Synthesis of mixed-linker Zr-MOFs for emerging contaminant adsorption and photodegradation under visible light

Metal organic frameworks (MOFs) modified with mixed-linker strategy have shown advantages in various fields. In this article, a mixed-linker Zr-MOFs, PCN-134, was successfully prepared according to the thermodynamic method and used as a photocatalytic adsorbent for the removal of diclofenac (DF), a representative emerging contaminant. In PCN-134, the connection between Zr6 clusters and benzene tribenzoate forms 2D layers, which are supported by TCPP (tetrakis(4-carboxyphenyl)porphyrin) to form a 3D layer-pillar framework. Since Zr-MOFs allows the partial absence of linkers leading to structural defects while maintaining its framework integrity, the defect density and properties of PCN-134 can be controlled. We synthesized a series of PCN-134s with different TCPP ratios and found the DF uptake of PCN30 was the highest among them due to a delicate balance between porosity and defects. The selected PCN-134 (PCN30) exhibited excellent adsorption performance for the removal of DF from water. The adsorption process is affected by pH and ionic strength. The kinetic data can be well described by pseudo-second-order model. The maximum adsorption capacity of DF is about 2.04 mmol/g, calculated by Langmuir isotherm model. Under visible light irradiation, PCN-134 exhibited high photocatalytic activity in DF degradation experiments. The formation of 1O2 indicated PCN-134 was dominated by type II photosensitization reaction, and the corresponding productivity was higher than that of single-linker PCN-224. The electron transform between TCPP linker and Zr6 clusters also contributed to the photodegradation of DF. The improved adsorptive and photocatalytic performance was mainly owing to the opened porous structure, extra defect sites, effective energy transfer and the synergistic effect between organic linkers. Further, as PCN-134 can be efficiently recycled, it is a promising photocatalytic adsorbent for wastewater treatment.