Showing papers on "Polyoxometalate published in 1998"

Highly Efficient Utilization of Hydrogen Peroxide for Selective Oxygenation of Alkanes Catalyzed by Diiron-Substituted Polyoxometalate Precursor

Abstract: The Keggin-type iron-substituted polyoxometalates have been used as catalysts for the oxygenation of alkanes in homogeneous reaction media using hydrogen peroxide as an oxygen donor. The efficiency and activity for the utilization of hydrogen peroxide greatly depended on the iron centers and diiron-substituted [γ-SiW10{Fe(OH2)}2O38]6- showed the highest efficiency of hydrogen peroxide utilization and conversion. Such a structure dependency of the catalysis is significant, and the remarkable catalytic performance of diiron-substituted polyoxometalate may be related to the catalysis by methane monooxygenase. It is remarkable that the efficiency of hydrogen peroxide utilization to oxygenated products reached up to ca. 100% for the oxygenation of cyclohexane catalyzed by diiron-substituted [γ-SiW10{Fe(OH2)}2O38]6- polyoxometalate. Not only cyclohexane but also n-hexane, n-pentane, and adamantane were catalytically oxygenated with high efficiency of hydrogen peroxide utilization. It was also suggested that dii...

Molecular Oxygen Activation by a Ruthenium-Substituted “Sandwich” Type Polyoxometalate

Abstract: The ruthenium-substituted “sandwich” type polyoxometalate [WZnRuIII2(XW9O34)2]11- where X = ZnII or CoII has been shown to be unique in its ability to catalyze the selective hydroxylation of adaman...

Selective transition-metal catalysis of oxygen delignification using water-soluble salts of polyoxometalate (POM) anions. part II. Reactions of α-[SiVW11O40]5- with phenolic lignin-model compounds

Abstract: When heated under anaerobic conditions, aqueous solutions of the polyoxometalate anion α-(SiVW 11 O 40 ] 5- (POM ox , 1), supplied as the salt K 5 [SiVW 11 O 40 ].12H 2 O, selectively delignify unbleached mixed-pine kraft pulps. During delignification, 1 is reduced hy residual lignin to α-[SiVW 11 O 40 ] 6- (POM red 2) : α-[SiVW 11 O 40 ] 5- + 1e- → α-[SiVW 11 O 40 ]6-, E o = +0.69V vs. NHE. Model studies suggest that phenolic (hydroxylated phenyl) groups present in residual lignin are substrates for the reduction of 1. Reactions of 1 with phenolic arylglycerol β-aryl ethers and other phenolic lignin models suggest that anaerobic POM degradation of residual lignin may occur via sequential single-electron oxidation reactions of phenolic substructures, first to cyclohexadienyl radicals and subsequently to cyclohexadienyl cations. Hydrolysis of these cationic intermediates results in alkyl-side-chain cleavage, a likely route to polymer degradation.

Charge Transfer Salts Based on Polyoxometalates and Seleno-Substituted Organic Donors. Synthesis, Structure, and Magnetic Properties of (BEST)3H[PMo12O40]·CH3CN·CH2Cl2 (BEST = Bis(ethylenediseleno)tetrathiafulvalene)

Abstract: Electrochemical oxidation of the tetrathiafulvalene (TTF) type organic donor bis(ethylenediseleno)tetrathiafulvalene (BEST) in the presence of the Keggin polyoxometalate [PMo12O40]3- affords the radical salt formulated as (BEST)3H[PMo12O40] (crystal data: triclinic, space group P1 with a = 13.056(1) A, b = 13.957(1) A, c = 22.302(3) A, α = 97.019(9)°, β = 94.17(1)°, γ = 95.847(9)°, and Z = 2). This is the first salt of a selenium-containing donor with a polyoxometalate cluster. The structure of this organic/inorganic hybrid consists of layers of the organic donors that alternate with polyoxometalate layers in the c direction. The organic molecules, which are completely ionized, form two types of layers with unprecedented packing arrangements and many intra- and interlayer side-on short contacts which give rise to a marked 3D electronic character. The magnetic susceptibility measurements indicate that the BEST+ radicals are strongly antiferromagnetically coupled and that the polyanion has been reduced by...

Selective Transition-Metal Catalysis of Oxygen Delignification Using Water-Soluble Salts of Polyoxometalate (POM) Anions.

Abstract: (POMred, 2) : α-[SiVW11 O40] 5+ 1e—> α-[SiVW 11 O40] 6-. E0= + 0.69 V vs. NHE, Model studies suggest that phenolic (hydroxylated phenyl) groups present in residual lignin are substrates for the reduction of 1. Reactions of 1 with phenolic arylglycerol β-aryl ethers and other phenolic lignin models suggest that anaerobic POM degradation of residual lignin may occur via sequential single-electron oxidation reactions of phenolic substructures, first to cyclohexodienyl radicals and subsequently to cyclohexadienyl cations. Hydrolysis of these cationic intermediates results in alkyl-side-chain cleavage, a likely route to polymer degradation.

