Methylbenzyl Radicals Research Articles

Towards a predictive kinetic model of 3-ethyltoluene: Evidence concerning fuel-specific intermediates in the flow reactor pyrolysis with insights into model implications

To reveal insights into high temperature kinetics of dialkylaromatics, a pyrolysis investigation of 3-ethyltoluene in a flow reactor together with its reaction kinetics are presented in this work. Concentrations and chemical structures of specific species covering temperature range from 796 to 1383K at the pressure of 30 and 760 Torr were recorded and quantified by using synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry (VUV-PI-MBMS). Important C8 and C9 fuel-specific intermediates relevant to primary decomposition of 3-ethyltoluene and isomerization of methylbenzyl and ethylbenzyl radicals were detected and identified. The kinetic model interpreting high temperature pyrolysis chemistry of 3-ethyltoluene was developed and reasonably predicted the measurements in this work. The model analyses reveal that the methyl-dissociated reaction from the ethyl group of 3-ethyltoluene is dominant in the fuel decomposition at low pressure, while the fuel is mainly consumed by hydrogen abstraction reactions at atmospheric pressure. The experimental observations of three methylbenzyl isomers, o-xylylene, p-xylylene, styrene and benzocyclobutene provide evidence for the relationships between products involving isomerization of methylbenzyl radicals, formation of xylylenes and decomposition of o-xylylene. The fuel structure effects of 3-ethyltoluene and m-xylene are revealed by comparing the pyrolysis behaviors in both cases. It has been found that the m-methylbenzyl-generating channel in the 3-ethyltoluene pyrolysis improved the reaction reactivity initially. Furthermore, the fuel with longer substituent ethyl group facilitates the formation of cycloalkenes and aromatics.

Jet-cooled spectroscopy of the m-ethylbenzyl radical in a corona-excited supersonic expansion.

The vibronic structure of m-ethylbenzyl radical has been investigated in the gas phase for the first time in a corona-excited supersonic expansion (CESE). Assignment of the m-ethylbenzyl radical has been reported by comparing the spectra from m-xylene and m-ethyltoluene. The theoretical calculation, Density functional theory, of the D0 and D1 state structures of the m-ethylbenzyl radical was computed. Conclusive assignment of vibronic emission spectrum has been analyzed with an aid of Franck-Condon simulation in the D0-D1 transition.

Visible emission spectroscopy of jet-cooled chloro-substituted 2-methylbenzyl radicals

Four isomeric chloro-substituted 2-methylbenzyl radicals were generated from the corona discharge of 3- and 4-chloro-o-xylenes seeded in a large amount of He carrier gas using a technique of corona excited supersonic expansion coupled with a pinhole-type glass nozzle. The observed visible vibronic emission spectra were analyzed to identify the radical species produced in the discharge, from which the production of the 3-, 4-, 5-, and 6-chloro-2- methylbenzyl radicals was confirmed. In addition, we determined the D1 → D0 electronic transition energies and vibrational mode frequencies in the D0 state of the isomeric chloro-substituted 2-methylbenzyl radicals.

Reaction mechanism and modeling study for the oxidation by SO2 of o‐xylene and p‐xylene in Claus process

AbstractClaus process, comprising of a furnace and a catalytic unit, is used to produce sulfur from H2S. The aromatic contaminants (benzene, toluene, and xylenes) in H2S feed form soot, and clog and deactivate the catalysts. Xylenes are known to be the most damaging ones. Therefore, there is a need to oxidize them in the furnace to enhance catalyst life. This article presents a kinetics study on the oxidation of o‐ and p‐xylene radicals by SO2 (an oxidant that is already present in the furnace) using density functional theory and a composite method. The mechanism begins with H‐abstraction from xylenes to form xylyl radicals, followed by exothermic addition of SO2 to them. The breakage of OS bond in the xylyl‐SO2 adducts leads to the loss of SO molecule, while the remaining O atom on them helps in their oxidation. The isomerization study shows that less‐stable dimethylphenyl radicals have a high tendency to isomerize to resonantly stabilized methylbenzyl radicals. However, methylbenzyl radicals have lower reactivity toward SO2 than dimethylphenyl radicals. The reaction rate constants were found using transition state theory. The reactor simulations reveal that p‐xylene has lower reactivity toward SO2 than o‐xylene, and CO, SO, and CHO are the main by‐products of oxidation.

