Keggin-type Polyoxometalate Research Articles

Preparation of polyoxometalate-doped aminosilane-modified silicate hybrid as a new barrier of chem-bio toxicant

Nanohybrid membranes based on the Keggin-type polyoxometalate (POM) H5PV2Mo10O40 and aminosilane-modified silicate (Ormosil and Ormosil(NR4+Cl−)) hybrids were synthesized as a new barrier to protect against simulants of chemical and biological toxicant. The 31P NMR and XPS results indicated that POM was bound to the Ormosil and Ormosil(NR4+Cl−) hybrids after impregnation. The antibacterial effects of the hybrids and hybrid-impregnated fabrics against Gram-negative and Gram-positive bacteria were investigated with zone of inhibition, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and plate-counting method. The MIC/MBC values of Ormosil(NR4+Cl−)/POM and Ormosil/POM against bacteria were 0.267/2.67 and 2.67/26.7, respectively, and the percentage reduction of bacteria was approximately 100% after 20 laundry cycles of their fabrics. The reaction products and mechanisms of the adsorptive degradation of 2-chloroethylethylsulfide (CEES) by hybrids were investigated with 13C NMR. The results of this study showed that POM-doped Ormosil systems are capable of destroying bacteria and CEES.

Surface-grafted Europium and Erbium Complexes of the 12-Tungstosilicate Heteropolyoxometalate: A Synthetic and Structural Investigations

The hydrothermal synthesis of two new inorganic-organic hybrid materials in which PDA (PDA = 1,10-phenanthroline-2,9-dicarboxylate) is connected to a well-defined Keggin-type polyoxometalate anion (POM) and a Ln(III) cation (POM = [SiW12O40]4−; Ln = Eu3+ (1), Er3+ (2)), are reported. These hybrids are characterized by elemental analyses, IR spectra and single-crystal X-ray diffraction. Single-crystal X-ray analysis reveals that both compounds have been assembled by hydrogen bonding and anion-π interactions among POM, PDA and lattice water units into a three-dimensional (3D) supramolecular structure. Compound 1 is composed of two different cationic layers of Eu(III) clusters with the POMs located in cavities that are created by the Eu-PDA groups. Compound 2 consists of [SiW12O40]4– Keggin-type POMs, where a single {W3O13} triad is decorated by a trinuclear Er(III)-complex.

Restricting Polyoxometalate Movement Within Metal-Organic Frameworks to Assess the Role of Residual Water in Catalytic Thioether Oxidation Using These Dynamic Composites

Comprised of high valent metal centers connected via oxide bonds, polyoxometalates (POMs) have a rich history in redox reactions. These discrete metal oxide clusters often are immobilized on heterogeneous supports to increase stability and allow for recyclability for catalytic reactions. One such support is the metal–organic framework (MOF), NU-1000. When POMs are installed into NU-1000, they are first sited in the mesoporous channel. Upon application of heat and removal of some physisorbed water, the POMs migrate to the microporous channel, where some active sites are blocked by the MOF linkers, resulting in lowered catalytic activity. To restrict this movement from the mesopore, we report here the use of two MOFs, naphthalene dicarboxylate-modified NU-1000 (NU-1000-NDC) and NU-1008, as supports for the Keggin-type polyoxometalate (POM), H3PW12O40. Upon the application of heat, these frameworks were found to hinder or prevent the movement of the POM between channels by blocking the aperture(s) connecting the mesoporous and microporous channels. The composite POM@MOF materials were used for the oxidation of a mustard gas simulant, 2-cholorethyl ethyl sulfide, using H2O2 as the oxidant. The POM@MOF catalysts exhibit enhanced reactivity when compared to the POM or MOF alone, more than doubling the initial rates. The activity trend in PW12@NU-1000-NDC matched that in PW12@NU-1000, where the scCO2-activated sample poised the POM in the mesopores to give greater substrate accessibility and enhance the reaction rate compared to the heated sample where the POMs are poised in the micropores. Contrary to these observed trends, PW12@NU-1008 prohibits POM migration and performs superior when water has been removed at elevated temperatures as the active sites are more accessible in the mesoporous channel.

