Covalent organic framework membrane based on regulation and control of multifunctional catalyst as well as preparation method and application of covalent organic framework membrane

Abstract

The invention discloses a covalent organic framework membrane based on regulation and control of a multifunctional catalyst, the membrane comprises a functional layer and a base membrane, and the functional layer is composed of covalent organic framework nanosheets and cellulose nanomaterials. The preparation process comprises the following steps: synthesizing covalent organic framework nanosheet dispersion liquid from two monomers by a water-oil phase transfer polymerization method under the regulation and control of a multifunctional catalyst, blending and dispersing the nanosheet dispersion liquid and cellulose nanomaterial dispersion liquid in a volume ratio of (10-20): 1, and carrying out suction filtration on the surface of a base membrane through a vacuum-assisted layer-by-layer self-assembly process. The synthesized nanosheet has high radius-thickness ratio and crystallinity due to multiple regulation and control effects of the multifunctional catalyst, and a compact and defect-free covalent organic framework film can be assembled. The method is simple and easy to operate, and has good universality and repeatability. The covalent organic framework membrane is used for pervaporation seawater desalination, and shows excellent performance and long-period operation stability.

Description

Technical Field

[0001] The invention relates to the field of membrane separation, and in particular to a method for preparing a covalent organic framework membrane regulated by a multifunctional catalyst and its application. Background Art

[0002] With the growth of global population and economic and social development, the shortage of freshwater resources has become one of the major challenges facing contemporary society. Although about 70% of the earth's surface is covered by water, 97.5% of it is salt water, and freshwater resources account for only about 2.5%, and freshwater that can be directly used by humans is even more scarce. In order to alleviate the water crisis, in addition to saving water and recycling wastewater, desalination of abundant salt water resources (such as seawater) has become the first choice for developing new water resources. Seawater desalination technology is mainly divided into two categories: thermal method and membrane method. Traditional thermal methods (such as multi-stage flash evaporation and multi-effect distillation) use thermal energy to drive brine phase change to achieve separation, but high energy consumption and high cost limit its wide application. Reverse osmosis technology in the membrane method uses a semipermeable membrane to separate water molecules and salt ions, with low energy consumption and high desalination efficiency. However, under high salt concentration conditions, reverse osmosis technology needs to overcome extremely high osmotic pressure, resulting in a significant increase in operating costs, which limits its application in high-concentration brine treatment. The thermal-membrane coupling technology that has emerged in recent years combines the advantages of both, showing comprehensive advantages in energy consumption and adaptability. Among them, pervaporation technology has attracted more and more attention due to its high selectivity and wide adaptability. Pervaporation technology uses the chemical potential difference between salt ions and water in the feed liquid as the driving force, and uses hydrophilic membrane as the medium. It has the advantages of ultra-high salt retention rate (>99%), strong anti-pollution ability and strong universality, showing good application prospects.

[0003] High-performance membrane materials are the core of membrane technology. Covalent Organic Framework (COF) materials have become ideal candidate materials for high-performance seawater desalination membranes due to their regular and orderly pore structure, high porosity and strong designability, but their insoluble and infusible properties lead to poor processability. COF nanosheets prepared by common methods currently have small lateral dimensions and low diameter-to-thickness ratios. They are prone to disordered stacking during the assembly process, resulting in tortuous mass transfer channels and difficulty in reducing the generation of boundary defects, which is not conducive to the preparation of dense and defect-free COF membranes. Larger-sized highly crystalline COF nanosheets are ideal building blocks for the preparation of high-performance COF membranes. Structural defects can be reduced by stacking layers during the assembly process. High crystallinity helps maintain long-range ordered mass transfer channels in the membrane and reduces the mass transfer resistance of water molecules. Summary of the invention

[0004] In view of the prior art, in order to improve the processability of COF and give full play to its advantages in the separation process, the present invention provides a covalent organic framework membrane based on multifunctional catalyst regulation. By introducing a multifunctional catalyst in the COF synthesis process, the multifunctional catalyst is simultaneously used to promote the reaction, reduce nanosheet defects and inhibit the accumulation of nanosheet layers, and a high aspect ratio and high crystallinity COF nanosheet is prepared for membrane preparation. This method expands the diversity of COF material synthesis and is expected to inspire the design of advanced catalysts to synthesize COF materials and other crystalline framework materials. The membrane is composed of a covalent organic framework functional layer and a base membrane, and its preparation is simple and controllable. The prepared COF membrane is used for pervaporation seawater desalination and has high permeability, high selectivity and high stability.

