Covalent Organic Framework Membranes Research Articles

Covalent organic framework membranes reconstructed pore channels through metal-phenolic complexation for dye/salt separation

Covalent organic frameworks (COFs) characterized by high porosity, ordered channels, and remarkable chemical stability are potential candidates for the fabrication of nanofiltration separation membranes. In general, COFs membranes, generated through the process of interfacial polymerization, exhibit superior permeance without comparatively poor selectivity toward small molecules. Hereby, it was addressed with a simplified route to construct an extra selective layer on the COFs membranes. Concretely, COFs (TpPa/HPAN) membranes were first fabricated via interfacial polymerization on the hydrolyzed polyacrylonitrile (HPAN) membranes by utilizing monomers of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa). Then, a selective layer of metal-phenolic complex was constructed on the TpPa/HPAN membranes by electrostatic adsorption and metal-phenolic complexation. The introduction of a metal-phenolic complex selective layer masked the intrinsic pore size of TpPa and enhanced the hydrophilicity and electronegativity of the membrane, which improved the rejection property of the membranes for dyes larger than 350 Da. The fabricated M-TpPa/HPAN membranes showed exceptional performance in dye desalination with a water permeance of 92.1 L‧m-2‧h-1‧bar-1, rejections of Congo red and Eriochrome black T (EBT) over 99%, 78.6% of acid fuchsin rejection, 85.3% of acid orange 7 rejection, low rejection of salts (e.g., 6.2% for NaCl rejection), and the NaCl/EBT separation factor up to 243. Additionally, the M-TpPa/HPAN membranes maintained excellent stability during long-term filtration. The strategy of building an extra selection layer with eco-friendly, cost-effective, and scaled-up methods is also extensible for the other COFs and framework-based membranes.

Vapor/Vapor‐Solid Interfacial Growth of Covalent Organic Framework Membranes on Alumina Hollow Fiber for Advanced Molecular Separation

Covalent organic frameworks (COFs), known for their chemical stability and porous crystalline structure, hold promises as advanced separation membranes. However, fabricating high‐quality COF membranes, particularly on industrial‐preferred hollow fiber substrates, remains challenging. This study introduces a novel vapor/vapor‐solid (V/V‐S) method for growing ultrathin crystalline TpPa‐1 COF membranes on the inner lumen surface of alumina hollow fibers (TpPa‐1/Alumina). Through vapor‐phase monomer introduction onto polydopamine‐modified alumina at 170 °C and 1 atm, efficient polymerization and crystallization occur at the confined V‐S interface. This enables one‐step growth within 8 h, producing 100 nm thick COF membranes with strong substrate adhesion. TpPa‐1/Alumina exhibits exceptional stability and performance over 80 h in continuous cross‐flow organic solvent nanofiltration (OSN), with methanol permeance of about 200 L m‐2 h‐1 bar‐1 and dye rejection with molecular weight cutoff (MWCO) of approximately 700 Da. Moreover, the versatile V/V‐S method synthesizes two additional COF membranes (TpPa2Cl/Alumina and TpHz/Alumina) with different pore sizes and chemical environments. Adjusting the COF membrane thickness between 100‐500 nm is achievable easily by varying the growth cycle numbers. Notably, TpPa2Cl/Alumina demonstrates excellent OSN performance in separating the model active pharmaceutical ingredient glycyrrhizic acid (GA) from dimethyl sulfoxide (DMSO), highlighting the method’s potential for large‐scale industrial applications.

Vapor/Vapor-Solid Interfacial Growth of Covalent Organic Framework Membranes on Alumina Hollow Fiber for Advanced Molecular Separation.

Covalent organic frameworks (COFs), known for their chemical stability and porous crystalline structure, hold promises as advanced separation membranes. However, fabricating high-quality COF membranes, particularly on industrial-preferred hollow fiber substrates, remains challenging. This study introduces a novel vapor/vapor-solid (V/V-S) method for growing ultrathin crystalline TpPa-1 COF membranes on the inner lumen surface of alumina hollow fibers (TpPa-1/Alumina). Through vapor-phase monomer introduction onto polydopamine-modified alumina at 170 °C and 1 atm, efficient polymerization and crystallization occur at the confined V-S interface. This enables one-step growth within 8 h, producing 100 nm thick COF membranes with strong substrate adhesion. TpPa-1/Alumina exhibits exceptional stability and performance over 80 h in continuous cross-flow organic solvent nanofiltration (OSN), with methanol permeance of about 200 L m-2 h-1 bar-1 and dye rejection with molecular weight cutoff (MWCO) of approximately 700 Da. Moreover, the versatile V/V-S method synthesizes two additional COF membranes (TpPa2Cl/Alumina and TpHz/Alumina) with different pore sizes and chemical environments. Adjusting the COF membrane thickness between 100-500 nm is achievable easily by varying the growth cycle numbers. Notably, TpPa2Cl/Alumina demonstrates excellent OSN performance in separating the model active pharmaceutical ingredient glycyrrhizic acid (GA) from dimethyl sulfoxide (DMSO), highlighting the method's potential for large-scale industrial applications.

