Covalent organic framework membrane based on multifunctional catalyst regulation and preparation method and application thereof

Abstract

The application discloses a kind of covalent organic framework films based on multifunctional catalyst regulation, the film includes functional layer and base film, the functional layer is composed of covalent organic framework nanosheet and cellulose nanomaterial.Its preparation process includes that two kinds of monomers are synthesized covalent organic framework nanosheet dispersion under the regulation of multifunctional catalyst by water-oil phase transfer polymerization method, nanosheet dispersion and cellulose nanomaterial dispersion are blended and dispersed with the volume ratio of 10-20:1, and are filtered on the surface of base film by vacuum assisted layer-by-layer self-assembly process.The multiple regulation of multifunctional catalyst makes the synthesized nanosheet have high aspect ratio and crystallinity, which helps to be assembled into dense and defect-free covalent organic framework film.The application is simple and easy to operate, and has good universality and repeatability.The covalent organic framework film is used for pervaporation seawater desalination, and shows excellent performance and long-period running stability.

Description

TECHNICAL FIELD

[0001] The present application 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 applications thereof. BACKGROUND

[0002] With the growth of global population and the development of economic society, the shortage of fresh water resources has become one of the major challenges faced by contemporary society. Although about 70% of the earth's surface is covered by water, 97.5% of which is salt water, and only about 2.5% is fresh water, and the fresh water available for direct use by humans is even more scarce. In order to alleviate the water crisis, in addition to water saving and wastewater recycling, desalination of salt water resources (such as seawater) with abundant reserves has become the first choice for developing new water resources. Seawater desalination technology is mainly divided into thermal method and membrane method. The traditional thermal method (such as multi-stage flash evaporation and multi-effect distillation) uses heat energy to drive the phase change of salt water to realize separation, but the high energy consumption and high cost limit its wide application. Reverse osmosis technology in membrane method uses semi-permeable membrane to separate water molecules and salt ions, which has lower energy consumption and higher desalination efficiency. However, in the case of high salt concentration, reverse osmosis technology needs to overcome very high osmotic pressure, resulting in a substantial increase in operating cost, which limits its application in high-concentration salt water treatment. In recent years, the emerging thermal-membrane coupling technology combines the advantages of both, showing comprehensive advantages in energy consumption and adaptability. Among them, the 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 a hydrophilic membrane as the medium, which has ultra-high salt rejection rate (> 99%), strong anti-pollution ability and strong universality, showing good application prospect.

[0003] High-performance membrane materials are the core of membrane technology. Covalent organic framework (COF) materials have become an ideal candidate for high-performance seawater desalination membranes due to their regular and ordered pore structure, high porosity and strong designability. However, due to its insoluble and infusible characteristics, the processability of COF is poor. The commonly used method for preparing COF nanosheets has small lateral size and low aspect ratio, which is easy to cause disordered accumulation in the process of assembling into a membrane, 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. Large-size high-crystalline COF nanosheets are ideal building units for preparing high-performance COF membranes, which can reduce structural defects through layer-by-layer stacking in the assembly process, and high crystallinity helps to maintain long-range ordered mass transfer channels in the membrane and reduce the mass transfer resistance of water molecules. SUMMARY

[0004] In order to improve the processability of COF, give full play to its advantages in the separation process, the application provides a kind of covalent organic framework film based on multifunctional catalyst regulation. By introducing multifunctional catalyst in the synthesis process of COF, the effects of promoting reaction, reducing nanosheet defects and inhibiting nanosheet interlayer accumulation are exerted, and high aspect ratio and high crystallinity COF nanosheets are prepared for film 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 crystal framework materials. The film is composed of a covalent organic framework functional layer and a base film, and is simple and controllable to prepare. The prepared COF film has high permeability, high selectivity and high stability for pervaporation seawater desalination.

