Highly active and selective Cu/SiO2 catalysts prepared by the urea hydrolysis method in dimethyl oxalate hydrogenation

Article abstract of DOI:10.1016/j.catcom.2011.04.019

Cu/SiO₂ catalysts have been successfully prepared via urea hydrolysis method. The catalysts have been systematically characterized by X-ray diffraction, high-resolution transmission electron microscopy, N ₂-physisorption and H₂ temperature-programmed reduction. The results demonstrated the presence of copper nanoparticles and their high dispersion on the SiO₂ support. Catalysts with different copper loadings were prepared, and their performances in the hydrogenation of dimethyl oxalate to ethylene glycol were studied. A 100% conversion of dimethyl oxalate and maximum 98% selectivity of ethylene glycol were reached with 15.6 wt.% copper loading at 200 °C and 2 MPa. Furthermore, under the same reaction conditions, the catalyst can maintain the selectivity of 90% when the reduction temperature reduced from 350 °C to 200 °C. The high activity and selectivity over the catalyst may be ascribed to the homogenously distribution of copper nanoparticles on the large surface.

Full text of DOI:10.1016/j.catcom.2011.04.019

Catalysis Communications 12 (2011) 1246–1250
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly active and selective Cu/SiO2 catalysts prepared by the urea hydrolysis method
in dimethyl oxalate hydrogenation
⁎
Shurong Wang , Xinbao Li, Qianqian Yin, Lingjun Zhu, Zhongyang Luo
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cu/SiO2 catalysts have been successfully prepared via urea hydrolysis method. The catalysts have been
systematically characterized by X-ray diffraction, high-resolution transmission electron microscopy, N2-
physisorption and H2 temperature-programmed reduction. The results demonstrated the presence of copper
nanoparticles and their high dispersion on the SiO2 support. Catalysts with different copper loadings were
prepared, and their performances in the hydrogenation of dimethyl oxalate to ethylene glycol were studied. A
100% conversion of dimethyl oxalate and maximum 98% selectivity of ethylene glycol were reached with
15.6 wt.% copper loading at 200 °C and 2 MPa. Furthermore, under the same reaction conditions, the catalyst
can maintain the selectivity of 90% when the reduction temperature reduced from 350 °C to 200 °C. The high
activity and selectivity over the catalyst may be ascribed to the homogenously distribution of copper
nanoparticles on the large surface.
Received 19 February 2011
Received in revised form 13 April 2011
Accepted 17 April 2011
Available online 22 April 2011
Keywords:
Cu/SiO2
Urea
Ethylene glycol
Dimethyl oxalate
Hydrogenation
Nanoparticle
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Therefore, it is an ideal alternative catalyst material for the hydrogena-
tion of DMO. A considerable body of work has been devoted to
Ethylene glycol (EG) is an important chemical material, which is
widely used in polyester and dynamite manufacture and in antifreeze
production. Currently, EG is mainly synthesized by petroleum-based
ethylene oxide hydrolysis, which is sharply influenced by the price of
crude oil and strongly limited by the huge energy and water
consumptions of the production process [1]. Hence, considerable global
research effort has been directed towards the development of an
alternative and sustainable approach for EG synthesis, and theprocessof
indirect synthesis from syngas, which is produced by the gasification of
coal and biomass, is becoming ever more attractive [2,3].
The process involves two steps: initial coupling of CO with methyl
nitrite ester toformdimethyl oxalate (DMO), and then hydrogenation of
the DMO to form EG [4,5]. Industrial-scale production of DMO has been
realized, and so increasing attention has been paid to the development
of highly active and stable catalysts for use in the hydrogenation of DMO
to EG. A great deal of work has been carried out in this area. In early
work, ruthenium-based homogeneous catalysts were used for the
hydrogenation of oxalates [6,7]. The yield of EG reached 95% under an H2
pressure of 7 MPa at 100 °C over a Ru-based homogeneous catalyst.
