Hierarchically Porous Metal–Organic Framework Monoliths Synthesized from Sacrificial Zirconium-Based Templates

As a highly promising candidate for hydrogen storage, crucial to vehicles powered by fuel cells, metal–organic frameworks (MOFs) have attracted the attention of chemists in recent decades. H2 uptak...

A series of MnOx-CeO2 with different Mn contents was prepared using CeBTC-MOF as the sacrificial template. These constituted a new kind of porous crystalline materials assembled by cerium as metal ions and 1, 3, 5-benzenetricarboxylic acid as organic ligands. The composite oxides exhibited good redox properties and were tested as catalysts in the oxidation of toluene. To obtain insight into the structure-activity relationship of the catalysts, the samples were characterized using powder X-ray diffraction (XRD), nitrogen adsorption-desorption, thermogravimetric analysis (TG), elemental analysis (EA), inductively coupled plasma-optical emission spectrometry (ICP-OES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), and UV-Vis diffuse reflectance spectroscopy. Studies of the CeBTC-MOF template showed that the metal-organic framework could be completely decomposed at a calcination temperature of 300 ℃. Therefore, CeBTC-MOF decomposed and generated CO2 and H2O during the calcination process. The gas molecule spilled out from the structure to form the interior void space. The spilling out could be controlled by varying the calcination temperature. This regulated the quantity and size of the interior void, which in turn made the surface area controllable. The secondary building unit of CeBTC-MOF was oxidized to nano-sized crystalline particles, which exhibited outstanding interfacial contact. SEM and TEM results showed that the composite oxides prepared by pyrolysis of the CeBTC-MOF template exhibited rod-shaped nanocrystalline particles. While introducing Mn into MOF, part of Mn entered the ceria lattice to form solid solution and the remaining Mn was dispersed on CeO2 surface. The elemental mappings revealed a well-proportioned distribution of Mn, which confirmed the successful formation of bimetallic metal oxides using the MOF-template method. All the samples exhibited sizes and shapes similar to their parent MOFs. As for catalytic activity, all the composite oxides showed better performances than pure CeO2 for catalytic oxidation of toluene. This could be attributed to higher concentration of oxygen vacancies, which was characterized by Raman spectroscopy. In addition, the XPS results indicated that Mn4+/(Mn2++Mn3+), Ce4+/Ce3+, Olatt (lattice oxygen), and Osur(surface oxygen) all participated in the redox process during catalytic oxidation of toluene, which helped elucidate the mechanism at a micro level. Interestingly, the catalytic activity did not improve further when the Mn content of the composite oxides reached 5%. This could be ascribed to two different states of the dispersed Mn: monolayer dispersion state and crystalline phase. The strong interaction between ceria oxides and dispersed Mn species played an important role in affecting catalytic activity. The results showed the presence of a monolayer dispersion threshold (6.2%), confirmed by XPS characterization, which was in accordance with all the characterization results; it was proved that this threshold had a significant impact on the catalytic activity. When the dispersed Mn content was lower than the monolayer dispersion threshold, Mn reacted with the surface CeO2 in the form of an incorporation model, leading to charge transfer and higher concentration of oxygen vacancies, which in turn effectively promoted the catalytic performance. When the dispersed Mn content exceeded the monolayer dispersion threshold, Mn3O4 was formed on the CeO2 surface; this disrupted the promotion of catalytic activity, which explains the same catalytic activity of all the samples (5% MnOx-CeO2, 8% MnOx-CeO2, and 10% MnOx-CeO2). This successful formation of bimetallic metal oxides using CeBTC-MOF template indicated that composite oxide synthesis was feasible using the MOF template method. To obtain high catalyst performance of these composite oxides, it was important to control the metal content at the level of the monolayer dispersion threshold.

An interfacial etching approach was developed for the synthesis of monodisperse and ultrasmall thiolated palladium nanoclusters (Pd NCs) using Zr‐UiO‐66‐NH2 metal–organic frameworks (MOFs) as sacrificial templates. The Pd NCs were originally synthesized inside the cavities of the MOFs (Pd@UiO‐66‐NH2). The Pd NCs released from the MOFs have a strikingly small size with narrow distribution (1.1±0.1 nm), amounting to a cluster size of ca. 40 Pd atoms. The 1H NMR spectrum indicates that thiol is the only capping agent for these Pd NCs. We derived the composition of the thiolated Pd NCs using thermogravimetric analysis (TGA) and inductively coupled plasma mass spectrometry (ICP‐MS) analysis. Moreover, the Pd NCs size can be tuned by using MOF templates with different cavity sizes. The thiolated Pd NCs are catalytically active in a model Suzuki–Miyaura coupling reaction.

