A method for preparing oxygen-18methanol

Abstract

The present invention proposes a method for preparing oxygen-18 methanol, comprising the following steps: ethylene and an oxidant react with heavy oxygen water H2 in an absorber. 18 O contact, and is oxidized to 18 O formaldehyde and formic acid, then add an appropriate amount of reducing agent to reduce oxygen 18 formaldehyde and oxygen 18 formic acid to oxygen 18 methanol, and finally purify the aqueous solution by distillation to obtain high-purity oxygen 18 methanol CH3 18 OH products. Among them, the oxidant used in the ethylene oxidation process is 18 O3, produced by an ozone generator using pure 18 O2 is used as raw material for preparation, and the reducing agent used in the reduction process is one or more of sodium hydride, lithium aluminum hydride or sodium borohydride. 18 O2 as raw material, heavy oxygen water H2 18 O is the solvent, and the whole process does not contain 16 O materials to ensure the abundance of the product. At the same time, the overall reaction conditions are mild and the reaction can be carried out at lower temperature and pressure. The process is simple, highly operable and has good economic benefits.

Description

Technical Field

[0001] The present invention relates to the technical field of isotope-labeled compound synthesis, and in particular to a method for preparing oxygen-18methanol. Background Art

[0002] Isotope-labeled reagents are chemical reagents that contain labeled isotopes of one or more elements in their molecules, exploiting their traceability for analytical purposes. By labeling specific sites in compounds with isotopes, related substances can be tracked, detected, and measured. They are tools used to study chemical and biological processes. Currently, isotope-labeled reagents have a wide range of applications in biology, chemistry, medicine, and environmental science.

[0003] Common methanol isotope-labeled reagents include deuterated methanol, carbon-13 methanol, and oxygen-18 methanol. Chinese patent document CN111116313B discloses a method for preparing deuterated methanol. This method involves passing a mixture of deuterium, carbon monoxide, and nitrogen into a reactor containing a catalyst. The mixture reacts at a pressure of 5 MPa to 7 MPa and a temperature of 210°C to 290°C. After the reaction is complete, the deuterated methanol is condensed to obtain liquid deuterated methanol. This liquid can then be distilled to obtain high-purity deuterated methanol.

[0004] Oxygen 18 methanol CH3 18 OH refers to the product obtained by replacing the oxygen atom with oxygen-18 atom in the methanol molecule. It is an O isotope labeling reagent. The current synthesis process of oxygen-18 methanol generally uses methyl orthoformate and heavy oxygen water to hydrolyze to obtain oxygen-18 methanol. The methyl orthoformate in this synthesis process contains active oxygen atoms. During the hydrolysis process, these oxygen atoms will react with CH3 18 The heavy oxygen atoms in OH are exchanged, so that CH3 18 The abundance of OH is insufficient, making it difficult to obtain high-purity products. Summary of the Invention

[0005] In view of this, the present invention proposes a method for preparing oxygen-18methanol to solve the problems of insufficient abundance and low purity in the existing oxygen-18methanol preparation process. 16 O's materials come into contact to ensure the abundance of the product.

[0006] The technical solution of the present invention is achieved as follows: The present invention provides a method for preparing oxygen-18methanol, comprising the following steps:

[0007] S1, heavy oxygen H2 18 O and organic solvent are added into the absorber. The temperature in the absorber is 20-100℃. The absorber is evacuated with a vacuum pump to separate ethylene and ozone. 18 O3 is connected to the absorber inlet, ethylene and ozone18 O3 reacts with heavy oxygen H2 in the absorber 18 O contacts and mixes, controls the gauge pressure in the absorber to 10-300 kPa, and absorbs for 20 min-2 h to obtain the absorption liquid;

[0008] S2, stirring the absorption liquid, and causing ethylene to undergo oxidation reaction in the presence of a catalyst, the reactor temperature is 20-100°C, the pressure in the reactor is 0-3 bar, and the reaction is carried out for 20-28 hours to obtain a mixed solution of 18-formaldehyde and 18-formic acid;

[0009] S3, adding a reducing agent to the formaldehyde obtained after oxidation and the formic acid solution, stirring and reacting for 20-40 minutes at a reaction temperature of 20-50° C. and a reaction pressure of 0-1 bar to obtain a reduced product;

[0010] S4, distilling the reduced product, collecting the fraction at 64-66°C to obtain 18-methanol.

