Postsynthetic Modification of a Metal–Organic Framework for Stabilization of a Hemiaminal and Ammonia Uptake

Comparison of Surface-Bound and Free-Standing Variations of HKUST-1 MOFs: Effect of Activation and Ammonia Exposure on Morphology, Crystallinity, and Composition

Metal-organic frameworks (MOFs) are extremely porous, crystalline materials with high surface area for potential use in gas storage, sequestration, and separations. Toward incorporation into structures for these applications, this study compares three variations of surface-bound and free-standing HKUST-1 MOF structures: surface-anchored MOF (surMOF) thin film, drop-cast film, and bulk powder. Herein, effects of HKUST-1 ammonia interaction and framework activation, which is removal of guest molecules via heat, are investigated. Impact on morphology and crystal structure as a function of surface confinement and size variance are examined. Scanning probe microscopy, scanning electron microscopy, powder X-ray diffraction, Fourier-transform infrared spectroscopy, and energy dispersive X-ray spectroscopy monitor changes in morphology and crystal structure, track ammonia uptake, and examine elemental composition. After fabrication, ammonia uptake is observed for all MOF variations, but reveals dramatic morphological and crystal structure changes. However, activation of the framework was found to stabilize morphology. For activated surMOF films, findings demonstrate consistent morphology throughout uptake, removal, and recycling of ammonia over multiple exposures. To understand morphological effects, additional ammonia exposure experiments with controlled post-synthetic solvent adsorbates were conducted utilizing a HKUST-1 standard powder. These findings are foundational for determining the capabilities and limitation of MOF films and powders.

Mixed-Valence Cobalt(II/III) Metal-Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH₃) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt(II/III) metal-organic framework (FJU-56) with tris-(4-tetrazolyl-phenyl)amine (H₃L) ligand was synthesized for colorimetric sensing toward ammonia. The activated FJU-56 demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH₃ or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the FJU-56 framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing.

Metal-Organic Frameworks with Internal Urea-Functionalized Dicarboxylate Linkers for SO2 and NH3 Adsorption

Introduction of a urea R-NH-CO-NH-R group as a seven-membered diazepine ring at the center of 4,4'-biphenyl-dicarboxylic acid leads to a urea-functionalized dicarboxylate linker (L1^2-) from which four zinc metal-organic frameworks (MOFs) could be obtained, having a {Zn₄(μ₄-O)(O₂C-)₆} SBU and IRMOF-9 topology (compound [Zn₄(μ₄-O)(L1)₃], 1, from dimethylformamide, DMF) or a {Zn₂(O₂C-)₄} paddle-wheel SBU in a 2D-network (compound [Zn₂(L1)₂(DEF)₂·2.5DEF], 2, from diethylformamide, DEF). Pillaring of the 2D-network of 2 with 4,4'-bipyridine (bipy) or 1,2-bis(4-pyridyl)ethane (bpe) gives 3D frameworks with rhombohedrally distorted pcu-a topologies ([Zn₂(L1)₂(bipy)], 3 and [Zn₂(L1)₂(bpe)], 4, respectively). The 3D-frameworks 1, 3, and 4 are 2-fold interpenetrated with ∼50% solvent-accessible volume, albeit of apparently dynamic porous character, such that N₂ adsorption at 77 K does not occur, while H₂ at 77 K (up to ∼1 wt %) and CO₂ at 293 K (∼5 wt %) are adsorbed with large hystereses in these flexible MOFs. The urea-functionalized MOF 3 exhibits an uptake of 10.9 mmol g^-1 (41 wt %) of SO₂ at 293 K, 1 bar, which appears to be the highest value observed so far. Compounds 3 and 4 adsorb 14.3 mmol g^-1 (20 wt %) and 17.8 mmol g^-1 (23 wt %) NH₃, respectively, which is at the top of the reported values. These high uptake values are traced to the urea functionality and its hydrogen-bonding interactions to the adsorbents. The gas uptake capacities follow the specific porosity of the frameworks, in combination with pore aperture size and affinity constants from fits of the adsorption isotherms.

