Method for preparing esters by alkoxycarbonylation

Abstract

The present invention relates to a process for preparing esters by alkoxycarbonylation. The subject of the invention is a process for preparing esters by alkoxycarbonylation of C2- to C20-hydrocarbons having at least one multiple bond, preferably at least one olefinic double bond, wherein the homogeneous catalyst system used is separated from the product mixture by means of membrane separation and recycled to the reaction zone. In one embodiment of the invention, the ester thus formed is converted into another ester by transesterification.

Description

Technical Field

[0001] The present invention relates to a process for preparing esters by alkoxycarbonylation of C2- to C16-hydrocarbons having at least one multiple bond, preferably at least one olefinic double bond, wherein the homogeneous catalyst system used is separated from the product mixture by membrane separation and recycled to the reaction zone. In one embodiment of the invention, the ester thus formed is converted into another ester by transesterification. Background Art

[0002] Alkoxycarbonylation is the reaction of hydrocarbon with at least one multiple bond, preferably at least one olefinic double bond, with carbon monoxide and alcohol to generate corresponding ester. Metal-ligand complexes are usually used as catalysts at this, and the metal-ligand complexes are homogeneously dissolved in the reaction mixture, so they are also present in the product mixture. In order to be able to separate the desired ester from the product mixture, it is advantageous to first remove at least most of the homogeneously dissolved catalyst from the product mixture. On the other hand, the cost of the catalyst used is relatively high, so for economic reasons, the catalyst should also be recovered.

[0003] Various methods are known for separating the homogeneously dissolved catalyst from the remaining reaction mixture. For example, WO 2013 / 107902 A1 discloses a method in which, after alkoxycarbonylation, the homogeneously dissolved catalyst is separated by means of a membrane that is impermeable to molecules with a molecular weight of 1 kDa and above, whereby the catalyst is retained and enriched in the retentate. According to WO 2013 / 107902 A1, membrane materials based on polyimide or polydimethylsiloxane are preferred.

[0004] However, the materials disclosed in WO2013 / 107902A1 have the following disadvantages: they generally do not have sufficient long-term stability, especially for acids (which are usually part of the catalyst system as cocatalysts), and may become unstable under various loads. In addition, these membrane materials have a negligible retention rate for the hydrocarbons or solvents used (which may also be reactants in the alkoxycarbonylation, such as the alcohols used). Neither alcohols nor hydrocarbons are enriched in the retentate. Thus, downstream post-treatment and / or purification steps, such as subsequent distillation steps, must be run more complicatedly to separate the solvent and / or reactants from the product. Summary of the invention

[0005] The object of the present invention arising from this is to provide an alkoxycarbonylation process in which the homogeneous catalyst is separated off using an acid-stable membrane material which can also be used for a relatively long time. A further object is to provide an alkoxycarbonylation process in which during the membrane separation preferably the products formed are passed through and at least part of the reactants used, in particular the alcohol, is at least partially retained.

[0006] The underlying object of the invention is achieved by a method according to claim 1. Preferred embodiments are given in the dependent claims.

[0007] The present invention comprises a process for preparing esters by carbonylation of C2- to C16-hydrocarbons, wherein the process comprises the following steps:

[0008] a) reacting a C2- to C16-hydrocarbon having at least one olefinic double bond with carbon monoxide and with an alcohol in the presence of a homogeneous catalyst system in a reaction zone (carbonylation) to obtain a liquid product mixture, the alcohol being a monohydric or polyhydric alcohol having 1 to 50 carbon atoms (two or more OH groups) or a mixture of two or more monohydric and / or polyhydric alcohols and, when it is a monohydric alcohol, used in a molar ratio to the C2- to C16-hydrocarbon (alcohol:C2- to C16-hydrocarbon) of 2 to 20 or, when it is a polyhydric alcohol, used in a molar ratio to the hydrocarbon used (hydrocarbon:polyol) of 2 to 20, the 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, the liquid product mixture comprising at least the esters formed by the reaction, the homogeneous catalyst system, low boilers, and / or high boilers and unreacted alcohol;

[0009] b) carrying out a membrane separation for separating the homogeneous catalyst system from the liquid product mixture, whereby the homogeneous catalyst system and also unreacted hydrocarbons and / or unreacted alcohol, preferably unreacted alcohol, are enriched in the retentate, and the ester formed in step a) is enriched in the permeate, wherein an acid-stable (OSN-capable) (organic solvent nanofiltration) membrane material is used as membrane material, i.e. a membrane material which is stable for at least 300 hours in the presence of an acid of the catalyst system and which has at least one separating-active layer, and the retentate is recycled into the reaction zone;

[0010] c) working up the permeate by means of one or more separation steps selected from thermal separation, extraction, crystallization or membrane separation, preferably by means of one or more distillation steps, for separating off the esters formed in step a) and for separating off unreacted C2 to C16 hydrocarbons having at least one olefinic double bond and / or unreacted alcohol, and recycling the unreacted C2 to C16 hydrocarbons having at least one olefinic double bond and / or unreacted alcohol into the reaction zone of step a.

[0011] The hydrocarbon used in the reaction in step a) must have at least one multiple bond. Preferably, the hydrocarbon has at least one olefinic double bond, particularly preferably the hydrocarbon has an olefinic double bond. In principle, there is no restriction on the carbon number of the compound with at least one multiple bond, preferably at least one olefinic double bond. However, only the carbonylation of C2- to C16- hydrocarbon with at least one multiple bond, preferably at least one olefinic double bond is industrially important. In a preferred embodiment of the present invention, C3- to C12- hydrocarbon can be used, which preferably has at least one olefinic double bond. These particularly include normal olefins, isoolefins, cycloolefins and aromatic olefins with 2-16 carbon atoms, preferably 3-12 carbon atoms.