Combining computational screening and machine learning to explore MOFs and COFs for methane purification

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have great potential to be used as porous adsorbents and membranes to achieve high-performance methane purification. Although the continuous increase in the number and diversity of MOFs and COFs is a great opportunity for the discovery of novel adsorbents and membranes with superior performances, evaluating such a vast number of materials in the quickest and most effective manner requires the development of computational approaches. High-throughput computational screening based on molecular simulations has been extensively used to identify the most promising MOFs and COFs for methane purification. However, the enormous and ever-growing material space necessitates more efficient approaches in terms of time and effort. Combining data science with molecular simulations has recently accelerated the discovery of optimal MOF and COF materials for methane purification and revealed the hidden structure–performance relationships. In this perspective, we highlighted the recent developments in combining high-throughput molecular simulations and machine learning to accurately identify the most promising MOF and COF adsorbents and membranes among thousands of candidates for separating methane from other gases including acetylene, carbon dioxide, helium, hydrogen, and nitrogen. After providing a brief overview of the topic, we reviewed the pioneering contributions in the field and discussed the current opportunities and challenges that we need to direct our efforts for the design and discovery of adsorbent and membrane materials.

MXene/COFs membrane with sandwich structure for rapid salt/dye separation by layer-by-layer electrophoretic deposition

Covalent organic frameworks (COFs) have evolved into a transformative category of nanofiltration materials due to their intrinsic pore channels and high porosity, sparking a burgeoning interest in fully harnessing the potential of COFs for applications in nanofiltration. Herein, a sandwich structured MXene/COFs membrane was fabricated by the technique of layer-by-layer electrophoretic deposition (LBL EPD), with COFs nanospheres (COFs NPs) serving as the intermediate layer. During the EPD process, controlling the deposition time allows for convenient regulation of each layer's thickness, enabling rapid production of defect-free MXene/COFs membranes. The ultra-thin MXene layer offers excellent film-forming properties for COFs NPs, whereas the intermediate layer of COFs NPs plays a primary sieving function. Moreover, the pore-rich COFs NPs, the ultra-thin MXene layer, and the COFs NPs layer with abundant slits guarantee short and successive mass transport pathways. Under optimal conditions, the MXene/COFs membrane provided a permeance as high as 200.2 L‧m−2‧h−1‧bar−1 and a rejection rate of Congo red (CR) as high as 98.9 %. This exceptional performance holds steadfast even within pH ranges of 7–11 and during long-term operation. Meanwhile, MXene/COFs membranes exhibit commendable salt/dye separation performance, achieving an excellent separation factor of 121.1 for NaCl/CR. Moreover, the membrane's exceptional robustness and mechanical strength further facilitate its stable utilization under high transmembrane pressure (1–5 bar). To summarize, MXene/COFs membranes, characterized by their excellent nanofiltration and salt/dye separation performance, highlight the advantages of COFs within the nanofiltration field. This study introduces an innovative alternative approach to designing COFs membranes for highly efficient and high-precision molecular separations.

Covalent organic framework membranes modified by end-capping monomers for organic solvent nanofiltration

Covalent organic frameworks (COFs) with excellent solvent stability, high porosity, and well-designed pore size, are competitive candidates for organic solvent nanofiltration (OSN). To manipulate their structures, pre-synthetic and post-synthetic modifications are commonly used. Nevertheless, pre-synthetic modifications may alter the stacking fashion of COF nanosheets; post-synthetic modifications are constrained by limited pre-functionalities and can hardly achieve complete conversion. In contrast, defect engineering is a facile method to modify the structure of COFs through defects generated by the breakage of linkages or the usage of end-capping molecules. In this study, we adopted defect engineering to improve the OSN performance of TpPa-COF membranes constructed of 2,4,6-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa) monomers with end-capping molecules—aniline (An). The membrane synthesized with 30 percent of An molecules (TpPaAn-30/HPAN membrane) maintained a continuous COF layer with a thickness of ∼20 nm but lower density or more free channels, compared to the control membrane. The optimized TpPaAn-30/HPAN membrane had improved permeance for various organic solvents (e.g., 31.8 L m−2 h−1 bar−1 for methanol, fourfold of the control membrane) but maintained rejection towards methyl blue (>90 %). It also allowed the passage of rhodamine B but blocked methyl blue when filtrating a mixed-dye methanolic solution. Herein, the usage of end-capping monomers is proven to be an efficient method to introduce transport channels and improve the separation performance of COF membranes.

