Direct synthesis of dimethyl carbonate from carbon dioxide and methanol using supported copper (Ni, V, O) catalyst with photo-assistance

Article abstract of DOI:10.1016/j.molcata.2007.08.028

The photo-catalytic effect of a copper modified (Ni, V, O) semiconductor complex catalyst on the direct synthesis of dimethyl carbonate (DMC) from CO₂ and CH₃OH was investigated. The synthesized catalysts were fully characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), Ultraviolet visible drift reflection spectra (UV-vis DRS) and transmission electron microscopy (TEM). The nano-scale catalyst particles were observed with TEM and light absorbance was then predicted by UV-vis DRS spectra. The pressure and temperature dependencies of the photo-catalytic activity were studied. The results demonstrated that the catalytic activity was enhanced with the assistance of ultraviolet (UV) irradiation compared with the pure thermal and surface catalytic reaction under the same reaction conditions.

Full text of DOI:10.1016/j.molcata.2007.08.028

Available online at www.sciencedirect.com
Journal of Molecular Catalysis A: Chemical 278 (2007) 92–96
Direct synthesis of dimethyl carbonate from carbon dioxide
and methanol using supported copper (Ni, V, O) catalyst
with photo-assistance
a
a
a
b
a,b,∗
X.J. Wang , M. Xiao , S.J. Wang , Y.X. Lu , Y.Z. Meng
a
State Key Laboratory of Optoelectronic Materials and Technologies/Institute of Optoelectronic and Functional Composite Materials,
Sun Yat-Sen University, Guangzhou 510275, PR China
b
Department of Chemistry & Medicinal Chemistry Program, Office of Life Sciences, National University of Singapore,
3
Science Drive 3, Singapore 117543, Republic of Singapore
Received 15 June 2007; received in revised form 21 August 2007; accepted 21 August 2007
Available online 2 September 2007
Abstract
The photo-catalytic effect of a copper modified (Ni, V, O) semiconductor complex catalyst on the direct synthesis of dimethyl carbonate (DMC)
from CO and CH OH was investigated. The synthesized catalysts were fully characterized by temperature programmed reduction (TPR), X-ray
2
3
diffraction (XRD), Ultraviolet visible drift reflection spectra (UV–vis DRS) and transmission electron microscopy (TEM). The nano-scale catalyst
particles were observed with TEM and light absorbance was then predicted by UV–vis DRS spectra. The pressure and temperature dependencies
of the photo-catalytic activity were studied. The results demonstrated that the catalytic activity was enhanced with the assistance of ultraviolet
(
©
UV) irradiation compared with the pure thermal and surface catalytic reaction under the same reaction conditions.
2007 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis; Catalysis; Carbon dioxide; Dimethyl carbonate
1
. Introduction
route reports high DMC yield, but it has the disadvantages of
high cost of the starting materials and difficulty in the product
separation [6].
Dimethyl carbonate (DMC) has received much attention as
an environment friendly intermediate and a non-toxic substitute
for poisonous and corrosive phosgene and dimethyl sulfate in the
reaction of carbonylation and methylation [1,2]. Further, DMC
can also be used as a good solvent, an alkylation agent and a fuel
additive in place of MTBE to reduce the tail gas pollution [3,4].
The traditional DMC synthesis route takes use of phosgene,
an intense toxic substance and co-produces hydrochloric acid,
which will erode the equipment seriously [3,4]. The oxidative
carbonylation of methanol with carbon monoxide and oxygen
is another route for the synthesis of DMC, which is dangerous
and potentially explosive, however, to use carbon monoxide and
oxygen mixture as starting materials [5]. The transesterification
Recently, one step synthesis of DMC from CO2 and CH3OH
has become attractive due to the low cost of the starting materi-
als and its alleviating the global climate change caused in part
by the increasing CO2 emissions [7–9]. However, DMC yield
in this route is relatively low due to the fact that CO2 is highly
thermodynamically stable and kinetically inert and due to the
deactivation of catalysts induced by water formation in the reac-
tion process [8,9]. Therefore, the removal of water from the
reaction system and the activation of CO2 are two major obsta-
cles in the direct synthesis of DMC from methanol and CO2. The
dehydrates and additives has been applied to the system, such as
CaCl2 [10], 2,2-dimethoxy propane (DMP) [11] acetals [7,12]
and molecular sieves [13] to minimize the water concentration
in the reaction system.
CO2 activation has long been a challenge for synthetic
chemists to convert CO2 into the industrially useful com-
pounds. Various catalysts have been used to activate CO2 for
the direct synthesis of DMC from CO2 and methanol, such as
∗
Corresponding author at: State Key Laboratory of Optoelectronic Mate-
rials and Technologies/Institute of Optoelectronic and Functional Composite
Materials, Sun Yat-Sen University, Guangzhou 510275, PR China.
