EP0040776B1 - Process for conjugating polyunsaturated fatty acids or fatty acid mixtures - Google Patents

Abstract

A process for the conjugation of the double bonds of polyunsaturated fatty acids or mixtures of fatty acids containing polyunsaturated fatty acids comprising treating said fatty acids with SO2 in the presence of substoichiometric amounts of soap-forming bases in a closed reaction vessel or in a reaction vessel equipped with a reflux condenser, at a temperature range of from 170 DEG C. to 260 DEG C.; particularly wherein the process is performed in the presence of from 0.5 to 25 mol % of SO2, 0.5 to 25 mol % of soap-forming alkali metal and/or alkaline earth metal compounds, and 0.05 to 2% by weight, based on the fatty acid starting material, of water.

Description

The conjugation of polyunsaturated fatty acids, such as those that occur or are present in natural fatty acid mixtures, plays an important technical role. Such fatty acid mixtures are obtained from the splitting of more unsaturated fats, for example from soybean oil, sunflower oil, linseed oil, tall oil and the like. These fatty acid mixtures contain, as conjugatable compounds, above all the double unsaturated linoleic acid and the triple unsaturated linolenic acid. The double bonds of these acids are separated from each other by methylene groups.

The conjugation of these polyunsaturated fatty acids brings them into a more chemically reactive form. In conjugated form, they have been proven to be of great advantage in their main field of application, namely in the drying oils. Conjugated double bonds can enter into the Diels-Alder reaction. In general, there is an expanded field of application for the conjugated fatty acids.

The processes known hitherto and used in the art for conjugating fatty acids are relatively complex, they require expensive catalysts or are not very environmentally friendly. For example, iodine or nickel catalysts are proposed. According to DE-A-2 261 517, complicated organic sulfur compounds are used. US-A-2 350 583 describes conjugation using the fatty acid sodium salts. In this process, which is used on an industrial scale, the conjugated fatty acid with mineral acid has to be released from its sodium salt in a second step, a full equivalent of an inorganic salt being obtained as a by-product.

Occasionally, there is evidence in the literature that gaseous S0 ₂ can be used for conjugation. S0 ₂ alone provides demonstrably poor degrees of conjugation in addition to a relatively large amount of polymer fatty acid. For example, a fatty acid mixture from sunflower oil with a content of approx. 60% linoleic acid is converted into a product with only 10% fatty acids with conjugated double bonds and 12% polymer fatty acids by passing S0 ₂ at 225 "'C for 4 hours.

US-A-3 257 377 describes the production of emulsifiers from tall oil for the polymerization of unsaturated compounds. For this purpose, tall oil with sulfur dioxide is first used in the temperature range of about 200 ° C to 320 ° C to disproportionate the rosin resin acids and conjugate unsaturated fatty acids and then in the presence of an alkaline compound at 250 ° C to 330 ° C for further disproportionation and dimerization of the conjugated fatty acids treated. The alkaline compound can also be added in the first stage of the treatment with sulfur dioxide. The use of 2 to 5% by weight of the alkaline compound, for example alkali metal hydroxide, carbonate or sulfide, is proposed. The S0 ₂ is passed in the gaseous state through the tall oil heated to high temperatures. The rework shows that even with the omission of the dimerization step, only small amounts of distillable monomeric fatty acids with conjugated double bonds are formed.

From US-A-2 418 454 a process for the conjugation of polyunsaturated fatty acids and fatty acid esters is known, in which the starting material is heated to 150 to 350 ° C. with substoichiometric amounts of alkali in the presence of sulfur dioxide, essentially with exclusion is worked by water.

The invention aims to provide a method which enables the conjugation of polyunsaturated fatty acids or fatty acid mixtures with SO ₂ in a reproducible manner and with high yields of conjugated polyunsaturated fatty acids, but works with only limited amounts of base. The process of the invention described below makes it possible, for example, to use almost all of the available linoleic acid in a period of 4 hours at 225 ° C. in the presence of only 0.2 to 0.3% by weight of alkali, based on the fatty acid mixture used The conditions of process duration and temperature have hitherto also been necessary for the sufficient conjugation of the fully saponified fatty acid.

The invention accordingly relates to a process for conjugating polyunsaturated fatty acids or fatty acid mixtures by treatment with S0 ₂ at elevated temperatures in the presence of substoichiometric amounts of soap-forming bases, this process being characterized in that the treatment of the fatty acid starting material in a reaction vessel in the presence of 0. 05-2% by weight, preferably 0.05 to 1.5% by weight of water, based on the fatty acid starting material, in the temperature range from 170 to 260 ° C.

