GB2183636A - Process for producing 2-6-Di (2-hydroxy-2-propyl)naphthalene - Google Patents

Abstract

2,6-Di (2-hydroxy-2-propyl)naphthalene is prepared by (i) oxidizing 2,6-diisopropylnaphthalene by molecular oxygen in an aqueous alkaline solution; (ii) separating and recovering said aqueous alkaline solution from the thus obtained reaction mixture; (iii) washing with water and, under heating, dissolving in benzene or an alkylbenzene having side chain(s) of from 1 to 3 carbon atoms the solid material remaining after step (ii); and (iv) cooling the thus formed solution, thereby separating 2,6-di(2-hydroxy-2-propyl)naphthalene. e

Description

SPECIFICATION "Process for producing 2,6-di(2-hydroxy-2-propyl) naphthalene" The present invention relates to a process for producing highly pure 2,6-di(2-hydroxy-2-propyl)naphthalene (hereinafter referred to as 2,6-DHPN) which is a raw material of 2,6-naphthalenediol utilized as a monomer of aromatic polyesters, particularly the aromatic polyester having a capability of forming liquid crystal.

In recent years, various polymers have been developed as engineering plastics, and of these polymers, aromatic polyester, particularly the aromatic polyester having a capability of forming liquid crystal has attracted one's attention.

As the representative of the aromatic polyester having a capability of forming liquid crystal, a three component polymer obtained from the two components selected from terephthalic acid, hydroquinone and p-hydroxybenzoic acid, and a monomer of naphthalene derivatives as a third component has been known.

Of the monomers of naphthalene derivatives, 2,6-naphthalenediol is an attractive monomer because the liquid crystalline polymer derived therefrom is excellent in physical properties and terephthaiic acid which is cheaply available can be used as the comonomer thereof.

Although 2,6-naphthalenediol can be obtained by the process formerly developed by the present inventors (Patent Application No. 8613759), wherein 2,6-DHPN is oxidized by hydrogen peroxide in acetonitrile or dioxane and in the presence of an inorganic acid, 2,6-DHPN used as the starting substance in the process should be highly pure and at the same time, should be easily and effectively produced in an industrial scale.

Hitherto, as a method for converting an isopropyl-substituted aromatic compound represented by the following formula (I) into a 2-hydroxy-2-propyl-substituted aromatic compound represented by the following formula (II), various methods have been known as follows:

wherein Ar represents an aromatic ring and n represents an integer of 1 or 2.

For instance, (1) Japanese Patent Publication No. 39-19355 (1964) discloses a method for obtaining (2-hydroxy-2-propyl)benzene of a purity of 95% by oxidizing cumene in an aqueous 50% solution of sodium hydroxide, separating an organic layer by liquid-liquid phase separation from the reaction mixture and distilling the organic layer. However, in the case of applying this method to the oxidation of 2,6-diisopropylnaphthalene, the formed 2,6-DHPN separates from the reaction mixture, and because the specific gravity thereof is close to 1, liquid-liquid separation is impossible at all. In addition, since 2,6-DHPN boils at a high temperature, a partial dehydration occurs in the course of distillation thereof, thereby reducing the yield of 2,6-DHPN.

On the other hand, (2) although Japanese Patent Publication No. 39-21242 (1964) discloses the oxidation of -isoprnpylnaphthalene as an example, concerning the separation of the reaction products, there is only a general description of carrying out liquid-liquid separation and there are no disclosure therein concerning the separation of 1S-(2-hydroxy-2-propyl)naphthalene.

In addition, in the method disclosed in Japanese Patent Application Laying-Open (KOKAI) No. 58-162539 (1983), relating to the oxidation of cumene, in order to separate the oxidation products the reaction mixture is extracted with ether after neutralizing the reaction mixture. However, it is impossible to separate the product from the unreacted substance because the dissolving power of ether is too large.

Furthermore, German Patent No. 1233839 discloses a method wherein p-diisopropylbenzene is oxidized in bromobenzene and in the presence of substantially anhydrous caustic alkali and p-di(2-hydroxy-2-propyl)benzene is separated by cooling the thus obtained reaction product.

