For the practical uses of crystalline porous materials, eligible chemical stability is of great concern. Covalent organic frameworks (COFs), a novel class of organic crystalline porous materials composed by light elements, have attracted wide attention of researchers. However, the insufficient chemical stability has come out as a significant block for the further study and application of these materials.

Until now, most of COFs were connected by some limited kinds of dynamic covalent linkages including boronate ester, boroxine, imine, imide et al. Yet most of these linkages are not strong enough and are easy to be attacked by nucleophilic reagent (such as water) under certain conditions like acid or base. The exploration of a chemical stable linkage in COFs is one of the fundamental solutions for the stability issue.

Polyarylether (PAE) is a highly stable engineered plastic with high heat resistance, excellent chemical stability, good flame-retardant properties and high mechanical robustness, have been widely used in broad application prospects. The ether bonds, especially in diphenyl structures, are extraordinary robust. However, no successful introduction of this kind of bonds into COFs has been reported before.

Based on this idea, we synthesized a serious of polyarylether-based covalent organic frameworks (PAE-COFs). Through the condensation of the triangular building unit 2,3,6,7,10,11-hexahydroxytriphenylene hydrate (HHTP) with two liner linkage tetrafluoroterephthalonitrile (TFTPN) and 2,3,6,7-tetrafluoroanthraquinone (TFAQ), JUC-505 with micropores and JUC-506 with mesopores were acquired (JUC = Jilin University China).

These materials exhibited high crystalline, abundant pore structures and outstanding chemical stability. Their structures and porosities can be reserved even after treatment of some harsh conditions including boiling water, strong acids (12 M HCl, 18 M H₂SO₄ and 40% HF) and bases (14 M NaOH and 5 M MeONa), oxidation (chromic acid solution) and reduction (2.4 M LiAlH₄) medium. For comparison, we chose some of the most stable and representative CPMs including the zeolites, metal-organic frameworks (MOFs) and COFs, and none of these materials can endure all the conditions.

Functional group is crucial for the performance and potential applications of materials. Thanks to their extraordinary chemical stabilities of the frameworks, PAE-COFs can be served as a preeminent platform for post-modification even under some harsh conditions and abundant functional group can be decorated on the framework. In our work, cyano functionalized JUC-505 was selected as the parent material and two derivative COFs with different functional groups, JUC-505-COOH with carboxyl groups and JUC-505-NH₂ with amino groups, were acquired by refluxing in NaOH (20% in ethanol/water = 1/1) and LiAlH₄ (2.4 M in THF), respectively. These frameworks showed exceptional performances for antibiotics removal from water over a wide pH range.

To learn more details, please read our article entitled “Chemically stable polyarylether-based covalent organic frameworks” in Nature Chemistry (https://doi.org/10.1038/s41557-019-0238-5).