METHOD FOR THE ALKOXYCARBONYLATION OF DI-ISOBUTEN AND A C4 TO C7 OLEFIN AND SUBSEQUENT HYDROLYSIS

Description

[0001] The present invention relates to a process for the production of carboxylic acids or their salts by hydrolysis or saponification of esters formed by alkoxycarbonylation of diisobutene and a C4 to C7 olefin 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 group 8 to 10 of the periodic table of elements or a compound thereof, a phosphorus-containing ligand, and an acid.

[0002] The production of carboxylic acids in industrial chemistry is largely achieved via hydroformylation to synthesize an aldehyde, followed by oxidation of the aldehyde to the carboxylic acid. Although the production of carboxylic acids via hydroformylation with subsequent oxidation has been an industrially established and proven process for decades, it still exhibits potential for improvement (see DE 100 10 771 C1 or EP 1 854 778 A1). One problem with this synthesis route is the use of transition metal-containing catalyst systems in the hydroformylation, which are typically expensive or require costly production. Another problem is the formation of byproducts in both the hydroformylation and the subsequent oxidation process, which can be observed even when no catalyst is used and / or the reaction conditions are comparatively mild.Another problem is the increased need for safety equipment during the oxidation of aldehydes, since oxygen, either in pure form or from air, is introduced into the organic reaction mixture during this step. The resulting explosive atmosphere requires separate and more complex monitoring and control. Document EP 3 750 865 A1 discloses a process for the alkoxycarbonylation of di-isobutene. Document CA 2 973 451 A1 discloses a process for the alkoxycarbonylation of an olefin mixture.

[0003] Di-isobutene is a technically relevant product obtained by the dimerization of isobutene. Di-isobutene 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) with a mass distribution TMP1 : TMP2 in the range of approximately 78:22 to 81 : 19 (equilibrium distribution). Technical mixtures containing C4 olefins include light naphtha fractions from refineries, C4 fractions from FC or steam crackers, mixtures from Fischer-Tropsch syntheses, mixtures from butane dehydrogenation, and mixtures produced by metathesis or other technical processes. C5 olefins, i.e., pentenes, are contained in light naphtha fractions from refineries or crackers. The higher olefins can be obtained, in particular, by oligomerization reactions.Both di-isobutene and C4 to C7 olefins can be converted by alkoxycarbonylation to valuable products such as the esters formed during alkoxycarbonylation.

[0004] The problem with such processes is that separate production facilities must be built or constructed, and these facilities require considerable effort to operate. Since the markets for petrochemical products are sometimes quite volatile, operating separate production facilities for each of the aforementioned olefins is hardly economically viable. Another disadvantage is that resource-efficient operation of multiple production facilities is virtually impossible, as all facilities require maintenance. This not only involves economic and personnel costs but also requires significant amounts of energy, such as electricity or heat transfer fluids.

[0005] The object of the present invention was therefore to provide a process for the production of carboxylic acids or their salts that does not exhibit the aforementioned problems. In particular, both diisobutene and C4 to C7 olefins should be able to be converted to esters via alkoxycarbonylation in a more resource-efficient manner. Furthermore, an alternative synthetic route should be obtained by which the desired carboxylic acids or their salts can be accessed relatively easily.

[0006] The underlying problem could be solved by the method described in claim 1. Preferred embodiments are specified in the dependent claims.

[0007] According to the invention, the process for producing carboxylic acids or their salts comprises at least the following steps: a. Providing a di-isobutene stream containing 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene, and providing an olefin stream containing the C4 to C7 olefin; b. Alkoxycarbonylation of di-isobutene and the C4 to C7 olefin with an alcohol and carbon monoxide in the presence of a homogeneous catalyst system comprising at least one metal of group 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, yielding a liquid product mixture comprising at least the esters formed by the alkoxycarbonylation, the homogeneous catalyst system, unreacted olefins (i.e., unreacted di-isobutene and unreacted C4 to C7 olefins), and unreacted alcohol; c.Separation of the homogeneous catalyst system from the liquid product mixture to obtain a crude product mixture comprising at least the esters formed by the alkoxycarbonylation, the unreacted olefins, and the unreacted alcohols; d. Distillative work-up of the crude product mixture in at least one distillation column to separate the unreacted alcohols and the unreacted olefins to obtain an ester mixture containing the formed esters, preferably separating the unreacted alcohol and unreacted olefins and recycling them to the alkoxycarbonylation in step b; e. Hydrolysis or saponification of the esters 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 acids or their salts, the eliminated alcohol, and the unreacted esters; and f.Separation of the carboxylic acids or their salts formed in step e in at least one separation process step, selected from thermal separation, for example distillation, extraction, crystallization and membrane separation.

