Influence of Zeolite Framework Geometry Structure on the Stability of the [ZnOZn]2+ Cluster by Periodical Density Functional Theory

The location of the [ZnOZn]2+ cluster in zeolites has been studied by the periodical d. functional method for a chosen compn. of two Si atoms substituted by two Al atoms in the same D6R unit cell of the chabazite framework. This leads to different arrangements for Al pair in two ring configurations: six- and eight-membered. The cationic cluster is most stabilized when placed in the eight-membered ring. There are two factors playing a major role in the stabilization of this site: the accommodation of framework stress and the electrostatic interaction provided by the oxygen atoms of the framework. In the case of the six-membered ring, the stability is markedly influenced by the electrostatic interaction due to the close proximity of the Zn cations to the ring plane. On the other hand, both factors are important for the stability of the [ZnOZn]2+ site in eight-membered rings. However, accommodation of the framework stress seems to be the main factor for the different stabilization results within the eight-membered family.