Green and efficient recrystallization and water extraction combined process for purifying L-lactide

Abstract

The invention belongs to the technical field of high polymer materials, and relates to synthesis and preparation of polylactic acid, in particular to a research on a green, friendly and efficient process for purifying L-lactide. A recrystallization method is combined with a water extraction method, crude lactide, ethyl acetate and other solvents are uniformly mixed, heating reflux is performed for a period of time, cooling, suction filtration and drying are performed, a molten intermediate product and ultrapure water are uniformly mixed and stirred, suction filtration and drying are performed after a period of time, and the high-purity L-lactide crystal can be obtained. The preparation method disclosed by the invention is simple, few in steps, simple and convenient to operate, green and environment-friendly. The prepared L-lactide has high optical purity and chemical purity, and high molecular weight polylactic acid is obtained through ring opening polymerization.

Description

[Technical field]

[0001] The invention belongs to the technical field of polymer materials, and specifically relates to a process research of purifying L-lactide by combining recrystallization with water extraction. [Background technology]

[0002] With the increasing popularity and extent of plastic use, a large amount of plastic waste exists for a long time and decomposes into microplastics, endangering water bodies, soil and food chains. Polylactic acid has good biodegradability and can be decomposed into carbon dioxide and water by microorganisms under appropriate environmental conditions, without causing long-term pollution to the environment. It has become a potential alternative to solve the problem of plastic pollution. At the same time, polylactic acid also has good mechanical strength and chemical stability, and can be widely used in civil engineering, medical treatment, agriculture, planting and other fields.

[0003] At present, the demand for polylactic acid is increasing rapidly. The traditional method of synthesizing polylactic acid by direct polycondensation of lactic acid has a low molecular weight and high production cost, which cannot meet the production demand. In industry, polylactic acid is usually formed by ring-opening polymerization of lactide. The key to the ring-opening polymerization method lies in the preparation, purification and polymerization process of lactide. The refining of lactide is particularly important. To synthesize high molecular weight polylactic acid, L-lactide with high chemical purity and high optical purity is required. Removing lactic acid, lactic acid dimer, lactic acid trimer, M-lactide, water, etc. from L-lactide can effectively increase the molecular weight of polylactic acid in the synthetic product.

[0004] Lactide has a high boiling point, a high freezing point, and heat sensitivity. Lactide with insufficient purity may contain a small amount of water and lactic acid. After re-hydrolysis, it will form lactic acid that is difficult to store stably for a long time, and impurities will reduce the molecular weight of the polymerization product. By purifying and separating crude lactide, the content of L-lactide in crude lactide can be effectively increased, and high molecular weight polylactic acid can be obtained by ring-opening polymerization. Therefore, it is of great significance to find a green and efficient technology for purifying L-lactide. Although lactide ring-opening polymerization shows excellent economic benefits, its purification and refining is still challenging.

[0005] The present invention adopts an environmentally friendly recrystallization combined with water extraction strategy to purify L-lactide, simplifies the synthesis steps, and reduces the introduction of impurities. The prepared L-lactide has good optical purity and chemical purity. [Summary of the invention]

[0006] [Technical issues to be solved]

[0007] In view of the deficiencies in the prior art, the present invention provides a process for purifying L-lactide by combining recrystallization with water extraction.

[0008] The purpose of the present invention is to uniformly mix crude lactide with a solvent such as ethyl acetate, heat and reflux for a period of time, cool and filter to dry, uniformly mix the product with ultrapure water and stir, filter and dry after a period of time, so as to obtain high-purity L-lactide crystals.

[0009] Technical solution:

[0010] A process for purifying L-lactide by recrystallization combined with water extraction comprises the following steps:

[0011] 1. Add ethyl acetate or other solutions to the crude lactide to be purified, stir and condense under reflux at 60-80°C, then cool in an ice bath, pour the intermediate product into a suction funnel and filter, repeat 3-5 times, then put it into a vacuum drying oven at 30-50°C and dry it, take out the intermediate product for later use.

[0012] The solvents that can be used for recrystallization in step 1 include ethyl acetate, methyl ethyl ketone, benzene or alkylbenzene, lower alcohols, ketones, lipids or mixtures thereof, among which ethyl acetate is the most commonly used.

[0013] Wherein, a condenser is used in step 1, and the condenser is a serpentine condenser.

[0014] Preferably, the mass ratio of the crude lactide described in 1 to the solution of ethyl acetate or the like is 3:1-4:1.

[0015] 2. Heat the intermediate product obtained in step 1 until it is melted, add the melted intermediate product to ultrapure water, and stir for 40-50 minutes at 20-35°C, wherein the mass ratio of the intermediate product to ultrapure water is 1:1-1:1.5. Pour the product into a suction funnel for suction filtration, and dry the product in a vacuum drying oven at 30-50°C after suction filtration.

[0016] Another object of the present invention is to provide a high-purity L-lactide product.

[0017] Purity detection method of L-lactide after purification

[0018] The purified L-lactide was weighed and ultrasonically dispersed in a methanol-ethanol mixed solution, and then filtered into a chromatographic bottle using a filter membrane, and then detected by liquid chromatography; the test results showed that compared with crude lactide, the L-lactide prepared by the present invention had higher optical purity and chemical purity.