[0008] The process according to the invention thus relates to the simultaneous reaction of di-isobutene and C4 to C7 olefins in a single common reaction zone with subsequent hydrolysis or saponification of the esters to the corresponding carboxylic acids or their salts. Such a process has a multitude of advantages.

[0009] The described process allows for flexible responses to markets, particularly those requiring small production volumes. Furthermore, only one production plant is needed, which can be operated more efficiently and thus more resource-conservingly, even under fluctuating market demands. The unique boiling sequence also makes it possible to separate the products of each olefin used, while the reactants can be returned to the reaction directly or after further processing.

[0010] The di-isobutene stream provided in step a contains 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 di-isobutene stream is at least 60 mol%, preferably at least 70 mol%, based on the total di-isobutene stream. Such streams can be di-isobutene streams produced by dimerization from isobutene or isobutene-containing hydrocarbon mixtures, for example, according to the process disclosed in EP 1 360 160 B1. Furthermore, the di-isobutene streams to be used here can be obtained as unreacted residual streams from carbonylation processes, for example, in alkoxycarbonylation or hydroformylation.

[0011] In addition to the diisobutene stream, an olefin stream containing the C4 to C7 olefin used in the process according to the invention is provided in step a. In a preferred embodiment of the present invention, an olefin stream containing C4 olefins is used, particularly preferably a C4 olefin stream. Such streams are known to those skilled in the art and are available on an industrial scale.

[0012] Olefin streams containing C4 olefins include, for example, light naphtha fractions from refineries, C4 fractions from FC or steam crackers, mixtures from Fischer-Tropsch syntheses, mixtures from butane dehydrogenation, or streams generated by metathesis or other industrial processes. For example, suitable C4 olefin streams for the process according to the invention can be obtained from the C4 fraction of a steam cracker. C5 olefins, i.e., pentenes, are contained in light naphtha fractions from refineries or crackers. C6 olefins can be obtained, for example, by the dimerization of propene. C7 olefins can be obtained, for example, by the dimerization of propylene and butene.

[0013] The streams provided in step a, i.e., the di-isobutene stream and the C4 to C7 olefin stream, are directed to the alkoxycarbonylation in step b. These streams can be directed individually and separately to the alkoxycarbonylation in step b or mixed beforehand. Preferably, the di-isobutene stream and the C4 to C7 olefin stream are mixed before the alkoxycarbonylation in step b. In a particularly preferred embodiment of the present invention, the di-isobutene stream, the C4 to C7 olefin stream, the alcohol, and the homogeneous catalyst system are even mixed before the alkoxycarbonylation in step b, particularly in a suitable mixing vessel. If there is a recycling stream for the reaction, e.g., by recirculation of the catalyst system, this recycling stream can also be directed to the mixing vessel.

[0014] The diisobutenes, i.e., 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene, and the C4 to C7 olefin are reacted with carbon monoxide (CO) and an alcohol in step b to form an ester. The number of carbon atoms in the ester increases by one carbon atom from the carbon monoxide compared to the original olefin, as well as by the number of carbon atoms from the alcohol. Thus, the diisobutenes (8 carbon atoms) yield an ester with 9 carbon atoms in the acid moiety, plus the carbon atoms from the alcohol in the alcohol moiety. Similarly, the C4 to C7 olefins yield esters with 5 to 8 carbon atoms in the acid moiety.

[0015] The carbon monoxide can be provided directly as a feed mixture in step b of the alkoxycarbonylation or by adding a carbon monoxide-containing gas selected from synthesis gas, water gas, producer gas and other carbon monoxide-containing gases.

[0016] It is also possible to provide the carbon monoxide by first separating the carbon monoxide-containing gas into its components in a manner known to those skilled in the art and then directing the carbon monoxide to the reaction zone. The carbon monoxide may still contain a certain proportion of hydrogen or other gases, because complete separation is hardly achievable.

