Catalytic reduction of carbon dioxide into methanol over copper under hydrothermal conditions

Article abstract of DOI:10.1016/j.cattod.2012.06.013

A novel method for the catalytic synthesis of methanol from CO₂ over Cu in the presence of Zn under hydrothermal conditions was investigated. The results showed Cu acts as an active catalyst in conversion of CO₂ into methanol. The methanol yield of 11.4% was achieved at 350 °C for 3 h with Cu 70 mmol, Zn 60 mmol, HCl 2.0 M and water filling 50%. The plausible mechanism for the hydrothermal conversion of CO₂ into methanol was also discussed.

Full text of DOI:10.1016/j.cattod.2012.06.013

Catalysis Today 194 (2012) 25–29
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Catalytic reduction of carbon dioxide into methanol over copper under
hydrothermal conditions
a
b
b
a
a,∗
Zhibao Huo , Mingbo Hu , Xu Zeng , Jun Yun , Fangming Jin
a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel method for the catalytic synthesis of methanol from CO2 over Cu in the presence of Zn under
hydrothermal conditions was investigated. The results showed Cu acts as an active catalyst in conversion
of CO2 into methanol. The methanol yield of 11.4% was achieved at 350 C for 3 h with Cu 70 mmol, Zn
Received 31 January 2012
Received in revised form 16 May 2012
Accepted 10 June 2012
◦
6
0 mmol, HCl 2.0 M and water filling 50%. The plausible mechanism for the hydrothermal conversion of
Available online 20 July 2012
CO2 into methanol was also discussed.
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Methanol
CO2
Hydrothermal chemistry
Catalyst
Copper
1
. Introduction
have still considerable drawbacks, including the low yield, strict
reaction condition, and/or high cost. Therefore, to avoid that, devel-
opment of a new method to effectively change CO2 into methanol
was increasing demand.
Over the past several decades, CO2 hydrogenation to methanol
is commonly used because of its abundance, low cost, nontoxic-
ity and high potential as a renewable source. It is well known that
CO2 has high thermodynamic stability and low reactivity, no mat-
ter what methods are reported for methanol synthesis, the use of
catalysts is essential. More importantly, catalytic reduction of CO2
into methanol has been proved technically feasible. Developing
mild methods to catalytically activate CO2 represents a challenge
of both academic and practical importance. Cu-based catalysts like
Global warming caused by the increase in atmospheric CO2 is
threatening human and earth’s survival. One of the main scien-
tific and technological challenges facing the humanity is to reduce
the concentration of CO . Methods for CO2 reduction are urgently
2
needed to reduce CO2 emissions and to minimize global warming.
The utilization of the CO2 as a carbon source to produce valuable
and useful chemicals could be regarded as the most potential way
to solve this problem, and it is attracting more research interests
worldwide. A number of research works related to CO2 utilization,
such as methanol, have been reported [1,2].
Methanol is a very important organic chemical because of its
wide utilization. Methanol has been often found as an antifreeze,
solvent, fuel and as a raw material for production of formalde-
hyde, flavors, dyes, medicines, gunpowder and also as a common
chemical feedstock for acetic acid, methyl tert-butyl ether, and
chloromethane, etc. [3–6]. In recent years, synthesis of methanol
is attracting interest due to the industrial importance, and much
attention has been paid to development efficient methods for the
synthesis of methanol, such as photochemical reduction [7–11],
electrochemical reduction [12–15], and catalytic hydrogenation
reduction [16–26]. Although these reactions have proven to be
extremely efficient for the synthesis of methanol, most of them
Cu/ZnO or Cu/ZnO/Al O have been often utilized for the methanol
2
3
synthesis from syngas because Cu element was active (Eq. (1)).
It is, however, needed to add metal oxides as adsorbed species
[16–26]. Recently, hydrothermal treatment in conversion CO2 or
biomass into value-added chemicals has received much attention
and become one of the most promising ways of utilizing because of
its unique advantages [27–34], such as fast reaction and green sol-
vent. Previously, we and other groups have reported some research
results in hydrothermal conversion of CO into value-added chem-
2
icals. For example, (1) methane was formed via nickel-catalyzed
conversion of CO2 (Eq. (2)) [35]. (2) CO2 was reduced to formic
acid over Cu in the presence of Fe under hydrothermal conditions
(Eq. (3)) [36,37]. Encouraged by these findings above, we thought
∗ Corresponding author. Tel.: +86 21 54742283; fax: +86 21 54742283.
E-mail address: fmjin@sjtu.edu.cn (F. Jin).
that CO2 hydrogenation would be converted to the methanol using
transition metal catalysts under hydrothermal conditions.
0
920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.06.013

