A method for catalytic depolymerization of polylactic acid to recover methyl lactate in one pot
By using a catalytic system formed by neutral nitrogen ligands and organolithium reagents under mild conditions, the problems of high reaction temperature and complex catalysts in the polylactic acid alcoholysis reaction of the prior art are solved, achieving efficient production of methyl lactate, simplifying the process and reducing costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- UNIV OF JINAN
- Filing Date
- 2026-03-13
- Publication Date
- 2026-05-29
AI Technical Summary
Existing technologies for preparing methyl lactate from polylactic acid alcoholysis generally suffer from problems such as high reaction temperature, high energy consumption, complex catalyst use, and numerous byproducts, which limit their efficient recovery and recycling.
A catalytic system is formed in situ in the reaction system using neutral nitrogen ligands and organolithium reagents. The reaction of polylactic acid and methanol is catalyzed in a one-pot process under mild conditions to produce methyl lactate, avoiding the need for pre-synthesis and separation of catalysts.
It achieves highly selective and efficient production of methyl lactate under mild conditions, simplifies the catalyst usage process, reduces process complexity and cost, and has good prospects for industrialization.
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Abstract
Description
Technical Field
[0001] This invention relates to the field of polylactic acid (PLA) recovery technology, and specifically to a one-pot catalytic depolymerization method for recovering methyl lactate from PLA. Background Technology
[0002] Polylactic acid (PLA), as a biodegradable bio-based material, is widely used in various fields due to its excellent biocompatibility and environmental performance. However, although PLA can degrade under specific environmental conditions, the natural degradation process usually makes it difficult to achieve high-value closed-loop recycling, resulting in limited resource utilization efficiency. This degradation method cannot directly return it to new raw materials or useful resources, lacking recycling value. To achieve high-value recycling of waste PLA, chemical recycling has become a key research direction. Its main methods include hydrolysis to recover lactic acid, alcoholysis to prepare lactate esters, and obtaining lactide through thermal cracking or catalytic depolymerization for repolymerization. These technical pathways provide diversified closed-loop solutions for waste PLA, helping to promote its industrial recycling process.
[0003] While hydrolysis for recovering lactic acid is theoretically simple, it generates numerous impurities, posing challenges in practical operation. Thermal pyrolysis, which decomposes polylactic acid (PLA) into lactide at high temperatures, requires secondary polymerization for reformation, making control difficult. In contrast, alcoholysis for recovering methyl lactate (MLL) occurs under milder reaction conditions, efficiently recovering MLL and finding wide application potential and economic benefits in the synthesis of pharmaceuticals, fragrances, and other polymers. ACS Catal 2019, 9, 409-416 reported a method for recovering MLL from PLA via alcoholysis using a Zn(II)imine-monophenol catalytic system. At 130°C with a 16 wt% catalyst, PLA conversion reached 100%, but the MLL yield was only 63%, with the remainder converted to low-molecular-weight oligomers or other intermediates. The overall catalytic efficiency of this system is low, and the catalyst synthesis is complex, limiting its economic viability. Polymers2022,14(9),1763 investigated various commercial catalysts and combinations thereof for the alcoholysis of PLA to produce methyl lactate, including Zn(OAc)2, Mg(OAc)2, 4-(dimethylamino)pyridine and triazole bicycloalkanes (TBD), among which the combination of Zn(OAc)2 and TBD showed the strongest catalytic activity, with a conversion rate of 100% at 130 °C, of which the yield of methyl lactate was 81%, and the remainder was oligomers.
[0004] Existing technologies for the alcoholysis of polylactic acid (PLA) to methyl lactate generally suffer from the following problems: First, they typically require high temperatures, resulting in high energy consumption and hindering rapid conversion under mild conditions; second, some systems rely on pre-prepared or separated organometallic catalysts, leading to complex catalyst usage processes and increased process burden; and third, under strong reaction conditions, byproducts such as oligomers are easily generated, affecting the selectivity and subsequent separation of the target product, methyl lactate. Therefore, developing a method for the rapid and highly selective alcoholysis of PLA under mild conditions without the need for pre-preparation of complex metal complexes is of great significance. Summary of the Invention
[0005] In view of the above-mentioned prior art, the purpose of this invention is to provide a one-pot catalytic depolymerization method for recovering methyl lactate from polylactic acid. This method eliminates the need for pre-synthesizing, separating, and purifying organometallic catalysts. Instead, it utilizes a neutral nitrogen ligand and an organolithium reagent to form an effective catalytic system in situ within the reaction system, thereby rapidly catalyzing the alcoholysis reaction of polylactic acid and methanol under mild conditions, highly selectively producing methyl lactate, and showing high industrialization potential.