[0017] The alcohol used in step b is preferably a monohydric alcohol with 1 to 4 carbon atoms, in particular methanol, ethanol, propanol, or butanol. This then yields esters with the corresponding chain length of the alcohol used. For example, when methanol is used, the methyl ester is formed.

[0018] The alcohol used in step b, preferably a monohydric alcohol with 1 to 4 carbon atoms, in particular methanol, ethanol, propanol, or butanol, is added in a molar ratio to the total amount of all olefins, i.e., the diisobutenes and the C4 to C7 olefins, alcohol:olefin in the range of 10:1 to 1:1, preferably 8:1 to 1.5:1, and particularly preferably 7:1 to 2:1. The alcohol is thus added—relative to the olefins used—in at least the same molar amount, but preferably in a molar excess.

[0019] 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 from group 8 to 10 of the periodic table of elements (PSE) or a compound thereof, a phosphorus-containing ligand, and an acid as a co-catalyst. It is understood that a suitable catalyst must be capable of alkoxycarbonylating all olefins, i.e., the diisobutenes and the C4 to C7 olefins. The content of the metal from group 8 to 10 of the periodic table of elements, in particular palladium, in the reaction solution during the alkoxycarbonylation in step b is preferably 100 to 500 ppm, more preferably 150 to 450 ppm, and particularly preferably 180 to 350 ppm.

[0020] The metal from groups 8 to 10 of the periodic table is preferably palladium. The palladium is preferably used in the form of a precursor compound coordinated by the 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)dichloropalladium(II) [Pd(CH₃CN)₂Cl₂], and palladium(cinnamyl) dichloride [Pd(cinnamyl)Cl₂]. Preferably the compounds [Pd(acac) 2 ] or [Pd(OAc) 2 ] are used.The metal concentration of palladium in step b is preferably between 0.01 and 0.6 mol%, preferably between 0.03 and 0.3 mol%, and particularly preferably between 0.04 and 0.2 mol% based on the amount of substance of the hydrocarbon used.

[0021] Suitable phosphorus-containing ligands for the catalyst system according to the invention preferably have a bidatate structure. Preferred phosphorus-containing ligands for the catalyst system according to the invention are benzene-based diphosphine compounds, such as those disclosed, for example, in EP 3 121 184 A2. The ligands can be combined with the palladium in a pre-reaction, so that the palladium-ligand complex is introduced to the reaction zone, or added in situ to the reaction and combined there with the palladium. The molar ratio of ligand to metal for the described reaction in step b can be 1:1 to 10:1, preferably 2:1 to 6:1, and particularly preferably 3:1 to 5:1.

[0022] The homogeneous catalyst system further comprises an acid, which is preferably a Brønsted or a Lewis acid. Preferably, aluminum triflate, aluminum chloride, aluminum hydride, trimethylaluminum, tris(pentafluorophenyl)borane, boron trifluoride, boron trichloride, or mixtures thereof are used as the Lewis acid. Of the Lewis acids mentioned, aluminum triflate is preferred. The Lewis acid is preferably added in a molar ratio of Lewis acid to ligand of 1:1 to 20:1, preferably 2:1 to 15:1, and particularly preferably 5:1 to 10:1.

[0023] Suitable Brønsted acids preferably have an acid strength of pKa ≤ 5, particularly preferably an acid strength of pKa ≤ 3. The specified acid strength pKa refers to the pKa value determined under standard conditions (25°C, 1.01325 bar). In the case of a polyprotic acid, the acid strength pKa, within the scope of this invention, refers to the pKa value of the first protolysis step. The Brønsted acid is preferably added in a Brønsted acid : ligand molar ratio of 1 : 1 to 15 : 1, preferably 2 : 1 to 10 : 1, particularly preferably 3 : 1 to 5 : 1.

[0024] Suitable Brønsted acids include, in particular, perchloric acid, sulfuric acid, phosphoric acid, methylphosphonic acid, or sulfonic acids. Examples of suitable sulfonic acids are methanesulfonic acid, trifluoromethanesulfonic acid, tert-butanesulfonic 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. Sulfuric acid is preferred.

