The effect of solution chemistry on the preparation of MgAl2O4 by hydrothermal-assisted sol-gel processing

Preparation of magnesium aluminate spinel powder by hydrothermal-assisted sol-gel processing from MgAl₂(OCH₂CH₂OR)₈, RCH₃ (1), CH₂CH₂OCH₃ (2), MgAl₂[OCH(CH₃)₂]₈ (3) and MgAl₂(O-^sBu)₈ (4) in toluene and parent alcohol has been investigated. Coordination status of aluminum atom in precursors was determined by ²⁷Al NMR and correlation between coordination number of aluminum and development of spinel phase in hydrothermal-assisted sol-gel processing has been studied. The gels obtained from hydrothermal-assisted hydrolysis of magnesium-aluminum alkoxides that contain six-coordinated aluminum atoms in solution (1 and 2) after calcination at 700 °C resulted in the formation of pure spinel phase, whereas in similar hydrolysis and calcination processes of precursors that contain four-coordinated aluminum (3 and 4) spinel phase forms along with some Al₂O₃ and MgO. Selected powders obtained from hydrothermal-assisted sol-gel processing were characterized by thermal analysis (TGA/DSC), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Results indicate that the coordination status of aluminum in the precursor is very crucial for the formation of pure phase spinel. The morphology of prepared spinels was studied by SEM and the results showed that the solvent in hydrothermal-assisted sol-gel processing has a marked effect on the morphology of the resulting MgAl₂O₄. In hydrothermal-assisted sol-gel processing of aluminum-magnesium alkoxides in hydrophobic solvent, spherical particles are formed, while in the parent alcohol, non-spherical powders are formed.