Electrochemical organic synthesis

The present invention relates to an electrochemical process for synthesizing carboxylic acids by reduction of gaseous oxides of carbon in which a gas transfer electrode is used as the cathode. The gas transfer electrodes are preferably used as hydrophobic gas transfer electrodes. In carrying out the process it is particularly preferred to use porous, hydrophobic gas transfer electrodes made from an electrocatalyst e.g. carbon, bound in a polymer such as polyethylene or polytetrafluoroethylene (PTFE). In the case of some reactions another electro-catalyst may be added to the carbon/polymer mixture. The process is particularly suited to producing acids such as formic acid and oxalic acid.

Electrochemical methods of synthesising organic compounds are known. For 
example, aqueous solutions of carbon dioxide can be electrochemically 
reduced to solutions of formate ions at low current densities. These prior 
art methods have always employed submerged electrodes and usually require 
high overvoltage which in turn therefore requires them to compete with one 
of the following hydrogen evolution reactions. 
EQU 2H.sub.3 O.sup.+ +2e.sup.- --H.sub.2 +2H.sub.2 O (acidic medium) 
EQU 2H.sub.2 O+2e.sup.- --H.sub.2 +20H.sup.- (basic medium) 
Hence, it is conventional to choose an electrode material on which the rate 
of hydrogen evolution is slow. Examples of such materials include mercury, 
lead and thallium. Since the rate of hydrogen evolution is pH dependent, 
it is also preferred to carry out the process in a neutral medium to 
minimise the adverse effects of the competitive reactions. Use of neutral 
media also enhances the solubility of carbon dioxide. A summary of results 
reported previously is given in Table 1 below together with the relevant 
references. 
TABLE 1 
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Reaction 
Current 
Current 
Voltage 
Density 
Efficiency CO.sub.2 Pressure 
Electrode 
vs SCE 
mA/cm.sup.2 
% HCOOH 
pH Electrolyte Atm Reference 
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Mercury -1.5 0.01 98 7 0.1 M NaHCO.sub.3 
1 1 
Mercury -1.95 
1.0 7 0.1 M NaHCO.sub.3 
1 1 
Mercury -1.2 0.14 8.1 1.4 
N/10 LiCl/HCl 1 2 
Mercury -1.7 0.59 60 4.6 
N/5 CH.sub.3 COOLi/CH.sub.3 COOH 
1 2 
Mercury -1.8 0.29 100 6.7 
N/10 LiHCO.sub.3 
1 2 
Rotating Copper 
-2.4 2.0 81.5 7-9 
10% Na.sub.2 SO.sub.4 
1 3 
amalgam 
Rotating Copper 
-2.4 5.0 32.8 7-9 
10% Na.sub.2 SO.sub.4 
1 3 
Rotating indium 
-1.95 
20 85 6 0.05 M Li.sub.2 CO.sub.3 
10 4 
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References: 
1 Ryu, J., Anderson, T.N. and Eyring, H., J Phys Chem, 76, 3278, 1972. 
2 Paikm W., Anderson, T.N. and Eyring, H., Electrochimica Acta, 14, 1217, 
1969. 
3 Udupa, K.S., Subramanian, G.S. and Udupa, H.V.K., Electrochimica Acta, 
16, 1593, 1971. 
4 Ko, K., Ikeda, S. and Okabe, M., Dendi Kagaku Oyobi Kogyo Butsari 
Kagaky, 48, 247, 1980. 
SCE -- Saturated Calomel Electrode 
From the results above it can be seen that the current density realised is 
dependent on mass transfer of dissolved carbon dioxide to the electrode 
surface. In the last three references in Table 1 the mass transfer 
limitation has been eased to some extent and relatively higher current 
densities achieved by increasing the solubility of carbon dioxide by 
raising the pressure above the electrolyte and/or by rotating the 
electrode at high speed. However, neither of these expedients are 
commercially attractive. Moreover, to make the process economically viable 
the current densities reported in the first five results in Table 1 at low 
carbon dioxide pressure must be increased at least by two orders of 
magnitude and it would also be desirable to reduce the reaction 
overvoltage. 
