A method for efficiently catalyzing the synthesis of lactide

Abstract

The present invention relates to a method for efficiently catalytically synthesizing lactide, comprising: (1) adding lactic acid oligomers and a depolymerization catalyst into a reaction kettle, heating and heating, adding a protonated solvent to carry out a depolymerization reaction, and collecting crude lactide; (2) purifying and refining the crude lactide obtained in step (1) to obtain a lactide product that meets polymerization grade requirements. The method of the present invention uses a metal compound with strong alkalinity as a catalyst, and by adding a suitable protonated solvent, the racemization of lactide in the synthesis process can be suppressed, thereby accelerating the depolymerization reaction rate while ensuring product quality and yield.

Description

Technical Field

[0001] The invention belongs to the technical field of degradable materials, and in particular relates to a method for efficiently catalyzing and synthesizing lactide. Background Art

[0002] At present, commercial biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene succinate-adipate (PBSA), polybutylene terephthalate-adipate (PBAT), etc. Among them, PLA is currently the most widely used and has the most outstanding application prospects. It not only has the basic properties of general polymer materials, but also has better processing properties, physical and mechanical properties and biodegradability. It can be widely used in the packaging industry, textile industry, agricultural industry, consumer goods market, etc., and is considered to be the most likely biodegradable material to replace petroleum-based polyester. At present, the global PLA production capacity is about 500,000 tons, and the output is more than 200,000 tons. The main manufacturers include NatureWorks in the United States, Corbin Purac in the Netherlands, Mitsubishi in Japan, Zhejiang Hisun, etc., among which NatureWorks and Corbin Purac in the Netherlands account for about 70% of the global production capacity. There are not many production lines that have been built and put into production in China. Zhejiang Haizheng is currently the largest one. With the gradual implementation of the domestic plastic ban, my country's PLA production capacity expansion has accelerated. As of the end of November 2020, according to incomplete statistics, there are at least 1.53 million tons of PLA projects under construction or planned in China.

[0003] Industrialized polylactic acid synthesis is mainly obtained through the ring-opening polymerization of lactide: the first step is to prepare lactide from lactic acid; the second step is to prepare polylactic acid through the ring-opening polymerization of lactide. The molecular weight of PLA obtained in this process can reach 100,000 to 1 million. Among them, lactide is the key to the entire synthesis process, and the process barriers are relatively high. It is usually prepared through polycondensation and depolymerization processes under a catalyst, high temperature, and high vacuum system, and this process is prone to racemization and high-temperature carbonization of lactide. This is mainly because the depolymerization process mainly occurs in the "back-biting" ester catalytic reaction on the lactic acid oligomer molecular chain. The specific process is that under the action of catalyst, high temperature and high vacuum, the carbonyl group on the lactic acid oligomer chain is activated, and the hydroxyl group at the beginning of the chain attacks the positively charged carbonyl group, so that the ester bond breaks ("positive bite" process), forming L / D-lactide; however, in the presence of alkaline oxides or at too high a temperature, the carboxylic acid anion at the end of the lactic acid oligomer attacks the chiral carbon atom on the unit adjacent to the lactic acid unit, thereby breaking the bond between the methyl carbon and the ester oxygen bond ("back bite" process), and the configuration is reversed to obtain meso-lactide ( m -lactide), as shown in formula (1). m-The presence of lactide, on the one hand, will affect the optical purity of lactide, and then affect the ring-opening polymerization process of lactide, making the molecular weight of the obtained PLA low; on the other hand, it will destroy the regularity of the PLA structure, reduce its crystallinity and mechanical properties.

[0004]

[0005] Therefore, the crude lactide obtained by depolymerization usually needs to be purified and refined through processes such as solvent recrystallization, water extraction, distillation, and melt crystallization to reduce the m-lactide in the product. However, due to the similar physical and chemical properties of L-lactide and m-lactide, and the high freezing point, boiling point, and thermal sensitivity of lactide itself, it is difficult to separate them, and the overall yield is low, only about 40%-60%, and the overall economic efficiency is also low. Therefore, the racemization of the lactide synthesis process is a key factor affecting the quality and yield of lactide, and it is also the focus and difficulty of current lactide technology research at home and abroad.

