Methods for the preparation of C6-12 saturated aliphatic carboxylic acids

Abstract

C for color stability 6~12 A method for preparing saturated aliphatic carboxylic acids, comprising: (1) oxidizing a corresponding aldehyde with molecular oxygen to obtain a crude saturated aliphatic carboxylic acid in a liquid mixture; (2) removing molecular oxygen from the crude saturated aliphatic carboxylic acid mixture; and (3) isolating the saturated aliphatic carboxylic acid from the oxygen-depleted mixture by distillation as a color-stable product.

Description

[Technical Field]

[0001] The present invention relates to a method for preparing saturated aliphatic carboxylic acids having 6 to 12 carbon atoms by oxidation of the corresponding aldehyde with molecular oxygen (molecular enzyme), which is tempered under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa for 4 hours, resulting in saturated aliphatic carboxylic acids with a very low tendency to darken, in high purity.

[0002] Furthermore, the present invention also relates to 2-ethylhexanoic acid that has a very low tendency to darken when tempered under inert gas conditions at a temperature of 225° C. and a pressure of 0.1 MPa for 4 hours. [Background technology]

[0003] Saturated aliphatic carboxylic acids are globally important intermediates with a wide range of applications. They can be used as such, but are typically further processed into metal salts, esters, amides, anhydrides, acid chlorides, and other derivatives. Generally, they are important intermediates for the production of a wide range of compounds, such as metal salts and metal soaps, perfumes, fragrances, pharmaceutical and agricultural ingredients, cosmetic ingredients, plasticizers, paints, coating additives, coolants, lubricants, or catalysts for polymer processing. 6~12 A very important representative of saturated aliphatic carboxylic acids is 2-ethylhexanoic acid, which is used primarily in the form of its derivatives, such as its metal salts or esters, for use as a thickener and drier for alkyd resins and paints, as a catalyst in polyurethane foam production, as a stabilizer and / or plasticizer for PVC, or as a corrosion and wear inhibitor for lubricants. 6~12 Esters of saturated aliphatic carboxylic acids, such as n-heptanoic acid, n-octanoic acid, 2-ethylhexanoic acid, n-nonanoic acid, or 3,5,5-trimethylhexanoic acid, are also often used as lubricants.

[0004] An important and widely used method for the preparation of saturated aliphatic carboxylic acids having 6 to 12 carbon atoms is the oxidation of the corresponding aldehyde with molecular oxygen in the liquid phase, with or without catalysts or additives. This general synthetic route is described, for example, in J. Kubitschke et al., "Carboxylic acids, aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry, 2014, Wiley-VCH Verlag GmbH & Co. KGaA, DOI: 10.1002 / 14356007.a05_235.pub2, Chapter 4.2.1 "Aldehyde oxidation." The C obtained by the above-mentioned oxidation 6~12 The saturated aliphatic carboxylic acid is then purified, usually by distillation, to give C 2 carboxylic acid in preferably pure form. 6~12 A saturated aliphatic carboxylic acid is obtained.

[0005] Since most applications require a highly transparent and colorless product, not only is high chemical purity, preferably well above 99 wt.-% (weight %), but also a very low APHA color number is desired.

[0006] US 5,504,229 describes the preparation of 2-ethylhexanoic acid by oxidation of 2-ethylhexanal in the presence of potassium 2-ethylhexanoate as a selectivity improving additive, and subsequent distillation to obtain purified 2-ethylhexanoic acid, in which potassium 2-ethylhexanoate is enriched in the bottom product and recycled back to the oxidation stage. According to Example 1, the obtained 2-ethylhexanoic acid showed a low APHA color number of 4.

[0007] CN 109438216 describes the preparation of 2-ethylhexanoic acid by a multi-step process in which n-butylaldehyde is subjected to aldol condensation to give 2-ethyl-3-hexenal, which is then hydrogenated to 2-ethylhexanal. The inventors found that after the hydrogenation step, a small amount of 2-ethyl-3-hexenal remains, which is likely oxidized together with 2-ethylhexanal in the oxidation step to produce crude 2-ethylhexanoic acid contaminated with 2-ethyl-3-hexenoic acid, which cannot be subsequently separated from the 2-ethylhexanoic acid by distillation. CN 109438216 teaches that the 2-ethyl-3-hexenal remaining after hydrogenation can be oxidized to 2-ethyl-3-hydroxyhexanal in the presence of an acid catalyst with water in an oxidation step, which can then be separated and removed from the 2-ethylhexanoic acid by distillation. According to Examples 1 to 5, 2-ethylhexanoic acid can be obtained with a purity of up to 99.91 wt.% and an APHA color number as low as 3.

[0008] According to the present invention, freshly distilled C 6~12 It was recognized that the low APHA color number of a saturated aliphatic carboxylic acid does not guarantee that such a low APHA color number will remain the same upon longer storage, nor does it guarantee a color-stable product utilizing the respective carboxylic acid. 6~12 It has been recognized that saturated aliphatic carboxylic acids typically tend to darken over time during storage, when exposed to heat stress, and / or in the products to which they are typically applied. Furthermore, in accordance with the present invention, it has been recognized that such darkening can be caused by the presence of peroxides.

[0009] It is known from the state of the art that the oxidation of aldehydes with oxygen first forms peracids, which then further oxidize the aldehydes to produce 2 moles of carboxylic acid per mole of intermediate peracid. Such a mechanism is described, for example, in JH Teles et al., "Oxidation" in Ullmann's Encyclopedia of Industrial Chemistry, 2015, Wiley-VCH Verlag GmbH & Co. KGaA, DOI: 10.1002 / 14356007.a18_261.pub2, Chapter 5.4.1 "Secondary reactions of radicals, peroxides, and other intermediates" in combination with Chapter 2.2.8 "Carboxylic acids, saturated". Thus, peracids also react with the oxygen of each aldehyde to produce 2 moles of carboxylic acid per mole of intermediate peracid. 6~12 Apparently, peracids are formed as intermediates during oxidation to saturated aliphatic carboxylic acids. Peracids are highly reactive molecules. Even if only small amounts of these remain in the crude carboxylic acid product after oxidation and are not properly separated and removed, C 6~12 Saturated aliphatic carboxylic acids can lead to undesirable properties such as lower color stability.

[0010] CN 108047027 addresses the decomposition of such peroxides formed in the oxidation of 3,5,5-trimethylhexanal to 3,5,5-trimethylhexanoic acid (isononanoic acid). It was found that, although the concentration of peroxides was fairly low, it was still high enough to cause problems in the rectification column because peroxides could accumulate there due to the difference in boiling points between the peroxides and other components. Furthermore, the CN application describes the use of relatively low concentrations of homogeneous catalysts to decompose peroxides in the prior art. However, such homogeneous catalysts are very difficult to separate and remove, and they also pose the risk of slagging, clogging, and explosion in the distillation column. To avoid such problems, the CN application teaches heterogeneously catalyzing the decomposition of peroxides over a metal-organic framework catalyst before entering the rectification column. It teaches that it is important to decompose peroxides only at low temperatures between 20 and 70°C, because otherwise side reactions, such as decarboxylation, would occur, reducing yield and purity. Moreover, it decomposes very quickly, so it can be used for 5-40 hours. -1 It is emphasized that this can be carried out at high space velocities, which are associated with short residence times of 1.5 to 12 minutes.

[0011] However, the use of metal-organic framework catalysts as peroxide decomposition catalysts generally has disadvantages. First, these metal-organic framework catalysts are very complicated to prepare. Second, the organic molecules that make up the framework are susceptible to oxidation, especially in the presence of peracids and the reactive radicals generated in their decomposition. In the presence of carboxylic acids, metal-organic frameworks are known to lose activity and / or framework metals as a result of bleeding. These leached metals then cause the same problems in the rectification column as metals used as homogeneous catalysts. Furthermore, all these factors contribute to the short lifespan of metal-organic framework catalysts, thus further increasing the process complexity by disposing of used catalysts and providing fresh ones.

[0012] In PCT application number PCT / EP2020 / 087,952 (based on the priority of EP application number 20150845.4), C 3~5 After oxidation of the aldehyde, C 3~5 It was found that the peracid remaining in the crude product of saturated aliphatic carboxylic acid could be largely decomposed by heat treatment before distillation purification. 3~5 Saturated aliphatic carboxylic acids exhibit only very low contents of active oxygen, which in turn exhibit very low contents of peroxides, e.g., peracids. The content of active oxygen is a quantitative measure of the amount of reactive oxygen capable of oxidizing easily oxidizable compounds, e.g., oxidizing iodide(1-) salts to iodine(0) or iron(II) to iron(III).

[0013] According to the present invention, C as proposed in EP application no. 20150845.4 3~5 Saturated aliphatic carboxylic acids are converted to the corresponding C 3~5 The thermal treatment for the production by oxidation of aldehydes is C 6~12 Saturated aliphatic carboxylic acids are converted to the corresponding C 6~12 It has been found that oxidation of aldehydes is not successful, at least not within a reasonable time frame of only a few hours. 6~12 The peracid is decomposed by such heat treatment and distilled C 6~12 Although saturated aliphatic carboxylic acids may exhibit low APHA color numbers, their content of active oxygen is still high and they tend to darken over time during storage, when exposed to heat stress, and in the products in which they are typically applied. Summary of the Invention [Problem to be solved by the invention]

[0014] Therefore, the object of the present invention was to find a method for preparing saturated aliphatic carboxylic acids having 6 to 12 carbon atoms by oxygen oxidation of the corresponding aldehyde, which can produce the respective saturated aliphatic carboxylic acids in high yield and high purity, and which do not or at least have a very low tendency to darken, especially when the product is exposed to thermal stress over time during storage and / or after its usual application, such as as an additive in polymer production.The method will also be easy to operate, safe to carry out, function stably over long operating times, and produce saturated aliphatic carboxylic acids with constant high quality. [Means for solving the problem]

