Platinum-silica-alumina catalyst cyclization

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

Over platinum-on-carbon catalyst at relatively low temperature (310°C), C5-cyclization of alkylbenzenes probably proceeds by direct closure of the ring between the carbon atoms of the side-chain and the benzene ring, bypassing dehydrogenation to olefins (25-27). However, at higher temperatures and on platinum-alumina or platinum-on-silica C5-dehydro-cyclization could involve olefinic intermediates (7,13, 28). 

Cyclization of alkylbenzenes is much slower over silica-alumina than over the platinum-containing catalysts. To clarify the successive steps of cyclization, Csicsery performed a set of experiments with 4-phenyl-1-butene, a dehydrogenation product of -butylbenzene (28). 

For 1-hexene isomerization and for acid catalyzed Cg aromatic reactions all molecular sieves were evaluated in their calcined, powdered state. For the study of Cg aromatics, selected SAPO molecular sieves were aluminum exchanged or steam treated as noted in Table IV. For bifunctional catalysts used in paraffin cyclization/isomerization and ethylbenzene-xylene interconversions, the calcined molecular sieve powder was mixed with platinum-loaded chlorided gamma alumina powder. These mixtures were then bound using silica sol and extruded to form 1/16" extrudates which were dried and calcined at 500°C. The bifunctional catalysts were prepared to contain about 0.54 platinum and about 40 to 504 SAPO molecular sieve in the finished catalysts. 

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