A novel approach to reduction of CO2 into methanol by water splitting with aluminum over a copper catalyst

Article abstract of DOI:10.1039/c5ra02872h

A novel method of water splitting for CO₂ reduction into methanol was proposed by directly using commercially available and non-precious Al and Cu powder as a reductant and catalyst, respectively. A 22% yield of methanol was obtained from CO₂. This technique is simple and environmentally friendly, and also provides an example of a multitude of possible chemical reactions for CO₂ conversion. This journal is

Full text of DOI:10.1039/c5ra02872h

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
A novel approach to reduction of CO2 into
methanol by water splitting with aluminum over a
Cite this: RSC Adv., 2015, 5, 31450
copper catalyst†
a
a
b
Lingyun Lyu, Fangming Jin, Heng Zhong, Hongjuan Chenc and Guodong Yaoa
Received 14th February 2015
Accepted 17th March 2015
*
*
DOI: 10.1039/c5ra02872h
www.rsc.org/advances
A novel method of water splitting for CO2 reduction into methanol hydrogen base on Cu-based catalysts.4 However, the hydrogen
was proposed by directly using commercially available and non- sources for CO2 hydrogenation are generally from fossil fuel,
precious Al and Cu powder as a reductant and catalyst, respectively. and also the hydrogen storage, transportation and purity
A 22% yield of methanol was obtained from CO2. This technique is requirement are the other technical obstacles. In addition, a
simple and environmentally friendly, and also provides an example of a complex or precious catalyst is generally needed.5 Hydrothermal
multitude of possible chemical reactions for CO2 conversion.
chemistry has received much attention in organic chemical
synthesis and biomass conversion due to the unique inherent
properties of high temperature water, including a high ion
product, a low dielectric constant, fast reaction rate and envi-
ronmental benign property. We have conducted many
researches involving biomass conversion under hydrothermal
reactions.6 On the other hand, hydrothermal reactions have
played an important role in the formation of fossil fuel in the
earth's crust and deep-sea hydrothermal vent such as the abiotic
synthesis of hydrocarbons from the dissolved CO2.7 Thus, CO2
reduction in the simulated hydrothermal vent system should be
a very desirable research. With this in mind, recently, we turned
our attention from biomass conversion into the much more
challenging CO2 reduction under hydrothermal conditions with
metal Fe as a reductant. Fe was selected because hydrothermal
uid enriches reduced ferrous-bearing mineral, which is
considered to generate reduction conditions by the reaction
with hot water.8 Our previous results with Fe as a reductant have
demonstrated the formation of formic acid from CO2.9 These
ndings inspired us to further investigated CO2 reduction with
other metals such as Zn, Mn and Al, and it was found that the
relative efficiency of CO2 reduction into formic acid was Al > Zn
> Mn > Fe.10 In these reactions, methanol formation was also
conrmed when Zn was used with Cu as a catalyst, however, the
yield of methanol was very low,11 and therefore the reactions
require further investigation for CO2 utilization. Since Al has a
better performance than Zn for CO2 reduction into formic acid,
an enhancement of methanol production from CO2 can be
expected by using Al instead of Zn. Moreover, a redox process of
ZnO/Zn, FeO/Fe3O4 and Mn(IV)/Mn(II) with solar energy have
been reported, even for MgO/Mg, its redox can be achieved by
solar power concentration using laser technology.12 The redox of
MgO/Mg suggests that the reduction of AlxOy would be much
The concentration of CO2 in the atmosphere is increasing at
an accelerating rate from 312 ppm in 1955 to 400 ppm in 2014,
and the high CO2 level is the primary source of the greenhouse
effect.1 Thus, developing a sustainable approach to CO2 utili-
zation as a cheap and abundant carbon source to produce
organic chemicals would be an especially fascinating tech-
nology, which can lead to a carbon-neutral fuel cycle in
theory.2 Many promising methods for CO2 reduction have
been proposed. Among these methods, water splitting for CO2
reduction can be considered as a promising approach because
water can be used as an in situ liquid hydrogen source and
then hydrogen storage, transportation and purity are not
required.
