Method for the preparation of C3-5 saturated aliphatic carboxylic acids

Abstract

A process for preparing a saturated aliphatic carboxylic acid having 3 to 5 carbon atoms by oxidation of a corresponding aldehyde with oxygen, the process comprising: (a) converting the corresponding aldehyde with oxygen at a temperature of 40 to 150°C and an oxygen partial pressure of 0.001 to 1 MPa to obtain a mixture containing a saturated aliphatic carboxylic acid and 2 mol-% or less of the corresponding aldehyde based on the saturated aliphatic carboxylic acid; (b) heat-treating the mixture obtained in step (a) in a liquid phase at a temperature of 80 to 250°C and a pressure of 0.1 to 2 MPa abs for 0.25 to 100 hours; and (c) distilling the mixture obtained in step (b) in a distillation apparatus to obtain a distillate containing 90 wt.-% or more of the saturated aliphatic carboxylic acid based on the distillate and having an active oxygen content of 0 to 25 wt.-ppm.

Description

[Technical field]

[0001] The present invention relates to a process for preparing saturated aliphatic carboxylic acids having 3 to 5 carbon atoms by oxidation of the corresponding aldehydes with oxygen, whereby the saturated aliphatic carboxylic acids are obtained in high purity with respect to active oxygen-containing compounds. [Background technology]

[0002] Saturated aliphatic carboxylic acids are globally important intermediates with a wide range of applications.They can be used as such, but are typically further processed into metal salts, esters, amides, acid anhydrides, acid chlorides and other synthetic components.Generally, they are important intermediates for the manufacture of various compounds, such as metal salts and metal soaps, flavors, fragrances, drug synthesis, cosmetic ingredients, plasticizers, paints, coating additives, coolants, lubricants or catalysts for polymer processing.Propionic acid, as one very important representative of saturated aliphatic carboxylic acids, is mainly used as a preservative for animal feed and food for human consumption.

[0003] A widely used and important method for the preparation of saturated aliphatic carboxylic acids having 3-5 carbon atoms is the oxidation of the corresponding aldehyde with molecular oxygen in the liquid phase, with or without a catalyst. The preparation of propionic acid by oxidation of propionaldehyde is described as an important route, for example, in U.-R. Samel et al., "Propionic Acid and Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry, 2017, Wiley-VCH Verlag GmbH & Co. KGaA, DOI: 10.1002 / 14356007.a22_223.pub4, Chapter 4.2 "Oxidation of Propanal".

[0004] CN108707071 describes a specific process for the production of propionic acid by oxidation of propionaldehyde, in which the propionaldehyde is first contacted with a Co-containing oxidation catalyst at 30-50°C in a high-pressure reactor, and then the reactor is heated to 85-110°C while pressurizing with oxygen. The reactor is then held under these conditions for a reaction time of 2-3 hours, followed by distillation of the reaction mixture to obtain propionic acid.

[0005] It is known from the state of the art that the oxidation of aldehydes with oxygen first forms a peracid, which then further oxidizes the aldehyde to produce 2 moles of carboxylic acid per mole of intermediate peracid. Such a mechanism is described, for example, in JH Teles et al., "Oxidation" in Ullmann's Encyclopedia of Industrial Chemistry, 2015, Wiley-VCH Verlag GmbH & Co. KGaA, DOI: 10.1002 / 14356007.a18_261.pub2, Chapter 2 "Organic Chemical Production by Oxidation". It is therefore clear that peroxides arise as intermediates during the oxidation of aldehydes with oxygen, and also remain as by-products in the carboxylic acids produced by the oxidation of aldehydes. Peroxides are very reactive molecules and also tend to explode at higher concentrations. It is therefore advisable to also pay attention to the formation of peroxides when oxidizing aldehydes with oxygen.

[0006] DE 1,071,685 discusses the preparation of short-chain saturated aliphatic carboxylic acids having 2-4 carbon atoms by oxidation of the respective aldehydes. It shows that without a catalyst, the oxidation reaction can collapse at low temperatures around 30° C., resulting in the formation of an explosive mixture containing unreacted aldehydes and excess oxygen. In addition, the peracids initially formed by the addition of O2 to the aldehydes will also accumulate due to their extremely low reactivity at such low temperatures. This can lead to dangerous concentrations of explosive peroxides. Higher reaction temperatures above 70° C. will accelerate the oxidation process and further increase the reactivity of the peroxides, but also lead to an increase in undesired by-products due to increased non-selective thermal decomposition of the peroxides. Finally, DE 1,071,685 teaches the use of alkali hydroxide catalysts to accelerate the reaction at lower temperatures and avoid the formation of a gas phase. Thus, the risk of forming dangerous amounts of peroxides can be avoided.

[0007] US 3,579,575 teaches another method of reducing the concentration of peroxides. It first describes that the oxidation of branched or olefinically unsaturated short-chain aldehydes having 4-5 carbon atoms in the presence of an oxidation catalyst results in relatively low yields and produces undesirable yellow carboxylic acids. It further describes that such inconveniences can be avoided by carrying out the oxidation in the absence of a catalyst. However, in such cases, the peroxide concentration can become very high and cause serious accidents, such as explosions. To avoid this risk, the US patent proposes carrying out the oxidation in the presence of a sufficient amount of water to form a separate aqueous liquid phase. It is assumed that the polar peroxide radicals are mainly directed towards the aqueous phase at the interface between the organic and aqueous phases, so that the hydrocarbon chains remain on the side of the organic phase. Thus, the peroxides will decompose in the aqueous phase, preventing the native oxygen resulting from such decomposition from reacting uncontrollably with the hydrocarbon chains.

[0008] Although the concentration of peroxides is significantly reduced by such measures, diluting the reaction mixture with water is disadvantageous. Firstly, the additional components would increase the reaction volume, requiring a larger reactor. Secondly, the aqueous phase would need to be separated, which would require a separator or settling tank as additional equipment. The separated aqueous phase would naturally contain some carboxylic acid, which would result in losses in the total yield, which would be disposed of as chemical waste, and would require additional labor. Thirdly, the corrosivity of carboxylic acids in the presence of water is much higher than in the absence of water. When water is present, composite corrosion-resistant construction materials are required. Last but not least, even if the aqueous phase is separated off, the organic phase would also contain some water dissolved therein. This would make the purification of carboxylic acids by distillation more complicated. There is also a risk that the purified carboxylic acid would still contain a higher amount of water than it would contain without the addition of water in the oxidation. Since water is usually a specific by-product of pure acids, the addition of water requires additional labor to obtain a product according to specification.

