Construction of covalent organic frameworks (COF) membranes on the polymeric substrate was challenging by interfacial polymerization (IP) process. In this study, COF-LZU1 membranes were fabricated by IP on the preheated PSf substrates to investigate the influence of substrate properties (i.e., pore size and porosity) on the COF-LZU1 layer formation. The substrates with tunable pore size (18.61 to 14.53 nm) and porosity (2.05 % to 0.42 %) could be obtained with the pretreating temperature of 20 °C to 50 °C. Preheated substrate altered monomer storage and diffusion, leading to the variations in crosslinking degree (14.89 % to 87.63 %) and observed thickness (142.9–575.9 nm). The preheated substrates endowed better growth conditions by inhibiting the COF-LZU1 intrusion into the substrates, and optimal COF 40°C membrane had water flux of 6.57 L m −2 h −1 , and Na 2 SO 4 rejection ratio of 90.9 %. The rejection ratio of pharmaceuticals by COF-LZU1 membranes was low due to adsorption. Correlation analysis showed that the C N bond content was unfavorable for absorption ( P < 0.05), and the increase of crosslinking and crystallinity of the COF-LZU1 layer will be beneficial to boost the pharmaceutical rejection. This work provided a facile method and valuable insight for the COF-LZU1 membrane fabrication to enhance desalination and pharmaceutical removal.
Covalent organic frameworks (COFs) are attractive innovative crystalline porous materials that have immense potential for the production of high-performance nanofiltration membranes. Although the conventional water-organic interfacial polymerization process has been broadly applied to fabricate COFs composite membranes on polymeric substrates, it cannot be applicable for COFs constructed by poorly water-soluble aromatic amine monomers . Herein, we report the fabrication of triphenylamine-based COFs composite membranes via an oligomer-triggered interfacial polymerization (OT-IP) process on a hydrolyzed polyacrylonitrile (HPAN) substrate, in which the interfacial reaction occurred between an acetic acid aqueous solution and an organic solution of 1,3,5-triformylphloroglucinol (Tp) in n-hexane and tris(4-aminophenyl)amine (TAPA) in mesitylene premixed in a short time. The Tp-TAPA oligomers generated during the premixing were characterized in terms of morphology and molecular weight. The TpTAPA film formation during the OT-IP process was in situ observed through an optical contact angle measurement device. The TpTAPA/HPAN composite membranes displayed a water permeance of 68.1 L⋅m −2 ⋅h −1 ·bar −1 , and the rejections of different dye (EBT, CR) and drug (DG, AG) molecules above 92.0%. In addition, the composite membrane showed excellent stability in acidic solution and continuous filtration for 75 h. The results indicated that the OT-IP approach is a feasible and efficient approach to fabricate triphenylamine-based COF composite membranes with water-insoluble monomers.
The nanofiltration (NF) membrane with better hydrophilicity, uniform pore size distribution, and dually charged properties is highly desirable to improve the pharmaceutical rejection, especially for the neutral and positively charged pharmaceuticals. Herein, a TpPa membrane was first in-situ crystallized via p-toluenesulfonic acid (PTSA)-mediated interfacial catalytic polymerization (ICP) strategy using 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa), followed by the post-functionalization with polyethylene-imine (PEI) to narrow the pore size, improve hydrophilicity, and tailor membrane charges to enhance pharmaceutical rejection. PTSA was used as a catalyst to enhance the crystallinity of the TpPa membrane. The PEI introduction narrowed the pore radius from 0.382 ± 0.50 nm to 0.272 ± 0.33 nm, improved surface hydrophilicity from 74.8° to 37.0°, and shifted surface charge from −19.25 mV to 11.15 mV. This PEI-functionalized TpPa (TpPa-PEI) layers exhibited heterogeneous charges on both sides with a positively charged top and negatively charged bottom. MgCl 2 rejection increased from 13.7 % to 83.0 % without sacrificing water permeance. Additionally, pharmaceutical rejection and the water permeance of the optimal TpPa-PEI membrane exceeded those of the TpPa IP -PEI membrane fabricated without PTSA by about 3.3 and 1.3 times, respectively. Furthermore, compared to the pristine TpPa membranes, the substantially enhanced electropositivity of the optimal TpPa-PEI membrane led to 3–5 times increase in positively charged pharmaceutical rejection (94.1 % for propranolol, 97.2 % for sulpiride, and 71.2 % for metformin). The synergy between the negatively charged TpPa-PEI bottom layers and the reduced pore size maintained a sulfadiazine rejection of 62.1 %. Mechanistic study further revealed that PEI penetrated 100 nm into the TpPa layer and cross-linked with the aldehyde groups, leading to tailored chargeability, improved hydrophilicity, and reduced pore size of the membranes. Via a PEI cross-linking strategy, sub-nanometer channels of crystalline COF layers can be rationally designed, featuring precisely tailored chargeability and hydrophilicity, promising functionalization of the membrane pores and remarkably robust for water reuse.
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