Dependence of Boehmite Thermal Evolution on Its Atom Bond Lengths and Crystallite Size

The thermal properties of boehmite with a crystallite size between 1 and 27 nm were analyzed by thermogravimetry and differential thermal analysis, and correlated with its crystallography, crystallite morphology, and the atom bond lengths determined by refining its crystalline structure with the Rietveld method. Boehmiteʹs thermal evolution depended on atom bond lengths: the dehydratation temperature was determined by the interaction between an aluminum atom and the oxygen atom of hydroxyl. After dehydratation, the number of hydroxyls in crystals was larger than expected from stoichiometry, because oxygen atoms on crystal surfaces perpendicular to the (020) plane reacted with hydrogen and hydroxyls in environment. The transition temperature from boehmite into GAMMA-alumina was determined by the hydrogen bond in boehmite. The transformation temperature from transitional aluminas into ALPHA-Al2O3 also varied with boehmiteʹs crystallite size; however, it was not determined by the amount of hydroxyls in transitional aluminas, but probably by the aluminum–oxygen bonds in boehmite.