Controllably oxidized copper flakes as multicomponent copper-based catalysts for the Rochow reaction

Article abstract of DOI:10.1039/c3ra46970k

The metallic Cu flakes prepared by milling metallic Cu powder were controllably oxidized in air at different temperatures to obtain the Cu-based catalysts containing multicomponents of Cu, Cu₂O, and CuO. These catalysts are explored 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. The samples were characterized by X-ray diffraction, temperature-programmed reduction, thermogravimetric analysis, oxidimetry analysis, particle size analysis, transmission electron microscopy, and scanning electron microscopy. Compared to the metallic Cu powder and Cu flakes, the controllably oxidized Cu flakes containing Cu, Cu₂O, and CuO species show much higher M2 selectivity and silicon conversion in the Rochow reaction. The enhanced catalytic performance may stem from the larger interfacial contact among the gas MeCl, solid Si particles, and solid Cu-based catalyst flakes, as well as the synergistic effect among the different Cu species. The work would be helpful to the development of novel Cu-based catalysts for organosilane synthesis.

Full text of DOI:10.1039/c3ra46970k

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Controllably oxidized copper flakes as
multicomponent copper-based catalysts for the
Rochow reaction
Cite this: RSC Adv., 2014, 4, 7826
Shaomian Liu,ab Yingli Wang,b Yongxia Zhu,b Guangna Wang,b Zailei Zhang,b
c
a
b
*
*
Hongwei Che, Lihua Jia and Fabing Su
The metallic Cu flakes prepared by milling metallic Cu powder were controllably oxidized in air at different
temperatures to obtain the Cu-based catalysts containing multicomponents of Cu, Cu2O, and CuO. These
catalysts are explored 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. The samples were characterized by X-ray diffraction, temperature-programmed reduction,
thermogravimetric analysis, oxidimetry analysis, particle size analysis, transmission electron microscopy,
and scanning electron microscopy. Compared to the metallic Cu powder and Cu flakes, the controllably
oxidized Cu flakes containing Cu, Cu2O, and CuO species show much higher M2 selectivity and silicon
conversion in the Rochow reaction. The enhanced catalytic performance may stem from the larger
interfacial contact among the gas MeCl, solid Si particles, and solid Cu-based catalyst flakes, as well as
the synergistic effect among the different Cu species. The work would be helpful to the development of
novel Cu-based catalysts for organosilane synthesis.
Received 23rd November 2013
Accepted 7th January 2014
DOI: 10.1039/c3ra46970k
www.rsc.org/advances
ðCH3Þ SiCl
ðCH3Þ 3SiCl2
2
1. Introduction
CH3SiCl3
CH3HSiCl2
copper catalyst
ƒƒƒƒƒƒƒƒ!
The aky metals such as Al,1 Ni,2 Zn,3 Au,4 Cu,5,6 and alloys,7,8 which
are generally produced by ball milling in industry,1 are widely
applied in electrode material,5,9 pigments,3,10 and catalysts.11 This is
because of their two-dimension structural characteristics, which
endow these materials with some special properties, such as good
adhesive ability,10,12 excellent conductivity,5,9 strong light reec-
tion3,10 and commendable catalytic performance.11,13 It is known
that the catalytic performance of a heterogeneous catalyst is
strongly affected by contact area between reactants and metal
catalyst particles with a specic morphology and/or small particle
size. Compared with the bulk counterparts, the metal akes with
even the same volume normally have much more active surface,
thus conducive to catalysis.
