Synthetic and analytical considerations for the preparation of amorphous metal-organic frameworks

Metal-organic frameworks (MOFs) are hybrid porous materials presenting several tuneable properties, allowing them to be utilised for a wide range of applications. To date, focus has been on the preparation of novel crystalline MOFs for specific applications. Recently, interest in amorphous MOFs (aMOFs), defined by their lack of correlated long-range order, is growing. This is due to their potential favourable properties compared to their crystalline equivalents, including increased defect concentration, improved processability and gas separation ability. Direct synthesis of these disordered materials presents an alternative method of preparation to post-synthetic amorphisation of a crystalline framework, potentially allowing for the preparation of aMOFs with varying compositions and structures, and very different properties to crystalline MOFs. This perspective summarises current literature on directly synthesised aMOFs, and proposes methods that could be utilised to modify existing syntheses for crystalline MOFs to form their amorphous counterparts. It outlines parameters that could discourage the ordering of crystalline MOFs, before examining the potential properties that could emerge. Methodologies of structural characterisation are discussed, in addition to the necessary analyses required to define a topologically amorphous structure.

Defect-enabling zirconium-based metal-organic frameworks for energy and environmental remediation applications

This comprehensive review explores the diverse applications of defective zirconium-based metal-organic frameworks (Zr-MOFs) in energy and environmental remediation. Zr-MOFs have gained significant attention due to their unique properties, and deliberate introduction of defects further enhances their functionality. The review encompasses several areas where defective Zr-MOFs exhibit promise, including environmental remediation, detoxification of chemical warfare agents, photocatalytic energy conversions, and electrochemical applications. Defects play a pivotal role by creating open sites within the framework, facilitating effective adsorption and remediation of pollutants. They also contribute to the catalytic activity of Zr-MOFs, enabling efficient energy conversion processes such as hydrogen production and CO2 reduction. The review underscores the importance of defect manipulation, including control over their distribution and type, to optimize the performance of Zr-MOFs. Through tailored defect engineering and precise selection of functional groups, researchers can enhance the selectivity and efficiency of Zr-MOFs for specific applications. Additionally, pore size manipulation influences the adsorption capacity and transport properties of Zr-MOFs, further expanding their potential in environmental remediation and energy conversion. Defective Zr-MOFs exhibit remarkable stability and synthetic versatility, making them suitable for diverse environmental conditions and allowing for the introduction of missing linkers, cluster defects, or post-synthetic modifications to precisely tailor their properties. Overall, this review highlights the promising prospects of defective Zr-MOFs in addressing energy and environmental challenges, positioning them as versatile tools for sustainable solutions and paving the way for advancements in various sectors toward a cleaner and more sustainable future.

Emerging Nanomaterials toward Uranium Extraction from Seawater: Recent Advances and Perspectives

Nuclear energy holds great potential to facilitate the global energy transition and alleviate the increasing environmental issues due to its high energy density, stable energy output, and carbon-free emission merits. Despite being limited by the insufficient terrestrial uranium reserves, uranium extraction from seawater (UES) can offset the gap. However, the low uranium concentration, the complicated uranium speciation, the competitive metal ions, and the inevitable marine interference remarkably affect the kinetics, capacity, selectivity, and sustainability of UES materials. To date, massive efforts have been made with varying degrees of success to pursue a desirable UES performance on various nanomaterials. Nevertheless, comprehensive and systematic coverage and discussion on the emerging UES materials presenting the fast-growing progress of this field is still lacking. This review thus challenges this position and emphatically focuses on this topic covering the current mainstream UES technologies with the emerging UES materials. Specifically, this review elucidates the causality between the physiochemical properties of UES materials induced by the intellectual design strategies and the UES performances and further dissects the relationships of materials-properties-activities and the corresponding mechanisms in depth. This review is envisaged to inspire innovative ideas and bring technical solutions for developing technically and economically viable UES materials.

Tuning the pore chemistry of Zr-MOFs for efficient metal ion capture from complex streams

Metal-organic frameworks (MOFs) have shown promise for adsorptive separations of metal ions. Herein, MOFs based on highly stable Zr(IV) building units were systematically functionalized with targeted metal binding groups. Through competitive adsorption studies, it was shown that the selectivity for different metal ions was directly tunable through functional group chemistry.

Open-Framework Vanadate as Efficient Ion Exchanger for Uranyl Removal

The elimination of uranium from radioactive wastewater is crucial for the safe management and operation of environmental remediation. Here, we present a layered vanadate with high acid/base stability, [Me2NH2]V3O7, as an excellent ion exchanger capturing uranyl from highly complex aqueous solutions. The material possesses an indirect band gap, ferromagnetic characteristic and a flower-like morphology comprising parallel nanosheets. The layered structure of [Me2NH2]V3O7 is predominantly upheld by the H-bond interaction between anionic framework [V3O7]nn- and intercalated [Me2NH2]+. The [Me2NH2]+ within [Me2NH2]V3O7 can be readily exchanged with UO22+. [Me2NH2]V3O7 exhibits high exchange capacity (qm = 176.19 mg/g), fast kinetics (within 15 min), high removal efficiencies (>99%), and good selectivity against an excess of interfering ions. It also displays activity for UO22+ ion exchange over a wide pH range (2.00-7.12). More importantly, [Me2NH2]V3O7 has the capability to effectively remove low-concentration uranium, yielding a residual U concentration of 13 ppb, which falls below the EPA-defined acceptable limit of 30 ppb in typical drinking water. [Me2NH2]V3O7 can also efficiently separate UO22+ from Cs+ or Sr2+ achieving the highest separation factors (SFU/Cs of 589 and SFU/Sr of 227) to date. The BOMD and DFT calculations reveal that the driving force of ion exchange is dominated by the interaction between UO22+ and [V3O7]nn-, whereas the ion exchange rate is influenced by the mobility of UO22+ and [Me2NH2]+. Our experimental findings indicate that [Me2NH2]V3O7 can be considered as a promising uranium scavenger for environmental remediation. Additionally, the simulation results provide valuable mechanistic interpretations for ion exchange and serve as a reference for designing novel ion exchangers.

