Direct synthesis of alkyldialkoxysilanes by the reaction of silicon, alcohol and alkene using a high-pressure flow reactor

Article abstract of DOI:10.1016/S0022-328X(00)00681-1

In the reaction of silicon, methanol and ethylene, use of a high-pressure flow reactor operating at 240°C at 0.86 MPa of ethylene and 0.18 MPa of methanol gave a 33% selectivity for ethyldimethoxysilane, which was much higher than that using an atmospheric-pressure flow reactor (8%) or using an autoclave (4%). The selectivity increased proportionally with the ethylene pressure below 0.5 MPa. This indicates that the addition of ethylene or methanol to a surface silylene intermediate determines the selectivity and that the ethylene addition is a first-order reaction. Above 0.5 MPa the slope of the selectivity change was smaller. By feeding propylene instead of ethylene, n- and iso-propyldimethoxysilanes were obtained as organosilanes with 15 and 2% of selectivity, respectively. The reaction of silicon, ethanol and ethylene resulted in 28% selectivity for ethyldiethoxysilane at 0.99 MPa of ethylene and 0.21 MPa of ethanol at 240°C.

Full text of DOI:10.1016/S0022-328X(00)00681-1

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 619 (2001) 235–240
Direct synthesis of alkyldialkoxysilanes by the reaction of silicon,
alcohol and alkene using a high-pressure flow reactor
Masaki Okamoto a, Jun-ichi Komai a, Miwa Uematsu a, Eiichi Suzuki a,*, Yoshio Ono 1
a Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Received 24 March 2000; received in revised form 17 July 2000; accepted 4 September 2000
Abstract
In the reaction of silicon, methanol and ethylene, use of a high-pressure flow reactor operating at 240°C at 0.86 MPa of ethylene
and 0.18 MPa of methanol gave a 33% selectivity for ethyldimethoxysilane, which was much higher than that using an
atmospheric-pressure flow reactor (8%) or using an autoclave (4%). The selectivity increased proportionally with the ethylene
pressure below 0.5 MPa. This indicates that the addition of ethylene or methanol to a surface silylene intermediate determines the
selectivity and that the ethylene addition is a first-order reaction. Above 0.5 MPa the slope of the selectivity change was smaller.
By feeding propylene instead of ethylene, n- and iso-propyldimethoxysilanes were obtained as organosilanes with 15 and 2% of
selectivity, respectively. The reaction of silicon, ethanol and ethylene resulted in 28% selectivity for ethyldiethoxysilane at 0.99
MPa of ethylene and 0.21 MPa of ethanol at 240°C. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Direct synthesis; Organosilane; Metallic silicon; Alcohol; Alkene; Copper catalyst
To explain the formation of trimethoxysilane in the
silicon–methanol reaction and the formation of
organosilane in the presence of ethylene or butadiene,
the reaction mechanism involving silylene intermediate
(I) was proposed. Eq. (1) shows the mechanism of
trimethoxysilane formation.
1. Introduction
large amount of dimethyldichlorosilane,
A
a
monomer for silicone polymers, is industrially synthe-
sized from metallic silicon and methyl chloride using a
copper catalyst [1]. However, there is an environmental
demerit in this reaction; the reactant, methyl chloride,
and the products including hydrogen chloride as by-
products are corrosive and harmful. Taking into ac-
count of the necessity of an anticorrosive reactor
system and an environment, new methods of organosi-
lane synthesis without halides are highly beneficial.
Methanol reacts with metallic silicon to afford
trimethoxysilane using copper catalysts [2,3], such as
copper(I) chloride [4,5]. We have found that the reac-
tion of silicon with methanol in the presence of ethylene
gives ethyldimethoxysilane (selectivity 8%) together
with trimethoxysilane [6]. Additionally, use of butadi-
ene instead of ethylene resulted in the formation of
1-methoxysilacyclopent-3-ene.
