Adsorptive removal of heavy metals from aqueous solutions: Progress of adsorbents development and their effectiveness

Increased levels of heavy metals (HMs) in aquatic environments poses serious health and ecological concerns. Hence, several approaches have been proposed to eliminate/reduce the levels of HMs before the discharge/reuse of HMs-contaminated waters. Adsorption is one of the most attractive processes for water decontamination; however, the efficiency of this process greatly depends on the choice of adsorbent. Therefore, the key aim of this article is to review the progress in the development and application of different classes of conventional and emerging adsorbents for the abatement of HMs from contaminated waters. Adsorbents that are based on activated carbon, natural materials, microbial, clay minerals, layered double hydroxides (LDHs), nano-zerovalent iron (nZVI), graphene, carbon nanotubes (CNTs), metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs) are critically reviewed, with more emphasis on the last four adsorbents and their nanocomposites since they have the potential to significantly boost the HMs removal efficiency from contaminated waters. Furthermore, the optimal process conditions to achieve efficient performance are discussed. Additionally, adsorption isotherm, kinetics, thermodynamics, mechanisms, and effects of varying adsorption process parameters have been introduced. Moreover, heavy metal removal driven by other processes such as oxidation, reduction, and precipitation that might concurrently occur in parallel with adsorption have been reviewed. The application of adsorption for the treatment of real wastewater has been also reviewed. Finally, challenges, limitations and potential areas for improvements in the adsorptive removal of HMs from contaminated waters are identified and discussed. Thus, this article serves as a comprehensive reference for the recent developments in the field of adsorptive removal of heavy metals from wastewater. The proposed future research work at the end of this review could help in addressing some of the key limitations facing this technology, and create a platform for boosting the efficiency of the adsorptive removal of heavy metals.

Flattening of a Bent Sulfonated MOF Linker: Impact on Structures, Flexibility, Gas Adsorption, CO2/N2 Selectivity, and Proton Conduction

Rational design of organic building blocks provides opportunities to control and tune various physicochemical properties of metal-organic frameworks (MOFs), including gas handling, proton conduction, and structural flexibility, the latter of which is responsible for new adsorption phenomena and often superior properties compared to rigid porous materials. In this work, we report synthesis, crystal structures, gas adsorption, and proton conduction for a flexible two-dimensional cadmium-based MOF (JUK-13-SO3H-SO2) containing a new sulfonated 4,4'-oxybis(benzoate) linker with a blocking SO2 bridge. This two-dimensional (2D) MOF is compared in detail with a previously reported three-dimensional Cd-MOF (JUK-13-SO3H), based on analogous, but nonflat, SO2-free sulfonated dicarboxylate. The comprehensive structure-property relationships and the detailed comparisons with insights into the networks flexibility are supported by five guest-dependent structures determined by single-crystal X-ray diffraction (XRD), and corroborated by spectroscopy (IR, 1H NMR), powder XRD, and elemental/thermogravimetric analyses, as well as by volumetric adsorption measurements (for N2, CO2, H2O), ideal adsorbed solution theory (IAST), density-functional theory (DFT+D) quantum chemical and grand-canonical Monte Carlo (GCMC) calculations, and electrochemical impedance spectroscopy (EIS) studies. Whereas both dynamic MOFs show moderate proton conductivity values, they exhibit excellent CO2/N2 selectivity related to the capture of CO2 from flue gases (IAST coefficients for 15:85 mixtures are equal to ca. 250 at 1 bar and 298 K). The presence of terminal sulfonate groups in both MOFs, introduced using a unique prechlorosulfonation strategy, is responsible for their hydrophilicity and water-assisted proton transport ability. The dynamic nature of the MOFs results in the appearance of breathing-type adsorption isotherms that exhibit large hysteresis loops (for CO2 and H2O) attributed to strong host-guest interactions. Theoretical modeling provides information about the adsorption mechanism and supports interpretation of experimental CO2 adsorption isotherms.

