Effects of Next-Nearest-Neighbor Aluminum Location on the Brønsted Acidity of HY Zeolites

The distribution of aluminum (Al) on the H-form zeolite framework strongly affects the Brønsted acidity, resulting in the corresponding acid-catalyzed reactivity of zeolite catalysts. In the present work, the effects of next-nearest-neighbor (NNN) Al locations and numbers on the acidity for the specific Brønsted acid sites (BAS) inside the channel and on the external surface of the HY zeolites with two Si/Al ratios were investigated using density functional theory calculations. The Gibbs free energy of ammonia adsorption at the BAS site with different local NNN Al environments was used to characterize the Brønsted acidity. It has been found that an NNN Al atom on the β cage slightly enhances the acidity, while NNN Al atoms at the hexagonal column and super cages significantly decrease the acidity. With the increasing number of NNN Al atoms, the acidity of the specific BAS becomes weaker. The effects of the NNN Al location and numbers on the Brønsted acidity are further confirmed using pyridine, indole, and quinoline as probe molecules