GB1603620A

GB1603620A - Process for the production of esters

Info

Publication number: GB1603620A
Application number: GB2435577A
Authority: GB
Original assignee: BP Chemicals Ltd
Current assignee: BP Chemicals Ltd
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-11-25

Description

(54) PROCESS FOR THE PRODUCTION OF ESTERS (71) We, BP CHEMICALS LIMITED, of Britannic House, Moor Lane, London, EC2Y 9BU, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statements: The present invention relates to a process for the production of esters and, in particular, to an esterification process in which the extent of reaction is determined by measurement of the electrical conductivity of the esterification mixture.

Esters are produced by the reaction of an organic acid with an alcohol in the presence of a catalyst which is generally a strong mineral acid, such as sulphuric acid or toluene-parasulphonic acid. Generally, on an industrial scale, the objective is to achieve acid conversions of at least 99% molar; otherwise, since excess acid in the crude ester product is subsequently neutralised by the addition of strong base prior to disposal of the plant effluent, the process economics are adversely affected. It is therefore of the utmost importance that the progress of the reaction is accurately monitored, especially in the region of 99% molar acid conversion. The analytical method currently employed involves determining the acidity of aliquots of the esterification mixture by titration with an aqueous solution of a strong base. Apart from being time consuming in themselves, such analytical procedures frequently result in unnecessary prolongation of the esterification reaction before it can be established that the required conversion to ester has been achieved.

It has now been found that the progress of an esterification reaction can be accurately monitored by measurement of the electrical conductivity of the esterification mixture.

Thus the present invention provides a process for the production of esters by reacting in the liquid phase and in the presence of an esterification catalyst a carboxylic acid or carboxylic acid anhydride with an alcohol or a phenol under esterification reaction conditions characterised in that the percentage molar conversion of the carboxylic acid or carboxylic acid anhydride is measured by determination of the electrical conductivity of the esterification reaction mixture.

The carboxylic acid or carboxylic acid anhydride may be for example:a monobasic acid or a monobasic acid anhydride containing up to 20 carbon atoms, e.g.

alkanoic acids, such as myristic, palmitic and stearic acids, and anhydrides thereof, an alkenoic acid or an alkenoic acid anhydride such as oleic acid or the anhydride thereof, or derivatives of such alkanoic and alkenoic acids such as ricinoleic acid; an aliphatic dibasic acid or an aliphatic dibasic acid anhydride containing from 3 to 20, preferably up to 10 carbon atoms, such as adipic, azelaic and sebacic acids and anhydrides thereof; tribasic aliphatic acids or tribasic aliphatic acid anhydrides such as citric acid and anhydrides thereof; monobasic aromatic acids or monobasic aromatic acid anhydrides such as those containing up to 10 carbon atoms, e.g. benzoic acid and anhydrides thereof; dibasic aromatic acids or dibasic aromatic acid anhydrides such as phthalic acids and phthalic anhydride; tribasic aromatic acids or their anhydrides such as hemimellitic, trimellitic and trimesic acids and their anhydrides.

The preferred acids and anhydrides are o-phthalic acid, o-phthalic acid anhydride, adipic acid and sebacic acid.

The alcohol or phenol may be for example: a monohydric alcohol containing up to 20 carbon atoms, preferably an alkanol containing from 4 to 14 carbon atoms e.g. butanol, isoheptanol, iso-octanol, 2-ethylhexanol, nonanol, decanol, tridecanol and mixtures of alcohols, containing, for example, 7 to 9 carbon atoms such as are obtained from olefin mixtures by the OXO process; a dihydric alcohol containing up to 20 carbon atoms, e.g. monoethylene glycol, diethylene glycol, triethylene glycol, mono-, di-, and tri-propylene glycol, the butylene glycols and 2,2,4-trimethyl-pentane diol; a trihydric alcohol such as glycerol; an aliphatic cyclic alcohol containing up to 10 carbon atoms such as cyclohexanol; a derivative, preferably an ether derivative of a dihydric or trihydric alcohol, e.g. a lower alkyl ether derivative such as 2-butoxy ethanol; a monohydric phenol containing up to 10 carbon atoms such as phenol itself and; a dihydric phenol such as catechol, resorcinol, hydroquinone and pyrogallol.

