Ester-crosslinked phenolphthalein-based polyimide film material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of membrane separation, in particular to an ester group cross-linked phenolphthalein-based polyimide membrane material as well as a preparation method and application thereof. The invention provides an ester group cross-linked phenolphthalein-based polyimide film material. The structural formula of the ester group cross-linked phenolphthalein-based polyimide film material is shown as a formula I, the phenolphthalein diamine is selected, phenolphthalein contains a large number of benzene rings, so that the polymer has excellent thermal stability, meanwhile, the-COOH group can be subjected to ester group crosslinking, the polyimide film has good mechanical performance, meanwhile, the gas permeability and selectivity of the polyimide film can be further improved, and the gas separation performance and the plasticizing resistance of the polyimide film are improved. The ester group cross-linked phenolphthalein-based polyimide membrane material can be used for gas separation, especially for separation of CO2 and methane in natural gas purification. # imgabs0 #

Description

Technical Field

[0001] The present application relates to the field of membrane separation technology, and in particular to an ester-crosslinked phenolphthalein-based polyimide membrane material and a preparation method and application thereof. Background Art

[0002] Removing high concentrations of CO2 from natural gas remains a major challenge for gas separation technology. As a green separation technology, gas separation membranes have the advantages of low cost, low energy consumption, simple operation, high selectivity, high efficiency and easy scale-up production. Among them, polyimide gas separation membranes have high gas selectivity, excellent thermal stability, good mechanical properties and good film-forming properties, and are considered to be a very promising gas separation material. However, traditional polyimide membranes have low gas permeability and are prone to plasticization when separating gases with high concentrations of soluble gases (ethane, ethylene, propane, propylene, CO2 and CH4, etc.). Plasticization refers to a phenomenon in which the mobility and free volume of polymer segments increase with the increase of feed side pressure, resulting in an increase in gas separation membrane permeability and a sharp decrease in selectivity.

[0003] The purpose of the present application is to improve the technical problems of low gas permeability, low gas selectivity and easy plasticization in the prior art, so as to improve the separation performance of the polyimide gas separation membrane. Summary of the invention

[0004] In view of the problems existing in the prior art, the present application provides an ester-crosslinked phenolphthalein-based polyimide membrane material and its preparation method and application. The present application selects phenolphthalein-based diamine, and the phenolphthalein group contains a large number of benzene rings, so that the polymer has excellent thermal stability. At the same time, it contains -COOH groups that can undergo ester-based crosslinking, so that the polyimide membrane has good mechanical properties, and at the same time, it can further improve its gas permeability and selectivity, and improve gas separation performance and anti-plasticization ability. The ester-crosslinked phenolphthalein-based polyimide membrane material described in the present application can be used for gas separation, separation of CO2 and methane in natural gas purification, separation of oxygen and nitrogen in air, separation of CO2 and nitrogen, separation of helium and N2, separation of helium and methane, separation of helium and neon, separation of hydrogen and methane, separation of ethane and ethylene, separation of propane and propylene; especially separation of CO2 and methane in natural gas purification.

[0005] In a first aspect, the present application provides an ester-crosslinked phenolphthalein-based polyimide membrane material, the structural formula of which is shown in Formula I:

[0006]

[0007] Wherein, R1, R2, R3, R4, R5, R6 are each independently H, C1-C4 alkyl; R7 is C1-C4 alkyl; R8 is each independently C1-C4 alkyl, s is 0, 1, 2, 3 or 4; R9 is C1-C6 alkylene; m, n represent the number of repeating units, each independently 10-800;

[0008] Each The same or different groups independently represent the groups formed after the dianhydride monomer reacts with the amino group.

[0009] In one embodiment, the dianhydride monomer is selected from pyromellitic anhydride optionally substituted with a C1-4 alkyl group or Any one, combination of two or more, R 10 Selected from direct bond, -C(CF3)2-, -CH2-, -O-, -CO-.

[0010] In one set of embodiments, the dianhydride monomer is selected from any one, two or more combinations of the following structures optionally substituted with a C1-C4 alkyl group:

[0011] PMDA, pyromellitic anhydride

[0012] MPDA, trimellitic anhydride

[0013] BPDA, biphenyltetracarboxylic dianhydride

[0014] 6FDA, hexafluoroisopropylphthalic anhydride

[0015] BTDA, Benzophenone tetracarboxylic dianhydride

[0016] ODPA, oxydiphthalic anhydride.