Novel Metal-Organic Framework (MOF) Based Composite Material for the Sequestration of U(VI) and Th(IV) Metal Ions from Aqueous Environment

The combination of magnetic nanoparticles and metal-organic frameworks (MOFs) has demonstrated their prospective for pollutant sequestration. In this work, a magnetic metal-organic framework nanocomposite (Fe₃O₄@AMCA-MIL53(Al) was prepared and used for the removal of U(VI) and Th(IV) metal ions from aqueous environment. Fe₃O₄@AMCA-MIL53(Al) nanocomposite was characterized by TGA, FTIR, SEM-EDX, XRD, HRTEM, BET, VSM (vibrating sample magnetometry), and XPS analyses. A batch technique was applied for the removal of the aforesaid metal ions using Fe₃O₄@AMCA-MIL53(Al) at different operating parameters. The isotherm and kinetic data were accurately described by the Langmuir and pseudo-second-order models. The adsorption capacity was calculated to be 227.3 and 285.7 mg/g for U(VI) and Th(IV), respectively, by fitting the equilibrium data to the Langmuir model. The kinetic studies demonstrated that the equilibrium time was 90 min for each metal ion. Various thermodynamic parameters were evaluated which indicated the endothermic and spontaneous nature of adsorption. The collected outcomes showed that Fe₃O₄@AMCA-MIL53(Al) was a good material for the exclusion of these metal ions from aqueous medium. The adsorbed metals were easily recovered by desorption in 0.01 M HCl. The excellent adsorption capacity and the response to the magnetic field made this novel material an auspicious candidate for environmental remediation technologies.

Au3+/Au⁰ Supported on Chromium(III) Terephthalate Metal Organic Framework (MIL-101) as an Efficient Heterogeneous Catalystfor Three-Component Coupling Synthesis of Propargylamines

Post-synthesis modification is a useful method for the functionalization of metal–organic frameworks (MOFs). A novel catalyst Au@MIL-101-ED-SA (ED = ethylenediamine, SA = salicylaldehyde), containing coexisting Au³⁺ ions and Au⁰ nanoparticles, was prepared successfully by post-synthesis modification with ethylenediamine, salicylaldehyde and gold. Gold nanoparticles supported on MIL-101 (Au@MIL-101) were prepared successfully by the impregnation method. Au@MIL-101-ED-SA and Au@MIL-101 were characterized by N₂ adsorption–desorption, X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectrometry. Au@MIL-101-ED-SA and Au@MIL-101 were applied as environmentally friendly catalysts in the three-component coupling reaction of aldehydes, amines, and alkynes for the preparation of diverse propargylamines. Au@MIL-101-ED-SA contained a fraction of cationic gold (Au³⁺/Au⁰ = 0.9) and showed higher catalytic activity than Au@MIL-101, which was prepared by the impregnation method. Furthermore, the reactions were performed under heterogeneous conditions and the novel catalyst was successfully recycled for four consecutive runs.

Advances in Solid-State Transformations of Coordination Bonds: From the Ball Mill to the Aging Chamber

Controlling the formation of coordination bonds is pivotal to the development of a plethora of functional metal-organic materials, ranging from coordination polymers, metal-organic frameworks (MOFs) to metallodrugs. The interest in and commercialization of such materials has created a need for more efficient, environmentally-friendly routes for making coordination bonds. Solid-state coordination chemistry is a versatile greener alternative to conventional synthesis, offering quantitative yields, enhanced stoichiometric and topological selectivity, access to a wider range of precursors, as well as to molecules and materials not readily accessible in solution or solvothermally. With a focus on mechanochemical, thermochemical and “accelerated aging” approaches to coordination polymers, including pharmaceutically-relevant materials and microporous MOFs, this review highlights the recent advances in solid-state coordination chemistry and techniques for understanding the underlying reaction mechanisms.

MOFs vs. zeolites: carbonyl activation with M(iv) catalytic sites

Comparative review of the catalytic performance of isolated tetravalent metals as Lewis acid sites in zeolite and MOF materials.

