We have investigated the isomeric C6H6 product distributions of the self-reaction of propargyl (C3H3) radicals at two nominal pressures of 25 and 50 bar over the temperature range 720-1350 K. Experiments were performed using propargyl iodide as the radical precursor in a high-pressure single-pulse shock tube with a residence time of 1.6-2.0 ms. The relative yields of the C6H6 products are strongly temperature dependent, and the main products are 1,5-hexadiyne (15HD), 1,2-hexadiene-5-yne (12HD5Y), 3,4-dimethylenecyclobutene (34DMCB), 2-ethynyl-1,3-butadiene (2E13BD), fulvene, and benzene, with the minor products being cis- and trans-1,3-hexadiene-5-yne (13HD5Y). 1,2,4,5-Hexatetraene (1245HT) was observed below 750 K but the concentrations were too low to be quantified. The experimentally determined entry branching ratios are: 44% 15HD, 38% 12HD5Y, and 18% 1245HT, which is efficiently converted to 34DMCB. Following the initial recombination step, various C6H6 isomers are formed by thermal rearrangement. The experimentally observed concentrations for the C6H6 species are in good agreement with earlier experiments on 15HD thermal rearrangement.
The pyrolysis of 1,5-hexadiyne has been studied in a high-pressure single pulse shock tube to investigate the mechanisms involved in the production of benzene from propargyl radicals. Analysis of the reaction products by gas chromatography and matrix isolation Fourier transform infrared spectroscopy has positively identified six linear C 6 H 6 species and two cyclic C 6 H 6 species. Of these species cis-1,3-hexadien-5-yne and trans-1,3hexadiene-5-yne have been unambiguously identified for the first time and provide vital information concerning a low-temperature route to benzene that does not involve the formation of fulvene; however, the data also provide support for two high-temperature paths from propargyl radicals to benzene via fulvene. Thus experimental evidence has been gained that supports two different routes to benzene formation. The mechanisms and rate coefficients that have been obtained in this work are discussed.
An iterative physically bounded Gauss-Newton (PGN) method has been formulated to estimate unknown kinetic parameters from experimental measurements. A physically bounded approach is adopted to reduce the size of the search space and ensure search within physically meaningful ranges of kinetic rates. First-order sensitivity information of state variables, with respect to unknown rate parameters, is computed simultaneously with the integration of the governing ordinary differential equations (ODEs). Optimal kinetic parameters are obtained by iteratively solving the Gauss-Newton update equations within the physical parameter range. Four different reaction systems, including both simple and complex networks, have been utilized for validating the performance of the proposed method. Comparisons with other state-of-the-art algorithms showed its efficiency, robustness, and accuracy. The methodology was also applied to analyze ethane oxidation based on the widely cited GRI-Mech 3.0 mechanism to demonstrate the algorithm's performance in large-scale practical applications. The model predictions have been greatly improved by determining the optimal pre-exponential factors for five unimolecular reactions from experimental data obtained at elevated pressures.
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