Functionalization of Defective Zr Metal-Organic Frameworks for Water Decontamination: Mechanistic Insight into the Competitive Roles of -NH2 and -SH Sites in Removal of As(III) Species

Direct removal of trivalent arsenic, As(III), arsenite, or H3AsO3, is a great challenge in accessing clean sources of water. Different methodologies and materials were applied in this regard, but among them, direct removal of As(III) species using a metal-organic framework (MOF)-based adsorbent shows a great deal of potential. Although some studies were conducted on As(III) removal using MOFs, studies of functional groups are still quite rare. For this purpose, three novel functionalized defective Zr-MOFs, using UiO-66 [Zr6(OH)4O4(BDC)6, where BDC2- = benzene-1,4-dicarboxylate], were fabricated to investigate the competitive or cooperative roles of the free -NH2 and/or -SH site in the removal of As(III). UiO-66 was functionalized with monocarboxylate linkers, including glycine (Gly, NH2-CH2-COOH), cysteine [Cys, SH(CH2)-NH2(CH)-COOH], and mercaptopropionic acid [Mer, SH-(CH2)2-COOH]. Gly@UiO-66, Cys@UiO-66, and Mer@UiO-66 were applied for the direct removal of As(III) species. Although Cys@UiO-66 is functionalized with both amine and thiol functional groups, Gly@UiO-66 has a higher adsorption capacity (301.4 mg g-1) with respect to As(III), which is among the best reported values. This is due to the fact that (1) the affinity of amine sites in Gly@UiO-66 for As(III) is higher than that of thiol sites in Mer@UiO-66 and (2) Cys@UiO-66 has a very small surface area compared to that of Gly@UiO-66. Mechanistic studies using X-ray photoelectron spectroscopy and vibrational spectroscopy reveal that not only the functionalization and chemical nature of the function but also other parameters such as the protonation-deprotonation mechanisms and chemical state of the function are other critical factors for designing a functional MOF-based adsorbent with high affinity for and maximum capacity with respect to the target analyte.

Probing Structural Defects in MOFs Using Water Stability

Defects in the crystal structures of metal-organic frameworks (MOFs), whether present intrinsically or introduced via so-called defect engineering, can play strong roles in the properties of MOFs for various applications. Unfortunately, direct experimental detection and characterization of defects in MOFs are very challenging. We show that in many cases, the differences between experimentally observed and computationally predicted water stabilities of MOFs can be used to deduce information on the presence of point defects in real materials. Most computational studies of MOFs consider these materials to be defect-free, and in many cases, the resulting structures are predicted to be hydrophobic. Systematic experimental studies, however, have shown that many MOFs are hydrophilic. We show that the existence of chemically plausible point defects can often account for this discrepancy and use this observation in combination with detailed molecular simulations to assess the impact of local defects and flexibility in a variety of MOFs for which defects had not been considered previously.

Synthesis of pH responsive malononitrile functionalized metal organic framework MIL-100(Fe) for efficient adsorption of uranium U(VI) from real-life alkaline leach liquor

The porous framework of MIL-100(Fe) was functionalized using malononitrile (MN), through an in-situ Knoevenagel condensation reaction to introduce abundant -CN groups on the surface of the developed adsorbent. The resultant MN-functionalized MIL-100(Fe) exhibited excellent Uranium (U(VI)) removal capacity (i.e., 270 mg/g) at highly alkaline pH (⁓ 10). Different coexisting cations and anions show negligible influence on the U-removal and it was 92.1-99.7 % in presence of different co-ions, with the concentration from 10 to 50 mg/L. Moreover, MIL-100(Fe)_MN showed extremely selective U removal from the actual alkaline leach liquor (⁓ 97 %), without any pH adjustment and leaching of the constituent Fe. The surface-grafted -CN groups were predominantly active towards the coordinative interactions with the U(VI) ionic moieties, as evident from the XPS and FTIR analysis. The MIL-100(Fe)_MN adsorbent was also subjected to five consecutive adsorption-desorption cycles, with >90 % U removal after 5th cycle. Moreover, the regenerated MIL-100(Fe)_MN was structurally and functionally resilient, as observed from the morphological and crystallographic analysis. A convection-pore diffusion based transport model was used to analyze the optimized mass transfer parameters. Overall, the present study highlights the simple design and development of malononitrile-functionalized MIL-100(Fe) as an efficient and selective adsorbent for U(VI) removal from U-rich alkaline leach liquor.

