Immobilization Of Polyoxometalates Research Articles

Polyoxometalate‐Soft Matter Composite Materials: Design Strategies, Applications, and Future Directions

AbstractMolecular metal oxides or polyoxometalates (POMs) offer unrivaled properties in areas ranging from catalysis and energy conversion through to molecular electronics, biomimetics, and theranostics. While POMs are ubiquitous metal oxide model systems studied in most areas of chemistry and materials science, their technological deployment is often hampered by their molecular nature, as this leads to increased degradation, leaching, and loss of reactivity, particularly when harsh applications, such as water electrolysis, thermal catalysis or highly basic/acidic reaction solutions are targeted. Therefore, immobilization of POMs on heterogeneous substrates has recently become a central theme in POM research. While early studies focused mainly on metal oxide and semiconductor supports, more recently, POM integration in soft matter matrices including polymers, conductive polymers, hydrogels, and stimuli‐responsive matrices leads to breakthroughs in multifunctional composite design. This Review summarizes pioneering experimental and theoretical progress in this emerging field over the last decade, highlighting current challenges that need to be overcome to allow a more widespread technological deployment and providing the authors’ view of some of the most promising future directions of the research field. In addition, the current understanding of correlations between structure (on the molecular, nano‐ and microscale) and reactivity in POM polymer composites is summarized, so that rational materials design can be further developed.

Origin of the Boosting Effect of Polyoxometalates in Photocatalysis: The Case of CO2 Reduction by a Rh-Containing Metal–Organic Framework

The immobilization of polyoxometalates (POMs) near catalytic centers of metal–organic frameworks (MOFs) has been reported as an advantageous strategy to boost their photocatalytic activity toward strategic reactions such as CO2 reduction (CO2RR) or hydrogen evolution (HER), although the reasons for such enhancement are still poorly understood. Unveiling the role of POM guests in the reaction mechanisms is therefore a key step toward the development of the next generation of multicomponent catalytic materials with optimal photocatalytic performances. Here, we elucidate the remarkable role of encapsulated [PW12O40]3– (PW12) polyoxometalates in boosting the photocatalytic activity of the Rh-functionalized UiO-67 MOF toward CO2RR and HER by combining theoretical density functional theory and microkinetic modeling approaches with experimental photophysical and spectroscopic techniques. First, we characterized in detail the reaction mechanism for CO2RR and HER catalyzed by the PW12-containing Rh-functionalized MOF, using [Ru(bpy)3]2+ as the photosensitizer (PS) and triethanolamine (TEOA) as the sacrificial electron donor in acetonitrile. Our results reveal that the encapsulated POMs act as efficient electron reservoirs, which quench [Ru(bpy)3]+─the photogenerated reduced form of the PS─and transfer the electrons to the Rh catalytic sites of the MOF. Notably, this is shown to favor the regeneration of the oxidized PS over its unproductive degradation, boosting the turnover numbers of the photocatalytic system. Such a mechanism can explain not only the higher formate and H2 product yields in the POM-containing catalyst but also the experimentally observed higher impact on the HER pathway than that on the CO2RR one, as the source of protons is generated in the reductive quenching of the photoexcited PS by TEOA. Finally, our computational exploration was extended to a whole variety of POMs, which allowed establishing relationships between their redox potentials and the activity of the related POM-containing catalytic materials. The optimal activity is reached when both the ability of the POM to accept electrons and that of its reduced form to reduce the Rh catalyst are simultaneously maximized, leading to a volcano plot whereby POMs with a moderate redox potential display the highest impact on photocatalytic performances.

