A micro-bubble space template strategy assisted ultrathin polyamide composite membrane and a preparation method and application thereof

Abstract

The application discloses a kind of micron bubble space template strategy assisted ultra-thin polyamide composite membrane and its preparation method and application.First, in neutral / acid aqueous solution, add micron bubble precursor, and utilize the reaction between precursor and neutral / acid aqueous solution or the self-decomposition of precursor to generate micron bubble;Then, cover organic phase solution above the above aqueous solution, under the assistance of ultrasonic wave, promote the transmission of micron bubble from aqueous solution to organic phase solution;Finally, under the free interface polymerization process, utilize the steric hindrance effect of micron bubble on interface organic phase side, effectively limit the diffusion path and diffusion rate of water phase monomer in interface reaction zone, build the polyamide separation layer with ultra-thin structure, effectively reduce the transmission resistance of water molecule across membrane and shorten the mass transfer path of water molecule, significantly improve the water flux of composite membrane.

Description

Technical Field

[0001] This invention discloses an ultrathin polyamide composite membrane prepared with the assistance of a micron-sized bubble space template strategy, belonging to the field of separation membrane preparation and application technology. Background Technology

[0002] For thin-layer composite membranes, the ideal polyamide separation layer should be as thin as possible while maintaining high selectivity. The preparation of such ultrathin polyamide layers typically involves introducing nanomaterials or modifiers, or even constructing an intermediate layer. Zhang et al. introduced sodium 4-hydroxybenzenesulfonate into an aqueous solution, utilizing the electrostatic interaction between the negatively charged sulfonic acid groups of this organic salt and the protonated amine monomer, as well as the hydrogen bonding between the hydroxyl groups of the organic salt and the amine monomer, to synergistically slow down the diffusion rate of the aqueous monomer, thus forming an ultrathin polyamide separation layer (Journal of Membrane Science 680 (2023) 121739). Patent CN 116371222 B, "An Ultrathin Composite Polyamide Nanofiltration Membrane, Its Preparation Method, and Its Application," constructs a superhydrophilic, amino-rich intermediate layer using three-dimensional porous melamine particles. The hydrogen bonding between the aqueous monomer and the porous melamine intermediate layer inhibits the diffusion of the aqueous monomer, resulting in an ultrathin polyamide separation layer that significantly improves the water flux of the nanofiltration membrane. The successful implementation of these strategies is mainly attributed to their alteration of the diffusion state of monomers throughout the aqueous system, rather than the fine-tuning of the diffusion of aqueous monomers in the interfacial reaction zone.

[0003] Because aqueous monomers typically have a high partition coefficient in organic solutions, interfacial polymerization reactions mainly occur on the organic phase side of the interface between the two phases. Therefore, in the rational design of ultrathin polyamide separation layers, the key lies in specifically adjusting the diffusion rate of aqueous monomers on the organic phase side of the interface, thereby achieving precise control of the interfacial polymerization reaction. In this design process, the template method, with its spatial confinement effect, provides a feasible path to achieve this goal. However, considering the nanoscale size of the polyamide separation layer, it is necessary to strictly control the distance between the template material and the interfacial reaction zone, and avoid structural defects in the polyamide separation layer caused by the self-deposition of the template material. The complex issues between the template material and the two-phase interface make this idea seem difficult to realize. Under these constraints, bubbles, which are commonly used as soft templates for functional nanomaterials, have attracted our attention. In fact, research on controlling the structure of polyamide separation layers through bubbles has been reported. These studies usually involve pretreating the aqueous solution using physical or chemical methods to pre-construct nanobubbles in the aqueous solution, or generating nanobubbles in situ during interfacial polymerization, thereby controlling the structure of the polyamide separation layer. However, nanobubbles in aqueous solutions or at interfaces are prone to generating nanocavities in the polyamide separation layer during upward transport, thereby increasing the thickness of the separation layer. This strongly conflicts with the idea of ​​using templates to design ultrathin polyamide separation layers. Summary of the Invention

