JPH07116084B2 - Method for oxidizing secondary alkyl group-substituted naphthalene compounds - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention oxidizes secondary alkyl group-substituted naphthalenes with molecular oxygen to produce hydroperoxides and / or carbinols of naphthalenes in high yield. On how to do.

Technical Background of the Invention and Problems Thereof A secondary alkyl group-substituted naphthalene, for example, diisopropylnaphthalene is oxidized to diisopropylnaphthalene dihydroperoxide (hereinafter sometimes abbreviated as DHP), which is acid-decomposed with an acidic catalyst. Thus, dihydroxynaphthalene can be produced. Further, diisopropylnaphthalenedicarbinol produced when oxidizing diisopropylnaphthalene can be converted into DHP by oxidizing it with hydrogen peroxide. This dihydroxynaphthalene is industrially useful as a raw material for synthetic resins, synthetic fibers, pharmaceuticals, agricultural chemicals, dyes and the like.

By the way, in U.S. Pat.No. 4,503,262, diisopropylnaphthalene is dissolved in an organic solvent, and a heavy metal salt catalyst,
For example, in a method for producing DHP by oxidizing with molecular oxygen in the presence of cobalt organic acid, in particular, an aliphatic hydrocarbon solvent having 5 to 14 carbon atoms, for example, n-heptane is used as the organic solvent. It is described that the reaction rate, the yield and the purity of the target dihydroperoxide can be improved by the above method, but the reaction rate and the yield are not always sufficient.

Also, U.S. Pat.No. 2,751,418 describes that when oxidizing a secondary alkyl group-substituted aromatic hydrocarbon with molecular oxygen to obtain a hydroperoxide, palladium, platinum, osmium, iridium and ruthenium are used. , A method using a noble metal catalyst such as rhodium has been shown. However, the catalyst disclosed in the publication is a so-called heterogeneous catalyst in which a noble metal is supported on a solid carrier such as alumina or a solid catalyst having a large catalyst surface area such as colloidal palladium is dispersed in a reaction solution. Is used as. And in the examples of the publication, p-cymen,
Although the oxidation of alkylbenzenes such as sec-butylbenzene is shown, no specific examples are given for secondary alkyl group substituted naphthalenes such as diisopropylnaphthalene. Further, in the oxidation reaction shown in the examples, a weak base such as sodium hydrogen carbonate or sodium carbonate is used as a base, and therefore the above-mentioned oxidation reaction is carried out only in a region where the pH value of the reaction solution is low.

Further, in British Patent No. 714,545, in oxidizing an aromatic hydrocarbon substituted with a secondary alkyl group with molecular oxygen to form a hydroperoxide, the oxidation reaction is
Although it is described that the oxidation rate is improved by using a reactor or a stirring blade whose surface is covered with copper, silver or gold, this publication specifically discloses that alkylbenzene such as cumene or diisopropylben is used as the surface. There are only disclosed examples in which the oxidation is carried out using a reactor coated with copper, and the effect is still insufficient.

In addition, British Patent No. 760,367 describes cumene as copper formate,
Although a method of obtaining a hydroperoxide by oxidation in the presence of silver acetate has been disclosed, its effect is still insufficient.

The present inventors have examined the method disclosed in the above-mentioned publication in oxidizing the secondary alkyl group-substituted naphthalene compound, and even if any of the methods disclosed in the above-mentioned publication is adopted, the secondary alkyl group is It has been found that the oxidation reaction of substituted naphthalenes cannot be carried out at a satisfactory rate.

Therefore, it has been desired to develop a method for producing a high yield of hydroperoxides and / or carbinols of naphthalene by oxidizing secondary alkyl group-substituted naphthalene with molecular oxygen.

An object of the present invention is to solve the problems associated with the prior art as described above, and to oxidize secondary alkyl group-substituted naphthalenes with molecular oxygen to obtain hydroperoxides and / or carbides. An object of the present invention is to provide a method capable of obtaining hydroperoxides or carbinols as a desired product in a high yield and a high selectivity as compared with a conventional method in producing the nors.

SUMMARY OF THE INVENTION A method for oxidizing secondary alkyl group-substituted naphthalenes according to the present invention is a liquid-phase oxidation of secondary alkyl group-substituted naphthalenes with molecular oxygen to produce hydroperoxides and / or carbinols. In this case, the reaction is carried out in the coexistence of an indane-based aromatic hydrocarbon and / or a tetralin-based aromatic hydrocarbon.

