GC headspace analysis of intermediates in the sol-gel synthesis of magnesium silicates

The first stage reactions associated with the sol-gel synthesis of amorphous precursors for magnesium silicates such as forsterite and enstatite were examined. Gas chromatographic (GC) analysis of the headspace of a mixture of Mg(OMe)2 and Si(OEt)4 in methanol revealed that transesterification had occurred to form Si(OMe)4, as the major product, and Si(OMe)3(OEt); a Mg:Si molar ratio as low as 0.03 in a 0.2M solution would cause this exchange within 3.5 h. Mg(OMe)2 induced catalytic alkoxide exchange in the mixed alcohol alkoxide solutions of TMOS in ethanol and TBOS in methanol as well. A minimum of 0.0012 mol Mg(OMe)2 per mol TEOS was needed to form the exchange series of TEOS, Si(OMe)(OEt)3, Si(OMe)2(OEt)2, Si(OMe)3(OEt), and TMOS within 1.5 h. The exchange was an equilibrium reaction that depended on solvent composition, and favored methoxy substitution. Other compounds found to catalyze the transesterification of the TEOS in methanol were NaOMe, LiOMe, Ca(OMe)2, HCl, and NH4OH; the alkoxides were the most effective.