Recovery of trimethylchlorosilane from its azeotropic mixture with SiCl4

Article abstract of DOI:10.1023/A:1015511327423

Reaction of tetraethoxysilane with the azeotropic mixture Me₃SiCl-SiCl₄ in the presence of certain cyclic (tetrahydrofuran, dioxane) and acyclic (diethyl ether) ethers, ethanol, or dimethylformamide was studied with the aim of SiMe₃Cl recovery.

Full text of DOI:10.1023/A:1015511327423

Russian Journal of Applied Chemistry, Vol. 74, No. 12, 2001, pp. 2091 2094. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 12, 2001,
pp. 2031 2034.
Original Russian Text Copyright
2001 by Voronkov, Baryshok, Kuznetsova.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Recovery of Trimethylchlorosilane
from Its Azeotropic Mixture with SiCl4
M. G. Voronkov, V. P. Baryshok, and G. A. Kuznetsova
Favorskii Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, Russia
Received July 30, 2001
Abstract Reaction of tetraethoxysilane with the azeotropic mixture Me SiCl SiCl in the presence of cer-
3
4
tain cyclic (tetrahydrofuran, dioxane) and acyclic (diethyl ether) ethers, ethanol, or dimethylformamide was
studied with the aim of SiMe Cl recovery.
3
Trimethylchlorosilane is one of the most important
organosilicon monomers. It is used to incorporate
inert terminal groups into oligoorganosiloxanes to be
applied as heat- and frost-resistant oils, lubricants,
liquid dielectrics, heat carriers, hydrophobizing com-
ponents, lacquers, and enamels; also, it is widely used
as trimethylsilylating agent in organic and organo-
metallic chemistry [1].
Both pathways, a and b, are accelerated in the pres-
ence of HCl. As a rule, the contribution of pathway b
becomes appreciable at elevated temperatures.
Reaction of SiCl with Si(OEt) at 150 160 C
4
4
yields a mixture of OEt Cl exchange products [8 10].
In the presence of 0.24 wt % ethanol, this reaction is
76 97% complete in 20 70 h at molar ratio SiCl4 :
Si(OEt) = 1 : 3 and 20 22 C [11].
4
Direct synthesis of methylchlorosilane by reaction
of CH Cl with the Si Cu contact mass yields up to
A procedure has been developed for separation of
a technical azeotropic mixture Me SiCl SiCl con-
3
1
0% of the azeotropic mixture Me SiCl SiCl [2].
3 4
3
4
The majority of methods proposed for its separation
azeotropic distillation with acetonitrile; partial hy-
taining MeSiHCl and HSiCl impurities by binding
2
3
[
SiCl with tetraethoxysilane to form ethoxychloro-
4
drolysis or alcoholysis; chemical binding of SiCl in
4
silanes in a selective reaction in the presence of initi-
ators, with subsequent fractionation of the reaction
mixture [12, 13].
the form of solid complexes with tertiary aliphatic and
aromatic amines, formamide, and dimethylformamide
(DMF), insoluble in the reaction medium; fluorination
in aqueous solution] have a number of drawbacks: low
To this end, a technical azeotropic mixture was
yield of Me SiCl, strongly corrosive medium, gela-
brought into reaction with Si(OEt) in the presence of
3
4
tion, and formation of abundant by-products whose
utilization requires sophisticated procedures [3 5].
0.2 10 wt % DMF, tetrahydrofuran (THF), dioxane,
diethyl ether, or ethanol. The reaction was performed
at 20 25 C or at refluxing (60 65 C) for 2 72 h
Simple procedures for Me SiCl recovery from the
3
until conversion of SiCl into ethoxychlorosilanes and
azeotropic mixture Me SiCl SiCl have been devel-
4
3
4
-
siloxanes was complete:
oped only recently. They are based on binding of SiCl4
with organylalkoxysilanes, ethyl silicate, and bottoms
from its synthesis in the presence of alcohols, traces
of moisture, metal salts, and acid catalysts [6, 7].
SiCl4 + (EtO) Si
Cl4 nSi(OEt)
n
4
+
[Cl Si(OEt)3 O] SiCl (OEt) ,
3 k p
(2)
m
m
k
p
The reaction between alkoxy- and chlorosilanes
occurs either as exchange of an alkoxy group for a
halogen atom [scheme (1), a] or as heterofunctional
condensation [scheme (1), b] [1]:
where n, m, k, p = 1 3.
With the above initiators, it was possible to recover
6
88% of Me SiCl from the reaction mixture. In the
3
a
b
presence of DMF, the mixture undergoes partial tar-
ring due to low thermal stability of DMF SiCl com-
*
Si X + Si OR RX + Si O Si* .
*
X Si + ROSi
4
(1)
plexes [14].
1
070-4272/01/7412-2091 $25.00 2001 MAIK Nauka/Interperiodica