However, the utility of this kind of catalyst proved to be limited due to
problems of corrosion and separation. On the other hand, copper shows
good performance in the hydrogenation of esters to alcohols, displaying
high hydrogenation activity and a low tendency for C\C bond cleavage.
comparing the performances of Cu-based catalysts supported on
different carriers (SiO2, Al2O3, ZnO, La2O3, and mesoporous materials
such as HMS and SBA-15) [8–12]. Due to the weak acidic and basic
properties of SiO2, Cu/SiO2 shows the highest yield of EG in the
hydrogenation of DMO. Moreover, catalyst prepared by the homoge-
neous deposition–precipitation method has been shown to have a
higher activity than that prepared by an impregnation method [13].
However, to the best of our knowledge, the preparations of Cu/SiO2
catalysts by the aforementioned precipitation method have employed
ammonia as a precipitant, whereas the influence of urea, which is also
commonly used as a precipitant, has yet to be studied. Catalysts
prepared by the urea hydrolysis method have a slow precipitation rate
due to the slow decomposition of urea at temperatures of 90–100 °C,
and this results in greater dispersion of the catalytically active
component. In the present work, Cu/SiO2 catalyst prepared by the
urea hydrolysis method has been investigated. The purpose of this study
has been to reveal the effect of the urea hydrolysis method on the
texture and structure of Cu/SiO2 catalysts, and to evaluate the
performance of the prepared catalysts in the hydrogenation of DMO
to EG.
2. Experimental
2.1. Catalyst preparation
Cu/SiO2 catalysts were prepared by the urea hydrolysis method
described below. The requisite quantity of Cu(NO3)2·3H2O (depending
⁎
Corresponding author. Fax: +86 57187951616.
E-mail address: srwang@zju.edu.cn (S. Wang).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.04.019

S. Wang et al. / Catalysis Communications 12 (2011) 1246–1250
1247
on the desired copper content of the catalyst) wasdissolved in deionized
water (300 mL). The solution was then mixed with an appropriate
quantity of urea (the mole ratio of urea to copper was 3:1), and the
mixture was stirred until the urea had dissolved. Subsequently, silica sol
was added to the solution, and the mixture was stirred for a further 4 h.
It was then heated and kept at 90 °C to decompose the urea. When the
pH of the suspension reached 6.5, the heating process was terminated.
The mixture was filtered and the collected solid was washed with
deionized water (500 mL), dried at 120 °C overnight, and calcined in air
at 350 °C for 4 h. The calcined samples were crushed and sieved to 40–
60 mesh. Catalysts with three different copper loadings were prepared,
which we denote as 5 U, 20 U, and 40 U, respectively, where the
numbers 5, 20, and 40 represent the copper loadings (see Table 1).
formed due to the high copper content in the catalyst. In contrast to
that of the 40 U catalyst, the patterns of 5 U and 20 U do not exhibit an
obvious CuO diffraction peak, indicating that the crystalline CuO was
highly dispersed on the SiO2 and was too small to be detected. The
40 U catalyst reduced at 350 °C exhibited four main peaks that can be
assigned to Cu2O at 2θ=36.8°, Cu (111) at 2θ=43.3°, Cu (200) at
2θ=50.4° and Cu (220) at 2θ=74.1° [13]. The peaks of Cu and Cu2O
detected in the reduced 20 U were much weaker and broader than the
peaks in the 40 U. However, there were no visible Cu and Cu2O peaks
in the reduced 5 U, suggesting that the particles were well dispersed
and their sizes were quite small in the 5 U catalyst. The Cu crystallite
size calculated by Scherrer–Warren equation increased from b3 nm to
12.2 nm with the increase of the copper loading (summarized in
Table 1).
2.2. Catalyst characterization
Fig. 2 shows the typical HRTEM images of the calcined and reduced
Cu/SiO2 catalysts. The particle sizes in the calcined samples were in
the range of 3–5 nm in the 5 U and 20 U catalysts. However, a wire-
like structure was observed in the 40 U catalyst, suggesting that a
silicate phase was produced, which is in good agreement with the
results of XRD measurement. The histograms of the particle size
distribution for the reduced catalysts were inserted in the top-right
corner of the HRTEM images. The average particle size of Cu
(summarized in Table 1) was calculated as
The BET surface areas, pore volumes, and pore size distributions of
the prepared catalysts were determined by measuring nitrogen
adsorption–desorption isotherms using a Quantachrom-Autosorb-1-C
apparatus. The real content of copper in the catalyst was analyzed by
inductively coupled plasma atomic emission spectrometry (ICP-AES),
using a Thermo iCAP 6000 device.
