Direct reaction between silicon and methanol over Cu-based catalysts: Investigation of active species and regeneration of CuCl catalyst

Article abstract of DOI:10.1039/c8ra03125h

When a CuCl/Si mixture was pretreated at 200-240 °C in a N₂ atmosphere, trimethoxysilane was predominantly formed in the direct reaction of silicon with methanol. When the pretreatment temperatures were raised to 260-340 °C, tetramethoxysilane was favorably formed. The Cu_xSi_yCl_z species catalyzed the reaction between silicon and methanol to trimethoxysilane. Chlorination of the spent CuCl/Si mixture promoted the reaction between silicon and methanol to form both trimethoxysilane and tetramethoxysilane due to the recovery of the CuCl phase and the exposure of the metallic Cu⁰ phase. When Cu₂O, CuO, and Cu⁰ were used as the catalysts, tetramethoxysilane was formed as the main product.

Full text of DOI:10.1039/c8ra03125h

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PAPER
Direct reaction between silicon and methanol over
Cu-based catalysts: investigation of active species
and regeneration of CuCl catalyst
Cite this: RSC Adv., 2018, 8, 19317
a
a
a
b
a
Aili Wang,
and Tongjie Hu
Mingming Zhang, Hengbo Yin, * Shuxin Liu, Mengke Liu
a
ꢀ
When a CuCl/Si mixture was pretreated at 200–240 C in a N
2
atmosphere, trimethoxysilane was
predominantly formed in the direct reaction of silicon with methanol. When the pretreatment
ꢀ
temperatures were raised to 260–340 C, tetramethoxysilane was favorably formed. The Cu
x
Si
y
Cl
z
species catalyzed the reaction between silicon and methanol to trimethoxysilane. Chlorination of the
spent CuCl/Si mixture promoted the reaction between silicon and methanol to form both
trimethoxysilane and tetramethoxysilane due to the recovery of the CuCl phase and the exposure of the
Received 12th April 2018
Accepted 21st May 2018
DOI: 10.1039/c8ra03125h
0
0
metallic Cu phase. When Cu
2
O, CuO, and Cu were used as the catalysts, tetramethoxysilane was
rsc.li/rsc-advances
formed as the main product.
between silicon and methanol over a CuCl catalyst is still worthy
of investigation.
1
. Introduction
Direct synthesis of alkoxysilanes and alkylalkoxysilanes via the
Tetramethyoxysilane was formed as a byproduct in the direct
reaction between silicon and an alcohol or alkene has attracted reaction between silicon and methanol over a CuCl catalyst. It
5,7
1
great interest from researchers. Among the products, trime- was also formed as the main product when the reaction was
0
8,10
thoxysilane and tetramethoxysilane as coupling agents instead carried out over metallic Cu and Cu
O catalysts.
It was
2
of organochlorosilanes, are widely used for the synthesis of suggested that the formation of tetramethoxysilane originated
2
–4
organosilicon products.
from the reaction between methanol and trimethoxysilane in
0
8
Synthesis of trimethoxysilane via the direct reaction between series catalyzed by metallic Cu active sites. The detailed
silicon and methanol can be catalyzed by cuprous chloride catalytic process is unclear and worthy of further investigation.
(
CuCl) in a xed-bed or a slurry phase reactor. Pretreatment
In the present work, the reactions between silicon and
0
temperature can signicantly affect the formation of trime- methanol over CuCl, Cu
2
O, CuO, and bulk metallic Cu catalysts
Si phase formed at and the regeneration of CuCl catalyst were investigated. The
3
5,6
thoxysilane. It was suggested that the Cu
ꢀ
the pretreatment temperature of 350 C was the active site for chemical structures of these catalysts aer pretreatment and
the formation of trimethoxysilane. However, it was found that reaction were investigated by XRD, XPS, and XAES techniques.
although the Cu
Si phase was not detected at pretreatment The active sites and reaction mechanisms over these catalysts
5,7
3
ꢀ
temperatures below 280 C, a high selectivity of trimethox- were discussed below.
