Process for preparation of carboxylic acids or salts thereof by alkoxycarbonylation of diisobutylene using pre-distillation and subsequent hydrolysis

Abstract

The present invention relates to a process for the preparation of carboxylic acids or salts thereof by alkoxycarbonylation of diisobutylene using pre-distillation and subsequent hydrolysis. The invention relates to a method for producing carboxylic acids or salts thereof by hydrolysis or saponification of esters produced by alkoxycarbonylation of diisobutylene. In the process, the diisobutylene stream used for alkoxycarbonylation is subjected to distillation prior to alkoxycarbonylation in order to enrich 2, 4, 4-trimethylpent-1-ene in the stream to be alkoxycarbonylated. The 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

Technical Field

[0001] The present invention relates to a process for preparing carboxylic acids or salts thereof by hydrolysis or saponification of esters produced by alkoxycarbonylation of diisobutylene. In the process, the diisobutylene stream used for alkoxycarbonylation is distilled before the alkoxycarbonylation in order to enrich 2,4,4-trimethylpent-1-ene in the stream to be alkoxycarbonylated. The alkoxycarbonylation is carried out with 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 Elements or a compound thereof, a phosphorus-containing ligand and an acid.

[0002] The production of carboxylic acid in industrial chemistry mainly produces aldehyde by hydroformylation, and aldehyde is subsequently oxidized to carboxylic acid for carrying out. Although the production of carboxylic acid by hydroformylation and subsequent oxidation is a complete and verified method for decades in industry, it still has the potential for improvement (referring to DE 10010771 C1 or EP 1854778 A1). A problem of this synthetic route is that hydroformylation uses a catalyst system containing transition metals, which is usually expensive or must be produced at high cost. Another problem is that by-products will be formed in hydroformylation and subsequent oxidation processes, even if catalyst and / or reaction conditions are relatively mild, by-products have also been observed. Another problem is the increased demand for safety equipment in the oxidation process of aldehyde, because in this step oxygen is supplied to the organic reaction mixture in pure form or by air. The resulting explosive atmosphere must be monitored and regulated individually and more difficultly. Background Art

[0003] Diisobutene is a technically relevant product obtained by dimerization of isobutene. Diisobutene 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 of about 78:22 to 81:19 (equilibrium distribution). This mixture can be converted into more valuable products in carbonylation processes in particular. In this case, in particular in carbonylation processes such as methoxycarbonylation (the reaction product here is 3,5,5-trimethylhexanoic acid methyl ester) or hydroformylation (the reaction product here is 3,5,5-trimethylhexanal), the internal olefin TMP2 has a significantly lower reactivity than the terminal olefin TMP1. Due to insufficient stability of the catalyst or economic factors (e.g. the size of the reactor), it is generally not possible to fully adjust the reaction conditions or residence times in these carbonylation reactions in order to react the terminal TMP2 completely. Summary of the invention

[0004] The object of the present invention is therefore a process for preparing carboxylic acids or salts thereof which does not have the above-mentioned problems. In particular, the alkoxycarbonylation should be carried out with a diisobutylene stream having a high proportion of 2,4,4-trimethylpent-1-ene. In addition, an alternative synthesis route for preparing carboxylic acids or salts thereof should be obtained, by means of which the desired carboxylic acids or salts thereof can be developed relatively easily.

[0005] This object is achieved by the method according to the invention according to independent claim 1. Preferred embodiments are described in the dependent claims.

[0006] The present invention describes a method for preparing a carboxylic acid or a salt thereof, wherein the method comprises at least the following steps:

[0007] a. supplying a diisobutene stream comprising 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene to at least one distillation column and separating the diisobutene stream into at least one overhead stream which is enriched in 2,4,4-trimethylpent-1-ene compared with the diisobutene stream used and comprises more than 85% by weight of 2,4,4-trimethylpent-1-ene and a residual stream which is depleted in 2,4,4-trimethylpent-1-ene compared with the diisobutene stream used;

[0008] b. supplying the top stream to an alkoxycarbonylation, whereby 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene are reacted in a reaction zone 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 to obtain a liquid product mixture comprising at least esters formed by the alkoxycarbonylation, the homogeneous catalyst system, unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene and unreacted alcohol;

[0009] c. removing the homogeneous catalyst system from the liquid product mixture to obtain a crude product mixture comprising at least the ester formed by alkoxycarbonylation, unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene and unreacted alcohol;

[0010] d. working up the crude product mixture by distillation in at least one distillation column to remove unreacted alcohol and unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene, wherein unreacted alcohol and unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene are removed and recycled to the alkoxycarbonylation in step b;

[0011] e. hydrolyzing or saponifying the ester obtained in 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, the decomposed alcohol and unreacted ester; and

[0012] f. isolating the carboxylic acid or its salt formed in step e in at least one separation process step selected from thermal separation such as distillation, extraction, crystallization and membrane separation.

