Covalent organic framework polycrystalline compact membranes, methods of making and applications thereof

Abstract

The application provides a covalent organic framework polycrystalline dense membrane and a preparation method and application thereof. The preparation method comprises the following steps: adding an acid catalyst, an amine compound and a carbonyl compound into a casting solution or a base film prepared from the casting solution, performing an interfacial polymerization reaction for 1 min to 30 min to generate a covalent organic framework, and solidifying to obtain the covalent organic framework polycrystalline dense membrane. The application also provides the covalent organic framework polycrystalline dense membrane prepared by the above preparation method and application of the covalent organic framework polycrystalline dense membrane as a selective separation nanofiltration membrane. The covalent organic framework polycrystalline dense membrane provided by the application has a sub-nanometer pore size and high crystallinity, and can realize effective retention and selective separation of inorganic salts.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nanofiltration membrane preparation, and particularly relates to a covalent organic framework polycrystalline dense membrane and a preparation method and application thereof. BACKGROUND

[0002] Covalent organic frameworks (COFs) are a new class of porous crystalline polymers, which are considered as ideal candidate materials for constructing NF membranes due to their tunable, long-range ordered pore size (0.7-5.0 nm), easy modification and the ability to use intrinsic uniform nanopores as transport channels to improve the pore size uniformity of the membrane. However, COFs are usually synthesized by solvothermal method under extreme reaction conditions such as inert gas atmosphere, aggressive solvent, high temperature and pressure, and the obtained product is generally non-processable and insoluble solid crystalline powder, which seriously hinders its further application in membrane separation. Although the preparation of COFs membranes (covalent organic framework membranes) has been realized through some strategies (such as non-solvent induced phase separation method (NIPS), interfacial polymerization method (IP) and the like), due to the large intrinsic pore size of COFs, poor film-forming property and negligible rejection rate of small molecule salts, the existing researches on COFs membranes are all focused on the preparation of ultrafiltration and loose nanofiltration membranes for dye rejection, and there is a lack of research on small molecule salt rejection membranes. Moreover, the synthesis of COFs membranes with rejection function requires a long reaction time of more than 48 h, which limits the batch production and wide application of COFs membranes; and there are a large number of defects in the membrane growth process, which hinders the formation of complete and continuous membrane structure.

[0003] Generally speaking, the discontinuous COFs-based membrane cannot meet the potential of COFs pore structure for membrane separation. Therefore, it is difficult to synthesize COFs-based membranes with high permeability and selectivity by using the non-solvent induced phase separation method. The IP method has certain advantages in preparing large-area, continuous and defect-free membranes, and can be applied to the preparation of commercial nanofiltration / reverse osmosis membranes, but there are problems of low crystallinity, long reaction time and low salt rejection rate in the COFs membranes prepared on the surface of organic substrates by using the IP method.

[0004] Therefore, it is necessary to provide a covalent organic framework membrane which can effectively solve the problems of large pore size, disorder or low crystallinity, defect in membrane structure, low inorganic salt rejection and poor separation effect of monovalent salt and divalent salt in the covalent organic framework membrane prepared on the surface of an organic substrate. SUMMARY

[0005] In order to solve the above problems, the purpose of the present application is to provide a covalent organic framework polycrystalline dense membrane and a preparation method and application thereof. The covalent organic framework polycrystalline dense membrane provided by the present application has a sub-nanometer pore size and high crystallinity, and can realize effective rejection and selective rejection of inorganic salts.

[0006] To achieve the above object, the present application provides a preparation method of covalent organic framework polycrystalline dense membrane, which comprises the following modes:

[0007] Mode 1: a base film is prepared by using a casting solution, an acid catalyst solution and an amine compound solution are coated on the surface of the base film, then the solution is left to stand, and then a carbonyl compound solution is coated, an interfacial polymerization reaction is carried out to generate a covalent organic framework, and solidification is carried out to obtain the covalent organic framework polycrystalline dense membrane;

[0008] Alternatively, mode 2: a base film is prepared by using a casting solution, an amine compound solution is coated on the surface of the base film, then the solution is left to stand, and then a mixed solution of an acid catalyst and a carbonyl compound is coated, an interfacial polymerization reaction is carried out to generate a covalent organic framework, and solidification is carried out to obtain the covalent organic framework polycrystalline dense membrane;

[0009] Alternatively, mode 3: an acid catalyst, an amine compound and a carbonyl compound are added to a casting solution, an interfacial polymerization reaction is carried out to generate a covalent organic framework, and heating and drying are carried out to obtain the covalent organic framework polycrystalline dense membrane;

[0010] In the above mode 1, mode 2 and mode 3, the mass concentration ratio of the acid catalyst, the amine compound and the carbonyl compound is (20-100):(20-80):1; and the time of the interfacial polymerization reaction is 1-30 min.

[0011] In the present application, the acid catalyst catalyzes the interfacial polymerization reaction of the amine compound and the carbonyl compound, and in the reaction process, synchronous polymerization, crystallization and stacking of the crystalline layer can be realized, and after heating and solidification, a covalent organic framework polycrystalline dense membrane is finally generated.

[0012] In the present application, the amine compound refers to an organic amine containing two or more amino groups, and the carbonyl compound refers to a ketone and / or an aldehyde containing two or more carbonyl groups. The covalent organic framework polycrystalline dense membrane is a membrane material having a covalent organic framework at least on the surface. The existing covalent organic framework membrane (also referred to as COFs membrane) formed by an amine compound and a carbonyl compound has the problems of large pore size, low crystallinity, easy defects between COFs crystals, and poor desalination capacity and selectivity of the membrane. In the above preparation method, on the one hand, the acid catalyst can undergo a reversible protonation-deprotonation reaction through interaction (forming a hydrogen bond) with the amine compound, and this reaction can compete with the Schiff base reaction to slow down the Schiff base reaction rate of the amine compound and the carbonyl compound, increase the reversibility of the Schiff base reaction, prolong the crystallization and growth time of the covalent organic framework (COFs) crystal grains, make the crystal grains grow more fully, have smaller pore size, and reduce the intercrystalline defects; on the other hand, the acid catalyst can form a hydrogen bond with the carbonyl group through the interaction between the carbonyl group and the hydrogen of the acid catalyst, which can reduce the steric hindrance of the carbonyl group and the amine group, and increase the reaction rate of the amine compound and the carbonyl compound. The salt increases the electrophilicity of the carbonyl group, and the carbonyl compound promotes the deprotonation of the amine compound after being added, thereby promoting the Schiff base reaction between the carbonyl group and the amine group, improving the reaction degree and the utilization rate of the monomer. The synergistic effect of the two enables the formation of a dense and crystalline COFs film on the base film surface. The COFs film obtained has a pore size that can reach a sub-nanometer level and has a high separation selectivity for divalent salt (such as Na2SO4) and monovalent / divalent salt (such as NaCl / Na2SO4).

[0013] In the presence of the acidic catalyst, the reaction process of the amine compound and the carbonyl compound can be as follows:

[0014]

[0015] Specifically, taking the amine compound as p-phenylenediamine, the carbonyl compound as triformylphloroglucin, and the acidic catalyst as p-toluenesulfonic acid as an example, the covalent organic framework formation process of one amino group in p-phenylenediamine can be as follows (R is benzyl):

[0016]

[0017] According to a specific embodiment of the present application, the acidic catalyst can specifically include acids, Lewis acids, and the like. Among them, the Bronsted acid can include one or a combination of two or more of acetic acid, p-toluenesulfonic acid, n-hexanoic acid, n-octanoic acid, n-decanoic acid, 4-hydroxybenzenesulfonic acid hydrate, sulfamic acid, trifluoroacetic acid, phosphoric acid, 4-aminobenzenesulfonic acid, and trifluoromethylsulfonic acid; and the Lewis acid can include one or a combination of two or more of scandium triflate, indium triflate, ytterbium triflate, and zinc triflate.

