Aromatic hydroxycarboxylic acids and dicarboxylic acids are important items of commerce. For instance o-hydroxybenzoic acid (salicylic acid) is used as a chemical intermediate, for instance to make aspirin, while p-hydroxybenzoic acid (PHBA) is used to make parabens and is also used as a monomer in making polyesters, while dicarboxylic acids are important as monomers. Traditionally aromatic hydroxycarboxylic acids are manufactured using the Kolbe-Schmitt reaction, which is a reaction of an alkali metal salt of an aromatic hydroxy compound with carbon dioxide, usually under elevated temperature and pressure.
The Kolbe-Schmitt reaction has been a standard procedure for the preparation of aromatic hydroxy acids for over 100 years, see for instance A. S. Lindsey, et al., Chem. Rev., vol. 57, p. 583-620 (1957) incorporated by reference herein. However, this process is complex and difficult to run, involving several manufacturing steps, which adds to the cost of the final product. Since the initial product of the carboxylation reaction is a dialkali metal salt of the aromatic hydroxycarboxylic acid, substantial cost is usually incurred for the use of compounds such as NaOH or KOH which are subsequently discarded (as sodium or potassium salts), since the free aromatic hydroxycarboxylic acid (or dicarboxylic acid) is usually isolated by reacting the dialkali metal salt with a strong acid. It is hence desirable to develop an improved Kolbe-Schmitt process for the manufacture of these compounds. Dicarboxylic acids are also sometimes available as their dialkali metal salts, and it is often desirable to convert these to the dicarboxylic acids themselves, and generate an alkali from the alkali metals present in the salts.
It is known that the salts of diacids or aromatic hydroxycarboxylic acids can be electrodialyzed to form the free dicarboxylic acid or aromatic hydroxycarboxylic acid and the alkali metal hydroxide. However, when one tries to completely electrodialyze these compounds to these final products, as one approaches complete electrolysis, the voltage increases and the current efficiency decreases rapidly and the process may become uneconomic. Therefore, it would be desirable to have another economical method for isolating the free aromatic hydroxycarboxylic acid or dicarboxylic acid from its dialkali metal salt, while at the same time being able to recycle the alkali metals in the process in an economical fashion.
Japanese Patent Application 40-11492 describes the electrodialysis of an alkali metal salt of terephthalic acid to terephthalic acid and an alkali metal hydroxide.
Japanese Patent Application 64-9954 describes the electrodialysis of an alkali metal salt of hydroxybenzoic acid.
None of the above references describes a partial electrodialysis followed by a treatment with a strong acid to effect isolation of a dicarboxylic acid or an aromatic hydroxycarboxylic acid.
Electrodialysis of salts of organic compounds is in general known, and generally requires only relatively simple equipment. However, if one attempts to electrodialyze an aqueous solution of the mono- or dipotassium salt of PHBA, one finds that before free PHBA is obtained, the voltage required to effect electrolysis greatly increases and the electrolysis essentially stops (see Comparative Example 1). However it has now been found that if this electrodialysis is done at elevated temperatures, good results can be obtained.