E-mail address: mengyzh@mail.sysu.edu.cn (Y.Z. Meng).
1
381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.08.028

X.J. Wang et al. / Journal of Molecular Catalysis A: Chemical 278 (2007) 92–96
93
Cu-Ni/VSO [8], V2O /H3PO4 [9], ZrO2-CeO2 [11], BuSn
cursor of (Ni, V, O) semiconductor complex supported on SiO2
(Ni 2 wt%).
5
(
OMe)2 [14,15], metal(IV)tetra-alkoxide [16], magnesium
dialkoxide [17], potassium carbonate [18], ZrO2 [19], etc.
In recent decades semiconductor catalysts have drawn much
attention due to their photo-catalytic activity of initiating
redox reactions [20–23]. Both photo-catalytic (ꢀG < 0) and
photo-synthetic (ꢀG > 0) processes have widely introduced
the light-driven redox reactions with semiconductor catalysts
2.2.2. Copper modified catalyst
The above catalyst precursor was treated in its turn as fol-
lows: impregnated stoichiometrically into the solution of cupric
ammonia, aged in room temperature for 24 h, carried out the
surface reaction at 363 K for 24 h, dried at 383 K for 6 h and cal-
cined at 723 K in air for 5 h. Prior to use the product was reduced
under the atmosphere of by 5%H2/N2 mixture at 623 K for 2 h to
form the copper modified semiconductor complex catalyst (Cu
1 wt%).
[
24,25]. The photo-reduction of CO2 has long been investigated
in the past to produce formaldehyde, formic acid, methanol,
methane and oxalate, etc. with SrTiO3, TiO2 (pure or doped),
SiC, CdS, ZnO, WO3 and BaTiO3 semiconductor photocatalysts
[
20,23,24]. UV and visible light photoenergies have been used
to break the thermodynamic limitations of some thermodynami-
cally unfavorable reactions that have been proved to be effective,
like water splitting [20,23,24,26]. In this paper, we introduce a
copper doped (Ni, V, O) semiconductor complex as the catalyst
into photosynthesis of DMC from CO2 and methanol directly in
the continuous flow fixed-bed reactor to enhance the CO2 acti-
vation and DMC yield. Moreover, the continuous flow fixed-bed
reactor can remove the produced water and build a water-free
reaction system to favor the DMC formation.
2.3. Catalyst characterization
2.3.1. Temperature programmed reduction (TPR)
The semiconductor complex catalysts copper (Ni, V, O)
were reduced in the temperature programmed reduction mode
of Quantachrom ChemBET 3000 apparatus using a quartz U-
shaped reactor at temperature range from room temperature to
1073 K at a heating rate of 8 K/min, in 10%H2/Ar atmosphere.
The sample weight was 50 mg.
2
. Experimental
2.3.2. X-ray diffraction (XRD)
X-ray analysis was carried out on a D/Max-IIIA powder
◦
◦
2
.1. Materials
diffractometer in a step mode between 3 and 60 2θ using Cu
K␣1 radiation.
Vanadium pentoxide, hydrochloric acid, ammonia solution,
nickel nitrate, cupric nitrate and dried methanol with analytical
pure were purchased from commercial sources. Carbon dioxide
was supplied by Shanghai SGIG Co., China with purity higher
than 99.99%. Silica gel, commercially available from Qingdao
Ocean Chemical Factory, China, was in 100–200 mesh with a
2.3.3. Transmission electron microscopy (TEM)
The catalyst sample was examined by using transmission
electron microscopy (JEM-2010HR operated at 200 kV).
2.3.4. UV–vis spectra
2
specific surface area of 360 m /g.
UV–vis diffuse reflectance spectra were recorded by a spec-
trophotometer (UV-2501PC range from 190 to 900 nm) using
barium sulphate as reference sample.
