A METHOD OF OXIDISING SECONDARY ALKYL SUBSTITUTED NAPHTHALENES

Abstract: A method of oxidizing secondary alkyl substituted naphthalenes with molecular oxygen in a liquid phase to hydro­ peroxides, carbinols or mixtures of these, which comprises: oxidizing the secondary alkyl substituted naphthalenes in the presence of an aromatic hydrocarbon having a fused ring which contains at least one methylene group therein in amounts of not more than about 1000 ppm based on the secondary alkyl substituted naphthalene used. A process of producing isopropylnaphthols is also disclosed, which comprises: oxidizing diisopropylnaphthalenes with molecular oxygen in a liquid phase to diisopropylnaph­ thalene monohydroperoxides in the presence of (a) either an aromatic hydrocarbon having a fused ring which contains at least one methylene group therein, or a paladium catalyst soluble in the reaction mixture, and (b) an organic polar compound such as acetonitrile; and then acid-decomposing the diisopropylnaphthalene monohydroperoxide to the isopropyl­ naphthol.

Description: This invention relates to a method of oxidizing secondary alkyl substituted naphthalenes. More particularly, the invention relates to a method of producing naphthalene hydroperoxides and/or carbinols in high yields by oxidizing secondary alkyl substituted naphthalenes with molecular oxygen in a liquid phase. It is already known that dihydroxynaphthalenes are produced by the oxidation of a secondary alkyl substituted naphthalenes such as diisopropylnaphthalenes to diisopropylnaphthalene dihydroperoxides, and the subsequent acid-decomposition of the diisopropylnaphthalene dihydroperoxide with an acid catalyst. Further, the diisopropylnaphthalene dicarbinols by-produced in the oxidation of diisopropylnaphthalenes are converted to diisopropylnaphthalene dihydroperoxides by the oxidation with hydrogen peroxide. The dihydroxynaphthalenes are useful as raw materials for the production of synthetic resins, synthetic fibers, medicines, agricultural chemicals or dyestuffs, as is well known. In U.S. Patent No. 2,751,418 there is described a process of the catalytic oxidation of secondary alkyl substituted aromatic hydrocarbons with molecular oxygen to hydroperoxides in the presence of a noble metal catalyst such as palladium, platinum, osmium, iridium, ruthenium, or rhodium. The nobel metal catalyst therein is used in a finely divided form, and the metal is, for example, supported on a solid carrier material such as alumina, or the metal is used in a colloidal form such as colloidal palladium. This type of solid or heterogeneous catalyst is disadvantageous from the standpoint of industrial feasibility of the process. Further, in the U.S. Patent No. 2,751,418, the oxidation of alkylbenzenes such as p-cymene or sec-butylbenzene is specifically disclosed, however, nothing is described about the oxidation of secondary alkyl substituted naphthalenes such as diisopropylnaphthalenes. As another aspect of the process disclosed therein, the oxidation reaction is carried out in the presence of a weak base such as sodium hydrogen carbonate or sodium carbonate, and thus only at low pH ranges. In U.S. Patent No. 4,503,262 there is described that diisopropylnaphthalenes are oxidized with molecular oxygen to diisopropylnaphthalene dihydroperoxides in organic solvents in the presence of heavy metal catalysts such as organic acid salts of cobalt, and that the use of aliphatic hydrocarbons of 5-14 carbons as a solvent improves the oxidation rate, and the yield and purity of the obtained dihydroperoxides. However, the improvement is limited and has been found insufficient. A further method of the catalytic oxidation of secondary alkyl substituted aromatic hydrocarbons with molecular oxygen to hydroperoxides is also known, in which the reaction is carried out in a reaction vessel having a inner surface coated with copper, silver or gold, to improve the oxidation rate, as is disclosed in British Patent No. 714,545. However, in this reference also, the oxidation of alkylbenzenes such as cumene or sec-butylbenzene is specifically disclosed, but nothing is described about the oxidation of secondary alkyl substituted naphthalenes such as diisopropylnaphthalenes. A still further method is disclosed in British Patent No. 760,367, in which cumene is oxidized in the presence of copper acetate or silver acetate to hydroperoxides. But the improvement in these methods has been still insufficient. The present inventors have studied the application of the above-mentioned prior processes to the oxidation of secondary alkyl substituted naphthalenes, and found that in any prior process the oxidation of secondary alkyl substituted naphthalenes does not proceed at a feasible rate. The inventors have made intensive investigations to solve the problems as mentioned above, and found that the oxidation of secondary alkyl substituted naphthalenes with molecular oxygen in a liquid phase in the presence of an aromatic