Low-Temperature Pyrolysis-Catalysis Coupled System for Efficient Tetrachlorobenzene Removal: Condition Optimization and Decomposition Mechanism

The decomposition of tetrachlorobenzene (TeCB) emitted from simulated fly ash was studied in a low-temperature pyrolysis-catalysis coupled system. The influences of catalyst support, active component type, active phase-to-assistant ratio, vanadium loading, and catalyst calcination temperature on TeCB conversion were comprehensively investigated. The optimal catalyst composition and preparation conditions were determined through single-factor experiments. Moreover, the effects of reaction temperature, space velocity, and pollutant initial concentration on TeCB decomposition efficiency were explored by orthogonal experiments. The possible mechanism for PeCB decomposition over prepared V₂O₅-WO₃/TiO₂ catalysts was proposed based on reaction-byproduct composition and distribution. It is found that the TeCB decomposition efficiency is positively correlated with reaction temperature while being negatively correlated with gas hourly space velocity (GHSV) and TeCB initial concentration. GHSV has the most significant effect on TeCB decomposition, followed by reaction temperature and TeCB concentration. Under optimal conditions of space velocity of 600 h^-1, reaction temperature of 350 °C, and TeCB initial concentration of 0.5 volume percent, the TeCB conversion can reach up to 94.1%. The key substeps of TeCB decomposition are aromatic pollutant adsorption, nucleophilic substitution, and intermediate electrophilic substitution. The TeCB molecules were first adsorbed on catalyst surface and then decomposed into low-chlorination aromatics and aromatic and aliphatic hydrocarbons as phenolates, benzoquinone, aldehydes, and carboxylic acids.