Chemical pathways for the formation of benzofuran and dibenzofuran in combustion

Understanding and predicting the formation of polycyclic aromatic compounds (PACs) and their role in the formation of high molecular mass compounds is still an unresolved topic in combustion. PACs characteristics, such as chemical composition, size, and presence of side chains, play an important role not only in terms of environmental and health impact, but also when developing models that describe the formation of nanoparticles and soot. In this paper, we report on a detailed analysis of the reaction pathways describing the chemistry of furan-embedded PACs using ab initio G3-type electronic structure calculations leading to the formation of benzofuran and dibenzofuran from benzene and biphenyl. The 82 elementary reactions, identified in this work, contain unexplored pathways involving triplet oxygen atom and hydroxyl radical addition reactions. A protocol for improving the calculations of reaction energetics from ab initio compound methods is proposed, which consists of the thorough usage of IRCmax scheme to identify the transition state structure and an energy correction (similar to 0.2 kcal/mol) to the empirical term in G3 formula for systems with open-shell singlet type of electronic configurations. Based on these ab initio calculations, temperature dependent reaction rate constants are calculated according to statistical theories, together with thermodynamics data. Branching ratio analysis based on steady-state approximation is carried out to illustrate the relative importance of the new pathways in an ethylene premixed flame. Results show that the newly discovered benzofuran formation pathways can play a relative important role when in presence of phenol or phenoxyl radicals at various locations in the flame. (C) 2019 Published by Elsevier Inc. on behalf of The Combustion Institute.