display some properties that are completely different from Microcrystalline boehmites were synthesized by aging amorthe well crystallized material.
phous precursors derived from base hydrolysis of aluminum ni-
Microcrystalline boehmites are of primary interest betrate solutions at 85ΠC. The susceptibility of the resultant boehmite cause of their technical importance in the preparation of gels to form a colloidal dispersion by reaction with dilute nitric coatings and controlled porosity catalysts with enhanced acid, i.e., their peptizability, was determined based on their particle physical properties (3-10). This is due to the susceptibility size measured by dynamic light scattering. Three different classes shown by some boehmite gels to peptization by dilute acids of peptizabilities were obtained by modifying parameters during at room temperature. Aluminum trihydroxides are not exthe neutralization step and were related to the characteristics of pected to have this property (3,4). Boehmite gels or xerothe boehmite gel and its precursors before aging. X-ray diffraction, thermal analysis, solid state 27 Al NMR, and transmission electron gels (powders) can be peptized to form either metastable microscopy techniques were applied to the characterization of the sols or less homogeneous pastes (high solid to liquid ratio). materials. The preparation of peptizable boehmites was favored In the latter, the plastic properties of the molded mass and by low temperature hydrolysis where the amorphous AlO(OH) compacting of its structure is assured, and thus the desired precursor, after aging, readily converted to boehmite crystallites pore structure and mechanical strength of the final alumina that did not aggregate into secondary particles. Nonpeptizable product can be achieved (5-7). On the other hand, colloidal boehmites were obtained at hydrolysis ratios (OH/Al ΓΊ 3) at high dispersions produced by acidic peptization of boehmite sustemperature, leading to boehmite crystallites which condensed to pension are required for a sol-gel process that yields porous large strongly interconnected polycrystalline fibrils. Partly peptizasolid spheres (4, 8-10). In most cases, the relationship beble boehmite was obtained at high temperatures but lower OH/Al tween peptization conditions and textural/physical characratios. Under this condition, the fast crystallization of boehmite teristics of the final product has been studied empirically.
in the neutralization step was retarded, thus hindering strong intercrystallite aggregation. This could be correlated to the presence The particles of the boehmite gel, as produced, are agof a significant portion of tetrahedrally coordinated aluminum in glomerates of crystallites. The homogeneous dispersion of the precursor. A brief discussion on the mechanisms of formation these aluminas is achieved by reducing the agglomerates to of the aluminum gel precursors is also presented. α§ 1997 Academic a sufficiently small particle size such that the particles remain Press suspended as a colloid. For some boehmites this is readily accomplished by using noncomplexing, usually monovalent acids such as nitric acid (3,4, 11