Structure Of Polyoxometalates Research Articles

Capped Polyoxometalate-Based Metal-Organic Complex with Mixed-Valence CuI/CuII for Synergistic Catalytic Synthesis of p-Benzoquinone.

The design and synthesis of efficient and environmentally friendly heterogeneous catalysts for the oxidation of phenols to benzoquinones are of great significance. Considering the highly catalytic activity of mixed-valence metals and polyoxometalates (POMs), a three-dimensional polyoxometalate-based metal-organic complex with mixed-valence CuI/CuII, [Cu2ICu2II(pyca)4(OH)3(PMo12O40Cu2II)(H2O)4]·3.5H2O (1, Hpyca = 2-pyrazinecarboxylic acid) has been successfully synthesized under solvothermal conditions. Surprisingly, complex 1 not only combines the desired mixed-valent CuI/CuII and POM sites but also contains a rare double Cu-capped POM structure. Due to the ternary synergistic effect of CuI, CuII, and POM, complex 1 exhibits excellent catalytic activity in the oxidation reaction of 2,3,6-trimethylphenol to the corresponding p-benzoquinone. The conversion and selectivity can reach 98 and 99% within 5 min, respectively, and the turnover frequency (TOF) value is as high as 8167 h-1, which is superior to most catalysts.

Fabrication of a Multi-Functional Separator Incorporating Crown ether- Polyoxometalate Supramolecular Compound for Lithium-Sulfur Batteries.

Recently, research on polyoxometalates (POMs) has gained significant momentum. Owing to their properties as electronic sponges, POMs catalyst harbor substantial potential in lithium-sulfur battery research. However, POMs undergo a transformation into reduced heteropoly blue (HPB) during electrochemical reactions, which then dissolve into the electrolyte, resulting in catalyst loss. In this research, we amalgamated 18-crown-6 (CR6) with K3PW12O40, (KPW), synthesized a novel POM-based supramolecular compound, and integrated it with graphene oxide (GO) to fabricate a multi-functional composite polypropylene (PP) separator KPW-CR6/GO/PP. The crown ether array was employed to immobilize POM and construct ion transport channels, thereby enhancing the Li+ migration rate and capturing polysulfides. Subsequently, leveraging the stable structure and redox properties of POM, the polysulfide is catalyzed to transform and inhibit the shuttle effect, thereby protecting the Li anode. The lithium-sulfur batteries with the Crown ether-POM supramolecular compound separators, exhibit enhanced capacity and stability (1073.3 mAh g-1 at 1.0 C, and 81.5 % retention rate after 250 cycles). The battery (S loading: 3.2 mg cm-2) presents an initial specific discharge capacity of 543.4 mAh g-1 at 0.5 C, with 89.8 % of the capacity retained after 160 cycles. This underlines the practical application potential of Crown ether-POM supramolecular materials in Li-S batteries.

Biomimetic supported polyoxometalate structures designed and its applications for the ultra-deep oxidative desulfurization of fuels

Inspired by the fact that human nose cilia can effectively contact with particles to prevent their entry into our body, this kind novel biomimetic structures are designed and synthesized to enable the loading efficiency between the polyoxometalate catalytic species (POMs) and the carrier, thereby improving their loading rate and promoting their catalytic performance. The mullite whiskers-uniformed cordierite (CMW) is used as a carrier to support POMs, in which mullite whiskers regarding as the cilia in our nose, this kind catalyst could not only overcome the shortcomings of simple POMs with low specific surface and easy to self-agglomerate, but also could greatly increase their contact probability with the carrier. At the same time, the advantage of the cordierite block structure also greatly conducive to their actual application value on the reusing operation. A series of Keggin-type POM-loaded bulk materials, POM@CMW, [VimAm]Br@POM@CMW, [DVim]Br@POM@CMW, have been developed with various ionic liquid-based POMs (ionic liquid = ILs, chosen these two kinds of [VimAm]Br and [DVim]Br) to a comparative study. The oxidative desulfurization reaction (ODS) for fuels was performed using the obtained materials as catalysts and H2O2 as oxidant, without the additional extractants for the existing of ILs in catalyst. Through experimental evaluations, [DVim]Br@POM@CMW was found the excellent efficiency on the Sulfur removal, with an DBT removal of 96.34 % at 70 °C for 120 min. According to the FT-IR and SEM results after the reaction, the catalyst could retain their original structure and the dibenzothiophene (DBT) removal could still be maintained above 95 % after 7 reuses. Furthermore, the oxidation reactivity of different substrates was in the following order: DBT > 4,6-dimethyldibenzothiophene (4,6-DMDBT) > thiophene (BT) and the high activity in actual diesel oil could be also exhibited. The enhanced reaction activity, excellent structural stability, and recyclability can be attributed to the abundance of active sites and outstanding mechanical properties provided by cordierite/mullite whiskers as a carrier, indicating their potential in the novel biomimetic-structured POMs-loaded catalysts.