[0005] In order to solve the above technical problems, the application provides a kind of covalent organic framework film based on multifunctional catalyst regulation, which is composed of a COF functional layer at the top and a base film at the bottom. The COF functional layer includes COF nanosheets and cellulose nanomaterials. The COF nanosheets are synthesized by hydroxyl monomers and fluorine-containing monomers through water-oil phase transfer polymerization method under the regulation of multifunctional catalyst, and then mixed and matched with the cellulose nanomaterials in proportion to form a dispersion liquid. The dispersion liquid is vacuum assisted self-assembly process to filter the above dispersion liquid on the base film to obtain the covalent organic framework film.

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

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

[0008] Step one, synthesis of COF nanosheet regulated by multifunctional catalyst, comprising: dissolving a certain amount of multifunctional catalyst in deionized water, adding hexahydroxytriphenyl, wherein the concentration of the hexahydroxytriphenyl is 0.0015 mmol / mL, the equivalent is 1, and the multifunctional catalyst is added in an amount of 200 times the total amine site number in the small molecule amine; ultrasonic dispersion to form a catalyst-water phase monomer solution; dissolving fluorine-containing aromatic nitrile monomer in o-dichlorobenzene to form an oil phase monomer solution; first, a certain volume of oil phase monomer solution is added to the reaction bottle, then the same volume of catalyst-water phase monomer solution is slowly and uniformly dropped on the oil phase, and the reaction bottle is sealed; the reaction system is placed in an oven at 80-90 DEG C for 72-144 h, the water phase is dark brown and is collected with a dropper, and the collected water phase solution is dialyzed in deionized water for 24-72 h to obtain a COF nanosheet dispersion;

[0009] Step two, preparation of COF film based on multifunctional catalyst regulation, comprising: configuring cellulose nanomaterials into a dispersion liquid of 0.5 mg / mL, taking the COF nanosheet dispersion liquid and cellulose nanomaterial dispersion liquid obtained in step one according to a volume ratio of 10-20:1, diluting with deionized water, and ultrasonic treatment for 10 min to obtain a casting solution, and the casting solution is vacuum assisted self-assembly method is filtered to the base film, and the covalent organic framework film is obtained after drying.

[0010] Further, the preparation method of the application, wherein:

[0011] In the synthesis of COF nanosheet of step one, the aliphatic small molecule amine having multiple primary amine groups and secondary amine groups includes one of diethylene triamine, triethylene tetramine, tetraethylene pentamine, spermine and spermidine; the dialysis uses a dialysis bag with a molecular weight cut-off of 50000.

[0012] In the preparation of the covalent organic framework film of step two, the cellulose nanomaterial is one of cellulose nanofiber, cellulose nanowhisker, sulfonated cellulose nanofiber, sulfonated cellulose nanowhisker, carboxylated cellulose nanofiber and carboxylated cellulose nanowhisker; the base film is one of hydrophilic polytetrafluoroethylene membrane, polyacrylonitrile ultrafiltration membrane and carboxylated polyacrylonitrile ultrafiltration membrane; the drying condition is drying at room temperature for 12 h.

[0013] Compared with the prior art, the present application has the beneficial effects that: a multifunctional catalyst is introduced to play multiple roles in regulating the COF synthesis process, to improve the processability, and to synthesize COF nanosheets with high aspect ratio and high crystallinity, thereby preparing high-quality seawater desalination membranes. Specifically, the multifunctional catalyst deprotonates the water-phase monomers, promotes the occurrence of aromatic nucleophilic substitution reaction, and realizes the in-plane polymerization of COF; the primary amine groups in the multifunctional catalyst can connect with the monomer residues that are not completely reacted in the nanosheet, thereby reducing the defects of the nanosheet; and the secondary amine in the chain of the multifunctional catalyst can weaken the out-of-plane π-π stacking of the nanosheets through the electrostatic repulsion effect, thereby maintaining the morphology of the nanosheet, and 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 application is simple, controllable and universal. When the covalent organic framework membrane prepared by the present application is used for seawater desalination by pervaporation, the permeation flux is 138.74-198.35 kg m -2 h -1 , the sodium chloride salt ion rejection rate is 99.95%-99.99%, the treatment of 50℃, 3.5wt% of the feed liquid, the separation performance is stable within 120h, as shown in Figure 6 . BRIEF DESCRIPTION OF DRAWINGS