[0017] In one group of embodiments, each of Formula I represent the same group.

[0018] In one set of embodiments, m and n are each independently 20-500.

[0019] In one set of embodiments, s is preferably 0.

[0020] In one group of embodiments, R1, R2, R3, R4, R5, and R6 are each independently H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl. R7 is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl. R8 is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl. R9 is selected from ethylene, n-propylene, isopropylene, n-butyl, isobutyl, tert-butyl, n-pentylene, and n-hexylene.

[0021] In one group of embodiments, R1, R2, R3, R4, R5, and R6 are each independently H, methyl, and ethyl; R7 is selected from methyl, ethyl, n-propyl, and n-butyl; R8 is selected from methyl and ethyl; and R9 is selected from ethylene, n-propylene, n-butylene, n-pentylene, and n-hexylene.

[0022] In one set of embodiments, s is preferably 0.

[0023] In one embodiment, the ester-crosslinked phenolphthalein-based polyimide membrane material is selected from one or more of the following:

[0024] (i) R1-R6 are all H;

[0025] (ii) R1=-CH3, R4=-CH3, R2, R3, R5, R6 are all H;

[0026] (iii) R7=-CH3;

[0027] (iv) R7=-CH2CH3;

[0028] (v) R7=-CH2CH2CH3;

[0029] (vi) R7=-CH2CH2CH2CH3;

[0030] (vii) R9=-CH2CH2-;

[0031] (viii) R9=-CH2CH2CH2-;

[0032] (ix) R9=-CH2CH2CH2CH2-;

[0033] (x) R9=-CH2CH2CH2CH2CH2CH2-;

[0034] (xi)R9=-CH2CH2CH2CH2CH2CH2-.

[0035] In one embodiment, the ester-crosslinked phenolphthalein-based polyimide membrane material, Formula I is selected from any one of the following:

[0036]

[0037]

[0038] In a second aspect, the present application provides a method for preparing an ester-crosslinked phenolphthalein-based polyimide film material according to the first aspect, comprising the following steps: (1) nitrating a phenolphthalein monomer to obtain a dinitrate monomer; (2) ring-opening reduction of the dinitrate monomer to obtain a diamine monomer; (3) polycondensing and imidizing the diamine monomer and the dianhydride monomer to obtain a polyimide; (4) using the polyimide to form a film; and (5) using an ester-based crosslinking agent, diol OH-R9-OH, to perform an ester-based crosslinking treatment, wherein the reaction route is as follows:

[0039]

[0040] In one group of embodiments, the preparation method comprises the following steps: (1) after the phenolphthalein monomer and the nitrating agent react in a solvent, cold water is added to precipitate, washed, and dried to obtain a dinitrate monomer; (2) the dinitrate monomer is dissolved in a solvent, a catalyst is added, nitrogen is introduced to exclude air, H2 is introduced to react, the catalyst is filtered out after the reaction, the mixture is cooled and crystallized, washed, and dried to obtain a diamine monomer; (3) the diamine monomer is dissolved in a solvent, a dianhydride monomer is added under nitrogen protection to carry out a condensation reaction, and then an acid anhydride and an alkaline catalyst are added to carry out an imidization reaction. After the reaction is completed, the mixture is poured into an alcohol solvent to precipitate, filtered, washed, and dried to obtain a polyimide; (4) the polyimide is dissolved in a solvent to obtain a casting solution, filtered, formed into a film on a plane, and dried to obtain a polyimide film; (5) the polyimide film is placed in a mixed solution of an ester-based crosslinking agent diol and an alcohol solvent, soaked, dried to remove the alcohol solvent, heated for crosslinking, and then dried to remove excess ester-based crosslinking agent to obtain an ester-based crosslinked phenolphthalein-based polyimide film.

[0041] In one group of embodiments, in the step (1), the nitrating agent is selected from yttrium nitrate, sodium nitrate, preferably yttrium nitrate hexahydrate. The solvent for the nitrating reaction is acetic acid or sulfuric acid. The amount of the nitrating agent is 0.5-5 equivalents of phenolphthalein monomer, preferably 1-3 equivalents, more preferably 1.5 equivalents. The reaction temperature of the nitrating reaction is 10-50°C, preferably room temperature, such as 15-35°C, 20-30°C. The drying temperature is 40-120°C, preferably 60-100°C, preferably vacuum drying.