[0015] The inventors have surprisingly discovered a process for preparing saturated aliphatic carboxylic acids having 6 to 12 carbon atoms by oxidation of the corresponding aldehyde with molecular oxygen, which process comprises the steps of: (a) converting a corresponding aldehyde with molecular oxygen at a temperature of 0 to 120°C and an oxygen partial pressure of 0.02 to 2 MPa to obtain a liquid mixture containing a saturated aliphatic carboxylic acid, a corresponding aldehyde in an amount of 2 mol-% or less based on the saturated aliphatic carboxylic acid, and molecular oxygen; (b) removing molecular oxygen from the liquid mixture obtained in step (a) to a content of 10 wt.-ppm or less based on the liquid mixture; (c) distilling the mixture obtained in step (b) in a distillation apparatus equipped with a purification column, from which a purified distillate containing at least 95 wt.-% saturated aliphatic carboxylic acids based on the distillate is obtained; We have found a method including: DETAILED DESCRIPTION OF THE INVENTION

[0016] Saturated aliphatic carboxylic acids with 6 to 12 carbon atoms are hereinafter referred to simply as C 6~12These are called saturated aliphatic carboxylic acids, and the methods for their preparation have been found to be very suitable. They may be linear or branched, substituted or unsubstituted. 6~12 Saturated aliphatic carboxylic acids contain, in addition to carbon and hydrogen, one or more heteroatoms, examples of which include halogens. 6~12 Saturated aliphatic carboxylic acids are preferred. Preferred examples of these, divided by the number of carbon atoms, are as follows: C6: Hexanoic acid, 2-methylpentanoic acid, 3-methylpentanoic acid, 4-methylpentanoic acid, 2,3-dimethylbutanoic acid, and 3,3-dimethylbutanoic acid C7: Heptanoic acid and 2-methylhexanoic acid C8: Octanoic acid, 2-methylheptanoic acid, 2-ethylhexanoic acid, 2-ethyl-4-methylpentanoic acid, and 2-propylpentanoic acid C9: Nonanoic acid and 3,5,5-trimethylhexanoic acid C 10 About Decanoic Acid, 2-Propylheptanoic Acid, and 2-Propyl-4-methylhexanoic Acid C 11 About undecanoic acid and 2-methyldecanoic acid C 12 About dodecanoic acid and 2-butyloctanoic acid

[0017] From the above list, hexanoic acid, 2-methylpentanoic acid, heptanoic acid, octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid, 2-propylheptanoic acid, and dodecanoic acid are more preferred. 8~12 Saturated aliphatic carboxylic acids, among which are octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid, 2-propylheptanoic acid and dodecanoic acid. 2-Ethylhexanoic acid is very particularly preferred.

[0018] In the first step of the process according to the invention, designated step (a), the corresponding aldehyde is oxidized with oxygen. 6~12 For saturated aliphatic carboxylic acids, the aldehydes are n-hexanal for hexanoic acid, 2-methylpentanal for 2-methylpentanoic acid, n-heptanal for heptanoic acid, n-octanal for octanoic acid, 2-ethylhexanal for 2-ethylhexanoic acid, n-nonanal for nonanoic acid, 3,5,5-trimethylhexanal for 3,5,5-trimethylhexanal for 3,5,5-trimethylhexanoic acid, n-decanal for decanoic acid, 2-propylheptanal for 2-propylheptanoic acid, and n-dodecanal for dodecanoic acid.

[0019] C 6~12 Aldehydes can usually be easily prepared by a variety of methods, depending on the availability of raw materials. One typical preparation method is the hydroformylation of the corresponding alkene, which has one carbon atom less than the desired aldehyde. An example is the hydroformylation of 1-pentene to n-hexanal. Another typical preparation method is the aldol condensation of an intermediate alkenal, followed by hydrogenation to the desired aldehyde. An example is the aldol condensation of n-butanal to 2-ethyl-2-hexenal, followed by hydrogenation to 2-ethylhexanal. Last but not least, a third typical preparation method is the dehydrogenation of the corresponding alcohol. An example of this method is the dehydrogenation of 1-decanol to n-decanal.

[0020] The aldehyde intended to be oxidized can be used in diluted or pure form. When the aldehyde is used in diluted form, the diluent will preferably be a compound that is inert to oxidation with oxygen, stable to the carboxylic acid produced, and easily separated from the carboxylic acid by distillation. The need to separate the aforementioned diluent from the carboxylic acid can be avoided if the corresponding carboxylic acid is used as the diluent. However, it is preferable not to intentionally dilute the aldehyde, since the diluent increases the reaction volume, thus reducing the space-time yield, and may even cause contamination of the desired carboxylic acid if a diluent other than the corresponding carboxylic acid is used. Preferably, the aldehyde is applied as a highly concentrated compound, preferably with an aldehyde content of 80 to 100 wt.%, more preferably 80 to 100 wt.%, particularly preferably 95 to 100 wt.%, and very particularly preferably 99 to 100 wt.%.

[0021] In addition to preparing only one specific saturated aliphatic carboxylic acid, the method of the present invention also provides 6~12 It can also be applied to the preparation of mixtures of saturated aliphatic carboxylic acids. When such mixtures are prepared, it is particularly important to select C carboxylic acids having similar carbon numbers. 6~12 Mixtures of saturated aliphatic carboxylic acids are preferred. However, unmixed C 6~12 It is usually more preferable to prepare saturated aliphatic carboxylic acids.

[0022] The preparation of 2-ethylhexanoic acid by oxidation of 2-ethylhexanal is particularly preferred.

[0023] The oxidation of the aldehydes is carried out with molecular oxygen, which can be used in pure form or diluted with other gases, for example in the form of air, O2 / N2 mixtures or mixtures with other inert gases.

[0024] The oxidation reaction can be carried out with or without an oxidation catalyst and / or with or without a selectivity-improving additive. When an oxidation catalyst is used, it is a homogeneous catalyst. Examples of homogeneous oxidation catalysts include salts of transition metals from Groups 6 to 11 of the Periodic Table of the Elements, preferably first-row salts of these groups, most preferably Mn, Fe, or Co. When a selectivity-improving additive is used, it is also homogeneous. Examples of selectivity-improving additives include salts of alkali metals, alkaline earth metals, and salts of transition metals from Group 12 of the Periodic Table of the Elements, preferably Na, K, Mg, Ca, Zn, or Cd, most preferably K or Na, and particularly preferably K. The salt can be selected from any salt soluble in the reaction mixture, with carboxylates, hydroxides, carbonates, and bicarbonates being preferred. The concentration of the homogeneous oxidation catalyst metal can vary over a wide range, with a metal content of 0.0001 to 0.1 wt.% of the reaction mixture being typical. The concentration of the homogeneous selectivity improving metal can also vary over a wide range, but a typical metal content is 0.01 to 5 wt. %, preferably ≥ 0.02 wt. % (≥ 0.02 wt. %, more preferably ≥ 0.05 wt. %, and preferably ≤ 2 wt. % (≤ 2 wt. %, more preferably ≤ 1 wt. %, and particularly preferably ≤ 0.5 wt. % of the reaction mixture. It is also possible to use a homogeneous catalyst metal and a homogeneous selectivity improving metal simultaneously.

[0025] Depending on the nature of the aldehyde, the presence of an oxidation catalyst, particularly a selectivity-improving metal, affects the extent and type of by-products. For example, alpha-branched aldehydes such as 2-ethylhexanal tend to produce more formate as a by-product in the absence of any selectivity-improving metal. However, in the presence of a selectivity-improving metal, particularly a sodium or potassium salt, and most particularly a potassium salt, alpha-branched aldehydes produce much less undesired formate. Therefore, for alpha-branched aldehydes, it is preferred to carry out the oxidation in the presence of a selectivity-improving metal, preferably a sodium or potassium salt, and particularly preferably a potassium salt. On the other hand, linear aldehydes such as n-hexanal or n-decanal already form only very small amounts of formate even in the absence of a selectivity-improving metal, and therefore the addition of a selectivity-improving metal is not, or only slightly, relevant to selectivity. Therefore, for linear aldehydes, it is preferred to carry out the oxidation in the absence of a selectivity-improving metal.

[0026] As for homogeneous catalytic metals, although they increase the reaction rate they have a detrimental effect on selectivity, so it is preferred to carry out the oxidation in the absence of added homogeneous catalytic metals.

[0027] Regardless of the presence or absence of an oxidation catalyst, the oxidation reaction is carried out at a temperature of 0 to 120°C and an oxygen partial pressure of 0.02 to 2 MPa. It is preferred to carry out the reaction at a temperature of ≥ 10°C (≥ 10°C), more preferably ≥ 20°C, particularly preferably ≥ 30°C, and preferably ≤ 100°C (≤ 100°C), more preferably ≤ 80°C, and particularly preferably ≤ 60°C. Regarding the oxygen partial pressure, it is preferred to carry out the reaction at an oxygen partial pressure of ≥ 0.05 MPa (≥ 0.05 MPa), more preferably ≥ 0.1 MPa, particularly preferably ≥ 0.11 MPa, and preferably ≤ 1.5 MPa, more preferably ≤ 1 MPa. The oxygen partial pressure can be easily determined by measuring the total pressure and multiplying it by the concentration of O in vol.-% determined by any suitable method known in the state of the art.

[0028] The oxygen partial pressure can vary over a wide range depending on the oxygen content of the oxygen source, but the total pressure in step (a) is typically in the range of 0.01 to 5 MPa abs. The oxidation reaction is preferably carried out at a total pressure of 0.1 MPa abs or higher, more preferably 0.2 MPa abs or higher. It is preferably carried out at a total pressure of 4 MPa abs or lower, more preferably 3 MPa abs or lower.

[0029] Under these conditions, the aldehyde is almost entirely in the liquid phase, and the oxidation reaction also takes place in the liquid phase.

[0030] The oxidation of the aldehyde in step (a) is typically carried out in a reaction device (reactor) either batchwise, semi-continuously, or continuously. In a continuous operation, the aldehyde and oxygen are continuously fed to the reaction device, and a sufficient flow of the reaction mixture is continuously removed. The process conditions for continuous operation, including the residence time, are selected to achieve the desired conversion. In a batch operation, the reaction device is charged with the addition of aldehyde and oxygen and replenished as needed. After the desired conversion is achieved, the mixture is removed from the reaction device. A semi-continuous operation is characterized by adding the aldehyde and oxygen together or intermittently to the reaction device over a specific period of time while the oxidation reaction is already occurring. After a time, for example when the reaction device is more or less full, the addition is stopped, and the mixture is removed from the reaction device after the desired conversion is achieved.