[0011] Based on the above technical solution, preferably, the organic solvent in step S1 is one of carbon tetrachloride, chloroform or dichloromethane, and the volume ratio of the organic solvent to heavy oxygen water is 0-10:1.

[0012] On the basis of the above technical solution, preferably, the ozone in step S1 18 O3 ozone generator uses pure 18 O2 is used as raw material for preparation, and the ozone 18 O3 purity ≥95%.

[0013] On the basis of the above technical solution, preferably, in step S1, ethylene and ozone 18 The molar ratio of O3 is 1:0.8-1.3.

[0014] On the basis of the above technical solution, preferably, the step S1 is performed in an absorber, the absorber is a jet absorber or a bubbling absorber, and the absorption gas-liquid volume ratio in the absorber is 1:1-10.

[0015] On the basis of the above technical solution, preferably, the reaction in step S2 is carried out in a jet reactor or a stirred tank reactor, and the absorber can be arranged inside or outside the reactor.

[0016] Specifically, the oxidation process of ethylene in step S2 of the present invention is: ozone 18 O3 attacks the carbon-carbon double bond of ethylene C2H4 to generate a five-membered cyclic intermediate. After rearrangement, the carbon-carbon double bond of the intermediate breaks, forming a carbon-oxygen double bond to obtain two oxygen-18 formaldehyde molecules (HCH 18 O) and an oxygen 18O2 molecules, and finally oxygen 18 formaldehyde molecules (HCH 18 O) is further oxygenated 18 O2 molecules are oxidized to produce oxygen 18 formic acid molecules (HC 18 O 18 OH).

[0017] On the basis of the above technical solution, preferably, the catalyst in step S2 is added together with the absorbent in step S1, the mass ratio of the catalyst to the absorbent is 0-1:20, and the catalyst is one or more of platinum, zinc, and palladium.

[0018] On the basis of the above technical solution, preferably, the reducing agent in step S3 is one or more of sodium hydride, lithium aluminum hydride or sodium borohydride.

[0019] Based on the above technical solution, preferably, the molar ratio of the reducing agent to ozone in step S3 is 1:0.2-1.2.

[0020] Specifically, the reduction process of the present invention is to use a reducing agent to convert oxygen 18 formaldehyde molecules (HCH 18 O) is reduced to a hydroxyl group to give an oxygen 18 methanol molecule (CH3 18 OH), oxygen 18 formic acid molecules (HC 18 O 18 The carboxyl group in OH) is first reduced to aldehyde and then further reduced to hydroxyl to obtain oxygen 18 methanol molecule (CH3 18 OH). Metal hydride reducing agents have strong hydrogen transfer ability and can reduce aldehydes and carboxylic acids to alcohols. The reaction conditions are mild, the side reactions are few, and the yield is high. Therefore, the reducing agent is selected from one or more of sodium hydride, lithium aluminum hydride, or sodium borohydride.

[0021] Based on the above technical solution, preferably, the isotopic abundance of oxygen-18methanol obtained in step S4 is greater than 99.9%.

[0022] The method for preparing oxygen-18methanol of the present invention has the following beneficial effects compared with the prior art:

[0023] (1) This application is based on 18 O2 as raw material, heavy oxygen water H2 18 O is the solvent, and the reaction is not 16 O materials, reducing the risk of isotope abundance dilution, oxygen 18 methanol CH3 18 The O element in OH can only come from 18 O2 or heavy oxygen H2 18 O, thus ensuring the abundance of the product.

[0024] (2) This application adjusts the ratio of raw materials in each step, namely ethylene and ozone 18 The molar ratio of O3, the volume ratio of gas and liquid absorbed in the absorber, and the ratio of reducing agent to ozone 18 The molar ratio of O3 is such that the final prepared oxygen 18 methanol CH3 18 OH has high purity and is 18 O2 calculation has a higher finished product yield.