Construction of dopamine supported Mg(Ca)Al layered double hydroxides with enhanced adsorption properties for uranium

A novel dopamine-supported Mg(Ca)Al layered double hydroxide composite was synthesized by co-precipitation method. The existence of Ca²⁺ and dopamine could promote the capture of uranium on the layered double hydroxides. In batch experiments, the composite exhibited good uranium removal performance, including high adsorption capacity (687.3 mg/g), strong anti-interference and good reusability (the removal percentage was still higher than 90 % after five cycles). At low initial uranium concentration, the uranium removal percentage on the composite exceeded 99.7 % and the residual concentration of uranium in the solution was <0.03 mg/L, reaching the limited standard of the World Health Organization. The studies of adsorption kinetics and isotherm indicated that the uranium adsorption behavior on the composite conformed to the pseudo-second-order kinetic and Langmuir isotherm models, suggesting that the process was a monolayer adsorption dominated by chemical adsorption. Furthermore, the high-efficiency uranium adsorption on the Mg(Ca)Al layered double hydroxide was mainly attributed to the strong complexation between the active sites (-OH and -NH₂) and uranium, the precipitation of interlayer intercalation ions (CO₃^2- and OH^-) to uranium and the ion exchange of Ca²⁺ to uranium. Due to these advantages, the dopamine-supported Mg(Ca)Al layered double hydroxide composite is expected to be used as fine adsorbent to remove uranium from wastewater.

Unveiling Structural Diversity of Uranyl Compounds of Aprotic 4,4'-Bipyridine N,N'-Dioxide Bearing O-Donors

As an aprotic O-donor ligand, 4,4'-bipyridine N,N'-dioxide (DPO) shows good potential for the preparation of uranyl coordination compounds. In this work, by regulating reactant compositions and synthesis conditions, diverse coordination assembly between uranyl and DPO under different reaction conditions was achieved in the presence of other coexisting O-donors. A total of ten uranyl-DPO compounds, U-DPO-1 to U-DPO-10, have been synthesized by evaporation or hydro/solvothermal treatment, and the possible competition and cooperation of DPO with other O-donors for the formation of these uranyl-DPO compounds are discussed. Starting with an aqueous solution of uranyl nitrate, it is found that an anionic nitrate or hydroxyl group is involved in the coordination sphere of uranyl in U-DPO-1 ((UO₂)(NO₃)₂(H₂O)₂·(DPO)), U-DPO-2 ((UO₂)(NO₃)₂(DPO)), and U-DPO-3 ((UO₂)(DPO)(μ₂-OH)₂), where DPO takes three different kinds of coordination modes, i.e. uncoordinated, monodentate, and biconnected. The utilization of UO₂(CF₃SO₃)₂ in acetonitrile, instead of an aqueous solution of uranyl nitrate, precludes the participation of nitrate and hydroxyl, and ensures the engagement of DPO ligands (4-5 DPO ligands for each uranyl) in a uranyl coordination sphere of U-DPO-4 ([(UO₂)(CF₃SO₃)(DPO)₂](CF₃SO₃)), U-DPO-5 ([UO₂(H₂O)(DPO)₂](CF₃SO₃)₂) and U-DPO-6 ([(UO₂)(DPO)_2.5](CF₃SO₃)₂). Moreover, when combined with anionic carboxylate ligands, terephthalic acid (H₂TPA), isophthalic acid (H₂IPA), and succinic acid (H₂SA), DPO works well with them to produce four mixed-ligand uranyl compounds with similar structures of two-dimensional (2D) networks or three-dimensional (3D) frameworks, U-DPO-7 ((UO₂)(TPA)(DPO)), U-DPO-8 ((UO₂)₂(DPO)(IPA)₂·0.5H₂O), U-DPO-9 ((UO₂)(SA)(DPO)·H₂O), and U-DPO-10 ((UO₂)₂(μ₂-OH)(SA)_1.5(DPO)). Density functional theory (DFT) calculations conducted to probe the bonding features between uranyl ions and different O-donor ligands show that the bonding ability of DPO is better than that of anionic CF₃SO₃ ^-, nitrate, and a neutral H₂O molecule and comparable to that of an anionic carboxylate group. Characterization of physicochemical properties of U-DPO-7 and U-DPO-10 with high phase purity including infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and luminescence properties is also provided.

Theoretical Investigation of Catalytic Water Splitting by the Arene-Anchored Actinide Complexes

Actinide complexes, which could enable the electrocatalytic H₂O reduction, are not well documented because of the fact that actinide-containing catalysts are precluded by extremely stable actinyl species. Herein, by using relativistic density functional theory calculations, the arene-anchored trivalent actinide complexes (^Me,MeArO)₃ArAn (marked as [AnL]) with desirable electron transport between metal and ligand arene are investigated for H₂ production. The metal center is changed from Ac to Pu. Electron-spin density calculations reveal a two-electron oxidative process (involving high-valent intermediates) for complexes [AnL] (An = P-Pu) along the catalytic pathway. The electrons are provided by both the actinide metal and the arene ring of ligand. This is comparable to the previously reported uranium catalyst (^Ad,MeArO)₃mesU (Ad = adamantine and mes = mesitylene). From the thermodynamic and kinetic perspectives, [PaL] offers appreciably lower reaction energies for the overall catalytic cycle than other actinide complexes. Thus, the protactinium complex tends to be the most reactive for H₂O reduction to produce H₂ and has the advantage of its experimental accessibility.

