Controllable wet synthesis of multicomponent copper-based catalysts for Rochow reaction

Article abstract of DOI:10.1039/c5ra10999j

This work aims to provide a facile, low-cost and scalable method for the preparation of multicomponent Cu-Cu₂O-CuO catalysts, which are of high interest to the organosilane industry. A series of submicrometer-sized and Cu-based catalysts containing CuO, Cu₂O and Cu, or some combination of them, were synthesized by a simple low-temperature wet chemical method using CuSO₄·5H₂O as the precursor and N₂H₄·H₂O as a reducing agent. The samples were characterized by X-ray diffraction, thermogravimetric analysis, temperature-programmed reduction, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy techniques. It was observed that the composition of the samples could be tailored by varying the amount of reducing agent at a given reaction temperature and time. These catalysts were then tested in the Rochow reaction, using silicon powder and methyl chloride (MeCl) as reactants to produce dimethyldichlorosilane (M2), which is the most important organosilane monomer in the industry. Compared with bare CuO and Cu particles, the ternary CuO-Cu₂O-Cu catalyst displayed much improved M2 selectivity and Si conversion, which can be attributed to the smaller copper particle size and the synergistic effect among the different components in the CuO-Cu₂O-Cu catalyst. This catalyst preparation method is expected to yield efficient and low-cost copper catalysts for the organosilane industry.

Full text of DOI:10.1039/c5ra10999j

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Page 1 of 27
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DOI: 10.1039/C5RA10999J
Controllable Wet Synthesis of Multicomponent Copper-Based Catalysts for
Rochow Reaction
Yanzhao Zhai,a, b Yongjun Ji,*, a Guangna Wang,a
, a
Yongxia Zhu,a Hezhi Liu,a Ziyi Zhongc and Fabing Su
*
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing, China 100190
b
School of Chemical and Environmental Engineering, China University of Mining and
Technology(Beijing)，Beijing, China 100083
c School of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang
Drive, Singapore 637459
*To whom correspondence should be addressed. Eꢀmail address: yjji@ipe.ac.cn (Y. Ji),
fbsu@ipe.ac.cn (F. Su), Tel.: +86ꢀ10ꢀ82544850, Fax: +86ꢀ10ꢀ82544851.
1

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Abstract
This work aims to provide a facile, lowꢀcost and scalable method for the preparation of the
multicomponent CuꢀCu2OꢀCuO catalysts which are highly interested by the organosilane industry. A
series of submicrometerꢀsized and Cuꢀbased catalysts containing CuO, Cu2O and Cu or some of
them have been synthesized by a simple lowꢀtemperature wet chemical method using CuSO4·5H2O
as the precursor and N2H4·H2O as reducing agent. The samples were characterized by Xꢀray
diffraction, thermogravimetric analysis, temperatureꢀprogrammed reduction, Xꢀray photoelectron
spectroscopy, transmission electron microscopy, and scanning electron microscopy techniques. It
was observed that the composition of the samples could be tailored by varying the amount of
reducing agent at the given reaction temperature and time. These catalysts were then tested for the
Rochow reaction using silicon powder and methyl chloride (MeCl) as reactants to produce
dimethyldichlorosilane (M2), which is the most important organosilane monomer in the industry.
Comparing with the bare CuO and Cu particles, the ternary CuOꢀCu2OꢀCu catalyst displayed much
improved M2 selectivity and Si conversion, which can be attributed to the smaller copper particle
size and the synergistic effect among the different components in the CuOꢀCu2OꢀCu catalyst. This
catalyst preparation method is expected to yield efficient and lowꢀcost copper catalysts for
organosilane industry.
