Showing papers on "Polyoxometalate published in 2020"

Tuning the electrophilicity of vanadium-substituted polyoxometalate based ionic liquids for high-efficiency aerobic oxidative desulfurization

Abstract: With increasing stringent standard of sulfur content in fuels, deep desulfurization with high-efficiency is necessary for ecological and environmental demands. Herein, vanadium-substituted polyoxometalate-based ionic liquids with tunable electrophilicity were facilely synthesized through ion exchange method, and applied in the aerobic oxidative desulfurization. The experimental results indicated that both imidazole cation and polyoxometalate anion played a vital role in the desulfurization process. The prepared materials have a good performance on the degradation of aromatic sulfur compounds, which can reach up to 98.9 % under optimal conditions. Moreover, the structure-function relationship was also investigated in detail, as well as the proposed mechanism.

Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction.

Abstract: The Keggin-type polyoxometalate (POM) PW12O403– and the catalytic complex Cp*Rh(bpydc)Cl2 (bpydc = 2,2′-bipyridine-5,5′-dicarboxylic acid) were coimmobilized in the Zr(IV) based metal organic frame...

Synthesis of polyoxometalate encapsulated in UiO-66(Zr) with hierarchical porosity and double active sites for oxidation desulfurization of fuel oil at room temperature

Abstract: Catalytic oxidative desulfurization (ODS) of fuel oil has received much attention. For this, the synthesis of a highly efficient ODS catalyst has been strongly pursued. Polyoxometalates (POMs) as active centers have been studied for ODS reactions. However, generally the obtained POM catalysts exhibited ordinary catalytic ODS performance mainly due to the role of single active sites. Moreover, the synthesis needs to be completed with the addition of solvent. In this work, we report a new approach to prepare highly efficient ODS catalysts with POM sites. This process can be simply realized by the combination of a common POM (H3PW12O40) and a stable metal–organic framework (MOF) material UiO-66(Zr) without the addition of solvent. More importantly, it is found that polyoxometalate encapsulated in UiO-66(Zr) possesses hierarchical porosity and double active sites. As a result, the catalyst exhibited extraordinary catalytic activity by using H2O2 as an oxidant in the ODS reactions of benzothiophene, dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene. The turnover frequency (TOF) for the removal of DBT reached 293 h−1 at room temperature, which is much higher than that of the reported catalysts. Such outstanding catalytic performance could be mainly attributed to easy accessibility of active sites and the existence of two types of active centers in polyoxometalate species and defective UiO-66(Zr).

Bimetallic manganese-vanadium functionalized N,S-doped carbon nanotubes as efficient oxygen evolution and oxygen reduction electrocatalysts

Abstract: The electrochemical oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are two critical processes for energy conversion technologies, including water electrolysis, fuel cells, and metal-air batteries. For technological implementation, both reactions require highly active and stable electrocatalysts. Here, we report the simultaneous functionalization of carbon nanotubes with bimetallic manganese-vanadium OER/ORR catalytic sites, and nitrogen/sulfur atoms to increase electrical conductivity. A bottom-up fabrication route using polyoxometalate metal precursors enables facile and scalable deposition of bimetallic catalysts on the carbon nanotubes. Electrocatalytic OER/ORR studies show the high activity and stability of the composite under alkaline aqueous conditions, and comparable performance to commercial Pt/C (20 wt.%) was observed. Initial mechanistic analyses shed light on the effects of the bimetallic functionalization as well as the N/S-doping of the carbon nanotubes. In future, the use of heterometallic polyoxometalate precursors could allow the variation of metal types and atomic ratios, which could lead to well-defined bimetallic composites for various electrochemical processes.

