Preparation method for high-purity lactide and use of high-purity lactide in polylactic acid synthesis

Abstract

Disclosed in the present invention are a preparation method for high-purity lactide and the use of the high-purity lactide in polylactic acid synthesis. The preparation method comprises the following steps: (1) subjecting a lactate A and a long-chain alkanol to dealcoholization in the presence of a catalyst at 120-190°C to generate a lactic acid long-chain alkyl ester B; (2) continuously subjecting a lactic acid long-chain alkyl ester B mixed system obtained in step (1) to reduced pressure distillation under the conditions of 200-260°C and 0.1-0.5 kPa, so as to obtain a distillate; and (3) firstly subjecting the distillate in step (2) to solid-liquid separation to obtain a solid lactide phase, and then subjecting the solid phase to liquid-liquid extraction, so as to obtain lactide; or directly subjecting the distillate in step (2) to liquid-liquid phase separation, so as to obtain a high-purity lactide. The preparation method of the present invention has a simple and convenient process flow and a low operation cost, and the purity of the obtained lactide is higher and is not lower than 98%.

Description

A method for preparing high-purity lactide and its application in polylactic acid synthesis Technical Field

[0001] The present invention belongs to the technical field of lactide, and in particular relates to a method for preparing high-purity lactide and application thereof in the synthesis of polylactic acid. Background Art

[0002] High-purity lactide is an important monomer for the synthesis of high-molecular-weight PLA. Its current synthesis routes are divided into two categories according to the synthesis steps: (1) one is a two-step method, in which lactic acid / lactate is dehydrated / dealcoholized to synthesize oligomers (oligomerization stage), and the oligomers are depolymerized under high temperature and vacuum conditions to form lactide (depolymerization stage); (2) the other is a one-step method, in which lactic acid or lactate is directly cyclized to synthesize lactide under the action of a catalyst.

[0003] One-step synthesis of lactide is a new process proposed in recent years, and it is still in the basic research stage. De Clercq et al. (De Clercq R, et al. Catalytic gas-Phase Production of Lactide from Renewable Alkyl Lactates. Angew. Chem. Int. Ed. 2018, 57, 3074.) use alkyl lactate as raw material and utilize TiO2 / SiO2 catalyst to synthesize lactide in one-step gas phase, but the catalyst preparation used in this process is complex and has poor stability, which cannot meet the requirements of industrial application. Dusselier et al. (Dusselier M, et al. Shape-selective zeolite catalysis for bioplastics production. Science 349, 78-80 (2015).) use acidic zeolite molecular sieve as catalyst to propose a process for the liquid phase synthesis of lactide from lactic acid, but the molecular sieve catalyst of this process is easily deactivated, and the dewatering solvent used is toxic solvents such as toluene and xylene, which does not meet the requirements of green environmental protection. Compared with the two-step method, the one-step process has certain limitations and its industrial application prospects are unclear.

[0004] The purity of crude lactide obtained from the traditional two-step lactide synthesis process is generally 80%-90%. This is because during the oligomerization process, as the polycondensation reaction proceeds, oligomers with molecular weights ranging from several thousand to tens of thousands are formed, causing the system viscosity to gradually increase. Small molecular impurities such as lactic acid or lactate esters and their dimers and trimers cannot be completely removed during the oligomerization stage and are distilled out of the system along with the lactide during the depolymerization stage, resulting in a low purity lactide product. Furthermore, under the high temperature conditions of depolymerization, oligomers with molecular weights of tens of thousands become highly viscous in the reaction system, leading to poor heat and mass transfer and difficult reactor operation. Furthermore, as the depolymerization process proceeds, the system becomes increasingly viscous and carbonized, making the depolymerization reaction difficult and the removed lactide yellowish in color, resulting in a double loss in yield and quality.

[0005] To obtain high-purity lactide, which meets the requirements for high-molecular-weight PLA production, it is necessary to remove as many small-molecule impurities as possible from the product. Currently, methods for removing small-molecule impurities fall into two categories: one is during the synthesis phase, where immobilized reagents that react with the small-molecule end groups are added to the system to convert small molecules such as lactic acid and lactic acid dimers into high-boiling-point impurities. Reaction conditions are controlled to prevent these high-boiling-point impurities from co-distilling with the lactide, thereby reducing the presence of small-molecule impurities in the lactide. The other is during the production phase, where impurities are removed from the crude lactide product through washing, recrystallization, extraction, and a combination of these methods.

