Membrane catalytic material for the preparation of polylactic acid by continuous flow open ring polymerization in air atmosphere

Abstract

The application provides a copper porphyrin metal organic framework film catalytic material and application thereof in preparation of polylactic acid. A copper porphyrin metal organic framework nanosheet dispersion liquid is obtained by using a solvothermal method, then a copper porphyrin metal organic framework film with different thicknesses is prepared on a porous substrate by using a reduced pressure suction filtration method, and is sequentially subjected to drying treatment and used as a film reactor to catalyze ring-opening polymerization of different lactide molecules to prepare polylactic acid in an air environment without protection of an inert atmosphere. The copper porphyrin metal organic framework film catalytic material has the following advantages in preparation of polylactic acid: the reaction can be carried out at room temperature without protection of an inert gas; the reaction time is short (< 10 minutes), and the reaction conversion rate can reach 98.5 %; the copper porphyrin metal organic framework film catalytic material is suitable for ring-opening polymerization of lactide with different optical activities to prepare polylactic acid; and the reaction is a continuous flow phase reaction, and separation of the catalyst and the product is not needed.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of membrane catalysis, in particular to a copper porphyrin metal-organic framework membrane catalytic material, a preparation method thereof and application thereof in preparing polylactic acid by continuous flow ring-opening polymerization in an air environment. BACKGROUND

[0002] Polylactic acid refers to a high molecular polyester material prepared by polymerization of lactide or lactic acid as a main raw material, which has good thermal stability, mechanical properties and physical properties. Because it can be completely degraded by microorganisms in nature after use, it has good biodegradability and biocompatibility, and is currently widely used in disposable packaging materials, agricultural fields, biomedical fields and the like.

[0003] At present, the methods for preparing polylactic acid mainly include two categories of direct polymerization of lactic acid polycondensation and ring-opening polymerization of lactide. The direct polymerization is difficult to remove the water generated by lactic acid polycondensation, while the ring-opening polymerization of lactide is carried out in a more atom-economical manner. In recent years, the research on the catalysts for the ring-opening polymerization of lactide mainly focuses on metal catalysts and organic molecular catalysts (Angew. Chem. Int. Ed. 2019, 58, 14679-14685; Chem. Commun., 2011, 47, 3105-3107), but these catalysts almost all need inert atmosphere protection and harsh reaction conditions of high or low temperature, which leads to relatively harsh reaction conditions of the ring-opening polymerization and also faces the problem of catalyst removal. SUMMARY

[0004] In order to improve the defect that the reaction conditions of the existing ring-opening polymerization for preparing polylactic acid are complicated and harsh, the present application provides a copper porphyrin metal-organic framework membrane catalytic material for preparing polylactic acid by ring-opening polymerization, a preparation method thereof and use thereof. The membrane catalytic material of the present application is obtained by preparing a copper porphyrin metal-organic framework nanosheet dispersion liquid by a solvothermal method, and further preparing a membrane catalytic material with one-dimensional confined channels by vacuum filtration. The reactants pass through the membrane catalytic material in a continuous flow phase reaction mode driven by a pressure difference, and a high-efficiency chain growth reaction is carried out in the one-dimensional confined channels. The product flows out with the flow phase and is separated from the system, and finally the rapid ring-opening polymerization for preparing polylactic acid is realized in an air environment at room temperature without inert atmosphere. The reaction conversion rate can be as high as 98.5%, and the molecular weight and configuration of polylactic acid can be controlled.

[0005] The present application provides a preparation method of a copper porphyrin metal-organic framework membrane catalytic material, which comprises the following steps: preparing a copper porphyrin metal-organic framework dispersion liquid, preparing a membrane on a porous substrate by a vacuum filtration method, and drying the obtained membrane to obtain the membrane catalytic material.

[0006] According to an embodiment of the present application, the preparation method comprises the following steps: dispersing copper porphyrin metal organic framework into a solution, and then filtering the solution through a porous substrate to form a film, and then drying the film under negative pressure after the solution is filtered out; and then taking off the film and continuing to dry the film, thereby obtaining the film catalytic material.

[0007] According to an embodiment of the present application, the film catalytic material is used for catalyzing the polymerization reaction of lactide.

[0008] According to an embodiment of the present application, the material of the porous substrate is a polytetrafluoroethylene filter membrane.

[0009] According to an embodiment of the present application, the copper porphyrin metal organic framework dispersion solution is a copper porphyrin metal organic framework nanosheet dispersion solution. According to an embodiment of the present application, the copper porphyrin metal organic framework nanosheet has a sheet diameter of >200 nm. According to an embodiment of the present application, the copper porphyrin metal organic framework nanosheet has a thickness of 2 nm.