(1)
Methanol reacts with surface silylene (I) on the sili-
con surface to form surface species (II), which is con-
verted to species (III) by further reaction with
methanol. Two SiꢀCu bonds in the species (III) are
simultaneously cleaved by attack of methanol to give
trimethoxysilane. Ethylene or butadiene attacks surface
silylene (I) to form silacyclopropane species (IV) or
silacyclopent-3-ene species (V), and then finally
ethyldimethoxysilane or 1-methoxysilacyclopent-3-ene
is formed, respectively (Eq. (2)).
* Corresponding author. Fax: +81-3-57342878.
E-mail address: esuzuki@o.cc.titech.ac.jp (E. Suzuki).
1 National Institution for Academic Degrees, Otsuka, Bunkyo-ku,
Tokyo 112-0012, Japan.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00681-1

236
M. Okamoto et al. / Journal of Organometallic Chemistry 619 (2001) 235–240
to remove oxide overlayers. The silicon grains and
copper(I) chloride grains (99.9% purity, 45–63 mm)
were mixed vigorously in a small vial. The mixture
containing 9 mmol of silicon and 10 wt % (as copper
metal) of the copper catalyst was placed in a stainless-
steel tubing (i.d. 9.3 mm) in a high-pressure fixed-bed
flow reactor system. An inner wall of the reactor was
coated with a thin silicon oxide layer to prevent the
formation of copper alloy by the reaction of stainless
steel with copper and the by-reactions of reactants
and/or products over stainless steel. Prior to the reac-
tion, the silicon–catalyst mixture was heated in a he-
lium stream at 240°C for 1 h at atmospheric pressure,
and then the flow was changed to a mixture of helium
and ethylene and the total pressure was raised to 1.5
MPa by controlling a back-pressure regulator. Finally,
methanol was fed into the reactor by a high-pressure
liquid pump through a preheating zone for vaporizing
methanol. The products were trapped for 15 min in
ethylbenzene cooled by ice. Using 3-methylpentane as
an internal standard, the products were analyzed every
15 min by a gas chromatograph equipped with a 2 m
long glass column packed with SE-30 and a thermal
conductivity detector.
(2)
Adding allyl propyl ether instead of ethylene resulted
in a 38% selectivity of allyldimethoxysilane [7]. This
strongly indicates the intermediacy of silylene (I).
In the synthesis of ethyldimethoxysilane from silicon,
methanol
and
ethylene,
the
selectivity
for
ethyldimethoxysilane is low (8%) using an atmospheric-
pressure flow reactor. To improve the selectivity for
ethyldimethoxysilane, the liquid-phase reaction in an
autoclave had been studied [8]. Ethyldimethoxysilane
and ethyltrimethoxysilane were formed as organosilanes
together with trimethoxysilane and tetramethoxysilane.
The liquid-phase reaction leads to a higher selectivity
for the organosilanes (26%) than that obtained using an
atmospheric-pressure flow reactor. In the liquid-phase
reaction, ethyldimethoxysilane is converted into
ethyltrimethoxysilane by the reaction with unreacted
methanol over metallic copper and/or a wall of the
reactor, because a contact time of ethyldimethoxysilane
with methanol is much longer than that in a continuous
flow reaction.
Gas-phase kinetics of the addition reaction of dihy-
drosilylene to ethylene has been studied by a couple of
groups [9]. They have reported that the reaction rate of
the addition is proportional to the ethylene pressure
controlled below an atmospheric pressure. This strongly
suggests that the selectivity for ethyldimethoxysilane
increases proportionally to the ethylene pressure in the
reaction of silicon, methanol and ethylene.
A selectivity, a yield and a silicon conversion are
defined by the following equations.