hydrocarbon which has a ring fused thereto containing at least one methylene group therein provides desired hydroperoxides and/or carbinols in higher selectivities and in higher yields than in any known process. Therefore, it is an object of the invention to provide a novel process of oxidizing secondary alkyl substituted naphthalenes with molecular oxygen in a liquid phase to hydroperoxides and/or carbinols in higher selectivities and in higher yields than in the prior arts. According to the invention, there is provided a method of oxidising secondary alkyl substituted naphthalenes with molecular oxygen in a liquid phase to hydroperoxides, carbinols or mixtures of these, which comprises: oxidizing the secondary alkyl substituted naphthalene in the presence of an aromatic hydrocarbon having a fused ring which contains at least one methylene group therein in amounts of not more than about 1000 ppm based on the secondary alkyl substituted naphthalene used. In the method of the invention, the secondary alkyl substituted naphthalene has one or more secondary alkyls preferably of 3 or 4 carbons, and is exemplified by β -isopropylnaphthalene, β -sec-butylnaphthalene, 2,6-diisopropylnaphthalene, 2,7-diisopropylnaphthalene, 2,4-diisopropylnaphthalene, 2,6-di(sec-butyl)naphthalene or 1,7-di(sec-butyl)naphthalene, among which are preferred isopropylnaphthalenes or diisopropylnaphthalenes, and most preferred is 2,6-diisopropylnaphthalene. A base may be or may not be used in the method of the invention, however, it is usually preferred that the reaction be carried out in the presence of a base. The base used may be any alkali metal compound known in the art as usable, such as hydroxides, carbonates, phosphates and acetates, e.g., sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, potassium phosphate, sodium acetate or potassium acetate. The alkali metal compound is used usually as an aqueous solution in the reaction. The alkali metal compound may be used as a mixture. Alkaline earth metal compounds are also usable as a base, which include calcium hydroxide, magnesium hydroxide and strontium hydroxide, among these is preferred calcium hydroxide. The alkaline earth metal compound may be used singly or as a mixture with another alkaline earth metal compound or an alkali metal compound. In the oxidation reaction, the secondary alkyl substituted naphthalene, their oxidation products and the aqueous alkaline solution are fully emulsified usually with aid of an effective agitation, however, an emulsifier known in the art such as stearic acid may be used, if desired. The concentration of the alkali or alkaline earth metal compound in an aqueous solution is preferably not more than about 20 % by weight. The aqueous alkaline solution is used in the reaction in amounts of about 5-80 % by weight, preferably in amounts of about 20-70 % by weight, based on the reaction mixture. When the amount of the solution is less than about 5 % by weight based on the reaction mixture, an organic mixtures of the unreacted secondary alkyl substituted naphthalenes and the oxidation products thereof is poorly dispersible in the aqueous alkaline solution, to render the mixture insufficiently emulsified, and this adversely affects the oxidation reaction. When the amount of the aqueous alakline solution is more than about 80 % by weight, too, the reaction mixture is insufficiently emulsified. The aqueous alkaline solution is maintained usually at a pH of not less than about 7, preferably a pH of not less than about 12 in the oxidation reaction. The oxidation reaction is carried out in the presence of an aromatic hydrocarbon having a fused ring which contains at least one methylene group therein, which will be hereinafter often referred to as the methylene containing aromatic hydrocarbon. The aromatic hydrocarbon is composed preferably of 9-14 carbons, and is exemplified by an indane structure aromatic hydrocarbon which includes indene, indane and fluorene, and tetralin structure aromatic hydrocarbon which includes tetralin, 9,10-dihydrophenanthrene and 9,10-dihydroanthracene. The methylene containing aromatic hydrocarbon may be used singly or as a mixture of two or more. Tetralin, 9,10-dihydroanthracene or fluorene is particularly preferred in the invention. The aromatic hydrocarbon is used in amounts of not more than about 1000 ppm, preferably of about 5-1000 ppm, most preferably of about 10-500 ppm, based on the secondary alkyl substituted naphthalene used. When the aromatic hydrocarbon is used in amounts of more than about 1000 ppm based on the secondary alkyl substituted naphthalene used, the oxidation of the aromatic hydrocarbon undesirably predominates over the oxidation of the secondary alkyl substituted naphthalene used, whereas when the aromatic hydrocarbon is used in amounts of less than about 5 ppm, substantially no improvement is attained in the selectivity of the reaction and the yield of the desired products. The aromatic hydrocarbon may be either