Modeling graphene oxide transport and retention in biochar

Experimental data from fixed-bed column studies and a numerical model based on convection-dispersion equations were used to describe transport and retention of Graphene Oxide (GO) in sand, biochar (BC), and BC modified with nanoscale zero-valent iron (BC-nZVI). Three blocking functions, namely no blocking, site-blocking, and depth-dependent blocking, were used to analyze GO transport and retention behavior in each media as a function of Ionic Strength (IS). An inverse modeling approach was implemented to determine the attachment coefficient (K_a) and maximum solid-phase retention capacity (S_max). The Langmuirian attachment model with site-blocking function effectively described experimental GO breakthrough curves (R² ~ 0.70-0.99) compared to other models, indicating the importance of introducing a limit on the attachment capacity of the media. The K_a values for BC and BC-nZVI were significantly higher than sand, attributable to high porosity, roughness, and surface chemical properties. The models predicted an increasing trend in K_a (0.065 to 0.615 min^-1) in BC with increasing IS (0.1 to 10 mM), while K_a values decreased (2.26 to 0.349 min^-1) for BC-nZVI. A consistent increase in S_max was observed for both BC and BC-nZVI with increasing IS. Scenario analysis was conducted to further understand the effect of influent IS, GO concentration, and treatment depth. BC-nZVI exhibited a higher K_a and S_max and as a result, higher GO retention than BC at lower IS (0.1 and 1.0 mM). BC-nZVI had a relatively lower K_a (0.349 min^-1) at 10 mM IS, however, it outperformed BC when GO retention capacities are compared over a longer period attributable to a higher S_max (6.47). Complete GO breakthrough occurred in a 5 cm media after 350 and 465 days for BC and BC-nZVI, respectively at 10 mM IS and influent concentration of 0.1 mg·L^-1. GO breakthrough time increased with increasing treatment depth, however, the relation was non-linear.

Metal-Organic Framework-Based Materials for Adsorption and Detection of Uranium(VI) from Aqueous Solution

The steady supply of uranium resources and the reduction or elimination of the ecological and human health hazards of wastewater containing uranium make the recovery and detection of uranium in water greatly important. Thus, the development of effective adsorbents and sensors has received growing attention. Metal-organic frameworks (MOFs) possessing fascinating characteristics such as high surface area, high porosity, adjustable pore size, and luminescence have been widely used for either uranium adsorption or sensing. Now pertinent research has transited slowly into simultaneous uranium adsorption and detection. In this review, the progress on the research of MOF-based materials used for both adsorption and detection of uranium in water is first summarized. The adsorption mechanisms between uranium species in aqueous solution and MOF-based materials are elaborated by macroscopic batch experiments combined with microscopic spectral technology. Moreover, the application of MOF-based materials as uranium sensors is focused on their typical structures, sensing mechanisms, and the representative examples. Furthermore, the bifunctional MOF-based materials used for simultaneous detection and adsorption of U(VI) from aqueous solution are introduced. Finally, we also discuss the challenges and perspectives of MOF-based materials for uranium adsorption and detection to provide a useful inspiration and significant reference for further developing better adsorbents and sensors for uranium containment and detection.

Strategies for induced defects in metal-organic frameworks for enhancing adsorption and catalytic performance

Metal-organic frameworks (MOFs) have emerged among porous materials. The designable structure and specific functionality make them stand out for diverse applications. In conceptual MOF, the metal ions/clusters and organic ligands...

Tailoring Defect Density in UiO-66 Frameworks for Enhanced Pb(II) Adsorption

Defect engineering of metal organic frameworks offers potential prospects for tuning their features toward particular applications. Herein, two series of defective UiO-66 frameworks were synthesized via changing the concentration of the linker and synthetic temperature of the reaction. These defective materials showed a significant improvement in the capability of Pb(II) removal from wastewater. This strategy for defect engineering not only created additional active sites, more open framework, and enhanced porosity but also exposed more oxygen groups, which served as the adsorption sites to improve Pb(II) adsorption. A relationship among degree of defects, texture features, and performances for Pb(II) removal was successfully developed as a proof-of-concept, highlighting the importance of defect engineering in heavy metal remediation. To investigate the kinetic and adsorption isotherms, we performed adsorption experiments influenced by the time and concentration of the adsorbate, respectively. For the practicality of the materials, the most significant parameters such as pH, temperature, adsorbent concentration, selectivity, and recyclability as well as simulated natural surface water were also examined. This study provides a clue for the researchers to design other advanced defective materials for the enhancement of adsorption performance by tuning the defect engineering.

Defect Level and Particle Size Effects on the Hydrolysis of a Chemical Warfare Agent Simulant by UiO-66

Defect engineering in metal-organic frameworks (MOFs) has recently become an area of significant research due to the possibility of enhancing material properties such as internal surface area and catalytic activity while maintaining stable 3D structures. Through a modulator screening study, the model Zr⁴⁺ MOF, UiO-66, has been synthesized with control of particle sizes (100-1900 nm) and defect levels (2-24%). By relating these properties, two series were identified where one property remained constant, allowing for independent analysis of the defect level or particle size, which frequently change coincident with the modulator choice. The series were used to compare UiO-66 reactivity for the hydrolysis of a chemical warfare agent simulant, dimethyl 4-nitrophenylphosphate (DMNP). The rate of DMNP hydrolysis displayed high dependence on the external surface area, supporting a reaction dominated by surface interactions. Moderate to high concentrations of defects (14-24%) allow for the accessibility of some interior MOF nodes but do not substantially promote diffusion into the framework. Individual control of defect levels and particle sizes through modulator selection may provide useful materials for small molecular catalysis and provide a roadmap for similar engineering of other zirconium frameworks.

