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Effects of double promoters on direct synthesis of triethoxysilane in gasЦsolid stirred fluidized bed.

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Full Paper
Received: 5 December 2010
Revised: 13 February 2011
Accepted: 14 February 2011
Published online in Wiley Online Library: 20 April 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1794
Effects of double promoters on direct synthesis
of triethoxysilane in gas–solid stirred fluidized
bed
Zhang Lei, Hao Sue, Yang Chunhui∗ , Li Ji, Yang Kai, Hu Chenfa and Ge Shibin
The reaction of silicon with ethanol was carried out in a gas–solid stirred fluidized bed, with cuprous chloride prepared by
a wet process as the catalyst. The effect of hydrogen fluoride and ethyl chloride addition on formation of triethoxysilane in
the direct process was described and discussed. The catalytic activity was evaluated using online gas chromatography, X-ray
diffraction and scanning electron microscopy to study the contact mass. The results show that the promoters hydrogen fluoride
and ethyl chloride not only supplemented anions, but also depressed the catalytic activity of metallic copper. The comparison
of technical-grade silicon and polycrystalline silicon showed many differences in reactivity, reaction rate and induction period.
Furthermore, the yield, selectivity of triethoxysilane and reaction rate were improved significantly with additions of hydrogen
c 2011 John Wiley & Sons, Ltd.
fluoride and ethyl chloride. Copyright Supporting information may be found in the online version of this article.
Keywords: direct synthesis; triethoxysilane; effects of double promoters; contact mass; gas–solid stirred fluidized bed
Introduction
Triethoxysilane is an important chemical from which various
organosilicon compounds can be derived.[1 – 10] Manufacture of
triethoxysilane is carried out by a reaction between trichlorosilane
and ethanol. However, this process has disadvantages, such as the
high cost of trichlorosilane, difficulty in product purification and
corrosion of the reaction apparatus. In order to overcome these
problems, the direct method, a vapor-phase reaction that makes
use of a fixed-bed reactor, was developed by Rochow.[11] The main
reaction of the direct process reads as follows:
Cu
Si + 3C2 H5 OH = HSi(OC2 H5 )3 + H2
(1)
508
Thereafter, the reaction was studied extensively in all kinds of
reactors. The advantages of the direct method over esterification
of trichlorosilane are its lower cost and environmental impact.
Work has been conducted on a modification of the direct method,
which makes use of a thermally stable and high-boiling solvent,
the process of which is carried out mainly in a slurry reactor.[12 – 24]
Compared with the vapor-phase reaction, the slurry method is
more expensive and more troublesome. In addition, the use of
a high-boiling-point solvent makes separation of the product
from the colloidal mixture, as well as its purification, difficult.
The mechanism and technology development based on direct
synthesis of trimethoxysilane have been studied extensively.[25 – 29]
Many Japanese researchers have developed a direct method
without solvent, which is a process that involves the reaction of
silicon with methanol in a gas–solid fixed bed. However, thus far,
there are no systematic studies on the synthesis technology and
the effect of promoters on direct synthesis triethoxysilane in a
gas–solid stirred fluidized bed.
In this work, triethoxysilane was synthesized using cuprous
chloride (CuCl) as catalyst. Hydrogen fluoride (HF) and ethyl
Appl. Organometal. Chem. 2011, 25, 508–513
chloride (C2 H5 Cl) were used as promoters for the reaction, so
the reaction rate and the yield of the synthesis were improved. In
order to confirm the effect of promoters, we used scanning electron
microscopy combined with energy dispersive spectrometry (SEMEDS) and X-ray diffraction (XRD) to investigate contact mass. Some
important problems of direct synthesis of triethoxysilane in the
gas–solid stirred fluidized bed were investigated systematically.
We also investigated the reaction rate, selectivity and yield of
triethoxysilane. The following two problems were investigated:
(1) the mode of promoter action; and (2) the reason why the
reaction maintains high activity only for a short time, after which
it declines sharply.
