Catalytic properties of γ-alumina. Ab initio molecular orbital studies of clusters of chlorinated γ-alumina

Abstract

Quantum-chemical studies of the surface-site geometries for aluminium–oxo (hydroxo) and chloro aluminium–oxo (hydroxo) cluster models derived from the spinel structure of γ-alumina are reported. The ab initio MO calculations which use an SV-321G basis indicate that a stabilisation of the LUMO energy occurs with a narrowing of the energy gap, E_gap′, by the substitution of chlorine for oxygen or hydroxy groups in γ-alumina cluster models containing tetrahedral aluminium environments. The studies also show that a hierarchy of potential Brønsted and Lewis character is exhibited with Al_tet—O—Al_tet > Al_tet—O—Al_oct > Al_oct—O—Al_oct′, indicative of a relationship between morpho-electronic factors and Lewis acidity at surface adsorption sites. The Brønsted acid character of the cluster is generated by the location of a near-neighbour tetrahedral aluminium environment. Substitution of chlorine for oxygen into the cluster model enhances the Brønsted acid character for the aluminium–oxo (hydroxo) cluster. The presence of octahedral aluminium–oxo (hydroxo) environments reduces both the Brønsted and Lewis acid character, respectively, for an aluminium–oxo (hydroxo) cluster.