[0006] The first type of method is to improve the synthesis process of oligomerization and depolymerization, usually using an immobilization agent to fix small molecular impurities in the system. For example, CN102775380A discloses a process for preparing lactide by fixing free acid with polyhydroxy / polyamino compounds. Lactic acid is reacted with a polyhydroxy / polyamino compound with a boiling point of 250-550°C at 0.5-3.0 kPa and 90-180°C to produce high-boiling-point hydroxylactic acid oligomers. The oligomers are then cracked at 180-230°C and 0.4-0.8 kPa to obtain lactide with a purity of 92.7%-98.6%. CN115745947A uses hydroxyl immobilization reagents such as inositol and pentaerythritol with L-lactic acid to synthesize L-lactide through pre-polymerization, polycondensation, and chain-scission esterification at 120-140°C, 140-160°C, and 200-250°C. The patent document does not mention the purity of the crude lactide, but the yield is reduced by 40-50% after purification. The polyhydroxy compounds convert small-molecule impurities such as lactic acid dimers into high-boiling-point substances that cannot be distilled out of the system with lactide under cracking conditions, thereby improving the purity of the lactide. However, the polyhydroxy immobilization reagents themselves have high viscosity and are unstable. Repeated heating within the system deactivates the reagent end groups, making them unable to immobilize small-molecule impurities. Regeneration of the deactivated reagent is difficult, and the amount of residue in the system gradually increases with the number of cycles. CN106892893A discloses a process for synthesizing lactide by an improved oligomerization and depolymerization method. The process involves adding an isocyanate compound to a lactic acid oligomer (Mw = 1000-8000 g / mol) and reacting at 20-230°C. The reaction is then carried out at 180-240°C and -0.09-0.1 MPa to obtain crude lactide, wherein the free acid content of the crude ester is 10-100 mmol / kg. An immobilized agent can effectively reduce the content of small molecule impurities such as free acid in the obtained crude lactide, but the agent itself is also introduced into the lactide product during the cracking stage. The subsequent secondary separation of lactide and the immobilized agent still needs to be considered. Currently, there is no low-energy, easy-to-operate separation method for lactide and immobilized agents.

[0007] The second method involves purifying the crude lactide product obtained using conventional oligomerization and depolymerization methods. Purification methods for crude lactide include water washing, recrystallization, extraction, and a combination of these methods. CN114437019A washes crude lactide with water at 20-40°C with stirring for 10-60 minutes, followed by dehydration, vacuum drying (10-40°C), and vacuum distillation (150-200°C, 1-10 kPa) to obtain refined lactide with a purity of approximately 97%-99% and a yield of 78%-85%. Water washing effectively removes impurities, but it also causes hydrolysis of the target product, resulting in a lower yield. CN109400574A mixes crude lactide with an ε-caprolactone solvent at 60-150°C, melts it, and then cools and crystallizes it to obtain pure lactide. The yield is no less than 65.0%, and the free carboxyl group content does not exceed 91.2 mmol / kg. The recrystallization method has a low yield and can contain residual solvent (the residual ε-caprolactone content in Example 5 is 1.8 wt%). CN114507209A discloses a purification method that combines water washing with dual solvent extraction. Crude lactide is crushed or melted and then rapidly extracted with water at low temperature. A low-carbon alcohol solvent and a water-insoluble organic solvent (chloroform) are then added sequentially for extraction. The purified lactide has a purity of 99% and a yield of 90%-93%. Low-temperature water extraction can alleviate the hydrolysis of lactide, but the chloroform used in the extraction is a highly toxic organic solvent and does not meet environmental protection requirements. The above-mentioned water washing and recrystallization purification processes are both extremely energy-intensive separation operations.

[0008] In summary, existing lactide synthesis technologies utilize improved oligomerization and depolymerization processes combined with subsequent purification methods such as washing and recrystallization of the crude lactide product to produce high-purity lactide. However, both the improvements in the synthesis stage and the purification of the crude product suffer from technical challenges such as lengthy processes, complex operations, and high energy consumption. This results in high lactide production costs and is the primary reason why PLA currently struggles to achieve large-scale application as a biodegradable material. Therefore, the field of lactide synthesis technology urgently needs new processes to produce high-purity lactide, particularly low-cost processes that are easily scalable.

[0009] Summary of the Invention

[0010] In view of the above problems, the present invention establishes a new method for the synthesis and separation of lactide with a simple process and low energy consumption to solve the problems existing in the current process of oligomerization and depolymerization to produce lactide.

[0011] To achieve the above object, the technical solution of the present invention is as follows: A method for preparing high-purity lactide, comprising the following steps:

[0012] (1) Lactic acid ester A and long-chain alkanol are dealcoholized at normal pressure in the presence of a catalyst at 120-190° C. to produce long-chain alkyl lactate B;

[0013] (2) the mixed system of the long-chain alkyl lactate B obtained in step (1) is further subjected to reduced pressure distillation at 200-260° C. and 0.1-0.5 kPa to obtain a distillate;

[0014] (3) Obtaining high-purity lactide by one of the following methods:

[0015] Method 1: The distillate obtained in step (2) is first subjected to solid-liquid separation to remove most of the long-chain alkanols to obtain a solid phase containing lactide, which is then mixed with an extraction solvent and subjected to liquid-liquid extraction to obtain high-purity lactide;

[0016] Method 2: directly subjecting the distillate obtained in step (2) to liquid-liquid separation to obtain high-purity lactide;

[0017] Wherein, the lactate ester A is an alkyl lactate, and the long-chain alkanol is selected from one of C20-30 normal alkanols.

