Tellurium compounds catalyst

Although gravimetric methods have been used traditionally for the determination of large amounts of tellurium, more accurate and convenient volumetric methods are favored. The oxidation of teUurium(IV) by ceric sulfate in hot sulfuric acid solution in the presence of chromic ion as catalyst affords a convenient volumetric method for the determination of tellurium (32). Selenium(IV) does not interfere if the sulfuric acid is less than 2 N in concentration. Excess ceric sulfate is added, the excess being titrated with ferrous ammonium sulfate using o-phenanthroline ferrous—sulfate as indicator. The ceric sulfate method is best appHed in tellurium-rich materials such as refined tellurium or tellurium compounds. 

Organic surface treatments, on titanium dioxide pigments, 25 26 Organic syntheses, 13 412—113 acid and base catalysts for, 24 182 advantages of fermentation over, 11 6 enzymes as catalysts for, 10 307 high pressure in, 13 438—139 ozone use in, 17 810-811 silylation in, 22 695-696 Organic tellurium compounds, 24 414-415, 422... 

On the scale of most other commercial metals, the production of elemental tellurium is relatively limited—approximately j million pounds annually. Commercial tellurium is marketed at a purity of about 99.7%, although much purer forms are obtainable—up to 99.999%. The application of tellurium and tellurium compounds as catalysts is expanding. Small quantities are used in various electronic components, including solar cells, infrared detectors, emitters, and thermoelectric generators. Tellurium also is sometimes used as a dopant for semiconductor... 

The selectivity in the formation of 1,4-diacetoxy-2-butene (1,4-DAB) is considerably enhanced when tellurium compounds are used as cocatalysts. Thus a heterogeneous catalyst, prepared by impregnation of Pd(N03)2 and Te02 dissolved in HN03 over active charcoal (Pd/Te = 10), can be used for the oxidation of butadiene (by 02 in AcOH at 90 °C) to 63% trans-l,4-DAB, 25% cis-1,4-DAB and 12% 3,4-diacetoxy-l-butene. Conventional soluble catalysts such as Pd(OAc)2/Li(OAc) are much less selective in the formation of 1,4-DAB 429 The gas-phase 1,4-diacetoxylation of butadiene in the presence of Pd-Te catalysts is currently being industrially developed by Mitsubishi and BASF 430... 

When the reaction was carried out in chloroform in the presence of ethyl carbamate with boron trifluoride-etherate as catalyst, /J-ethoxycarbonylaminoalkyl phenyl tellurium oxides were formed. These tellurium oxides were not purified but reduced with hydrazine to the corresponding tellurium compounds. ... 

Azobenzene or l,2-di(4 -pyridyl)ethene was used as a reduction catalyst. The generated benzenetellurolate was employed in the synthesis of cyanophenyl phenyl tellurium compounds and of benzoylphenyl phenyl tellurium compounds. ... 

It was noted that cross-linked polystyrene-telluric acid (a product of TeCl4 polycondensation with a copolymer of divinylbenzene and styrene and subsequent hydrolysis in an alkaline medium) is an active catalyst of olefin epoxidation by hydrogen peroxide [210]. At 333 K and in dioxane and terf-butanol solutions, the above catalyst quantitatively yielded epoxides from a wide range of unsaturated compounds such as aromatic and aliphatic hydrocarbons, alcohols and their derivatives. A plausible mechanism for the catalytic properties of a tellurium compound during polycondensation was discussed. 

Hydrogenation Catalysts. The key to catalytic hydrogenation is the catalyst, which promotes a reaction which otherwise would occur too slowly to be useful. Catalysts for the hydrogenation of nitro compounds and nitriles are generally based on one or more of the group VIII metals. The metals most commonly used are cobalt, nickel, palladium, platinum, rhodium, and mthenium, but others, including copper (16), iron (17), and tellurium... 

Tellurium tetrachloride is an efficient catalyst in the Knoevenagel reaction of non-enoUz-able aldehydes with active methylene compounds. ... 

Among DASF derivatives examined, the compound 32 prepared from the diselenide 2 and cyclohexene oxide was revealed to be the best catalyst for this addition, giving up to 94 % ee. It is noteworthy that the sulfur (33) and tellurium analogues (34) of 32 also catalyzed the reaction to afford the alcohol, but with lower enantioselectivity (52% and 46% ee, respectively). Related compounds 35 and 36 do not act at all as a catalyst for the reaction, indicating that the presence of both hydroxyl and dimethylamino groups in 32 are indispensable to act as an efficient asymmetric catalyst. Typical results of enantioselective addition of diethylzinc to aldehydes other than benzaldehyde catalyzed by 32 are also summarized in Table 4. 

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