2
.2. Catalyst preparation
2
.2.1. Catalyst precursor of (Ni, V, O) semiconductor
2.4. Direct synthesis of DMC with photo-assistance from
CH3OH and CO2
complex
V2O was added into HCl (38 wt%) at 363 K for 1 h to
5
form dark blue VOCl3 solution. Silica gel was pre-treated at
The title synthesis of DMC was carried out in the photo-
reactor as illustrated in Fig. 1. The setup comprised a
photo-reactor, a CO2 mass flow controller, a HLPC syringe
pump, a six-way valve, a back-pressure regulator and a gas
chromatography (GC). The six-way valve was placed into an
oven. The oven, heater and reactor were each equipped with a
thermometer, measured by thermocouples with an accuracy of
3
63 K for 2 h and the required liquid volume for stoichiometric
impregnation was measured. Silica gel was impregnated stoi-
chiometrically into VOCl3 solution, aged at room temperature
for 24 h and carried out the surface reaction at 363 K for another
2
4 h. Ammonia solution was added into the reaction system
to neutralize excessive acid at room temperature. The diluted
ammonia solution (1:10 vol.%) and distilled water was used to
◦
± 1 C. The system pressure was determined by pressure sensor
−
wash several times and get completely rid of the Cl ion retained
and controlled by the back-pressure regulator with an accuracy
of ± 0.01 MPa.
in the system, which was verified by 0.1 mol/L AgNO3 solution.
The mixture was filtered, dried at 383 K for 6 h and calcined at
During the experiment, the catalyst was first loaded into the
reactor and the reactor was sealed and purged using CO2 gas flow
for 10 min to exhaust the air inside. Prior heating up CO2 was
chargedintothereactortoacertainpressureandtheflowratewas
measured and controlled by the mass flow controller. When the
heater, reactor and oven were heated to the desired temperature,
the methanol was pumped into the system by the HPLC syringe
7
23 K in air for 5 h to produce the complex of V2O dispersed
5
on SiO2 (V2O 8.88 wt%). The complex was impregnated stoi-
5
chiometrically into the solution of nickel ammonia, aged in room
temperature for 24 h, and finally carried out the surface reaction
at 363 K for 24 h. The produced product was dried at 383 K for
6
h and calcined at 723 K in air for 5 h to yield the catalyst pre-

9
4
X.J. Wang et al. / Journal of Molecular Catalysis A: Chemical 278 (2007) 92–96
Fig. 3. XRD patterns of SiO2 and the copper modified (Ni, V, O) complex
catalyst.
Fig. 1. Schematic diagram of apparatus for photo-assistant synthesis of DMC.
(
(
(
(
1) CO2 cylinder; (2) pressure reducer valve; (3) filter; (4) pressure regulator;
5) mass flow controller; (6) buffer; (7) one-way valve; (8) methanol carrier;
9) HLPC syringe pump; (10) three-way inlet wave; (11) blender; (12) heater;
13) photo-reactor; (14) six-way valve; (15) cooler; (16) cooling separator; (17)
◦
is shifted to lower temperature of 400 C, indicating that NiO
particles were well dispersed on SiO2. The broad peak appeared
◦
between 450 and 540 C indicates that NiO reacted with V O
and produced the catalyst precursor of semiconductor complex
2
5
cut-off valve; (18) back-pressure regulator; (19) gas chromatography.
(
2
Ni, V, O). The reduction peak of CuO particles was reduced to
pump and simultaneously the ultrahigh pressure mercury lamp
◦ ◦
49 C and a new peak appeared at 438 C (curve 5), demon-
(
365 nm wavelength) was turned on with a power of 125 W. An
strating that CuO particles were also well dispersed within the
semiconductor complex (Ni, V, O) under the experimental con-
ditions.
Fig. 3 shows that the amorphous XRD patterns of the copper
modified (Ni, V, O) semiconductor complex catalysts, which is
similar to the pattern of SiO2. This indicates that the catalyst
was well dispersed on the surface of SiO2 particles.
As further disclosed in TEM of Fig. 4 for the copper modified
electric fan was used to remove the excessive heat accumulated
by the UV irradiation. The resulting mixture was introduced
into the on-line gas chromatograph (GC7890F) to analyze the
reaction composition with a flame ionization detector (FID).
The condensed liquid from the cut-off valve was collected and
analyzed by the gas chromatograph mass spectrometer (GCMS-
QP2010 plus) to confirm the formation of DMC.
(
Ni, V, O) semiconductor complex catalyst, the brighter grayish
3
. Results and discussion
portions correspond to SiO2 support, whereas the darker dots are
the catalyst particles with nano-scale of 4–8 nm. These facts are
also consistent with the findings in both TPR profiles and XRD
patterns. The energy diffraction spectrometer (EDS) suggests
that these darker dots include Cu, Ni and V elements. In a nano-
crystal, the wave function of the charge carrier spreads over
the whole semiconductor particle and the charge carriers can
accomplish reactions with species present at the surface without
diffusion, therefore, the semiconductor catalyst exhibits highly
catalytic activity [24].
3
.1. Investigation of catalyst structure
Fig. 2 presents the TPR profiles for different samples. Curves
1
, 2, 3 shows the reduction peaks for the pure V2O , NiO and
5
◦
CuO dispersed on SiO2 at 559, 443 and 280 C, respectively.
