Carbon dioxide thermal system: An effective method for the reduction of carbon dioxide

Article abstract of DOI:10.1039/b100183n

When carbon dioxide is in the supercritical state and reduced by Fe₃O₄, multicarbon bearing hydrocarbon molecules such as phenol (rather than CO or formate usually formed in electrochemical or photochemical techniques) can be obtaine

Full text of DOI:10.1039/b100183n

Carbon dioxide thermal system: an effective method for the reduction
of carbon dioxide
Q. Chen*ab and Y. Qianc
a Structure Research Laboratory, University of Science & Technology of China (USTC), Hefei 230026,
P.R. China. E-mail: cqw@ustc.edu.cn
b Department of Materials & Engineering, USTC, Hefei 230026, P.R. China
c
Department of Chemistry, USTC, Hefei 230026, P.R. China
Received (in Cambridge, UK) 4th January 2001, Accepted 19th June 2001
First published as an Advance Article on the web 12th July 2001
When carbon dioxide is in the supercritical state and
reduced by Fe3O4, multicarbon bearing hydrocarbon mole-
cules such as phenol (rather than CO or formate usually
formed in electrochemical or photochemical techniques) can
be obtained, the reduction yield is improved remarkably and
the transformation yield for CO2 to phenol can reach
7.6%.
(GC/FID) (Ohkura, GC-202 with Porapak R column packing)
was used to determine the content of the hydrocarbon molecules
formed.
A typical reaction used 8.0 g CO2, 22 g Fe3O4 and 0.5 ml
H2O, in which CO2 was in excess for the oxidation of Fe2+ in
Fe3O4, to ensure a high pressure in the system when it was
heated to appropriate temperatures. It was found that the degree
of transformation in carbon dioxide reduction increases with
increasing temperature and time, and reaches a maximum value
after 2.5 h treatment, then remains nearly constant. The total
yield of ethanol and acetic acid was around 4.2% (mol ratio)
when the reaction was carried out at 200 °C for 2.5 h and was
decreased with an increase of temperature and the amount of
H2O included. However, temperatures higher than 300 °C
yielded aromatic compounds with phenol and diphenyl ether
being the main products with ethanol, acetic acid and acet-
aldehyde also present. Fig. 1(a) depicts the mass spectrum of
standard phenol while that of phenol formed in the experiment
The reduction of carbon dioxide has been extensively studied
using electrochemical1,2 and photochemical3–5 reactions in light
of the problems of global warming and depletion of fossil
fuels.6–8 Results obtained are still not satisfactory as to useful
valuable reaction products and reaction rates need to be further
improved. In addition the detailed reduction mechanism could
not be established in most cases. At present, much interest has
focused on the use of 14-membered transition-metal micro-
cycles in CO2 electrochemical reduction,9–12 however, the
catalysis efficiency for these transition-metal complexes is poor
and the catalyst is rapidly destroyed by hydrogenation and/or
carboxylation of the microcycles.13 On the other hand, although
research to optimise the reaction products is active, multicarbon
containing hydrocarbon molecules such as products containing
benzene rings have never been obtained. CO is often the main
reduction product while formate may also be formed depending
on the reaction condition.9,14 Photochemical CO2 reduction has
2+
been carried out in a catalytic system using Ru(bpy)3 as the
sensitizer, cobalt or nickel macrocycles as the electron relay
catalysts, and ascorbate as the sacrificial reductive quencher.3,4
These systems, however, also produce CO. Furthermore, the
rate of CO2 reduction is limited by the low mass transfer of CO2
both in electrochemical and photochemical techniques. Hence,
the conversion of CO2 to useful products by a simple effective
method is clearly an interesting and important topic in CO2
chemistry. Here we report that, when CO2 is in the supercritical
state and reduced with Fe3O4 powder, it can be transformed to
phenol and diphenyl ether. In some cases, we can obtain
ethanol, acetaldehyde and acetic acid. This novel reduction
method could allow studies of continuous reduction systems for
practical industrial applications.
An autoclave (flexible Au/Ti) capable of heating the system
up to 400 °C was used. A sufficient amount of solid CO2,
freshly made from high purity CO2 gas (99+%), was placed in
an autoclave to ensure that the CO2 is in a supercritical state at
high temperatures. An appropriate amount of Fe3O4 (chemical
purity reagent) and a small amount of water obtained from the
Milli-Q water purification system were placed into the auto-
clave (50 ml), which was heated to 100–350 °C for 1–3 h and
then rapidly cooled to room temperature naturally. The vapor
phase was sampled and analyzed by GC–MS, (Shimadzu, GC-
MS-QP-1100EX) to detect hydrocarbons. An appropriate
amount of water was placed into the autoclave, then the product
was collected, and filtered. After filtration, the filtrate was
analyzed by GC–MS and quantified by gas chromatography
(GC), while solid products were examined by X-ray diffraction
(XRD). A gas chromatograph with a flame ionization detector
Fig. 1 Mass spectrum of phenol: (a) standard sample, (b) phenol formed via
CO2 reduction.
1402
Chem. Commun., 2001, 1402–1403
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b100183n

