Sustainable process for producing 1,1,4,4-tetramethoxy-2-butene

Abstract

The present invention relates to a process for producing 1,1,4,4-tetramethoxy-2-butene from 2,5-dimethoxy-2,5-dihydrofuran using methanol in the presence of an acidic ion exchanger as a catalyst, wherein the molar ratio of 2,5-dimethoxy-2,5-dihydrofuran to methanol is 1:45 or greater. The present invention also relates to a process for separating 1,1,4,4-tetramethoxy-2-butene, methanol, and 2,5-dimethoxy-2,5-dihydrofuran from each other and from HO, wherein a basic compound, preferably a basic metal salt or a basic ion exchanger, is present. Both processes have high selectivity and space-time yield, and produce less waste compared to known processes, making them suitable for industrial scale operation in batch or continuous mode and sustainable. Further objects of the present invention are the use of a basic compound in the distillation of a mixture comprising HO, 1,1,4,4-tetramethoxy-2-butene and 2,5-dimethoxy-2,5-dihydrofuran, and the use of an acidic ion exchanger as a catalyst in the reaction of 2,5-dimethoxy-2,5-dihydrofuran with methanol to form 1,1,4,4-tetramethoxy-2-butene, wherein the molar ratio of 2,5-dimethoxy-2,5-dihydrofuran to methanol is 1:45 or greater.

Description

Detailed Description of the Invention

[0001] [Summary of the Invention] The present invention relates to a process for producing 1,1,4,4-tetramethoxy-2-butene from 2,5-dimethoxy-2,5-dihydrofuran using methanol in the presence of an acidic ion exchanger as a catalyst, wherein the molar ratio of 2,5-dimethoxy-2,5-dihydrofuran to methanol is 1:45 or more. The present invention further relates to a process for separating 1,1,4,4-tetramethoxy-2-butene, methanol and 2,5-dimethoxy-2,5-dihydrofuran from each other and from H2O, in which a basic compound, preferably a basic metal salt or a basic ion exchanger, is present.

[0002] Both processes can be carried out on an industrial scale in batch units or in continuous mode and are sustainable because they have high selectivity and space-time yield and produce less waste compared to known processes.

[0003] A further object of the present invention is the use of a basic compound in the distillation of a mixture comprising H2O, 1,1,4,4-tetramethoxy-2-butene and 2,5-dimethoxy-2,5-dihydrofuran and the use of an acidic ion exchanger as a catalyst in the reaction of 2,5-dimethoxy-2,5-dihydrofuran with methanol to 1,1,4,4-tetramethoxy-2-butene, wherein the molar ratio of 2,5-dimethoxy-2,5-dihydrofuran to methanol is 1:45 or more.

[0004] [Background of the Invention] 1,1,4,4-Tetramethoxy-2-butene (hereinafter referred to as "C4-diacetal") is an important intermediate in the synthesis of, for example, 2,7-dimethyl-2,4,6-octatriene-1,8-dialdehyde (so-called "C 10 -dialdehyde"), which is necessary for the chemical synthesis of carotenoids.

[0005] As shown in Figure 1, C4-diacetal can be produced by reacting 2,5-dimethoxy-dihydrofuran (hereinafter referred to as "DMDF") with methanol in the presence of a catalyst. The undesirable by-products are PMB (1,1,2,4,4-pentamethoxybutane) or DMB (4,4-dimethoxy-but-2-enal) (see also Figure 1).

[0006] In the process according to EP-A-1099676, in this reaction, a solid catalyst having an acid center is used. Unfortunately, PMB is produced in an amount of at least 1.5% based on the amount of starting material DMDF (see Example 5).

[0007] In the processes described in WO 2006 / 108664 and EP 581097, it is necessary to use trialkyl orthoformate as the acetalizing agent, which makes these processes more expensive and thus uneconomical and not sustainable.

[0008] As already mentioned above, all these known processes have certain disadvantages. Therefore, an object of the present invention is to provide a process without such disadvantages.

[0009] Furthermore, in order to reduce the environmental impact on waste, there is an increasing demand to improve the efficiency and sustainability of industrial chemical processes.