Experimental Section
Materials and Methods
Technical-grade silicon, Sitech (Any ang DX Silicon Industry Co.
Ltd), and semiconductor-quality polycrystalline silicon, Sipure
(Wu xi Zhong Cai Technology Co. Ltd), were used, and their
impurities are listed in Table 1. Other materials used in the
reaction included ethanol (Sinopharm Chemical Reagent Co. Ltd,
99.8%), ethyl chloride (Ji Hua Chemical Company, 99%), HF (KAISN
Fluorochemical Co. Ltd, 99%) and CuCl prepared by the wet
process, as described in Harada and Yamada.[15]
∗
Correspondence to: Yang Chunhui, School of Chemical Engineering and
Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001,
People’s Republic of China. E-mail: yangchh@hit.edu.cn
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, People’s Republic of China
c 2011 John Wiley & Sons, Ltd.
Copyright Effects of double promoters on direct synthesis of triethoxysilane
Table 1. Impurities of the two Si samples
Impurity
Fe
Al
Ca
Pb
Sn
Mn
Ni
Cu
V
Cr
Sitech (wt%)
Sipure (ppbw)
0.6899
0.1470
0.0682
0.0007
0.4533
0.1792
0.0845
0.3412
0.1098
0.2965
19.5
7.78
9.88
0.79
2.02
0.27
1.81
2.17
0.07
0.10
The reaction rate, selectivity and yield of triethoxysilane are
defined as follows:
Figure 1. The rate of product formation rate vs reaction time in the
reaction conditions (1) Sitech with C2 H5 OH/C2 H5 Cl/HF); (2) Sitech with
C2 H5 OH/C2 H5 Cl; and (3) Sipure with C2 H5 OH/C2 H5 Cl.
reaction rate = Fx/W
where F = C2 H5 OH flow rate, x = C2 H5 OH conversion and
W = contact mass weight.
Selectivity(%)
Amount of the product (g)
× 100%
Sum of amounts of silicon-containing products (g)
Amount of the product (mol)
Yield(%) =
Amount of siliconcharged in the reactor (2 mol)
× 100%
=
Catalytic Experiments
Appl. Organometal. Chem. 2011, 25, 508–513
Activity vs Time Dependence on the Different Reaction
Conditions
The reactivity of the two silicon samples was investigated. Figure 1
shows the time vs rate of product formation. The highest reaction
rate and yield were obtained by using Sitech as the Si source
and HF and C2 H5 Cl as promoters (2 h induction period) [Fig. 1(1)].
The profiles for Sitech /C2 H5 OH/C2 H5 Cl and Sipure /C2 H5 OH/C2 H5 Cl
[Fig. 1(2), (3)] are very different from those in Fig. 1(1). In the
case of Sitech /C2 H5 OH/C2 H5 Cl [Fig. 1(2)], the rate increased sharply
immediately after ethanol was introduced, then decreased sharply
after the maximum was achieved; the yield was very low. In the case
of Sipure /C2 H5 OH/C2 H5 Cl [Fig. 1(3)], the reaction required a longer
induction period and had a lower maximum rate. Figures 1(2)
and 1(3) show that the induction periods of Sitech and Sipure differ
from each other. This is because of the difference in the number
and formation rate of active sites. According to most researchers,
the active site is formed by the reaction between CuCl and Si,
which produces finely divided Cu that diffuses into the Si matrix
to form the active site. However, we believe the active site is
formed, not only by the reaction between CuCl and Si, but also
by the reaction between CuCl and impurities in the Sitech . In
Fig. 1(3), the reaction has a ∼5 h induction period when Sipure
is used as the reactant; however the induction period is shorter
with Sitech . In this experiment, both technical-grade silicon and
semiconductor-quality polycrystalline silicon were washed by HF
solution before being ground to powder, so the effect of SiO2 layer
is not crucial. The only difference between the two silicon sources
is the impurities content. The impurities can promote reactivity
of reduction. The effects of impurities have two aspects. First, the
CuCl can be reduced by impurities (Fe Al Ca) to form active copper
atoms. In this aspect the impurities is most effective, because of
the lower redox potential and the stronger reduction. The redox
potentials (E ) of Fe, Al, Ca and Si are −0.4089, −1.68, −2.869
and −0.857, respectively, and the redox potential of Cu/Cu+ is
0.1787. Therefore, both impurities and Si can reduce CuCl, but the
reduction ability of impurities (Al and Ca) is higher than that of Si.
