GB2269588A

GB2269588A - Hydroxylation of aromatic hydrocarbons

Info

Publication number: GB2269588A
Application number: GB9217398A
Authority: GB
Inventors: Miranda Service
Original assignee: Solvay Interox Ltd
Current assignee: Solvay Interox Ltd
Priority date: 1992-08-15
Filing date: 1992-08-15
Publication date: 1994-02-16
Also published as: GB9217398D0

Abstract

Hydroxylation of aromatic hydrocarbons is prone to polyhydroxylation and the production of tarry by-products. Selective monohydroxylation of aromatics hydrocarbons can be obtained by reacting a limited amount of hydrogen peroxide with the aromatic in solution in a compatible organic solvent and in the presence of a catalyst that is at least partly soluble in the reaction medium and is the salt of a heteropolyacid of general formula: Q3+vPMnVvO40 in which Q represents a compatible organic cation, M represents molybdenum or tungsten, v is an integer which is up to 3, and n is an integer such that n + v = 12. A preferred organic cation comprises cetyl pyridinium. Preferably the reaction medium comprises acetonitrile.

Description

Hydroxylation of Aromatic Compounds The present invention concerns a process for the hydroxylation of aromatic compounds. More specifically the present invention concerns a process for the selective hydroxylation of aromatic hydrocarbons.

Aromatic hydrocarbons form a widely available and relatively cheap starting material for the production of many industrially important compounds and intermediates. One particularly desirable group of intermediates comprise monohydroxylated aromatic hydrocarbons, and in particular, phenols. These are often produced by the direct hydroxylation of the aromatic hydrocarbons. Unfortunately, it has often been found that the introduction of one hydroxy group increases the reactivity of the aromatic compound towards further hydroxylation, and so makes the selective production of the monohydroxylated compound difficult.This further hydroxylation tends to produce a mixture of compounds, which means that it is necessary to.undertake a separation process to obtain the desired product, and can also result in the production of unwanted tarry byproducts, which reduce the yields achievable and can make separation of the product and recycling the catalyst difficult. Consequently, it is desirable to provide a process for selectively monohydroxylating aromatic hydrocarbons.

One suitable reagent for hydroxylation is hydrogen peroxide, usually in the presence of a catalyst to increase its activity. Amongst suitable catalysts are heteropolyacids.

In Chemical Abstracts Vol 103, abstract No 195777j, Xu, Bo et al describe the investigation of the mechanism of the direct hydroxylation of benzene using a heteropolyacid of formula H3+nPMo12 nVnO40 plus hydrogen peroxide. In a trial of this catalyst it was found to have not only a low activity, giving poor yields but also to give significant amounts of nonmonohydroxylated product. It would therefore be advantageous to identify a catalyst having either or both of improved activity and improved selectivity to monohydroxylation.

It is an object of the present invention to provide a process for the selective hydroxylation of aromatic hydrocarbons with hydrogen peroxide having improved activity and selectivity compared to the heteropolyacid catalysed processes of the prior art.

According to the present invention, there is provided a process for the selective hydroxylation of aromatic hydrocarbons in which the aromatic hydrocarbon is dispersed in a compatible organic reaction medium and brought into contact with hydrogen peroxide in the presence of a catalyst characterised in that the catalyst, which is at least partly soluble in the reaction medium, is selected from salts of heteropolyacids of general formula Q3+v PMnVvo4o in which Q represents a compatible organic cation, M represents molybdenum or tungsten, v is an integer which is up to 3, and m is an integer such that n + v = 12 and no more than a limited amount of hydrogen peroxide is employed.

Aromatic hydrocarbons which can be selectively monohydroxylated by the process according to the present invention can be any such compound which contains at least one aromatic ring, and at least one aromatic carbon bonded to a hydrogen atom. In many embodiments, the aromatic compound will contain up to 4 such rings.

The aromatic hydrocarbon can be substituted with any substituent that does not react under the conditions of the process according to the present invention. In many cases, such substituents will be selected from one or more of halogen and alkoxy groups.

A preferred aromatic compound having no substituents is benzene.

By selecting the composition of the heteropolyacid catalyst in accordance with the description provided herein, it has been found possible to observe extremely low levels of polyhydroxylation.

