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

Starting from gelatinous aluminum chloride hydroxide, the transformation process toward α‐Al 2 O 3 was examined using 27 Al NMR , both in the liquid and solid states, as a main analytical tool. By increasing the hydrolysis ratio ( h , defined as [ OH − ]/[Al 3+ ]) of the starting aqueous precursor up to h = 2.5, the transition temperature to the final product, α‐Al 2 O 3 , decreased to as low as 500°C. In this case, the structural change from amorphous alumina to α‐Al 2 O 3 took place without intermediate transition Al 2 O 3 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 δ‐[Al 2 O 8 Al 28 ( OH ) 56 (H 2 O) 24 ] 18+ (δ‐Al 30 ). δ‐Al 30 species were found in the solution phase and became predominant after drying. We conclude that the lower temperature synthesis of α‐Al 2 O 3 became possible due to preformation of polymerized AlO 6 construction units in the precursor, reducing the energy barrier for the nucleation of the final α‐Al 2 O 3 phase.