Keywords: Controlled synthesis; Wet chemical method; Copperꢀbased catalysts; Rochow reaction
2

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1. Introduction
The Rochow reaction, which was discovered in the 1940s,1 is still the most economical route to
produce silicone monomers in the organosilane industry. In this reaction, gaseous methyl chloride
(MeCl) reacts with silicon (Si) in the presence of Cuꢀbased catalysts and a trace amount of promoters
as following:2,3
(CH3 )3SiCl
(CH3 )2SiCl2
CH3SiCl3
Copper catalyst
Si + MeCl → CH3HSiCl2
(1)
o
300ꢀ350 C
(CH3 )2HSiCl
other low boilers
residue
Among the products from the above Rochow reaction, dimethyldichlorosilane ((CH3)2SiCl2, M2)
is the most important monomer used for silicon rubber production in organosilane industry, and thus
a high M2 yield is highly desired. Besides the accurate control of reaction conditions and selection of
proper reactors, development of highly efficient catalyst systems is crucial for a high M2 selectivity
and Si conversion. As reported，the primary catalysts used in the Rochow reaction are Cuꢀbased
catalysts including metallic Cu,1 CuCl,4 Cu2O,5 CuO,6 CuꢀSi alloy7 and CuꢀCu2OꢀCuO,8 together
with some catalyst promoters.2,6,9ꢀ11 In recent years, our group has developed a number of cupreous
and copper oxide structures, such as flowerꢀlike ZnO grown on urchinꢀlike CuO microspheres,12
porous cubic Cu microparticles,13 mesoporous Cu2O microspheres,5 shapeꢀcontrolled Cu2O
microparticles,14 dandelionꢀlike CuO microspheres,15 flowerꢀlike CuO microspheres,16 and CuCl
microcrystals with different morphologies17 etc., for M2 synthesis, and found that the particle
morphology and size as well as the catalyst composition impact on the catalytic properties
significantly.
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Among various Cuꢀbased catalysts, the multicomponent Cuꢀbased catalysts containing Cu, Cu2O,
and CuO are of great interest due to their superior catalytic performances in Rochow reaction. For
instance, Khitouni et al prepared such kind of catalysts through a highꢀenergy mechanical milling
process using Cu, Cu2O, and CuO as the precursors.18 Although their method is simple, it still lacks
effective controls on the solid reaction among Cu, Cu2O, and CuO during the mechanical milling
process. Therefore, to meet industrial application, more effective methods for preparing
multicomponent Cuꢀbased catalysts should be developed. More recently, we found that the
multicomponent CuꢀCu2OꢀCuO catalyst can also be synthesized by partial reduction of CuO
nanoparticles in H2/N2 gas19 or by controlled oxidation of copper flakes in O2/N2 gas.20 These
methods, however, still suffer from the low yields and use of highꢀcost equipment together with
intensive energy consumption (high temperature), which are troublesome and thus limit their further
application. On the other hand, we have noticed that there are scarce reports concerning about the
preparation of multicomponent Cuꢀbased catalysts via wet chemical approaches.
Herein, we report a facile and low cost preparation of the ternary CuOꢀCu2OꢀCu catalyst by wet
o
chemical method with obvious advantages such as mild reaction condition (70 C), largeꢀscale
production capability and easy operation. Most importantly, the composition of the samples could be
tailored by simply varying the amount of N2H4·H2O as reducing agent. The prepared ternary
CuOꢀCu2OꢀCu catalyst exhibited highly improved M2 selectivity and Si conversion in the Rochow
reaction in comparison to those of sole CuO and Cu microparticles, demonstrating the importance of
the synergistic catalytic effect among the multicomponents. The resulting ternary CuOꢀCu2OꢀCu
using the present method is a promising catalyst for industrial Rochow reaction.
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2. Experimental
2.1. Material synthesis
The samples were synthesized using a wet chemical reduction method as shown in Fig.1. In a
typical preparation, 37.5 g of copper (II) sulfate pentahydrate (CuSO4·5H2O, A.R., Sinopharm
Chemical Reagent Co., Ltd., China) was dissolved in 300.0 mL of deionized water to get a clear
o
solution with a copper concentration of 0.5 M. Upon heating to 70 C under magnetic stirring,
sodium hydroxide (NaOH, A.R., Sinopharm Chemical Reagent Co., Ltd.) aqueous solution (6 M)
was added dropwise until the pH value of the solution reached 7.5. The obtained mixed gel was
maintained for 10 min at this temperature to give a black precipitate. Subsequently, the desired
amounts of hydrazine hydrate (N2H4·H2O) used as the reductant were introduced dropwise. After
stirred for another 90 min, the reaction mixture was cooled down to room temperature naturally. The
resulting solid was collected by filtration, washed with deionized water and ethanol for several times,
and finally dried in a vacuum at 60 oC for 8 h. The synthesis was carried out by fixing the Cu amount
at 9.5 g while varying the amount of N2H4·H2O added. The sample compositions were determined by
XRD analysis, and the prepared samples are thus denoted as CuO, CuOꢀCu2O, CuOꢀCu2OꢀCu,
Cu2OꢀCu and Cu according to their measured compositions when the amount of N2H4·H2O was 0,
1.5, 2.5, 3.5 and 5.0 mL, respectively.
