Method for isomerizing vitamin A acetate

Abstract

The invention provides a method for isomerizing vitamin A acetate, which comprises the following steps: extracting and separating a vitamin A acetate isomerization reaction solution into an extract phase containing the vitamin A acetate and a raffinate phase containing a homogeneous catalyst, and recycling the raffinate phase instead of a fresh homogeneous catalyst to the isomerization reaction of the vitamin A acetate, the total content of the vitamin A acetate cis-isomer and the total trans-isomer in the raffinate phase is controlled to be 0.1-2wt%. According to the method, the solute content in the catalyst phase is controlled, so that the catalyst is prevented from being separated out, the catalyst phase is recycled, and the use cost of the catalyst is reduced.

Description

Technical Field

[0001] The invention belongs to the field of chemical intermediate synthesis, and particularly relates to a method for isomerizing vitamin A acetate. Background Art

[0002] Vitamin A acetate is an important feed additive. It can be used in the feed industry to increase animal growth rate and immunity. Studies have shown that different vitamin A isomers have different bioavailability, among which the all-trans isomer has the highest bioavailability. Therefore, the current industrial production process of vitamin A has an isomerization reaction step to convert the cis isomer into the all-trans isomer.

[0003] The mainstream process for isomerization reaction in the industry is generally a homogeneous metal-catalyzed reaction, that is, the catalyst is dissolved in the reaction system for catalytic reaction, and then the catalyst is removed by extraction, adsorption, crystallization, etc. after the reaction. However, these methods of removing the catalyst will make the catalyst unable to be reused, and the unit consumption and production cost of the catalyst will be greatly increased.

[0004] Therefore, developing a stable, efficient, and catalyst-applicable vitamin A acetate isomerization method is of great significance for reducing the cost of vitamin A production. Summary of the invention

[0005] Based on this, the inventors of this application have studied in detail the reasons why the catalyst cannot be reused. Among them, the adsorption method uses activated carbon, resin and other methods to chelate the catalyst. The activated carbon and resin after chelation cannot dissociate the catalyst again, so the catalyst cannot be recycled and applied; the crystallization method removes the catalyst and causes the catalyst to remain in the vitamin A product. Most of the catalyst remains in the crystallization mother liquor. If you want to reuse the catalyst, you need to add a separation process. This process causes product pollution and adds a new process flow, which greatly increases the complexity of the process. Compared with the first two methods, the extraction method separates the catalyst and vitamin A by the principle that vitamin A and the catalyst have different solubility in two solvents. The separation process is simple and the catalyst removal rate is high. This is a more friendly way for industrial production. However, in order to improve the recovery rate of vitamin A in the existing process, the solvent containing the catalyst is greatly reduced relative to the catalyst solubility, resulting in the precipitation of the catalyst, which cannot be applied and can only be used once. On the basis of discovering this core problem, the inventors of this application successfully realized the application of the catalyst solution by regulating the components in the solution containing the catalyst to increase the solubility of the catalyst in the solution and avoid precipitation, thereby completing the present invention.

[0006] The object of the present invention is to provide a stable and efficient vitamin A acetate isomerization method in which the catalyst can be applied.

[0007] To achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

[0008] A method for isomerizing vitamin A acetate comprises the steps of extracting and separating a vitamin A acetate isomerization reaction liquid into an extraction phase containing vitamin A acetate and a raffinate phase containing a homogeneous catalyst, and recycling the raffinate phase to the vitamin A acetate isomerization reaction instead of a fresh homogeneous catalyst, wherein the total content of cis-isomers and all-trans-isomers of vitamin A acetate in the raffinate phase is controlled to be 0.1-2wt%.

[0009] In a preferred embodiment, a method for isomerization of vitamin A acetate comprises the following steps:

[0010] 1) a raw material containing a cis isomer of vitamin A acetate, an all-trans isomer and a retinoid impurity is dissolved in a solvent A, and an isomerization reaction is carried out in the presence of a fresh homogeneous catalyst, whereby the cis isomer is converted into the all-trans isomer to obtain an isomerization reaction liquid;

[0011] 2) The isomerization reaction liquid is extracted with solvent B, wherein the cis-isomer and all-trans-isomer of vitamin A acetate are transferred to the solvent B phase, which is called the extraction phase, and the homogeneous catalyst remains in the solvent A phase, which is called the raffinate phase;

[0012] 3) controlling the total content of cis-isomer and all-trans-isomer of vitamin A acetate in the raffinate phase to 0.1-2%, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, etc., preferably 0.5-1%, and recycling the raffinate phase to the isomerization reaction instead of fresh homogeneous catalyst.