[0025] The alkoxycarbonylation in step b is preferably carried out at a temperature in the range of 60 to 120 °C, more preferably in the range of 65 to 110 °C, and particularly preferably in the range of 70 to 100 °C. The alkoxycarbonylation in step b is further preferably carried out at a carbon monoxide pressure of 10 to 35 bar, more preferably 12.5 to 30 bar, and particularly preferably 15 to 25 bar.

[0026] The alkoxycarbonylation in step b takes place in a suitable reaction zone. The reaction zone comprises at least one reactor, but can also consist of two or more reactors arranged in parallel or in series. The at least one reactor can be selected from the group consisting of a stirred tank reactor, a loop reactor, a jet-loop reactor, a bubble column reactor, or combinations thereof. If several reactors are present, they can be identical or different.

[0027] The alkoxycarbonylation described above in step b yields a liquid product mixture comprising at least the esters formed by the alkoxycarbonylation, the homogeneous catalyst system, unreacted olefins, and unreacted alcohols. Additionally, the liquid product mixture may contain low-boiling byproducts such as dimethyl ether or formic acid and / or high-boiling components such as ligand degradation products.

[0028] The resulting product mixture is fed to the subsequent step c to separate the homogeneous catalyst system from the liquid product mixture. Before adding the product mixture, low-boiling components, such as some of the unreacted alcohol and / or the low-boiling byproducts, can be removed, for example, by distillation. The alcohol can be recycled to step b, i.e., the alkoxycarbonylation. The low-boiling byproducts can be purged from the process to prevent their accumulation. The separation of the homogeneous catalyst system yields a crude product mixture that includes at least the esters formed by the alkoxycarbonylation, unreacted olefins, and at least some of the unreacted alcohols.

[0029] The separation of the homogeneous catalyst system to obtain the crude product mixture in step c can be carried out using various separation methods, for example, thermal separation or membrane separation. Such methods are familiar to those skilled in the art. Preferably, the separation of the homogeneous catalyst system in step c within the scope of the present invention is carried out by membrane separation. As is known, membrane separation yields a retentate and a permeate. The homogeneous catalyst system will be concentrated in the retentate. The permeate then represents the aforementioned crude product mixture and is passed to the subsequent step d, the distillative work-up.

[0030] According to the invention, it is preferred that the retentate is recycled to the alkoxycarbonylation in step b or to the reaction zone where the alkoxycarbonylation is carried out. This allows the catalyst system to be reused. In the preferably continuous execution of the claimed process, a catalyst cycle is thus created where, if any, only minor process-related catalyst losses need to be compensated for. If, according to the preferred embodiment, the diisobutene stream, the olefin stream, the alcohol, and the homogeneous catalyst system are mixed before the alkoxycarbonylation in step b, particularly in a suitable mixing vessel, the retentate is fed to the mixing vessel. During the recycling of the retentate, a purge stream can also be used, which removes inert alkanes, low-boiling byproducts (e.g.,Ether), which may contain possible degradation products of the catalyst system or other impurities such as traces of water or nitrogen, should be removed to avoid accumulation in the reaction zone(s).

[0031] Any suitable membrane material can be used for membrane separation. Preferably, an OSN membrane material (OSN = Organic Solvent Nanofiltration) is used in the membrane separation step c of the process according to the invention. Such a membrane material preferably consists of at least one separation-active layer (also: active separation layer) and a substructure on which the separation-active layer is located. More preferably, the membrane material according to the invention consists of at least one separation-active layer and a substructure.

[0032] The membrane material, consisting of at least a separation-active layer and a substructure, should be acid-stable to prevent damage from the acid present in the liquid product mixture. Within the scope of the present invention, the term "acid-stable" means that the membrane material remains stable for at least 300 hours in the presence of the acid in the catalyst system and that the separation efficiency is maintained.

[0033] The substructure preferably has a porous structure that is permeable to the permeate passing through the separation layer. The substructure has a stabilizing function and serves as a support for the separation layer. In principle, the substructure can consist of any suitable porous material. Such materials are familiar to those skilled in the art. However, the material must be acid- and base-stable. The substructure can also consist of the same material as the separation layer. Preferred materials for the substructure are plastics such as polypropylene (PP), polyethylene (PE), or non-condensation polymers that cannot be hydrolyzed or alcoholically cleaved, such as polysulfones, polytetrafluoroethylene (PTFE), polyethersulfone (PES), polyvinylidene fluoride (PVDF), or polyacrylonitrile (PAN).

