Detailed Modeling of Low-Temperature Propane Oxidation: 1. The Role of the Propyl + O₂ Reaction

Abstract

Accurate description of reactions between propyl radicals and molecular oxygen is an essential prerequisite for modeling of low-temperature propane oxidation because their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The CBS-QB3 level of theory was used to construct potential energy surfaces for n-C(3)H(7) + O(2) and i-C(3)H(7) + O(2). High-pressure rate constants were calculated using transition state theory with corrections for tunneling and hindered rotations. These results were used to derive pressure- and temperature-dependent rate constants for the various channels of these reactions under the framework of the Quantum Rice-Ramsperger-Kassel (QRRK) and the modified strong collision (MSC) theories. This procedure resulted in a thermodynamically consistent C(3)H(7) + O(2) submechanism, which was either used directly or as part of a larger extended detailed kinetic mechanism to predict the loss of propyl and the product yields of propylene and HO(2) over a wide range of temperatures, pressures, and residence times. The overall good agreement between predicted and experimental data suggests that this reaction subset is reliable and should be able to properly account for the reactions of propyl radicals with O(2) in propane oxidation. It is also demonstrated that for most conditions of practical interest only a small subset of reactions (e.g., isomerization, concerted elimination of HO(2), and stabilization) controls the oxidation kinetics, which makes it possible to considerably simplify the mechanism. Moreover, we observed strong similarities in the rate coefficients within each reaction class, suggesting the potential for development of relatively simple rate constant estimation rules that could be applied to analogous reactions involving hydrocarbon radicals that are too large to allow accurate detailed electronic structure calculations.

Flow reactor studies of non-equilibrium plasma-assisted oxidation of n -alkanes

“…The current understanding of this reaction is that it proceeds through a barrierless pathway where its equilibrium constant is highly temperature-dependent and favours RO 2 at lower temperatures (T < 700 K) [29]. At increasingly higher pressures, the effectiveness of this reaction increases as the collisional stabilization of RO 2 increases [30]. The intricacies of this type of reaction are quite complex and have been theoretically studied in general for alkyl radicals [29], with few experimental and theoretical studies examining the propyl + O 2 system specifically [31][32][33].…”

Mechanism and kinetics of low-temperature oxidation of a biodiesel surrogate−methyl acetate radicals with molecular oxygen

“…These radicals can react with molecular oxygen to form chemically activated peroxy radicals which can undergo the stabilization, isomerization, and dissociation reactions to form back reactants and/or bimolecular products. Similar to the analogous alkyl systems [18,19,38], these systems are expected to be more complicated (due to the presence of heterogeneous O atom in the ester functional group) and to play a central role in low-temperature combustion of the title fuel [39].…”

“…The composite CBS-QB3 method by Petersson and coworkers [17] was selected because of its capability of predicting thermodynamic properties to ''chemical accuracy'', which is normally defined as within *1 kcal/mol of experimental data. It is worth mentioning that the method has shown to be the effective method for analogous alkyl?O 2 systems [18,19]. Moreover, the method was also intensively used to study thermodynamics and kinetics of similar and/or larger oxygenated systems.…”

“…Note that other empirical and/or ab initio-based methods (e.g., Group Additivity [30,31] and Reaction Class Transition State Theory [32]) to obtain reliable high-pressure rate constants on the fly were extended to the kinetics of the oxygenated species [33,34]. The high-pressure rate constants for the barrierless recombination of MA radicals with O 2 were not calculated in this work but derived from similar alkyl?O 2 systems [18,19].…”

“…Typically, they have the structure of a methyl ester group attached to a long hydrocarbon chain of about [16][17][18][19] carbon atoms (C [16][17][18][19] H x -C(=O)O-CH 3 ). Due to their large size and their chemical/physical complexity (e.g., the introduction of the heterogeneous O atom compared to hydrocarbon fuels), detailed kinetic study on these biodiesel molecules is challenging both experimentally and theoretically.…”