[0010] [Detailed Description of the Invention] These needs are met by the present invention, which is a process ("Process 1") for producing 1,1,4,4-tetramethoxy-2-butene (=C4-diacetal), comprising the following steps: a) Optionally providing 2,5-dimethoxy-2,5-dihydrofuran containing 1,1,4,4-tetramethoxy-2-butene; b) In the presence of an acidic ion exchanger as a catalyst, optionally in the presence of water, until up to 79% of the 2,5 - dimethoxy - 2,5 - dihydrofuran reacts to give 1,1,4,4 - tetramethoxy - 2 - butene, reacting the 2,5 - dimethoxy - 2,5 - dihydrofuran provided in step a), optionally in the presence of 1,1,4,4 - tetramethoxy - 2 - butene, with methanol to obtain a mixture containing H2O and 1,1,4,4 - tetramethoxy - 2 - butene, wherein the molar ratio of 2,5 - dimethoxy - 2,5 - dihydrofuran to methanol is 1:45 or more, c) Optionally, adding a basic compound to the mixture containing H2O, methanol, 1,1,4,4 - tetramethoxy - 2 - butene and 2,5 - dimethoxy - 2,5 - dihydrofuran, d) Distilling the said mixture containing H2O, methanol, 1,1,4,4 - tetramethoxy - 2 - butene, 2,5 - dimethoxy - 2,5 - dihydrofuran in the presence of the basic compound to separate these compounds from each other, a process in which 1,1,4,4 - tetramethoxy - 2 - butene, methanol and unreacted 2,5 - dimethoxy - 2,5 - dihydrofuran are obtained, comprising, wherein the basic compound is either added to the mixture in step c) or is present during the distillation in step d), a process ("Process 1").

Brief Description of the Drawings

[0011]

Figure 1

[0012] In a preferred embodiment of Process 1, the unreacted 2,5 - dimethoxy - 2,5 - dihydrofuran obtained in step d) is recycled back to step a) or b) respectively, thus increasing the sustainability of the process.

[0013] A single step is disclosed in more detail below.

[0014] [Step a)] Preferably, the amount of C4-diacetal is less than 1 mol%, more preferably less than 0.5 mol%, and most preferably less than 0.1 mol% with respect to the amount of DMDF.

[0015] [Step b)] Preferably, the reaction is carried out in continuous mode in a reactor, as known to those skilled in the art. Particularly preferred are fixed-bed or tube reactors or tube bundle reactors.

[0016] The reaction is preferably carried out at a temperature in the range of 0 to 50 °C, more preferably in the range of 10 to 30 °C, most preferably in the range of 15 to 25 °C and / or preferably at atmospheric pressure.

[0017] [Catalyst] In a preferred embodiment, the catalyst used in step b) is an acidic ion exchanger having a concentration of acidic sites of at least 2.5 eq / kg, preferably at least 3.0 eq / kg, more preferably at least 4.0 eq / kg, and most preferably 5.0 eq / kg. When using an acidic ion exchanger having a concentration of acidic sites lower than 2.5 eq / kg, the selectivity and conversion are still high, but the reaction time becomes longer (see Example 1F compared to Examples 1A - 1E).

[0018] In a more preferred embodiment, the catalyst is an acidic ion exchanger containing sulfonic acid groups and having a concentration of acidic sites of at least 2.5 eq / kg, preferably at least 3.0 eq / kg, more preferably at least 4.0 eq / kg, and most preferably 5.0 eq / kg.

[0019] Surprisingly, it has been found that the use of an acidic ion exchanger having the above-mentioned preferred tendencies with a specific particle size distribution and a specific water retention capacity results in an even higher selectivity for the product, i.e., C4-diacetal.

[0020] Such a preferred catalyst is an acidic ion exchanger having a particle size distribution of 400 μm or more and a water retention capacity of less than 60%, preferably in the range of 40 to 60%, more preferably in the range of 50 to 60%, or an acidic ion exchanger having a particle size distribution of less than 400 μm and a water retention capacity of more than 60%, preferably in the range of 60 to 80%, more preferably in the range of 60 to 75%.

[0021] The acidic ion exchanger showed high activity over a long period. Nevertheless, when these activities are decreased, the acidic ion exchanger can be reactivated by washing with a polar organic solvent, preferably by washing with methanol, or by washing with an aprotic organic solvent, or by washing with an aprotic inorganic solvent such as an acid.

[0022] Those skilled in the art will select the amount of the catalyst, i.e., the bed volume, according to the amount of the reaction mixture.