[0009] In CN 108047027, the concentration of peroxides formed in the oxidation of 3,5,5-trimethylhexanal to 3,5,5-trimethylhexanoic acid (isononanoic acid) is relatively low in the reaction stage, so that explosion risk is not an issue here, but in the rectification column, the relatively low concentration was found to be high enough to still cause problems, since peroxides can accumulate therein due to the difference in boiling point between the peracids and other components. Furthermore, the CN application describes that in the prior art, relatively low concentrations of homogeneous catalysts are used to decompose the peroxides. However, such homogeneous catalysts are very difficult to separate and remove, and also cause the risk of slagging, clogging and explosion in the distillation column. To avoid such problems, the CN application teaches heterogeneously catalyzing the decomposition of peroxides on a metal-organic framework catalyst before entering the rectification column. It is taught that it is important to decompose the peroxide at a low temperature of 20-70°C, otherwise side reactions such as decarboxylation will occur, reducing the yield and purity. Moreover, the decomposition is very fast, so it takes only 5-40 hours. -1 It is emphasized that the process can be carried out at high space velocities of 1.5 to 12 minutes, which is associated with short residence times of 1.5 to 12 minutes.

[0010] Despite the fact that the CN application is specifically aimed at the production of isononanoic acid, the use of metal-organic framework catalysts as peroxide decomposition catalysts is generally disadvantageous. First of all, these metal-organic framework catalysts are very complex to manufacture. Secondly, the organic molecules that make up the framework are susceptible to oxidation, especially in the presence of peracids and the reactive radicals generated in their decomposition. In the presence of carboxylic acids, metal-organic frameworks are known to lose activity and / or framework metals as a result of bleeding. These leached metals then cause the same problems in the rectification column as metals used as homogeneous catalysts. Moreover, all these factors contribute to the short life of metal-organic framework catalysts, thus further increasing the process complexity by disposing of the used catalysts and providing fresh ones.

[0011] With regard to the production of propionic acid by oxidation of propionaldehyde, it has been recognized by the present invention that a high conversion rate leads to an increase in the concentration of perpropionic acid. Perpropionic acid has a boiling point only slightly lower than propionic acid and is extremely stable, making it difficult to obtain pure propionic acid using very low concentrations of perpropionic acid. Perpropionic acid is a strong oxidizing agent that can cause significant problems in downstream processing of propionic acid. It is known that propionic acid with a high content of perpropionic acid, for example 100 wt.-ppm or more, leads to undesirable coloration when such propionic acid is processed into an ester product. The above-mentioned behavior is also true in principle for the production of C4 and C5 carboxylic acids, respectively. Summary of the Invention [Problem to be solved by the invention]

[0012] Therefore, C 2 O 3 does not have peracids and other peroxides or has them at least in a very low content. 3~5 It is desirable to provide a carboxylic acid.

[0013] The object of the present invention was to discover a method for preparing saturated aliphatic carboxylic acids having 3 to 5 carbon atoms by oxygen oxidation of the corresponding aldehydes, which is capable of producing the respective saturated aliphatic carboxylic acids in high yield and high purity, in particular free of peracids and other peroxides or at least with a very low content of them.The method will also be easy to operate, function stably over long operating times, and produce saturated aliphatic carboxylic acids with constant high quality. [Means for solving the problem]

[0014] A process for preparing a saturated aliphatic carboxylic acid having 3 to 5 carbon atoms by oxidation of the corresponding aldehyde with oxygen, comprising the steps of: (a) converting the corresponding aldehyde with oxygen at a temperature of 40 to 150°C and an oxygen partial pressure of 0.001 to 1 MPa to obtain a mixture containing a saturated aliphatic carboxylic acid and 2 mol-% or less of the corresponding aldehyde based on the saturated aliphatic carboxylic acid; (b) heat treating the mixture obtained in step (a) in a liquid phase at a temperature of 80 to 250° C. and a pressure of 0.1 to 2 MPa abs for 0.25 to 100 hours; and (c) distilling the mixture obtained in step (b) in a distillation apparatus to obtain a distillate having a saturated aliphatic carboxylic acid content of 90 wt.-% or more (weight %) and an active oxygen content of 0 to 25 wt.-ppm based on the distillate. The present inventors have found a method including the steps of:

[0015] The saturated aliphatic carboxylic acids for which this preparation method is highly suitable contain 3 to 5 carbon atoms. The carboxylic acids may also contain heteroatoms, e.g. halogens, instead of hydrogen, but unsubstituted carboxylic acids are preferred. 3~5Examples of saturated aliphatic carboxylic acids include propionic acid, n-butyric acid, 2-methylpropionic acid (isobutyric acid), n-pentanoic acid (valeric acid), 3-methylbutyric acid, 2-methylbutyric acid and 2,2-dimethylpropionic acid (pivalic acid). Preferably, the non-alpha branched carboxylic acids propionic acid, n-butyric acid, n-pentanoic acid and 3-methylbutyric acid, and particularly preferably propionic acid, are prepared.

[0016] In the first step of the process according to the invention, designated step (a), the corresponding aldehyde is oxidized with oxygen. 3~5 According to the saturated aliphatic carboxylic acid, the corresponding aldehyde is propionaldehyde, n-butyraldehyde, 2-methylpropionaldehyde (isobutyraldehyde), n-pentanaldehyde (valeraldehyde), 3-methylbutyraldehyde, 2-methylbutyraldehyde or 2,2-dimethylpropionaldehyde (pivalaldehyde). According to the preferred preparation of non-alpha branched carboxylic acid, propionaldehyde, n-butyraldehyde or n-pentanaldehyde are the corresponding aldehyde.

[0017] The aldehydes can be used in diluted or pure form. If the aldehydes are used in diluted form, the diluent will preferably be a compound that is inert to oxidation with oxygen, stable towards the carboxylic acid produced and easily separated from the carboxylic acid by distillation. It is preferred not to intentionally dilute the aldehydes, since diluents increase the reaction volume and therefore reduce the space-time yield and may even cause contamination of the desired carboxylic acid. Preferably, the aldehydes are applied as very concentrated compounds, preferably with an aldehyde content of 80-100 wt.-%, more preferably 80-100 wt.-%, particularly preferably 95-100 wt.-% and very particularly preferably 99-100 wt.-%.

[0018] In addition to the preparation of only one specific saturated aliphatic carboxylic acid, the method of the present invention can also be applied to the preparation of a mixture of saturated aliphatic carboxylic acids.Mixtures of technical interest are, for example, a mixture of n-pentanoic acid and 2-methylbutyric acid, where n-pentanaldehyde and 2-methylbutyraldehyde are used, or a mixture of n-butyric acid and isobutyric acid, where n-butyraldehyde and isobutyraldehyde are used.

