Method for preparing lactide by cleaving polylactic acid catalyzed by lithium amine imine

Abstract

The application discloses a method for preparing lactide by cracking polylactic acid with lithium amine imine catalyst, and belongs to the technical field of chemical recycling of polylactic acid. The method comprises the following steps: dissolving polylactic acid in a solvent, adding methanol and a lithium amine imine catalyst to perform a pre-depolymerization reaction; removing the solvent by evaporation, then increasing the temperature to perform depolymerization, and recovering lactide monomers under reduced pressure. The method generates oligomers by pre-depolymerization to serve as endogenous reaction media, thereby reducing the depolymerization temperature and improving the depolymerization efficiency. The method has the advantages of small catalyst dosage, no heavy metal residue, simple post-treatment, high lactide yield, good selectivity and the like, and can be used for high-value recycling of polylactic acid waste and substandard products.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling of polylactic acid, and more specifically to a method for preparing lactide by catalytic cracking of polylactic acid using lithium imine. Background Technology

[0002] With the restriction and banning of non-degradable plastic products, biodegradable materials have been further developed to replace traditional non-degradable materials. Polylactic acid (PLA), due to its biodegradability and biocompatibility, is widely used in catering, packaging, medicine, and agricultural films, becoming one of the largest-volume biodegradable materials, with an annual production capacity of 200,000 to 300,000 tons. The principle is as follows: agricultural starch is fermented to produce lactic acid, which is then polycondensed to form low-polyester, followed by pyrolysis to generate lactide monomers. Under the catalysis of tin(II) alkoxide initiators formed in situ from stannous octoate and alcohols, the lactide monomers undergo ring-opening polymerization to produce PLA. This process is widely used because of its strong controllability and ability to produce high molecular weight PLA that is difficult to achieve by polycondensation methods. High molecular weight PLA combines the properties of practical plastics and fibers, and under controlled conditions, it can be composted to ultimately produce water and carbon dioxide. These advantages make it an ideal material in the field of biodegradable materials. However, composting as a treatment method has inherent defects, leading to a "waste" of the material's inherent properties—bond energy loss.

[0003] Currently, the chemical recovery of polylactic acid (PLA) mainly relies on methods such as thermal decomposition or catalytic cracking to prepare lactide. Traditional processes generally use metal oxides such as zinc oxide, tin oxide, and magnesium oxide as catalysts to crack PLA at high temperatures of 180–250°C to obtain lactide. For example, CN115403554B reported a route that uses fatty alcohol chain scission, followed by the addition of a catalyst and purification through vacuum distillation and multiple solvent crystallizations. This route can obtain purified lactide from waste PLA, but it involves high reaction temperatures (180–220°C), high energy consumption, and is prone to partial racemization of the product, reducing the optical purity of the lactide and hindering its use in the repolymerization of high-molecular-weight PLA.

[0004] Patent CN115160289B reports a process for obtaining racemic lactide by depolymerizing random PLA to racemic lactide using a catalyst formed in situ with MgCl2 and bipyridine ligand under heating and reduced pressure (220°C, 1 mbar) after heating. Although this process achieves a high yield, it suffers from significant racemization problems at high temperatures (L≈42%, D≈43%, meso≈15% in the L / D / meso mixture).

[0005] CN116655583A reports a zinc / magnesium composite catalytic system that can cleave PLA to produce lactide at 200-300℃. The process requires a high temperature to maintain catalytic activity, and side reactions occur to generate byproducts such as malonate and lactate, resulting in low lactide selectivity (about 90%).

[0006] Studies have found that adding high-boiling-point solvents can solve the high-temperature racemization of lactide. The literature Angew. Chem., Int. Ed. 2022, 61(33), No. e202204531 reports a chemical recovery method for polylactic acid. This study introduced exogenous solvents DMF or γ-valerol at 140℃, adding 0.1 equivalents of stannous octoate to recover lactide. However, the introduction of high-boiling-point solvents increased the difficulty of subsequent lactide purification, and the reproductive toxicity of DMF and the use of large amounts of stannous octoate limited its application in the food and medical fields.