[0018] It should be noted that step (2) is to directly continue to distill under reduced pressure the mixed system of the long-chain alkyl lactate B after dealcoholization at normal pressure in step (1).

[0019] The present invention is further configured such that when method 1 is adopted in step (3), the long-chain alkanol in step (1) is correspondingly selected from one of C20-30 normal alkanols.

[0020] The present invention is further configured such that when step (3) adopts method 2, the long-chain alkanol in step (1) is correspondingly selected from one of C26-30 normal alkanols.

[0021] The present invention is further configured such that, in step (1), the molar ratio of lactate A to the long-chain alkanol is (1:2)-(20:1).

[0022] The present invention is further configured such that, in step (1), the catalyst is selected from one of anhydrous SnCl2, Sn(CH3)2Cl2, Sn(Oct)2, tetrabutyl titanate, and zinc lactate.

[0023] The present invention is further configured such that the mass ratio of the catalyst to lactate A in step (1) is 1:(100-12000).

[0024] The present invention is further configured such that the dealcoholization product long-chain alkyl lactate B in step (1) has a system viscosity of 2.0-6.0 mPa·s under reduced pressure distillation conditions of 200-260° C. and 0.1-0.5 kPa.

[0025] The present invention is further configured such that, in step (3), the temperature for solid-liquid separation is 75-90°C.

[0026] The present invention is further configured such that the temperature of the liquid-liquid extraction in mode 1 of step (3) is 110-150°C.

[0027] The present invention is further configured such that the temperature of the liquid-liquid phase separation in mode 2 of step (3) is 110-150°C.

[0028] The present invention is further configured such that, in method 1 of step (3), the extraction solvent used for the liquid-liquid extraction is selected from a combination of one or more of sliced ​​paraffin, fully refined paraffin, and Fischer-Tropsch wax.

[0029] The present invention is further configured such that, in method 1 in step (3), the mass ratio of the solid phase to the extraction solvent is 1:2-1:5.

[0030] The principle of the above technical solution of the present invention is as follows: (1) long-chain alkanol is introduced into the system to undergo an ester exchange reaction with lactic acid ester, and after removing the low-carbon alcohol, long-chain alkyl lactate B is generated, and further cyclization reaction occurs under high temperature and vacuum conditions to generate lactide. The long carbon chain of the long-chain alkanol can disperse the lactic acid ester at the physical level, reducing the probability of its oligomerization. At the same time, the terminal hydroxyl group can undergo ester exchange with the lactic acid ester to chemically block the end, further inhibiting the self-polymerization of the lactic acid ester, regulating the ratio between the long-chain alkanol and the lactic acid ester, and controlling the long-chain alkyl lactate B within a certain molecular weight, so that the viscosity of the reaction system is several mPa·s, thereby greatly reducing the generation of small molecular impurities in the reaction system. At the same time, due to the low viscosity of the reaction system, the reaction operation is simple and easy to scale up. (2) The physical property differences between lactide, long-chain alkyl lactate B and long-chain alkanol in the product are cleverly utilized, and a simple, efficient and low-energy separation method is used to separate and obtain high-purity lactide. First, based on the melting point differences between lactide, long-chain alkanol, and long-chain alkyl lactate B, solid-liquid separation is used to remove most of the long-chain alkanol, yielding a solid lactide phase. Then, utilizing the solubility differences between lactide, long-chain alkanol, and long-chain alkyl lactate B in the extraction solvent, liquid-liquid extraction is performed to obtain 98-99% pure lactide. For long-chain alkanols with 26-30 carbon atoms, the distillate from vacuum distillation can also be directly subjected to liquid-liquid separation to produce high-purity lactide.

[0031] The present invention provides high-purity lactide prepared according to the above preparation method.

[0032] The present invention also provides an application of the high-purity lactide, which is applied to the preparation of polylactic acid by ring-opening polymerization of lactide.

[0033] Furthermore, the preparation method of polylactic acid includes the following steps: melting the high-purity lactide in a reaction system, then adding a catalyst and reacting at 120-180°C for a period of time, adding a terminator or placing in ice water to quench the reaction to obtain a polymer product; wherein the catalyst is a tin catalyst, and the catalyst amount is 0.2wt%-0.4wt% of the high-purity lactide; the reaction time is 12-24h; and the terminator includes isopropyl alcohol and benzoic acid.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) The long-chain alkanol introduced in the present invention undergoes an ester exchange reaction with lactic acid ester, which, on the one hand, physically disperses the lactic acid ester and reduces the probability of its oligomerization. At the same time, the terminal hydroxyl groups can undergo ester exchange with lactic acid ester to chemically cap the end groups, further inhibiting the self-polymerization of lactic acid ester, thereby avoiding the production of lactic acid oligomers with a molecular weight of tens of thousands in the conventional oligomerization and depolymerization process, greatly reducing the viscosity of the system and the small molecular impurities in the lactide product. At the same time, the reduction in viscosity also reduces the reaction coking, making the reaction operation and process scale-up easier.