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

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

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

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

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

[0019] Figure 6 is a long-period running stability diagram of the membrane 2 obtained in Example 2. DETAILED DESCRIPTION

[0020] The application provides a covalent organic framework film based on multifunctional catalyst regulation, and the design concept is as follows: a multifunctional catalyst is used to regulate the synthesis of covalent organic framework nanosheets with high aspect ratio and high crystallinity, so that the material has good processability and film-forming property; then, the nanosheets and auxiliary cellulose nanomaterials are assembled on a polytetrafluoroethylene base film through a vacuum-assisted self-assembly method, so that the covalent organic framework film is obtained. Since the nanosheets have high crystallinity and high aspect ratio, the nanosheets are helpful to the assembly of a dense and defect-free film, the vertical channels in the film reduce the water mass transfer resistance in the separation process, and the introduction of the multifunctional catalyst enhances the hydrophilicity of the film, so that a high permeation flux in the process of pervaporation seawater desalination is realized. In addition, the performance of the film can be further optimized by adjusting the monomer type, monomer concentration, catalyst concentration, reaction time and the like.

[0021] The covalent organic framework film is composed of a COF functional layer at the top and a base film at the bottom, the COF functional layer comprises COF nanosheets and cellulose nanomaterials, the COF nanosheets are synthesized from a hydroxyl monomer and a fluorine-containing monomer through a water-oil phase transfer polymerization method under the regulation of a multifunctional catalyst, and then the COF nanosheets and the cellulose nanomaterials are proportionally blended and matched into a dispersion liquid through a vacuum-assisted self-assembly process, so that the covalent organic framework film is obtained.

[0022] The multifunctional catalyst deprotonates the hydroxyl monomer and reacts with the fluorine-containing monomer to promote the growth of the COF nanosheets in the process of synthesizing the COF nanosheets; the multifunctional catalyst has a plurality of primary amine groups which react with the base monomer to connect the unreacted sites of the hydroxyl monomer in the generated nanosheets, so as to reduce the defects in the nanosheets; and the multifunctional catalyst inhibits the interlayer π-π stacking of the nanosheets through self-charging, so that the nanosheets with high aspect ratio and high crystallinity are synthesized.

[0023] In the application, the hydroxyl monomer is hexahydroxytriphenyl, and the fluorine-containing monomer is a fluorine-containing aromatic nitrile.

[0024] The multifunctional catalyst is an aliphatic small molecule amine with a plurality of primary amine groups and secondary amine groups, and the aliphatic small molecule amine with a plurality of 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 nanofibers, cellulose nanowhiskers, sulfonated cellulose nanofibers, sulfonated cellulose nanowhiskers, carboxylated cellulose nanofibers and carboxylated cellulose nanowhiskers.

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

[0027] The technical solutions of the present application are further described in detail below in combination with specific examples and the accompanying tables. The specific examples described are only used to explain and illustrate the present application, and do not limit the present application.

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

[0029] Step one, synthesis of multifunctional catalyst-regulated COF nanosheet, including:

[0030] 1-1) Dissolve 424 mg of diethylenetriamine in 40 mL of deionized water, after stirring uniformly, add 20 mg of 2,3,6,7,10,11-hexahydroxytriphenyl monomer, ultrasonic for 30 min to make it disperse uniformly, form diethylenetriamine-aqueous monomer solution.

[0031] 1-2) Dissolve 18.5 mg of 2,4,6-trifluoroterephthalonitrile monomer in 40 mL of o-dichlorobenzene, stir uniformly to form an oil phase monomer solution.

