Differences between MFI- and MEL-Type Zeolites in Paraffin Hydrocracking

A crit. evaluation of published paraffin hydroconversion data shows that MEL-type zeolites preferentially hydrocrack paraffins where two Me groups are sepd. by a methylene group, whereas MFI-type zeolites prefer paraffins with geminal Me groups (preferably at the central carbon atom). Due to this difference in hydrocracking pathway, MEL-type zeolites will hydroisomerize a higher percentage of the feed than MFI-type zeolites at low temp., while the reverse is true at high temp. The free energies of adsorption calcd. by means of configurational bias Monte Carlo (CBMC) mol. simulations are used to explain these differences in selectivity. They show that the MEL- and MFI-type zeolites favor the formation and hydrocracking of the di-Me paraffins that have a shape commensurate with that of their pores. They indicate that the higher paraffin hydroisomerization selectivity of the MEL-type zeolites can also be explained by their higher selectivity for adsorbing linear rather than branched paraffins at high paraffin loading. At low paraffin loading this difference in adsorption selectivity disappears. Both temp. and loading effects could resolve a disparity in the literature between n-decane and n-heptane hydroisomerization selectivity data. (c) 2001 Academic Press.