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

Article abstract of DOI:10.1002/anie.201506838

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.

Full text of DOI:10.1002/anie.201506838

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201506838
German Edition: DOI: 10.1002/ange.201506838
Catalytic Depolymerization
Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from
Biogenic, Green, and Sustainable Sources
Richard M. Laine,* Joseph C. Furgal, Phi Doan, David Pan, Vera Popova, and Xingwen Zhang
Abstract: The direct depolymerization of SiO2 to distillable
alkoxysilanes has been explored repeatedly without success for
85 years as an alternative to carbothermal reduction (19008C)
to Simet, 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-pentane-
diolato) 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-pentane-
diolato) 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
simple diol ethylene glycol (antifreeze) was used, we observed
the initial production of Si(OCH2CH2OH)4, which on heating
was converted into
a polymer approximated as Si-
(OCH2CH2O)2; this polymer could be mixed with EtOH
and a trace amount of an acid to produce Si(OEt)4 in 60%
(unoptimized) yield.
Although coal and crude oil make up less than 0.01% of
the Earthꢀs crust, their utility to society is enormous given that
they serve as the basis for much of the worldꢀs fuel, for most
organic materials, ranging from plastic bags, food packaging,
and fibers for textiles to major components in flat panel
displays. In contrast, silicon (as silica, SiO2) lies just below
carbon in the periodic table, offers many chemical-bonding
similarities, makes up more than 40% of the Earthꢀs minerals,
and yet has much less impact on our society, despite being
important for applications ranging from solar cells and
silicone rubbers to potential drug analogues.[1–5]
À
single Si C bonds.
One reason for the limited impact of silicon compounds is
that silicon–silicon and silicon–carbon double bonds are very
hard to synthesize unless they are stabilized sterically. There-
fore, silicon compounds can not be polymerized readily by the
methods used for carbon-based compounds. Another reason
is that the silicon–oxygen bond (534 kJmolÀ1) is one of the
strongest found in nature. Thus, most silicon-containing
compounds and materials are produced from Simet (met =
metallurgical grade, ca. 98% purity)), which is made by the
carbothermal reduction of silica in a high-temperature
capital-equipment- and energy-intensive process [see
Eqs. (1–7); Me2SiCl2 is a silicone/polysiloxane precursor].[6–10]
The much higher purities required for photovoltaic-grade
(Sipv) and electronics-grade silicon (Sieg) necessitate addi-
tional processing steps, typically those of the Siemens process
[Eqs. (5) and (6) 99.9999% pure which generate the by-
product HCl (normally recycled).
R
HA is a by-product of rice production that is often
produced coincidently with electricity, whereby the energy
derives from plant-fixed CO2. Thus, the production of RHA
can be energy-positive, sustainable, and offers a low carbon
footprint. We report herein the first examples of the direct,
base-catalyzed depolymerization of silica simply by treatment
with low-cost, hindered diols to produce spirocyclic alkoxy-
silanes that can be distilled, in high purity, directly from the
reaction mixture. This process can be seen as the culmination
of over 85 years of effort targeting the same or similar
objectives. The resulting spirocyclic alkoxysilanes can be used
to produce fumed, colloidal, or precipitated silica or as
À
precursors to compounds containing Si C bonds. When the
[*] R. M. Laine
Department of Materials Science and Engineering
University of Michigan
Ann Arbor, MI 48109-2136 (USA)
E-mail: talsdad@umich.edu
1900ꢀ
C
SiO þ 2 C
2 CO þ Si
ð1Þ
ð2Þ
ƒƒƒ!
2
met
Cu=Sn catalyst=300ꢀ
C
Simet þ 3 HCl
HSiCl þ H ðby-product SiCl Þ
ƒƒƒƒƒƒƒƒƒƒ!
3
2
4
J. C. Furgal, P. Doan
Department of Chemistry, University of Michigan (USA)
cat:
Simet þ 4 EtOH ðor 4 MeOHÞ
SiðOEtÞ ½or SiðOMeÞ4Š þ 2 H2
ƒ!
