Al-Pillared saponites. Part 1.—IR studies
Abstract
Al-Pillared saponites have been prepared from natural Na-exchanged Ballarat saponite. Acidic properties of these saponites have been studied by IR spectroscopy using pyridine as a probe molecule. Al-Pillared saponites exhibit both Brønsted and Lewis acidity. Lewis sites are evidenced in the parent Na-exchanged clay as well as in the pillared clays. Brønsted acid sites are not evidenced in the parent Na-exchanged saponite but Al-pillared saponites exhibit Brønsted acidity as shown by a pyridinium band at 1549 cm–1. Brønsted acid sites have been related to OH groups at 3740–3720 cm–1 and 3597–3594 cm–1. The same wavenumbers are evidenced for ν(OH) stretchings in acidified saponite and in Al-pillared saponites. It is assumed that an acidic attack of Si—O—Al linkages which may provide Brønsted sites occurs on the clay sheets during the pillaring process.