Anderson polyoxometalate built-in covalent organic frameworks for enhancing catalytic performances

Abstract: Covalent organic frameworks (COFs) have functional synthons, diverse structures and feature-rich functionality. Herein, the combination of a linear linker NH2–{MMo6}–NH2 with 4-connected tetrahedral tetrakis(4-formylphenyl)silicon building units produced three isostructural complexes M-Anderson-COFs with a 3-fold interpenetrated diamondoid network. Among them, Mn-Anderson-COF exhibited the highest catalytic activity in the photodegradation of RhB and MB in water. The addition of different scavengers demonstrated that 1O2 and H2O2 were primary reactive species to degrade the dyes. Moreover, using M-Anderson-COF as a catalyst, organic sulfides were almost completely oxidized with 100% sulfoxide/sulfone selectivity.

H5PV2Mo10O40 Polyoxometalate Encapsulated in NU-1000 Metal-Organic Framework for Aerobic Oxidation of a Mustard Gas Simulant

Abstract: The immobilization of H5PV2Mo10O40 polyoxometalates (POMs) in the in the mesoporous channel-type metal–organic framework (MOF), NU-1000, via simple impregnation method is reported here. Characteriz...

A Chemically Soldered Polyoxometalate Single-Molecule Transistor

Abstract: Polyoxometalates have been proposed in the literature as nanoelectronic components, where they could offer key advantages with their structural versatility and rich electrochemistry. Apart from a few studies on their ensemble behaviour (as monolayers or thin films), this potential remains largely unexplored. We synthesised a pyridyl-capped Anderson-Evans polyoxometalate and used it to fabricate single-molecule junctions, using the organic termini to chemically "solder" a single cluster to two nanoelectrodes. Operating the device in an electrochemical environment allowed us to probe charge transport through different oxidation states of the polyoxometalate, and we report here an efficient three-state transistor behaviour. Conductance data fits a quantum tunnelling mechanism with different charge-transport probabilities through different charge states. Our results show the promise of polyoxometalates in nanoelectronics and give an insight on their single-entity electrochemical behaviour.

Catalytic oxidative desulphurization of gasoline using amphiphilic polyoxometalate@polymer nanocomposite as an efficient, reusable, and green organic–inorganic hybrid catalyst

Abstract: To preparation of ultra-clean gasoline fuel, a new amphiphilic nanocomposite (TBA-SiWMn@PVA) has been successfully synthesized by supporting sandwich-type silicotungstate polyoxometalate ((...

Polyoxometalate-Based Photoactive Hybrid: Uncover the First Crystal Structure of Covalently Linked Hexavanadate-Porphyrin Molecule

Abstract: Polyoxometalates (POMs)-porphyrin hybrids can serve as multifunctional materials with fascinating photocatalytic and photovoltaic properties. However, most previous POM-porphyrin hybrids are synthe...

A Series of Polyoxometalate-Based Metal–Bis(pyridyl-tetrazole) Complexes with High Electrocatalytic Activity for Hydrogen Evolution Reaction in Alkaline and Acid Media

Abstract: In this work, a succession of Keggin-type polyoxometalate (POM)-based metal–bis(pyridyl-tetrazole) complexes, {Cu₂(3-bptzp)₃(H₂O)₄[SiW₁₂O₄₀]}·H₂O (1), {Cu₂(3-bptzpe)₂(H₂O)₈[SiW₁₂O₄₀]}·4H₂O (2), and {Cu₂(3-bptzh)₃(H₂O)₆[SiW₁₂O₄₀]} (3) (3-bptzp = 1,4-bis(5-(3-pyridyl)tetrazolyl)-butane; 3-bptzpe = 1,4-bis(5-(3-pyridyl)tetrazolyl)-pentane; 3-bptzh = 1,4-bis(5-(3-pyridyl)tetrazolyl)-hexane), were prepared and structurally characterized, which exhibited different architectures. The title complexes were used directly as electrocatalysts for hydrogen evolution reaction in alkaline and acidic media. The electrocatalytic activity for generating hydrogen was reinforced by the introduction of POMs. Complex 1 possesses the highest electrocatalytic activity for hydrogen evolution reaction with a low overpotential of 59.4 mV, achieving the current density of 10 mA·cm–² in 0.1 M KOH. The different structures of the title complexes showed effects on the hydrogen evolution performance. For 1, the insertion of POMs between the ordered 2D metal–organic layers induced the enhancement of electrocatalytic activity.

