Preparation and application of a flower-like Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite membrane

Abstract

The application discloses a kind of flower-shaped Mg-Al LDH modified polydimethylsiloxane / polyether sulfone composite film preparation and application thereof.Polyether sulfone is dissolved in N,N-dimethylacetamide, after blade film, film is immersed in deionized water.The PDMS is dissolved in isooctane solvent, then different concentrations of LDH are added to continue stirring until completely dispersed, and the crosslinking agent and catalyst are added according to the mass ratio of 10:2:1, and the wet film immersion coating method is used to prepare the coating.Modified composite film is obtained after coating.The application is mainly applied to the preparation of gas-liquid two-phase gas separation to obtain high CO2 / O2 selectivity, high gas permeation composite membrane material.The method is simple in preparation process, and the raw materials are cheap and easy to obtain, and the composite membrane modified by LDH can effectively improve the gas permeability and selectivity of the material, providing a new preparation idea for preparing good CO2 gas separation membrane.

Description

Technical Field

[0001] This invention belongs to the field of gas-liquid two-phase technology in biomedicine, and relates to the preparation and application of a flower-shaped Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite membrane. Background Technology

[0002] Mg-Al layered bimetallic hydroxide (LDH) is a hydrotalcite compound. The structure of LDH consists of charged magnesia-like layers, and its general chemical formula is [Mg...]. Ⅱ 1-x Al Ⅲ x (OH)] x+ [(X m- ) x / m ·mH2O] x- The layered structure of LDH consists of a positively charged metal hydroxide main layer and interlayer anions. Therefore, its physicochemical properties can be precisely adjusted by modifying the Mg / Al ratio. Furthermore, the metal -OH groups on the surface of Mg-Al LDH contain basic sites, which can form metal -HCO3 with CO2, and the CO32- anions in the LDH layers... 2- It can act as a mobile carrier for CO2 transport and also promote CO2 transfer. Therefore, Mg-Al LDH is a good CO2 selective modified filler.

[0003] Polyethersulfone (PES) is a commonly used material for preparing support layers in gas separation membranes due to its excellent mechanical stability, chemical stability, and porosity. However, its microporous surface structure results in low gas selectivity in gas separation applications. Polydimethylsiloxane (PDMS) polymer is a silicone rubber material with good CO2 selectivity, high density, and strong film-forming properties, making it suitable as a coating to form defect-free composite membranes on PES surfaces. Bilal Haider et al., in their paper "Highly permeable innovative PDMS coated polyethersulfone membranes embedded with activated carbon for gas separation," described the preparation of a separation membrane with PDMS as the functional layer and PES as the support layer. PES, with its high mechanical strength, high heat resistance, and excellent alkali resistance, is a potential support layer. However, due to the intrinsic properties of PDMS, the upper limit of the CO2 separation performance of the composite membrane is relatively low; therefore, unmodified PDMS composite membranes have certain defects. Therefore, this invention introduces Mg-Al LDH material with good CO2 selectivity channels, explores the synthesis of flower-shaped LDH by changing reaction conditions, does it into PDMS to form a hybrid coating, and explores the effect of flower-shaped LDH on the CO2 gas permeability and selectivity of the composite membrane. Summary of the Invention

[0004] To address the application limitations of the PDMS materials described above, this invention proposes a simple modification method using a hydrothermal synthesis reaction. This involves adding different concentrations of urea and hydrolyzing the resulting NH4 at high temperatures. + Adjusting the pH with OH- reduces the charge of edge groups and lowers Coulomb repulsion under highly alkaline synthesis conditions, leading to edge-edge aggregation rather than substrate-edge growth. This alters the crystal growth path of LDH, resulting in the synthesis of flower-like Mg-Al LDH. Then, PES, which is physicochemically stable, non-toxic, and does not change its composition upon contact with both gas and liquid phases, is used as the composite membrane support material. A mixed matrix composite membrane is prepared using PDMS doped with flower-like Mg-Al LDH as the coating. This invention utilizes phase inversion and wet coating methods to prepare composite membranes, successfully addressing the issue of the inherent softness of PDMS material. Furthermore, the modified PDMS coating results in a thinner membrane layer, achieving good gas permeability and selectivity.