[0005] In order to solve the above technical problems, the present invention proposes a covalent organic framework membrane based on multifunctional catalyst regulation, which is composed of a COF functional layer on the top and a base membrane on the bottom, wherein the COF functional layer includes COF nanosheets and cellulose nanomaterials, wherein the COF nanosheets are synthesized from hydroxyl monomers and fluorine-containing monomers by a water-oil phase transfer polymerization method under the regulation of a multifunctional catalyst, and then mixed with the cellulose nanomaterials in proportion to form a dispersion, and the dispersion is filtered onto the base membrane through a vacuum-assisted self-assembly process to obtain a covalent organic framework membrane.

[0006] The multifunctional catalyst is an aliphatic small molecule amine having multiple primary amine groups and secondary amine groups; the hydroxyl monomer is hexahydroxytriphenyl, and the fluorine-containing monomer is a fluorine-containing aromatic nitrile; during the synthesis of COF nanosheets, the multifunctional catalyst deprotonates the hydroxyl monomer and reacts with the fluorine-containing monomer to promote the growth of COF nanosheets; the primary amine group of the multifunctional catalyst reacts with the hydroxyl monomer to connect the unreacted sites of the hydroxyl monomer in the generated nanosheets, thereby reducing defects in the nanosheets; the multifunctional catalyst inhibits the interlayer π-π stacking of the nanosheets through its own charging effect, thereby synthesizing nanosheets with high aspect ratio and high crystallinity.

[0007] The preparation method of the above-mentioned covalent organic framework membrane mainly includes two steps: COF nanosheet synthesis regulated by a multifunctional catalyst and vacuum-assisted layer-by-layer self-assembly. The specific steps are as follows:

[0008] Step 1, synthesis of COF nanosheets regulated by a multifunctional catalyst, comprising: dissolving a certain mass of the multifunctional catalyst in deionized water, adding hexahydroxytribenzene, wherein the hexahydroxytribenzene concentration is 0.0015 mmol / mL, the equivalent is 1, and the multifunctional catalyst is added with an equivalent of 200 to the total number of amine sites in the small molecule amine; fully ultrasonically dispersing to form a catalyst-aqueous phase monomer solution; dissolving a fluorinated aromatic nitrile monomer in o-dichlorobenzene at a concentration of 0.002 mmol / mL to form an oil phase monomer solution; first adding a certain volume of the oil phase monomer solution to a reaction bottle, then slowly and evenly dropping the same volume of the catalyst-aqueous phase monomer solution drop by drop on the oil phase, and sealing the reaction bottle; placing the above system in an oven at 80-90° C. for reaction for 72-144 hours, the aqueous phase is dark brown and is collected with a dropper, and the collected aqueous phase solution is dialyzed in deionized water for 24-72 hours to obtain a COF nanosheet dispersion;

[0009] Step 2, preparation of COF membrane based on multifunctional catalyst regulation, comprising: configuring cellulose nanomaterial into a 0.5 mg / mL dispersion, taking the COF nanosheet dispersion and cellulose nanomaterial dispersion obtained in step 1 at a volume ratio of 10 to 20:1, diluting with deionized water, ultrasonically treating for 10 minutes to obtain a casting liquid, filtering the casting liquid onto the base membrane by a vacuum-assisted self-assembly method, and obtaining a covalent organic framework membrane after drying.

[0010] Furthermore, the preparation method of the present invention, wherein:

[0011] In the synthesis of COF nanosheets in step 1, the aliphatic small molecule amine having multiple primary amine groups and secondary amine groups includes one of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, spermine and spermidine; and the dialysis uses a dialysis bag with a molecular weight cutoff of 50,000.

