WO2001058845A1

WO2001058845A1 - Process for the preparation of aromatic carboxylic acids

Info

Publication number: WO2001058845A1
Application number: PCT/EP2001/001181
Authority: WO
Inventors: Carlo Fumagalli; Francesco Minisci; Roberto Pirola
Original assignee: Lonza S.P.A.
Priority date: 2000-02-11
Filing date: 2001-02-05
Publication date: 2001-08-16
Also published as: ITMI20000237A1; AU2001233740A1; IT1317828B1; ITMI20000237A0

Abstract

Aromatic carboxylic acids are produced by oxidation of the corresponding aryl alkyl ketones with molecular oxygen in the presence of a catalyst comprising a manganese salt. The oxidation takes place under mild conditions and the starting materials are easily accessible.

Description

Process for the Preparation of Aromatic Carboxylic Acids

The invention relates to a process for the production of aromatic carboxylic acids by oxidation of aryl alkyl ketones with molecular oxygen.

Many aromatic carboxylic acids and their derivatives are of industrial interest as fine chemicals or starting materials for .polymers. The simplest compounds of this group (benzoic acid and the isomeric phthalic acids) are produced by oxidation of methyl aromatics such as toluene and xylenes with oxygen. These starting materials are available in large amounts by reforming of petrol hydrocarbons. Preferably the oxidation is carried out with catalysts based on carboxylates of cobalt and manganese in the presence of bromine. It is also possible to oxidize alkyl aromatics with alkyl groups other than methyl, for example ethyl, isopropyl or tert-butyl. This method is less suitable for the production of more complex aromatic carboxylic acids, mainly for two reasons:

• With the exception of the very simple ones, the alkyl aromatics are not directly available from petrochemical sources. It is necessary to introduce the alkyl groups through electrophilic alkylation which is generally a process of low chemo- and regioselectivity. The introduction of an alkyl group increases the reactivity of the aromatic ring, which favours polysubstitution. Consequently, the production of more complex alkyl aromatics in an economic way constitutes a problem which is not easily solved on an industrial scale.

• The oxidation of alkyl aromatics takes place under rather drastic conditions (150-200 °C, 20-50 bar), making other functional groups which may be present in the aromatic compound prone to oxidative degradation.

On the other hand, the electrophilic acylation of aromatic compounds is much more selective than the alkylation. The regioselectivity is generally much higher and the introduction of an acyl group in an aromatic compound renders it less susceptible to further substitution. Consequently, in polycyclic systems such as naphthalene or biphenyl the introduction of a second acyl group takes place exclusively in the non-substituted ring. Furthermore, methods for the acylation of aromatic compounds being much simpler and more economic than the tradit onal ones requiring acyl chlorides and aluminium chloride as catalyst have been develop d recently. Instead of acyl chlorides, the corresponding acid anhydrides (EP-A-0 355 983, EP-A-0 167 286) or carboxylic acids (J Mol. Catal. 1994, 93, 169) have been used as starting materials. Aluminium chloride as catalyst has been replaced by other Lewis acids such as ferric salts (Synthesis 1972, 553), protic acids in homogeneous (US-A-4 560 789, EP-A-0 167 286) or heterogeneous phase (J. Org. Chem. 1986, 51, 1567, 1997) or zeolites (J. Mol. Catal. 1987, 40, 231).

The oxidation of acyl aromatics, however, notably the very simple and in general the less expensive ones, such as the acetyl derivatives, under mild conditions to the corresponding carboxylic acids can be a method of considerable industrial interest for the production of aromatic carboxylic acids.

Consequently, the technical problem to be solved by the present invention was to provide a method of oxidizing acyl aromatics to the corresponding aromatic carboxylic acids under mild conditions, using an inexpensive catalyst and a cheap oxidant, preferably molecular oxygen or air.

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

It has been found that using salts of manganese, in particular manganese(n), as catalysts and molecular oxygen as oxidant, it is possible to oxidize unsubstituted or substituted aryl alkyl ketones to the corresponding unsubstituted or substituted mono- or polybasic aromatic carboxylic acids. As manganese salts, common salts such as acetate, chloride, nitrate or sulfate may be employed.

Preferably, the process of the invention is used to prepare aromatic carboxylic acids of the general formula

[R-]_mA[-COOH]„ (I),

wherein A is a mono- or polycyclic aromatic residue, R at each occurence is independently selected from the group consisting

groups, C₂_₄-acyloxy groups, carboxy and halogens, m is an integer from 0 to 10 and n is an integer from 1 to 4, with the proviso that the sum m + n does not exceed the number of hydrogens present in the corresponding unsubstituted aromatic hydrocarbon, by oxidizing aryl alkyl ketones of the general formula

[R-]_mA[-C(=O)CHR'R"]_B (II),

wherein R, m and n are as defined above and R' and R" at each occurence are independently selected from the group consisting of hydrogen,

and phenyl.

