Mechanisms of product formation from the pyrolytic thermal degradation of catechol Slawomir Lomnicki, Hieu Truong, Barry Dellinger * Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA article info Article history: Received 28 September 2007 Received in revised form 14 March 2008 Accepted 17 March 2008 Available online 21 July 2008 Keywords: Combustion by-products Persistent free radical (PFR) Semiquinone Cyclopentadiene Phenoxyl radical abstract Catechol has been identified as one of the most abundant organic products in tobacco smoke and a major molecular precursor for semiquinone type radicals in the combustion of biomass material. The high-tem- perature gas-phase pyrolysis of catechol under hydrogen-rich and hydrogen-lean conditions was studied using a fused-silica tubular flow reactor coupled to an in-line GC/MS analytical system. Thermal degra- dation of catechol over temperature range of 250–1000 °C with a reaction time of 2.0 s yielded a variety products including phenol, benzene, dibenzofuran, dibenzo-p-dioxin, phenylethyne, styrene, indene, anthracene, naphthalene, and biphenylene. Ortho-benzoquinone which is typically associated with the presence of semiquinone radicals was not observed and is proposed to be the result of fast decomposition reactions that lead to a variety of other reaction products. This is in contrast to the decomposition of hydroquinone that produced para-benzoquinone as the major product. A detailed mechanism of the degradation pathway of catechol is proposed. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Catechol (CT) has been identified as one of the most abundant organic chemical in tobacco smoke and a product of combustion of any type of biomass (Kallianos et al., 1968; Schlotzhauer et al., 1982; Carmella et al., 1984; Dellinger et al., 2001). Catecholic groups are major components of the molecular structure of lignin, which comprises the woody component of biomass materials (Hayashi and Namura, 1966; Troughton et al., 1972; Dorrestijn et al., 2000; Hays et al., 2005). Exposure to CT has been reported to induce high-blood pressure and upper respiratory tract irritation as well as kidney damage and convulsions in high-doses (Vasil’ev and Matlina, 1972; Flickinger, 1976; Sax and Lewis, 1988; Van Duursen et al., 2004). Studies of cigarette smoke suggest that CT contributes to lung cancer and DNA damage through the formation of biologically active semiquinone radicals (Pryor et al., 1983a,b; Borish et al., 1985; Leanderson and Tagesson, 1990, 1992; Bermu- dez et al., 1994; Li and Trush, 1994; Pryor, 1994; Stone et al., 1995; Dellinger et al., 2000; Dellinger et al., 2001; Squadrito et al., 2001). In biological systems, radicals can attack proteins and DNA that lead to their destruction and diminished capacity to carry out nor- mal biochemical cell processes. Thermal degradation of CT may also lead to formation of other by-products including dibenzo-p-dioxins with significant health and environmental impacts. Recently, Wornat et al., reported a study of the pyrolysis of CT using GC-FID and HPLC for product analysis (Wornat et al., 2001; Ledesma et al., 2002, 2003; Marsh et al., 2004); however, the focus of their research was the forma- tion of polycyclic aromatic hydrocarbons. We present the results of our research on the thermal degrada- tion of CT at a gas-phase reaction time of 2.0 s over a temperature range of 250–1000 °C using a high-temperature flow reactor equipped with an in-line GC–MS analytical system. Based on the results, we propose a mechanism for decomposition of CT and for- mation of organic by-products. 2. Methods and materials The pyrolysis of CT was studied by using a high-temperature, flow reactor analytical system referred to in the archival literature as the System for Thermal Diagnostic Studies (STDS) (Rubey and Grant, 1988; Striebich and Rubey, 1990; Striebich et al., 1991). The STDS consists of a high-temperature fused-silica flow reactor that is 35-cm long with a 1 cm inside diameter in a helical config- uration. This flow reactor is contained within a high-temperature furnace with a maximum operating temperature of 1200 °C. The furnace is housed inside a GC oven (Varian, CP 3800) that is con- trolled at a constant temperature of 200 °C to facilitate transport of gas-phase reactants and products. The reactor effluent is trans- ported through a heated temperature-controlled transfer line (deactivated silica-lined stainless steel tube) to the head of the cap- illary column of a GC–MS System where it is cryogenically trapped 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.03.064 * Corresponding author. Tel.: +1 225 578 6759; fax: +1 225 578 0276. E-mail addresses: barryd@lsu.edu, barry.dellinger@chemserv.chem.lsu.edu (B. Dellinger). Chemosphere 73 (2008) 629–633 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere