Preparation method and application of eutectic solvent catalyst

Abstract

The invention relates to a preparation method and application of a deep eutectic solvent catalyst. Stannous salicylate and polyhydric alcohol form a deep eutectic solvent (DES), so that the form and solubility of the catalyst are effectively improved, and an efficient composite catalytic system is formed. Under the condition of extremely low catalyst dosage, the catalytic system can realize rapid polymerization of lactide to prepare high-molecular-weight polylactic acid. The method is simple to operate, low in cost and suitable for industrial requirements of efficiently preparing polylactic acid in a reactive extrusion process.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and specifically to a preparation method and application of a deep eutectic solvent catalyst. Background Art

[0002] With the increasingly serious pollution problems caused by non-degradable petroleum-based plastics, biodegradable polyester materials have gradually attracted wide attention. As a biodegradable material derived from bio-based sources, polylactic acid (PLA) has been widely used in fields such as packaging, textiles, agricultural films, and drug sustained release due to its excellent mechanical properties, processability, and bioabsorbability, and is one of the most promising alternative materials.

[0003] Currently, the industrial production of polylactic acid is mainly achieved through the ring-opening polymerization method of lactide to obtain high-molecular-weight and high-quality polylactic acid. The reaction devices include batch reactors, plug flow reactors, pipeline circulation reactors, reactive distillation columns, and reactive extruders, etc. Among them, the reactive extrusion process has efficient mass transfer and heat transfer characteristics due to the strong shear mixing effect of the extruder screw, which greatly improves the lactide polymerization reaction rate and enables the polymerization process to be completed within a few minutes. However, the production efficiency of the reactive extrusion process is limited by the residence time of the material in the extruder, and its efficient operation depends on an ultra-high-efficiency catalytic system.

[0004] Tin-based catalysts have been widely used in the ring-opening polymerization reaction of lactide due to their excellent catalytic activity. Among them, stannous octanoate, as a liquid catalyst, is easy to disperse and dissolve in lactide and has a high catalytic efficiency. Currently, many patents have reported catalytic systems based on stannous octanoate. For example, in patent US5378801, stannous octanoate is used to catalyze the reactive extrusion of lactide to prepare polylactic acid; in patent US6166169, a composite catalytic system of stannous octanoate and triphenylphosphine is used, and polylactic acid with a weight average molecular weight of 1.6×10 5 Da can be synthesized in the extruder after staying for 7 minutes, but its production efficiency is only 0.7 kg / h, and there is still room for improvement.

[0005] Improving the catalytic efficiency of the catalytic active center is the key to further enhancing the production efficiency of reactive extrusion. Adjusting the ligand structure of the metal center can change its acidic environment and thus affect the catalytic performance. As a cheap and easily available natural hydroxy acid, salicylic acid can enhance the acidity of the tin(II) center due to the strong electron-withdrawing effect of its hydroxyl group, and at the same time helps to stabilize the reaction intermediate. However, after salicylic acid coordinates with tin(II), a stable crystal structure is formed, and the resulting stannous salicylate is in the form of solid powder and is difficult to disperse and dissolve in lactide, showing low catalytic activity. Summary of the Invention

[0006] Based on this, it is necessary to provide a preparation method and application of a deep eutectic solvent catalyst, aiming to solve the problems existing in the prior art.

[0007] To achieve the above object, the present invention provides a technical solution:

[0008] A preparation method of a deep eutectic solvent catalyst, the preparation method of the deep eutectic solvent catalyst comprising the steps of:

[0009] Mix stannous salicylate with a polyol, and heat and stir until the system is clear and transparent to obtain the deep eutectic solvent catalyst;

[0010] In some embodiments, the stirring time is 2 h to 8 h.

[0011] The polyol includes at least one of glycerol, ethylene glycol, 1,3-propanediol, 1,2-butanediol, 1,2-pentanediol, glycerol, trihydroxyethane, xylitol, and dulcitol.

[0012] In some embodiments, the metal salicylate includes stannous salicylate;

[0013] The structural formula of stannous salicylate is as follows:

[0014]

[0015] In some embodiments, the metal salicylate includes zinc salicylate.

[0016] In some embodiments, the molar ratio of stannous salicylate to the polyol is 1:(1.3 to 1:8).

[0017] In some embodiments, the reaction temperature is 120 °C to 200 °C.

[0018] In some embodiments, the preparation steps of stannous salicylate include:

[0019] S100. Dissolve sodium salicylate in water to obtain an aqueous solution of sodium salicylate;

[0020] S200. Dissolve stannous salt in an alcohol solvent to obtain an alcohol solution of stannous salt.