Polyoxometalate/hydroquinone dual redox electrolyte for hybrid energy storage systems

A dual redox-active (proton-conductive) electrolyte composed of the mixture of two different soluble redox species is considered here for application in hybrid (battery-type) energy storage devices operating like high-power supercapacitor-type systems with an enhanced specific energy. The Keggin-type polyoxometalate (phosphotungstic acid), undergoing fast (electrochemically reversible) one-electron redox processes has been selected to support the negative electrode. In order to develop the potential difference in the cell, the hydroquinone, capable of oxidation/reduction according to the fast two-electron mechanism has been adopted and its electroactivity was predominantly confined to the positive electrode. It enabled us to design a hybrid charge storage cell with the operating voltage of 0.8 V and the specific energy of 20 Wh kg−1 (per total mass of both electrodes). In the constant power discharge mode, 13 Wh kg−1 of the energy has been preserved at the power of 1 kW kg−1 during discharging down to ½ of the maximum voltage which is consistent with the fast charging/discharging dynamics of the proposed hybrid system. A mixed hierarchical micro-porous (predominantly mesoporous) carbon of high (>2 cm3g-1) total pore volume and the BET surface area approaching 1000 m2 g−1 has been selected as the electrode material permitting electrical double layer charging, supporting electroactivity of redox species and unimpeded mass transport. Also, the normalization of electrical parameters against the total mass of electrodes and electrolyte resulted in a ca. five-fold increase of discharge capacity and specific energy in comparison to the performance of a simple electrical double-layer capacitor.

Jahn-Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations.

Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)-dinitrogen complexes [PW11O39M(N2)] n- (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N2 molecule, N-N bond length, N-N stretching frequency, and the NBO charge on the coordinated N2 moiety indicate that MoII-, TcII-, WII-, ReII-, and OsII-POM complexes are significant for binding and activation of the inert N2 molecule. The degree of the N2 activation can be classified into the "moderately activated" category according to Tuczek's sense [ J. Comput. Chem. 2006 , 27 , 1278 ]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N2 molecule in these POM-dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn-Teller distorted effects lead to an effective orbital mixture between σ2s* orbital of N2 and d z2 orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N2 into NH3 has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors.

Distinctive Trend of Metal Binding Affinity via Hydration Shell Breakage in Nanoconfined Cavity

We report a distinctive trend in the binding affinity of metal cations when they interact with lacunary Keggin-type polyoxometalate K7[α-PW11O39] clusters in aqueous media. The binding affinity of metal cations follows the special trends of Li+ > Na+ > K+ for monovalent cations and Mg2+ > Ca2+ > Sr2+ > Ba2+ for divalent cations, completely reversed from the common trends of Li+ < Na+ < K+ and Mg2+ < Ca2+ < Sr2+ < Ba2+ for macroion–counterion ion-pair formation in dilution solution. Isothermal titration calorimetry and NMR measurements confirm that the hydration shells of metal cations are significantly broken during the interaction in the nanoscale cavity, making the binding affinity stronger for the cations with smaller ionic sizes instead of hydrated sizes. The same phenomenon is also observed for the lacunary Dawson-type clusters, which contain well-defined cavity in their structures as well, demonstrating the universality of this reverse trend and its connection to nanoconfinement effect in small subm...

Polyoxometalate-based metal-organic frameworks for boosting electrochemical capacitor performance

