Super-hydrophobic PVDF-UiO-66-PFOC composite membrane as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of membrane separation, and discloses a super-hydrophobic PVDF-UiO-66-PFOC composite membrane as well as a preparation method and application thereof.The super-hydrophobic PVDF-UiO-66-PFOC composite membrane is characterized in that a PVDF hollow fiber membrane is used as a substrate, and carboxyl active sites are introduced through alkali treatment and oxidation treatment; a UiO-66-NH2 metal organic framework layer with CO2 affinity is grown on the surface of the membrane through an in-situ hydrothermal synthesis method, so that the surface roughness and the CO2 adsorption capacity are improved; furthermore, a specific fluorination reagent PFOC (perfluorooctanoyl chloride) is grafted to the amino group of the UiO-66-NH2 through nucleophilic addition reaction, so that a chemically bonded super-hydrophobic fluorination layer is formed, and the surface energy is remarkably reduced. The prepared composite membrane has high wettability resistance, high CO2 selectivity and long-term operation stability. The composite membrane can be widely applied to CO2 capture systems in the fields of coal-fired power plants, chemical plants and the like, has high absorption efficiency, long-acting wetting resistance and operation stability, is different from an MOF modified membrane for oil-water separation or general gas separation in the prior art, and has remarkable economic and environment-friendly benefits.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, and relates to a superhydrophobic PVDF-UiO-66-PFOC composite membrane, its preparation method and application. Background Technology

[0002] Polyvinylidene fluoride (PVDF) membranes are widely used in membrane separation due to their excellent chemical stability, mechanical strength, and hydrophobicity. However, traditional PVDF hollow fiber membranes still face problems such as insufficient hydrophobicity and easy wetting during long-term operation, leading to liquid phase wetting of the membrane pores, significantly increased mass transfer resistance, and decreased separation efficiency. Especially in the process of CO2 absorption, membrane wetting can severely reduce mass transfer flux and operational stability, limiting its industrial application. Therefore, how to improve the anti-wetting performance of PVDF hollow fiber membranes through superhydrophobic modification while avoiding pore blockage to maintain air permeability has become an urgent technical challenge.

[0003] The core characteristic of superhydrophobic surfaces is a static contact angle greater than 150°. To improve hydrophobicity, existing technologies mainly rely on the synergistic effect of micro / nano-scale rough structures and low surface energy materials. Common superhydrophobic modification methods include fluorosilane coating (Chinese patent CN107556477A), chemical etching (Chinese patent CN107321583A), plasma treatment (Chinese patent CN108978173A), and layer-by-layer self-assembly (Chinese patent CN107503228A). To simultaneously endow membranes with specific separation functions (such as CO2 selectivity) and structural stability, researchers have begun to explore the introduction of metal-organic frameworks (MOFs) into membranes. For example, Chinese patent CN120662134A discloses a method for in-situ growth of UiO-66-NH2 and grafting of PIL onto a PET membrane. Its modification endpoint is the grafting of PIL to improve CO2 / N2 selectivity, without any improvement to the membrane's hydrophobicity, and it does not provide any solution to the core challenge in the membrane absorption process—the liquid phase wetting problem. Existing technologies include methods for modifying PVDF hollow fiber membranes by combining MOFs with fluorides. For example, patents CN115155335A and CN116617877A disclose the preparation of hydrophilic and oleophobic membranes based on UiO-66-NH2 or NH2-MIL-88B(Fe) grafted with fluorine-containing reagents for oil-water separation. However, these methods aim to achieve hydrophilicity and oleophobicity to separate oil-water emulsions, with the membrane structure focusing on liquid retention rather than efficient gas permeation. Furthermore, they utilize MOF physical deposition, resulting in poor stability, and fail to address the challenges of achieving the necessary superhydrophobicity and anti-wetting properties, as well as high CO2 mass transfer efficiency, required for CO2 capture membranes.

[0004] Therefore, there is an urgent need to develop a PVDF composite membrane that can balance superhydrophobicity and antiwetting properties with high CO2 mass transfer efficiency. Summary of the Invention

[0005] To address the problems of low membrane wetting, low CO2 absorption efficiency, and poor stability of modified layers in existing technologies, this invention provides a superhydrophobic PVDF-UiO-66-PFOC composite membrane, its preparation method, and its application. Using polyvinylidene fluoride (PVDF) as a substrate, a CO2-affinity-based UiO-66-NH2 metal-organic framework material is grown in situ and grafted with a specific fluorinating agent PFOC (perfluorooctanoyl chloride). The resulting composite membrane achieves high anti-wetting properties, high CO2 selectivity, and long-term operational stability.

