Effects of Mo promoters on the Cu-Fe bimetal catalysts for the DMC formation from CO2 and methanol

Article abstract of DOI:10.1016/j.cclet.2013.02.001

The Mo-promoted Cu-Fe bimetal catalysts were prepared and used for the formation of dimethyl carbonate (DMC) from CO₂ and methanol. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), laser Raman spectra (LRS), energy dispersive spectroscopy (EDS) and temperature programmed desorption (TPD) techniques. The experimental results demonstrated that the Mo promoters can decrease the reducibility and increase the dispersion of Cu-Fe clusters. The concentration balance of base-acid sites can be readily adjusted by changing the Mo content. The moderate concentration balance of acid and base sites was in favor of the DMC formation. Under optimal experimental conditions, the highest methanol conversion of 6.99% with a DMC selectivity of 87.7% can be obtained when 2.5 wt% of Mo was loaded.

Full text of DOI:10.1016/j.cclet.2013.02.001

Chinese Chemical Letters 24 (2013) 307–310
Contents lists available at SciVerse ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Effects of Mo promoters on the Cu–Fe bimetal catalysts for the DMC formation
from CO2 and methanol
a
a
b
a,
Ying-Jie Zhou a, Min Xiao a, , Shuan-Jin Wang , Dong-Mei Han , Yi-Xin Lu , Yue-Zhong Meng
*
*
a The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province/State Key Laboratory of Optoelectronic Materials and Technologies,
Sun Yat-sen University, Guangzhou 510275, China
b Department of Chemistry & Medicinal Chemistry Program, Office of Life Sciences, National University of Singapore, Singapore 117543, Republic of Singapore
A R T I C L E I N F O
A B S T R A C T
Article history:
The Mo-promoted Cu–Fe bimetal catalysts were prepared and used for the formation of dimethyl
carbonate (DMC) from CO2 and methanol. The catalysts were characterized by X-ray diffraction (XRD),
temperature programmed reduction (TPR), laser Raman spectra (LRS), energy dispersive spectroscopy
(EDS) and temperature programmed desorption (TPD) techniques. The experimental results
demonstrated that the Mo promoters can decrease the reducibility and increase the dispersion of
Cu–Fe clusters. The concentration balance of base–acid sites can be readily adjusted by changing the Mo
content. The moderate concentration balance of acid and base sites was in favor of the DMC formation.
Under optimal experimental conditions, the highest methanol conversion of 6.99% with a DMC
selectivity of 87.7% can be obtained when 2.5 wt% of Mo was loaded.
Received 31 December 2012
Received in revised form 21 January 2013
Accepted 24 January 2013
Available online 14 March 2013
Keywords:
Dimethyl carbonate
Carbon dioxide
Catalysis
ß 2013 Min Xiao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Bimetal catalyst
1. Introduction
CO2 because of their unusual electronic structures and ability to
enhance catalytic properties [19]. In our previous study, we have
The incorporation of carbon dioxide (CO2) into industrially
useful organic products is receiving significant attention as CO2 is
abundant, inexpensive and non-toxic [1]. One of the promising
reactions in this area is the direct synthesis of dimethyl carbonate
(DMC) from CO2 and methanol [2]. DMC has increasingly attracted
interest for its benign nature and wide applications. It is used as an
alternative to the notorious phosgene in several organic transfor-
mations [3], starting materials for polycarbonates resin, electrolyte
of lithium-ion batteries, an octane booster in gasoline and so on [4].