High-nuclearity cobalt(II)-containing polyoxometalate anchored on nickel foam as electrocatalyst for electrochemical water oxidation studies

Polyoxometalates (POMs) are known for their unique structural properties and have attracted widespread attention in the field of storage devices, energy conversion materials, catalysis, electronics and molecular sensing. This article focuses on the anchoring of high nuclearity 16-cobalt(II)-containing POM that is, Na30.5K1.5[{Co4(OH)3PO4}4(A-α-GeW9O34)4]٠116H2O (Co16-GeW9) using 3-aminopropyl triethoxysilane (APTS) as chemical linker over nickel foam (NiF) and is tested as electrocatalyst for water oxidation studies. The fabricated POM-based electrode (Co16-GeW9@NiF) has been characterized by UV–visible spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. Co16-GeW9 @NiF has been tested for its electrocatalytic water oxidation activity in 1 M KOH (pH 14) and sodium phosphate buffer (pH 7) with NaNO3 as a supporting electrolyte. The catalyst exhibited excellent electrocatalytic oxygen evolution reaction (OER) activity with overpotential of 370 mV at 10 mA/cm2 and a Tafel slope of 84 mV/dec in 1 M KOH, while overpotential of 530 mV at 10 mA/cm2 and a Tafel slope of 153 mV/dec in sodium phosphate buffer. The immobilization of POM over NiF could generate a way for developing efficient electrocatalysts for water oxidation in both neutral and basic pH.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

Immobilization of polyoxometalates via in-situ protonation and self-gelation of PEG-b-PDMAEMA-b-PTEPM triblock copolymer and its application in selective oxidation

To enrich the building block and function as well as the preparation methodology of metallopolymer system, functioned inorganic polyoxometalate clusters were introduced by using their intrinsic acidity induced multiple interactions with triblock copolymers. In the designed triblock copolymer PEG-b-PDMAEMA-b-PTEPM, the POMs clusters can be immobilized through in-situ protonation of the PDMAEMA block induced electrostatic interaction and the self-gelation of the PTEPM block, while the PEG block can improve the water solubility and dispersion of the final polyoxometalate-based hybrid catalyst (PHC). The selective oxidation activity of PHC is comparable with the homogeneous POMs catalyst as demonstrated by the model reaction of the selective oxidation of methyl phenyl sulfide (MPS) to methyl phenyl sulfoxide (MPSO). By optimizing the ratio between PHC and oxidant H2O2, the selectivity for MPSO can achieve as high as 99% with conversion above 95%. Furthermore, the multiple interactions between POMs and the polymer substrate also enhance the stability of PHC and enable the recycling of PHC. Therefore, the methodology of in-situ protonation and self-gelation by the acidity of POMs provides a facile and general strategy to prepare robust POMs-based metallopolymers, paving a new avenue towards cluster-based materials and devices.

Immobilization of polyoxometalates on protonated graphitic carbon nitride: A highly efficient and reusable catalyst for the synthesis of pyridine-N-oxides

N-oxides represent a significant class of compounds with increasing value due to their extensive applications in chemistry and biology. Herein, a series of heterogeneous catalysts were prepared based on the impregnation-precipitation method. In this strategy, protonated graphitic carbon nitride (pg-C3N4) was prepared first, and Cs3PW12O40 was immobilized over the surface of pg-C3N4 to obtain CsPW-CN composites. The prepared CsPW-CN composites achieved better catalytic activities than the pure Cs3PW12O40 in the N-oxidation of pyridine, and obtained 99% yield of pyridine-N-oxide in aqueous medium with H2O2 as a mild oxidant. Based on the characterizations of the catalyst morphology, structure, and chemical composition, the intimate interaction between Cs3PW12O40 and pg-C3N4 was verified. Meanwhile, the occurrence of the unique semi-embedded structure was an expected derivation of the pg-C3N4. Furthermore, the prepared CsPW-CN-1 composite was readily recovered and yielded 88.3% of the pyridine-N-oxide after 4 runs. This work could potentially provide a well-defined CsPW-CN composite for the N-oxidation of pyridine with a sustainable approach.

