Patent GB-837007-A (GB)
Title: Improvements in and relating to a process for the production of phthalic acids
Inventor: WHITFIELD GORDON HOWARD
IPC: C07C51/43
Filed: 1956-08-20
Abstract: The invention comprises a process for separating o-phthalic acid from iso and/or terephthalic acids or oxidation products of mono and dialkyl aromatic hydrocarbons or oxygenated derivatives thereof, yielding on oxidation the above acids, which comprises decreasing the partial pressure of water vapour over the liquid, preferably in the presence of a solvent, whereby the phthalic anhydride formed remains in solution, separating off insoluble tere and iso phthalic acid and/or other insoluble oxidation products, hydrolysing the phthalic anhydride by treatment with at least the stoichiometric amount of water required to form o-phthalic acid, which is then separated from the liquid.  The mixture may be suspended in the solvent, heated, and after the aqueous vapour pressure over the liquid is decreased the insoluble material filtered off, and the filtrate hydrolysed with water. Alternatively, the mixture may be heated and then located with a solvent in which the phthalic anhydride dissolves, the insoluble material filtered off and the o-phthalic acid obtained by hydrolysing the filtrate with water and separated by filtering. The mixture containing the o-phthalic acid and/or o-phthalic anhydride may be one produced by the oxidation of e.g. benzene substituted in the o-position with groups selected from <FORM:0837007/IV (b)/1> where R is H or alkyl, R1 is alkyl, and <FORM:0837007/IV (b)/2> where R11 and R111=H or alkyl or aryl or any closely related oxygenated derivative of any such compounds, e.g. those containing -CHO, -CH2OR1 (R1=H, alkyl, acyl or aroyl), -COCH3, or hydroperoxy groups, or by the vapour phase oxidation of naphthalene.  The oxidation may be carried out as in the parent Specification or by means of other oxidizing agents such as nitric acid.  The process of the invention may also be applied to the oxidation products of xylenes or their closely related oxygenated derivatives, or the halomethylation product of toluene using such other oxidizing agents.  The compounds containing oxidizable groups may be prepared by known methods of alkylation, acylation or carbonylation with Friedel-Crafts reagents.  The adjustment of the partial pressure of the water over the liquid and the hydrolysis step are carried out as in the parent Specification, preferably in the presence of mixtures of polar and non-polar solvents, e.g. a mixture of benzene or o-dichlorbenzene with an aliphatic acid or using one of the specified solvents of the parent Specification.  When the process of the invention is carried out in the absence of a solvent and the mixture comprises the nitric acid oxidation product of xylenes, the mixture is subjected to dehydrating conditions to yield phthalic anhydride, insoluble tere- and iso-phthalic acids are filtered off at temperatures sufficiently above the melting point of phthalic anhydride, the anhydride hydrolysed and the insoluble o-phthalic acid separated.  When the process of the invention is applied to a mixture of o- and p-autoxidizable disubstituted benzenes, substantially pure terephthalic acid may be recovered from the filtration before hydrolysis and substantially pure o-phthalic acid by the filtration after hydrolysis.  Pure iso-phthalic acid may be obtained by an analogous method.  Phthalic anhydride may be prepared from o-phthalic acid, and purified as described in the parent Specification. Examples are given of the separation of o-phthalic acid from its admixture with (1) terephthalic acid, (2) terephthalic and iso-phthalic acids, (3) benzoic acid and o-xylene.