[0005] The technical solution of the present invention:

[0006] A flower-like Mg-Al LDH-modified polydimethylsiloxane / polyethersulfone composite membrane is disclosed, wherein polyethersulfone serves as the porous support layer of the composite membrane; PDMS modified with flower-like LDH doped with different mass parameters serves as the functional layer; PDMS is dissolved at room temperature, then LDH of different concentrations is added, followed by crosslinking with a crosslinking agent; the treated modified PDMS coating solution is then dip-coated onto a pre-prepared polyethersulfone base membrane. The modified PDMS acts as a separation coating in the composite membrane, and its thickness can be controlled by dip-coating. The polyethersulfone base membrane has a loose, finger-like porous structure in cross-section with large internal cavities and micropores on its dark surface. The modified PDMS, as the functional coating, is dense and exhibits uniform LDH dispersion.

[0007] A method for preparing a flower-like Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite film, comprising the following steps:

[0008] (1) PES porous support layer: The phase inversion method was used, with N,N-dimethylformamide (DMF) as solvent, 15 wt.% of PES and 5 wt.% of polyethylene glycol (PEG) were added to the total solution, and PEG was used as pore-forming agent to prepare the base film solution. The solution was ultrasonically allowed to stand for 2 hours. The membrane was coated using a tabletop flatbed membrane coating machine. After coating, the membrane was immersed in deionized water for 24 hours to carry out phase inversion and obtain the PES porous support layer.

[0009] (2) Preparation of flower-like Mg-Al LDH: Magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, and urea were dissolved in deionized water at a molar ratio of 2:1:10 to prepare Mg 2+ A 0.67 mol / L solution was prepared. The solution was placed in a reactor and subjected to hydrothermal crystallization in an oven at 110 °C for 24 h. During the reaction, urea hydrolysis created a highly alkaline environment, reducing the charge of edge groups and lowering Coulomb repulsion, thus altering the crystallization process and causing crystals to grow in different directions. After the reaction, the crystals were washed three times with deionized water and anhydrous ethanol by centrifugation. The resulting precipitate was dried in a vacuum oven at 70 °C for 12 h and then ground to obtain flower-like Mg-Al LDH.

[0010] (3) Preparation of PDMS coating composite film with LDH doped: Prepare an isooctane solution of PDMS with a concentration of 5wt.% and add flower-shaped Mg-Al LDH to the isooctane solution of PDMS and stir until completely dispersed. Control the mass ratio of flower-shaped Mg-Al LDH to PDMS to be 2%-10%. Add crosslinking agent and catalyst according to the mass ratio of PDMS, crosslinking agent tetraethyl silicate and catalyst dibutyltin dilaurate of 10:2:1. Defoaming is performed by ultrasonication to obtain a uniform coating liquid. The coating liquid is coated on the surface of PES porous support layer by wet coating to form a uniform liquid film on the surface of PES porous support layer. Dry at 85℃ for 2h to obtain flower-shaped Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite film.

[0011] The above-mentioned flower-shaped LDH-modified polydimethylsiloxane / polyethersulfone composite membrane is used as a membrane material for gas exchange at the gas-liquid interface.

[0012] The beneficial effects of this invention: This invention is mainly used to prepare composite membrane materials with high CO2 / O2 selectivity and good gas permeability. First, a polyethersulfone membrane material with certain mechanical strength is prepared via a phase inversion method. This material, as the base membrane of the composite membrane, can provide support for the functional layer membrane material with poor mechanical properties, thereby overcoming the limitations of the functional coating material itself. Then, a flower-shaped LDH-modified polydimethylsiloxane functional layer is prepared by blending LDH with PDMS. LDH contains metal-OH basic sites, which can form metal-HCO3 with CO2, improving the adsorption of CO2 by the composite membrane. Furthermore, the CO32- anions between the LDH layers... 2- The ability to act as a mobile carrier for CO2 transport also facilitates CO2 transfer. LDH-doped PDMS coatings, as dense membrane materials, can improve problems such as gas embolism and feed leakage during composite membrane applications. Furthermore, the readily available and inexpensive raw materials of this invention enable the large-scale application of this method.