It should be noted that curve 3 has a peak shoulder at 263 C,
which is due to the reduction of the well-dispersed CuO particles
compared with the peak shoulder at 280 C for the bigger CuO
◦
◦
particles. From curve 4 we can see that the NiO reduction peak
Fig. 5 shows the UV–vis drift reflection spectra (DRS) for
the V2O supported on SiO2, the catalyst precursor (Ni, V, O)
5
semiconductor complex and the copper modified (Ni, V, O)
semiconductor complex catalyst, respectively. There exists a
cross-point at 510 nm between curves 1 and 2. It can be seen
that for the wavelength less than 510 nm the light reflectivity
of catalyst precursor (Ni, V, O) semiconductor complex has
decreased due to the formation of solid solution of catalyst pre-
cursor (Ni, V, O) semiconductor complex which increases the
absorbance of UV–vis light. By comparing curve 2 with 3, one
can also see that the reflectivity increases slightly from about 7
to 12% for the wavelength less than cross-point 446 nm, while
the reflectivity decreases greatly from about 45% to below 15%
for the wavelength over 446 nm, suggesting that the introduc-
tion of copper can increase the absorbance of catalyst in visible
Fig. 2. TPR profiles of different samples.

X.J. Wang et al. / Journal of Molecular Catalysis A: Chemical 278 (2007) 92–96
95
Fig. 4. TEM image of copper modified (Ni, V, O) complex catalyst.
light range. Coupled semiconductor photo-catalyst with copper
doping provides a potential way to increase the efficiency of a
photo-catalytic process by increasing the charge separation and
extending the energy range of photo-excitation for the system
and by the improved trapping of electron to inhibit electron–hole
recombination during illumination [20].
UV irradiation appears a little lower than that without UV
irradiation, while the DMC yield with UV irradiation exhibits
33–56% higher than that without UV irradiation. Both yield and
selectivity of DMC with or without UV irradiation increased
with increasing pressure. DMC yield and selectivity without
UV irradiation seems more sensitive to the pressure change in
case the system pressure goes up over 6 atm. Presumably, this
is resulted from different reaction mechanisms. However, the
yield and selectivity of DMC with UV irradiation shows little
dependence on the pressure, increasing only 16% when system
pressure increases from 1 to 12 atm. Considering the relationship
among the DMC yield, selectivity and the operation pressure, it
is preferred to perform the DMC synthesis under the pressure of
1 atm.
3
.2. Photo-catalytic synthesis of DMC from CO2 and
CH3OH
Fig. 6 shows the pressure dependence of DMC synthesis
on different catalytic system, with (curves 2 and 3) or with-
out (curves 1 and 4) UV irradiation, at the constant temperature
◦
of 120 C. Under the same pressure, the DMC selectivity with
Fig. 5. UV–vis DRS spectra of (1) V2O5 supported on SiO2, (2) catalyst pre-
cursor (Ni, V, O) complex and (3) copper modified (Ni, V, O) complex catalyst.
Fig. 6. The dependence of DMC yield and selectivity on pressure with (curves
2 and 3) or without (curves 1 and 4) UV irradiation.

9
6
X.J. Wang et al. / Journal of Molecular Catalysis A: Chemical 278 (2007) 92–96
DMC from methanol and CO2 was successfully carried out with
and without the UV radiation. The experimental results showed
that the nano-scale catalyst particles had a size of 4–8 nm. The
UV–vis DRS spectra of the catalyst showed its feature of good
light absorbance. UV irradiation reduced the reaction pressure
to 1 atm and the DMC yield increased 57% compared with pure
thermal surface catalytic reaction. The optimized operation con-
◦
dition profile for the direct DMC synthesis was at 120–140 C
and under 1 atm with the assistance of UV light irradiation.
Acknowledgements
Fig. 7. The dependence of DMC yield and selectivity on temperature with
The authors would like to thank Guangdong Province Sci &
Tech Bureau (Key Strategic Project Grant No. 2006B12401006)
and Guangzhou Sci & Tech Bureau (2005U13D2031) for finan-
cial support of this work.
(
curves 2 and 3) or without (curves 1 and 4) UV irradiation.
Fig. 7 presents the temperature dependence of DMC syn-
thesis on different catalytic systems, with (curves 2 and 3) or
without (curves 1 and 4) UV irradiation, under the constant
pressure of 1 atm. At the same temperature, the DMC selectivity
with UV irradiation shows a little lower than that without UV
irradiation, while the DMC yield with UV irradiation exhibits
References
[
[
[
1] Y. Ono, Appl. A: Gen. 155 (1997) 133.
