GB771086A - Improvements in or relating to the production of benzene carboxylic acids - Google Patents

Abstract

Benzene polycarboxylic acids are obtained from benzene homologues by (1) chlormethylation, with chlormethylether or with formaldehyde and hydrogen chloride, to give a mono- or di-chlormethyl body and obtain a derivative of benzene containing at least three alkyl and chlormethyl groups; (2) oxidation of the product with nitric acid at atmospheric pressures to oxidize the chlormethyl groups and give a chlorine-free intermediate; (3) oxidation of this intermediate with nitric acid at elevated pressure to oxidize the residual alkyl groups. Nitric acid of 38-47 per cent strength may be employed in stage 2 and 30-38 per cent strength in stage 3, and the same acid, weakened by the reaction, may be used without separation in both stages. An oxygen-containing gas may be passed through during reaction. The mono- and di-chlormethylated bodies are prepared together if a substantial excess of formaldehyde is used, and then, (1) they may be oxidized together, the acids obtained being separated by fractional crystallization; or (2) the crystals of dichlormethyl body which separate on cooling may be oxidized separately; (3) the mono-chlormethyl body may be separated by distillation and separately oxidized or further chlormethylated to give dichlormethyl body. Terephthalic acid is obtained as a by-product when crude xylene containing ethylbenzene is used as starting material. In examples: (1) m-xylene is chlormethylated, and the oily crystal layer separated and filtered, the crystals of dichlormethyl body being oxidized as above to pyromellitic acid, and the oily liquid mixture is either oxidized to pyromellitic and trimellitic acids, separated by fractional crystallization, or fractionally distilled and the separated mono-and di-chlormethyl bodies are oxidized; (2) crude xylene is treated as under (1), terephthalic acid being obtained as a by-product; (3) 2,4-dichlormethyl toluene is oxidized with nitric acid to give a chlorine-free methyl-phthalic acid, which is further oxidized to trimellitic acid; (4) pseudo-cumene is chlormethylated, and the crystals of dichlormethyl body oxidized with nitric acid to benzene pentacarboxylic acid; (5) durene or isodurene is similarly converted to mellitic acid; (6) crude xylene is monochlormethylated and the monochlormethyl compound is oxidized with nitric acid in two stages.