Method for directionally depolymerizing polycaprolactone into caprolactone monomer

Abstract

The invention discloses a method for directionally depolymerizing polycaprolactone into a caprolactone monomer, and belongs to the technical field of directional depolymerization of caprolactone. According to the invention, efficient closed-loop depolymerization of polycaprolactone is realized, and generation of dimers is reduced. According to the polycaprolactone depolymerization method provided by the invention, the polycaprolactone can be directionally depolymerized into the caprolactone monomer without a high vacuum degree condition, the depolymerization condition is relatively mild, the process is efficient, and a new way is developed for chemical recovery of waste plastics while the environmental and economic benefits of recycling of the polycaprolactone are improved. The obtained depolymerization product is relatively high in purity, relatively high in conversion rate and low in dimer generation amount, and the depolymerization product can be directly and efficiently recycled.

Description

Technical Field

[0001] The present invention relates to a method for the directional depolymerization of polycaprolactone to caprolactone monomer, belonging to the technical field of the directional depolymerization of caprolactone. Background Art

[0002] Polycaprolactone is a biodegradable polyester derived from petroleum. Due to its good biocompatibility and non-toxicity, it plays an important role in packaging, the automotive industry, 3D printing, and the biomedical field. In recent years, with the increasing use of polycaprolactone, its waste treatment has become a major challenge. Currently, its recycling methods mainly include biodegradation, physical recycling, and chemical recycling. However, it usually takes 2 - 4 years for polycaprolactone to be completely biodegraded and transformed in the natural environment, which is not realistic for the timely treatment and recycling of a large amount of polycaprolactone plastic waste. In addition, the efficiency of reuse through physical recycling is relatively low, and the mechanical properties of the polymer will weaken after recycling, which is a process of downgrading utilization. Chemical recycling of waste plastics can not only convert mixed plastics that cannot be processed by traditional mechanical recycling, or even contaminated waste, into recycled plastics or high-value chemicals with properties comparable to the original plastics, but also plays an important role in eliminating plastic pollution, reducing the consumption of fossil resources, and promoting the circular economy. At the same time, it also avoids the carbon emission problem caused by plastic incineration. Therefore, converting waste polycaprolactone into value-added chemicals through chemical recycling is a relatively efficient solution.

[0003] The chemical recycling method of depolymerizing polycaprolactone in a closed loop to caprolactone monomer maximizes its recycling efficiency. Although there have been certain progresses in this field currently, there are still some difficulties. Currently, the methods for depolymerizing to caprolactone are all achieved through vacuum distillation. The requirements of high temperature and high vacuum degree make the process energy-consuming. Therefore, it is of great significance to develop a depolymerization method that does not require high vacuum and is under relatively mild conditions. In addition, dimers are usually generated during the closed-loop recycling process of polycaprolactone. As a macrocyclic monomer, the ring tension of the dimer is small, and it is difficult to achieve controlled polymerization. If a large amount of dimers are generated, it is difficult to effectively reuse the depolymerization products. Therefore, how to control the generation of dimers during the depolymerization process is also an urgent problem to be solved. Summary of the Invention

[0004] In order to solve the problems of the efficient closed-loop depolymerization of polycaprolactone and the reduction of dimer generation, the present invention provides a method for the directional depolymerization of polycaprolactone to caprolactone monomer.

[0005] The technical solution of the present invention:

[0006] One of the objectives of the present invention is to provide a method for the depolymerization of polycaprolactone to caprolactone monomers, which is as follows: in the presence of a solvent and under heating conditions, polycaprolactone is depolymerized using a catalyst to obtain ε-caprolactone monomers.

[0007] Further defined, the catalyst is a metal aluminum complex prepared from trimethylaluminum and a ligand as raw materials;

[0008] The structural formula of the ligand is as follows:

[0009]

[0010] Wherein, R1, R2, and R3 are alkyl groups, aromatic groups, or halogen groups, and the binaphthyl skeleton is in the R configuration or the S configuration.

