GB719635A - Manufacture of substantially pure unsaturated nitriles - Google Patents

Abstract

Substantially pure unsaturated nitriles are made by reacting in the vapour phase a ,b -unsaturated aliphatic aldehydes or substances yielding such compounds under the reaction conditions at elevated temperatures in the presence of ammonia, molecular oxygen, and an oxidation catalyst and removing substantially completely from the reaction product impurities consisting of unreacted aldehyde and/or hydrogen cyanide and thereafter fractionating the reaction product to recover the nitrile. Allyl and a -methyl allyl alcohol, b -alkoxy propionaldehyde, diallyl ether, acrolein acetal, trialkoxy propane and b -alkoxy isobutyraldehyde are specified as substances which yield a ,b -unsaturated aliphatic aldehydes under the reaction conditions. The term reaction product includes the gaseous mixture from the high temperature reactor, this mixture after scrubbing with dilute acid to remove ammonium, the solution of the gaseous mixture in e.g. water, and the crude nitrile-oil phase which separates on preliminary distillation of such a solution. The reaction product may be treated with an alkali to neutralize the cyanide or polymerize the unreacted aldehyde or the two impurities may be condensed together and the resulting product removed. Alkaline compounds mentioned are hydroxides, carbonates and cyanides of alkali metals, oxides and hydroxides of alkaline earth metals and basic oxides or hydroxides which remove hydrogen cyanide from gas mixtures, e.g. alkaline ferrous sulphate and p ferric or ferrous oxides or hydroxides. The alkaline compounds may be used as aqueous solutions or as solids. Preferred solids are alkaline earth metal oxides and hydroxides, e.g. calcium and barium hydroxides. Treatment with a solid alkali is preferably effected on the dry gaseous efluent before scrubbing with acid. Preferably concentrated solutions of alkali are used 35-40 per cent caustic soda and saturated solutions of calcium or barium hydroxide being specified. The gaseous reaction mixture is advantageously treated only once with alkaline scrubbing liquid. The nitrile may be recovered from the alkali-treated gas mixture by adsorption in water with subsequent distillation. The condensation of aldehyde with hydrogen cyanide is suitably carried out on the nitrile-oil phase by allowing it to stand at a low temperature optionally in the presence of an alkali as a catalyst. Alkaline catalysts specified are sodium or potassium hydroxides or cyanides in quantities, e.g. of up to 25 mg. equivalents of alkali per 100 mol. of solution. Preferably the temperature during condensation should be below 20 DEG C. and any considerable amount of a separate aqueous phase should be absent. Additional quantities of aldehyde or cyanide may be added to give an excess of that reactant. The condensation product may be separated by distillation or solvent extraction. The distillation should be effected at low temperatures, e.g. below 100 DEG C. and preferably below 70 DEG C., to avoid decomposition of the cyanhydrin. Preferably the oil is neutralized or made slightly acid before distillation. Rapid flash distillation is a preferred method, the condensation product remaining in the residue. During distillation diluents, e.g. water, toluene, xylene or unsaturated product nitrile may advantageously be present. The condensation product may be decomposed to give hydrogen cyanide and unsaturated aldehyde the latter being used in the main reaction. Acrylo- and methacrylo - nitriles are specified products. Examples are provided in which acrolein is reacted with air and the gaseous reaction product treated with aqueous caustic soda, sodium carbonate or barium hydroxide solutions, with solid barium hydroxide or calcium oxide or scrubbed with dilute sulphuric acid and water distilled and the nitrile oil phase separated and allowed to stand optionally in the presence of sodium hydroxide solution the final product being obtained by flash distillation in a climbing film evaporator. Specification 709,337 is referred to.