A Novel Amide-functionalized Covalent Organic Framework for Selective Dye Adsorption

Organic dyes are widely applied for cloth dyeing process, paper printing, cosmetics, and food processing industries. The toxicity of industrial dyes poses serious threats to water and human health. Thus, several methods such as adsorption, degradation, and photocatalytic oxidation or reduction have been proposed to remove dyes from water. Among them, with low cost and high adsorption efficiency, porous materials become the most common materials to remove organic dyes from aqueous solutions. Notably, covalent organic frameworks (COFs) are a kind of novel crystalline porous polymers with regular pores, modifiable frameworks, high specific areas and excellent stability. Because of their outstanding properties, COFs have been used in many fields, such as gas adsorption and separation, heterogeneous catalysis, semiconductors and sensors. Especially the introduction of the nitrogen-containing functional groups to the COF skeletons provides abundant active sites to adsorb specific dyes in the field of dye adsorption. Inspired by this, a novel amide-functionalized two-dimensional COF (termed as JUC-578) was successfully prepared through Schiff-based condensation reaction of 4,4'-diaminobenzanilide (DABA) and 1,3,5-benzenetricarboxaldehyde (BT). A series of characterizations proved that JUC-578 has high crystallinity, uniform morphology and opening one-dimensional mesoporous pores. Especially the morphology of JUC-578, which was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), showed a clear spherical structure with uniform morphology distribution and smooth surface. Moreover, we studied the reversible adsorption of JUC-578 on dyes from aqueous solutions by UV-Vis spectrophotometry. The results show that JUC-578 can selectively adsorbed cationic dyes and the structure remained stable after cycling, which was attributed to the electrostatic interaction between the electron donor nitrogen in the skeleton and the electron-deficient dyes as well as other weak interactions (hydrogen bonding, coupling, etc.). Meanwhile, the crystallinity and ordered pores of JUC-578 are also important factors for reversible dye adsorption. These results indicate that COFs as functionalized porous materials have great potential in dealing with environmental problems.