[0013] An advantage of the process is that the diisobutene stream is distilled before the alkoxycarbonylation, since this makes it possible to increase the proportion of 2,4,4-trimethylpent-1-ene in the top stream of the at least one distillation column compared to the stream used. As a result, proportions of more than 85% by weight, preferably more than 90% by weight, more preferably more than 95% by weight of 2,4,4-trimethylpent-1-ene can be achieved in the stream, which is advantageous for the downstream alkoxycarbonylation due to the higher reactivity of 2,4,4-trimethylpent-1-ene.

[0014] Step a of the process according to the invention involves the distillation of a diisobutene stream comprising 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene. In this process, the diisobutene stream is separated into at least one top stream which is enriched in 2,4,4-trimethylpent-1-ene compared with the diisobutene stream used, and a residual stream which is depleted in 2,4,4-trimethylpent-1-ene compared with the diisobutene stream used, the top stream preferably containing more than 85% by weight of 2,4,4-trimethylpent-1-ene, further preferably more than 90% by weight of 2,4,4-trimethylpent-1-ene, particularly preferably more than 95% by weight of 2,4,4-trimethylpent-1-ene.

[0015] The diisobutylene stream used in step a comprises 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene. Such a stream may be a diisobutylene stream, which may be prepared by dimerization of isobutylene or an isobutylene-containing hydrocarbon mixture (such as those disclosed in EP1360160 B1). In addition, the stream to be used here may be produced as an unreacted residual stream in a carbonylation process, such as in methoxycarbonylation or in hydroformylation. It has already been mentioned that the input stream may be supplied to isomerization and / or distillation. The designation of steps a and b in the present claims therefore does not constitute a priority or time sequence. It is obvious that both steps must be carried out and that each step is fed by the corresponding other step. Therefore, which step is considered to be carried out first is secondary.

[0016] The distillation in step a of the process according to the invention is carried out in at least one distillation column. Distillation columns are known in principle to those skilled in the art. The distillation unit according to the invention preferably comprises at least one distillation column, more preferably at least two distillation columns. In the case where more than one distillation column is present in the distillation unit, the following description of the characteristics of the distillation columns always applies.

[0017] The at least one distillation column preferably has internals to cope with the separation task. Suitable internals are familiar to the person skilled in the art. Particularly suitable here are random packings or structured packings, such as those known to the person skilled in the art under the trade names of In a preferred embodiment of the present invention, the at least one distillation column comprises at least 50 theoretical plates, preferably at least 70 theoretical plates, more preferably at least 90 theoretical plates. If two or more distillation columns are present, these distillation columns may have the same or different numbers of theoretical plates.

[0018] The operating parameters of the at least one distillation column follow the separation task and the design of the distillation column. In the process according to the invention, it is preferred that the at least one distillation column is operated under reduced pressure, particularly preferably at a pressure of 0.2 to 0.9 bar. Within the scope of the present invention, reduced pressure is present as long as operation is carried out at an ambient pressure of less than about 1 bar, i.e. the atmospheric pressure prevailing at the respective location. If two or more distillation columns are present, these can be operated at the same or different pressures.

[0019] It is further preferred that the temperature in the bottom of the at least one distillation column is in the range from 50 to 100° C. If two or more distillation columns are present, these can be operated at the same or different temperatures.

[0020] Another parameter for the design of a distillation column is the reflux ratio. The reflux ratio is the ratio of reflux (recycle) to distillate (drawn condensate, in this case also the top stream K). The reflux is the condensed part of the top stream that is returned to the distillation column. According to the invention, it is preferred that the reflux ratio of the at least one distillation column in the distillation in step a is in the range of 5 to 15.

[0021] In the at least one distillation column, the distillation according to the invention in step a produces a top stream which preferably contains at least 85% by weight of 2,4,4-trimethylpent-1-ene, further preferably at least 90% by weight of 2,4,4-trimethylpent-1-ene, particularly preferably at least 95% by weight of 2,4,4-trimethylpent-1-ene. In addition, the distillation provides a residual stream which is depleted in 2,4,4-trimethylpent-1-ene compared to the diisobutene stream used.

[0022] The residual stream obtained in addition in the distillation of step a can be withdrawn as a bottom stream of the at least one distillation column or as a side stream of the at least one distillation column. If two distillation columns are present, the residual stream is obtained in the last distillation column, regardless of whether it is withdrawn as a side stream or as a bottom stream. If the residual stream can be withdrawn as a side stream, it can in principle be arranged at the same height as the inlet of the diisobutene stream used or below the inlet of the diisobutene stream used. It is obvious here that the inflow and outflow must be arranged at a sufficient distance from each other. Whether it is withdrawn as a side stream or as a bottom stream, the residual stream preferably contains 80% to 92% by weight of 2,4,4-trimethylpent-2-ene.

[0023] If a residual stream from the distillation in step a is withdrawn as a bottom stream of the at least one distillation column, a purge stream is preferably withdrawn from the residual stream which comprises high boilers formed during the isomerization, for example dimers or oligomers of 2,4,4-trimethylpent-2-ene and / or 2,4,4-trimethylpent-1-ene. The removed high boilers can be incinerated to generate energy or can be hydrogenated to produce valuable alkanes.

