Tungsten-niobium oxides

The Structural Chemistry of Bismuth-Tungsten-Molybdenum Oxides and Bismuth-Tungsten-Niobium Oxides... 

Bis(cyclopentadienyl)bis(pentafluorophenyl)zirconium, 3824 Bis(cyclopentadienyldinitrosylchromium), 3269 Bis(cyclopentadienyl)hexafluoro-2-butynechromium, 3629 Bis(cyclopentadienyl)lead, 3288 Bis(cyclopentadienyl)magnesium, 3271 Bis(cyclopentadienyl)manganese, 3272 Bis(cyclopentadienyl)niobium tetrahydroborate, 3318 Bis(cyclopentadienyl)pentafluorophenylzirconium hydroxide, 3696 Bis(cyclopentadienyl)phenylvanadium, 3698 Bis(cyclopentadienyl)titanium selenate, 3287 Bis(cyclopentadienyl)tungsten diazide oxide, 3279 Bis(cyclopentadienyl)vanadium diazide, 3280 Bis(cyclopentadienyl)zirconium, 3290 Bis(l,2-diaminoethane)diaquacobalt(III) perchlorate, 1787 Bis(l,2-diaminoethane)dichlorocobalt(III) chlorate, 1780 Bis(l,2-diaminoethane)dichlorocobalt(III) perchlorate, 1781 c/i-Bisf 1,2-diaminoethane)dinitrocobalt(III) iodate, 1782 Bis(l,2-diaminoethane)dinitrocobalt(III) perchlorate, 1778 Bis(l,2-diaminoethane)hydroxooxorhenium(V) perchlorate, 1785 Bis(l, 2-diaminopropane)-ds-dichlorochromium(III) perchlorate, 2609 1,10-Bis(diazonio)decaboran(8)ate, 0197... 

Tungsten(VI) oxide tetrachloride is a common impurity in tungsten(VI) chloride. By anaiogy with niobium(V) oxide trichloride (Section 9.2.2), phosgene might have been expected to convert WOCi to WClj, according to ... 

Some metal oxide catalysts are activated by thermal reduction with hydrogen or carbon monoxide. For example, the catalytic activity of molybdenum oxide and tungsten oxide for the metathesis reaction of olefins is very much enhanced by their slight reduction (1). The catalytic activity for butene isomerization and ethene oligomerization appears on niobium oxide by its... 

Cl4Nb02CgH,g, Niobium, tetrachlorobis-(tetrahydrofuran)-, 29 120 CI4OW, Tungsten tetrachloride oxide,... 

Alloying iron with nickel and chromium is necessary to mitigate corrosion above 900 °C. At higher temperatures, 2% A1 addition increases the alloy oxidation resistance [19,20]. The oxidation rate of austenitic stainless steels (Types 309 and 310) decreases with the addition of nickel in the alloy, while the addition of tungsten, niobium, tantalum, and molybdenum increase mechanical strength, as shown in Fig. 11.7 [3]. 

A timeline for the development of olefin metathesis, adapted from a review by Grubbs, is shown in Figure 21.3. Olefin metathesis is more than 50 years old. " It was first conducted with ill-defined rhenium, molybdenum, and tungsten systems generated from perrhenate, aluminum oxide, - and tetraethyl lead as additive, from molybdenum oxide on p-TiO and tetramethyltin as additive, ° or from tungsten phenoxides supported on niobium oxide and silicon oxide activated with alkylaluminum reagents. The temperatures for these processes are hi, but the catalysts are relatively inexpensive and can be long lived. These are the types of catalysts that have been used for the synthesis of commodity chemicals by olefin metathesis. 

Sheng and Zajacek [182] have shown that vanadium complexes are somewhat superior catalysts to molybdenum. Both give good selectivity to amine oxide but reaction rates are faster when vanadium complexes are used. Compounds of tungsten, niobium and tantalum were poorer catalysts whereas chromium, cobalt, manganese and iron complexes were ineffective. 

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). 

The catalysts of this process are vanadium, molybdenum, tungsten, niobium, chromium, and titanium compounds. The yield of oxide calculated per olefin is close to 100%, and that per hydroperoxide reaches 85—95% and depends on the catalyst, temperature, and depth of conversion. 

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

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