Recent Progress in Metal-Organic Frameworks and Their Derived Nanostructures for Energy and Environmental Applications

Layered double hydroxide-derived bimetallic-MOF as a promising platform: Urea-coupled water oxidation and supercapattery-driven water electrolyzer

Developing a two-dimensional (2D) ultrathin metal-organic framework plays a significant role in energy conversion and storage systems. This work introduced a facile strategy for engineering ultrathin NiMn-MOF nanosheets on Ni foam (NF) via in situ conversion from NiMn-layered double hydroxide (LDH). The as-synthesized LDH-derived NiMn-MOF (LDH-D NiMn-MOF) nanosheet exhibited an overpotential of 350 mV to drive a current density of 100 mA cm-2 during oxygen evolution reaction (OER) owing to its better redox activity, hierarchical architecture, and intercalating ability. The similar effective catalytic trend was noticed during the urea-assisted water oxidation process. The developed catalyst required only a potential of 1.39 V vs. RHE at 100 mA cm-2 towards urea oxidation reaction (UOR). Moreover, the urea-assisted overall water-splitting voltage was found to be 1.5 V at the current density of 10 mA cm-2. Furthermore, the same catalyst was explored as an energy-storage material for supercapattery application with an aerial specific capacity value of 2613.9 mC cm-2 at 1 mA cm-2 which was found to be 1.5 times higher than NiMn-LDH (1724.3 mC cm-2). Additionally, an aqueous asymmetric supercapattery device was fabricated which demonstrated the best electrochemical performance and provided a maximum energy density of 64.1 Wh kg-1 at a power density of 493 W kg-1 with 77.8 percent capacity retention after a continuous run of 8000 cycles at 10 mA cm-2 current density. Hence, the multifaceted properties of energy conversion and storage of LDH-D NiMn-MOF outline its performance in real-world applications.

Emerging insights into the application of metal-organic framework (MOF)-based materials for electrochemical heavy metal ion detection

Heavy metal ions are one of the main sources of water pollution, which has become a major global problem. Given the growing need for heavy metal ion detection, electrochemical sensor stands out for its high sensitivity and efficiency. Metal-organic frameworks (MOFs) have garnered much interest as electrode modifiers for electrochemical detection of heavy metal ions owing to their significant specific surface area, tailored pore size, and catalytic activity. This review summarizes the progress of MOF-based materials, including pristine MOFs and MOF composites, in the electrochemical detection of various heavy metal ions. The synthetic methods of pristine MOFs, the detection mechanisms of heavy metal ions and the modification strategies of MOFs are introduced. Besides, the diverse applications of MOF-based materials in detecting both single and multiple heavy metal ions are presented. Furthermore, we present the current challenges and prospects for MOF-based materials in electrochemical heavy metal ion detection.

Mixed-Linker Zr-Metal-Organic Framework with Improved Lewis Acidic Sites for CO2 Fixation Reaction Catalysis

Applying the mixed-linker strategy in synthesizing metal-organic frameworks (MOFs) has drawn considerable attention as a heterogeneous catalyst owing to their easy synthesis and different functional ligands in their frameworks. Following this strategy, we have developed a mixed linker Zr(IV)-based MOF, [Zr6O4(OH)4(FUM)n(PZDC-NO2)6-n] (PZDC-NO2 = 4-nitro-3,5-pyrazoledicarboxylic acid, FUM = fumaric acid) denoted as MOF-801(PZDC-NO2) synthesized via this strategy which possess an electron-withdrawing group (-NO2) on secondary linkers. The MOF-801(PZDC-NO2) has been fully characterized via various analyses, such as Fourier transform infrared, powder X-ray diffraction, 13C/1H nuclear magnetic resonance, XPS, TGA, and N2 adsorption/desorption, SEM, EDX, etc. By considering the concurrent existence of acid-base active sites and the synergistic role of these sites, this mixed-linker MOF was used as a catalyst for the cycloaddition reaction of CO2 and epoxides under mild without-solvent conditions. MOF-801(PZDC-NO2) displays significant catalytic performance by producing the highest catalytic conversion of epoxide to cyclic carbonate (93%) with a turnover number of 130.7 in 8 h reaction time and 100 °C temperature under low-pressure CO2 pressure. The mixed-linker Zr-MOF exhibits exceptional stability and reusability, maintaining its structure and functionality after consecutive cycles of utilization. Finally, the reaction mechanism was further investigated by density functional theory calculations. The total energy of the reactants, intermediates, and products involved in the process.

Boosting Photocatalytic Hydrogen Production by MOF-Derived Metal Oxide Heterojunctions with a 10.0 % Apparent Quantum Yield

Photocatalytic hydrogen production offers an alternative pathway to establish a sustainable energy economy, utilizing the Earth's natural sunlight and water resources to address environmental concerns associated with fossil fuel combustion. While numerous photoactive materials exhibit high potential for generating hydrogen from water, the synergy achieved by combining two different materials with complementary properties in the form of heterojunctions can significantly enhance the rate of hydrogen production and quantum efficiency. Our study describes the design and generation of the metal-organic framework-derived (MOF) metal oxide heterojunction herein referred to as RTTA, composed of RuO2/N,S-TiO2. The RuO2/N,S-TiO2 is generated through the pyrolysis of MOFs, Ru-HKUST-1, and the amino-functionalized MIL-125-NH2 in the presence of thiourea. Among the various RTTA materials tested, RTTA-1, characterized by the lowest RuO2 content, exhibited the highest hydrogen evolution rate, producing 10,761 μmol ⋅ hr-1 ⋅ g-1 of hydrogen with an apparent quantum yield of 10.0 % in pure water containing glycerol. In addition to RTTA-1, we generated two other MOF-derived metal oxide heterojunctions, namely ZTTA-1 (ZnO/N,S-TiO2) and ITTA-1 (In2O3/N,S-TiO2). These heterojunctions were tested for their photocatalytic activity, leading to apparent quantum yields of 0.7 % and 0.3 %, respectively. The remarkable photocatalytic activity observed in RTTA-1 is thought to be attributed to the synergistic effects arising from the combination of metallic properties inherent in the metal oxides, complemented by the presence of suitable band alignment, porosity, and surface properties inherited from the parent MOFs. These properties enhance electron transfer and restrict hole movement. The photocatalytic efficiency of RTTA-1 was further demonstrated in actual water samples, producing hydrogen with a rate of 8,190 μmol ⋅ hr-1 ⋅ g-1 in tap water, and 6,390 μmol ⋅ hr-1 ⋅ g-1 in river water.

Flexible electronics for heavy metal ion detection in water: a comprehensive review

Flexible electronics offer a versatile, rapid, cost-effective and portable solution to monitor water contamination, which poses serious threat to the environment and human health. This review paper presents a comprehensive exploration of the versatile platforms of flexible electronics in the context of heavy metal ion detection in water systems. The review overviews of the fundamental principles of heavy metal ion detection, surveys the state-of-the-art materials and fabrication techniques for flexible sensors, analyses key performance metrics and limitations, and discusses future opportunities and challenges. By highlighting recent advances in nanomaterials, polymers, wireless integration, and sustainability, this review aims to serve as an essential resource for researchers, engineers, and policy makers seeking to address the critical challenge of heavy metal contamination in water resources. The versatile promise of flexible electronics is thoroughly elucidated to inspire continued innovation in this emerging technology arena.

Durable and recyclable MOF@polycaprolactone mixed-matrix membranes with hierarchical porosity for wastewater treatment

With the fast-growing global water crisis, the development of novel technologies for water remediation and reuse is crucial. Industrial wastewater especially contains various toxic pollutants that pose an additional threat to the environment; thus, efficient removal of such contaminants can ensure safe reprocessing of industrial wastewater, thereby alleviating the demand for fresh water. Herein, we describe a novel and efficient approach for preparing porous polycaprolactone (PCL) membranes with a hierarchical architecture via a simple solvent/non-solvent methodology. A mixed-matrix membrane (MMM) was further constructed utilizing an amine-functionalized metal-organic framework as the sorbent filler nanoparticles and PCL as the polymer support matrix (MOF@PCL) for wastewater treatment applications. The MOF@PCL MMM demonstrated homogeneous morphology as well as exceptional performance towards the removal of both cationic (methylene blue, MB) and anionic (methyl orange, MO) organic dyes, where the maximum adsorption capacities reached 309 mg g-1 and 208 mg g-1, respectively. Kinetic and thermodynamic investigations revealed that the adsorption process was endothermic with a fast intraparticle diffusion rate constant. The MOF@PCL MMM also displayed excellent mechanical stability and recyclability, where the removal efficiency was maintained after 10 cycles.

