Process for the preparation of aluminum salts of a fatty acid

Description

[0001] The present invention relates to a process for producing aluminum salts of a fatty acid and to aluminum salts of a fatty acid obtainable by such a process.

[0002] Aluminum salts of fatty acids are also known as aluminum soaps. They possess hydrophobic properties and are characterized by a waxy consistency. Due to the trivalent nature of the aluminum ions they contain, aluminum salts of fatty acids can exist as mono-, di-, or trifatty acid aluminum salts. In addition to the fatty acid anions, the mono- or difatty acid aluminum salts can contain hydroxide ions as additional counterions for the trivalent aluminum ion. The hydroxide ions contained in the mono- or difatty acid aluminum salts impart dispersing properties to the aluminum salts of the fatty acid. This makes the aluminum salts of fatty acids suitable for thickening oils and can be used in the pharmaceutical or cosmetics industry, for example, in the production of cream or ointment bases.

[0003] The industrial production of aluminum salts of fatty acids is primarily carried out by the precipitation process ("double reaction"). In a first reaction step, a sodium salt of a fatty acid is formed in an aqueous solution. In a second reaction step, the aluminum salt of the fatty acid is precipitated by adding aluminum sulfate or chloride. A disadvantage of this process is that the sodium sulfate or chloride salts formed as byproducts are not easily separated from the aluminum salt of the fatty acid. Complete or almost complete leaching of the byproducts is only possible with great effort.

[0004] This is particularly disadvantageous with regard to the use of aluminum salts of fatty acids in the pharmaceutical industry. Aluminum salts of fatty acids, such as aluminum stearates, are used in the pharmaceutical industry, for example, in the production of capsules, inhalation sprays, or ointments. According to the European Pharmacopoeia, (Pharmacopoeia Europaea, For example, according to the European Chemicals Agency (EC) No. 127 / 2008 (Ph. Eur.), the chloride content in aluminum stearate is limited to 1000 ppm, and the sulfate content to 5000 ppm. Since the byproducts sodium sulfate and sodium chloride formed in the previously described precipitation process can only be removed from the product with great effort, the required low chloride and sulfate contents cannot be achieved economically.

[0005] Loncar et al., APTEFF34(2003) 1-148 describes the preparation and characterization of aluminum stearates.

[0006] US 2,469,041 describes another process for the production of aluminum salts of fatty acids. Here, the aluminum alcoholates of low-molecular-weight alcohols are first prepared. These are then reacted directly with the desired fatty acids in an oil matrix. The byproducts of the reaction—low-molecular-weight alcohols—are evaporated. However, the disadvantage of the process described is that it is only suitable for in suitable for in-situ production of lubricating greases.

[0007] It is therefore the object of the present invention to provide a process for the preparation of aluminum salts of a fatty acid which overcomes at least some of the problems known from the prior art.

[0008] The object of the invention is achieved by a process for the preparation of aluminum salts of a fatty acid, comprising the following steps: a) mixing a fatty acid and an aqueous solution of a strong base to produce an aqueous base-fatty acid mixture, b) mixing the aqueous base-fatty acid mixture with an aluminum source to produce an aqueous base-fatty acid-aluminum mixture, c) mixing the aqueous base-fatty acid-aluminum mixture with an acid, and d) separating the resulting aluminum salts of the fatty acid.

[0009] In step a) of the process according to the invention for producing aluminum salts of a fatty acid, an aqueous base-fatty acid mixture is prepared by mixing a strong base with water. This results in an aqueous solution of a strong base. A fatty acid is then added to the previously formed aqueous solution of the strong base. The fatty acid is dissolved in the aqueous base solution. The aqueous base-fatty acid mixture from step a) preferably comprises a deprotonated fatty acid.

[0010] In step b) of the process according to the invention for producing aluminum salts of a fatty acid, an aqueous base-fatty acid-aluminum mixture is prepared by adding an aluminum source to the aqueous base-fatty acid mixture from step a) and mixing it with the mixture. Preferably, the aluminum source is dissolved in the aqueous base-fatty acid mixture from step a). The aluminum source from step b) is preferably an aqueous solution of the aluminum source.

[0011] In step c) of the process according to the invention for producing aluminum salts of a fatty acid, the aqueous base-fatty acid-aluminum mixture from step b) is admixed with an acid. The acid is mixed with the aqueous base-fatty acid-aluminum mixture.

