Metal occupancy and its influence on thermal stability of synthetic saponites

Abstract

Due to high surface acidity and thermal stability, saponite has important applications in various fields. In this study, saponites with different Mg/Zn ratios (0–6) and a fixed Si/Al ratio of 5.43 were synthesized under hydrothermal condition. The resultant synthetic saponites were characterized by using XRD, FTIR, TG, SEM and MAS NMR. The characterization results demonstrated that, with a decrease of Mg/Zn ratio, both the crystallinity and layer stacking order of saponite were increased, and rose-like morphology was observed in Zn-saponite. Al3+ preferentially occupied tetrahedral sites in Si-O tetrahedral sheet in all synthetic saponites. A decrease of Mg/Zn ratio in octahedral sheet could improve the substitution extent of Al3+ for Si4+ and resulted in an increase of Al(IV)/Al(VI) ratio in the resultant saponite, which might be a crucial factor to control the crystal growth of saponite. The dehydroxylation temperature of saponite depended strongly on the Mg/Zn ratio and the replacement of Al3+ for Mg2+ and Zn2+ in octahedral sheets, due to their different bond strength. A ‘one-to-one’ mode was proposed for the substitution of Al3+ for Mg2+ and Zn2+ in octahedral sheets of the synthetic saponites.