The structure of acid treated sepiolites: small-angle X-ray scattering and multi MAS-NMR investigations

Structural changes in the chain silicate sepiolite brought about by acid treatment were studied by X-ray diffraction, gas adsorption and MAS-NMR methods. It was established that as the amount of iron and aluminum extracted from the mineral is increased, the specific surface area of the sample is also increased (from 195.2 to 306.6 m2/g) and the originally microporous structure is transformed into a mesoporous one. The ion exchange capacity of sepiolite (0.486 mequ/g) may be completely eliminated by acid treatment. The changes in the surface fractal dimension Ds (from 2.35 to 2.03) and the correlation length lc (from 24.9 nm to 17.6 nm) were followed by SAXS measurements. 29Si− and 27Al−MAS−NMR studies yield information on changes occurring in the structure of the mineral, i.e., on the arrangement of Si and Al atoms in the lattice. During acid treatment the clay structure is progressively transformed into amorphous silica–alumina.