nanochannels is crucial for biological membranes and artificial membrane systems. Covalent organic frameworks (COFs) are emerging crystalline polymer materials with regular and adjustable nanochannels, and have become an ideal material platform for ion transport membranes. However, the nanochannel size of most COF is larger than that of ions, which usually results in non-selective transmission of ions, making it difficult for COF membranes to achieve ion interception. Existing technologies mainly endow COF membranes with ion interception capabilities by reducing the size of nanochannels, but this is mostly at the expense of membrane permeability and is facing a huge bottleneck.
Recently, Tianjin UniversityProfessor Wu Hong, Professor Jiang Zhongyi team developed an ionic COF membrane (ionic COF membranes, iCOFMs), with a large number of sulfonic acid groups distributed on the nanochannel wall, which can provide Ultra-high charge density, and then use localized electrostatic interaction to control ion transport. Research on has found that the overlap of the electric double layer in the charged nanochannel can hinder the entry of co-ions, while compressing the ion transfer path, ultimately inhibiting counter-ion penetration into through charge balance. Therefore, the strongly charged, large-sized nanochannels within iCOFMs not only achieve ion trapping, but also maintain the inherent high water permeability of COF materials. This study confirmed the possibility of controlling ion transfer based on COF membranes, and is expected to be used in fields such as water purification, ion separation, sensing and energy conversion . Relevant work was published in the latest issue of "ACS Nano" under the title "Charged Nanochannels in Covalent Organic Framework Membranes Enabling Efficient Ion Exclusion".
Figure 1. Conceptual diagram of ion transport in iCOFM charged nanochannels
Specifically, the research team chose charged amino monomers as building blocks to design ionic COF membranes (iCOFMs), with densely arranged charged groups within the framework. Strongly charged nanochannels can be formed to control ion transport. In addition, this strategy does not require shrinking the nanochannel size and can ensure fast water transfer (Fig. 1a). Therefore, the confined electrostatic interaction of high-density charged groups within the nanochannel inhibits salt ion transport, while water molecules can still freely pass through the large-sized nanochannel of iCOFMs (Figure 1b).
Figure 2. Preparation and characterization of iCOFM
[Preparation and structural characterization of iCOFM]
iCOFM is assembled from sulfonic acid-type iCOF nanoflakes. The obtained iCOF nanosheets have a high aspect ratio (103) and crystallinity, and then strong iCOFMs can be assembled, with the film thickness adjustable from nanometers to micrometers (Figure 2a and b). The above-mentioned iCOF nanosheets are tightly stacked to form layered iCOFM, exhibits two-dimensional orientation characteristics in SAED and GIWAXS characterization (Figure 2c and d) , which can provide vertical nanochannels for efficient mass transfer .
Nitrogen adsorption experiments and molecular interception experiments show that the nanochannel size of iCOFM is approximately 1.34 nm (dry state) and 1.41 nm (wet state) respectively, both of which are close to the theoretical value of iCOF (approximately 1.4 nm). Electrically driven ion transfer experiments showed that the charged groups on the channel wall greatly affected the internal ion transfer, causing the ion conductance value to deviate significantly from the bulk value of the solution, showing typical nanofluid characteristics (Figure 2g). This surface charge-dominated ion transport produces plateau conductance values at low ion concentrations (10-6-10-5M), corresponding to a surface charge density of approximately 200 mC m-2, the highest value reported so far for charged nanochannels (Figure 2h).
Figure 3. Ion transfer characteristics
[Ion transfer behavior of iCOFM]
Researchers tested the ion transmembrane transfer characteristics of commercial polyamide membrane NF90 (DOW®) and iCOFM respectively. As shown in Figure 3a, the size exclusion mechanism dominates the ion transport behavior of the NF90 membrane, as it is more sensitive to salts with large size ions (such as MgCl2 in Mg2+ or Na2SO4 in SO42-) than salts with small size ions. (e.g., NaCl) exhibit lower permeability.But the transfer characteristics of iCOFM (Figure 3b) are completely different from those of NF90: the permeability of MgCl2 and MgSO4 is significantly higher than that of NaCl and LiCl, although the former has a larger Mg2+ ion size, while the latter has Na+ and Li+ The ion size is smaller. In addition, the permeability of Na2SO4 is lower than that of MgSO4, which indicates that ions are also affected by cations during their passage through the nanochannel. As shown in Figure 3c, by testing the penetration rate of Na2SO4 with different ionic strengths, the characteristic parameter of the electrostatic interaction distance, Debye length, can be matched to the scale of the nanochannel: when the double electric layers in the nanochannel do not overlap, almost all ions It can pass through iCOFM with water molecules without hindrance. When the double electric layer overlaps, the ions in the channel will be affected by electrostatic interactions and the transmission will be blocked, confirming the important influence of electrostatic interactions on localized ion transmission (Figure 3d) .