[0036] (2) Based on the difference in melting point and solubility between the product lactide and other components of the reaction system, the separation of lactide from other substances can be completed through two low-energy-consumption and easy-to-operate process flows, solid-liquid separation and liquid-liquid extraction, or through simple liquid-liquid phase separation, to obtain high-purity lactide with a purity of 98% to 99% and a yield of 85% to 95%. The process is simple, the operating cost is low, and the energy consumption is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG1 is a schematic diagram of the process flow of the present invention;

[0038] FIG2 is a hydrogen nuclear magnetic resonance spectrum of the lactide product prepared in Example 1. DETAILED DESCRIPTION

[0039] The present invention is further described below by way of examples. The following examples are only used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0040] Referring to FIG1 , the present invention provides a method for preparing high-purity lactide, comprising the following steps:

[0041] (1) Lactic acid ester A and long-chain alkanol are dealcoholized at normal pressure in the presence of a catalyst at 120-190° C. to produce long-chain alkyl lactate B;

[0042] (2) the mixed system of the long-chain alkyl lactate B obtained in step (1) is further subjected to reduced pressure distillation at 200-260° C. and 0.1-0.5 kPa to obtain a distillate;

[0043] (3) Obtaining high-purity lactide by one of the following methods:

[0044] Method 1: The distillate obtained in step (2) is first subjected to solid-liquid separation to remove most of the long-chain alkanols to obtain a solid phase containing lactide, which is then subjected to liquid-liquid extraction to obtain high-purity lactide;

[0045] Method 2: The distillate obtained in step (2) is directly subjected to liquid-liquid separation to obtain high-purity lactide.

[0046] The technical solution of the present invention is further illustrated below with reference to specific embodiments.

[0047] Example 1

[0048] Accurately weigh 29.86 g of 1-eicosanol into a 100 ml flask and melt it at 80°C. Then add 0.0362 g of anhydrous SnCl2 and 5.21 g of methyl L-lactate. Stir at atmospheric pressure and begin to heat the mixture. React at 120-190°C while distilling off methanol. The reaction stops when no more methanol is distilled off. The liquid in the flask is eicosanyl lactate. The reaction system was then evacuated to 0.5 kPa, and the reaction temperature was gradually increased to 230° C. for reduced pressure distillation. The reaction time was started from a reaction temperature of 200° C., the reaction was continued for 2.5 hours, and 32.28 g of distillate was collected. The distillate was solid after being cooled to room temperature and was further heated to 75° C. for solid-liquid separation to obtain 5.41 g of solid product. The solid product and 16.23 g of Fischer-Tropsch wax were subjected to liquid-liquid extraction at 120° C. to finally obtain 3.14 g of lactide with a yield of 85.60%. The purity of the extracted product was measured by gas chromatography external standard method to be 98.32%. The lactide product prepared in this example is a white needle-shaped crystal. Its H NMR spectrum is shown in FIG2 . The chemical shift of the lactide methyl hydrogen atom (-CH3) is 1.65 ppm, and the chemical shift of the methine hydrogen atom (-CH-) is 5.05 ppm, which is consistent with the report in the literature (Guocai W, et al. Preparation of high purity lactide using a high-boiling-point alcohol immobilization method [J]. Industrial & Engineering Chemistry Research, 2018, 57 (22): 7711-7716.). No other miscellaneous peaks were observed.

[0049] 200 mg of the prepared lactide product was accurately weighed and added to a 10 ml Schlenk tube. After complete melting, 0.6 mg of Sn(Oct)2 was added and the mixture was allowed to react at 120°C for 12 hours. The reaction was then quenched by adding isopropanol to obtain a polymer. The polymer was dissolved in dichloromethane, precipitated, and washed with excess anhydrous ethanol to obtain the final polylactic acid product. GPC analysis of the obtained polylactic acid product revealed a weight-average molecular weight of 98.5 kg / mol and a molecular weight distribution index of 1.32.

[0050] Example 2

[0051] Accurately weigh 32.66g of 1-docosanol into a 100ml flask and melt at 80°C. Then, add 0.0425g of anhydrous SnCl2 and 10.41g of methyl L-lactate. Stir under normal pressure and begin heating. Reaction proceeds at 120-190°C while distilling off methanol until no more methanol distills out. The liquid in the flask is now docosyl lactate. The reaction system is then evacuated to 0.3kPa, while the temperature is gradually raised to 250°C for vacuum distillation. Starting at 220°C, the reaction continues for 2 hours. Collect 35.67g of distillate, which solidifies upon cooling to room temperature. It was further heated to 80°C for solid-liquid separation to obtain 10.08 g of solid product. The solid product and 40.32 g of sliced ​​paraffin were subjected to liquid-liquid extraction at 130°C to finally obtain 6.45 g of lactide with a yield of 88.39%. The purity of the extracted product was measured by gas chromatography external standard method to be 98.75%.

