Method for catalytically synthesizing lactide by using kaolin-based catalyst

Abstract

The invention discloses a method for catalytically synthesizing lactide by using a kaolin-based catalyst, which comprises the following steps: (1) preparing oligomeric lactic acid: carrying out lactic acid dehydration polymerization reaction by using L-lactic acid or D-lactic acid or DL-lactic acid as a raw material to obtain oligomeric lactic acid of which the weight-average molecular mass of an oligomer is 1000-3000Da; and (2) depolymerizing the oligomeric lactic acid: carrying out depolymerization reaction on the oligomeric lactic acid obtained in the step (1) under the action of a kaolin-based catalyst to obtain crude lactide. The raw materials of the catalyst are easy to obtain, the cost is low, the regeneration process is simple, the reaction temperature of depolymerization cyclization is low, the reaction time is short, and the cost can be greatly reduced in industrial production.

Description

Technical Field

[0001] The present invention relates to the technical field of deep processing of lactic acid, and more specifically to a method for synthesizing lactide by catalysis of kaolin-based catalyst. Background Art

[0002] There are two methods for producing polylactic acid in the prior art, namely, a one-step method and a two-step method. Since the one-step method directly generates polylactic acid from lactic acid, the viscosity of the material is large in the later stage during the industrial production process, and the water in the reaction system is not easy to remove, and the molecular weight of the polylactic acid generally produced is low. The two-step method, also known as the ring-opening polymerization method, first generates lactide and then catalyzes the ring-opening polymerization of lactide to generate polylactic acid. This method can obtain polylactic acid with a high relative molecular weight and is currently the most important process route for industrial production.

[0003] However, lactide production is a bottleneck in the production of high molecular weight polylactic acid, which usually requires two steps. First, lactic acid is dehydrated and polymerized to generate oligomeric lactic acid, and then the oligomeric lactic acid is depolymerized and cyclized under the action of a catalyst to obtain lactide. Currently, the commonly used catalysts for the depolymerization and cyclization of oligomeric lactic acid are stannous octoate and stannous chloride, which have the disadvantages of being expensive, difficult to regenerate, and having a high reaction temperature. Summary of the invention

[0004] Therefore, the technical problem to be solved by the present invention is to provide a method for catalyzing the synthesis of lactide with a kaolin-based catalyst having readily available catalyst raw materials, low cost and simple regeneration process.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] A method for synthesizing lactide using a kaolin-based catalyst comprises the following steps:

[0007] (1) preparing oligomeric lactic acid: using L-lactic acid, D-lactic acid or DL-lactic acid as raw material, carrying out lactic acid dehydration polymerization reaction to obtain oligomeric lactic acid having an oligomer weight average molecular weight of 1000-3000Da;

[0008] (2) Depolymerization of oligomeric lactic acid: The oligomeric lactic acid obtained in step (1) is subjected to a depolymerization reaction under the action of a kaolin-based catalyst to obtain crude lactide.

[0009] In the above-mentioned method for synthesizing lactide catalyzed by a kaolin-based catalyst, the temperature of the lactic acid dehydration polymerization reaction in step (1) is 140-160° C., the dehydration polymerization reaction is first carried out at an operating pressure of 10-40 kPa for 2-4 hours, and then at an operating pressure of 5-10 kPa for 1-2 hours.

[0010] In the above method for synthesizing lactide using a kaolin-based catalyst, the temperature of the depolymerization reaction in step (2) is 180-190° C. and the operating pressure is 0.1-1 kPa.

[0011] In the above method for synthesizing lactide using a kaolin-based catalyst, the amount of the kaolin-based catalyst used in step (2) is 1-2‰ of the mass of the oligolactic acid.

[0012] In the above method for synthesizing lactide catalyzed by a kaolin-based catalyst, the preparation method of the kaolin-based catalyst in step (2) is as follows:

[0013] (2-1) Kaolin and H 2 SO 4 The solutions are mixed, heated and stirred for activation, left to stand and cool, and then the acid solution is removed by vacuum filtration, and the filter cake is washed with distilled water until the filtrate is close to neutral, and then dried and calcined to obtain pretreated kaolin;

[0014] (2-2) respectively preparing a solution of a zinc precursor and a tin precursor with a solvent, then adding the pretreated kaolin, stirring and mixing evenly, adding ammonia water to adjust the pH under vigorous stirring, continuing stirring, and placing for aging; filtering and washing until the filtrate is close to neutral, drying and dehydrating the filter cake to obtain kaolin loaded with zinc oxide and tin oxide;

[0015] (2-3) Add kaolin loaded with zinc oxide and tin oxide to H 2 SO 4 The solution was stirred and immersed, then filtered, the filter cake was dried, and then calcined under nitrogen atmosphere;

[0016] (2-4) The product obtained in step (2-3) is immersed in a chitosan suspension, heated and stirred, and filtered and dried to obtain a kaolin-based composite catalyst.

