Surface hydroxyl groups tetrahedral aluminum ions

Zecchina (123) mentioned the appearance of two complex groups of OH-stretching bands at 3800 to 3780 cm-1 and 3740 to 3690 cm-1, respectively, on surfaces of rj- and 7-Al2 03. The first group of bands was assigned to hydroxyls belonging to the coordination sphere of tetrahedral Al3+ ions, and the second group to hydroxyls shared by an octahedral and a tetrahedral aluminum ion. 

Lewis acidity, arising from electron deficient centers, does not exist in silica. In alumina this acidity is associated with surface aluminum ions which are coordinated in a tetrahedral manner. Such sites may be generated either by removal of water coordinated to an aluminum center or by dehydration of two adjacent hydroxyl groups above 500°C. 

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