Keggin-type Polyoxometalate Research Articles

Nickel substituted polyoxometalates in layered double hydroxides as metal-based nanomaterial of POM–LDH for green catalysis effects

Three nickel substituted Keggin-type polyoxometalates, α-[SiW9O37{Ni(H2O)}3]−10 (denoted as SiW9Ni3), was intercalated into Zn3Al based Layered Double Hydroxide (Zn3Al-LDH) by the selective ion-exchange technique. The as-synthesized nanocomposite, SiW9Ni3@Zn3Al, was used as heterogeneous nanoreactor to promote the synthesis of drug-like aminoimidazopyridine small molecule skeletons via the well-known Ugi-type Groebke–Blackburn–Bienaymé reaction (GBB 3-CRs) in the absence of any acid/additive and under mild and solvent-free conditions. A synergistic catalytic effect between SiW9Ni3 polyoxometalate and Zn3Al-LDH precursors is evidenced by a higher catalytic property of the SiW9Ni3@Zn3Al composite compared to the individual constituents separately. Lewis/Bronsted acidity of the SiW9Ni3 polyoxometalate and Zn3Al-LDH precursors appear to be essential for the catalytic performance of the composite. Furthermore, the catalytic performance of SiW9Ni3@Zn3Al was also tested in GBB 3-CRs synthesis of amino imidazothiazole under mild and solvent-free conditions.

Towards cobalt-containing polyoxometalate-based electrolytes for redox flow batteries operating under mild conditions

Sustainability and safety of Redox Flow Batteries (RFBs) are some of the limitations that hinder the further implementation of this technology. To overcome the disadvantages of conventional RFBs, new electrolyte formulations are being proposed. Herein we report on two cobalt(II)-containing Keggin-type polyoxometalates (POMs) working as electroactive species. These are the plenary [α-CoW12O40]6− (CoW12) anion and the cobalt(II)-monosubstituted [α-Co(H2O)SiW11O39]6− (CoSiW11) tungstosilicate. During the battery performance, cobalt-containing POMs play the role of both positive and negative electrolyte, because the ability of Co (CoIIWVI/CoIIIWVI) and W (CoIIWVI/CoIIWV) centres to be oxidized and reduced, respectively. These redox processes are separated by 1.7 V (CoW12) and 1.9 V (CoSiW11) thus, both POMs supply with a suitable operational voltage for RFBs. The reversibility of the reactions was examined by a kinetic study. Among the formulations tested, those accomplished in aqueous 1 M HAc/LiAc and 1 M LiAc revealed high stability while operating under mild pH conditions. Moreover, the nature of the selected supporting electrolytes (SEs) avoids the occurrence of gassing side reactions. In addition, the physicochemical characterization of the electrolytes showed high conductivity (ca. 60 mS cm−1) and low viscosity (ca. 1.4 mPa s) for highly concentrated electrolytes (up to 120 mM). Therefore, the developed electrolytes exhibit promising properties for their direct incorporation in RFBs, including high energy density (21.86 and 24.44 W h L−1 for CoW12 and CoSiW11 respectively) and enhanced safety.

Intercalation Pseudocapacitance of Cation-Exchanged Molybdenum-Based Polyoxometalate for the Fast and Stable Zinc-Ion Storage.

Recently, intercalation pseudocapacitance has received significant interest as an abnormal charge storage mechanism owing to the battery-like intercalation energy storage into the bulk electrodes and the fast charge storage kinetics of electrochemical capacitors. However, intercalation pseudocapacitance of molybdenum-based polyoxometalates (POMs) for high-performance Zn ion battery (ZIB) cathodes is yet to be exploited. Herein, we demonstrate the fast and reversible intercalation pseudocapacitance of vanadium-substituted Keggin-type molybdenum-based POMs (XPMoV), where H of HPMoV is replaced by X cations (X = Li, Na, K, or Rb). This cation exchange allows cation-exchanged XPMoV to exhibit the morphological evolution into an anisotropic rodlike structure and to achieve a pillar effect on the improved chemical and structural integrity. Despite the micron-size rod morphology and the contracted lattice of (100) plane, the intercalation pseudocapacitance kinetics of XPMoV was dominated by the fast surface-confined electrochemistry and became highly reversible after the 1st cycle activation process by co-intercalation of Li+ and Zn2+ ions. Therefore, the ZIB with the KPMoV cathode delivered a high rate capability of 74.0 mAh g-1 at 20,000 mA g-1 and 87% capacity retention over 2000 cycles at 1000 mA g-1, far exceeding HPMoV and other Mo-based cathodes. This study paves the way to design the fast and reversible intercalation pseudocapacitance of POMs and the cation exchange chemistry into the improved (electro)chemical and structural integrity.