[0018] In some specific embodiments, the acidic catalyst can improve the reaction degree of the amine compound and the carbonyl compound, and also promote the formation of a small-pore, dense COFs film through the staggered stacking of COFs units. The acidic catalyst includes one or a combination of two or more of acetic acid, p-toluenesulfonic acid, n-hexanoic acid, n-octanoic acid, n-decanoic acid, 4-hydroxybenzenesulfonic acid hydrate, sulfamic acid, trifluoroacetic acid, phosphoric acid, 4-aminobenzenesulfonic acid, trifluoromethylsulfonic acid, scandium triflate, indium triflate, ytterbium triflate, and zinc triflate. Preferably, the acidic catalyst includes p-toluenesulfonic acid or scandium triflate.

[0019] According to specific embodiments of the present application, the amine-based compound includes one or more than two combinations of p-phenylenediamine, dimethyl-p-phenylenediamine, hydrazine, benzidine, 3,3'-diaminobenzidine, 1,3,5-tris(4-aminophenyl)benzene, 2,2'-bipyridine-5,5'-diamine, 4,4'-azobenzene diamine, and triaminoguanidine hydrochloride.

[0020] According to specific embodiments of the present application, the carbonyl-based compound includes one or more than two combinations of triformylphloroglucinol, trimesylform, p-xylylene glycol, 2,5-dihydroxy-p-xylylene glycol, 2,3-dimethoxy-p-xylylene glycol, and glyoxal.

[0021] In the above preparation method, by adjusting the amount of the acid catalyst relative to the amine-based compound, the proportion of the amine-based compound combined with the acid catalyst and the amine-based compound participating in the Schiff base reaction can be adjusted, the formation and growth of COFs can be controlled, and thus the thickness, pore size, crystallinity, and surface defect condition of the COF film formed by the amine-based compound and the carbonyl-based compound can be controlled, ultimately achieving the adjustment of the permeability, inorganic salt retention, and salt selectivity of the COF polycrystalline dense film. In some specific embodiments, the mass ratio of the acid catalyst to the carbonyl-based compound is generally controlled to be (20-100):1, and can be further controlled to be (60-100):1.

[0022] According to specific embodiments of the present application, when the mass ratio of the acid catalyst to the carbonyl-based compound is less than 20:1, the COF material in the COF film is in an amorphous state; when the mass ratio of the acid catalyst to the amine-based compound is greater than or equal to 20:1 and less than or equal to 100:1, the COF material in the COF polycrystalline dense film is in a crystalline state.

[0023] According to specific embodiments of the present application, the mass ratio of the amine-based compound to the carbonyl-based compound can be specifically controlled to be 20:1, 40:1, 60:1, 80:1, 100:1, etc.

[0024] According to specific embodiments of the present application, the mass concentration of the acid catalyst in the solution of the acid catalyst is generally controlled to be 5% or less (i.e., greater than 0% and less than or equal to 5%), for example, can be controlled to be 1%-5%, 3%, etc. The mass concentration of the amine-based compound in the solution of the amine-based compound is generally controlled to be 2% or less. The mass concentration of the carbonyl-based compound in the solution of the carbonyl-based compound is generally controlled to be 5% or less, for example, 0.05%.

[0025] According to the specific embodiments of the present application, in the mode 1, the solution of the acidic catalyst can be coated onto the surface of the base film prior to the solution of the amino compound, or can be coated onto the surface of the base film simultaneously with the solution of the amino compound. Further, a mixed solution of the acidic catalyst and the amino compound can be coated onto the surface of the base film. In the mixed solution of the acidic catalyst and the amino compound, the mass concentration of the acidic catalyst is controlled to be less than 5%, and the mass concentration of the amino compound is controlled to be less than 2%.

[0026] According to the specific embodiments of the present application, in the mode 2, the solution of the acidic catalyst can be added onto the surface of the base film prior to the solution of the carbonyl compound, or can be coated onto the surface of the base film containing the amino compound simultaneously with the solution of the carbonyl compound. Further, a mixed solution of the acidic catalyst and the carbonyl compound can be coated onto the surface of the base film containing the amino compound. In the mixed solution of the acidic catalyst and the carbonyl compound, the mass concentration of the acidic catalyst is controlled to be less than 5% (further less than 2%), and the mass concentration of the carbonyl compound is controlled to be less than 5% (further less than 0.05%).

[0027] According to the specific embodiments of the present application, the solution of the acidic catalyst and the solution of the amino compound can be aqueous solutions, respectively.

[0028] According to the specific embodiments of the present application, the solution of the carbonyl compound can be an oil solution.

[0029] According to the specific embodiments of the present application, the solvent of the solution of the carbonyl compound includes a water-immiscible organic solvent, for example, one or more than two combinations of n-hexane, cyclohexane, n-heptane, n-heptanol, toluene, mesitylene, xylene, dichloromethane.

[0030] According to the specific embodiments of the present application, in the mode 1 and the mode 2, the standing time is generally controlled to be 30s-1min.

[0031] According to the specific embodiments of the present application, the interfacial polymerization reaction is a process catalyzed by the acidic catalyst to generate a covalent organic framework. By controlling the time of the interfacial polymerization reaction, the formation and growth process of the COFs, the compactness and thickness of the COFs film, and further the permeability, retention and salt selectivity of the COFs film can be adjusted. Specifically, in the early stage of the interfacial polymerization reaction, the Schiff base reaction of the amino compound and the carbonyl compound can occur at a moderate reaction rate to form a compact COFs film. With the extension of the reaction time, the self-limiting effect of the COFs film formation hinders the further diffusion of the reaction monomers (the amino compound and the carbonyl compound), thus slowing down the increasing speed of the thickness of the COFs film. This effect can allow the COFs film formed after a certain reaction time to maintain a relatively thin thickness.

[0032] In the specific embodiments of the present application, in both way 1 and way 2, the carbonyl compound is generally added after the acid catalyst and the amine-based compound are added. After the acid catalyst and the amine-based compound are coated on the surface of the base film, the acid catalyst and the amine-based compound form weak hydrogen bonds (for example, the hydrogen bond length formed by p-toluenesulfonic acid and p-phenylenediamine is about ), each amino group is surrounded by three acid catalyst molecules, forming a chain-like one-dimensional layered structure, so as to obtain a porous matrix containing an acid catalyst layer and an amine-based compound layer; after the addition of the carbonyl compound, the hydrogen bond between the acid catalyst and the amine-based compound can be broken, and the carbonyl compound replaces the connection between the acid catalyst and the amine-based compound, so as to make the covalent organic framework material grow along the layered structure in a regular direction, and the obtained covalent organic framework has high crystallinity.

[0033] According to the specific embodiments of the present application, in both way 1 and way 2, the time of the interfacial polymerization reaction is 1-30 min. Specifically, when the time of the interfacial polymerization reaction is less than 1 min, the covalent organic framework material in the COF film obtained is amorphous; when the time of the interfacial polymerization reaction is greater than or equal to 1 min and less than or equal to 30 min, the covalent organic framework material in the covalent organic framework polycrystalline dense film is crystalline. In some specific embodiments, the time of the interfacial polymerization reaction can be specifically 1-15 min, 10-15 min, 5-30 min, 15 min, etc.

[0034] According to the specific embodiments of the present application, in both way 1 and way 2, the temperature of the solidification is 60-80°C, for example, 80°C, and the time of the solidification is 3-5 min, for example, 5 min. It can be understood that, in addition to coating, other ways such as impregnation can also be used to add the acid catalyst, the amine-based compound and the carbonyl compound to the base film.

[0035] According to the specific embodiments of the present application, in both way 1 and way 2, the base film is formed by a casting solution, and the preparation process of the base film can be: pouring the casting solution on the surface of a glass plate, and scraping the film to obtain the base film. The thickness of the base film is generally controlled to be 80-100 μm. In some specific embodiments, for the base film with a thickness of 80-100 μm, the solution amount of the acid catalyst solution, the amine-based compound solution and the carbonyl compound solution participating in the reaction can be 10-15 mL, respectively.

[0036] In some embodiments, the base film can be a common base film such as a polysulfone porous base film, and correspondingly, the casting solution for preparing the polysulfone porous base film can include polysulfone (hereinafter referred to as PSF), polyvinylpyrrolidone (hereinafter referred to as PVP), and 1-methyl-2-pyrrolidone (hereinafter referred to as NMP). Further, the mass ratio of PSF, PVP, and NMP can be controlled to be 15:8:77.

