Confined-Coordination Induced Intergrowth of Metal-Organic Frameworks into Precise Molecular Sieving Membranes.

Metal-organic framework (MOF) membranes with rich functionality and tunable pore system are promising for precise molecular separation; however, it remains a challenge to develop defect-free high-connectivity MOF membrane with high water stability owing to uncontrollable nucleation and growth rate during fabrication process. Herein, we report on a confined-coordination induced intergrowth strategy to fabricate lattice-defect-free Zr-MOF membrane towards precise molecular separation. The confined-coordination space properties (size and shape) and environment (water or DMF) were regulated to slow down the coordination reaction rate via controlling the counter-diffusion of MOF precursors (metal cluster and ligand), thereby inter-growing MOF crystals into integrated membrane. The resulting Zr-MOF membrane with angstrom-sized lattice apertures exhibits excellent separation performance both for gas separation and water desalination process. It was achieved H2 permeance of ~1200 GPU and H2/CO2 selectivity of ~67; water permeance of ~8 L ⋅ m-2 ⋅ h-1 ⋅ bar-1 and MgCl2 rejection of ~95 %, which are one to two orders of magnitude higher than those of state-of-the-art membranes. The molecular transport mechanism related to size-sieving effect and transition energy barrier differential of molecules and ions was revealed by density functional theory calculations. Our work provides a facile approach and fundamental insights towards developing precise molecular sieving membranes.

Nanochannel Stability of Chemically Converted Graphene Oxide Membranes.

Chemically converted graphene oxide laminate membranes, which exhibit stable interlayered nanochannels in aqueous environments, are receiving increasing attention owing to their potential for selective water and ion permeation. However, how the molecular properties of conversion agents influence the stabilization of nanochannels and how effectively nanochannels are stabilized have rarely been studied. In this study, mono-, di-, and tri-saccharide molecules of glucose (Glu), maltose (Glu2), and maltotriose (Glu3) are utilized, respectively, to chemically modify graphene oxide (GO). The aim is to create nanochannels with different levels of stability and investigate how these functional conversion agents affect the separation performance. The effects of the property differences between different conversion agents on nanochannel stabilization are demonstrated. An agent with efficient chemical reduction of GO and limited intercalation in the resulting nanochannel ensures satisfactory nanochannel stability during desalination. The stabilized membrane nanochannel exhibits a permeance of 0.69 L m-2 h-1 bar-1 and excellent Na2SO4 rejection of 96.42%. Furthermore, this optimized membrane nanochannel demonstrates enhanced stability under varying external conditions compared to the original GO. This study provides useful information for the design of chemical conversion agents for GO nanochannel stabilization and the development of nanochannel membranes for precise separation.

Fabrication of polydopamine/rGO Membranes for Effective Radionuclide Removal.

In this work, a novel polydopamine/reduced graphene oxide (PDA/rGO) nanofiltration membrane was prepared to efficiently and stably remove radioactive strontium ions under an alkaline environment. Through the incorporation of PDA and thermal reduction treatment, not only has the interlayer spacing of graphene oxide (GO) nanosheets been appropriately regulated but also an improved antiswelling property has been achieved. The dosage of GO, reaction time with PDA, mass ratio of PDA to GO, and thermal treatment temperature have been optimized to achieve a high-performance PDA/rGO membrane. The resultant PDA/rGO composite membrane has exhibited excellent long-term stability at pH 11 and maintains a steady strontium rejection of over 90%. Moreover, the separation mechanism of the PDA/rGO membrane has been systematically investigated and determined to be a synergistic effect of charge repulsion and size exclusion. Results have indicated that PDA/rGO could be considered as a promising candidate for the separation of Sr2+ ions from nuclear industry wastewater.

Thin continuous membrane coating with high surface energy for comprehensive antifouling seawater distillation.

Membrane distillation (MD) has prominent advantages such as treating high-salinity wastewater with a low-grade thermal energy, high salt rejection, and zero discharge. However, organic fouling and mineral scaling are two major challenges for hydrophobic MD membranes when used for practical applications. Commonly, improving organic fouling- and mineral scaling-resistance require oppositely enhanced wetting properties of membrane, thus is difficult to simultaneously realize dual resistance with one membrane. Here, we proposed to use underwater thermodynamically stable high-surface-energy coating to modify the hydrophobic membrane with Janus structures comprising different surface energy. The underlayered structure meets the hydrophobicity requirements of the MD membrane, while the coating layer realizes dual resistance to organic and inorganic foulants. Theoretical analysis and experimental proof reveal that the membrane with the high-surface-energy coating layer outperforms the pristine one with approximately 10 times of longevity. This strategy provides a new way for the use of high-surface-energy materials in versatilely fouling-resistant MD process.