[0031] Preferred operations for step (a) are batch and continuous operations, with continuous operations being especially preferred.

[0032] The reaction device in which the oxidation of the aldehyde in step (a) takes place may comprise one or more reactors. In principle, suitable reactors include devices suitable for carrying out exothermic gas-liquid reactions and devices that can be operated discontinuously, semi-continuously, or continuously. For discontinuous processes, stirred autoclaves or autoclaves with jet loop mixing are, for example, suitable. For semi-continuous processes, stirred vessels, trickle bed reactors, and bubble column reactors are possible examples. For continuous processes, stirred vessels, trickle bed reactors, bubble column reactors, jet loop reactors, and cascades of the aforementioned reactors are suitable examples. Preferred examples of suitable reactors are described in detail in WO 2009 / 024,446 and WO 2009 / 024,549. When using a reactor cascade, for example, in a continuous process, 2 to 5, preferably 2 to 4, and particularly preferably 2 to 3 reactors are connected in series.

[0033] It is preferred to use a reactor that allows for intensive (violent) gas-liquid mixing and good distribution of oxygen in the liquid reaction mixture.

[0034] Due to the formation of a considerable amount of heat of reaction due to oxidation, it is necessary to remove heat from the reaction zone. Depending on the concentrations of aldehyde and oxygen fed into the reactor, in a continuous process it may be sufficient to remove heat from the reaction mixture alone and to control the temperature in the reactor by adding cold fresh aldehyde. However, at higher concentrations of aldehyde and oxygen contents equal to or greater than that of air, it is usually necessary to cool the reaction liquid in the reactor. Such cooling can be achieved, for example, by externally cooling the outer wall of the reactor, by cooling pipes through which a coolant flows within the reactor, or by using an external heat exchanger in an external loop.

[0035] Overall chemical formula

[0036] [ka] (Wherein R is C 5~11 Aldehydes and C 5~11 C for carboxylic acid 5~11 (representing a group) According to the OCR standard, 0.5 mol of oxygen O2 is stoichiometrically required to oxidize an aldehyde to a carboxylic acid. Although it is possible to carry out the oxidation in an oxygen deficit, resulting in partial conversion and the presence of residual aldehyde in the reaction mixture, it is preferred to apply oxygen in stoichiometric or superstoichiometric amounts. To ensure sufficient conversion on the one hand and limit the gas load on the other, the oxidation reaction is preferably carried out at a molar ratio of oxygen to aldehyde of 0.5 to 1. This is more preferably at least 0.51, particularly preferably at least 0.52, more preferably at most 0.7, particularly preferably at most 0.6, and very particularly preferably at most 0.58.

[0037] For continuous oxidation processes, the use of a reactor cascade has been shown to be particularly advantageous, since it allows for the stepwise addition of oxygen. The major advantages of such stepwise addition of oxygen are better control of the heat of reaction and, in particular, a small gas fraction in each cascade stage. Therefore, it is particularly preferred to use such a reactor cascade of two to three reactors, preferably with about 70 to 95% of the total amount of aldehyde and of the total oxygen being fed to the first reactor, and the remaining 5 to 30% of the oxygen being fed either entirely to the second reactor or further divided into two parts to the second and third reactors, where the fraction for the third reactor will preferably be a smaller fraction.

[0038] Even if oxygen is applied in a stoichiometric or superstoichiometric amount, it would take a very long time to achieve nearly 100% aldehyde conversion. This would unnecessarily block the reactor or make it too large. Therefore, in step (a), it is advantageous to convert the aldehyde until a residual aldehyde amount of 2 mol-% or less based on the saturated aliphatic carboxylic acid is achieved. Depending on the nature of the aldehyde, the concentration of the oxygen-containing gas provided to the reaction device, and the process conditions, a residual aldehyde amount of 2 mol-% or less based on the saturated aliphatic carboxylic acid is generally achieved after a reaction time of 0.1 to 5 hours.

[0039] The composition of the reaction mixture of step (a) in terms of the content of saturated aliphatic carboxylic acids and the corresponding aldehydes can usually be determined by gas chromatography.

[0040] The mixture obtained in step (a) preferably contains at most 1.5 mol-%, more preferably at most 1 mol-%, particularly preferably at most 0.5 mol-%, very particularly preferably at most 0.3 mol-% of the corresponding aldehyde, based on the saturated aliphatic carboxylic acid, and preferably at least 0.05 mol-%, particularly preferably at least 0.1 mol-%.

[0041] Considering the reaction time, it is preferably 0.2 hours or more, more preferably 0.3 hours or more, and more preferably 0.5 hours or more, and more preferably 8 hours or less, more preferably 4 hours or less, and particularly preferably 3 hours or less.

[0042] After the mixture containing the above-mentioned saturated aliphatic carboxylic acid and the corresponding aldehyde in an amount of 2 mol % or less based on the saturated aliphatic carboxylic acid is obtained in step (a), the reaction liquid is then preferably separated from the remaining oxygen-containing gas phase. In a batch process, this can be achieved, for example, by simply venting the oxygen-containing gas phase. In a continuous process, this can be achieved, for example, by simply removing the liquid reaction mixture from the reactor.

[0043] Regarding the possible by-products of the separated reaction solution, it is known from the state of the art that in the oxidation of aldehydes with oxygen, peracids are first formed, which then further oxidize the aldehydes to produce 2 moles of carboxylic acid per mole of intermediate peracid. The reaction steps are shown below:

[0044] [ka] (Wherein R is C 5~11 (representing the C group). Such acids are also 6~12 It is believed that peracids are formed in the oxidation step (a) of the preparation of saturated aliphatic carboxylic acids, and the added aldehydes are usually not completely converted but remain in small amounts in the liquid reaction mixture. Peracids have a high oxidation potential and, if not properly separated and removed, can cause darkening of the carboxylic acid over time during storage, when the product is exposed to thermal stress, and / or in the products to which they are typically applied. However, peracids, along with other compounds with high oxidation potential, can easily be characterized as so-called "active oxygen," which is a quantitative measure of the amount of reactive oxygen. The term "active oxygen" is already known and used in the state of the art and is described, for example, in A. Uhl et al., "Peroxy Compounds, Organic," in Ullmann's Encyclopedia of Industrial Chemistry, 2017, Wiley-VCH Verlag GmbH & Co. KGaA, DOI: 10.1002 / 14356007.a19_199.pub2, Chapter 10, "Analytical Determination." The amount of active oxygen in a sample is generally determined by adding a fixed amount of an easily oxidizable compound, such as iodide(1-) or iron(II) salts, to a fixed amount of sample. The reactive oxygen present oxidizes the oxidizable compound, and the amount of oxidized oxidizable compound is then determined by titration.

[0045] Because the boiling points of peracids are usually not significantly different from those of the corresponding acids and therefore usually cannot be easily separated from the corresponding acids, and based on the finding that peracids as impurities in distilled acids will cause darkening over time during storage and when exposed to heat stress, as well as in products to which the acids are normally applied, such peracids are also C 6~12 It was initially assumed that saturated aliphatic carboxylic acids were responsible for the darkening of the C 3~5 Based on the teachings in EP Application No. 20150845.4 regarding the preparation of saturated aliphatic carboxylic acids, it is believed that peracids are easily decomposed by heat treatment, and this heat treatment is taught to be carried out before distillation purification. However, according to the present invention, such heat treatment is not carried out by distillation. 6~12 It was found that the preparation of saturated aliphatic carboxylic acids was not successful, at least not within a reasonable time frame of only a few hours. First, C prepared according to step (a) 6~12 The active oxygen content in the crude saturated aliphatic carboxylic acid product is only slightly reduced by such heat treatment. Second, the C 6~12 The peracid is decomposed by such heat treatment and distilled C 6~12 Although saturated aliphatic carboxylic acids may exhibit low APHA color numbers, their content of active oxygen is still high and they tend to darken over time during storage, when exposed to heat stress, and in the products in which they are typically applied.

[0046] Surprisingly, in the distillation of the non-thermally treated product as well as in the subsequent distillation of the thermally treated product, most of the components responsible for the active oxygen content were enriched in the bottom fraction, with only a small, but still significant, portion being in the distilled C 6~12 It was found that the same compounds were found in saturated aliphatic carboxylic acids. Both findings are consistent with the 6~12This led to the important conclusion that the active oxygen content in the crude saturated aliphatic carboxylic acid product was mainly due to components other than peracids. A more detailed study of the nature of these other components revealed that they were primarily alkyl hydroperoxides of the general formula (4)

[0047] [ka] (wherein, depending on the aldehyde applied, R 1 C has a COOH group 1~11 represents a group, and R 2 is C 1~10 group or H, R 3 is C 1~5 group or H, where R 1 , R 2 and R 3 The total number of carbon atoms in the hydroperoxy group is 5 to 11. The hydroperoxy group is a group consisting of all carbon atoms except for the COOH group, regardless of whether the carbon atoms are primary, secondary, or tertiary. 6~12 It may be located at any carbon atom of the saturated aliphatic carboxylic acid, however, tertiary carbon atoms and carbon atoms alpha to the COOH group are particularly prone to peroxidation.

[0048] If not properly separated and removed, alkyl hydroperoxides can cause darkening of carboxylic acids over time during storage, when exposed to heat stress, and / or in the products in which they are typically applied.