[0037] According to the embodiments of the present application, the method for preparing the polysulfone porous base film includes mixing PSF, PVP, and NMP in a certain proportion to form a casting solution, standing, coating to form a film, water bath heating, and obtaining the polysulfone porous base film. In some embodiments, the standing time is generally controlled to be more than 24 h, the water bath heating temperature is generally 60°C, and the heating time is generally 30 min.

[0038] According to the embodiments of the present application, in method 3, the acidic catalyst, the amine-based compound, and the carbonyl compound can be directly added to the casting solution, and the interfacial polymerization reaction occurs during the standing process, and then heating and drying are performed to obtain the covalent organic framework polycrystalline dense film.

[0039] According to the embodiments of the present application, in method 3, the heating and drying temperature is 60-80°C, for example, 80°C, and the heating and drying time is 3-5 min, for example, 5 min.

[0040] According to the embodiments of the present application, the method for preparing the covalent organic framework polycrystalline dense film includes:

[0041] 1. Pouring the casting solution on a glass plate and scraping the film to obtain a base film;

[0042] 2. Coating the mixed solution of the acidic catalyst and the amine-based compound on the surface of the base film, standing for 30 s-1 min, then coating the carbonyl compound solution, and performing the interfacial polymerization reaction for 1 min-30 min to generate the covalent organic framework, or coating the acidic catalyst solution, the amine-based compound solution, and the carbonyl compound solution on the surface of the base film, standing for 30 s-1 min, and performing the interfacial polymerization reaction for 1 min-30 min to generate the covalent organic framework; wherein the mass ratio of the acidic catalyst, the amine-based compound, and the carbonyl compound is (20-100):(20-80):1;

[0043] 3. Curing at 60-80°C for 3-5 min to obtain the covalent organic framework polycrystalline dense film.

[0044] Alternatively, the method for preparing the covalent organic framework polycrystalline dense film can include:

[0045] The acid catalyst, amine-based compound and carbonyl compound added into the casting solution are subjected to interfacial polymerization to generate the covalent organic framework, wherein the mass ratio of the acid catalyst, amine-based compound and carbonyl compound in the casting solution is (20-100):(20-80):1, and the covalent organic framework polycrystalline dense film is obtained by heating and drying at 60-80℃ for 3-5 min.

[0046] The application further provides a covalent organic framework polycrystalline dense film prepared by the above preparation method.

[0047] According to the specific embodiments of the application, the covalent organic framework film has a dense structure and a small pore size, and the pore size is 0.34-1.2 nm, for example, controllable to 0.34-0.5 nm.

[0048] According to the specific embodiments of the application, the water flux of the covalent organic framework polycrystalline dense film can reach 11.4-37.13 L m -2 h -1 .

[0049] According to the specific embodiments of the application, the 90% retention rate of the covalent organic framework polycrystalline dense film corresponds to a retention molecular weight of less than or equal to 8500 Da, and further less than or equal to 100 Da, and the covalent organic framework polycrystalline dense film has high retention capacity and selective separation capacity for inorganic salts. In some specific embodiments, the retention rate of the covalent organic framework polycrystalline dense film for Na2SO4 can reach 19.70-91.80%; the retention rate for MgSO4 can reach 12.9-81.4%; the retention rate for NaCl can reach 11-51.4%; the separation factor for NaCl / Na2SO4 can reach 1.03-6.71, for example, 1.44-3.71; and the separation factor for NaCl / MgSO4 can reach 1.02-2.97.

[0050] The application further provides the use of the covalent organic framework polycrystalline dense film as a selective separation nanofiltration membrane. The covalent organic framework polycrystalline dense film has good permeability, strong retention capacity for divalent salts, and good selectivity for the separation of monovalent ion salts and divalent ion salts.

[0051] The application has the following beneficial effects:

[0052] The preparation method provided by the application slows down the reaction rate of the amine monomer and the aldehyde ketone monomer by adding an acidic catalyst in the reaction system, using the interaction between small molecule acids and amine monomers to improve the reversibility of the overall reaction, and then regulating the growth, assembly and crystallization of COFs, effectively improving the crystallinity of the COFs film and reducing the pore size of the COFs film, so that a dense COFs film with high bivalent salt retention rate and good monovalent / bivalent salt separation effect is obtained, and the density and salt selectivity of the covalent organic framework polycrystalline film are strengthened.

[0053] The crystallinity, Na2SO4 retention rate, Mg2SO4 retention rate and NaCl retention rate of the covalent organic framework polycrystalline dense film provided by the application are obviously improved compared with when no catalyst is added. The preparation method of the application has simple reaction system composition and simple operation, and realizes the preparation of a crystalline COFs film with sub-nanometer pore size. The COFs film has a Na2SO4 retention rate of 91.4-91.8% and a NaCl retention rate of 42.3-51.4%, has high monovalent / bivalent salt (NaCl / Na2SO4) separation selectivity (6.7), and has obvious improvement effect on bivalent salt retention performance and monovalent / bivalent salt separation, and the separation performance is much better than that of the existing COFs-based film, which can inspire the design of the COFs film structure for effective ion separation and provide the possibility of recovering concentrated brine and separating monovalent and bivalent ions by the COFs-based film. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 The flowchart of the covalent organic framework polycrystalline dense film preparation method of Examples 2 to 8 and Comparative Example 1 is shown.

[0055] Figure 2 The front view photos of the film samples of Examples 1 to 8 and Comparative Example 1 and Comparative Example 2 are shown.

[0056] Figure 3 The back view photos of the film samples of Examples 1 to 8 and Comparative Example 1 and Comparative Example 2 are shown.

[0057] Figures 4a to 4g The surface SEM photos of the film samples of Example 1, Comparative Example 1, Example 2, Example 3, Example 4, Example 6 and Example 8 are shown in sequence. Figures 4a to 4g The size marked in the middle is the COFs layer thickness on the surface of the base film.

[0058] Figure 5 The TEM cross-sectional photos of Comparative Example 1, Example 3, Example 4 and Example 8 are shown. Figure 5 The size marked in the middle is the COFs layer thickness on the surface of the base film.

[0059] Figure 6Photographs of Example 3 film sample after treatment with DMAc solution.

[0060] Figure 7 Particle size distribution plots for Example 3, Example 4, Example 6, and Example 8.

[0061] Figure 8 Infrared spectra of Example 1, Example 2, Example 3, Example 4, Comparative Example 1, Example 8 film samples.

[0062] Figure 9 XPS spectra of Example 1, Example 2, Example 3, Example 4, Comparative Example 1, Example 8 film samples.

[0063] Figure 10a and Figure 10b XRD plots and crosslinking degree vs. crystallinity plots of Example 2, Example 3, Example 4, Comparative Example 1, Example 8 film samples.

[0064] Figure 11a and Figure 11b Test results of the effect of acid catalyst loading on water permeability and salt rejection and separation performance of covalent organic framework polycrystalline dense membranes.

[0065] Figure 12 Test results of the effect of acid catalyst loading on hydrophilic / hydrophobic properties of covalent organic framework polycrystalline dense membranes.

[0066] Figure 13a and Figure 13b Test results of the effect of interfacial polymerization reaction time on water permeability and salt rejection and separation performance of covalent organic framework polycrystalline dense membranes.

[0067] Figure 14 Test results of the effect of interfacial polymerization reaction time on hydrophilic / hydrophobic properties of covalent organic framework polycrystalline dense membranes.

[0068] Figure 15 Schematic diagrams of the conventional COFs membrane generation process and PTSA catalyzed interfacial polymerization to generate COFs membranes.

[0069] Figure 16a and Figure 16b Test results of the effect of PTSA acid catalyst loading on pore size distribution and molecular weight cut-off of covalent organic framework polycrystalline dense membranes.

[0070] Figure 17a and Figure 17b Test results of the effect of interfacial polymerization reaction time on pore size distribution and molecular weight cut-off of covalent organic framework polycrystalline dense membranes.

[0071] Figure 18Zeta potential chart of Example 2, Example 3, Example 4, Comparative Example 1, Example 8. DETAILED DESCRIPTION

[0072] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present application, the technical solutions of the present application will be described in detail below, but it should not be understood as limiting the scope of the present application.