Selective Transition-Metal Catalysis of Oxygen Delignification Using Water-Soluble Salts of Polyoxometalate (POM) Anions. Part I. Chemical Principles and Process Concepts

Abstract: Polymeric metal-oxide anion complexes (polyoxometalates) may provide a means for using transition metals and oxygen (O 2 ) to selectively delignify wood or wood pulp in effluent-free (closed) mills. The development of this chemistry. and associated process concepts, evolved from the observation that certain fungi use transition metals (Cu. Fe and Mn) to selectively transfer electrons from lignin to oxygen. Chemical, physicochemical and engineering issues pertaining to the development of aqueous transition-metal systems for selective delignification, and for mill closure, are outlined in this brief introduction to the title technology.

Electrocatalysis with a Dirhodium-Substituted Polyoxometalate Anchored in a Xerogel-Based Composite: Application to the Oxidation of Methionine and Cystine at Physiological pH

Abstract: Replacing a tungsten oxide group of phosphotungstic acid with a Rh(II) acetate dimer yielded a species, Rh2POM, that catalyzed the electrochemical oxidation of L-methionine (L-Met), L-cystine, and As(III) The electrocatalytic activity of Rh2POM was greater than that of the simple Rh(II) acetate dimer in homogeneous solution A conducting carbon-composite electrode (CCE) comprising Rh2POM and carbon powder in a silica matrix prepared by sol-gel chemistry was catalytically active over the pH range 2–10 The CCEs produced voltammetric responses toward L-Met and L-cystine that were pH independent over the above range and were stable for at least 4 months under dry storage If passivated, the catalytic activity was reproducibly generated by polishing For example, a relative standard deviation of 29 % (5 points) was obtained when the surface was polished between cyclic voltammetric experiments on 50 mM L-Met at pH 74 A least squares fit of flow injection amperometric data obtained over the range 5–156 μM L-Met (7 points) yielded the following: slope, 085 μA mM−1; intercept, 005 μA; and r / 0995 The detection limit was 23 μM

Radical salts of the organic donor BET-TTFwith polyoxometalate clusters

Abstract: The synthesis, structures and physical characterizations of the first radical ion salts of the organic donor BET-TTF1 with polyoxometalate clusters ([M6O19 ]2– ; M=WVI , MoVI , 2) are reported. The 2:1 salts (BET-TTF+)2 [M6O19 ]2– are formed by pairs of the BET-TTF+ radical cations surrounded by polyanions so as to form a 3D packing of anions and cations with short intermolecular contacts between the cation pairs. The synthesis, physical properties and main structural features of the first radical ion salts of the organic donor BET-TTF 1 with the polyoxometalate cluster [SiW12O40 ]4–3 are also reported. The 4:1 salt (BET-TTF+)4 [SiW12O40 ]4– is formed by a 2D hexagonal packing of anions and by isolated BET-TTF+ pairs. The compounds are insulators and diamagnetic.

A process for the epoxidation of alkenes

Abstract: The invention provides a process for the catalytic epoxidation of alkene comprising contacting a transition metal substituted polyoxometalate and molecular oxygen with alkene.

A novel optical charge-transfer complex between an organic substrate and a polyoxometalate, alpha-H6P2W18O62middot phen middot10H2O

Abstract: The new complex alpha-H6P2W18O62middot 6phen middot 10H2O, consisting of the heteropolytungstophosphate α-H6P2W18 O62middotxH2O and organic substrate 1,10-phenanthroline (phen) has been synthesized and spectroscopically characterized. There appears to be a sizeable electron-transfer interaction between the electron-rich aromatic organic moiety and the inorganic acid, both in the solid state and in solution, and this interaction is thought to be responsible for the formation of the title compound, which exhibits second-order and third-order nonlinear optical responses of Isample2 omega=0.6IKDP2 omega and χ(3)=3.35*10-11esu, respectively.

Oxidation of Hydrocarbons with Hydrogen Peroxide Catalyzed by Zinc Containing Polyoxometalate

Abstract: Zinc heteropolyanion, PZnMo2W9O395−, was found to be an efficient catalyst for alkene epoxidation and oxidation of alkylaromatic compounds with 30% H2O2. Epoxide selectivities in the range of 80 to 100% was observed.

Syntheses and IR Spectra of New Charge Transfer Polyoxometalates

Abstract: Eleven new charge transfer polyoxometalates have been synthesized from four heteropoly acids with Dawson structure and three heteroaromatic compounds.IR spectra studies indicate that the Dawson structure of the heteropolyanion has a little change after a charge transfer polyoxometalate was formed,and that charge transfer interaction is present between organic donor and heteropolyanion.

Oxidation of Hydrocarbons with Hydrogen Peroxide Catalyzed by Zinc Containing Polyoxometalate.

Abstract: Zinc heteropolyanion, PZnMo2W9O395−, was found to be an efficient catalyst for alkene epoxidation and oxidation of alkylaromatic compounds with 30% H2O2. Epoxide selectivities in the range of 80 to 100% was observed.