In recent years, a variety of catalysts for this reaction have
been investigated, such as organometallic compounds [5], metal
(IV) tetra alkoxide [6], potassium carbonate [7], zirconia-based
[8–10] catalysts, cerium-based [11,12] catalysts, heteropoly
compounds [13], H3PO4–V2O5 [14], Cu–Ni bimetallic [15–18]
supported on different carriers. However, these catalysts suffer
from tedious preparation procedures or agglomeration of active
components, which might restrict their effective use and wide
application from the industrial point of view. Transition group
elements have drawn considerable attention to interact with
investigated the Cu–Fe bimetal catalysts in the direct DMC
formation from CO2 and methanol. It was found that catalysts
with moderate molar ratios of acid–base sites were in favor of
the DMC formation [20]. However, the DMC yields were not very
satisfactory, and the development of catalyst that combined
large amount of acid and base sites with desirable catalytic
activity was necessary. On the other hand, the addition of a
second or third metal component to a catalyst will improve the
dispersion or alter the electronic structure of a catalyst. More
importantly, the presence of
a promoter can modify the
adsorption characteristics of the catalyst surface, change the
reducibility of the catalyst or in certain cases alter the catalytic
performance [21]. There have been many papers reporting the
effects of Mo as a structural promoter on catalytic properties of
Fe-based catalysts [22–24]. The addition of Mo has beneficial
effects on the stabilization and dispersion of iron on the catalyst
support. Moreover, the intrinsic acid–base sites on molybdenum
oxides could enhance the reactions. Therefore, it could be
expected that the addition of molybdenum to a Cu–Fe catalyst
might have potential advantages for the DMC formation from
CO2 and methanol. In this contribution, the Mo-promoted Cu–Fe
bimetal catalysts were prepared and evaluated for the DMC
formation from CO2 and methanol. The effects of Mo contents on
the activity and structure of the catalysts are reported.
* Corresponding authors.
E-mail addresses: stsxm@mail.sysu.edu.cn (M. Xiao),
mengyzh@mail.sysu.edu.cn (Y.-Z. Meng).
1001-8417/$ – see front matter ß 2013 Min Xiao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.02.001

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Y.-J. Zhou et al. / Chinese Chemical Letters 24 (2013) 307–310
2. Experimental
2.1. Catalyst preparation
2.3. Catalytic reactions
Catalytic activity tests were performed in a continuous tubular
fixed-bed micro-reactor with catalyst weight of 2 g. Pure N2 was
fed into the reactor to exhaust the air inside before the catalyst bed
was heated to the desired temperature. Methanol was introduced
by passage of the CO2 carrier gas through a saturator maintained at
35 8C. Methanol-saturated CO2 stream was heated in a pre-heater
before it was passed into the reactor. The formation of DMC from
CO2 and methanol was carried out subsequently under the
following reaction conditions: pressure (P): 0.6 MPa, temperature
(T): 120 8C, space velocity (SV): 360 hÀ1. All the effluent gases were
analyzed by gas chromatographs (GC) equipped with a flame
ionization detector (FID). The condensed liquid collected by the
cooling separator was analyzed by a gas chromatograph mass
spectrometer (GCMS-QP2010 plus) to confirm the DMC formation
in this reaction. The main reaction products of CO2 reacting with
methanol over the catalysts were DMC and H2O, and there were
several by-products such as CH2O, CO and dimethyl ether (DME).
The detailed description of the reactor and product analysis have
been provided elsewhere [20].
Mo-promoted Cu–Fe-based catalysts were prepared in two
steps. First, Mo was loaded on silica (China GBS Hightech &Industry
Co., Ltd., HL-380) using wet impregnation methods. Appropriate
amounts of (NH4)6Mo7O24Á4H2O were dissolved in deionized water
and impregnated over silica. The samples were then dried in a
rotary evaporator and calcined at 500 8C for 5 h to yield the
xMo/SiO2 (x = 1, 2.5, 4, 5.5 or 7 wt%). Afterward, the Cu–Fe bimetal
(CuO + Fe2O3 = 15 wt%, Cu:Fe = 3:2) were loaded on xMo/SiO2
using the citric acid method [25]. An aqueous solution of citric
acid and the metal precursors such as iron nitrate, copper nitrate
were impregnated over xMo/SiO2, followed by heating at 60 8C for
1 h until the mixture became gelatinous. The as-prepared samples
were dried at 120 8C overnight before being subjected to
calcinations at 500 8C in air for 4 h. All catalysts were reduced
in a hydrogen stream at 600 8C for 4 h, prior to each catalytic
measurement.
