Novel synthesis of Al13-cluster based alumina materials

Abstract

The polyoxocation Al₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺, known as an Al₁₃ cluster, has been synthesized by controlled hydrolysis of aluminum(III) aqueous solution at 70 ^oC, and pH between 5.0 and 6.0. The formation of the polyoxocation Al₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺ in solution was experimentally confirmed by ²⁷Al NMR. The Al₁₃ cluster-containing powder was obtained by precipitating the polyoxoaluminate ion from the solution by the addition of a suitable salt, e.g. sodium sulphate or ammonia oxalate. The obtained powder was characterized by TGA, XRD, and SEM. It was found that the sulfate-precipitated powder had three different crystal structures, i.e. monoclinic fiber, monoclinic rectangle shaped crystal and tetrahedral shaped crystal, while the oxalate powder obtained from the oxalate precipitation was a spherical-particles powder with particle size around 200 nm. XRD of the Al₁₃-sulfate showed that the powder consisted of a cubic phase with a unit cell size of 17.9Å. After calcination of the Al₁₃-sulfate and Al₁₃-oxalate salts at 1000 °C, the powder was converted to α-alumina. The α-alumina powder obtained from the oxalate route was about 100 to 200 nm in diameter.