[0012] In the preparation of the covalent organic framework membrane in step 2, the cellulose nanomaterial is one of cellulose nanofibers, cellulose nanowhiskers, sulfonated cellulose nanofibers, sulfonated cellulose nanowhiskers, carboxylated cellulose nanofibers, and carboxylated cellulose nanowhiskers; the base membrane is one of a hydrophilic polytetrafluoroethylene membrane, a polyacrylonitrile ultrafiltration membrane, and a carboxylated polyacrylonitrile ultrafiltration membrane; and the drying condition is drying at room temperature for 12 hours.

[0013] Compared with the prior art, the beneficial effects of the present invention are: the introduction of multifunctional catalysts plays multiple roles to regulate the COF synthesis process, improve the processability, synthesize COF nanosheets with high aspect ratio and high crystallinity, and thus prepare high-quality seawater desalination membranes. Specifically, the multifunctional catalyst promotes the occurrence of aromatic nucleophilic substitution reaction by deprotonating the aqueous phase monomers, thereby realizing the in-plane polymerization of COF; the primary amine groups it has can be connected with the unreacted monomer residues in the nanosheets, thereby reducing the defects of the nanosheets; and the secondary amines in its chain can weaken the out-of-plane π-π stacking of the nanosheets through the charged repulsion effect, maintain the nanosheet morphology, thereby realizing the synthesis of nanosheets with high aspect ratio and high crystallinity, which is conducive to the preparation of thin and dense high-performance COF membranes. The COF membrane preparation method of the present invention is simple, the process is controllable and has universal applicability. The covalent organic framework membrane prepared by the present invention is used for pervaporation seawater desalination. When the feed temperature is 40°C and the NaCl liquid concentration is 3.5wt%, the permeation flux is 138.74-198.35kg m -2 h -1 The sodium chloride ion rejection rate is 99.95% to 99.99%. When treating 50°C, 3.5wt% feed liquid, the separation performance remains stable within 120h. Figure 6 shown. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a surface electron microscope image of the film 1 obtained in Example 1.

[0015] Figure 2 This is a surface electron microscope image of the film 2 obtained in Example 2.

[0016] Figure 3 This is a surface electron microscope image of the film 3 obtained in Example 3.

[0017] Figure 4 This is the surface electron microscope image of the membrane 4 obtained in Comparative Example 1.

[0018] Figure 5 is a comparison chart of permeation flux and salt rejection of the membranes of Examples 1-3 and the comparative example membrane in pervaporation seawater desalination;

[0019] Figure 6 This is a schematic diagram of the long-term operation stability of membrane 2 obtained in Example 2. DETAILED DESCRIPTION

[0020] The present invention proposes a covalent organic framework membrane based on multifunctional catalyst regulation. The design concept is: using multifunctional catalyst to regulate the synthesis of covalent organic framework nanosheets with high aspect ratio and high crystallinity, giving the material good processability and film-forming properties, and then assembling the nanosheets and auxiliary cellulose nanomaterials on a polytetrafluoroethylene-based membrane by vacuum-assisted self-assembly to obtain a covalent organic framework membrane. Since the nanosheets have high crystallinity and high aspect ratio, they are helpful to assemble into dense and defect-free membranes. The vertically penetrating channels in the membrane reduce the mass transfer resistance of water molecules in the separation process, and the introduction of multifunctional catalysts enhances the hydrophilicity of the membrane, achieving high permeation flux in pervaporation seawater desalination. In addition, the performance of the membrane can be further optimized by adjusting conditions such as monomer type, monomer concentration, catalyst concentration, reaction time, etc.

[0021] The covalent organic framework membrane is composed of a COF functional layer on the top and a base membrane on the bottom. The COF functional layer includes COF nanosheets and cellulose nanomaterials. The COF nanosheets are synthesized from hydroxyl monomers and fluorine-containing monomers through a water-oil phase transfer polymerization method under the regulation of a multifunctional catalyst, and then mixed with the cellulose nanomaterials in proportion to form a dispersion. The dispersion is filtered onto the base membrane through a vacuum-assisted self-assembly process to obtain the covalent organic framework membrane.

