Showing papers on "Tungstate published in 2005"

Modified zirconia solid acid catalysts for organic synthesis and transformations

Abstract: A series of sulfate, molybdate and tungstate promoted ZrO 2 catalysts were prepared by wet impregnation method. To incorporate these promoters to Zr(OH) 4 , sulfuric acid, ammonium heptamolybdate, and ammonium metatungstate were used as precursors, respectively. Further, a Pt promoted Mo-ZrO 2 catalyst was also prepared separately by impregnating with hexachloroplatinic acid. The surface and bulk properties of various promoted ZrO 2 catalysts were investigated by means of X-ray powder diffraction, BET surface area, ammonia-TPD, and Raman spectroscopy techniques. The unpromoted ZrO 2 when calcined at 873 K exists in the crystalline form with monoclinic phase dominating over the tetragonal phase. Incorporation of various promoters into Zr(OH) 4 shows a strong influence on the bulk and the surface properties. Addition of promoters enhanced the tetragonal zirconia phase and the surface acidity. In the case of Pt/Mo-ZrO 2 catalyst, a complete tetragonal ZrO 2 phase is observed. The ammonia-TPD results indicate that the impregnated sulfate ions show a strong influence on the acidity of ZrO 2 , which is followed by molybdate. The prepared catalysts were evaluated for various organic synthesis and transformation reactions in the liquid phase. All catalysts exhibit good catalytic activity for synthesis of diphenylureas, coumarines and 1,5-benzodiazepines, acylation of alcohols, phenols and amines, and protection of carbonyl compounds. In particular, the sulfate and molybdate promoted catalysts exhibited excellent catalytic activity.

Surface characterization and catalytic activity of sulfate-, molybdate- and tungstate-promoted Al2O3-ZrO2 solid acid catalysts

Abstract: Alumina–zirconia mixed oxide-supported sulfate-, molybdate- and tungstate-promoted solid acid catalysts were synthesized and characterized by various techniques. The Al–Zr hydroxide gel was obtained by a co-precipitation method from their corresponding nitrate salts by hydrolysis with aqueous ammonia. To the Al–Zr-hydrous mixed oxide support, aqueous solutions of sulfuric acid, ammonium heptamolybdate and ammonium metatungstate were added and the mixtures were refluxed at 110 °C, followed by evaporation of the water, drying and calcination at 650–750 °C. The surface and bulk properties of the catalysts were examined by using X-ray diffraction, BET surface area, TGA/DTA, ammonia-TPD and XPS techniques. The XRD results reveal that Al2O3–ZrO2 mixed oxide calcined at 650 °C is in amorphous or poorly crystalline state exhibiting broad diffraction lines due to tetragonal ZrO2 phase. At 750 °C calcination, a better crystallization of the zirconia tetragonal phase is observed. The XRD results further indicate that incorporation of alumina into zirconia stabilizes the tetragonal phase. The TGA–DTA studies provide information that there are at least two types of sulfate species with different thermal stabilities in the case of SO42−/Al2O3–ZrO2 catalyst. The TPD study reveals that the SO42−/Al2O3–ZrO2 catalyst exhibits an ammonia desorption peak maximum at 600 °C indicating super-acidic nature of the catalyst. The XPS peak intensities and the corresponding binding energies indicate that sulfate ion interacts with the support more strongly than other promoters. All characterization results provide information that the impregnated sulfate ions show a relatively strong influence on the physicochemical properties of the Al2O3–ZrO2 mixed oxide. The prepared catalysts were evaluated for acetylation of alcohols and amines with acetic anhydride in the liquid phase. In line with physicochemical characteristics, the SO42−/Al2O3–ZrO2 exhibits better product yields under very mild reaction conditions.

On the active site in H3PW12O40/SiO2 catalysts for fine chemical synthesis

Abstract: The surface environment and structural evolution of silica supported phosphotungstic acid (H3PW12O40) catalysts have been investigated as a function of acid loading. H3PW12O40 clusters are deposited intact upon the silica surface, adopting a Stranksi-Krastanov growth mode forming a two-dimensional adlayer which saturates at 45wt% acid. Intimate contact with the silica support perturbs the local chemical environment of three tungstate centres, which become inequivalent with those in the remaining cluster, suggesting an adsorption mode involving three terminal W==O groups. Above the monolayer, H3PW12O40 clusters form three-dimensional crystallites with physico-chemical properties indistinguishable from those in the bulk heteropoly acid. These H3PW12O40/SiO2 materials are efficient for the solventless isomerisation of α-pinene under mild reaction conditions. Activity scales directly with the number of accessible perturbed tungstate sites at the silica interface; these are the active species.

