Keggin Polyoxometalate Research Articles

Hybrid dimers based on metal-substituted Keggin polyoxometalates (metal = Ti, Ln) for cyanosilylation catalysis.

Six novel organic-inorganic hybrid polyoxometalate derivatives, K[(H2O)4(3-Hpic)2Ce][(H2O)5(3-Hpic)2Ce][PW10Ti2O40]·11H2O 1, K[(H2O)4(3-Hpic)2Nd][(H2O)5(3-Hpic)2Nd][PW10Ti2O40]·12H2O 2, K[(H2O)4(3-Hpic)2Sm][(H2O)5(3-Hpic)2Sm][PW10Ti2O40]·8H2O 3, H[(H2O)7(4-Hpic)La][(4-Hpic)2LaPW11O39]·14.5H2O 4, H[(H2O)7(4-Hpic)Ce][(4-Hpic)2CePW11O39]·7.5H2O 5, and K(4-Hpic)0.5[(H2O)6(4-Hpic)Nd][(4-Hpic)2NdPW11O39]·10H2O 6, (3-Hpic = 3-picolinic acid; 4-Hpic = 4-picolinic acid) have been synthesized and characterized by elemental analysis, IR, solid state UV-vis spectroscopy, thermal gravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. Isostructural compounds 1-3 are hybrid dimers that consist of two Ti-substituted Keggin-type polyoxoanions [PW10Ti2O40]7- and four Ln-3-Hpic coordination groups. The strong hydrogen bonds in 1-3 can make these dimers yield a 1D supramolecular chain. Compounds 4-6 are also dimers constructed from two lanthanide-substituted Keggin-type polyoxoanions [PW11LnO39]4- and four 4-Hpic ligands, which are further linked by two Ln-4-Hpic groups to form a bi-supporting structure. Then these subunits are joined together by strong hydrogen-bonding interactions between polyoxoanions and coordinated water molecules to produce a 2D supramolecular framework. All these compounds as heterogeneous Lewis acid-base catalysts show high activity and high selectivity for the cyanosilylation of different carbonyl compounds under solvent-free conditions, and can be recycled without any obvious inactivation.

A PTA@MIL-101(Cr)-diatomite composite as catalyst for efficient oxidative desulfurization

Developing new materials that can efficiently reduce the level of sulfur contents in fossil oils is a pressing issue due to environmental problems. Polyoxometalates (POMs) have been used as catalysts to remove organosulfur compounds via the process of oxidative desulfurization for a long time. Here, a new catalytic material, POMs@MIL-101(Cr)-diatomite, has been obtained by restricting MIL-101(Cr) encapsulating Keggin POMs into mesoporous diatomite, which can overcome the obstacle about the poor solubility of POMs in the overall nonpolar environment. The catalyst was characterized by N2 adsorption-desorption, SEM, XRD, IR and XPS. The desulfurization rate could be up to 98.6%, using H2O2 as the oxidant. Furthermore, the catalyst was reused three times with no structure change under crystal analysis.

Photoelectric active hybrid film based on RuII terpyridyl complex and EuIII substituted Keggin polyoxometalate of [Eu(BW11O39)2]15−

The hybrid film consisting of alternating layers of EuIII substituted Keggin polyoxometalate anion [Eu(BW11O39)2]15− (EuBW) and RuII complex cation of [Ru(L)2]2+ {L=4′-(4-(imidazol-1-yl)phenyl)-2,2′:6′,2″-terpyridine}, abbreviated as RuL1, was prepared on quartz and indium-tin oxide (ITO)-coated glass substrates by electrostatically self-assembled method, and studied by UV–vis absorption spectroscopy, cyclic voltammetry, permeability of the film to the redox probe [Fe(CN)6]3−, electrochemical impedance spectroscopy and photoelectrochemical measurements. The uniform depositions of the two components into the films were evidenced by uniform growths of the UV–vis absorption peaks. The ITO electrodes modified with the hybrid film showed stable electrochemical property in relation to EuBW anion and RuL1. The significant cathodic photocurrent generation was observed upon visible light irradiation with a maximum photocurrent density of 8.47μA/cm2 being achieved for a 5-layer film of (EuBW/RuL1)5 at an applied potential of −0.3V versus saturated calomel electrode. The effects of substitution atoms in MBW (M=EuIII vs. ZnII) and experimental conditions including applied bias potentials, layer numbers of the films and irradiated light intensities on photocurrent generation were discussed.

