Pair sites on Al3O nodes of the metal-organic framework MIL-100: Cooperative roles of defect and structural vacancy sites in methanol dehydration catalysis

Enhanced removal of perfluorooctanoic acid by aluminum-based metal-organic frameworks prepared by bauxite residue

Upcycling solid waste into advanced adsorbents is a sustainable approach in the field of waste valorization and wastewater treatment. In this study, we developed a phase-controlled synthesis method for a single phase of an aluminum-based metal-organic framework (MOF) using an aluminum source (Al3+) in red mud (RM), and demonstrated its potential for aqueous perfluorooctanoic acid (PFOA) removal. By optimizing the pre-treatment process, the selective extraction of aluminum ion from RM was achieved. Subsequently, three distinct aluminum-based MOFs (i.e., MIL-53(Al), MIL-96(Al), and MIL-100(Al)) were synthesized by controlling the hydrothermal synthesis conditions and using specific organic linkers (terephthalic acid and trimesic acid). For MOFs based on trimesic acid, the initial Al3+: trimesic acid ratio and duration of hydrothermal synthesis exerted an observable influence on the formation of the second building unit of the MOF. By manipulating these factors, we could precisely control isolated MIL-96(Al) and MIL-100(Al). The PFOA adsorption results revealed a remarkable increase in the adsorption capacity (Qmax: 131.58 mg/g) on MIL-100(Al) compared with that on MIL-96(Al). This was due to its large surface area (1189.15 m2/g) and the presence of numerous hydrophilic sites favorable for interaction with the carboxylic group of PFOA. Furthermore, a computational investigation revealed that in addition to direct Lewis acid-base interaction between PFOA and aluminum sites, the major mechanism involved the formation of a complex induced by ion exchange between coordinated NO3- and PFOA anions.

Defect-derived catalytic sites in Ce/Zr-UiO-66 for degradation of hexachlorobenzene

It is of great significance to develop catalysts for the degradation of hexachlorobenzene from the industrial thermal process. In this paper, formic acid was used as a modulator to generate defect sites in Ce/Zr-UiO-66 with intrinsic Brønsted acidity. The defective formate ligands were removed through methanol-water vapor treatment to expose additional open metal sites with Lewis acidity. The intrinsic Brønsted acid sites of the resulting Ce/Zr-UiO-66-FA-P achieved a hexachlorobenzene degradation efficiency of 99.5% at 250 °C. The generated Lewis acid sites facilitated the C-C cleavage of degradation intermediates. More than 95.0% of the final products were CO2/CO, coupled with chlorinated alkanes/alkenes, which outperformed other benchmark metal oxide catalysts. The Ce/Zr-UiO-66-FA-P catalyst maintained its catalytic activity in the model industrial flue gas and humid environment. The degradation pathway of hexachlorobenzene was tracked using in situ FT-IR spectra.

Achieving Robust α-Alumina Nanofibers by Ligand Confinement Coupled with Local Disorder Tuning

As high-performance thermal protection and structure enhancement materials, oxide ceramic fibers have become indispensable in numerous areas, ranging from deep-sea exploration to supersonic aircraft. However, under extreme energy input, abnormal grain growth and inevitable vermiculate structure would break the fiber integrity, causing catastrophic structure failure. Nowadays, the design of nanoceramics brings potential answers for strengthening of mechanical properties, but with the diameter downsized to the nanoscale, the increasing structural susceptibility of ceramic fiber to phase transformation and grain growth becomes a huge barrier. Here, we propose a strong carboxylic ligand confinement strategy by the combination of formic and acetic acids to control the inorganic colloid growth for fabricating robust α-alumina nanofibers. The rapid hydrolysis and coordination of the carboxylate groups with aluminum together with subsequent concentration synergistically promote the formation of small and compact precursor colloids, laying a solid foundation for suppressing abnormal grain growth and achieving refined alumina grain structure. The local disorder induced by silica and boron oxide surrounding α-alumina grains imparts excellent mechanical properties and flexibility with no fractures observed even after 500 buckling cycles and a wide range of temperatures from -196 to 1100 °C, providing an enlightening paradigm for ceramic fiber strengthening.

Chemistry of Formate and Water Ligands on Metal Oxide Cluster Nodes of Metal-Organic Framework hcp Hf-UiO-66: Keys to Understanding Reactivity of Paired μ2-OH and Defect Sites

Many metal-organic frameworks (MOFs) incorporate nodes that are metal oxide clusters, and ligands that have been observed on these nodes include formates, acetates, water, hydroxyl groups, and others, all of which are potentially important in affecting reactivities for applications in separations, catalysis, and sensing. Formate is a common node ligand, arising from formic acid used as a modulator and from N,N-dimethylformamide used as a solvent in MOF syntheses. Yet only little work has been reported characterizing the reactivities of node formate ligands. Infrared spectra reported here show that formate bonds to two types of sites on the paired Hf6O8 nodes of hcp UiO-66, namely, defect and μ2-OH sites. Quantifying the number of formate ligands by 1H NMR spectroscopy of digested samples showed an almost equal number of formate ligands on the two sites, indicating the likelihood that they neighbor each other. These formate ligands interact with water molecules, reversibly switching their bonding from bidentate to monodentate. The formates on μ2-OH sites of hcp Hf-UiO-66 interact much more strongly with water than those on defect sites of the same node, and both interact more strongly than isolated defect sites of Hf-UiO-66. Correspondingly, the catalytic activities of hcp UiO-66 determined as turnover frequencies on each site are approximately twofold higher than those on UiO-66, bolstering the inference that methanol dehydration is catalyzed by a node defect site and a neighboring node μ2-OH site. The results show how MOFs, with their well-defined node structures, provide unprecedented opportunities to understand details of reactivities and catalysis on metal oxide clusters, in contrast to bulk metal oxide surfaces.