[0013] This invention primarily focuses on testing the CO2 permeability of flower-shaped LDH morphology. Due to its 3D morphology, flower-shaped LDH exhibits isotropic CO2 transport and adsorption. Furthermore, the 3D structure exposes more -OH sites, and the openness of the internal channels significantly reduces resistance to CO2 transport, thus facilitating CO2 adsorption and transport. Flower-shaped LDH achieves good CO2 and O2 separation even at ambient pressure. For the ambient pressure environment within biological systems, LDH materials are particularly advantageous for separating CO2 and O2 gaseous components in gas-liquid two-phase media. Attached Figure Description

[0014] Figure 1The images show the SEM, XRD, and Fourier transform infrared (FTIR) spectra of two different morphologies of LDH, where (A) is the SEM image of sheet-like LDH; (B) is the SEM image of flower-like LDH; (C) is the Fourier transform infrared (FTIR) spectrum of sheet-like Mg-Al LDH and flower-like Mg-Al LDH; and (D) is the XRD spectrum of sheet-like LDH and flower-like LDH.

[0015] Figure 2 The images show the SEM images of two morphologies of unmodified and modified PDMS / PES composite films with LDH. (A) Surface image of 5wt.% PDMS / PES composite film; (B) Surface image of 5wt.% PDMS / PES composite film with sheet-like LDH doping; (C) Surface image of 5wt.% PDMS / PES composite film with flower-like LDH doping; (D) Cross-sectional image of 5wt.% PDMS / PES composite film; (E) Cross-sectional image of 5wt.% PDMS / PES composite film with sheet-like LDH doping; (F) Cross-sectional image of 5wt.% PDMS / PES composite film with flower-like LDH doping.

[0016] Figure 3 These are CO2 gas permeability diagrams for flower-shaped and sheet-shaped LDH-modified PDMS / PES composite membranes.

[0017] Figure 4 This is a simulation diagram of the orbital energy level potential of PDMS / PES composite films modified with flower-shaped and sheet-shaped LDH.

[0018] Figure 5 These are molecular dynamics (MD) simulations of flower-shaped and sheet-shaped LDH-modified PDMS / PES composite films. In the MD simulation, (A) is the mean square displacement (MSD) simulation of CO2 in the flower-shaped and sheet-shaped LDH-modified composite films, and (B) is the radial distribution function (DFT) simulation of CO2 in the flower-shaped and sheet-shaped LDH-doped PDMS coatings. Detailed Implementation

[0019] The specific embodiments of the present invention are further described below with reference to the accompanying drawings and technical solutions.

[0020] Example:

[0021] The method for preparing flower-shaped LDH-modified polydimethylsiloxane / polyethersulfone composite membrane includes the following steps:

[0022] (1) PES porous support layer: The phase inversion method was used, with N,N-dimethylformamide (DMF) as solvent and polyethylene glycol (PEG) as pore-forming agent. The total solution mass was 60g, including 9g of PES, 3g of PEG and 48g of DMF. The base film solution was prepared and ultrasonically allowed to stand for 2h. The film was coated using a tabletop flatbed film coating machine. After coating, the film was immersed in deionized water for 24h to carry out phase inversion and obtain the PES porous support layer.

[0023] (2) Preparation of flower-like Mg-Al LDH: 10.256 g magnesium nitrate hexahydrate, 7.5026 g aluminum nitrate nonahydrate, and 12.012 g urea were dissolved in 60 mL of deionized water to prepare Mg 2+ A 0.67 mol / L solution was placed in a reactor and subjected to hydrothermal crystallization in an oven at 110°C for 24 hours. During the reaction, urea hydrolyzed to create a highly alkaline environment, reducing the charge of edge groups and lowering Coulomb repulsion, thus altering the crystallization process and causing crystals to grow in different directions. After the reaction, the crystals were washed three times with deionized water and anhydrous ethanol by centrifugation. The resulting precipitate was dried in a vacuum oven at 70°C for 12 hours and then ground to obtain flower-shaped LDH.

