Analogous Metal-organic Frameworks Research Articles

Kinetic and thermodynamic studies of neutral dye removal from water using zirconium metal-organic framework analogues

A series of isostructural water stable UiO-66-X (X = H, NH2, NO2), one of the most well-known metal-organic framework (MOF) materials, was synthesized by solvothermal synthesis. Their dye adsorption capacity toward a neutral dye, Phenol Red (PR), was investigated for the first time in this research in order to elucidate how the functional group of the organic linker contribute to the adsorption capacity. The operational parameters, i.e. pH, adsorbent concentration, initial dye concentration, contact time and temperature were monitored. The maximum adsorption capacities of UiO-66, UiO-66-NO2 and UiO-66-NH2 were measured to be 19 mg/g, 11 mg/g and 27 mg/g, respectively. The adsorption kinetics of all three adsorbents were similar and fitted best with pseudo-second-order kinetics model. Adsorption isotherms of the adsorbents can be explained by Freundlich model which suggests that the adsorption of PR on the adsorbents can be both physisorption and chemisorption. Thermodynamic studies indicated that the adsorption process in all studied adsorbents was spontaneous, endothermic and entropic-driven which was confirmed by the obtained thermodynamic variables. Functional group of the organic ligand was found to contribute significantly to the adsorption capacity of MOFs. The adsorbent with a functional group that can create more intermolecular forces will enhance the adsorption capacity. Experimentally, UiO-66-NH2 showed the highest adsorption capacity when compared to UiO-66 and UiO-66-NO2.

Boosting Interfacial Charge-Transfer Kinetics for Efficient Overall CO2 Photoreduction via Rational Design of Coordination Spheres on Metal-Organic Frameworks.

The recombination of electron-hole pairs severely detracts from the efficiency of photocatalysts. This issue could be addressed in metal-organic frameworks (MOFs) through optimization of the charge-transfer kinetics via rational design of structures at atomic level. Herein, a pyrazolyl porphyrinic Ni-MOF (PCN-601), integrating light harvesters, active catalytic sites, and high surface areas, has been demonstrated as a superior and durable photocatalyst for visible-light-driven overall CO2 reduction with H2O vapor at room temperature. Kinetic studies reveal that the robust coordination spheres of pyrazolyl groups and Ni-oxo clusters endow PCN-601 with proper energy band alignment and ultrafast ligand-to-node electron transfer. Consequently, the CO2-to-CH4 production rate of PCN-601 far exceeds those of the analogous MOFs based on carboxylate porphyrin and the classic Pt/CdS photocatalyst by more than 3- and 20-fold, respectively. The reaction avoids the use of hole scavengers and proceeds in a gaseous phase which can take full advantage of the high gas uptake of MOFs. This work demonstrates that the rational design of coordination spheres in MOF structures not only reconciles the contradiction between reactivity and stability but also greatly promotes the interfacial charge transfer to achieve optimized kinetics, providing guidance for the design of highly efficient MOF photocatalysts.

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.

MOF-mimetic molecules: carboxylate-based supramolecular complexes as molecular metal–organic framework analogues

This review covers MOF-like molecules, an emerging class of potentially porous, soluble, and discrete cages.

Contributions of metalloporphyrin linkers and Zr6 nodes in gas adsorption on a series of bioinspired zirconium-based metal-organic frameworks: A computational study

In this study, gases adsorption on a series of mesoporous zirconium-based metal-organic framework analogues [PCN-222(M) where M = Mg, Fe, Co, Ni, Cu, and Zn)] has been evaluated by quantum mechanical calculations. These bioinspired frameworks are one of the most stable MOF materials, comprising the metalloporphyrin linkers/building blocks and the Zr6 nodes. The metalloporphyrins, open-zirconium coordination sites, and terminal hydroxyl groups on the nodes have been considered for the interactions. The structural and electronic properties of the complexes were investigated via the atoms in molecules (AIM), natural bond orbital (NBO), and harmonic oscillator model of aromaticity (HOMA) index. The results showed that the iron-porphyrin is an appropriate candidate for the ammonia capture. It is observed that the hydrogen bond interactions of H2O with the hydroxyl groups of node are stronger than the others, while the interactions of NH3 with the open-zirconium coordination sites are stronger than the others. The all results indicated that the PCN-222(Fe) is selective for adsorption of NH3 over the other gases.

Conformal Ultrathin Film Metal-Organic Framework Analogues: Characterization of Growth, Porosity, and Electronic Transport.

