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/γ-Al2O3 was admixed 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 1H 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. 139La 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 29Si and 27Al 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 Brønsted acid sites in these small cages during the thermal dehydration of lanthanum cations (Hirschler–Plank mechanism). A similar effect was found in HNaY and HKY zeolites: The presence of the bulkier potassium cations causes a significant diminution of the accessible Brønsted 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 Brønsted acid sites, regardless of how these sites were generated. Copyri ght 2001 Academic Press.

The factors influencing the length of the induction period in ethylbenzene disproportionation over large pore zeolites were systematically investigated using LaNaY zeolites with different degrees of lanthanum exchange. The catalytic behavior was related to the number and strength of the Bronsted acid sites in these catalysts as determined by additional non-catalytic techniques. Mechanistic models which may account for the occurrence of the induction period, the deficit of diethylbenzenes during this period and the distribution of the diethylbenzene isomers are discussed.