MgO: An excellent catalyst support for CO oxidative coupling to dimethyl oxalate

Article abstract of DOI:10.1039/c4cy00245h

Pd/MgO catalysts are found, for the first time, to be extraordinarily active and stable for CO oxidative coupling to dimethyl oxalate. A series of Pd/MgO catalysts with Pd loadings of 0.1, 0.3, 0.5, 1 and 2 wt% were prepared by a wet impregnation method and systematically characterized by XRD, TEM, ICP, UV-DRS, H₂-TPR and CO₂-TPD. It has been demonstrated that the amount of Pd loading has a pronounced effect on the catalytic activity for CO oxidative coupling to dimethyl oxalate. CO conversion increases with the increase of the Pd loading due to high dispersion and similar sizes of Pd nanoparticles, as well as, the increase in number of surface active sites. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4cy00245h

Volume 4 Number 7 July 2014 Pages 1839–2162
Catalysis
Science &
Technology
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ISSN 2044-4753
PAPER
Zhong-Ning Xu, Guo-Cong Guo et al.
MgO: an excellent catalyst support for CO oxidative coupling to
dimethyl oxalate

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Technology
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MgO: an excellent catalyst support for CO
oxidative coupling to dimethyl oxalate
Cite this: Catal. Sci. Technol., 2014,
4, 1925
Si-Yan Peng,ab Zhong-Ning Xu, a Qing-Song Chen,a Zhi-Qiao Wang,a Yumin Chen,a
*
a
a
a
*
Dong-Mei Lv, Gang Lu and Guo-Cong Guo
Pd/MgO catalysts are found, for the first time, to be extraordinarily active and stable for CO oxidative
coupling to dimethyl oxalate. A series of Pd/MgO catalysts with Pd loadings of 0.1, 0.3, 0.5, 1 and 2 wt%
were prepared by a wet impregnation method and systematically characterized by XRD, TEM, ICP,
UV-DRS, H2-TPR and CO2-TPD. It has been demonstrated that the amount of Pd loading has a
pronounced effect on the catalytic activity for CO oxidative coupling to dimethyl oxalate. CO conversion
increases with the increase of the Pd loading due to high dispersion and similar sizes of Pd nanoparticles,
as well as, the increase in number of surface active sites.
Received 25th February 2014,
Accepted 16th April 2014
DOI: 10.1039/c4cy00245h
www.rsc.org/catalysis
oxidative coupling to DMO remains controversial. Pd based
catalysts supported on other supports with high activity for CO
1. Introduction
Dimethyl oxalate (DMO) is an important chemical raw material
and intermediate for the syntheses of oxalic acid, oxamide,
dyes, pharmaceuticals, etc.1 More importantly, hydrogenation
of DMO can also be used to synthesize ethylene glycol (EG),2–5
which is a considerable chemical feedstock with a global
demand of about 25 million tons each year. Commercial EG is
mainly produced from ethylene oxide hydrolysis.6 Considering
the soaring price of crude oil and the depletion of petroleum
resources, a new EG synthesis technology called coal to ethyl-
ene glycol (CTEG) developed by our institute is attracting grow-
ing interest because of its green and atom economy.7,8 CTEG
includes three main processes: 1) elimination of a small amount
of hydrogen gas in CO separated from coal-derived synthesis
gas; 2) CO oxidative coupling to DMO: 2CH3ONO + 2CO →
(COOCH3)2 + 2NO; and 3) hydrogenation of DMO to EG. Among
these, CO oxidative coupling to DMO is the crucial step to realize
the conversion of inorganic C1 to organic C2 in CTEG.9
Pd/α-Al2O3 has been widely considered as an active cata-
lyst for CO oxidative coupling to DMO.10–14 However, there
are still several problems to be solved despite considerable
efforts that have been dedicated to the study of the CO oxida-
tive coupling process. To date, the Pd loading of an industrial
catalyst for CO oxidative coupling to DMO is relatively high at
2 wt% (the state of the art), resulting in a greatly increased
cost of production. Moreover, the catalytic mechanism of CO
oxidative coupling to DMO are rather limited. Pd catalysts with
different supports for CO oxidative coupling to DMO have been
summarized by Uchiumi et al., and the results have demon-
strated that Pd catalysts supported on NaY zeolites, silica,
activated alumina, and activated carbon show much lower activ-
ities compared to Pd/α-Al2O3.15 Zhao et al.1 reported a high effi-
ciency Pd based catalyst supported on carbon nanofibers for
the synthesis of DMO, but the Pd loading of the catalyst was
still high (1 wt%). It is well established that the support effect
has a significant influence on many heterogeneous catalytic
reactions, such as CO oxidation,16,17 methane combustion,18
hydrogenation of nitrobenzene,19 ethylene oxidation,20 and
acrylonitrile decomposition.21 Acid–base and redox properties
of the support and metal–support interactions can greatly affect
the catalytic performance of supported catalysts. Therefore,
designing efficient Pd based nanocatalysts by making use of
the support effect is necessary and important from the points
of view of both fundamental study and industry application.