[0006] The technical solution of this invention is: The first aspect of this invention provides a method for preparing a superhydrophobic PVDF-UiO-66-PFOC composite membrane, comprising the following steps: (1) Activation treatment of PVDF hollow fiber membrane: The PVDF hollow fiber membrane was subjected to alkali treatment and oxidative grafting treatment in sequence to obtain activated PVDF hollow fiber membrane. Carboxyl groups (-COOH) were introduced on the membrane surface to provide specific binding sites for subsequent MOF in situ growth. (2) In-situ growth of UiO-66-NH2: The activated PVDF hollow fiber membrane in step (1) is immersed in a precursor solution containing zirconium chloride (ZrCl4), 2-aminoterephthalic acid and formic acid. A UiO-66-NH2 crystal layer is grown in situ on the surface of the activated PVDF hollow fiber membrane by hydrothermal synthesis to obtain a PVDF-UiO-66-NH2 membrane. The UiO-66-NH2 crystal layer has CO2 selective adsorption capacity and amino sites adapted to PFOC grafting. (3) PFOC grafting modification: The PVDF-UiO-66-NH2 membrane obtained in step (2) is immersed in a mixed solvent containing perfluorooctanoyl chloride (PFOC), triethylamine and n-hexane to carry out a nucleophilic addition reaction, and the fluorine is grafted onto the amino group of UiO-66-NH2 to form an amide bond-linked fluorinated layer, thus obtaining the PVDF-UiO-66-PFOC composite membrane.

[0007] Preferably, the alkaline treatment in step (1) specifically involves using a mixed solution containing 0.8-1.2 mol / L NaOH and 0.05wt%-0.15wt% tetrabutylammonium bromide (TBAB) and reacting at 50℃-80℃ for 10-20 minutes. In the alkaline treatment, TBAB acts as a phase transfer catalyst to promote the reaction between NaOH and the surface of the PVDF hollow fiber membrane; the temperature range of 50℃-80℃ has been optimized to ensure carboxyl grafting efficiency while avoiding membrane pore collapse. The amount of TBAB, reaction temperature, and time have been synergistically optimized to maximize the introduction of carboxyl sites without damaging the pore structure of the PVDF hollow fiber membrane, ensuring uniform and dense growth of UiO-66-NH2 crystals.

[0008] Preferably, the conditions for the oxidative grafting treatment in step (1) are as follows: using a mixed solution containing 0.2 mol / L - 0.5 mol / L HNO3, 0.1 w / v% - 0.15 w / v% cerium ammonium sulfate, and 4 v / v% - 6 v / v% acrylic acid, and reacting at 20℃ - 45℃ for 1-2 hours. The HNO3 concentration, cerium ammonium sulfate dosage, and reaction conditions are optimized for PVDF hollow fiber membrane modification to avoid membrane degradation and ensure carboxyl grafting efficiency, providing sufficient anchoring points for MOF in-situ growth.

[0009] Preferably, in the precursor solution described in step (2), the amount of formic acid added is 30-120 equivalents (eq), based on the amount of 2-aminoterephthalic acid, and the hydrothermal synthesis conditions are 100℃-140℃ for 10-12 hours. The amount of formic acid added (30-120 eq) is a key parameter for controlling the size and distribution of UiO-66-NH2 crystals. The reaction temperature of 100℃-140℃ can balance the crystal growth rate and membrane structure compatibility, ensuring that the composite membrane has both CO2 adsorption capacity and gas permeability. Further preferably, the amount of formic acid added is 60-120 eq. Formic acid, as a regulator, can control the size of UiO-66-NH2 crystals (200-500nm) and avoid crystal agglomeration blocking the membrane pores; the hydrothermal temperature of 100℃-140℃ can balance the crystal growth rate and membrane structure compatibility. The preferred amount of formic acid is 120 equivalents. At this amount, the UiO-66-NH2 crystals are uniform in size (about 300 nm) and have the best surface roughness (Ra=114.3 nm), providing physical support for superhydrophobicity. The PFOC concentration of 1.5 mL / 115 mL n-hexane can ensure high hydrophobicity (contact angle ≥160°) while avoiding excessive coverage of the fluorine chain, which would increase the mass transfer resistance of CO2.

