Exploring the chemical kinetics on oxygen addition reactions of o-xylyl radical at the low temperature

Abstract o-Xylene oxidation displays an autoignition behavior similar to alkanes at low temperatures. This paper presents a detailed investigation of the chemical kinetics of oxygen additions with the o-xylyl radical that control the ignition reactivity of o-xylene at low temperatures. High-level electronic structure calculations, transition state theory, and master equation simulations are combined to predict the rate coefficients of main elementary reactions. For the initially formed o-methylbenzylperoxy (ROO) in the first oxygen addition o-xylyl + O2, its isomerization to o-hydroperoxymethyl-benzyl (QOOH) proceeds with a much smaller branching ratio compared to the counterpart ROO → QOOH in alkanes. Despite the slow formation of QOOH, the absence of fast dissociation pathways enable QOOH concentration to build up. QOOH that has the unpaired electron located on a side-chain carbon can readily react with a second molecular oxygen. The QOOH + O2 reaction then efficiently leads to the growth of the radical pool through a highly chain-branching reaction sequence QOOH + O2 → 2-hydroperoxymethyl-benzaldehyde + OH → 1,2-diformylbenzene + 2OH + H. The predicted oxygen addition kinetics offers a good explanation for the alkane-like autoignition behavior of o-xylene. Meanwhile, as the key intermediate to chain branching, the lower yield of QOOH results in its lower ignition reactivity. The present study shows that classical low temperature scheme is also valid for benzylic-type hydrogens and radicals of o-xylene where the transferred hydrogen from the ortho-methyl chain facilitates the isomerization ROO ↔ QOOH. It is also easy to deduce that for m- and p-xylenes, where no such isomerization step is available, little reactivity should be expected at the low temperature.