Polyoxometalate-based metal-organic frameworks (POMOFs) possess promising applications as capacitors. Herein, we report the syntheses, structures and electrochemical properties of five copper-containing POMOFs: [CuI4H2(btx)5(PW12O40)2]·2H2O (1), [CuIICuI3(H2O)2(btx)5(PWVI10WV2O40)]·2H2O (2), [CuI6(btx)6(PWVI9WV3O40)]·2H2O (3), [CuI4H2(btx)5(PMo12O40)2]·2H2O (4) and [CuIICuI3(btx)5(SiMoVI11MoVO40)]·4H2O (5) (btx = 1,4-bis(triazol-1-ylmethyl) benzene) with potential applications as capacitors. Compounds 1–3 contain the same Keggin-type polyoxometalate (POM) although with different oxidation states, allowing the analysis of the effect of the electronic population on the capacitance performance of this Keggin-type POM. Compounds 1/4 and 2/5 present the same microstructure but different chemical composition, allowing the analysis of the effect of the chemical composition on the capacitance performance of these isostructure POMOFs. Compound 4 shows the highest specific capacitance (237.0 F g−1 at 2 A g−1) with capacitance retention of about 92.5% after 1000 cycles at a high charge/discharge current density of 10 A g−1. Such superior performance is comparable with state-of-the-art MOF-based and POM-based supercapacitor electrode materials. More important, this work demonstrates that the design and syntheses of POMOFs by tuning single active site (SAS) could guide the development of the new generation of electrode materials for electrochemical capacitors in the near future.

Two new supramolecular compounds with reversible skeleton structure changes during dehydration and rehydration: Synthesis and characterization

Two novel supramolecular compounds of Keggin-type polyoxometalate (POM), [Cr(apca)2(H2O)2]4[KBW12O40]·9H2O (1), [Al(apca)2(H2O)2]8[SiW12O40]2·13H2O (2); apca = 3-aminopyrazine-2-carboxylic acid), were synthesized in aqueous solution and characterized by single-crystal X-ray diffraction technique, elemental analysis, TG analyses, IR, X-ray powder diffraction and fluorescence spectroscopy. In compound 1 [BW12O40]5− anion acting as octadentate ligand coordinates to two K ions through weak interaction, forming an infinite anionic chain extending along c–axis. The chains locate in the channels formed by stacking of the complex cations [M(apca)2(H2O)2]+. Hydrogen bonds combine the chains, cations and lattice water molecules forming a 3D framework. Besides the hydrogen bonds there exists π-π interaction between the complex cations. Compound 2 has a similar structure except without the K+ ion. They show a high proton conductivity of 1.70 × 10−3 S cm−1 and 9.30 × 10−3 S cm−1 at 77° C under 97% RH, respectively. Compound 1-2 showed reversible skeleton structure changes upon dehydration and rehydration, which had been established and monitored by X-ray powder diffraction.

Transition metal-substituted Keggin-type polyoxometalates as catalysts for adipic acid production

The adipic acid (AA) production was carried out in two stages: oxidation of cyclohexanone (-one) by Keggin-type polyoxometalate (POM), followed by oxidation of this latter by hydrogen peroxide. The process lasts 20 h and the temperature is maintained at 90 °C. AA is then recovered by cold crystallization (4 °C). The POMs have as formula HMPMo12O40 (M:Co, Ni, Mn, Cu or Zn). The materials were characterized by FT-IR and UV–Vis spectroscopies and by thermogravimetric analysis. The purity of adipic acid was confirmed by FT-IR and 13C and 1H NMR analysis. The effects of POM composition, catalyst/-one molar ratio and the cyclohexanol addition to -one on adipic acid yield were examined. The whole catalysts were found to be effective toward cyclohexanone oxidation and the highest yield (53%) was obtained with HZnPMo12O40 system for a catalyst/-one molar ratio of 1.89 × 10−3. The alcohol addition to -one has a negative effect on adipic acid formation.

Self-Assembly of Short Chain Poly- N-isopropylacrylamid Induced by Superchaotropic Keggin Polyoxometalates: From Globules to Sheets.