The process according to the invention enables the perfect reproduction of high reaction yields of conjugated polyunsaturated fatty acids. Compared to the prior art, there is an important and unforeseeable improvement. If, for example, one works in an open reaction vessel without observing certain precautionary measures, which will be described in the following, then according to the experience on which the invention is based, reproducible results with regard to the yield of conjugated fatty acids cannot be set. When working in a closed reaction vessel and in the specified temperature range, however, the desired high yields of fatty acids with conjugated double bonds are set repeatedly. The participation of water or water is highly likely to assume traces in the reaction, which is ensured by the measures of the invention and will be discussed in detail. The water content in the reaction mixture is accordingly at least about 0.05% by weight, based on the fatty acid starting material.

The water content of the reaction zone is preferably not more than 1.5% by weight and in particular not more than 1% by weight. Areas of water content of 0.1 to 1% by weight are particularly suitable. In general, at least about 0.2% by weight of water should be present in the reaction mixture, so that the range from 0.2 to 1% by weight is particularly suitable in practice. All these percentages by weight relate to the starting material containing the unsaturated fatty acids.

It has surprisingly been found that the presence of even small amounts of water in the reaction mixture apparently has a decisive influence on the desired formation of the fatty acids with conjugated double bonds. When carrying out the process according to the invention, it must be taken into account that initially small amounts of water are present in the reaction mixture or form therein. The fatty acid mixture thus introduces a very low water content (<0.1%), water of reaction arises in particular from salt formation and possibly a limited formation of anhydride. However, it should also be taken into account that the process according to the invention operates in the temperature range far above the boiling point of the water and that the miscibility of the reaction mixture with water is extremely low at these high temperatures. If, for example, S0 _{2 is passed} through the hot approach without special precautionary measures, the water in the S0 ₂ stream can be discharged in the shortest possible way. The content of fatty acids with conjugated double bonds in the reaction product can then, for example after 4 hours at 225 ° C., be below 10%.

In contrast, according to the invention, care is taken to ensure that the prescribed amount of water is present in the reaction mixture during the entire period of conjugation. The desired water can be added, batchwise and / or continuously supplemented or - in particular by means of reflux cooling - recycled. In a further embodiment of the invention, these measures enable working in the open reaction container. According to the invention, however, it can be particularly preferred to carry out the conjugation in a closed reaction container, in which case the reaction mixture is preferably operated under autogenous pressure. In this way, the maintenance of the desired concentration of water in the reaction mixture can be ensured in a particularly simple manner.

In general, it can be sufficient to work without the addition of extraneous water and only to use the amounts of water to promote the desired conjugation, which are formed in the course of the reaction and / or are introduced into the reaction mixture by the reactants. It has been shown that in this way water contents in the range of at least 0.1% by weight, based on the fatty acid mixture, are set in practice. With these quantities of water, taking into account the further process parameters described below, the technical solution to the object of the invention is achieved.

An important further parameter of the process according to the invention is the amount of the soap-forming base which is added to the reaction mixture. The amount of base, based on the fatty acids introduced, is usually in the range from 0.5 to 25 mol%, preferably in the range from 1 to 12.5 mol%. Working with amounts of 1.5 to 10 mol% is particularly suitable. Corresponding alkali and / or alkaline earth metal compounds are particularly preferred as soap-forming bases, the alkali metal compounds being the more important representatives. Any soap-forming components from the groups mentioned can be used. The hydroxides or oxides are particularly suitable. Preferred representatives of these reactants are therefore the hydroxides of sodium, potassium or lithium and the oxides or hydroxides of barium or magnesium. NaOH is a particularly suitable representative of this group. It can be used in amounts of 0.1 to 5% by weight, preferably in amounts of 0.2 to less than 2% by weight, based on the fatty acids used.

S0 ₂ can be added to the reaction mixture in gaseous form or, if desired, can be passed through the reaction mixture at least during a period of the reaction. In this last case in particular, however, it must be ensured, as described above, that the water content of the reaction mixture does not drop below the required minimum levels.

The amount of SO ₂ dissolved in the fatty acid is preferably at least about 0.2% by weight, in particular at least about 0.3% by weight. Usually, not not more than 7.5 wt .-% SO ₂ and preferably more than 3 wt .-% S0 ₂ - put to use - in each case based on the initial charge fatty acid mixture. Particularly preferred ranges for the SO ₂ - expressed in mol% and based on the starting material presented - are the following: 0.5 to 25 mol%, preferably 1 to 12.5 mol% and in particular 1.5 to 8 mol% % SO ₂ .

In addition to or instead of the free S0 ₂ , it is also possible to use salts which split off S0 ₂ under the reaction conditions. For example, sodium sulfite, sodium disulfite or even sodium dithionite are suitable. In these cases, however, at least as much of the salt must always be used that sufficient S0 _{2 is} released. The amount of metal cations present for the saponification of fatty acids is normally higher than is absolutely necessary.

