Preparation method and application of a deep eutectic solvent catalyst

Abstract

The application relates to a preparation method and application of a eutectic solvent catalyst, and the patent effectively improves the form and solubility of the catalyst by forming a eutectic solvent (DES) from stannous salicylate and a polyhydric alcohol, and forms a high-efficiency composite catalyst system. Under the condition of extremely low catalyst consumption, the catalyst system can realize the rapid polymerization of propiolactone, and high-molecular-weight polylactic acid is prepared. The method is simple in operation and low in cost, and is suitable for the industrial demand of efficiently preparing polylactic acid in a reaction extrusion process.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a method for preparing and applying a eutectic solvent catalyst. Background Technology

[0002] With the increasing severity of pollution problems caused by non-degradable petroleum-based plastics, biodegradable polyester materials have gradually attracted widespread attention. Polylactic acid (PLA), as a biodegradable material derived from bio-based materials, has been widely used in packaging, textiles, agricultural mulch films, and drug delivery systems due to its excellent mechanical properties, processability, and bioabsorption, making it one of the most promising alternative materials.

[0003] Currently, the industrial production of polylactic acid (PLA) is mainly achieved through the ring-opening polymerization of lactide to obtain high molecular weight, high-quality PLA. The reaction equipment includes batch reactors, plug flow reactors, tubular circulating reactors, reactive distillation columns, and reactive extruders. Among these, reactive extrusion, due to the strong shear mixing action of the extruder screw, possesses highly efficient mass and heat transfer characteristics, greatly increasing the lactide polymerization rate and allowing the polymerization process to be completed within minutes. However, the production efficiency of reactive extrusion is limited by the residence time of the material within the extruder, and its efficient operation relies on a highly efficient catalytic system.

[0004] Tin-based catalysts are widely used in the ring-opening polymerization of lactide due to their excellent catalytic activity. Among them, stannous octoate, as a liquid catalyst, is easily dispersed and dissolved in lactide and exhibits high catalytic efficiency. Currently, many patents have reported catalytic systems based on stannous octoate. For example, patent US5378801 describes the extrusion preparation of polylactic acid using stannous octoate as a catalyst; patent US6166169 describes the synthesis of polylactic acid with a weight average molecular weight of 1.6 × 10⁻⁶ after a 7-minute residence time in an extruder using a composite catalytic system of stannous octoate and triphenylphosphine. 5 Da's polylactic acid has a production efficiency of only 0.7 kg / h, which has room for improvement.

[0005] Improving the catalytic efficiency of the active site of the catalyst is key to further enhancing the efficiency of reactive extrusion production. Adjusting the ligand structure of the metal center can alter its acidic environment, thereby affecting catalytic performance. Salicylic acid, as an inexpensive and readily available natural hydroxy acid, can enhance the acidity of the tin(II) center due to the strong electron-withdrawing effect of its hydroxyl group, while also helping to stabilize the reaction intermediate. However, after coordination of salicylic acid with tin(II), a stable crystal structure is formed, resulting in stannous salicylate in the form of a solid powder, which is difficult to disperse and dissolve in lactide, exhibiting low catalytic activity. Summary of the Invention

[0006] Therefore, it is necessary to provide a method for preparing a eutectic solvent catalyst and its application, aiming to solve the problems existing in the prior art.

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

[0008] A method for preparing a eutectic solvent catalyst, the method comprising the steps of:

[0009] Stannous salicylate is mixed with a polyol and heated and stirred until the system is clear and transparent to obtain a eutectic solvent catalyst.

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

[0011] The polyols include at least one of glycerol, ethylene glycol, 1,3-propylene glycol, 1,2-butanediol, 1,2-pentanediol, glycerol, trihydroxyethane, xylitol, and sweet alcohol.

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

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

[0014]

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

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

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

[0018] In some embodiments, the preparation steps of the 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 of these embodiments, the concentration of the alcoholic solution of the 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. Add the alcoholic solution of the stannous salt to the aqueous solution of the sodium salicylate, and a white precipitate is generated. Filter and dry the white precipitate to obtain the stannous salicylate.

[0025] The stannous salicylate catalyst was constructed because the presence of hydroxyl groups in the salicylic acid ligand promotes the acidity of the tin center, thus the catalyst has greater potential to exhibit high catalytic activity.

[0026] This invention also provides the application of a eutectic solvent catalyst in the synthesis of polylactic acid.

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

[0028] A eutectic solvent catalyst is added to lactide to induce a ring-opening polymerization reaction. The temperature is raised to 140℃~180℃, and polylactic acid is obtained after the reaction is completed.

