Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic, Green, and Sustainable Sources

Abstract

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.

Green routes to silicon-based materials and their environmental implications

“…It was found that sterically hindered diols showed even better conversion and catalytic activity to spirosiloxanes (tetra-coordinated silanes) (Frye 1969). It was found that among those, 2methyl-2,4-pentanediol, the hydrogenated product of base-catalyzed acetone condensation, which is also nonbio-accumulative and biodegradable, showed excellent conversions to Si(2-methyl-2,4-pentanediolato) 2 in 98 % isolated yield from fumed silica (350 m 2 /g) and >60 % conversion of high surface area RHA (see below) to starting materials in less than 24 h (Figure 7) (Laine et al 2016a(Laine et al , 2016b. This product is fractionally distilled from the reaction pot and separated from residual diol performing a hexane/water extraction.…”

“…The derivation of alkoxysilanes from RHA is mentioned briefly in Section 2.2, but it is important to detail here. Laine et al, Fukaya et al and others have set out to use RHA as a source material for the low-temperature catalytic forming of alkoxysilanes, the "Holy Grail" of silicon chemistry (Laine et al 2016a). The most common methods to acomplish this conversion have been to take acid pretreated RHA, a base catalyst (i.e.…”

“…Table 3 shows the maximum total dissolution obtained from a high surface area RHA over the course of 24 h. Though the yields are generally lower than that from pure amorphous fumed silica (surface area 350 m 2 /g), these are obtained without first removing the remaining 10-15 % carbon content and are from green sources. Figure 16 outlines the possible outcomes from the Laine diol methodology with precipitated/fumed silica and R-alkoxysilanes being major targets (Laine et al, 2014;Laine et al, 2016a). Since it is possible to make R-trialkoxysilane derivatives directly from RHA derived materials, the next section will explore the synthesis of silsesquioxane and octasilicate based structural motifs.…”

“…Laine et al 2016a) 71 % (60 % TEOS)(Fukaya et al 2017) 27 %(Laine et al 2014) General schematic for the conversion of biogenic silica to reactive silicon-based building blocks.…”

A Systematic Study on Bio-Based Hybrid Aerogels Made of Tannin and Silica

“…In contrast to that, the utilization of a glycol-based silica precursor namely tetrakis(2-hydroxyethyl)orthosilicate (EGMS), as proposed by Huesing et al [ 9 , 10 , 11 ], is beneficial as it is water-soluble and, hence, can easily be hydrolyzed/condensed under neutral and aqueous conditions without any co-solvent. Furthermore, it was recently shown that EGMS can easily be obtained from biogenic, sustainable sources, i.e., silica from rice hull ash [ 12 ].…”