[0012] In step d) of the process according to the invention for producing aluminum salts of a fatty acid, the resulting aluminum salts of the fatty acid are separated from the remaining components of the acidified aqueous base-fatty acid-aluminum mixture from step c). The aluminum salts of the fatty acid can be separated from the remaining components of the acidified aqueous base-fatty acid-aluminum mixture by a physical separation process such as filtration or centrifugation. Separation is preferably carried out by filtration. The aluminum salts of the fatty acid constitute the filter cake. To improve the separation of the resulting aluminum salts of the fatty acid from the remaining components, the filter cake can be washed with water, preferably distilled water. During separation by centrifugation, the aluminum salts of the fatty acid form the precipitate.

[0013] The aluminum salts of a fatty acid produced by the process according to the invention can be mono-, di-, or trifatty acid aluminum salts.

[0014] The fatty acids used in the process according to the invention for producing aluminum salts of a fatty acid preferably originate from natural sources. The fatty acids can be obtained, for example, from palm or palm kernel oil, coconut oil, tallow, castor oil, but also from rapeseed or soy. They can be saturated or unsaturated fatty acids.

[0015] The fatty acids used in the process according to the invention are preferably fatty acids with chain lengths of C6 to C28 or mixtures thereof. More preferably, the fatty acid is selected from the group consisting of stearic acid, palmitic acid, and mixtures of these fatty acids.

[0016] In one embodiment of the process according to the invention, the fatty acid is stearic acid.

[0017] Commercially available stearic acid is predominantly obtained from tallow or palm oil. Depending on its origin, commercially available stearic acid does not usually consist of pure stearic acid (octadecanoic acid), but rather has a fatty acid distribution typical of its origin. Strictly speaking, commercially available stearic acid is often a fatty acid mixture that includes stearic acid. If the stearic acid is obtained from tallow, it contains approximately 65% ​​by weight octadecanoic acid and 25% by weight palmitic acid (hexadecanoic acid). If the stearic acid is obtained from palm oil or palm kernel oil, it can have a content of 50% or 65% by weight octadecanoic acid. Commercially available stearic acid can also contain significantly less than 50% by weight octadecanoic acid. The hexadecanoic acid content is correspondingly higher.

[0018] In one embodiment of the process according to the invention, stearic acid obtained from tallow and having an octadecanoic acid content of 50 to 70 wt.% is used as the fatty acid. In another embodiment, stearic acid obtained from palm oil and having an octadecanoic acid content of 40 to 70 wt.% is used.

[0019] In another embodiment, stearic acid with a purity of >80 wt.% is used. Stearic acid with a purity of ≥98 wt.% may also be used.

[0020] The strong base from step a) is preferably an alkali hydroxide. More preferably, it is sodium hydroxide or potassium hydroxide. Most preferably, the strong base is sodium hydroxide. The concentration of the strong base in the aqueous solution of the strong base obtained by mixing the strong base with water is preferably 0.2 mol / L or more. The concentration is preferably 1.0 mol / L or less. More preferably, the concentration is between 0.3 mol / L and 0.8 mol / L, particularly preferably between 0.35 mol / L and 0.75 mol / L.

[0021] The pH of the aqueous base-fatty acid mixture from step a) is preferably in the range between 7.0 and 12.0, more preferably between 8.0 and 11.0.

[0022] The aluminum source used in step b) is preferably an alkali aluminate or aluminum powder. More preferably, the aluminum source is sodium aluminate or aluminum powder. Even more preferably, the aluminum source is sodium aluminate. Alkali aluminates are salts of the aluminum acid HAlO 2 × H 2 O, in which aluminum forms a complex anion [Al(OH) 4 ] -< with hydroxide ion as ligand, as well as salts in which the anion is present as a condensate of the aluminate ion. The general composition of such compounds is M[Al(OH) 4] , where M is an alkali cation, preferably Na +< . Fully condensed, anhydrous compounds have the general composition MAlO 2 with AlO 2 -< as the anion, where M is an alkali cation, preferably Na +< . In step b) of the process according to the invention, commercially available NaAlO 2 can be used as the aluminum source.

[0023] The concentration of the aluminum source in the aqueous base-fatty acid-aluminum mixture in step b) is preferably between 50 mmol / L and 600 mmol / L, more preferably between 100 mmol / L and 500 mmol / L, each measured as aluminum. Methods for the quantitative determination of aluminum are known to those skilled in the art. The aluminum content can be determined, for example, by means of atomic absorption spectrometry (AAS). This determines the total amount of aluminum (total aluminum).