[0006] To achieve the above objectives, the present invention adopts the following technical solution:
[0007] A first aspect of the present invention provides a one-pot catalytic depolymerization method for recovering methyl lactate from polylactic acid, the method comprising:
[0008] Polylactic acid, amine nitrogen ligand and methanol were mixed, dissolved in a solvent, and then n-butyllithium solution was added to carry out depolymerization reaction; after the reaction was completed, the solvent was removed by vacuum distillation, and the crude product was dried under vacuum to obtain methyl lactate product.
[0009] Preferably, the amine nitrogen ligand includes diimine compounds, 2,2-bipyridine, or tetramethylethylenediamine.
[0010] Preferably, the diimine compounds include compounds having the following structural formula:
[0011] ,
[0012] Where R is a substituent, and can be hydrogen, methyl, ethyl, or isopropyl.
[0013] Preferably, the polylactic acid includes substandard polylactic acid produced during the polylactic acid production process or recycled polylactic acid.
[0014] Preferably, the molar ratio of the amine nitrogen ligand to the carbonyl group in polylactic acid is 0.001~0.1:1; and the molar ratio of the carbonyl group in polylactic acid to methanol is 1:2~6.
[0015] Preferably, the number of moles of repeating ester bonds in the polylactic acid is calculated by dividing the mass of polylactic acid by 72; each repeating unit corresponds to one ester carbonyl group.
[0016] Preferably, the molar ratio of the amine nitrogen ligand to the carbonyl group in polylactic acid is 0.005:1; and the molar ratio of the carbonyl group in polylactic acid to methanol is 1:4.
[0017] Preferably, the solvent is selected from dichloromethane, tetrahydrofuran, toluene, and benzene; the ratio of the solvent to polylactic acid is 10-20 mL: 1 g.
[0018] Preferably, the solvent is dichloromethane; the ratio of the solvent to polylactic acid is 15 mL: 1 g.
[0019] Preferably, the n-butyllithium solution is a hexane solution of n-butyllithium; the concentration of the n-butyllithium solution is 1 mol / L; and the molar ratio of n-butyllithium to the amine nitrogen ligand is 1:1.
[0020] Preferably, the depolymerization reaction is carried out at a temperature of 15~35℃ for a time of 6~600s.
[0021] Preferably, the depolymerization reaction is carried out at a temperature of 25°C for 10 seconds.
[0022] Preferably, the temperature for the vacuum drying is 40°C and the pressure is 50 kPa.
[0023] A second aspect of the present invention provides the application of the above-described method in improving the depolymerization efficiency of polylactic acid.
[0024] The beneficial effects of this invention are:
[0025] 1. This invention provides a method for preparing methyl lactate by catalytic alcoholysis of polylactic acid using amine nitrogen ligands, which has the advantages of mild reaction conditions, high depolymerization efficiency, and high yield of methyl lactate.
[0026] 2. This invention uses a one-pot in-situ method to construct a catalytic system, eliminating the need for pre-synthesis, separation, and purification of organometallic catalysts. The process is simple, easy to operate, and reduces the cost of catalyst use and process complexity.
[0027] 3. The method described in this invention does not require prior separation and purification of sensitive catalytic components, reducing dependence on harsh operating conditions and demonstrating good practicality and industrial application prospects. Attached Figure Description
[0028] Figure 1 : 1H NMR spectrum of α-diimine ligand in Example 1;
[0029] Figure 2 : The 1H NMR spectrum of the 2,6-diimine pyridine ligand in Example 2;
[0030] Figure 3 : 1H NMR spectrum of 2-iminepyridine ligand in Example 3;
[0031] Figure 4 Example 5: 1H NMR spectrum of methyl lactate synthesized in Example 5;
[0032] Figure 5 Gas chromatogram of methyl lactate, the product in Example 5;
[0033] Figure 6 NMR spectrum of crude polylactic acid depolymerized product in Comparative Example 2. Detailed Implementation
[0034] 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.
[0035] As described in the background section, existing catalysts for the degradation of polylactic acid into methyl lactate generally suffer from problems such as low reactivity, high reaction temperature, and long reaction time.
