Aluminum ion occupancy in the structure of synthetic saponites: Effect on crystallinity

Abstract

Two series of saponites with fixed (Si+Al)/Mg and Si/Mg ratios, respectively, were synthesized by using hydrothermal methods. The obtained products were characterized by XRD, XRF, ²⁷Al, and ²⁹Si MAS NMR, SEM, and TEM. XRD patterns showed that well-ordered saponites were obtained in the initial Si/Al ratio range of 5.43-7.89. Beyond this Si/Al ratio range, poorly crystallized saponites were obtained with small crystallized particles, which can be seen from TEM images. When intercalating saponite with surfactant, the intercalated products displayed strong and well-ordered (00l) reflections, indicating that layered saponite has been successfully synthesized in the present study. ²⁷Al MAS NMR spectra demonstrated that well-crystallized synthetic saponites had a higher Al(IV)/Al(VI) ratio than the poorly crystallized samples, which is an important factor affecting the crystallinity of synthetic saponite. A one-to-one substitution (i.e., 1 Al³+ → 1 Mg²⁺) actually occurred in the octahedral sheet and this substitution had a negative effect on the crystallinity of the synthetic saponites. After grafting the synthetic saponites with silane, the decreased intensity of the 29Si NMR signal at -86 ppm and the increased intensity of Q³ Si(0Al) and Q³ Si(1Al) signals strongly suggested that the signal at ca. -86 ppm corresponded to Q² Si at the layer edges of saponite.