2.2. Catalyst characterizations
3. Results and discussion
Powder X-ray diffraction (XRD) measurements were performed
on a Rigaku D-Max 2200 diffractometer using graphite-filtered Cu
3.1. Catalytic activity
K
radiation (l = 0.154178 nm) at 40 kV and 30 mA. The crystallite
a
size of the particles was estimated from the XRD peak widths using
the Scherer equation. The temperature programmed reduction
(TPR) profiles of calcined catalyst precursors were recorded on a
Quantachrom Chem-BET 3000 apparatus. About 50 mg sample
placed in a quartz U-shaped tube was pretreated at 300 8C for 1 h in
a flow of N2 (60 mL/min) to remove the adsorbed water and other
contaminants followed by cooling to 30 8C. The reducing gas 5%
H2/N2 was passed through the samples at a flow rate of 80 mL/min,
with a rate of temperature rise of 8 8C/min to 750 8C. The signal of
H2 consumption by the samples with increasing temperatures was
recorded by a thermal conductivity detector (TCD). The laser
Raman spectra (LRS) of the catalysts was obtained on a laser Raman
apparatus (Renoshaw inVia) with a 514.5 nm line of an Ar+ laser as
the excitation source. The as-prepared catalysts were analyzed by
an energy dispersive spectroscopy (EDS) in order to confirm the
elements on the catalyst surface. The samples were placed on
conductive double-faced tape and then placed in a scanning
electron microscopy (JSM-5600LV system of JEOL) equipped with
an energy dispersive X-ray detector. The accelerating voltage was
15 kV. The temperature programmed desorption (TPD) experi-
ments were carried out on a Quantachrom Chem-BET 3000
apparatus. A flow (50 mL/min) of adsorbate gas (pure CO2 or NH3)
was introduced into the U-shape quartz filled with 100 mg of
reduced catalyst at 50 8C for 1 h, and then the degassing process
was performed at a heating rate of 8 8C/min under N2 flow
(30 mL/min), and CO2 or NH3 desorbed was detected by a TCD.
The catalytic performances and EDS results of the Mo-promoted
3Cu–2Fe/SiO2 catalysts in the DMC formation from CO2 and
methanol are shown in Table 1. The addition of Mo element could
significantly improve the catalytic performance of the Cu–Fe
bimetal catalysts, and with an increase of Mo element loading
contents up to 2.5 wt%, the methanol conversion increased to a
maximum methanol conversion of 6.99% with a DMC selectivity of
87.7%. Further increasing Mo loading contents led to a decrease in
methanol conversion. Compared to the unpromoted 3Cu–2Fe/SiO2,
the DMC selectivity was improved at low Mo contents, but
gradually decreased with the increase of Mo element contents. The
EDS results of the catalysts were slightly lowered than the nominal
loaded contents in the synthetic route, but probably within the
range of experimental error.
3.2. Catalyst structure characterizations
The presence of any crystalline phases in the catalysts
characterized by XRD is shown in Fig. 1. When the Mo-promoted
catalyst precursor was calcined (Fig. 1(a)), copper and iron oxides
showed highly crystallized monoclinic CuO, rhombohedral
a-
Fe2O3 and orthorhombic MoO3. For all catalysts with various Mo
loading contents (Fig. 1(b)–(g)), three main peaks located at
around
characteristic diffraction of metallic Cu, while two broad peaks
located at around 2 = 35.58, 62.58 were assigned to the diffraction
2u = 43.28, 50.48 and 74.18 were attributed to the
u
Table 1
The dependence of catalytic performances and surface properties on Mo contents over the catalystsa.
Mo contents (%)
Methanol conversion (%)
DMC selectivity (%)
DMC yield (%)
EDSb
Cu
Fe
Mo
0
4.72
5.59
6.99
6.24
5.87
5.31
4.08
5.05
6.13
5.35
4.96
4.42
86.5
90.4
87.7
85.8
84.6
83.3
7.11
7.03
6.95
6.84
6.71
6.38
4.08
4.04
3.89
3.78
3.67
3.78
0
1
0.91
2.32
3.86
5.29
6.73
2.5
4
5.5
7
a
All catalysts were reduced in H2 stream at 600 8C for 4 h. Reaction conditions: catalyst weight: 2 g; P = 0.6 MPa, T = 120 8C, SV =360 hÀ1; time on stream: 7 h. The data are
the peak values obtained with time on stream.
b
Five parts of each sample was analyzed by EDS. The data were the average values.