[0019] The preparation of propionic acid by oxidation of propionaldehyde is particularly preferred.

[0020] The oxidation of the aldehydes is carried out with oxygen, which can be used in pure form or diluted with other gases, for example in the form of air, O2 / N2 mixtures or mixtures with other inert gases.

[0021] The oxidation reaction can be carried out with or without an oxidation catalyst and / or with or without a selectivity improving additive. When oxidation catalysts are used, these are homogeneous catalysts. Examples of homogeneous oxidation catalysts include salts of transition metals from groups 6 to 11 of the periodic table of the elements, preferably the first row salts of these groups, most preferably Mn, Fe or Co salts. When selectivity improving additives are used, these are also homogeneous. Examples of selectivity improving additives include salts of alkali metals, alkaline earth metals and salts of transition metals from group 12 of the periodic table of the elements, preferably Na, K, Mg, Ca, Zn or Cd salts, most preferably K or Na salts. The salts can be selected from any salt soluble in the reaction mixture, although carboxylates, hydroxides, carbonates and bicarbonates are preferred. The concentration of homogeneous oxidation catalyst metals can vary over a wide range, but a metal content of 0.0001 to 0.1 wt.-% of the reaction mixture is a typical content. The concentration of the homogeneous selectivity improving metal can also vary over a wide range, but a metal content of 0.1 to 5 wt.-% of the reaction mixture is typical. It is also possible to use homogeneous catalyst metals and homogeneous selectivity improving metals simultaneously.

[0022] Depending on the nature of the aldehyde, the presence of an oxidation catalyst, especially the presence of a selectivity improving metal, affects the extent and type of by-products. For example, alpha-branched aldehydes such as isobutyraldehyde or 2-methylbutyraldehyde tend to produce more formate as a by-product in the absence of any selectivity improving metal. However, in the presence of a selectivity improving metal, especially in the presence of a salt of sodium or potassium, most especially in the presence of a potassium salt, alpha-branched aldehydes produce much less undesirable formate. Thus, for alpha-branched aldehydes, it is preferred to carry out the oxidation in the presence of a selectivity improving metal, preferably in the presence of a salt of sodium or potassium, especially preferably in the presence of a potassium salt. On the other hand, linear aldehydes such as propionaldehyde, n-butyraldehyde and n-pentanaldehyde already form only small amounts of formate in the absence of a selectivity improving metal, so that the addition of a selectivity improving metal is not or only slightly relevant to the selectivity. Thus, for linear aldehydes, it is preferred to carry out the oxidation in the absence of a selectivity improving metal.

[0023] As for homogeneous catalytic metals, although these increase the reaction rate they have a detrimental effect on selectivity, so it is preferred to carry out the oxidation in the absence of added homogeneous catalytic metals.

[0024] Regardless of the presence or absence of an oxidation catalyst, the oxidation reaction is carried out at a temperature of 40 to 150°C and an oxygen partial pressure of 0.001 to 1 MPa. It is preferable to carry out the reaction at a temperature of 50°C or higher, more preferably 60°C or higher, particularly preferably 70°C or higher, and preferably 120°C or lower, more preferably 100°C or lower, and particularly preferably 80°C or lower. As for the oxygen partial pressure, it is preferable to carry out the reaction at an oxygen partial pressure of 0.005 MPa or higher, more preferably 0.01 MPa or higher, particularly preferably 0.02 MPa or higher, and preferably 0.8 MPa or lower, more preferably 0.5 MPa or lower. The oxygen partial pressure can be easily determined by measuring the total pressure and multiplying it by the concentration of O2 in vol.-% determined by any suitable method known in the current state of the art.

[0025] The oxygen partial pressure can vary in a wide range depending on the oxygen content of the oxygen source, but the total pressure in step (a) is usually in the range of 0.05 to 5 MPa abs. The oxidation reaction is preferably carried out at a total pressure of 0.08 MPa abs or more, more preferably 0.09 MPa abs or more, and particularly preferably 0.1 MPa abs or more. It is preferably carried out at a total pressure of 4 MPa abs or less, more preferably 3 MPa abs or less, and particularly preferably 2.5 MPa abs or less.

[0026] Under these conditions, the aldehyde is almost entirely in the liquid phase and the oxidation reaction also takes place in the liquid phase.

[0027] The oxidation of the aldehyde in step (a) is typically carried out in a reaction device, either batchwise, semi-continuously or continuously. In a continuous operation, the aldehyde and oxygen are continuously fed to the reaction device and a sufficient flow of the reaction mixture is continuously removed. The process conditions of the continuous operation, including the residence time, are selected to achieve the desired conversion. In a batch operation, the reaction device is charged with the addition of aldehyde and oxygen and replenished if necessary. After the desired conversion is achieved, the mixture is removed from the reaction device. A semi-continuous operation is characterized by the addition of aldehyde and oxygen together or intermittently to the reaction device for a certain time while the oxidation reaction is already taking place. After a time, for example when the reaction device is more or less full, the addition is stopped and the mixture is removed from the reaction device after the desired conversion is achieved.

[0028] Preferred procedures for step (a) are batch and continuous procedures, with continuous procedures being especially preferred.

[0029] The oxidation reaction is typically carried out in a reaction device. The reaction device may include one or more reactors. In principle, suitable reactors for use in the reaction device according to the invention include those suitable for carrying out exothermic gas-liquid reactions and those that can be operated discontinuously, semi-continuously or continuously. For discontinuous processes, stirred autoclaves or autoclaves with jet loop type mixing are suitable, for example. For semi-continuous processes, stirred vessels, trickle bed reactors and bubble column reactors are mentioned as possible examples. For continuous processes, stirred vessels, trickle bed reactors, bubble column reactors, jet loop type reactors and cascades of the abovementioned reactors are mentioned as suitable examples. Preferred examples of suitable reactors are described in detail in WO2009 / 024,446 and WO2009 / 024,549. When using a reactor cascade, for example in a continuous process, 2 to 5, preferably 2 to 4 and particularly preferably 2 to 3 reactors are connected in sequence.

[0030] It is preferred to use a reactor that allows for intensive (violent) gas-liquid mixing and good distribution of oxygen in the liquid reaction mixture.

[0031] Due to the formation of a considerable amount of reaction heat by oxidation, it is necessary to remove heat from the reaction zone. Depending on the concentrations of aldehyde and oxygen fed into the reactor, in a process carried out continuously, it may be sufficient to remove heat only from the reaction mixture and to control the temperature in the reactor by adding fresh aldehyde at low temperature. However, at higher concentrations of aldehyde and oxygen contents equal to or greater than that of air, it is usually necessary to cool the reaction liquid in the reactor. Such cooling can be carried out, for example, by external cooling of the outer wall of the reactor or by cooling tubes through which a coolant flows inside the reactor.

