Hydroisomerization and Hydrocracking of Alkanes. 5. Hydroisomerization and Hydrocracking of N-Hexane and N-Heptane on PtHY Catalysts. Effect of the Distribution of Metallic and ACID Sites.

Abstract

The activity, the stability and the selectivity of a series of bifunctional PtHY catalysts containing 0.02 to 1.5 wt% platinum and having Si/Al atomic ratios of 3 or 9 were compared for n-hexane and n-heptane transformations at 250°C, 1 atm, pH 2 /P alkane = 9. The balance between the two functions was characterized by nPt/nA (nPt: number of accessible platinum atoms, nA: number of strong acid sites). Qualitatively the change as a function of nPt/nA of the catalytic characteristics is the same with n-hexane and with n-heptane: for low values of nPt/nA the activity per acid site is low, the stability is weak and monobranched isomers M, dibranched isomers B and cracking products C are formed directly from the reactant while for very high values the activity is optimal, the stability perfect and M, B, C are formed through a step-by-step process (“ideal” hydroisomerization catalyst). Quantitatively big differences exist. In particular in order to be “ideal”, a PtHY catalyst must have a much lower nPt/nA ratio for n-hexane isomerization. Moreover the reactivity of n-hexane is much lower than that of n-heptane. These results are interpreted in the light of the types of carbenium ions and of their rearrangement and scissions implied in n-hexane and n-heptane transformations.