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Y.-J. Zhou et al. / Chinese Chemical
T
Letters 24 (2013) 307–310
309
Fig. 1. XRD patterns of 3Cu–2Fe/SiO2 with varying Mo contents before and after
being reduced at 600 8C: (a) catalyst precursor calcined at 500 8C, (b) 3Cu–2Fe/SiO2,
(c) 3Cu–2Fe–1Mo/SiO2, (d) 3Cu–2Fe–2.5Mo/SiO2, (e) 3Cu–2Fe–4Mo/SiO2, (f) 3Cu–
2Fe–5.5Mo/SiO2, (g) 3Cu–2Fe–7Mo/SiO2.
Fig. 2. TPD–NH3 profiles of 3Cu–2Fe/SiO2 with varying Mo contents: (a) 3Cu–2Fe/
SiO2 (b) 3Cu–2Fe–1Mo/SiO2, (c) 3Cu–2Fe–2.5Mo/SiO2, (d) 3Cu–2Fe–4Mo/SiO2, (e)
3Cu–2Fe–5.5Mo/SiO2, (f) 3Cu–2Fe–7Mo/SiO2.
of Fe3O4 species. There was a peak related to metallic Fe
(2u = 44.68) in the XRD pattern of 3Cu–2Fe/SiO2 (Fig. 1(b)) but
and Fig. 3 (TPD–CO2), respectively. There is one broad peak of NH3
desorption at 150–400 8C in the TPD–NH3 profiles over all
catalysts, corresponding to one type of acid sites. The peak areas
of 3Cu–2Fe/SiO2 are lower than that of all Mo-promoted catalysts.
The addition of Mo element can effectively enhance the acid sites
of the 3Cu–2Fe/SiO2. The peak areas of NH3 desorption decreased
with increased Mo contents in 3Cu–2Fe/SiO2, indicating that the
concentration of acid sites declined at higher Mo contents. In the
TPD–CO2 profiles(Fig.3),thepeaksataround300 8Cand460 8Cwere
broader and more intense compared to that at 120 8C. This reveals
the abundant and strong base sites on the surface of catalysts. With
the increase of Mo contents in catalysts, the amount of CO2 desorbed
at 120 8C increased progressively, while desorption at 300 8C and
460 8C did not change significantly. According to the XRD (Fig. 1)
studies, there existed metallic Cu, Fe3O4 and MoO2 on the surface of
the Mo-promoted 3Cu–2Fe/SiO2 catalysts. The composition of the
catalysts implied that there were three types of active centers on the
surfaceofcatalysts: metal sites(Cu),Lewisacidsites(Mon+, Fen+)and
Lewis base sites (Mo–O, Fe–O). The enhanced acidity of the
none in the Mo promoted catalysts (Fig. 1(c)–(g)). This demon-
strates that the strong interactions between Cu–Fe and Mo/SiO2
may hinder the reduction of the metal oxides to metallic particles.
These can also be observed in TPR and LRS (See Figs. S1 and S2 in
supporting information). No Mo phase was observed in the XRD
patterns of the Mo-promoted 3Cu–2Fe/SiO2 at lower Mo contents
(Fig. 1(c)–(e)), indicating that Mo elements were highly dispersed
on the SiO2. Increasing Mo contents to above 5.5 wt% (Fig. 1(f)–(g))
led to new diffraction peaks ascribed to MoO2 (2u = 36.58),
suggesting the reduction of MoO3 to MoO2 in H2 stream at
600 8C occurred. It was reported that the mixture of Fe2O3 and
MoO3 can form a ferric molybdate (Fe2(MoO4)3) when calcined
above 470 8C [23]. However, in this study, the molybdenum oxide
was well dispersed on SiO2 so that it was difficult to confirm the
existence of ferric molybdate in the Mo-promoted 3Cu–2Fe/SiO2.
