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 preparation and application of hydrazone-linked 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 building block and intramolecular hydrogen bonding as knot in the network. The proof-of-concept design was demonstrated by exploring 1,3,5-triformylphloroglucinol (Tp) and oxalyldihydrazide (ODH) as precursors to synthesize a novel COF material (TpODH), in which the different organic building units are combined through hydrazone bonds to form 2D 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. However, another COF synthesized with 1,3,5-triformylbenzene (TFB) and ODH exhibited less crystallinity and low special surface area (94 m2/g). Representatively, the resulting TpODH afforded Cu (II) and Hg (II) capacities of 324 and 1692 mg g-1, respectively, which exceeded most COFs previously reported. Moreover, the FT-IR and XPS 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.