ysilane (ca. 98%) was obtained. Recently, researchers suggested
that the Cu Si phase itself was not a reactive intermediate, but
3
2
. Experimental
the precursor composed of Cu–Si complex played an important
8,9
role in the reaction. For the direct reaction between silicon 2.1. Materials
and methanol over CuCl catalyst, the active species are still
Methanol (99.5%), cuprous chloride (97%), cuprous oxide
contradictory. On the other hand, the reaction rate between
silicon and methanol over a CuCl catalyst in a xed-bed reactor
rapidly decreases in a short reaction time period. The practical
use of a CuCl catalyst is limited. Therefore, direct reaction
(
(
95%), copper oxide (99%), and bulk metallic copper powder
150 mm) were of reagent grade and were purchased from
Sinopharm Chemical Reagent Co., Ltd. Silicon powder (>99.5%)
was supplied by Jiangsu Hongda New Materials Co., Ltd. The
materials were used as received without further purication.
a
Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
2.2. Characterization
2
12013, China. E-mail: yin@ujs.edu.cn; Tel: +86-511-88787591
b
XRD patterns of the samples were recorded on a XRD-6100Lab
diffractometer with a graphite monochromator using the Cu
School of Chemistry and Chemical Engineering, Mianyang Normal University,
Mianyang 621000, China
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Ka radiation (l ¼ 1.54056 A) at the scanning speed of 2 min
Paper
˚
ꢀ
ꢁ
1
.
X-ray photoelectron spectra (XPS) and X-ray Auger Electron
Spectra (XAES) of the samples were obtained on an X-ray
photoelectron spectroscopy/ESCA spectrometer (K-ALPHA,
Thermo Fisher Scientic) using Al Ka radiation (1486.68 eV).
The binding energies were calculated with respect to C 1s peak
of contaminated carbon at 284.6 eV.
2.3. Catalytic text
Given amounts of silicon powder and catalyst (CuCl, Cu
2
O, CuO
0
or bulk metallic Cu ) were mixed in a high-speed, multi-
function crusher (2000 rpm) for 2 min. The particle sizes of
the catalyst/Si mixture ranged from 50 to 200 mesh. The reac-
tion was carried out in a xed-bed reactor with diameter and
length of 1.8 cm and 50 cm. 20 mL of glass beads (diameter, 3
mm) were added on the top of catalyst/Si bed to evaporate
2
methanol feed into gas phase. A N stream (99.999%) with
ꢁ
1
a ow rate of 20 mL min was introduced when the catalyst/Si
mixture was pretreated at different temperatures. For the cata-
lytic reaction between silicon and methanol, methanol with
ꢁ1
a ow rate of 6 mL h was pumped into the reactor with
a constant ow pump (TBP 1010, Tauto). The resulting reaction
products were condensed and collected in an ice-water trap. The
reaction products were analyzed by gas chromatography (Agi-
lent 7890A) with a capillary column (SE-54, 0.32 ꢂ 30) and an
FID detector.
3
. Results and discussion
Fig. 1 XRD spectra of the CuCl/Si mixtures after pretreatment and
reaction. (a) The CuCl/Si mixtures were pretreated at different
3.1. Structures of CuCl/Si mixtures aer pretreatment and
ꢀ
ꢁ1
temperatures of 200–340 C for 2 h in a N stream (20 mL min ). (b)
2
reaction
After pretreating at a given temperature, the CuCl/Si mixtures reacted
ꢀ
3
.1.1. XRD analysis. The effects of pretreatment and reac- with methanol at 220, 240, and 260 C for 1 h, respectively. The
reaction conditions: CuCl/Si mixture, 40 g; CuCl/Si weight ratio,
tion on the chemical structures of the CuCl/Si mixtures were
investigated by ex situ XRD technique. The XRD spectra of the
mixtures pretreated at different temperatures in a N stream
2
ꢁ1
8
: 100; methanol flow rate, 6 mL h . The insets are the amplified XRD
peaks of CuCl phase.
show that the characteristic peaks of silicon appeared at (2q)
ꢀ
4
7.3, 56.1, 69.1, and 76.4 , respectively, which were consistent
with those of the standard silicon sample (JCPDs 27-1402)
(
Fig. 1a). The main characteristic peaks of the CuCl catalyst were
ꢀ
overlapped by those of silicon. A weak peak at (2q) 33.1
ascribed to that of the standard CuCl sample (JCPDs 06-0344)
was observed for the mixtures pretreated at the lower temper-
ꢀ
atures of 200–240 C (Fig. 1a, inset). The peak intensities of the
CuCl phase decreased with pretreatment temperature, indi-
ꢀ
cating that pretreatment at 200–240 C caused the reaction
between CuCl and silicon to form surface Cu–Si–Cl complexes.