It has now been found that these problems can be mitigated by using gas 
transfer electrodes of the type conventionally used in fuel cells. 
Accordingly the present invention relates to a non-photoreductive 
electrochemical process for synthesising carboxylic acids by reduction of 
gaseous oxides of carbon characterised in that a gas transfer electrode 
which is not a photosensitive electrode having a p-type semi-conductor 
material on the surface thereof is used as the cathode. 
Gas transfer electrodes, also referred to as called gas diffusion 
electrodes, are well known. Hitherto such electrodes have been used for 
power generation in fuel cells for the oxidation of hydrogen and the 
reduction of oxygen. 
The gas transfer electrodes are used as cathodes in the process of the 
present invention. Most preferably, the gas transfer electrodes are used 
as hydrophobic gas transfer electrodes. In carrying out the process of the 
present invention any of the conventional hydrophobic gas transfer 
electrodes may be used. It is particularly preferred to use porous, 
hydrophobic gas transfer electrodes made from an electrocatalyst eg 
carbon, bound in a polymer such as a polyolefin eg polyethylene, polyvinyl 
chloride or polytetrafluoroethylene (PTFE). In the case of some reactions 
another electrocatalyst may be used. 
Electro-catalytic mixtures that may suitably be used include carbon/tin 
(powder) mixtures, carbon/strontium titanate mixtures, carbon/titanium 
dioxide mixtures and silver powder/carbon mixtures. Graphite may be used 
in place of carbon in such electro-catalytic mixtures. All these 
electrocatalysts are rendered hydrophobic by binding in a polymer such as 
polyethylene or polytetrafluoroethylene (PTFE). The specific catalysts 
chosen for a given reaction will depend upon the nature of the reactants, 
the electrolyte used and the products desired. 
The reactions which may be used to synthesise various organic compounds 
according to the process of the present invention include reduction of 
carbon dioxide and carbon monoxide to the corresponding acids, aldehydes 
and alcohols. Specifically, formic and oxalic acids may be produced by the 
reduction of carbon dioxide in this manner. 
The solvent used as electrolyte for a given reaction will depend upon the 
nature of the reactants and the products desired. Both protic and aprotic 
solvents may be used as electrolytes. Specific examples of solvents 
include water, strong mineral acids and alcohols such as methanol and 
ethanol which represent protic solvents, and alkylene carbonates such as 
propylene carbonate which represent aprotic solvents. The solvents used as 
electrolytes may have other conventional supporting electrolytes eg sodium 
sulphate, sodium chloride and alkyl ammonium salts such as triethyl 
ammonium chloride. 
The electrolytic reaction is suitably carried out at temperatures between 
0.degree. and 100.degree.C. 
Taking the specific example of carbon dioxide as a reactant, it is possible 
to control the reaction to yield a desired product by selecting the 
appropriate catalyst and electrolyte. 
For example, if a carbon/tin catalyst is used in a protic solvent such as 
ethanol, the major product is formic acid. The carbon/tin electrode 
produced formic acid at a current density of 149 mA/cm.sup.2 with a 
current efficiency of 83% and an electrode potential of -1644 mV vs SCE. 
When these results are compared with those of the prior art summarised in 
Table 1 above, the surprising nature of the invention will be self 
evident. 
The gas transfer electrodes of the present invention may be used either in 
a flow-through mode or in a flow-by mode. In a flow-through mode 
sufficient gas pressure is applied to the gas side of the electrode to 
force gas through the porous structure of the electrode into the 
electrolyte. In a flow-by mode, less pressure is applied to the gas side 
of the electrode and gas does not permeate into the electrolyte. 
The present invention is further illustrated with reference to the 
following Examples. 
