Title of Invention

METHOD OF STABILIZING ALIPHATIC C3-C14 ALDEHYDES

Abstract

The present method makes it possible to stabilize aliphatic C3-C14-aldehydes against polymerization and autocondensation. It comprises the addition of alkaline substances to the aldehydes wherein the alkaline substances used are alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates or alkaline earth metal carboxylates and are added to the aldehyde to be stabilized in amounts of 0.05-20 ppm, based on the aldehyde.

Full Text

1A The present invention relates to a method of stabilizing aldehydes against polymerization and autocondensation. Owing to their high reactivity, aldehydes have a tendency toward polymerization and autocondensation. Polymerization forms mainly trimeric products. For example, isobutyraldehyde forms 2,4,6-triisopropyl-1,3,5-trioxane, although other aliphatic aldehydes having 3-14 carbon atoms also polymerize to form the cyclic trimeric aldehyde (trialkyltrioxane). The trimerization is catalyzed by chemical substances such as chlorine or bromine, phosphorus pentoxide, sulfuric acid, hydrogen sulfide, hydrogen chloride, hydrogen fluoride, boron trifluoride, aluminum chloride or zinc chloride. In the presence of such acid compounds, the polymerization of the aldehydes commences spontaneously. If the concentration of the acid compound is sufficiently high, this results in formation of crystalline trimeric aldehydes within a few minutes. At concentrations of up to 10 ppm of the acid compound, trimer formation occurs somewhat more slowly over a period of a few days. In addition, low temperatures, i.e. temperatures of about 0°C or below, or UV light promote the polymerization of the aliphatic aldehydes. A further problem is the tendency of the aldehydes to undergo the aldol condensation in the presence of alkaline substances. Owing to their transformation into such relatively high molecular weight compounds, aldehydes cannot be stored for an indefinite period. Although the polymerization and autocondensation products of the aldehydes redissociate at relatively high temperature, their formation stands in the way of unrestricted industrial use of the aldehydes. Efforts are therefore made to prevent the formation of relatively high molecular weight products from the aldehydes. - 2 - This is possible over a limited period of time by preparing and storing the aldehydes in highly pure form. However, the purification operations required for this are so complicated that they are not feasible for commercial preparation of the aldehydes. It is known that the polymerization and autocondensation reactions can be suppressed by addition of certain substances. In practice, these substances must meet a series of requirements if the aldehyde is to be able to be employed without restriction in a wide variety of applications. Among these is the requirement that the substance concerned has to remain active over a prolonged period even in a low concentration and also has to cause no interference in the processing of the aldehyde as a result of chemical reactions. Stabilizers which have been described for isobutyraldehyde are, for example, mercaptobenzimid-azole and 2,2-methylenedi(4-methyl-6-tert-butylphenol). However, these stabilizers are effective for only an insufficient time. Thus, according to DE-A-29 05 267, when 100 ppm of mercaptobenzimidazole are added to isobutyraldehyde, a considerable degree of trimerization is again observed after the stabilized aldehyde has been stored for only 5 weeks. According to another method, a solution of diphenylamine in ethanol is added to the aldehydes to prevent polymerization. However, this procedure likewise does not ensure that the polymerization is suppressed over a prolonged period of time. It is known from DE-A-29 05 267 and DE-A 29 17 789 that isobutyraldehyde and other aliphatic aldehydes having 3-14 carbon atoms can be stabilized against - 3 - polymerization and autocondensation by addition of triethanolamine or dimethylethanolamine. When using these