[0042] In one group of embodiments, in step (2), the catalyst is a transition metal catalyst, such as palladium carbon, platinum carbon, and Raney nickel. The palladium carbon is 10% Pd / C. The amount of the catalyst is 2-20wt% of the dinitration monomer, preferably 5-10wt%. The solvent for the reaction is alcohol or ether, preferably methanol or ethanol. The reaction temperature is 20-80°C, preferably 40-70°C, and more preferably 55-65°C. The drying temperature is 20-60°C, preferably 40-45°C, preferably vacuum drying.

[0043] In one group of embodiments, in the step (3), the molar ratio of the diamine monomer to the dianhydride monomer is 1:2-2:1, preferably 1:1. The solvent for the polycondensation reaction is selected from aprotic polar solvents, such as N-methyl-pyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide; the solution concentration of the polycondensation reaction is 10-50wt%, preferably 25-35wt%. The temperature of the polycondensation is -10-10°C, preferably 0-5°C. The anhydride is selected from acetic anhydride, propionic anhydride, butyric anhydride, valeric anhydride, preferably acetic anhydride, and the amount of the anhydride is 3-7 equivalents of the dianhydride monomer, preferably 5 equivalents. The alkaline catalyst is an organic amine, preferably a tertiary amine, such as pyridine and triethylamine; the amount of the alkaline catalyst is 2-3 equivalents of the dianhydride monomer, preferably 2.5 equivalents. The temperature of the imidization reaction is 10-50°C, preferably room temperature, such as 15-35°C, 20-30°C. The alcohol solvent is methanol, ethanol, etc. The drying is preferably vacuum drying. The drying procedure is preferably 50-70°C for 20-28h, 80-100°C for 10-16h, 110-130°C for 10-16h, 140-160°C for 20-28h.

[0044] In one group of embodiments, in the step (4), the solvent is selected from aprotic polar solvents, such as N-methyl-pyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide. The solid content of the casting liquid is 5-10wt%, preferably 8wt%. The filtration is carried out using a polytetrafluoroethylene filter membrane, and the pore size of the filter membrane is preferably 0.45-1μm. The film formation refers to film formation on a plane by casting or scraping, such as film formation on a glass plate or a culture dish. The drying is preferably vacuum drying. The drying procedure is 50-70℃20-28h, 80-100℃10-16h, 110-130℃10-16h, 140-160℃20-28h. The obtained polyimide membrane is a dense membrane with uniform thickness and smooth surface, and the membrane thickness is 50-100μm, preferably 70-80μm.

[0045] In one embodiment, in step (5), the alcohol solvent is methanol, ethanol, etc. The volume ratio of the ester-based cross-linking agent to the alcohol solvent is 1:2-1:10, preferably 1:3-1:6, 1:4-1:5. Preferably, the alcohol solvent is removed by drying at room temperature, cross-linking is performed at 100-150°C for 8-16h, and excess ester-based cross-linking agent is removed by drying at 100-150°C; more preferably, the alcohol solvent is removed by drying at room temperature for 4h, cross-linking is performed at 120°C for 12h, and excess ester-based cross-linking agent is removed by drying at 120°C; preferably, excess ester-based cross-linking agent is removed by vacuum drying.

[0046] In a third aspect, the present application provides a use of the ester-crosslinked phenolphthalein-based polyimide membrane material described in the first aspect, which is used for gas separation.

[0047] In one group of embodiments, the gas separation refers to the separation of CO2 and methane in natural gas purification, the separation of oxygen and nitrogen in air, the separation of CO2 and nitrogen, the separation of helium and N2, the separation of helium and methane, the separation of helium and neon, the separation of hydrogen and methane, the separation of ethane and ethylene, and the separation of propane and propylene. It is preferably used for the separation of CO2 and methane in natural gas purification.

[0048] Compared with the prior art, the beneficial technical effects achieved by this application are:

[0049] The present application uses the low-priced and easily available phenolphthalein monomer, and the lactone ring is ring-opened and reduced as the diamine monomer. The phenolphthalein group contains a large number of benzene rings, which makes the polymer have excellent thermal stability.

