Porous (CuO)xZnO hollow spheres as efficient Rochow reaction catalysts

Article abstract of DOI:10.1039/c6ce00173d

Nowadays, how to achieve both high dimethyldichlorosilane selectivity and silicon conversion in the Rochow reaction still remains a major challenge in the organosilane industry, in which silicon and chloromethane are converted into methylchlorosilanes on Cu-based catalysts mixed with ZnO promoter. Therefore, this calls for the development of outstanding catalysts with both high activity and selectivity for the Rochow reaction and also for a deep fundamental understanding of the catalytic mechanism. In this work, we designed and synthesized a series of copper oxide-zinc oxide catalysts ((CuO)_xZnO (0 ≤ x ≤ 49)) with a distinct porous hollow spherical structure for the reaction. These porous hollow spherical catalysts composed of CuO and ZnO nanoparticles were prepared through co-adsorption of Cu²⁺ and Zn²⁺ in the interior and outer surfaces of the hydrothermally synthesized carbonaceous spheres, followed by a new hydrothermal treatment and calcination in air. The catalytic properties of the (CuO)_xZnO hollow spheres for dimethyldichlorosilane synthesis via the Rochow reaction was investigated, and a deeper understanding of the catalytic mechanism was obtained. As compared to pure CuO hollow spheres, the prepared (CuO)₁₉ZnO hollow spheres exhibited much higher dimethyldichlorosilane selectivity and silicon conversion, which are clearly related to the synergistic electronic effect between Cu and ZnO and to the distinct catalyst structures which allow intimate contact of the reactant molecules with the active component and the efficient transport of the molecules. This work opens a new way for the fabrication of efficient and integrated Cu-based catalysts for the Rochow reaction.

Full text of DOI:10.1039/c6ce00173d

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Page 1 of 34
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DOI: 10.1039/C6CE00173D
Porous (CuO)xZnO hollow spheres as efficient Rochow reaction catalyst
Zailei Zhang,*,a Yongjun Ji,a Jing Li,a Yongxia Zhu,a Ziyi Zhongb and Fabing Su*,a
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese
Academy of Sciences, Beijing, China 100190,
b
School of Chemical & Biomedical Engineering, Nanyang Technological University (NTU), 62
Nanyang Drive, Singapore 637459, Singapore
*To whom correspondence should be addressed. E-mail address: zhangzl@ipe.ac.cn, fbsu@ipe.ac.cn,
Tel.: +86-10-82544850, Fax: +86-10-82544851.
1

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Abstract
Nowadays, how to achieve both high dimethyldichlorosilane selectivity and silicon conversion in
Rochow reaction still remains as the major challenge in organosilane industry, in which silicon and
chloromethane are converted into methylchlorosilanes on the Cu-based catalysts mixed with ZnO
promoter. Therefore, it calls for development of outstanding catalysts with both high activity and
selectivity for the Rochow reaction, and also for a deep fundamental understanding to the catalytic
mechanism. In this work, we designed and synthesized a series of copper oxide-zinc oxide ((CuO)xZnO
(0 ≤ x ≤ 49)) with a distinct porous hollow spherical structure for the reaction. These porous hollow
spherical catalysts composed of CuO and ZnO nanoparticles were prepared through co-adsorption of
Cu2+ and Zn2+ in the interior and outer surface of the hydrothermally synthesized carbonaceous spheres,
followed with a new hydrothermal treatment and calcination in air. The catalytic property of the
(CuO)xZnO hollow spheres for dimethyldichlorosilane synthesis via the Rochow reaction was
investigated, and a deeper understanding to the catalytic mechanism was obtained. As compared to
pure CuO hollow spheres, the prepared (CuO)19ZnO hollow spheres exhibited much higher
dimethyldichlorosilane selectivity and silicon conversion, which are clearly related to the synergistic
electronic effect between Cu and ZnO, and to the distinct catalyst structures which allows intimate
contact of the reactant molecules with the active component and the efficient transportation of the
molecules. This work opens a new way for fabrication of efficient and integrated Cu-based catalysts for
Rochow reaction.
Keywords: copper oxide; zinc oxide; porous hollow spheres; Rochow reaction; synergistic electronic
effect
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1. Introduction
Hybrid nanomaterials, such as metal–metal, metal–metal oxide and metal oxide–metal oxide, are of
great interest in heterogeneous catalysis, mainly because of the possibility for providing the multiple
functionalities such as synergistic effect.1-5 Recently it has revealed in literature that the catalytic
properties of the hybrid materials like Au@Ag core-shell nanoparticles,6 Pd-Cu2O composites,1
Fe3O4@Cu2-xS core-shell nanoparticles,7 hierarchical TiO2 nanowire@MoS2 nanosheets,8 and
BiFeO3/TiO2 nanotube arrays9 are closely related not only to their component, particle size and shape,
but also to the potential synergistic effect.10-12 In particular, when there is presence of synergistic
interaction between the metal oxide–metal oxide components, their catalytic performances could be
altered significantly.13-15 CuO is a typical p-type semiconductor with a narrow band gap (Eg = 1.2
eV),16, 17 while ZnO is a well-known n-type semiconductor (Eg = 3.37 eV).18, 19 A quantum chemical
study suggests that the Cu sites in ZnO clusters show unique electron affinity properties, e.g., being
able to accept a fraction of the electron density from anionic clusters.20 A density functional theory
study indicates that, when strongly interacting with ZnO, some Cu atoms become positively charged.21
A kinetic model, which was developed to evaluate the kinetic parameters of the reduction process from
computer simulation based on the temperature-programmed reduction, indicates that, comparing with
that of the CuO reduction, the activation energy for the reduction of CuO-ZnO becomes apparently
lower.22 It has been indicated that ZnO/CuO composites with p-n heterojunction structures,23 including
ZnO-CuO composites,24, 25 CuO nanoparticles coated on tetrapod-like ZnO,26 and ZnO/CuO core-shell
nanowires27 exhibited improved performance in catalytic reaction. Furthermore, it is reported that the
synthesized porous and hollow spherical catalysts with multiple components, low densities and high
surface areas showed enhanced catalytic activity.28-31
As we know, Rochow reaction was discovered in 1940s32 and is widely used to synthesize
methylchlorosilanes directly by reacting silicon with chloromethane over the Cu-based catalysts.
