Electrochemical method for preparing 1, 2, 3, 4-butanetetracarboxylic acid tetraalkyl ester

Abstract

The present invention relates to an electrochemical process for the preparation of tetraalkyl 1, 2, 3, 4-butane tetracarboxylic acid esters. The invention relates to an electrochemical method for producing tetraalkyl 1, 2, 3, 4-butanetetracarboxylic acid esters containing alkyl groups having 1 to 6 carbon atoms. The method comprises electrohydrodimerization of a dialkyl maleate containing an alkyl group having 1 to 6 carbon atoms in a reactant solution containing an alcohol and a conductive salt.

Description

Technical Field

[0001] The present invention relates to an electrochemical process for preparing tetraalkyl 1,2,3,4 - butanetetracarboxylates having alkyl groups of 1 to 6 carbon atoms. The process comprises electrohydrodimerization of dialkyl maleates having alkyl groups of 1 to 6 carbon atoms in a reactant solution containing an alcohol and a conductive salt. Background Art

[0002] Tetraalkyl 1,2,3,4 - butanetetracarboxylates are esters known in the chemical industry and have the following general structure

[0003]

[0004] wherein all four groups R each represent an alkyl group. For example, these esters can be used as plasticizers.

[0005] Tetraalkyl 1,2,3,4 - butanetetracarboxylates can in principle be prepared by chemical and electrochemical methods. The chemical route proceeds by synthesizing 1,2,3,4 - butanetetracarboxylic acid and subsequently esterifying with an alcohol to obtain the corresponding tetraalkyl 1,2,3,4 - butanetetracarboxylate. The electrochemical route proceeds by hydrodimerization of dialkyl maleates occurring at the cathode. Some such methods have been described in the patent literature, for example in EP 0816533 A2, WO 97 / 26389A1, WO 02 / 42249A1 or JPH05156478A1.

[0006] The known methods have the disadvantage that they either cannot be operated economically or cannot be operated sustainably on an industrial scale. Alternative routes for preparing the relevant tetraalkyl 1,2,3,4 - butanetetracarboxylates are also to be provided. Summary of the Invention

[0007] Accordingly, the object of the present invention is to provide an economic and sustainable method for preparing tetraalkyl 1,2,3,4 - butanetetracarboxylates. The object is achieved by the method described in claim 1. Preferred embodiments are set forth in the dependent claims.

[0008] A process for preparing tetraalkyl 1,2,3,4 - butanetetracarboxylate containing an alkyl group having 1 to 6 carbon atoms, preferably an alkyl group having 2 to 5 carbon atoms, and particularly preferably an alkyl group having 5 carbon atoms, according to the present invention is carried out by electrohydrodimerization with a reactant solution in at least one reaction zone comprising an anode and a cathode. The reactant solution contains a dialkyl maleate containing an alkyl group having 1 to 6 carbon atoms, preferably an alkyl group having 2 to 5 carbon atoms, at least one monohydric alcohol having 1 to 6 carbon atoms, preferably having 2 to 5 carbon atoms, and a conductive salt. Wherein the dialkyl maleate undergoes electrohydrodimerization at the cathode to obtain tetraalkyl 1,2,3,4 - butanetetracarboxylate, and the anode and cathode used are boron - doped diamond electrodes.

[0009] The advantage of using boron - doped diamond electrodes as the electrode material is that the cell voltage during the reaction process is lower than when using known electrode materials such as graphite or glassy carbon. The electrical energy consumption of electrolysis is proportional to the cell voltage. Therefore, by using boron - doped diamond electrodes, the process according to the present invention can be carried out in a more energy - efficient manner.

[0010] The dialkyl maleate containing an alkyl group having 1 to 6 carbon atoms, preferably an alkyl group having 2 to 5 carbon atoms, and particularly preferably an alkyl group having 5 carbon atoms used in the electrohydrodimerization reacts according to a known mechanism. The dialkyl maleate used can be obtained on an industrial scale.

[0011] In a preferred embodiment of the present invention, a dialkyl maleate containing an alkyl group having 5 carbon atoms each, i.e., dialkyl pentyl maleate, is used. In the process according to the present invention, tetraalkyl 1,2,3,4 - butanetetracarboxylate containing an alkyl group having 5 carbon atoms each, i.e., tetra - pentyl 1,2,3,4 - butanetetracarboxylate, is thus formed.