[0012] In addition to at least one olefinic double bond, the above hydrocarbons may contain one or more other functional groups. Examples of suitable functional groups are carboxyl, thiocarboxyl, sulfo, sulfinyl, carboxylic anhydride groups, imide groups, carboxylate groups, sulfonate groups, carbamoyl, sulfamoyl, cyano, carbonyl, thiocarbonyl, hydroxyl, mercapto, amino, ether groups, thioether groups, aryl, heteroaryl or silyl and / or halogen substituents.

[0013] Particularly preferred hydrocarbons used in step a) of the process according to the invention have only one olefinic double bond, in particular n-olefins and isoolefins having 2 to 16 carbon atoms, preferably 3 to 12 carbon atoms. The hydrocarbons used are preferably unsubstituted.

[0014] According to the present invention, in step a), the used and above-mentioned hydrocarbon with olefinic double bonds is reacted with carbon monoxide (CO) and alcohol to generate the corresponding ester. Here, carbon monoxide can be provided directly in the form of a feed mixture, or provided 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 separating the gas containing carbon monoxide into its components in advance in a manner known to those skilled in the art and passing carbon monoxide into the reaction zone. Carbon monoxide may still contain a certain proportion of hydrogen or other gases at this point, because it is almost impossible to achieve complete separation.

[0015] The alcohol used in the reaction in step a) is a monohydric or polyhydric alcohol (two or more OH groups) with 1 to 50 carbon atoms, preferably 1 to 15 carbon atoms, particularly preferably 1 to 10 carbon atoms, or a mixture of two or more monohydric and / or polyhydric alcohols. In a preferred embodiment, the polyhydric alcohol is a dihydric alcohol, a trihydric alcohol or a tetrahydric alcohol with the above-mentioned carbon number, preferably a dihydric alcohol or a trihydric alcohol. Suitable alcohols for the reaction in step a) are methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 2-propanol, the tert-butyl alcohol, 3-pentanol, 2-propylheptanol, cyclohexanol, phenol or a mixture thereof, preferably ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 2-propanol, the tert-butyl alcohol, 3-pentanol, 2-propylheptanol.

[0016] When the alcohol used in step a) is a monohydric alcohol, it is used in a molar ratio to the hydrocarbon used (monohydric alcohol: hydrocarbon) of 2 to 20, preferably 3 to 10, particularly preferably 4 to 6. The monohydric alcohol is therefore added in a molar excess, based on the hydrocarbon used. Thus, the alcohol can serve both as reactant for the carbonylation and as solvent. When the alcohol used in step a) is a polyhydric alcohol, it is used in a molar ratio to the hydrocarbon used (hydrocarbon: polyhydric alcohol) of 2 to 20, preferably 3 to 10, particularly preferably 4 to 8. The polyhydric alcohol is therefore added in a molar deficit, based on the hydrocarbon used.

[0017] The reaction in step a) according to the present invention is carried out in the presence of a homogeneous catalyst system comprising at least one metal of Groups 8-10 of the Periodic Table of Elements (PSE) or a compound thereof, a phosphorus-containing ligand and an acid as a promoter.

[0018] The metal of Group 8-10 of the PSE is preferably palladium. Palladium is preferably used in the form of a precursor compound as a palladium compound coordinated by a phosphorus-containing ligand. An example of a palladium compound that can be used as a precursor compound is palladium chloride [PdCl 2 ], palladium(II) acetylacetonate [Pd(acac) 2 ], palladium(II) acetate [Pd(OAc) 2 ], dichloro-(1,5-cyclooctadiene)palladium(II) [Pd(cod) 2 Cl 2 ], bis(dibenzylideneacetone)palladium(0) [Pd(dba) 2 ], tris(dibenzylideneacetone)dipalladium(0) [Pd 2 (dba) 3 ], bis(acetonitrile)-dichloropalladium(II) [Pd(CH 3 CN) 2 Cl 2 ], (cinnamyl) palladium dichloride [Pd(cinnamyl)Cl 2The compound [Pd(acac) 2 ] or [Pd(OAc) 2 The metal concentration of palladium in step a) is preferably 0.01 to 0.6 mol %, preferably 0.03 to 0.3 mol %, particularly preferably 0.04 to 0.2 mol %, based on the substance amount of hydrocarbon used.

[0019] Suitable phosphorus-containing ligands of the catalyst system according to the present invention preferably have a bidentate structure. Preferred phosphorus-containing ligands of the catalyst system according to the present invention are phenyldiphosphine compounds, such as those disclosed in EP3121184A2. The ligand can be combined with palladium in a pre-reaction, thereby directing the palladium-ligand-complex to the reaction zone, or added to the reaction in situ and combined with palladium there. The molar ratio of the ligand: metal used for the reaction in step a) can be 1: 1 to 10: 1, preferably 2: 1 to 6: 1, and particularly preferably 3: 1 to 5: 1.

[0020] The homogeneous catalyst system further comprises an acid, in particular a Bronsted acid or a Lewis acid. The Lewis acid used can be a Lewis acid having an LAU value greater than 25, preferably an LAU value of 29. The LAU value is a new method for determining the strength of a Lewis acid (JR Gaffen et al., Chem, Vol. 5, No. 6, pp. 1567-1583). 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, aluminum trifluoromethanesulfonate is preferably used. The Lewis acid is preferably added in a molar ratio of a Lewis acid to a ligand of 1:1 to 20:1, preferably 2:1 to 15:1, and particularly preferably 5:1 to 10:1.

[0021] Suitable Bronsted acids preferably have an acid strength of pKs≤5, particularly 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 polyprotic acids, within the scope of the present invention, the acid strength pKs refers to the pKs of the first proton transfer 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, particularly preferably 3:1 to 5:1.

[0022] The Bronsted acids used may in particular be perchloric acid, sulfuric acid, phosphoric acid, methylphosphonic acid or sulfonic acid. 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.

[0023] The reaction and / or carbonylation of the hydrocarbons having olefinic double bonds used in step a) is preferably carried out at a temperature of 25 to 140° C., further preferably at a temperature of 80 to 130° C. and particularly preferably at a temperature of 90 to 120° C. The pressure in step a) can be 5 to 60 bar, preferably 10 to 40 bar, particularly preferably 15 to 30 bar.