Engineered Catalyst Based on MIL-68(Al) with High Stability for Hydrogenation of Carbon Dioxide and Carbon Monoxide at Low Temperature

Today, the importance of decreasing and converting COx gases from the atmosphere into value-added chemicals by catalytic hydrogenation reactions has become one crucial challenge. In the current work, to facilitate the hydrogenation of COx, several mesoporous alumina catalysts with high efficiency and stability were synthesized using the MIL-68(Al) platform, a nanoporous MOF with a high surface area as a precatalyst, encapsulating nickel or nickel-iron nanoparticles (NPs). After removing the organic linker of MIL-68(Al) by calcination in air, two types of catalysts, promoted and unpromoted, were obtained with various loads of nickel and iron. A set of analyses (PXRD, BET-N2, TEM, FE-SEM, ICP-OES, EDX-map, CO2-TPD, H2-TPR, and H2-TPD) were performed to evaluate the physicochemical properties of catalysts. Based on the analysis results, the promoted catalyst had smaller particles and pores due to the effective and uniform distribution of nickel NPs. Also, H2-TPR and CO2-TPD results in samples containing Fe promoter demonstrated the facilitation of the reduction process and the adsorption and activation of CO2, respectively. The results of CO2 methanation indicated an improved catalytic performance for promoted samples, especially at low temperatures (200-300 °C), compared to unpromoted catalysts. 5Fe·15Ni@Al2O3 MIL-68(Al) catalyst displayed the best performance compared to other catalysts, with a conversion of 92.4% and selectivity of 99.6% at 350 °C and GHSV = 2500 h-1. Moreover, the 5Fe·15Ni@Al2O3 MIL-68(Al) catalyst facilitated the CO2 methanation reaction by reducing the activation energy to 42.5 kJ mol-1 compared with other reported catalysts. Both types of catalysts performed 100% hydrogenation of CO to CH4 with full selectivity at 250 °C and exhibited high stability for at least 100 h at 300 °C. Notably, such high significant catalytic performance is only achieved by the usage of the "MOFs templating strategy" due to the high surface area for the effective distribution of NPs, the strong metal-support interaction, and the formation of nickel aluminate species, preventing the sintering of NPs.

Controlled Encapsulation of Gold Nanoparticles into Zr-Metal-Organic Frameworks with Improved Detection Limitation of Volatile Organic Compounds via Surface-Enhanced Raman Scattering

Volatile organic compounds (VOCs) have harmful effects on human health and the environment but detecting low levels of VOCs is challenging due to a lack of reliable biomarkers. However, incorporating gold nanoparticles (Au NPs) into metal-organic frameworks (MOFs) shows promise for VOC detection. In this study, we developed nanoscale Au@UiO-66 that exhibited surface-enhanced Raman scattering (SERS) activity even at very low levels of toluene vapors (down to 1.0 ppm) due to the thickness of the shell and strong π-π interactions between benzenyl-type linkers and toluene. The UiO-66 shell also increased the thermal stability of the Au NPs, preventing aggregation up to 550 °C. This development may be useful for sensitive detection of VOCs for environmental protection purposes.

Temperature-dependent tailoring of the pore structure based on MOF-derived carbon electrodes for electrochemical capacitors

The pore structures of carbon play a critical role in the charge storage process of electrochemical capacitors; however, the involvement of other varying characteristics, such as electrical conductivity and surface functionalities, complicate the research of the pore size effects on various electrochemical phenomena. In this study, by carbonizing MOF-5 at a selected temperature range of 500-700 °C, a series of MOF-derived carbon materials were obtained with pore size distribution concentrated in different size ranges while admitting similar results in the graphitization degree and surface functionalities. The related morphological changes of ZnO were systematically investigated by changing the carbonization temperature and dwelling time, demonstrating a "from thin to thick, from inside to outside" growth routine of ZnO crystals. With the pore size approximated as the sole variable, the as-assembled electrochemical capacitors present a linear relationship between the 1-10 nm pores and the impedance resistance, which for the first time demonstrate how 1-10 nm pores is beneficial to ion diffusion. The results of this study not only provide a useful approach to manipulating the pore structure in carbon electrodes but also pave the way to establish the numerical relationship between the pore structure and various phenomena in electrochemistry or other related areas.