Synthesis of calcium-aluminum-layered double hydroxide and a polypyrrole decorated product for efficient removal of high concentrations of aqueous hexavalent chromium

To efficiently remove high concentrations of hexavalent chromium (Cr(VI)), calcium-aluminum-layered double hydroxide (CaAl-LDH, denoted as CAL), and polypyrrole-modified CAL (CAL-PPy) were prepared by hydrothermal and in situ polymerization methods, respectively. The chemical structure, morphology, and elemental results indicated that the chain-like polypyrrole was decorated with hexagonal CAL. The specific surface area of CAL-PPy increased from 8.746 m²/g to 24.24 m²/g. The adsorption performances of CAL and CAL-PPy for aqueous Cr(VI) were investigated using batch equilibrium experiments. The decontamination process of aqueous Cr(VI) (100 mg/L) reached the equilibrium state within 50 min, and the kinetic data met the pseudo-second-order kinetic equation. The Langmuir model described the isothermal data properly, and the obtained theoretical adsorption capacity of CAL for Cr(VI) at 318 K was 34.06 mg/g, while that of CAL-PPy was 66.14 mg/g. The removal mechanisms involved electrostatic attraction, surface complexation, anion exchange, and reduction to low-toxicity Cr(III). Therefore, CAL and CAL-PPy have underlying applications in treating real wastewater containing Cr(VI).

Metal-organic frameworks as superior porous adsorbents for radionuclide sequestration: Current status and perspectives

Efficient separation of hazardous radionuclides from radioactive waste remains a challenge to the global acceptance of nuclear power due to complex nature of the waste, high radiotoxicities and presence of large number of interfering elements. Sorption of radioactive elements from liquid phase, gas phase or their solid particulates on various synthetic organic, inorganic or biological sorbents is looked as one of the options for their remediation. In this context, highly porous materials, termed as metal-organic frameworks (MOFs), have shown promise for efficient capturing of various types of radioactive elements. Major advantages that have been advocated for the application of MOFs in radionuclide sorption are their excellent chemical stability, and their large surface area due to abundant functional groups, and porosity. In this review, recent developments on the application of MOFs for radionuclide sequestration are briefly discussed. Focus has been devoted to address the separation of few crucial radioactive elements such as Th, U, Tc, Re, Se, Sr and Cs from aqueous solutions, which are important for liquid radioactive waste management. Apart from these radioactive metal ions, removal of radionuclide bearing gases such as I₂, Xe, and Kr are also discussed. Aspects related to the interaction of MOFs with the radionuclides are also discussed. Finally, a perspective for comprehensive investigation of MOFs for their applications in radioactive waste management has been outlined.

Multifunctional Two-Dimensional Metal-Organic Frameworks for Radionuclide Sequestration and Detection

Two lanthanide-containing porous coordination polymers, [Ln₂(bpdc)₆(phen)₂]·nH₂O (1) and [Ln₂(bpdc)₆(terpy)₂]·3H₂O (2) (Ln = Pr, Nd, or Sm-Dy; bpdc: 2,2'-bipyridine-5,5'-dicarboxylic acid; phen: 1,10-phenanthroline; and terpy: 2,2':6',2″-terpyridine), have been hydrothermally synthesized and structurally characterized by powder and single-crystal X-ray diffraction. Crystallographic analyses reveal that compounds 1 and 2 feature Ln³⁺-containing dimeric nodes that form a porous two-dimensional (2D) and nonporous three-dimensional (3D) framework, respectively. Each material is stable in aqueous media between pH 3 and 10 and exhibits modest thermal stability up to ∼400 °C. Notably, a portion of the phen and bpdc ligands in 1 can be removed thermally, without compromising the crystal structure, causing the surface area and pore volume to increase. The optical properties of 1 and 2 with Gd³⁺, Sm³⁺, Tb³⁺, and Eu³⁺ are explored in the solid state using absorbance, fluorescence, and lifetime spectroscopies. The analyses reveal a complex blend of metal and ligand emission in the materials containing Sm³⁺ and Tb³⁺, while those featuring Eu³⁺ are dominated by intense metal-based emission. Compound 1 with Eu³⁺ shows promise for the capture and detection of the uranyl cation (UO₂)²⁺ from aqueous media. In short, uranyl capture is observed at pH 4, and the adsorption thereof is detectable via vibrational and fluorescence spectroscopies and colorimetrically as the off-white color of 1 turns yellow with uptake. Finally, both 1 and 2 with Eu³⁺ produce bright red emission upon irradiation with Cu Kα X-ray radiation (8.04 keV) and are candidate materials for applications in solid-state scintillation.

Metal-Organic-Framework Based Functional Materials for Uranium Recovery: Performance Optimization and Structure/Functionality-Activity Relationships

Uranium recovery has profound significance in both uranium resource acquisition and pollution treatment. In recent years, metal-organic frameworks (MOFs) have attracted much attention as potential uranium adsorbents owing to their tunable structural topology and designable functionalities. This review explores the research progress in representative classic MOFs (MIL-101, UiO-66, ZIF-8/ZIF-67) and other advanced MOF-based materials for efficient uranium extraction in aqueous or seawater environments. The uranium uptake mechanism of the MOF-based materials is refined, and the structure/functionality-property relationship is further systematically elucidated. By summarizing the typical functionalization and structure design methods, the performance improvement strategies for MOF-based adsorbents are emphasized. Finally, the present challenges and potential opportunities are proposed for the breakthrough of high-performance MOF-based materials in uranium extraction.