[0017] The method for synthesizing lactide catalyzed by kaolin-based catalyst comprises the following steps: in step (2-1): kaolin and 10% by mass H 2 SO 4 The solution is mixed according to a solid-liquid ratio of 1: (10-15) g / mL, activated by stirring at a constant temperature of 85-95°C for 4-6 hours, and after standing to cool, the acid solution is removed by vacuum filtration, the filter cake is washed with distilled water until the filtrate is close to neutral, and then placed at 110-140°C for drying for 6-12 hours, and finally calcined at 300-500°C for 3-5 hours to obtain pretreated kaolin.

[0018] The method for synthesizing lactide catalyzed by a kaolin-based catalyst comprises the following steps: in step (2-2): a zinc precursor and a tin precursor are prepared into solutions with a solvent respectively, and then pretreated kaolin is added, stirred for 1-2 hours, and then ammonia water is added under vigorous stirring to adjust the pH to 9-10, and stirring is continued for 2-5 hours, and the mixture is allowed to age for 12-24 hours; after suction filtration and washing until the filtrate is close to neutral, the filter cake is dried at 110-120°C.

[0019] The method for synthesizing lactide catalyzed by the kaolin-based catalyst comprises the following steps: taking the dried filter cake, adding 1-2 mol / L H 2 SO 4 The solution was stirred and immersed for 3-5 hours and then filtered. The filter cake was dried at 110°C for 3 hours and then calcined at 500°C for 3-5 hours under nitrogen atmosphere.

[0020] The method for synthesizing lactide catalyzed by a kaolin-based catalyst comprises the following steps: in step (2-4): immersing the product obtained in step (2-3) in a chitosan suspension having a pH of 4-6 and a mass fraction of 10-20%, heating and stirring at 50-80° C. for 0.5-2 h, and filtering and drying to obtain a kaolin-based composite catalyst.

[0021] In the above method for synthesizing lactide using a kaolin-based catalyst, in step (2-2): the zinc precursor and the tin precursor are sulfates, nitrates, acetates or chlorides, preferably the zinc precursor is zinc acetate and the tin precursor is tin tetrachloride.

[0022] The technical solution of the present invention achieves the following beneficial technical effects:

[0023] 1. Modify kaolin with sulfuric acid to leach aluminum from kaolin, which is beneficial for zinc ions and tin ions to enter the pores of kaolin in subsequent reactions and greatly increase the roughness of the kaolin surface. In subsequent reactions, more zinc hydroxide, tin hydroxide and chitosan can be adsorbed on the surface for dense accumulation. Calcination can further remove adsorbed water in kaolin.

[0024] 2. The addition of ammonia water reacts with the zinc ions and tin ions in the solution to generate zinc hydroxide and tin hydroxide. On the other hand, it uses alkali to corrode the kaolin structure, which is conducive to more zinc ions and tin ions entering the pores of kaolin to occupy space.

[0025] 3. After impregnating kaolin loaded with zinc oxide and tin oxide with sulfuric acid, drying and calcining, a catalyst with high catalytic activity and reusable properties can be obtained.

[0026] 4. In a solution with a pH of 4-6, chitosan will be hydrolyzed to generate low molecular weight chitosan, which is more conducive to entering the pores of kaolin and combining with the transition metal ions exposed on the inner wall of the pores through the amino groups on it, so that the hydroxyl groups on it can be esterified with oligomeric lactic acid during the depolymerization of oligomeric lactic acid catalyzed by kaolin-based catalysts to achieve end group chemical closure, thereby preventing oligomeric lactic acid from self-polymerizing during the depolymerization process to form higher molecular weight oligomeric lactic acid, thereby improving the yield of lactide and the purity of crude lactide, and at the same time reducing the viscosity of the depolymerization reaction mixture to prevent carbonization and coking during the depolymerization reaction.

