Polyoxometalate (POM)-Layered Double Hydroxides (LDH) Composite Materials: Design and Catalytic Applications

Assessing the Influence of Confinement on the Stability of Polyoxometalate-Functionalized Surfaces: A Soft Sequential Immobilization Approach for Electrochromic Devices

A functionalization process has been developed and the experimental conditions optimized allowing the immobilization of first-row transition metal (Mn+) containing polyoxometalates (POMs) with the formula [M(H2O)P2W17O61](10-n)- on transparent indium-tin oxide (ITO) electrodes for electrochromic applications. Both flat ITO grafted with 4-sulfophenyl moieties and sulfonate-functionalized vertically oriented silica films on ITO have been used as electrode supports to evaluate possible confinement effects provided by the mesoporous matrix on the stability of the modified surfaces and their electrochromic properties. Functionalization involved a two-step sequential process: (i) the immobilization of hexaaqua metallic ions, such as Fe(H2O)63+, onto the sulfonate-functionalized materials achieved through hydrogen bonding interactions between the sulfonate functions and aqua ligands (water molecules) coordinated to the metallic ions facilitating and stabilizing the attachment of the metallic ions to the sulfonated surfaces; (ii) their coordination to [P2W17O61]10- species to generate "in situ" the target [Fe(H2O)P2W17O61]7- moieties. Comparison of the characterized surfaces clearly evidenced a significant improvement in the long-term stability of the nanostructured [Fe(H2O)P2W17O61]7--functionalized silica films compared to the less constrained flat [Fe(H2O)P2W17O61]7--modified ITO electrodes for which a rapid loss of [P2W17O61]10- species was observed. Concordantly, the [Fe(H2O)P2W17O61]7- POM confined in the mesoporous films coated on ITO gave rise to much better and stable electrochromic properties, with a transmittance modulation of 40% at 515 nm.

High stability and selectivity of butterfly pea flower extract-NiAl LDH-based catalysts in the tetracycline degradation

Layered double hydroxide (LDH) is an applicable material that can be modified in various ways. Modifications using natural extracts fulfill the principles of "green chemistry." The preparation of butterfly pea flower extract (BPE)-modified NiAl LDH was completed using the calcination and restacking method. The characteristics of the prepared composites were identified through analysis of functional groups, crystal phase, bandgap energy, surface area and surface morphology. Fourier transform-infrared (FT-IR) characterization revealed that the active group of the catalyst is -OH except for NiAl layered double oxide (LDO), which has the metal oxide-like functional groups. X-ray diffraction patterns expressed a typical layered material structure of NiAl LDH dan NiAl LDH-BPE, but not for NiAl LDO and NiAl LDO-BPE. Introducing BPE into NiAl LDH and NiAl LDO effectively decreased the bandgap energy and changed the surface morphology. The prepared catalysts were applied in a batch system with pH 5 to degrade tetracycline (TC). NiAl LDO demonstrated the highest activity as a catalyst in TC degradation, with a 93.61% degradation rate. In contrast, NiAl LDO-BPE demonstrated the highest structural stability in TC degradation and repeated use, with an initial degradation percentage of 82.58% and a fifth regeneration percentage of 71.4%.

Polyoxometalate-Intercalated Tremella-Like CoNi-LDH Nanocomposites for Electrocatalytic Nitrite-Ammonia Conversion

The electrocatalytic reduction of NO2- (NO2RR) holds promise as a sustainable pathway to both promoting the development of emerging NH3 economies and allowing the closing of the NOx loop. Highly efficient electrocatalysts that could facilitate this complex six-electron transfer process are urgently desired. Herein, tremella-like CoNi-LDH intercalated by cyclic polyoxometalate (POM) anion P8W48 (P8W48/CoNi-LDH) prepared by a simple two-step hydrothermal-exfoliation assembly method is proposed as an effective electrocatalyst for NO2- to NH3 conversion. The introduction of POM with excellent redox ability tremendously increased the electrocatalytic performance of CoNi-LDH in the NO2RR process, causing P8W48/CoNi-LDH to exhibit large NH3 yield of 0.369 mmol h-1 mgcat-1 and exceptionally high Faradic efficiency of 97.0% at -1.3 V vs the Ag/AgCl reference electrode in 0.1 M phosphate buffer saline (PBS, pH = 7) containing 0.1 M NO2-. Furthermore, P8W48/CoNi-LDH demonstrated excellent durability during cyclic electrolysis. This work provides a new reference for the application of POM-based nanocomposites in the electrochemical reduction of NO2- to obtain value-added NH3.