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Fig. 1 Schematic illustration for the preparation process
2.2. Characterization
Xꢀray diffraction (XRD) patterns were recorded on a PANalytica X'Pert PRO MPD using the
Kα radiation of Cu (λ = 1.5418 Å) and checked with the card number of Joint Committee on Powder
Diffraction Standards (JCPDS). The crystallite size was calculated using the Scherrer equation, in
which the shape factor of K is 0.90. Moreover, the 2θ value of instrumental broadening is 0.06,
which was subtracted in the test process. The microscopic feature of the samples was observed by
fieldꢀemission scanning electron microscopy (SEM) (JSMꢀ7001F, JEOL, Tokyo, Japan) with
energyꢀdispersive spectroscopy (EDS) (INCA XꢀMAX, JEOL, Oxford, England) and transmission
electron microscopy (TEM) (JEMꢀ2010F, JEOL, Tokyo, Japan). The specific surface area was
determined according to the Brunauer−Emmett−Teller (BET) method in the relative pressure range
of 0.05ꢀ0.2. Thermal gravimetric (TG) analysis was carried out on an EXSTAR TG/DTA 6300 (Seiko
Instruments, Japan) with a heating rate of 10 °C/min in air (200 mL/min). H2ꢀtemperature
programmed reduction (H2ꢀTPR) measurements were carried out on automated chemisorption
analyzer (ChemBET pulsar TPR/TPD, Quantachrome). Upon loading of about 0.10 g of sample into
a quartz Uꢀtube, the sample was degassed at 150 °C for 30 min under helium. When the temperature
decreased to 30 °C, the gas was changed to 9.9 % O2/He. Finally, the sample was heated from 30 to
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900 °C at a rate of 10 °C/min in 9.9 % O2/He with a gas flow of 30 mL/min. Xꢀray photoelectron
spectroscopy (XPS) analysis was carried out on an ESCALAB 250Xi from Thermo Scientific
Corporation using AlKa Xꢀray radiation.
2.3. Catalytic Measurement
The evaluation of catalyst was carried out on a homemade lab fixedꢀbed reactor. 10.0 g Si
powder (20ꢀ50 mesh, provided by Jiangsu Hongda New Material Co., Ltd.) and 1.0 g of prepared
catalyst together with 0.10 g of Zinc (Zn, A.R., Sinopharm Chemical Reagent Co., Ltd.) used as a
promoter were mixed homogeneously to form a contact mass, which was loaded in the glass reactor.
The reactor system was purged with high purity N2 for 0.5 h followed by heating to 325 °C within 1
h under a N2 flow rate of 25 mL/min. Subsequently, it was switched to the MeCl gas with a flow rate
of 25 mL/min to react with Si at 325 °C. After 24 h, the reaction was ceased. The gas products were
cooled with a circulator bath controlled at 5 °C by a programmable thermal circulator (GDH series,
Ningbo xinzhi biological technology Co., LTD) and then identified by gas chromatography mass
spectrometry (GC−MS) (QP2010, SHIMADZU), which are composed of methyltricholorsilane
(CH3SiCl3, M1), dimethyldichlorosilane ((CH3)2SiCl2, M2), trimethylchlorosilane ((CH3)3SiCl, M3),
methyldichlorosilane (CH3SiHCl2, M1H), dimethylchlorosilane ((CH3)2SiHCl, M2H), low boiler
(LB) and high boiler (HB). They were quantitative analyzed on a gas chromatograph (Agilent
Technologies 7890A, Thermal conductivity detector, KBꢀ201 column). Following the previously
reported method,13 the selectivity of the products was calculated by the peak area ratio (in percentage)
and the Si conversion was obtained from the difference of contact masses (before and after reaction),
as expressed as follows:
weight (contact mass before reaction)ꢀweight (contact mass after reaction)
Conversion of Si (CꢀSi) =
×100 % (2)
weight (Si before reaction)
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3. Results and discussion
3.1. Characterizations of Cuꢀbased catalysts
Fig. 2a shows the XRD patterns of all the samples, which have high diffraction intensities,
indicating their good crystallinity. In the absence of N2H4·H2O, diffraction peaks at 2θ values of