Si þ MeCl
(1)
300ꢀ350ꢁC
ðCH3Þ HSiCl
2
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 in organosilane industry, and
thus a high M2 yield is the most desired. Besides the accurate
control of reaction conditions and selection of proper reactors,
the development of highly efficient Cu-based catalyst is critical
for a high M2 selectivity and Si conversion. It has been reported
that Cu-based catalysts, including metallic Cu,14,16 Cu2O,17,18
CuO,17 CuCl,19,20 Cu–Si alloy,21,22 and Cu–Cu2O–CuO,23 are active
in the presence of promoter additives.24–26 In organosilane
industry, the Cu-based catalysts containing Cu, Cu2O, and CuO
prepared via high-energy mechanical milling have been widely
applied.23 Recently, various cupreous and copper oxide struc-
tures, including ower-like ZnO grown on urchin-like CuO
microspheres,25 porous cubic Cu microparticles,27 mesoporous
Cu2O microspheres,28 shape-controlled Cu2O microparticles,29
dandelion-like CuO microspheres,30 ower-like CuO micro-
spheres,31 CuCl microcrystals with different morphologies,32
and Cu@Cu2O core–shell microspheres,33 have been developed
in our group as model Cu-based catalysts to explore the
catalytic properties for M2 synthesis. It is found that surface
The Rochow reaction, which was discovered in 1940s,14 is
commercially employed to produce methylchlorosilanes
(MCSs). In this reaction, gas methyl chloride (MeCl) reacts with
solid silicon (Si) powder in the presence of Cu-based catalysts
and a trace amount of promoters as following Formula (1):15
aCollege of Chemistry and Chemical Engineering, Qiqihaer University, Qiqihaer,
Heilongjiang Province, 161006, China. E-mail: jlh29@163.com; su@mail.ipe.ac.
cn; Fax: +86-10-82544851; Tel: +86-10-82544850
bState Key Laboratory of Multiphase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing, 100190, China
cCollege of Equipment Manufacturing, Hebei University of Engineering, Hebei
province, 056038, China
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morphology, pore structure, particle size, and catalyst compo- calculated using the Debye–Scherrer equation. The microscopic
sition have signicant effects on the Rochow reaction. However, feature of the sample was observed by eld-emission scanning
all the above Cu-based catalysts are always in the micrometer electron microscopy (SEM) (JSM-7001F, JEOL, Tokyo, Japan)
size and the synergistically catalytic effect between different with energy-dispersive spectroscopy (EDS) (INCA X-MAX, JEOL,
components is not fully understood yet. More recently, Liu Oxford, England) and transmission electron microscopy (TEM)
et al.,34 prepared nanosized Cu-based catalysts with multicom- (JEM-2010F, JEOL, Tokyo, Japan). Thermogravimetric (TG)
ponent of Cu, Cu2O, and CuO by partial reduction of CuO analysis was carried out on an EXSTAR TG/DTA 630ꢀ01(Seiko
ꢁ
nanoparticles in hydrogen. However, a large scale manufacture Instruments, Japan) with a heating rate of 5.0 C min in air
of such catalysts seems impossible considering the preparation (200 mL minꢀ1). H2-temperature programmed reduction (TPR)
cost. On the other hand, these nanosized catalysts are hardly measurements were carried out on an automated chemisorp-
stored for a long time due to the easy surface oxidation of the tion analyzer (ChemBET pulsar TPR/TPD, Quantachrome).
reduced Cu nanoparticles when exposed to air. As we know, Upon loading of about 0.1 g of sample into a quartz U-tube, the
the oxidation of Cu could generate surface copper oxide lm, samples was degassed at 150 ꢁC for 30 min under helium. When
which may avoid above problems. In addition, metallic Cu and the temperature was dropped to 30 ꢁC, the gas was changed to
alloyed Cu akes have been used in laboratory for MCS 9.9% H2/He. Finally, the sample was heated from 30 to 900 ꢁC at
synthesis.16 Combining these ideas, it is expected to generate 10 ꢁC minꢀ1 under the H2/He ow (30 mL minꢀ1). The particle
aky multicomponent Cu–Cu2O–CuO catalysts via partially size distribution (PSD) was measured by laser particle size
oxidation of the Cu ake as Cu-based catalysts for the Rochow analyzer (BT-9300Z, Bettersize Instruments Ltd., China). 1.0 g
reaction.
sample was added to sample cell containing deionized water.
