A process for the preparation of saturated aliphatic C 6-12 carboxylic acids

Abstract

A process for preparing a color stable saturated aliphatic C 6‑12 carboxylic acid wherein (1) a corresponding aldehyde is oxidized with molecular oxygen to obtain a crude saturated aliphatic carboxylic acid in a liquid mixture, (2) the molecular oxygen is removed from the crude saturated aliphatic carboxylic acid mixture, and (3) the saturated aliphatic carboxylic acid is isolated as a color stable product from the molecular oxygen depleted mixture by distillation.

Description

[0001] This invention relates to a method for preparing saturated aliphatic carboxylic acids having 6-12 carbon atoms by oxidizing the corresponding aldehyde with molecular oxygen, wherein the saturated aliphatic carboxylic acid is obtained with high purity by tempering at 225°C and 0.1 MPa under an inert gas atmosphere for 4 hours with a very low tendency to darken.

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

[0003] Saturated aliphatic carboxylic acids are important intermediates with wide applications globally. They can be used directly, but are usually further processed into metal salts, esters, amides, acid anhydrides, acyl chlorides, and other derivatives. In general, they are important intermediates in the production of various compounds such as metal salts and metal soaps, flavorings, fragrances, pharmaceutical and agrochemical ingredients, cosmetic ingredients, plasticizers, paint and coating additives, coolants, lubricants, or catalysts for polymer processing. Saturated aliphatic C 6-12 A very important representative of carboxylic acids is 2-ethylhexanoic acid. It is primarily used in its derivative forms, such as its metal salts or esters, as a drying agent and thickener in alkyd resins and paints, as a catalyst in polyurethane foam manufacturing, as a PVC stabilizer and / or plasticizer, or as a wear-resistant agent and corrosion inhibitor in lubricants. Saturated aliphatic C 6-12 Esters of carboxylic acids, such as those of heptanoic acid, octanoic acid, 2-ethylhexanoic acid, nonanoic acid, or 3,5,5-trimethylhexanoic acid, are also frequently used as lubricants.

[0004] A widely used and important method for producing saturated aliphatic carboxylic acids with 6-12 carbon atoms is the oxidation of the corresponding aldehyde with molecular oxygen in the liquid phase, with or without a catalyst or any additives. This general synthetic route is described, for example, in J. Kubitschke et al., “Carboxylic acids, aliphatic,” 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 saturated aliphatic C atoms obtained by the above oxidation… 6-12 Carboxylic acids are then typically purified by distillation to obtain the preferred pure form of saturated aliphatic C. 6-12 carboxylic acid.

[0005] Since most applications require highly transparent and colorless products, it is necessary not only to have a chemical purity significantly higher than 99% by weight, but also to have a very low APHA color number.

[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 selective modifier, followed by distillation to obtain purified 2-ethylhexanoic acid, wherein potassium 2-ethylhexanoate is enriched in the bottom product and recycled to the oxidation stage. According to Example 1, the obtained 2-ethylhexanoic acid exhibits a low APHA color number of 4.

[0007] CN 109438216 discloses a multi-step method for preparing 2-ethylhexanoic acid, wherein n-butyraldehyde undergoes aldol condensation to obtain 2-ethyl-3-hexanal, which is then hydrogenated to 2-ethylhexanal. The inventors realized that after the hydrogenation step, a small amount of 2-ethyl-3-hexanal remains, which is oxidized together with 2-ethylhexanal in the oxidation step, resulting in crude 2-ethylhexanoic acid contaminated with 2-ethyl-3-hexenoic acid, which cannot be separated from 2-ethylhexanoic acid by distillation. CN 109438216 teaches the conversion of the remaining 2-ethyl-3-hexanal after hydrogenation to 2-ethyl-3-hydroxyhexanal in the presence of an acidic catalyst, which can be oxidized to 2-ethyl-3-hydroxyhexanoic acid in the oxidation step and separated from 2-ethylhexanoic acid by distillation. According to Examples 1-5, 2-ethylhexanoic acid with a purity of up to 99.91% by weight and a low APHA color number of up to 3 can be obtained.

[0008] According to the present invention, freshly distilled saturated aliphatic C 6-12 A low APHA color number in carboxylic acids does not guarantee that this low APHA color number will remain unchanged during prolonged storage, nor does it guarantee the color stability of the corresponding carboxylic acid used in the product. It is believed that saturated aliphatic C... 6-12 Carboxylic acids tend to darken over time when subjected to thermal stress during storage and / or in products where they are routinely used. Furthermore, according to the invention, this darkening can be caused by the presence of peroxides.

[0009] It is known from existing technology that aldehydes are initially oxidized with oxygen to form peroxy acids, which are then further oxidized to produce 2 moles of carboxylic acids per mole of the intermediate peroxy acid. This mechanism is described, for example, in JHTeles et al., “Oxidation,” Ullmann’s Encyclopedia of Industrial Chemistry, 2015, Wiley-VCH Verlag GmbH & Co. KGaA, DOI:10.1002 / 14356007.a18_261.pub2, Section 5.4.1 “Secondary reactions of radicals, peroxides, and other intermediates” and Section 2.2.8 “Carboxylic acids, saturated.” Therefore, it is clear that peroxy acids also serve as intermediates in the oxidation of the corresponding aldehyde to the corresponding saturated aliphatic C4 by oxygen. 6-12 In the process of carboxylic acid oxidation, peroxy acids are highly reactive molecules. Even small amounts of them may remain in the crude carboxylic acid product after oxidation and, if not properly separated, saturated aliphatic C... 6-12 Carboxylic acids can also lead to undesirable properties, such as poor color stability.

[0010] CN 108047027 relates to the decomposition of 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 was relatively low, it was still high enough to cause problems in the distillation column, as the peroxide could accumulate due to the boiling point difference between the peroxy acid and other components. Furthermore, this Chinese application describes the use of low-concentration homogeneous catalysts in the prior art to decompose peroxides. However, such homogeneous catalysts are very difficult to separate and also pose a risk of slagging, clogging, and explosion in the distillation column. To avoid such problems, this Chinese application teaches the heterogeneous catalytic decomposition of peroxides on a metal-organic framework catalyst before they enter the distillation column. It is crucially taught that the peroxides are decomposed only at low temperatures of 20-70°C, otherwise side reactions such as decarboxylation will occur and reduce the product and purity. Furthermore, it is emphasized that the decomposition is very rapid and can therefore be completed in 5-40 hours. -1 This is conducted at high altitudes, involving short stays of 1.5-12 minutes.

[0011] However, the use of metal-organic framework catalysts as peroxide decomposition catalysts is generally disadvantageous. First, these metal-organic framework catalysts are very complex to produce. Second, the organic molecules that form the framework are readily oxidized, especially in the presence of peroxy acids and the reactive groups generated during their decomposition. In the presence of carboxylic acids, metal-organic frameworks are known to lose activity due to leaching and / or the framework metals. These leached metals subsequently cause the same problems in the distillation column as metals used as homogeneous catalysts. Furthermore, all these factors contribute to the short lifespan of metal-organic framework catalysts and thus add further complexity to the process by disposing of spent catalysts and providing new catalysts.

[0012] PCT application number PCT / EP2020 / 087,952 (based on priority of EP application number 20150845.4) recognizes saturated aliphatic C 3-5 C in crude carboxylic acid products 3-5 The peroxyacids remaining after aldehyde oxidation can be decomposed primarily through heat treatment before distillation purification. Distilled saturated aliphatic C 3-5 Carboxylic acids show very low levels of reactive oxygen species, which in turn indicates very low levels of peroxides such as peroxy acids. The content of reactive oxygen species is a quantitative measure of the amount of reactive oxygen species capable of oxidizing easily oxidized compounds, such as salts of iodides (1-) to iodine (0) or salts of iron (II) to iron (III).

[0013] According to the present invention, the method proposed in European Patent Application No. 20150845.4 for oxidizing the corresponding C 3-5 Aldehyde production saturated aliphatic C 3-5 This heat treatment of carboxylic acids does not oxidize the corresponding C 6-12 Aldehydes successfully produced saturated aliphatic C 6-12 Carboxylic acids, at least not within a reasonable timeframe of just a few hours. Although possible C 6-12 Peroxyacids may decompose through this heat treatment, and distilled saturated aliphatic C 6-12 Carboxylic acids exhibit low APHA color numbers, but their reactive oxygen species content remains high, and they tend to darken over time when subjected to thermal stress during storage and in products where they are routinely used.

[0014] Therefore, the object of the present invention is to find a method for preparing saturated aliphatic carboxylic acids having 6-12 carbon atoms by oxidizing the corresponding aldehyde with oxygen, which can produce the corresponding saturated aliphatic carboxylic acids in high yield and high purity, particularly that during storage, when the product is subjected to thermal stress and / or after its conventional application (e.g., as an additive in polymer preparation), it does not tend, or at least has a very low tendency, to darken over time. The method should also be easy to operate, safe in performance, and stable under long-term operation, thereby producing the saturated aliphatic carboxylic acid with a constant high quality.

[0015] We have surprisingly discovered a method for preparing saturated aliphatic carboxylic acids with 6-12 carbon atoms by oxidizing the corresponding aldehydes with molecular oxygen, including:

[0016] (a) A liquid mixture containing the saturated aliphatic carboxylic acid and the corresponding aldehyde and molecular oxygen by converting the corresponding aldehyde with molecular oxygen at a temperature of 0-120°C and an oxygen partial pressure of 0.02-2 MPa.

[0017] (b) Remove molecular oxygen from the liquid mixture obtained in step (a) to a concentration ≤10 ppm by weight based on the liquid mixture; and

[0018] (c) The mixture obtained in step (b) is distilled in a distillation apparatus containing a purification column and a purified distillate is taken out based on the presence of ≥95% by weight of the saturated aliphatic carboxylic acid in the distillate.

