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)(3)Al, alumatrane, and Al(Acac)(3)] and inorganic alumina [AICl(3) and Al(NO3)(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 approximate to60 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 approximate to 12 m(2)/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 delta(*) that could only be successfully identified and quantified by Rietveld refinement. Because the delta 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 degreesC, the dominant phase becomes theta-Al2O3 that was clearly identified by Al-27 MAS NMR using ab initio calculations of the Al-27 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.