[0010] Preferably, the concentration of 2-aminoterephthalic acid and the concentration of ZrCl4 in the precursor solution in step (2) are 5-10 mmol / L.

[0011] Preferably, in step (3), the volume ratio of PFOC, triethylamine, and n-hexane in the mixed solvent is 2-3 mL: 1-2 mL: 70-120 mL, and the nucleophilic addition reaction is carried out at room temperature for 1-1.5 hours. The catalytic conditions of triethylamine and the reaction parameters of PFOC and PVDF-UiO-66-NH2 have been optimized to achieve uniform grafting of fluorine chains, avoid membrane pore blockage, and ensure the chemical bonding stability between the fluorinated layer and the MOF layer, thereby improving the long-term anti-wetting properties of the composite membrane.

[0012] The second aspect of the present invention provides a superhydrophobic PVDF-UiO-66-PFOC composite membrane obtained by the above preparation method. The composite membrane has a three-layer composite structure: a PVDF hollow fiber membrane as a substrate, on which a UiO-66-NH2 crystal layer is grown in situ, and the outermost layer is a perfluorooctanoyl (PFOC) fluorinated layer grafted onto the amino group of the UiO-66-NH2 through amide bonds.

[0013] The third aspect of this invention provides the application of the above-mentioned superhydrophobic PVDF-UiO-66-PFOC composite membrane in CO2 absorption.

[0014] This invention addresses the core problems of traditional PVDF hollow fiber membranes in CO2 capture—low wetting, low mass transfer efficiency, and poor stability—by in-situ growth of a targeted combination of UiO-66-NH2 and PFOC on the surface of activated PVDF hollow fiber membranes and optimizing process parameters. This distinguishes it from existing MOF-modified membranes used for oil-water separation and general gas separation. The main reasons are as follows: Using PVDF hollow fiber membrane as a substrate is a common substrate in the field of CO2 capture. After alkali + oxidation treatment, carboxyl groups can be introduced to adapt to the in-situ growth of UiO-66-NH2.

[0015] The intermediate layer is a UiO-66-NH2 crystal layer grown on the PVDF surface by in-situ hydrothermal synthesis. The UiO-66-NH2 crystal layer has a stable structure, strong adhesion to the PVDF hollow fiber membrane, and is not easy to fall off. It has a high specific surface area and CO2 affinity, and can selectively adsorb CO2 to improve mass transfer efficiency. The surface is rich in amino groups, which can undergo nucleophilic addition reactions with PFOC to form stable chemical bonds.

[0016] A stable hydrophobic fluorinated layer is formed by grafting PFOC onto the amino group of UiO-66-NH2 via nucleophilic addition reaction. The selected PFOC has a high fluorine chain density, which can significantly reduce surface energy and ensure superhydrophobicity, resulting in a water droplet contact angle >150°, thus providing excellent anti-wetting performance during CO2 capture. The PFOC has moderate reactivity and can react efficiently with the amino group at room temperature without damaging the pore structure of the PVDF hollow fiber membrane. It is chemically grafted onto the amino group of MOF via amide bonds. Compared with coating, this chemical bonding method has stronger stability and is suitable for the long-term operation requirements of CO2 capture.

[0017] The advantages and beneficial effects of this invention are: (1) The composite membrane of this invention exhibits excellent superhydrophobicity and long-lasting anti-wetting properties. The static contact angle reaches 166.37°, far exceeding the superhydrophobic standard of 150°. After a 60-hour stability test, the contact angle decreased by only 25.28%, indicating that the fluorinated layer is firmly attached through chemical bonding and is not easily damaged or peeled off. During CO2 absorption, it effectively prevents the absorbent from wetting the membrane pores, avoiding a sharp increase in mass transfer resistance caused by liquid filling the pores.

[0018] (2) The CO2 mass transfer rate of the composite membrane of the present invention reaches 0.33 mol·m -2 ·s -1 Compared to the original PVDF hollow fiber membrane, this represents a significant improvement. It offers greater processing capacity per unit membrane area, allowing for smaller equipment sizes or increased throughput, while reducing investment and operating costs.