It is desirable and practical to develop high performance
Pd based catalysts supported on other supports for CO oxida-
tive coupling to DMO. In this work, we firstly discovered an
excellent MgO support and further developed a low Pd load-
ing (ca. 0.5 wt%) Pd/MgO catalyst with high activity, selectiv-
ity and stability for CO oxidative coupling to DMO.
2. Experimental
2.1. Catalyst preparation
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002,
PR China. E-mail: gcguo@fjirsm.ac.cn; Fax: +86 591 83714946;
Tel: +86 591 83512502
b Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Chinese
Academy of Sciences, Fuzhou, Fujian 350002, PR China
A series of Pd/MgO catalysts with 0.1, 0.3, 0.5, 1 and 2% (by
weight) Pd loadings were prepared by a wet impregnation
method and by controlling the amount of the palladium ace-
tate precursor. The MgO support was impregnated with an
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acetone solution of palladium acetate under vigorous mag-
netic stirring and then the mixed solution was heated at 50 °C
until the acetone was evaporated completely. The sample was
calcined at 400 °C for 4 h to form PdO/MgO. Prior to the activ-
ity evaluation, the calcined sample was reduced in a hydrogen
atmosphere to obtain Pd/MgO. The calcined samples were
denoted as 0.1%-PdO/MgO, 0.3%-PdO/MgO, 0.5%-PdO/MgO,
1%-PdO/MgO and 2%-PdO/MgO, and the reduced catalysts were
thus denoted as 0.1%-Pd/MgO, 0.3%-Pd/MgO, 0.5%-Pd/MgO,
1%-Pd/MgO and 2%-Pd/MgO.
Other supported Pd catalysts such as Pd/α-Al2O3, Pd/TiO2,
Pd/γ-Al2O3 and Pd/AC (activated charcoal) were prepared by the
same method for comparison. The theoretical Pd loadings of
these catalysts were 0.5 wt%, and the actual Pd loadings were
measured by Inductively Coupled Plasma (ICP) spectroscopy.
(50 mL min−1) for 1 h, followed by purging with helium for
30 min, and then heated to 800 °C by ramping at 10 °C min−1
under flowing helium.
2.3. Activity evaluation
The activity of the catalysts for CO oxidative coupling to DMO
was evaluated in a fixed-bed quartz tubular reactor. The catalysts
(200 mg) were placed in the center of the quartz tubular reactor.
The reactant gases (28% CO, 20% CH3ONO, 4% Ar and N2 bal-
ance) were passed through the reactor at a gas hourly space
velocity (GHSV) of 3000 h−1. The catalytic activity tests were
performed under atmospheric pressure. The composition of the
reactant gases and reaction products was monitored using an
on-line Shimadzu GC-2014 gas chromatograph equipped with a
thermal conductivity detector and a flame ionization detector.
