Introduction of Flexibility into a Metal–Organic Framework to Promote Hg(II) Capture through Adaptive Deformation

Palladium(II) Modulation Enhances the Water Stability and Aqueous 99TcO4-/ReO4- Removal Performance of Metal-Organic Frameworks

Improving the water stability of metal-organic frameworks (MOFs) is essential for their use in water pollution treatment and environmental remediation, though it remains technically challenging. Herein, we report a novel cationic MOF constructed with [Th6O4(OH)4(COO)12] units and [CoN4·Cl2] units possessing a ftw-type topology (denoted as 1-Th-Co). 1-Th-Co itself exhibited poor water stability but excellent stability following a palladium(II) modulation strategy. Experimental studies reveal that Co(II) ions in 1-Th-Co were replaced by Pd(II) ions through cation exchange in N,N-diethylformamide (yielding 1-Th-Pd). The planar PdN4 units in 1-Th-Pd were responsible for improving the water stability of the framework. As a result, 1-Th-Pd offered excellent stability, fast adsorption kinetics, and high removal ratios for 99TcO4- and ReO4- (as a 99TcO4- surrogate) in contaminated water. When used in packed columns, 1-Th-Pd can dynamically capture ReO4- from groundwater. This work provides a new avenue for improving the water stability of MOFs, offering new vistas for the decontamination of aqueous solutions containing 99TcO4- and ReO4-.

Functionalization of Defective Zr-MOFs for Water Decontamination: Mechanistic Insight into the Competitive Roles of -NH2 and -SH Sites in the Removal of Hg(II) Ions

Functional metal-organic frameworks (MOFs), especially those based on sulfur and nitrogen atoms, were frequently applied for the removal of Hg(II) ions. However, a systematic study on the cooperative or competitive roles of -SH and -NH2 functions in the presence of secondary mechanisms (proton transfer and redox) is still rare. In this work, the UiO-66 framework (Zr6(OH)4O4(BDC)6, BDC2- = benzene-1,4-dicarboxylate) was decorated with functional monocarboxylate linkers including glycine (Gly), mercaptopropionic acid (Mer), and cysteine (Cys). Due to the molecular similarity of these functional linkers, the coordination affinity between the amine and thiol sites with Hg(II) ions can be compared, and the effect of proton transfer and redox mechanisms on the possible thiol···Hg(II) and amine···Hg(II) interactions can be investigated. The results show that the Cys@UiO-66 framework can adsorb 1288 mg g-1 of Hg(II), while Mer@UiO-66 and Gly@UiO-66 can adsorb 593 and 313 mg g-1 at pH = 7 and 500 ppm, respectively. This is due to the facts that both the amine and the thiol functions of the Cys@UiO-66 framework show synergism in Hg(II) removal, and the secondary mechanisms reduce the affinity of thiol in Mer@UiO-66 and amine in Gly@UiO-66 frameworks in the removal process of Hg(II) ions. Free -SH sites in Mer@UiO-66 undergo a redox convert to -SO3H groups, and free protonated -NH2 sites in Gly@UiO-66 do not fully deprotonate during Hg(II) removal. Yet, in the case of Cys@UiO-66, free protonated -NH2 sites are fully deprotonated, and free SH sites did not convert to -SO3H groups during Hg(II) removal. These observations show that the redox and proton transfer mechanisms can negatively affect the adsorption capacity of functional MOFs containing free -SH and -NH2 groups. So, not only the functionalization but also control over secondary mechanisms in the removal process are necessary parameters to improve the affinity between functional MOFs and Hg(II) ions.

Water-stable metal-organic frameworks (MOFs): rational construction and carbon dioxide capture

Metal-organic frameworks (MOFs) are considered to be a promising porous material due to their excellent porosity and chemical tailorability. However, due to the relatively weak strength of coordination bonds, the stability (e.g., water stability) of MOFs is usually poor, which severely inhibits their practical applications. To prepare water-stable MOFs, several important strategies such as increasing the bonding strength of building units and introducing hydrophobic units have been proposed, and many MOFs with excellent water stability have been prepared. Carbon dioxide not only causes a range of climate and health problems but also is a by-product of some important chemicals (e.g., natural gas). Due to their excellent adsorption performances, MOFs are considered as a promising adsorbent that can capture carbon dioxide efficiently and energetically, and many water-stable MOFs have been used to capture carbon dioxide in various scenarios, including flue gas decarbonization, direct air capture, and purified crude natural gas. In this review, we first introduce the design and synthesis of water-stable MOFs and then describe their applications in carbon dioxide capture, and finally provide some personal comments on the challenges facing these areas.