[0019] (3) After 60 hours of continuous operation, the mass transfer rate of the composite membrane of the present invention remains at 0.206 mol·m⁻¹. -2 ·s -1 The performance degradation is far lower than that of the comparative model, exhibiting excellent operational stability. This extends the service life and replacement cycle of membrane modules, reduces maintenance frequency, and enhances the reliability of industrial applications. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the preparation process of the PVDF-UiO-66-PFOC composite membrane of the present invention.

[0021] Figure 2 The images show the AFM diagrams of the PVDF hollow fiber membrane and the PVDF-UiO-66-NH2 composite membrane with different formic acid contents of the present invention, wherein the PVDF hollow fiber membrane is shown in (a) and the membranes contain 0 eq formic acid (b), 30 eq formic acid (c), 60 eq formic acid (d), 90 eq formic acid (e), and 120 eq formic acid (f).

[0022] Figure 3 This is a graph showing the effect of formic acid content on the contact angle of PVDF-UiO-66-NH2 composite membrane and PVDF-UiO-66-PFOC composite membrane according to the present invention.

[0023] Figure 4 This is a graph showing the effect of formic acid content on the air permeability of PVDF-UiO-66-NH2 composite membrane and PVDF-UiO-66-PFOC composite membrane according to the present invention.

[0024] Figure 5 shows the changes (a) and the degree of decrease (b) of the contact angle of the PVDF hollow fiber membrane and PVDF-UiO-66-PFOC with different formic acid contents in the immersion experiment of the present invention.

[0025] Figure 6This figure shows the effect of gas phase flow rate on the CO2 mass transfer efficiency and removal rate of PVDF hollow fiber membrane and PVDF-UiO-66-PFOC composite membrane at a liquid phase flow rate of 15 mL / min.

[0026] Figure 7 This figure shows the effect of liquid phase flow rate on CO2 mass transfer efficiency and removal rate of PVDF hollow fiber membrane and PVDF-UiO-66-PFOC composite membrane at a gas phase flow rate of 200 mL / min.

[0027] Figure 8 This is a graph showing the change in CO2 mass transfer efficiency of the PVDF hollow fiber membrane and the PVDF-UiO-66-PFOC composite membrane during long-term operation of the present invention.

[0028] Figure 9 SEM images of PVDF-UiO-66-NH2 composite membranes with formic acid contents of 0 eq(a), 30 eq(b), 60 eq(c), 90 eq(d), and 120 eq(e) and PVDF-UiO-66-PFOC composite membranes with formic acid contents of 0 eq(f), 30 eq(g), 60 eq(h), 90 eq(i), and 120 eq(j). Detailed Implementation

[0029] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0030] Example 1 A method for preparing a superhydrophobic PVDF-UiO-66-PFOC composite membrane includes the following steps: (1) Activation treatment of PVDF hollow fiber membranes: The PVDF hollow fiber membrane was immersed in a mixed solution containing 1 mol / L NaOH and 0.1 wt% TBAB, treated at 60°C for 15 minutes, and washed with deionized water. It was then transferred to a solution containing 0.4 mol / L HNO3, 0.1 wt% (NH4)4Ce(SO4)2, and 5 v / v% acrylic acid, reacted at 35°C for 1 hour, and dried to obtain the activated PVDF hollow fiber membrane (PVDF-COOH membrane).

[0031] (2) In-situ growth of UiO-66-NH2: 2 mmol of 2-aminoterephthalic acid, 2 mmol of ZrCl4, and 120 eq (based on the molar amount of 2-aminoterephthalic acid) of formic acid were dissolved in 240 mL of ethanol and ultrasonically dispersed. A PVDF-COOH membrane was immersed in the above solution and reacted in a hydrothermal reactor at 120 °C for 12 hours. After washing and drying, a PVDF-UiO-66-NH2 membrane was obtained.

[0032] (3) PFOC grafting modification: 1.5 mL of PFOC was dissolved in 115 mL of n-hexane, 1.67 mL of triethylamine was added, and the mixture was immersed in a PVDF-UiO-66-NH2 membrane. The nucleophilic addition reaction was carried out at room temperature for 1.5 hours. After washing with ethanol and drying, the PVDF-UiO-66-PFOC composite membrane was obtained.

[0033] Example 2 The only difference from Example 1 is that the amount of formic acid added in step (2) is 30 eq.

[0034] Example 3 The only difference from Example 1 is that the amount of formic acid added in step (2) is 60 eq.

[0035] Example 4 The only difference from Example 1 is that the amount of formic acid added in step (2) is 90 eq.

