Theoretical Studies of the Electrostatic Effect on Carbocations Adsorbed over Zeolitic Sites

Theoretical studies, at MNDO level, of carbocations adsorbed over a zeolite active site were carried out, taking into account the effect of electrostatic fields. Clusters 1 and 2 were placed between two electrostatic charges of opposite sign. The distance from each charge to the center of the oxygen atom bearing the proton or alkyl group was varied in order to find the minimum in energy. The ionicity, expressed by the decrease in bond order, slightly increased with the inclusion of the charges. A more pronounced effect was observed for a positive charge of +2. The energy of the system decreased with the introduction of electrostatic charges. This fact may be partly explained by the electrostatic interaction between the opposed charges and also to an observed increase in the Si–-O–-Al bond angle, decreasing the steric repulsions between the substituents and the acid site. The bond orders remained high, indicating a predominant covalent character for the adsorption of the carbocations. The calculations are in agreement with experimental data of exchange of polyvalent cations, which pointed to an increase in the acid strength of the zeolite, due to decrease in the electron density in the aluminum atom. This revised version was published online in June 2006 with corrections to the Cover Date.