Dehydrogenations oxidative, over platinum

The conversion of cyclohexanes to aromatics is a classical dehydrogenation reaction which will readily take place on many transition metals and metal oxides. On chromia-alumina Herington and Eideal (S) have demonstrated the occurrence of cyclo-olefin intermediate products. Weisz and Swegler 25) have demonstrated the effect on benzene yield of allowing early diffusional escape of cyclo-olefin from the porous catalyst particle. Prater et al. 26) have developed evidence that cyclohexene occurs as a quasi-intermediate in aromatization catalysis over platinum catalyst also, although at a smaller concentration, because of a larger ratio of effective rate constants fe/Zci in the scheme... 

Dehydration of cyclopentanemethanol over aluminum oxide at 320° tgave an olefin mixture whence hydrogenation afforded a mixture (94%) of [14C]methylcyclopentane with about 34% of [14C]cyclohexane. The mixed cycloalkanes were transformed into [14C]cyclo-hexane by Nenitzescu and Cantuniari s method,154 and dehydrogenation on a platinum-charcoal catalyst at 360° then yielded [14C1]benzene. 

Keresszegi, C., Mallat, T., Grunwaldt, J., etal. (2004). A Simple Discrimination of the Promoter Effect in Alcohol Oxidation and Dehydrogenation over Platinum and Palladium, J. Catal., 225, pp. 138-146. 

Reactions over chromium oxide catalysts are often carried out without the addition of hydrogen to the reaction mixture, since this addition tends to reduce the catalytic activity. Thus, since chromium oxide is highly active for dehydrogenation, under the usual reaction conditions (temperature >500°C) extensive olefin formation occurs. In the following discussion we shall, in the main, be concerned only with skeletally distinguished products. Information about reaction pathways has been obtained by a study of the reaction product distribution from unlabeled (e.g. 89, 3, 118, 184-186, 38, 187) as well as from 14C-labeled reactants (89, 87, 88, 91-95, 98, 188, 189). The main mechanistic conclusions may be summarized. Although some skeletal isomerization occurs, chromium oxide catalysts are, on the whole, less efficient for skeletal isomerization than are platinum catalysts. Cyclic C5 products are of never more than very minor impor-... 

In the discussion of the subject Balandin mentions (15) that Fischer previously postulated that methylene radicals may be produced as an intermediate in the formation of hydrocarbons by his method (116). This mechanism of carbon deposition on platinum supported on oxides of nickel and chromium (oxidized nichrome) through the intermediate formation of methylenes was thought by Balandin to be similar to the mechanism of dehydrogenation over this type of catalyst in that both occur on the boundaries of platinum-nickel and of platinum-chromia and were brought in agreement by him with his multiplet theory (26). 

In some cases a catalyst consists of minute particles of an active material dispersed over a less active substance called a support. The active material is frequently a pure metal or metal alloy. Such catalysts are called supported catalysts, as distinguished from unsupported catalysts, whose active ingredients are major amounts of other substances called promoters, which increase the activity. Examples of supported catalysts are the automobile-muffler catalysts mentioned above, the platinum-on-alumina catalyst used in petroleum reforming, and the vanadium pentoxide on silica used to oxidize sulfur dioxide in manufacturing sulfuric acid. On the other hand, the platinum gauze for ammonia oxidation, the promoted iron for ammonia synthesis, and the silica-alumina dehydrogenation catalyst used in butadiene manufacture typify unsupported catalysts. 

The heat delivered by the formation of platinum oxide measured by DTA and the dehydrogenation rate of cyclohexane over these platinum modified catalytic membranes show that e pH value of impregnating solutions... 

The hydrogenation of benzene to cyclohexane and the oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone is described in Section 9.2.2.1. This mixture is dehydrogenated to yield phenol. Details of the process have not been disclosed but the mixture is probably passed over a platinum on carbon catalyst at about 380°C. 

A good example of the bifunctionality is seen in Table 4.30, showing the products in the conversion of methylcyclopentane over SiOz — AI2O3, Pt/SiOz and the mechanical mixture of SiOz — AlzOz, and Pt/SiOz. An acidic oxide is not active for dehydrogenation. Dehydrogenation to methylcyclopentene or to methylcyclopentadiene occurs on the platinum. The acid centers are necessary to isomerize cyclopentene to a six-membered ring. Thus, benzene can be obtained only when both acidic and metal components are present. 

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