[0049] Alkyl hydroperoxides of saturated aliphatic carboxylic acids are known to have boiling points significantly higher than the respective saturated aliphatic carboxylic acids. The following list shows the boiling points of several 2-hydroperoxy acids compared to the boiling points of the respective acids. These boiling points at atmospheric pressure were estimated using SciFinder, an electronic database of chemical and bibliographic information provided by the American Chemical Society, and are as follows:

[0050] 195±8℃ 2-methylpentanoic acid 295±23℃ 2-methyl-2-hydroperoxypentanoic acid

[0051] 239±3℃ Octanoic acid 327±25℃ 2-hydroperoxyoctanoic acid

[0052] 306±10℃ 2-ethyldecanoic acid 373±25℃ 2-Ethyl-2-hydroperoxydecanoic acid

[0053] Such a large difference in boiling points should facilitate easy separation by distillation. The high content of active oxygen in the bottom fraction found in the above study also strongly suggests this. However, based on that, a small but significant portion of the active oxygen may also be present in the distilled C 6~12 It is even more surprising that this was found in saturated aliphatic carboxylic acids, where slippage of the respective hydroperoxy acids during distillation is highly unlikely due to their high boiling points.

[0054] To be more precise regarding the information value of the active oxygen content, the method for measuring it will be described in more detail. According to the present invention, the determination of active oxygen is preferably carried out by oxidation of iodide (1-). This analytical method is called iodometric titration and is well known to those skilled in the art. However, it will be briefly described below.

[0055] In iodometric titration, a certain amount of aqueous potassium iodide in acetic acid is added to a certain amount of sample at room temperature and stirred to oxidize iodide (I-) to elemental iodine. The amount of potassium iodide added is related to the expected amount of active oxygen and can be estimated by preliminary measurements. For iodometric measurements, the amount of iodide (I-) added must be slightly greater than the amount that will be oxidized to elemental iodine. The elemental iodine is then titrated with sodium thiosulfate to determine the amount of elemental iodine formed by the previous oxidation. Starch is typically used as an indicator, which is purple as long as elemental iodine is present and becomes colorless when all elemental iodine has been reduced to iodide. Alternatively, a platinum electrode can be used. Based on the amount of potassium iodide added and the amount of elemental iodine formed by oxidation, the amount of oxidized iodide (I-) can be calculated. Formal equation:

[0056] [ka] and more detailed formulas for peracids and alkyl hydroperoxides

[0057] [ka] According to the formula, two moles of iodide (1-) react with one mole of active oxygen atom in the presence of acetic acid. The active oxygen is represented by "O" in formula (5) and is part of the peroxo group in formulas (6) and (7). It is reduced to water. In formulas (6) and (7), "Ac" represents an acetyl group, and the groups R and R 1 , R 2 and R 3 have the meanings as defined in equations (2) / (3) and (4), respectively. The active oxygen content of a sample is the weight fraction of active oxygen atoms relative to the weight of the sample, and is expressed in wt.-% or wt.-ppm.

[0058] The content of active oxygen is calculated by converting active oxygen into hydroperoxides. 6~12Assuming that the active oxygen is singly bonded to a saturated aliphatic carboxylic acid, it can be easily converted to the equivalent peroxide compound content. This can be done by multiplying the measured active oxygen content by the ratio of the molar mass of the hydroperoxy acid to the molar mass of the oxygen atom. For example, for 2-ethylhexanoic acid, the multiplier is 176.2 / 16.0 = 11.0125.

[0059] For completeness, the amount of active oxygen in a carboxylic acid-containing sample can be determined essentially by physical methods, e.g. 13 It is also noted that the solubility can be determined by C-NMR.

[0060] However, in the present invention, active oxygen is understood as the mass of oxygen present in the sample that is capable of oxidizing iodide (1-) to elemental iodine in an aqueous acetic acid medium at room temperature and atmospheric pressure.

[0061] The active oxygen content of the mixture obtained in step (a) is typically 0.02 to 1 wt.-% of the mixture, preferably 0.03 wt.-% or more, more preferably 0.05 wt.-% or more, and preferably 0.8 wt.-% or less, more preferably 0.5 wt.-% or less.

[0062] C 6~12 After intensive research into the surprising behavior of crude product mixtures of saturated aliphatic carboxylic acids, it was surprisingly discovered that the presence of molecular oxygen in the crude product mixture was mainly due to the distilled C 6~12 It has been found that saturated aliphatic carboxylic acids account for a significant amount of active oxygen content.

[0063] In the oxidation step (a), a portion of the molecular oxygen remains unreacted in the liquid mixture as physically dissolved molecular oxygen, depending on its partial pressure, the temperature of the liquid mixture, and the chemical composition of the liquid mixture. Its concentration in the liquid mixture obtained by step (a) is usually greater than 10 wt.-ppm to less than 1 wt.-% of the liquid mixture. Because a small amount of molecular oxygen remains unreacted in the liquid mixture, a content of less than 10 wt.-ppm of the liquid mixture is usually not achieved, and a content of more than 1 wt.-% is not practically achieved due to its limited solubility. Since the above-mentioned concentrations of molecular oxygen relate to the liquid mixture obtained in step (a), the values ​​refer to the reaction conditions at the end of the conversion just before the liquid mixture leaves the reactor. Therefore, the measurement of the molecular oxygen concentration in the liquid mixture is preferably carried out under the above-mentioned conditions just before the liquid mixture leaves the reactor. However, it may alternatively be measured at a lower pressure, e.g., atmospheric pressure, and the measurement value may be corrected taking into account widely accepted laws, such as Henry's law for pressure dependence. The molecular oxygen concentration in the liquid mixture obtained in step (a) is preferably at least 20 wt.-ppm, more preferably at least 50 wt.-ppm, with an upper limit usually at most 1 wt.-% (or at most 10,000 wt.-ppm in ppm), preferably at most 5,000 wt.-ppm, more preferably at most 2,500 wt.-ppm, particularly preferably at most 1,000 wt.-ppm and very particularly preferably at most 750 wt.-ppm.

[0064] The concentration of molecular oxygen in the liquid mixture obtained by step (a) can be easily measured. Useful devices are optical sensors, which are often used, for example, in water analysis. A possible example is an optical fluorescence sensor. Optical fluorescence sensors are state of the art, and those skilled in the art know how to calibrate and use them. Such sensors are highly specific for oxygen and highly sensitive to it. They can even measure very low concentrations, down to 0.01 wt.-ppm. Such optical fluorescence sensors can also be used for in-line measurements, since they are somewhat heat and pressure resistant.

[0065] C of the liquid obtained by step (a) 6~12 Having explained that the crude product mixture of saturated aliphatic carboxylic acids contains hydroperoxides and at most very small amounts of each as by-products, as well as unconverted dissolved molecular oxygen, it is important to note that the term active oxygen does not include molecular oxygen. This is due to the fact that molecular oxygen reacts only very slowly under the conditions used for measuring active oxygen in samples containing hydroperoxides; furthermore, it is good practice to carry out the measurement under inert gas to minimize any interference by molecular oxygen. As an advantageous consequence of this, active oxygen and molecular oxygen can be measured and evaluated separately.

[0066] Based on all the above surprising findings, the C of the liquid obtained by step (a) 6~12 The oxygen molecules present in the crude product mixture of saturated aliphatic carboxylic acids are distilled to produce C 6~12It has then been found that darkening of the carboxylic acids over time during storage, when exposed to thermal stress, and / or in the products to which they are normally applied can be avoided or at least greatly reduced if the saturated aliphatic carboxylic acids are removed in a subsequent step, designated step (b), to a content of 10 wt.-ppm or less relative to the liquid mixture before isolation. For completeness, it is mentioned that there may be further steps between steps (a) and (b), such as, for example, a reduction or increase in temperature or pressure, or the removal of low boilers by distillation.

[0067] There are several possibilities for removing molecular oxygen in the liquid crude product mixture down to a content of 10 wt.-ppm or less. One possibility is to allow the mixture to react for a long time, allowing the dissolved molecular oxygen to react with the aldehydes still present. This can be carried out, for example, in a pipe through which the mixture flows, allowing the intended residence time, or in one or more residence time vessels connected in series. Although such a procedure does not require the addition of additional compounds or the adjustment of specific conditions, such as temperature increase, it is not a preferred method because it requires a long time, typically much longer than 10 hours, and even more than 50 or even 100 hours.

[0068] A preferred possibility for removing molecular oxygen to a content of 10 wt.-ppm or less is to strip the liquid crude product mixture obtained in step (a) with an inert gas.

[0069] In principle, stripping can be carried out in a vessel also called a stripper, where the liquid crude product mixture can be contacted with an inert gas in order to transfer dissolved oxygen molecules to the gas phase of the inert gas and remove it together with the gas phase. In a continuous preparation process, the liquid crude product mixture obtained in step (a) is continuously fed to a stripper, where an inert gas can be passed through the mixture. In principle, such a stripper can be of various shapes so that essentially any type of vessel known in the state of the art for contacting a gas with a liquid phase can be used, but so-called stripping columns are preferred. Stripping columns are characterized by their length to average diameter ratio being greater than 1, preferably 5 or more, more preferably 8 or more, and preferably 15 or less. An inert gas is fed to the lower region of the stripping column and bubbled up to the upper region, while the liquid crude product mixture can be fed to the lower region to cause a cocurrent flow or to the upper region to cause a countercurrent flow. C, including stripping in step (b) in countercurrent flow, 6~12 Continuous preparation of saturated aliphatic carboxylic acids is preferred. Whether stripping is performed cocurrently or countercurrently, it is preferred to improve contact between the gas and liquid phases. This can be easily achieved by random or structured packing or trays within the stripper. Such internals for promoting gas / liquid contact are known to those skilled in the art and can be readily selected.

[0070] Alternatively, molecular oxygen can also be removed by spraying the liquid crude product mixture into a free-space device, such as the top of a vertical column, where an inert gas is fed countercurrently from the bottom of the column.

[0071] For discontinuous or semi-continuous preparation processes, the liquid crude product mixture can be held, for example, in a reactor in which an inert gas is passed through the liquid crude product mixture obtained after the conversion of the aldehyde in step (a) has taken place and, preferably, after the gas phase of the molecular oxygen-containing oxidizing gas has been discharged. It is also possible to transfer the liquid crude product mixture from the reactor of step (a) to another vessel, also called a stripper, in which an inert gas is passed through the mixture. For such a stripper, the above remarks regarding the continuous preparation process apply analogously, except that the liquid crude product mixture is usually the stationary phase.

