A method for preparing MOF functionalized nanofiber composite proton exchange membrane

Abstract

The application discloses a method for preparing MOF functionalized nanofiber composite proton exchange membrane, and belongs to the field of polymer membrane materials. The proton exchange membrane is obtained through spinning, hydrolysis, in-situ growth and impregnation steps. The spun polymer material is polyacrylonitrile, the hydrolysis process is to immerse the polyacrylonitrile nanofiber membrane in a sodium hydroxide aqueous solution for a certain time, the in-situ growth process is to grow UiO-66-NH2 on the hydrolyzed polyacrylonitrile nanofiber, and the impregnation process is to fill the pores of the polyacrylonitrile nanofiber membrane after in-situ growth of MOF with a Nafion solution. Compared with the prior art, the application provides a method for preparing a polyacrylonitrile nanofiber membrane with in-situ growth of UiO MOF, and the method is used as a reinforcing body to be combined with Nafion to prepare a proton exchange membrane. The method combines the in-situ growth and electrospinning technologies, improves the compatibility of the MOF composite nanofiber and Nafion, and widens the application of the MOF in the field of proton exchange membranes.

Description

Technical Field

[0001] This invention belongs to the field of polymer membrane materials, and particularly relates to a method for preparing MOF functionalized nanofiber composite proton exchange membranes by introducing active sites on the surface of polyacrylonitrile nanofibers based on hydrolysis. Background Technology

[0002] With the development of nanofiber technology, combining nanofibers with high specific surface area with perfluorosulfonic acid polymers (such as Nafion) to prepare proton exchange membranes has become a research hotspot. Metal-organic frameworks (MOFs), as a rapidly developing class of novel porous materials, especially the UiO series, possess advantages such as high specific surface area, tunable pore structure, and strong stability. Combining MOFs with nanofibers and Nafion to prepare high-performance proton exchange membranes shows great promise.

[0003] There are two main ways to combine MOF with nanofibers: (1) direct electrospinning, in which the pre-formed MOF is added to the polymer precursor solution and the composite nanofiber membrane is obtained directly by electrospinning. This method is simple to operate and has few limitations, but the MOF particles are easy to aggregate and cause blockage during spinning. Also, since the diameter of MOF particles is smaller than that of the fiber, they are usually covered inside the fiber and cannot play a role. For example, in the study of Journal of Power Sources, 2020, 450, 227592, the synthesized MOF particles (UiO-66-NH2) were added to the precursor solution of sulfonated polyether sulfone for spinning. The maximum loading of MOF was only 8 wt%. When the loading reached 10 wt%, due to aggregation and blockage, the various properties of the proton exchange membrane began to decline; (2) in-situ growth, in which the fiber membrane obtained by spinning is immersed in a solution containing metal salts and organic ligands, and MOF is grown in-situ on the fiber surface under high temperature and high pressure. This method significantly reduces the degree of aggregation while increasing the MOF loading and improves the compatibility between MOF and polymer fibers, making it a more promising preparation method. However, MOFs are difficult to grow on ordinary polymer surfaces, and it is usually necessary to embed MOF growth seeds in the fibers, such as adding metal salts or organic ligands to the spinning precursor solution to promote MOF growth.

[0004] Polyacrylonitrile (PAN) is often used as a reinforcing agent in combination with Nafion to prepare proton exchange membranes due to its excellent mechanical properties, good spinnability, and low cost. Currently, research on combining MOFs with PAN fibers and Nafion to prepare proton exchange membranes is in its early stages, and methods using electrospinning and in-situ growth are even rarer. The synthesis of UiO-type MOFs requires the use of polar solvents such as DMF and DMAc, which PAN is soluble in, and the subsequent growth process must be carried out under harsh high-temperature and high-pressure conditions, thus significantly limiting the development of this field. For example, in the study of J. Mater. Chem. A, 2014, 2, 2110-2118, in order to prepare MOF membranes for gas separation, PAN hollow fibers were subjected to alkaline hydrolysis to give them -COO- on the surface, which served as seeds for MOF growth. However, due to solvent limitations, only CuBTC and ZIF-type MOFs with water or alcohol as solvents could be grown subsequently. However, CuBTC has poor compatibility with Nafion due to the presence of rigid six-membered rings, while ZIF has weaker water stability than UiO. Therefore, neither of them is the best choice for preparing proton exchange membranes. In response, a study published in Ind. Eng. Chem. Res. 2017, 56, 49, 14502–14506 added 2-aminoterephthalic acid (BDC-NH2) as a seed to the PAN spinning precursor solution, and then used acetone as a solvent to grow UiO-66-NH2 on the fiber surface. However, this method uses BDC-NH2 as the active site, which is easily oxidized and degraded in air. Furthermore, BDC-NH2 partially dissolves in acetone during growth, resulting in poor MOF morphology. In addition, if the PAN / UiO-66-NH2 fiber membrane prepared by this method is directly composited with Nafion, the proton conductivity will decrease due to the poor compatibility between PAN and Nafion.

