Kinetic Suppression of Dioxin Formation by Silver Chloride: A Temperature-Dependent Mechanistic and Free-Energy...

The formation of polychlorinated dibenzo-p-dioxins (PCDDs), particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is a significant environmental issue due to its toxicity and persistence. This study presents a comprehensive mechanistic investigation of silver chloride (AgCl) as a kinetic suppressor for dioxin formation, using density functional theory (B3LYP/def2-TZVP). Seven AgCl-2,4,5-trichlorophenol (TCP) complexes were identified, stabilized via σ- and π-coordination motifs, with M2 being the most exergonic (-57.84 kJ mol^-1). NBO charge analysis revealed Ag's role as a Lewis acid in mediating bond-breaking and bond-forming events. The mechanism involves two HCl eliminations, oxygen-bridge formation, and final dioxin ring closure, with AgCl regenerated at the end. Gibbs free energy profiles show the process is exergonic across all temperatures (298-1200 K), but kinetically limited even at high temperatures (900-1200 K). The AgCl-mediated pathway exhibits higher effective barriers (≥385 kJ mol^-1) compared to CuCl-mediated pathways (≈304-310 kJ mol^-1 at 900 K), confirming AgCl's stronger suppression effect. The same mechanism applies to 2-chlorophenol, but with reduced toxicity in non-chlorinated dioxin products. These findings position AgCl as an effective kinetic suppressor for dioxin formation in post-combustion systems.