[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 to 60 min.

[0031] In some embodiments, the polylactic acid has a molecular weight of 50,000 Da to 300,000 Da.

[0032] The core of this invention lies in the principle of generating DES by utilizing the interaction between metal salts and alcohols. This principle is not limited to a specific metal. Stannous salicylate is only one preferred embodiment; 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 behavior of the metal ions to optimize the performance of the DES.

[0033] The beneficial effects of this invention are:

[0034] This patent effectively improves the morphology and solubility of the catalyst by forming a eutectic solvent (DES) with salicylic acid metal salt and polyol, thus creating a highly efficient composite catalytic system. This system enables the rapid polymerization of lactide to produce high molecular weight polylactic acid (PLA) with extremely low catalyst dosage. This method is simple to operate, low in cost, and suitable for the industrial demand for efficient PLA preparation in reactive extrusion processes. Attached Figure Description

[0035] Figure 1 Infrared spectra of stannous salicylate, glycerol, and their eutectic solvents;

[0036] Figure 2The 1H NMR spectrum of the prepared DES-catalyzed polylactic acid synthesis. Detailed Implementation

[0037] To better illustrate the purpose, technical solution, 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 experimental methods used are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0039] Example 1

[0040] S100, Preparation of stannous salicylate:

[0041] Dissolve 0.1 mol 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] An aqueous solution of sodium salicylate was added dropwise to an alcoholic solution of stannous chloride dihydrate, resulting in a white precipitate. The precipitate was filtered and washed three times with ultrapure water. After filtration and drying, stannous salicylate was obtained, weighing 11.8 g. The yield was calculated to be 60%.

[0044] S200, Preparation of eutectic solvent catalysts:

[0045] The stannous salicylate and polyol (glycerol) prepared in step S100 were added to a single-necked flask (the molar ratio of stannous salicylate to glycerol was 1:4), heated to 180°C and stirred for 3 hours to prepare a eutectic solvent catalyst.

[0046] S300, The prepared eutectic solvent catalyst is used to catalyze the ring-opening polymerization of lactide:

[0047] 0.2 mol (28.8 g) of lactide was placed in a 50 mL single-necked flask, and the eutectic solvent catalyst prepared in step S200 was added to the flask. The molar ratio of lactide to catalyst (calculated as tin) monomer was 2000:1. Nitrogen gas was purged three times, and the temperature was raised to 180 °C under normal pressure. The reaction was carried out for 20 min to obtain a product with a weight-average molecular weight of 1.78 × 10⁻⁶. 5 Da, a polylactic acid with a molecular weight distribution of 1.8. The product is white and transparent.

[0048] FTIR spectra as follows Figure 1 As shown, the synthesized polylactic acid 1 HNMR spectra as follows Figure 2 As shown.

[0049] Note: In this embodiment, stannous salicylate reacting with alcohol to form DES is used as an example; however, the technical solution of the present invention is also applicable to other metal salts. Based on the principle of the present invention, metal salts with similar reactivity to stannous salicylate (e.g., 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] The other steps are the same as in Example 1, except that in step S200, glycerol is replaced with ethylene glycol.

[0052] Example 3

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

[0054] Example 4

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

[0056] Example 5

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

[0058] Example 6

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

[0060] Example 7

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

[0062] Example 8

[0063] The other steps are the same as in Example 1, except that in step S200, glycerol is replaced with sweet alcohol.

[0064] Comparative Example 1

[0065] The other steps are the same as 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 in the examples, except that Comparative Example 2 does not include step S200. Instead, the stannous salicylate and glycerol (the molar ratio of stannous salicylate to glycerol is 1:4) obtained in step S100 are used as catalysts in step S300. 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. Add stannous salicylate and glycerol (the molar ratio of stannous salicylate to glycerol is 1:4) to the single-necked flask. The molar ratio of lactide to catalyst (calculated as tin) monomer is 2000:1. Replace the nitrogen gas three times, raise the temperature to 180 °C under normal pressure, and react for 20 min to obtain polylactic acid.

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

[0070] Table 1 Catalytic performance test results of catalysts prepared from different polyols

[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, the system quickly becomes viscous, catalyzing the polymerization of lactide. High molecular weight polylactic acid can be prepared after the reaction lasts for 20 minutes.

[0073] In Comparative Example 1, stannous salicylate and 1,4-butanediol could not form DES. When this mixture was added to lactide, stannous salicylate could not be dispersed, the system became opaque, and the reaction rate was extremely low. Even after 20 minutes of reaction, the system remained fluid.