ꢀ
CuCl + Si / Cu–Si–Cl complex + SiCl (200–240 C)
(1)
4
Fig. 2 XRD spectra of the CuCl/Si mixtures under different reaction
ꢀ
There was no CuCl peak at (2q) 33.1 detected at the
ꢀ
conditions. (a) The CuCl/Si mixture was pretreated at 200 C for 2 h in
ꢀ
ꢁ1
pretreatment temperatures of 260–340 C. The XRD peak a N2 stream (20 mL min ) and then reacted with methanol at 220,
ꢀ
ꢀ
appearing at 43.3 ascribed to the characteristic peak of the 240, and 260 C for 1 h, respectively. (b) The CuCl/Si mixture reacted
with methanol at 240 ꢀC for 12 h. (c) After reacting for 12 h, the spent
0
standard metallic Cu (JCPDs 04-0836) was detected at 260–
ꢀ
40 C. The intensities of the metallic Cu peaks slightly
increased with pretreatment temperature. The intensity ratios
of metallic Cu (111) peak to Si (220) peak were in a range of
ꢁ1
ꢀ
0
CuCl/Si mixture was chlorinated with HCl (40 mL min ) at 280 C for
3
1
.5 h. Reaction conditions: CuCl/Si weight ratio of 8 : 100, 40 g;
ꢁ1
methanol flow rate, 6 mL h
.
0
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Fig. 4 XAES of Cu LMM of the bulk CuCl, reacted CuCl/Si, and chlo-
rinated CuCl/Si samples. The CuCl/Si mixture (40 g, CuCl/Si weight
ꢁ
1
ꢀ
ratio of 8 : 100) reacted with methanol (6 mL h ) at 240 C for 12 h,
denoted as reacted CuCl/Si. After reacting for 12 h, the spent CuCl/Si
ꢁ1
ꢀ
mixture was chlorinated with HCl (40 mL min ) at 280 C for 1.5 h,
denoted as chlorinated CuCl/Si.
ꢀ
1
2CuCl + 7Si ¼ 4Cu Si + 3SiCl (280–340 C)
(3)
3
4
Aer taking part in the reaction between silicon and meth-
anol, the CuCl phase was only observed in the CuCl/Si mixture
ꢀ
0
pretreated at 200 C (Fig. 1b, inset). The metallic Cu phase
(
3
JCPDs 04-0836) appeared in the mixtures pretreated at 200–
ꢀ
40 C, whereas the Cu Si phase (JCPDs 51-0916) appeared in
3
ꢀ
the mixtures pretreated at 280–340 C (Fig. 1b). The results
revealed that CuCl took part in the reaction and the metallic Cu
0
3
phase was formed in the reaction process. The Cu Si phase was
formed due to the pretreatment at high temperature rather than
via the reaction.
ꢀ
When the CuCl/Si mixture was pretreated at 200 C for 2 h in
ꢀ
a N atmosphere and then reacted at 220, 240, and 260 C for
2
0
1
h, respectively, the metallic Cu phase appeared and the CuCl
ꢀ
phase remained (Fig. 2a). Aer reacting at 240 C for a longer
time period of 12 h, metallic Cu was formed, but the CuCl
0
phase disappeared (Fig. 2b), indicating that the CuCl phase
took part in the reaction. When the spent CuCl/Si mixture was
chlorinated in an HCl stream, in addition to the presence of the
Fig. 3 XPS of Si 2p, Cl 2p, and Cu 2p of the bulk Si, bulk CuCl, reacted
CuCl/Si, and chlorinated CuCl/Si samples. Reaction conditions: CuCl/
0
metallic Cu phase, the CuCl phase appeared again (Fig. 2c).
ꢁ1
Si weight ratio of 8 : 100, 40 g; methanol flow rate, 6 mL h . The
The chlorination converted the copper species to CuCl.