A new method for the preparation of MgAl layered double hydroxide-copper metal-organic frameworks structures: application to electrocatalytic oxidation of formaldehyde

In this research, we present a novel design protocol for the in-situ synthesis of MgAl layered double hydroxide-copper metal-organic frameworks (LDH-MOFs) nanocomposite based on the electrocoagulation process and chemical method. The overall goal in this project is the primary synthesis of para-phthalic acid (PTA) intercalated MgAl-LDH with Cu (II) ions to produce the paddle-wheel like Cu-(PTA) MOFs nanocrystals on/in the MgAl-LDH structure. The physicochemical properties of final product; Cu-(PTA) MOFs/MgAl-LDH, were characterized by the surface analysis and chemical identification methods (SEM, EDX, TEM, XRD, BET, FTIR, CHN, DLS, etc.). The Cu-(PTA) MOFs/MgAl-LDH nanocomposite was used to modification of the carbon paste electrode (CPE); Cu-(PTA) MOFs/MgAl-LDH/CPE. The electrochemical performance of Cu-(PTA) MOFs/MgAl-LDH/CPE was demonstrated through the utilization of electrochemical methods. The results show a stable redox behavior of the Cu (III)/Cu (II) at the surface of Cu-(PTA) MOFs/MgAl-LDH/CPE in alkaline medium (aqueous 0.1 M NaOH electrolyte). Then, the Cu-(PTA) MOFs/MgAl-LDH/CPE was used as a new electrocatalyst toward the oxidation of formaldehyde (FA). Electrochemical data show that the Cu-(PTA) MOFs/MgAl-LDH/CPE exhibits superior electrocatalytic performance on the oxidation of FA. Also the diffusion coefficient, exchange current density (J°) and mean value of catalytic rate constant (Kcat) were found to be 1.18 × 10-6 cm2 s-1, 23 mA cm-2 and 0.4537 × 104 cm3 mol-1 s-1, respectively. In general, it can be said the Cu-(PTA) MOFs/MgAl-LDHs is promising candidate for applications in direct formaldehyde fuel cells.

High-Performance Novel Polyoxometalate-LDH Nanocomposite-Modified Thin-Film Nanocomposite Forward Osmosis Membranes: A Study of Desalination and Antifouling Performance

Numerous investigations have focused on creating effective membranes for desalination in order to alleviate the water scarcity crisis. In this study, first, LDH nanoplates were synthesized and utilized to alter the surface of thin-film composite (TFC) membranes in the course of this investigation. Following that, a simple technique was used to produce a novel nanocomposite incorporating LDH layers and Na14(P2W18Co4O70)·28H2O polyoxometalate nanoparticles, resulting in the creation of a fresh variety of thin-film nanocomposite (TFN). The performance of all of the membranes acquired was examined in the process of forward osmosis (FO). The impact of the compounds that were prepared was assessed on the hydrophilicity, topology, chemical structure, and morphology of the active layer of polyamide (PA) through analysis methods such as atomic force microscopy (AFM), energy-dispersive X-ray (EDX), FTIR spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and water contact angle (WCA) goniometry. After evaluating the outcomes of both modified membrane types, it was observed that the membrane equipped with the nanocomposite modifier at a concentration of 0.01 wt % exhibited the highest water flux, measuring 46.6 LMH and selectivity of 0.23 g/L. This membrane was thus considered the best option. Furthermore, the membrane's ability to prevent fouling was examined, and the findings revealed an enhancement in its resistance to fouling in comparison to the filler-free membrane.

Intercalation of Ionic Liquids into LDH Structures for Microwave-Accelerated Polymerizations

Microwave-accelerated ring-opening polymerization (ROP) of cyclic esters catalyzed by ionic liquid (IL) anions, intercalated into layered double hydroxides (LDHs), has been recently described as a fast and environmentally friendly synthetic way to prepare biodegradable polyester/LDH nanocomposites. However, to observe this synergistic catalytic effect between microwaves and IL anions and to achieve a homogeneous structure of the final polymer nanocomposite, the IL anions must be efficiently intercalated inside the LDH structure. Herein, we investigate the effects of various metal compositions of M2+/Al3+ LDHs (M = Mg, Co, and Ca) and different LDH synthetic routes (one-step direct coprecipitation, two-step coprecipitation/anion exchange, and two-step urea/anion exchange) on the intercalation efficiency of trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate IL. The most effective IL anion intercalation was observed for Ca2+/Al3+ LDH prepared using the two-step method consisting of coprecipitation and subsequent anion exchange. After optimization, this synthetic pathway led to the production of LDHs with intercalated IL anions and a reduced amount of intercalated water (<0.6 wt %). The catalytic ability of thus optimized LDH particles was demonstrated on the microwave-assisted ROP of ε-caprolactone, showing rapid progress of polymerization. Within minutes, the polycaprolactones with an average molecular mass in the range of 20 000-50 000 g/mol containing fully delaminated and exfoliated LDH nanoparticles were obtained.