Two hybrids based on Keggin polyoxometalates and dinuclear copper(II) complexes: syntheses, structures and electrocatalytic properties

By introducing mixed-ligands en and ox, $$\hbox {Cu}^{2+}$$ and different polyoxotungstates as synthons, two new polyoxotungstate-based inorganic-organic hybrid compounds $$\{[\hbox {Cu}_{2}\hbox {(en)}_{2}\hbox {(ox)}]\hbox {[HPW}_{12}\hbox {O}_{40}]\}\cdot \hbox {(en)}_{2}\cdot \hbox {2H}_{2}\hbox {O}$$ (1) and $$\{[\hbox {Cu}_{2}\hbox {(en)}_{2}\hbox {(ox)}]$$ [ $$\hbox {H}_{3}\hbox {BW}_{12}\hbox {O}_{40}]\}\cdot \hbox {(en)}_{2}\cdot \hbox {2H}_{2}\hbox {O}$$ (2) (en = ethylenediamine and ox = oxalate), were obtained in identical hydrothermal conditions and further characterized by elemental analyses, IR spectroscopy and single-crystal X-ray diffraction. Structural analyses revealed that both compounds are isostructural, and show one-dimensional (1D) chain constructed by $$\hbox {[XW}_{12}\hbox {O}_{40}]^{\mathrm{n}-}$$ (X = P 1, B 2) Keggin-type polyoxoanions and $$[\hbox {Cu}_{2}\hbox {(en)}_{2}\hbox {(ox)}]^{2+}$$ dinuclear copper subunits. The electrochemical experiments indicated that 1-based carbon paste electrode possesses high catalytic efficiency and selectivity towards reduction of $$\hbox {H}_{2}\hbox {O}_{2}$$ , and thus 1 has potential to detect $$\hbox {H}_{2}\hbox {O}_{2}$$ . SYNOPSIS Two new hybrids based on polyoxoanions and dinuclear copper complexes have been synthesized and characterized. The results of electrocatalytic experiments indicate that the hybrid-based electrode possesses high catalytic efficiency and selectivity towards reduction of $$\hbox {H}_{2}\hbox {O}_{2}$$ .

A Ag-La heterometallic 2D layer based on mono-lacunary Keggin polyoxometalate: Synthesis, structure, and photocatalytic property

A new two-dimensional (2D) purely inorganic 4d-4f heterometallic compound [La(H2O)8][Ag{La(H2O)6}2{SiW11La(H2O)4O39}2]·13H2O (1), based on mono-lacunary Keggin-type {α-SiW11O39} cluster (abbreviated to SiW11), was conventional synthesized and characterized by elemental analysis, IR, TG, and single crystal X-ray diffraction. In compound 1, two mono-substituted {SiW11La(H2O)4O39}2 clusters (abbreviated to SiW11La) polymerize together to form a dimer. And the dimers are linked by Ag+ and La3+ heterometallic cations displaying a 2D network. Furthermore, the photocatalytic experiment indicates that the title compound presents excellent photocatalytic activity towards degradation of rhodamine B (RhB) dye.

Assembly of the first bivanadium-capping saturated Keggin type arsenovanadotungstate compound

A new hybrid compound based on bivanadium-capping saturated Keggin polyoxometalates, [Cu24(trz)16Cl4][AsW12O40(VO)2]·2H2O (CuAsW12V2) (trz=1,2,3-triazole), has been hydrothermally synthesized and structurally characterized by routine techniques. The new compound represents the first example of the bivanadium-capping saturated Keggin POMs in the W/O/V system. In addition, to improve the photocatalytic activity of the CuAsW12V2 compound, polypyrrole (PPy) was loaded on its surface through a facile chemical oxidation polymerization process forming the PPy/CuAsW12V2 composite. Under the visible light, the PPy/CuAsW12V2 composite as catalysts exhibit better photocatalytic activity and stabilization for the degradation of Rhodamine-B (RhB) than that of CuAsW12V2 matrix.

Development and Optimization of Integrated Photoelectrochemical Energy Storage Cells Using Ion Soft-Landing