[0023] Any acidic ion exchanger having one or more of the above preferred characteristics can be used without problems in the process of the present invention.

[0024] The most preferred acidic ion exchanger has all of the above preferred characteristics.

[0025] [Amount of methanol] In step b), the molar ratio of 2,5-dimethoxy-2,5-dihydrofuran to methanol is preferably in the range of 1:45 to 1:100, more preferably in the range of 1:50 to 1:90, even more preferably in the range of 1:60 to 1:80, and most preferably in the range of 1:70 to 1:80.

[0026] [Conversion rate] Preferably, the reaction is carried out until up to 75% of the DMDF has reacted to obtain C4-diacetal, more preferably until 20 - 72% of the DMDF has reacted to obtain C4-diacetal, even more preferably until 35 - 70% / 40 - 65% / 45 - 60% of the DMDF has reacted respectively to obtain C4-diacetal, and most preferably until 50 - 60% of the DMDF has reacted to obtain C4-diacetal.

[0027] [Absence of acetalizing agents other than methanol] In a preferred embodiment of the present process, there is no acetalizing agent other than methanol. The use of trialkyl orthoformate is particularly excluded because methyl formate is formed thereby increasing the waste generated in the process, in particular the use of trialkyl orthoformate used in the processes of European Patent No. 581097 and International Publication No. WO 2006 / 108664 pamphlet. In the process of the present invention, since there is no acetalizing agent other than methanol, the amount of waste is minimized.

[0028] Since H2O is formed during the reaction promoting the back reaction to the starting materials and the reaction to by-product DMB, the amount of H2O is preferably below a certain level in order to still guarantee a sufficient conversion rate. The amount of water when stopping the reaction in step b) is preferably in the range of 0 - 100 mol%, more preferably in the range of 20 - 80 mol%, and most preferably in the range of 30 - 65 mol% with respect to the amount of 2,5-dimethoxy-2,5-dihydrofuran.

[0029] The combination of steps c) and d) is also an invention. Accordingly, the present invention relates to a process ( "Process 2") for separating 1,1,4,4-tetramethoxy-2-butene (= C4-diacetal), methanol and 2,5-dimethoxy-2,5-dihydrofuran (= DMDF) from a mixture containing H2O, methanol, 1,1,4,4-tetramethoxy-2-butene and 2,5-dimethoxy-2,5-dihydrofuran, comprising the following steps: i) Providing a mixture comprising H2O, methanol, a C4-diacetal, DMDF and optionally a basic compound; ii) Optionally, adding a basic compound to the mixture; iii) Distilling the mixture comprising H2O, methanol, a C4-diacetal and DMDF in the presence of the basic compound to separate methanol, the C4-diacetal and DMDF from one another which process (''Process 2'') also relates to, wherein the basic compound is either already present in the mixture provided in step i) or the basic compound is added in step ii).

[0030] [Processes c and d) / Processes i and ii) respectively] The starting mixture preferably contains less than 70 mol% DMDF, more preferably in the range of 20 - 70 mol%, even more preferably in the range of 30 - 60 mol%, most preferably in the range of 40 - 50 mol% DMDF, based on the total amount of C4-diacetal and DMDF.

[0031] The amount of C4-diacetal in the starting mixture is preferably more than 30 mol%, more preferably in the range of 30 - 80 mol%, even more preferably in the range of 40 - 70 mol%, most preferably in the range of 50 - 60 mol%, based on the total amount of C4-diacetal and DMDF.

[0032] The amount of water in the starting mixture is preferably more than 30 mol%, more preferably in the range of 30 - 80 mol%, even more preferably in the range of 40 - 70 mol%, most preferably in the range of 50 - 60 mol%, based on the total amount of C4-diacetal, H2O and DMDF.

[0033] Preferably, the basic compound is a heterogeneous basic compound, more preferably a basic metal salt or a basic ion exchanger or a mixture thereof, and most preferably the basic compound is a basic ion exchanger. Advantageously, the basic compound can be recycled.

[0034] Generally, any basic salt soluble in methanol is suitable. Such salts are, for example, alkalis, alkaline earths and ammonium salts soluble in methanol. Preferred examples of such salts are potassium carbonate, sodium carbonate and diammonium hydrogen phosphate, with potassium carbonate and sodium carbonate being preferred. Mixtures of these salts, especially mixtures of these preferred salts, can also be used.