Porous metal-organic framework (MOF) materials: design strategy, synthesis, sensing and catalysis Many different kinds of organic (antibiotics, explosives, and dye, etc.) and inorganic (cationic and anion) pollutants in water can enter the human body through the food chain, which will lead to irreversible damage to human health (Srivastava et al., 2004; Joarder et al., 2015; Li et al., 2020) . The rapid and sensitive identification or capture of organic/inorganic pollutants is therefore of great significance for protecting ecosystems and human health (Fu et al., 2018; Li et al., 2021; Tang et al., 2023) . Porous metal-organic frameworks (MOFs) are synthesized by metal ions/clusters and various organic ligands via coordination bonds (Islamoglu et al., 2017) . They have been proven to hold outstanding properties, such as ultrahigh specific surface area, high porosity, and adjustable porous structures. Thus, MOFs may show a lot of potential for adsorbing and sensing environmental pollutants and are thought to be the most promising sorption and sensing materials (Rowsell and Yaghi, 2006; Mallick et al., 2015; Xing et al., 2021) . This Research Topic on "Porous metal-organic framework (MOF) materials: design strategy, synthesis, sensing and catalysis" includes recent studies on the characteristics and different applications of porous functional MOFs. The Research Topic consists of four original research papers from eight different institutions. Using the metallic cobalt ions and π-conjugated amide-functionalized ligands, Yan et al. prepared two nano-MOFs TMU-50 and TMU-51, both of which displayed dual interpenetrated frameworks and distinct luminescent properties. The authors studied the effects of some important parameters on the morphology and size of the nanostructures during the synthesis process, including the initial reagent concentration, ultrasonic power, and time. The BET results showed that TMU-51 is a non-porous motif, whereas the TMU-50 is a porous structure. Based on the unique porosity of TMU-50, it could be used for sensitively detecting nitroaromatics (NP)

Metal-organic frameworks (MOFs) derived transition metal oxides exhibit enhanced performance in energy conversion and storage. In this work, porous hollow Co3O4 with N-doped carbon coating (Co3O4/N-C) polyhedrons have been prepared using cobalt-based MOFs as a sacrificial template. Assembled from tiny nanoparticles and N-doped carbon coating, Co3O4/N-C composite shortens the diffusion length of Li+/Na+ ions and possesses an enhanced conductivity. And the porous and hollow structure is also beneficial for tolerating volume changes in the galvanostatic discharge/charge cycles as lithium/sodium battery anode materials. As a result, it can exhibit impressive cycling and rating performance. At 1000 mA g-1, the specific capacities maintaine stable values of ∼620 mAh g-1 within 2000 cycles as anodes in lithium ion battery, while the specific capacity keeps at 229 mAh g-1 within 150 cycles as sodium ion battery anode. Our work shows comparable cycling performance in lithium ion battery but even better high-rate cycling stability as sodium ion battery anode. Herein, we provide a facile method to construct high electrochemical performance oxide/N-C composite electrode using new MOFs as sacrificial template.

Porous Metal-Organic Frameworks: Promising Materials for Methane Storage

Porous Metal–Organic Frameworks as New Drug Carriers

New Strategies for Novel MOF-Derived Carbon Materials Based on Nanoarchitectures

Metal-Organic Frameworks for solar energy utilization

Core-shell motif construction: Highly graphitic nitrogen-doped porous carbon electrocatalysts using MOF-derived carbon@COF heterostructures as sacrificial templates

Hierarchical metal-organic framework (MOF) pore engineering

Defect modulation of MOF-derived ZnFe2O4/CNTs microcages for persulfate activation: Enhanced nonradical catalytic oxidation

Molecular recognition, an important process in biological and chemical systems, governs the diverse functions of a variety of enzymes and unique properties of some synthetic receptors. Because molecular recognition is based on weak interactions between receptors and substrates, the design and assembly of synthetic receptors to mimic biological systems and the development of novel materials to discriminate different substrates for selective recognition of specific molecules has proved challenging. The extensive research on synthetic receptors for molecular recognition, particularly on noncovalent complexes self-assembled by hydrogen bonding and metal-organic coordination, has revealed some underlying principles. In particular, these studies have demonstrated that the shapes of the supramolecular receptors play significant roles in their specific and selective recognition of substrates: receptors can offer concave surfaces that complement their convex targets. This Account describes our research to develop a synthetic molecular recognition platform using porous metal-organic frameworks (MOFs). These materials contain functional pores to direct their specific and unique recognition of small molecules through several types of interactions: van der Waals interactions of the framework surface with the substrate, metal-substrate interactions, and hydrogen bonding of the framework surface with the substrate. These materials have potential applications for gas storage, separation, and sensing. We demonstrate a simple strategy to construct a primitive cubic net of interpenetrated microporous MOFs from the self-assembly of the paddle-wheel clusters M(2)(CO(2))(4) (M = Cu(2+), Zn(2+), and Co(2+)) with two types of organic dicarboxylic acid and pillar bidentate linkers. This efficient method allows us to rationally tune the micropores to size-exclusively sort different small gas molecules, leading to the highly selective separation and purification of gases. By optimizing the strong interactions between open metal sites within porous MOFs and gas molecules such as hydrogen and acetylene, we have developed several MOF materials with extraordinary acetylene storage capacity at room temperature. We have also immobilized Lewis acidic and basic sites into luminescent porous MOFs to recognize and sense neutral and ionic species. Using the strategy to systematically immobilize different open metal sites within porous MOFs from the metalloligand precursors, we have developed the first microporous mixed-metal-organic framework (M'MOF) with enhanced affinity for hydrogen molecules, which successfully separated D(2) from H(2) using kinetic isotope quantum molecular sieving. Because we can functionalize the pores to direct their specific recognition of small molecules, the emerging porous MOFs serve as novel functional materials for gas storage, separation, heterogeneous catalysis, and sensing.

