Covalently linked MOF@COF direct Z-scheme heterojunction for visible light-driven photocatalytic degradation of flotation agents

It is necessary to find a green, highly efficient and cost-effective way to treat the water pollution caused by flotation agents that are widely used in mineral processing, coal desulfurization and water treatment. Herein, by wrapping an amino group-containing MOF core with a visible light active triazine COF through imine bonds, a covalently linked MOF@COF composite was prepared and found to work as a direct Z-scheme heterojunction photocatalyst for highly efficient visible light-driven degradation of flotation agents. As prepared MOF@COF not only inherits the merits of parent MOF and COF such as large specific surface area, porous framework, good crystallinity, and strong visible light-harvesting capability, but also shows other attractive advantages including robust stability, highly efficient photogenerated electron-hole pair separation and enhanced redox ability due to its covalently linking mode. After carefully selecting the COF content in the composite and optimizing reaction conditions, the resultant MOF@COF composite was successfully used for the photocatalytic degradation and mineralization of flotation agent pollutes in a wide concentration range, giving an apparent rate constant that was 3.2 and 3.5 times higher than those of parent MOF and COF, respectively. In view of systematical characterization and experimental analysis, the underlying reaction mechanism concerning Z-scheme heterojunction was proposed. This work provides a promising way to prepare high-performance photocatalytic materials for the degradation and removal of flotation agent pollutants using inexhaustible solar energy, and to broaden the applications of MOF and COF-based materials.