Polylactic acid depolymerization catalyst as well as preparation method and application thereof

Abstract

The invention provides a polylactic acid depolymerization catalyst as shown in a formula (1) as well as a preparation method and application thereof. According to the polylactic acid depolymerization catalyst, a polyhydroxy side chain structure is introduced, so that the problems of high system viscosity and low product evaporation speed in a polylactic acid degradation process are solved. Meanwhile, the catalyst is a solid at normal temperature, has better storage stability compared with a monohydroxyl catalyst, does not need low-temperature storage, and can be directly mixed with the raw material PLA by using powder in the use process, so that the contact surface of the catalyst and the raw material is increased, and the depolymerization time can be shortened. # imgabs0 #

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst preparation, and specifically relates to a polylactic acid depolymerization catalyst and a preparation method and application thereof. Background Art

[0002] As people's requirements for environmental protection become higher and higher, the demand for biodegradable plastics is also gradually increasing. Due to its biodegradability and good mechanical properties, the use of polylactic acid (PLA) products is increasing year by year. Currently, polylactic acid is widely used in disposable packaging, textile fibers, 3D printing and other fields.

[0003] At present, the main synthesis methods of polylactic acid are one-step method and two-step method. Both methods are to produce lactic acid (LA) by fermentation of corn starch. Then the lactic acid is dehydrated into oligolactic acid, then cracked into lactide, and finally purified and polymerized into polylactic acid. However, the two-step method has the disadvantages of high cost, low yield and low efficiency, resulting in high cost of corresponding polylactic acid products. At the same time, the degradation conditions of polylactic acid are relatively harsh. It can be rapidly degraded under composting and aerobic environment at 60°C, and the degradation rate in nature is slow (it takes 10-20 years to degrade in seawater), which is not conducive to environmental protection. The proposal of a circular society has increased the demand for later waste utilization, but the physical reuse of PLA currently has poor thermal stability, so its performance deteriorates quickly and cannot be reused. The repolymerization of chemically recovered monomers can circumvent the problem of performance degradation. However, hydrolysis and alcoholysis both have the problems of high cost and poor economic efficiency. Pyrolysis of PLA to recover lactide is currently a better solution with low cost and high economic efficiency for chemical recycling, but this method currently has the problems of poor selectivity and high difficulty in cracking. Summary of the invention

[0004] In order to improve the above technical problems, the present invention provides a structure shown in formula (1):

[0005]

[0006] Wherein, n is an integer of 2-100; m is 2 or 3;

[0007] M is Fe, Al, Zn, Mg or Sn;

[0008] When M is Fe or Al, m is 3; when M is Zn, Mg or Sn, m is 2;

[0009] X and L are the same and are halogen;

[0010] R is imidazole, an imidazole derivative, thiophene or a thiophene derivative;

[0011] The imidazole derivative is imidazole substituted by one, two, three or four Ra;

[0012] The thiophene derivative is thiophene substituted with one, two, three or four Rb;

[0013] Ra and Rb are the same or different and are independently selected from C 1-6 alkyl;

[0014] R 2 C is replaced by one, two or more Rs 1-6 A straight or branched alkyl group, wherein Rs is

[0015] According to an embodiment of the present invention, n is an integer of 3-50, preferably an integer of 5-30, for example, 10 or 15, such as 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30;

[0016] X and L are Cl, Br or I;

[0017] R 2 C is replaced by one, two or three Rs 1-3 A straight or branched alkyl group, wherein Rs is

[0018] According to an embodiment of the present invention, R is One of them.

[0019] The present invention also provides a method for preparing the structure represented by the above formula (1), comprising the following steps:

[0020] (S1) a polyhydroxyl-containing alkyl halide Quaternization reaction with compound R to obtain polyhydroxy-containing onium halide

[0021] (S2) the polyhydroxy-containing onium halide obtained in step (S1) MX with metal ion halides m Carry out Lewis acid-base neutralization reaction to obtain polyhydroxy-containing onium salt

[0022] (S3) the onium salt obtained in step (S2) Mixed with lactic acid, subjected to dehydration esterification reaction or added to lactide for ring-opening polymerization reaction to obtain the structure shown in formula (1);

[0023] Among them, R 2 ' is the above R 2 The structure in which the Rs substituent is changed to OH;

[0024] R, M, X, L and m are as defined above.