[0073] Example 1

[0074] The present embodiment provides a base film, and a preparation method of the base film comprises:

[0075] The PSF (molecular weight cut-off 35000 Da), PVP, NMP are mixed according to a mass ratio of 15:8:77, and magnetically stirred for 24 h until the solution is uniform and transparent, i.e. a casting solution is obtained. The casting solution is left to stand for 24 h or more for degassing, the air humidity is adjusted to 75-77 RH%, the casting solution is quickly poured on one end of a clean and dry glass plate, a doctor blade with a thickness of 140 μm is used to scrape the film, and after staying for 30 s, it is quickly placed in a water bath at 60℃ for heating for 30 min, and then washed with deionized water to obtain a polysulfone porous base film (PSf base film), which is placed in deionized water at 4℃ for standby. The area of the base film is 43 cm 2 , and the thickness is 80-100 μm.

[0076] Example 2

[0077] The present embodiment provides a covalent organic framework polycrystalline dense film, as shown in Figure 1 , and a preparation method of the covalent organic framework comprises the following steps:

[0078] 1. Pouring a mixed aqueous solution of p-phenylenediamine (hereinafter referred to as Pa) and p-toluenesulfonic acid (hereinafter referred to as PTSA) on the PSf base film in the base film Figure 1 , standing for 30 s, pouring off the excess solution, and removing the excess water droplets with a rubber roller to obtain a polysulfone porous base film containing a Pa and PTSA layer;

[0079] In the mixed solution of p-phenylenediamine and p-toluenesulfonic acid, the mass concentration of p-phenylenediamine is 2 wt.%, and the mass concentration of p-toluenesulfonic acid is 1 wt.%.

[0080] 2. Pour the 0.05% trifurated phloroglucinol (hereinafter referred to as Tp) solution in n-hexane on the surface of the polysulfone porous base film treated in step 1, and stand for 15 min. The standing process is the interfacial polymerization process, which can generate covalent organic frameworks. Pour out the excess solution to obtain the polysulfone porous base film with COFs layer. Then, the polysulfone porous base film with COFs layer is cured at 80°C for 5 min, washed thoroughly with deionized water, and stored in deionized water at 4°C for 12 h to obtain the covalent organic framework polycrystalline dense film (i.e. the crystalline COFs film in the application). Figure 1 In the above process, the residence amount of the mixed solution of the acidic catalyst and the amine-based compound and the carbonyl compound solution in the base film is 15 mL, and the mass concentration ratio of the acidic catalyst, the amine-based compound, and the carbonyl compound participating in the reaction is 20:40:1.

[0081] Example 3

[0082] The covalent organic framework polycrystalline dense film provided in the example is prepared by the same method as in Example 2, except that the mass concentration of p-toluenesulfonic acid in the mixed aqueous solution of p-phenylenediamine and p-toluenesulfonic acid in step 1 of the example is 3 wt.%, i.e. the mass ratio of the acidic catalyst, the amine-based compound, and the carbonyl compound is 60:40:1.

[0083] Example 4

[0084] The covalent organic framework polycrystalline dense film provided in the example is prepared by the same method as in Example 2, except that the mass concentration of p-toluenesulfonic acid in the mixed aqueous solution of p-phenylenediamine and p-toluenesulfonic acid in step 1 of the example is 5 wt.%, i.e. the mass ratio of the acidic catalyst, the amine-based compound, and the carbonyl compound is 100:40:1.

[0085] Comparative Example 1

[0086] The covalent organic framework polycrystalline dense film provided in the example is prepared by the same method as in Example 2, except that p-toluenesulfonic acid is not added in step 1 of the example.

[0087] Example 5

[0088] The covalent organic framework polycrystalline dense film provided in the example is prepared by the same method as in Example 2, except that the reaction time of step 2 of the example is 1 min.

[0089] Example 6

[0090] The covalent organic framework polycrystalline dense film provided in the example is prepared by the same method as in Example 2, except that the reaction time of step 2 of the example is 5 min.

[0091] Example 7

[0092] This embodiment provides a covalent organic framework polycrystalline dense film, the preparation method of which is basically the same as that of Example 2, the only difference being that the reaction time of step 2 in this embodiment is 10 min.

[0093] Example 8

[0094] This embodiment provides a covalent organic framework polycrystalline dense film, the preparation method of which is basically the same as that of Example 2, the only difference being that the reaction time in step 2 of this embodiment is 30 min.

[0095] Comparative Example 2

[0096] This comparative example provides a nanofiltration membrane, the preparation method of which is basically the same as that of Example 2, the only difference being that in step 1 of this comparative example, only p-toluenesulfonic acid is added, and p-phenylenediamine is not added (which can be regarded as the concentration of Pa in the solution being 0%). Although the standing time after coating with the trialdehyde phloroglucinol solution in step 2 is 15 min, since PTSA and Tp do not react, the reaction time of interfacial polymerization can be regarded as 0 min. The membrane formed in this comparative example does not have a covalent organic framework.

[0097] The main experimental parameters of Examples 1 to 8, Comparative Examples 1 and 2 are summarized in Table 1.

[0098] Table 1

[0099] Pa concentration (wt%) Tp concentration (wt%) PTSA concentration (wt%) Reaction time Example 1 0 0 0 0 min Example 2 2 0.05 1 15 min Example 3 2 0.05 3 15 min Example 4 2 0.05 5 15 min Comparative Example 1 2 0.05 0 15 min Example 5 2 0.05 3 1 min Example 6 2 0.05 3 5 min Example 7 2 0.05 3 10 min Example 8 2 0.05 3 30 min Comparative Example 2 0 0.05 3 15 min

[0100] Test Example 1

[0101] This test case provides characterization of the membrane surface and back surface morphology of Examples 1 to 8, Comparative Examples 1 and 2.

[0102] Figure 2 The above are front-facing photographs of each sample. Figure 3 This is a photo of the back of the above sample. Figure 2 , Figure 3 In this example, diagrams a0 and a1 correspond to Example 1, diagrams b0 and b1 correspond to Example 2, diagrams c0 and c1 correspond to Example 1, diagrams d0 and d1 correspond to Example 2, diagrams e0 and e1 correspond to Example 3, diagrams f0 and f1 correspond to Example 4, diagrams g1 and g2 correspond to Example 5, diagrams h0 and h1 correspond to Example 6, diagrams i0 and i1 correspond to Example 7, and diagrams j0 and j1 correspond to Example 8.

[0103] The base film without COFs is white, and the COFs is orange red. The color change of the COFs film surface can be used to know the change of the film thickness and the arrangement of the structure. The surface of the coating solution is marked as the front surface, and the opposite surface is marked as the back surface. From the front surface to the back surface, the color of the COFs film is orange red, yellow, and colorless, respectively. Figure 2 , Figure 3 As can be seen from the above, with the increase of the amount of PTSA, the back of the COFs film changes from yellow to colorless, indicating that PTSA promotes the formation of COFs layer on the surface of the PSf base film. In addition, with the increase of the amount of acidic catalyst, the color of the front surface of the film gradually changes from deep red to orange red, indicating that the amount of COFs formed is reduced. Secondly, the film of Example 8 shows a deeper orange yellow color, which reveals that the extension of the reaction time can lead to the sufficient reaction of the amine compound and the carbonyl compound to form more COFs crystals on the surface of the base film.

[0104] Figures 4a to 4g , Figure 5 The SEM and TEM characterization of the front surface morphology of the COFs film is carried out. Figures 4a to 4g The surface SEM photograph and the cross-sectional SEM photograph of the above sample are shown in the following. Figure 4a Corresponding to Example 1 (PSf), Figure 4b Corresponding to Comparative Example 1 (TpPa -15min ), Figure 4c Corresponding to Example 2 (TpPa -1wt.％-15min ), Figure 4d Corresponding to Example 3 (TpPa -3wt.％-15min ), Figure 4e Corresponding to Example 4 (TpPa -5wt.％-15min ), Figure 4f Corresponding to Example 6 (TpPa -3wt.％-5min ), Figure 4g Corresponding to Example 8 (TpPa -3wt.％-30min ).