[0022] During the synthesis of COF nanosheets, the multifunctional catalyst deprotonates the hydroxyl monomer and reacts with the fluorine-containing monomer to promote the growth of COF nanosheets; the multiple primary amine groups of the multifunctional catalyst react with the base monomer to connect the unreacted sites of the hydroxyl monomer in the generated nanosheets, thereby reducing defects in the nanosheets; the multifunctional catalyst inhibits the interlayer π-π stacking of the nanosheets through its own charging effect, thereby synthesizing nanosheets with a high aspect ratio and high crystallinity.

[0023] In the present invention, the hydroxy monomer is hexahydroxytriphenyl, and the fluorine-containing monomer is fluorine-containing aromatic nitrile.

[0024] The multifunctional catalyst is an aliphatic small molecule amine having multiple primary amine groups and secondary amine groups, and the aliphatic small molecule amine having multiple primary amine groups and secondary amine groups includes but is not limited to one of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, spermine and spermidine.

[0025] The cellulose nanomaterial is one of cellulose nanofiber, cellulose nanowhisker, sulfonated cellulose nanofiber, sulfonated cellulose nanowhisker, carboxylated cellulose nanofiber and carboxylated cellulose nanowhisker.

[0026] The base membrane is one of a hydrophilic polytetrafluoroethylene membrane, a polyacrylonitrile ultrafiltration membrane and a carboxylated polyacrylonitrile ultrafiltration membrane.

[0027] The technical solution of the present invention is further described in detail below in conjunction with specific examples and appendices. The specific implementation examples described are only used to explain the present invention and are not intended to limit the present invention.

[0028] Example 1: Preparation of a covalent organic framework membrane regulated by a multifunctional catalyst, the steps are as follows:

[0029] Step 1: Synthesis of COF nanosheets regulated by a multifunctional catalyst, including:

[0030] 1-1) Dissolve 424 mg of diethylenetriamine in 40 mL of deionized water, stir evenly, add 20 mg of 2,3,6,7,10,11-hexahydroxytriphenyl monomer, and ultrasonicate for 30 min to disperse evenly to form a diethylenetriamine-aqueous phase monomer solution.

[0031] 1-2) Dissolve 18.5 mg of tetrafluorophenylene dihydrazine monomer in 40 mL of o-dichlorobenzene and stir evenly to form an oil phase monomer solution.

[0032] 1-3) First, add the oil phase monomer solution prepared in step 1-2) into a 100 mL borosilicate wire-mouthed reagent bottle, then slowly and evenly drop the diethylenetriamine-water phase monomer solution prepared in step 1-1) on top of the oil phase, seal the borosilicate wire-mouthed reagent bottle, and place the above system in an oven at 80°C to react for 120 hours. The aqueous phase is dark brown and is collected with a dropper.

[0033] 1-4) The collected aqueous phase solution was dialyzed in deionized water for 24 h using a dialysis bag with a molecular weight cutoff of 50,000 to obtain a COF nanosheet dispersion.

[0034] Step 2: Preparation of COF membrane based on multifunctional catalyst regulation, including:

[0035] The sulfonated cellulose nanowhisker colloid was dispersed in deionized water to prepare a 0.5 mg / mL dispersion. 1 mL of the COF nanosheet dispersion obtained in step 1 and 100 μL of the sulfonated cellulose nanowhisker dispersion (volume ratio of 10:1) were dispersed in 20 mL of deionized water and ultrasonically treated for 10 min to obtain a casting solution. The casting solution was filtered onto a hydrophilic polytetrafluoroethylene-based membrane with a pore size of 0.22 μm by a vacuum-assisted self-assembly method and dried at room temperature for 12 hours to obtain a COF membrane based on multifunctional catalyst regulation. This membrane is denoted as membrane 1. The surface electron microscopy image of membrane 1 is shown in FIG. Figure 1 shown.

[0036] Membrane 1 was used for pervaporation desalination of seawater, treating 3.5 wt% NaCl solution at 40 °C with a permeate flux of 138.74 kg m -2 h -1 , the salt rejection rate is 99.96%, as shown in Table 1.

