Method for preparing high-purity lactide through double-solvent phase change extraction

Abstract

The invention relates to the field of chemistry and chemical engineering, in particular to a method for preparing high-purity lactide through double-solvent phase change extraction. A compound containing oxygen-containing functional groups such as alcohol, ether, ketone, carboxylic acid and amide is selected as a hydrogen bond acceptor solvent, and water is selected as a hydrolysis solvent. Wherein the hydrogen bond acceptor solvent is combined with free acid impurities (hydrogen bond donors) in the crude lactide to form a deep eutectic solvent, and the water hydrolyzes meso-lactide into lactic acid retaining D, L-lactide. In the process, the whole system is subjected to heating, melting, blending, cooling, crystallizing and separating out, finally, solid-phase high-purity D, L lactide is subjected to separation and drying treatment, the chemical purity is improved to 99% or above from 40%-95%, and the optical purity is improved to 98% or above. According to the scheme, the operation is simple and convenient, the separation efficiency is high, the method is environment-friendly, the energy consumption is remarkably reduced, the defects of traditional high-temperature distillation and repeated recrystallization are avoided, and an efficient and environment-friendly new scheme is provided for industrial purification of lactide.

Description

Technical Field

[0001] The present invention relates to the field of chemistry and chemical engineering, and more particularly to a method for preparing high-purity lactide by dual-solvent phase-change extraction. This method is suitable for the industrial purification of lactide, significantly improving its chemical and optical purity, and is therefore widely applicable to the production of polylactic acid (PLA) and its derivatives. Background Art

[0002] Industrial lactide production typically uses lactic acid as a raw material, which is dehydrated to produce oligomeric lactic acid, which is then thermally decomposed to produce lactide. This process typically yields a mixture of three optical isomers: D-lactide, L-lactide, and meso-lactide. Furthermore, the raw materials and reaction conditions may also result in the formation of free acid, residual oligomeric lactic acid, and other impurities. These byproducts can affect the purity of the lactide, with the presence of meso-lactide, in particular, significantly reducing the optical purity.

[0003] In order to obtain high-purity lactide, the traditional process usually uses the following methods for purification:

[0004] 1) High-temperature distillation: Distillation can significantly improve the chemical purity of lactide by separating it from impurities such as oligolactic acid and free acid. However, this method has disadvantages such as high energy consumption (distillation temperatures are generally above 150°C), complex equipment (high temperatures place strict demands on distillation equipment materials), and difficulty in improving optical purity (the optical activity of meso-lactide and D,L-lactide is difficult to separate at high temperatures).

[0005] 2) Recrystallization: Recrystallization selectively precipitates the target isomer from a lactide mixture by adjusting the solubility of the solvent system. This method can improve the optical purity of lactide to a certain extent, but it has drawbacks such as low efficiency (requiring multiple crystallizations, which is time-consuming and labor-intensive), high product loss (each recrystallization results in a certain amount of product loss), and high solvent consumption (using large amounts of organic solvents, which not only increases costs but also may cause environmental pollution).

[0006] 3) Extraction and Separation: Extraction technology has been explored in recent years for lactide purification. For example, selective solvents are used to dissolve and separate free acid impurities and meso-lactide, retaining D,L-lactide of high optical purity. However, traditional extraction methods often require multiple operations, resulting in low solvent recovery rates and limited industrial application.

[0007] To address the aforementioned issues, researchers at home and abroad have proposed several patented technologies for lactide purification. Patent CN103128359A discloses a process for purifying lactide using solvent extraction. This patent employs an organic solvent system to remove free acid impurities and meso-lactide from lactide through repeated extraction, ultimately yielding high-purity D,L-lactide. However, this method requires multiple extractions, resulting in high solvent consumption; it has limited effect on improving optical purity and still requires subsequent crystallization. Patent CN101278991A discloses a method for purifying lactide using continuous distillation. This patent employs segmented continuous distillation technology to separate impurities from lactide by controlling distillation temperature and pressure. This patent significantly improves chemical purity and is suitable for large-scale industrial production. However, it has limited impact on optical purity, and the distillation process is energy-intensive, placing stringent requirements on equipment. Patent CN109562476A reports a multi-solvent recrystallization process for purifying lactide. This patent utilizes two or more solvent systems for multiple recrystallizations, optimizing the solvent ratio to increase the optical purity of D,L-lactide. While this patent achieves improved optical purity and relatively low operating temperatures, it still suffers from the drawback of requiring a high number of recrystallizations. Summary of the Invention

[0008] This invention provides a method for preparing high-purity lactide by dual-solvent phase-change extraction. Through a designed dual-solvent system and phase-change process, the chemical and optical purity of lactide are significantly improved. This method combines the selective binding properties of a deep eutectic solvent with the efficient separation effects of a hydrolysis reaction, overcoming the shortcomings of traditional high-temperature distillation, multiple recrystallizations, and conventional extraction techniques. It offers the advantages of high separation efficiency, ease of operation, low energy consumption, and environmental friendliness.

