On the correlation between kinetic rate constants in the auto-ignition process of some oxygenates and their cetane number: a quantum chemical study

This theoretical study investigates the chemical reactivity of several oxygenates with molecular oxygen in the context of the use of oxygenates as Diesel fuel additives. The general objective consists in testing the hypothesis according to which the cetane number (CN) of a pure compound fuel depends essentially on the rate of initiation of the homogeneous gas phase reaction of this compound with oxygen. To this end theoretical calculations of rate constants for this initiation step were conducted for a series of organic compounds. These molecules were selected to be small enough that these calculations are feasible and because reliable measurements of their CNs were available. Ab initio molecular orbital calculations are carried out with both Hartree-Fock and density functional theory (HF-DFT) methods in conjunction with the 6-311G(p,d) basis set. Structural, energetic, vibrational spectra, thermodynamic and kinetic parameters of the auto-ignition process are evaluated for the gas-phase initiation reactions RH + O-2 - R + O2H, where RH is dimethyl ether, diethylether, methanol, methylal or propane (propane is taken as a reference). The initiation step reaction rate constants at the temperatures 298.15 and 1000 K are calculated using the kinetic model based on the conventional transition state theory (TST) method. In both levels of theory used, a fair correlation is observed between calculated rate constants of the hydrogen atom abstraction process and experimental values of the CN of these pure compounds. Nonetheless, a better correlation is obtained using the HF-DFT method for rate constant calculations at high temperature which is relevant to operating conditions of Diesel engines. The correlation obtained in this work yields a first confirmation of the hypothesis according to which only the rate of the initiation step in the auto-ignition sequences of reactions determines the value of the CN of the pure compounds. (C) 2003 Elsevier Science B.V. All rights reserved.