Preparation method of polyamide solvent-resistant nanofiltration membrane with high crosslinking degree

Abstract

The invention discloses a preparation method of a high-crosslinking-degree polyamide solvent-resistant nanofiltration membrane, and belongs to the technical field of water and solvent treatment. Polyacrylonitrile (PAN) is used as a base membrane, an iron oxyhydroxide (FeOOH) middle layer is prepared through in-situ hydrolysis, then polyamine monomers and high-activity acyl chloride monomers react to synthesize a polyamide separation layer, and finally the polyamide solvent-resistant composite nanofiltration membrane with the adjustable crosslinking degree is prepared. The method has the advantages that the process is simple and mature, the conditions are green and mild, the surface hydrophilicity of the middle layer can be reasonably regulated and controlled through hydrolysis process parameters, the thickness of the polyamide separation layer is reduced, and the membrane flux is improved; meanwhile, due to the use of the rigid polyamine monomer, the prepared solvent-resistant nanofiltration membrane has a high degree of crosslinking, is endowed with good solvent resistance, can stably remove small molecular pollutants (the molecular weight is 200-1000Da) in solvents such as ethanol, acetone and tetrahydrofuran, and has a relatively wide application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of water and solvent treatment, and particularly relates to a preparation method of a polyamide solvent-resistant nanofiltration membrane with high crosslinking degree. Background Art

[0002] The membrane separation process is an efficient and environmentally friendly separation process. It is a high-tech that integrates multiple disciplines, showing various characteristics in terms of physical, chemical, and biological properties and having many advantages. As a membrane separation technology that applies nanofiltration to the organic phase, organic solvent nanofiltration is not only efficient but also green and environmentally friendly. By means of a simple pressure-driven method, it can achieve the separation of organic mixtures at the molecular level. Currently, solvent-resistant nanofiltration membranes are widely used in the field of water treatment, covering aspects such as hard water softening, seawater desalination, and heavy metal wastewater treatment. Now, with the continuous development of solvent-resistant nanofiltration membranes, their application fields have gradually expanded to industries including petrochemical, fine chemical, food processing, and medicine and health that involve organic solvents.

[0003] Polyacrylonitrile (PAN) is a polymer with excellent chemical stability, being not easily hydrolyzed and having strong antioxidant ability. PAN also has excellent film-forming properties. Acrylonitrile monomers are easy to copolymerize with a variety of other monomers, and it has good tolerance to most organic solvents (such as alcohols and hydrocarbons) and weak acids / weak bases, making it suitable for handling separation tasks in complex chemical environments. It maintains stable performance within a wide pH range (pH 2 - 10) and is suitable for scenarios such as wastewater treatment that require chemical cleaning. The polar cyano group (-CN) of PAN gives it natural hydrophilicity, reducing the adsorption of hydrophobic pollutants, increasing the flux, and reducing fouling. Therefore, PAN separation membranes are currently a kind of polymer membrane material with a relatively high degree of commercialization.

[0004] In recent years, composite nanofiltration membranes with an interlayer structure have received extensive attention. The preparation method of the interlayer is to uniformly deposit or coat materials onto the substrate membrane by methods such as surface coating, in-situ growth, co-deposition, or covalent bonding before preparing the polyamide selective layer by interfacial polymerization, and then carry out interfacial polymerization to prepare a polyamide composite nanofiltration membrane with an interlayer structure. Research shows that the introduction of interlayer materials can increase the adsorption of amine monomers by the substrate, control the release rate of amine monomers during the interfacial polymerization reaction, thereby improving the conditions of the interfacial polymerization reaction and facilitating the formation of a polyamide separation layer with a higher degree of crosslinking; on the other hand, the introduction of the interlayer structure can optimize the water molecule transport channels, enabling the composite nanofiltration membrane with an interlayer structure to exhibit relatively excellent performance in terms of permeation flux. Interfacial polymerization is the most widely used method for preparing composite nanofiltration membranes. The composite nanofiltration membranes prepared by the interfacial polymerization method show high rejection performance for solutes with a molecular weight greater than 200 Da and multivalent ions due to their pore size range and surface charge properties, and can reduce energy consumption and save energy because the operating pressure required is relatively low.