2092
VORONKOV et al.
Conditions and isolated products of the reaction of Si(OEt) with SiCl in an industrial azeotropic mixture with Me SiCl
4
4
3
Reaction conditions
Yield of indicated exchange product, g
Yield
of 99%
azeotrope (EtO) Si
4
initiator,
wt %
ethoxychloro- Me SiCl,
3
T,
C
, h
EtOSiCl3 (EtO) SiCl2 (EtO) SiCl (EtO) Si
2
3
4
polysiloxanes
%
g
THF:
1.0
1.0
1.0
2.0
2.0
60 65 10
60 65 15
10.15
10.15
10.15
5.08
15.6
15.6
15.6
7.8
3.4
4.7
4.5
1.8
3.9
3.1
8.9
15.6
14.9
4.6
5.1
0.6
0.6
0.6
0.5
0.2
66
70
80
80
88
1.8
3.6
1.5
1.8
25
60 65
25
72
5
72
1.1
0.5
2.0
5.08
7.8
3.6
Dioxane:
1
1
2
2
.0
.0
.0
.0
60 65 15
10.15
10.15
5.08
15.6
15.6
7.8
2.2
1.0
1.5
1.0
6.2
6.3
1.4
2.3
6.3
11.7
3.9
6.8
2.4
3.6
2.6
0.6
1.2
0.3
0.2
78
68
84
72
25
60 65
25
72
5
72
5.08
7.8
5.0
Ethanol:
0
0
.2
.2
60 65 10
60 65 15
10.15
10.15
15.6
15.6
1.0
0.75
4.2
3.95
12.8
13.8
3.1
1.5
1.2
2.25
68
71
DMF:
.0
5
60 65 18
25.00
11.5
0.9
1.6
4.4
2.9
3.5
72
Reaction (2) performed in the presence of 0.2 wt %
ethanol, 1 wt % THF or dioxane, or 2 wt % diethyl
ether mainly yields triethoxychlorosilanes, with rela-
tively low content of siloxanes originating from het-
erofunctional condensation (see table). Raising the
concentration of THF or dioxane to 2 wt % notice-
ably decreases the yield of ethoxychlorosilanes and
favors formation of oligosiloxanes even at 20 25 C
In this connection, formation of intermediate asso-
ciates like Me SiOMe 2HCl, decomposing with
3
cleavage of the Si O bond, was suggested:
Me SiOMe 2HCl
Me SiCl + HOMe + HCl. (4)
3
3
In a solvent forming associates with HCl, the equi-
librium of reaction (4) is significantly shifted to the
left. This fact is nicely consistent with the previously
found [13] catalytic activity of ethanol in reaction (3).
(
see table). At THF content as high as 10 wt %,
Me SiCl is also involved in the heterofunctional con-
3
densation.
However, addition of acyclic (Et O) and cyclic (THF,
2
dioxane) ethers, capable of forming oxonium com-
plexes with HCl [16], must enhance the catalytic ef-
fect of HCl. In the case of THF, the oxonium com-
plex can transform into 4-chloro-1-butanol whose
capability for etherification of type (3b) is considera-
bly lower than that of ethanol. This fact could de-
crease the content of HCl in the reaction mixture and
hinder reaction (3). However, when THF is added in
an amount as large as 10 wt %, the reaction is not
Initiation of exchange between alkoxy groups and
chlorine atoms in reactions of chlorosilanes with
alkoxysilanes under the action of HCl (usually present
in chlorosilanes because of their partial hydrolysis
by atmospheric moisture) is believed to occur in two
stages [1]:
Si OEt + HCl
Si Cl + HOEt,
(3a)
(3b)
inhibited; moreover, more inert Me SiCl is involved
3
in the process.
*
*
Si Cl + HOEt
Si OEt + HCl.
Nucleophilic substitution at the silicon atom is
accelerated in the presence of strong nucleophiles
(DMF, HMPA, F , RCOO , Cl ) and is a first-order
reaction with respect to the initial silane, alcohol, and
activating nucleophile [17, 18]. The suggested reac-
tion mechanism involves formation of an intermediate
The initial rate of the reaction between trimethyl-
methoxysilane and SiCl in the presence of HCl or
4
hydroxyl-containing compounds ROH (R = H, MeO,
PhCO, Et Si) releasing HCl in the reaction with SiCl ,
3
4
is proportional to squared HCl concentration [15].
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 12 2001