Selectivity (%)
Amount of the product (mmol)
Sum of amounts of silicon-containing products (mmol)
=
×100
Yield (%)=
Amount of the product (mmol)
Amount of the silicon charged in the reactor (9 mmol)
×100
Silicon conversion (%)=
In this work, a high-pressure flow reactor was used
for the reaction of metallic silicon with alcohol and
alkene at an elevated alkene pressure in order to deter-
mine the dependence of the selectivity for organosilane
on alkene pressure and to synthesize organosilanes with
a high selectivity. Furthermore, the effects of reaction
conditions on the selectivity and the yield of organosi-
lane were examined. When propylene as an alkene was
used, the formation of two kinds of propyldimethoxysi-
lanes was expected. The effects of temperature and
pressure were examined and the mechanism of propy-
ldimethoxysilane formation was discussed.
Sum of amounts of silicon-containing products (mmol)
Amount of the silicon charged in the reactor (9 mmol)
×100
3. Results and discussion
3.1. Ethyldimethoxysilane synthesis from silicon,
methanol and ethylene
3.1.1. Time course of the reaction
After the mixture of silicon and the catalyst was
heated at 240°C for 1 h in a helium stream, the reaction
of silicon, methanol and ethylene was carried out at
240°C. The total pressure was 1.0 MPa consisting of
0.15 MPa of methanol, 0.72 MPa of ethylene and 0.13
MPa of helium as a balance gas. Ethyldimethoxysilane
2. Experimental
Silicon grains (99.9% purity, 45–63 mm) were washed
by 46% HF solution for 1 h at room temperature (r.t.)

M. Okamoto et al. / Journal of Organometallic Chemistry 619 (2001) 235–240
237
Even comparing with the sum of ethyldimethoxysilane
and ethyltrimethoxysilane selectivities (26%) using an
autoclave [8], a 30% selectivity for ethyldimethoxysilane
using a high-pressure flow reactor was higher.
In our previous work, it has been reported that ethyl-
trimethoxysilane and tetramethoxysilane are secondary
products formed by the reaction of ethyldimethoxysi-
lane and trimethoxysilane with unreacted methanol
over copper metal, respectively [8]. Low selectivities for
ethyltrimethoxysilane and tetramethoxysilane indicate
that use of the flow reactor depresses the secondary
reactions.
We have found that the reaction of silicon with
methanol at a higher reaction temperature (above
300°C) gives methyldimethoxysilane together with
trimethoxysilane and that formaldehyde is a reaction
intermediate for methyldimethoxysilane formation [10].
Formation of a small amount of methyldimethoxysi-
lane was observed also in the silicon–methanol–ethyl-
ene reaction. It is plausible that the reaction at a high
pressure leads to the formation of formaldehyde even at
a low temperature and the consequent formation of
methyldimethoxysilane.
and trimethoxysilane were formed together with small
amounts of ethyltrimethoxysilane, methyldimethoxysi-
lane and tetramethoxysilane. Fig. 1 shows the time
courses of formation rates of the products and a cumu-
lative silicon conversion. All the rates increased with
reaction time toward maxima around 1 h and then
decreased. The reaction stopped at 4.25 h and the
silicon conversion reached 63%. A 30% selectivity for
ethyldimethoxysilane was obtained, the product distri-
bution being trimethoxysilane (61%), ethyltrimethoxysi-
lane (4%), methyldimethoxysilane (3%) and tetra-
methoxysilane (2%). A much higher selectivity for
ethyldimethoxysilane using a high-pressure flow reactor
was obtained compared to 8 and 4% using an atmo-
spheric-pressure flow reactor [6] and an autoclave [8],
respectively. In the liquid-phase reaction, ethyldime-
thoxysilane was converted into ethyltrimethoxysilane
by the reaction of ethyldimethoxysilane with methanol.
3.1.2. Dependence of the selecti6ity for
ethyldimethoxysilane on ethylene pressure
The effect of ethylene pressure is shown in Fig. 2.
Ethylene pressure was varied from 0 to 1.25 MPa, while
methanol pressure was kept at 0.15 MPa. Thus, total
pressure was also changed from 1.0 to 1.4 MPa. With
increasing ethylene pressure, ethyldimethoxysilane se-
lectivity proportionally increased below 0.5 MPa.
Above 0.5 MPa, the slope of the selectivity was smaller
and the selectivity was 36% at 1.25 MPa. However, the
silicon conversion gradually decreased with ethylene
pressure. The yield of ethyldimethoxysilane increased
and reached 19% at 0.72 MPa of ethylene pressure
above, which it slightly decreased.
Kinetic studies of silylene addition reaction to ethyl-
ene have revealed that the addtion is a first-order
reaction with respect to ethylene pressure at a low
pressure [9]. As shown in Fig. 2, the selectivity for
ethyldimethoxysilane proportionally increased with eth-
ylene pressure below 0.5 MPa. According to Eqs. (1)
and (2), the selectivity is governed by the reaction of
surface silylene with ethylene or methanol. The linear
relation between the selectivity for ethyldimethoxysilane
and the ethylene pressure shows that the reaction of
surface silylene with ethylene is a first-order reaction.
This agrees with the literature of the kinetic study [9],
indicating that surface silylene reacts with ethylene
according to the same mechanism as vapor-phase
silylene. On the other hand, above 0.5 MPa of ethylene
pressure, the selectivity nonlinearly increased. At a
higher pressure, ethylene probably adsorbed and
oligomerized on the surface and consequently the selec-
tivity may decrease.
Fig. 1. Changes in the silicon conversion (a) and formation rates of
trimethoxysilane (b), ethyldimethoxysilane (c), ethyltrimethoxysilane
(d), methyltrimethoxysilane (e) and tetramethoxysilane (f) with reac-
tion time in the reaction of silicon, methanol and ethylene. The
pretreatment was carried out at 240°C for 1 h. The reaction tempera-
ture was 240°C. Methanol and ethylene pressures were 0.15 and 0.72
MPa, respectively.
Fig. 2. Effect of ethylene pressure on the silicon conversion (a), the
selectivity (b) and yield (c) of ethyldimethoxysilane. The pretreatment
was carried out at 240°C for 1 h. The reaction temperature was
240°C. Methanol pressure was 0.15 MPa.

238
M. Okamoto et al. / Journal of Organometallic Chemistry 619 (2001) 235–240
temperatures between 180 and 240°C. The results are
shown in Fig. 3. The silicon conversion, the selectivity
and the yield of ethyldimethoxysilane increased with
increasing temperature, while ethyltrimethoxysilane se-
lectivity decreased. We have reported that te-
tramethoxysilane is formed by a secondary reaction of
trimethoxysilane with methanol over metallic copper in
the silicon–methanol reaction [6]. It is highly plausible
that ethyldimethoxysilane is a sole primary product as
an ethylene-added organosilane and that a methoxyla-
tion of ethyldimethoxysilane with methanol results in
the formation of ethyltrimethoxysilane [8]. The sum of
ethyldimethoxysilane and ethyltrimethoxysilane selec-
tivities was almost constant around 33%. This means
that the selectivity for ethylene-added organosilanes
does not depend on the reaction temperature.
The effect of the reaction temperature above 240°C
was examined with no pretreatment. As shown in Fig.
4, the silicon conversion declined to 34% at 280°C and
the selectivity and the yield of ethyldimethoxysilane
were also low at 280°C. At 240 and 260°C, the sum of
ethyldimethoxysilane and ethyltrimethoxysilane selec-
tivity was almost same, while at 280°C it slightly de-
creased. The selectivity for secondary products, i.e.
ethyldimethoxysilane and tetramethoxysilane, increased
with the reaction temperature. Figs. 3 and 4 indicate
Fig. 3. Effect of reaction temperature between 180 and 240°C on the
silicon conversion (a), the selectivity (b) and the yield (c) of
ethyldimethoxysilane, and the selectivity for ethyltrimethoxysilane
(d). The pretreatment was carried out at 240°C for 1 h. Methanol and
ethylene pressures were 0.15 and 0.72 MPa, respectively.
that
a
high selectivity and
a
high yield of
ethyldimethoxysilane are obtained at 240°C.