[0010] According to an embodiment of the present application, the copper porphyrin metal organic framework is a metal organic framework formed by a porphyrin molecule (such as tetra(4-carboxyphenyl) porphine, tetra(4-carboxyphenyl) porphyrin zinc (II), tetra(4-carboxyphenyl) porphyrin copper (II)) and copper ions.

[0011] According to an embodiment of the present application, the thickness of the film catalytic material is 3 μm-25 μm, such as 4 μm-24 μm, for example, 4 μm, 4.2 μm, 4.5 μm, 5 μm, 5.4 μm, 6 μm, 8 μm, 10 μm, 15 μm, 20 μm, 24 μm. According to an embodiment of the present application, the concentration of the copper porphyrin dispersion solution is 0.02 mg / mL-2 mg / mL, such as 0.05 mg / mL, 0.08 mg / mL, 0.1 mg / mL, 0.2 mg / mL, 0.3 mg / mL, 0.5 mg / mL, 0.8 mg / mL, 1 mg / mL. By adjusting the concentration of the copper porphyrin dispersion solution or the volume of the dispersion solution filtered out, the thickness of the film catalytic material can be adjusted.

[0012] According to an embodiment of the present application, the copper porphyrin metal organic framework and the copper porphyrin metal organic framework dispersion solution can be prepared by a solvothermal method.

[0013] According to an embodiment of the present application, the preparation method of the copper porphyrin metal organic framework dispersion liquid is as follows: dispersing the copper porphyrin metal organic framework in solvent-1, to obtain the copper porphyrin dispersion liquid. According to an embodiment of the present application, the solvent-1 is an organic solvent that can uniformly disperse the copper porphyrin metal organic framework, such as dimethyl sulfoxide. According to an embodiment of the present application, the copper porphyrin metal organic framework can be dispersed in the solvent by ultrasonic treatment; preferably, the power of the ultrasonic treatment is 50W-200W, such as 80W, 100W, 120W, 150W, 180W; preferably, the time of the ultrasonic treatment is 5min-60min, such as 10min, 20min, 30min, 40min, 50min.

[0014] According to an embodiment of the present application, the preparation process of the copper porphyrin metal organic framework is as follows: reacting the tetrakis(4-carboxyphenyl) porphyrin solution with the copper nitrate solution, the surfactant in solvent-2, to obtain the copper porphyrin metal organic framework. According to an embodiment of the present application, the surfactant is selected from benzoic acid or polyvinylpyrrolidone. According to an embodiment of the present application, the tetrakis(4-carboxyphenyl) porphyrin solution is a N,N-dimethylformamide solution of tetrakis(4-carboxyphenyl) porphyrin; and / or, the concentration of the tetrakis(4-carboxyphenyl) porphyrin solution is 0.1mmol / L-2mmol / L, such as 0.5mmol / L. According to an embodiment of the present application, the copper nitrate solution is an aqueous solution of copper nitrate trihydrate; and / or, the concentration of the copper nitrate solution is 5mmol / L-20mmol / L, such as 12.5mmol / L. According to an embodiment of the present application, the solvent-2 is an organic solvent that can dissolve the tetrakis(4-carboxyphenyl) porphyrin solution, the copper nitrate solution, the surfactant, such as N,N-dimethylformamide. According to an embodiment of the present application, the molar ratio of tetrakis(4-carboxyphenyl) porphyrin to copper nitrate is 1-5:10-50, such as 2:25.

[0015] According to an embodiment of the present application, the tetrakis(4-carboxyphenyl) porphyrin solution, the copper nitrate solution, the surfactant, and the solvent-2 are stirred and mixed uniformly, heated for reaction, cooled down after the reaction, and centrifuged to obtain the precipitate, which is the copper porphyrin metal organic framework. According to an embodiment of the present application, the heating reaction temperature is the reflux temperature, such as 90℃; and / or, the heating reaction time is 2h-8h, such as 5h. According to an embodiment of the present application, the centrifugation parameter condition is: the centrifugal speed is 10000rpm, and the centrifugation time is 20 minutes. According to an embodiment of the present application, after the centrifugation to obtain the precipitate, an alcoholic solvent (such as ethanol) is optionally further added to the precipitate, mixed uniformly, and then centrifuged to obtain the precipitate after removing the impurities; preferably, the centrifugation parameter condition is: the centrifugal speed is 8000rpm, and the centrifugation time is 20 minutes; preferably, after obtaining the precipitate after removing the impurities, the alcoholic solvent is removed, and the copper porphyrin metal organic framework can be obtained after drying.