[0011] Even further defined, the preparation method of the catalyst is: trimethylaluminum and 1 - 3 molar equivalents of the ligand are stirred and reacted in a toluene or dichloromethane solvent. After the reaction is completed, the solvent is removed to obtain the catalyst.

[0012] Further defined, the number average molecular weight of polycaprolactone is 10 3 ~10 6 g / mol.

[0013] Further defined, the solvent is one or a mixture of more of MeCN, DMSO, DMF, DMI, toluene, chlorobenzene, fluorobenzene, tetrahydrofuran, chloroform, and dichloromethane.

[0014] Even further defined, the solvent is MeCN.

[0015] Further defined, the concentration of the polycaprolactone repeating unit in the solvent is 0.01 - 10 mol / L.

[0016] Further defined, the catalyst addition amount is 0.1 - 20% of the molar amount of the polycaprolactone repeating unit.

[0017] Further defined, the heating temperature is 80 - 260 °C, and the time is 1 - 48 h.

[0018] Another objective of the present invention is to provide ε-caprolactone monomers obtained by depolymerization using the above method. Specifically, the obtained ε-caprolactone monomers can be directly reused without purification.

[0019] Beneficial effects:

[0020] (1) The method for depolymerizing polycaprolactone provided by the present invention can depolymerize polycaprolactone into caprolactone monomers directionally without high vacuum conditions. The depolymerization conditions are relatively mild and the process is efficient. While improving the environmental and economic benefits of the recycling of polycaprolactone, it opens up a new way for the chemical recycling of waste plastics. The process of the present invention for the directional depolymerization of polycaprolactone to caprolactone under mild conditions benefits from the specific selection of the catalyst and the reaction solvent. The confined structure of the catalyst enables it to selectively reduce the activation energy of the path for depolymerization to caprolactone monomers and block the generation path for depolymerization to dimers. Polar aprotic solvents such as acetonitrile act on the catalyst and the polymer through their high polarity, dielectric constant, and weak coordination ability, inducing a reduction in the Gibbs free energy of the depolymerization process to promote ring-closure back-biting, and further strengthening the selective activation effect of the catalyst on the depolymerization paths of monomers and dimers. Through the synergistic effect of the active center of the metal catalyst and the microenvironment of the polar aprotic solvent above, the monomer selectivity control of the polycaprolactone depolymerization process is realized.

[0021] (2) The depolymerization products obtained by the method for depolymerizing polycaprolactone provided by the present invention have high purity, high conversion rate, and low dimer generation amount, and the depolymerization products can be directly reused efficiently. Description of the Drawings

[0022] Figure 1 1H NMR spectra for comparing the contents of caprolactone and dimer in the depolymerization products obtained in Example 1 and Example 3. Detailed Embodiments

[0023] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention is provided in conjunction with the examples of the specification.

[0024] Many specific details are set forth in the following description to facilitate a thorough understanding of the present invention, but the present invention may be practiced in other ways different from those described herein. Those skilled in the art can make similar generalizations without departing from the spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0025] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or alternative embodiments that exclude each other with other embodiments.

[0026] The experimental methods used in the following examples are all conventional methods unless otherwise specified. The materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in this field. Without special instructions, those skilled in the art can obtain them through commercial channels, and the purity of the solid and liquid reagents used is analytical pure.

[0027] Example 1:

[0028] The depolymerization of polycaprolactone was carried out using trimethylaluminum as a catalyst. The specific process is as follows:

[0029] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and AlMe3 (1 M in hexane, 12.5 μL, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, polycaprolactone repeating unit concentration 0.05 mol / L), and stir and react at 150 °C for 16 h to obtain the depolymerization product ε-CL.

[0030] The obtained product was analyzed by NMR. The results showed that the depolymerization conversion rate of polycaprolactone was 88%, the selectivity of caprolactone monomer was 51%, and the selectivity of dimer was 49%. This indicates that although the depolymerization of polycaprolactone was achieved in the solvent, the selectivity of caprolactone monomer in the depolymerization product was relatively low.

