GB2072656A - Purification of organic compounds - Google Patents

Abstract

A process for reducing the concentration of a coloured impurity contained in a composition comprises contacting together the composition and a borohydride at an acid pH and distilling the resultant mixture. The composition may be a mixture of fatty acids, for instance, tall oil.

Description

SPECIFICATION Purification of organic compounds This invention reiates to the purification of organic compounds and in particular to processes for reducing the concentration of relatively dark coloured impurities in a composition comprising at least one desired compound and one or more such impurities.

It is frequently desired to improve, that is to say lighten, the colour of organic compounds which are intended for various industrial applications. It is known that treatment of many organic compounds or compositions with sodium borohydride can result in an improvement of the colour of the compounds or compositions. It has, however, always been supposed that the treatment with sodium borohydride must be effected in a system with an alkaline, or at the very least a neutral, pH.

This belief that the use of sodium borohydride to reduce or eliminate dark coloured impurities can only be effected at neutral or alkaline pH is not unreasonable, particularly since at an acid pH the borohydride decomposes with the evolution of hydrogen.

By way of illustration, where the compound to be purified is a carboxylic acid, then treatment with sodium borohydride would have involved the conversion of the acid to the corresponding salt in aqueous solution, the treatment with sodium borohydride, and the subsequent reacidification and the separation of the acid from the aqueous solution. This separation procedure can be difficult since often emulsions, whether water-in-fat or fat-in-water emulsions, may well be formed and these emulsions have to be "broken" before separation of the acid can be performed. The separated acid then has to be dried and distilled. It is at this final distillation stage that the colour improvement is normally observed.It is assumed that the coloured impurities are carbonyl compounds or derivatives thereof and that these impurities are converted by the sodium borohydride to products which are not distilled over with the carboxylic acid.

According to the present invention there is provided a process for reducing the concentration of an impurity contained in a composition comprising a desired compound and the impurity, the impurity being darker in colour than the desired compound, the process comprising contacting together the composition and a borohydride at an acid pH and distilling the resultant mixture.

Thus it has now been discovered, contrary to all established practice, that treatment with a borohydride can be effected at an acid pH. As will be appreciated from the above, the benefits of this are substantial. In the case of the purification of a carboxylic acid, the borohydride can be added to the acid, in the absence of water, and the resultant mixture can be distilled to produce directly the purified final product.

Preferably the borohydride is sodium borohydride.

The composition to be purified can be, for example, a mixture of fatty acids (by which is meant saturated or unsaturated carboxylic acids). Preferably the fatty acids are aliphatic acids having from 8 to 24 carbon atoms, more preferably from 14 to 20 carbon atoms and most preferably from 16 to 18 carbon atoms. A particularly preferred acid is an aliphatic carboyxlic acid having 18 carbon atoms.

Again by way of example, a composition known as tall oil is obtained as the waste product of the pulping of wood to make paper. Tall oil contains a mixture of fatty acids, these acids being predominantly unsaturated carboxylic acids with numbers of carbon atoms ranging from 14 to 24, predominantly 16 to 18, and a major ingredient being an acid or acids with 18 carbon atoms.

Crude tall oil is a dark coloured composition and it is distilled in order to obtain a crude fatty acid mixture which is brown in colour. Such a mixture of fatty acids has a variety of uses, for instance in the preparation of varnishes for paints or surface coatings, for printing inks, and for the preparation of soap and detergents.

Particularly for paints, surface coatings and light coloured printing inks, there is a need to reduce the darkness of the fatty acid mixture. Prior to the present invention, treatment with sodium borohydride would involve the above-mentioned complicated procedure of saponification, sodium borohydride treatment, and subsequent re-acidification separation of the fatty acids, drying and final distillation. The present invention allows a fatty acid mixture to be obtained, having the desired light colour, by direct treatment of the mixture with sodium borohydride, the mixture being simultaneously or subsequently distilled.

The amount of sodium borohydride used will depend on the amount of impurities present which are reactive with sodium borohydride. These impurities may include not only dark coloured impurities but also light coloured impurities which may also be reactive with the sodium borohydride. The amount of sodium borohydride used will also depend on the rate of its decomposition under the treatment conditions. In practice, the amount of sodium borohydride to be used will normally be ascertained after appropriate trials have been made in order to obtain a product having the desired lightness of colour. However, it has been found that only small amounts of sodium borohydride are required in order considerably to reduce the colouration of compositions such as the above-mentioned crude fatty acid mixture obtained from tall oil.A preferred amount of sodium borohydride is from 0.01 to 0.5% by weight (based on the weight of the fatty acid mixture). More preferably the amount of sodium borohydride is from 0.02 to 0.05% by weight It has been found that the optimum sodium borohydride amount is about 0.03% for the crude fatty acid mixture obtained from tall oil. Thus the colour of the product obtained becomes progessively lighter as the sodium borohydride concentration is increased up to this optimum amount. Above this amount no further substantial improvement is normally observed.

It is found that the product obtained as a result of the process of the present invention not only has a lighter colour than the composition before treatment but in addition the colour stability of the product is good and at least comparable with products obtained by treatment of sodium borohydride at an alkaline pH.

After mixing the sodium borohydride with the composition to be treated, the resultant mixture may be heated prior to distillation. For instance the mixture may be heated at a temperature of from 50 to 800C for about one to three hours under normal pressure. It has been found that at a temperature above about 80"C the evolution of hydrogen becomes substantial in the case of the treatment of crude fatty acid mixture obtained from tall oil.

The distillation can be carried out immediately after the above-mentioned heat treatment or alternatively may be carried out some time, perhaps many days, after the end of the heat treatment without any deterioration of the final product.