[0006] In the article “Efficient synthesis of lactide with low racemization catalyzed by sodium bicarbonate and zinc lactate”, Zhang Quanxing et al. used zinc lactate and NaHCO 3 The combined system catalyzed the synthesis of lactide from lactic acid. It was found that when the two components were used as catalysts together, the depolymerization reaction was mainly "back-biting" within the first 4.0 hours, and the m-lactide content in the crude product was low. However, as the reaction time increased, the concentration of carboxyl anions in the reactants increased relatively, the frequency of carboxyl anions attacking chiral carbon atoms increased, and the m-lactide content in the crude product gradually increased. At the same time, it was found that NaHCO 3 As a basic oxide, it can accelerate the reaction rate in the catalytic depolymerization process, but it is also easy to cause racemization. 3 The scheme of increasing the addition amount of zinc lactate can reduce the racemization of lactide.

[0007] CN110105325A discloses a process for synthesizing lactide by catalysis of alkali metal compounds, comprising: mixing alkali metal compounds with lactic acid oligomers, then undergoing cracking reaction, and obtaining lactide by vacuum distillation; the alkali metal compound used is one or more of alkali metal carbonate, alkali metal bicarbonate, and alkali metal hydroxide. The reaction is carried out at 150°C-230°C and vacuum degree of 2-10torr for 2-5 hours. Compared with the prior art, the alkali metal compound of the invention reacts and dissolves rapidly after being added to the system, and there is no problem of catalyst insolubility and clogging pipelines; the alkali metal compound used is cheap and easy to obtain, and the amount of catalyst used is small, the lactide yield of the reaction product is >80%, the reaction is rapid, and it is easy to implement industrially. However, although the use of such catalysts can accelerate the reaction rate and ensure the product yield, it will cause the optical purity of the crude lactide product to be low, that is, the m-lactide content is high, which increases the burden and cost of the subsequent refining and purification process.

[0008] Therefore, high temperature and catalyst are two important factors that induce the racemization of lactide. In order to achieve the control of racemization in the lactide synthesis process, the existing technology often chooses to lower the reaction temperature, reduce the catalyst concentration or dosage, and select a weakly alkaline or non-alkaline catalyst. However, at the expense of the reaction rate or product yield, the m-lactide content in the product is still greater than 5%, which affects the reaction efficiency of the lactide synthesis process and the economic efficiency of the product. Summary of the invention

[0009] In view of the shortcomings of the prior art, the present invention provides a method for efficiently catalyzing the synthesis of lactide. The present invention uses a metal compound with strong alkalinity as a catalyst and adds a suitable protonated solvent to inhibit the racemization of lactide during the synthesis process, thereby accelerating the depolymerization reaction rate while ensuring product quality and yield.

[0010] The present invention provides a method for efficiently catalyzing and synthesizing lactide, comprising the following steps:

[0011] (1) Adding lactic acid oligomers and a depolymerization catalyst into a reactor, heating the reactor, and then adding a protonated solvent to carry out a depolymerization reaction, and collecting crude lactide;

[0012] (2) Purifying the crude lactide obtained in step (1) to obtain a lactide product that meets polymerization grade requirements.

[0013] In the above method, the molecular weight of the lactic acid oligomer in step (1) is 800-4000, preferably 1200-2800. The lactic acid oligomer is prepared by dehydration and polycondensation process using L-lactic acid or / and D-lactic acid as raw materials. The dehydration process mainly removes free water in the lactic acid, which can be carried out under normal pressure or reduced pressure. The reaction temperature of the polycondensation process is 140-170°C, the vacuum degree is 1000-2000Pa, and the reaction time is 0.5-4.0h.

