Kinetics of reaction RH + O2 = R + HO2 (RH = normal C1–C14 alkanes): A DFT investigation

Abstract

Alkane dehydrogenation by O2 molecule is a one of the key elementary reactions for combustion, while the reliable reaction rates are very limited. Here UM06-2X/ccpv-dz calculations are carried out on reaction RH + O2 = R + HO2 (RH = normal C1–C141Abbreviations:‘Cn’ and ‘Cn alkane’ stand for normal alkanes with n carbon atoms. C1 for methane, C10 for decane, etc.But ‘H(Cn)’ stands for the H atom on the nth carbon atom of a normal alkane as shown in Fig. 1.A stands for the whole pre-exponential part in a rate constant expression. It is A in Arrhenius equation and is A’*Tn in modified Arrhenius equation. The accent sign is to indicate that the A’ is only a part of the pre-exponential factor. A’’ is used in the more complicated expression in Section 3.4.Atheo is calculated pre-exponential part: Atheo=k/[exp-Ea/RT], where k and Ea are both calculated values.1 alkanes), the rate constants of these reactions at 500–2000 K are then simulated with canonical variational transition state theory and compared with those from experiments and kinetic models. The parameters of Arrhenius equation were fitted using the calculated rate constants and reaction barriers. For primary H, we found that the barrier decreases from C1 to C5, and then does not change obviously from C5 to C14. The barriers of the secondary H are smaller, and are all similar from C3 to C14. Consequently, the rate constants of the primary H of C5–C14 are slightly larger than C1–C4 at low temperature, and the secondary H have larger rate constants than the primary H. The barrier is not always the only decisive factor to judge the trends of the rate constants. We found that the pre-exponential factor for secondary H increases dramatically from C10 to C14, resulting a higher rate constants than other smaller alkanes. We also found that the pre-exponential factor is highly dependent on temperature, therefore the rate constant is better fitted by Arrhenius equations with Tn term. We believe these studies could provide valuable information for increasing the reliability of the current combustion model of alkanes, as well as constructing new combustion model for long alkanes.