Powder X-ray diffraction (XRD) analysis of the prepared catalysts
was carried out on a D/max-Ra X-ray diffractometer with a Cu-Kα
radiation source operated at 40 kV and 30 mA. HRTEM images were
obtained using a Philips Tecnai G2F30 transmission electron
microscope.
∑nid3i
dTEM
=
∑nid2i
Temperature-programmed reduction (TPR) was conducted using an
AutoChem II 2920 instrument. A calcined catalyst sample (30 mg) was
purged with Ar at 100 °C for 30 min and then cooled to room temperature.
The reduction was carried out with 10% H2/Ar (40 mL/min), and the
sample was heated at a rate of 2 °C/min up to 600 °C. The amount of H2
consumed was monitored by a thermal conductivity detector.
where ni is the number of particles having a characteristic diameter di
(within a given diameter range) [14]. Highly dispersed and nano-size
Cu particles were detected in all the catalysts prepared by urea
hydrolysis method. The average sizes of Cu nanoparticles were 4 and
7.6 nm for 5 U and 20 U, respectively. These sizes of Cu nanoparticles
were in good accordance with the unsupported copper nanoparticles
(diameter=3 1.5 nm) and the supported copper nanoparticles on
activated carbon (diameter=6 2 nm) prepared by Francisco et al.
[15,16]. The average particle size of 40 U (17.5 nm) was larger than
the former two.
The physical properties and copper loadings of the prepared Cu/SiO2
catalysts are provided in Table 1. The BET surface area increased from
229 to 336 m2/g when the copper loading was increased from 4.3 to
36.7 wt.%. This might be induced by the formation of copper
phyllosilicate [17]. The N2 adsorption–desorption isotherms of the 5 U,
20 U and 40 U catalysts are shown in Fig. 3A. All of them revealed a type
IV Langmuir adsorption isotherm according to the IUPAC classification,
while the hysteresis loop changed from H1-type to H3-type with copper
2.3. Catalytic activity test
The reactions were carried out in a continuous fixed-bed reactor.
1 g catalyst (about 3 mL) was sandwiched with quartz sand and
packed in a steel tube reactor with an inner diameter of 8 mm. A
thermocouple was inserted into the center of the catalytic bed to
monitor the reaction temperature. Before the reaction, the catalyst
was activated with pure hydrogen at 350 °C for 4 h at a ramping rate
of 4 °C/min. After cooling to the reaction temperature, a solution of
12.5 wt.% DMO in methanol was fed into the preheater by means of a
syringe pump, vaporized, and mixed with the required amount of H2
at 200 °C. The liquid products were condensed and analyzed on an
Agilent 7890 GC equipped with a flame-ionization detector (FID).
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of the as-prepared and reduced Cu/SiO2 catalysts
are shown in Fig. 1. The peak of SiO2 in the 40 U catalyst was shifted
and broadened, suggesting that amorphous SiO2 and silicate were
Table 1
Physical properties and Cu loadings of the prepared Cu/SiO2 catalysts.
a
Catalyst
Cu loading
(wt.%)
SBET
Vpore
Dpore
(nm)
dCu (nm)
(m2 g−1
)
(cm3 g−1
)
b
TEM
XRD
5 U
20 U
40 U
4.3
15.6
36.7
229
319
336
0.75
0.68
0.86
10.4
6.9
9.3
4
7.6
17.5
b 3
5.1
12.2
a
Measured by ICP-AES.
Calculated according to Scherrer–Warren equation, Cu(111).
