Method for extracting, separating and purifying crude lactide based on eutectic solvent assisted phase change

Abstract

The invention relates to the technical field of chemical separation and purification, in particular to a method for separating and purifying crude lactide based on deep eutectic solvent assisted phase change extraction, which comprises the steps of preparation of a deep eutectic solvent system and precipitation of lactide crystals, wherein the hydrogen bond acceptor is an oxygen-containing functional group, and the hydrogen bond donor is a free acid impurity in crude lactide. The method has the advantages that a compound containing an oxygen functional group is selected as a hydrogen bond acceptor, the compound and free acid impurities in crude lactide form a eutectic solvent system, the eutectic solvent system is heated to form a homogeneous solution, then the homogeneous solution is cooled, lactide is subjected to phase change and is separated out in a crystal form, and finally high-purity lactide is obtained through separation and drying operation.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical separation and purification, and particularly relates to a method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction. Background Art

[0002] Lactide is a key monomer for the production of polylactic acid (PLA), and its purity directly affects the molecular weight, mechanical properties, and degradation characteristics of PLA. However, crude lactide often contains free acid impurities such as lactic acid monomers and oligomers (such as dimers and trimers), which can interfere with the polymerization process and reduce the quality of the final product. Therefore, how to efficiently separate and purify lactide has become an important issue in industrial production.

[0003] Existing separation and purification technologies mainly include: distillation method, water extraction method, and recrystallization method.

[0004] The distillation method is a classic method for separating components in crude lactide based on the boiling point differences of each component. For example, high-vacuum distillation technology (such as patent CN102875522B) reduces the risk of lactide pyrolysis by reducing the system pressure and improves the separation efficiency at the same time. However, this method relies on high-vacuum equipment (pressure as low as 300 - 3000 Pa), with high equipment investment and energy consumption, and may still cause lactide thermal decomposition or oxidation discoloration at high temperatures (135 - 155°C), affecting the product purity and yield. In addition, the distillation method has strict requirements for operating conditions (such as temperature gradient and pressure control), and it is difficult to achieve industrial continuous production.

[0005] The hydrolysis method purifies by selectively hydrolyzing impurities (such as lactic acid oligomers), but it has significant defects. Since lactide itself is a cyclic dimer of lactic acid, the hydrolysis reaction easily causes its own ring-opening hydrolysis, reducing the yield of the target product. In addition, hydrolysis by-products (such as lactic acid monomers and short-chain oligomers) need to be additionally treated, increasing the process complexity and cost.

[0006] The recrystallization method realizes the crystallization separation of lactide through solvent selection and is a commonly used method in laboratories. For example, patent CN105646440A uses a water-ethanol double-solvent combination with melt crystallization to improve the purity by gradient cooling. However, this method requires multiple recrystallization operations (such as alternating the use of ethanol and ethyl acetate), with a large solvent consumption (mass ratio up to 20:1), and flammable and explosive organic solvents (such as toluene and n-hexane) bring safety and environmental hazards. In addition, meso-lactide in crude lactide is difficult to crystallize and is prone to residue, and requires additional distillation or composite solvent treatment, further increasing the cost and process complexity. Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies in the prior art and provide a method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction. By selecting a compound with an oxygen-containing functional group as the hydrogen bond acceptor, a eutectic solvent system is formed with the free acid impurities in the crude lactide. After heating to form a homogeneous solution, the temperature is then lowered to cause the lactide to undergo a phase change and precipitate in the form of crystals. Finally, through separation and drying operations, high-purity lactide is obtained.

[0008] The present invention is achieved through the following technical solutions: On the one hand, a method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction is provided. This method includes the preparation of a eutectic solvent system and the precipitation of lactide crystals; among them, the hydrogen bond acceptor is a compound with an oxygen-containing functional group, and the hydrogen bond donor is the free acid impurity in the crude lactide.

[0009] Furthermore, the oxygen-containing functional group is selected from any one or several of compounds containing alcohol, ether, ketone, ester, carboxylic acid, or amide functional groups; The free acid impurities in the crude lactide are any one or several of lactic acid monomer, lactic acid dimer, and lactic acid trimer.

[0010] Even further, the oxygen-containing functional group is selected from at least one of amyl alcohol, diisopentyl ether, acetophenone, ethyl lactate, and N,N-dimethylformamide.

[0011] Furthermore, the precipitation of the lactide crystals is by cooling precipitation. The prepared eutectic solvent system is cooled to -10°C - 70°C at a rate of 2 - 10°C / min.

