Hydroisomerization and hydrocracking of n-heptane on Pth zeolites. Effect of the porosity and of the distribution of metallic and acid sites.

Abstract

The catalytic properties of PtHY, PtHZSM5 and PtH mordenite catalysts are controlled by the number of their acid sites (n A, number of acid sites on which the adsorption heat of NH 3 is greater than 100 kJ mol −1) and their hydrogenating sites (n Pt). The activity of PtHY catalysts changes with n Pt/n A as can be expected from the classical bifunctional mechanism : it increases with n Pt/n A for the low values of this ratio, then reaches a plateau for n Pt/n A ⩾ 0.03. For low values of n Pt/n A, monobranched, multibranched isomers and cracking products are formed simultaneously but, for n Pt/n A ⩾ 0.17, n-heptane transforms successively into the three families of products. With PtHZSM5 catalysts, the same trend can be observed concerning the changes of the activity as a function of n Pt/n A. However the changes in the reaction scheme as a function of n Pt/n A are different : cracking remains the main reaction even at high values of n Pt/n A ; multibranched isomers are found in very low concentration. This can be attributed to a slower migration of the olefinic intermediates. Moreover, particular selectivities are found which are explained by a good matching between some intermediates and the space available near the acid sites. With PtH mordenite, the activity changes differently with n Pt/n A. In particular, it decreases for n Pt/n A > 0.015. This can be attributed to a partial pore blockage by platinum or by coke, leaving only a small part of the catalyst active. This is confirmed by the fact that the selectivities are the same as those of a PtHY zeolite with a n Pt/n A ratio 10–20 times lower.