Patent GB-816248-A (GB)
Title: A process for the preparation of cyclic dibasic acids
IPC: C07C51/41
Filed: 1956-12-07
Abstract: The invention comprises a process for the preparation of cyclic dibasic carboxylic acids, from reaction mixtures obtained by heating salts of other cyclic carboxylic acids under conditions, under which salts of said dibasic acids are obtained, by treating a solution of said mixture with carbon dioxide under pressure, so precipitating an acid salt of the cyclic dicarboxylic acid or a mixture of the acid salt and the free dicarboxylic acid, treating the solid mixture so obtained, preferably at elevated temperature, in some cases after separation of at least a part of the liquid and, if necessary, after addition of fresh solvent, with the cyclic carboxylic acid of which the salt was employed as initial starting material, or with an acid salt of said acid, separating the liberated dibasic acid and evaporating the liquid phase remaining, if desired, after combining it with any liquid removed from said solid-liquid mixture.  Specified cyclic dibasic acids which may be prepared by this process are terephthalic acid, naphthalene dicarboxylic acids, diphenyldicarboxylic acids, cyclohexane, cyclopentane, or pyridine dicarboxylic acids, 4-hydroxybenzoic acid and hydroxynaphthalene carboxylic acids. The acids may also contain further nuclear substituents, e.g. halogen or alkyl.  Examples of suitable starting materials for the disproportionation process are the potassium or thallous salts of benzoic, phthalic, isophthalic, hemimellitic, trimellitic, trimesic, mellophanic, prehnitic, pyromellitic, mellitic, naphthalic, 2-hydroxynaphthalene 1-carboxylic, salicylic and diphenic acid.  Mixtures of salts of benzencarboxylic acids, e.g. of those which have been obtained by the oxidation of dialkyl benzenes, e.g. crude xylene, with air or nitric acid or by the oxidative degradation of higher ring systems, or during the treatment of graphite, bituminous or brown coal, peat, wood, lignin coal extracts, tars, pitches, coke and asphalts with nitric acid. Mixtures of the acids or their anhydrides with the potassium or thallium hydroxides or carbonates may also be used. The heat treatment may be carried out at 250-450 DEG  C. at atmospheric or superatmospheric pressure in the presence of carbon dioxide or nitrogen, in a rotating tubular furnace or in a fluidized bed, in the presence of the oxides, carbonates, halides or organic salts (e.g. the carboxylic acid serving as starting material) of zinc, cadmium, or divalent iron.  When the starting materials have been prepared by a nitric acid oxidation it is preferable to use also small quantities of the hydrides or carbides of potassium or sodium.  The cyclic dibasic acids are liberated from their salts in two stages.  In the first stage carbon dioxide is passed into a solution of the reaction mixture under pressure, e.g. up to about 30 atmospheres, preferably between about - 10 DEG  C. and the boiling-point of the solvent, such as water, methanol, ethanol, acetone, dioxan or tetrahydrofuran. Any impurities remaining undissolved may be filtered off before the carbon dioxide treatment.  Part of the catalyst which is separated in this filtration may be recovered by digesting the filter residue with a hot solution of the cyclic carboxylic acid, the salt of which was used as the starting substance, and washing with hot water.  The catalyst may then be recovered by evaporation.  Alternatively the catalysts may be separated by extraction with an organic solvent such as carbon disulphide. Small quantities, e.g. about 0.1 equivalents of the acid to be obtained, of the cyclic carboxylic acid, the salt of which served as starting material, may be added to the mixture before or during the carbon dioxide treatment.  The mixture treated with carbon dioxide may be freed from at least part of the liquid present, advantageously under pressure of carbon dioxide, e.g. by evaporation, filtration or centrifugation, with the aid of a rotary pressure filter or a centrifugal separator.  The liquid which is removed may be extracted to recover dissolved cyclic carboxylic acid and the extract added to the concentrated mixture. The content of dicarboxylic acid and its acid salt in the mixture should preferably be 20-40 per cent by weight after separation of the liquid.  The concentrated mixture, preferably after heating to boiling temperature, is then treated with the cyclic carboxylic acid, the salts of which have served as starting materials for the original heat treatment.  The amount of acid added is preferably so large that it corresponds at least to a quantity equivalent to the dibasic acid to be separated.  The liberated dibasic acid is then separated off by suitable means.  The remaining liquid phase is then evaporated, if desired together with the liquid separated from the mixture before the addition of the carboxylic acid, and/or the liquid containing the catalyst, and the dry residue obtained may be used for the preparation of the dibasic acid. Examples are given of the preparation of terephthalic acid from dipotassiumphthalate; (2) oxidation of xylene; (3) potassium benzoate, and of the preparation of 4-hydroxy benzoic acid from the neutral potassium salt of salicylic acid.