Fig. 1. XRD patterns of the as-prepared and reduced Cu/SiO2 catalysts. (a) 5 U, (b) 20 U,
(c) 40 U, (d) 5 U after reduction, (e) 20 U after reduction, and (f) 40 U after reduction.
b

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S. Wang et al. / Catalysis Communications 12 (2011) 1246–1250
(A)
(B)
10 nm
10 nm
35
30
25
20
15
10
5
(C)
(D)
0
2.5
3.5
4.5
5.5
Particle size / nm
20 nm
10 nm
45
40
35
30
25
20
15
10
5
16
14
12
10
8
6
4
2
0
(E)
(F)
12
8
10
14
16
18
0
4
6
8
10
12
Particle size / nm
Particle size / nm
10 nm
10 nm
Fig. 2. HRTEM images of the as-prepared and reduced catalysts. (A) 5 U, (B) 20 U, (C) 40 U, (D) 5 U after reduction, (E) 20 U after reduction, and (F) 40 U after reduction.
loading increasing. These results inferred that therewere a large amount
of mesopores in three catalysts and the pores shape changed from
cylindrical channels to slit-like channels. Furthermore, the pore size
distributions curves obtained from the BJH desorption branch are
shown in Fig. 3B. It showed that a bimodal pore structure was formed in
the 20 U catalyst.
Fig. 4 illustrates the H2-TPR profiles of the calcined Cu/SiO2
catalysts with different copper loading. The observation of a narrow
reduction peak indicated that the copper particles were uniformly
dispersed on the SiO2 support. Meantime, the low reduction
temperature (centered at 170 °C) suggested the weak interaction
between copper oxide and SiO2. The slight shift of different copper
loading catalysts may be caused by different copper particle sizes and
different copper oxide dispersions. Besides the main reduction peak,
there was a shoulder peak at 185 °C for the catalysts of 40 U, which
should be ascribed to a double reduction step: Cu2+ →Cu1+ →Cu0
[12]. It was in good agreement with the result of XRD. Furthermore,
the stability of the catalyst can be significantly improved due to the

S. Wang et al. / Catalysis Communications 12 (2011) 1246–1250
1249
3.2. Catalytic performance
The DMO hydrogenation proceeds according to the following
reactions:
CH3OOCCOOCH3 þ 2H2→HOCH2COOCH3 þ CH3OH
ð1Þ
ð2Þ
ð3Þ
HOCH2COOCH3 þ 2H2→HOCH2CH2OH þ CH3OH
HOCH2CH2OH þ H2→CH3CH2OH þ H2O
The catalytic performances of the prepared Cu/SiO2 catalysts in the
gas-phase hydrogenation of DMO to EG are shown in Fig. 5A and B. The
DMO conversion was increased with increasing copper loading, climbing
from 50% to 100% when the copper content reached 15.6 wt.%. The
selectivity for EG on the 20 U and 40 U catalysts also shows a dramatic
improvement compared with that produced by the low copper loading of
the 5 U catalyst. This can be mainly attributed to the dramatic increase of
catalyst surface area and the increase of copper content in the catalyst,
which may increase the number of active sites. The 20 U catalyst showed
the highest selectivity for EG (90%) under the same reaction conditions: a
reaction temperature of 200 °C, an H2/DMO molar ratio of 200, LHSV
maintained at 0.8 h−1, and system pressure maintained at 2 MPa.
However, the selectivity for EG was only 80% when the copper loading
was 36.7 wt.%. Differences in the pore structures, particle sizes, particle
dispersions, and the phases of copper and SiO2 resulted in different
catalytic performances for the 20 U and 40 U catalysts.
Meantime, as shown in Fig. 5B, at an increased H2/DMO molar ratio
(260), the conversion of DMO on the 20 U catalyst was maintained at
100% as the temperature was varied from 195 to 215 °C. The selectivity
for EG first increased and reached 98% when the reaction temperature
was raised to 200 °C, but then decreased to 84% at 215 °C, indicating that
the hydrogenation of DMO over Cu/SiO2 for EG production is a
temperature-sensitive reaction. The selectivity of EG obtained here
was higher than that the results obtained by using deposition–
precipitation method (94%) or incipient wet impregnation (92%)
[12,13], and no less than that obtained by using ammonia-evaporation
method (the selectivity reached 95% and 98% respectively) [11,17].