Uniform titania-supported Ce(iii) carbonate cluster catalysts for degradation of reactive oxygen species

We report the synthesis of uniform 2.5 ± 0.4 nm diameter Ce(iii) carbonate clusters deposited on the surface of TiO₂ nanoparticles and characterize them using HAADF-STEM and EELS, as well as UV-Vis and FTIR spectroscopies. This material is a highly proficient catalytic antioxidant for the degradation of photocatalytically generated reactive oxygen species (ROS). We observed an unusual U-shaped pH-dependence in its photoprotection catalytic activity, with an optimum function in the near-neutral pH range of 7.7 ± 0.7. This sharp pH dependence is not observed in previously reported bulk Ce(iii) carbonate materials, and it is also not a consequence of Ce(iii) carbonate cluster decomposition. However, it is consistent with a tandem reaction sequence consisting of a biomimetic superoxide dismutase and catalase function, which is dependent on a balance of protons and hydroxide anions for function. Our dissolution-deposition approach for synthesizing nanoscale Ce(iii) carbonate clusters on TiO₂ should be generalizable to other carbonates and metal-oxide supports.

Insight into the In-Situ Encapsulation-Reassembly Strategy To Fabricate PW12@NiCo-LDH Acid-Base Bifunctional Catalysts

Acid-base bifunctional catalysts have attracted increasing attention due to the improved overall efficiency of synthetic reactions. Herein, we reported the successful fabrication of a PW₁₂@NiCo-LDH acid-base bifunctional catalyst by using the in-situ encapsulation-reassembly strategy. The evolution process of morphology and structure was monitored carefully by various time-dependent characterizations. X-ray absorption fine structure (XAFS) and density functional theory (DFT) calculations demonstrated that the terminal oxygen of PW₁₂ in PW₁₂@NiCo-LDH preferred to assemble with the oxygen vacancies on NiCo-LDH. When applied for deacetalization-Knoevenagel condensation, the PW₁₂@NiCo-LDH displayed >99% conversion of benzaldehyde dimethyl acetal (BDMA) and >99% yield of ethyl α-cyanocinnamate (ECC). Moreover, PW₁₂@NiCo-LDH can be recycled at least 10 cycles without obvious structural change, which can be attributed to the confinement of PW₁₂ into the NiCo-LDH nanocage. Such excellent catalytic activity of PW₁₂@NiCo-LDH was benefited from the short mass transfer pathway between acid sites and base sites, which was caused by the stable assembly between PW₁₂ and NiCo-LDH.

Recent Advances in Confining Polyoxometalates and the Applications

Polyoxometalates (POMs) are widely used in catalysis, energy storage, biomedicine, and other research fields due to their unique acidity, photothermal, and redox features. However, the leaching and agglomeration problems of POMs greatly limit their practical applications. Confining POMs in a host material is an efficient tool to address the above-mentioned issues. POM@host materials have received extensive attention in recent years. They not only inherent characteristics of POMs and host, but also play a significant synergistic effect from each component. This review focuses on the recent advances in the development and applications of POM@host materials. Different types of host materials are elaborated in detail, including tubular, layered, and porous materials. Variations in the structures and properties of POMs and hosts before and after confinement are highlighted as well. In addition, an overview of applications for the representative POM@host materials in electrochemical, catalytic, and biological fields is provided. Finally, the challenges and future perspectives of POM@host composites are discussed.

Layered Double Hydroxides for Sustainable Agriculture and Environment: An Overview

Agricultural practices in modern society have a detrimental impact on the health of the ecosystem, environment, and consumers. The significantly high usage rate of chemicals causes serious harm, and the sector demands the development of innovative materials that can foster improved food production and lessen ecological impacts. The majority of layered double hydroxides (LDH) are synthetic. At the same time, some of them occur in the form of natural minerals (hydrotalcite), which have recently emerged as favorable materials and provided advanced and ingenious frontiers in various fields of agriculture through practical application possibilities that can replace conventional agricultural systems. LDH can exchange anions intercalated between the layers in the interlayer structure, and there is evidence that atmospheric carbon dioxide and moisture can completely break down LDH over time. Due to certain unique properties such as tunable structure, specific intercalation chemistry, pH-dependent stability, as well as retention of the guest molecules within interlayers and their subsequent controlled release, LDHs are increasingly investigated as materials to enhance yield, quality of crops, and soil in recent times. This review aims to present the current research progress in the design and development of LDH-based materials as nanoscale agrochemicals to illustrate its relevance in making agro-practices more sustainable and efficient. Specific emphasis is given to the functionality of these materials as effective materials for the slow release of fertilizers and plant growth factors as well as adsorption of toxic agrochemical residues and contaminants. Relevant research efforts have been briefly reviewed, and the potential of LDH as new generation green materials to provide solutions to agricultural problems for improving food productivity and security has been summarized.