[0014] In the above method, the catalyst in step (1) is a metal compound with relatively strong alkalinity, such as at least one of an alkali metal compound and an alkaline earth metal compound, and specifically can be at least one of an oxide, hydroxide, carbonate, bicarbonate, etc. of an alkali metal or / and an alkaline earth metal. Preferably, the oxide of the alkali metal or alkaline earth metal is one or more of calcium oxide, barium oxide, magnesium oxide, etc.; the hydroxide is one or more of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, barium hydroxide, etc.; the carbonate is one or more of sodium carbonate, calcium carbonate, magnesium carbonate, barium carbonate, potassium carbonate, etc.; the bicarbonate is one or more of sodium bicarbonate, calcium bicarbonate, magnesium bicarbonate, barium bicarbonate, potassium bicarbonate, etc.

[0015] In the above method, the amount of the catalyst used in step (1) is 1.0%-10% of the weight of the lactic acid oligomer.

[0016] In the above method, the lactic acid oligomer and the catalyst are mixed in step (1) and then heated to 100-160°C.

[0017] In the above method, the protonated solvent in step (1) is at least one of a diol or a diamine having a carbon number of not less than 12, preferably at least one of a diol or a diamine having a carbon number of 12-18, more preferably at least one of dodecanediamine, tetradecanediamine, hexadecanediamine, tetradecanediol, hexadecanediol, etc.

[0018] In the above method, the amount of the protonated solvent in step (1) is 0.1%-10% of the amount of the lactic acid oligomer, preferably 0.5%-5%.

[0019] In the above method, the protonated solvent in step (1) is melt-treated before being added, and the melting temperature is 80-160°C, preferably 100-160°C.

[0020] In the above method, the depolymerization reaction temperature in step (1) is 190-240°C, the vacuum degree is 200-1500Pa, and the reaction time is 1.0-3.0h.

[0021] In the above method, the crude lactide obtained in step (1) has an L-lactide content of 78%-86%, an m-lactide content of 3.5%-6.0%, an L-lactic acid content of 2.0%-5.0%, and a dimer and trimer content of 2.0%-6.5%.

[0022] In the above method, the single-pass conversion rate of lactic acid oligomers in the depolymerization process in step (1) can reach more than 94%.

[0023] In the above method, the undepolymerized lactic acid oligomers and protonated solvent at the bottom of the kettle in step (1) can be directly mixed with fresh lactic acid oligomer raw materials and used for crude lactide preparation, or can be separated and reused from the protonated solvent and lactic acid after hydrolysis.

[0024] In the above method, the refining process in step (2) can be carried out in the form of solvent water extraction or distillation. Through a single separation and refining process, the requirements of polymerization-grade lactide monomer can be met. The product yield of the purification process is not less than 80%, and the chemical purity and optical purity of the product can reach more than 99%. The m-lactide content is not more than 2.0%, which meets the requirements of polymerization-grade lactide monomer.

[0025] In the above method, after step (2) is completed, the total single-pass yield of the product in the entire lactide synthesis process can reach more than 80%.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) Based on the use of a highly alkaline metal compound as a catalyst, the present invention adds a long carbon chain protonated solvent to suppress the "backbiting" process that occurs during the depolymerization process, thereby reducing the probability of racemization. Compared with the traditional preparation process using alkali metal or alkaline earth metal compounds as catalysts, the degree of racemization of the obtained crude product can be reduced by more than 50%.

[0028] (2) The use of a long carbon chain protonated solvent solves the problem that although alkali metal or alkaline earth metal compound catalysts can increase the reaction rate in the process of synthesizing lactide, they are very likely to cause lactide racemization, thereby affecting the application of such catalysts. The present invention can not only use such catalysts to increase the reaction rate, but also reduce the racemization of lactide, and the product has a higher optical purity.

[0029] (3) The crude product prepared by the method of the present invention does not need to be refined by complex processes such as melt crystallization. A lactide product that meets the polymerization grade requirements can be obtained by only one-step distillation or water extraction process, further reducing the cost of subsequent separation and refining. DETAILED DESCRIPTION

[0030] The method and its effects of the present invention are further illustrated by the following examples. The examples are implemented based on the technical solution of the present invention, and detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the following examples.