[0021] In some embodiments, the concentration of the alcohol solution of stannous salt is 0.1 mol / L to 1 mol / L;

[0022] In some embodiments, the alcohol solvent includes at least one of methanol, ethanol, propanol, and water;

[0023] In some embodiments, the stannous salt includes at least one of stannous chloride dihydrate, stannous chloride, stannous nitrate, and stannous sulfate.

[0024] S300. Drop the alcoholic solution of the stannous salt into the aqueous solution of sodium salicylate to form a white precipitate. Filter and dry the white precipitate to obtain stannous salicylate.

[0025] Construct a stannous salicylate catalyst. Due to the presence of hydroxyl groups in the salicylic acid ligand, the acidity of the tin center is promoted, so this catalyst has greater potential to exhibit high catalytic activity.

[0026] The present invention also provides an application of a deep eutectic solvent catalyst in the synthesis of polylactic acid.

[0027] In some embodiments, the specific steps for synthesizing polylactic acid include:

[0028] Add a deep eutectic solvent catalyst to lactide, and carry out a ring-opening polymerization reaction. Heat the temperature to 140°C - 180°C, and polylactic acid is obtained after the reaction ends.

[0029] In some embodiments, the molar ratio of lactide to the metal in the catalyst is (200 - 10000):1.

[0030] In some embodiments, the reaction time of the ring-opening polymerization reaction is 10 min - 60 min.

[0031] In some embodiments, the molecular weight of the polylactic acid is 50000 Da - 300000 Da.

[0032] The core of the present invention lies in the principle of generating DES by the interaction between metal salts and alcohols. This principle is not limited to specific metals. Stannous salicylate is only a preferred embodiment, and other metal salts (such as zinc salicylate) are theoretically equally applicable, and the reaction conditions can be adjusted according to the electronic properties and coordination behaviors of metal ions to optimize the performance of DES.

[0033] Advantages of the present invention:

[0034] In this patent, by forming a deep eutectic solvent (DES) from a metal salicylate and a polyol, the morphology and solubility of the catalyst are effectively improved, and an efficient composite catalytic system is formed. At an extremely low catalyst dosage, this catalytic system can achieve rapid polymerization of lactide and prepare high-molecular-weight polylactic acid. This method is simple to operate, low in cost, and suitable for the industrial demand of efficiently preparing polylactic acid in a reactive extrusion process. Description of the Drawings

[0035] Figure 1 Infrared spectra of stannous salicylate, glycerol and the deep eutectic solvent formed by them;

[0036] Figure 21H NMR spectrum of the poly(lactic acid) synthesized by the prepared DES catalyst. Detailed implementation manners

[0037] To better illustrate the objectives, technical solutions, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0038] In the embodiments, unless otherwise specified, the test methods used are all conventional methods, and the materials, reagents, etc. used, unless otherwise specified, can all be obtained from commercial channels.

[0039] Example 1

[0040] S100. Preparation of stannous salicylate:

[0041] Dissolve 0.1 mol of sodium salicylate (16.01 g) in 500 mL of water to obtain an aqueous solution of sodium salicylate;

[0042] Dissolve 0.05 mol of stannous chloride dihydrate (11.28 g) in 50 mL of methanol to obtain an alcoholic solution of stannous chloride dihydrate;

[0043] Drop the aqueous solution of sodium salicylate into the alcoholic solution of stannous chloride dihydrate to obtain a white precipitate. Filter and wash it 3 times with ultrapure water, then filter and dry to obtain stannous salicylate. Weigh it to be 11.8 g, and calculate the yield to be 60%.

[0044] S200. Preparation of deep eutectic solvent catalyst:

[0045] Take the stannous salicylate prepared in step S100 and polyol (glycerol) and add them to a single-necked flask (the molar ratio of stannous salicylate to glycerol is 1:4). Heat to 180 °C and stir for 3 h to prepare a deep eutectic solvent catalyst.

[0046] S300. Use the prepared deep eutectic solvent catalyst to catalyze the ring-opening polymerization of lactide:

[0047] Take 0.2 mol (28.8 g) of lactide and put it into a 50 mL single-necked flask, and add the deep eutectic solvent catalyst prepared in step S200 to the single-necked flask. Among them, the monomer molar ratio of lactide to the catalyst (calculated by tin) is 2000:1. Replace nitrogen 3 times, and under normal pressure, heat to 180 °C and react for 20 min to obtain poly(lactic acid) with a weight-average molecular weight of 1.78×10 5 Da and a molecular weight distribution of 1.8. The product color is white and transparent.

