Mechanisms of the Formation of Polychlorinated Benzenes and Phenols by Heterogeneous Reactions of C₂ Aliphatics

Persistent aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo- p -dioxins and furans (PCDD/F), are formed during thermal reactions in combustion systems, such as municipal waste incineration (MWI). Aliphatic products of incomplete combustion (PICs) can react to form simple aromatics that are intermediates or precursors to PAHs and PCDD/F. Products from 600 °C heterogeneous combustion reactions of C 2 aliphatics, benzene, and hexachlorobenzene were compared in order to initiate further understanding of the mechanisms of aromatic formation from simple aliphatics. Reaction results (1) suggest that the C 2 aliphatics share related thermal decomposition pathways; (2) confirm that Cu plays a critical role in the catalytic action of MWI fly ash; and (3) suggest that Al 2 O 3 plays a unique catalytic role in the formation of aromatic rings from aliphatics. Comparisons of congener distribution patterns of chlorinated benzenes and chlorinated phenols and additional volatile and semivolatile compounds provided the basis for a proposed set of pathways that we believe are important in the formation of aromatics from short-chain aliphatics. CuO and fly ash promote the chlorination of aliphatic intermediates followed by ring formation, whereas Al 2 O 3 selectively catalyzes the formation of nonchlorinated aromatics followed by chlorination.