Brønsted Acidity of Isomorphously Substituted ZSM-5 by B, Al, Ga, and Fe. Density Functional Investigations

The Brønsted acidity of isomorphously substituted ZSM-5 by B, Al, Ga, and Fe has been studied at the B3LYP level of density functional theory. On the basis of the calculated proton affinity, natural charge on the acidic proton, and the adsorption energy of NH 3, the Brønsted acidity increases in the order: B−(OH)−Si < Fe−(OH)−Si < Ga−(OH)−Si < Al−(OH)−Si, in agreement with the experiment. In both Al and Ga modified clusters, the adsorbed NH 3 becomes ammonium (NH 4 + ) stabilized by two N−H···O hydrogen bonds, while the physisorbed NH 3 is stabilized by one N···H−O hydrogen bond in Fe and B substituted clusters. It is also found that NH 3 adsorption changes the B coordination sphere.