[0025] According to an embodiment of the present invention, in step (S1), the polyhydroxyl-containing halogenated alkane It is one of 3-chloro-1,2-propanediol, 2-chloro-1,3-propanediol, 3-bromo-1,2-propanediol and 2-bromo-1,3-propanediol.

[0026] According to an embodiment of the present invention, in step (S1), the compound R is 1-methylimidazole, 2-methylimidazole, 3-methylimidazole, 4-methylimidazole, 1-thiophene or 2-methylthiophene.

[0027] According to an embodiment of the present invention, in step (S1), the reaction is carried out under an inert gas atmosphere, such as nitrogen or argon.

[0028] According to an embodiment of the present invention, in step (S1), the reaction process is: first, in a nitrogen atmosphere, a polyhydroxyl-containing alkyl halide is reacted with Add to compound R, and perform the first reaction heating during the addition process; then heat up for the second heat preservation reaction. The first heating temperature is 50-90°C and the heating time is 1-40h; the second heating temperature is 100-170°C and the heating time is 10-40h. Preferably, the first and second heating are performed under normal pressure.

[0029] Exemplarily, the first heating temperature is 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C; the first heating time is 6h, 7h, 8h, 10h, 15h, 20h, 30h or 40h.

[0030] Exemplarily, the second heating temperature is 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, 165°C or 170°C; and the second heating time is 10h, 20h, 30h or 40h.

[0031] According to an embodiment of the present invention, in step (S1), the speed of introducing the inert gas is 0.05-0.4 L / min, preferably 0.1-0.3 L / min, exemplarily 0.1 L / min, 0.2 L / min or 0.3 L / min.

[0032] According to an embodiment of the present invention, in step (S1), the polyhydroxyl-containing halogenated alkane The molar amount added is 0.8 to 1.2 times that of compound R. Exemplarily, it is 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15 or 1.2 times.

[0033] According to an embodiment of the present invention, in step (S2), the metal ion halide is FeCl 3 SnCl 2 、ZnCl 2 , FeBr 3 SnBr 2 、ZnBr 2 One of them.

[0034] According to an embodiment of the present invention, in step (S2), the molar amount of the metal ion halide added is 0.8 to 1.2 times that of compound R. Exemplarily, it is 0.8, 0.9, 1.0, 1.05, 1.1, 1.15 or 1.2 times.

[0035] According to an embodiment of the present invention, in step (S2), the reaction temperature is 60-120°C, exemplarily, the reaction temperature is 60°C, 80°C, 100°C, 110°C, 120°C, and the reaction time is 10-40h, exemplarily, the reaction time is 10h, 20h, 40h.

[0036] According to an embodiment of the present invention, in step (S2), the reaction is carried out under an inert gas, such as nitrogen or argon.

[0037] According to an embodiment of the present invention, in step (S3), the molar amount of lactic acid or lactide is 5-100 times the hydroxyl group in the product of step (S1), illustratively 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90 or 100 times.

[0038] According to an embodiment of the present invention, in step (S3), the lactic acid / lactide needs to be dehydrated to a moisture content of less than 100 ppm before feeding.

[0039] According to an embodiment of the present invention, in step (S3), the reaction temperature is 80-150° C., and the reaction time is 5-40 h.

[0040] According to an embodiment of the present invention, in step (S3), when lactic acid is added, the dehydration esterification step is: dehydration at 80°C, 100°C, 120°C and 130°C for 1-6h respectively.