Preferably the alcohol, or phenol is reacted with the acid in an amount in excess of that required for complete reaction, i.e. the stoichiometric amount. The excess may be up to 50%, preferably between 10 and 30% molar, over the stoichiometric amount.

The esterification catalyst may suitably be a strong mineral acid such as sulphuric acid or toluene-para-sulphonic acid or an amphoteric catalyst such as a tetra-alkyl titanate, of which tetra-isopropyl and tetra-isobutyl titanates are representative, or a tin compound, such as stannous oxalate. Preferably the esterification catalyst is sulphuric or toluene-parasulphonic acid. The amount of the acid esterification catalyst employed may be in the range 0.001 to 0.1, preferably 0.003 to 0.01 moles per mole of carboxylic acid or carboxylic acid anhydride present in the reaction mixture.

The esterification reaction conditions may be a temperature in the range 1200 to 230"C, preferably 1500 to 1800C, and atmospheric or superatmospheric, preferably atmospheric pressure.

Preferably the esterification reaction time is sufficient to achieve at least 99% molar conversion of the acid, the % molar conversion being measured by determination of the electrical conductivity of the reaction mixture. The precise value of the electrical conductivity of the esterification mixture corresponding to 99% molar conversion of the acid will depend to some extent on the type and amount of alcohol, acid or anhydride thereof and catalyst employed in the reaction. In the case of the production of diethyl phthalate from phthalic anhydride in the presence of sulphuric acid as catalyst it has been found that the conversion of phthalic anhydride to diethyl phthalate is inversely proportional to the amount of water present in the reaction mixture. Thus, an increase in conversion of phthalic anhydride to ester of from 90 to 99.5%, using 0.12% w/w H2SO4 as catalyst at 1500C/1 bar, is accompanied by a decrease in water content from about 0.2 to 0.01% w/w. This decrease in the water content of the reaction mixture also gives a corresponding decrease in its electrical conductivity from about 1.5 x 10.6 mhos cml to 1.04 x 10.6 mhos cam~'. It is therefore preferred to react phthalic anhydride with ethyl alcohol in the presence of sulphuric acid as catalyst for a time sufficient to reduce the electrical conductivity of the reaction mixture to a value less than or equal to 1.04 x 10.6 mhos cm~l.

The correlation between acid conversion and the electrical conductivity of the reaction mixture may be determined for other combinations of acid, alcohol and catalyst in a similar manner.

In a typical industrial scale esterification reaction, acid or anhydride catalyst and alcohol are fed to an esterification reactor which is then brought to the esterification temperature.

The water formed in the reaction, together with some unconverted alcohol, is removed overhead until at least 99% of the acid or anhydride is converted to ester. Alcohol is added continuously to the reaction mixture to replace the alcohol consumed in the esterification reaction in addition to the alcohol removed overhead.

The electrical conductivity of the reaction mixture may be measured by direct insertion of a probe, connected to a suitable measuring apparatus, into the esterification reaction mixture in the reactor or into the reactor's associated pipework. A suitable measuring apparatus is a Lock Conductivity Bridge, Type MCB.2. Preferably the electrical conductivity is measured continuously.

On achieving that value of the electrical conductivity corresponding to at least 99% molar conversion of the acid or anhydride, any residual acid or anhydride in the ester product may be neutralised by the addition of alkali, either in solid form or in solution.

The invention will now be illustrated by reference to the following Examples.

Example 1 An equimolar mixture of phthalic anhydride and ethanol, together with 0.2% w/w of sulphuric acid catalyst, was heated to 1500C/1 bar in a stirred reactor which was connected to an overhead condenser and distillate receiver. Additional ethanol was added at the rate of 0.1 kg per kilogram of reaction mixture per hour, to replace the ethanol converted to ester and also the unconverted ethanol which was taken overhead with the reaction water.

