GB762879A - Process for purifying lactams - Google Patents

Abstract

A lactam is purified by passing the impure lactam in aqueous solution over both anion and cation exchangers. The anion and cation exchangers may be used in any order and they may also be used in the form of a mixed bed. Any non-ionogenic impurities which may be present may be removed by using a surfaceactive adsorption agent, e.g. activated carbon or a bleaching earth, in the form of a filter bed before or after passing the lactam solution through the ion exchangers. Suitable cation exchangers are those containing strongly acid groups such as sulphonated polymerization and polycondensation products, e.g. sulphonated polystyrene. Weaker cation exchange agents may also be used. Suitable anion exchange agents are quaternary ammonium bases of high molecular weight. The concentration of the aqueous lactam solution should preferably not exceed 30 per cent and the lactam may be recovered from the treated aqueous solution by evaporation preferably under reduced pressure. The non-ionogenic impurities present in the impure lactam may also be removed by previously subjecting a solution of the lactam in an inert organic solvent medium to an oxidizing treatment so as to oxidize the impurities selectively, e.g. by the process disclosed in Specification 757,406. In examples: (1) a 50 per cent aqueous solution of impure caprolactam is passed successively through a column filled with Dowex-50 and a column filled with Dowex-2 and the purified solution is then neutralized by passing through a pH correction filter and then evaporated to recover the purified lactam; (2) as in (1) except that the aqueous solution of the lactam is of 20 per cent concentration and the pH correction filter is stated to be Dowex-50; (3) a 20 per cent aqueous solution of impure caprolactam is passed successively through columns filled with Dowex-2, Dowex-50 and previously washed activated carbon and the purified solution then evaporated; (4) crude caprolactam is purified by passing a 20 per cent aqueous solution thereof through a column filled with a mixture of equal parts by volume of Dowex-2 and Dowex-50; (5) a 20 per cent solution in benzene of the lactam obtained by the Beckmann transformation of cyclohexanone oxime is boiled with a saturated aqueous solution of KMnO4 and after distilling off the water as an azeotrope with benzene the solution is filtered and extracted with water to give a 20 per cent aqueous solution of the lactam which is then passed successively through columns filled with activated carbon, Dowex-50, Dowex-2, and Dowex-50.