Organic reactions that take place under acid catalysis can be
very efficiently carried out using clay catalysts, which may
function as Bronsted or Lewis acids or both. The Lewis acidity
is due to AI3+ and Fe3+ at the crystal edges, and can be further
enhanced by exchange of the interlayer cations Na+ and Ca2+ by
AP+ ions on treating the clay with AICl3 solution.
The Bronsted acid character of clays arises mainly due to the
dissociation of the intercalated water molecules coordinated to
cations (1). Higher levels of Bronsted acidity are achieved when
highly polarizing ions such as Cr3+ have exchanged for N a +,
Ca2+, etc. present in the natural clay, and the water content is
low. Besides the total acidity,