[0005] The degree of crosslinking is of great significance to the permeability of nanofiltration membranes. An appropriate degree of crosslinking can ensure that the membrane material has specific pore sizes and pore structures, thereby achieving efficient substance separation effects; and a suitable degree of crosslinking helps to enhance the anti-fouling ability and chemical stability of the membrane material, extending the service life of the membrane; moreover, an appropriate degree of crosslinking can guarantee the separation efficiency of the membrane material for specific substances and achieve the goal of efficient recovery of target substances. If the degree of crosslinking is too high, the pore size of the membrane will become smaller and the permeability will decrease accordingly. On the contrary, if the degree of crosslinking is too low, the pore size of the membrane may be too large, thus affecting the separation effect; and when the degree of crosslinking is too low, the membrane material may swell and deform during use, affecting its stability; moreover, when the degree of crosslinking is too low, the separation effect of the membrane material on the target substance may also be unsatisfactory. It can be said that the degree of crosslinking of the separation layer has a very significant impact on the performance of solvent-resistant nanofiltration membranes, especially in terms of solvent resistance performance, but at present, there are not many reports on this aspect.

[0006] Therefore, it is of great research value to develop a polyamide nanofiltration membrane with a high degree of crosslinking and a certain flux. Its purpose is to solve the problem that polymer membrane materials are prone to swelling and further improve the separation performance of organic solvent nanofiltration. Summary of the Invention

[0007] Aiming at the problems in the prior art, the present invention provides a preparation method of a polyamide solvent-resistant nanofiltration membrane with a high degree of crosslinking, which can solve the problems such as easy swelling, poor stability, and complex preparation process existing in conventional nanofiltration membranes.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions:

[0009] A preparation method of a highly cross-linked polyamide solvent-resistant nanofiltration membrane is characterized as follows:

[0010] (1) The nanofiltration membrane comprises a support layer, an intermediate layer, and a polyamide separation layer;

[0011] (2) The support layer is a hydrolyzed polyacrylonitrile ultrafiltration membrane substrate;

[0012] (3) The intermediate layer is obtained by in-situ growth method. The ferric chloride solution is coated on the surface of the carrier, and a ferric hydroxide oxide film layer is formed at a specific temperature;

[0013] (4) First, the upper surface of the obtained ferric hydroxide oxide composite membrane is immersed in an aqueous solution, and then immersed in an organic phase solution to carry out an interfacial polymerization reaction to prepare the separation layer. Finally, the membrane is placed in an oven for heating and curing to obtain a highly cross-linked polyamide solvent-resistant nanofiltration membrane.

[0014] The alkali modification of polyacrylonitrile, that is, the thermal alkali treatment of polyacrylonitrile, aims to form a cross-linked structure connected by N atoms through hydrolysis to enhance the solvent resistance.

[0015] In the in-situ growth stage, the hydrolyzed polyacrylonitrile membrane is immersed in a ferric chloride / hydrochloric acid solution, so that ferric ions are adsorbed on the membrane surface and in the membrane pores. The ferric ions are hydrolyzed in the aqueous solution to form ferric hydroxide, and then further nucleate and grow in the growth solution prepared with ferric trichloride and hydrochloric acid. Finally, a ferric hydroxide oxide nanoparticle layer is grown on the surface of the polyvinylidene fluoride membrane. In the interfacial polymerization stage, due to the presence of the ferric hydroxide oxide nanoparticle layer, the separation layer obtained after the interfacial polymerization reaction of the aqueous monomer and the organic monomer can stably adhere to the surface of the polyacrylonitrile membrane, and at the same time, the permeation performance of the nanofiltration membrane is increased.

[0016] Preferably, in step (2), the hydrolysis conditions are that the concentration of the potassium hydroxide solution is 1-2 mol / L, the temperature is 60-100 °C, and the hydrolysis time is 30-60 min.

[0017] Preferably, in step (3), the mass concentration of the ferric chloride solution is 0.5-1.5%, the temperature is 60-80 °C, and the in-situ growth time is 8-16 h.

