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 these materials have been measured in the Bronsted acid catalysed rearrangement of alpha-pinene to camphene, and the Lewis acid catalysed rearrangement of camphene hydrochloride to isobornyl chloride, following thermal activation at temperatures from 75 to 350 degrees C. The surface acidities of the ion-exchanged clays have been measured using a microcalorimetric method involving ammonia adsorption, and through the infrared spectra of adsorbed pyridine. The results show that maximum Bronsted acidity is generated on thermal activation at approximately 150 degrees C and maximum Lewis acidity at 250-300 degrees C. A good correlation has been found between the surface acidities and the catalytic activities of the ion-exchanged clays in both reactions. A significant result is the relatively low surface Lewis acid strength of Al3+-exchanged clays, for which a possible explanation is proposed.