Superhydrophobic MOFs with enhanced catalytic activity for chemical fixation of CO2

A general approach to prepare superhydrophobic MOFs (denoted as MOFs-CF3) through a post-decorating strategy for highly efficient chemical fixation of CO2 was demonstrated. The enhanced catalytic activity of MOFs-CF3 is attributed to a synergistic effect between the Lewis acid sites of MOFs and modification of the electron-withdrawing trifluoromethyl group, which resulted in a high CO2 enrichment capacity. The possible mechanism of cycloaddition catalyzed by the MOFs-CF3 catalyst was also proposed.

A robust ultra-microporous cationic aluminum-based metal-organic framework with a flexible tetra-carboxylate linker

Al-based cationic metal-organic frameworks (MOFs) are uncommon. Here, we report a cationic Al-MOF, MIP-213(Al) ([Al₁₈(μ₂-OH)₂₄(OH₂)₁₂(mdip)₆]6Cl·6H₂O) constructed from flexible tetra-carboxylate ligand (5,5'-Methylenediisophthalic acid; H₄mdip). Its crystal structure was determined by the combination of three-dimensional electron diffraction (3DED) and high-resolution powder X-ray diffraction. The structure is built from infinite corner-sharing chains of AlO₄(OH)₂ and AlO₂(OH)₃(H₂O) octahedra forming an 18-membered rings honeycomb lattice, similar to that of MIL-96(Al), a scarce Al-polycarboxylate defective MOF. Despite sharing these structural similarities, MIP-213(Al), unlike MIL-96(Al), lacks the isolated μ₃-oxo-bridged Al-clusters. This leads to an ordered defective cationic framework whose charge is balanced by Cl^- sandwiched between two Al-trimers at the corner of the honeycomb, showing strong interaction with terminal H₂O coordinated to the Al-trimers. The overall structure is endowed by a narrow quasi-1D channel of dimension ~4.7 Å. The Cl^- in the framework restrains the accessibility of the channels, while the MOF selectively adsorbs CO₂ over N₂ and possesses high hydrolytic stability.

Rational Synthesis of a Stable Rod MOF for Ultrasensitive Detection of Nitenpyram and Nitrofurazone in Natural Water Systems

Overuse of nitenpyram and nitrofurazone in agricultural products poses enormous risks to ecosystems, and effective detection and quantification of these residual pollutants are of great concern. Although several strategies have been established for detecting nitenpyram and nitrofurazone in water, searching for a new sensor material with great sensitivity, selectivity, and recyclability remains challenging. Here, we design and synthesize a stable metal-organic framework (MOF) (Zn-CPTA) by employing an organic linker based on the coordination features of benzene-1,4-dicarboxylate and picolinic acid. Zn-CPTA is a 3D framework built from Zn-O-Zn chains called rod secondary building units, which contains 1D open channels modified by uncoordinated carboxyl O atoms and exhibits impressive chemical stability in aqueous solutions within a pH range from 2 to 12. Especially, fluorescent Zn-CPTA can quickly and sensitively detect nitenpyram and nitrofurazone in aqueous solutions with a high quenching constant and low detection limit (LOD) (K_SV values for nitenpyram and nitrofurazone are 1.67 × 10⁴ and 1.02 × 10⁵ M^-1 with LOD of 0.625 and 0.126 μM, respectively), as well as outstanding selectivity and recyclability. Notably, the LOD value is the lowest among the reported MOFs used for nitrofurazone detection. Besides, experiments and density functional theory calculations are combined to explain the quenching mechanism. Finally, the practical application of Zn-CPTA was further explored in real environment samples with satisfactory recoveries.

Multifunctional nanomaterials and nanocomposites for sensing and monitoring of environmentally hazardous heavy metal contaminants

