Process for alkoxycarbonylation and subsequent hydrolysis of diisobutylene and C4 to C7 olefins

Abstract

The present invention provides a process for the alkoxycarbonylation of diisobutylene and C4 to C7 olefins in a common reaction zone. Alkoxycarbonylation is carried out with an alcohol and carbon monoxide in the presence of a homogeneous catalyst system comprising at least one metal of groups 8 to 10 of the Periodic Table of Elements or a compound thereof, a phosphorus-containing ligand and an acid.

Description

[0001] The present invention relates to a process for producing carboxylic acids or salts thereof by hydrolysis or saponification of esters produced by alkoxycarbonylation of diisobutylene and C4 to C7 olefins in a common reaction zone. The alkoxycarbonylation is carried out with an alcohol and carbon monoxide in the presence of a homogeneous catalyst system comprising at least one metal from Groups 8 to 10 of the Periodic Table of the Elements or a compound thereof, a phosphorus-containing ligand and an acid.

[0002] The production of carboxylic acid in industrial chemistry is mainly achieved by hydroformylation to produce aldehydes, followed by aldehyde oxidation to obtain carboxylic acid. Although the production of carboxylic acid by hydroformylation and subsequent oxidation has been an industrially established and proven method for decades, there is still potential for improvement (see DE 100 10 771C1 or EP 1 854 778 A1). A problem with this synthetic route is that hydroformylation uses a catalyst system containing transition metals, which is usually expensive or has high production costs. Another problem is the formation of by-products in its hydroformylation and downstream oxidation process, which are also obvious even if catalysts and / or reaction conditions are relatively mild. Another problem is the requirement for the improvement of safety equipment in the oxidation of aldehydes, because in this step, oxygen is supplied to the organic reaction mixture in pure form or by air. The explosive atmosphere produced requires separate monitoring and control, which increases complexity.

[0003] Diisobutylene is an industrially relevant product obtained by dimerization of isobutylene. Diisobutylene consists of the isomers 2,4,4-trimethylpent-1-ene (hereinafter also referred to as TMP1) and 2,4,4-trimethylpent-2-ene (hereinafter also referred to as TMP2), and the mass distribution of TMP1: TMP2 is in the range of about 78: 22 to 81: 19 (equilibrium distribution). Industrial mixtures containing C4 olefins are light petroleum fractions from refineries, C4 fractions from FC crackers or steam crackers, mixtures from Fischer-Tropsch synthesis, mixtures from butane dehydrogenation, and mixtures from metathesis or other industrial processes. C5 olefins, i.e., pentenes, are present in light petroleum fractions from refineries or crackers. Higher olefins can be obtained, in particular, by oligomerization. Both diisobutylene and C4 to C7 olefins can be converted into valuable products, such as esters formed in alkoxycarbonylation, by alkoxycarbonylation.

[0004] The problem with these methods is that special production equipment must be provided or built, and the operation of these equipment must bear the resulting expenses. Since the market for petrochemical products is quite variable in some cases, it is almost impossible to operate a special production equipment for each of the listed olefins economically. Another disadvantage is that since all equipment requires maintenance, it is almost impossible to operate the resources of each production equipment efficiently. This requires not only economic and personnel costs, but also a certain amount of energy, such as electricity or heat transfer medium.

[0005] The object of the present invention is therefore to provide a process for producing carboxylic acids or their salts which does not present the above-mentioned problems. It should be possible in particular to convert diisobutene and C4 to C7 olefins into esters by alkoxycarbonylation in a more resource-efficient manner. In addition, alternative synthesis routes should be provided which make it possible to obtain the desired carboxylic acids / salts thereof relatively easily.

[0006] The basic object is achieved by the method according to claim 1. Preferred embodiments are indicated in the dependent claims.

[0007] According to the present invention, the method for producing a carboxylic acid or a salt thereof comprises at least the following steps:

[0008] a. providing a diisobutylene stream containing 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene and providing an olefin stream containing C4 to C7 olefins;

[0009] b. alkoxycarbonylating diisobutylene and C4 to C7 olefins with alcohols and carbon monoxide in the presence of a homogeneous catalyst system comprising at least one metal of Groups 8 to 10 of the Periodic Table of Elements or a compound thereof, a phosphorus-containing ligand and an acid in a common reaction zone to obtain a liquid product mixture comprising at least esters formed by alkoxycarbonylation, the homogeneous catalyst system, unreacted olefins (i.e., unreacted diisobutylene and unreacted C4 to C7 olefins), and unreacted alcohol;

[0010] c. removing the homogeneous catalyst system from the liquid product mixture to obtain a crude product mixture, the crude product mixture comprising at least an ester formed by alkoxycarbonylation, unreacted olefin and unreacted alcohol;

[0011] d. subjecting the crude product mixture to a distillation treatment in at least one distillation column to remove the unreacted alcohol and the unreacted olefin to obtain an ester mixture containing the formed esters, wherein the unreacted alcohol and the unreacted olefin are preferably removed and recycled to the alkoxycarbonylation in step b;

[0012] e. hydrolyzing or saponifying the ester obtained from step d in the presence of an acidic catalyst or a saponifying agent to obtain a reaction mixture comprising at least a carboxylic acid or a salt thereof, an eliminated alcohol and unreacted ester; and

[0013] f. removing the carboxylic acid or salt thereof formed in step e in at least one separation process step selected from, for example, distillation, extraction, thermal separation by crystallization and membrane separation.

[0014] Thus, the process according to the invention involves the simultaneous reaction of diisobutylene and C4 to C7 olefins in a single common reaction zone, followed by hydrolysis or saponification of the ester to the corresponding carboxylic acid or its salt. Such a procedure has many advantages.

[0015] The method described enables flexible response to the market, in particular to small production volumes. In addition, only one production plant is required, and even when market demand fluctuates, the plant can be operated more efficiently and thus in a more resource-saving manner. The specific boiling sequence also makes it possible to separate the resulting products of the olefins used respectively, while the reactants can be returned to the reaction directly or after further processing.

[0016] The diisobutene stream provided in step a comprises 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene. In a preferred embodiment, the proportion of 2,4,4-trimethylpent-1-ene in the diisobutene stream is at least 60 mol%, preferably at least 70 mol%, based on the total diisobutene stream. This stream can be, for example, a diisobutene stream produced by dimerization of isobutene or an isobutene-containing hydrocarbon mixture according to the process disclosed in EP 1 360 160 B1. In addition, the diisobutene stream used here can be obtained as an unreacted residual stream in a carbonylation process, for example in an alkoxycarbonylation or in a hydroformylation.

[0017] Step a comprises not only providing a diisobutylene stream, but also providing an olefin stream containing C4 to C7 olefins for the process of the present invention. In a preferred embodiment, the present invention uses an olefin stream comprising C4 olefins, particularly preferably a C4 olefin stream. The corresponding stream is known to those skilled in the art and can be obtained on a large industrial scale.

[0018] Olefin streams comprising C4 olefins include, for example, light petroleum fractions from an oil refinery, C4 fractions from a FC cracker or steam cracker, mixtures from Fischer-Tropsch synthesis, mixtures from butane dehydrogenation, or streams from metathesis or from other industrial processes. For example, C4 olefin streams suitable for the process of the present invention can be obtained from the C4 fraction of a steam cracker. C5 olefins, i.e., amylenes, are present in light petroleum fractions from an oil refinery or cracker. C6 olefins can be obtained, for example, by dimerization of propylene. C7 olefins can be obtained, for example, by dimerization of propylene and butenes.

[0019] The stream provided in step a, i.e. the diisobutylene stream and the C4 to C7 olefin stream, is passed to the alkoxycarbonylation in step b. The stream can be sent to the alkoxycarbonylation in step b individually and separately or premixed. The diisobutylene stream and the C4 to C7 olefin stream are preferably mixed before the alkoxycarbonylation in step b. A particularly preferred embodiment of the present invention in fact provides that before the alkoxycarbonylation in step b, the diisobutylene stream, the C4 to C7 olefin stream, the alcohol and the homogeneous catalyst system are mixed, particularly in a suitable mixing vessel. If there is a recycle stream to the reaction, i.e., for example, by recycling the catalyst system, the recycle stream can also be passed to the mixing vessel.