[0032] 1-3) First, the oil phase monomer solution prepared in step 1-2) is added to a 100 mL high borosilicon neck reagent bottle, then the diethylenetriamine-aqueous monomer solution prepared in step 1-1) is slowly and uniformly dropped on the oil phase drop by drop, the high borosilicon neck reagent bottle is sealed, and the reaction system is placed in an 80℃ oven for reaction for 120 h, the aqueous phase is dark brown and is collected with a dropper.

[0033] 1-4) The collected aqueous solution is dialyzed in deionized water for 24 h with a dialysis bag with a molecular weight cut-off of 50000 to obtain a COF nanosheet dispersion.

[0034] Step two, preparation of multifunctional catalyst-regulated COF membrane, including:

[0035] The sulfonated cellulose nanowhisker colloid is dispersed in deionized water to prepare a dispersion with a concentration of 0.5 mg / mL. 1 mL of the COF nanosheet dispersion obtained in step one and 100 μL of the sulfonated cellulose nanowhisker dispersion (volume ratio of 10:1) are dispersed in 20 mL of deionized water, and ultrasonic treatment is performed for 10 min to obtain a casting solution. The casting solution is vacuum assisted self-assembly method is filtered onto a hydrophilic polytetrafluoroethylene membrane with a pore size of 0.22 μm, and dried at room temperature for 12 h to obtain a multifunctional catalyst-regulated COF membrane. The membrane is denoted as membrane 1, and the surface electron microscope image of membrane 1 is shown in Figure 1 .

[0036] The membrane 1 is used for pervaporation seawater desalination, and a 3.5wt% NaCl solution is treated at 40℃, the permeation flux is 138.74 kg m -2 h -1 , the salt rejection rate is 99.96%, as shown in Table 1.

[0037] Example 2: Preparation of multifunctional catalyst-regulated covalent organic framework membrane, basically the same as the steps of Example 1, the only difference is that in step 1-1) preparation of catalyst-aqueous monomer solution, the multifunctional catalyst used is changed from 424 mg of diethylenetriamine to 451 mg of triethylenetetramine, to prepare a triethylenetetramine-aqueous monomer solution; finally get multifunctional catalyst-regulated COF membrane. The membrane is recorded as membrane 2, the surface electron microscope image of membrane 2 is shown in Figure 2 .

[0038] Membrane 2 is used for seawater desalination by pervaporation, and a 3.5wt% NaCl solution is treated at 40℃, the permeation flux is 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 membrane 2 treats 50℃, 3.5wt% feed liquid, and the separation performance remains stable within 120h, with long-period operation stability.

[0039] Example 3: Preparation of multifunctional catalyst-regulated covalent organic framework membrane, basically the same as the steps of Example 1, the only difference is that in step 1-1) preparation of catalyst-aqueous monomer solution, the multifunctional catalyst used is changed from 424 mg of diethylenetriamine to 467 mg of tetraethylenepentamine, to prepare a tetraethylenepentamine-aqueous monomer solution; finally get multifunctional catalyst-regulated COF membrane. The membrane is recorded as membrane 3, the surface electron microscope image of membrane 3 is shown in Figure 3 .

[0040] Membrane 3 is used for seawater desalination by pervaporation, and a 3.5wt% NaCl solution is treated at 40℃, the permeation flux is 171.79 kg m -2 h -1 , the salt rejection rate is 99.99%, as shown in Table 1.

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

[0042] The preparation process is basically the same as that of Example 1, the only difference is that in step 1-1), diethylenetriamine is changed to 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 monomer solution; finally get the COF membrane recorded as comparative membrane, the surface electron microscope image of comparative membrane is shown in Figure 4 .