Typically a crude fatty acid mixture obtained from tall oil is heated at a temperature of around 70"C for about one hour and the resultant mixture is then distilled. However, the temperature and duration of this heating step prior to.distillation may be varied over wide ranges, and it is probably more important to ensure an intimate mixing of the sodium borohydride with the composition to be treated prior to the distillation step. Indeed it is thought that no less satisfactory results may be obtained by omitting the heating step prior to distillation where such intimate mixing is achieved.

With regard to the distillation itself, this is, in the case where the composition is a crude fatty acid mixture obtained from tall oil, a vacuum distillation.

The addition of sodium borohydride may be made directly to the reaction vessel as a powder or in the form of a concentrated slurry in the composition to be treated. Preferably an air ejector is fitted to the reaction vessel to disperse evolved hydrogen. The capacity of the air ejector used depends upon the quantity of sodium borohydride added as well as the temperature of the composition being treated.

A process in accordance with the present invention can be conducted either continuously or in a batchwise manner. It is found that the sodium borohydride selectively reduces the colour forming impurities without adversely affecting the yield or the properties of the fatty acid mixture, in the case where the mixture derives from tall oil. A small amount of sodium metaborate is formed as a by-product which remains in the residue after distillation.

Embodiments of the present invention will now be described by way of examples. In the following Examples the colour stabilities of the fatty acid mixture were determined by heating 209 samples in a glass tube having a diameter of 20mm by placing the tube in an oven held at 200"C for one hour. Colour measurements were made using the Gardner 1963 scale.

Example 1 (a) For comparative purposes, 1200 parts by weight of a crude fatty acid mixture (obtained from Tall Oil) were heated to 70"C, held at this temperature for one hourwithout additives then vacuum distilled. A small amount of forerunnings and a main fraction were collected.

(b) 1200 parts by weight of the same crude fatty acid mixture as in (a) were heated to 700C. 0.03% sodium borohydride powder was mixed in and reacted at this temperature for one hour. A vacuum distillation was then carried out in a similar manner two (a).

Forerun Main fraction Residue (a) (I) (a) (b) (a) (b) % Weight 5.3 5.8 85.1 84.8 9.6 9.4 Acid Value - - 187.0 187.0 159.3 120.0 % Fatty Acid - - 89.1 88.4 76.9 73.8 % Rosin Acid - - 5.4 6.2 18.8 21.0 % Unsaps - - 5.3 5.2 4.1 5.1 Iodine Value - - 154.2 153.1 - Gardner Colour 9 6 7 5 - Colour Stability - - 9 6 - (1 hour at 200"C) Example 2 In this Example pretreatment was kept to a minimum. Sodium borohydride was mixed with the same fatty acid mixtures as in Example 1 for 30 minutes at 20"C when it was then completely dispersed. A vacuum distillation at 1600C was carried out as in the first Example.

% Sodium Forerun Main Fraction Borohydride 1 0.01 0.02 0.05 0 0.01 0.02 0.05 %Weight 5.4 5.6 5.4 5.8 87.1 87.4 86.6 87.3 Gardner Colour 9 7 7 6 7 6 6 5 Colour Stab- - - - - 9 8 7 6 ility (1 hour at 200 C Example 3 A concentrated slurry of 0.3 parts sodium borohydride was made up at room temperature in 60 parts of the same fatty acid mixture as in Example 1. This was added to a further 1140 parts of fatty acid mixture held at 60 C,giving a sodium borohydride level of 0.025%. The mixture was stirred at this temperature for 3 hours. A vacuum distillation was carried out, this time taking forerunnings and two main fractions Forerun 2nd Fraction 3rd Fraction (a) (b) (a) (b) (a) (b) %Weight 5.7 5.8 42.0 42.0 42.6 43.6 Gardner Colour 9 7 8 7 7 5 Colour Stability - - - - 8 6 (1 hour at 200"C)

Claims (15)

1. A process for reducing the concentration of an impurity contained in a composition comprising a desired compound and the impurity, the impurity being darker in colour than the desired compound, the process comprising contacting together the composition and a borohydride at an acid pH and distilling the resultant mixture.

2. A process according to claim 1 wherein the borohydride is sodium borohydride.

3. A process according to claim 1 or claim 2 wherein the composition comprises a mixture of saturated and/or unsaturated carboxylic acids.

4. A process according to claim 3 wherein the carboxylic acids are aliphatic acids having from 8 to 24 carbon atoms.

5. A process according to claim 4 wherein the aliphatic acids have from 14 to 20 carbon atoms.

6. A process according to claim 5 wherein the aliphatic acids have from 16 to 18 carbon atoms.

7. A process according to any of the preceding claims wherein the composition includes an aliphatic carboxylic acid having 18 carbon atoms.

8. A process according to any of the preceding claims wherein the composition is tall oil.

9. A process according to any of the preceding claims wherein the borohydride is used in an amount of from 0.01 to 0.5% by weight based on the weight of the composition.

10. A process according to any of the preceding claims wherein the borohydride is used in an amount of from 0.02 to 0.05% by weight based on the weight of the composition.

11. A process according to any of the preceding claims wherein the composition is tall oil and the borohydride is sodium borohydride used in an amount of about 0.03% by weight based on the weight of the tall oil.

12. A process according to any of the preceding claims wherein the mixture of composition and borohydride is heated prior to distillation.

13. A process according to claim 12 wherein the mixture is heated at a temperature of from 50 to 800C for from one to three hours under normal pressure prior to distillation.

14. A process for reducing the concentration of an impurity contained in a composition comprising a desired compound and the impurity, the process being substantially as described herein.

15. A process according to claim 1 and substantially as described in any of the specific Examples.