added as it is to the reaction mixture at the outset of the reaction, or added as it is continuously and gradually in portions during the reaction or as a solution in an organic solvent inactive to the oxidation reaction such as n-decane. In the oxidation reaction of the invention, it is preferred that the reaction be carried out in the presence of a catalyst composed of a palladium compound which is soluble in the reaction mixture. The preferably used catalyst includes inorganic palladium compounds such as palladium chloride, palladium sulfate, palladium nitrate, sodium tetrachloropalladate(II), ammonium tetrachloropalladate(II), and organic palladium compounds such as palladium acetylacetonate, palladium oxalate or palladium acetate, and among these are particularly preferred palladium chloride, palladium nitrate or palladium acetate. The palladium catalyst is used in the reaction in amounts of not less than about 0.5 ppm as metallic palladium based on the secondary alkyl substituted naphthalene used. Herein the specification, the reaction mixture means a solution of a mixture of the secondary alkyl substituted naphthalene unreacted if any, oxidation products thereof, i.e., hydroperoxides and/or carbinols, the base and water. Namely, the reaction mixture means a solution of a mixture present at any instant throughout the oxidation reaction of the secondary alkyl substituted naphthalene used. Therefore, when the reaction system contains no insolubles, the entire of the reaction system is a reaction mixture. More specifically, when an aqueous alkaline solution is used in the reaction, the reaction mixture is composed of two phase of a water-immiscible or insoluble organic phase and an aqueous phase. The palladium catalyst used is at the outset of the reaction dissolved in amounts of not less than about 5 ppm based on the secondary alkyl substituted naphthalene used in the reaction mixture as hereinbefore described, and in this case, the palladium catalyst may be dissolved either in the water-immiscible organic phase or in the aqueous phase, or in the both, in total in amounts of not less than about 5 ppm based on the secondary alkyl substituted naphthalene used. Meanwhile, when a powdery base is used, not as an aqueous solution thereof, the reaction mixture is composed only of an organic phase. In this case, a palladium catalyst soluble in the organic phase is used, such as palladium acetylacetonate. When the reaction is carried out batchwise, the palladium catalyst is dissolved in the reaction mixture in amounts of not less than about 5 ppm based on the secondary alkyl substituted naphthalene used at the outset of the reaction. When the reaction is carried out continuously, the palladium catalyst is so used as to be present in amounts of not less than about 5 ppm based on the total amount of the unreacted secondary alkyl substituted naphthalene circulated to a reaction zone and the secondary alkyl substituted naphthalene anew added to the reaction zone. In practicing the invention, it is preferred that the palladium catalyst be first added to a solvent in which the catalyst is soluble to prepare a catalyst solution of a suitable concentration, and the solution is fed into a reaction vessel together with a secondary alkyl substituted naphthalene and a base. This method assures the presence of the catalyst in amounts exactly predetermined at the outset of the reaction. However, if desired, the catalyst solution may be added to the reaction mixture continuously or intermittently in small portions after the reaction has started. In this case, an aqueous alkaline solution of the catalyst is preferably used. In the invention, the solvent used for the preparation of the palladium catalyst solution may be water, aqueous alkaline solutions, aqueous acidic solutions or hydrocarbons, and more specifically, the solvent is selected depending upon the palladium compound used as a catalyst. For instance, when palladium chloride is used as a catalyst, an aqueous acidic or alkaline solution is preferred as a solvent, since palladium chloride is scarcely soluble in neutral water. The aqueous acidic or alkaline solution may be, for example, hydrochloric acid or sodium hydroxide solution, respectively. When palladium sulfate is used as a catalyst, either almost neutral water, an aqueous acidic solition or an alkaline solution may be used as a solvent. When an organopalladium compound such as palladium acetylacetonate is used as a catalyst, since an organopalladium compound is usually soluble in hydrocarbons such as alkylbenzenes or secondary alkyl substituted naphthalenes, such hydrocarbons are preferably used as a solvent. The concentration of a palladium compound in the catalyst solution thus prepared is not specifically limited in the invention, so far as the concentration of the catalyst in the reaction mixture is maintained as being not less than about 0.5 ppm as metallic palladium based on the secondary alkyl substituted naphthalene