[0031] Example 2:

[0032] The depolymerization of polycaprolactone was carried out using 1 equivalent of ligand complex as a catalyst. The specific process is as follows:

[0033] (1) The catalyst was prepared by reacting trimethylaluminum with 1 equivalent of ligand (R1, R2, R3 = H) in toluene.

[0034] Specific preparation method: In the glove box, dissolve the ligand (49.2 mg, 0.1 mmol, 1.0 eq.) in 0.9 mL of toluene and add it to a 4 mL glass bottle. Dropwise add AlMe3 (0.1 mL, 0.1 mmol, 1.0 eq., 1 M) into the reaction system and stir at room temperature. Bubbles are generated during the reaction. After overnight reaction, no more bubbles are generated. Take samples during the reaction for NMR detection. After the hydroxyl peak of the ligand disappears and the peak shapes are clear, dropwise add the obtained yellow solution into n-hexane for precipitation (20 mL × 3), centrifuge to collect the precipitate, and dry it under vacuum overnight to obtain a yellow solid with a quantitative yield, and store it at -20 °C.

[0035] (2) Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (6.65 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, polycaprolactone repeat unit concentration 0.05 mol / L). Stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0036] The obtained product was analyzed by NMR. The results showed that the depolymerization conversion rate of polycaprolactone was 87%, the selectivity for caprolactone monomer was 56%, and the selectivity for dimer was 44%.

[0037] Example 3:

[0038] The depolymerization of polycaprolactone was carried out using a 2-equivalent ligand complex as the catalyst. The specific process is as follows:

[0039] The difference between this example and Example 2 is that the ligand dosage in the catalyst preparation process is 2 equivalents, and the other process steps and parameter settings are the same as those in Example 2.

[0040] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.6 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, polycaprolactone repeat unit concentration 0.05 mol / L). Stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0041] The obtained depolymerization product was analyzed by NMR. The results showed that the depolymerization conversion rate of polycaprolactone was 87%, and the selectivity for caprolactone monomer was 98%. In this example, the selectivity for dimer was controlled within 2%, indicating that good control of dimer formation was achieved through the optimization of the catalyst.

[0042] Example 4:

[0043] The depolymerization of polycaprolactone was carried out using a 3-equivalent ligand complex as the catalyst. The specific process is as follows:

[0044] The difference between this example and Example 2 is that the ligand dosage in the catalyst preparation process is 3 equivalents, and the other process steps and parameter settings are the same as those in Example 2.

[0045] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and catalyst (18.5 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, polycaprolactone repeating unit concentration 0.05 mol / L). Stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0046] Perform NMR analysis on the obtained product, and the depolymerization conversion rate of polycaprolactone is 87%, and the selectivity of caprolactone monomer is 98%.

[0047] Example 5:

[0048] Use 2 equivalent ligand complex as the catalyst for the depolymerization of polycaprolactone. The specific process is as follows:

[0049] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and catalyst (5 mg, 5 μmol, 2 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, polycaprolactone repeating unit concentration 0.05 mol / L). Stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0050] Perform NMR analysis on the obtained product, and the depolymerization conversion rate of polycaprolactone is 83%, and the selectivity of caprolactone monomer is 98%. It is proved that good selectivity control can be achieved at different catalyst equivalents.

[0051] Example 6:

[0052] Use 2 equivalent ligand complex as the catalyst for the depolymerization of polycaprolactone. The specific process is as follows:

[0053] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and catalyst (2.5 mg, 2.5 μmol, 1 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, polycaprolactone repeating unit concentration 0.05 mol / L). Stir and react at 150 °C for 20 h to obtain the depolymerization product ε-CL.

[0054] Perform NMR analysis on the obtained product, and the depolymerization conversion rate of polycaprolactone is 85%, and the selectivity of caprolactone monomer is 98%. It is proved that good selectivity control can be achieved at different catalyst equivalents.

[0055] Example 7:

[0056] The depolymerization of polycaprolactone was carried out using 2 equivalents of ligand complex as the catalyst, and the specific process is as follows:

[0057] Weighed polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.6 mg, 12.5 μmol, 5 mol% equiv.) were added to a 10 mL Schlenk flask, and then acetonitrile solvent (5 mL, polycaprolactone repeat unit concentration 0.05 mol / L) was added. The mixture was stirred and reacted at 160 °C for 18 h to obtain the depolymerization product ε-CL.