[0024] The acid used in step c) of the process according to the invention is preferably an organic acid, an inorganic acid, or a mixture thereof. The organic acid is preferably a monocarboxylic acid, more preferably formic acid or acetic acid, even more preferably acetic acid. The inorganic acid is preferably selected from the group consisting of phosphoric acid, hydrochloric acid, nitric acid, sulfuric acid, or one of their derivatives. In a preferred embodiment, the inorganic acid is phosphoric acid. Hydrochloric acid or sulfuric acid can be used in step c). In this case, amounts of acid are used which result in the chloride content in the product being at most 1000 ppm and / or the sulfate content in the product being at most 5000 ppm.

[0025] In a preferred embodiment of the process according to the invention, the acid used in step c) is an organic acid, preferably a monocarboxylic acid, more preferably formic acid or acetic acid, even more preferably acetic acid.

[0026] After mixing the aqueous base-fatty acid-aluminum mixture with the acid in step c), the pH of the acidified aqueous base-fatty acid-aluminum mixture is in the range between 3.5 and 7.0. Preferably, the pH is between 4.5 and 7.0. More preferably, the pH is between 5.0 and 6.0 or between 5.3 and 5.8.

[0027] In one embodiment of the process according to the invention, the fatty acid and the aqueous solution of a strong base from step a) are mixed with an aromatic carboxylic acid, preferably benzoic acid and / or derivatives of benzoic acid. Mixing with the aromatic carboxylic acid preferably takes place before the aluminum source is added to the aqueous base-fatty acid mixture. More preferably, the aromatic carboxylic acid is mixed with the aqueous solution of the strong base before the fatty acid is added. The aluminum salts of a fatty acid formed in this embodiment are so-called aluminum complex soaps. In addition to the fatty acid anions, these also contain anions of the aromatic carboxylic acids. The aluminum complex soaps are particularly suitable for the production of lubricating grease.

[0028] Surprisingly, it has been found that aluminum salts of a fatty acid produced by the process according to the invention have a low content of sodium chloride and sodium sulfate, as well as other salts. If acetic acid is used as the acid in step c) of the process according to the invention, an alkali metal acetate is formed as a by-product of the aluminum salts of the fatty acid, which is easily washed out. For example, aluminum stearate produced by the process according to the invention has a chloride content of at most 1000 ppm and a sulfate content of at most 5000 ppm and thus meets the requirements of the European Pharmacopoeia. Furthermore, the surprisingly low total sodium content of the aluminum salts of the fatty acids obtained by the process according to the invention also proves to be advantageous.

[0029] The invention further relates to aluminum salts of a fatty acid, which are obtainable by the process according to the invention for producing aluminum salts of a fatty acid. The aluminum salts of a fatty acid preferably have a chloride content of at most 1000 ppm and / or a sulfate content of at most 5000 ppm. The chloride content is determined according to the European Pharmacopoeia regulation specified in the monograph on aluminum stearate (European Pharmacopoeia, Edition 10.0 (2019): "Monographs A", 'Aluminium Stearate' and "Methods of Analysis", '2.4.4 Chlorides'). The sulfate content is determined according to the European Pharmacopoeia regulation specified in the monograph on aluminum stearate (European Pharmacopoeia, Edition 10.0 (2019): "Monographs A", 'Aluminium Stearate' and "Methods of Analysis", '2.4.13 Sulfates').

[0030] Unless otherwise stated, all volumes are measured at 23 °C. Unless otherwise stated, the term "ppm" refers to the mass fraction. A chloride content of the aluminum salts of a fatty acid of 1000 ppm or less therefore means that 1 kg of product contains 1000 mg or less of chloride.

[0031] The invention is further illustrated by the following examples: The sodium content of the products obtained in Examples 1 to 5 was determined by X-ray fluorescence analysis. The total amount of sodium was determined (total sodium). Example V: Experiments on the production of low-chloride and low-sulfate aluminum fatty acids (comparative examples)

[0032] Aluminum stearate was obtained on an industrial scale by reacting stearic acid with sodium hydroxide solution and precipitating it with aluminum sulfate. It is a white solid that is insoluble in water and floats to the surface. The product contains a high amount of water after filtration.