Defect Engineering in Metal‒Organic Frameworks as Futuristic Options for Purification of Pollutants in an Aqueous Environment

Clean water scarcity is becoming an increasingly important worldwide issue. The water treatment industry is demanding the development of novel effective materials. Defect engineering in nanoparticles is among the most revolutionary of technologies. Because of their high surface area, structural diversity, and tailorable ability, Metal‒Organic Frameworks (MOFs) can be used for a variety of purposes including separation, storage, sensing, drug delivery, and many other issues. The application in wastewater treatment associated with water stable MOF‒based materials has been an emerging research topic in recent decades. Defect engineering is a sophisticated technique used to manufacture defects and to change the geometric framework of target compounds. Since MOFs have a series of designable structures and active sites, tailoring properties in MOFs by defect engineering is a novel concept. Defect engineering can excavate hidden active sites in MOFs, which can lead to better performance in many fields. Therefore, this technology will open new opportunities in water purification processes. However, there has been little effort to comprehensively discuss this topic. In this review, we provide an overview of the development of defect engineered MOFs for water purification processes. Furthermore, we discuss the potential applications of defect engineered materials.

Facile fabrication of ion-imprinted Fe3O4/carboxymethyl cellulose magnetic biosorbent: removal and recovery properties for trivalent La ions

An Fe₃O₄/carboxymethyl cellulose (Fe₃O₄/CMC) magnetic biosorbent was prepared using the ion-imprinting technology, where La(iii) was used as the template ion. The morphology and structure of Fe₃O₄/CMC were characterized by SEM, FTIR and XRD. It is found that nano Fe₃O₄ with inverse spinel structure can distribute in CMC and endow the composite with good magnetic properties. The adsorption performance such as adsorption capacity, influence of pH and initial concentration were fully explored. The prepared Fe₃O₄/CMC is revealed to have good adsorption properties with Q _max of 61.5 mg g^-1, in line with the pseudo-second-order kinetic model. When handling the multi-ion coexistence solution of Cu(ii), Ni(ii) and Cd(ii), Fe₃O₄/CMC shows high selective adsorption for La(iii). Meanwhile, cycling experiments find that the adsorption capacity is only slightly reduced (less than 5%) after 5-time reuse. Good adsorption properties, high selectivity and easy recovery give the newly-synthesized Fe₃O₄/CMC biosorbent broad application potential in the treatment of La(iii)-containing wastewater.

Interface-Constrained Layered Double Hydroxides for Stable Uranium Capture in Highly Acidic Industrial Wastewater

Low acid endurance of layered double hydroxides (LDHs) limits their uranium(VI) [U(VI)] adsorption capability from harsh industrial wastewater. Here, we demonstrate magnesium-cobalt LDHs (Mg-Co LDHs) anchored in situ onto the pore channel of dendritic fibrous nanosilica (DFNS) via an interface-constrained strategy. The synergy of Mg-Co LDHs and DFNS not only improves the endurance of the Mg-Co LDH under harsh acidic conditions but also increases the number of active sites of DFNS. Thus, DFNS@Mg-Co LDH shows a high U(VI) uptake capacity (1143 mg g^-1) at pH = 3 and C₀ = 598.7 mg L^-1, which is about 4.8-fold higher than that of pristine DFNS. The DFNS@Mg-Co LDH exhibits excellent U(VI) uptake in various background water circumstances due to its acid endurance and highly selective adsorption. This interface-constrained strategy provides LDH materials with durability under extremely acidic conditions along with a high adsorption capacity, which is promising for uranium capture from various water fields.

Carboxylated UiO-66 Tailored for U(VI) and Eu(III) Trapping: From Batch Adsorption to Dynamic Column Separation

U(VI) and Eu(III), as representative elements of the hexavalent actinide and trivalent lanthanides (always as a chemical analogue for trivalent actinide), respectively, have attracted more and more attentions due to the widespread use of nuclear energy. Much effort has been focused on developing versatile materials for their uptake from aqueous solution. For the first time, we report here UiO-66 and its mono- (UiO-66-COOH) and di-carboxyl (UiO-66-2COOH) functional derivatives as robust adsorbents for efficient U(VI) and Eu(III) removal. It is found that the introduction of carboxyl groups greatly reduces the surface charge of UiO-66, thus guaranteeing excellent adsorption capacity at low pH. At pH = 3, for example, the adsorption capacity of UiO-66-2COOH for U(VI) and Eu(III) is more than 100 and 60 mg/g, respectively, while almost no adsorption occurs for pristine UiO-66. At pH = 4, both UiO-66-COOH and UiO-66-2COOH show high performance on U(VI) and Eu(III) removal. UiO-66-COOH has adsorption capacities of 80 and 43 mg/g for U(VI) and Eu(III), respectively, while the values for UiO-66-2COOH reach 150 and 80 mg/g, respectively. Also, all these materials achieve adsorption equilibrium within 100 min. More importantly, combining the needs of practical applications and the characteristics of high stability, high porosity, and excellent adsorption performance of UiO-66-2COOH, dynamic adsorption column experiments were successfully conducted; ∼99% U(VI)/Eu(III) can be efficiently adsorbed, and >90% adsorbed U(VI)/Eu(III) can be re-collected with dilute nitric acid solution, even after four adsorption-desorption cycles. The findings of this work demonstrate the application potential of metal-organic framework materials to remove radionuclides from environmental samples or nuclear waste liquids.