The development of efficient energy conversion and storage technologies is important to meet increasing demand. Solar energy is a valuable renewable and sustainable resource utilized in numerous energy technologies. However, the inherent variability of solar energy necessitates the addition of efficient energy storage components (e.g. batteries, flow cells, etc.) to achieve a continuous supply of electricity. Development of integrated systems that combine solar energy conversion and storage in a single electrochemical device is a promising approach that may potentially improve the efficiency of solar energy utilization. A photoelectrochemical system capable of converting and storing solar energy relies on photo-induced reactions of redox-active species.(1, 2) Polyoxometalates (POMs) are a class of stable anionic metal oxides clusters with multi-electron redox activity.(3) POMs undergo multiple redox transitions by utilizing either electrons or photons and exhibit a color change during certain transitions (photo- and electro chromic).(4) For example, the photo-induced reduction of POM as a photo catalyst has been used in water oxidation and hybrid fuel cell applications.(5, 6) In this work, we developed an integrated photo electrochemical energy storage (IPES) cell with the ability to convert and store solar energy in a single device using POM’s photochromic and redox properties, and explored factors that affect the performance of the IPES cell. Electrode surfaces prepared using conventional solution based methods often contain both electrochemically active ions and inactive counter ions that can hinder electron transfer processes. Previously, we demonstrated that a complete elimination of strongly coordinating counter cations substantially improved the performance of redox-supercapacitors prepared by soft landing of mass-selected POM anions.(7) Ion soft landing enables deposition of uniform layers of electroactive ions with well-defined charge state, composition and kinetic energy onto electrode surfaces, something not possible using other preparation techniques.(8, 9) Herein, we prepared IPES cells with POM anions deposited onto the anode using either ion soft landing or solution based deposition for comparison. The IPES cells consist of (i) Keggin POMs deposited on an indium-tin oxide (ITO) photoanode using either spin casting and or ion soft landing (ii) an ionic liquid membrane that contains ferrocene methanol as a model redox molecule (iii) a glassy carbon cathode. Our preliminary results with the IPES cells irradiated by direct solar light confirmed that the IPES cell generates useful current and potential due to the photo-induced redox conversion of POM at the anode and the counter redox reaction at the cathode. Additionally, in-situ UV-Vis spectroscopy confirmed the photo chromic transition of POM in the IPES cell from a clear to dark blue color upon irradiation. The design of the IPES cell will be presented and its performance under different experimental conditions will be discussed.

Polyoxometalate-Incorporated Metallapillararene/Metallacalixarene Metal-Organic Frameworks as Anode Materials for Lithium Ion Batteries.

A series of remarkable crystalline compounds containing metallapillararene/metallacalixarene metal-organic frameworks (MOFs), [Ag5(pyttz)3·Cl·(H2O)][H3SiMo12O40]·3H2O (1), [Ag5(trz)6][H5SiMo12O40] (2), [Ag5(trz)6][H5GeMo12O40] (3), and [Ag5(trz)6][H4PW12O40] (4) (pyttz = 3-(pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, trz = 1,2,4-triazole), have been obtained by using a simple one-step hydrothermal reaction of silver nitrate, pyttz for 1 and trz for 2-4, and Keggin type polyoxometalates (POMs). Crystal analysis reveals that Keggin POMs have been successfully incorporated in the windows of the metallapillararene/metallacalixarene MOFs in compounds 1-4. In addition, the Keggin silicomolybdenate-based hybrid compounds 1 and 2 were used as anode materials in lithium ion batteries (LIBs), which exhibited promising electrochemical performance with the first discharge capacities of 1344 mAh g-1 for 1 and 1452 mAh g-1 for 2, and this stabilized at 520 mAh g-1 for 1 and 570 mAh g-1 for 2 after 100 cycles at a current density of 100 mA g-1. The performances are better than that of (NBu4)4[SiMo12O40] matrix and commercial graphite anodes.

The First Organic–Inorganic Hybrid Compound Based on Polyoxotungstates and Alkali-tris(imidazolyl) Segments with Electrocatalytic Activity

The first organic–inorganic hybrid compound based on the Keggin polyoxometalate and alkali-N-heterocycle ligand [Na4(tib)2(H2O)2(α-HBW12O40)]·2H2O (1) (tib = 1,3,5-tris(1-imidazoly)benzene) was hydrothermally synthesized by utilizing a pH-dependent approach in the POM/Cu/tib reaction systems. X-ray structural analyses reveal that compound 1, formed in pH 5.2, possesses a (3,4,6)-connected 2D net with the (42·5)(46)(33·46·52·64) topology. In addition, electrochemical and electrocatalytic properties of compound 1 were studied by cyclic voltammograms. Compound 1 displayed electrocatalytic activities toward reduction of nitrite.

