An ionic covalent organic framework composite membrane, a preparation method and application thereof in separating dyes and salts

Abstract

This invention relates to an ionic covalent organic framework composite membrane, its preparation method, and its application in the separation of dyes and salts, belonging to the field of polymer materials and their preparation technology. The invention prepares an ionic covalent organic framework composite membrane using an improved casting-precipitation-evaporation method for separating dye and salt wastewater. The composite membrane can achieve the separation of salt and dye mixtures, exhibiting a high dye rejection rate (~97%) and a low salt retention rate (<5%), with a water permeability reaching 68 L / m². ‑2 h ‑1 bar ‑1 Furthermore, the obtained membrane exhibits good stability and antifouling properties, avoiding the problem of membrane performance degradation due to membrane layer separation. This invention solves the trade-off between selectivity and permeability in traditional polymer membranes, as well as the problem of poor mechanical properties and fragility of stand-alone membranes in practical applications. Therefore, it has certain application value in purifying dye wastewater and recovering salt from dye wastewater to supplement costs.

Description

An ionic covalent organic framework composite membrane, its preparation method, and its application in separating dyes and salts. Technical Field

[0001] This invention belongs to the field of polymer materials and their preparation technology, specifically relating to an ionic covalent organic framework composite membrane, its preparation method, and its application in separating dyes and salts. Background Technology

[0002] Covalent organic frameworks (COAs) possess high porosity, low mass density, excellent heat resistance, and tunable structure, making them ideal membrane materials. Traditional methods for preparing COAs composite membranes include interfacial polymerization and vacuum filtration. However, these methods often produce independent, self-supporting COAs membranes with poor mechanical properties and difficulties in direct membrane formation. This invention employs a casting-precipitation-evaporation method to prepare a high-performance COAs composite membrane. The designed composite membrane consists of an ionic COAs active layer and a support layer. An active layer is grown in situ on the surface of a hydrolyzed polyacrylonitrile support membrane via a Schiff base reaction. The physical and chemical properties of the membrane surface are simply treated with heat. The ionic COAs composite membrane prepared by this invention has significant application value in the separation of dyes and salts. Summary of the Invention

[0003] The purpose of this invention is to provide an ionic covalent organic framework composite membrane with certain antifouling properties and the ability to separate dyes and salts, and its preparation method. This ionic covalent organic framework composite membrane is prepared by mixing polyacrylonitrile, a pore-forming agent, and a solvent in a specific ratio to form a casting solution, which is then coated onto a nonwoven fabric using a solvent-free phase inversion method. The membrane is then heat-treated in an alkaline environment (1.5–2.5M sodium hydroxide aqueous solution) at 40–60°C for 1–2 hours to hydrolyze it, obtaining a carboxyl-rich hydrolyzed polyacrylonitrile support membrane. A positively charged ionic covalent organic framework active layer is then grown in situ on the surface of the support membrane via a Schiff base reaction. This invention aims to solve the trade-off between selectivity and permeability in traditional polymer membranes and the problems of poor mechanical properties and fragility of self-supporting membranes in practical applications. Therefore, it has certain application value in separating dyes and salts.

[0004] This invention first prepares a hydrolyzed polyacrylonitrile supported membrane, and the membrane preparation process is as follows:

[0005] 1) Preparation of polyacrylonitrile supported membrane: Polyacrylonitrile, polyvinylpyrrolidone and dimethylformamide were mixed, with the concentrations of polyacrylonitrile and polyvinylpyrrolidone in the mixed solution being 10-25 wt% and 3-18 wt%, respectively; the mixture was magnetically stirred until the reaction solution was a pale yellow, transparent, and viscous state, and then allowed to stand at room temperature until no bubbles were present; the obtained casting solution was coated onto a nonwoven fabric plate with pores of 100-300 μm, and after 15-30 seconds it was immersed in deionized water at 20℃-30℃, the deionized water was replaced every 6-10 hours, and after 20-30 hours, a polyacrylonitrile membrane was obtained;

[0006] 2) Immerse the polyacrylonitrile membrane obtained in step 1) in a 1.5-2.5M sodium hydroxide solution and heat treat it at 40-60℃ for 1-2 hours; after taking it out, wash it with deionized water until the washing water is neutral, and dry it naturally at room temperature to obtain a hydrolyzed polyacrylonitrile supported membrane.