[0035] The basic ion exchanger includes a strong anion exchange resin containing a hydroxy group or the corresponding salt.

[0036] The amount of the basic compound is preferably in the range of 0.01 to 1% by weight, more preferably in the range of 0.02 to 0.75% by weight, still more preferably in the range of 0.03 to 0.5% by weight, and most preferably in the range of 0.03 to 0.3% by weight, based on the total weight of the reaction mixture containing methanol, H2O, C4-diacetal and DMDF.

[0037] Larger amounts of the basic compound can be used, but the process sustainability decreases due to increased waste.

[0038] The presence of the basic compound prevents losses of the desired product (=C4-diacetal) and unreacted starting material (=DMDF) during distillation of the mixture. The presence of the basic compound can reduce the losses of both compounds of C4-diacetal and DMDF to an amount less than 7 mol%, preferably less than 5 mol%.

[0039] [Step iii] Step iii) is preferably carried out at a temperature in the range of 20 to 180 °C, more preferably in the range of 30 to 150 °C, most preferably in the range of 40 to 110 °C and / or preferably at a pressure in the range of 0.1 to 1013 mbar, more preferably in the range of 0.5 to 500 mbar, most preferably in the range of 1 to 100 mbar.

[0040] The present invention includes any combination of one or more of the above preferred embodiments of Processes 1 and 2.

[0041] A further embodiment of the present invention is the use of the basic compound as defined above in the distillation of a mixture comprising H2O, 1,1,4,4 - tetramethoxy - 2 - butene and 2,5 - dimethoxy - 2,5 - dihydrofuran, and the use of an acidic ion exchanger as a catalyst in the reaction of 2,5 - dimethoxy - 2,5 - dihydrofuran with methanol to 1,1,4,4 - tetramethoxy - 2 - butene, wherein the molar ratio of 2,5 - dimethoxy - 2,5 - dihydrofuran to methanol is 1:45 or more.

[0042] Here, the present invention is further illustrated by the following non - limiting examples.

[0043] [Examples] The following acidic ion exchangers are tested.

[0044] [Acidic Ion Exchanger] Amberlyst 15WET (commercially available from Dupont Water Solutions - DDP Specialty Products Germany GmbH&Co.KG, Neu - Isenburg, Germany): macroporous, particle size distribution: 400 - 1200 μm, water retention capacity: 52 - 57%, concentration of acidic sites: 5 equivalents per kg of ion exchanger Amberlyst 46 (commercially available from Dupont Water Solutions - DDP Specialty Products Germany GmbH&Co.KG, Neu - Isenburg, Germany): macroporous, particle size distribution: 400 - 1200 μm, water retention capacity: 26 - 36%, concentration of acidic sites: 1 equivalent per kg of ion exchanger Dowex 88H (commercially available from Dupont Water Solutions - DDP Specialty Products Germany GmbH&Co.KG, Neu-Isenburg, Germany): macroporous, particle size distribution: 300 - 1200 μm, water retention capacity: 42 - 48%; concentration of acidic sites: 5 equivalents per kg of ion exchanger, Dowex 50WX4 50 - 100 mesh (commercially available from Dupont Water Solutions - DDP Specialty Products Germany GmbH&Co.KG, Neu-Isenburg, Germany): gel, particle size distribution: 200 - 400 μm, water retention capacity: 64 - 72%; concentration of acidic sites: 5 equivalents per kg of ion exchanger, Dowex 50WX4 16 - 25 mesh (commercially available from Dupont Water Solutions - DDP Specialty Products Germany GmbH&Co.KG, Neu-Isenburg, Germany): gel, particle size distribution: 400 - 1200 μm, water retention capacity: 64 - 72%; concentration of acidic sites: 5 equivalents per kg of ion exchanger, Dowex 50WX4 200 - 400 mesh (commercially available from Dupont Water Solutions - DDP Specialty Products Germany GmbH&Co.KG, Neu-Isenburg, Germany): gel, particle size distribution: 37 - 74 μm, water retention capacity: 64 - 72%; concentration of acidic sites: 5 equivalents per kg of ion exchanger.

[0045] [Basic ion exchanger] Amberlyst A26 (commercially available from Dupont Water Solutions - DDP Specialty Products Germany GmbH&Co.KG, Neu-Isenburg, Germany): particle size distribution: 560 - 700 μm, water retention capacity: 66 - 75%, surface area: 30 m 2 / g (nitrogen BET method), pore diameter: 290 Å.