The applications of conventional sensors are limited by the long response time, high cost, large detection limit, low sensitivity, complicated usage and low selectivity. These sensors are nowadays replaced by Nanocomposite-based modalities and nanomaterials which are known for their high selectivity and physical and chemical properties. These nanosensors effectively detect heavy metal contaminants in the environment as the discharge of heavy metals into natural water as a result of human activity has become a global epidemic. Exposure to these toxic metals might induce many health-related complications, including kidney failure, brain injury, immune disorders, muscle paleness, cardiac damage, nervous system impairment and limb paralysis. Therefore, designing and developing novel sensing systems for the detection and recognition of these harmful metals in various environmental matrices, particularly water, is of extremely important. Emerging nanotechnological approaches in the past two decades have played a key role in overcoming environmentally-related problems. Nanomaterial-based fabrication of chemical nanosensors has widely been applied as a powerful analytical tool for sensing heavy metals. Portability, high sensitivity, on-site detection capability, better device performance and selectivity are all advantages of these nanosensors. The detection and selectivity have been improved using molecular recognition probes for selective binding on different nanostructures. This study aims to evaluate the sensing properties of various nanomaterials such as metal-organic frameworks, fluorescent materials, metal-based nanoparticles, carbon-based nanomaterials and quantum dots and graphene-based nanomaterials and quantum dots for heavy metal ions recognition. All these nano-architectures are frequently served as effective fluorescence probes to directly (or by modification with some large or small biomolecules) sense heavy metal ions for improved selectivity. However, efforts are still needed for the simultaneous designing of multiple metal ion-based detection systems, exclusively in colorimetric or optical fluorescence nanosensors for heavy metal cations.

Water-Stable Carborane-Based Eu3+/Tb3+ Metal-Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding

Luminescent lanthanide metal-organic frameworks (Ln-MOFs) have been shown to exhibit relevant optical properties of interest for practical applications, though their implementation still remains a challenge. To be suitable for practical applications, Ln-MOFs must be not only water stable but also printable, easy to prepare, and produced in high yields. Herein, we design and synthesize a series of m CB-Eu _y Tb _1-y (y = 0-1) MOFs using a highly hydrophobic ligand mCBL1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane. The new materials are stable in water and at high temperature. Tunable emission from green to red, energy transfer (ET) from Tb³⁺ to Eu³⁺, and time-dependent emission of the series of mixed-metal m CB-Eu _y Tb _1-y MOFs are reported. An outstanding increase in the quantum yield (QY) of 239% of mCB-Eu (20.5%) in the mixed mCB-Eu_0.1Tb_0.9 (69.2%) is achieved, along with an increased and tunable lifetime luminescence (from about 0.5 to 10 000 μs), all of these promoted by a highly effective ET process. The observed time-dependent emission (and color), in addition to the high QY, provides a simple method for designing high-security anticounterfeiting materials. We report a convenient method to prepare mixed-metal Eu/Tb coordination polymers (CPs) that are printable from water inks for potential applications, among which anticounterfeiting and bar-coding have been selected as a proof-of-concept.

Experimental and Theoretical Assessment of Water Sorbent Kinetics

The kinetics of water adsorption in powder sorbent layers are important to design a scaled-up atmospheric water capture device. Herein, the adsorption kinetics of three sorbents, a chromium (Cr)-based metal-organic framework (Cr-MIL-101), a carbon-based material (nanoporous sponges/NPS), and silica gel, have been tested experimentally, using powder layers ranging from ∼0 to 7.5 mm in thickness, in a custom-made calibrated environmental chamber cycling from 5 to 95% RH at 30 °C. A mass and energy transfer model was applied onto the experimental curves to better understand the contribution of key parameters (maximum water uptake, kinetics of single particles, layer open porosity, and particle size distribution). Open porosity (i.e., the void-to-particle ratio in the sorbent layer) shows the highest influence to improve the kinetics. Converting the sorbent kinetics data into a daily yield of captured water demonstrated (i) the existence of an optimal open porosity for each sorbent, (ii) that thinner layers with moderate open porosity performed respectively better than thicker layers with high open porosity, and (iii) that high maximum water uptake and fast single-particle kinetics are not necessarily predictive of high daily water yield.