2
6
Z. Huo et al. / Catalysis Today 194 (2012) 25–29
General methodsf or catalytic hydr ogen ation of CO2 into CH OH .
2.3. Analytical methods
3
Cu-based catalysts
CO2
+
H2
CH OH
(1)
After the reactions, the gas was collected and analyzed by
gas chromatography equipped with thermal conductivity detector
(GC/TCD). The remaining reaction mixture was filtered, and then
3
Our previous work. Hydrothermal conversion CO2 into chemicals.
◦
the precipitate was dried in isothermal oven at 70 C for 24 h after
Cat. Ni/ Zn reductant
washing with distilled water. The liquid sample analyzed with high
performance liquid chromatography (HPLC), GC-FID, the precipi-
tate was determined by X-ray diffractometer (XRD).
CO2
CO2
+
+
H O
CH4
(2)
(3)
2
Hydrothermal condition
Cat. Cu/ Fe reductant
H O
HCOOH
2
Hydrothermal condition
3. Results and discussion
This work : Hydrothermal conversion of CO into CH OH (f irst example reported).
Initially, we screened the reaction conditions for the hydrother-
mal conversion of CO2 into methanol. The reaction was carried out
with 2.0 M NaOH at 350 C for 2 h in the presence of 70 mmol Cu
and 60 mmol Zn in H2O (water filling: 50%). It was found that no
methanol formation was observed in the presence of NaOH, while
the reaction of CO2 proceeded to give desired methanol in the
presence of HCl. Therefore, we continued to optimize the initially
reaction conditions for the formation of methanol from CO2.
2
3
Cat. Cu/ Zn reductant
◦
CO2
+
H O
CH OH
(4)
2
3
Hydrothermal condition
Scheme 1. Equations.
Here, we focused our attention in the conversion of CO2 into
methanol under hydrothermal conditions (Eq. (4)). In this process,
water acts not only as an excellent solvent, but also as a source
of hydrogen generated by reduction of metal reductants [38], so
it would be advantageous to avoid the use of gaseous hydrogen,
which is not easy to obtain with less energy cost, as reducing agent
to the reaction system. Among transition metals we investigated,
Cu catalyst showed significant activity, which Zn acts as an efficient
reductant for methanol formation. Therefore, Zn and Cu, two com-
mercially available and inexpensive metals, were chosen to be a
reductant and a catalyst under the hydrothermal reactions. To the
best of our knowledge, there has been no example for production
of methanol from CO2 over Cu as a catalyst and Zn as a reductant
under hydrothermal reaction conditions (Scheme 1).
3
.1. The effect of Cu on methanol yields
◦
First, we carried out the experiments with 2.0 M HCl at 350 C for
h in the presence of 70 mmol Cu and 60 mmol Zn in H O (water
2
2
filling: 50%) to examine the effect of Cu and Zn in hydrothermal
reaction. After the hydrothermal reaction, solid residual was ana-
lyzed by XRD, as illustrated in Fig. 1. Cu showed no apparent change
and still existed in solid samples while Zn was converted into ZnO.
Some unknown phases in the precipitates after hydrothermal reac-
tion were also observed. It is just speculative for some unknown
phases to be CuO or Cu O according to limited experimental data.
From the results, it is clarified that Zn acts as a reductant, and Cu has
catalytic activity for reduction of CO2 into methanol as a catalyst.
2
Effect of Cu amount on the formation of methanol from CO was
2
◦
performed at 350 C for 2 h with the different ratios of Zn/NaHCO .
3
2
. Experimental
The results are shown in Fig. 2. The profile of the reaction of CO2
monitored by HPLC, indicated that the yield of methanol increased
by changing Cu amount from 20 to 70 mmol in all cases. Fur-
2.1. Experimental materials
thermore, the use of ratio of Zn/NaHCO = 1:1 gave higher yield
3
In this study, NaHCO3 was used as the source of CO2 to sim-
plify the experiments. Experiments were conducted using a batch
reactor system and the schematic drawing can be found elsewhere
of methanol as compared to ratio of Zn/NaHCO = 1.5:1. A small
3
increase on the yield of methanol was observed in the presence
of Cu amount from 20 to 40 mmol. Interestingly, the Cu catalyst
showed significantly catalytic activity for the methanol produc-
tion and the yields of methanol increased as Cu amount increased
[
39]. The reactor vessel was made of a piece of stainless steel 316
tubing (9.525 mm (3:8 in.) o.d., 1-mm wall thickness and 120-mm
long) with end fittings; one was a Swagelok cap and the other was
a valve made by Autoclave Engineers Inc. with a reducing union,
providing the inner volume of 5.7 ml.
Cu
ZnO
2.2. Experimental procedure
A typical experimental procedure is as follows. Firstly, the
desired amount of different concentrations of HCl solution was
loaded into the reactor, then Cu-powder (catalyst), Zn-powder
(
reductant), NaHCO3 (CO2 source) were loaded into the reaction
chamber in sequence to avoid the quick reaction with HCl and the
reactor was sealed. The reactor was put into the salt bath, which had
◦
been preheated to the desired temperature (250–350 C). During
the reaction, the reactor was shaken while being kept horizontally
to enhance the mixture and heat transfer. After the desired reaction
time, the reactor was taken out of the salt bath and put into a cold
water bath to quench the reaction. The reaction time was defined
as the duration the reactor was kept in the salt bath. Filling ratio
was defined as the ratio of the volume of the solution put into the
reactor and the inner volume of the reactor.
1
0
30
50
70
90
2
-Theta (deg)
◦
Fig. 1. The XRD pattern of precipitates after hydrothermal reaction (T: 350 C; time:
2 h; Cu: 70 mmol; Zn: 60 mmol; NaHCO3: 40 mmol; HCl: 2.0 M; water filling: 50%).