[0018] During the in-situ growth stage, the mass concentration of the ferric chloride solution is 0.5-1.5%, the in-situ growth temperature is 60-80°C, and the in-situ growth time is 8-16h. The reason is that when the mass concentration of the ferric chloride solution is lower than 0.5%, the growth temperature is lower than 60°C, and the growth time is lower than 8h during the in-situ growth stage, it is not conducive to the nucleation and growth of iron oxyhydroxide crystals. When the mass concentration of the ferric chloride solution is higher than 1.5%, the growth temperature is higher than 80°C, and the growth time is higher than 16h during the in-situ growth stage, it will lead to the aggregation and accumulation of iron oxyhydroxide nanoparticles. At the same time, the higher growth temperature also causes waste of energy. It should be noted that the mass concentration of the ferric chloride solution during the in-situ growth stage can be 0.7%, 0.9%, 1.1%, 1.3% or any point value within the above range. The in-situ growth temperature can also be 65°C, 70°C, 75°C or any point value within this range. And the in-situ growth time can be 10 hours, 12 hours or any value within the above interval. Those skilled in the art can make corresponding adjustments to these parameters according to the actual reaction conditions.

[0019] Preferably, during the interfacial polymerization process, the mass concentration of the aqueous phase is 0.1-2%, the soaking time of the aqueous phase is 2-20 minutes, the mass concentration of the organic phase is 0.01-0.2%, the soaking time of the organic phase is 1-5 minutes, and the heating and curing time is 1-10 minutes.

[0020] There is a reason for limiting the mass concentrations of the aqueous phase and the organic phase. Because only when the aqueous phase monomer and the organic phase monomer are within the above suitable ranges, the two will undergo an interfacial polymerization reaction, and then a complete, defect-free and appropriately thick separation layer can be obtained. If the aqueous phase and the organic phase are too much or too little, it will lead to an overly thick separation layer or defects on the surface of the separation layer, which will reduce the performance of the nanofiltration membrane. It should be understood that the concentrations of the aqueous phase and the organic phase can also take any point value within the above range, and those skilled in the art can make corresponding adjustments according to different actual reaction conditions.

[0021] Preferably, the aqueous phase solute is any one of m-phenylenediamine, piperazine, melamine, TAPA, TAPB, TAPPE; the organic phase solute is any one of trimesoyl chloride, 2-chlorobenzoyl chloride, 2,3,4,5-tetrafluorobenzoyl chloride, and the solvent is n-hexane.

[0022] For water-insoluble aqueous phase monomers, 1-2 drops of glacial acetic acid can be added to dissolve them.

[0023] The selection of the aqueous phase monomer is not limited to those listed in the above embodiments. Based on their own common sense, those skilled in the art can make reasonable selections and adjustments within the scope of the art, and it is also feasible to use other substances as the aqueous phase monomer.

[0024] Due to the above technical solutions, the present invention has the following beneficial effects:

[0025] 1. The present invention provides a highly cross-linked polyamide solvent-resistant nanofiltration membrane prepared by in-situ growth method and interfacial polymerization. This preparation process has the advantages of simple operation, high efficiency, and easy scale-up.

[0026] 2. Due to the presence of the iron oxide hydroxide nanoparticle layer, the separation layer after interfacial polymerization can be stably attached to the surface of the polyacrylonitrile membrane in the polyamide solvent-resistant nanofiltration membrane prepared by the above method, avoiding the shedding of the separation layer. At the same time, the natural hydrophilicity of the iron oxide hydroxide nanoparticle layer greatly improves the flux of the polyamide solvent-resistant nanofiltration membrane. The iron oxide hydroxide nanoparticle layer can disperse the operating stress, reduce the damage to the membrane structure caused by repeated swelling-shrinking cycles, and extend the service life.

[0027] 3. For the polyamide solvent-resistant nanofiltration membrane finally obtained by the above method, its cross-linking degree is controllable. The high cross-linking degree can reduce the swelling or dissolution of polymer chains in solvents (such as resisting the erosion of strongly polar solvents), expand the application range of the membrane in harsh chemical environments (such as the treatment of industrial wastewater containing organic solvents), and provide an effective method guidance for the preparation of solvent-resistant nanofiltration membranes.

