GB2188927A - Oxidation of furfural to 2-furoic acid - Google Patents

Abstract

A process for selectively oxidising furfural to 2-furoic acid employs concentrated hydrogen peroxide in the presence of a secondary or tertiary amine promoter, preferably in an amount of at least 0.8 moles per mole of furfural, and advantageously employing an amine promoter having a pKa of at least 9. Especially preferred promoters are triethylamine and tripropylamine. It is possible also to employ the N-oxide or N-hydroxide derivative of respectively the secondary or tertiary amines. The reaction is preferably carried out at 40-60 DEG C for 4-8 hours during which the hydrogen peroxide is introduced gradually. The reaction can be carried out in hydrocarbon, chlorinated hydrocarbon or low molecular weight aliphatic ester or alcoholic solvents, but is preferably carried out in a solvent-free process.

Description

SPECIFICATION Selective oxidation process The present invention relates to the selective oxidation of furfural to furoic acid, and in particular to a process employing a peroxygen compound.

2-Furoic acid has traditionally been made by a Cannizaro reaction but this route inevitably produces an approximately equimolaryield of furfuryl alcohol mixed with furoic acid. Accordingly, such a route cannot be regarded as selective. In the late 1950's it was suggested that hydrogen peroxide could oxidise furfural to furoic acid when dissolved in pyridine but article disclosed that the reaction took three days, which may explain why the route was discarded, especially when considered in the knowledge that alkaline hydrogen peroxide can oxidatively open up a furan ring. More recently, oxygen gas has been employed to oxidise furfural in the presence of a silver/copper catalyst, but this is a heterogenous reaction which can be hazardous.

Thirdly, and again recently it has been proposed to use alkaline sodium hydrochlorite solution to oxidise furfural, but again this is a potentially hazardous reaction, the reaction products contain tars and the effluent is polluted with chlorides and/or oxychlorine species. Accordingly there remains a need for a selective process for the production of furoic acid which avoids or ameliorates the hazards and/or is carried out in conventionally acceptable reaction period.

According to the present invention there is provided a process for the selective oxidation offurfural to 24uroic acid in which furfural and concentrated hydrogen peroxide solutions are brought into contact at a temperature of up to 70CC in the presence of a promotional amount of a secondary or tertiary aliphatic, cycloaliphatic or alicyclic amine or N-oxygenated derivative thereof. Advantageously, by the use of such a promoter the oxidation occurs without the formation of significant by-products, and in particular without ring opening ortarformation.

The amine promoter preferably has a pKa of at least 7 and more preferably of at least 9. In general its pKa is not higher than 12 and many convenient ones have a pKa of 10.5 to 11.5.

Aliphatic or cycloaliphatic Promoters often satisfy the formula R1R2R3N in which R1, R2 and B3 are each selected from alkyl groups containing from C1 to C20 atoms or cycloalkyl groups, forming a tertiary amine and B3 alternatively may be hydrogen forming a secondary amine. Otherwise, the groups R,, R2 and R3 can be the same or different. Especially preferred promoters are those in which the alkyl groups are ethyl or propyl. The longer alkyl groups i.e. at least C3 can be linear or branched. Promoters of especial interest include triethylamine and tripropylamine.

Additionally, the alkyl groups R1 and/or R2 can be substituted by an aryl or alkaryl group such as phenyl or a C1-C4 substituted phenyl group. Preferably not more than one aryl substituent is present. Examples of suitable compounds include dimethyl benzylamine and diethyl,2-phenyl-ethylamine. The cycloalkyl group is normally cyclohexyl.

Alternatively the promoter can have the formula R4NR3 in which R4 represents an aliphatic diradical, preferably C4 or C5 completing an alicyclic amine with the nitrogen atom. The ring carbon atoms can themselves each be substituted by an alkyl, aryl, alkaryl, aralkyl or cycloalkyl group but R4 preferably contains not more than 20 carbon atoms in total. Examples include piperidine and N-methyl or N-ethyl piperidine.