[0025] (3) The preparation conditions in this application are flexible and diverse and are not restricted. For example, during the oxidation process, a relatively simple conventional reactor can be used to carry out the reaction preparation at normal pressure, or a pressure reactor can be used for preparation. The absorber can be set inside the reactor or outside the reactor. Moreover, the conditions of the entire reaction process are mild, and the reaction can be carried out at lower temperature and pressure. The process is simple and highly operable. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 This is the hydrogen nuclear magnetic resonance spectrum of oxygen-18methanol prepared in Example 2 of the present invention.

[0028] Figure 2 This is the gas chromatogram of oxygen-18methanol prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0029] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] Example 1

[0031] This embodiment provides a method for preparing oxygen-18methanol, which specifically includes the following steps:

[0032] S1, fill the jet absorber with 500ml of heavy oxygen H2 18 O is used as the absorbent, and the absorber is evacuated with a vacuum pump. Then the circulation pump of the jet absorber is started, and the heating system is started to maintain the temperature in the jet absorber at 20°C.18 O2 is decompressed and then produced into ozone through an ozone generator 18 O3, ozone purity is 95%, the unreacted oxygen discharged from the ozone generator is recycled and reused, open the ethylene cylinder, and mix the ethylene with the ozone generated by the ozone generator. 18 O3 is connected to the inlet of the jet absorber, and the ethylene and ozone are controlled by the mass flow meter. 18 The volume flow ratio of O3 is 1:0.8, which controls the absorption gas-liquid volume ratio of ethylene and ozone. 18 The ratio of the volume of O3 to the volume of the absorbent is 1:1, and the pressure gauge pressure in the absorber is controlled to be 10KPa. Ethylene and ozone are mixed with heavy oxygen water H2 in the absorber. 18 O is contacted and mixed, and the mixture is absorbed for 20 minutes to obtain an absorption liquid.

[0033] S2, stirring the absorption liquid to cause ethylene oxidation reaction, the pressure in the reactor is 0.5 bar, the reaction temperature is maintained at 20 ° C. After the reaction is carried out for 28 hours, the gas source is turned off.

[0034] S3, the solution obtained after the oxidation reaction is introduced into a stirred reactor, sodium borohydride is added, sodium borohydride and ozone are added 18 The molar ratio of O3 was 1:0.2, the reaction was stirred for 20 minutes, the reduction reaction temperature was 20°C, and the reaction pressure was 0.1 bar.

[0035] S4, introducing the solution obtained after the reduction reaction into a distillation tower for distillation, using stainless steel wire mesh packing, a reflux ratio of 1:20, a distillation pressure of 100KPa, and collecting the fraction at 64-66°C to obtain oxygen-18 methanol.

[0036] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 95%, the abundance is 99.93%, and the purity is 90%.

[0037] Example 2

[0038] This embodiment provides a method for preparing oxygen-18methanol, which specifically includes the following steps:

[0039] S1, fill the jet absorber with 500ml of heavy oxygen H2 18 O and 2500ml of carbon tetrachloride CCl4 as absorbent, and use a vacuum pump to evacuate the absorber, then start the circulation pump of the jet absorber and start the heating system to maintain the temperature in the jet absorber at 60℃. 18 Ozone is produced by an ozone generator after the O2 is decompressed 18 O3, ozone purity is 95%, open the ethylene cylinder, and mix ethylene with ozone generated by the ozone generator. 18O3 is connected to the inlet of the jet absorber, and the ethylene and ozone are controlled by the mass flow meter. 18 The volume flow ratio of O3 is 1:1, which controls the absorption gas-liquid volume ratio of ethylene and ozone. 18 The ratio of the volume of O3 to the volume of the absorbent is 1:5, and the pressure in the absorber is controlled to be 150KPa. Ethylene and ozone are mixed with heavy oxygen water H2 in the absorber. 18 O is contacted and mixed, and the mixture is absorbed for 1 hour to obtain an absorption liquid.