Detailed Description of the Invention Hereinafter, the method for oxidizing secondary alkyl group-substituted naphthalenes according to the present invention will be described more specifically.

In the present invention, the secondary alkyl group-substituted naphthalene is liquid-phase oxidized with molecular oxygen to produce hydroperoxides and / or carbinols.

In this case, as the secondary alkyl group-substituted naphthalene,
Specifically, β-isopropylnaphthalene, β-sec-
Butylnaphthalene, 2,6-diisopropylnaphthalene,
2,7-diisopropylnaphthalene, 2,4-diisopropylnaphthalene, 2,6-di (sec-butyl) naphthalene, 1,7
-Di (sec-butyl) naphthalene and the like can be exemplified, and among these, isopropylnaphthalene and diisopropylnaphthalene are preferable, and 2,6-diisopropylnaphthalene is particularly preferable.

In carrying out the oxidation reaction according to the present invention, the presence of a base is not always necessary, but it is usually preferable to carry out the oxidation in the presence of a base, and in this case, an alkali metal compound is preferably used. As such an alkali metal compound, specifically, conventionally known compounds such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, potassium phosphate, potassium acetate, sodium acetate can be used. . The alkali metal compound is usually used as an aqueous solution.

The concentration of these alkali metal compounds in the aqueous solution is preferably 20% by weight or less. The amount of the aqueous base solution used in the reaction mixture preferably accounts for 5 to 80% by weight of the reaction mixture, and more preferably 20 to 70% by weight. When the amount of the aqueous base solution used is less than 5% by weight of the reaction mixture, the unreacted secondary alkyl group-substituted naphthalene oils and their oxidation products and the reaction solution consisting of the aqueous base solution are not well dispersed. , The emulsified state becomes insufficient, which adversely affects the oxidation reaction. On the other hand, when the amount of the aqueous base solution used is more than 80% by weight,
It is not preferable because the emulsified state of the reaction system deteriorates. Also,
In the oxidation reaction, the pH of the aqueous base solution is usually maintained at 7 or higher, preferably 12 or higher.

The secondary alkyl group-substituted naphthalene and its oxidation product, which are the raw materials, and the aqueous base solution can usually be sufficiently emulsified by mechanical stirring, but if necessary, for example, stearic acid or the like can be used. You may stir in the presence of the conventionally known emulsifier.

As the base, an alkaline earth metal hydroxide such as calcium hydroxide, magnesium hydroxide or strontium hydroxide can also be used. Particularly, calcium hydroxide is preferable. These alkaline earth metal oxides may be used alone or in combination with the alkali metal compound.

In the present invention, the oxidation reaction of the secondary alkyl group-substituted naphthalene compound is carried out in the presence of an indane aromatic hydrocarbon, a tetralin aromatic hydrocarbon, or both of them.

Specific examples of the indane-based aromatic hydrocarbon used in the present invention include indane, indene, fluorene, and their derivatives.

Further, as the tetralin aromatic hydrocarbon, specifically, tetralin, 9,10-dihydrophenanthrene, 9,10
-Dihydroanthracene or derivatives thereof are used. Of these, tetralin and 9,10-dihydroanthracene are particularly preferable.

Such an indane-based aromatic hydrocarbon and / or tetralin-based aromatic hydrocarbon is an amount of 1000 ppm or less, preferably 5-1000 ppm, more preferably 10-500 ppm, based on the secondary alkyl group-substituted naphthalene as a raw material. It is desirable to be present in an amount. The amount of the indane-based aromatic hydrocarbon and / or the tetralin-based aromatic hydrocarbon is 10% with respect to the secondary alkyl group-substituted naphthalene.
When present in an amount exceeding 00 ppm, it is not preferable because the oxidation of these indane-based aromatic hydrocarbons and / or tetralin-based aromatic hydrocarbons proceeds preferentially over the oxidation of secondary alkyl group-substituted naphthalenes, Meanwhile, 5 ppm
When the amount is less than, the indane aromatic hydrocarbon and /
Alternatively, it is not preferable because the effect of adding the tetralin aromatic hydrocarbon cannot be recognized.