Immobilization of Polyoxometalate onto Modified Magnetic Nanoparticles: A New Catalyst for the Synthesis of Dihydropyranopyrazole Derivatives

AbstractA new nanocatalyst Fe3O4@carbon@polyoxometalate (Fe@C@Mo6O18) consisting of a cluster of Fe3O4nanoparticles as core, a carbon layer as inner shell, and a Lindqvist type polyoxometalate as the outer shell was prepared. The prepared nanocatalyst structure was characterized by energy dispersive X‐ray (EDX), scanning electron microscopy (SEM), vibration sample magnetometer (VSM), thermogravimetric analysis (TGA), and Fourier transform infrared (FT‐IR) spectroscopy techniques and their catalytic activity was evaluated for the synthesis of dihydropyranopyrazole derivatives. The results were excellent regarding the high yield of the products and short reaction time. The new nanocatalyst showed good recoverability with a significant loss of activity and selectivity within successive runs. Most importantly, these materials can be easily recovered by simple magnetic separation.

Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides.

Immobilization of polyoxometalates (POMs) onto oxides is relevant to many applications in the fields of catalysis, energy conversion/storage, or molecular electronics. Optimization and understanding the molecule/oxide interface is crucial to rationally improve the performance of the final molecular materials. We herein describe the synthesis and covalent grafting of POM hybrids with remote carboxylic acid functions onto flat Si/SiO2 substrates. Special attention has been paid to the characterization of the molecular layer and to the description of the POM anchoring mode at the oxide interface through the use of various characterization techniques, including ellipsometry, AFM, XPS, and FTIR. Finally, electron transport properties were probed in a vertical junction configuration and energy level diagrams have been drawn and discussed in relation with the POM molecular electronic features inferred from cyclic-voltammetry, UV-visible absorption spectra, and theoretical calculations. The electronic properties of these POM-based molecular junctions are driven by the POM LUMO (d-orbitals) whatever the nature of the tether or the anchoring group.

Property Characterization and Mechanism Analysis of Polyoxometalates-Functionalized PVDF Membranes by Electrochemical Impedance Spectroscopy.

Polyoxometalates (POMs) has proved its advantage in constructing high-performance nanocomposite membranes such as catalytic membranes, adsorptive membranes, and forward osmosis membranes. However, it is challenging or tedious to characterize its distribution and effect on the membrane structures due to the equipment resolution limitation, discrete nano-scaled structures of POMs, and limited doping amount compared to the polymeric membrane matrix. In this paper, POMs-functionalized polyvinylidene fluoride (PVDF) membranes were fabricated by phase inversion combined with the sol-gel method, and electrochemical impedance spectroscopy (EIS) was utilized to analyze the nanocomposite membrane intrinsic properties. Through adjusting the additives in the sol-forming process, a set of membranes with varied intrinsic properties were developed accordingly. The wetting degree of the membranes related to the hydrophilic nature of the membrane surfaces had a crucial influence on the impedance measurements at the early stage. Through EIS analysis, it was demonstrated that the amination of the membrane matrix through (3-aminopropyl)trimethoxysilane (APTMS) treatment and the immobilization of POMs through electrostatic attraction would not generate new pore structures into the membrane and only alter the membrane surface roughness and composition. To my knowledge, it is the first time that EIS was utilized to characterize the hydrophilicity of the membranes and pore structures of the POMs-modified membranes. Our findings indicate that EIS can provide valuable information for probing the structures of other nano-materials-incorporated membranes.