[0024] The reaction in the described step a) is carried out in a suitable reaction zone. The reaction zone for the reaction comprises at least one reactor, but may also be composed of two or more reactors. 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 there are a plurality of reactors, the reactors may be identical or different.

[0025] A liquid product mixture is provided by the reaction in the above-mentioned step a), which comprises at least the ester formed by the reaction, a homogeneous catalyst system, unreacted alcohol A and possible other components such as low-boiling substances, such as low-boiling by-products such as ethers, and / or high-boiling substances and / or unreacted hydrocarbons. The product mixture is then supplied to the membrane separation in the subsequent step b). In addition, in the reaction in step a), waste gas can also be taken out from the reaction zone, and the waste gas is at least composed of non-reactive impurities such as nitrogen, hydrogen, alkanes and low-boiling by-products (such as ethers already mentioned). The impurities and low-boiling by-products can accumulate, and in the long run reduce the partial pressure of the reaction gas (CO), thereby slowing down the reaction rate.

[0026] In the subsequent step b), the product mixture is supplied to the membrane separation to separate the homogeneous catalyst system from the product mixture. Through the membrane material according to the present invention, the homogeneous catalyst system and unreacted hydrocarbons and / or unreacted alcohols are enriched in the retentate, and the ester formed in step a) is enriched in the permeate. The permeate comprising the formed ester is then supplied to the subsequent step c). The retentate containing the enriched homogeneous catalyst system is then recycled to the reaction zone. When recycling the retentate, a purge stream can also be taken out, which can include inert alkanes, low-boiling by-products (such as ethers), possible decomposition products of the catalyst system or other impurities introduced by the hydrocarbon stream used, such as trace amounts of water or nitrogen, to avoid accumulation in one or more reaction zones. The recycling of the retentate ensures that the catalyst system produced in the retentate during membrane separation is recycled to the reaction. Thus, the catalyst loss caused by deposition or deactivation is minimized and the method cost is more favorable. Catalyst losses can generally not be completely avoided, but the reduction in losses mentioned results in less catalyst having to be compensated by supplying fresh catalyst.

[0027] Membrane separation is based on the semipermeability of the membrane material, which is permeable for certain substances and impermeable for other substances. The membrane material used in step b) of the method according to the invention is an OSN membrane material (OSN=organic solvent nanofiltration). Such a membrane material preferably consists of at least a relatively thin separation-active layer (also referred to as: active separation layer) and an optional thicker backing on which the separation-active layer is located. Preferably, the membrane material according to the invention consists of at least a separation-active layer and a backing. One or more intermediate layers may be present between the separation-active layer and the backing. In a preferred embodiment, the membrane material consists only of a separation-active layer and a backing. The membrane material consisting of at least a separation-active layer and a backing should be acid-stable so that the membrane material is not damaged by acids present as co-catalysts in the liquid product mixture. In the context of the present invention, the term "acid-stable" means that in the presence of an acid in the catalyst system, in particular a Brønsted acid with a pKs≤5, particularly preferably a pKs≤3, or a Lewis acid with an LAU value of greater than 25, preferably with an LAU of 29, the membrane material is stable for at least 300 hours and is not destroyed, so that an actual separation effect is no longer possible.

[0028] In particular, the backing has a porous structure that is permeable to permeates passing through the separation active layer. The backing has a stabilizing function and serves as a carrier for the separation active layer. The backing can in principle consist of any suitable porous material. However, the prerequisite is that the material is stable to acids and bases. The backing can consist of the same material as the separation active layer.

[0029] The separation active layer according to the present invention is preferably composed of a PAEK polymer (polyaryletherketone). PAEK is characterized in that, within the repeating unit, the aromatic group is alternately connected by an ether functional group and a ketone functional group. The preferred separation active layer according to the present invention is composed of PEEK (polyetheretherketone). As a separation active layer, it is particularly preferred to use a PEEK polymer with a sulfonation degree of less than 20%, and particularly preferably a sulfonation degree of less than 10%. The corresponding PEEK polymer and its preparation are described in WO2015 / 110843A1 or J. daSilva Burgal et al.; Journal of Membrane Science, Vol. 479 (2015), pp. 105-116. It has been surprisingly shown that the material is particularly stable, especially for acids as co-catalysts of homogeneous catalyst systems. In addition, the PEEK material according to the present invention is characterized in that, when used as a separation active layer, it allows the formed ester to pass preferentially, while the reactant alcohol used itself is at least partially retained and thus enriched in the retentate. As a result, the subsequent workup of the remaining liquid product mixture can be carried out more economically and more permanently, since less alcohol has to be separated off in comparison with known membrane materials.

[0030] The membrane separation in step b) is preferably carried out at a temperature of 25 to 100° C., more preferably 30 to 80° C., particularly preferably 40 to 70° C. In order to bring the product mixture to the preferred prevailing temperature during 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, thereby heating another stream in the method according to the invention. Optionally, there is also a degassing step between the reaction zone in step a) and the membrane separation in step b) in order to remove volatile compounds such as carbon monoxide and / or residual unreacted impurities such as nitrogen, hydrogen, alkanes and low-boiling by-products (such as the ethers already mentioned) from the product mixture in advance. Here, the product mixture is first depressurized at the partial pressure of dissolved components such as carbon monoxide to degas it, so as to then increase the pressure specified in the membrane separation again.

[0031] In step b), the transmembrane pressure (TMP) can be 10 to 60 bar, preferably 15 to 55 bar, particularly preferably 20 to 50 bar (relative). The permeate-side pressure can here be above atmospheric pressure up to 15 bar, preferably 3 to 7 bar, whereby the pressure on the retentate side is then obtained according to the TMP. In a preferred embodiment, at the pressure ratio, in particular at the permeate-side pressure, it should be noted that the pressure is adjusted according to the hydrocarbon used, the alcohol used and the temperature present in order to avoid evaporation after penetration through the membrane, since the entire mode of operation can become unstable as a result. In principle, the same applies to dissolved components, such as carbon monoxide, which can optionally be removed by the degassing step already mentioned.