[0028] The present invention uses the interfacial polymerization method to prepare the membrane. This method has a simple and mature process, mild and green conditions, and can effectively avoid the dangers and environmental hazards in polymer synthesis. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The following further describes the present invention according to the drawings.

[0030] Figure 1 It is a schematic diagram for the preparation of the polyamide solvent-resistant nanofiltration membrane.

[0031] Figure 2 It is a schematic diagram of the structures of various aqueous monomers.

[0032] Figure 3 It is a schematic diagram of the reaction of piperazine and trimesoyl chloride in Example 1.

[0033] Figure 4 It is a schematic diagram of the reaction of TAPA and trimesoyl chloride in Example 3.

[0034] Figure 5 It is an electron micrograph of the hydrolyzed polyacrylonitrile ultrafiltration membrane prepared in Example 1.

[0035] Figure 6 It is an electron micrograph of the surface of the polyacrylonitrile nanofiltration membrane loaded with an iron oxide hydroxide layer prepared in Example 1.

[0036] Figure 7 It is a front electron micrograph of the polyacrylonitrile nanofiltration membrane prepared in Example 1. Detailed implementation mode

[0037] To make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below.

[0038] See the appendix Figure 1 In the present invention, polyacrylonitrile (PAN) is used as the base membrane, and an iron oxyhydroxide (FeOOH) intermediate layer is prepared by in-situ hydrolysis. Then, a polyamide separation layer is synthesized by reacting a polyamine monomer with a highly active acyl chloride monomer, and finally a polyamide solvent-resistant composite nanofiltration membrane with adjustable crosslinking degree is prepared.

[0039] Example 1

[0040] The preparation steps of the potassium hydroxide solution are as follows: Take 42 g of potassium hydroxide, place it in 500 ml of deionized water and dissolve it, stirring continuously until it is uniform. After the potassium hydroxide is completely dissolved, the resulting solution is properly sealed for subsequent use.

[0041] The preparation method of the polyacrylonitrile hydrolyzed membrane is as follows: First, cut a polyacrylonitrile base membrane with a certain thickness to make it of appropriate size. Then, immerse the cut polyacrylonitrile base membrane in the potassium hydroxide solution prepared above. Next, put it into an oven at 60 °C and let it stand for 30 minutes, and then take it out. The taken-out membrane is repeatedly rinsed with a large amount of deionized water, and the number of rinsing times is controlled at 3-5 times. Finally, let it dry naturally at room temperature, and thus a polyacrylonitrile hydrolyzed membrane can be obtained.

[0042] The preparation steps of the in-situ growth solution are as follows: First, dissolve 9 g of ferric chloride hexahydrate in 500 ml of deionized water, and then perform ultrasonic treatment for 20 min. In addition, take 1.5 ml of concentrated hydrochloric acid and add it to 250 ml of deionized water, stir and mix until it is in a uniform state. Then mix the obtained ferric chloride solution with the dilute hydrochloric acid, and then perform ultrasonic treatment for 20 min again to make it disperse evenly.

[0043] The preparation process of the iron oxyhydroxide layer is as follows: Immerse the prepared polyacrylonitrile hydrolyzed membrane in the in-situ growth solution, then place it in an oven and carry out mineralization treatment at 80 °C, and take it out after 12 hours. The taken-out membrane is rinsed three times with deionized water, and finally dried naturally at room temperature, so that a polyacrylonitrile hydrolyzed membrane loaded with an iron oxyhydroxide layer can be obtained. The electron micrograph of the hydrolyzed polyacrylonitrile ultrafiltration membrane is as Figure 5 shown.

[0044] The preparation method of the aqueous solution is as follows: Take 0.5 g of piperazine, dissolve it in 100 ml of deionized water, and then perform ultrasonic operation for 15 min, so that an aqueous solution can be obtained.

[0045] The preparation steps of the oil-phase solution are as follows: Take 0.1 g of trimesoyl chloride and dissolve it in 100 ml of n-hexane, and then perform ultrasonic treatment for 20 min. After these operations, the oil-phase solution is obtained.

