GB818268A - Process for the thermal rearrangement of salts of aromatic carboxylic acids - Google Patents

Abstract

The invention comprises a process for the thermal re-arrangement of salts of benzene carboxylic acids to produce terephthalic acid, by heating a mixture containing alkali metal ions, cadmium or zinc ions as catalyst, anions of a benzene carboxylic acid, fluorine ions and carbonate ions, in such proportions that there are at least 0.25 mol. of the cadmium or zinc ion per mol. of carboxylic acid salt, and at least two fluorine ion equivalents and two carbonate ion equivalents per mol. of cadmium or zinc, the carbonate ions if desired being partially replaced by equivalent amounts of halide ions. Preferable amounts of catalyst are from 0.35-1 mol. of cadmium fluoride per mol. of alkali metal salt of benzene carboxylic acid. The reaction mixture may also contain diluents, e.g. alkali metal chlorides, fluorides or carbonates, in an amount not generally exceeding 50 per cent by weight of the reaction mixture. The anions and cations may be added in the form of various compounds provided that the final mixture contains requisite percentage composition of the ions. Other cations may also be present provided that they are not catalytically active, e.g. magnesium and calcium and they are preferably present in quantities not greater than 2 equivalents per mol. of the cadmium or zinc. The starting materials may be mono, di or polycarboxylic salts of benzene, e.g. those containing 3-6 carboxyl groups per molecule, and may be introduced into the reaction mixture as such or in the form of the components from which they are formed, e.g. mixtures of the free acids, their anhydrides or chlorides with alkali metal carbonates. Tri and tetra carboxylic acids may be formed as by-products. The constituents of the reaction mixture are preferably mixed as uniformly as possible, for example, by grinding the separate constituents in ball mills, swing mills or rotary mills, and then mixing together. Alternatively the mixture may be allowed to react in a compact state, by previously compacting, e.g. by pressure or sintering. The constituents may also be mixed in the form of aqueous solutions or suspensions and then dried, e.g. in a drum drier or atomizer. The reaction is carried out at temperatures between 300 DEG C. and the decomposition temperature of the carboxylic acid salts present, preferably up to 550 DEG C. Inert gases, e.g. carbon dioxide, hydrogen, nitrogen, methane, ethane, propane or butane may be present and the reaction charge is preferably free from water as far as possible. The reaction may be carried out under pressure or in vacuo, as well as under normal pressure. Examples are given of the preparation of terephthalic acid using different mixtures of the following starting materials: potassium, cadmium, and zinc benzoates, and carbonates; potassium, cadmium, calcium and sodium fluorides and sodium carbonate.