[0027] 5. The depolymerization and cyclization reaction temperature of the method of the present invention is low and the reaction time is short, which can greatly reduce the cost in industrial production; after the catalyst is deactivated, it can be regenerated according to the catalyst preparation method of the present invention. DETAILED DESCRIPTION

[0028] Example 1

[0029] The method for synthesizing lactide catalyzed by a kaolin-based catalyst in this embodiment comprises the following steps:

[0030] (1) Preparation of oligomeric lactic acid: Using L-lactic acid, D-lactic acid or DL-lactic acid as raw materials, a lactic acid dehydration polymerization reaction is carried out to obtain oligomeric lactic acid with an oligomer weight average molecular weight of 1000-3000Da; the temperature of the lactic acid dehydration polymerization reaction is 140°C, and the dehydration polymerization reaction is first carried out at an operating pressure of 10-40 kPa for 4 hours, and then at an operating pressure of 5-10 kPa for 1 hour.

[0031] (2) Depolymerization of oligomeric lactic acid: A kaolin-based catalyst is used as a catalyst, and the amount of kaolin-based catalyst used is 1‰ of the mass of the oligomeric lactic acid; the depolymerization and cyclization of the oligomeric lactic acid is catalyzed at a reaction temperature of 180°C and an operating pressure of 0.1-0.5 kPa. After the reaction is continued for 1 hour, the crude lactide collected is tested and the lactide content is 94%, the meso-lactide content is 1.8%, and the lactide yield is 86%.

[0032] The preparation method of the kaolin-based catalyst is as follows:

[0033] (2-1) Kaolin and 10% by mass H 2 SO 4 The solution was mixed at a solid-liquid ratio of 1:10 g / mL, activated at 85°C for 4 h with constant stirring, and then cooled by vacuum filtration to remove the acid solution. The filter cake was washed with distilled water until the filtrate was close to neutral, and then dried at 110°C for 6 h, and finally calcined at 300°C for 3 h to obtain pretreated kaolin.

[0034] (2-2) zinc acetate and tin tetrachloride are prepared into solutions with solvents respectively, and then pretreated kaolin is added, stirred for 1 hour, and then ammonia water is added under vigorous stirring to adjust the pH to 9, and the stirring is continued for 2 hours, and the mixture is aged for 12 hours; after suction filtration and washing until the filtrate is close to neutral, the filter cake is dried and dehydrated at 110° C. to obtain kaolin loaded with zinc oxide and tin oxide;

[0035] (2-3) Take kaolin loaded with zinc oxide and tin oxide, add 1 mol / L H 2 SO 4 The solution was stirred and immersed for 3 h and then filtered. The filter cake was dried at 110 °C for 3 h and then calcined at 500 °C for 3 h under nitrogen atmosphere.

[0036] (2-4) The product obtained in step (2-3) is immersed in a chitosan suspension having a pH value of 4 and a mass fraction of 10%, and heated and stirred at 50° C. for 0.5 h. After filtering and drying, a kaolin-based composite catalyst is obtained.

[0037] Example 2

[0038] The method for synthesizing lactide catalyzed by a kaolin-based catalyst in this embodiment comprises the following steps:

[0039] (1) Preparation of oligomeric lactic acid: Using L-lactic acid, D-lactic acid or DL-lactic acid as raw materials, a lactic acid dehydration polymerization reaction is carried out to obtain oligomeric lactic acid with an oligomer weight average molecular weight of 1000-3000Da; the temperature of the lactic acid dehydration polymerization reaction is 160°C, and the dehydration polymerization reaction is first carried out at an operating pressure of 10-40 kPa for 2 hours, and then at an operating pressure of 5-10 kPa for 2 hours.

[0040] (2) Depolymerization of oligomeric lactic acid: A kaolin-based catalyst was used as a catalyst. The amount of kaolin-based catalyst was 2‰ of the mass of oligomeric lactic acid to catalyze the depolymerization and cyclization of oligomeric lactic acid. The reaction temperature was 190°C and the operating pressure was 0.5-1 kPa. After 1 hour of reaction, the crude lactide collected was tested and the lactide content was 97%, the meso-lactide content was 1%, and the lactide yield was 90%.

[0041] The kaolin-based catalyst is used as a catalyst to catalyze the depolymerization and cyclization of oligomeric lactic acid to obtain crude lactide. The temperature of the depolymerization and cyclization reaction is 190°C and the operating pressure is 0.5-1kPa. The amount of the kaolin-based catalyst is 2‰ of the mass of the oligomeric lactic acid.