High and Reversible Ammonia Uptake in Mesoporous Azolate Metal-Organic Frameworks with Open Mn, Co, and Ni Sites

A series of new mesoporous metal-organic frameworks (MOFs) made from extended bisbenzenetriazolate linkers exhibit coordinatively unsaturated metal sites that are responsible for high and reversible uptake of ammonia. Isostructural Mn, Co, and Ni materials adsorb 15.47, 12.00, and 12.02 mmol of NH3/g, respectively, at STP. Importantly, these near-record capacities are reversible for at least three cycles. These results demonstrate that azolate MOFs are sufficiently thermally and chemically stable to find uses in recyclable sorption, storage, and potentially separation of chemically challenging and/or corrosive gases, especially when designed to exhibit a high density of open metal sites.

Tuning the properties of the metal–organic framework UiO-67-bpy via post-synthetic N-quaternization of pyridine sites

N-Quaternization of pyridine sites in UiO-67-bpy affords a cationic MOF UiO-67-bpy-Me with faster and improved anionic dye and CO₂adsorption, and extended visible-light absorption properties, which make UiO-67-bpy-Me more efficient in photodegrading organic dyes.

A MOF-membrane based on the covalent bonding driven assembly of a NMOF with an organic oligomer and its application in membrane reactors

Fabrication of a covalent bonding driven UiO-66-MOF-based membrane, which is generated from the self-assembly of NMOFs with organic oligomer SBUs, and its application in membrane reactors are reported.

SO3H-functionalized metal organic frameworks: an efficient heterogeneous catalyst for the synthesis of quinoxaline and derivatives

The high dispersed –SO₃H groups in the framework of MIL-101-Cr–NH–RSO₃H ensure high catalytic activity for the condensation of 1,2-diamines with benzil. The catalyst exhibits good stability, general applicability and excellent recycling performance.

Light driven mesoscale assembly of a coordination polymeric gelator into flowers and stars with distinct properties

Control over the self-assembly process of porous organic-inorganic hybrids often leads to unprecedented polymorphism and properties. Herein we demonstrate how light can be a powerful tool to intervene in the kinetically controlled mesoscale self-assembly of a coordination polymeric gelator. Ultraviolet light induced coordination modulation via photoisomerisation of an azobenzene based dicarboxylate linker followed by aggregation mediated crystal growth resulted in two distinct morphological forms (flowers and stars), which show subtle differences in their physical properties.

Combination of Optimization and Metalated-Ligand Exchange: An Effective Approach to Functionalize UiO-66(Zr) MOFs for CO2 Separation

The strategy to functionalize water-stable metal-organic frameworks (MOFs) in order to improve their CO2 uptake capacities for efficient CO2 separation remains limited and challenging. We herein present an effective approach to functionalize a prominent water-stable MOF, UiO-66(Zr), by a combination of optimization and metalated-ligand exchange. In particular, by systematic optimization, we have successfully obtained UiO-66(Zr) of the highest BET surface area reported so far (1730 m(2)  g(-1) ). Moreover, it shows a hybrid Type I/IV N2 isotherm at 77 K and a mesopore size of 3.9 nm for the first time. The UiO-66 MOF underwent a metalated-ligand-exchange (MLE) process to yield a series of new UiO-66-type MOFs, among which UiO-66-(COONa)2 -EX and UiO-66-(COOLi)4 -EX MOFs have both enhanced CO2 working capacity and IAST CO2 /N2 selectivity. Our approach has thus suggested an alternative design to achieve water-stable MOFs with high crystallinity and gas uptake for efficient CO2 separation.

A Study on the Condensation Reaction of 4-Amino-3,5-dimethyl-1,2,4-triazole with Benzaldehydes: Structure and Spectroscopic Properties of Some New Stable Hemiaminals

Studies on the stable hemiaminals and Schiff bases formation in the reaction of substituted benzaldehydes with primary 3,5-dimethyl-1,2,4-triazole 4-amine were carried out under neutral conditions. These products were investigated by IR, Raman, MS, ¹H- and ¹³C-NMR spectra as well as by X-ray crystallography. The effect of reaction conditions: temperature, polarity of the solvents utilized, substrate concentration and the ortho and para benzaldehyde substituents on the yield of products was also examined.