stabilizers, good stabilization over a prolonged period can be achieved even when relatively low concentrations are employed. It is stated, for example, that 10 ppm of the ethanolamines mentioned, based on the aldehyde, can rule out the formation of high molecular weight compounds by polymerization and autocondensation, e.g. in the presence of oxygen, for a period of 30 weeks. When 20-100 ppm, based on the aldehyde, are added, the stabilizers suppress the formation of the trimer or the aldol condensation product on storage of the aldehyde without particular precautions for a period of about 1 year. However, these stabilizers have the disadvantage that they can be removed again from the aldehydes only with a considerable outlay in terms of distillation. JP 45 012282 B4 likewise addresses the problem of stabilizing isobutyraldehyde against the formation of trimers. It is stated that the treatment of isobutyraldehyde with an aqueous alkali solution is completely ineffective. Stabilization can only be achieved if the alkaline substance is added to the isobutyraldehyde as a solid or in the form of a very concentrated aqueous solution; in the latter case, it is critical that the amount of water is below the saturation limit of the isobutyraldehyde. Alkaline compounds used are alkali metal compounds (carbonate salts, bicarbonate salts, silicates and fatty acid salts), alkaline earth metal compounds (oxides, hydroxides, carbonate salts, bicarbonate salts and fatty acid salts) and ammonia or ammonium carbonate. However, the amounts of alkaline compound added are very large. In the case of sodium hydrogen carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, calcium carbonate and calcium hydroxide, sodium acetate and magnesium oxide as stabilizers, - 4 -500 ppm are in each case added to the isobutyraldehyde, an amount which is regarded as small in the context of JP 45 012282 B4. Although the use of such large amounts of the alkaline stabilizer suppresses trimer formation, problems caused by the alkali-catalyzed aldol condensation of the isobutyraldehyde which occurs to an increasing extent are unavoidable. Furthermore, the addition of the alkaline substance as solid to relatively large amounts of aldehyde, e.g. stored in tanks, is associated with the problem of completely dissolving, dispersing and uniformly distributing the alkaline substance throughout the total aldehyde volume. It is therefore an object of the invention to provide an improved method which makes it possible to prevent polymerization and autocondensation reactions of the aldehydes for as long as possible a time. This object is achieved by a method in which aliphatic C3-C14-aldehydes are stabilized by addition of alkaline substances, which comprises adding alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates or alkaline earth metal carboxylates as alkaline substances to the aldehyde to be stabilized in amounts of 0.05-20 ppm, preferably 0.05-5 ppm, particularly preferably 0.05-2.5 ppm, based on the aldehyde. The process of the invention is notable for the fact that the stabilizers used are effective even in extremely low concentrations. Even 0.05 ppm of the stabilizer prevent the formation of high molecular weight compounds by polymerization or aldol condensation on storage of the aldehyde, even at low temperatures, without further precautions for a period of several weeks. It should also be emphasized that the substances used for stabilizing the aldehydes do not - 5 - interfere in the further processing of the aldehydes. If removal of the alkaline substance before further processing of the aldehydes is nevertheless desired, this can be achieved by simple distillation in which the alkaline substance remains in the distillation bottoms. It is particularly noteworthy that the stabilizers, despite the fact that they are alkaline, trigger no aldol condensation reaction in the aldehydes. Preferred alkali metal hydroxides are sodium and potassium hydroxide, and the preferred alkaline earth metal hydroxide is calcium