Herein, we report the preparation of partially oxidized Cu Aer 3 min of in situ ultrasonication, the PSD data and curve
ake as Cu-based catalysts for the Rochow reaction by dry were automatically given. Oxidimetry analysis was used for
milling and controllable oxidation processes. These catalysts determine the content of Cu, Cu2O, and CuO components
can be easily scaled up in industry. Compared with metallic Cu within a sample. All used chemicals with an analytical
powder and Cu akes, the controllably oxidized Cu akes con- grade were purchased from Sinopharm Chemical Reagent Co.,
taining Cu, Cu2O, and CuO species show much higher M2 Ltd. China. A sample of 0.3 g was putted into a ask containing
selectivity and Si conversion because of the improved contact 120.0 mL acetonitrile (CH3CN, 95%), followed by adding 40.0
between catalyst particles and Si powders, and the synergistic mL of 18.5 wt% diluted HCl to form a mixture. Aer vigorously
effect among different Cu species. Our work would be helpful rocking for 20 seconds and immediate ltration, 160.0 mL
for developing novel Cu-based catalysts.
deionized water was added to the ltrated solution and then
titrated with 0.02 mol Lꢀ1 KMnO4 ($99.5%). The content of
Cu2O in the sample can be calculated by the amount of
consumed KMnO4 (Formula (2)). The residual solid precipitate
aer above ltration was dissolved in 20.0 mL of 0.1 mol Lꢀ1
FeCl3 solution kept at 50 ꢁC in water bath, and then cooled
down to room temperature (Formula (3)). The solution was
added with 20.0 mL of 0.7 mol Lꢀ1 MnSO4 solution and 100.0
mL deionized water, and again titrated with 0.02 mol Lꢀ1
KMnO4 solution. The content of Cu is further calculated
through the amount of ferrous iron derived from amount of
consumed KMnO4 (Formula (4)). The content of CuO is thus the
result of total amount of sample subtracting the amount of Cu
and Cu2O.
2. Experimental
2.1. Material synthesis
20.0 g of Cu powders (denoted Cu-p, C.P., Beijing General
Research Institute of Mining & Metallurgy, China) and 2.0 wt%
of stearic acid (C18H36O2, C.P., Shijiazhuang Xinghong Rubber
and Plastic Additives Sales Co, Ltd., China)35 were mixed and
mechanically ball-milled for 2 h in a planet-ball-grinding
machine (QM-3SP04, Nanjing NanDa Instrument Plant, China).
The milling process was carried out at 600 rpm in a Teon
container using 8 mm diameter stainless steel balls with ball-to-
powder weight ratio of 30 : 1. The obtained Cu ake powders
were denoted Cu-f. In the oxidation process, 10.0 g of Cu-f
was placed in a square crucible, which was then put in a
corundum tube in a tube furnace. The samples were oxidized in
air (50.0 mL minꢀ1) with ramp rate of heating at 5 ꢁC minꢀ1 at
different temperatures of 200, 300, 400, and 500 ꢁC for 10 min,
naturally cooled down to room temperature in a pure N2 ow,
and were denoted Cu–Cu2O-f, Cu–Cu2O–CuO-f, Cu2O–CuO-f,
and CuO-f, respectively.