The conversion of CO, the selectivity to DMO and the
space-time yields (STY) of DMO were calculated using the fol-
lowing formulas:
2.2. Catalyst characterization
Powder X-ray diffraction (XRD) patterns were measured on a
glass wafer using a Rigaku MiniFlex II diffractometer with a
Cu-Kα X-ray source (λ = 1.5406 Å) at a scan speed of 5° min−1
(2θ). The X-ray tube was operated at 40 kV and 30 mA. Sam-
ples for transmission electron microscopy (TEM) observations
were prepared by drying a drop of diluted ethanol dispersion
of Pd/MgO catalysts on copper grids. Images were obtained by
TEM (Tecnai G2 F20) carried out at 200 kV. ICP elemental analy-
sis measurements were carried out using an Ultima 2 plasma
emission spectrometer from Jobin Yvon. UV-visible diffuse
reflectance spectra (UV-DRS) were measured using a PE Lambda
950 spectrophotometer equipped with a diffuse reflectance
accessory and were recorded in the range of 400–850 nm using
BaSO4 as the reference sample. Temperature programmed
reduction (TPR) and temperature programmed desorption
(TPD) of CO2 experiments were carried out using an Altamira
AMI-300 instrument equipped with a thermal conductivity
detector (TCD). Prior to conducting the H2-TPR experiment,
100 mg of sample placed in a quartz U-tube was first heated to
400 °C for 1 h under a flow of argon (30 mL min−1). After
cooling to room temperature, the sample was exposed to 10%
H2–Ar mixture (30 mL min−1) and then heated to 500 °C at a
rate of 10 °C min−1. The basic properties of the catalysts were
examined by the CO2-TPD technique. In a typical experiment,
100 mg of sample was treated with helium at 400 °C for 2 h to
remove the adsorbed impurities. After cooling to 25 °C under a
helium flow, the sample was exposed to 40% CO2–He mixture
Conversion of CO (%) = (1 − ([Ar]in/[Ar]out)/([CO]in/[CO]out)) × 100%
Selectivity to DMO (%) = (SDMO × R-FDMO)/(SDMO × R-FDMO
+ SDMC × R-FDMC) × 100%
STY of DMO (g L−1 h−1) = conversion of CO
× selectivity to DMO × GHSV of CO
× 118.09 g mol−1/(2 × 22.4 L mol−1)
where [Ar]in and [Ar]out are the concentrations of Ar at the
inlet and outlet, [CO]in and [CO]out are the concentrations of
CO at the inlet and outlet, respectively, SDMO and SDMC are
the peak areas of dimethyl oxalate and dimethyl carbonate
and R-FDMO and R-FDMC are the relative correction factors of
dimethyl oxalate and dimethyl carbonate, respectively.
3. Results and discussion
3.1 Catalytic activity
The evaluation of the catalytic performance for CO oxidative
coupling to DMO was carried out using a home-made cata-
lytic evaluation device. Table 1 summarizes the catalytic per-
formance for CO oxidative coupling to DMO over Pd/MgO,
Table 1 CO oxidative coupling to DMO on different catalystsa
Catalysts
Actual Pd loading (%)
Conversionb (%)
Selectivityc (%)
STYd (g L−1 h−1
)
Pd/MgO
Pd/α-Al2O3
Pd/TiO2
Pd/γ-Al2O3
Pd/AC
0.42
0.46
0.43
0.41
0.42
63
56
26
20
15
97
94
90
88
89
1353
1166
518
390
296
a
Reaction conditions: 200 mg of the catalyst, theoretical Pd loading of 0.5 wt%, gas hourly space velocity (GHSV) of 3000 h−1, reactants
b
c
d
CO/CH3ONO volume ratio: 1.4, 0.1 MPa, 130 °C. Conversion of CO. Selectivity to DMO. STY represents the space-time yield in grams of
DMO per liter of the catalyst per hour (g L−1 h−1).
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Pd/α-Al2O3, Pd/TiO2, Pd/γ-Al2O3 and Pd/AC catalysts, which
were prepared by the same method and evaluated under
the same reaction conditions. It is interesting to find that
Pd/MgO exhibits remarkable catalytic performance for CO oxi-
dative coupling to DMO, even exceeding that of Pd/α-Al2O3,
whereas other supported catalysts show much lower activities
for this reaction. The CO oxidative coupling activity is in the
order of Pd/MgO > Pd/α-Al2O3 > Pd/TiO2 > Pd/γ-Al2O3 > Pd/AC,
which demonstrates that the catalytic activity for CO oxidative
coupling to DMO is strongly dependent on the nature of the
support materials.
Fig. 1 shows CO conversion over Pd/MgO catalysts with
different Pd loadings as a function of reaction temperature.
