BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates in general to the production of acetoxyaryl carboxylic acids. More particularly, the present invention relates to the production of acetoxyaryl carboxylic acids from di-isopropylaryls through an autoxidation catalytic process.

2. Description of the Related Art Phenolic compounds, such as phenol and hydroquinone, act to inhibit free radical reactions in general, and particularly oxidation reactions. As such, phenolic compounds, including derivatives thereof such as p-tert-butylcatechol and natural phenolic compounds such as tocopherols and rosemary oil extracts, are used commercially to inhibit spoiling and rancidity of foods. That is, by inhibiting oxidation, foods can be preserved.

The conversion of isopropyl benzene (cumene) to phenol by oxidation to the corresponding cumyl hydroperoxide, followed by"Hock"rearrangement to yield phenol and acetone is an important method for the production of phenol. Di-isopropyl-aryls can undergo similar reactions to produce the corresponding aryl diols. For example, by applying heat, p-diisopropyl benzene can be oxidized to the corresponding di- hydroperoxide by the following reaction I.

p-isopropyl benzene is first oxidized to the corresponding monohydroperoxide. After accumulation of the monohydroperoxide product, further oxidation produces the di- hydroperoxide product. Reaction I is generally run as a continuous process. The output of the reactor contains approximately 60% p-diisopropyl benzene, 30% monohydroperoxide and 10% di-hydroperoxide. The di-hydroperoxide is separated from the p-diisopropyl benzene and the monohydroperoxide. The p-diisopropyl benzene and monohydroperoxide are recirculated. The di-hydroperoxide is then rearranged as shown in the following reaction II.

The major products of the"Hock"rearrangement of reaction II are hydroquinone and acetone. Reaction II proceeds in the presence of an acid catalyst. For this purpose, an acidic resin such as Amberlyst 15 Tll, an exchange resin sold by Roam and Haas, may be used. Although reaction II shows the rearrangement of the di-hydroperoxide, it is important to note that the monohydroperoxide could also be rearranged to produce 1 mole of p-isopropyl phenol and 1 mole of acetone. However, further catalytic oxidation from p-isopropylphenol to hydroquinone generally does not occur because the free phenol group serves as a radical trap and inhibits such further oxidation.

Liquid crystal polyesters are high performance plastics used where high strength, good dimensional stability and high heat distortion temperatures are required.

Acetoxyaryl acids and their corresponding hydroxy acids are important and valuable intermediates in the preparation of liquid crystal polyesters. Acetoxyaryl acids are also valuable in the field of preservatives.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a use for the monohydroperoxides produced in the oxidation of diisopropyl benzenes.

It is a further object of the present invention to provide alternative methods for producing acetoxyaryl acids.

It is a still further object of the present invention to provide a method for oxidizing alkylphenols to the corresponding acetoxyaryl carboxylic acid.

These and other objects are accomplished by providing a method for producing acetoxyaryl carboxylic acids that converts a dialkylaryl to a monohydroperoxide of the dialkylaryl, converts the monohydroperoxide of the dialkylaryl to an alkylaryl acetate, and oxidizes the alkylaryl acetate to the acetoxyaryl carboxylic acid in the presence of cobalt and bromine catalysts. If the alkyl groups are isopropyl groups, the method converts a monohydroperoxide of a diisopropylaryl to an isopropylaryl acetate and oxidizes the isopropylaryl acetate to the acetoxyaryl carboxylic acid in the presence of cobalt and bromine catalysts.

The isopropylaryl acetate may be isolated prior to the oxidation step.

Alternatively, the isopropylaryl acetate may not be isolated prior to the oxidation substep.

However, acetone is produced in the step of converting the monohydroperoxide of the diisopropylaryl to the isopropylaryl acetate, and the acetone should be removed prior to the oxidation step. The aryl group may be a phenyl group or a naphthyl group. The oxidation step may by conducted in the presence of cobalt, bromine and manganese catalysts and more preferably in the presence of cobalt, bromine, manganese and potassium catalysts. The cobalt concentration should be within the range of from 0.01 molar to 0.05 molar and more preferably within the range of from 0.02 molar to 0.03 molar.

It is also possible to produce the acetoxyaryl carboxylic acid from an isopropylaryl alcohol. In this case, the isopropylaryl alcohol is converted to an isopropylaryl acetate and the isopropylaryl acetate is oxidized to the acetoxyaryl carboxylic acid in the presence of cobalt and bromine catalysts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described with reference to preferred embodiments and examples which are given by way of example only, not limitation.

According to a preferred embodiment of the present invention, the monohydroperoxide of p-diisopropyl benzene (p-isopropylcumyl hydroperoxide) may be converted to p-acetoxy benzoic acid. One source of the monohydroperoxide of p- diisopropyl benzene is the process for producing hydroquinone. The invention will be described with reference to p-diisopropyl benzene, p-isopropylcumyl hydroperoxide, p- acetoxy benzoic acid and the related intermediates. However, in addition to the para isomer, the ortho and meta isomers can also be used. Also, alkyl groups other than the isopropyl group can be used. Further, other aryls, such as naphthalene, can be used instead of the benzene aryl. For example, the various isomers of diisopropyl naphthalene, including 2,6-diisopropyl naphthalene, may be converted to the corresponding isomer of acetoxy naphthoic acid, including 6-acetoxy-2-naphthoic acid.