[0040] S2, stirring the absorption liquid, and causing ethylene to undergo oxidation reaction under the action of catalyst platinum particles. The catalyst platinum particles are added together with the absorbent in step S1. The mass ratio of catalyst platinum particles to absorbent is 0.5:20. The pressure in the reactor is 1 bar. The reaction temperature is maintained at 60°C. After the reaction is carried out for 24 hours, the gas source is turned off.

[0041] S3, the solution obtained after the oxidation reaction is introduced into a stirred reactor, lithium aluminum hydride, lithium aluminum hydride and ozone are added 18 The molar ratio of O3 was 1:0.7, the reaction was stirred for 30 minutes, the reduction reaction temperature was 35°C, and the reaction pressure was 0.5 bar.

[0042] S4, introducing the solution obtained after the reduction reaction into a distillation tower for distillation, using stainless steel wire mesh packing, a reflux ratio of 1:20, a distillation pressure of 100KPa, and collecting the fraction at 64-66°C to obtain oxygen-18 methanol.

[0043] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 97%, the abundance is 99.95%, and the purity is 92%.

[0044] Product characterization and analysis

[0045] Nuclear magnetic resonance analysis: NMR frequency 400MHz, injection volume 5mg, diluted with 0.6mlCDCl3 and then automatically injected, such as Figure 1 As shown, the characteristic chemical shifts of oxygen 18 methanol are 3.42 and 4.02, and the characteristic peak at 4.02 is - 18 OH characteristic peak, in addition there are a few miscellaneous peaks derived from impurities in the reaction products.

[0046] Gas chromatography analysis: The oxygen-18methanol prepared in Example 2 was analyzed by gas chromatography using a hydrogen flame detector, nitrogen as the carrier gas, and an HP-5 non-polar chromatographic column. The vaporizer temperature was 130°C, the detector temperature was 130°C, the chromatographic column temperature was 75°C, the heating rate was 10°C / min, the temperature was raised to 130°C, and the temperature was maintained for 5 minutes. Figure 2 As shown, under this condition, the retention time of 18-methanol is 16.78 min, and its purity is 92% calculated by peak area percentage integration.

[0047] Example 3

[0048] This embodiment provides a method for preparing oxygen-18methanol, which specifically includes the following steps:

[0049] S1, fill the jet absorber with 500ml of heavy oxygen H2 18 O and 5000ml of carbon tetrachloride CCl4 are used as absorbents, and the absorber is evacuated with a vacuum pump. Then the circulation pump of the jet absorber is started, and the heating system is started to maintain the temperature in the jet absorber at 100℃. 18 Ozone is produced by an ozone generator after the O2 is decompressed 18 O3, ozone purity is 95%, open the ethylene cylinder, and mix ethylene with ozone generated by the ozone generator. 18 O3 is connected to the inlet of the jet absorber, and the ethylene and ozone are controlled by the mass flow meter. 18 The volume flow ratio of O3 is 1:1.3, the volume ratio of the absorbed gas and liquid, that is, the ratio of the volume of ethylene and ozone to the absorbent is 1:10, and the pressure in the absorber is controlled to be 300KPa. Ethylene and ozone are mixed with heavy oxygen water H2 in the absorber. 18 O is brought into contact and mixed, and absorbed for 2 h to obtain a saturated solution.

[0050] S2, stirring the absorption liquid, and causing ethylene to undergo oxidation reaction under the action of catalyst platinum particles. The catalyst platinum particles are added together with the absorbent in step S1. The mass ratio of catalyst platinum particles to absorbent is 1:20. The pressure in the reactor is 3 bar. The reaction temperature is maintained at 100°C. After the reaction is carried out for 20 hours, the gas source is turned off.

[0051] S3, the solution obtained after the oxidation reaction is introduced into a stirred reactor, lithium aluminum hydride, lithium aluminum hydride and ozone are added 18 The molar ratio of O3 was 1:1.2. The reaction was stirred for 40 minutes, the reduction reaction temperature was 50°C, and the reaction pressure was 1 bar.

