Selective electrochemical oxidation of organic compounds

The present invention relates to a method and electrochemical cell useful for the selective electrochemical oxidation of aryl-compounds including aromatic and polynuclear aromatic hydrocarbons such as benzene, naphthalene and anthracene or their derivatives such as phenols and naphthols. The anodic electrode of the cell includes a first foraminous or porous layer of a hydrophobic material; a second foraminous or porous layer which includes an oxidation catalyst; and an electrical current collector in contact with the second layer. As a result of the special chemical properties and porosity of the first and second layers of the anode, and because of careful control of the pressure differential between the electrolyte solution and the aryl-compound, an active interface is formed by the electrolyte solution and aryl-compound between the first and second layers or in the second layer of the anode thereby providing for very selective controlled oxidation of the aryl-compound with excellent current efficiencies.

BACKGROUND OF THE INVENTION 
This invention relates to electrochemical oxidation. More particularly, 
this invention relates to the selective electrochemical oxidation of 
aryl-compounds and their derivatives to quinoid compounds. 
Known methods for the electrochemical oxidation of organic compounds 
include dissolving or suspending the organic compounds to be oxidized in 
an aqueous electrolyte solution and passing this mixture through an 
electrochemical cell. Such methods have the inherent disadvantage that the 
organic compounds may be at least partially oxidized to an oxidation level 
beyond that of the desired product due primarily to the cell design. The 
product selectivity and current efficiency of such electrochemical methods 
may be lowered and undesired byproducts can be formed. 
SUMMARY 
In general, the present invention provides an electrochemical cell and a 
method for the selective electrochemical oxidation of aryl-compounds and 
their derivatives to quinoid compounds. 
The electrochemical cell for oxidizing an aryl-compound includes a cell 
body forming a compartment to hold an aqueous electrolyte solution; an 
anodic electrode including a first foraminous or porous layer of a 
hydrophobic material, a second foraminous or porous layer with an 
oxidation catalyst dispersed therein, and a current collector in 
electrical contact with the second layer, the second layer positioned to 
provide contact with the aqueous electrolyte solution; a cathodic 
electrode positioned to provide contact with the aqueous electrolyte; 
means for transporting the aryl-compound through the first layer to the 
second layer of the anodic electrode; means for maintaining a pressure 
differential between the aqueous electrolyte solution and the 
aryl-compound sufficiently low to prevent substantial bulk intermixing of 
the aryl-compound and aqueous electrolyte solution or flow of either the 
electrolyte solution or the aryl-compound through the anodic electrode 
whereby a substantially uniform interface of the aryl-compound and the 
aqueous electrolyte solution is formed at the boundary between the first 
and second layers or in the second layer of the anodic electrode; means 
for removing the quinoid oxidation product from the cell; and means for 
supplying an electrical current between the cathodic and anodic 
electrodes. 
The method for the selective electrolytic oxidation of an aryl-compound to 
a quinoid compound includes the steps of disposing an aqueous electrolyte 
solution in a compartment of an electrochemical cell with the electrolyte 
solution contacting a cathodic and an anodic electrode, the anodic 
electrode including a first foraminous or porous layer of hydrophobic 
material, a second foraminous or porous layer with an oxidation catalyst 
dispersed therein, and a current collector in electrical contact with the 
second layer, the first layer positioned to contact the aryl-compound and 
the second layer positioned tocontact the aqueous electrolyte solution; 
transporting the aryl-compound through the first hydrophobic layer to the 
second layer of the anodic electrode; maintaining a pressure differential 
between the aqueous electrolyte solution and the aryl-compound 
sufficiently low to prevent substantial bulk intermixing of the 
aryl-compound and aqueous electrolyte solution or flow of either the 
electrolyte solution or the aryl-compound through the anodic electrode 
whereby a substantially uniform interface between the aryl-compound and 
the aqueous electrolyte solution is formed at the boundary between the 
first and second layers or in the second layer of the anodic electrode; 
supplying an electrical current between the cathodic and anodic 
electrodes; and removing the quinoid oxidation product from the cell. 
As defined herein aryl-compounds and their derivatives include, but are not 
limited to, aromatic compounds, polynuclear aromatic compounds, 
substituted aromatic and polynuclear aromatic compounds, and the like. In 
addition, depending on the operation of the cell, the design of the anodic 
electrode such as the porosity of first and second layers, and the 
preferential solubility of the quinoid oxidation product, it is possible 
to remove the oxidation product on either the organic or the aqueous 
electrolyte side of the cell. 