[0036] During the experiment, it was discovered that the neutral nitrogen ligand and the organolithium reagent do not simply mix physically, but can form lithium coordination species with suitable reactivity in situ within the reaction system. The coordination effect of the neutral nitrogen ligand on the lithium center can regulate the electronic and spatial environments of the lithium center, enabling it to both promote methanol activation to form alkoxide-type active species and avoid excessive reactivity of the uncoordinated organolithium reagent, which could lead to non-selective side reactions. Therefore, the resulting active system can rapidly attack the ester bond sites in the polylactic acid (PLA) molecular chain under relatively mild conditions, promoting the breakage of the PLA backbone and the formation of methyl lactate. Compared with existing technologies that rely on high temperatures to drive the reaction, this invention achieves efficient in-situ construction of active species through ligand regulation, allowing the reaction to be completed near room temperature in a short time while maintaining high selectivity for the target product.
[0037] 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.
[0038] 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.
[0039] Example 1
[0040] Synthesis of α-diimine (R is isopropyl): 2.15 g of 2,3-butanedione and 8.86 g of 2,6-diisopropylaniline were placed in a 100 mL round-bottom flask. 40 mL of ethanol was added to the flask, and 0.5 g of trifluoroacetic acid was added to catalyze the reaction. The mixture was heated under reflux for 24 h. After the reaction was completed, the mixture was quickly transferred to a -18 °C freezer for crystallization. The solid was then filtered and washed three times with 5 mL of icy ethanol to obtain the solid. The obtained solid was transferred to a 40 °C vacuum drying oven and dried for 24 h to obtain a pale yellow solid, which is the product.
[0041] Example 2
[0042] Synthesis of 2,6-diimine pyridine (R = isopropyl): 0.125 g of 2,6-pyridinedicarboxaldehyde was placed in a 100 mL round-bottom flask and dissolved in 20 mL of anhydrous ethanol. Then, 0.36 g of 2,6-diisopropylaniline and 1 drop of glacial acetic acid were added sequentially. The mixture was heated under reflux for 6 h, and after cooling to room temperature, a solid precipitated. The product was then filtered using a sintered glass funnel connected to a vacuum filter and washed three times with 2 mL of glacial ethanol to obtain a solid. The obtained solid was transferred to a vacuum drying oven at 40 °C and dried for 24 h. The resulting pale yellow solid was the product.
[0043] Example 3
[0044] Synthesis of 2-iminepyridine (R is isopropyl): 5.36 g of pyridine-2-carboxaldehyde and 8.86 g of 2,6-diisopropylaniline were weighed and placed in a 100 mL round-bottom flask. 40 mL of ethanol was added to the flask, and the mixture was heated under reflux for 6 h. After cooling to room temperature, a solid was obtained and washed three times with 2 mL of ice-cold ethanol. The solid was then transferred to a vacuum drying oven at 40 °C and dried for 24 h. The resulting yellow solid was the product.
[0045] Example 4
[0046] Synthesis of β-diimine: 10.3 mL (100 mmol) of acetylacetone and 18 mL (200 mmol) of aniline were placed in a round-bottom flask, and 50 mL of anhydrous ethanol was added. 8.3 mL of concentrated hydrochloric acid was slowly added dropwise under vigorous stirring. The mixture was heated under reflux for 48 h to remove the solvent. The resulting solid was neutralized with 20 mL of triethylamine and then extracted with dichloromethane to remove the solvent, yielding a yellow powder. Recrystallization from ethanol yielded 16.1 g of yellow crystals.
[0047] Example 5
[0048] In a nitrogen atmosphere, 144 g of recovered polylactic acid and 4 g (0.01 mol) of α-diimine ligand (R substituent is isopropyl) were added to 2160 mL of dichloromethane. The polylactic acid was completely dissolved using a stirrer to ensure the solution was clear and homogeneous. Then, 324 mL of methanol was added. At room temperature (25 °C), under stirring, 10 mL of butyllithium (0.01 mol, 1 mol / L hexane solution) was added to the solution below the liquid surface. The mixture was stirred for 10 seconds, and the reaction was confirmed to be complete by GC. The solvent was removed by vacuum distillation, and the crude product was dried under vacuum (40 °C, 50 kPa) to obtain 206 g of methyl lactate, with a purity of 99.5% (based on the theoretical yield of methyl lactate) (gas chromatography).
[0049] Examples 6 to 15
[0050] The differences from Example 5 are as follows: the type of polylactic acid, the type of solvent, the amount of solvent added, the ligand (containing different substituents), the amount of ligand added, and the reaction time are different. The differences are shown in Table 1.
