FTIR spectra of hydroxyls and dehydroxylation kinetics mechanism in montmorillonite

Abstract

The application of Fourier transform infrared (FTIR) spectroscopy to the analysis of the hydroxyl groups bands' intensities of montmorillonite from Texas shows four regions of intensity loss rate for thermally shocked samples at 290 673 K. However, what is surprising is the persistence of very weak water stretching (∼3470 cm−1) and bending (∼1628 cm−1) vibrations at 553<T<773 K. It is speculated that this water, formed because of dehydroxylation, is trapped in the hexagonal cavities of the dehydrated montmorillonite lattice. However, conclusive evidence will require surface-sensitive spectroscopic measurements as this water could also be adsorbed on the external surfaces of processed samples. In the range 773<T<823 K, the main dehydroxylation of the AlAl-OH group results, and this reaction induces structural transformations in the montmorillonite lattice. FTIR measurements at 803 K for 0<t< 25 h were used to determine the kinetics mechanism of dehydroxylation in montmorillonite from Texas. The experimental data was tested, using diffusion controlled as well as six decomposition models to ascertain the kinetics mechanism of the AlAl-OH group's dehydroxylation. It appears that the dehydroxylation process can be described by the contracting spherical movement model rather than by a diffusion controlled model, suggesting surface nucleation, growth over the surface, and then advancement of the dehydroxylated/hydroxylated interface toward the center of the montmorillonite particles.