Abstract
Boehmite aqueous suspensions were acidified (peptized) with dilute nitric acid at room temperature to produce colloidal sol systems. These were characterized by27Al nuclear magnetic resonance spectroscopy. Two resonances in the octahedral region of the spectrum were identified. A narrow resonance was assigned to the hexaaquo ion[formula]and its monomeric hydrolyzed species, whose behavior is very similar to that of Al3+aqueous systems under self or forced hydrolysis. The other broader resonance shows a second-order quadrupolar induced chemical shift. Based on its field-dependent behavior, this peak has an isotropic chemical shift of 11.7 ppm and a quadrupolar coupling constant of 2.9 MHz. We attribute it to27Al nuclei in the colloidal particles of boehmite present in the system, with an estimated correlation time of 35 ns. When the sol–gel transition occurs, this resonance decreases in intensity, giving rise to a very broad signal at about the same frequency, tentatively assigned to larger derived structures from aggregation of the colloidal particles. This transformation was reversible at high temperature. The acidic peptization of the studied boehmite was characterized as a partial dissolution process. The amount of Al3+released to solution is nearly proportional to the total alumina content in the suspension and to the acid to aluminum molar ratio.
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