Process for the electroreduction of aromatic carboxylic acids

Disclosed herein is a process for the electroreduction of aromatic acids comprising reducing an aromatic carboxylic acid electrolytically in an aqueous acidic solution by the use of a material of a low oxygen overvoltage as the anode for oxygen generation, thereby producing the corresponding benzyl alcohol in high yield in a single electrolytic apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The invention relates to a process for producing benzyl alcohols from the 
corresponding aromatic carboxylic acids. 
Among benzyl alcohols, there are many compounds useful as intermediates for 
agricultural chemicals and pharmaceuticals or as perfumes. However, it has 
not been successful to date to produce them at low costs on an industrial 
scale. 
2. Prior Art of the Invention 
It is well known to reduce aromatic carboxylic acids electrolytically in 
aqueous acidic solutions. For instance, it has been proposed to submit 
aromatic carboxylic acids to electroreduction at 70.degree. C. in the 
presence of sodium amalgam and 15 wt. % sulfuric acid [Bericht 38, 1752 
(1905)]. However, this process gives so low yield that it has not been 
adopted industrially. 
The present inventors have previously proposed to reduce m-hydroxybenzoic 
acid or esters thereof electrolytically at a pH of 4 or less in an aqueous 
solution or in a water-soluble organic solvent (Japanese Patent Laid-Open 
No. 234987/1985). 
The inventors have also proposed to use a cation-exchange membrane as a 
diaphragm and add a quaternary ammonium salt as a supporting electrolyte 
in the electroreduction (Japanese Patent Laid-Open No. 243293/1985). 
The inventors have further proposed an electroreduction process for 
producing high purity p-xylylene glycol from other benzoic acids than 
m-hydroxybenzoic acid, for example, from terephthalic acid (Japanese 
Patent Laid-Open No. 297482/1987). 
In addition, the inventors have proposed an electroreduction process of 
m-phenoxybenzoic acid as a benzoic acid other than m-hydroxybenzoic acid 
(Japanese Patent Laid-Open Nos. 17188/1988 and 192883/1988). 
The electroreduction of these aromatic carboxylic acids are usually carried 
out in an aqueous acidic solution. No particular restrictions are imposed 
on the aqueous acidic solution so far as the acidic substance contained 
therein is inert to the electrolytic reaction at the cathode. Costwise, 
however, it is generally desirable to use mineral acids, and hence 
sulfuric acid has been particularly used from the standpoint of material 
and yield. 
Cathode materials used in the electrolytic reaction include those involving 
high hydrogen overvoltages, specifically, zinc, lead, cadmium, mercury, 
and the like. 
In the opposing anode, it is common to use metallic materials that are not 
corroded in aqueous sulfuric acid solutions or otherwise do not affect the 
cathodic reaction adversely even if they are taken into solution as 
metallic ions. Platinum, lead oxide, lead and lead alloys are being 
generally used. 
However, the foregoing anode materials including platinum, lead oxide, lead 
and lead alloys have high oxygen overvoltages at usual supply voltages 
(0.1 A/dm.sup.2 -100 A/dm.sup.2) and therefore their oxidizing abilities 
are high. Consequently, when the electrolytic reaction of aromatic 
carboxylic acids is carried out in a single electrolytic cell in which the 
anode and cathode compartment are not separated from each other, oxidation 
as well as reduction take place simultaneously, resulting in a significant 
decrease in the yield of the intended benzyl alcohols. 
In order that the oxidation reaction on the surface of the anode is 
suppressed and the intended product is obtained in a high yield, it has 
been necessary to conduct the electrolytic reaction in an anode-cathode 
separated electrolytic cell in which the anode compartment is separated 
from the cathode compartment by a diaphragm like an ion-exchange membrane, 
and feed the raw material only to the cathode compartment. 
