GB573247A - Improvements in or relating to the manufacture of unsaturated organic compounds - Google Patents

Abstract

An unsaturated nitrile, carboxylic acid or ester is made by thermal decomposition in the vapour phase of a lower fatty acid ester of a hydroxy-nitrile, a hydroxy-carboxylic acid or an esterified hydroxy-carboxylic acid in the presence of free lower fatty acid in addition to that produced from the ester undergoing decomposition. The lower fatty acid is defined as one containing not more than 4 carbon atoms. Preferably, the esterification and thermal decomposition of the ester are combined in a single operation. Thus, the hydroxy compound is heated with lower fatty acid and/or anhydride until part or all is esterified, the ester vaporized with the free fatty acid, heated to yield the unsaturated compound and free acid, part or all of the latter being separated and returned to the starting mixture. Acrylonitrile may be prepared from ethylene or acetaldehyde cyanhydrin acetate or in a single operation from the cyanhydrins themselves, crotonic nitrile from propionaldehyde cyanhydrin acetate or b -acetoxy-butyric nitrile or the corresponding unesterified hydroxy compounds, methacrylic nitrile from acetone cyanhydrin acetate or b -acetoxy-isobutyric nitrile, and methyl acrylate from methyl a -acetoxy-propionate or methyl lactate. Instead of acetic, another lower fatty acid such as propionic may be used as the esterifying acid and the free acid. In converting ethylene or acetaldehyde cyanhydrin into acrylonitrile in a single operation, only 1-1.5 molar proportions of acetic anhydride or 2 of acetic acid per 2 molar proportions of cyanhydrin need be used for the acetylation. The thermal decomposition of the cyanhydrin acetate may be effected at about 500 DEG -600 DEG C. or lower if catalysts are used, e.g. at 400 DEG -450 DEG C. with silica gel, and metaphosphoric acid or its aluminium salt, alone or on carriers such as charcoal or pumice. The reactants may be heated in a still-base above which is a hydrolysis tube whose upper end feeds into a fractionating column, the top of which is connected to a condenser and the bottom of which feeds back to the still-base. In the single-stage process, the mixture may be kept just boiling to begin and the pyrolysis tube cool. The latter, which may be packed with inert material or catalyst, is then heated, the fractionating column temperature being such that water and acrylonitrile pass over, usually with some acetic acid, the rest of the acid and any unchanged ester being returned. The amount of acid returned may be 3-5 times the weight of the ester entering the pyrolysis tube. The distillate may be fractionated to obtain a water-acrylonitrile azeotrope which separates into a layer consisting of a solution of the nitrile in water from which it can be salted out and an upper layer of acrylonitrile with a little water. The process may be made continuous by introducing fresh reactants into the still-base. In examples: (1) ethylene cyanhydrin acetate is boiled in a copper apparatus, the pyrolysis tube being at 520 DEG -550 DEG C. When substantial amounts of acetic acid are being returned, the temperature in the still-base falls to about 165 DEG C.; (2) ethylene cyanhydrin and acetic anhydride are heated for 1 hour at about 165 DEG C. in a glass apparatus. The pyrolysis tube is then heated to 500 DEG C.; (3) acetic acid with a trace of sulphuric acid is used in place of acetic anhydride; (4) ethylene cyanhydrin and acetic anhydride are fed dropwise into the still-base at 160 DEG -170 DEG C., the pyrolysis tube being at 530 DEG -550 DEG C. Acetaldehyde cyanhydrin may replace ethylene cyanhydrin in the examples, the temperature in the still-base being about 180 DEG C.