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

[0075] In another aspect, this invention also investigated the catalytic performance of water eutectic solvent catalysts prepared under different molar ratios of stannous salicylate to glycerol:

[0076] Based on 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 results for different molar ratios of glycerol and stannous salicylate.

[0078] Proportion Reaction status Weight-average molecular weight (Da) Molecular weight distribution Conversion rate 1:1 DES cannot be formed — — — 1:1.3 Forming DES 183300 1.8 96.8％ 1:1.5 Forming DES 168200 1.6 97.50％ 1:1.8 Forming DES 155100 1.9 98.20％ 1:2 Forming DES 247100 1.68 96.80％ 1:3 Forming DES 223200 1.76 98％ 1:4 Forming DES 178000 1.9 95.60％ 1:5 Forming DES 154000 1.82 96％ 1:10 Forming DES 68200 1.93 96.5％ 1:20 Forming DES 22100 1.82 97.2％

[0079] Table 2 shows that the formation of DES by stannous salicylate and glycerol in different ratios was further investigated. 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 allowed DES to form, and further increasing the amount of glycerol also resulted in DES formation. After DES formation, the catalyst exhibited excellent catalytic activity, which is attributed to the improved dispersion and solubility of the catalyst. However, increasing the amount of alcohol in the DES resulted in an overall decreasing trend in the molecular weight of polylactic acid synthesized.

[0080] The third aspect of this invention also investigates the catalytic performance of different catalyst (calculated as tin) molar ratios to lactide monomers and the reaction conditions at different temperatures:

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

[0082] Table 3. Catalytic performance test results of different catalyst (calculated as tin) dosages relative to the molar ratio of lactide monomers.

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

[0084] As shown in Table 3, different amounts of DES catalyst exhibit good dispersibility, solubility, and catalytic activity in lactide. With the increase of catalyst dosage, the reaction rate increases, but the molecular weight of synthesized polylactic acid decreases. This is due to the increase in the content of polyol in the reaction system. The molecular weight of polylactic acid is influenced by the molar ratio of lactide to alcohol, which also indicates that polyol not only acts as a dispersant for stannous salicylate but also plays the role of an initiator.

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

[0086] Table 4: Test results of DES catalytic performance at different reaction temperatures

[0087]

[0088]

[0089] As shown in Table 4, DES can dissolve rapidly in lactide at different reaction temperatures and exhibits excellent catalytic activity. High molecular weight polylactic acid (PLA) is obtained when the lactide conversion reaches approximately 95%. The final PLA molecular weight does not differ significantly at different reaction temperatures, and is largely influenced by the molar ratio of lactide monomer to initiator glycerol. However, the polymerization rate of lactide increases with increasing temperature.

[0090] It should be noted that the specific parameters or reagents in the above embodiments are specific or preferred embodiments under the concept of the present invention, and not limitations thereof; 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 by, The preparation method of the eutectic solvent catalyst comprises the following steps: mixing the metal salicylate with the polyhydric alcohol, and heating and stirring until the system is clear and transparent to obtain the eutectic solvent catalyst; the polyhydric alcohol comprises at least one of ethylene glycol, 1,3-propanediol, 1,2-butanediol, 1,2-pentanediol, glycerol, xylitol and sweet alcohol; the metal salicylate is stannous salicylate; the structure of the stannous salicylate is as follows: ； the molar ratio of the metal salicylate to the polyhydric alcohol is 1: (1.3-20); the reaction temperature is 120-200 DEG C.

2. The application of the eutectic solvent catalyst prepared by the preparation method of claim 1 in the synthesis of polylactic acid.

3. Use of the deep eutectic solvent catalyst according to claim 2 in the synthesis of polylactic acid, characterized in that, The specific steps of the synthesis of polylactic acid comprise: adding the eutectic solvent catalyst into lactide to occur ring-opening polymerization, heating to 140-180 DEG C, and obtaining polylactic acid after the reaction is completed.

4. Use of the deep eutectic solvent catalyst according to claim 3 in the synthesis of polylactic acid, characterized in that, the molar ratio of lactide to the metal in the catalyst is (200-10000):

1.

5. Use of the deep eutectic solvent catalyst according to claim 3 in the synthesis of polylactic acid, characterized in that, the reaction time of the ring-opening polymerization is 10-60 min.

6. Use of the deep eutectic solvent catalyst according to claim 3 in the synthesis of polylactic acid, characterized in that, the molecular weight of the polylactic acid is 50000-300000 Da.

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