Fig. 1 also presents the changes in diffraction peaks of the active
phases with different Mo loading contents. The addition of Mo to
3Cu–2Fe/SiO2 catalysts significantly induced the broadening of the
diffraction peaks of Cu and Fe species owing to the decrease in the
size of the crystalline particles. This indicates a better dispersion of
Cu–Fe clusters in the Mo-promoted catalysts compared to the
unpromoted catalysts, which can also be observed from the TPR
curves (see Fig. S1 in supporting information). The diffraction
peaks became sharper when the Mo loading increased, suggesting
that the Cu–Fe particles became larger with increased Mo contents.
The crystalline particle sizes that are estimated from Fig. 1 using
Scherrer equation are shown in Table 2. Apparently, Mo promoter
can improve the dispersions of Cu–Fe phases on the catalyst, but
excessive Mo loading inhibits the dispersion of Cu–Fe clusters.
The acid–base properties of the Mo-promoted 3Cu–2Fe/SiO2
investigated by the TPD technique are shown in Fig. 2 (TPD–NH3)
[F(ig._3)TD$FIG]
Table 2
Crystalline particle sizes of the 3Cu–2Fe/SiO2 with different Mo loading contents.
Mo loading (wt%)
0
1
2.5
4
5.5
7
a
Fe3O4 (nm)
14.4
15.3
16.3
17.8
18.9
19.1
21.6
20.4
23.5
21.8
Cub (nm)
25.0
Fig. 3. TPD–CO2 profiles of 3Cu–2Fe/SiO2 with varying Mo contents: (a) 3Cu–2Fe/
SiO2 (b) 3Cu–2Fe–1Mo/SiO2, (c) 3Cu–2Fe–2.5Mo/SiO2, (d) 3Cu–2Fe–4Mo/SiO2, (e)
3Cu–2Fe–5.5Mo/SiO2, (f) 3Cu–2Fe–7Mo/SiO2.
a
2
u
values are 35.58 and 62.58.
b
2
u values are 43.28, 50.48 and 74.18.

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Y.-J. Zhou et al. / Chinese Chemical Letters 24 (2013) 307–310
Mo-promoted Cu–Fe/SiO2 are due to the presence of Lewis acid sites
Mon+, and the enhanced basicity of the catalysts are the results of
Lewis base sites Mo–O. The TPD results suggested that the
introduction of the Mo afforded both extra base sites and extra
acid sites for the 3Cu–2Fe/SiO2, and it provided more base sites
instead of acid sites with higher Mo loading contents.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2013.02.001.
References
According to the reaction mechanisms [15,26], methanol was
activated on Lewis base sites to form the methoxy species and
Lewis acid sites to form methyl species. The methoxy species first
reacted with horizontally adsorbed CO2 on base sites to produce
the methoxy carbonate anion, which may further react with
methyl species to generate the DMC. That is to say, more base sites
are beneficial for the activation of CO2 while both addition of both
Lewis acid sites and Lewis base sites favors the activation of
methanol. The acid and base sites of catalysts play significant roles
in the catalytic performance in the DMC formation from CO2 and
methanol. Attempts were made to correlate the catalytic activity
(Table 1) and acid–base properties (Figs. 2 and 3) of the catalysts.
All Mo-promoted 3Cu–2Fe/SiO2 exhibited better catalytic activity
for the direct DMC formation than the unpromoted catalysts. This
implies that the catalytic performance improved with an increased
total number of acid and base sites. However, a no-linear
correlation between catalytic activity and acid–base concentra-
tions was observed. The catalytic performances of Mo-promoted
Cu–Fe/SiO2 first increased and then decreased with increased
concentrations of base sites, but the correlation between catalytic
performances and the number of acid sites present a reverse trend.
Among the catalysts tested, 3Cu–2Fe–2.5Mo/SiO2 with neither the
largest basicity nor the largest acidity showed the best catalytic
performance. In conclusion, the catalysts own a large amount of
acid and base sites with proper ratios of acid sites to base sites
favors the DMC formation.
