PROCESS FOR THE PREPARATION OF FORMIC ACID

Abstract: A thermally decomposable formate salt is produced from the formate salt of a first base by contacting the formate salt of the first base with a second, weaker and less volatile base under condi­ tions where the second base displaces the first base. The thermally decomposable formate salt can subsequently be decomposed to formic acid and the second base thereby provid­ ing overall a method of preparing formic acid from the formate salt of the first base.

Description: The invention relates to a process for the preparation of thermally decomposable base formates and also to a process for the preparation of formic acid involving decomposition of said base formates. In GB-A-1 028 930 there is described a process for the production of addition compounds containing formic acid by reacting 1 mol of a tertiary organic base with at least 2 mols of a highly concentrated formic acid. In EP-A-0 095 321 there is described a process for the production of a formate salt of a nitrogenous base which comprises reacting hydrogen and carbon dioxide with the nitrogenous base using a Group VIII transition metal catalyst and using an alcoholic solvent. Formic acid can be obtained from the formate salt or addition compound by hydrolysis with an acid stronger than formic acid but this has the disadvantage that a salt of the base is produced. It is therefore an object of the present invention to provide an improved process for the preparation of formic acid from the aforesaid salts or addition compounds. According to the present invention is provided A process for the preparation of formic acid from a first base formate which is either a salt or an addition compound of formic acid combined with a first base which is a C₁ to C₁₀ trialkylamine characterised in that the process comprises the steps of: (1) contacting the first base formate with a second base of general formula: where R₁ is a monovalent hydrocarbon group containing 1 to 12 carbon atoms and R₂ is either hydrogen or R₁ group, which is (I) weater than the first base and (II) is less volatile than the first base hunder conditions to cause the second base to displace the first base and thereby form a second base formate which is thermally decomposable at a temperature higher than the boiling point of the first base; (2) separating, by distillation, the first base from the second base formate, and (3) thermally decomposing the second base formate to yield formic acid and the second base. Conveniently the second base is a high boiling compound, for example boiling above 180°C since this facilates separation from the formic acid. The second base has a pK a in the range 4.0 to 9.0 and is a compound of the general formula: where R₁ is a monovalent hydrocarbon group containing 1 to 12 carbon atoms and R₂ is a hydrogen atom or an R₁ group, the total number of carbon atoms on R₁ and R₂ conveniently being not more than 20 and preferably from 4 to 12. Suitable hydrocarbon radicals in the imidazole derivatives (I) are, in general, alkyl of 1 to 8 carbon atoms, cyclopentyl, cyclohexyl, phenyl and methyphenyl. Amongst the above, imidazole derivatives where R₁ is n-1-alkyl of 4 to 10 carbon atoms and R₂ is hydrogen or methyl are particularly suitable. Examples of such compounds are 1 - (n - 1 - butyl) - imidazole (pKa 5.9), 1 - (n - 1 - pentyl) - imidazole (pKa 5.9), 1 - (n - 1 - decyl) -imidazole (pKa 5.75), 1 - (n - 1 - butyl) - 2 - methylimidazole (pKa 7.0) and 1 - (n - 1 - pentyl) - 2 - methylimidazole (pKa 6.85). For a definitiion of the pKa values, which are a measure of the base strengh, reference may be made, for example to Landoldt-Börnstein, 6th edition, 7th part volume II, page 900 et seq. The first base is a C₁ to C₁₀ trialkylamine. Examples of suitable trialkylamines are trimethylamine, triethylamine, tripopylamine and tributylamine. Depending on the identity of the first base, the second base is selected so that (1) the second base in wealker than the first (2) the second base formate is thermally decomposable at a temperature higher than the boiling point of the first base. (3) the second base is less volatile than the first base. Desirably a molar excess of the second base is employed. The invention is illustrated by reference to the accompanying figure which is a drawing of a continuous process for operating the present invention. A feed comprising an aqueous solution of triethylammonium formate obtained as the product from a process in which triethylamine was reacted with carbon dioxide and hydrogen in a mixture of water and alcohol as solvent and using a Group VIII transition metal catalyst as described in EP-A-0 095 321 is passed by line 2 into column 4 from which column triethylamine, alcohol and water are removed as overheads via line 6. The triethylammonium formate in the form of a base adduct with a base:formate ratio of 1:2 to 1:3 is withdrawn from the base of column 4 via line 8 (to which line recycled N-butyl imidazole is added as a 0.5 to 1.0 molar excess with respect to formate) and passed to column 10. In this column at 150°C and 220 mbar mercury pressure the base interchange reaction occurs, the triethylamine being displaced from the base formate and removed overhead at 39-40°C via line 12 and the formate salt of the weak base in the form of N-butyl imidazolinium formate withdrawn from the base of column 10 via line 14 and passed to column 16 where it is decomposed into anhydrous formic acid which is removed as overheads via line 18 and N-butyl imidazole which is recycled via line 20 to line 8. The invention is illustrated by the following examples. The distillation experiments given in Examples 1 and 2 were carried out using a 20 plate 25.4 mm Oldershaw column. Example 1 A solution of free triethylamine (81 g), triethylammonium formate (351.5 g, formic acid:amine ratio 1:1) in water (225 g) was distilled to remove triethylamine and water. At atmospheric pressure two fractions were taken off; the first, a 10% w/w aqueous triethylamine azeotrope (18.3 g) at a head temperature of 76°C, reflux ratio 1:1, then a 25% aqueous triethylamine heterogeneous azeotrope (64.2 g) over 89-94°C head temperature, reflux ratio 2:1. At reduced pressure (220 mbar) a further two main fractions were removed; a 46% aqueous triethylamine heterogeneous azeotrope (18.6 g) at a head temperature of 37°C and water fraction (164.0 g) at 60°C, reflux ratio 1:1. This left a residue consisting of an amine formate adduct (185.5 g, formic acid:amine ratio 3:1) which contained 1% w/w water. To 182.5 g of this residue, 600 g of N-butyl imidazole was added. The remaining triethylamine (75 g) was distilled from the mixture at 39-40°C head temperature, 220 mbar reflux ratio 1:1. Gas chromatographic analysis of the residue (690 g) showed it to contain n-butyl imidazolinium formate (384 g) in n-butyl imidazole (i.e. formic acid equivalent 110 g). The residue was that distilled to give formic acid overhead at 26-29°C, base temperature 147°C, 47.6 mbar, reflux ratio 4:1. Example 2 N-butyl imidazole (250 g) was added to a solution of triethylammonium formate (147 g, formic acid to amine ratio 1:1) in water (50 g). The triethylamine/water azeotrope (109 g; 10% w/w water) was distilled from the mixture at a head temperature of 77°C, reflux ratio 2:1, followed by a water fraction at 28-30°C, 46.7 mbar, reflux ratio 4:1. Finally aqueous formic acid (61% w/w acid) was taken off overhead at 30-31°C, base temperature 147°C, 46.7 mbar reflux ratio 4:1. Example 3 N-methyl imidazole (25 g) was added to trimethylammonium formate (18 g, acid to base ratio 2.5:1). Trimethylamine (4.5 g) was distilled from the solution using a Vigreaux column (20 mm diameter) at a base temperature of 108-150°C. Gas chromatographic analysis of the residue (37.4 g) showed it to contain 25 g of N-methyl imidazolium formate (acid to base ratio 1:1) dissolved in N-methyl imidazole (11.4 g).

