CHARACTERIZATION OF PEPTIZED BOEHMITE SYSTEMS - AN AL-27 NUCLEAR-MAGNETIC-RESONANCE STUDY

Boehmite aqueous suspensions were acidified (peptized) with dilute nit ric acid at room temperature to produce colloidal soi systems. These w ere characterized by Al-27 nuclear magnetic resonance spectroscopy. Tw o resonances in the octahedral region of the spectrum were identified. A narrow resonance was assigned to the hexaaquo ion Al(H2O)(6)(3+) ac id its monomeric hydrolyzed species, whose behavior is very similar to that of Al3+ aqueous systems under self or forced hydrolysis. The oth er broader resonance shows a second-order quadrupolar induced chemical shift, Based on its field-dependent behavior, this peak has an isotro pic chemical shift of 11.7 ppm and a quadrupolar coupling constant of 2.9 MHz. We attribute it to Al-27 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 i n intensity, giving rise to a very broad signal at about the same freq uency, tentatively assigned to larger derived structures from aggregat ion of the colloidal particles. This transformation was reversible at high temperature. The acidic peptization of the studied boehmite was c haracterized as a partial dissolution process. The amount of Al3+ rele ased to solution is nearly proportional to the total alumina content i n the suspension and to the acid to aluminum molar ratio. (C) 1995 Aca demic Press, Inc.