ꢀ
8H+ + 5Cu+ + MnO4 / 5Cu2+ + Mn2+ + 4H2O
(2)
(3)
(4)
Cu + 2Fe3+ / Cu2+ + 2Fe2+
ꢀ
8H+ + 5Fe2+ + MnO4 / 5Fe3+ + Mn2+ + 4H2O
2.3. Catalytic measurement
2.2. Characterization
The evaluation of catalyst was carried out with a typical MCS lab
X-ray diffraction (XRD) analysis were performed with a PAN- xed-bed reactor. 10.0 g of Si powder (20–50 mesh, provided
alytica X'Pert PRO MPD using the Ka radiation of Cu (l ¼ 1.5418 by Jiangsu Hongda New Material Co., Ltd.) and 1.0 g of
˚
A) and checked with the card number of Joint Committee on prepared product catalyst together with 0.1 g of zinc (Zn, A.R.,
Powder Diffraction Standards (JCPDS). The crystallite size was Sinopharm Chemical Reagent Co., Ltd.) used as a promoter were
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homogeneously mixed to form a contact mass, which was loaded in which correspond to the lattice plane of (110), (200) and (220) of
the glass reactor. The reactor system was purged with puried N2 for pure Cu phase (JCPDS no. 01-070-3039), respectively. Accord-
0.5 h followed by heating to 325 ꢁC within 1 h under a N2 ow rate of ingly, the XRD pattern of Cu-f shows the same diffraction peaks
25 mL minꢀ1. Subsequently, N2 was turned off, and MeCl gas with a as those in Cu-p but with broadened shape and decreased
ow rate of 25 mL minꢀ1 was introduced into the reactor to react intensity, suggesting a degraded crystal structure, which is due
with Si followed by heating to 325 ꢁC. Aer a given period of 24 h, to the decrease of grain size during milling.8 For Cu–Cu2O-f, two
the reaction was stopped. The gas product was cooled into liquid new peaks appeared at 2q values of 36.7 and 42.7ꢁ, which
phase with the circulator bath controlled at 5 ꢁC by a programmable correspond to the lattice planes of (111) and (200) of cubic Cu2O
thermal circulator (GDH series, Ningbo Xinzhi Biological Tech- (JCPDS no. 00-001-1142), respectively, suggesting the formation
nology Co., Ltd., China). The products in the liquid solution were of a bi-component material of Cu and Cu2O. The diffraction
quantitatively analyzed on an Agilent Technologies gas chromato- peaks of Cu–Cu2O–CuO-f at 2q values of 33.5, 35.5, 38.2, 48.7,
graph (GC-7890A) equipped with a KB-210 column (60 m) and a 54.2, 58.3, 62.5, 66.4, 68.2, 73.4, and 75.6ꢁ are observed, which
TCD detector. Gas chromatography system was used for identi- correspond to the lattice planes of (110), (ꢀ111), (111), (ꢀ202),
cation of the products, which are mainly comprised of methyltri- (020), (202), (ꢀ113), (ꢀ311), (220), (311), and (111) of mono-
chlorosilane (CH3SiCl3, M1), M2, trimethylchlorosilane ((CH3)3SiCl, clinic CuO (JCPDS no. 03-065-2309), respectively. From
M3), methyldichlorosilane (CH3SiHCl2, M1H), dimethyl- Cu–Cu2O–CuO-f to Cu2O–CuO-f, the diffraction peaks of Cu are
chlorosilane ((CH3)2SiHCl, M2H), low boiler (LB) and high boiler gradually weakened until completely disappeared with
(HB). The selectivity of the products was calculated by the peak area the increase of the oxidation temperature. Further increasing
ꢁ
ratio (in percentage). The waste contact mass (residual solid aer the temperature to 500 C, the XRD pattern of CuO-f suggests
reaction) containing unreacted Si powder, Cu compounds, and the formation of pure CuO. The grain size of Cu, Cu2O, and CuO
promoters were weighed to calculate Si conversion, which was that in each sample was calculated based on the 2q values of 35.8,
the ratio of weight difference of contact mass (before and aer 36.7, and 43.4ꢁ, respectively, and compiled in Table 1, in which,
reaction) and weight of Si (before reaction) (Formula (5)).
it can be seen that the grain size of Cu in Cu-p is 32.5 nm,
weightcontact mass before reaction ꢀ weight
Si before reaction
contact mass after reaction
Conversion of Si ðCSiÞ ¼
ꢂ 100%
(5)
weight
slightly larger than that in Cu-f (28.9 nm). In Cu–Cu2O-f and Cu–
Cu2O–CuO-f, the grain size of Cu decreases gradually from 28.5
to 28.0 nm. This may because vacancies or vacancy clusters in
Cu grain boundaries move easily along grain boundaries
towards the surfaces and then disappear rapidly. As a result, the
atomic arrangement tends fast towards an ordered arrange-
ment, and thus the grain size of Cu becomes small, with the
increasing oxidation temperature.36,37 The grain size of Cu2O in
Cu–Cu2O-f, Cu–Cu2O–CuO-f, and Cu2O–CuO-f is 28.3, 34.3, and
39.7 nm, respectively. For Cu–Cu2O–CuO-f, Cu2O–CuO-f, and
CuO-f, the grain size of CuO increases gradually from 38.3 to
54.7 nm with the increase of the oxidation temperature.38,39
Fig. 1b shows the TG curves of all the samples measured in air.