A new inorganic 2D framework based on Bi-bismuth-capping Keggin polyoxometalate with photodegradation and selective absorption of organic dyes

A new purely inorganic 2D framework, {PMo12O40Bi2(H2O)2}·4H2O(1), was synthesized under solvothermal condition and characterized by infrared(IR) spectroscopy, thermal gravimetric analysis, UV-Vis spectroscopy, powder X-ray diffraction and single-crystal X-ray diffraction. Structural analysis reveals that compound 1 consists of Keggin unit {PMo12O40} with two bismuth atoms capping two opposite pits, leading to the bi-bismuth-capping Keggin polyoxometalate. Compound 1 not only displays excellent photodegradation activity for methyl orange(MO), but also has a high absorption capacity for rhodamine B(RhB) and methylene blue(MB). Moreover, compound 1 is extremely stable and easily separated from the reaction system for reuse.

Synthesis and structure of an unprecedented meso-helixes dominated by bivanadyl-capped Keggin POMs

A new bivanadyl-capped Keggin polyoxometalates based hybrid containing the left- and right-handed helical chains, [Cu12(trz)8][PMo7VIMo5VO40(VO)2]·H2O (1) (trz=1,2,3-triazole), has been hydrothermally synthesized and structurally characterized by routine techniques. It is interesting that the left- and right-handed metal-organic helical chain are constructed from the rigid [Cu5(trz)4] subunits with the same structure but different coordination directions, which is induced and stabilized by [PMo12O40(VO)2]4− polyoxoanions. Note that the [PMo12O40(VO)2]4− polyoxoanions link with fourteen Cu ions in the new compound, represents the highest linking number of Keggin POM clusters to date, and dominates to the formation of helical structures. In addition, the electrochemical properties of the new compound have been investigated.

Photoresponsive Nanosheets of Polyoxometalates Formed by Controlled Self‐Assembly Pathways

AbstractAnionic Keggin polyoxometalates (POMs) and ether linkage‐enriched ammonium ions spontaneously self‐assemble into rectangular ultrathin nanosheets in aqueous media. The structural flexibility of the cation is essential to form oriented nanosheets; as demonstrated by single‐crystal X‐ray diffraction measurements. The difference in initial conditions exerts significant influence on selecting for self‐assembly pathways in the energy landscape. Photoillumination of the POM sheets in pure water causes dissolution of reduced POMs, which allowed site‐specific etching of nanosheets using laser scanning microscopy. By contrast, photoetching was suppressed in aqueous AgNO3 and site‐selective deposition of silver nanoparticles occurred as a consequence of electron transfer from the photoreduced POMs to Ag+ ions on the nanosheet surface.

Photoresponsive Nanosheets of Polyoxometalates Formed by Controlled Self‐Assembly Pathways

Anionic Keggin polyoxometalates (POMs) and ether linkage-enriched ammonium ions spontaneously self-assemble into rectangular ultrathin nanosheets in aqueous media. The structural flexibility of the cation is essential to form oriented nanosheets; as demonstrated by single-crystal X-ray diffraction measurements. The difference in initial conditions exerts significant influence on selecting for self-assembly pathways in the energy landscape. Photoillumination of the POM sheets in pure water causes dissolution of reduced POMs, which allowed site-specific etching of nanosheets using laser scanning microscopy. By contrast, photoetching was suppressed in aqueous AgNO3 and site-selective deposition of silver nanoparticles occurred as a consequence of electron transfer from the photoreduced POMs to Ag+ ions on the nanosheet surface.

Selective Liquid‐Phase Sorption of Small Aliphatic Normal Alcohols on a Flexible Assembly Composed of Keggin Polyoxometalate

Single crystals of Na3[Fe3O(O2CCH2OCH3)6(H2O)3][α‐SiW12O40]·13.5H2O (1a) and their nanoparticles (2a) have been synthesized by self‐assembly of the constituent ions and a one‐pot sonochemical reaction, respectively. Their corresponding activated phases, 1b and 2b, were obtained by desorption of crystallization water through a thermal process. The sorption capacity of 2b for methanol vapor, owing to the lack of well‐defined pores, was significantly less than 1b, which was confirmed by thermogravimetric analysis. Liquid‐phase sorption of small aliphatic alcohols (C1–C4) was applied to characterize the sorption behavior of 1b. The lattice flexibility was revealed by unit‐cell volume calculation for 1b after sorption of each alcohol by using powder X‐ray diffraction data. Also, investigation of the sorption isotherms, fitting experimental kinetic data to the Lagergren and Ho models, and calculation of the activation energies for sorption of each alcohol were carried out. Finally, the sorption selectivity of 1b towards the alcohols was determined.

