Acidic properties of ZSM-5 zeolite modified with Ba2+, Al3+ and La3+ ion-exchange

The acidity of a ZSM-5 zeolite was modified with Ba2+, Al3+ and La3+ ion-exchange. The ion-exchange process was repeated three times and after each ion-exchange cycle the samples were calcined. The acidity of the modified zeolites was characterized by IR and MAS NMR spectroscopy. The number of strong Bronsted acid sites decreases rapidly with increasing degree of barium ion-exchange since the effect of Plank-Hirschler mechanism is quite negligible due to the relatively low electrostatic potential of Ba2+ cations. However, the splitting of water molecules resulting in the formation of new Bronsted acid sites and acidic M(OH)(2)(2+) groups is more probable in La3+ and Al3+ ion-exchanged zeolites. The formation of new Bronsted acid sites was observed in the IR spectra of highly Ba2+ exchanged zeolite after the chemisorption of pyridine. This may be due to the formation of pyridinium ions via the reaction of base pyridine with unsplitted water molecules coordinated to Ba2+ ions. The catalytic activity of the modified zeolites was measured in an n-hexane cracking reaction. Zeolites with a high degree of Al3+ and La3+ ion-exchange were found to be slightly more active than unmodified zeolite, in accordance with the formation of new acid sites. The aluminium and lanthanum ion-exchange enhanced slightly the formation of C-5 and olefinic products and decreased the formation of C-3 and aromatic products.