[0024] (3) Preparation of LDH-doped PDMS-LDH / PES composite membrane: Prepare a 5 wt.% PDMS isooctane solution. Dissolve 2 g PDMS in 38 g isooctane, and then add 2%, 4%, 6%, 8%, and 10% LDH (based on the mass of PDMS) to the PDMS solution and stir until completely dispersed. Add 400 mg tetraethyl silicate as a crosslinking agent and 200 mg dibutyltin dilaurate as a catalyst for crosslinking for 30 min. Defoaming is performed by ultrasonication to obtain a uniform coating solution. The coating solution is then wet-coated onto the surface of a PES porous support substrate to form a uniform liquid film on the surface of the PES porous support substrate. The film is dried at 85 °C for 2 h to obtain a flower-shaped LDH-doped PDMS-LDH / PES composite membrane.

[0025] Table 1. Preparation details of 0-10 wt.% flower-shaped LDH-modified PDMS coating solution

[0026]

[0027] The doped flower-shaped LDH has a good promoting effect on CO2 transport on the PDMS surface. The flower-shaped LDH has a 3D morphology, which is isotropic in CO2 transport and adsorption. At the same time, the 3D structure can expose more -OH sites, and the openness of the internal channels of the 3D structure greatly reduces the resistance to CO2 transport, which is beneficial to CO2 adsorption and transport.

[0028] Comparative example:

[0029] A method for preparing sheet-like LDH-modified polydimethylsiloxane / polyethersulfone composite membrane material includes the following steps:

[0030] (1) PES porous support layer: PES porous base film is prepared by phase inversion method and flat plate film scraping machine.

[0031] (2) Preparation of flake-shaped Mg-Al LDH: 1.28 g magnesium nitrate hexahydrate, 0.94 g aluminum nitrate nonahydrate, and 1.35 g urea were dissolved in 50 mL of deionized water to prepare Mg-Al LDH flakes. 2+ A 0.01 mol / L solution was placed in a sealed reaction vessel and then placed in an oven at 150°C for hydrothermal crystallization for 16 hours. The low urea concentration and low hydrolysis pH during the hydrothermal reaction caused crystals to grow along the substrate edge. After the reaction, the precipitate was washed three times with deionized water and anhydrous ethanol, and then dried in a vacuum oven at 70°C for 12 hours. After the reaction, the precipitate was washed three times with deionized water and anhydrous ethanol by centrifugation, dried in a vacuum oven at 70°C for 12 hours, and then ground to obtain flake-like LDH.

[0032] (3) Preparation of PDMS-LDH / PES composite film doped with LDH: Prepare a 5 wt.% PDMS isooctane solution. Dissolve 2 g PDMS in 38 g isooctane, and then add 2%, 4%, 6%, 8%, and 10% of sheet-like LDH (based on the mass of PDMS) to the PDMS solution and stir until completely dispersed. Add 400 mg tetraethyl silicate as a crosslinking agent and 200 mg dibutyltin dilaurate as a catalyst for crosslinking for 30 min. Defoaming is performed by ultrasonication to obtain a uniform coating solution. The coating solution is then wet-coated onto the surface of a PES porous support substrate to form a uniform liquid film on the surface of the PES porous support substrate. The film is dried at 85 °C for 2 h to obtain a sheet-like LDH-doped PDMS-LDH / PES composite film.

[0033] Characterization and Result Analysis of Composite Membrane Materials

[0034] 1. The flower-like Mg-Al LDH of the prepared examples and the sheet-like Mg-Al LDH of the comparative examples were characterized by field emission scanning electron microscopy. The crystal structure of the prepared Mg-Al LDH was analyzed by X-ray diffraction (XRD), and the characteristic functional groups of the prepared Mg-Al LDH were characterized by Fourier transform infrared spectroscopy (ATR). The results are as follows: Figure 1As shown, Figures (A) and (B) illustrate the scanning electron microscopy analysis of the LDH synthesized in the examples and comparative examples, demonstrating the successful synthesis of the flower-like LDH of the examples and the plate-like LDH of the comparative examples. Figure (C) shows the FTIR spectra of the flower-like LDH of the examples and the plate-like LDH of the comparative examples, revealing the presence of characteristic peaks of LDH, indicating the successful synthesis of the LDH in the examples and the comparative examples. Figure (D) shows the XRD patterns of the flower-like LDH of the examples and the plate-like LDH of the comparative examples. After comparison with the standard card, there was no significant difference in the characteristic peaks between the XRD patterns of the examples and the comparative examples. However, due to the 3D structure of the flower-like LDH, there was a change in crystallinity, most notably a slight shift at around 33°. Therefore, the flower-like and plate-like LDH can be distinguished from each other from the XRD patterns.