Evolution of morphology, structure, and performance of the COF membranes with monomer concentrations: Enabling effective desalination and kinetics analysis

Fabricating crystalline covalent organic framework (COF) membrane with sub-nanopores for desalination remains challenging. Monomer (p-phenylenediamine (Pa) and 1,3,5-triformylphloroglucinol (Tp)) concentration was crucial to tailor the diffusion-reaction process in the p-toluenesulfonic acid (PTSA)-mediated interfacial catalytic polymerization process for crystalline TpPa-COF membrane fabrication. By increasing monomer concentration, membrane surface color evolved from light yellow to dark orange. TpPa-COF layers with adjustable thickness (90.47–436 nm) transformed from amorphous to crystalline with crystalline and cross-linking degree increasing from 16.47 % to 73.90 %, and 66.40 % to 87.60 %, respectively. Pore size decreased from 1.69 to 0.17 nm, while Na2SO4 rejection significantly increased from 26.7 % to 90.4 %. A 3-day filtration test for RO concentrate demonstrated the stability and robust desalination performance with SO42− rejection ratio of 74.3 ± 1.08 % and Mg2+ rejection ratio of 65.6 ± 1.33 %. Diffusion behavior experiments demonstrated that diffusion coefficient for Pa-PTSA complex (6.49 × 10−5–8.87 × 10−5 cm2 s−1, Pa > 0.05 wt%) higher than Tp (3.53 × 10−6–6.39 × 10−6 cm2 s−1, Tp > 0.006 wt%) by one order of magnitude promoted structural crystallinity and regulated assembly at the interface, resulting in TpPa-COF layer with sub-nanopores. This study highlighted the importance of monomer concentration on the COF membrane formation, providing valuable insights for the highly crystalline COF membrane fabrication.

Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms.

Conjugated and processable self-standing vinylene-linked covalent organic framework membranes (COFMs) are highly demanding for photonics and optoelectronics. In this work, we have fabricated the first cyclotriphosphazene (CTP) cored vinylene-linked self-standing COFM (CTP-PDAN). For comparison purposes, we have successfully fabricated the imine-linked congener (CTP-PDA). Leveraging the inherent nonlinear optical (NLO) response of the CTP core, both membranes were directly mounted to evaluate NLO parameters using the open-aperture (OA) Z-scan technique. Direct measurement of NLO responses on membranes is advantageous and free from solvent and scattering effects, making it a more practical approach compared to the conventional dispersion mode. The OA Z-scan transmission yields a reverse saturable absorption signature exhibiting a higher NLO absorption coefficient (β) of 58.37 cm/GW for CTP-PDAN, compared to that of the imine-linked CTP-PDA COFM (β = 8.5 cm/GW). These results can be correlated to the efficient conjugation through the vinylene linkage in CTP-PDAN compared to the imine linked congener.

Anionic covalent organic framework membranes for the removal of per- and polyfluoroalkyl substances with enhanced selectivity

Nanofiltration (NF) is an effective technology for removing per- and polyfluoroalkyl substances (PFAS), a group of emerging pollutants. However, conventional NF membranes often reject high levels of minerals, leading to low salt/PFAS selectivity. Herein, we introduce the use of continuous ionic covalent organic framework (COF) membranes as a novel approach to achieve effective PFAS removal with high selectivity. Thin TpPa-SO3H COF selective membrane layers were fabricated on polymer supports using a scalable counter-diffusion interfacial polymerization (IP) method to form thin film composite (TFC) membranes. These membranes feature a highly negative surface charge and ordered pore channels with suitable pore sizes, leading to impressive PFAS rejection rates, with over 99% rejection for perfluorooctanesulfonic acid (PFOS) and 90-95% rejection for perfluorooctanoic acid (PFOA) and two short-chain PFAS. Moreover, the TpPa-SO3H membranes allowed the passage of scale-forming salts, enabling selective PFAS removal with a high salt/PFAS selectivity. Furthermore, these continuous ionic COF membranes exhibited a high water permeance of 19.9-37.5 LMH/bar, outperforming commercial membranes like NF270 and other lab-made membranes.

