Conversion of CFC-13 to trifluoroacetic acid by electrochemical reaction with carbon dioxide

Article abstract of DOI:10.1246/cl.2002.444

The electrochemical conversion of CFC-13 (CClF₃) and carbon dioxide into trifluoroacetic acid (TFAA) was attempted at a Ag wire electrode under high pressure. TFAA was efficiently produced and the current efficiency was slightly dependent on the potential during electrolysis and pressure of co-existing carbon dioxide.

Full text of DOI:10.1246/cl.2002.444

444
Chemistry Letters 2002
Conversion of CFC-13 to Trifluoroacetic Acid by Electrochemical Reaction with Carbon Dioxide
Noriyuki Sonoyamaà and Tadayoshi Sakata
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8502
(Received November 20, 2001; CL-011173)
The electrochemical conversion of CFC-13(CClF 3) and
carbon dioxide into trifluoroacetic acid (TFAA) was attempted at
a Ag wire electrode under high pressure. TFAA was efficiently
produced and the current efficiency was slightly dependent on the
potential during electrolysis and pressure of co-existing carbon
dioxide.
electrodes, respectively. The electrolyte solution was 50 ml of
dry acetonitrile (Kanto Chemical Co., Inc.) containing 0.1 M
tetra(n-butyl ammonium) bromide (Wako Pure Chemical In-
dustries, Ltd.) that was dried in a vacuum oven at 110 ꢀC for 3
days. Before electrolysis, dissolved oxygen was removed from
the electrolyte solution by bubbling purified Ar gas for at least
20 min. 10 atm of CFC-13was introduced into the autoclave and
then further compressed carbon dioxide was introduced. Electro-
lysis started 1 h after the introduction of the gases for saturation of
the electrolyte solution with the gases. Electrolyses were carried
out at least twice with an error of at most 10% in current
efficiency, where the current efficiency is defined as the ratio of
the charge used for the formation of each product to the total
charge passed during electrolysis. Analysis of products in the gas
and liquid phases was carried out by gas chromatography and 19F-
NMR, respectively.
Figure 1 shows the current-voltage (C-V) curves at a Ag
electrode under the atmospheric pressure of Ar, carbon dioxide
and CFC-13gas. Under CFC-13, the current began to flow around
À1:0 V, whereas the current started flowing from À1:5 V under
the carbon dioxide atmosphere. Considering the result that the
current was hardly observed even at À2:0 V under the Ar gas,
these cathodic current under CFC-13and carbon dioxide is the
reduction current of these gases. This reduction potential hardly
affected by the increase in the pressure. These results indicate that
CFC-13can be selectively reduced in the potential range from
À1:0 to À1:5 V. Current efficiencies for the product formation in
electrolysis at the Ag wire electrode at the various potentials are
summarized in Table 1. The condition was that the initial 10 atm
of CFC-13was introduced into the autoclave and then carbon
dioxide was introduced until the total pressure reached 20 atm
(the initial partial pressure of each gas was 10 atm). Trifluor-
oacetic acid (TFAA) was obtained with high selectivity (69.6–
Chlorofluorocarbons (CFCs) are known to cause depletion of
the ozone layer in the stratosphere1;2 and the production of the
specified CFCs has been discontinued since 1995. However, large
amounts of CFCs are still in use as the refrigerants in refrigerators
and air conditioners. These CFCs in use should be recovered and
converted into recyclable compounds. One of the most reasonable
CFC treatment methods would be to replace chlorine in CFCs,
that gives CFCs the potential for ozone layer depletion, with other
atoms without removing fluorine atoms from CFCs. We have
carried out the electrochemical conversion of CFCs to hydro-
fluorocarbons (HFCs) that have no potential for depleting the
ozone layer and therefore are considered as the substitute of
CFCs.3;4 Although HFCs are recyclable as the substitute of CFCs,
the development of the next generation refrigerants has been
attempted.5 From the long-term point of view, a method of CFC
conversion to other valuable products would be needed. The
electrochemical formation of tetrafluoroethylene, the monomer
of Teflon, from CFC-12 (CCl2F2) was reported by several
groups.6;7 This method would be very useful for the recycle of
CFC-12, because this method produces a recyclable industrial
material from CFC-12. In the case of mono-chloro-CFCs, such as
CFC-13(CClF 3), dimer products are not able to be recycled as the
monomer of plastics because no double bond is produced by the
dimerization of mono-chloro-CFCs with selective dechlorina-
tion. On the other hand, it is well known that carbanions attack
carbon dioxide and efficiently forms carboxylic acids. If fluoro
carbanions can be electrochemically produced from mono-
chloro-CFCs by the electrochemical method, mono-chloro-CFCs
can be converted to fluorocarboxylic acids via the reaction with