Functional MOF-Based Materials for Environmental and Biomedical Applications: A Critical Review

Over the last ten years, there has been a growing interest in metal-organic frameworks (MOFs), which are a unique category of porous materials that combine organic and inorganic components. MOFs have garnered significant attention due to their highly favorable characteristics, such as environmentally friendly nature, enhanced surface area and pore volume, hierarchical arrangements, and adjustable properties, as well as their versatile applications in fields such as chemical engineering, materials science, and the environmental and biomedical sectors. This article centers on examining the advancements in using MOFs for environmental remediation purposes. Additionally, it discusses the latest developments in employing MOFs as potential tools for disease diagnosis and drug delivery across various ailments, including cancer, diabetes, neurological disorders, and ocular diseases. Firstly, a concise overview of MOF evolution and the synthetic techniques employed for creating MOFs are provided, presenting their advantages and limitations. Subsequently, the challenges, potential avenues, and perspectives for future advancements in the utilization of MOFs in the respective application domains are addressed. Lastly, a comprehensive comparison of the materials presently employed in these applications is conducted.

Detection and Sorption of Heavy Metal Ions in Aqueous Media by a Fluorescent Zr(IV) Metal-Organic Framework Functionalized with 2-Picolylamine Receptor Groups

Increasing global environmental pollution due to heavy metal ions raises the importance of research on new multifunctional materials for simultaneous detection and removal of these contaminants from water resources. In this study, we report a microporous 8-connected Zr⁴⁺ metal-organic framework (MOF) based on a terephthalate ligand decorated with a chelating 2-picolylamine side group (dMOR-2), which shows highly efficient fluorescence sensing and sorption of heavy metal cations. We demonstrate by detailed fluorescence studies the ability of a water-dispersible composite of dMOR-2 with polyvinylpyrrolidone for real-time detection of Cu²⁺, Pb²⁺, and Hg²⁺ in aqueous media. The limits of detection were found to be below 2 ppb for these species, while the system's performance is not affected by the presence of other potentially competitive ions. In addition, sorption studies showed that a composite of dMOR-2 with calcium alginate (dMOR-2@CaA) is an excellent sorbent for Pb²⁺ and Cu²⁺ ions with capacities of 376 ± 15 and 117 ± 4 mg per gram of dMOR-2@CaA, respectively, while displaying the capability for simultaneous removal of various heavy metal ions in low initial concentrations and in the presence of large excesses of other cationic species. Structural and spectroscopic studies with model ligands analogous to our material's receptor unit showed chelation to the 2-picolylamine moiety to be the main binding mode of metal ions to dMOR-2. Overall, dMOR-2 is shown to represent a rare example of a MOF, which combines sensitive fluorescence detection and high sorption capacity for heavy metal ions.

A Novel Coordination Polymer as Adsorbent Used to Remove Hg(II) and Pb(II) from Water with Different Adsorption Mechanisms

Under the hydrothermal condition, a new type of two-dimensional coordination polymer ([Cd(D-Cam)(3-bpdb)]_n, Cd-CP) has been constructed. It is composed of D-(+)-Camphoric-Cd(II) (D-cam-Cd(II)) one-dimensional chain and bridging 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (3-bpdb) ligands. Cd-CP has a good removal effect for Hg(II) and Pb(II), and the maximum adsorption capacity is 545 and 450 mg/g, respectively. Interestingly, thermodynamic studies have shown that the adsorption processes of Hg(II) and Pb(II) on Cd-CP use completely different thermodynamic mechanisms, in which the adsorption of Hg(II) is due to a strong electrostatic interaction with Cd-CP, while that of Pb(II) is through a weak coordination with Cd-CP. Moreover, Cd-CP has a higher affinity for Hg(II), and when Hg(II) and Pb(II) coexist, Cd-CP preferentially adsorbs Hg(II).

