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 <20 nm and corresponding surface areas of 60 m2\g at rates of 50 g\h. In contrast, the nitrate appears to melt during combustion rather than volatilize, forming large, hollow particles typical of a spray pyrolysis process 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 *, that could only be successfully identified and quantified by Rietveld refinement. Because the 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 -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.