[0050] The lactone ring of the phenolphthalein monomer is opened and reduced to a carboxyl group, which can cause ester cross-linking, thereby obtaining an excellent three-dimensional network structure and avoiding the destruction of the polymer membrane structure, thereby maintaining the good mechanical properties of the polyimide gas separation membrane.

[0051] After ester crosslinking, the carboxyl groups on the phenolphthalein groups react with the hydroxyl groups on the diols to form ester groups, increasing the distance between the chains and improving the gas permeability of the membrane. At the same time, after ester crosslinking, a more rigid polymer skeleton is formed, which makes the polymer membrane have better size screening performance and improves selectivity.

[0052] The ester-crosslinked phenolphthalein-based polyimide membrane described in the present application increases gas permeability and selectivity, and improves the plasticization resistance and gas separation performance of the polymer membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Exemplary embodiments of the present application are described in more detail with reference to the accompanying drawings.

[0054] Figure 1 This is the reaction formula for the nitration, ring-opening reduction and reaction with 6FDA to synthesize polyimide in the present application of phenolphthalein monomer.

[0055] Figure 2 3,3'-dinitrophenolphthalein in Example 1 of the present application 1 H-NMR spectrum (DMSO-d6).

[0056] Figure 3 The phenolphthaleinyl diamine 2-(bis(3-amino-4-hydroxyphenyl)methyl)benzoic acid (AHPBA) in Example 1 of the present application is 1 H-NMR spectrum (DMSO-d6).

[0057] Figure 4 is the 6FDA-AHPBA polyimide in Example 1 of the present application 1 H-NMR spectrum (DMSO-d6).

[0058] Figure 5 This is the reaction formula for ester cross-linking of the polyimide in Example 1 of the present application.

[0059] Figure 6 The polyimide films prepared in Example 1 of the present application: a is a display of the flexibility of the PI film; b is a PI film; c is an ethylene glycol cross-linked polyimide film PI-EG film.

[0060] Figure 7 This is a comparison chart of the anti-plasticization performance of the uncross-linked polyimide original film and the ethylene glycol cross-linked polyimide film in Example 1 of the present application.

[0061] Figure 8 This is a comparison chart of infrared spectra of the uncrosslinked polyimide original film and the ethylene glycol crosslinked polyimide film in Example 1 of the present application. DETAILED DESCRIPTION

[0062] The present application is further described below in conjunction with embodiments; however, the specific embodiments described herein are only used to illustrate and explain the present invention and do not limit the scope of the present application. Many modifications and changes are obvious to those of ordinary skill in the art without departing from the scope and spirit of the embodiments described.

[0063] Unless otherwise stated, all reactants used in the examples were obtained from commercial sources; the instruments and equipment used in the synthesis experiments and product analysis and detection were conventional instruments and equipment commonly used in organic synthesis.

[0064] The endpoints and any values ​​of the ranges disclosed in this application are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0065] Example 1

[0066] The phenolphthalein monomer undergoes nitration, ring-opening reduction, and polymerization with 6FDA to synthesize polyimide. The reaction formula is as follows: Figure 1 shown.

[0067] Weigh 1.0 g of phenolphthalein, dissolve it in 30 mL of acetic acid and stir evenly, add 1.2 g of yttrium nitrate hexahydrate, react for 2 hours, pour into 50 mL of ice-cold deionized water, precipitate a yellow precipitate, filter it, stir the solid with deionized water for 1 hour, filter it again, and dry the obtained solid in a vacuum drying oven at 60°C. The yield is 92%. The obtained 3,3'-dinitrophenolphthalein 1 H-NMR spectrum (DMSO-d6) Figure 2 shown.