[0013] In view of the difficulty that the catalyst cannot be applied in the homogeneous catalytic isomerization process of vitamin A acetate in the industry, the inventors of this application have studied in detail the relationship between the catalyst, vitamin A acetate, reaction solvent, and extraction solvent, and found that the solvent dissolved with trace amounts of vitamin A acetate is conducive to improving the solubility of the catalyst in the solvent, so that the catalyst phase can remain homogeneous, and can be well reused for isomerization reaction, effectively solving the difficulty that the homogeneous isomerization catalyst cannot be applied. Further research found that the solvent dissolved with the catalyst is conducive to the stability of vitamin A, so that the trace amount of vitamin A acetate introduced with the catalyst reuse during the application process will not be further lost, but the higher content of vitamin A acetate was found to be deteriorated, so it is necessary to control the content of vitamin A acetate when the catalyst is reused to maintain the overall recovery rate of vitamin A acetate.

[0014] In the present invention, there is no particular limitation on the method for controlling the content of vitamin A acetate in the catalyst phase, for example, it can be achieved by adjusting the extraction conditions (temperature, feed amount, extraction stage, etc.) in combination with the extraction process. Of course, the content of vitamin A acetate can also be controlled by combining simple extraction with other processes, such as adsorption, etc., which should also be within the scope of protection of the present invention.

[0015] In the present invention, the cis isomers of vitamin A acetate in the raw material include 11-cis isomer, 9-cis isomer and 13-cis isomer.

[0016] In the present invention, the raw material contains 2-40% 11-cis isomer, 0.2-20% 9-cis isomer, 0.1-10% 13-cis isomer, 3-30% retinoid impurity and the rest are all-trans isomers in terms of mass percentage.

[0017] In the present invention, the solvent A is selected from one or more of acetonitrile, propionitrile, butyronitrile, N,N-dimethylformamide, and dimethyl sulfoxide, preferably acetonitrile.

[0018] In the present invention, the mass ratio of the solvent A to the raw material is (0.5-50):1, for example 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 12:1, 15:1, 18:1, 20:1, 23:1, 25:1, 28:1, 30:1, 35:1, 40:1, 45:1, 50:1, etc., preferably (4-10):1.

[0019] In the present invention, the homogeneous catalyst is selected from one or more of tetrakis(triphenylphosphine)palladium, palladium chloride, palladium acetate, palladium nitrate, palladium nitrate hydrate, di(acetylacetonate)palladium, bis(acetonitrile)palladium chloride, bis(triphenylphosphine)palladium dichloride, ruthenium trichloride, ruthenium acetate, tris(triphenylphosphine)carbonyldihydroruthenium, triphenylphosphine ruthenium chloride, triphenylphosphine rhodium chloride, rhodium acetate, rhodium octanoate, dicarbonyl acetylacetonate rhodium, iridium trichloride, iridium tetrachloride and iridium acetate, preferably palladium chloride.

[0020] In the present invention, the mass ratio of the homogeneous catalyst to the raw material is (0.00001-0.0005):1, for example, 0.00002:1, 0.00003:1, 0.00004:1, 0.00005:1, 0.00006:1, 0.00007:1, 0.00008:1, 0.00009:1, 0.0001:1, 0.00015:1, 0.0002:1, 0.00025:1, 0.0003:1, 0.00035:1, 0.0004:1, 0.00045:1, 0.0005:1, etc., preferably (0.00005-0.0001):1.

[0021] In the present invention, the isomerization reaction temperature is 30-90°C, for example, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, etc., preferably 50-60°C; the reaction time is 1-10h, for example, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, etc., preferably 3-5h.

[0022] In the present invention, the solvent B is selected from one or more of n-hexane, cyclohexane, n-pentane, cyclopentane, n-heptane and n-octane, preferably n-hexane.