For Cu-p, the weight is gradually increased to about 24.5 wt% at
3. Results and discussion
3.1. Characterization
Fig. 1a shows the XRD patterns of all the samples. Diffraction
peaks of Cu-p at 2q values of 43.3, 50.5 and 74.2ꢁ are observed,
ꢁ
800 C, close to the theoretical value (25.0 wt%), indicating the
high purity of metallic Cu. For Cu-f, in the temperature range of
230–250 ꢁC, a weight loss of 0.7 wt% close to the amount of
residual stearic acid used in milling process can be attributed to
its decomposition, which is conrmed by the TG analysis of pure
stearic acid (not shown here). The weight increase of Cu-f is about
23.5 wt% at 800 ꢁC, slightly less than its theoretical value (24.3 wt
%) considering the presence of stearic acid in this sample. A
similar weight loss in the temperature range of 230–250 ꢁC is also
seen for Cu–Cu2O-f because its oxidation was carried out at 200
ꢁC, less than the decomposition temperature of stearic acid.
However, this weight loss disappeared when the oxidation
Fig. 1 XRD patterns (a), TG curves (b), H2-TPR curves (c), and PSD
curves (d) of all the samples.
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Table 1 The physical parameters of all the samples
Cua (nm) Cu2Oa (nm) CuOa (nm) Cub (wt%) Cu2Ob (wt%) CuOb (wt%) TM (ꢁC) STPR
Particle sizee (mm)
c
d
Sample
Cu-p
Cu-f
32.5
28.9
28.5
0.0
0.0
28.3
34.3
39.7
0.0
0.0
0.0
0.0
38.3
45.1
54.7
100.0
100.0
53.0
27.9
0.0
0.0
0.0
47.0
40.9
39.1
0.0
0.0
0.0
0.0
31.2
60.9
100.0
—
—
389.9
387.7
380.1
376.3
—
—
31.2
21.7
Cu–Cu2O-f
Cu–Cu2O–CuO-f 28.0
Cu2O–CuO-f
CuO-f
6959.6 20.7
14 348.3 20.4
22 017.9 20.2
26 662.5 17.5
0.0
0.0
0.0
a
The grain size of Cu, Cu2O, and CuO of all samples calculated from the XRD patterns. b The weight percent of Cu, Cu2O, and CuO determined by
oxidimetry. c Temperatures at the peak maximum from H2-TPR patterns. d The hydrogen consumption peak area of samples from H2-TPR patterns.
e
The particle size value of the catalysts from laser particle analyzer.
temperature is beyond to 300 ꢁC because stearic acid was (Fig. 2e–l), Cu–Cu2O–CuO-f (Fig. 2g–h), Cu2O–CuO-f (Fig. 2i–j),
decomposed during the oxidation process. For Cu–Cu2O-f and and CuO-f (Fig. 2k–l) is around 40–80 mm, similar to that of Cu-f.
Cu–Cu2O–CuO-f, their weight increase is about 18.3 and 11.4 wt The morphology of Cu–Cu2O-f has no signicant difference
%, respectively, which is due to the oxidation of Cu and Cu2O to from that of Cu-f, but the surface of Cu–Cu2O–CuO-f shows a
CuO, in good agreement with the values calculated based on the grain-like appearance and Cu2O–CuO-f has a blade-like struc-
oxidimetry result (18.5 and 11.7 wt%). For Cu2O–CuO-f, the ture. Furthermore, the surface of CuO-f exhibits a clear ruffle-
weight increase derived from the oxidation of Cu2O to CuO is 4.2 like microstructure aer a higher oxidation temperature
wt%, and thus the calculated Cu2O content in this sample is (500 ꢁC), which is generated from the full introduction of oxygen
about 38.8 wt%, consistent with the oxidimetry result (39.1 wt% and may indicate that oxidation of CuO-f is well processed.
in Table 1). It is seen that there is no visible weight change for
sample CuO-f, suggesting it is nearly pure CuO.
