Neutral alkoxysilanes from silica

Silica (SiO2) is found to react readily with ethylene glycol (EGH(2)) to Form neutral glycoxysilanes in the presence of catalytic amounts of high-boiling organic amines, such as triethylenetetramine (TETA), trishydroxymethyleneaminomethane [H2NC(CH2OH)(3), THAMH(3)], and triethanolamine [N(CH2CH2OH)(3), TEAH(3)]. Kinetic studies show that these amines offer similar catalytic efficiencies although their pK(b) values differ by 3 orders of magnitude. In addition, silica dissolution is found to be pseudo-zero order in silica. These kinetic data can be explained by a rate-limiting step involving release of free base from an intermediate pentacoordinated silicate coincident with the formation of a tetraalkoxysilane. The products from these reactions were characterized by H-1, C-13, and Si-29 solution and solid-state NMR, thermal gravimetric analysis, and mass spectroscopy. Depending on the type and amount of base used, different products form: either neutral terraalkoxysilanes, 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(CH2O)(3)Si- (OCH2CH2O). Comparative studies demonstrate that Group I metal hydroxides also catalyze silica dissolution in ethylene glycol with better catalytic efficiencies than the amine bases. The products of silica dissolution using Group I metal hydroxide catalysts were also identified by Si-29 solution NMR and mass spectroscopy and found to consist primarily of Si(OCH2CH2OH)(4) and its oligomeric derivatives.