Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction

Abstract

•An alternative Li+/Mg2+ separation mechanism by membrane is proposed•Oligoethers facilitate Li+ transport but obstruct other ions from entering•The oligoethers on the COF act as relay parts to transport Li+ by charge repulsion•Pore-environment engineering represents a new addition to enhance membrane properties With lithium batteries expected to increase over the coming decades, access to this reserve is imperative for future energy storage devices. Substantial effort has been expended to develop membrane-based technologies to extract lithium from salt-lake brines, but the very similar reactivities of Li+ and Mg2+ make this a challenging task. Herein, we show how two-dimensional covalent organic frameworks (COFs) possess all the necessary traits to create ideal membranes for Li extraction. The COF active layer with orderly aligned oligoethers in the pore channels affords a high Li+/Mg2+ separation factor of 64, going into a top rank in terms of extraction efficiency. The lithiophilic oligoethers show a pumping feature that accelerates the Li+ transfer in the pore channels, defeating the trade-off between selectivity and permeability. These findings represent a promising addition to the rapidly increasing arsenal of energy material harvesting. As the most sophisticated separation system, biological membranes have served as natural prototypes for the design of artificial membranes because they provide high permeability and solute selectivity, owing to the presence of specialized ion channels. However, developing stable and selective artificial ion channels remains a formidable challenge. Herein, we demonstrate the construction of lithium nanochannels in two-dimensional covalent organic frameworks (COFs) to create biomimetic membranes. Implanted lithiophilic oligoethers conferred specificity and facilitated Li+ diffusion along the pore pathway of the COF. The ion channel characteristics were indicated by reversal potential measurements, showing that the relative permeability decreased in the order Li+ > K+ > Na+ > Ca2+ > Mg2+. The Li+ transfer was enhanced, while other ions were obstructed, allowing for high selectivity and permeability. A Li+/Mg2+ separation factor of 64 was achieved, confirming high lithium affinity. This study may serve as a design principle to develop selective artificial membranes for effective ion separation. As the most sophisticated separation system, biological membranes have served as natural prototypes for the design of artificial membranes because they provide high permeability and solute selectivity, owing to the presence of specialized ion channels. However, developing stable and selective artificial ion channels remains a formidable challenge. Herein, we demonstrate the construction of lithium nanochannels in two-dimensional covalent organic frameworks (COFs) to create biomimetic membranes. Implanted lithiophilic oligoethers conferred specificity and facilitated Li+ diffusion along the pore pathway of the COF. The ion channel characteristics were indicated by reversal potential measurements, showing that the relative permeability decreased in the order Li+ > K+ > Na+ > Ca2+ > Mg2+. The Li+ transfer was enhanced, while other ions were obstructed, allowing for high selectivity and permeability. A Li+/Mg2+ separation factor of 64 was achieved, confirming high lithium affinity. This study may serve as a design principle to develop selective artificial membranes for effective ion separation. The desire to mimic cell membranes with meticulous control over ion transport has attracted significant research interest for decades.1Tu Y.-M. Song W. Ren T. Shen Y.-X. Chowdhury R. Rajapaksha P. Culp T.E. Samineni L. Lang C. Thokkadam A. et al.Rapid fabrication of precise high-throughput filters from membrane protein nanosheets.Nat. Mater. 2020; 19: 347-354Google Scholar, 2Gouaux E. 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Mater. 