The present invention relates to an improved method of manufacturing 2-hydroxynaphthalene-6-carboxylic acid by the reaction of .beta.-naphtholpotassim and carbon dioxide.
2-hydroxynaphthalene-6-carboxylic acid is an important raw material for every kind of aromatic polyesters and indispensable especially to the production of a liquid crystal polymer excellent in workability and fluidity, and a resin or fiber having good elasticity and heat resistivity.
There had been known many methods of production for 2-hydroxynaphthalene-6-carboxylic acid, for example, the U.S. Pat. No. 1593816 (1926), old one, and the Japanese patent KOKAI Nos. 95939/1982 and 212139/1982, relatively new disclosures.
By such known methods, however, it had not been possible to produce 2-hydroxynaphthalene-6-carboxylic acid at a high yield and at a high selective ratio of (term "selective ratio" means the ratio of 2-hydroxy naphthalene-6-carboxylic acid to (2-hydroxynaphthalene-6-carboxylic acid plus 2-hydroxynaphthalene-3-carboxylic acid) in the present specification.)
In our early researches for establishing a method of producing 2-hydroxynaphthalene-6-carboxylic acid from .beta.-naphthol on an industrial scale, it had been found that the product was obtainable at relatively good yields and at high selective ratios when the reaction was made to take place by selecting a carbon dioxide pressure suitable for the reaction temperature as removing free .beta.-naphthol arised therein from the reaction system by means of overflow with the reaction medium (reference: Patent Publication No. 35911/1984). It is understood that, in the above-mentioned Kolbe-Schmitt reaction, the intermediate formed by bonding of a potassium atom to an aromatic nucleus of .beta.-naphthol is decomposed in the presence of water to return the .beta.-naphthol. Therefore, it had been generally considered that this reaction must be carried out by first reacting .beta.-naphthol and a potassium source in an aqueous solution, second the .beta.-naphtholpotassium obtained therefrom is dehydrated, and third reacted with carbon dioxide. And the degree of this dehydration has been considered to bear upon the yields and selective ratios of the product.