Effects of Synthesis Conditions on the Thermal Stability of a Texas Montmorillonite Expanded with [Al13O4(OH)24(H2O)12]7+ Cations

Abstract

Smectites, when expanded with metal clusters, can generate layered microporous solids with an appropriate acidity, pore size, and thermal stability required for catalysts used in conventional chemical (Pinnavaia, 1983) and petrochemical processes (Figueras, 1988; Occelli, 1989). The detailed description of the ion-exchange reactions that produce pillared interlayered clay (PILC) and the conditions necessary to generate thermally stable PILC are still a subject of some debate and controversy. Investigations of the hydrolysis reactions of aqueous solutions of aluminum salts (Hsu and Bates, 1964; Baes and Mesmer, 1976; Bottero et al., 1980, 1982; Brosset et al., 1954; Bailar, 1956) showed a complicated, pH-sensitive equilibrium between Al(H2O)63+, [Al2(OH)2(OH2)4]4+, [Al3(OH)4(OH2)9]5+, and the Keggin ion [Al13O4(OH)24(H2O)12]7+ (=Al13) Dilution and refluxing of the aluminum chlorhydrol (ACH) solution affects the concentration of the Keggin species in the pillaring solution (Bottero et al., 1982). Nonetheless, in PILC, the Keggin ion may not be the sole ion in the phyllosilicate interlayer (Schoonheydt et al., 1994). Upon heating, Keggin ions may undergo condensation reactions in the interlayer with the formation of higher polymers such as the Al24-polycation (Fu et al., 1991). Moreover, the [Al41O32(OH)48(H2O)24]11+ complex has been isolated from aged ACH solution (Fitzgerald, 1988), and its presence in certain PILC samples cannot be excluded. The micropores of calcined Al13-pillared montmorillonites and volumes computed by multiplying the number of pillars present by the density of bulk alumina have been studied previously ( e.g., Maes et al., 1995). The calculated volume was ~47% larger than the …