2021; : 2009970https://doi.org/10.1002/adfm.202009970Google Scholar In contrast to the current nanofiltration membranes, which are optimized empirically, COFs, with the advantage of high modularity, can potentially form active layers with designable pore structures and tunable functionality. Further, the discogens of 2D COFs are arranged in a columnar fashion, owing to the strong π-π interactions between aromatic cores and the aligned lithiophilic functionalities oriented in close proximity, offering unidirectional pathways for swift ion diffusion and enhanced communication between adjacent ions in the queue (Figure 1). In this contribution, we show that COF-based membranes implanted with oligoethers can indeed provide an ion-diffusion pathway. The transport of Li ions is accelerated by rapid and reversible coordination with the ether moieties, thereby differentiating Li ions from other ions. Theoretical and spectroscopic studies were carried out to explain the observed selective extraction efficiency based on the interaction and dynamic exchange between oligoethers and Li ions. The synergistic effect between the densely populated lithiophilic sites and the 1D channels promotes significantly faster ion transport rates and allows the use of thicker films. The performance is very stable, as reflected by the fact that the selectivity is retained under the same conditions for at least 40 h. Our studies indicate that pore-environment engineering by introducing lithiophilic functionalities could be a promising strategy to optimize the separation performance of membranes that circumvent the issues of current ones that require trade-offs in properties. We selected 1,3,5-tris(4-aminophenyl)benzene (TAB) as the base for the construction of COF membranes because the resulting COFs have not only been proven to be stable under a wide range of conditions, but also possess high crystallinity as well as geometrical compatibility with various aldehyde counterparts, enabling us to optimize the composition within a single COF family (Figures 2A–2D ).49Xu H. Gao J. Jiang D. Stable, crystalline, porous, covalent organic frameworks as a platform for chiral organocatalysts.Nat. Chem. 2015; 7: 905-912Google Scholar,50Sun Q. Tang Y. Aguila B. Wang S. Xiao F.-S. Thallapally P.K. Al-Enizi A.M. Nafady A. Ma S. Reaction environment modification in covalent organic frameworks for catalytic performance enhancement.Angew. Chem. Int. Ed. 2019; 58: 8670-8675Google Scholar To construct lithium channels, we paired TAB with an aldehyde monomer bearing oligo(ethylene oxide) chains, 2,5-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)terephthalaldehyde (4EO) (Figure 2B), as previous X-ray diffraction study revealed that in a polyethylene oxide-Li complex, there are four ether oxygens coordinated to each Li ion.51Ratner M.A. Shriver D.F. Ion transport in solvent-free polymer.Chem. Rev. 1988; 88: 109-124Google Scholar To reveal the role of oligoether moieties and exclude the impact of channel congestion resulting from the introduced substituted group, TAB was also paired with 2,5-bis(heptyloxy)terephthalaldehyde (OHep) for comparison (Figure 2C). To confer stability and processibility to the COF-based membranes, we grew COF active layers on polyacrylonitrile (PAN) ultrafiltration membranes via interfacial polymerization (Figure S1).52Dong R. Zhang T. Feng X. Interface-assisted synthesis of 2D materials: trend and challenges.Chem. Rev. 2018; 118: 6189-6235Google Scholar The choice of PAN as the substrate was made because it is flexible, which can increase the operability, and it is hydrophilic and negatively charged, which can lower the transmembrane energy of cations. To facilitate the reaction at the interface of PAN, an aqueous solution of acetic acid and amine monomers and an organic phase with aldehyde monomers were separately introduced into two sides of a diffusion cell, with the resulting membranes denoted as COF-4EO-PAN and COF-OHep-PAN (Figure S2). The characterization results of COF-4EO-PAN are discussed here, while the corresponding results for COF-OHep-PAN are detailed in the supplemental information (Figures S3–S7). The morphologies of the as-prepared COF-4EO-PAN were examined by scanning electron microscopy (SEM). Compared with the pristine PAN, the SEM images of COF-4EO-PAN showed several notable features (Figure S8). Cross-sectional SEM images of pristine PAN and COF membranes revealed distinct regions with different morphologies. COF-4EO-PAN was composed of multiple nanosheets, which were stacked layer by layer into a highly regular and lamellar structure with a thickness of approximately 1 μm (Figures 2E, S9, and S10). The top view of the SEM image of COF-4EO-PAN revealed that the PAN support was completely covered with a layer of COF material. The Fourier transform infrared (FTIR) spectra of both the TAB and the dialdehyde monomers, and the free-standing COF membrane, which was achieved by dissolving the PAN support in dimethylformamide, are shown in Figure S10. In contrast to the spectra of TAB and 4EO, no peaks were observed in the primary amine region (3,440 and 3,350 cm−1) or at 1,685 cm−1, corresponding to the carbonyl group, in the spectrum of the COF membrane. Together with the appearance of the characteristic C=N band at 1,614 cm−1, this suggests the formation of a COF layer and that no detectable monomers are trapped in the membrane (Figure S11).53Sun Q. Aguila B. Lan P.C. Ma S. Tuning pore heterogeneity in covalent organic frameworks for enhanced enzyme accessibility and resistance against denaturants.Adv. Mater. 