Making Heterometallic Metal–Metal Bonds in Keggin-Type Polyoxometalates by a Six-Electron Reduction Process

Polyoxometalates (POMs) represent a promising class of molecular electron reservoirs. However, their multielectron reduction gives rise to intricate physical-chemical phenomena that must be fully understood for their future use in energy-storage devices. Herein, we show that bulk electrolysis of the archetypal Keggin-type POM [Si(WVI2MoVIO10)(WVI3O10)3]4- in aqueous solution leads to the six-electron-reduced derivative [Si(WIV2MoIVO7(H2O)3)(WVI3O10)3]4- (notated SiW11Mo-VI') in which the mixed-metal triad acts as a storage unit for six electrons and six protons. X-ray diffraction analysis and multinuclear NMR (183W and 95Mo) studies reveal that this electron-rich species represents the first example of POMs containing heterometallic metal-metal bonds between addenda centers. This electron-rich POM can be further reduced through multielectronic events, while its full oxidation restores the structure of the oxidized parent ion. Remarkably, the formation of SiW11Mo-VI' results from a fast clustering process compared to that observed for the entirely W-based analogue, revealing that the formation of metal-metal bonds in the mixed-metal Mo/W POM is facilitated because the reaction rate is not limited by a slow disproportionation step. Last, we evaluate the supramolecular properties of SiW11Mo-VI' using a method based on the cloud-point measurement of a nonionic surfactant. This investigation demonstrates that the clustering process has dramatic consequences on the solution behavior of the POM, canceling its superchaotropic character due to a local structuring effect of the hydration shell. These fundamental results pave the way for applications using the massive electron-storage properties of mixed-metal POMs.

Heteroatom-Modulated Assembly of Hexalanthanoid-Containing Polyoxometalate-Based Coordination Networks.

Two series of lanthanoid (Ln)-containing polyoxometalates (POMs) {[Ln6(ampH)4(H2O)24-n(ampH2)n(PW11O39)2]·21H2O (Ln = Tb, n = 0 (1), Ln = Er, n = 1 (2)) and K2[Ln6(ampH)4(H2O)22(SiW11O39)2]·23H2O (Ln = Tb (3), Er (4)) (ampH2 = (aminomethyl) phosphonic acid)} have been synthesized with tri-lacunary Keggin-type POMs containing different types of heteroatoms. Compounds 1 and 2 display neutral organic-inorganic hybrid POM molecules containing {Ln6(ampH)4} ({Ln6}) cores sandwiched by two {PW11O39} units. By changing the heteroatoms from PV to SiIV, the extended 2D networks of 3 and 4 were successfully isolated where the adjacent {Ln6} clusters were connected by {SiW11O39} moieties. Luminescence performances and magnetic properties of 1-4 have been systematically surveyed. The solid-state fluorescence spectra of 1-4 display characteristic emissions of Ln components resulting from the 4f-4f transitions, and energy transfer from the POM segments to Ln3+ centers in 1 and 3 has been observed based on the lifetime decay behaviors. Furthermore, all compounds can be utilized as electrocatalysts toward reduction of nitrite with high stability.