[0016] According to an embodiment of the present application, the vacuum degree of the negative pressure state is 0.3 atm-1.0 atm, for example, 0.5 atm, 0.6 atm, 0.8 atm; and / or, the drying time under the negative pressure state is 5 h-20 h, for example, 12 h.

[0017] According to an embodiment of the present application, the continuous drying time after removing the film is 10 h-50 h, for example, 24 h; and / or, the continuous drying temperature is 60℃-90℃, for example, 80℃.

[0018] According to an embodiment of the present application, the loading amount of copper porphyrin in the film catalytic material is 0.5 mg-5 mg, preferably 1 mg-3 mg, for example, 1.2 mg, 1.4 mg, 1.5 mg, 1.8 mg, 2.0 mg, 2.5 mg.

[0019] The present application also provides a copper porphyrin metal organic framework film catalytic material prepared by the above preparation method.

[0020] According to an embodiment of the present application, the copper porphyrin metal organic framework is a metal organic framework formed by a porphyrin molecule (for example, tetra(4-carboxyphenyl)porphine, tetra(4-carboxyphenyl)porphyrin zinc(II), tetra(4-carboxyphenyl)porphyrin copper(II)) and copper ions. According to an embodiment of the present application, the thickness of the film catalytic material is 3 μm-25 μm, for example, 4 μm-24 μm, for example, 4 μm, 4.2 μm, 4.5 μm, 5 μm, 5.4 μm, 6 μm, 8 μm, 10 μm, 15 μm, 20 μm, 24 μm. The thickness of the film catalytic material can be adjusted by adjusting the concentration of the copper porphyrin dispersion or the volume of the dispersion filtrate.

[0021] The present application also provides an application of the above film catalytic material in preparing polylactic acid.

[0022] The present application also provides a method for preparing polylactic acid, which comprises performing a polymerization reaction in the presence of the above film catalytic material.

[0023] According to an embodiment of the present application, the method is specifically: using lactide as a reaction raw material, performing a polymerization reaction to prepare polylactic acid; preferably, the method is specifically: using lactide as a reaction raw material, performing a polymerization reaction through a film catalytic material in the presence of an initiator and a cocatalyst to prepare polylactic acid. According to an embodiment of the present application, the method is specifically: using a lactide solution and a mixed solution containing an initiator and a cocatalyst as a reaction liquid, pushing the reaction liquid through the above film catalytic material under the action of a pressure difference, and performing a polymerization reaction to prepare polylactic acid.

[0024] According to embodiments of the present application, the lactide is selected from any one of L-lactide, D-lactide, meso-lactide, or a mixture thereof; for example, the lactide is selected from meso-lactide, rac-lactide. According to embodiments of the present application, the lactide solution is a dimethyl sulfoxide solution of lactide; and / or, the concentration of the lactide solution is 0.05 g / mL - 0.8 g / mL, for example, 0.1 g / mL, 0.2 g / mL, 0.23 g / mL, 0.3 g / mL, 0.5 g / mL.

[0025] According to embodiments of the present application, the co-catalyst is an organic base catalyst; preferably, the organic base catalyst is selected from guanidine catalysts. According to embodiments of the present application, the guanidine catalyst is selected from 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). According to embodiments of the present application, the initiator is a monohydric alcohol; for example, selected from one, two or more of isopropyl alcohol, benzyl alcohol, 2-methoxyethanol, 1-naphthalenemethanol. According to embodiments of the present application, the mixed solution comprising the initiator and the co-catalyst is a dimethyl sulfoxide solution comprising the initiator and the co-catalyst.

[0026] According to embodiments of the present application, the amount of the co-catalyst is 0.1% - 5% of the mass of the lactide, for example, 0.2%, 0.5%, 0.8%, 1%, 2%, 3%, 4%. According to embodiments of the present application, the volume to mass ratio of the initiator and the co-catalyst is 0.1 - 20 μL: 10 - 50 mg, preferably, 1 - 20 μL: 20 - 50 mg, for example, (2 μL or 2.5 μL or 3 μL or 5 μL or 8 μL or 10 μL or 12.5 μL or 15 μL or 18 μL): (15 mg or 20 mg or 25 mg or 30 mg or 33.4 mg or 35 mg or 40 mg or 45 mg).