[0032] Overall chemical formula

[0033] [ka] (Wherein, R is C3~5 Aldehydes and C 3~5 Carboxylic Acid C 2~4 (representing a group) According to the method of the present invention, 0.5 mol of oxygen O2 is stoichiometrically required to oxidize an aldehyde to a carboxylic acid. Although it is possible to carry out the oxidation with a shortage of oxygen, resulting in partial conversion and the presence of residual aldehyde in the reaction mixture, it is preferable to apply oxygen in stoichiometric or superstoichiometric amounts. In order to ensure sufficient conversion on the one hand and to limit the gas load on the other hand, the oxidation reaction is preferably carried out with a molar ratio of oxygen to aldehyde of 0.5 to 1. This is more preferably carried out with a molar ratio of oxygen to aldehyde of 0.51 or more, particularly preferably 0.52 or more, more preferably 0.7 or less, particularly preferably 0.6 or less, very particularly preferably 0.58 or less.

[0034] For continuous oxidation processes, the use of a reactor cascade has been shown to be particularly advantageous since it allows for a stepwise addition of oxygen. The great advantage of such a stepwise addition of oxygen is a better control of the heat of reaction and a smaller amount of by-products, especially due to a lower oxygen concentration in each reactor. It is therefore particularly preferred to use such a reactor cascade of 2 to 3 reactors, preferably where about 70 to 95% of the total amount of aldehyde and of the total oxygen is fed to the first reactor, and the remaining 5 to 30% of the oxygen will either enter the second reactor in its entirety or be further divided into two parts into the second and third reactors, where the fraction for the third reactor will be the smaller fraction.

[0035] Even if oxygen is applied in stoichiometric or superstoichiometric amounts, it would take a very long time to achieve nearly 100% aldehyde conversion. This would unnecessarily block the reactor or make it too large. Therefore, in step (a), it is advantageous to convert the aldehyde until a residual aldehyde amount of 2 mol-% or less based on the saturated aliphatic carboxylic acid is achieved. Depending on the nature of the aldehyde, the concentration of the oxygen-containing gas provided to the reaction device, and the process conditions, a residual aldehyde amount of 2 mol-% or less based on the saturated aliphatic carboxylic acid is generally achieved after a reaction time of 0.1 to 5 hours.

[0036] The mixture obtained in step (a) preferably contains 1.5 mol-% or less, more preferably 1 mol-% or less, and preferably 0.1 mol-% or more, more preferably 0.2 mol-% or more, particularly preferably 0.5 mol-% or more of the corresponding aldehyde, based on the saturated aliphatic carboxylic acid.

[0037] In terms of reaction time, the reaction time is preferably 0.2 hours or more, more preferably 0.3 hours or more, and more preferably 0.5 hours or more, and more preferably 4 hours or less, and more preferably 3 hours or less.

[0038] However, even if the added aldehyde is not completely converted during step (a), meaning that unconverted aldehyde is still present, the reaction mixture contains, in addition to the desired saturated aliphatic carboxylic acid, by-products with high oxidation potential formed by peroxidation. Such by-products are represented by the following reaction scheme:

[0039] [ka] (Wherein, R is C 2~4 (representing a group) As shown in the figure, for example, oxygen as O2 is completely inserted into an aldehyde to form a peracid. In addition to the above-mentioned peracids, other by-products with high oxidation potential, such as alkyl hydroperoxides, may also be formed, but usually in small amounts compared to the peracids. Since their harmful effects are mainly based on their oxidation potential itself, such by-products are generally characterized by their content of so-called "active oxygen", as a quantitative measure of the amount of reactive oxygen that they will supply to easily oxidizable compounds. The term "active oxygen" is already known and used in the current state of the art and is described, for example, in A. Uhl et al., "Peroxy Compounds, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, 2017, Wiley-VCH Verlag GmbH & Co. KGaA, DOI: 10.1002 / 14356007.a19_199.pub2, Chapter 10 "Analytical Determination". The amount of active oxygen in a sample is generally determined by adding a fixed amount of an easily oxidizable compound, such as iodide(1-) or iron(II) salts, which is then oxidized by a compound with a higher oxidation potential, to a fixed amount of the sample.

[0040] According to the invention, the determination of active oxygen is preferably carried out by oxidation of iodide (1-). This analytical method is called iodometric titration and is well known to those skilled in the art. However, it will be briefly described below.

[0041] In iodometric titration, a certain amount of potassium iodide in acetic acid is added to a certain amount of sample at room temperature and stirred to oxidize iodide (1-) to elemental iodine. The amount of potassium iodide added is related to the expected amount of active oxygen and can be estimated by preliminary measurements. For iodometric titration measurements, the amount of iodide (1-) added must be slightly more than the amount that is oxidized to elemental iodine. The elemental iodine is then titrated with sodium thiosulfate to determine the amount of elemental iodine formed by the previous oxidation. Starch is typically used as an indicator, which is purple as long as elemental iodine is present and becomes colorless when all elemental iodine has been reduced to iodide. Alternatively, a platinum electrode can be used. Based on the amount of potassium iodide added and the amount of elemental iodine formed by oxidation, the amount of oxidized iodide (1-) can be calculated. The formal formula is

[0042] [ka] and a more detailed formula

[0043] [ka] According to the formula (3), two moles of iodide (1-) react with one mole of active oxygen atom in the presence of acetic acid. The active oxygen is represented as "O" in formula (3) and is part of the peroxo group in formula (4). It is reduced to water. In formula (4), R is C 2~4 "Ac" stands for acetyl group, and "Ac" stands for acetyl group. The active oxygen content of a sample is the weight fraction of active oxygen atoms relative to the weight of the sample, expressed as wt.-% or wt.-ppm. The active oxygen content can be easily converted to the equivalent peracid content by multiplying the active oxygen content by the ratio of the molar mass of the peracid to the molar mass of oxygen atoms, assuming that the active oxygen is singly bound in the peracid. In the case of perpropionic acid, the multiplication factor is 90.0 / 16.0=5.625.

[0044] For completeness, the amount of active oxygen in a carboxylic acid-containing sample can be determined essentially by physical methods, e.g. 13 It is also mentioned that the carbon signal of the peracid group appears at approximately 170 ppm, while the carbon signal of the acid group appears at approximately 180 ppm.

[0045] However, in the present invention, active oxygen is understood as the mass of oxygen present in the sample capable of oxidizing iodide (1-) to elemental iodine in an aqueous acetic acid medium at room temperature and atmospheric pressure.