[0023] In the present invention, extraction is a continuous process, and the feed mass ratio of the isomerized reaction liquid and the solvent B is (0.3-3):1, for example, 0.4:1, 0.5:1, 0.8:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, etc., preferably (0.5-1):1.

[0024] In the present invention, the extraction temperature is 10-40°C, for example, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, etc., preferably 15-20°C.

[0025] In the present invention, the number of extraction stages is 3-8, such as 3, 4, 5, 6, 7, 8, etc., preferably 4-6. The number of extraction stages is an industry term, and for intermittent extraction, it corresponds to the number of extractions, and for continuous extraction, it corresponds to the number of extractions corresponding to the number of theoretical plates.

[0026] In the present invention, the raffinate phase can be directly reused as a catalyst solution.

[0027] Compared with the prior art, the present invention has the following positive effects:

[0028] The present invention, through in-depth research on the relationship between the catalyst, vitamin A, reaction solvent and extraction solvent, invents a stable and efficient vitamin A acetate homogeneous catalytic isomerization method with applicable catalysts. The catalyst can be applied ≥10 times, which is a very meaningful industrial application method. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0030] Liquid chromatography analysis: Agilent 1260 liquid chromatograph, chromatographic column Sphersorb C18 column The UV-visible spectrometer was Hitachi L7420, the chromatography workstation data processing system was Chomatopac C-RIA, and the stationary phase was Zorbax-SIL.

[0031] Chromatographic conditions: mobile phase is methanol / isopropanol = 8 / 1 (v / v) mixture, detection temperature is 45°C, flow rate is 0.8 mL / min, wavelength is 328 nm. Qualitative and quantitative analysis of product composition is performed.

[0032] Specifications and sources of some reagents in the examples and comparative examples

[0033]

[0034]

[0035] Example 1

[0036] Step 1: Fresh Catalyst Reaction

[0037] 600g VA raw material (11-cis isomer 40.03%, 9-cis isomer 0.51%, 13-cis isomer 10.02%, all-trans isomer 46.34%, retinoid impurity 3.10%) and 2400g acetonitrile were added to a 10L reactor, heated to 60°C under stirring, 30mg palladium chloride was added as a catalyst, and the reaction was carried out at 60°C for 3 hours. After the reaction was completed, the temperature was lowered to 15°C and a sample was taken for liquid phase analysis. After deducting the solvent, the VA product composition was 0.31% of 11-cis isomer, 0.61% of 9-cis isomer, 2.23% of 13-cis isomer, 93.75% of all-trans isomer, and 3.10% of retinoid impurity. The above process was repeated for 2 batches to collect materials.

[0038] Step 2: Catalyst application

[0039] A 4-stage rotary disc extraction tower was built, the reaction liquid was fed from the bottom of the extraction tower at a feed rate of 20 g / min, the extraction solvent n-hexane was fed from the top of the extraction tower at a feed rate of 6 g / min, the extraction phase (n-hexane phase) was collected from the top of the tower, and the raffinate phase (acetonitrile phase) was collected in the bottom of the tower. The total content of cis-isomer and all-trans-isomer of vitamin A acetate in the acetonitrile phase was 0.1%, 2400 g of the raffinate phase was added to a 10 L reactor, and then 600 g of VA raw material (11-cis isomer 40.03%, 9-cis isomer The reaction solution was heated to 60°C for 3 hours. After the reaction was completed, the temperature was lowered to 15°C and samples were taken for liquid phase analysis. After deducting the solvent, the composition of the VA product was 0.31% for 11-cis isomer, 0.60% for 9-cis isomer, 2.23% for 13-cis isomer, 93.76% for all-trans isomer, and 3.10% for retinoid impurities. The reaction results were basically consistent with those of the fresh catalyst.

[0040] Repeat step 2 and continue to apply the catalyst for different times. After the reaction is completed, the product composition is as follows:

[0041]

[0042]

[0043] The application experiment shows that the raffinate phase obtained in this process was used as a catalyst 10 times, and the reaction results were basically consistent with those of the fresh catalyst.

