Characterization of pillared layered structures

Recent progress in characterizing pillared layered materials, and especiall y PILCs (pillared interlayered clays) are reviewed in terms of the pillarin g process. Smectite platelet aggregation and the influence of pillaring age nts on it (especially Al-13-Keggin ion) are described, and the aqueous Ails species themselves. It is established that "Al-13" solutions contain dimer ic and higher oligomeric species, that Ga3+ substitution into Keggin ion (t o give [GaO4Al12(OH)(24)(OH2)(12)](7+)) has a stabilizing effect, which in turn thermally stabilizes PILCs derived from it. Chemical derails of the me chanism during precursor formation are still obscure, but Al-27 and Si-29 M AS-NMR have clarified cross-linking mechanisms during calcination (confirmi ng a tetrahedral-inversion mechanism for beidellite). Montmorillonite and b eidellite give micro- and mesoporous PILCs with bimodal micro to ultra-micr opore (6-10 Angstrom) distribution. Recent XAFS results provide structural data for pillaring (in "chromia"-pillared group IV phosphates) and also dir ect evidence for cross-linking. The complex interplay between platelet/coll oid ordering and micro-meso porosity in PILCs, and new porosity determinati on methods, are described, as also catalytic ones for monitoring acidity/sh ape-selectivity. Small- (air-gases, hydrocarbons) and medium-sized (e.g., p yrene) molecules sorption, and AFM investigations are discussed. Both indic ate that pillar density, and even strain removal from TOT smectite layers o n pillaring, are measurable. Prospects are outlined for applying these adva nces in characterization methods to newer sol-gel prepared PLS.