Al-pillared saponites. Part 3.—Effect of parent clay layer charge on the intercalation–pillaring mechanism and structural properties

Abstract

A series of synthetic saponites (smectite clays with tetrahedral substitution) have been intercalated by the Al₁₃ polycation followed by pillaring (anchoring of the pillars to the layers on calcination to 500 °C).

Even the clays with the highest layer charge have been successfully intercalated and pillared, but the amount of Al intercalated never exceeds one Al₁₃ per six unit cells. This limit appears to be due to steric constraints at the interface between the intercalating solution and the delaminated clay. Furthermore, there is a competition between flocculation and Al₁₃ intercalation: low-layer-charge saponites flocculate quickly and polycation intercalation proceeds only slowly thereafter. The ordering (followed by X-ray diffraction) and surface area of intercalated and pillared samples are well correlated with the amount of Al₁₃ pillars in the interlayers.

²⁹ Si NMR chemical shifts have been correlated with the layer charge: ²⁹Si spectra undergo systematic changes for pillared samples owing to the anchoring of the pillars to the clay layers.

²⁷ Al NMR and IR reveal that the Al₁₃ pillars keep their basic structure on heating to 500 °C, although they lose some terminal water ligands, leaving five-coordinated Als that constitute Lewis acidic centres. Brønsted acidity is also present, owing to the remaining H₂O and OH groups on the pillars, and possibly also to Si—OH—Al groups formed in the tetrahedral sheets.

These observations allow us to present a schematic picture of the reactions involved in saponite clay pillaring.