[0020] Step b comprises reacting diisobutylene (i.e. 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene) and C4 to C7 olefins with carbon monoxide (CO) and alcohol to obtain esters. Therefore, the number of carbon atoms in the ester is increased by 1 carbon atom from carbon monoxide compared to the olefins used, and the number of carbon atoms in the alcohol used is increased. Therefore, diisobutylene (8 carbon atoms) produces an ester having 9 carbon atoms in the acid portion of the ester plus the carbon atoms of the alcohol in the alcohol portion of the ester. Then the C4 to C7 olefins are converted into esters having 5 to 8 carbon atoms in the acid portion accordingly.

[0021] In the alkoxycarbonylation of step b, carbon monoxide can be provided directly in the form of an input mixture, or by adding a carbon monoxide-containing gas selected from synthesis gas, water gas, generator gas (generator gas) and other carbon monoxide-containing gases. Carbon monoxide can also be provided by first separating the carbon monoxide-containing gas into its components in a manner known to those skilled in the art and passing the carbon monoxide to the reaction zone. Carbon monoxide may still contain a certain proportion of hydrogen or other gases, because complete separation is almost impossible.

[0022] The alcohol used in the reaction of step b is preferably a monohydric alcohol having 1 to 4 carbon atoms, in particular methanol, ethanol, propanol or butanol. This then gives an ester having a chain length corresponding to the alcohol used. For example, the use of methanol results in the formation of methyl esters.

[0023] The alcohol used in step b, preferably a monohydric alcohol having 1 to 4 carbon atoms, in particular methanol, ethanol, propanol or butanol, is used in a molar ratio to the total amount of all olefins, i.e. diisobutene and C4 to C7 olefins (alcohol:olefin in the range of 10:1 to 1:1, preferably 8:1 to 1.5:1, particularly preferably 7:1 to 2:1). The alcohol is therefore added in at least the same molar amount, but preferably in a molar excess, based on the olefins used.

[0024] The reaction according to the invention in step b is carried out in the presence of a homogeneous catalyst system comprising at least one metal of Groups 8 to 10 of the Periodic Table of Elements (PTE) or a compound thereof, a phosphorus-containing ligand and an acid as a promoter. It goes without saying that a suitable catalyst must be able to alkoxycarbonylate all olefins, i.e. diisobutylene and C4 to C7 olefins. In the alkoxycarbonylation reaction solution of step b, the content of metals of Groups 8 to 10 of the Periodic Table of Elements, in particular palladium, is preferably 100 to 500 ppm, further preferably 150 to 450 ppm, particularly preferably 180 to 350 ppm.

[0025] The Group 8 to 10 metal of PTE is preferably palladium. Palladium is preferably used in the form of a precursor compound as a palladium compound coordinated by a phosphorus-containing ligand. Examples of palladium compounds that can be used as precursor compounds are palladium chloride [PdCl2], palladium (II) acetylacetonate [Pd(acac)2], palladium (II) acetate [Pd(OAc)2], dichloro (1,5-cyclooctadiene) palladium (II) [Pd(cod)2Cl2], bis (dibenzylideneacetone) palladium (0) [Pd(dba)2], tris (dibenzylideneacetone) dipalladium (0) [Pd2(dba)3], bis (acetonitrile) dichloro palladium (II) [Pd(CH3CN)2Cl2], (cinnamyl) dichloro palladium [Pd(cinnamyl)Cl2]. Preferably, the compound [Pd(acac)2] or [Pd(OAc)2] is used. The concentration of palladium metal in step b is preferably between 0.01 and 0.6 mol %, preferably between 0.03 and 0.3 mol %, more preferably between 0.04 and 0.2 mol %, based on the molar amount of hydrocarbon used.

