Formation of dibenzo-p-dioxin and dibenzofuran from catechol in relevance to combustion of biomass
Pyrolysis of biomass constitutes a critical front in the pursuit of utilising renewable energy at a wider scale. The complex chemical and physical processes occurring during thermal treatment of biomass are in the centre of a mounting research interest. Great deal of this research aims to understand emission of pollutants from thermal utilisation of biomass.1 The intricate chemical composition of biomass constituents makes it a daunting task to assign formation of organic pollutants to a specific chemical reaction. To overcome complexity pertinent to structure of biomass, model compounds or “surrogates” are often used to mimic characteristics of the real fuel, biomass. The most discussed compounds in this context are dihydroxylated benzenes, catechol (o-dihydroxybenzene and hydroquinone (p-dihydroxybenzene). Catechol (CT hereafter) is a major product from the combustion of any type of biomass.2 Decomposition of CT was found to form various species, including phenol, benzene, naphthalene, and polycylic aromatic hydrocarbons.3 Dellinger’s group has thoroughly investigated thermal decomposition of CT.4-5 In addition to previously identified products, they found that, thermal degradation of CT leads to the formation of dibenzo-p-dioxin (DD) and dibenzofuran (DF). Using electro paramagnetic resonance (EPR), Khachatryan et al.5 detected the formation of o-semiquinone (o-SQ) as the most prominent initial intermediate from decomposition of the CT molecule via fission of one of its phenolic O-H bonds. Along the same line of enquiry, we have shown in a theoretical study, that main exit channel of the o-SQ radical is through CO/ring-contraction mechanism leading to the formation of cyc-C5H4OH radical.6 Based on our reported reaction constant for the latter channel, a life time of the o-SQ radical falls around 2200 s. The relatively long lifetime of the o-SQ supports its categorisation as an environment persistent free radical (EPFR).5 Thus, the o-SQ radical could serve as a building block for the formation of DD and DF. Truong et al. suggested a pathway for the evolution of DD and DF from pyrolysis of the CT molecule based on the well-established analogous mechanism of the chlorophenol systems.7,8 However, this proposed mechanism does not take into account a plausible role of the OH group in facilitating the occurrence of prominent reaction steps; most notably ring-closure reaction and initial coupling modes. In this contribution, we demonstrate, for the first time, a comprehensive mechanism for the formation of DD and DF (and their hydrolxylated derivatives) from catechol. Results presented herein should provide an insight into emission of DD and DF from thermal processing of biomass.
|Publication Type:||Journal Article|
|Murdoch Affiliation:||School of Engineering and Information Technology|
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