Near-Room-Temperature Synthesis of Alkoxysilanes and H2 via Mechanochemical Ball Milling

Alkoxysilanes are important precursors of silicone materials for which various synthesis methods have been reported. However, these approaches are not without problems, and hence, halogen-free, HF-free, and room-temperature reactions producing alkoxysilanes are required. In this study, we demonstrated a direct low-temperature (∼40 °C) alkoxysilane synthesis method that did not involve halogens or HF. Si, Cu, and ethanol were milled in a planetary ball mill for 2 h, and a 50% yield of tetraethoxysilane (TEOS) was obtained; in addition, analysis revealed negligible contamination (<50 ppm) and a TEOS purity of 99.9%. Gas-, liquid-, and solid-phase mechanochemical products were identified. The alkoxysilane yield depended significantly on the mechanochemical pre-treatment, and seven transition metals were investigated as catalysts. Specifically, mechanochemical pre-treatment of a Si–Cu mixture produced silicide (Cu3Si), which was responsible for the high yields. The alkoxysilane yields and silicide formation mechanism were examined by calculating the local temperature upon impact between colliding balls. We also observed high-volume H2 production and demonstrated that the stainless-steel milling medium acted as a mechanocatalyst. Thus, a near-room-temperature, halogen-free green one-pot synthesis method, involving milling Si and Cu powders in alcohol, was discovered and demonstrated as a novel strategy for high-purity alkoxysilane and H2 production.