Heterogeneous catalyst chromia

Surface Vanadate, Molybdate and Tungstate Species The pure and mixed oxides and the salts of vanadium, molybdenum and tungsten in their higher oxidation states are used widely as heterogeneous catalysts, for selective oxidation as well as for acid catalysis. Similarly, supported chromia and rhenium oxides find wide application in different catalytic processes. 

Similarly, chromia-pillared montmorillonite catalyst reportedly [39] catalyzes the selective oxidation of alcohols with TBHP and titanium-montmorillonite in conjunction with tartaric acid esters functions as a heterogeneous catalyst for asymmetric epoxidations with TBHP [40]. 

Heterogeneous Catalysts Used for Large-Scale Syntheses of Selected Chlorohydrocarbons and Fluorohydrocarbons Fluorinated Chromia and eta-Alumina... 

Many different reaction schemes, as described in Figure 7.5, are considered and both homogeneous and heterogeneous catalysts are featured. Fluorinated chromia appears as the catalyst for the catalytic fluorination step that leads to the intermediate, CF3C1C=CH2, HFO-1233xf it is apparent that catalyst deactivation is a problem and for this reason addition of a basic molecule such as di-isopropylamine to the vapor feed is recommended. 

Best SA, Squires RG, Walton RA The X-ray photoelectron spectra of heterogeneous catalysts. II. The chromia-silica catalyst system, J Cata 47(3) 292—299, 1977. 

Phillips (1) A process for polymerizing ethylene and other linear olefins and di-olefins to make linear polymers. This is a liquid-phase process, operated in a hydrocarbon solvent at an intermediate pressure, using a heterogeneous oxide catalyst such as chromia on silica/ alumina. Developed in the 1950s by the Phillips Petroleum Company, Bartlesville, OK, and first commercialized at its plant in Pasadena, TX. In 1991, 77 reaction fines were either operating or under construction worldwide, accounting for 34 percent of worldwide capacity for linear polyethylene. 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

Although the first technical plants for CFC manufacturing used the Swarts catalyst exclusively, heterogeneously catalysed processes are competitive in the situations described above. Metal(III) oxides, especially chromia and alumina, are frequently used as solid catalysts. Moreover, they have often been used mixed with traces of other, usually metal(II), oxides, to prepare catalysts that have perceived advantages. 

The derivation of a mechanism for a chemical reaction is by its very nature an uncertain process, being dependent critically on the nature and extent of the experimental evidence. Mechanisms that have at their heart a surface process or processes are even more uncertain and when the constraints imposed by the manipulation of HF are also taken into account, it is not surprising that there have been relatively few mechanistic studies made of heterogeneous catalytic fluorination. However a catalytic process cannot be said to be understood fully without a mechanism based on the experimental evidence available and such studies are helpful in the design of the next generation of catalysts. In most cases the work described below involves chromia or y-alumina based catalysts that have been pretreated according to the methods described above. Studies involving C2 and Q compounds are described in turn. 

This mechanism has many features in common with that proposed by Burwell (12) for the heterogeneous catalytic hydrogenation on a chromia gel catalyst (Reaction 27). 

C. M. Cunningham and H. L. Johnston explained the zero-order kinetics of the heterogeneous liquid phase o-hydrogen conversion on chromia-alumina catalysts on the basis of the selective adsorption of o-hydrogen pointed out by Y. Sandler. This led us to the successful preparation of 95 %... 

In brief, the catalysts under consideration are highly porous, poorly crystallized, heterogeneous mixtures of various oxides of alumina and chromia whose structures are of considerable complexity. In the following sections we will briefly describe the physical and chemical techniques one can use to elucidate the structures of these catalysts, and then we will discuss some relevant properties of chromia, alumina, and chromia-alumina solid solutions, before turning to a consideration of the catalysts themselves. 

Additionally, metal fluorides are extremely sensitive to traces of water. Probably due to a combination of specific surface area and susceptibility to hydrolysis, heterogeneous acid catalysts for use in large-scale fluorination processes are usually based on either chromia or 7-alumina, which become fluorinated in situ. 

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