[0052] Example 3

[0053] 13.06g of 1-docosanol was accurately weighed and added to a 100ml flask. The mixture was melted at 80°C, followed by the addition of 0.0021g of Sn(Oct)2 and 21.24g of methyl L-lactate. The mixture was stirred under atmospheric pressure and the temperature was raised. The reaction was continued at 120-190°C while methanol was distilled off. The reaction ceased when no more methanol was distilled off. The liquid in the flask was now docosyl lactate. The reaction system was then evacuated to 0.3kPa, and the temperature was gradually raised to 250°C for vacuum distillation. The viscosity of the system under these vacuum distillation conditions was measured to be 2.5mPa·s. The reaction was continued for 1.5 hours, starting at a reaction temperature of 220°C. 27.20g of distillate was collected, which solidified upon cooling to room temperature. It was further heated to 80°C for solid-liquid separation to obtain 17.35g of solid product. The solid product and 52.05g of sliced ​​paraffin were subjected to liquid-liquid extraction at 130°C to finally obtain 14.12g of lactide with a yield of 95.12%. The purity of the extracted product was measured by gas chromatography external standard method to be 99.05%.

[0054] 200 mg of the lactide product prepared in this example was accurately weighed and added to a 10 ml Schlenk tube. After complete melting, 0.4 mg of Sn(Oct)2 was added. After reacting at 140°C for 24 hours, the Schlenk tube was placed in ice water to quench the reaction, yielding a polymer. The polymer was dissolved in dichloromethane, precipitated, and washed with excess anhydrous ethanol to yield the final polylactic acid product. GPC analysis of the polylactic acid product revealed a weight-average molecular weight of 145 kg / mol and a molecular weight distribution index of 1.45.

[0055] Example 4

[0056] Accurately weigh 17.73g of 1-tetracosanol into a 100ml flask and melt at 85°C. Then add 0.0603g of Sn(CH3)2Cl2 and 12.05g of ethyl L-lactate. Stir under normal pressure and begin heating. Reaction proceeds at 120-190°C while distilling off methanol until no more methanol is distilled off. The liquid in the flask is tetracosanol lactate. The reaction system is then evacuated to 0.2kPa, and the temperature is gradually raised to 250°C for vacuum distillation. Starting at 230°C, the reaction is continued for 2.5 hours. 23.19g of distillate is collected, which solidifies upon cooling to room temperature. It was further heated to 90°C for solid-liquid separation to obtain 8.71g of solid product. The solid product and 43.55g of Fischer-Tropsch wax were subjected to liquid-liquid extraction at 110°C to finally obtain 6.58g of lactide with a yield of 88.02%. The purity of the extracted product was measured by gas chromatography external standard method to be 98.33%.

[0057] Example 5

[0058] 6.53g of 1-docosanol was accurately weighed and added to a 100ml flask. The mixture was melted at 80°C, followed by the addition of 0.0021g of Sn(Oct)2 and 21.24g of methyl L-lactate. The mixture was stirred under atmospheric pressure and the temperature was raised. The reaction was continued at 120-190°C while methanol was distilled off. The reaction ceased when no more methanol was distilled off. The liquid in the flask was now docosyl lactate. The reaction system was then evacuated to 0.3kPa, and the temperature was gradually raised to 250°C for vacuum distillation. The viscosity of the system under these vacuum distillation conditions was measured to be 5.1mPa·s. The reaction was continued for 1.5h, starting at a reaction temperature of 220°C. 20.50g of distillate was collected, which solidified upon cooling to room temperature. It was further heated to 80°C for solid-liquid separation to obtain 16.87 g of solid product. The solid product and 50.61 g of fully refined paraffin were subjected to liquid-liquid extraction at 120°C to finally obtain 13.98 g of lactide with a yield of 93.50%. The purity of the extracted product was measured by gas chromatography external standard method to be 98.33%.

[0059] Example 6

[0060] Accurately weigh 5.97g of 1-eicosanol into a 100ml flask and melt it at 80°C. Then, add 0.0042g of tetrabutyl titanate and 21.24g of methyl L-lactate. Stir under normal pressure and begin heating. Reaction proceeds at 120-190°C while distilling off methanol until no more methanol distills out. The liquid in the flask is eicosanyl lactate. The reaction system is then evacuated to 0.5kPa, and the temperature is gradually increased to 230°C for vacuum distillation. The viscosity of the system under these vacuum distillation conditions is measured to be 4.5mPa·s. Starting at a reaction temperature of 200°C, the reaction is continued for 1.5 hours, and 20.05g of distillate is collected. The distillate solidifies upon cooling to room temperature. It was further heated to 75°C for solid-liquid separation to obtain 15.21 g of solid product. The solid product and 45.63 g of sliced ​​paraffin were subjected to liquid-liquid extraction at 140°C to finally obtain 13.75 g of lactide with a yield of 92.70%. The purity of the extracted product was measured by gas chromatography external standard method to be 99.12%.

