The Lewis Acid Sites of Aluminas and SAs

The catalytic activity of transitional aluminas (y-, T)-, 5-, 6-AI2O3) are undoubtedly mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as weU as to the high ionicity of the surface Al—O bond [101]. Alumina s Lewis sites have been well characterized by adsorption of several probes. They are the strongest among metal oxides. The number of such very strong Lewis sites present on transitional alumina surfaces depend on the dehydroxylation degree (depending on the activation temperature) and on the particular phase and preparation. 

Although it is clear that surface Lewis acid sites on alumina are due to coordi-natively unsaturated AP ions, it is not fully clear what is the coordination of such surface ions. Most authors agree that at least three different types of Lewis acid sites (weak, medium, strong) exist on transitional aluminas, arising in some way from the two or three coordinations of the ions in the bulk spinel-type structure. 

Adsorbate CO Pivalonitrile Pyridine Ammonia Adsorbing site type 

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