[0004] In view of this, the present invention proposes a method for preparing ultrathin polyamide composite membranes using a microbubble spatial template strategy. First, a microbubble precursor is added to a neutral or acidic aqueous solution, and microbubbles are generated through the reaction between the precursor and the neutral / acidic aqueous solution, or through the self-decomposition of the precursor. Next, an organic phase solution is placed on the surface of the aqueous solution, and under ultrasonic assistance, the microbubbles are rapidly transported from the aqueous solution to the organic phase solution, achieving loading of microbubbles in the organic phase solution. Finally, under a free interface polymerization process, the steric hindrance of the microbubbles on the organic phase side is utilized to effectively limit the diffusion path and diffusion rate of aqueous monomers in the interfacial reaction zone, constructing an ultrathin polyamide separation layer. This effectively reduces the transmembrane transport resistance of water molecules and shortens the mass transfer path, significantly enhancing the permeation and separation performance of the polyamide composite membrane.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: an ultrathin polyamide composite membrane prepared with the assistance of a micron bubble space template strategy, comprising a porous support layer and an ultrathin polyamide separation layer on the porous support layer. The ultrathin polyamide separation layer is prepared by interfacial polymerization reaction between an aqueous solution containing polyamines and an organic solution containing polyacrylamide chlorides and micron bubbles. The surface of the ultrathin polyamide separation layer contains collapsed bubble-like structures.

[0006] Preferably, the thickness of the ultrathin polyamide separation layer is 24-35 nm.

[0007] A method for preparing ultrathin polyamide composite films using a micron-sized bubble space template strategy includes the following steps: (1) Preparation of ultrathin polyamide composite membrane: A porous support layer is placed at the bottom of the reaction vessel. Under the free interface polymerization strategy, an aqueous solution containing polyamine, an organic buffer solution, and an organic solution containing polyacrylamide chloride and micron bubbles are added sequentially to the surface of the support layer to carry out the interfacial polymerization reaction. The height of the aqueous solution is 2-10 mm, preferably 3-7 mm, the height of the organic buffer solution is 0.05-0.3 mm, preferably 0.1-0.2 mm, and the height of the organic solution is 1-10 mm, preferably 1-5 mm. (2) Post-treatment of polyamide composite membrane: After the interfacial polymerization reaction is completed, the solution between the polyamide separation layer and the support layer is extracted through the support layer through the bottom outlet of the reaction vessel, so that the polyamide separation layer is deposited on the surface of the support layer to form a primary composite membrane. Then, an organic solvent, which is the same as the organic phase solvent in the organic phase solution, is used to rinse the surface of the primary composite membrane and the membrane is heat-treated to form a stable polyamide composite membrane.

[0008] Adding an organic buffer solution above the aqueous solution not only helps avoid the impact of direct diffusion of the organic phase during interfacial polymerization on the uniformity of the separation layer, but also promotes the rapid and uniform dispersion of microbubbles at the water-organic phase interface, thus fully utilizing their spatial template function. The organic buffer solution is the same as the organic solvent in the organic phase solution.

[0009] The specific preparation method of the organic phase solution containing polyacrylamide chloride and microbubbles in step (1) is as follows: First, different types of microbubble precursors are added to a neutral or acidic aqueous solution. The precursor concentration in the neutral or acidic aqueous solution is 0.5-1.5 wt.%, preferably 0.6-1.0 wt.%. Next, the organic phase solution is covered on the surface of the above-mentioned neutral or acidic aqueous solution, with a volume ratio of aqueous solution to organic phase solution of 1:1-1:5. Subsequently, ultrasonic treatment is performed 2-6 times, each time for 1-5 seconds, with an ultrasonic power of 200-400 W and a frequency of 20-50 kHz, to promote the vigorous transport of microbubbles from the neutral or acidic aqueous solution to the organic phase solution, thereby achieving the loading of microbubbles in the organic phase solution. Of course, microbubbles can also be loaded in the organic phase solution in other ways, such as using a microbubble generator to aerate the organic phase solution.

[0010] In the first implementation method, a microbubble precursor reacts with a neutral aqueous solution to form microbubbles. The microbubble precursor includes, but is not limited to, any one or more of the following: borohydrides (such as lithium borohydride, sodium borohydride, potassium borohydride), hydrides (such as lithium hydride, sodium hydride, potassium hydride, calcium hydride, magnesium hydride, aluminum hydride), carbides (such as sodium carbide, potassium carbide, calcium carbide, aluminum carbide), sodium superoxide, and potassium superoxide. After the precursor is added to the neutral aqueous solution, an organic phase solution is immediately added to the surface of the aqueous solution, followed by ultrasonic treatment to intensify the reaction between the precursor and the neutral aqueous solution, promote the transport of microbubbles from the neutral aqueous solution to the organic phase solution, and achieve loading of microbubbles in the organic phase solution. This allows the microbubbles in the organic phase solution to control the interfacial polymerization reaction.