[0105] Figure 5 The TEM cross-sectional photographs of the sample films of Comparative Example 1, Example 3, Example 4 and Example 8 are shown in the following, b2 corresponds to Comparative Example 1 (TpPa -15min ), d2 corresponds to Example 3 (TpPa -3wt.％-15min ), e2 corresponds to Example 4 (TpPa -5wt.％-15min ), and g2 corresponds to Example 8 (TpPa -3wt.％-30min ).

[0106] Except for the sample of Comparative Example 1, the crystalline layer can be observed on the surface of the PSf base film after the growth of COFs, which is consistent with the obvious crystalline layer observed in the SEM cross-sectional image. At the same time, it should be pointed out that with the increase of the amount of PTSA catalyst, the COFs crystals tend to present a large size flaky structure.

[0107] In addition to the rich micro-COFs fragments contained in the film of Example 2, some large size plate-like crystals appeared on the surface of PSf in both Example 3 and Example 4 samples, and the COFs crystal size of Example 3, Example 4 samples increased relative to Example 2, indicating that the PTSA catalyst can promote the growth of COFs crystals by slowing down the crystallization rate.

[0108] In addition, from Figures 4a to 4g It can be seen that the base film of Example 1 has many large pores, but after the growth of COFs, the surface pores of the base film are gradually covered by some particles, indicating that the generated COFs oligomers or particles grow slowly, not only can form a continuous COFs layer on the surface, but also can penetrate into the pores of the base film.

[0109] In order to further confirm the distribution of COFs on the base film, the COFs film prepared in the example was treated with DMAc solution to etch the PSf base film. Figure 6 The photos of the film samples of Comparative Example 1 and Example 3 after treatment with DMAc solution. Figure a corresponds to Comparative Example 1 (TpPa -15min ), Figure b corresponds to Example 3 (TpPa -3wt.％-15min ).

[0110] Referring to Figure 6 , the a) in the DMAc solution Figure 6 immersed in the film of Comparative Example 1 presents a deeper orange color than the b) in the DMAc solution Figure 6 immersed in the film of Example 3, indicating that the COFs layer at the bottom of the film of Comparative Example 1 collapses with the removal of the PSf base film, further revealing that without the addition of acidic catalyst, COFs tends to penetrate into the pores of the base film and conformal growth along the PSf base film skeleton to form the film of Comparative Example 1. At the same time, referring to Figures 4c to 4e , after the addition of PTSA catalyst, the thickness of the COFs layer gradually decreases, and when the PTSA concentration increases from 1wt% to 5wt%, the thickness of the COFs layer on the surface of the film decreases from about 357nm to about 277nm, which is consistent with the change result in Figure 5 .

[0111] The above characterization results show that for lower PTSA dosage, these newly formed, fine COFs fragments formed along the pore wall exist loosely with many voids, which will promote the further diffusion and reaction of amine-based compounds and carbonyl compounds monomers, thereby forming a relatively thick COFs selective layer assembled by fragmented COFs aggregates; while higher PTSA dosage will promote COFs crystals to tend to turn into large size plate-like structure, such as Figure 4d and Figure 4e, and further prevent the COFs crystals from penetrating into the PSf substrate, resulting in the growth of a continuous and dense COFs layer on the surface of the PSf substrate. Meanwhile, this dense structure will greatly reduce the transport speed of the reactants to inhibit further reaction, thereby forming an ultra-thin COFs layer assembled by large-size COFs sheets.

[0112] In addition, the particle size distribution of the COFs crystals in Example 3, Example 4, Example 6 and Example 8 was obtained by analyzing the SEM surface morphology by Image software, and the results are as follows Figure 7 . Figure 7 In the above table, a corresponds to Example 3, b corresponds to Example 4, c corresponds to Example 6, and d corresponds to Example 8. The average size of the COFs crystal grains in the film of Example 3 was measured to be 1.28 μm, and the average size of the COFs crystal grains in the film of Example 4 was 1.02 μm, which was slightly smaller than that of Example 3, which may be related to the decrease in the amount of effective Pa monomer for reaction with Tp due to the increase in the amount of PTSA. By comparing the test results of Example 3, Example 6 and Example 8, it can be seen that by adjusting the reaction time of interfacial polymerization, the grain size of COFs can also be adjusted.

[0113] In addition, the COFs film containing nanoscale crystals (Example 2, Figure 4c ) has more defects than the COFs film containing large-size crystals (Example 3 and Example 4, Figure 4d and Figure 4e ), indicating that the COFs film containing large-size sheet structure has a dense COFs layer. Meanwhile, as the reaction time increases from 5 min to 30 min, the thickness of the COFs layer increases from about 254 nm (Example 6) to about 382 nm (Example 8), which is because prolonging the reaction time for growing COFs provides more opportunities for the reaction of Pa and Tp, which helps to increase the crosslinking degree of the COFs film, reduce the effective pore size of the COFs film, and heal the defects of the COFs layer.

[0114] Test Example 2

[0115] This test example provides the analysis results of the molecular structure of the COFs film. Figure 8 is the infrared spectrum test result of the film sample, wherein the a sample is Example 1 (PSf substrate), the b sample is Comparative Example 1 (TpPa -15min ), the c sample is Example 2 (TpPa -1wt.％-15min ), the d sample is Example 3 (TpPa -3wt.％-15min ), the e sample is Example 8 (TpPa -3wt.％-30min ), and the f sample is Example 4 (TpPa -5wt.％-15min ).

[0116] Except for sample a corresponding to Example 1, the infrared spectra of other samples did not show the characteristic peak of enol form (-OH (3292 cm⁻¹)). -1 ) and C = N (1614 cm -1 The stretching vibration peak of ) exists, and C = C(1582cm) exists. -1 ), C=O(1607cm) -1 ), CN (1237cm) -1 The stretching vibration peak indicates that samples b to f synthesized ketone-structured COFs layers in situ in the base film of Example 1 via Schiff base reaction.

[0117] Comparative Example 1 and Example 2 samples contained CH=O (1661 cm⁻¹) of Tp monomer. -1 The stretching vibration peak indicates that, under conditions of no PTSA or low PTSA dosage, residual Tp monomers did not react with Pa to form COFs. The results from Comparative Example 1 and Examples 2, 3, 4, and 8 show that, with the same amount of reactant monomers (amine and carbonyl compounds), the absence of an acid catalyst leads to an increase in the amount of carbonyl compounds remaining after the reaction, indicating that using an acid catalyst can improve the utilization rate and reaction extent of both amine and carbonyl compounds.

[0118] Samples from Examples 4 and 8 were stored at 3430 cm⁻¹ -1 The characteristic peak is attributed to N + -H···O - Hydrogen bonds of (PTSA-amine salt). When the amount of PTSA and the reaction time increase, the CH=O stretching vibration peak of the Tp monomer disappears, indicating that the Tp monomer fully participates in the reaction and thus forms a dense COF film.

[0119] right Figure 8 The products characterized in the table were subjected to XPS testing, and the results are summarized in Table 2 and 3. Figure 9 Compared with the film in Comparative Example 1, the COFs film prepared after adding PTSA showed a significant increase in N element (NH group derived from Pa) content and a decrease in S element content. Considering the limitation of the maximum XPS detection depth of about 10 nm, this result indicates that the addition of PTSA can promote the formation of more COFs crystals on the surface of the PSf base film.

[0120] In addition, the degree of cross-linking (DNC) of the COFs layer was calculated based on the C / N ratio detected from the XPS spectrum. The C / N ratio of the COFs film decreased with the increase of the PTSA content, which indicated that the degree of cross-linking of the COFs film increased (e.g., from 30.93% of the comparative example 1 film sample to 81.20% of the example 4 film sample). In addition, the example 8 film sample exhibited a higher degree of cross-linking, which indicated that a more compact COFs film was formed in a longer reaction time, which was also consistent with the results of SEM and FT-IR.

[0121] Table 2

[0122] C1s (%) O1s (%) N1s (%) C / N S2p (%) DNC (%) Example 1 81.64 12.64 3.16 25.84 2.56 - Comparative Example 1 78.49 13.59 6.19 12.68 1.73 30.93 Example 2 77.11 14.46 6.97 11.06 1.46 52.53 Example 3 72.88 18.34 8.08 9.01 0.71 79.87 Example 4 74.59 16.56 8.37 8.91 0.48 81.20 Example 8 73.52 17.11 8.76 8.39 0.61 88.13

[0123] The calculation method of the above-mentioned degree of cross-linking (DNC) is as follows:

[0124]

[0125] wherein (C / N) is measured based on the XPS test, the C / N of the completely cross-linked COFs structure is 7.5, and the C / N of the shortest completely polymerized (amorphous) structure is 15.