[0072] An inert gas suitable for stripping is a substance that is gaseous under the conditions under which stripping is carried out, does not react with the liquid crude product mixture, and is essentially free of molecular oxygen. The allowable concentration of molecular oxygen in the inert gas is C 6~12 Although it may depend on the nature of the saturated aliphatic carboxylic acid and the conditions under which the liquid crude product mixture is stripped, the molecular oxygen content of the inert gas is preferably 5 vol.-ppm or less, more preferably 2 vol.-ppm or less, and particularly preferably 1 vol.-ppm or less. Small amounts of molecular oxygen may be present in the inert gas as impurities, for example, resulting from their preparation or purification. Possible inert gases include nitrogen, hydrogen, carbon dioxide, carbon monoxide, nitrous oxide, and noble gases, such as helium, neon, argon, and krypton. The inert gas may be provided in pure form or as a mixture of two or more different inert gases. Due to its availability, nitrogen is preferred.

[0073] The stripping in step (b) can be carried out in a wide temperature and pressure range. It is preferably carried out at a temperature of 0 to 150°C and a pressure of 0.0001 to 10 MPa abs. More preferably, the stripping is carried out at a temperature within the range between the temperature at which the liquid crude product mixture leaves the reactor in step (a) and the temperature at which the stripped mixture is fed to the distillation apparatus in step (c). Stripping is particularly preferably carried out at a temperature above 25°C, particularly preferably below 100°C, very particularly preferably below 60°C, which is advantageous in terms of avoiding or at least minimizing undesired reactions between molecular oxygen and the liquid mixture and in particular the already formed carboxylic acid. 6~12 This is particularly advantageous for crude product mixtures of saturated aliphatic carboxylic acids. With regard to pressure, stripping is more preferably carried out at a pressure within the range between the pressure at which the liquid crude product mixture leaves the reactor in step (a) and the pressure at which the stripped mixture is fed to the distillation apparatus in step (c). The wide pressure range mentioned above already indicates excellent flexibility in the selection of pressure. Depending on the facilities and equipment available at the plant construction site, the advantages of stripping under vacuum conditions or above atmospheric pressure may be effective. Stripping under vacuum conditions of 0.0001 to <0.1 MPa abs is usually preferred, since, when sufficient vacuum power is available, the amount of stripping gas required is small and stripping is highly efficient. Alternatively, stripping at 0.1 MPa abs or above has the advantage that no vacuum power is required. However, stripping at above atmospheric pressure is preferably carried out at 0.2 MPa abs or below, since the volume of inert gas is still large enough to be particularly advantageously bubbled through the liquid mixture.

[0074] The amount of inert gas applied to remove a specific amount of molecular oxygen by stripping can vary within a wide range. Surprisingly, it has been found that stripping is highly efficient, and that a small molar amount of inert gas per mole of removed molecular oxygen is already sufficient. Therefore, stripping in step (b) is preferably carried out at a molar ratio between the inert gas fed to the stripper and the molecular oxygen removed of 0.25 mol / mol or more. However, the molar ratio between the inert gas fed to the stripper and the molecular oxygen removed is preferably 1 mol / mol or less, more preferably 4 mol / mol or less. The amount of inert gas per unit time will be sufficiently low so as not to exceed the flooding point of the stripper. Based on the geometry of the stripper, the amount of liquid crude product mixture fed to the stripper in the case of continuous operation, process conditions such as temperature and pressure, and the physical properties of the mixture, a person skilled in the art can calculate or otherwise experimentally determine the maximum allowable amount of inert gas per unit time in each case.

[0075] Due to the low molar ratio between the inert gas fed to the stripper and the molecular oxygen being removed, stripping in a continuously operated stripping column can usually be carried out within 1 to 10 minutes, preferably 2 minutes or more, preferably 5 minutes or less. Stripping in a batch operation, where the liquid crude product mixture is already placed in a device that is then stripped with an inert gas, usually requires more time. For example, stripping in a batch-operated bubble column will usually require from several minutes to several hours to achieve the desired removal of molecular oxygen.

[0076] By the inventive removal of molecular oxygen from the liquid mixture obtained in step (a), the molecular oxygen content is reduced in step (b) to 10 wt.-ppm or less, preferably 5 wt.-ppm or less, more preferably 2 wt.-ppm or less, particularly preferably 1 wt.-ppm or less, and very particularly preferably 0.5 wt.-ppm or less. The molecular oxygen content may also be reduced below its detection limit, although a small amount of 0.05 wt.-ppm or more typically remains. The concentration of molecular oxygen in the liquid mixture obtained in step (b) can be easily measured in a manner similar to that already described for step (a).

[0077] The liquid obtained in step (b) is depleted of oxygen molecules. 6~12 The crude product mixture of saturated aliphatic carboxylic acids is then transferred to step (c). For completeness, it is mentioned that the transfer may include further steps between steps (b) and (c), such as a decrease or increase in temperature or pressure.

[0078] In step (c), the mixture obtained in step (b) is distilled in a distillation apparatus equipped with a purification column, from which a purified distillate containing at least 95 wt.-% saturated aliphatic carboxylic acids is obtained. The term purification column refers to a C 6~12 This refers to a distillation column from which a purified distillate of saturated aliphatic carboxylic acid is obtained. In addition to the purification column, the distillation apparatus may also include additional distillation columns, such as a distillation column for removing light-boiling components and a column for working up high-boiling components. The total number of distillation columns is not limited, but is usually 1 to 5, preferably 1 to 4, more preferably 1 to 3, and particularly preferably 1 to 2. In addition to the distillation columns themselves and their internal components, the term distillation apparatus also includes their peripherals, such as conduits, heat exchangers, reboilers, condensers, reflux drums, etc.

[0079] C 6~12 The distillative separation of the saturated aliphatic carboxylic acid can usually be carried out continuously or discontinuously.

[0080] In the case of a discontinuous or semi-continuous oxidation process in step (a), it is usually advantageous to also carry out steps (b) and (c) discontinuously. For such discontinuous distillations, the distillation apparatus preferably comprises only one distillation column, which at the same time constitutes a purification column. In this case, low boilers are first distilled off overhead, and C 6~12 Saturated aliphatic carboxylic acids are subsequently obtained as a further fraction. In order to minimize the number of apparatuses required and to simplify the process, it may also be preferable to carry out the removal of molecular oxygen and the subsequent distillation in one and the same distillation apparatus.

[0081] In the case of a continuous oxidation process in step (a), continuous and discontinuous removal of molecular oxygen in step (b) and distillation in step (c) can be advantageous, and both are essentially equally preferred. However, the advantages of continuous distillation are usually more effective for larger production volumes, and the advantages of discontinuous distillation are more effective for smaller production volumes. Typically, continuous distillation is more effective for production of more than 1000 tons of C per year. 6~12 Discontinuous distillation is preferred for plants with a production capacity of saturated aliphatic carboxylic acids, while discontinuous distillation is preferred for plants with a respective production capacity of 1000 tons per year or less. For discontinuous distillation, the distillation apparatus preferably comprises only a single distillation column, while continuous distillation is usually carried out either in a single distillation column or in the interconnection of several distillation columns. For discontinuous distillation, the explanations in the preceding paragraphs consequently apply. For continuous distillation, it can be carried out, for example, in one distillation column which simultaneously constitutes a purification column, in which low boilers are distilled off at the top and C 6~12 Saturated aliphatic carboxylic acids are removed as a side stream. High boilers are removed as a bottom stream. Continuous distillation can also be carried out in two or more interconnected distillation columns. When two interconnected distillation columns are used, low boilers are usually separated off in the first distillation column, and C 6~12 The bottoms containing saturated aliphatic carboxylic acids and high boilers are transferred to a second distillation column constituting a purification column, where C6~12 Saturated aliphatic carboxylic acids are taken off at the top, and high boilers remain as bottom products. In a further undesirable variant with two interconnected distillation columns, saturated aliphatic carboxylic acids are taken off at the top of the first distillation column together with low boilers, and are thus already separated from high boilers in the first distillation column, but then must be purified in a second distillation column, which would require energy-intensive evaporation of additional saturated aliphatic carboxylic acids. Instead of using two interconnected distillation columns, a dividing wall column or an equivalent Petlyuk structure can also be used. For completeness, the use of three or more distillation columns is also mentioned, for example, to separate light boilers into different fractions. Two or more different C 6~12 When saturated aliphatic carboxylic acids are prepared together, this is usually 6~12 The use of three or more distillation columns may also be interesting, since one separate purification column is required for saturated aliphatic carboxylic acids.

[0082] Whether only one distillation column or two or more interconnected distillation columns are used, they may, and preferably do, have separation efficiency-aiding internals, such as structured packing, random packing, or trays. The number of separation stages required depends primarily on the separation task, in particular the difference in boiling points of the saturated aliphatic carboxylic acids relative to the boiling points of the low boilers and high boilers, and the desired purity of the saturated aliphatic carboxylic acids.

[0083] Regarding the operating conditions in step (c), it has been found that higher temperatures increasingly facilitate the formation of undesired by-products, such as the formation of anhydrides, and therefore, 6~12 It is recommended that saturated aliphatic carboxylic acids not be exposed to temperatures above 170°C. 6~12The temperature of the distillation column in which the saturated aliphatic carboxylic acid is treated is preferably 170°C or less, and in particular the temperature of the purification column is preferably 170°C or less, more preferably 150°C or less, and particularly preferably 130°C or less. This can be easily achieved by selecting an appropriate pressure. 6~12 Since even saturated aliphatic carboxylic acids have boiling points at atmospheric pressure above 190°C, the distillation of step (c) is preferably carried out at 0.1 to 99 kPa abs, which corresponds to C 6~12 This relates to all distillation columns in which saturated aliphatic carboxylic acids are treated. The distillation is preferably carried out at a pressure of 0.5 kPa abs or more, particularly preferably 1 kPa abs or more, more preferably 50 kPa abs or less, particularly preferably 20 kPa abs or less. When two or more distillation columns are used, each distillation column may be operated at a different pressure. Considering the above pressure range, the distillation in step (c) is usually carried out at a temperature of 0°C or more, preferably 25°C or more, more preferably 40°C or more.