[0005] Therefore, a method is needed to introduce active sites on the surface of PAN fibers to grow UiO-66-NH2 in situ, and then combine it with Nafion to prepare a proton exchange membrane. This method requires that the polymer fibers are insoluble in the solvent required for growing UiO-66-NH2, and that the active sites and MOFs be stable, thereby improving proton conductivity while solving the problem of poor compatibility between the composite fibers and Nafion. Summary of the Invention

[0006] This invention aims to provide a method for introducing active sites on the surface of PAN fibers via hydrolysis to obtain MOF-functionalized nanofiber mats through in-situ growth of UiO-66-NH2, followed by combining these nanofiber mats with Nafion to prepare MOF-functionalized nanofiber composite proton exchange membranes. This method embeds -CONH2 and -COOH into PAN fibers as UiO-66-NH2 growth seeds through simple hydrolysis. The generated -CONH2 and -COOH act as additional proton conductors, improving proton conductivity and enhancing the compatibility between PAN and Nafion. The -NH2 on UiO-66-NH2, in addition to its own proton conductivity, can combine with -SO3H on the Nafion side chains to form acid-base pairs, simultaneously improving fiber-Nafion compatibility and lowering the energy barrier for proton conduction, thereby further enhancing proton conductivity. The porous structure of MOF reduces methanol permeability. PAN, as a reinforcing agent, effectively improves the mechanical properties of the proton exchange membrane and reduces its swelling rate. Therefore, the prepared proton exchange membrane exhibits excellent comprehensive performance.

[0007] The technical solution for preparing the proton exchange membrane according to the present invention includes the following steps:

[0008] S1: Prepare PAN spinning precursor solution, and prepare PAN fiber membrane by electrospinning;

[0009] S2: The PAN fiber membrane is hydrolyzed in an aqueous sodium hydroxide solution to convert -CN into -CONH2 and -COOH;

[0010] S3: The hydrolyzed PAN fiber membrane was placed in an acetone solution of metal salt and organic ligands, and UiO-66-NH2 was grown in situ under high temperature and high pressure.

[0011] S4: The Nafion solution is cast onto PAN / UiO-66-NH2 fiber felt, so that the Nafion fills the pores of the fiber felt, and finally heat-treated to form a film;

[0012] Preferredly, a PAN spinning precursor solution is prepared. The specific process of preparing PAN fiber membrane by electrospinning is as follows: 5 ml of 10 wt% PAN / DMF electrospinning precursor solution is prepared, and electrospinning is carried out at a spinning advance rate of 0.8-1 ml / h, a voltage of 14-16 kV, and a receiving distance of 15-20 cm to prepare PAN fiber membrane.

[0013] Preferably, the PAN fiber membrane obtained in S1 is hydrolyzed in an aqueous sodium hydroxide solution to convert -CN to -CONH2 and -COOH. Specifically, the PAN fiber membrane is placed in a 15-20 wt% aqueous sodium hydroxide solution and hydrolyzed at 50°C for 2-3 hours. Then, it is washed with deionized water and dilute hydrochloric acid until neutral and dried at 80°C for 12 hours to obtain the hydrolyzed PAN fiber membrane.

[0014] Preferredly, the process of in-situ growth of UiO-66-NH2 under high temperature and high pressure in the hydrolyzed PAN fiber membrane obtained in S2 is as follows: 1 mmol of ZrCl4 and 1 mmol of BDC-NH2 are dissolved in 20 ml of acetone, 0.1 ml of acetic acid is added dropwise, and the mixture is sonicated for 30 min. After complete dissolution, the hydrolyzed PAN fiber membrane is placed in the mixture and reacted at 90-100℃ for 24 h. After cooling to room temperature, the membrane is ultrasonically washed with fresh acetone for 1 h, and this process is repeated 2-3 times. Subsequently, the membrane is vacuum dried at 60℃ for 12 h to obtain PAN / UiO-66-NH2 fiber mat.

[0015] Preferredly, the Nafion solution was cast onto the PAN / UiO-66-NH2 fiber felt obtained in S3, so that the Nafion filled the pores of the fiber felt. The final heat treatment to form the film was as follows: 5 wt% of commercially available Nafion aqueous alcohol solution was evaporated to obtain 10 wt% Nafion aqueous alcohol solution. This solution was carefully cast onto the PAN / UiO-66-NH2 fiber felt placed in an ultraflat culture dish and impregnated at room temperature for 24 h. Then, it was heat-treated at 120-140 °C for 3 h to obtain the final MOF functionalized nanofiber composite proton exchange membrane. Attached Figure Description

[0016] Figure 1 The image shown is the XRD pattern of PAN / UiO-66-NH2 in an embodiment of the present invention.