Fig. 5 shows the effect of total pressure. The ratio of
ethylene to methanol pressure maintained 0.72:0.15.
The selectivity increased with total pressure and 33%
selectivity was obtained at 1.5 MPa. The silicon conver-
sion also increased to 63% and then slightly decreased
above 1.0 MPa. At 1.2 MPa the maximum yield of
ethyldimethoxysilane (20%) was obtained, the silicon
conversion and the ethyldimethoxysilane selectivity be-
ing 62 and 33%, respectively.
Fig. 4. Effect of reaction temperature above 240°C on the silicon
conversion (a), the selectivity (b) and yield (c) of ethyldimethoxysi-
lane, and the selectivity for ethyltrimethoxysilane (d). No pretreat-
ment was carried out. Methanol and ethylene pressures were 0.15 and
0.72 MPa, respectively.
3.2. n-Propyldimethoxysilane synthesis from silicon,
methanol and propylene
The reaction of silicon, methanol and propylene was
carried out using the high-pressure reactor. The
changes of the silicon conversion and formation rates
of products with reaction time are shown in Fig. 6.
n-Propyldimethoxysilane,
iso-propyldimethoxysilane
and n-propyltrimethoxysilane as propylene-added
products were formed together with trimethoxysilane,
Fig. 5. Effect of total pressure on the silicon conversion (a), the
selectivity (b) and yield (c) of ethyldimethoxysilane. The pretreatment
was carried out at 240°C for 1 h. The reaction temperature was
240°C. The ratio of ethylene to methanol pressure was 0.72:0.13.
tetramethoxysilane,
methyldimethoxysilane
and
methyltrimethoxysilane. The n-propylsilane was a main
product among the propylsilanes. n-Propyldimethoxysi-
lane selectivity was 14%, the silicon conversion being
59%.
Fig. 7 shows the effect of propylene pressure. The
n-propyldimethoxysilane selectivity increased in pro-
portion to propylene pressure and attained 14% at 0.6
3.1.3. Effects of reaction conditions on the selecti6ity
for ethyldimethoxysilane
After the silicon–catalyst mixture was pretreated at
240°C for 1 h, the reaction was carried out at various

M. Okamoto et al. / Journal of Organometallic Chemistry 619 (2001) 235–240
239
Fig. 8. Effect of reaction temperature on the silicon conversion (a),
the selectivity (b) and yield (c) of n-propyldimethoxysilane. The
pretreatment was carried out for 1 h at the same as the reaction
temperature. Methanol and propylene pressures were 0.10 and 0.60
MPa, respectively.
Fig. 6. Changes in the silicon conversion (a) and formation rates of
trimethoxysilane (b), n-propyldimethoxysilane (c), iso-propy-
ldimethoxysilane (d), tetramethoxysilane (e), n-propyltrimethoxysi-
lane (f), methyldimethoxysilane (g) and methyltrimethoxysilane (h)
with reaction time in the reaction of silicon, methanol and propylene.
The pretreatment was carried out at 240°C for 1 h. The reaction
temperature was 240°C. Methanol and propylene pressures were 0.10
and 0.60 MPa, respectively.
however, an increase of temperature decreased the ratio
to 4 at 280°C. When methanol pressure was reduced to
a half (52 kPa) at 240°C, both n-propyldimethoxysilane
selectivity and the silicon conversion increased to 15
and 82%, respectively. A 12% of n-propyldimethoxysi-
lane yield was obtained.
The speculated mechanism of propyldimethoxysi-
lanes formation is shown in Eq. (3).