For carrying out the method, it is necessary to carry out the oxidation in a specific solvent. Further, in the case of applying the method to the oxidation of 2,6-diisopropyinaphthalene, since 2,6-DHPN has a high melting point and is slightly soluble in solvents, it is very difficult to carry out the separation and purification of 2,6-DHPN by the disclosed method, namely, the disclosed method is not practical for producing 2,6-DHPN.

In this connection, in the reaction wherein an isopropyl-substituted aromatic compound is oxidized in an aqueous alkali solution to obtain a 2-hydroxy-2-propyl-substituted aromatic compound, it is inevitable to be accompanied by the unreacted starting substance and by-products in addition to the object product, and accordingly, the separation and purification of the product are indispensable.

As has been described above, it has been difficult to obtain 2,6-DHPN advantageously in an industrial scale by applying the known method of oxidizing the isopropylbenzene and diisopropylbenzene into 2-hydroxy-2propylbenzene and di (2- hydroxy-2-propyl) benzene, respectively, to the oxidation of 2,6-diisopropyl - naphthalene.

As a result of the present inventors' studies on the process for effectively producing highly pure 2,6-DHPN, it has been found by the present inventors that highly pure 2,6-DHPN can be effectively obtained by the process comprising the steps of (1) oxidizing 2,6-diisopropylnaphthalene in an aqueous solution of an alkali, (2) removing the naphthalene compounds having carboxyl group(s) from the thus obtained reaction mixture, (3) recrystallizing the thus obtained mixture with benzene or an alkylbenzene having side chain(s) of from 1 to 3 carbon atoms, thereby obtaining 2,6-DHPN and in addition, (4) feeding back the oxidation intermediates such as 2-(2- hydroxy-2-propyl) -6- isopropylnaphthalene, etc. and the unreacted 2,6-diisopropylnaphthalene in the mother liquor of the recrystallization to the oxidation step, and on the basis of these findings, the present inventors have attained the present invention.

In an aspect of the present invention, there is provided a process for producing 2,6-di(2-hydroxy-2-propyl)naphthalene comprising oxidizing 2,6-diisopropylnaphthalene by molecular oxygen in an aqueous solution of an alkali, separating and recovering said aqueous solution of an alkali from the thus obtained reaction mixture, washing the solid material obtained by separating the aqueous solution of an alkali with water to remove the naphthalene compounds having carboxyl group(s), which are by-produced in the step of oxidation, dissolving the thus washed solid material in benzene or an alkylbenzene having side chain(s) of from 1 to 3 carbon atoms under heating and cooling the thus formed solution, thereby separating 2,6-di(2- hydroxy-2-propyl) naphthalene.

The present invention relates to a process for effectively producing highly pure 2,6-DHPN, which process comprises the steps of (1) oxidizing 2,6-diisopropylnaphthalene in an aqueous solution of an alkali by molecular oxygen, (2) separating and recovering the aqueous solution of an alkali from the thus obtained reaction mixture, (3) washing the solid material obtained by separating the aqueous solution of an alkali with water, thereby removing the naphthalene compounds having carboxyl group(s), which are by-produced in the step of the oxidation, (4) dissolving the thus washed solid material into benzene or an alkylbenzene having side chain(s) of from 1 to 3 carbon atoms under heating and thereafter (5) cooling the thus obtained solution, thereby separating 2,6-DHPN.

According to the the present invention, in the first place, 2,6-diisopropylnaphthalene is oxidized by molecular oxygen in an aqueous solution of an alkali. As the aqueous solution of an alkali used herein, an aqueous 20 to 70% by weight, preferably from 30 to 50% by weight solution of sodium hydroxide or potassium hydroxide is preferable.

In the case where the concentration of the aqueous solution of an alkali is lower than 20% by weight, the rate of oxidation of 2,6-diisopropylnaphthalene into the naphthalene compounds having carboxyl group(s) (hereinafter referred to as naphthalenecarboxylic acids) is raised, thereby reducing the yield of the object product of 2,6-DHPN, and accordingly such a situation is not preferable.

On the other hand, in the case where the concentration of the aqueous solution of an alkali is higher than 70% by weight, there is no practical advantage in improving the yield of 2,6-DHPN.

The amount of the aqueous solution of an alkali used herein is preferably from 1 to 20 ml, more preferably from 2 to 10 ml to 1 g of the starting substance, 2,6-diisopropylnaphthalene, and in the case of using an amount below 1 ml to 1 g of 2,6-diisopropylnaphthalene, it is difficult to obtain a favourable contact between 2,6-diisopropylnaphthalene and oxygen by stirring.

