A thermo-FTIR-spectroscopy analysis of Al-pillared smectites differing in source of charge, in KBr disks

Abstract

The reactions occurring by the clay framework during the thermal treatment of montmorillonite, beidellite, saponite, hectorite and Laponite saturated with the Keggin ion [Al IVO 4Al VI 12(OH) 24(H 2O) 12] 7+) were investigated by gradual heating KBr disks of the clay samples up to 600°C and recording their FTIR spectra at each 50° increment. During the thermal treatment the Keggin ion undergoes dehydration, dehydroxylation and rearrangements, the latter is seen from changes in the Al IV–O and Al VI–O vibrations. From 500°C clays dehydroxylate and are transformed into new phases with some silanol groups on the surfaces of the tetrahedral sheets. This reaction occurs at lower temperatures in the Keggin-treated smectites compared with Li- or Na-exchanged smectites. At 550°C, there is a condensation reaction of the dehydroxylated Keggin cation with Si–O-groups on the surface of the tetrahedral sheet, giving rise to new bands at 614 and 402 cm −1. This condensation reaction increases considerably at 600°C. The framework of the heated pillared clay preserves the layer structure of the precursor smectite. The adsorption of water by the pillared clays becomes very intense after heating the disks to 600°C.