DFT studies on the location and acid strength of Brönsted acid sites in MCM-22 zeolite

Abstract

Density functional theory was applied to study the location and the acid strength of Brönsted acid sites in MCM-22 zeolite. The calculations were performed based on cluster model by using of B3LYP hydride functional and 6-31G** basis set. The replacements of Si by Al at eight inequivalent tetrahedral crystallographic sites and the corresponding Si(OH)Al sites were examined by calculating the (Al,H)/Si substitution energy, deprotonation energy, hydroxyl stretching vibtational frequency and the adsorption energy of some basic probe molecules on the acid centers. According to the calculated results it was predicated that the most favorable sites for framework Al substitutions in MCM-22 were T1, T3, and T4 sites and the most preferable Brönsted acid sites were on Al1(O3H)Si4, Al4(O3H)Si1, and Al3(O11H)Si2 bridged groups. The first two acid sites located on the 12-membered ring in the supercages, and the third located in the10-membered ring sinusoidal channels. Evaluating of acid strength by the calculated ammonia adsorption energies revealed that Al1(O3H)Si4 and Al4(O3H)Si1 sites display the similar acidity, and that stronger than Al3(O11H)Si2. The calculated hydroxyl vibrational frequencies of three acid sites were 3628 cm −1, 3618 cm −1, and 3592 cm −1(corrected with scaling factor 0.948), respectively, consistent with the reported FTIR experimental data. It was clarified that the calculations of deprotonation energy and hydroxyl stretching frequency were strongly dependent on the lattice configuration surrounding the Si(OH)Al acid center, therefore, it was obscure using these properties to characterize the acidities from site to site. Nevertheless, adsorption energy of probe molecule on zeolite cluster with adequate size would reflect practically the acid strength corresponding to different zeolite structures.