Entrapment vs. immobilization of polyoxometalates into silica and titania aerogels: Application in heterogeneous oxidation catalysis

Silica and titania aerogels were successfully used as immobilization and entrapment media for phosphomolybdic and phosphotungstic acids. The hybrid materials were investigated in the heterogeneous oxidation catalysis of phenyl methyl sulfide. The characterization of the hybrid catalysts confirmed the immobilization and entrapment of the polyoxometalates (POM) and their dispersion onto the surface of the solid support. The entrapment of these clusters was found to affect less the surface properties of the inorganic network compared to the immobilization process, thus allowing smoother access to the active sites of the polyoxometalate molecules. The catalytic activity of the POM clusters was widely maintained after immobilization and incorporation, where the chemical nature of the surface functional groups was found to affect the behavior of the catalyst. The fastest oxidation of methyl phenyl sulfide was observed for the reactions catalyzed by POM-incorporated silica aerogels, coupled with a high selectivity towards the methyl phenyl sulfoxide.

Immobilization of polyoxometalate on aminosilane-modified silicate: Synthesis, characterization and antibacterial performance

This paper presents studies on the immobilization of the polyoxometalate (H5PV2Mo10O40) on aminosilanemodified silicate as a new barrier of biological toxicant. The polyoxometalate-doped aminosilane-modified silicate (Ormosil/POM hybrid and Ormosil + POM blend) were synthesized by an in situ sol–gel route under acidic conditions. The physical properties of the samples were examined using NMR, SEM, XPS and TGA spectroscopy. The results indicated that POM was immobilized in channels by covalent linkages with the Ormosil of the Ormosil/POM hybrid, and that POM was incorporated into the Ormosil after impregnation from the Ormosil + POM blend. The covalent bond resulted from the reaction of Ormosil with POM, in which Si species were inserted into the vacancy of lacunary POM to form Si–O–Mo bonds. The antibacterial effects of samples were investigated with zone of inhibition testing, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The Ormosil/POM hybrid and Ormosil + POM blend possessed excellent antibacterial abilities and the antibacterial performance of the Ormosil/POM hybrid was better than that of the Ormosil + POM blend.

Cellulose nanocrystal shelled with poly(ionic liquid)/polyoxometalate hybrid as efficient catalyst for aerobic oxidative desulfurization

Immobilization of polyoxometalates (POMs) plays significant roles in their industrial applications. In this work, POM-supported catalyst has been developed with cellulose nanocrystal (CNC) as a sustainable carrier and employed in the aerobic oxidative desulfurization (AODS) of fuels. The catalyst was constructed by grafting a poly(ionic liquid) (PIL) onto CNCs and subsequently exchanging the counterions of the PIL into [Co(OH)6Mo6O18]3−, a kind of POM anionic clusters. The resulting CNC@PIL@POM was systematically characterized and utilized as a catalyst in the AODS of thiophenic compounds including benzothiophene (BT), dibenzothiophene (DBT), and 4, 6-dimethyldibenzothiophene (4, 6-DMDBT), with oxygen contained in air as oxidizing agent. The catalyst could achieve complete conversion of DBT in model diesel under mild condition and be reused five times without apparent losing of activity. As for the desulfurization of a real diesel, it was demonstrated that almost all of the original sulfides were completely converted. Based on these results, it is expected that the core-shell POM-supported catalyst may serve as an efficient, separable and sustainable catalyst for AODS of fuels.

Immobilization of polyoxometalate in a cage-based metal–organic framework towards enhanced stability and highly effective dye degradation

Due to their large surface areas, tunable functionality, and diversiform building blocks, metal–organic frameworks (MOFs) have arisen great attention in the last few decades. Nevertheless, most of MOFs show poor water stability, which seriously hinders their further applications. In this study, through the immobilization of polyoxometalate in a water unstable cage-based MOF material {[(Cu4Cl)(CPT)4(H2O)4]·3NO3·5NMP·3.5H2O} (1), in which HCPT = 4-(4-carboxyphenyl)-1,2,4-triazole and NMP = N-methyl pyrrolidone), via a simple one-pot solvothermal reaction, a new hybrid material {[(Cu4Cl)(CPT)4]·(HSiW12O40)·31H2O} (2) can be fabricated. The hybrid material 2 shows a nearly identical network structure to that of 1 but possesses much higher water and chemical stability. Moreover, the visible light dye degradation performance for 2 in water has been illustrated.