[0007] The preparation method of the ionic covalent organic framework composite membrane of the present invention comprises the following steps:

[0008] p-Phenylenediamine and 1,3-diaminoguanidine hydrochloride, in a molar ratio of 1:1, were added to 1–3 mL of acetonitrile and 1–3 mL of deionized water, respectively. The two solutions were then mixed and added to 0.05–0.15 mL of acetic acid, followed by sonication to obtain a homogeneous amine solution. The concentrations of p-phenylenediamine and 1,3-diaminoguanidine hydrochloride in the amine solution were 2.70–21.6 g / L and 3.15–25.2 g / L, respectively, with preferred concentrations of 5.40–10.80 g / L and 9.45–12.60 g / L, respectively. Then, terephthalaldehyde was added to dichloromethane and sonicated to obtain a homogeneous aldehyde solution. The aldehyde solution contains terephthalaldehyde at a concentration of 6.71–53.64 g / L, preferably 13.40–26.80 g / L. The aldehyde solution is poured into the amine solution to obtain a mixed solution, with a molar ratio of terephthalaldehyde to 1,3-diaminoguanidine hydrochloride of 3–5:1. The color of the mixed solution changes from light yellow to dark. The mixed solution is then poured into a petri dish containing a dried hydrolyzed polyacrylonitrile support membrane and heat-treated at 50–70°C for 20–24 hours. After the membrane cools naturally to room temperature, it is repeatedly soaked, rinsed, and dried with deionized water to obtain an ionomer-based covalent organic framework membrane.

[0009] This invention prepares an ionic covalent organic framework composite membrane using an improved casting-precipitation-evaporation method. First, an ionic covalent organic framework active layer is grown in situ on the surface of a hydrolyzed polyacrylonitrile (PPI) supported membrane via a Schiff base reaction. Then, a simple heat treatment process is used to accelerate the reaction rate and alter the physical and chemical properties of the support layer surface. During the Schiff base reaction, aldehyde and amine monomers condense to form a covalent organic network that covers the support layer surface. Excess amino groups combine with carboxyl groups on the PPI support layer. Furthermore, the negatively charged support layer and the positively charged active layer interact electrostatically, resulting in a tighter bond between the covalent organic framework active layer and the support layer. This modifies the upper surface of the hydrolyzed PPI, altering its surface physicochemical properties and avoiding the membrane performance degradation caused by membrane separation in practical applications.

[0010] The ionic covalent organic framework composite membrane prepared by this invention is mainly used for the separation of dyes and salts, achieving a separation time of ~68 L / h. -1 m -2 bar -1 (Figure 4b) The membrane exhibits high flux and high rejection rates for dyes of different molecular weights, with a rejection rate of 97% for tiger's blood dye (Figure 4b). Furthermore, it achieves low salt rejection rates (<6%) for four types of salts: sodium sulfate, magnesium sulfate, sodium chloride, and magnesium chloride (Figure 4a). Due to its hydrophilic, highly smooth surface, the composite membrane also possesses excellent antifouling properties, achieving a flux recovery rate of 96.1% after 360 minutes of cyclic fouling with bovine serum albumin. Attached Figure Description

[0011] Figure 1 shows the infrared spectra of the ionic covalent organic framework composite membrane (iCOFMs) and the hydrolyzed polyacrylonitrile supported membrane (HPAN) prepared in Example 1.

[0012] As shown in Figure 1, the ionic covalent organic framework membrane at 1614 cm⁻¹ -1 The strong -C=N- stretching vibration at the site indicates that the reaction between the aldehyde and amine groups occurred successfully.

[0013] Figure 2 shows scanning electron microscope (SEM) images of the surface (a) and cross-section (b) of the covalent organic framework composite membrane.

[0014] As can be seen from the figure, the covalent organic framework active layer completely covers the surface of the supporting membrane, resulting in a relatively dense layer and large ridges. Measurements of the membrane cross-section clearly show that the thickness of the selective layer is approximately 177 nm.