Methanol is one of chemical fuels from CO2 reduction,
which has been used as a transportation fuel in either modied
internal combustion engines or direct methanol fuel cells, and
also as a raw material for the synthesis of important platform
chemicals for methyl tert-butyl ether and chloromethane, etc.3
In recent decades, various processes have exemplied the
concept that methanol can be produced from CO2 and
aSchool of Environmental Science and Engineering, State Key Lab of Metal Matrix
Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai
200240, China. E-mail: fmjin@sjtu.edu.cn; Fax: +86-21-54742283; Tel: +86-21-
54742283
bResearch Center for Advance Science and Technology, The University of Tokyo, Tokyo
153-8904, Japan. E-mail: zhongheng@hotaka.t.u-tokyo.ac.jp
cState Key Laboratory of Pollution Control and Resources Reuse, College of
Environmental Science and Engineering, Tongji University, 1239 Siping Road,
Shanghai 200092, China
† Electronic supplementary information (ESI) available: Experimental details,
characterization data. See DOI: 10.1039/c5ra02872h
31450 | RSC Adv., 2015, 5, 31450–31453
This journal is © The Royal Society of Chemistry 2015

View Article Online
Communication
RSC Advances
easier than MgO when using solar energy because Mg is more CO2 into methanol. Thus, the overall reaction of the reduction
active than Al. Thus, a circulation of Al would be achieved by of CO2 with Al in water can be explained as eqn (1):
combining solar energy.
To the best of our knowledge, the hydrothermal reduction of
CO2 into methanol with Al as a reductant was not yet reported
(1)
previously. In the present article, we report the discovery of
hydrothermal conversion of CO2 to methanol with two
commercially available and non-precious metal powders of Al
and Cu directly working as a reductant and a catalyst, respec-
tively. In this process, water not only acts as an excellent greener
solvent, but also as a greener hydrogen sources. Further,
neither elaborately prepared catalysts nor expensive reagents
were used.
Subsequently, the liquid samples and gaseous samples aer
the reactions were analyzed by GC/FID, GC/MS, HPLC and
GC/TCD, respectively. Results showed that the major product
was methanol with a few amount of formic acid as well as trace
quantity of acetic acid and methane (see Fig. S3–S5†).
Although methanol could be produced from CO2 with Al on
the Cu catalyst under hydrothermal conditions, formic acid was
formed mainly as a competitive product of methanol. To
enhance the selectivity of methanol, the effect of H+ concen-
tration on the yield of methanol and formic acid was investi-
First, various transition-metals such as Fe, Zn, Mn and Al
were tested as a reductant for the reduction of NaHCO3, as a
source of CO2, with Cu-based catalysts. The results are
summarized in Table 1. Among all metals investigated, the
reaction did not give the desired methanol in the absence of Fe
or Mn as a reductant (entry 1–7). Interestingly, methanol was
formed when using Zn or Al as a reductant (entry 8 and 10) with
Cu as a catalyst. Evidently, the methanol yield with Al (1.5%)
was higher than that with Zn (0.6%), which was in accordance
with their activities. Moreover, no formation of methanol was
observed when using Al with CuO or using only Cu without Al
(entry 11 and 12). Thus, only for the case of using both Al (as a
reductant) and Cu (as a catalyst), CO2 can be reduced into
methanol. Additionally, in order to test whether Cu2+ can act as
a catalyst in the conversion of CO2 into methanol, a reaction
with CuSO4 instead of Cu powder was conducted. The results
showed that no methanol was produced (entry 13), indicating
that metallic Cu rather than Cu2+ acted as a catalyst in the
reduction of CO2 into methanol. Analysis for solid residues aer
reactions by XRD showed that Al was oxidized to AlO(OH) and
most of Cu still existed in Cu(0) (see Fig. SI-1†), conrming the
reductant role of Al and catalytic role of Cu in the reduction of
gated by adding hydrochloric acid from 0 to 2.0 mol Lꢀ1
,
because some research has reported that a weak acidic system
was adverse to formic acid formation in CO2 hydrogenation.9b,10b
As shown in Fig. 1, the yield of formic acid decreased sharply
with an increase in the initial concentration of hydrochloric
acid, while the yield of methanol increased greatly from 1.5% to
5.4%, suggesting that an acidic system was favorable for the
production of methanol. However, when the concentration of
hydrochloric acid exceeded 1.2 mol Lꢀ1, the yield of methanol
decreased. In order to further conrm the effect of H+ concen-
tration on the production of methanol, reactions with sulfuric
acid instead of hydrochloric acid with the concentration of
0.6 mol Lꢀ1 (H+ concentration of 1.2 mol Lꢀ1) was conducted. As
a result, the yield of methanol was 4.8% (see the red point in
Fig. 1), which was close to the one obtained with hydrochloric
acid in the same H+ concentration of 1.2 mol Lꢀ1. The fact that a
low pH is favorable for the production of methanol suggests
ꢀ
that methanol comes from CO2 rather than HCO3 because a
low pH is favorable for the formation CO2 in the equilibrium of
ꢀ
2ꢀ
CO2, HCO3 and CO3 in water solution.