[0126] Figure 9 For Figure 8 The C1s scan results of the samples, a sample is example 1 (PSf-based film), b sample is comparative example 1 (TpPa -15min ), c sample is example 2 (TpPa -1wt.％-15min ), d sample is example 3 (TpPa -3wt.％-15min ), e sample is example 8 (TpPa -3wt.％-30min ), and f sample is example 4 (TpPa -5wt.％-15min ).

[0127] In the C1s spectrum, the PSf-based film and the COFs film contain carbon-related peaks of C=C / C-C (284.8 eV), C=O (286.9-288.9 eV), and C-N (286.2 eV), which is consistent with the infrared. All the COFs films exhibit a π-π (291.5 eV) peak, which indicates that π-π stacking occurs between the COFs layers, and the example 8 film sample has a higher π-π (291.5 eV) peak intensity, which reveals that the formed COFs layer on the surface of the PSf-based film undergoes π-π stacking, thereby effectively reducing the pore size of the COFs film. Compared with the comparative example 1 film sample, the C-N peak area of the example 4 film sample increases (from 24.22% to 26.46%), which indicates that the addition of PTSA can effectively promote the Schiff base reaction of Tp and Pa monomers on the surface of the base film to form a compact COFs film.

[0128] Test Example 3

[0129] This test example provides analytical results on the crystallinity of polycrystalline dense films with covalent organic frameworks. The test results can be found in [link to test results]. Figure 10a and Figure 10b .from Figure 10a It can be seen that, compared with the membrane sample of Comparative Example 1, the membrane samples of Examples 2, 3, 4, and 8 showed a new diffraction peak at 3.0°, which corresponds to the (100) crystal plane, indicating that the addition of the acidic catalyst is beneficial to the formation of crystalline COFs membranes on the substrate surface. According to the CIF card, the COFs formed by Tp and Pa ( Figure 10a The standard peak position of the (100) crystal plane in the TpPa sim is 4.7°. The diffraction peak of the (100) crystal plane in the above examples shifted from 4.7° to 3° in the simulated COFs, which can be attributed to the formation of large-size COFs crystals. In addition, the half-width at half-maximum (FWHM) of the diffraction peak is related to the crystal quality. The smaller the FWHM, the greater the crystal quality. The FWHM of the (100) crystal plane peak was obtained by fitting a Gaussian function. The half-width at half-maximum of the (100) crystal plane decreased from 0.43° (Example 2) to 0.28° (Example 3), and then increased to 0.38° (Example 4). Therefore, the film of Example 3 has a larger mass of COFs crystals.

[0130] In addition, the crystallinity of the COF film was further evaluated using the density method; the results are shown in [reference needed]. Figure 10b With increasing acid catalyst concentration, the crystallinity initially increased and then decreased (from 21.45% to 71.84% and then decreased to 62.95%), consistent with the FWHM results. These results further confirm that the amount of PTSA has a significant impact on the excellent crystallinity of COFs membranes. Higher PTSA catalyst concentrations (>5 wt.%) preferentially promote the protonation-deprotonation reaction of Pa monomers while slowing down the Schiff base reaction between Tp and Pa, resulting in insufficient reaction between Pa and Tp monomers at the interface. Consequently, the crystallinity of the membrane in Example 4, with an acid catalyst concentration as high as 5 wt.%, is poor.

[0131] The method for calculating Xc using the density method mentioned above is as follows:

[0132] Where ρ is the measured density of the sample, ρ c ρ is the crystalline density of crystalline COFs powder. a Comparative Example 1 (TpPa) is amorphous -15min The density of the membrane.

[0133] Figure 10a , Figure 10b The data on crystallinity and crosslinking degree are summarized in Table 3.

[0134] Table 3

[0135] Comparative Example 1 Example 2 Example 3 Example 4 Crystallinity, % 0 21.45 71.84 62.96 Crosslinking degree, % 30.93 52.53 79.87 81.2

[0136] Test Example 4

[0137] This test example provides an analytical test of the effect of acid catalyst dosage on the membrane permeation selectivity. The water flux of covalent organic framework polycrystalline dense membranes from Comparative Example 1 (0 wt.%), Example 2 (1 wt.%), Example 3 (3 wt.%), and Example 4 (5 wt.%) were tested, as well as the rejection rates of these membranes for Na₂SO₄ (1 g / L), Mg₂SO₄ (1 g / L), and NaCl (1 g / L). The test conditions were: room temperature 23-25°C, cross-flow filtration, and a feed pressure of 5 bar. The test results are summarized in... Figure 11a and Figure 11b middle.

[0138] Test Example 5

[0139] This test example provides an analytical test of the effect of PTSA catalyst dosage on the hydrophilicity and hydrophobicity of COFs membranes. The water droplet contact angle of the COFs membranes in Comparative Example 1, Example 2, Example 3, and Example 4 were characterized, and the test results are summarized below. Figure 12 From. Figure 12 It can be seen that the covalent organic framework polycrystalline dense membrane provided by the present invention is hydrophilic. With the increase of catalyst dosage, the hydrophilicity of the COFs membrane is enhanced, and the contact angle decreases from 74.9° to 63.6°.

[0140] from Figure 11a It can be seen that, without the addition of a catalyst, the water flux of the COFs membrane in Comparative Example 1 is 37.1 L / m³. -2 h -1 It also exhibits a low Na₂SO₄ rejection rate (19.7%), which may be due to the relatively large pore size (approximately 1.8 nm) and numerous defects in the COF membrane. With increasing PTSA catalyst dosage, the membrane water flux decreased to 15.6 L / m². -2 h -1 However, the rejection rates of Na₂SO₄ increased from 38.6% to 91.8%, MgSO₄ from 12.9% to 81.2%, and NaCl from 11.0% to 51.3%. The decrease in water flux and the increase in rejection rates demonstrate that the compactness of the COF membrane has been improved. See also... Figure 11bCompared with Comparative Example 1, the selectivity of NaCl / Na2SO4 and NaCl / MgSO4 of Example 2 to Example 4 increased from 1.20 to 6.70 and from 1.07 to 2.59, respectively, indicating that a high-quality COFs membrane was formed, and the addition of the acidic catalyst improved the separation precision of the covalent organic framework polycrystalline dense membrane. The selectivity of NaCl / MgSO4 was lower than that of NaCl / Na2SO4 mainly due to the Donnan exclusion effect of the negatively charged membrane surface.

[0141] In Figure 11a and Figure 11b , when the catalyst concentration was 3 wt.%, the water flux and salt retention of the membrane sample of Example 3 changed sharply compared with those of Comparative Example 1 and Example 2, which may be due to the change of the COFs layer structure and thickness Figures 4b to 4d . The increase in divalent salt retention and monovalent / divalent salt separation performance and the decrease in water flux are related to the formation of a high-crosslinking-degree (dense) COFs membrane Figure 10a and Figure 10b . When the PTSA concentration is low, the formed partial COFs particles tend to penetrate into the pore walls of the PSf base membrane instead of forming a continuous COFs layer on the surface of the base membrane, resulting in a COFs membrane with higher water and salt permeability at this time Figure 11a ; when the PTSA concentration is high (≥ 3 wt.%), PTSA slows down the COFs crystallization rate, forms a large number of large-size COFs crystals on the surface of the base membrane, and further increases the crosslinking degree of the COFs membrane, reduces the membrane pore size and the selectivity layer defect, thus resulting in higher divalent salt retention and monovalent / divalent separation factor Figure 4d and Figure 4e . And, referring to Figure 12 , the addition of PTSA enhances the hydrophilicity of the COFs membrane, and the contact angle decreases from 74.9° to 63.6°, indicating that the decrease in membrane pore size is the main reason for the decrease in water flux of the membrane. Considering that the COFs membrane prepared at a PTSA concentration of 3 wt% has a higher salt retention rate (91.4% Na2SO4) and a moderate water flux (18.7 L m -2 h -1 ), it is indicated that the preparation conditions of Example 3 are conducive to the improvement of the comprehensive performance of the COFs membrane. Figure 11a 、 Figure 11b and Figure 12 The data results are summarized in Table 4.

