Oxycarbides

A non-acidic isomerization catalyst system has unexpectedly emerged from recent studies by French workers [4] in the area of Mo-oxycarbides. Although at an early stage of development, these new materials exhibit high selectivities for the isomerization of paraffins such as n-heptane. An alternative non-carbenium ion mechanistic route to achieve isomerization of higher alkanes could potentially overcome some of the limitations of conventional solid acid based catalyst systems. 

In one other example, Raman spectroscopy was employed along with FTIR spectroscopy, XPS, elemental analysis, TGA, SEM and transmission electron microscopy (TEM) to follow the compositional and structure variations of polymethylsilsesquioxane samples pyrolysed at different temperatures in an atmosphere of nitrogen [56]. At 900°C the main product was silica, with formation too of some silica oxycarbide and amorphous carbon, with Raman spectroscopy showing complementary evidence for presence of both the minor species. 

Fig. 1 illustrates the types of reaction pathway which have been proposed to be of importance in the decomposition reaction. In practice, where additional processes such as reforming of DME may possibly occur, the reaction scheme could be much more complex than that shown. For example, on the basis of the product spectrum over molybdenum oxycarbide dimers reported in NaY,20 the following pathway has... 

The optimum Co content regarding activity was 50%, with an optimal carburization temperature of 600 °C. Though slightly more active than Cu-Zn-Al initially, the catalyst exhibited a faster deactivation rate after 300 min. Based on incomplete carburization, the authors ascribed the active component to an amorphous Co-Mo oxycarbide phase. 

Zank, G. A. Preceramic Polymer-Derived Silicon Oxycarbides. In Silicon-Containing Polymers. The Science and Technology of Their Synthesis and Applications Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 697-726. 

Interestingly, molybdenum has been deposited from [Mo(CO)6] on thin aluminum films. At 700 K the precursor decomposition provides an alumina layer and then a molybdenum oxycarbide film with the simultaneous loss of CO and CO2. 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Chemical vapor deposition is used industrially to deposit protective hard coatings on metal objects. Common coatings are titanium carbide (TiC), titanium nitride (TiN), titanium oxycarbide (TiCxOy), titanium carbonitride (TiCxNy), titanium oxycarbonitride (TiCxNyOz).91 Coatings based on TiC... 

The thermal stability of Sm2(Fei Co )i7Cy decreases with carbon content. This is probably due to the formation of a stable oxycarbide when oxygen impurities are present in the synthesis atmosphere ... 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

Figure 20.8 (a and b) show a comparison of the Pt// -zeolite catalyst which activated molybdenum oxycarbide catalysts, prepared from both Mo2C and Mo03, for the isomerization of n-heptane and n-octane at elevated pressure. For the platinum catalyst the chain length is important, as explained above, with the isomerization selectivity obtained for n-octane dropping more quickly with increasing conversion than that for n-heptane over this plantinum catalyst the isomerization of n-hexane can be... 

It is proposed that the high selectivity to isomers even at high conversion is obtained because the formation of n-hydrocarbon from the monomethyl isomers, i.e. the reverse reaction, occurs at a higher rate over the molybdenum oxycarbide catalysts than the bifunctional Pt/jS-zeolite catalyst. Over the Pt/jS-zeolite catalyst this reverse reaction would involve... 

The particle diameter D is related to the full width at half maximum A by the Debye-Scherrer equation D = 0.9 XIA cos0, where 20 is the diffraction angle and X is the X-ray wavelength. Table 27.1 lists the particle size and lattice plane spacing calculated using the strongest (h,k,l) peak for the Fe, W, Mo carbides, nitrides, oxynitrides and oxycarbides. It is important to note that the calculated particle size using the Debye-... 

As illustrated by Figure 27.6(d), the denitrogenation of quinoline is found to be a more difficult reaction. In the absence of sulfur, the oxynitride and the oxycarbide exhibit little activity for denitrogenation of quinoline in the absence of sulfur, while the hematite does not show any activity whatsoever (not shown). At the reaction conditions studied, the quinoline rapidly converted into tetrahydroquinoline, as evidenced by the high conversion rate indicated in Table 27.3, which reacted more slowly to form propylaniline, tetrahydroquinoline and decahydroquinoline. Only a small fraction of propylbenzene is detected as a nitrogen free product, accounting for less than 1% HDN. Sulfur addition as DMDS results in a moderate increase in HDN activity (4% HDN) for both Mo2NaO>, and... 

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