Stable Hemiaminals: 2-Aminopyrimidine Derivatives

Stable hemiaminals can be obtained in the one-pot reaction between 2-aminopyrimidine and nitrobenzaldehyde derivatives. Ten new hemiaminals have been obtained, six of them in crystal state. The molecular stability of these intermediates results from the presence of both electron-withdrawing nitro groups as substituents on the phenyl ring and pyrimidine ring, so no further stabilisation by intramolecular interaction is required. Hemiaminal molecules possess a tetrahedral carbon atom constituting a stereogenic centre. As the result of crystallisation in centrosymmetric space groups both enantiomers are present in the crystal structure.

Metal-Organic Frameworks via Emissive Metal-Carboxylate Zwitterion Intermediates

Pyridinemethanol-carboxylate esters form octahedral complexes with Zn(NO₃ )₂ in aqueous DMF that subsequently undergo hydrolysis at elevated temperatures to form metal-carboxylate zwitterions. In situ deprotonation of the hydroxy group leads to thermally robust, neutral MOFs. This stepwise synthesis can be controlled by temperature and is made possible by the subtle difference in reactivity of the functional groups.

Impact of the Nature of the Organic Spacer on the Crystallization Kinetics of UiO-66(Zr)-Type MOFs

The influence of the constitutive dicarboxylate linkers (size, functional group) over the crystallization kinetics of a series of porous Zr metal-organic frameworks with the UiO-66 topology has been investigated by in situ time-resolved energy dispersive X-ray diffraction (EDXRD). Both large aromatic spacers (2,6-naphthalene-, 4,4'-biphenyl- and 3,3'-dichloro-4,4'-azobenzene-dicarboxylates) and a series of X-functionalized terephthalates (X=NH2 , NO2 , Br, CH3 ) were investigated in dimethylformamide (DMF) at different temperatures and compared with the parent UiO-66. Using different crystallization models, rate constants and further kinetic parameters (such as activation energy) have been extracted. Finally, the impact of the replacement of the toxic DMF by water on the crystallization kinetics was studied through the synthesis of the functionalized UiO-66-NO2 solid.

Tackling poison and leach: catalysis by dangling thiol-palladium functions within a porous metal-organic solid

Self-standing thiol (-SH) groups within a Zr(IV)-based metal-organic framework (MOF) anchor Pd(II) atoms for catalytic applications: the spatial constraint prevents the thiol groups from sealing off/poisoning the Pd(II) center, while the strong Pd-S bond precludes Pd leaching, enabling multiple cycles of heterogeneous catalysis to be executed.

Charged functional group effects on a metal–organic framework for selective organic dye adsorptions

The quaternary N-alkyl ammonium salt functionality was introduced into MOF pores through a postsynthetic strategy, which allows a selective dye adsorption depending on its charge.

Active site engineering in UiO-66 type metal–organic frameworks by intentional creation of defects: a theoretical rationalization

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 increases by using synthesis modulators as trifluoroacetate (TFA) or hydrochloric acid (HCl), which can be removed post-synthetically.

Towards multifunctional MOFs – transforming a side reaction into a post-synthetic protection/deprotection method

Controlled formylation/deformylation of the important amino-substituted metal–organic framework UiO-66-NH₂ allowed for the first successful post-synthetic protection–modification–deprotection sequences in MOFs.

Conformation-selective coordination-driven self-assembly of a ditopic donor with PdII acceptors

Different binding conformations of 3-(5-(pyridin-3-yl)-1H-1,2,4-triazol-3-yl)pyridine (L) yielded a self-assembled 3D cube and 2D macrocycles selectively depending on the nature of acceptors. Selection of a particular conformation of the donor L by a specific metal acceptor during self-assembly was corroborated well by a theoretical study.