Guided Synthesis of a Mo/Zn Dual Single-Atom Nanozyme with Synergistic Effect and Peroxidase-like Activity

We present a facile route towards a dual single-atom nanozyme composed of Zn and Mo, which utilizes the non-covalent nano-assembly of polyoxometalates, supramolecular coordination complexes as the metal-atom precursor, and a macroscopic amphiphilic aerogel as the supporting substrate. The dual single-atoms of Zn and Mo have a high content (1.5 and 7.3 wt%, respectively) and exhibit a synergistic effect and a peroxidase-like activity. The Zn/Mo site was identified as the main active center by X-ray absorption fine structure spectroscopy and density functional theory calculation. The detection of versatile analytes, including intracellular H₂ O₂ , glucose in serum, cholesterol, and ascorbic acid in commercial beverages was achieved. The nanozyme has an outstanding stability and maintained its performance after one year's storage. This study develops a new peroxidase-like nanozyme and provides a robust synthetic strategy for single-atom catalysts by utilizing an aerogel as a facile substrate that is capable of stabilizing various metal atoms.

Polyoxometalate-based materials in extraction, and electrochemical and optical detection methods: A review

Polyoxometalates (POMs) as metal-oxide anions have exceptional properties like high negative charges, remarkable redox abilities, unique ligand properties and availability of organic grafting. Moreover, the amenability of POMs to modification with different materials makes them suitable as precursors to further obtain new composites. Due to their unique attributes, POMs and their composites have been utilized as adsorbents, electrodes and catalysts in extraction, and electrochemical and optical detection methods, respectively. A survey of the recent progress and developments of POM-based materials in these methods is therefore desirable, and should be of great interest. In this review article, POM-based materials, their properties as well as their identification methods, and analytical applications as adsorbents, electrodes and catalysts, and corresponding mechanisms of action, where relevant, are reviewed. Some current issues of the utilization of these materials and their future prospects in analytical chemistry are discussed.

Research progress on photocatalytic reduction of CO2 based on LDH materials

Converting CO₂ to renewable fuels or valuable carbon compounds is an effective way to solve the global warming and energy crisis. Compared with other CO₂ conversion methods, photocatalytic reduction of CO₂ is more energy-saving, environmentally friendly, and has a broader application prospect. Layered double hydroxide (LDH) has attracted widespread attention as a two-dimensional material, composed of metal hydroxide layers, interlayer anions and water molecules. This review briefly introduces the basic theory of photocatalysis and the mechanism of CO₂ reduction. The composition and properties of LDH are introduced. The research progress on LDH in the field of photocatalytic reduction of CO₂ is elaborated from six aspects: directly as a catalyst, as a precursor for a catalyst, and by modification, intercalation, supporting with other materials and construction of a heterojunction. Finally, the development prospects of LDH are put forward. This review could provide an effective reference for the development of more efficient and reasonable photocatalysts based on LDH.

The Curing Kinetics of Multiscale [Ni(EDTA)]-2 Intercalated Zn-Al Layered Double Hydroxides: Glass Fiber–Epoxy Composite Prepreg

In the present research, the effect of Zn2Al layered double hydroxides (LDH) and nickel (II)-EDTA complex intercalated LDH (LDH-[Ni(EDTA)]-2) on the cure kinetics of glass fiber/epoxy prepreg (GEP) was explored using nonisothermal differential scanning calorimetry (DSC). The results showed that LDH caused a shift in the cure temperature toward lower temperatures while accelerating the curing of epoxy prepregs. The use of LDH-[Ni(EDTA)]-2 more profoundly influenced the acceleration of the curing process. The curing kinetics of prepregs was assessed through the differential isoconversional Friedman (FR) technique and the integration method of Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS). A decrease was detected in the $E_{\alpha }$ value of glass fiber/LDH-[Ni(EDTA)]-2/epoxy (GELP) and glass fiber/LDH-[Ni(EDTA)]-2/epoxy (GELNiP) prepregs at small cure degrees relative to GEP, suggesting the catalytic effect of LDH or LDH-[Ni(EDTA)]-2 on the initial epoxy/amine reaction. Furthermore, LDH-[Ni(EDTA)]-2 performed better due to the catalyst role of nickel (II). Moreover, the activation energy exhibited lower reliance on the degree of conversion in the cases of GELP and GELNiP rather than pure epoxy prepregs. An autocatalytic model was used to evaluate the curing behavior of the system. Based on the results, the curing reaction of the epoxy prepreg can be described by the autocatalytic Šesták-Berggren model even after the incorporation of LDH or LDH-[Ni(EDTA)]-2. The kinetic parameters of the autocatalytic model (such as $E_{\alpha }$ , $A$ , $m$ , $n$ ) and the equations explaining the curing behavior of prepregs were introduced as well whose predictions were in line with the experimental findings.