[0011] As a second approach, a microbubble precursor reacts with an acidic aqueous solution to form microbubbles. The microbubble precursor includes, but is not limited to, any one or more of carbonates, bicarbonates, percarbonates, sulfites, bisulfites, sodium hydrosulfide, potassium hydrosulfide, and pure metals (such as zero-valent copper and zero-valent iron). After adding the precursor to the acidic aqueous solution, an organic phase solution is immediately added to the surface of the solution, followed by ultrasonic treatment to intensify the reaction between the precursor and the acidic aqueous solution. This promotes the transport of microbubbles from the acidic aqueous solution to the organic phase solution, achieving loading of microbubbles in the organic phase solution. This allows the microbubbles in the organic phase solution to control the interfacial polymerization reaction.

[0012] As a third implementation method, microbubble precursors self-decompose in neutral aqueous solutions to generate microbubbles. These precursors include, but are not limited to, any one or more of hydrogen peroxide, sodium peroxide, calcium peroxide, and sodium percarbonate. After adding the precursor to the neutral aqueous solution, an organic phase solution is immediately added to the surface of the solution, followed by ultrasonic treatment to accelerate the self-decomposition of the precursor in the neutral aqueous solution. This promotes the transport of microbubbles from the neutral aqueous solution to the organic phase solution, achieving loading of microbubbles in the organic phase solution. This allows the microbubbles in the organic phase solution to control the interfacial polymerization reaction.

[0013] The pH value of the acidic aqueous solution is 1-6, and the adjustment method includes, but is not limited to, adding any one or more of hydrochloric acid, acetic acid, phosphoric acid, nitric acid, sulfuric acid, citric acid, and perchloric acid.

[0014] In step (1), the porous support layer is a commonly used support layer material for preparing organic composite membranes, including but not limited to any one of polysulfone, polyester, polycarbonate, polyacrylonitrile, and polyvinylidene fluoride.

[0015] The polyamine in step (1) includes, but is not limited to, piperazine, 4-aminopiperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, triethylenetetramine, adamantyl diamine, 2,6-diaminotoluene, polyethyleneimine, ethylenediamine, β-cyclodextrin, diethylenetriamine, 3,5-dihydroxybenzoic acid, N,N-dimethylm-phenylenediamine, 3,5-dihydroxy-4-methylbenzoic acid, 4,4',4'',4'',4'''-methyltetraethyltetra(phenyl-1,2-diamine), 4,6-diaminoresorcinol dihydrochloride, diethylamino-β-cyclodextrin, and 3,5-diamino-N-(4-aminophenyl)benzamide, and the concentration of the polyamine in the aqueous solution is 0.1-5.0 wt.%, preferably 0.3-3.0%. The organic phase solvent is a n-alkanes, including but not limited to any one of n-pentane, n-hexane, n-heptane, n-octane, n-decane, n-undecane, and n-dodecane; the polyacryl chlorides include but are not limited to any one or more of isophthaloyl chloride, naphthalenedioxyyl chloride, phthaloyl chloride, terephthaloyl chloride, biphenyldioxyyl chloride, trimesoyl chloride, cyclohexyltrioxyyl chloride, trimesoyl trimesoyl chloride, 3,5-bis(sulfonylamino)benzoyl chloride, cyclopentanetetracarboxylic acid chloride, 1,2,3,4-cyclobutanetetracarboxylic acid chloride, 2,4,4',6-biphenyltetracarboxylic acid chloride, 2,2',4,4',6,6'-biphenylhexamethyl chloride, 5-isocyanate isopeptide chloride, 5-(1-pyrrolidinyl)-1,3-isophthaloyl chloride, and 5-(dichlorophosphoryl)isophthaloyl chloride, and the concentration of the polyacryl chloride in the organic phase solution is 0.01-0.4%. wt.%, preferably 0.05-0.2 wt.%.

[0016] The interface aggregation time in step (1) is 10-180 s, preferably 45-90 s.

[0017] In step (1), the polyamide composite membrane includes, but is not limited to, any one of nanofiltration membrane, forward osmosis membrane, and reverse osmosis membrane.

[0018] In step (2), the heat treatment temperature is 50-90 ℃ and the time is 1-5 min.