[0033] The sulfur content was measured by X-ray fluorescence analysis. Due to the molar mass of 32 g / mol for sulfur and 96 g / mol for sulfate, the sulfate content is three times higher than the sulfur content. Additionally, the water content of the aluminum stearate must be taken into account. This is on the order of 70 wt% and was measured for each sample. sample Treatment Sulfate content (calculated) [ppm] Source material 19000 Approach 1 Wash 3 times for 900 s with 60 parts by weight of tap water, 60 °C, 10 s pressing of the filter 17700 Approach 2 1 wash for 900 s with 60 parts by weight of tap water, 60 °C, 10 s pressing of the filter 17700 Approach 3 Wash once for 900 s with 60 parts by weight of tap water, 20 °C, 10 s pressing of the filter 19100 Approach 4 Wash once for 900 s with 60 parts by weight of deionized water, 20 °C, press the filter for 10 s 15500 Approach 5 Wash once for 600 s with 100 parts by weight of tap water, 20 °C, 10 s pressing of the filter 17100 Approach 6 Wash once for 600 s with 100 parts by weight of tap water, 20 °C, without pressing the filter 18300 Approach 7 Wash once for 600 s with 100 parts by weight of tap water, 20 °C, without pressing the filter, 18300 Approach 8 1 wash for 600 s with 100 parts by weight of tap water, 20 °C, 10 s pressing of the filter, 19100 Approach 9 1 wash for 1200 s with 100 parts by weight of tap water, 20 °C, 10 s pressing of the filter, 19300

[0034] A sufficient reduction in the sulfate content could not be achieved by washing under different conditions. Example 1: Synthesis of an aluminum mono-di-stearate

[0035] 1.5 L of distilled water are placed in a beaker and heated to 80 °C. 43 g of solid sodium hydroxide solution are then dissolved in the water. It is slowly saponified by adding 270 g of liquid vegetable stearic acid (70 °C). The stearic acid has a content of 66 wt.% stearic acid and 27 wt.% palmitic acid. 64.5 g of sodium aluminate, which was previously dissolved in 250 g of water, are then slowly added. After stirring for 15 minutes, the mixture is acidified to a pH of approximately 5.5 with 210 g of 60% acetic acid. After stirring for a further 10 minutes, the product is filtered through a Buchner filter and washed several times with distilled water.

[0036] Washed with water. Drying takes place in an oven at 80 °C to a water content of < 1.5%. The product has the following analytical data: • Aluminum content: 5.6 wt% • free fatty acid: 1.6 wt.% • Sodium content: < 10 ppm • Water content: 0.7 wt.% Example 2: Synthesis of an aluminum di-tri-stearate

[0037] Place 2 liters of distilled water in a beaker and heat to 70-80°C. 31.6 g of solid sodium hydroxide solution is then dissolved in the water. It is slowly saponified by adding 185 g of liquid tallow-based stearic acid (70°C). 24.6 g of sodium aluminate, previously dissolved in 180 g of water, are then slowly added. After stirring for 15 minutes, the mixture is acidified to a pH of approximately 5.5 with 84 g of 60% acetic acid. After stirring for another 10 minutes, the product is filtered through a Buchner filter and washed several times with distilled water. It is dried in a drying cabinet at 80°C to a water content of <1.5%. The product has the following analytical data: • Aluminum content: 3.9 wt.% • free fatty acid: 12.2 wt.% • Sodium content: < 10 ppm • Water content: 1.1 wt.% Example 3: Synthesis of an aluminum tri-stearate

[0038] Place 1.5 L of distilled water in a beaker and heat to 70-80°C. Then, 29.2 g of solid sodium hydroxide solution is dissolved in the water. It is slowly saponified with the addition of 194 g of liquid vegetable stearic acid (70°C). The stearic acid has a content of 55% by weight stearic acid and 45% by weight palmitic acid. Then, 21.2 g of sodium aluminate, previously dissolved in 250 g of water, is slowly added. After stirring for 15 minutes, the mixture is acidified to a pH of approximately 5.5 with 73 g of 60% acetic acid. After stirring for another 10 minutes, the product is filtered through a Buchner filter and washed several times with distilled water. It is dried in a drying cabinet at 80°C to a water content of <1.5%. The product has the following analytical data: • Aluminum content: 3.4 wt% • free fatty acid: 18.3 wt.% • Sodium content: < 10 ppm • Water content: 0.8 wt.% Example 4: Synthesis of an aluminum mono-di-stearate