Analyzing Stabilities of Metal-Organic Frameworks: Correlation of Stability with Node Coordination to Linkers and Degree of Node Metal Hydrolysis

Among the important properties of metal-organic frameworks (MOFs) is stability, which may limit applications, for example, in separations and catalysis. Many MOFs consist of metal oxo cluster nodes connected by carboxylate linkers. Addressing MOF stability, we highlight connections between metal oxo cluster chemistry and MOF node chemistry, including results characterizing Keggin ions and biological clusters. MOF syntheses yield diverse metal oxo cluster node structures, with varying numbers of metal atoms (3-13) and the tendency to form chains. MOF stabilities reflect a balance between the number of node-linker connections and the degree of node hydrolysis. We summarize literature results showing how MOF stability (the temperature of decomposition in air) depends on the degree of hydrolysis/condensation of the node metals, which is correlated to their degree of substitution with linkers. We suggest that this correlation may help guide the discovery of stable new MOFs, and we foresee opportunities for progress in MOF chemistry emerging from progress in metal oxo cluster chemistry.

Characterization, Structure, and Reactivity of Hydroxyl Groups on Metal-Oxide Cluster Nodes of Metal-Organic Frameworks: Structural Diversity and Keys to Reactivity and Catalysis

Among the most stable metal-organic frameworks (MOFs) are those incorporating nodes that are metal oxide clusters with frames such as Zr6 O8 . This review is a summary of the structure, bonding, and reactivity of MOF node hydroxyl groups, emphasizing those bonded to nodes containing aluminum and zirconium ions. Hydroxyl groups are often present on these nodes, sometimes balancing the charges of the metal ions. They arise during MOF syntheses in aqueous media or in post-synthesis treatments. They are identified with infrared and 1 H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, paired µ2 -OH, and aqua groups on nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolated µ2 -OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g., µ3 -OH) are mere spectators in catalysis. There are similarities between MOF node hydroxyl groups and those on the surfaces of bulk metal oxides, zeolites, and enzymes, but the comparisons are mostly inexact, and much remains to be understood about MOF node hydroxyl group chemistry. It is posited that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts.

Functionalization strategies of metal-organic frameworks for biomedical applications and treatment of emerging pollutants: A review

One of the representative coordination polymers, metal-organic frameworks (MOFs) material, is of hotspot interest in the multi field thanks to their unique structural characteristics and properties. As a novel hierarchical structural class, MOFs show diverse topologies, intrinsic behaviors, flexibility, etc. However, bare MOFs have less desirable biofunction, high humid sensitivity and instability in water, restraining their efficiencies in biomedical and environmental applications. Thus, a structural modification is required to address such drawbacks. Herein, we pinpoint new strategies in the synthesis and functionalization of MOFs to meet demanding requirements in in vitro tests, i.e., antibacterial face masks against corona virus infection and in wound healing and nanocarriers for drug delivery in anticancer. Regarding the treatment of wastewater containing emerging pollutants such as POPs, PFAS, and PPCPs, functionalized MOFs showed excellent performance with high efficiency and selectivity. Challenges in toxicity, vast database of clinical trials for biomedical tests and production cost can be still presented. MOFs-based composites can be, however, a bright candidate for reasonable replacement of traditional nanomaterials in biomedical and wastewater treatment applications.

Synergy of Paired Brønsted-Lewis Acid Sites on Defects of Zr-MIL-140A for Methanol Dehydration

As a common defect-capping ligand in metal-organic frameworks (MOFs), the hydroxyl group normally exhibits Brønsted acidity or basicity, but the presence of inherent hydroxyl groups in the MOF structure makes it a great challenge to identify the exact role of defect-capping hydroxyl groups in catalysis. Herein, we used hydroxyl-free MIL-140A as the platform to generate terminal hydroxyl groups on defect sites via a continuous post-synthetic treatment. The structure and acidity of MIL-140A were properly characterized. The hydroxyl-contained MIL-140A-OH exhibited 4.6-fold higher activity than the pristine MIL-140A in methanol dehydration. Spectroscopic and computational investigations demonstrated that the reaction was initiated by the respective adsorption of two methanol molecules on the terminal-OH and the adjacent Zr vacancy. The dehydration of the adsorbed methanol molecules then occurred in the Brønsted-Lewis acid site co-participated associative pathway with the lowest energy barrier.

Active sites, kinetics, and inhibiting species in the catalytic dehydration of methanol over MIL-100(Cr)

Structure–property relationships over highly-uniform, isolated MIL-100(Cr) nodes are investigated. Brønsted acid-mediated dehydration of methanol is used as a probe reaction to decipher acid site properties, and the data point to the prevalence of an associative mechanism.

Fabricating defect-rich metal-organic frameworks via mixed linker-induced crystal transformation

Defect-rich hcp UiO-66-NO₂ was synthesized via mixed linker-induced crystal transformation from fcu UiO-66-NO₂/NH₂. The defect concentration and porosity of hcp UiO-66-NO₂ can be fine-tuned by varying the BDC-NH₂/BDC-NO₂ ratio, which in turn endowed hcp UiO-66-NO₂ with superior catalytic performance in the ring-opening reaction of epoxides with alcohols.