In-situ detoxification of schedule-I chemical warfare agents utilizing Zr(OH)4@W-ACF functional material for the development of next generation NBC protective gears

Chemical warfare agents (CWAs) have become a pivotal concern for the global community and spurred a wide spectrum of research for the development of new generation protective materials. Herein, a highly effective self-detoxifying filter consisting of in-situ immobilized Zirconium hydroxide [Zr(OH)4] over woven activated carbon fabric [Zr(OH)4@W-ACF] is presented for the removal of CWAs. It was prepared to harness the synergistic effect of high surface area of W-ACF, leads to high dispersion of CWAs and high phosphilicity and reactivity of [Zr(OH)4]. The synthesized materials were characterized by ATR-FTIR, EDX, SEM, TEM, XPS, TGA, and BET surface area analyzer. The kinetics of  in-situ degradation of CWAs over Zr(OH)4@W-ACF were studied and found to be following the first-order reaction kinetics. The rate constant was found to be 0.244 min-1 and 2.31 × 10-2 min-1 for sarin and soman, respectively over Zr(OH)4@W-ACF. The potential practical applicability of this work was established by fabricating Zr(OH)4@W-ACF as reactive adsorbent layer for protective suit, and found to be meeting the specified criteria in terms of air permeability, tearing strength and nerve agent permeation as per TOP-08-2-501A:2013 and IS-17380:2020. The degradation products of CWAs were analyzed with NMR and GC-MS. The combined properties of dual functional textile with reactive material are expected to open up new exciting avenues in the field of CWAs protective clothing and thus find diverse application in defence and environmental sector.

Rational design of carborane-based Cu2-paddle wheel coordination polymers for increased hydrolytic stability

A new unsymmetric carborane-based dicarboxylic linker provided a 1D Cu₂-paddle wheel coordination polymer (2) with much higher hydrolytic stability than the corresponding 2D Cu₂-paddle wheel polymer (1), obtained from a related more symmetrical carborane-based linker. Both 1 and 2 were used as efficient heterogeneous catalysts for a model aza-Michael reaction but only 2 can be reused several times without significant degradation in catalytic activity.

Synthesis of Al-Based Metal-Organic Framework in Water With Caffeic Acid Ligand and NaOH as Linker Sources With Highly Efficient Anticancer Treatment

In this study, novel nanostructures of aluminum base metal-organic framework (Al-MOF) samples were synthesized using a sustainable, non-toxic, and cost-effective green synthesis route. Satureja hortensis extract was used as an effective source of linker for the development of the Al-MOF structures. The Fourier-transformed infrared (FTIR) spectrum confirmed the presence of characterization bonds related to the Al-MOF nanostructures synthesized by the green synthesis route. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed that the sample synthesized by Na2-CA was composed of multilayers, although it was agglomerated, but it had dispersed and occurred in spherical particles, indicating active organic matter. N2 adsorption/desorption isotherms demonstrated the significant porosity of the Al-MOF samples that facilitate the high potential of these nanostructures in medical applications. The anticancer treatment of Al-MOF samples was performed with different concentrations using the MTT standard method with untreated cancer cells for 24 and 48 h periods. The results exhibited the significant anticancer properties of Al-MOF samples developed in this study when compared with other MOF samples. Thus, the development of a novel Al-MOF and its application as a natural linker can influence the anticancer treatment of the samples. According to the results, the products developed in this study can be used in more applications such as biosensors, catalysts, and novel adsorbents.

Removal of organic pollutants from aqueous solution using metal organic frameworks (MOFs)-based adsorbents: A review

The development of metal organic frameworks (MOFs) has recently drawn a lot of scientific interest in water treatment due to the unique properties such as tunable porosities, large pore volumes, hierarchical structures, excellent adsorption and regeneration performances. MOFs represent an eco-friendly alternative to conventional adsorbents especially for the adsorptive removal of noxious organic pollutants from aqueous solution. Advanced MOFs' performances are justified by the introduction of functional groups, magnetic moieties, and specific foreign materials onto MOFs. This however leads to increase in the manufacturing costs of MOFs and consequently possess a huge challenge in large-scale applications. This review hence critically discusses the recent progresses in the development of MOFs-based adsorbents for the removal of selected organic pollutants (e.g., dyes, antibiotics and pesticides) from aqueous solution. Furthermore, major interaction mechanisms between MOFs and organic pollutants in response to numerous experimental conditions, such as pH, temperature, coexisting ions are put forward. Finally, some recommendations in support for designing MOFs with improved adsorption performances are also highlighted.