The chemistry and applications of hafnium and cerium(iv) metal-organic frameworks

The coordination connection of organic linkers to the metal clusters leads to the formation of metal-organic frameworks (MOFs), where the metal clusters and ligands are spatially entangled in a periodic manner. The immense availability of tuneable ligands of different length and functionalities gives rise to robust molecular porosity ranging from several angstroms to nanometres. Among the large family of MOFs, hafnium (Hf) based MOFs have been demonstrated to be highly promising for practical applications due to their unique and outstanding characteristics such as chemical, thermal, and mechanical stability, and acidic nature. Since the report of UiO-66(Hf) and DUT-51(Hf) in 2012, less than 200 Hf-MOFs (ca. 50 types of structures) have been reported. Besides, tetravalent cerium [Ce(iv)] has been proven to be capable of forming similar topological MOF structures to Zr and Hf since its first discovery in 2015. So far, ca. 40 Ce(iv) MOFs with 60% having UiO-66-type structure have been reported. This review will offer a holistic summary of the chemistry, uniqueness, synthesis, and applications of Hf/Ce(iv)-MOFs with a focus on presenting the development in the Hf/Ce(iv)-clusters, topologies, ligand structures, synthetic strategies, and practical applications of Hf/Ce(iv)-MOFs. In the end, we will present the research outlook for the development of Hf/Ce(iv)-MOFs in the future, including fundamental design of Hf/Ce(iv)-clusters, defect engineering, and various applications including membrane development, diversified types of catalytic reactions, irradiation absorption in nuclear waste treatment, water production and wastewater treatment, etc. We will also present the emerging computational approaches coupled with machine-learning algorithms that can be applied in screening Hf and Ce(iv) based MOF structures and identifying the best-performing MOFs for tailor-made applications in future practice.

Graphene oxide transport and retention in biochar media

This study explores the use of biochar (BC), an inexpensive filtration media, for removing graphene oxide (GO) contaminants from the aquatic subsurface environments. Mass balance approaches and column dissection tests were used to analyze the retention behavior of GO in a series of model fixed-bed columns as a function of ionic strength (IS) and flowrate. The column based on the biochar media (BC) displayed 3.6-fold higher retention compared to the quartz sand (control). To overcome the challenges of unfavorable electrostatic interactions between GO and BC, we used a facile functionalization strategy to modify the BC surfaces with nanoscale zero-valent iron (BC-nZVI). The BC-nZVI (5:1, w/w) retained 2.6-fold higher amounts of GO compared with bare biochar. Furthermore, the performance of BC-nZVI increased with decreasing values of IS, attributed to the attachment of GO to nZVI where nZVI was partially dissolved by the presence of higher chloride ion at high IS. A better GO retention (86%) at higher IS was observed in BC where the GO was primarily retained due to the higher aggregation via straining.

An active and stable multifunctional catalyst with defective UiO-66 as a support for Pd over the continuous catalytic conversion of acetone and hydrogen

The one-pot synthesis of methyl isobutyl ketone (MIBK) and methyl isobutyl methanol (MIBC) from acetone and hydrogen is a typical cascade reaction comprised of aldol condensation-dehydration-hydrogenation. Pd loss and aggregation during long term operation are typical problems in industrial application. In this paper, an active and stable catalyst was achieved with defective UiO-66 as a support for Pd, which was synthesized with the ratio 15 : 1 of ZrOCl2·8H2O to ZrCl4 as Zr-precursors. The resultant Pd catalyst remained active for at least 1000 h with a MIBK + MIBC selectivity of 84.87-93.09% and acetone conversion of 45.26-53.22% in a continuous trickle-bed reactor. Besides the increased Brønsted acid amount generated by the defect sites was favorable for the activity, the cavity confinement in the UiO-66 (R = 15 : 1) structure also efficiently prevented Pd loss and aggregation during the long term run. The contrast of the characterization of the fresh and used Pd/UiO-66 (R = 15 : 1) indicated that the deactivation of the catalyst was attributed to carbonaceous accumulation on the catalyst surface, which could be easily regenerated by calcination. This work supplied a new alternative for the design and utilization of industrial catalysts for MIBK and MIBC synthesis.

Synthetic strategies to incorporate Ru-terpyridyl water oxidation catalysts into MOFs: direct synthesis vs. post-synthetic approach

Incorporating molecular catalysts into metal-organic frameworks (MOFs) is a promising strategy for improving their catalytic longevity and recyclability. In this article, we investigate and compare synthetic routes for the incorporation of the potent water oxidation catalyst Ru(tda)(pyCO2H)2 (tda = 2,2':6',2''-terpyridine-6,6''-dicarboxylic acid, pyCO2H = iso-nicotinic acid) as a structural linker into a Zr-based UiO-type MOF. The task is challenging with this particular metallo-linker because of the equatorial dangling carboxylates that can potentially compete for Zr-coordination, as well as free rotation of the pyCO2H groups around the HO2CpyRupyCO2H axis. As a consequence, all attempts to synthesize a MOF with the metallo-linker directly under solvothermal conditions led to amorphous materials with the Ru(tda)(pyCO2H)2 linker coordinating to the Zr nodes in ill-defined ways, resulting in multiple waves in the cyclic voltammograms of the solvothermally obtained materials. On the other hand, an indirect post-synthetic approach in which the Ru(tda)(pyCO2H)2 linker is introduced into a preformed edba-MOF (edba = ethyne dibenzoic acid) of UiO topology results in the formation of the desired material. Interestingly, two distinctly different morphologies of the parent edba-MOF have been discovered, and the impact that the morphological difference has on linker incorporation is investigated.