However, use of the anode-cathode separated electrolytic cell in the 
industrialization of the electrolytic reaction unavoidably increases the 
power cost due to the resistance of the diaphragm, increases the cost of 
expendables as the diaphragm, and causes mass transfer from the cathode 
compartment to the anode compartment during the reaction. The transfer 
loss leads to a reduction in the yield, while the two solution tanks 
required for the anode and cathode renders the apparatus complicated. 
Further, the installation of the diaphragm has imposed a restriction on 
the shapes of the electrolytic cell and the electrodes. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an improved process for the 
production of benzyl alcohols in high yields from the corresponding 
aromatic carboxylic acids by their electroreduction in an aqueous acidic 
solution in a single electrolytic cell in which the anode compartment is 
not separated from the cathode compartment, without using a processwise 
complex anode-cathode separated electrolytic cell. 
The inventors have made intensive investigations to attain the above 
object. As a result, it has been found that when the electrolysis is 
conducted in a single electrolytic cell, in which the anode and cathode 
compartments are not divided, by lowering its anode potential, the 
oxidation and decomposition of organic matters (i.e., aromatic carboxylic 
acids used as the raw material and/or benzyl alcohols which are organic 
matters or reduction products derived from the raw material, benzaldehydes 
as an intermediate, radical anion intermediates, etc.) by the anode are 
suppressed, and the intended corresponding benzyl alcohols can be obtained 
with high selectivities. 
According to the invention, when an aromatic carboxylic acid is reduced 
electrolytically in an aqueous acidic solution to produce the 
corresponding benzyl alcohol, use of an anode material involving a low 
oxygen overvoltage as the anode material for oxygen generation allows the 
reaction to proceed in a single electrolytic apparatus, in which the anode 
is not isolated from the cathode by a diaphragm, in a yield as high as in 
the conventional process in which the compartments are separated from each 
other by a diaphragm. 
When employed industrially, the process proves to be very useful from a 
practical viewpoint because the electrolysis is effected in a simple 
single apparatus provided with a diaphragm only in the electrolytic 
chamber.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
The invention is described in detail hereinbelow. FIG. 3 illustrates the 
relationship between the anode potential and the benzyl alcohol 
selectivity in the cathodic reduction of the corresponding benzoic acids 
in a single electrolytic cell under the control of the anode potential at 
a fixed level. As clearly seen from FIG. 3, the lower the anode potential, 
the higher is the benzyl alcohol selectivity. As the anode potential is 
increased, the selectivity is reduced. At a point where the anode 
potential exceeds +1.1 V at 25.degree. C. in a 15 wt. % aqueous sulfuric 
acid solution (based on a mercury sulfate electrode; +1.7 V, when 
expressed in a standard hydrogen electrode potential), a sudden decrease 
of the benzyl alcohol selectivity is observed in the reduction of any 
benzoic acids. 
The inventors have thus found that it is desirable to conduct the reaction 
at an anode potential of not higher than +1.1 V in order to attain a high 
selectivity in a single electrolytic cell. However, in using anode 
materials inherently involving high oxygen overvoltages (not less than 
+1.1 V at a practical current density of from 0.1 A/dm.sup.2 to 100 
A/dm.sup.2), such as platinum, lead oxide, lead and lead alloys, which are 
generally used stably in aqueous acidic solutions, if the electrolytic 
reaction is conducted in an aqueous acidic solution by maintaining the 
anode potential at as low as +1.1 V or less, the current density must be 
reduced to far less than 0.1 A/dm.sup.2, resulting in a prolonged reaction 
time and a lowered efficiency of the electrolytic cell. 
The inventors have also found that by using electrode materials having 
inherent oxygen overvoltages of +1.1 V or less at a practical current 
density for electrolytic reactions (commonly in the order of from 0.1 
A/dm.sup.2 to 100 A/dm.sup.2) as the anode, it becomes possible to design 
an industrially practicable process using a single electrolytic cell in 
which the anode compartment is not separated from the cathode compartment. 
