Synthesis of Pyrene-Based Covalent Organic Frameworks for Photocatalytic Tetracycline Degradation

Pyrene-based covalent organic frameworks (sp2c-COF and Py-NH2–COF) were successfully prepared via a solvothermal route. Upon the irradiation of visible light, COFs were used to degrade tetracycline hydrochloride (TC) in aqueous solution. The results showed that sp2c-COF exhibited much better photocatalytic activity than Py-NH2–COF. 73.5 and 3.1% TC could be degraded within 90.0 min with the initial concentration of 20 mg/L via sp2c-COF and py-NH2–COF under visible-light irradiation, respectively. The influence of bond connection types on the visible-light catalytic ability of COFs was explored. Electrochemical impedance spectroscopy (EIS) results proved that sp2c-COF had a higher photogenerated electron–hole separation efficiency. X-ray photoelectron spectroscopy (XPS) and Mott–Schottky plots revealed that the band gaps of sp2c-COF and Py-NH2–COF were 1.98 and 2.28 eV, respectively, resulting in sp2c-COF having a higher electron carrier transport efficiency. Importantly, sp2c-COF exhibits good reusability, affording 70.3% degradation efficiency even after 5 successive recycles. Active species trapping experiments and electron paramagnetic resonance (EPR) and electrochemical analyses were performed to consult the proposed degradation mechanism. This work indicates that the COF with C═C bonds could improve electron hole separation and electron carrier transport more effectively than the COF with C═N bonds in the photocatalytic process, which led to a more effective way of transmission of π conjugation across the two-dimensional (2D) lattice and thus enhanced the electron delocalization. So it was more beneficial for the degradation of tetracycline.