[0084] Very low pressures, below 1 kPa abs, lead to low distillation temperatures, typically below 100°C, and such low distillation temperatures greatly reduce the harmful effects of molecular oxygen, making distillation at very low pressures particularly advantageous for pure C 6~12 It may be a particular option to obtain saturated aliphatic carboxylic acids, or alternatively to tolerate a slightly higher content of molecular oxygen. However, such very low pressure variants are not preferred options, as the distillation rate decreases at these very low pressures, quickly resulting in either very low throughput or the need for undesirably large column diameters.

[0085] Based on the above findings and relationships, the purification column is preferably operated at a pressure of 0.1 to 99 kPa abs and a temperature of 0 to 170°C.

[0086] C of the liquid obtained by step (a) 6~12The oxygen molecules present in the crude product mixture of saturated aliphatic carboxylic acids are distilled to produce C 6~12 As a theoretical consequence of the surprising finding that darkening of carboxylic acids when exposed to thermal stress over time during storage and / or in the products to which they are normally applied can be avoided or at least greatly reduced if saturated aliphatic carboxylic acids are removed in step (b) to a content of 10 wt.-% or less relative to the liquid mixture before isolation, it is self-evident that the introduction of molecular oxygen after removal of molecular oxygen in step (b), including transfer to step (c), during and after the distillation in step (c) will not produce the desired results. 6~12 If the equipment in which saturated aliphatic carboxylic acids are treated is sealed or at least largely sealed under operating conditions, the intrusion of oxygen molecules can be avoided or at least reduced to a very low level. This is particularly advantageous for parts that operate under vacuum, in particular distillation columns, and especially purification columns. A high degree of sealing of the columns used, in particular vacuum distillation columns, but also strippers when operated at reduced pressure, can be achieved by various technical means known to those skilled in the art. Such means are, for example, the use of high-quality seals or even welded joints with low leakage rates, the use of inert gas outlet flange protection, or minimizing the number of flanges. As a general rule, the more such measures are implemented, the lower the leakage rate of the distillation column will be.

[0087] In step (c), it is desirable to avoid any intrusion of oxygen molecules. 6~12 The small amount of oxygen molecules that enter the crude product mixture of saturated aliphatic carboxylic acids is C 6~12This does not significantly impair the purity of the saturated aliphatic carboxylic acid distillate, and usually still makes it possible to obtain a low APHA color number within the specified range after the tempering procedure specified in step (c). As a result, a large distillation column of industrial size, for example designed for distillation of 1000 kg per hour or more, can usually tolerate a larger absolute amount of invaded molecular oxygen per unit time than a small laboratory-sized distillation column, for example handling only 1 kg per hour or less. Furthermore, the ratio between the surface of the sealant and the internal volume of the distillation column is much smaller in the case of a large industrial-sized column than in a small-scale laboratory column. According to the present invention, the oxygen-depleted C of step (b) can be obtained. 6~12 It has been found that the amount of molecular oxygen entering the distillation apparatus in step (c) in the range of the amount of molecular oxygen supplied to the distillation apparatus by the crude product mixture of saturated aliphatic carboxylic acids is acceptable.

[0088] C 6~12 All parts of the distillation apparatus in step (c) in which the saturated aliphatic carboxylic acid is treated are advantageously sealed to the extent reasonably possible, but in particular the sealing of the purification column is 6~12 The purity of the saturated aliphatic carboxylic acid distillate is more sensitive. This applies particularly to purification columns operated under vacuum conditions. The tightness, or leak rate, of a distillation column can be determined by measuring the amount of molecular oxygen that has entered during distillation and comparing these values ​​with the amount of molecular oxygen supplied to the distillation column by the feed stream. For vacuum columns, this can be done by measuring the amount of off-gas from the vacuum unit and the concentration of molecular oxygen therein.

[0089] The purification column operated at a pressure of 0.1 to 99 kPa abs and a temperature of 0 to 170°C is preferably sealed so that the amount of oxygen molecules removed by the off-gas of the vacuum unit is at least 2 times or less than the amount of oxygen molecules supplied to the distillation apparatus by the mixture obtained in step (b). More preferably, the amount of oxygen molecules removed by the off-gas of the vacuum unit is 1.8 times or less, particularly preferably 1.5 times or less, and the lower limit is 1 time or more, of the amount of oxygen molecules supplied to the distillation apparatus by the mixture obtained in step (b). For discontinuous distillation, the term "amount" refers to the absolute amount during discontinuous distillation, while for continuous distillation, the term "amount" refers to the amount per unit time. C, where the amount of oxygen molecules supplied to the distillation apparatus is depleted, 6~12 The amount of molecular oxygen in the crude product mixture of saturated aliphatic carboxylic acids can be easily determined as already mentioned above by measuring the concentration of molecular oxygen, for example using an optical molecular oxygen-sensitive sensor, and taking into account the total amount of the product mixture in the case of discontinuous distillation, or by relating it to the amount of product mixture fed to the purification column per unit time in the case of continuous distillation. The amount of molecular oxygen removed by the off-gas of the vacuum unit of the purification column can be easily determined by measuring the concentration of molecular oxygen in the off-gas of the vacuum unit, preferably on the pressure side, and determining the amount of off-gas, for example using a flow meter. The total amount is calculated for discontinuous distillation, and the relative amount per unit time for continuous distillation.

[0090] Because the above-mentioned methods for determining the seal or leak rate of a vacuum column require the amount of off-gas in addition to the concentration of molecular oxygen in the column off-gas, it can be difficult to obtain a reliable value for the amount of off-gas, as the amount can be very small and therefore difficult to measure. Therefore, an alternative measurement method was developed that does not require the amount of column off-gas. This alternative method takes into account that a vacuum column that is not completely sealed will draw some ambient air into the column. Since air contains argon in a fixed ratio to molecular oxygen, the concentration of argon in the column off-gas can be used as an indirect measure of the concentration of molecular oxygen based on air intrusion. The concentration of molecular oxygen in air is 20.95 vol.-% and the concentration of argon is 0.93 vol.-%, which corresponds to an Ar / O2 molar ratio of 0.0444. An argon-free, oxygen-depleted C 6~12 A completely sealed column fed with a crude product mixture of saturated aliphatic carboxylic acids will have an Ar / O2 molar ratio of 0 (zero) due to the absence of argon in the column off-gas. Conversely, a leaky column fed with an argon- and molecular oxygen-free product mixture will have an Ar / O2 molar ratio of 0.0444, the theoretical value for air. Depending on the amount of oxygen fed to the distillation column with the product mixture, the Ar / O2 molar ratio of the leaky column will be lower than that of pure air. If the amount of oxygen introduced is the same as the amount of molecular oxygen fed to the distillation column with the oxygen-depleted product mixture, the Ar / O2 molar ratio will be 0.0222. The more sealed the column, the lower the Ar / O2 molar ratio, and vice versa; a molecular oxygen concentration in the column off-gas of 0 vol.-% is by definition associated with a completely sealed distillation column.

[0091] According to this alternative measurement method, a purification column operated at a pressure of 0.1 to 99 kPa abs and a temperature of 0 to 170°C is preferably sealed so that the molar ratio n(Ar off-gas) / n(O off-gas) in the off-gas of the vacuum unit of the purification column is 0 to 0.0222, where n(Ar off-gas) is the amount of argon molecules in the off-gas corrected by the amount of argon molecules that may have been supplied to step (c) by the mixture obtained in step (b), and n(O off-gas) is the amount of oxygen molecules in the off-gas. More preferably, the molar ratio n(Ar off-gas) / n(O off-gas) is 0.0197 or less, particularly preferably 0.0148 or less. For discontinuous distillation, the term amount refers to the absolute amount during discontinuous distillation, while for continuous distillation, the term amount relates to the amount per unit time. For measuring the amount of oxygen molecules in the off-gas of the vacuum unit, the measurement method described above can be applied. The amount of argon molecules in the off-gas of the vacuum unit is easily measured, for example, by gas chromatographic analysis of the pressure-side off-gas. For example, a portable gas chromatograph can be used. If the mixture obtained in step (b) already contains argon, whatever the reason, its amount can also be measured by gas chromatographic analysis and used to correct the n(Ar off-gas) value.

[0092] Furthermore, in accordance with the present invention, C is preferably used in the distillation column, particularly when the distillation column is not a divided wall column. 6~12 The location of the feed point where the feed is added to the purification column relative to the location of the sampling point where the saturated aliphatic carboxylic acid is removed is also C 6~12 It was found that this affected the tempering stability of saturated aliphatic carboxylic acids. When the feed point of a non-divided-wall distillation column is located above the sampling point, C 6~12Saturated aliphatic carboxylic acids are usually less stable to tempering than the reverse. Therefore, in step (c), it is preferred that the feed to the purification column that is not a divided-wall column is added at a position lower than the position at which the purified distillate is taken out. If the total external height of the purification column is taken as 100%, the distance between the horizontal level of the feed point and the horizontal level of the sampling point higher than the feed point is preferably 1% or more, more preferably 2% or more, particularly preferably 3% or more, and preferably 90% or less, more preferably 80% or less of the external height of the purification column. For a divided-wall column as a purification column, the relative positions of the feed point and the sampling point are C 6~12 Saturated aliphatic carboxylic acids have a minor influence on tempering stability.

[0093] Based on the above description of the distillation apparatus and its operation, one skilled in the art will be able to determine the C 6~12 Depending on the properties of the saturated aliphatic carboxylic acid, the distillation apparatus can be designed and appropriate operating conditions can be determined.

[0094] The above method is C for the distillate. 6~12 C, which has a high purity with a saturated aliphatic carboxylic acid content of 95 wt.% or more and has a very low APHA color number even after being tempered under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa for 4 hours. 6~12 It is possible to prepare saturated aliphatic carboxylic acids. As shown in numerous figures, the purified distillate containing 95 wt.-% or more saturated aliphatic carboxylic acids based on the distillate has an APHA color number of 0 to 10 after tempering under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa for 4 hours. 6~12The content of saturated aliphatic carboxylic acids is preferably 98 wt.-% or more, more preferably 99 wt.-% or more, particularly preferably 99.5 wt.-% or more, and very particularly preferably 99.8 wt.-% or more, based on the distillate. Typical by-products include formates, carboxylic acids with a low carbon number, alcohols, ketones, and water. For the sake of clarity, it is noted that the APHA color number refers to the well-established and well-known Hazen scale, which is based on DIN EN ISO 6271 and ASTM D1209, respectively.