33.5°, 35.5°, 38.2°, 48.7°, 54.2°, 58.3°, 62.5°, 66.4°, 68.2°, 73.4°, and 75.6° are observed for the
product, which correspond respectively to the lattice planes of (110), (−111), (111), (−202), (020),
(202), (−113), (−311), (220), (311), and (004) of monoclinic CuO (JCPDS No. 03ꢀ065ꢀ2309). As
compared to those of CuO, the sample prepared with 1.5mL of N2H4·H2O has additional diffraction
peaks at 2θ=36.7° and 42.7°, which are attributed to the (111) and (200) planes of cubic Cu2O
(JCPDS No. 00ꢀ001ꢀ1142), suggesting the formation of a bicomponent system of CuO and Cu2O. By
increasing the amount of N2H4·H2O to 2.5 mL, in addition to the above CuOꢀCu2O diffraction peaks,
new peaks at 2θ values of 43.3°, 50.5°, and 74.2° are appeared for the sample, which are assigned to
the lattice planes of (111), (200), and (220) of cubic Cu (JCPDS No.01ꢀ070ꢀ3039), revealing that the
sample contains CuO, Cu2O and Cu. With further increase of N2H4·H2O amount up to 3.5 mL or 5.0
mL, the XRD patterns show that the obtained samples contain Cu2O and Cu or pure metallic Cu. The
nanocrystal sizes of CuO, Cu2O, and Cu for each sample were calculated using the Scherrer formula
based on the diffraction peaks at 2θ values of 35.5°, 36.7,°and 43.3°, respectively, and these data are
compiled in Table 1. For bare CuO, the nanocrystal has a mean size of 18 nm, while in the samples
of CuOꢀCu2O and CuOꢀCu2OꢀCu, the nanocrystal size of CuO is decreased from 18 to 17 nm. On the
other hand, the nanocrystal size of Cu2O in CuOꢀCu2O is 45 nm, which is smaller than that in
CuOꢀCu2OꢀCu (51 nm) but slightly larger than that in Cu2OꢀCu (41 nm). Different from CuO and
Cu2O, the nanocrystal size of Cu in CuOꢀCu2OꢀCu (35 nm), Cu2OꢀCu (46 nm), and Cu (51 nm) is
increased progressively with the increasing amount of N2H4·H2O. We once treated CuO
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nanoparticles by partial reduction in H2/N2 mixed gas with the purpose to obtain the multicomponent
Cuꢀbased catalysts, and also observed this trend of size change.19
Fig. 2b shows the TG curves of all the samples measured in air. It is seen that there is no visible
weight change for CuO sample, suggesting the formation of nearly pure CuO phase. The CuOꢀCu2O
o
sample shows a weight increase of 4.5 wt% in a wide temperature range (200ꢀ420 C) which is
derived from the oxidation of Cu2O to CuO. Thus, the estimated content of Cu2O in this sample is
about 40.2 wt%. On the other hand, the CuOꢀCu2OꢀCu and Cu2OꢀCu samples exhibit 11.2 and 17.1
wt% weight increase, respectively, corresponding to the oxidation of Cu2O and Cu to CuO. In the
case of Cu, the weight increase reaches 25.2 wt%, close to the theoretical value of the oxidation of
Cu into CuO (25.0 wt%), confirming that it is metallic Cu.
Fig. 2c shows the H2ꢀTPR curves of all the samples. The hydrogen consumption peak areas
(STPR) and peak temperatures (TM) are summarized in Table 1. As observed, with the decrease of the
CuO content in the samples following the order of CuO, CuOꢀCu2O, CuOꢀCu2OꢀCu, Cu2OꢀCu and
Cu, the reduction temperature and hydrogen consumption are also slightly decreased. For the
reduced samples, the reduction peaks move slightly to a lower temperature region with the increase
of amount of reducing agent. Using the ratio of hydrogen consumption peak area to the mole number
of O in CuO as the reference and based on the above TG analysis, it can calculate that the content of
CuO in CuOꢀCu2O and CuOꢀCu2OꢀCu samples is about 59.4 and 15.7 wt%, respectively. For
Cu2OꢀCu, the content of Cu2O is 40.6 wt%. Therefore, the above results indicate that the contents of
Cu, Cu2O, and CuO in the final products can be well tuned by adjusting the amount of N2H4·H2O in
the preparation.