[0046] The preparation process of the polyamide nanofiltration membrane is as follows: First, take a polyacrylonitrile membrane with a hydroxyl iron oxide layer loaded on its surface, and immerse the upper part of it in the aqueous solution obtained previously for 10 min. Then, immerse the front side of the membrane in a 0.2% trimesoyl chloride - n-hexane solution for 2 min to carry out an interfacial polymerization reaction. Finally, place this membrane in an oven for 5 min of heating and curing operation, and thus the polyamide nanofiltration membrane is obtained. Figure 3 The reaction schematic diagram of piperazine and trimesoyl chloride is shown. Figure 6 It is the electron micrograph of the surface of the prepared polyacrylonitrile nanofiltration membrane loaded with a hydroxyl iron oxide layer. The electron micrograph of this polyamide nanofiltration membrane is as Figure 7 shown, and its ethanol flux for Congo red is 14 L / (m 2 ·h·bar), and the rejection rate reaches 95%.

[0047] Example 2

[0048] Preparation of potassium hydroxide solution: The same as in Example 1.

[0049] Preparation of polyacrylonitrile hydrolyzed membrane: The same as in Example 1.

[0050] Preparation of in-situ growth solution: The same as in Example 1.

[0051] Preparation of hydroxyl iron oxide layer: The same as in Example 1.

[0052] Preparation of aqueous solution: Dissolve 0.5 g of m-phenylenediamine in 100 ml of deionized water and ultrasonicate for 15 min to obtain the aqueous solution.

[0053] Preparation of oil-phase solution: The same as in Example 1.

[0054] Preparation of polyamide nanofiltration membrane: The same as in Example 1, except that the aqueous monomer used is m-phenylenediamine. For the obtained polyamide nanofiltration membrane, its ethanol flux for Congo red is 13 L / (m 2 ·h·bar), and the rejection rate is 96%.

[0055] Example 3

[0056] Preparation of potassium hydroxide solution: The same as in Example 1.

[0057] Preparation of polyacrylonitrile hydrolyzed membrane: The same as in Example 1.

[0058] Configuration of in-situ growth solution: The same as in Example 1.

[0059] Preparation of iron oxyhydroxide layer: The same as in Example 1.

[0060] Preparation of aqueous solution: Dissolve 0.5 g of TAPA in 100 ml of deionized water and sonicate for 15 min to obtain an aqueous solution.

[0061] Preparation of oil-phase solution: The same as in Example 1.

[0062] Preparation of polyamide nanofiltration membrane: The same as in Example 1, except that the aqueous monomer used is TAPA. The obtained polyamide nanofiltration membrane shows the reaction schematic diagram of TAPA and trimesoyl chloride. The ethanol flux of Congo red through the polyamide nanofiltration membrane obtained in this example is 11 L / (m Figure 3 ·h·bar), and the rejection rate is 98%. 2 ·h·bar), and the rejection rate is 98%.

[0063] Example 4

[0064] Configuration of potassium hydroxide solution: The same as in Example 1.

[0065] Preparation of hydrolyzed polyacrylonitrile membrane: The same as in Example 1.

[0066] Configuration of in-situ growth solution: The same as in Example 1.

[0067] Preparation of iron oxyhydroxide layer: The same as in Example 1.

[0068] Preparation of aqueous solution: Dissolve 0.5 g of TAPPE in 100 ml of deionized water and sonicate for 15 min to obtain an aqueous solution.

[0069] Preparation of oil-phase solution: The same as in Example 1.

[0070] Preparation of polyamide nanofiltration membrane: The same as in Example 1, except that the aqueous monomer used is TAPPE. The ethanol flux of Congo red through the obtained polyamide nanofiltration membrane is 10 L / (m 2 ·h·bar), and the rejection rate is 99%.

[0071] For the above examples, the other test results are shown in the following table:

[0072] Test solution: All are ethanol solutions of Congo red.

[0073] Flux unit: L / (m 2 ·h·bar).

