Alumina-silica cracking catalyst owes its activity primarily t o a number of acid sites located in the catalyst surface. These sites are formed in the preparation of catalyst by the condensation of the surface hydroxyl groups of the incompletely polymerized silica hydrogel with the hydroxyl groups of the hydrolyzed aluminium ions. The formation of acid is explained by the known tendency of the aluminium atom t o acquire a pair of electrons, enhanced by the displacement of electrons in the A1-0-Si bonds towards the silicon atom carrying four positive charges. The electron pair is donated by water, the hydroxyl group becomes part of the structure, and the hydrogen ion is held by electrostatic attraction. The activity of the catalyst is thus reduced t o the same general basis as the activity of aluminium halides.The acidity (acid strength) of the surface of the catalyst has been shown t o be in the range of strong acids known t o promote hydrocarbon reactions. A linear relationship was found between the activity of the catalyst for polymerization of propylene and the number of acid sites in the surface. A more complex relationship was observed in the cracking of isopropylbenzene.

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