Ethanol feed containing 6.8% w/w of water was added initially over 4.5 hours and the ethanol used subsequently contained ca. 0.4% w/w water. Samples of the reaction mixture were removed at appropriate intervals for acidity, water and conductivity determinations and the results are given below.

ESTERIFICATION REACTION MIXTURE % Conversion Esterification of PA to Duration (h) Ester (Based Water Content Conductivity on Acidity) (% w/w) (x l06 mhos elm4) 5.3 0.32 1.60 88.2 6.3 0.136 1.44 93.7 8.3 0.043 1.20 98.1 10.3 0.017 1.10 99.3 12.3 0.007 1.04 99.7 A conversion of PA to ester of 99.5%, which is a most desirable economic target conversion, was thus achieved after a batch time of ca. 12 hours and this was readily identifiable by the above change in electrical conductivity of the reaction mixture.

Example 2 The procedure described in Example 1 above was repeated except that the initial feed of ethanol contained 1.71% w/w of water. This feed was used for 4.5 hours and subsequently an ethanol feed containing 0.4% w/w of water was used. The results obtained from monitoring the reaction mixture for acidity, water and conductivity are given below.

ESTERIFICATION REACTION MIXTURE % Conversion Esterification of PA to Duration (h) Ester (Based Water Content Conductivity on Acidity) (% w/w) (x l0-6 mhos cm-i) 4.25 0.16 1.45 91.3 5.25 0.12 1.36 94.5 6.25 0.039 1.20 97.4 7.25 0.016 1.15 98.5 8.25 0.017 1.13 99.2 9.25 0.015 1.10 99.6 10.25 0.012 1.0 99.9 A conversion of PA to ester of 99.5etc. was thus achieved after an esterification time of ca. 9.25 hours.

Plots of conductivity against ester conversion based on the results from both Examples 1 and 2 are given in Figure 1. These show that attainment of an esterification conversion of 99.5% can be identified by decrease in conductivity of the reaction mixture to 1.04 x 10-6 mhos cam~.

Example 3 Acetic acid (38.25 moles), ethylene glycol (19.6 moles), benzene 388 g) and toluene-parasulphonic acid (6.6 g) were charged to a Pyrex (Registered Trade Mark) glass round-bottom flask (5 1), fitted with a stirrer, a 17-plate Oldershaw column (50 mm diameter) and an overhead decanter. The reaction mixture was heated to reflux temperature (initially to ca.

1200C), and reaction water was collected overhead.

The conversion of acetic acid to glycol ester was monitored by measurement of acidity and electrical conductivity of samples which were withdrawn periodically from the esterification reactor. These results are given in Figure 2.

After 6 hours, ca. 85% conversion of acid to ester was achieved. A further quantity of acetic acid (2 moles) was added subsequently to the esterifier contents, and the reaction was continued for a further 1.5 hours, which increased the conversion to 94.2%. Over this period, the conductivity of the reaction mixture decreased from 2.6 to 1.3 micromhos cm~' which indicated clearly the required end-point of the esterification reaction. Pure ethylene glycol diacetate was obtained subsequently by a vacuum heading and tailing distillation of the crude reaction product.

Example 4 Oxalic acid (3.5 moles), ethanol (7 moles of ethanol; water azeotrope) and sulphuric acid (4 g) were charged to a Pyrex glass round-bottom flask (1 1), fitted with a stirrer, a 10-plate Oldershaw column (30 mm diameter) and an overhead product collection system. This mixture was heated to reflux temperature (viz. ca. 94"C). Additional ethanol azeotrope (feed rate ca. 630 cm3 h-l) was fed into the reaction mixture, and the unconverted ethanol together with the liberated reaction water, was taken overhead using a reflux ratio of 1:1.

After 6 hours. a conversion of oxalic acid to ester of 71% (as determined by acidity measurements) was achieved.

The reaction was continued subsequently using anhydrous ethanol (feed rate ca. 330 cm3 h-'), and after a further 4 hours reaction, virtually 100% conversion of oxalic acid to ester was achieved. During these last four hours, the conductivity of the reaction mixture decreased from 1 to 0.13 micromhos cm~l, which indicated clearly the required end-point of the reaction (see Figure 3). Pure diethyl oxalate was obtained from the crude reaction product (after neutralisation with sodium carbonate) by a vacuum heading and tailing distillation.

