An ONIOM study on the distribution, local structure and strength of Brِnsted acid sites in FER zeolite

Density functional theory was employed to investigate the accommodation of aluminum as well as the locations and strengths of Brönsted acid in FER zeolite. In order to describe the structural and energetic properties of H-FER zeolite effectively and reasonably, two-layered ONIOM schemes (B3LYP/6-31G (d, p): am1) optimization on 52T clusters have been tested. The replacements of Si by Al at crystallographic sites were examined by calculating the bond lengths, substitution energies, deprotonation energies, O–H stretching frequencies, and the adsorption energy of the probe molecules on the acid centers which was obtained with a basis set superposition error (BSSE) correction. The calculated results indicate that the most stabilized Brönsted acid (B-acid) site was Al4–O6–Si2, while Al1–O4–Si1 and Al4–O7–Si3 were unlikely to be occupied because of their less stabilities. Evaluation of acid strength by the calculated deprotonation energies revealed that Al3–O7–Si4 site displayed the strongest acidity, and Al4–O6–Si2 site exhibited the weakest acidity. It was also found that B-acid sites, whose O atoms are situated on the surfaces of the 10-membered channel or at the intersection of two channels, show the high frequencies and fairly good stability. The results of adsorption energies before and after BSSE correction are generally aligns with each other. The calculated hydroxyl vibrational frequencies of acid sites were around 3600 cm−1, and in accordance with the available experimental data.