[0042] The preparation method of the kaolin-based catalyst is as follows:

[0043] (2-1) Kaolin and 10% by mass H 2 SO 4The solution was mixed at a solid-liquid ratio of 1:15 g / mL, activated at 95°C for 6 h with constant stirring, and then cooled by vacuum filtration to remove the acid solution. The filter cake was washed with distilled water until the filtrate was close to neutral, and then dried at 140°C for 12 h, and finally calcined at 500°C for 5 h to obtain pretreated kaolin.

[0044] (2-2) respectively preparing a solution of zinc acetate and tin tetrachloride with a solvent, then adding the pretreated kaolin, stirring for 2 hours, adding ammonia water to adjust the pH to 10 under vigorous stirring, continuing stirring for 5 hours, and aging for 24 hours; filtering and washing until the filtrate is close to neutral, drying and dehydrating the filter cake at 120° C. to obtain kaolin loaded with zinc oxide and tin oxide;

[0045] (2-3) Take kaolin loaded with zinc oxide and tin oxide, add 2 mol / L H 2 SO 4 The solution was stirred and immersed for 5 h and then filtered. The filter cake was dried at 110 °C for 3 h and then calcined at 500 °C for 5 h under nitrogen atmosphere.

[0046] (2-4) The product obtained in step (2-3) is immersed in a chitosan suspension having a pH value of 6 and a mass fraction of 20%, and heated and stirred at 80° C. for 2 h. After filtering and drying, a kaolin-based composite catalyst is obtained.

[0047] Example 3

[0048] The method for synthesizing lactide catalyzed by a kaolin-based catalyst in this embodiment comprises the following steps:

[0049] (1) Preparation of oligomeric lactic acid: Using L-lactic acid, D-lactic acid or DL-lactic acid as raw materials, a lactic acid dehydration polymerization reaction is carried out to obtain oligomeric lactic acid with an oligomer weight average molecular weight of 1000-3000Da; the temperature of the lactic acid dehydration polymerization reaction is 150°C, and the dehydration polymerization reaction is first carried out at an operating pressure of 10-40 kPa for 3 hours, and then at an operating pressure of 5-10 kPa for 1 hour.

[0050] (2) Depolymerization of oligomeric lactic acid: A kaolin-based catalyst was used as a catalyst, and the amount of kaolin-based catalyst used was 1.5‰ of the mass of the oligomeric lactic acid; the depolymerization and cyclization of the oligomeric lactic acid was catalyzed at a reaction temperature of 185°C and an operating pressure of 0.3-0.8 kPa. After the reaction was continued for 1 hour, the crude lactide collected was tested and the lactide content was 98%, the meso-lactide content was 0.5%, and the lactide yield was 91%.

[0051] The preparation method of the kaolin-based catalyst is as follows:

[0052] (2-1) Kaolin and 10% by mass H2 SO 4 The solution was mixed at a solid-liquid ratio of 1:12 g / mL, activated at 90°C for 5 h with constant stirring, and then cooled by vacuum filtration to remove the acid solution. The filter cake was washed with distilled water until the filtrate was close to neutral, and then dried at 120°C for 8 h, and finally calcined at 400°C for 4 h to obtain pretreated kaolin.

[0053] (2-2) respectively preparing a solution of zinc acetate and tin tetrachloride with a solvent, then adding the pretreated kaolin, stirring for 2 hours, adding ammonia water to adjust the pH to 10 under vigorous stirring, continuing stirring for 3 hours, and aging for 16 hours; filtering and washing until the filtrate is close to neutral, and drying and dehydrating the filter cake at 115° C. to obtain kaolin loaded with zinc oxide and tin oxide;

[0054] (2-3) Take kaolin loaded with zinc oxide and tin oxide, add 1.5 mol / L H 2 SO 4 The solution was stirred and immersed for 5 h and then filtered. The filter cake was dried at 110 °C for 3 h and then calcined at 500 °C for 4 h under nitrogen atmosphere.

[0055] (2-4) The product obtained in step (2-3) is immersed in a chitosan suspension having a pH value of 5 and a mass fraction of 15%, and heated and stirred at 70° C. for 1 h. After filtering and drying, a kaolin-based composite catalyst is obtained.

