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(CH 2 CH 2 O) 3 Al, alumatrane, and Al(Acac) 3 ] and inorganic alumina [AlCl 3 and Al(NO 3 ) 3 ·9H 2 O] precursors dissolved in 1:1 ethanol/THF, aerosolized with O 2 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 m 2 /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 m 2 /g. AlCl 3 appears to volatilize easily but does not hydrolyze rapidly in the flame leading to mixtures of alumina and recovered AlCl 3 . 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-Al 2 O 3 that was clearly identified by 27 Al MAS NMR using ab initio calculations of the 27 Al 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.