Nowadays, this process is still the most economical route for the synthesis of the methylchlorosilanes,
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which are the primary monomeric intermediates used for manufacture of silicone products. It has
demonstrated that metallic Cu33 and its compounds such as Cu2O,34 CuO,35 CuCl,36 and their hybrid
materials are active for Rochow reaction with assistance of promoters,37 and Zn and ZnO seem to be
the most effective promoters to assist the Cu-based catalysts in improving the dimethyldichlorosilane
selectivity as well as the silicon conversion.38 In our lab, we have studied catalysts such as flower-like
ZnO grown on urchin-like CuO microspheres,39 and the commercial catalyst added with urchin-like
ZnO microspheres promoter 40 for the Rochow reaction. However, it is still lack of a clear picture of the
synergistic interaction between the ZnO and Cu-based catalysts. Also, the observed catalytic
performance is not satisfactory enough.
In continuation of our work on Cu-based catalysts for Rochow reaction, here we report the design
and synthesis of novel porous copper oxide-zinc oxide ((CuO)xZnO (0 ≤ x ≤ 49)) catalysts with a
distinct hollow spherical structure. These (CuO)xZnO catalysts containing both CuO and ZnO
nanoparticles were synthesized through annealing carbonaceous microspheres/Cu2+Zn2+ in air.
Comparing with low silicon conversions of the bare CuO (4.3%) and ZnO (0%) hollow sphere catalysts,
much higher silicon conversions are observed on these (CuO)xZnO hollow spheres, e.g., 78.2% on
CuOZnO, 70.2% on (CuO)4ZnO, 64.6% on (CuO)9ZnO, 53.3% on (CuO)19ZnO, and 38.2% on
(CuO)49ZnO. More importantly, a significant increase in dimethyldichlorosilane selectivity was
achieved on the obtained (CuO)19ZnO hollow spheres (the selectivity was 88.5% based on a silicon
conversion of 53.3%, while those of the commercial catalysts were 82.6% and 42.6% respectively). It
is believed that in the porous hollow spherical structure there is closer contact among gaseous
chloromethane, solid catalyst and solid silicon, faster gas transportation, as well as stronger synergistic
electronic effect between Cu and ZnO, which lead to the enhanced formation of dimethyldichlorosilane
on the silicon surface from chloromethane. This work shows the Cu-based catalysts for Rochow
reaction still can be improved if their structure is properly manipulated.
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2. Experimental section
2.1 Materials synthesis
In a typical synthesis, carbonaceous spheres (CS) were first prepared by a modified hydrothermal
method reported previously.41 Glucose (12 g, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in
deionized water (70 mL) and glycol (5 mL) to form a homogeneous slurry, which was subsequently
o
sealed in a stainless-steel autoclave and heated at 180 C for 24 h. The CS were collected by
o
centrifugation, washed with distilled water and absolute ethanol, and finally dried in vacuum at 80 C
for 24 h. Subsequently, 100 mg of CS was added into 100 mL of glycol followed by sonication to reach
a high dispersion. After that, Cu(CH3COO)2 (0.5 mmol), Zn(CH3COO)2 (0.5 mmol) (Sinopharm
Chemical Reagent Co., Ltd. A. R.) were added into the above mixture with stirring. The obtained
o
mixture was then transferred into a round bottom flask and kept refluxing at 120 C for 8 h in an oil
bath, followed with a natural cooling down to room temperature. The product (CS/Cu2+Zn2+) was
collected by centrifugation and washed with ethanol. In order to obtain (CuO)xZnO (0 ≤ x ≤ 49) porous
o
hollow spheres, the CS/Cu2+Zn2+ product was further annealed in air at 500 C for 2 h with a slow
heating rate of 2 oC min-1.
2.2 Materials characterization
X-ray diffraction patterns (XRD) were recorded on a PANalytical X'Pert PRO MPD using the Cu Kα
radiation of (λ = 1.5418 Å). The microscopic feature of the samples was characterized by field-
emission scanning electron microscopy (FESEM) with an energy-dispersive X-ray spectrometer (EDX)
(JSM-7001F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM) with an EDX (JEM-
2010F, JEOL, Tokyo, Japan) operated at 300 kV. Thermal gravimetric (TG) analysis was carried out
on an EXSTAR TG/DTA 6300 (Seiko Instruments, Japan) at a heating rate of 2 °C min-1 in air. The
pore nature of the samples was investigated using physical adsorption of N2 at the liquid-nitrogen
temperature (–196 °C) on an automatic volumetric sorption analyzer (NOVA3200e, Quantachrome).