Polyoxometalate-based crystalline materials as a highly sensitive electrochemical sensor for detecting trace Cr(vi).

Abstract: It is crucial to find a convenient and sensitive method for quantitative determination of heavy metal chromium(vi) ions. Developing crystalline materials coupled with polyoxometalates as an electrochemical sensor is a promising approach to address the above issues. Here we reported two reductive polyoxometalate-based crystalline compounds with the formula of (H2bpp)2[Na4Fe(H2O)7][Fe(P4Mo6O31H6)2]·2H2O (1) and (H2bpp)6(bpp)2[Fe(P4Mo6O31H8)2]2·13H2O (2) (bpp = 1,3-bi(4-pyridyl)propane). Structural analysis indicated that both two compounds were composed of inorganic polyanionic clusters and organic protonated bpp cations. The difference lies in the arrangement mode of the inorganic moiety: crystal 1 shows a unique three-dimensional (3-D) inorganic porous skeleton, while crystal 2 consists of isolated 0-D polyanionic clusters. When used as electrochemical sensors in the determination of trace Cr(vi), crystal 1 shows a broad linearity range (2-2610 μM) with a low limit of detection (LOD) of 0.174 μM (9 ppb), which is superior to that of compound 2 (a LOD of 0.33 μM) and meets the standard of Cr(vi) in drinking water set by the WHO (less than 0.962 μM or 50 ppb). Importantly, crystal 1 showed benign selectivity to Cr(vi) in the presence of various heavy metal ions and good reproducibility in a real water sample, which prove its strong anti-interference ability. In addition, experimental results showed that the spatial arrangement of polyanionic clusters could affect the final electrochemical behavior of crystalline materials. This work provides some insights into the design of cost-effective POM-based electrochemical sensors at the molecular level.

Efficient biodiesel production from oleic acid using metal–organic framework encapsulated Zr-doped polyoxometalate nano-hybrids

Abstract: According to the need of sustainable development, solid catalysts are critical materials for green biodiesel production. In this study, a hydrothermal method was used to develop a reusable and highly active Fe-BTC and UiO-66 metal–organic framework encapsulated Zr-doped polyoxometalate nano-hybrid catalysts (ZrSiW/Fe-BTC and ZrSiW/UiO-66) for the acid-catalyzed esterification of oleic acid with methanol. The structural and compositional characterization of the synthesized catalysts were characterized using various techniques such as Fourier transform infrared spectrometry (FT-IR), X-ray diffraction (XRD), nitrogen adsorption–desorption, scanning electron microscopy (SEM), and thermogravimetric (TG) and temperature programmed desorption (NH3-TPD) analysis. The as-prepared ZrSiW/UiO-66 had excellent catalytic activity in the esterification of oleic acid with methanol, higher than that of ZrSiW/Fe-BTC catalyst, owing to the fact that ZrSiW/UiO-66 nano-hybrids have high acidity, large specific surface area and pore volumes, and relatively large average pore sizes. Furthermore, the reusability and stability of nano-hybrids were also evaluated, and it was found that the stability of ZrSiW/UiO-66 catalyst was better.