Cathode materials for lithium-sulfur battery: a review

Lithium-sulfur batteries (LSBs) are considered to be one of the most promising candidates for becoming the post-lithium-ion battery technology, which would require a high level of energy density across a variety of applications. An increasing amount of research has been conducted on LSBs over the past decade to develop fundamental understanding, modelling, and application-based control. In this study, the advantages and disadvantages of LSB technology are discussed from a fundamental perspective. Then, the focus shifts to intermediate lithium polysulfide adsorption capacity and the challenges involved in improving LSBs by using alternative materials besides carbon for cathode construction. Attempted alternative materials include metal oxides, metal carbides, metal nitrides, MXenes, graphene, quantum dots, and metal organic frameworks. One critical issue is that polar material should be more favorable than non-polar carbonaceous materials in the aspect of intermediate lithium polysulfide species adsorption and suppress shuttle effect. It will be also presented that by preparing cathode with suitable materials and morphological structure, high-performance LSB can be obtained.

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A Manganese(II) 3D Metal–Organic Framework with Siloxane-Spaced Dicarboxylic Ligand: Synthesis, Structure, and Properties

A new metal–organic framework {[Mn4(Cx)3(etdipy)5]·2ClO4}n (1) was prepared via the complexation of manganese ion from a Mn(ClO4)2 source with 1,3-bis(carboxypropyl)tetramethyldisiloxane (Cx) and 1,2-di(4-pyridyl)ethylene (etdipy) in the presence of 2,4-lutidine as a deprotonating agent. The single-crystal X-ray diffraction analysis revealed a dense 3D framework structure. The presence in the structure of flexible tetramethyldisiloxane moieties, which tend to orient themselves at the interface with the air, gives the compound a highly hydrophobic character, as indicated by the result of the water vapor sorption analysis in the dynamic regime, as well as the shape and stability of the water droplet on the crystalline mass of the compound. The compound is an electrical insulator, and due to its hydrophobicity, this characteristic is unaffected by environmental dampness. The thermal analysis indicated thermal stability up to about 300 °C and an unusual thermal transition for an MOF structure, more precisely a glass transition at 24 °C, the latter also being attributed to the flexible segments in the structure. The magnetic studies showed dominant antiferromagnetic interactions along the metal ion chain in compound 1.

Metal-Organic Frameworks and Their Composites for Environmental Applications

From the point of view of the ecological environment, contaminants such as heavy metal ions or toxic gases have caused harmful impacts on the environment and human health, and overcoming these adverse effects remains a serious and important task. Very recent, highly crystalline porous metal-organic frameworks (MOFs), with tailorable chemistry and excellent chemical stability, have shown promising properties in the field of removing various hazardous pollutants. This review concentrates on the recent progress of MOFs and MOF-based materials and their exploit in environmental applications, mainly including water treatment and gas storage and separation. Finally, challenges and trends of MOFs and MOF-based materials for future developments are discussed and explored.

High resolution solid state NMR in paramagnetic metal-organic frameworks

We study the metal-organic framework (MOF) ZIF-67 with ¹H and ¹³C nuclear magnetic resonance (NMR). In addition to the usual orbital chemical shifts, we observe spinning sideband manifolds in the NMR spectrum due to hyperfine interactions of the paramagnetic cobalt with ¹H and ¹³C. Both orbital and paramagnetic chemical shifts are in good agreement with values calculated from first principles, allowing high-confidence assignment of the observed peaks to specific sites within the MOF. Our measured resonance shifts, line shapes, and spin lattice relaxation rates are also consistent with calculated values. We show that molecules in the pores of the MOF can exhibit high-resolution NMR spectra with fast spin lattice relaxation rates due to dipole-dipole couplings to the Co²⁺ nodes in the ZIF-67 lattice, showcasing NMR spectroscopy as a powerful tool for identification and characterization of "guests" that may be hosted by the MOF in electrochemical and catalytic applications.

