Role of singlet oxygen in combustion initiation of aromatic fuels

Application of singlet oxygen in oxy-fuel systems reduces the activation energy of the initiation reactions, accelerating the chain-branching mechanism and decreasing the overall ignition temperature. However, the underlining reaction mechanism of the surface-generated singlet oxygen O2 1Δg that reacts with fuel surrogates (i.e., toluene) in the gas media remains poorly explored. Herein, comprehensive mechanistic and thermo-kinetic accounts underpinning the reaction of the simplest alkylbenzene, namely, toluene, with singlet oxygen in the gas phase are reported. In analogy to reaction of singlet oxygen with benzene, the titled reaction branches into several opening channels. The 1,4 cycloaddition and ene type reactions of toluene with singlet oxygen affords p-quinonemethide (4-methylenecyclohexa-2,5-dienone) and o-quinonemethide (6-methylenecyclohexa-2,4-dienone), respectively (i.e., very reactive intermediates). The initiation of the para channel follows a concerted mechanism through an enthalpic barrier of 34.5 kJ mol–1 with a fitted reaction rate coefficient of k(T) = 1.51 × 10–15 exp(−34 500/(RT)) cm3 molecule–1 s–1. A corresponding value for the formation of o-quinonemethide amounts to 47.6 kJ mol–1 and k(T) = 8.31 × 10–14 exp(−42 600/RT) cm3 molecule–1 s–1. Moreover, the relative reactivity of singlet oxygen, based on the reaction rate constants, follows the order of OH > H > CH3 > 1O2 > HO2 > 3O2. These indicate that the presence of singlet oxygen considerably lowers the activation energy of the initiation channels, resulting in an energetically improved combustion process. In addition, the result illustrates that the reported meta route (2+2 cycloaddition) in the catalytic reaction of toluene with metal oxides occurs when the metal oxide promotes triplet to singlet oxygen and positions the adsorbed molecule of 1O2 parallel to one of the sides of the aromatic ring of the benzene molecule