[0058] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was 93%, and the selectivity of caprolactone monomer was 97%. Good control of depolymerization selectivity can be achieved at different temperatures.

[0059] Example 8:

[0060] The depolymerization of polycaprolactone was carried out using 2 equivalents of ligand complex as the catalyst, and the specific process is as follows:

[0061] Weighed polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.6 mg, 12.5 μmol, 5 mol% equiv.) were added to a 10 mL Schlenk flask, and then acetonitrile solvent (5 mL, polycaprolactone repeat unit concentration 0.05 mol / L) was added. The mixture was stirred and reacted at 135 °C for 18 h to obtain the depolymerization product ε-CL.

[0062] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was 80%, and the selectivity of caprolactone monomer was 99%. Good control of depolymerization selectivity can be achieved at different temperatures.

[0063] Example 9:

[0064] The depolymerization of polycaprolactone was carried out using 2 equivalents of ligand complex as the catalyst, and the specific process is as follows:

[0065] Weighed polycaprolactone pellets (molecular weight 13200 g / mol, molecular weight distribution 1.28) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.6 mg, 12.5 μmol, 5 mol% equiv.) were added to a 10 mL Schlenk flask, and then acetonitrile solvent (5 mL, polycaprolactone repeat unit concentration 0.05 mol / L) was added. The mixture was stirred and reacted at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0066] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was 89%, and the selectivity of caprolactone monomer was 98%. This catalytic system can achieve the depolymerization of polycaprolactone with different molecular weights.

[0067] Example 10:

[0068] The depolymerization of polycaprolactone was carried out using 2 equivalents of the ligand complex as the catalyst. The specific process is as follows:

[0069] Weighed polycaprolactone pellets (molecular weight 63200 g / mol, molecular weight distribution 1.98) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.6 mg, 12.5 μmol, 5 mol% equiv.) were added to a 10 mL Schlenk flask, and then acetonitrile solvent (5 mL, polycaprolactone repeating unit concentration 0.05 mol / L) was added. The reaction was stirred at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0070] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was 84%, and the selectivity of caprolactone monomer was 98%. This catalytic system can achieve the depolymerization of polycaprolactone with different molecular weights.

[0071] Example 11:

[0072] The depolymerization of polycaprolactone was carried out using 2 equivalents of the ligand complex as the catalyst. The specific process is as follows:

[0073] The catalyst was prepared by reacting trimethylaluminum with 2 equivalents of the ligand (R1, R2, R3 = H) in toluene.

[0074] Weighed polycaprolactone waste (molecular weight 26200 g / mol, molecular weight distribution 1.79) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.6 mg, 12.5 μmol, 5 mol% equiv.) were added to a 10 mL Schlenk flask, and then acetonitrile solvent (5 mL, polycaprolactone repeating unit concentration 0.05 mol / L) was added. The reaction was stirred at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0075] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was 86%, and the selectivity of caprolactone monomer was 98%. This catalytic system can achieve the depolymerization of polycaprolactone from different sources.

[0076] Example 12:

[0077] The depolymerization of polycaprolactone was carried out using 2 equivalents of the ligand complex as the catalyst. The specific process is as follows:

[0078] Weigh polycaprolactone for 3D printing products (molecular weight 25400 g / mol, molecular weight distribution 1.58) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.6 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, the concentration of polycaprolactone repeating units is 0.05 mol / L), and stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0079] Perform NMR analysis on the obtained product, and the depolymerization conversion rate of polycaprolactone is 83%, and the selectivity of caprolactone monomer is 98%. This catalytic system can achieve the depolymerization of polycaprolactone from different sources.

[0080] Example 13:

[0081] The difference between this example and Example 3 lies in the ligand (R1 = methyl, R2, R3 = H) during the catalyst preparation process, and the remaining process steps and parameter settings are the same as those in Example 3.