[0027] According to an embodiment of the present application, the pressure difference is 0.1 atm-0.9 atm, for example 0.3 atm, 0.5 atm, 0.75 atm, 0.9 atm. According to an embodiment of the present application, the reaction solution is subjected to polymerization reaction through the one-dimensional confined channel of the above-mentioned membrane catalytic material. According to an embodiment of the present application, the above-mentioned membrane catalytic material is placed in the reduced pressure suction filtration device, in particular, placed in the middle of the filter cup and filter core of the reduced pressure suction filtration device. According to an embodiment of the present application, the product containing polylactic acid flows out with the solvent; preferably, the solvent is the solvent in the lactide solution; preferably, the solvent is the solvent in the mixed solution containing initiator and co-catalyst; preferably, the solvent is dimethyl sulfoxide. According to an embodiment of the present application, the average pore size of the one-dimensional confined channel is 0.9 nm. According to an embodiment of the present application, the reaction solution is added to the filter cup of the reduced pressure suction filtration device containing the above-mentioned membrane catalytic material, and under the driving of the pressure difference, the reaction solution is subjected to polymerization reaction through the one-dimensional confined channel of the above-mentioned membrane catalytic material, and the product flows out with the solvent.

[0028] According to an embodiment of the present application, the method or reaction is carried out at room temperature (for example 22°C). According to an embodiment of the present application, the method or reaction does not require inert atmosphere protection (the inert gas is one of nitrogen and argon). According to an embodiment of the present application, the method or reaction is carried out in an air environment. According to an embodiment of the present application, the method or reaction is carried out in a continuous flow phase reaction. According to an embodiment of the present application, the reaction time of the method is less than 10 min.

[0029] According to an embodiment of the present application, the number average molecular weight of the polylactic acid is 8000 g / mol-20000 g / mol, for example 9000 g / mol, 9861 g / mol, 10000 g / mol, 10799 g / mol, 11000 g / mol, 11968 g / mol, 11983 g / mol, 12000 g / mol, 13000 g / mol, 14000 g / mol, 15000 g / mol, 16000 g / mol, 17000 g / mol, 17163 g / mol;

[0030] and / or, the weight average molecular weight of the polylactic acid is 10000 g / mol-35000 g / mol, for example 10000 g / mol, 11000 g / mol, 11930 g / mol, 12000 g / mol, 13000 g / mol, 13174 g / mol, 14000 g / mol, 14492 g / mol, 14720 g / mol, 15000 g / mol, 18000 g / mol, 20000 g / mol, 25000 g / mol, 30000 g / mol, 31724 g / mol, 35000 g / mol;

[0031] and / or, the molecular weight distribution coefficient of the polylactic acid is 1.10-1.90, for example, 1.15, 1.20, 1.21, 1.22, 1.23, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.84, 1.90.

[0032] Advantages

[0033] The application provides a copper porphyrin metal organic framework film catalytic material and application thereof in preparation of polylactic acid. A copper porphyrin metal organic framework nanosheet dispersion liquid is obtained by a solvothermal method, then a copper porphyrin metal organic framework film with different thicknesses is prepared on a porous substrate by a reduced pressure suction filtration method, and is sequentially subjected to drying treatment and used as a film reactor to catalyze ring-opening polymerization of different lactide molecules to prepare polylactic acid in an air environment without inert gas protection.

[0034] The copper porphyrin metal organic framework film catalytic material provided by the application has the following advantages in preparation of polylactic acid:

[0035] 1. The reaction can be carried out at room temperature without inert gas protection;

[0036] 2. The reaction time is short (<10 minutes), and the reaction conversion rate can reach 98.5%;

[0037] 3. The reaction is suitable for ring-opening polymerization of lactide with different optical activities to generate polylactic acid;

[0038] 4. The reaction is a continuous flow phase reaction, and separation of the catalyst and the product is not needed. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a schematic diagram of a film catalytic process, reaction liquid penetrates through the copper porphyrin metal organic framework film under the driving of a pressure difference, and the product flows out of the film system with the solvent. The reaction conversion rate is calculated by quantitative analysis of the liquid collected on the film by nuclear magnetic resonance hydrogen spectrum.

[0040] Figure 2 is a description of Example 5, wherein 2a is a scanning electron microscope image of the cross section of the copper porphyrin metal organic framework film, 2b is an infrared spectrum diagram, and 2c is an X-ray diffraction diagram of the copper porphyrin metal organic framework film, indicating that the interlayer spacing of the film is 2d is a continuous flow polymerization reaction flux diagram of the film, 2e is a nuclear magnetic resonance hydrogen spectrum diagram, and 2f is a gel permeation chromatogram of the product polylactic acid. DETAILED DESCRIPTION

[0041] The technical solutions of the present application will be further described in detail below in combination with specific examples. It should be understood that the following examples are only illustratively described and explained, and should not be interpreted as limiting the scope of protection of the present application. Any technology achieved based on the above description of the present application is covered within the scope intended to be protected by the present application.