In the present invention, the indane-based aromatic hydrocarbon and / or the tetralin-based aromatic hydrocarbon may be charged to the reaction system at the start of the reaction as they are, or may be continuously fed to the reaction system in small amounts, depending on the case. May be obtained by dissolving indane-based aromatic hydrocarbon and / or tetralin-based aromatic hydrocarbon in a hydrocarbon such as n-decane which is inactive against oxidation and continuously supplying it to the reaction system.

Further, in the present invention, in the oxidation reaction of the secondary alkyl group-substituted naphthalene compound, as the catalyst, a metal compound soluble in the reaction mixture solution of the secondary alkyl group-substituted naphthalene compound is used among the palladium-based metal compounds. Is preferred. As such a soluble palladium compound, specifically, an inorganic palladium compound such as palladium chloride, palladium sulfate, palladium nitrate, sodium palladium chloride, or ammonium palladium chloride, or palladium acetylacetonate, palladium oxalate, palladium acetate, or the like. Examples of the organic palladium compound include palladium chloride, palladium nitrate, and palladium acetate.

The palladium compound soluble in the reaction mixed solution of the secondary alkyl group-substituted naphthalene described above is a reaction mixed solution in an amount of 0.5 ppm or more with respect to the raw material secondary alkyl group-substituted naphthalene, as a metal-converted amount. It is preferable that the oxidative reaction is carried out by dissolving in a solution. This will be described in detail below.

In the present invention, a reaction mixed solution of a secondary alkyl group-substituted naphthalene in the present specification means a hydroperoxide, a carbinol, and an unreacted naphthalene produced by oxidation of a base and a secondary alkyl group-substituted naphthalene. A mixed solution containing an oxidation reaction product such as, that is, existing between the start of the oxidation reaction by charging the raw materials necessary for performing the oxidation reaction according to the present invention into the reactor and the end of the reaction. It means a mixed solution. Therefore, when there is no insoluble matter in the reaction mixture, the whole amount corresponds to the reaction mixed solution according to the present invention. The reaction mixed solution will be described in more detail. When an aqueous solution of the above-mentioned alkali metal compound is used as the base, the reaction mixed solution forms an oil-water mixture consisting of an oil phase and an aqueous phase. . In the present invention, the palladium compound as the catalyst described above is used for the secondary alkyl group-substituted naphthalene starting material.
It is used by dissolving it in the reaction mixture in an amount of 0.5 ppm or more,
In this case, the palladium compound may be dissolved in either the oil phase or the water phase. For example, the palladium compound as a catalyst may be dissolved only in the oil phase or only in the water phase, or may be dissolved in both the oil phase and the water phase. In short, the palladium compound as a catalyst is dissolved in either the oil phase, the water phase, or both phases, and the total amount of the dissolved amount is 0.5 ppm or more based on the secondary alkyl group-substituted naphthalene. Then, the oxidation reaction is carried out.

In the present invention, it is also possible to use the above-mentioned alkaline earth palladium hydroxide as a base in a powder form without using water, and in this case, the above-mentioned reaction mixed solution is only the oil phase, but at this time, the catalyst is used. An organic palladium compound such as palladium acetylacetonate can be used as the palladium compound of 1), and the reaction can be carried out by dissolving this in the oil phase.

In the present invention, it is preferable to use the palladium compound dissolved in the above-mentioned reaction mixed solution in an amount of 0.5 ppm or more with respect to the secondary alkylnaphthalene, and this point will be further described. When the oxidation reaction is carried out batchwise, the amount of the palladium compound is dissolved in the secondary alkylnaphthalene as a raw material in an amount of 0.5 ppm or more at the time of charging, and the reaction is carried out. When the oxidation reaction is performed continuously,
The amount of the palladium compound is a total amount obtained by adding the amount of unreacted secondary alkyl group-substituted naphthalene recycled to the reaction section and the amount of fresh secondary alkyl group-substituted naphthalene newly additionally supplied. The oxidation reaction is carried out so that the amount becomes 0.5 ppm or more.

