A zinc coordination polymer catalyst for depolymerizing polylactic acid to recover lactide, and a microwave-assisted preparation method and application thereof

Abstract

The application belongs to the technical field of waste polylactic acid monomer recovery, and discloses a zinc coordination polymer catalyst for depolymerizing polylactic acid to recover propiolactone, a microwave-assisted preparation method and application thereof, and the preparation method comprises the following steps: mixing a zinc salt, a formate and an organic solvent, and then performing reaction under microwave conditions, and then performing filtration and separation to obtain a zinc coordination polymer catalyst, and the repeat structure unit of the zinc coordination polymer catalyst is [Zn(HCOO)2]. Compared with the prior art, the application has the advantages of simple operation, advanced process, short reaction time and high yield. The zinc coordination polymer catalyst has good activity in polylactic acid depolymerization, the purity of the monomer propiolactone is 99.5%, and the recovery rate can reach 99.0%. Compared with zinc oxide, zinc chloride and other catalysts, the zinc coordination polymer catalyst has the advantages of low reaction temperature, short reaction time and small catalyst consumption in the catalytic depolymerization process, and has good application prospect in the field of waste polylactic acid resin monomer recovery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of waste polylactic acid monomer recovery, and particularly relates to a zinc coordination polymer catalyst for polylactic acid monomer recovery and microwave-assisted preparation and application thereof. BACKGROUND

[0002] Polylactic acid (PLA) is a biodegradable polyester material with excellent biocompatibility, processability and mechanical properties. In addition to being applied in the fields of biological medicine such as surgical sutures, drug release packaging agents and tissue repair materials, it can also be applied in the fields of disposable products such as tableware and food packaging materials, agricultural mulching films, textiles and 3D printing. With the wide application of PLA in various fields, although it has the ability to degrade in natural environments such as water bodies and soil, the long degradation period of PLA waste may cause new pollution. Therefore, it is of great significance to recover lactide monomers from PLA waste.

[0003] Catalytic depolymerization of PLA to recover lactide monomers is a very effective method for the recycling of PLA waste, and the catalyst plays a crucial role. The most common catalysts include zinc, tin and strong base catalysts. In the process of catalytic depolymerization of PLA to recover lactide monomers, Wang Qinggang et al. used zinc chloride as catalyst, and the recovery rate of lactide monomers was 97.2% after 9 hours of reaction at 200℃ and 1 millipascal vacuum (CN115160287); Liu Xiong et al. used stannous octoate as catalyst to depolymerize PLA, and found that the recovery rate of lactide monomers could reach 96.5% after 1.5 hours of vacuum distillation at 195℃ (CN115403554); Xu Guangqiang et al. used 1,5,7-triazabicyclo[4.4.0]dec-5-ene as catalyst, and the recovery rate of lactide was 98.0% after 4 hours of vacuum distillation at a higher temperature (220℃) (CN115160288). The above-mentioned homogeneous catalysts of zinc, tin and strong base have low activity, long depolymerization time, high depolymerization temperature, low monomer recovery rate and non-recyclable catalysts. In 2022, Hou Cuiping et al. used molecular sieve SBA-15 supported zinc oxide heterogeneous catalyst to depolymerize PLA, and the recovery rate of lactide monomers was 89.0% after 4 hours of vacuum distillation at 200℃, and the recovery rate of the recovered catalyst remained unchanged after reuse (CN118026990). Therefore, it is of great scientific significance and application value to develop recyclable and high-activity depolymerization catalysts for the depolymerization and recovery of lactide from PLA waste.