The other effect of impurities is as Gillot et al. reported: ‘aluminum
alone is an important activator of the formation of the Cu3 Si phase,
and of its decomposition. The Ca and Fe, as additional impurities
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
509
Before the experiment, the two silicon samples were ground to a
powder with average particle size of 40–50 µm using a jet mill.
The CuCl catalyst was freshly prepared by the wet process. The
powder and catalyst were dried for 10 h in a tube furnace at 100 ◦ C
under a nitrogen atmosphere.
The silicon grains (56 g) and CuCl (5.6 g) were mixed in a small
vial with vigorous vibration to form the so-called ‘contact mass’.
Reaction of the contact mass with ethanol was carried out in
a stirred fluidized bed reactor. The reactor was composed of a
glass tube (70 mm diameter, 400 mm length) with a mechanical
stirrer at the top. Contact mass was fed to the base of the
reactor. Ethanol mixtures were prepared with the corresponding
mole ratios: C2 H5 OH/C2 H5 Cl/HF (1000 : 0.3 : 10), C2 H5 OH/C2 H5 Cl
(1000 : 0.3) and C2 H5 OH/HF (100 : 1). The flow rate of the reactant
was 20 mmol/min. The ethanol mixtures were then fed into the
gasifier by a constant-flow pump; subsequently, the ethanol
mixture steam–gas was mixed with nitrogen using a volume
flow controller. This reaction mixture was pumped into the stirred
fluidized bed. The temperature was maintained at 240–250 ◦ C
using a thermostat. The effluent gas was analyzed every 30 min
with an online gas chromatograph equipped with a 2 m SE-54
column and a flame ionization detector.
The morphology, element composition, and distribution of the
contact mass surface were investigated using an SEM (Apollo 300)
equipped with EDS (Oxford-Inca). XRD patterns were recorded on
a Bruker-D8 Advance operating at 40 kV and 40 mA.
Results and Discussion
Z. Lei et al.
(a)
(b)
(c)
(d)
Figure 2. Effects of the various promoters on the rate of triethoxysilane formation, and the selectivity for triethoxysilane with time. (a) Sitech with
C2 H5 OH/C2 H5 Cl; (b) Sitech with C2 H5 OH/HF; (c) Sitech with C2 H5 OH/C2 H5 Cl/HF; (d) Sipure with C2 H5 OH/C2 H5 Cl. Reaction temperature: 245 ◦ C; stirring rate,
800 rpm. Curve (1) represents the rate of triethoxysilane formation; curve (2) represents the selectivity for triethoxysilane; curve (3) represents the rate of
formation tetraethoxysilane vs time.
to Al, could reduce the effect of aluminum on the decomposition
of the Cu3 Si phase’.[30]
Effect of Double Promoters on Reaction Rate and Selectivity
of Triethoxysilane
510
Figure 2 shows the selectivity and rate of triethoxysilane at
four different conditions. The selectivity of the unpromoted
reaction [Fig. 2(a) and (d)] is comparable to that promoted by
HF [Fig. 2(b) and 2(c)]. Without HF promotion, the selectivity
of triethoxysilane rises with the reaction rate, and decreases
sharply after the maximum rate is achieved. With HF as promoter,
triethoxysilane was formed with high selectivity throughout the
reaction. The yields of triethoxysilane were 17, 40, 42, and 18%
[Fig. 2(a–d)].
