An ONIOM study of the distribution of skeletal aluminum and Brønsted acidity in ZSM-48 zeolite

Abstract

A density functional theory (DFT) study was performed on distribution of skeletal aluminum and Brønsted acid (B-acid) sites as well as the acid strength in ZSM-48 zeolite. The correlation between Si / Al molar ratio and the general acid strength was also investigated. The calculations were performed based on 51T and 90T cluster models by using two-layered ONIOM schemes (B3LYP/6-31G (d,p): AM1) method. The former 51T cluster is used for the calculation of single- Al substitution and the latter is for the multi- Al substitution study. The properties of Si / Al substitution energy, deprotonation energy, bridging hydroxyl stretching vibration frequency and adsorption energy for the probe molecule were calculated and used to measure Brønsted acid location and strength. As the result shows, T2 is the most readily to be replaced by Al and the Brønsted acids prefer to be formed at Al 2– O 7– Si 3 site. Al 2– O 6– Si 2 is the highest in Brønsted acid strength. Besides, the acid strength weakens with the decrease of Si / Al .