WHAT WE CLAIM IS: 1. A process for the production of esters by reacting in the liquid phase and in the presence of an esterification catalyst a carboxylic acid or a carboxylic acid anhydride with an alcohol or a phenol under esterification reaction conditions, characterised in that the percentage molar conversion of the carboxylic acid or carboxylic acid anhydride is measured by determination of the electrical conductivity of the esterification reaction mixture.

2. A process according to claim 1 wherein the acid or acid anhydride is a monobasic acid or an anhydride of a monobasic acid containing up to 20 carbon atoms.

3. A process according to claim 2 wherein the monobasic acid or anhydride of a monobasic acid is myristic, palmitic or stearic acid or an anhydride thereof.

4. A process according to claim 1 wherein the acid or acid anhydride is oleic acid or the anhydride thereof.

5. A process according to claim 1 wherein the acid or acid anhydride is an aliphatic dibasic acid or an anhydride of an aliphatic dibasic acid containing from 3 to 20 carbon atoms.

6. A process according to claim 5 wherein the dibasic acid or dibasic acid anhydride is adipic, azelaic or sebacic acid or an anhydride thereof.

7. A process according to claim 1 wherein the acid or acid anhydride is a tribasic aliphatic acid or a tribasic acid anhydride.

8. A process according to claim 7 wherein the tribasic aliphatic acid or tribasic aliphatic acid anhydride is citric acid or an anhydride thereof.

9. A process according to claim 1 wherein the acid or acid anhydride is a monobasic aromatic or a monobasic acid anhydride.

10. A process according to claim 9 wherein the monobasic aromatic acid or monobasic aromatic acid anhydride is benzoic acid or the anhydride thereof.

11. A process according to claim 1 wherein the acid or acid anhydride is a dibasic aromatic acid or a dibasic aromatic acid anhydride.

12. A process according to claim 11 wherein the acid or acid anhydride is phthalic acid or phthalic anhydride.

13. A process according to claim 1 wherein the acid or acid anhydride is a tribasic aromatic acid or a tribasic aromatic acid anhydride.

14. A process according to claim 13 wherein the tribasic acid or tribasic acid anhydride is hemimellitic, trimellitic or trimesic acid or an anhydride thereof.

15. A process according to claim 1 wherein the acid or acid anhydride is o-phthalic acid, o-phthalic acid anhydride, adipic acid or sebacic acid.

16. A process according to any one of the previous claims wherein the alcohol is a monohydric alcohol containing up to 20 carbon atoms.

17. A process according to claim 16 wherein the alcohol is an alkanol containing from 4 to 14 carbon atoms.

18. A process according to claim 17 wherein the alkanol is butanol, isoheptanol, iso-octanol, 2-ethylhexanol, nonanol. decanol or tridecanol.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. acetic acid (2 moles) was added subsequently to the esterifier contents, and the reaction was continued for a further 1.5 hours, which increased the conversion to 94.2%. Over this period, the conductivity of the reaction mixture decreased from 2.6 to 1.3 micromhos cm~' which indicated clearly the required end-point of the esterification reaction. Pure ethylene glycol diacetate was obtained subsequently by a vacuum heading and tailing distillation of the crude reaction product. Example 4 Oxalic acid (3.5 moles), ethanol (7 moles of ethanol; water azeotrope) and sulphuric acid (4 g) were charged to a Pyrex glass round-bottom flask (1 1), fitted with a stirrer, a 10-plate Oldershaw column (30 mm diameter) and an overhead product collection system. This mixture was heated to reflux temperature (viz. ca. 94"C). Additional ethanol azeotrope (feed rate ca. 630 cm3 h-l) was fed into the reaction mixture, and the unconverted ethanol together with the liberated reaction water, was taken overhead using a reflux ratio of 1:1. After 6 hours. a conversion of oxalic acid to ester of 71% (as determined by acidity measurements) was achieved. The reaction was continued subsequently using anhydrous ethanol (feed rate ca. 330 cm3 h-'), and after a further 4 hours reaction, virtually 100% conversion of oxalic acid to ester was achieved. During these last four hours, the conductivity of the reaction mixture decreased from 1 to 0.13 micromhos cm~l, which indicated clearly the required end-point of the reaction (see Figure 3). Pure diethyl oxalate was obtained from the crude reaction product (after neutralisation with sodium carbonate) by a vacuum heading and tailing distillation. WHAT WE CLAIM IS:

1. A process for the production of esters by reacting in the liquid phase and in the presence of an esterification catalyst a carboxylic acid or a carboxylic acid anhydride with an alcohol or a phenol under esterification reaction conditions, characterised in that the percentage molar conversion of the carboxylic acid or carboxylic acid anhydride is measured by determination of the electrical conductivity of the esterification reaction mixture.

19. A process according to claim 17 wherein the alcohol is the mixture of alcohols

containing from 7 to 9 carbon atoms obtained from olefin mixtures by the OXO process.

20. A process according to any one of claims 1 to 15 wherein the alcohol is a dihydric alcohol containing up to 20 carbon atoms.

21. A process according to claim 20 wherein the dihydric alcohol is monoethylene glycol, diethylene glycol, triethylene glycol, mono-, di- or tri-propylene glycol, butylene glycol or 2,2,4-trimethyl-pentane diol.

22. A process according to any one of claims 1 to 15 wherein the alcohol is a trihydric alcohol.

23. A process according to claim 22 wherein the trihydric alcohol is glycerol.

24. A process according to any one of claims 20 to 23 wherein the alcohol is an ether derivative of a dihydric or trihydric alcohol.

25. A process according to claim 24 wherein the ether derivative is 2-butoxy ethanol.

26. A process according to any one of claims 1 to 15 wherein the phenol is phenol itself.

27. A process according to any one of claims 1 to 15 wherein the phenol is catechol, resorcinol, hydroquinone or pyrogallol.

28. A process according to any one of the previous claims wherein the amount of alcohol or phenol reacted with the acid or anhydride is in excess of the stoichiometric amount required for complete reaction.

29. A process according to claim 28 wherein the excess is between 10 and 30% molar.

30. A process according to any one of the previous claims wherein the esterification catalyst is sulphuric acid or toluene-para-sulphonic acid.

31. A process according to any one of claims 1 to 29 wherein the esterification catalyst is an amphoteric catalyst.

32. A process according to claim 31 wherein the amphoteric catalyst is tetra-isopropyl or tetra-isobutyl titanate or stannous oxalate.

33. A process according to claim 30 wherein the amount of esterification catalyst employed is in the range 0.001 to 0.1 moles per mole of carboxylic acid or carboxylic acid anhydride present in the reaction mixture.

34. A process according to claim 33 wherein the amount of esterification catalyst employed is in the range 0.003 to 0.01 moles per mole of carboxylic acid or carboxylic acid anhydride.

35. A process according to any one of the preceding claims wherein the esterification reaction temperature is in the range 120 to 230"C.

36. A process according to claim 35 wherein the temperature is in the range 150 to 1800C.

37. A process according to any one of the previous claims wherein the esterification reaction pressure is atmospheric pressure.

38. A process according to any one of the previous claims wherein the esterification reaction time is sufficient to achieve at least 99% molar conversion of the acid or acid anhydride, the % molar conversion being measured by determination of the electrical conductivity of the reaction mixture.

39. A process according to any one of claims 11, 12, 15, 16, 28 to 30 and 33 to 38 wherein diethyl phthalate is produced by reacting phthalic anhydride with ethyl alcohol in the presence of sulphuric acid as catalyst for a time sufficient to reduce the electrical conductivity of the reaction mixture to a value less than, or equal to 1.04 x 10.6 mhos cm~1.

40. A process according to any one of the previous claims wherein the electrical conductivity is measured continuously.

41. A process for the production of an ester substantially as hereinbefore described with reference to the Examples.

42. Esters whenever produced by a process as claimed in any one of the preceding claims.