[0031] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be purchased from biochemical reagent stores.

[0032] The lactic acid used in the embodiment of the present invention is heat-resistant grade L-lactic acid with a lactic acid content of 88% or more, and an optical purity of not less than 99.0%.

[0033] The present invention adopts Malvern Viscotek OMNISEC GPC / SEC gel chromatograph to analyze the molecular weight of lactic acid oligomers. The conventional calibration method is adopted, polystyrene (PS) is used as the internal standard, the chromatographic column model is T3000, the size is 300mmL×8.0mm, the column temperature is 40℃, the flow rate is 1.0mL / min, the sample concentration is 2-5mg / mL, and the single injection volume is 500μL.

[0034] The present invention adopts Agilent high performance liquid chromatograph to analyze the chemical purity of lactide, ultraviolet detector, phosphoric acid and acetonitrile as mobile phase, chromatographic column model is ZORBAX SB-Aq, column length is 250mm, column inner diameter is 4.6mm, internal filler particle size is 5μm, detection wavelength is 200nm, column temperature is 40℃, flow rate is 1mL / min, injection volume is 5μL.

[0035] The present invention adopts Agilent gas chromatograph to analyze the content of different optical isomers of lactide, selects CYCLOSIL-B model chromatographic column, vaporization chamber temperature 250°C, detector temperature 280°C, hydrogen flame ionization detector, column temperature program heating initial temperature 100°C, hold 5min, heat to 140°C at a rate of 4°C / min, hold 7min, heat to 200°C at a rate of 8°C / min, hold 20min, carrier gas N 2 Flow rate: 1.4 mL / min, hydrogen flow rate: 30 mL / min, air flow rate: 400 mL / min, injection volume: 0.5 μL.

[0036] The yield Y of the lactide purification process and the yield Y of the entire preparation and purification process 总 The calculation formula is as follows:

[0037]

[0038] in, m 0 is the mass of crude lactide, y 0is the purity of L-lactide in crude lactide, m is the mass of lactide product, M is the mass of lactide that a certain amount of lactic acid oligomers can theoretically be converted into, that is, the mass of lactic acid oligomers.

[0039] The specific rotation of the sample was analyzed by WZZ-2S automatic polarimeter to characterize the optical purity of the sample. The specific rotation of pure L-lactide was -278, and the specific rotation of pure D-lactide was +278. m -The specific rotation of lactide is 0, and the calculation formula of the sample optical purity X is as follows;

[0040]

[0041] Among them, α 纯物质 Represents the specific rotation of pure lactide, α 被测样品 Indicates the specific optical rotation of the substance being measured.

[0042] Example 1

[0043] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 600g of L-lactic acid (lactic acid content of about 88.0%) and add it to a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the system pressure at about 50kPa. Start heating under vacuum and gradually heat to 110-120℃. Dehydrate for 2h. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the pressure of the system to about 1.0kPa. The temperature of the feed liquid is gradually raised to 160℃ and reacted for 3h. At this time, a polycondensation reaction occurs between lactic acid molecules, and the water generated by the reaction in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2214.

[0044] Take 500g of the above-mentioned lactic acid oligomers and 10g of calcium oxide, add them into a tank reactor, heat to 140°C, add 5.0g of dodecane diamine melted at 120°C, control the vacuum degree of the depolymerization reaction at 500Pa, the reaction temperature at 200°C, and react for 1.0h to obtain crude L-lactide. The product was analyzed to have an L-lactide content of 85.3%, an m-lactide content of 4.1%, an L-lactic acid content of 3.3%, and a dimer and trimer content of 5.1%. The single-pass conversion rate of lactic acid oligomers during the depolymerization process was 95.4%.