Fig. 3 shows the TEM images of all the aky samples. It is
seen that the lattice plane distance of Cu-f is 0.21 nm (Fig. 3a),
Fig. 1c shows the H2-TPR proles of all the samples. The corresponding to the (111) plane distance of cubic Cu derived
hydrogen consumption peak areas and peak temperatures (TM) from the XRD pattern. The TEM image of Cu–Cu2O in Fig. 3b
are compiled in Table 1. It can be seen that, with the increase of shows the presence of two different lattice plane distances of
the CuO content in the samples (Cu–Cu2O-f, Cu–Cu2O–CuO-f, 0.21 and 0.24 nm, corresponding to that of Cu (111) and Cu2O
Cu2O–CuO-f, and CuO-f), the reduction temperature is slightly (111) respectively. Similarly, Cu–Cu2O–CuO-f (Fig. 3c) has three
decreased but the hydrogen consumption is increased. For the lattice plane distances of 0.21, 0.24, and 0.25 nm, correspond-
oxidized samples, the reduction peaks move forward to a slightly ing to that of Cu (111), Cu2O (111), and CuO (ꢀ111), respec-
lower temperature region with the increase of the oxidation tively. In Cu2O–CuO-f (Fig. 3d), there are two lattice plane
temperature. This may be because of the formation of more distances of 0.24 and 0.25 nm, corresponding to that of Cu2O
defects in the oxidized products at a higher temperature.40 Using (111) and CuO (ꢀ111), respectively. For CuO-f (Fig. 3e), the lattice
the ratio of hydrogen consumption peak area to the mole number plane distance is around 0.25 nm, indicating the presence of CuO
of O in CuO-f as the reference and based on the above TG anal- (ꢀ111). These results suggest that the prepared Cu-based
ysis, it can calculate that the content of CuO in Cu–Cu2O–CuO-f samples are composed of polycrystals while each multi-compo-
and Cu2O–CuO-f samples are about 60.4 and 31.1 wt%, respec- nent particle consists of different Cu component crystals.
tively, in good agreement with the above oxidimetry result (60.9
and 31.2 wt% in Table 1). For Cu–Cu2O-f, the content of Cu2O is
46.7 wt%, consistent with the oxidimetry result (47.0 wt% in
Table 1). Therefore, the contents of Cu, Cu2O, and CuO within the
3.2. Controllable oxidation mechanism
The oxidation process of Cu akes is proposed as depicted in
Fig. 4. In the oxidation process, it is normally accepted that
products can be well tuned by controlling the oxidation temper-
copper oxidation involves the migration of electrons and
ature. Fig. 1d shows the PSD curves of all the samples. The
cations to the reaction zone.41 It hypothesizes that a very strong
electric eld is set up through the oxide between the Cu and the
absorbed O at the gas interface, which enables the Cu ions to
move through the oxide to the reaction zone (Formula (6) and
(7)). The possible oxidation steps are shown as the following
equations:
median diameter of Cu-p, Cu-f, Cu–Cu2O-f, Cu–Cu2O–CuO-f,
Cu2O–CuO-f, and CuO-f at the maximum peak position is 31.2,
21.7, 20.7, 20.4, 20.2, and 17.5 mm, indicating the particle size is
slightly decreased with the increase of the oxidation temperature,
which may be caused by different levels of damage and disloca-
tion during the oxidation process, resulting in more defects
which may be active for the Rochow reaction.23
Fig. 2(a and b) present the SEM images of the Cu-p, which
show the branch-like morphology with a wide size range of 30–
120 mm. The SEM images of Cu-f in Fig. 2(c and d) show that the
Cu-f akes with a narrow ake diameter range of 40–80 mm have
a smooth surface and a uniform aky structure with a thickness
between 0.5 and 0.8 mm. The ake diameter of Cu–Cu2O-f
Cu / Cu+ + eꢀ
O2 / O + O
(6)
(7)
(8)
(9)
2Cu+ + O + 2eꢀ / Cu2O
Cu2O + O / 2CuO
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Fig. 3 TEM images of Cu-f (a), Cu–Cu2O-f (b), Cu–Cu2O–CuO-f (c),
Cu2O–CuO-f (d), and CuO-f (e).