Even at normal pressure in an open vessel, a sufficient SO _z concentration is usually established when S0 _{2 is introduced} and / or when S0 ₂ salts are added. It offers everything However, from this point of view the volatile S0 _{2 also} works in a closed vessel to prevent unnecessary SO ₂ losses. A pressure of approximately 3 bar builds up, for example, using 0.5% SO ₂ or 1.5% Na ₂ S0 ₃ , with which the process according to the invention can be carried out in normal technical metal stirred vessels. In this way, the small amounts of water released are retained under control or in the batch.

Isolated certain polyunsaturated fatty acids or corresponding fatty acid mixtures can be used as the starting material. A preferred starting material are natural fatty acid mixtures, such as those obtained when splitting more unsaturated fats, in particular vegetable fats, for example corresponding fatty acid mixtures from splitting soybean oil, sunflower oil, safflower oil, linseed oil, tall oil and the like. The content of polyunsaturated fatty acids is usually at least 30% by weight, often at least 40% by weight. In practice, the content of polyunsaturated components in such fatty acid mixtures is frequently from 35 to 75% by weight, the process according to the invention can be particularly expedient for those feedstocks of natural origin which are depleted in saturated components and correspondingly more than 50% by weight. have unsaturated components.

The process temperature for the conjugation under the conditions according to the invention is between 170 ° C and 260 "C, preferably between 180" and 240 ° C. The duration of the process is usually between 1 and 10 hours, the process period of 1 to 6 hours being particularly important. By varying the process parameters, it is possible on the one hand to ensure rapid and complete conjugation, but on the other hand to largely suppress the formation of polymeric reaction products.

However, the conjugation according to the invention is always accompanied by a restricted oligomerization of the fatty acids. It is possible to keep the formation of dimer fatty acids below 15%, even below 10%. These dimer fatty acids can in turn be used in a variety of ways. The monomeric conjuene fatty acids and oligomerization products formed can be separated from one another by distillation. However, this separation is not required for some applications. The formation of the dimer fatty acids is a result of the special constitution of the conjuene fatty acids formed in the process according to the invention.

The products of the invention are distinguished by a comparatively high trans-trans content of the fatty acids with conjugated double bonds. This is a particular advantage of the method according to the invention. The trans-trans constitution is characterized by the increased ability to form diesters adducts. For example, GB-A-1 141 690 describes a process according to which corresponding trans-trans-conjugated acids can be produced from cis-trans-conjugated fatty acids in an additional second process step. The conjugation of the fully saponified fatty acids according to the prior art predominantly yields cis-trans fatty acid and only approximately one tenth of the fatty acid with conjugated double bonds in the trans-trans form. In contrast, the method of the invention provides about six tenths of the fatty acids with conjugated double bonds in the trans-trans form, about one third in the cis-trans form and less than one tenth in the cis-cis form. This ratio of cis-trans to trans-trans is not far from the equilibrium of 29:71 (J.R. Chipault et al .: J. Am. Oil Chem. 37, 176 ff (1960)). The previously described formation of a maximum of 10 to 15% dimer fatty acid is a side effect that can hardly be suppressed in the production of a predominantly trans-trans-conjugated fatty acid in one step from linoleic acid in the temperature range from approx. 200 "C. to 250" C.

• 64% Linolsäure
• 27% Ölsäure
• 2% Stearinsäure
• 5% Palmitinsäure
• 1%Arachinsäure
• 1 % Myristinsäure

In the following examples, a fatty acid mixture from sunflower oil is used which had been separated off at 5 ° C. by the sunflower oil fatty acid crosslinking process, see Ullmann, Encyclopedia of Industrial Chemistry, 4th edition (1976), volume 11, page 537. The saturated Parts of the product are partially separated:

• 64% linoleic acid
• 27% oleic acid
• 2% stearic acid
• 5% palmitic acid
• 1% arachidic acid
• 1% myristic acid

Here and in the following, the% data are% by weight, unless stated otherwise. The analysis for determining the conjugation in percent is carried out using conventional ultraviolet techniques.

example 1

10 g of sodium hydroxide were added to 1000 g of fatty acid, 64% linoleic acid. The batch was heated to 225 ° C. in a three-necked flask with a reflux condenser, thermometer and gas inlet tube while stirring and introducing SO ₂ (approx. 50 g / hour). This temperature was held for 4 hours, but the SO ₂ feed was stopped after only 0.5 hours / 225 ° C. In the reaction space Water content of about 0.5 wt .-%, which is kept by reflux and early termination of the SO ₂ stream during the entire period in the reaction chamber. The product was mixed with 150 ml of 10% sulfuric acid and then washed with water almost neutral. The high vacuum distillation at approx. 0.1 mbar (via Claisen attachment) provided 84% distillate and 16% residue up to the bottom temperature 230 ° C. The content of fatty acids with conjugated double bonds in the distillate was 47%. The color measurement in the Lovibondtintometer® 5 ¼ "cuvette showed: yellow 3, red 0.5.