Although the mechanism of direct synthesis has been studied
extensively,[31 – 33] reports have not been unanimous. The process
of direct synthesis of methylchlorosilane is similar to that of
triethoxysilane in having a ‘Rochow contact’ as catalyst. Most
scientists agree that Cu3 Si formation is accomplished by the
reaction between Si and CuCl, which results in finely divided
Cu and gaseous SiCl4 , and the reaction between Cu and methyl
chloride to regenerate CuCl.
In direct synthesis of methylchlorosilane, CH3 Cl not only
functions as a reactant, but also compensates for the anions
wileyonlinelibrary.com/journal/aoc
detached by SiCl4 . In the direct synthesis of triethoxysilane, the
reaction rate, selectivity and yield are lower when only ethanol is
used. The reason can be described by the mechanism in Scheme 1.
The function of HF seems to be to supplement the anion levels.
Fluoride furnished by the promoter HF reacts with metallic copper
to form CuF2 , which reacts with Si to regenerate the active Si–Cu
amorphous intermetallics. This results in a reaction of higher
reaction rate, selectivity and yield of triethoxysilane. Without HF
as promoter, the concentration of anions is not sufficient to
react with copper. Therefore, large amounts of inactive metallic
copper form, but no active site is formed, so the reaction breaks
down.
The rate of triethoxysilane formation changes upon addition
of HF. This is depicted in Fig. 3, which provides additional direct
evidence in favor of the above proposal. The effect of HF on reaction
rate is shown in Fig. 3. The reaction used C2 H5 OH/C2 H5 Cl/HF. At
3.5 h, the feed was replaced by pure ethanol, and the reaction
rate sharply decreased, then stopped quickly. At 4.5 h, the
feed was switched back to C2 H5 OH/C2 H5 Cl/HF, and the reaction
resumed.
The effect of ethyl chloride is significant because of its major
effect in inhibiting formation of by-products. We found that
metallic copper was formed during the reaction process, while
simultaneously, the selectivity for triethoxysilane and the reaction
rate decreased. This phenomenon indicates that metallic copper
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 508–513
Effects of double promoters on direct synthesis of triethoxysilane
(a)
(b)
Scheme 1. Suggestions on the mechanism of the direct synthesis triethoxysilane. (A) Without HF as promoter; (B) with HF as promoter.
Surface Analysis of Contact Mass
Figure 3. Change in the rate of triethoxysilane formation upon addition of
HF. Reaction temperature, 245 ◦ C; stirring rate, 800 rpm.
Appl. Organometal. Chem. 2011, 25, 508–513
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
511
can catalyze conversion of the triethoxysilane to tetraethoxysilane.
This result is in agreement with previous studies.[29] The
rate of formation of tetraethoxysilane was at a maximum at
7.5 h, whereas the rate and selectivity of triethoxysilane both
declined, as shown in Fig. 2(c)(3). Figure 2(c)(3) shows that the
formation rate of triethoxysilane declines sharply at 7.5 h when
tetraethoxysilane formation rate reaches the maximum. This
is because, at the end of the reaction, excessive inactive
metallic copper and depleted levels of the Si–Cu active site
remain.
When ethyl chloride was not used as promoter [Fig. 2(b)], the
formed metallic copper catalyzed the dehydrogenation of ethanol
to form aldehyde. At the same time, 1,1-diethoxyethane was
formed through condensation of the aldehyde. The triethoxysilane
was accompanied by the aldehyde, 1,1-diethoxyethane and other
derivatives from the ethanol dehydrogenation. Thus, ethyl chloride
is an inhibitor for the catalysis mediated by metallic copper in
ethanol dehydrogenation.
Figure 4 shows the change in morphology, element composition
and distribution of the contact mass surface with respect to
reaction time. The surface morphology and element composition
differ significantly, especially for the elemental composition.
