membrane separation is widely used in the separation field due to its simplicity, efficiency, energy saving, environmental protection and other advantages. Various polymer materials have been developed for separation membranes. Among them, polyamide has become one of the widely used film materials due to its high chemical stability, strong mechanical strength and inherent hydrophilicity. However, traditional polyamide films are amorphous and have a large number of unevenly distributed free volume pores, which reduces the permeability of the film. COFs have rich ordered pores and high porosity , adjustable pore size and controllable functionalization, making COF-based membranes have obvious advantages in permeability selectivity. Therefore, based on the characteristics of polyamides and COFs, a new type of polyamide film has excellent properties such as high permeability and high retention rate of organic dye .
As shown in Scheme 1, this paper uses potassium hydrogen persulfate (KHSO5) oxidized imine-linked COFs to synthesize amide-linked COFs. In addition, the authors also prepared imine-COF membranes and amine-COF membranes for comparison.
Scheme 1: Synthesis of imine-COF, amide-COF and amine-COF layers
Figure 1 reveals the chemical information of imine-COF, amide-COF and amine-COF. As shown in Figure 1a and b, the FT-IR spectrum of imine-COF shows that the N-H (3217 cm−1) and C =O (1712 cm−1) peaks disappear, and the imine peak appears at 1626 cm−1, indicating successful monomer polymerization. The FT-IR spectrum of amide-COF shows that the peak corresponding to the C=N stretching vibration band disappears, and a C=O peak appears at 1667 cm−1, confirming the transition from imine bond to amide bond. The 13C NMR spectrum of Figure 1c can also be proved. For amine-COF, the C-N peaks at 49 ppm in FT-IR demonstrated the successful synthesis of amine-COF. Thermal stability of the three COFs was studied through thermogravimetric analysis and found that they all had good thermal stability. Among them, amide-COF loses weight by 10% at the highest temperature (556°C), and the residual amount remains above 70% at 800°C. Fig. 1d obtained the crystallinity of the COF layer by PXRD measurement. The three COFs show diffraction peaks at nearly the same location because they have similar frame structures. The results show that oxidation and reduction treatments do not cause damage to the frame structure of the precursor imine-COF. To evaluate the porosity of the COF layer, the N2 adsorption-desorption experiment was performed at 77 K, and the BET surface areas of imine , amide and amine-COF were calculated to be 1035, 984 and 79m2 g−1, respectively (Fig. 1e). The BET surface area of amine-COF was found to be much lower than that of the other two COFs, which is attributed to its flexible amine bond . The pore size distribution curves of the three COFs were calculated by non-local density functional theory (NLDFT) (Fig. 1f), and the distribution of imine-COF and amide-COF was very narrow. This shows that the pores in the COF layer are uniform in height, which is conducive to improving separation selectivity.
Figure 1: Structural characterization of imine-COF, amide-COF and amine-COF layers by (a, b) FT-IR spectroscopy, (c) solid-state 13C-CPMAS NMR spectroscopy, (d) PXRD, (e) N2 adsorption-desorption measurement and (f) pore size distribution analysis.
Figure 2 tests the microstructure and characteristics of the amide-COF layer. As shown in the scanning electron microscope (SEM) diagram in Figure 2a, the amide-COF layer exhibits a smooth layer structure. After these layers were further assembled into films by pressure-induced layer stacking, the surface SEM images of the amide-COF film showed clear local protrusions induced by layer stacking (Fig. 2b−e), but no significant defects were found on the surface of the resulting laminated film. Figure 2f shows that the prepared film thickness is uniform. The uniform membrane ensures stable performance under strong impact of water or dye molecules during the test. According to the above SEM diagram and the digital diagram of the film (Fig. 2g), it can be seen that the prepared film is defect-free and uniform, and can be used directly for water treatment. Slight fluctuations were observed in the AFM graph of Figure 2h. Slight inhomogeneity will not significantly destroy the uniformity of the film as a whole, but it will improve the surface roughness of the film, thereby improving the hydrophilicity and water flux of the film. The contact angle (CA) test of amide-COF membrane further demonstrated its hydrophilicity (Fig. 2i).
Figure 2: Morphological characteristics of amide-COF: (a) Surface SEM map ( scale bar 1 mm) and (b−e) Surface SEM map (substrates are 200, 50, 10 and 5 μm, respectively), (f) Section SEM map, (g) digital map, (h) AFM map, and (i) water contact angle.
Figure 3a The mechanical properties of amide-COF membrane were studied by nanoindentation technology and compared with imine and amine COF membranes. As shown in Figure 3b, none of the curves showed significant mutations as the load increased, indicating that no sudden local plastic deformation of occurred during the experiment. Quantitative nanomechanical properties (Fig. 3c), such as Young's modulus (Er) and hardness (H), were calculated from the load-depth curve obtained in Fig. 3b. The modulus is related to the number and strength of inter-molecular interactions, and the hardness indicates the irreversible sliding of the layer. Among the three membranes (Er=13.79MPa and H=3.33MPa of imine-COF membrane; Er=37.50MPa and H=6.67MPa of amine-COF membrane), the amide-COF membrane showed the highest two parameter values (Er=104.50MPa, H=21.52MPa), indicating that the amide-COF membrane had the best mechanical properties. Figure 3d is used to perform a relaxation experiment to further compare the mechanical response of the membrane. Under the same load, the amide-COF membrane has the smallest depth, indicating that it has the highest resistance to long-term external force toughness. This shows that the channels within the amide-COF membrane are highly resistant to the separation process, and the solvent transport and solute separation capabilities will remain constant to a certain extent.
