Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K

Abstract

Phenols are demonstrated precursors of “dioxins” ‐ polychlorinated dibenzo‐p‐dioxins (DDs) and dibenzofurans (DFs) ‐ in thermal processes, especially incineration. Heterogeneous catalysis, depending on conditions, can play an important role, but mere gas‐phase combination of phenolic entities to ultimately DD and/or DF is always possible. The present paper addresses the fundamental role of phenol itself. Phenol has long been known to give DF upon pyrolysis and in similar thermal reactions. In the liquid phase under oxidative conditions it yields five condensation products (A‐E); this clearly occurs through the dimerization of two phenoxy (PhO) radicals, followed by enolisation/rearomatisation. Our study shows that in the gas phase, at the lower T end, such dimers are also formed, but still with very little DF. That DF, indeed, is almost the only condensation product at elevated temperatures is substantiated by thermochemical‐kinetic analysis (favouring the pathway of ortho‐C/ortho‐C combination of two PhO radicals), as well as by results obtained with two plausible intermediates, viz. 2,2′‐dihydroxybiphenyl (A) and 2‐phenoxyphenol (C). Mechanisms for the requisite enolisation and dehydration steps leading to DF are discussed.

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds. Cinnamyl alcohol

“…The persistent radicals at 800 °C most probably are O-centered which are more resistant to self-recombination than carbon centered radicals [36–38] and hence stable to annihilation (Fig. 2d).…”

Phenols from pyrolysis and co-pyrolysis of tobacco biomass components

“…For instance, phenol affects liver enzymes, lungs, kidneys, and the cardiovascular system, while m-cresol attacks the nervous systems (Talhout et al, 2011). Phenoxy radicals produced from further degradation of phenol (Khachatryan et al, 2008), on the other hand can combine to form dibenzo-p-dioxin/dibenzofuran (DD/F) which is one of the most toxic environmental pollutants (Berho and Lesclaux, 1997;Wiater et al, 2000;Khachatryan et al, 2003;Evans and Dellinger, 2005). The p-hydroxyl phenoxy radical known in literature as neutral semi-quinone radical has been reported in cigarette smoke and shown to be highly active in generation of reactive oxygen species (ROS) in biological systems (Pryor et al, 1998;Dellinger et al, 2000).…”

Ab initio study of the formation and degradation reactions of chlorinated phenols

“…Phenoxyl radicals are known to be resistant to oxidation [3,4] and chlorinated phenoxyl radicals may even be less reactive. In fact, chlorinated phenoxyl radicals are thought by some to be so stable that they only react with other radicals or themselves, the self-reaction resulting in the formation of PCDD/Fs [5–10]. In addition, it has recently been noticed that substituted phenoxyl radicals were environmentally persistent when associated with copper oxide-containing particles [11].…”