Z. Huo et al. / Catalysis Today 194 (2012) 25–29
27
1
1
1
4
2
0
8
6
4
2
0
12
Zn/NaHCO =1.5
3
Zn/NaHCO =1.5
3
Zn/NaHCO =1
3
Zn/NaHCO =1
10
3
8
6
4
2
0
2
0
30
40
50
60
70
2
00
250
300
350
400
Amount of Cu (mmol)
o
Temperature ( C)
◦
Fig. 2. Effect of amount of Cu on the yields of methanol (T: 350 C; time: 2 h; Zn:
0 mmol; NaHCO3: 40 mmol (or 60 mmol); HCl: 2.0 M; water filling: 50%).
6
Fig. 4. Effect of temperature on the yield of methanol (time: 2 h; Cu: 50 mmol; Zn:
0 mmol; NaHCO3: 40 (or 60) mmol; water filling: 50%; HCl: 2.0 M).
6
from 40 to 70 mmol to respectively reach the maximum yields of
.5% and 10.4% for methanol. The highest yield of methanol was
obtained at 2 h when Cu (70 mmol) was added in the case of ratio
that HCl concentration had a greater effect on the yield of methanol
under hydrothermal conditions.
6
of Zn/NaHCO = 1:1.
3
3.3. The effects of reaction temperature and time on methanol
yields
3.2. The effect of HCl concentration on methanol yields
It is known that CO2 has high thermodynamic stability and low
To examine the effect of concentration of HCl on yield of
reactivity. Therefore, high reaction temperature is favorable to con-
version of CO2. Experiments were performed to examine the effect
of reaction temperature on the yields of methanol from CO2 at
methanol, experiments were carried out by varying HCl concentra-
◦
tion from 0 to 3 M at 350 C for 2 h in the presence of 50 mmol Cu and
0 mmol Zn. The results are shown in Fig. 3, the yield of methanol
remarkably increased as the concentration of HCl increased from
to 2.0 M. Between the HCl concentration of 0.5 M and 1.2 M,
◦
6
different reaction temperatures from 250 to 350 C for 2 h. The
results in Fig. 4 showed that the yields of methanol remarkably
◦
0
increased as the temperature increased from 250 to 350 C. The
◦
the yield of methanol was almost constant. The yield respectively
reached a maximum value of 4.2% for methanol at an HCl con-
centration of 2.0 M. The yield was decreased drastically with the
further increase of HCl concentration after attaining a peak level at
the concentration of 2.0 M. Probably, it is because of the catalyst
deactivation caused by chlorine. It has been reported that a trace
of chlorine can lead to a great loss of the activity of Cu-based cata-
lyst [5]. The optimum HCl concentration was 2.0 M to achieve the
reaction temperature was 350 C to achieve the maximum value of
10.4% methanol from CO2, respectively. In addition, in the case of
ratio of Zn/NaHCO3, the ratio of Zn/NaHCO3 = 1:1 gave higher yield
of methanol as compared to ratio of Zn/NaHCO3 = 1.5:1. It means
that large amount of NaHCO3 can improve the yield of methanol
under hydrothermal conditions.
Experiments were performed by changing the reaction time
◦
from 1 to 6 h with Cu 70 mmol, Zn 60 mmol and HCl 2.0 M at 350 C
maximum yield of methanol from CO . The present study indicated
to examine the effect of reaction time. Variation of the yields of
methanol is shown in Fig. 5. The time profile of the reaction of CO2
2
5
4
3
2
1
0
1
1
1
1
8
6
4
2
Methanol
Zn/NaHCO =1
3
10
8
6
4
0
.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1
2
3
4
5
6
+
Concentration of H (mol/L)
Reaction time (h)
◦
◦
Fig. 3. Effect of concentration of HCl on the yield of methanol (T: 350 C; time: 2 h;
Fig. 5. Effect of reaction time on the yield of methanol (T: 350 C; Cu: 70 mmol; Zn:
Cu: 50 mmol; Zn: 60 mmol; NaHCO3: 40 mmol; water filling: 50%).
60 mmol; NaHCO3: 40 (or 60) mmol; water filling: 50%; HCl: 2.0 M).

