Polyoxometalate

About: Polyoxometalate is a research topic. Over the lifetime, 3493 publications have been published within this topic receiving 94123 citations. The topic is also known as: POM.

Polyoxoanion with Octahedral Germanium(IV) Hetero Atom: Synthesis, Structure, Magnetism, EPR, Electrochemistry and XPS Studies on the Mixed-Valence 14-Vanadogermanate [GeVV12VIV2O40]8−

Abstract: The mixed-valence 14-vanadogermanate [GeVV 12VIV 2O40]8− (1) has been synthesized and characterized in solution by 51V-NMR, UV–vis and electrochemistry and in the solid state by IR, magnetism, EPR, XPS and elemental analysis. Single-crystal X-ray analysis was carried out on K2Na6[GeVV 12VIV 2O40]·10H2O (KNa-1), which crystallizes in the orthorhombic system, space group Immm, with a=10.9623(3) A, b=11.6205(3) A, c=20.2658(5) A, and Z=2. Polyanion 1 is composed of a central GeIVO6 octahedron which is surrounded by a total of 14 VO6 octahedra. Vanadium-51 NMR in solution results in three peaks with intensity ratio of 8:4:2 which is in complete agreement with the solid state structure. The presence of two VIV centers was established by UV–vis, electrochemistry, magnetism, EPR, XPS and elemental analysis. Electrochemistry revealed that the two VIV-centers in 1 are oxidized through a single well-defined step, which does not split with changes in scan rate or pH. Polyanion 1 is also an active two-electron oxidation catalyst for the coenzyme NADH at pH 8, unprecedented in polyoxometalate chemistry. Magnetic susceptibility, magnetization and EPR data on KNa-1 complement the X-ray and electrochemistry results by confirming the presence of two unpaired electrons per molecule of 1. The two VIV ions possessing the spin are very weakly coupled, essentially acting as two well-isolated S=1/2 ions. The observed g-value of 1.977 from EPR and magnetic susceptibility measurements is consistent with literature reported value for a VIV ion, suggesting a possible ground state of $$3d_{x^{2}-y^{2}}.$$ XPS measurements on KNa-1 also confirmed the coexistence of VV and VIV in 1.

Amorphous Cobalt Oxide Nanoparticles as Active Water‐Oxidation Catalysts

Abstract: Artificial photosynthesis requires the practical use of efficient, robust, and economical water-oxidation catalysts (WOCs) for chemical-fuel production. The synthesis of amorphous cobalt oxide nanoparticles (ca. 2nm) is reported as a WOC with a turnover frequency up to 8.6s(-1) in the photocatalytic Ru(bpy)(3)(2+)-Na2S2O8 system (bpy=2,2-bipyridyl). This activity is unprecedented in heterogeneous cobalt-based WOCs and is even comparable to that of a state-of-the-art homogeneous cobalt-based polyoxometalate catalyst. With the help of experimental and computational X-ray absorption spectroscopy, the atomic structure of the synthesized amorphous cobalt oxide nanoparticles was characterized, and it consists of a one-dimensional chain of dimeric edge-sharing CoO6 octahedra. Theoretical calculations suggest that this structure was able to promote O-O bond coupling, unlike crystalline cobalt oxide WOCs, which led to the enhanced water-oxidation activity.

Efficiency of Polyoxometalate-Based Mesoporous Hybrids as Covalently Anchored Catalysts.

Abstract: Polyoxometalate (POM) hybrids have been covalently immobilized through the formation of amide bonds on several types of mesoporous silica. This work allows the comparison of three POM-based mesoporous systems, obtained with three different silica supports in which either the organic functions of the support (amine vs carboxylic acid) and/or the structure of the support itself (SBA-15 vs mesocellular foams (MCF)) were varied. The resulting POM-based mesoporous systems have been studied in particular by high resolution transmission electronic microscopy (HR-TEM) in order to characterize the nanostructuration of the POMs inside the pores/cells of the different materials. We thus have shown that the best distribution and loading in POMs have been reached with SBA-15 functionalized with aminopropyl groups. In this case, the formation of amide bonds in the materials has led to the nonaggregation of the POMs inside the channels of the SBA-15. The catalytic activity of the anchored systems has been evaluated through the epoxidation of cyclooctene and cyclohexene with H2O2 in acetonitrile. The reactivity of the different grafted POMs hybrids has been compared to that in solution (homogeneous conditions). Parallels can be drawn between the distribution of the POMs and the activity of the supported systems. Furthermore, recycling tests together with catalyst filtration experiments during the reaction allowed us to preclude the hypothesis of a significant leaching of the supported catalyst.

Visible-LED-light-driven photocatalytic synthesis of N-heterocycles mediated by a polyoxometalate-containing mesoporous zirconium metal-organic framework

Abstract: A mesoporous metal-organic framework with photothermal properties, namely PCN-222, was solvothermally synthesized from meso-tetra(4-carboxyphenyl)porphyrin and zirconium chloride employing both benzoic acid (BA) and trifluoroacetic acid (TFA) as modifiers. The MOF material subsequently served as a porous support for a polyoxometalate (POM), H3PW12O40, via a facile impregnation method which rendered a novel porous POM@PCN-222 composite. The solid was characterized by FT-IR, PXRD, SEM/EDX, TGA/DSC, ICP-OES, UV–Vis DRS, cyclic voltammetry (CV), and BET surface area. The one-pot synthesis of N-heterocycles (pyridine derivatives) was investigated utilizing the hybrid material via one-pot pseudo four-component reaction between aromatic aldehydes, methyl acetoacetate and ammonium acetate promoted under visible LED light irradiation in the presence of molecular oxygen as green oxidant. Products were selectively formed in good yields in the presence of the recyclable heterogeneous solid. Remarkably, POM@PCN-222 showed a superior performance for this procedure as compared to both unfunctionalized MOF and POM. The photosensitizer and photothermal properties of the porphyrin linkers combined with Lewis acidic sites derived from PW12 and Zr6-nodes were responsible for the observed excelling performance. To understand the mechanism, control investigations, electron paramagnetic resonance (EPR) analysis and FT-IR reaction monitoring were performed. The work discloses, for the first time, a simple and environmentally friendly approach for the direct production of pyridines via one-pot thermo-photocatalytic approach using a novel POM-modified MOF in the absence of any chemical additive.