Patent GB-728511-A (GB)
Title: Production of formic acid from its salts
IPC: C07C51/02
Filed: 1952-03-21
Abstract: In a process for the production of formic acid from its alkali metal and ammonium salts, an aqueous solution or suspension of the salt of formic acid is reacted with a mineral acid or acid salt which liberates free formic acid and the free formic acid is extracted with a solvent which readily dissolves formic acid, has a boiling point substantially different from that of formic acid, does not form an azeotropic mixture with formic acid and is insoluble or only sparingly soluble in water or aqueous formic acid or has had its power of dissolving water or aqueous formic acid reduced by the addition of a further solvent which is miscible with the first solvent and is completely insoluble in water or aqueous formic acid and the formic acid is recovered from the extract by fractional distillation.  Solvents mentioned include carboxylic acid esters of monohydric and polyhydric alcohols, e.g. ethyl, isopropyl and isoamyl carboxylic esters and the carboxylic esters of glycols which may be partially etherified, e.g. methyl-, ethyl- and butyl-glycol formate or methoxybutanol formate, ethers such as aliphatic ethers, e.g. diethyl ether and di-isopropyl ether or cyclic ethers, preferably glycol ethers, e.g. tetrahydrofuran and its methyl derivatives, e.g. a -methyl-tetrahydrofurane, ketones such as ethylmethyl ketone or cyclohexanone and tertiary alcohols such as tertiary amyl alcohol or pinacone.  Mixtures of solvents may be used. Further solvents which may be added to the extraction medium to reduce the power of dissolving water or aqueous formic acid include, e.g. mono- or di-chlorobenzene which may be added to cyclohexanone, carbon tetrachloride or benzene which may be added to ethyl methyl ketone and isopropyl formate, benzene or an aliphatic hydrocarbon which may be added to tetrahydrofurane. Residual extraction medium in the extracted aqueous reaction solution may be removed by heating or extraction with a water-immiscible solvent.  The presence of the mineral acid alkali-metal- or ammonium salt formed exerts a salting out effect and this can be increased by the use of acid salts of mineral acids, e.g. sodium formate may be reacted with sodium bisulphate or acid sodium phosphate.  The process may be carried out using a mixture of solvent and non-solvent such that formic acid containing water is extracted and dehydrating the acid obtained by azeotropic distillation.  In examples: sodium formate in aqueous solution is reacted with sulphuric acid and the mixture is extracted with isopropyl formate, the extract is distilled to remove the isopropyl formate with water from the formic acid and after separation of the water the isopropyl formate is recycled (1); a similar aqueous reaction mixture containing formic acid is extracted in countercurrent with a mixture of n-propyl formate and tetrahydrofurane and the solvent mixture is removed together with the water by distillation of the extract (2); in a similar process a mixture of tetrahydrofurane and isopropyl formate is employed such that on distillation of the solvents from the extract an aqueous solution of formic acid is obtained which is dehydrated by azeotropic distillation with n-propyl formate which is recycled (3); a similar process is carried out using a mixture of tetrahydrofurane and benzene as solvent (4); and in a similar process sodium bisulphate is employed instead of sulphuric acid (5).  Specification 604,760 is referred to.