4
R. M. Laine, D. Pan
Department of Macromolecular Science and Engineering
University of Michigan (USA)
ð3Þ
ð4Þ
ð5Þ
Cu=Sn cat:=300ꢀ
C
Simet þ MeCl
MeSiCl þ Me SiCl
ƒƒƒƒƒƒƒƒ!
3
2
2
V. Popova
Mayaterials Inc
Ann Arbor, MI 48108 (USA)
disproportionation cat:
4 HSiCl3
SiH þ 3 SiCl
ƒƒƒƒƒƒƒƒƒƒƒ !
4
4
hot wire=rod
X. Zhang
H þ HSiCl =SiCl ðor SiH Þ
ƒƒƒƒƒƒ!
2
3
4
4
Department of Chemistry, Harbin Institute of Technology
Harbin 150001 (China)
ð6Þ
ð7Þ
Sipv=Sieg ð6 À 9 9s purityÞ þ HCl ðrecycledÞ
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506838.
>900 ꢀ
C
SiCl4 þ 2 H2=O2
SiO ðfumed silicaÞ þ 4 HCl
ƒƒƒ!
2
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Because all chlorosilanes and HCl gas are corrosive, toxic,
and polluting, such production processes, including those used
to produce fumed silica [Eq. (7)], require expensive and
extensive safeguards, which add to the overall cost of the final
products. Since Simet is a kinetic product (SiC is the
thermodynamic product), its synthesis requires electric arc
furnace processing at approximately 19008C, thus adding to
the overall cost even for the production of Si(OEt)4 or
Si(OMe)4. Likewise, precipitated silicas are most commonly
made by the high-temperature fusion of sand with sodium
carbonate, followed by dissolution and precipitation with
H2SO4:
powder, but most of these studies focused on some form of
the reaction depicted in Equation (11) to generate the
hexacoordinated triscatecholato silicate I.[12–19]
The key to the success of this reaction is that silicon,
unlike carbon, can form five and six bonds, and thus the
À
original Si O bond strength of tetrahedral silicon is dimin-
1300 ꢀ
C
SiO2 þ Na2CO3
Na SiO þ CO
ð8Þ
ð9Þ
ƒƒƒ!
2
3
2
ished. Unfortunately, I cannot be distilled and is so stable that
it is water-soluble and would have to be treated with H2SO4 to
produce ppt SiO2. From a practical perspective, this process,
while offering a low-temperature route to ppt SiO2, would
require three moles of catechol per mole of ppt SiO2 or about
330 g of catechol to produce 60 g of ppt SiO2 and simulta-
neously 280 g of Na2SO4. These features are quite unattrac-
tive, although no CO2 would be produced.
Na2SiO3 þ H2SO4 ! Na2SO4 þ SiO2 þ H2O
As can be seen, each mole of Na2SiO3 produced releases
one mole of CO2 and requires one mole of H2SO4 to produce
one mole of precipitated silica and one mole of Na2SO4, which
must be disposed of. Thus the production of precipitated (ppt)
silicas, such as those used as fillers in polymers (in tires, for
example), as the abrasive in toothpaste, or in vacuum
insulation panels, also requires high temperatures and gen-
erates unwanted by-products, in particular, CO2 and
Na2SO4.[11]
Reactions (1–7) begin with SiO2, which is reduced to the
metal (e.g. Simet) and then reoxidized, often to some form of
SiO2, including fumed silica. This approach is illogical, and
because all these processes are equipment- and energy-
intensive, it is unreasonably costly. Beginning in the early
1930s, repeated attempts were made to develop low-temper-
ature, low-cost methods of depolymerizing silica and thus
alternate routes to silicon-containing compounds as well as
precipitated silica. The establishment of such a process, as
outlined in Equation (10), can be considered a “grand
A search for a simpler reagent than catechol led us to try
ethylene glycol to promote silica depolymerization according
to Equations (12) and (13).[20,21] The depolymerization mech-
anism again builds on expansion of the coordination sphere
around silicon.