Molecular Recognition on the Multisite Binding Surface of the Keplerate {Mo72Fe30} Giving Supramolecular Texturing and Modulating the Function of Guest Molecules.

Molecular recognition underlies structure formation in supramolecular architectures either in materials or in living systems. Here, we used the nanoscale nontoxic Keplerate-type polyoxometalate (POM) {Mo72Fe30} as a template for the recognition of two different guest molecules [tetracycline (TC) and doxorubicin (DOX)] on the textured surface. By means of single crystal X-ray analysis and X-ray photoelectron spectroscopy (XPS), we revised the key features of the {Mo72Fe30} structure, showcasing the guest dimolybdenum units' {Mo2} location under the hexagonal pores and dynamic exchange of these units during dissolution in an aqueous medium. Based on the clarified POM structure, we demonstrated how the small differences between the TC and the DOX molecules can be recognized by the Keplerate surface, revealing the nature of the binding sites─{Mo6}/{FeO6} for TC and {FeO6} for DOX. Furthermore, using the Monte-Carlo method, we calculated the statistical distribution of the guest molecules in the stoichiometric compounds {Mo72Fe30}@TC12 and {Mo72Fe30}@DOX12, displaying the supramolecular ordering of the DOX species and randomization of the TC as a result of different coordinations to the POM surface. The produced {Mo72Fe30}@TC12 and {Mo72Fe30}@DOX12 associates were evaluated for bioactivity, showing how their interaction with POM can modulate the biological function of guest molecules.

Configurational Isomerism in Bimetallic Decametalates.

In this work, we report on the development of a computational algorithm that explores the configurational isomer space of bimetallic decametalates with general formula MxM10-x'O28q. For x being a natural number in the range of 0 to 10, the algorithm identifies 318 unique configurational isomers. The algorithm is used to generate mixed molybdenum(VI)-vanadium(V) systems MoxV10-xO288- for x=0,1,2, and 3 that are of experimental relevance. The application of the density functional theory (DFT) effectively predicts stability trends that correspond well with empirical observations. In dimolybdenum-substituted decavanadate systems, we discover that a two-electron reduction preferentially stabilizes a configurational isomer due to the formation of metal-metal bonding. The particular polyoxometalate structure is of interest for further experimental studies.

Heteroelements in polyoxometalates: a study on the influence of different group 15 elements on polyoxometalate formation.

In the field of polyoxometalate (POM) chemistry, different heteroelements are integrated into the cage-like structures, to obtain different structural types of so-called heteropolyanions (HPAs). While it is generally accepted, that some elements favor certain types of structure, a systematic study is still missing. In this article, we present a systematic investigation of the influence of the group 15 elements nitrogen, phosphorous, arsenic, and antimony on the formation of different POM structure types. Our study is comprised of DFT calculations and corresponding experimental structural analysis. In this context, the DFT study establishes the thermodynamics of formation of different coordination geometries with various heteroelements on two POM structure types, the Keggin and the Anderson-Evans structures. Our POM synthesis experiments were performed at two different pH values (1 and 5) and resulted in a variety of heteropolytungstates, which were identified and characterized by elemental analysis as well as single crystal X-ray diffraction and vibrational spectroscopy. With these methods, we were able to establish a clear trend, showing that heavier elements lead to formation of different structure types than lighter elements. These results signify a large step towards a better understanding of POM formation specifically with respect to the choice of heteroelement.