[0004] Coordination polymer is a new inorganic-inorganic hybrid material self-assembled by metal ions and organic ligand molecules through coordination, which has structural diversity, tunability and unsaturated metal active sites, and has been widely used in alkylation, condensation, ester exchange, polymerization and various oxidation reactions. Coordination polymers, especially zinc-based coordination polymers with good biocompatibility, have been widely used in catalyzing ring-opening polymerization of lactide to prepare PLA. However, there is no literature report on the application of coordination polymers to catalyze the depolymerization of degradable polyesters. It has broad application space to develop new coordination polymers to quickly and efficiently catalyze the depolymerization of waste PLA to recover monomers, and also provides a new direction for the monomer recycling research of waste PLA. SUMMARY

[0005] This section is intended to summarize some aspects of the embodiments of the present application and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of the specification of the present application to avoid obscuring the purpose of this section, the abstract and the title, and such simplifications or omissions cannot be used to limit the scope of the present application.

[0006] The purpose of the present application is to provide a method for preparing a zinc coordination polymer catalyst by microwave and its application in depolymerization of polylactic acid to recover lactide.

[0007] The purpose of the present application can be achieved by the following technical solutions.

[0008] A preparation method of a zinc coordination polymer catalyst, comprising:

[0009] Mixing zinc salt, formate and organic solvent, and reacting under microwave conditions to obtain a zinc coordination polymer catalyst after filtration and separation, and the repeat structure unit is [Zn(HCOO)2];

[0010] The microwave conditions include: microwave output power is 200-800W, and microwave frequency is 2000-3000MHz; the reaction conditions include: reaction temperature is 100-180℃, and reaction time is 5-90 minutes.

[0011] Further, the zinc salt includes one or more of zinc chloride, zinc acetate and zinc nitrate.

[0012] Further, the formate includes one or more of potassium formate, calcium formate, sodium formate and magnesium formate.

[0013] Further, the molar ratio of the zinc salt to the formate is 1:2-6.

[0014] Further, the solvent is one or a mixed solution of ethanol, methanol, N,N-dimethylformamide and N-methylformamide.

[0015] Further, the amount of solvent corresponding to 1 mmol of zinc salt is 4-60 ml.

[0016] A zinc coordination polymer catalyst is prepared by the method as described above.

[0017] The application of a zinc coordination polymer catalyst, the catalyst is applied to catalyze the depolymerization of polylactic acid to recover lactide monomer, comprising:

[0018] The zinc coordination polymer and polylactic acid are mixed in a ball mill for 20 minutes, and then the mixture is put into a Schlenk reaction tube to collect lactide monomer by vacuum distillation.

[0019] The reaction conditions include a reaction temperature of 175-215 DEG C, a reaction time of 10-300 minutes, and a vacuum degree of 5-1000 Pa.

[0020] Further, the mass ratio of polylactic acid to zinc coordination polymer is 100:0.1-10, the purity of lactide monomer is 99.5%, and the recovery rate can reach 99.0%, which has good application prospect in the field of waste polylactic acid resin monomer recovery.

[0021] Compared with the prior art, the application has the following characteristics:

[0022] The preparation method of the zinc coordination polymer is simple, the process is advanced, the reaction time is short, and the yield is as high as 96.1%. The zinc coordination polymer catalyzes the depolymerization of polylactic acid and has good activity, the purity of lactide monomer is 99.5%, and the recovery rate can reach 99.0%. Compared with zinc oxide and zinc chloride, the zinc coordination polymer has the advantages of low reaction temperature, short reaction time and small amount of catalyst in the catalytic depolymerization process, and has good application prospect in the field of waste polylactic acid resin monomer recovery. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is an infrared spectrum diagram of the zinc coordination polymer in Example 1;

[0024] Figure 2 It is a powder diffraction diagram of the zinc coordination polymer in Example 1;

[0025] Figure 3 It is a thermogravimetric diagram of the zinc coordination polymer in Example 1;

[0026] Figure 4 It is a nuclear magnetic hydrogen spectrum diagram of lactide recovered by the zinc coordination polymer catalytic depolymerization of polylactic acid in Example 10. DETAILED DESCRIPTION

[0027] The application will be described in detail below with reference to the drawings and specific embodiments. The embodiments are implemented on the premise of the technical solutions of the application, and detailed implementation modes and specific operation processes are given, but the protection scope of the application is not limited to the following embodiments.