[0009] A method for preparing high-purity lactide by dual-solvent phase change extraction comprises the following steps:

[0010] S1: A hydrogen bond acceptor solvent mixed with water is added to the crude lactide as an extractant;

[0011] S2: Heat and stir for 0.1-3h to form a homogeneous liquid.

[0012] S3: Cooling the solution obtained in step S2 to allow lactide to crystallize;

[0013] S4: solid-liquid separation and drying to obtain lactide with a chemical purity of ≥99% and an optical purity of ≥98%.

[0014] Preferably, the hydrogen bond acceptor solvent is a compound containing oxygen functional groups such as alcohol, ether, ketone, carboxylic acid, or amide, and is selected from one or more of cyclohexanol, diisoamyl ether, propiophenone, and N,N-dimethylformamide.

[0015] Preferably, the hydrogen bond acceptor forms a deep eutectic solvent with lactic acid and its oligomers through hydrogen bonding.

[0016] Preferably, the amount of water used in step S1 is 5-50 wt% of the hydrogen bond acceptor solvent, and the amount of the extractant added is 1-20 wt% of the crude lactide.

[0017] Preferably, in step S2, the temperature is raised in the range of 80-140°C.

[0018] Preferably, in step S3, the temperature is lowered to a range of -10-40°C.

[0019] Preferably, the solid-liquid separation in step S4 is performed by centrifugation or filtration technology.

[0020] Preferably, in step S4, the drying method adopts normal pressure or negative pressure drying, the vacuum degree range is 0-95kPa, and the temperature is 10-60°C.

[0021] Preferably, in step S4, the chemical purity refers to the total content of D-lactide, L-lactide, and meso-lactide ≥ 99%, and the optical purity refers to the amount of D, L-lactide in the total lactide ≥ 98%.

[0022] In addition: In the steps of preparing high-purity lactide: 1. Selective solvent binding: Utilizing the formation principle of low eutectic solvents, compounds containing oxygen-containing functional groups are selected as hydrogen bond acceptor solvents to combine with free acid impurities (hydrogen bond donors) in crude lactide, thereby achieving efficient separation of impurities.

[0023] 2. Optical selective hydrolysis: By adding water as the hydrolysis solvent, meso-lactide is selectively hydrolyzed to lactic acid under specific conditions while retaining D,L-lactide.

[0024] 3. Phase change process to enhance separation: Through the phase change extraction process of heating to melt and mix, and cooling to induce crystallization, efficient crystallization and precipitation of lactide is achieved, thereby further improving its purity.

[0025] The present invention has the following beneficial effects:

[0026] The dual-solvent phase-change extraction method for preparing high-purity lactide provided by the present invention offers significant advantages. First, a deep eutectic solvent system is used to selectively separate free acid impurities and meso-lactide, avoiding the multiple operations required by traditional extraction and distillation methods and significantly improving separation efficiency. Second, a hydrolysis process converts meso-lactide into lactic acid while retaining D,L-lactide, significantly improving optical purity. This technology eliminates the need for high-temperature distillation or multiple recrystallizations, avoiding the use of large amounts of organic solvents, reducing energy consumption and environmental pollution. It is also simple to operate, and the resulting lactide has a chemical purity of ≥99% and an optical purity of ≥98%, meeting the requirements of high-end polylactic acid (PLA) production. This solution offers comprehensive advantages in separation efficiency, energy consumption, environmental friendliness, and improved optical purity, providing a new, highly efficient solution for industrial lactide purification. This technical solution is particularly suitable for industrial production, and the resulting lactide has high chemical purity (≥99%) and high optical purity (≥98%), meeting the stringent raw material requirements for polylactic acid (PLA) production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 This is a flow chart of the dual-solvent phase change extraction process of the present invention.

[0029] Figure 2 It is the Fourier transform infrared spectrum of lactic acid of the present invention.

[0030] Figure 3 This is the Fourier infrared spectrum of N,N-dimethylformamide of the present invention.