Tuning the Wettability of Metal-Organic Frameworks via Defect Engineering for Efficient Oil/Water Separation

Zirconium-based metal-organic frameworks (MOFs) have attracted interest due to their chemical and thermal stabilities and structural tunability. In this work, we demonstrate the tuning of the wettability of a UiO-66 structure via defect-engineering for efficient oil/water separation. UiO-66 crystals with controlled levels of missing-linker defects were synthesized using a modulation approach. As a result, the hydrophilicity of the defect-engineered UiO-66 (d-UiO-66) can be varied. In addition, a thin layer of hydrophilic d-UiO-66 was successfully fabricated on a series of stainless steel meshes (d-UiO-66@mesh), which exhibited excellent superhydrophilic and underwater superoleophobic properties and displayed interesting separation performance for various oil/water mixtures.

Confined Heteropoly Blues in Defected Zr-MOF (Bottle Around Ship) for High-Efficiency Oxidative Desulfurization

The Keggin-type polyoxometalates (POMs) are effective catalysts for oxidative desulfurization (ODS) and confining these POMs in metal-organic frameworks (MOFs) is a promising strategy to improve their performances. Herein, postsynthetic modification of POMs confined in MOFs by adding thiourea creates more unsaturated metal sites as defects, promoting ODS catalytic activity. Additional modification by confining 1-butyl-3-methyl imidazolium POMs in MOFs is performed to obtain higher ODS activity, owing to the affinity between electron-rich thiophene-based compounds and electrophilic imidazolium compounds. The ODS catalytic activities of four Zr-MOF-based composites (bottle around ship) including phosphomolybdate acid (PMA)/UiO-66, [Bmim]₃ PMo₁₂ O₄₀ /UiO-66, PMA/Thiourea/UiO-66, and [Bmim]₃ PMo₁₂ O₄₀ /Thiourea/UiO-66 are therefore investigated in detail. In order to explore the catalytic mechanism of these MOF composites, their microstructures and electronic structures are probed by various techniques such as X-ray diffraction, thermogravimetric analysis, Fourier transform infrared, Raman, scanning electron microscope, transmission electron microscope, BET, X-ray photoelectron spectroscopy, EPR, UV-vis, NMR spectra, and H₂ -temperature-programmed reduction. The results reveal that phosphomolybdate blues and imidazolium phosphomolybdate blues with different Mo⁵⁺ /Mo⁶⁺ ratios with the Keggin structure are confined in defected UiO-66 for all four composites. This approach can be applied to design and synthesize other POMs/MOFs composites as efficient catalysts.

Rational Construction of Porous Metal-Organic Frameworks for Uranium(VI) Extraction: The Strong Periodic Tendency with a Metal Node

Although metal-organic frameworks (MOFs) have been reported as important porous materials for the potential utility in metal ion separation, coordinating the functionality, structure, and component of MOFs remains a great challenge. Herein, a series of anionic rare earth MOFs (RE-MOFs) were synthesized via a solvothermal template reaction and for the first time explored for uranium(VI) capture from an acidic medium. The unusually high extraction capacity of UO₂²⁺ (e.g., 538 mg U per g of Y-MOF) was achieved through ion-exchange with the concomitant release of Me₂NH₂⁺, during which the uranium(VI) extraction in the series of isostructural RE-MOFs was found to be highly sensitive to the ionic radii of the metal nodes. That is, the uranium(VI) adsorption capacities continuously increased as the ionic radii decreased. In-depth mechanism insight was obtained from molecular dynamics simulations, suggesting that both the accessible pore volume of the MOFs and hydrogen-bonding interactions contribute to the strong periodic tendency of uranium(VI) extraction.

In Situ Ligand Formation-Driven Synthesis of a Uranyl Organic Framework as a Turn-on Fluorescent pH Sensor

A uranium-based metal-organic framework, [(UO₂)(H₂DTATC)] (HNU-39, H₄DTATC = 5,5'-(9,10-dihydroxy-4a,9,9a,10-tetrahydroanthracene-9,10-diyl)diisophthalic acid) was successfully prepared by a hydrothermal method. The structure of HNU-39 comprises UO₈ hexagonal bipyramids linked by doubly protonated DTATC ligands, forming a ribbon arrangement. It is worth noting that the DTATC ligand was transformed in situ from 5,5'-(anthracene-9,10-diyl)diisophthalic acid (H₄DPATC) during the synthesis of HNU-39. Research on fluorescence properties has shown that HNU-39 exhibits fluorescence turn-on response under alkaline conditions and could be used as a potential pH sensor. Moreover, HNU-39 can also be successfully applied for pH sensing in real samples from a sewage treatment plant. The sensing mechanism can be interpreted as OH^- ions reacting with the protons in the organic ligand of HNU-39.

Grafting functional groups in metal–organic frameworks for U(vi) sorption from aqueous solutions

Recent highlights of the organic groups-functionalized MOFs for uranium extraction from aqueous solution are discussed.