GB772635A - A process for preparing a phthalic acid by oxidizing di-isopropylbenzene - Google Patents

Abstract

Phthalic acids such as terephthalic acid are prepared by oxidizing the corresponding diisopropylbenzene with oxygen or an oxygencontaining gas in a liquid medium consisting mainly of an aliphatic carboxylic acid or mixture of such acids in which the di-isopropyl-benzene is soluble, water being removed from the reaction mixture during the reaction to maintain the water content of the reaction mixture below 5 per cent by weight. Catalysts such as compounds of heavy metals which can exist in different valency states such as manganese or cobalt or mixtures of such compounds with or without barium compounds may be present. Aliphatic acids mentioned are acetic, propionic and butyric acids. The methods described to maintain the water content below 5 per cent are (i) passing more oxygen or oxygen-containing gas through the reaction mixture than is necessary to effect oxidation, thus removing water vapour in the surplus gas, (ii) adding to the reaction mixture either at the commencement or during the reaction, a water-binding reagent which does not interfere with the course of the reaction, (iii) adding to the reaction mixture a component which at reaction temperatures distils off as an azeotropic mixture with the water to be removed, or (iv) withdrawing reaction mixture continuously or intermittently from the reaction vessel, isolating the desired reaction product and recycling the remaining liquid after reduction of the water content (a) by addition of a water-binding reagent or (b) by distillation with or without use of an azeotroping liquid. Waterbinding reagents used in methods (ii) and (iva) above are anhydrides or mixed anhydrides of those aliphatic carboxylic acids having 2 to 4 carbon atoms per molecule such as acetic anhydride or mixtures of such anhydrides. An apparatus for conducting the process of method (ivb) without an azeotroping liquid is described. Examples illustrate the oxidation of p-di-isopropylbenzene in acetic acid as solvent with air in the presence of cobalt acetate and manganese acetate as catalyst under pressure and at elevated temperature, (1) wherein acetic anhydride is added to the mixture at the beginning of the reaction, and after reaction terephthalic acid is filtered from the mixture, further p-di-isopropylbenzene, catalyst, solvent and acetic anhydride being added to the filtrate and the oxidation being repeated several times, the results being compared with a series of similar oxidations in which no acetic anhydride is used; (2) wherein reaction mixture is continuously withdrawn from the reaction vessel, terephthalic acid is filtered therefrom and water is removed from the filtrate by distillation, the distillation bottoms being recycled to the reaction and fresh p-di-isopropylbenzene, solvent and catalyst being added intermittently. Specification 737,439 is referred to.