Zn–Al Layered Double Hydroxides Synthesized on Aluminum Foams for Fluoride Removal from Water

Fluoride excess in water represents an environmental issue and a risk for human health since it can cause several diseases, such as fluorosis, osteoporosis, and damage of the nervous system. Layered double hydroxides (LDHs) can be exploited to remove this contaminant from water by taking advantage of their high ion-exchange capability. LDHs are generally mixed with polluted water in the form of powders, which then cause the problem of uneasy separation of the contaminated LDH sludge from the purified liquid. In this work, Zn–Al LDH films were directly grown in situ on aluminum foams that acted both as the reactant and substrate. This method enabled the removal of fluoride ions by simple immersion, with ensuing withdrawal of the foam from the de-contaminated water. Different LDH synthesis methods and aluminum foam types were investigated to improve the adsorption process. The contact time, initial fluoride concentration, adsorbent dosage, and pH were studied as the parameters that affect the fluoride adsorption capacity and efficiency. The highest absorption efficiency of approximately 70% was obtained by using two separate growth methods after four hours, and it effectively reduced the fluoride concentration from 3 mg/L to 1.1 mg/L, which is below the threshold value set by WHO for drinking water.

Recent Progress on Transition Metal Based Layered Double Hydroxides Tailored for Oxygen Electrode Reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), namely, so-called oxygen electrode reactions, are two fundamental half-cell reactions in the energy storage and conversion devices, e.g., zinc–air batteries and fuel cells. However, the oxygen electrode reactions suffer from sluggish kinetics, large overpotential and complicated reaction paths, and thus require efficient and stable electrocatalysts. Transition-metal-based layered double hydroxides (LDHs) and their derivatives have displayed excellent catalytic performance, suggesting a major contribution to accelerate electrochemical reactions. The rational regulation of electronic structure, defects, and coordination environment of active sites via various functionalized strategies, including tuning the chemical composition, structural architecture, and topotactic transformation process of LDHs precursors, has a great influence on the resulting electrocatalytic behavior. In addition, an in-depth understanding of the structural performance and chemical-composition-performance relationships of LDHs-based electrocatalysts can promote further rational design and optimization of high-performance electrocatalysts. Finally, prospects for the design of efficient and stable LDHs-based materials, for mass-production and large-scale application in practice, are discussed.

Functionalized polyoxometalate microspheres ensure selective adsorption of phosphoproteins and glycoproteins

Lacunary polyoxometalate (POM), [PW₉O₃₄]^9-, grafts with a boronic acid group attached via an organosilane bridge assemble into microspheres, PW₉-Si-APBA. The oxygen-rich and hydrophilic surface of POM facilitates the binding of phosphate groups in phosphoproteins and glycans in glycoproteins. While the metal-oxo in POM provides π-π interactions with the phosphate groups of phosphoproteins, the boronic acid group specifically binds to glycoproteins via the cis-diols of glycans. Therefore, these multi-driving forces ensure the selective adsorption of phosphoproteins and glycoproteins by PW₉-Si-APBA microspheres in biological sample matrixes, even in the presence of very high protein abundance, i.e., BSA, at mass ratio of β-ca/IgG/OVA/BSA = 1 : 1 : 1 : 200.

Photocatalytic remediation of organic waste over Keggin-based polyoxometalate materials: A review

Photocatalytic remediation of industrial water pollution has courted intense attention lately due to its touted green approach. In this respect, Keggin-based polyoxometalates (POMs) as green solid acids in photocatalytic reaction possess superior qualities, viz. unique photoinduced charge-transfer properties, strong photooxidative-photoreductive ability, high chemical and thermal stability, and so forth. Unfortunately, it suffers from a large bandgap energy, low specific surface area, low recoverability, and scarce utilization in narrow absorption range. Therefore, the pollutant degradation performance is not satisfactory. Consequently, multifarious research to enhance the photocatalytic performance of Keggin-based POMs were reported, viz. via novel modifications and functionalizations through a variety of materials, inclusive of, inter alia, metal oxides, transition metals, noble metals, and others. In order to advocate this emerging technology, current review work provides a systematic overview on recent advancement, initiated from the strategized synthetic methods, followed by hierarchical enhancement and intensification process, at the same time emphasizes on the fundamental working principles of Keggin-based POM nanocomposites. By reviewing and summarizing the efforts adopted global-wide, this review is ended with providing useful outlooks for future studies. It is also anticipated to shed light on producing Keggin-based POM nanocomposites with breakthrough visible- and solar-light-driven photocatalytic performance against recalcitrant organic waste.

