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.
Keywords: depolymerization; materials science; silicon; sustainable chemistry.
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