PLATINUM ZEOLITE CATALYST FOR REFORMING NORMAL-HEXANE - KINETIC AND MECHANISTIC CONSIDERATIONS

A platinum/L-zeolite-reforming catalyst exhibits higher activity and selectivity for converting n-hexane into benzene than other Pt catalyst. The reaction pathways indicate that for all catalysts, e.g., Pt/K L or Pt/K Y, benzene is formed as a primary product by one-six-ring closure and methylcyclopentane is formed as a primary product via one-five-ring closure. The ratio for one-six to one-five-ring closure, however, is about two times greater for the Pt/K L than for the Pt/K Y, or other platinum catalysts. The preference for the one-six-ring closure in L zeolite appears to be related to the optimum pore size of the L zeolite. In addition to an increased selectivity for one-sixring closure, the Pt/K L-zeolite catalyst also displays increased reactivity. For example, the turnover frequency of the Pt/K L-zeolite catalyst is 10 times higher for formation of benzene and 3.3 times higher for formation of methylcyclopentane compared with the Pt/K Y-zeolite catalyst. Although the Pt/K L is more reactive than Pt/K Y, the apparent activation energies, 54 kcal/mol for one-sixring closure and 39 kcal/mol for one-five-ring closure, are the same for both catalysts. Differences in reactivity are associated with an increase in the preexponential term for the Pt/K L catalyst. The increased aromatics selectivity for Pt/K L is consistent with the confinement model which proposes that n-hexane is adsorbed as a six-ring pseudo-cycle resembling the transition state for one-sixring closure. © 1991.