Alternatively when R1 and/or R2 are alkyl they can also be substituted by a non-primary amino group of formula -NR3R5, where B5 is selected from C1-C20 alkyl group optionally aryl or alkaryl substituted or cycloalkyl groups. In such promoters the amine groups are preferably separated by a C2to C5 linearalkylene diradical.

Examples include 1 ,2-bis(ethylamino) ethane, 1 ,2-bis(dimethylamine) ethane and 1 ,2-bis(diethylamino) ethane. The tertiary amines can be oxidised in situ to form N-oxides and the secondary amines oxidised to form N-hydroxides and such oxygenated derivatives are also useful in the invention process although the amines themselves have somewhat greater activity.

It is preferable to employ at least 0.5 moles of promoter per mole of furfural and particularly at least 0.8 moles per mole. The amount of promoter is normally not more than 2 moles per mole furfural and often not more than 1.5 moles. Very good results have been obtained in the range of 0.9 to 1.2 moles per mole furfural.

It is believed that the promoter can form a complex or adduct with the furoic acid and thus the presence of a molar excess of promoter over product beneficially counteracts a downward drift in acidity of the mixture which would otherwise occur as furoic acid is produced.

The most preferred promoter is triethylamine. However, in some embodiments in which the furoic acid product is subsequently separated from the promoter by its extraction into aqueous solution it may be advantageous to select an amine such as tripropylamine or a higher molecular weight amine on account of their lower water solubility, but this must also be balanced against the tendency for the activity of the promoter to increase as the weight of the promoter changes from trioctylamine to triethylamine.

The amount of hydrogen peroxide to employ is preferably at least a stoichiometric amount based on the furfural, i.e. at least one mole per mole, and more preferably a slight excess is used, for example an excess of 5% to 20% molar excess. By so doing, sufficient hydrogen peroxide is present to effect oxidation of the furfural whilst minimising the extent of oxidation of the amine promoter. Higher excesses of hydrogen peroxide are usable but at the expense of comparatively impaired hydrogen peroxide usage. The concentration of hydrogen peroxide to use is normally at least 35% w/w and preferably at least 50% w/w. Of the normally available commercial grade that of about 65-70% w/w is the most preferable. Higher concentrations yet of up to 85% w/w are usable also, but are not widely available or readily transported.It will be recognised that the most preferred practical combination of amounts and concentrations of hydrogen peroxide is from 0.9 to 1.2 moles of hydrogen peroxide per mole furfural, added in the form of a 60-75% w/w solution.

The reaction is preferably carried out by introducing hydrogen peroxide into a mixture of the amine promoter and furfural. All the hydrogen peroxide can be added in a single shot at least in theory and always provided that the scale of preparation of furoic acid is sufficiently small and appropriate safety precautions are observed strictly, including the provision of adequate cooling means. However, for various reasons including enhanced safety, it is much preferred that the hydrogen peroxide be added progressively, that is to say either incrementally or continuously over an extended period of time. The use of smaller increments and increasing the period of time for the introduction of the oxidant both lead to a reduced oxidation of the amine promoter and thus retain its activity better.Accordingly, it is preferred to introduce the oxidant over a period of at least one hour and especially up to 5 hours, the most preferred period being from 2 to 5 hours. After all the hydrogen peroxide is introduced, it is desirable to permit the reagents to remain in contact for a suitable period to enable the remainder of the reaction to take place. The post-introduction reaction period is typically not more than 8 hours and in practice it is selected in conjunction with the length of the introduction period.

The two in summation form the total reaction period, which is thus in practice at least 1 hour and is often from 4to 8 hours.

The reaction is preferably carried out at a temperature of at least 300C, in many instances at up to 700C and especially from 40 to 65"C.

The reaction can be carried out in the absence of or presence of an inert solvent. By inert herein is meant that the solvent does not react to any significant extent with any of furfural, furoic acid or hydrogen peroxide.