Here and in the following, mono- or polycyclic aromatic residues are for example those derived from benzene, naphthalene, biphenyl, anthracene, phenanthrene, pyrene, perylene etc..

are for example methyl, ethyl, propyl, isopropyl, butyl, isobutyl, seobutyl and tert-butyl.

groups are composed of Cι__₄-alkyl and oxygen. C₂_₄-Acyloxy groups are acetoxy, propionyloxy, butyryloxy and isobutyryloxy. Halogens are fluorine, chlorine, bromine and iodine.

Especially preferred starting materials are aryl alkyl ketones (II) wherein A is a benzene, naphthalene or biphenyl system.

Also especially preferred as starting materials are aryl alkyl ketones (II) wherein both R' and R" are hydrogen.

Also especially preferred is the oxidation of aryl alkyl ketones (II) wherein n is 1.

The process of the invention is preferably conducted in a solvent which comprises at least one C₂_₆-alkanoic acid, for example acetic acid, propionic acid, butyric acid, isobutyric acid, valeric (pentanoic) acid or caproic (hexanoic) acid, acetic acid being especially preferred.

Preferably the catalyst comprises manganese nitrate, particularly in an amount of 0.001 to 5 mol% based on the aryl alkyl ketone. It has been found that manganese nitrate is more active than other common salts of manganese. In another preferred embodiment, the catalyst comprises manganese acetate, preferably manganese(π) acetate, and a catalytic amount of at least one compound selected from the group consisting of nitric acid, alkali nitrates and alkali nitrites. Most preferably, the amount of manganese acetate is 0.001 to 5 mol% and the amount of nitric acid, alkali nitrate or alkali nitrite is 0.001 to 6 mol%, each based on the aryl alkyl ketone. When combined with these additives, the catalytic activity of manganese acetate or other manganese salts is comparable to -that of manganese nitrate.

In another preferred embodiment, a cobalt and/or copper salt, most preferably cobalt nitrate or copper nitrate, is present as co-catalyst. More preferably, the co-catalyst is present in an amount of 0.001 to 4 mol%, based on the aryl alkyl ketone. These co-catalysts increase the oxidation rate, leaving the selectivity — which is very high in any case — essentially unaffected. Thus it is possible to carry out the oxidation under even milder conditions of temperature and pressure.

Preferably the molecular oxygen is used either in pure form or in a gaseous mixture containing at least 5 vol% oxygen, the remainder being inert gas(es). A preferred gaseous mixture is ordinary air.

The process of the invention is preferably carried out under a pressure of about 1 bar (i. e., ambient/atmospheric pressure) to 25 bar.

The reaction temperature is preferably between 40 °C and 120 °C.

The weight ratio of solvent and aryl alkyl ketone is preferably between 2:1 and 20: 1.

An important characteristic of the catalytic systems for the oxidation of aryl alkyl ketones according to the present invention resides in the much milder conditions in comparison with the oxidation of alkyl aromatics, where these catalytic systems have minor efficacy. On the other hand, those catalysts comprising salts of transition metals and bromine which are more efficient in the oxidation of alkyl aromatics do not give satisfactory results in the oxidation of aryl alkyl ketones under mild conditions as disclosed herein. When used in the oxidation of acyl-alkyl aromatics, they predominantly lead to the oxidation of the alkyl group, while the catalysts according to the present invention selectively accomplish the oxidation of the acyl group.

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

Example 1 Benzoic acid

Acetophenone (25 mmol), acetic acid (25 ml), manganese(n) nitrate (0.5 mmol) and cobalt(π) nitrate (0.5 mmol) were stirred for 6 h in oxygen atmosphere at 90 °C and ambient pressure. The solvent was distilled off and the residue analyzed by GC after esterifϊcation. The conversion was found to be 96% and the selectivity of the oxidation to benzoic acid 94%.

Examples 2-10 Benzoic acid

The procedure of Example 1 was repeated with various manganese and cobalt salts and reaction temperatures. In each experiment, the amounts of acetophenone, acetic acid, manganese salt (if present) and cobalt salt (if present) were 25 mmol, 25 ml, 0.5 mmol and 0.5 mmol, respectively. The reaction time was 6 h and the pressure 1 bar, with the exception of one experiment, where oxygen was replaced by air under 8 bar pressure. The conditions and results are summarized in Table 1. Table 1

*⁾ comparative example ** 8 bar pressure

Example 11 Benzoic acid

The procedure of Example 1 was repeated with copper(n) nitrate instead of cobalt(n) nitrate at 75 °C instead of 90 °C. The conversion was 66%, the selectivity 96%.

Example 12 Benzoic acid

The procedure of Example 3 was repeated with a lower amount of manganese(π) nitrate (0.1 mmol) at 100 °C. The conversion was 43%, the selectivity 92%.

Examples 13-15 Benzoic acid

The procedure of Example 12 was repeated with various additives. Additives and results are summarized in Table 2. Table 2

*^} Vol.-%

Examples 16-27

The procedure of Example 1 was repeated at 100 °C reaction temperature using various aryl alkyl ketones instead of acetophenone. The starting materials, products, conversions and selectivities are summarized in Table 3.

