Toluene and benzyl decomposition mechanisms: elementary reactions and kinetic simulations

Abstract

The high temperature decomposition kinetics of toluene and benzyl were investigated by combining a kinetic analysis with the ab initio/master equation study of new reaction channels. It was found that similarly to toluene, which decomposes to benzyl and phenyl losing atomic hydrogen and methyl, also benzyl decomposition proceeds through two channels with similar products. The first leads to the formation of fulvenallene and hydrogen and has already been investigated in detail in recent publications. In this work it is proposed that benzyl can decompose also through a second decomposition channel to form benzyne and methyl. The channel specific kinetic constants of benzyl decomposition were determined by integrating the RRKM/master equation over the C(7)H(7) potential energy surface. The energies of wells and saddle points were determined at the CCSD(T) level on B3LYP/6-31+G(d,p) structures. A kinetic mechanism was then formulated, which comprises the benzyl and toluene decomposition reactions together with a recently proposed fulvenallene decomposition mechanism, the decomposition kinetics of the fulvenallenyl radical, and some reactions describing the secondary chemistry originated by the decomposition products. The kinetic mechanism so obtained was used to simulate the production of H atoms measured in a wide pressure and temperature range using different experimental setups. The calculated and experimental data are in good agreement. Kinetic constants of the new reaction channels here examined are reported as a function of temperature at different pressures. The mechanism here proposed is not compatible with the assumption often used in literature kinetic mechanisms that benzyl decomposition can be effectively described through a lumped reaction whose products are the cyclopentadienyl radical and acetylene.

Kinetic modeling study of benzene and PAH formation in laminar methane flames

“…As mentioned, this mechanism was validated in previous investigations of laminar flames [50,53,61,64,65]. The aromatic formation submechanism was improved based on our previous investigations [53,56,66], recent theoretical calculations [43,44,51,67] and other aromatic mechanisms [54,57,[68][69][70], mainly including HACA (hydrogen-abstraction acetylene-addition) pathways [39] and the recombination reactions of resonantly stabilized radicals with small flame species [37,53,[68][69][70].…”

“…Later, they updated their PAH mechanism based on the further validations of ethane and ethylene flames [41,42]. Their main conclusion is that three PAH formation pathways show similar importance: HACA mechanism [38,39], the addition of small molecules to aromatic molecules/radicals [43,44], and the combination reactions of aromatic molecules and radicals [45,46]. According to their extensive model analysis, these reaction pathways give main contributions in PAH formation at T > 1550 K, while part of the reaction pathways proceeds in the reverse direction or achieving equilibrium when the flame temperature is lower than 1500 K.…”

“…There are no comprehensive kinetic modeling investigations on laminar methane flames, including the validations under the premixed, counter flow and coflow diffusion flame conditions. Based on the recent progress in the combustion kinetics of methane [36], and the efforts on the combustion of benzene [49][50][51][52], toluene [43,[53][54][55] and naphthalene [44,46,[56][57][58][59], a detailed kinetic model was developed for the combustion and PAH formation in laminar methane flames. This model was validated against the experimental data of laminar flames in various previous investigations, including the premixed flames [22][23][24], and the counter flow flames [60], whose operative conditions are reported in Table 1.…”

Gas‐Phase Synthesis of the Benzyl Radical (C6H5CH2)

“…During the last decade, particular focus has been directed to the role of the C 7 H 7 radicals, including benzyl (C 6 H 5 CH 2 ), o-, m-, and p-tolyl (2-, 3-, and 4-tolyl; C 6 H 4 CH 3 ), and cycloheptatrienyl (C 7 H 7 ) radicals (Scheme 1). [7][8][9] The benzyl radical (C 6 H 5 CH 2 ) has been proposed to yield indene (C 9 H 8 ) upon reaction with acetylene (C 2 H 2 ). [10,11] Indene may further produce indenyl radical(s).…”

“…Owing to the potential key role of the benzyl (C 6 H 5 CH 2 ) radical, which is both aromatic and resonantly stabilized, in the formation of PAHs carrying fivemembered rings, reaction mechanisms to distinct C 7 H 7 isomers involving the phenyl radical (C 6 H 5 ), fulvenallene (C 7 H 6 ), 1-ethynyl-cyclopentadiene (C 7 H 6 ), and the propargyl radical (C 3 H 3 ) have been explored computationally. [7,8,12,13] However, the formation of C 7 H 7 isomers (among them the thermodynamically most stable benzyl (C 6 H 5 CH 2 ) radical) via the bimolecular reaction of ubiquitous dicarbon molecules (C 2 ) with C 5 H 8 isomers such as 2-methyl-1,3-butadiene (isoprene, C 5 H 8 ; X 1 A') has never been reported. The dicarbon molecule is abundant in hydrocarbon flames and in the interstellar medium while the 2-methyl-1,3-butadiene can be formally derived from 1,3-butadiene (C 4 H 6 ) by replacing the hydrogen atom at the C2 carbon atom by a methyl group.…”