RECOVERY OF TRIMETHYLCHLOROSILANE FROM ITS AZEOTROPIC MIXTURE WITH SICL4 2093
with a five-coordinate Si atom and its subsequent
attack by a nucleophile (Nu):
heated for 10 h at 60 65 C and fractionated. Yield of
Me SiCl 3.3 g (66%), purity 99%.
3
(
b) A mixture of 25.0 g of Me SiCl SiCl azeo-
3 4
X Nu
X
.
trope, 11.5 g of Si(OEt)4, and 1.83 g (5.0 wt %) of
DMF was heated to reflux for 18 h and cooled to
room temperature; the liquid phase was separated by
decanting and fractionated.
.
Nu
.
Si X + Cat
Si
Si
Si Nu + X ,
5)
Cat
Cat
Cat
(
(
c) A mixture of 17.9 g of Me SiCl SiCl azeo-
where X = H, OR, NR , Cl.
3 4
2
trope, 10.7 g of Si(OEt) , and 6.0 g of the complex
SiCl 2H(O)CNMe was heated to reflux for 4 h.
After cooling to room temperature, the liquid phase
was separated from the precipitate (4.9 g) by decant-
ing and distilled to give 5.9 g (67%) of Me SiCl, bp
4
Presumably, DMF in reaction (2), forming a com-
4
2
plex with SiCl , enhances the nucleophilicity of the
4
Cl atoms. However, THF, dioxane, and diethyl ether
are quite inert relative to SiCl and Si(OEt) [19, 20].
4
4
3
A common property of ethers (and alcohols) is their
capability to form oxonium complexes with hydrogen
halides. In such complexes, the nucleophilicity of
chloride ions is enhanced, and their exchange with
ethoxy groups is activated. Presumably, reaction (2)
involves formation of a six-membered cyclic transi-
tion state in which cleavage of the Si O bond (i) yields
5
5 56 C; 4.9 g of (EtO)SiCl , bp 102 105 C; 3.7 g
3
of (EtO) SiCl , bp 137 139 C; and 5.8 g of (EtO)
SiCl, bp 156 158 C.
2
2
3
At other reaction times and temperatures and with
other amounts of catalysts, the mixtures were worked
up similarly. The experimental conditions and reaction
products are listed in the table.
exchange products, and cleavage of the O CH Me
2
bond (ii), heterofunctional cyclization products:
Reaction of Me SiCl SiCl4 azeotrope with
3
Si(OEt) in the presence of 10 wt % THF. A mix-
4
ture of 5.08 g of Me SiCl SiCl azeotrope, 7.8 g of
3
4
Si(OEt) , and 10 wt % THF was heated at 60 65 C
4
for 10 h. The resulting mixture was analyzed by GC
MS. Component content, wt %: Me SiCl 7.7, THF
3
8
.5, (EtO) SiCl 5.5, (EtO) SiCl 58.5, Si(OEt) 15.0,
2 2 3 4
(EtO) SiOMe3 3.2, (EtO) Si(OSiMe ) 1.5, and
3 2 3 2
EtOSi(OSiMe ) and unidentified products <0.5.
3
3
Reaction of SiCl with Si(OEt) in the presence
4
4
of Et O. (a) A mixture of 4.25 g of SiCl , 5.2 g of
2
4
Si(OEt) , and 2 wt % Et O was heated at 60 65 C
4
2
EXPERIMENTAL
for 10 h and then analyzed by GC MS. Component
content, wt %: Et O 0.2, EtOSiCl 1.1, (EtO) SiCl
2
3
2
2
The azeotropic mixture SiCl Me SiCl obtained
4
3
1
14.9, (EtO) SiCl 76.7, Si(OEt) 5.1, and ethoxychlo-
3
4
from synthesis of chlorosilanes contained 49.07%
Me SiCl, 37.05% SiCl , 12.00% MeSiHCl , 0.77%
ropolysiloxanes 2.0.
3
4
2
SiH Cl , 0.74% SiHCl , and 0.21% Me SiOSiMe .
(b) A mixture of 1.7 g of SiCl , 6.24 g of Si(OEt) ,
4 4
2
2
3
3
3
and 2 wt % Et O was heated at 60 65 C for 10 h and
2
Chromatographic analysis was performed with a
Hewlett Packard HP 5890 device (EI, 70 eV, HP
then analyzed by GC MS. Component content, wt %:
Et O 1.6, EtOSiCl 22.4, (EtO) SiCl 2.5, Si(OEt)
2
3
2
2
4
5971 A mass-selective detector, 50-m Ultra-2 column
6
9.0, Cl SiOSi(OEt)3 2.7, and unidentified com-
3
coated with polymethylsilicone containing 5% phenyl
groups, vaporizer temperature 250 C, column temper-
ature 50 280 C).
pounds 1.8.
CONCLUSION
Reaction of Si(OEt) with SiCl in azeotropic
4
4
mixture with Me SiCl in the presence of initiator.
Treatment of the azeotropic mixture Me SiCl SiCl
3
3
4
(
a) A mixture of 10.15 g of Me SiCl SiCl azeo-
with Si(OEt) at 25 65 C in the presence of 0.2
3
4
4
trope, 15.6 g of Si(OEt) , and 1.0 wt % THF was
10 wt % cyclic (THF, dioxane) or acyclic (diethyl
ether) ethers, ethanol, or dimethylformamide allows
isolation of 99% pure trimethylchlorosilane in up to
88% yield.
4
1
Sibirskii Silikon Joint-Stock Company (Usol’e-Sibirskoe,
Irkutsk oblast).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 12 2001

2094
VORONKOV et al.
10. Kumada, M., J. Inst. Polytech. Osaka City Univ.,
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 12 2001