Dye contaminated wastewater treatment through metal–organic framework (MOF) based materials

A complete discussion of MOFs and MOF composites such as MOF-based membranes, magnetic MOFs, and metal–organic gels (MOGs) used for dye removal along with their adsorption efficiency has been done.

Nickel phosphonate-derived Ni2P@N-doped carbon co-catalyst with built-in electron-bridge for boosting the photocatalytic hydrogen evolution

To construct photocatalytic systems that can efficiently convert solar energy to hydrogen energy, numerous studies have been focused on transition metal phosphides (TMPs) co-catalysts, which display low overpotential and cost...

Methanation of CO2 Using MIL-53-Based Catalysts: Ni/MIL-53–Al2O3 versus Ni/MIL-53

MIL-53 and the MIL-53–Al2O3 composite synthesized by a solvothermal procedure, with water as the only solvent besides CrCl3 and benzene-1,4-dicarboxylic acid (BDC), were used as catalytic supports to obtain the novel MIL-53-based catalysts Ni(10 wt.%)/MIL-53 and Ni(10 wt.%)/MIL-53–Al2O3. Ni nanoparticle deposition by an adapted double-solvent method leads to the uniform distribution of metallic particles, both smaller (≤10 nm) and larger ones (10–30 nm). MIL-53–Al2O3 and Ni/MIL-53–Al2O3 show superior thermal stability to MIL-53 and Ni/MIL-53, while MIL-53–Al2O3 samples combine the features of both MIL-53 and alumina in terms of porosity. The investigation of temperature’s effect on the catalytic performance in the methanation process (CO2:H2 = 1:5.2, GHSV = 4650 h−1) revealed that Ni/MIL-53 is more active at temperatures below 300 °C, and Ni/MIL-53–Al2O3 above 300 °C. Both catalysts show maximum CO2 conversion at 350 °C: 75.5% for Ni/MIL-53 (methane selectivity of 93%) and 88.8% for Ni/MIL-53–Al2O3 (methane selectivity of 98%). Stability tests performed at 280 °C prove that Ni/MIL-53–Al2O3 is a possible candidate for the CO2 methanation process due to its high CO2 conversion and CH4 selectivity, corroborated by the preservation of the structure and crystallinity of MIL-53 after prolonged exposure in the reaction medium.

Slow Magnetic Relaxation in {[CoCxAPy)] 2.15 H2O}n MOF Built from Ladder-Structured 2D Layers with Dimeric SMM Rungs

We present the magnetic properties of the metal-organic framework {[CoCxAPy]·2.15 H2O}n (Cx = bis(carboxypropyl)tetramethyldisiloxane; APy = 4,4`-azopyridine) (1) that builds up from the stacking of 2D coordination polymers. The 2D-coordination polymer in the bc plane is formed by the adjacent bonding of [CoCxAPy] 1D two-leg ladders with Co dimer rungs, running parallel to the c-axis. The crystal packing of 2D layers shows the presence of infinite channels running along the c crystallographic axis, which accommodate the disordered solvate molecules. The Co(II) is six-coordinated in a distorted octahedral geometry, where the equatorial plane is occupied by four carboxylate oxygen atoms. Two nitrogen atoms from APy ligands are coordinated in apical positions. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization measurements on an isostructural compound {[Zn0.8Co0.2CxAPy]·1.5 CH3OH}n (2). The results show that the Co(II) ion has orthorhombic anisotropy with the hard-axis direction in the C2V main axis, lying the easy axis in the distorted octahedron equatorial plane, as predicted by the ab initio calculations of the g-tensor. Magnetic and heat capacity properties at very low temperatures are rationalized within a S* = 1/2 magnetic dimer model with anisotropic antiferromagnetic interaction. The magnetic dimer exhibits slow relaxation of the magnetization (SMM) below 6 K in applied field, with a tlf ≈ 2 s direct process at low frequencies, and an Orbach process at higher frequencies with U/kB = 6.7 ± 0.5 K. This compound represents a singular SMM MOF built-up of Co-dimers with an anisotropic exchange interaction.