Side-Chain-Dependent Functional Intercalations in Graphene Oxide Membranes for Selective Water and Ion Transport.

Subnanometer interlayer space in graphene oxide (GO) laminates is desirable for use as permselective membrane nanochannels. Although the facile modification of the local structure of GO enables various nanochannel functionalizations, precisely controlling nanochannel space is still a challenge, and the roles of confined nanochannel chemistry in selective water/ion separation have not been clearly defined. In this study, macrocyclic molecules with consistent basal plane but varying side groups were used to conjunct with GO for modified nanochannels in laminates. We demonstrated the side-group dependence of both the angstrom-precision tunability for channel free space and the energy barrier setting for ion transport, which challenges the permeability-selectivity trade-off with a slightly decreased permeance from 1.1 to 0.9 L m-2 h-1 bar-1 but an increased salt rejection from 85% to 95%. This study provides insights into the functional-group-dependent intercalation modifications of GO laminates for understanding laminate structural control and nanochannel design.

When self-assembly meets interfacial polymerization.

Interfacial polymerization (IP) and self-assembly are two thermodynamically different processes involving an interface in their systems. When the two systems are incorporated, the interface will exhibit extraordinary characteristics and generate structural and morphological transformation. In this work, an ultrapermeable polyamide (PA) reverse osmosis (RO) membrane with crumpled surface morphology and enlarged free volume was fabricated via IP reaction with the introduction of self-assembled surfactant micellar system. The mechanisms of the formation of crumpled nanostructures were elucidated via multiscale simulations. The electrostatic interactions among m-phenylenediamine (MPD) molecules, surfactant monolayer and micelles, lead to disruption of the monolayer at the interface, which in turn shapes the initial pattern formation of the PA layer. The interfacial instability brought about by these molecular interactions promotes the formation of crumpled PA layer with larger effective surface area, facilitating the enhanced water transport. This work provides valuable insights into the mechanisms of the IP process and is fundamental for exploring high-performance desalination membranes.

Two-Dimensional Interlayer Space Induced Horizontal Transformation of Metal-Organic Framework Nanosheets for Highly Permeable Nanofiltration Membranes.

Laminar membranes comprising graphene oxide (GO) and metal-organic framework (MOF) nanosheets benefit from the regular in-plane pores of MOF nanosheets and thus can support rapid water transport. However, the restacking and agglomeration of MOF nanosheets during typical vacuum filtration disturb the stacking of GO sheets, thus deteriorating the membrane selectivity. Therefore, to fabricate highly permeable MOF nanosheets/reduced GO (rGO) membranes, a two-step method is applied. First, using a facile solvothermal method, ZnO nanoparticles are introduced into the rGO laminate to stabilize and enlarge the interlayer spacing. Subsequently, the ZnO/rGO membrane is immersed in a solution of tetrakis(4-carboxyphenyl)porphyrin (H2 TCPP) to realize in situ transformation of ZnO into Zn-TCPP in the confined interlayer space of rGO. By optimizing the transformation time and mass loading of ZnO, the obtained Zn-TCPP/rGO laminar membrane exhibits preferential orientation of Zn-TCPP, which reduces the pathway tortuosity for small molecules. As a result, the composite membrane achieves a high water permeance of 19.0 L m-2  h-1  bar-1 and high anionic dye rejection (>99% for methyl blue).

Deformation constraints of graphene oxide nanochannels under reverse osmosis.

Nanochannels in laminated graphene oxide nanosheets featuring confined mass transport have attracted interest in multiple research fields. The use of nanochannels for reverse osmosis is a prospect for developing next-generation synthetic water-treatment membranes. The robustness of nanochannels under high-pressure conditions is vital for effectively separating water and ions with sub-nanometer precision. Although several strategies have been developed to address this issue, the inconsistent response of nanochannels to external conditions used in membrane processes has rarely been investigated. In this study, we develop a robust interlayer channel by balancing the associated chemistry and confinement stability to exclude salt solutes. We build a series of membrane nanochannels with similar physical dimensions but different channel functionalities and reveal their divergent deformation behaviors under different conditions. The deformation constraint effectively endows the nanochannel with rapid deformation recovery and excellent ion exclusion performance under variable pressure conditions. This study can help understand the deformation behavior of two-dimensional nanochannels in pressure-driven membrane processes and develop strategies for the corresponding deformation constraints regarding the pore wall and interior.

Zwitterionic Copolymer-Regulated Interfacial Polymerization for Highly Permselective Nanofiltration Membrane.