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Prior to the measurement, the sample was degassed at 200 ºC for 12 h under vacuum. The specific
surface area was determined according to the Brunauer–Emmett–Teller (BET) method in the relative
pressure range of 0.05–0.2. The pore size distribution (PSD) curve was derived from the Barrett–
Joyner–Halenda (BJH) method using the adsorption branch. The pore size was estimated from the
maximum position of the BJH-PSD curve. X-ray photoelectron spectroscopy (XPS) analysis was
carried out on an ESCALAB 250Xi from Thermo Scientific Corporation using AlKa X-ray radiation.
Temperature programmed reduction (TPR) measurements were carried out on an automated
chemisorption analyzer (ChemBET pulsar TPR/TPD, Quantachrome). 0.10 g of (CuO)xZnO porous
hollow spheres was initially loaded in a quartz U-tube. Prior to the measurement, the sample was
degassed at 200 ºC for 4 h under helium. When the temperature was dropped to 20 ºC, the gas was
switched to 9.9 % H2/Ar. Finally, the sample was heated from 20 to 500 °C under the flow of 9.9 %
H2/Ar at 30 mL/min. The actual content of Cu and Zn in porous hollow spheres was determined by an
inductively coupled plasma optical emission spectrometry (ICP-OES).
2.3. Catalytic measurement
The evaluation of the catalyst was carried out with a typical lab fixed-bed reactor.42 The silicon powder
(20-50 mesh, provided by Jiangsu Hongda New Material Co., Ltd.) and the prepared (CuO)xZnO
catalysts with a ratio of 20:1 were mixed homogeneously to form a contact mass, which was loaded in
the glass reactor. The reactor system was purged with purified N2 for 0.5 h followed by heating to 325
oC within 1 h under a N2 flow rate of 25 mL/min. Subsequently, N2 was turned off, and CH3Cl gas at a
o
flow rate of 25 mL/min was introduced into the reactor to react with Si followed by heating to 325 C.
After a given period of 24 h, the reaction was stopped. The gas product was cooled into liquid phase
within a circulator bath controlled at -5 oC by a programmable thermal circulator (GDH series, Ningbo
xinzhi biological technology Co., LTD). The waste contact mass (solid residue after reaction),
containing unreacted Si powder, and (CuO)xZnO compounds were weighed for calculating Si
conversion. The products in the liquid solution were quantitatively analyzed on an Agilent
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Technologies (GC-7890A) gas chromatograph equipped with KB-201 column (60 m) and TCD
detector. Gas chromatography system was used for identification of the products, which was mainly
comprised 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). The selectivity of the
products was calculated by the peak area ratio (in percentage). The spent contact mass (residual solid
after reaction) containing unreacted Si powder, Cu, and ZnO compounds were weighed to calculate Si
conversion, which was that the ratio of weight difference of contact mass (before and after reaction)
and weight of Si (before reaction) (formula 1).
weightcontactmassbefore reaction - weight contactmassafterreaction
weight Sibefore reaction
Conversionof Si (CSi ) 
 100%
(1)
The Rochow reaction is described in the following equation (formula 2).
3. Results and discussion
The formation procedures of the (CuO)xZnO (0 ≤ x ≤ 49) porous hollow spheres are illustrated in Fig. 1.
First of all, carbonaceous spheres (CS) (see Fig. 1a and SEM and TEM images in Fig. S1), which
contain about 88.2 wt% of carbon43 obtained by the modified hydrothermal method, are well dispersed
into an glycol solution. The prepared CS possess a large amount of functional groups in the interior and
outer surface, thus offering an ideal chemical environment for the adsorption of Cu2+ and Zn2+
cations.41, 44 Subsequently, the metal precursors (Cu(CH3COO)2 and Zn(CH3COO)2) are added, and the
solution is heated up to 120 oC in an oil bath and kept stirring and refluxing at this temperature for 8 h
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to form the CS/Cu2+Zn2+ spheres (Fig. 1b) by adsorbing Cu2+ and Zn2+ cations in both the interior and
outer surface of CS.45 At last, hollow (CuO)xZnO spheres (Fig. 1c) are obtained through a annealing
treatment in air. In the transformation, the formation of the porous hollow volume should be due to the
release of functional groups in the CS template and the complete combustion of the CS template, and
the (CuO)xZnO nanoparticles are formed by decomposition of the Cu2+ and Zn2+ precursors. It should
be pointed out that the strategy demonstrated here can be easily extended to prepare hollow spheres of
other porous metal oxides or mixed metal oxides. The amounts of the Cu(CH3COO)2 and
Zn(CH3COO)2 precursors, the actual content of Cu and Zn in the final porous hollow spheres obtained
by the ICP-OES measurement, and the reaction conditions used for synthesis of all the samples are
compiled in Table 1.
