Surface Acidity and Catalytic Activity

The catalytic activity of clay minerals is linked to their capacity to act as solid acids whose strength is commonly expressed in terms of the Hammett acidity function, H_O. The surface acidity of clay catalysts may be of the Brønsted or Lewis type. Brønsted acidity essentially arises from dissociation of water molecules associated with the exchangeable cations. Thus, the proton-donating ability of clay minerals is influenced by the water content of the system and the ionic potential of the counterion. The principal Lewis acid (electron-accepting) sites are identifiable with ferric ions in the layer structure and under-coordinated aluminum ions exposed at particle edges. Under certain conditions, exchangeable transition metal ions (e.g., Cu²⁺, Fe³⁺) in smectites can also act as Lewis acids. The coexistence of Brønsted and Lewis acid sites offers scope for a synergistic interaction between sites. The total concentration of acid sites may be measured by n-butylamine titration in the presence of Hammett indicators, while the type of acid sites can be assessed by FTIR spectroscopy of adsorbed pyridine. Examples are given of organic conversions, catalyzed by Brønsted and Lewis acid sites, or through a Brønsted-Lewis acid site combination.