We show here for the first time that short chain poly( N-isopropylacrylamide) (PNIPAM), one of the most famous thermoresponsive polymers, self-assembles in water to form (i) discrete nanometer-globules and (ii) micrometric sheets with nm-thickness upon addition of the well-known Keggin-type polyoxometalate (POM) H3PW12O40 (PW). The type of self-assembly is controlled by PW concentration: at low PW concentrations, PW adsorbs on PNIPAM chains to form globules consisting of homogeneously distributed PWs in PNIPAM droplets of several nm in size. Upon further addition of PW, a phase transition from globules to micrometric sheets is observed for PNIPAMs above a polymer critical chain length, between 18 and 44 repeating units. The thickness of the sheets is controlled by the PNIPAM chain length, here from 44 to 88 repeating units. The PNIPAM sheets are electrostatically stabilized PWs accumulated on each side of the sheets. The shortest PNIPAM chain with 18 repeating units produces PNIPAM/PW globules with 5-20 nm size but no sheets. The PW/PNIPAM self-assembly arises from a solvent mediated mechanism associated with the partial dehydration of PW and of the PNIPAM, which is related to the general propensity of POMs to adsorb on neutral hydrated surfaces. This effect, known as superchaotropy, is further highlighted by the significant increase in the lower critical solubilization temperature (LCST) of PNIPAM observed upon the addition of PW in the mM range. The influence of the POM nature on the self-assembly of PNIPAM was also investigated by using H4SiW12O40 (SiW) and H3PMo12O40 (PMo), i.e. changing the POM's charge density or polarizability in order to get deeper understanding on the role of electrostatics and polarizability in the PNIPAM self-assembly process. We show here that the superchaotropic behavior of POMs with PNIPAM polymers enables the formation and the shape control of supramolecular organic-inorganic hybrids.

N‐Doped Carbon Wrapped Polyoxometalate Derived from POM‐IL Hybrid: A Heterogeneous Catalyst for the Synthesis of Coumarin Derivatives under Solvent‐Free Conditions

Vanadium substituted Keggin type polyoxometalate loaded N‐doped carbon was prepared from polyoxometalate ionic liquid hybrid by carbonization process. The decomposition of organic part generates various functional groups leaving polyoxometalate part intact. The catalyst was characterized by elemental analysis, FTIR, MAS 13C NMR, XPS and electron microscopy studies. The presence of carboxylic acids, aromatic and hetero‐aromatic groups in the catalyst was confirmed by 13C NMR. The high resolution C1s and O1s XPS analysis further confirmed the presence of these groups. Various oxidation states of nitrogen were confirmed by deconvoluted N1s XPS spectra. Pyridine adsorption study confirms the presence of Bronsted and Lewis acid sites in the catalyst. Layered structure of the catalyst was confirmed by HR TEM analysis. The hybrid material displayed excellent catalytic activity towards the synthesis of coumarin derivatives under solvent‐free conditions.

Oxidative Depolymerization of Cellulolytic Enzyme Lignin over Silicotungvanadium Polyoxometalates.

The aim of this study was to explore the catalytic performance of the oxidative depolymerization of enzymatic hydrolysis lignin from cellulosic ethanol fermentation residue by different vanadium substituted Keggin-type polyoxometalates (K5[SiVW11O40], K6[SiV2W10O40], and K6H[SiV3W9O40]). Depolymerized products were analyzed by gel permeation chromatography (GPC), gas chromatography–mass spectrometer (GC/MS), and two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance (2D HSQC NMR) analysis. All catalysts showed an effective catalytic activity. The best result, concerning the lignin conversion and lignin oil production, was obtained by K6[SiV2W10O40], and the highest yield of oxidative depolymerization products of 53 wt % was achieved and the main products were monomer aromatic compounds. The HSQC demonstrated that the catalysts were very effective in breaking the β-O-4 structure, the dominant linkage in lignin, and the GPC analysis demonstrated that the molecular of lignin was declined significantly. These results demonstrate the vanadium substituted silicotungstic polyoxometalates were of highly active and stable catalysts for lignin conversion, and this strategy has the potential to be applicable for production of value-added chemicals from biorefinery lignin.