Figure 4. Ion transfer mechanism of iCOFM
[Ion transfer mechanism of iCOFM]
The research team conducted molecular dynamics simulations to obtain microscopic information about the nanofluids within iCOFM, thereby deepening the understanding of electrostatically dominated ion transport. The average potential energy (PMF) of the anions and water molecules of the four salts passing through the few-layer iCOFM was first measured. As shown in Figure 4a, both the anion and water molecules need to overcome the energy barrier to enter the iCOFM nanochannel. Probability distribution of ions in the nanochannel visualized the ion transport pathways within the iCOFM (Figure 4b and c) and found that there are significantly separated anion/cation transport pathways inside, which is extremely consistent with the ion transport pathways within the biocharged nanochannel OmpF. resemblance. Notably, the anion transport path is significantly compressed by electrostatic repulsion, resulting in lower ion currents. In addition, although the cation transfer path is close to the charged wall, smaller cations are also distributed in a small amount inside the channel and can also interact with anions to hinder ion transfer. Based on the above experimental and simulation results, we can summarize the three key factors of the iCOFM ion interception mechanism: (1) energy barrier at the entrance of the nanochannel; (2) compressed transfer path of anions; (3) coupling between anions and cations (Figure 4d).
Figure 5. Membrane performance evaluation
[Separation performance of iCOFM]
Considering that the ion rejection performance and high water permeability of iCOFM are expected to be used in the nanofiltration separation process, the research team systematically investigated its nanofiltration in a cross-flow filtration system. Filtration and separation performance. The results show that the iCOFM with the best comprehensive performance has a rejection rate of sodium sulfate of about 93%, a water flux of about 120 L m-2h-1 (operating pressure 1.0 bar, cross-flow rate 1.6 L min-1), and a permeability of It can be up to 600 times that of the existing nanofiltration membrane (Figure 5a). Based on the electrostatic interception mechanism, iCOFM can remove charged borate ions up to more than 70%, and its comprehensive boron removal performance is better than existing reverse osmosis , nanofiltration and forward osmosis technologies (Figure 5b-c). Considering the long-term stability problems faced by two-dimensional layered membranes, the team investigated the long-period nanofiltration performance of iCOFM in the cross-flow filtration process. During an operation cycle of up to 200 hours, iCOFM can maintain stable water flux and ion rejection rate (Figure 5d).
[Summary]
In summary, this work developed ionic covalent organic framework membranes (iCOFMs) for efficient ion interception. The ordered arrangement of sulfonic acid groups in the regular nanochannels produces a high charge density (about 200 mC m-2), which exceeds the currently reported biological and artificial charged nanochannels. Experimental results and molecular simulations confirm that the overlapping electric double layers in charged COF nanochannels can hinder the entry of ions and compress their transfer paths through localized electrostatic interactions, thereby achieving ion trapping. The optimized iCOFM has ultra-high water permeability (up to 600 times that of existing nanofiltration membranes), a rejection rate of 93.5%, and a boron removal rate of over 70%. The team envisions that the above-mentioned COF membrane with charged nanochannels and its electrostatically dominated ion transport are expected to change the structural design paradigm of artificial nanochannel membranes.In addition, the confined ion transport mechanism based on framework materials is expected to inspire the design of new nanofluid membranes and device platforms and be applied to scenarios such as molecule/ion transport, separation, sensing, and energy conversion.
The first authors of this paper are You Xinda, a doctoral graduate of Tianjin University (now an associate professor of Fujian Agriculture and Forestry University), Cao Li, a doctoral graduate of Tianjin University (now a postdoctoral fellow of King Abdullah University of Science and Technology) and Dr. Liu Yawei ( Chinese Academy of Sciences Institute of Process Engineering ), this work was funded by the National Natural Science Foundation of China (21878215) and the Zhejiang Provincial Key R&D Plan Project (2021C03173).
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Original link:
https://pubs.acs.org/doi/10.1021/acsnano.2c04767
Source: Frontiers of Polymer Science