carbon dioxide that is also a greenhouse gas. This method will be
useful for the recycle of CFCs, because fluorocarboxylic acids are
also recyclable as industrial materials. In this letter, we attempted
electrochemical conversion of CFC-13to trifluoroacetic acid
using a Ag electrode, that is known to selectively break the C-Cl
bond of CFC-134, in the presence of carbon dioxide.
All electrolyses were carried out in the stainless steel
autoclave. The structure of the autoclave was described else-
where.3 As the working and counter electrodes, Ag (length (L)
50 cm, diameter (D) 1 mm) and Al (L 30 cm, D 1 mm) wires were
used, respectively. A Ag wire (L 10 cm, D 0.5 mm) was used as a
reference electrode. For the measurement of the current-potential
curves, a Ag wire (L 3cm, D 1 mm) and a Pt wire (L 30 cm,
diameter 0.3mm) were used as the working and counter
Figure 1. The current-voltage curves at the Ag electrode in
acetonitrile. The scan rate was 50 mV sÀ1
.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
445
Table 1. The current efficiency for the product formation at the
various potentials in electrolysis of CFC-13in the presence of
carbon dioxide at the Ag electrode
efficiency for the product formation on the initial partial pressure
of carbon dioxide co-existing with CFC-13was plotted under the
condition that the initial partial pressure of CFC-13was 10 atm at
À1:5 V vs Ag. Although trifluoromethane was produced with a
current efficiency of about 10% in the lower pressure region, the
selectivity of TFAA hardly depended on the pressure of co-
existing carbon dioxide. The dimer products of CFC-13also
could not be detected. These results indicate that the TFAA
formation efficiently proceeds even under the condition that the
concentration of carbon dioxide in the electrolyte solution is
lower. The mechanism of this reaction is presumed as follows.
Accepted electron from the Ag electrode localizes in the Cl atom
Potential Current eficiency/%
aTFAA CHF3 CO CH4 formic H2 total
acid
À1.0
À1.25
À1.5
À2.0
69.6
12.3 bnd nd
nd nd
0.1 12.0 93.7
0.4 18.0.9103
0.2 6.1 97.1
0.2 18.9 98.3
75.310.2
85.2
72.5
5.6
nd nd
2.0 4.7 nd
The initially introduced partial pressure of CFC-13and carbon
dioxide were 10 atm. aTrifluoroacetic acid. b Not detected.
À
of CFC-13. After the elimination of the ClÀ anion, CF3 anion
radical is produced on the surface of the Ag electrode byÀthe
85.2% in the current efficiency). Other by-products were
hydrogen, trifluoromethane (HFC-23) and the carbon dioxide
reduction products (carbon monoxide and formic acid). The
current efficiency for the TFAA formation was not significantly
dependent on the potential during electrolysis and the carbon
dioxide reduction products were the minor ones even at À2:0 V,
where a certain reduction current of carbon dioxide was observed
in the C-V curve (Figure 1). These results suggest that CFC-13is
selectively reduced at the Ag electrode even in the presence of
carbon dioxide. In Figure 2, the dependence of the current
ꢁ
reduction of CF3 radical. Aymard et al. also stipulated CF3 as
the intermediate of the electrochemical reduction of CF3Br.8
À
CF3 anion radical nucleophilically attacks carbon dioxide to
form TFAA. If sufficient amount of carbon dioxide is not supplied
to the surface of the electrode, CF3À anion radical would abstract
hydrogen from the solvent molecule to form trifluoromethane, or
attack CFC-13and dimer products are formed, because an anion
radical is generally very reactive intermediate. Contrary to this
consideration, the current efficiency for the TFAA formation was
hardly dependent on the partial pressure of co-existing carbon
À
dioxide. This result seems to suggest that CF3 anion radical is
stabilized on the surface of the Ag electrode and selectively
attacks carbon dioxide.
We thank Prof. Toshio Fuchigami and Dr. Hideki Ishii for the
NMR measurements and their kind advice. We are thankful for
the helpful discussion with Dr. Hidetoshi Tokuyama. This study
was partly supported by ‘‘Ground Research Announcement for
Space Utilization’’ promoted by NASDA and Japan Space
Forum.
References
1
2
M. J. Molina and F. S. Rowland, Nature, 249, 810 (1974).
M. J. Prather and R. T. Watson, Nature, 344, 729 (1990).
3N. Sonoyama and T. Sakata, Environ. Sci. Technol., 32, 3 75
(1998).
4
N. Sonoyama and T. Sakata, Environ. Sci. Technol., 32, 4005
(1998).
5
6
A. Sekiya and S. Misaki, CHEMTECH, 26, 44 (1996).
N. S. Stepanova, M. M. Gol’din, and L. G. Feoktistov, Soviet
Electrochem., 12, 1070 (1976).
7
8
A. Schizodimou, G. Kyriacou, and Ch. Lambrou, J. Electro-
anal. Chem., 471, 26 (1999).
Figure 2. Relationship between the current efficien-
cies for product formations and the initial partial
pressure of co-existing carbon dioxide at the Ag
electrode at À1:5 V vs Ag in acetonitrile.
´ ´
F. Aymard, J.-Y. Nedelec, and J. Perichon, Tetrahedron Lett.,
´
35, 8623(1994).