[0012] In this context, the term "pentyl" should be understood to mean that the esters of the present invention can contain different pentyl isomers: n - pentyl (1 -, 2 - or 3 - pentyl), 2 - methylbutyl, 3 - methylbutyl, 2 - methylbut - 2 - yl, 3 - methylbut - 2 - yl, 2,2 - dimethylpropyl, especially 2 - methylbutyl and / or 3 - methylbutyl and / or n - pentyl. Therefore, the term "pentyl" does not refer to a single specific C5 alkyl group in principle. Thus, the tetra - pentyl 1,2,3,4 - butanetetracarboxylate preferably prepared according to the present invention can contain only 2 - methylbutyl, only 3 - methylbutyl, only n - pentyl, or a mixture of 2 - methylbutyl and / or n - pentyl and / or 3 - methylbutyl.

[0013] In addition to the dialkyl maleate, the reactant solution further comprises at least one monohydric alcohol having 1 to 6 carbon atoms, preferably 1 to 5 carbon atoms. Preferred alcohols are methanol and pentanol. Pentanol may be a mixture of different isomeric pentanols, for example, a mixture of 2-methylbutanol and / or 3-methylbutanol and / or 1-pentanol. In the electrohydrodimerization according to the present invention, the alcohol is used as a solvent because the dialkyl maleate and the conductive salt can be dissolved in the alcohol used. The alcohol can also be used simultaneously as a reactant for the anodic reaction. In principle, a mixture of two or more monohydric alcohols having different carbon chain lengths can also be used. Thereby, a mixed ester containing different alkyl groups can be formed. However, it is preferred to use only one alcohol. However, as already described, the one alcohol may also represent a mixture of different isomers having the same number of carbon atoms.

[0014] In principle, it is also conceivable that the dialkyl maleate used and the alcohol used contain different alkyl groups having different numbers of carbon atoms. However, according to the present invention, it is preferred that the number of carbon atoms of the alkyl group of the dialkyl maleate is the same as the number of carbon atoms of the monohydric alcohol. Thus, if tetraamyl 1,2,3,4-butanetetracarboxylate is to be prepared from diamyl maleate, the solvent used is pentanol.

[0015] An influencing factor in the electrohydrodimerization according to the present invention is the concentration of the dialkyl maleate based on the amount of the alcohol. A higher concentration of the dialkyl maleate can have a positive effect on the yield, selectivity and current efficiency of the tetraalkyl 1,2,3,4-butanetetracarboxylate to be formed. In a preferred embodiment of the present invention, the concentration of the dialkyl maleate is 0.5 to 4 moles per liter of monohydric alcohol, preferably 1 to 3 moles per liter of monohydric alcohol.

[0016] The reactant solution used in the electrohydrodimerization according to the present invention further comprises a conductive salt. The conductive salt ensures that the solution has sufficient conductivity during the electrochemical reaction. In principle, various conductive salts suitable for the reactant solution and the reaction can be used. Corresponding conductive salts are known to those skilled in the art in principle.

[0017] Conductive salts useful for electrohydrodimerization particularly include those having a tetraalkylammonium cation, an alkali metal cation and an anion selected from aromatic substituted sulfonate, alkyl sulfonate, acetate, perchlorate, tetrafluoroborate, tetraphenylborate, bromide, iodide, phosphate, phosphonate, sulfate, alkyl sulfate, hexafluorophosphate. Examples of suitable conductive salts include tetrabutylammonium p-toluenesulfonate and sodium acetate.

[0018] The concentration of the conductive salt can also be an influencing factor in the electrohydrodimerization according to the present invention. In particular, a low concentration of the conductive salt itself is economically advantageous, but it increases the cell voltage and thus has an adverse effect. In the context of the present invention, the concentration of the conductive salt is preferably 0.05 to 0.4 mol / L of a monohydric alcohol, more preferably 0.1 to 0.4 mol / L of a monohydric alcohol.

[0019] The reactant solution must contain at least dialkyl maleate, a monohydric alcohol, and a conductive salt. In a preferred embodiment of the present invention, the reactant solution in the electrohydrodimerization according to the present invention further contains a cosolvent. The use of a cosolvent results in a decrease in the cell voltage. In the electrohydrodimerization according to the present invention, the suitable cosolvent does not react at the cathode. The cosolvent is preferably selected from acetonitrile, dimethyl sulfoxide, tetrahydrofuran, dioxane, propylene carbonate, N,N-dimethylformamide, organic carbonates (such as dimethyl carbonate), dichloromethane, chloroform, and acetone.