[0046] The active oxygen content of the mixture obtained in step (a) is typically 0.02 to 1 wt.-% of the mixture, preferably 0.03 wt.-% or more, more preferably 0.05 wt.-% or more, and preferably 0.8 wt.-% or less, more preferably 0.5 wt.-% or less.

[0047] The composition of the reaction mixture of step (a) in terms of the content of saturated aliphatic carboxylic acids and the corresponding aldehydes can usually be determined by gas chromatography.

[0048] After the mixture containing the saturated aliphatic carboxylic acid and the corresponding aldehyde in an amount of 2 mol-% or less based on the saturated aliphatic carboxylic acid is obtained in step (a), the reaction liquid and the remaining oxygen-containing gas are preferably separated, for example by venting the oxygen-containing gas phase in a batch process, or by removing the liquid reaction mixture from the reaction device in a continuous process.

[0049] Surprisingly, it has been found that the active oxygen content of the mixture obtained in step (a) can be significantly reduced if, in a subsequent step, named step (b), the mixture is heat treated in the liquid phase at a temperature of 80-250° C. and a pressure of 0.1-2 MPa abs for 0.25-100 h. In order to avoid the formation of further active oxygen-containing compounds, the heat treatment is particularly preferably carried out without the addition of further oxygen.

[0050] Essentially, a typical reactor for step (b) is a residence time vessel in which the liquid mixture is held at a certain temperature range and a certain pressure range for a certain time. Depending on whether the process for the preparation of saturated aliphatic carboxylic acids is carried out discontinuously or semi-continuously on the one hand, or continuously on the other hand, different types of reactors are advantageous.

[0051] For discontinuous or semi-continuous preparation processes, the mixture obtained in step (a) can, for example, be kept in the reactor in which the conversion of the aldehydes took place, where the conditions for the heat treatment according to step (b) can be adjusted. As already mentioned above, typical reactors for discontinuous or semi-continuous oxidation of aldehydes are mainly autoclaves. However, it is of course also possible to transfer the reaction mixture obtained in step (a) to another residence time vessel, for example a stirred or unstirred tank.

[0052] In a continuous preparation process, the reaction mixture obtained in step (a) is typically fed continuously through a residence time vessel, which is usually a separate vessel. Without being limited in scope, the residence time vessel can be, for example, a tubular reactor or a cascade of stirred tank reactors. These geometries are selected such that the desired residence time is achieved during the continuous flow of the reaction mixture therethrough. In the context of the present invention relating to step (b), the term tubular reactor also encompasses elongated tubes whose length is significantly longer than their diameter, generally also described as pipes due to their large length / diameter ratio. Although the length / diameter ratio is not a mandatory parameter for the present invention, the tubular reactor preferably has a length / diameter ratio of 1 to 25, more preferably 2 or more and more preferably 15 or less. Back-mixing is advantageously avoided or at least suppressed, since the effect of the heat treatment would be counteracted or reduced. One means for avoiding or suppressing back-mixing is, for example, to select the geometry of the tubular reactor such that more or less plug flow occurs. Even if the geometry of the tubular reactor is not sufficient to promote sufficient plug flow, suitable plug flow can still be achieved by providing internals, such as perforated plates or random packings such as rings, Raschig rings, Pall rings, Ralu rings, Torus saddles, etc. When using a cascade of stirred tank reactors, it is recommended to use a cascade with at least three stirred tank reactors to reduce backmixing. Due to the increasing complexity of the residence time vessel with increasing number of stirred tanks, it is preferred to use less than 10, more preferably less than 8, and especially preferably less than 5 stirred tanks.

[0053] In a preferred embodiment of the present invention, the preparation of saturated aliphatic carboxylic acids is carried out continuously and the heat treatment in step (b) is carried out in a tubular reactor, more preferably such a tubular reactor is substantially free of internals.

[0054] The heat treatment can be carried out with or without a cracking catalyst. When a cracking catalyst is used, it is usually a homogeneous catalyst, but heterogeneous catalysts are not excluded. Examples of homogeneous cracking catalysts include soluble salts of transition metals of the first row of groups 6-11, preferably groups 6-9, and more preferably groups 6-9 of the periodic table of the elements. Particularly preferably, the homogeneous cracking catalyst contains Mn, Fe or Co, and very particularly preferably Mn. The concentration of the homogeneous cracking catalyst metal can vary in a wide range, but a metal content of 0.0001-0.1 wt.-% relative to the mixture to be treated is a typical content. Although cracking catalysts enhance the decomposition of active oxygen-containing compounds, they generally have the disadvantage that further components must be added to the mixture to be treated and then separated in order to obtain the saturated aliphatic carboxylic acids free of the homogeneous catalyst in step (c). In the present invention, it has been found that under the specific conditions of temperature between 80 and 250° C., pressure between 0.1 and 2 MPa abs, and decomposition time between 0.25 and 100 hours, decomposition occurs sufficiently and consistently even without the addition of a homogeneous catalyst.

[0055] The heat treatment for decomposing the active oxygen-containing compound is carried out at a temperature of 80 to 250°C and a pressure of 0.1 to 2 MPa abs. It is preferably carried out at a temperature of 90°C or higher, more preferably 100°C or higher, and preferably 200°C or lower, more preferably 180°C or lower. As for the pressure, it is generally preferred to carry out the heat treatment at a pressure at least equal to or more preferably higher than the vapor pressure of the saturated aliphatic carboxylic acid at the treatment temperature in order to keep the saturated aliphatic carboxylic acid almost completely in the liquid phase. Particularly preferably, the pressure is 1.2 times or more the vapor pressure of the saturated aliphatic carboxylic acid at the treatment temperature. In particular, the heat treatment in step (b) is carried out at a temperature of 100 to 180°C.

[0056] Since the initial concentration of active oxygen-containing compounds is relatively low, the adiabatic temperature increase is usually less than 10 K, and therefore generally no need to remove the heat of reaction. The heat treatment is therefore usually carried out adiabatically or quasi-adiabatically. It can also be carried out isothermally or by varying, for example increasing, the temperature during the heat treatment.

[0057] As already mentioned above, the heat treatment is particularly preferably carried out without the addition of additional oxygen.If the process is carried out continuously and the heat treatment is carried out in a tubular reactor, there is usually no gas phase, since the liquid reaction mixture obtained in step (a) completely fills the space of the tubular reactor.If the heat treatment is carried out in a manner in which a gas phase is present, as in the case of heat treatment in a stirred vessel, it is advantageous to apply a gas phase that is free or substantially free of oxygen.Preferably, the gas phase mainly contains an inert gas, such as nitrogen, a noble gas or a mixture thereof.

