Neutral alkoxysilanes from silica

Silica (SiO2) is found to react readily with ethylene glycol (EGH(2)) to Fo rm neutral glycoxysilanes in the presence of catalytic amounts of high-boil ing organic amines, such as triethylenetetramine (TETA), trishydroxymethyle neaminomethane [H2NC(CH2OH)(3), THAMH(3)], and triethanolamine [N(CH2CH2OH) (3), TEAH(3)]. Kinetic studies show that these amines offer similar catalyt ic efficiencies although their pK(b) values differ by 3 orders of magnitude . In addition, silica dissolution is found to be pseudo-zero order in silic a. These kinetic data can be explained by a rate-limiting step involving re lease of free base from an intermediate pentacoordinated silicate coinciden t with the formation of a tetraalkoxysilane. The products from these reacti ons were characterized by H-1, C-13, and Si-29 solution and solid-state NMR , thermal gravimetric analysis, and mass spectroscopy. Depending on the typ e and amount of base used, different products form: either neutral terraalk oxysilanes, such as Si(OCH2CH2OH)(4) and its soluble oligomers, or neutral pentacoordinate silanes, such as N(CH2CH2O)(3)S iOCH(2)CH(2)OH and H3N+C(CH 2O)(3)Si- (OCH2CH2O). Comparative studies demonstrate that Group I metal hy droxides also catalyze silica dissolution in ethylene glycol with better ca talytic efficiencies than the amine bases. The products of silica dissoluti on using Group I metal hydroxide catalysts were also identified by Si-29 so lution NMR and mass spectroscopy and found to consist primarily of Si(OCH2C H2OH)(4) and its oligomeric derivatives.