[0045] The undepolymerized components at the bottom of the kettle (including lactic acid oligomers, calcium oxide and dodecane diamine) were mixed with fresh lactic acid oligomers and continued to participate in the depolymerization reaction process. The conditions were the same as above. The crude lactide obtained had an L-lactide content of 85.0%, an m-lactide content of 5.1%, an L-lactic acid content of 3.1%, a dimer and a trimer content of 4.8%, and a single-pass conversion rate of lactic acid oligomers in the depolymerization process of 94.5%. It can be seen that the influence of the recycled substrate on the content of m-lactide and the lactic acid conversion rate in the lactide product obtained in the depolymerization process is relatively small, and the product yield and quality can still be guaranteed after three cycles.

[0046] The crude lactide obtained in the above preparation process is purified and refined. After three-stage distillation in series, the refined lactide product can be obtained after removing the light and heavy components. Its chemical purity is 99.2%, chemical purity is 99.1%, and the m-lactide content is 1.0%. The product quality meets the requirements of polymerization grade. The product yield of the crude lactide purification and refining process can reach 82.7%. In the entire lactide synthesis process, the total product yield can reach 81.4%.

[0047] Example 2

[0048] The preparation of lactic acid oligomers was the same as in Example 1.

[0049] Take 500g of lactic acid oligomers and 10g of calcium oxide, add them into a kettle reactor, heat to 160℃, add 5.0g of hexadecane diamine melted at 140℃, control the vacuum degree of the depolymerization reaction at 500Pa, the reaction temperature at 200℃, and react for 1.0h to obtain a crude L-lactide product. Analysis shows that the L-lactide content of this product is 83.1%, the m-lactide content is 4.7%, the L-lactic acid content is 4.0%, and the dimer and trimer content is 5.7%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 94.6%.

[0050] The undepolymerized components at the bottom of the kettle (including lactic acid oligomers, calcium oxide and hexadecane diamine) are mixed with fresh lactic acid oligomers and continue to participate in the depolymerization reaction process under the same conditions as above. The crude lactide obtained has an L-lactide content of 82.4%, an m-lactide content of 4.9%, an L-lactic acid content of 3.9%, and a dimer and trimer content of 5.8%. After three cycles, the product yield and quality can still be guaranteed.

[0051] The lactide purification and refining process is the same as in Example 1. The chemical purity of the lactide product obtained is 99.1%, the chemical purity is 99.0%, the m-lactide content is 1.5%, the product quality meets the requirements of polymerization grade, and the product yield of the crude lactide purification and refining process can reach 83.4%. In the entire lactide synthesis process, the total product yield can reach 81.0%.

[0052] Example 3

[0053] The preparation of lactic acid oligomers was the same as in Example 1.

[0054] Take 500g of lactic acid oligomers and 10g of calcium oxide, add them into a kettle reactor, heat to 140℃, add 5.0g of tetradecane diamine melted at 140℃, control the vacuum degree of depolymerization reaction at 500Pa, the reaction temperature at 200℃, react for 1.0h, and obtain a crude L-lactide product. Analysis shows that the L-lactide content of this product is 83.7%, the m-lactide content is 4.6%, the L-lactic acid content is 4.3%, and the dimer and trimer content is 5.3%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 94.4%.

[0055] The undepolymerized components at the bottom of the kettle (including lactic acid oligomers, calcium oxide and tetradecane diol) are mixed with fresh lactic acid oligomers and continue to participate in the depolymerization reaction process under the same conditions as above. The crude lactide obtained has an L-lactide content of 82.8%, an m-lactide content of 5.3%, an L-lactic acid content of 4.1%, and a dimer and trimer content of 4.5%. After three cycles, the product yield and quality can still be guaranteed.

[0056] The lactide purification and refining process is the same as that in Example 1. The chemical purity of the lactide product obtained is 99.2%, the optical purity is 99.1%, the m-lactide content is 1.3%, the product quality meets the requirements of polymerization grade, and the product yield of the crude lactide purification and refining process can reach 83.1%. In the entire lactide synthesis process, the total product yield can reach 80.7%.

