Hydroisomerization of n-hexane over platinum mazzite and platinum mordenite catalysts kinetics and mechanism

Abstract

A kinetic study of n-hexane isomerization was carried out at 250°C under high pressure (13–48 bar, p H 2 p n-hexane from 3 to 27) on three PtHMAZ catalysts without extraframework aluminium species ( Si Al=10, 15 and 30 ) and on a commercial PtHMOR catalyst. Under standard conditions (30 bar, p H 2 p n-hexane =9 ), PtHMAZ catalysts with Si Al ratios of 15 and 30 are 1.6 times more active than the PtHMOR catalyst, which can be related to the greater strength of their acid sites. With PtHMAZ and PtHMOR, methylpentanes, 2,3-dimethylbutane and low amounts of 2,2-dimethylbutane appear as primary products. The apparent direct formation of dimethylbutane is explained by the unidirectional pore structure of these zeolite catalysts. 2,3-dimethylbutane isomerization into 2,2-dimethylbutane is slower on PtHMOR than on the PtHMAZ catalysts whose pore size is larger. A kinetic bifunctional model, in which the limiting step is the isomerization of olefinic intermediates on the acid sites, accounts for the quantitative changes in the reaction rate versus hydrogen and n-hexane pressures. This provided that a concentration effect in the zeolite pores by a physical adsorption process is considered.