used. However, it is advantageous to use a solution which contains a palladium compound in concentrations of about 10-1000 ppm as metallic palladium since the concentration of the palladium catalyst in the reaction mixture is readily adjusted to not less than about 0.5 ppm as metallic palladium based on the secondary alkyl substituted naphthalene used when such a catalyst solution is used. The process in accordance with the invention uses a homogeneous palladium catalyst, or a catalyst dissolved in the reaction mixture in the reaction, and thus the process has an advantage in that a high oxidation reaction rate is achieved in the presence of only a very small amount of a catalyst. On the contrary, according to a prior process in which a heterogeneous catalyst, or metallic palladium or palladium supported on a solid carrier material dispersed in the reaction mixture is used, the oxidation rate is usually slow, and it is therefore necessary to use much amounts of a palladium catalyst to increase the reaction rate. It is preferred that the oxidation reaction of the invention be effected in the presence of a palladium catalyst in amounts of not less than about 0.5 ppm as metallic palladium based on the secondary alkyl substituted naphthalene used, as hereinbefore described, since when the catalyst amount is less than about 0.5 ppm as metallic palladium based on the secondary alkyl substituted naphthalene used, the oxidation rate is not scarcely improved. From the industrial standpoint, the use of a palladium catalyst in amounts of not less than about 2 ppm is preferred. An upper limit is not specically limited, but the amount of the catalyst used is usually up to about 1000 ppm as metallic palladium based on the secondary alkyl substituted naphthalene used from the standpoint of process economy. Further the pH of the reaction mixture is preferably not less than about 7. The pH of the reaction mixture may be measured as follows. An amount of 10-20 ml of the reaction mixture is sampled and left standing, and the pH of the resulting aqueous phase is directly measured. When a difficulty is attended in the separation of the aqueous phase from the organic phase, methyl isobutyl ketone saturated with water is added to the sampled reaction mixture, and the pH of the solution can be measured. In the process of the invention, it is especially preferred that the pH of the reaction mixture be not less than about 12 to remarkably increase the oxidation rate. In this regard, the use of a strong base such as sodium hydroxide rather than a weak base such as sodium carbonate is preferred in the invention. However, the reaction may be carried out at a pH of less than about 12 in the presence of a weak base such as sodium carbonate. When this method is employed, it is preferred that a palladium compound be used usually in amounts of not less than about 50 ppm as metallic palladium based on the secondary alkyl substituted naphthalene used, to attain an oxidation rate substantially the same as in the reaction at a high pH region. In the process of the invention, molecular oxygen is used as an oxidant, and usually air is used. Although the amount of the molecular oxygen used is not specifically limited, but usually in the range of about 5-15 Nl/hour as air in relation to 100 g of the secondary alkyl substituted naphthalene used. The process of the invention may be carried out either batchwise or in a continuous manner. The reaction is effected usually at temperatures ranging from about 80°C to about 150°C, preferably from about 90°C to about 130°C, usually for a period of about 6-40 hours, although depending upon the other reaction conditions such as reaction temperatures. The reaction is usually carried out under elevated pressures, but may be carried out under normal or reduced pressures, if necessary. In the process of the invention, the use of reaction initiator is preferred. The reaction initiator used includes, for example, an azobis initiator such as α, α-azobis(cyclohexane-1-carbonitrile), or a hydroperoxide as an oxidation product of naphthalenes or their derivatives. The amount of the reaction initiator used is usually in the range of about 0.005-1 parts by weight per 100 parts by weight of the reaction mixture fed. According to the process of the invention, the secondary alkyl substituted naphthalene is oxidized to provide hydroperoxides and/or carbinols. When 2,6-diisopropylnaphthalene is used as a starting material, there are produced hydroperoxides which include 2,6-bis(2-hydroperoxy-2-propyl)naphthalene (referred to also as 2,6-diisopropylnaphthalene dihydroperoxide), 2-(2-hydroxy-2-propyl)-6-(2-hydroperoxy-2-propyl)naphthalene and 2-isopropyl-6-(2-hydroperoxy-2-propyl)naphthalene, and/or carbinols which include 2,6-bis(2-hydroxy-2-propyl)naphthalene and 2-isopropyl-6-(2-hydroxy-2-propyl)naphthalene. The composition of the reaction products may be determined by liquid chromatography. By way of example, an alcohol is added