[0019] Furthermore, the application of ultrathin polyamide composite membranes prepared with the assistance of a micron-bubble space template strategy in separation processes. These separation processes are suitable for nanofiltration, forward osmosis, and reverse osmosis membranes, such as liquid-liquid separation and gas-gas separation.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) Compared with the method of adding regulators or nanomaterials, the micron bubble space template strategy proposed in the present invention is simple and low in cost, avoids the rough adjustment of the monomer diffusion rate in the entire aqueous system, and achieves fine control of the diffusion path and diffusion rate of aqueous monomer in the interface reaction zone through the spatial steric hindrance of micron bubbles on the organic phase side of the interface; (2) Compared with the sacrificial intermediate layer strategy, the present invention effectively avoids the damage that the etching process may cause to the fine structure of the polyamide separation layer, and ensures the compactness of the separation layer; (3) Compared with the general bubble control strategy, the present invention achieves the loading of micron bubbles in the organic phase solution through the pretreatment method, and effectively limits the thickness of the polyamide separation layer by using the micron bubble space template, breaking the general understanding of controlling the nano cavity structure in the separation layer by bubbles, and opening up a new path for the combination of bubbles and polyamide separation layer; (4) The micron bubble space template strategy proposed in the present invention has strong universality. It can not only be applied to micron bubble precursors of various reaction types, but also be flexibly applied to the preparation of various ultrathin polyamide composite films, and has high practical application value. Attached Figure Description

[0021] Figure 1 For Comparative Example 1, SEM images of the surface and cross-section of a polyamide reverse osmosis (RO) membrane prepared without the introduction of micron-sized bubbles.

[0022] Figure 2 Example 2: SEM images of the surface and cross-section of an ultrathin polyamide RO membrane prepared by H2 micron bubbles.

[0023] Figure 3 SEM images of the cross-section of ultrathin polyamide RO membranes prepared by H2 micron bubbles, as shown in Examples 1 and 3.

[0024] Figure 4 For comparative example 2, SEM images of the surface and cross-section of the polyamide RO membrane prepared by H2 nanobubbles.

[0025] Figure 5 Example 4 shows SEM images of the surface and cross-section of an ultrathin polyamide RO membrane prepared by CO2 micron-sized bubbles.

[0026] Figure 6 Example 5 shows SEM images of the surface and cross-section of an ultrathin polyamide RO membrane prepared by O2 micron-sized bubbles.

[0027] Figure 7 The images show cross-sectional SEM images of the polyamide nanofiltration (NF) membranes prepared by H2 micron bubbles in Comparative Example 3 without the introduction of micron bubbles and Example 6. Detailed Implementation

[0028] This invention provides a method for preparing an ultrathin polyamide composite membrane using a microbubble spatial template strategy. First, a microbubble precursor is added to a neutral or acidic aqueous solution. Microbubbles are generated through the reaction between the precursor and the neutral / acidic aqueous solution, or through the self-decomposition of the precursor. Next, an organic phase solution is placed on the surface of the aqueous solution. Under ultrasonic assistance, the microbubbles are vigorously transported from the aqueous solution to the organic phase solution, achieving loading of microbubbles in the organic phase solution. Finally, in a free interface polymerization process, the steric hindrance of the microbubbles on the organic phase side effectively restricts the diffusion path and diffusion rate of the aqueous monomer in the interfacial reaction zone, promoting the formation of an ultrathin polyamide separation layer (24-35 nm thick), and significantly enhancing the permeation and separation performance of the polyamide composite membrane. The technical solution will be described in detail below through embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention and do not represent all embodiments.

[0029] Comparative Example 1: A method for preparing a polyamide RO membrane, specifically comprising: First, connect the outlet at the bottom of the reaction vessel to a vacuum pump. Next, place a polysulfone support layer inside the reaction vessel (approximately 40 mm in radius). Then, sequentially pour an aqueous solution containing 2.0 wt.% m-phenylenediamine (MPD) (approximately 2.4 mm in height), 0.5 mL of n-hexane buffer (approximately 0.1 mm in height), and a n-hexane solution containing 0.1 wt.% trimesoyl chloride (TMC) (approximately 1 mm in height) into the device. The interfacial polymerization reaction takes approximately 60 s. Subsequently, the reaction vessel is filtered using a vacuum pump to remove the solution between the polysulfone support layer and the polyamide separation layer, allowing the prepared polyamide separation layer to deposit on the surface of the polysulfone support layer. Finally, the reverse osmosis composite membrane is removed by a pull-out method, rinsed with n-hexane, and heat-treated in an 80 °C oven for 3 min.

