Characterization of pillared layered structures

Recent progress in characterizing pillared layered materials, and especially PILCs (pillared interlayered clays) are reviewed in terms of the pillaring process. Smectite platelet aggregation and the influence of pillaring agents on it (especially Al(13)-Keggin ion) are described, and the aqueous Ails species themselves. It is established that "Al(13)" solutions contain dimeric and higher oligomeric species, that Ga(3+) substitution into Keggin ion (to give [GaO(4)Al(12)(OH)(24)(OH(2))(12)](7+)) has a stabilizing effect, which in turn thermally stabilizes PILCs derived from it. Chemical derails of the mechanism during precursor formation are still obscure, but (27)Al and (29)Si MAS-NMR have clarified cross-linking mechanisms during calcination (confirming a tetrahedral-inversion mechanism for beidellite). Montmorillonite and beidellite give micro- and mesoporous PILCs with bimodal micro to ultra-micropore (6-10 Angstrom) distribution. Recent XAFS results provide structural data for pillaring (in "chromia"-pillared group IV phosphates) and also direct evidence for cross-linking. The complex interplay between platelet/colloid ordering and micro-meso porosity in PILCs, and new porosity determination methods, are described, as also catalytic ones for monitoring acidity/shape-selectivity. Small- (air-gases, hydrocarbons) and medium-sized (e.g., pyrene) molecules sorption, and AFM investigations are discussed. Both indicate that pillar density, and even strain removal from TOT smectite layers on pillaring, are measurable. Prospects are outlined for applying these advances in characterization methods to newer sol-gel prepared PLS.