Molecular oxygen used in the oxidation is pure oxygen or a gaseous mixture prepared by diluting pure oxygen with an inert gas.

Although the oxidation reaction proceeds more rapidly as the partial pressure of oxygen increases, in practice, the partial pressure of oxygen of higher than 0.2kg/cm2G, preferably from 0.5 to 30 kg/cm2G is sufficient. In addition, in the case of using molecular oxygen after mixing with an inert gas, the total pressure of the gaseous mixture is not particularly restricted, however, generally the reaction proceeds rapidly under the total pressure of from 1 to 30 kg/cm2G.

Although the reaction rate and the reaction time period are decided by considering the other reaction conditions, the reaction temperature is preferably from 60 to 200"C, more preferably from 80 to 150"C and the reaction time period is preferably from 1 to 50 hours, more preferably from 10 to 30 hours.

Hitherto, in the oxidation of cumene in the presence of an alkali, there has been a method of carrying out the reaction in the presence of a catalytic amount of manganese naphthenate (Japanese Patent Publication No.

39-19355 (1964)), however, in the reaction according to the present invention, the presence of a salt of heavy metal such as manganese and cobalt is not favorable, because the presence of such heavy metal causes the increase of the by-products and the coloration of 2,6-DHPN.

In addition, concerning the material of the reaction vessel, those which are corroded by a concentrated aqueous solution of an alkali should be avoided, and it is preferable to use a reaction vessel in which the part exposed to the concentrated aqueous solution of an alkali is made of nickel.

In the oxidation of 2,6-diisopropylnaphthalene, the first step of oxidation of 2,6-diisopropylnaphthalene into 2- (2-hydroxy-2-propyl) -6-isopropylnaphthalene (hereinafter referred to as mono(hydroxypropyl) naphthalene) proceeds rapidly, however, the second step of oxidation of mono(hydroxypropyl)naphthalene into 2,6-DHPN proceeds slowly and it is practically difficult to complete the oxidation of 2,6-diisopropylnaphthalene into 2,6-DHPN.

Accordingly, the reaction mixture of the oxidation reaction contains intermediates such as mono(hydroxypropyl) -naphthalene and by-products such as 2-acetyl-6- isopropylnaphtha lene, 2-acetyl-6-(2hydroxy-2-propyl)naphthalene and an alkali salt of the naphthalenecarboxylic acids, etc. as well as the object substance, 2,6-DHPN, and the unreacted 2,6-diisopropylnaphthalene.

In this connection, the naphthalenecarboxylic acids, 2-acetyl-6-isopropyinaphthalene and 2-acetyl-6-(2 hydroxy-2-propyl)naphthalene are the by-products which could not be converted into 2,6-DHPN. The 2,6-DHPN, 2,6-diisopropylnaphthalene, intermediates and by-products are solids at ordinary temperature and do not dissolve in a concentrated aqueous solution of an alkali, and accordingly, in the present invention, 2,6-DHPN, the intermediates and the by-products are recovered by subjecting the reaction mixture to solid-liquid separation, thereby removing the aqueous solution of an alkali. After adjusting the concentration of the thus recovered aqueous solution of an alkali to 20 to 70% by weight, the thus adjusted aqueous solution of an alkali can be reused in the step of oxidizing 2,6-diisopropylnaphthalene.

In the solid material obtained by the solid-liquid separation, several compounds mentioned above are contained, and accordingly, in the present invention, the object product, 2,6-DHPN, is isolated from the solid material according to the following steps of procedure.

Namely, although the naphthalenecarboxylic acids present in the solid material do not dissolve in a concentrated aqueous solution of an alkali, they dissolve in a dilute aqueous solution of an alkali, and accordingly, the naphthalenecarboxylic acids can be separated by washing the solid material with water. The water-washing is preferably carried out until the filtrate becomes neutral.

After drying the washed solid material obtained by separating the naphthalenecarboxylic acids, the dried solid material is dissolved in benzene or an alkylbenzene by heating to from 70 to 150 C, preferably from 100 to 130"C, and on cooling the thus obtained solution to from 10 to 40"C, preferably from 20 to 30"C, substantially only 2,6-DHPN is precipitated. The thus precipitated 2,6-DHPN is separated and collected.