[0015] Figure 3 shows the effect curves of adding different contents of 1,3-diaminoguanidine hydrochloride (a) and different heat treatment times (b) on the membrane separation performance of the membrane prepared in Example 2.

[0016] As shown in Figure 3a, when the 1,3-diaminoguanidine hydrochloride content is too low (concentration 3.15–6.30 g / L), the resulting membrane is discontinuous and defective, thus resulting in a very high water flux (141 L·m). -2 ·h -1 ·bar -1 This corresponds to a 1,3-diaminoguanidine hydrochloride concentration of 3.15 g / L; when the concentration is 6.08 g / L, the water flux is 66 L·m. -2 ·h -1 ·bar -1 The rosin rejection rate is very low (29%, corresponding to a 1,3-diaminoguanidine hydrochloride concentration of 3.15 g / L; at a concentration of 6.08 g / L, the rejection rate is 59%). When the 1,3-diaminoguanidine hydrochloride content is too high (concentration 15.75–25.2 g / L), the aldehyde monomer is completely consumed and the Schiff base reaction can no longer occur. Therefore, the membrane flux and dye rejection rate remain unchanged, and the excess amine monomer is removed during the washing process after membrane formation, resulting in waste. Therefore, a 1,3-diaminoguanidine hydrochloride concentration of 9.45–12.6 g / L is the optimal addition amount. Figure 3b shows that if the heat treatment time is too short (less than 20 hours), the reaction is incomplete, resulting in an uneven membrane and surface defects. Therefore, the water flux and rosin rejection rate are unstable. Thus, a heat treatment time of 20–24 hours is the optimal time.

[0017] Figure 4 is a bar graph of the separation performance of the ionic covalent organic framework composite membrane prepared in Example 3, including (a) the separation performance bar graph of different salts, (b) the separation performance bar graph of different dyes, (c) the separation performance bar graph of the tiger cinnabar / salt mixed system, and (d) the separation effect bar graph of tiger cinnabar solution of different concentrations mixed with 1 g / L sodium chloride.

[0018] As shown in Figure 4(a), due to the negative charge on the surface of the composite membrane, the electrostatic interaction with divalent cations is greater than that with monovalent cations. Furthermore, SO42- 2- The hydrated ionic radius is greater than Cl -This results in a higher rejection rate for sulfates compared to hydrochlorides. Therefore, the salt rejection order is MgSO4 (5.13%) > MgCl2 (5.07%) > Na2SO4 (3.23%) > NaCl (2.48%). As shown in Figure 4(b), by comparing dyes with similar molecular weights but different charges, such as methyl orange (negative charge, 327.3 Da, retention rate 23.7%) and methylene blue (positive charge, 319.9 Da, retention rate 17.5%), and dyes with the same charge but different molecular weights, such as methylene blue (negative charge, 799.8 Da, retention rate 80.0%), Congo red (two negative charges, 696.66 Da, retention rate 87.5%), and rose benzalkonium chloride (negative charge, 1072.8 Da, retention rate 97.5%), it can be seen that the composite membrane retains dyes of different molecular weights through the Donnan effect and molecular sieving, but salt can pass through the membrane, thus effectively separating salt from dye wastewater. As shown in Figure 4(c), in the tiger roxene / salt mixed binary system, the high dye rejection rate (82–97%) and low salt rejection rate (1.24–4.98%) indicate that the ionic covalent organic framework membrane can effectively separate salt from dye wastewater. As shown in Figure 4(d), after increasing the dye concentration, the high dye rejection rate (96–98%) and low salt rejection rate (1.24–4.25%) also demonstrate that the ionic covalent organic framework membrane can effectively separate salt from dye wastewater.

[0019] Figure 5(a) is a line graph showing the repulsion effect of the membrane on cinnabar under different operating pressures; (b) is a bar graph showing the cyclic separation performance of the membrane; (c) is a bar graph showing the retention effect of the membrane on cinnabar at different ultrasonic times (0, 30, and 60 minutes); and (d) is the infrared spectrum of the composite membrane after ultrasonic treatment.