[1] E. Leino, P. Maki-Arvela, V. Eta, et al., Conventional synthesis methods of short-
chain dialkylcarbonates and novel production technology via direct route from
alcohol and waste CO2, Appl. Catal. A – Gen. 383 (2010) 1–13.
[2] N.S. Isaacs, B. O’Sullivan, C. Verhaelen, High pressure routes to dimethyl carbonate
from supercritical carbon dioxide, Tetrahedron 55 (1999) 11949–11956.
[3] Y. Ono, Catalysis in the production and reactions of dimethyl carbonate, an
environmentally benign building block, Appl. Catal. A – Gen. 155 (1997) 133–166.
[4] D. Delledonne, F. Rivetti, U. Romano, Developments in the production and
application of dimethylcarbonate, Appl. Catal. A – Gen. 221 (2001) 241–251.
[5] J. Kizlink, Synthesis of dimethyl carbonate from carbon dioxide and methanol in
the presence of organotin compounds, Collect. Czech. Chem. Commun. 58 (1993)
1399–1402.
[6] J. Kizlink, I. Pastucha, Preparation of dimethyl carbonate from methanol and
carbon dioxide in the presence of Sn(IV) and Ti(IV) alkoxides and metal acetates,
Collect. Czech. Chem. Commun. 60 (1995) 687–692.
[7] M.M. Du, Q.X. Li, W.T. Dong, T. Geng, Y.J. Jiang, Synthesis of glycerol carbonate
from glycerol and dimethyl carbonate catalyzed by K2CO3/MgO, Res. Chem.
Intermed. 38 (2012) 1069–1077.
[8] K. Tomishige, T. Sakaihori, Y. Ikeda, K. Fujimoto, A novel method of direct
synthesis of dimethyl carbonate from methanol and carbon dioxide catalyzed
by zirconia, Catal. Lett. 58 (1999) 225–229.
[9] Y. Ikeda, T. Sakaihori, K. Tomishige, K. Fujimoto, Promoting effect of phosphoric
acid on zirconia catalysts in selective synthesis of dimethyl carbonate from
methanol and carbon dioxide, Catal. Lett. 66 (2000) 59–62.
[10] K. Tomishige, Y. Furusawa, Y. Ikeda, M. Asadullah, K. Fujimoto, CeO2–ZrO2 solid
solution catalyst for selective synthesis of dimethyl carbonate from methanol and
carbon dioxide, Catal. Lett. 76 (2001) 71–74.
[11] M. Aresta, A. Dibenedetto, C. Pastore, et al., Cerium(IV)oxide modification by
inclusion of a hetero-atom: a strategy for producing efficient and robust nano-
catalysts for methanol carboxylation, Catal. Today 137 (2008) 125–131.
[12] M. Aresta, A. Dibenedetto, C. Pastore, et al., Influence of Al2O3 on the perfor-
mance of CeO2 used as catalyst in the direct carboxylation of methanol to
dimethylcarbonate and the elucidation of the reaction mechanism, J. Catal. 269
(2010) 44–52.
[13] L.A. Allaoui, A. Aouissi, Effect of the Bronsted acidity on the behavior of CO2
methanol reaction, J. Mol. Catal. A – Chem. 259 (2006) 281–285.
[14] X.L. Wu, M. Xiao, Y.Z. Meng, Y.X. Lu, Direct synthesis of dimethyl carbonate on
H3PO4 modified V2O5, J. Mol. Catal. A – Chem. 238 (2005) 158–162.
[15] S.H. Zhong, H.S. Li, J.W. Wang, X.F. Xiao, Cu–Ni/V2O5–SiO2 catalyst for the direct
synthesis of dimethyl carbonate from carbon dioxide and methanol, Acta Phys.
Chim. Sin. 16 (2000) 226–231.
[16] X.J. Wang, M. Xiao, S.J. Wang, Y.X. Lu, Y.Z. Meng, Direct synthesis of dimethyl
carbonate from carbon dioxide and methanol using supported copper (Ni, V, O)
catalyst with photo-assistance, J. Mol. Catal. A – Chem. 278 (2007) 92–96.