Fig. 2 shows the XRD results of the air-calcined porous CuO, ZnO, and (CuO)xZnO hollow spheres
obtained through the adsorption of Zn2+ and Cu2+ on the outer and interior surface of CS at various
Cu/Zn atomic ratios. With only Cu(CH3COO)2 precursor, monoclinic symmetry CuO (JCPDS No. 05-
0661)46 is obtained. Upon further addition of the Zn(CH3COO)2 precursor, besides the above CuO
diffraction peaks, several other peaks assigned to hexagonal ZnO (JCPDS No.070-2551)47 are also
observed, suggesting the formed CuOZnO, (CuO)4ZnO, (CuO)9ZnO, (CuO)19ZnO, (CuO)49ZnO
samples are composed of both CuO and ZnO phases. Similarly, the pure ZnO sample is obtained by
only adding Zn(CH3COO)2 precursor. No other obvious crystalline impurities can be observed in these
hollow spherical samples. The calculated average crystallite diameters of single phase CuO and ZnO
using the classical Scherrer formula based on the peak at 38.7o for CuO and 36.5o for ZnO, are 10.8 nm
and 9.2 nm, respectively. Similarly, the average crystallite diameters of CuO and ZnO in CuOZnO,
(CuO)4ZnO, (CuO)9ZnO, (CuO)19ZnO, and (CuO)49ZnO samples based on the same two peaks are 10.8
nm and 9.4 nm, 10.7 nm and 9.4 nm, 10.7 nm and 8.9 nm,10.7 nm and 8.7 nm, and 10.7 nm and 8.7 nm,
respectively, indicating that these porous hollow spheres are composed of small CuO and ZnO
nanoparticles.
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The SEM and TEM images of the porous ZnO and CuO hollow spheres are shown in Fig. 3. The
obvious ZnO hollow spheres with thin shells and diameters between 300‒500 nm are obtained after
only adding Zn(CH3COO)2 precursor, as shown in SEM image of Fig. 3a, TEM image of Fig. 3b, and
its EDX spectra of Fig. 3c. Comparing with the thin-shell ZnO hollow spheres, the distinct CuO hollow
spheres with relatively thicker shells are synthesized after merely adding Cu(CH3COO)2 precursor
(SEM image of Fig. 3d, TEM image of Fig. 3e, and its EDX spectra of Fig. 3f). Obviously, the hollow
structure can be easily distinguished from both these SEM and TEM images.
When the Zn(CH3COO)2 and Cu(CH3COO)2 precursors are added together at a ratio of 1:4, through
the co-adsorption of Cu2+ and Zn2+ on the surface of CS, thicker shell (CuO)4ZnO hollow spheres with
a diameter of about 300‒500 nm are obtained, as shown in SEM image of Fig. 4a, TEM image of Fig.
4b, and its EDX spectra of Fig. 4c. Upon further increasing the Zn(CH3COO)2 precursor until the
Zn(CH3COO)2 and Cu(CH3COO)2 precursors ratio reaches 1:1, as shown in Fig. 4d and 4e, there is no
obvious change in morphology, which are still hollow structure but with an increased shell thinkness.
The high-resolution TEM image in the inset of Fig. 4g indicates that the lattice fringe spacing within
one nanoparticles is about 0.28 nm, corresponding to the interplanar distance of (100) plane in ZnO,
while the one at 0.25 nm within another nanoparticle, corresponding to the interplanar distance of (-111)
planes in CuO. The SEM image, EDX spectra, the corresponding elemental mapping images, and the
line scan along the yellow line of Cu and Zn of CuOZnO hollow spheres show the homogeneous
distribution of Zn and Cu elements (Fig. 4f, 4h and 4i).
The morphologies of (CuO)9ZnO (Fig. 5a and 5b), (CuO)19ZnO (Fig. 5c and 5d) and (CuO)49ZnO
(Fig. 5e and 5f), which are obtained by adding Cu(CH3COO)2 and Zn(CH3COO)2 precursors with
atomic ratios of 9:1, 19:1, and 49:1, respectively, have no big change as compared with that of
CuOZnO and (CuO)4ZnO hollow spheres. However, the shells for these porous hollow spheres
becomes thicker with the increasing amount of Zn(CH3COO)2 precursor. From these characterization
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results, we can conclude that (CuO)xZnO porous hollow spheres composed of fine ZnO and CuO
nanoparticles have been successfully prepared.