Targeted Grafting Metal-Organic Frameworks on Polyoxometalates for Efficient Water Oxidation

In recent years, due to the diverse and tunable microenvironment of metal centers, Metal-organic framework materials (MOFs) have shown great advantages in heterogeneous catalysis.[1,2] Studies have found that in some oxidation scenarios, taking water oxidation as an example, the instability of is framework limits its practical application under severe conditions. Therefore, many efforts have focused on improving the catalytic stability of MOFs materials. [3-5]The MOFs-based electro-catalytic systems developed for oxygen evolution under alkaline conditions in our work are discussed in the following three sections. First, based on the loss of active sites faced by MOFs during OER and the high oxidation potential due to insufficient electrical conductivity, a kind of nanoscale inorganic materials with oxygen-enriched surfaces --polyoxometalates (POMs), was selected as a composite modifier. Subsequently, a two-dimensional nanomaterial (MOFs@POMs) with high catalytic activity and ultra-high stability was synthesized on the basis of MOFs using a facile solvothermal method to immobilize the active sites of MOFs through the M-O ionic bond between POM and MOFs. Furthermore, we found that the structure of POMs plays a key role in the formation of this unique superstructure, which leads to a catalytic active site that can be persistently maintained during the OER process. Based on this, a scheme for the immobilization of MOFs active sites by POMs is proposed to synthesize MOFs composites with ultra-high OER stability based on both POMs and MOFs materials. The assembly mechanism of the composites during the synthesis process is proposed, and the intrinsic reason for the modulation of the active site of MOFs by POMs and its long-lasting immobilization is demonstrated, which enables the MOFs materials to maintain excellent catalytic activity throughout the great OER catalytic process. This study successfully demonstrates that the stability challenges faced in the OER process of MOFs can be solved, which may provide a helpful solution for the application of MOFs-related OER catalysts.

Localized structural and compositional effects on the kinetic performance of Ni-modified polyoxometalate catalysts for ethylene oligomerization

Ethylene oligomerization is an important step for upgrading light olefins to liquid fuels or value-added chemicals such as butene monomers. Ethylene oligomerization catalyzed by mono-Ni substituted polyoxometalate (POM) catalysts is promising due to the observed selectivity toward linear butene products and stability of these well-defined materials. In this work, two approaches to tailor the catalytic properties of isolated Ni sites in POMs were experimentally tested, both of which perturbed the surrounding molecular environment of the active sites. First, structural modifications were performed to alter the size of the polyoxometalates, evaluating between Wells-Dawson and Keggin structures while maintaining the same elemental components. When comparing these two structures, it was observed that the Ni active sites were kinetically independent from the effect of POM size, even when the POM consisted of the same elements. This was evidenced by the identical product distributions and kinetic parameters observed with both structures. However, a localized kinetic effect was observed when Keggin POM structures containing different internal heteroatoms (e.g, P, Si, Al) were included in close proximity to the Ni active site. Specifically, a notable periodic trend was observed between the electronegativity of internal heteroatoms and the measured apparent activation energy for the catalyzed ethylene coupling reaction to butene.

Hybrid Sub-1 nm Nanosheets of Co-assembled MnZnCuOx and Polyoxometalate Clusters as Anodes for Li-ion Batteries.

Transition metal oxide (TMO) anode materials in lithium-ion batteries (LIBs) usually suffer from serious volume expansion leading to the pulverization of structures, further giving rise to lower specific capacity and worse cycling stability. Herein, by introducing polyoxometalate (POM) clusters into TMOs and precisely controlling the amount of POMs, the MnZnCuOx -phosphomolybdic acid hybrid sub-1 nm nanosheets (MZC-PMA HSNSs) anode is successfully fabricated, where the special electron rich structure of POMs is conducive to accelerating the migration of lithium ions on the anode to obtain higher specific capacity, and the non-covalent interactions between POMs and TMOs make the HSNSs possess excellent structural and chemical stability, thus exhibiting outstanding electrochemical performance in LIBs, achieving a high reversible capacity (1157 mAh g-1 at 100 mA g-1 ) and an admirable long-term cycling stability at low and high current densities.

