New evidence for microstructural evolution model of acid-activated montmorillonite

Ca-montmorillonite samples from Choushan treated at various acid concentrations were studied using chemical analysis, XRD, 29Si and 27Al MAS NMR, to investigate the microstructure of the activated montmorillonites.With the increase of acid concentration and dissolution of cations of montmorillonite, the intensity of d(001) decreased obviously and significant changes of the microstructure of Si and Al occurred. There were two new types of structural units of Si atoms formed: (SiO)3SiOH units and Q4(0AI)units. For Al atoms, in the course of activation, the removal of one of a pair of octahedral aluminium atoms from montmorillonite removed two hydroxyl groups and left the other aluminum of the pair in the four-fold coordination. The 27Al signal at δ 54.0 corresponding to Al? arose from the fourcoordinated Al in the octahedral sheet. This study first confirmed, by 2gsi and 27AI MAS NMR spectra, that the model of microstructural evolution for activated montmorillonite postulated by Thomas et al. (1950) is correct.``