Platinum-silica-alumina catalyst

Catforming a process for reforming naphtha using a platinum-silica-alumina catalyst which permits relatively high space velocities and results in the production of high-purity hydrogen. 

Fio. 3. Isomerization of 2-methyIbioyclo[2.2.1]heptane (12) at 250 , vapor phase, in the presence of platinum-silica-alumina catalyst. 

Polyaromatics react under hydrocracking conditions to undergo partial or complete saturation of the aromatic rings, together with isomerization and cracking of intermediate and perhydro products. Phenanthrene, for example, can lead to tetralin and methyl cyclohexane as the principal products. With a platinum/ silica-alumina catalyst,67 the major products are isomerized perhydrophenan-threnes, which includes some adamantanes and cracked products, the total number of products exceeding 100. 

From this beginning, an extensive study of the isomerization of n-heptane was made with platinum on silica-alumina catalysts. Figure 2 shows curves plotted from the data obtained illustrating the total isomer yield versus conversion and the temperatures that produced these conversions. The conversion-isomer yield curve follows closely the 45° theoretical yield line, goes through a maximum at about 65% isomer yield, and then drops sharply because of cracking. The temperature at which the maximum yield of isomers was obtained was about 660° F. 

This mechanistic interpretation is based on the assumption that, once formed, five- or six-membered products of dehydrocyclization do not undergo interconversion. As discussed above, isomerizations are extremely slow at 317°C for tetralin to methylindan and methylindan to tetralin over alumina, silica-alumina, platinum-on-alumina, and platinum-on-silica-alumina catalysts (22, 23). 

Support. In multiphase catalysts, the active catalytic material is often present as the minor component dispersed upon a support sometimes called a carrier. The support may be catalyticaliy inert but it may contribute to the overall catalytic activity. Certain bifunctional catalysts ( 1.2.8) constitute an extreme example of this. In naming such a catalyst, the active component should be listed first, the support second and the two words or phrases should be separated by a solidus, for example, platinum/silica or platinum/silica-alumina. The solidus is sometimes replaced by the word on, for example, platinum on alumina. 

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. 

In contrast, we shall see that in a paraflhi isomerization system a platinum on silica-alumina catalyst is a multifunctional, specifically, a hifunc-tional catalyst the platinum sites catalyze distinctly different reactions and reaction steps than do the silica-alumina sites neither catalyze the reactions of the other component furthermore, both tj ies of reactions are relevant to accomplish the over-all reactions of the desired conversion system. 

When this reaction rate was measured by the rate of molar gas formation in a Schwab type differential reactor 28, 28a) over silica-alumina catalyst, this rate was found not greatly affected by the introduction of a platinum component into the catalyst mass the observed gas formation rate remained controlled by the acidic activity alone. Yet, an analysis of the gas produced, and of the liquid product, showed a shift in product composition... 

Platinum-silica-alumina (1% Pt by wt) was prepared by impregnation of a commercial cracking catalyst (6% alumina) with chloroplatinic acid solution, drying, and reduction under hydrogen. 

The hydroisomerization reaction is not only of industrial importance but is also of theoretical interest. The catalysts reported for this reaction consist of a hydrogenation component, such as nickel, platinmn, etc, deposited on acidic supports, such as silica-alumina ) or platinum on alumina containing halogen 2). A detailed study of the hydroisomerization reaction as a function of catalyst composition and experimental conditions has been reported ). Ciapetta (3a) studied the hydroisomerization of ethylcyclohexane over nickel on silica-alumina catalyst and reported that isomerization was the primary reaction and that the isomers consisted of dimethylcyclohexanes and of trimethylcyclopentanes. The dimethylcyclo-hexanes were stated to be composed of the 1,1- and 1,2-dimethylcyclo-hexanes, the latter predominating, and possibly of small amounts of 1,3-and 1,4-dimethylcyclohexanes. 

When minute particles of an active material are dispersed on a less active substance to produce a catalytic effect, such catalysts are called supported catalysts. The active material is usually a pure metal or a metal alloy. Examples of supported catalysts are the platinum on-alumina catalyst used in petroleum reforming and vanadium pentoxide on silica catalyst used in oxidation of sulphur dioxide. 

Isomerization of alkylcyclopentanes may also occur on the platinum catalyst surface or on the silica/alumina. For example, methylcyclopen-tane isomerizes to cyclohexane ... 

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

In addition to this work on charcoal- and silica-supported catalysts and on evaporated platinum films, a number of studies have been made on alumina-supported platinum catalysts (e.g., 111-114, 81,115) in which the aim has been the study of reactions at the platinum alone. In these cases, one cannot automatically dismiss the possibility of participation of the alumina support (i.e., of dual function behavior of the catalyst) because it is known that alumina may have acidic properties, particularly when retained halogen is present. In general terms, there is no immediate answer to this problem because the nature of this sort of catalyst wall be much dependent on the details of catalyst history, preparation, and use. However, there can be little doubt that in many experimental studies using plati-num/alumina, and in which the assumption has been made that the alumina support is inert, this assumption is essentially valid. For instance, one may note the inert alumina used by Davis and Venuto (111) and the justification provided by Gault et al. (116) for the inertness of the alumina used in a substantial body of previous work irrespective of whether the catalyst was... 

Catforming [Catalytic reforming] A catalytic reforming process using a platinum catalyst on a silica/alumina support. Developed by the Atlantic Refining Company and first operated in 1952. 

Pentafining A process for isomerizing pentane in a hydrogen atmosphere, using a platinum catalyst supported on silica-alumina. Developed by the Atlantic Richfield Company. 

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