Constructing large loadings of dual pathways with Ti3C2Tx-CDs in thin film nanocomposite membrane for enhanced organic permeation
Graphical abstract
Introduction
The sharp rise of organic solvents in the environment stemming from anthropogenic emissions brings about serious pollution and waste. With its low cost, energy efficiency, and flexibility, membrane-based separation technology can greatly contribute to the recovery, separation, and disposal of organic solvents [[1], [2], [3], [4], [5]]. Among the substantial amount of membranes, thin-film composite (TFC) membranes hold great promise owing to their sophisticated architecture—a thin separating layer produced on the top of a porous supporting layer via interfacial polymerization [[6], [7], [8], [9], [10], [11]]. Nevertheless, the simultaneous realization of acceptable permeability and rejection of TFC membranes poses a huge challenge, which significantly impedes the development and practical application of these membranes [12]. A general approach is to fabricate TFN membranes by incorporating nanoparticles. To date, a variety of nanoparticles, such as zeolites [13], titanium dioxide [14], graphene oxide [15], MXenes [16], carbon dots [17], and covalent−organic frameworks [18] have been successfully incorporated into the separating layer of TFN membranes. The separation performances of TFN membranes are boosted as these incorporated nanoparticles simply entitle TFN membranes with modified structures and properties. Unfortunately, most TFN membranes exhibit low permeation for nonpolar solvents because of the hydrophilicity of polymer matrix [19,20]. This makes it fail to apply in practical organic solvent systems, containing both polar and nonpolar solvents. For example, the process of solvent recovery in the dewaxing of lube oil, lube oil is the solute while mixtures of butanone (polar) and toluene (nonpolar) are solvents [21]. It inevitably causes intensified energy and cost with common TFN membranes.
A strategy of dual-pathway nanostructure offers the potential to solve these issues by simultaneously constructing nonpolar and polar solvent pathways. For instance, Mao et al. obtained high permeation (3.94 L m−2 h−1 bar−1) for polar solvent (isopropanol), as well as increased permeation (2.52 L m−2 h−1 bar−1) for nonpolar solvent (toluene) through incorporating CDs into the skin layer [22]. The hydrophobic inner cavities of CDs provided a highway for nonpolar solvents, whereas the hydrophilic polymer matrixes for polar solvents. Liu et al. reported that integrating hydrophobic polydimethylsiloxane in hydrophilic polymer matrixes efficiently elevated the n-heptane permeation up to 7.86 L m−2 h−1 bar−1, meanwhile, maintained the isopropanol permeation as high as 3.0 L m−2 h−1 bar−1 [23]. The presence of both hydrophilic and hydrophobic regions in these membranes facilitated the transfer of polar and nonpolar solvents. Despite advances, a significant challenge is to achieve large loadings of dual pathways to improve the permeation of polar and nonpolar solvents noticeably, which generally sacrifices the mechanical stability and gives rise to the cost due to the high concentration of nanoparticles [24,25]. Moreover, it leads to the formation of agglomeration, thus the deterioration of separation performance [[26], [27], [28]].
Recently, Ti3C2Tx nanosheets, an emerging kind of two dimensional (2D) nanosheets materials, have gained increasing popularity as novel nanoparticles to fabricate membranes due to their favorable thermostability, flexibility, and dispersibility in water [[29], [30], [31], [32], [33]]. The unique 2D transfer channels by stacking Ti3C2Tx nanosheets endow these membranes with fast and selective transfer of molecules, benefitting to the acquisition of high permeation [[34], [35], [36], [37], [38]]. Interestingly, Ti3C2Tx nanosheets possess abundant –OH groups which render Ti3C2Tx promising candidates for anchoring large loadings of functional materials by facile modification. Besides, modifying Ti3C2Tx nanosheets can limit them to the possibility of self-alignment into a compact layered structure. In our previous work, functionalized Ti3C2Tx nanosheets were prepared by grafting functional groups, then incorporated into polyethyleneimine and polydimethylsiloxane matrixes to fabricate TFN membranes, respectively [16]. The results presented that these nanosheets resulted in a strong effect on the solvent transfer property of TFN membranes under the operating pressures of 4 and 10 bar. However, the application of Ti3C2Tx nanosheets to construct TFN membranes with dual-pathway nanostructures has not been explored yet.
In this work, a “one-stone-two-birds” strategy was innovatively proposed. Modifying Ti3C2Tx nanosheets by anchoring functional materials can not only construct large loadings of dual pathways in TFN membranes but also overcome the drawbacks of self-alignment into a compact layered structure when using bare Ti3C2Tx. In detail (Fig. 1), novel building blocks which subtly synergize the superiorities of two materials were designed for the first time: anchoring CDs on Ti3C2Tx nanosheets (Ti3C2Tx-CDs). Then, TFN membranes with large loadings of dual pathways were fabricated by incorporating Ti3C2Tx-CDs in hydrophilic PEI matrixes. The physical and chemical structures of these membranes were studied in detail. Solvent permeation and solute rejection were tested to evaluate the nanofiltration performance.
Section snippets
Materials
CDs (α-CDs, β-CDs, and γ-CDs) were supplied by Chengdu Xiya Reagent Research Center. PEI and trimesoyl chloride (TMC) were obtained from Alfa Aesar (China). Polyacrylonitrile (PAN) support (molecular weight cutoff (MWCO), 50 kDa) was purchased from Zhongkeruiyang Membrane Engineering & Technology Co. Ltd (China). Isopropanol, n-hexane, n-heptane, sodium hydroxide, ammonium hydroxide, and hydrazine hydrate were acquired from Kewei Chemistry Co. Ltd (China). Polyethylene glycol (PEG) (Mws 200,
Characterization of Ti3C2Tx and Ti3C2Tx-CDs
Here, α-CDs, β-CDs, and γ-CDs anchored Ti3C2Tx (Ti3C2Tx-CDs) were synthesized by grafting CDs on the surface of Ti3C2Tx nanosheets. TEM image (Fig. 3a) reveals that Ti3C2Tx nanosheets have a lateral dimension of several micrometers demonstrating a typical sheet structure [39]. AFM images (Fig. 3b and c) show that Ti3C2Tx nanosheets possess a smooth surface with the thickness of 4.5 nm. After the grafting procedure, Ti3C2Tx-β-CDs retains the original shape as that of Ti3C2Tx nanosheets in Fig. 3
Conclusion
In summary, Ti3C2Tx-CDs can construct large loadings of dual pathways in TFN membranes. The abundant –OH groups of Ti3C2Tx nanosheets render quantities of CDs to be firmly and uniformly anchored on Ti3C2Tx nanosheets, then, forming large loadings of dual pathways. The hydrophobic internal cavities of CDs allow for the fast transfer of nonpolar solvents, while hydrophilic PEI matrixes facilitate polar solvents, leading to highly enhanced permeation. Specifically, the n-heptane permeation of
CRediT authorship contribution statement
Lan Hao: Investigation, Methodology, Writing - original draft. Zexu Chi: Investigation, Visualization. Qingbai Chen: Formal analysis, Investigation. Haoqin Zhang: Conceptualization, Supervision. Jianyou Wang: Supervision, Writing - review & editing.
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
This work was supported by Natural Science Foundation of Henan Province (182300410276), National Key Research and Development Program of China (2017YFC0404003), Tianjin Special Project of Ecological Environment Management Science and Technology (18ZXSZSF00050), and Tianjin Key Research and Development Program (19YFZCSF00760).
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