[0082] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.7 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, the concentration of polycaprolactone repeating units is 0.05 mol / L), and stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0083] Perform NMR analysis on the obtained product, and the depolymerization conversion rate of polycaprolactone is 85%, and the selectivity of caprolactone monomer is 98%. Different ligand structures can achieve good control of the depolymerization process and selectivity.

[0084] Example 14:

[0085] The difference between this example and Example 3 lies in the ligand (R2 = methyl, R1, R3 = H) during the catalyst preparation process, and the remaining process steps and parameter settings are the same as those in Example 3.

[0086] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.7 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, the concentration of polycaprolactone repeating units is 0.05 mol / L), and stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0087] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was 87%, and the selectivity of caprolactone monomer was 98%. Different ligand structures can achieve good control of the depolymerization process and selectivity.

[0088] Example 15:

[0089] The difference between this example and Example 3 lies in the ligand (R2 = Cl, R1, R3 = H) during the catalyst preparation process, and the remaining process steps and parameter settings are the same as those in Example 3.

[0090] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (12.7 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add acetonitrile solvent (5 mL, polycaprolactone repeat unit concentration 0.05 mol / L), and stir and react at 150 °C for 18 h to obtain the depolymerization product ε-CL.

[0091] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was 86%, and the selectivity of caprolactone monomer was 98%. Different ligand structures can achieve good control of the depolymerization process and selectivity.

[0092] Comparative Example 1

[0093] The difference between this example and Example 2 is that the solvent used during the polycaprolactone depolymerization process is toluene, and the specific process is as follows:

[0094] Weigh polycaprolactone pellets (molecular weight 35200 g / mol, molecular weight distribution 1.65) (28 mg, 0.25 mmol, 1 equiv.) and the catalyst (6.65 mg, 12.5 μmol, 5 mol% equiv.) and add them to a 10 mL Schlenk flask. Then add toluene solvent (5 mL, polycaprolactone repeat unit concentration 0.05 mol / L), and stir and react at 150 °C for 18 h.

[0095] The obtained product was analyzed by NMR, and the depolymerization conversion rate of polycaprolactone was less than 5%. In the toluene solvent, it is difficult to achieve the conversion of polycaprolactone.

[0096] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Anyone familiar with this technology can make various modifications and decorations without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be defined by the claims.

Claims

1. A method for the directional depolymerization of polycaprolactone to caprolactone monomers, characterized in that, Under the conditions of the presence of a solvent and heating, polycaprolactone is depolymerized using a catalyst to obtain ε-caprolactone monomers.

2. The method according to claim 1, wherein The catalyst is a metal aluminum complex prepared from trimethylaluminum and a ligand as raw materials; The structural formula of the ligand is as follows: Among them, R1, R2, and R3 are alkyl groups, aromatic groups, or halogen groups, and the binaphthyl skeleton is in the R configuration or S configuration.

3. The method according to claim 2, wherein The preparation method of the catalyst is: stirring and reacting trimethylaluminum with 1 - 3 molar equivalents of the ligand in a toluene or dichloromethane solvent, and removing the solvent after the reaction is completed to obtain the catalyst.

4. The method according to claim 1, wherein The number-average molecular weight of polycaprolactone is 10 3 ~10 6 g / mol.

5. The method according to claim 1, wherein The solvent is one or a mixture of more of MeCN, DMSO, DMF, DMI, toluene, chlorobenzene, fluorobenzene, tetrahydrofuran, chloroform, and dichloromethane.

6. The method according to claim 5, characterized in that, The solvent is MeCN.

7. The method according to claim 1, wherein The concentration of the polycaprolactone repeating unit in the solvent is 0.01 - 10 mol / L.

8. The method according to claim 1, characterized in that, The addition amount of the catalyst is 0.1 - 20% of the molar amount of the polycaprolactone repeating unit.

9. The method according to claim 1, wherein The heating temperature is 80 - 260 °C, and the time is 1 - 48 h.

10. The ε-caprolactone monomer depolymerized by the method according to any one of claims 1 to 9, characterized in that, Reuse can be directly achieved without purification.

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