Tungsten vanadates

Metallic taste, 11 565 Metallic tungsten, 25 374 Metallic Type II superconductors, critical current density value in, 23 822 Metallic vanadates, 25 513 Metalliding, 15 251 Metalliferous oxides deposits of, 17 689-690 in ocean basins, 17 693 Metalliferous sulfide deposits, 17 690-691 Metalliferous sulfides, in ocean basins, 17 693-694... 

A general method for the separation of vanadium from arsenic, molybdenum, phosphorus, chromium, uranium, tungsten, and silicon, consists in precipitating these metals as their respective lead salts and digesting the precipitate with potassium carbonate, whereupon all the lead salts are decomposed with the exception of the lead vanadate.5... 

Point defects in the form of cation vacancies () were introduced by Aykan et al. (93-95) into molybdates, tungstates, and vanadates with scheelite-type crystal structures. The authors studied the catalytic properties of more than 30 scheelite-structure phases represented by the formula A1 x< xM04 (M = molybdenum, tungsten, and/or vanadium and A may include Li, Na, K, Ag, Ca, Sr, Ba, Cd, Pb, Bi, and/or arare earth element in quantities appropriate to achieve charge balance for the normal oxidation states). It was found that the defects can be introduced... 

Materials with significant mobility of di- and trivalent cations (Mg2+, Ca2+, Sr2+, Cd2+, Zn2+, Al3+, Sc3+, Ho3+, Y3+) the family of /3-alumina, M2+Zr4(P04)6, several vanadates and tungstenates of the corresponding metals such as Al2(W04)3, and their derivatives. In this case, however, the information available in literature is often scarce and additional studies of the ionic conduction mechanisms are often required. 

Bismuth vanadate can be produced by either a wet process or high-temperature calcination. By the first method, an acidic solution of bismuth nitrate, Bi(N03)3. is mixed with an alkaline solution of sodium vanadate, Na3V04. The resultant gel is filtered, washed, and converted to crystalline form by low-temperature calcination at 200-500°C. Multiple-phase pigments can be created by coprecipitation of bismuth vanadate with molybdenum, tungsten, or niobium compounds. 

Surface Vanadate, Molybdate and Tungstate Species The pure and mixed oxides and the salts of vanadium, molybdenum and tungsten in their higher oxidation states are used widely as heterogeneous catalysts, for selective oxidation as well as for acid catalysis. Similarly, supported chromia and rhenium oxides find wide application in different catalytic processes. 

Cr(II) may be used to carry out all the reactions of Ti(III), but usually under milder conditions. Applications of Cr(II) as a reductant have been reviewed. The applications include Sn(IV) chloride in the presence of catalysts such as Sb(V) or Bi(III), Sb(V) in 20% HCl at elevated temperatures, Cu(II), silver, gold, mercury, bismuth, iron, cobalt, molybdenum, tungsten, uranium, dichromate, vanadate, titanium, thallium, hydrogen peroxide, oxygen in water and gases, as well as organic compounds such as azo, nitro, and nitroso compounds and quinones. Excess Cr(II) in sulfuric acid solution reduces nitrate to ammonium ion. The reduction is catalyzed by Ti(IV), which is rapidly reduced to Ti(III). 

Transition metal catalysts not only increase the reaction rate but may also affect the outcome of the oxidation, especially the stereochemistry of the products. Whereas hydrogen peroxide alone in acetonitrile oxidizes alkenes to epoxides [729], osmic acid catalyzes syn hydroxylation [736], and tungstic acid catalyzes anti hydroxylation [737]. The most frequently used catalysts are titanium trichloride [732], vanadium pentoxide [733,134], sodium vanadate [735], selenium dioxide [725], chromium trioxide [134], ammonium molybdate [736], tungsten trioxide [737], tungstic acid [737],... 

If the trace element and the collector have opposite chemical properties (acidic, basic), the co-precipitation may be the result of formation of chemical compounds. For example, traces of germanium or vanadium form germanates or vanadates in co-precipitation with Fe(III)-, A1-, or La- hydroxides, while traces of tungsten or molybdenum, on co-precipitation with Fe(III) hydroxide, form the corresponding Fe(III) tungstate or molybdate. 