[0051] Table 1 Results of different depolymerization conditions for polylactic acid
[0052] Example solvent solvent / ml Methanol / ml ligands Ligand amount / mmol Reaction time / s Product yield / % Product purity / % 6 <![CDATA[CH2Cl2]]> 2160 324 2,6-Diiminepyridine (R: isopropyl) 10 20 98.5 99.5 7 <![CDATA[CH2Cl2]]> 2160 324 2-Iminepyridine (R: isopropyl) 5 20 93.7 99.2 8 Toluene 2160 324 2,2-Bipyridine 10 30 94.6 99.0 9 benzene 2160 324 TMEDA 10 6 99.4 99.4 10 <![CDATA[CH2Cl2]]> 2880 162 TMEDA 2 600 95.4 99.2 11 <![CDATA[CH2Cl2]]> 1440 486 TMEDA 200 6 97.4 99.3 12 <![CDATA[CH2Cl2]]> 2880 324 2,6-Diiminepyridine (R: ethyl) 10 30 98.0 99.4 13 THF 2160 324 2,6-Diiminepyridine (R: hydrogen) 10 70 98.9 98.9 14 <![CDATA[CH2Cl2]]> 2160 324 2-Iminepyridine (R: hydrogen) 10 40 92.7 97.2 15 <![CDATA[CH2Cl2]]> 1440 162 α-Diimine (R: hydrogen) 10 120 94.1 96.1
[0053] 1. Example 10 was a reaction at 15°C; the conditions for the other examples were the same as those for Example 5.
[0054] 2. Example 11 involves a reaction at 35°C; the other conditions are the same as those in Example 5.
[0055] Comparative Example 1
[0056] The difference from Example 5 is that the amine nitrogen ligand used is different, and the α-diimine ligand is replaced with an equal amount of β-diimine.
[0057] The structural formula of β-diimine is:
[0058] R stands for isopropyl.
[0059] The target product, methyl lactate, was not obtained under the above reaction conditions, indicating that the β-diimine ligand did not exhibit effective catalytic activity under these conditions.
[0060] Furthermore, experimental results show that different nitrogen ligand structures have a significant impact on catalytic activity. The catalytic effect of the method of the present invention does not come from the simple superposition of organolithium reagents or nitrogen ligands, but from the synergistic coordination between a specific neutral nitrogen ligand and the lithium center.
[0061] Comparative Example 2
[0062] The difference from Example 5 is that an equal amount of LiHMDS was used to replace the α-diimine ligand. Under the above reaction conditions, the yield of methyl lactate was only 36.7%, indicating that LiHMDS alone could not achieve the catalytic effect described in this invention. This demonstrates that the technical effect of this invention does not solely originate from the lithium source itself, but rather from the synergistic effect between the neutral nitrogen ligand and the lithium source.
[0063] 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 one-pot catalytic depolymerization method for recovering methyl lactate from polylactic acid, characterized in that, The method is as follows: Polylactic acid, amine nitrogen ligand and methanol were mixed, dissolved in a solvent, and then n-butyllithium solution was added to carry out depolymerization reaction; after the reaction was completed, the solvent was removed by vacuum distillation, and the crude product was dried under vacuum to obtain methyl lactate product; The amine nitrogen ligands include diimine compounds, 2,2-bipyridine, or tetramethylethylenediamine.
2. The method according to claim 1, characterized in that, The diimine compounds include compounds having the following structural formula: , or , where R is a substituent of hydrogen, methyl, ethyl, or isopropyl.
3. 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.
4. The method according to claim 1, characterized in that, The molar ratio of the amine nitrogen ligand to the carbonyl group contained in polylactic acid is 0.001-0.1:1; the molar ratio of the carbonyl group contained in polylactic acid to methanol is 1:2-6.
5. The method according to claim 1, characterized in that, The solvent is selected from dichloromethane, tetrahydrofuran, toluene, and benzene; the ratio of the solvent to polylactic acid is 10-20 mL: 1 g.
6. The method according to claim 1, characterized in that, The n-butyllithium solution is a hexane solution of n-butyllithium; the concentration of the n-butyllithium solution is 1 mol / L; the molar ratio of n-butyllithium to the amine nitrogen ligand is 1:
1.
7. The method according to claim 1, characterized in that, The depolymerization reaction is carried out at a temperature of 15~35℃ for a time of 6~600s.
8. The method according to claim 1, characterized in that, The vacuum drying temperature is 40℃ and the pressure is 50kPa.
9. The application of the method according to any one of claims 1 to 8 in improving the depolymerization efficiency of polylactic acid.