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

[0044] Table 1

[0045] Membrane Permeate flux (kg m -2 h -1 )]]> Salt rejection (%) 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 Comparative membrane 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 the 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. The membrane is dense and uniform, the permeation flux at low temperature can reach 198.35 kg m-2h-1 under the condition of salt ion rejection > 99.9%. -2 h -1 And the comparative examples are difficult to synthesize nanosheet building units with high aspect ratio by using ordinary catalysts, and the disordered stacking in the membrane assembly process increases the mass transfer resistance of water molecules, so it is difficult to achieve good performance of pervaporation seawater desalination, such as Figure 5 As shown in the above, it can be seen that the covalent organic framework membrane based on multifunctional catalyst regulation has broad application potential.

[0047] Although the present application has been described in conjunction with the accompanying drawings, the present application is not limited to the specific embodiments described above, and the specific embodiments described above are only illustrative but not restrictive. Those skilled in the art can make many improvements and changes under the inspiration of the present application without departing from the purpose of the present application, and these all belong to the protection of the present application.

Claims

1. A covalent organic framework membrane regulated by a multifunctional catalyst, characterized in that, The membrane consists of a top COF functional layer and a bottom base membrane. The COF functional layer includes COF nanosheets and cellulose nanomaterials. The COF nanosheets are synthesized by water-oil phase transfer polymerization of hydroxyl monomers and fluorine-containing monomers under the control of a multifunctional catalyst. They are then blended with the cellulose nanomaterials in a certain proportion to form a dispersion. The dispersion is then filtered onto the base membrane through a vacuum-assisted self-assembly process to obtain a covalent organic framework membrane. The multifunctional catalyst is an aliphatic small molecule amine with multiple primary 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 monomers and reacts them with fluorine-containing monomers, promoting the growth of COF nanosheets. The primary amine groups of the multifunctional catalyst react with the hydroxyl monomers, connecting unreacted sites of the hydroxyl monomers 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.

2. A method for preparing a covalent organic framework membrane according to claim 1, characterized in that, Includes the following steps: Step 1, synthesis of COF nanosheets, includes: A certain mass of multifunctional catalyst was dissolved in deionized water, and hexahydroxytriphenyl was added, wherein the concentration of hexahydroxytriphenyl was 0.0015 mmol / mL and the equivalent was 1. The multifunctional catalyst was added with the equivalent of 200 total amine sites in the small molecule amine. The catalyst-aqueous monomer solution was formed by thorough ultrasonic dispersion. A fluorinated aromatic nitrile monomer was dissolved 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 flask. Then, the same volume of catalyst-aqueous phase monomer solution is slowly and evenly dropped dropwise onto the oil phase. The reaction flask is then sealed. The reaction system is placed in an oven at 80-90℃ for 72-144 hours. The aqueous phase turns dark brown and is collected with a dropper. The collected aqueous phase solution is dialyzed in deionized water for 24-72 hours to obtain a COF nanosheet dispersion. Step 2, preparation of covalent organic framework membranes, including: Cellulose nanomaterials were prepared into a dispersion of 0.5 mg / mL. The COF nanosheet dispersion and the cellulose nanomaterial dispersion obtained in step one were taken at a volume ratio of 10 to 20:1, diluted with deionized water, and ultrasonically treated for 10 min to obtain a casting solution. The casting solution was filtered onto the base membrane by vacuum-assisted self-assembly and dried to obtain a covalent organic framework membrane.

3. The preparation method according to claim 2, characterized in that, In step one, the aliphatic small molecule amine having multiple primary 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 one, dialysis is performed using a dialysis bag with a molecular weight cutoff of 50,000.

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

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

7. The preparation method according to claim 2, characterized in that, In step two, the drying conditions are drying at room temperature for 12 hours.

8. An application of a covalent organic framework membrane, characterized in that, When the covalent organic framework membrane prepared according to any one of claims 2 to 7 is used for seawater desalination by pervaporation, the permeation flux is 138.74–198.35 kg m³ when the feed temperature is 40°C and the NaCl feed concentration is 3.5 wt%. -2 h -1 The sodium chloride ion rejection rate is 99.95% to 99.99%, and the separation performance remains stable within 120 hours when treating a 3.5 wt% feed solution at 50℃.

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