[0037] Example 2: Preparation of a covalent organic framework membrane regulated by a multifunctional catalyst, the steps are basically the same as those of Example 1, except that in step 1-1) in preparing a catalyst-aqueous phase monomer solution, the multifunctional catalyst used is changed from 424 mg of diethylenetriamine to 451 mg of triethylenetetramine, to prepare a triethylenetetramine-aqueous phase monomer solution; finally, a COF membrane regulated by a multifunctional catalyst is obtained. This membrane is referred to as membrane 2, and the surface electron microscope image of membrane 2 is shown in FIG. Figure 2 shown.

[0038] Membrane 2 was used for pervaporation desalination of seawater, treating 3.5 wt % NaCl solution at 40 °C with a permeate flux of 198.35 kg m -2 h -1 , the salt rejection rate is 99.98%, as shown in Table 1. Figure 6 It is shown that when membrane 2 treats a 50°C, 3.5 wt% feed solution, the separation performance remains stable within 120 hours, and has long-term operation stability.

[0039] Example 3: Preparation of a covalent organic framework membrane regulated by a multifunctional catalyst, the steps are basically the same as those of Example 1, except that in step 1-1) in preparing the catalyst-aqueous phase monomer solution, the multifunctional catalyst used is changed from 424 mg of diethylenetriamine to 467 mg of tetraethylenepentamine, to prepare a tetraethylenepentamine-aqueous phase monomer solution; and finally a COF membrane regulated by a multifunctional catalyst is obtained. This membrane is denoted as membrane 3, and the surface electron microscope image of membrane 3 is as follows Figure 3 shown.

[0040] Membrane 3 was used for pervaporation desalination of seawater, treating 3.5 wt% NaCl solution at 40 °C with a permeate flux of 171.79 kg m -2 h -1 , the salt rejection rate is 99.99%, as shown in Table 1.

[0041] Comparative Example: Preparation of a covalent organic framework membrane regulated by a common catalyst, the steps are as follows:

[0042] The preparation process is basically the same as that of Example 1, except that in step 1-1), diethylenetriamine is replaced with 1248 mg of triethylamine (which is an aliphatic small molecule amine without multiple primary amine groups and secondary amine groups) to prepare a triethylamine-aqueous phase monomer solution; the COF film finally obtained is recorded as a comparative film, and the surface electron microscopy image of the comparative film is as follows Figure 4 shown.

[0043] The seawater desalination performance and water contact angle data of the above-mentioned example membrane and comparative example membrane are shown in Table 1.

[0044] Table 1

[0045] membrane <![CDATA[Permeation flux (kg m -2 h -1 )]]> Salt rejection rate (%) Water contact angle (°) Membrane 1 138.74 99.95 72.19 Membrane 2 198.35 99.98 58.12 Membrane 3 171.79 99.99 47.73 Contrast film 68.42 99.53 75.70

[0046] By comparing the examples with the comparative examples, it can be seen that the COF membrane prepared by multifunctional catalyst regulation has good performance due to the reduction of structural defects, the maintenance of high crystallinity and the good construction of long-range ordered channels. On the basis of salt ion rejection rate of >99.9%, the permeation flux at low temperature can reach 198.35 kg m -2 h -1 In the comparative example, since it is difficult to synthesize nanosheet building units with high diameter-to-thickness ratio using common catalysts, the disordered stacking during membrane assembly leads to increased mass transfer resistance of water molecules, making it difficult to achieve good pervaporation seawater desalination performance. Figure 5 As shown. It can be seen that the covalent organic framework membrane based on multifunctional catalyst regulation proposed in the present invention has broad application potential.

[0047] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can make many improvements and changes without departing from the purpose of the present invention, which are all within the protection of the present invention.