The organic cation Q is desirably an onium cation and particularly an ammonium or phosphonium cation. It is preferable to select a cation that is commensurate in size with the heteropolyacid anion, to at least a reasonable extent. In accordance therewith, it is particularly suitable for the cation to contain at least 8 carbon atoms and preferably from about 15 to about 30 carbons. The carbons may be distributed evenly as for example in tetra ethyl or tetrabutyl ammonium or one or two of the alkyl substituents can contain a disproportionate number of carbons, as in a long chain substituent of from eg 9 to 18 carbons with the remaining alkyl substituents being short chain, such as ethyl of methyl. Two or more of the alkyl substituents can combine to form with the hetero atom, eg nitrogen, a heterocycle, such as pyridinium.An especially convenient range of onium cations comprises alkyl pyridinium cations in which the alkyl contains from 12 to 18 linear carbons, such as cetyl pyridinium.

It will be understood that the catalyst is described herein in terms of the heteropolyacid salt that is introduced into the reaction mixture, and that during the reaction period, a fraction of the heteropolyacid may become transformed in situ to species different from that introduced. The present invention specifically includes the use of any active species which is so derived in situ, even if that species does not accord with the general formula given hereinabove. The catalyst employed can comprise recycled catalyst from a previous hydroxylation process, or catalyst recovered from any other process employing a catalyst according to the general formula above.

It is desirable to choose a suitable temperature at which to carry out the hydroxylation reaction. The temperature selection takes into account the need to dissolve the substrate and at least a proportion of the heteropolyacid catalyst in the reaction medium. In practice it has been found that a temperature in the vicinity of at least 400C is advantageous.

The upper limit comprises the reflux temperature for the medium, which naturally varies depending upon the nature of that medium. In practice, the reaction is often effected at a temperature of from 50 to 800C. In many operational embodiments, the amount of catalyst is such that it is completely dissolved when the reaction is conducted at a temperature of at least 500C.

A suitable solvent in which the reaction can be carried out has been found to comprise low molecular weight aliphatic carboxylic acids, in particular acetic acid. A preferred class of solvents comprises low molecular weight nitriles and in particular acetonitrile. Mixtures of the two aforementioned classes of solvent can be employed.

One especially desirable combination of catalyst, solvent and reaction temperature comprises the use of a salt in which the cation is a C10 to C18 pyridinium ion and the anion contains tungsten or especially molybdenum in a very thigh ratio to vanadium, such as about 11: 1, the solvent comprises acetonitrile as the major proportion and the temperature is from 50 to 800C.

The best results according to the present invention have been obtained when the substrate had been converted in a proportion of from about 5 to 25% and preferably from about 10 to about 20%. Accordingly, in one most desirable aspect of the invention, the reagents are preferably selected so as to achieve such a conversion, or the reaction quenched when monitoring of the reagents and products indicates that a conversion in that range has been attained.

It is desirable, in processes according to the present invention, to employ at least 0.1 mmole and up to 100 mmole of catalyst salt per mole of substrate, and preferably from 0.5 to 5 mmole per mole of substrate. In some processes, it has been convenient to employ substrate significantly in a weight excess to catalyst, such as at least 2.5:1 and significant selective hydroxylation has been obtained using a substrate:catalyst weight ratio selected in the range of from 5:1 to 15:1.

The concentration of substrate is normally selected within the range of 0.1 to 2 moles per litre, taking into account such factors as the temperature and composition of the reaction medium.

One other variable comprises the mole ratio of hydrogen peroxide to substrate. In processes according to the present invention, no more than a limited amount of hydrogen peroxide is employed, with the specific intention of reducing the extent to which over-oxidation of substrate may occur.

Generally, it is desirable to restrict the ratio of hydrogen peroxide:substrate to about 4:1 or less. Significant yields of monohydroxylated product have been obtained when the mole ratio of peroxide consumed to substrate present initially fell in the range of from about 1:1 to about 2:1. Unreacted substrate may be recovered from the reaction medium and employed in a subsequent reaction. It will be recognised that the lower preferred limit for hydrogen peroxide:substrate will depend at least partly upon the extent to which a user is willing to recycle unreactec substrate. It is preferable for at least 5% substrate to be reacted in each cycle.

