Aliphatic carboxylic acids and their derivatives are compounds of particular industrial interest. In particular, dicarboxylic acids such as adipic acid, 1, 8-octanedioic acid (suberic acid) or 1,12-dodecanedioic acid are valuable starting materials for the synthesis of polyamides and other polymeric materials.

Several syntheses of dicarboxylic acids starting from cyclic ketones are known in the art, some of them being performed on a commercial scale. The most commonly used oxidant is nitric acid, although some other oxidants have been investigated, too. The nitric acid oxidation has several drawbacks, resulting mainly from the corrosivity of nitric acid and the byproducts formed, in particular nitrous oxide which has been found to cause environmental problems.

The technical problem to be solved by the present invention was to provide a selective and high-yield process for the production of aliphatic carboxylic or dicarboxylic acids from aliphatic or cycloaliphatic ketones using a non-corrosive and environmentally friendly oxidant.

According to the present invention, this problem has been solved by the process of claim 1.

It has been found that aliphatic carboxylic acids of the general formulae Rl-COOH (Ia) and R2-COOH (Ib) wherein Rl is tertiary C4-20-alkyl and R2 is selected from hydrogen and CI_6-alkyl, or Rl and R2 together are-(CH2), < with n = 3 to 10, can be prepared by oxidizing ketones of the general formula

wherein R'and R2 are as defined above, with molecular oxygen in the presence of a soluble manganese (tri) compound.

Here and hereinbelow, Cl_6-alkyl is to be understood as any linear or branched primary, secondary or tertiary alkyl group having 1 to 6 carbon atoms, in particular groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, neopentyl, hexyl and so on.

Tertiary Co20-alkyl is to be understood as any tertiary alkyl group having 4 to 20 carbon atoms, in particular groups such as tert-butyl (1,1-dimethylethyl) or tert-pentyl (1,1-di- methylpropyl).

The process according to the invention is applicable to the oxidation of open-chain (aliphatic) ketones as well as alicyclic ketones, the former yielding a pair of mono- carboxylic acids and the latter yielding a dicarboxylic acid.

In a preferred embodiment, Rl and R2 together are-(CH2) r and n is an integer selected from 3,4,5,6 and 10, corresponding to the alicyclic ketones cyclopentanone, cyclo- hexanone, cycloheptanone, cyclooctanone and cyclododecanone as starting materials and the dicarboxylic acids glutaric acid (1,5-pentanedioic acid), adipic acid (1,6-hexanedioic acid), pimelic acid (1,7-heptanedioic acid), suberic acid (1,8-octanedioic acid) and 1,12-dodecanedioic acid as products.

In another preferred embodiment, Rl is tert-butyl and R2 is hydrogen, the starting material being methyl tert-butyl ketone and the products (Ia) and (Ib) pivalic acid and formic acid, respectively.

Preferably, the soluble manganese (tri) compound is manganese (lI) nitrate. Other manganese (It) salts such as manganese (n) acetate may also be used.

More preferably, manganese (n) nitrate is used in an amount of 0.1 to 4 mol%, based on the amount of ketone (II).

The conversion rate and selectivity of the oxidation may be further improved by adding at least one compound selected from the group consisting of nitric acid, soluble cobalt (tut) compounds and soluble copper (ici) compounds in an amount of 0.1 to 5 mol%, based on the amount of ketone (II).

Preferably, the oxidation reaction is carried out in a solvent comprising at least one C2-5-alkanoic acid. More preferably, acetic acid is used as solvent.

In a preferred embodiment, the molecular oxygen is used either in essentially pure form or in a gaseous mixture containing up to 90 vol% of nitrogen, e. g., ordinary air. Preferably, the total pressure is about 1 bar (i. e., ambient pressure) to 20 bar, more preferably about 1 bar to 5 bar.

The reaction temperature is preferably 0 °C to 100 °C, more preferably 20 °C to 70 °C. As the oxidation is exothermic, external cooling may be necessary to avoid thermal excursions.

Preferably, the oxidation reaction is carried out in a solvent, the weight ratio solvent/ketone being from 4 to 20 (i. e., 4: 1 to 20: 1). Besides the preferred alkanoic acids, any solvent which is essentially inert under the reaction conditions may be employed.

The invention is further illustrated by the following non-limiting examples.

Example 1 Adipic Acid A solution containing 25 mmol of cyclohexanone, 0.5 mmol of manganese (l8) nitrate, 0.5 mmol of cobalt (n) nitrate and 1.5 mmol of nitric acid in 25 ml of acetic acid was stirred for 5 h at 40 °C in an oxygen atmosphere under ambient pressure. Subsequently, the acetic

acid was distilled off. The residue was found to consist mainly of adipic acid with some glutaric acid and a small amount of unreacted starting material. The conversion of cyclo- hexanone was found to be 97.5%, the selectivity of the oxidation to adipic acid being 93.4%.

Examples 2-8, Comparative Examples 1-3 Adipic acid The procedure of Example 1 was repeated with different catalysts (without addition of nitric acid) at 20 °C or 40 °C. The conditions and results are shown in Table 1.

Table 1 Example No. Catalyst [mmol] Temperature [°C] Conversion [%] Selectivity 2 Mn(NO3)2/0.5 20 68.7 92.2 Co (NO3) 2/0 5 3 Mn(NO3)2/0.5 20 34.6 81.7 4 Mn(OAc)2/0.5 20 9.1 83.1 5 Mn (N03) 2/0. 5 40 90. 6 89. 1 Co (NO30 2 0. 5 6 Mn(OAc)2/0.5 40 19.3 90.1 Co (OAc) 2/0. 5 Mn (NO3) 2/0 5 40 91. 6 89. 5 Cu (N03) 2/0. 5 8 Mn(OAc)2/0.5 40 17.1 91.2 Cu (OAc) 2/0. 5 C 1 Co(NO3)2/0.5 20 6.2 80.3 C 2 Co (OAc) 2/0 5 20 trace C 3 Cu (NO3) 2/0 5 20 trace

Example 9 Adipic acid The procedure of Example 1 was repeated using 50 mmol of cyclohexanone instead of 25 mmol. The conversion of cyclohexanone was 95.7%, the selectivity 91.8%.

Example 10 Adipic acid The procedure of Example 1 was repeated using air at 5 bar instead of oxygen at 1 bar. The conversion of cyclohexanone was 95.7%, the selectivity 92.6%.

Examples 11-14 The procedure of Example 1 was repeated using several different ketones as starting materials and a reaction temperature of 60 °C. The results are compiled in Table 2.

Table 2 Example No. Ketone Conversion [%] Product Selectivity [%] 11 Cyclopentanone 98. 2 Glutaric acid 91. 7 12 Cycloheptanone 99. 1 Pimelic acid 92. 6 13 Cyclooctanone 97. 8 Suberic acid 93. 4 14 Cyclododecanone 98. 6 1, 12-Dodecane-92. 3 dioic acid

Example 15 Pivalic acid The procedure of Example 1 was repeated using methyl tert-butyl ketone instead of cyclo- hexanone and 70 °C reaction temperature. The conversion was 89.6%, the selectivity 96.4%. Formic acid was formed as second product (Ib).