New tungstate fluorophosphate glasses

Abstract: A new class of tungstate fluorophosphate glasses was identified in the NaPO 3 –BaF 2 –WO 3 ternary system. The variation of several physical properties was determined with respect to chemical composition. Characteristic temperatures, density and refractive index increase as tungsten oxide content increases. The optical transmission range and specially the energy bandgap depend of the WO 3 amount. No crystallization could be observed for the most WO 3 concentrated vitreous samples (⩾20% molar). Color and optical properties of the glasses depend of the melting time because of the presence of reduced tungsten species like W 5+ and W 4+ . In addition, photodarkening is observed in tungsten rich glass samples under UV laser illumination and this phenomenon can be reversible by heat treatment near the glass transition temperature.

Ferroelectric and Ferroelastic Phase Transitions in Molybdates and Tungstates of Monovalent and Bivalent Elements

Abstract: Ferroelectric and ferroelastic phase transitions in molybdates and tungstates of monovalent and bivalent elements are reviewed. The nature and peculiarities of these transitions are briefly discussed. Most of interesting phase transitions are observed in compounds, containing monovalent ions, while among compounds of bivalent elements, only perovakites have properties of interest. Some molybdates and tungstates under consideration are improper ferroelectrics and ferroelastics, some of them have non-reversible, but reorienting, spontaneous polarization, some have modulated phases. There are indications on possibility of the ferroelectric state formation as a result of a reconstructive phase transition. Some crystal chemical problems are briefly discussed.

Binary Molybdates and Tungstates of Monoand Trivalent Elements as Possible Ferroelastics and Ferroelectrics

Abstract: Phase transitions in binary molybdates and tungstates of mono- and trivalent elements, as well as some of their physical properties are reviewed with a special attention to ferroelastic and ferroelectric behavior. There are many compounds with phase transitions of the displacement type and reconstructive ones. The author tried to pick out only compounds with the displacement-type transitions. This allowed the identification of many compounds interesting for physics of ferroelasticity and ferroelectricity among the structure families of scheelite, glaserite, palmierite, wolframite etc. So far, they have been studied in the main (better or worse) by chemists. Now physisists have to study them.

Local order around tungsten atoms in tungstate fluorophosphate glasses by X-ray absorption spectroscopy

Abstract: X-ray absorption spectroscopy was used to study the local environment of tungsten atoms in NaPO 3 –BaF 2 –WO 3 glasses and the results were compared with crystalline references Na 2 WO 4 and WO 3 . XANES measurements at the W-L 1 edge allowed to determine a distorted octahedral environment of tungsten atoms in these glasses similar to the local order of tungsten in monoclinic WO 3 . Extended X-ray absorption fine structure (EXAFS) has been used as a local probe to monitor the effect of WO 3 concentration on the tungsten environment. Based on an analysis of the EXAFS data, we proposed a three-shell model of oxygen atoms around tungsten as in monoclinic WO 3 . With increasing WO 3 concentration, it was found that R 2 decreases from 1.96 to 1.92 A whereas R 3 increases from 2.07 to 2.12 A.

Measurements of Raman gain coefficient for barium tungstate crystal

Abstract: The Raman gain coefficient of a barium tungstate crystal was measured in the 532 nm to 1064 nm spectral region. The experimentally obtained data were fitted with the empirical equation, which takes into account the resonance electronic transitions.

Influence of cations on the vibrational spectra and structure of [WO4] complexes in molten tungstates

Abstract: The vibrational spectra and structure of isolated [WO4]2− anions in molten alkali, alkaline-earth, rare-earth, Pb, Bi, Zn, Sc, and Cd tungstates are studied by high-temperature Raman spectroscopy. The degenerate modes of the [WO4]2− anion in K-Gd tungstate melts are found to be split, which is interpreted in terms of the symmetry of the cation environment of the [WO4]2− anion in the melts. The frequency of the fully symmetric stretching mode (ν1) of the [WO4]2− group in molten tungstates is examined in relation to the polarizing power P and electronegativity γ of the cation. The origin of the nonmonotonic variation of ν1 with P and γ for the tungstate melts studied is discussed.