The temperature dependence of CO conversion indicates that
the activity of Pd/MgO catalysts for CO oxidative coupling to
DMO increases gradually as the temperature is increased
from 90 to 150 °C. Moreover, the amount of Pd loading
also has a significant influence on the catalytic activity for
CO oxidative coupling to DMO. By and large, CO conversion
increases with the increase of Pd loading, but not in propor-
tion to it. However, the high Pd loading will result in a greatly
increased cost of catalysts due to the expensive price and
extreme shortage of the Pd noble metal. The 0.5%-Pd/MgO
catalyst exhibits high activity and selectivity with 63% CO con-
version and 97% selectivity to DMO under the same reaction
conditions as those for the industrial catalyst. Moreover, CO
conversion can reach up to 68.7% at 150 °C, close to the theo-
retical maximum value of 71.4%. Therefore, a Pd loading of
around 0.5 wt% is identified as the optimum content from
both catalytic performance and economical points of view.
The stability investigation of the 0.5%-Pd/MgO catalyst was
carried out with a duration lifetime test for 120 h at 130 °C.
As shown in Fig. 2, the excellent catalytic performance can be
stable over 120 h, which lays a good foundation for further
extending the long-term stability. Furthermore, Pd/MgO was
prepared via a wet impregnation method, which is flexible
in controlling the Pd loading and suitable for its large-scale
production.
Fig. 2 Space-time yield of DMO over a 0.5%-Pd/MgO catalyst (200 mg)
at 130 °C for 120 h.
3.2 Catalyst characterization
Fig. 3 presents the TEM images and the size distribution his-
tograms of Pd/MgO catalysts. Obviously, the Pd nanoparticles
of all catalysts are highly dispersed on the surface of the
MgO support, suggesting that the Pd nanoparticles did not
aggregate with the increase of the Pd loading. The number of
Pd nanoparticles increased significantly with the increase of
the Pd loading, but the average size of the Pd nanoparticles
remained nearly unchanged. The average sizes of the Pd nano-
particles over all catalysts are in the range of 4.1–4.5 nm. It
should be mentioned that the size statistics of the 0.1%-Pd/MgO
catalyst cannot be obtained since few Pd nanoparticles can be
observed (Fig. 3a). However, the size of the existing Pd nano-
particles over the 0.1%-Pd/MgO catalyst is similar to those of
other catalysts. In addition, the size distributions of Pd nano-
particles over all catalysts are very narrow and uniform and
show no marked differences. Therefore, the number of active
sites will increase as the Pd loading increases from 0.1 to 2 wt%,
which contributes mainly to the increasing catalytic activity
for CO oxidative coupling to DMO (Fig. 1). It should be noted
that the morphology and the size distribution of Pd nano-
particles of 0.5%-Pd/MgO after the durability test for 120 h
(Fig. 3f) are very similar to those of the fresh catalyst (Fig. 3c).
This implies that neither aggregation nor sintering of the Pd
nanoparticles of 0.5%-Pd/MgO occurred during the lifetime
evaluation process, which illustrates the high stability of the
0.5%-Pd/MgO catalyst.
Fig. 4 shows the XRD patterns of Pd/MgO catalysts with
different Pd loadings. The sharp diffraction peaks are
observed for all of the five catalysts at 2θ = 37.1, 43.1, 62.4,
74.8, and 78.6°, which are assigned to the crystalline
phase of the MgO support.22 No Pd diffraction peaks are
observed for the 0.1%-Pd/MgO, 0.3%-Pd/MgO, 0.5%-Pd/MgO
and 1%-Pd/MgO catalysts, indicating that the Pd nano-
particles are highly dispersed on the surface of MgO or that
the amounts of Pd loading are too small to be detected.
When the Pd loading is raised to 2 wt%, there is a new weak
diffraction peak at 40° observed for the 2%-Pd/MgO catalyst,
corresponding to the face-centered cubic (fcc) Pd (111) char-
acteristic diffraction.
Fig. 1 Conversion of CO over Pd/MgO catalysts with different Pd
loadings for CO oxidative coupling to DMO at different reaction
temperatures.
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Fig. 3 TEM images of 0.1%-Pd/MgO (a), 0.3%-Pd/MgO (b), 0.5%-Pd/MgO (c), 1%-Pd/MgO (d), 2%-Pd/MgO (e), and 0.5%-Pd/MgO after 120 h of
lifetime evaluation (f). The insets correspond to the size distributions of Pd nanoparticles supported on MgO.
Fig. 4 XRD patterns of Pd/MgO catalysts with different Pd loadings.