[0057] Example 4

[0058] The preparation of lactic acid oligomers was the same as in Example 1.

[0059] Take 500g of lactic acid oligomers and 10g of calcium oxide, add them into a kettle reactor, heat to 150℃, add 10.0g of tetradecane diol melted at 140℃, control the vacuum degree of depolymerization reaction at 500Pa, the reaction temperature at 200℃, react for 1.0h, and obtain a crude L-lactide product. Analysis shows that the L-lactide content of this product is 82.1%, the m-lactide content is 5.6%, the L-lactic acid content is 4.6%, and the dimer and trimer content is 5.9%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 94.0%.

[0060] The undepolymerized components at the bottom of the kettle (including lactic acid oligomers, calcium oxide and hexadecanediol) are mixed with fresh lactic acid oligomers and continue to participate in the depolymerization process under the same conditions as above. The crude lactide obtained has an L-lactide content of 81.4%, an m-lactide content of 5.9%, an L-lactic acid content of 4.3%, and a dimer and trimer content of 5.9%. After three cycles, the product yield and quality can still be guaranteed.

[0061] The lactide purification and refining process is the same as that in Example 1. The chemical purity of the lactide product obtained is 99.1%, the optical purity is 99.0%, the m-lactide content is 1.8%, the product quality meets the requirements of polymerization grade, and the product yield of the crude lactide purification and refining process can reach 82.5%. In the entire lactide preparation and refining process, the total product yield can reach 80.1%.

[0062] Example 5

[0063] The preparation of lactic acid oligomers was the same as in Example 1.

[0064] Take 500g of lactic acid oligomer and 10g of sodium hydroxide, add them into a kettle reactor, heat to 140℃, and add 5.0g of dodecane diamine melted at 120℃, control the vacuum degree of depolymerization reaction at 500Pa, the reaction temperature at 200℃, and react for 1.0h to obtain a crude L-lactide product. The L-lactide content of this product is 84.2% after analysis. m The content of -lactide was 3.9%, the content of L-lactic acid was 3.0%, the content of dimer and trimer was 5.8%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 95.3%.

[0065] The undepolymerized components at the bottom of the kettle (including lactic acid oligomers, sodium hydroxide and dodecane diamine) are mixed with fresh lactic acid oligomers and continue to participate in the depolymerization reaction process under the same conditions as above. The crude lactide obtained has an L-lactide content of 83.6%, an m-lactide content of 4.3%, an L-lactic acid content of 3.8%, and a dimer and trimer content of 4.5%. After three cycles, the product yield and quality can still be guaranteed.

[0066] The lactide purification and refining process is the same as that in Example 1. The chemical purity of the lactide product obtained is 99.2%, the optical purity is 99.1%, the m-lactide content is 1.1%, the product quality meets the requirements of polymerization grade, and the product yield of the crude lactide purification and refining process can reach 83.2%. In the entire lactide synthesis process, the total product yield can reach 81.0%.

[0067] Example 6

[0068] The preparation of lactic acid oligomers was the same as in Example 1.

[0069] Take 500g of lactic acid oligomer and 13g of sodium carbonate, add them into a kettle reactor, heat to 140℃, add 5.0g of dodecane diamine melted at 120℃, control the vacuum degree of depolymerization reaction at 500Pa, the reaction temperature at 200℃, react for 1.0h, and obtain a crude L-lactide product. The L-lactide content of this product is 85.8% after analysis. m The content of -lactide is 3.8%, the content of L-lactic acid is 3.9%, the content of dimer and trimer is 5.4%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 95.9%.

[0070] The undepolymerized components at the bottom of the kettle (including lactic acid oligomers, sodium carbonate and dodecane diamine) are mixed with fresh lactic acid oligomers and continue to participate in the depolymerization reaction process under the same conditions as above. The crude lactide obtained has an L-lactide content of 85.3%, an m-lactide content of 4.2%, an L-lactic acid content of 4.1%, and a dimer and trimer content of 4.5%. After three cycles, the product yield and quality can still be guaranteed.