[0095] According to the low APHA color number after the tempering procedure described above, the freshly distilled untempered C 6~12 The APHA color number of saturated aliphatic carboxylic acids is also very low, typically 0 to 10, preferably 5 or less, and more preferably 3 or less.

[0096] The tempering procedure described above is 6~12 Saturated aliphatic carboxylic acids are tempered under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa for 4 hours, simulating heat stress situations under defined and reproducible conditions and as such being a more or less reliable indication of color stability during storage, when exposed to heat stress and / or after their normal application in the manufacture of products.

[0097] More specifically, the so-called tempering test is carried out as follows: 6~12A sample containing saturated aliphatic carboxylic acids is placed in a three-necked round-bottom flask equipped with a magnetic stirrer, a condenser, and a glass frit for bubbling inert gas (approximately 0.2 Nl / min, preferably argon 5.0) through the sample. At ambient temperature, the sample is sparged with inert gas for 30 minutes under stirring. The sparge is then reduced to a minimum, and the flask is lowered into a preheated oil bath maintained at 225°C. After 4 hours, the sparge rate is increased slightly, and the heating bath is removed. It is important that the sparge rate is fast enough to prevent air from being drawn into the flask during cooling. Once the flask has cooled to ambient temperature, the sample is removed and the APHA color number is measured. Measurements can be performed, for example, using a 10 mL cuvette in a Hach LICO® 620 colorimeter. The instrument is pre-calibrated against distilled water.

[0098] Refined C after tempering according to the tempering test described above 6~12 The APHA color number of the saturated aliphatic carboxylic acid distillate is preferably 8 or less, more preferably 5 or less. Although an APHA color number of 0 can be achieved, it is often 1 or greater.

[0099] C obtained in step (c) 6~12 Discoloration during tempering tests of saturated aliphatic carboxylic acid distillates is mainly caused by the presence of small amounts of peroxides. 6~12 The content of active oxygen in the saturated aliphatic carboxylic acid distillate is a further indicator of its color stability. According to the present invention, the purified distillate obtained in step (c) preferably has an active oxygen content of 0 to 100 ppm by weight, more preferably 50 ppm by weight or less, particularly preferably 10 ppm by weight or less, and preferably 1 ppm by weight or more, based on the distillate.

[0100] As already mentioned before, each of steps (a) to (c) can be carried out continuously or discontinuously, while for step (a) semi-continuous operation is a further option. Such discontinuous or semi-continuous processes are usually more adaptable and easier to operate when production volumes are small, so that for smaller production volumes, C 6~12 It may be advantageous to produce saturated aliphatic carboxylic acids in a discontinuous or semi-continuous process. On the other hand, a continuous process in which steps (a) to (c) are carried out continuously has the advantage that it is efficient and runs stably as soon as the process is started. This therefore makes it possible to produce saturated aliphatic carboxylic acids in a larger production volume of more than 1000 tons per year. 6~12 It is a preferred option for the continuous preparation of saturated aliphatic carboxylic acids, especially for saturated aliphatic carboxylic acids produced in large quantities, such as octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid, 2-propylheptanoic acid, and dodecanoic acid.

[0101] In a typical embodiment for the continuous preparation of 2-ethylhexanoic acid, liquid 2-ethylhexanal, aqueous potassium hydroxide as a selectivity-improving additive, and gaseous oxygen are continuously fed into a jet loop reactor operated at a temperature ranging from 30 to 60°C and an oxygen partial pressure ranging from 0.11 to 1 MPa abs. The reactor is externally cooled by an external heat exchanger to remove the heat generated by the exothermic oxidation reaction. The reaction mixture obtained by step (a) above still contains unconverted 2-ethylhexanal in an amount of 0.1 to 2 mol-% relative to the 2-ethylhexanoic acid, and molecular oxygen in a concentration of 20 to 500 wt.-ppm relative to the liquid mixture, depending on the oxygen partial pressure, temperature, and conversion rate. The resulting mixture is fed to a stripping column and stripped at 40-60°C under near atmospheric pressure or vacuum conditions with a nitrogen stream containing 0.5-4 moles of nitrogen per mole of oxygen to obtain a mixture depleted in oxygen molecules containing 10 wt.-ppm or less of the liquid mixture. The depleted mixture is then continuously fed into a distillation apparatus operated under vacuum conditions, equipped with a low-boiling point column for separating and removing low-boiling points, such as water, residual 2-ethylhexanal, and other low-boiling by-products, and a purification column to which the bottom stream of the low-boiling point column is fed and from which purified 2-ethylhexanoic acid is withdrawn as a side stream. Optionally, the two columns may also be combined into a single divided-wall column. The 2-ethylhexanoic acid obtained by the distillation step (c) described above has a high purity and contains 99.5 wt.-% or more of 2-ethylhexanoic acid. After being tempered under inert gas conditions at a temperature of 225° C. and a pressure of 0.1 MPa for 4 hours, it exhibits an APHA color number of only 10 or less.

[0102] In a typical embodiment for the discontinuous preparation of 2-ethylhexanoic acid, 2-ethylhexanoic acid as a diluent solvent and potassium hydroxide as a selectivity-improving additive are placed in a temperature-controlled stirred tank, and 2-ethylhexanal and molecular oxygen or a molecular oxygen-containing gas mixture are simultaneously fed into the stirred tank at a total pressure of 0.1 to 10 MPa abs for 0.1 to 5 hours while controlling the temperature at 25 to 60°C. After the desired liquid level is achieved in the stirred tank, the addition of 2-ethylhexanal is stopped, and molecular oxygen is replenished to maintain the partial pressure and bring the conversion rate to the desired value. After the desired conversion rate is reached, the addition of molecular oxygen is also stopped, and, while stirring in the tank is still continued, nitrogen is added for stripping at 40 to 60°C and near atmospheric pressure or vacuum conditions. Typically, a nitrogen stream of 0.5 to 100 moles of nitrogen per mole of dissolved oxygen is fed over a period of 10 minutes to 2 hours to obtain an oxygen-depleted mixture containing 10 wt.-ppm or less of the liquid mixture. The oxygen-depleted mixture is then fed to a batch vacuum distillation column to obtain a fraction containing purified 2-ethylhexanoic acid after removal of low boilers, or continuously fed to a continuously operated distillation unit from which purified 2-ethylhexanoic acid is extracted. The resulting purified 2-ethylhexanoic acid is highly pure, containing 99.5 wt.-% or more of 2-ethylhexanoic acid. After tempering under inert gas conditions at 225°C and 0.1 MPa for 4 hours, it exhibits an APHA color number of only 10 or less.

[0103] In addition to the above-mentioned methods, 2-ethylhexanoic acid was found to have an APHA color number of 0 to 10 after 4 hours of tempering under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa. This was surprising insofar as even 2-ethylhexanoic acid produced by methods other than oxidation, such as those described in the prior art by dehydrogenating 2-ethylhexanol with NaOH to give sodium 2-ethylhexanoate followed by acidification with H2SO4, did not result in tempering-stable 2-ethylhexanoic acid that would pass the above-mentioned tempering test, even though this method is believed to avoid the formation of peroxides.

[0104] The process of the present invention allows the preparation of saturated aliphatic carboxylic acids having 6 to 12 carbon atoms by oxidation of the corresponding aldehydes with oxygen in high yield and purity, and in particular with high color stability, such that the APHA color number remains very low even after a 4-hour tempering procedure under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa. 6~12 The saturated aliphatic carboxylic acid also has a very low content of active oxygen, and therefore a very low content of peroxides, such as peracids, hydroperoxides, and other peroxides. The method is also easy to operate, functions stably over a long period of operation, and can produce saturated aliphatic carboxylic acids with consistent high quality. 6~12 Saturated aliphatic carboxylic acids can be stored for long periods and can be subjected to heat or applied in the manufacture of further products with no or at least a very low tendency to darken.

[0105] Furthermore, the process makes it possible to utilize highly pure and tempering stable 2-ethylhexanoic acid. [Example]

[0106] Measurement of active oxygen by iodometric titration The content of active oxygen in the samples was measured by iodometric titration. Below is a general description of how the measurements were performed.

[0107] Approximately 5 g of sample is weighed to the nearest 0.1 mg, placed in a reaction vial, flushed with argon, and 40 ml of a 1:1 acetic acid / chloroform mixture is added to dissolve the sample. The reaction vial is equipped with a condenser and placed in a stirring heating block preheated to 80 °C. A gentle argon stream is passed through the condenser to prevent air from entering. After the temperature equilibrates, 5.0 mL of saturated potassium iodide solution (approximately 60.0 g of potassium iodide dissolved in 100 mL of deionized water) is added through the condenser, and the mixture is boiled under reflux for 10 minutes. In the next step, 40.0 mL of deionized water is added, and the sample solution is titrated with 0.01 M thiosulfate solution using a platinum electrode as the endpoint.

[0108] Measurement of molecular oxygen The molecular oxygen content of the samples was measured using a highly accurate and O2-sensitive optical fluorescence sensor suitable for measuring molecular oxygen in carboxylic acids. In the example, an optical sensor named FDO® 925 from WTW was used. The measurement data were verified by further measurements using a galvanic cell oxygen analyzer, also known as a Hersch cell.

[0109] APHA color number measurement The APHA color number of the samples was measured using a colorimeter pre-calibrated against distilled water. In the example, a Hach Lico® 620 colorimeter was used with a 10 mL cuvette.

[0110] Tempering test description The so-called tempering test simulates heat stress situations under defined and reproducible conditions and shows the color stability of a sample during storage, heat stress and / or after normal application in the manufacture of the product. Below is a description of how the tempering test was performed.