[0032] The economic viability of a membrane separation process can be determined greatly by the service life of the membrane material used, so the service life or stability of the membrane can also be a criterion for selecting a suitable membrane material. A minimum service life of about half a year is assumed here. This can become critical for processes in which the product is enriched in the permeate (such as the present process), since the required membrane area increases with the total capacity of the process.

[0033] This results in three criteria for the service life or stability of the membrane, namely the technical integrity of the membrane, the retention rate of the membrane (corresponding to the separation performance of the membrane, see the following definition) and the permeability of the membrane (see the following definition). In this case, defects with regard to the technical integrity, such as dissolution of the membrane, the formation of defects or holes or a significant shrinkage or expansion of the membrane area, will affect the retention rate of the membrane (reduction of the retention rate) and the permeability (increase of the permeability) and ensure that the membrane can no longer be pressurized to the mentioned pressure and / or the transmembrane pressure cannot be set.

[0034] In membrane technology, in order to characterize the permeability or separation performance of a membrane, the retention rate R of the membrane for a specific component of a substance mixture is defined according to the following formula (1):

[0035] R = 1 − w (I)P / w (I)R (1)

[0036] where w (I)P represents the mass proportion of the observed component in the permeate, and w (I)R Represents the mass proportion of the observed component in the retentate of the membrane. Thus, the rejection can take values ​​from 0 to 1 and is therefore usually given in %. A rejection of 0% means that the observed component permeates through the membrane unhindered, so that the mass proportion of the component in the retentate is the same as in the permeate. On the other hand, a rejection of 100% means that the observed component is completely retained by the membrane, but this is technically almost impossible to achieve.

[0037] In addition to the rejection, the so-called permeability of the membrane according to the following formula (2) is also decisive for characterizing its permeation capacity:

[0038] P = m' / (A * TMP) (2),

[0039] Where m' is the mass flow rate of the permeate, A is the area of ​​the membrane, and TMP is the applied transmembrane pressure. It is usually expressed in units of kg / (h * m 2 The permeability is therefore a characteristic value normalized for the membrane area and the set TMP.

[0040] In terms of characterizing the stability of the membrane, the relative change in permeability P can be defined according to the following formula (3): Rel :

[0041] P Rel =P t=x / P t=0 (3)

[0042] Where P t=x represents the permeability at time t = x, and P t=0 represents the initial permeability at time point t=0 (it may also be another time reference point, provided that t=x>t=y).

[0043] In addition, the membrane can be characterized by its selectivity for a specific component. Selectivity refers to the ratio of the concentration of a specific component i before and after the membrane. The selectivity is calculated according to the following formula (4):

[0044] S i = C P_i / C F_i (4)

[0045] Among them C F_irepresents the observed concentration of component i in the feed (here, the liquid product mixture) reaching the membrane, while C P_i represents the concentration of component i observed in the permeate.

[0046] In a preferred embodiment of the present invention, after being used for 300 hours in the membrane separation method in step b) described herein, the membrane material according to the present invention has the following two characteristics:

[0047] • the membrane rejection R for the homogeneous catalyst system is at least 75%, preferably 85%, particularly preferably 90%, and deteriorates by not more than 30 percentage points, preferably not more than 20 percentage points, particularly preferably not more than 10 percentage points; or

[0048] •Relative permeability P rel The value is 30%-300%, preferably 50%-150%, particularly preferably 75%-125%.

[0049] Therefore, the membrane should not lose its technical integrity. Examples of loss of technical integrity are dissolution of the membrane, formation of defects / holes and / or a significant shrinkage or a significant increase in the membrane area. The membrane material according to the invention should be stable in particular to acids present in amounts of 0.1% to 5% by weight, without losing its technical integrity and having the two features mentioned above.

[0050] In the subsequent step c), the permeate from the membrane separation (step b)) is subjected to a separation process selected from thermal separation (e.g. distillation), extraction, crystallization or further membrane separation to separate the ester formed in step a) from the remaining permeate. The separation process is preferably distillation. The corresponding process conditions are known to the person skilled in the art.

[0051] It may happen that in the separation method used in step c), in particular in the distillation, not only the ester formed is separated from the permeate, but also any high boilers formed, for example high-boiling by-products which may have been produced in the reaction in step a). In order to remove these high boilers, the process according to the invention can have a purification step, i.e. the ester formed is purified by separating it from the high boilers contained in the permeate by means of thermal separation, extraction, crystallization or membrane separation. Preferably, a thermal separation method is used, particularly preferably a further distillation, to purify the ester formed. The process conditions are known to the person skilled in the art.

[0052] In a preferred embodiment, the permeate obtained in step c) and containing at least unreacted alcohol and / or unreacted hydrocarbons, from which most of the esters formed in step a) have been removed, is subjected to a recycle component separation. Here, unreacted alcohol and / or unreacted hydrocarbons are separated from the remaining permeate, especially the low boilers contained there, by means of thermal separation, extraction, crystallization or membrane separation. Preferably, a thermal separation method is used, particularly preferably a further distillation, to separate unreacted alcohol and / or unreacted hydrocarbons from the remaining permeate. The process conditions are known to those skilled in the art. The unreacted alcohol and / or unreacted hydrocarbons obtained here can then be recycled to the reaction zone.

[0053] The esters formed by the process according to the invention can be transesterified in two further subsequent process steps d) and e). In the transesterification, the part of the ester corresponding to the first alcohol used in step a) is replaced by the second alcohol. The transesterification is carried out after the above-mentioned step c), optionally after a possible purification step, and comprises the following steps:

[0054] d) in a second reaction zone, transesterifying the ester formed in step a) with a second alcohol, wherein the second alcohol is different from the alcohol used in step a), to obtain a second product mixture, which comprises at least the ester with the second alcohol, the dissociated first alcohol and the unreacted second alcohol;

[0055] e) separating the formed ester with the second alcohol from the remaining second product mixture, in particular from the dissociated first alcohol, by means of one or more separation steps selected from thermal separation, extraction, crystallization or membrane separation, preferably by thermal separation and / or by means of membrane separation, particularly preferably by means of one or more distillation steps, and recycling the dissociated first alcohol to the reaction zone of step a), and recycling the unreacted second alcohol to the second reaction zone.