[0071] The lactide purification and refining process is the same as that in Example 1. The chemical purity of the lactide product obtained is 99.4%, the optical purity is 99.1%, the m-lactide content is 1.2%, the product quality meets the requirements of polymerization grade, and the product yield of the crude lactide purification and refining process can reach 82.9%. In the entire lactide synthesis process, the total product yield can reach 80.8%.

[0072] Example 7

[0073] The preparation of lactic acid oligomers was the same as in Example 1.

[0074] Take 500g of lactic acid oligomer and 10g of potassium bicarbonate, add them into a kettle reactor, heat to 140℃, and add 5.0g of dodecane diamine melted at 120℃, control the vacuum degree of depolymerization reaction at 500Pa, the reaction temperature at 200℃, and react for 1.0h to obtain a crude L-lactide product. The L-lactide content in the product is 86.0% after analysis. m The content of -lactide is 3.5%, the content of L-lactic acid is 4.2%, and the content of dimer and trimer is 5.4%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 95.9%.

[0075] The undepolymerized components at the bottom of the kettle (including lactic acid oligomers, sodium hydroxide and dodecane diamine) are mixed with fresh lactic acid oligomers to continue the depolymerization reaction process under the same conditions as above. The crude lactide obtained has an L-lactide content of 85.1%, an m-lactide content of 3.7%, an L-lactic acid content of 4.5%, and a dimer and trimer content of 5.7%. After three cycles, the product yield and quality can still be guaranteed.

[0076] The lactide purification and refining process is the same as that in Example 1. The chemical purity of the lactide product obtained is 99.3%, the optical purity is 99.1%, the m-lactide content is 1.1%, the product quality meets the requirements of polymerization grade, and the product yield of the crude lactide purification and refining process can reach 84.1%. In the entire lactide preparation and refining process, the total product yield reaches 81.7%.

[0077] Comparative Example 1

[0078] The same as Example 1, except that dodecadiamine was not added during the lactide preparation process. The crude lactide product obtained had an L-lactide content of 74.2% and an m-lactide content of 12.1%. The single-pass conversion rate of lactic acid oligomers during the depolymerization process was 82.1%.

[0079] The purification of lactide was the same as in Example 1, and the chemical purity of lactide was 94.2%. m -The lactide content is 4.5%, and the product yield of the crude lactide purification process is 65.3%. During the entire lactide synthesis process, the total product yield is 60.9%, which cannot meet the requirements of polymerization-grade monomers and still needs to be further refined in conjunction with the melt crystallization process.

[0080] Comparative Example 2

[0081] The same as Example 1, except that the same amount of protonated solvent ethanol was added instead of dodecanediamine during the lactide preparation process. During the reaction, ethanol failed to inhibit the racemization of lactide, and the crude lactide product obtained had an L-lactide content of 76.2%. m -Lactide content is 11.4%.

[0082] The purification and refining of lactide was the same as in Example 1, and the chemical purity of the lactide product was 95.7%. m -The lactide content is 4.2%, and the product yield of the crude lactide purification process is 67.3%. During the entire lactide synthesis process, the total product yield is 62.8%, which cannot meet the requirements of polymerization-grade monomers and still needs to be refined in conjunction with the melt crystallization process.

[0083] Comparative Example 3

[0084] The same as Example 1, except that: during the preparation of lactide, the same amount of tetracarbonyl diamine was added instead of dodecaamine. During the reaction, tetracarbonyl diamine failed to inhibit the racemization of lactide. The L-lactide content in the obtained crude lactide product was 77.9%. m - Lactide content is 10.5%.

[0085] The purification and refining of lactide was the same as in Example 1, and the chemical purity of the lactide product was 98.3%. m-The lactide content is 3.1%, and the product yield of the crude lactide purification process is 73.2%. The total product yield of the entire lactide synthesis process is 65.5%, which cannot meet the requirements of polymerization-grade monomers and still needs to be refined in conjunction with the melt crystallization process.