[0019] In the following text, it will be referred to as saturated aliphatic C. 6-12 Carboxylic acids, and the discovered production methods are highly suitable for their production. Saturated aliphatic carboxylic acids having 6-12 carbon atoms can be straight-chain or branched, and substituted or unsubstituted. Substituted saturated aliphatic C 6-12 Carboxylic acids contain one or more heteroatoms in addition to carbon and hydrogen; halogens are mentioned as examples. Unsubstituted saturated aliphatic C atoms... 6-12 Carboxylic acids are preferred. Preferred examples, classified by the number of carbon atoms, are:

[0020] For C6: hexanoic acid, 2-methylvaleric acid, 3-methylvaleric acid, 4-methylvaleric acid, 2,3-dimethylbutyric acid and 3,3-dimethylbutyric acid;

[0021] For C7: heptanoic acid and 2-methylhexanoic acid;

[0022] For C8: octanoic acid, 2-methylheptanoic acid, 2-ethylhexanoic acid, 2-ethyl-4-methylvaleric acid and 2-propylvaleric acid;

[0023] For C9: nonanoic acid and 3,5,5-trimethylhexanoic acid;

[0024] For C 10 : Decanoic acid, 2-propylheptanoic acid and 2-propyl-4-methylhexanoic acid;

[0025] For C 11 Undecanoic acid and 2-methyldecanoic acid; and

[0026] For C 12 Dodecanoic acid and 2-Butyloctanoic acid.

[0027] Of the above list, hexanoic acid, 2-methylvaleric 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. Saturated aliphatic C is particularly preferred. 8-12 Carboxylic acids, including octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid, 2-propylheptanoic acid, and dodecanoic acid. 2-Ethylhexanoic acid is particularly preferred.

[0028] In the first step of the method of the invention, referred to as step (a), the corresponding aldehyde is oxidized by oxygen. Regarding the more preferred saturated aliphatic C... 6-12 Carboxylic acids, these aldehydes are n-hexanoal for hexanoic acid, 2-methylpentanal for 2-methylpentanal for heptanoic acid, n-heptanal for octanoic acid, n-octanal for 2-ethylhexanoic acid, n-nonanal for nonanoic acid, 3,5,5-trimethylhexanoal for 3,5,5-trimethylhexanoic acid, n-decanal for decanoic acid, 2-propylheptanal for 2-propylheptanic acid, and n-dodecanoal for dodecanoic acid.

[0029] C 6-12 Aldehydes can generally be readily prepared by a variety of methods, depending on the availability of the primary product. A typical method involves the hydroformylation of the corresponding alkene having one less carbon atom than the desired aldehyde. For example, the hydroformylation of 1-pentene to n-hexanal is mentioned. Another typical method involves the condensation of an aldol to an enal as an intermediate, followed by hydrogenation to the desired aldehyde. For example, the aldol condensation of n-butyraldehyde to 2-ethyl-2-hexanal and its subsequent hydrogenation to 2-ethylhexanal are mentioned. Last but not least, a third typical method involves the dehydrogenation of the corresponding alcohol. For this method, the dehydrogenation of 1-decanol to n-decanal is taken as an example.

[0030] The aldehyde to be oxidized can be used in diluted or purified form. If the aldehyde is used in diluted form, the diluent should preferably be a compound that is inert to oxidation with oxygen, stable to the produced carboxylic acid, and easily separated from the carboxylic acid by distillation. If the corresponding carboxylic acid is used as the diluent, the aforementioned necessity of separating the diluent from the carboxylic acid can be avoided. However, since the diluent increases the reaction volume and thus reduces the space-time yield, and if other diluents besides the corresponding carboxylic acid are used, additional contamination of the desired carboxylic acid may occur, it is preferable not to intentionally dilute the aldehyde. Preferably, the aldehyde is used as a highly concentrated compound having an aldehyde content of preferably 80-100% by weight, more preferably 80-100% by weight, particularly preferably 95-100% by weight, and very particularly preferably 99-100% by weight.

[0031] Besides preparing only one specific saturated aliphatic carboxylic acid, the method of the present invention can also be used to prepare saturated aliphatic C 6-12A mixture of carboxylic acids. When preparing such a mixture, saturated aliphatic C atoms having the same number of carbon atoms are particularly preferred. 6-12 A mixture of carboxylic acids. However, it is generally preferred to prepare an unmixed saturated aliphatic C. 6-12 carboxylic acid.

[0032] 2-Ethylhexanoic acid is particularly preferred to be prepared by oxidizing 2-ethylhexanal.

[0033] The oxidation of this aldehyde is carried out using molecular oxygen. It can be used in pure form or diluted with other gases, such as air, an O2 / N2 mixture, or a mixture with other inert gases.

[0034] The oxidation reaction can be carried out with or without an oxidation catalyst and / or with or without selective modifiers. If an oxidation catalyst is used, it is a homogeneous catalyst. Examples of homogeneous oxidation catalysts include salts of transition metals in Groups 6-11 of the periodic table, preferably salts from the first row of these groups, and most preferably Mn, Fe, or Co salts. If selective modifiers are used, they are also homogeneous. Examples of selective modifiers include salts of alkali metals, alkaline earth metals, and transition metals in Group 12 of the periodic table, preferably salts of Na, K, Mg, Ca, Zn, or Cd, most preferably salts of K or Na, and particularly preferably salts of K. These salts can be selected from any salt soluble in the reaction mixture, but carboxylates, hydroxides, carbonates, and bicarbonates are preferred. The concentration of the metal in the homogeneous oxidation catalyst can vary over a wide range, but a typical concentration is 0.0001-0.1% by weight based on the reaction mixture. The concentration of the homogeneous selectivity-enhancing metal can also vary over a wide range, but based on the reaction mixture, a metal content of 0.01-5% by weight, preferably ≥0.02% by weight, more preferably ≥0.05% by weight, preferably ≤2% by weight, more preferably ≤1% by weight, and particularly preferably ≤0.5% by weight is typical. Homogeneous catalyst metal and homogeneous selectivity-enhancing metal can also be used simultaneously.

[0035] Depending on the nature of the aldehyde, the presence of an oxidation catalyst, and particularly the presence of a selective modifier metal, will affect the range and type of byproducts. For example, in the absence of a selective modifier metal, α-branched aldehydes such as 2-ethylhexanal tend to produce more formate esters as byproducts. However, in the presence of a selective modifier metal, especially in the presence of sodium or potassium salts, and most particularly in the presence of potassium salts, α-branched aldehydes produce far fewer unwanted formate esters. Therefore, it is preferable to oxidize α-branched aldehydes in the presence of a selective modifier metal, preferably in the presence of sodium or potassium salts, and particularly preferably in the presence of potassium salts. On the other hand, linear aldehydes such as n-hexanal or n-decanal have already formed only trace amounts of formate esters in the absence of a selective modifier metal, so the addition of a selective modifier metal is irrelevant or only slightly relevant to selectivity. Therefore, it is preferable to oxidize linear aldehydes in the absence of a selective modifier metal.

[0036] For homogeneous catalytic metals, they increase the reaction rate, but they have an adverse effect on selectivity. Therefore, oxidation is preferably carried out in the absence of added homogeneous catalytic metals.

[0037] Regardless of the presence of an oxidation catalyst, the oxidation reaction is carried out at a temperature of 0-120°C and an oxygen partial pressure of 0.02-2 MPa. Preferably, it is carried out at a temperature of ≥10°C, more preferably ≥20°C, particularly preferably ≥30°C and preferably ≤100°C, more preferably ≤80°C, particularly preferably ≤60°C. For the oxygen partial pressure, it is preferably carried out at an oxygen partial pressure of ≥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 readily determined by measuring the total pressure and multiplying it by the O2 concentration, measured in volume % by any suitable method known in the art.

[0038] Although the partial pressure of oxygen can vary over a wide range based on the oxygen content of the oxygen source, the total pressure in step (a) is typically in the range of 0.01-5 MPa abs. The oxidation reaction is preferably carried out at a total pressure of ≥0.1 MPa abs, more preferably ≥0.2 MPa abs. It is more preferably carried out at a total pressure of ≤4 MPa abs, more preferably ≤3 MPa abs.

[0039] Under these conditions, the aldehyde is almost entirely in the liquid phase and the oxidation reaction also takes place in the liquid phase.

[0040] The oxidation of the aldehyde in step (a) is typically carried out in a batch, semi-continuous, or continuous manner in a reaction apparatus. In continuous operation, the aldehyde and oxygen are continuously fed into the reaction apparatus and a sufficient stream of the reaction mixture is continuously removed. The process conditions for continuous operation—including residence time—are selected in a manner that achieves the desired conversion. In batch operation, the reaction apparatus is loaded, and the added aldehyde and oxygen are replenished if necessary. After the desired conversion is achieved, the mixture is removed from the reaction apparatus. Semi-continuous operation is characterized by the addition of the aldehyde and oxygen together or intermittently to the reaction apparatus over a specific period of time, while the oxidation reaction is already underway. After a period of time, for example, if the reaction apparatus is more or less full, the feeding is stopped, and the mixture is removed from the reaction apparatus after the desired conversion is achieved.

[0041] The preferred operations in step (a) are batch operations and continuous operations, with continuous operations being particularly preferred.

[0042] The reaction apparatus for the aldehyde oxidation in step (a) may include one or more reaction devices. Suitable reaction devices generally include those suitable for gas-liquid exothermic reactions and capable of discontinuous, semi-continuous, or continuous operation. For discontinuous methods, stirred autoclaves or autoclaves with jet circulation mixing are suitable, for example. For semi-continuous methods, stirred vessels, trickle bed reactors, and bubble cap reactors are mentioned as possible examples. For continuous methods, stirred vessels, trickle bed reactors, bubble cap reactors, jet circulation reactors, and cascades of the above reactors are mentioned as suitable examples. Preferred examples of suitable reactors are described in more detail in WO 2009 / 024,446 and WO 2009 / 024,549. If reactor cascading is used, as in continuous methods, for example, 2-5, preferably 2-4, and particularly preferably 2-3 reactors are connected in series.

[0043] It is preferable to use reaction equipment that allows for thorough gas-liquid mixing and good distribution of oxygen in the liquid reaction mixture.