[0142] The calculation method of the above-mentioned NaCl / Na2SO4 (a(NaCl / MgSO4)) and NaCl / MgSO4 (a(NaCl / MgSO4)) separation factors is as follows:

[0143]

[0144]

[0145] Table 4

[0146] Comparative Example 1 Example 2 Example 3 Example 4 Water flux (Lm -2 h -1 )]]> 37.13 36.05 18.65 15.60 Na2SO4 rejection (%) 19.69 38.60 91.40 91.80 Mg2SO4 rejection (%) 9.56 12.9 80.6 81.20 NaCI rejection rate (%) 3.29 11.00 42.30 51.40 NaCl / Na2SO4 separation factor 1.20 1.44 6.70 5.92 NaCl / MgSO4 separation factor 1.06 1.02 2.97 2.58 Contact angle (°) 74.93 72.22 71.18 63.60

[0147] Test Example 6

[0148] This test example provides an analysis test of the influence of the reaction time of interfacial polymerization after adding an acidic catalyst on the permeation properties of the covalent organic framework polycrystalline dense membrane. The water fluxes of Example 5 (1 min), Example 6 (5 min), Example 7 (10 min), Example 3 (15 min), and Example 8 (30 min) were measured, and the rejection rates of these samples to Na2SO4 (1 g / L), MgSO4 (1 g / L), and NaCl (1 g / L) were tested under the same conditions as in Test Example 4, and the test results are summarized in Table 4. Figure 13a and Figure 13b .

[0149] Test Example 7

[0150] This test example provides an analysis test of the influence of the reaction time of interfacial polymerization after adding a PTSA catalyst on the hydrophilicity and hydrophobicity of the COF membrane. The water drop contact angles of the COF membranes of Example 5, Example 6, Example 7, Example 3, and Example 8 were characterized, and the test results are summarized in Table 5. It can be seen from Table 5 that the covalent organic framework polycrystalline dense membrane provided by the present application is hydrophilic, and the hydrophilicity of the COF membrane decreases with the increase of the reaction time, and the contact angle increases from 67.07° to 74.12°. Figure 14 Figure 14

[0151] Figure 13a , Figure 13b and Figure 14 The test data of Table 5.

[0152] Table 5

[0153]

[0154]

[0155] In summary, Figure 13a , Figure 13b and Figure 14 It can be seen that with the increase of the reaction time from 1 to 5 min, the water flux decreases from 36.70 L m -2 h -1 to 22.4 L m -2 h -1 ​​The rejection rates of Na2SO4increased from 56.10% to 70.51%, the rejection rates of MgSO4increased from 36.30% to 40.5%, the rejection rates of NaCl increased from 18% to 22.6%, and the separation selectivity of monovalent salt / divalent salt (NaCl / Na2SO4) increased from 1.29 to 2.63.

[0156] When the reaction time was short (< 15 min), the COFs membrane generated on the surface of the base membrane only covered part of the surface pores of the base membrane of Example 1, resulting in some defects in the formed COFs membrane, and the membrane exhibited weak negative electricity (the electrical test results are referred to Figure 18 ), thus causing a low rejection rate of salt. When the reaction time was further prolonged to 15 min, the water flux was further reduced to 18.7 L m -2 h -1 , however, the rejection rate of Na2SO4increased to 91.4%, the rejection rate of NaCl was 42.3%, and the separation selectivity of monovalent salt / divalent salt (NaCl / Na2SO4) reached 6.7, indicating that a longer reaction time allowed the Tp and Pa monomers to fully react to form a more dense COFs membrane. When the reaction time was further prolonged to 30 min, the salt rejection (91.6% Na2SO4) and the separation selectivity of monovalent salt / divalent salt (6.2 NaCl / Na2SO4) changed slightly, indicating that when the reaction time was prolonged to a certain extent, the self-limiting effect of the formation of the COFs membrane hindered the further diffusion of the reaction monomers.

[0157] The change in the hydrophilicity of the membrane was consistent with the change in the water flux, when the reaction time increased from 1 to 30 min, the contact angle increased from 67.07° to 74.12°, and the water flux of the membrane decreased to 11.4 L m -2 h -1 , due to the fact that prolonging the reaction time was conducive to increasing the thickness of the COFS layer Figure 4d , Figure 4f and Figure 4g , which in turn led to an increase in the water transmission resistance.

[0158] Figure 15 A schematic diagram of the process of catalyzing the interface polymerization of COFs by PTSA is shown in FIG. 1, wherein a is the COFs generation process without adding an acidic catalyst, and b is the COFs generation process with the addition of an acidic catalyst. The COFs crystal first forms a nucleus in an amorphous structure through initial polymerization, and then undergoes subsequent growth and transformation processes to form a crystalline state. This process requires a reversible reaction of protonation to fully crystallize the COFs framework, thereby improving the structural regularity and density of the COFs. As shown in FIG. 1, the COFs crystal first forms a nucleus in an amorphous structure through initial polymerization, and then undergoes subsequent growth and transformation processes to form a crystalline state. This process requires a reversible reaction of protonation to fully crystallize the COFs framework, thereby improving the structural regularity and density of the COFs. As shown in FIG. 1, Figure 15As shown in b, the sulfonic acid group of PTSA can be linked to the amine compound via hydrogen bonding, protonating the primary nitrogen atom in the amine compound to form an amine salt. Upon the addition of Tp, the amine salt is deprotonated and forms an imine bond via a Schiff base reaction. Simultaneously, the protonation and deprotonation reactions of the amine salt can slow down the diffusion rate of Pa into the reaction system by competing with the Schiff base reaction, increasing the reversibility of the Schiff base reaction. This promotes the full growth of COFs grains and the acquisition of high-quality COFs crystals, ultimately forming large-sized COFs crystals that accumulate and assemble into a dense film on the film surface. SEM, TEM, FTIR, XRD, and XPS results also confirm this. Figures 4a to 4g , Figure 5 , Figure 7 , Figure 8 , Figure 9 and Figures 10a to 10b However, as Figure 15 As shown in a, without PTSA, the faster interfacial reaction rate leads to greater heat generation in the water / organic interface region. Convection occurs during interfacial polymerization, causing COF crystals to enter the pores within the base film, preventing the formation of a continuous and dense COF layer.

[0159] Test Example 8

[0160] This test provides an analytical assay of the effect of PTSA dosage on the pore size distribution and molecular weight cutoff of covalent organic framework polycrystalline dense membranes. The pore size distribution and molecular weight cutoff of samples from Comparative Example 1, Example 2, Example 3, and Example 4 were measured. The molecular weight cutoff and pore size distribution were obtained by measuring the retention rates of neutral probe solutes PEGs (200, 6000, 10000, and 20000 Da) at 5 bar, with a neutral probe concentration of 50 mg / L. -1 The test results are summarized in... Figure 16a , Figure 16b And in Table 6. The molecular weight cutoff in Table 6 is the molecular weight corresponding to a cutoff rate of 90%.

[0161] Table 6

[0162] Comparative Example 1 Example 2 Example 3 Example 4 Pore size (nm) 1.30 1.02 0.36 0.34 Pore size distribution Sp 0.93 0.76 0.22 0.25 Molecular weight cut-off (Da) 19501 8204 117 121

[0163] Test Example 9

[0164] This test example provides an analytical test of the effect of reaction time (Tp) on the pore size distribution and molecular weight cutoff of a covalent organic framework polycrystalline dense membrane after the addition of PTSA. The pore size distribution and molecular weight cutoff were measured in Examples 3, 5 to 8. The test conditions were the same as in Example 8, and the test results are summarized in… Figure 17a , Figure 17b And in Table 7.

[0165] Table 7

[0166] Example 5 Example 6 Example 7 Example 3 Example 8 Pore size (nm) 1.20 0.50 0.38 0.36 0.34 Pore size distribution Sp 0.75 0.33 0.23 0.22 0.25 Molecular weight cut-off (Da) 6536 354 191 117 83

[0167] From Figure 16a It can be seen that the molecular weight cut-off of the comparative example 1 membrane (corresponding to the molecular weight at which the retention rate is 90%) is 19501 Da, and the molecular weight cut-off of the membrane decreases from 8204 Da to 117 Da as the concentration of the catalyst increases from 1 wt.% to 5 wt.%.