UV-DRS investigations were carried out for the characteri-
zation of the PdO species. Fig. 5a presents the UV-DRS pro-
files of the pure MgO support and the calcined PdO/MgO
samples with various PdO loadings. For the pure MgO sup-
port, there are no absorption bands observable above 400 nm,
while for PdO/MgO, there is only one broad absorption band
with a maximum at about 620 nm, which can be assigned to
the optical absorption edge of PdO.23 The Kubelka–Munk
function (α/S = (1 – R)2/2R) is often used to evaluate the band
Fig. 5 (a) UV-DRS profiles of the pure MgO support and the calcined
samples (PdO/MgO). (b) Tauc plots constructed from (a) according to
the Kubelka–Munk function α/S = (1–R)2/2R; the ordinate values (α/S)
gap of a semiconductor material in terms of the Tauc plot,24
which is constructed by plotting the Kubelka–Munk function.
The determined band gap of the PdO increases with the
have been normalized.
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increase of PdO loading (Fig. 5b), indicating a blue shift of
the absorption edge. There is a significant difference (0.16 eV)
between the band gaps of 0.1%-PdO/MgO and 2%-PdO/MgO.
The UV-DRS results suggest that the coordination environment
of the palladium species was changed due to the interaction
between PdO and the MgO support, which was significantly
affected by the PdO loading.
The results of the TPR experiments for the calcined sam-
ples (PdO/MgO) are presented in Fig. 6. A single peak was
observed for all of the calcined samples and the intensity
increased gradually along with the increase of the PdO load-
ing. This reveals that the PdO species are reduced to metallic
Pd and the amounts of H2 consumed are closely associated
with the PdO loading. Moreover, it should be noted that the
reduction peak position of the maxima shifts towards higher
temperatures from 73–93 °C as the PdO loading increases.
This shift can be interpreted as the result of the enhance-
ment of the interaction between PdO species and the MgO
support, based on the characterization of UV-DRS. The strong
interaction between PdO species and the MgO support can
effectively prevent the aggregation and sintering of Pd nano-
particles during the hydrogen reduction process at high tem-
perature, which can be verified by the TEM images (Fig. 3).
Fig. 7 displays the TPD profiles of CO2 adsorbed on the
pure MgO support and Pd/MgO catalysts with different Pd
loadings. Several CO2 desorption peaks are observed for all
samples which can be classified into three types of basic sites
with different strengths, i.e., “weak” (CO2 desorption between
27 and 147 °C), “medium” (CO2 desorption between 147 and
377 °C), and “strong” (CO2 desorption above 377 °C) basic-
ity.25 The “weak” basic sites are probably associated with
Brønsted basicity and most likely originate from the lattice-
bound OH groups. The “medium” and “strong” sites are
probably correlated to Lewis basicity originating from the
three- and four-fold-coordinated O2− anions.25 The CO2-TPD
profiles of Pd/MgO catalysts are similar to that of the pure
MgO support, indicating that the surface basicity sites of
the MgO support remained unchanged during the prepara-
tion of catalysts. A similar phenomenon was also observed by
Shen et al.26
Fig.
7
CO2-TPD profiles of the pure MgO support and Pd/MgO
catalysts with different Pd loadings.
4. Conclusions
In summary, we firstly found that basic MgO can serve as an
excellent support for Pd catalysts for CO oxidative coupling to
DMO. The amount of Pd loading significantly affects the cat-
alytic performance for CO oxidative coupling to DMO. The
activity of the Pd/MgO catalyst increases with the increase of
the Pd loading because of the high dispersion and similar
sizes of the Pd nanoparticles, as well as the increase in
number of surface active sites. The low Pd loading (ca. 0.5%)
Pd/MgO catalyst exhibits excellent activity, selectivity and
stability with 63% CO conversion and 97% selectivity to DMO
after more than 120 h on stream at 130 °C. This work pro-
vides insight into designing high-performance catalysts for
CO oxidative coupling to DMO by choosing appropriate sup-
ports. Further experimental and theoretical studies are still
required to gain more insight into the origin of the role of
the supports.
Acknowledgements
We gratefully acknowledge financial support from the 973
Program (2011CBA00505, 2013CB933200), the NSF of China
(21003126, 21203200, 21303202), and the NSF of Fujian
Province (2013J05034).
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