The highꢀresolution XPS spectra in Fig. 2d show the binding energies of Cu 2p3/2. The peaks at
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934.2 eV are characteristic of Cu (II), whereas the ones at 932.9 eV are attributed to Cu (0) or Cu
(I).21,22 Unfortunately, from XPS data alone, it is difficult to distinguish clearly the Cu (0) and Cu (I)
species due to the effects of crystal size and surface coverage on the binding energy.23 However, by
combining with the XRD results shown in Fig. 2a, we can judge whether the sample contains Cu (0)
or Cu (I), or both of them. In other words, we can conclude that the sample actually contains CuO,
Cu2O and Cu.
★
Cu
125
120
115
110
105
100
Cu
a
b
★
★
◆
Cu
2
OꢀCu
◆
◆
◆
◆
Cu
2
OꢀCu
★
◆
—Cu
—Cu
O
CuOꢀCu
2
OꢀCu
2
CuOꢀCu OꢀCu
2
▲—CuO
▲
◆
▲
CuOꢀCu
2
O
CuOꢀCu O
2
▲
▲▲
CuO
CuO
10 20 30 40 50 60 70 80 90
100 200 300 400 500 600 700
o
A
Temperature ( C)
2 Theta (deg.)
Cu
c
d
2P3/2
Cu OꢀCu
2
2+
CuOꢀCu OꢀCu
2
0 or 1+
CuOꢀCu O
2
CuO
500
938
936
934
932
930
100
200
300
400
Temperature (oC)
Binding energy (eV)
Fig. 2 XRD patterns (a), TG curves (b), and H2ꢀTPR curves (c) of all the samples, and XPS spectra
of Cu 2p for the CuOꢀCu2OꢀCu sample (d).
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Table 1 The physical chemistry parameters of all the samples
Cua
(nm)
ꢀ
Cu2Oa CuOa
Weight increaseb
TMc
(oC)
SBET
d
c
Samples
STPR
(nm)
ꢀ
(nm)
18
18
17
ꢀ
(wt%)
ꢀ
(m2/g)
15.3
10.2
3.6
CuO
CuOꢀCu2O
CuOꢀCu2OꢀCu
Cu2OꢀCu
Cu
264.2
258.5
241.2
214.5
ꢀ
53460.8
34482.3
23146.7
9435.0
ꢀ
ꢀ
45
51
41
ꢀ
4.5
35
46
51
11.2
17.1
25.2
3.4
ꢀ
2.5
aCalculated from the XRD patterns, standard errors: crystal size, 0.5 nm.
bGiven by TG analysis.
cObtained from H2ꢀTPR curves using integrated areas under curves presented.
dCalculated with BET method.
Fig. 3 shows the SEM images of all the samples. In agreement with the XRD results, the
samples all appear to be well crystallized. Fig. 3a presents the SEM image of the CuO, which reveals
that the sample is made up of a large quantity of different sheetꢀlike submicrometer structures.
However, after addition of a small amount of N2H4·H2O, the SEM image shows that the obtained
CuOꢀCu2O sample is composed of premature polyhedron structures, on the surface of which are
covered with sheetꢀlike substance (Fig. 3b), implying that the CuO crystallites are partially reduced
to Cu2O and attached to the surface simultaneously during the growth of the polyhedron structures,
which is coincide with the previous studies.24ꢀ26 A further increase in the amount of N2H4·H2O leads
the obvious changes in the morphology of CuOꢀCu2OꢀCu (Fig. 3c), which consist of sphere, cube,
octahedron and other polyhedral with the size ranging from approximately 0.05ꢀ2 ꢁm. Fig. 3d
displays the SEM image of Cu2OꢀCu, in which the majority of the crystals have the cube and
octahedron shapes, and their sizes are in the range of 100ꢀ500 nm. The Cu sample as shown in Fig.
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3e exhibits a block–like shape with the size range of 0.1ꢀ3 ꢁm.
Fig. 3 SEM images of CuO (a), CuOꢀCu2O (b), CuOꢀCu2OꢀCu (c), Cu2OꢀCu (d), and Cu (e).
