INTERMEDIATES IN THE SOL-GEL SYNTHESIS OF FORSTERITE

A precursor to the technologically important material forsterite, prep ared by the reaction of 30% H2O2 and water with Si(OMe)4 (TMOS) and Mg (OMe)2 in methanol, was analyzed by several methods to characterize in termediates. Headspace analysis showed that most of the TMOS was consu med after addition of an equimolar amount of water, in the presence of excess Mg(OMe)2. Iodimetric analysis revealed that peroxide was prese nt in the sol and xerogel stages at approximately 56% and 25% of its o riginal concentration, respectively. Si-29 NMR spectra of the initial stages in the synthesis displayed unprecedented peaks in the range -76 .7 to -81.3 ppm. Although H2O2 was needed for formation of several of these peaks, Si-29 NMR spectra of triphenylsilyl hydroperoxide and rel ated model compounds provided evidence against a silyl hydroperoxide i n the sol. Raman spectroscopic analysis of the xerogel that formed upo n removal of solvent, indicated that MgO2 had not formed. Si-29 NMR an alysis of mixtures of 30% H2O2 with TMOS and Mg(OMe)2 in low concentra tion showed products of condensation exclusively. The catalytic activi ty caused by Mg(OMe)2 Was comparable to that of common sol-gel catalys ts HCl and NH4OH, and other alkoxides (LiOMe and NaOMe). The forsterit e precursor synthesis occurred in two distinct mechanistic stages base d on the concentration, M, of Mg(OMe)2: hydrolysis of TMOS, and possib le formation of magnesium hydroperoxides occurred when M was high init ially, followed by condensation of Si species (viz., base catalysis) w hen M was low.