[0036] Example 5 The only difference from Example 1 is that the hydrothermal temperature in step (2) is 100°C.

[0037] Example 6 The only difference from Example 1 is that the hydrothermal temperature in step (2) is 140°C.

[0038] Comparative Example 1 The only difference from Example 1 is step (2). The PVDF-COOH membrane was immersed in 40 mL of methanol solution containing 2 mmol Zn(NO3)2·6H2O and 8 mmol 2-methylimidazole, stirred at room temperature for 12 hours, washed and dried to obtain the PVDF-ZIF-8 membrane.

[0039] Comparative Example 2 The only difference from Example 1 is step (3), in which the PVDF-UiO-66-NH2 membrane is immersed in an aqueous solution containing 1 mg / mL PFHx (perfluorooctanoic acid), shaken at room temperature for 24 hours, washed and dried to obtain the PVDF-UiO-66-PFHx composite membrane.

[0040] Comparative Example 3 The only difference from Example 1 is that formic acid is not added in step (2).

[0041] Performance testing and analysis The following tests were performed on the original PVDF hollow fiber membrane (original membrane), the composite membranes of the examples, and the comparative examples: Hydrophobicity testing was conducted using a static contact angle test, which measured the initial static contact angle of the membrane and the static contact angle after immersion for 60 hours.

[0042] CO2 absorption and long-term stability experiment: The initial mass transfer rate of the membrane, the mass transfer rate after 60 h of operation, and the CO2 removal rate were tested at a gas phase flow rate of 200 mL / min and a liquid phase flow rate of 15 mL / min.

[0043] The test data is shown in Table 1.

[0044] Table 1

[0045] The analysis based on the table data and attached figures is as follows: Figure 2 AFM images of PVDF hollow fiber membranes and PVDF-UiO-66-NH2 composite membranes with different formic acid contents are shown, where PVDF hollow fiber membrane (a), formic acid addition is 0 eq (b), 30 eq (c), 60 eq (d), 90 eq (e), and 120 eq (f). The images allow for a direct observation of the surface microstructure of the original PVDF membrane and the PVDF-UiO-66-NH2 composite membranes with different formic acid contents. UiO-66-NH2 crystals grow uniformly and densely on the PVDF substrate surface, forming a micro / nano-scale rough structure. The surface roughness of the membrane changes with the formic acid content.

[0046] Figure 3 The graph shows the effect of formic acid content on the contact angle of PVDF-UiO-66-NH2 and PVDF-UiO-66-PFOC composite membranes, clearly illustrating the trend of contact angle variation with formic acid content. Through the nucleophilic addition reaction of perfluorooctanoyl chloride (PFOC) with the amino group of UiO-66-NH2, fluorine chains were successfully grafted onto the membrane surface, significantly reducing surface energy. The results show that at a formic acid content of 120 eq, the initial contact angle of the composite membrane reaches 166.37°, meeting the superhydrophobic standard (contact angle > 150°), an improvement of 86.9% compared to the original membrane (89.01°). This data indicates that the composite membrane significantly enhances hydrophobicity through synergistic micro / nano roughening structure and low surface energy modification, effectively suppressing membrane pore wetting.

[0047] Figure 4This figure shows the effect of formic acid content on the air permeability of PVDF-UiO-66-NH2 composite membranes and PVDF-UiO-66-PFOC composite membranes. The figure visually illustrates the changes in air permeability of the two composite membranes with increasing formic acid content, revealing the influence of UiO-66-NH2 crystal growth and PFOC grafting on air permeability, as well as the air permeability coefficient and maximum pore size of the PVDF-UiO-66-PFOC composite membrane at different formic acid contents. The results show that when the formic acid content is 120 eq, the air permeability coefficient of the composite membrane is 0.157 mL·m. -2 ·s -1 ·Pa -1 Compared to the original membrane (0.236 mL·m -2 ·s -1 ·Pa -1 The 33.5% decrease indicates that the in-situ growth of UiO-66-NH2 did not significantly block the membrane pores, and the membrane structure maintained reasonable permeability, balancing hydrophobicity and mass transfer efficiency.