Fig. 4a presents the TEM image of the CuOꢀCu2OꢀCu sample, which consists of cube,
octahedron, sphere and other polyhedral, in full agreement with the above SEM result (Fig. 3c). The
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highꢀresolution transmission electron microscope (HRTEM) image of CuOꢀCu2O in Fig. 4b shows
the presence of two different interplanar distances of 0.25 and 0.24 nm, corresponding to the (111)
planes of CuO and Cu2O, respectively. Moreover, two sets of diffraction spots in the fast Fourier
transform (FFT) patterns further verify the coexistence of the two structures. Similarly, the
CuOꢀCu2OꢀCu sample (Fig. 4c) possesses three apparent lattice spacings, in which the one at 0.21
nm corresponds to the (111) plane of Cu, and the other two indexed to the (111) planes of CuO and
Cu2O remain the same (0.25 and 0.24 nm) as CuOꢀCu2O. In the case of Cu2OꢀCu (Fig. 4d), two
lattice plane distances of 0.24 and 0.21 nm are observed, which are characteristic of the (111) facets
of Cu2O and CuO, respectively. These results prove that the prepared Cuꢀbased samples are
composed of polycrystals, while within each multiꢀcomponent particle there are different Cu
component crystals, supporting the successful synthesis of Cuꢀbased catalysts with controllable
composition by varying the amount of reducing agent using the wet chemical method.
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Fig. 4 TEM image of CuOꢀCu2OꢀCu (a), HRTEM images of CuOꢀCu2O (Top and down insets show
the corresponding FFT pattern of CuO (ꢀ111) and Cu2O (111), respectively) (b), CuOꢀCu2OꢀCu (c),
and Cu2OꢀCu (d).
3.2. Reduction Mechanism.
The reactions involved in the CuO synthesis and their reduction process are shown in the
following equations:
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CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4
Cu(OH)2 → CuO+ H2O
(3)
(4)
(5)
(6)
CuO+ N2H4 → 2Cu2O+ N2 + 2H2O
Cu2O+ N2H4 → 2Cu + N2 + H2O
The chemical reaction between CuSO4 and NaOH yields cupricꢀhydroxide (Cu(OH)2) (Formula
(3)), which is a metastable phase. Upon the heating in a constantꢀtemperature (70 oC) water bath
under magnetic stirring, it is decomposed into more stable cupricꢀoxide (CuO) (Formula (4)),27
which has been also confirmed previously by Cudennec and Lecerf et al.28 Upon addition of
N2H4·H2O, which is employed as a reducing agent, CuO is reduced into Cu2O and Cu,
accompanying with production of N2 and H2O (formulas (5) and (6)).24,25 In addition, we also found
that a trace of NH3 as a byꢀproduct will also be generated. The formation of various products with
distinct morphologies is illustrated in Scheme 1. As mentioned above, Cu(OH)2 is not stable. When
o
heated at 70 C for 10 min, it is transformed into black CuO solely, which has the sheetꢀlike
morphology and submicrometer size. Upon addition of a small amount of N2H4·H2O, an obvious
change in the product morphology and size is observed (Fig. 3b in SEM image). Clearly, there
should be a dissolutionꢀrecrystallization process starting from the solid precursor.26 During the
reduction process, the added N2H4·H2O reacts with the free Cu2+ ions via coordination and reduction
steps. With progressive dissolution of CuO to yield Cu2+ and subsequent reduction of Cu2+ to Cu+,
the CuO particles are gradually dissolved and recrystallization.25,26 When the amount of N2H4·H2O
reaches 1.5 ml, part of CuO is transformed into Cu2O, forming nanocrystals of both CuO and Cu2O
with premature polyhedron structures. Continuously increasing the amount of N2H4·H2O to 2.5 ml,
CuO is completely transformed into Cu2O, and even part of the yielded Cu2O is further converted
into Cu. Thus, a coexistence of CuO, Cu2O, and Cu grains with the structure of sphere, cube,
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octahedron and other polyhedral is obtained. It should be noted that the presence of small amounts of
ions such as Na+, OH−, etc. has a great influence on the morphology of Cu2O product.26 Finally, with
a further increase of N2H4·H2O amount to 3.5 ml or 5.0 ml, as there is surplus of reducing agent, the
generated Cu2O is converted into Cu2OꢀCu or pure Cu totally. Therefore, by tuning the supply of
N2H4·H2O, it is able to control the content of CuO, Cu2O, and Cu in the final Cuꢀbased products, as
proved by above comprehensive characterizations with XRD, TG, TPR, XPS and TEM techniques.