Table 1 Effect of reductants and catalysts on the yield of methanola,b
Then, the reaction characteristics of the reduction of CO2
into methanol with Al and Cu, and also the parameters design
for getting a high yield of methanol were investigated in the
Entry
Reductant
Catalyst
Yield (%)
1
—
—
0
2
3
4
5
6
7
8
9
10
11
12
13
Fe
Fe
Fe
Mn
Mn
Mn
Zn
Zn
Al
Cu
0
0
0
0
0
0
0.6
0
1.5
0
CuO
Cu2O
Cu
Cu2O
CuO
Cu
—
Cu
Cu
CuO
CuSO4
—
Al
Al
0
0
a
Reaction conditions: 350 ꢁC, 2 h, 35% of water lling, 1 mmol
NaHCO3, 6 mmol metal, 0.5 g Cu, or 40 mmol NaHCO3, 60 mmol Zn,
40 mmol Cu (entries 8 and 9), or 40 mmol NaHCO3, 40 mmol Al, 50
mmol Cu (entries 10 and 11), or 20 mmol NaHCO3, 60 mmol Al, 50
b
mmol Cu, 0.08 mol Lꢀ1 CuSO4 (entries 12 and 13). The yield of
Fig. 1 Effect of hydrochloric acid concentration on the yield of
methanol (temperature: 350 ꢁC, time: 2 h, NaHCO3: 40 mmol, Al:
40 mmol, Cu: 50 mmol, water filling: 50%).
methanol is dened as the percentage of methanol to the initial
NaHCO3 on a carbon basis (see eqn S1†).
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 31450–31453 | 31451

View Article Online
RSC Advances
Communication
22%, which was higher than that with NaHCO3. These results
conrmed again the methanol formation was from CO2.
Additionally, a reaction with directly using gaseous CO2 and
H2 with Cu catalyst was conducted. As a result, a methanol
ꢁ
yield of 20.3% was obtained at 350 C for 2 h, which demon-
strates the suitability of using CO2, H2 with Cu catalyst to
obtain methanol under hydrothermal conditions.
Generally, a complex catalyst is needed in the traditional
hydrogenation of CO2 into methanol. The observed highly
catalytic activity of a general Cu powder under hydrothermal
conditions suggests that some intermediates might act as a
synergetic catalyst with Cu in the formation of methanol from
CO2. To examine this assumption, the solid residues aer
reactions were further analyzed by XRD. Unexpectedly, a small
amount of Cu2O was detected (Fig. SI-2(a)†). The formation of
Cu2O was probably because of the oxidation of Cu in high
temperature water, which was supported by the fact that a few
of Cu2O was detected by XRD (see Fig. SI-2(b)†), and the grain
diameter of Cu2O was less than 100 nm in a further experiment
with Cu directly reacted with NaHCO3 and HCl at 350 ꢁC.
