GB774902A - Process for the preparation of terephthalic acid - Google Patents

Abstract

A process for preparing terephthalic acid comprises reacting para-di-isopropylbenzene with molecular oxygen in the presence of a catalytic amount of a heavy metal compound and a lower aliphatic mono-carboxylic acid of 1 to 6 carbon atoms in the molecule, at a temperature in the range of 150 DEG C. to 275 DEG C. and a pressure in the range of 300 p.s.i.g. to 1500 p.s.i.g. such that a liquid phase of the lower acid is maintained. The reaction may be carried out in the presence of water. The process may be operated batchwise. The reaction temperature may be regulated by allowing volatilization of the lower acid to take place. Water and acid vapours may be removed during the reaction or partially condensed and recycled. The para-di-isopropyl benzene starting material may be a mixture containing some meta-di-isopropyl-benzene and lower or higher alkyl benzenes. In addition the mixture may contain saturated aliphatic hydrocarbons relatively resistant to oxidation. Isophthalic acid, if present in the reaction product, is removed by washing with glacial acetic acid. Pure oxygen gas or mixtures such as air may be employed in the process. In examples oxygen is passed into a mixture containing para-di-isopropyl benzene with some metaisomer, acetic acid and manganese cumate with agitation at elevated temperature and such a pressure that the reaction mixture contains an aqueous phase, the crude terephthalic acid is filtered off and washed with acetic acid and water, isophthalic acid is obtained on distillation of the acetic acid washings and the crude terephthalic acid is esterified directly with methanol in the presence of hydrochloric, sulphuric or phosphoric acid, optionally after washing with methanol, ethanol or propanol or the terephthalic acid is converted to the acid chloride with thionyl chloride and subsequently esterified (1); caproic acid is employed instead of acetic acid in a similar process and cobalt cumate is employed as catalyst in a comparative process (2); and water is added to reaction mixtures in processes similar to (1), (2), (3) and (4), the pressure being such that a separate aqueous phase is present. Metal carboxylate catalysts are prepared by reacting the metal oxide or other compound with the appropriate acid or dissolving the latter in caustic soda solution and adding the metal acetate. The catalyst is washed with water, and dried in air and then over calcium chloride at low pressure. The manganese carboxylate catalyst may be the salt of a monocarboxylic acid having from 2 to 10 carbon atoms, preferably an acid formed in the reaction system. Manganese cumate may be prepared by dissolving cumic acid in sodium hydroxide solution and adding manganese acetate. Specification 762,129 is referred to.