[0034] The separating layer according to the invention preferably consists of a PAEK polymer (polyaryletherketone). PAEK is characterized by the fact that aryl groups within the repeating unit are alternately linked via an ether functionality and a ketone functionality.

[0035] A separating layer preferred according to the invention consists of PEEK (polyetheretherketone). PEEK polymers with a sulfonation degree of less than 20%, and particularly preferably with a sulfonation degree of less than 10%, can be used as the separating layer. The corresponding PEEK polymers and their preparation are described in WO 2015 / 110843 A1.

[0036] The membrane separation in step c is preferably carried out at a temperature in the range of 25 to 100°C, more preferably in the range of 30 to 80°C, and particularly preferably in the range of 40 to 70°C. To bring the product mixture to the prevailing temperature preferred for the membrane separation, the product mixture can be cooled. In addition to active cooling using a cooling medium, cooling can also be achieved via a heat exchanger, where thermal energy is transferred to another stream, thereby cooling the product mixture and heating the other stream.

[0037] The transmembrane pressure (TMP) during membrane separation in step c is preferably in the range of 10 to 60 bar, more preferably in the range of 15 to 55 bar, and particularly preferably in the range of 20 to 50 bar. The permeate-side pressure can be above atmospheric pressure and preferably up to 15 bar, more preferably 3 to 7 bar. The retentate-side pressure is calculated from the difference between the TMP and the permeate-side pressure. In a preferred embodiment, care should be taken with regard to the pressure conditions, and especially the permeate-side pressure, to ensure that the pressure is adjusted according to the hydrocarbon, the alcohol, and the ambient temperature used, so as to prevent evaporation after passing through the membrane. Evaporation could lead to unstable operation.

[0038] In the subsequent step d, the crude product mixture or permeate from the membrane separation is processed by distillation in at least one distillation column to separate the unreacted alcohols and olefins, i.e., unreacted diisobutenes and unreacted C4 to C7 olefins. This yields an ester mixture containing the esters formed. The ester mixture thus contains the esters formed from the diisobutene and the C4 to C7 olefins.

[0039] During the distillative work-up of the crude product mixture or permeate in step d, the unreacted alcohol and unreacted olefins, i.e., unreacted diisobutenes and unreacted C4 to C7 olefins, collect at the top of the at least one distillation column. The mixture of esters formed consequently collects at the bottom of the at least one distillation column. The overhead stream, which contains the unreacted alcohol and unreacted olefins separated in the at least one distillation column, can be recycled to the alkoxycarbonylation in step b or to the reaction zone. If a mixture of the input components is present prior to the alkoxycarbonylation, the overhead stream is naturally directed to the mixing point. This enables continuous operation of the process according to the invention with the highest possible yield.A purge can be taken from the recycled headstream to remove low-boiling byproducts from the process.

[0040] The distillative work-up to separate the unreacted alcohols and olefins in step d can be carried out in a single distillation column. It would be conceivable to perform this process in multiple distillation columns, but this would require significantly more equipment. Therefore, it is preferable to perform the distillative work-up in step d in a single distillation column.

[0041] The pressure in the distillation column during the distillative work-up in step d is preferably in the range of 0.3 to 2 bar, more preferably in the range of 0.4 to 1 bar, and particularly preferably in the range of 0.5 to 0.7 bar. The temperature at the bottom of the distillation column during the distillative work-up in step d is preferably in the range of 80 °C to 160 °C. The temperature at the top of the distillation column during the distillative work-up in step d is preferably in the range of 30 to 80 °C. Furthermore, it is preferred that the reflux ratio in the distillation column is between 1 and 2. The distillation column for the separation in step d preferably comprises 10 to 30 theoretical stages. The distillation column may contain high-performance structured packings. Such high-performance structured packings are known to those skilled in the art.

[0042] As mentioned, distillation yields an ester mixture containing the esters formed from diisobutene and the C4 to C7 olefin. To obtain both esters as pure substances as possible, a further distillation step can be carried out to separate them from the ester mixture. The esters from the C4 to C7 olefin will collect at the top of the distillation column, and the esters from diisobutene will collect at the bottom.