3.1.2. XPS and XAES analyses. The surface chemical
structures of the bulk CuCl, bulk Si, reacted CuCl/Si, and
chlorinated CuCl/Si were detected using the ex situ XPS tech-
nique. The X-ray photoelectron spectra of Si 2p, Cl 2p, and Cu
ꢀ
mixture reacted at 240 C for 12 h, denoted as reacted CuCl/Si. After
ꢁ1
reacting for 12 h, the mixture was chlorinated with HCl (40 mL min
at 280 C for 1.5 h, denoted as chlorinated CuCl/Si.
)
ꢀ
5
.1 : 100 to 12.7 : 100. When the pretreatment temperatures 2p are shown in Fig. 3.
ꢀ
ꢀ
were 280–340 C, the XRD peaks at 44.5 (012) and 45.1 (300)
ascribed to those of the Cu
Si phase (JCPDS 51-0916) were Si, and chlorinated CuCl/Si samples were 98.9, 98.8, and
observed. The intensity ratios of Cu
Si (012) and (300) peaks to 99.2 eV, respectively (Fig. 3a). The Si 2p3/2 peak of the reacted
Si (220) peak ranged from 2.5 : 100 to 4.8 : 100 and from CuCl/Si mixture shied by 0.1 eV to a lower binding energy as
.8 : 100 to 6.6 : 100, respectively. The results revealed that high compared to that of the bulk Si, revealing that there was an
pretreatment temperature caused the reaction between CuCl interaction between bulk Si and CuCl during the reaction
The binding energies of Si 2p3/2 of the bulk Si, reacted CuCl/
3
3
2
0
11
and silicon to form metallic Cu and Cu
3
Si phases as follows.
process. However, chlorination caused a positive shi of the Si
2
p
3/2 peak of the chlorinated CuCl/Si mixture, probably due to
ꢀ
+
4
CuCl + Si ¼ 4Cu + SiCl
4
(260–340 C)
(2) surface-interacted Cu being chlorinated to CuCl.
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Table 1 Surface copper components based on Cu LMM deconvolution and surface atomic ratios of Cl to Cu based on XPS analysis
Kinetic energies (eV)
2+
+
0
a
2+
+
0
b
Samples
Cu
Cu
Cu
Peak area ratios , Cu : Cu : Cu
Atomic ratios , Cl : Cu
Bulk CuCl
Reacted CuCl/Si
Chlorinated CuCl/Si
918
917.7
918
916
916
916
—
918.7
918.7
7.7 : 1 : 0
0.3 : 1 : 1.4
2.4 : 1 : 0.7
1 : 1
0.3 : 1
0.8 : 1
a
2+
+
0
b
Peak area ratios among Cu , Cu , and Cu were calculated by the deconvolution of Cu LMM XAES. Atomic ratios of Cl to Cu were obtained by
XPS analysis.
with peak positions at 916, 917.7–918, and 918.7 eV corre-
+
2+
0
sponding to Cu , Cu , and Cu species, and the XAES peaks
+
2+
were deconvoluted into three symmetrical peaks of Cu , Cu ,
and Cu species by using a XPSPEAK41 soware. Considering
0
that a shoulder peak appeared at 912 eV, an extra Gaussian–
Lorentzian band with the peak position at ca. 912 eV was used to
eliminate the effect of other orbital electrons on the XAES.
According to the deconvolution results, the surface copper
2
+
+
species of the bulk CuCl were composed of Cu and Cu
components (Fig. 4). The surface copper species of the reacted
CuCl/Si and chlorinated CuCl/Si mixtures were composed of
Scheme 1 Reactions between CuCl and bulk Si and the chlorination
reaction.
2
+
+
0
Cu , Cu , and Cu components. Aer taking part in the reac-
0
tion, in addition to the formation of the metallic Cu compo-
2
+
ꢁ
nent, the surface Cu content of the reacted CuCl/Si mixture
was obviously less than that of the bulk CuCl, but the surface
The binding energies of Cl 2p3/2 and Cl 2p1/2 of Cl of the
bulk CuCl, reacted CuCl/Si, and chlorinated CuCl/Si samples
were ca. 198.7 and 200.3 eV, respectively (Fig. 3b). According to
the deconvolution of the Cl 2p3/2 and Cl 2p1/2 peaks, the area
ratios of the Cl 2p3/2 peak to the Cl 2p1/2 peak for the three
samples were 1 : 0.8, 1 : 1.9, and 1 : 0.8, respectively, revealing
that there was an interaction between Si and Cl during the
reaction process. Aer chlorination, the CuCl phase was
recovered because the bulk CuCl and chlorinated CuCl/Si
+
Cu content of the reacted CuCl/Si mixture was larger than that
2
+
of the bulk CuCl, indicating that Cu was reduced to copper
species with lower valence states (Table 1). Aer chlorination,
2
+
+
the total content of surface Cu and Cu species of the chlori-
nated CuCl/Si mixture increased as compared to that of the
2
+
reacted CuCl/Si mixture. Considering that the surface Cu
species of the chlorinated CuCl/Si mixture could be attributed
+
to the oxidation of Cu during the sample characterization
samples had the same area ratios of Cl 2p3/2 to Cl 2p1/2
.