2
8
Z. Huo et al. / Catalysis Today 194 (2012) 25–29
1
2
0
8
6
4
2
0
Zn + H
2
O
ZnO + H
2
Zn/NaHCO =1.5
3
Zn/NaHCO =1
3
1
CH
3
OH, H
2
O
O
H
, CO
2
2
Cu
Zn
H
C
H
C
H
H
H
H H
H H H H
Cu
O O O
H
O O O
Zn
Cu
Zn
3
0
35
40
45
50
55
H H
C
Water filling (%)
H
◦
Fig. 6. Effect of water filling on the yield of methanol (T: 350 C; Cu: 70 mmol; Zn:
0 mmol; NaHCO3: 40 (or 60) mmol; HCl: 2.0 M).
O O O
H H H
Cu
6
Zn
indicated that the yields increased with reaction time increased in
the first 3 h. The highest yield of methanol, 11.4%, was obtained
when the reaction time was 3 h. The yield of methanol had a
small change and decreased a little from 3 to 4 h. The formation of
methanol reached to plateau after 4 h and the yield did not almost
increase significantly even at a prolonged reaction time. Therefore,
longer reaction time influenced little for the yield of methanol.
Fig. 7. Plausible mechanism for the formation of methanol from CO2 over Cu.
activity of the catalyst for methanol synthesis from CO . Liu [5]
2
reported that ZnO can act synergistically with Cu to catalyze the
synthesis of methanol, and ZnO was a good hydrogenation catalyst
that activates hydrogen by heterogeneous splitting. According to
these findings, the proposed path of reduction of CO into methanol
2
with Cu in the presence of Zn under hydrothermal conditions is
illustrated in Fig. 7. Initially, CO2 is adsorbed on ZnO, and subse-
quent undergoes stepwise hydrogenation to methoxide species,
with atomic hydrogen being supplied by spillover from Cu. The
final step is the hydrolysis of methoxide groups on ZnO to obtain
3
.4. The effect of water filling on methanol yields
The effect of filling rate on the yield of methanol was also
investigated at different water filling from 35% to 50% for 3 h
when ratio of Zn/NaHCO3 is 1:1 and 1.5:1. As shown in Fig. 6, the
yields of methanol increased as the water filling increased from
methanol along with H O produced as a co-product of methanol
2
formation.
3
5% to 50% in all cases. In the case of ratio of Zn/NaHCO = 1.5:1,
3
the yield of methanol increased from 0.6% (water filling: 35%) to
.96% (water filling: 50%). The optimum water filling was 50% to
achieve the maximum value of 11.4% methanol from CO . The
3
5. Conclusions
2
ratio of Zn/NaHCO = 1:1 showed higher yield than that of ratio of
A new chemical process for the conversion of CO into methanol
3
2
Zn/NaHCO = 1.5:1.
using Zn as a reductant and Cu as a catalyst in mild hydrothermal
conditions has been developed. The effect of various experiment
parameters was studied and the methanol yield of 11.4% was
3
4
. Proposed mechanism for catalytic conversion of CO2 into
◦
obtained at 350 C for 3 h with Cu 70 mmol, Zn 60 mmol, water
methanol in the presence of Cu
filling of 50% and HCl 2.0 M. The proposed mechanism for the for-
mation of methanol from CO2 under hydrothermal conditions was
investigated. The present study is not only helpful for future prac-
tical application in controlling the global “Green-house Effect”, but
also useful for alternative fuels or carbon resources.
After reaction, the gas samples were collected for analysis by
using GC–TCD. The experimental results are shown in Table 1. Only
H2 and CO2 were detected in gas samples, and almost no CO and
CH4 was detected, which indicated that the reactions concerning
CO and CH4 did not occur. Compared with the two experiments,
CO2 was nearly used up in experiment 1.
Acknowledgments
The mechanism for methanol synthesis from CO /H2 has been
2
proposed by a number of researchers. For example, Fisher and
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (No. 21077078),
the National High Technology Research and Development Pro-
gram of China (863 program; No. 2009AA063903), The Program
for Professor of Special Appointment (Eastern Scholar) at Shanghai
Institutions of Higher Learning.
Bell [40,41], have investigated the Cu/ZrO /SiO2 catalysts by in
2
situ infrared spectroscopy and suggested Cu supported on zirconia
is exceptionally active for methanol synthesis from CO . There-
2
fore, the addition of zirconia to Cu/SiO progressively enhances the
2
Table 1
The percentage of H2 and CO2 for gas samples after the reactions.
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