[0026] Suitable phosphorus-containing ligands for the catalyst system according to the invention preferably have a bidentate structure. Preferred phosphorus-containing ligands for the catalyst system according to the invention are benzene-based diphosphine compounds, such as disclosed in EP 3 121 184 A2, for example. The ligand can be combined with palladium in a preliminary reaction, so that the palladium-ligand complex is passed into the reaction zone, or added to the reaction in situ and combined with palladium there. The molar ratio of ligand: metal for the reaction described in step b1 can be: 1 to 10: 1, preferably 2: 1 to 6: 1, particularly preferably 3: 1 to 5: 1.

[0027] The homogeneous catalyst system further comprises an acid, in particular Acid or Lewis acid. The Lewis acid used is preferably aluminum trifluoromethanesulfonate, aluminum chloride, aluminum hydride, trimethylaluminum, tri(pentafluorophenyl)borane, boron trifluoride, boron trichloride or a mixture thereof. Among the Lewis acids mentioned, preference is given to using aluminum trifluoromethanesulfonate. The Lewis acid is preferably added in a molar ratio of Lewis acid:ligand of 1:1 to 20:1, preferably 2:1 to 15:1, particularly preferably 5:1 to 10:1.

[0028] Suitable The acid preferably has an acid strength of pKa ≤ 5, more preferably an acid strength of pKa ≤ 3. The acid strength pKa relates to the pKa determined under standard conditions (25° C., 1.01325 bar). For polyacids, the acid strength pKa in the context of the present invention relates to the pKa of the first proton transfer step. The acid is preferably used in a ratio of 1:1 to 15:1, preferably 2:1 to 10:1, particularly preferably 3:1 to 5:1. The acid:ligand was added in a molar ratio.

[0029] Used The acid can specifically be perchloric acid, sulfuric acid, phosphoric acid, methylphosphonic acid or sulfonic acid. Suitable sulfonic acids are, for example, methanesulfonic acid, trifluoromethanesulfonic acid, tert-butylsulfonic acid, p-toluenesulfonic acid (PTSA), 2-hydroxypropane-2-sulfonic acid, 2,4,6-trimethylbenzenesulfonic acid and dodecylsulfonic acid. Particularly preferred acids are sulfuric acid, methanesulfonic acid, trifluoromethanesulfonic acid and p-toluenesulfonic acid. The acid is preferably sulfuric acid.

[0030] The alkoxycarbonylation in step b is preferably carried out at a temperature in the range of from 60° C. to 120° C., further preferably from 65° C. to 110° C., particularly preferably from 70° C. to 100° C. The alkoxycarbonylation in step b is further preferably carried out at a carbon monoxide pressure of from 10 to 35 bar, preferably from 12.5 to 30 bar, more preferably from 15 to 25 bar.

[0031] The alkoxycarbonylation in step b is carried out in a suitable reaction zone. The reaction zone for the reaction comprises at least one reactor, but may also consist of two or more reactors arranged in parallel or in series. At least one reactor may be selected in particular from the group consisting of a stirred tank reactor, a loop reactor, a jet loop reactor, a bubble column reactor or a combination thereof. If more than one reactor is used, the reactors may be identical or different.

[0032] The alkoxycarbonylation in step b above obtains a liquid product mixture comprising at least the ester formed by the alkoxycarbonylation, the homogeneous catalyst system, unreacted olefins and unreacted alcohol. In addition, the liquid product mixture may contain low-boiling by-products, such as dimethyl ether or formic acid and / or high-boiling components such as ligand decomposition products.

[0033] The product mixture thus obtained is supplied to subsequent step c to remove the homogeneous catalyst system from the liquid product mixture. The supply of the product mixture can be carried out after removing the low-boiling components, for example, after removing, for example, a portion of unreacted alcohol and / or low-boiling by-products by distillation. Alcohol can be recycled to step b, i.e. alkoxycarbonylation. The low-boiling by-products can be discharged from the method under the condition of purging so that they do not accumulate in the method. Removing the homogeneous catalyst system obtains a crude product mixture, which at least comprises an ester formed by alkoxycarbonylation, unreacted olefins and at least a portion of unreacted alcohol.