17
process in air, it was reasonable to suggest that chlorination
The Cu 2p3/2 and Cu 2p1/2 peaks of the bulk CuCl, reacted
CuCl/Si, and chlorinated CuCl/Si samples were doublet (Fig. 3c).
The presence of a satellite peak at ca. 942.5 eV for the three
+
increased the surface Cu content.
The XPS analysis revealed that the surface atomic ratio of Cl
to Cu in the reacted CuCl/Si mixture was lower than that in the
bulk CuCl (Table 1). Chlorination of the reacted CuCl/Si mixture
increased the surface atomic ratio of Cl to Cu. It could be
explained as follows. During the reaction process, CuCl crys-
2
+
12
samples indicated the presence of Cu species. The Cu 2p3/2
and Cu 2p1/2 peaks at ca. 935 and 955 eV were ascribed to those
2
+
of Cu species, whereas the Cu 2p3/2 and Cu 2p1/2 peaks at
+
9
32.4 and 952.3 eV were ascribed to those of Cu and/or metallic
0
13–16
tallites reacted with silicon to form Cu
x
Si
y
Cl
z
species. Further-
Cu species. The XPS analysis revealed that the three
samples contained Cu , Cu and/or Cu species. The presence
of the Cu species is observed because CuCl is easily oxidized
ꢁ
2
+
+
0
more, Cl from the Cu
Si
x
Cl
y
z
species reacted with Si and/or
2
+
methanol, removing it from the active species. This process
ꢁ
caused the decrease in Cl content of Cu
Si
x y
Cl
z
0
active species.
when it is exposed to air during the ex situ sample character-
ꢁ
With the consumption of Cl , metallic Cu was formed.
However, chlorination treatment caused the formation of CuCl
17
ization process.
+
0
Considering that Cu and metallic Cu have similar binding
energies, it is difficult to distinguish them based on Cu 2p
spectra. X-ray excited Auger Electron Spectroscopy (XAES) was
and probably recovered the Cu
x
Si
y
Cl
z
active species. The reac-
tions are suggested in Scheme 1.
2
+
+
used to determine the surface compositions of the Cu , Cu ,
0
0
13,16,18–20
3.2. Chemical structures of Cu
mixtures aer pretreatment and reaction
3.2.1. XRD analysis. When Cu O and CuO were used as the
Cu species are centered at 916, 918, and 918.7 eV, respec- catalysts, Cu O and CuO phases existed in the pretreated
Therefore, we chose Gaussian–Lorentzian bands samples, respectively (Fig. 5a and b). However, aer reacting at
2
O/Si, CuO/Si, and Cu /Si
and Cu species.
The Cu LMM XAES of the bulk CuCl,
reacted CuCl/Si, and chlorinated CuCl/Si samples are shown in
+
2+
Fig. 4. Literatures report that the XAES peaks of Cu , Cu , and
2
0
2
13,16,18–20
tively.
ꢀ
0
2
20–260 C for 3 h, the metallic Cu phase was formed with the
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2
Fig. 6 The XPS of (a) Si 2p and (b) Cu 2p of the reacted Cu O/Si,
reacted CuO/Si, and reacted Cu/Si mixtures.
0
2
Fig. 5 XRD spectra of (a) Cu O/Si, (b) CuO/Si, and (c) Cu /Si mixtures
after pretreatment and reaction. These mixtures were pretreated at
ꢀ
ꢁ1
2
2
40 C for 2 h in a N
2
stream (20 mL min ) and then reacted at 220,
ꢀ
40, and 260 C for 1 h, respectively. The reaction conditions: catalyst/
0
Si, 40 g; weight ratios of Cu
: 100; methanol flow rate, 6 mL h
2
O, CuO, and Cu to Si, 5 : 100, 5 : 100, and
ꢁ1
8
.