Hybrid Sub‐1 nm Nanosheets of Co‐assembled MnZnCuOx and Polyoxometalate Clusters as Anodes for Li‐ion Batteries

AbstractTransition metal oxide (TMO) anode materials in lithium‐ion batteries (LIBs) usually suffer from serious volume expansion leading to the pulverization of structures, further giving rise to lower specific capacity and worse cycling stability. Herein, by introducing polyoxometalate (POM) clusters into TMOs and precisely controlling the amount of POMs, the MnZnCuOx‐phosphomolybdic acid hybrid sub‐1 nm nanosheets (MZC‐PMA HSNSs) anode is successfully fabricated, where the special electron rich structure of POMs is conducive to accelerating the migration of lithium ions on the anode to obtain higher specific capacity, and the non‐covalent interactions between POMs and TMOs make the HSNSs possess excellent structural and chemical stability, thus exhibiting outstanding electrochemical performance in LIBs, achieving a high reversible capacity (1157 mAh g−1 at 100 mA g−1) and an admirable long‐term cycling stability at low and high current densities.

Synthesis, structure, and catalytic activity of polyoxometalate (C7H15N4)2[Co(H2O)6][C6H12N2CoMo6O24]·4H2O in the azide–alkyne cycloaddition reaction

A new polyoxomolybdate with a formula of (C7H15N4)2 [Co(H2O)6] [C6H12N2CoMo6O24]·4H2O (1) was synthesized and then it was characterized using IR, EDX and X-ray crystallography. The unit cell of 1 consists of two [Co(H2O)6]2+ and one N-methyluroptropine (C7H15N4)+ cations, and one polyanion [C6H12N2CoMo6O24]4- and four lattice water molecules. In the polyanion [C6H12N2CoMo6O24]4-, molybdenum metals are surrounded by two terminal and one bridging oxygen atoms and also one oxygen of the trimethanolamine ligand is coordinated to each Mo metal. Cobalt metal in the center of the polyanion is also bonded to four bridged oxygen atoms and two oxygen atoms of the trimethanolamine ligand. This novel polyoxomolybdate was used as an efficient catalyst for the azide–alkyne cycloaddition reaction to produce various 1,2,3-triazoles in high yields (34–99%).

Polyoxometalates (POMs) with Ion/Electron‐Sponge Properties and Abundant Active Sites as Emerging Electrode Materials for Secondary Batteries: A Review

AbstractFaced with the increasingly severe energy and environment issues, eco‐friendly and efficient energy storage/transformation technology has become a research hotspot. Due to the ion/electron‐sponge properties and superior catalytic performance, polyoxometalates (POMs) have broad prospects in improving the performance of electrodes and batteries. In this paper, the structural characteristics and physical/chemical properties of POMs are introduced, and the recent research progresses of POM‐electrodes for various battery systems are reviewed. By analyzing the relationship between the composition, structure and electrochemical performance of POMs, the advantages and challenges of applying POMs as electrode materials are explained, and the practical modification strategies for improving the performance of POM‐electrode are proposed, aiming to provide reference for the further investigation and application of POMs in energy storage field.

Atomic structural changes in the formation of transition metal tungstates: the role of polyoxometalate structures in material crystallization.

Material nucleation processes are poorly understood; nevertheless, an atomistic understanding of material formation would aid in the design of material synthesis methods. Here, we apply in situ X-ray total scattering experiments with pair distribution function (PDF) analysis to study the hydrothermal synthesis of wolframite-type MWO4 (M : Mn, Fe, Co, Ni). The data obtained allow the mapping of the material formation pathway in detail. We first show that upon mixing of the aqueous precursors, a crystalline precursor containing [W8O27]6- clusters forms for the MnWO4 synthesis, while amorphous pastes form for the FeWO4, CoWO4 and NiWO4 syntheses. The structure of the amorphous precursors was studied in detail with PDF analysis. Using database structure mining and an automated modelling strategy by applying machine learning, we show that the amorphous precursor structure can be described through polyoxometalate chemistry. A skewed sandwich cluster containing Keggin fragments describes the PDF of the precursor structure well, and the analysis shows that the precursor for FeWO4 is more ordered than that of CoWO4 and NiWO4. Upon heating, the crystalline MnWO4 precursor quickly converts directly to crystalline MnWO4, while the amorphous precursors transform into a disordered intermediate phase before the crystalline tungstates appear. Our data show that the more disordered the precursor is, the longer the reaction time required to form crystalline products, and disorder in the precursor phase appears to be a barrier for crystallization. More generally, we see that polyoxometalate chemistry is useful when describing the initial wet-chemical formation of mixed metal oxides.