[0007] Therefore, there is an urgent need to develop a novel catalytic system with low temperature, high efficiency, and strong stereoselectivity to achieve efficient depolymerization of polylactic acid and preparation of lactide, thereby promoting the closed-loop recycling and green manufacturing of biodegradable materials. Summary of the Invention

[0008] To address the problems of high reaction temperature, large catalyst dosage, numerous side reactions, and complex post-processing in existing polylactic acid (PLA) pyrolysis for lactide recovery, this invention provides a method for preparing lactide from PLA using lithium imine-catalyzed pyrolysis. This method first pre-depolymerizes PLA in the presence of methanol to generate oligolactic acid (OLA). Then, the solvent used in the pre-depolymerization stage is removed, followed by heated pyrolysis, and lactide is recovered by distillation under reduced pressure. The generated oligomers serve as both a reaction substrate and an endogenous reaction medium in the subsequent pyrolysis stage, which helps to lower the pyrolysis temperature, improve depolymerization efficiency, and reduce the use of exogenous high-boiling-point solvents. Under preferred conditions, this method can obtain lactide products with high yield and high selectivity, and effectively suppresses racemization side reactions.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] (1) Pre-depolymerization: After dissolving polylactic acid raw material in a solvent to form a uniform solution, a certain amount of methanol is added, followed by the addition of lithium imine catalyst to carry out the pre-depolymerization reaction.

[0011] (2) Pyrolysis: Evaporate the solvent, raise the reaction temperature, and depolymerize while recovering lactide monomer.

[0012] Preferably, the polylactic acid in step (1) includes substandard polylactic acid produced during the polylactic acid production process or recycled polylactic acid.

[0013] Preferably, the lithium amineimide catalyst in step (1) has the following formula, where R is isopropyl, ethyl, methyl, or hydrogen, preferably isopropyl;

[0014]

[0015] Preferably, in step (1), the lithium amineimide catalyst is a toluene solution of lithium amineimide with a concentration of 1 mol / L.

[0016] Preferably, the solvent in step (1) is dichloromethane, chloroform, tetrahydrofuran, or toluene, with dichloromethane or chloroform being more preferred.

[0017] Preferably, the solvent in step (1) is a dry solvent with a water content of less than 50 ppm;

[0018] Preferably, the ratio of solvent to polylactic acid in step (1) is 5~30mL:1g;

[0019] Preferably, in step (1), the ratio of methanol to carbonyl groups in polylactic acid is 0.1 to 0.15:1, and more preferably 0.125:1;

[0020] Furthermore, the methanol is anhydrous methanol with a water content of less than 50 ppm;

[0021] Furthermore, the molar ratio of lithium amineimide to carbonyl groups in polylactic acid is 0.01-0.05:1, preferably 0.02:1;

[0022] Furthermore, the pre-depolymerization reaction time is 0.1-30 minutes;

[0023] Furthermore, the depolymerization reaction time is 1-6 hours;

[0024] Furthermore, the depolymerization reaction temperature is 120-200°C. o C;

[0025] Furthermore, the vacuum pressure for the depolymerization reaction is 20–40 kPa;

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) This invention develops a highly efficient amine-nitrogen ligand lithium catalytic system for catalytic depolymerization of polylactic acid to recover lactide. This catalytic system has high catalytic activity, requires a small amount of catalyst, and does not contain heavy metal components, which can effectively avoid the problem of metal residue. At the same time, the catalyst structure is tunable, has good stability, and has good green and environmentally friendly characteristics.

[0028] (2) No exogenous solvent is required during the depolymerization stage. The oligomers generated in the system participate in the reaction as both the reaction medium and the reaction substrate, achieving self-solubilization of the system, reducing the cost of subsequent separation and purification, and showing potential for process scale-up applications.

[0029] (3) This catalytic system can achieve rapid depolymerization of polylactic acid in a short time, obtain highly selective lactide products, and significantly reduce energy consumption. Attached Figure Description

[0030] Figure 1 1H NMR spectrum of lithium amineimide (R is isopropyl);

[0031] Figure 2 The 1H NMR spectrum of lactide, the product in Example 2;

[0032] Figure 3 Liquid phase spectrum of lactide, the product in Example 2. Detailed Implementation

[0033] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0034] As described in the background section, existing catalysts for the degradation and recovery of lactide from polylactic acid generally suffer from technical problems such as high reaction temperature, low selectivity, and cumbersome post-processing.

[0035] Therefore, the purpose of this invention is to provide a rapid and efficient catalyst and method for the catalytic degradation and recovery of lactide from polylactic acid. This invention utilizes an amine-imine lithium catalytic system, first pre-depolymerizing the lactide into oligomers, which serve as both reactants and reaction media. Under the amine-imine lithium catalytic system, efficient and mild depolymerization to prepare lactide is achieved. The entire process is free of heavy metal catalysts, requires only a small amount of catalyst, and reduces cumbersome post-processing, showing promising prospects for industrial production.

[0036] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0037] The test materials used in the embodiments of this invention are all conventional test materials in the art and can be purchased through commercial channels.

