Preparation of Alumina Catalyst Supports

Because of the amphoteric character of aluminium, it is soluble in both acidic and basic solvents. In acidic solutions below pH = 2 aluminium will be present as solvated Al3+ ions, while in basic solutions above pH = 12 it is present in the form of aluminate ions (AlO -). 

Starting from an aqueous acidic Al3+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (A10(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. (X-AI2O3 is formed at higher temperatures. 

If the initial microcrystalline boehmite gel slurry is aged at 40°C it is converted 

It is also possible to start the precipitation from a basic AlO - solution by addition of an acid. At pH below about 11 precipitation of a gel-like bayerite occurs, which can be treated as indicated above. 

Finally it is also possible to obtain a crystalline boehmite precipitate directly, by simultaneous addition of a basic and an acidic solution to a reactor to obtain precipitation in the pH range between 6 and 8. 

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