In the next place, on distilling the organic solvent off from the solution from which 2,6-DHPN has been separated, a mixture mainly containing mono(hydroxypropyl)-naphthalene is obtained, and accordingly in the present invention, the thus obtained mono(hydroxypropyl)naphthalene is fed back to the step of oxidation of 2,6-diisopropylnaphthalene, thereby reusing in conversion to 2,6-DHPN.

As the alkylbenzene, those having 1 to 2 side chains of from 1 to 3 carbon atoms, for instance, toluene, xylene, cumene, etc. may be exemplified. In addition, since the solubility of 2,6-DHPN into benzene and these alkylbenzenes highly depends upon the temperature thereof, it is preferable to dissolve the solid material by heating to from 70 to 150 C, more preferably from 100 to 130"C. The recovery of 2,6-DHPN in recrystallization becomes over 90% by doing so.

Benzene or an alkylbenzene is preferably used in an amount of from 1 to 30 ml, more preferably from 2 to 5 ml, to 1 9 of the solid material.

In the present invention, the removal of the naphthalenecarboxylic acids after separating and recovering the aqueous solution of an alkali and the treatment by benzene or an alkylbenzene may be carried out in that order, however, both steps may be carried out at the same time.

Namely, water and benzene or an alkylbenzene are added to the solid material obtained by separating the aqueous solution of an alkali and the thus formed mixture is heated under stirring, thereby dissolving the naphthalenecarboxylic acids into the aqueous layer and the other compounds into the organic layer to effect the separation. After separating the aqueous layer and washing the organic layer with hot water, the organic layer is cooled to separate 2,6-DHPN.

As has been described, according to the present invention, the yield of 2,6-DHPN which is the object substance of the present invention can be increased by feeding back the intermediates which are present in the reaction mixture obtained by oxidizing 2,6-diisopropylnaphthalene to the oxidation step, thereby subjecting again to oxidation.

Furthermore, by the use of benzene or an alkylbenzene as the solvent for recrystallization, it is able to obtain 2,6-DHPN at a high purity.

Besides, since the unreacted 2,6-diisopropylnaphthalene in the reaction mixture obtained by oxidizing 2,6-diisopropylnaphthalene by molecular oxygen in an aqueous solution of an alkali, 2,6-DHPN, the intermediates and by-products scarcely dissolve in the concentrated aqueous solution of an alkali, it is possible to separate and recover the aqueous solution of an alkali and to repeatedly use thereof.

The present invention will be explained more precisely while referring to the non-limitive examples as follows.

Example 1 Into a 1.5 e nickel autoclave provided with a powerful stirrer, a reflux condenser and a gas inlet tube, 250 g of 2,6-diisopropylnaphthalene, 240 g of sodium hydroxide and 560 g of water were introduced and the thus introduced substances were stirred for 20 hours at 140"C while supplying gaseous oxygen to the substances at a rate of 5 t/hour and holding the inner pressure of the autoclave at 10 kg/cm2G.

In the next place, the autoclave was cooled to take out the content thereof, and the aqueous solution of sodium hydroxide was recovered in an amount of 642 g by filtration of the content. On acid-treatment of the thus recovered alkali solution by dilute sulfuric acid, no turbidity was observed in the alkali solution showing that organic substances were scarcely contained in the alkali solution.

Thereafter, the solid material obtained by the filtration was washed with water until the filtrate became neutral.

On adjusting the pH of the washings to 3 by using dilute sulfuric acid, a white precipitate was obtained. The precipitate was extracted with ethyl acetate to obtain 23.3 g of naphthalenecarboxylic acids.

In the next place, by drying the thus washed solid material, 248 g of a white, dried cake were obtained. After dissolving the cake in 3000 ml of toluene by heating to 110"C, the thus obtained solution was cooled to room temperature to precipitate needle-like crystals. By collecting the needle-like crystals on a filter and drying the thus collected crystals, 142 g of 2,6-DHPN of a purity of 97.9% were obtained. By recrystallizing the thus obtained, 2,6-DHPN from 430 ml of toluene, 136 g of nearly pure 2,6-DHPN were obtained.

