Aluminum chlorohydrate III: Conversion to aluminum hydroxide

Area of Science:

• Materials Science
• Inorganic Chemistry
• Mineralogy

Background:

• Aluminum hydroxide polymorphs, such as bayerite and gibbsite, are crucial in various industrial applications.
• Understanding the formation mechanisms of these polymorphs is key to controlling their properties.
• Aluminum chlorohydrate serves as a precursor in the synthesis of aluminum hydroxide.

Purpose of the Study:

• To investigate the formation pathways of bayerite and gibbsite from aluminum chlorohydrate solutions.
• To elucidate the role of solution chemistry, specifically the hydroxyl to aluminum ratio and dilution, in polymorph selection.
• To relate the observed polymorph formation to the structure of the Al13O4(OH)24(H2O)7+ complex.

Main Methods:

• Titration of aluminum chlorohydrate solutions with sodium hydroxide to systematically alter the hydroxyl to aluminum ratio.
• Controlled dilution of aluminum chlorohydrate solutions with deionized water.
• Characterization of the precipitated aluminum hydroxide polymorphs (e.g., using X-ray diffraction, microscopy).

Main Results:

• Bayerite readily forms when the hydroxyl to aluminum ratio in aluminum chlorohydrate solutions reaches 3 via sodium hydroxide titration.
• Gibbsite formation is induced by the dilution of aluminum chlorohydrate solutions with water.
• Both conversion pathways are mechanistically linked to the specific structure of the Al13O4(OH)24(H2O)7+ polycation.

Conclusions:

• Solution chemistry, specifically pH and concentration, dictates the precipitation of bayerite versus gibbsite.
• The Al13O4(OH)24(H2O)7+ complex acts as a fundamental building block or precursor in the formation of both aluminum hydroxide polymorphs.
• This study provides insights into the controlled synthesis of specific aluminum hydroxide polymorphs.