[0061] 200 mg of the lactide product prepared in this example was accurately weighed and added to a 10 ml Schlenk tube. After complete melting, 0.8 mg of SnCl2 was added and the mixture was allowed to react at 130°C for 24 h. The reaction was then quenched by adding isopropanol to obtain a polymer. The polymer was dissolved in dichloromethane, precipitated and washed with excess anhydrous ethanol to obtain a polylactic acid product. GPC analysis of the obtained polylactic acid product revealed a weight-average molecular weight of 135 kg / mol and a molecular weight distribution index of 1.54.

[0062] Example 7

[0063] Accurately weigh 21.95g of 1-triacontol into a 100ml flask and melt it at 100°C. Then, add 0.1205g of anhydrous SnCl2 and 12.05g of L-ethyl lactate. Stir under normal pressure and begin heating. The reaction is carried out at 120-190°C while ethanol is distilled off. The reaction stops when no ethanol is distilled off. The liquid in the flask is triacontol lactate. The reaction system is then evacuated to 0.1kPa, and the reaction temperature is gradually raised to 260°C for vacuum distillation. Starting at 240°C, the reaction is continued for 2.5 hours. 27.89g of distillate is collected, which solidifies upon cooling to room temperature. The distillate is subjected to liquid-liquid phase separation at 130°C, ultimately yielding 6.24g of lactide, with a yield of 84.00%. The purity of the separated product, as measured by gas chromatography with an external standard method, is 98.95%.

[0064] 200 mg of the lactide product prepared in this example was accurately weighed and added to a 10 ml Schlenk tube. After complete melting, 0.6 mg of SnCl2 was added and the mixture was allowed to react at 180°C for 24 h. Benzoic acid was then added to quench the reaction to obtain a polymer. The polymer was dissolved in dichloromethane, precipitated and washed with excess anhydrous ethanol to obtain a polylactic acid product. GPC analysis of the obtained polylactic acid product revealed a weight-average molecular weight of 154 kg / mol and a molecular weight distribution index of 1.68.

[0065] Example 8

[0066] Accurately weigh 41.08g of 1-octacosanol into a 100ml flask and melt it at 100°C. Then, add 0.0319g of anhydrous SnCl2 and 10.62g of L-methyl lactate. Stir under normal pressure and begin heating. The reaction is carried out at 120-190°C while distilling methanol. The reaction stops when no more methanol is distilled out. The liquid in the flask is now octacosanol. The reaction system is then evacuated to 0.1kPa, and the reaction temperature is gradually raised to 260°C for vacuum distillation. Starting at 240°C, the reaction is continued for 2.5 hours, and 48.20g of distillate is collected. After cooling to room temperature, the distillate solidifies. Liquid-liquid phase separation of the distillate at 150°C yields 6.18g of lactide, with a yield of 82.93%. The purity of the separated product, as measured by gas chromatography with an external standard method, is 98.65%.

[0067] Example 9

[0068] 4.90g of 1-docosanol was accurately weighed and added to a 100ml flask and melted at 80°C. Then, 0.0153g of Sn(Oct)2 and 31.23g of methyl L-lactate were added. The mixture was stirred under normal pressure and the temperature was raised. The reaction was continued at 120-190°C while methanol was distilled off. The reaction ceased when no more methanol was distilled off. The liquid in the flask was now docosyl lactate. The reaction system was then evacuated to 0.3kPa, and the temperature was gradually raised to 250°C for vacuum distillation. The viscosity of the system under these vacuum distillation conditions was measured to be 5.8mPa·s. The reaction was continued for 1.5 hours, starting at a reaction temperature of 220°C. 26.45g of distillate was collected, which solidified upon cooling to room temperature. It was further heated to 80°C for solid-liquid separation to obtain 22.45 g of solid product. The solid product and 67.35 g of sliced ​​paraffin were subjected to liquid-liquid extraction at 130°C to finally obtain 20.67 g of lactide with a yield of 94.62%. The purity of the extracted product was measured by gas chromatography external standard method to be 98.96%.

[0069] Example 10

[0070] Accurately weigh 15.31g of 1-hexacosanol into a 100ml flask and melt it at 90°C. Then, add 0.0482g of zinc lactate and 24.11g of ethyl L-lactate. Stir under normal pressure and begin heating. The reaction proceeds at 120-190°C while distilling off ethanol until no more ethanol is distilled out. The liquid in the flask is now hexacosanol lactate. The reaction system is then evacuated to 0.2kPa, and the temperature is gradually raised to 250°C for vacuum distillation. Starting at 230°C, the reaction is continued for 1.5 hours. 29.20g of distillate is collected, which solidifies upon cooling to room temperature. The distillate is subjected to liquid-liquid separation at 110°C, yielding 13.31g of lactide, with a yield of 88.70%. The purity of the separated product, as determined by gas chromatography with an external standard method, is 98.01%.

