Identification of reactive sites in calcined kaolinite and montmorillonite from a combination of chemical methods and solid-state nmr spectroscopy

Abstract

Kaolinite and montmorillonite have been examined at heating temperatures from 500–1100 °C. For each sample, the degrees of dissolution in acidic and basic media have been determined, using a 1.0 vol.% HF solution and an 8.0 M NaOH solution, respectively. The solid residues from these experiments are analyzed along with the calcined starting materials by 27Al and 29Si MAS NMR. A comparison of these spectra, before and after dissolution, enables a clear differentiation of the silicon and aluminium environments that are present in each sample, providing direct information about the aluminate and silicate species which are dissolved under basic and acidic conditions, i.e., identification of the active sites in calcined clays. Moreover, this procedure facilitates the structural assignment of the different silicon environments observed by 29Si NMR, shedding light on the dehydroxylation process and on the structural changes that occur for kaolinite and montmorillonite upon heat treatment.