Furthermore, since the low reduction temperature disclosed in the
TPR measurement, it is suggested that the process of reduction for the
prepared Cu/SiO2 catalysts can be operated under a temperature of
200 °C. The reduction temperature is very important for the copper-
based catalyst, which has a significant effect on catalyst stability in the
industrial application. Aggregation and growth of the copper particles in
Cu/SiO2 catalyst can be effectively inhibited by operating at low
reduction temperature. So the performances of the 20 U catalyst reduced
at low temperature of 200 °C were tested, and the results are shown in
Fig. 5C. It demonstrated that the conversion of DMO reached 100% and
the selectivity of EG can reached 90%, suggesting that the Cu/SiO2
catalyst prepared via urea hydrolysis method may extend the catalyst
life-time. The changes of the catalytic performance with temperature
variation were in good accordance with the 20 U catalyst which was
reduced at 350 °C. What's more, it was also confirmed that the
hydrogenation of DMO to EG over Cu/SiO2 catalyst was a temperature-
sensitive reaction.
Fig. 3. Nitrogen adsorption–desorption isotherms (A) and pore size distributions (B) of
the prepared catalysts. (a) 5 U, (b) 20 U, and (c) 40 U.
relatively low temperature at which the reduction is conducted,
which reduces the aggregation and growth of the copper particles. We
suggest that the Cu/SiO2 catalyst prepared by urea hydrolysis
represents an effective material for overcoming the bottleneck of
Cu/SiO2 catalyst stability in the industrial production of EG by DMO
hydrogenation.
4. Conclusions
The presented work has demonstrated that the Cu/SiO2 catalysts
prepared by urea hydrolysis method have nano-size and highly
dispersed copper particles. Coupled with an efficient copper loading,
high surface area and bimodal pore structure, the prepared catalysts
showed high catalytic performance in the gas-phase hydrogenation of
Fig. 4. H2-TPR of the prepared Cu/SiO2 catalysts (a) 5 U, (b) 20 U, and (c) 40 U.

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S. Wang et al. / Catalysis Communications 12 (2011) 1246–1250
that the hydrogenation of DMO for EG production over Cu/SiO2
catalyst was a temperature-sensitive reaction.
Furthermore, the influences of the reduction temperature at
350 °C and 200 °C on the catalyst performance were also discussed.
It was found that the catalyst also exhibited a high DMO conversion
(100%) and EG selectivity (90%) when the reduction temperature was
reduced to 200 °C. The relatively low reduction temperature of the
copper-based catalyst would suggest that the method of Cu/SiO2
catalyst preparation via urea hydrolysis is an effective way to
overcome the bottleneck of Cu/SiO2 catalyst stability in the industrial
production of EG by DMO hydrogenation.
Acknowledgement
The authors are grateful for financial support from the Doctoral
Program of Higher Education of China (20090101110034), the Zhejiang
Provincial Natural Science Foundation of China (R1110089), the Interna-
tional Science and Technology Cooperation Program (2009DFA61050),
the National High Technology Research and Development Program of
China (2009AA05Z407), and the National Basic Research Program of
China (2007CB210200).
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Fig. 5. (A) Catalytic performances of the prepared Cu/SiO2 catalysts with different
copper mass concentration in the hydrogenation of DMO. Reaction conditions:
p=2 MPa, T=200 °C, H2/DMO=200 mol/mol, and LHSV=0.8 h−1. (B) Variations in
catalytic activity of the 20 U catalyst reduced at 350 °C with reaction temperature.
Reaction conditions: p = 2 MPa, H2/DMO = 260 mol/mol, and LHSV =0.8 h− 1
.
(C) Variations in catalytic activity of the 20 U catalyst reduced at 200 °C with reaction
temperature. Reaction conditions are the same with (B).
DMO for EG production. The catalyst with a 15.6 wt.% copper loading
showed the highest DMO conversion (100%) and selectivity for EG
(98%) under the optimized reaction conditions. Besides, it was found