A Carboxyl‐Functionalized Covalent Organic Framework Synthesized in a Deep Eutectic Solvent for Dye Adsorption

Abstract Instead of using organic solvents, a deep eutectic solvent (DES) composed of tetrabutylammonium bromide and imidazole (Bu 4 NBr/Im) was employed as a solvent for the first time to synthesize covalent organic frameworks (COFs). Due to the low vapor pressure of the Bu 4 NBr/Im‐based DES, a new carboxyl‐functionalized COF (TpPa‐COOH) was synthesized under environmental pressure. The as‐synthesized TpPa‐COOH has open channels, and the DES can be removed completely from the pores. The dye adsorption performance of TpPa‐COOH was examined for three organic dyes with similar molecular sizes: one anionic dye (eosin B, EB) and two cationic dyes (methylene blue, MB and safranine T, ST). TpPa‐COOH showed an excellent selective adsorption effect on MB and ST. The electronegative keto form in TpPa‐COOH might help to form electrostatic and π–π interactions between the π‐stacking frameworks of TpPa‐COOH and the positive plane MB and ST molecules. The adsorption isotherms of MB and ST on TpPa‐COOH were further investigated in detail, and the equilibrium adsorption was well modeled by using a Langmuir isotherm model. Together with hydrogen bonding, TpPa‐COOH showed higher adsorption capacity for ST than for MB (1135 vs. 410 mg g −1 ). These results could provide a guidance for the green synthesis of adsorbents in removing organic dyes from wastewater.