Theoretical study of the enhanced Bronsted acidity of Zn2+-exchanged zeolites

The effect of Zn2+ exchange on the Bronsted acidity of a protonic zeolite has been studied by the ab initio DFT (density functional theory) approach using the BLYP generalized gradient approximation. Three different zeolite cluster models have been compared: two 6"T" models (two 4"T" rings with an oxygen atom bridge) with Si/Al=1 and Si/Al=2 and a 4"T" model (ring form) with Si/Al=1. The Bronsted acidity has been probed by computation of the acetonitrile adsorption and the cluster deprotonation energy. The presence of Zn2+ does not affect the cluster Bronsted acidity but it creates a very strong Lewis site (Zn2+) in all models studied. On the other hand, the presence of ZnOH+ enhanced the Bronsted site acidity in the case of the 6T model with Si/Al=1. This enhancement is due to a change in cluster geometry and position of OH group in ZnOH+ upon acetonitrile adsorption.