Claims

1. A method for efficiently catalyzing the synthesis of lactide, characterized in that The steps include: (1) Adding lactic acid oligomers and a depolymerization catalyst into a reaction kettle, heating the reaction kettle, adding a protonated solvent for depolymerization reaction, and collecting crude lactide; the molecular weight of the lactic acid oligomers is 800-4000; the catalyst is at least one of an alkali metal compound and an alkaline earth metal compound; the protonated solvent is at least one of a diol and a diamine having 12 to 18 carbon atoms; the depolymerization reaction temperature is 190-240° C., the vacuum degree is 200-1500 Pa, and the reaction time is 1.0-3.0 h; (2) The crude lactide obtained in step (1) is purified and refined by solvent water extraction or distillation, and a lactide product that meets polymerization grade requirements is obtained through a single separation and purification process.

2. The method according to claim 1, characterized in that: The molecular weight of the lactic acid oligomer in step (1) is 1200-2800.

3. The method according to claim 1 or 2, characterized in that: The lactic acid oligomers described in step (1) are prepared from L-lactic acid and / or D-lactic acid as raw materials through a dehydration and polycondensation process.

4. The method according to claim 3, characterized in that: The dehydration process is mainly to remove free water in lactic acid, using normal pressure or reduced pressure; the reaction temperature of the polycondensation process is 140-170°C, the vacuum degree is 1000-2000Pa, and the reaction time is 0.5-4.0h.

5. The method according to claim 1, characterized in that: The catalyst in step (1) is at least one of an oxide, hydroxide, carbonate and bicarbonate of an alkali metal or / and an alkaline earth metal.

6. The method according to claim 5, characterized in that: The oxides of the alkali metal and alkaline earth metal are one or more of calcium oxide, barium oxide, and magnesium oxide; the hydroxides are one or more of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, and barium hydroxide; the carbonates are one or more of sodium carbonate, calcium carbonate, magnesium carbonate, barium carbonate, and potassium carbonate; and the bicarbonates are one or more of sodium bicarbonate, calcium bicarbonate, magnesium bicarbonate, barium bicarbonate, and potassium bicarbonate.

7. The method according to claim 1, characterized in that: The amount of the catalyst used in step (1) is 1.0%-10% of the amount of the lactic acid oligomer.

8. The method according to claim 1, characterized in that: In step (1), the lactic acid oligomer and the catalyst are mixed and heated to 100-160°C.

9. The method according to claim 1, characterized in that: The protonated solvent in step (1) is at least one of dodecanediamine, tetradecanediamine, hexadecanediamine, tetradecanediol, and hexadecanediol.

10. The method according to claim 1 or 9, characterized in that: The amount of the protonated solvent used in step (1) is 0.1%-10% of the amount of the lactic acid oligomer.

11. The method according to claim 10, characterized in that: The amount of the protonated solvent used in step (1) is 0.5%-5% of the amount of the lactic acid oligomer.

12. The method according to claim 1, characterized in that: The protonated solvent in step (1) is melted before being added, and the melting temperature is 80-160°C.

13. The method according to claim 12, characterized in that: The melting temperature is 100-160℃.

14. The method according to claim 1, characterized in that: The crude lactide obtained in step (1) has a composition of 78%-86% L-lactide, 3.5%-6.0% m-lactide, 2.0%-5.0% L-lactic acid, and 2.0%-6.5% dimer and trimer.

15. The method according to claim 1, characterized in that: The undepolymerized lactic acid oligomers and protonated solvent at the bottom of the kettle in step (1) are directly recycled and mixed with fresh lactic acid oligomer raw materials, or are separated and recycled from the protonated solvent and lactic acid after hydrolysis.

16. The method according to claim 1, characterized in that: After purification in step (2), the yield of the product is not less than 80%, the chemical purity and optical purity of the product are both above 99%, and the m-lactide content is not more than 2.0%.

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