Nanocarbon-Enhanced 2D Photoelectrodes: A New Paradigm in Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) water splitting systems are highly promising for converting solar energy into clean and sustainable chemical energy. In such PEC systems, an integrated photoelectrode incorporates a light harvester for absorbing solar energy, an interlayer for transporting photogenerated charge carriers, and a co-catalyst for triggering redox reactions. Thus, understanding the correlations between the intrinsic structural properties and functions of the photoelectrodes is crucial. Here we critically examine various 2D layered photoanodes/photocathodes, including graphitic carbon nitrides, transition metal dichalcogenides, layered double hydroxides, layered bismuth oxyhalide nanosheets, and MXenes, combined with advanced nanocarbons (carbon dots, carbon nanotubes, graphene, and graphdiyne) as co-catalysts to assemble integrated photoelectrodes for oxygen evolution/hydrogen evolution reactions. The fundamental principles of PEC water splitting and physicochemical properties of photoelectrodes and the associated catalytic reactions are analyzed. Elaborate strategies for the assembly of 2D photoelectrodes with nanocarbons to enhance the PEC performances are introduced. The mechanisms of interplay of 2D photoelectrodes and nanocarbon co-catalysts are further discussed. The challenges and opportunities in the field are identified to guide future research for maximizing the conversion efficiency of PEC water splitting.

Polyoxometalate-functionalized macroporous microspheres for selective separation/enrichment of glycoproteins

Glycoproteins always participate in various biological processes. Selective separation and enrichment of glycoproteins are of great significance for the research of pathogenesis. Herein, macroporous polymer microspheres were fabricated, and further functionalized by polyoxometalate. Thus, a simple, efficient and highly selective approach was constructed for glycoprotein enrichment from a complex matrix. The as-prepared material shows promise as a potential adsorbent in bio-separation and downstream clinical applications.

Polyoxometalates encapsulated into hollow double-shelled nanospheres as amphiphilic nanoreactors for an effective oxidative desulfurization

Although some catalytic hollow nanoreactors have been fabricated in the past, the encapsulated active species focus on metal nanoparticles, and a method for polyoxometalate (POM)-containing hollow nanoreactors has seldom been developed. Herein, we report a synthetic strategy towards POM-based amphiphilic nanoreactors, where the hollow mesoporous double-shelled SiO₂@C nanospheres were used to encapsulate Keggin-type H₃PMo₁₂O₄₀ (PMo₁₂). The outer hydrophobic carbon shell was beneficial for the enrichment of the organic substrate around the nanoreactor and simultaneously prevented the deposition of POMs on the outer surface of the nanoreactor. The inner hydrophilic silica cavity was modified by two types of organosilanes, which not only created an amphiphilic cavity environment but also acted as an anchor to mobilize PMo₁₂. As the POM nanoreactor had the hydrophilic@hydrophobic SiO₂@C shell and an amphiphilic cavity, both dibenzothiophene (DBT) and H₂O₂ could smoothly diffuse into the nanosized cavity, where the DBT was effectively oxidized (conversion: >99%) by the immobilized PMo₁₂ under mild conditions. Importantly, the control experiments indicated that the confined effect of nanoreactor, amphiphilic SiO₂@C double-shell, unique cavity environment, and mesoporous channels accounted for an excellent catalytic performance. Moreover, the nanoreactor was robust and could be reused for five cycles without loss of activity.

Cu-Al mixed oxide-catalysed multi-component synthesis of gluco- and allofuranose-linked 1,2,3-triazole derivatives

A series of carbohydrate-linked 1,2,3-triazole derivatives were synthesized in good yields from glucofuranose and allofuranose diacetonides using as key step a three-component 1,3-dipolar azide-alkyne cycloaddition catalysed by a Cu-Al mixed oxide. In this multi-component reaction, Cu-Al mixed oxide/sodium ascorbate system serves as a highly reactive, recyclable and efficient heterogeneous catalyst for the regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles. The reported protocol has significant advantages over classical CuI/N,N-diisopropylethylamine (DIPEA) or CuSO4/sodium ascorbate conditions in terms of efficiency and reduced synthetic complexity. In addition, the selective deprotection of synthesized di-O-isopropylidene derivatives was also carried out leading to the corresponding mono-O-isopropylidene products in moderate yields. Some of the synthesized triazole glycoconjugates were tested for their in vitro antimicrobial activity using the disc diffusion method against Gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis), Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa), as well as fungus (Aspergillus niger) and yeast (Candida utilis). The results revealed that these compounds exhibit moderate to good antimicrobial activity mainly against Gram-negative bacteria.

