Neutral Alkoxysilanes from Silica

Richard Laine

R. TRLFBYC H. Cheng and D.R. Treadwell (2000)

J. Am. Chem. Soc., 122:10063-72.

Silica (SiO2) is found to react readily with ethylene glycol (EGH2) to form neutral glycoxysilanes in the presence of catalytic amounts of high-boiling organic amines, such as triethylenetetramine (TETA), trishydroxymethyleneaminomethane [H2NC(CH2OH)3, THAMH3], and triethanolamine [N(CH2CH2OH)3, TEAH3]. Kinetic studies show that these amines offer similar catalytic efficiencies although their pKb 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 1H, 13C, and 29Si 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 tetraalkoxysilanes, such as Si(OCH2CH2OH)4 and its soluble oligomers, or neutral pentacoordinate silanes, such as N(CH2CH2O)3SiOCH2CH2OH and H3N+C(CH2O)3Si-(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 29Si solution NMR and mass spectroscopy and found to consist primarily of Si(OCH2CH2OH)4 and its oligomeric derivatives.