Self-organization of unprecedented POMCPs with four-fold helixes based on the highest connected Keggin POMs

A new Keggin polyoxometalate based inorganic-organic hybrid compound, [Ag5(trz)4·(H2O)2][H2PW12O40] (1), was successfully synthesized and characterized. Single crystal X-ray analysis reveals that the title compound possesses fourfold meso-helical structure fabricated by POM-Ag inorganic chains, then the 1D metal-organic chains insert into the 3D POM-Ag framework using Ag as their common junction obtaining the whole structure with {4}8{48·896·1216} topolopy. The title compound represents the most complex helical conformation (fourfold meso-helix) among the Keggin based hybrids' materials to date, and [PW12O40]3− polyanions in the title compound also exhibit the highest coordination number so far, which dominates the formation of fourfold meso-helix.

Third-order NLO properties of ultrathin films containing cationic phthalocyanine and Keggin polyoxometalates fabricated using layer-by-layer deposition from aqueous solution

Third-order optical nonlinearities of ultrathin films derived from cationic phthalocyanine and Keggin polyoxometalates can be well tuned and enhanced.

Polyoxometalates as artificial nucleases: hydrolytic cleavage of DNA promoted by a highly negatively charged ZrIV-substituted Keggin polyanion.

A highly negatively charged binuclear ZrIV-substituted Keggin polyoxometalate [{α-PW11O39Zr(μ-OH)(H2O)}2]8- (ZrK 2 : 2) has been shown to promote the hydrolytic cleavage of phosphoester bonds in the supercoiled plasmid pUC19 DNA under physiological pH and temperature, giving relaxed and linear forms of pUC19 as hydrolysis products. The interaction between ZrK 2 : 2 and DNA was experimentally proven by circular dichroism (CD) spectroscopy and 31P diffusion ordered NMR spectroscopy.

Synthesis and photo-/electro-catalytic properties of Keggin polyoxometalate inorganic–organic hybrid layers based on d10 metal and rigid benzo-diazole/-triazole ligands

Unusual hepta- and nona-nuclear copper/silver complexes were introduced into different Keggin cluster leading to 2-D hybrid layers, which exhibit merit photo- and electro-catalytic properties.

In situ solid-state electrochemistry of mass-selected ions at well-defined electrode–electrolyte interfaces

Molecular-level understanding of electrochemical processes occurring at electrode-electrolyte interfaces (EEIs) is key to the rational development of high-performance and sustainable electrochemical technologies. This article reports the development and application of solid-state in situ thin-film electrochemical cells to explore redox and catalytic processes occurring at well-defined EEIs generated using soft-landing (SL) of mass- and charge-selected cluster ions. In situ cells with excellent mass-transfer properties are fabricated using carefully designed nanoporous ionic liquid membranes. SL enables deposition of pure active species that are not obtainable with other techniques onto electrode surfaces with precise control over charge state, composition, and kinetic energy. SL is, therefore, demonstrated to be a unique tool for studying fundamental processes occurring at EEIs. Using an aprotic cell, the effect of charge state ([Formula: see text]) and the contribution of building blocks of Keggin polyoxometalate (POM) clusters to redox processes are characterized by populating EEIs with POM anions generated by electrospray ionization and gas-phase dissociation. Additionally, a proton-conducting cell has been developed to characterize the oxygen reduction activity of bare Pt clusters (Pt30 ∼1 nm diameter), thus demonstrating the capability of the cell for probing catalytic reactions in controlled gaseous environments. By combining the developed in situ electrochemical cell with ion SL we established a versatile method to characterize the EEI in solid-state redox systems and reactive electrochemistry at precisely defined conditions. This capability will advance the molecular-level understanding of processes occurring at EEIs that are critical to many energy-related technologies.

Tuning the Selectivity and Reactivity of Metal‐Substituted Polyoxometalates as Artificial Proteases by Varying the Nature of the Embedded Lewis Acid Metal Ion

The hydrolysis of horse heart cytochrome c (cytC) protein by two isostructural Keggin‐type polyoxometalates (POMs), (Me2NH2)10[Ce(α‐PW11O39)2] (Ce1–K2) and (Et2NH2)10[Zr(PW11O39)2] (Zr1–K2), which differ in the nature of the embedded Lewis acid metal ion, has been investigated. In the presence of Ce1–K2, selective hydrolysis of cytC was observed at the Trp60‐Lys61 and Gly78‐Thr79 peptide bonds at pH 7.4 and 37 °C. However, the isostructural Zr1–K2 exhibited a lower reactivity and different selectivity, cleaving cytC at the Asp3‐Val4, Asp51‐Ala52 and Gly78‐Thr79 peptide bonds. Different spectroscopic techniques were used to verify the molecular interactions between cytC and each metal‐substituted Keggin POM to elucidate the role of the Lewis acid metal ion in directing the selectivity of protein hydrolysis.