Polyoxometalate-based graphitic carbon nitride for reduction of toxic nitro aromatic compounds in water

The present study focuses on the development of an impressive way to eliminate toxic nitro aromatic compounds as hazardous industrial pollutants using polyoxometalate-based composites. For this purpose, transition metal substituted Keggin-type polyoxometalates were integrated on modified protonated graphitic carbon nitride (POMs@g-C3N4) via an electrostatic self-assembly strategy. These stable compounds are characterized by different techniques and used in the reduction of toxic nitro aromatic compounds. Among Cr, Mn, Fe, Co, Ni, Cu, and Zn, the composite containing copper substituted Keggin-type polyoxometalate (CuPOM@g-C3N4) presents the most respectable catalytic activity in these reduction reactions. Moreover, this efficient catalytic system is applied in the reduction of different toxic nitro aromatic compounds into the amino aromatic analogues with appropriate conversions and reaction times. Kinetic studies are also used to determine their apparent rate constants. This process is easy, low-cost, non-toxic, high potential catalytic performance, and environmental friendliness that could be accomplished in water as solvent and at room temperature.

Mechanism of melanogenesis inhibition by Keggin-type polyoxometalates.

Abnormal melanin overproduction can result in hyperpigmentation syndrome in human skin diseases and enzymatic browning of fruits and vegetables. Recently, our group found that Keggin-type polyoxometalates (POMs) can efficiently inhibit tyrosinase activity. However, it remains unclear whether Keggin-type POMs exhibit optimal effects in vivo. Additionally, the inhibitory effect and mechanism of action of POMs on cellular tyrosinase activity and melanogenesis have been rarely reported. Here we demonstrate that our screened and synthesised PMo11Zn and GaMo12 show superior inhibitory effects on melanin formation as well as inhibition of cellular tyrosinase activity compared to other Keggin-type POMs. Intriguingly, we reveal that Keggin-type POMs competitively bind to tyrosinase mainly through more interactions with Cu2+ ions and the amino acid residue is capable of forming van der Waals, cation-π and hydrogen bonds, resulting in a reversible non-covalent complex formation. Our findings provide valuable insights into the design, synthesis and screening of polyoxometalates as multifunctional metallodrugs and food preservatives against hyperpigmentation.

Facile fabrication and characterization of rare earth complexes based on Keggin-type polyoxometalate with highly efficient activity for photocatalytic degradation of MO

Rare earth complexes based on Keggin-type polyoxometalate for photocatalytic degradation of MO. The degradation rate reached more than 95% within 15 min.

Keggin-type polyoxometalate-based crown ether complex for lithium-sulfur batteries.

A novel supramolecular complex Li3Cl[(HPW12O40)(H24C12O6)3(CH3CN)2] {CR-PW12} was confirmed first to apply as a sulfur host in lithium-sulfur batteries. The {CR-PW12}@S cathode exhibits a reversible capacity of 1120 mA h g-1 at 1.0 C and excellent cycle stability.

Five new multifunctional inorganic–organic hybrids based on Keggin-type polyoxometalates and a 3,5-di(1H-imidazol-1-yl)benzoic acid ligand

The tripodal ligand 3,5-di(1H-imidazol-1-yl)benzoic acid was used to synthesize five new inorganic–organic hybrid materials based on Keggin-type POMs, which is of great significance in the field of environmental monitoring.

A study of the antioxidant properties of Keggin-type polyoxometalates.

In this paper, we used extracellular reactive oxygen radical scavenging assay and cellular antioxidant assay to investigate the antioxidant ability of Keggin-type polyoxometalates on the inside and outside of cells influenced by three different factors: heteroatom substitution, transition metal substitution and the number of vanadium substitutions. The results showed that the IC50 values of heteroatomic (P, Si, Ga) polyoxometalates on superoxide anion radical scavenging were 1.32 ± 0.00047 mg mL-1, 17.49 ± 2.47500 mg mL-1, and 66.99 ± 20.0227 mg mL-1, respectively. In comparison, PMo12 had the best ability to scavenge free radicals, and the SOD activity of PMo12 at 12.5 μmol L-1 increased by 50% compared with that of the unspiked drug, which played an antioxidant role; the scavenging effect of superoxide anion radicals of PMo11Mn in transition metals (Fe, Mn, Cu) instead of polyoxometalate (IC50 value 1.18 ± 0.0008 mg mL-1) was lower than that of unsubstituted PMo12 (IC50 value 1.32 ± 0.00047 mg mL-1), where PMo11Mn was nearly 1.5 times higher than PMo11Cu administration to reduce the number of cells by half; the PMo11V, PMo10V2, PMo9V3, PMo8V4, PMo7V5 hydroxyl radical scavenging rates (IC50 values) were 0.19 ± 0.0011 mg mL-1, 0.22 ± 0.0027 mg mL-1, 0.03 ± 0.0014 mg mL-1, 0.04 ± 0.0008 mg mL-1, and 0.11 ± 0.0005 mg mL-1, respectively, and in comparison, scavenging of PMo9V3 radicals was more effective and they acted as an antioxidant. Therefore, they can be used as good antioxidants in biological and pharmaceutical applications and play an important role in the treatment of tumours, cancer, Alzheimer's disease and other diseases.

