Sol-Gel Synthesis of Transparent Alumina Gel and Pure Gamma Alumina by Urea Hydrolysis of Aluminum Nitrate

Abstract

γ-Alumina was synthesized by a sol-gel method with the aluminum ion hydrolysis control performed by urea. The initial saturated Al3+/urea solution presented urea coordinated with the aluminum ion, as shown in the 13C NMR and 27Al NMR spectra and longitudinal relaxation times, T1, from the latter. The substitution of water molecules in the Al3+ coordination shell by urea controlled the hydrolysis process and provided an extensive nucleation during the initial steps of the aluminum hydroxide formation due to urea thermolysis at 90°C. The resulting sol, composed of Al(OH)3 nanoparticles, coalesced and became a transparent gel permeated by a solution of urea and the polycation ion [Al13O4(OH)24(H2O)12]7+. The freshly prepared gel was transformed, under heating at 300°C, directly to γ-alumina, characterized by FTIR, 27Al-MAS-NMR and SBET techniques, without δ- or θ-phases, as a consequence of the high degree of homogeneity of the γ-alumina precursor.