Multinuclear NMR Study of Aluminosilicate Sol-Gel Synthesis Reasons for the Homogeneous Gelation Using the Prehydrolysis Method

Aluminosilicate gels have been utilized in numerous optical, dielectric, and catalytic applications. These applications require a high degree of molecular homogeneity, but reproducible means of achieving this remain unobtainable for many compositions. In this work, reaction occurring in one of the most successful solgel processes are examined by means of both liquid and solid 13C, 29 si, 17, and Al NMR. The synthesis process studied was the method, optimized by Krol, of prehydrolyzing tetraethylorthosilicate, TEOS, in ethanol, and subsequently adding aluminum sec-butoxide, and then further water. After prehydrolysis and addition of aluminum sec-butoxide, an aluminosilicate precursor is formed in which the aluminum is tetrahedrally coordinated to four silicate ligands. After adding further water, gelation is accompanied by the expansion of the tetrahedral aluminum to octahedral coordination this apparently. occurs by nucleophilic attack of silanol groups on the aluminum in the aluminosilicate precursor. At sufficiently low water to silicon molar ratios, where transparent gels and longer gel times result, this coordination expansion is preceded by condensation of the silicate ligands to form Si-O-Si.