Thin-film formation and transport properties of two copper-paddlewheel metal-organic framework (MOF) -based systems (MOF-14 and MOF-399) are investigated for their potential integration into electrochemical device architectures. Thin-film analogs of these two systems are fabricated by the sequential, alternating, solution-phase deposition of the inorganic and organic ligand precursors that result in conformal films via van der Merwe-like growth. Atomic force microscopy reveals smooth film morphologies with surface roughnesses determined by the underlying substrates and linear film growth of 1.4 and 2.2 nm per layer for the MOF-14 and MOF-399 systems, respectively. Electrochemical impedance spectroscopy is used to evaluate the electronic transport properties of the thin films, finding that the MOF-14 analog films demonstrate low electronic conductivity, while MOF-399 analog films are electronically insulating. The intrinsic porosities of these ultrathin MOF analog films are confirmed by cyclic voltammetry redox probe characterization using ferrocene. Larger peak currents are observed for MOF-399 analog films compared to MOF-14 analog films, which is consistent with the larger pores of MOF-399. The layer-by-layer deposition of these systems provides a promising route to incorporate MOFs as thin films with nanoscale thickness control and low surface roughness for electrochemical devices.

Insight into the influence of framework metal ion of analogous metal–organic frameworks on the adsorptive removal performances of dyes from water

Abstract Two water stable metal–organic frameworks (MOFs) [M4(ᴅ-cam)4(bpzpip)3(H2O)]·DMAc·3H2O (1, M = Ni; 2, M = Co) were hydro(solvo)thermally prepared from the reactions of MCl2·6H2O (M = Ni, Co), ᴅ-camphoric acid (ᴅ-H2cam), and N,N′-bis(pyraz-2-yl)piperazine (bpzpip) in N,Nʹ-dimethylacetamide (DMAc)–H2O at 150 °C. Ni MOF 1 and Co MOF 2 are isomorphous and isostructural, both of which adopt a three-dimensional (3D) (4,6)-connected framework with (42.52.72)(42.57.65.8) topology. Dye removal properties of 1 and 2 toward methyl orange (MO), malachite green (MG), and methyl blue (MyB) showed that 1 selectively adsorbed only MyB from water with saturated adsorption capacity of 216 mg/g, while 2 selectively captured MyB and MG from water with saturated adsorption capacities of 671 and 405 mg/g, respectively, in several adsorption–desorption cycles. The Langmuir isotherm model and the pseudo-second-order kinetics model have interpreted the adsorption isotherm and adsorption kinetics, respectively, very well. Most importantly, this study figures out the specific influence of framework metal ion (Ni2+ and Co2+) on the crystal surface charge (zeta potential = −1.67 mV for 1 versus −5.81 mV for 2) that greatly affects the dye removal properties including adsorption rate and capacity as well as adsorbate from water.

13C pNMR of “crumple zone” Cu(II) isophthalate metal-organic frameworks

NMR spectroscopy of paramagnetic materials (pNMR) has the potential to provide great structural insight, but many challenges remain in interpreting the spectra in detail. This work presents a study of a series of structurally analogous metal-organic frameworks (MOFs) based on 5-substituted isophthalate linkers and Cu(II) paddlewheel dimers, of interest owing to their “crumple zone” structural rearrangement on dehydration/rehydration. 13C MAS NMR spectra reveal a wide variation in the observed resonance position for chemically similar C species in the different MOFs but, despite this, resonances are overlapped in several cases. However, by considering a combination of the integration of quantitative spectra, the resonance position as a function of temperature and T1 relaxation measurements, the spectra can be fully assigned. It is also demonstrated that the prototypical MOF in this series, STAM-1, displays a crumple zone rearrangement on dehydration, similar to the well-characterised 5-ethoxyisophthalate MOF (STAM-17-OEt) although, while the materials have similar local C environments, dehydrated STAM-1 exhibits less long-range order.

Engineering Copper Carboxylate Functionalities on Water Stable Metal–Organic Frameworks for Enhancement of Ammonia Removal Capacities

Functionalization of copper carboxylate groups on a series of UiO-66 metal organic framework (MOF) analogues and their corresponding impact on humid and dry ammonia adsorption behavior were studied. Relative locations of possible carboxylic acid binding sites for copper on the MOF analogues were varied on ligand and missing linker defect sites. Materials after copper incorporation exhibited increased water vapor and ammonia affinity during isothermal adsorption and breakthrough experiments, respectively. The introduction of copper markedly increased ammonia adsorption capacities for all adsorbents possessing carboxyl binding sites. In particular, the new MOF UiO-66-(COOCu)2 displayed the highest ammonia breakthrough capacities of 6.38 and 6.84 mmol g–1 under dry and humid conditions, respectively, while retaining crystallinity and porosity. Relative carboxylic acid site locations were also found to impact sorbent stability, as missing linker defect functionalized materials degraded under humid conditions ...

In situ hybridization of enzymes and their metal-organic framework analogues with enhanced activity and stability by biomimetic mineralisation.