[0031] Figure 4 It is a Fourier transform infrared spectrum of the deep eutectic solvent formed by lactic acid and N,N-dimethylformamide of the present invention.

[0032] Figure 5 It is the Fourier infrared spectrum of cyclohexanol of the present invention.

[0033] Figure 6 It is a Fourier transform infrared spectrum of the deep eutectic solvent formed by lactic acid and cyclohexanol of the present invention. DETAILED DESCRIPTION

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0035] Example:

[0036] A method for preparing high-purity lactide by dual-solvent phase change extraction comprises the following steps:

[0037] S1: A hydrogen bond acceptor solvent mixed with water is added to the crude lactide as an extractant;

[0038] S2: Heat and stir for 0.1-3h to form a homogeneous liquid.

[0039] S3: Cooling the solution obtained in step S2 to allow lactide to crystallize;

[0040] S4: solid-liquid separation and drying to obtain lactide with a chemical purity of ≥99% and an optical purity of ≥98%.

[0041] In the above method: 1. Material and solvent selection

[0042] Hydrogen bond acceptor solvents are selected from compounds containing oxygen-containing functional groups such as alcohols, ethers, ketones, carboxylic acids, and amides, such as cyclohexanol, diisoamyl ether, propiophenone, and N,N-dimethylformamide. These solvents can bind to free acid impurities in crude lactide to form a deep eutectic solvent, enabling extraction and separation. Ultrapure water is used as the hydrolysis solvent for the selective hydrolysis of meso-lactide. The raw material is crude lactide with a chemical purity of 40%-95% and an optical purity of 70-90%, which often contains meso-lactide, free acid impurities, and oligomeric lactic acid.

[0043] 2. Process flow (see attached flow chart Figure 1 )

[0044] (1) Mixing solvent to prepare extractant and mixing with crude lactide

[0045] A hydrogen bond acceptor solvent is mixed with a certain amount of water to form an extractant, wherein the amount of the extractant added is 1-20 wt% of the crude lactide, and the proportion of water is 5-50 wt% of the hydrogen bond acceptor solvent. The extractant is added to the crude lactide and mixed thoroughly to form an initial reaction system.

[0046] (2) Heat and stir thoroughly

[0047] The mixture is gradually heated to 80-140°C and stirred until the system forms a uniform, stable liquid state. Stirring is maintained for 0.1-3 hours. This process selectively separates the free acid impurities and meso-lactide through the dissolution and extraction effects of the solvent.

[0048] (3) Cooling to precipitate lactide

[0049] The homogeneous liquid obtained in step S2 is gradually cooled to -10-40°C to induce crystallization of D,L-lactide while impurities remain in the liquid phase. During this process, lactide is efficiently separated in solid form.

[0050] (4) Solid-liquid separation and drying

[0051] After separating the precipitated solid from the liquid by centrifugation or filtration, the solid is dried under normal or negative pressure at a temperature of 10-60°C and a vacuum range of 0-95 kPa. The resulting product is high-purity lactide with a chemical purity of ≥99% and an optical purity of ≥98%. This method utilizes a deep eutectic solvent system in conjunction with a hydrolysis reaction to efficiently separate free acid impurities and meso-lactide in a single process flow, avoiding the inefficiencies associated with multiple operations in conventional techniques. During the hydrolysis process, meso-lactide is converted to lactic acid while retaining the optical activity of D,L-lactide, significantly improving optical purity (≥98%). This method avoids the use of traditional high-temperature distillation processes, reducing energy consumption; eliminates the use of large amounts of organic solvents, minimizing environmental pollution; and allows for solvent recycling, further improving resource utilization. The process is highly industrially feasible, requires minimal equipment, and is suitable for large-scale industrial production. The product quality is superior, with the chemical purity of the obtained lactide ≥99% and the optical purity ≥98%, which can meet the requirements of high-end polylactic acid production.

