Theoretical Studies of Carbocations Adsorbed over a Large Zeolite Cluster. Implications on Hydride Transfer Reactions

A semiempirical MNDO study of simple alkylcarbenium ions (Me, Et, i -Pr, t -Bu) on a large and more realistic cluster, comprising a hexagonal prism and a sodalite unit (3) and simulating different adsorption sites on zeolite Y, was carried out. On going from H to bulky alkyl groups, there is an increasing tendency to stretch the Al−O bond length and to decrease the Si−O−Al bond angle. Nevertheless, the proton and the alkyl groups are covalently bonded to the framework, as expressed by the high bond orders, near unity. Adsorption on site O 4, located in the sodalite, is energetically disfavored by 2−4 kcal/mol relative to adsorption on site O 1, in the hexagonal prism. The MNDO calculations on cluster 3 showed a reasonable agreement with ab initio calculations of carbenium ions adsorbed on smaller clusters 1 and 2, except when adsorption on O 4 is considered, indicating that steric strain, due to the crystalline structure, plays an important role. Adsorption on Si−O−Si sites is about 45 kcal/mol higher in energy than the correspondent adsorption on Si−O−Al sites. This result may explain the observed experimental dependence of hydride transfer reactions with the structural Si/Al ratio, also suggesting the participation of the zeolite structure in the transition state.