[0048] The FTIR spectrum is as Figure 1 shown, and the 1 1H NMR spectrum of the synthesized poly(lactic acid) is as Figure 2 shown.

[0049] Note: In this embodiment, stannous salicylate and alcohol are used to form DES as an example; however, the technical solution of the present invention is equally applicable to other metal salts. According to the principle of the present invention, metal salts with reaction activities similar to stannous salicylate (such as zinc salicylate) can also be used to prepare DES, and the specific effects and reaction conditions can be adjusted according to the actual situation.

[0050] Example 2

[0051] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with ethylene glycol.

[0052] Example 3

[0053] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with 1,3-propanediol.

[0054] Example 4

[0055] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with 1,2-butanediol.

[0056] Example 5

[0057] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with 1,2-pentanediol.

[0058] Example 6

[0059] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with trihydroxyethane.

[0060] Example 7

[0061] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with xylitol.

[0062] Example 8

[0063] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with dulcitol.

[0064] Comparative Example 1

[0065] Other steps are the same as those in Example 1, except that in step S200, glycerol is replaced with 1,4-butanediol.

[0066] Comparative Example 2

[0067] The other steps are the same as those in the embodiment, except that, in Comparative Example 2, step S200 is not included, and the stannous salicylate and glycerol (the molar ratio of stannous salicylate to glycerol is 1:4) prepared in step S100 are directly used as catalysts for step S300, and the specific steps of step S300 are as follows:

[0068] Take 0.2 mol (28.8 g) of lactide and put it into a 50 mL single-necked flask, and add stannous salicylate and glycerol (the molar ratio of stannous salicylate to glycerol is 1:4) into the single-necked flask, wherein the molar ratio of lactide to catalyst (calculated as tin) monomer is 2000:1, replace nitrogen 3 times, heat to 180°C under normal pressure, react for 20 minutes, and obtain polylactic acid.

[0069] The catalytic performance tests of the catalysts prepared in Examples 1 to 8 and Comparative Examples 1 to 2 are shown in Table 1.

[0070] Table 1 Catalytic performance test table of different polyol preparation catalysts

[0071]

[0072] As shown in Table 1, in Examples 1-8, stannous salicylate can form DES with these polyols. These DES catalysts have good solubility in lactide. Within a few minutes of reaction time, the system quickly becomes viscous and catalyzes the polymerization of lactide. High molecular weight polylactic acid can be prepared after the reaction is carried out for 20 minutes.

[0073] In Comparative Example 1, stannous salicylate and 1,4-butanediol cannot form DES. When the mixture is added to lactide, stannous salicylate cannot be dispersed, the system is opaque, and the reaction rate is extremely low. When the reaction is carried out for 20 minutes, the reaction system still has fluidity.

[0074] In Comparative Example 2, glycerol and stannous salicylate were added separately, and no deep eutectic solvent (DES) was formed. The catalyst was not dissolved, and the catalytic effect was poor. The reaction phenomenon was the same as that in Comparative Example 1.

[0075] On the other hand, the present invention also explores the catalytic performance of the water deep eutectic solvent catalyst prepared under different molar ratios of stannous salicylate to glycerol:

[0076] On the basis of Example 1, the molar ratio of stannous salicylate to glycerol in step S200 was changed, and the test results are shown in Table 2.

[0077] Table 2 Catalytic performance test table of different molar ratios of glycerol and stannous salicylate

[0078] Ratio Reaction condition Weight-average molecular weight (Da) Molecular weight distribution Conversion rate 1:1 Unable to form DES — — — 1:1.3 Form DES 183300 1.8 96.8％ 1:1.5 Form DES 168200 1.6 97.50％ 1:1.8 Form DES 155100 1.9 98.20％ 1:2 Form DES 247100 1.68 96.80％ 1:3 Form DES 223200 1.76 98％ 1:4 Form DES 178000 1.9 95.60％ 1:5 Form DES 154000 1.82 96％ 1:10 Form DES 68200 1.93 96.5％ 1:20 Form DES 22100 1.82 97.2％

[0079] As can be seen from Table 2, the formation of DES with different ratios of stannous salicylate to glycerol was further explored. When the molar ratio of stannous salicylate to glycerol was 1:1, DES could not be formed. Increasing the amount of alcohol to a ratio of 1:1.3 enabled the formation of DES, and further increasing the amount of glycerol could form DES. After the formation of DES, the catalyst exhibited excellent catalytic activity, which was attributed to the improved dispersion solubility of the catalyst. However, as the amount of alcohol in DES increased, the molecular weight of the catalytically synthesized polylactic acid showed an overall decreasing trend.