Suitable classes of solvents include hydrocarbons, both aliphatic/cycloaliphatic and aromatic, chlorinated hydrocarbons and low molecular weight aliphatic esters. The reaction can also be carried out in the presence of low molecular weight saturated aliphatic alcohols.

The aliphatic/cycloaliphatic hydrocarbons are most advantageously saturated and can comprise linear or non-linear hydrocarbons including pentane, hexane dimethylbutane, heptane and others in many cases having a molecular weight of from 72 to 300 and can comprise a mixture of various molecular weights and/or extent of branching. The aromatic hydrocarbons are often substituted around a benzene or naphthalene nucleus by one or more low molecular weight aliphatic residues such as methyl or ethyl, and suitable examples include toluene, xylene and mesitylene. Alternatively, aliphatic or aromatic fractions from the distillation of oil fractions can be used.

The chlorinated hydrocarbons include chloropropane, dichloromethane, ethylene dichloride, propylene dichloride, and the corresponding trichlorides, or mixtures thereof.

The aliphatic esters conveniently contain up to about 10 carbon atoms and preferably at least 3. Preferred esters include methyl, ethyl and propyl acetate and ethyl or propyl propionate or butyrate. The aliphatic aicohols are preferably ethanol or isopropanol, but can be a linear or branched alcohol up to hexanol.

The amount of such solvents to use is normally at least the combined weight of furfural and amine promoter and often from 125 to 200% of the combined weight. It will be recognised that the aforementioned solvents boil over a wide range of temperatures and also that some of them are sufficiently miscible with the reactants and reaction products in the relative amounts of each as used for the reaction mixture to be a single phase.

These solvents include ethylene dichloride and dichloromethane.

Such solvents are of particularvalue in embodiments in which the solution is boiled during the course of introduction of the hydrogen peroxide and the subsequent reaction period which results in evaporative removal of solvent and water from the reaction vessel. The solvent/water vapour mixture can be cooled and settled so that the immiscible liquids separate into layers, the upper organic layer being recycled to the same reaction vessel or to a subsequent batch.

It is preferable to carry out the reaction in either solvent4ree conditions or employing the inert hydrocarbon or chlorinated hydrocarbon as solvent for co-removal of water that is either introduced within the hydrogen peroxide solution or generated by the oxidation. By so carrying out the reaction it is possible to achieve one or more of better yields of furoic acid, or a simpler process to operate and control or improved recovery and recylability of the promoter.

Many of the aforementioned solvents including the hydrocarbons, the esters and the higher molecular weight chlorinated hydrocarbons are less than completely miscible with the reaction products and thus a two phase mixture results. Conveniently, it has been found thatfuroic acid partitions predominantly in favour of the aqueous phase, so that it can be readily saparated from the organic phase containing some residual promoter and furfural, for which latter mixture can be recycled to a subsequent batch.

Although many solvents can be used, reaction in the absence of any inert solvent is particularly preferred in that it combined high product yield with optimum space yield of product.

In a modification of the present invention process, it has been found that the yield of furoic acid is increased if atri-alkyl phosphate is employed in conjunction with the N-oxide promoter. It is beneficial to employ at least 0.1 moles per mole furfural and especially at least 0.2 up to 0.5 moles per mole furfural. The three alkyl groups, which may be the same dr different often contain in total at least 8 carbon atoms and usually up to 24 carbon atoms. They may be linear or branched and suitable examples include n-butyl and 2-ethyl-hexyl. A particularly suitable trialkyl phosphate is tributyl phosphate and especially in combination with triethylamine N-oxide.

