In Situ Synthesis of Defect-Engineered MOFs as a Photoregenerable Catalytic Adsorbent: Understanding the Effect of LML, Adsorption Behavior, and Photoreaction Process

Defect-engineering is an exciting strategy for the modification of metal-organic frameworks (MOFs), which can go beyond the limit of conventional MOFs, tailor material properties, and incorporate multiple functionalities. Herein, based on the large mixed-linker approach, we successfully integrated tetrakis(4-carboxyphenyl)porphyrin (TCPP) into stable UiO-66 via an in situ one-pot synthetic method and used the obtained material for the removal of diclofenac (DF). TCPP@UiO-66 maintained the structure, excellent stability, and porosity of UiO-66. The defect density significantly affected the phase purity, crystallite morphology, and properties of TCPP@UiO-66s. Owing to the delicate balance between defects, stability, and porosity, TCPP@UiO-66(25%) was the optimal material in our system. The pseudo-second-order kinetic model and the Sips isothermal model described the adsorption of DF onto defect-engineered MOFs, and the adsorption capacity was 590 mg/g. Electrostatic interaction, Lewis acid-base interaction, π-π interaction, hydrogen bonding, and anion-π interaction were possible adsorption mechanisms. Moreover, under simulated sunlight irradiation, TCPP@UiO-66(25%) was catalytically active for the degradation of DF with a removal efficiency of 99%. It displayed good recyclability during three reaction cycles. The result of electron spin resonance revealed the generation of 1O2, implying the occurrence of type II photosensitization reaction. Meanwhile, the first-order rate constants of DF photodegradation after the addition of scavengers confirmed that h+ is also a key reactive species. Both the energy transfer from TCPP to triplet oxygen and the electron transfer from TCPP to Zr clusters contributed to the degradation of DF. The degradation byproducts of DF were monitored by three-dimensional excitation-emission matrix (3D EEM). Therefore, TCPP@UiO-66(25%) was an attractive photoregenerable catalytic adsorbent for the effective removal of DF. Combining the advantages of the parent framework and the functional linker, our strategy expands the functionality of the stable MOFs for potential applications in environmental remediation.