ChemInform Abstract: Immobilization of Polyoxometalates in Crystalline Solids for Highly Efficient Heterogeneous Catalysis

AbstractReview: 109 refs.

Immobilization of Polyoxometalate in the Metal-Organic Framework rht-MOF-1: Towards a Highly Effective Heterogeneous Catalyst and Dye Scavenger.

A series of three remarkable complexes [PMo12O40]@[Cu6O(TZI)3(H2O)9]4·OH·31H2O (H3TZI = 5-tetrazolylisophthalic acid; denoted as HLJU-1, HLJU = Heilongjiang University), [SiMo12O40]@[Cu6O(TZI)3(H2O)9]4·32H2O (denoted as HLJU-2), and [PW12O40]@[Cu6O(TZI)3(H2O)6]4·OH·31H2O (denoted as HLJU-3) have been isolated by using simple one-step solvothermal reaction of copper chloride, 5-tetrazolylisophthalic acid (H3TZI), and various Keggin-type polyoxometalates (POMs), respectively. Crystal analysis of HLJU 1−3 reveals that Keggin-type polyoxoanions have been fitted snuggly in the cages of rht-MOF-1 (MOF: metal−organic framework) with large cell volume in a range of 87968−88800 Å3 and large pore volume of about 68%. HLJU 1–3 exhibit unique catalytic selectivity and reactivity in the oxidation of alkylbenzene with environmental benign oxidant under mild condition in aqueous phase as well as the uptake capacity towards organic pollutants in aqueous solution.

Immobilization of polyoxometalates in crystalline solids for highly efficient heterogeneous catalysis.

Polyoxometalates (POMs) are a unique class of molecular metal-oxygen clusters with attractive architecture and tunable properties. Due to their strong acidity, redox chemistry, photoactivity, charge distribution and multielectron transformation, POMs have been used as efficient catalysts in a variety of chemical reactions. To meet the requirement of sustainable chemistry, great effort has been focused on immobilization of the active POMs on different solid supports to realize heterogeneous catalysis. This short review summarizes the recent progress on immobilization of POM moieties in crystalline solids with defined crystal structures, including organic-inorganic hybrid materials, POM-based inorganic crystalline solids and POM-encapsulated metal-organic frameworks (POM@MOFs), and their catalytic properties in oxidation, hydrolysis, cyanosilylation, photocatalysis and electrocatalysis. As illustrated in the text, these crystalline solids exhibit interesting catalytic properties, such as high activity, stability and selectivity, and simple recovery and easy recycling, which are much superior to those of the corresponding constituent species in most cases.

Immobilization of polyoxometalates in the Zr-based metal organic framework UiO-67.

The encapsulation of polyoxometalates within the large pores of the Zr(iv) biphenyldicarboxylate UiO-67 metal-organic framework has been achieved, for the first time, by direct solvothermal synthesis. The resulting POM@UiO-67 composite materials were fully characterized by XRPD, IR, MAS NMR, N2 porosimetry measurements and cyclic voltammetry.

Functionalization and post-functionalization: a step towards polyoxometalate-based materials

Polyoxometalates (POMs) have remarkable properties and a great deal of potential to meet contemporary societal demands regarding health, environment, energy and information technologies. However, implementation of POMs in various functional architectures, devices or materials requires a processing step. Most developments have considered the exchange of POM counterions in an electrostatically driven approach: immobilization of POMs on electrodes and other surfaces including oxides, embedding in polymers, incorporation into Layer-by-Layer assemblies or Langmuir-Blodgett films and hierarchical self-assembly of surfactant-encapsulated POMs have thus been thoroughly investigated. Meanwhile, the field of organic-inorganic POM hybrids has expanded and offers the opportunity to explore the covalent approach for the organization or immobilization of POMs. In this critical review, we focus on the use of POM hybrids in selected fields of applications such as catalysis, energy conversion and molecular nanosciences and we endeavor to discuss the impact of the covalent approach compared to the electrostatic one. The synthesis of organic-inorganic POM hybrids starting from bare POMs, that is the direct functionalization of POMs, is well documented and reliable and efficient synthetic procedures are available. However, as the complexity of the targeted functional system increases a multi-step strategy relying on the post-functionalization of preformed hybrid POM platforms could prove more appealing. In the second part of this review, we thus survey the synthetic methodologies of post-functionalization of POMs and critically discuss the opportunities it offers compared to direct functionalization.