Herreros et al. deduced that the pentacoordinated silicate
in Equation (12) is the primary intermediate in the ethylene
glycol mediated synthesis of silicalite zeolites.[22] Likewise,
Kinrade et al. proposed that the formation of similar penta-
coordinated silicates by sugars allows plants to transport silica
within the plant and thus forms the basis for biosilification
processes.[23]
We were able to demonstrate that the reaction in
Equation (12) can be promoted catalytically by an alkali
base [Eq. (14)].[24,25] Our proof-of-principle studies were
carried out with fumed silica (350 m2 gÀ1), which defeats the
overall objective of the “grand challenge”. However, these
studies were important, as they established that: 1) the
reaction in Equation (14) is first order in the base concen-
tration and surface area; 2) the activation energy for the
reaction is approximately 60 kJmolÀ1; and 3) the reaction is
faster with amorphous rather than crystalline silica.[22]
Unfortunately, glycoxysilane (GS) does not distill; instead,
it polymerizes to Si(eg)2 on heating and is thus difficult to
purify.
challenge” for silicon chemists. The ability to distill the
resulting product should enable the direct production of
silicon-containing materials of very high purity, including
precipitated (ppt) SiO2, directly from any silica source at low
temperatures, thus greatly reducing energy costs, the carbon
footprint, and the need for high capital-equipment invest-
ments. For example, high-purity ppt and colloidal silica are
used in applications ranging from edible products (e.g.
toothpaste) and polishing aids for planarizing silicon wafers
to the production of high-purity silica for optical applications
(lenses, gratings, photonic band gap materials) and the
production of crucibles for growing electronics-grade silicon
boules.[6,7,10]
Thus, following initial studies reported by Rosenheim
et al. in 1931,[12] a number of researchers have previously
explored SiO2 depolymerization.[13–19] A wide range of SiO2
feedstocks were used, from amorphous silica to quartz
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We attempted to dissolve a set of biogenic and mineral
silica sources by using four diols: ethylene glycol (EGH2), 1,4-
butanediol, 2-methyl-2,4-pentanediol, and 2,2,4-trimethyl-
1,3-pentanediol (Table 1). The first two diols were not
expected to form spirocyclic alkoxysilanes through base-
catalyzed silica dissolution, whereas the latter two were likely
candidates. Two other hindered diols were also tested, 2-
methyl-1,3-propanediol and 1,3-butanediol, which also gave
spirocyclic alkoxysilanes that distilled over. We were not able
to fully characterize these compounds because of their
tendency to gel; however, peaks were observed in their 29Si
NMR spectra in the À62 to À65 and À70 to À73 ppm regions,
which we have previously associated with simple spirocyclic
alkoxysilanes and ring-opened polymer analogues.[25,30]
The silica sources explored included RHA, DE, Celite
(low-surface-area DE), vermiculite (iron magnesium alumi-
nosilicate, ca. 10–12 wt% silica), ashed rice hulls (A-RH), and
fumed silica. All materials except fumed silica and the high-
surface-area A-RH were first boiled in dilute HCl (24 h,
5 wt%), washed with water and acetone, and then oven dried
As a consequence, we sought other amorphous silica
sources with high surface areas and identified rice hull ash
(RHA) and diatomaceous earth (DE) as potential replace-
ments for fumed silica.[26,27] RHA is produced in 250 kton per
year quantities in the USA alone, is mostly amorphous, and
offers specific surface areas (SSAs)
of 20–85 m2 gÀ1. The samples used in
Table 1: Percent SiO2 depolymerization with different diols.[a]
our study contained 70–90 wt%
silica with 5–25 wt% carbon and
5 wt% minerals, which could be
removed readily by washing with
dilute HCl.[28] We were also able to
obtain a sample of rice hulls that
had been ashed at ꢁ 6008C (A-RH)
SiO2
SSA
EGH2
(b.p. 1978C)
HO(CH2)4OH
(b.p. 2358C)
2-methyl-2,4-
pentanediol
(b.p. 1978C)
2,2,4-trimethyl-1,3-
pentanediol
[m2gÀ1
]
(b.p. 2328C)
Celite
vermiculite
RHA
RHA
DE
1
4
12%
2.5%
20%
13%
3%
23%
4%
3%
1.5%
–
12%
26
85
23
350
230
24%
40%[b]
4%
40%[b]
16%
to produce
a
material with
18%
3%
98%[c]
–
fumed SiO2
A-RH
98%[c]
98%[c]
98%[c]
60%
> 95 wt% silica and with SSAs of
approximately 230 m2 gÀ1. DE is
available from multiple sources
with SSAs ranging from 1–
70 m2 gÀ1 and is mostly amorphous.