2019; 31: 1900008Google Scholar Moreover, the peak ascribed to the aldehyde group at around 170 ppm disappeared in the solid-state 13C nuclear magnetic resonance (NMR) spectrum of the free-standing COF-4EO membrane, further supporting the aforementioned claim (Figure S12). The powder X-ray diffraction (PXRD) pattern of the free-standing COF membrane showed several prominent diffraction peaks suggestive of its high crystallinity (Figure 2F). To determine the structure, we used Materials Studio, which revealed that the experimental powder patterns were well matched with the optimized p6m symmetric structure model in eclipsed AA stacking (Tables S2–S4). The porosity of the membrane was evaluated by N2 sorption isotherms, showing that COF-4EO possessed a Brunauer-Emmett-Teller surface area of 837 m2 g−1 and a pore size of 2.34 nm (Figure S13). To gain insight into the extent of the alignment of the COF layer on PAN, we performed grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, which indicated that COF-4EO-PAN had a (001) direction perpendicular to the substrate (Figure 2G).54Colson J.W. Woll A.R. Mukherjee A. Levendorf M.P. Spitler E.L. Shields V.B. Spencer M.G. Park J. Dichtel W.R. Oriented 2D covalent organic framework thin films on single-layer graphene.Science. 2011; 332: 228-231Google Scholar Therefore, we successfully prepared a COF active layer with its 2D plane flat on the substrate, where the oligo(ethylene oxide) chains vertically line the pore walls. To demonstrate the ion channel characteristics of the resulting membranes, we investigated the relative permeativity of various ions by measuring the reversal potentials. Tests were carried out on a bi-ionic system separated by the COF membranes. A MgCl2 solution was introduced on the cis side, and various metal chlorides, such as NaCl, KCl, LiCl, MgCl2, or CaCl2, were placed on the trans side (facing the COF layer). To evaluate the relative permeativity of cations exclusively, the concentrations of Cl ions were kept the same. From the x intercepts of the current traces plotted against voltages, the reversal potentials were obtained.55Sparreboom W. van den Berg A. Eijkel J.C.T. Principles and applications of nanofluidic transport.Nat. Nanotechnol. 2009; 4: 713-720Google Scholar When the trans side was filled with MgCl2, the resulting current-voltage curves passed almost through the origin, suggesting that both COF-4EO-PAN and COF-OHep-PAN showed nearly equal permeabilities on both sides. Reversal potentials of 21.2, 17.7, 8.8, and 2.5 mV were observed for LiCl, KCl, NaCl, and CaCl2, respectively, indicating a higher permeability to Li+ over other cations (Figure 3A). In contrast, COF-OHep-PAN exhibited an ion transport selectivity trend of K+ > Na+ > Li+ > Mg2+ > Ca2+, in agreement with their intrinsic ion transmission efficiency (Figure 3B).10Lu J. Zhang H. Hou J. Li X. Hu X. Hu Y. Easton C.D. Li Q. Sun C. Thornton A.W. et al.Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks.Nat. Mater. 2020; 19: 767-774Google Scholar These results verified the high activity of the oligoether-mediated transport of Li ions.22Gadjourova Z. Andreev Y.G. Tunstall D.P. Bruce P.G. Ionic conductivity in crystalline polymer electrolytes.Nature. 2001; 412: 520-523Google Scholar, 23Webb M.A. Jung Y. Pesko D.M. Savoie B.M. Yamamoto U. Coates G.W. Balsara N.P. Wang Z.-G. Miller III, T.F. Systematic computational and experimental investigation of lithium-ion transport mechanisms in polyester-based polymer electrolytes.ACS Cent. Sci. 2015; 1: 198-205Google Scholar, 24Zhang G. Hong Y.