Dehydroxylation mechanisms in Al3+/Fe3+ dioctahedral phyllosilicates by quantum mechanical methods with cluster models

Atomic models involving the dehydroxylation process of dioctahedral phyllosilicates without interlayer charge were used to calculate energies and explore the reaction paths of the possible mechanisms of this reaction at a quantum mechanical level. The geometrical features and electronic structure of a molecular cluster model of two edge-sharing octahedrally coordinated cations coupled to a ring of six silicate tetrahedra was evaluated by ab initio molecular orbital calculations with Hartree-Fock approximation. Two dehydroxylation mechanisms are considered. One mechanism involves two contiguous hydroxyl that are on an octahedron shared edge that joins a pair of octahedral cations. The other model considered involves OH loss from across an octahedral vacant. The substitution effect of Al3+ by Fe3+ in the octahedral sheet on the activation energy and structural transformations is compared by minimization of the critical points of the Potential Energy Surface (PES) for the reactant, transition state and product along the reaction path of the dehydroxylation process. The calculated energy differences and vibration frequencies are according to previous experimental results. The dehydroxylation mechanism involving OH across the octahedral hole, is less energetically favorable and is endothermic.