[0044] Example 2

[0045] Step 1: Fresh Catalyst Reaction

[0046] 600gVA raw material (25.15% of 11-cis isomer, 7.76% of 9-cis isomer, 0.10% of 13-cis isomer, 59.49% of all-trans isomer, 7.50% of retinoid impurities) and 6000g of butyronitrile were added to a 10L reactor, heated to 50°C under stirring, 60mg of ruthenium trichloride was added as a catalyst, and the reaction was reacted at 50°C for 5 hours. After the reaction was completed, the temperature was lowered to 10°C and samples were taken for liquid phase analysis. After deducting the solvent, the VA product composition was 0.25% of 11-cis isomer, 3.55% of 9-cis isomer, 1.76% of 13-cis isomer, 86.94% of all-trans isomer, and 7.50% of retinoid impurities. The above process was repeated for 2 batches to collect materials.

[0047] Step 2: Catalyst application

[0048] A rotary disc extraction tower with 8 stages was built, and the reaction liquid was fed from the bottom of the extraction tower at a feed rate of 20 g / min. The extraction solvent n-heptane was fed from the top of the extraction tower at a feed rate of 20 g / min. The extraction phase (n-heptane phase) was collected from the top of the tower, and the raffinate phase (nitrile phase) was collected in the bottom of the tower. The total content of cis-isomer and all-trans-isomer of vitamin A acetate in the nitrile phase was 1.02%. 6000 g of the raffinate phase was added to a 10L reactor, and VA raw materials (11-cis isomer 25.15%, 9-cis isomer 25.25%, 11-cis isomer 25.30%, 11-cis isomer 25.60%, 11-cis isomer 25.70%, 11-cis isomer 25.9 ... The reaction solution was heated to 50°C for 5 hours. After the reaction was completed, the temperature was lowered to 10°C and samples were taken for liquid phase analysis. After deducting the solvent, the composition of the VA product was 0.24% for 11-cis isomer, 3.55% for 9-cis isomer, 1.77% for 13-cis isomer, 86.94% for all-trans isomer and 7.50% for retinoid impurities. The reaction results were basically consistent with those of the fresh catalyst.

[0049] Example 3

[0050] Step 1: Fresh Catalyst Reaction

[0051] 600gVA raw material (11-cis isomer 33.66%, 9-cis isomer 13.62%, 13-cis isomer 0.96%, all-trans isomer 40.86%, retinoid impurity 10.90%) and 300g acetonitrile were added to a 5L reactor, heated to 50°C under stirring, 6mg palladium chloride was added as a catalyst, and the reaction was reacted at 90°C for 1 hour. After the reaction was completed, the temperature was lowered to 40°C and samples were taken for liquid phase analysis. After deducting the solvent, the VA product composition was 0.89% of 11-cis isomer, 4.45% of 9-cis isomer, 3.39% of 13-cis isomer, 80.37% of all-trans isomer, and 10.90% of retinoid impurity. Repeat the above process for 2 batches to collect materials.

[0052] Step 2: Catalyst application

[0053] A rotary disc extraction tower with 6 stages was built, and the reaction liquid was fed from the bottom of the extraction tower at a feed rate of 20 g / min, and the extraction solvent n-hexane was fed from the top of the extraction tower at a feed rate of 10 g / min. The extract phase (n-hexane phase) was collected from the top of the tower, and the raffinate phase (acetonitrile phase) was collected in the bottom of the tower. The total content of cis-isomer and all-trans-isomer of vitamin A acetate in the acetonitrile phase was 2.00%. 300 g of the raffinate phase was added to a 5L reactor, and then VA raw materials (11-cis isomer 33.66%, 9-cis isomer 13 .62%, 13-cis isomer 0.96%, all-trans isomer 40.86%, retinoid impurities 10.90%), the reaction solution was heated to 90°C for 1 hour, and after the reaction was completed, the temperature was lowered to 10°C and samples were taken for liquid phase analysis. After deducting the solvent, the composition of the VA product was 0.87% of 11-cis isomer, 4.43% of 9-cis isomer, 3.32% of 13-cis isomer, 80.48% of all-trans isomer, and 10.90% of retinoid impurities. The reaction results were basically consistent with those of the fresh catalyst.