[0039] Place 1.5 L of distilled water in a beaker and heat to 80°C. 43 g of solid sodium hydroxide solution are then dissolved in the water. The mixture is slowly saponified by adding 270 g of liquid vegetable stearic acid (70°C). The stearic acid has a content of 66% by weight stearic acid and 27% by weight palmitic acid. 64.5 g of sodium aluminate, previously dissolved in 250 g of water, are then slowly added. After stirring for 15 minutes, the mixture is acidified to a pH of approximately 5.5 with 82.3 g of 85% phosphoric acid. After stirring for another 10 minutes, the product is filtered through a Buchner filter and washed several times with distilled water. It is dried in a drying cabinet at 80°C to a water content of <1.5%. The product has the following analytical data: • Aluminum content: 5.2 wt.% • free fatty acid: 2.2 wt.% • Sodium content: < 10 ppm • Water content: 1.2 wt.% Example 5: Synthesis of an aluminum complex soap

[0040] Place 1.5 L of distilled water in a beaker and heat to 80°C. 43 g of solid sodium hydroxide solution are then dissolved in the water. The solution is slowly saponified by adding 26.8 g of solid benzoic acid. 210 g of liquid vegetable stearic acid are then added. 64.5 g of sodium aluminate, previously dissolved in 250 g of water, are then slowly added. After stirring for 15 minutes, the mixture is acidified to a pH of approximately 5.5 with 210 g of 60% acetic acid. After stirring for another 10 minutes, the product is filtered through a Buchner filter and washed several times with distilled water. It is dried in a drying cabinet at 80°C to a water content of < 1.5%. The product has the following analytical data: • Aluminum content: 5.3 wt% • free fatty acid: 3.2 wt.% • Sodium content: < 10 ppm • Water content: 1.2 wt.%

Claims

1. A process for the production of aluminum salts of a fatty acid, comprising the following steps: a) mixing a fatty acid and an aqueous solution of a strong base to prepare an aqueous base / fatty acid mixture, b) mixing said aqueous base / fatty acid mixture with an aluminum source to prepare an aqueous base / fatty acid / aluminum mixture, c) mixing said aqueous base / fatty acid / aluminum mixture with an acid, and d) separating the produced aluminum salts of the fatty acid.

2. The process according to claim 1, wherein said aluminum salts of a fatty acid are mono-, di- or tri-fatty acid aluminum salts.

3. The process according to either of claims 1 or 2, wherein said fatty acid is a fatty acid having a chain length of C6 to C28, or a mixture of fatty acids, preferably stearic acid, palmitic acid, or a mixture of such fatty acids.

4. The process according to any of claims 1 to 3, wherein said fatty acid is stearic acid.

5. The process according to any of claims 1 to 4, wherein said strong base is an alkali hydroxide, preferably sodium hydroxide or potassium hydroxide, more preferably sodium hydroxide.

6. The process according to any of claims 1 to 5, wherein said aluminum source is an alkali aluminate or aluminum powder, preferably sodium aluminate or aluminum powder, more preferably sodium aluminate.

7. The process according to any of claims 1 to 6, wherein the acid used in step c) is an organic acid, an inorganic acid, or a mixture of both.

8. The process according to claim 7, wherein said organic acid preferably is a monocarboxylic acid, more preferably formic acid or acetic acid.

9. The process according to either of claims 7 or 8, wherein said inorganic acid is preferably phosphoric acid, hydrochloric acid, nitric acid, sulfuric acid, or any of their derivatives, more preferably phosphoric acid.

10. The process according to any of claims 1 to 9, wherein the separating of the produced aluminum salts of the fatty acid is effected by a physical separation method, preferably by filtration.

11. The process according to claim 1, wherein the fatty acid and the aqueous solution of a strong base are mixed with an aromatic carboxylic acid, preferably benzoic acid.

12. The process according to any of claims 1 to 11, wherein the pH value of said aqueous base / fatty acid mixture is within a range of from 7 to 12.

13. The process according to any of claims 1 to 12, wherein the pH value of said acidified aqueous base / fatty acid / aluminum mixture is within a range of from 3.5 to 7.

14. Aluminum salts of a fatty acid obtainable by a process according to any of claims 1 to 13, wherein the aluminum salts of a fatty acid have a content of at most 1000 ppm chloride and 5000 ppm sulfate.