[0041] As an exemplary embodiment of the present invention, the preparation of the structure represented by formula (1) comprises the following steps:

[0042] (1) Place 1-methylimidazole in a flask at 50°C under N 2Slowly add 3-chloro-1,2-propanediol dropwise under atmosphere for 1-3 hours, heat to 80°C and keep warm for 25-40 hours, then heat to 110°C and keep warm for 10-20 hours, then cool to room temperature and wash with ethyl acetate until there is no impurity, to obtain a halogenated imidazolium salt containing multiple hydroxyl groups;

[0043] (2) Add anhydrous ferric chloride to the product of step (1) at 80° C. for 30 h to obtain a polyhydroxy-containing imidazolium halogenide complex (FeCl 4 ) – (Im-CH 2 -CH-OH-CH 2 -OH) + ;

[0044] (3) At room temperature, lactic acid is added to the polyhydroxy-containing halogenated imidazolium salt complex of step (2) to carry out a dehydration esterification reaction by gradually increasing the temperature or adding lactide to carry out a ring-opening reaction.

[0045] The present invention also provides the use of the structure shown in formula (1) as a catalyst in catalyzing the depolymerization of polylactic acid.

[0046] According to an embodiment of the present invention, the depolymerization process is: adding the structure represented by formula (1) into PLA to carry out cyclodepolymerization reaction to obtain lactide.

[0047] According to an embodiment of the present invention, the molar ratio of the terminal hydroxyl group to the PLA ester bond in the structure shown in formula (1) is 1:(10-50), and the cyclization depolymerization product is continuously distilled out under the conditions of oil pump decompression to 0-300 Pa and 100-250° C. with stirring.

[0048] The present invention provides a method for preparing lactide by depolymerizing polylactic acid, comprising: subjecting the polylactic acid to a depolymerization reaction in the presence of the structure of the above formula (1).

[0049] According to an embodiment of the present invention, the depolymerization reaction is carried out at 0-300Pa.

[0050] According to an embodiment of the present invention, the depolymerization reaction is carried out at 100-250°C, for example, at 180-220°C.

[0051] According to an embodiment of the present invention, the chemical purity of lactide prepared by the method is above 92%, such as above 95%.

[0052] According to an embodiment of the present invention, the content of L-LA prepared by the method is above 95%, such as above 96%.

[0053] According to an embodiment of the present invention, the content of D-LA prepared by the method is less than 4%, such as less than 3% <2% or less than 1%.

[0054] Beneficial effects of the present invention

[0055] The polylactic acid depolymerization catalyst provided by the present invention improves the problem of high system viscosity and slow product evaporation speed during polylactic acid degradation by introducing a polyhydroxy side chain structure (it is well known to those skilled in the art that the polylactic acid cyclization depolymerization product lactide needs to be evaporated by distillation. When the system viscosity is too large, the system with high viscosity wraps the small molecule product lactide, causing lactide to be difficult to evaporate, making the reaction stirring difficult, and the energy consumption is significantly increased. At the same time, due to the poor stability of lactide at high temperature, it is easy to decompose and cause optical racemization, greatly reducing the yield of high optical purity lactide, resulting in a substantial increase in production costs); the present invention can adjust the compatibility of the catalyst and the depolymerized product by designing the length of the side chain, so that the viscosity of polylactic acid can be rapidly reduced during the depolymerization process, ensuring that lactide is smoothly and quickly evaporated. At the same time, the polyhydroxy catalyst of the present invention is solid at room temperature, and has better storage stability than the monohydroxy catalyst (viscous liquid at room temperature), and does not need to be stored at low temperature. During use, the powder can be directly used to mix with the raw material PLA, which increases the contact surface between the catalyst and the raw material and shortens the depolymerization time.

[0056] The synthesis method of the present invention ensures the reaction activity of imidazole in the first step of quaternary ammonium salt reaction by low-temperature pre-reaction and then heating aging, ensures that the reaction is thorough and the color of the system is lighter, and effectively improves the activity of the catalyst ligand. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is the gas phase spectrum of lactide recovered in Example 1.

[0058] Figure 2 It is the liquid phase spectrum of the lactide recovered in Example 1.

[0059] Figure 3 This is the gas phase spectrum of lactide recovered in Comparative Example 1.

[0060] Figure 4 This is the liquid phase spectrum of lactide recovered in Comparative Example 1. DETAILED DESCRIPTION

[0061] The technical scheme of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only exemplary descriptions and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are included in the scope that the present invention is intended to protect.

