Single-Layered Nanosheets of Covalent Triazine Frameworks (CTFs) by Mild Oxidation for Molecular-Sieving Membranes

Abstract

Covalent triazine framework (CTF) nanosheets featured with uniform intrinsic nanoporosity and excellent stability are promising building blocks for fast, selective membranes. However, it remains challenging to produce ultrathin CTF nanosheets, significantly hindering the development of CTF-based membranes. Herein, we develop a mild oxidation strategy to exfoliate CTFs, enabling the preparation of highly permeable membranes with stacked CTF nanosheets as the selective layers. The interlamellar spacing of CTF is effectively expanded following the mechanism of “proton donating–accepting” in which dimethyl sulfoxide (DMSO) works as a soft oxidant, leading to ultrathin CTF nanosheets with the assistance of ultrasonication. Furthermore, oxygen-containing functional groups are also introduced onto the CTF nanosheets through mild oxidation, improving surface hydrophilicity. The CTF nanosheet can be stacked onto porous substrates by vacuum filtration to form composite membranes with the thickness of the stacked CTF nanosheets down to ∼30 nm. Thus-obtained membranes exhibit impressive dye separation performances with both high water permeance and high rejection. This work provides not only an efficient method to synthesize ultrathin CTF nanosheets but also a process to prepare fast but selective membranes for molecular separations.

Covalent Organic Framework Membranes for Efficient Chemicals Separation

“…Similar to this study, Wang and coworker adopted a mild oxidation strategy to exfoliate powdered CTF into ultrathin nanosheets and then restacked them into functional membranes for molecular separation. [177] A simple and economical azobenzene template approach also was developed to exfoliate TpAD (a β-ketoenamine-linked COF) into larger scale nanosheets that were then restacked into COF membranes by vacuum filtration for highly efficient water treatment. [178] Moreover, some facile bottom-up synthesis strategies such as interfacial crystallization [119] and salt template [179] have been applied to prepare COFs nanosheets, which were then deposited onto porous supports by vacuum filtration for dyes removal.…”

“…These membranes showed a water permeance over 89 L m À2 h À1 bar À1 and nearly complete rejection for dye molecules with molecular weight larger than 800 Da (Figure 25b). [179] Otherwise, other pure COF membranes based on the triazine-based COF nanosheets [177] and β-ketoenamine-linked COF nanosheets [178] were also explored, which exhibited desired dye separation performances. In additional, some COF hybrid membranes that were derived from the combination of COF nanosheets and other 1D and 2D materials also perform well in the separation of organic micropollutants.…”

“…[202] Therefore, the contributions of adsorption, as compared with that of sieving (including size exclusion and electrostatic repulsion), should be strictly examined. In other words, some indispensable experiments, [47,90,101,103,104,126,127,[177][178][179][180]185,203] such as static adsorption and continuous long-term stability with large feed volumes, should be conducted to quantificationally calculate the adsorption capacity of COF membranes, which is beneficial to figure out that which one separation behavior (adsorption or sieving) plays a dominate role in the membrane separation process. In addition, the separation mechanism of COF membranes should be discreetly explored in the future.…”

Covalent Triazine Framework Nanosheets for Efficient Energy Storage and Conversion

“…After that, Xu et al [35] synthesized layered CTFs crystals with a smooth surface and lateral sizes over tens of micrometers via interfacial polymerization[Fig. 2 Recently, Wang et al [36] obtained CTFs nanosheets with a thickness of 1.1 nm through the mild oxidation of layered CTFs prepared by interfacial polymerization with dimethyl sulfoxide(DMSO) as a soft oxidant based on the mechanism of -proton donating-accepting‖. The interlamellar spacing of Vol.36…”

Immobilized Covalent Triazine Frameworks Films as Effective Photocatalysts for Hydrogen Evolution Reaction

“…7,[10][11][12][13] There are only a few reports for the preparation of CTFs based lms, and all of them were prepared by the superacid catalytic method. 4,6,[20][21][22][23] The thickness, lateral size, and crystalline nature of such lms, however, is di cult to control and it is still a challenge to fabricate CTFs lms with large lateral size, and controllable thickness under mild conditions.…”