[0048] Figure 5 Figure (a) shows the changes in contact angle (and the degree of decrease) of PVDF hollow fiber membranes and PVDF-UiO-66-PFOC composite membranes with different formic acid contents during immersion experiments, and Figure (b) clearly shows the trend of contact angle changes over time for the original PVDF membrane and PVDF-UiO-66-PFOC composite membranes with different formic acid contents during the immersion experiment, as well as the comparison of the degree of decrease in their contact angles. Figure 5 The data clearly demonstrates that the amount of formic acid added (30-120 eq) is the key to controlling the hydrophobic stability of the composite membrane. When the formic acid is ≥60 eq, the uniform growth of UiO-66-NH2 crystals can be controlled, and the fluorinated layer chemically bonded by PFOC can be combined to enable the composite membrane to simultaneously possess "superhydrophobic initial properties" and "low-attenuation long-term stability", thus precisely solving the technical pain point of easy wetting of traditional PVDF hollow fiber membranes.

[0049] Figure 6 This figure shows the effect of gas phase flow rate on the CO2 mass transfer efficiency and removal rate of the original PVDF membrane and the PVDF-UiO-66-PFOC composite membrane. It illustrates the changes in CO2 mass transfer rate and removal rate of the original and composite membranes at different gas phase flow rates, with a gas phase flow rate of 200 mL / min. -1 The liquid flow rate is 15 mL / min. -1 Under the specified conditions, the highest mass transfer rate of the composite membrane was 0.33 mol·m⁻¹. -2 ·s -1 Compared to the original membrane (0.19 mol·m -2 ·s -1The CO2 removal rate increased by 73.7% from 31.46% to 55.29%. This performance improvement is attributed to the selective adsorption of CO2 by UiO-66-NH2 and the anti-wetting properties of the composite membrane.

[0050] Figure 7 The effect of liquid phase flow rate on the CO2 mass transfer efficiency and removal rate of PVDF membrane and PVDF-UiO-66-PFOC composite membrane is shown. The change in CO2 mass transfer rate of the PVDF-UiO-66-PFOC composite membrane during continuous 60 hours of operation can be observed. The initial mass transfer rate of the composite membrane is 0.33 mol·m⁻¹. -2 ·s -1 After running for 60 hours, it decreased to 0.206 mol·m -2 ·s -1 The decrease was 37.6%, but it was still significantly higher than the original membrane (0.136 mol·m). -2 ·s -1 The results indicate that although the hydrophobicity of the composite membrane decreases due to the hydrolysis of some amide bonds during long-term operation, its overall anti-wetting performance and CO2 absorption efficiency are still superior to those of the traditional PVDF hollow fiber membrane.

[0051] Figure 8 This figure shows the changes in CO2 mass transfer efficiency of the original PVDF membrane and the PVDF-UiO-66-PFOC composite membrane during long-term operation. The figure compares the changes in CO2 mass transfer rate of the original PVDF membrane and the PVDF-UiO-66-PFOC composite membrane during 60 hours of operation, visually reflecting the long-term operational stability of the composite membrane.

[0052] Figure 9 SEM images of the PVDF-UiO-66-NH2 composite membranes with formic acid contents of 0 eq(a), 30 eq(b), 60 eq(c), 90 eq(d), and 120 eq(e), and the PVDF-UiO-66-PFOC composite membrane with formic acid contents of 0 eq(f), 30 eq(g), 60 eq(h), 90 eq(i), and 120 eq(j) are shown. The surface SEM images of the PVDF-UiO-66-PFOC composite membrane (formic acid content 120 eq) are also presented. It is evident that UiO-66-NH2 crystals grow uniformly and densely on the PVDF substrate surface, forming a micro-nano-scale rough structure. After PFOC grafting, there is no significant change in the membrane surface morphology, indicating that the fluorine chain modification did not disrupt the crystal framework. This structure provides physical support for superhydrophobicity while preserving the integrity of the membrane pores.

[0053] Comparative Example 1 showed decreased hydrophobicity, CO2 mass transfer efficiency, and stability due to poor interfacial compatibility between ZIF-8 and PVDF, leading to easy particle shedding. The water contact angle was only 135.2°±3.1°, lacking a roll-off angle advantage, and the gas-liquid interface was easily wetted by the absorbent, increasing mass transfer resistance.

[0054] Comparative Example 2 had a water contact angle of 102.6°, which decreased to 60.1° after 60 hours of immersion (the design in this application maintains 124.3°), indicating poor hydrophobic durability. The CO2 permeation flux was 920 Barrer, with a CO2 / N2 selectivity of only 24.3, indicating weak CO2 adsorption selectivity due to the short hydrophobic chain of PFHx. After 60 hours of continuous operation, the mass transfer rate decreased by 47.6%, indicating weak grafting forces between PFHx and UiO-66-NH2, leading to easy desorption and loss.