[0012] Furthermore, this method also includes the separation and drying of the lactide crystals; among them, the separation method is centrifugation, and the centrifugation rate is 2000 - 4000 rpm; the drying method is vacuum drying or forced air drying, and the drying temperature is 25°C - 80°C.

[0013] Furthermore, the content of lactide in the crude lactide is 60% - 95%.

[0014] Furthermore, this method also includes the recovery treatment of the separated eutectic solvent system, and the hydrogen bond acceptor is separated and recovered through water washing and alkali washing steps to realize its recycling.

[0015] Furthermore, the purity of the lactide separated by this method reaches more than 98.5%.

[0016] Beneficial effects: In the continuous process of actual production, the initial temperature of the obtained crude lactide is usually between 100 - 150 °C, and the content is usually greater than 80%. Therefore, a hydrogen bond acceptor can be directly added thereto to form a deep eutectic solvent. Then, through the steps of cooling crystallization, separation, and drying, a high-purity lactide product is obtained. Compared with traditional distillation, the energy consumption is significantly reduced, and further reactions of the obtained lactide product at high temperatures can be avoided; compared with the rectification method, this method avoids the secondary heating of the crude lactide and the complex process flow.

[0017] Generally speaking, compared with the existing lactide separation technologies, the present invention ingeniously utilizes the characteristics of impurities in the crude lactide to form a deep eutectic solvent, completely separating the free acid impurities originally mixed in the crude lactide in a liquid form. The whole process is simple to operate, with mild conditions, effectively reducing energy consumption and carbon dioxide emissions, reducing the use of organic solvents, reducing environmental pollution and treatment costs, reducing the pyrolysis risk of lactide, and increasing the product yield.

[0018] The present invention utilizes the characteristics of the deep eutectic solvent to achieve efficient separation of lactide and impurities, with a purity of over 98%, meeting the requirements for lactide as an intermediate to synthesize PLA. Moreover, the content of the present invention is easy to realize industrial production, and can fully contribute to the development of the biodegradable plastic industry in China. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram showing the formation process of the deep eutectic solvent (DES) of the present invention; Figure 2 It is an analysis diagram of Fourier transform infrared spectroscopy (FTIR) of the present invention; Figure 3 It is a flow chart showing the separation and purification of crude lactide by deep eutectic solvent-assisted phase change extraction of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the following further details the present invention with reference to specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] A method for separating and purifying crude lactide by deep eutectic solvent-assisted phase change extraction, as Figure 3 shown, the preparation of the deep eutectic solvent; Select a compound containing an alcohol, ether, ketone, ester, carboxylic acid or amide functional group as a hydrogen bond acceptor (such as pentanol, diisopentyl ether, phenylacetone, ethyl lactate, N,N-dimethylformamide or a combination thereof), and mix it with the free acid impurities (such as lactic acid monomer, lactic acid dimer, lactic acid trimer, etc.) in the crude lactide. Heat the mixture to 70°C to 180°C to form a homogeneous eutectic solvent system.

[0022] Precipitation of lactide; Cool the eutectic solvent system to -10°C to 70°C, and use the temperature reduction to trigger the precipitation of lactide crystals to achieve preliminary separation.

[0023] Separation and drying of crystals; Separate the lactide crystals by centrifugation, filtration or precipitation methods, and further remove residual impurities and moisture by blowing drying or vacuum drying. The drying temperature is from room temperature to 80°C. The purity of the separated and dried lactide crystals can be increased to more than 98%.

[0024] Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The experimental methods not specified in the following examples are usually carried out under conventional conditions or according to the conditions recommended by the manufacturer. Unless otherwise stated, all percentages, ratios, proportions or parts are by weight.

[0025] The reagents and raw materials used in the examples and comparative examples of the present invention can be obtained through commercial channels without special instructions.

[0026] Example 1 A method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction, the method comprising the following steps: Step 1) Formation of a eutectic solvent system; Take 10 g of crude lactide (lactide content is 82.30%), add 0.8 g of pentanol; heat and stir well at 140°C to form a homogeneous solution; Step 2) Precipitation of lactide crystals; Cool the solution to 30°C, and lactide crystals precipitate; Step 3) Separation and drying of lactide crystals; Centrifuge at 3500 rpm for 30 minutes to collect the crystal part; dry the crystals by blowing at room temperature for 8 hours, and the purity of the obtained lactide product is 98.88%.

[0027] On the basis of the above scheme, the separated eutectic solvent system is recovered, and the hydrogen bond acceptor is recycled by separation through water washing and alkali washing separation.