The following Examples were carried out in a three compartment cell 
comprising a reference Standard Calomel Electrode compartment from which 
extended a Luggin Capillary into a cathode compartment housing the gas 
diffusion cathode and an anode compartment housing a platinum anode. The 
cathode and anode compartments were separated by a cation exchange 
membrane to prevent reduction products formed at the cathode being 
oxidised at the anode. The porous gas diffusion cathode was placed in 
contact with the electrolyte in each case. Analytical grade carbon dioxide 
was passed on the dry side of the electrode surface. 
The PTFE bonded porous gas diffusion cathodes of the present invention were 
based on carbon. Finely divided Raven 410 carbon (corresponding to 
Molacco, 23 m.sup.2 /g medium resistivity from Columbian Carbon, Akron, 
Ohio, USA) and Vulcan XC72 (230 m.sup.2 /g conductive carbon black from 
Cabot Carbons, Ellesmere Port, Cheshire, UK) were used in the Examples. 
The carbon was slurried with a PTFE dispersion (Ex ICI GPI) and, where 
indicated, an additional metal or compound, and water. The slurry was 
pasted onto a substrate which was a lead-plated twill weave nickel mesh. 
The pasted substrate was cured by heating under hydrogen for one hour at 
300.degree. C. unless otherwise stated. 
Analyses of carboxylic acid content both in aqueous and in aprotic 
solutions were done using either ion-exchange liquid chromatography or 
high performance liquid chromatography. 
The details of elecrocatalysts, electrolytes and reaction conditions used 
and results achieved are shown below. All percentages referred to are by 
weight.

EXAMPLES 1-4 
Electrode Fabrication and Electrochemical Testing 
Vulcan XC72 carbon was mixed with an appropriate amount of PTFE dispersion 
("Fluon", GP1, from ICI) and distilled water to form a slurry. This slurry 
was repeatedly applied onto a lead-plated nickel mesh or copper mesh 
current collector until on visual examination all the perforations were 
fully covered with the catalyst mixture. After drying in an oven at 
100.degree. C. for 10 minutes, the electrode was compacted, using a metal 
rod which was rolled over the electrode several times until the catalyst 
mixture was firmly imbedded on the the gauze substrate. The electrode was 
finally cured under hydrogen at 300.degree. C. for 1 hour. 
The resulting electrodes were mounted in a cylindrical glass holder which 
has a gas inlet and an outlet connected to a water manometer. The holder 
was then positioned in the cell in a floating mode at a carbon dioxide 
pressure of about 2 cm of water in order to keep one side of the electrode 
dry. The electrodes were finally used for electrolysis at a constant 
potential (shown in Table 2 below) for 90 minutes in aqueous sodium 
chloride solution (25% w/v) and at room temperature. 
TABLE 2 
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Average 
current 
Weight efficiency 
Weight of of Constant 
Average (%) for 
Ex- Vulcan XC72 
PTFE potential 
current formic 
am- carbon (mg/ Vs SCE density acid 
ple (mg/cm.sup.2) 
cm.sup.2) 
(volts) 
(mA/cm.sup.2) 
production 
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1 34.9 42 -2.00 128 21.4 
2 69.5 125.3 -1.8 46 36.8 
3 87.2 41.8 -1.8 102 76.1 
4 80 38.4 -2.0 113 40.2 
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EXAMPLE 5 
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Catalyst: 23.8% Raven 410 Carbon, 28.6% PTFE and 
47.6% tin 
powder (150 microns) 
Potential: -1644 vs SCE 
Current Density: 
150 mA/cm.sup.2 
Electrolyte: 
5% aqueous solution of sodium chloride 
pH: 4-5 at room temperature (22.5.degree. C.) 
Efficiency: 
83% for formic acid 
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EXAMPLE 6 
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Catalyst: 71.5% Raven 410 Carbon, 28.5% PTFE 
Potential: -1767 mV vs SCE 
Current Density: 
115 mA/cm.sup.2 
Electrolyte: 5% aqueous solution of sodium sulphate 
pH: 3.5-5 at room temperature (20-22.5.degree. C.) 
Efficiency: 43% for formic acid 
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