Although little experimental data is available, numerous patents have been issued for the vapor phase catalytic oxidation of various other derivatives containing the benzene nucleus, as well as heterocyclic compounds Thus, fluorene (diphenyl methane) is oxidized to fluorenone with air in the presence of a catalyst containing iron vanadate or other suitable metal salt of the fifth or sixth group of the periodic system at a temperature of 360° to 400°.1,2 Maleic acid and anhydride are formed by the catalytic oxidation of compounds of the furan series, such as furan, furfural alcohol, furfural, methyl furfural, hydroxymethylfurfural, pyromucic acid or mixtures, with air over catalysts of molybdenum, vanadium, or other metals.133 Dimethyl benzaldehyde is formed by oxidizing pseudocumene with air at 550° C. in the presence of a tungsten oxide catalyst. Molybdenum, vanadium, or tantalum oxide catalysts may also be used to form aromatic aldehydes from o-, m-, or p-xylenes, mesitylene, p-cymene, or o-chlorotoluene by air oxidation. Times of contact of 0.3 to 0.4 seconds... 

Chromiimi(lI) and titanium(II[I) are very powerful reducing agents, but they are readily air-oxidized and difficult to handle. The standard potential of the former is -0.41 V (Cr3+/Cr2+) and that of the latter is 0.04 V (TiO"+/Ti +). The oxidized forms of copper, iron, silver, gold, bismuth, uranium, tungsten, and other metals have been titrated with chromium(II). The principal use of Ti " " is in the titration of iron(in) as well as copper(II), tin(IV), chromate, vanadate, and chlorate. 

Luckey et al. (1975) indicated that comparison of the toxicity of metal compounds is more useful vhen the LD50 value is expressed in mmol kg instead of the usual mg kg k Metals with small differences in atomic mass can show large variances in specific gravity, which influences the toxicity. For example, tungsten and metavanadate are equally toxic if the LD50 is expressed in mmol kg but vanadate is three-fold... 

The work was strongly inspired by Union Carbide s Ethoxene process, a route for manufacturing ethylene from ethane and oxygen by oxidative dehydrogenation. The first catalysts consisted of molybdenum, vanadium, and niobium oxides. The selectivity for ethylene was very high but, unfortunately, the conversion of ethane was low ( 10%). Therefore, scientists at the time focused on the co-production of ethylene and acetic acid. A catalyst consisting of molybdenum, vanadium, niobium, calcium, and antimony supported on a molecular sieve was developed (63% selectivity to acetic acid, 14% selectivity to ethylene, and 3% conversion of ethane). In addition, Rhone-Poulenc (catalyst vanadium oxide or vanadyl pyrophosphate) and BP (catalyst combination of rhenium and tungsten) patented processes for the production of acetic acid from ethane. Very efficient catalysts were also disclosed by Hoechst (molybdenum vanadate, promoted with Nb, Sb, Ca, and Pd, 250-280 °C, 15 bar, 86% selectivity to acetic add at 11% conversion of ethane per pass) and Sabic (phosphorus-modified molybdenum-niobium vanadate, 260 °C, 14 bar, 50% selectivity to acetic acid at 53% conversion of ethane). 

Analogous reactions may be assumed for the ammonium tungstates and vanadates. Therefore, all of these cases involve catalytic effects stemming from the formation, action and regeneration of molybdenum, tungsten or vanadium percompounds. 

Solutions of alkali molybdate or vanadate containing 2000 y Mo or 5(X) y V per drop do not react in the same manner as tungstate however even smaller amounts of these metallo acids prevent or impair the tungsten-oxine reaction. Nevertheless, the following procedure will reveal 100 y tungsten in the presence of 500 y molybdenum. 

Tungstate Vanadate acid Compare tungsten Compare vanadium (4) Compounds 0.1 455... 

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