[0071] Example 11

[0072] 13.16g of 1-triacontol was accurately weighed and added to a 100ml flask. The mixture was melted at 100°C, followed by the addition of 0.0080g of Sn(CH3)2Cl2 and 15.93g of methyl L-lactate. The mixture was stirred under normal pressure and the temperature was raised. The reaction was allowed to proceed at 120-190°C while methanol was distilled off. The reaction ceased when no more methanol was distilled off. The liquid in the flask was triacontol lactate. The reaction system was then evacuated to 0.1kPa, and the temperature was gradually raised to 260°C for vacuum distillation. The reaction was continued for 1.5 hours, starting at 240°C. 22.39g of distillate was collected, which solidified upon cooling to room temperature. The distillate was subjected to liquid-liquid separation at 130°C, yielding 10.66g of lactide, with a yield of 95.08%. The purity of the separated product, as determined by gas chromatography with an external standard method, was 98.35%.

[0073] Example 12

[0074] Accurately weigh 17.73g of 1-tetracosanol into a 100ml flask and melt it at 85°C. Then add 0.0011g Sn(Oct)2 and 10.62g of L-methyl lactate. Stir and heat at normal pressure. React at 120-190°C while distilling methanol. Stop the reaction when no more methanol is distilled. The liquid in the flask is tetracosanol lactate. Then evacuate the reaction system to 0.2kPa and gradually increase the reaction temperature to 250°C for vacuum distillation. Start the reaction at 230°C and react for 2.5 hours. Collect 23.95g of distillate, which solidifies after cooling to room temperature. It was further heated to 85°C for solid-liquid separation to obtain 8.07 g of solid product. The solid product and 32.28 g of Fischer-Tropsch wax were subjected to liquid-liquid extraction at 150°C to finally obtain 6.45 g of lactide with a yield of 86.76%. The purity of the extracted product was measured by gas chromatography external standard method to be 98.89%.

[0075] Example 13

[0076] Accurately weigh 4.48g of 1-eicosanol into a 100ml flask and melt at 80°C. Then, add 0.0064g of Sn(Oct)2 and 31.87g of methyl L-lactate. Stir under normal pressure and begin heating. Reaction proceeds at 120-190°C while distilling off methanol until no more methanol distills out. The liquid in the flask is eicosanyl lactate. The reaction system is then evacuated to 0.5kPa, and the reaction temperature is gradually increased to 230°C for vacuum distillation. Starting at 200°C, the reaction is continued for 1.5 hours. 25.89g of distillate is collected, which solidifies upon cooling to room temperature. It was further heated to 75°C for solid-liquid separation to obtain 22.31g of solid product. The solid product and 44.62g of Fischer-Tropsch wax were subjected to liquid-liquid extraction at 130°C to finally obtain 20.92g of lactide with a yield of 93.17%. The purity of the extracted product was measured by gas chromatography external standard method to be 98.25%.

[0077] Comparative Example 1

[0078] 0.0021g Sn(Oct)2 and 21.24g methyl L-lactate were accurately weighed and added to a 100ml flask. The mixture was stirred under normal pressure and the temperature was raised. The reaction was carried out at 120-190°C while methanol was distilled off. The reaction stopped when no more methanol was distilled out. The yellow viscous liquid in the flask was lactic acid oligomers. The reaction system was then evacuated to 0.5kPa, and the reaction temperature was gradually raised to 230°C for vacuum distillation. Starting at a reaction temperature of 200°C, the reaction was continued for 1.5 hours. A light yellow liquid gradually distilled out, namely crude lactide, totaling 11.27g. The purity of the crude lactide was determined by gas chromatography with an external standard method to be 85.10%, and the calculated yield was 65.23%. The lactide product obtained in this comparative example was a yellow, slightly moist solid, which was significantly lower in purity and yield than the lactide product prepared in Example 1.

[0079] Comparative Example 2

[0080] 6.53g of 1-docosanol was accurately weighed and added to a 100ml flask. The mixture was melted at 80°C, followed by the addition of 0.0021g of Sn(Oct)2 and 21.24g of methyl L-lactate. The mixture was stirred at atmospheric pressure and the temperature was raised. The reaction was allowed to proceed at 120-190°C while methanol was distilled off. The reaction ceased when no more methanol was distilled off. The liquid in the flask was now docosyl lactate. The reaction system was then evacuated to 0.3kPa, and the temperature was gradually raised to 250°C for vacuum distillation. The reaction was started at 220°C and the reaction was continued for 1.5 hours. 20.12g of distillate was collected, which solidified upon cooling to room temperature. The distillate was further heated to 80°C for solid-liquid separation, yielding 17.01g of a solid product. The purity of the product after solid-liquid separation was 80.60% as determined by gas chromatography with an external standard method, resulting in a calculated yield of 93.25%.