[0038] Example 1

[0039] Synthesis of the amine imine ligand (isopropyl): In a 250 mL round-bottom flask, o-fluorobenzaldehyde (4.24 mL, 40 mmol), n-hexane (30 mL), and anhydrous sodium sulfate (2 g) were added sequentially, followed by the slow dropwise addition of an equimolar amount of 2,6-diisopropylaniline (7.60 mL, 40 mmol). The reaction system was sealed and stirred at room temperature for 8 hours. After the reaction was complete, solid impurities were separated by filtration, and the filter cake was thoroughly washed with n-hexane (3 × 10 mL). The filtrates were combined, and the solvent was removed by rotary evaporation, finally yielding an orange-yellow solid product a (9.4 g, 83%), which was sealed and stored for later use. Under standard anhydrous and oxygen-free operating conditions, the baked ampoules were purged three times with nitrogen. After cooling, dry 2,6-diisopropylaniline (3 mL, 16 mmol) and 10 mL of anhydrous tetrahydrofuran (THF) were added sequentially. Under liquid nitrogen-ethanol bath cooling, a 10 mL (16 mmol) solution of n-butyllithium was slowly added dropwise. The reaction system was then slowly heated to room temperature and stirred continuously for 8 hours to obtain a 2,6-diisopropylaniline lithium salt solution. In a separately baked ampoule, intermediate a (4.5 g, 16 mmol) and 10 mL of anhydrous THF were added and dissolved completely. The THF solution of a was then slowly added to the lithium salt solution via a three-way valve. After addition, the reaction was stirred at room temperature for 12 hours. After the reaction was complete, 3 mL of deionized water was added to quench the reaction, and the organic phase was separated. The aqueous phase was extracted with ethyl acetate (15 mL × 3), and the combined organic layers were washed with saturated brine and dried over anhydrous sodium sulfate. The solvent was removed by rotary evaporation under reduced pressure, and 10 mL of pre-cooled methanol was added for recrystallization, precipitating a solid product. The solid was collected by filtration and dried in a vacuum drying oven for 4 hours to finally obtain a white solid product—an amine imine ligand.

[0040] Synthesis of the lithium amineimide complex (isopropyl): The amineimide ligand (isopropyl) (0.89 g, 2 mmol) was placed in a Schlenk flask, baked under vacuum, and purged three times with gas. The mixture was then cooled to room temperature and purged with nitrogen. Subsequently, 20 mL of dry n-hexane was added, and the mixture was heated until the ligand was completely dissolved. Under continuous nitrogen purging, the Schlenk flask was cooled in a liquid nitrogen-ethanol bath, and a solution of n-butyllithium (n-BuLi) (1.3 mL, 2.1 mmol) was slowly added dropwise. After the addition was complete, the reaction system was slowly heated to room temperature and reacted for 12 hours. After the reaction was complete, the lithium complex precipitated, was filtered under vacuum, and washed three times with dry n-hexane to obtain a yellow solid product, lithium amineimide (0.79 g, 88%). The above lithium amineimide complex was dissolved in toluene to prepare a 1 mol / L solution.

[0041] Example 2

[0042] Under a nitrogen atmosphere, 72g of PLLA waste (72g, model: 4032D) was dissolved in 1L of dichloromethane and stirred to form a homogeneous solution. 5 mL of MeOH was added, followed by 20 mL of lithium amineimide (R is isopropyl, 0.02mol) solution. After reacting for 5 min, the solvent was evaporated. The temperature was slowly raised to 140℃ and the reaction was stirred while maintaining a vacuum pressure of 20 kPa for distillation until no product was distilled off (the reaction took about 2 h). The lactide monomer was obtained, and the yield was 96.2%, with an L:D:meso ratio of 99.2:0.8:0 (determined by liquid chromatography).

[0043] Example 3

[0044] Under a nitrogen atmosphere, 72g of PLLA waste (model: 4043D) was dissolved in 1L of chloroform and stirred to form a homogeneous solution. 5mL of MeOH was added, followed by 10mL of lithium amineimide (R is methyl, 0.01mol) solution. After reacting for 10min, the solvent was evaporated. The temperature was slowly raised to 120℃ and the reaction was stirred while maintaining a vacuum pressure of 20 kPa for distillation until no product was distilled off (reaction time was about 5h). Lactide monomer was obtained, and the yield was 94.8%, with an L:D:meso ratio of 99.6:0.4:0 (determined by liquid chromatography).