[0046] [Examples 1A - H: Acid - Catalyzed Conversion of DMDF to C4 - Diacetal] The reactor is filled with 50 mm of glass wool and then an acidic ion exchanger (= catalyst) is added according to Tables 1 - 3. Then, methanol is pumped through the catalyst bed until the solvent becomes colorless. A solution of DMDF (408 mmol) in methanol (1250 ml, 31.2 mol) is prepared and pumped through the catalyst bed at various temperatures (11 - 21 °C) and flow rates (1 - 24 mL / min). Samples are taken after flowing through three bed volumes under constant conditions. The starting solution and the samples are analyzed by gas chromatography.

[0047] [Table 1]

[0048] [Table 2]

[0049] [Table 3]

[0050] [Examples 2A - E: Acid - Catalyzed Conversion of DMDF to C4 - Diacetal] The reactor is filled with 50 mm of glass wool and the acidic ion exchanger Amberlyst 15WET (= catalyst) is added. Then, methanol is pumped through the catalyst bed until the solvent becomes colorless. A solution of DMDF (408 mmol) in methanol (1250 ml, 31.2 mol) is prepared and pumped through the catalyst bed at 21 °C and a flow rate of 4 mL / min. Samples are taken after a certain period. The starting solution and the samples are analyzed by gas chromatography. As shown in Table 4 below, the catalyst showed a constant high selectivity even after more than 70 hours.

[0051] [Table 4]

[0052] [Examples 3A - E: Acid - Catalyzed Conversion of a Mixture of DMDF and C4 - Diacetal to C4 - Diacetal] The reactor is filled with 50 mm of glass wool and an acidic ion exchanger Amberlyst 15WET (= catalyst) is added. Then, methanol is pumped into the catalyst bed until the solvent becomes colorless. A solution of DMDF (408 mmol) and various amounts of C4 - diacetal (0 - 142.8 mmol) in methanol (1250 ml, 31.2 mol) is prepared and pumped through the catalyst bed of Amberlyst 15WET at various flow rates (2 - 14 mL / min) at 20 °C. Samples are taken after a certain period. The starting solution and the samples are analyzed by gas chromatography. The results are shown in Table 5.

[0053] [Table 5]

[0054] The selectivity remains high in the presence of C4 - diacetal.

[0055] [Examples 4A - B: Acid - Catalyzed Conversion of a Mixture of DMDF, C4 - Diacetal and H2O to C4 - Diacetal] The reactor is filled with 50 mm of glass wool and an acidic ion exchanger Amberlyst 15WET (= catalyst) is added. Then, methanol is pumped into the catalyst bed until the solvent becomes colorless. A solution of DMDF (408 mmol), C4 - diacetal (142.8 mmol) and H2O (0 or 1224 mmol) in methanol (1250 ml, 31.2 mol) is prepared and pumped through the catalyst layer of Amberlyst 15WET at a residence time of 18 minutes at 22 °C. Samples are taken after a certain period. The starting solution and the samples are analyzed by gas chromatography. The results are shown in Table 6.

[0056] [Table 6]

[0057] When water is present in an amount three times that of DMDF, the conversion of DMDF to C4-diacetal decreases.

[0058] [Examples 5A - D: Stability Tests of DMDF and C4-Diacetal in the Presence of Water during Distillation] A 2000 mL round-bottom flask was filled with DMDF (245 mmol), C4-diacetal (245 mmol), and H2O (0 or 1000 mmol) in methanol (1000 ml, 25 mol), and further basic compounds were added according to Table 7. The reaction mixture was evaporated to a certain amount at 70 °C / 700 mbar. The evaporated solution and the starting solution were analyzed by gas chromatography. The results are shown in Table 7.

[0059] [Table 7]

[0060] 5A - 5D are the starting solutions.

[0061] "ev" means that the corresponding sample was evaporated until the amounts shown in Table 7 remained.

[0062] Sample 5A (887 g), for example, originally contained 249 mmol of C4-diacetal and 245 mmol of DMDF, and the rest was methanol. When the solvent was evaporated (= sample "5A(ev)"), 245 mmol of C4-diacetal (98.4% of the original amount) and 242 mmol of DMDF (98.8% of the original amount) could be recovered.