[0035] 2. Material characterization of the flower-like and sheet-like LDH-doped PDMS-LDH / PES composite films prepared in Examples 1-6 and Comparative Examples 1-6 by field emission scanning electron microscopy. First, the composite films were cryogenically quenched in liquid nitrogen to observe their cross-sectional morphology. The treated composite films were then fixed onto the sample stage using conductive adhesive and sputtered with gold to observe their surface and cross-sectional morphology. The characterization results are as follows: Figure 2 As shown, (A) and (D) represent the surface and cross-sectional views of the PDMS / PES composite membranes prepared in Example 1 and Comparative Example 1, respectively. The dense PDMS layer has a smooth and flat surface, and the PDMS coating thickness is approximately 2.7 μm, forming a uniform and stable functional coating. (B) and (E) represent the PDMS-LDH / PES composite membrane prepared in the comparative example. The surface of the composite membrane shows obvious sheet-like LDH morphology. However, due to the large difference in size between the vertical and horizontal directions of the sheet-like LDH, there are more horizontally distributed sheet-like LDHs in the PDMS, which is not conducive to CO2 transport. (C) and (F) represent the PDMS-LDH / PES composite membrane prepared in the example. The surface of the composite membrane shows obvious flower-like LDH morphology, and the 2.7 μm coating thickness can just cover the 2.6 μm LDH. Due to the 3D morphology of the flower-like LDH, there is no problem of uneven arrangement in the PDMS coating, which can promote CO2 transport from each direction.

[0036] 3. The pure gas permeability of the composite membrane materials in the examples and comparative examples was determined using a gas permeability testing device. The composite membranes were placed in the gas testing device, ensuring a consistent membrane area, and the water bath was kept at a constant temperature. Pressure was applied to the coating to maintain the transmembrane pressure at 0.1 MPa. The upper and lower gas paths of the pipeline were purged with pure gas to ensure a single flowing gas. The gas permeability of CO2 and O2 was measured using a soap bubble flow meter, respectively. (1 GPU = 10^64 GHz) -6 cm 3 / cm 2The permeation rate, expressed as scmHg, is determined by the following equation:

[0037]

[0038] Where J is the gas permeability (GPU); ΔV is the volume of gas passing through the flow meter, which is 0.1 cm³. 3 T is room temperature (25℃); A is the membrane area, calculated to be 0.785 cm². 2 t represents the time it takes for the gas to pass through the flow meter, in seconds; Δp represents the test pressure difference, in MPa. Each data point was measured in triplicate, and the average value was taken. The gas permeability of the examples and comparative examples is shown below. Figure 3 As shown, comparing the flower-shaped PDMS-LDH / PES composite membrane prepared in the example with the sheet-like PDMS-LDH / PES composite membrane prepared in the comparative example, the addition of flower-shaped LDH significantly improved the CO2 permeation rate of the composite membrane. Furthermore, the CO2 permeability increased with increasing doping concentration of flower-shaped LDH, reaching a maximum of approximately 1700 GPU, breaking the intrinsic gas permeability limit of PDMS. This is because the spatial structure of flower-shaped LDH is fixed and its CO2 transport is isotropic; stacking them does not affect the LDH's transport efficiency. In contrast, the sheet-like PDMS-LDH / PES composite membrane in the comparative example showed a significantly reduced CO2 transport effect. This is because the sheet-like LDH accumulated in large quantities during the doping process, affecting the transport rate and resulting in a maximum CO2 transport rate of only 1100 GPU.