[0020] The electrohydrodimerization according to the present invention is carried out in a suitable reaction zone, which may include one or more reactors. Since an electrochemical reaction takes place here, it is well known that the reactor must include an anode and a cathode. In the context of the present invention, the reactor may also be referred to as an electrolytic cell.

[0021] The desired reaction for the formation of the target product 1,2,3,4-butanetetracarboxylic acid tetraalkyl ester from dialkyl maleate occurs at the cathode. Naturally, oxidation takes place simultaneously at the anode. For example, the monohydric alcohol can be oxidized at the anode to give an aldehyde. Thus, if methanol is used as the monohydric alcohol, formaldehyde will be formed. Butyraldehyde is formed when butanol is used, and valeraldehyde is formed when pentanol is used as the monohydric alcohol. In particular, valeraldehyde is an important raw material for synthesis in the chemical industry and thus represents a valuable product. Therefore, when pentanol is used, two different valuable products are produced, namely the co-production of valeraldehyde and 1,2,3,4-butanetetracarboxylic acid tetra-n-pentyl ester. When using dialkyl maleates with different alkyl groups, such as di(2-methylbutyl / n-pentyl) maleate in pentanol, 2-methylbutanol is also released to a low extent by pentanol substitution and then 2-methylbutyraldehyde is obtained by anodic oxidation.

[0022] The anode and the cathode can be composed of known materials, such as metals or carbon-based materials. In the context of the present invention, the anode and the cathode employed are boron-doped diamond electrodes.

[0023] When using boron-doped diamond electrodes, the distance between the anode and the cathode can preferably be adjusted to at least 1 mm (using a suitable plastic frame as a spacer).

[0024] The arrangement of the anode and cathode relative to each other in the one or more electrolytic cells is in principle not limited to a specific arrangement. It is obvious that the arrangement is chosen such that the lowest possible cell voltage is generated. In a preferred embodiment of the invention, the anode and cathode are arranged parallel to each other in a plane in the electrolytic cell.

[0025] The electrohydrodimerization according to the invention can in principle be configured as a batch process or a continuous process. In the case of a batch process, it is well known that the reactant solution is filled into the one or more reactors or the one or more electrolytic cells, the reaction takes place, and then the product mixture is removed. If the electrohydrodimerization is operated in continuous mode, fresh reactant solution must be metered continuously into the reactor or electrolytic cell and the resulting product solution removed therefrom.

[0026] In both the case of a batch process and continuous mode, there is a continuous flow through the reactor or through the electrolytic cell. This is used in particular for the mass transfer towards or away from the electrodes. In this context, the flow rate is the volume flowing through the reactor or electrolytic cell per unit time.

[0027] In principle, it must be assumed that a higher flow rate improves the mass transfer towards and away from the electrodes. In the context of the present invention, the flow rate is preferably 50 to 700 l / h per 100 cm 2 of electrode surface area, preferably 100 to 500 l / h per 100 cm 2 of electrode surface area.

[0028] As high a temperature as possible is favorable for a low cell voltage, but leads to increased requirements on the materials and, in the case of short-chain alcohols, a significant increase in the vapor pressure. Therefore, the temperature to be set is a compromise between these two preconditions. Thus, in the context of the present invention, it is preferred to carry out the electrohydrodimerization at a temperature of 20 °C to 80 °C, preferably 25 °C to 65 °C. It is further preferred to carry out the electrohydrodimerization at a pressure of 0.5 to 3 bar, preferably 0.75 to 2 bar.

[0029] The electrochemical parameters are also important influencing factors in the electrohydrodimerization according to the invention. The current density in the electrohydrodimerization is preferably 1 to 25 mA / cm 2 , preferably 2 to 15 mA / cm 2 , particularly preferably 4 to 10 mA / cm 2Further preferably, only the stoichiometric amount of electric charge is provided for the electrochemical reaction. In this case, it is shown as the electric charge per mole of dialkyl maleate. In the context of the present invention, the electric charge is preferably 1 to 1.5 F / mol of dialkyl maleate, preferably 1 to 1.1 F / mol of dialkyl maleate. Very particularly preferably, the reaction only requires the stoichiometric amount of electric charge, i.e., the electric charge is 1 F / mol of dialkyl maleate.