Porphyrin Based 2D-MOF Structures as Dual-Kinetic Sorafenib Nanocarriers for Hepatoma Treatment

The existing clinical protocols of hepatoma treatment require improvement of drug efficacy that can be achieved by harnessing nanomedicine. Porphyrin-based, paddle-wheel framework (PPF) structures were obtained and tested as dual-kinetic Sorafenib (SOR) nanocarriers against hepatoma. We experimentally proved that sloughing of PPF structures combined with gradual dissolving are effective mechanisms for releasing the drug from the nanocarrier. By controlling the PPF degradation and size of adsorbed SOR deposits, we were able to augment SOR anticancer effects, both in vitro and in vivo, due to the dual kinetic behavior of SOR@PPF. Obtained drug delivery systems with slow and fast release of SOR influenced effectively, although in a different way, the cancer cells proliferation (reflected with EC50 and ERK 1/2 phosphorylation level). The in vivo studies proved that fast-released SOR@PPF reduces the tumor size considerably, while the slow-released SOR@PPF much better prevents from lymph nodes involvement and distant metastases.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Coordination sphere hydrogen bonding as a structural element in metal-organic Frameworks

In the design of new metal-organic frameworks, the constant challenges of framework stability and structural predictability continue to influence ligand choice in favour of well-studied dicarboxylates and similar ligands. However, a small subset of known MOF ligands contains suitable functionality for coordination sphere hydrogen bonding which can provide new opportunities in ligand design. Such interactions may serve to support and rigidity the coordination geometry of mononuclear coordination spheres, as well as providing extra thermodynamic and kinetic stabilisation to meet the challenge of hydrolytic stability in these materials. In this perspective, a collection of pyrazole, amine, amide and carboxylic acid containing species are examined through the lens of (primarily) inner-sphere hydrogen bonding. The influence of these interactions is then related to the overall structure, stability and function of these materials, to provide starting points for harnessing these interactions in future materials design.

Metal-Organic Framework (MOF) Derived Recyclable, Superhydrophobic Composite of Cotton Fabrics for the Facile Removal of Oil Spills

Marine oil spill cleanup is one of the major challenges in recent years due to its detrimental effect on our ecosystem. Hence, the development of new superhydrophobic oil absorbent materials is in high demand. The third-generation porous materials, namely metal-organic frameworks (MOFs), have drawn great attention due to their fascinating properties. In this work, a superhydrophobic MOF with UiO-66 (SH-UiO-66) topology was synthesized strategically with a new fluorinated dicarboxylate linker to absorb oil selectively from water. The fully characterized superhydrophobic MOF showed extreme water repellency with an advancing water contact angle (WCA) of 160° with a contact angle hysteresis (CAH) of 8°. The newly synthesized porous MOF (S_BET = 873 m² g^-1) material with high WCA found its promising application in oil/water separation. The superhydrophobic SH-UiO-66 MOF was further used for the in-situ coating on naturally abundant cotton fiber to make a superhydrophobic MOF@cotton composite material. The MOF-coated cotton fiber composite (SH-UiO-66@CFs) showed water repellency with a WCA of 163° and a low CAH of 4°. The flexible superhydrophobic SH-UiO-66@CFs showed an oil absorption capacity more than 2500 wt % for both heavy and light oils at room temperature. The superoleophilicity of SH-UiO-66@CFs was further exploited to separate light floating oil as well as sedimentary heavy oil from water. SH-UiO-66@CFs material can also separate oil from the oil/water mixture by gravity-directed active filtration. Hence, the newly developed MOF-based composite material has high potential as an oil absorbent material for marine oil spill cleanup.

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.

An excellent thermostable dual-functionalized 3D fsx-type Cd(ii) MOF for the highly selective detection of Fe3+ ions and ten nitroaromatic explosives

A 3D fsx-type thermostable MOF was designed and synthesized for the highly selective detection of Fe3+ ions and ten nitroaromatic explosives.