[0020] As shown in Figure 5(a), the rejection rate of argentine tinctoria only fluctuated slightly when the operating pressure increased from 2 bar to 6 bar. Furthermore, the membrane was run at 3 bar for 50 minutes, then washed in ethanol solution for 10 minutes, and the argentine tinctoria solution was filtered again, and this cycle was repeated. As shown in Figure 5(b), the normalized flux of the membrane reached 100% and the rejection rate of argentine tinctoria reached approximately 97% within the first 180 minutes; after 200 minutes, both flux and rejection rate began to gradually decrease. To further evaluate the membrane stability, we also subjected the membrane to 16 kHz ultrasonic treatment for 30 minutes and 60 minutes, respectively. After ultrasonic treatment, as shown in Figure 5(c), the water flux of the membrane remained essentially unchanged, while the rejection rate of argentine tinctoria decreased by approximately 8%. The infrared spectrum in Figure 5(d) also indicates that the membrane structure remained unchanged after ultrasonic treatment. All the above tests demonstrate that the composite membrane has good stability.

[0021] Figure 6(a) is a line graph showing the change in water flux with pure water and bovine serum albumin as feed solutions alternately; (b) is a bar graph showing the flux recovery rate (PRR), reversible scaling rate (Rr), irreversible scaling rate (Rir), and total scaling rate (Rt) of the membrane calculated from the flux after the experiment in Example 5.

[0022] The calculation formula is as follows:

[0023] PRR% = F w,2 / F w,1 × 100% (1)

[0024] Rr% = ( F w,2 - F P ) / F w,1 × 100% (2)

[0025] Rir% = ( F w,1 - F w,2 ) / F w,1 × 100% (3)

[0026] Rt%=Rr%+Rir%=(F w,1 - F p ) / F w,1 × 100% (4)

[0027] In the formula, F w,1 F is the water flux when pure water is used as the feed after pre-compression. p F is the water flux when bovine serum albumin aqueous solution replaces pure water as feed. w,2 The water flux is the amount of water fed into the membrane after it has been cleaned with water and contaminated with bovine serum albumin.

[0028] As shown in Figure 6(a), after 60 minutes, replacing the pure water with bovine serum albumin (BSA) caused a sharp drop in water flux due to the large molecular weight (66446 Da) and negative charge of BSA, which clogged the pores. After rinsing the membrane with water and then passing pure water again, the flux recovered somewhat. Because the composite membrane is negatively charged and has a hydrophilic surface, scaling is less likely to occur on its surface. Figure 6(b) shows that after 210 minutes of circulation, the membrane flux recovery rate (PRR) reached 96.5%, the reversible scaling rate (Rr) was 42.9%, the irreversible fouling rate (Rir) was 3.49%, and the total fouling rate (Rt) was 46.4%. After 360 minutes of circulation, the membrane flux recovery rate (PRR) reached 96.1%, the reversible scaling rate (Rr) was 41.0%, the irreversible fouling rate (Rir) was 3.88%, and the total fouling rate (Rt) was 44.9%. This indicates that our membrane has a certain degree of anti-fouling effect. Detailed Implementation

[0029] Example 1: Preparation of ionic covalent organic framework composite membrane

[0030] Add 1.5 g (11 wt%) of polyacrylonitrile, 0.546 g (4 wt%) of polyvinylpyrrolidone and 12.23 mL (85 wt%) of dimethylformamide to a glass vial and stir with a magnetic stirrer until the liquid in the vial is a pale yellow, transparent, and viscous liquid. Then let it stand at room temperature until no air bubbles are present. Pour the resulting casting solution onto a nonwoven fabric plate with a gap of 200 μm, coat it for 20 seconds, and then immediately immerse it in deionized water at 25 °C. Change the water every 8 hours, and obtain a polyacrylonitrile membrane after 24 hours.

[0031] The polyacrylonitrile membrane was immersed in a sodium hydroxide solution (2M) and heat-treated at 50°C for 1 hour. Then, the polyacrylonitrile membrane was washed with deionized water until the rinse water was neutral, and then naturally dried at room temperature to obtain the hydrolyzed polyacrylonitrile supported membrane.

