Liquid-Feed Flame Spray Pyrolysis of Metalloorganic and Inorganic Alumina Sources in the Production of Nanoalumina Powders

Liquid-feed flame spray pyrolysis (LFFSP) of metalloorganic N(CH2CH2O)3Al, alumatrane, and Al(Acac)3 and inorganic alumina AlCl3 and Al(NO3)3·9H2O precursors dissolved in 1:1 ethanol/THF, aerosolized with O2 and ignited can produce quite different alumina nanopowders during the ensuing combustion process. The metalloorganics appear to volatilize and combust easily to give nano-alumina, with particle sizes 70 nm and surface areas of ≈12 m2/g. AlCl3 appears to volatilize easily but does not hydrolyze rapidly in the flame leading to mixtures of alumina and recovered AlCl3. The resulting nanopowders consist of a mixture of transition alumina phases, primarily δ*, that could only be successfully identified and quantified by Rietveld refinement. Because the δ phase is not typically made as a high-surface-area material or in large quantities, it offers the opportunity to serve as a novel catalyst support. On heating to 1000 °C, the dominant phase becomes θ-Al2O3 that was clearly identified by 27Al MAS NMR using ab initio calculations of the 27Al NMR parameters derived from the X-ray structure. At present, the exact mechanism(s) whereby particles nucleate and grow, and phases form from the species generated during combustion, remains unknown.