[0030] Example 1: A method for preparing ultrathin polyamide RO membranes using an H2 micron bubble space template strategy is as follows: First, 0.6 wt.% sodium borohydride was added to a neutral aqueous solution, and a hexane solution containing 0.1 wt.% TMC was transferred to the surface of the neutral aqueous solution. The mixture was sonicated for 5 seconds and repeated twice to intensify the reaction between sodium borohydride and the aqueous solution, promoting the transport of H2 microbubbles from the neutral aqueous solution to the organic phase solution (hexane solution containing 0.1 wt.% TMC), thus achieving the loading of H2 microbubbles in the organic phase solution. The sonication power was 300 W and the frequency was 40 kHz. The hexane solution containing H2 microbubbles and 0.1 wt.% TMC was then aspirated from the top of the reaction solution and poured onto a 0.5 mL hexane buffer solution (approximately 0.1 mm high) following the method of Comparative Example 1. The subsequent experimental steps and conditions were the same as in Comparative Example 1, except that the hexane solution containing 0.1 wt.% TMC was replaced with a hexane solution containing H2 microbubbles and 0.1 wt.% TMC.

[0031] Examples 2 and 3: In Examples 2 and 3, the concentrations of sodium borohydride in the neutral aqueous solutions were 0.8 wt.% and 1.0 wt.%, respectively, and the subsequent experimental steps and conditions were the same as in Example 1.

[0032] Comparative Example 2: A method for preparing polyamide RO membranes using an H2 nanobubble spatial template strategy, specifically as follows: First, 0.4 wt.% sodium borohydride was added to a neutral aqueous solution, and a hexane solution containing 0.1 wt.% TMC was transferred to the surface of the neutral aqueous solution. The mixture was sonicated for 5 seconds and repeated twice to intensify the reaction between sodium borohydride and the aqueous solution, promoting the transport of H2 nanobubbles from the neutral aqueous solution to the organic phase solution, thus achieving the loading of H2 nanobubbles in the organic phase solution. The sonication power was 300 W and the frequency was 40 kHz, resulting in a hexane solution containing H2 nanobubbles and 0.1 wt.% TMC. The subsequent experimental steps and conditions were the same as in Example 1, except that the hexane solution containing 0.1 wt.% TMC was replaced with a hexane solution containing H2 nanobubbles and 0.1 wt.% TMC.

[0033] Example 4: A method for preparing ultrathin polyamide RO membranes using a CO2 micron-sized bubble space template strategy is as follows: First, the pH of the pure aqueous solution was adjusted to approximately 2 using hydrochloric acid. Then, 0.8 wt.% sodium bicarbonate was added to the acidic aqueous solution, and a hexane solution containing 0.1 wt.% TMC was transferred to the surface of the aqueous solution. The solution was sonicated for 5 seconds and repeated twice to intensify the reaction between sodium bicarbonate and the acidic aqueous solution, promoting the transport of CO2 microbubbles from the acidic aqueous solution to the organic phase solution. This achieved the loading of CO2 microbubbles in the organic phase solution, resulting in a hexane solution containing CO2 microbubbles and 0.1 wt.% TMC. The subsequent experimental steps and conditions were the same as in Example 1, except that the hexane solution containing 0.1 wt.% TMC was replaced with a hexane solution containing CO2 microbubbles and 0.1 wt.% TMC.

[0034] Example 5: A method for preparing ultrathin polyamide RO membranes using an O2 micron bubble space template strategy is as follows: First, 0.8 wt.% sodium percarbonate was added to a neutral aqueous solution, and a hexane solution containing 0.1 wt.% TMC was transferred to the surface of the aqueous solution. The mixture was sonicated for 5 seconds and repeated twice to intensify the decomposition of sodium percarbonate and promote the transport of O2 microbubbles from the neutral aqueous solution to the organic phase solution, thereby achieving the loading of O2 microbubbles in the organic phase solution and obtaining a hexane solution containing O2 microbubbles and 0.1 wt.% TMC. Except that the hexane solution containing 0.1 wt.% TMC was replaced with a hexane solution containing O2 microbubbles and 0.1 wt.% TMC, the subsequent experimental steps and conditions were the same as in Example 1.

[0035] Comparative Example 3: A method for preparing a polyamide NF film: Compared with Comparative Example 1, the aqueous solution of this comparative example does not contain MPD, but contains 0.4 wt.% piperazine, and all other aspects are the same.