Synthesis and characterization of nanophased silver tungstate

Abstract: Silver tungstate (Ag2WO4) nanoparticles in two different morphologies are prepared by controlling the reaction kinetics of aqueous precipitation. X-ray diffraction studies reveal that the silver tungstate nanoparticles are in the α-phase. SEM images show the rod-like and fiber-like morphologies of the nanoparticles with high aspect ratios. The TGA and DTA studies show the high thermal stability of the nanorods. The average crystallite sizes (20–30 nm) of the rod-like silver tungstate estimated from TEM is consistent with the XRD results

Tungstate polyoxometalates as active components of molecular devices

Abstract: Polyoxometalates are a class of well-defined metal oxygen clusters mostly known for their catalytic properties. However, their electronic and optical properties have been used in device applications such as electrochromic displays, dopants for conductive polymers, gas and chemical sensors, capacitors, and electrochemical cells. We fabricate nanodevices based on a composite poly(methyl methacrylate) H3SiW12O40 system and we investigate the effects of electrode material, electrode distance, and molecular concentration on the electronic transport characteristics. It is found that in the case of electrode distances smaller than 50 nm, tunneling effects appear, which are discussed using tunneling theory models. These effects are primarily dependent on the electrode distance and molecular concentration, and less on the electrode material.

Isotropic zero CTE reinforced composite materials

Abstract: A reinforced composite material, having isotropic thermal expansion properties and a low coefficient of thermal expansion over at least the temperature range of from about 0° C. to at least about 150° C., which composite material comprises in combination a first continuous phase comprising a three dimensional preformed bonded powder material reinforcement, including a bonding agent, and in which the bonded powder material is chosen from the group consisting of zirconium tungstate, hafnium tungstate, zirconium hafnium tungstate, and mixtures of zirconium tungstate and hafnium tungstate, and a second continuous phase matrix material chosen from the group consisting of aluminium, aluminium alloys in which aluminium is the major component, magnesium, magnesium alloys in which magnesium is the major component, titanium, titanium alloys in which titanium is the major component, engineering thermoplastics and engineering thermoplastics containing a conventional solid filler.

Theoretical investigation of the luminescence centres in PbWO4 and CdWO4 crystals

Abstract: Electronic structures of possible centres of luminescence in the perfect lead (PbWO4) and cadmium tungstate (CdWO4) crystals are calculated using cluster approach. Electronic states of Pb2+, and Cd2+ cations, WO 4 2 - and WO 6 6 - are obtained on the basis of partial densities of electronic states calculated for several clusters containing from 72 to 104 atoms of the crystals. Calculated energies of one-electron transitions in these centers are compared with experimental data on luminescence and luminescence excitation. Obtained results evidence in favor of a tungstate nature of intrinsic luminescence in the PbWO4 and CdWO4 crystals . I ntrinsic luminescence probably originates from the tungstate groups WO 4 2 - in scheelite-type, and from the WO 6 6 - groups in the raspite-type PbWO4 crystals. Tungstate groups WO 6 6 - are assumed to be the centres of excitation of blue–green and yellow luminescence bands in CdWO4 crystals.

Calculations of the electronic transition energies in the system of luminescence centers of lead, cadmium and zinc tungstate crystals

Abstract: The electronic structures of lead, cadmium and zinc tungstates are ab-initio calculated in molecular cluster approach. Ground and excited electronic states of luminescence centers of zinc tungstate crystal ZnWO4 which include Zn2+ cations and WO66– tungstate groups are evaluated. Calculations confirm the assumption that optical properties of ZnWO4 crystals in the spectral region of fundamental absorption edge are determined by electronic transitions in tungstate groups. Calculated electronic structures of the scheelite-type PbWO4, the wolframite-type PbWO4, CdWO4, and ZnWO4 crystals are compared. It is shown that the domination of the electronic transitions in tungstate groups in the region of fundamental absorption edge is the common feature of the investigated wolframite-type crystals. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Catalysts containing tungstate for the synthesis of alkylmercaptane and method for the production thereof

Abstract: The invention relates to a catalyst containing alkali tungstate for the synthesis of alkylmercaptanes from alkanols and hydrogen sulphide, in addition to a method for the production of said catalyst, wherein the molar ratio of alkali to tungstan is < 2:1.

Predicted and real habits of flux grown potassium lutetium tungstate single crystals

Abstract: The laser host crystals of KLu(WO4)2(KLuW) with large dimensions up to 43 × 35 × 16 mm3 have been grown along b, a*, c, and [111] crystallographic directions, respectively, from the melt by the to...

Solid state features of electrocrystallized tungstate films

Abstract: X-ray diffraction, scanning electron microscopy, and infra-red spectroscopy are applied to study the evolution of films electrodeposited from acidic tungstate solutions. Structural inhomogeneity is found to be responsible for the difference in rechargeability of films of different thickness. Voltammetric responses demonstrate pronounced sensitivity to the nature of crystalline phases, thus throwing light on the defects of the lattice features of nonstoichiometric W(V)–W(VI) oxides. One crystalline phase observed in the films under study and attributed to the layered nonstoichiometric oxohydroxide was never reported for oxotungstate films fabricated by other techniques. This phase is believed to be special to electrocrystallized films and to keep some structural features of dissolved isopolytungstate molecular precursors.