PROCESS FOR THE PREPARATION OF C3-5 SATURATED ALIPHATIC CARBOXYLIC ACIDS

Abstract

A process for the preparation of a saturated aliphatic carboxylic acid with 3 to 5 carbon atoms by oxidation of the corresponding aldehyde with oxygen in which (a) the corresponding aldehyde is converted with oxygen at a temperature of 40 to 150° C. and an oxygen partial pressure of 0.001 to 1 MPa to obtain a mixture containing the saturated aliphatic carboxylic acid and ≤2 mol-% of the corresponding aldehyde with respect to the saturated aliphatic carboxylic acid, (b) the mixture obtained in step (a) is thermally treated in the liquid phase at a temperature of 80 to 250° C. and a pressure of 0.1 to 2 MPa abs for 0.25 to 100 hours, and (c) the mixture obtained in step (b) is distilled in a distillation apparatus to obtain a distillate containing ≥90 wt.-% of the saturated aliphatic carboxylic acid and having an active oxygen content of 0 to 25 wt.-ppm based on the distillate.

Claims

1.-11. (canceled)

12. A process for the preparation of a saturated aliphatic non-alpha-branched carboxylic acid with 3 to 5 carbon atoms by oxidation of the corresponding aldehyde with oxygen, which comprises (a) converting the corresponding aldehyde with oxygen at a temperature of 40 to 150° C. and an oxygen partial pressure of 0.001 to 1 MPa to obtain a mixture containing the saturated aliphatic carboxylic acid and ≤2 mol-% of the corresponding aldehyde with respect to the saturated aliphatic carboxylic acid, (b) thermally treating the mixture obtained in step (a) in the liquid phase at a temperature of 80 to 250° C. and a pressure of 0.1 to 2 MPa abs for 0.25 to 100 hours, and (c) distilling the mixture obtained in step (b) in a single distillation column to obtain a distillate containing ≥90 wt.-% of the saturated aliphatic carboxylic acid and having an active oxygen content of 0 to 25 wt.-ppm based on the distillate.

13. The process according to claim 12, wherein the saturated aliphatic non-alpha-branched carboxylic acid is prepared by converting in step (a) propionaldehyde, n-butyraldehyde or n-pentanaldehyde and conducting the oxidation in the absence of salts of alkali metals, of alkaline earth metals and of transition metals of group 12 of the Periodic Table of the Elements.

14. The process according to claim 12, wherein the saturated aliphatic non-alpha-branched carboxylic acid is propionic acid and the aldehyde is propionaldehyde.

15. The process according to claim 12, wherein the active oxygen content of the mixture obtained in step (a) is 0.02 to 1 wt.-% based on the mixture.

16. The process according to claim 12, wherein the preparation of the saturated aliphatic non-alpha-branched carboxylic acid is performed continuously, and the thermal treatment in step (b) is performed in a tubular reactor.

17. The process according to claim 12, wherein the thermal treatment in step (b) is performed at a temperature of 100 to 180° C.

18. The process according to claim 12, wherein the thermal treatment in step (b) is performed for 0.25 to 5 hours.

19. The process according to claim 12, wherein the active oxygen content of the thermally treated mixture obtained in step (b) is 0 to 100 wt.-% based on the thermally treated mixture.

20. The process according to claim 12, wherein a distillate containing ≥99 wt.-% of the saturated aliphatic non-alpha-branched carboxylic acid based on the distillate is obtained.

21. The process according to claim 12, wherein a distillate containing the saturated aliphatic non-alpha-branched carboxylic acid has an active oxygen content of 0 to 10 wt.-ppm based on the distillate.

22. The process according to claim 12, wherein the saturated aliphatic non-alpha-branched carboxylic acid is prepared continuously.

Description

EXAMPLES

[0074] Iodometry

[0075] Approximately 5 g of the sample, weighed to the nearest 0.1 mg, are placed in a normed reaction vial, flushed with argon and dissolved in 40 ml of a 1:1 acetic acid/chloroform mixture. The reaction vial is provided with a cooler and placed in a stirring heating block, that is already pre-heated to 80° C. A weak argon flow is passed through the cooler to cover the sample surface. This is necessary to avoid the ingress of atmospheric oxygen. Afterwards, 5.0 mL of a saturated potassium iodide solution (ca. 60.0 g potassium iodide dissolved in 100 mL deionized water) are added through the cooler and the mixture is boiled under reflux for 10 min. In the next step, 40.0 mL deionized water are added, and the sample solution is titrated with a 0.01 M thiosulfate solution while using a platinum electrode.