Although further evidence is needed, these results suggest that
the high methanol yield with Cu might be attributed to a
synergistically catalytic role of the interface between Cu and
Cu2O because Cu is a conductive material and Cu2O is a good
semiconductor material with narrow band, the interface
between Cu and Cu2O would be the active site for the deeper six-
electron-reduction product methanol from CO2. Work along
these lines is now in progress.
Fig. 2 Effect of the amount of Al and Cu on the methanol yield
(350 ꢁC, 2 h, 40 mmol NaHCO3, 1.2 mol Lꢀ1 HCl, 50% of water filling).
presence of hydrochloric acid concentration of 1.2 mol Lꢀ1
.
First, the effect of the initial amounts of Cu and Al was studied.
As shown in Fig. 2, the methanol yield increased evidently with
the increase in the initial amount of Cu, when the ratio of
Cu/NaHCO3 increased to 0.5, the yield of methanol verged to a
constant. For the effect of Al amount, the yield of methanol
increased clearly with the increase in the amount of Al, which is
probably because a stronger reduction condition or a large
amount of hydrogen can improve the CO2 reduction. Consid-
ering the cost saving, although the yield of methanol still
increased with the increase in the initial amount of Al,
Al/NaHCO3 ¼ 1 : 1 was chosen in further investigations.
In the following experiments, the effects of some important
parameters such as reaction temperature, reaction time, water
lling and NaHCO3 amount on the formation of methanol were
investigated. As shown in Fig. 3, the increase in the reaction
temperature, time and water lling (pressure) were favorable for
the conversion of CO2 into methanol, and also an increase in
the initial amount of NaHCO3 led to an increase in the yield of
methanol. The highest yield of methanol reached to 18.5%,
which occurred at 350 ꢁC for 7 h with water lling 50% and
NaHCO3 40 mmol. Further, the result obtained with gaseous
Conclusions
In summary, we have developed an efficient method for reducing
CO2 into six-electron-reduction product methanol in water with
commercially available and non-precious metals of Al and Cu,
which directly worked as a reductant and a catalyst, respectively. A
high methanol yield of 22% was obtained from CO2. The proposed
method is simple, green, and environmentally friendly because an
environmental benign solvent of water and the hydrogen from
water are used in the proposed process. Furthermore, a carbon
cycle could be achieved by coupling geochemistry, specically the
metal-based water splitting for CO2 reduction, with solar driven
thermochemistry for reducing metal oxide into metal.
ꢁ
CO2 at 350 C for 2 h showed that the yield of methanol was
Acknowledgements
The authors thank the nancial support of the National Natural
Science Foundation of China (no. 21277091), the State Key
Program of National Natural Science Foundation of China (no.
21436007), Key Basic Research Projects of Science and Tech-
nology Commission of Shanghai (no. 14JC1403100).
Notes and references
Fig. 3 Effect of reaction temperature, time, water filling and the initial
amount of NaHCO3 on methanol production (40 mmol Al, 50 mmol
Cu, 1.2 mol Lꢀ1 HCl; (A) 2 h, 50% of water filling, 40 mmol NaHCO3; (B)
350 ꢁC, 50% of water filling, 40 mmol NaHCO3; (C) 350 ꢁC, 2 h,
40 mmol NaHCO3; (D) 350 ꢁC, 2 h, 50% of water filling).
1 (a) D. M. Xiang, D. Magana and R. B. Dyer, J. Am. Chem. Soc.,
2014, 136, 14004; (b) J. Hansen, M. Sato, G. Russell and
P. Kharecha, Philos. Trans. R. Soc., A, 2013, 371, 20120294.
31452 | RSC Adv., 2015, 5, 31450–31453
This journal is © The Royal Society of Chemistry 2015