In the present invention, it is preferable to use the following method as a method of dissolving the palladium compound in the above-mentioned reaction mixed solution in an amount of 0.5 ppm or more with respect to the secondary alkylnaphthalene. That is, a method is adopted in which a palladium compound is first dissolved in a soluble solvent to prepare a catalyst solution having an appropriate concentration, and the solution is charged into an oxidation reactor together with a base and a secondary alkyl group-substituted naphthalene compound and mixed. By doing so, it is possible to surely dissolve the palladium compound in the reaction mixture solution in a predetermined amount from the start of the reaction to carry out the oxidation reaction. Such a catalyst solution may be charged by reacting the whole amount of the catalyst solution before starting the reaction, or if necessary, an appropriate amount of the catalyst solution may be continuously or discontinuously added after the reaction is started. It is also possible to carry out the reaction by feeding to. In this case, it is preferable to use a catalyst in which a palladium compound is dissolved in a basic aqueous solution as the catalyst.

Specific examples of the soluble solvent used for dissolving the palladium compound include water, an aqueous base solution, an aqueous acid solution, and a hydrocarbon solvent. Among them, the aqueous base solution is preferable for the reasons described above. Which one is preferably used as the soluble solvent depends on what one chooses as the palladium compound used. For example, when palladium chloride is used as a catalyst, palladium chloride is difficult to dissolve in neutral water, and therefore, water whose pH is made acidic or alkaline by adding an acid or base such as hydrochloric acid or sodium hydroxide to water,
That is, it is preferable to use an aqueous acid solution or an aqueous base solution. When using palladium sulfate as a catalyst, palladium sulfate can be dissolved in any of near-neutral water, acid aqueous solution, and basic aqueous solution. When an organopalladium compound such as palladium acetylacetonate is used as a catalyst, the organopalladium is usually dissolved in a hydrocarbon solvent such as alkylbenzene or a secondary alkyl group-substituted naphthalene which is an oxidizing raw material. Hydrogen solvents can be used as soluble solvents.

In the catalyst solution thus obtained, the concentration of the dissolved palladium compound is not particularly limited. That is, when the catalyst solution was charged into a reactor together with a base and a secondary alkyl group-substituted naphthalene as a reaction raw material to carry out an oxidation reaction, the amount of the palladium compound dissolved in the reaction mixed solution during the oxidation reaction was converted to palladium. The concentration of the palladium compound in the catalyst solution is not particularly limited, as long as the amount can be 0.5 ppm or more with respect to the secondary alkyl group-substituted naphthalene as a raw material, and the palladium compound is usually converted to a metal. Displayed as 10-1000ppm
When a catalyst solution having a concentration of is prepared and used, the amount of the palladium compound dissolved in the reaction mixture solution is easily adjusted to 0.5 ppm described above.
The above amount can be used.

In the present invention, the oxidation of the secondary alkyl group-substituted naphthalene is carried out by dissolving the palladium compound in the reaction mixture solution as described above, and the dissolved amount thereof is 0.5 ppm or more with respect to the naphthalene as the raw material. On the other hand, palladium as a catalyst is used as a metal or dispersed without being dissolved in a reaction mixed solution like a supported solid catalyst, in the conventional method for use as a so-called heterogeneous catalyst, The oxidation rate of primary alkyl group-substituted naphthalenes is slow, and in order to improve the rate by this method, an extremely large amount of heterogeneous catalyst must be used.
On the other hand, the method in which the catalyst is dissolved in the reaction mixture solution as in the present invention has an advantage that a small amount is sufficient.
In addition, the amount of the palladium compound dissolved in the reaction mixture solution, relative to the secondary alkyl group-substituted naphthalene raw material
When it is less than 0.5 ppm, the oxidation rate of naphthalene compounds is not improved so much, so that the oxidation is carried out at 0.5 ppm or more, preferably 2 ppm or more. In the present invention, the upper limit of the amount of the catalyst used is not particularly limited, but for economic reasons, it is usually used at 1000 ppm or less, which is preferable.

The pH of the reaction mixture solution is usually preferably 7 or more, and particularly preferably 12 or more. Of the reaction mixture
A pH of 12 or more is preferable because the oxidation rate can be remarkably increased even if the amount of the palladium compound used as a catalyst, that is, the amount of the palladium compound dissolved in the reaction mixture solution is reduced.

The pH value of the reaction mixture solution here is 10 to 20 ml of the reaction mixture solution.
It can be carried out by sampling, allowing to stand and then directly measuring the pH of the aqueous phase that separates. When oil-water separation is difficult, the pH can be measured by adding methyl isobutyl ketone saturated with water to the reaction mixture solution.