[0056] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for synthesizing lactide using a kaolin-based catalyst, characterized in that: The steps include: (1) preparing oligomeric lactic acid: using L-lactic acid, D-lactic acid or DL-lactic acid as raw material, carrying out lactic acid dehydration polymerization reaction to obtain oligomeric lactic acid having an oligomer weight average molecular weight of 1000-3000Da; (2) Depolymerization of oligomeric lactic acid: The oligomeric lactic acid obtained in step (1) is subjected to a depolymerization reaction under the action of a kaolin-based catalyst to obtain crude lactide.

2. The method for synthesizing lactide using a kaolin-based catalyst according to claim 1, characterized in that: The temperature of the lactic acid dehydration polymerization reaction in step (1) is 140-160° C. The dehydration polymerization reaction is first carried out at an operating pressure of 10-40 kPa for 2-4 hours, and then at an operating pressure of 5-10 kPa for 1-2 hours.

3. The method for synthesizing lactide using a kaolin-based catalyst according to claim 1, characterized in that: The temperature of the depolymerization reaction in step (2) is 180-190° C. and the operating pressure is 0.1-1 kPa.

4. The method for synthesizing lactide using a kaolin-based catalyst according to claim 1, characterized in that: The amount of kaolin-based catalyst used in step (2) is 1-2‰ of the mass of the oligomeric lactic acid.

5. A method for synthesizing lactide catalyzed by a kaolin-based catalyst according to any one of claims 1 to 4, characterized in that: The preparation method of the kaolin-based catalyst in step (2) is as follows: (2-1) Mixing kaolin with H2SO4 solution, heating and stirring to activate, standing and cooling, removing the acid solution by vacuum filtration, washing the filter cake with distilled water until the filtrate is close to neutral, and then drying and calcining to obtain pretreated kaolin; (2-2) respectively preparing a solution of a zinc precursor and a tin precursor with a solvent, then adding the pretreated kaolin, stirring and mixing evenly, adding ammonia water to adjust the pH under vigorous stirring, continuing to stir, and placing for aging; After filtering and washing until the filtrate is close to neutral, the filter cake is dried and dehydrated to obtain kaolin loaded with zinc oxide and tin oxide; (2-3) adding kaolin loaded with zinc oxide and tin oxide into a H2SO4 solution, stirring and impregnating, filtering, drying the filter cake, and then calcining under a nitrogen atmosphere; (2-4) The product obtained in step (2-3) is immersed in a chitosan suspension, heated and stirred, and filtered and dried to obtain a kaolin-based composite catalyst.

6. The method for synthesizing lactide using a kaolin-based catalyst according to claim 5, characterized in that: In step (2-1): kaolin is mixed with a 10% H2SO4 solution at a solid-liquid ratio of 1: (10-15) g / mL, and activated by stirring at a constant temperature of 85-95°C for 4-6 hours. After standing and cooling, the acid solution is removed by vacuum filtration, and the filter cake is washed with distilled water until the filtrate is close to neutral, and then dried at 110-140°C for 6-12 hours, and finally calcined at 300-500°C for 3-5 hours to obtain pretreated kaolin.

7. The method for synthesizing lactide using a kaolin-based catalyst according to claim 5, characterized in that: In step (2-2): respectively prepare a solution of a zinc precursor and a tin precursor with a solvent, then add the pretreated kaolin, stir for 1-2 hours, add ammonia water to adjust the pH to 9-10 under vigorous stirring, continue stirring for 2-5 hours, and age for 12-24 hours; filter and wash until the filtrate is close to neutral, and dry the filter cake at 110-120°C.

8. The method for synthesizing lactide using a kaolin-based catalyst according to claim 5, characterized in that: In step (2-3): take the dried filter cake, add 1-2 mol / L H2SO4 solution according to the solid-liquid ratio of 1: (15-20) g / mL, stir and soak for 3-5 hours, then filter, dry the filter cake at 110°C for 3 hours, and then calcine at 500°C for 3-5 hours under nitrogen atmosphere.

9. The method for synthesizing lactide using a kaolin-based catalyst according to claim 5, characterized in that: In step (2-4): the product obtained in step (2-3) is immersed in a chitosan suspension having a pH of 4-6 and a mass fraction of 10-20%, and heated and stirred at 50-80°C for 0.5-2h, and then filtered and dried to obtain a kaolin-based composite catalyst.

10. The method for synthesizing lactide using a kaolin-based catalyst according to claim 5, characterized in that: In step (2-2): the zinc precursor and the tin precursor are sulfate, nitrate, acetate or chloride, preferably the zinc precursor is zinc acetate and the tin precursor is tin tetrachloride.