It is an object of this invention to provide a method and an 
electrochemical cell for the selective oxidation of aryl-compounds. It is 
a further object of this invention to provide means whereby a desired 
oxidation product is protected from further oxidation prior to recovery. 
It is a further object of this invention to provide an electrochemical 
apparatus characterized by high selectivity and current efficiencies. It 
is a further object of this invention to provide a method and an 
electrochemical cell for the selective oxidation of benzene or phenol to 
para-benzoquinone, naphthalene or naphthol, to 1,4-naphthoquinone, and 
anthracene to 9,10-anthroquinone. Other objects of the invention will be 
apparent to those skilled in the art from the more detailed description 
which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description illustrates the manner in which the principles of 
the present invention are applied, but is not to be construed as in any 
sense limiting the scope of the invention. 
More specifically, FIG. 1 illustrates a schematic representation of an 
electrochemical cell 1 made according to present invention. 
The electrochemical cell 1 comprises cell walls 2; an anodic electrode 3; a 
cathodic electrode 7; a compartment 4, including a baffle or separator 10, 
for holding an aryl-compound; electrolyte solution compartments 5 and 8 
separated by a porous membrane or layer 6; and electrical leads 9 attached 
to the anodic electrode 3 and cathodic electrode 7. 
The anodic electrode 3 is formed of a first foraminous or porous layer 3a 
of a hydrophobic material, a second foraminous or porous layer 3b which 
includes an oxidation catalyst, and a current collector 3c separating the 
first layer 3a from and in electrical contact with the second layer 3b. 
The hydrophobic material forming the first layer 3a is preferably porous 
polytetrafluoroethylene. The second layer 3b may be formed from a 
composition of hydrophobic material and any well known oxidation catalyst, 
but is preferably formed from a composition of polytetrafluoroethylene and 
lead dioxide. The current collector 3c is preferably a conductive metal 
screen which is, most preferably, made of lead. 
The second layer 3b of the anodic electrode 3 is further characterized as 
having a high degree of porosity and large specific-surface area. The 
specific-surface area is preferably between about twenty and about thirty 
square meters per gram. Also, the composition of the second layer 3b is 
preferably between about eighty and about ninety percent lead dioxide, and 
between about ten and about twenty percent polytetrafluoroethylene by 
weight. Layer 3b is preferably fabricated by forming a mixture of about 
eighty percent lead dioxide, about ten percent polytetrafluoroethylene, 
and about ten percent granular sodium by weight and then calendering, 
sintering, water-leaching, and drying the layer using techniques which are 
well known for producing a porous teflon sheet. The composition of the 
dried layer 3b produced in this manner would be about eighty-nine percent 
lead dioxide and about eleven percent polytetrafluoroethylene by weight. 
The lead dioxide used in the composition of layer 3b may be freshly 
prepared using the procedure described by L. C. Newell and R. D. Maxson, 
Inorganic Synthesis, Volume 1, p. 45, which is incorporated by reference. 
Lead dioxide prepared in this manner is characterized as spongy and 
porous, with a specific surface area of between about twenty and about 
forty square meters per gram. 
The cathodic electrode 7 of cell 1 may be characterized as chemically inert 
to the aqueous electrolyte solution in compartments 5 and 8 when an 
electrical current is passed between the electrodes 3 and 7. The electrode 
7 is preferably made from platinum or lead, and most preferably, is made 
from lead. In addition, the cell 1 may include a porous layer 6 and a 
separator 10. Layer 6 may be formed from any inert porous material such as 
porous Teflon or a glass frit, and is utilized to help minimize the threat 
of an explosive mixture of hydrogen and oxygen forming in case of an 
uncontrolled cell voltage upset. Separator 10 may be formed of any 
material that is chemically inert to the aryl-compound being oxidized, the 
solvent used to transport to aryl-compound, or the oxidation products, and 
will provide for more efficient separation and recovery of the oxidation 
products. The cell walls 2 may also be characterized as chemically inert 
or resistant to the aryl-compound, solvent, or electrolytic solution which 
they contact. For example, the separator 10 and cell walls 2 around 
compartment 4 may be formed from heavy metals or polyvinylester resins, 
and the cell walls 2 around compartments 5 and 8 may be formed from 
polytetrafluoroethylene resins, polyvinylidene fluoride resins or titanium 
metal. 
The supporting aqueous electrolyte solution disposed in compartments 5 and 
8 may include an acid, salt, or a mixture of both. Preferably, the 
electrolyte solution is formed from an acid, and more preferably, the acid 
is an inorganic acid such as sulfuric acid. The most suitable 
concentration of the sulfuric acid electrolyte solution is between about 
three and about seven percent by weight, and the acid solution may be 
further saturated with lead sulfate to minimize the loss of lead dioxide 
from layer 3b of the anode electrode 3. 
