A thermo-XRD study of Al-pillared smectites differing in source of charge, obtained in dialyzed, non-dialyzed and washed systems

The mechanism of Al-pillaring of the following smectites: montmorillonite, beidellite, saponite, hectorite and Laponite was investigated. For this purpose the products of three different preparation techniques that had been applied on the Keggin-ion smectite suspensions were studied by thermo-XRD analysis. These techniques included: (a) dialysis of the clay-Keggin ion suspensions; (b) aging of the smectites in the Keggin ion solution, and (c) separation between the clay and the supernatant after 24 h and washing the smectite to remove excess Na, Al and Cl. In the first stage after treating the smectite with the Keggin ion solution, the Keggin ion dissociated into smaller units. The adsorption of Al by the smectites took place in the first 24 h, but was very high relative to the cation exchange capacity of the smectites, indicating that the adsorbed species were hydroxy and/or chloro Al complexes. Thermo-XRD analysis showed that these adsorbed cations were monomers or very small oligomers. Large three-dimensional oligomers, which are essential for the formation of thermally stable pillared clays, were obtained inside the interlayer space after 7 days of dialysis or after thoroughly washing the smectites. At 600°C, the Al oxocation condensed with the silicate layer and the heated smectite did not swell in water. Pillared clays were obtained with montmorillonite, beidellite and saponite after 7 days of dialysis or repeated washings (in 5 days). Under the conditions used in the present study ([OH]/[Al] ratio of 2.3 and Al concentration 0.07 M) Laponite and hectorite did not form thermally stable pillared clays. (C) 2000 Elsevier Science B.V. All rights reserved.