Example 2a

1000 g of fatty acid, 64% linoleic acid, were heated with 17 g of sodium sulfite in an autoclave at 225 ° C. for 4 hours. The water content of the reaction mixture is about 0.3%. After the addition of 150 ml of 10% sulfuric acid, the product was washed with water almost neutral and distilled. 83% distillate and 17% residue were obtained.

Distillate analysis:

53% fatty acids with conjugated double bonds thereof

Residue analysis:

Example 2b

Instead of Na ₂ SO ₃ , 0.5% SO ₂ and 0.6% NaOH are used and 200 ° C is chosen instead of 225 ° C. Now 91.6% distillate, 41.5% fatty acids with conjugated double bonds and 8.4% residue were obtained.

Example 3

According to Example 2, corresponding tests are carried out with various catalysts. The work is carried out in 100 g batches without pressure. The reaction mixture is heated to 225 ° C. for 4 hours with the possibility of reflux (reflux condenser without cooling liquid; water dripped back into the reaction vessel over the entire heating period). The reaction product is distilled without prior washing. The yields of fatty acids with conjugated double bonds and residue obtained are summarized in the following Table 1 - here, as in the previous examples, the percentages are percentages by weight.

In a parallel experiment, 1.0% by weight of NaOH as a catalyst and gaseous SO ₂ (about 10 l / h) are used. There are obtained 47.9% fatty acids with conjugated double bonds at 26.7% residue. However, if the same experiment is repeated without the addition of NaOH, only 10.5% fatty acids with conjugated double bonds and 11.8% residue are obtained.

In a further parallel experiment, 0.2% instead of 1.0% NaOH is used as the catalyst. 42.5% fatty acids with conjugated double bonds and 16.7% residue are obtained. However, if the same experiment is repeated in an open flask without the possibility of condensation for the water released, only 6.1% fatty acids with conjugated double bonds and 12.7% residue are obtained.

Comparative example

Gaseous SO ₂ is passed in an amount of 0.7% / hour for 2 hours at 265 ° C. through the fatty acid mixture according to Example 1, in which 1.0% by weight of NaOH had been dissolved. It works without reflux cooling and return of water. (Reflux cooler without cooling liquid; at 265 ° C., all of the water was largely discharged through the SO _2. ) The process product is washed with dilute sulfuric acid and with water and then distilled. A distillate with only 6.7% by weight of fatty acids with conjugated double bonds is obtained. The distillation residue is 10.5%.

If, under otherwise identical conditions, the sulfur dioxide is passed through the fatty acid mixture (with a content of 1.0% by weight of NaOH) in an amount of 0.63% by weight / hour for 4 hours at 225 ° C., then the distillate contains only 18.3% by weight of fatty acids with conjugated double bonds with a distillation residue of 8.5% by weight.

Claims (6)

1. A process for conjugating polyunsaturated fatty acids or fatty acid mixtures by treatment with S0₂ at elevated temperatures in the presence of sub-stoichiometric quantities of soap-forming bases, characterized in that the treatment of the fatty acid starting material is carried out in a reaction vessel in the presence of from 0.05 to 2% by weight and preferably in the presence of from 0.05 to 1.5% by weight of water, based on the fatty acid starting material, and at a temperature in the range from 170 to 260'C.

2. A process as claimed in Claim 1, characterized in that the treatment is carried out in the presence of from 0.5 to 25 mole percent, preferably from 1.0 to 12 mole percent, of SO₂, and from 0.5 to 25 mole percent, preferably from 1 0 to 125 mole percent, of soap forming alkali and/or alkaline-earth compounds, based respectively on the fatty acid starting material.

3. A process as claimed in Claims 1 and 2, characterized in that the treatment is carried out in the presence of from 1.5 to 8 mole percent of S0₂, from 1.5 to 10 mole percent of soap-forming alkali compounds and from 0.2 to 1% by weight of water, based respectively on the fatty acid starting material, at temperatures in the range from 180 to 240" C and over a period of from 1 to 6 hours.

4. A process as claimed in Claims 1 to 3, characterized in that salts which give of S0₂ at the reaction temperature, more particularly corresponding alkali salts, are used in addition to or instead of S0₂ introduced in gaseous form into the reaction vessel.

5. A process as claimed in Claims 1 to 4, characterized in that a fatty acid mixture containing at least 30% by weight and preferably 40% by weight and more of polyunsaturated fatty acids is used, natural fatty acid mixtures in particular being subjected to the process.

6. A process as claimed in Claims 1 to 5, characterized in that, where the treatment is carried out in an open reaction vessel, a water content of at least 0.05% by weight is maintained in the reaction mixture during the reaction.