[0058] Depending on the nature of the active oxygen-containing compounds, their amount, the treatment conditions, e.g. the treatment temperature, and the active oxygen content to be achieved, the residence time under the heat treatment conditions ranges from 0.25 to 100 hours. In general, a low active oxygen content of the mixture obtained in step (a), a high temperature during the heat treatment, up to 250°C, and a higher target active oxygen content tolerance at the end of the heat treatment will result in a shorter residence time, while a high active oxygen content of the mixture obtained in step (a), a low temperature during the heat treatment, and a lower target active oxygen content tolerance at the end of the heat treatment will result in a longer residence time. It has been recognized by the present invention that the treatment temperature generally has a significant effect on the rate of decomposition of the active oxygen-containing compounds. In the course of the present invention, tests using, for example, a propionic acid-containing system have shown that it takes 6 hours at 100°C to decompose 90% of the active oxygen-containing compounds, while the same degree of decomposition is already achieved after 0.5 hours at 130°C.

[0059] The heat treatment in step (b) is preferably carried out for 0.1 hours or more, more preferably 0.25 hours or more, and preferably 100 hours or less, more preferably 10 hours or less, particularly preferably 5 hours or less. In particular, the heat treatment in step (b) is carried out for 0.25 to 5 hours.

[0060] The active oxygen content of the heat-treated mixture obtained in step (b) is typically between 0 and 100 wt.-ppm, preferably below 75 wt.-ppm, more preferably below 50 wt.-ppm, relative to the heat-treated mixture. It is here clearly emphasized that the active oxygen content of the heat-treated mixture obtained in step (b) can already be very low, depending on the active oxygen content of the mixture obtained in step (a) and the conditions under which the heat treatment in step (b) was carried out.

[0061] The heat-treated mixture obtained in step (b) is then distilled in a distillation apparatus in step (c) to obtain a distillate containing 90 wt.-% or more of saturated aliphatic carboxylic acids and having an active oxygen content of 0 to 25 wt.-ppm based on the distillate.

[0062] The distillation apparatus can be a single distillation column and an interconnection of several distillation columns. Usually, low and high boiling components are separated from the desired saturated aliphatic carboxylic acid. This can be done either in one distillation column, where the low boilers are separated off at the top, the high boilers are separated off as a bottom product, and the saturated aliphatic carboxylic acid is taken off as a side stream, or in two interconnected distillation columns. When two interconnected distillation columns are used, the low boilers are usually separated off in the first distillation column, and the bottom residue containing the saturated aliphatic carboxylic acid and the high boilers is transferred into the second distillation column, where the saturated aliphatic carboxylic acid is taken off at the top and the high boilers remain as a bottom product. In a further non-preferred variant with two interconnected distillation columns, the saturated aliphatic carboxylic acids are taken off at the top of the first distillation column together with the low boilers and are thus already separated from the high boilers in the first distillation column, but then have to be purified in the second distillation column, which would require an energy-intensive evaporation of further saturated aliphatic carboxylic acids.

[0063] Distillation columns known in the art can be used, whether only one distillation column or two or more interconnected distillation columns are used. They usually have a bottom evaporator, a top condenser and internals. The body of the column can be equipped with, for example, structured packing, random packing or trays. The number of separation stages required depends mainly on the separation task, in particular the difference in boiling points of the saturated aliphatic carboxylic acids relative to the boiling points of the low boilers and high boilers, and the purity of the saturated aliphatic carboxylic acids that is targeted. Distillation columns can be easily designed using the skills of a person skilled in the art.

[0064] When two or more saturated aliphatic carboxylic acids are prepared together in the preparation process, the number of distillation columns required is usually equal to or one more than the number of different saturated aliphatic carboxylic acids being purified.

[0065] The distillative separation of the saturated aliphatic carboxylic acid can be carried out continuously and discontinuously. In the case of a continuous oxidation process in step (a), it is usually advantageous to carry out steps (b) and (c) continuously as well. Conversely, in the case of a discontinuous oxidation process in step (a), it is usually advantageous to carry out steps (b) and (c) discontinuously as well.

[0066] In addition to the design of the distillation columns, these operating conditions can also be easily determined using the skills of a person skilled in the art. For practical reasons, the distillation columns are advantageously designed and operated in such a way that the evaporation of the mixture fed into the respective distillation column can be carried out by using so-called "16 bar steam" (1.6 MPa steam), which is usually available on the premises of a chemical plant, and the concentration of the overhead product can be carried out using cooling water. This results in a preferred evaporation temperature of less than 180°C and a preferred concentration temperature of more than 40°C. Furthermore, within the above mentioned data, it is advantageous to design and operate the distillation columns in such a way that no or only little heating or cooling of the feed streams is required. As regards the distillation column feeding the thermally treated mixture obtained in step (b), it is preferably designed in such a way that, in addition to the use of 16 bar steam and cooling water, it can be operated directly at or approximately at a pressure at which the temperature at the feed point in the column is close to, preferably within ±10 K and more preferably within ±5° K, the temperature of the stream that ends the thermal treatment. Furthermore, it is also advantageous to operate the distillation column feeding the heat-treated mixture obtained in step (b) at the same or approximately the same temperature at which the heat-treated mixture was obtained. It is specifically mentioned that the adaptation of the temperature and pressure of the above-mentioned distillation column to those of the heat-treated mixture can in many cases be easily achieved by already selecting the temperature and pressure of the heat treatment in step (b) in such a way that the heat-treated mixture can be easily fed to the distillation in step (c) without significant temperature and pressure adaptations.

[0067] The above-described process allows the preparation of saturated aliphatic carboxylic acids in high purity, with a content of saturated aliphatic carboxylic acids of 90 wt.-% or more and an active oxygen content of 0 to 25 wt.-ppm, based on the distillate. The content of saturated aliphatic carboxylic acids is preferably 95 wt.-% or more, more preferably 98 wt.-% or more, particularly preferably 99 wt.-% or more and very particularly preferably 99.5 wt.-% or more, based on the distillate.

[0068] Typical by-products include formates, lower carbon carboxylic acids, alcohols, ketones and water.

[0069] The active oxygen content of the distilled saturated aliphatic carboxylic acid obtained in step (c) is preferably 20 wt.-ppm or less, more preferably 15 wt.-ppm or less, particularly preferably 10 wt.-ppm or less, very particularly preferably 8 wt.-ppm or less. An active oxygen content of 0 wt.-% can also be achieved, but is often 1 wt.-ppm or more.

