Effect of framework Si/Al ratio and extra-framework aluminum on the catalytic activity of Y zeolite

The effect of the Si/Al ratio of the framework on the intrinsic activity of Brønsted acid sites in Y zeolite was investigated by monomolecular cracking of propane and physicochemical characterization. Samples with different Si/Al ratios were obtained by synthesis and by post-synthesis treatments. Two samples with Si/Al ratios of 3.3 and 3.6 exhibited characteristic features similar to those of commercial Y zeolite with a Si/Al ratio of 2.6, but they were more stable. An ultrastable Y (HUSY) was treated with ammonia and ethylenediaminetetraacetic acid (EDTA) to remove extra-framework aluminum (EFAl). The rates per gram increased with Si/Al ratio, but the activation energies did not depend on the Si/Al ratio or on the presence of EFAl. This implies that the intrinsic activity of the Brønsted acid sites that participate in the reaction is identical in all the samples. The enhanced activity of the samples with Si/Al ratios up to 5.0 is attributed to the creation of more isolated Brønsted acid sites, which are active in this demanding reaction. The removal of EFAl did not affect the intrinsic catalytic activity but increased the number of catalytically active framework Brønsted acid sites.

Not all tetrahedrally coordinated aluminum atoms contribute to active Brønsted acid sites in the monomolecular cracking of propane. Steaming and synthesis of high-silicon zeolite Y results in more isolated acid sites that contribute to activity. The intrinsic activity of the active sites is independent of synthesis and treatment procedures. Figure optionsDownload as PowerPoint slide