is shown in Fig. 1(b). The peak at m/z 94 in Fig. 1 is assigned to
the molecular ion, C6H6O+. Almost all of the corresponding
peaks in Fig. 1(a) and (b) appear at the same positions,
indicating that phenol formed in the reaction. On the basis of
this result, the formation of diphenyl ether in the present system
can be rationalized in terms of phenol production in the carbon
dioxide thermal reduction system. Crystals of phenol can be
obtained by slowly heating the aqueous filtrate. A scanning
electron microscope image of the sample shows the sample
consists of typical grains with size 0.2 3 0.3 mm (not shown).
The total yield of phenol was nearly 7.6%. The GC–MS
spectrum (not shown) shows no detectable organic compounds
in the starting CO2 gas ruling out contamination in the starting
material. GC analysis showed ethane, propane and isobutane,
were the main products in the vapor phase, while no CO in the
vapor phase or formic acid in aqueous solution was detected
(the main products in electrochemical and photochemical
reduction techniques). These results indicate that the mecha-
nism of the reduction in our reaction is different from that for an
electrochemical reduction process. Carbon dioxide is a low-
energy molecule, with the standard potential of the CO/CO2·2
couple in an aprotic solvent such as N,NA-dimethylformamide
ity of CO2 can be changed by controlling its density, the
dielectric constant of CO2 ranging from 1 to 1.6.15 The
increased polarity in a high pressure system is favorable for CO2
absorption on the surface of Fe3O4 particles, and may accelerate
the electron transfer from FeII ions to an intermediate due to the
changed adsorption energy level in the energy gap of Fe3O4
semiconductor. Further details about the mechanism will be
discussed later. The most significant feature is that the reductant
used for CO2 reduction is easily obtained and this reaction can
be performed in a continuous autoclave, which could lead to
practical applications.
In conclusion, we have reported, for the first time, the
reduction of carbon dioxide in the supercritical state. Valuable
products such as ethanol, acetaldehyde, acetic acid and,
especially, phenol can be obtained. The maximum transforma-
tion ratio for CO2 to phenol was found to be as high as 7.6% at
temperatures > 300 °C. The reduction is suggested to occur on
the surface of the Fe3O4 particles and to occur via a
multielectron reductive coupling of a pair of or several
carbonyls to produce an intermediate. This might open a route
for industrial reduction of CO2, which previously was only
viable via an electrochemical technique.
+
(DMF) containing a non ion-pairing counter cation (NEt4 ),
being as negative as 22.2 V vs. SCE.13 The potential for the
Fe3+/Fe2+ redox couple is not as negative as that for the CO2
reduction to CO2· anion radical. Hence reduction most probably
occurs on the surface of Fe3O4 particles, and a surface-mediated
process may be involved. The XRD pattern of the solid product
shows the coexistence of Fe2O3 and Fe3O4. This is possibly due
to the surface layer of Fe3O4 being oxidized by carbon dioxide.
Furthermore, hydrocarbon molecules containing more than one
carbon atom are the products, suggesting that the mechanism of
this reaction could involve multielectron reductive coupling of
a pair of or several carbonyls to produce an intermediate bound
to the surface of the solid Fe3O4 particles (at FeII sites). The
presence of a small amount of water is required in this reaction
system as this is the source of hydrogen. Too much water,
however, is unfavorable for the reduction of carbon dioxide. It
has also been found that the transformation ratio increases with
a decrease of Fe3O4 particle size, which suggests the Fe3O4
surface area seems to have an influence on the reaction. All
these results suggest that adsorption, formation of intermediates
and hydrolysis processes are involved in the reaction. More
work should be carried out to establish the reduction mechanism
more fully.
Notes and references
1 J. L. Roberts and D. T. Sawyer, J. Electroanal. Chem., 1965, 9, 1.
2 P. G. Russel, N. Novac, S. Srinivasan and M. Sterrinberg, J. Electro-
chem. Soc., 1977, 124, 1329.
3 J. L. Grant, K. Goswami, L. O. Spreer, J. W. Otvos and M. Calvin,
J. Chem. Soc., Dalton Trans., 1987, 2105.
4 C. A. Craig, L. O. Spreer, J. W. Otvos and M. Calvin, J. Phys. Chem.,
1990, 94, 7957.
5 G. M. Brown, B. S. Brunschwing, C. Creutz, J. F. Endicott and N. Sutin,
J. Am. Chem. Soc., 1979, 101, 1298.
6 T. Inone, A. Fujishima, S. Konishi and K. Honda, Nature, 1979, 277,
637.
7 P. G. Jessop, T. Ikariya and R. Nayori, Nature, 1994, 368, 231.
8 K. R. Thampi, J. Kiwi and M. Gratzel, Nature, 1987, 327, 506.
9 B. Fisher and R. Eisenberg, J. Am. Chem. Soc., 1980, 102, 7361.
10 G. B. Balazs and F. C. Anson, J. Electroanal. Chem., 1992, 322, 325.
11 M. H. Schmidt, G. M. Miskelly and N. S. Lewis, J. Am. Chem. Soc.,
1990, 112, 3420.
12 C. A. Kelly, Q. G. Mulazzani, M. Venturi, E. L. Blinn and M. A.
Rodgers, J. Am. Chem. Soc., 1995, 117, 4911.
13 I. Bhugun, D. Lexa and J.-M. Saveant, J. Phys. Chem., 1996, 100,
19 981.
14 T. Ogata, S. Yanagida, B. S. Brunschwig and E. Fujita, J. Am. Chem.
Soc., 1995, 117, 6708.
15 B. D. Drake and R. L. Smith, Jr., J. Supercrit. Fluids, 1990, 3, 162.
It is known that the rate of CO2 reduction is limited by the low
mass transfer of CO2 both in electrochemical and photo-
chemical techniques. Under supercritical conditions, the polar-
Chem. Commun., 2001, 1402–1403
1403