In the electrolytic reaction in an aqueous acidic solution, oxygen 
generation due to the electrolysis of water and oxidation-decomposition of 
aromatic carboxylic acids used as the raw material and/or of organic 
compounds derived from the raw material takes places competitively at the 
anode. Then, use of an anode material of a low oxygen overvoltage allows 
the electrolysis of water at the anode to take place at a low potential so 
that oxygen generates there preferentially. Thus, it becomes possible to 
control the oxidation-decomposition of organic matters by the anode, which 
is responsible for the decrease in the yield of the intended benzyl 
alcohols. The invention has been completed on the basis of these findings. 
The invention offers an electroreduction process of aromatic carboxylic 
acids which comprises, in the electroreduction of aromatic carboxylic 
acids in an aqueous acidic solution to produce the corresponding benzyl 
alcohols, conducting the reaction under the control of the 
oxidation-decomposition of organic matters by the use of an anode material 
involving a low oxygen overvoltage as the anode material for oxygen 
generation. 
No particular limitations are placed on the aromatic carboxylic acids used 
as the raw material in the invention, so far as they are reducible in a 
conventional electrolytic cell consisting of two compartments in which a 
diaphragm is provided. They may include, for example, benzoic acid, 
hydroxybenzoic acid, alkoxybenzoic acids, phenoxybenzoic acid, 
aminobenzoic acid, alkylbenzoic acids, isophthalic acid and 
hydroxymethylbenzoic acid. 
As the anode material involving a low oxygen overvoltage, which is useful 
in the practice of the invention, it is advisable to use a metallic oxide 
which is preferably stable in an aqueous acidic solution and the oxygen 
overvoltage of which is not higher than +1.1 V at a supply current density 
of from 0.1 A/dm.sup.2 to 100 A/dm.sup.2 on the basis of a mercury sulfate 
electrode (measured at 25.degree. C. in a 15 wt. % aqueous sulfuric acid 
solution; not higher than +1.7 V, when expressed in a standard hydrogen 
electrode potential). The anode may also include a metallic substrate 
having its surface covered by the oxide. 
As the oxide may be mentioned the oxides of Group VIII, platinum group, 
such as ruthenium oxide, rhodium oxide, palladium oxide, osmium oxide, 
iridium oxide, platinum oxide, tin oxide, tantalum oxide and cobalt oxide, 
and mixtures thereof. 
It is preferable from an industrial viewpoint to use DSE (Dimensionally 
Stable Electrode) formed by coating the surface of a metallic titanium 
substrate with the oxide of a platinum group metal as the chief component. 
In the invention, there is no particular restriction imposed on the aqueous 
acidic solution, so far as it is of an acidic substance inert to the 
electrolytic reaction at the cathode. However, mineral acids may 
preferably be used in view of cost. Among others, sulfuric acid is 
favorable from the standpoint of material and yield, and hence a 1 to 50 
wt. % aqueous sulfuric acid is generally used. 
It is also preferable in the invention to add a quaternary ammonium salt, 
an aprotic polar solvent or both of them into the electrolytic reaction 
solution, for the purpose of increasing the solubility of aromatic 
carboxylic acids used as the raw material. 
The quaternary ammonium salt is represented by the formula (I): 
##STR1## 
wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are individually a lower 
alkyl group and X represents the acid radical of p-toluenesulfonic acid, 
sulfuric acid, hydrochloric acid or hydrobromic acid. Among these ammonium 
salts are tetraethylammonium p-toluenesulfonate, tetramethylammonium 
p-toluenesulfonate, tetrapropylammonium p-toluenesulfonate, 
tetrabutylammonium p-toluenesulfonate, and the sulfates, chlorides and 
bromides substituted for these p-toluenesulfonates. These salts are added 
to the aqueous acidic solution in an amount up to a high of 60 wt. %. 