Recent Advances in Metal-Organic Frameworks Derived Nanocomposites for Photocatalytic Applications in Energy and Environment

Solar energy is a key sustainable energy resource, and materials with optimal properties are essential for efficient solar energy-driven applications in photocatalysis. Metal-organic frameworks (MOFs) are excellent platforms to generate different nanocomposites comprising metals, oxides, chalcogenides, phosphides, or carbides embedded in porous carbon matrix. These MOF derived nanocomposites offer symbiosis of properties like high crystallinities, inherited morphologies, controllable dimensions, and tunable textural properties. Particularly, adjustable energy band positions achieved by in situ tailored self/external doping and controllable surface functionalities make these nanocomposites promising photocatalysts. Despite some progress in this field, fundamental questions remain to be addressed to further understand the relationship between the structures, properties, and photocatalytic performance of nanocomposites. In this review, different synthesis approaches including self-template and external-template methods to produce MOF derived nanocomposites with various dimensions (0D, 1D, 2D, or 3D), morphologies, chemical compositions, energy bandgaps, and surface functionalities are comprehensively summarized and analyzed. The state-of-the-art progress in the applications of MOF derived nanocomposites in photocatalytic water splitting for H2 generation, photodegradation of organic pollutants, and photocatalytic CO2 reduction are systemically reviewed. The relationships between the nanocomposite properties and their photocatalytic performance are highlighted, and the perspectives of MOF derived nanocomposites for photocatalytic applications are also discussed.

Synthesis and characterization of defective UiO-66 for efficient co-immobilization of arsenate and fluoride from single/binary solutions

Here, we aimed to synthesize UiO-66 architected fumaric acid mediated lanthanum (La-fum), zirconium (Zr-fum), and cerium (Ce-fum) metal-organic frameworks (MOFs) for co-immobilizations of both arsenate and fluoride from both single and binary systems. The crystalline behavior of Zr-fum MOF was the lowest compared to the other two forms, due to the fact that it required a modulator support as the nucleus growth nature of zirconium moiety is different. The Langmuir maximum adsorption densities of arsenate (fluoride) were 2.689 (4.240), 1.666 (2.255), and 2.174 (4.155) mmol/g for La-fum, Zr-fum, and Ce-fum, respectively and these adsorption densities were found to have record-high values compared with the existing materials in the literature. The arsenate and fluoride adsorption on the MOF materials were confirmed by XPS, PXRD and FTIR studies. The arsenate adsorption mechanism on La-fum and Ce-fum through monodentate complexation confirmed using the distinguished K-edge shell distance in EXAFS studies. The arsenate and fluoride-sorbed materials were recycled using 0.01 M HNO₃ and were further utilized for six consecutive cycles for both arsenate and fluoride adsorption indicated the feasibility of the materials. This kind of facile and easy solvothermal synthesized MOFs could pave a way towards the removal of toxins in a practical wastewater as these have superior adsorption properties, stability and reusability.

Removal of low Sb(V) concentrations from mining wastewater using zeolitic imidazolate framework-8

Wastewater generated during mining remains a significant source of antimony pollution, because techniques to quickly and efficiently remove antimony from wastewater do not exist. In this study, zeolitic imidazolate framework-8 (ZIF-8), a specific type of Metal Organic Frameworks (MOFs), was successfully used to remove trace levels (1 mg L^-1) of Sb(V) with a high removal efficiency when the ZIF-8 dose was 0.5 g L^-1. Scanning electron microscopy-X-ray energy dispersive spectrometry (SEM-EDS) indicated that Sb(V) was adsorbed onto the ZIF-8surface. The powder X-ray diffraction (XRD) pattern of ZIF-8 before and after adsorption of Sb(V) indicated that ZIF-8 was successfully synthesized, and remained structurally stable after Sb(V) was adsorbed. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) both suggested complexation of zinc on ZIF-8 with Sb(V), where removal of Sb(V) by ZIF-8 followed the Langmuir adsorption isotherm with pseudo second-order kinetics. Thus, a possible removal mechanism was proposed which involved Sb(V) complexing with the zinc hydroxyl groups on ZIF-8 (Zn-OH-Sb). Practically, ZIF-8, could remove 78.6% of Sb(V) from a mining wastewater containing 20 μg L^-1 Sb(V). Furthermore, ZIF-8 could be remain active after repeated uses and could still remove and 42.3% of Sb(V) from wastewater containing 1 mg L^-1) Sb(V) even when the ZIF-8 was reused five time. This indicated that ZIF-8 had potential for practical removal of Sb(V) from mining wastewaters.