[0068] Weigh 10g of 3,3'-dinitrophenolphthalein and dissolve it in 20mL of ethanol. Pour the solution into the reactor, add 1g of 10% Pd / C catalyst, install the reactor, fill the reactor with 0.5MPa N2, open the exhaust port to release N2, repeat 3 times, expel the air in the reactor, and finally fill the reactor with 0.5MPa N2 again. Wait for 30min and observe the pressure change. If the pressure remains unchanged, the reactor has good air tightness and can continue the reaction. The pressure decreases and the reactor is checked for air leakage. Fill the reactor with 0.5MPa H2, open the exhaust port to release H2, repeat 3 times, fill the reactor with 1.5MPa H2, start the reaction at 60°C, when the pressure in the reactor drops to 0.5MPa, fill H2 again to increase the pressure to 1.5MPa, repeat the above steps until the pressure no longer decreases, the reaction is over, use a suction filtration device to filter, pour the solution into a beaker, seal it and put it in a freezer for 48 hours, crystals will appear on the wall of the beaker, filter the crystals on the wall of the beaker and wash them. Dry the washed product in a 45°C vacuum drying oven until the product is completely dry. The obtained phenolphthaleinyl diamine 2-(di(3-amino-4-hydroxyphenyl)methyl)benzoic acid (AHPBA) 1 H-NMR spectrum (DMSO-d6) Figure 3 shown.

[0069] Take 3.50g (350.37g / mol, 10mmol) of 2-(bis(3-amino-4-hydroxyphenyl)methyl)benzoic acid (AHPBA) into a 100mL three-necked flask, add 18mL NMP, stir for 30min in a low-temperature ice bath and under nitrogen protection, add 4.44g (444.24g / mol, 10mmol) of 6FDA twice, with a solid content of 30wt%, continue stirring and react for 24h to form a high-viscosity polyamic acid solution, remove the ice bath, add 5mL acetic anhydride and 5mL anhydrous pyridine. Continue to react at room temperature for 24h to obtain a polyimide solution with high viscosity, pour it into methanol for precipitation, filter, wash with methanol, then remove the residual solvent, and gradually increase the temperature: 60℃24h, 90℃12h, 120℃12h, 150℃24h, to obtain 8.42g of dried polyimide. 6FDA-AHPBA polyimide 1 H-NMR spectrum (DMSO-d6) Figure 4 shown.

[0070] Take 1.00g of dried polyimide powder and dissolve it in DMF, stir until it is completely dissolved, the solid content of the casting liquid is 8.0wt%, and filter it with a 0.45 or 1μm polytetrafluoroethylene filter membrane. Inject the filtered solution into a flat glass culture dish, and then put it into a vacuum oven and increase the temperature gradually: 60℃24h, 90℃12h, 120℃12h, 150℃24h to obtain a polyimide dense film with uniform thickness and smooth surface, with a film thickness between 70 and 80μm.

[0071] The dried polyimide membrane was immersed in a methanol-ethylene glycol mixed solution (methanol to ethylene glycol volume ratio of 4:1) for 8 hours, then dried at room temperature for 4 hours to remove methanol, placed in a 120°C oven for crosslinking for 12 hours, and finally placed in a 120°C vacuum oven for vacuum drying for 24 hours to remove the remaining ethylene glycol, to obtain an ester-crosslinked phenolphthalein-based polyimide gas separation membrane. The reaction formula for ester-crosslinking of the polyimide membrane is shown in 5.

[0072] Comparison of polymer membrane images before and after ester cross-linking treatment Figure 6 -b- Figure 6 -c. Table 1 shows the gas permeability and gas selectivity of the polyimide membrane synthesized by AHPBA and 6FDA and the polyimide membrane cross-linked by ethylene glycol ester. PI-original refers to the polyimide membrane synthesized by AHPBA and 6FDA, which is not cross-linked. The molecular weight of PI-original is 30705. PI-EG refers to the polyimide membrane cross-linked by ethylene glycol ester.

[0073] Table 1

[0074]

[0075] As can be seen from Table 1, the permeability increases after ester cross-linking, which is mainly because the carboxyl groups on the phenolphthalein group react with the hydroxyl groups on the ethylene glycol to form ester groups, and the chain spacing increases. The increase in selectivity is due to the formation of a more rigid polymer skeleton after ester cross-linking, which makes the polymer membrane have better size screening performance.

[0076] For condensable gas CO2, the permeation isotherm of glassy polymer may show an upward trend due to the plasticization caused by the swelling of polymer film under high pressure. Figure 7 It can be seen that when the CO2 pressure is 30atm, the polyimide membrane cross-linked with ethylene glycol does not show plasticization phenomenon, which proves that the ester-cross-linked polyimide gas separation membrane exhibits excellent anti-plasticization performance.

