Catalysts, bifunctional reforming component

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. 

The catalysts on which these reactions occur are bifunctional that is, they possess two different types of catalytic activity. In addition to catalyzing hydrogenation and dehydrogenation reactions, they also catalyze hydrocarbon rearrangements of a type commonly observed in acid catalysis. These two distinct types of activity are associated with two different components of the catalyst, at least under conditions typical of catalytic reforming. 

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. 

A well-known cooperation effect is due to bifunctional catalysis in catalytic reforming, both Pt and alumina carry out part of the catalytic work. Several other types of cooperation have been mentioned in the last years, e.g. between catalysts within zeolitic structures and another component (1-3) or between separate oxide phases in oxidation or related reactions (4-12). Explanations for these cooperations often rest on spill over processes (13-15). 

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