-l. Nishiyama Y. Bai S. Kitagawa S. Horike S. Accumulation of glassy poly(ethylene oxide) anchored in a covalent organic framework as a solid-state Li+ electrolyte.J. Am. Chem. Soc. 2019; 141: 1227-1234Google Scholar, 25Guo Z. Zhang Y. Yu D. Li J. Li S. Shao P. Feng X. Wang B. Fast ion transport pathway provided by polyethylene glycol confined in covalent organic frameworks.J. Am. Chem. Soc. 2019; 141: 1923-1927Google Scholar, 26Xie Z. Wang B. Yang Z. Yang X. Yu X. Xing G. Zhang Y. Chen L. Stable 2D heteroporous covalent organic frameworks for efficient ionic conduction.Angew. Chem. Int. Ed. 2019; 58: 15742-15746Google Scholar, 27Xu Q. Tao S. Jiang Q. Jiang D. Ion conduction in polyelectrolyte covalent organic frameworks.J. Am. Chem. Soc. 2018; 140: 7429-7432Google Scholar To investigate the separation of lithium and magnesium, ion transport kinetics measurements were conducted to further demonstrate the ability of COF-4EO-PAN to selectively transport Li+ over Mg2+ across the membrane. The experiments were conducted at room temperature using a homemade diffusion cell, wherein the feed and permeate chambers were filled with an aqueous salt solution and deionized water, respectively. The ion concentration in the permeate chamber was analyzed at different time intervals by ion chromatography (Figure S14), with each point measured three times in parallel. As shown in Figure 4, the concentrations of both Li+ and Mg2+ ions increased linearly over time, and the slope for Li+ was much steeper than that of Mg2+, indicative of the higher permeability to Li+ over Mg2+ across the membrane. A Li+/Mg2+ separation factor of 12 was obtained by dividing the slopes. In contrast, COF-OHep-PAN afforded a Li+/Mg2+ separation factor of only 3 under otherwise identical conditions (Figures S15 and S16). To understand the chemical basis of the binding selectivity of COF-4EO-PAN toward Li+ over Mg2+, quantum density functional theory computations were performed. Calculations of the truncated fragments shown in Figure S17 were performed using M062X exchange and correlation functions, and a 6-311G∗ basis was used for all atoms. The binding free energies for Li+ and Mg2+ to the oligoether were computed using the quasi-chemical method and showed that the oligoether moiety exhibits a higher binding affinity toward Li+ over Mg2+ by 55.5 kJ mol−1 in aqueous solution (Table S5). We performed X-ray photoelectron spectroscopy (XPS) to analyze the interaction between Li+ and the oligoethers. The binding energy of lithium species in Li+@COF-4EO (55.9 eV) is lower than that in LiCl (56.6 eV), reflecting the electron transfer from the ethylene oxide moiety to the Li ions (Figure S18). To further understand the Li+ transport processes in COF-4EO-PAN, the dynamic behavior of Li ions in the COF channels was studied by static solid-state 7Li NMR spectroscopy. We collected the spectra of LiCl and [email protected] (COF-OHep is the corresponding powder form of COF-OHep-PAN) as references to show the spectroscopic behavior of Li species in a solid matrix with minimal mobility and in a porous material with no lithium binding sites, which gave rise to very broad peaks centered at −1.53 ppm. In contrast, a narrower 7Li NMR signal was detected for COF-4EO, suggestive of the weakened solid-state couplings and hence the higher mobility (Figure S19).24Zhang G. Hong Y.-l. Nishiyama Y. Bai S. Kitagawa S. Horike S. Accumulation of glassy poly(ethylene oxide) anchored in a covalent organic framework as a solid-state Li+ electrolyte.J. Am. Chem. Soc. 2019; 141: 1227-1234Google Scholar From the characterization results, we concluded that the oxygen atoms in the oligoether moieties replace the oxygen atoms in water and coordinate with the Li ion. The oligoether moieties thus act like surrogate water, with the energetic costs of dehydration balanced by the energy gains from coordination with oxygen atoms (in bulk aqueous solution there are four and six water molecules coordinated with each Li or Mg ion, respectively, in the first hydration shell).56Marcus Y. Effect of ions on the structure of water: structure making and breaking.Chem. Rev. 2009; 109: 1346-1370Google Scholar This process may be further facilitated by the hydrophobic COF active layer, as revealed by the contact-angle measurements (Figure S20). The alkyl chain exhibited no specific affinity for Li ions, thus giving rise to an inferior selective permeation. However, the mechanism of the conduction of Li ions through the COF channels was still unclear, as the characterization results implied that a single Li ion would be held very tightly within the membrane. To understand this, we evaluated the Li+ concentration in COF-4EO-PAN by measuring the transmembrane ionic conductance. The membrane was squeezed between two reservoirs containing symmetric salt solutions of various concentrations. The measured ionic conductance deviated from the bulk values, suggesting that the Li ions are enriched in the pore channels (Figure 5A).11Ding L. Xiao D. Lu Z. Deng J. Wei Y. Caro J. Wang H. Oppositely