The aprotic polar solvent may include acetonitrile, 
N,N'-dimethylimidazolidinone, N,N'-dimethylformamide, 
N-methyl-2-pyrrolidone and sulfolane. These polar solvents are added in an 
amount up to a high of 10 fold by weight based on the aqueous acidic 
solution used. 
The concentration of the aromatic carboxylic acid in the aqueous acidic 
solution generally ranges from 1 wt. % to 40 wt. %. 
In the invention, the electroreduction is carried out at a temperature in 
the range of from 20.degree. C. to 100.degree. C. of the electrodes for 
use in the electrolysis, the cathode employs a material involving a high 
hydrogen overvoltage, specifically, zinc, lead, cadmium or mercury. 
In the electroreduction of the invention, the current density may 
preferably be in the range of from 1 A/dm.sup.2 to 30 A/dm.sup.2. 
Theoretically, the reaction entails 4-electron reduction so that it could 
have been accomplished by a quantity of electricity of 4 Fr/mol. However, 
a quantity of electricity of from 8 Fr/mol to 20 Fr/mol is required to 
complete the reaction, because the current efficiency is about 20% to 80%. 
No particular limitation is imposed on the electrolytic apparatus used in 
the invention. Basically, however, it is usual to employ a method 
comprising charging an electrolyte (2) in a beaker-type electrolytic cell 
(1) in which no diaphragm is provided, fixing an anode (3) and a cathode 
(4) therein and conducting a current therethrough from a DC power supply 
(see FIG. 1). 
In the case of a filter press-type electrolytic apparatus, it is feasible 
to use a single electrolytic apparatus in which both of the anode-cathode 
electrolytic chamber and a solution tank (5) are not divided, as seen in 
FIG. 2. 
Further, upon the electrolytic reaction in an aqueous solution, oxygen 
evolves at the anode on account of the electrolysis of water, whereas at 
the cathode, hydrogen generation occurs by the electrolysis of water as a 
side reaction, in addition to the reduction of a benzoic acid used as the 
raw material. Thus, there is a danger of explosion due to the mixing of 
oxygen and hydrogen. 
As a means to avoid the danger of explosion, it is preferable, for example, 
to blow forcibly an inert gas like nitrogen or argon into the electrolytic 
cell (1) or the electrolytic chamber (6) and a gas-liquid separator (8), 
reduce the pressure in the electrolytic call or chamber, and withdraw the 
gases to the outside of the system effectively. 
It is also possible to use a process in which hydrogen and oxygen in the 
electrolytic cell or chamber are diluted so that the reaction can be 
carried out under the lower explosion limit of hydrogen, an apparatus as 
illustrated in FIG. 4 in which the electrolytic cell itself is not divided 
but a diaphragm (7) is provided in the section involving the danger of the 
mixing of the generated gases with a view toward the prevention of the 
mixing of oxygen and hydrogen, and a filter press-type single apparatus as 
shown in FIG. 5 in which one solution tank is not divided and a diaphragm 
is provided only in the electrolytic chamber to isolate the two gases. 
In the electroreduction of aromatic carboxylic acids, when the reaction is 
carried out in an anode-cathode separated electrolytic apparatus, a 
cation-exchange membrane is commonly used as a diaphragm so as to minimize 
the transfer of organic matters from the cathode to the anode and, in 
addition, to make smooth the transfer of protons from the anode to the 
cathode, with the aim of improving the rate of recovery of the intended 
products in the cathode compartment. However, the cation exchange membrane 
is expensive and entails the problem of instability in long-term 
operations. 
However, in the case of the diaphragm useful in the practice of the 
invention, it is not necessary to consider the transfer of organic matters 
from the cathode to the anode and that of protons from the anode to the 
cathode. Therefore, it is possible to use neutral polymer membranes, 
asbestos diaphragms, glass diaphragms, etc. which are inexpensive and 
stable as well as are capable of separation of the generated gases at the 
anode and cathode. 