[0043] The present process is suitable for the alkoxycarbonylation of di-isobutene and C4 olefins with an alcohol, preferably an alcohol with 1 to 4 carbon atoms, followed by hydrolysis or saponification. Certain combinations of olefins and alcohols are particularly preferred within the scope of the present invention: In a preferred embodiment, the process relates to the alkoxycarbonylation of di-isobutene and a C4 olefin, i.e., 1-butene, cis- and / or trans-2-butene, isobutene, or mixtures thereof, with methanol. The ester formed from di-isobutene during alkoxycarbonylation with methanol is 3,5,5-trimethylhexanoic acid methyl ester. From the various butenes, valeric acid methyl ester, 2-methylbutyric acid methyl ester, and / or 3-methylbutyric acid methyl ester can be formed by alkoxycarbonylation with methanol. If a mixture of butenes is used, a mixture of the aforementioned esters is obtained accordingly.

[0044] In a preferred embodiment, the process relates to the alkoxycarbonylation of di-isobutene and a C4 olefin, i.e., 1-butene, cis- and / or trans-2-butene, isobutene, or mixtures thereof, with ethanol. The ester formed from di-isobutene upon alkoxycarbonylation with ethanol is ethyl trimethylhexanoate. From the various butenes, ethyl valerate, ethyl 2-methylbutyrate, and / or ethyl 3-methylbutyrate can be formed by alkoxycarbonylation with ethanol. If a mixture of butenes is used, a mixture of the aforementioned esters is obtained accordingly.

[0045] The esters produced in step b, separated in step d, and, if necessary, purified, are then subjected to hydrolysis or saponification in step e. The acidic catalyst or saponifying agent used in this process cleaves the ester group, yielding a carboxylic acid or a carboxylic acid salt, and allowing the recovery of the alcohol bound during esterification in step b. Thus, the saponification process produces a reaction mixture comprising at least the carboxylic acids or their salts, the released alcohol, and unreacted esters.

[0046] It has already been mentioned that the process according to the invention yields a mixture of esters containing both the esters obtained from the diisobutene and those obtained from the C4 to C7 olefin. During the hydrolysis or saponification in step e, this ester mixture can be used to obtain the respective carboxylic acids in a single reaction zone. However, it is also possible to separate the esters obtained from the diisobutene and those obtained from the C4 to C7 olefin beforehand, with the hydrolysis or saponification taking place separately in their respective reaction zones. Simultaneous hydrolysis of both esters has the advantage of lower equipment costs and is therefore preferred according to the invention.

[0047] The alcohol recovered by hydrolysis or saponification in step e can be separated from the resulting mixture in a subsequent process step and returned to the reaction zone in step b.

[0048] Hydrolysis is a well-known chemical reaction in which an ester is converted into a carboxylic acid with the elimination of an alcohol, using an acidic catalyst. The typical conditions for hydrolysis are known to those skilled in the art.

[0049] The hydrolysis in step e takes place in the presence of an acidic heterogeneous or homogeneous catalyst. Known homogeneous catalysts are acidic compounds such as Brønsted 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. Known heterogeneous catalysts are acidic compounds such as cation exchangers, acidic H-zeolites, acidic functionalized metal oxides with silica, and heteropoly acids. Suitable heterogeneous catalysts include, in particular, Amberlyst®< 15, Amberlyte®< IR 120 (H), HClO 4 -SiO 2 , 3-propylsulfonic acid functionalized silica, Nafion®< -SiO2, Aciplex®< -SiO 2 , 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).

[0050] The reaction mixture from the hydrolysis in step e is subjected in the subsequent step f to at least one separation process selected from the group consisting of thermal separation, for example distillation, extraction, crystallization, or further membrane separation, in order to separate the carboxylic acid or its salt formed in step e from the remaining reaction mixture. The separation process is preferably distillation. The corresponding process conditions are known to those skilled in the art. Multi-stage distillation can also be carried out.

[0051] Saponification is a well-known chemical reaction in which an ester is converted into a carboxylic acid salt with the elimination of an alcohol, using a basic or enzymatic saponifying agent. The typical conditions for saponification are known to those skilled in the art.