[0028] Example 2 A method for separating and purifying crude lactide by low eutectic solvent-assisted phase change extraction, the method comprising the following steps: Step 1) Formation of a low eutectic solvent system; Take 10 g of crude lactide (lactide content is 70.25%), add 1 g of ethyl lactate, heat the mixture to 100 °C, and stir well to form a uniform low eutectic solvent system; Step 2) Precipitation of lactide crystals; Cool the solution to 20 °C, and lactide crystals gradually precipitate; Step 3) Separation and drying of lactide crystals; Centrifuge at 3000 rpm for 30 minutes to collect the crystal part; place the crystals in a vacuum dryer at 40 °C for 12 h to obtain high-purity lactide; the lactide content of the obtained lactide product is 98.96%.

[0029] On the basis of the above scheme, the separated low eutectic solvent system is recycled, and the hydrogen bond acceptor is recycled through water washing and alkali washing separation.

[0030] Example 3 A method for separating and purifying crude lactide by low eutectic solvent-assisted phase change extraction, the method comprising the following steps: Step 1) Formation of a low eutectic solvent system; Take 12 g of crude lactide (lactide content is 85.10%), add 0.8 g of diisopentyl ether, heat the mixture to 110 °C, and stir well to form a uniform low eutectic solvent system; Step 2) Precipitation of lactide crystals; Cool the solution to 15 °C, and lactide crystals gradually precipitate; Step 3) Separation and drying of lactide crystals; Centrifuge at 4000 rpm for 45 minutes to collect the crystal part; place the crystals in a vacuum dryer at 35 °C for 10 h to obtain high-purity lactide; the lactide content of the obtained lactide product is 98.76%.

[0031] On the basis of the above scheme, the separated low eutectic solvent system is recycled, and the hydrogen bond acceptor is recycled through water washing and alkali washing separation.

[0032] Example 4 A method for separating and purifying crude lactide by low eutectic solvent-assisted phase change extraction, the method comprising the following steps: Step 1) Formation of a low eutectic solvent system; Take 15 g of crude lactide (lactide content is 60.45%), add 2 g of phenylacetone, heat the mixture to 130 °C, and stir well to form a homogeneous eutectic solvent system; Step 2) Precipitation of lactide crystals; Cool the solution to 10 °C, and lactide crystals gradually precipitate; Step 3) Separation and drying of lactide crystals; Centrifuge at 2000 rpm for 30 minutes to collect the crystal part; Place the crystals in a blast dryer at 50 °C for 8 h to obtain high-purity lactide; The lactide content of the obtained lactide product is 98.95%.

[0033] On the basis of the above scheme, the separated eutectic solvent system is recycled, and the hydrogen bond acceptor is recycled through water washing and alkali washing separation.

[0034] Example 5 A method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction, the method comprising the following steps: Step 1) Formation of eutectic solvent system; Take 8 g of crude lactide (lactide content is 78.50%), add 0.6 g of N,N-dimethylformamide, heat the mixture to 120 °C, and stir well to form a homogeneous eutectic solvent system; Step 2) Precipitation of lactide crystals; Cool the solution to 5 °C, and lactide crystals gradually precipitate; Step 3) Separation and drying of lactide crystals; Centrifuge at 3000 rpm for 40 minutes to collect the crystal part; Place the crystals in a vacuum dryer at 30 °C for 16 h to obtain high-purity lactide; The lactide content of the obtained lactide product is 99.12%.

[0035] On the basis of the above scheme, the separated eutectic solvent system is recycled, and the hydrogen bond acceptor is recycled through water washing and alkali washing separation.

[0036] Example 6 A method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction, the method comprising the following steps: Step 1) Formation of eutectic solvent system; Take 20 g of crude lactide (lactide content is 90.20%), add 1 g of ethyl lactate and 1 g of diisopentyl ether, heat the mixture to 150 °C, and stir well to form a homogeneous eutectic solvent system; Step 2) Precipitation of lactide crystals; Cool the solution to 25 °C, and lactide crystals gradually precipitate; Step 3) Separation and drying of lactide crystals; Centrifuge at 4000 rpm for 50 minutes to collect the crystal part; place the crystals in a vacuum at 40 °C for 10 h to obtain high-purity lactide; the lactide content of the obtained lactide product is 98.67%.

[0037] On the basis of the above scheme, the eutectic solvent system after separation is recycled, and the hydrogen bond acceptor is recycled through separation by steam washing and alkali washing.