Table 3

Example 28 4-Acetoxybenzoic acid

4-Acetoxyacetophenone (25 mmol), acetic acid (25 ml), acetic anhydride (2.5 ml), manganese(π) nitrate (0.5 mmol) and cobalt(π) nitrate (0.5 mmol) were stirred for 6 h in oxygen atmosphere at 100 °C and ambient pressure. The solvents were distilled off and the residue analyzed. The conversion was found to be 98% and the selectivity 96%.

Example 29

1,5-NaphthalenedicarboxyIic acid

1,5-Diacetylnaphthalene (25 mmol), acetic acid (25 ml), manganese(π) nitrate (0.5 mmol) and cobalt(π) nitrate (0.5 mmol) were stirred for 6 h in oxygen atmosphere at 100 °C and 6 bar. The solvent was distilled off and the residue analyzed. The conversion was found to be 94% and the selectivity 96%.

Example 30 4,4'-Biphenyldicarboxylic acid

4,4'-Diacetylbiphenyl (25 mmol), acetic acid (25 ml), manganese(ll) nitrate (0.5 mmol) and cobalt(n) nitrate (0.5 mmol) were stirred for 6 h in oxygen atmosphere at 100 °C and 6 bar. The solvent was distilled off and the residue analyzed. The conversion was found to be 96% and the selectivity 97%.

Example 31 Benzoic acid

Acetophenone (70 mmol), acetic acid (25 ml), manganese(π) nitrate (1.4 mmol) and cobalt(π) nitrate (1.4 mmol) were stirred for 6 h in oxygen atmosphere at 100 °C and ambient pressure. The solvent was distilled off and the residue analyzed. The conversion was found to be 92% and the selectivity 94%.

Example 32

4-Acetoxybenzoic acid

4-Acetoxyacetophenone (100 mmol), acetic acid (73 ml), acetic anhydride (19 ml) and manganese(n) nitrate (2.2 mmol) were stirred for 6 h at 100 °C and 4 bar pressure, feeding through a flow meter and a pipe 10 Nl/h air. The solvents were distilled off and the residue analyzed. The conversion was found to be 99% and the selectivity 98%.

Claims

1. A process for the preparation of unsubstituted or substituted mono- or polybasic aromatic carboxylic acids, comprising the oxidation of a corresponding aryl alkyl ketone with molecular oxygen in the presence of a catalyst comprising at least one salt of manganese.

2. The process of claim 1 wherein the aromatic carboxylic acid has the general formula

[R-]_mA[-COOH]„ (I),

wherein A is a mono- or polycyclic aromatic residue, R at each occurence is independently selected from the group

groups,

groups, carboxy and halogens, m is an integer from 0 to 10 and n is an integer from 1 to 4, with the proviso that m + n does not exceed the number of hydrogens present in the corresponding unsubstituted aromatic hydrocarbon, and the aryl alkyl ketone has the general formula

[R-]_mA[-C(=O)CHR'R"]„ (II),

wherein R, m and n are as defined above and R' and R" at each occurence are independently selected from the group consisting of hydrogen, Ci-^-alkyl and phenyl.

3. The process of claim 2, wherein A is a benzene, naphthalene or biphenyl system.

4. The process of claim 2 or 3, wherein R' and R" are hydrogen.

5. The process of any one of claims 2 to 4, wherein n is 1.

6. The process of any one of claims 1 to 5, wherein the oxidation takes place in a solvent comprising at least one C₂_₆-alkanoic acid.

7. The process of any one of claims 1 to 6, wherein the catalyst comprises manganese nitrate.

8. The process of claim 7, wherein manganese nitrate is present in an amount of 0.001 to 5 mol%, based on the aryl alkyl ketone.

9. The process of any one of claims 1 to 6, wherein the catalyst comprises manganese acetate and a catalytic amount of at least one compound selected from the group consisting of nitric acid, alkali nitrates and alkali nitrites is present.

10. The process of claim 9, wherein the amount of manganese acetate is 0.001 to 5 mol% and the amount of nitric acid, alkali nitrate or alkali nitrite is 0.001 to 6 mol%, based on the aryl alkyl ketone.

11. The process of any one of claims 1 to 10, wherein a cobalt and/or copper salt is present as co-catalyst.

12. The process of claim 1 1, wherein the co-catalyst is present in an amount of 0.001 to 4 mol%, based on the aryl alkyl ketone.

13. The process of any one of claims 1 to 12, wherein the molecular oxygen is used in pure form or in a gaseous mixture containing at least 5 vol% oxygen, the remainder being inert gas(es).

14. The process of any one of claims 1 to 13, wherein the oxidation takes place under a pressure of 1 to 25 bar.

15. The process of any one of claims 1 to 14, wherein the oxidation takes place at a temperature of 40 to 120 °C.

16. The process of any one of claims 6 to 15, wherein the solvent/aryl alkyl ketone ratio is 2:1 to 20:1.