The structure and composition of these hollow spherical samples were further characterized by
temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), thermal
gravimetric (TG) and N2 adsorption-desorption analysis. Fig. 6a shows the H2 consumption curves of
all the samples. As observed, the order of reduction temperature for all the samples is CuO >
(CuO)49ZnO > (CuO)19ZnO > (CuO)9ZnO > (CuO)4ZnO > CuOZnO, suggesting that ZnO is able to
promote the reduction of CuO, which is well agreement with the literature reports.22, 39, 48, 49 Fig. 6b
shows the Cu2p XPS spectra of CuO and CuOZnO hollow spheres. The former has two strong peaks at
934.2 eV for Cu 2p3/2 and 953.8 eV for Cu 2p1/2, and one shakeup satellite (indicated as “Sat.”),
indicating the oxidation state of Cu2+ in the CuO hollow spheres.50 However, the position of the main
peak, corresponding to Cu 2p3/2, shifts to higher binding energy for the CuOZnO hollow spheres. This
clearly indicates a modification of the electron density in CuO species due to the interaction between
the CuO and ZnO nanoparticles,51 consistent with the literature report.50 In the Zn 2p spectrum shown
in Fig. 6c, two strong peaks at 1022.1 eV and 1045.1 eV assigned to Zn 2p3/2 and Zn 2p1/2 respectively
are observed, indicating the oxidation state of Zn2+ in all the CuOZnO hollow spheres.52 However, the
position of the main peak, corresponding to Zn 2p3/2, is shifted to higher binding energy for CuOZnO
hollow spheres, further confirming the modification of the electron density in ZnO species due to the
strong interaction between the CuO and ZnO nanoparticles. Fig. 6d presents N2 adsorption/desorption
isotherms of all samples. These isotherms with a hysteresis loop in the relative pressure range of 0.6-
1.0 belong to the type IV. The BET surface areas and pore volume for CuO, CuOZnO, (CuO)4ZnO,
(CuO)9ZnO, (CuO)19ZnO, and (CuO)49ZnO are 19.6, 22.8, 24.7, 23.9, 22.1, 21.4 m2/g, and 0.083,
0.089, 0.088, 0.089, 0.086, 0.085 cm3/g obtained from the volume of nitrogen adsorbed at the relative
pressure of 0.98, respectively. The PSD curves in Fig. 6e show that the pore sizes are distributed in the
range of 10-50 nm, suggesting the formation of mesoporous structure, probably because of the
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aggregation of CuO and ZnO nanoparticles when forming the porous hollow structure. The TG curve
of CS/Cu2+Zn2+ spheres in air reveals that there is a weight loss of ∼68.9 wt% from 100 to 800 °C (Fig.
6f); therefore, the content of CuO and ZnO in the mixture is calculated to be about 31.1 wt%. However,
from the EDX analysis, it seems that these porous spheres still contain trace amount of carbon species
after the annealing treatment in air.
The catalytic performances of CuO, ZnO, (CuO)xZnO porous hollow spheres and the commercial
catalysts for Rochow reaction are displayed in Fig. 7 and Table 2. It can be seen that (CuO)xZnO
hollow spheres exhibit quite high Si conversions, which are 78.2% for CuOZnO, 70.2% for (CuO)4ZnO,
64.6% for (CuO)9ZnO, 53.3% for (CuO)19ZnO, and 38.2% for (CuO)49ZnO, much higher than those of
the pure CuO (4.3%), ZnO (0%) and the several reported Cu-based catalysts (Table 3). As
dimethyldichlorosilane (M2) is the most valuable precursor in organosilane industry, attention is
mainly paid to its selectivity. As shown in Table 2, the order of M2 selectivity is (CuO)49ZnO (88.6%) >
(CuO)19ZnO (88.5%) > (CuO)9ZnO (75.2%) > (CuO)4ZnO (58.6%) > CuOZnO (8.7%). These results
indicate that the proper ratios of CuO and ZnO, such as 19:1 and 49:1, can improve the M2 selectivity
significantly. As compared with the commercial catalysts added with ZnO, whose M2 selectivity is
82.6%, the synthesized (CuO)19ZnO catalyst shows even higher M2 selectivity (88.5%) based on a
higher Si conversion (53.3%), while the commercial catalyst added with ZnO has a Si conversion of
only 46.2%. This catalytic performance enhancement should be related to the abundant porous
structure in (CuO)19ZnO, which could provide enlarged contact interface for interaction of the gas
chloromethane, the solid catalyst and the solid Si, and faster gas transportation within the pore system.
Also, as indicated both by TPR and by XPS analyses, the presence of the synergistic interaction
between ZnO and Cu nanoparticles should contribute to this enhanced M2 selectivity. It is reported that,
by interacting with ZnO, Cu atoms became more positively charged and could enhance the formation
of methylchlorosilanes.20, 21 In addition, it is found that the Si conversion rises up with increase of the
ZnO content and reaches its highest point with CuOZnO, over which 78.2% Si conversion is achieved.
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Meanwhile, the Si conversion is increased from 38.2% over (CuO)49ZnO to 78.2% over CuOZnO , but
the M2 selectivity is decreased from 88.6% to 8.7% with this ZnO content increment. In addition, the
Rochow reaction at various temperatures over (CuO)19ZnO hollow spheres catalyst was also
investigated. From Table 4, it is seen that the Si conversion is increased while M2 selectivity is
decreased with the rise of the reaction temperature, achieving the highest M2 selectivity (90.6%) at 295
o
oC but the highest Si conversion (80.4%) at 345 C. Since the catalytic durability is essential for
evaluation of the catalytic performance of porous hollow spherical catalysts, we thus conducted the
lifetime test for (CuO)19ZnO porous hollow spheres at 325 oC for 60 h at a gas flow of 25 mL/min. Fig.
8 shows the selectivity of M1, M2, M3, M1H, M2H, LB and HB. It can be seen that the M2 selectivity
remains above 80% over a period of 50 h, but decreases to about 70% after the 60h reaction. After the
reaction, the measured SEM image in Fig. S2 shows that the contact mass is composed of many
microparticles with (CuO)19ZnO porous hollow spherical catalyst.
