Multinuclear NMR study of aluminosilicate sol-gel synthesis using the prehydrolysis method

Tetraethoxysilane (TEOS), water and aluminum sec-butoxide (Al(OBu^s)₃) are combined in several stages of prehydrolysis technique, and reaction intermediates at each stage were examined by means of both liquid and solid nuclear magnetic resonance of ¹³C, ²⁹Si, ¹⁷O, and ²⁷Al nuclei. The spectra indicate that when Al(OBu^s)₃ is added to a prehydrolyzed TEOS solution an aluminosilicate precursor is formed in which the aluminum is tetrahedrally coordinated by four silicate ligands. When further water is added, gelation is accompanied by the expansion of aluminum coordination; the spectra indicate that this occurs by the nucleophilic attack of silanol groups. At water contents for which gels of low transparency result, this coordination expansion is accompanied by the formation of new siloxane linkages, but at water contents so high that opaque gels result, the coordination expansion proceeds much more quickly and no new siloxane linkages are observed.