View Article Online
Communication
RSC Advances
2 (a) V. S. Thoi, N. Kornienko, C. G. Margarit, P. Yang and
C. J. Chang, J. Am. Chem. Soc., 2013, 135, 14413; (b)
N. J. Brown, J. Weiner, K. Hellgardt, M. S. Shaffer and
C. K. Williams, Chem. Commun., 2013, 49, 11074.
3 (a) G. A. Olah, Catal. Lett., 2004, 93, 1; (b) X. M. Liu, G. Q. Lu
and Z. F. Yan, Ind. Eng. Chem. Res., 2003, 42, 6518; (c)
E. E. Ortelli, J. Wambach and A. Wokaun, Appl. Catal., A,
2001, 216, 227; (d) G. A. Olah, A. Goeppert, M. Czaun and
G. K. S. Prakash, J. Am. Chem. Soc., 2013, 135, 648.
7 (a) F. Jin, X. Zeng, Z. Jing and H. Enomoto, Ind. Eng. Chem.
Res., 2012, 51, 9921; (b) E. L. Shock and M. D. Schulte, J.
Geophys. Res., 1998, 103, 28513; (c) A. Yamaguchi,
M. Yamamoto, K. Takai, T. Ishii, K. Hashimoto and
R. Nakamura, Electrochim. Acta, 2014, 141, 311.
8 (a) F. Ji, H. Zhou and Q. Yang, Origins Life Evol. Biospheres,
2008, 38, 117; (b) M. T. McCollom and J. S. Seewald, Chem.
Rev., 2007, 107, 382.
9 (a) B. Wu, Y. Gao, F. Jin, J. Cao, Y. Du and Y. Zhang, Catal.
Today, 2009, 148, 405; (b) H. Takahashi, L. H. Liu,
Y. Yashiro, K. Ioku, G. Bignall and N. Yamasaki, J. Mater.
Sci., 2006, 41, 1585.
¨
¨ ¨
4 (a) F. Guder, E. Frei, U. M. Kucukbayrak, A. Menzel,
R. Thomann, R. Luptak and M. Zacharias, ACS Appl. Mater.
Interfaces, 2014, 6, 1576; (b) L. C. Grabow and
M. Mavrikakis, ACS Catal., 2011, 1, 365; (c) M. Lim, 10 (a) F. Jin, Y. Gao, Y. Jin, Y. Zhang, J. Cao, Z. Wei and
S. Kang, H. Chae, J. Bae and K. Jun, Ind. Eng. Chem. Res.,
2009, 48, 10448; (d) F. Pontzen, W. Liebner, V. Gronemann,
M. Rothaemel and B. Ahlers, Catal. Today, 2011, 171, 242.
R. L. Smith Jr, Energy Environ. Sci., 2011, 4, 881; (b) L. Lyu,
X. Zeng, J. Yun, F. Wei and F. Jin, Environ. Sci. Technol.,
2014, 48, 6003.
5 (a) M. S. Spencer, Top. Catal., 1999, 8, 259; (b) J. F. Deng, 11 Z. Huo, M. Hu, X. Zeng, J. Yun and F. Jin, Catal. Today, 2012,
Q. Sun, Y. L. Zhang, S. Y. Chen and D. Wu, Appl. Catal., A, 194, 25.
1996, 139, 75; (c) J. H. Lee, S. W. Kim, B. S. Ahn and 12 (a) P. Haueter, S. Moeller, R. Palumbo and A. Steinfeld, Sol.
D. J. Moon, J. Nanosci. Nanotechnol., 2015, 15, 400.
6 (a) J. F. Dubreuil and M. Poliakoff, Pure Appl. Chem., 2006, 78,
1971; (b) N. Akiya and P. E. Savage, Chem. Rev., 2002, 102,
2725; (c) N. Akiya and P. E. Savage, Ind. Eng. Chem. Res.,
2001, 40, 1822.
Energy, 1999, 67, 161; (b) S. Uchida, SolarPACES, Sept 11–14,
2012, I-11; (c) M. Galvez, G. Loutzenhiser, I. Hischier and
A. Steinfeld, Energy Fuels, 2008, 22, 3544; (d) E. Alonso,
C. Hutter, M. Romero, A. Steinfeld and J. Aguilar, Energy
Fuels, 2013, 27, 4884.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 31450–31453 | 31453