hydroxide. Preferred alkali metal carbonates are sodium and potassium carbonate and preferred alkaline earth metal carbonates are magnesium and calcium carbonate. As alkali metal carboxylate, particular preference is given to using sodium butyrate. The alkaline substances are usually used as a 0.01-1M, preferably 0.05-0.5M and in particular 0.1-0.25M, aqueous solution. In particular cases it can also prove to be advantageous to add the alkaline substances, in particular the alkali metal carboxylates and among these preferably sodium butyrate, as solids. Examples of aldehydes which can be stabilized by the method of the invention are: propanal, n- and i-butanal, n- and i-pentanal, n- and i-hexanal, n- and i-heptanal, n- and i-octanal, n- and i-nonanal, n- and i-decanal, undecanal, dodecanal, lauric aldehyde, methylnonyl aldehyde (MNA), tridecyl aldehyde and myristyl aldehyde. These aldehydes may contain up to 3% by weight, preferably from 0.5 to 2% by weight and in particular from 0.75 to 1.25% by weight, of water. - 6 - In one embodiment of the method of the invention, the stabilizer is initially charged in the form of the aqueous solution and the aldehyde, which may likewise contain water, is added thereto. Conversely, the aqueous solution of the stabilizer can also be added to the anhydrous or water-containing aldehyde. Examples: Examples 1-3: The sodium hydroxide used for stabilization is placed, as a 0.1M aqueous solution, in the appropriate amount in polyethylene bottles, subsequently admixed with the appropriate amount of aldehyde and blanketed with nitrogen. The isobutyraldehyde in Examples 1 and 2 contains, initially, 2% of deionized water in each case and the n-butyraldehyde in Example 3 contains 1% of deionized water. The polyethylene bottles are subsequently shaken on a rotary shaker for 2 0 minutes to achieve optimum mixing. In Example 2, the bottle is shaken for the entire 4 weeks of the experiment. The bottles are stored in the absence of light for the respective times of the experiments. Sampling after the various storage times is in each case carried out with addition of 100 ppm of triethanolamine to ensure that the respective sample remains in the same state as at the time of sampling. The samples are analyzed by means of gas chromatography. All work is carried out under nitrogen. - 7 - gable 1: Trimer formation [% by weight] Example 1 Example 2 Example 3 i-Butyraldehyde i-Butyraldehyde with continual shaking n-Butyraldehyde NaOH Days None 0.17 ppm 0.5 ppm None 0.2- ppm 2 ppm None 0.1 ppm 0.17 ppm 0 0.025 0.005 0.063 7 0.305 0.169 0.048 0.125 0.063 0.063 14 1.611 0.284 0.085 28 2.317 0.434 0.087 Examples 4-10 : The sodium hydroxide, potassium hydroxide or sodium carbonate used for stabilization is placed, at the amount indicated in Table 2 and in the form of an aqueous solution having the concentration indicated in the table, in polyethylene bottles, subsequently admixed with the appropriate amount of n-butyraldehyde and blanketed with nitrogen. The polyethylene bottles are subsequently shaken on a rotary shaker for 2 0 minutes to achieve optimum mixing. The bottles are stored in the absence of light for the respective times of the experiments. Sampling after the various storage times is in each case carried out with addition of 100 ppm of triethanolamine to ensure that the respective sample remains in the same state as at the time of sampling. The samples are analyzed by means of gas chromatography. All work is carried out under nitrogen. - 8 -Table 2: Ex. Addition Analysis Sampling after (h) Initial 48 h 120 h [% by weight] 4 1% of deionized water n-C4-al Trim C4-al Kl-Trim C4-al K2 S Tetramer S Aldol 99.928 99.856 0.051 0.010 0.009 0.004 99.545 0.279 0.054 0.045 0.004 5 1% of deionized water + 0.17 ppm of NaOH ( = 0.0017% strength solution) n-C4-al Trim C4-al Kl Trim C4-al K2 S Tetramer S Aldol 99.928 99.924 0.002 0.004 99.919 0.002 0.004 6 0.5% of deionized water + 0.17 ppm of NaOH (=0.0034% strength solution) n-C4-al Trim C4-al Kl Trim C4-al K2 S Tetramer S Aldol 99.928 99.921 99.914 7 0.25% of deionized water + 0.17 ppm of NaOH (=0. 