SKELETAL ISOMERIZATION OF HEXANE OVER PT H-BETA ZEOLITES - IS THE CLASSICAL MECHANISM CORRECT/

The isomerization of n-hexane occurs over Pt-loaded H-beta zeolites in a highly selective manner. At 250 degrees C, with 45 Torr n-hexane an d 2.0 x 10(3) Torr H-2 over a 0.65% Pt/H-beta zeolite, the isomerizati on activity was 3.7 mu mol g(-1) s(-1), and the isomerization selectiv ity was 99%. Both the activity and the selectivity were stable with re spect to time on stream. Pure H-beta zeolite was only about 10% as act ive 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 an d the isomerization selectivity. Moreover, the reaction of 1-hexene ov er H-beta zeolite did not result in the formation of any observable he xene isomers, These results, together with the fact that the equilibri um hexene concentration is very small (4.9 x 10(-4) Torr) under the re action conditions, suggest that hexene produced by the dehydrogenation of hexane at the metal component is not directly involved in the isom erization reaction. An alternate mechanism is proposed in which an aci d-catalyzed chain reaction involving methyl shifts and hydrogen transf er is responsible for the isomerization of hexane. The chain is termin ated when an alkene reacts with a C6H13+ species that is bonded to the zeolite. The principal role of the Pt is to hydrogenate alkenes so th at their steady-state concentration remains negligibly small. The prox imity of the Pt particles and the acid sites is important in order to prevent an increase in the local concentration of alkenes. (C) 1998 Ac ademic Press.