[0168] From Figure 17a It can be seen that the molecular weight cut-off decreases from 354 Da to 117 Da as the reaction time increases from 1 min to 15 min; when the reaction time increases to 30 min, the molecular weight cut-off of the membrane is further reduced to 83 Da.

[0169] From Figure 16b It can be seen that the pore radius of the comparative example 1 membrane sample is 0.65 nm, and the pore size is 1.3 nm, and the pore size and pore size distribution of this sample are lower than the theoretical pore size value of 1.8 nm and the wider pore size distribution (Sp = 0.93), which can be attributed to the π-π stacking effect between COFs. At the same time, as the concentration of PTSA catalyst increases from 1 wt.% to 5 wt.%, the pore size of the membrane decreases from 1.02 nm to 0.34 nm, indicating that the addition of PTSA can slow down the crystallization rate of COFs, and then form large-size COFs crystals on the surface of the membrane, which are assembled into a membrane with fewer defects, smaller pore size and more dense structure (). Figures 4a to 4g At the same time, the pore size distribution of the membrane after adding the acidic catalyst is more concentrated, indicating that the uniformity of the pore size of the COFs membrane is improved Figure 17a and Figure 17b , thereby improving the separation precision of the COFs membrane.

[0170] Test Example 10

[0171] This test example provides the potential test results of the covalent organic framework dense crystalline membrane.

[0172] Figure 18 Zeta potential of Example 2 (TpPa -1wt.％-15min ), Comparative Example 1 (TpPa -15min ), Example 3 (TpPa -3wt.％-15min ), Example 4 (TpPa -5wt.％-15min ), Example 8 (TpPa -3wt.％-30min ). The results show that all the tested membranes are negatively charged under neutral conditions.

[0173] In summary Figure 17a , Figure 17b and Figure 18As can be seen from the table, when the concentration of PTSA is 3 wt.%, the molecular weight cut-off gradually decreases from 354 Da to 117 Da as the reaction time increases from 1 min to 15 min, and further increases to 30 min, the molecular weight cut-off decreases to 83 Da. In addition, as the reaction time increases from 1 min to 5 min, the membrane pore size decreases from 1.20 nm to 0.50 nm, and further increases the reaction time to 30 min, the membrane pore size decreases to 0.34 nm. The above results show that when the reaction time is 1 min, the formation of a small amount of COFs units leads to a large number of defects in the COFs membrane Figure 4c ), and further prolonging the reaction time (≥5 min) can form more large-size COFs units on the surface of the base film, and these COFs units accumulate at the interface to form sub-nanometer-sized pores. In addition, Figure 18 The tested COFs membranes have similar zeta potentials under neutral conditions Figure 18 ).

[0174] In combination with Figure 16a , Figure 16b , Figure 17a , Figure 17b The test results of the molecular weight cut-off of the membrane samples show that increasing the amount of acidic catalyst and prolonging the reaction time can promote the reduction of the pore size of the COFs membrane, and the reduction of the pore size can improve the desalination ability and the separation selectivity of NaCl / Na2SO4 of the covalent organic framework dense crystalline membrane.

[0175] The above results show that the covalent organic framework polycrystalline dense membrane provided by the present application has a COFs layer on the surface, has the structural characteristics of few defects, small pore size and densification. The covalent organic framework polycrystalline dense membrane has high retention rate for divalent salt, and has good monovalent / divalent salt separation effect.

Claims

1. A method for preparing a covalent organic framework polycrystalline dense membrane, the method comprising: preparing a base film using a casting solution, coating an acid catalyst solution and an amine compound solution on the surface of the base film, standing, then coating a carbonyl compound solution, and performing interfacial polymerization to generate a covalent organic framework, and solidifying to obtain the covalent organic framework polycrystalline dense membrane; or, preparing a base film using a casting solution, coating an amine compound solution on the surface of the base film, standing, then coating an acid catalyst solution and a carbonyl compound solution, and performing interfacial polymerization to generate a covalent organic framework, and solidifying to obtain the covalent organic framework polycrystalline dense membrane; or, adding an acid catalyst, an amine compound and a carbonyl compound to a casting solution, performing interfacial polymerization to generate a covalent organic framework, and heating and drying to obtain the covalent organic framework polycrystalline dense membrane; wherein the amine compound comprises an organic amine having two or more amino groups, and the carbonyl compound comprises an aldehyde and / or a ketone having two or more carbonyl groups; the mass ratio of the acid catalyst, the amine compound and the carbonyl compound is (60-100):(20-80):1; and the interfacial polymerization is performed for 5-30 min. The covalent organic framework polycrystalline dense membrane has a pore size of 0.34-0.50 nm.

2. The production method according to claim 1, wherein, The acid catalyst comprises a Bronsted acid and / or a Lewis acid.

3. The method of making according to claim 2, wherein, The Bronsted acid comprises one or more than two of acetic acid, p-toluenesulfonic acid, n-hexanoic acid, n-octanoic acid, n-decanoic acid, 4-hydroxybenzenesulfonic acid hydrate, sulfamic acid, trifluoroacetic acid, phosphoric acid, 4-aminobenzenesulfonic acid, and trifluoromethylsulfonic acid.

4. The production method according to claim 2, wherein The Lewis acid comprises one or more than two of scandium triflate, indium triflate, ytterbium triflate, and zinc triflate.

5. The production method according to claim 2, wherein, The acid catalyst comprises p-toluenesulfonic acid or scandium triflate.

6. The production method according to claim 1, wherein The amine compound comprises one or more than two of p-phenylenediamine, dimethyl-p-phenylenediamine, hydrazine, benzidine, 3,3'-diaminobenzidine, 1,3,5-tris(4-aminophenyl)benzene, 2,2'-bipyridine-5,5'-diamine, and 4,4'-azobenzene diamine.

7. The production method according to claim 1, wherein The carbonyl compound comprises one or more than two of triformylphloroglucin, trimesylformaldehyde, p-xylene dicarboxaldehyde, 2,5-dihydroxy-p-xylene dicarboxaldehyde, 2,3-dimethoxy-p-xylene dicarboxaldehyde, and glyoxal.

8. The production method according to claim 1, wherein The acid catalyst solution and the amine compound solution are aqueous solutions; and the carbonyl compound solution is an oil phase solution.

9. The production method according to claim 8, wherein The solvent of the carbonyl compound solution comprises a water-immiscible organic solvent.

10. The production method according to claim 8, wherein The solvent of the carbonyl compound solution comprises one or more than two of n-hexane, cyclohexane, n-heptane, n-heptanol, toluene, mesitylene, xylene, and dichloromethane.

11. The method of making according to claim 1, wherein, The interfacial polymerization is performed for 5-15 min.

12. The method of making according to claim 1, wherein, The interfacial polymerization is performed for 10-15 min.

13. The method of making according to any one of claims 1, 11-12, wherein, The solidification is performed at a temperature of 60-80℃ for 3-5 min. The temperature of the heating drying is 60-80℃; the time of the heating drying is 3-5 minutes.

14. The production method according to claim 13, wherein The temperature of the solidification is 80℃.

15. The method of making according to claim 13, wherein, The time of the solidification is 5 minutes.

16. The method of making according to claim 13, wherein, The temperature of the heating drying is 80℃.

17. The method of making according to claim 13, wherein, The time of the heating drying is 5 minutes.

18. A covalent organic framework polycrystalline dense membrane obtained by the method of any one of claims 1-17.

19. The covalent organic framework polycrystalline dense film of claim 18, wherein, The covalent organic framework polycrystalline dense membrane has a molecular weight cut-off of less than or equal to 8500 Da.

20. The covalent organic framework polycrystalline dense film of claim 18, wherein, The covalent organic framework polycrystalline dense membrane has a molecular weight cut-off of less than or equal to 100 Da.

21. Use of the covalent organic framework polycrystalline dense membrane of any one of claims 18-20 as a selective separation nanofiltration membrane.

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