Decoupling synthesis and crystallization of covalent organic frameworks (COFs) for more crystalline layer targeting high efficiency dye separation

Abstract

Covalent organic frameworks (COFs) are crystalline porous materials composed of organic building blocks linked by covalent bonds, which have attracted great attention for membrane–based separation applications. However, fabrication of COF membranes with high quality and performance remains a challenge, especially for the β–ketoenamine linked COFs. In the conventional IP methods, the synthesis process (the Schiff–base reaction of the monomers and the tautomerism of the oligomers) and the crystallization process (the self–assembly of the oligomers) of the COFs materials occur simultaneously on the surface of the support membrane, which can result in low crystallinity, high defect density, and poor separation performance of the COF layer. In this study, a novel strategy for fabricating large-area, defect–free COFs composite membranes by interfacial polymerization (IP) was presented to overcome this problem. By incorporating both the amine and aldehyde monomers in one organic phase, and dissolving organic acid catalyst in another phase (the water), the synthesis process mostly take place in the bulk of the organic phase, while the crystallization occurs on the surface of the support membrane under the influence of the organic acid catalyst. This strategy effectively decoupled the synthesis process with the crystallization, providing a stable growth region for crystalline layer, ultimately leading to the formation of a thin, defect–free and highly ordered COF layer on the support membrane. The strategy was demonstrate by synthesis of triformylphloroglucinol phenylenediamine (TpPa)–Ⅱ/Poly(m–phenylene isophthalaminde) (PMIA) composite membrane. We characterize the structure and morphology of the TpPa–Ⅱ/PMIA composite membrane by various analytical techniques to verify our hypothesis, and evaluate its separation performance using pure water permeance and dyes retention rate as indicators. The results show that the TpPa–Ⅱ/PMIA composite membrane has an 80 nm thick top-layer with regular COF structure and orderly pores. It exhibits an excellent dye separation performance, with a (pure water permeance) PWP of 105.6 L·m−2·h−1·bar−1 and CR rejection of 99.2 %, which surpasses most of the TpPa–type COFs composite membranes reported in literature, as well as the reference (TpPa–I/PMIA membrane) in this work. Moreover, the TpPa–Ⅱ/PMIA composite membrane maintains both its performance and structural stability during a 240–hour cross–flow filtration of the dye solution, demonstrating its excellent potential in the dye wastewater treatment.