A novel polyoxovanadate-based Co-MOF: highly efficient and selective oxidation of a mustard gas simulant by two-site synergetic catalysis

Abstract: Two novel polyoxovanadate-based metal–organic frameworks (MOFs), [Co(bib)]{V2O6} (V–Co-MOF) and [Ni(en)(bib)]{V2O6}·2H2O (V–Ni-MOF) (bib = 1,4-bis(1H-imidazoly-1-yl)benzene, en = ethylenediamine) were facilely synthesized under mild hydrothermal conditions. Single-crystal X-ray diffraction analysis shows that the V sites in both compounds adopt {VO4} tetrahedral coordination geometries, and the Co center in the V–Co-MOF presents a four-coordinated distorted tetrahedron configuration (coordinatively unsaturated metal sites, CUMS), while the Ni center in the V–Ni-MOF exhibits six-coordinated octahedral geometry (coordinatively saturated metal sites, CSMS). Given that the CUMS can generally be used as active sites for catalytic reactions, we explored the catalytic activities of these two compounds for the oxidation of a mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES). The experimental results indicate that they can catalyze the oxidation of CEES to give the only nontoxic product, 2-chloroethyl ethyl sulfoxide (CEESO). Significantly, the V–Co-MOF exhibits higher catalytic activity; it converts 100% of CEES in 10 min, whereas V–Ni-MOF converts only 47.5% of CEES under identical conditions. Researching the mechanism of the catalytic reaction revealed that the excellent catalytic performance of the V–Co-MOF was attributed to the two-site synergetic effect: (1) the oxidant H2O2 interacts with the V site to produce peroxovanadium with higher oxidation activity; (2) the S atom in CEES coordinates with the four-coordinated Co(II) center to obtain 2-chloroethyl ethyl sulfonium cation (CEES+), which makes the CEES more easily oxidize to CEESO based on the oxidation mechanism of peroxovanadium and shortens the molecular size distance between CEES and the obtained peroxovanadium, thereby greatly improving the rate of the catalytic reaction. To our knowledge, this is the first dual-active-site polyoxometalate-based MOF catalyst for catalysing the oxidative detoxification of CEES.

An Ultrastable, Small {Ag 7 } 5+ Nanocluster within a Triangular Hollow Polyoxometalate Framework

Abstract: Small Agn nanoclusters (n<10) have been emerging as promising materials as sensing, biolabeling, and catalysis because of their unique electronic states and optical properties. However, studying synthesis, structure determination, and exploration of their properties remain major challenges as a result of the low stability of small Ag nanoclusters. Herein, we synthesized an atomically precise face-centered-cubic-type small {Ag7 }5+ nanocluster supported by a novel triangular hollow polyoxometalate (POM) framework [Si3 W27 O96 ]18- . The cluster showed unique {Ag7 }5+ -to-POM charge transfer bands in both visible and UV light regions. Furthermore, this small {Ag7 }5+ nanocluster exhibited an unprecedented ultrastability in solution, despite having exposed Ag sites that can be accessed by small molecules, such as O2 , water, and solvents.

Efficient Electrocatalytic Water Oxidation by Fe(salen)-MOF Composite: Effect of Modified Microenvironment.

Abstract: An efficient and robust heterogeneous electrocatalyst, FSWZ-8 ((Fe-(salen)(OH) + H4[SiW12O40]·HCl)@ZIF-8) for oxygen evolution reaction (OER) at the neutral pH, was developed by coencapsulation of Fe-salen (i.e., Fe(salen)Cl) and SiW12 (i.e., H4[SiW12O40]) inside the cavity of zeolitic imidazolate framework-8 (ZIF-8) material by an in situ synthesis. Here ZIF-8 functions as a host, Fe-salen as the active catalyst, and SiW12 helps in the charge transport by lowering the overall electrical resistance of the resulting composite system. High turnover frequency (∼5 s-1) and high Faradaic efficiency (∼94%) make the concerned composite an efficient catalyst toward water oxidation. This is the first report of one of the simplest known metal complexes, Fe-salen, to perfrom electrocatalytic OER as a heterogeneous catalyst in the neutral pH. This work also highlights the benefits of coencapsulation of the Keggin polyoxometalate (POM) along with the active catalyst Fe-salen species. Encapsulation of SiW12 results in (i) faster formation of FSWZ-8 composite, (ii) higher loading of Fe-salen, and, most importantly, (iii) lowering of required overpotential for electrochemical OER by more than 150 mV. The Keggin POMs, located as discrete molecular oxides inside the cavity of ZIF-8 as well as on the surface of ZIF-8, facilitate electrical charge conduction in the ZIF-8 matrix and lower the overall charge-transfer resistance.