[0036] Example 6: A method for preparing ultrathin polyamide NF films using an H2 micron bubble space template strategy: Compared to Example 1, the aqueous solution in this example does not contain MPD, but contains 0.4 wt.% piperazine, and all other aspects are the same.

[0037] Test Example 1: The performance testing process for polyamide RO membranes is as follows: The water flux and salt rejection rate of the RO membrane were tested in a cross-flow filtration unit. The operating temperature was 25 °C, the operating pressure was 2 MPa, the feed solution was 2000 ppm NaCl solution, and the effective filtration area of ​​the RO membrane was 12.56 cm². 2The operation time should be maintained for at least 1 hour to keep the water flux stable.

[0038] Test Example 2: The performance testing process for the polyamide NF membrane was as follows: The water flux and salt rejection rate of the NF membrane were tested in a cross-flow filtration device. The operating temperature was 25 ℃, the operating pressure was 0.5 MPa, the feed solution was 1000 ppm Na₂SO₄ solution, and the effective filtration area of ​​the NF membrane was 12.56 cm². 2 The operation time should be maintained for at least 1 hour to keep the water flux stable.

[0039] Table 1 shows the water flux and salt rejection rate of the ultrathin polyamide RO membranes prepared with the assistance of H2, CO2, and O2 microbubble space template strategies. The results of Comparative Example 1 and Examples 1-3 show that the water flux of the prepared RO membranes is significantly improved under the steric hindrance of the H2 microbubble template. With increasing sodium borohydride concentration, the RO membrane water flux initially increases and then decreases; this change may be related to subtle differences in the thickness of the polyamide separation layer. Example 2 exhibits the highest water flux, which is 2.22 times that of Comparative Example 1. Careful comparison reveals that the salt rejection rate of the polyamide RO membrane prepared using the H2 microbubble space template strategy is slightly increased. This may be related to the restriction of amine monomer diffusion into the organic phase by the H2 microbubbles, causing the amine monomers to accumulate in the interfacial reaction zone below the microbubbles, resulting in a denser polyamide separation layer structure.

[0040] Furthermore, the microbubble space template strategy proposed in this invention has strong universality. By adjusting the way microbubbles are generated from the precursor, the structure and performance of the RO membrane can also be improved. In Examples 4 and 5, the microbubble precursors were replaced with sodium bicarbonate and sodium percarbonate, respectively. CO2 microbubbles formed by the reaction of sodium bicarbonate with acidic aqueous solution, and O2 microbubbles generated by the self-decomposition of sodium percarbonate in neutral aqueous solution, were used to load two different types of microbubbles into the organic phase solution, effectively limiting the diffusion path and diffusion rate of the amine monomer on the organic phase side of the interface. Compared with Comparative Example 1, the RO membrane prepared accordingly also exhibited high water flux and maintained a relatively reasonable salt rejection rate. Table 2 shows the data correspondence between water flux and salt rejection rate of the ultrathin polyamide NF membrane prepared with the assistance of the H2 microbubble space template strategy. As can be seen from Table 2, the H2 microbubble space template strategy proposed in this invention is also applicable to the preparation of high-performance NF membranes. Among them, Example 6 exhibited a high water flux, approximately three times that of Comparative Example 3.