Engineering polyoxometalate-intercalated layered double hydroxides for catalytic applications

Polyoxometalate-intercalated layered double hydroxide (POM-LDH) nanocomposites have received considerable attention in recent years because such nanocomposites not only inherit the intrinsic properties of POMs and LDHs but also exert significant synergistic effects during the catalytic process. In this frontier article, we present the latest advances on the POM-LDH nanocomposites ranging from new synthetic methods to catalytic applications. By making use of the host layer modification method and exfoliation assembly method, the as-prepared POM-LDH nanocomposites show a wide range of catalytic applications. The challenges and future opportunities are also discussed by highlighting some creative work on related POM- or LDH-based materials.

Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry

Polyoxometalates (POMs) are molecular metal-oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self-assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM-cation interactions in solution, the resulting solid-state compounds, and behavior and properties that emerge from these POM-cation interactions. We will explore how application-inspired research has exploited cation-controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM-cation interactions.

Ionic liquid-functionalized LDH as catalytic-initiating nanoparticles for microwave-activated ring opening polymerization of ε-caprolactone

Layered double hydroxides with ionic liquid as highly active catalytic-initiating system for microwave-assisted ring opening polymerization of ε-caprolactone.

Highlights on the Catalytic Properties of Polyoxometalate-Intercalated Layered Double Hydroxides: A Review

Layered double hydroxides (LDH) are an extended class of two-dimensional anionic materials that are known for their unique lamellar structure, versatile composition, and tunable properties. The layered architecture allows the intercalation between the positively charged sheets of a vast variety of anionic species, including oxometalates and polyoxometalates (POM). The hybrid composites that were developed using POM and LDH show great advantages when compared to both parent materials causing the appearance of new functionalities, which may lead to remarkable contributions in many areas of application, especially in catalysis. The current review paper emphases all of the crucial works already existing in literature that are related to the large group of POM-LDH solids and their use as catalysts for fine organic synthesis. The new trends in the development of the POM-LDH catalysts are highlighted based on the overview of 121 scientific articles that were published between 1984 and 2019. The main topics are focused primarily on the synthesis, characterization, and the catalytic applications of different LDH systems hosting polyoxometalates with low, medium, and high nuclearity. The intense exploration of the POM-LDH field has led to the obtaining of countless effective catalysts used in various types of reactions, from condensation, esterification, halodecarboxylation, to oxidation and epoxidation.

Novel Ti4+-Chelated Polyoxometalate/Polydopamine Composite Microspheres for Highly Selective Isolation and Enrichment of Phosphoproteins

Selective isolation and enrichment of phosphoproteins play critical roles for identification of biomarkers in biological applications. Herein, a kind of polyoxometalate (P₅W₃₀)/polydopamine (PDA) composite microspheres is readily synthesized via an in situ polymerization way, followed by immobilization of Ti⁴⁺ on the surface of the microspheres to obtain P₅W₃₀/PDA-Ti⁴⁺. Due to metal affinity and π stacking interaction, this novel material exhibits high selectivity to β-casein (β-ca), and the theoretical maximum adsorption capacity is as high as 1250 mg g^-1, fitting well with the Langmuir model. The captured β-ca can be collected by using Britton-Robinson (B-R) buffer at pH 7.0, and a recovery of 81.5% is acquired. The enrichment factor is over 150 at a mass ratio of BSA/β-ca = 100:1, indicating that phosphoproteins can be purified by P₅W₃₀/PDA-Ti⁴⁺. Moreover, the application of P₅W₃₀/PDA-Ti⁴⁺ as sorbent in real biological samples has been investigated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, and the consequences show that this kind of material is able to selectively isolate phosphoproteins from complex samples such as drinking milk and chicken egg white.