[0030] In order to obtain a tetraalkyl 1,2,3,4-butane tetracarboxylate in which the alkyl group has 2 or more carbon atoms, preferably 5 carbon atoms, the respective tetraalkyl 1,2,3,4-butane tetracarboxylate can initially be prepared from dimethyl maleate or diethyl maleate by the method of the present invention, and subsequently the ester is transesterified with a suitable alcohol.

[0031] Therefore, the subject matter of the present invention is also a method in which tetramethyl 1,2,3,4-butane tetracarboxylate or tetraethyl 1,2,3,4-butane tetracarboxylate is prepared by electrohydrodimerization, and then tetramethyl 1,2,3,4-butane tetracarboxylate or tetraethyl 1,2,3,4-butane tetracarboxylate is transesterified with at least one monohydric alcohol having 3 to 6 carbon atoms, preferably 5 carbon atoms, to obtain a tetraalkyl 1,2,3,4-butane tetracarboxylate containing an alkyl group having 3 to 6 carbon atoms, preferably 5 carbon atoms.

[0032] Transesterification itself is a method known to those skilled in the art, in which, according to the present teachings, long-chain alcohols displace methanol (when dimethyl maleate is used) or ethanol (when diethyl maleate is used) from the ester. The transesterification reaction with a monohydric alcohol having 2 to 6 carbon atoms, preferably 5 carbon atoms, is preferably carried out in the presence of one or more catalysts, for example, where Bronsted or Lewis acids or bases are used as catalysts. Particularly suitable catalysts that have been found are sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, metals or their compounds. Examples of particularly preferred metal catalysts are tin powder, tin(II) oxide, tin(II) oxalate, titanates, such as tetra-isopropyl titanate or tetra-butyl titanate, and zirconates, such as tetra-butyl zirconate, as well as sodium methoxide and potassium methoxide.

[0033] The transesterification can be carried out in a typical reactor known to those skilled in the art under conventional process conditions. The method is preferably carried out at a temperature equal to or higher than the boiling point of the alcohol formed in the reaction so that the alcohol can be distilled out of the reaction mixture. The transesterification is preferably carried out at a temperature of 100°C to 300°C, preferably 120°C to 270°C, especially 140°C to 250°C. The internal pressure is preferably 0.1 to 20 or 15 bar, especially 0.1 to 10 bar. Detailed Description

[0034] The present invention will be described below with reference to embodiments. The specific exemplary embodiments are intended to illustrate, but not limit, the subject matter of the present invention.

[0035] Embodiment

[0036] Experimental apparatus:

[0037] For electrohydrodimerization, an electrolytic cell with a planar parallel arrangement of an anode plate and a cathode plate is used, where the area of each of the anode plate and the cathode plate is 100 cm 2 . The materials used for the anode and the cathode are glassy carbon / glassy carbon, graphite / graphite, or boron-doped diamond / boron-doped diamond. When using glassy carbon and boron-doped diamond, a PTFE spacer frame with a thickness of 1 mm is used, and when using graphite, a PTFE spacer frame with a thickness of 2 mm is used (short circuit occurs at 1 mm and electro-synthesis is not possible). The electrodes are connected to a power supply and an additional voltmeter.

[0038] The effluent from the upper region of the cell is fed into a glass intermediate storage vessel with a temperature-adjustable outer jacket, and the outer jacket is in turn connected to a thermostat. The outlet of the intermediate storage vessel is connected to the suction side of a peripheral gear pump. The inflow fed from the pressure side of the pump into the lower region of the cell.

[0039] Embodiment 1 (not according to the present invention):

[0040] In this experiment, glassy carbon was used as the electrode material. 74 g of bis(2-methylbutyl / n-pentyl) maleate (289 mmol) and 24 g (58 mmol) of tetrabutylammonium p-toluenesulfonate were dissolved in 150 ml of pentanol and subsequently charged into the intermediate storage vessel of the experimental apparatus after sampling. After starting the pump circulation at 280 l / h and adjusting the temperature-controlled jacket to 50 °C, a current of 600 mA (current density 6 mA / cm 2 ) was applied. The experimental duration was 13 hours. During this period, a charge quantity of 0.29 F was supplied (corresponding to 1.01 F / mol of bis(2-methylbutyl / n-pentyl) maleate). The cell voltage was 7.2 V at the start of the experiment and rose to 7.5 V at the end of the experiment. After the experiment, 212 g of the product electrolyte solution was obtained, which was analyzed by gas chromatography. The solution contained 38.4 g of tetra(2-methylbutyl / n-pentyl) 1,2,3,4-butanetetracarboxylate (yield 52%, current efficiency 51%), 13.3 g of bis(2-methylbutyl / n-pentyl) succinate (yield 18%), and 20.7 g of bis(2-methylbutyl / n-pentyl) maleate (corresponding to a conversion of 72%).