[0056] In step d), the actual transesterification is carried out, that is, the first alcohol actually bound in step a) is dissociated and the second alcohol is bound. To this end, the ester formed in step a) is reacted in a reaction zone with a second alcohol different from the first alcohol. In a particularly preferred embodiment, the second alcohol used in the transesterification is an alcohol with a higher boiling point than the first alcohol used in step a). In the transesterification, the second alcohol is preferably added in excess to promote the transesterification reaction.

[0057] The second alcohol used in the transesterification in step d) is preferably a monohydric or polyhydric alcohol (two or more OH groups) having 1 to 50 carbon atoms, more preferably 1 to 15 carbon atoms, particularly preferably 1 to 10 carbon atoms, or a mixture of two or more monohydric and / or polyhydric alcohols, provided that the first alcohol used in step a) is different from the second alcohol. In a preferred embodiment, the polyol is a dihydric alcohol, trihydric alcohol or tetrahydric alcohol having the above-mentioned number of carbon atoms, preferably a dihydric alcohol or a trihydric alcohol. Suitable alcohols for the reaction in step a) are methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 2-propanol, tert-butanol, 3-pentanol, 2-propylheptanol, cyclohexanol, phenol, pentaerythritol, neopentyl glycol, trimethylolpropane, dipentaerythritol or mixtures thereof, preferably ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 2-propanol, tert-butanol, 3-pentanol, 2-propylheptanol.

[0058] The transesterification in step d) is preferably carried out under acid or base catalysis. The acid used can be a Bronsted acid or a Lewis acid.

[0059] Suitable Bronsted acids for the transesterification in step d) are perchloric acid, sulfuric acid, phosphoric acid, methylphosphonic acid or sulfonic acids, for example methanesulfonic acid, trifluoromethanesulfonic acid, tert-butanesulfonic acid, p-toluenesulfonic acid (pTSA), 2-hydroxypropane-2-sulfonic acid, 2,4,6-trimethylbenzenesulfonic acid or dodecylsulfonic acid. The Bronsted acid used is preferably sulfuric acid or sulfonic acid, particularly preferably sulfuric acid. Metals or their compounds, for example tin powder, tin(II) oxide, tin(II) oxalate, titanates such as tetraisopropyl orthotitanate or tetraisobutyl orthotitanate, and zirconium esters such as tetrabutyl zirconate and sodium and potassium methoxide can also be used.

[0060] A suitable Lewis acid for the transesterification in step d) is titanium(IV) isopropoxide, Bu 2 SnO, BuSn(O)OH or aluminum triflate. Lewis acids preferably used are titanium(IV) isopropoxide and aluminum triflate.

[0061] Suitable bases for the transesterification in step d) are alkali metals, alkali metal alkoxides, alkali metal or alkaline earth metal acetates or oxides, alkali metal or alkaline earth metal alkoxides such as NaEtOH or MgEtOH, or alkali metal carbonates such as K 2 CO 3 or Cs 2 CO 3 However, it is also possible to use basic ion exchangers or NaOH. Preference is given to using Na or Mg alkoxides such as NaEtOH or MgEtOH.

[0062] The acid-catalyzed transesterification is preferably carried out at a temperature of 60-220° C., more preferably 100-210° C., particularly preferably 130-200° C. The reaction is preferably carried out above the boiling point of the first alcohol to be cleaved, so as to remove the cleaved first alcohol directly from the reaction mixture and thus promote the shift of the equilibrium to the product side. The second alcohol is preferably added to the ester formed in step a) in a significant excess, for example 30:1.

[0063] The base-catalyzed transesterification is preferably carried out at a temperature of 20-100°C.

[0064] The transesterification described gives rise to a second product mixture which comprises at least esters with the second alcohol, the dissociated first alcohol and unreacted second alcohol.

[0065] In the subsequent step e), the second ester formed in step d) is separated from the remaining second product mixture. The separation is carried out by means of thermal separation, preferably distillation and / or by means of membrane separation, in particular with the above-mentioned membrane materials. The corresponding process conditions are known to those skilled in the art.

[0066] It may happen that in the separation method used in step e), in particular in the distillation, not only the ester formed is separated from the remaining second product mixture, but also any high boilers formed, for example high-boiling by-products which may have been produced in the reaction in step c1). In order to remove these high boilers, the process according to the invention can have a purification step, i.e. the ester formed in step c1) is purified by separating the ester formed from the high boilers present by means of thermal separation, extraction, crystallization or membrane separation. Preferably, a thermal separation method is used, particularly preferably a further distillation, to purify the ester formed. The process conditions are known to the person skilled in the art.

[0067] The esters obtained according to the invention can be used as reactants for the preparation of further products. Important reactions of the esters obtained are hydrogenation to form alcohols, hydrolysis or saponification to form carboxylic acids or carboxylate salts, or aminolysis to form amides. DETAILED DESCRIPTION

[0068] Example:

[0069] MET pool (Zell) (intermittent operation mode):

[0070] The experiments were carried out in a commercially available Evonik MET type METcell dead-end intermittent filter cell with the commercially available membranes listed in Tables 1 to 3 and a PEEK-acetone membrane according to the invention. The PEEK-acetone membrane according to the invention was prepared similarly to the publication J. da Silva Burgal et al.; Journal of Membrane Science, Vol. 479 (2015), pp. 105-116 (see also WO 2015 / 110843 A1).

[0071] The experiment was conducted under the following conditions: 56.7cm 2 Active membrane area of ​​20 bar, transmembrane pressure of 20 bar, separation temperature of 25° C. (50° C. for PEEK), stirrer speed of 250 rpm.