Efficient separation of CO2/CH4 by covering ultrathin ionic liquid film on COF membrane

Covalent organic framework (COF) membranes show great potential for gas separation, but their large pore size make it hard to effectively separate gases with similar kinetic diameters. A promising strategy is to cover the surface of COF membranes with an ultrathin film of ionic liquids (ILs) to improve the selectivity of gas permeation. Here, we investigated the CO2/CH4 separation performance of composite membranes composed of COF (CTF-1) and 20 kinds of ILs, respectively, using molecular simulations. The [C4mim][PF6]/COF membrane is found to exhibit the best separation performance, with CO2 permeability up to 7.93 × 104 GPU and CO2/CH4 selectivity of 12.33 when the IL thickness is 8 Å. By studying the microstructure of the composite membrane, it was found that there is a relatively dispersed distribution of cations and anions on the COF surface, which is beneficial to the diffusion of gas molecules. The gas permeation process results reveal that CO2 rapidly achieves equilibrium in both the adsorption and dissolution layers simultaneously, thereby enhancing the permeation rate of gas molecules. The analysis of interaction energy and PMF indicates that the improved selectivity is due to the stronger interaction between IL and CO2 than that for CH4.

Leaf-inspired hybrid membrane assembled by covalent organic framework and cellulose nanocrystals for efficient desalination

Covalent organic framework (COF) membranes hold great promise in molecular separations. However, COF membranes fabricated from COF nanosheets only often suffer from boundary defects with undesirable separation performance due to the small lateral size and weak interlayer interaction of COF nanosheets. Inspired by the 2D lamellar structure of leaf where 1D veins distributed in the leaflets render them integrity and robustness, vein-mimic 1D sulfonated cellulose nanocrystals (S–CNC) were assembled with the leaflet-mimic COF nanosheets to fabricate S–CNC–COF hybrid membranes. The hydrogen bonding between crystalline COF nanosheets and hydrophilic S–CNC could effectively eliminate boundary defects. More importantly, S–CNC with high rigidity could ensure the oriented assembly of COF nanosheets during the membrane formation process. With ordered structure, high hydrophilicity, and reduced mass transport resistance, the optimal S–CNC–COF membrane exhibits an excellent water flux of 314.93 kg m−2 h−1 and rejection of NaCl (99.99%). This leaf-inspired strategy sets up a platform for designing advanced hybrid COF membranes for molecular separations.

Boosting Osmotic Energy Harvesting from Organic Solutions by Ultrathin Covalent Organic Framework Membranes.

Extracting osmotic energy from waste organic solutions via reverse electrodialysis represents a promising approach to reuse such industrial wastes and helps to mitigate the ever-growing energy needs. Herein, a molecularly thin membrane of covalent organic frameworks is engineered via interfacial polymerization to investigate its ion transport behavior in organic solutions. Interestingly, a significant deviation from linearity between ion conductance and reciprocal viscosity is observed, attributed to the nanoscale confinement effect on intermolecular interactions. This finding suggests a potential strategy to modulate the influence of apprarent viscosity on transmembrane transport. The osmotic energy harvesting of the ultrathin membrane in organic systems was studied, achieving an unprecedented output power density of over 84.5 W m-2 at a 1000-fold salinity gradient with a benign conversion efficiency and excellent stability. These findings provide a meaningful stepping stone for future studies seeking to fully leverage the potentials of organic systems in energy harvesting applications.

Scalable, Interfacially Synthesized, Covalent-Organic Framework (COF)-Based Thin-Film Composite (TFC) Hollow Fiber Membranes for Organic Solvent Nanofiltration (OSN).

Covalent organic frameworks have great potential for energy-efficient molecular sieving-based separation. However, it remains challenging to implement COFs as an alternative membrane material due to the lack of a scalable and cost-effective fabrication mechanism. This work depicts a new method for fabricating a scalable in situ COF hollow fiber (HF) membrane by an interfacial polymerization (IP) approach at room temperature. The 2D COF film was constructed on a polyacrylonitrile HF substrate using aldehyde (1,3,5-trimethylphloroglucinol, Tp) and amine (4,4'-azodianiline (Azo) and 4,4',4″-(1,3,5-triazine- 2,4,6-triyl) trianiline (Tta)) as precursors. The COF membrane on the PAN substrate showed 99% rejection of Direct red-80 with remarkable solvent permeance. The rejection analysis revealed that the structural aspects of the solute molecule play a major role in rejection rather than the molecular weight. We further optimized the precursor concentrations to improve the permeation performance of the resulting membrane. The durability study reveals excellent stability of the membrane toward organic solvents. This study also demonstrated the easy scalability of the membrane fabrication approach. The approach was further extrapolated to fabricate a cation-based COF membrane. These charged membranes exhibited an enhanced rejection performance. Finally, this approach can facilitate industrially challenging molecular sieving applications using COF-based membranes.