Anderson-type polyoxometalate-based hybrid with high flame retardant efficiency for the preparation of multifunctional epoxy resin nanocomposites

Abstract: Transition metal oxides based catalytic carbonization have been proved to be a promising way to enhance polymer flame retardancy, but finding better way to introduce organic phosphorus and transition metal oxide hybrids into polymer matrix to form nanocomposites is still a great challenge. In this work, an organic phosphorus and metal oxide hybrid compound named P–MnMo6 was prepared through combining 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) with polyoxometalate (POM). The as-prepared P–MnMo6 had a center of [MnMo6O18]3- and was used as the precursor of transition metal oxides to improve the flame retardancy of epoxy resin (EP). With the aid of the organic modification, the addition of P–MnMo6 not only maintained the transparency of EP but also enhanced the dynamic mechanical properties. Notably, the thermogravimetric analysis showed that the char yield of EP/P–MnMo6 composite increased significantly. Besides, epoxy with 8 wt% addition of P–MnMo6 passed the vertical burning grade (UL-94) V-0 rating with the limiting oxygen index (LOI) value of 30.4%. Cone calorimeter test indicated the peak heat release rate of EP/P–MnMo6-4 decreased by 41%. Meanwhile, smoke products and CO were greatly suppressed. In addition, the potential flame retardancy mechanism of P–MnMo6 in epoxy matrix was proposed.

Chelation-Assisted Selective Etching Constructionof Hierarchical Polyoxometalate-Based Metal–Organic Framework

Abstract: A chelation-assisted selective etching (CASE) strategy is developed to construct a hierarchical polyoxometalate-based metal–organic framework (POM@MOF). The selected chelator, ethylenediaminetetraa...

Visible‐Light Photochemical Reduction of CO2 to CO Coupled to Hydrocarbon Dehydrogenation

Abstract: Research on the photochemical reduction of CO2 , initiated already 40 years ago, has with few exceptions been performed by using amines as sacrificial reductants. Hydrocarbons are high-volume chemicals whose dehydrogenation is of interest, so the coupling of a CO2 photoreduction to a hydrocarbon-photodehydrogenation reaction seems a worthwhile concept to explore. A three-component construct was prepared including graphitic carbon nitride (g-CN) as a visible-light photoactive semiconductor, a polyoxometalate (POM) that functions as an electron acceptor to improve hole-electron charge separation, and an electron donor to a rhenium-based CO2 reduction catalyst. Upon photoactivation of g-CN, a cascade is initiated by dehydrogenation of hydrocarbons coupled to the reduction of the polyoxometalate. Visible-light photoexcitation of the reduced polyoxometalate enables electron transfer to the rhenium-based catalyst active for the selective reduction of CO2 to CO. The construct was characterized by zeta potential, IR spectroscopy, thermogravimetry, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). An experimental Z-scheme diagram is presented based on electrochemical measurements and UV/Vis spectroscopy. The conceptual advance should promote study into more active systems.

Polyoxometalate-pillared metal–organic frameworks synthesized by surfactant-assisted strategy and incorporated with carbon nanotubes for energy storage

