(38g) Thermal Chemistry of Alkylaromatics Reconsidered

Many hydrocarbon resources (coal, crude oil, lignin) and refinery streams contain a large number of alkyl aromatic linkages. The breaking of these linkages during pyrolysis or hydrothermal treatment is a key step in converting heavy molecules into lighter, often more valuable, transportation fuels or chemicals. Therefore the chemistry of alkylaromatic model compounds have been extensively studied, to better understand this reactivity. In literature models, much of this reactivity has been attributed to retroene reactions, a type of concerted elementary step reaction, but quantum chemistry calculations indicate that the hypothesized reactions have very high barriers and so much lower rates than previously thought. After this type of reaction is removed from a published model for hexylbenzene, the kinetic model predictions are found to be very sensitive to the thermodynamic parameters of a few key molecules and radicals. Here we present a new model where the thermochemistry of the alkylbenzenes, alkylbenzene radicals, aliphatic radicals, and styrene were calculated using the CBS-QB3 quantum chemistry method. The rate coefficient of a key beta scission reaction was also calculated. The results of these calculations have led to an overall improvement in the predictive accuracy of the kinetic model for the pyrolysis of hexylbenzene. The conversion of hexylbenzene and selectivity of major products such as toluene, ethylbenzene, and styrene can be predicted within a factor of four without any prior knowledge from experimental data.