PH-modulated ion transport and amplified redox response of Keggin-type polyoxometalates through vertically-oriented mesoporous silica nanochannels

Cyclic voltammetry has been applied to characterize mass transport of a Keggin-type polyoxometalates ([SiW12O40]4-) through vertically-aligned mesoporous silica thin films in aqueous media. The results indicate a drastic influence of pH and the ionic strength which may be rationalized in terms of electrostatic interactions and confinement effects. Diffusion of such bulky anionic electroactive species is considerably restricted at pH values higher than the point of zero charge of the silica for which the walls of the nanochannels are negatively charged. On the other hand, the positive charge density generated in more acidic solutions favors the ingress of [SiW12O40]4- species through the mesochannels, enhancing considerably the electrochemical response, except at high ionic strength due to competition for the binding sites. The accumulation of POM anions into the silica films can be even promoted by functionalization of the silica surface with propylammonium groups. In summary, pH tuning induced gating of the ionic flux through the silica nanopores, which can be somewhat modulated by the ionic strength by varying the thickness of the electrical double layer generated at the silica walls surface.

Non-corrosive heteropolyacid-based recyclable ionic liquid catalyzed direct dehydrative coupling of alcohols with alcohols or alkenes

A non-corrosive, recyclable and efficient heterogeneous catalyst based on Keggin-type polyoxometalate, i.e., [NMPH]H3[SiW12O40] was developed for the direct dehydrative coupling of alcohols with alcohols (or alkenes) to synthesize various polysubstituted olefins in good to excellent yields. Furthermore, this reaction could be scaled up and the catalyst could be used for seven runs without significant loss of activity. The kinetic competition experiment shows that the C–H bond cleavage might be involved in the rate-determining step.

Immobilization of polyoxometalate-based sulfonated ionic liquids on UiO-66-2COOH metal-organic frameworks for biodiesel production via one-pot transesterification-esterification of acidic vegetable oils

In accordance with the need of green and cleaner production, the aim of the present study is to fabricate an organic-inorganic hybrid recyclable catalyst with both Brönsted and Lewis acid sites for the one-pot production of biodiesel from the low-cost acidic oil feedstocks. To achieve this, the UiO-66-2COOH metal-organic framework materials (MOFs) were firstly functionalized with Keggin-type polyoxometalate (POM) acids through the in-situ preparation strategy, and then the sulfonated acidic ionic liquids (AILs) were further incorporated into the prepared POM/UiO-66-2COOH composites by pairing polyoxometalate anions with functionalized IL cations. The as-prepared AILs/POM/UiO-66-2COOH solid catalyst features a high surface area and an increased acidity, thus combining the merits of Brönsted and Lewis acids together with the heterogeneous microenvironment of MOFs. The solid acid catalysts were fully characterized by several techniques, and the results demonstrated that the polyoxometalate-based sulfonated AILs were virtually immobilized on the UiO-66-2COOH MOFs, and furthermore the porous crystalline structure of the UiO-66-2COOH material was almost unaltered after the immobilization processes. The solid catalyst had excellent catalytic activities toward the simultaneous transesterification and esterification of simulated low-cost oils with long-term stability and good reusability, and moreover still remained considerable activities even with the existence of 9 wt% free fatty acid and 3 wt% water in the low-cost oils. Therefore, this solid catalyst shows potential applications for the one-step biodiesel production especially from the low-cost feedstocks which usually contain high amounts of free fatty acid and water.

Controlled synthesis of silver nanoparticles from polyoxometalates-immobilized poly(4-vinylpyridine) brushes

ABSTRACT This contribution reports the immobilization of polyoxometalate (POM) into poly(4-vinyl pyridine) (P4VP) brushes and the controlled reduction of silver ions, in-situ generating metal nanoparticles in the brushes. P4VP brushes were straightforwardly created by UV-assisted photopolymerization of 4-vinyl pyridine (4VP) on silicon or glass substrates. Phosphotungstic acid (H3PW12O40), one of the most widely used Keggin-type POM was anchored onto these pyridine moieties through electrostatic interaction, leading to the P4VP/POM hybrid brushes. The immobilized POM was further reduced to heteropolyblue, which could be used to generate silver nanoparticles in a controlled fashion. AFM, UV-vis and IR characterization indicate that P4VP brushes not only provide an efficient platform in the controlled preparation of Ag nanoparticles, but also efficiently disperse and stabilize POM, thus preventing the aggregation of the generated Ag nanoparticles.

