Chromia catalytic properties

Catalytic Properties of Chromia-Pillared Montmorillonite and Preliminary Study Results... 

An investigation of chromia supported on alumina or silica as catalyst for partial oxidation of paraxylene [144] revealed influence of the oxidation states and degree of oligomerization of ehannium on the catalytic activity. Superior catalytic properties were obtained when chromium was supported on alumina. 

The present chapter has as its primary objective the description of the molecular configurations of chromium in chromia-alumina catalysts, and the possible relationships of these configurations to the catalytic properties of chromia-alumina. Emphasis, however, will be placed on the solid state physical chemistry of these catalysts, rather than on the details of their catalytic behavior, since this latter subject is of sufficient complexity to warrant a separate treatment. 

The physical-chemical methods used to study the chromia-alumina catalyst system have provided a coherent picture of its electronic structure. This picture has emerged in spite of the great complexities of the properties exhibited by the system. The most fruitful lines for future investigation will probably be a simultaneous study of the catalytic and physical-chemical properties of various types of chromia-alumina samples. When sufficient data of this type become available, it should be possible to formulate an over-all theory to account for the physical-chemical and catalytic properties of chromia-alumina catalysts. 

El-Shobaky, G.A. Ghozza, A M., and Mohamed, G.M., Surface and catalytic properties of gamma-irradiated chromia/alumina solids. Adsorpt. Sci. Technol., 15(6). 465-476 (1997). 

The presence of oxidation states in chromia other than -Fill is well known under some circumstances, their presence can materially affect catalytic properties. In the search for very active catalysts that could increase the kinetics of the thermodynamically unfavorable reaction CH2CICF3 + HF, possible correlations between conversion of CH2CICF3 to CH2FCF3 and the behavior of the unfluorinated precursor have been examined [32]. Chromia samples... 

Promoter deposition through different mechanisms can account for different catalyst properties. In particular, chromate depositing as chromia does not easily redissolve but, zinc oxide does redissolve once the leach front passes and the pH returns to the bulk level of the lixiviant. Therefore, chromate can provide a more stable catalyst structure against aging, as observed in the skeletal copper system. Of course, promoter involvement in catalyst activity as well as structural promotion must be considered in the selection of promoters. This complexity once again highlights the dependence of the catalytic activity of these materials on the preparation conditions. 

The catalytic and structural properties of two chromia-pillared montmorillonites were compared in an effort to establish structure-reactivity relationships in these materials. The basal spacings of pillared products, prepared by reaction of Na+-montmorillonite with base-... 

In an effort to more fully elucidate the structure and reactivity of metal oxide pillared clays, we have been investigating the structure-reactivity properties of chromia-pillared derivatives (17). In the following sections, we provide an example of the structure-catalytic reactivity properties of chromia-pillared montmorillonites. Also, we report our initial efforts to structurally characterize the intercalated chromia aggregates by Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. Unlike previously reported metal oxide pillared clays, chromia-pillared clay exhibits strong K-edge absorption and fine structure suitable for determination of metal-oxygen bond distances in the pillars. 

On the basis of temperature programmed reduction and oxidation (TPR-TPO) measurements, it was proposed that the calcination of chromia used as a catalyst for the selective fluorination of CF3CH2C1 into CF3CH2F depends strongly on the gas used to calcine the precursor [50], Furthermore, the fluorination of CF3CH2C1 depends directly on the number of reversibly oxidizible chromium atoms in the catalysts. The oxidation/reduction properties are related closely to the atmosphere of pre-treatment and a linear relationship between the catalytic activity and the hydrogen uptake during the second reduction step has been found [51],... 

The effect of Ca loading on the acid-base and redox properties of chromia catalysts supported on alumina has been investigated by microcalorimetry of NH3 adsorption and TPR. This alkaline promoter strongly decreases the acidity of the chromia catalyst, particularly suppressing the medium and strong acid sites. No clear correlations were found between the surface acidic properties and the catalytic behavior of the investigated samples in the oxidative dehydrogenation of isobutene, while clear trends were observed between reducibihty and catalytic activity [52]. 

It will be apparent from the topics described in the sections above that fluorinated chromia has subtleties that are not completely understood and in some catalytic systems have so far been described incompletely. Some factors are clear however. The surface properties depend crucially on the extent of surface prefluorination. Although many laboratory studies have employed CFC or better HFC reagents, such surfaces are normally lightly fluorinated only, and in the case of CFCs will inevitably contain surface chloride species that complicate interpretation of the catalysis. For large-scale applications, anhydrous HF is the prefluorination (and fluorination) reagent of choice. The process is slow and, even at the surface level, is probably never complete. This is beneficial, since complete conversion to a chromium(in) fluoride layer, it is generally agreed, would result in a catalyst whose activity was very low or which was inactive. 

Synthetic adsorbents such as silica, chromia, nnd alumina gels are expensive, but various other metals can be introvluced for improving the catalytic, adsorptive, or regenerative properties of the gels or catalysts. Likewise, various chemicals can be added to the natural adsorbents for enhancing their efficiency. 

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