[0032] Acetonitrile (1 mL) was added to a beaker containing p-phenylenediamine (16.2 mg, 0.15 mmol), and deionized water (1 mL) was added to a beaker containing 1,3-diaminoguanidine hydrochloride (18.9 mg, 0.15 mmol). The two solutions were mixed and added to acetic acid (0.05 mL), and sonicated for 30 seconds to obtain a homogeneous amine solution (the concentration of p-phenylenediamine in the amine solution was 8.1 g / L, and the concentration of 1,3-diaminoguanidine hydrochloride was 9.45 g / L). Dichloromethane (4 mL) was added to a beaker containing terephthalaldehyde (80.48 mg, 0.6 mmol), and sonicated for 30 seconds to obtain a homogeneous aldehyde solution (20.12 g / L). The aldehyde solution was poured into the amine solution to obtain a mixed solution, which changed from light yellow to dark yellow. Finally, the mixed solution was poured directly into a petri dish containing a hydrolyzed polyacrylonitrile support membrane that had been naturally dried at room temperature, and then heat-treated in a 60°C oven for 24 hours. After the membrane was removed, it was allowed to cool to room temperature naturally. It was then repeatedly soaked, rinsed, and dried with deionized water to obtain an ionic covalent organic framework membrane.

[0033] Example 2: Optimal Reaction Conditions for Film Formation

[0034] Based on Example 1, different membranes were prepared under the same conditions by varying the amount of each monomer added. The membrane containing 6.30 mg 1,3-diaminoguanidine hydrochloride, 5.40 mg p-phenylenediamine, and 26.82 mg terephthalaldehyde was named iCOF-0 (1,3-diaminoguanidine hydrochloride concentration: 3.15 g / L); the membrane containing 9.45 mg 1,3-diaminoguanidine hydrochloride, 8.10 mg p-phenylenediamine, and 40.23 mg terephthalaldehyde was named iCOF-1 (1,3-diaminoguanidine hydrochloride concentration: 4.73 g / L); the membrane containing 12.60 mg 1,3-diaminoguanidine hydrochloride, 10.80 mg p-phenylenediamine, and 53.66 mg terephthalaldehyde was named iCOF-2 (1,3-diaminoguanidine hydrochloride concentration: 6.30 g / L); and the membrane containing 18.90 mg... The membrane containing 1,3-diaminoguanidine hydrochloride, 16.20 mg p-phenylenediamine, and 80.48 mg terephthalaldehyde was named iCOF-3 (1,3-diaminoguanidine hydrochloride concentration was 9.45 g / L); the membrane containing 25.20 mg 1,3-diaminoguanidine hydrochloride, 21.6 mg p-phenylenediamine, and 107.30 mg terephthalaldehyde was named iCOF-4 (1,3-diaminoguanidine hydrochloride concentration was 12.60 g / L); the membrane containing 31.50 mg 1,3-diaminoguanidine hydrochloride, 27.00 mg p-phenylenediamine, and 134.10 mg terephthalaldehyde was named iCOF-5 (1,3-diaminoguanidine hydrochloride concentration was 15.75 g / L); and the membrane containing 50.40 mg... The membrane containing 1,3-diaminoguanidine hydrochloride, 43.20 mg p-phenylenediamine, and 214.56 mg terephthalaldehyde salt was named iCOF-6 (the concentration of 1,3-diaminoguanidine hydrochloride was 25.20 g / L).

[0035] Based on Example 1, the heat treatment time was varied, and the effects of heat treatment time on membrane separation efficiency were explored by selecting 8, 12, 16, 20, and 24 hours.

[0036] Example 3: Separation Performance Test

[0037] Using a cross-flow filtration device, the membrane separation performance was measured at 2 bar pressure with either 5 mg / L dye or 2 g / L salt (sodium chloride, sodium sulfate, magnesium chloride, or magnesium sulfate) as feed. Subsequently, the membrane selectivity was measured with a 5 mg / L tiger's blood dye mixed with a 1 g / L sodium chloride solution as feed.

[0038] Example 4: Stability Test

[0039] The membrane's dye rejection rate was measured under different pressures. Cyclic stability testing of the membrane involved filtration of dye at 3 bar for 50 minutes, followed by immersion in an ethanol solution for 10 minutes. Membrane stability was determined by calculating the water flux recovery rate and dye rejection rate in each cycle. Furthermore, the membrane's separation performance for tiger's blood dye was tested after ultrasonic treatment at 16 kHz for 30 and 60 minutes, and this was used to evaluate membrane stability.