[0024] If the residual stream from the distillation in step a is drawn off as a side stream from the at least one distillation column, a stream comprising high boilers formed during the isomerization (e.g. dimers or oligomers of 2,4,4-trimethylpent-2-ene and / or 2,4,4-trimethylpent-1-ene) is preferably obtained at the bottom of the at least one distillation column. The removed high boilers can be incinerated to generate energy or can be hydrogenated to produce valuable alkanes.

[0025] As is known, the distillation column is heated at the bottom to cope with the separation task. The energy can be supplied here by means of hot steam which is usually available at chemical production sites. For this purpose, a portion of the bottom product is passed through a heat exchanger (reboiler), where it is heated and then returned to the bottom of the distillation column. In order to reduce the energy requirement and / or CO2 emissions, the distillation in step a can be designed with heat integration. Heat integration means that the energy generated or present in the process is used elsewhere. In the present case, the (released) condensation energy generated at the top of at least one distillation column of the distillation unit is particularly suitable. In a preferred embodiment of the present invention, heat integration is carried out in step a so that at least a portion of the condensation energy at the top of the column is used to heat the bottom product.

[0026] Another option for heat integration is so-called vapor compression, in which at least part of the vapor (the top stream) is compressed, i.e. pressurized to a higher level and optionally heated. The vapor thus compressed and optionally heated is supplied to a heat exchanger (reboiler) to heat the bottom product. For this purpose, the heat of condensation of the vapor is utilized.

[0027] Another option is to use a heat pump. The heat pump is operated with a working medium such as n-butane or water. The condensation energy is therefore first transferred to the working medium in a heat exchanger in this case and from there to the distillation column bottom product in another suitable heat exchanger. The working medium is usually conveyed to the heat exchanger at the bottom of the column via a compressor. In principle, a two-stage or multi-stage heat pump with more than one compressor stage can also be used. In the case of a two-stage heat pump, there is not only one working medium, but two working media, wherein the energy exchange between the first and second working media also takes place in the heat exchanger. In the case of a multi-stage configuration, there are correspondingly more working media.

[0028] The top stream obtained from the distillation in step a is supplied to the subsequent step b of the alkoxycarbonylation and reacted there. In a preferred embodiment, the residual stream also obtained from the distillation in step a is supplied to an isomerization, in which 2,4,4-trimethylpent-2-ene is at least partially isomerized to 2,4,4-trimethylpent-1-ene using a heterogeneous catalyst based on zeolites or ion exchange resins. The isomerization can thus be carried out with a residual stream which is depleted in 2,4,4-trimethylpent-1-ene compared to the diisobutene stream used in step a and is removed from the distillation in step a.

[0029] The advantage of carrying out the isomerization before the distillation in step a is the use of a heterogeneous catalyst system. Such a catalyst system does not have to be removed from the isomerization stream and remains in the reaction vessel or reactor. The catalyst systems based on zeolites or ion exchange resins preferred according to the invention can also achieve high conversions.

[0030] Isomerization can be carried out in principle in any suitable reactor. Isomerization can be carried out in a single reactor or in a plurality of reactors connected in parallel or in series. It can also be carried out in batches or continuously. Isomerization is preferably carried out in one or more continuously operated reactors conventionally used in solid / liquid contact reactions. When a continuously operated flow reactor is used, a fixed bed is usually but not exclusively used. When a fixed bed flow reactor is used, the liquid can flow upward or downward. In most cases, it is preferred that the liquid flows downward. In addition, the reactor can be operated under product recirculation or in a straight-through manner. A concept different from a fixed bed reactor is, for example, a reactor in which an ion exchanger or a zeolite is suspended in the liquid phase.

[0031] As reactors for the isomerization, tubular reactors or tube bundle reactors can be used, in particular reactors with an internal tube diameter of 10 to 60 mm. The aspect ratio of the catalyst bed can be varied by means of the geometric dimensions of the reactor or by means of its filling level. With the same contact quantity and load (LHSV), different empty tube velocities can thus be achieved and the heat transfer to the cooling medium can be influenced in a targeted manner.

[0032] The cooling of the tubes of the reactor (whether a tubular reactor or a tube bundle reactor) can be achieved by means of a cooling medium (e.g., cooling water or an endothermic process fluid for heat integration) via a heat exchanger in the shell space of the reactor or an external recirculation system. In particular, when a liquid heating medium is used, the shell side is configured to make the temperature gradient adjacent to all tubes as uniform as possible. The technical measures required for this purpose are known to those skilled in the art and are described in the literature (installation of baffles, disc-ring configurations, input / output of heat carriers at different points in the reactor, etc.). Preferably, the reaction medium and the heat carrier are transported through the reactor tube and the reactor jacket in a cocurrent manner, more preferably from top to bottom. A preferred embodiment is described in, for example, DE 102006040433A1.

[0033] The isomerization of 2,4,4-trimethylpent-2-ene to 2,4,4-trimethylpent-1-ene occurs exothermically, ie it is carried out with release of energy, which leads to an increase in the temperature of the reaction mixture. In order to limit the temperature increase, the input stream can be diluted, for example by recycling the product.