2] A.A. Shaikh, S. Sivaram, Chem. Rev. 76 (1996) 951.
3] P. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 99 (1999) 475–494.
5
3–63% higher than that without UV irradiation. This demon-
[4] S. Neil Isaaca, B. O’Sullivan, et al., Tetrahedron 55 (1999) 11949–11956.
[
5] U. Romano, R. Tesei, M.M. Mauri, P. Rebora, Ind. Eng. Chem. Prod. Res.
Dev. 19 (1980) 396.
strated the effectiveness of UV irradiation or photocatalysis for
the reaction. Both yield and selectivity of DMC with or with-
out UV irradiation increased with increasing temperature from
[
[
[
6] T.S. Zhao, Y.Z. Han, Y.H. Sun, Fuel Process. Technol. 27 (1998) 457–463.
7] J.C. Choi, T. Sakakura, T. Sako, J. Am. Chem. Soc. 121 (1999) 3793–3794.
8] X.L. Wu, Y.Z. Meng, M. Xiao, Y.X. Lu, J. Mol. Catal. A: Chem. 249 (2006)
93–97.
◦
◦
1
00 to 120 C, followed by leveling off from 120 to 140 C
◦
and finally decreased sharply from 140 to 160 C. It is apparent
that the optimized operation temperature appears to be 130 C.
At low temperature, the increasing of temperature favors the
synthesis of DMC, however, too high temperature (>140 C)
leads to DMC decomposition. The increased temperature accel-
erates the recombination of photoelectrons and holes generated
by UV irradiation which could be another reason for the lower
activity. Overall, under the optimized operation conditions of
normal pressure, 130 C and 125 W UV irradiation, the conver-
sion of CH3OH was up to 4.04% with the DMC selectivity of
◦
[9] X.L. Wu, M. Xiao, Y.Z. Meng, Y.X. Lu, J. Mol. Catal. A: Chem. 238 (2005)
58–162.
1
[
[
[
10] Q. Jiang, T. Li, F. Liu, Chin. J. Appl. Chem. 16 (5) (1999) 115–116.
11] K. Tomishige, K. Kunimori, Appl. Catal. A: Gen. 237 (2002) 103–109.
12] J.C. Choi, L.N. He, et al., Green Chem. 4 (2002) 230–243.
◦
[13] Z.S. Hou, B. Han, Z. Liu, T. Jiang, G. Yang, GreenChem. 4(2002)467–471.
[
[
[
[
[
14] J. Kizlink, Collect. Czech. Chem. Commum. 58 (1993) 1399.
15] K. Ko, O. Fujumaro, Jpn. Kakai Tokkyo Koho 95 (1995) 224011.
16] J. Kizlink, I. Pastucha, Collect. Czech. Chem. Commun. 60 (1995) 687.
17] X. Gui, F. Cao, D. Liu, D. Fang, J. Chem. Eng. Chin. Univ. 12 (1998) 152.
18] S. Fang, K. Fujimoto, Appl. Catal. A 142 (1996) L1.
◦
8
5.4% over copper (Ni, V, O) catalyst. The major by-products
are methoxyl methalic acid and carbon monoxide in this reaction
27]. Presumably, methoxyl methalic acid is resulted from the
[19] K. Taek Jung, T. Alexic Bell, Topics Catal. 20 (2002) 97.
[
[
20] A.L. Linsebigier, G.G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735–758.
21] K. Szacilowski, W. Maoyk, A. Drzewiecka-Matuszek, M. Brindell, G.
Stochel, Chem. Rev. 105 (2005) 2647–2694.
[
un-completely reaction between carbon dioxide and methanol,
while carbon monoxide is produced from the cleavage of the
C O bond of the CO2 anion radical.
[
[
22] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341–357.
23] A.V. Emeline, V.A. Otroshchenko, V.K. Ryabchuk, N. Serpone, J. Pho-
tochem. Photobiol. C: Photochem. Rev. 3 (2003) 203–224.
24] A. Mills, S.L. Hunte, J. Photochem. Photobiol. A: Chem. 108 (1997) 1–35.
25] N. Serpone, E. Borgarello, E. Pelizzetti, J. Electrochem. Soc.: Electrochem.
Sci. Technol. 135 (1988) 2760–2766.
•
−
[
[
4
. Conclusions
[
26] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen,
Nature 440 (2006) 295.
Copper modified (Ni, V, O) semiconductor complex catalysts
can be characterized by TPR technology. The direct synthesis of
[27] E.C.Y. Inn, J.M. Heimerl, J. Atmos. Sci. 28 (1971) 838–841.