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.

A Carboxyl-Functionalized Covalent Organic Framework Synthesized in a Deep Eutectic Solvent for Dye Adsorption

Instead of using organic solvents, a deep eutectic solvent (DES) composed of tetrabutylammonium bromide and imidazole (Bu₄ NBr/Im) was employed as a solvent for the first time to synthesize covalent organic frameworks (COFs). Due to the low vapor pressure of the Bu₄ NBr/Im-based DES, a new carboxyl-functionalized COF (TpPa-COOH) was synthesized under environmental pressure. The as-synthesized TpPa-COOH has open channels, and the DES can be removed completely from the pores. The dye adsorption performance of TpPa-COOH was examined for three organic dyes with similar molecular sizes: one anionic dye (eosin B, EB) and two cationic dyes (methylene blue, MB and safranine T, ST). TpPa-COOH showed an excellent selective adsorption effect on MB and ST. The electronegative keto form in TpPa-COOH might help to form electrostatic and π-π interactions between the π-stacking frameworks of TpPa-COOH and the positive plane MB and ST molecules. The adsorption isotherms of MB and ST on TpPa-COOH were further investigated in detail, and the equilibrium adsorption was well modeled by using a Langmuir isotherm model. Together with hydrogen bonding, TpPa-COOH showed higher adsorption capacity for ST than for MB (1135 vs. 410 mg g^-1 ). These results could provide a guidance for the green synthesis of adsorbents in removing organic dyes from wastewater.

Critical Aspects of Metal-Organic Framework-Based Materials for Solar-Driven CO2 Reduction into Valuable Fuels

Photoreduction of CO2 into value-added fuels is one of the most promising strategies for tackling the energy crisis and mitigating the "greenhouse effect." Recently, metal-organic frameworks (MOFs) have been widely investigated in the field of CO2 photoreduction owing to their high CO2 uptake and adjustable functional groups. The fundamental factors and state-of-the-art advancements in MOFs for photocatalytic CO2 reduction are summarized from the critical perspectives of light absorption, carrier dynamics, adsorption/activation, and reaction on the surface of photocatalysts, which are the three main critical aspects for CO2 photoreduction and determine the overall photocatalytic efficiency. In view of the merits of porous materials, recent progress of three other types of porous materials are also briefly summarized, namely zeolite-based, covalent-organic frameworks based (COFs-based), and porous semiconductor or organic polymer based photocatalysts. The remarkable performance of these porous materials for solar-driven CO2 reduction systems is highlighted. Finally, challenges and opportunities of porous materials for photocatalytic CO2 reduction are presented, aiming to provide a new viewpoint for improving the overall photocatalytic CO2 reduction efficiency with porous materials.

Removal of Heavy Metals from Wastewaters: A Challenge from Current Treatment Methods to Nanotechnology Applications

Removing heavy metals from wastewaters is a challenging process that requires constant attention and monitoring, as heavy metals are major wastewater pollutants that are not biodegradable and thus accumulate in the ecosystem. In addition, the persistent nature, toxicity and accumulation of heavy metal ions in the human body have become the driving force for searching new and more efficient water treatment technologies to reduce the concentration of heavy metal in waters. Because the conventional techniques will not be able to keep up with the growing demand for lower heavy metals levels in drinking water and wastewaters, it is becoming increasingly challenging to implement technologically advanced alternative water treatments. Nanotechnology offers a number of advantages compared to other methods. Nanomaterials are more efficient in terms of cost and volume, and many process mechanisms are better and faster at nanoscale. Although nanomaterials have already proved themselves in water technology, there are specific challenges related to their stability, toxicity and recovery, which led to innovations to counteract them. Taking into account the multidisciplinary research of water treatment for the removal of heavy metals, the present review provides an updated report on the main technologies and materials used for the removal of heavy metals with an emphasis on nanoscale materials and processes involved in the heavy metals removal and detection.