Abstract: The application of surfactant or surfactant-assisted media to grow polyoxometalate (POM)-based MOFs (POMOFs) is extremely scarce. In this work, for the first time, we use water-polyethylene glycol (PEG) 400 mixed media to synthesize two new crystalline POM-pillared three-dimensional porous arrays, [Co7(Trz)12(H2O)8][HPX12O40(VO)2]·12H2O (X = Mo for Co-PMo and X = W for Co-PW, Trz = 1,2,4-triazole). By employing a simple sonication-driven functionalization strategy, Co-PMo/CNTs (PCNT-n) nanocomposites with increased effective sites, enhanced specific surface area and excellent conductivity are fabricated. The resulting PCNT-2 nanocomposite shows an excellent reversible capacity of 820 mA h g−1 after 100 cycles and outstanding rate capability as an anode for lithium batteries. After the initial decay, the PCNT-2 electrode can bear a current density of 2000 mA g−1 over 450 cycles without obvious capacity loss. Meanwhile, the mechanism of lithium storage in PCNT-2 is discussed using X-ray photoelectron spectroscopy (XPS). Moreover, the PCNT-2 nanocomposite exhibits the hybrid behavior of battery and supercapacitor according to the electrochemical kinetic analyses. This might open new avenues for the design of crystalline POM-based materials or their nanocomposites to achieve high-performance energy storage.

Synthesis, Assembly, and Sizing of Neutral, Lanthanide Substituted Molybdenum Blue Wheels {Mo90Ln10}.

Abstract: Polyoxometalate Molybdenum blue (MBs) complexes typically exist as discrete multianionic clusters and are composed of repeating Mo building units. MB Wheels, such as {Mo176} and {Mo154} are made fr...

Various Anderson-type polyoxometalate-based metal–organic complexes induced by diverse solvents: assembly, structures and selective adsorption for organic dyes

Abstract: Six new Anderson-type polyoxometalate (POM)-based metal-organic complexes (MOCs), namely, {HCu(HPCAP)2[CrMo6(OH)6O18]}·2H2O (1), {Zn4(PCAP)2[CrMo6(OH)6O18]2(H2O)12}·4H2O (2), {Zn3(PCAP)2[CrMo6(OH)5O19](H2O)6}·6H2O (3), {Ni3(PCAP)2[NiMo6(OH)5O19](H2O)6}·8H2O (4), {Cu3(PCAP)2[AlMo6(OH)5O19](H2O)6}·6H2O (5), and {Co3(HPCAP)2[AlMo6(OH)6O18](H2O)10}[AlMo6(OH)6O18]·6H2O (6) (HPCAP = 3-(2-pyridinecarboxylic acid amido)pyridine), were constructed from diverse metal ions (CuII/ZnII/NiII/CoII), the HPCAP ligand and various Anderson-type POM anions under hydrothermal and solvothermal conditions and structurally characterized by single-crystal X-ray diffraction, IR spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). Crystal analysis shows that complex 1 has a 3D supramolecular structure based on the CrMo6 anions and the 1D metal-organic ribbon chains [Cu(HPCAP)2] via hydrogen bonds. Complex 2 is a 2D supramolecular layer composed of two kinds of dinuclear metal clusters of [Zn2(CrMo6)2] and [Zn2(PCAP)2]. Complexes 3-5 show similar 3D frameworks. In 3-5, [M1Mo6]3- (M1 = Cr for 3, Ni for 4, and Al for 5) anions are bridged by MII ions (M = Zn for 3, Ni for 4, and Cu for 5) to give a 1D M-M1Mo6 inorganic chain; the adjacent 1D inorganic chains are connected through two directions of [M2(PACP)2] rings to form a 3D framework. It should be noted that in complex 4, the NiMo6 anions are in situ generated from the NiII ion and Mo7O246- anions, which is rarely observed in POM-based MOCs. Complex 6 shows a 1D ladder-like supramolecular double chain, which is composed of 1D Co-AlMo6 inorganic chains, [Co2(HPCAP)2] dual-core rings and discrete AlMo6 polyanions interlinked by hydrogen bonding interactions. These structural diversities show that solvents play key roles in the construction of various architectures. All of the title complexes have excellent adsorption activities for the cationic dyes methylene blue (MB) and gentian violet (GV); in particular, complex 5 is capable of selectively adsorbing MB and GV from the mixtures of MB and MO and GV and MO, respectively. In addition, the electrochemical properties of complexes 1, 4, and 5 are also studied.