Example 1

[0076] (Preparation of Crude Propionic Acid)

[0077] Crude propionic acid was produced in a technical plant with a production capacity of around 4 tons propionaldehyde per hour by oxidation of propionaldehyde with air. The technical plant comprised three reactors which were connected in series. The first two reactors were fully back-mixed and nearly isothermal jet-loop reactors and the third reactor was an adiabatic sieve plate tower with a nearly ideal tube characteristic. The oxidation was performed with a slight excess of air, providing 105 to 110% of the theoretical amount of O.sub.2 based on propionaldehyde, or in other words, providing a molar ration of oxygen to propionaldehyde of 0.525 to 0.55. Propionaldehyde was only fed to the first reactor and air only to the first two reactors with a distribution of approximately 90% of the total amount for the first reactor and the remaining 10% for the second reactor. All three reactors were operated at a temperature of 75° C. and a pressure of 2.2 MPa abs. The propionaldehyde conversion at the outlet of the third reactor was >99% based on the amount of propionaldehyde fed into the first reactor.

[0078] The crude propionic acid obtained at the exit of the third reactor contained 98.4 wt.-% propionic acid and 650 wt.-ppm of active oxygen, measured by iodometry as described above. It was filled, stored and shipped under an inert gas atmosphere.

Example 2

Comparative Example

[0079] A sample of 1 kg of the crude propionic acid of example 1 was distilled in a batch distillation apparatus containing a 2 m column packed with mesh rings. The column was operated at a top pressure of 0.123 MPa abs and propionic acid distilled off overhead and the fraction boiling at 147.5±0.5° C. was collected. The recovered propionic acid fraction had a propionic acid content of 99.8 wt.-%, analyzed by gas chromatography, and contained 26 wt.-ppm active oxygen, measured by iodometry as described above.

[0080] The propionic acid fraction obtained by the above-mentioned batch distillation showed the same propionic acid content and the same active oxygen content as the propionic acid fraction obtained in the technical plant by distillation in the commercial distillation tower of technical size.

Example 3

Example According to the Invention

[0081] Another sample of 1 kg of the crude propionic acid of example 1 was heated in a glass flask under a nitrogen atmosphere to 105° C. and kept under these conditions for 6 hours. By this procedure, the active oxygen content decreased from 650 wt.-ppm of the crude propionic acid to 89 wt.-ppm at the end of the thermal treatment. The thermally treated mixture was then distilled in the same batch distillation apparatus and under the same conditions as described in example 2. The recovered propionic acid fraction had a propionic acid content of 99.8 wt.-%, analyzed by gas chromatography, and contained 4 wt.-ppm active oxygen, measured by iodometry as described above.

Example 4

Example According to the Invention

[0082] A further sample of 1 kg of the crude propionic acid of example 1 was admixed with manganese propionate to achieve a Mn concentration in the mixture of 0.34 wt.-ppm. The mixture was then heated in a glass flask under a nitrogen atmosphere to 100° C. and kept under these conditions for 6 hours. By this procedure, the active oxygen content decreased from 650 wt.-ppm of the crude propionic acid to 22 wt.-ppm at the end of the thermal treatment. The thermally treated mixture was then distilled in the same batch distillation apparatus and under the same conditions as described in example 2. The recovered propionic acid fraction had a propionic acid content of 99.8 wt.-%, analyzed by gas chromatography, and contained 3 wt.-ppm active oxygen, measured by iodometry as described above.

[0083] Comparative example 2 shows that without the inventive thermal treatment process, the active oxygen content of the distilled propionic acid amounts to significant 26 wt.-ppm, whereas the inventive thermal treatment process performed before the final distillation enables an active oxygen content of the distilled propionic acid of very low 4 wt.-ppm in inventive example 3 and very low 3 wt.-ppm in inventive example 4.

[0084] The examples also show that a homogeneously catalyzed decomposition by manganese propionate leads to nearly the same result as the non-catalyzed decomposition. Thus, it is shown that the inventive process does already perform in the absence of a catalyst.