[0070] In order to maintain a very low content of active oxygen and a very low content of oxygen dissolved in the saturated aliphatic carboxylic acid, it is advantageous to store, handle and transport the purified saturated aliphatic carboxylic acid under a blanket of an inert gas, such as nitrogen, a noble gas or mixtures thereof.

[0071] As already mentioned before, each step (a) to (c) can be carried out continuously or discontinuously, while for step (a) semi-continuous operation is a further option. For smaller production volumes, it may be advantageous to produce saturated aliphatic carboxylic acids in a discontinuous or semi-continuous process, since such discontinuous or semi-continuous processes are usually more adaptable and easier to operate when production volumes are small. On the other hand, a continuous process in which steps (a) to (c) are carried out continuously has the advantage that it is efficient as soon as the process is started and runs stably. It is therefore a preferred option for the continuous preparation of saturated aliphatic carboxylic acids. This is particularly true for saturated aliphatic carboxylic acids that are produced in large quantities, such as propionic acid, n-butyric acid and n-pentanoic acid. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] In a typical embodiment for the continuous preparation of propionic acid, liquid propionaldehyde and gaseous oxygen are continuously fed into a jet loop reactor operated at a temperature in the range of 60-80°C and an oxygen partial pressure in the range of 0.1-0.5 MPa. To remove the heat generated by the exothermic oxidation reaction, the reactor is cooled externally by an external heat exchanger. The reaction mixture obtained by step (a) above still contains unconverted propionaldehyde in an amount of 0.1-2 mol-% relative to the propionic acid and further contains active oxygen in an amount of 0.02-1 wt.-% relative to the mixture. It is continuously passed through a heat exchanger, in which it is heated to a temperature in the range of 100-180°C, and fed into a compartmented tubular reactor, through which the reaction mixture flows in plug flow for 0.25-2 hours. The tubular reactor is operated at a total pressure of 0.1-1 MPa abs. The heat-treated mixture obtained by the above step (b) has a significantly reduced active oxygen content of only 1-100 wt.-ppm. The heat-treated mixture is then continuously fed into a distillation column in which residual high and low boilers such as propionaldehyde and acetic acid are separated off and purified propionic acid is taken off as a side stream. The propionic acid obtained by the above step (c) has a high purity and contains more than 99 wt.-% propionic acid and only 1-25 wt.-ppm active oxygen relative to the distillate.

[0073] In a typical embodiment for the discontinuous preparation of 2-methylpropionic acid (isobutyric acid), a regulated (heatable and coolable) autoclave is charged with a mixture of equal volumes of liquid 2-methylpropionaldehyde (isobutyraldehyde) and 2-methylpropionic acid, and 4 wt.-% potassium isobutyrate is added. The reactor is sealed, heated with stirring to a temperature in the range of 60-80° C., and pressurized with nitrogen to a pressure of 1-2.5 MPa abs. The reaction is started by adding oxygen, while controlling the pressure, so that the additional oxygen pressure is up to 10% of the initial nitrogen pressure. The reaction is continued until almost no oxygen is absorbed and the total pressure is essentially constant. The reactor is then depressurized, flushed with nitrogen until the off-gas contains less than 10 vol.-ppm oxygen, and repressurized with nitrogen. The reaction mixture obtained by the above step (a) still contains unconverted 2-methylpropionaldehyde in an amount of 0.1-2 mol-% based on 2-methylpropionic acid and further contains active oxygen in an amount of 0.02-1 wt.-% based on the mixture. It is then heat treated in an autoclave at a temperature in the range of 100-150°C and a total pressure of 0.1-1 MPa abs for 0.25-2 hours. The autoclave is then depressurized. The heat treated mixture obtained by the above step (b) has a significantly reduced active oxygen content of only 1-100 wt.-ppm. The heat treated mixture is then transferred into a batch distillation apparatus and distilled. The purified 2-methylpropionic acid fraction has a high purity and contains more than 99 wt.-% 2-methylpropionic acid and only 1-25 wt.-ppm active oxygen based on the distillate.

[0074] The method of the present invention allows the preparation of saturated aliphatic carboxylic acids having 3 to 5 carbon atoms by oxygen oxidation of the corresponding aldehydes, in high yield and high purity, in particular without peracids and other peroxides or at least with a very low content of them.The method is also easy to operate, works stably over a long operation time, and can produce saturated aliphatic carboxylic acids with constant high quality. EXAMPLES

[0075] Iodimetry Approximately 5 g of sample is weighed to the nearest 0.1 mg, placed in a standard reaction vial, flushed with argon and dissolved in 40 ml of a 1:1 acetic acid / chloroform mixture. The reaction vial is equipped with a condenser and placed in a stirring heating block already preheated to 80 °C. A weak argon stream is passed through the condenser to cover the sample surface. This is necessary to avoid the ingress of atmospheric oxygen. Then, 5.0 mL of saturated potassium iodide solution (approximately 60.0 g of potassium iodide dissolved in 100 mL of deionized water) is added through the condenser and the mixture is boiled under reflux for 10 min. In the next step, 40.0 mL of deionized water is added and the sample solution is titrated with a 0.01 M thiosulfate solution using a platinum electrode.

[0076] [Example 1] (Preparation of Crude Propionic Acid) Crude propionic acid was produced by oxidation of propionaldehyde with air in a technical plant with a production capacity of around 4 tons of propionaldehyde per hour. The technical plant had three reactors connected in series. The first two reactors were fully backmixed and nearly isothermal jet loop type reactors, while the third reactor was an adiabatic sieve plate tower with nearly ideal tube characteristics. The oxidation was carried out with a slight excess of air, providing 105-110% of the theoretical amount of O2 relative to the propionaldehyde, in other words a molar ratio of oxygen to propionaldehyde of 0.525-0.55. Propionaldehyde was fed only to the first reactor, and air was fed only to the first two reactors, with a distribution of approximately 90% of the total amount to the first reactor and the remaining 10% to the second reactor. All three reactors were operated at a temperature of 75° C. and a pressure of 2.2 MPa abs. The propionaldehyde conversion at the outlet of the third reactor was >99% relative to the amount of propionaldehyde fed into the first reactor.

[0077] The crude propionic acid obtained at the outlet of the third reactor contained 98.4 wt.-% propionic acid and 650 wt.-ppm active oxygen as determined by iodometric titration as described above, and was packed, stored and shipped under an inert gas atmosphere.

[0078] [Example 2] (Comparative Example) A 1 kg sample of the crude propionic acid of Example 1 was distilled in a batch distillation apparatus containing a 2 m column packed with mesh rings. The column was operated at a maximum pressure of 0.123 MPa abs, the propionic acid was distilled overhead, and the fraction boiling at 147.5±0.5° C. was collected. The recovered propionic acid fraction had a propionic acid content of 99.8 wt.-% when analyzed by gas chromatography and contained 26 wt.-ppm active oxygen when measured by iodometric titration as described above.