[0052] S4, introducing the solution obtained after the reduction reaction into a distillation tower for distillation, using stainless steel wire mesh packing, a reflux ratio of 1:20, a distillation pressure of 10KPa, and collecting the fraction at 64-66°C to obtain oxygen-18 methanol.

[0053] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 78%, the abundance is 99.95%, and the purity is 91%.

[0054] Example 4

[0055] This embodiment provides a method for preparing oxygen-18methanol. The specific operating steps are the same as those in Example 2, except that:

[0056] In S1, ethylene and ozone are controlled by mass flow meters. 18 The volume flow ratio of O3 is 1:0.5.

[0057] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 97%, the abundance is 99.92%, and the purity is 89%.

[0058] Example 5

[0059] This embodiment provides a method for preparing oxygen-18methanol. The specific operating steps are the same as those in Example 2, except that:

[0060] In S1, ethylene and ozone are controlled by mass flow meters. 18 The volume flow ratio of O3 is 1:1.5.

[0061] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 91%, the abundance is 99.95%, and the purity is 92%.

[0062] Example 6

[0063] This embodiment provides a method for preparing oxygen-18methanol. The specific operating steps are the same as those in Example 2, except that:

[0064] In S1, the absorption gas-liquid volume ratio, that is, the ratio of the volume of ethylene and ozone to the volume of the absorbent, is controlled to be 1:0.8.

[0065] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 88%, the abundance is 99.94%, and the purity is 92%.

[0066] Example 7

[0067] This embodiment provides a method for preparing oxygen-18methanol. The specific operating steps are the same as those in Example 2, except that:

[0068] In S1, the absorption gas-liquid volume ratio, that is, the ratio of the volume of ethylene and ozone to the volume of the absorbent, is controlled to be 1:12.

[0069] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 90%, the abundance is 99.95%, and the purity is 89%.

[0070] Example 8

[0071] This embodiment provides a method for preparing oxygen-18methanol. The specific operating steps are the same as those in Example 2, except that:

[0072] In S3, lithium aluminum hydride is added, lithium aluminum hydride and ozone 18The molar ratio of O3 is 1:0.15. The yield of finished product of oxygen 18 methanol is 18 The O3 content is 81%, the abundance is 99.91%, and the purity is 87%.

[0073] Embodiment 9

[0074] This embodiment provides a method for preparing oxygen-18methanol. The specific operating steps are the same as those in Example 2, except that:

[0075] In S3, lithium aluminum hydride is added, lithium aluminum hydride and ozone 18 The molar ratio of O3 is 1:1.5. 18 Lithium aluminum hydride with an O3 molar ratio of 1:1.5.

[0076] The yield of finished product of oxygen 18 methanol is 18 The O3 content is 73%, the abundance is 99.93%, and the purity is 88%.

[0077] Analyzing all the above embodiments, it can be seen that: in all the embodiments, the reaction process does not contain 16 O materials are in contact, thus ensuring that the abundance of oxygen 18 methanol products is greater than 99.9%; ethylene and ozone 18 If the volume flow rate ratio of O3, i.e. the molar ratio, is too large, the purity of the product will be reduced; if it is too small, the yield will be reduced; if the absorption gas-liquid volume ratio, i.e. the ratio of the volume of ethylene and ozone to the volume of the absorbent, is too large, the yield will be reduced; if it is too small, the product purity will be reduced; 18 Too large or too small a molar ratio of O3 will result in reduced purity and yield.

[0078] Comparative Example 1

[0079] Comparative Example 1 provides a method for preparing oxygen-18methanol, which specifically comprises the following steps:

[0080] S1, add 75g of tributyl orthoformate, 50g of diethylene glycol diethyl ether and H2 into the flask. 18 Add 5g of hydroxybenzoic acid (NH4PO4) and stir to mix thoroughly. Then, add 25ml of 0.1mol / L hydrochloric acid dropwise using a dropping funnel. Stir vigorously at room temperature for 40 minutes. Once the reaction mixture becomes a homogeneous solution, drain it and add it to a small glass distillation column. Distill at 101.3kPa with a reflux ratio of 10:1, and collect the distillate at 64-66°C.

