Catalysts, bifunctional reforming platinum

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

Investigations of the isomerization of alkanes in recent years have provided evidence that the reaction can occur on certain metals, notably platinum, in the absence of a separate acidic component in the catalyst (20-22). While it has been shown that a purely metal-catalyzed isomerization process can occur, the findings do not challenge the commonly accepted mode of action of bifunctional reforming catalysts in which separate metal and acidic sites participate in the reaction. The available data at conditions commonly employed with commercial reforming catalysts indicate that a purely metal-catalyzed process does not contribute appreciably to the overall isomerization reaction on a bifunctional catalyst. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles ( 1.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to crea te the concept of polyfunctional catalysis. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

The reaction takes place on the catalyst housed in three stationary beds in the reactor. The catalyst used for the l-hexene isomerization reaction is a commercial E-302 reforming catalyst, supplied by Engelhard corporation. The bifunctional catalyst is composed of 0.6 wt% Platinum supported on 1/16" right cylindrical gamma-alumina extrudates. To minimize external mass-transfer resistances and to achieve CSTR behavior, the fluid phase containing the reactants is kept mixed by an impeller powered by a 0.75 hp MagneDrive assembly that can provide stirring speeds up to 3,000 rpm. Unconverted reactant, product and the SCF medium exit via a port located at the top of the reactor. 

Platinum on alumina-reforming catalysts, whether modified by additional components or not, are referred to as bifunctional. Separate and distinct reactions occur on the platinum site and on the alumina. The platinum typically performs dehydrogenation and hydrogenolysis, whereas the acidic alumina isomerizes, cy-clizes, and cracks. 

Bifunctional Catalysts. The complete reforming reaction sequence requires both metal-catalyzed dehydrogenation and acid-catalyzed isomerization and cyclization. It is clear that both acid and metal functionalities must exist on a satisfactory reforming catalyst. Reforming catalysts are often referred to as bifunctional, meaning that both metal and acid functions exist on the catalyst. Modern catalysts are platinum on alumina, typically modified by a number of additional elements. The platinum supplies the dehydrogenation function, and the alumina supplies the acidic function. The acidity of the alumina is enhanced through the adsorption of chloride. 

Straight-run gasoline is composed primarily of alkanes and cycloalkanes with only a small fraction of aromatics, and has a low ON of about 50. The ON is improved by catalytic reforming of n-paraffins and cycloalkanes into branched alkanes and aromatics. The main reactions are isomerization (w- to iso-), cycli-zation, dehydrogenation, and dehydrocyclization. The bifunctional catalyst has an acidic function to catalyze isomerization and cyclization and a dehydrogenation function that requires an active metal site. Typically, platinum is used as the metal and AI2O3 for the acidity. 

One common reforming catalyst is platinum on alumina. Platinum on alumina (AljOj) (see SEM photo below) is a bifunctional catalyst that can be prepared by exposing alumina pellets to a chioroplatinic acid solution, drying, and then heating in air at 775 K to 875 K for several hours. Next, the material is exposed to hydrogen at temperatures around 725 K to 775 K to produce very small clusters of Pt on alumina. These clusters have sizes on the order of 10 A. 

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