As described above, when the pH of the reaction mixed solution is 12 or more, the oxidation reaction rate can be increased. In this sense, it is preferable to use a strong base such as sodium hydroxide as the base used in the oxidation reaction because the pH of the reaction mixed solution can be increased as compared with the case where a weak base such as sodium hydrogen carbonate is used. .

As the base, for example, using sodium carbonate or the like, when performing the oxidation reaction with the pH value of the reaction mixed solution less than 12, the amount of the palladium compound dissolved in the reaction mixed solution, as the amount converted to metal, When the secondary alkyl group-substituted naphthalene as a raw material is subjected to an oxidation reaction in an amount of usually 50 ppm or more, the oxidation rate can be increased to the same extent as when the pH value is increased.

In the present invention, molecular oxygen is used as an oxidant,
As molecular oxygen, air is usually sufficient. The amount of molecular oxygen used is usually 5 to 15 Nl / hr in terms of air with respect to 100 g of the secondary alkyl group-substituted naphthalene compound, but is not particularly limited. The oxidation reaction can be carried out either batchwise or continuously.

In the present invention, the reaction temperature is usually 80 to 150 ° C, preferably 90 to 1
It is 30 ° C., and the reaction time is usually 6 to 40 hours, although it varies depending on the conditions such as the reaction temperature. The reaction is usually carried out under pressure, but if necessary, it may be carried out under normal pressure or reduced pressure.

In the oxidation reaction of secondary alkyl group-substituted naphthalenes, a reaction initiator is preferably used. For example,
Hydroperoxide which is an oxidation product of α, α-azobis (cyclohexane-1-carbonitrile) or naphthalene can be used as a reaction initiator, and the amount thereof is usually 0.005 to 1 part by weight with respect to 100 parts by weight of the reaction mixture. It is a department.

In the present invention, the secondary alkyl group-substituted naphthalene compound is oxidized as described above to obtain a hydroperoxide compound and / or a carbinol compound. As the obtained oxidation product, when 2,6-diisopropylnaphthalene is used, 2,6-bis (2-hydroperoxy-2-propyl) naphthalene [2,6-diisopropylnaphthalene dihydroperoxide] Abbreviated as DHP], 2-
(2-Hydroxy-2-propyl) -6- (2-hydroperoxy-2-propyl) naphthalene [abbreviated as HHP], 2-isopropyl-6- (2-hydroperoxy-2-propyl) naphthalene [abbreviated as MHP] ] And other hydroperoxides are formed, and 2,6-bis (2-hydroxy-2-propyl) naphthalene [abbreviated as DCA], 2
-Carbinols such as isopropyl-6- (2-hydroxy-2-propyl) naphthalene [abbreviated as MCA] are produced.

To determine the composition of the reaction product by the oxidation reaction, for example, alcohol and the like are added to the mixed solution after the reaction and analyzed by liquid chromatography as a uniform solution. Reaction products such as DHP, HHP, DCA, MHP and MCA can be quantified.

The total amount of hydroperoxides in the oxidation reaction mixture (means the total amount of DHP, HHP, MHP, etc.,
-HPO) can be determined by a known iodometric analysis method for the organic layer.

After the completion of the oxidation reaction, a solvent such as methyl isobutyl ketone is added to the reaction mixture if necessary and oil-water separation is performed, whereby an oil phase containing hydroperoxides and carbinols can be obtained. If necessary, the solvent is distilled off from the obtained oil phase, whereby an acid decomposition raw material for acid decomposition to obtain dihydroxynaphthalene can be obtained.

Effects of the Invention According to the method for oxidizing secondary alkyl group-substituted naphthalene compounds according to the present invention, a specific amount of indane-based aromatic hydrocarbon and / or tetralin-based aromatic hydrocarbon is allowed to coexist in the reaction system. Hydroperoxides and carbinols that are oxidation products can be obtained in high yield.

Further, in the present invention, when oxidizing a secondary alkyl group-substituted naphthalene compound, a certain amount of an indane-based aromatic hydrocarbon and / or a tetralin-based aromatic hydrocarbon is allowed to coexist in the reaction system, and palladium soluble in the reaction mixture solution is used. The presence of the compound as a catalyst can remarkably increase the oxidation reaction rate of the secondary alkyl group-substituted naphthalenes.

