Electronic and structural properties of reduced-charge montmorillonites

Abstract

Solid state silicon ( 29 Si ) and aluminum ( 27 Al ) nuclear magnetic resonance (NMR) spectroscopies were applied to a series of reduced-charge montmorillonites (RCMs) to discern changes in electronic and structural properties that are induced by Li fixation. Room temperature 29 Si MAS NMR spectra revealed a consistent chemical shift to more negative values and increased line width of the main Q 3(0Al) Si resonance with increasing levels of Li fixation in the RCM series. A decreased line width of the octahedral Al (Al [6]) environment was observed and may be attributed to formation of a more uniform electronic environment surrounding Al [6] as charge reduction occurs. No appreciable changes in the tetrahedral Al (Al [4]) peak were observed for the series, except for line broadening. Correlations of 29 Si NMR chemical shifts with layer charge and infrared-active structural vibrations indicated that distortions in the Si–O–T bond angles (T=tetrahedral Si or Al) occurred, with the mean Si–O–T bond angle increasing, following charge reduction. These results are interpreted as evidence of a redistribution both of layer charge and an abatement of the fit between octahedral and tetrahedral sheets following Li fixation and charge reduction. Our results are consistent with the formation of pyrophyllite-like character in reduced-charge montmorillonite.