A possible first step in the conversion of the monohydroperoxide to the acetoxyaryl acid is to convert the monohydroperoxide to an alcohol. This is done by Hock rearrangement, and for p-isopropylcumyl hydroperoxide is performed according to the following reaction III.

According to reaction III, p-isopropylcumyl hydroperoxide is converted to p- isopropylphenol and acetone in the presence of an acid catalyst such as an acidic resin. A possible next step in the production of the acetoxyaryl acid is to convert the alkylphenol to the corresponding alkylaryl acetate. This may be done by reacting acetic anhydride with the alkylphenol. For p-isopropylphenol, the reaction proceeds as shown in the The products of reaction IV are p-isopropylphenyl acetate and acetic acid. It is important to note that it is not necessary to derive the alkylphenol from reactions I and III. Other routes for producing the alkylphenol are certainly available. Further, the monohydroperoxide can be converted directly to the alkyl aryl acetate, without an alkylphenol intermediate. Referring to the following reaction V, the monohydroperoxide, (p-isopropylcumyl hydroperoxide in our example) is converted directly to the alkylphenyl acetate (here p-isopropylphenyl acetate), acetone and acetic acid by reaction with acetic anhydride in the presence of a strong acid.

Reaction V proceeds in a manner similar to the Hock rearrangement discussed above.

That is, the monohydroperoxide product is rearranged in an acetic acid-acetic anhydride solution in the presence of a strong acid, preferably an acidic resin such as the Amberlyst 15TM ion exchange resins sold by Rohm and Haas.

After conversion to the alkylphenyl acetate, this product is converted to the acetoxyaryl carboxylic acid by the following reaction VI.

Reaction VI is a catalytic oxidation reaction at an elevated temperature. The isopropyl group is converted to a carboxylic acid to produce carbon dioxide and water. Before reaction VI, the alkylphenyl acetate product of reaction V may be isolated from the acetone and acetic acid products. Alternatively, it may be possible to simply remove the acetone, perhaps by heating. Further, it may be possible to proceed directly to the acetoxyaryl carboxylic acid without isolation.

The temperature at which reaction VI is performed is within the range of from 50°C to 180°C, more preferably within the range of from 100°C to 150°C, and most preferably between 110°C and 145°C. Reaction VI is carried out in acetic acid and acetic anhydride solvents. Since the oxidation is likely carried out at temperatures above the normal boiling point of the reactants/solvent mixture, the reaction pressure must be at least high enough to maintain the reaction in a liquid state. For example, a pressure of 90 psi gauge may be used.

The primary oxidant is molecular oxygen, which may be supplied as pure oxygen, or as air. The use of pure oxygen should be generally be avoided for safety reasons, although oxygen may be diluted with inert gases other than nitrogen, if desired.

Regardless of the oxygen source, the concentration of oxygen in the off gas should be maintained at or below 8 %.

As mentioned above, acetic acid is used as a solvent for the oxidation in reaction VI. The amount of acetic acid is not critical, but is chosen to permit efficient handling of the mixture. In addition to acetic acid, acetic anhydride may also used as a solvent. The amount of acetic anhydride can be varied widely.

The catalyst system employs a mixture of Co++, Mn++, an alkali metal such as sodium or potassium, and a source of Br. In choosing catalysts, a combination of cobalt and bromine is preferable, a combination of cobalt, bromine and manganese is more preferable, and a combination of cobalt, bromine, manganese and potassium is most preferable. The cobalt and manganese ions are conveniently supplied as the acetates.

The bromide ion may be derived either from a bromide salt or from hydrogen bromide or hydrobromic acid.

The amounts of cobalt, bromine, manganese and alkali metal may be varied among themselves. However, as a general guideline, it has been found that molar ratios of 1: 1: 3: 8 for cobalt: bromine: manganese: alkali metal are effective. The molar ratio of Co++ to alkylaryl acetate should be between 0.01 and 0.1 and preferably between 0.02 and 0.05. Similarly, the ratios of Mn++ and Br to alkylaryl acetate should be between . 01 and 0.1, preferably between 0.04 and 0.1. A portion or all of the alkali metal may be supplied as an acetate. The alkali metal (sodium or potassium) should be present in at least a molar ratio of 0.01 moles alkali metal to 1 mole of alkylaryl acetate reactant. The cobalt concentration should be within the range of from 0.01 molar to 0.05 molar and more preferably within the range of from 0.02 molar to 0.03 molar.

EXAMPLES The examples that follow are intended to illustrate the process of the invention and are not intended to limit the scope of the invention.