[0044] Because this oxidation generates a significant amount of heat, heat removal from the reaction zone is necessary. Depending on the concentrations of the aldehyde and oxygen supplied to the reactor, in continuous processes, heat removal using only the reaction mixture and control of the reactor temperature by adding fresh aldehyde at a low temperature are sufficient. However, for higher concentrations of aldehyde and oxygen content at or above the air level, cooling of the reaction liquid in the reactor is generally required. This cooling can be achieved, for example, through an externally cooled outer wall of the reactor, through cooling pipes within the reactor through which the coolant flows, or through an external heat exchanger in an external loop.

[0045] Based on the overall chemical equation:

[0046]

[0047] Where R represents C 5-11 Aldehydes and C 5-11 C of carboxylic acids 5-11 The oxidation of the aldehyde to the carboxylic acid requires 0.5 mol of oxygen (O2) in stoichiometric amounts. Although the oxidation can be carried out under oxygen-deficient conditions—resulting in partial conversion and residual aldehyde in the reaction mixture—it is preferable to apply oxygen in stoichiometric or superstoichiometric amounts. To ensure sufficient conversion on the one hand and limit gas load on the other, the oxidation reaction is preferably carried out at an oxygen / aldehyde molar ratio of 0.5–1. More preferably, it is carried out at an oxygen / aldehyde molar ratio of ≥0.51, particularly preferably ≥0.52, and more preferably ≤0.7, particularly preferably ≤0.6, and very particularly preferably ≤0.58.

[0048] It has been shown that the use of reactor cascades is particularly advantageous for continuous oxidation processes because it allows for the gradual addition of oxygen. A significant advantage of this gradual oxygen addition is better control of the heat of reaction, especially the smaller gas proportions in the corresponding cascade stages. Therefore, the use of a reactor cascade of 2-3 reactors is particularly preferred, whereby approximately 70-95% of the total amount of the aldehyde and all of the total oxygen is preferably fed into the first reactor, and the remaining 5-30% of the oxygen is either fed entirely into the second reactor or further divided into two portions fed into the second and third reactors, wherein the proportion in the third reactor is preferably smaller.

[0049] Even with stoichiometric or superstoichiometric application of oxygen, achieving near 100% aldehyde conversion requires a very long time. This would unnecessarily clog the reaction apparatus or make it very large. Therefore, it is advantageous to convert the aldehyde in step (a) until a residual aldehyde content of ≤2 mol% relative to the saturated aliphatic carboxylic acid is achieved. Depending on the nature of the aldehyde, the concentration of the oxygen-containing gas supplied to the reaction apparatus, and the process conditions, a residual aldehyde content of ≤2 mol% relative to the saturated aliphatic carboxylic acid is typically achieved after a reaction time of 0.1–5 hours.

[0050] The composition of the reaction mixture in step (a) can usually be determined by gas chromatography in terms of the content of the saturated aliphatic carboxylic acid and the corresponding aldehyde.

[0051] The mixture obtained in step (a) preferably contains ≤1.5 mol%, more preferably ≤1 mol%, particularly preferably ≤0.5 mol%, very particularly preferably ≤0.3 mol%, and preferably ≥0.05 mol%, particularly preferably ≥0.1 mol%, of the corresponding aldehyde relative to the saturated aliphatic carboxylic acid.

[0052] Regarding the reaction time, a time of ≥0.2 hours is preferred, more preferably ≥0.3 hours, and especially preferably ≥0.5 hours. Furthermore, a time of ≤8 hours is preferred, more preferably ≤4 hours, and particularly preferably ≤3 hours.

[0053] After obtaining a mixture containing a saturated aliphatic carboxylic acid and a corresponding aldehyde of ≤2 mol% relative to the aforementioned saturated aliphatic carboxylic acid in step (a), it is preferable to then separate the reaction liquid from the remaining oxygen-containing phase. In a batch process, this can be achieved, for example, by simply discharging the oxygen-containing phase, while in a continuous process, it can be achieved, for example, by simply removing the liquid reaction mixture from the reaction apparatus.

[0054] Regarding the possible byproducts of the separated reaction liquids, it is known from the prior art that peroxyacid is initially formed during the oxidation of aldehyde with oxygen, followed by further oxidation of the aldehyde to produce 2 moles of carboxylic acid / moles of the intermediate peroxyacid. The reaction steps are as follows.

[0055]

[0056] Where R represents C 5-11 Group. It is expected that such acids will also be used in the preparation of saturated aliphatic C groups. 6-12 The aldehyde is formed in the oxidation step (a) of the carboxylic acid, and although the added aldehyde is not usually completely converted, it is expected to be retained to some extent in the liquid reaction mixture. Peroxy acids have high oxidation potentials and, if not properly separated, can cause the carboxylic acid to darken over time during storage when the product is subjected to thermal stress and / or in products where they are used conventionally. However, peroxy acids, along with other components having high oxidation potentials, are readily characterized as so-called “reactive oxygen species,” which is a quantitative measure of the amount of reactive oxygen. The term “reactive oxygen species” is known and used in the prior art and is described, for example, in A. Uhl et al., “Organic Peroxy Compounds,” 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 reactive oxygen species in a sample is typically determined by adding a specified amount of an easily oxidizable compound, such as iodine salt (1-) or iron salt (II), to a specified amount of the sample. The present reactive oxygen species oxidize the oxidizable compound, and the amount of oxidized oxidizable compound is then determined by titration.

[0057] Because the boiling points of peroxy acids typically do not deviate significantly from those of their corresponding acids, they are usually not easily separated from them. Furthermore, based on the fact that peroxy acids, as impurities in distilled acids, cause darkening over time during storage under thermal stress and in products containing acids in conventional applications, it was initially assumed that such peroxy acids also contribute to the darkening of saturated aliphatic C... 6-12 The reason for the darkening of carboxylic acids. Based on European patent application number 20150845.4 regarding the preparation of saturated aliphatic C. 3-5 The teachings of carboxylic acids, which have already been mentioned in the introductory summary of the prior art, anticipate that peroxy acids are readily decomposed by heat treatment, which is taught to be carried out prior to distillation purification. However, according to the present invention, it is recognized that such heat treatment is suitable for saturated aliphatic C... 6-12 The preparation of carboxylic acids was unsuccessful, at least within a reasonable timeframe of only a few hours. First, the saturated aliphatic C20 prepared according to step (a) was... 6-12 The reactive oxygen content in the crude carboxylic acid product is reduced only slightly by this heat treatment. Secondly, although possible C 6-12 Peroxyacids may decompose and distill saturated aliphatic Cs due to this heat treatment. 6-12 Carboxylic acids exhibit low APHA color numbers, but their reactive oxygen species content remains high, and they tend to darken over time during storage, when subjected to thermal stress, and in products where they are routinely used.

[0058] Surprisingly, it was found that in the subsequent distillation of the heat-treated product and the distillation of the untreated product, most of the components that cause the high reactive oxygen species content were enriched in the bottom fraction, and only in the distilled saturated aliphatic C4 fraction. 6-12 Small but still noteworthy fractions were found in carboxylic acids. These two findings led to important conclusions regarding saturated aliphatic C... 6-12 The reactive oxygen species content in crude carboxylic acid products is mainly caused by components other than peroxy acids. Further research into the properties of these other components reveals that they are primarily alkyl hydroperoxides with the general formula (4):

[0059]

[0060] Where R 1 C represents a carbon atom with a COOH group. 1-11 Group, R 2 C represents 1-10 Group or H, R 3 C represents 1-5 Group or H, while R 1 R 2 and R 3 The sum of carbon atoms in the aldehyde is 5-11, depending on the type of aldehyde being applied. The hydroperoxy group can be located at a saturated aliphatic carbon atom. 6-12In carboxylic acids, any carbon atom, whether primary, secondary, or tertiary, is susceptible to peroxidation, except in the COOH group. However, tertiary carbon atoms and carbon atoms at the α-position of the COOH group are particularly prone to peroxidation.

[0061] Alkyl hydroperoxides, when stored under thermal stress and / or in products where they are routinely used, can cause carboxylic acids to darken over time if not properly separated.

[0062] Alkyl hydroperoxides of saturated aliphatic carboxylic acids are known to have significantly higher boiling points than their corresponding saturated aliphatic carboxylic acids. The table below shows a comparison of the boiling points of some 2-hydroperoxy acids with their corresponding acids. Their boiling points at atmospheric pressure have been estimated using SciFinder, an electronic database of chemistry and bibliographic information provided by the American Chemical Society.

[0063] For 2-methylvaleric acid, the temperature is 195±8℃;

[0064] For 2-methyl-2-hydroperoxyvalerate, the temperature is 295±23℃;

[0065] The temperature for caprylic acid is 239±3℃;

[0066] For 2-hydroperoxyoctanoic acid, the temperature is 327±25℃;

[0067] For 2-ethyldecanoic acid, the temperature is 306±10℃; and

[0068] The temperature for 2-ethyl-2-hydroperoxydecanoic acid is 373±25℃.

[0069] This significant difference in boiling points should facilitate separation by distillation. The high reactive oxygen content in the bottom fraction found in the aforementioned investigation is also strong evidence. However, based on this, what is even more surprising is the presence of reactive oxygen species in the distilled saturated aliphatic C... 6-12 Small, but still noteworthy, reactive oxygen species can still be found in carboxylic acids. Due to their high boiling points, slippage of the corresponding hydroperoxyacids during distillation is highly unlikely.