[0062] Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0063] The calculation method of depolymerization yield in the following examples and comparative examples is:

[0064]

[0065] Example 1

[0066] 20 g of 1-methylimidazole was put into a flask, and a condenser and a separatory funnel (with 28 g of 3-chloro-1,2-propylene glycol) were connected. The flask was replaced with nitrogen for 5 minutes to ensure that the material was in a nitrogen atmosphere, and nitrogen was continuously introduced at a ventilation rate of 0.3 L / min. The temperature was raised to 80°C, and 3-chloro-1,2-propylene glycol was gradually added dropwise. The addition was completed within 2 hours. After the addition, nitrogen was continuously introduced at a ventilation rate of 0.2 L / min. The temperature was kept for 30 hours, and the temperature was raised to 110°C and kept for 15 hours. The temperature was cooled to room temperature, and the mixture was washed and purified with ethyl acetate to obtain a hydroxyl-containing 1-methylimidazolium chloride salt;

[0067] Take 45g of 1-methylimidazolium chloride containing hydroxyl group, 50℃N 2 Under the protection of the reactor, 37.88 g of anhydrous ferric chloride was added, and the temperature was gradually raised to 110 ° C. The reaction was kept warm for 30 hours to obtain a polyhydroxy-containing 1-methylimidazolium chloride complex [FeCl 4 ] - [Im-CH 2 -CHOH-CH 2 OH] + .

[0068] At 80°C, 1.08 g [FeCl 4 ] - [Im-CH 2 -CHOH-CH 2 OH] + It was mixed with 8.29 g of anhydrous lactic acid (water content 80 ppm) and gradually dehydrated and esterified at 80°C, 100°C, 120°C, and 130°C for 4 h each. [FeCl 4 ] - [Im-CH 2 -CH-poly(L-LA) 10 -OH-CH 2 -poly(L-LA) 10 -OH] + In the structure of formula (1), m is 3 and n is 10.

[0069] The prepared [FeCl 4 ] - [Im-CH 2 -CH-poly(L-LA) 10 -OH-CH 2-poly(L-LA) 10 -OH] + Catalyze the cyclization and depolymerization of PLLA. Add 0.6g of the above catalyst and 50g of PLLA, stir under the condition of oil pump decompression to 200Pa and 200℃, the viscosity of the reaction system drops significantly from 300mPa*s to 125mPa*s, and the cyclization and depolymerization products are continuously distilled out. The depolymerization yield is 96% after 1h. The results of gas chromatography test are as shown in the attached Figure 1 As shown, the recovered product is high-purity lactide with a chemical purity of 95%; the results of liquid chromatography test are as shown in the attached Figure 2 As shown, the L-LA content in the recovered product is 99%, and the D-LA content is <1%.

[0070] Example 2

[0071] 20 g of 1-methylimidazole was put into a flask, and a condenser and a separatory funnel (with 28 g of 3-chloro-1,2-propylene glycol) were connected. The flask was replaced with nitrogen for 5 minutes to ensure that the material was in a nitrogen atmosphere, and nitrogen was continuously introduced at a ventilation rate of 0.1 L / min. The temperature was raised to 80°C, and 3-chloro-1,2-propylene glycol was gradually added dropwise. The addition was completed within 2 hours. After the addition, nitrogen was continuously introduced at a ventilation rate of 0.2 L / min. The temperature was kept for 30 hours, and the temperature was raised to 110°C and kept for 15 hours. The temperature was cooled to room temperature, and the mixture was washed and purified with ethyl acetate to obtain a hydroxyl-containing 1-methylimidazolium chloride salt;

[0072] Take 45g of 1-methylimidazolium chloride containing hydroxyl group, 50℃N 2 Under the protection of the reactor, 39.5 g of anhydrous stannous chloride was added, and the temperature was gradually raised to 110 ° C. The reaction was kept warm for 30 hours to obtain a polyhydroxy-containing 1-methylimidazolium chloride complex [SnCl 3 ] - [Im-CH 2 -CHOH-CH 2 OH] + .

[0073] At 80°C, 1.08 g [SnCl 3 ] - [Im-CH 2 -CHOH-CH 2 OH] + It was mixed with 5.09 g of anhydrous lactic acid (water content 80 ppm) and dehydrated and esterified stepwise at 80°C, 100°C, 120°C, and 130°C for 4 h each. [SnCl 3 ] - [Im-CH 2 -CH-poly(L-LA) 10 -OH-CH2 -poly(L-LA) 10 -OH] + In the structure of formula (1), m is 2 and n is 10.