The aryl-compounds disposed in compartment 4 which may be selectively 
oxidized according to the present invention include aromatic and 
polynuclear aromatic hydrocarbons such as benzene, naphthalene and 
anthracene, and their derivatives such as phenols and naphthols. 
Preferably, the aryl-compounds to be oxidized are benzene, naphthalene, 
anthracene, and phenol; and the quinoid compounds produced are 
para-benzoquinone, 1,4-naphthoquinone, and 9,10-anthroquinone. In carrying 
out the oxidation process in the cell 1, the difference in electrical 
potential across the cell electrodes 3 and 7 is controlled to provide 
oxygen from electrolytically decomposed water at a rate sufficient to 
selectively produce the desired oxidation products. If the potential is 
too low, conversion and current efficiencies may be poor, and if the 
potential is too high, undesirable oxidation byproducts may be formed. If 
necessary or desirable, an inert solvent may be used to form a solution 
with the aryl-compound to be oxidized. The solvents selected should be 
chemically inert under the reaction condition of cell 1, and for example, 
may include methylene chloride, hexane, diethyl ether, and mixtures of 
these or other similar solvents. 
Cell 1 is used to produce selective oxidation products by first placing the 
desired organic compound in compartment 4 and the supporting aqueous 
electrolyte in compartments 5 and 8. An electrical current is then passed 
through cell 1 by connecting electrodes 3 and 7, through electrical leads 
9, to the positive and negative terminals, respectively, of a suitable 
battery or other power source, not shown. As electrolysis proceeds, the 
organic compound or solution thereof diffuses through the layer 3a and 
screen 3c into layer 3b of the anodic electrode 3. At the same time, the 
electrolyte solution diffuses through the layer 3b and contacts the 
organic compound to form an interface within the porous layer 3b. 
Selective electrochemical oxidation of the organic compound takes place at 
this interface. This interface is first formed as a result of the special 
chemical properties of layers 3a and 3b and is carefully maintained by 
controlling the pressures of the aqueous electrolyte solution and 
aryl-compound in compartments 4 and 5 such that there is no substantial 
pressure differential between the two compartments. There are many well 
known methods of measuring and controlling liquid pressures, such as pump 
and valve systems, that can be used with cell 1, with the final selection 
dependent on the specific needs of the user's application. 
The oxidation product formed in cell 1 then diffuses back into compartment 
4 and is removed from the cell 1 as shown in FIG. 1. The oxidation product 
may then be separated from any inert solvent or residual aryl-compound by 
conventional known techniques such as fractional distillation or 
fractional crystallization, and the remaining solvent and aryl-compound, 
along with fresh aryl-compound, may be recycled back to compartment 4 in 
cell 1 as shown in FIG. 1. In like manner, the electrolyte solution may be 
removed from and returned to compartments 5 and 8, respectively, as shown 
in FIG. 1, thereby providing for control of the electrolyte solution 
concentration and removal of any impurities. Also, depending on the 
operating conditions of cell 1, the design features of the layers 3a and 
3b such as porosity, and the chemical properties of the oxidation product, 
it is possible, although not preferred, to remove the product with the 
electrolyte solution. 
The prevention of bulk mixing of the aryl-compound or solution thereof and 
the electrolyte solution by control of pressure in cell 1 and design of 
layers 3a and 3b is an important feature of the present invention. As 
previously noted, the known technology utilizes a solution or a suspension 
of an organic compound in the supporting electrolyte solution which is 
transported through an electrochemical cell to electrolytically oxidize 
the compound. In the present cell, bulk contact between the aryl-compound 
or solution thereof and the electrolyte solution is restricted to the 
interface within the anodic electrode 3, thereby preventing intermixing 
and further over-oxidation of the desired product to an undesirable higher 
oxidation state. This result is made possible by control of pressures in 
cell 1 and by the construction of the anodic electrode 3, whereby the 
electrical current transmitted to layer 3b of the electrode 3 is uniformly 
distributed from layer 3c through the pores of the layer 3b to the 
aryl-compound or solution thereof and the electrolyte solution at the 
liquid-liquid interface. A second important feature of the anodic 
electrode 3 is that, by virtue of its structure, the aryl-compound, or 
solution of the aryl-compound is restricted from diffusing through the 
layer 3b into the electrolyte solution. These unique structural features 
of the anodic electrode 3 and the cell 1 beneficially permit the use of 
high current densities which result in high current efficiencies, as well 
as provide the basis for highly selective oxidation of the aryl-compounds 
by electrolysis. 