[0034] The one or more reactors used in the isomerization can be operated adiabatically, polytropically (polytrop) or almost isothermally. Almost isothermally means that the temperature at any point in the reactor is at most 10° C. higher than the temperature at the reactor inlet. In the case of adiabatic operation of the reactor, it is generally meaningful to connect a plurality of reactors in series and to provide cooling between the reactors. Reactors suitable for polytropic or almost isothermal operation are, for example, the tube bundle reactors already mentioned as well as stirred tank reactors and loop reactors.

[0035] The process can be carried out at a fairly mild temperature. The isomerization is preferably carried out at a temperature of 25 to 90°C, preferably 30 to 80°C, more preferably 35 to 70°C. In addition, the isomerization of the present invention can be carried out at a pressure equal to or greater than the vapor pressure of the input stream mixture and / or the reaction mixture at the corresponding reaction temperature, preferably at a pressure greater than 0 bar but less than 40 bar. Further preferably, the isomerization is carried out in a liquid phase. It should be clear that in this case, the pressure and temperature should be selected so that the input stream is present in a liquid phase, or can be present in a liquid phase.

[0036] In the one or more reactors, various zeolite-based or ion exchange resin-based catalysts can also be used for isomerization. For example, a mixture of ion exchange resins of different reactivity can be used. The reactors can also contain catalysts of different activities, for example arranged in layers. If more than one reactor is used, each reactor can be filled with the same or different one or more zeolite-based or ion exchange resin-based catalysts.

[0037] As heterogeneous catalysts, zeolite-based or ion exchange resin-based catalysts can be used for isomerization. Suitable zeolites and ion exchange resins are widely available on the market. It has been found that zeolite-based catalysts preferably have a Si:Al ratio of 40:1 to 200:1. It has also been found that, as ion exchange resin-based catalysts, H-type and partially neutralized forms of styrene-divinylbenzene types are very suitable.

[0038] When the catalyst is a zeolite, some zeolites have proven to be particularly advantageous, such as β- and γ-zeolites. The zeolite is therefore preferably selected from Z-β-H-25, Z-β-H-38, Z-β-H-360, Z-Mor-H-20, ZYH-60, ZYH-80, Z-β-H, Z-CFG-1, Z-β-ammonium-38, Z-CBV 760CY (1.6), Z-CBV 780CY (1.6), CP 814E CY (1.6), CBV 500CY (1.6), H-CZB-150 and mixtures thereof.

[0039] As ion exchange resins, ion exchange resins made, for example, by sulfonation of phenol / aldehyde condensates or by sulfonation of copolymers of aromatic vinyl compounds can be used. Examples of aromatic vinyl compounds used to produce the copolymers are: styrene, vinyltoluene, vinylnaphthalene, vinylethylbenzene, methylstyrene, vinylchlorobenzene, vinylxylene and divinylbenzene. Particularly preferably, ion exchange resins made by sulfonation of copolymers formed by the reaction of styrene and divinylbenzene are used for isomerization. Ion exchange resins can be produced in gelatinous, macroporous or spongy forms. The properties of these resins, in particular the specific surface area, porosity, stability, swelling or shrinkage and exchange capacity can be varied as known by the production method.

[0040] Preferred ion exchange resins of the styrene-divinylbenzene type are sold, inter alia, under the trade names CT 151 and CT 275 from Purolite, CT 151 from Rohm & Haas, CT 275 ... 15. 35. IR-120, 200, Dowex M-31 from Dow, Lanxess K 2621, K-2431.

[0041] The pore volume of the ion exchange resins which can be used as catalysts, in particular those of the preferred styrene-divinylbenzene type, is preferably from 0.3 to 0.9 ml / g, more preferably from 0.5 to 0.9 ml / g. The pore volume can be determined, for example, by adsorption techniques. The particle size of the ion exchange resin is preferably from 0.3 mm to 1.5 mm, more preferably from 0.5 mm to 1.0 mm. Narrower or broader particle size distributions can be selected. Thus, for example, ion exchange resins with very uniform particle sizes (monodisperse resins) can be used.

[0042] The ion exchange resins which can be used as catalysts for the isomerization can be present as partially neutralized ion exchange resins. For this purpose, the ion exchange resins can be treated with acids or bases as described in EP 1360160 B1.

[0043] From the optional isomerization, an isomerized stream is obtained, wherein the proportion of 2,4,4-trimethylpent-2-ene in the isomerized stream is lower and the proportion of 2,4,4-trimethylpent-1-ene is higher than in the residual stream used. The isomerized stream can be supplied at least partly to the distillation in step a, preferably the entire isomerized stream is supplied to the distillation in step a.

[0044] The process may further comprise providing an input stream with which the process is fed. The input stream for feeding the process and comprising 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene may be introduced into the distillation unit in step a in addition to the isomerization stream and / or introduced into the isomerization in addition to the residual stream.