Recent advances in the application of water-stable metal-organic frameworks: Adsorption and photocatalytic reduction of heavy metal in water

Heavy metals pollution in water is a global environmental issue, which has threatened the human health and environment. Thus, it is important to remove them under practical water environment. In recent years, metal-organic frameworks (MOFs) with water-stable properties have attracted wide interest with regard to the capture of hazardous heavy metal ions in water. In this review, the synthesis strategy and postsynthesis modification preparation methods are first summarized for water-stable MOFs (WMOFs), and then the recent advances on the adsorption and photocatalytic reduction of heavy metal ions in water by WMOFs are reviewed. In contrast to the conventional adsorption materials, WMOFs not only have excellent adsorption properties, but also lead to photocatalytic reduction of heavy metal ions. WMOFs have coupling and synergistic effects on the adsorption and photocatalysis of heavy metal ions in water, which make it more effective in treating single pollutants or different pollutants. In addition, by introducing appropriate functional groups into MOFs or synthesizing MOF-based composites, the stability and ability to remove heavy metal ions of MOFs can be effectively enhanced. Although WMOFs and WMOF-based composites have made great progress in removing heavy metal ions from water, they still face many problems and challenges, and their application potential needs to be further improved in future research. Finally, this review aims at promoting the development and practical application of heavy metal ions removal in water by WMOFs.

Benchmark Acetylene Binding Affinity and Separation through Induced Fit in a Flexible Hybrid Ultramicroporous Material

Structural changes at the active site of an enzyme induced by binding to a substrate molecule can result in enhanced activity in biological systems. Herein, we report that the new hybrid ultramicroporous material sql-SIFSIX-bpe-Zn exhibits an induced fit binding mechanism when exposed to acetylene, C2 H2 . The resulting phase change affords exceptionally strong C2 H2 binding that in turn enables highly selective C2 H2 /C2 H4 and C2 H2 /CO2 separation demonstrated by dynamic breakthrough experiments. sql-SIFSIX-bpe-Zn was observed to exhibit at least four phases: as-synthesised (α); activated (β); and C2 H2 induced phases (β' and γ). sql-SIFSIX-bpe-Zn-β exhibited strong affinity for C2 H2 at ambient conditions as demonstrated by benchmark isosteric heat of adsorption (Qst ) of 67.5 kJ mol-1 validated through in situ pressure gradient differential scanning calorimetry (PG-DSC). Further, in situ characterisation and DFT calculations provide insight into the mechanism of the C2 H2 induced fit transformation, binding positions and the nature of host-guest and guest-guest interactions.

Progress in Metal-Organic Frameworks Facilitated Mercury Detection and Removal

Metal Organic Frameworks (MOFs) are noted as exceptional candidates towards the detection and removal of specific analytes. MOFs were reported in particular for the detection/removal of environmental contaminants, such as heavy metal ions, toxic anions, hazardous gases, explosives, etc. Among heavy metal ions, mercury has been noted as a global hazard because of its high toxicity in the elemental (Hg0), divalent cationic (Hg2+), and methyl mercury (CH3Hg+) forms. To secure the environment and living organisms, many countries have imposed stringent regulations to monitor mercury at all costs. Regarding the detection/removal requirements of mercury, researchers have proposed and reported all kinds of MOFs-based luminescent/non-luminescent probes towards mercury. This review provides valuable information about the MOFs which have been engaged in detection and removal of elemental mercury and Hg2+ ions. Moreover, the involved mechanisms or adsorption isotherms related to sensors or removal studies are clarified for the readers. Finally, advantages and limitations of MOFs in mercury detection/removal are described together with future scopes.

Functionalized Metal-Organic Frameworks for Hg(II) and Cd(II) Capture: Progresses and Challenges

Mercury and cadmium are deemed to be the most harmful heavy metal ions for elimination due to their persistent bio-accumulative and bio-expanding toxic effects. Although many technologies have been developed for capturing Hg(II) and Cd(II) ions from aqueous solution, developing efficient and practical capature technology remains a big challenge. Metal-organic frameworks (MOFs) have been considered as the most promising adsorbents for Hg(II) and Cd(II) removal due to their high porosity and easy functionalization, and various of MOF-based adsorbents based on different synthetic strategies have been prepared and studied. In this article, the progresses of MOF-based absorbents for Hg(II) and Cd(II) capture are described according to the synthetic strategies and the types of functional groups, and the comparison and practical analysis of various adsorbents are also presented.