Avoiding carbothermal reduction: Distillation of alkoxysilanes from biogenic, green, and sustainable sources

The direct depolymerization of SiO₂ to distillable alkoxysilanes has been explored repeatedly without success for 85 years as an alternative to carbothermal reduction (1900 °C) to Si_met, followed by treatment with ROH. We report herein the base-catalyzed depolymerization of SiO₂ with diols to form distillable spirocyclic alkoxysilanes and Si(OEt)₄. Thus, 2-methyl-2,4-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, or ethylene glycol (EGH₂) react with silica sources, such as rice hull ash, in the presence of NaOH (10 %) to form H₂O and distillable spirocyclic alkoxysilanes [bis(2-methyl-2,4-pentanediolato) silicate, bis(2,2,4-trimethyl-1,3-pentanediolato) silicate or Si(eg)₂ polymer with 5-98 % conversion, as governed by surface area/crystallinity. Si(eg)₂ or bis(2-methyl-2,4-pentanediolato) silicate reacted with EtOH and catalytic acid to give Si(OEt)₄ in 60 % yield, thus providing inexpensive routes to high-purity precipitated or fumed silica and compounds with single Si-C bonds. No detours: The base-catalyzed depolymerization of SiO₂ from different sources with diols led directly to distillable alkoxysilanes, including spirocyclic compounds, thus providing inexpensive routes to high-purity silica and compounds with single Si-C bonds (see scheme): The alkoxysilanes could be converted either into Si(OEt)₄ by treatment with EtOH and a catalytic amount of acid or into high-purity precipitated (ppt) or fumed silica.