Light-induced dissolution and concomitant crystallization of a Keggin-type polyoxometalate mimicking a naturally occurring phenomenon.

A suspension of a yellow polycrystalline compound [PPh4]3[PMoVI12O40] in N-methylformanilide (NMF) (in which it is not soluble), on irradiation with sunlight, initiates dissolution via its reduction followed by its crystallization leading to the isolation of single crystals of compound [PPh4]4[PMoVMoVI11O40]·3CH3(C6H5)NCHO (1). Compounds [PPh4]3[PMoVI12O40]·1.75 CH3(C6H5)NCHO (2) and [PPh4]3[PMoVI12O40]·2CH3(C6H5)NCHO (3), each containing an oxidized Keggin anion, are obtained at two different temperatures when the corresponding mother liquor is kept in the dark.

Rare earth inorganic-organic hybrid compounds based on Keggin-type polyoxometalate {SiW12} with fast-responsive photochromism and switchable luminescence properties

Four rare earth inorganic-organic hybrid compounds based on Keggin-type polyoxometalate {SiW12} with general formula [{RE(DMF)4(H2O)4RE(DMF)6}2{SiW12O40}3] (RE = La(1), Pr(2), Sm(3), Eu(4), DMF = N,N-dimethylformamide) were synthesized by the conventional solution method. Structure analyses indicate that four compounds are isostructural and are all built from three [SiW12O40]4– polyoxoanions and DMF ligands linked by RE3+. Compounds 1–4 are extremely sensitive to UV light and present excellent photochromic properties, in which the coloring and fading time of samples are both no more than 2 min, and the reversible coloring-fading process can be repeated for at least 10 cycles with little fatigue, suggesting that compounds 1–4 can be a series of fast-responsive and durable photochromic materials. Moreover, the solid-state photoluminescence spectra of compounds 3(Sm) and 4(Eu) display strong characteristic emissions of rare earth ions based on f–f transitions. Meanwhile, compound 4(Eu) has a longer fluorescence lifetime up to 566.74 μs. Notably, compounds 3(Sm) and 4(Eu) exhibit the switchable luminescence behavior induced by photochromism, which allows them to be used as potential molecular switches.

Boosting Simultaneous Uranium Decorporation and Reactive Oxygen Species Scavenging Efficiency by Lacunary Polyoxometalates.

The chemical toxicity and the oxidative stress induced by the internal exposure of uranium is responsible for the long-term adverse effect of in vivo contamination of uranium. An agent with simultaneous removal capability of uranium and excess reactive oxygen species (ROS) is highly desired. Herein, the lacunary Keggin-type polyoxometalate (POM) is demonstrated to selectively bind with uranyl ions in the presence of excess essential divalent ions and exhibits a compelling ROS scavenging efficiency of 78.8%. In vivo uranium decorporation assays illustrate the uranium sequestration efficiencies of 74.0%, 49.4%, and 37.1% from kidneys by prophylactic, prompt, and delayed administration of lacunary POM solution, respectively. The superior ROS quenching and uranium removal performance in comparison with all reported bifunctional agents endow lacunary polyoxometalates as novel agents to effectively protect people from injuries caused by the internal exposure of actinides.