Whilst the invention process is particularly suitable for carrying out in a batch/recycle fashion, it is possible to carry it out alternatively in a continuous fashion employing a water-immiscible solvent in which the reagents and promoter in the aforementioned ratios are introduced continuously into a body of reaction mixture in a reaction vessel at such rates that their residence time in the vessel enables a substantial proportion of the furfural to be reacted to furoic acid, withdrawing from the reaction vessel at a corresponding rate a part of the reaction mixture into a settling vessel in which it separates into an aqueous phase containing a major proportion of the furoic acid and an organic phase, withdrawing the organic phase to the reaction vessel optionally after analysis for residual furfural, furoic acid, promoter and hydrogen peroxide, and collecting the aqueous phase for recovery of furoic acid therefrom. The term continous herein contemplates also frequent intermittent introductions that approximate to continuous. Residence times can conveniently be 6 to 12 hours for example. The solvent is particularly a hydrocarbon or propylene dichloride.

The use of the promoters herein described permits the oxidation of furfural to furoic acid to be accelerated selectively, avoiding wasteful rupture of the furan ring.

The furoic acid product can be recovered from the reaction mixture by its extraction into aqueous solution and especially an aqueous caustic solution. Advantageously, upon basification a substantial part of the amine separates from the furoic acid forming a separate phase which can be recycled to the reaction vessel. Furoic acid then can be precipitated by acidification of the aqueous phase with a mineral acid.

Having described the invention in general terms, specific embodiments will now be described more fully by way of example only.

In each of the Examples, the concentration of residual furfural and furoic acid products in the reaction mixture were determined by an hplc technique.

Examples 1 to 6 In each of these Examples, furfural (309) and organic base (0.3 moles approx) were dissolved in ethanol (40g except Ex 3 where it was 829) at 50"C and aqueous hydrogen peroxide solution (70% w/w, 17g) was introduced slowly over a period of 1.5 hours. The stirred reaction mixture was maintained at 500C for a further 4.5 hours whereupon it was analysed to determine the residual amount offurfural and amount offuroic acid product. The residual available oxygen (avox) was also measured and expressed as the corresponding amount of 70% hydrogen peroxide solution. The results are summarised in Table 1 below. In Example 6, tributyl phosphate (0.063 moles, 16.7g) was present throughout in the reaction mixture.

It was found in each Example that the residual amount of furfural corresponded to difference between the initial charge and that converted to furoic acid, showing that selective oxidation of furfural had occurred.

TABLE 1 Ex Organic Base Yield of H202 No Furoic acid Left 1 Triethylamine 22.5 64.3 0.9 2 Tripropylamine 21.3 60.6 1.0 3 Trioctylamine 19.2 54.9 2.0 4 Dicyclohexylamine 13.5 38.6 0.05 5 Triethylamine N-oxide 19.3 55 7.3 6 Triethylamine N-oxide 27.7 79.1 3.5 From Table 1 it can be seen that the lower molecular weight alkylamines are a little more efficient than their higher molecular weight homologues and that the alkylamine is likewise more effective than the corresponding N-oxide but that the effectiveness of the latter can be enhanced by the presence of trialkyl phosphate.

Examples 7to 9 In these Examples, there was added to a mixture of furfural (309, 0.31 moles approx) and triethylamine (309, 0.3 moles as 70% or 35% w/w solutions) over a period of 1.5 hours and the mixture continued to be stirred for a further 4.5 hours. The temperature throughout the addition and reaction periods was maintained at 40"C or 30"C as specified in Table 2 below. At the end of the reaction period, the mixture was analysed forfuroic acid and furfural.

TABLE 2 Ex No Reaction Conc Furoic Acid Yield Temp C H202 g % w/w 7 50 70 27.5 78.6 8 30 70 28.8 82 9 30 35 27.2 78 From Table 2 it can be seen that the improved yield is obtained at 30"C using 70% w/w hydrogen peroxide compared with using either a higher reaction temperature of 50"C or concentrated hydrogen peroxide of 35% w/w. By a comparison with Table 1, Example 1, it will be observed that all these results were markedly better than in the presence of ethanol.