General Experimental Methods P-isopropylphenyl acetate was prepared by the partial oxidation of p-diisopropyl benzene. More specifically, p-diisopropyl benzene and p-isopropylphenyl acetate were fed to a columnar oxidation vessel, near the top of the vessel. Air was fed by means of a sparger, near the bottom of the oxidation vessel. The oxidation vessel was maintained at 80°C and approximately seven atmospheres total pressure. The temperature was maintained by removing vapor from the top of the oxidizer and returning condensable material to the oxidizer. The conversation of the oxygen feed was calculated to be about 75%.

Following partial oxidation, the crude oxidizer product from the oxidation vessel was fed to a rearrangement esterification reactor. The crude oxider product contained p- diisopropyl benzene monohydroperoxide, p-diisopropyl benzene dihydroperoxide and p- isopropylphenyl acetate hydroperoxide. The rearrangement reactor operated in an acetic acid-acetic anhydride solution in the presence of a strong acid Amberlyst 15TM ion exchange resin. The resin was supplied to the reactor as beads. Excess acetic anhydride was also fed to the reactor. The reactor had a screened filter leg to allow liquid product to flow therethrough, but not the Amberlyst 15 TI ion exchange resin beads.

The product from the rearrangement reactor was fed to the mid-point of a first distillation column operated at 1 atmosphere total pressure and a temperature ranging from 225-240°C. The stream taken overhead consisted primarily of acetone, acetic acid and acetic anhydride. The underflow from the column was fed to the mid-point of a second distillation column maintained at between 0.5 and 1 atmospheres total pressure and a temperature ranging from 225 to 300°C. The overhead stream contained p- isopropylphenyl acetate and unreacted p-diisopropyl benzene.

Example 1 A solution of 16.9 g (. 095 mole) of p-isopropylphenyl acetate in 105 g of acetic acid and 74 g (. 725 mole) of acetic anhydride was mixed with 0.70 g (. 003 mole) of cobaltous acetate, 2.11 g (. 008 mole) of manganous acetate, 0.35 g (. 003 mole) of potassium bromide, and 1.8 g (. 02 mole) of potassium acetate and then charged to a 1.0 L Hastalloy autoclave. The autoclave was sealed. The contents were stirred vigorously, pressurized to 300 psi and heated to 145°C for four hours. While stirring and maintaining the elevated pressure and temperature, a 1: 1 mixture of nitrogen and air was passed through the autoclave at a rate of 3.5 mole/h. The autoclave was equipped with an apparatus for measuring the amount of oxygen in the off-gas for safety purposes.

After the four hour period, the autoclave was allowed to cool and the contents thereof was discharged. Gas chromatography analysis of the contents showed only a trace of isopropylphenyl acetate. The only aromatic products observed were p-acetoxy benzoic acid and p-hydroxy benzoic acid in a 16: 1 ratio. The yield of p-acetoxy benzoic acid was calculated to be 65%.

Comparative Example 1 A mixture of 125 g of acetic acid, 88 g (. 862 mole) of acetic anhydride, 2.5 g (. 014 mole) of cobaltous acetate, 7.5 g (. 043 mole) of manganous acetate, 1.25 g (. 01 mole) of potassium bromide, and 6.25 g (. 064 mole) of potassium acetate was charged to a 1 L titanium autoclave equipped with a condenser for off-gas, a reactant feed system, and an inlet to permit continuous gas purging with an air-nitrogen mixture. A gas purge (one part air to two parts nitrogen) was supplied at the rate of 3.5 mol/hr. Then, the autoclave was stirred vigorously, pressurized to 300 psi gauge, and heated to 180°C.

When the temperature reached 170°C, 20 g (. 112 mole) of p-isopropylphenyl acetate was fed from a syringe pump at the rate of 20 g/hr. Heating was continued until the temperature reached 180°C, and the 180°C temperature was maintained for 4 hours.

After this, the autoclave was allowed to cool and the product was discharged. Only a trace of acetoxy benzoic acid was observed. All isopropylphenyl acetate was consumed.

Comparative Example 2 The procedure of Comparative Example 1 was repeated. However, no potassium was used in Comparative Example 2, and hydrobromic acid was used instead of potassium bromide and potassium acetate. The mixture used in Comparative Example 1 was substituted for 20 g (. 112 mole) of p-isopropylphenyl acetate, 128 g of acetic acid, 90 g (. 882 mole) of acetic anhydride, 2.5 g (. 014 mole) of cobaltous acetate, 7.5 g (. 043 mole) of manganous acetate, and 1.8 g (. 011 mole) of 48% hydrobromic acid. The mixture was heated and maintained at 180°C for four hours at which time the autoclave is allowed to cool and the product discharged. No acetoxy benzoic acid was observed in the product. A trace of isopropylphenyl acetate was observed.

While the invention has been described in connection with the preferred embodiments and examples, it will be understood that modifications within the principles outlined above will be evident to those skilled in the art. Thus, the invention is not limited to the preferred embodiments and examples, but is intended to encompass such modifications.