[0045] A flexible supply of the input stream is advantageous here. This allows the process to be adapted to the respective given conditions, for example, depending on the input stream used. If the ratio of 2,4,4-trimethylpent-2-ene to 2,4,4-trimethylpent-1-ene in the input stream is close to the equilibrium distribution, this stream can be supplied directly to the distillation. If the ratio of 2,4,4-trimethylpent-2-ene to 2,4,4-trimethylpent-1-ene in the input stream deviates from the equilibrium distribution, this stream can first be supplied to the isomerization. This makes it possible to integrate into existing production plants in a simple manner depending on the composition of the input stream.

[0046] As described above, the top stream obtained from the distillation in step a is supplied to the subsequent step b of alkoxycarbonylation and reacted therein.

[0047] Diisobutylene, i.e. 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene, reacts in step b with carbon monoxide (CO) and an alcohol to form an ester. This increases the number of carbon atoms in the ester by 1 carbon atom from the carbon monoxide and the number of carbon atoms in the alcohol used compared to the diisobutylene used. Thus, 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 is produced from diisobutylene (8 carbon atoms).

[0048] Carbon monoxide can be provided directly in step b in the form of a raw material mixture, or by adding a gas containing carbon monoxide selected from synthesis gas, water gas, producer gas and other gases containing carbon monoxide. Carbon monoxide can also be provided by first separating the gas containing carbon monoxide into its components in a manner known to those skilled in the art and feeding the carbon monoxide into the reaction zone. The carbon monoxide may still contain a certain proportion of hydrogen or other gases, since it is almost impossible to achieve complete separation.

[0049] 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. In this way, esters of the alcohol used with the corresponding chain length are obtained. For example, when methanol is used, methyl esters are formed.

[0050] 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 (alcohol:diisobutene) of 10:1 to 1:1, preferably 8:1 to 1.5:1, more preferably 7:1 to 2:1 to the total amount of all diisobutene. The alcohol is thus added in at least the same molar amount, based on the diisobutene used, but preferably in a molar excess.

[0051] The reaction in step b according to the present invention 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 (PSE) or a compound thereof, a phosphorus-containing ligand and an acid as a promoter. In the reaction solution of the alkoxycarbonylation process of step b, the content of the metal of Groups 8 to 10 of the Periodic Table of Elements (especially palladium) is preferably 100 to 500 ppm, further preferably 150 to 450 ppm, particularly preferably 180 to 350 ppm.

[0052] The metal of the 8th to 10th group of PSE is preferably palladium. Palladium is preferably used as a precursor compound of a palladium compound, which is 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 from 0.01 to 0.6 mol %, preferably from 0.03 to 0.3 mol %, more preferably from 0.04 to 0.2 mol %, based on the substance amount of hydrocarbon used.

[0053] Suitable phosphorus-containing ligands of the catalyst system according to the present invention preferably have a bidentate structure. Preferred phosphorus-containing ligands for the catalyst system according to the present invention are diphosphine compounds based on benzene, such as disclosed in EP 3121184 A2. The ligand can be combined with palladium in a pre-reaction, thereby the palladium-ligand complex is fed into the reaction zone, or added to the reaction in situ and combined with palladium there. The molar ratio of the ligand to the metal for the reaction described in step b can be 1: 1 to 10: 1, preferably 2: 1 to 6: 1, more preferably 3: 1 to 5: 1.

[0054] The homogeneous catalyst system further comprises an acid, in particular a Brønsted acid or a Lewis acid. As Lewis acids, aluminum trifluoromethanesulfonate, aluminum chloride, aluminum hydride, trimethylaluminum, tris(pentafluorophenyl)borane, boron trifluoride, boron trichloride or mixtures thereof are preferably used. Among the Lewis acids mentioned, aluminum trifluoromethanesulfonate is preferably used. The Lewis acid is preferably added in a Lewis acid:ligand ratio of 1:1 to 20:1, preferably 2:1 to 15:1, particularly preferably 5:1 to 10:1.

[0055] Suitable Bronsted acids preferably have an acid strength of pKs≤5, more preferably an acid strength of pKs≤3. The acid strengths pKs given are pKs values ​​determined under standard conditions (25° C., 1.01325 bar). For polyacids, the acid strength pKs is related within the scope of the present invention to the pKs of the first proton dissociation step. The Bronsted acid is preferably added in a molar ratio of Bronsted acid:ligand of 1:1 to 15:1, preferably 2:1 to 10:1, more preferably 3:1 to 5:1.

[0056] As Bronsted acids, perchloric acid, sulfuric acid, phosphoric acid, methylphosphonic acid or sulfonic acid can be used in particular. Suitable sulfonic acids are, for example, 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. The acid is preferably sulfuric acid.

[0057] The alkoxycarbonylation in step b is preferably carried out at a temperature of 60 to 120° C., further preferably 65 to 110° C., particularly preferably 70 to 100° C. The alkoxycarbonylation in step b is further preferably carried out at a carbon monoxide pressure of 10 to 35 bar, preferably 12.5 to 30 bar, more preferably 15 to 25 bar.

[0058] 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. The at least one reactor may be particularly selected from a stirred tank reactor, a loop reactor, a jet loop reactor, a bubble column reactor or a combination thereof. If a plurality of reactors are present, the reactors may be identical or different from each other.