[0063] However, when water was present in the starting solution and no basic compound was present (see Examples 5B and 5B(ev) respectively), only 150 mmol of C4-diacetal (60.3% of the original amount) and 215 mmol of DMDF (87.8% of the original amount) could be recovered, so that the target product (= C4-diacetal) and the unreacted starting material (= DMDF) were significantly lost.

[0064] This loss could be prevented by the presence of a basic compound - see Examples 5C / 5C(ev) and 5D / 5D(ev) respectively.

[0065] When 0.5 g of K2CO3 was present in the starting solution 5C, 244 millimoles of C4-diacetal (99.2% of the original amount) and 245 millimoles of DMDF (100.4% of the original amount * ) could be recovered after evaporation.

[0066] When 2.0 g of the basic ion exchanger Amberlyst A26 was present in the starting solution 5D, 237 millimoles of C4-diacetal (96.7% of the original amount) and 244 millimoles of DMDF (100% of the original amount) could be recovered after evaporation.

[0067] *In the presence of a basic compound, the by-product DMB reacts to give DMDF, and more DMDF is recovered than originally present.

Claims

1. A process for producing 1,1,4,4 - tetramethoxy - 2 - butene, comprising the following steps: a) Optionally providing 2,5 - dimethoxy - 2,5 - dihydrofuran containing 1,1,4,4 - tetramethoxy - 2 - butene; b) Reacting the 2,5 - dimethoxy - 2,5 - dihydrofuran provided in step a), optionally in the presence of 1,1,4,4 - tetramethoxy - 2 - butene which is optionally present, with methanol in the presence of an acidic ion exchanger as a catalyst and optionally in the presence of water, until 35 - 70% of the 2,5 - dimethoxy - 2,5 - dihydrofuran has reacted to obtain 1,1,4,4 - tetramethoxy - 2 - butene, to obtain a mixture containing H ２ 2O and 1,1,4,4 - tetramethoxy - 2 - butene, wherein the molar ratio of 2,5 - dimethoxy - 2,5 - dihydrofuran to methanol ranges from 1:45 to 1:100; c) Optionally adding a basic compound to the mixture containing H ２ 2O, methanol, 1,1,4,4 - tetramethoxy - 2 - butene and 2,5 - dimethoxy - 2,5 - dihydrofuran; d) Distilling the mixture containing H ２ 2O, methanol, 1,1,4,4 - tetramethoxy - 2 - butene and 2,5 - dimethoxy - 2,5 - dihydrofuran in the presence of the basic compound to separate the compounds from each other, to obtain 1,1,4,4 - tetramethoxy - 2 - butene, methanol and unreacted 2,5 - dimethoxy - 2,5 - dihydrofuran; The basic compound is either added to the mixture in step c) or is present during the distillation in step d), and the unreacted 2,5 - dimethoxy - 2,5 - dihydrofuran obtained in step d) is recycled back to step a) or b) respectively.

2. The catalyst used in step b) is an acidic ion exchanger having a concentration of acidic sites of at least 2.5 equivalents / kg, and / or the catalyst used in step b) is an acidic ion exchanger containing a sulfonic acid group and having a concentration of acidic sites of at least 2.5 equivalents / kg, the method according to claim 1.

3. The acidic ion exchanger has a particle size distribution of 400 μm or more and a water retention capacity of less than 60%, the method according to claim 1 or 2.

4. The acidic ion exchanger has a particle size distribution of less than 400 μm and a water retention capacity of more than 60%, the method according to claim 1 or 2.

5. The reaction in step b) is carried out at a temperature in the range of 0 to 50 °C, the method according to any one of claims 1 to 4.

6. The reaction is carried out until 40 to 65% of 2,5-dimethoxy-2,5-dihydrofuran has reacted to give 1,1,4,4-tetramethoxy-2-butene (step b)), the method according to any one of claims 1 to 5.

7. In step b), the molar ratio of 2,5-dimethoxy-2,5-dihydrofuran to methanol is in the range of 1:50 to 1:90, the method according to any one of claims 1 to 6.

8. There is no acetalizing agent other than methanol, the method according to any one of claims 1 to 7.

9. The amount of water when stopping the reaction in step b) is in the range of 0 to 100 mol% with respect to the amount of 2,5-dimethoxy-2,5-dihydrofuran, the method according to any one of claims 1 to 8.

Images