[0054] Example 4

[0055] Step 1: Fresh Catalyst Reaction

[0056] 600g VA raw material (2.03% 11-cis isomer, 20.04% 9-cis isomer, 2.90% 13-cis isomer, 45.02% all-trans isomer, 30.01% retinoid impurity) and 30000g acetonitrile were added to a 100L reactor, heated to 30°C under stirring, 300mg palladium chloride was added as a catalyst, and the reaction was carried out at 30°C for 10 hours. After the reaction was completed, the temperature was lowered to 20°C and samples were taken for liquid phase analysis. After deducting the solvent, the VA product composition was 0.12% 11-cis isomer, 5.12% 9-cis isomer, 1.89% 13-cis isomer, 62.86% all-trans isomer, and 30.01% retinoid impurity. The above process was repeated for 2 batches to collect materials.

[0057] Step 2: Catalyst application

[0058] A three-stage rotary disc extraction tower was built, and the reaction liquid was fed from the bottom of the extraction tower at a feed rate of 20 g / min. The extraction solvent n-hexane was fed from the top of the extraction tower at a feed rate of 60 g / min. The extract phase (n-hexane phase) was collected from the top of the tower, and the raffinate phase (acetonitrile phase) was collected in the bottom of the tower. The total content of cis-isomer and all-trans-isomer of vitamin A acetate in the acetonitrile phase was 0.50%. 30,000 g of the raffinate phase was added to a 100 L reactor, and then 600 g of VA raw material (2.03% of 11-cis isomer and 2.03% of 9-cis isomer) was added. The reaction solution was heated to 30°C for 10 hours. After the reaction was completed, the temperature was lowered to 20°C and samples were taken for liquid phase analysis. After deducting the solvent, the composition of the VA product was 0.12% for 11-cis isomer, 5.14% for 9-cis isomer, 1.88% for 13-cis isomer, 62.85% for all-trans isomer and 30.01% for retinoid impurities. The reaction results were basically consistent with those of the fresh catalyst.

[0059] Comparative Example 1

[0060] The isomerized reaction liquid obtained in Example 1 was selected as a raw material, and a rotary disc extraction tower with 9 stages was built. The reaction liquid was fed from the bottom of the extraction tower at a feed rate of 20 g / min, and the extraction solvent n-hexane was fed from the top of the extraction tower at a feed rate of 70 g / min. The extract phase (n-hexane phase) was collected from the top of the tower, and the raffinate phase (acetonitrile phase) was collected from the bottom of the tower. Catalyst precipitation was found at the interface between the two phases of the extraction tower. The total content of cis-isomer and all-trans-isomer of vitamin A acetate in the acetonitrile phase was 0.07%. 2400 g of the raffinate phase was added to a 10L reactor, and 600 g of V A raw material (11-cis isomer 40.03%, 9-cis isomer 0.51%, 13-cis isomer 10.02%, all-trans isomer 46.34%, retinoid impurities 3.10%), the reaction solution was heated to 60°C for 3 hours, and after the reaction was completed, the temperature was lowered to 15°C and samples were taken for liquid phase analysis. After deducting the solvent, the composition of the VA product was 15.33% of the 11-cis isomer, 0.99% of the 9-cis isomer, 8.87% of the 13-cis isomer, 69.62% of the all-trans isomer, and 5.19% of the retinoid impurities.

[0061] Compared with Example 1, the extraction conditions were changed so that the vitamin A content in the catalyst phase was lower than the specified conditions. The catalyst precipitated during the extraction process, and the conversion rate of the cis isomer was greatly reduced when the catalyst phase was used in a stacked manner, and the content of vitamin A-like impurities increased.

[0062] Comparative Example 2

[0063] The isomerized reaction liquid obtained in Example 1 was selected as a raw material, and a rotary disc extraction tower with a stage number of 2 was built. The reaction liquid was fed from the bottom of the extraction tower at a feed rate of 20 g / min, and the extraction solvent n-hexane was fed from the top of the extraction tower at a feed rate of 3 g / min. The extract phase (n-hexane phase) was collected from the top of the tower, and the raffinate phase (acetonitrile phase) was collected from the bottom of the tower. The total content of cis-isomer and all-trans-isomer of vitamin A acetate in the acetonitrile phase was 3.15%. 2400 g of the raffinate phase was added to a 10L reactor, and then 600 g of VA raw material (11- The reaction solution was heated to 60°C for 3 hours, and after the reaction was completed, the temperature was lowered to 15°C and samples were taken for liquid phase analysis. After deducting the solvent, the composition of the VA product was 0.28% for 11-cis isomer, 0.55% for 9-cis isomer, 2.25% for 13-cis isomer, 87.95% for all-trans isomer, and 8.97% for retinoid impurities.

