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 tetrah edral sites of a zeolite framework on the properties of Bronsted acid sites is investigated by computational techniques. We used the density functiona l theory (DFT) based acidity descriptor, "local softness", and a new acidit y descriptor, "relative electrophilicity" to predict the acidity sequence o f the zeolite clusters. The finite difference approximation is used to calc ulate local softness. The relative electrophilicity is found to be a suitab le descriptor of acidity of zeolite clusters. The trend of acidity derived from local softness values depends on several factors such as basis sets, g eometry of zeolite clusters, and different population analyses used to calc ulate them. The acidity trend derived from the relative electrophilicity fo llows the experimental trend, and it is not affected by the choice of basis sets or cluster models.