The invention is illustrated specifically by reference to the following 
examples. In the examples, "%" signifies wt. %. The oxygen overvoltage of 
the anode was measured at 25.degree. C. in a 15 wt. % aqueous sulfuric 
acid by the use of a mercury sulfate electrode as a reference electrode. 
EXAMPLE 1 
In a 100-ml cylindrical single electrolytic cell were charged 25 g of a 15 
wt. % aqueous sulfuric acid and 1 g of m-hydroxybenzoic acid. A 6-cm.sup.2 
lead plate and a 6-cm.sup.2 titanium plate having its surface coated with 
iridium oxide (oxygen overvoltage: +0.95 V vs Hg.sub.2 SO.sub.4) were used 
as a cathode and an anode, respectively. 
While maintaining the electrolytic cell at 50.degree. C., 1 A of a constant 
DC was allowed to flow for 2 hours (10.3 Fr/mol). 
Thereafter, the reaction solution was analyzed by liquid chromatography 
(HPLC). The analysis revealed that the rate of residual m-hydroxybenzoic 
acid was 2.5% and the yield of m-hydroxybenzyl alcohol was 86.9%, based on 
the feed m-hydroxybenzoic acid (current efficiency: 37.9%). 
EXAMPLE 2 
An experiment was conducted in much the same manner as in Example 1, except 
that a titanium plate coated with iridium oxide and tantalum oxide (oxygen 
overvoltage: +0.88 V vs Hg.sub.2 SO.sub.4) was used as the anode in place 
of the titanium plate coated with iridium oxide. 
The reaction solution was analyzed by HPLC. The rate of residual 
m-hydroxybenzoic acid was found to be 2.1%, while the yield of 
m-hydroxybenzyl alcohol was 88.5%, based on the feed m-hydroxybenzoic acid 
(current efficiency: 38.0%). 
EXAMPLE 3 
An experiment was conducted in much the same manner as in Example 1, except 
for the use of 0.88 g of benzoic acid in place of the m-hydroxybenzoic 
acid as the reaction raw material. 
The reaction solution was analyzed by HPLC. The analysis revealed that the 
rate of residual benzoic acid was 0.9% and the yield of benzyl alcohol was 
91.5%, based on the feed benzoic acid (current efficiency: 38.5%). 
EXAMPLE 4 
An experiment was conducted in much the same manner as in Example 1, except 
that 0.78 g of m-phenoxybenzoic acid was used in place of the 
m-hydroxybenzoic acid as the reaction raw material, 1 g of 
tetraethylammonium p-toluenesulfonate being added, and 1 A of a constant 
DC was allowed to flow for 1.5 hours (15.4 Fr/mol). 
The reaction solution was analyzed by HPLC. The analysis revealed that the 
rate of residual m-phenoxybenzoic acid was 1.1% and the yield of 
m-phenoxybenzyl alcohol was 90.5%, based on the feed m-phenoxybenzoic acid 
(current efficiency: 25.7%). 
EXAMPLE 5 
An experiment was conducted in much the same manner as in Example 1, except 
for the use of 0.99 g of o-aminobenzoic acid in place of the 
m-hydroxybenzoic acid as the reaction raw material. 
The reaction solution was analyzed by HPLC. The analysis revealed that the 
rate of residual o-aminobenzoic acid was 0.5% and the yield of 
o-aminobenzyl alcohol was 81.2%, based on the feed o-aminobenzoic acid 
(current efficiency: 38.6%). 
EXAMPLE 6 
An experiment was conducted in much the same manner as in Example 1, except 
that 1.54 g of 2,3,4-trimethoxybenzoic acid was used in place of the 
m-hydroxybenzoic acid as the reaction raw material and 5 g of acetonitrile 
was added. 