Fig. 7 The XAES of Cu LMM of the reacted Cu/Si, reacted CuO/Si, and
reacted Cu O/Si samples.
2
disappearance of the Cu
2
O and CuO phases, indicating that
0
Si samples are shown in Fig. 6. The Si 2p3/2 peaks of the reacted
Cu O/Si, reacted CuO/Si, and reacted Cu/Si samples slightly
shied by 0.1 eV to a lower binding energy as compared with
that of the bulk Si sample (Fig. 6a), indicating that there was
a weak interaction between copper species and Si surface.
The binding energies of Cu 2p3/2 and Cu 2p1/2 of the reacted
Cu O and CuO were reduced to metallic Cu in the reaction
2
0
process. When metallic Cu was used as the catalyst, aer
pretreatment and reaction, the metallic Cu phase did not
change (Fig. 5c). No other phases, such as Cu–Si compounds,
were detected by XRD analysis.
2
0
3.2.2. XPS and XAES analyses. The Si 2p and Cu 2p spectra
Cu O/Si, reacted CuO/Si, and reacted Cu/Si mixtures were ca.
of the bulk Si, reacted Cu O/Si, reacted CuO/Si, and reacted Cu/
2
2
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Table 2 Surface Cu components based on Cu LMM deconvolution
copper species with lower valence states. The ratios of surface
+
0
Cu to Cu were inuenced by their copper precursors
Table 2).
Kinetic
energies (eV)
(
+
0
a
+
0
Samples
Cu
Cu
Peak area ratios , Cu : Cu
3.3. Direct reaction of silicon with methanol over CuCl
catalyst
Reacted Cu/Si
Reacted Cu
Reacted CuO/Si
916
915.6
915.6
918.7
918.6
918.5
6.8 : 1
14.7 : 1
4.8 : 1
2
O/Si
3.3.1. Effect of pretreatment temperature. The results of
the reaction between silicon and methanol over a CuCl catalyst
pretreated in a N2 atmosphere at different temperatures are
listed in Table 3. Trimethoxysilane and tetramethoxysilane were
detected as the main products. The conversion of methanol
increased upon increasing the pretreatment temperature,
a
+
0
Peak area ratios of Cu to Cu were calculated by the deconvolution of
Cu LMM XAES.
0
9
32.4 and 952.3 eV, respectively, indicating that metallic Cu
+
and/or Cu species were present (Fig. 6b). The presence of probably due to the possibility that high pretreatment temper-
a weak satellite peak at 942.5 eV for the reacted Cu
O/Si and ature promoted the interaction between CuCl and silicon,
reacted CuO/Si mixtures indicated that there was a trace providing more active sites for the reaction between silicon and
amount of Cu species present. However, the absence of the methanol. Interestingly, it was found that the trimethoxysilane
satellite peak in the reacted Cu/Si mixture indicated that no selectivity at the pretreatment temperatures of 200–240 C was
2
2
+
ꢀ
2
+
ꢀ
Cu species existed.
higher than that at 260–340 C. When the pretreatment
ꢀ
To ascertain the chemical states of the surface copper temperature was more than 280 C, the trimethoxysilane
species, the XAES peaks were deconvoluted (Fig. 7). Using selectivity was less than the tetramethoxysilane selectivity.
Gaussian–Lorentzian bands with the peak positions at ca. 916 Combining the XRD, XPS, and XAES analyses, it was suggested
+
0
and 918.7 eV for Cu and Cu and at ca. 912 eV for other orbital that the Cu
x y z
Si Cl species formed in the pretreatment process
electrons, the XAES peaks were deconvoluted into three played an important role for the formation of trimethoxysilane.
+
0
13,16,18–20
symmetrical peaks.
According to the deconvolution results, the surface copper the formation of tetramethoxysilane.