The structure and composition of the contact masses after the reaction with (CuO)4ZnO catalyst were
further characterized by XRD, SEM, elemental mapping, and SEM-EDX analysis, because both the Cu
and ZnO phases can be better observed during the reaction using (CuO)4ZnO than that using
(CuO)9ZnO, (CuO)19ZnO, and (CuO)49ZnO catalysts. Fig. 9a shows the XRD patterns of the contact
masses with (CuO)4ZnO + Si and CuO after the induction period and reaction for 24 h. It can be seen
that, after the induction period, the contact masses are composed of Si, Cu, and ZnO, but no CuO
species, probably due to the reaction of chlorosilane with the lattice oxygen from CuO that causes the
formation of Cu in the induction period. As a promoter, ZnO remains unchanged in the reaction. The
CuxSi species in the contact masses is also observed, but a much higher intensity of CuxSi is only
observed in the case of (CuO)4ZnO + Si, implying that the (CuO)4ZnO catalyst is more active in
generating CuxSi than pure CuO. In addition, the XRD patterns of the contact masses with CuOZnO,
(CuO)4ZnO, (CuO)9ZnO, (CuO)19ZnO, (CuO)49ZnO after a 24h reaction in Fig. 9b indicate that all
these samples are composed of Si, Cu, CuxSi, and ZnO phases after the reaction. Comparing with the
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XRD peaks shown in Fig. 9b, the content of CuxSi is increased with the rise of the ZnO content. Fig. 9c
shows the SEM image, elemental mapping images of Si, Cu, Zn, and SEM-EDX spectra of contact
masses during reaction using (CuO)4ZnO as the catalyst, which prove that most of the Cu and Zn
elements are distributed in the pores of the Si particle, suggesting the occurrence of etching process
during the reaction. The elemental mapping images of Si, Cu, Zn, and SEM-EDX spectra after reaction
in Fig. 9d indicate the uniform distribution of the elements Si, Cu, and Zn on the reaction interface,
suggesting the reaction interface is mainly composed of Si, Cu, and Zn elements.
Based on the above results, a detailed Rochow reaction process over the (CuO)xZnO hybrid catalysts
is proposed, which is illustrated in Fig. 10. Firstly, CuO is converted to Cu in the induction period by
extracting lattice oxygen in CuO by CH3Cl (Fig. 10a), which is proven by the XRD patterns shown in
Fig. 9a (the contact masses for (CuO)4ZnO + Si and CuO + Si after the induction period), and there is
non-formation of methylchlorosilanes in this induction period. Secondly, Cu diffuses to the interior
layer of Si forming an interface, on which the main products ((CH3)2SiCl2 and CH3SiCl3) and by-
products are generated from Si and CH3Cl (Fig. 10b). From the experimental and characterization
results, the entire interface can be depicted as that in Fig. 10c, which is mainly composed of Si, Cu,
CuxSi, and ZnO phases. In Rochow reaction, CuxSi is normally suggested as the key catalytic active
species, from which, M2 is formed.53, 54 The CuxSi species formed between the Cu-based catalyst and
the Si interface is an indicator of activity for a Cu-based catalyst.55 The formation procedures of
(CH3)2SiCl2 reported in literature56-58 are as follows: the CuxSi composed of electropositivity Cu+ and
electronegativity Si– first initiates CH3–Cl bond cleavage to generate electronegativity Cl– and
+
+
electropositivity CH3 , then the Cu+ site absorbs the Cl–, and Si– absorbs CH3 . Subsequently the
electronegativity Cl– transfers to and combines with electronegativity Si– to form (CH3)2SiCl2, as
shown in Fig. 10d. In general, this reported catalytic mechanism doesn’t consider and reveal the role of
Zn-based promoter in this catalytic reaction. Furthermore, the interaction details between the Cu-based
catalysts and the Zn-based promoter are not very clear yet, and there are no very specific studies on this
13

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interaction. Fig. 10e shows our newly proposed formation process for (CH3)2SiCl2 with appropriate
ZnO promoter. Similarly, the Cu atoms connected with ZnO atoms and Si atoms initiate the CH3–Cl
+
bond cleavage to generate electronegativity Cl– and electropositivity CH3 , then the electropositivity
+
Cu+ absorbs electronegativity Cl–, and the electronegativity Si– absorbs the electropositivity CH3 . At
last, electronegativity Cl– transfers to and combines with Si– to form (CH3)2SiCl2 via an exothermic
reaction. Theoretical studies have showed that Cu atoms can strongly interact with ZnO atoms and
become more positively charged generating synergistic electronic effect.20, 21 When a lot of ZnO is
added, the positively charged Cu atoms can absorb more electronegativity Cl– but less electropositivity
+
CH3 , mainly forming CH3SiCl3 (Fig. 10f), which is in line with the experiment results shown in Table
2. Probably, the high Si conversion achieved by the reaction between Si and CH3Cl needs larger
interface among Cu, ZnO and Si and stronger synergistic electronic effect among Cu, ZnO and Si,
while the high M2 selectivity requires suitable amount of ZnO promoter which generates the
synergistic electronic effect. This work not only primarily outlines the component-property relationship
of the (CuO)xZnO catalysts for the Rochow reaction, but also develops a high efficient Cu-based
catalysts with the high Si conversion and high M2 selectivity.
4. Conclusions
In conclusion, we have demonstrated a hard templating approach to preparing a series of (CuO)xZnO (0
≤ x ≤ 49) porous hollow spheres with various CuO/ZnO atomic ratios, such as CuOZnO, (CuO)4ZnO,
(CuO)9ZnO, (CuO)19ZnO, (CuO)49ZnO. Comparing with the commercial catalysts and pure CuO or
ZnO porous hollow spheres, the (CuO)19ZnO porous hollow spheres exhibit much higher silicon
conversion and dimethyldichlorosilane selectivity in Rochow reaction, mainly because of two reasons:
(1) in the porous hollow spheres, the porous structure allows closer contact between the catalysts and
the reactant molecules and the enhanced gas transportation; (2) the presence of the synergistic
electronic effect between Cu and ZnO can promote to the generation of active immediate of CuxSi.
