Tungsten oxides on alumina

Rhenium oxide supported on alumina is easily reduced compared to tungsten oxide on alumina, denoting a smaller interaction between carrier and promoter. ... 

Raman spectra of tungsten oxide on alumina catalysts show a broad band at about 970 cmwhich is attributed to a polymeric, octahedrally co-ordinated tungsten compound.No crystalline WO3 is observed. The difference between WO3/AI2O3 and W03/Si02 catalysts is evident from their colour. The former are white, while WOJS1O2 catalysts are yellow, the yellow colour being attributed to crystalline W03. ° ... 

Table I. Reduction Behavior of Tungsten Oxide on Alumina...

Whereas the Mobil process starts with syn gas based methyl alcohol, Olah s studies were an extension of the previously discussed electrophilic functionalization of methane and does not involve any zeolite-type catalysts. It was found that bifunctional acidic-basic catalysts such as tungsten oxide on alumina or related supported transition metal oxides or oxyfluorides such as alumina or related supported transition metal oxides or oxyfluorides such as tantalum or zirconium oxyfluoride are capable of condensing methyl chloride, methyl alcohol (dimethyl ether), methyl mercaptan (dimethyl sulfide), primarily to ethylene (and propylene) (equation 65) . 

The catalysts developed for hydrotreating include cobalt, molybdenum, nickel, vanadium and tungsten oxides on alumina. Cobalt and molybdenum oxides on alumina catalysts are in greater use today, being highly selective, easy to regenerate and relatively resistant to poisons. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

Table 18.4 Selectivity (%) of tungsten carbide on alumina compared to the supported tungsten oxide precursor in FT reaction...

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

The response of the supported nickel oxide on alumina to an in situ experiment is very different from that presented earlier for the supported tungsten oxide, as shown in Figure 13. The Raman band of the supported nickel oxide on alumina is not affected by the removal of the moisture present on the sample surface (compare Figures 13(a) and 13(d)) and suggests that water molecules do not coordinate to the supported nickel oxide. 

Figure 8. Model of the transformations observed for tungsten oxide on an alumina film by controlled atmosphere electron microscopy.

The predominant process for manufacture of aniline is the catalytic reduction of nitroben2ene [98-95-3] ixh. hydrogen. The reduction is carried out in the vapor phase (50—55) or Hquid phase (56—60). A fixed-bed reactor is commonly used for the vapor-phase process and the reactor is operated under pressure. A number of catalysts have been cited and include copper, copper on siHca, copper oxide, sulfides of nickel, molybdenum, tungsten, and palladium—vanadium on alumina or Htbium—aluminum spinels. Catalysts cited for the Hquid-phase processes include nickel, copper or cobalt supported on a suitable inert carrier, and palladium or platinum or their mixtures supported on carbon. 

Catalysts used for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of heavy oil fractions are largely based on alumina-supported molybdenum or tungsten to which cobalt or nickel is added as a promoter [11]. As the catalysts are active in the sulfided state, activation is carried out by treating the oxidic catalyst precursor in a mixture of H2S and H2 (or by exposing the catalyst to the sulfur-containing feed). The function of hydrogen is to prevent the decomposition of the relatively unstable H2S to elemental sulfur, which would otherwise accumulate on the surface of the... 

Both silicon oxide and alumina slurries can be efficiently removed on PECVD TEOS oxide or silicon nitride substrates in a conventional SCI or in a SCI without any water peroxide in the case of outcropping tungsten (see Fig. 5). When water peroxide is not present to continuously regrow a protective oxide layer, OH species can etch the silicon. In the latter case, the backside of the wafer must therefore be protected with a nitride or oxide layer to avoid a severe silicon roughening effect. Nevertheless to achieve the same particle removal efficiency obtained with a scrubber, power mega-sonics also have to be used (see Fig. 18). 

Immobilized complexes anchored to the surface of oxide supports may serve as active metathesis catalysts. In fact, Mo(CO)6 on alumina was reported in the first paper on metathesis by Banks and Bailey.1 Molybdenum and tungsten hexacarbo-nyls and alkylmolybdenum and alkyltungsten complexes have been studied most.6,24,26 Anchored organomolybdenum complexes are particularly effective catalysts, even at temperatures below 0°C however, they quickly lose activity.41-43... 

Moffat, Johnson, and Clark86)found the propylene disproportionation reaction on tungsten oxide-silica catalyst to be limited by interphase diffusional effects in spite of calculations which predict that no diffusional limitation should occur. They postulate that widely separated and very active sites could have their inherent activity limited by localized film diffusional effects which are functions of Reynolds and Schmidt numbers. Activity of cobalt molybdate-alumina was not limited by interphase or prediffusional effects. 

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