[0064] Compared with Example 1, the extraction conditions were changed so that the vitamin A content in the catalyst phase was higher than the specified range, and the content of vitamin A impurities in the product increased significantly when the catalyst phases were used in combination.

[0065] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention.

Claims

1. A method for isomerizing vitamin A acetate, characterized in that: The method comprises the steps of extracting and separating the vitamin A acetate isomerization reaction liquid into an extraction phase containing vitamin A acetate and a raffinate phase containing a homogeneous catalyst, and recycling the raffinate phase to the vitamin A acetate isomerization reaction instead of a fresh homogeneous catalyst, wherein the total content of the cis-isomer and the all-trans-isomer of the vitamin A acetate in the raffinate phase is controlled to be 0.1-2wt%.

2. The method according to claim 1, characterized in that The following steps are involved: 1) a raw material containing a cis isomer of vitamin A acetate, an all-trans isomer and a retinoid impurity is dissolved in a solvent A, and an isomerization reaction is carried out in the presence of a fresh homogeneous catalyst, whereby the cis isomer is converted into the all-trans isomer to obtain an isomerization reaction liquid; 2) The isomerization reaction liquid is extracted with solvent B, wherein the cis-isomer and all-trans-isomer of vitamin A acetate are transferred to the solvent B phase, which is called the extraction phase, and the homogeneous catalyst remains in the solvent A phase, which is called the raffinate phase; 3) controlling the total content of cis-isomer and all-trans-isomer of vitamin A acetate in the raffinate phase to be 0.1-2 wt %, and recycling the raffinate phase to the isomerization reaction instead of fresh homogeneous catalyst.

3. The method according to claim 2, characterized in that The vitamin A acetate cis isomer in the raw material includes at least one of 11-cis isomer, 9-cis isomer or 13-cis isomer.

4. The method according to claim 2 or 3, characterized in that In terms of mass percentage, the raw material contains 2-40% 11-cis isomer, 0.2-20% 9-cis isomer, 0.1-10% 13-cis isomer, 3-30% retinoid impurities, and the rest are all-trans isomers.

5. The method according to claim 2, characterized in that The solvent A is selected from one or more of acetonitrile, propionitrile, butyronitrile, N,N-dimethylformamide, and dimethyl sulfoxide, preferably acetonitrile; Preferably, the mass ratio of the solvent A to the raw material is (0.5-50):1, preferably (4-10):

1.

6. The method according to claim 1 or 2, characterized in that: The homogeneous catalyst is selected from one or more of tetrakis(triphenylphosphine)palladium, palladium chloride, palladium acetate, palladium nitrate, palladium nitrate hydrate, di(acetylacetonate)palladium, bis(acetonitrile)palladium chloride, bis(triphenylphosphine)palladium dichloride, ruthenium trichloride, ruthenium acetate, tri(triphenylphosphine)carbonyldihydroruthenium, triphenylphosphine ruthenium chloride, triphenylphosphine rhodium chloride, rhodium acetate, rhodium octanoate, dicarbonyl acetylacetonate rhodium, iridium trichloride, iridium tetrachloride and iridium acetate; preferably palladium chloride; Preferably, the mass ratio of the homogeneous catalyst to the raw material is (0.00001-0.0005):1, preferably (0.00005-0.0001):

1.

7. The method according to claim 6, characterized in that The reaction temperature of the isomerization reaction is 30-90°C, preferably 50-60°C; the reaction time is 1-10h, preferably 3-5h.

8. The method according to claim 2, characterized in that The solvent B is selected from one or more of n-hexane, cyclohexane, n-pentane, cyclopentane, n-heptane and n-octane, preferably n-hexane.

9. The method according to claim 2 or 8, characterized in that The extraction is a continuous process, and the feed mass ratio of the isomerization reaction liquid to the solvent B is (0.3-3):1, preferably (0.5-1):

1.

10. The method according to claim 9, characterized in that Extraction temperature 10-40°C, preferably 15-20°C; Preferably, the number of extraction stages is 3-8, preferably 4-6.

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