Fig. 4 Schematic illustration of the oxidation reaction process of Cu
flakes.
occurs by the formation of vacant cationic sites at the metal–
oxide interface, followed by their diffusion to the oxide–oxygen
interface.42 The external-oxide scale formed on a Cu surface
grows at the scale–gas interface by outward Cu-ion diffusion
(Formula (8) and (9)). Two copper oxides (Cu2O and CuO) are
stable in the copper–oxygen system.43 In the temperature range
ꢁ
of 250–350 C, the values reported for the relative amounts of
cuprous and cupric oxides show large discrepancies. Grain
grown on Cu is oxidized at 300 ꢁC, in which Cu oxidation is
contributed from lattice diffusion and grain boundary diffu-
sion. The rate-determining step in this temperature range is the
outward diffusion of Cu in Cu2O. The same process is also
suitable to the description of formation of Cu–Cu2O–CuO-f
Fig. 2 SEM images of Cu-p (a and b), Cu-f (c and d), CuO–Cu2O-f (e and
f), CuO–Cu2O–Cu-f (g and h), CuO–CuO-f (i and j), and CuO-f (k and l).
For Cu–Cu2O-f, it is generally believed that cuprous oxide (Fig. 4b). For the formation of CuO on Cu2O during oxidation, it
accounts predominantly for the adherent layer formed below is believed that the growth of Cu2O is predominant at 350–
250 ꢁC (Fig. 4a). In this process, the transfer of Cu ions actually 500 ꢁC, which are attributable to the predominant growth of
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Cu2O by outward diffusion of Cu cations, just as the grain big difference over the various prepared catalysts are also
boundary diffusion does at 350–500 ꢁC. For Cu2O–CuO-f, fast detected in the products.
grain boundary diffusion in the high range of temperature leads
Fig. 5 displays the XRD patterns of the waste contact masses
ꢁ
to blade growth on Cu oxidized at 400 C. Oxide blades grow aer reaction, which is the solid mixture of the unreacted Si, the
predominantly by surface diffusion of cations along a tunnel Cu-based catalyst, and trace promoter Zn. Aer the reaction, all
centered on the core of a dislocation (Fig. 4c). For CuO-f, the the waste contact masses are composed of Si and Cu, but no
lattice diffusion contributes to Cu oxidation at high tempera- Cu2O and CuO species are detected in Fig. 5a. The formation of
tures. Thus, as the temperature increases, lateral lattice diffu- Cu may originate from the reaction of MeCl with the lattice
sion becomes relatively more favorable, and oxide growth leads oxygen of the Cu-based catalyst.17 An enlarged view of the XRD
to a ruffle-like microstructure. As there are some residues of Cu patterns in the range of 40–50ꢁ (Fig. 5b) shows the presence of
cations, which penetrate the Cu2O lattices following the layer- Cu3Si and Cu6.69Si species, suggesting the formation of alloyed
by-layer fashion from inner to outside, ultimately, Cu2O is CuxSi active components through reaction of Cu and Si via
converted to CuO completely (Fig. 4d). The composition of the diffusion during the reaction. In the Rochow reaction, Cu3Si is
supercial oxide layer on Cu undergoes large variations over the normally suggested as the key catalytic active species,45,46 by
temperature range of 0–500 ꢁC.44 As explained, lattice, grain which MCSs, especially M2, are formed. Cu3Si is normally
boundary, and surface diffusion can contribute to the surface created between the Cu catalyst and the Si interface at elevated
growth of oxide grain, blade and ruffle-like microstructure in temperatures.18 The amounts of Cu3Si can substantially affect
the oxidation process. Therefore, by turning the temperature, it the Si conversion and M2 selectivity.19 Although the intensity of
is able to control the content of Cu, Cu2O, and CuO, and the Cu peaks for all the waste contact masses are almost similar, a
surface morphology for the Cu-based catalysts.
much higher intensity of CuxSi is observed for Cu–Cu2O–CuO-f
than the other samples, suggesting that Cu–Cu2O–CuO-f is
more active in generating CuxSi than the other samples. This
may be due to the proper ternary composition of Cu–Cu2O–CuO
akes which creates a stronger synergistic effect and more
contacts between the catalysts and the solid Si, thus promoting
the formation of active CuxSi phases, which is the key step to
enhance the catalytic activity for M2 production.