[0034] In step c, the homogeneous catalyst system is removed to obtain a crude product mixture and can be carried out by means of various separation methods, for example, by thermal separation or by membrane separation. Suitable methods are familiar to those skilled in the art. In the context of the present invention, the removal of the homogeneous catalyst system in step c is preferably carried out by membrane separation. As is well known, membrane separation produces retentate and permeate. The homogeneous catalyst system will accumulate in the retentate. In this case, the permeate constitutes the aforementioned crude product mixture and is sent to the subsequent step d distillation process.

[0035] According to the present invention, the retentate is preferably recycled to the alkoxycarbonylation / reaction zone of alkoxycarbonylation in step b). This allows the catalyst system to be reused. In the preferred continuous execution of the claimed method, this produces a catalyst cycle, wherein only a small amount of catalyst loss associated with the method needs to be compensated at most. If, according to a preferred embodiment, before the alkoxycarbonylation in step b, the diisobutylene stream, the olefin stream, the alcohol and the homogeneous catalyst system are mixed in a suitable mixing vessel, the retentate is sent to the mixing vessel. In the recycling of the retentate, a purge stream that may contain inert alkanes, low-boiling by-products (such as ethers), possible decomposition products of the catalyst system or other impurities such as trace water or nitrogen can be taken out in addition to avoid accumulation in the reaction zone.

[0036] Any suitable membrane material can be used for membrane separation. Preferably, an OSN (organic solvent nanofiltration) membrane material is used in the membrane separation of step c of the method of the present invention. Such a membrane material preferably consists of at least a separation active layer (also referred to as: active separation layer) and a substructure on which the separation active layer is present. The membrane material of the present invention preferably consists of at least a separation active layer and a substructure.

[0037] The membrane material consisting of at least the separation active layer and the substructure should be acid-stable so that the membrane material is not damaged by the acid present in the liquid product mixture. In the context of the present invention, the term "acid-stable" means that the membrane material is stable in the presence of the acid of the catalyst system for at least 300 h and maintains separation performance.

[0038] The substructure preferably has a porous structure, and the porous structure is permeable for the permeate that has passed through the separation active layer. The substructure has a stabilizing function and is used as the support of the separation active layer. The substructure can be composed of any suitable porous material in principle. Suitable materials are familiar to those skilled in the art. However, the prerequisite is that the material is stable for acid and alkali. The substructure can be composed of the same material as the separation active layer. The preferred material for the substructure includes plastics, such as polypropylene (PP), polyethylene (PE) or non-condensation polymers that are not easy to hydrolyze or alcoholysis cracking, such as polysulfone, polytetrafluoroethylene (PTFE), polyethersulfone (PES), polyvinylidene fluoride (PVDF) or polyacrylonitrile (PAN).

[0039] The separation active layer according to the invention is preferably composed of a PAEK (polyaryletherketone) polymer. PAEK has the following specific feature: within the repeating unit, the aromatic groups are alternately connected via ether functions and ketone functions. The separation active layer preferably according to the invention is composed of PEEK (polyetheretherketone). As the separation active layer, it is particularly preferred to use a PEEK polymer with a sulfonation degree of less than 20%, particularly preferably a sulfonation degree of less than 10%. The corresponding PEEK polymers and their production are described in WO 2015 / 110843 A1.

[0040] The membrane separation in step c is preferably carried out at a temperature in the range of from 25° C. to 100° C., further preferably from 30° C. to 80° C., particularly preferably from 40° C. to 70° C. In order to bring the product mixture to the prevailing temperature preferably for membrane separation, the product mixture can be cooled. In addition to active cooling using a coolant, cooling can also be achieved by a heat exchanger, wherein thermal energy is transferred to another stream, thereby cooling the product mixture and heating the other stream.

[0041] The transmembrane pressure (TMP) in the membrane separation in step c is preferably in the range of from 10 to 60 bar, further preferably in the range of from 15 to 55 bar, particularly preferably in the range of from 20 to 50 bar. The permeate side pressure here can be higher than atmospheric pressure, preferably up to 15 bar, preferably 3 to 7 bar. The difference between TMP and the permeate side pressure gives the retentate side pressure. In a preferred embodiment, particularly in the case of pressure ratio and permeate side pressure, care should be taken to ensure that the pressure is set according to the temperature in the hydrocarbon used, the alcohol used and the system, thereby avoiding evaporation after passing through the membrane. Evaporation may cause unstable operation.