We report herein the first exam-
ples of the base-catalyzed synthesis
of distillable spirocyclic alkoxysi-
[a] Standard reaction conditions: SiO2 (0.3 mol), NaOH (0.03 mol), 200 mL distilled at the boiling point
of the solvent (as noted) at atmospheric pressure, 4–8 h. [b] The reaction was carried out with 630 g of
RHA (75 wt% silica by TGA, or 7.87 mol SiO2), EGH2 (7 L), and NaOH (10 mol%); the distillation was
continued for 24 h, see experimental details in the Supporting Information.
lanes directly from these biogenic silica sources. The use of
sterically hindered diols is key to the formation of spirocyclic
alkoxysilanes.[29,30] Of the commercially available hindered
diols, 2-methyl-2,4-pentanediol, the hydrogenated product of
base-catalyzed acetone condensation, is the lowest cost diol
available and forms some of the most stable spirocyclic
alkoxysilanes, Kemmitt and Milestone.[30] 2-Methyl-2,4-pen-
tanediol and 2,2,4-trimethyl-1,3-pentanediol permit the
extension of the reaction in Equation (14) to the formation
of spirocyclic alkoxysilanes.
(1158C, 24 h) prior to use to eliminate basic components that
could also catalyze silica dissolution in the absence of added
base; however, this procedure was not used for the large-scale
RHA experiment described below. The isolated products and
undissolved silica samples were characterized by mass spec-
troscopy, multinuclear NMR spectroscopy, XRD, BET surface-
area measurement, and thermogravimetric analysis (TGA;
see Table S1 and Figure S1 in the Supporting Information.
With the exception of vermiculite, all sources were
amorphous silica. In general, the amount of SiO2 that
dissolved was related to the specific surface area of the
source and the reaction temperature. The low silicon content
and crystallinity of vermiculite were probably at least partially
responsible for its poor dissolution rates as compared to those
of the other silica sources.
The depolymerization rates for 2-methyl-2,4-pentanediol
(b.p. 2008C) were slightly lower than for EGH2. Mass spectral
analysis suggests that the diol “cracks” to produce isopropa-
nol and acetone as the major products rather than the
spirocycle II. Both DE and Celite probably have highly acidic
sites that account for the observed cracking products. The
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ashed rice hulls gave the second-highest degree of dissolution
under standard conditions, as might be expected with SSAs of
approximately 230 m2 gÀ1.
Although we could not isolate Si(eg)2 as a spirocyclic
alkoxysilane, the highest-intensity ion peak at m/z 149 in the
EI mass spectrum of GS probably corresponds to the
spirocyclic alkoxysilane (MW= 148 for 28Si) and suggests
that in the absence of a solvent this compound is stable.
Furthermore, SGS can be isolated quantitatively from the
stoichiometric reaction in Equation (12), and its crystal
structure has been reported.[20] Thus, the pentacoordinated
spirocyclic alkoxysilane must be quite stable. Surprisingly,
SGS is insoluble in EtOH (pKa = 15.9) but soluble in MeOH
(pKa = 15.5), in which EGH2 (pKa = 15.1) is exchanged for
MeOH.[20] Thus, SGS was originally recrystallized from
MeOH with an excess of EGH2. One might envision that
Si(eg)2 is then a mixed ring and ring-opened polymer. In the
presence of excess EtOH or MeOH, acid-catalyzed elimina-
tion of ring strain may drive the partial replacement of EGH2,
whereas complete replacement occurs simply by displace-
ment of the equilibrium towards TEOS and TMOS in the
presence of excess EtOH or MeOH. The insolubility of the
polymer itself in EtOH or MeOH may also provide a second
driving force for the formation of TEOS and TMOS.