[0070] To obtain a more accurate understanding of the reactive oxygen species (ROS) content, the measurement method is described in more detail. According to the present invention, the determination of ROS is preferably performed by the oxidation of iodide (1-). This analytical method is known as iodometry and is well-known to those skilled in the art. However, it is also roughly explained below:

[0071] In iodometric titration, a predetermined amount of potassium iodide in an aqueous solution of acetic acid is added to a predetermined amount of sample at room temperature and stirred to oxidize the iodide (1-) to elemental iodine. The amount of potassium iodide added is related to the expected amount of reactive oxygen species and can be estimated by preliminary measures. For this iodometric measurement, the amount of iodide (1-) added must be slightly higher than the amount oxidized to elemental iodine. The elemental iodine is then titrated with sodium thiosulfate to determine the amount of elemental iodine formed by the preceding oxidation. Starch is commonly used as an indicator, which is purple in the presence of elemental iodine and becomes colorless when all elemental iodine is 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 (1-) can be calculated. According to the formal equation:

[0072] 2I - +2H + +"O"→I2+H2O (5)

[0073] And more detailed equations for peroxy acids and alkyl hydroperoxides:

[0074]

[0075] Two moles of iodide (1-) react with one mole of reactive oxygen atom in the presence of acetic acid. The reactive oxygen atom is represented by “O” in formula (5) and is part of the peroxy 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, R… 1 R 2 and R 3 They have the meanings described for equations (2) / (3) and (4), respectively. The reactive oxygen content of the sample is the weight fraction of reactive oxygen atoms based on the weight of the sample and is expressed as % or ppm by weight.

[0076] The content of reactive oxygen species can be readily converted into the equivalent content of peroxide compounds, assuming that reactive oxygen species are bound only as hydroperoxides in saturated aliphatic C464 compounds. 6-12 In carboxylic acids, the conversion can be made by multiplying the measured reactive oxygen species content by the ratio of the molar mass of hydroperoxy acid to the molar mass of oxygen atoms. For example, for 2-ethylhexanoic acid, the multiplication factor is 176.2 / 16.0 = 11.0125.

[0077] For completeness, it should also be mentioned that the amount of reactive oxygen species in carboxylic acid-containing samples can also be largely determined by physical methods such as... 13 C-NMR determination.

[0078] However, in this invention, active oxygen should be 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.

[0079] The active oxygen content of the mixture obtained in step (a) is typically 0.02-1% by weight, preferably ≥0.03% by weight, more preferably ≥0.05% by weight, and preferably ≤0.8% by weight, more preferably ≤0.5% by weight.

[0080] In the study of saturated aliphatic C 6-12 Further investigation into the surprising behavior of mixtures of crude carboxylic acid products revealed that the presence of molecular oxygen in the mixture primarily leads to the distillation of saturated aliphatic C... 6-12 Carboxylic acids contain a significant amount of reactive oxygen species.

[0081] In oxidation step (a), a portion of molecular oxygen, depending on its partial pressure, the temperature of the liquid mixture, and the chemical composition of the liquid mixture, remains unreacted as physically dissolved molecular oxygen in the liquid mixture. Its concentration in the liquid mixture obtained in step (a) is typically >10 wt ppm to ≤1 wt%, based on the liquid mixture. A concentration of ≤10 wt ppm based on the liquid mixture is generally unattainable because a small amount of molecular oxygen remains unreacted in the liquid mixture, and a concentration of >1 wt% is practically impossible due to its limited solubility. Since the above-mentioned concentration of molecular oxygen is related to the liquid mixture obtained in step (a), this value refers to the reaction conditions at the end of the conversion before the liquid mixture leaves the reaction apparatus. Therefore, the measurement of the molecular oxygen concentration in the liquid mixture is preferably performed under the above conditions immediately before the liquid mixture leaves the reaction apparatus. However, it can also be measured alternatively at lower pressures, such as atmospheric pressure, with the measured value corrected for accepted laws (e.g., pressure-related Henry's law). The concentration of molecular oxygen in the liquid mixture obtained in step (a) is preferably ≥20 wt ppm, more preferably ≥50 wt ppm. Regarding its upper limit, it is generally ≤1% by weight (or ≤10,000 ppm by weight, expressed on a ppm scale), preferably ≤5,000 ppm by weight, more preferably ≤2,500 ppm by weight, particularly preferably ≤1,000 ppm by weight, and very particularly preferably ≤750 ppm by weight.

[0082] The concentration of molecular oxygen in the liquid mixture obtained in step (a) can be readily determined. Useful devices include, for example, optical sensors commonly used in water analysis. As a possible example, optical fluorescence sensors are mentioned. Optical fluorescence sensors are prior art, and those skilled in the art know how to calibrate and use them. These sensors are highly specific and sensitive to oxygen. They can even measure extremely low concentrations as low as 0.01 ppm by weight. Because such optical fluorescence sensors are to some extent heat and pressure resistant, they can also be used for online measurements.

[0083] This explains the liquid saturated aliphatic C obtained through step (a). 6-12The crude carboxylic acid product mixture contains hydroperoxides and at most trace amounts of the corresponding byproducts, as well as unconverted dissolved molecular oxygen. It is worth noting that the term "reactive oxygen species" does not include molecular oxygen. This is because molecular oxygen reacts very slowly under the conditions used to determine reactive oxygen species in samples containing hydroperoxides. Furthermore, it is preferable to perform the determination under an inert gas atmosphere to minimize interference from molecular oxygen. The advantage of this is that reactive oxygen species and molecular oxygen can be determined and evaluated separately.

[0084] Based on all the above surprising findings, it was subsequently discovered that during storage, when subjected to thermal stress and / or in products where they are routinely used, darkening of carboxylic acids can be avoided or at least significantly reduced if the liquid saturated aliphatic C obtained in step (a) is applied in a subsequent step, referred to as step (b). 6-12 Molecular oxygen present in the crude carboxylic acid product mixture was removed to a content of ≤10 ppm by weight based on the liquid mixture, followed by distillation to separate saturated aliphatic C. 6-12 Before carboxylic acids. For completeness, it is mentioned that there may be other steps between steps (a) and (b), including, for example, lowering or raising the temperature or pressure, or removing low-boiling substances by distillation.

[0085] There are several possibilities for removing molecular oxygen from a liquid crude product mixture to a content of ≤10 ppm by weight. One possibility is to allow the mixture to be given a long time, allowing the dissolved molecular oxygen to react with the remaining aldehydes. This can be done, for example, in a pipe through which the mixture flows and the desired residence time can be achieved, or in one or more residence time vessels connected in series. Although such a procedure does not require the addition of more compounds or the adjustment of specific conditions (e.g., increased temperature), it is not the preferred method because it typically requires well over 10 hours, and even more than 50 or even more than 100 hours.

[0086] As a preferred possibility for removing molecular oxygen to a content ≤10 ppm by weight, the liquid crude product mixture obtained in step (a) is mentioned for stripping with an inert gas.

[0087] Essentially, stripping can be carried out in a container, also known as a stripper, in which the liquid crude product mixture can be contacted with an inert gas to transfer dissolved molecular oxygen to the gas phase of the inert gas and remove it together with the gas phase. For continuous preparation methods, the liquid crude product mixture obtained in step (a) is continuously fed into a stripper, in which the inert gas passes through the mixture. In principle, such stripping columns can have different shapes, so in principle any kind of container known in the art for contacting the gas with the liquid phase can be used, but so-called stripping columns are preferred. A stripping column is characterized by a length-to-average diameter ratio >1, preferably ≥5, more preferably ≥8, and preferably ≤15. While the inert gas is fed into the lower region of the stripping column to bubble to the upper region, the liquid crude product mixture can be fed into the lower region, resulting in co-current flow, or into the upper region, resulting in counter-current flow. Preferably, the stripping in step (b) is carried out continuously to prepare saturated aliphatic C under counter-current flow. 6-12 Carboxylic acids. Whether stripping is carried out in co-current or counter-current flow, it is beneficial for improving the contact between the gas and liquid phases. This can be easily achieved using random or structured packing or trays within the stripping column. Such internals for promoting gas / liquid contact are well known to those skilled in the art and can be readily selected.

[0088] Alternatively, molecular oxygen can be removed by injecting a mixture of liquid crude products into the top of a device with free space, such as a vertical column, where an inert gas is supplied countercurrently from the bottom.

[0089] For discontinuous or semi-continuous preparation methods, the liquid crude product mixture can, for example, be maintained in a reactor where the aldehyde conversion in step (a) occurs and an inert gas is passed through the resulting liquid crude product mixture, preferably after the gas phase of the oxidizing gas containing molecular oxygen has been vented. The liquid crude product mixture can also be transferred from the reactor of step (a) to another vessel, also called a stripper, in which an inert gas passes through the mixture. For such a stripper, the above description of continuous preparation methods applies in a similar manner, except that the liquid crude product mixture is typically a stationary phase.

[0090] Inert gases suitable for stripping are substances that are gaseous under stripping conditions, do not react with the liquid crude product mixture, and are substantially free of molecular oxygen. Although the permissible concentration of molecular oxygen in the inert gas may depend on the saturated aliphatic C4O4... 6-12The properties of carboxylic acids and the conditions for stripping the liquid crude product mixture are considered, but the inert gas preferably has a molecular oxygen content of ≤5 ppm by volume, more preferably ≤2 ppm by volume, and particularly preferably ≤1 ppm by volume. For example, a small amount of molecular oxygen may be present in the inert gas as an impurity from their preparation or purification. Possible inert gases include nitrogen, hydrogen, carbon dioxide, carbon monoxide, nitrous oxide, and rare gases such as helium, neon, argon, and krypton. Inert gases can be provided in pure form and in mixtures of two or more different inert gases. Nitrogen is preferred due to its availability.

[0091] The stripping in step (b) can be carried out over a wide range of temperature and pressure. Preferably, it is carried out at a temperature of 0-150°C and a pressure of 0.0001-10 MPa abs. More preferably, it 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 into the distillation apparatus in step (c). Stripping is particularly preferably carried out at ≥25°C, particularly preferably ≤100°C, and very particularly preferably ≤60°C, which is suitable for liquid saturated aliphatic C 6-12 Mixtures of crude carboxylic acid products are particularly advantageous because undesirable reactions between molecular oxygen and the liquid mixture, especially between already formed carboxylic acids, are avoided or at least minimized. Regarding pressure, it is more preferable to carry out the stripping at the pressure at which the liquid crude product mixture leaves the reactor apparatus in step (a) and at the pressure range where the stripped mixture is fed into the distillation apparatus in step (c). The wide pressure range described above demonstrates considerable flexibility in pressure selection. Depending on the facilities and infrastructure available at the plant site, the advantages of stripping under vacuum conditions or at atmospheric pressure and higher pressures may prevail. Stripping is generally preferred under vacuum conditions of 0.0001 to <0.1 MPa abs, where sufficient vacuum power is available, because the required amount of stripping gas is low and the stripping efficiency is high. Alternatively, stripping at ≥0.1 MPa abs has the advantage of not requiring vacuum power. However, stripping at atmospheric pressure or higher pressures is preferably carried out at ≤0.2 MPa abs, because the volume of inert gas is still high enough to advantageously bubble through the liquid mixture.

