How the Most Neglected\nResidual Species in MOF-Based\nCatalysts Involved in Catalytic Reactions to Form Toxic Byproducts

In recent years, multifarious new materials have been\ndeveloped\nfor environmental governance. Thereinto, metal organic framework (MOF)-based\ncatalysts have been widely employed for heterogeneous catalysis because\nof their high porosity to confine noble metal particles faraway from\naggregation. However, the potential reactions between residual species\nfrom the material synthesis process and target pollutants, which could\nform highly toxic byproducts, are often neglected. Herein, we took\nthe widely used Zr-MOF, UiO-66, with highly thermal stability supported\nPd catalysts as the example to investigate how the residual species\nin catalysts are involved in aromatic volatile organic compounds (VOCs)\ndegradation reaction. The results showed that residual Cl species\noriginated from the ZrCl<sub>4</sub> metal precursor participated\nin the VOC degradation reaction, leading to the production of various\nchlorine-containing byproducts, even the hypertoxicity dioxin precursor,\ndichlorobenzene. Meanwhile, the chlorination mechanism for the formation\nof chlorine-containing byproducts was revealed by density functional\ntheory calculation. Furthermore, the highly efficient residual Cl\nremoval approaches are proposed. Importantly, the migration and transformation\nof residual Cl during the degradation of five benzene series VOCs\nare comprehensively studied and elucidated. We anticipate that these\nfindings will raise alarm about the neglected issue of residual species\nin MOF-based catalysts for heterogeneous catalysis, especially environmentally\nfriendly catalysis.