[0028] Example 1

[0029] A method for preparing a zinc coordination polymer catalyst by a microwave method:

[0030] Zinc chloride (136 mg, 1 mmol), potassium formate (336 mg, 4 mmol) and N,N-dimethylformamide (25 mL) were added into a 100 mL polytetrafluoroethylene-lined reaction kettle, and then the reaction kettle was placed in a microwave reactor, the microwave output power was set to 400 W, the microwave frequency was 2450 MHz, the reaction temperature was 140°C, and the reaction was ended after 30 minutes of reaction, and then filtration and drying were performed to obtain a white powder with a yield of 96.1% (149.2 mg, based on Zn).

[0031] Infrared data (KBr, cm -1 ): 3236m, 1637s, 1617s, 1558m, 1461m, 1418m, 1384m, 1338m, 1188w, 1153m, 1043w, 848w, 799w, 617m, 514m, 447m, see Figure 1 (Instrument model: Nicolet ESP 460)

[0032] Example 2

[0033] A method for preparing a zinc coordination polymer catalyst by a microwave method,

[0034] Zinc chloride (136 mg, 1 mmol), potassium formate (504 mg, 6 mmol) and N,N-dimethylformamide (60 mL) were added into a 100 mL polytetrafluoroethylene-lined reaction kettle, and then the reaction kettle was placed in a microwave reactor, the microwave output power was set to 400 W, the microwave frequency was 2450 MHz, the reaction temperature was 180°C, and the reaction was ended after 30 minutes of reaction, and then filtration and drying were performed to obtain a white powder with a yield of 89.2% (138.3 mg, based on Zn).

[0035] Example 3

[0036] A method for preparing a zinc coordination polymer catalyst by a microwave method,

[0037] Zinc acetate (183 mg, 1 mmol), calcium formate (520 mg, 4 mmol) and N,N- dimethylformamide (25 mL) were added to a 100 mL polytetrafluoroethylene- lined reaction vessel, which was then placed in a microwave reactor, and the microwave output power was set to 200 W, the microwave frequency to 2000 MHz, and the reaction temperature to 100 °C. After 90 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 51.6% (73.2 mg, based on Zn).

[0038] Example 4

[0039] A method of preparing a zinc complex polymer catalyst using a microwave method,

[0040] Zinc nitrate (189 mg, 1 mmol), sodium formate (272 mg, 4 mmol) and N,N- dimethylformamide (25 mL) were added to a 100 mL polytetrafluoroethylene- lined reaction vessel, which was then placed in a microwave reactor, and the microwave output power was set to 800 W, the microwave frequency to 3000 MHz, and the reaction temperature to 140 °C. After 30 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 83.3% (128.9 mg, based on Zn).

[0041] Example 5

[0042] A method of preparing a zinc complex polymer catalyst using a microwave method,

[0043] Zinc chloride (136 mg, 1 mmol), potassium formate (168 mg, 2 mmol) and N,N- dimethylformamide (4 mL) were added to a 100 mL polytetrafluoroethylene- lined reaction vessel, which was then placed in a microwave reactor, and the microwave output power was set to 400 W, the microwave frequency to 2450 MHz, and the reaction temperature to 180 °C. After 5 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 65.5% (101.1 mg, based on Zn).

[0044] Example 6

[0045] A method of preparing a zinc complex polymer catalyst using a microwave method,

[0046] Zinc chloride (136 mg, 1 mmol), magnesium formate (600 mg, 4 mmol) and N- methylformamide (25 mL) were added to a 100 mL polytetrafluoroethylene- lined reaction vessel, which was then placed in a microwave reactor, and the microwave output power was set to 400 W, the microwave frequency to 2450 MHz, and the reaction temperature to 140 °C. After 30 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 74.4% (114.9 mg, based on Zn).