Claims: 1. A process for the preparation of formic acid from a first base formate which is either a salt or an addition compound of formic acid combined with a first base which is a C₁ to C₁₀ trialkylamine characterised in that the process comprises the steps of: (1) contacting the first base formate with a second base of general formula: where R₁ is a monovalent hydrocarbon group containing 1 to 12 carbon atoms and R₂ is either hydrogen or R₁ group, which is (i) weaker then the first base and (ii) is less volatile then the first base, under conditions to cause the second base to displace the first base and thereby form a second base formate which is thermally decomposable at a temperature higher than the boiling point of the first base; (2) separating, by distillation, the first base from the second base formate, and (3) thermally decomposing the second base formate to yield formic acid and the second base. 2. A process as claimed in claim 1 characterised in that the second base obtained by decomposing the second base formate in step (3) is recycled to step (1). 3. A process as claimed in either claim 1 or claim 2 characterised in that the first base formate used as starting material is one obtained by reaction of carbon dioxide and hydrogen in the presence of a C₁ to C₁₀ trialkylamine and that it is first treated to at least partly remove water and, optionally some of the C₁ to C₁₀ triaklylamine, and is then contacted with the second base in step (1) and that the C₁ to C₁₀ trialkylamine is recycled to the formate salt-forming reaction stage involving carbon dioxide and hydrogen. 4. A process as claimed in claim 1 characterised in that in the formula (I) R₁ is n-1-alkyl of 4 to 10 carbon atoms and R₂ is either hydrogen or methyl. 5. A process according to either claim 1 or claim 3 wherein the compound of formula (I) is 1-(n-1-butyl)-imidazole, 1-(n-1-pentyl)-imidazole, 1-(n-1-decyl)-imidazole, 1-(n-1-butyl)-2-methylimidazole or 1-(n-1-pentyl)-2-methylimidazole.

Inventors: ANDERSON, JEFFREY JOHN, HAMLIN, JOHN EDWARD
Applicants: BP CHEMICALS LIMITED
Application year: 1984
Main classification: C07C51
IPC codes: C07C53, C07C51, C07C67