[0042] Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0043] Example 1

[0044] (1) Take 40 mL of N,N-dimethylformamide solution of tetra(4-carboxyphenyl) porphyrin (0.5 mmol / L) in a beaker, and sequentially add 20 mL of copper nitrate trihydrate aqueous solution (12.5 mmol / L), 4.5 g of benzoic acid and 200 mL of N,N-dimethylformamide, stir for 10 minutes at room temperature to mix uniformly, then heat to 90°C, and keep at 90°C for 5 hours. After the reaction is completed and the solution is cooled to room temperature, collect the precipitate by centrifugation (centrifugal speed is 10000 rpm, centrifugal time is 20 minutes). Add 150 mL of ethanol to the precipitate to remove impurities, mix and then centrifuge (centrifugal speed is 8000 rpm, centrifugal time is 20 minutes) to collect the precipitate. Repeat the operation three times, and then place the obtained precipitate in a vacuum drying dish to remove ethanol. After vacuum drying for 12 hours, take out the precipitate, add 225 mL of dimethyl sulfoxide, and ultrasonically treat for 30 minutes at 120 W ultrasonic power to obtain a copper porphyrin metal organic framework nanosheet dispersion (dispersion concentration is about 0.2 mg / mL), and mark the dispersion as Cu-MOF. According to the atomic force microscope image of the nanosheet, the thickness of the copper porphyrin metal organic framework nanosheet is 2 nm, and the flake diameter is >200 nm.

[0045] (2) Take 7 mL of Cu-MOF dispersion, and prepare a film on a polytetrafluoroethylene substrate by a reduced pressure suction filtration method. After the dispersion in the filter cup above the device is suctioned dry, continue to dry under negative pressure (vacuum degree is 0.6 atm) for 12 hours, and then take out the film and transfer to an 80°C oven for drying for 24 hours to obtain a film catalytic material, which is marked as Cu-MOF-7. The thickness of Cu-MOF-7 is about 4.2 μm as characterized by a scanning electron microscope, and the average pore size of the one-dimensional confined channel perpendicular to the substrate is about 0.9 nm as characterized by a nitrogen adsorption-desorption test.

[0046] (3) The Cu-MOF-7 is used as a membrane catalytic material to catalyze ring-opening polymerization of meso-lactide to prepare polylactic acid. The specific operation is as follows: 1.152 g of meso-lactide is dissolved in 5 mL of dimethyl sulfoxide; 12.5 μL of benzyl alcohol and 33.4 mg of 1,5,7-triazabicyclo[4.4.0]dec-5-ene are dissolved in 1 mL of dimethyl sulfoxide; 3 mL of the dimethyl sulfoxide solution of meso-lactide is taken and added to 0.2 mL of the dimethyl sulfoxide mixed solution of benzyl alcohol and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a reaction liquid; then, the reaction liquid is added to a filter cup of a reduced-pressure suction filtration device into which the Cu-MOF-7 membrane is previously placed, and the reaction liquid is pushed by a pressure difference (0.75 atm) to pass through the one-dimensional confined channel of the Cu-MOF-7 membrane to perform a polymerization reaction, and the product flows out with dimethyl sulfoxide. The reaction time is 4.6 minutes. After the dissolved product is collected with deuterated chloroform, the composition is analyzed by nuclear magnetic resonance hydrogen spectrum, and the conversion rate is calculated. After the product is purified with tetrahydrofuran (purified with methanol), the molecular weight and the molecular weight distribution of the product are analyzed by gel permeation chromatography.

[0047] The reaction result is that the conversion rate is 89.8% at room temperature without inert atmosphere protection, the number average molecular weight of the polymer is 10799 g / mol, the weight average molecular weight of the polymer is 13174 g / mol, and the molecular weight distribution coefficient is 1.22.

[0048] Example 2

[0049] The other operations are the same as in Example 1, except that:

[0050] 9 mL of the Cu-MOF dispersion liquid is taken to prepare a membrane on a polytetrafluoroethylene substrate by a reduced-pressure suction filtration method. After the dispersion liquid in the filter cup above the device is suctioned dry, the membrane is continuously dried under a negative pressure (vacuum degree 0.6 atm) for 12 hours, and then taken out and transferred to an 80°C oven for drying for 24 hours to obtain a membrane catalytic material, which is marked as Cu-MOF-9. The thickness of the Cu-MOF-7 is about 5.4 μm, which is characterized by a scanning electron microscope, and the average pore size of the one-dimensional confined channel perpendicular to the substrate is about 0.9 nm, which is characterized by nitrogen adsorption and desorption test.

