GB730519A - Improvements in the production of acetic anhydride - Google Patents

Abstract

Acetic anhydride is obtained by treating dimethyl ether or methyl acetate with carbon monoxide under a pressure exceeding 200 atmospheres, e.g. between 400 and 800 atmospheres and at a temperature between 100 DEG and 250 DEG C. in an anhydrous liquid medium containing as catalyst cobalt bromide or iodide or a mixture thereof. Instead of using cobalt bromide or iodide as catalyst there may be used a mixture of cobalt metal or a cobalt compound with bromine or iodine or with a bromine or iodine compound such as an alkyl bromide or iodide. It is preferred to carry out the reaction in the presence of a solvent for the catalyst. Specified solvents are the lower N-alkylated pyrrolidones, particularly N-methyl pyrrolidone, acetic acid and acetic anhydride. The process may be carried out batchwise or continuously and it is not necessary to carry out the reaction to completion since the unchanged starting material may be used again in the reaction and the catalyst recovered may also be used again without further treatment. The carbon monoxide may be used in concentrated form or mixtures of carbon monoxide and hydrogen containing at least 60 per cent by volume of carbon monoxide may be used. Examples are given in which methyl acetate is converted using (1) cobalt iodide, and (2) a mixture of cobalt bromide and ethyl iodide respectively as catalyst and an example is also given in which dimethyl ether is converted in the presence of a mixture of cobalt bromide and ethyl iodide as catalyst. In each case the carbon monoxide used contains a minor amount of hydrogen and the reaction is carried out in the presence of a solvent which in the case of (1) is either N-methyl pyrrolidone or acetic anhydride and in the case of (2) is N-methyl pyrrolidone. In the example using dimethyl ether as starting material the solvent is N-methyl pyrrolidone, acetic anhydride, or acetic acid.ALSO:Catalysts for the production of acetic anhydride by treating dimethyl ether or methyl acetate with carbon monoxide at elevated temperature and pressure in an anhydrous liquid medium (see Group IV (b)) comprise either cobalt bromide or iodide or a mixture thereof or a mixture of cobalt metal or a cobalt compound with bromine or iodine or with a bromine or iodine compound such as an alkyl bromide or iodide. Examples include the use of a mixture of cobalt bromide and ethyl iodide as catalyst.