[0092] The amount of inert gas used to remove a specific amount of molecular oxygen by stripping can vary over a wide range. Surprisingly, stripping has been found to be highly efficient, and a low molar amount of inert gas per molar amount of molecular oxygen to be removed is sufficient. Therefore, the stripping in step (b) is preferably carried out at a molar ratio of inert gas fed into the stripper to molecular oxygen to be removed ≥ 0.25 mol / mol. However, the molar ratio of inert gas fed into the stripping column to molecular oxygen to be removed is preferably ≤ 1 mol / mol, more preferably ≤ 4 mol / mol. The amount of inert gas used per unit time should be sufficiently low to not exceed the flooding point of the stripping column. Based on the geometry of the stripping column, the amount of liquid crude product mixture fed into the stripping column in continuous operation, process conditions such as temperature and pressure, and the physical properties of the mixture, those skilled in the art can calculate or experimentally determine the maximum permissible amount of inert gas per unit time under each condition.

[0093] Based on the low molar ratio between the inert gas supplied to the stripping tower and the molecular oxygen to be removed, stripping in a continuously operating stripping tower can typically be carried out within 1-10 minutes, preferably ≥2 minutes, and more preferably ≤5 minutes. In batch operations, where the liquid crude product mixture is already placed in the equipment and then stripped with an inert gas, stripping typically requires more time. For example, stripping in a batch bubble cap tower can often take several minutes to even several hours to achieve the desired molecular oxygen removal.

[0094] By means of the present invention, molecular oxygen is removed from the liquid mixture obtained in step (a), and the content of molecular oxygen is reduced to ≤10 ppm by weight, preferably ≤5 ppm by weight, more preferably ≤2 ppm by weight, particularly preferably ≤1 ppm by weight, and very particularly preferably ≤0.5 ppm by weight in step (b). The content of molecular oxygen can even be reduced below its detection limit, but usually a small amount of ≥0.05 ppm by weight will still remain. The concentration of molecular oxygen in the liquid mixture obtained in step (b) can be easily determined in the same manner as described in step (a).

[0095] Then the liquid, molecularly oxygen-depleted saturated aliphatic C obtained in step (b) 6-12 The mixture of crude carboxylic acid products is transferred to step (c). For completeness, references to the transfer may include other steps between steps (b) and (c), such as a decrease or increase in temperature or pressure.

[0096] In step (c), the mixture obtained in step (b) is distilled in a distillation apparatus containing a purification column, and a purified distillate containing ≥95% by weight of saturated aliphatic carboxylic acids is collected. The term purification column refers to the distillation unit in which saturated aliphatic C... 6-12A distillation column for purifying the distillate of carboxylic acids. In addition to the purification column, the distillation apparatus may also include other distillation columns, such as those for removing light-boiling components or for post-treatment of high-boiling components. There is no limit to the total number of distillation columns, but typically 1-5 are used, preferably 1-4, more preferably 1-3, and particularly preferably 1-2. Besides the distillation columns themselves and their internals, the term distillation apparatus also includes their periphery, such as piping, heat exchangers, reboilers, condensers, reflux tanks, etc.

[0097] Saturated aliphatic C 6-12 The distillation separation of carboxylic acids can usually be carried out continuously or discontinuously.

[0098] In the case of a discontinuous or semi-continuous oxidation process in step (a), it is generally advantageous to also perform steps (b) and (c) discontinuously. For such discontinuous distillation, the distillation apparatus preferably contains only one distillation column that simultaneously constitutes the purification column. In this column, low-boiling compounds are first separated from the top, and then saturated aliphatic C20 is obtained as another fraction. 6-12 Carboxylic acids. It may also be advantageous to carry out the removal of molecular oxygen and subsequent distillation in the same distillation unit to minimize the number of required equipment and simplify the process.

[0099] In the case of a continuous oxidation process in step (a), both continuous and discontinuous removal of molecular oxygen in step (b) and distillation in step (c) may be advantageous and both are substantially equally preferred. However, the advantages of continuous distillation are generally applicable to larger capacities, while the advantages of discontinuous distillation are generally applicable to smaller capacities. Typically, for an annual production of saturated aliphatic C... 6-12 For equipment with a capacity of >1000 tons of carboxylic acids, continuous distillation is preferred, while for equipment with a corresponding capacity of ≤1000 tons per year, discontinuous distillation is preferred. For discontinuous distillation, the distillation unit preferably contains only a single distillation column, whereas continuous distillation is typically carried out in a single distillation column or in an interconnection of multiple distillation columns. The explanations in the preceding paragraphs apply accordingly to discontinuous distillation. Regarding continuous distillation, for example, this can be carried out in a single distillation column, which happens to constitute a purification column and in which low-boiling-point compounds are separated from the top of the column and saturated aliphatic C... 6-12 Carboxylic acids are taken as a side stream. High-boiling-point substances are taken as a bottom stream. Continuous distillation can also be carried out in two or more interconnected distillation columns. If two interconnected distillation columns are used, low-boiling-point substances, containing saturated aliphatic C4 compounds, are typically separated in the first distillation column. 6-12 The bottoms and high-boiling points of the carboxylic acid distillation column are transferred to a second distillation column, which constitutes the purification column and is saturated with aliphatic C4 compounds. 6-12Carboxylic acids are removed from the top of the column, leaving the high-boiling fraction as the bottom product. In another, less desirable variation with two interconnected distillation columns, saturated aliphatic carboxylic acids are discharged from the top of the first distillation column along with the low-boiling fraction and are thus separated from the high-boiling fraction in the first distillation column, but subsequent purification is required in the second distillation column, which necessitates further energy-intensive evaporation of the saturated aliphatic carboxylic acids. Besides using two interconnected distillation columns, indirect wall columns or equivalent Petlyuk configurations can also be used. For completeness, the use of three or more distillation columns is also mentioned, for example, to further separate the light-boiling fractions into different fractions. If two or more different saturated aliphatic Cs are prepared together... 6-12 For carboxylic acids, it may also be meaningful to use three or more distillation columns, because for each saturated aliphatic C... 6-12 Carboxylic acids typically require a separate purification tower.

[0100] Whether using a single distillation column or an interconnection of two or more distillation columns, they can and preferably be equipped with internals that promote separation efficiency, such as structured packing, random packing, or trays. The required number of separation stages depends primarily on the separation objective, particularly the difference in boiling points between saturated aliphatic carboxylic acids and low- and high-boiling substances, as well as the target purity of the saturated aliphatic carboxylic acids.

[0101] Regarding the operating conditions in step (c), it is recommended not to use saturated aliphatic C. 6-12 Carboxylic acids are exposed to temperatures exceeding 170°C because, according to the present invention, higher temperatures increasingly favor the formation of unwanted byproducts, such as acid anhydrides. Therefore, the treatment of saturated aliphatic C... 6-12 The temperature inside the distillation column for carboxylic acids is preferably ≤170°C, especially in the purification column, preferably ≤170°C, more preferably ≤150°C, and particularly preferably ≤130°C. This can be easily achieved by selecting a suitable pressure. Even the lowest boiling point saturated aliphatic C... 6-12 Carboxylic acids also have boiling points above 190°C at atmospheric pressure. The distillation in step (c) is preferably carried out at 0.1-99 kPa abs, which involves the treatment of saturated aliphatic C... 6-12 All distillation columns for carboxylic acids. Distillation is preferably carried out at pressures ≥0.5 kPa abs, particularly preferably ≥1 kPa abs, more preferably ≤50 kPa abs, and particularly preferably ≤20 kPa abs. If more than one distillation column is used, each column can operate at different pressures. Considering the above pressure ranges, the distillation in step (c) is generally carried out at temperatures ≥0°C, preferably ≥25°C, and more preferably ≥40°C.

[0102] The extremely low pressure (≤1 kPa abs) results in typically low distillation temperatures (≤100°C), and such low distillation temperatures significantly reduce the harmful effects of molecular oxygen. Therefore, distillation at very low pressures may yield exceptionally pure saturated aliphatic C. 6-12 A specific choice of carboxylic acid, or perhaps an option that tolerates slightly higher molecular oxygen content. However, this extremely low pressure variant is not the preferred choice because at these extremely low pressures, the distillation rate drops rapidly, resulting in very low production or the need for unfavorable large-diameter columns.

[0103] Based on the above findings and relationships, the purification tower is preferably operated at a pressure of 0.1-99 kPa abs and a temperature of 0-170°C.

[0104] As a logical consequence of this surprising discovery, during storage, when subjected to thermal stress and / or in products where they are routinely used, the darkening of carboxylic acids over time can be avoided or at least significantly reduced if the liquid saturated aliphatic C obtained in step (a) is... 6-12 In step (b), the molecular oxygen present in the crude carboxylic acid product mixture is removed to a content of ≤10% by weight based on the liquid mixture, and then the saturated aliphatic C is separated by distillation. 6-12 Carboxylic acids, needless to say, will have the opposite effect if molecular oxygen permeates after the removal of molecular oxygen in step (b), including during or after the distillation in step (c), including the transfer to step (c). If saturated aliphatic C... 6-12 Carboxylic acid apparatuses are sealed, or at least largely sealed, under operating conditions to prevent or at least reduce the permeation of molecular oxygen to very low levels. This is particularly desirable for components operating under vacuum, especially distillation columns, and especially purification columns. High sealing of the columns used, particularly vacuum distillation columns and stripping columns (if operating under reduced pressure), can be achieved through various techniques known to those skilled in the art. These techniques include, for example, using high-quality seals or even welded joints with low leakage rates, flushing flange protectors with inert gases, or minimizing the number of flanges. Generally, the more of these techniques implemented, the lower the leakage rate of the distillation column.