The reaction solution was analyzed by HPLC. The analysis revealed that the 
rate of residual 2,3,4-trimethoxybenzoic acid was 5.2% and the yield of 
2,3,4-trimethoxybenzyl alcohol was 82.6%, based on the feed 
2,3,4-trimethoxybenzoic acid (current efficiency: 36.8%). 
EXAMPLE 7 
In a filter press-type single electrolytic apparatus equipped with a 300-ml 
solution tank as shown in FIG. 5 were charged 250 g of a 15 wt. % aqueous 
sulfuric acid and 10 g of m-hydroxybenzoic acid. 
A lead plate with an effective area of 50 cm.sup.2 and a titanium plate 
with an effective area of 50 cm.sup.2 which is coated on its surface with 
iridium oxide (oxygen overvoltage: +0.88 V vs Hg.sub.2 SO.sub.4) were used 
as a cathode and an anode, respectively. Between the two electrodes, an 
asbestos diaphragm was provided to prevent the generated gases at the 
electrodes from mixing. 
While maintaining the electrolytic cell at 50.degree. C., 5 A of a constant 
DC was allowed to flow for 4 hours (10.3 Fr/mol). 
Thereafter, the reaction solution was analyzed by liquid chromatography 
(HPLC). The analysis revealed that the rate of residual m-hydroxybenzoic 
acid was 2.0% and the yield of m-hydroxybenzyl alcohol was 88.6%, based on 
the feed m-hydroxybenzoic acid (current efficiency: 38.0%). 
COMATIVE EXAMPLE 1 
An experiment was conducted in much the same manner as in Example 1, except 
for the use of a platinum plate (oxygen overvoltage: +1.3 V vs Hg.sub.2 
SO.sub.4) in place of the iridium oxide as the anode. 
The reaction solution was analyzed by HPLC. The analysis revealed that the 
rate of residual m-hydroxybenzoic acid was 0.2% and the yield of 
m-hydroxybenzyl alcohol was 17.2%, based on the feed m-hydroxybenzoic 
acid. 
COMATIVE EXAMPLE 2 
An experiment was conducted in much the same manner as in Example 1, except 
for the use of a lead plate (oxygen overvoltage: +1.4 V vs Hg.sub.2 
SO.sub.4) in place of the iridium oxide as the anode. 
The reaction solution was analyzed by HPLC. The analysis revealed that the 
rate of residual m-hydroxybenzoic acid was 0.6% and the yield of 
m-hydroxybenzyl alcohol was 5.9%, based on the feed m-hydroxybenzoic acid. 
COMATIVE EXAMPLE 3 
An experiment was conducted in much the same manner as in Comparative 
Example 1, except for the use of 0.78 g of benzoic acid in place of the 
m-hydroxybenzoic acid as the reaction raw material. 
The reaction solution was analyzed by HPLC. The analysis revealed that the 
rate of residual benzoic acid was 1.2% and the yield of benzyl alcohol was 
26.2%, based on the feed benzoic acid. 
REFERENCE EXAMPLE 
Using a H-type electrolytic cell which is divided into two electrolytic 
chambers, each having a volume of 100 ml, by a diaphragm called Celemion 
CMV (trade name; a cation exchange membrane manufactured by Asahi Glass 
Co.), 50 g of a 15% aqueous sulfuric acid was charged in each of the 
chambers. A 6-cm.sup.2 lead plate and a 6-cm.sup.2 platinum plate (oxygen 
overvoltage: +1.3 V vs Hg.sub.2 SO.sub.4) were used as a cathode and an 
anode, respectively. 
While maintaining the electrolytic cell at 50.degree. C., 2 g of 
m-hydroxybenzoic acid was charged in the cathode chamber and 1 A of a 
constant DC was allowed to flow for 4 hours (10.3 Fr/mol). 
The cathode solution was analyzed by HPLC. The analysis revealed that the 
residual rate of the feed m-hydroxybenzoic acid was 2.3% and the yield of 
m-hydroxybenzyl alcohol was 83.0% (current efficiency: 37.9%).