[0059] The alkoxycarbonylation in step b above obtains a liquid product mixture, which contains at least the ester formed by the alkoxycarbonylation, the homogeneous catalyst system, unreacted diisobutylene 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene, 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.

[0060] The product mixture thus obtained is supplied to subsequent step c so as to remove the homogeneous catalyst system from the liquid product mixture. Before the product mixture is fed, low boiling components, such as a portion of unreacted alcohol and / or low boiling by-products can be removed. The alcohol can be recycled to step b, i.e. recycled to the alkoxycarbonylation. The low boiling by-products can be discharged from the process in a purge mode so that they can not be enriched in the process. Removing the homogeneous catalyst system obtains a crude product mixture, which at least comprises an ester formed by the alkoxycarbonylation, unreacted diisobutylene and at least a portion of unreacted alcohol.

[0061] Removing the homogeneous catalyst system in step c to obtain a crude product mixture can be achieved by various separation methods, for example by thermal separation or by membrane separation. Suitable methods are well known to those skilled in the art. Removing the homogeneous catalyst system in step c is preferably achieved by membrane separation within the scope of the present invention. As known, membrane separation produces a retentate and a permeate. The homogeneous catalyst system is enriched in the retentate. In this case, the permeate is the previously mentioned crude product mixture and is introduced into the subsequent step d, the distillation post-treatment.

[0062] According to the present invention, preferably, the retentate is recycled to the alkoxycarbonylation in step b or to the reaction zone in which the alkoxycarbonylation is carried out. This enables the catalyst system to be reused. In the preferred continuous implementation of the claimed method, this produces a catalyst cycle, wherein only a small amount of catalyst loss associated with the process needs to be compensated at most. According to a preferred embodiment, before the alkoxycarbonylation in step b, more particularly when the diisobutylene stream, alcohol and homogeneous catalyst system are mixed in a suitable mixing vessel, the retentate is supplied to the mixing vessel. In the recycling of the retentate, a purge stream that may contain inert alkanes, low-boiling by-products (e.g. ethers), possible catalyst system decomposition products or other impurities (e.g. traces of water or nitrogen) may be additionally extracted to avoid accumulation in one or more reaction zones.

[0063] Any suitable membrane material can be used for the membrane separation. Preferably, an OSN membrane material (OSN=organic solvent nanofiltration) is used in the membrane separation in step c of the process according to the invention. Such a membrane material preferably consists of at least a separation active layer (also called active separation layer) and a substructure on which the separation active layer is present. The membrane material according to the invention preferably consists of at least a separation active layer and a substructure.

[0064] The membrane material consisting of at least the separation active layer and the underlying structure should be acid-stable so that the membrane material is not destroyed by the acid present in the liquid product mixture. The term "acid-stable" within the scope of the present invention means that the membrane material is stable in the presence of the acid of the catalyst system for at least 300 hours and maintains separation performance.

[0065] The substructure preferably has a porous structure, which is permeable to the permeate that has passed through the separation active layer. The substructure has a stabilizing function and serves as a carrier for the separation active layer. The substructure can in principle be composed of any suitable porous material. Suitable materials are well known to those skilled in the art. However, a prerequisite is that the material is stable to acids and alkalis. The substructure can also be composed of the same material as the separation active layer. The preferred material for the substructure is a plastic, such as polypropylene or a PAEK polymer (PAEK means polyaryletherketone). PAEK polymers are particularly PEEK (polyetheretherketone), more preferably PEEK polymers having a sulfonation degree of less than 20%, more preferably PEEK polymers having a sulfonation degree of less than 10%.

[0066] The separation active layer according to the invention is preferably composed of a PAEK (polyaryletherketone) polymer. PAEK is characterized in that in 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%, and particularly preferably a PEEK polymer with a sulfonation degree of less than 10%. The corresponding PEEK polymers and their production are described in WO 2015 / 110843A1.

[0067] The membrane separation in step c is preferably carried out at a temperature of 25° C. to 100° C., further preferably 30° C. to 80° C., and particularly preferably 40° C. to 70° C. In order to bring the product mixture to a preferred advantageous temperature for 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, whereby another stream is heated in the process according to the invention.

[0068] The transmembrane pressure (TMP) in the membrane separation of step c is preferably 10 to 60 bar, further preferably 15 to 55 bar, particularly preferably 20 to 50 bar. The permeate side pressure 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, in the case of pressure ratios and in particular permeate side pressures, care should be taken to ensure that the pressure is set according to the hydrocarbon used, the alcohol used and the temperature in the system so as to avoid evaporation after passing through the membrane. Evaporation may lead to unstable operation.

[0069] In the subsequent step d, the crude product mixture or the permeate from the membrane separation is worked up by distillation in at least one distillation column to remove unreacted alcohol and unreacted diisobutylene. An ester product comprising the formed ester is obtained. The ester product comprises the ester formed from diisobutylene, i.e. 3,5,5-trimethylhexanoic acid alkyl ester, wherein the alkyl group contains 1 to 4 carbon atoms, depending on the alcohol used.

