Purification of acrylates by distillation

摘要

In order to prevent polymerization during the purification of a lower alkyl acrylate by distillation, the operation is conducted in the presence of an aqueous solution of a metallic nitrite and/or a mixture of nitrogen oxides, and the distilled ester is treated to remove free nitrogen oxides if present. Suitable nitrites are those of sodium, lithium, potassium, barium, calcium, and strontium. If nitrogen oxides are used in the distillation, these are prepared by the action of sulphuric or hydrochloric acid on a metallic nitrite. The process may be used in the purification of methyl or ethyl acrylates or the corresponding alpha-alkyl acrylates, from methanol or ethanol and other impurites, by extractive distillation using water as extractant. The distilled ester must be freed from nitrogen dioxide, if present, for otherwise it undergoes polymerization. This removal of NO2 may be achieved by washing the ester with dilute aqueous sodium sulphite or carbonate solution, or by blowing an inert gas through it, or by vacuum treatment. The distilled ester may also be treated for removal of NO2 by the addition of a phenolic compound, preferably a phenolic polymerization inhibitor such as hydroquinone or pyrogallol.ALSO:In order to prevent polymerization during distillation of a lower alkyl acrylate, the operation is conducted in the presence of an aqueous solution of a metallic nitrite and/or a mixture of nitrogen oxides, and the distilled ester is treated to remove free nitrogen oxides if present. Suitable metallic nitrites are those of sodium, lithium, potassium, barium, calcium, and strontium. If nitrogen oxides are used in the distillation, these are prepared by the action of sulphuric or hydrochloric acid on a metallic nitrite. The process may be used in the purification of methyl or ethyl acrylates or the corresponding alpha-alkyl acrylates, from methanol or ethanol and other impurities, by extractive distillation using water as extractant. The distilled ester must be freed from nitrogen dioxide, if present, for otherwise it undergoes rapid polymerization. This removal of NO2 may be achieved by washing the ester with dilute aqueous sodium sulphite or carbonate solution, or by blowing an inert gas through it, or by vacuum treatment. The distilled ester may also be treated for removal of NO2 by the addition of a phenolic compound, preferably a phenolic polymerization inhibitor such as hydroquinone or pyrogallol. In an example, a feed containing approximately 50 per cent methyl acrylate together with methanol and methyl methoxypropionate was fed at ca. 2 gallons per hour into a 40 plate bubble-cap column of stainless steel. A solution of sodium nitrite (3 gms./litre), adjusted to pH 6.0, was fed to the top of the column at 12 galls./hr., and at a temperature of p 44 DEG C. The pressure at the head of the column was 250 mm. Hg. and at the kettle 332 mm. The distillate separated into two layers; and the decanted methyl acrylate layer contained less than 0.5 per cent methanol and approx. 0.01 per cent nitrous acid. The apparatus ran for over 420 hours without any serious polymerization occurring in the still or in the distillate.