GB771991A - Production of aliphatic acids - Google Patents

Abstract

Propionic acid is produced by oxidizing in the liquid phase a paraffinic hydrocarbon consisting substantially of hydrocarbons having 6-10 carbon atoms and containing a terminal unbranched carbon chain of 3 or 4 carbon atoms, but not containing a secondary butyl group, with a gas containing or consisting of molecular oxygen, at a temperature of less than 200 DEG C. and in the absence of a hydrogen halide catalyst, and recovering the propionic acid produced. Where the hydrocarbon has a terminal unbranched chain of 4 carbon atoms, high yields of butyric acid may also be recovered. Suitable starting materials include n-hexane, n-heptane and 2-methylpentane. The oxidation may be performed with air or mixtures poorer or richer in oxygen, and part of the molecular oxygen may be in the form of ozone. Superatmospheric pressure may be necessary to maintain the reactants in the liquid phase. It is preferred to use paraffins which are normally liquid at 50 DEG C. and atmospheric pressure. Temperatures of 100 DEG C. to less than 200 DEG C., preferably 130 DEG C. to less than 200 DEG C. may be used. An oxidation catalyst, other than a hydrogen halide catalyst, may be present. Compounds of manganese, cobalt, nickel, vanadium or copper are suitable. The catalysts may be in the form of oil soluble salts of organic acids or the catalyst metal may be added in the form of an anion, e.g. a vanadate. The process may be operated batchwise or continuously. It is preferred to remove continuously or intermittently a part of the reaction mixture, to cool this material to below the reaction temperature, whereby two layers separate, to return the upper layer of unchanged hydrocarbon to the reaction zone and to remove the lower layer as product. Part of the product removed may be replaced by fresh hydrocarbon to maintain a constant volume of liquid in the reactor. The acids produced may be separated by neutralization followed by extraction of the salts and regeneration of the acids, or by partition between suitable solvents without neutralization, or by a combination of extraction and distillation steps. It is preferred to distil the lower aqueous acid layer to remove volatile non-acid oxidation products and then to distil off the lower aliphatic acids and water from the higher boiling oxidation products. The wet acids may be freed from water by distillation with a water entrainer and then fractionated to obtain formic, acetic, propionic and butyric acids in separate fractions. Detailed examples are given of the oxidation of (1) 2-methylpentane, (2) n-hexane and (3) n-heptane using a manganese naphthenate catalyst. The oxidations are effected in a stainless steel reactor fitted with a stirrer, a bottom air inlet, a line for removing gas and provision for withdrawing continuously a part of the liquid product through a cooler to a separator, from which the lower layer is removed as product and the upper layer returned to the reactor. The product removed from the separator is replaced by fresh hydrocarbon. The product is distilled as previously described to obtain a non-acid fraction and separate fractions of formic, acetic, propionic and butyric acids. Specification 743,991 is referred to.