The swelling properties of Al-pillared clays, obtained from five different smectites, were studied using X-ray diffraction.
These clays, the dioctahedral beidellite and montmorillonite and the trioctahedral saponite, hectorite and laponite differ
in source of isomorphic substitution and represent a series of decreasing basicity along the siloxane plane. An Al oxyhydroxy
cation was inserted between the layers to form the respective pillared clays and these clays were heated incrementally to
600°C. The XRD peaks at each stage of heating were recorded as well as the same samples subsequently wetted. Basal spacings
of each clay at each stage of dehydration ↭d rehydration indicated that the swelling of tetrahedrally substituted saponite
and beidellite was indeed restricted, compared with the other three clays. This was attributed to greater basicity of the
oxygen plane of beidellite and saponite due to tetrahedral substitution of Si by Al, resulting in an increase in the strength
of hydrogen bonds between either water or the interlayer polyhydroxy cation and the clay.
The data from the XRD analyses helped in addition, to clarify the thermal transformations of the Keggin ion itself. According
to the changes in thed-spacings of the pillared clays it was concluded that the Keggin ion lost its structural water at ∼200°C and dehydroxylated
in a range beginning at 350°C. Between 500 to 600°C this polymer cation, which is thought to form the Al2O3 oxide, did not rehydrate.