Gels from a double alkoxide: (BuO)2-Al-O-Si-(OEt)3

Abstract

NMR study (27Al,29Si) of the double Si-Al precursor (BuO)2Al-O-Si(OEt)3 in alcoholic solution shows molecular associations resulting from acid-base interactions with O → Al ≡ bondings. Gels have been prepared in the H2O, HOiPr, (BuO)2Al-O-Si(OEt)3 ternary phase diagram for a large range of oxide concentrations. For all the compositions, the very fast hydrolysis of Al-OR groups leads to an amphoteric sol. Nevertheless, the gelation time is governed by the initial water concentration, showing that the hydrolysis of Si-OR groups is the rate-determining process for the gelation. The apparent activation energy is about 20 kcal mol−1. The relative hardness of gels increases when increasing the precursor concentration. After drying, the oxide skeleton of the xerogel is relatively dense, but the open porosity remains high up to about 1000° C. At this temperature, samples turn into optically clear and dense glass-ceramics, formed of mullite crystallites in amorphous silica.