Hereinafter, the method of the present invention will be specifically described with reference to Examples.
The present invention is not limited to these examples.

Example 1 In a 500 ml autoclave made of SUS21L equipped with a stirrer, a cooling pipe, a sampling port, a blowing pipe, and a thermowell, 50.0 g of 2,6-diisopropylnaphthalene,
4.5% NaOH aqueous solution 100.0g, PdCl ₂ 1% sulfuric acid aqueous solution 10.0g
(Pd1200pp for 2,6-diisopropylnaphthalene
m), 0.05 g of 9,10-dihydroanthracene (1000 ppm with respect to 2,6-diisopropylnaphthalene) and 0.20 g of azobiscyclohexanecarbonitrile as an initiator at once, and equipped with an autoclave in an oil bath,
After the temperature was raised to 100 ° C., the inside of the system was pressurized to 5 kg / cm ² G with air, and then air was fed at the same pressure at a flow rate of 20 l / hour to carry out a reaction for 12 hours. After adding 50.0 g of methyl isobutyl ketone to the reaction product to separate oil and water, the oil layer was diluted with methanol and analyzed by liquid chromatography.

The results are shown in Table 1.

Comparative Example 1 In Example 1, PdCl ₂ 1% sulfuric acid aqueous solution and 9,10-
The reaction was carried out for 12 hours in the same manner as in Example 1 except that dihydroanthracene was not used.

The results are shown in Table 1.

Example 2 The reaction was performed for 12 hours in the same manner as in Example 1 except that the 1% sulfuric acid aqueous solution of PdCl ₂ was not used.

The results are shown in Table 1.

Comparative Example 2 The reaction was carried out for 12 hours in the same manner as in Example 1 except that 9,10-dihydroanthracene was not used.

The results are shown in Table 1.

Example 3 In Example 1, 4.5% KHO was used instead of 4.5% KOH aqueous solution.
The reaction was carried out for 12 hours in the same manner as in Example 1 using an aqueous solution (100.0 g).

The results are shown in Table 1.

Example 4 In Example 1, instead of PdCl ₂ 1% sulfuric acid aqueous solution, P
The reaction was performed for 12 hours in the same manner as in Example 1 using 10.0 g of d (NO ₃ ) ₂ 1% sulfuric acid aqueous solution (Pd 920 ppm with respect to 2,6-diisopropylnaphthalene).

The results are shown in Table 1.

Example 5 The reaction was carried out in the same manner as in Example 1 except that 0.05 g of tetralin was used instead of 9,10-dihydroanthracene in Example 1 for 12 hours.

The results are shown in Table 1.

Example 6 The reaction was carried out for 12 hours in the same manner as in Example 1 except that the Pd concentration relative to 2,6-diisopropylnaphthalene was changed to 3 ppm.

The results are shown in Table 1.

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Claims (4)

[Claims] 1. An indane-based aromatic hydrocarbon and / or a tetralin-based aromatic compound for producing hydroperoxides and / or carbinols by liquid-phase oxidation of secondary alkyl group-substituted naphthalenes with molecular oxygen. A method for oxidizing secondary alkyl group-substituted naphthalenes, which comprises oxidizing secondary alkyl group-substituted naphthalenes in the presence of a hydrocarbon. 2. The method according to claim 1, wherein the indane aromatic hydrocarbon and / or the tetralin aromatic hydrocarbon is present in an amount of 1000 ppm or less based on the secondary alkyl group-substituted naphthalene. For oxidizing secondary alkyl group-substituted naphthalenes. 3. The secondary alkyl group as a raw material, wherein the oxidation reaction of the secondary alkyl group-substituted naphthalene is calculated as the amount of the palladium compound soluble in the reaction mixture solution of the secondary alkyl group-substituted naphthalene converted to metal. The method for oxidizing secondary alkyl group-substituted naphthalenes according to claim 1, which is carried out by dissolving the naphthalenes in an amount of 0.5 ppm or more based on the substituted naphthalenes. 4. The method for oxidizing secondary alkyl group-substituted naphthalenes according to claim 1, wherein the oxidation reaction is carried out with the pH value of the reaction mixture solution being 12 or more.