Hierarchically porous metal-organic frameworks: synthesis strategies, structure(s), and emerging applications in decontamination

Metal-organic frameworks (MOFs) with high porosity have received much attention as promising materials for many applications owing to their unique properties. However, to date, most of the reported MOFs have microporous structures, which slow down diffusion/mass transfer and limit the accessibility of bulky molecules to its internal surface. Thus, it is crucial to develop an efficient way to create larger pores (mesoporous and/or macroporous) into microporous MOFs to form hierarchical porous metal-organic frameworks (HP-MOFs), which facilitate the diffusion and mass transfer of guest molecules. HP-MOFs are excellent and promising candidates for environmental applications under the background of environmental contaminations. In this review paper, we are primarily focusing on the latest progress in the preparation of HP-MOFs by employing template-assisted and template-free synthetic approaches for environmental cleaning applications. Particularly, the adsorptive purification of the most common toxic substances, including gases, dyes, heavy metal ions, and antibiotics from the environment using HP-MOFs as adsorbents is briefly discussed. The overall results clearly showed that the superiority of HP-MOFs compared with conventional microporous MOFs. Finally, we summarize the remaining challenges and provide personal perspectives on possible future development of HP-MOFs.

A Trinuclear Cobalt-Organic Framework: Solvatochromic Sensor towards CH2 Cl2 , and its Derivative as an Anode of Lithium-Ion Batteries with High Performance

Here, a porous cobalt-organic framework with pillared layer structures, namely [Co₃ OBA₃ PTD(H₂ O)₂ ⋅ 2 DMA⋅H₂ O]_n (1, H₂ OBA=4,4'-oxybis(benzoic acid); PTD=6-(pyridin-4-yl)-1,3,5-triazine-2,4-diamine), was fabricated by using cobalt trinuclear nodes, low-cost carboxylic linker, and accessible nitrogen heterocyclic ligands. This compound exhibited a highly efficient solvatochromism towards CH₂ Cl₂ within one minute and can be used 200 times at least. The corresponding dropper detector was assembled as a practical sensor. Meanwhile, the porous Co₃ O₄ was obtained by a simple but effective annealing treatment. Electrochemical measurements confirm that this Co₃ O₄ material derived from compound 1 shows high and stable lithium storage capabilities (1081.75 mA h g^-1 at 200 mA g^-1 after 115 cycles) and excellent rate properties.

Strong Interplay between Polymer Surface Charge and MOF Cage Chemistry in Mixed-Matrix Membrane for Water Treatment Applications

Despite the large number of reports on the utilization of highly microporous solids, most relevant are metal-organic frameworks (MOFs), in different demanding applications, the successful hybridization of MOFs and moldable polymer matrices into flexible, water-permeable membranes exhibiting strong entanglement of the MOF and the polymer matrix properties is still lacking. We describe herein an efficient pathway to construct a mixed-matrix membrane (MMM) comprising a water-stable metal-organic framework (UiO-66-NH₂), as the active sorbent, and cellulose acetate (CA), as the polymer matrix, to construct a flexible membrane for water treatment applications. The MOF@CA MMM demonstrated superior performance in terms of exceptional removal of organic dyes (both cationic and anionic species) as well as hexavalent Cr ions, compared to the control CA membrane. The recorded high uptake of the MOF@CA MMM for this wide array of contaminants demonstrated the accessibility of the MOF nanocages immobilized within the MMM, in contrast to the common perception that the polymer matrix might act as a physical barrier to block the accessibility of the MOF cages. The negative surface charge of the matrix exerted a notable action to affect the diffusion of the negatively charged contaminants to reach the active sorbent filler. Moreover, the formed membrane demonstrated high durability and recyclability with no detected loss of performance over numerous cycles. This approach outlines the ability to formulate one of the most water-stable MOFs, as exceptional microporous sorbent, into a usable membrane form compatible with real-life applications.