Application of hard-soft acid-base principle to study Bronsted acid sites in zeolite clusters: a quantum chemical study

The effect of isomorphous substitution of Si by Al, B, and Ga in the tetrahedral sites of a zeolite framework on the properties of Bronsted acid sites is investigated by computational techniques. We used the density functional theory (DFT) based acidity descriptor, "local softness", and a new acidity descriptor, "relative electrophilicity" to predict the acidity sequence of the zeolite clusters. The finite difference approximation is used to calculate local softness. The relative electrophilicity is found to be a suitable descriptor of acidity of zeolite clusters. The trend of acidity derived from local softness values depends on several factors such as basis sets, geometry of zeolite clusters, and different population analyses used to calculate them. The acidity trend derived from the relative electrophilicity follows the experimental trend, and it is not affected by the choice of basis sets or cluster models.