The surface Cu and metallic Cu species probably co-catalyzed
species of the reacted Cu/Si, reacted Cu
Si mixtures were mainly composed of Cu and metallic Cu
components. During the reaction process, Cu and Cu
2
O/Si, and reacted CuO/
3.3.2. Effect of CuCl loading. Considering that a lower
+
0
pretreatment temperature gave high selectivity of trimethox-
+
2+
ꢀ
ysilane, the CuCl/Si mixtures pretreated at 200 C were used to
species in the Cu
2
O/Si and CuO/Si mixtures were reduced to investigate the effect of CuCl loading on the reaction between
silicon and methanol. The reaction results are listed in Table 4.
a
Table 3 Direct reaction between silicon and methanol over CuCl catalyst pretreated at different temperatures
Pretreatment
temperatures ( C)
Reaction
temperatures ( C)
Methanol
conversions (%)
Trimethoxysilane
selectivities (%)
Tetramethoxysilane
selectivities (%)
ꢀ
ꢀ
200
220
240
260
280
300
340
220
0.5
94.2
91.6
81.5
91.3
88.7
82.6
90.4
87.7
88.3
52.9
48.0
42.6
31.1
42.5
46.9
30.3
36.9
44.7
5.8
5.8
8.4
18.5
8.7
11.3
17.4
9.6
12.3
11.7
47.1
52.0
57.4
68.9
57.5
53.1
69.7
63.1
55.3
94.2
93.2
90.5
2
2
40
60
89.3
97.2
1.1
220
2
2
40
60
90.7
94.5
92.4
97.1
94.1
96.8
98.0
99.1
95.6
97.6
99.6
96.0
99.1
99.7
97.0
100
220
2
2
40
60
220
2
2
40
60
220
2
2
40
60
220
2
2
40
60
220
2
2
40
60
6.8
9.5
99.9
a
ꢁ1
The reaction mixtures of silicon and CuCl were pretreated at different temperatures for 2 h in a N
2
stream with a ow rate of 20 mL min . The
ꢁ1
mixture of CuCl and silicon was 40 g, the CuCl/Si weight ratio was 8 : 100, and the methanol ow rate was 6 mL h
.
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Table 4 Direct reaction between silicon and methanol over CuCl catalyst with different CuCl loadings
Trimethoxysilane
Tetramethoxysilane
CuCl/Si weight
ratios
Reaction
temperatures ( C)
Methanol
conversions (%)
ꢀ
Selectivities (%)
Yields (%)
Selectivities (%)
Yields (%)
3
5
8
1
: 100
220
0.1
60.5
81.4
0.5
90.4
95.2
0.5
89.3
97.2
0.1
90.2
98.4
87.1
79.8
62.3
93.2
89.6
84.0
94.2
91.6
81.5
91.7
87.7
84.0
0.1
48.3
50.7
0.5
81.0
80.0
0.5
81.8
79.2
0.1
79.1
82.7
12.9
20.2
37.7
6.8
10.4
16.0
5.8
8.4
18.5
8.3
0.01
12.2
30.7
0.03
9.4
15.2
0.03
7.5
18.0
0.01
11.1
15.7
2
2
40
60
: 100
220
2
2
40
60
: 100
220
2
2
40
60
0 : 100
220
2
2
40
60
12.3
16.0
a
ꢀ
ꢁ1
The CuCl/Si mixtures were pretreated at 200 C for 2 h in a N
2
stream with a ow rate of 20 mL min . The CuCl/Si mixture was 40 g and the
ꢁ1
methanol ow rate was 6 mL h
.
of trimethoxysilane and tetramethoxysilane remained at ca.
40% and 60%, respectively.
The results revealed that without pretreatment, the reaction
rate between silicon and methanol increased gradually with
reaction time. The main product was trimethoxysilane. When
the spent CuCl/Si mixture was chlorinated with HCl, the reac-
tion rate between silicon and methanol rapidly increased,
x y z
probably due to the formation of more Cu Si Cl active sites and
0
the exposure of the metallic Cu active sites. However, aer
chlorination with HCl, the selectivity of trimethoxysilane was
one half that using a fresh CuCl/Si mixture, indicating that the
0
exposed metallic Cu probably promoted the formation of
tetramethoxysilane.