Association of Keplerate-Type Polyoxometalate {Mo72Fe30} with Tetracycline: Nature of Binding Sites and Antimicrobial Action

The association process between the tetracycline (TC) antibiotic molecule and Keplerate-type nanocluster polyoxometalate (POM) {Mo72Fe30} was studied in aqueous solution. The novel supramolecular ensemble {Mo72Fe30}@TC12.5 was produced, its composition and structure were revealed by means of elemental analysis (C, N, H) and vibrational spectroscopy (IR and Raman). Based on the spectral data, the POM structure’s integrity was confirmed and binding sites of TC with the Keplerate {Mo72Fe30} surface were found. Due to thermogravimetric analysis (TG) and in-situ Raman spectroscopy during the process of {Mo72Fe30}@TC12.5 thermal destruction, we showed a significant change in the phase composition of POM’s destruction products after association with TC. The antibacterial activity of the obtained complex {Mo72Fe30}@TC12.5 was examined. The experimental results allowed us to note the partial inhibition of TC’s antibacterial activity owing to the coordination of TC to FeIII centers, in turn, which hinders the participation of TC in coordination via Mg2+ of ribosomal subunits 30S in bacteria.

Polyoxometalates-based vesicles for application in biological systems

Functional materials with vesicle structure have been demonstrated to have wide application in biomedicine, catalysis and environmental protection. The characterization of inner constructions, the understanding of the formation mechanism and the exploiting of novel vesicles always drives the further exploitation of this territory. Polyoxometalates (POMs) based vesicles take advantages of the intrinsic properties of POMs like molecular structure diversity, high solubility, modifiable surfaces, and to have a unique class of novel vesicles with different sizes, high stability and functionoriented application potential. More importantly, the precise molecular structures of POMs provide great convenience to study the vesicles formation mechanism. In this review, the current research advancement on the self-assembly of POMs-based vesicles are summarized. We com-pendiously classify POMs-based vesicles into two categories depending on the formation driving force which are non-covalent interaction and covalent modification. Furthermore, the applications of POMs-based vesicles in biomedicine and biomolecular recognition are also introduced. Finally, the prospects and outlooks for engineering novel vesicles with ingenious design towards the bioengineering and pharmaceutical application are presented.

Development of TiO2 catalyst based on Anderson-polyoxometalates for efficient visible-light-driven photocatalytic oxidative desulfurization

Photocatalytic oxidative desulfurization (PODS) is a novel emerged promising method for effectively removing sulfur compounds under milder operating conditions. Herein, Anderson type polyoxometalate (NH4)4ZnH6Mo6O24 modified TiO2 nanomaterial (named Zn-Mo/TiO2) was designed to oxidize DBT to DBTO2 under visible light irradiation with H2O2, this catalyst was synthesized based on Anderson type polyoxometalate (NH4)4ZnH6Mo6O24 by a sol-hydrothermal method. Different experimental effects of Anderson type polyoxometalate modified TiO2 on the desulfurization conversion rate were investigated, including different (NH4)4ZnH6Mo6O24 contents, various catalyst dosage, additional sulfur-containing substrates and different oxygen sulfur ratios. 100 % sulfur removal can be achieved with (NH4)4ZnH6Mo6O24/TiO2 catalyst in only 30 min under room temperature. The considerable enhancement of photocatalytic activity is associated with a synergistic effect between the multi-metal structure of polyoxometalate and TiO2, which are conducive to promoting the transfer and separation of photogenerated electrons and holes.

Oxo-Replaced Polyoxometalates: There Is More than Oxygen.

