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The isomerization of n -hexane occurs over Pt-loaded H-β zeolites in a highly selective manner. At 250°C, with 45 Torr n -hexane and 2.0×10 3 Torr H 2 over a 0.65% Pt/H-β zeolite, the isomerization activity was 3.7 μmol g −1 s −1 , and the isomerization selectivity was 99%. Both the activity and the selectivity were stable with respect to time on stream. Pure H-β zeolite was only about 10% as active as the Pt-loaded sample, and the isomerization selectivity was 66%, with the remaining 34% caused by cracking reactions. The addition of 1-hexene by several means decreased both the overall reaction rate and the isomerization selectivity. Moreover, the reaction of 1-hexene over H-β zeolite did not result in the formation of any observable hexene isomers. These results, together with the fact that the equilibrium hexene concentration is very small (4.9×10 −4 Torr) under the reaction conditions, suggest that hexene produced by the dehydrogenation of hexane at the metal component is not directly involved in the isomerization reaction. An alternate mechanism is proposed in which an acid-catalyzed chain reaction involving methyl shifts and hydrogen transfer is responsible for the isomerization of hexane. The chain is terminated when an alkene reacts with a C 6 H + 13 species that is bonded to the zeolite. The principal role of the Pt is to hydrogenate alkenes so that their steady-state concentration remains negligibly small. The proximity of the Pt particles and the acid sites is important in order to prevent an increase in the local concentration of alkenes. |
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