Improvements in the production of dialkyl esters of iso- and/or terephthalic acid

In a process for the production of iso- and terephthalic acid dialkyl esters of lower alkanols with 1 to 4 carbon atoms at least one dialkyl benzene with two oxidisable side chains in the same positions as the carboxylic ester groups in the desired diester and with 1 to 4 carbon atoms in each side chain or at least one chlorine-substituted dialkyl benzene of the same alkyl chain length in which up to two hydrogen atoms attached to at least one carbon atom of at least one alkyl group are substituted by chlorine, is treated in an alkanolic solution which contains at least 60 to about 98% by weight of an alkanol with 1 to 4 carbon atoms with reference to the weight of the whole solution, with a gas containing molecular oxygen at a temperature between 120 DEG and 220 DEG C. and a pressure of at least 8 atmospheres up to about 500 atmospheres in the liquid phase. Starting materials specified include 1,3- and 1,4-cymenes, diisopropyl benzene, m- and p-xylyl chloride, xylylene dichloride, xylidene dichloride, m-and p-chlormethylcumenes, 1,3- and 1,4-chlorethylcumenes and 1,4-bis-(chlorethyl)-benzene. Catalysts specified include salts of manganese, cobalt or vanadium with acetic, butyric, ethyl-hexanoic, crotonic, oleic or naphthenic acids. As reaction promoters bromides of alkali metals e.g. potassium, of ammonium, and of heavy metals such as manganese, cobalt or nickel may be employed. The pressure employed should be sufficient to ensure the presence of the alcohol in the liquid phase. The process may be carried out batchwise or continuously. Initial materials recovered and such oxidation intermediates as toluic acid alkyl esters, diacetylbenzene, acetylbenzoic acid alkyl esters, p or m-bis(2)hydroxy-isopropylbenzene, aldehydic benzoic acid esters and toluic aldehyde are preferably recycled or further oxidised by themselves. The iso- and terephthalic acid esters may be converted to polyesters after recrystallisation and/or vacuum distillation. In examples: Dimethyl terephthalate is obtained by oxidising p-diisopropylbenzene in the presence of cobalt naphthenate, manganese oleate and methanol, part of the product being drawn off during the reaction and fresh methanolic diisopropylbenzene containing cobalt naphthenate and manganous stearate being supplied (1); dimethyl isophthalate is obtained by oxidation of m-diisopropylbenzene in the presence of manganese acetate (2); dimethylterephthalate is obtained by oxidation of p-diisopropyl benzene in the presence of cob