[0041] Embodiment 2 (not according to the present invention):

[0042] Graphite was used as the electrode material in this experiment. 74 g of bis(2-methylbutyl / n-pentyl) maleate (289 mmol) and 24 g (58 mmol) of tetrabutylammonium p-toluenesulfonate were dissolved in 150 mL of pentanol and subsequently charged into the intermediate storage container of the experimental apparatus after sampling. After starting the pump circulation at 280 l / h and adjusting the temperature-controlled jacket to 50 °C, a current of 600 mA (current density 6 mA / cm 2 ) was applied. The experimental duration was 13 hours. During this period, a charge quantity of 0.29 F was supplied (corresponding to 1.01 F / mol of bis(2-methylbutyl / n-pentyl) maleate). The battery voltage was 9.6 V at the start of the experiment and dropped to 9.2 V at the end. After the experiment, 211 g of the product electrolyte solution was obtained and analyzed by gas chromatography. The solution contained 27.5 g of tetra(2-methylbutyl / n-pentyl) 1,2,3,4-butanetetracarboxylate (yield 37%, current efficiency 37%), 14.3 g of bis(2-methylbutyl / n-pentyl) succinate (yield 19%), and 31.9 g of bis(2-methylbutyl / n-pentyl) maleate (corresponding to a conversion of 57%). The solution also contained n-valeraldehyde (valeraldehyde) and 2-methylbutyraldehyde, which are anode products, as opposed to the above-mentioned cathode products.

[0043] Example 3 (not according to the present invention):

[0044] Graphite was used as the electrode material in this experiment. 74 g of bis(2-methylbutyl / n-pentyl) maleate (289 mmol) and 24 g (58 mmol) of tetrabutylammonium p-toluenesulfonate were dissolved in 150 mL of pentanol and 27 mL of acetonitrile and subsequently charged into the intermediate storage container of the experimental apparatus after sampling. After starting the pump circulation at 280 l / h and adjusting the temperature-controlled jacket to 50 °C, a current of 600 mA (current density 6 mA / cm 2 ) was applied. The experimental duration was 13 hours. During this period, a charge quantity of 0.29 F was supplied (corresponding to 1.01 F / mol of bis(2-methylbutyl / n-pentyl) maleate). The battery voltage was 5.8 V at the start of the experiment and rose to 6.6 V at the end. After the experiment, 224 g of the product electrolyte solution was obtained and analyzed by gas chromatography. The solution contained 34.1 g of tetra(2-methylbutyl / n-pentyl) 1,2,3,4-butanetetracarboxylate (yield 46%, current efficiency 46%), 12.6 g of bis(2-methylbutyl / n-pentyl) succinate (yield 17%), and 17.3 g of bis(2-methylbutyl / n-pentyl) maleate (corresponding to a conversion of 77%).

[0045] Example 4 (according to the present invention):

[0046] In this experiment, boron-doped diamond was used as the electrode material. 74 g of bis(2-methylbutyl / n-pentyl) maleate (289 mmol) and 24 g (58 mmol) of tetrabutylammonium p-toluenesulfonate were dissolved in 150 ml of pentanol and subsequently loaded into the intermediate storage container of the experimental setup after the sample was taken out. After starting the pump circulation at 280 l / h and adjusting the temperature-controlled jacket to 50 °C, a current of 600 mA (current density 6 mA / cm 2 ) was applied. The duration of the experiment was 13 hours. During this period, a charge quantity of 0.29 F was supplied (corresponding to 1.01 F / mol of bis(2-methylbutyl / n-pentyl) maleate). The cell voltage was 5.7 V at the start of the experiment and rose to 6.2 V at the end of the experiment. After the experiment, 210 g of the product electrolyte solution was obtained and analyzed by gas chromatography. The solution contained 35.6 g of tetra(2-methylbutyl / n-pentyl) 1,2,3,4-butanetetracarboxylate (yield 48%, current efficiency 48%), 14.9 g of bis(2-methylbutyl / n-pentyl) succinate (yield 20%), and 20.7 g of bis(2-methylbutyl / n-pentyl) maleate (corresponding to a conversion of 72%).

[0047] The results of Examples 1 to 4 are compared with each other in Table 1 below.