Performance evaluation of CuBTC composites for room temperature oxygen storage

Oxygen is commonly separated from air using cryogenic liquefaction. The inherent energy penalties of phase change inspire the search for energy-efficient separation processes. Here, an alternative approach is presented, where we determine whether it is possible to utilise simpler, stable materials in the right process to achieve overall energy efficiency. Adsorption and release by Metal-Organic Frameworks (MOFs) are an attractive alternative due to their high adsorption and storage capacity at ambient conditions. Cu-BTC/MgFe2O4 composites were prepared, and magnetic induction swing adsorption (MISA) used to release adsorbed oxygen quickly and efficiently. The 3 wt% MgFe2O4 composites exhibited an oxygen uptake capacity of 0.34 mmol g-1 at 298 K and when exposed to a magnetic field of 31 mT, attained a temperature rise of 86 °C and released 100% of adsorbed oxygen. This water vapor stable pelletized system, can be filled and emptied within 10 minutes requiring around 5.6 MJ kg-1 of energy.

Hydrophobic Metal-Organic Frameworks: Assessment, Construction, and Diverse Applications

Tens of thousands of metal-organic frameworks (MOFs) have been developed in the past two decades, and only ≈100 of them have been demonstrated as porous and hydrophobic. These hydrophobic MOFs feature not only a rich structural variety, highly crystalline frameworks, and uniform micropores, but also a low affinity toward water and superior hydrolytic stability, which make them promising adsorbents for diverse applications, including humid CO2 capture, alcohol/water separation, pollutant removal from air or water, substrate-selective catalysis, energy storage, anticorrosion, and self-cleaning. Herein, the recent research advancements in hydrophobic MOFs are presented. The existing techniques for qualitatively or quantitatively assessing the hydrophobicity of MOFs are first introduced. The reported experimental methods for the preparation of hydrophobic MOFs are then categorized. The concept that hydrophobic MOFs normally synthesized from predesigned organic ligands can also be prepared by the postsynthetic modification of the internal pore surface and/or external crystal surface of hydrophilic or less hydrophobic MOFs is highlighted. Finally, an overview of the recent studies on hydrophobic MOFs for various applications is provided and suggests the high versatility of this unique class of materials for practical use as either adsorbents or nanomaterials.

Two isomeric In(iii)-MOFs: unexpected stability difference and selective fluorescence detection of fluoroquinolone antibiotics in water

The fluorescence of the two new In(iii)-MOFs is selectively quenched by the fluoroquinolones, including ciprofloxacin, one of the most widely used antibiotics worldwide.

Recent advances in the shaping of metal–organic frameworks

In this review, the recent advances in the shaping of MOFs are overviewed, and some promising strategies recently developed are highlighted, including templated shaping, self-shaping, shaping on substrates, and shaping with sacrificial materials.

Adsorption of hydrogen arsenate and dihydrogen arsenate ions from neutral water by UiO-66-NH2

The metal-organic framework (MOF) UiO-66-NH₂ was synthesized with different substrate to solvent ratios and its morphology, surface area, pore distributions, and NMR, XRD, and TGA-FTIR patterns were obtained. Adsorption tests at pH 7 and 25 °C showed that the produced UiO-66-NH₂ has a hydrogen arsenate adsorption capacity of 76.9 mg/g. With the affinity onto Zr clusters, this MOF also can adsorb phosphate ions from water. Treatment with 1-4 M hydrochloric acid (HCl) protonated the amine groups in the MOF. Treatment with 1 M HCl at 25 °C for 6 h maximized the adsorption capacity of UiO-66-NH₂ to 161.3 mg/g, such that the protonated amine groups accounted for 53.7% of the adsorption of arsenate from the water. The use of excessively strong acid at elevated temperature reduced the adsorption capacity.

Multifunctional Behavior of Sulfonate-Based Hydrolytically Stable Microporous Metal-Organic Frameworks

An isostructural pair of extremely rare, permanently microporous sulfonate-based metal-organic frameworks (MOFs) having a novel topology has been reported here by integration of rationally chosen building units. The compounds bear polar sites in the pore surfaces and exhibit selective adsorption of CO₂, which features among the highest reported uptakes in the domain of organosulfonate-based MOFs. The compounds also exhibit multifunctionality for C₆-cyclic hydrocarbon separation and selective detection of neurotransmitter nitric oxide. Such multifunctional behavior on the basis of permanent porosity has been rarely observed for sulfonate-based MOFs. The efficacy of the synthesis approach is further highlighted by the resistance over a wide pH range and promising feasibility of reticular chemistry in porous organosulfonate-based systems.