In one experiment with ethylene glycol, after dissolution,
carbon-enriched RHA was filtered off, the liquid volume was
reduced by vacuum evaporation, and the solution was cooled,
thus causing sodium glycolato silicate [SGS, Eq. (12)] to
precipitate. This SGS was recovered and recycled (see the
Supporting Information) to catalytically dissolve the silica in
RHA (26 m2 gÀ1) in a second reaction, thus enabling the
dissolution of 24 wt% of the silica, which is essentially the
same amount as that found in Table 1. In a second scaled
experiment, a mixture of 630 g of RHA (85 m2 gÀ1, 7.87 mol
SiO2), NaOH (10 mol%), and EGH2 (7 L) with a silica
content of 75 wt% (as determined by TGA), gave (40 Æ 3)%
silica dissolution after distillation for 20 h, as determined by
TGA of the recovered RHA.
Vermiculite is a common aluminosilicate mineral with no
free SiO2 available for dissolution, yet some dissolution was
observed. We have not characterized the product(s), although
some alumina dissolution may occur concurrently, as Al–EG
complexes have been reported previously.[31]
We have also successfully investigated the formation of
À
The distillation of 2-methyl-2,4-pentanediol and II oc-
curred at nearly the same temperature, thus making isolation
and purification somewhat problematic. However, we were
pleasantly surprised to find that both II and 2-methyl-2,4-
pentanediol are soluble in hexane; the diol is also water-
soluble. Hence, simply washing hexane solutions of the
recovered, distilled mixture or the reaction filtrate removed
the diol and left pure II, which could be recovered readily and
redistilled at about 2008C to give a product with much higher
purity. Simple rotary evaporation led to II as a liquid that
slowly crystallized on cooling.
Compound III and the parent diol are also soluble in
hexane, but the diol is not water-soluble; however, III could
be isolated simply by washing with MeOH. Both spirocyclic
alkoxysilanes could be distilled to higher purity. The forma-
tion of II and III are the first examples of the direct
conversion of biogenic silica into a distillable alkoxysilane.
Because Si(OEt)4 (TEOS) and Si(OMe)4 (TMOS) repre-
sent optimal products, we explored the use of II as a precursor
to TEOS and TMOS, and found it straightforward to generate
TEOS and TMOS in 65 and 40% (unoptimized) yield
through acid-catalyzed exchange (see the Supporting Infor-
mation). Next, we explored the direct synthesis of TEOS from
Si(eg)2 [Eq. (14)]. We were able to successfully synthesize
TEOS [Eq. (17)] in (55 Æ 3)% (unoptimized) yield under
similar conditions. Thus, we have now succeeded in meeting
the “grand challenge” noted above.
Si C bonds without first having to produce silicon metal
[compare Eq. (4)]. Our results will be reported elsewhere, as
well as the use of II or TEOS to produce fumed silica, thus
avoiding the entire route from Simet to SiCl4.
In conclusion, we are now able to effect base-catalyzed
depolymerization of silica to form readily isolated, distillable,
spirocyclic alkoxysilanes and/or TEOS without first carbo-
thermally reducing SiO2 to Si metal. The spirocyclic alkoxy-
silanes and TEOS/TMOS can be distilled to high purity and
thereafter combusted or treated with water and trace acid to
provide access to high-purity ppt or fumed silica (Scheme 1).
If we only consider precipitated silica, then our process does
not produce CO2 or Na2SO4 by-products, thus making it
a green, low carbon footprint,[32] low-temperature, and low-
cost route to high-purity ppt SiO2. Finally, the resulting
carbon-enriched RHA, when treated with dilute HCl, pro-
vides a high-purity starting material for direct carbothermal
reduction to produce silicon metal with 99.9999% purity
without further purification.[32–34]
Acknowledgements
This research was supported in part by a grant from the NSF
(DMR 1105361). We thank Professor Jorge Cervantes
Jµuregui and his group at Guanajuato University, Mexico
for graciously providing the high-surface-area ashed rice
hulls. We also thank Patrick McDonell for a preliminary life
cycle analysis of this process.[32]
Several of the spirocyclic alkoxysilanes synthesized by
Frye undergo reversible ring-opening polymerization.[29]
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How to cite: Angew. Chem. Int. Ed. 2016, 55, 1065–1069
Angew. Chem. 2016, 128, 1077–1081
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Received: July 23, 2015
Revised: October 19, 2015
Published online: December 3, 2015
Angew. Chem. Int. Ed. 2016, 55, 1065 –1069
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