By incorporating Cytochrome c (peroxidase, Cyt c) into a skeleton of its corresponding synthetic MOF analogue (peroxidase mimic, CuBDC), approximately 12-fold catalytic efficiency (kcat/KM) enhancement is observed compared to free Cyt c. Meanwhile, the shield endowed by CuBDC prevents encapsulated enzymes from deactivation by trypsin digestion, thermal treatment and long-term storage in vitro. This concept of combining enzymes and their MOF mimics with enhanced enzymatic activity and stability may provide new insights into the design of highly active, stable enzyme-MOF composite catalysts and holds promise for applications in biocatalysis, biosensing and drug delivery systems.

Crystal Dynamics in Multi-stimuli-Responsive Entangled Metal-Organic Frameworks.

An understanding of solid-state crystal dynamics or flexibility in metal-organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single-crystallinity. In this article, detailed experimental studies on a twofold entangled MOF (f-MOF-1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f-MOF-1, by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f-MOF-1 a and f-MOF-1 b, could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO2 adsorption profiles. DFT-based calculations provide an insight into the selective and gated adsorption behavior with CO2 of f-MOF-1 b. The gate-opening threshold pressure of CO2 adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (-CH2 -CH2 -) in f-MOF-1 to an azo (-N=N-) functionality in an analogous MOF, f-MOF-2. The modulation of functionality has an indirect influence on the gate-opening pressure owing to the difference in inter-net interaction. The framework of f-MOF-1 is highly responsive toward CO2 gas molecules, and these results are supported by DFT calculations.

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.

Mechanical tunability via hydrogen bonding in metal-organic frameworks with the perovskite architecture.

Two analogous metal-organic frameworks (MOFs) with the perovskite architecture, [C(NH2)3][Mn(HCOO)3] (1) and [(CH2)3NH2][Mn(HCOO)3] (2), exhibit significantly different mechanical properties. The marked difference is attributed to their distinct modes of hydrogen bonding between the A-site amine cation and the anionic framework. The stronger cross-linking hydrogen bonding in 1 gives rise to Young's moduli and hardnesses that are up to twice those in 2, while the thermal expansion is substantially smaller. This study presents clear evidence that the mechanical properties of MOF materials can be substantially tuned via hydrogen-bonding interactions.

Effect of central metal ions of analogous metal-organic frameworks on the adsorptive removal of benzothiophene from a model fuel

Liquid phase adsorption of benzothiophene (BT) has been studied over CuCl2-loaded analogous metal-organic frameworks (MOFs), metal-benzenedicarboxylates (Me-BDCs, Me: Al, Cr and V), to understand the effect of central metal ions on the adsorptive removal of BT from a model fuel. Among the central metal ions (Al3+, Cr3+ and V3+) of the Me-BDCs only V3+ was oxidized by the loaded CuCl2 (or Cu2+) at ambient condition resulting in V4+ and Cu+ species. Different from the CuCl2-loaded Al- and Cr-BDCs, the CuCl2/V-BDC adsorbed BT remarkably well compared to the virgin V-BDCs which suggests a specific favorable interaction (π-complexation) between the obtained Cu+ in the CuCl2/V-BDC and BT.

ChemInform Abstract: Analogous Porous Metal—Organic Frameworks: Synthesis, Stability and Application in Adsorption

AbstractReview: 114 refs.

Analogous porous metal–organic frameworks: synthesis, stability and application in adsorption

So far, a huge number of metal–organic frameworks (MOFs) have been synthesized and studied very widely for various applications like gas adsorption/storage, separation, catalysis, drug delivery, luminescence, magnetism, etc. Some of the MOFs are isomorphous, isostructural or isoreticular in topologies having nearly similar (analogous) framework structures. On the other hand, some of the MOFs also have very similar structures with different functional groups via direct synthesis or post-modification. In this highlight, MOFs having very similar structures will be classified into three categories: (1) analogous MOFs with different metallic components; (2) analogous MOFs with different linkers; (3) analogous MOFs with different functional groups. Moreover, various MOFs with very similar structures composed of different metallic, organic or functional groups will be compared especially with regard to their synthesis kinetics, chemical/thermal stability and their applications in the adsorption of hydrogen, acetylene, propylene, carbon dioxide and sulfur-containing compounds, and so on. The synthesis rate and chemical stability of analogous MOFs depend on the lability and inertness, respectively, of metal ions. On the other hand, thermal stability may be explained with the bond strength of metal–oxygen in common oxides. The thermal or chemical stability of analogous MOFs having extra functional groups depends on the functional groups tagged on the linkers; however, no comprehensive explanation is available. Adsorption depends strongly on the property of the metallic or organic moiety of analogous MOFs, and important parameters (size, binding strength, ionic character, density, redox ability, softness and acidity of the metal ions; length, polarity, and hydrophobicity of the linkers) for adsorption can be suggested. Based on the analysis of the reported results, it can be concluded that the metallic, organic or functional groups of analogous MOFs have dominant roles in the synthesis, stability and adsorption even though contradictory results were also reported. Understanding the effects of metallic, organic or functional moieties of very similar MOFs on the synthesis, stability and adsorption will lead to a new way to develop MOF materials that have various commercial applications.