[0038] On the basis of the above scheme, as Figure 1 shown, this figure shows the formation mechanism and phase change process of the eutectic solvent. At room temperature, lactic acid (hydrogen bond donor) and hydrogen bond acceptor (such as pentanol, ethyl lactate, N,N-dimethylformamide, etc.) do not fully form due to intermolecular hydrogen bonding, and lactic acid will exist in the form of crystals. By heating to 70 °C to 180 °C, the system reaches a homogeneous liquid state, and at this time, a stable hydrogen bond network is formed between the hydrogen bond donor and the acceptor. Subsequently, when the temperature is lowered to -10 °C to 70 °C, the system maintains a homogeneous liquid state, confirming the formation of the eutectic solvent. At the same time, lactide selectively precipitates due to its reduced solubility. This process realizes the synergistic effect of solvent system phase change and target product separation through temperature control. That is to say, at room temperature, the hydrogen bond donor and the hydrogen bond acceptor will not completely mix into a liquid. Only by heating, then allowing it to mix evenly and then cooling, can it form a homogeneous mixture, a homogeneous liquid, and this homogeneous liquid is called the eutectic solvent. In addition, it should be noted that the physical and chemical properties of lactic acid are basically the same as those of lactic acid dimers and trimers, and the separation and purification of lactic acid dimers and trimers cannot be achieved worldwide at present. Therefore, lactic acid can be used to reflect the eutectic solvent formation ability of lactic acid, lactic acid dimers, and lactic acid trimers.

[0039] As Figure 2 shown, this figure compares the infrared spectral characteristics of lactic acid (Lactic acid, LA), N,N-dimethylformamide (N,N-Dimethylformamide, DMF), and the eutectic solvent (Deep eutectic solvent, DES) formed by the two: lactic acid (LA) has a significant peak near 3500 cm -1 corresponding to the stretching vibration of the hydroxyl group (-OH) in lactic acid. There is no significant peak in DMF near 3500 cm -1 , while the intensity of the hydroxyl stretching vibration peak of the DES formed by the two blends significantly decreases or even disappears at 3500 cm -1 . This indicates that a hydrogen bond is formed between the hydroxyl hydrogen of lactic acid and the O atom in DMF, thus confirming the formation of DES.

[0040] The physical photo and infrared spectrum analysis jointly confirm that lactic acid can be used as a hydrogen bond donor to form a deep eutectic solvent with a hydrogen bond acceptor.

[0041] In summary, the present invention utilizes the unique selectivity of the deep eutectic solvent system to separate lactide from impurities through simple heating, cooling, and separation operations, avoiding the cumbersome processes of multiple recrystallizations and high-temperature rectification. The process conditions of this method are mild, it does not rely on high-energy-consuming equipment, and the selected hydrogen bond acceptor is a low-toxicity compound with a small dosage and good environmental friendliness. Moreover, the purity of the crude lactide (70% - 95%) can be increased to over 98%, which can be directly applied to the production of polylactic acid, significantly improving the performance of PLA products.

[0042] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for separating and purifying crude lactide by low eutectic solvent-assisted phase change extraction, which is characterized in that The method includes the preparation of a deep eutectic solvent system and the precipitation of lactide crystals; wherein, the hydrogen bond acceptor is an oxygen-containing functional group, and the hydrogen bond donor is the free acid impurity in crude lactide.

2. The method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction according to claim 1, characterized in that The oxygen-containing functional group is selected from any one or more of compounds containing alcohol, ether, ketone, ester, carboxylic acid or amide functional groups; The free acid impurities in the crude lactide are selected from any one or more of lactic acid monomer, lactic acid dimer and lactic acid trimer.

3. The method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction according to claim 1, wherein, The oxygen-containing functional group is selected from at least one of amyl alcohol, diisopentyl ether, phenylacetone, ethyl lactate, N,N-dimethylformamide.

4. The method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction according to claim 1, wherein The precipitation of the lactide crystals is by cooling precipitation, and the prepared deep eutectic solvent system is cooled to -10°C - 70°C at a rate of 2 - 10°C / min.

5. The method for separating and purifying crude lactide based on deep eutectic solvent-assisted phase change extraction according to claim 1, wherein The method also includes the separation and drying of the lactide crystals; wherein, the separation method is centrifugation, and the centrifugation rate is 2000 - 4000 rpm; the drying method is vacuum drying or air drying, and the drying temperature is 25°C - 80°C.

6. The method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction according to claim 1, wherein, The content of lactide in the crude lactide is 60% - 95%.

7. The method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction according to claim 1, wherein, The method also includes the recovery treatment of the separated deep eutectic solvent system, and the hydrogen bond acceptor is separated and recovered through water washing and alkali washing steps to realize its recycling.

8. The method for separating and purifying crude lactide based on eutectic solvent-assisted phase change extraction according to any one of claims 1-7, characterized in that, The purity of the lactide separated by this method reaches more than 98.5%.

Images