[0105] While it is desirable to avoid any infiltration of molecular oxygen in step (c), the molecular oxygen-depleted saturated aliphatic C30 treated in step (c) is still a concern. 6-12 In terms of the amount of crude carboxylic acid product mixture, the low amount of infiltrated molecular oxygen does not significantly impair saturated aliphatic C. 6-12The purity of the carboxylic acid distillate, and typically allows for a low APHA color number within a specified range even after the tempering procedure specified in step (c). Therefore, large distillation columns, for example, designed for a technical scale of 1000 kg / h or more distillation, are generally able to withstand a greater absolute amount of permeated molecular oxygen per unit time than small laboratory-scale distillation columns that process, for example, only 1 kg / h or less. Furthermore, the ratio between the seal surface and the internal volume of the distillation column is much lower for large industrial-scale columns than for small-scale laboratory columns. According to the invention, it is recognized that the amount of molecular oxygen permeating into the distillation apparatus of step (c) is significantly lower than that of the molecular oxygen-depleted saturated aliphatic C4 distillate obtained through step (b). 6-12 The amount of molecular oxygen supplied to the distillation apparatus for the crude carboxylic acid product mixture is within acceptable limits.

[0106] Although it deals with saturated aliphatic C 6-12 All parts of the distillation apparatus for carboxylic acid step (c) should be advantageously and reasonably sealed, specifically the purification column should be well-sealed for saturated aliphatic C 6-12 The purity of carboxylic acid distillates is more sensitive. This is particularly true for purification columns operating under vacuum conditions. The airtightness, or in other words, leakage rate of a distillation column can be determined by measuring the amount of molecular oxygen that permeates during distillation and comparing these values ​​with the amount of molecular oxygen supplied to the distillation column via the feed stream. For vacuum columns, this can be accomplished by measuring the exhaust gas volume of the vacuum unit and the concentration of molecular oxygen therein.

[0107] Preferably, the purification column, operating at a pressure of 0.1-99 kPa abs and a temperature of 0-170°C, is sealed at least such that the amount of molecular oxygen removed from the exhaust gas of its vacuum unit is ≤ twice the amount of molecular oxygen supplied to the distillation apparatus from the mixture obtained in step (b). More preferably, the amount of molecular oxygen removed from the exhaust gas of its vacuum unit is ≤1.8, particularly preferably ≤1.5, and with respect to a lower limit ≥ one time the amount of molecular oxygen supplied to the distillation apparatus from the mixture obtained in step (b). For discontinuous distillation, the term "quantity" represents an absolute amount in the discontinuous distillation process, while for continuous distillation, the term "quantity" refers to the amount per unit time. The molecular oxygen-depleted saturated aliphatic C2O3 supplied to the distillation apparatus... 6-12The molecular oxygen content of the crude carboxylic acid product mixture can be readily determined by measuring the molecular oxygen concentration as described above, for example, by using an optically sensitive molecular oxygen sensor, and considering the total amount of the product mixture in the case of discontinuous distillation, or, in the case of continuous distillation, by relating it to the amount of the product mixture fed into the purification column per unit time. The amount of molecular oxygen removed from the exhaust gas of the vacuum unit of the purification column can be readily determined by measuring the molecular oxygen concentration of the exhaust gas of the vacuum unit, preferably on the pressure side, and by determining the exhaust gas volume, for example, using a flow meter. For discontinuous distillation, the total amount is calculated, and for continuous distillation, the relative amount per unit time is calculated.

[0108] Because the methods described above for determining the airtightness or leakage rate of a vacuum tower require both the concentration of molecular oxygen in the exhaust gas and the amount of exhaust gas, it can sometimes be difficult to obtain a reliable value for the exhaust gas quantity, as it can sometimes be very low and difficult to measure. Therefore, an alternative method that does not require the amount of exhaust gas has been developed. This alternative method takes into account that a vacuum tower, which is not absolutely sealed, will draw some surrounding air into the tower. Since the ratio of argon to molecular oxygen in the air is fixed, the concentration of argon in the exhaust gas can be used as an indirect measure of the molecular oxygen concentration based on air permeation. The molecular oxygen concentration in the air is 20.95% by volume, and the argon concentration is 0.93% by volume, corresponding to an Ar / O2 molar ratio of 0.0444. The feed is argon-free and molecular oxygen-depleted saturated aliphatic C 6-12 An absolutely sealed column for a crude carboxylic acid product mixture results in an Ar / O2 molar ratio of 0 (zero) because there is no argon in the column exhaust gas. Conversely, a leaky column with a product mixture fed without argon and molecular oxygen results in an Ar / O2 molar ratio of 0.0444, which is the theoretical value for air. The Ar / O2 molar ratio for a leaky column will be lower than the value for pure air, depending on the amount of molecular oxygen supplied to the distillation column along with the product mixture. If the amount of molecular oxygen permeating is the same as the amount of molecular oxygen supplied to a distillation column with a molecular 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, and by definition, a molecular oxygen concentration of 0% by volume in the column exhaust gas is associated with an absolutely sealed distillation column.

[0109] According to this alternative determination method, the purification tower, preferably operated at a pressure of 0.1-99 kPa abs and a temperature of 0-170°C, is at least sealed such that the molar ratio n(Ar waste gas) / n(O2 waste gas) in the exhaust gas of the vacuum unit of the purification tower is 0-0.0222, where n(Ar waste gas) is the molecular weight of argon in the exhaust gas, corrected by the molecular weight of argon that may be supplied to step (c) by the mixture obtained in step (b), and n(O2 waste gas) is the molecular weight of molecular oxygen in the exhaust gas. More preferably, the molar ratio n(Ar waste gas) / n(O2 waste gas) ≤ 0.0197, particularly preferably ≤ 0.0148. For discontinuous distillation, the term "quantity" represents the absolute amount in the discontinuous distillation process, while for continuous distillation, the term "quantity" refers to the amount per unit time. The above-described determination method can be applied to the measurement of molecular oxygen in the exhaust gas of the vacuum unit. The molecular weight of argon in the exhaust gas of the vacuum unit can be easily determined, for example, by gas chromatography analysis of the exhaust gas on the pressure side. For example, a portable gas chromatograph can be used. If the mixture obtained in step (b) already contains argon, for whatever reason, its amount can also be determined by gas chromatography and used for the correction of the n(Ar exhaust) value.

[0110] Furthermore, according to the present invention, it is understood that the location of the feed point into the purification tower is relative to the location of the saturated aliphatic C45 sample. 6-12 The location of the carboxylic acid sampling point also affects saturated aliphatic C. 6-12 The tempering stability of carboxylic acids, especially if the distillation column is not an indirect-wall column. If the feed point of the distillation column (not an indirect-wall column) is higher than the sampling point, then saturated aliphatic C... 6-12 The tempering stability of carboxylic acids is generally lower than that of the opposite. Therefore, it is preferable to add the feed to the purification column (which is not an indirect-wall column) in step (c) at a point lower than the extraction point of the purified distillate. Taking the total external height of the purification column as 100%, the distance between the feed point level and the sampling point level, which is higher than the feed point level, is preferably ≥1%, more preferably ≥2%, particularly preferably ≥3%, and preferably ≤90%, more preferably ≤80%, based on the external height of the purification column. For an indirect-wall column as a purification column, the relative positions of the feed point and the sampling point are important for saturated aliphatic C 6-12 The tempering stability of carboxylic acids is minimally affected.

[0111] Based on the above description of the distillation apparatus and its operation, those skilled in the art can determine the appropriate method based on saturated aliphatic C... 6-12 The properties of carboxylic acids are used to design distillation apparatus and determine suitable operating conditions.

[0112] The above method can prepare saturated aliphatic C with high purity and a content of ≥95% by weight based on the distillate. 6-12Carboxylic acids and saturated aliphatic Cs exhibiting extremely low APHA color numbers, even after tempering for 4 hours under inert gas conditions at 225°C and 0.1 MPa. 6-12 Carboxylic acids. Expressed as a number, the purified distillate, after tempering for 4 hours under inert gas conditions at 225°C and 0.1 MPa, exhibits an APHA color number of 0-10 based on the presence of ≥95% by weight of saturated aliphatic carboxylic acids. Saturated aliphatic C 6-12 The content of carboxylic acids is preferably ≥98% by weight, more preferably ≥99% by weight, particularly preferably ≥99.5% by weight, and very particularly preferably ≥99.8% by weight, based on distillate. Typical byproducts include formate esters, carboxylic acids with low carbon numbers, alcohols, ketones, and water. For clarity, references to APHA color number refer to the well-known Hazen color number established according to DIN EN ISO 6271 and ASTM D1209, respectively.

[0113] Based on the low APHA color number after the above tempering procedure, freshly distilled, untempered saturated aliphatic C... 6-12 The APHA color number of carboxylic acids is also very low, usually 0-10, preferably ≤5, and more preferably ≤3.

[0114] Saturated aliphatic C 6-12 The above tempering procedure, in which carboxylic acids are tempered for 4 hours at 225°C and 0.1 MPa under inert gas conditions, simulates a thermal stress scenario under defined and reproducible conditions and provides a more or less reliable indicator of color stability during storage when subjected to thermal stress and / or after routine application in product manufacturing.

[0115] More specifically, the so-called tempering test is conducted as follows: 40.0-60.0g of a mixture containing saturated aliphatic C... 6-12 The carboxylic acid sample was placed in a three-necked round-bottom flask equipped with a magnetic stirrer, a condenser, and a glass frit for bubbling an inert gas (approximately 0.2 Nl / min, preferably argon 5.0) through the sample. At ambient temperature, the sample was bubbled with the inert gas for 30 minutes while stirring. Bubbling was then reduced to a minimum, and the flask was lowered to a preheated oil bath maintained at 225°C. After 4 hours, the bubbling rate was slightly increased, and the heating bath was removed. It is important that the bubbling rate be high enough to prevent air from being drawn into the flask during cooling. When the flask had cooled to ambient temperature, the sample was removed, and the APHA color number was measured. For example, a 10 mL cuvette can be used in Hach... Measurements were taken in a 620 colorimeter. The instrument was pre-calibrated with distilled water.