[0040] Example 5: Anti-pollution test

[0041] Dynamic antifouling experiments were conducted using bovine serum albumin (BSA) as the standard contaminant. Before the experiment, the membrane was pre-pressurized with deionized water for 30 minutes. Then, pure water was filtered at 2 bar for 60 minutes. Under the same conditions, a 0.1 g / L BSA solution was used instead of pure water for 90 minutes of filtration. Next, the membrane was immersed in deionized water for 90 minutes and repeatedly rinsed to ensure complete removal of contaminants from the membrane surface. Finally, the membrane was filtered with pure water for 60 minutes, and the flux of the pure water was measured again.

[0042] The cross-flow filtration devices used in the above separation experiments were all small membrane plate testing machines (TYLG-17) purchased from Shandong Bona Group.

Claims

1. A method for preparing an ionic covalent organic framework composite membrane, comprising the following steps: 1) Preparation of a polyacrylonitrile-supported membrane: Polyacrylonitrile, polyvinylpyrrolidone, and dimethylformamide are mixed, with the concentrations of polyacrylonitrile and polyvinylpyrrolidone in the mixed solution being 10-25 wt% and 3-18 wt%, respectively; the mixture is magnetically stirred until the reaction solution is a pale yellow, transparent, and viscous state, and then allowed to stand at room temperature until no bubbles are present; the resulting casting solution is coated onto a nonwoven fabric plate with pores of 100-300 μm, and after 15-30 seconds, it is immersed in deionized water at 20℃-30℃, and the deionized water is replaced every 6-10 hours. After 20-30 hours, a polyacrylonitrile membrane is obtained; 2) The polyacrylonitrile membrane obtained in step 1) is immersed in a 1.5-2.5 M sodium hydroxide solution and heat-treated at 40-60℃ for 1-2 hours; after removal, it is washed with deionized water until the washing water is neutral, and then naturally dried at room temperature to obtain a hydrolyzed polyacrylonitrile-supported membrane; 3) The molar ratio is... A 1:1 mixture of p-phenylenediamine and 1,3-diaminoguanidine hydrochloride was added to acetonitrile and deionized water, respectively. The two solutions were then mixed and added to acetic acid. The mixture was sonicated to obtain a homogeneous amine solution with concentrations of 2.70–21.6 g / L for p-phenylenediamine and 3.15–25.2 g / L for 1,3-diaminoguanidine hydrochloride. Terephthalaldehyde was then added to dichloromethane and sonicated to obtain a homogeneous aldehyde solution with a concentration of 6.71–53 g / L. 0.64 g / L; Pour the aldehyde solution into the amine solution to obtain a mixed solution, so that the molar ratio of terephthalaldehyde to 1,3-diaminoguanidine hydrochloride is 3-5:

1. The color of the mixed solution changes from light yellow to dark. Then pour the mixed solution into a petri dish containing the dried hydrolyzed polyacrylonitrile support membrane from step 2), and heat-treat at 50-70℃ for 20-24 hours. After the membrane cools naturally to room temperature, repeatedly soak and rinse it with deionized water and dry it to obtain an ion-type covalent organic framework membrane.

2. The method for preparing an ionic covalent organic framework composite membrane as described in claim 1, characterized in that: The concentrations of p-phenylenediamine and 1,3-diaminoguanidine hydrochloride in the amine solution were 5.40–10.80 g / L and 9.45–12.60 g / L, respectively, while the concentration of terephthalaldehyde in the aldehyde solution was 13.40–26.80 g / L.

3. An ionic covalent organic framework composite membrane, characterized in that: It is prepared by the method described in claim 1 or 2.

4. The application of the ionic covalent organic framework composite membrane according to claim 3 in the separation of dyes and salts.

5. The application of the ionic covalent organic framework composite membrane as described in claim 4 in the separation of dyes and salts, characterized in that: The salt is one or more of sodium sulfate, magnesium sulfate, sodium chloride, and magnesium chloride, and the dye is one or more of methyl orange, methylene blue, methylene blue, Congo red, and rose benzene.

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