[0077] The preferred embodiments of the present application are described in detail above; however, the present application is not limited to the specific details in the above embodiments. Within the technical concept of the present application, a variety of simple modifications can be made to the technical solution of the present application, and these simple modifications all fall within the protection scope of the present application.

[0078] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this application will not further describe various possible combinations.

[0079] In addition, the various implementation modes of the present application may be arbitrarily combined, and as long as they do not violate the concept of the present application, they should also be regarded as the contents disclosed in the present application.

Claims

1. An ester-crosslinked phenolphthalein-based polyimide film material, characterized in that: Its structural formula is shown in Formula I, Wherein, R1, R2, R3, R4, R5, R6 are each independently H, C1-C4 alkyl; R7 is C1-C4 alkyl; R8 is each independently C1-C4 alkyl, s is 0, 1, 2, 3 or 4; R9 is C1-C6 alkylene; m, n represent the number of repeating units, each independently 10-800, preferably 20-500; Each The same or different groups independently represent the groups formed after the dianhydride monomer reacts with the amino group.

2. The ester-crosslinked phenolphthalein-based polyimide film material according to claim 1, characterized in that: The dianhydride monomer is selected from pyromellitic anhydride optionally substituted by a C1-C4 alkyl group or Any one, combination of two or more, R 10 Selected from direct bond, -C(CF3)2-, -CH2-, -O-, -CO-.

3. The ester-crosslinked phenolphthalein-based polyimide film material according to claim 2, characterized in that: The dianhydride monomer is selected from any one, two or more combinations of the following structures optionally substituted by a C1-4 alkyl group: PMDA, pyromellitic anhydride MPDA, Polysorbate 40 diacid BPDA, biphenyltetracarboxylic dianhydride 6FDA, hexafluoroisopropylphthalic anhydride BTDA, Benzophenone tetracarboxylic dianhydride ODPA, oxydiphthalic anhydride.

4. The ester-crosslinked phenolphthalein-based polyimide film material according to any one of claims 1 to 3, characterized in that: Each of the formula I represent the same group.

5. The ester-crosslinked phenolphthalein-based polyimide film material according to any one of claims 1 to 4, characterized in that: in, R1, R2, R3, R4, R5, and R6 are each independently H, methyl, or ethyl; R7 is selected from methyl, ethyl, n-propyl, or n-butyl; R8 is selected from methyl and ethyl; R9 is selected from ethylene, n-propylene, n-butylene, n-pentylene, or n-hexylene.

6. The ester-crosslinked phenolphthalein-based polyimide film material according to any one of claims 1 to 5, characterized in that: Choose one of the following: (i) R1-R6 are all H; (ii) R1=-CH3, R4=-CH3, R2, R3, R5, R6 are all H; (iii) R7=-CH3; (iv) R7=-CH2CH3; (v) R7=-CH2CH2CH3; (vi) R7=-CH2CH2CH2CH3; (vii) R9=-CH2CH2-; (viii) R9=-CH2CH2CH2-; (ix) R9=-CH2CH2CH2CH2-; (x) R9=-CH2CH2CH2CH2CH2CH2-; (xi)R9=-CH2CH2CH2CH2CH2CH2-.

7. The ester-crosslinked phenolphthalein-based polyimide film material according to claim 1, characterized in that: Formula I is selected from the following:

8. The method for preparing the ester-crosslinked phenolphthalein-based polyimide film material according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) Phenolphthalein monomer is nitrated to obtain dinitro monomer; (2) dinitro monomer is ring-opened and reduced to obtain diamine monomer; (3) diamine monomer and dianhydride monomer are polycondensed and imidized to obtain polyimide; (4) polyimide is used to prepare film; (5) ester crosslinking treatment is performed using ester crosslinking agent diol OH-R9-OH, and the reaction route is as follows:

9. The preparation method according to claim 8, characterized in that: The following steps are involved: (1) After the phenolphthalein monomer and the nitrating agent react in a solvent, cold water is added to precipitate, which is washed and dried to obtain a dinitrile monomer; (2) The dinitrile monomer is dissolved in a solvent, a catalyst is added, nitrogen is introduced to exclude air, H2 is introduced to react, the catalyst is filtered out after the reaction, the mixture is cooled to crystallize, washed and dried to obtain a diamine monomer; (3) The diamine monomer is dissolved in a solvent, a dianhydride monomer is added under nitrogen protection to carry out a condensation reaction, and then an acid anhydride and an alkaline catalyst are added to carry out an imidization reaction. After the reaction is completed, the mixture is poured into an alcohol solvent to precipitate, filtered, washed and dried to obtain a polyimide; (4) The polyimide is dissolved in a solvent to obtain a casting solution, which is filtered to form a film on a plane, and dried to obtain a polyimide film; (5) The polyimide film is placed in a mixed solution of an ester-based crosslinking agent diol and an alcohol solvent, soaked, dried to remove the alcohol solvent, heated for crosslinking, and then dried to remove excess ester-based crosslinking agent to obtain an ester-based crosslinked phenolphthalein-based polyimide film.

10. The preparation method according to claim 8 or 9, characterized in that: In the step (1), the nitrating agent is yttrium nitrate or sodium nitrate, and the solvent is acetic acid or sulfuric acid; preferably, the amount of the nitrating agent is 0.5-5 equivalents of phenolphthalein monomer, more preferably 1-3 equivalents.

11. The preparation method according to claim 8 or 9, characterized in that: In the step (2), the catalyst is a transition metal catalyst, and the solvent is an alcohol or an ether; preferably, the catalyst is palladium carbon, platinum carbon, or Raney nickel, and the solvent is selected from methanol and ethanol.

12. The preparation method according to claim 11, characterized in that: In the step (2), the palladium carbon is 10% Pd / C, and its usage is preferably 2-20wt% of the dinitration monomer, and more preferably 5-10wt%.

13. The preparation method according to claim 8 or 9, characterized in that: In the step (3), the molar ratio of the diamine monomer to the dianhydride monomer is 1:2-2:1, the solvent for the polycondensation reaction is selected from a non-protonic polar solvent, the acid anhydride is selected from acetic anhydride, propionic anhydride, butyric anhydride, and valeric anhydride, the alkaline catalyst is an organic amine, and the alcohol solvent is selected from methanol and ethanol; preferably, the molar ratio of the diamine monomer to the dianhydride monomer is 1:1, the solvent is selected from N-methyl-pyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide, the acid anhydride is acetic anhydride, and the alkaline catalyst is a tertiary amine.

14. The preparation method according to claim 8 or 9, characterized in that: In the step (4), the solvent is selected from a non-protonic polar solvent, and the solid content of the casting liquid is 5-10%; preferably, the solvent is selected from N-methyl-pyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide, and the solid content of the casting liquid is 8%.

15. The preparation method according to claim 8 or 9, characterized in that: In the steps (3)-(4), the drying procedure is 50-70°C for 20-28h, 80-100°C for 10-16h, 110-130°C for 10-16h, and 140-160°C for 20-28h.

16. The preparation method according to claim 8 or 9, characterized in that: In the step (4), the thickness of the obtained polyimide film is 50-100 μm, preferably 70-80 μm.

17. The preparation method according to claim 8 or 9, characterized in that: In the step (5), the alcohol solvent is selected from methanol and ethanol, and the volume ratio of the ester-based cross-linking agent to the alcohol solvent is 1:2-1:10; preferably, the volume ratio of the ester-based cross-linking agent to the alcohol solvent is 1:4-1:

5.

18. The preparation method according to claim 8 or 9, characterized in that: In the step (5), the alcohol solvent is dried at room temperature, cross-linked at 100-150°C for 8-16 hours, and excess ester-based cross-linking agent is dried at 100-150°C; preferably, cross-linked at 120°C for 12 hours, and excess ester-based cross-linking agent is dried at 120°C.

19. Use of the ester-crosslinked phenolphthalein-based polyimide membrane material according to any one of claims 1 to 7 for gas separation.

20. The use according to claim 19, characterized in that The gas separation refers to the separation of CO2 and methane in natural gas purification, the separation of oxygen and nitrogen in the air, the separation of CO2 and nitrogen, the separation of helium and N2, the separation of helium and methane, the separation of helium and neon, the separation of hydrogen and methane, the separation of ethane and ethylene, and the separation of propane and propylene.

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