Superhydrophilic molybdenum phosphide quantum dots on porous carbon matrix for boosting hydrogen evolution reaction

Fabricating low-cost, earth-enriched and high-performance electrocatalysts for hydrogen evolution reaction (HER) is significant but challenging for the development of a sustainable hydrogen economy. Herein, we report superhydrophilic molybdenum phosphide quantum dots on porous carbon matrix (MoP-QDs@PC) using polyoxometalates (POMs)-contained metal organic frameworks (MOFs) as precursors, where Keggin-type PMo12 POMs clusters are confined within the zirconium-based UiO-66 MOFs (denoted as PMo12@UiO-66). We demonstrate that the as-formed MoC species by in-situ confined carbonization of PMo12@UiO-66 can be converted into superhydrophilic MoP-QDs by further phosphorization, while the porous carbon matrix can be converted into rich P and O co-doped carbon matrix, which greatly increase the wettability and the amount of catalytic active sites exposed to the electrolytes. The as-prepared MoP-QDs@PC electrocatalyst exhibits excellent HER performance, which outperforms commercial 20% Pt/C electrocatalyst when overpotential (η) higher than 197 mV and 233 mV under alkaline and acidic conditions, respectively. Depending on the diversity of POMs and MOFs, the current feasible and reliable MoP-QDs@PC electrocatalysts from PMo12@UiO-66 precursor would inspire more effective strategies to economical, efficient and stable electrocatalysts for hydrogen evolution.

Developing ethyl lauroyl arginate antimicrobial films to combat Listeria monocytogenes in cured ham

Widespread use of plastics poses a serious environmental hazard to our planet and should be substituted by eco-friendly and biodegradable alternatives, simultaneously reducing waste of perishable food products and the risk of transmission of pathogenic microbes. In our study, we describe how the water solubility of the antimicrobial surfactant ethyl lauroyl arginate (LAE) can be reduced through complexation with a Keggin-type polyoxometalate (POM), K8[SiW11O39]. The POM-LAE complex, LAE7K[SiW11O39], was effective against Listeria monocytogenes (L. monocytogenes) and Escherichia coli (E. coli) with minimal inhibitory concentrations (MICs) of 32 and 64 μg/mL, respectively, with the important finding that the concentrations of LAE required to inhibit bacterial growth were as much as two times lower in the POM-LAE complex, compared with LAE on its own. In addition, our results demonstrate that POM-LAE is both an effective inhibitor of biofilm formation and is also able to destroy pre-formed biofilms of L. monocytogenes and E. coli at MIC concentrations. Further, POM-LAE was incorporated into carboxymethyl cellulose (CMC) films that were able to reduce 7–8 log (CFU/mL) of L. monocytogenes at concentrations of 5–10% POM-LAE. In vivo assays of the POM-LAE-CMC films with cured ham prevented initialk bacterial growth with a 0.77 log significative reduction in bacterial counts. Overall, this work provides new alternatives for the development of antimicrobial biodegradable films for ready-to-eat (RTE) foods prone to contamination with pathogenic bacteria, such as L. monocytogenes, while also circumventing practical issues related to the incorporation of LAE into active packaging films.

Construction of polyoxometalate-based metal−organic frameworks through covalent bonds for enhanced visible light-driven coupling of alcohols with amines

Broad spectral response and rapid carrier transport are essential in making efficient photocatalysts. In this work, we utilized Keggin-type polyoxometalates (POMs) [CoIIW12O40]6– as metal-to-metal charge transfer (MMCT) chromophores and assembled them into metal–organic frameworks (MOFs) to construct visible-light-responsive, noble metal-free crystalline POM-based MOFs (POMOFs) (CoW-1 and CoW-2). The precise introduction of MMCT chromophores extending the light absorption of POMOFs to the visible region and improving the solar energy utilization efficiency, meanwhile, the Cu-O-W covalent bonds constructed in CoW-1 makes the framework structure more robust, reduces the interfacial contact resistance, facilitates the electron transfer, suppresses the recombination of the electron-hole pairs, improves the charge carriers separation efficiency, and boosts the quantum efficiency, thus achieving high catalytic activity in the coupling reaction of benzyl alcohol with aniline under visible light irradiation (>400 nm). In the presence of CoW-1, the reaction conversion yield was measured as 92.6 % with turnover frequency reaching 374.4 h−1, and the apparent quantum yield at 595 nm was calculated as 17.5 %, besides, CoW-1 also exhibited high catalytic stability and reusability. To the best of our knowledge, this is the first work describing the enhanced photocatalytic performance of POMOFs based on the synergistic effect of MMCT and covalent linkage.