[0116] The purified saturated aliphatic C after tempering in the above tempering test 6-12The APHA color number of the carboxylic acid distillate is preferably ≤8, more preferably ≤5. Although the APHA color number can reach 0, it is usually ≥1.

[0117] Due to the saturated aliphatic C obtained in step (c) 6-12 The discoloration of carboxylic acid distillate during tempering tests is mainly due to the presence of trace amounts of peroxides. Untempered saturated aliphatic C... 6-12 The active oxygen content in carboxylic acid distillate is another indicator of its color stability. According to the present invention, the active oxygen content of the purified distillate obtained in step (c) is preferably 0-100 ppm by weight, more preferably ≤50 ppm by weight, particularly preferably ≤10 ppm by weight, and preferably ≥1 ppm by weight based on the distillate.

[0118] As previously mentioned, steps (a)-(c) can be performed continuously or discontinuously, and semi-continuous operation is also an additional option for step (a). For lower production volumes, it may be advantageous to produce this saturated aliphatic C using discontinuous or semi-continuous methods. 6-12 Carboxylic acids are preferred because discontinuous and semi-continuous methods are generally more flexible and easier to operate for small-scale production. On the other hand, continuous methods, in which steps (a)-(c) are carried out sequentially, have the advantage of being more efficient and stable once started. Therefore, for higher capacities of >1000 tons / year, the preferred option is the continuous production of this saturated aliphatic C. 6-12 Carboxylic acids. This is particularly applicable to 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.

[0119] In a general embodiment for the continuous preparation of 2-ethylhexanoic acid, liquid 2-ethylhexanal, an aqueous solution of potassium hydroxide as a selective modifier, and gaseous oxygen are continuously fed into a jet-circulating reactor operating at a temperature in the range of 30-60°C and an oxygen partial pressure in the range of 0.11-1 MPa abs. To remove the heat generated by this exothermic oxidation reaction, the reactor is externally cooled by an external heat exchanger. The reaction mixture obtained by step (a) above still contains 0.1-2 mol% of unconverted 2-ethylhexanal relative to 2-ethylhexanoic acid and molecular oxygen at a concentration of 20-500 ppm by weight, depending on the oxygen partial pressure, temperature, and conversion rate based on the liquid mixture. It is then fed into a stripping column and stripped at 40-60°C and approximately atmospheric pressure or vacuum with a nitrogen stream of 0.5-4 mol nitrogen per mol of molecular oxygen to obtain a mixture depleted of molecular oxygen based on ≤10 ppm by weight. The oxygen-depleted mixture is then continuously fed into a distillation apparatus operated under vacuum. This apparatus contains a low-boiling-point column for separating low-boiling-point substances such as water, residual 2-ethylhexanal, and other low-boiling-point byproducts, and a purification column fed from the bottom of the low-boiling-point column, with the purified 2-ethylhexanoic acid discharged as a side stream. Optionally, these two columns can also be combined into a single partitioned-wall column. The 2-ethylhexanoic acid obtained in the distillation step (c) above is of high purity, containing ≥99.5% by weight. After tempering for 4 hours under inert gas conditions at 225°C and 0.1 MPa, it exhibits an APHA color number ≤10.

[0120] In a general embodiment for the discontinuous preparation of 2-ethylhexanoic acid, 2-ethylhexanoic acid as a diluent and potassium hydroxide as a selective modifier are placed in a temperature-controlled stirred vessel. Simultaneously, 2-ethylhexanal and molecular oxygen or a mixture of gases containing molecular oxygen are fed into the stirred vessel at a total pressure of 0.1-10 MPa abs over 0.1-5 hours, while the temperature is controlled at 25-60°C. Once the desired liquid level is reached in the stirred vessel, the addition of 2-ethylhexanal is stopped, and molecular oxygen is added to maintain the partial pressure and achieve the desired conversion. After the desired conversion is achieved, the addition of molecular oxygen is also stopped, and nitrogen is added instead. Stripping is performed at 40-60°C and approximately atmospheric pressure or vacuum, while stirring continues in the vessel. Typically, a nitrogen stream of 0.5-100 mol of nitrogen per mol of dissolved molecular oxygen is supplied over a period of 10 minutes to 2 hours to obtain a molecular oxygen-depleted mixture containing ≤10 ppm by weight of the liquid mixture. The oxygen-depleted mixture was then placed in a batch vacuum distillation column. After removing low-boiling components, a fraction containing purified 2-ethylhexanoic acid was obtained, or it could be continuously fed into a continuously operated distillation unit from which purified 2-ethylhexanoic acid was extracted. The resulting purified 2-ethylhexanoic acid was of high purity, containing ≥99.5% by weight. After tempering for 4 hours under inert gas conditions at 225°C and 0.1 MPa, it showed an APHA color number of ≤10.

[0121] In addition to the methods described above, 2-ethylhexanoic acid with an APHA color number of 0-10 was obtained after tempering at 225°C and 0.1 MPa for 4 hours under inert gas conditions. This is surprising because even when 2-ethylhexanoic acid is produced by methods other than oxidation processes as described in the prior art, such as by dehydrogenating 2-ethylhexanol to sodium 2-ethylhexanoate with NaOH followed by acidification with H2SO4, temper-stable 2-ethylhexanoic acid that meets the above tempering test requirements is not produced, although peroxide formation is not expected in this method.

[0122] The method of this invention can prepare saturated aliphatic carboxylic acids with 6-12 carbon atoms by oxidizing the corresponding aldehydes with oxygen, with high yield and high purity, and particularly high color stability, such that even after tempering at 225°C and 0.1 MPa for 4 hours under inert gas conditions, the APHA color number remains very low. Saturated aliphatic C 6-12 Carboxylic acids also possess very low levels of reactive oxygen species, and therefore very low levels of peroxides such as peroxy acids, hydroperoxides, and other peroxides. This method is also readily operable and stable under long-term operation, thus producing this saturated aliphatic carboxylic acid with consistently high quality. Saturated aliphatic C 6-12 Carboxylic acids can be stored for extended periods, exposed to heat, or used in the production of other products without darkening or at least have a very low tendency to darken.

[0123] Furthermore, this method can yield high-purity and temper-stable 2-ethylhexanoic acid. Example

[0124] Determination of reactive oxygen species by iodometric titration

[0125] The content of reactive oxygen species in a sample is determined by iodometric titration. A general description of how this determination is performed is given below.

[0126] Weigh approximately 5g of the sample, to the nearest 0.1mg, into a reaction flask, purge with argon, and add 40ml of a 1:1 acetic acid / chloroform mixture to dissolve the sample. The reaction flask is equipped with a cooler and placed in a preheated stirring block at 80°C. A weak argon flow is passed through the cooler to prevent air ingress. After temperature equilibration, add 5.0ml of saturated potassium iodide solution (approximately 60.0g of potassium iodide dissolved in 100ml of deionized water) through the cooler, and boil the mixture under reflux for 10 minutes. In the next step, add 40.0ml of deionized water and titrate the sample solution with 0.01M thiosulfate solution while using a platinum electrode as an endpoint indicator.

[0127] Determination of molecular oxygen

[0128] The molecular oxygen content in the sample was determined using an optical fluorescence sensor with high precision and high O2 sensitivity, suitable for measuring molecular oxygen in carboxylic acids. In this example, a sensor from WTW named [insert sensor name here] was used. 925 optical sensor.

[0129] The measured data were cross-checked using additional measurements from a galvanic cell oxygen analyzer (also known as a Hersch cell).

[0130] APHA color number determination

[0131] The APHA color number of the sample was determined using a colorimeter pre-calibrated with distilled water. In the example, a colorimeter from Hach with 10 mL cuvettes was used. A colorimeter of size 620.

[0132] Description of tempering test

[0133] The so-called tempering test simulates a thermal stress scenario under defined and reproducible conditions to demonstrate the color stability of a sample during storage, under thermal stress, and / or after routine use in product manufacturing. A description of how the tempering test is performed is given below.

[0134] Place 40.0–60.0 g of sample 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, argon 5.0) through the sample. At ambient temperature, bubble the sample with inert gas for 30 minutes while stirring. Then reduce the bubbling to a minimum, just enough to prevent air from being drawn into the flask, and lower the flask to a preheated oil bath maintained at 225°C. After 4 hours, slightly increase the bubbling rate and remove the heating bath. The slightly increased bubbling rate must be high enough to prevent air from being drawn into the flask during cooling. When the flask has cooled to ambient temperature, remove the sample and measure the APHA color number as described above.

[0135] Example 1 (Preparation of crude 2-ethylhexanoic acid)

[0136] According to step (a) of the present invention, crude 2-ethylhexanoic acid is produced. 2-Ethylhexanoic acid is produced in a technical plant with a capacity of approximately 3.75 tons of 2-ethylhexanoic acid per hour by continuous oxidation of 2-ethylhexaldehyde with pure oxygen at a temperature of 30-60°C and a pressure of 0.25 MPa abs, in the presence of 0.4 wt% potassium ions in the reaction mixture. The technical plant contains three reactors connected in series, with the addition of oxygen feed distributed among these three reactors. 2-Ethylhexaldehyde and potassium salt are added to the first reactor.

[0137] The crude 2-ethylhexanoic acid obtained at the outlet of the third reactor contained 92.4 wt% 2-ethylhexanoic acid and 281 wt% reactive oxygen species. The APHA color number was >1000. Before depressurization, the molecular oxygen content at the outlet of the third reactor was 600 wt% ppm. Even after cooling to ambient temperature and depressurization to ambient pressure, the crude 2-ethylhexanoic acid still contained 210 wt% molecular oxygen species. The measured values ​​are summarized in Table 1.