Scheme 1 Graphic description for the reduction mechanism.
3.3. Catalytic property of Cuꢀbased catalysts
Table 2 summarizes the catalytic performance of all the samples for M2 synthesis via the
Rochow reaction. As shown, the CuO sample achieves a M2 selectivity of 66.1% and Si conversion
of 26.4%. For CuOꢀCu2O, the corresponding values become 68.7 and 27.2%, respectively. In the
case of CuOꢀCu2OꢀCu, a markedly improved M2 selectivity of 80.0% and Si conversion of 51.4%
are achieved, higher than those of Cu2OꢀCu (71.8% M2 selectivity and 32.0% Si conversion). For
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sole Cu, both the M2 selectivity and the Si conversion are decreased obviously again. Also, the
characterization data of the reaction product organo silanes via GC are provided (See Table S1ꢀS5
and Fig. S1ꢀS5 in Supporting Information). It should be pointed out that the error bars for both
conversion and selectivity were ±0.1% and all the catalytic data were obtained by at least three
repeated experiments using the same silicon. As we known, M2 is the most desirable monomer in
organosilane industry, the above results demonstrate that the prepared catalysts possessing three
components of Cu, Cu2O, and CuO have the best M2 selectivity and yield among all the samples,
probably because of the synergistic effect among Cu, Cu2O, and CuO, which promotes the catalytic
activity for the Rochow reaction.5 In addition, byproducts such as M1, M3, M1H, M2H, LB, and HB
with a negligible difference over the different prepared catalysts are also detected except that on the
Cu sample.
Table 2 Catalytic performances of all the catalysts for Rochow reactiona
Product Composition (%)
Samples
M2
CꢀSi
(%)
M1
M2
M3
M1H
M2H
LBR
HBR
yield
17.5
18.7
41.1
23.0
8.8
CuO
12.5
11.3
66.1
68.7
80.0
71.8
47.3
1.4
1.0
1.5
1.0
1.0
14.9
9.2
0.8
0.4
0.9
1.1
0.9
0.3
0.4
0.1
0.1
0.3
4.0
9.0
0.2
3.4
5.6
26.4
27.2
51.4
32.0
18.7
CuOꢀCu2O
CuOꢀCu2OꢀCu 10.1
7.2
Cu2OꢀCu
Cu
11.4
21.9
11.2
23.0
a Reaction conditions: cat., 1.0 g; catalyst: Si (mass ratio)= 1:10; flow rate of CH3Cl, 25 mL minꢀ1;
temp., 325 oC; time, 24 h.
Fig. 5 displays the XRD patterns of the waste contact masses after reaction, which contain the
unreacted Si, the Cuꢀbased catalyst, and trace amount of promoter Zn. As shown in Fig. 5a, all the
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waste contact masses are composed of Si and Cu, but lack of Cu2O and CuO species. The formation
of Cu may originate from the reaction of MeCl with the lattice oxygen of the Cuꢀbased catalyst.9 An
enlarged view of the XRD patterns in the range of 40ꢀ50° (Fig. 5b) shows the presence of ηꢀCu3Si
and Cu6.69Si species, suggesting the formation of alloyed CuxSi active components by reaction of Cu
and Si via diffusion during the reaction. The Cu3Si alloy, generally produced between the Cu catalyst
and the Si interface at elevated temperatures,29 is normally regarded as the key catalytic active
species in the Rochow reaction,30,31 by which Methylchlorosilanes (MCSs), especially M2, are
formed. The amounts of Cu3Si can substantially affect the Si conversion and M2 selectivity.6 The
intensity of Cu3Si peaks observed for CuOꢀCu2OꢀCu is much higher than that of the other samples,
suggesting that CuOꢀCu2OꢀCu is more active in generating Cu3Si than the other samples. This may
be because the ternary composition of CuOꢀCu2OꢀCu has a stronger synergistic effect and closer
contact between the catalysts and the solid Si, thus promoting the formation of more active Cu3Si
phases, which is the key step to enhance the catalytic activity for M2 production.
