NMR study on reaction processes from aluminum chloride hydroxides to alpha alumina powders

Starting from gelatinous aluminum chloride hydroxide, the transformation process toward alpha-Al2O3 was examined using Al-27 NMR, both in the liquid and solid states, as a main analytical tool. By increasing the hydrolysis ratio (h, defined as [OH-]/[Al3+]) of the starting aqueous precursor up to h = 2.5, the transition temperature to the final product, alpha-Al2O3, decreased to as low as 500 degrees C. In this case, the structural change from amorphous alumina to alpha-Al2O3 took place without intermediate transition Al2O3 phases. Examining the process of networking during the transition from aqueous sol-through the state of xerogel-to final anhydrous oxide by nuclear magnetic resonance (NMR) revealed the presence of highly polymeric species mainly ascribed to delta-[Al2O8Al28(OH)(56)(H2O)(24)](18+) (delta-Al-30). delta-Al-30 species were found in the solution phase and became predominant after drying. We conclude that the lower temperature synthesis of alpha-Al2O3 became possible due to preformation of polymerized AlO6 construction units in the precursor, reducing the energy barrier for the nucleation of the final alpha-Al2O3 phase.