Electronic and structural properties of montmorillonite - a quantum chemical study

The importance of geometry and its relation with property in montmorillonite have been studied in terms of octahedral aluminum and tetrahedral silicon present in the framework. Quantum chemical (QC) calculations were performed using MNDO (modified neglect of differential overlap) and DFT (density functional theory) is reported. Cluster calculations were performed on two octahedral aluminum sites present in the unit cell of montmorillonite to show the distinguishability between the two octahedral aluminums. The energy difference between two aluminum octahedral cluster was 1.12 eV. This was further justified by the calculation of magnesium substitution energy for two different positions of aluminum. This difference in two octahedral aluminum sites was mainly due to the typical orientation of the bridging hydroxyl groups attached with the octahedral aluminums. Calculations were also performed on tetrahedral silica hexagons with and without aluminum substitution to predict the Bronsted acid strength of the surface both by MNDO and DMOL QC methods; which shows that aluminum substitution is an unfavorable process in this type of clay. The above structure property relationship in case of montmorillonite will further help in explaining the catalytic activity of the material. (C) 1998 Elsevier Science B.V. All rights reserved.