AL-PILLARED SAPONITES .3. EFFECT OF PARENT CLAY LAYER CHARGE ON THE INTERCALATION-PILLARING MECHANISM AND STRUCTURAL-PROPERTIES

A series of synthetic saponites (smectite clays with tetrahedral substitution) have been intercalated by the Al-13 polycation followed by pillaring (anchoring of the pillars to the layers on calcination to 500 degrees C). Even the clays with the highest layer charge have been successfully intercalated and pillared, but the amount of Al intercalated never exceeds one Al-13 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 Al-13 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 Al-13 pillars in the interlayers. Si-29 NMR chemical shifts have been correlated with the layer charge: Si-29 spectra undergo systematic changes for pillared samples owing to the anchoring of the pillars to the clay layers. Al-27 NMR and IR reveal that the Al-13 pillars keep their basic structure on heating to 500 degrees C, although they lose some terminal water ligands, leaving five-coordinated Als that constitute Lewis acidic centres. Bronsted 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.