[0055] Industrial applications The composite membrane of this invention is specifically designed for CO2 capture systems in coal-fired power plants, chemical plants, and other fields. Compared with existing technologies, it has the following application advantages: ① Compared with MOF modified membranes used for oil-water separation (such as Chinese patents CN115155335A and CN116617877A), this composite membrane combines superhydrophobicity with selective CO2 adsorption capacity, improving mass transfer efficiency by 73.7%, and can be directly adapted to gas-liquid contact scenarios for CO2 capture; ② Compared with membranes used for general gas separation (such as CN120662134A), this composite membrane has optimized the modification process for PVDF membranes, resulting in better long-term operational stability (mass transfer rate decrease of only 36.4% after 60 hours); ③ The process parameters are clear and easy to scale up industrially. Key parameters such as formic acid addition and reaction temperature have been optimized to avoid performance fluctuations in industrial production, resulting in significant economic and environmental benefits.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the inventive concept, and these all fall within the protection scope of the present invention.

Claims

1. A method for preparing a superhydrophobic PVDF-UiO-66-PFOC composite membrane, characterized in that, Includes the following steps: (1) Activation treatment of PVDF hollow fiber membrane: The PVDF hollow fiber membrane is subjected to alkali treatment and oxidative grafting treatment in sequence to obtain activated PVDF hollow fiber membrane; (2) In-situ growth of UiO-66-NH2: The activated PVDF hollow fiber membrane in step (1) is immersed in a precursor solution containing zirconium chloride, 2-aminoterephthalic acid and formic acid. UiO-66-NH2 crystal layer is grown in-situ on the surface of the activated PVDF hollow fiber membrane by hydrothermal synthesis to obtain PVDF-UiO-66-NH2 membrane; (3) PFOC grafting modification: The PVDF-UiO-66-NH2 membrane obtained in step (2) is immersed in a mixed solvent containing PFOC, triethylamine and n-hexane to carry out a nucleophilic addition reaction, and the fluorine is grafted onto the amino group of UiO-66-NH2 to form an amide bond-linked fluorinated layer, thus obtaining the PVDF-UiO-66-PFOC composite membrane.

2. The preparation method according to claim 1, characterized in that, The conditions for the alkali treatment in step (1) are as follows: use a mixed solution containing 0.8-1.2 mol / L NaOH and 0.05wt%-0.15wt% tetrabutylammonium bromide, and react at 50℃-80℃ for 10-20 minutes.

3. The preparation method according to claim 1, characterized in that, The conditions for the oxidative grafting treatment in step (1) are as follows: use a mixed solution containing 0.2 mol / L - 0.5 mol / L HNO3, 0.1 w / v% - 0.15 w / v% cerium ammonium sulfate and 4 v / v% - 6 v / v% acrylic acid, and react at 20℃ - 45℃ for 1-2 hours.

4. The preparation method according to claim 1, characterized in that, In the precursor solution described in step (2), based on the amount of 2-aminoterephthalic acid, the amount of formic acid added is 30-120 equivalents (eq), and the hydrothermal synthesis conditions are 100℃-140℃ for 10-12 hours.

5. The preparation method according to claim 1, characterized in that, The concentration of 2-aminoterephthalic acid and the concentration of ZrCl4 in the precursor solution in step (2) are 5-10 mmol / L.

6. The preparation method according to claim 1, characterized in that, In step (3), the volume ratio of PFOC, triethylamine, and n-hexane in the mixed solvent is 2-3 mL: 1-2 mL: 70-120 mL.

7. The preparation method according to claim 1, characterized in that, The nucleophilic addition reaction was carried out at room temperature for 1-1.5 hours.

8. A superhydrophobic PVDF-UiO-66-PFOC composite membrane obtained by the preparation method according to any one of claims 1-7, characterized in that, The composite membrane has a three-layer composite structure: a PVDF hollow fiber membrane as the substrate, on which a UiO-66-NH2 crystal layer is grown in situ, and the outermost layer is a perfluorooctanoyl fluorinated layer grafted onto the amino group of the UiO-66-NH2 through amide bonds.

9. The application of the superhydrophobic PVDF-UiO-66-PFOC composite membrane as described in claim 8 in CO2 absorption.

Images