0068% strength solution) n-C4-al Trim C4-al Kl Trim C4-al K2 S Tetramer S Aldol 99.928 99.918 0.002 0.004 99.907 0.002 0.007 8 1% of deionized water + 0.17 ppm of KOH n-C4-al Trim C4-al Kl Trim C4-al K2 S Tetramer 99.928 99.923 99.919 - 9 - • (=0.0017% strength solution) S Aldol 0.002 0.004 0.005 9 1% of deionized water + 500 ppm of Na2CO3 (=4.8% strength solution) n-C4-al Trim C4-al Kl Trim C4-al K2 S Tetramer S Aldol 99.928 99.888 99.869 10 1% of deionized water + 500 ppm of NaOH (=4.8% strength solution) n-C4-al Trim C4-al Kl Trim C4-al K2 S Tetramer S Aldol 99.928 spontaneous commencement of aldolization - 10 -The abbreviations used in the table have the following meanings: n-C4-al : n-Butyraldehyde Trim C4-al K1 : 2,4,6-Tri-n-propyl-l,3,5- trioxane with eee or aaa configuration Trim C4-al K2 : 2,4,6-Tri-n-propyl-l,3,5- trioxane with eea or aae configuration S Tetramer ": " Tetrameric polymerization product of n-butyraldehyde S Aldol : Total of aldol condensation products Examples 11-14: In all Examples 11-14, the n-butyraldehyde containing 1% by weight of water is first acidified to an acid content of 1 ppm by addition of sulfuric acid. Example 11 is a blank to which no alkaline substance is added as stabilizer. In Examples 12 and 13, 20 ppm and 10 ppm, respectively, of solid sodium butyrate are subsequently added to the n-butyraldehyde; in Example 14, 10 ppm of sodium hydroxide are subsequently added to the n-butyraldehyde. The bottle, which is blanketed with nitrogen, is subsequently shaken on a rotary shaker for 2 0 minutes to ensure optimum mixing. The bottles are stored in the absence of light for the respective times of the experiments. 250 ml samples from each of the bottles are taken after the different storage times in each case with addition of 100 ppm of triethanolamine to ensure that the respective sample remains in the same state as at the time of sampling. All work is carried out under nitrogen. The samples are analyzed by means of gas chromatography. Table 3: Sampling after (h) Example % by weight 11 Blank (with acid) 12 20ppm of Na butyrate 13 l0ppm of Na butyrate 14 l0ppm of NaOH (0.05M soln.) 24 n-C4-al 96.189 99.707 99.752 99.838 Trim.C4-al K1 2.873 0.191 0.154 0.089 Trim.C4-al K2 0.552 0.037 0.031 0.018 S Tetramer 0.346 0.026 0.021 0.014 S Aldol 0.009 0.006 0.006 0.010 48 n-C4-al 93.160 99.651 99.710 99.836 Trim.C4-al Kl 5.246 0.224 0.181 0.089 Trim.C4-al K2 1.002 0.045 0.036 0.017 S Tetramer 0.554 0.033 0.030 0.015 S Aldol 0.008 0.012 0.010 0.014 72 n-C4-al 91.441 99.639 99.705 99.815 Trim.C4-al Kl 6.642 0.223 0.171 0.090 Trim.C4-al K2 1.247 0.044 0.035 0.019 S Tetramer 0.621 0.030 0.031 0.021 S Aldol 0.013 0.017 0.013 0.016 120 n-C4-al 86.996 99.653 99.717 99.816 Trim.C4-al Kl 10.296 0.213 0.173 0.096 Trim.C4-al K2 1.877 0.042 0.033 0.018 S Tetramer 0.778 0.027 0.023 0.014 S Aldol 0.017 0.021 0.014 0.018 11 - 12 - Examples 15-18: Example 15 is a blank to which no alkaline substance is added as stabilizer. In Examples 16, 17 and 18, 0.5 ppm of solid sodium butyrate, solid calcium butyrate and sodium hydroxide are subsequently added as a 0.05M solution to the n-butyraldehyde. The bottle, which is blanketed with nitrogen, is subsequently shaken on a rotary shaker for 2 0 minutes to ensure optimum mixing. The bottles are stored in the absence of light for the respective times of the experiments. 250 ml samples from each of the bottles are taken after the different storage times in each case with addition of 100 ppm of triethanolamine to ensure that the respective sample remains in the same state as at the time of sampling. All work is carried out under nitrogen. The samples are analyzed by means of gas chromatography. Examples 19-21: Example 19 is a blank to which no alkaline substance is added as stabilizer. In Examples 20 and 21, 0.5 ppm and 0.25 ppm, respectively, of sodium hydroxide is subsequently added as a 0.1M solution to the n-butyraldehyde. The bottle, which is blanketed with nitrogen, is subsequently shaken on a rotary shaker for 20 minutes to ensure optimum mixing. The bottles are stored in the absence of light for the respective times of the experiments. 