Examples lotto 13 In each of these Examples a solution offurfural (30g), triethylamine (weight TEA as specified) dichloromethane (1 00g) was boiled under reflex during the period of introduction of aqueous hydrogen peroxide solution (179,70% w/w) and the subsequent reaction periods specified in Table 3 below. The reaction mixture was cooled and its furoic acid content assessed, as well as its residual hydrogen peroxide content.

TABLE 3 Ex H202 Reaction Weight Yield of Furoic H202 No add her Period her TEA g g % Leftg 10 1.5 4 30 29.8 85.2 0.5 11 1.5 4 10 13.9 39.7 8.8 12 4 5 30 31.6 90.3 0.5 13 4 2 35 32.6 93 0.3 From Table 3, it can be seen that the removal of some water from the reaction mixture by boiling and the increase in the period of introduction of hydrogen peroxide, i.e. decreasing its rate of introduction, both result in an improved rate and extent of furoic acid production. It can also be seen that in the presence of only 0.3 moles amine base per mole furfural the rate of furoic acid production was substantially slowed although the residual hydrogen peroxide demonstrates that substantial further production would occur using a longer reaction period.

Example 14 Into a suitable vessel equipped with stirring, thermometer and reflux condenser was charged 609 (0.625 moles) fu furfural together with 70g (0.693 motes) of triethylamine and the mixture warmed to 500C. To this solution was added 349 (0.70 moles) of 70% H202 progressively over 4 hours maintaining the temperature at 55"C +/- 1"C. Stirring was continued for a further 3 hours at 55"C +/- 1"C. After a total reaction time of 7 hours 150g of 15% NaOH solution was added with stirring. Upon standing the mixture separated out into two phases. The aqueous phase (lower) was drained off into another vessel and 2259 of water was added.This aqueous phase was cooled to 200C and 60.259 of concentrated H2SO4 added maintaining a temperature of 20-25 C. When all the sulphuric acid had been added the solution was cooled to 200C, the solid which precipitated out was filtered off and dried in an oven at 40-500C overnight. The product was 52.129 of 96% furoic acid. The upper phase, 95.4% triethylamine was available for recycling.

Analysis by HPLC of the liquor at the end of the reaction period but before basification indicated that the selectivity of the reaction was 100%, i.e. 2furoic acid was the only reaction product and that 90% of the furfural had reacted.

Example 15 Example 14was repeated but employing additionally 2009 dichloromethane solvent and the mixture was refluxed. Substantially the same yield of furoic acid and 100% selectivity of reaction was obtained indicating that reactivity was as good but space yield impaired by the addition of solvent.

Example 16 Example 15 was repeated employing 200g ethylene dichloride as solvent. The yield and selectivity of the reaction stage were respectively 80% and 89%.

Example 17 Into a suitable vessel equipped with stirring, thermometer and reflux condenser was charged 60g (0.625 moles) furfural together with 709 (0.693 moles) of triethylamine and 200g propylene dichloride and the mixture warmed to 50"C. To this solution was added 349 (0.70 moles) of 70% H202 over 4 hours maintaining the temperature at 55"C +/- 1"C. The mixture separated into 2 phases and phases were analysed forfuroic acid content by HPLC. It was found that 89% of the furfural had reacted at approximately 100% selectivity and that the aqueous:organic partition coefficient for furoic acid was 17.2:1.

Example 18 In this Example, Example 17 was repeated but employing ethyl acetate, instead of propylene dichloride. The respective reaction yield, selectivity and aqueous :organic partition coefficient for furoic acid were 82%, 81% and 11.7:1 respectively.

Example 19 In this Example, Example 17 was repeated but employing heptane instead of propylene dichloride. The respective reaction yield, selectivity and aqueous:organic partition coefficient for furoic acid were 98%, 83% and 00:1 respectively.

Example 20 In this Example, Example 17 was repeated but employing toluene instead of propylenedichioride. The respective reaction yield, selectivity and aqueous:organic partition coefficient for furoic acid were 91 89% and 55:1 respectively.