[0074] Use [SnCl 3 ] - [Im-CH 2 -CH-poly(L-LA) 10 -OH-CH 2 -poly(L-LA) 10 -OH] + Catalyze the cyclization and depolymerization of PLLA. 0.6g of the above catalyst was fused with 50g of PLLA, and stirred at 200°C under the condition of oil pump decompression to 200Pa. The viscosity of the system dropped significantly from 300mPa*s to 90mPa*s, and the cyclization and depolymerization products were continuously distilled out. The depolymerization yield was 98% after 0.5h, and the purity of the recovered product lactide was 96% after gas phase detection. The L-LA content in the recovered product was 97% after liquid phase detection, and the D-LA content was <2%.

[0075] Example 3

[0076] 16.8 g of 1-thiophene was put into a flask, and a condenser and a separatory funnel (with 22.1 g of 3-chloro-1,2-propylene glycol) were connected. The flask was replaced with nitrogen for 5 minutes to ensure that the material was in a nitrogen atmosphere, and nitrogen was continuously introduced at a ventilation rate of 0.1 L / min. The temperature was raised to 80°C, and 3-chloro-1,2-propylene glycol was gradually added dropwise. The addition was completed within 2 hours. After the addition, nitrogen was continuously introduced at a ventilation rate of 0.3 L / min. The temperature was kept for 30 hours, and the temperature was raised to 110°C and kept for 15 hours. The temperature was cooled to room temperature, and the mixture was washed and purified with ethyl acetate to obtain a hydroxyl-containing 1-thiazolium chloride salt;

[0077] At 50℃N 2 Under the protection of the reactor, 38.9 g of 1-thiazolium chloride containing hydroxyl groups was taken, 32.4 g of anhydrous ferric chloride was added, and the temperature was gradually raised to 110 ° C. The reaction was kept warm for 30 hours to obtain a polyhydroxyl-containing 1-thiazolium chloride complex [FeCl 4 ] - [Thi-CH 2 -CH-poly(L-LA) 10 -OH-CH 2 -poly(L-LA) 10 -OH] + .

[0078] At 80 °C, 3.57 g [FeCl 4 ] - [Thi-CH2 -CH-poly(L-LA) 10 -OH-CH 2 -poly(L-LA) 10 -OH] + It was mixed with 18.02 g of anhydrous lactic acid (water content 80 ppm) and gradually dehydrated and esterified at 80°C, 100°C, 120°C, and 130°C for 4 h each. [FeCl 4 ] - [Thi-CH 2 -CH-poly(L-LA) 10 -OH-CH 2 -poly(L-LA) 10 -OH] + In the structure of formula (1), m is 3 and n is 10.

[0079] Use [FeCl 4 ] - [Thi-CH 2 -CH-poly(L-LA) 10 -OH-CH 2 -poly(L-LA) 10 -OH] + Catalyze the cyclization and depolymerization of PLLA. 0.6g of the above catalyst was fused with 50g of PLLA, and stirred at 200°C under the condition of oil pump decompression to 200Pa. The viscosity of the system dropped significantly from 300mPa*s to 150mPa*s, and the cyclization and depolymerization products were continuously distilled out. After 0.6h, the depolymerization yield was 96%, and the purity of the recovered product lactide was 95% after gas phase detection. The L-LA content in the recovered product was 98% by liquid phase detection, and the D-LA content was <1%.