[0047] Example 7

[0048] A method for preparing a zinc complex polymer catalyst using a microwave method,

[0049] Zinc chloride (136 mg, 1 mmol), calcium formate (520 mg, 4 mmol), and methanol (25 mL) were added to a 100 mL polytetrafluoroethylene-lined reaction vessel, and then the reaction vessel was placed in a microwave reactor, and the microwave output power was set to 200 W, the microwave frequency was set to 2450 MHz, and the reaction temperature was set to 140°C. After 30 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 79.6% (122.7 mg, based on Zn).

[0050] Example 8

[0051] A method for preparing a zinc complex polymer catalyst using a microwave method,

[0052] Zinc chloride (136 mg, 1 mmol), calcium formate (520 mg, 4 mmol), and methanol (25 mL) were added to a 100 mL polytetrafluoroethylene-lined reaction vessel, and then the reaction vessel was placed in a microwave reactor, and the microwave output power was set to 200 W, the microwave frequency was set to 2450 MHz, and the reaction temperature was set to 140°C. After 30 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 79.6% (122.7 mg, based on Zn).

[0053] Comparative Example 1: Effect of microwave treatment

[0054] A method for preparing a zinc complex polymer catalyst:

[0055] Zinc chloride (136 mg, 1 mmol), calcium formate (520 mg, 4 mmol), and methanol (25 mL) were added to a 100 mL polytetrafluoroethylene-lined reaction vessel, and then the reaction vessel was placed in a microwave reactor, and the microwave output power was set to 200 W, the microwave frequency was set to 2450 MHz, and the reaction temperature was set to 140°C. After 30 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 79.6% (122.7 mg, based on Zn).

[0056] Comparative Example 2

[0057] A method for preparing a zinc complex polymer catalyst:

[0058] Zinc chloride (136 mg, 1 mmol), calcium formate (520 mg, 4 mmol), and methanol (25 mL) were added to a 100 mL polytetrafluoroethylene-lined reaction vessel, and then the reaction vessel was placed in a microwave reactor, and the microwave output power was set to 200 W, the microwave frequency was set to 2450 MHz, and the reaction temperature was set to 140°C. After 30 minutes of reaction, the reaction was terminated, and the product was dried after filtration to obtain a white powder in a yield of 79.6% (122.7 mg, based on Zn).

[0059] Example 9

[0060] Characterization of the zinc coordination polymer catalyst of Example 1.

[0061] (1) Characterization of phase purity of the zinc coordination polymer catalyst

[0062] The phase purity of the zinc coordination polymer can be characterized by X-ray powder diffractometer. The diffraction pattern of the zinc coordination polymer catalyst prepared by microwave method was determined by Rigaku D / Max-2500 (Japan Rigaku) and compared with the theoretical value (Cu Ka, fixed monochromator, tube voltage 40 kV, tube current 100 mA, scanning speed 8 ° / min). The results showed that the zinc coordination polymer prepared by microwave method had similar diffraction peaks with the theoretical value, indicating that it had reliable phase purity, which provided a guarantee for its application as a catalyst for depolymerization of polylactic acid. Figure 2 : Schematic diagram of powder diffraction of zinc coordination polymer.

[0063] (2) Characterization of thermal stability of the zinc coordination polymer catalyst

[0064] The thermal stability of the zinc coordination polymer can be characterized by thermogravimetric analyzer. The zinc coordination polymer prepared by microwave method was determined by NETZSCH / TG209F3 thermogravimetric analyzer (Germany) (nitrogen atmosphere, temperature range 50-600 ℃, heating rate 10 ℃ / min). The thermogravimetric curve showed that the coordination skeleton of the zinc coordination polymer prepared by microwave method was stable to 225 ℃, then began to collapse and rapidly lost weight to 340 ℃, and then the weight loss became slow. Figure 3 : Thermogravimetric analysis of zinc coordination polymer.