[0074] Table 1

[0075]

[0076] Table 2

[0077]

[0078] Based on the above embodiments, the advantages of the present invention are as follows: (1) For the iron oxyhydroxide intermediate layer obtained by hydrolysis, the thickness and hydrophilicity of the intermediate layer can be reasonably regulated through hydrolysis process parameters, making the aqueous monomer easier to spread, reducing the thickness of the separation layer, and improving the nanofiltration membrane flux; (2) Polyamine monomers such as tris(4-aminophenyl)amine (TAPA) and N,N,N',N'-tetrakis(p-aminophenyl)-p-phenylenediamine (TAPPE) can form a separation layer with a higher crosslinking degree and more excellent solvent resistance through interfacial polymerization reaction with trimesoyl chloride (TMC), compared with commercial difunctional monomers (such as m-phenylenediamine and piperazine); (3) The prepared solvent-resistant nanofiltration membrane has good solvent resistance and can stably remove small molecule pollutants (with molecular weights between 200 and 1000 Da) in solvents such as ethanol, acetone, and tetrahydrofuran, showing a broad application prospect.

[0079] The present invention verifies the technical effects achieved by the technical solution of the present invention through the above embodiments. It can be clearly seen that these described embodiments are only a part of the embodiments of the present invention, rather than all embodiments, and do not cover all choices of the technical solution. For all raw material selections, concentration, temperature, time and other process conditions defined by the technical solution of the present invention, although not described through embodiments, have the same technical effects as the shown embodiments. It should be further noted that according to the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts are within the protection scope of the present invention.

Claims

1. A method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane, Its features are as follows: (1) The organic solvent resistant nanofiltration membrane comprises a support layer, an intermediate layer and a polyamide separation layer; (2) The support layer is a hydrolyzed polyacrylonitrile ultrafiltration membrane base membrane; (3) The intermediate layer is obtained by an in-situ growth method, where a ferric chloride solution is coated on the surface of the carrier to form an iron oxyhydroxide film at a specific temperature; (4) The upper surface of the obtained iron oxyhydroxide composite membrane is first immersed in an aqueous solution and then immersed in an organic solution to perform an interfacial polymerization reaction to prepare a polyamide separation layer. Finally, the membrane is placed in an oven for heating and curing to obtain a highly cross-linked polyamide solvent-resistant nanofiltration membrane.

2. The method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane according to claim 1, characterized in that: The hydrolysis conditions in step (2) are: the concentration of the potassium hydroxide solution is 1-2 mol / L, the temperature is 60-100° C., and the hydrolysis time is 30-60 min.

3. The method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane according to claim 1, characterized in that: In step (3), the mass concentration of the ferric chloride solution is 0.5-1.5%, the temperature is 60-80° C., and the in-situ growth time is 8-16 hours.

4. The method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane according to claim 1, characterized in that: In the interfacial polymerization process in step (4), the mass concentration of the water phase is 0.1-2%, the immersion time of the water phase is 2-20 minutes, the mass concentration of the organic phase is 0.01-0.2%, the immersion time of the organic phase is 1-5 minutes, and the heating curing time is 1-10 minutes.

5. The method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane according to claim 1, characterized in that: In step (4), the aqueous phase solute is any one of piperazine, m-phenylenediamine, melamine, TAPA, TAPB, and TAPPE; the organic phase solute is any one of trimesoyl chloride, 2-chlorobenzoyl chloride, and 2,3,4,5-tetrafluorobenzoyl chloride; and the solvent is n-hexane.

6. The method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane according to claim 1, characterized in that: The in-situ growth solution is obtained by the following configuration method: dissolving ferric chloride in deionized water, followed by ultrasonic treatment; the aqueous phase solution is obtained by the following configuration method: dissolving an aqueous phase solute in deionized water, followed by ultrasonic treatment; The oil phase solution is obtained by the following preparation method: dissolving the organic phase solute in the organic solvent and then performing ultrasonic treatment.

7. The method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane according to claim 6, characterized in that: The preparation of the aqueous solution includes a step of adding glacial acetic acid.

8. The method for preparing a highly cross-linked polyamide solvent-resistant nanofiltration membrane according to claim 3, characterized in that: The mass concentration of the ferric chloride solution can be 0.7%, 0.9%, 1.1% or 1.3%; the temperature is 65° C., 70° C. or 75° C.; and the in-situ growth time is 10 hours or 12 hours.

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