[0080] Example 4

[0081] 20 g of 1-methylimidazole was put into a flask, and a condenser and a separatory funnel (with 28 g of 3-chloro-1,2-propylene glycol) were connected. The flask was replaced with nitrogen for 5 minutes to ensure that the material was in a nitrogen atmosphere, and nitrogen was continuously introduced at a ventilation rate of 0.1 L / min. The temperature was raised to 80°C, and 3-chloro-1,2-propylene glycol was gradually added dropwise. The addition was completed within 2 hours. After the addition, nitrogen was continuously introduced at a ventilation rate of 0.3 L / min. The temperature was kept for 30 hours, and the temperature was raised to 110°C and kept for 15 hours. The temperature was cooled to room temperature, and the mixture was washed and purified with ethyl acetate to obtain a hydroxyl-containing 1-methylimidazolium chloride salt;

[0082] Take 45g of 1-methylimidazolium chloride containing hydroxyl group, 50℃N 2Under the protection of the reactor, 39.5 g of anhydrous stannous chloride was added, and the temperature was gradually raised to 110 ° C. The reaction was kept warm for 30 hours to obtain a polyhydroxy-containing 1-methylimidazolium chloride complex [SnCl 3 ] - [Im-CH 2 -CHOH-CH 2 OH] + .

[0083] At 80°C, 1.08 g [SnCl 3 ] - [Im-CH 2 -CHOH-CH 2 OH] + Mix with 7.635g of anhydrous lactic acid (water content 80ppm), and start dehydration esterification step by step, dehydration at 80℃, 100℃, 120℃, and 130℃ for 4h each. [SnCl 3 ] - [Im-CH 2 -CH-poly(L-LA) 15 -OH-CH 2 -poly(L-LA) 15 -OH] + In the structure of formula (1), m is 2 and n is 15.

[0084] Use [SnCl 3 ] - [Im-CH 2 -CH-poly(L-LA) 15 -OH-CH 2 -poly(L-LA) 15 -OH] + Catalyze the cyclization and depolymerization of PLLA. 0.6g of the above catalyst was fused with 50g of PLLA, and stirred at 200°C under the conditions of oil pump decompression to 200Pa. The viscosity of the system dropped significantly from 300mPa*s to 110mPa*s, and the cyclization and depolymerization products were continuously distilled out. After 0.5h, the depolymerization yield was 98.2%, and the purity of the recovered product lactide was 97.3% after gas phase detection. The L-LA content in the recovered product was 96% by liquid phase detection, and the D-LA content was <3%.

[0085] Comparative Example 1

[0086] 1.64 g 1-methylimidazole and 1.89 g 3-chloro-1-propanol were added to a round-bottom flask, and the mixture was stirred and reacted for 48 h in a nitrogen atmosphere in an oil bath at 100 °C. After washing with ethyl acetate and drying, 1-hydroxypropyl-3-methylimidazolium chloride (Cl – +Im-CH 2-CH 2 -CH 2 -OH).

[0087] 3.53 gCl – +Im-CH 2 -CH 2 -CH 2 -OH and 2.54gFeCl 3 Add to the round-bottom flask, stir and react at 25°C in a nitrogen atmosphere for 4 h to obtain [FeCl 4 ] - [Im-CH 2 -CH 2 -CH 2 -OH] + .

[0088] 3.03 g [FeCl 4 ] - [Im-CH 2 -CH 2 -CH 2 -OH] + 14.41 g L-lactide (water content 80 ppm) was added to a round-bottom flask and stirred in a nitrogen atmosphere at 140 °C oil bath for 4 h to obtain [FeCl 4 ] - [Im-CH 2 -CH 2 -CH 2 -poly(L-LA) 10 -OH] + .

[0089] Prepared [FeCl 4 ] - [Im-CH 2 -CH 2 -CH 2 -poly(L-LA) 10 -OH] + Catalyze the cyclization and depolymerization of PLLA. 0.6g of the above catalyst was fused with 50g of PLA, and stirred at 200°C under the condition of oil pump decompression to 200Pa. The viscosity of the system dropped from 300mPa*s to 240mPa*s, and the cyclization and depolymerization product was distilled out. The depolymerization yield after 1h was 96%. The purity of the recovered lactide was 96.3% after gas phase detection. The test results are as follows: Figure 3 The liquid phase of the recovered product detected L-LA content of 92.7%, D-LA content> 5%, the test results are as follows Figure 4 shown.

