Surface: active oxygen alumina, silica acid sites

Activated alumina is also widely used as a desiccant because of the same advantages for which silica gel is used. Unlike silica gel, which is amorphous, activated alumina is crystalline. Oxygen vacancies (defects) are easily formed on its surfaces, thus alumina has both Lewis and Brpnsted acid sites. The surface chemistry, as well as the pore structure of activated alumina, can be modified, for example, by treatment with acid (HCl or HF) or alkaline (to alter the acidity) and controlled thermal treatment (to tailor the pore structure). As a result, activated alumina is more versatile than sihca gel and has been applied more often as a sorbent. 

---