Fig. 6a shows the SEM images of the Si particles aer the
reaction using Cu–Cu2O–CuO-f as the catalyst. It can be seen
that the smooth region on the surface of Si particle stems from
the unreacted Si, and coarse or cavity-like region is from reacted
Si, suggesting the occurrence of etching process during the
reaction, consistent with the so-called anisotropic etching
reaction mechanism.28 The element mapping images show the
distribution of the elements Si (Fig. 6b) and Cu (Fig. 6c) on the
surface of the waste contact mass. The element Si image clearly
shows reacted (dark yellow) and unreacted (bright yellow) zones
of Si particle. Similarly, the element Cu image presents a
uniform distribution on the reacted or etched Si surface, sug-
gesting the occurrence of catalytic reaction between Si particles
and Cu-based catalysts. Thus, the Rochow reaction can be
schematically illustrated in Fig. 6d. In the process, a large
quantity of Si is fully mixed with a small quantity of partially
oxidized Cu akes before the reaction. With Cu diffusing into
3.3. Catalytic property
Table 2 shows the catalytic performance of all the samples as
model Cu-based catalysts for M2 synthesis via the Rochow
reaction. It can be seen that the M2 selectivity and Si conversion
of Cu-p is 5.7 and 1.6%, respectively, while that of Cu-f increases
to 54.0 and 16.3%, respectively. This is because the ake
structure and higher surface area of Cu-f offer better contact
between the catalysts and the Si powders,11 which can improve
the catalytic property. For Cu–Cu2O-f, the M2 selectivity and Si
conversion rises up to 62.5 and 24.8%, respectively, and more
importantly, Cu–Cu2O–CuO-f shows much increased values of
82.0 and 32.4% in the rst test (its values are reproducible
during 3 tests), respectively, higher than those of Cu2O–CuO-f
(80.1% M2 selectivity and 30.7% Si conversion). For CuO-f, both
M2 selectivity and Si conversion decrease slightly. These results
demonstrate that the prepared catalysts possessing three
components of Cu, Cu2O, and CuO have the best catalytic
performance among all the samples tested possibly because of
the synergistic effect among Cu, Cu2O, and CuO, which
promotes the catalytic activity for the Rochow reaction.33 The
byproducts such as M1, M3, M1H, M2H, LB, and HB without a
Table 2 Catalytic performance of all the catalysts for Rochow reaction
Product composition (%)
Sample
M1 M2 M3 M1H M2H LB HB C–Si (%)
Cu-p
Cu-f
Cu–Cu2O-f
67.6 5.7 1.9 22.5 0.1
26.7 54.0 2.9 12.1 0.3
23.5 62.5 1.8 10.3 0.5
0.7 1.5 1.6
0.6 3.4 16.3
0.7 0.7 24.8
0.3 0.6 32.4
0.3 0.6 32.5
0.3 0.7 32.1
0.3 0.6 30.7
0.3 0.7 28.3
Cu–Cu2O–CuO-f1st 10.2 82.0 1.9 4.4 0.6
Cu–Cu2O–CuO-f2nd 10.3 82.3 1.7 4.2 0.6
Cu–Cu2O–CuO-f3rd 10.9 81.8 1.5 4.1 0.7
Cu2O–CuO-f
CuO-f
10.5 80.1 1.6 6.2 0.7
10.9 78.4 1.2 7.8 0.7
Fig. 5 XRD patterns of waste contact masses (a) and enlarged view in
the 2q range of 40–50ꢁ (b).
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In summary, we have demonstrated that the multicomponent
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