[0090] The above is an explanation of the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. The structure shown in formula (1): in, n is an integer from 2 to 100; m is 2 or 3; M is Fe, Al, Zn, Mg or Sn; When M is Fe or Al, m is 3; when M is Zn, Mg or Sn, m is 2; X and L are the same and are halogen; R is imidazole, an imidazole derivative, thiophene or a thiophene derivative; The imidazole derivative is imidazole substituted by one, two, three or four Ra; The thiophene derivative is thiophene substituted with one, two, three or four Rb; Ra and Rb are the same or different and are independently selected from C 1-6 alkyl; R 2 C is replaced by one, two or more Rs 1-6 A straight or branched alkyl group, wherein Rs is 2. The structure according to claim 1, It is characterized in that n is an integer from 3 to 50; X and L are Cl, Br or I; R 2 C is replaced by one, two or three Rs 1-3 A straight or branched alkyl group, wherein Rs is 3. The structure according to claim 1 or 2, It is characterized in that R is One of them.

4. A method for preparing the structure according to any one of claims 1 to 3, It is characterized in that The steps include: (S1) a polyhydroxyl-containing alkyl halide Quaternization reaction with compound R to obtain polyhydroxy-containing onium halide (S2) the polyhydroxy-containing onium halide obtained in step (S1) MX with metal ion halides m Carry out Lewis acid-base neutralization reaction to obtain polyhydroxy-containing onium salt (S3) the onium salt obtained in step (S2) Mixed with lactic acid, subjected to dehydration esterification reaction or added to lactide for ring-opening polymerization reaction to obtain the structure shown in formula (1); Among them, R 2 ' is R as described in any one of claims 1 to 3 2 The structure in which the Rs substituent is changed to OH; R, M, X, L and m have the meanings as defined in any one of claims 1-3.

5. The preparation method according to claim 4, It is characterized in that In step (S1), the polyhydroxyl-containing alkyl halide It is one of 3-chloro-1,2-propanediol, 2-chloro-1,3-propanediol, 3-bromo-1,2-propanediol and 2-bromo-1,3-propanediol; Preferably, in step (S1), the compound R is 1-methylimidazole, 2-methylimidazole, 3-methylimidazole, 4-methylimidazole, 1-thiophene or 2-methylthiophene; Preferably, in step (S1), the reaction is carried out under an inert gas atmosphere, such as nitrogen or argon; Preferably, in step (S1), the reaction process is: firstly, in a nitrogen atmosphere, a polyhydroxyl-containing alkyl halide is reacted with Add to compound R, and perform the first reaction heating during the adding process; then heat up and perform the second heat preservation reaction; wherein the first heating temperature is 50-90°C, and the heating time is 1-40h; the second heating temperature is 100-170°C, and the heating time is 10-40h; Preferably, the first and second heating are performed under normal pressure.

6. The preparation method according to claim 5, It is characterized in that In step (S2), the metal ion halide is FeCl 3 SnCl 2 、ZnCl 2 , FeBr 3 SnBr 2 、ZnBr 2 One of; Preferably, in step (S2), the molar amount of the metal ion halide added is 0.8 to 1.2 times that of compound R; Preferably, in step (S2), the reaction temperature is 60-120°C; Preferably, in step (S2), the reaction is carried out under an inert gas.

7. The preparation method according to any one of claims 4 to 6, It is characterized in that In step (S3), the molar amount of lactic acid or lactide is 5-100 times the hydroxyl group in the product of step (S1); Preferably, in step (S3), the lactic acid / lactide is first dehydrated to a moisture content of less than 100 ppm before feeding; Preferably, in step (S3), the reaction temperature is 80-150°C; Preferably, in step (S3), when lactic acid is added, the dehydration esterification step is: dehydration at 80°C, 100°C, 120°C and 130°C for 1-6h respectively.

8. Use of the structure represented by formula (1) according to any one of claims 1 to 3 as a catalyst in catalyzing the depolymerization of polylactic acid.

9. The use according to claim 8, It is characterized in that The depolymerization process is as follows: adding the structure shown in formula (1) into PLA to carry out cyclization depolymerization reaction to obtain lactide.

10. A method for preparing lactide by depolymerizing polylactic acid, It is characterized in that include: The polylactic acid is subjected to a depolymerization reaction in the presence of the structure of formula (1) according to any one of claims 1 to 3.

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