[0051] The Cu-MOF-9 is used as a membrane catalytic material to catalyze ring-opening polymerization of meso-lactide to prepare polylactic acid. The reaction result is that the conversion rate is 98.5% at room temperature without inert atmosphere protection, the number average molecular weight of the polymer is 11968 g / mol, the weight average molecular weight of the polymer is 14720 g / mol, and the molecular weight distribution coefficient is 1.23.

[0052] Comparative Example 1

[0053] Take 40 ml of tetrakis (4-carboxyphenyl) porphyrin N, N-dimethylformamide solution (0.5 mmol / L) in a beaker, add 20 ml of copper nitrate trihydrate aqueous solution (12.5 mmol / L) in turn, 4.5 g of benzoic acid and 200 ml of N, N-dimethylformamide, stir for 10 minutes at room temperature to make them mix evenly, then heat to 90°C, keep at 90°C for 5 hours. After the reaction is completed and the solution is cooled to room temperature, collect the precipitate by centrifugation (centrifugal speed is 10000 rmp, centrifugal time is 20 minutes). Add 150 ml of ethanol to the precipitate to remove impurities, mix well and centrifuge (centrifugal speed is 8000 rmp, centrifugal time is 20 minutes) to collect the precipitate. Repeat the operation three times, then place the obtained precipitate in a vacuum drying dish to remove ethanol. After vacuum drying for 12 hours, take out the precipitate, add 225 ml of dimethyl sulfoxide and ultrasonic treat for 30 minutes at 120 W ultrasonic power to obtain a copper porphyrin metal organic framework nanosheet dispersion (dispersion concentration is about 0.2 mg / mL), mark the dispersion as Cu-MOF.

[0054] Take 0.4 mg of Cu-MOF powder as a catalytic material to catalyze the ring-opening polymerization of meso-lactide to prepare polylactic acid, the specific operation is as follows: dissolve 1.152 g of meso-lactide in 5 mL of dimethyl sulfoxide; dissolve 12.5 μL of benzyl alcohol and 33.4 mg of 1,5,7-triazabicyclo[4.4.0]dec-5-ene in 1 mL of dimethyl sulfoxide; take 3 mL of dimethyl sulfoxide solution of meso-lactide, add 0.2 mL of dimethyl sulfoxide mixed solution of benzyl alcohol and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as the reaction solution; add 2 mL of Cu-MOF dispersion to the reaction solution, stir for 24 hours at room temperature without inert atmosphere protection to carry out the polymerization reaction. After the reaction is completed, take 100 μL of sample, add deuterated chloroform, analyze its composition by nuclear magnetic resonance hydrogen spectrum, and calculate the conversion rate. Take 300 μL of sample, purify it with methanol, add tetrahydrofuran to make it completely dissolved, and analyze its molecular weight and molecular weight distribution by gel permeation chromatography.

[0055] The reaction result is: the conversion rate is 65% at room temperature without inert atmosphere protection, the nuclear magnetic resonance hydrogen spectrum shows that the content of isotactic polylactic acid increases compared with the membrane-limited reaction (example 1), the number average molecular weight of the polymer is 9562 g / mol, the weight average molecular weight of the polymer is 16351 g / mol, and the molecular weight distribution coefficient is 1.71.

[0056] Example 3

[0057] Other operations are the same as in example 1, the only difference is:

[0058] The above Cu-MOF-7 is used as a membrane catalytic material to catalyze ring-opening polymerization of rac-lactide to prepare polylactic acid, and the specific steps are as follows: 0.576 g of L-lactide and 0.576 g of D-lactide are dissolved in 5 mL of dimethyl sulfoxide, and the dimethyl sulfoxide solution is marked as rac-lactide dimethyl sulfoxide solution; 6.5 μL of benzyl alcohol and 33.4 mg of 1,5,7-triazabicyclo[4.4.0]dec-5-ene are dissolved in 1 mL of dimethyl sulfoxide; 3 mL of the rac-lactide dimethyl sulfoxide solution is added to 0.2 mL of the mixed dimethyl sulfoxide solution of benzyl alcohol and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a reaction solution; then, the reaction solution is added to a filter cup of a reduced-pressure suction filtration device into which the Cu-MOF-7 membrane is previously placed, and under the push of a pressure difference (0.75 atm), the reaction solution passes through the one-dimensional confined channel of the Cu-MOF-7 membrane to perform polymerization reaction, and the product flows out with dimethyl sulfoxide, and the reaction time is 5 minutes. After the dissolved product is collected with deuterated chloroform, the composition is analyzed by nuclear magnetic resonance hydrogen spectrum, and the conversion rate is calculated. After the product is dissolved and purified with tetrahydrofuran, the molecular weight and the molecular weight distribution are analyzed by gel permeation chromatography.

