Antimonate catalysts

Fig. 3. Mechanism of selective ammoxidation and oxidation of propylene over antimonate catalysts (31).

The surface transformations of propylene, allyl alcohol and acrylic acid in the presence or absence of NHs over V-antimonate catalysts were studied by IR spectroscopy. The results show the existence of various possible pathways of surface transformation in the mechanism of propane ammoxidation, depending on the reaction condition and the surface coverage with chemisorbed NH3. A surface reaction network is proposed and used to explain the catalytic behavior observed in flow reactor conditions. 

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

INS has been used to probe the binding of reactants and intermediates to an iron antimonate catalyst [96]. The spectra were assigned with reference to the spectra of compounds of known structure. 

These spectra aid the interpretation of the mode of binding of adsorbed allyl species formed in the selective oxidation of hydrocarbons over metal oxide catalysts. The INS spectra of allyl iodide adsorbed by an iron antimonate catalyst at 293 K, and after heating to 353 K, were different from the spectrum of allylpalladium chloride and consistent with the allyl binding to the catalyst through the double bond there was no evidence for a ri -allyl (3) [96]. 

Partial oxidation of toluene to benzaldehyde over vanadium antimonate catalysts doped with titanium. The influence of the antimony content over the deactivation process. 

In a previous work [7] we have studied the changes produced by the replacing of antimony sites by titanium on the structure of a vanadium antimonate catalyst and on its activity and selectivity levels during the vapour phase oxidation of toluene. 

The extensive use of promoter elements has been employed with antimonate catalysts just as it has been with molybdate catalysts. This has been used to good effect on the Fe-Sb-0 catalyst in the development of several generations of commercial propylene ammoxidation catalysts (47). As seen from structural analyses, the major crystalline phase present in the catalyst is FeSb04, having the statistical rutile (Ti02) structure (44). A second antimony oxide phase, Q -Sb204, is also present in smaller amounts relative to the rutile phase. The presence of the antimony oxide phase is necessary for optimal activity and selectivity (48). 

The additive elements used to enhance the performance of the Fe-Sb-0 catalyst either enter the iron antimonate rutile phase to form a solid solution (49,50) or they form separate rutile phases (44). The promoter elements that produce the best performing iron antimonate-based ammoxidation catalysts are copper, molybdenum, tungsten, vanadium, and tellurium. Copper serves as a structural stabilizer for the antimonate phase by forming a rutile-related solid solution (23). Molybdenum, tungsten, and vanadium promote the redox properties of iron antimonate catalysts (51). They provide redox stability and prevent reductive deactivation of the catalyst, especially under conditions of low oxygen partial pressure (see above). The tellurium additive produces a marked enhancement of the selectivity of iron antimonate catalyst. How the tellurium additive functions to increase selectivity is not clear, but the presumption is that it must directly modify the active site. In fact, it is likely that it can actually serve as the site of selective oxidation because in its two prevalent oxidation states Te + and Te +, tellurium possesses the requirements for the selective (amm)oxidation site, a-hydrogen abstraction, and 0/N insertion (see below). 

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