Polyoxometalate Nanocone Nanoreactors: Magnetic Manipulation and Enhanced Catalytic Performance

Rationally designed assembly structures are of high scientific and technological importance for the development of multifunctional materials and devices. Among various types of nanobuilding blocks, polyoxometalates (POMs) are a class of metal oxide nanoclusters that offer rich structural versatility and enormous applications. The recent progresses in the synthesis of POM-based amphiphilic units have revolutionized POM chemistry, and consequently a variety of robust and well-defined assembly architectures with tunable properties have been developed, including one-dimensional wires and fibers, two-dimensional thin-films and disks, and threedimensional vesicles, spheres, tubes, and flowers. Although, these assembly structures hold great promise for the design of new functional materials, in reality they have been less explored for their potential use in various scientific fields. POMs have been extensively applied as catalysts for the oxidation of a variety of compounds such as alkenes, alcohols, and sulfides. Particularly, the oxidation of sulfides to sulfones, which is environmentally highly important, yet brings up a key challenge. To meet this challenge, many strategies in both homogeneous and heterogeneous catalytic reaction systems have been developed. The homogeneous approaches utilize the POM catalyst in bare form or in combination with a phase-transfer reagent, which generally have drawbacks associated with difficult catalyst separation and recovery. The heterogeneous approaches involve encapsulation with specific cations, immobilization of POM into silica or polymer matrices, or microemulsion formation. Although, heterogeneous systems provide easier catalyst recovery, generally it is based on filtration, which may be highly cumbersome on an industrial scale. In addition, the immobilization of POMs in supporting matrices involves complicated and lengthy catalyst preparation processes. Therefore, alternative multifunctional and superior approaches are needed. Recently, we reported well-defined, robust Keggin ion based nanocones, obtained by a simple and fast synthesis technique at room temperature. Herein, we report the functionalization of nanocones with magnetite nanocrystals and their controlled manipulation in the reaction system. For example, we applied these nanocones for the catalytic oxidation of sulfides in which they act as nanoreactors to provide enhanced efficiency, selectivity, and easier recovery under an external magnetic field. Figure 1 illustrates the concept of the nanocone nanoreactors. According to the accepted mechanism for the increased efficiency of the POM hybrid building block (DODA)3PW12O40 (DODA= dimethyldioctadecylammo-

Influence of the Functionalization Degree of Multiwalled Carbon Nanotubes on the Immobilization of Polyoxometalates and Its Effect on their Electrochemical Behavior

Hybrid materials based on the anchoring polyoxometalate particles POM on several multiwalled carbon nanotubes (NTs) matrices with different oxidation degree are characterized and electrochemically studied, in order to correlate the dispersion degree of POM particles on these matrices with their electrochemical behavior. These hybrids synthesized with soluble PMo12 particles resulted in truly homogenous hybrids and are analyzed by XRD, SEM, EDS, TEM, and N2 adsorption isotherms in order to confirm the presence of their components and their dispersion properties. Higher dispersion of PMo12 nanoparticles is the main factor contributing to an improved electrochemical behavior in these types of hybrid materials, making hybrid NT-S-PMo12 with this property the one resulting in the best electrochemical performance where the redox contribution of PMo12 particles are clearly observed as well as the improved double layer contribution of its nanotube matrix (NT-S, highly oxidized).