An ab initio quantum chemical investigation on the effect of the magnitude of the T-O-T angle on the Broensted acid characteristics of zeolites

Abstract

Ab initio molecular orbital calculations are used to monitor the effect of increasing SiOAl angle (SiOAl) on the acidic properties of bridged hydroxyl groups in zeolites. A 3-21G basis set is employed in the calculations, and the bridged hydroxyl group is modeled via an aluminosiloxane unit. Increasing angle SiOAl is shown to give rise to decrease in the stretching frequency of the OH group (anti v/sub OH/) with a corresponding decrease in the ionicity of the OH bond. The decrease in ionicity of the OH bond indicates that increasing angle SiOAl gives rise to a decrease in acidity. The decreased strength of the OH bond as evidenced by the decreased anti v/sub OH/ value suggests that the OH bond is unstable when situated at a high angle SiOAl link with the decreased polarity of the bond tending to favor homolytic dissociation over heterolytic dissociation. It is also shown that angle SiOH exhibits decreased flexibility with increasing angle SiOAl, suggesting that the SiOH bending frequency should be greater for zeolites with higher TOT angles.