Antibiotic Adsorption by Metal-Organic Framework (UiO-66): A Comprehensive Kinetic, Thermodynamic, and Mechanistic Study.

Bacterial antibiotic resistance has been deemed one of the largest modern threats to human health. One of the root causes of antibiotic resistance is the inability of traditional wastewater management techniques, such as filtration and disinfection, to completely eliminate residual antibiotics from domestic and industrial effluents. In this study, we examine the ability of UiO-66; a metal-organic framework (MOF); in removing the antibiotic Doxycycline from aqueous environments. This study's findings suggest that UiO-66 was able to remove nearly 90% of the initial Doxycycline concentration. To correlate the isothermal data, Langmuir and Freundlich models were used. It was determined that the Langmuir model was best suited. Pseudo-first and -second order models were examined for kinetic data, where the pseudo-second order model was best suited-consistent with the maximum theoretical adsorption capacity found by the Langumir model. Thermodynamic analysis was also examined by studying UiO-66 adsorption under different temperatures. Mechanisms of adsorption were also analyzed through measuring adsorption at varying pH levels, thermogravimetric analysis (TGA), Infrared spectroscopy (IR) and Brunauer-Emmet-Teller (BET). This study also explores the possibility of recycling MOFs through exposure to gamma radiation, heat, and heating under low pressure, in order for UiO-66 to be used in multiple, consecutive cycles of Doxycycline removal.

Amidation, Esterification, and Thioesterification of a Carboxyl-Functionalized Covalent Organic Framework.

Three new post-synthetic modification reactions, namely amidation, esterification, and thioesterification, were demonstrated on a novel highly crystalline two-dimensional covalent organic framework (COF), COF-616, bearing pre-installed carboxyl groups. The strategy can be used to introduce a large variety of functional groups into COFs and the modifications can be carried out under mild reaction conditions, with high yields, and an easy work-up protocol. As a proof of concept, various chelating functionalities were successfully incorporated into COF-616 to yield a family of adsorbents for efficient removal of several contaminants in the water.

Reticular Synthesis of Multinary Covalent Organic Frameworks.

Hexagonal hexaminophenyl benzene, tetragonal tetrakis(4-aminophenyl) ethane, and trigonal 1,3,5-tris(p-formylphenyl)benzene were all joined together by imine linkages to yield a 2D porous covalent organic framework with unprecedented tth topology, termed COF-346. Unlike the 5 simple existing 2D topologies reported in COFs, COF-346 has 3 kinds of vertices and 2 kinds of edges and is constructed with linkers of 3 kinds of connectivity, and thus represents a higher degree of complexity in COF structures. The success in crystallizing COF-346 was based on precisely chosen geometry and metrics of the linkers and error correction offered by dynamic imine formation. We also report two additional related COFs: a crystalline, porous COF, termed COF-360 with a rare kgd topology, as well as the first crystalline, porous COF with defected tth topology, termed COF-340.

3D Covalent Organic Frameworks of Interlocking 1D Square Ribbons.

A new mode of mechanical entanglement in extended structures is described where 1D organic ribbons of corner-sharing squares are mutually interlocked to form 3D woven covalent organic framework-500, COF-500. Reaction of aldehyde-functionalized tetrahedral Cu(PDB)2PO2Ph2 complexes (PDB = 4,4'-(1,10-phenanthroline-2,9-diyl)dibenzaldehyde) with rectangular tetratopic ETTBA (4',4‴,4''''',4''''‴-(ethene-1,1,2,2-tetrayl)tetrakis([1,1'-biphenyl]-4-amine)) linkers through imine condensation, yielded a crystalline porous metalated COF, COF-500-Cu, with pts topology. Upon removal of the Cu(I) ions, the individual 1D square ribbons in the demetalated form (COF-500) are held together only by mechanical interlocking of rings, which allows their collective movement to produce a narrow-pore form, as evidenced by nitrogen adsorption and solid-state photoluminescence studies. When exposed to tetrahydrofuran vapor, the interlocking ribbons can dynamically move away from each other to reopen up the structure. The structural integrity of COF-500 is maintained during its dynamics because the constituent square ribbons cannot part company due to spatial confinement imparted by their interlocking nature.

Bioinspired Metal-Organic Framework Catalysts for Selective Methane Oxidation to Methanol.

Particulate methane monooxygenase (pMMO) is an enzyme that oxidizes methane to methanol with high activity and selectivity. Limited success has been achieved in incorporating biologically relevant ligands for the formation of such active site in a synthetic system. Here, we report the design and synthesis of metal-organic framework (MOF) catalysts inspired by pMMO for selective methane oxidation to methanol. By judicious selection of a framework with appropriate topology and chemical functionality, MOF-808 was used to postsynthetically install ligands bearing imidazole units for subsequent metalation with Cu(I) in the presence of dioxygen. The catalysts show high selectivity for methane oxidation to methanol under isothermal conditions at 150 °C. Combined spectroscopies and density functional theory calculations suggest bis(µ-oxo) dicopper species as probable active site of the catalysts.

Crystalline Dioxin-Linked Covalent Organic Frameworks from Irreversible Reactions.

Triangular 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and linear tetrafluorophthalonitrile (TFPN) or 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile (TFPC) were linked by 1,4-dioxin linkages to form crystalline 2D covalent organic frameworks, termed COF-316 and -318. Unlike the condensation reactions commonly used to crystallize the great majority of COFs, the reactions used in this report are based on nucleophilic aromatic substitution reactions (SNAr) that are considered irreversible. Our studies show that the reactivity of TFPN and TFPC with HHTP is enhanced by the nitrile substituents leading to facile reactions of planar building units to yield the present 1,4-dioxin linked COFs. Because these reactions are irreversible, the resultant frameworks have high chemical stability in both acid and base. This has led to postsynthetic modifications of COF-316 by reactions necessitating extreme conditions to covalently install functionalities not otherwise accessible. We also report the permanent porosity of these COFs.

Impact of Disordered Guest-Framework Interactions on the Crystallography of Metal-Organic Frameworks.

It is a general and common practice to carry out single-crystal X-ray diffraction experiments at cryogenic temperatures in order to obtain high-resolution data. In this report, we show that this practice is not always applicable to metal-organic frameworks (MOFs), especially when these structures are highly porous. Specifically, two new MOFs are reported here, MOF-1004 and MOF-1005, for which the collection of the diffraction data at lower temperature (100 K) did not give data of sufficient quality to allow structure solution. However, collection of data at higher temperature (290 K) gave atomic-resolution data for MOF-1004 and MOF-1005, allowing for structure solution. We find that this inverse behavior, contrary to normal practice, is also true for some well-established MOFs (MOF-177 and UiO-67). Close examination of the X-ray diffraction data obtained for all four of these MOFs at various temperatures led us to conclude that disordered guest-framework interactions play a profound role in introducing disorder at low temperature, and the diminishing strength of these interactions at high temperatures reduces the disorder and gives high-resolution diffraction data. We believe our finding here is more widely applicable to other highly porous MOFs and crystals containing highly disordered molecules.