BRONSTED AND LEWIS-ACID CATALYSIS WITH ION-EXCHANGED CLAYS

An acid-treated montmorillonite clay has been ion-exchanged with Al3+, Fe3+, Cu2+, Zn2+, Ni2+, Co2+, and Na+. The catalytic activities of th ese materials have been measured in the Bronsted acid catalysed rearra ngement of alpha-pinene to camphene, and the Lewis acid catalysed rear rangement of camphene hydrochloride to isobornyl chloride, following t hermal activation at temperatures from 75 to 350 degrees C. The surfac e acidities of the ion-exchanged clays have been measured using a micr ocalorimetric method involving ammonia adsorption, and through the inf rared spectra of adsorbed pyridine. The results show that maximum Bron sted acidity is generated on thermal activation at approximately 150 d egrees C and maximum Lewis acidity at 250-300 degrees C. A good correl ation has been found between the surface acidities and the catalytic a ctivities of the ion-exchanged clays in both reactions. A significant result is the relatively low surface Lewis acid strength of Al3+-excha nged clays, for which a possible explanation is proposed.