[0041] Table 1. Water flux and salt rejection of RO membranes prepared in Comparative Examples 1-2 and Examples 1-5 Comparative Example 1 <![CDATA[H2-0]]> 41.8 98.0 128 ± 11 Comparative Example 2 <![CDATA[H2-0.4]]> 71.7 98.7 292 ± 22 Example 1 <![CDATA[H2-0.6]]> 80.8 98.5 31 ± 1 Example 2 <![CDATA[H2-0.8]]> 92.9 98.4 27 ± 1 Example 3 <![CDATA[H2-1.0]]> 77.6 98.4 33 ± 2 Example 4 <![CDATA[CO2-0.8]]> 83.5 98.3 28 ± 1 Example 5 <![CDATA[O2-0.8]]> 94.9 98.2 25 ± 1 Table 2. Water flux and salt rejection rate of the NF membranes prepared in Comparative Example 3 and Example 6 Comparative Example 3 <![CDATA[H2-0]]> 18.7 97.7 70 ± 5 Example 6 <![CDATA[H2-0.6]]> 55.3 98.0 31 ± 5 Figure 1 The RO membrane surface in Comparative Example 1 is relatively smooth, and the polyamide separation layer thickness is approximately 128 nm. After loading micron-sized bubbles into the organic phase through pretreatment, and with the assistance of an H2 micron-sized bubble space template, Figure 2 The RO membrane surface exhibits a collapsed, bubble-like polyamide structure. Compared to Comparative Example 1, the polyamide separation layer thickness in Example 2 is reduced by approximately 80%, to only 26 nm. The formation of this ultrathin polyamide separation layer significantly reduces the transmembrane transport resistance of water molecules and shortens the mass transfer path, which is beneficial for improving the water flux of the RO membrane. With decreasing (Example 1) or increasing (Example 3) sodium borohydride concentration, the size of the H2 micron-sized bubbles on the organic phase side of the interface changes accordingly, thereby affecting the transinterfacial diffusion path and diffusion rate of amine monomers, resulting in slight differences in the polyamide separation layer thickness. Figure 3 The polyamide separation layer thicknesses in Examples 1 and 3 were significantly lower than those in Comparative Example 1. When sodium borohydride was excessively reduced to generate nanobubbles... Figure 4 The thickness of the separation layer in Comparative Example 2 is about an order of magnitude greater than that in Examples 1-3, and the separation layer contains a large number of nanocavity structures. The formation of these structures is mainly attributed to the weak confinement effect of nanobubbles on the diffusion of amine monomers. Although the discrete nanocavities in the separation layer of Comparative Example 2 are conducive to the rapid transport of water molecules, Table 1 shows that its water flux is still lower than that of Examples 1-3, which have ultrathin separation layer structures, due to the influence of the separation layer thickness.

[0042] Furthermore, the strategy of controlling the polyamide separation layer structure through microbubbles is independent of the gas type. By replacing the precursors with sodium bicarbonate and sodium percarbonate, respectively, and with the assistance of CO2 and O2 microbubbles, Figure 5 and 6 The polyamide release layers in Examples 4 and 5 exhibit morphological features similar to those in Example 2, namely collapsed bubble-like polyamide structures. Furthermore, the thicknesses of both polyamide release layers are relatively low, approximately 28 and 25 nm, respectively, confirming the universality of the micron-bubble space template strategy.

[0043] More importantly, the micron-bubble spatial template strategy proposed in this invention is also applicable to the preparation of ultrathin NF films. When micron-bubbles are not introduced into the organic phase solution, Figure 7 a shows that the polyamide separation layer thickness of the NF membrane in Comparative Example 3 is approximately 70 nm. Conversely, under the confinement of the H2 micron-sized bubble space template on the organic phase side of the interface, Figure 7 b shows that the thickness of the polyamide separation layer in Example 6 is significantly reduced to approximately 31 nm.

[0044] The above results indicate that by utilizing the reaction between the precursor and neutral / acidic aqueous solution, or by generating microbubbles through the self-decomposition of the precursor, and promoting the loading of microbubbles in the organic phase solution through a pretreatment process, the thickness of the polyamide separation layer can be effectively limited, thus enabling the preparation of ultrathin polyamide composite membranes. This is beneficial for improving the water flux of the composite membrane and maintaining its salt rejection rate. This control strategy is applicable to microbubbles with various gas properties and various types of polyamide composite membranes.

Claims

1. A method for preparing an ultrathin polyamide composite film assisted by a micron-sized bubble space template strategy, characterized in that, Includes the following steps: (1) Preparation of ultrathin polyamide composite membrane: The porous support layer is placed at the bottom of the reaction vessel. Under the free interface polymerization strategy, an aqueous phase solution containing polyamine, an organic buffer solution, and an organic phase solution containing polyacrylamide chloride and micron bubbles are added sequentially to the surface of the support layer to carry out the interface polymerization reaction. (2) Post-treatment of polyamide composite membrane: After the interfacial polymerization reaction is completed, the solution between the polyamide separation layer and the support layer is extracted through the support layer through the bottom outlet of the reaction vessel, so that the polyamide separation layer is deposited on the surface of the support layer to form a primary composite membrane. Then, the surface of the primary composite membrane is rinsed with organic solvent and the membrane is heat-treated to form a stable polyamide composite membrane. Step (1) The method for preparing the organic phase solution containing polyacrylamide chloride and microbubbles is as follows: First, add different types of microbubble precursors to a neutral or acidic aqueous solution, with a precursor concentration of 0.5-1.5 wt.% in the neutral or acidic aqueous solution; then, cover the surface of the above neutral or acidic aqueous solution with the organic phase solution, with a volume ratio of aqueous solution to organic phase solution of 1:1-1:5; subsequently, sonicate 2-6 times, each time for 1-5 seconds, with an ultrasonic power of 200-400 W and a frequency of 20-50 kHz.

