Preparation and properties of layered silica and layered alumino-silica hydrate from natural apophyllite

摘要

Crystalline layered silica hydrates were preparedfrom a natural apophyllite pretreated with 0.1 mol.dm~-3hydrochloric acid at 5beg or 25beg C. In these treatments, aproduct with higher crystallinity was obtained by acidic treatmentat a lower temperature. The two silica hydrates had the differentlocal silica structure, and were found to correspond to the silicareported as an H-apophyllite and a Silica-AP, respectively. Thesilica hydrates have a fundamental silica sheet structure ofapophyllite, and the structure is destroyed by heating above 200begC due to condensation of silanol groups in the interlayer spaces.On the other hand, a crystalline layered alumino-silica hydrate wasalso obtained from a natural apophyllite pretreated with analuminum chloride solution. Formation of four different hydratephases depended mainly upon pH of the reacting solution and/ortemperature in the drying process. The introduced aluminum ionwas coordinated tetrahedrally and occupied mainly at the interlayerof the silica sheets. On heating, these alumino-silicates maintainedtheir crystal structures up to ca. 400beg C. These results suggestedthat the introduced aluminum ion should prevent the basal silicasheet distance from narrowing and the silanol groups fromcondensing. In consequence, the sheet structure of these alumino-silicates were more stabilized thermally than that of the silica hydrate.