How to achieve complete elimination of Cl-VOCs: A critical review on byproducts formation and inhibition strategies during catalytic oxidation

In catalytic oxidation of Cl-VOCs, H2O, CO2, and HCl are anticipated final products, while Cl2 with strong oxidability, CO and organics without complete mineralization are considered as byproducts. The particularity of Cl substitution in Cl-VOCs further brings higher possibility in byproducts formation with higher toxicity. Furthermore, polychlorinated byproducts as predominant precursors for PCDDs and PCDFs are commonly detected, which becomes the biggest potential risk. This review systematically summarizes multiple byproducts distribution in effluent gas and surficial accumulated compounds. The migration pathways of Cl species during catalytic oxidation include HCl desorption assisted by Brønsted acidic sites, Cl2 formation via Deacon reaction, chlorination into organic byproducts, and deposition on catalyst surface. Various strategies for inhibiting byproducts formation are explored based on the critical factors for byproducts formation with target to provide guidance to the development of catalysts for Cl-VOC efficient and complete elimination. Rapid transformation of intermediates without participating chlorination process is the crucial strategy. Improving further oxidation with rational redox property and preventing chlorination with suitable acidity and specific structures are two directions for inhibit byproducts formation. However, it is still challenging to reveal the migration and evolution routes of Cl species and rational design of catalysts to achieve actually complete elimination of Cl-VOCs with no secondary pollution.