[0048] Table 1: Summary of the results of Examples 1 to 4

[0049]

[0050]

[0051] * = tetra(2-methylbutyl / n-pentyl) 1,2,3,4-butanetetracarboxylate

[0052] ** = according to the present invention

[0053] As can be seen from the summary in Table 1, the lowest average cell voltage was achieved when using the boron-doped diamond electrode. Therefore, the electrical energy consumption for electrohydrodimerization was the lowest compared to all other experiments.

Claims

1. A process for preparing tetraalkyl 1,2,3,4 - butanetetracarboxylate containing an alkyl group having 1 to 6 carbon atoms by electrohydrodimerization with a reactant solution in at least one reaction zone comprising an anode and a cathode, said reactant solution comprising a dialkyl maleate containing an alkyl group having 1 to 6 carbon atoms, at least one monohydric alcohol having 1 to 6 carbon atoms, and a conductive salt, wherein the dialkyl maleate undergoes electrohydrodimerization at the cathode to obtain tetraalkyl 1,2,3,4 - butanetetracarboxylate, and the anode and cathode employed are boron - doped diamond electrodes.

2. The process according to claim 1, wherein the number of carbon atoms of the alkyl group of the dialkyl maleate is the same as the number of carbon atoms of the monohydric alcohol.

3. The process according to claim 1 or 2, wherein a conductive salt having a tetraalkylammonium cation, an alkali metal cation and an anion selected from aromatic - substituted sulfonate, alkyl sulfonate, acetate, perchlorate, tetrafluoroborate, tetraphenylborate, bromide, iodide, phosphate, phosphonate, sulfate, alkyl sulfate, hexafluorophosphate is used.

4. The process according to any one of the preceding claims, wherein the tetraalkyl 1,2,3,4 - butanetetracarboxylate and the dialkyl maleate each contain an alkyl group having 5 carbon atoms, preferably each contains an alkyl group selected from 2 - methylbutyl, 3 - methylbutyl and n - pentyl.

5. The process according to any one of the preceding claims, wherein the monohydric alcohol used is methanol, butanol or pentanol, preferably 1 - pentanol.

6. The process according to any one of the preceding claims, wherein a cosolvent selected from the following is additionally used in the electrohydrodimerization: acetonitrile, dimethyl sulfoxide, tetrahydrofuran, dioxane, propylene carbonate, N,N - dimethylformamide, organic carbonates (such as dimethyl carbonate), dichloromethane, chloroform and acetone.

7. The process according to any one of the preceding claims, wherein the electrohydrodimerization is carried out at a temperature of 20 °C to 80 °C, preferably 25 °C to 65 °C.

8. The process according to any one of the preceding claims, wherein the electrohydrodimerization is carried out at a pressure of 0.5 to 3 bar, preferably 0.75 to 2 bar.

9. The method according to any one of the preceding claims, wherein the electrohydrodimerization is carried out in an electrolytic cell, and the flow rate in the electrolytic cell is 50 to 700 l / h per 100 cm 2 of electrode surface area, preferably 100 to 500 l / h per 100 cm 2 of electrode surface area.

10. The process according to any one of the preceding claims, wherein during the process, the monohydric alcohol having 1 to 6 carbon atoms undergoes anodic reaction to obtain an aldehyde.

11. The process according to claim 10, wherein when pentanol is used as the monohydric alcohol, pentanal and also 2 - methylbutanal are formed.

12. The process according to any one of the preceding claims, wherein tetra - methyl 1,2,3,4 - butanetetracarboxylate is prepared in the electrohydrodimerization, and then the tetra - methyl 1,2,3,4 - butanetetracarboxylate is transesterified with at least one monohydric alcohol having 2 to 6 carbon atoms to obtain tetraalkyl 1,2,3,4 - butanetetracarboxylate containing an alkyl group having 2 to 6 carbon atoms.

13. The method according to any one of claims 1 to 12, wherein tetraethyl 1,2,3,4-butanetetracarboxylate is prepared in the electrohydrodimerization, and then the tetraethyl 1,2,3,4-butanetetracarboxylate is transesterified with at least one monohydric alcohol having 3 to 6 carbon atoms to obtain a tetraalkyl 1,2,3,4-butanetetracarboxylate containing an alkyl group having 3 to 6 carbon atoms.

14. The method according to claim 12 or 13, wherein the transesterification is carried out at a temperature of 100 °C to 300 °C, preferably 120 °C to 270 °C.

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