A new supramolecular compound based on Keggin-type phosphomolybates with mixed-ligand and mixed-valence Cu(I/II): [Cu(I)(phen)2]2{[Cu(II)]2(phen)4PTA}[H3PMo8VIMo4VO40

With an aim to obtain a new supramolecular compound based on Keggin-type polyoxometalates with mixed-ligand: [Cu(I)(phen)2]2{[Cu(II)]2(phen)4PTA}[H3PMoV4MoVI8O40] (phen = 1,10-phenanthroline, PTA = terephthalic acid) has been synthesized under hydrothermal conditions and structurally characterized by elemental analyses, IR, X-ray single crystal diffraction, TG analyses and XPS. In the title compound, the Keggin-type phosphomolybates anions, butterfly-shaped [Cu(I)(phen)2]+ subunits and dumbbell-shaped {[Cu(II)]2(phen)4PTA}2+ subunits are alternately connected to form chiral helical chains via hydrogen bonds along b axis, and the adjacent chiral helical chains are connected in the way of sharing butterfly-shaped [Cu(I)(phen)2]+ subunits and dumbbell-shaped {[Cu(II)]2(phen)4PTA}2+ subunits to form 3D supramolecular reticular framework. As expected, the title compound with mixed-ligands (phen and PTA) has been successfully synthesized, inconceivably, the title compound also contains mixed-valence Cu (Cu(I) in butterfly-shaped [Cu(I)(phen)2]+ subunits; Cu(II) in dumbbell-shaped {[Cu(II)]2(phen)4PTA}2+ subunits). The preliminary analyses show that mixed-valence Cu(I/II) may be due to the reduction of Cu(II) (from CuCl2) to Cu(I) (in butterfly-shaped [Cu(I)(phen)2]+ subunits) by N-containing compound. The successful results of this work provide a new idea that supramolecular materials with attractive structures and novel properties, which could be designed and synthesized by introducing of mixed-ligands into the supramolecular materials. Additionally, the electrocatalytic property of the title compound to H2O2 has been studied.

Electrocatalytic oxidation of water at a polyoxometalate nanoparticle modified gold electrode.

Spherical polyoxometalate nanoparticles, [HPMo]NPs, were synthesized from a very well known Keggin-type polyoxometalate [H3PMo12O40] in the presence of sodium dodecyl sulphate (SDS) and polyvinyl pyrrolidine (PVP) in aqueous medium and characterized by UV-Vis spectroscopy and Transmission Electron Microscopy (TEM). The [HPMo]NPs were used to modify a gold working electrode and they were characterized by SEM, EDX, elemental mapping, cyclic voltammetry and electrochemical impedance spectroscopy and applied for the electrocatalytic oxidation of water in a phosphate buffer solution at neutral pH. The modified electrode showed excellent electrocatalytic activity towards oxidation of water at an impressively low overpotential ∼350 mV with a high current density of around 1.7 mA cm−2, good stability under exhaustive electrolysis conditions and also showed long term stability.

Keggin-type polyoxometalate cluster as an active component for redox-based nonvolatile memory

Keggin-type polyoxometalate (POM) cluster based non-volatile memory has been investigated, and the molecular reconfiguration induced by the reduction process of POM molecules is proposed to initialize the resistive switching behavior.

Optical and Electrical Properties of Monolacunary Keggin-Type Polyoxometalate/Star-Shaped Polycarbazole Nanocomposite Film

Polyoxometalates (POM), also known as polyanionic metal oxide clusters, have recently become one of the focus researches in science and technology. In this study, a nanocomposite film electrochemically formed by combining the Keggin type polyoxometalate (K7- xNaxPW11O39·14H2O) (POM) and star shaped carbazole derivative (N2, N4, N6-tris (9-ethyl-9H-carbazol-3-yl)-1,3,5-triazine-2,4,6-triamine denoted as T3AC. A synergistic interaction between carbazole and POM derivative has been noticed for the first time in the literature. Optical properties of the POM/T3AC solution have been changed due to this interaction which not observed in other electroactive monomers such as pyrrole, thiophene, indole etc. More importantly electrical and optical properties of the nanocomposite structure synthesized electrochemically from this solution have been improved.