[17] J. Bian, M. Xiao, S.J. Wang, Y.X. Lu, Y.Z. Meng, Direct synthesis of DMC from CH3OH
and CO2 over V-doped Cu–Ni/AC catalysts, Catal. Commun. 10 (2009) 1142–1145.
[18] J. Bian, M. Xiao, S.J. Wang, et al., Highly effective synthesis of dimethyl carbonate
from methanol and carbon dioxide using a novel copper–nickel/graphite bime-
tallic nanocomposite catalyst, Chem. Eng. J. 147 (2009) 287–296.
[19] I. Omae, Aspects of carbon dioxide utilization, Catal. Today 115 (2006) 33–52.
[20] Y.J. Zhou, S.J. Wang, M. Xiao, et al., Novel Cu–Fe bimetal catalyst for the formation
of dimethyl carbonate from carbon dioxide and methanol, RSC Adv. 2 (2012)
6831–6837.
4. Conclusion
Mo-promoted Cu–Fe based catalysts can be used in the DMC
formation from CO2 and gaseous methanol. The introduction of Mo
substantially increased the dispersion of Cu–Fe clusters on
support, whilst the strong interactions between the metal oxides
and Mo shifted the reduction to higher temperatures. TPD results
showed that the concentration of base sites increased with
increased Mo contents, and suitable concentration balance of acid
and base sites favors DMC formation. Under the optimal reaction
conditions (120 8C, 0.6 MPa, SV = 360 hÀ1), the introduction of Mo
significantly enhanced the catalytic activity of the Cu–Fe bimetal
catalysts. The highest methanol conversion of 6.99% with a DMC
selectivity of 87.7% can be obtained when 2.5 wt% Mo was loaded.
[21] G. Jacobs, T.K. Das, Y.Q. Zhang, et al., Fischer–Tropsch synthesis: support, loading,
and promoter effects on the reducibility of cobalt catalysts, Appl. Catal. A – Gen.
233 (2002) 263–281.
Supplementary material
The reduction behaviors and the information about interactions
of active metals with support of catalysts characterized by TPR and
LRS are presented in supporting information.
[22] W. Ma, E.L. Kugler, J. Wright, D.B. Dadyburjor, Mo–Fe catalysts supported on
activated carbon for synthesis of liquid fuels by the Fischer–Tropsch process:
effect of Mo addition on reducibility, activity, and hydrocarbon selectivity, Energy
Fuels 20 (2006) 2299–2307.
[23] S.D. Qin, C.H. Zhang, J. Xu, et al., Effect of Mo addition on precipitated Fe catalysts
for Fischer–Tropsch synthesis, J. Mol. Catal. A – Chem. 304 (2009) 128–134.
[24] R.M.M. Abbaslou, J. Soltan, A.K. Dalai, Iron catalyst supported on carbon nano-
tubes for Fischer–Tropsch synthesis: effects of Mo promotion, Fuel 90 (2011)
1139–1144.
[25] K. Faungnawakij, Y. Tanaka, N. Shimoda, et al., Hydrogen production from
dimethyl ether steam reforming over composite catalysts of copper ferrite spinel
and alumina, Appl. Catal. B – Environ. 74 (2007) 144–151.
[26] M. Aresta, A. Dibenedetto, E. Fracchiolla, et al., Mechanism of formation of organic
carbonates from aliphatic alcohols and carbon dioxide under mild conditions
promoted by carbodiimides. DFT calculation and experimental study, J. Org.
Chem. 70 (2005) 6177–6186.
Acknowledgments
The authors would like to thank the China High-Tech Develop-
ment 863 Program (No. 2009AA03Z340), Guangdong Province
Universities and Colleges Pearl River Scholar Funded Scheme (2010),
Guangdong Province Sci & Tech Bureau (Key Strategic Project Nos.
2008A080800024 and 10151027501000096), and Chinese Univer-
sities Basic Research Founding for financial support of the work. Y. L.
thanks National University of Singapore for financial support.