[0079] The propionic acid fraction obtained by the above-mentioned batch distillation showed the same propionic acid content and the same active oxygen content as the propionic acid fraction obtained in a technical plant by distillation in a commercial distillation column of technical size.

[0080] [Example 3] (Example according to the present invention) Another sample of 1 kg of the crude propionic acid of Example 1 was heated to 105° C. under nitrogen atmosphere in a glass flask and maintained under these conditions for 6 hours. This procedure reduced the active oxygen content from 650 wt.-ppm in the crude propionic acid to 89 wt.-ppm at the end of the heat treatment. The heat-treated mixture was then distilled in the same batch distillation apparatus under the same conditions as described in Example 2. The recovered propionic acid fraction had a propionic acid content of 99.8 wt.-% when analyzed by gas chromatography and contained 4 wt.-ppm active oxygen when measured by iodometric titration as described above.

[0081] [Example 4] (Example according to the present invention) A further sample of 1 kg of the crude propionic acid of Example 1 was mixed with manganese propionate to achieve a Mn concentration of 0.34 wt.-ppm in the mixture. The mixture was then heated to 100° C. under nitrogen atmosphere in a glass flask and maintained under these conditions for 6 hours. This procedure reduced the active oxygen content from 650 wt.-ppm in the crude propionic acid to 22 wt.-ppm at the end of the heat treatment. The heat-treated mixture was then distilled in the same batch distillation apparatus under the same conditions as described in Example 2. The recovered propionic acid fraction had a propionic acid content of 99.8 wt.-% when analyzed by gas chromatography and contained 3 wt.-ppm active oxygen when measured by iodometric titration as described above.

[0082] Comparative Example 2 shows that without the heat treatment method of the present invention, the distilled propionic acid has a significant active oxygen content of 26 wt.-ppm, while the heat treatment method of the present invention prior to the final distillation allows the distilled propionic acid to have a very low active oxygen content of 4 wt.-ppm in Example 3 and 3 wt.-ppm in Example 4.

[0083] The examples also show that the decomposition homogeneously catalyzed by manganese propionate gives almost the same results as the uncatalyzed decomposition, thus demonstrating that the process of the present invention can be carried out already in the absence of a catalyst. Some embodiments are given below. Item 1 1. A process for preparing saturated aliphatic carboxylic acids having 3 to 5 carbon atoms by oxidation of the corresponding aldehydes with oxygen, comprising the steps of: (a) converting the corresponding aldehyde with oxygen at a temperature of 40 to 150°C and an oxygen partial pressure of 0.001 to 1 MPa to obtain a mixture containing a saturated aliphatic carboxylic acid and 2 mol-% or less of the corresponding aldehyde based on the saturated aliphatic carboxylic acid; (b) heat treating the mixture obtained in step (a) in a liquid phase at a temperature of 80 to 250° C. and a pressure of 0.1 to 2 MPa abs for 0.25 to 100 hours; and (c) distilling the mixture obtained in step (b) in a distillation apparatus to obtain a distillate having a saturated aliphatic carboxylic acid content of 90 wt.-% or more and an active oxygen content of 0 to 25 wt.-ppm based on the distillate. A method comprising: Section 2 Item 3. The method according to item 1, wherein the saturated aliphatic carboxylic acid is propionic acid and the aldehyde is propionaldehyde. Section 3 Item 3. The method according to item 1 or 2, wherein the mixture obtained in step (a) has an active oxygen content of 0.02 to 1 wt.% based on the mixture. Section 4 The method according to any one of items 1 to 3, wherein the preparation of the saturated aliphatic carboxylic acid is carried out continuously and the heat treatment in step (b) is carried out in a tubular reactor. Section 5 The method according to any one of items 1 to 4, wherein the heat treatment in step (b) is carried out at a temperature of 100 to 180 ° C. Section 6 The method according to any one of items 1 to 5, wherein the heat treatment in step (b) is carried out for 0.25 to 5 hours. Section 7 The method according to any one of items 1 to 6, wherein the active oxygen content of the heat-treated mixture obtained in step (b) is 0 to 100 wt.-% based on the heat-treated mixture. Section 8 8. The method according to any one of items 1 to 7, wherein a distillate containing 99 wt.-% or more saturated aliphatic carboxylic acids based on the distillate is obtained. Section 9 Item 9. The method according to any one of items 1 to 8, wherein the distillate containing saturated aliphatic carboxylic acids has an active oxygen content of 0 to 10 wt.-ppm based on the distillate. Section 10 10. The method according to any one of claims 1 to 9, wherein the saturated aliphatic carboxylic acid is prepared continuously.

Claims

1. A process for preparing propionic acid by oxidation of propionaldehyde with oxygen in the absence of the addition of a homogeneous catalytic metal comprising a salt of a transition metal of groups 6 to 11 of the periodic table of the elements, comprising: (a) converting propionaldehyde with oxygen at a temperature of 40-150° C. and an oxygen partial pressure of 0.001-1 MPa to obtain a mixture containing propionic acid and 2 mol-% or less of propionaldehyde relative to propionic acid; (b) heat treating the mixture obtained in step (a) in liquid phase at a temperature of 80-250° C. and a pressure of 0.1-2 MPa abs for 0.25-100 hours; and (c) distilling the mixture obtained in step (b) in a single distillation column to obtain a distillate containing at least 90 wt.-% propionic acid and having an active oxygen content of 0 to 25 wt.-ppm based on the distillate. A method comprising:

2. 2. The method of claim 1, wherein the active oxygen content of the mixture obtained in step (a) is 0.02-1 wt.-% based on the mixture.

3. The method according to claim 1 or 2, wherein the preparation of propionic acid is carried out continuously and the heat treatment in step (b) is carried out in a tubular reactor.

4. The method according to any one of claims 1 to 3, wherein the heat treatment in step (b) is carried out at a temperature of from 100 to 180°C.

5. The method according to any one of claims 1 to 4, wherein the heat treatment in step (b) is carried out for 0.25 to 5 hours.

6. 6. The method according to claim 1, wherein the active oxygen content of the heat-treated mixture obtained in step (b) is between 0 and 100 wt.-ppm based on the heat-treated mixture.

7. The method of claim 1, wherein the distillate containing propionic acid has an active oxygen content of 0 to 10 wt.-ppm relative to the distillate.

8. The method according to claim 1, wherein the propionic acid is prepared continuously.

Images