Vibronic Assignments of Isomeric Trimethylbenzyl Radicals : Revisited

The vibronic emission spectra of isomeric trimethylbenzyl radicals were reassigned based on substituent effect on electronic transition energy as well as ab initio calculation of the benzyl radical. The electronic transition energy of three isomeric jet-cooled trimethylbenzyl radicals produced by corona discharge of 1,2,3,5tetramethylbenzene were analyzed using the empirical data of three isomeric methylbenzyl radicals. Calculated data were obtained by summing up the shifts measured from methylbenzyl radicals as well as taking the position and alignment of substituents on the benzene ring into account. The revised assignments show better agreement with observation, and rationalize the validity of the model developed to explain the substituent effect on electronic transition energy of benzyl radicals.

Spectroscopic Evidence of Jet-Cooled p-Chloro-α-Methylbenzyl Radical in Corona Excitation

We report the first spectroscopic evidence of the jet-cooled p-chloro-<TEX>${\alpha}$</TEX>-methylbenzyl radical. The visible vibronic emission spectrum was recorded from the corona discharge of precursor p-chloro-ethylbenzene seeded in a large amount of inert carrier gas helium using a pinhole-type glass nozzle coupled with a technique of corona excited supersonic expansion. From the comparison with the vibronic spectrum of the p-chlorobenzyl radical, we identified the evidence of formation of the jet-cooled p-chloro-<TEX>${\alpha}$</TEX>-methylbenzyl radical in the corona discharge of precursor p-chloro-ethylbenzene.

Decomposition of Methylbenzyl Radicals in the Pyrolysis and Oxidation of Xylenes

Alkyl benzyl radicals are important initial products in thermal and combustion reactions of substituted aromatic fuels. The decomposition reactions of the three isomeric methylbenzyl radicals, formed as primary products in xylene combustion, are studied theoretically and are shown to be significantly more complex than previously reported. Thermochemical properties are calculated using the G3X and G3SX model chemistries, with isodesmic and atomization work reactions. G3X atomization calculations reproduce heats of formation for the 14 reference species in the work reactions to a mean unsigned error of 0.23 kcal mol(-1), and maximum error of 0.70 kcal mol(-1), slightly outperforming the G3SX method. For the target molecules the isodesmic and atomization heats of formation agree to within 0.20 kcal mol(-1), on average. We posit that this study approaches the crossover point at which atomization calculations offer improved accuracy over isodesmic ones, for these closed-shell hydrocarbons. Our results suggest that m-xylylene is not the decomposition product of m-methylbenzyl, as was previously reported. Instead, the m-methylbenzyl radical decomposes to p-xylylene (and perhaps some of the less stable o-xylylene) via a ring-contraction/methylene-migration (RCMM) mechanism, with activation energy of around 70 kcal mol(-1). At higher temperatures m-methylbenzyl is predicted to also decompose to 2- and 3-methylfulvenallene + H, with activation energy of around 84 kcal mol(-1). The o-methylbenzyl radical is shown to primarily decompose to o-xylylene + H with bond dissociation energy of 67.3 kcal mol(-1), with fulvenallene + CH3 proposed as a minor product set. Finally, the p-methylbenzyl radical decomposes solely to p-xylylene + H with bond dissociation energy 61.5 kcal mol(-1). Rate expressions are estimated for all reported reactions, based on thermochemical kinetic considerations, with further modeling along with detailed experiments needed to better refine rate constants and branching ratios for methylbenzyl radical decomposition. These calculated reaction mechanisms and rate constants for methylbenzyl radical decomposition are consistent with the experimental data. Our results help explain the ignition behavior of the xylenes, and should lead to improved kinetic models for combustion of these and other alkylated aromatic hydrocarbons.

Reassignment of Isomeric Dimethylbenzyl Radicals Generated by Corona Discharge of 1,2,4-Trimethylbenzene

Isomeric dimethylbenzyl radicals were identified by analysis of the vibronic emission spectrum recorded from the corona excited supersonic expansion through a pinhole-type glass nozzle of precursor 1,2,4-trimethylbenzene seeded in a large amount of carrier gas helium. The visible spectrum showed several vibronic bands reflecting the characteristics of the band shape of each isomer generated. An analysis based on monosubstituted methylbenzyl radicals confirmed that all three of the dimethylbenzyl isomers were formed in the corona discharge of precursor as a result of the removal of a hydrogen atom from the methyl group at the different substitution position. For the three isomeric products, the D 1 → D 0 electronic transition and the vibrational mode frequencies in the ground electronic state were determined by comparing with those from both an ab initio calculation and the known data of the precursor.