[0072] To determine the retention rate, the marker 4,4'-di-tert-butyl-2,2'-bipyridyl was added to the study mixture at 0.01% by weight. The markers given were selected so that the structure and molar mass represent the ligands commonly used in methoxycarbonylation (e.g. EP 3121186 A2) and correspond to the catalytic amount at low concentrations. In addition, 0.5% by weight of aluminum trifluoromethanesulfonate (based on the total weight of the mixture) was added as an acid.

[0073] The METcell was filled with 200 mL of the above mixture and brought to operating temperature (= separation temperature). It was then pressurized with nitrogen up to an operating pressure of 20 bar. A total of 100 mL was taken out of the METcell as permeate, wherein the weight of the permeate was continuously recorded. At the end of the experiment, i.e. after 100 mL of permeate had been drawn off from the METcell, samples of the retentate and permeate were used for GC and HPLC analysis. The permeabilities given are the average values ​​of the 100 mL of permeate collected (results see Table 1).

[0074] The retention rate was determined by HPLC-UV on a C18 column (results are shown in Table 2), and the alcohol selectivity was determined by GC-FID (results are shown in Table 3). The methods are known to those skilled in the art.

[0075] Example 1

[0076] Different membrane materials were investigated with a mixture of 43% by weight of methanol and 57% by weight of methyl octanoate, corresponding to a molar ratio of 4:1. As already mentioned, a marker (0.01% by weight of 4,4'-di-tert-butyl-2,2'-bipyridine) and an acid (0.5% by weight of aluminum trifluoromethanesulfonate) were also present in the mixture.

[0077] Table 1: Comparison of permeability

[0078]

[0079] *No data points due to material damage,

[0080] **Data point does not exist.

[0081] Table 2: Comparison of membrane retention rates (for markers)

[0082]

[0083] *No data points due to material damage,

[0084] **Data point does not exist

[0085] ***Measured at 22 hours.

[0086] Table 3: Comparison of alcohol selectivity (here for methanol)

[0087]

[0088] *No data points due to material damage,

[0089] **Data point does not exist.

[0090] These tables show that the comparative examples of membrane DuraMem 300 and PuraMem S sometimes show an extreme increase in permeability over time. In the case of PuraMem S, it is found that the membrane material has lost its technical integrity over time according to the membrane rejection value. In contrast, DuraMem 300 shows a relatively constant rejection value, but shows an increase in unacceptable permeability after 300 hours, so there is only a small selectivity to methanol rejection. Membrane GMT oNF-2 does not last 300 hours, but is destroyed due to insufficient acid stability. Membrane Solsep NF 010206 S has shown an increase in unacceptable permeability after 70 hours and has not shown a special selectivity (~1) to methanol in addition. Membrane Solsep NF 030306F shows a value of >1 for the selectivity of methanol and therefore shows no selectivity to methanol. In addition, an unacceptable increase in permeability is found in the case of this membrane.

[0091] In contrast, the membrane PEEK-acetone according to the invention also exhibits a good permeability with only a slight increase after 300 hours, a good rejection and a good selectivity (<1) for methanol. Methanol is therefore also enriched in the retentate.

[0092] Example 2

[0093] PEEK membranes were studied with mixtures of different alcohols and esters. In addition, markers (0.01% by weight of 4,4'-di-tert-butyl-2,2'-bipyridine) and acids (0.5% by weight of aluminum trifluoromethanesulfonate) as already mentioned in Example 1 were present in the mixture. Similar to Example 1, esters and alcohols were used in such a molar ratio that there were always about 4 alcohol groups per ester group, i.e., the molar ratio was matched to the number of the respective functional groups. Studies were conducted with esters (methyl propionate, isopropyl propionate, methyl tetradecanoate, octyl butyrate, octyl octanoate, octyl isovalerate and benzyl propionate), diesters (dimethyl adipate) and alcohols (methanol, isopropanol, octanol, benzyl alcohol and 2-ethylhexanol). The esters were selected so that they were composed of olefins with different chain lengths and different numbers of olefinic double bonds. In addition, in terms of potential alcohols (C1-C8), the esters were changed. The experiments were conducted under the following conditions: 56.7 cm 2 Active membrane area of ​​40 bar, transmembrane pressure of 40 bar, separation temperature of 50°C, stirrer speed of 250 rpm.

[0094] Table 4: Comparison of alcohol selectivity using PEEK membranes

[0095]

[0096] **Data point does not exist.

[0097] It has been shown that good selectivities can be achieved for the alcohol used even when using different alcohols and different esters.

[0098] Table 5: Comparison of membrane retention rates (for markers) using PEEK membranes

[0099]

[0100] **Data point does not exist;

[0101] ***Data point does not exist yet.

[0102] It has been shown that, for all the alcohol and ester mixtures used, good results can also be achieved with regard to the membrane retention of the markers during this time.

[0103] Table 6: Comparison of permeability using PEEK membrane

[0104]

[0105] **Data point does not exist.

[0106] The permeability of some is very low, which is due to, for example, longer carbon atom chains or the size of the general molecule. Despite the low permeability, good retention (for the marker) and good selectivity (for methanol) can be achieved. In addition, as can be seen from the table, there is no significant increase in permeability during 300 hours.

