2 and [amineH]+Br- salts that are produced during the amine grafting step.

Bimetallic-atom-hybridization-driven catalytic reaction kinetics and solar utilization to accelerate norfloxacin degradation

The high level of norfloxacin (NOR) in aquatic environment causes bacterial drug resistance and perilous effects on human and aquatic life because of the greater toxicity and the oxidation-refractory property. Herein, we propose a series of porous metal-organic frameworks (MOFs) via a developed solvothermal process to accelerate NOR degradation, in which the solar response and catalytic activity is sharply enhanced through an electronic hybridization between Fe3+ and Co2+ metal center. The time-resolved photoluminescence spectra disclose that electrostatic potential induced by electronegativity difference in bimetallic sites leads to an efficient separation of electron-hole pairs. As a result, the optimized bimetallic MOF sample makes the NOR concentration go down to ∼20 % within 30 min under sunlight radiation, keeping an unchanged activity after 9 cycles. Our findings provide new inspirations for regulating the photogenic carrier dynamics via bimetallic hybridization strategy in accelerating NOR degradation.

Thermochemical Investigation of Oxyanion Coordination in a Zirconium-Based Metal-Organic Framework.

Porous materials possess high internal surface areas and void fractions that make them valuable in several applications, including gas storage, heterogeneous catalysis, and water purification. Despite the plentiful effort allocated to porous materials research annually, few methods exist to directly monitor and characterize chemical events occurring within a pore's confines. The crystalline nature of zeolites, covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) permit structural characterization by X-ray diffraction; yet, quantifying the thermodynamics of chemical processes and transformations remains tedious and error ridden. Herein, we employ isothermal titration calorimetry (ITC) to determine the full thermodynamic profile of oxyanion adsorption in a zirconium-based MOF, NU-1000. To further validate this method, which we recently introduced to the field, we replicated ITC experiments as bulk adsorption measurements to demonstrate the correlation between the extracted stoichiometric parameter from ITC thermograms and the MOF uptake capacity. Moreover, based on the calculated association constants, we accurately predicted which analytes might be able to displace others. For example, dihydrogen phosphate can displace selenate and sulfate because of its higher association constant (ΔGphosphate = -5.41 kcal/mol; ΔGselenate = -4.98 kcal/mol; ΔGsulfate = -4.77 kcal/mol). We monitored the exchange processes by titrating oxyanion-functionalized MOF samples with a more strongly binding analyte.

XGBoost: An Optimal Machine Learning Model with Just Structural Features to Discover MOF Adsorbents of Xe/Kr.

The inert gases Xe and Kr mainly exist in the used nuclear fuel (UNF) with the Xe/Kr ratio of 20:80, which it is difficult to separate. In this work, based on the G-MOFs database, high-throughput computational screening for metal–organic frameworks (MOFs) with high Xe/Kr adsorption selectivity was performed by combining grand canonical Monte Carlo (GCMC) simulations and machine learning (ML) technique for the first time. From the comparison of eight classical ML models, it is found that the XGBoost model with seven structural descriptors has superior accuracy in predicting the adsorption and separation performance of MOFs to Xe/Kr. Compared with energetic or electronic descriptors, structural descriptors are easier to obtain. Note that the determination coefficients R2 of the generalized model for the Xe adsorption and Xe/Kr selectivity are very close to 1, at 0.951 and 0.973, respectively. In addition, 888 and 896 MOFs have been successfully predicted by the XGBoost model among the top 1000 MOFs in adsorption capacity and selectivity by GCMC simulation, respectively. According to the feature engineering of the XGBoost model, it is shown that the density (ρ), porosity (ϕ), pore volume (Vol), and pore limiting diameter (PLD) of MOFs are the key features that affect the Xe/Kr adsorption property. To test the generalization ability of the XGBoost model, we also tried to screen MOF adsorbents on the CO2/CH4 mixture, it is found that the prediction performance of XGBoost is also much better than that of the traditional machine learning models although with the unbalanced data. Note that the dimension of features of MOFs is low while the quantity of MOF samples in database is very large, which is suitable for the prediction by model such as XGBoost to search the global minimum of cost function rather than the model involving feature creation. The present study represents the first report using the XGBoost algorithm to discover the MOF adsorbates.