High Pressure Pyrolysis of Toluene. 2. Modeling Benzyl Decomposition and Formation of Soot Precursors

Laser-based sensing in the long-wavelength mid-infrared: chemical kinetics and environmental monitoring applications

We present chemical kinetics and environmental monitoring applications in the long-wavelength mid-infrared (LW-MIR) region using a new diagnostic that exploits a widely tunable light source emitting in the LW-MIR. The custom-designed laser source is based on a difference-frequency generation (DFG) process in a nonlinear orientation-patterned GaAs crystal. The pump laser, an external-cavity quantum cascade laser, is tuned in a continuous-wave (cw) mode, while the signal laser, a C O ₂ gas laser, is operated in a pulsed mode with a kilohertz repetition rate. The idler wavelength can be tuned between 11.58 (863.56c m ^-1) and 15.00 µm (666.67c m ^-1) in a quasi-cw manner. We discuss the unique prospective applications offered by probing the LW-MIR region for chemical kinetics and environment-monitoring applications. We showcase the potential of the DFG laser source by some representative applications.

Formation of polycyclic aromatic hydrocarbons, benzofuran, and dibenzofuran in fuel-rich oxidation of toluene using a flow reactor

Recently, polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) have been attracting considerable attention owing to their high toxicity. Understanding their formation mechanism during combustion processes is important to control their emission. However, there are few studies that have quantitatively investigated OPAH formation in the fuel-rich oxidation of hydrocarbons, despite the availability of several studies on PAH formation. In this study, benzofuran and dibenzofuran as OPAHs were quantified in the fuel-rich oxidation of toluene using a flow reactor at atmospheric pressure in a temperature range of 1050-1350 K at equivalence ratios from 3.0 to 12.0 and residence times from 0.2 to 1.5 s. In addition to benzofuran and dibenzofuran, 4 types of monocyclic aromatic hydrocarbons and 19 types of PAHs were also evaluated. The experimental data obtained in this study were compared with those of the ethylene oxidation performed in our previous study. The existing kinetic model for PAH growth was modified based on several theoretical studies to predict the behavior of OPAHs with furan structures. The modified model showed significant improvements in the prediction of benzofuran and dibenzofuran formation. Based on the rate of production and sensitivity analysis using the modified model, the dominant reaction pathways of benzofuran and dibenzofuran were investigated.

Thermal Decomposition of Benzyl Radicals: Kinetics and Spectroscopy in a Shock Tube

Understanding the mechanism of high-temperature reactions of aromatic hydrocarbons and radicals is essential for the modeling of hydrocarbon growth processes in combustion environments. In this study, the thermal decomposition reaction of benzyl radicals was investigated using time-resolved broadband cavity-enhanced absorption spectroscopy behind reflected shock waves at a postshock pressure of 100 kPa and temperatures of 1530, 1630, and 1740 K. The transient absorption spectra during the decomposition were recorded over the spectral range of 282-410 nm. The spectra were contributed by the absorption of benzyl radicals and some transient and residual absorbing species. The temporal behavior of the absorption was analyzed using a kinetic model to determine the rate constant for benzyl decomposition. The obtained rate constants can be represented by the Arrhenius expression k₁ = 1.1 × 10¹² exp(-30 500 K/T) s^-1 with an estimated logarithmic uncertainty of Δlog₁₀ k = ±0.2. Kinetic simulation of the secondary reactions indicated that fulvenallenyl radicals are potentially responsible for the transient absorption that appeared around 400 nm. This assignment is consistent with the available spectroscopic information of this radical. Possible candidates for the residual absorbing species are presented, suggesting the potential importance of ortho-benzyne as a reactive intermediate.

Gas phase formation of cyclopentanaphthalene (benzindene) isomers via reactions of 5- and 6-indenyl radicals with vinylacetylene

The reaction of indenyl radicals with vinylacetylene leads to cyclopentanaphthalene at low temperature.

How to add a five-membered ring to polycyclic aromatic hydrocarbons (PAHs) – molecular mass growth of the 2-naphthyl radical (C10H7) to benzindenes (C13H10) as a case study

The reaction of aryl radicals with allene/methylacetylene leads to five-membered ring addition in PAH growth processes.

Reactions of o-benzyne with propargyl and benzyl radicals: potential sources of polycyclic aromatic hydrocarbons in combustion

The kinetics and mechanisms of the reactions of o-benzyne with propargyl and benzyl radicals have been investigated computationally. The possible reaction pathways have been explored by quantum chemical calculations at the M06-2X/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level and the mechanisms have been investigated by the Rice-Ramsperger-Kassel-Marcus theory/master-equation calculations. It was found that the o-benzyne associates with the propargyl and benzyl radicals without pronounced barriers and the activated adducts easily isomerize to five-membered ring species. Indenyl radical and fluorene + H were predicted to be dominantly produced by the reactions of o-benzyne with propargyl and benzyl radicals, respectively, with the rate constants close to the high-pressure limits at temperatures below 2000 K. The related reactions on the two potential energy surfaces, namely, the reaction between fulvenallenyl radical and acetylene and the decomposition reactions of indenyl and α-phenylbenzyl radicals were also investigated. The high reactivity of o-benzyne toward the resonance stabilized radicals suggested a potential role of o-benzyne as a precursor of polycyclic aromatic hydrocarbons in combustion.