[0052] As an implementation method: N,N-dimethylformamide-water system. This embodiment provides a method for preparing high-purity lactide by dual-solvent phase change extraction, which is as follows: Weigh 5g of N,N-dimethylformamide (DMF), add 0.5g of deionized water, and mix well as an extractant. Add 100g of crude lactide to a stirring container, slowly add the above-mentioned extractant, and keep stirring to ensure that the crude lactide and the extractant are fully mixed. Gradually heat the mixture to 110°C while keeping stirring. At this point, the system gradually forms a uniform and stable liquid, and the free acid impurities and part of the meso-lactide in the crude lactide are dissolved by DMF to form a low eutectic solvent. Continue stirring for 1 hour to ensure that the extractant and the impurities fully react and the reaction is complete. Gradually cool the system from 110°C to 5°C, and control the cooling rate to 1°C / min using a cooling device to avoid the crystallization process being too fast and causing uneven crystal particles. During the cooling process, D,L-lactide gradually crystallized and precipitated as a solid, while impurities (such as meso-lactide and free acid) remained in the mother liquor. The precipitated solid was separated from the mother liquor by centrifugation at 4000 rpm for 10 minutes. The solid was transferred to a vacuum drying oven and dried at 50°C and 80 kPa for 6 hours to obtain high-purity lactide. Liquid chromatography analysis showed that the chemical purity increased from 85% to 99.2%, and polarimetry analysis showed that the optical purity increased from 80% to 98.2%. The total lactide yield was 92%.

[0053] As a possible implementation method: Preparation of a cyclohexanol-water system: Weigh 5g of cyclohexanol, add 0.6g of deionized water, and mix thoroughly to serve as the extractant. Add 100g of crude lactide to a stirred vessel, slowly add the extractant, and maintain stirring until thoroughly mixed. Gradually heat the mixture to 100°C and maintain stirring for 1 hour to allow the free acid impurities and some meso-lactide in the crude lactide to form a deep eutectic solvent with the cyclohexanol. Cool the mixture to 8°C at a controlled cooling rate of 1.5°C / min. D,L-lactide crystallizes, while impurities remain in the liquid phase. After removing the mother liquor by centrifugation at 4000 rpm for 10 minutes, the solid is dried at 50°C under a vacuum of 70 kPa for 6 hours to obtain lactide with a chemical purity of 98.8%, an optical purity of 97.5%, and an overall yield of 90%.

[0054] As a possible implementation method: Preparation of propiophenone-water system: Weigh 6g of propiophenone, add 0.8g of deionized water, and mix well as an extractant. Add 100g of crude lactide to a stirring container, slowly add the extractant, stir thoroughly, and heat the mixture to 100°C. Keep stirring for 1 hour to make the solution uniform and the impurities fully dissolved. The system is gradually cooled to 7°C, and D,L-lactide precipitates in the form of crystals. After removing the mother liquor by centrifugation at 4000rpm for 10 minutes, the solid is transferred to a vacuum drying oven and dried at 50°C and a vacuum degree of 75kPa for 5 hours to obtain lactide with a chemical purity of 98.7% and an optical purity of 97.3%, with a total yield of 89%.

[0055] As a possible implementation method: Preparation of a diisoamyl ether-water system: Weigh 7g of diisoamyl ether, add 0.7g of deionized water, and mix well as an extractant. Place 100g of crude lactide in a stirred container, add the extractant, and heat to 85°C. Maintain stirring for 1 hour until the system forms a homogeneous liquid. Then cool to 10°C, and D,L-lactide gradually crystallizes and precipitates, leaving impurities in the mother liquor. After removing the mother liquor by centrifugation at 4000rpm for 10 minutes, the solid is dried at 50°C and 80kPa vacuum for 6 hours to obtain lactide with a chemical purity of 98.6% and an optical purity of 97.4%, for a total yield of 90%.

[0056] As a possible implementation method: Preparation of N,N-dimethylacetamide-water system: Weigh 5g of N,N-dimethylacetamide (DMAc), add 0.6g of deionized water, and mix well as an extractant. Add 100g of crude lactide to a stirring container, gradually add the extractant and mix thoroughly. Heat the mixture to 105°C and keep stirring for 1 hour. The free acid impurities and part of the meso-lactide in the crude lactide will dissolve to form a low eutectic solvent. Cool the system to 6°C to induce crystallization of D,L-lactide. After centrifugation at 4000rpm for 10 minutes, the crystals were dried at 50°C and vacuum at 75kPa for 5 hours to obtain lactide with a chemical purity of 98.8% and an optical purity of 97.2%, with a total yield of 89%.

[0057] As a possible implementation method: Preparation of a butanone-water system: Weigh 6g of butanone, add 0.5g of deionized water, and mix as the extractant. Place 100g of crude lactide in a stirred container, add the extractant, and heat to 75°C. Stir for 1 hour to fully dissolve the impurities. Then cool to 8°C to allow D,L-lactide to crystallize. After removing the mother liquor by centrifugation at 4000 rpm for 10 minutes, the crystals are dried at 50°C and 70kPa vacuum for 6 hours to obtain lactide with a chemical purity of 98.5% and an optical purity of 97.0%, for a total yield of 88%.