The pyrolysis of 2-, 3-, and 4-methylbenzyl radicals behind shock waves

The thermal decomposition of 2-, 3-, and 4-methylbenzyl radicals was studied behind reflected shock waves. α -Bromo- ortho ( meta - and para )-xylenes were used as precursors for a series of experiments with temperatures ranging from 1150 to 1600 K and pressures between 1.5 and 4 bar. Mixtures of 1–5.5 ppm of the precursor diluted in argon were used for the investigations. Initiated by the fast thermal dissociation of the precursor CH 3 C 6 H 4 CH 2 Br→CH 3 C 6 H 4 CH 2 +Br (R3) the methylbenzyl radicals subsequently decomposed. 2-methylbenzyl→ o -CH 2 C 6 H 4 CH 2 +H (R2 o ) 3-methylbenzyl→ m -CH 2 C 6 H 4 CH 2 +H (R2 m ) 4-methylbenzyl→ p -CH 2 C 6 H 4 CH 2 +H (R2 P ) We followed the rate of H-atom formation by using atom resonance absorption spectroscopy (H-ARAS) at 121.6 nm. We report the first directly measured rate coefficients for the decomposition of methylbenzyl radicals: k 2o =5×10 15 exp {−(310±4)kJ mol −1 / RT } s −1 , k 2m =5×10 15 exp{−(340±4) kJ mol −1 / RT } s −1 , and k 2p =5×10 15 exp{−(295±4) kJ mol −1 / RT } s −1 These rate coefficients are about a factor of 4 below the high-pressure limit, and the accuracy is estimated to be 30%. The complete H-atom concentration-time profiles were successfully modeled using a simple mechanism.

Mechanisms for the formation of exhaust hydrocarbons in a single cylinder spark-ignition engine, fueled with deuterium-labeled ortho-, meta- and para-xylene

Combustion studies in engines have investigated the chemistry leading to the formation in the exhaust of aromatic hydrocarbons from deuterium-labeled isomeric xylenes. These fuels were: ortho-xylene-d 0 and ortho-xylene-d 10 (1:1); para-xylene-d 0 and para-xylene-d 10 (1:1); and meta-xylene-2,4,5,6-d 4. Isotopic distributions within the exhausted hydrocarbons establish the postflame chemistry involved. There is an isotope effect in the consumption of residual fuel in the postflame region. The residual fuel from each experiment exhibits minimal H-D exchange. Toluene is an intermediate in the formation of ethylbenzene, and is produced through X • atom (X • = H or D) displacement of methyl radicals from the xylene fuel. Benzene is formed by direct demethylation, but there are other routes. Styrene from o- and p-xylene fuels is formed intramolecularly, probably involving xylylene and methylcycloheptatetraene intermediates. Ethyltoluene is formed by combination of methyl and methylbenzyl radicals.

Polymerization of dec-1-ene—II. Product analysis shows that the free radical process is an oligomerization, and reveals mechanistic details

The products from the free radical polymerization of dec-1-ene have been analyzed and characterized in an effort to specify the details of what has previously (part I) been shown to be essentially a degradative chain transfer mechanism. Vapour phase osmometry and gel permeation chromatography show that the products are oligometric, and the latter technique shows an abnormally high dimer component in the distribution, a finding consistent with the mechanism proposed. NMR analysis shows the absence of groups corresponding to initiator fragments in the oligomers and, on this basis, it is proposed that initiation occurs by transfer to monomer to yield non-allylic radicals. Gas chromatographic analysis coupled with mass spectrometry has revealed the presence of four distinct “dimer” types, each in several isomeric forms: 1. (1) dimer monoenes resulting from the propagation reaction; 2. (2) dimer dienes resulting from the recombination of allylic radicals; 3. (3) methyl-benzyl radical recombination products; and 4. (4) combination products resulting from interaction of a methyl-benzyl radical and an allylic radical. Detailed mechanisms for total oligomeric product formation are proposed on the basis of these results.

Reactions of the radical cations of methylated benzene derivatives in aqueous solution

The radical cations of methylated benzene decompose in acid solution into the corresponding methylbenzyl radical and a proton. The rate constant for this reaction decreases by three orders of magnitude as the number of methyl groups increases from one to five. The rate constants can be correlated with the ionization potential of the parent compound. In neutral solution the reverse reaction to the acid-catalyzed OH adduct conversion occurs and the radical cations react with water to form the OH adduct. In slightly alkaline solution the radical cations of the higher methylated benzenes (n greater than or equal to 3) react with hydroxide ions forming the OH adduct.

Hydrogen Transfer by Thiols

THE thermal decomposition of α,α′-azoethylbenzene in inert solvents at 125° C. leads, apart from a quantitative production of nitrogen, to methylbenzyl radicals which combine to meso and racemic 2,3-diphenylbutane. Disproportionate of the radicals does not occur since ethylbenzene could not be detected (1).