Influence of the nature of residual alkali cations on the catalytic activity of zeolites X, Y, and EMT in their Bronsted acid forms

Lanthanum-exchanged zeolites X, Y, and EMT, containing either sodium or potassium as residual cations, were used as catalysts in two acid-catalyzed hydrocarbon reactions, viz. the disproportionation of ethylbenzene and the isomerization of n-octane (for the latter reaction, Pd/gamma -Al2O3 was admired to the acid zeolites to make the catalysts bifunctional). All pairs of LaM-type zeolites (where M stands for Na or K) showed remarkable activity differences, the LaK form being always less active than the LaNa form. IR and H-1 NMR spectroscopy revealed that, for a given zeolite type and degree of lanthanum exchange, the concentration of bridging hydroxyl groups in the large cages was at least 20% lower than that for the LaK form. La-139 NMR spectroscopy indicated that the concentration of lanthanum ions in the large cages of the as-exchanged LaK forms was always lower than that in the corresponding LaNa forms. Surprisingly, already in the as-exchanged LaK forms, some La3+ migration into the small cages appears to occur, and this was corroborated by framework strains indicated by Si-29 and Al-27 NMR. It is proposed that potassium cations in the large cages cause an enhanced migration of lanthanum cations into the small cages and a preferential formation of Bronsted acid sites in these small cages during the thermal dehydration of lanthanum cations (Hirschler-Plank mechanism). A similar effect was found in HNaY and HXY zeolites: The presence of the bulkier potassium cations causes a significant diminution of the accessible Bronsted acid sites in the large cages and of the catalytic activity. Overall, one must conclude from these results that in zeolites possessing both large and small cages, the nature of the residual alkali cations can exert a pronounced influence on the local distribution of the Bronsted acid sites, regardless of how these sites were generated. (C) 2001 Academic Press.