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Neutral Alkoxysilanes from Silica

2000; American Chemical Society; Volume: 122; Issue: 41 Linguagem: Inglês

10.1021/ja001885h

1943-2984

Hengqin Cheng, Ryo Tamaki, Richard M. Laine, Florence Babonneau, Yoshiki Chujo, David R. Treadwell,

Layered Double Hydroxides Synthesis and Applications

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.