In practice, the present invention is carried out for reasons of convenience and safety by forming a solution of substrate and the catalyst in the selected reaction medium at the selected reaction temperature and thereafter introducing the selected amount of hydrogen peroxide solution over a significant introduction period, preferably gradually. A convenient peroxide introduction period is often chosen from the range of 15 to 75 minutes. Thereafter, it is desirable for the reaction mixture to be maintained with agitation at the reaction temperature for a further reaction period such that the overall period of introduction and reaction is from about 2 to 7 hours.

Having described the present invention in a general way, a specific embodiment thereof is described hereinafter in greater detail by way of example only.

Example. Hvdroxvlation of Benzene (Cetylpyridinium)4PVMo1 1040 (0.29, 0.07mmol), benzene (2.459, 27mmol and acetonitrile (40ml) were stirred at 600C in a flask fitted with a condenser. Hydrogen peroxide (4.59 of 35%w/w aq solution, 50mmol) was added dropwise over 4 hours. The reaction was continued over a further 2 hours, and then allowed to cool to room temperature. Analysis by HPLC showed 17.1% conversion of benzene with a selectivity to phenol of 76.1%. Only trace quantities of dihydroxy species were noted.

ComDarison. Use of a Prior Art Catalvst The procedure of the Example was repeated except that the catalyst employed was H4PVMo1 1040 (0.29). Analysis by HPLC showed that the maximum yield of phenol was about 6%, with the formation of a significant amount of non-monohydroxylated product, having a peak area on the HPLC of approximately the same area as for phenol.

It can therefore be seen that the catalyst according to the present invention demonstrates superior performance in both yield and selectivity to that of the prior art catalyst.

Claims

Claims.

1. A process for the selective hydroxylation of aromatic hydrocarbons in which the aromatic hydrocarbon is dispersed in a compatible organic reaction medium and brought into contact with hydrogen peroxide in the presence of a catalyst characterised in that the catalyst, which is at least partly soluble in the reaction medium, is selected from salts of heteropolyacids of general formula Q3+v PMnVv 40 in which Q represents a compatible organic cation, M represents molybdenum or tungsten, v is an integer which is up to 3, and m is an integer such that n + v = 12 and no more than a limited amount of hydrogen peroxide is employed.

2. A process according to claim 1, characterised in that n = 11 in the general formula for the heteropolyacid.

3. A process according any preceding claim, characterised in that the organic cation contains at least 8 carbon atoms.

4. A process according to claim 3, characterised in that the organic cation contains from about 15 to about 30 carbons.

5. A process according to any preceding claim characterised in that the organic cation is an ammonium or phosphonium cation.

6. A process according to claim 5, characterised in that the organic cation is an alkyl pyridinium cation.

7. A process according to any preceding claim, characterised in that the aromatic hydrocarbon contains up to 4 aromatic rings.

8. A process according to claim 7, characterised in that the aromatic hydrocarbon is benzene.

9. A process according to any preceding claim, characterised in that the organic reaction medium comprises one or more low molecular weight aliphatic carboxylic acids and/or low molecular weight nitriles.

10. A process according to claim 9, characterised in that the low molecular weight aliphatic carboxylic acid is acetic acid, and the low molecular weight nitrile is acetonitrile.

11. A process according to any preceding claim, characterised in that the reaction temperature is at least 400C.

12. A process according to claim 11, characterised in that the reaction temperature is from about 50 to about 800C.

13. A process according to any preceding claim, characterised in that the substrate is converted in a proportion of from about 5 to 25%.

14. A process according to any preceding claim, characterised in that the mole ratio of catalyst salt to substrate is from about 0.1 mmole and up to about 100 mmole of catalyst salt per mole of substrate.

15. A process according to claim 14, characterised in that the mole ratio of catalyst salt to substrate is from about 0.5 to about 5 mmole per mole of substrate.

16. A process according to any preceding claim, characterised in that hydrogen peroxide is introduced in a mole ratio to substrate of 4: 1 or less.

17. A process for the selective monohydroxylation of aromatic hydrocarbons substantially as described herein with reference to the Example.

18. A process for the selective monohydroxylation of aromatic hydrocarbons substantially as described herein with respect to any novel feature or combination of features.