The presence of oxo-ligands is one of the main required characteristics for polyoxometalates (POMs), although some oxygen ions in a metallic environment can be replaced by other nonmetals, while maintaining the POM structure. The replacement of oxo-ligands offers a valuable approach to tune the charge distribution and connected properties like reducibility and hydrolytic stability of POMs for the development of tailored compounds. By assessing the reported catalytic and biological applications and connecting them to POM structures, the present review provides a guideline for synthetic approaches and aims to stimulate further applications where the oxo-replaced compounds are superior to their oxo-analogues. Oxo-replacement in POMs deserves more attention as a valuable tool to form chemically activated precursors for the synthesis of novel structures or to upgrade established structures with extraordinary properties for challenging applications.

Recent Advances of Anderson-Type Polyoxometalates as Catalysts Largely for Oxidative Transformations of Organic Molecules.

Anderson-type ([XM6O24]n−) polyoxometalates (POMs) are a class of polymetallic-oxygen cluster inorganic compounds with special structures and properties. They have been paid extensive attention by researchers now, due to their chemical modification and designability, which have been widely applied in the fields of materials, catalysis and medicine. In contemporary years, the application of Anderson-type POMs in catalytic organic oxidation reaction has gradually shown great significance for the research of green catalytic process. In this paper, we investigate the application of Anderson-type POMs in organic synthesis reaction, and these works are summarized according to the different structure of POMs. This will provide a new strategy for further investigation of the catalytic application of Anderson-type POMs and the study of green catalysis.

Modification of Keggin anion structure with ion beams—A new spectroscopic insights into the effects of keV‐ and MeV‐ion beam irradiation on 12‐tungstophosphoric acid

Ion beam irradiation is a versatile tool for structural modification and engineering of new materials. In this study, 12-tungstophosphoric acid (WPA) films of different thickness were spin-coated on platinized silicon substrate and irradiated with low energy hydrogen ions (10 keV) and swift heavy ions (Bi, Xe, and V) with energies up to 710 MeV. The different energy/fluence combinations allowed controllable structural changes that were investigated in detail using Raman and Infrared spectroscopy. For 120-nm-thick WPA samples, the irradiation led to the decrease of intensity of the skeletal and W-Oc-W bands of Keggin anion in order: Bi < V < Xe (for their applied energy/fluence combination). Also, symmetry change of Keggin anion similar to the one observed in the case of Keggin anions interacting with the supports was observed. For the selected ion beam irradiation parameters, xenon ion beam induced transformation of WPA to polytungstate. For 20-μm-thick WPA samples, the irradiation with hydrogen ion beam induced changes of skeletal vibrations and increased individualistic behavior of Keggin anions. As the fluence increased, the amount of the Keggin anions partially transformed to bronze also increased. Irradiation with vanadium also caused transformation to bronze-like structure but with higher ratio of terminal W=Od bonds. The overall results show clear correlation between degree of structural modification of WPA and the calculated displacement per atom value. These results open possibilities for engineering new catalytically active structures of polyoxometalates with the help of ion beams.

Exploring the Ligand Functionality, Electronic Band Gaps, and Switching Characteristics of Single Wells–Dawson‐Type Polyoxometalates on Gold

AbstractThe miniaturization, high performance, energy efficiency, and new added functionalities are the essential drivers of modern information data storage and processing technologies. Polyoxometalates (POMs) characterized by atomically well‐defined structures with discrete energy levels and the ability to undergo redox transformations are viewed as promising active components for the integration into the next‐generation (beyond‐CMOS) hybrid nanoelectronics. Herein, new fundamental insights into the application of organically augmented POMs on conducting surfaces are offered. Three key findings resulting from scanning probe investigations combined with integral spectroscopic methods used to explore tris(alkoxo)‐ligated, vanadium‐containing Wells‐Dawson‐type POM structures on Au(111) are reported on. First, it is shown how the (OCH2)3C–R ligands, depending on the structurally exposed R group (R = CH2SMe and NHCOC6H4SMe), influence the self‐assembly behavior of the synthesized POMs on gold. Second, the impact of the employed (OCH2)3C–R ligands and the determined assembly characteristics on the relative position of POM's electronic band structure against the Fermi level of the gold surface are explained. Third, the on‐surface conductance switching of single POM structures due to external electrical stimuli is demonstrated. The author's experimental efforts enable to discover highly sought‐after multi‐level resistive switching orchestrated by electrically accessible V(3d) states in the POM single‐molecules at room temperature in a narrow voltage range.