Example 21 In this Example, Example 14 was repeated but employing about 0.7 moles of tripropylamine instead of triethylamine. In the reaction mixture, HPLC analysis indicated 99% reaction of the furfural and 91 % selectivity.

Claims (31)

1. A process for the selective oxidation of furfural to 24uroic acid in which furfural and concentrated hydrogen peroxide solutions are brought into contact at a temperature of up to 70"C in the presence of a promotional amount of a secondary or tertiary aliphatic cycloaliphatic or alicyclic amine or N-oxygenated derivative thereof.

2. A process according to claim 1 in which the promoter has a pKa of at least 7.

3. A process according to claim 2 in which the promoter has a pKa of 9-12.

4. A process according to claim 3 in which the promoter has a pKa of 10.5 to 11.5.

5. A process according to any preceding claim in which the promoter satisfies the formula R, R2, R3, N in which at least two, R,,R2 and B3 are each selected from alkyl groups containing C2 to C20 atoms and cycloalkyl groups and the third may alternatively be hydrogen.

6. A process according to claim 5 in which the promoter is triethylamine or tripropylamine.

7. A process according to any preceding claim in which at least 0.8 moles promoter is employed per mole furfural.

8. A process according to claim 7 in which up to 2 moles promoter is employed per mole furfural.

9. A process according to claim 7 in which 0.9 to 1.2 moles promoter are employed per mole furfural.

10. A process according to any preceding claim in which at least one mole of hydrogen peroxide is employed per mole furfural.

11. A process according to claim 10 in which 1.05 to 1.20 moles hydrogen peroxide are employed per mole furfural.

12. A process according to any preceding claim in which the concentration of hydrogen peroxide employed is at least 35% w/w aqueous solution.

13. A process according to claim 12 in which the concentration of hydrogen peroxide employed is 65 to 70% w/w.

14. A process according to any preceding claim in which the hydrogen peroxide is introduced progressive ly over an extended period into a mixture of the amine promoter and furfural.

15. A process according to claim 14 in which the hydrogen peroxide is introduced over a period of 2 to 5 hours.

16. A process according to any preceding claim in which the reactants are permitted to react for a period of up to 8 hours after the hydrogen peroxide has been introduced.

17. A process according to any preceding claim in which the period during which hydrogen peroxide is introduced and the post-introduction reaction period in summation total from 4 to 8 hours.

18. A process according to any preceding claim which is carried out at 30 to 70"C.

19. A process according to claim 18 which is carried out at 40 to 600C.

20. A process according to any preceding claim which is carried out in the presence of an inert solvent.

21. A process according to claim 20 in which the solvent is a hydrocarbon, chlorinated hydrocarbon or low molecular weight aliphatic ester or alcohol.

22. A process according to claim 20 or 21 in which the solvent is immiscible with water and thereby forms a second phase in the reaction mixture, and is selected from those solvents having an aqeous:solvent partition coefficient for furoic acid of at least 10:1.

23. A process according to claim 22 in which the solvent is an aromatic or aliphatic carbon, or is propylene dichloride.

24. A process according to claim 20 in which the solvent is co-removed together with water by boiling the reaction mixture.

25. A process according to claim 24 in which the solvent is dichloromethane.

26. A process according to any of claims 20 to 25 in which the amount of solvent employed is from 125 to 200% of the combined weight of furfural and amine promoter.

27. A process according to any of claims 1-19 carried out in the absence of a solvent.

28. A process according to any preceding claim in which the furoic acid product in a subsequent step is extracted into an aqueous alkaline phase or the aqueous phase is basified, whereby amine which had been adducted to the furoic acid is released and forms a physically separable phase.

29. A process for the oxidation of furfural to 2-fu roic acid substantially as described herein with respect to any one of the Examples.

30. A process for the oxidation of furfuralto 2-furoic acid substantially according to claim 1 and employing any of the novel features or alone or in combination with any other feature described herein.

31. Furoic acid whenever obtained by a process according to any preceding claim.