Relationship between acidity and catalytic reactivity of faujasite zeolite: A periodic DFT study

- Relationships between acidity and reactivity of faujasites were studied by DFT calculations.
- Multinuclear EFAl species enhance catalytic activity of vicinal acid sites.
- Promoting effect of EFAl varies depending on the acidity probe/catalytic mechanism.
- Cationic EFAl species stabilize anionic lattice in ion pair-type transition states.
- Lattice aluminum density in low-silica zeolites influences acid strength.

The fundamental aspects of Brønsted acidity and catalytic reactivity of faujasite-type zeolites were investigated by periodic DFT calculations. The adsorption energies of ammonia and pyridine on the Brønsted acid site (BAS) were used to determine the acidity. It is demonstrated that the acid strength of zeolite materials increases with rising Si/Al ratio (low-silica faujasite), and then levels off at high Si/Al ratio (high-silica faujasite). The presence of multinuclear extra framework Al (EFAl) in the sodalite cages substantially enhances the Brønsted acidity. The catalytic reactivity of faujasite toward protolytic propane cracking correlates well with the characterized acidity by base adsorption. However, for H/D exchange reaction of benzene the presence of EFAl species can induce deviations between the measured acidity and the reactivity of faujasite catalysts, indicating that acidity and reactivity are not always directly correlated.

150