to the reaction mixture, and the resultant solution is subjected to chromatographic analysis to determine the composition of the products qualitatively. After the reaction, the reaction mixture, if necessary after the addition thereto of, for example, methyl isobutyl ketone, is separated into an aqueous layer and an organic oily layer, in which the hydroperoxides and/or carbinols produced are contained. Thus, the total amount of the hydroperoxides produced in the reaction may be determined by the iodometric analysis of the organic layer. The removal of solvents from the reaction mixture provides a material which gives dihydroxynaphthalenes by acid decomposition. As set forth above, according to the process of the invention, not only a high oxidation rate but also a high yield of the desired oxidation products of the secondary alkyl substituted naphthalene used, that is, hydroperoxides and carbinols, are achieved. In particular, both of the alkyls of the secondary alkyl substituted naphthalene are oxidized in the oxidation. The use of a palladium catalyst remarkably enhances the oxidation rate. The invention will now be more fully described with reference to examples, which however are not to be construed as limiting to the invention. Example 1 In a 500 ml capacity autoclave made from SUS 21L provided with a stirrer, a cooling tube, a sampling opening, an inlet tube and a thermowell were placed 50.0 g of 2,6-diisopropylnaphthalene, 100.0 g of a 4.5 % aqueous sodium hydroxide solution, 10.0 g of a 1 % aqueous sulfuric solution of PdCl₂ (1200 ppm based on palladium of 2,6-diisopropylnaphthalene), 0.05 g of 9,10-dihydroanthracene (1000 ppm based on 2,6-diisopropylnaphthalene) and 0.20 g of an initiator (azobiscyclohexanecarbonitrile). The autoclave was heated to 100°C in an oil bath, and was pressurized to 5 kg/cm²G with air, and then the oxidation was carried out under the pressure for 12 hours while air was fed thereinto at a rate of 20 litres per hour. After the completion of the reaction, 50.0 g of methyl isobutyl ketone was added to the reaction mixture to separate an organic layer from an aqueous layer. The organic layer was diluted with methanol and analized by liquid chromatography. The results are shown in Table 1. Comparative Example 1 Palladium chloride and 9,10-dihydroanthracene were not used and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Example 2 Palladium chloride was not used and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Comparative Example 2 9,10-Dihydroanthracene was not used and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Example 3 An amount of 100.0 g of a 4.5 % aqueous potassium hydroxide solution was used in place of the sodium hydroxide solution, and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Example 4 An amount of 10.0 g of a 1% aqueous sulfuric acid solution of Pd(NO₃)₂ was used (920 ppm as palladium based on 2,6-diisopropylnaphthalene) in place of the PdCl₂ solution, and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Example 5 An amount of 0.05 g of tetralin was used in place of 9,10-dihydroanthracene and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Example 6 Palladium chloride was used in amounts of 3 ppm based on the diisopropylnaphthalene and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Example 7 An amount of 0.05 g of fluorene was used in place of 9,10-dihydroanthracene and otherwise in the same manner as in Example 1, the reaction was carried out for 12 hours. The results are shown in Table 1. Example 8 An amount of 100.0 g of a 4.5 % aqueous potassium hydroxide solution was used in place of the potassium acetate solution and otherwise in the same manner as Example 1, the reaction was carried out for 12 hours. The results are shown in Table 2.

Claims: A process for producing secondary alkyl substituted naphthalene hydroperoxides, carbinols or mixtures of these, which comprises: oxidising a secondary alkyl substituted naphthalene with molecular oxygen in a liquid phase in the presence of an aromatic hydrocarbon having a fused ring which contains at least one methylene group therein in amounts of not more than 1000 ppm based on the secondary alkyl substituted napthalene used. A process according to claim 1 wherein the oxidation reaction is carried out in the present of a palladium catalyst soluble in the reaction mixture in amounts of not less than 0.5 ppm based on the secondary alkyl substituted naphthalene used. A process according to claim 1 or claim 2 wherein the oxidation reaction is carried out at a pH of not less than 7. A process according to claim 3 wherein the oxidation reaction is carried out at a pH of not less than 12. A process according to any one of claims 1 to 4 which comprises oxidising an isopropyl- or diisopropylnaphtalene.

Inventors: HIROKANE, NOBUYA, ISHIGURO, MASAHARU, MIKI, HISAYA
Applicants: MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1988
Main classification: C07C409
IPC codes: C07C39, C07C409, C07C37