By distilling off the toluene from the remaining toluene solution after separating 2,6-DHPN, a mixture comprising mainly the oxidation intermediate, 2- (2- hydroxy-2-propyl) -6- isopropylnaphthalene (hereinafter referred to as the mixture of intermediates) was obtained in an amount of 84.8 g. The composition of the mixture of intermediates was as follows.

7.8% by weight of 2,6-diisopropylnaphthalene, 0.5% by weight of 2-acetyl-6-isopropylnaphthalene, 86.1% by weight of 2- (2-hydroxy-2-propyl) -6-isopropylnaphthalene, 5.1% by weight of 2,6-DHPN and 0.5% by weight of 2-acetyl-6- (2-hydroxy-2- propyl) naphthalene.

Accordingly, the yields of the products obtained by the oxidation reaction are as follows, respectively.

2.7% by mol of 2,6-diisopropylnaphthalene, 0.2% by mol of 2-acetyl-6-isopropylnaphthalene, 28.7% by mol of 2- (2-hydroxy-2-propyl) -6- isopropylnaphthalene, 49.1% by mol of 2,6-DHPN, 0.2% by mol of 2-acetyl-6- (2- hydroxy-2-propyl) naphthalene and 9.0% by mol of naphthalenecarboxylic acids.

Since the naphthalenecarboxylic acids were obtained as a mixture, the mean molecular weight thereof was presumed to be 220.

Example 2 Into a 200 ml nickel autoclave provided with a powerful stirrer, a reflux condenser and a gas inlet tube, 10 g of 2,6-diisopropylnaphthalene and 100 g of an aqueous 50% solution of sodium hydroxide were introduced, and the content in the autoclave was stirred for 22 hours at 120"C while holding the inner pressure of the autoclave at 5 kg/cm2G and supplying a gaseous mixture of 21% of oxygen and 79% of nitrogen thereinto at a rate of 5 t/hour.

In the next place, after recovering 77 g of the aqueous solution of sodium hydroxide by filtering the reaction mixture, the thus obtained solid material was washed with 450 ml of water to obtain 23 g of a white and wet cake. After drying the wet cake, it was dissolved in 120 ml of toluene by heating to 110"C, and the thus formed solution was cooled to room temperature to precipitate needle-like crystals. By filtering and drying the needle-like crystals, 4.57 g of 2,6-DHPN of a purity of 96.5% were obtained. The crystals contained nothing other than 2-(2- hydroxy-2-propyl) -6-isopropylnaphthalene as the impurity.

By distilling off the toluene from the remaining toluene solution after separating 2,6-DHPN, the mixture of intermediates having the following composition was obtained in an amount of 4.55 g.

13.4% by weight of 2,6-diisopropylnaphthalene, 0.2% by weight of 2-acetyl-6-isopropyl naphthalene, 79.0% by weight of 2-(2-hydroxy-2-propyl)-6-isopropylnaphthalene, 6.9% by weight of 2,6-DHPN and 0.5% by weight of 2-acetyl-6-(2-hydroxy-2-propyl)-naphthalene.

In addition, on adjusting the pH of the washings to 3, a white precipitate was obtained. By extracting the precipitate with ethyl acetate, 0.8 g of naphthalenecarboxylic acids was obtained.

Example 3 Into the same autoclave as used in Example 2, 12 g of 2,6-diisopropylnaphthalene, 60 g of sodium hydroxide and 60 g of water were introduced, and the content in the autoclave was stirred for 22 hours at 120 C while holding the inner pressure of the autoclave at 5 kg/cm2G and supplying oxygen thereinto at a rate of 3 t/hour.

In the next place, after cooling the autoclave, the content therein was talen out and filtered to recover 90 g of the aqueous solution of sodium hydroxide. The solid material remaining on the filter was dissolved by heating to 100"C in 500 ml of water and 150 ml of xylene and the aqueous layer was separated from the organic layer. The organic (xylene) layer was washed two times with each 100 ml of hot water. On leaving the xylene layer to cool to room temperature, needle-like crystals were precipitated. By collecting the needle-like crystals and drying them, 7.18 g of 2,6-DHPN of a purity of 98.9% were obtained in a yield of 51.4% bymol.

