How the Most Neglected Residual Species in MOF-Based Catalysts Involved in Catalytic Reactions to Form Toxic Byproducts

In recent years, multifarious new materials have been developed for environmental governance. Thereinto, metal organic framework (MOF)-based catalysts have been widely employed for heterogeneous catalysis because of their high porosity to confine noble metal particles faraway from aggregation. However, the potential reactions between residual species from the material synthesis process and target pollutants, which could form highly toxic byproducts, are often neglected. Herein, we took the widely used Zr-MOF, UiO-66, with highly thermal stability supported Pd catalysts as the example to investigate how the residual species in catalysts are involved in aromatic volatile organic compounds (VOCs) degradation reaction. The results showed that residual Cl species originated from the ZrCl4 metal precursor participated in the VOC degradation reaction, leading to the production of various chlorine-containing byproducts, even the hypertoxicity dioxin precursor, dichlorobenzene. Meanwhile, the chlorination mechanism for the formation of chlorine-containing byproducts was revealed by density functional theory calculation. Furthermore, the highly efficient residual Cl removal approaches are proposed. Importantly, the migration and transformation of residual Cl during the degradation of five benzene series VOCs are comprehensively studied and elucidated. We anticipate that these findings will raise alarm about the neglected issue of residual species in MOF-based catalysts for heterogeneous catalysis, especially environmentally friendly catalysis.