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

Abstract

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 O4, located in the sodalite, is energetically disfavored by 2−4 kcal/mol relative to adsorption on site O1, 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 O4 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 energ...