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This work shows that the Cu-based catalysts with this novel porous hollow structure should be a
promising catalyst for Rochow reaction.
Acknowledgment
The authors gratefully acknowledge the supports from the National Natural Science Foundation of
China (no. 51402299 and 51272252) and Hundred Talents Program of the Chinese Academy of
Sciences. Z. Zhong is mainly working in the Institute of Chemical and Engineering Sciences (ICES)
and also holds an adjunct position in Nanyang Technological University (NTU) in Singapore. He
would like to thanks ICES for supporting of this collaboration.
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List of Tables
Table 1. The synthesis conditions used for preparation of (CuO)xZnO (0 ≤ x ≤ 49) porous hollow
spheres and the actual content of Cu and Zn in porous hollow spheres.
Table 2. Catalytic performance of all the catalysts for Rochow reaction at 325 oC.
Table 3. A comparison of the Si conversion for (CuO)19ZnO and CuOZnO catalysts with several
reported Cu-based catalyst.
Table 4. Catalytic performance of (CuO)19ZnO catalysts at different reaction temperatures for Rochow
reaction.
List of the Figure Captions
Fig. 1 Schematic illustration of the formation procedures for the carbonaceous spheres (CS) (a)
adsorption Cu2+ and Zn2+ forming CS/Cu2+Zn2+ spheres (b), and CS combustion and Cu2+ and Zn2+
precursors oxidation forming (CuO)xZnO porous hollow spheres after annealing in air (c).
Fig. 2. XRD patterns for ZnO, CuOZnO, (CuO)4ZnO, (CuO)9ZnO, (CuO)19ZnO, (CuO)49ZnO, and
CuO porous hollow spheres.
Fig. 3 SEM (a, d), TEM (b, e) and EDX spectra (c, f) images of porous ZnO (a, b. c) and CuO (d, e, f)
hollow spheres after calcination in air.
Fig. 4 SEM (a), TEM (b) images and its EDX spectra (c) of (CuO)4ZnO hollow spheres, SEM (d),
TEM (e) images, its EDX spectra (f), high-resolution TEM image (g), and elemental mapping images
of Cu, Zn, and O elements (h) and the line scan along the yellow line of Cu and Zn (i) for CuOZnO
hollow spheres after calcination in air.
Fig. 5 SEM (a, c, e), and TEM (b, d, f) images of (CuO)9ZnO (a, b), (CuO)19ZnO (c, d), and
(CuO)49ZnO (e, f) porous hollow spheres after calcination in air.
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Fig. 6 H2-TPR curves for all the samples (a), XPS spectra: Cu 2p for the CuO, CuOZnO hollow
spheres (b), Zn 2p for the ZnO, CuOZnO hollow spheres (c), N2 adsorption-desorption isotherms (d),
and BJH-PSD curves (e) of CuO and (CuO)xZnO hollow spheres samples, and TG curve for
CS/Cu2+Zn2+ spheres in air (f).
Fig. 7 Selectivity of dimethyldichlorosilane (M2) and conversion of Si for (CuO)xZnO and commercial
catalysts for Rochow reaction. Error bars, standard deviation of triplicated wells. Commercial catalyst
named Coca. The catalytic measurements were independently repeated three times to obtain three M2
selectivity and Si conversion for each catalyst.
Fig. 8 Lifetime test of (CuO)19ZnO porous hollow spheres: selectivity of M1, M2, M3, M1H, M2H, LB
and HB.
Fig. 9 XRD patterns of the contact masses with for (CuO)4ZnO and CuO hollow spheres after the
induction period and the 24h reaction (a), XRD patterns of the contact masses with CuOZnO,
(CuO)4ZnO, (CuO)9ZnO, (CuO)19ZnO, (CuO)49ZnO after the 24h reaction (b), SEM image, elemental
mapping images of Si, Cu, Zn, and SEM-EDX spectra of contact masses during reaction (c), and SEM
image, elemental mapping images of Si, Cu, Zn, and SEM-EDX spectra of the reaction interface
composed of Si, Cu, and ZnO (d).
Fig. 10 The illustration of the catalytic mechanism using (CuO)xZnO mixture: the catalytic reaction
processes (a and b), the component of the interface (c), the formation processes of (CH3)2SiCl2 in
literature reports (d), the proposed formation processes of (CH3)2SiCl2 with appropriate ZnO promoter
(e), and the proposed formation processes of CH3SiCl3 with a lot of ZnO promoter (f).
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Table 1.
Samples
Cu(CH3COO)2 Zn(CH3COO)2 Adsorpti 500 oC
Cu (wt%) Zn (wt%)
(mmol)
(mmol)
on time (in air) (h) From ICP From ICP
(h)
ZnO
0
0.5
0.5
8
8
2
2
0
100
CuOZnO
0.5
49.3
50.7
(CuO)4ZnO
(CuO)9ZnO
(CuO)19ZnO
(CuO)49ZnO
CuO
0.5
0.5
0.5
0.5
0.5
0.125
0.0556
0.0263
0.0102
0
8
8
8
8
8
2
2
2
2
2
79.6
89.7
94.9
98.3
100
20.4
10.3
5.1
1.7
0
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Table 2.
