Fabrication of Hydrazone-Linked Covalent Organic Frameworks Using Alkyl Amine as Building Block for High Adsorption Capacity of Metal Ions

There are several researches on the preparation and application of hydrazone-linked covalent organic frameworks (COFs), and all of them generally necessitate rigid aromatic amines. Herein, we report a strategy for design and synthesis of COF with flexible alkyl amine as a building block and intramolecular hydrogen bonding as a knot in the network. The proof-of-concept design was demonstrated by exploring 1,3,5-triformylphloroglucinol and oxalyldihydrazide (ODH) as precursors to synthesize a novel COF material (TpODH), in which different organic building units are combined through hydrazone bonds to form two-dimensional porous frameworks. It should be pointed that irreversible enol-to-keto tautomerism and intramolecular N-H···O═C hydrogen bonding of TpODH would enhance the crystallinity and chemical stability, leading to large specific surface area of 835 m² g^-1. However, another COF synthesized with 1,3,5-triformylbenzene and ODH exhibited less crystallinity and low special surface area (94 m² g^-1). Representatively, the resulting TpODH afforded Cu(II) and Hg(II) capacities of 324 and 1692 mg g^-1, respectively, which exceeded that of most COFs previously reported. Moreover, the Fourier-transform infrared and X-ray photoelectron spectroscopy spectra analyses were taken to demonstrate the adsorption mechanism. These results suggested that the materials could be applied to the removal of metallic ions in the future.