[0111] A 40.0-60.0 g sample is placed in a three-neck round-bottom flask equipped with a magnetic stirrer, a condenser, and a glass frit for bubbling inert gas (Argon 5.0 at approximately 0.2 Nl / min) through the sample. At ambient temperature, the sample is sparged with inert gas for 30 minutes under stirring. The sparge is then reduced to a minimum just enough to prevent air from being drawn into the flask, and the flask is lowered into a preheated oil bath held at 225°C. After 4 hours, the sparge rate is increased slightly, and the heating bath is removed. The slightly increased sparge rate should be fast enough to prevent air from being drawn into the flask during cooling. Once the flask has cooled to ambient temperature, the sample is removed and the APHA color number is measured as described above.

[0112] [Example 1] (Preparation of crude 2-ethylhexanoic acid) Crude 2-ethylhexanoic acid was produced according to step (a) of the present invention. Crude 2-ethylhexanoic acid was produced by continuous oxidation of 2-ethylhexanal with pure oxygen in the presence of 0.4 wt.% potassium ions in the reaction mixture at temperatures between 30 and 60°C and a pressure of 0.25 MPa abs in a technical plant with a production capacity of approximately 3.75 tons of 2-ethylhexanoic acid per hour. The technical plant was equipped with three reactors connected in series, and the oxygen feed was divided among the three reactors. 2-Ethylhexanal and potassium salt were added to the first reactor.

[0113] The crude 2-ethylhexanoic acid obtained at the outlet of the third reactor contained 92.4 wt.-% 2-ethylhexanoic acid and 281 wt.-ppm active oxygen. The APHA color index was measured to be greater than 1000. The molecular oxygen content at the outlet of the third reactor before decompression was 600 wt.-ppm. After cooling to ambient temperature and decompression to ambient pressure, the crude 2-ethylhexanoic acid still contained 210 wt.-ppm molecular oxygen. The measured values ​​are summarized in Table 1.

[0114] [Example 2] (Comparative Example) A 1 kg sample of crude 2-ethylhexanoic acid from Example 1 was distilled in a pilot-plant-sized batch distillation apparatus containing a 2 m column packed with mesh rings. The column was operated at a maximum pressure of 1 kPa abs, and the 2-ethylhexanoic acid was distilled overhead, collecting the fraction boiling at 102.5 ± 0.5 °C. The recovered 2-ethylhexanoic acid had a 2-ethylhexanoic acid content of 99.38 wt.-% and contained 11 wt.-ppm active oxygen when analyzed by gas chromatography. Its APHA color number was 2. The measurements are summarized in Table 1.

[0115] The distilled sample was then tempered under the conditions of the tempering test described above, and the APHA color number was measured. The APHA color number after the tempering test was 40. This high value indicates that the color stability of 2-ethylhexanoic acid treated according to the state of the art was not at all satisfactory.

[0116] [Example 3] (Example of the present invention) Another sample of 1 kg of crude 2-ethylhexanoic acid from Example 1 was first stripped with high-purity nitrogen (purity grade 5.0) at ambient temperature and pressure until it contained only 2 wt.-ppm molecular oxygen. This procedure relates to step (b) of the present invention. This sample was then distilled in a batch distillation apparatus similar to that used in Example 2, containing a 2 m column packed with mesh rings, specially cared for by carefully applying high-vacuum silicone grease to all glass joints to minimize air leakage into the column. The column was operated at a maximum pressure of 1 kPa abs, and 2-ethylhexanoic acid was distilled overhead, collecting the fraction boiling at 102.5 ± 0.5 °C. The recovered 2-ethylhexanoic acid had a 2-ethylhexanoic acid content of 99.99 wt.-% and contained only 2 wt.-ppm active oxygen when analyzed by gas chromatography. Its APHA color number was 0, and the active oxygen content was below the detection limit (<1 ppm). The measurements are summarized in Table 1.

[0117] The distilled sample was then tempered under the conditions of the tempering test described above, and the APHA color number was measured. The APHA color number after the tempering test was only 7, according to step (c). This low value indicates that the removal of molecular oxygen to 10 wt.-ppm or less in step (b) of the present invention enables the preparation of 2-ethylhexanoic acid with high color stability.

[0118] [Example 4] (Effect of molecular oxygen on APHA color number after tempering test) A 500 ml sample of purified 2-ethylhexanoic acid obtained in Example 3 was filled into a 1-liter flask and immersed in a tempering bath at 40°C without taking any precautions to exclude contact with air. Starting at time "0", a flow of 10 NL / h of air was bubbled through the sample via a glass frit. Samples were removed at regular intervals and analyzed for their content of active oxygen and molecular oxygen, as well as their APHA color number before and after the tempering test. The results are summarized in Table 2.

[0119] Because it took some time to transfer the sample from Example 3 to the experimental setup for Example 4, the molecular oxygen content rose from less than 1 wt.-ppm at the beginning of Example 4, indicated by time "0," to 14 wt.-ppm. Over time, the molecular oxygen content increased to 54 wt.-ppm after 5 hours (Sample No. 3). Along with this increase, the active oxygen content and the APHA color number after the tempering procedure also increased, while the APHA color number of the untempered sample remained at a value of 0. Because molecular oxygen itself cannot be measured by iodometric titration, the increase in the active oxygen content indicates that some of the molecular oxygen was converted to active oxygen. After 5 hours (Sample No. 3), the molecular oxygen content remained constant at 54 wt.-ppm, which is considered the saturated concentration of molecular oxygen in 2-ethylhexanoic acid. Nevertheless, the active oxygen content and the APHA color number after the tempering test still increased with the time air was bubbled through the sample. This indicates that molecular oxygen was continuously converted into active oxygen, which together with molecular oxygen was responsible for the high APHA color number after the tempering procedure.

[0120] [Example 5] (Effect of drastic reduction of oxygen molecules on APHA color number after tempering test) After Example 4, the air sparge was stopped after 24 hours (i.e., after sample number 5 was removed) and replaced with a 24-hour sparge of 10 NL / h nitrogen (purity grade 5.0). After 24 hours of nitrogen sparging, a sample designated sample number 6 was removed and analyzed as in Example 4.

[0121] Nitrogen sparging had little to no effect on the active oxygen content, which decreased only slightly from 31 wt.-ppm in Sample No. 5 to 28 wt.-ppm in Sample No. 6. However, the amount of molecular oxygen decreased significantly, from 64 wt.-% in Sample No. 5 to only 14 wt.-ppm in Sample No. 6, a reduction of about fourfold. Concomitantly, the APHA color number after tempering also decreased, from 112 in Sample No. 5 to 71 in Sample No. 6. This demonstrates that the APHA color number after tempering depends on both the molecular oxygen content and the active oxygen content. The significant decrease in the molecular oxygen content of Sample No. 6 also resulted in a decrease in the APHA color number after tempering.

[0122] [Table 1]

[0123] [Table 2]

Claims

1. 1. A process for preparing saturated aliphatic carboxylic acids having 6 to 12 carbon atoms by oxidation of the corresponding aldehyde with molecular oxygen, comprising: (a) converting a corresponding aldehyde with molecular oxygen at a temperature of 0-120°C and an oxygen partial pressure of 0.02-2 MPa to obtain a liquid mixture containing a saturated aliphatic carboxylic acid, a corresponding aldehyde in an amount of 2 mol-% or less based on the saturated aliphatic carboxylic acid, and molecular oxygen; (b) removing molecular oxygen from the liquid mixture obtained in step (a) to a content of 10 wt.-ppm or less based on the liquid mixture; (c) distilling the mixture obtained in step (b) in a distillation apparatus equipped with a purification column, from which a purified distillate containing at least 95 wt.-% saturated aliphatic carboxylic acids based on the distillate is obtained; A method comprising:

2. 2. The method of claim 1, wherein the saturated aliphatic carboxylic acid is 2-ethylhexanoic acid and the aldehyde is 2-ethylhexanal.

3. 3. The method according to claim 1 or 2, wherein the content of molecular oxygen in the liquid mixture obtained in step (a) is greater than 10 wt.-ppm and less than or equal to 1 wt.-% of the liquid mixture.

4. 4. The method according to claim 1, wherein in step (b) molecular oxygen is removed by stripping with an inert gas.

5. 5. The process according to claim 4, wherein the preparation of saturated aliphatic carboxylic acids is carried out continuously and the stripping is carried out countercurrently.

6. 6. The method according to claim 4, wherein the inert gas has an oxygen molecular content of 5 vol.-ppm or less.

7. 7. The method according to any one of claims 4 to 6, wherein in step (b) the removal of molecular oxygen is carried out at a temperature of 0 to 150°C and a pressure of 0.0001 to 10 MPa abs.

8. 8. The method according to any one of claims 1 to 7, wherein in step (b) molecular oxygen is removed in step (b) to 2 wt.-ppm or less.

9. 9. The method according to any one of claims 1 to 8, wherein in step (c) the purification column is operated at a pressure of 0.1 to 99 kPa abs and a temperature of 0 to 170°C.

10. 10. The method of claim 9, wherein the amount of molecular oxygen removed by the off-gas of the vacuum unit of the purification column is not more than twice the amount of molecular oxygen supplied to the distillation apparatus by the mixture obtained in step (b).

11. The molar ratio n(Ar off-gas) / n(O) in the off-gas of the vacuum unit of the purification column 2 off-gas) is between 0 and 0.0222, where n(Ar off-gas) is the amount of argon molecules in the off-gas corrected by the amount of argon molecules that may have been supplied to step (c) by the mixture obtained in step (b), and n(O 2 10. The method of claim 9, wherein the amount of oxygen in the off-gas is the amount of oxygen in the off-gas.

12. 12. The process of any one of claims 1 to 11, wherein in step (c) the feed to the purification column that is not a divided wall column is added at a lower point than the point at which the purified distillate is removed.

13. 13. The method according to any one of claims 1 to 12, wherein the purified distillate obtained in step (c) contains at least 99.5 wt.-% saturated aliphatic carboxylic acids.

14. 14. The process according to any one of claims 1 to 13, wherein the purified distillate has an APHA colour number of 0 to 10 after being tempered under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa for 4 hours.

15. 15. The method according to any one of claims 1 to 14, wherein the purified distillate obtained in step (c) has an active oxygen content of 0 to 100 wt.-ppm relative to the distillate.

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