Kinetics and thermochemistry of cyclohexadienes reactions with hydroxyl radicals

Abstract

Cyclohexadienes (CHDs) are important intermediates in the pyrolysis and oxidation of many C 6 cyclic hydrocarbons. Under combustion relevant conditions, these dienes may either eliminate H 2 directly or lose H atoms sequentially to produce the first aromatic ring, i.e., benzene. This is one of the major reaction pathways for the formation of benzene from C 6 cyclic hydrocarbons. Despite its importance in PAH and soot forma- tion process, the chemistry of CHD has attracted less attention from the scientific community. In this work, we have investigated hydrogen abstraction reactions of 1,3- and 1,4-CHD by OH radicals using a shock tube and UV laser absorption over the temperature range of 900 – 1100 K and pressures of ∼1 –3 atm. Reaction kinetics was followed by monitoring OH radicals near 307 nm. At high temperatures, both isomers of CHDs are found to exhibit comparable reactivity with OH radicals. These reactions show a weak positive temper- ature dependence. Measured rate coefficients can be represented by the following Arrhenius expressions in units of cm 3 molecule −1 s −1 : k 1 ( 1 , 3 −CHD + OH ) = 1 . 76 ×10 −10 exp (−1761 K T )k 2 ( 1 , 4 −CHD + OH ) = 2 . 23 ×10 −10 exp (−1813 K T )To gain further insights into these reactions, various ab initio methods were employed to compute relevant thermochemical data. This study reports the first high-temperature kinetic data for cyclohexadienes which will be beneficial to improve the performance of the oxidation kinetic models of C6 cyclic hydrocarbons. © 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.