[0042] In the subsequent step d, the crude product mixture / permeate from the membrane separation is treated by distillation in at least one distillation column to remove unreacted alcohol and unreacted olefins, i.e. unreacted diisobutylene and unreacted C4 to C7 olefins. This gives an ester mixture comprising the formed esters. Thus, the ester mixture contains esters formed from diisobutylene and C4 to C7 olefins.

[0043] In the distillative treatment of the crude product mixture / permeate in step d, unreacted alcohol and unreacted olefins, i.e. unreacted diisobutylene and unreacted C4 to C7 olefins, are obtained at the top of at least one distillation column. The mixture of esters thus formed accumulates at the bottom of at least one distillation column. The overhead stream containing the unreacted alcohol and unreacted olefins removed in at least one distillation column can be recycled to the alkoxycarbonylation / to the reaction zone in step b). If the components used are mixed before the alkoxycarbonylation, the overhead stream is supplied to the mixing according to definition. This allows the process of the present invention to be operated continuously with the highest possible yield. A purge can be taken from the recycled overhead stream to discharge low-boiling by-products from the process.

[0044] The distillation treatment for removing unreacted alcohol and unreacted olefin in step d can be carried out in a distillation column. It is possible to carry out the distillation treatment in two or more distillation columns to remove the unreacted alcohol and unreacted olefin in step d. However, this will require significantly higher equipment costs. Therefore, it is preferred that the distillation treatment in step d is carried out in a single distillation column.

[0045] The pressure in the distillation column in the distillation treatment in step d is preferably in the range of from 0.3 to 2 bar, more preferably in the range of from 0.4 to 1 bar, particularly preferably in the range of from 0.5 to 0.7 bar. The temperature of the bottom of the distillation column in the distillation treatment in step d is preferably in the range of from 80° C. to 160° C. In the distillation treatment in step d, the temperature of the top of the distillation column is preferably in the range of from 30° C. to 80° C. In addition, it is preferred that the reflux ratio in the distillation column is between 1 and 2. The distillation column used for the removal in step d preferably comprises 10 to 30 theoretical plates. The distillation column may contain a high-performance structured packing. Suitable high-performance structured packings are known to those skilled in the art.

[0046] As described above, the distillation process results in an ester mixture comprising esters formed from diisobutylene and C4 to C7 olefins. In order to obtain both esters in the highest possible purity, a further distillation step may be carried out to separate the esters of the ester mixture from one another. The esters from the C4 to C7 olefins will be obtained at the top of the distillation column and the esters from the diisobutylene will be obtained at the bottom of the distillation column.

[0047] The present process is suitable for the alkoxycarbonylation of diisobutylene and C4 olefins with alcohols, preferably alcohols having 1 to 4 carbon atoms, and subsequent hydrolysis or esterification. In the context of the present invention, certain combinations of olefins and alcohols are particularly preferred:

[0048] In a preferred embodiment, the method involves the alkoxycarbonylation of diisobutylene and C4 olefins (i.e. 1-butene, cis- and / or trans-2-butene, isobutylene or a mixture thereof) with methanol. The ester formed in the alkoxycarbonylation of diisobutylene with methanol is 3,5,5-trimethylhexanoic acid methyl ester. The alkoxycarbonylation of different butenes with methanol can form methyl valerate, methyl 2-methylbutyrate and / or methyl 3-methylbutyrate. Therefore, the use of a mixture of butenes also results in a mixture of esters.

[0049] In a preferred embodiment, the method involves the alkoxycarbonylation of diisobutylene and C4 olefins (i.e. 1-butene, cis- and / or trans-2-butene, isobutylene or a mixture thereof) with ethanol. The ester formed by diisobutylene and ethanol in the alkoxycarbonylation reaction is 3,5,5-trimethylethyl hexanoate. The alkoxycarbonylation of different butenes with ethanol can form ethyl valerate, ethyl 2-methylbutyrate and / or ethyl 3-methylbutyrate. Therefore, the use of a mixture of butenes also results in a mixture of the esters.

