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 subst itution) have been intercalated by the Al-13 polycation followed by pi llaring (anchoring of the pillars to the layers on calcination to 500 degrees C). Even the clays with the highest layer charge have been suc cessfully 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 s olution and the delaminated clay. Furthermore, there is a competition between flocculation and Al-13 intercalation: low-layer-charge saponit es flocculate quickly and polycation intercalation proceeds only slowl y thereafter. The ordering (followed by X-ray diffraction) and surface area of intercalated and pillared samples are well correlated with th e amount of Al-13 pillars in the interlayers. Si-29 NMR chemical shift s have been correlated with the layer charge: Si-29 spectra undergo sy stematic changes for pillared samples owing to the anchoring of the pi llars to the clay layers. Al-27 NMR and IR reveal that the Al-13 pilla rs keep their basic structure on heating to 500 degrees C, although th ey lose some terminal water ligands, leaving five-coordinated Als that constitute Lewis acidic centres. Bronsted acidity is also present, ow ing to the remaining H2O and OH groups on the pillars, and possibly al so to Si-OH-Al groups formed in the tetrahedral sheets. These observat ions allow us to present a schematic picture of the reactions involved in saponite clay pillaring.