Formation of PCDD/Fs in Oxidation of 2-Chlorophenol on Neat Silica Surface

Abstract

This contribution studies partial oxidation of 2-chlorophenol on surfaces of neat silica at temperatures of 250, 350, and 400 °C; i.e., temperatures that frequently lead to catalytic formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) from their precursors. We have identified 2,6-dichlorophenol (2,6-DCPh), 2,4-dichlorophenol (2,4-DCPh), and 2,4,6-trichlorophenol (2,4,6-TriCPh), but have detected no chlorinated benzenes (CBzs). The detected chlorinated and nonchlorinated DD/Fs comprise dibenzo-p-dioxin (DD), 1- and 2-monochlorodibenzo-p-dioxin (1-, 2-MCDD), 1,6-, 1,9-, 1,3-dichlorodibenzo-p-dioxin (1,6-, 1,9-, 1,3-DCDD), 4-monochlorodibenzofuran (4-MCDF), and 4,6-dichlorodibenzofuran (4,6-DCDF) at the reaction temperatures of 350 and 400 °C. However, at a lower reaction temperature, 250 °C, we have detected no PCDD/Fs. We have demonstrated that neat silica surfaces catalyze the generation of PCDD/Fs from chlorophenols at the upper range of the catalytic formation temperature of PCDD/F. The present finding proves the generation of PCDD/Fs on particles of fly ash, even in the absence of transition metals.

Formation of phenoxy-type Environmental Persistent Free Radicals (EPFRs) from dissociative adsorption of phenol on Cu/Fe and their partial oxides

“…In Table 4, we assemble literature reported activation barriers for the fission of phenolic O-H bond over metallic and metal oxide surfaces (Altarawneh et al, 2009b;Honkela et al, 2012;Li et al, 2015;Assaf et al, 2016;Mosallanejad et al, 2016;Pan et al, 2019). It is evident that the clean Fe and Cu surfaces and their partially oxidized configuration require higher activation energies to transform phenol into a surface-bound phenolate.…”

“…Seemingly unreacted silica surfaces also facilitate rupture of the phenolic's O-H bond (Mosallanejad et al, 2016). In our recent theoretical studies (Assaf et al, 2016), we illustrated thermo-kinetics parameters for the dissociative adsorption of phenol yielding adsorbed phenolate species over surfaces of dehydroxylated alumina and OH-alumina clusters.…”

Optimization of Binary Mixtures of Biodiesel and Fossil Diesel for Clean Energy Combustion

“…Radical species formed from thermal degradation events have generally been referred to as environmentally persistent free radicals ( EPFRs ) and represent a class of reactive species that have prolonged life time even under ambient environmental conditions [41]. These radicals may be precursors for a group of environmentally persistent volatile organic pollutants such as phenols, dioxins and benzofurans which can easily be converted to the most toxic environmental pollutants ever known-polychlorinated dioxins and polychlorinated dibenzo furans ( PCDD∕Fs ) in presence of small amounts of chlorine and a transition metals such as iron or copper and/or their corresponding oxides [41,42].…”

Mechanism and thermal rate constants for complete series reactions of bromochlorophenols with H

“…Dioxins are formed from halogenated phenols via two general mechanisms: homogeneous gas-phase mechanism and heterogeneous metal mediated mechanism. Although over 70% of all dioxins are formed through surface-mediated reactions over catalytic surfaces in the post-ame, cool zone of combustion systems at the temperature range of 200À600 C, [33][34][35][36][37] the homogeneous gas-phase mechanism also make a signicant contribution to the dioxin formations at high temperatures (>600 C). 38,39 Studies have shown that the radical-radical coupling of halogenated phenoxy radicals plays a considerable role in the homogeneous gas-phase formation of dioxins and furans.…”