ꢀ
Fig. 8 Reaction between silicon and methanol at 240 C with
ꢁ
1
a methanol flow rate of 6 mL h . Reaction conditions: (a) The CuCl/Si 3.4. Direct reaction of silicon with methanol over Cu O,
2
0
mixture was not pretreated before reaction. The amount of CuCl/Si
CuO, and metallic Cu catalysts
mixture was 40 g with a CuCl/silicon weight ratio of 8 : 100. (b) The
0
ꢁ1
When Cu O, CuO, and bulk metallic Cu were used as the
catalysts, tetramethoxysilane was formed as the main product.
spent CuCl/Si mixture was chlorinated with HCl (40 mL min ) at
2
ꢀ
2
80 C for 1.5 h before reaction.
The Cu
2
O and CuO catalysts exhibited higher catalytic activities
for the reaction between silicon and methanol than the bulk
0
The reaction results showed that upon increasing the CuCl metallic Cu catalyst (Table 5). This suggested that the surface
+
0
loading and reaction temperature, the conversion of methanol Cu /Cu species co-catalyzed the reaction between silicon and
increased. When the CuCl loadings were 5–10% at the reaction methanol to form tetramethoxysilane.
ꢀ
temperatures of 240–260 C, the trimethoxysilane yield of ca.
It was interesting to discover that a small amount of methyl
formate was formed as the by-product over the Cu O and CuO
.3.3. Reaction life time and HCl treatment. Without catalysts. While using the bulk metallic Cu as the catalyst,
80% was obtained.
2
0
3
pretreatment, the methanol conversion increased with pro- a small amount of trimethoxysilane was formed as the by-
0
longing reaction time during the rst 4 h (Fig. 8a). The meth- product. According to the crystallite sizes of the metallic Cu ,
2
anol conversion remained at ca. 70% for 5 h. Then the methanol the high catalytic activities over the Cu O and CuO catalysts
0
conversion decreased rapidly to 5.5% in 3 h. The selectivities of were due to the formation of small-sized metallic Cu crystal-
0
trimethoxysilane and tetraethoxysilane remained at ca. 85% lites in the reaction. The small-sized metallic Cu crystallites
and 15% during the reaction time period, respectively.
also catalyzed the formation of methyl formate via the inter-
21
When the spent CuCl/Si mixture was chlorinated with HCl, molecular dehydrogenation reaction of methanol.
the conversion of methanol increased to 95% during the rst
2
CH
3
OH ¼ CH
3
OOCH + 2H
2
(4)
1
h and remained at above 99% for 9 h (Fig. 8b). Then the
methanol conversion decreased to 7.7% in 3 h. The selectivities
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4. Conclusions
In the direct reaction of silicon with methanol over a CuCl
catalyst, pretreatment temperature affected the evolution of the
0
Cu
x
Si
y
Cl
z
species, metallic Cu , and Cu
3
Si phases. The Cu
Si
x y
Cl
z
species appeared at a lower pretreatment temperature, whereas
the Cu
3
Si phase appeared at a pretreatment temperature of
ꢀ
280 C or higher.
Aer pretreating CuCl/Si (5 : 100–10 : 100) mixtures at the
ꢀ
pretreatment temperature of 200 C, trimethoxysilane was
predominantly formed with a yield ca. 80% at methanol
conversion of more than 89% and reaction temperatures of
ꢀ
2
40–260 C. The Cu
x y
Si Cl
z
species catalyzed the formation of
trimethoxysilane. Aer pretreating the CuCl/Si mixture at
ꢀ
a pretreatment temperature of above 280 C, tetramethoxysilane
+
0
was favorably formed. The surface Cu and metallic Cu phases
catalyzed the formation of tetramethoxysilane.
Chlorination of the spent CuCl/Si mixture promoted the
reaction between silicon and methanol to the formation of tri-
methoxysilane and tetraethoxysilane, probably due to the
recovery of the Cu
x
Si
metallic Cu species.
When the direct reaction was catalyzed over Cu
y z
Cl active species and the exposure of
0
2
O and CuO
catalysts, the Cu O and CuO phases were reduced to the
2
0
metallic Cu phase during the reaction process. Tetramethox-
ysilane was formed as the main product with a selectivity of
ꢀ
more than 90% at the reaction temperatures of 220–260 C.
Cu
2
O and CuO catalysts exhibited higher catalytic activities for
0
the direct reaction than the bulk metallic Cu catalyst. The
surface Cu and metallic Cu co-catalyzed the formation of
tetraethoxysilane.
+
0
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (21506078) and China Post-
doctoral Science Foundation (2016M601739).
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