Claims

1. A covalent organic framework membrane regulated by a multifunctional catalyst, characterized in that: The membrane is composed of a COF functional layer on the top and a base membrane on the bottom, wherein the COF functional layer includes COF nanosheets and cellulose nanomaterials, wherein the COF nanosheets are synthesized from hydroxyl monomers and fluorine-containing monomers by a water-oil phase transfer polymerization method under the control of a multifunctional catalyst, and then mixed with the cellulose nanomaterials in proportion to form a dispersion, and the dispersion is filtered onto the base membrane by a vacuum-assisted self-assembly process to obtain a covalent organic framework membrane; The multifunctional catalyst is an aliphatic small molecule amine having multiple primary amine groups and secondary amine groups; The hydroxyl monomer is hexahydroxytriphenyl, and the fluorine-containing monomer is fluorine-containing aromatic nitrile; During the synthesis of COF nanosheets, the multifunctional catalyst deprotonates the hydroxyl monomer and reacts with the fluorine-containing monomer to promote the growth of COF nanosheets; the primary amine group of the multifunctional catalyst reacts with the hydroxyl monomer to connect the unreacted sites of the hydroxyl monomer in the generated nanosheets, thereby reducing defects in the nanosheets; the multifunctional catalyst inhibits the interlayer π-π stacking of the nanosheets through its own charging effect, thereby synthesizing nanosheets with a high aspect ratio and high crystallinity.

2. A method for preparing a covalent organic framework membrane according to claim 1, characterized in that: The following steps are involved: Step 1: Synthesis of COF nanosheets, including: A certain amount of multifunctional catalyst is dissolved in deionized water, and hexahydroxytriphenylene is added, wherein the concentration of the hexahydroxytriphenylene is 0.0015 mmol / mL, the equivalent is 1, and the multifunctional catalyst is added with an equivalent of 200 total amine sites in the small molecule amine; and the catalyst-water phase monomer solution is formed by sufficient ultrasonic dispersion; Dissolving a fluorinated aromatic nitrile monomer in o-dichlorobenzene at a concentration of 0.002 mmol / mL to form an oil phase monomer solution; First, a certain volume of oil phase monomer solution is added to the reaction bottle, and then the same volume of catalyst-water phase monomer solution is slowly and evenly dripped on the oil phase, and the reaction bottle is sealed; the above system is placed in an oven at 80-90°C for reaction for 72-144 hours, and the water phase is dark brown and is collected with a dropper. The collected water phase solution is dialyzed in deionized water for 24-72 hours to obtain a COF nanosheet dispersion; Step 2: Preparation of a covalent organic framework membrane, comprising: The cellulose nanomaterial is configured into a 0.5 mg / mL dispersion, the COF nanosheet dispersion obtained in step 1 and the cellulose nanomaterial dispersion are taken in a volume ratio of 10 to 20:1, diluted with deionized water, and ultrasonically treated for 10 minutes to obtain a casting solution, and the casting solution is filtered onto the base membrane by a vacuum-assisted self-assembly method, and a covalent organic framework membrane is obtained after drying.

3. The preparation method according to claim 2, characterized in that: In step 1, the aliphatic small molecule amine having a plurality of primary amine groups and secondary amine groups includes one of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, spermine and spermidine.

4. The preparation method according to claim 2, characterized in that: In step 1, a dialysis bag with a molecular weight cut-off of 50,000 was used for dialysis.

5. The preparation method according to claim 2, characterized in that: In step 2, the cellulose nanomaterial is one of cellulose nanofiber, cellulose nanowhisker, sulfonated cellulose nanofiber, sulfonated cellulose nanowhisker, carboxylated cellulose nanofiber, and carboxylated cellulose nanowhisker.

6. The preparation method according to claim 2, characterized in that: In step 2, the base membrane is one of a hydrophilic polytetrafluoroethylene membrane, a polyacrylonitrile ultrafiltration membrane, and a carboxylated polyacrylonitrile ultrafiltration membrane.

7. The preparation method according to claim 2, characterized in that: In step 2, the drying condition is drying at room temperature for 12 hours.

8. An application of a covalent organic framework membrane, characterized in that: The covalent organic framework membrane prepared by the preparation method according to any one of claims 2 to 7 is used for pervaporation desalination of seawater. When the feed temperature is 40°C and the NaCl solution concentration is 3.5 wt%, the permeation flux is 138.74-198.35 kg m -2 h -1 The sodium chloride ion rejection rate is 99.95% to 99.99%. When treating a 50°C, 3.5wt% feed liquid, the separation performance remains stable within 120 hours.

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