[0058] As a possible implementation method: Preparation of hexanol-water system: Weigh 7g of hexanol, add 0.4g of deionized water, and mix well as an extractant. Add 100g of crude lactide to a reaction vessel, slowly add the extractant and mix thoroughly. Heat the mixture to 70°C and keep stirring for 0.5 hours to dissolve the impurities in the crude lactide. The system is then cooled to 10°C, and D,L-lactide precipitates in the form of crystals. After centrifugation at 4000rpm for 10 minutes, the crystals are dried at 50°C and a vacuum of 75kPa for 4 hours to obtain lactide with a chemical purity of 98.3% and an optical purity of 97.1%, with a total yield of 87%.

[0059] In the above examples, the use of alcohols and amides can effectively improve the purity of the product and obtain a higher yield. Figure 2-6 Fourier transform infrared spectra of lactic acid, N,N-dimethylformamide, cyclohexanol, and their formation into a deep eutectic solvent are shown. The figure shows that before the formation of the deep eutectic solvent, hydroxyl-containing compounds (such as alcohols and acids) exhibit a significant stretching vibration peak at 3400 cm-1. However, after the formation of the deep eutectic solvent, this peak decreases significantly, indicating that the deep eutectic solvent is formed through hydrogen bonding. The formation of the deep eutectic solvent strengthens the binding between the hydrogen bond acceptor extractant and the free acid impurity, allowing the free acid to remain in the liquid phase after cooling and crystallization, ultimately achieving phase change extraction and separation. In addition, it should be noted that the article "Application of Deep Eutectic Solvents in Polymer Synthesis" (DOI: 10.7536 / PC220306) points out that the formation of deep eutectic solvents requires hydrogen bond donors and hydrogen bond acceptors to be formed under heating conditions. In the present invention, the free acid impurities act as hydrogen bond donors and the added extractant acts as a hydrogen bond acceptor. The process of heating and stirring provides the conditions for the formation of the deep eutectic solvent. Combined with the infrared spectrum data, the formation of the deep eutectic solvent can be confirmed.

[0060] The above-mentioned technical solution, by combining a deep eutectic solvent system, selective hydrolysis, and phase-change extraction separation techniques, achieves efficient separation while significantly improving the chemical and optical purity of lactide. This method overcomes the limitations of traditional high-temperature distillation, multiple recrystallizations, and conventional extraction processes, offering significant advantages in technological innovation, environmental friendliness, industrial feasibility, and final product quality. It is a more efficient, green, and sustainable method for lactide purification. This technology will effectively promote the upgrading of lactide industrial production, lay a solid foundation for the widespread application of polylactic acid materials, and also have important significance for the development of green chemical industry and polymer materials.

[0061] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing high-purity lactide by dual-solvent phase change extraction, characterized in that: The following steps are involved: S1: A hydrogen bond acceptor solvent mixed with water is added to the crude lactide as an extractant; S2: Heat and stir for 0.1-3h to form a homogeneous liquid; S3: Cooling the solution obtained in step S2 to allow lactide to crystallize; S4: solid-liquid separation and drying to obtain lactide with a chemical purity of ≥99% and an optical purity of ≥98%.

2. The method according to claim 1, characterized in that The hydrogen bond acceptor solvent is a compound containing oxygen functional groups such as alcohol, ether, ketone, carboxylic acid, or amide, and is selected from one or more of cyclohexanol, diisoamyl ether, propiophenone, and N,N-dimethylformamide.

3. The method according to claim 1, characterized in that The hydrogen bond acceptor forms a deep eutectic solvent with lactic acid and its oligomers through hydrogen bonding.

4. The method according to claim 1, wherein In step S1, the amount of water used is 5-50 wt% of the hydrogen bond acceptor solvent, and the amount of the extractant added is 1-20 wt% of the crude lactide.

5. The method according to claim 1, wherein In step S2, the temperature is raised to 80-140°C.

6. The method according to claim 1, characterized in that In step S3, the temperature is lowered to -10-40°C.

7. The method according to claim 1, characterized in that In step S4, the solid-liquid separation is performed by centrifugation or filtration technology.

8. The method according to claim 1, characterized in that In step S4, the drying method adopts normal pressure or negative pressure drying, the vacuum degree range is 0-95kPa, and the temperature is 10-60°C.

9. The method according to claim 1, characterized in that In step S4, the chemical purity refers to the total content of D-lactide, L-lactide, and meso-lactide being ≥99%, and the optical purity refers to the amount of D and L-lactide in the total lactide being ≥98%.

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