[0081] S2, weigh 16g of lithium aluminum hydride, add it to 400ml of diethylene glycol diethyl ether solution to dissolve it, prepare an absorption liquid, introduce the distillate in the distillation tower in step S1 into the absorption liquid, place the absorption liquid in an ice-water bath, stir and react for 30 minutes, after suspended matter appears in the solution, add 8ml of water and 50ml of ethylene glycol, and continue stirring and reacting for 30min.

[0082] S3, introducing the solution obtained after the reaction in step S2 into another distillation tower for distillation. The reflux ratio is controlled to be 10:1, and the distillation is carried out at 101.3 kPa. The fraction at 64-66° C. is collected to obtain 18-methanol.

[0083] The yield of finished product of oxygen 18 methanol is based on H2 18 The O content is 84%, the abundance is 99%, and the purity is 90%.

[0084] Analysis of Example 2 and Comparative Example 1 shows that: Example 2 is compared with Comparative Example 1 because the reaction is not carried out with 16 O materials, thereby ensuring that the abundance of oxygen-18methanol products is greater than 99.9%; the entire reaction process is simple, and only one distillation is required, thereby improving the purity of the product while ensuring a higher yield.

[0085] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing oxygen-18methanol, characterized in that: The steps include: S1, heavy oxygen H2 18 O and organic solvents act as absorbents to absorb ethylene and ozone 18 O3, absorption temperature is 20-100℃, absorption pressure is 10-300KPa, absorption time is 20min-2h to obtain absorption liquid; S2, stirring the absorption liquid and causing ethylene to undergo oxidation reaction in the presence of a catalyst, the reaction temperature being 20-100°C, the reaction pressure being 0.5-3 bar, and the reaction being carried out for 20-28 hours to obtain a mixed solution of 18-oxygen formaldehyde and 18-oxygen formic acid; the catalyst being one or more of platinum, zinc, and palladium; S3, adding a reducing agent to the mixed solution of oxygen-18 formaldehyde and oxygen-18 formic acid, stirring and reacting for 20-40 minutes, the reaction temperature is 20-50°C, and the reaction pressure is 0.1-1 bar to obtain a reduced product; The reducing agent is one or more of sodium hydride, lithium aluminum hydride or sodium borohydride; S4, distilling the reduced product, collecting the fraction at 64-66°C to obtain 18-methanol.

2. The method for preparing oxygen-18methanol according to claim 1, wherein: In step S1, the organic solvent is one of carbon tetrachloride, chloroform or dichloromethane, and the volume ratio of the organic solvent to heavy oxygen water is 0-10:

1.

3. The method for preparing oxygen-18methanol according to claim 1, wherein: In step S1, ozone 18 O3 is produced by an ozone generator using pure 18 O2 is used as raw material for preparation, and the ozone 18 O3 purity ≥95%.

4. The method for preparing oxygen-18methanol according to claim 1, wherein: In step S1, ethylene and ozone 18 The molar ratio of O3 is 1:0.8-1.

3.

5. The method for preparing oxygen-18methanol according to claim 1, wherein: The step S1 is performed in an absorber, which is a jet absorber or a bubbling absorber, and the absorption gas-liquid volume ratio in the absorber is 1:1-10.

6. The method for preparing oxygen-18methanol according to any one of claims 1 to 4, characterized in that: The reaction in step S2 is carried out in a jet reactor or a stirred tank reactor, and the absorber is arranged inside or outside the reactor.

7. The method for preparing oxygen-18methanol according to claim 1, wherein: The catalyst in step S2 is added together with the absorbent in step S1, and the mass ratio of the catalyst to the absorbent is 0-1:

20.

8. The method for preparing oxygen-18methanol according to claim 7, characterized in that: In step S3, the reducing agent and ozone 18 The molar ratio of O3 is 1:0.2-1.

2.

9. The method for preparing oxygen-18methanol according to claim 1, wherein: The isotopic abundance of oxygen-18methanol obtained in step S4 is greater than 99.9%.

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