[0039] 4. Using the Dmol3 program in the Materials Studio (MS) software package, the optimization and electronic properties of the LDH-doped PDMS composite films of the examples and comparative examples were calculated based on density functional theory (DFT). The generalized gradient approximation and the Perdew-Burke-Ernzerhof (PBE) function were used to describe electron exchange and related effects, with dnd and basfilev4.4 as the basis. Molecular dynamics (MD) simulations and geometry optimizations were performed in an NVT ensemble at 298 K using MS's Forcite tool and COMPASS force field, with a total simulation time of 500 ps and a time step of 1 fs, to demonstrate the transfer and adsorption of CO2 / O2 gases in the film materials of the examples and comparative examples. The simulation results are as follows: Figure 4 and Figure 5 As shown, Figure 4 This diagram shows the potential energy level orbitals of the embodiment and the comparative example, both at the high energy level E. HOMO There is almost no difference in the orbits, but the embodiments are at low energy level E. LUMOThe lower orbital value results in a significantly smaller ΔE compared to the comparative example, indicating that less energy is required for the flower-shaped LDH to transport CO2. The PDMS-LDH / PES composite membrane doped with flower-shaped LDH has a regulated ability to rapidly adsorb CO2, which is beneficial for CO2 transport. Figure 5 The figures show molecular dynamics simulations of LDH-modified PDMS / PES composite membranes in the examples and comparative examples. Figure (A) shows that the mean square displacement (MSD) of CO2 transport is significantly faster in the PDMS composite membrane doped with flower-shaped LDH than in the PDMS composite membrane doped with sheet-like LDH, indicating that flower-shaped LDH has a stronger effect on CO2 transport. Figure (B) shows that, compared to the comparative example, the flower-shaped LDH in the examples can induce CO2 to form a short-range ordered distribution in the membrane material. This localized ordering significantly promotes the dissolution and diffusion of CO2 in the flower-shaped LDH-PDMS / PES composite system.

[0040] This invention optimizes and modifies the performance of a dense functional layer using a composite membrane. The presence of the PES base membrane provides mechanical support for the coated Mg-Al LDH-modified PDMS layer. Testing showed that the prepared flower-like LDH-modified PDMS layer provides excellent CO2 gas permeability for gas-liquid two-phase separation. Furthermore, molecular dynamics (MSD) simulations theoretically and indirectly confirmed the effective role of flower-like Mg-Al LDH in improving the CO2 gas permeability of the composite membrane, greatly strengthening the theoretical basis and providing new insights for the research of gas separation membranes.

Claims

1. A method for preparing a flower-like Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite film, characterized in that, The morphology of LDH was controlled by changing the reaction temperature and pH value, as follows: (1) A porous polyethersulfone support layer was prepared by phase inversion method; (2) Preparation of flower-like Mg-Al LDH: Magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, and urea were dissolved in deionized water to prepare Mg 2+ A solution with a concentration of 0.67 mol / L was prepared; the solution was subjected to hydrothermal crystallization; after the reaction was completed, the solution was washed by centrifugation with deionized water and anhydrous ethanol, and the resulting precipitate was dried and ground to obtain flower-shaped Mg-Al LDH. (3) Preparation of flower-shaped Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite membrane: Prepare a 5 wt.% polydimethylsiloxane isooctane solution, add flower-shaped Mg-Al LDH to the PDMS isooctane solution and stir until completely dispersed; add PDMS, crosslinking agent tetraethyl silicate, and catalyst dibutyltin dilaurate in a mass ratio of 10:2:1, and ultrasonically defoam to obtain a uniform coating liquid; apply the coating liquid to the surface of the PES porous support layer by wet coating to form a uniform liquid film on the surface of the PES porous support layer, and dry to obtain a flower-shaped Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite membrane; the mass ratio of flower-shaped Mg-Al LDH to PDMS is 2%-10%.

2. The preparation method according to claim 1, characterized in that, The molar ratio of magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, and urea is 2:1:

10.

3. The preparation method according to claim 1, characterized in that, The hydrothermal crystallization temperature was 110℃, and the time was 24 hours.

4. The preparation method according to claim 1, characterized in that, The drying conditions were: drying in a vacuum at 70°C for 12 hours.

5. The preparation method according to claim 1, characterized in that, The drying conditions are: drying at 85℃ for 2 hours.

6. A flower-shaped Mg-Al LDH modified polydimethylsiloxane / polyethersulfone composite membrane obtained by the preparation method according to any one of claims 1-5 is used as a membrane material for gas exchange at the gas-liquid interface.

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