Metal organic frameworks (MOFs) have numerous practical applications including hydrogen storage, drug delivery, separation processes and sensing. Sensing is particularly important in the mineral processing sector, where the success of mineral leaching processes depends upon the development of reliable and efficient methods for the detection of ferric ions (Fe). Here, we examine the atomic interactions between a hydrated ferric ion and an europium-based MOF with a view to the development of a sensing device capable of determining the ferric ion concentration in an analyte solution. Using a continuum approach together with the Lennard-Jones potential, we determine the Lennard-Jones (van der Waals) interaction energy and force between a hydrated ferric ion and a MOF pore. The hydrated ferric ion is modelled as a point at the centre of two concentric spheres, while the MOF pore is modelled by a semi-infinite cylinder. The model predicts that it is energetically more favourable for the hydrated ferric ion to reside inside the MOF pore and that the greatest attractive force is experienced at the entrance of the MOF pore. However, the hydrated ferric ion is unlikely to enter the MOF pore because it is geometrically larger than the free space inside. This means that in practice the hydrated ferric ion will remain on the surface of the MOF crystal, considerably simplifying ion removal. This property will be useful for washing and reusing the sensor.

Chlorine is an essential element of chemical industries, and especially has paramount importance in polymer industry. A typical route for chlorine generation is electrolysis of brine solution where chlorine is generated at the anode; producing caustic soda and hydrogen at cathode. However, the relevant state of the art membrane reactors is one of the most energy intensive processes in industry. Moreover, the membrane technology is believed to have reached the theoretical limit, and energy consumption cannot be reduced further. In the present chlor-alkali technology, sodium chloride solution is electrolyzed at a constant current density, and the decomposition voltage ΔE of the overall reaction is 2.186 V as per thermodynamic calculation. If the hydrogen evolution reaction at the cathode side is replaced with oxygen reduction reaction (ORR), the decomposition voltage will be significantly reduced, and, as a consequence, the energy consumption will be reduced simultaneously. Introduction of such oxygen depolarized cathode results in the following chemical reactions: Anode: 2Cl- —— Cl2+2e- (E=1.358 V vs. NHE) Cathode: 4Na+ + 2H2O + O2 +4e- —— 4NaOH (E=0.401 V vs. NHE) Overall Reaction: 4NaCl + 2H2O + O2 —— 4NaOH + 2Cl2 (ΔE=-0.957 V) Even though the voltage requirement can be minimized by oxygen depolarized cathode, there is still significant overpotential loss from the cathodic reaction. This is due to the fact that ORR is kinetically a very sluggish reaction. Platinum is the best catalyst known for ORR in acid media. However, Pt is poisoned in concentrated NaOH solution resulting in significant shifts in onset potential and limiting current values. Early in 1970s, heating of non-noble metal along with carbon and a source of nitrogen atoms in presence of ammonia resulted in formation of electrocatalysts with ability to reduce oxygen. Since then, there has been a gradual development of such “non-PGM” catalysts, where PGM stands for Precious Group Metals. The introduction of metal organic frameworks (MOFs) as sacrificial templates has brought a revolution in electrocatalysis research. Owing to the large surface area, preferable porosity and high nitrogen content, the non-PGM catalysts obtained from MOF precursors have shown excellent ORR activity. However, most of the MOF synthesis methods are based on the traditional solution reactions, which need either excessive amount of ligand or a huge amount of solvent, or both. Thus, the low yielding synthesis route and comprehensive separation methods of MOF seriously constrain the application of the MOF based catalysts for ORR. In the presentation, we will be discussing about the new in-house synthesized MOF based catalyst using solid-state reaction. In addition to advantages like simple preparatory route and high reproducibility, this catalyst has high yield and scalability which are uncommon traits for the more conventional MOF based catalysts. This catalyst has high ORR onset potential even in extreme corrosive conditions such as concentrated alkali and acid. Herein, we propose a facile, simplified approach to obtain the MOF based catalysts for ORR with low cost and high reproducibility. The as-prepared catalysts have been tested as oxygen depolarized cathodes in chlor-alkali electrolysis cells and have been operated at significantly lower voltages compared to commercial catalysts like Pt/C and Ag/C.