[0045] Example 4

[0046] Under a nitrogen atmosphere, 72g of PLLA waste (model: 6400D) was dissolved in 375ml of chloroform and stirred to form a homogeneous solution. 4.1mL of MeOH was added, followed by 50ml of lithium amineimide (R is hydrogen, 0.05mol) solution. After reacting for 10min, the solvent was evaporated. The temperature was slowly raised to 180℃ and the reaction was stirred while maintaining a vacuum pressure of 40KPa for distillation until no product was distilled off (reaction time was about 1h). Lactide monomer was obtained, and the yield was 93.5%, with an L:D:meso ratio of 92.4:4.6:3.0 (determined by liquid chromatography).

[0047] Example 5

[0048] Under a nitrogen atmosphere, 72g of PLLA waste (model: 6201D) was dissolved in 2160ml of tetrahydrofuran and stirred to form a homogeneous solution. 5mL of MeOH was added, followed by 20ml of lithium amineimide (R is methyl, 0.02mol) solution. After reacting for 5min, the solvent was evaporated. The temperature was slowly raised to 120℃ and the reaction was stirred while maintaining a vacuum pressure of 30KPa for distillation until no product was distilled off (reaction time was about 2.5h). Lactide monomer was obtained, and the yield was 95.1% with an L:D:meso ratio of 98.8:1.2:0 (determined by liquid chromatography).

[0049] Example 6

[0050] Under a nitrogen atmosphere, 72g of PLLA waste (model: 4032D) was dissolved in 1L of toluene and stirred to form a homogeneous solution. 5.0mL of MeOH was added, followed by 20mL of lithium amineimide (R is ethyl, 0.02mol) solution. After reacting for 5min, the solvent was evaporated. The temperature was slowly raised to 140℃ and the reaction was stirred while maintaining a vacuum pressure of 20KPa for distillation until no product was distilled off (the reaction took about 2h). Lactide monomer was obtained, and the yield was 95.8% with an L:D:meso ratio of 98.9:1.1:0 (determined by liquid chromatography).

[0051] Comparative Example 1

[0052] The difference from Example 2 is that an equimolar amount of LiHMDS was used instead of the lithium amine imine catalyst, while all other conditions remained the same. After the reaction, no obvious product was detected, indicating that LiHMDS itself could not effectively catalyze the cleavage of polylactic acid to lactide under these conditions, suggesting that the amine imine coordination environment plays a crucial role in catalytic activity.

[0053] Comparative Example 2

[0054] The difference from Example 2 is that methanol was not added, while all other conditions remained the same. The results showed that without methanol pre-depolymerization, the polylactic acid pyrolysis efficiency was significantly reduced, with lactide yield only 3%. This indicates that methanol pre-depolymerization is beneficial for generating oligomeric intermediates and promoting the subsequent pyrolysis process.

[0055] Comparative Example 3

[0056] The difference from Example 2 is that an equimolar amount of stannous octoate was used instead of the lithium amineimide catalyst, while all other conditions remained the same. After the reaction, no obvious products were detected, indicating that under the same conditions, the conventional tin catalytic system could not achieve the cracking effect of the lithium amineimide catalytic system of this invention.

[0057] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing lactide by catalytic cracking of polylactic acid using lithium imine, characterized in that, The method is as follows: (1) Pre-depolymerization: Dissolve polylactic acid raw material in solvent to form a homogeneous solution, then add a certain amount of methanol and lithium imine catalyst to carry out pre-depolymerization reaction; (2) Pyrolysis: Evaporate the solvent, raise the reaction temperature, and simultaneously recover the lactide monomer liquid under vacuum during pyrolysis; The molecular structural formulas of the lithium amineimide catalysts are as follows: Where R represents isopropyl, ethyl, methyl, or hydrogen.

2. The method according to claim 1, characterized in that, The polylactic acid includes substandard polylactic acid produced during the polylactic acid production process or recycled polylactic acid.

3. The method according to claim 1, characterized in that, The solvent is dichloromethane, chloroform, tetrahydrofuran, or toluene.

4. The method according to claim 1, characterized in that, The ratio of the solvent to polylactic acid is 5~30mL:1g.

5. The method according to claim 1, characterized in that, The ratio of methanol to carbonyl groups in polylactic acid is 0.1~0.15:

1.

6. The method according to claim 1, characterized in that, The molar ratio of lithium amineimide to carbonyl groups in polylactic acid is 0.01 to 0.05:

1.

7. The method according to claim 1, characterized in that, The pre-depolymerization reaction time is 0.1 to 30 minutes.

8. The method according to claim 1, characterized in that, The depolymerization reaction time is 1-6 hours, and the depolymerization reaction temperature is 120-200°C. o C.

9. The application of the method according to any one of claims 1 to 8 in improving the depolymerization efficiency of polylactic acid.

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