★
—Cu
◆
—Si
▲
—SiO2
◆
a
b
◆
Cu
◆
■
ꢀCu3Si
◆
★
★
◆
▲
★
▲
▲
o
ꢀCu6.69Si
■
Cu
CuOꢀCu
CuOꢀCu
2
OꢀCu
Cu
Cu OꢀCu
■
o
2
OꢀCu
2
2
O
CuOꢀCu OꢀCu
2
CuOꢀCu
CuO
2
O
CuO
40 41 42 43 44 45 46 47 48 49 50
10 20 30 40 50 60 70 80 90
2 Theta (deg.)
2 Theta (deg.)
Fig. 5 XRD patterns of waste contact masses (a) and enlarged view in the 2θ range of 40-50° (b).
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Fig. 6a shows the SEM images of the Si particles after the reaction using CuOꢀCu2OꢀCu as the
catalyst. The smooth region on the surface of Si particle stems from the unreacted Si, while coarse or
cavityꢀlike region from reacted Si, suggesting the occurrence of etching process during the reaction,
consistent with the soꢀcalled anisotropic etching reaction mechanism.14 The element mapping images
show the distribution of the elements Si (Fig. 6b), Cu (Fig. 6c) and C (Fig. 6d) on the surface of the
waste contact mass. The element Si mapping image clearly shows the reacted (dark yellow) and
unreacted (bright yellow) zones of Si particle, while both Cu (dark cyan) and C (red) distribute
uniformly on the reacted or etched Si surface, indicating the occurrence of catalytic reaction between
Si particles and Cuꢀbased catalysts. As shown in Scheme 2, a mechanism for catalytic cycle is
proposed. The ternary CuꢀCu2OꢀCuO catalyst is first transformed to Cu* (active species) via the
reaction of MeCl with the lattice oxygen, which then reacts with Si to form alloyed CuxSi active
intermediate. Further, MeCl is adsorbed on their surface and transformed into M2, and meanwhile
Cu* is released again.
The above results allow us to describe graphically the possible Rochow reaction process in the
presence of Cuꢀbased catalyst (Scheme 3). First, Si is mixed with the reduced Cu particles in
submicrometer size before the reaction. With diffusion of Cu into the Si, the CuxSi alloy is gradually
formed at the Cu catalyst and the Si interface at the elevated temperatures, which further reacts with
the gas MeCl to form MSCs. As the reaction proceeds further, more elemental copper is produced
while the Cu in the CuxSi alloy is reduced until the deactivation of the catalyst. At the same time,
MeCl may be transformed into carbon deposition (C) initiated by the metallic Cu, leading to the
deactivation of the contact mass. The presence of the multicomponent particles structure should
enhance the gas diffusion into the CuꢀSi contact area, resulting in the formation of much more active
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species. Nevertheless, the real synergistic effect is not yet fully understood at present and should be
further explored.
Fig. 6 SEM images of Si particle after reaction (a), elemental mapping images of Si (b), Cu (c) and C
(d).
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Scheme 2 A proposed catalytic mechanism for catalytic cycle.
Scheme 3 Schematic illustration of the process of Rochow reaction.
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This method is simple without use of any complicated and delicate equipment, and the reaction
conditions are mild, as the reaction temperature is 70 °C only. More importantly, this method is
readily scalable. Our experiment results showed that about 10 g of products could be obtained at one
time by using a 500 mL beaker (not shown), indicating its great potentiality for industrial application.
4. Conclusions
In summary, we have demonstrated the controllable synthesis of the Cuꢀbased catalysts with
variable CuO, Cu2O and Cu contents by a wet chemical method, in which the key is to control the
amount of N2H4·H2O used as the reducing agent. Various characterizations indicate that the reduction
of CuO is a successive reaction that yields Cu2O, and Cu, which in turn provides the possibility to
finely tune the catalyst composition via varying the amount of reducing agent. Comparing with the
synthesized CuO and Cu particles, the multicomponent CuOꢀCu2OꢀCu catalyst shows much higher
M2 selectivity and Si conversion in the Rochow reaction, proving the significance of the synergistic
effect among the multicomponent. It is expected that this ternary CuOꢀCu2OꢀCu catalysts will have
great potential in the industrial Rochow reaction.
Acknowledgements
The authors gratefully acknowledge the financial supports from the National Natural Science
Foundation of China (no. 51272252and21206172). Z. Z is grateful to the kind supports of both
Nanyang Technological University (NTU) and Institute of Chemical Engineering and Sciences
(ICES) under Agency for Science, Technology and Research (A*STAR).
Supplementary material
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Supplementary data associated with this article can be found, in the online version, at…
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