FORMATION OF PEPTIZABLE BOEHMITES BY HYDROLYSIS OF ALUMINUM NITRATE IN AQUEOUS-SOLUTION

Microcrystalline boehmites were synthesized by aging amorphous precurs ors derived from base hydrolysis of aluminum nitrate solutions at 85 d egrees C. The susceptibility of the resultant boehmite gels to form a colloidal dispersion by reaction with dilute nitric acid, i.e., their peptizability, was determined based on their particle size measured by dynamic light scattering. Three different classes of peptizabilities were obtained by modifying parameters during the neutralization step a nd were related to the characteristics of the boehmite gel and its pre cursors before aging. X-ray diffraction, thermal analysis, solid state Al-27 NMR, and transmission electron microscopy techniques were appli ed to the characterization of the materials. The preparation of peptiz able boehmites was favored by low temperature hydrolysis where the amo rphous AIO(OH) precursor, after aging, readily converted to boehmite c rystallites that did not aggregate into secondary particles. Nonpeptiz able boehmites were obtained at hydrolysis ratios (OH/AI > 3) at high temperature, leading to boehmite crystallites which condensed to large strongly interconnected polycrystalline fibrils. Partly peptizable bo ehmite was obtained at high temperatures but lower OH/AI ratios. Under this condition, the fast crystallization of boehmite in the neutraliz ation step was retarded, thus hindering strong intercrystallite aggreg ation. This could be correlated to the presence of a significant porti on of tetrahedrally coordinated aluminum in the precursor. A brief dis cussion on the mechanisms of formation of the aluminum gel precursors is also presented. (C) 1997 Academic Press.