Interfacial polymerization of self-standing ABC-stacking Covalent organic framework membranes for dye separation

Covalent organic framework (COF)-based membranes are burgeoning candidates for separation technologies owing to their well-ordered channel structures. However, it is still a significant challenge to obtain high-quality COF membranes with a mild and convenient preparation strategy. In this paper, we adopted a simple liquid-liquid interfacial polymerization method to manufacture a self-standing COF membrane with decent crystallinity at room temperature. Aldehyde monomer and catalyst were dissolved in water as the bottom layer and the amine monomer was dissolved in ethyl acetate as the top layer, and new COF membranes were subsequently obtained by interfacial polymerization in the two-phase interface. The synthetic conditions including precursor and catalyst dosage were further optimized, and thus-synthesized PDA-TAPB-ABC membrane displayed a stable rejection to MB of 97.57% with a high water permeance of up to 101. 32 L m−2h−1bar−1. Its dyes separation performance was much better than their AA-stacking counterpart. Moreover, the prepared membrane endowed favorable chemical stability and long-term durability.

Covalent Organic Framework Membranes from Oppositely Charged Nanosheets toward Efficient Desalination.

Fabricating covalent organic framework (COF) membranes through the pre-assembly of nanosheets with different properties may open a novel avenue to the fabrication of advanced 2D membranes. Herein, COF membranes are fabricated using oppositely-charged COF nanosheets (CONs). Negatively-charged CONs and positively-charged CONs are pre-assembled through simple physical mixing, yielding the CONs with an aspect ratio of exceeding 10000, which are assembled into three kinds of COF membranes. The optimal membranes exhibit the highest desalination performance with permeation flux of 132.66 kg m-2 h-1, salt rejection of 99.99%, and superior long-term operation stability.

Porous covalent organic framework nanofibrous membrane for excellent enrichment and ultra-high sensitivity detection of trace organochlorine pesticides in water

Developing adsorbents with high performance and long service life for effective extracting the trace organochlorine pesticides (OCPs) from real water is attracting numerous attentions. Herein, a self-standing covalent organic framework (COF-TpPa) membrane with fiber morphology was successfully synthesized by using electrospun nanofiber membranes as template and employed as solid-phase microextraction (SPME) coating for ultra-high sensitivity extraction and analysis of trace OCPs in water. The as-synthesized COF-TpPa membrane exhibited a high specific surface area (800.83 m2 g−1), stable nanofibrous structure, and excellent chemical and thermal stability. Based on the COF-TpPa membrane, a new SPME analytical method in conjunction with gas chromatography-mass spectrometry (GC–MS) was established. This proposed method possessed favorable linearity in concentration of 0.05–2000 ng L−1, high sensitivity with enrichment factors ranging from 2175 to 5846, low limits of detection (0.001–0.150 ng L−1), satisfactory precision (RSD < 10 %), and excellent repeatability (>150 cycles), which was better than most of the reported works. Additionally, the density functional theory (DFT) calculations and XPS results demonstrated that the outstanding enrichment performance of the COF-TpPa membrane was owing to synergistic effect of π–π stacking effects, high specific surface area and hydrogen bonding. This work will expect to extend the applications of COF membrane to captures trace organic pollutants in complex environmental water, as well as offer a multiscale interpretation for the design of effective adsorbents.