[0050] The ester produced in step b and removed and optionally purified in step d is then subjected to hydrolysis or saponification in step e. The acidic catalyst used or the saponifying agent used cleaves the ester group, thereby forming a carboxylic acid or a carboxylate salt and allowing recovery of the alcohol bound in the ester formation in step b. The saponification thus yields a reaction mixture comprising at least the carboxylic acid or its salt, the eliminated alcohol and the unreacted ester.

[0051] As described above, the process according to the invention forms a mixture of esters containing esters formed from diisobutylene and esters formed from C4 to C7 olefins. In the hydrolysis or saponification in step e, this ester mixture can be used to obtain the corresponding carboxylic acid in a reaction in a single reaction zone. However, the esters obtained from diisobutylene and the esters obtained from C4 to C7 olefins can also be separated from each other in advance, and the hydrolysis or saponification is carried out separately from each other in the respective dedicated reaction zones. The simultaneous hydrolysis of the two esters has the advantage of reducing equipment costs and is preferred according to the invention.

[0052] The alcohol recovered from the hydrolysis or saponification in step e can be removed from the resulting mixture in a subsequent process step and recycled to the reaction zone in step b.

[0053] Hydrolysis is a known chemical reaction in which an ester is converted into a carboxylic acid using an acidic catalyst with elimination of an alcohol. Typical conditions for hydrolysis are known to those skilled in the art.

[0054] The hydrolysis in step e is carried out in the presence of an acidic heterogeneous or homogeneous catalyst. Known homogeneous catalysts are acidic compounds such as Acids, in particular HCl, H2SO4, phosphoric acid, p-toluenesulfonic acid and 4-dodecylbenzenesulfonic acid, or Lewis acids, in particular AlCl3, ZnCl2, HfCl4.2THF and Al(OTf)3.

[0055] Well-known heterogeneous catalysts include acidic compounds such as cation exchangers, acidic H-zeolites, acidic functionalized metal oxides with silica and heteropolyacids. Suitable heterogeneous catalysts are in particular 15. IR 120(H), HClO4-SiO2, 3-propylsulfonic acid functionalized silica, Cs2.5H0.5PW12O40, H-ZSM5, H-ZSM-5-C18, Z-beta-H-25, Z-beta-H-38, Z-beta-H-150, Z-beta-H-360, ZYH-60, ZYH-80 or CZB 150 (BEA 150) (Clariant).

[0056] In the subsequent step f, the reaction mixture from the hydrolysis in step e is subjected to at least one separation process step selected from the group consisting of: thermal separation such as distillation, extraction, crystallization, or further membrane separation, in order to remove the carboxylic acid or its salt formed in step e from the remaining reaction mixture. The separation process is preferably distillation. Suitable process conditions are known to those skilled in the art. Multistage distillation can also be performed.

[0057] Saponification is a known chemical reaction in which an ester is converted into a carboxylate by eliminating an alcohol using an alkaline or enzymatic saponifying agent. Typical conditions for saponification are known to those skilled in the art.

[0058] The saponification in step e is carried out in the presence of a saponifying agent. Known saponifying agents include alkaline compounds such as potassium hydroxide, potassium carbonate (bicarbonate), sodium hydroxide, sodium carbonate (bicarbonate) or amine compounds. It is also possible to carry out saponification with an enzymatic saponifying agent, in particular an esterase.

[0059] In a subsequent step f of removing the carboxylic acid or its salt formed in step e from the remaining reaction mixture, the reaction mixture from the saponification in step e is subjected to at least one separation process step selected from the group consisting of, for example, distillation, extraction, thermal separation by crystallization or further membrane separation. The separation process is preferably distillation. Suitable process conditions are known to those skilled in the art. It is also possible to carry out a multi-stage distillation.

[0060] The alcohol used in step b and recovered via hydrolysis or saponification can also be removed in at least one separation process step and recycled to the reaction zone. During the recycling, a purge stream can be withdrawn, for example, to remove by-products from the process. Furthermore, if the two esters of diisobutene and C4 to C7 olefins are hydrolyzed or saponified together, the two carboxylic acids or their salts can also be separated from one another.