2. The method for preparing ultrathin polyamide composite films assisted by the micron-bubble space template strategy according to claim 1, characterized in that, The height of the aqueous solution is 2-10 mm, the height of the organic buffer solution is 0.05-0.3 mm, and the height of the organic solution is 1-10 mm.

3. The method for preparing ultrathin polyamide composite films assisted by the micron-bubble space template strategy according to claim 1, characterized in that, The polyamines include any one or more of piperazine, 4-aminopiperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, triethylenetetramine, adamantyl diamine, 2,6-diaminotoluene, polyethyleneimine, ethylenediamine, β-cyclodextrin, diethylenetriamine, 3,5-dihydroxybenzoic acid, N,N-dimethylm-phenylenediamine, 3,5-dihydroxy-4-methylbenzoic acid, 4,4',4'',4'',4'''-methyltetraethyltetra(phenyl-1,2-diamine), 4,6-diaminoresorcinol dihydrochloride, diethylamino-β-cyclodextrin, and 3,5-diamino-N-(4-aminophenyl)benzamide, with a concentration of 0.1-5.0%. wt.%; Polyacrylamide chlorides include any one or more of isophthaloyl chloride, naphthalenedioxyyl chloride, phthaloyl chloride, terephthaloyl chloride, biphenyldioxyyl chloride, trimesoyl chloride, cyclohexanetrioyl chloride, trimesoyl trioxide chloride, 3,5-bis(sulfonylamino)benzoyl chloride, cyclopentanetetracarboxylate chloride, 1,2,3,4-cyclobutanetetracarboxylate chloride, 2,4,4',6-biphenyltetracarboxylate chloride, 2,2',4,4',6,6'-biphenylhexamethyl chloride, 5-isocyanate isopeptide chloride, 5-(1-pyrrolyl)-1,3-isophthaloyl chloride, and 5-(dichlorophosphoryl)isophthaloyl chloride, with a concentration of 0.01 - 0.4 wt.%.

4. The method for preparing ultrathin polyamide composite films assisted by the micron-bubble space template strategy according to claim 1, characterized in that, In step (2), the organic solvent and the organic buffer solution are the same as the organic phase solvent in the organic phase solution. The organic phase solvent is a n-alkanes, including any one of n-pentane, n-hexane, n-heptane, n-octane, n-decane, n-undecane, and n-dodecane.

5. The method for preparing ultrathin polyamide composite films assisted by the micron-bubble space template strategy according to claim 1, characterized in that, Micron bubble precursors react with neutral aqueous solutions to form micron bubbles. Micron bubble precursors include any one or more of borohydrides, hydrides, carbides, sodium superoxide, and potassium superoxide. Alternatively, micron bubble precursors react with acidic aqueous solutions to form micron bubbles. Micron bubble precursors include any one or more of carbonates, bicarbonates, percarbonates, sulfites, bisulfites, sodium hydrosulfide, potassium hydrosulfide, and pure metals. Alternatively, microbubble precursors can self-decompose in neutral aqueous solutions to generate microbubbles. Microbubble precursors include any one or more of hydrogen peroxide, sodium peroxide, calcium peroxide, and sodium percarbonate.

6. The method for preparing an ultrathin polyamide composite film assisted by a micron-bubble space template strategy according to claim 1, characterized in that, The porous support layer material includes any one of polysulfone, polyester, polycarbonate, polyacrylonitrile, and polyvinylidene fluoride; the polyamide composite membrane includes any one of nanofiltration membrane, forward osmosis membrane, and reverse osmosis membrane.

7. A micron-bubble space template-assisted ultrathin polyamide composite film prepared by the method of any one of claims 1-6, characterized in that, It includes a porous support layer and an ultrathin polyamide separation layer on the porous support layer. The ultrathin polyamide separation layer is prepared by interfacial polymerization reaction between an aqueous solution containing polyamines and an organic solution containing polyacrylamide chlorides and micron-sized bubbles. The surface of the ultrathin polyamide separation layer contains collapsed bubble-like structures, and the thickness of the ultrathin polyamide separation layer is 24-35 nm.

8. The polyamide composite membrane prepared by the method of any one of claims 1-6, or the application of the polyamide composite membrane of claim 7 in the separation process.

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