250 ml samples from each of the bottles are taken after the different storage times in each case with addition of 100 ppm of triethanolamine to ensure that the respective sample remains in the same state as at the time of sampling. All work is carried out under nitrogen. The samples are analyzed by means of gas chromatography. Table 4: Sampling after (h) Example % by weight 15 Blank 16 0.5 ppm of Na butyrate/solid 17 0.5 ppm of Ca butyrate/solid 18 0.5 ppm of NaOH (0.05M soln.) 24 n-C4-al 99.886 99.950 99,899 99.946 Trim.C4-al K1 0.040 0.031 ' Trim.C4-al K2 0.008 0.006 S Tetramer 0.005 0.004 S Aldol 0.006 0.004 0.006 0.006 72 n-C4-al 99.802 99.939 99.888 99.934 Trim.C4-al K1 0.097 0.033 0.002 Trim.C4-al K2 0.019 0.007 S Tetramer 0.014 0.003 S Aldol 0.006 0.008 0.006 0.008 120 n-C4-al 99.652 99.944 99.897 99.940 Trim.C4-al K1 0.212 0.031 Trim.C4-al K2 0.040 0.006 S Tetramer 0.030 0.003 S Aldol 0.005 0.007 0.006 0.007 13 Table 5: Sampling after (h) Example % by weight 19 Blank 20 0.5 ppm of NaOH (0.1M solution) 21 0.25 ppm of NaOH (0.1M solution) 24 n-C4-al 99.633 99.954 , 99.907 Trim.C4-al K1 0.241 0.006 0.035 Trim.C4-al K2 0.46 0.007 S Tetramer 0.032 S Aldol 0.004 0.006 0.005 72 n-C4-al 99.023 99.951 99.899 Trim.C4-al K1 0.696 0.005 0.036 Trim.C4-al K2 0.132 0.007 S Tetramer 0.096 0.004 S Aldol 0.006 0.008 0.007 96 n-C4-al 98.815 99.953 99.913 Trim.C4-al K1 0.871 0.004 0.030 Trim.C4-al K2 0.165 0.006 S Tetramer 0.110 0.004 S Aldol 0.008 0.005 14 15 WE CLAIM 1. A method of stabilizing aliphatic C3-C14-aldehydes by addition of alkaline substances, which comprises adding alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates or alkaline earth metal carboxylates as alkaline substances to the aldehyde to be stabilized in amounts of 0.05-20 ppm, preferably 0.05-5 ppm, particularly preferably 0.05-2.5 ppm, based on the aldehyde. 2. The method as claimed in claim 1, wherein alkali metal hydroxides used are sodium or potassium hydroxide, the alkaline earth metal hydroxide used is calcium hydroxide, alkali metal carbonates used are sodium or potassium carbonate, alkaline earth metal carbonates used are magnesium or calcium carbonate or the alkali metal carboxylate used is sodium butyrate. 3. The method as claimed in claim 1 or 2, wherein the alkaline substance is used as a 0.01-1M, preferably 0.05-0.5M and in particular 0.1-0.25M, aqueous solution. 4. The method as claimed in one or more of claims 1 to 3, wherein aliphatic C3-C14-aldehydes used are propanal, n- and i-butanal, n- and i-pentanal, n-and i-hexanal, n- and i-heptanal, n- and i-octanal, n- and i-nonanal, n- and i-decanal, undecanal, dodecanal, lauric aldehyde, methylnonyl aldehyde (MNA), tridecyl aldehyde or myristyl aldehyde. 5. The method as claimed in claim 4, wherein the aldehydes contain up to 3% by weight, preferably from 0.5 to 2% by weight and in particular from 0.75 to 1.25% by weight, of water. 1. 16 6. The method as claimed in one or more of claims 1 to 5, wherein the alkaline substance is initially charged in the form of the aqueous solution and the aldehyde, which may likewise contain water, is added thereto. 7. The method as claimed in one or more of claims 1 to 6, wherein the aqueous solution of the alkaline substance "is added to the anhydrous or water-containing aldehyde. of L S DAVAR & CO. . Applicants' Agent. Dated this 21 st day of DECEMBER 1998. The present method makes it possible to stabilize aliphatic C3-C14-aldehydes against polymerization and autocondensation. It comprises the addition of alkaline substances to the aldehydes wherein the alkaline substances used are alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates or alkaline earth metal carboxylates and are added to the aldehyde to be stabilized in amounts of 0.05-20 ppm, based on the aldehyde.

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