[0052] The saponification in step e takes place in the presence of a saponifying agent. Known saponifying agents are basic compounds such as potassium hydroxide, potassium (hydrogen) carbonate, sodium hydroxide, sodium (hydrogen) carbonate, or amine compounds. Saponification with an enzymatic saponifying agent, especially esterases, is also possible.

[0053] The reaction mixture from the saponification in step e is subjected in the subsequent step f to at least one separation process selected from the group consisting of thermal separation, for example distillation, extraction, crystallization, or further membrane separation, in order to separate the carboxylic acids or their salts formed in step e from the remaining reaction mixture. The separation process is preferably distillation. The corresponding process conditions are known to those skilled in the art. Multi-stage distillation can also be carried out.

[0054] In at least one separation step, the alcohol used in step b and recovered by hydrolysis or saponification can also be separated and recycled to the reaction zone. A purge stream can be extracted during recycling to remove byproducts from the process. Furthermore, the two carboxylic acids or their salts can also be separated from each other, provided that the two esters of diisobutene and the C4 to C7 olefin were jointly subjected to hydrolysis or saponification.

Claims

1. Process for producing carboxylic acids or salts thereof, wherein the process comprises at least the following steps: a. providing a diisobutene stream containing 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene and providing an olefin stream containing the C4 to C7 olefin; b. alkoxycarbonylation of diisobutene and the C4 to C7 olefin with an alcohol and carbon monoxide in the presence of a homogeneous catalyst system comprising at least one metal of group 8 to 10 of the periodic table of the 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 the esters 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 comprising at least the esters formed by the alkoxycarbonylation, the unreacted olefins and unreacted alcohols; d. distillative processing of the crude product mixture in at least one distillation column to remove the unreacted alcohols and the unreacted olefins to obtain an ester mixture containing the esters formed; e. hydrolysis or saponification of the esters obtained from step d in the presence of an acidic catalyst or a saponification agent to obtain a reaction mixture comprising at least the carboxylic acids or salts thereof, the eliminated alcohol and unreacted esters; and f. removing the carboxylic acids or salts thereof formed in step e in at least one separation process step selected from thermal separation, for example distillation, extraction, crystallization and membrane separation.

2. Process according to Claim 1, wherein the alkoxycarbonylation in step b is performed at a temperature of 60°C to 120°C, preferably 65°C to 110°C, particularly preferably 70°C to 100°C.

3. Process according to Claim 1 or 2, wherein the alkoxycarbonylation in step b is performed at a carbon monoxide pressure of 10 to 35 bar, preferably 12.5 to 30 bar, particularly preferably 15 to 25 bar.

4. Process according to any of the preceding claims, wherein the content of the metal of group 8 to 10 of the periodic table of the elements, in particular of 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. Process according to any of the preceding claims, wherein the alcohol is a monohydric alcohol having 1 to 4 carbon atoms, especially methanol, ethanol, propanol or butanol.

6. 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. Process according to any of the preceding claims, wherein the removal of the homogeneous catalyst system in step c is effected by membrane separation.

8. Process according to Claim 7, wherein the homogeneous catalyst system accumulates in the retentate.

9. Process according to any of the preceding claims, wherein the diisobutene stream, the olefin stream, the alcohol and the homogeneous catalyst system are initially mixed in a mixing vessel before they are passed into the reaction zone.

10. Process according to any of the preceding claims, wherein the distillative processing in step d is carried out in a single distillation column.

11. Process according to Claim 10, wherein the pressure in the distillation column is in the range from 0.3 to 2 bar, preferably in the range from 0.4 to 1 bar, particularly preferably in the range from 0.5 to 0.7 bar.

12. Process according to Claim 10 or 11, wherein the temperature in the bottom of the distillation column is in a range from 80°C to 160°C.

13. Process according to any of Claims 10 to 12, wherein the temperature at the top of the distillation column is in a range from 30°C to 80°C.

14. Process according to any of the preceding claims, wherein a C4 olefin and methanol are employed, as a result of which the ester mixture obtained is a mixture of methyl 3,5,5-trimethylhexanoate and methyl valerate, methyl 2-methylbutyrate, methyl 3-methylbutyrate or a mixture thereof.

15. Process according to any of the preceding claims, wherein in step d unreacted alcohol and unreacted olefins are separated and recycled to the alkoxycarbonylation in step b.