[0080] In the third aspect of the present invention, the catalytic performance at different molar ratios of the catalyst (calculated as tin) to lactide monomer and the reaction conditions at different temperatures were also explored:

[0081] Based on Example 1, using the DES formed when the ratio of stannous salicylate to glycerol was 1:2 as the catalyst, the molar ratio of the DES catalyst (calculated as tin) to the lactide monomer was changed, and the test results are shown in Table 3.

[0082] Table 3 Catalytic performance test table of different dosages of the catalyst (calculated as tin) and the molar ratio of the lactide monomer

[0083] Ratio Weight-average molecular weight Molecular weight distribution Conversion rate Reaction time 1:10000 315200 1.69 0.98 60 min 1:4000 283300 1.98 0.978 60 min 1:2000 247100 1.68 0.968 40 min 1:1000 189100 1.75 0.99 20 min 1:500 115400 1.79 0.985 10 min 1:200 53900 2.02 0.982 10 min

[0084] As can be seen from Table 3, different dosages of the DES catalyst showed good dispersion solubility and catalytic activity in lactide. As the catalyst dosage increased, the reaction rate accelerated, but the molecular weight of the synthesized polylactic acid decreased. This was due to the increase in the content of polyol in the reaction system. The molecular weight of polylactic acid was affected by the molar ratio of lactide to alcohol, which also indicated that polyol not only acted as a dispersant for stannous salicylate but also played the role of an initiator.

[0085] Based on Example 1, using the DES formed when the ratio of stannous salicylate to glycerol was 1:2 as the catalyst, with the catalyst-to-monomer ratio of 1:2000, the reaction temperature was changed, and the test results are shown in Table 4.

[0086] Table 4: Catalytic performance test table of DES at different reaction temperatures

[0087]

[0088]

[0089] As can be seen from Table 4, at different reaction temperatures, DES can be rapidly dissolved in lactide and exhibits excellent catalytic activity. When the lactide conversion reaches about 95%, high molecular weight polylactic acid is obtained. At different reaction temperatures, the final molecular weight of polylactic acid varies little. The molecular weight is generally affected by the molar ratio of lactide monomer to initiator glycerol, but the polymerization rate of lactide increases with the increase of temperature.

[0090] It should be noted that the specific parameters or some reagents in the above embodiments are specific embodiments or preferred embodiments under the concept of the present invention, rather than limitations thereto; those skilled in the art can make adaptive adjustments within the concept and protection scope of the present invention.

Claims

1. A method for preparing a deep eutectic solvent catalyst, characterized in that: The preparation method of the deep eutectic solvent catalyst comprises the steps of: The metal salicylate and the polyol are mixed, and heated and stirred until the system becomes clear and transparent, thereby obtaining a low eutectic solvent catalyst; The polyol includes at least one of glycerol, ethylene glycol, 1,3-propylene glycol, 1,2-butylene glycol, 1,2-pentanediol, glycerol, trihydroxyethane, xylitol and sweet alcohol.

2. The method for preparing a deep eutectic solvent catalyst according to claim 1, characterized in that: The metal salicylate includes stannous salicylate; The structural formula of the stannous salicylate is as follows:

3. The method for preparing a deep eutectic solvent catalyst according to claim 1, characterized in that: The metal salicylate includes zinc salicylate.

4. The method for preparing a deep eutectic solvent catalyst according to claim 1, characterized in that: The molar ratio of stannous salicylate to polyol is 1:(1.3-20).

5. The method for preparing a deep eutectic solvent catalyst according to claim 1, characterized in that: The reaction temperature is 120°C to 200°C.

6. Use of a deep eutectic solvent catalyst prepared by the method for preparing a deep eutectic solvent catalyst according to any one of claims 1 to 5 in synthesizing polylactic acid.

7. Use of the deep eutectic solvent catalyst according to claim 6 in the synthesis of polylactic acid, characterized in that: The specific steps of synthesizing polylactic acid include: A low eutectic solvent catalyst is added to lactide to cause a ring-opening polymerization reaction. The temperature is raised to 140°C to 180°C. Polylactic acid is obtained when the reaction is completed.

8. Use of the deep eutectic solvent catalyst according to claim 7 in the synthesis of polylactic acid, characterized in that: The molar ratio of lactide to the metal in the catalyst is (200-10000):

1.

9. Use of the deep eutectic solvent catalyst according to claim 7 in the synthesis of polylactic acid, characterized in that: The reaction time of the ring-opening polymerization is 10 min to 60 min.

10. Use of the deep eutectic solvent catalyst according to claim 7 in the synthesis of polylactic acid, characterized in that: The molecular weight of the polylactic acid is 50000Da to 300000Da.

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