[0065] Example 10

[0066] The performance of the zinc coordination polymer prepared by microwave method in Example 1 for catalyzing depolymerization of polylactic acid to recover lactide monomer was studied.

[0067] Polylactic acid (L-polylactic acid (PLLA)) (4 g) and zinc coordination polymer (0.2 g) were mixed in a ball mill for 20 minutes, and then the mixture was put into a Schlenk reaction tube, the temperature was heated to 190 ℃, and the depolymerization product was collected by distillation under reduced pressure (30 Pa) for 60 minutes, and after drying, lactide monomer 3.96 g was obtained, with a purity of 99.5% and a yield of 99.0%.

[0068] Example 11

[0069] The zinc coordination polymer prepared by microwave method in Example 1 was used to catalyze depolymerization of polylactic acid to recover lactide, compared with Example 10, the only difference was that the reaction temperature was 175 ℃ and the reaction time was 300 minutes; the rest was the same as Example 10.

[0070] The obtained lactide monomer was 2.16 grams, with a purity of 96.3%, and a yield of 54.2%.

[0071] Example 12

[0072] The zinc coordination polymer prepared by the microwave method in Example 1 was used to catalyze the depolymerization of polylactic acid to recycle lactide, and compared with Example 10, the difference was only that the reaction temperature was 215°C, and the reaction time was 10 minutes; the rest was the same as Example 10.

[0073] The obtained lactide monomer was 2.96 grams, with a purity of 98.0%, and a yield of 74.4%.

[0074] Example 13

[0075] The zinc coordination polymer prepared by the microwave method in Example 1 was used to catalyze the depolymerization of polylactic acid to recycle lactide, and compared with Example 10, the difference was only that the amount of zinc coordination polymer was 0.4 grams; the rest was the same as Example 10. The obtained lactide monomer was 3.68 grams, with a purity of 99.1%, and a yield of 92.1%.

[0076] Example 14

[0077] The zinc coordination polymer prepared by the microwave method in Example 1 was used to catalyze the depolymerization of polylactic acid to recycle lactide, and compared with Example 10, the difference was only that the amount of zinc coordination polymer was 0.004 grams, and the vacuum degree was 5 Pa; the rest was the same as Example 10.

[0078] The obtained lactide monomer was 2.56 grams, with a purity of 97.6%, and a yield of 64.1%.

[0079] Example 15

[0080] The zinc coordination polymer prepared by the microwave method in Example 1 was used to catalyze the depolymerization of polylactic acid to recycle lactide, and compared with Example 10, the difference was only that the vacuum degree was 1000 Pa, and the reaction time was 300 minutes; the rest was the same as Example 10.

[0081] The obtained lactide monomer was 3.28 grams, with a purity of 98.4%, and a yield of 82.3%.

[0082] Example 16

[0083] Characterization of the purity of the lactide monomer.

[0084] The purity of lactide monomer was characterized by nuclear magnetic resonance spectrometer. The purity of lactide monomer was determined by AVANCE III 400M nuclear magnetic resonance spectrometer (using tetramethylsilane (TMS, δ = 0 ppm) as internal standard, and deuterated chloroform (CDCl3, δ = 7.26 ppm) as solvent). The nuclear magnetic resonance hydrogen spectrum showed that the recovered lactide monomer recovered by zinc coordination polymer catalytic depolymerization of polylactic acid had high purity. The recovered lactide monomer in Example 10 was determined by nuclear magnetic resonance hydrogen spectrum, and the results are shown in Figure 1. 1 The purity of the lactide monomer was quantitatively analyzed by HNMR and was 99.5%. Figure 4 The nuclear magnetic hydrogen spectrum of the lactide recovered by zinc coordination polymer catalytic depolymerization of polylactic acid.

[0085] Comparative Example 3

[0086] Zinc formate was used to catalyze the depolymerization of polylactic acid to recover lactide, and the catalytic reaction process was the same as that in Example 10.