[0138] Example 2 (Comparative Example)

[0139] One kg of crude 2-ethylhexanoic acid sample from Example 1 was distilled in a pilot-scale batch still containing a 2 m column packed with mesh rings. The column was operated at a top pressure of 1 kPa abs, and 2-ethylhexanoic acid was distilled off at the top, with the boiling fraction collected at 102.5 ± 0.5 °C. The recovered 2-ethylhexanoic acid, analyzed by gas chromatography, showed a content of 99.38% by weight and contained 11 ppm by weight of active oxygen. Its APHA number was 2. The measurements are summarized in Table 1.

[0140] The distilled sample was then tempered under the tempering test conditions described above, and the APHA color number was measured. The APHA color number after tempering was 40. This high value indicates that the color stability of 2-ethylhexanoic acid treated according to existing technology is far from sufficient.

[0141] Example 3 (based on an embodiment of the present invention)

[0142] Another sample of crude 2-ethylhexanoic acid from Example 1 (1 kg) was first stripped with high-purity nitrogen (purity grade 5.0) at ambient temperature and pressure until the molecular oxygen content was only 2 ppm by weight. This procedure relates to step (b) of the invention. The sample was then distilled in the same batch distillation apparatus used in Example 2, containing a 2 m column packed with mesh rings, with particular care taken to minimize air leakage into the column by carefully lubricating all glass joints with high-vacuum silicone grease. The column was operated at a top pressure of 1 kPa abs, and 2-ethylhexanoic acid was distilled off at the top of the column, collecting the boiling fraction at 102.5 ± 0.5 °C. The recovered 2-ethylhexanoic acid was analyzed by gas chromatography and showed a 2-ethylhexanoic acid content of 99.99% by weight and contained only 2 ppm by weight of reactive oxygen species. Its APHA color number was 0 and the reactive oxygen species content was below the detection limit (<1 ppm). The measurements are summarized in Table 1.

[0143] The distilled sample was then tempered under the tempering test conditions described above, and the APHA color number was measured. The APHA color number after tempering was only 7 (according to step (c)). This low value indicates that the removal of molecular oxygen to ≤10 ppm by weight in step (b) of the present invention can prepare highly color-stable 2-ethylhexanoic acid.

[0144] Example 4 (The effect of molecular oxygen on the APHA color number after tempering test)

[0145] 500 ml of the purified 2-ethylhexanoic acid sample obtained in Example 3 was placed in a 1-liter flask and immersed in a 40°C tempering bath without any precautions to prevent contact with air. Starting at time "0", an airflow of 10 NL / h was bubbled through the sample via the glass frit. Samples were taken periodically to analyze the content of active oxygen, molecular oxygen, and APHA color number before and after the tempering test. The results are summarized in Table 2.

[0146] Because transferring the samples from Example 3 to the experimental setup of Example 4 takes some time, the molecular oxygen content increased from <1 ppm by weight to 14 ppm by weight at the start of Experiment 4, indicated by time "0". Over time, the molecular oxygen content increased to 54 ppm by weight (sample 3) after 5 hours. With this increase, the reactive oxygen content and APHA color number also increased after the tempering procedure, while the APHA color number of the untempered sample remained at 0. The increase in reactive oxygen content indicates that a portion of the molecular oxygen is converted into reactive oxygen, since molecular oxygen itself cannot be determined by iodometric titration. After 5 hours (sample 3), the molecular oxygen content remained at a constant value of 54 ppm by weight, which is the expected saturation concentration of molecular oxygen in 2-ethylhexanoic acid. Nevertheless, the reactive oxygen content and APHA color number after the tempering test still increased with increasing air bubbling time through the sample. This indicates that molecular oxygen is continuously converted into reactive oxygen and, together with the molecular oxygen, contributes to the high APHA color number after the tempering procedure.

[0147] Example 5 (The effect of molecular oxygen depletion on APHA color number after tempering test)

[0148] According to Example 4, air bubbling was stopped after 24 hours (i.e., after sampling sample 5) and replaced with bubbling with nitrogen (purity grade 5.0) at 10 NL / h for 24 hours. After 24 hours of nitrogen bubbling, a sample named sample 6 was taken and analyzed as in Example 4.

[0149] Bubbling with nitrogen had almost no effect on the reactive oxygen species (ROS) content, decreasing only slightly from 31 ppm by weight in sample 5 to 28 ppm by weight in sample 6. However, the amount of molecular oxygen decreased significantly from 54 ppm by weight in sample 5 to only 14 ppm by weight in sample 6. This is a decrease of approximately four times. Consequently, the APHA color number after tempering also decreased from 112 in sample 5 to 71 in sample 6. This indicates that the APHA color number after tempering depends on both the molecular oxygen content and the ROS content. With the significant decrease in molecular oxygen content in sample 6, the APHA color number after tempering also decreased.

[0150] Table 1: Examples 1-3

[0151]

[0152]

Claims

1. A method for preparing saturated aliphatic carboxylic acids having 6-12 carbon atoms by oxidizing the corresponding aldehyde with molecular oxygen, comprising: (a) The corresponding aldehyde is converted by 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 saturated aliphatic carboxylic acid and the corresponding aldehyde and molecular oxygen at a relative 2 mol% of the saturated aliphatic carboxylic acid. (b) Remove molecular oxygen from the liquid mixture obtained in step (a) to a concentration ≤10 ppm by weight based on the liquid mixture; and (c) The mixture obtained in step (b) is distilled in a distillation apparatus containing a purification column and a purified distillate is taken out based on the presence of ≥95% by weight of saturated aliphatic carboxylic acids in the distillate.

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

3. The method of claim 1, wherein, based on the liquid mixture, the content of molecular oxygen in the liquid mixture obtained in step (a) is >10 ppm by weight to ≤1 ppm by weight.

4. The method of claim 2, wherein, based on the liquid mixture, the content of molecular oxygen in the liquid mixture obtained in step (a) is >10 ppm by weight to ≤1 ppm by weight.

5. The method according to any one of claims 1-4, wherein molecular oxygen is removed in step (b) by stripping with an inert gas.

6. The method of claim 5, wherein the preparation of saturated aliphatic carboxylic acids is carried out continuously, and stripping is performed in countercurrent flow.

7. The method of claim 5, wherein the inert gas has a molecular oxygen content of ≤5 ppm by volume.

8. The method of claim 6, wherein the inert gas has a molecular oxygen content of ≤5 ppm by volume.

9. The method of claim 5, wherein the inert gas is nitrogen, hydrogen, carbon dioxide, carbon monoxide, nitrous oxide, helium, neon, argon, krypton, or a mixture of two or more of such inert gases.

10. The method according to any one of claims 6-8, wherein the inert gas is nitrogen, hydrogen, carbon dioxide, carbon monoxide, nitrous oxide, helium, neon, argon, krypton, or a mixture of two or more of such inert gases.

11. The method of claim 5, wherein the removal of molecular oxygen in step (b) is carried out at a temperature of 0-150°C and a pressure of 0.0001-10 MPa abs.

12. The method according to any one of claims 6-9, wherein the removal of molecular oxygen in step (b) is carried out at a temperature of 0-150°C and a pressure of 0.0001-10 MPa abs.

13. The method according to any one of claims 1-4, 6-9 and 11, wherein in step (b), molecular oxygen is removed to ≤2 ppm by weight.

14. The method of claim 12, wherein in step (b), molecular oxygen is removed to ≤2 ppm by weight.

15. The method according to any one of claims 1-4, 6-9, 11 and 14, wherein in step (c), the purification column is operated at a pressure of 0.1-99 kPa abs and a temperature of 0-170°C.

16. The method of claim 13, wherein in step (c), the purification tower is operated at a pressure of 0.1-99 kPa abs and a temperature of 0-170°C.

17. The method of claim 15, wherein the amount of molecular oxygen extracted from the exhaust gas of the vacuum unit of the purification tower is ≤ twice the amount of molecular oxygen supplied to the distillation apparatus from the mixture obtained in step (b).

18. The method of claim 16, wherein the amount of molecular oxygen extracted from the exhaust gas of the vacuum unit of the purification tower is ≤ twice the amount of molecular oxygen supplied to the distillation apparatus from the mixture obtained in step (b).

19. The method of claim 15, wherein the molar ratio n(Ar waste gas) / n(O2 waste gas) in the exhaust gas of the vacuum unit of the purification tower is 0-0.0222, wherein n(Ar waste gas) is the molecular weight of argon in the exhaust gas, corrected by the molecular weight of argon that may be supplied to step (c) from the mixture obtained in step (b), and n(O2 waste gas) is the molecular weight of molecular oxygen in the exhaust gas.

20. The method of claim 16, wherein the molar ratio n(Ar waste gas) / n(O2 waste gas) in the exhaust gas of the vacuum unit of the purification tower is 0-0.0222, wherein n(Ar waste gas) is the molecular weight of argon in the exhaust gas, corrected by the molecular weight of argon that may be supplied to step (c) from the mixture obtained in step (b), and n(O2 waste gas) is the molecular weight of molecular oxygen in the exhaust gas.

21. The method according to any one of claims 1-4, 6-9, 11, 14 and 16-20, wherein in step (c) the feed is added to the purification column, which is not an indirect wall column, at a point lower than the take-off point of the purified distillate.

22. The method according to any one of claims 1-4, 6-9, 11, 14 and 16-20, wherein the purified distillate obtained in step (c) contains ≥99.5% by weight of saturated aliphatic carboxylic acids.

23. The method of claim 21, wherein the purified distillate obtained in step (c) contains ≥99.5% by weight of saturated aliphatic carboxylic acids.

24. The method according to any one of claims 1-4, 6-9, 11, 14, 16-20 and 23, wherein the purified distillate has an APHA color number of 0-10 after being tempered for 4 hours under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa.

25. The method of claim 22, wherein the purified distillate, after being tempered for 4 hours under inert gas conditions at a temperature of 225°C and a pressure of 0.1 MPa, has an APHA color number of 0-10.

26. The method according to any one of claims 1-4, 6-9, 11, 14, 16-20, 23 and 25, wherein the purified distillate obtained in step (c) has an active oxygen content of 0-100 ppm by weight, based on the distillate.

27. The method of claim 24, wherein the purified distillate obtained in step (c) has an active oxygen content of 0-100 ppm by weight, based on the distillate.

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