Figure 3: (a) Schematic diagram of the force applied to the COF membrane with a standard Berkovich indenter. (b) Load (P)-depth (h) curve of COF film. (c) Histogram of modulus and hardness values. (d) Load (P)-depth (h) curve for relaxation experiments.
Figure 4 studies the performance of COF membranes for solvent penetration and dye separation. As shown in FIG. 4a, changing the COF layer content on the surface of the support film can adjust the thickness of the COF film. For polyamide-COF membrane, at a load of 20 mg, the flux of the dye solution was measured at 415.3L m−2h−1bar-1 at a load of 20 mg, while the retention rate of MB ( methylene blue ) was higher than 99.7% (Fig. 4b). Figure 4c is the permeability test of the membrane to different solvents. It was found that the flux of the organic solvents was greater than 50 L m−2h−1bar-1, which was roughly negatively correlated with the viscosity of the solvent. Next, the authors tested the retention rate of different dyes through amide-COF membranes (Fig. 4d). The results show that the introduction of COF layer significantly improves the dye separation ability of the membrane. In short, the water flux and MB retention rate of amide-COF membranes are at a high level (Fig. 4e). Compared with the separation performance of the three membranes,
found that there was no significant difference between amide-COF and imine-COF membranes in ordinary nanofiltration, while the membranes prepared by amine-COF have lower water permeability and retention rate of dyes (Fig. 4c, d). The reason is that the microstructure of the first two COFs is similar, the BET specific surface area is comparable, while the latter has a lower specific surface area. However, after the membrane is treated under harsh conditions, the membrane performance varies greatly, which is closer to the practical application environment. In the experiment, all COF layers were treated with ultrasound, strong acid and strong alkali respectively, and then the performance of the resulting membrane separated by MB was tested (Fig. 4f). The results show that under harsh environment, the membranes prepared by amide-COF layer can still maintain high separation performance, and the MB retention rates of ultrasonic, 12 M HCl and 6 M NaOH treated membranes were 97.3, 93.7 and 98.7%, respectively. In contrast, imine- and amine-linked COFs significantly reduced MB retention after ultrasound and 12 M HCl treatment. The poor performance of imine-COF and amine-COF membranes is due to their low stability in harsh environments, resulting in structural damage to COFs. In addition, the stability of the polyamide-COF membrane under acidic and alkaline conditions was verified using MB/water feed at different pH conditions (Fig. 4g). The results show that the permeability of and the intercept rate of under different pH conditions are basically the same, indicating that the polyamide-COF membrane has good stability. In addition, polyamide-COF membranes also exhibited high cyclic stability for MB separation at different pH conditions, which is revealed by maintaining high dye retention and flux during long continuous separations (Fig. 4h). These results show that amide-linked COFs are superior to their imine and amine-linked membranes as membrane materials.The high stability of the polyamide-COF membrane is attributed to the hydrogen bond interaction between molecules in the COF layer, which improves the stability of the membrane and gives it great advantages in water treatment under harsh conditions.
Figure 4: (a) Schematic diagram of the process of preparing the film by layer-by-layer COF layer. (b) Permeability and MB retention of amide-COF membranes prepared with different COF layer contents. (c) The permeability of the membrane to different solvents. (d) The membrane retention rate for different dyes. (e) Comparison of the properties of amide, amine and imine COF membranes with other membranes. (f) Membrane stability experiments under different harsh conditions. (g) Water permeability and MB retention of amide-COF membranes under different pH conditions. (h) Cycling properties of amide-COF membranes at different pHs.
In summary, a novel polyamide film based on amide-linked COF was prepared in this paper. Unlike the general amorphous polyamide film, the polyamide-COF film is crystalline, so the pore distribution in the film is orderly. The membrane has good solvent permeability and separation selectivity. Furthermore, the polyamide-COF film exhibits higher mechanical strength and stability compared to the imine-linked and amine-linked COF film with the same structural block, thanks to the strong hydrogen bonded interaction between the amide units. Because polyamide COFs combine polyamide and COF, the polyamide COF film can maintain high separation performance under harsh conditions.
Title: Polyamide Covalent Organic Framework Membranes for Molecular Sieving
Author: Ya Lu, Zhi-Bei Zhou, Qiao-Yan Qi, Jin Yao,* and Xin Zhao*
Quote: ACS Appl. Mater. Interfaces, 2022, 4, 37019−37027.
DOI: 10.1021/acsami.2c07753
