Abstract
The phenyl + ethylene (C6H5 + C2H4) reaction network was explored experimentally and theoretically to understand the temperature dependence of the reaction kinetics and product distribution under various temperature and pressure conditions. The flash photolysis apparatus combining laser absorbance spectroscopy (LAS) and time-resolved molecular beam mass spectrometry (MBMS) was used to study reactions on the C8H9 potential energy surface (PES). In LAS experiments, 505.3 nm laser light selectively probed C6H5 decay, and we measured the total C6H5 consumption rate coefficients in the intermediate temperature region (400–800 K), which connects previous experiments performed in high-temperature (pyrolysis) and low-temperature (cavity-ring-down methods) regions. From the quantum chemistry calculations by Tokmakov and Lin using the G2M(RCC5)//B3LYP method, we constructed a kinetic model and estimated phenomenological pressure-dependent rate coefficients, k(T, P), with the Arkane package in the reaction mechanism generator. The MBMS experiments, performed at 600–800 K and 10–50 Torr, revealed three major product peaks: m/z = 105 (adducts, mostly 2-phenylethyl radical, but also 1-phenylethyl radical, ortho-ethyl phenyl radical, and a spiro-fused ring radical), 104 (styrene, co-product with a H atom), and 78 (benzene, co-product with C2H3 radical). Product branching ratios were predicted by the model and validated by experiments for the first time. At 600 K and 10 Torr, the yield ratio of the H-abstraction reaction (forming benzene + C2H3) is measured to be 1.1% and the H-loss channel (styrene + H) has a 2.5% yield ratio. The model predicts 1.0% for H-abstraction and 2.3% for H-loss, which is within the experimental error bars. The branching ratio and formation of styrene increase at high temperature due to the favored formally direct channel (1.0% at 600 K and 10 Torr, 5.8% at 800 K and 10 Torr in the model prediction) and the faster β-scission reactions of C8H9 isomers. The importance of pressure dependence in kinetics is verified by the increase in the yield of the stabilized adduct from radical addition from 80.2% (800 K, 10 Torr) to 88.9% (800 K, 50 Torr), at the expense of styrene + H. The pressure-dependent model developed in this work is well validated by the LAS and MBMS measurements and gives a complete picture of the C6H5 + C2H4 reaction.
Highlights
In past decades, the formation of polycyclic aromatic hydrocarbons (PAHs) has attracted attention in astrochemical[1−4] and combustion[5−8] research
In efforts to minimize the yield of PAHs, numerous potential energy surfaces (PESs) involving PAH initiation and propagation have been calculated and applied to mechanisms used in industrial chemical processes, and species like styrene are important in PAH formation.[24−27] To validate the theoretical works, kinetics measurements and product quantification under different conditions have been designed and performed
The radical addition product, 2-phenylethyl radical, has been used as a carbon-centered radical interacting with DNA to study the metabolic activation of hydrazine derivatives.[28−31] Another radical adduct formed through 2phenylethyl radical isomerization is 1-phenylethyl radical, and it has been selected as a model species to react with various monomers to understand their reactivity toward the polystyrene radical.[32−35] In this reaction network, styrene can be generated through the well-skipping reaction from C6H5 + C2H4 and H-elimination from radical adducts
Summary
The formation of polycyclic aromatic hydrocarbons (PAHs) has attracted attention in astrochemical[1−4] and combustion[5−8] research. This chemistry is important in processes for making coke, carbon black, and carbon nanotubes and in a variety of deposit formation mechanisms.[9−12] The simplest aromatic radical, phenyl radical (C6H5), reacts with unsaturated hydrocarbons and forms a second ring through radical cyclization. The radical addition product, 2-phenylethyl radical, has been used as a carbon-centered radical interacting with DNA to study the metabolic activation of hydrazine derivatives.[28−31] Another radical adduct formed through 2phenylethyl radical isomerization is 1-phenylethyl radical, and it has been selected as a model species to react with various monomers to understand their reactivity toward the polystyrene radical.[32−35] In this reaction network, styrene can be generated through the well-skipping reaction from C6H5 + C2H4 and H-elimination from radical adducts
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