A highly permselective nanofiltration membrane was engineered via zwitterionic copolymer assembly regulated interfacial polymerization (IP). The copolymer was molecularly synthesized using single-step free-radical polymerization between 2-methacryloyloxyethyl phosphorylcholine (MPC) and 2-aminoethyl methacrylate hydrochloride (AEMA) (P[MPC-co-AEMA]). The dynamic network of P[MPC-co-AEMA] served as a regulator to precisely control the kinetics of the reaction by decelerating the transport of piperazine toward the water/hexane interface, forming a polyamide (PA) membrane with ultralow thickness of 70 nm, compared to that of the pristine PA (230 nm). Concomitantly, manipulating the phosphate moieties of P[MPC-co-AEMA] integrated into the PA matrix enabled the formation of ridge-shaped nanofilms with loose internal architecture exhibiting enhanced inner-pore interconnectivity. The resultant P[MPC-co-AEMA]-incorporated PA membrane exhibited a high water permeance of 15.7 L·m-2·h-1·bar-1 (more than 3-fold higher than that of the pristine PA [4.4 L·m-2·h-1·bar-1]), high divalent salt rejection of 98.3%, and competitive mono-/divalent ion selectivity of 52.9 among the state-of-the-art desalination membranes.

Chemically Converted Graphene Nanosheets for the Construction of Ion-Exclusion Nanochannel Membranes.

Water and ion transport in nanochannels is an intriguing topic that has been extensively investigated in several energy- and environment-related research fields. Recently developed two-dimensional (2D) materials are ideal building blocks for constructing confined nanochannels by self-stacking. Among these, graphene oxide (GO) is the most frequently employed as the starting material because of its excellent solution processability. Since solvation of the GO nanostructure usually impairs the function of nanochannels, in this study, chemically converted graphene was prepared using a one-step method, to simultaneously acquire the desired stability and functionality of the nanochannels. The confined channels with high charge densities are capable of excluding ∼90% NaCl solutes from water in a pressure-driven filtration process. This surpasses the performance of most GO desalination membranes reported in the literature. Thus, this study provides useful information for the feasible development of ion-exclusion nanochannel membranes based on the proposed nanochannel-confined charge repulsion mechanism.

Engineering Heterostructured Thin-Film Nanocomposite Membrane with Functionalized Graphene Oxide Quantum Dots (GOQD) for Highly Efficient Reverse Osmosis.

In this study, custom-tailored graphene oxide quantum dots (GOQD) were synthesized as functional nanofillers to be embedded into the polyamide (PA) membrane for reverse osmosis (RO) via interfacial polymerization (IP). The heterostructured interface-functionalization of amine/sulfonic decoration on GOQD (N/S-d-GOQD) takes place via the tuning of the molecular design. The embedded N/S-d-GOQD inside the PA matrix contributes to facilitating water molecules quick transport due to the more accessible capturing sites with higher internal polarity, achieving a nearly 3-fold increase in water permeance when compared to the pristine thin-film composite (TFC) membrane. Covalent bonding between the terminal amine groups and the acyl chloride of trimesoyl chloride (TMC) enables the formation of an amplified selective layer, while the sulfonic part assists in maintaining a robust membrane surface negative charge, thus remarkably improving the membrane selectivity toward NaCl. As a result, the newly developed TFN membrane performed remarkably high water permeance up to 5.89 L m-2 h-1 bar-1 without the compromising of its favorable salt (NaCl) rejection ratio of 97.1%, revealing a comparably high separation property when comparing to the state-of-the-art RO membranes, and surpassing the permeability-selectivity trade-off limits. Furthermore, we systematically investigated the GOQDs with different surface decorations but similar configurations (including 3 different nanofillers of pristine GOQD, amine decorated GOQD (N-d-GOQD), and N/S-d-GOQD) to unveil the underlying mechanisms of the swing effects of internal geometry and polarity of the embedded nanofillers on contributing to the uptake, and/or release of aqueous molecules within TFN membranes, providing a fundamental perspective to investigate the impact of embedded nanofillers on the formation of an IP layer and the overall transporting behavior of the RO process.

Controllable ion transport by surface-charged graphene oxide membrane.

Ion transport is crucial for biological systems and membrane-based technology. Atomic-thick two-dimensional materials, especially graphene oxide (GO), have emerged as ideal building blocks for developing synthetic membranes for ion transport. However, the exclusion of small ions in a pressured filtration process remains a challenge for GO membranes. Here we report manipulation of membrane surface charge to control ion transport through GO membranes. The highly charged GO membrane surface repels high-valent co-ions owing to its high interaction energy barrier while concomitantly restraining permeation of electrostatically attracted low-valent counter-ions based on balancing overall solution charge. The deliberately regulated surface-charged GO membranes demonstrate remarkable enhancement of ion rejection with intrinsically high water permeance that exceeds the performance limits of state-of-the-art nanofiltration membranes. This facile and scalable surface charge control approach opens opportunities in selective ion transport for the fields of water transport, biomimetic ion channels and biosensors, ion batteries and energy conversions.