(3)
Propylene reacts with surface silylene (I) to form
methylsilacyclopropane species (VI), which is attacked
by methanol to cleave either SiꢀC bond a or b in the
species (VI). Cleavage of bond a or b governs a selectiv-
ity for the propylsilanes. If bond a is cleaved, then
n-propylsilane is formed. On the other hand, cleavage
of bond b leads to the formation of the iso-propylsi-
lane. It has been reported that 1-methyl-1-phenyl-2-
hexylsilacyclopropane produced by the reaction of
methylphenylsilylene and 1-octene is methanolyzed to
form 2-(methylphenylmethoxysilyl)octane, an iso-
product; iso-/n-ratio is 7.0 [11]. When allyltrimethylsi-
lane is used instead of 1-octene, 1-trimethylsilyl-2-
(methylphenylmethoxysilyl)propane, which is an iso-
product, is mainly obtained [11]. These results do not
correspond with the result of methanolysis of species
(VI), probably because of difference between a gas-
phase and a surface methanolysis. We have reported
that the reaction of silicon, hydrogen chloride and
propylene for synthesizing propyldichlorosilanes gives
mainly iso-propyldichlorosilane [12]. Key steps of
organosilane formation in the silicon–HCl–propylene
and silicon–methanol–propylene reactions are same,
Fig. 7. Effect of propylene pressure on the silicon conversion (a), the
selectivity (b) and yield (c) of n-propyldimethoxysilane. The pretreat-
ment was carried out at 240°C for 1 h. The reaction temperature was
240°C. Methanol pressure was 0.10 MPa.
MPa. This result also indicates that the addition of
propylene to surface silylene is a first-order reaction.
The silicon conversion was almost constant around
60%. The ratio of n-propyl to iso-propyldimethoxysi-
lane did not depend on the pressure, and was around 9.
The effect of the reaction temperature is shown in
Fig. 8. The pretreatment of the silicon–catalyst mixture
was carried out for 1 h at the same temperature as the
reaction. The n-propyldimethoxysilane selectivity in-
creased with the temperature and reached 14% between
220 and 240°C and the formation of n-propy-
ldimethoxysilane almost stopped at 280°C. The silicon
conversion reached 59% at 240°C and then decreased.
The maximum of n-propyldimethoxysilane yield was
8% at 240°C. The ratio of n-propyl to iso-propy-
ldimethoxysilane significantly changed with the reaction
temperature. The ratio was very high (10) at 220°C,

240
M. Okamoto et al. / Journal of Organometallic Chemistry 619 (2001) 235–240
i.e. the reaction of surface silylene with propylene. The
selectivity for n- or iso-product is determined by a kind
of molecule attacking the species (VI) and the reaction
temperature. In the silicon–methanol–propylene reac-
tion, the iso-/n-ratio was constant when the propylene
pressure was changed. This result indicates that propyl-
ene addition is not the step for determination of the
selectivity for iso- or n-product. This is in agreement
with the reaction mechanism shown in Eq. (3).
an autoclave reactor. The examination of the relation
between the selectivity for alkyldimethoxysilane and
alkene pressure revealed that the addition of surface
silylene intermediate to alkene is a first-order reaction
and that the mechanisms of the reactions of alkene with
vapor-phase silylene and surface silylene are the same.
Use of propylene gave n-propyldimethoxysilane as a
main product among organosilanes, the selectivity for
n-propyldimethoxysilane being 15%. The silicon–etha-
nol–ethylene gave ethyldiethoxysilane with 28%
selectivity.
3.3. Ethyldiethoxysilane synthesis from silicon, ethanol
and ethylene
References
Ethyldiethoxysilane was obtained with a high selec-
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When the reaction was carried out at 0.99 MPa of
ethylene and 0.21 MPa of ethanol at 240°C, ethyldi-
ethoxysilane with a 28% selectivity was formed together
with triethoxysilane (61%), tetraethoxysilane (2%),
ethyltriethoxysilane (5%), diethylethoxysilane (3%) and
diethyldiethoxysilane (1%). The silicon conversion was
46% and the yield of ethyldiethoxysilane was 13%.
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Use of a high-pressure flow reactor enhanced the
selectivity for organosilane in the reaction of silicon,
alcohol and alkene. The reaction of silicon, methanol
and ethylene under high-pressure conditions resulted in
33% selectivity for ethyldimethoxysilane, which is much
higher than that under atmospheric conditions or using
.