Figure 4(1) depicts SEM images of the contact mass after 1 h
reaction. Here, pits were observed. The proportion by weight of
the pit site was 20% Cu and 70% Si. Figure 4(2) shows that, when
the reaction rate increased, the reactive area increased gradually.
The proportion of metallic copper also increased simultaneously
(71%). Figure 4(3) shows that, at the end of the reaction, there
were numerous free silicon surfaces and free copper surfaces,
which is not similar as the explanation of Ehrich,[34,35] who
ascribed the loss of free silicon surface to the failure of the
copper to diffuse into the silicon matrix to form Cu–Si surface
species.
The result of SEM-EDS confirms our hypothesis. When large
amounts of inactive metallic copper form, the reaction breaks
down. The direct synthesis of triethoxysilane differs from the
direct synthesis of methylchlorosilane. In the methylchlorosilane
synthesis, CH3 Cl can supply anions to form CuCl, which reacts
with Si to regenerate the active site. Thus, CH3 Cl is not only
a reactant, but also an anion compensator. In contrast, the
ethanol in direct synthesis of triethoxysilane is a reactant that
cannot supply anions. The result is that the concentration of
anions is not sufficient to react with copper. Therefore large
amounts of inactive metallic copper form, and the reaction breaks
down.
Contact mass was examined by XRD at different reaction stages
(Sitech reaction with C2 H5 OH/C2 H5 Cl at 245 ◦ C). The diffraction
peaks due to the metallic copper phase gradually increased in
intensity vs the reaction time [Fig. 5(a, c)]. The diffraction peaks
due to the silicon phase existed at all times [Fig. 5(a–c)]. At the
maximum reaction rate [Fig. 5(b)], there were hardly any diffraction
peaks due to the metallic copper phase and Cu3 Si. We can only
see the diffraction peaks due to the CuCl and the Si. This result
shows that there were numerous amorphous Si–Cu intermetallics
in the reaction system, which was the active site for the direct
synthesis. The active site formed from the active copper atoms
reduced by Si, or mainly because of the metallic impurity of Sitech .
When the active site was consumed by the ethanol, the elemental
Z. Lei et al.
Figure 5. XRD patterns of the contact mass formed by the reaction of Sitech
with C2 H5 OH/C2 H5 Cl at 245 ◦ C. (a) At the beginning of the reaction (1 h);
(b) at the beginning of the reaction (2 h); and (c) at the end of the reaction.
the HF promoter seems to act as an anion-supplementing agent.
The yield and the selectivity improved, especially the yield
(42%), by adding HF gas to the ethanol mixture (Nethanol : NHF =
100 : 1). The harmful catalysis of metallic copper was significantly
suppressed by adding ethyl chloride to the ethanol mixture
(Nethanol : Nethyl chloride = 1000 : 0.3). The possible reason for the
bad reaction effect was the lack of active copper atoms. The active
copper was derived from the reaction between silicon and CuCl or
metallic impurity coexistent with Sitech and CuCl.
Supporting information
Supporting information may be found in the online version of this
article.
References
Figure 4. SEM images of the contact mass formed by the reaction of Sitech
with C2 H5 OH/C2 H5 Cl at 245 ◦ C. (1) After 1 h, magnification 5000×. EDS
results of the nonreacted silicon surface (100% Si, point a), and the active
sites (pits, point b): 20% Cu and 70% Si. (2) After 2 h, magnification 3000×.
EDS results of the reactive area (pit) with bright areas (point a), mass
spectrum: 71% Cu, 7% Si, 21% Cl, and free silicon surface (point b) in
100% Si. (3) After 3.5 h, magnification 5000×. EDS results of the free silicon
surface (point a), 100% Si, and the deactivation metallic copper with bright
site (point b): >91% Cu.
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Conclusion
512
The function of HF and ethyl chloride as promoters and the
mechanism of the direct method were investigated. In summary,
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