Early Stages of Alumina Sol‐Gel Formation in Acidic Media: An 27Al Nuclear Magnetic Resonance Spectroscopy Investigation

Abstract

The effects of processing conditions on the formation of alumina sols from the hydrolysis of alkoxide precursors have been investigated using 27Al nuclear magnetic resonance (NMR) spectroscopy. In the range of acid ratios between ∼0.3 to 1.0 mol of HNO3 per mole of alumina sec‐butoxide, the NMR data show that sols processed at 20°C, low temperature (LT), differ greatly from those processsed at 90° to 95°C, high temperature (HT). In the LT sols, the aluminum is predominantly in the form of the polyoxycation species, Al13O4(OH)24(H2O)xn+ (where n is probably 7), rather than a folded defect boehmite structure as suggested by Pierre and Uhlmann. Furthermore, heating the LT sols at 90°C does not transform them to the HT form directly, but rather results in the growth of new tetrahedral and octahedral alumina species.