COF/MXene composite membranes compact assembled by electrostatic interactions: A strategy for H2/CO2 separation

H2 and CO2 separation in processes such as reforming hydrocarbons or gasifying fossil fuels using membranes has gained attention as an energy-efficient separation method. Covalent organic frameworks (COFs) are a promising class of organic porous crystalline materials, but their pore sizes (typically >0.5 nm) exceed the kinetic diameters of H2 (0.289 nm) and CO2 (0.33 nm). Herein, we aimed to improve the H2/CO2 selectivity of COF (TpPa-1) membranes by fabricating a two-dimensional composite membrane. This strategy involved the compact assembly of mesoporous COF (TpPa-1) nanosheets facilitated by non-porous MXene (Ti3C2Tx), driven by electrostatic interactions. The objective was to reduce the pore sizes of gas transport channels, enabling molecular sieving for H2/CO2 separation. Additionally, strong adsorptive interactions between CO2 and the COF/MXene composite membranes were leveraged to impede the transport of CO2 molecules. The results demonstrated that the 2D COF/MXene composite membranes with a Ti3C2Tx doping amount of 65 wt% achieved the highest H2/CO2 selectivity of 64.0 at 298 K, which was 5.67 times that of the pristine COF (TpPa-1) membrane. The H2 permeance reached 2.35 × 10−7 mol m−2 s−1 Pa−1, surpassing the latest Robeson upper bound. This innovative strategy not only presents a high-performance candidate for H2/CO2 separation but also provides inspiration for pore regulation of COF compactly assembled with non-porous MXene through electrostatic interactions.

Understanding the effect of layer spacing on water desalination through two-dimensional covalent organic framework membranes via molecular dynamics simulation

The desalination performance of covalent organic framework (COF) membrane can be tuned through adjusting its layer spacing, which can improve the water permeance while maintaining high ion rejection rate. However, the mechanism of the influence of COF membrane spacing on its seawater desalination performance is still unclear. In this work, the molecular dynamics (MD) simulation were performed to systematically investigate the influence of the layer spacing on desalination performance of AB-stacked MPCOF, including 0.34 nm, 0.44 nm, 0.54 nm and 0.64 nm. The findings indicated a decrease in the resistance of water molecules passing through the AB stacked MPCOF membrane, leading to a significant increase in water permeance as the layer spacing increases. The interlayer spacing of the COF membrane also significantly influenced its interactions with ions. When the layer spacing exceeded 0.44 nm, the ion rejection rate began to decrease. As a result, the AB-stacked MPCOF membranes with 0.44 nm possessed the superior desalination performance, which was much better than other conventional membranes. Our work explored the mechanisms of the effects of COF layer spacing on desalination performance, which might provide valuable suggestions for the optimization of subsequent COF desalination membranes.

Ultrathin PEI-functionalized carboxyl covalent organic framework membranes for efficient CO2/N2 separation

Covalent organic frameworks (COF) are deemed as disruptive membrane materials owing to their versatile functionalities, high stability and ordered nanochannels. However, the size mismatch between the COF intrinsic pore and gas molecules becomes a grand challenge when using COF membrane for carbon capture. Herein, we propose a new type of polymer-functionalized COF laminar membranes through grafting CO2-phlic polyethyleneimine (PEI) polymer onto carboxylic acid COF (cCOF) laminar membranes. The cCOF nanosheets with high density of carboxyl groups are prepared via modified single phase solution method and then assembled to fabricate membranes. Further, the cCOF membranes are functionalized with PEI using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-Hydroxysuccinimide (EDC/NHS) coupling chemistry. The introduction of abundant amino groups from PEI afford the affinity of membranes toward CO2. Meanwhile, the graft of the PEI polymers reduces the defects or pinholes on the membrane surface. As a result, the PEI functionalized cCOF membranes exhibit a high CO2 permeance of 1004 GPU and a CO2/N2 selectivity of 33.7, which reach the carbon capture target performance and show great prospect for treating real flue gas. Our approach of functionalizing cCOF membranes with tailored functionalities may inspire the design of molecule-selective COF membranes.

Effect of Tautomerization on Water Desalination by Covalent Organic Frameworks

Covalent organic frameworks (COFs) have gained increasing interest as promising building blocks for desalination membranes due to their inherent porosity. Efforts have been made to prepare COF membranes and explore their transport mechanisms, however the effect of tautomerization has not been reported. Using non-equilibrium molecular dynamics, we discover here that a mere 0.5-Å atomic displacement between two tautomers of an imine-based COF (TpHZ) can surprisingly lead to dramatic changes in the desalination behavior of their multilayers. The tautomeric transformation (from TpHZ-NH to TpHZ-OH) improves the water permeability by about 1.6–3 times. The interior resistance rather than the entrance resistance is found to dominate the water transport. The weaker adhesion and the smoother channel path of TpHZ-OH result in a much lower interior resistance. Furthermore, NaCl rejection increases from ∼ 89 % to 100 % due to the stronger sieving effect resulting from the reduced compensatory effect of TpHZ-OH for ion hydrations.