[0061] The present invention is explained below with reference to Examples. The Examples relate to preferred embodiments and should not be construed as limiting the present invention.

Claims

1. A method for producing a carboxylic acid or a salt thereof, wherein the method comprises at least the following steps: a. providing a diisobutylene stream containing 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene and providing an olefin stream containing C4 to C7 olefins; b. alkoxycarbonylating diisobutylene and C4 to C7 olefins with alcohols and carbon monoxide in a common reaction zone in the presence of a homogeneous catalyst system comprising at least one metal from Groups 8 to 10 of the Periodic Table of Elements or a compound thereof, a phosphorus-containing ligand and an acid, to obtain a liquid product mixture comprising at least the ester formed by the alkoxycarbonylation, the homogeneous catalyst system, unreacted olefins and unreacted alcohol; c. removing the homogeneous catalyst system from the liquid product mixture to obtain a crude product mixture, the crude product mixture comprising at least the ester formed by the alkoxycarbonylation, the unreacted olefin and the unreacted alcohol; d. subjecting the crude product mixture to a distillation treatment in at least one distillation column to remove the unreacted alcohol and the unreacted olefin to obtain an ester mixture containing the formed esters; e. hydrolyzing or saponifying the ester obtained from step d in the presence of an acidic catalyst or a saponifying agent to obtain a reaction mixture comprising at least the carboxylic acid or a salt thereof, the eliminated alcohol and the unreacted ester; and f. removing the carboxylic acid or salt thereof formed in step e in at least one separation process step selected from, for example, distillation, extraction, thermal separation by crystallization and membrane separation.

2. The process according to claim 1, wherein the alkoxycarbonylation in step b is carried out at a temperature of 60 to 120°C, preferably 65 to 110°C, particularly preferably 70 to 100°C.

3. The process according to claim 1 or 2, wherein the alkoxycarbonylation in step b is carried out at a carbon monoxide pressure of 10 to 35 bar, preferably 12.5 to 30 bar, particularly preferably 15 to 25 bar.

4. The process according to any one of the preceding claims, wherein the content of metals from Groups 8 to 10 of the Periodic Table of the Elements, in particular palladium, in the alkoxycarbonylation reaction solution in step b is 100 to 500 ppm, preferably 150 to 450 ppm, particularly preferably 180 to 350 ppm. 5 . The process according to claim 1 , wherein the alcohol is a monohydric alcohol having 1 to 4 carbon atoms, in particular methanol, ethanol, propanol or butanol.

6. The process according to any of the preceding claims, wherein the proportion of 2,4,4-trimethylpent-1-ene in the diisobutene stream is at least 60 mol%, preferably at least 70 mol%.

7. The process according to any one of the preceding claims, wherein the removal of the homogeneous catalyst system in step c is carried out by membrane separation.

8. The process of claim 7, wherein the homogeneous catalyst system accumulates in the retentate.

9. The process according to any one of the preceding claims, wherein the diisobutylene stream, the olefin stream, the alcohol and the homogeneous catalyst system are first mixed in a mixing vessel before they are passed into the reaction zone.

10. The process according to any one of the preceding claims, wherein the distillation treatment in step d. is carried out in a single distillation column. 11 . The process according to claim 10 , wherein the pressure in the distillation column is in the range from 0.3 to 2 bar, preferably from 0.4 to 1 bar, particularly preferably from 0.5 to 0.7 bar.

12. The method according to claim 10 or 11, wherein the temperature of the bottom of the distillation column is in the range from 80°C to 160°C.

13. The process according to any one of claims 10 to 12, wherein the temperature at the top of the distillation column is in the range from 30°C to 80°C.

14. The process according to any one of the preceding claims, wherein C4 olefins and methanol are used, with the result that the ester mixture obtained is a mixture of methyl 3,5,5-trimethylhexanoate and methyl valerate, methyl 2-methylbutyrate, methyl 3-methylbutyrate or mixtures thereof.

15. The process according to any one of the preceding claims, wherein in step d, unreacted alcohol and unreacted olefin are separated and recycled to the alkoxycarbonylation in step b.