[0059] The reaction results are as follows: under the condition of no inert atmosphere protection at room temperature, the conversion rate is 70%, the number average molecular weight of the polymer is 9861 g / mol, the weight average molecular weight of the polymer is 11930 g / mol, and the molecular weight distribution coefficient is 1.21.

[0060] Example 4

[0061] Other operations are the same as in Example 2, except that:

[0062] The above Cu-MOF-9 is used as a membrane catalytic material to catalyze ring-opening polymerization of rac-lactide to prepare polylactic acid, and the specific steps are as follows: 0.576 g of L-lactide and 0.576 g of D-lactide are dissolved in 5 mL of dimethyl sulfoxide, and the dimethyl sulfoxide solution is marked as rac-lactide dimethyl sulfoxide solution; 6.5 μL of benzyl alcohol and 33.4 mg of 1,5,7-triazabicyclo[4.4.0]dec-5-ene are dissolved in 1 mL of dimethyl sulfoxide; 3 mL of the rac-lactide dimethyl sulfoxide solution is added to 0.2 mL of the mixed dimethyl sulfoxide solution of benzyl alcohol and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a reaction solution; then, the reaction solution is added to a filter cup of a reduced-pressure suction filtration device into which the Cu-MOF-9 membrane is previously placed, and under the push of a pressure difference (0.75 atm), the reaction solution passes through the one-dimensional confined channel of the Cu-MOF-9 membrane to perform polymerization reaction, and the product flows out with dimethyl sulfoxide, and the reaction time is 4.7 minutes. After the dissolved product is collected with deuterated chloroform, the composition is analyzed by nuclear magnetic resonance hydrogen spectrum, and the conversion rate is calculated. After the product is dissolved and purified with tetrahydrofuran, the molecular weight and the molecular weight distribution are analyzed by gel permeation chromatography.

[0063] The reaction result is: the conversion rate is 85% under the condition of no inert atmosphere protection at room temperature, the number average molecular weight of the polymer is 11983 g / mol, the weight average molecular weight of the polymer is 14492 g / mol, and the molecular weight distribution coefficient is 1.21.

[0064] Comparative Example 2

[0065] The other operations are the same as those in Comparative Example 1, except that:

[0066] 0.4 mg of Cu-MOF powder was taken as the catalytic material to catalyze the ring-opening polymerization of rac-lactide to prepare polylactic acid. The specific steps are as follows: 0.576 g of L-lactide and 0.576 g of D-lactide were dissolved in 5 mL of dimethyl sulfoxide, marked as a dimethyl sulfoxide solution of rac-lactide; 6.5 μL of benzyl alcohol and 33.4 mg of 1,5,7-triazabicyclo[4.4.0]dec-5-ene were dissolved in 1 mL of dimethyl sulfoxide; 3 mL of the dimethyl sulfoxide solution of rac-lactide was taken and added to 0.2 mL of the dimethyl sulfoxide mixed solution of benzyl alcohol and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as the reaction liquid; 0.4 mg of Cu-MOF powder was added to the reaction liquid, and the polymerization reaction was carried out under stirring for 24 hours at room temperature without inert atmosphere protection. After the reaction was completed, 100 μL of the sample was added to deuterated chloroform, and its composition was analyzed by nuclear magnetic resonance hydrogen spectrum, and the conversion rate was calculated. 300 μL of the sample was purified with methanol, then tetrahydrofuran was added to completely dissolve it, and its molecular weight and molecular weight distribution were analyzed by gel permeation chromatography.

[0067] The reaction result is: the conversion rate is 46.5% under the condition of no atmosphere protection at room temperature, the content of isotactic polylactic acid increases compared with the membrane-limited reaction, the number average molecular weight of the polymer is 4766 g / mol, the weight average molecular weight of the polymer is 5863 g / mol, and the molecular weight distribution coefficient is 1.24.

[0068] Example 5

[0069] The other operations are the same as those in Example 2, except that:

[0070] The above Cu-MOF-9 is used as a film catalytic material to catalyze ring-opening polymerization of meso-lactide to prepare polylactic acid. The specific steps are as follows: 1.152 g of meso-lactide is dissolved in 5 mL of dimethyl sulfoxide. 2.5 μL of benzyl alcohol and 33.4 mg of 1,5,7-triazabicyclo[4.4.0]dec-5-ene are dissolved in 1 mL of dimethyl sulfoxide. 3 mL of the dimethyl sulfoxide solution of meso-lactide is taken and added to 0.2 mL of the dimethyl sulfoxide mixed solution of benzyl alcohol and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a reaction liquid. Subsequently, the reaction liquid is added to a filter cup of a reduced pressure suction filtration device into which Cu-MOF-7 film is previously placed. Under the push of a pressure difference (0.75 atm), the reaction liquid passes through the one-dimensional limited channel of the Cu-MOF-7 film to perform polymerization reaction, and the product flows out with dimethyl sulfoxide. The reaction time is 4.6 minutes. After the dissolved product is collected with deuterated chloroform, its composition is analyzed by nuclear magnetic resonance hydrogen spectrum, and the conversion rate is calculated. After the product is purified with tetrahydrofuran, its molecular weight and molecular weight distribution are analyzed by gel permeation chromatography.

[0071] The reaction result is that the conversion rate is 93.5% and the number average molecular weight of the polymer is 17163 g / mol, the weight average molecular weight of the polymer is 31724 g / mol, and the molecular weight distribution coefficient is 1.84 under the condition of no inert atmosphere protection at room temperature.

[0072] The above describes the embodiments of the present application. However, the present application is not limited to the above embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing polylactic acid, the method comprising: feeding a solution of lactide and a mixed solution containing an initiator and a cocatalyst as a reaction liquid, and feeding the reaction liquid through a membrane catalytic material under the driving of a pressure difference to perform a polymerization reaction, thereby preparing polylactic acid; the method for preparing the membrane catalytic material comprising the steps of: preparing a copper porphyrin metal organic framework dispersion liquid, and preparing a membrane on a porous substrate by a reduced pressure suction filtration method, and drying the obtained membrane, thereby obtaining the membrane catalytic material; the cocatalyst is an organic base catalyst; the organic base catalyst is selected from guanidine catalysts; the guanidine catalyst is selected from 1,5,7-triazabicyclo[4.4.0]dec-5-ene, 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, and 1,8-diazabicyclo[5,4,0]undec-7; the amount of the cocatalyst is 0.1%-5% of the mass of lactide; the volume-mass ratio of the initiator to the cocatalyst is 0.1 μL-20 μL:10 mg-50 mg; the reaction liquid is fed through a one-dimensional confined channel of the membrane catalytic material to perform the polymerization reaction; the method is performed at room temperature; the method is performed in an air environment; the reaction time of the method is less than 10 min.

2. The method of claim 1, wherein, the method for preparing the membrane catalytic material comprising the steps of: preparing a copper porphyrin metal organic framework dispersion liquid, and preparing a membrane on a porous substrate by a reduced pressure suction filtration method, and drying the obtained membrane, thereby obtaining the membrane catalytic material.

3. The method according to claim 1 or 2, characterized in that, the material of the porous substrate is a polytetrafluoroethylene filter membrane; and / or, the copper porphyrin metal organic framework dispersion liquid is a copper porphyrin metal organic framework nanosheet dispersion liquid; and / or, the copper porphyrin metal organic framework is a metal organic framework formed by a porphyrin molecule and a copper ion; and / or, the thickness of the membrane catalytic material is 3 μm-25 μm; and / or, the concentration of the copper porphyrin dispersion liquid is 0.02 mg / mL-2 mg / mL; and / or, the copper porphyrin metal organic framework and the copper porphyrin metal organic framework dispersion liquid are prepared by a solvothermal method.

4. The method of claim 3, wherein, the flake diameter of the copper porphyrin metal organic framework nanosheet is >200 nm; and / or, the thickness of the copper porphyrin metal organic framework nanosheet is 2 nm.

5. The method of claim 2, wherein, the vacuum degree of the negative pressure state is 0.3 atm-1.0 atm; and / or, the drying time under the negative pressure state is 5 h-20 h; and / or, the time for continuing to dry after the membrane is removed is 10 h-50 h; and / or, the temperature for continuing to dry is 60 ℃-90 ℃; and / or, the loading amount of copper porphyrin in the membrane catalytic material is 0.5 mg-5 mg.

6. The method of claim 1 or 2, wherein, the lactide is selected from any one of L-lactide, D-lactide, meso-lactide, or a mixture thereof; and / or, the initiator is selected from one, two or more of isopropyl alcohol, benzyl alcohol, 2-methoxyethanol, and 1-naphthalenemethanol; and / or, the pressure difference is 0.1 atm-0.9 atm.

7. The method according to claim 1 or 2, characterized in that, the reaction liquid is fed into a filter cup of a reduced pressure suction filtration device containing the membrane catalytic material, and the reaction liquid is fed through a one-dimensional confined channel of the membrane catalytic material under the driving of a pressure difference to perform a polymerization reaction, and the product flows out with the solvent.

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