[0087] The obtained lactide monomer was 0.64 grams, with a purity of 90.5%, and a yield of 16.7%.

[0088] Comparative Example 4

[0089] Zinc acetate was used to catalyze the depolymerization of polylactic acid to recover lactide, and the catalytic reaction process was the same as that in Example 10.

[0090] The obtained lactide monomer was 0.92 grams, with a purity of 92.7%, and a yield of 23.4%.

[0091] Comparative Example 5

[0092] Zinc nitrate was used to catalyze the depolymerization of polylactic acid to recover lactide, and the catalytic reaction process was the same as that in Example 10.

[0093] The obtained lactide monomer was 1.24 grams, with a purity of 95.2%, and a yield of 31.3%.

[0094] Comparative Example 6

[0095] Zinc oxide was used to catalyze the depolymerization of polylactic acid to recover lactide, and the catalytic reaction process was the same as that in Example 10.

[0096] The obtained lactide monomer was 2.56 grams, with a purity of 97.5%, and a yield of 64.5%.

[0097] Comparative Example 7

[0098] Zinc lactate was used to catalyze the depolymerization of polylactic acid to recover lactide, and the catalytic reaction process was the same as that in Example 10.

[0099] The obtained lactide monomer was 1.72 grams, with a purity of 96.0%, and a yield of 43.1%.

[0100] Comparative Example 8

[0101] Zinc chloride was used to catalyze depolymerization of polylactic acid to recover lactide. The catalytic reaction process was the same as Example 10.

[0102] Lactide monomer 1.92 g, purity 96.2%, yield 48.0%.

[0103] The foregoing description of the examples has been set forth to facilitate an understanding of the application for those of ordinary skill in the art, and is not intended to limit the application thereof. Various modifications of these examples as well as other examples could be readily made by those skilled in the art, and would be within the scope of the application, which is set forth only by the claims. Accordingly, the patentable scope of the application is defined only by the claims.

Claims

1. Use of a zinc complex polymerization catalyst, characterized in that: The zinc coordination polymer catalyst is applied to catalyze polylactic acid depolymerization to recover lactide monomers; The preparation method of the zinc coordination polymer catalyst comprises the following steps: mixing a zinc salt, a formate and an organic solvent, and reacting under microwave conditions, and then filtering and separating to obtain the zinc coordination polymer catalyst, wherein the repeated structural unit of the zinc coordination polymer catalyst is [Zn(HCOO)2].

2. Use according to claim 1, characterized in that: The microwave conditions comprise that the microwave output power is 200-800 W and the microwave frequency is 2000-3000 MHz; and the reaction conditions comprise that the reaction temperature is 100-180 DEG C and the reaction time is 5-90 minutes.

3. Use according to claim 1, characterized in that: The zinc salt comprises one or more of zinc chloride, zinc acetate and zinc nitrate; and the formate comprises one or more of potassium formate, calcium formate, sodium formate and magnesium formate.

4. The use according to claim 1, characterized in that: The molar ratio of the zinc salt to the formate is 1:2-6.

5. The use according to claim 1, characterized in that: The organic solvent is one or more of ethanol, methanol, N,N-dimethylformamide and N-methylformamide.

6. The use according to claim 1, characterized in that: The amount of the solvent corresponding to 1 mmol of the zinc salt is 4-60 ml.

7. The use according to claim 1, wherein The zinc coordination polymer and polylactic acid are mixed in a ball mill first, and then the mixture is put into a reaction device, and lactide monomers are collected through vacuum distillation.

8. Use according to claim 7, wherein the compound is ###0002### The mass ratio of polylactic acid to the zinc coordination polymer is 100:0.1-10; and the reaction conditions comprise that the reaction temperature is 175 DEG C-215 DEG C, the reaction time is 10-300 minutes, and the vacuum degree is 5-1000 Pa.

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