[0070] In the distillation post-treatment of the crude product mixture or permeate of step d, unreacted alcohol and unreacted diisobutylene are obtained at the top of the at least one distillation column. The ester product is thus produced at the bottom of the at least one distillation column. The top stream containing the unreacted alcohol and unreacted diisobutylene removed in the at least one distillation column is recycled to the alkoxycarbonylation of step b. If the components used are mixed before the alkoxycarbonylation, the top stream is of course supplied to the mixing. This allows continuous operation of the method of the present invention with the highest possible yield. A purge stream can be taken from the recycled top stream in order to discharge low-boiling by-products from the method.

[0071] The distillative work-up in step d for removing unreacted alcohol and unreacted diisobutylene can be carried out in a distillation column. It is conceivable that the distillative work-up in step d for removing unreacted alcohol and unreacted diisobutylene is carried out in a plurality of distillation columns. However, this means significantly higher costs in terms of equipment. Therefore, it is preferred that the distillative work-up in step d is carried out in a single distillation column.

[0072] In the distillation post-treatment of step d, the pressure in the distillation column is preferably 0.3 to 2 bar, further preferably 0.4 to 1 bar, particularly preferably 0.5 to 0.7 bar. In the distillation post-treatment of step d, the bottom temperature of the distillation column is preferably 80° C. to 160° C. In the distillation post-treatment of step d, the top temperature of the distillation column is preferably 30 to 80° C. In addition, it is preferred that the reflux ratio in the distillation column is 1 to 2. The distillation column used for removal in step d preferably contains 10 to 30 theoretical plates. The distillation column may contain high-performance structured packings here. Suitable high-performance structured packings are known to those skilled in the art.

[0073] The ester produced in step b and separated and optionally purified in step d is then subjected to hydrolysis or saponification in step e. The ester group is cleaved by the acidic catalyst used here or the saponifying agent used here, thereby producing a carboxylic acid or a carboxylate salt, and the alcohol bound in the ester formation in step b can be recovered. Therefore, the saponification produces a reaction mixture containing at least a carboxylic acid or a salt thereof, the decomposed alcohol, and an unreacted ester.

[0074] The alcohol recovered from the hydrolysis or saponification in step e can be removed from the resulting mixture in a subsequent process step and returned to the first reaction zone.

[0075] Hydrolysis is a known chemical reaction in which an ester is converted into a carboxylic acid with the aid of an acidic catalyst and decomposes into an alcohol. Typical hydrolysis conditions are known to those skilled in the art.

[0076] The hydrolysis in step e is carried out in the presence of an acidic heterogeneous or homogeneous catalyst. Known homogeneous catalysts are acidic compounds, for example Bronsted acids, in particular HCl, H2SO4, phosphoric acid, p-toluenesulfonic acid and 4-dodecylbenzenesulfonic acid, or Lewis acids, in particular AlCl3, ZnCl2, HfCl4.2THF, Al(OTf)3. Known heterogeneous catalysts are acidic compounds, for example cation exchangers, acidic H zeolites, acid-functionalized metal oxides with silicon dioxide and heteropolyacids. Suitable heterogeneous catalysts are in particular 15. IR 120(H), HClO4-SiO2, 3-propylsulfonic acid functionalized silica, -SiO2, -SiO2, Cs2.5H0.5PW12O40, H-ZSM5, H-ZSM-5-C18, Z-β-H-25, Z-β-H-38, Z-β-H-150, Z-β-H-360, ZYH-60, ZYH-80 or CZB 150 (BEA 150) (Clariant Company).

[0077] The reaction mixture from the hydrolysis in step e is subjected in a subsequent step f to at least one separation process step selected from thermal separation, for example distillation, extraction, crystallization or another 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. Suitable process conditions are known to those skilled in the art. Multistage distillation can also be carried out.

[0078] Saponification is a known chemical reaction in which an ester is converted into a carboxylate with the aid of an alkaline or enzymatic saponifying agent and the alcohol is decomposed. Typical conditions for saponification are known to those skilled in the art.

[0079] The saponification in step e is carried out in the presence of a saponifying agent. Known saponifying agents are alkaline compounds, such as potassium hydroxide, potassium (bi) carbonate, sodium hydroxide, sodium (bi) carbonate or amine compounds. It is also feasible to carry out saponification with an enzyme saponifying agent, in particular an esterase.

[0080] The reaction mixture from the saponification in step e is subjected in a subsequent step f to at least one separation process step selected from thermal separation, for example distillation, extraction, crystallization or another membrane separation, in order to separate the carboxylate 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 carried out.

[0081] In addition, the alcohol used in step b can also be separated off in the at least one separation process step and recycled to the reaction zone. During the return, a purge stream can be drawn off, for example in order to discharge by-products of the process. BRIEF DESCRIPTION OF THE DRAWINGS

[0082] refer to Figure 1 and Figure 2 The invention is described in more detail. The drawings show specific embodiments and are not intended to limit the subject matter of the invention.

