Sol-gel synthesis of transparent alumina gel and pure gamma alumina by urea hydrolysis of aluminum nitrate

gamma-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 C-13 NMR and Al-27 NMR spectra and longitudinal relaxation times, T-1, 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 90degreesC. 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 300degreesC, directly to gamma-alumina, characterized by FTIR, Al-27-MAS-NMR and S-BET techniques, without delta- or theta-phases, as a consequence of the high degree of homogeneity of the gamma-alumina precursor.