The Hydrolytic Stability and Degradation Mechanism of a Hierarchically Porous Metal Alkylphosphonate Framework

To aid the design of a hierarchically porous unconventional metal-phosphonate framework (HP-UMPF) for practical radioanalytical separation, a systematic investigation of the hydrolytic stability of bulk phase against acidic corrosion has been carried out for an archetypical HP-UMPF. Bulk dissolution results suggest that aqueous acidity has a more paramount effect on incongruent leaching than the temperature, and the kinetic stability reaches equilibrium by way of an accumulation of a partial leached species on the corrosion conduits. A variation of particle morphology, hierarchical porosity and backbone composition upon corrosion reveals that they are hydrolytically resilient without suffering any great degradation of porous texture, although large aggregates crack into sporadic fractures while the nucleophilic attack of inorganic layers cause the leaching of tin and phosphorus. The remaining selectivity of these HP-UMPFs is dictated by a balance between the elimination of free phosphonate and the exposure of confined phosphonates, thus allowing a real-time tailor of radionuclide sequestration. Moreover, a plausible degradation mechanism has been proposed for the triple progressive dissolution of three-level hierarchical porous structures to elucidate resultant reactivity. These HP-UMPFs are compared with benchmark metal-organic frameworks (MOFs) to obtain a rough grading of hydrolytic stability and two feasible approaches are suggested for enhancing their hydrolytic stability that are intended for real-life separation protocols.

The synergistic effect of oxygen and water on the stability of the isostructural family of metal-organic frameworks [Cr3(BTC)2] and [Cu3(BTC)2]

The instability of most prototype metal-organic frameworks (MOFs) in the presence of water has limited their industrial scale development. The sensitivity of certain MOFs to humid conditions has been vigorously studied and most of the carrier gas used to make the humid conditions is inert gas. However, a large amount of industrial activities are carried out in air containing 21% oxygen, so it is important to study the effect of oxygen on the hydrostability of MOFs for future industrial applications. In this work, we have studied the stability of M₃(BTC)₂ (M = Cu, Cr; BTC = 1,3,5-benzenetricarboxylate) under controlled environments (pure oxygen environment; water vapor environment and mixed O₂ and H₂O environment). The stability was evaluated using water vapor and oxygen adsorption isotherms combined with powder X-ray diffraction (PXRD) experiments and surface area analysis. Our research shows that Cr₃(BTC)₂ has a relatively high stability under a single atmosphere of either oxygen or water vapor (with Ar). Interestingly, when it was placed under the mixed O₂ and H₂O environment, it rapidly lost approximately 96% of its original surface area. Cu₃(BTC)₂ is more stable; however, it was also degraded, especially under a mixed O₂ and H₂O environment. The experiments show that water molecules and oxygen molecules have a synergistic effect on the stability of MOFs. Computational simulations were used to provide insight into the mechanism governing these trends in the stability of the materials studied.

Liquid/Liquid Interfacial Synthesis of a Click Nanosheet

A liquid/liquid interfacial synthesis is employed, for the first time, to synthesize a covalent two-dimensional polymer nanosheet. Copper-catalyzed azide-alkyne cycloaddition (CuAAC) between a three-way terminal alkyne and azide at a water/dichloromethane interface generates a 1,2,3-triazole-linked nanosheet. The resultant nanosheet, with a flat and smooth texture, has a maximum domain size of 20 μm and minimum thickness of 5.3 nm. The starting monomers in the organic phase and the copper catalyst in the aqueous phase can only meet at the liquid/liquid interface as a two-dimensional reaction space; this allows them to form the two-dimensional polymer. The robust triazole linkage generated by irreversible covalent-bond formation allows the nanosheet to resist hydrolysis under both acidic and alkaline conditions, and to endure pyrolysis up to more than 300 °C. The coordination ability of the triazolyl group enables the nanosheet to act as a reservoir for metal ions, with an affinity order of Pd²⁺ >Au³⁺ >Cu²⁺ .