Geometry-Modulated Self-Assembly Structures of Covalent Polyoxometalate–Polymer Hybrid in Bulk and Thin-Film States

This work reports the synthesis, characterization, and self-assembled structures of three series of Keggin-type polyoxometalate (POM)-based hybrid molecules KPOM-mPSn (m = 1, 2 and 3), in which one, two, or three polystyrene (PS) chains are covalently attached to a POM head, respectively. The classical “click” reaction was applied to prepare samples with predesigned molecular geometry, which were further purified by C18 reversed-phase high-performance liquid chromatography (RP-HPLC). Strong chemical incompatibility between inorganic POMs and organic PS chains drives the hybrid molecules to microphase separate into versatile nanostructures in both bulk and thin-film states. In thermally annealed bulk samples, four phase morphologies were found depending on both the overall molecular geometry and the volume fractions of PS tails, including lamellae (LAM), hexagonal packed cylinders (HEX), disordered micelles (DM), and body-centered cubic (BCC) packed spheres, as revealed by small-angle X-ray scattering (SAXS). In the thin-film state, LAM, HEX, and BCC structures were observed and proved by transmission electron microscopy (TEM) images and grazing incidence SAXS (GISAXS). This work expands the scope of self-assembled nanostructures of POM-based hybrid materials to spherical phases and provides a platform for the study of the basic physical principles of their self-assembly behavior.

Comparing Cs+ binding affinity of Keggin type polyoxometalate and sodium Tetrakis(4-florophenyl)borate in solution and from Cs-doped pure phase vermiculite

We assessed the aptitude of cesium (Cs+) binding by Keggin type polyoxometalates (POMs) and compared the results with the Cs+ binding by sodium tetrakis(4-fluorophenyl)-borate (Na-TFPB). In this work, we aimed to establish a system to treat radioactive Cs+ contaminated soil with POMs economically. We evaluated the effect of initial Cs+ concentration (0.1M) and precipitant (POMs and TFPB) concentrations (0.01M) on Cs+ precipitation. Our comparison of Cs+ precipitation by three different POMs and TFPB was obtained by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). We synthesized POMs molybdovanadophosphoric acid, H5PMo10V2O40 (MVPA), and silicotungstic acid, H4SiW12O40 (STA), and used commercially available phosphotungstic acid, H3PW12O40 (PTA), and TFPB. Cs-doped pure phase vermiculite was also used to demonstrate the extraction potential of Cs+ by TFPB, STA, and PTA. All the POMs and corresponding Cs-bound POMs were characterized by UV–visible spectroscopy, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray powder diffraction (XRD). In this simulation study, we demonstrated that the Cs+ removal by POMs is much more effective than TFPB and could be a promising method for the treatment of radiocesium contaminated soil.

Green complexation for heavy metals removal from wastewater by Keggin-polyoxometalates enhanced ultrafiltration.

The presence of heavy metals in wastewater has become a serious issue and a global concern for the environment and public health with rapid progress of modern textile industry. To minimize the health risks of heavy metals their complexation to a chelating agent constitute a promising process using membrane separation. We highlight for the first time the use of Keggin type-polyoxometalates (PW12) as complexing agent to eliminate heavy metals from synthetic textile wastewater. Indeed, filtration experiments were performed through the ultrafiltration organic regenerated cellulose membrane (3KDa). Effects of pressure (1-2.5 bar), PW12 concentration (10-50 mg·L-1), salt concentration (10-4-2 M) and pH value (2-12) on cadmium (Cd) and copper (Cu) removal were regularly explored. Experimental data showed that the addition of PW12 improves metal removal efficiency (up to 90%). The addition of NaCl salt significantly decreases the metals retention to 42%. The retention drop is probably due to the competition between Na+ and metals on complexation same negative sites of the PW12 and to the electric double-layer compressing. 24 full factorial design has been used to evaluate the most influencing parameters. The results obtained revealed that the maximum metal retention was 99% for both Cd and Cu.