[0017] Figure 2 This is a SEM image of PAN / UiO-66-NH2 in an embodiment of the present invention. Detailed Implementation

[0018] The preparation method of the MOF functionalized nanofiber composite proton exchange membrane provided by the present invention will be further described below through specific embodiments.

[0019] Example 1

[0020] (1) Prepare 5 ml of 10 wt% PAN / DMF electrospinning precursor solution, perform electrospinning, with a spinning advance rate of 1 ml / h, a voltage of 16 kV, and a receiving distance of 15 cm to prepare PAN fiber membrane.

[0021] (2) The PAN fiber membrane was placed in a 20wt% sodium hydroxide aqueous solution and hydrolyzed at 50°C for 3h. Then it was washed with deionized water and dilute hydrochloric acid until neutral and dried at 80°C for 12h to obtain the hydrolyzed PAN fiber membrane.

[0022] (3) Dissolve 1 mmol of ZrCl4 and 1 mmol of BDC-NH2 in 20 ml of acetone, add 0.1 ml of acetic acid, sonicate for 30 min, and after complete dissolution, place in the hydrolyzed PAN fiber membrane, react at 100 °C for 24 h, cool to room temperature, and sonicate wash with fresh acetone for 1 h, repeat 3 times, and then vacuum dry at 60 °C for 12 h to obtain PAN / UiO-66-NH2 fiber felt.

[0023] (4) Evaporate 5 wt% of commercially available Nafion aqueous alcohol solution to obtain 10 wt% Nafion aqueous alcohol solution. Carefully cast the solution onto PAN / UiO-66-NH2 fiber felt placed in an ultraflat culture dish and impregnate at room temperature for 24 h. Then heat treat at 140 °C for 3 h to obtain the final MOF functionalized nanofiber composite proton exchange membrane.

Claims

1. A method of preparing MOF functionalized nanofiber composite proton exchange membrane, characterized in that, Includes the following steps: S1: Prepare PAN spinning precursor solution, and prepare PAN fiber membrane by electrospinning; S2: The PAN fiber membrane is hydrolyzed in an aqueous sodium hydroxide solution to convert -CN into -CONH2 and -COOH; S3: The hydrolyzed PAN fiber membrane was placed in an acetone solution of metal salt and organic ligands, and UiO-66-NH2 was grown in situ under high temperature and high pressure. S4: The Nafion solution is cast onto PAN / UiO-66-NH2 fiber felt, so that the Nafion fills the pores of the fiber felt, and finally heat-treated to form a film; In step S1, the process of preparing PAN spinning precursor solution and electrospinning to prepare PAN fiber membrane is as follows: prepare 5 ml of 10 wt% PAN / DMF electrospinning precursor solution, perform electrospinning, with a spinning advance rate of 0.8-1 ml / h, a voltage of 14-16 kV, and a receiving distance of 15-20 cm to prepare PAN fiber membrane. In step S2, the process of hydrolyzing the PAN fiber membrane in an aqueous sodium hydroxide solution to convert -CN into -CONH2 and -COOH is as follows: the PAN fiber membrane is placed in a 15-20 wt% aqueous sodium hydroxide solution and hydrolyzed at 50°C for 2-3 hours. Then it is washed with deionized water and dilute hydrochloric acid until neutral and dried at 80°C for 12 hours to obtain the hydrolyzed PAN fiber membrane. In step S3, the process of in-situ growth of UiO-66-NH2 in a solution of metal salt and organic ligands in acetone and under high temperature and pressure is as follows: 1 mmol of ZrCl4 and 1 mmol of BDC-NH2 are dissolved in 20 ml of acetone, 0.1 ml of acetic acid is added dropwise, and the mixture is sonicated for 30 min. After complete dissolution, the hydrolyzed PAN fiber membrane is placed in the solution and reacted at 90-100℃ for 24 h. After cooling to room temperature, the membrane is ultrasonically washed with fresh acetone for 1 h, and this process is repeated 2-3 times. Then, the membrane is vacuum dried at 60℃ for 12 h to obtain PAN / UiO-66-NH2 fiber mat. In step S4, the Nafion solution is cast onto PAN / UiO-66-NH2 fiber felt, allowing Nafion to fill the pores of the fiber felt. The final heat treatment process for film formation is as follows: 5 wt% of commercially available Nafion aqueous alcohol solution is evaporated to obtain 10 wt% Nafion aqueous alcohol solution. This solution is carefully cast onto PAN / UiO-66-NH2 fiber felt placed in an ultraflat culture dish and immersed at room temperature for 24 hours. Subsequently, it is heat-treated at 120-140℃ for 3 hours to obtain the final MOF functionalized nanofiber composite proton exchange membrane.

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