Example 4 In this example, the aqueous solution of an alkali recovered from the reaction mixture and the mixture of intermediates were fed back to the oxidation step of 2,6-diisopropyinaphthalene for reuse.

Into the same autoclave as used in Example 2, 10 g of 2,6-diisopropylnaphthalene, 30 g of sodium hydroxide and 70 g of water were introduced and the content in the autoclave was stirred for 22 hours at 120"C while holding the inner pressure of the autoclave at 5 kg/cm2G and supplying gaseous oxygen thereto at a rate of 5 t/hour.

in the next place, the reaction mixture was treated in the same manner as in Example 2 to obtain 6.75 g of 2,6-DHPN of a purity of 97.5%, 2.48 g of the mixture of intermediates, 1.26 g of naphthalenecarboxylic acids and 80.6 g of the recovered aqueous solution of sodium hydroxide, respectively.

Thereafter, 7.91 g of 2,6-diisopropylnaphthalene and 19.4 g of an aqueous 30% solution of sodium hydroxide were added to 2.25 g of the thus obtained mixture of intermediates and 80.6 g of the thus recovered aqueous solution of sodium hydroxide, and the thus formed mixture was subjected to reaction in the same manner as in Example 2 to obtain, as the products of the second run, 6.80 g of 2,6-DHPN of a purity of 97.0%, 2.45 g of the mixture of intermediates, 1.32 g of naphthalenecarboxylic acids and 78.3 g of the recovered aqueous solution of sodium hydroxide.

In the next place, 7.99 g of 2,6-diisopropylnaphthalene and 21.7 g of the aqueous 30% solution of sodium hydroxide were added to 2.14 g of the mixture of intermediates and 78.3 g of the recovered aqueous dense solution of sodium hydroxide both of which were obtained in the second run, and the thus formed mixture was reacted under the same conditions as in Example 2. By treating the reaction mixture in the same manner as in Example 2, as the products of third run, 6.73 g of 2,6-DHPN of a purity of 97.3%, 2.49 g of the mixture of intermediates, 1.30 g of naphthalenecarboxylic acids and 78.2 g of the recovered aqueous solution of sodium hydroxide were obtained.

Claims (11)

1. A process for producing 2,6-di(2-hydroxy-2-propyl)naphthalene, which process comprises (i) oxidizing 2,6-diisopropylnaphthalene by molecular oxygen in an aqueous alkaline solution; (ii) separating and recovering said aqueous alkaline solution from the thus obtained reaction mixture; (iii) washing with water and, under heating, dissolving in benzene or an alkylbenzene having side chain(s) of from 1 to 3 carbon atoms the solid material remaining after step (ii); and (iv) cooling the thus formed solution, thereby separating 2,6-di(2-hydroxy-2-propyl)naphthalene.

2. A process according to claim 1, wherein said aqueeus alkaline solution recovered in step (ii) is recycled for use in step (i).

3. A process according to claim 1 or 2, wherein the material remaining after separation of the 2,6-di(2-hydroxy-2-propyl)naphthalene in step (iv) and after subsequent removal of said benzene or alkylbenzene is recycled for use in step (i).

4. A process according to any one of the preceding claims, wherein said aqueous alkaline solution is an aqueous 20 to 70% by weight solution of sodium hydroxide or potassium hydroxide.

5. A process according to any one of the preceding claims, wherein said alkylbenzene is toluene or xylene.

6. A process according to any one of the preceding claims, wherein step (i) is carried out in a reaction vessel made of nickel.

7. A process according to any one of the preceding claims, wherein in step (iii) the solid material remaining after step (ii) is first washed with water and then dissolved under heating in said benzene or alkylbenzene.

8. A process according to any one of claims 1 to 6, wherein in step (iii) the solid material remaining after step (iii) is simultaneously washed with water and dissolved under heating in said benzene or alkylbenzene.

9. A process for the preparation of 2,6-di(2-hydroxy-2-propyl)naphthalene, said process being substantially as hereinbefore described in any one of Examples 1 to 4.

10. 2,6-Naphthalenediol produced from 2,6-di(2-hydroxy-2-propyl)naphthalene prepared by a process as claimed in any one of the preceding claims.

11. A polymer obtained from 2,6-naphthalenediol as claimed in claim 10 and two of terephtahalic acid, hydroquinone and p-hydroxybenzoic acid.