Investigation of Thermal Behavior of Layered Double Hydroxides Intercalated with Carboxymethylcellulose Aiming Bio-Carbon Based Nanocomposites

Carboxymethylcellulose (CMC), a polymer derived from biomass, was intercalated into layered double hydroxides (LDH) composed by M2+/Al3+ (M2Al-CMC, M = Mg or Zn) and evaluated as precursors for the preparation of biocarbon-based nanocomposites by pyrolysis. M2Al-CMC hybrids were obtained by coprecipitation and characterized by X ray diffraction (XRD), vibrational spectroscopies, chemical analysis, and thermal analysis coupled to mass spectrometry. Following, pyrolyzed materials obtained between 500–1000 °C were characterized by XRD, Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Above 600 °C, Raman spectra of all samples showed the presence of graphitic carbon, which plays a role in the degree of crystallinity of produced inorganic phases (for comparison purposes, M2Al-CO3 materials were investigated after calcination in the same experimental conditions). XRD patterns of Mg2Al-CMC pyrolyzed between 600–1000 °C showed poorly crystallized MgO and absence of spinel reflections, whereas for Zn2Al-CMC, it was observed well crystallized nanometric ZnO at 800 °C, and ZnAl2O4 and γ-Al2O3 phases at 1000 °C. Above 800 °C, the carbothermic reaction was noticed, transforming ZnO to zinc vapour. This study opens perspectives for nanocomposites preparation based on carbon and inorganic (mixed) oxides through precursors having organic-inorganic interactions at the nanoscale domain.

Polyoxometalates as components of supramolecular assemblies

The non-covalent interaction of polyoxometalates (POMs) with inorganic- or organic-based moieties affords hybrid assemblies with specific physicochemical properties that are of high interest for both fundamental and applied studies, including the discovery of conceptually new compounds and unveiling the impact of their intra-supramolecular relationships on the fields of catalysis, molecular electronics, energy storage and medicine. This minireview summarises the recent advances in the synthetic strategies towards the formation of such non-covalent POM-loaded assemblies, shedding light on their key properties and the currently investigated applications. Four main emerging categories according to the nature of the conjugate are described: (i) POMs in metal-organic frameworks, (ii) POMs merged with cationic metal complexes, (iii) architectures generated with solely POM units and (iv) POMs assembled with organic molecular networks.

Catalytic use of layered materials for fine chemical syntheses

The present work reviews the catalytic use of layered solid materials for fine chemical syntheses with focus on layered double hydroxides, but including other classes of layered compounds of catalytic relevance.

Immobilization of a Ce(IV)-substituted polyoxometalate on ethylenediamine-functionalized graphene oxide for selective extraction of phosphoproteins

A sorbent for selective extraction of phosphoproteins was obtained by immobilization of a Ce(IV)-substituted polyoxometalate on ethylenediamine-functionalized graphene oxide (CeEGO). The resulting composites exhibit an adsorption capacity of 981 mg g^-1 for β-casein due to the synergistic effect of metal-affinity interaction between Ce(IV) and phosphate groups and π-stacking interaction between the polyoxometalate framework and the phosphate groups. The results of LC-MS and SDS-PAGE analysis show that the CeEGO composites can be applied to the extraction of phosphoproteins from protein mixture, and as little as 50 μg mL^-1 of the phosphoprotein β-casein can be detected by SDS-PAGE. It was also applied to the extraction of β-casein from spiked biological samples such as drinking milk, whole blood and swine heart tissue extract. Graphical abstract An efficient sorbent is obtained by immobilization of a Ce(IV)-substituted polyoxometalate on ethylenediamine-functionalized graphene oxide (CeEGO). The resulting composites exhibit highly selective capture capacity towards phosphoproteins due to the synergistic effect of metal-affinity interaction between Ce(IV) and phosphate groups and π-stacking interaction between the polyoxometalate framework and the phosphate groups.

Multi-Tasking POM Systems

Polyoxometalate (POM)-based materials of current interest are summarized, and specific types of POM-containing systems are described in which material facilitates multiple complex interactions or catalytic processes. We specifically highlight POM-containing multi-hydrogen-bonding polymers that form gels upon exposure to select organic liquids and simultaneously catalyze hydrolytic or oxidative decontamination, as well as water oxidation catalysts (WOCs) that can be interfaced with light-absorbing photoelectrode materials for photoelectrocatalytic water splitting.

Self-Assembly in Polyoxometalate and Metal Coordination-Based Systems: Synthetic Approaches and Developments

Utilizing new experimental approaches and gradual understanding of the underlying chemical processes has led to advances in the self-assembly of inorganic and metal–organic compounds at a very fast pace over the last decades. Exploitation of unveiled information originating from initial experimental observations has sparked the development of new families of compounds with unique structural characteristics and functionalities. The main source of inspiration for numerous research groups originated from the implementation of the design element along with the discovery of new chemical components which can self-assemble into complex structures with wide range of sizes, topologies and functionalities. Not only do self-assembled inorganic and metal–organic chemical systems belong to families of compounds with configurable structures, but also have a vast array of physical properties which reflect the chemical information stored in the various “modular” molecular subunits. The purpose of this short review article is not the exhaustive discussion of the broad field of inorganic and metal–organic chemical systems, but the discussion of some representative examples from each category which demonstrate the implementation of new synthetic approaches and design principles.