[0107] Example 3: Triplicate Test (3er Teststand) (Continuous Operation Mode)

[0108] The experiment was carried out in a continuously operating experimental device in which the permeate is completely recirculated in a closed loop. The device consists essentially of a high-pressure circuit with a flow (überströmten) that can be pressurized up to 60 bar and three flat-channel membrane cells. The circuit is fed from a tank with a capacity of 5 liters, which is filled with a feed solution that is mechanically mixed and blanketed with nitrogen. The feed solution is brought to the operating pressure of the membrane circuit by means of a membrane piston pump and thus enters the high-pressure area of ​​the experimental device. The high-pressure area of ​​the experimental device consists essentially of a liquid circuit operated by a circulation pump and three flat membrane test cells and the required sensors (e.g. pressure measurement, measurement of the flow rate in the circuit). The liquid stream that permeates through the membrane is led away from the membrane module as permeate and conveyed to the tank again. According to the Coriolis measuring principle, the amount of permeate is continuously recorded for all three membranes. The circulation in the high-pressure circuit is achieved by a centrifugal pump to ensure the required flow through the membrane. The flow is usually selected so high that the concentrations on the feed side and the retentate side of the membrane are approximately equal. (This is the case if the throughput greatly exceeds the permeate stream to be discharged.) The feed supplied to the high-pressure circuit by the high-pressure pump is also selected so that the volume flow of the feed stream exceeds the volume flow of the permeate by a multiple. The excess feed amount (the delivery flow of the high-pressure pump minus the sum of the permeates of the three membranes) is also recirculated to the storage tank. This recirculation is carried out via a mechanical prepressure regulator, which is also used to set the prepressure of the nanofiltration stage. The circuit is heated by a thermostat, thereby ensuring the given temperature for the separation. The experiments were carried out under the following conditions: active membrane area per module 84.5 cm 2 , transmembrane pressure 20 bar, separation temperature 40℃.

[0109] Different membrane materials were studied with a mixture of 43% by weight of methanol and 57% by weight of methyl octanoate (molar ratio 4:1). To determine the retention rate, the marker 4,4'-di-tert-butyl-2,2'-bipyridyl was added to the study mixture at 0.01% by weight, as in the previous examples. In addition, after a given run time, 0.5% by weight of aluminum trifluoromethanesulfonate (based on the total weight of the mixture) was added. The time point for adding the acid is defined as time t=0h in Tables 7 to 9 below. At a given sampling time, liquid samples of the respective permeate streams, feed and retentate were extracted.

[0110] The permeability was determined as described above (results in Table 7). The retention of the marker was at C 18 The on-column selectivity was determined by HPLC with a UV detector (260 nm) (results in Table 8), and the alcohol selectivity was determined by GC with a FID detector (results in Table 9).

[0111] Table 7: Comparison of penetration rates

[0112]

[0113] *No data points due to material damage.

[0114] Table 8: Comparison of membrane retention rates

[0115]

[0116] *No data points due to material damage.

[0117] Table 9: Comparison of alcohol selectivity (here for methanol)

[0118]

[0119] *No data points due to material damage.

[0120] The membrane GMT oNF-2 did not last for 300 hours, but was destroyed due to insufficient acid stability. All Solsep membranes showed an unacceptable increase in permeability after 100 hours and in other cases did not show a particular selectivity for methanol, but rather showed values ​​of selectivity for methanol>1, which means that methanol preferentially passed through the membrane and was not retained. Therefore, the objects according to the invention could not be achieved with the Solsep membranes. In contrast, the PEEK membrane according to the invention also showed good permeability after 100 hours, good retention and good selectivity (<1) for methanol. Therefore, methanol was also enriched in the retentate.

[0121] Example 4

[0122] Diisobutylene (DiB) was converted into 3,5,5-trimethylhexanoic acid methyl ester and recycled as catalyst by membrane separation. DiB is a mixture of two C8 isomers, 2,4,4-trimethylpent-1-ene and 2,4,4-trimethylpent-2-ene, in a ratio of about 80:20. The experiment was carried out in a continuously operated experimental device constructed as follows.

[0123] The equipment consists essentially of a 200 mL glass autoclave (=reactor) from Büchi (pressurizable up to 10 bar). The autoclave is supplied by a glass container filled with the reaction solution and covered with argon with the aid of an HPLC pump from Knauer. Another HPLC pump from Knauer pumps from the glass autoclave into a separate high-pressure circuit that can be pressurized up to 60 bar. The high-pressure circuit of the experimental equipment consists essentially of a liquid circuit operated by a circulation pump and a flat membrane test cell as well as the required sensors (e.g. pressure measurement, temperature measurement). The high-pressure circuit is circulated by a centrifugal pump to ensure the required flow through the membrane. The liquid stream that permeates through the membrane is drawn off from the membrane module as permeate and collected in a glass container covered with argon. The amount of permeate is continuously detected by means of a balance. The excess feed amount (retentate) is recirculated into the glass autoclave. This recirculation is carried out by a mechanical pre-pressure regulator, which is also used to adjust the pre-pressure of the nanofiltration stage. The circuit is heated by a thermostat, thereby ensuring a given temperature for separation. The experiment was carried out under the following conditions: active membrane area 84.5 cm 2 , transmembrane pressure 45 bar, separation temperature 25°C.

[0124] Samples can be taken at the feed stream to the high-pressure loop (feed), from the high-pressure loop (retentate) and from the permeate. For the analysis of the yield, 0.08 g of sample and 0.03 g of ethylbenzene (internal standard) are weighed and diluted with 0.25 g of acetonitrile. Further analysis is carried out with the aid of GC-FID. After pre-decomposition of samples of the permeate and retentate, the retention of ligands and metals is determined with the aid of ICP-OES. The analysis of the retention of acids is carried out with the aid of 19 F-NMR was performed.

[0125] Sample preparation and analysis for all three methods are known to those skilled in the art.

[0126] PEEK was used as membrane material. The PEEK-acetone membrane according to the invention was produced analogously to the publication J. da SilvaBurgal et al.; Journal of Membrane Science, Vol. 479 (2015), pp. 105-116 (see also WO 2015 / 110843 A1).

[0127] The contents of the glass autoclave were stirred to ensure almost perfect mixing. The pressure in the glass autoclave was regulated to 10 bar via a mechanical pre-pressure regulator. The glass autoclave was kept at the desired temperature with the aid of an oil bath with an external temperature measurement installed in the glass autoclave.

[0128] First, a glass autoclave and a high-pressure loop were filled with a basic reaction mass consisting of 56 wt% MeOH, 40 wt% DiB, 0.5 wt% 1,2-bis((tert-butyl(pyridin-2-yl)phosphinyl)methyl)benzene (ligand), 3.4 wt% aluminum triflate (acid), and 0.1 wt% [Pd(acac) 2 ] (metal), inerted with 10 bar argon and put into operation.