In one of the preferred modes of operating the cell 1, the anodic and 
cathodic potentials may be controlled individually and separately by using 
known anodic and cathodic probes to measure the anode and cathode 
electrode potentials relative to a standard reference electrode, not shown 
in FIG. 1. For example, in a process for oxidizing benzene to 
para-benzoquinone and naphthalene to 1,4-naphthoquinone, the potential 
difference across the cell 1 may be between about two and about four 
volts, and preferably, between about two-and-one-half volts and about 
three-and-one-half volts. However, the anodic potential of the anodic 
electrode 3 may be controlled between about 1.5 and about 1.7 volts with 
an anodic probe relative to a saturated calomel electrode, thereby 
providing a more sensitive control of the rate of oxygen formation and 
subsequent oxidation product selectivity at the interface of the 
aryl-compound and electrolyte solution. 
The present invention is further illustrated by means of the following 
examples: 
EXAMPLE 1 
Using an electrode and cell made according to the present invention and 
five percent by weight aqueous sulfuric acid solution as a supporting 
electrolyte, benzene was oxidized to para-benzoquinone. The cell 
temperature was ambient, the anodic potential was +1.5 volts relative to a 
saturated calomel electrode, and the anodic current density varied between 
fifteen and twenty-five milliamperes per square centimeter. 
Para-benzoquinone was produced with a ninety-five percent selectivity and 
a ninety percent current efficiency. 
EXAMPLE 2 
Using an electrode and cell similar to Example 1 and five percent by weight 
aqueous sulfuric acid solution saturated with lead sulfate as a supporting 
electrolyte, naphthalene was oxidized to 1,4-naphthoquinone. The cell 
temperature was ambient, the naphthalene was dissolved in hexane to 
provide a solution containing fifteen percent naphthalene by weight, the 
anodic potential was about +1.5 volts relative to a saturated calomel 
electrode, and the anodic current density varied between fifteen and 
twenty-five milliamperes per square centimeter. 1,4-Naphthoquinone was 
produced with about ninety-five percent selectivity and seventy percent 
current efficiency. 
EXAMPLE 3 
Using an electrode and cell similar to Example 1 and five percent by weight 
aqueous sulfuric acid solution saturated with lead sulfate as a supporting 
electrolyte, phenol was oxidized to para-benzoquinone. The cell 
temperature was ambient, the phenol was dissolved in benzene at a 
concentration of three-and-one-half weight percent, the anodic potential 
was +1.5 volts relative to a saturated calomel electrode, and the anodic 
current density varied between fifteen and twenty-five milliamperes per 
square centimeter. Para-benzoquinone was produced with a ninety-percent 
selectivity and an eighty-percent current efficiency. In general, the 
anode potential for this reaction was approximately fifty millivolts lower 
at the same current density compared to the reaction of benzene alone in 
Example 1. 
EXAMPLE 4 
Using an electrode and cell similar to Example 1, and five percent by 
weight aqueous sulfuric acid solution saturated with lead sulfate as a 
supporting electrolyte, phenol was oxidized to para-benzoquinone. The cell 
temperature was ambient. The phenol was dissolved in n-hexane in one run 
and in methylene chloride in a second run, at a concentration of three 
weight percent. The anodic potential was +1.58 volts relative to the 
saturated calomel electrode, and the anodic current density varied between 
fifteen and twenty-five milliamperes per square centimeter. 
Para-benzoquinone was produced in both runs with a one-hundred percent 
selectivity, and an eighty-five percent current efficiency for the 
n-hexane run and an eighty-two percent current efficiency for the 
methylene-chloride run. 
EXAMPLE 5 
Using an electrode and cell similar to Example 1, and five percent by 
weight aqueous sulfuric acid solution saturated with lead sulfate as a 
supporting electrolyte, anthracene was oxidized to 9,10-anthroquinone. The 
cell temperature was ambient. The anthracene was dissolved in hexane to a 
concentration of one percent by weight. The anodic potential was between 
+1.8 and +2.0 volts relative to the saturated calomel electrode. The 
cathodic potential was -1.2 volts relative to the saturated calomel 
electrode. The anodic current density was seven milliamperes per square 
centimeter. The 9,10-anthroquinone was produced at a current efficiency of 
about fifty percent and a selectivity of about ninety percent. 
While certain representative embodiments and details have been shown for 
the purpose of illustrating the present invention, it will be apparent to 
those skilled in the art that various changes and modifications can be 
made therein without departing from the spirit and scope of the invention.