Precisely Controlling Nanochannels of Graphene Oxide Membranes through Lignin-Based Cation Decoration for Dehydration of Biofuels.

Lignin-based cations introduced into graphene oxide (GO) have been found to bring about stabilization of the nanostructure and the active sites and to give rise to various interactions for subsequent modification with polyelectrolyte and nanospacers, with a view to precisely controlling the nanochannels of the GO-based membranes. The resulting membranes exhibited excellent performance in biofuel dehydration with water flux of 4000-6000 g m-2 h-1 , which exceeds that of the state-of-the-art polymeric and GO-based membranes.

Incorporating Graphene Oxide into Alginate Polymer with a Cationic Intermediate To Strengthen Membrane Dehydration Performance.

Two-dimensional graphene oxide (GO) in hybrid membranes provides fast water transfer across its surface due to the abundant oxygenated functional groups to afford water sorption and the hydrophobic basal plane to create fast transporting pathways. To establish more compatible and efficient interactions for GO and sodium alginate (SA) polymer chains, cations sourced from lignin are employed to decorate GO (labeled as cation-functionalized GO (CG)) nanosheets via cation-π and π-π interactions, providing more interactive sites to confer synergetic benefits with polymer matrix. Cations from CG are also functional to partially interlock SA chains and intensify water diffusion. And with the aid of two-dimensional pathways of CG, fast selective water permeation can be realized through hybrid membranes with CG fillers. In dehydrating aqueous ethanol solution, the hybrid membrane exhibits considerable performance compared with bare SA polymer membrane (long-term stable permeation flux larger than 2500 g m-2 h-1 and water content larger than 99.7 wt %, with feed water content of 10 wt % under 70 °C). The effects of CG content in SA membrane were investigated, and the transport mechanism was correspondingly studied through varying operation conditions and membrane materials. In addition, such a membrane possesses long-term stability and almost unchanged high dehydration capability.

Excellent Biofouling Alleviation of Thermoexfoliated Vermiculite Blended Poly(ether sulfone) Ultrafiltration Membrane.

Flux and antifouling properties of mixed matrix membranes (MMMs) are yet to attain satisfactory status. The objective of this study is to find a method for mitigating the biofouling of poly(ether sulfone) (PES) ultrafiltration membranes via blending of thermoexfoliated vermiculite (VMT). Flow cytometry analysis shows that the behaviors of Bacillus subtilis 168 as a Gram-positive bacterium and Escherichia coli DH5 alpha as a Gram-negative bacterium were different. Hence, cell property is a suspected contributory factor in biofilm formation. Accordingly, considering the local predominant bacterial strains, a regionally customized membrane could scientifically be an expert solution for biofouling mitigation. Fabricated composite membranes have shown a higher flux compared to control PES membrane. Among all composite membranes, the PES-VMT0.10 had the highest flux of 476.4 L/(m2 h) (LMH) before fouling, and the highest flux of 210.7 LMH after three cycles of usage. In addition, the rejection rate of the PES-VMT0.15 The bovine serum albumin (BSA) sample was >77%, while that of the PES-VMT0.10 was >84%. The results of the static BSA adsorption test and the bacterial attachment test indicated that the membranes with macro-roughness on their surface showed better antibiofouling resistance. The antifouling properties of the modified membranes were also improved, because of their optimal wettability. On one hand, the hydrophilicity of membranes caused damaging both Gram-positive and Gram-negative bacteria and bacteriocidal effect. On the other hand, BSA adsorption and bacterial attachment on the membrane surface were affected by pore diameter.

A ZIF-71 Hollow Fiber Membrane Fabricated by Contra-Diffusion.

As a subclass of metal-organic framework materials, zeolitic imidazolate frameworks (ZIFs) have exhibited great potential for numerous applications because of their special three-dimensional structure. Up to now, utilizing ZIF membranes for liquid separations is still limited because it is very difficult to select suitable materials and to fabricate integrated membranes. In this work, a modified contra-diffusion method was carried out to prepare ZIF-71 hollow fiber membranes. The metals Zn(2+) and the organic links imidazole would meet and react on the interface of ceramic hollow fiber through diffusion. The as-prepared ZIF-71 membrane exhibits good performance in separation of ethanol-water mixtures.