[0129] Then, the glass autoclave is heated to an internal temperature of 100° C. and the initial argon gas stream is changed to CO (reaction start). Through 15 h, the basic reaction material is pumped in circulation to start the system. After 15 h, 99% of the DiB in the feed is converted into TMH-ester. After 15 h, the circulation operation mode is terminated, and the permeate is collected in a separate glass container and fresh reaction solution (60 wt % MeOH and 40 wt % DiB) is metered in. A new permeate collection container is used for the experimental period listed in Table 1. Through the research period of 105 h, fresh catalyst, ligand or acid are not added. For each research period, the yield of permeability, TMH-ester and the retention rate of the components for the catalyst system are measured with the above-mentioned method.

[0130] Table 1: Continuous conversion of DiB to TMH esters

[0131]

[0132] * Yield at the end of the respective observed experimental period,

[0133] **Retention rate not determined (system startup).

Claims

1. A process for preparing esters by carbonylation of C2- to C16-hydrocarbons, wherein the process comprises the following steps: a) carbonylating a C2- to C16-hydrocarbon having at least one olefinic double bond with carbon monoxide and with an alcohol or phenol in the presence of a homogeneous catalyst system in a reaction zone to obtain a liquid product mixture, the alcohol or phenol being a monohydric or polyhydric alcohol or phenol having 1 to 50 carbon atoms or a mixture of two or more thereof and being used in a molar ratio of alcohol or phenol:C2- to C16-hydrocarbon of 2 to 20 when it is a monohydric alcohol or phenol or in a molar ratio of hydrocarbon:polyol of 2 to 20 when it is a polyhydric alcohol, the 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, the liquid product mixture comprising at least the ester formed by the reaction, the homogeneous catalyst system, low boilers, high boilers and unreacted alcohol or phenol; b) carrying out membrane separation for separating the homogeneous catalyst system from the liquid product mixture, whereby the homogeneous catalyst system and also unreacted hydrocarbons and / or unreacted alcohols or phenols are enriched in the retentate, and the esters formed in step a) are enriched in the permeate, wherein an acid-stable membrane material having an OSN organic solvent nanofiltration capacity, i.e. a membrane material which is stable for at least 300 hours in the presence of an acid of the catalyst system and has at least one separation-active layer, is used as the membrane material, and the retentate is recycled into the reaction zone, wherein the separation-active layer consists of polyetheretherketone; c) working up the permeate by means of one or more separation steps selected from thermal separation, extraction, crystallization or membrane separation for separating off the esters formed in step a) and for separating off unreacted C2 to C16 hydrocarbons having at least one olefinic double bond and / or unreacted alcohol or phenol, and recycling the unreacted C2 to C16 hydrocarbons having at least one olefinic double bond and / or the unreacted alcohol or phenol into the reaction zone of step a).

2. The method according to claim 1, wherein after being used for at least 300 hours in the membrane separation method in step b) as described herein, the membrane material according to the present invention has the following two characteristics: The membrane rejection R for the ligand of the homogeneous catalyst system is at least 75% and deteriorates by no more than 30 percentage points; or Relative permeability P rel The value is 30%-300%.

3. The process according to any one of the preceding claims, wherein the alcohol or phenol used in step a) is a monohydric or polyhydric alcohol or phenol having 1 to 15 carbon atoms or a mixture of two or more thereof.

4. The process according to claim 3, wherein the alcohol used in step a) is selected from the group consisting of methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 2-propanol, tert-butanol, 3-pentanol, 2-propylheptanol, cyclohexanol and mixtures thereof.

5. The process according to claim 1, wherein the C2- to C16-hydrocarbon having at least one olefinic double bond used in step a) is a normal or isoolefin having 2 to 16 carbon atoms.

6. The process according to any one of claims 1 to 2, wherein the metal of Group 8 to 10 of the Periodic Table of the Elements or a compound thereof in the homogeneous catalyst system in step a) is palladium or a compound thereof.

7. The process according to any one of claims 1 to 2, wherein the phosphorus-containing ligand in the homogeneous catalyst system has a bidentate structure.

8. The process according to any one of claims 1 to 2, wherein the acid of the catalyst system in step a) is a Bronsted acid with a pKs≤5 or a Lewis acid with an LAU value greater than 25.

9. The process according to any one of claims 1 to 2, wherein the acid of the catalyst system in step a) is a Bronsted acid or a Lewis acid, wherein the Bronsted acid is perchloric acid, sulfuric acid, phosphoric acid, methylphosphonic acid or a sulfonic acid, and wherein the Lewis acid is aluminum trifluoromethanesulfonate, aluminum chloride, aluminum hydride, trimethylaluminum, tris(pentafluorophenyl)borane, boron trifluoride, boron trichloride or a mixture thereof.

10. The process according to any one of claims 1 to 2, wherein the reaction zone for the reaction in step a) has at least one reactor.

11. The method according to any one of claims 1 to 2, wherein the method after the separation in step c) has the following further steps: d) in a second reaction zone, transesterifying the ester formed in step a) with a second alcohol, wherein the second alcohol is different from the alcohol used in step a), to obtain a second product mixture, which comprises at least the ester with the second alcohol, the dissociated first alcohol or phenol and the unreacted second alcohol, wherein the second alcohol is used in an equimolar amount or in excess; e) separating the formed ester with the second alcohol from the remaining second product mixture and from the dissociated first alcohol or phenol by means of one or more separation steps selected from thermal separation, extraction, crystallization or membrane separation, and recycling the dissociated first alcohol or phenol to the first reaction zone of step a) and recycling the unreacted second alcohol to the second reaction zone.

12. The process according to claim 11, wherein the second alcohol used in step e) is an alcohol having a higher boiling point than the first alcohol or phenol.

13. The method according to claim 11, wherein: A portion of the dissociated first alcohol or phenol is already removed from the second reaction zone and is recycled to the first reaction zone.

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