[0081] Comparative Example 3

[0082] Accurately weigh 6.53g of 1-docosanol into a 100ml flask and melt at 80°C. Then, add 0.0021g of Sn(Oct)2 and 21.24g of methyl L-lactate. Stir under normal pressure and begin heating. Reaction proceeds at 120-190°C while distilling off methanol until no more methanol distills out. The liquid in the flask is now docosyl lactate. The reaction system is then evacuated to 0.3kPa, and the temperature is gradually raised to 250°C for vacuum distillation. Starting at 220°C, the reaction continues for 1.5 hours. 20.50g of distillate is collected, which solidifies upon cooling to room temperature. It was further heated to 80° C. for solid-liquid separation to obtain 16.87 g of a solid product. The solid product was washed three times with ultrapure water to finally obtain 7.86 g of lactide with a yield of 49.26%. The purity of the washed product was measured by gas chromatography external standard method to be 92.15%.

[0083] Comparative Example 4

[0084] Accurately weigh 6.53g of 1-docosanol into a 100ml flask and melt it at 80°C. Then, add 0.0021g of Sn(Oct)2 and 21.24g of methyl L-lactate. Stir under normal pressure and begin heating. Reaction proceeds at 120-190°C while distilling off methanol until no more methanol is released. The liquid in the flask is now docosyl lactate. The reaction system is then evacuated to 0.3kPa, and the temperature is gradually raised to 250°C for vacuum distillation. Starting at 220°C, the reaction continues for 1.5 hours, collecting 20.58g of distillate. The distillate solidifies upon cooling to room temperature. It was further heated to 80° C. for solid-liquid separation to obtain 17.05 g of a solid product. The solid product was washed three times with ethanol to finally obtain 12.07 g of lactide with a yield of 77.36%. The purity of the product after alcohol washing was measured by gas chromatography external standard method to be 94.23%.

[0085] Comparative Example 5

[0086] Accurately weigh 6.53g of 1-docosanol into a 100ml flask and melt at 80°C. Then, add 0.0021g of Sn(Oct)2 and 21.24g of methyl L-lactate. Stir under normal pressure and begin heating. Reaction proceeds at 120-190°C while distilling off methanol until no more methanol distills out. The liquid in the flask is now docosyl lactate. The reaction system is then evacuated to 0.3kPa, while the temperature is gradually raised to 250°C for vacuum distillation. Starting at 220°C, the reaction continues for 1.5 hours, collecting 20.78g of distillate. The distillate solidifies upon cooling to room temperature. It was further heated to 80° C. for solid-liquid separation to obtain 17.75 g of a solid product. The solid product was recrystallized from ethyl acetate to finally obtain 10.14 g of lactide with a yield of 65.98%. The purity of the recrystallized product was measured by gas chromatography external standard method to be 95.67%.

[0087] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A method for preparing lactide, characterized in that: The steps include: (1) lactic acid ester A and long-chain alkanol are dealcoholized at normal pressure in the presence of a catalyst at 120-190° C. to produce lactic acid long-chain alkyl ester B; (2) the mixed system of the long-chain alkyl lactate B obtained in step (1) is further subjected to reduced pressure distillation at 200-260° C. and 0.1-0.5 kPa to obtain a distillate; (3) Producing lactide by one of the following methods: Method 1: The distillate obtained in step (2) is first subjected to solid-liquid separation to remove most of the long-chain alkanols to obtain a solid phase containing lactide, and the solid phase is then mixed with an extraction solvent to obtain lactide through liquid-liquid extraction; Method 2: directly subjecting the distillate obtained in step (2) to liquid-liquid separation to obtain lactide; Wherein, the lactate ester A is an alkyl lactate, and the long-chain alkanol is selected from one of C20-30 normal alkanols.

2. The method for preparing lactide according to claim 1, characterized in that: When step (3) adopts method 2, the long-chain alkanol in step (1) is selected from one of C26-30 normal alkanols.

3. The method for preparing lactide according to claim 1, characterized in that: In step (1), the molar ratio of lactate A to long-chain alkanol is (1:2)-(20:1).

4. The method for preparing lactide according to claim 1, characterized in that: In step (1), the catalyst is selected from one of anhydrous SnCl2, Sn(CH3)2Cl2, Sn(Oct)2, tetrabutyl titanate, and zinc lactate; and the mass ratio of the catalyst to lactate A is 1:(100-12000).

5. The method for preparing lactide according to claim 1, characterized in that: In step (3), the temperature for solid-liquid separation is 75-90°C.

6. The method for preparing lactide according to claim 1, characterized in that: In step (3), the temperature of the liquid-liquid extraction in method 1 is 110-150°C; the temperature of the liquid-liquid phase separation in method 2 is 110-150°C.

7. The method for preparing lactide according to claim 1, characterized in that: In method 1 of step (3), the extraction solvent used for the liquid-liquid extraction is selected from a combination of one or more of slice paraffin, fully refined paraffin, and Fischer-Tropsch wax.

8. The method for preparing lactide according to claim 1, characterized in that: In method 1 of step (3), the mass ratio of the solid phase to the extraction solvent is 1:2-1:

5.

9. Lactide prepared by the preparation method according to any one of claims 1 to 8.

10. A use of lactide as claimed in claim 9, characterized in that: It is used to prepare polylactic acid by ring-opening polymerization of lactide.

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