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

Instead of using organic solvents, a deep eutectic solvent (DES) composed of tetrabutylammonium bromide and imidazole (Bu4NBr/Im) was employed as a solvent for the first time to synthesize covalent organic frameworks (COFs). Due to the low vapor pressure of the Bu4NBr/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 pi-pi interactions between the pi-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.

Automated summary

A novel carboxyl-functionalized COF (TpPa-COOH) was successfully synthesized using a deep eutectic solvent (Bu4NBr/Im), showing excellent selective adsorption performance for methylene blue and safranine T. The adsorption isotherms of TpPa-COOH for the dyes were well modeled using a Langmuir isotherm, with a higher adsorption capacity observed for safranine T compared to methylene blue. These results could provide guidance for the green synthesis of adsorbents for wastewater treatment.