Sample
Product Composition (%)
CSi
M1
M2
M3
M1H
M2H
LBR
HBR
(%)
ZnO
CuO
0
0
0
0
0
0
0
0
9.7
80.2
8.7
3.8
1.8
3.1
2.3
1.6
1.7
3.4
2.4
1.0
0
2.1
4.5
2.3
0.2
1.8
1.7
1.5
1.2
1.0
11.2
3.6
0.3
1.2
1.7
1.6
0.4
2.2
2.2
2.0
2.0
2.0
1.5
2.4
2.2
4.3
CuOZnO
(CuO)4ZnO
(CuO)9ZnO
(CuO)19ZnO
(CuO)49ZnO
Coca
71.6
30.3
19.9
3.7
78.2
70.2
64.6
53.3
38.2
5.4
58.6
75.2
88.5
88.6
80.0
82.6
0.1
0.1
1.2
1.3
1.1
0.1
3.5
10.0
11.1
*Coca + ZnO
46.2
Note: methyltricholorsilane (M1), dimethyldichlorosilane (M2), trimethylchlorosilane (M3), methyldichlorosilane
(M1H), dimethylchlorosilane (M2H), low boiler (LB), high boiler (HB), and Si conversion (Csi), *Commercial
catalysts : ZnO hollow spheres = 19 : 1. Commercial catalyst named Coca.
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Table 3.
Samples
(CuO)19ZnO
CSi (%)
53.3
78.2
20.0
34.5
33.5
33.2
40.4
29.2
38.8
41.6
23.4
32.4
24.8
~12
M2 (%)
88.5
8.7
Ref.
This work
CuOZnO
This work
39
Bulk CuOZnO
69.6
85.5
67.3
40
33
59
34
42
35
60
61
62
63
37
38
Coca+Urchin-like ZnO
Porous Cu microparticles
Mesoporous Cu2O microspheres
Hexahedron Cu2O microparticles
Core–shell Cu@Cu2O microspheres
Flower-like CuO microparticles
Dandelion-like CuO microparticles
CuO-Cu2O-Cu nanoparticles
Cu-Cu2O-CuO microparticles
Tetrahedra CuCl microparticles
CuCl particles
62.6
75.7
77.6
83.2
86.7
74.5
82.0
81.3
~65
~20
CuCl particles
~80
o
Note: Ref [37] Si: catalyst: Zn = 100:10:0.06, temperature 300 C; Ref [38] Si: catalyst: Zn = 200:10:1, temperature
o
o
300 C; Ref [33,34,35,39,40,42,52,53,54,55,56] Si: catalyst: Zn = 100:10:1, temperature 325 C; this work Si:
(CuO)xZnO = 100:5, temperature 325 oC.
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Table 4.
Temperature (oC)
Product Composition (%)
CSi
M1
M2
M3
M1H
M2H
LBR
HBR
2.8
3.7
90.6
88.5
76.3
1.4
1.6
2.5
1.3
1.2
1.0
1.2
1.8
1.0
1.0
1.2
1.5
1.7
2.0
2.2
45.8
53.3
80.4
295
325
15.5
345
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Fig. 1
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Fig. 2
(101)
(002)
(100)
(112)
(103)
(110)
(102)
(201)
ZnO
CuOZnO
(CuO)4ZnO
(CuO)9ZnO
(CuO)19ZnO
(CuO)49ZnO
(-111)
(111)
(-311)
(202)
(-222)
(311)
(-202)
(020)
(220)
CuO
(-113)
(110)
(-313)
10 20 30 40 50 60 70 80 90
2Theta (Degree)
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Fig. 3
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Fig. 4
(a)
(d)
(b)
(e)
(c)
(f)
(CuO)4ZnO
CuOZnO
Cu
Zn
(g
)
(g)
(h)
(i)
ZnO (100)
0.28 nm
CuO (-111)
0.25 nm
5 nm
500 nm
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Fig. 5
(b)
(a)
(c)
(e)
(CuO)9ZnO
(CuO)19ZnO
(CuO)49ZnO
(d)
(f)
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Fig. 6
(a)
(b)
Cu 2p3/2
CuOZnO
CuOZnO
Sat.
Cu 2p1/2
(CuO)4ZnO
(CuO)9ZnO
(CuO)19ZnO
CuO
(CuO)49ZnO
CuO
960 955 950 945 940 935 930
100
200
300
400
500
Temperature (oC)
Binding Energy (eV)
140
(c)
(d)
120
100
80
60
40
20
0
(CuO)49ZnO 21.4m2/g
CuOZnO
ZnO
(CuO)19ZnO 22.1 m2/g
(CuO)9ZnO 23.9 m2/g
(CuO)4ZnO 24.7 m2/g
CuOZnO 22.8 m2/g
CuO 19.6 m2/g
1050
1040
1030
1020
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Binding Energy (eV)
(e)
(f)
120
100
80
60
40
20
0
CS/Cu2+Zn2+ spheres
68.9 wt%
(CuO)49ZnO
(CuO)19ZnO
(CuO)9ZnO
(CuO)4ZnO
CuOZnO
CuO
100 200 300 400 500 600 700 800
1
10
Pore size(nm)
100
Temperature (oC)
29