Low temperature synthesis of α-alumina from aluminum hydroxide hydrothermally synthesized using [Al(C2O4)x(OH)y] complexes

• α-Alumina has been produced through the gibbsite obtained using [Al(C2O4)x(OH)y] complex at 900 °C.
• Adding oxalic acid caused the morphology to change to a speared form.
• α-Alumina showed an egg-shaped morphology by adding 3.0 mol of oxalic acid.
• α-Alumina produced through [Al(C2O4)3] complex has higher density of 1.359 g cm−3.

This study describes the low temperature synthesis of α-alumina from aluminum hydroxide hydrothermally synthesized using [Al(C2O4)x(OH)y] complexes, in which Al3+ chelated by bidentate oxalate at various molar ratios. Aluminum hydroxide hydrothermally synthesized at 200 °C without oxalic acid exhibited a monoclinic boehmite (AlOOH) structure with hexagonal shape. However, adding oxalic acid caused the morphology to change to a spear-shaped boehmite/gibbsite mixed structure. Thermal treatment of the bohemite typed aluminum hydroxide-0 hydrothermally synthesized without oxalic acid produced a rhombohedral α-phased alumina with a rice-shaped at above 1200 °C. However α-alumina was obtained at lower temperature by the bohemite/gibbsite mixture typed aluminum hydroxide-1, -2, and -3 hydrothermally synthesized with oxalic acid. In particular, the aluminum hydroxide-3 having more gibbsite portion hydrothermally synthesized using 3.0 mol of oxalic acid was easily changed to the egg-shaped rhombohedral α-alumina at temperatures lower than 900 °C. On increasing the oxalic acid concentration, the density of α-alumina increased to 1.359 g cm−3.

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