[0019] In the L-lactide purification technology provided by the present invention, distillation is generally used to remove impurities present in crude lactide. However, on the one hand, the high boiling point and high freezing point of lactide increase the temperature and number of plates required for distillation, which will increase energy consumption and cause waste of resources; on the other hand, due to the thermosensitivity of lactide, it is easy to decompose at high temperature, resulting in an increase in impurities. The recrystallization method currently studied requires high-purity solvents, and multiple dissolution and crystallization take a long time; excessive lactic acid and polylactic acid will affect the efficiency of the water extraction method. Therefore, a greener and more efficient method is needed to purify and separate crude lactide.

[0020] The present invention provides a new recrystallization combined with water extraction purification process, one of the main purposes of which is to reduce the purification temperature, improve the process yield, and reduce the process energy consumption and production cost. The crude lactide is first recrystallized by a solution such as ethyl acetate to reduce the impurity content in the crude lactide, and then the lactide is further purified by water extraction. By adopting the process disclosed in the present invention, lactic acid, lactic acid dimer, lactic acid trimer, M-lactide, water, etc. in the crude lactide are removed to obtain a product with qualified optical purity and chemical purity.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0022] The preparation method of the invention is simple, has few steps, is easy to operate, and is green and environmentally friendly; the crude lactide is firstly recrystallized for rough fractionation to remove a large amount of impurities, especially lactic acid and polylactic acid, and then the L-lactide is further purified by water extraction. The L-lactide prepared by the invention has good optical purity and chemical purity, and high molecular weight polylactic acid can be obtained by ring-opening polymerization. The production and technical costs are significantly reduced, and high-purity L-lactide crystals are obtained.

Brief Description of the Drawings

[0023] Figure 1 It is the instrument used for the recrystallization method in technical solution 1.

[0024] Figure 2 The instrument used for the water extraction method in technical solution 2

[0025] Figure 3 is the optical purity of the obtained L-lactide. [Specific implementation method]

[0026] Combined with Figure 1-3 The present invention is further described with reference to the following examples.

[0027] Example

[0028] 1. Recrystallization method to purify crude lactide

[0029] Add 20g of crude lactide and 5g of ethyl acetate solvent into a 20ml round-bottom bottle, continue stirring and heating to 70℃. Then place in a water bath at 70℃ for condensation and reflux for 2h. After the reaction is completed, take out and cool in ice water. Filter with a suction funnel and wash with a small amount of ethyl acetate. Repeat three times and then filter and dry the product in a vacuum drying oven at 40℃ for 6h.

[0030] 2. Purification of crude lactide by water extraction

[0031] The intermediate product obtained in the previous step was heated to a molten state, added to ultrapure water of the same mass and stirred at 30°C for 40 minutes, and the product was poured into a suction funnel for filtration, and the product after filtration was placed in a vacuum drying oven at 40°C for 6 hours. The purity of the purified L-lactide was tested by liquid chromatography, with a pump flow rate of 1 mL / min and a wavelength of 228 nm.

Claims

1. A green and efficient recrystallization combined with water extraction process for purifying L-lactide, characterized in that Purifying L-lactide from crude lactide comprises the following steps: (1) Add crude lactide and solvents such as ethyl acetate into a round-bottom flask, place a rotor and zeolite, and continue stirring and heating at 60-80°C; (2) placing the reaction mixture in a 60-80°C water bath and condensing and reflux for 2 h; (3) After the mixture obtained in step (2) is cooled in an ice bath, it is filtered and washed with a small amount of ethyl acetate, and the process is repeated 3-5 times. The mixture is then placed in a vacuum drying oven at 30-50° C. to obtain an intermediate product. (4) heating the intermediate product obtained in step (3) until it is melted, adding the same mass of ultrapure water, and stirring at 20-35° C. for 40-50 minutes; (5) The mixture obtained in step (4) is filtered and washed with mother liquor in small amounts for multiple times, and then dried to obtain a product.

2. The method for purifying L-lactide according to claim 1, characterized in that: The mass ratio of the crude lactide and solvent such as ethyl acetate in step (1) is 3:1-4:

1.

3. The method for purifying L-lactide according to claim 1, characterized in that: The solvents that can be used for the recrystallization in step (1) include ethyl acetate, methyl ethyl ketone, benzene or alkylbenzene, lower alcohols, ketones, lipids or mixtures thereof, among which ethyl acetate is the most commonly used.

4. The method for purifying L-lactide according to claim 1, characterized in that: The water bath temperature in step (2) is 60-80°C.

5. The method for purifying L-lactide according to claim 1, characterized in that: The mass ratio of the intermediate product to ultrapure water in step (3) is 1:1-1:1.

5.

6. The method for purifying L-lactide according to claim 1, characterized in that: The drying temperature and time in step (3) are 30-50° C. for 4-6 hours.

7. The method for purifying L-lactide according to claim 1, characterized in that: The water bath temperature in step (4) is 20-35°C.

8. The method for purifying L-lactide according to claim 1, characterized in that: The drying temperature and time in step (5) are 30-50° C. for 6-8 hours.

9. High-purity L-lactide prepared according to the method of any one of claims 1 to 7.

10. The high-purity L-lactide according to claim 8, characterized in that: The L-lactide has good optical purity and chemical purity, and high molecular weight polylactic acid can be obtained through ring-opening polymerization.

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