[0083] Figure 1 An embodiment is shown in which in step a a diisobutene stream (1) is supplied to a distillation column (2). This top stream (3) is enriched in 2,4,4-trimethylpent-1-ene compared to the diisobutene stream used. The distillation also produces a residual stream (4) which is depleted in 2,4,4-trimethylpent-1-ene compared to the diisobutene stream used. The top stream (3) is subsequently supplied to the alkoxycarbonylation (5) in step b, whereby a product mixture (6) is obtained. This product mixture (6) is subjected to catalyst removal (7) in step c to provide a crude product mixture (8), which is subjected to distillation (9) in step d. The product (11) and unreacted alcohol and unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene are obtained here as stream (10), which is recycled to the alkoxycarbonylation (5). The product, alkyl 3,5,5-trimethylhexanoate, is then sent to a hydrolysis or saponification (14) and converted there into a carboxylic acid which is separated off as stream (16) in a downstream distillative separation (15).

[0084] Figure 2 A further embodiment is concerned in which an isomerization (12) is additionally present. Here, the diisobutene stream (13) used can be fed to the isomerization and / or distillation (2). This is indicated by the dashed line. The isomerization stream (1) then becomes the feed for the distillation (2). The isomerization is also fed with the residual stream (4) from the distillation, in order to convert the 2,4,4-trimethylpent-2-ene at least partially into 2,4,4-trimethylpent-1-ene there. DETAILED DESCRIPTION

[0085] The invention is described below with reference to examples. These are for illustrative purposes and do not limit the subject matter of the invention.

Claims

1. A method for preparing a carboxylic acid or a salt thereof, wherein the method comprises at least the following steps: a. supplying a diisobutene stream comprising 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene to at least one distillation column and separating the diisobutene stream into at least one overhead stream which is enriched in 2,4,4-trimethylpent-1-ene compared with the diisobutene stream used and comprises more than 85% by weight of 2,4,4-trimethylpent-1-ene and a residual stream which is depleted in 2,4,4-trimethylpent-1-ene compared with the diisobutene stream used; b. supplying the top stream to an alkoxycarbonylation, whereby 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene are reacted in a reaction zone 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 Elements or a compound thereof, a phosphorus-containing ligand and an acid to obtain a liquid product mixture, wherein the liquid product mixture comprises at least esters formed by the alkoxycarbonylation, the homogeneous catalyst system, unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene and unreacted alcohol; c. removing the homogeneous catalyst system from the liquid product mixture to obtain a crude product mixture comprising at least the ester formed by alkoxycarbonylation, unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene and unreacted alcohol; d. working up the crude product mixture by distillation in at least one distillation column to remove unreacted alcohol and unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene, wherein unreacted alcohol and unreacted 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene are removed and recycled to the alkoxycarbonylation in step b; e. hydrolyzing or saponifying the ester obtained in 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, the decomposed alcohol and unreacted ester; and f. isolating the carboxylic acid or its salt formed in step e in at least one separation process step selected from thermal separation such as distillation, extraction, crystallization and membrane separation.

2. The process according to claim 1, wherein the residual stream in step a is taken off as a bottom stream of the at least one distillation column or as a side stream of the at least one distillation column.

3. The process according to claim 1 , wherein the residual stream is supplied to an isomerization, wherein 2,4,4-trimethylpent-2-ene is at least partially isomerized to 2,4,4-trimethylpent-1-ene using a heterogeneous catalyst based on zeolites or based on ion exchange resins.

4. The process according to claim 3, wherein the isomerization results in an isomerized stream in which the proportion of 2,4,4-trimethylpent-2-ene is lower than in the residual stream and the proportion of 2,4,4-trimethylpent-1-ene is higher than in the residual stream.

5. The process according to claim 4, wherein the isomerized stream is at least partially supplied to the distillation in step a.

6. The process according to claim 3 , wherein the input stream for feeding the process and comprising 2,4,4-trimethylpent-2-ene and 2,4,4-trimethylpent-1-ene is supplied to the distillation unit in step a in addition to the isomerization stream and / or to the isomerization in addition to the residual stream.

7. The process according to any one of claims 3 to 6, wherein the isomerization is carried out at a temperature of 25 to 90°C, preferably 30 to 80°C, more preferably 35 to 70°C.

8. The method according to any one of claims 3 to 7, wherein the zeolite has a Si:Al ratio of 40:1 to 200:

1.

9. The process according to any one of claims 3 to 8, wherein the zeolite is selected from the group consisting of β-zeolite and γ-zeolite.

10. The process according to claim 3, wherein the ion exchange resins used are those which have been prepared by sulfonation of phenol / aldehyde condensates or by sulfonation of copolymers of aromatic vinyl compounds.

11. The process according to any one of the preceding claims, wherein the alkoxycarbonylation in step b is carried out at a temperature of 60 to 120°C, preferably 65 to 110°C, more preferably 70 to 100°C.

12. The process according to claim 1, 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, more preferably 15 to 25 bar.

13. 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.

14. The process according to claim 1, wherein the removal of the homogeneous catalyst system in step c is effected by membrane separation.

15. The process according to claim 1, wherein the diisobutene stream, the alcohol and the homogeneous catalyst system are first mixed in a mixing vessel before they are introduced into the reaction zone.

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