Two-dimensional metal−porphyrin framework membranes for efficient molecular sieving
Graphical abstract
Introduction
With the frequent dye emissions and continually increasing frequency of dye/salt mixture discharges, dyeing wastewater has caused many environmental disasters. As the by-products of dye production and dyeing processes, the salts coexisting in textile wastewater restrict further treatment and biodegradation of dye wastewater. The effective separation of dye/salt mixtures is a worldwide challenge with respect to water purification technologies [1,2]. Traditional separation methods for treating dye wastewater, such as adsorption, oxidation and chemical degradation, are not suitable for dye/salt mixtures, especially those containing small organic dye molecules and multivalent salts [[3], [4], [5], [6]]. Burgeoning membrane technology has been widely employed to separate a variety of dye/salt mixture systems [7,8]. Compared with traditional membranes, two-dimensional (2D) membranes have displayed great advantages of high-permeance and excellent selectivity [9,10], which were constructed from 2D nanomaterials of atom-thick, such as graphene oxide (GO) [11,12], transition metal dichalcogenides (TMDs) [[13], [14], [15]], graphitic carbon nitride (g-C3N4) [16,17], MXene [18,19], metal-organic frameworks (MOFs) [20] and others [21]. Among them, MOF nanosheets, which contain organic ligands and metal ions connected by coordination bonds, are showing tremendous potentials in next-generation separation membrane field due to their regular and highly tunable nanopore structures. Compared with GO membranes [22], which separate molecules primarily relying on the interlayer spacing, the 2D MOF membrane is endowed with ultrafast and efficient selective separation performance due to their additional regular in-plane nanopores and rigid skeletons. Although MOF membranes have been widely used in nanofiltration [23], desalination [24], as well as pervaporation for the separation of organic/water mixtures [25], only a few investigations have been conducted on 2D MOF membranes for sieving small molecules, especially molecules in solvent system. It may arise from the low permeance of these membranes because of the non-directional growth of MOFs leading to an ultra-compact filter layer and a minor aperture [20,26]. Besides, the solvent stability and material flexibility of MOF nanosheets pose a challenge to the separation performance of the resultant membranes [27]. Therefore, it is important to rationally design a 2D MOF membrane with remarkable performance to overcome the trade-off upper bound for dye/salt separation.
As a crucial member of the MOF family, crystalline porphyrin-based MOF nanosheets with thicknesses of only a few molecular layers were adopted to construct laminar membranes thanks to its high stability and crystallinity [28,29]. Recent investigations have indicated that in addition to the interspacing between adjacent nanosheets, the intrinsic vertical nanopores of 2D nanosheets can serve as transport channels, thus reducing the molecule transmission length and minimizing the frictional resistance towards water transport [10,30]. Zhang et al. [31], reported a stable 2D Al-MOF membrane that achieved a water permeance as great as 0.04 L m−2 h−1 bar−1 and exhibited excellent ion rejection levels of nearly 100%. Based on porphyrin coordination with copper, 2D Cu-TCPP nanosheets show high crystallinity [32] and optimum negative charge properties revealing its stability [33], as well as controllable lamella morphology [34]. However, the Cu-TCPP nanosheets obtained by conventional method have poor uniformity and large layer thickness, thus it is hard to assemble a uniform and integral nanomembrane. Peng et al. [35], found that the incorporation of Cu-TCPP nanosheets into sulfonated polystyrene (SPS) membranes as novel nanofillers resulted in efficient photo-switch proton conductive membranes. Furthermore, Wu et al. [36], found that filling the 2D MOF membrane interlayer spacing and defects with poly(N-vinyl caprolactam) polymers obtained an excellent water permeance. Although the great potential of 2D MOF membranes has been demonstrated, few studies have researched free-standing 2D lamellar Cu-TCPP membranes for dye/salt separation. In-situ growth of MOFs and the polymer-filled strategy for MOF membrane preparation are difficult to ensure that the intrinsic pores of MOFs and the interlayer channels work together. In addition, owing to the limited availability of methods to characterize hydrated membranes, the transport behaviors and separation mechanisms of porphyrin-based 2D MOF membranes are seldom discussed.
Herein, monolayer Cu-TCPP nanosheets were used to construct laminar membranes, which possessed superior performance for dye/salt separation, as shown in Fig. 1. The facile vacuum assistant filtration method was applied to ensure the interlayer structure of 2D Cu-TCPP. The membrane prepared from Cu-TCPP nanosheets with a layer spacing of ∼1.34 nm and an intrinsic pore size of ∼1.2 nm showed good permeance towards H2O (2.8 Å) and inorganic salts but high retentions for dyes and other macromolecules, breaking the restraint of the trade-off effect between permeance and selectivity. The effects of nanosheet lateral dimensions and thickness on the filtration performance of membranes were studied. Furthermore, the membrane showed excellent separation stability and antifouling performance in a cross-flow system, indicating the promising potentials for water purification applications.
Section snippets
Materials
Nylon microfiltration supports (mean pore size is 0.2 μm) were purchased from Whatman, Germany. Porous anodic alumina oxide (AAO) supports with a pore size of 200 nm were purchased from PUYUAN NANO, China. Cupper nitrate trihydrate [Cu(NO3)2·3H2O], formic acid, polyvinyl pyrrolidone (PVP, average Mw: 59000), tetrakis (4-carboxyphenyl) porphyrin (TCPP), bovine serum albumin (BSA, Mw = 67,000 Da), dimethyl formamide (DMF, analytical grade), ethanol (analytical grade), polyvinyl alcohol (PVA-210)
Morphologies and structures of Cu-TCPP nanosheets
The Cu-TCPP nanosheets of different lateral dimensions were obtained by fine-tuning the parameters of ultrasound time and centrifugal speeds. The TEM and AFM analyses presented in Fig. 2 show the microstructures and morphologies of large lateral dimension nanosheets (LNs) and small lateral dimension nanosheets (SNs). The difference in nanosheet lateral dimensions was confirmed by the TEM images in Fig. 2a and d. Moreover, it can be inferred from the almost transparent nanosheet that its
Conclusion
In summary, 2D Cu-TCPP laminar membranes were prepared successfully via vacuum filtration using a PVA-modified nylon substrate. The morphologies, structures and thicknesses of the lamellar membranes were adjusted by varying the lateral dimensions and loading amounts of the Cu-TCPP nanosheets, which allowed for tuning the membrane separation performance and antifouling properties. Under the optimum conditions, the Cu-TCPP SN membrane with a thickness of 300 nm possessed a water permeance of
Author statement
P. Cheng: Methodology, Investigation, Formal analysis, Data curation, Visualization, Writing - original draft, Writing - review & editing. Y.D. Huang: Investigation, Data Curation, Formal analysis. C. Wu: Investigation, Validation. X.-P. Wang: Investigation, Formal analysis. X.-R. Fu: Investigation, Formal analysis. P. Li: Investigation, Methodology. S.J. Xia: Writing - Review & Editing, Supervision, Project administration, Funding acquisition. Y.L. Liu: Investigation, Writing - review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge the funding support from National Natural Science Foundation of China (51778442).
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