Oxygen → Dimethyl terephthalate

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A process for the production of dimethyl terephthalate from p-xylene and methanol by oxidation in a reactor, in the liquid phase with atmospheric oxygen in the presence of dissolved heavy metal compounds as a catalyst, of a mixture of p-xylene and a fraction containing predominantly methyl p-toluate, which fraction is recycled into the oxidation, to obtain an oxidation product containing primarily p-toluic acid and monomethyl terephthalate, at a temperature of 140°-170° C. and under a pressure of 4-8 bar; by esterification of the oxidation product with liquid and subsequently vaporized methanol brought to an elevated pressure, at a temperature of 220°-280° C. and under a pressure of 20-25 bar in a reactor to obtain a raw ester containing primarily p-toluate and dimethyl terephthalate; by withdrawing the raw ester fraction and a methanol-containing vapor fraction from the esterification stage, by distillatory separation of the raw ester into a p-toluate fraction which is recycled into the oxidation, a raw dimethyl terephthalate fraction which is subjected to further processing by recrystallization in methanol, and a residual fraction which is optionally subjected to a thermal aftertreatment or a reactive treatment with methanol and is partially recycled into the process after a subsequent working-up step by distillation. In this process, the esterification of the oxidation product is conducted with a methanol-containing vapor, brought by compression to the elevated pressure and elevated temperature required for esterification, which methanol-containing vapor has been obtained from the filtrate of the recrystallization of the raw dimethyl terephthalate fraction by vaporization or also by rectification of the methanol-containing vapor fraction and optionally additional methanol-containing process streams, under a pressure lying below the pressure utilized in the esterification reactor.

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