The positions of inner hydroxide groups and aluminium ions in exfoliated kaolinite as indicators of the external chemical environment

Abstract

Kaolinite as a remarkable industrial raw material has notable structural features despite its simple chemical composition (Al₂O₃·2SiO₂·2H₂O). We report here a systematic development of a coordination chemical model for the [6Al-6(OH)] honeycomb-like unit of kaolinite's octahedral sheet, which was proposed to be the adsorption site for small molecules from earlier studies. The coordination environment of the Al³⁺ ions was completed with outer sphere groups from both octahedral and tetrahedral sheets. Dangling bonds were terminated by additional Al³⁺ and Si⁴⁺ ions with hydroxide and oxide groups from the second coordination sphere versus simple protonation. A cage of Na⁺ and Mg²⁺ ions rendered the computational model to be charge neutral. In this exfoliated kaolinite model, the inner hydroxide groups and the adjacent Al³⁺ ions have compositionally the most complete environments with respect to the crystal structure. Thus, their atomic positions were used as a benchmark for the level of theory dependence of the optimized structures. We evaluated the performance of a representative set of density functionals, basis sets, point-charges, identified pitfalls and caveats. Importantly, the structural changes during optimization of periodic and cluster models suggest pliability for the exfoliated kaolinite layers, which is influenced by the external chemical environment.