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

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.